Effect of Hot Rolling Process on Microstructure, Properties and Surface Quality of 420 MPa Grade Thin Steel Strip

The microstructure, mechanical properties and oxide scale evolution of 420 MPa grade thin steel strip under different hot rolling processes were analyzed by using optical microscope and scanning electron microscope. The results show that different cooling rate is the main reason for the difference in microstructure and mechanical properties of steel strip. Under low temperature rolling and rapid cooling process, the microstructure of the strip steel at edge area presents medium-temperature transition characteristics, the transverse structure is different obviously. The yield strength of the steel strip is 458 MPa, the elongation is 26.46%. Under high temperature rolling and slow cooling mode, the uniformity of transverse microstructure is improved, and the yield strength is decreased by 25 MPa, the elongation is increased by 2.50%. The process route of low temperature rolling and rapid cooling inhibits the high temperature oxidation of Fe₃O₄, and the thickness of iron oxide in the middle of strip steel is 1.13 μm less than that produced under high rolling temperature and low cooling rate process. Coiling temperature affects the low temperature transformation of FeO. When the coiling temperature is 561℃, the（Fe+Fe₃O₄） eutectoid structure is fine and its transformation is full. When the coiling temperature is 584 ℃, the transformation amount of eutectoid structure is decreased, while the amount of proeutectoid Fe₃O₄ and residual FeO is increased.