Experimental Study And FEM Simulation Of Dynamic Recrystallization Behavior Under Hot Compression For Bridge Steel

In recent ten years, the steel structure bridges have been widely used in engineering, due to their many advantages, breaking through the situation of their applications only in large-span steel bridges. With the rapid development of national economy in China, the input in Transportation Industry has been increasing and the demand for high performance bridge steels has been rising. Compared with the bridge steels of foreign countries, domestic ones have lower strength, poorer weldability and weather resistance. Therefore, it is necessary to optimize their chemical composition and processing parameters.In the producing processes of bridge steels, hot rolling plays a key role in determining the quality of the products. The optimization of processing parameters is dependent on the understanding of the law of microstructure evolutions during hot rolling, especially during dynamic recrystallization (DRX). DRX evolutions are often complex and difficult to predict and control. Due to the development of physical metallurgy and numerical simulation of hot forming, it is practical to simulate the DRX evolutions of bridge steels in hot work conditions. Because of the common features in deformation and microstructure evolutions between hot compression and hot rolling, establishing the mathematical models and simulating the microstructure evolutions of bridge steels under hot compression are help in optimizing thermo-mechanical parameters and providing theoretical foundation for predicting microstructure and properties of the products during hot compression and hot rolling.In the present paper, hot compression tests of advanced bridge steels HPS485wf and Q420qE were carried out and their flow stress models of DRX were established based on the creep equations. On the basis of quantitative metallographic analysis and phenomenological theory of DRX, the kinetic and grain size models of HPS485wf and Q420qE steel were established. Furthermore, by integrating the thermo- mechanical coupled finite element method (FEM) with mathematical models of microstructure evolutions, a program for microstructure evolutions of DRX during hot deformation based on the MARC FEM software platform was developed. Then, the program was applied to simulate the DRX microstructure evolutions of HPS485wf and Q420qE steel. The law of microstructure evolutions of DRX were revealed and the effect of thermo-mechanical parameters on DRX were analyzed.The results of numerical simulation are in good agreement with the experimental results and the law of DRX obtained from the simulation is verified by the experimental observation and DRX theory, which prove that the proposed numerical simulation method is proper, the established models for flow stress and microstructure evolutions are effective, the revealed critical conditions of DRX are accurate and the obtained law of effect of thermo-mechanical parameters on DRX is correct. These findings offer help to optimize the thermo-mechanical process parameters of HPS485wf and Q420qE steel and provide reference for microstructure control of other bridge steels.