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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.
Two kinds of 980 MPa 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 1 μm to 3 μm, and the size of NbC precipitates is from 15 nm to 40 nm. As to Nb-containing steel, the yield strength, tensile strength, yield ratio and elongation are 501 MPa, 1035 MPa, 0.48 and 17.5% respectively. It is slowly due to 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.