Research On The Structure And Performance Of GCr15 Bearing Steel During Hot Rolling And Spheroidization

The paper relies on the national 863 project-"Key technology development of major equipment used bearing steel" and carries out research to solve the problem met during the production of GCr15 bearing steel, which will be used in the railway at 200-250km per hour, and promote the nationalization of the high-speed railway bearing steel. Network carbide was easy to found at the grain boundry in GCr15 bearing steel after hot rolling and wearing water-cooled, which has great influence on the hardness, toughness and fatigue life. Besides, the size and distribution of the carbide of apheroidzed structure will affect its final organization and properties. To solve these problems, experiments of controlled rolling and controlled cooling were conducted to improve the rolling and cooling process in the worksite. On the promise of realizing the homogenization of heat-treatment microstructure, a series experiments of spheroidizing annealing has been conducted to seek for the appropriate annealing process. The main works involved as follows:(1) High temperature continuous cooling transformation experiment of GCr15 bearing steel was performed by Full Automatic Transformation Measuring Apparatus in order to analyse the types of organization transformation and the rules of organizational evolution under the condition of continuous cooling. Temperature range for secondary carbide precipitation is 700℃～900℃. The critical cooling rate to restrict the precipitation of secondary carbide is 8℃/s. As the rate of cooling increase, the precipitation of secondary carbide changed to half a mesh from tight mesh distribution, and finally precipitated dispersively.(2) Hot-forming and controlled cooling process were simulated by thermo-mechanieal simulator. Between 790～870℃, the amount of precipitation of secondary carbide first and then enlarged along with the increase of transformation finishing temperature, and the interlamellar spacing of pearlite gradually increased. Along with the increase of cooling rate, secondary carbide changed to half a mesh from mesh distribution, and then finally precipitate dispersively, and the pellet diameter and interlamellar spacing of pearlite reduced gradually. Pearlite pellets is tighter and presents semi-continuous carbide nertwork when the final cooling temperature is 600℃, as the final cooling temperature increases to 700℃, its microstructure presents typical lamellar pearlite and carbide network at the grain boundary was clearly.(3) Controlled-rolling and controlled-cooling experiment was conducted for GCrl5 bearing steel and analyzed the influence of different cooling process parameters on the microstructure and properties of the bearing steel. The pellet diameter and interlamellar spacing of pearlite increased along with the increasing of FRT, and reduced obviously with increasing of cooling rate, and the level of carbide network reduced clearly. The pellet diameter and interlamellar spacing of pearlite and the level of carbide network reduced with the reduce of the final cooling temperature. Impact toughness and hardness inverse with the pearlite lamellar spacing. A small number of degenerated pearlite generates when the final cooling temperature reached 650℃.(4) The impact of the process parameters during various stages of on the result of spheroidization was analyzed during the isothermal annealing experiment. Large carbide tends to appear under too high austenitizing temperature; with the increasing of holding time, the distribution of carbide particle size tends to be uniform; the average diameter of carbide gradually increased along with the isothermal temperature increasing and when the temperature is too low rod, carbide is easy to produce; the average diameter reaches to its maximum when the isothermal time is 2h, and the distribution is more uniform. The hardness inverses with the average diameter of carbide. The optimized isothermal annealing process is: heat to 800℃ and hold 5h, then cooled with the furnance to 720℃ to hold 2h, cooled to 650℃ slowly and air cooled. Carbide spheroidized sufficiently and distributed uniformly under this process.(5) Isothermal spheroidized annealing experiments were conducted for GC15 bearing steel with different TMCP processes, in order to analyze the spheroidization effect of the original microstructure. The carbide of sample which was water-cooled to 680℃ after the final rolling has a better spherical degree, and the average diameter is 0.32μm.