| Low carbon steel was often used as the outer covering material of automobile because of its good formability.However,it was necessary to maintain good formability of low carbon steel while improving its strength with the development trend of automobile lightweight.Compared with the traditional cold rolling process,the warm rolling process was more beneficial to improve the forming properties of steel,and has been well used in IF steel.However,dynamic strain aging would occur in low carbon steel during the warm rolling process,which was not conducive to the formation of shear bands and {111} texture,thus reducing its formability.The addition of alloying element Cr could delay and inhibit the dynamic strain aging,thus improving the formability of low carbon steel.Increasing carbon content can improve the strength of steel,it was extremely unfavorable to its formability,but appropriate heat treatment process could realize the optimization of formability and strength.Based on this,this paper designed two kinds of low carbon steel with different Cr content(low Cr and high Cr)through the intercritical quenching process.The effect of Cr on intercritical quenched microstructure and texture of low carbon steel was studied.The influence of phase transformation and recrystallization microstructure and texture of low carbon steel during intercritical quenching was studied.The main conclusions are as follows:(1)Cr and C were easy to form alloy carbides that were difficult to melt into austenite,which hinder the diffusion of C and slow down the formation rate of austenite,resulting in the increase of Ac1 temperature in high Cr sample.According to the precipitation type calculated by JMat Pro software,the carbide of low Cr warm rolled plate was M7C3,while that of high Cr warm rolled plate was M7C3 and M23C6.The increase of Cr content would reduce the content of solid dissolved carbon in low carbon steel,which led to the weakening of dynamic strain aging,the increase of shear bands and the enhancement of {111} deep drawing texture.(2)The morphology of martensite and texture transformation during quenching were affected by different heating rates.When the heating rate was 10 ℃/s,the morphology of martensite was island,and bainite was observed with increasing temperature.At 830 ℃,the γtexture was the strongest and the Goss texture was weak.When the heating rate was 100 ℃/s,the morphology of martensite was elongated,and network martensite appeared with increasing temperature.At 830 ℃,the γ and Goss textures were the strongest.(3)In the intercritical quenching,the sample with heating rate of 10 ℃/s has recrystallized completely when the temperature reached the critical region,so the austenite mainly nucleated at the grain boundary and finally transformed into martensite after water cooling.When the heating rate was 100 ℃/s and the temperature rose to the critical region,the recrystallization was not complete and there was still a small amount of pearlite.At this time,austenite nucleated in the pearlite.(4)During the quenching process,recrystallization nucleation was observed at three locations: shear band,grain boundary and deformation band.The recrystallization nucleation occurs first in shear bands and grain boundaries and the recrystallization nucleation of Goss grains was mainly in the shear band.Grain boundaries were mainly divided into α-deformed grain boundaries and γ-deformed grain boundaries.The recrystallized grain orientations on α-deformed grain boundaries included γ,Cube,Goss,and other orientations,while γ-recrystallized grain orientations were mainly on γ-deformed grain boundaries. |
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