Hot-rolled DP780 Manufacture And The Study Of Its Organization And Dynamic Performance

The automotive steel's need for high strength raised the demands for hot-rolled dual-phase steel with high strength and good formability,but dual phase steel with high strength was mainly produced by cold strip plant.Therefore,in this article,a special chemical composition steel was desighed and smelt,a hot-rolled dual-phase steel with780MPa tensile strength was developed by thermal mechanical control process?TMCP?.The influence of the cooling parameters on the microstructures and properties of the steel were studied.The experimental results showed that the control parameters was critical to the mechanical properties.After hot rolling,the steel was cooled to a desighed isothermal temperature in the salt bath and hold for a few minutes so that the austenite to ferrite transformation could proceed for a certain degree.When the temperature was too high or the holding time was too short,the content of martensite in dual phase steel would be too high,this made the steel with a high strength but insufficient elongation which was featured by A_50mm.when isothermal temperature increased,the strength and yield ratio of the dual-phase steel increased,but the values of A50mm and n decreased.While with the increase of isothermal holding time,the strength and yield ratio of dual-phase steel decreased,but the A_50c0c and n values increased.When the temperature was 633?and holding time was in the range of 5s-11s,the mechanical properties of dual-phase steel met the national standard.The properties of the steel were mainly determined by the microstructure.When the volume fraction of martensite?greater than 28 vt%?increased,the strength and yield ratio of the dual-phase steel were gradually increased,but the elongation A_50mm0mm and n values decreased.When the martensite islands were smaller and more dispersive,the the elongation A_50mm0mm of dual-phase steel was larger.Refined ferrite grains could increase the strength and yield ratio of dual-phase steel,while increased content of the martensite could increase the n value of dual-phase steel.The optimal volume fraction of martensite was 32.5-47.3vt%.