Micro-laminated and ultrafine-grained dual-phase steel plates generated via intercritical rolling followed by water quenching

Abstract

In this study, a kind of heterogeneous structured dual-phase (DP) steels with micro-laminated and ultrafine-grained microstructures are obtained via intercritical rolling followed by water quenching. The microstructures are tailored by changing rolling temperature, i.e., reheating temperature. We find that with the decrease in reheating temperature from 810 °C to 750 °C, martensite aspect ratio increases from 4.5 ± 3.5 to 6.1 ± 2.9, and martensite long axis elongates from 1.72 ± 0.09 μm to 2.86 ± 0.98 μm. Meanwhile, the size of ferrite grain surrounded by high-angle grain boundaries refines from 1.98 ± 2.32 μm to 1.47 ± 2.14 μm. The yield strength improves 78 MPa from 432 ± 3 MPa to 510 ± 6 MPa, and the tensile strength increases 114 MPa from 745 ± 5 MPa to 859 ± 1 MPa. An important find is that the strain hardening rate increases abnormally with the refinement in ferrite grain due to enhanced hetero-deformation induced (HDI) hardening. So, the downtrend of tensile ductility varies with the improvement in strength is nearly stopped when reheating temperature reduces from 780 °C to 750 °C. Therefore, better comprehensive mechanical properties of DP steels are obtained compared with the approximately equiaxed and coarse-grained DP steel plate. The microstructure evolution mechanism is discussed from the perspective of deformation-induced ferrite transformation and ferrite dynamic recovery. The change of HDI hardening is related to the microstructure feature of the tested DP steels.