Jukka Kömi

University of Oulu · Faculty of Technology

A low-alloyed ultrahigh-strength steel comprising CrNiMoWMnV was designed based on thermodynamic calculations and by controlling the microalloying elements to promote various strengthening mechanisms upon processing. The hot deformation behaviour and mechanism were correlated with the processing parameters, that is, strain rate and temperature. The...

The influence of martensite on the transformation rate of bainite as well as enhancement of various properties is well established, yet the combined influence of both the primary martensite and low temperature ausforming below the martensite start temperature (MS) on the characteristics of microstructural evolution comprising ultrafine/nanostructur...

In the present study, very high cycle fatigue (VHCF) characteristics and properties of a direct quenched and partitioned (DQ&P) medium carbon (0.4 wt.% C) steel having ultrahigh-strength (UTS > 2 GPa) was investigated using an ultrasonic-fatigue testing technique (∼19 kHz frequency) up to ∼1010 cycles at the load ratio (R) of −1 (fully reversed ten...

The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for a newly developed low-carbon CrNiMnB ultrahigh-strength steel with and without molybdenum addition. The time for 50% static recrystallization (t50%) over a wide range of strains and hot-deformation temperatures were obtained using...

The overwhelming impact of complex-phase microstructures on mechanical response in multiphase steels requires accurate constitutive properties of the individual phases. However, precise prediction of individual phase properties on their mechanical response is critical and sophisticated, which commonly requires multiscale characterizations and numer...

IDS is a thermodynamic-kinetic software combined with a microstructure tool developed to simulate the non-equilibrium solidification of steels. The present work introduces its main calculation module, SOL, and presents some essential results of that module, such as the formation of ferrite and austenite in different types of steels during their sol...

The novel processing concept of direct quenching and partitioning (DQ&P) has been explored with a medium-carbon (0.4 wt.% C) steel to evaluate and optimize the processing route for excellent property combinations. New compositional design approach was based on physical simulation studies aiming to understand the influence of varying silicon content...

The presence of acicular ferrite (AF) in the heat-affected zone (HAZ) of steels used offshore is generally seen as beneficial for toughness. In this study, the effects of varying fractions of AF (0-49 vol.%) were assessed in the simulated, unaltered and coarse-grained heat-affected zones (CGHAZ) of three experimental steels. Two steels were deoxidi...

Phase transformation in low alloyed Fe-C steels during quenching and partitioning (Q&P), though authentically attributed to carbon diffusion, is scarcely quantified experimentally in terms of microstructural evolution and lack of fundamental backing at a quantum mechanics level. Herein, we report on a combined in-situ high energy synchrotron X-ray...

An effort has been made to examine the high cycle fatigue (HCF) properties including crack propagation characteristics and related fracture mechanisms of submicron-grained (SG) Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling (MCR) and multiaxial forging (MAF). The HCF and other mechanical properties have been...

The high and very high cycle fatigue (VHCF) properties of ultrahigh-strength Ck45M steel processed by thermomechanical rolling integrated direct quenching were investigated. S–N tests with smooth and small drilled holes containing specimens as well as near-threshold fatigue crack growth measurements were performed up to 2 × 10^10 cycles using ultra...

Thermomechanical processing consisted of hot-rolling in the austenitic region with deformation both above and below the non-recrystallization temperature. Immediately after rolling, specimens were directly quenched in water to three different temperatures of 560 °C, 420 °C and room temperature. The first two samples, which were quenched to 560° C a...

A novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (DQ&P) was designed for developing tough, ductile, ultrahigh strength steels using 0.4 wt% carbon steels. A preliminary characterization of a laboratory-rolled, high-silicon DQ&P steel revealed an excellent combination of mechanical properties...

This work will be presented in the 19th International Conference on Textures of Materials (ICOTOM 19) which be held virtually on March 1 to 4, 2021. More info about the conference can be found here: http://icotom19.com

A medium-grained (~5 μm) Fe-Mn-Al-C steel was deformed under interrupted uniaxial tension until fracture (~50% strain) at room temperature. The corresponding microstructures were evaluated according to X-ray line profile analysis, based on dislocation-induced strain broadening, recounting interestingly opposing revelations for such steels. Planar (...

The underlying mechanism of dislocation substructure formation in a tensile deformed fine-grained high-Mn steel is reported using transmission electron microscopy. A cross-slip assisted dislocation truncation mechanism was revealed that formed strings of dislocation loops at early strain, which were also retained at fracture strain. Planar glide pr...

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurrin...

Density functional theory based computational study has been conducted in order to investigate the effect of substitution of Cr and Co components by Si on the structure, mechanical, electronic, and magnetic properties of CrCoNiFe. It is found that the presence of a moderate concentration of Si substitutes (up to 12.5 %) does not significantly reduc...

The effect of austenite deformation on carbon partitioning and transformation to athermal and isothermal martensite, and bainite during quenching and partitioning (QP) is described for three steel compositions: Fe-0.3C-0.6Si-1.1Al, Fe-0.3C-1.0Si and Fe-0.3C-0.5Si-0.5Al. Microstructures were characterized using SEM-EBSD, TEM and XRD. Austenite decom...

The effect of as-cast structure and macrosegregation on the mechanical properties of a direct-quenched low-alloy martensitic ultrahigh-strength aluminum killed and calcium treated steel cast at different superheats was studied. Samples from the castings were laboratory hot rolled with two different finishing rolling temperatures to distinguish the...

Stress relief treatments were carried out separately with a pneumatic chipping hammer, ultrasonic peening treatment, and heat treatment for metal active-gas welding (MAG) welded joints of 2205 duplex stainless steel. The effects of these methods on the residual stress, microstructure, mechanical properties and corrosion resistance of welded joints...

In this study, a set of thermodynamic, kinetic, and microstructure data is presented to simulate the non-equilibrium solidification of Fe-Al-Mn-Si-C alloys. The data were further validated with the experimental measurements and then used in a thermodynamic-kinetic software, IDS, to establish the effect of the alloying and cooling rate on the solidi...

Thermomechanical deformation is one of the most efficient and facile routes to tailor microstructure in structural materials for mechanical property enhancement. In this work, the Ce‐modified SAF2507 super duplex stainless steel (Ce‐SAF2507) was deformed at different levels from 30% to 90% at a cryogenic temperature (‐196 oC) to achieve superior me...

The kinetics of strain-induced martensite (SIM) formation in a Ti-bearing 201L stainless steel were evaluated and compared to the existing results of two conventional stainless steel grades; i.e., 201L and 304L AISI. The effects of strain rate and rolling pass reduction on the kinetics of SIM formation during cold rolling were investigated. The Ti-...

The effect of chromium content and prior hot deformation of the austenite on the continuous cooling transformation (CCT) diagram of a newly developed low-carbon bainitic steel has been studied using dilatometer measurements conducted on a Gleeble 3800 simulator with cooling rates ranging from 2-80 °C/s. After austenitization at 1100 °C, specimens w...

The effect of electroslag remelting (ESR) with CaF2-based synthetic slag on the microstructure and mechanical properties of three as-quenched martensitic/martensitic-bainitic ultrahigh-strength steels with tensile strengths in the range of 1250-2000 MPa was investigated. Ingots were produced both without ESR, using induction furnace melting and cas...

The effect of total applied strain (TAS) and finish forging temperature (FFT) on the microstructure and precipitation kinetics of a newly developed low‐cost, low‐alloy CrNiMoWMnV ultrahigh‐strength steel has been investigated. A Gleeble 3800 thermomechanical simulator was used to simulate the hot forging process and its influence on the precipitati...

Featured Application: The current results can be used as a guideline for the production of high-strength low-carbon bainitic steels with high impact toughness. Abstract: The effect of chromium content in the range of 1 wt.%-4 wt.% on the microstructure and mechanical properties of controlled-rolled and direct-quenched 12 mm thick low-carbon (0.04 w...

In the present study, at first flow behavior of Nb–Ti microalloyed and interstitial-free (IF) steels was investigated to know the effects of processing parameters on their microstructural evolution. Then, innovative 3-steps multiphase control rolling schedules have been designed to yield submicron size uniform grains structure and successfully achi...

The effect of grain size in the range 72 to 190 μm and carbon content in the range 0.105–0.073 wt.% on the intergranular corrosion of the austenitic stainless steel 301 has been investigated. Grain boundary chromium depletion has been studied directly using energy dispersive X-ray spectroscopy combined with scanning transmission electron microscopy...

The cleanliness of ultrahigh-strength steels (UHSSs) without and with electroslag remelting (ESR) using a slag with the composition of 70% CaF2, 15% Al2O3 and 15% CaO has been studied. Three experimental heats of UHSSs with different chemical compositions were designed, melted in an induction furnace and refined using ESR. Cast ingots were forged a...

Controlled cold rolling impacts on microstructure and mechanical properties of the rare earth Ce-modified SAF 2507 super duplex stainless steel (SDSS) are investigated towards a better application and development prospective over the pristine SDSS. Rolling was performed at room temperature with the deformation level in a range of 30%–90%. A large a...

The effect of microalloying elements on the prior austenite grain growth of slab materials during slab reheating has been investigated. The investigated materials were laboratory castings with two levels of carbon (0.05 and 0.09 wt.%) and different combinations of microalloying elements, such as V, Nb, Mo and Ti. Experimental results were compared...

A model for static recrystallization by Zurob et al. [1] has been fitted to experimental stress relaxation [2] data obtained on a low-alloyed steel using a Gleeble thermomechanical simulator. The model has been implemented as an algorithm that calculates the stress relaxation as a function of time, including physical descriptions of the recovery an...

Computer simulations of steel microstructural development provide a powerful tool, which can form the basis of mechanical property predictions. However, in order to create detailed understanding of the factors affecting the properties, the model should predict microstructural evolution during cooling. The present study compares the results of cellu...

The degree of sensitization in an austenitic stainless steel, has been measured using double loop electrochemical reactivation tests, and the measured values compared with predictions based on grain boundary chromium depletion characteristics obtained using the precipitation and diffusion modules of Thermo-Calc. In order to quantitatively predict C...

With the aim of improving the strength and impact toughness combination of two ultrahigh-strength quenched and tempered steels, the effect of high-temperature austenitization and quenching prior to conventional austenitization, quenching and tempering at 200 °C has been investigated. The CrNiMoWMnV steels concerned had carbon contents of 0.18 and 0...

The direct quenching of low-carbon steels after thermomechanical processing on hot strip mills is able to produce both strong and tough coiled plate without the need for subsequent tempering. The process is energy and time efficient with relatively low emissions when compared to conventional reheating, quenching and tempering. For some applications...

A large number of thermodynamic simulations has been used to design a new Nb-Ti dual stabilized ferritic stainless steel with excellent creep resistance at 1050°C through an optimal volume fraction of Laves (η) phase stabilized by the alloying elements Nb, Si and Mo. By raising the dissolution temperature of the phase, which also corresponds to the...

The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels (UHSSs) with different levels of C, Cr and Ni has been evaluated for the as-cooled and untempered condition. One UHSS had higher contents of C and Cr, while the other one had a higher Ni content. On the basis of dilatation curves, micr...

The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels (UHSSs) with different levels of C, Cr and Ni has been evaluated for the as-cooled and untempered condition. One UHSS had higher contents of C and Cr, while the other one had a higher Ni content. On the basis of dilatation curves, micr...

This article studied the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory control rolled steels based on grade 22MnB5. The constructed continuous cooling transformation diagrams revealed that an addition of Mo significantly increased the hardenability. Especially in the case of austenite compressed bel...

Electrochemical impedance spectroscopy (EIS) has been used to detect sensitization in austenitic stainless steels that are heat treated in the temperature regime 600-820 °C to produce different degrees of sensitization in the material. The tests were conducted at five different DC potentials in the transpassive region. The quantitative determinatio...

The influence of chemical composition and processing parameters on the microstructure and bendability of three thermomechanically rolled and direct‐quenched wear resistant steel plates is studied. Overall, the bendability of the steels is good, but there are exceptions that are given special attention. The prior austenite condition and final micros...

The direct quenching process is an energy- and resource-efficient process for making high-strength structural steels with good toughness, weldability, and bendability. This paper presents the results of an investigation into the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory rolled and direct-quenche...

A new experimental steel containing in weight percent 0.3C-2.0Mn-0.5Si-1.0Al-2.2Cr and 0.3C-1.9Mn-1.0Si-1.0Cr was hot rolled in a laboratory rolling mill and directly quenched within the martensite start and finish temperature range. It was then partitioned without reheating during slow furnace cooling to achieve tensile yield strengths over 1100 M...

In the direct quenching and partitioning (DQ&P) process, tough ultra-high-strength steel is made by combining thermomechanical processing with quenching and partitioning to obtain martensite toughened by thin films of retained austenite. The hot rolling stage with deformation and recrystallization between the rolling passes affects the state of the...

The segregation of alloying elements that occurs during the solidification of steel leads to microscale and macroscale microstructural heterogeneity that can cause anomalous mechanical behaviour. The centreline macrosegregation of a cast and its increased inclusion content are usually considered to be particularly detrimental in the case of convent...

Recent advances show that tough as-quenched ultra-high-strength steels in fully and partially martensitic conditions demand proper control of the effective coarse grain size, which is the key microstructural parameter controlling the toughness in the ductile-brittle transition region. The most effective way to reduce this grain size and texture com...

High strength and sufficient toughness are key requirements for modern high-performance structural steels. In an attempt to develop a suitable estimation of impact toughness transition temperatures for as-quenched steels, we investigated the determiners of low-temperature toughness with a group of thermomechanically rolled direct-quenched steels wi...

Using a novel TMR-DQP processing route, two ultrahigh-strength steels have been developed with yield strengths up to 1100 MPa combined with good uniform and total elongations and low-temperature impact toughness. Processing involved thermomechanically controlled rolling including significant reductions below the recrystallization stop temperature (...

The effect of shot blasting on the bendability of two tempered trial ultrahigh-strength steels has been studied by comparing the bending behaviour of otherwise identical plates with and without shot blasting. The yield strength of the studied 10 mm thick trial steel was 700 MPa and 7.5 mm trial steel 1100 MPa. The local microstructures below the di...

The cleanness, microstructure and mechanical properties of a newly developed CrNiMoWMnV ultrahigh-strength steel with and without electroslag refining (ESR) with 70% CaF2, 15% Al2O3 and 15% CaO have been studied. This steel was designed and melted in an air induction furnace followed by refining using ESR. Cast ingots with and without ESR were forg...

This paper presents a novel TFT (tuning fork test) stress corrosion testing method, which was developed for classifying martensitic high‐strength steels. The novel method was developed by applying finite element calculations to optimize a tuning fork geometry to enable accurate stress adjustment with simple inexpensive equipment. Different steels w...

Offshore steels for cold climate conditions require not only relatively high strength to improve the cost-efficiency of the structures, but also excellent toughness at low temperatures to guarantee the safety of the structures in harsh environments. The most challenging locations to fulfil both requirements are in the welded joints of these steels...

Offshore steels for the arctic conditions have an increasing demand due to the opening of new oil fields in the Arctic Ocean. However, the require-ments for these steels are extremely demanding, as they need to maintain the desired properties in harsh arctic conditions. Additionally, these require-ments need to be achieved also in heat-affected zon...

The microstructure evolution in Ti-Nb dual stabilized ferritic steel at high service temperature was simulated with heat treatments at 600 °C for up to 120 h. Thermodynamic calculations indicated that, in addition to conventional MX type carbides and nitrides containing Nb and Ti, heat treatment at high temperature can promote the formation of inte...

This research focuses on studying the dynamic energy absorption property of a micro-alloyed TWIP steel, which was proposed to act as a connection part between car front bumper and chassis in middle-class cars for vehicle safety. The studied TWIP steel was designed based on stacking fault energy of 25mJ/m2. The as-cast steel was deformed in hot and...