Influence Of Rolling And Cooling Processes On Microstructures And Mechanical Properties Of 500MPa Steel Strip For Automobiles

High-strength strip production is one of advantages of the thin slab casting and rollingproduction line. So it is a practical application to produce high-strength auto-used 550MPastrips with V-N micro-alloying by this process. During developing these new steel varieties,the influences of controlled rolling and controlled cooling (TMCP) processes includingheating temperature, reduction rate, rolling temperature, coiling mode, coiling temperature ontheir microstructures and properties were studied in detail in this work. The main conclusionsare as follows.(1) The influence of heating temperature on microstructures and properties of 550MPastrips for automobiles was studied. For the thick V-N micro-alloyed 550MPa automobile steelstrips, when the heating temperature reach to 1100℃, the transverse yield strength is 20MPahigher, the impact toughness at -20℃is 20J higher, and the average grain size is 1.1μmsmaller than that at the heating temperature 1130℃.(2) The influence of rolling temperature on microstructures and properties of 550MPastrips for automobiles was studied. When the rolling temperature is 820℃, the yield strengthis 40MPa higher, the elongation rate is 10% lower, the average grain size is almost sameness.But the cold bending property is not qualified at this rolling temperature. Meanwhile, thereare a lot of acicular ferrites in the microstructure.(3) The influence of reduction rate on microstructures and properties of 550MPa stripsfor automobiles was studied. When the accumulative reduction rate increases 9%, the yieldstrength is 20MPa higher, the elongation is 4% lower, the average grain size is almostsamensee.(4) The influence of coiling mode on microstructures and properties of 550MPa stripsfor automobiles was studied. Different cooling modes have different cooling rates. Themaximum continuous cooling rate at the tail reaches 65.9℃/s. The minimum interval coolingrate at the head reaches 37.1℃/s. Different cooling modes have little effects on themicrostructures and properties of 550MPa automotive steel strips.(5) The influence of coiling temperature on microstructures and properties of 550MPastrips for automobiles was studied. Compared with the coiling temperature 600℃, when coiling temperature is 550℃, the yield strength is 12.5MPa lower, the elongation is 1.25%higher, the average grain size is almost sameness , and the transverse impact toughness at alow temperature -20℃is 13.4J lower, and the longitudinal one is 14.3J lower.