Study On Microstructure And Properties Of Hot Forming Steel Based On Ultra-refinement Of Austenite Grain

With the development of the new generation steel, people are putting forward a higher demand for properties on the structural steel materials. Among them, the hot forming steel has become a focus of the lightweight in automotive industry because of its characteristics of the high strength and the obvious weight loss effect.However, the toughness and plasticity will decrease with the improvement of the strength and the service life of the structural component will reduce. Therefore, the fine-grained strengthing was obtained more and more attention which can improve the strength and toughness at the same time. In this paper, the rule of austenite transformation of Mn-B steel with two composition, the effect of the different heating process and initial microstructure on ultrafining austenite grains were studied by rapid induction heating and cold rolling. The effect of ultra-refinement of austenite grains on microstructure and properties were also studied.Firstly, rapid heating and quenching process was completed by Gleeble-3800 thermal simulation and rapid induction heating. The small and equiaxed austenite grains can be obtained when heated to 850 ~ 900 ℃ by austenite grains size analysing under different heating process. The results also indicated that the effect of the change of heating speed on final grain size is not obvious during the rapid induction heating process.Four different initial microstructure, ferrite, cold-rolled ferrite, ferrite+pearlite and cold-rolled ferrite+pearlite, were prepared by heat treatment and cold rolling.The rules of different initial microstructure were studied and fine austenite grains was obtained. The research suggested that the more advantageous initial microstructure to refine the austenite grains is ferrite+pearlite by analysing the quenching microstructure and final grain size.The strength and toughness were improved to some extent after the grain refinement, the tensile strength reached 970MPa、1100MPa、930MPa and 1090 MPa,respectively, and the biggest impact energy reached 18 J. The results indicated that with the decreasing of austenite grain size, the martensite lath width decreases, the crack propagation effect increases and the crack extension resistance improves,which improve the strength and toughness. In comparison, the microstructures toachieve the better balance between strength and toughness were cold-rolled ferrite and cold-rolled ferrite+pearlite.