Study On Microstructure And Mechanical Properties Of Advanced High Strength Steel For Automobile Under Different Strain Rates

One of the purposes of the development of high-strength steel materials for automobiles is to effectively solve the problem of reduced body safety caused by weight loss in the process of lightweight development of automobiles,and one of the important indicators to evaluate the safety of lightweight vehicles is lightweight body parts The response characteristics of strain rate and energy absorption in the process of withstanding external force impact.Moreover,in actual life,the strain rate span of automobile collision process is mostly from low strain rate to high strain rate,and the performance of high-strength steel for automobiles under high-speed strain remains to be studied.The high-strength steel Q&P980 and TRIP590,22MnB5 were used to perform Hopkinson pressure bar test and quasi-static tensile test,respectively,to study the mechanical properties of three advanced high-strength steels for automobiles under different compressive strain rates and tensile strain rates.Explore the importance of the mechanical properties of automotive high-strength steels in automobile crash safety performance,and combine the mechanical performance curves and microstructure diagrams of the research to obtain the following research results:?1?The structure of the Q&P980 steel plate after impact compression is mainly composed of ferrite and martensite.The two structures become very flat.The martensite structure includes the original martensite and the new martensite.The comparison of the original martensite Small,and the new martensite structure is mostly lathed,and the lath irregularity extends in different directions.The structure of TRIP590 steel plate after impact compression is that with the increase of strain rate,the ferrite structure gradually becomes larger,the larger ferrite structure covers the bainite structure,and the new martensite structure gradually increases.The structure of the 22MnB5 steel sheet after impact compression is all martensite,and the structure becomes more slender.The structure changes so that with the increase of strain rate,the distribution of martensite structure is still regionalized,but the scope of regionalization is slightly changed Big.?2?Under the experimental setting of 0s^-1,5000s^-1,10000s^-1,and 15000s^-11 impact compression strain rates,Q&P980 steel,TRIP590 steel,and 22MnB5 steel exhibit similar engineering stress-strain change curves,changes,or trends In order to increase rapidly,then slowly increase to the maximum stress value,and finally the stress drops sharply until it completely disappears.Among them,in the process of increasing strain rate,Q&P980 is greater than the maximum engineering stress that TRIP590 can achieve.On the one hand,it is because the content of ferrite in the structure of TRIP590 steel is more than Q&P980,and the plasticity and ductility of ferrite are better.During the impact compression process,large deformation occurs,and ferrite extends as a soft phase in different directions,absorbing more stress,resulting in stress dispersion;on the other hand,it can also be seen from the microstructure that Q&P980 steel contains a large amount of martensite structure,the martensite phase itself has high strength,it is not easy to deform or little deformation during impact compression,so that the ferrite cannot diffuse around on the martensite matrix,thus improving the Q&P980 steel The strength,the maximum engineering stress it can bear is also large.In addition,there is a certain amount of retained austenite in both types of steel.The TRIP effect when the retained austenite is deformed will increase the strength of the steel to varying degrees,but because the content of retained austenite in TRIP590 steel is less than Q&P980,the TRIP effect The resulting strengthening effect is also small,which ultimately results in the maximum engineering stress that TRIP590 can reach is lower than Q&P980.?3?From the experimental data,the true stress-strain curve of TRIP590 steel is closer than that of Q&P980 steel,the true stress-strain curve of 22MnB5 steel is closer than that of TRIP590 steel,and the yield platform is more and more obvious.The interpretation of the stress-strain curve is:First,there are certain defects in the steel.During the compression process,the defects are deformed by compression to form micro-cracks.In addition,there are a large number of dislocations in the matrix.When the dislocations move After meeting each other,they will cancel each other,which will reduce the dislocation density and thus the strength.Second,under the effect of dynamic load,a large number of dislocations or sub-grains are formed in the matrix,and the newly formed phases also hinder the movement of dislocations,resulting in steel The mechanical strength and structural stability are greatly increased.In the initial stage of deformation,the second cause is dominant.When the stress value rises to the maximum yield stress,the two factors work together.The flow stress of the material remains basically unchanged.The first cause of deformation ends up playing a leading role,leading to the material The stress drops rapidly.?4?The change process of strain energy of Q&P980 steel and TRIP590 steel can be divided into three stages.The first stage and the second stage are the process of storing strain energy,including the transformation of retained austenite to martensite and the TRIP effect.Energy,as well as the distortion energy generated by the deformation inside the sample.The third stage is the rebound process of the sample after the impact compression is completed.During this process,the strain energy is released,but the energy is not fully released,and some Saved inside the sample,the released energy has a small amount of heat dissipation in addition to the rebound of the distortion energy,and it is certain that the energy stored inside the sample due to the TRIP effect has not been released,because it does not A reverse reaction of the TRIP effect will occur.The difference from the previous two is that there is no residual austenite structure in the 22MnB5 steel,only martensite structure,so there is no strain energy stored in the 22MnB5 steel due to the TRIP effect,only the distortion energy brought by impact compression.?5?In the tensile experiment,when no heat treatment is performed,the tensile strength and strong plastic product of Q&P980 are significantly higher than 22MnB5 and TRIP590.The mechanical properties of the three steels do not change much at different tensile strain rates,and the appropriate heat treatment process Will improve the mechanical properties of automotive steel,respectively,the appropriate heat treatment process for the three steels.After heat treatment,the comprehensive mechanical properties of the three steels have been increased to different degrees,especially the high tensile strength and strong plastic product of Q&P980 steel make it have good strength and plastic mechanical properties,which can meet the needs of the car during driving.The greater the impact deformation,the higher the strength requirement.