Microstructure and Properties of 980MPa Ultrafine-grained Cold Rolled Dual-Phase Steels

摘要

Two kinds of 980MPa grade cold rolled dual phase steels have been developed by designing C-Si-Mn and C-Si-Mn-Nb alloy systems. The microstructure of martensite in Nb-free steel is consisted of lath martensite and twined martensite with the volume fraction of 67%. However, the main hard phase in Nb-containing one is twined martensit with the volume percent of 59%. The size of martensite islands in Nb-containing steel is from 1μm to 3μm, and the size of NbC precipitates is from 15nm to 40nm. As to Nb-containing steel, the yield strength, tensile strength, yield ratio and elongation are 501MPa, 1035MPa, 0.48 and 17.5% respectively. Futhermore, Nb-containing steel has higher working hardening rate value above the critical strain 6.5%. And it decreases slowly with increasing the strain. This is mainly because of ultrafine grain size and nano-precipitates in ferrite, which improves the compatibility of phases and reduces the stress concentration at the phase interface.