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The microstructural characteristics and low-temperature impact toughness of simulated reheat coarse grain heat affected zone (CGHAZ) of high strength medium-Mn steel were investigated based on thermal simulation. The results indicated that the impact energy of unaltered reheated CGHAZ (UA CGHAZ), supercritical reheated CGHAZ (SCR CGHAZ), intercritical reheated CGHAZ (ICR CGHAZ) and subcritical reheated CGHAZ (SR CGHAZ) at –40 °C was 38, 65, 26 and 9 J, respectively, and the microhardness was 376, 385, 400, and 329 HV, respectively, at welding heat input of 15 kJ/cm. Fine martensite lath microstructure and the high proportion of misorientation boundaries in SCR CGHAZ induced relatively higher impact toughness and microhardness. The microstructure of ICR CGHAZ had inferior uniformity and consisted of lath martensite with high dislocation density and lamellar twin martensite. Thereinto, twin martensite with higher hardness and less toughness deteriorated the ICR CGHAZ toughness. The segregation of Mn at grain boundaries reduced the adhesion between grain boundaries and led to severe embrittlement of SR CGHAZ. [ABSTRACT FROM AUTHOR]

Microstructure, texture and mechanical property of Fe-27Mn-3Si-4Al TWIP steels during asymmetric warm rolling were studied. It was found that asymmetric warm rolling strongly refined the microstructures and affected texture evolution of steels because of an additional shear strain in contrast to the symmetric warm rolling. The complete recrystallization occurred on the upper and central layer of sheet, and Goss and Brass textures was developed into γ-fiber and R-Copper {112} <110> textures when the velocity ratio increased to 1.2. The combination of applied asymmetry rolling at elevated reduction resulted in a high yield strength (706 MPa) due to the enhanced shear deformation on the one hand and a reasonably high elongation (26.3%) on the other hand which was attributable to the grain fragmentation at higher velocity ratio, while the significant enhancement of ductility (30.9%) was achieved by the higher dislocation activation due to the decrease of rolling reduction. [ABSTRACT FROM AUTHOR]

The microstructures and mechanical properties of 500 MPa grade gantry mounting structural steel at three different finish rolling temperature processes were investigated. It was found that the microstructures were polygonal ferrite, pearlite and nanometer-sized precipitates. With the decrease in finish rolling temperature, the tensile strength and impact energy significantly increased, and the cleavage fracture gradually transformed into ductile fracture mode. The specific strengthening mechanisms were discussed. Maximum tensile strength (690 MPa) and Charpy impact toughness (121.5 J) were observed at the finish rolling temperature of 860 °C. The formation of fine nanometer-sized precipitates with elliptical and rod-shaped was mainly through the mechanism of interphase precipitation and dispersion precipitation. The precipitates with less than ~ 10 nm in diameter and less than ~ 30 nm in row spacing primarily formed during γ/α phase transformation after hot rolling process. [ABSTRACT FROM AUTHOR]

Dynamic deformation behavior of a cold-rolled Fe-1.47Mn-1.40Si-0.21C-0.025Nb transformation-induced plasticity (TRIP)-aided steel was studied at strain rates in the range of 0.01-200 s−1. An understanding of dynamic deformation behavior is important from the perspective of automotive applications and there exists a gap in the knowledge. A novel approach of using a tensile specimen with variable cross section is proposed to realize the controllable strain experiment and study the dynamic deformation process. The study indicated that the increase of adiabatic temperature during dynamic tensile deformation increased the additional mechanical driving force, leading to increase in the stability of retained austenite (RA). The degree of transformation and the rate of transformation of RA during dynamic tensile straining was less than the quasi-static tensile deformation. The microstructure of experimental steel was mainly composed of polygonal ferrite, bainite and RA, and majority of large blocky RA at the ferrite grain boundaries transformed to martensite during the initial stage of dynamic deformation. With the increase of strain, RA with smaller average grain size transformed to martensite. At the end of deformation, only a few small granular RA that was too stable did not transform. With the increase of strain rate from 0.01 to 200 s−1, both yield strength and ultimate tensile strength (UTS) of the experimental steel increased monotonously. However, the total elongation (TEL) initially decreased and then increased. The experimental steel exhibited remarkable mechanical properties with the product of UTS and TEL (PSE) of 25.6 GPa% at strain rate of 200 s−1. The study underscores the significance of strain rate in designing TRIP steel. [ABSTRACT FROM AUTHOR]

We describe here microstructural characteristics and low temperature impact toughness relationship of simulated heat affected zone (HAZ) in low-C medium-Mn high strength steel based on thermal simulation. The results indicated that the low-temperature impact energy of coarse grained HAZ (CGHAZ) was decreased from 39 to 29 J with the increasing simulated welding heat input from 10 to 30 kJ/cm. The deterioration of toughness in CGHAZ was mainly attributed to coarse martensite lath. The low-temperature impact energy of CGHAZ, fine grained HAZ (FGHAZ), intercritical HAZ (ICHAZ) and subcritical HAZ (SCHAZ) at − 40 °C was 38, 92, 57 and 146 J, respectively, and the microhardness was 381, 399, 321 and 282 HV, respectively, at simulated welding heat input of 15 kJ/cm. High proportion of high misorientation boundaries and fine martensite lath microstructure in FGHAZ induced relatively good low temperature impact toughness and highest microhardness. The microstructure of ICHAZ comprises tempered martensite, retained austenite and non-uniform martensite, which reduced the impact toughness to some extent. The microstructure and properties of SCHAZ did not change significantly compared to the base metal because of lower peak temperature. [ABSTRACT FROM AUTHOR]

The impact of cooling rate on austenite stability and mechanical properties of a cold-rolled transformation-induced plasticity (TRIP) steel with nominal chemical composition of Fe-11Mn-4Al-0.2C (wt.%) was elucidated in the present study. Mechanical properties were obtained by tensile test and microstructure was analyzed by XRD, SEM, EMPA. Intercritical annealing in the range of 700~800 °C for 5min followed by either water-quenched or air-cooled, both led to a ferrite–austenite mixed microstructure. However, the austenite stability varied with the annealing temperature. The water-quenched and air-cooled samples annealed at 750 °C with medium austenite stability demonstrated significantly higher ductility than the samples annealed at 700 and 800 °C. The superior total elongation (TE) in 750 °C sample was not only contributed by TRIP effect of austenite, but also the cooperative deformation of ferrite. The air-cooled samples were characterized by higher austenite stability and ultimate tensile strength (UTS), but lower total elongation compared to the water-quenched samples, which is because the TRIP effect in the air-cooled samples without the cooperation of ferrite contributed primarily to work hardening and UTS, but less to TE. Moreover, the critical factor in controlling austenite stability of water-quenched sample was morphology, granular austenite showing the highest stability; while the dominant factor in austenite stability of the air-cooled sample was composition, enrichment of Mn and C improving austenite stability. [ABSTRACT FROM AUTHOR]

A comprehensive analysis and characterization of granular structure (GS) and granular bainite (GB) were conducted in a low-carbon alloy steel with the objective to elucidate the mechanisms that contributed to strength–ductility combination. Ferrite + martensite–austenite (M-A) islands were observed in both GS and GB, but a transformation mechanism produced a variation in the matrix and in the M-A size, distribution and morphology. The differences in the matrix and M-A islands were carefully studied by electron backscatter diffraction. Furthermore, tensile tests were interrupted at different strains to study the microstructure evolution during tensile straining and mechanical properties determined from samples tensile strained to fracture. It was observed that the specimen with GB exhibited higher tensile strength, ductility and strain hardening compared to GS. Dislocation strengthening of GB was more apparent than GS. The higher volume fraction and uniform distribution of M-A islands also contributed to the higher strength of GB. The improvement in ductility may partly be attributed to the sub-structural refinement. An increased presence of geometrically necessary dislocations array in GB also simultaneously enhanced strength and ductility. During deformation, GB exhibited a more uniform local strain distribution compared to GS because of superior plastic ability of M-A islands. Based on the experimental results and analysis, we propose models describing the different failure mechanism in GS and GB. [ABSTRACT FROM AUTHOR]