Research On The Partitioning Parameters And Organizational Control Of Cold Rolled High Strength Dual Phase Steel

A large amount of ultra high strength dual phase steel of cold rolling is used for fabrication of automobiles for weight lighting and emission reduction.The strength of the steel is very high,but the elongation is low,which leads to poor formability and limited application.Therefore,in this article,the continuous annealing after cold rolling of the high strength steel was replaced by the heat treatment of Mn and carbon partitioning.A certain amount of retained austenite is obtained in the final microstructure through this heat treatment,and the plasticity of DP980 steel is improved;the optimum partitioning procedure was successfully applied to hot galvanizing high strength steel of cold rolling.In DP980 steel,the samples were rapidly heated to 780? at rate of 100?/s for 150s of Mn partitioning,and were cooled at quenching rate for heat treatment of carbon partitioning.The microstructure shows a band structure,which is composition of ferrite,martensite and the retained austenite between martensite laths.The temperature of Mn partitioning above 830? leaded to the disappearance of the band,The grain size of ferrite and martensite island gradually grew with the increase of Mn partitioning time at 830?.The analysis result shows that a certain amount of Mn atoms are enriched in martensite islands by Mn partitioning.The heat treatment of carbon partitioning stabilizes the retained austenite in microstructure.When the time of carbon partitioning is 60s,12.5%of the retained austenite is obtained in the microstructure,leading to the increase of elongation to 18%.Also the yield and tensile strength are 851MPa and 131 0MPa respectively.The optimum heat treatment of Mn and carbon partitioning obtained in DP980 was applied for hot galvanizing high strength steel of cold rolling to replace the continuous annealing.The retained austenite of 11.7%is found in the heat treated microstructure when the carbon partitioning time is 30s.The properties of the steel are as fellows:18.5%elongation,722MPa yield strength and 1214MPa tensile strength.However,more precipitation carbides were found in the final microstructure as Si content of the steel is 0.35%wt which is too low to prohibit the precipition.