Microstructure And Mechanical Properties Of ST52-3G Steel With Isothermal Heat Treatment

With the development of industry,especially the development of automobile manufacturing industry,the demand for cold-rolled steel sheet has been increasing rapidly because cold-rolled steel plate has the characteristics of high functionality,abundant raw material production,relatively low production cost and mass production.It is difficult to replace in a short period of time.With the increasing performance of steel,the improvement of mechanical properties and performance prediction of steel has become a hot research.The heat treatment as a way to improve the steel structure and performance is necessary to make a in-depth study.In this paper,according to the user’s requirements for mechanical properties of cold-rolled steel plate and the actual situation of cold-rolled steel plate,ST52-3G steel was used to study for the regularity of the microstructures,especially the recrystallization of ferrite in different isothermal annealing temperature and holding time by Gleeble-1500 D thermal physical simulation test machine.Also,mechanical performance prediction model under different isothermal annealing temperatures and holding times was established for providing a reference for the actual production to produce a reasonable heat treatment process.The main contents of this paper include the following:ST52-3G steel cold-cold austenite continuous cooling phase transformation curve(CCT)was measured.According to the expansion curve measured by the expansion instrument of Gleeble-1500 D thermal simulation test machine,Ac1,Ac3 and phase transition points measured by tangent method and metallographic-hardness method,the CCT curve of ST52-3G steel was obtained.At the same time,Ac1 and Ac3 during rapid heating at 50 ℃/s were measured.The critical transition point for rapid heating is significantly higher than the standard condition.Isothermal heat treatment experiments were studied.The main parameters of the isothermal heat treatment process are the heating temperature and the holding time.The results show that the heating temperature of the ST52-3G steel between 500 ℃ ~ 550 ℃,the microstructure only recovery occur,but at 650 ℃~ 850 ℃ the ferrite recrystallization occur,last soaking at 950 ℃ follow by ferrite transformation,pearlite transformation and bainite phase change during the cooling process.And the empirical model of recrystallized ferrite grain size is established,and the fitting coefficient reach 0.9830.Mechanical model for isothermal heat treatment was established.The mechanical properties of ST52-3G steel were investigated by isothermal heat-treatment.The mechanical property models of ST52-3G steel were established based on the JMAK model on both the recrystallization and non-recrystallization part of the weighted average method.The results show that the average relative errors of the yield strength,tensile strength and tensile are respectively 0.9761,0.9175 and 0.9429,their average absolute errors are 14.54 MPa,23.44 MPa and 1.07%,respectively and the average relative errors are 2.47%,2.67% and 6.72% respectively too.So that the model has good accuracy,so as to provide theoretical reference for practice.