Study On Microstructure And Properties Of Asynchronous Hot Rolling-partitioning Integration Q&P Steel With Gradient Structure

With the worsening of the environment and the increasing shortage of energy,the problems of energy saving,safety and environmental protection have become the main problems that restrict the development of automobile industry.As a typical representative of the third generation advanced high strength steel(AHSS),Quenching and Partitioning(Q&P)steel has shown a very good application prospect in the field of automotive steel due to its advantages of low cost and high strength and excellent ductility.The gradient structure can be used to effectively avoid the performance mutation caused by the structural size mutation,and greatly improve the strength and ductility of the material.Therefore,the combination of the gradient structure and the Q&P process provides a new preparation process for the development of high strength low alloy steel with excellent comprehensive properties.In order to systematically research the influence of asynchronous rolling and Q&P process on microstructure and properties of asynchronous hot rolling-partitioning integration Q&P steel with gradient structure.In this paper,combining asynchronous hot rolling with Q&P process,the hot rolling-partitioning integration Q&P steel with gradient structure is developed,which exhibits excellent comprehensive mechanical properties.Moreover,the production procedures can be simplified and production efficiency can be improved by this new process.By observing and analyzing the microstructure and mechanical properties of gradient structural steel,the influences of gradient structure,original structure and phase composition on the microstructure evolution and mechanical properties of experimental steel are further revealed.The main contents and results are expressed below:(1)By changing hot-rolled deformation amount and asynchronous ratio,the pearl ite steel with gradient structure was obtained,and the influence of the gradient structure on the microstructure and properties of this steel was investigated.The experimental results showed that the mechanical properties of the pearlitic steel with gradient structure are better than conventional pearlitic steel.When the deformation amount was 72%and asynchronous ratio was 1.05,this experimental steel exhibited a good combination of high strength and ductility,the tensile strength,was 891MPa,elongation and product of strength and elongation(PSE)were 26.2%,and 23.3GPa·%,respectively.(2)The effect of asynchronous hot-rolled process on the microstructure and properties of Q&P steel was studied.When the deformation amount was 72%and asynchronous ratio was 1.05,the results showed that the thickness of gradient layer is about 50μm;the average grain size and volume fraction of retained austenite of the edge microstructure were about 1.57μm and 14.8%,respectively;as for the middle microstructure,the average grain size and volume fraction of retained austenite were about 2.20μm and 11.2%,respectively.At the same time,Compared with the finish rolling temperature at 910℃,the volume fraction of retained austenite was higher,the elongation was greatly increased,and the comprehensive mechanical property is better at 810℃.(3)To compare the influence of Q&P process on the microstructure and properties,the portioning at 260℃ of one-step Q&P process and at 400℃ of two-step Q&P process were carried out.The results showed that the tensile strength of the two-step Q&P process was lower than that of the one-step Q&P process,but the elongation greatly increased,and the comprehensive mechanical properties of the experimental steel are better.(4)In this study,the production mode of hot rolling-Q&P process integration was developed,which can make production process simplify,cost save and production efficiently improve.When the deformation amount,asynchronous ratio and finish rolling temperature were 72%,1.05 and 810℃,respectively,at the same time the heat treatment was two-step Q&P process,the asynchronous hot rolling-partitioning integration Q&P steel with gradient structure exhibited a good combination of high strength and ductility,the tensile strength was 878MPa,the elongation was 34%,and the PSE can reache 29.8GPa·%.