Studies On Oxidation Kinetics And Thermal Treatment Process Of Hot-rolled Steels

The final structure of oxide scales formed in the hot-rolled process of hot-rolled sheet steels depends on coiling temperature and cooling condition. Investigation of the structure of oxide scales on hot-rolled sheet steels and the phase transformation mechanism have great significance to the steel industry.The four types of hot-rolled sheet steels SS400,DC01,QstE500TM and SPHC produced by Bao Steel were oxidized in the range of 550-950℃in air for 30 min, and their corresponding oxidation kinetics were obtained. By fitting the thermal gravimetric curves, it was found that the oxidation kinetics of all steels followed parabolic law. The corresponding oxidation rate constant decreased as the order of:SPHC> QstE500TM> DC01> SS400, when the oxidation temperature was above 600-700℃.SS400 steel was oxidized at 800℃,700℃and 630℃in air for different time firstly, then heat treated in vacuum or high purity N2. The XRD quantitative analysis indicated that Fe3O4 and Fe2O3 in the oxide scales could be transferred to FeO when the initial oxidation temperature and subsequent thermal treatment temperature were the same, and the fraction of FeO increased according to a parabolic law, which showed that the control-step of the phase transformation was ion diffusion. The transformation rate constant was higher during thermal treatment in N2 than that in vacuum. Lower the heat-treat temperature, and thicker the initial FeO layer,smaller the phase transformation rate.SS400 and QstE500TM hot-rolled steels with initial oxide layer were thermally treated in different conditions, and four types of the oxide scale were obtained, including the reductive quick-cooling oxide layer, the reductive slow-cooling oxide layer, the proeutectoid reaction+Seam oxide layer, and the delay-eutectoid reaction oxide scale. The oxide surface hardness and oxide adhesion were determined by using a superficial hardness tester and scratch tester respectively. The results indicated that the hardness and adhesion of initial oxide layer decreased after heat treatment, but heat-treating condition had little effect on their surface hardness and oxide adhesion.