Study On Microstructure And Mechanical Properties Of Micro-carbon Deep Drawing Dual Phase Steel

With the continuous development of the automobile industry,more and more advanced high-strength steel is used in automobile bodies to improve the safety,economy and environmental quality of automobiles.For conventional deep drawing steels,a single ferrite structure is difficult to meet the high strength requirements.In this context,the introduction of a deep-drawn duplex steel with martensite as the strengthening phase has received wide attention.That is,the strength is increased on the basis of not affecting the deep-drawing performance as much as possible,and the requirements for the use of the inner and outer cover parts and the complex deep-drawn parts of the automobile are satisfied.This paper mainly introduces three kinds of dual-phase steels,Cr-Mo,Cu-Ni and Si-P.The recrystallization kinetics and recrystallization and phase transition competition of Cu-Ni and Si-P were studied,and the conflict between the hardening action of high alloy precipitates and carbides and the softening process of recrystallization process was revealed.The effects of elements and heat treatment on the formation of carbides,martensite and micro structure texture reveal the effects of annealing and over-aging on micro structure and mechanical properties.These were investigated by means of continuous annealing,hood annealing and salt bath annealing,using optics metallographic analysis(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD)and static tensile tests.Based on Johnson-Mell and Arrhenius formulas,the model of recrystallization volume fraction and isothermal temperature and isothermal time,annealing temperature and complete recrystallization time of Si-P test steel,In[-ln(1-f)]=-4372/T+0.69lnt-lnT+9.013,lnt=-6.954+8016.42/T,were established.Among them,the continuous annealing of Cu-Ni alloy has a r value of up to 1.54 and an elongation of about 25%.The strength and properties of the Si-P test steel are relatively good,the strength reaches 475 MPa,the r value is above 1.1,and the elongation is greater than 28%.Although the texture pretreatment process improves the strength near the ?-orientation line of the material,the texture after the two-steps annealing is weakened,and the r value is not significantly improved.Only the Si-P test steel increases the r value to 1.31.The increase in the overaging temperature of the salt bath greatly improved the elongation of the Cu-Ni test steel.After annealing at 760 0C for 240 s,the elongation at 350 0C over aging 420 s salt bath process reached 40.09%.The extension of the salt bath annealing time increases the strength of the Cu-Ni and Si-P test steels.The strength of the Si-P test steel after the salt bath test reached 500 MPa or more,which was increased by 60 MPa compared with the continuous retreat process,and the elongation was also increased to over 30-%,and the r value was also significantly improved(up to 1.39).