Study On The Technology Of Forging And Spheroidizing Annealing Of GCr15 Bearing Steel

This article focuses on the research and development of forging technology for bearing steel production technology of a domestic steel mill.Because the GCrl5 bearing steel produced by it does not meet the requirements of the bearing steel organization and performance requirements of the national standard high carbon chromium bearing steel GB/T18254-2016,The non-uniformity of the mesh is severely precipitated during processing,and the level of mesh carbides cannot meet the national standard requirements for high-carbon chromium bearing steel.When a large number of network carbides are present,it will reduce the impact toughness of the bearing steel,increase brittleness,increase quenching cracks,and reduce its service life.If the network carbides are particularly serious,they cannot be eliminated during subsequent heat treatment,which will cause serious Reduce the yield of bearing steel.Aiming at this large production phenomenon,this paper studies the forging process and spheroidizing annealing process of bearing steel,and focuses on the effects of forging amount,final forging temperature,post-forging cooling,and final cooling temperature on the structural properties of bearing steel.A process method that can suppress the network precipitation of carbide unevenness and obtain a refined and uniformly distributed pearlite structure has been developed.This paper mainly draws the following conclusions:(1)During continuous cooling experiments of GCr15 bearing steel,the phase change products in different temperature ranges are different,and are divided into cementite,pearlite and martensite.The critical cooling rate to suppress the precipitation of secondary carbides is 8?/s,and its precipitation temperature range is 698 to 912?;the critical cooling rate to complete pearlite transformation is 5?/s,and its precipitation temperature range is 500 to 758?.(2)When other influencing factors remain unchanged during forging:with the increase of the forging deformation,when the forging deformation reaches 80%,the sample structure after forging is fine and dense,and the precipitation of network carbides is not particularly obvious.The carbide grade is relatively low,which meets the requirements for bearing steel production.The pearlite layer spacing is fine,forming pearlite pellets.(3)With the decrease of the final forging temperature,the precipitation of secondary carbides gradually decreases,and finally the dots are finely and uniformly distributed.When the final forging temperature is lowered to 810?,it can be observed that in the microstructure,the reticular carbides are distributed in a dot shape,the secondary carbide reticular grade is lower,and the pearlite structure is fine and uniform.(4)In the cooling process after forging,with the increase of the cooling rate,the network structure of the secondary carbides precipitated along the austenite grain boundaries gradually becomes stripe,intermittent,and block,until the cooling rate is greater than 8?/s,the secondary carbides are distributed in spots,and the amount is small and uniformly precipitated.(5)In the rapid cooling stage,when the final cooling temperature is too high,the secondary carbides are distributed in a network,and the precipitation of the network carbides increases,and the grade does not meet the requirements of the national standard for bearing steel.When the final cooling temperature is 650?,the structure performance is the best,and the structure is fine-grained pearlite and lower-level network carbides.(6)GCr15 bearing steel samples are forged in different ways.Under the same forging ratio,there are still some differences in the structure and performance.In the case of other influencing factors being the same,in terms of the level of reticulated carbides,The multi-directional forged bearing steel sample can have a minimum grade of reticulated carbide structure of 1.0,and under the same conditions,the grade of unidirectional forged reticulated carbide is 2.0;in terms of the pearlite layer spacing,multi-directional forging requires The structure is smaller and denser than that of unidirectional forging;in terms of bearing steel performance,the hardness of multi-directional forging is greater than that of GCr15 bearing steel obtained by unidirectional forging.(7)Comprehensive comparison of different spheroidizing annealing processes shows that isothermal spheroidizing annealing processes are better than other spheroidizing annealing processes.The isothermal spheroidizing annealing process first heats the steel to an austenitizing temperature of 810? and keeps it for 5 hours,and then cools it to 700?with the furnace for 2 h,and then slowly cools it to 650? and takes out air cooling.The process of pearlite spheroidization can be divided into three processes:melting and decomposition of carbides?granulation of carbides?spheroidization of carbides.Finally,a spherical structure with good comprehensive performance is obtained.(8)The original tissue has a certain effect on the spheroidization effect.The structure of the forged piece after air cooling is relatively coarse,and the spherical carbides are relatively coarse after spheroidizing annealing.There are many large particles of carbides and the distribution is uneven,while the spheroidizing effect of water-cooled forgings is obvious.The spherical carbide particles are fine and uniform.Because when the pearlite structure is small,it is easy to melt and decompose during heating to austenitizing temperature,which results in the decomposition of carbide particles that are relatively small and uniform,which provides favorable conditions for the subsequent spheroidization process.Therefore,the finer spheroidization effect of the original tissue is better.