Research On Microstructures And Properties Of Low Carbon Si-Mn Steels Based On Dynamic Carbon Partitioning

The DP steels comprised of martensite and ferrite are the main steel for automobile panel.Because the edge of two phases is a weak combination area and it is easy to fracture.It can't meet the demand of high strength.Q&P process is a kind of new type heat treatment process.When the steel is during heating procedure,C atom can diffuse from martensite to autensite.The paper studies that low carbon Si-Mn steels are treated by DQ&P process,it can attain martensite,ferrite and retained autensite.So using the feature of the carbon partitioning process,let retained autensite keep stable,and then keep the grain boundary junction of matensite and autensite having rich carbon,and the grain boundary is not easy to fracture.The paper studies the effect of process parameters on the microstructure and the properties.The main results are as follows:(1)The thermal simulation experiment was carried out to investigate the effect of process parameters on the microstructure and the properties for 0.078%C test steels by a MMS-300 thermo-mechanical simulator.The effect of deformation temperature,coiling temperature and relaxation on the microstructure and the properties for the test steels was studied by microstructure observation and XRD phase analysis.The results showed that,deformation temperature had little influence on the microstructure and the properties for the test steels,and when deformed in 840? because of close to two-phase region,so the test steels had more ferrite.When the coiling temperature was above 300?,the test steels had more retained autensite.Compared with no relaxation,having relaxation was good for the test steels.The volume fraction of retained austenite reached the highest and it was 9.18%when coiling temperature was 360?.(2)The hot rolling experiment was carried out to investigate the effect of process parameters on the microstructure and the properties for 0.078%C test steels.The results showed that,when the deformation temperature was 880?,and the starting cooling temperature was 830?,and coiling temperature was 225 ?,the tensile strength of the test steels was 820MPa,and the total elongation was 24.13%,and the combination of elongation and tensile strength was 20160MPa%,so the test steels had good comprehensive properties.(3)The hot rolling experiment was carried out to investigate the effect of process parameter on the microstructure and the properties for 0.12%C test steels.The results showed that when the deformation temperature was 880?,the start cooling temperature was 760?and the coiling temperature was 330?,the microstructures of the test steels were comprised of martensite and ferrite.The tensile strength of the test steel was 875MPa,and the total elongation was 25.58%,and the combination of elongation and tensile strength was 22380MPa%,and then the retained austenite reached the highest and it was 14.67%.(4)Based on the chemical potential of the ideal solid solution,the thermodynamics of carbon atom partitioning was studied in coiling insulation of DQ&P process.By analyzing the curved surface on chemical potential of carbon atom in martensite and in retained autensite,it was concluded that chemical potential of carbon atom in martensite and chemical potential of carbon in retained autensite can be equal in coiling partitioning of DQ&P process.It can have many temperatures to meet the condition.