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The study concerns the influence of heating rate during continuous annealing on microstructure and mechanical properties of high-strength ferrite–martensite dual-phase (DP) steel. The study suggests that the superheat (ΔT), carbon diffusion and distribution were strongly influenced by increasing the heating rate, leading to different microstructure and mechanical properties of DP steel. The morphology of martensite was observed to shift from network along ferrite grain boundaries to banded structure to uniformly dispersed, and the average ferrite grain size was refined from 5.3 to 2.0 µm. When the heating rate was 300 °C/s, the steel exhibited excellent mechanical properties with tensile strength (UTS) of 982 MPa, total elongation (TEL) of 17.5%, strength–elongation balance (UTS × TEL) of 17.2 GPa%, because of finer microstructure, uniform martensite distribution and low carbon content of martensite. Moreover, the precipitation strengthening effect of NbC is an important reason for the high-strength DP steels. In addition, the variation in strain-hardening behavior and fracture mechanism of steel subjected to different heating rates is discussed in relation to the microstructure. [ABSTRACT FROM AUTHOR]

Ultrafine-grained dual-phase (UFG-DP) steel consisting of ferrite (1.2 μm) and martensite (1 μm) was uniquely processed via combination of hot rolling, cold rolling and continuous annealing of a low-carbon Nb-microalloyed steel. Room temperature tensile properties were evaluated and fracture mechanisms studied and compared to the coarse-grained (CG) counterpart. In contrast to the CG-DP steel, UFG-DP had 12.7% higher ultimate tensile strength and 10.7% greater uniform elongation. This is partly attributed to the increase in the initial strain-hardening rate, decrease in nanohardness ratio of martensite and ferrite. Moreover, a decreasing number of ferrite grains with {001} orientation increased the cleavage fracture stress and increased the crack initiation threshold stress with consequent improvement in ductility UFG-DP steel. [ABSTRACT FROM AUTHOR]

Two different morphologies of martensite in dual phase (DP) steel were obtained using two different processing routes. In one case, intermediate quenching (IQ) was adapted, where DP steel was water-quenched to obtain martensite phase, followed by inter-critical annealing. In the second case, the steel was cold rolled, followed by inter-critical annealing (CR-IA). For IQ and CR-IA steels, the inter-critical temperatures varied from 750 °C to 850 °C to obtain different volume fractions of martensite. An understanding of structure–property was obtained using a combination of scanning electron microscope (SEM), transmission electron microscope (TEM), and tensile tests. It was observed that fibrous martensite presented in IQ samples, gradually transformed to blocky martensite with increase in inter-critical temperature, resembling the CR-IA steels. The fibrous martensite encouraged martensite cracking, however, the martensite cracking was dramatically decreased in the IQ samples with increase in martensite fraction. The strain hardening behavior studied using the differential C – J model indicated multistage depending on the fraction of martensite. The low volume fraction of martensite in the DP steel provided high ductility–toughness combination and improved strain hardening ability due to the presence of soft ferrite phase in DP steel. Fibrous martensite in DP steel resulted in less strain hardening than blocky martensite, prior to exceeding a threshold volume fraction. The threshold value was significantly smaller for DP steel with blocky martensite. [ABSTRACT FROM AUTHOR]

Deep cryogenic treatment (DCT) provides an effective and convenient pathway to improve the comprehensive mechanical properties of multiphase steels. However, the transformation characteristics of retained austenite (RA) during the DCT and its effect on the precipitation behavior and bainitic transformation at the subsequent tempering stage are still unclear. In the present work, DCT is applied to a 1500 MPa grade ultrahigh-strength bainitic steel subjected to quenching and austempering treatment (QAT). The same region before and after DCT is selected and characterized by electron backscatter diffraction (EBSD), and the observations show that blocky RA located at the boundaries of primary martensite and prior austenite undergoes martensitic transformation. Dilatometry and transmission electron microscopy (TEM) characterization methods were used to elucidate the effects of tempering temperature and DCT on the competitive relationship between bainitic transformation and nanoscale carbide precipitation during tempering. Compared with QAT steel, the yield strength of QAT-C-T steels is significantly enhanced by 199–292 MPa, depending on the tempering temperature, due to the precipitation of nanoscale carbides and the elimination of unstable RA. EBSD characterization of RA at different tensile strains reveals that large blocky RA at prior austenite grain boundaries and fine RA between the bainite laths transform to martensite during uniform deformation and the necking stage, respectively. The evolution of RA during DCT and tensile strain demonstrates the correlation of its thermal and mechanical stabilities. • Effects of DCT and tempering on structure-properties of bainitic steel are studied. • DCT eliminates 5.5% blocky RA distributed at PAGB as observing same region via EBSD. • DCT affects both carbide precipitation and bainitic transformation during tempering. • DCT increases yield strength by eliminating unstable RA and promoting precipitation. • Thermal and mechanical stabilities of RA are interdependent in bainitic steel. [ABSTRACT FROM AUTHOR]

• Primary solidification was changed from δ -ferrite to γ phase with Ni content increased. • Microstructure in fusion zone was changed with Ni content increased. • Full martensite was obtained in the fusion zone of Al-Si coated PHS, with 60 μm Ni foil. In this present, hot-rolled Al-Si coated press hardened steel (PHS) with 1.5 mm thick was laser tailor welded using Ni foil as an interlayer. The effect of Ni foil thickness on the microstructure evolution of fusion zone (FZ) was studied. With the Ni foil thickness increased, Al had a uniform profile in the FZ but Ni had a segregation in the fusion boundary. JMatPro analysis results showed that the fraction of δ -ferrite at high temperature was decreasing and the primary solidification phase was changed from δ -ferrite to γ phase. The fusion boundary and FZ center had the similar solidification phase evolution during laser welding. The microstructure was the composition of δ -ferrite and lath martensite when the Ni foil was 30 μm, but it was full martensite with the Ni foil increased to 60 μm. Austenite was present in the fusion boundary with the Ni foil increased to 100 μm but it was still lath martensite in FZ center. [ABSTRACT FROM AUTHOR]