Article

Improvement of tensile mechanical properties in a TRIP- assisted steel by controlling of crystallographic orientation via HSQ&P processes

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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.

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... 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]. ...
Article
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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.
... 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. ...
Article
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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%).
... 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 ( . ...
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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]. ...
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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%.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.