Experiment And Simulation Research On The Structure Evolution Of GCr15 Bearing Steel During Heating Process

This paper is on the homogenization heating process before rolling of GCr15 bearing steel. The heating and structure evolution processes of the billets were studied in detail. The coupling numerical simulation models of billets temperature prediction and structure transformation, which can predict the structure transformation during heating process and provide theoretical basis of heating system optimization, were established. In the aspect of experimental research: the DIL805 type dilatometer was used to study the pearlite-to-austenite transformation kinetics during isothermal and uniform heating processes; the austenite grain growth kinetics on high temperatures were experimentally studied with a high temperature furnace; the large carbides dissolution at the billet core was also studied by isothermal experiments. In the aspect of numerical simulation: the temperature prediction model of the billets in the furnace was established; the three-dimensional cellular automata models of pearlite-to-austenite transformation and austenite grain growth were established;the dissolution of the billet core complex large carbides was also simulated with a two-dimensional diffusion model. At last, the billets temperature prediction model was coupled with the structure prediction models, and a structure prediction model during the before rolling heating process was established.Above all, the following innovative research conclusions are listed below:(1) The pearlite-to-austenite transformation of bearing steel billets was studied with a dilatometer. The experimental results shows that the pearlite-to-austenite transformation can be divided into two stages: the synergistic dissolution of ferrite and cementite and the dissolution of residual cementite. As the temperature of isothermal transformation or the heating velocity increase, the C concentration of austenite decreases and the volume fraction of residual cementite increases at the end of stage 1. The relationship between the key transformation temperature nodes and heating rate are also obtained. The dynamics and thermodynamics analysis show that the transformation is controlled by Cr diffusion below 805' C and controlled by C diffusion above 805 ?.(2) The austenite grain growth process was studied by metallographic method. The kinetic formula of austenite grain growth under high temperature condition is obtained. The influence of composition inhomogeneity on grain size distribution of austenite was also discussed. The results shows that the austenite grain size distribution satisfies the relationships described by the Log-normal function after heated at low temperatures for short times, and the austenite grain size distribution satisfies the relationships described by the Weibull function after heated at high temperatures for long times.(3) The dissolution of large carbides in the core of bearing steel was studied by metallographic method. Due to the large volume of massive carbides in the core of bearing steel and the large difference in shape between different carbides, it is difficult to describe the shape and size of carbides accurately with the traditional spherical simplification. By tracking the size changes in the heating process of single large carbides, a new linear relationship between the carbide area and the heating time is proposed based on theoretical analysis.(4) Three-dimensional cellular automata models of pearlite-to-austenite transformation was established. In the process of austenitizing, the angle between pearlite lamellar orientation and the austenite growth direction will lead to different growth rates of austenite in different directions. The pearlite-to-austenite transformation process was predicted by this model, and the anisotropy of the austenite grain growth velocity in the pearlite matrix was analyzed. Good simulation results were obtained using this model.(5) Three-dimensional cellular automata models of austenite grain growth was established. Since the austenite grain growth process is a curvature-driven process, the precise description of the three-dimensional grain boundary curvature is the basis of this model. Based on the previous research, a new expression of three-dimensional curvature was proposed,and then a three-dimensional grain growth cellular automata model was established. By comparing the calculation results of the model with the previous research and the experimental results, the new grain growth model was proved to be reasonable.(6) A two-dimensional diffusion model was established to simulate the dissolution of large carbides in the billet core. As the complex morphology of large carbides lead to the complexity of the dissolution process, it is very difficult to simplify the large carbides with simple geometries. The real large-scale carbides metallographic diagram is transformed into a two-dimensional physical model, and the dissolution model of large carbides with complex morphology is established on the basis of thermodynamics and kinetic parameters. The calculation results of this model were compared with the analytical model and the experimental results, showing that the new model can describe the dissolution process of carbides reasonably.(7) The prediction model which combines the billet temperature prediction model and structure evolution prediction model during before rolling heating process was established.Then the model is used to evaluate and optimize the heating systems of practical production in a factory. On the premise of ensuring that the billet temperature meets the rolling requirements and the core large carbides are completely dissolved, the heating system can shorten the average furnace time from 350min to 146.6min and increase the production efficiency by 1.39 times, and the optimization goal of saving 0.03GJ/t on average can be achieved.Although the study of this paper is for the heating process of GCr15 bearing steel, the research results also have a certain universal significance. The experiment and simulation results and methods can be extended to the research of other low-alloy high-carbon steels.