Abstract
Hot stamping process is widely used in the manufacture of the high strength automotive steel, mainly including the stamping and quenching process of the hot-formed steel. In the hot stamping process, the steel is heated above the critical austenitizing temperature, and then it is rapidly stamped in the mold and the quenching phase transition occurs at the same time. The quenching operation in the hot stamping process has a significant influence on the phase transition and mechanical properties of the hot-stamping steel. A proper quenching technique is quite important to control the microstructure and properties of an ultra-high strength hot-stamping steel. In this paper, considering the factors of the austenitizing temperature, the austenitizing time and the cooling rate, a coupled model on the thermal homogenization and phase transition from austenite to martensite in quenching process was established for production of ultra-high strength hot-stamping steel. The temperature variation, the austenite decomposition and martensite formation during quenching process was simulated. At the same time, the microstructure and the properties of the ultra-high strength hot-stamping steel after quenching at different austenitizing temperature were experimental studied. The results show that under the conditions of low cooling rate, the final quenching microstructure of the ultra-high strength hot-stamping steel includes martensite, residual austenite, bainite and ferrite. With the increase of the cooling rate, bainite and ferrite gradually disappear. While austenitizing at 930 °C, the tensile strength, yield strength, elongation and strength-ductility product of the hot-stamping steel are 1770.1 MPa, 1128.2 MPa, 6.72% and 11.9 GPa%, respectively.
Highlights
With the rapid development of the lightweight technology in the automobile industry, high-strength automobile steel gradually replaces the original low-strength automobile steel [1,2].Due to the difficulty of the cold forming technology, hot stamping technology has become the main technology of producing the high-strength automobile steel, which mainly includes the process of austenitizing, quenching and plastic forming [3]
The toughness and plasticity of the ultra-high strength steel are closely related to the quenching parameters in the hot stamping process, and the appropriate hot stamping process is the basis of the production of the high-quality automobile steel with ultra-high strength [7]
In order to improve the strength and plasticity of the hot stamping steel, it is of great significance to study the phase transition in the quenching process and its influence on the mechanical properties
Summary
With the rapid development of the lightweight technology in the automobile industry, high-strength automobile steel gradually replaces the original low-strength automobile steel [1,2]. In order to improve the strength and plasticity of the hot stamping steel, it is of great significance to study the phase transition in the quenching process and its influence on the mechanical properties. Wang et al [10] simulated the quenching process of 22MnB5 under different conditions in the hot stamping process, and found that the final microstructure composition of the hot-stamping steel is closely related to the cooling rate. In order to improve the strength and plasticity of ultra-high strength hot-stamping steel, it is very important to study the phase transition and the mechanical property variation in the hot stamping process. In this paper, considering the different austenitizing temperature and holding time, the coupled mathematical model of the thermal and phase transition is established for the quenching process of the hot-stamping steel under different cooling rates. The results provide the guidance for the quenching process of the ultra-high strength hot-stamping steel
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