Solidification Mechanism And Microstructure Tailoring By Heat Treatment Of High Chromium Steel For Rolls

High chromium steel used for roll materials usually contains high alloying elements such as chromium,carbon,vanadium and molybdenum.The solidification microstructure of high chromium steel contains M7C3-type carbides and can obtain high hardness and excellent wear resistance when reasonable heat treatment is employed to obtain a microstructure with tempered martensite and multi-type carbides.The control of solidification microstructure and heat treatment process are the key issue to obtain comprehensive performance.In this paper,the solidification mechanism and microstructure evolution of 1.3C-1 3.0Cr-2.0Mo-0.5V-1.0Ni-0.5Si1.0Mn high chromium steel were studied by means of confocal laser scanning microscope,SEM,EBSD,TEM etc.Thermodynamic simulation and experimental verification were conducted to study the solidification thermodynamics and microstructure evolution of high-chromium steel.The behavior of carbide precipitation and alloying element redistribution on retained austenite during austenitizing and tempering were systematically studied.With the help of simulated differential temperature heat treatment experiments,the design principle for differential temperature heat treatment process was finally obtained.The main work and results are as follows:(1)The precipitation and growth behavior of carbide during solidification and cooling was studied by thermodynamic simulation and high-temperature confocal laser scanning microscope in situ observation.Under non-equilibrium rapid solidification conditions,the cast microstructure consists of matrix austenite and primary precipitated M7C3 eutectic carbides and a small amount of martensite.At the beginning of solidification,dendritic austenite was first precipitated.Strips of M7C3 carbide and eutectic austenite were precipitated during the eutectic reaction stage,until the solidification was completed.(2)The phase transformation behavior of high chromium steel was studied by thermal dilatometer.The measured phase transformation temperature of Acl was 752℃ and Accm was 852℃ which was time dependent.The "C curve" of the pearlite transformation starting time was plotted to provide a design basis for avoiding pearlite formation.(3)The main newly precipitated phase in high chromium steel in solid solution state is secondary M7C3 carbides,and the size of carbides in austenite phase increases with the increase of solution quenching temperature.During lowtemperature solution quenching,the amount of secondary M7C3 carbide precipitation increases,and the size of the newly precipitated particles is small.The solution quenching process cannot dissolve the eutectic M7C3 carbide,and its network structure can only be improved by controlling solidification.(4)The secondary carbide precipitation behavior during tempering was investigated by the PRISMA module and DICTRA module in Thermal-calc software to perform the second phase precipitation kinetic simulation.It was found that the secondary M7C3 phase and M23C6 phase are the main secondary precipitation phases of high chromium steel during the tempering treatment.The desolation of M23C6 carbide in martensite is the main phase transformation behavior during tempering.The combined effect of secondary hardening of M23C6 carbide and strengthening of martensite affects the mechanical properties of the working layer of high chromium steel rolls.(5)The content of alloying elements in the martensite decreases with the precipitation of the M23C6 phase during tempering and the alloying elements of the residual austenite adjacent to it diffuse into the martensite,resulting in a decrease in the stability of the residual austenite.The low stability of residual austenite continues to transform into martensite during tempering and cooling,further increasing the hardness of the material.(6)A simulated differential heat treatment test study was conducted,the results showed that reducing the single tempering time and increasing the number of tempering cycles contributed to the increase in the total amount of carbide precipitation and the decrease in the residual austenite content.A non-linear relationship between the number of tempering cycles and the material hardness was observed:while the residual austenite content dropped below 5%,the overall hardness of the material started to decrease.The hardness of the steel was highest with a residual austenite content of 32.72%at the tempering heat treatment of 60h and 2 cycles.The hardness decreases slightly at 3 cycles tempering and the residual austenite content is 5.65%.In this thesis,the synergistic effect of M7C3 carbide and martensite phase transformation during-after austenitization of high chromium steel was revealed by a comprehensive thermodynamic simulation and experimental study on the carbide precipitation behavior of high chromium steel at high temperature austenitization.The synergistic mechanism of austenite stability,carbide precipitation and carbon diffusion were studied by multiple tempering,and the mechanism of microstructure evolution of M23C6 carbide precipitation from primary eutectic carbide to the original austenite grain center during multiple tempering was elucidated.Finally,based on the actual working conditions of industrialized,the temperature field of rolls during differential heat treatment was analyzed by means of numerical simulation,and the experimental study of simulated differential heat treatment was carried out by combining the phase change thermodynamic simulation and experimental results.According to the research results,the recommended heat treatment process combination for the working layer of high-chromium steel composite rolls is:2h quenching at 1080℃ and then 500℃ tempering three times,with 60 hours per time.