| As deteriorating global environmental problem,utilizing AHSS(Advanced high strength steels)to manufacture automobiles is a prominent trend in automobile industry.Dual-phase(DP)steel is one of the most prominent examples of AHSS,which has a larger amount of application in automobile than other steels.High strength DP steel was generally produced by cold rolling coupled with annealing.Howe’ver,if using hot-rolled DP steel to partly replace the cold-rolled DP steel,it will reduce the cycle of processing and production cost,and as well as improving the market competitiveness of steel enterprises.It has a wide application prospect.But how to optimize the composition and thermomechanical processing of hot-rolled DP steel,improving the quality of surface of plate,enhancing the compatibility of high strength hot-rolled steel and increasing the match degree of strength and plasticity,are still needed to further study.Additionally,there are still various open questions,particularly regarding the micro-deformation mechanism and the relationship between micro-failure and macro-failure.By using micro-DIC(Digital image correlation)combined with in-situ tensile test,the microstructure deformation and micro-failure mechanism of hot-rolled high strength DP steel have been investigated.The achievements and main research results are drawn as follows:(1)The chemical composition was designed and the effect of hot rolling on microstructures and mechanical properties was clarified.The first type of DP steel with martensite and ferrite was obtained after hot rolling followed by directly water quenching,with the tensile strength of 985-1040MPa.and the total elongation of 16.3-18.4%,which had the main feature of high strength and low plasticity.The constitution of the second type of DP steel was ferrite and martensite which incorporated a small amount of bainite in martensite after hot rolled following coiling.The tensile strength of this DP steel was 865 MPa,and the total elongation was 25.0%,which rendered the production of strength and elongation reach 21.7GPa.%.The third type of DP steel consisted of bainite and ferrite by hot rolling followed by fast cooling to transformation zone of bainite and holding for a while.The corresponding tensile strength was 615-675 MPa,and the total elongation was 37.2-38.0%,showing the low strength and good plasticity.By measuring the nanohardness of martensite and interface,the results showed strength of hard phase in DP steel determined the strength of the material.However,the high strength and plasticity could be obtained simultaneously when the material had good compatibility of strength of hard phase and soft phase.Moreover,the strength of interface between hard and soft phases determined the mode of fracture.The higher strength of the interface,the higher tendency of brittle fracture.(2)The effect of tempering on microstructure and mechanical properties of high strength DP steel was clarified.The optimized mechanical properties were obtained after tempering at 200 ℃ for 30 min:the tensile strength decreased a little,the total elongation increased from 16.32%to 25.92%,and the product of tensile strength and total elongation increased from 16.87GPa%to 24.62 GPa%.There were no changes in microstructure observed by SEM(scanning electron microscopy)after tempering at 200℃ for 30min.By using tensile test based on DIC,the main reason of the enhanced mechanical properties by low temperature tempering was explained.The primary explanation was that the compatibility of deformation was improved and the deformation was more homogeneous after low temperature tempering.(3)Comprehensive detection of fine microstructure before and after tempering was conducted by EBSD(Electron Back Scattered Diffraction),ECCI(Electron Channeling Contrast Imaging),and APT(Atom Probe Tomography),focusing on the quantification of segregation of alloy elements and the changes of transition carbides from the atomic scale.The relationship of microstructure and mechanical properities were analyzed as well.Low temperature tempering triggered the movement of dislocations to form low angle boundaries and the segregation of C and Nb atoms to dislocations in ferrite,hindering the movement of dislocations during deformation.Thus,the strength of ferrite remained unchanged.Meanwhile,the C atoms in martensite diffused to retained austenite and to lath boundaries and dislocations causing the decrease of content of C in martensite.Additionally,the residual stress in martensite was released after low temperature tempering.Considering the two aspects mentioned above,the nanohardness of martensite decreased by 16%.As the strength of martensite decreased while that of ferrite remained stable,the stress incompatibility released,which leaded to a more homogeneous deformation.(4)The micro-DIC coupled with in-situ tensile test was conducted to investigate the relationship between local strain distribution and micro structure which clarified the micro-deformation mechanism,and to quantify the local strain of martensite,which established two types of deformation of martensite.The region with low volume fraction of martensite(below 10%)deformed more homogeneously and the homogeneous deformation enhanced the deformation of martensite simultaneously.With the increasing of volume fraction of martensite,the complexity of the distribution of martensite and ferrite increased,which had a strong impact on the distribution of local strain.As the martensite particles were sited closely together with only small ferrite grains in between,in where the severe localization occurred,the microstructure deformed heterogeneously.If the region with high volume of martensite,a significant strengthening effect on the ferrite grains would be obtained from the surrounding martensite.Correspondingly,the plasticity of ferrite would be deteriorated which led to a poor plasticity of the material.As the lath distance of martensite varied,the localization would occur at lath with large lath distance during the deformation,resulting in a heterogeneous distribution of local strain in martensite.This phenomenon resembled the localization in the region between closely spaced martensite particles.In contrast,as the lath distance had no significant difference in martensite,the local strain mainly distributed in the small region which 1μm far away from the interface of martensite and ferrite.The local strain in it distributed gradiently from interface to the interior of martensite.(5)By using micro-DIC coupled with in-situ tensile test,the microstructure of fractured dual phase sample was characterized and three micro-failure mechanisms were posed,which nucleated in martensite,at the interface of ferrite and martensite and at the ferrite grain boundary respectively.Micro-failure in martensite was caused by stress concentration.It would propagate along the tip to form thin and long crack.However,the propagation had subtle effect on the local strain distribution in martensite.Micro-failure at interface was contributed by strain concentration.It would propagate a round void at the end of deformation.Micro-failure in ferrite grain boundary was caused by strain concentration as well and propagated along the ferrite grain boundary.It happened in DP steel infrequently and just happened at the later stage of deformation. |
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