Microstructure And Properties Of Cold Rolled DP780 Dual Phase Steel

In recent years,with the rapid development of China's automobile industry,the following problems,such as fuel shortage,environmental pollution and traffic safety were becoming increasingly serious.Therefore,the energy-saving,environmental protection,safety and other issues of automobiles are widely concerned by the state and automobile manufacturers.The research shows that a large number of advanced high strength steel used in automobiles had a good effect on solving the above problems.Dual phase steel was a kind of advanced high strength steel,which had the characteristics of high strength,good formability,high initial work hardening rate and good energy absorption.In order to simplify the heat treatment process of DP780 cold-rolled dual phase steel and improve the production efficiency,this paper proposes that the dual phase steel with high strength and toughness can be obtained by combined the cold rolling deformation before heat treatment with its heat treatment process.In this paper,DP780 cold rolled dual phase steel produced by a domestic steel plant was taken as the research object,the metallographic observation and microhardness test methods were used.The effects of different cold rolling deformation on the microstructure recovery,recrystallization and austenite transformation of DP780 cold rolled dual phase steel during heat treatment were studied.The results show that when the cold rolling deformation was 75%,the recovery,recrystallization and austenite transformation of dual phase steel could be effectively promoted.On this foundation,the recovery and recrystallization of ferrite,the evolution of austenization and the distribution of quenched microstructure and hardness of dual phase steel with 75% cold rolling deformation at different heating temperatures(690 ?,750 ?,810 ?,870 ?)and holding times were studied and analyzed.The results show that when the heat treatment temperature was 750 ?,after the completion of the ferrite recrystallization transformation,the austenization transformation would be carried out,and the island martensite dispersed around the ferrite could be obtained.Combined with MSC.MARC nonlinear elastic-plastic finite element software,the cold rolling model of the dual phase steel was established,and the reason of the uneven distribution of the thickness direction structure during the rolling deformation of the dual phase steel was further analyzed.On this foundation,the effects of different holding time(120 s,180 s,240 s,300 s,600 s,1800 s)on the microstructure and properties of dual phase steel were studied when the cold rolling deformation was 75% and the heating temperature was 750 ?.The comparative studied shows that the dual phase steel with better comprehensive mechanical properties could be obtained by quenched after holding time of 300 s.At the same time,the microstructure and properties of the dual phase steel with 75% cold rolling deformation and holding time was 300 s at different heating temperatures(690 ?,750 ?,810 ?,870 ?)were further analyzed.The results show that when the heating temperature was 750 ?,the island martensitic dual phase steel with dispersion distribution could be obtained,which was beneficial to obtain the dual phase steel with better comprehensive properties of strong plasticity.When the heating temperature was 810 ?,the martensite with higher volume fraction can be obtained,which was beneficial to the production of dual phase steel with higher tensile strength.Based on the above experimental studied,the optimized deformation and heat treatment process of DP780 cold rolled dual phase steel was determined as follows: 75% cold rolling deformation of dual phase steel before heat treatment ? Heat to 750 ? for 300 s ? rapid cooling ? dual phase steel products with martensite dispersed around ferrite,its tensile strength is 795 MPa,elongation is 13.92%,and product of strength and elongation is 11066.4/MPa%.Compared with the traditional heat treatment method,the heat treatment and cooling process of DP780 dual phase steel was greatly simplified,and the production efficiency of DP780 dual phase steel was effectively improved.