| Advanced high-strength steel is often used in the manufacturing process of automobile structural parts because of its higher strength.It can reduce the thickness of the automobile plate under the premise of ensuring the safety performance,so as to achieve the purpose of lightweight.However,there still has problems of automobile high-strength steel such as unsatisfactory cold rolling effect and poor plastic deformation ability at room temperature.The HS-Q&P process,which combines the hot stamping(HS)process with the Q&P process,can solve the problems of difficulty in forming high-strength steel at room temperature and poor plasticity of hot stamping steel,thereby obtaining high-strength steel with both strength and plasticity.HS-Q&P process has now been extensively researched and developed.At present,advanced high-strength steels are mainly used for collision avoidance and stress concentration components of automobiles.These components will undergo higher strain rates(10-103 s-1)and deform during vehicle collisions.Therefore,a correct and comprehensive evaluation of the dynamic mechanical properties of HS-Q&P steel under high-speed strain is of great significance to process improvement and driving safety.Therefore,this article uses HS-Q&P process to treat low-carbon silico-manganese steel,and uses Hopkinson pressure bar and universal tensile testing machine to determine the dynamic mechanical properties of HS-Q&P steel under different parameters,combined with OM,SEM,EBSD,XRD analysis and characterization to explore the relationship between organizational changes and mechanical properties.Reveal the strong plastic mechanism and dynamic mechanical deformation behavior of HS-Q&P steel under high strain rate.The research results are as follows:(1)With the increase of compression rate,the yield strength of HS-Q&P steel austenitizing and heated at 920?gradually increases,but the increase is smaller and more reflected in the edge.When the compression rate is 12000 s-1,the yield stress is the maximum,which is 1935MPa.The compression rate increased from 4000 s-1 to 12000 s-1,and the plasticity increased by26.7%.The true stress-strain curves at different rates are the same,and they are divided into three stages:a rapid ascent stage,a steady yield stage,and a descending stage.The initial stage of deformation is dominated by strain hardening,dislocation proliferation,and TRIP effect.In the middle of deformation,the TRIP effect and adiabatic heating effect work together to produce a platform stage where the dislocation can continue to move.The early TRIP effect is the main cause of plastic deformation.The microstructure of the test steel is mainly composed of acicular martensite,a small amount of ferrite and retained austenite.As the strain rate increases,the martensite boundary gradually becomes blurred,and the number of broken grain increases,refined martensite grains,and areas with large areas of unclear interface and inconspicuous gullies appear,and massive martensite increases.In the EBSD diagram,adiabatic shear bands and deformation twins are observed under the action of adiabatic heating.(2)The maximum yield stress of HS-Q&P steel heated by austenitizing at 800?also gradually rises and is divided into three stages:rising stage,steady yield stage,and falling stage.There will be a small drop during the steady yield phase,in this stage,the adiabatic heating plays a major role,and the TRIP effect is suppressed by increasing the stacking fault energy during the strain process.The microstructure of the test steel is mainly composed of lath martensite,ferrite,and retained austenite.As the strain rate increases,the martensite lath becomes thicker,the grain boundaries gradually becomes blurred,the martensite-ferrite boundaries begins to fuse,and the structure gradually becomes flat.There is a white fine martensite structure formed by martensite transformation,and there has different austenite grain profiles.(3)The dynamic mechanical properties and microstructure of HS-Q&P steel under two different processes were compared,and the results showed that the true stress-strain curve trends of the two process test steels under different compression rates were roughly the same,but they were also each has its advantages and disadvantages.The mechanical properties of the austenitizing process under low strain are slightly better than the two-phase austenitizing process,and the plasticity of the two-phase austenitizing process under high strain is even better.Although both are composed of martensite,ferrite,and retained austenite,their morphology and content are completely different,the two processes are quite different in microstructure.However,the two process have similar deformation behavior and strengthening mechanism,and have different advantages in the face of different compression rates,which proves that although the content of retained austenite is small,it is still a very important and common strengthening process.(4)The stress-strain curves of the above two processes are also similar in the fast stretch.The tensile strength first increases and then decreases;the elongation after fracture and the strong plastic product decrease with the increase of the strain rate.However,the maximum value of the decrease occurs when the stretching rate is the smallest,which is 11.16%,17.1 GPa·%,18.3 GPa·%,12.56%.Due to the existence of a large amount of martensite,the austenitizing process has a smaller plasticity than HS-Q&P steel that has been two-phase austenitizing.On the contrary,due to the existence of more ferrite structure,the plasticity of HS-Q&P steel with austenitizing process is lower than that of HS-Q&P steel with two-phase austenitizing process.This is all due to the slight difference in mechanical properties caused by the characteristics of the heat treatment process.The content and morphology of the two processes are completely different but they can have similar mechanical properties,which proves that the TRIP effect of retained austenite is the main strengthening effect in steel.(5)The microstructure and mechanical properties of HS-Q&P,Q&P+cold rolling process and HS-Q process are compared.Due to both Q&P+cold rolling and HS-Q processes will consume retained austenite during forming and partitioning,the mechanical properties of HS-Q&P steel are better.It proves that the HS-Q&P process can not only integrate the advantages of Q&P and hot stamping under quasi-static conditions,but also has better comprehensive mechanical properties in the dynamic stretching process than the other two processes. |
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