Evolution of Phase Transition and Mechanical Properties of Ultra-High Strength Hot-Stamped Steel During Quenching Process

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.

Non-Destructive Investigation of Intrinsic Magnetic Field of Austenitic Biomaterials by Magnetic Field Sensors

“…According to theoretical knowledge, austenitic structures are transformed into martensitic ones, the so-called phase transition. This transformation results in a local weakening of the biomaterial, which is thus more susceptible to the formation of corrosion and fatigue inhomogeneities [ 24 , 25 ]. Due to the significant change in the material magnetic properties, suitable sensors should be used to detect changes in the magnetic field and the transition in the properties of the biomaterial in a non-destructive way.…”

Medium-Mn steels for hot forming application in the automotive industry

“…Nam et al [76] described that stamping temperatures ranging from 500 to 700°C hardly influence the strength of the warm-stamped Nb-bearing 6Mn steel, in which nano-sized Nb carbides are dispersed in the fine martensitic structure. By contrast, the tensile strength of 22MnB5 steel is greatly affected by the deformation temperature because austenite with much lower stability could transform to ferrite and pearlite before die quenching at a low deformation temperature, leading to incomplete martensitic transformation [77].…”

Forming and Heat Treatment of Modern Metallic Materials

“…The division and classification of contributions is made on the basis of the main problems published in the text. The first can be characterized as materials for forming and their study, together with hot forming, where the papers are presented [1][2][3][4]. The second group consists of contributions devoted to shaping and unconventional shaping [5][6][7].…”

“…The strength followed a Hall-Petch relationship with the austenite grain size, but the austenite grain size has nothing to do with the impact toughness. One paper [4] considered the factors of the austenitizing temperature, the austenitizing time and the cooling rate, a coupled model on the thermal homogenization and established the phase transition from austenite to martensite in quenching process for production of ultra-high strength hot-stamping steel.…”

“…Liu et al's [4] paper focuses on the hot-pressing process, which is widely used in the production of high-strength automotive steel, including the process of pressing and hardening hot-formed steel. In the hot-pressing process, the steel is heated above the critical austenitization temperature, then rapidly pressed into a mold, and at the same time a hardening phase transition occurs.…”