| Currently,the requirements of the automotive industry for energy saving,emission reduction and safety performance improvement are becoming higher and higher with each passing day,which has promoted the in-depth researches and developments of automotive steel materials.Although tempering treatment of the quenched martensite can improve the elongation,its product of tensile strength and elongation still cannot meet the requirements of the third-generation automotive steel.The new quenching-partitioning process has provided new ideas for improving tensile strength and elongation of the martensitic steel.Quenching and partitioning(QP)process serves to heat high-strength steel to austenitizing temperature for heat preservation,and then to quench and cool it to quenching temperature(between M_sand M_f)in order to obtain the structures of untransformed austenite and martensite phases.The key of QP process is,through the diffusion of C,to increase the retained austenite content in the final structure in order to improve the elongation of the material.In this paper,a detailed analysis on the QP process of low-carbon 22Si Mn2Ti B and medium-carbon 30Cr Mn Si2Nb martensitic steels,and we propose the application of quenching and partitioning process to the hot stamping line,and a comparative analysis combining with quenching and tempering processes,have been made in exploring the mechanism of the role of the QP process in improving the tensile strength and elongation performances of ultra-high strength martensitic steels,which has clarified the structural evolution and its impact on performance.1.The austenite grain growth characteristics of the above two ultra-high strength steels are discussed by direct quenching(DQ),which has clarified the law of austenite grain growth under different alloy systems,and has determined the optimal austenitization system:the optimal austenitizing temperature range of 30Cr Mn Si2Nb is 930?980°C,and the optimal austenitizing temperature range of 22Si Mn2Ti B is 930?1030°C.2.In the quenching and tempering(QT)treatment process I of medium-carbon30Cr Mn Si2Nb steel,the diffraction peak of martensite moves to the left,on the whole,with the increase of the tempering time,and its half-peak width decreases,which indicate that the dislocation and internal stress in the lath martensite decrease with the increase of the tempering time.3.The research results of the quenching and tempering(QT)processes II of30Cr Mn Si2Nb and 22Si Mn2Ti B have showed that the temperature at which30Cr Mn Si2Nb strength began to drop obviously was 425°C,while the temperature at which 22Si Mn2Ti B strength began to drop obviously was 325°C.This difference had a decisive impact on the partitioning temperature.4.Hot stamping QP process results show:Compared with the direct quenching(DQ)process,retained part of retained austenite can significantly improve the plasticity of materials.The retained austenite content is the main structural unit that determines the tensile strength and elongation performance of the martensitic steel.5.Hot stamping QP process experiment results show that:For the medium-carbon30Cr Mn Si2Nb steel,by using two-step QP process(with the quenching temperature of280°C,for heat preservation for 10s,425°C QP 30s process),the product of tensile strength and elongation can reach 23.5GPa%,of which the tensile strength is 1351MPa and the elongation is 17.42%,the Product of tensile strength and elongation is 7.3GPa%higher than the direct quenching process..For the low-carbon 22Si Mn2Ti B steel,by using one-step 290°C QP 50s process,it is possible to obtain a relatively high product of tensile strength and elongation(18.9G Pa%),of which the tensile strength is 1468MPa and the elongation is 12.9%,the Product of tensile strength and elongation is 2.4GPa%higher than the direct quenching process.6.The alloy element of high strength steel had an obvious impact on its performance and the determination of heat treatment process parameters.The two high strength steels selected in this paper have different performances and different optimal partitioning processes due to their differences in the compositions of C,Nb and Ti.Element C mainly strengthens the basal body and expands the austenite phase area;Nb plays a role in suppressing partitioning of element C,which makes it necessary to increase the temperature appropriately;Ti plays an obvious role in refining austenite grains.By treating austenitizing deformation at 950oC,strength has been improved to a certain extent. |
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