| It is an effective way to achieve lightweighting,energy saving,emission reduction and safety enhancement of automobile by exploring steels with high strength and ductility,good service performance.The feature of the hot stamping steel is its mixed microstructure of martensite and retained-austenite, which can provide the properties of the third generation of automobile steel required by the industry.The influence of heat treatment process and chemical composition on microstructure and mechanical properties and the relationship between them of the newly designed steel was characterized, tested and investigated by means of optical microscopy(OM),transmissionelectron microscopy (TEM), X-ray diffraction (XRD) and SANS universal testing machine. The influence of alloy element on microstructure and mechanical properties of the three hot stamping steels (#94,#95and#98) through simulating hot stamping-carbon distribution process was characterized, tested and investigated by means of above-mentioned universal testing machine. The main achievements are as follows.(1) Considering the requirements of the hot-stamping process, hardenability and weldability, the chemical composition of the steel used was designed according to the roles of alloy elements in steel,three kind of hot-stamping steel (94#,95#,98#) were designed.(2) The critical points of temperature of98#steel were determined as follows: Ac1=698.5℃, Ac3=912.3℃, MS=451℃, Mf=291.5℃, and the critical cooling rate for obtaining martensitic is20～25℃/s.(3) All three steels (94#,95#,98#) tested under the simulated hot stamping-dynamic partitioning (HS-DP) process showed good mechanical propertieswith the product of strength and elongation values from20000MPa%to25321.52MPa%. Thus, it can be concluded that all the three steels are applicable to the existing hot stamping process. (4) The tensile strength of the three steels tested under the simulated process of hot stamping-online carbon partitioning(HS-OnP) was investigated and was found to be in the order:δb94>σb95>σb98. The elongation of the95#and98#steels were essentially unchanged when holding time increases,while the elongation of the94#steel first increased then decreased with holding time increases at the quenching/partition temperatures of300℃and350℃.Typical lath martensite was observed by TEM morphology in the samples of the three steels quenched at350℃and partitioning for1min.(5) The94#steel can be used for obtaining a combination of high strength and plasticity by precising control of processing conditions. The process window for the95#and98#steels was, however, relatively wider. Nevertheless, all the three steels are capable of achieving excellent mechanical properties via today’s hot stamping processing and equipment through dynamic or online carbon partitioning mechanisms. |
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