Thota Hemanth | Indian Institute of Technology Madras, Chennai (original) (raw)

Papers by Thota Hemanth

Intersections of shear bands in metastable austenites have been shown to be effective sites for s... more Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of E' (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shearband intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fcc ~bcc chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fcc, bcc, and hcp entropy differences through proper compositional control.

Etching martensite and bainite for quantitative metallography of dual-phase steels presents uniqu... more Etching martensite and bainite for quantitative metallography of dual-phase steels presents unique problems. Standard etching techniques usually accent grain boundaries which are then counted in area measurements. Some specimens must be tempered or overetched to develop the contrast needed to differentiate between martensite and ferrite; when this is done, bainite cannot be quantitatively differentiated from grain boundaries. Since image analysis equipment operates by distinguishing differences between various shades of gray, contrast is of the utmost importance. illustrates the typical contrast problem encountered with a 2% nital etchant.

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe... more Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15±30) wt.%Mn steels with additions of aluminium and silicon have been investigated. It is known that additions of aluminium increase the stacking fault energy f and therefore strongly suppress the 3 4 transformation while silicon decrease f and sustains the 3 4 transformation. The 3 4 phase transformation takes place in steels with f 420 mtam 2 . For steels with higher stacking fault energy twinning is the main deformation mechanism. Tensile tests were carried out at dierent strain rates and temperatures. The formation of twins, a-and 4-martensite during plastic deformation was analysed by optical microscopy, X-ray diraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The developed light weight high manganese TRIP (``transformation induced plasticity'') and TWIP (``twinning induced plasticity'') steels exhibit high¯ow stress (600±1100 MPa) and extremely large elongation (60±95%) even at extremely high strain rates of about 10 3 s À1 . Recent trends in the automotive industry towards improved safety standards and a reduced weight as well as a more rational and cost eective manufacturing have led to great interest in these high strength and``super tough'' steels. #

. Fe-55Mn-3Al-3Si mass% TWIP-steel: Yield stress R p0.2 , tensile strength R m , uniform elongati... more . Fe-55Mn-3Al-3Si mass% TWIP-steel: Yield stress R p0.2 , tensile strength R m , uniform elongation e un and total elongation e f as functions of test temperature.

Retained austenite characteristics and tensile properties in a 0.2C-1.5Si-1.5Mn, mass%, high-stre... more Retained austenite characteristics and tensile properties in a 0.2C-1.5Si-1.5Mn, mass%, high-strength cold-rolled "TRIP type bainitic steel" which was associated with the transformation induced plasticity (TRIP) of the retained austenite were investigated. The steel mainly consisted of bainitic ferrite lath matrix, blocky martensites and stable retained austenite films of 5-12 vol%. When austempered at temperatures above M S temperature, the steel possessed high tensile strength of 900 MPa, large total elongation of 15-20 % and large reduction of area of 40-60 %. The good ductility was mainly owing to uniform fine lath structure, initial martensite and the TRIP effect of retained austenite, as well as a small contribution of long range internal stress resulting from untransformed retained austenite films.

The effect of the substitution of silicon by aluminium on the mechanical properties and the micro... more The effect of the substitution of silicon by aluminium on the mechanical properties and the microstructure af cold rolled C-Mn-Si TRI P steels was investigated for different continuous annealing cycles, The mechanical properties were evaluated using tensile testing, It wasseen that the Al alloyed steel had very good mechanical properties with an improved formability comparedto the conventional C-Mn-Si TRIP steel. The strain hardening behaviour was studied in detail. All the investigated specimens showed a very high n value but their strain dependence was different. For the conventional C-Mn-Si TRIP steel the maximum n value was reached at low strain, while the Al substituted TRIP steel showed a gradual increase of the n value. The latter effect resulted in a larger uniform elongation for the C-Mn-Al-Si TRIP steel.

Therelationship betweensecond phasemorphologyand retained austenite morphologyand the influences ... more Therelationship betweensecond phasemorphologyand retained austenite morphologyand the influences of these two kinds of morpho[ogy on tensile properties of a 0.1 7C-1 .41 Si=2.00Mn (masso/o) TRIP-aided dual-phase steel have been investigated in a temperature range between 20 and 400"C.

A systematic experimental study has been conducted on ferrite recrystallization and intercritical... more A systematic experimental study has been conducted on ferrite recrystallization and intercritical austenite formation for two low-carbon steels with chemical compositions typically used for dual-phase and transformation-induced plasticity (TRIP) steels. Different initial heating rates, holding temperatures, and times were applied to the materials to examine the ferrite recrystallization and austenite formation kinetics. An Avrami model was developed to describe the isothermal ferrite recrystallization behavior and was applied successfully to the nonisothermal conditions. It was found that the initial heating rate affects the isothermal austenite formation kinetics for both the hot-rolled and cold-rolled materials albeit the effect is more pronounced for the cold-rolled material. This can be attributed to the interaction between the ferrite recrystallization and austenite formation processes. Furthermore, it was found that the distribution of austenite phase is also affected by the ferrite recrystallization process. When ferrite recrystallization is completed before the austenite formation (i.e., under sufficiently slow heating rate conditions), austenite is to a large extent randomly distributed in the ferrite matrix. On the other hand, incomplete recrystallization of ferrite due to higher heating rates leads to the formation of banded austenite grains. It is proposed that this observation is characteristic of simultaneous recrystallization and austenite formation where moving ferrite grain boundaries do not provide suitable sites for austenite nucleation.

The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) fo... more The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) for use in the automobile industry, including chemical composition design, microstructure and mechanical properties development during thermomechanical processing, production technology characterisation, potential applications and performance in service. AHSS steels are considered to be the major materials for future applications in this production sector. As opposed to the cold formable single phase deep-drawable grades, the mechanical properties of AHSS steels are controlled by many factors, including: phase composition and distribution in the overall microstructure, volume fraction, size and morphology of phase constituents, as well as stability of metastable constituents. The main feature of these steels is that they do not permit to rely on the well-established traditional microstructure-properties relationships. Therefore, many different alloy concepts and alternative processing routes are still under development by different steel producers for comparable steel grades.

We highlight the recent advance in strong transformation-induced plasticity (TRIP) steels, which ... more We highlight the recent advance in strong transformation-induced plasticity (TRIP) steels, which have higher alloying contents compared to the conventional TRIP steels for achieving tensile strength higher than 1 GPa. Given that the extraordinary strength-elongation balance is led by the characteristic mixture of coarse and submicron-sized grains of ferrite and austenite, diverse strategies of materials design to obtain unprecedented properties are reviewed. Challenges to achieve the vital engineering parameters other than tensile properties are discussed as well, which should be resolved to secure competitiveness over other emerging structural alternatives.

Ultrafine-grained duplex manganese-bearing steels fabricated by quenching and annealing demonstra... more Ultrafine-grained duplex manganese-bearing steels fabricated by quenching and annealing demonstrated excellent combinations of tensile elongation of 31-44% and tensile strength of 1-1.5 GPa and a three-stage work-hardening behavior. Their enhanced mechanical properties and work-hardening behavior were explained by their dynamic composition due to the strain induced phase transformation from large-fractioned austenite (>30%). It was suggested that the austenite volume fraction and its mechanical stability is the key to understand the phase transformation induced deformation behavior.

Intercritical annealing, flash process and tempering were innovatively combined to obtain high st... more Intercritical annealing, flash process and tempering were innovatively combined to obtain high strengthhigh ductility combination in 0.12Ce4.89Mn-1.57Al steel. The process referred as multi-step partitioning (MSP) was designed to accomplish the following objectives: (a) enrichment of austenite with Mn to enhance the stability of retained austenite, (b) transformation hardening during quenching in the flash process and (c) stress relaxation and carbon enrichment of retained austenite. The tensile strength of steel increased from~667 MPa in intercritically annealed steel to~986 MPa in flash processed steel. The product of strength and elongation of flash steel and tempered steel were 23.2 GPa% and 24.9 GPa%, respectively and higher than the intercritically annealed steel (21.3 GPa%). The high ductility, especially the uniform elongation of flash steel (16.2%) and tempered steel (19.4%) is attributed to~15e19% by volume of Mn-rich stable retained austenite and efficient TRIP (transformation induced plasticity) effect. Thermodynamic calculations enabled us to understand the partitioning behavior of alloying elements in MSP. C, Mn and Al reverse partitioning during the flash process led to increased stability of retained austenite. The unique distribution of chemical constituents contributed to two types of martensitic transformation during the flash process: (a) austenite / a 0 -martensite transformation dominated at high temperature and contributed to the formation of stacking faults and ε-martensite transformation and (b) austenite / ε-martensite / a 0 -martensite phase transformation dominated at lower temperature. The stability of retained austenite and interaction with stress concentration contributed to highly efficient TRIP effect in flash processed and tempered steel. The experiment findings were consistent with the diffusion-controlled transformation simulation analysis.

Differently heat treated samples of a low alloyed TRIP steel have been investigated using electro... more Differently heat treated samples of a low alloyed TRIP steel have been investigated using electron diffraction techniques in SEM and TEM. Aim was, first, to discriminate the microstructure constituents, austenite, ferrite, bainite, and martensite, second to gain information on the c-a phase transformation mechanisms and third to correlate the mechanical properties and the microstructure of the samples. Bainite always occurs in conjunction with an orientation gradient in the surrounding ferrite matrix. It consists of fine lamellae of ferrite and austenite which show a sharp Kurdjumov-Sachs orientation relationship with each other. This was interpreted in terms of a displacive bainite formation mechanism. The microstructure is formed by growth of c-grains during intercritical annealing and shrinking of these grains during the subsequent cooling without nucleation of new a-grains. The transformation first occurs reconstructively into ferrite and then, at lower temperature, displacively into bainite. The mechanical properties of differently heat treated samples are most strongly influenced by the amount and distribution of carbon in the retained austenite and by the degree of recovery in bainite and austenite.

The purpose of the present contribution is to review the current knowledge about the relationship... more The purpose of the present contribution is to review the current knowledge about the relationship between the micro-structure of cold rolled intercritically annealed low alloy TRIP-aided sheet steels and their mechanical properties from a materials engineering point of view. The focus is on their production in existing industrial lines and on their application in the manufacture of passenger cars with a body-in-white which offers an improved passive safety. The review aims to make clear that although low alloy TRIPaided sheet steel is by now starting to be an established structural material in BIW manufacturing, there is still room for the further optimization of the composition and the processing. In addition, there are still a number of problems related to their physical metallurgy that require a better fundamental understanding.

The good performance of supermartensitic stainless steels is strongly dependent on the volume fra... more The good performance of supermartensitic stainless steels is strongly dependent on the volume fraction of retained austenite at room temperature. The present work investigates the effect of secondary tempering temperatures on this phase transformation and quantifies the amount of retained austenite by X-ray diffraction and saturation magnetization. The steel samples were tempered for 1 h within a temperature range of 600-800°C. The microstructure was characterized using scanning electron microscopy and electron backscatter diffraction. Results show that the amount of retained austenite decreased with increasing secondary tempering temperature in both quantification methods.

In the present study, we fundamentally explore the reasons underlying differences in mechanical p... more In the present study, we fundamentally explore the reasons underlying differences in mechanical properties in hot-rolled 0.2C-1.6Al-6.1Mn-Fe TRIP steels subjected to different heat treatments. Comparing with austenite reverted transformation annealing (ART) process, quenching and tempering (Q&T) process was effective in obtain excellent mechanical properties. [Q&T (UTS: 902-1235 MPa, TEL: 18-42%]; [ART (UTS: 885-945 MPa, TEL: 13-28%)]. In the ART process, long time annealing led to excessive C and Mn enrichment in austenite, which rendered austenite too stable and deteriorated TRIP effect. Furthermore, long time annealing reduced dislocation density and led to low work-hardening rate. The Q&T process enabled appropriate enrichment of elements and hence desired stability for significant TRIP effect to be observed. Thus, the steel quenched from 625 ℃ exhibited best combination of mechanical properties (UTS: 1038 MPa, TEL: 42%, UTS ×TEL: 43.6 GPa%) 2 because of significant contribution of TRIP effect and high dislocation density in austenite.

The effect of the microstructural characteristics of retained austenite on its transformation sta... more The effect of the microstructural characteristics of retained austenite on its transformation stability in steel after Quenching and Partitioning (Q&P) was studied via interrupted tensile tests and Electron Backscatter Diffraction measurements on a pre-determined zone of a micro-tensile test sample. The evolution of the retained austenite fraction was obtained as a function of the plastic strain. The dependence of the austenite transformation stability on the corresponding grain size, morphology, and local crystallographic orientation was discussed. Furthermore, the importance of the parameters on the austenite stability was analysed and it was shown that the austenite grains rotated, in addition to being transformed, constituting therefore an additional contribution to the ductility of Q&P steel.

With a suite of multi-modal and multi-scale characterization techniques, the present study unambi... more With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100 MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800 MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels.

Intersections of shear bands in metastable austenites have been shown to be effective sites for s... more Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of E' (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shearband intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fcc ~bcc chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fcc, bcc, and hcp entropy differences through proper compositional control.

Etching martensite and bainite for quantitative metallography of dual-phase steels presents uniqu... more Etching martensite and bainite for quantitative metallography of dual-phase steels presents unique problems. Standard etching techniques usually accent grain boundaries which are then counted in area measurements. Some specimens must be tempered or overetched to develop the contrast needed to differentiate between martensite and ferrite; when this is done, bainite cannot be quantitatively differentiated from grain boundaries. Since image analysis equipment operates by distinguishing differences between various shades of gray, contrast is of the utmost importance. illustrates the typical contrast problem encountered with a 2% nital etchant.

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe... more Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15±30) wt.%Mn steels with additions of aluminium and silicon have been investigated. It is known that additions of aluminium increase the stacking fault energy f and therefore strongly suppress the 3 4 transformation while silicon decrease f and sustains the 3 4 transformation. The 3 4 phase transformation takes place in steels with f 420 mtam 2 . For steels with higher stacking fault energy twinning is the main deformation mechanism. Tensile tests were carried out at dierent strain rates and temperatures. The formation of twins, a-and 4-martensite during plastic deformation was analysed by optical microscopy, X-ray diraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The developed light weight high manganese TRIP (``transformation induced plasticity'') and TWIP (``twinning induced plasticity'') steels exhibit high¯ow stress (600±1100 MPa) and extremely large elongation (60±95%) even at extremely high strain rates of about 10 3 s À1 . Recent trends in the automotive industry towards improved safety standards and a reduced weight as well as a more rational and cost eective manufacturing have led to great interest in these high strength and``super tough'' steels. #

. Fe-55Mn-3Al-3Si mass% TWIP-steel: Yield stress R p0.2 , tensile strength R m , uniform elongati... more . Fe-55Mn-3Al-3Si mass% TWIP-steel: Yield stress R p0.2 , tensile strength R m , uniform elongation e un and total elongation e f as functions of test temperature.

Retained austenite characteristics and tensile properties in a 0.2C-1.5Si-1.5Mn, mass%, high-stre... more Retained austenite characteristics and tensile properties in a 0.2C-1.5Si-1.5Mn, mass%, high-strength cold-rolled "TRIP type bainitic steel" which was associated with the transformation induced plasticity (TRIP) of the retained austenite were investigated. The steel mainly consisted of bainitic ferrite lath matrix, blocky martensites and stable retained austenite films of 5-12 vol%. When austempered at temperatures above M S temperature, the steel possessed high tensile strength of 900 MPa, large total elongation of 15-20 % and large reduction of area of 40-60 %. The good ductility was mainly owing to uniform fine lath structure, initial martensite and the TRIP effect of retained austenite, as well as a small contribution of long range internal stress resulting from untransformed retained austenite films.

The effect of the substitution of silicon by aluminium on the mechanical properties and the micro... more The effect of the substitution of silicon by aluminium on the mechanical properties and the microstructure af cold rolled C-Mn-Si TRI P steels was investigated for different continuous annealing cycles, The mechanical properties were evaluated using tensile testing, It wasseen that the Al alloyed steel had very good mechanical properties with an improved formability comparedto the conventional C-Mn-Si TRIP steel. The strain hardening behaviour was studied in detail. All the investigated specimens showed a very high n value but their strain dependence was different. For the conventional C-Mn-Si TRIP steel the maximum n value was reached at low strain, while the Al substituted TRIP steel showed a gradual increase of the n value. The latter effect resulted in a larger uniform elongation for the C-Mn-Al-Si TRIP steel.

Therelationship betweensecond phasemorphologyand retained austenite morphologyand the influences ... more Therelationship betweensecond phasemorphologyand retained austenite morphologyand the influences of these two kinds of morpho[ogy on tensile properties of a 0.1 7C-1 .41 Si=2.00Mn (masso/o) TRIP-aided dual-phase steel have been investigated in a temperature range between 20 and 400"C.

A systematic experimental study has been conducted on ferrite recrystallization and intercritical... more A systematic experimental study has been conducted on ferrite recrystallization and intercritical austenite formation for two low-carbon steels with chemical compositions typically used for dual-phase and transformation-induced plasticity (TRIP) steels. Different initial heating rates, holding temperatures, and times were applied to the materials to examine the ferrite recrystallization and austenite formation kinetics. An Avrami model was developed to describe the isothermal ferrite recrystallization behavior and was applied successfully to the nonisothermal conditions. It was found that the initial heating rate affects the isothermal austenite formation kinetics for both the hot-rolled and cold-rolled materials albeit the effect is more pronounced for the cold-rolled material. This can be attributed to the interaction between the ferrite recrystallization and austenite formation processes. Furthermore, it was found that the distribution of austenite phase is also affected by the ferrite recrystallization process. When ferrite recrystallization is completed before the austenite formation (i.e., under sufficiently slow heating rate conditions), austenite is to a large extent randomly distributed in the ferrite matrix. On the other hand, incomplete recrystallization of ferrite due to higher heating rates leads to the formation of banded austenite grains. It is proposed that this observation is characteristic of simultaneous recrystallization and austenite formation where moving ferrite grain boundaries do not provide suitable sites for austenite nucleation.

The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) fo... more The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) for use in the automobile industry, including chemical composition design, microstructure and mechanical properties development during thermomechanical processing, production technology characterisation, potential applications and performance in service. AHSS steels are considered to be the major materials for future applications in this production sector. As opposed to the cold formable single phase deep-drawable grades, the mechanical properties of AHSS steels are controlled by many factors, including: phase composition and distribution in the overall microstructure, volume fraction, size and morphology of phase constituents, as well as stability of metastable constituents. The main feature of these steels is that they do not permit to rely on the well-established traditional microstructure-properties relationships. Therefore, many different alloy concepts and alternative processing routes are still under development by different steel producers for comparable steel grades.

We highlight the recent advance in strong transformation-induced plasticity (TRIP) steels, which ... more We highlight the recent advance in strong transformation-induced plasticity (TRIP) steels, which have higher alloying contents compared to the conventional TRIP steels for achieving tensile strength higher than 1 GPa. Given that the extraordinary strength-elongation balance is led by the characteristic mixture of coarse and submicron-sized grains of ferrite and austenite, diverse strategies of materials design to obtain unprecedented properties are reviewed. Challenges to achieve the vital engineering parameters other than tensile properties are discussed as well, which should be resolved to secure competitiveness over other emerging structural alternatives.

Ultrafine-grained duplex manganese-bearing steels fabricated by quenching and annealing demonstra... more Ultrafine-grained duplex manganese-bearing steels fabricated by quenching and annealing demonstrated excellent combinations of tensile elongation of 31-44% and tensile strength of 1-1.5 GPa and a three-stage work-hardening behavior. Their enhanced mechanical properties and work-hardening behavior were explained by their dynamic composition due to the strain induced phase transformation from large-fractioned austenite (>30%). It was suggested that the austenite volume fraction and its mechanical stability is the key to understand the phase transformation induced deformation behavior.

Intercritical annealing, flash process and tempering were innovatively combined to obtain high st... more Intercritical annealing, flash process and tempering were innovatively combined to obtain high strengthhigh ductility combination in 0.12Ce4.89Mn-1.57Al steel. The process referred as multi-step partitioning (MSP) was designed to accomplish the following objectives: (a) enrichment of austenite with Mn to enhance the stability of retained austenite, (b) transformation hardening during quenching in the flash process and (c) stress relaxation and carbon enrichment of retained austenite. The tensile strength of steel increased from~667 MPa in intercritically annealed steel to~986 MPa in flash processed steel. The product of strength and elongation of flash steel and tempered steel were 23.2 GPa% and 24.9 GPa%, respectively and higher than the intercritically annealed steel (21.3 GPa%). The high ductility, especially the uniform elongation of flash steel (16.2%) and tempered steel (19.4%) is attributed to~15e19% by volume of Mn-rich stable retained austenite and efficient TRIP (transformation induced plasticity) effect. Thermodynamic calculations enabled us to understand the partitioning behavior of alloying elements in MSP. C, Mn and Al reverse partitioning during the flash process led to increased stability of retained austenite. The unique distribution of chemical constituents contributed to two types of martensitic transformation during the flash process: (a) austenite / a 0 -martensite transformation dominated at high temperature and contributed to the formation of stacking faults and ε-martensite transformation and (b) austenite / ε-martensite / a 0 -martensite phase transformation dominated at lower temperature. The stability of retained austenite and interaction with stress concentration contributed to highly efficient TRIP effect in flash processed and tempered steel. The experiment findings were consistent with the diffusion-controlled transformation simulation analysis.

Differently heat treated samples of a low alloyed TRIP steel have been investigated using electro... more Differently heat treated samples of a low alloyed TRIP steel have been investigated using electron diffraction techniques in SEM and TEM. Aim was, first, to discriminate the microstructure constituents, austenite, ferrite, bainite, and martensite, second to gain information on the c-a phase transformation mechanisms and third to correlate the mechanical properties and the microstructure of the samples. Bainite always occurs in conjunction with an orientation gradient in the surrounding ferrite matrix. It consists of fine lamellae of ferrite and austenite which show a sharp Kurdjumov-Sachs orientation relationship with each other. This was interpreted in terms of a displacive bainite formation mechanism. The microstructure is formed by growth of c-grains during intercritical annealing and shrinking of these grains during the subsequent cooling without nucleation of new a-grains. The transformation first occurs reconstructively into ferrite and then, at lower temperature, displacively into bainite. The mechanical properties of differently heat treated samples are most strongly influenced by the amount and distribution of carbon in the retained austenite and by the degree of recovery in bainite and austenite.

The purpose of the present contribution is to review the current knowledge about the relationship... more The purpose of the present contribution is to review the current knowledge about the relationship between the micro-structure of cold rolled intercritically annealed low alloy TRIP-aided sheet steels and their mechanical properties from a materials engineering point of view. The focus is on their production in existing industrial lines and on their application in the manufacture of passenger cars with a body-in-white which offers an improved passive safety. The review aims to make clear that although low alloy TRIPaided sheet steel is by now starting to be an established structural material in BIW manufacturing, there is still room for the further optimization of the composition and the processing. In addition, there are still a number of problems related to their physical metallurgy that require a better fundamental understanding.

The good performance of supermartensitic stainless steels is strongly dependent on the volume fra... more The good performance of supermartensitic stainless steels is strongly dependent on the volume fraction of retained austenite at room temperature. The present work investigates the effect of secondary tempering temperatures on this phase transformation and quantifies the amount of retained austenite by X-ray diffraction and saturation magnetization. The steel samples were tempered for 1 h within a temperature range of 600-800°C. The microstructure was characterized using scanning electron microscopy and electron backscatter diffraction. Results show that the amount of retained austenite decreased with increasing secondary tempering temperature in both quantification methods.

In the present study, we fundamentally explore the reasons underlying differences in mechanical p... more In the present study, we fundamentally explore the reasons underlying differences in mechanical properties in hot-rolled 0.2C-1.6Al-6.1Mn-Fe TRIP steels subjected to different heat treatments. Comparing with austenite reverted transformation annealing (ART) process, quenching and tempering (Q&T) process was effective in obtain excellent mechanical properties. [Q&T (UTS: 902-1235 MPa, TEL: 18-42%]; [ART (UTS: 885-945 MPa, TEL: 13-28%)]. In the ART process, long time annealing led to excessive C and Mn enrichment in austenite, which rendered austenite too stable and deteriorated TRIP effect. Furthermore, long time annealing reduced dislocation density and led to low work-hardening rate. The Q&T process enabled appropriate enrichment of elements and hence desired stability for significant TRIP effect to be observed. Thus, the steel quenched from 625 ℃ exhibited best combination of mechanical properties (UTS: 1038 MPa, TEL: 42%, UTS ×TEL: 43.6 GPa%) 2 because of significant contribution of TRIP effect and high dislocation density in austenite.

The effect of the microstructural characteristics of retained austenite on its transformation sta... more The effect of the microstructural characteristics of retained austenite on its transformation stability in steel after Quenching and Partitioning (Q&P) was studied via interrupted tensile tests and Electron Backscatter Diffraction measurements on a pre-determined zone of a micro-tensile test sample. The evolution of the retained austenite fraction was obtained as a function of the plastic strain. The dependence of the austenite transformation stability on the corresponding grain size, morphology, and local crystallographic orientation was discussed. Furthermore, the importance of the parameters on the austenite stability was analysed and it was shown that the austenite grains rotated, in addition to being transformed, constituting therefore an additional contribution to the ductility of Q&P steel.

With a suite of multi-modal and multi-scale characterization techniques, the present study unambi... more With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100 MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800 MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels.