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Improvement of tensile mechanical properties in a TRIP- assisted steel by controlling of crystallographic orientation via HSQ&P processes
Abstract
The mechanical properties and grain orientation evolution have been investigated in a TRIP-assisted steel in search of enhanced strength and plasticity. A novel combined process (HSQ&P) of Hot Stamping (HS) and Quenching and Partitioning (Q&P) treatment with different initial straining temperatures has been conducted by thermo-mechanical simulation. The microstructure was analyzed in detail using scanning electron microscopy and electron backscattered diffraction. The results revealed that carbon partitioning from saturated martensite into retained austenite led to less distortion in the lattice due to less martensite tetragonality, which decreases stored energy. In addition, the predominance of {011} and {111} crystallographic textures, which are parallel to the normal direction of retained austenite and martensite grains formed in HSQ&P sample, enhanced ductility as it facilitates displacive phase transformation. Thus, it improves the tensile mechanical properties as verified by subsized tensile test. A novel thermo-mechanical process is being proposed for TRIP-assisted steels Based on these findings.
... Complex TMCP+Q&P cycles encompass a high number of parameters, which can modify the microstructure and the mechanical properties at room temperature. These parameters include: (i) austenitizing temperature (full or partial); (ii) isothermal [13][14][15][16]19] and nonisothermal [10][11][12]20,21] deformation at high temperatures, and; (iii) Q&P process, involving rapid cooling rates to optimum quenching temperature (OQT), in order to maximize the retained austenite fraction after the final cooling [24]. The OQT in turn can be affected by the previous hot deformation, which has not been accurately studied in the literature. ...
... Additionally, subsized tensile specimens with reduced gauge lengths were machined from the central area of the Gleeble samples after the thermomechanical processes. The details of the procedure for obtaining the mechanical properties can be found in [24]. ...
... It is notable that although HSQ&P(318) 750 sample exhibited the highest fraction of retained austenite (≈4.8%) among other samples, the predominance of non-compact cleavage {100} planes in both BCC and FCC phases, might deteriorate the mechanical properties. It should be stressed that the volume fraction of retained austenite measured by EBSD is lower than X-ray diffraction measurements (e.g., ≈4.8% by EBSD and ≈10.4% by XRD [24]) due to: the limits of the spatial resolution of the EBSD systems, as the smallest austenite laths cannot be resolved by EBSD but are detectable with X-ray diffraction; because of austenite transformation to martensite during mechanical preparation on the sample surface; and because of the penetration depth of X-rays is greater than that of electrons [24]. Consequently, EBSD measurements are known to provide underestimation in the determination of the volume fraction of retained austenite [46,70,71]. ...
A novel quenching and partitioning process (Q&P) including the hot stamping (HS) process was studied, using two stamping temperatures (750 °C and 800 °C) and two quenching temperatures (318 °C and 328 °C). This combination is here called Hot Stamping and Quenching and Partitioning process (HSQ&P). The partitioning step was performed at 400 °C for 100 s in all cycles. Microstructural features were comprehensively studied using electron backscattered diffraction and nanoindentation techniques. HSQ&P samples showed a good combination of ductility and high-strength due to the presence of: retained austenite, inter-critical ferrite with low stored internal strain energy, grain refinement via DIFT-effect (deformation induced ferrite transformation), martensite, and bainite. Significant internal stress relief was caused by carbon partitioning, which was induced by the DIFT-effect and the partitioning stage. This also led to a considerable stored energy, which was characterized by the Kernel average dislocation and geometrically necessary dislocation analysis. In addition, predominant {110}//strain direction crystallographic texture was identified, which promotes slip deformation and enhances the mechanical properties. Moreover, remarkable amounts of fine film-like retained austenite oriented along compact crystallographic directions (i.e., <111> and <112>) were observed. Finally, subsize tensile test verified the optimum mechanical behavior of HSQ&P specimens.
... The ultimate tensile strength (UTS) of classic C-Mn-Si(Al) TRIP-assisted steels typically lies in the range of 700-980 MPa, while a total elongation (TE) is 20-40 % [13]. Further increase in the level of steel properties can be attained by an additional alloying of steel with the appropriate adjustment of heat treatment mode [14][15][16][17]. Micro-adding of carbide-forming elements leads to increasing the steel's hardenability and strength through changing thermodynamics and kinetics of the transformation and employing different structural mechanisms [18][19][20]. ...
The article is aimed at studying the effect of austempering temperature below and above Ms temperature on the phase-structural state and mechanical properties of 0.2 wt.% C TRIP-assisted steel micro-added with Nb, V, Mo, Cr. The samples were austenitized at a temperature close to the Ac3 point (900 o C) and held at 300 o C (below Ms), 350 o C (close to Ms) and 400 o C (above Ms) for 5-20 min. The work was performed using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction, and tensile/impact testing. It was found that austempering at the aforementioned modes ensures the multiphase structure consisting of carbide-free bainite, tempered martensite, ferrite and retained austenite (in different combinations). The optimal was austempering at a temperature close to Ms which provided an advanced complex of tensile properties (PSE of 23.9 GPa⋅%) and V-notched impact toughness (95 J/cm 2). TRIP-effect contributed to these properties while the strain hardening process tended to be prolonged with increasing the austempering temperature.
... These processing techniques employ plastic deformation and heat treatment to primarily refine the grain size or achieve a preferential crystallographic orientation of grains. The effects of grain size [1][2][3] and crystallographic orientation [4][5][6] on the mechanical properties of ferrous materials have been studied extensively and have been established in the literature. However, a unanimous agreement on the effect of grain size and crystallographic orientation on corrosion resistance of ferrous materials is lacking. ...
This work investigates the role of grain size and recrystallization texture in the corrosion behavior of pure iron in 0.1 M sulfuric acid solution. Annealing heat treatment was applied to obtain samples with different average grain sizes (26, 53 and 87 µm). Optical microscopy, X-ray diffraction and electron backscatter diffraction techniques were used to characterize the microstructure. The EBSD data analysis showed ferrite phase with no inclusions and very low geometrically necessary dislocation density, indicating strain-free grains constituting all samples. The crystallographic texture analysis of the samples revealed that the 26 µm grain size sample had a high volume fraction of {111} oriented grains parallel to the sample surface, while other samples exhibited nearly random crystallographic texture. The electrochemical results from potentiodynamic polarization and electrochemical impedance spectroscopy showed a decrease in corrosion resistance from 87 µm to 53 µm grain size sample and then an increase for the 26 µm grain size sample. This increase was attributed to the dominant effect of recrystallization texture on the corrosion behavior of the sample. The cathodic hydrogen evolution reaction kinetics was found to play a decisive role in the corrosion behavior of iron.
... Han et al. [13] carried out hot forming and QP treatment on the commercial hot forming material 22MnB5, which improved the performance of the final parts. Ariza [14,15] introduced the QP process of TRIP800 steel into the hot forming process, and carried out thermal simulation tests at different strain temperatures. Through this new process, it ensured that there was enough residual austenite to produce TRIP effect at the end of forming. ...
In order to improve the plasticity of hot stamping parts, this paper combines the heat treatment process with the plastic forming of sheet metal, and creatively proposes a new process of hot stamping-carbon partitioning-intercritical annealing. The mechanical properties and microstructure are characterized under the newly proposed process, the quenching-partition (QP) process, and the intercritical annealing (IA) process, respectively. The new process firstly undergoes incomplete austenitizing treatment at 610 °C, then carries out distribution treatment while stamping at 300 °C, and finally conducts annealing treatment in critical zone at 680 °C in two-phase zone. The results show that a multi-phase refined microstructure composed of lath martensite, retained austenite, fresh martensite, and carbides are obtained by the new process. Most of the retained austenite is shaped in the thin film due to martensitic shear, in which carbon and manganese elements diffuse from martensite to austenite by heat treatment, thus stabilizing the retained austenite. Retained austenite with a volume fraction of 33.7% is obtained in the new process. The retained austenite with higher content and better stability is completely consumed during the stretching process, which gives full play to discontinuous TRIP effects, thus delivering the elongation of 36.8% and the product of strength and elongation (PSE) reached as high as 43.6 GPa%.
... They confirmed that carbon partitioning from martensite provides a more satisfactory explanation, although bainite formation during partitioning cannot be wholly excluded. Finally, Ariza et al. [4,6,12,13] proposed that the crystallographic orientations and interface boundaries between austenite and ferrite/martensite phase could influence the carbon partitioning mechanism. This paper studied the tempering behavior of a commercial medium carbon-silicon steel followed by tempering through dilatometry to design a novel Q&P path. ...
The constant demand for increasing the strength without ductility loss and production cost encourages industrial and academic societies to propose novel heat treatment processing of commercial steel grades. To improve the mechanical properties of commercial spring steel, a novel quenching and partitioning (Q&P) processing was designed to deliver a complex and desirable nanostructured multicomponent microstructure by controlling the carbon partitioning kinetics. Furthermore, the partitioning of excessive carbon from saturated martensite into untransformed austenite enhances the formation of transition carbides during tempering between 130 - 280 °C. Electron microscopy confirmed a complex multicomponent structure containing BCC tempered lath combined with retained austenite and nanocarbides particles within the tempered laths. Such multicomponent lath-type structure obtained by designed Q&P heat treatment on commercial carbon-silicon spring steel revealed localized mechanical resistance varying from 4.92 GPa for the QP-220-375-400 to 8.22 GPa for the QP-220-325-400 samples determined by nanoindentation test. Moreover, the tensile test showed high ultimate tensile strength and a yield strength up to 1400 MPa and 975 MPa, respectively, in the QP-220-375-400 sample due to a set of complex multicomponent lath-type refined structures designed by Q&P coupled with bainitic transformation, with good strain to fracture (∼0.12%).
... The dominance of green colour in KAM map (Fig. 11a) indicated higher stored energy due to lattice distortion and dislocation accumulation in smaller grains. The sample welded with the highest heat input (C70) is designated with sharper peak and dominance of blue colour in KAM corroborating larger grain size [56]. ...
Fabrication of superconductive radio frequency (SRF) cavity by electron beam welding (EBW) of pure Niobium (Nb) has been a topic of discussion for quite some time. The influence of beam current on the weld properties of Nb plates has been investigated in light of mechanical properties and microstructure. The widths of the fusion zone (FZ) and heat-affected zone (HAZ) of the welded samples increase with increasing the beam current. The hardness profile across the base metal (BM) to FZ followed a typical symmetrical pattern with a plateau across the FZ. The maximum drop of hardness at FZ compared to BM is only about 15%, making the EBW process a suitable one for the joining of Nb plates for the application of SRF cavity. The development of mechanical anisotropy of the welded joints due to differential heat transfer and the associated change in microstructure in FZ and HAZ has been evaluated by conducting tensile tests along different directions to the line of welding. Elongation of the welded joint is the maximum (~94%) for samples with the tensile direction parallel to the welding direction for a beam current of 70 mA. The Lankford constant (r-value) at different angles (0, 45, and 90°) to the tensile axes has been measured and correlated with crystallographic texture measured by Electron Backscattered Diffraction (EBSD). The dominance of the {554} <225> component for samples with larger heat input is found to be responsible for higher r-values, maximum formability, and lower Kernal average misorientation (KAM) profile. A heat transfer simulation has been used to simulate the dimensions of the weld pool and subsequently correlate its impact on various properties.
... It is relevant to develop alternative automobiles with outstanding mechanical performance, higher safety, less fuel consumption and lower cost for the automotive industry in the face of global problems such as energy dilemma and greenhouse effect (Ref 1). Transformation-induced plasticity (TRIP) steels have been extensively studied to obtain excellent properties of tensile strength, elongation, formability and energy absorption ability (Ref [1][2][3]. The typical microstructure of TRIP steels consists of ferrite, retained austenite (RA), bainite or martensite ( . ...
The determining significance of isothermal holding on microstructure and mechanical properties of a transformation-induced plasticity steel (Fe-1.67Mn-1.32Al-0.55Si-0.47C) was studied with multiple techniques including x-ray diffraction, scanning electron microscopy and transmission electron microscopy. The objective was to design an optimal isothermal holding treatment for medium-carbon TRIP steels with ultrahigh strength and high elongation. A critical analysis of the experimental observations is presented. After isothermal holding treatment, the microstructure mainly consisted of ferrite, bainite, retained austenite, and a small amount of martensite-austenite island. The volume fraction of RA first increased from 28 to 32% with the increase of temperature from 380 to 420°C, and then decreased dramatically to a minimum value of 23% with further increasing temperature to 450°C. However, carbon content in RA decreased from 1.42 to 1.18% with the increase of temperature. A tensile strength of 1250 MPa and a maximum elongation of 55% were obtained at 420°C because of the optimal combination of RA and carbon content. The highest yield strength of 660 MPa was obtained at 380°C and the highest tensile strength of 1480 MPa was obtained at 450°C, respectively. Less stable RA transformed to martensite, while RA with a high stability was retained during tensile straining.
... TRIP steels have an excellent combination of strength and ductility, due to a high work hardening rate caused by the transformation of retained austenite to martensite during plastic deformation. The martensite transformation of the retained austenite avoids localised deformation, leading to the comparatively large homogeneous elongation obtained in these steels [12,13]. The martensite transformation takes place during sheet metal forming and can be exploited when making sections of highly complicated shapes enabling the safety improvement by increasing the amount of absorbed collision energy. ...
... 9(a-d), indicated the geometrical dislocation density accumulation in grains or subgrains. It can be noticed that the rim sample has shifted to a higher misorientation angle due to severe straining leading to lattice distortion and dislocation accumulation in rim area compared to TMT core and MAHR steel [38]. However, the misorientation angles were reduced in MAHR steel for development of energetically homogenous microstructure. ...
... Nowadays, an increasing number of investigations attempt to integrate Q&P processing into the steel forming processes in order to save energy and reduce the necessity of reheating [12][13][14]. For example, partitioning has been performed after direct quenching from the hot stamping temperature [15][16][17]. Similarly, the concept of hot-rolling direct quenching and partitioning (HDQP) has been introduced to eliminate the conventional partitioning step in conventional single-step or double-step Q&P [18,19]. ...
The objective of Quenching and Partitioning (Q&P) processing is to enable the carbon diffusion from martensite to austenite, thereby increasing the austenite stability. However, it is recognized that carbon in martensite cannot be fully partitioned into austenite, indicating the potential for improvement. Retained austenite is subjected to compressive stresses subsequent to martensitic transformation. Compressive stresses in austenite oppose the volume expansion due to its carbon enrichment in the partitioning step, thereby restricting the extent of partitioning. In the present study, the effect of external tensile stress on the efficiency of carbon partitioning is investigated. For this purpose, a Fe-12.6Cr-2.1Si-0.46C steel was quenched from 1250 °C to-20 °C to form approximately 64 vol.% martensite. Subsequently, partitioning was done at 400 °C under tensile stresses up to 1040 MPa. Tensile stresses were applied at 250 °C and maintained during heating to 400 °C and the isothermal holding time of 30 min at 400 °C. The results confirm that external tensile stresses boost the carbon partitioning into austenite. The carbon enrichment of austenite increases at higher stress levels.
In this study, we investigate the microstructures and mechanical properties of quenching and partitioning (Q&P) steels prepared with hot-rolled (HR) and cold-rolled (CR) sheets at different annealing temperatures (partial austenitisation). The ferrite and retained austenite are present in laths and blocks in HR-Q&P samples but are mainly blocky in CR-Q&P samples. Compared with CR-Q&P, the HR-Q&P samples have higher content of retained austenite, better elongation and higher product of strength and elongation (PSE). The lath-shaped retained austenite in HR-Q&P samples with a higher ratio of surface to volume is conducive to homogenization of carbon in austenite grains, thus improving the thermal stability and content of retained austenite. The lath-shaped retained austenite transforms into martensite obviously until the strain exceeds 5% and exhibits higher mechanical stability than coarse blocky retained austenite. When annealed at 810°C, HR-Q&P and CR-Q&P samples present best PSEs. Indeed, the PSE of HR-810°C-Q&P sample is 62.8% higher than that of CR%-810°C-Q&P. The better elongation and higher PSE possessed by HR-810°C-Q&P sample are linked with the higher content of retained austenite and the presence of two kinds of retained austenite with different mechanical stability. The retained austenite with different mechanical stability in HR-810°C-Q&P sample, contributes to continuous transformation during tensile deformation, thereby improving the elongation. Meanwhile, the lath-shaped structures can delay the formation of voids and microcracks, thereby improving the elongation.
Three different compositions of C–Mn–Si–Al type Q&P steels, A, B and C, were subjected to thermo-mechanically controlled processing (TMCP) at two different rolling temperatures viz. 1075 °C and 1150 °C, using two different levels of strain 0.51 and 1.1, given in single pass, followed by direct quench and partitioning (DQP) to produce a martensite – austenite structure. It was observed that Si and Al significantly influenced the formation of prior austenite grains (PAGs) during TMCP and thereby the evolution of microstructure after the subsequent Q&P treatment. With the help of constitutive models, it was found that mainly dynamic recrystallization (DRX) along with/without meta-dynamic recrystallization (MRX) were likely to occur during TMCP and their effects would be retained by immediate quench from the high temperature of processing. Electron back-scattered diffraction (EBSD) results confirmed that the PAGs were essentially DRX grains. Rolling at the lower temperature produced finer PAGs, thereby favouring the formation of larger amount of retained austenite. Effect of strain, which enhanced the enrichment of carbon in the austenite, was more pronounced at the lower rolling temperature and at higher applied strain. More than the grain refinement, dislocation density was found to be the dominant strengthening mechanism in the martensite. A maximum dislocation density of 2.18 × 10¹⁵ m⁻² was observed for the steel rolled with 1.1 strain at 1150 °C. Higher rolling temperature favoured higher dislocation density in the steels, owing to the attainment of higher martensite carbon content. Consequently, TMCP with a higher rolling temperature favoured higher strength properties. In fact, the maximum strength of 1593 MPa, with yield strength of 1042 MPa and total elongation of 13% were achieved in Steel-A rolled at 1150 °C with 1.1 strain (1.1-DQP-1150 °C Steel A). After evaluating the different strengthening mechanisms operating in these steels, it was concluded that the maximum strength achieved in Steel A was primarily due to its alloy composition.
We systematically investigated the effects of varying α phase fraction on the mechanical properties and deformation mechanisms in a metastable β-51.1Zr-40.2Ti-4.5Al-4.2V (wt. %) alloy containing mixed α + β phase. Various heat treatments from single β phase regime to dual α+β phase regime were imposed in order to alter the volume fraction of α phase. The results show that the triggering stress increases and the total elongation decreases with the increasing α phase fraction. As the α phase fraction increases, the work hardening rate curve changes from an obvious parabolic evolution to a monotonically decreasing evolution and the work hardening effect is gradually reduced. Co-existence deformation mechanisms of kink bands, deformation-induced α′ martensite and {101‾1}α′ mechanical twinning were detected in low α phase fraction samples but not in high α phase fraction sample. In intermediate α phase fraction sample, only deformation-induced α′ martensite was activated. For the first time, the effects of α phase fraction on the deformation-induced α′ martensite, {101‾1}α′ mechanical twinning and kink bands were observed in metastable β-Zr alloy. These effects of varying α phase fraction on the mechanical properties and deformation mechanisms are discussed based on the β stability and the β domain size.
A high silicon, medium carbon cast steel was designed and heat-treated in order to develop microstructures composed of carbide-free bainite and small amounts of free ferrite, with the aim of obtaining high strength cast steels with improved ductility. Because of microsegregation, it was observed that ferrite present in partially austenitised samples is mostly present at the highly alloyed zones, creating an interconnected network even for low proportions of this phase. Despite the coarse solidification structure and marked microsegregation in the cast steel, the mechanical properties obtained for both fully bainitic and bainitic-ferritic microstructures largely satisfy the minimum standard requirements for high strength cast steels and are similar to those reported for wrought steels of similar microstructures.
In order to obtain the products with high strength and ductility, the hot stamping technology of TRIP steel has been getting more attention in recent years. In this study, the effects of cooling rate on the mechanical properties of a typical TRIP780 steel used in hot stamping process were investigated with tensile tests, bending tests and impact tests. It was found that even though a lower cooling rate for the quenching of TRIP780 steel led to the formation of the bainite and ferrite mixture phases, the mechanical properties of the material containing these phases were similar to those of the material containing martensite only. TRIP780 steel exhibited twice the elongation with a similarly high strength as compared with the conventional boron steel, and its mechanical properties are insensitive to cooling rate, which was approved by the hot stamping experiment of a U-cap part.
The enhancement of the fracture toughness is essential for opening the possible range of applications of advanced high-strength steels, while the focus in the literature is primarily on the strength–ductility compromise. A high fracture toughness is indeed needed for energy absorbing components as well as to limit edge cracking sensitivity during part forming. This study investigates the tensile properties and the fracture toughness of various quenched and partitioned microstructures. The fracture resistance is evaluated using double-edge notched tension tests. While the uniform elongation continuously increases with the retained austenite (RA) fraction, the fracture toughness shows a maximum at intermediate RA content. For the highest amount of RA, the relatively low fracture toughness is mainly attributed to the formation of brittle necklace of fresh blocky martensite in the fracture process zone due to a high stress triaxiality, inducing an intergranular fracture mode. For intermediate RA fraction, the RA morphology evolves from blocky to film type, leading to a transition from intergranular to ductile fracture mode, and the RA-to-martensite transformation contributes to a higher total work of fracture compared to tempered martensitic steel. A proper control of both the amount and morphology of RA during microstructure design is thus essential to generate the best compromise between tensile properties and fracture toughness.
Advanced high strength steels (AHSS), with yield strengths over 300 MPa and tensile strengths exceeding 600 MPa, are becoming more noticeable in vehicle manufacturing. A novel processing route of a TRIP-assisted steel was developed. Characterization and modelling techniques were used to establish correlations between processing, microstructure and mechanical properties. Quenching and partitioning (Q&P) and a novel process of hot straining (HS) and Q&P (HSQ&P) treatments have been applied to a TRIP-assisted steel in a Gleeble ®3S50 thermo-mechanical simulator. The heat treatments involved intercritical annealing at 800 oC and a two-step Q&P heat treatment with a partitioning time of 100 s at 400 oC. The effects of high-temperature isothermal deformation on the carbon enrichment of austenite, carbide formation and the strain-induced transformation to ferrite (SIT) mechanism were investigated. Carbon partitioning from supersaturated martensite into austenite and carbide precipitation were confirmed by means of atom probe tomography (APT). Austenite carbon enrichment was clearly observed in all specimens, and in the HSQ&P samples it was slightly greater than in Q&P, suggesting an additional carbon partitioning to austenite from ferrite formed by the SIT phenomenon. By APT, the carbon accumulation at austenite/martensite interface was clearly observed. The newly developed combined process is promising as the transformation induced plasticity can contribute to the formability and energy absorption, contributing to fill the gap of the third generation of high-strength steels.
In current work, the effect of different aging treatments on microstructure and tensile behavior of a novel designed maraging‐TRIP steel containing 12 wt% Mn is studied. Three distinct phases including thermally ϵ‐martensite, α′‐martensite, and reversed austenite are developed through aging. Thermally HCP‐ϵ lath martensite is nucleated at prior austenite grain and developed inside the grains during aging. However, increasing the aging temperatures led to disappearing the prior austenite grain boundaries due to reverse austenite transformation. A significant increasing in tensile strength and ductility is found in samples aged at 450 and 550 °C. Finally, the grain orientations studies show a good agreement with Kurdyumov–Sachs (K–S) orientation relationship between distinct phases developed in aged sample at 450 °C. The aging response of a maraging‐TRIP steel is studied in terms of phase transformation, texture evolutions, as well as mechanical properties. The results shows that HCP‐ϵ martensite is nucleated at prior austenite grains and developed inside the grains during aging, where the K–S orientation relationship is realized for the BCC‐α′ martensite, HCP‐ϵ martensite, and FCC‐γ austenite phases.
Quenching and partitioning (Q&P) and a novel combined process of hot straining (HS) and Q&P (HSQ&P) treatments have been applied to a TRIP-assisted steel in a Gleeble®3S50 thermomechanical simulator. The heat treatments involved intercritical annealing at 800 °C and a two-step Q&P heat treatment with a partitioning time of 100 seconds at 400 °C. The “optimum” quench temperature of 318 °C was selected according to the constrained carbon equilibrium (CCE) criterion. The effects of high-temperature deformation (isothermal and non-isothermal) on the carbon enrichment of austenite, carbide formation, and the strain-induced transformation to ferrite (SIT) mechanism were investigated. Carbon partitioning from supersaturated martensite into austenite and carbide precipitation were confirmed by means of atom probe tomography (APT) and scanning transmission electron microscopy (STEM). Austenite carbon enrichment was clearly observed in all specimens, and in the HSQ&P samples, it was significantly greater than in Q&P, suggesting an additional carbon partitioning to austenite from ferrite formed by the deformation-induced austenite-to-ferrite transformation (DIFT) phenomenon. By APT, the carbon accumulation at austenite/martensite interfaces was observed, with higher values for HSQ&P deformed isothermally (≈ 11 at. pct), when compared with non-isothermal HSQ&P (≈ 9.45 at. pct) and Q&P (≈ 7.6 at. pct). Moreover, a local Mn enrichment was observed in a ferrite/austenite interface, indicating ferrite growth under local equilibrium with negligible partitioning (LENP). © 2018 The Minerals, Metals & Materials Society and ASM International
Digital Image Correlation (DIC) is an important and widely used non-contact technique for measuring material deformation. Considerable progress has been made in recent decades in both developing new experimental DIC techniques and in enhancing the performance of the relevant computational algorithms. Despite this progress, there is a distinct lack of a freely available, high-quality, flexible DIC software. This paper documents a new DIC software package Ncorr that is meant to fill that crucial gap. Ncorr is an open-source subset-based 2D DIC package that amalgamates modern DIC algorithms proposed in the literature with additional enhancements. Several applications of Ncorr that both validate it and showcase its capabilities are discussed.
The capability of strain-induced ferrite transformation in refining the microstructure of a promising transformation-induced plasticity (TRIP) steel has been studied in the present investigation. This was performed employing the hot compression testing technique to deform the experimental TRIP steel at temperatures below the austenite-to-ferrite transformation temperature (i.e., Ac3). The strain-induced ferrite transformation (DIFT) has been identified as the most effective mechanism contributing to the grain refinement at temperatures (860 and 880 degrees C) just below the corresponding Ac3. The influence of this phenomenon on the mechanical properties of the material during and after deformation has been fully addressed. The results show that the DIET would lead to a work softening behavior during deformation. However, the grain refinement through DIET has significantly improved the mechanical properties of the deformed specimens characterized by the shear punch testing method.
The martensitic transformation behaviour of the metastable austenite phase in low-alloyed transformation-induced plasticity (TRIP) steels has been studied
in situ
using high-energy X-ray diffraction during deformation. The austenite stability during tensile deformation has been evaluated at different length scales. A powder diffraction analysis has been performed to correlate the macroscopic behaviour of the material to the observed changes in the volume phase fraction. Moreover, the austenite deformation response has been studied at the length scale of individual grains, where an in-depth characterization of four selected grains has been performed, including grain volume, local carbon concentration and grain orientation. For the first time, a high-resolution far-field detector was used to study the initial and evolving structure of individual austenite grains during uniaxial tensile deformation. It was found that the austenite subgrain size does not change significantly during tensile deformation. Most austenite grains show a complete martensitic transformation in a single loading step.
When a single crystal deforms by glide which is unevenly distributed over the glide surfaces the lattice becomes curved. The constant feature of distortion by glide on a single set of planes is that the orthogonal trajectories of the deformed glide planes (the c-axes in hexagonal metals) are straight lines. This leads to the conclusion that in polygonisation experiments on single hexagonal metal crystals the polygon walls are planes, while the glide planes are deformed into cylinders whose sections are the involutes of a single curve. The analysis explains West's observation that the c-axes in bent crystals of corundum are straight lines. For double glide on two orthogonal sets of planes there is a complete analogy between the geometrical properties of the distorted glide planes and those of the "slip-lines" in the mathematical theory of plasticity. More general cases are discussed and formulae are derived connecting the density of dislocations with the lattice curvatures. For a three-dimensional network of dislocations the "state of dislocation" of a region is shown to be specified by a second-rank tensor, which has properties like those of a stress tensor except that it is not symmetrical.
Hot stamping of steel sheets using water or nitrogen cooling media was studied on a laboratory scale. Sheets of grade 22MnB5 boron steels in three different thicknesses were investigated and the results of experimental hot stamping tests were considered. Microstructural analysis, linear and surface hardness profiling as well as tensile tests of formed samples were carried out. After hot stamping, mostly fully martensitic microstructures, which yield ultra high strength levels, were produced. It is concluded that die cooling media, i.e., water or nitro- gen, have a significant effect on material properties after hot stamping. Using liquid nitrogen as coolant in the punch instead of water in- creases yield strength by 50 to 65MPa. Moreover, the evolution of the temperature and force during the hot stamping process was simu- lated by using a coupled thermomechanical FEM program. The results of numerical simulation and experimental results are in good agreement.
The carbon supersaturation of bainitic ferrite was investigated by means of atom probe tomography in three steels with different carbon and silicon contents, to elucidate the effect of transformation temperature and the reaction velocity on the mechanisms controlling bainite formation with and without the interference of cementite precipitation. Results indicated no difference in the growth mechanism over the temperature range investigated. These results provide new evidence that the bainite transformation is essentially martensitic in nature.
Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bain-itic-austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austem-pering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation.
We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spec-troscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe 50 Mn 30 Co 10 Cr 10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe 20 Mn 20 Ni 20-Co 20 Cr 20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10e35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects.
A novel two-step intercritical annealing process was designed for an ultra-low carbon medium manganese steel plate. Excellent mechanical properties with yield strength of 590 MPa, tensile strength of 840 MPa, total elongation of 28.5% and high impact energy of 106 J at −80 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite together with a small amount of martensite after the two-step intercritical annealing. Both lath-like and blocky retained austenite with volume fraction of ~25% and relatively poor stability were obtained. The submicron-sized lath-like retained austenite exhibited Nishiyama-Wassermann (N-W) orientation relationship with the neighboring martensitic ferrite lath. The fine grain size played a crucial role in stabilizing austenite during phase transformation by significantly lowering Ms temperature and increasing the elastic strain energy. The overall stability of retained austenite during deformation was considered to be mainly governed by the chemical composition of the studied steel. The mechanism of toughening was elucidated. The superior low-temperature toughness was associated with TRIP effect of metastable retained austenite, which relieved the local stress concentration, enhanced the ability to plastic deformation and delayed the initiation and propagation of microcracks.
Thermomechanical simulation of quenching, hot stamping, and quenching and partitioning processes of a high-strength TRIP-assisted steel were carried out in a Gleeble®3S50 thermo-mechanical simulator, coupled to the synchrotron X-ray diffraction line. The microstructures and mechanical properties were analyzed using Field Emission Gun Scanning Electron Microscopy (FEG-SEM), X-ray diffraction, and nanoindentation. The microstructures of thermomechanical treated specimens were modeled using the Object Oriented Finite Element (OOF) technique. The modeled microstructures were then fed into a finite element model to predict the mechanical behavior. By using a reverse algorithm method, the elasto-plastic mechanical properties of different microconstituents were determined. This was done through the analysis of instrumented nanoindentation loading-penetration curves. Tensile properties of the thermomechanical processed steels were measured by tensile testing of subsized specimens cut from samples processed on the Gleeble®3S50. The comparison between the experimental results and those of the reverse algorithm and the OOF modeled microstructure showed quite good agreement.
In this study, fast-heating (300 °C/s) was applied to achieve the intercritical annealing of a cold-rolled quenching and partitioning (Q&P) steel with a wide range of soaking temperature (770-850 °C) and soaking time (0-120 s) for austenitization. The dilatometry and microstructural analysis revealed that in contrast to the conventional heating rate (5 °C/s), a fast-heating rate led to an accelerated transformation and grain refinement of the prior austenite in the Q&P samples. The microstructures of the Q&P treated samples subjected to different intercritical annealing conditions were studied in detail by various material characterization techniques including electron microscopy, electron probe micro-analysis, and X-ray diffraction. The working hardening behavior and the mechanical stability of the retained austenite were discussed on the basis of the typical stress-strain curves. The statistics of the ultimate tensile strength vs. total elongation of each sample under the orthogonal annealing conditions suggest that, for the investigated steel, the fast-heating process improved the strength with approximately 90 MPa on average within the elongation ranged from 17 to 27%.
Transmission electron microscopy and in situ synchrotron high-energy X-ray diffraction were used to investigate the martensitic transformation and lattice strains under uniaxial tensile loading of Fe-Mn-Si-C-Nb-Mo-Al Transformation Induced Plasticity (TRIP) steel subjected to different thermo-mechanical processing schedules. In contrast with most of the diffraction analysis of TRIP steels reported previously, the diffraction peaks from the martensite phase were separated from the peaks of the ferrite-bainite α-matrix. The volume fraction of retained γ-austenite, as well as the lattice strain, were determined from the diffraction patterns recorded during tensile deformation. Although significant austenite to martensite transformation starts around the macroscopic yield stress, some austenite grains had already experienced martensitic transformation. Hooke’s Law was used to calculate the phase stress of each phase from their lattice strain. The ferrite-bainite α-matrix was observed to yield earlier than austenite and martensite. The discrepancy between integrated phase stresses and experimental macroscopic stress is about 300 MPa. A small increase in carbon concentration in retained austenite at the early stage of deformation was detected, but with further straining a continuous slight decrease in carbon content occurred, indicating that mechanical stability factors, such as grain size, morphology and orientation of the retained austenite, played an important role during the retained austenite to martensite transformation.
We report an in-situ study of the deformation-induced rotation and transformation of austenite grains in a low-carbon steel treated by the quenching and partitioning process using electron back-scattered diffraction and uniaxial tension experiments. It was found that retained austenite could be classified into four types according to different locations in the microstructure: retained austenite at triple edges, twinned austenite, retained austenite distributed between martensite and retained austenite embedded completely in a single ferrite. The results showed that at the early stage of deformation, the retained austenite at the triple edges and twinned austenite transformed easily, while the retained austenite at the boundaries between martensite and that embedded completely in a single ferrite rotated with no transformation; and did not transform until a large deformation was provided. This phenomenon implies that the retained austenite at the boundaries between martensite and that embedded completely in a single ferrite are more capable of resisting deformation. From the observations of Schmid factor maps and the texture of retained austenite, it can be concluded that the rotation of retained austenite followed a particular slip plane and slip direction. Moreover, the rotation of retained austenite could improve the ductility of the material. In comparison with the film-like retained austenite distributed between martensite, the retained austenite embedded completely in a single ferrite could resist a larger rotation angle, i.e. the latter could contribute more to the ductility of the steel. In addition, from the analysis of kernel average misorientation that the strain distribution mainly concentrated near the α - γ phase boundaries and in the interior of martensite, and the rotation angles and dislocation density of austenite increase with increasing strain.
The quenching and partitioning (Q&P) process is studied in Ti-bearing low-carbon steel. Detailed characterization of the microstructural evolution is performed by means of optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results indicate that the investigated steel subjected to the Q&P process forms a multiphase microstructure of primarily lath martensite, with small amounts of plate-type martensite and retained austenite. The distribution and morphology of the retained austenite are observed; moreover the relationship between the phase fraction of the retained austenite, its carbon concentration, and the partitioning conditions is established. Carbides preferentially precipitate within the plate-type martensite at first, and gradually form in the martensitic laths over time during the partitioning step. Additionally, titanium precipitations contribute to both the refinement of prior austenite grains and the improvement of strength by precipitation strengthening. The results of mechanical properties testing indicate that the samples partitioned at 400 °C exhibit a superior combination of strength and elongation, with products of the two properties ranging between 19.6 and 20.9 GPa%. Based on analysis of work hardening behavior it is determined that the higher ductility is closely related to the higher phase fraction and/or stability of retained austenite.
A quenching–long partitioning (Q–LP) heat treatment has been proposed for large-scale industrial production of steel components. Compared to the conventional quenching–tempering (Q–T) and quenching–partitioning (Q–P) process, the steel treated by air-quenching from austenitizing temperature and 60 min-partitioning at 200 °C exhibits the best combination of strength and toughness. With a nearly unchanged product of strength and elongation of around 18 GPa%, the room-temperature impact toughness has been significantly improved from 84 J/cm2 for the air Q–T treatment to 104 J/cm2 for the Q–LP treatment, which is attributed to the more retained austenite and the effective relief of the microstresses. The multi-phase microstructure of the Q–LP treated steel contains martensite, bainite and retained austenite exhibiting two different morphologies, i.e., one is the inter-lath plates, and the other is the austenite films within the bainite plates.
Cold rolled sheets of a low carbon quenching and partitioning (Q&P) steel grade were subjected to heat treatment cycles, which were designed by dilatometric experiments and optimised with respect to the quenching temperature, partitioning temperature and partitioning time. Characterisation of the retained austenite was carried out by electron backscattered diffraction, whereas the carbides were studied by scanning electron microscopy (SEM) and differential scanning calorimetry. The mechanical properties were evaluated by tensile testing and linked with retained austenite fractions and carbon contents, determined by X-ray diffraction. Conclusions are drawn concerning the influence of the kinetics of partitioning on the microstructure in terms of optimal austenite fraction in the martensitic matrix, its C content and ensuing mechanical properties.
A method involving the decomposition of the X-ray diffraction (XRD) peaks for the single wavelengths Kα1 and Kα2 was used to quantify the amount of retained austenite at levels lower than 5% in low-carbon high-manganese steels. By applying this method, it was possible to use the two main peaks of austenite (γ) and the two main peaks of ferrite (α) in the calculations, despite the partial overlapping of the (111)γ and (110)α peaks. The diffraction peaks were modeled with the Pearson VII equation using a nonlinear least-squares optimization technique. This allowed the integrated intensities of the XRD peaks to be calculated using only the Kα1 side. The method was used to measure the levels of retained austenite in samples of a metal-inert gas steel welding rod cooled at the rates of 10 °C/s and 1.6 °C/s. The accuracy of the method was determined by performing six measurements in different directions in both the longitudinal and the transverse section of the 1.6 °C/s sample.
This paper examines the relationship between as-formed microstructure and mechanical properties of a hot stamped boron steel used in automotive structural applications. Boron steel sheet metal blanks were austenized and quenched at cooling rates of 30 degrees C/s, 15 degrees C/s and 10 degrees C/s within a Gleeble thermal-mechanical simulator. For each cooling rate condition, the blanks were simultaneously deformed at temperatures of 600 degrees C and 800 degrees C. A strain of approximately 0.20 was imposed in the middle of the blanks, from which miniature tensile specimens were extracted. Depending on the cooling rate and deformation temperature imposed on the specimens, some of the as-quenched microstructures consisted of predominantly martensite and bainite, while others consisted of martensite, bainite and ferrite. Optical and SEM metallographraphic techniques were used to quantify the area fractions of the phases present and quasi-static (0.003 s(-1)) uniaxial tests were conducted on the miniature tensile specimens. The results revealed that an area fraction of ferrite greater than 6% led to an increased uniform elongation and an increase in n-value without affecting the strength of the material for equivalent hardness levels. This finding resulted in improved energy absorption due to the presence of ferrite and showed that a material with a predominantly bainitic microstructure containing 16% ferrite (with 257 HV) resulted in a 28% increase in energy absorption when compared to a material condition that was fully bainitic with a hardness of 268 HV. Elevated strain rate tension tests were also conducted at 10 s(-1) and 80 s(-1) and the effect of strain rate on the ultimate tensile strength (sigma(UTS)) and yield strength (sigma(Y)) was shown to be moderate for all of the conditions. The true stress versus effective plastic strain (flow stress) curves generated from the tensile tests were used to develop the "Tailored Crash Model II" (TCM II) which is a strain rate sensitive constitutive model that is a function of effective plastic strain, true strain rate and area fraction of martensite, bainite and ferrite. The model was shown to accurately capture the hardening behaviour and strain rate sensitivity of the multiphase material conditions examined.
Mechanism of failure by hydrogen induced cracking (HIC) in pipeline steel has not been extensively investigated in the past. In the present work, an API X70 pipeline steel was electrochemically charged with hydrogen for different durations in order to find crack nucleation and propagation sites. After 3 h charging, suitable regions for crack initiation and propagation were found. These regions were studied by color metallography, EDS and EBSD techniques. The results brought out that HIC cracks nucleated from regions rich of manganese sulphide inclusions, some complex carbonitride precipitates such as (Ti, Nb, V) (C, N) and further propagated through the segregation area of some elements, such as manganese, carbon, silicon and sulfur. It is worth-mentioning that all these potential sites for crack nucleation and propagation appeared at the center of cross section of the specimens. EBSD measurements were carried out at the center of cross section in as-received and hydrogen-charged specimens in order to find a pattern between microstructural parameters (texture, grain boundary nature and Taylor factor) and probability of HIC cracking. The results showed that fine grain colonies (less than 3.5 μm in length) with dominant ND||<001> orientations were prone to intergranular HIC crack propagation. The grain boundaries identified between two grains with a mismatch in Taylor factor were more susceptible to intergranular fracture while transgranular fracture occurred in fragmented grains with high and similar Taylor factor that were less likely to yield. HIC cracking occurred in a wide range of orientations such as ND||<123>, ND||<100>, ND||<112>, ND||<110> and even ND||<111>; however, role of high angle grain boundaries and type of fracture would be of great importance in crack propagation.
The orientation dependence of the austenite-to-martensite transformation during uniaxial tensile testing was modelled using the phenomenological theory of martensite crystallography and the mechanical driving force. It was validated experimentally by means of electron backscatter diffraction measurements on a pre-defined zone of a quenched and partitioned steel during interrupted tensile tests. A close match is obtained between the predictions of the model and the experimental observations. (C) 2014 International Union of Crystallography
Three different heat treatment processes have been proposed as a fundamental method to produce three kinds of TRIP-aided steels with polygonal ferritic matrix (F-TRIP), bainitic matrix (B-TRIP) and martensitic matrix (M-TRIP) in a newly designed low alloy carbon steel. By means of dilatometry study and detailed characterization, the relationships among transformation, microstructure and the resulting mechanical behavior were compared and analyzed for the three cases. The work hardening of the samples was evaluated by calculating the instantaneous n value as a function of strain. The M-TRIP sample exhibits the highest strength with the highest work hardening rate at low strains and subsequent rapid descending at high strains. In contrast, the B-TRIP sample has relatively high continuously constant work hardening behavior over strain levels greater than 0.067. The difference in work hardening behavior corresponds directly to the rate of the retained austenite–martensitic transformation during straining, which can be attributed to the carbon content, the morphology of the retained austenite and the matrix microstructure in the respective TRIP-aided samples.
Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel
sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation.
The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the
material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels
up to about 20%. A comparison of the XRD and EBSD results revealed that the retained austenite grains at the surface have a stronger
tendency to transform than the austenite grains in the bulk of the material. The deformation-induced changes of individual austenite
grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have
different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded
austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite
grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite
grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain
within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation
behavior is also a significant factor contributing to enhancement of the ductility.
The unique impact of two types of ferrite, intercritical ferrite and δ-ferrite on austenite stability and deformation behavior of hot-rolled Fe–11Mn–4Al–0.2C transformation-induced plasticity (TRIP) steel were studied. Each of the two ferrites exhibited their respective role in enhancing the stability of austenite, contributing to superior ductility. An optimized quenching at 800 °C and tempering at 200 °C adopted on the as-hot-rolled steel led to a ferrite–austenite mixed microstructure that was characterized by excellent combination of tensile strength of 1082 MPa and elongation of 35%, and a three-stage work hardening behavior.
We report the mechanistic explanation of the variation in Lüders strain in fine-grained transformation-induced plasticity-assisted steel. The austenite stability is demonstrated to have a profound influence on the Lüders strain. Furthermore, it is shown unambiguously using a thermodynamic analysis that the transformation of austenite is strain-induced. The work results in a generic method of distinguishing the cause of martensitic transformation during tensile tests, given that both stresses and strains are necessarily present beyond the yield point.
Assuming a spherical nucleating particle, the effect of particle size and surface properties upon nucleation efficiency is investigated. A general result is derived which is then applied to the condensation, sublimation, and freezing of water on foreign nuclei. The size effect is found to become important in the range 100–1000 A of particle radius. For particles larger than this, nucleation efficiency is substantially independent of size, while for smaller particles the efficiency is very greatly reduced.
The results of a study of using neutron diffraction for determining the retained austenite content of TRIP steels are presented. The study covers a wide area of materials, deformation modes (uniaxial, biaxial and plane strain), strains, and the retained austenite content as a result of these variables. It was determined using basic principles of statistics that a minimum of two reflections (hkl) for each phase is necessary to calculate a phase mass fraction and the associated standard deviation. Texture from processing the steel is the largest source of uncertainty. Through the method of complete orientation averaging described in this paper, the texture effect and with it the standard deviation of the austenite mass fraction can be substantially reduced, regardless of the type or severity of the texture.
The goal of the quench and partition (Q&P) process for steel heat treatment is to enrich austenite with carbon during a partitioning treatment after initial quenching below the martensite start temperature (Ms). Two proposed mechanisms for austenite carbon enrichment during partitioning include carbon transport from martensite and/or the formation of carbide-free bainite. Theoretical calculations show experimentally measured austenite fractions are difficult to explain based upon a mechanism involving solely bainite formation. Carbon partitioning from martensite provides a more satisfactory explanation, although the formation of bainite during partitioning cannot be completely excluded.
The theory of the direct comparison X-ray method of phase analysis is extended to correct for preferred orientation effects. Texture parameters are defined to assess the type and intensity of preferred orientation using data from diffractometer patterns. The analysis is illustrated with results obtained on three austenitic stainless steels.
Controlled rolling processes are designed to produce a desired microstructure via the control of hot rolling without subsequent heat treatment. Critical to the success of controlled rolling are the stages that occur after hot deformation, when the steel is cooled to room temperature. This can be divided into two parts: the run out table, where the material is allowed to cool relatively rapidly to a pre-determined temperature, and ''coiling'' at which point the rolled material is coiled, thus showing down the cooling rate considerably. Controlled rolling schedules generally finish by coiling the steel at temperatures below the bainite transformation start temperature (B-s). Any changes in coiling conditions (temperature and time) in this region can result in variations in bainite characteristics (morphology, size, carbide precipitation, etc.). This in turn, may affect the state of the retained austenite and, consequently, the mechanical properties of Si-Mn TRIP steels, which have bainite as the dominant microconstituent. The effect of changes in the bainite transformation conditions were investigated using two grades of Si-Mn TRIP steels, including one a containing Nb as a microalloy addition. The results reveal that the retained austenite volume fraction was strongly influenced by both bainite formation temperature and hold time. The highest values of total elongation (46 and 33%) and formability index (61180 and 40260 MPa .%) were observed for an intermediate hold time (5 min) and temperature (400 degrees C), respectively. These findings are explained by considering the effect of the bainite transformation on the state of the retained austenite.
The main emphasis of this study has been placed on thermomechanical (TM) processing simulations of Mn–Si transformation-induced plasticity (TRIP)-aided steel using press forging. In order to rationalize the retained austenite volume fraction in multiphase structure, three TM schedules were employed at experiment where austenite different conditioning was considered. The choice of applied strain and thermal parameters had strong impact on austenite conditioning and consequently on the kinetics of isothermal ferrite and bainite transformation. The different multiphase structure characteristics were obtained after TM, with different volume fraction of retained austenite. Modification of structural characteristics consequently influenced the mechanical behaviour of TRIP steel. The present work also reports on the results received from in situ neutron diffraction experiments focused on monitoring of retained austenite transformation during mechanical incremental straining and evaluation of the lattice strains redistribution in present phases.
Usage of high strength steels may reduce the weight of automobiles and improve the crash safety and low down the gas emissions. Besides cold forming, hot stamping has gained much interest for the production of car body components. Boron alloyed steels have been the point of focus for the materials choice in hot stamping. In this paper, four high strength non-boron alloyed steels were hot stamped using water and nitrogen cooling media. Microstructural analyses, lateral and surface hardness profiling as well as tensile tests of hot stamped samples were performed. These steels provided yield strength (Y.S.) values of 600–1100 MPa and ultimate tensile strength (U.T.S.) values of 900–1400 MPa. Increasing cooling rates, i.e. by using nitrogen cooled punch (NCP) during hot stamping resulted in mostly martensitic microstructure and maximum strength, while hot stamping using water cooled punch (WCP) resulted in maximum formability index due to presence of some ferrite phase.
Hot stamping is a technique to produce ultra high strength automobile components. The common material used in hot stamping process is coated and/or uncoated 22MnB5 boron alloyed steel. Ferritic-pearlitic microstructure in as-delivered sheets is transformed to fully lath martensitic after hot stamping. In the present research, hot stamping under water or nitrogen cooling media was investigated using different boron alloyed steel grades. Microstructural analyses, linear and surface hardness profiling as well as tensile tests of hot stamped samples were performed. Various microstructures of fully bainitic and/or fully martensitic were produced. The resulting microstructures provided yield strengths of 650–1370 MPa and tensile strengths of 850–2000 MPa. There is an optimum carbon equivalent content for which the highest formability index value, UTS × A25, is achieved. Using a nitrogen cooled punch resulted in higher yield strength without significant changes in ultimate tensile strength. It is concluded that a wide range of B-bearing steels having an extended carbon equivalent range with an acceptable formability index value can be used by increasing the cooling rate in the die assembly.
In situ three-dimensional (3-D) X-ray diffraction experiments have been performed at a synchrotron source on low-alloyed multiphase TRIP steels containing 0.25 wt.% Si and 0.44 wt.% Al and produced with different bainitic holding times, in order to assess the influence of the bainitic transformation on the thermal stability of individual austenite grains with respect to their martensitic transformation. A detailed characterization of the austenite grain volume distribution at room temperature was performed as a function of the prior bainitic holding time. In addition, the martensitic transformation behaviour of individual metastable grains was studied in situ during cooling to a temperature of 100 K. Both the carbon content and the grain volume play a key role in the stability of the austenite grains below 15 μm3, while the carbon content exerts the dominant effect in the stability of the bigger grains. Measurements also suggest that the tetragonality of the thermally formed martensite is suppressed.
Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced
plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental
temperatures between 848K and 948K (575°C and 675°C) for 1 week to enrich austenite in manganese. The resulting microstructures
are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent
on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that
display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using
in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between
annealing conditions help explain the observed strain hardening behaviors.
The “quenching and partitioning” process is a new heat treatment for the development of multiphase steels with improved mechanical properties. In this work, a partial austenitization followed by Q&P paths, at which the partitioning step is effectuated at a temperature equal to the quenching temperature, has been applied to a low-carbon steel. The resulting multiphase microstructures have been investigated by optical microscopy using bright field and differential interference contrast, electron backscatter diffraction, X-ray diffraction and magnetic measurements. This group of techniques has led to a complete identification of the microstructural constituents: ferrite present during the partial austenitization, epitaxial ferrite formed during cooling, martensite and retained austenite. The analysis of the results has shown a significant relevance of the epitaxial ferrite in the retention of austenite, whereas the carbon partitioning from martensite to austenite has played a minor role.
A novel concept for the heat treatment of martensite, different to customary quenching and tempering, is described. This involves quenching to below the martensite-start temperature and directly ageing, either at, or above, the initial quench temperature. If competing reactions, principally carbide precipitation, are suppressed by appropriate alloying, the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase, thereby increasing the stability of the residual austenite upon subsequent cooling to room temperature. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering, and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Another approach that has been used to produce austenite-containing microstructures is by alloying to suppress carbide precipitation during the formation of bainitic structures, and interesting comparisons can be made between the two approaches. Moreover, formation of carbide-free bainite during the Q&P partitioning treatment may be a reaction competing for carbon, although this could also be used constructively as an additional stage of Q&P partitioning to form part of the final microstructure. Amongst the ferrous alloys examined so far are medium carbon bar steels and low carbon formable TRIP-assisted sheet steels.
The martensitic reaction is treated as a strain transformation with shear and dilatational displacements, respectively parallel and normal to the habit plane. When external forces are acting, the resulting effect on the Ms temperature is calculated from the mechanical work done on or by the transforming region as the resolved shear and normal components of the applied stress are carried through the corresponding transformation strains. This energy term is added algebraically to the chemical free energy change of the reaction, to compute the alteration in temperature at which the critical value of the thermodynamic driving force is attained to initiate the transformation. The transformation is aided by shear stresses, but may be aided or opposed by the normal stress component depending on whether the latter is tensile or compressive. The above criterion for the action of applied stress has successfully predicted the quantitative change in the Ms temperature of iron-nickel and iron-nickel-carbon alloys under uniaxial tension, uniaxial compression and hydrostatic pressure. Ms is raised by tension, less so by compression and is lowered by hydrostatic pressure.
Deformation induced ferrite transformation (DIFT) is a kind of solid state transformation induced through deformation, which can be applied to be as the effective method to produce fine or ultrafine ferrite grains. This paper reviews the research progress in the theory and application of DIFT from five aspects: evidence and study methods, thermodynamics and kinetics, transformation mechanisms, factors influencing DIFT, application of DIFT in production of fine grained C–Mn steel and ultrafine-grained microalloyed steel.
Silicon steel is a soft magnetic material that is used in electrical power transformers, motors and generators. It has a high silicon content of about 3.2 mass%, which increases the electrical resistivity of iron and, therefore, reduces eddy current losses. Grain-oriented silicon steel that is used for non-rotating applications, i.e. transformers, is characterised by a strong preferred crystallographic orientation. In iron the easiest directions of magnetisation are the 001 crystal directions. In grain-oriented silicon steel the Goss orientation, i.e. the {110}001 orientation, is technologically realised to minimise magnetic losses in electrical transformers. The strong Goss texture of grain-oriented silicon steel is the result of a complex processing scheme, i.e. of a long microstructural and textural inheritance chain. The origin of the evolution of the final Goss orientation is in the hot rolling stage, where the Goss orientation develops below the sheet surface due to shear deformation. The particular
importance of this Goss-containing subsurface layer was demonstrated by experiments in which the removal of this layer resulted in incomplete secondary recrystallisation. In the cold rolled material, the fraction of the Goss orientation, considering a misorientation of up to 15�°, is about 1% as measured using electron backscatter diffraction (EBSD). This means that the Goss component is too weak to be detected by X-ray diffraction, as used in earlier studies. In the subsequent primary annealing step, the material recrystallises and the Goss component slightly increases. In the final secondary high-temperature annealing process, normal grain growth is inhibited by particles. However, some of the Goss grains that are present in the recrystallised material grow abnormally. This gives rise to a sharp Goss texture with an average misorientation from the exact Goss orientation of about 3–7�°. The exact
mechanisms of inheritance of the Goss orientation through the process chain and details of abnormal Goss grain growth in the secondary annealing stage are still unclear.
Here a FeSi single crystal with an initial {1 1 0}〈0 0 1〉 orientation, also referred to as Goss orientation, was cold rolled up to a thickness reduction of 89%. Most of the crystal volume rotated into the two symmetrical {1 1 1}〈1 1 2〉 orientations. However, a weak Goss component remained in the highly strained material, even though the Goss orientation is mechanically unstable under plane strain loading. Two types of Goss-oriented regions were discernable in the material subjected to 89% reduction. It appeared that these two types of Goss regions have different origins. Goss grains that were found aligned in shear bands form during straining. A second type of Goss region was found between microbands where the initial Goss orientation was retained.
A model is developed to describe the endpoint of carbon partitioning between quenched martensite and retained austenite, in the absence of carbide formation. The model assumes a stationary α/γ interface, and requires a uniform chemical potential for carbon, but not iron, in the two phases, leading to a metastable equilibrium condition identified here as “constrained paraequilibrium” or CPE. The model is explained with example calculations showing the characteristics of the constrained paraequilibrium condition, and applications are discussed with respect to new microstructures and processes, including a new “quenching and partitioning,” or Q&P process, to create mixtures of carbon-depleted martensite, and carbon-enriched retained austenite. Important new implications with respect to fundamental elements of the bainite transformation are also discussed.
The stability of the retained austenite has been studied in situ in low-alloyed transformation-induced-plasticity (TRIP) steels using high-energy X-ray diffraction during tensile tests at variable temperatures down to 153 K. A detailed powder diffraction analysis has been performed to probe the austenite-to-martensite transformation by characterizing the evolution of the phase fraction, load partitioning and texture of the constituent phases simultaneously. Our results show that at lower temperatures the mechanically induced austenite transformation is significantly enhanced and extends over a wider deformation range, resulting in a higher elongation at fracture. Low carbon content grains transform first, leading to an initial increase in average carbon concentration of the remaining austenite. Later the carbon content saturates while the austenite still continues to transform. In the elastic regime the probed {h k l} planes develop different strains reflecting the elastic anisotropy of the constituent phases. The observed texture evolution indicates that the austenite grains oriented with the {2 0 0} plane along the loading direction are transformed preferentially as they show the highest resolved shear stress. For increasing degrees of plastic deformation the combined preferential transformation and grain rotation results in the standard deformation texture for austenite with the {1 1 1} component along the loading direction. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between carbon concentration in the austenite, grain orientation, load partitioning and temperature.