Effect of Solid-Solution Second-Phase Particles on the Austenite Grain Growth Behavior in Nb-Ti High-Strength if Steel
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EFFECT OF SOLID-SOLUTION SECOND-PHASE PARTICLES ON THE AUSTENITE GRAIN GROWTH BEHAVIOR IN Nb-Ti HIGH-STRENGTH IF STEEL H. M. Zhang,a,b,1 R. Chen,a H. B. Jia,a Y. Li,b and Z. Y. Jiang
UDC 539.4
c
The effect of the solid solution of second-phase particles on the austenite grain growth behavior at different heating temperatures and holding times of Nb-Ti high-strength IF steel is examined. The experimental results showed that the solid solubility of Nb in austenite increased significantly with temperature, dispersion and fine second-phase particles continued to dissolve in austenite. The pinning effect weakened and the austenite grains increased slowly. When temperature reached 1050 °C, a large number of second-phase particles (NbC, TiC, etc.) gradually dissolved in austenite. The solid solubility of microalloyed elements in austenite used for testing steel was different. To make these elements dissolved completely and, at the same time, obtain fine-grained austenite, heating temperature needs to be kept between 1050–1100°C and the holding time limited to 30–40 min. The austenite grain growth model is constructed based on the experimental data. The calculated values are in agreement with the experimental results. Keywords: second-phase particle, grain growth, solid solution, austenite, high-strength IF steel. Introduction. Light weight can be very useful in the automotive industry because it can reduce fuel consumption and gas emission over the past ten years, because of safety requirements, the weight of vehicles has been increasing [1, 2]. IF steel has been widely used for the external panels of cars because of its excellent formability but, the traditional production method of IF steel requires the addition of strengthening alloy elements such as Si, Mn or P, in order to strengthen the solid solution, but this may deteriorate the surface quality and decrease the draw-ability of the steel. In 2007, JFE Steel developed a new type of high performance high strength and super fine grain IF steel (SFG) for the exposed panels of the automobile body and it gives an excellent press-formability with high tensile strength (up to 440 MPa) and low yield strength, and higher r-value than that of conventional IF steel [3, 4]. In the rolling process, the heating system of steel is important for making the rolling process possible. In addition, it also has a significant effect on the solid solution conditions of low-alloyed elements and austenite grain size of the testing steel [5–7]. Much of the research had examined the grain growth of austenite during the heating process. Sha and Sun [8] investigated the grain growth behavior of coarse-grain austenite in an Nb-V-Ti micro-alloy steel. They proposed that the Ti-rich carbonitrides retain its strong pinning effect on the grain boundary of coarse-grained austenite below 1250°C. Yu and Sun [9] also pointed out that the austenite grain of 0.015% Nb steel a
School of Material and Metallurgy, University of Science and Technology Liaoning, Anshan, Liaoning, China ( [email protected]).
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