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The effects of different quenching methods on the microstructure and mechanical properties of 30MnB5NbTi hot stamping steel are investigated, and the quenched microstructure is characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The mechanical properties are evaluated by uniaxial tensile test. The results reveal that, in comparison to die quenching, oil quenching, or air cooling, water‐quenched samples (tempered after water quenching, die quenching and oil quenching) exhibit the highest ultimate tensile strength (UTS) and yield strength (YS), reaching 2052 and 1422 MPa, respectively. Various quenching methods enable multi‐level strength control for the same steel grade, achieving a strength control range of ≈1000 MPa. The microstructure of samples quenched by water, die, and oil is fully martensite, and martensite exhibited block and lath morphology. With the increase of cooling rate, martensite laths become finer and the prior austenite size decreased. The microstructure of air‐cooled samples is martensite, ferrite, and retained austenite. The strengthening mechanisms of different quenched samples are calculated. The results show that dislocation strengthening and precipitation strengthening are the two dominant strengthening mechanisms in 30MnB5NbTi hot stamping steel. Therefore, the properties of hot stamping steel can be improved by changing the cooling rate by designing physical dies. [ABSTRACT FROM AUTHOR]

Herein, the effect of strain rate (10−4–10−1 s−1) on the microstructural evolution and deformation behavior of medium‐Mn transformation‐induced plasticity (TRIP) steels is investigated. The steel after intercritical annealing exhibits an austenite–martensite–ferrite triplex microstructure with a relatively high austenite fraction (66.1%). A negative strain rate dependence of ultimate tensile strength (UTS) and significant serrated behavior are found during the room temperature tensile test. The total elongation (TE) attains a maximum value of 32.0% at the strain rate of 10−2 s−1 and then decreases with increasing strain rate. The evolution of strain‐induced martensitic transformation (SIMT) as a function of strain rate is analyzed considering the adiabatic heating during deformation. The different degrees of SIMT under different strain rates lead to a fracture transition between quasicleavage fracture and ductile fracture. The delamination cracks originating from the interface of martensite and austenite are closely related with the quantity and the local strain of the martensite–austenite interface. [ABSTRACT FROM AUTHOR]

In view of the poor applicability of medium Mn steel, this study designed an application-friendly medium Mn steel with 4 wt% Mn, Fe–4Mn-1.5Al-0.5Si-0.2C-0.05Nb. The tested steel achieves high mechanical properties with the ultimate tensile strength >1000 MPa and the total elongation >30%. Through mechanical properties testing and microstructure characterization and theoretical calculation, the influence of intercritical annealing (IA) temperature on the microstructure evolution and mechanical properties of cold-rolled steel was studied. The highest austenite volume fraction (19.8%) and the best comprehensive mechanical properties (yield strength: 653 MPa; ultimate tensile strength: 1084 MPa; total elongation: 30.2%) were achieved in the sample annealed at 720 °C through reasonable microstructure construction and coupling of various strengthening mechanisms. The response relationship between strengthening and toughening mechanism at the microscale, microstructure characteristics at the mesoscale, and mechanical properties at the macroscale were obtained. Among them, solid solution strengthening, grain boundary strengthening and dislocation strengthening play a significant role in the multi-mechanism coupling effect of yield strength. In the α+γ+α' triplex phase system, the increase of IA temperature leads to the increase of austenite and martensite fraction, resulting in a linear negative correlation between tensile strength and ferrite fraction as the other phase. In the plastic deformation, the TRIP effect of partial austenite and dislocation multiplication of ferrite contribute to the dominant work hardening effect, in which the former mainly plays a role in the first half of the deformation stage, while the latter plays a uniform role in the whole deformation stage. [ABSTRACT FROM AUTHOR]

The microstructure characteristics and yielding behavior of two Fe–4Mn–2Al-0.2C steels with different Nb contents, i.e. 0.02 wt% and 0.1 wt%, were investigated in the present study. The results show that the increase of Nb content will bring about 50–100 MPa more fine-grain strengthening effect. But on the other hand, Nb promotes the precipitation of cementite and delays recrystallization, so that the stability of austenite decreases and presents a single lathy morphology. The partial martensitic transformation of austenite in the cooling process promotes the diffusion of carbon from martensite to retained austenite, and causes the segregation of carbon at the austenite phase boundaries, leading to the Lüders effect. The steel with 0.02 wt% Nb undergoes a more persistent TRIP effect during the deformation process due to two austenite morphologies with higher volume fractions and exhibits excellent comprehensive mechanical properties with the yield strength of 570 MPa, ultimate tensile strength of 1011 MPa, total elongation of 35.6% and product of strength and elongation of 36 GPa·% after intercritical annealing at 725 °C for 30 min without the appearance of Lüders effect. Thus, this paper is of great importance to the effect of Nb contents on microstructure and yielding behavior of medium Mn steels. [ABSTRACT FROM AUTHOR]

A multi-heterogeneous austenite morphologies consisting of equiaxed + coarse lath-like + fine lamellar austenite grains is constructed in 3Mn steel through warm rolling in the dual-phase region and tempering. Lower tempering temperature induces a more uniform distribution of multi-heterogeneous austenite morphologies and effectively preserves the stacking fault network formed during warm rolling, which catches exceptional mechanical properties (ultimate tensile strength of 1591 MPa and total elongation of 24.5 %) as well as suppresses plastic instability behaviors including yield plateau and stress serrations in 3Mn steel. [Display omitted] [ABSTRACT FROM AUTHOR]

Nb, V, and Ti are alloying elements that are often added to Marine steel to improve the mechanical properties. In this paper, the microstructure characteristics and impact fracture mechanisms of Nb and V–Ti micro-alloyed offshore platform steels were investigated. The results show that Nb plays a better role in prior austenite grain refinement and precipitation behavior, while V/Ti is better in bainite/martensite laths refinement and stabilizing retained austenite. In terms of mechanical properties, Nb steel exhibits better yield strength, ultimate tensile strength, and Vickers hardness, while V–Ti steel performs better in total elongation and low-temperature impact toughness. The fracture surface of V–Ti steel exhibited a mixed fracture mode with the ductile and cleavage region while that of Nb steel reveal a cleavage failure without any ductile features. The ductile fractures were deduced from retained austenite deformation and the cleavage fracture areas were mainly correlated with martensite and bainite deformation. When the impact load was applied, the cracks first nucleated between lathy martensite and lathy bainite and then propagated along with the interface. Some cracks terminated after encountering M/A constituents and produced the secondary cracks, and some would encounter grain boundaries and formed microcracks. • Nb micro-alloy element could retard austenite grain-growing more effectively than V–Ti during TMCP. • Nb steel exhibits better strength and hardness while V–Ti steel performed better in total elongation and impact toughness. • Cracks first nucleated between lathy martensite and lathy bainite and expanded along with the interface, terminated at M/A or grain boundaries. [ABSTRACT FROM AUTHOR]

The microstructural evolution and the high-temperature tensile properties of a novel Nb-Ti-N alloyed austenitic heat-resistant cast steel were investigated during the isothermal aging at 900 °C for up to 1000 h. The mechanisms of enhancing strength-ductility synergy and the high-temperature tensile fracture behaviors after the aging process were analyzed in detail through tensile tests at 900 °C. Obtained results show that the high-density dislocation regions and the Cr segregation, formed around the intragranular primary (Nb,Ti)(C,N), provided numerous nucleation sites for submicron-sized secondary M 23 C 6 precipitates. The density of secondary M 23 C 6 significantly increased, while the coarsening rate was notably reduced during the aging process. Simultaneously, the uniform dispersed precipitation of nano-sized secondary (Nb,Ti)C was promoted by massive dislocations, which exhibited high resistance to coarsening during long-term aging. The improvement of high-temperature strength was attributed to the precipitation strengthening effects induced by the submicron-sized M 23 C 6 and nano-sized (Nb,Ti)C particles. Additionally, the enhancement of ductility during aging was mainly due to the more effective stress transmission and the ameliorated interface relationship between primary (Nb,Ti)(C,N) and austenitic matrix, which was facilitated by the abundant precipitation of secondary phases. The current findings indicate that high-density dislocations constructed by the intragranular primary (Nb,Ti)(C,N) induce the extensive dispersion of secondary strengthening phases, which enhanced both ultimate tensile strength of 158 MPa and elongation of 45.3% at 900 °C after isothermal aging for 1000 h. • High-density dislocations are constructed by the intragranular primary (Nb,Ti)(C,N). • High-density dislocations provide nucleation sites and induce the extensive dispersion of secondary M 23 C 6 and (Nb,Ti)C. • The secondary M 23 C 6 and (Nb,Ti)C obstruct the movement of dislocations and enhance the high-temperature strength. • The enhancement of ductility was due to the more effective stress transmission and the ameliorated interface relationship. [ABSTRACT FROM AUTHOR]

[Display omitted] • The introduction and retention of austenite during warm rolling is beneficial to the improvement of mechanical properties. • Warm rolling in dual-phase region and tempering induces the formation of multi-heterogeneous austenite morphologies. • The formation of superlattice structures is attributed to the construction of stacking fault networks. Facing the current lack of clarity in temperature selection mechanisms for warm rolling in medium Mn steel, three warm rolling methods were conducted in 3Mn steel according to the rolling temperatures in the dual-phases region or ferrite region. Subsequently, the effect of heat treatment on the austenite morphologies and tensile properties of warm-rolling samples was studied. Rolling in dual-phase regions (750 °C) led to heterogeneous austenite morphology composed of equiaxed and lamellar grains while rolling in the ferrite region (600 °C) resulted in significant cementite precipitation and the absence of retained austenite. Warm rolling promoted the formation of dislocation cells in the lath-like grains, and they did not evolve into grain boundaries completely because low deformation temperature didn't trigger the complete dynamic recrystallization. Tempering after rolling provided a significant enhancement in plasticity through the enrichment of C and Mn in austenite and the formation of multiple heterogeneous austenite morphologies with equiaxed, coarse lath-like and fine lamellar grains. In addition, superlattice structures in the matrix are formed through the formation of stacking fault networks and the spinodal decomposition. Ultimately, the exceptional properties were achieved in 3Mn steel, with an ultimate tensile strength of 1409 MPa and a total elongation of 27.3 %. [ABSTRACT FROM AUTHOR]

This work explores the carbide evolution behavior and mechanical property evolution mechanisms of intercritical-annealed 3Mn steel during the tempering process by comparing different tempering times (5/10/20 min). The results showed that the elongation of 3Mn steel increased from 19.5% to 26.5% after tempering while maintaining a tensile strength above 1250 MPa. It is also found that carbide precipitation during tempering is first followed by dissolution. The mechanical properties exhibited abnormal degradation at tempering for 10 min, with a tensile strength of 1265 MPa and a total elongation of 16.0%. However, the optimal mechanical properties were achieved at tempering for 20 min, with a tensile strength of 1286 MPa and a total elongation of 26.5%. During the initial stage of tempering, a transformation occurs from M 3 C to stable M 7 C 3 , with the presence of numerous dislocations and grain boundaries promoting the precipitation and growth of abundant carbides. This leads to a reduction in the C and Mn content in the matrix, consequently decreasing the austenite content and stability, resulting in the transformation of austenite to the BCC phase during tempering instead of cooling. The decreased austenite content and stability lead to reduced ductility. Subsequently, dislocations were annihilated during tempering, suppressing carbide precipitation, and the over-precipitated carbides dissolved under thermodynamic driving force, leading to an increase in C and Mn content in the remaining austenite, thus enhancing its stability. The precipitation and dissolution of carbides during tempering, along with the partitioning of elements between carbides and austenite, strengthen austenite stability and improve mechanical properties. • During tempering, M 3 C was transformed into M 7 C 3 as well as the carbides precipitated first and then dissolved. • During tempering, dissolution of carbieds leads to the partitioning of C and Mn to retained ausetnite, which strengthens the stability of austenite. • Tempering promotes the product of strength and elongation from 26.7GPa∙% to 34.1 GPa∙%. [ABSTRACT FROM AUTHOR]

Isothermal Bainite Transformation (IBT) generates heterogeneous grain structures, leading to increased strain-hardening ability and inhibiting yield plateau and stress serrations. IA-steel and IBT-steel have ultimate tensile strengths of 1306 MPa, 1354 MPa, and elongations of 20.0%, 19.2%, respectively. IBT effectively suppresses plastic instability while maintaining mechanical properties. [ABSTRACT FROM AUTHOR]