Research On Grain Microstructure Evolution And Heat Treatment Process Of Q235B Steel Based On Hot Rolling Process

With the rapid development of heavy equipment industry, demands for annular parts in wind power, ship, railway and other industries are increasing,the size are more and more large, quality requirements are also getting higher and higher. In recent years, the casting-rolling compound forming process was used widely in many fields such as the wind power flanges and bearing rings.This new process is based on the casted ring blank to roll and obtain the ring product directly. The quality of the finished ring is not only related with the rolling parameters, but the heating schedule before the process of rolling and the heat treatment after the process are also play a key role. Therefore, based on the microstructure and mechanical properties of the final rings, the heating schedule before the process and the heat treatment process after rolling were studied, and the key problems were solved as follow:(1)The grain growth behavior of casting ring in the heating state. Taking Q235 B steel which is casted by centrifuge process as studying object, the austenitic grain size by the means of heating experimental samples with different temperatures and holding different time was investigated and the high temperature austenitic microstructure was observed with metallurgical microscope. Finally, we can obtain the rule of grain growth by analyzing the morphology of austenitic grains. The grain growth models of centrifugal casting Q235 B steel under isothermal condition and non-isothermal condition are established.(2)CA model for normal grain growth of as-cast materials was established.The normal grain growth CA model of centrifugal casting Q235 B steel was established, and the microstructure evolution during the process of grain growth was obtained. Through the analysis of the change of microstructure evolution,grain number, the influence of temperature on grain size, grain size distribution and the number of the edges of the grain distribution, verifying the accuracy of the model.Besides, the accuracy of the model is verified through the comparison of experimental data and the calculated value. The establishment of CA modelprovides a model reference for the numerical simulation of the subsequent ring rolling.(3)Heat treatment process after ring rolling. Effect of heat treatment process on microstructure and mechanical properties of Q235 B steel flange based on cast-rolling compound forming technology was investigated by means of experiment. The optimum heat treatment process was discussed. The result shows that: quenching at 900? is more favorable to the uniformity of austenite composition, which is beneficial to obtain the martensite structure with higher strength and more uniform structure.Through the research on the above mentioned problems in this paper, the centrifugal casting Q235 B steel was studied as the research object. Finally, the following important conclusions were made: the heating rule of Q235 B steel before rolling was heating temperature between 1030~1100? and holding 1h;the process parameter of heat treatment was quenching at 900? and holding 1h,600? tempering for 2h after cooling in 10%NaCl aqueous solution. After the process, the microstructure of rolled rings is more uniform and the mechanical properties are improved significantly.