Study On Carbonitride Evolution And Toughness Of High Nitrogen Martensitic Stainless Steel

At present, research on quantitative characterization and evolution law of carbonitride and strength-toughness properties in steel has little focus.Therefore, 40Crl5Mo2VN bearing steel is regard as the research steel. Put forward and optimize the characterization of carbonitride and inclusion in steels, Respectively based on the strength-toughness properties of solidification process, hot working process, heat-treatment process. To reveal the relationship of carbonitride evolution and the strength-toughness properties, expectations for the necessary theoretical support of industrial preparation and control carbonitride on high-nitrogen martensite stainless bearing steels.Tested after the optimization of heat-treatment, the main microstructure are tempered martensite, plenty of tiny carbonitride as well as few residual austenite. The width of martensite lath mostly 300nm, which microstructure is meticulous. Residual austenite in the form of thin layers and blocks, the volume fraction less than 4%. It can provide a certain toughness. There are two types of precipitated phase which M23C6 and Cr2(CN).The M23C6 average size are more than 200nm, Nitrides morphology are spherical dispersion distribution, mostly concentrated in about 50nm size, maximum size less than 100nm.Smelted by combined process-non-vacuum induction melting and electroslag remelting. Then forging into Φ200mm steel ingot, the grain size is about 11.34μm.The tensile strength and yield strength are 2016MPa and 1733MPa, but the impact toughness is just 2J. When the steels are processed by homogenizing forging into Φ160mm, the grain size grows up to 11.34μm. The tensile strength and yield strength are fall to 1918MPa and 1694MPa. The impact toughness increase less. Smelted by non-vacuum induction melting and electroslag remelting+ ESR continuous directional solidification and three times in the upsetting forging, which grain size is about 11.53μm, the tensile strength and yield strength are 2056MPa and 1734MPa, impact absorbing energy is 6J, the impact toughness improved significantly.Research on three kinds of preparation technology which distribution characteristics of carbonitride, it found refine the grain boundary of thick carbonitride by homogenizing forging, but has not changed species distribution of the chain and agglomerated carbonitride. Possessd by ESR continuous directional solidification and three times in the upsetting forging, the purity of steel and carbonitride distribution have improved. Its inclusion are least and has none of chain inclusions, size refinement (2.31um).The chain and agglomerated carbonitride reduce considerably. The carbonitride distribute dispersion and uniform in size. The increase of impact toughness is by the increase of purity caused by ESR continuous directional solidification and more diffuse distribution of carbonitride caused by homogenizing forging.By investigated the relationship of carbonitride evolution and the change of strength-toughness property in different stages of the heat-treatment. With the Quenching state to the final state, the impact energy from 9J dropped to 4J, tensile strength increased by 20.5%, yield strength increased by 39.6%. Tested steel quenching to one-time cryogenic, the change of strength-toughness property is main due to a substantial reduction in the volume fraction of the residual austenite (18.09% to 6.5%), The strengthening of martensite phase transformation plays a main role. One-time cryogenic state to the final state, the change of strength-toughness property mainly by a large number of small dispersion (<0.4μm) carbonitride precipitation. There has no significant change in chains and agglomerated carbonitride, In accordance with the quantitative statistical result, the numerical relationship between the increment of strength and the particle size of carbonitirdes precipitates could be expressed as follows:TSPO=1.36xl03·f1/2·D-1/2 or YSPO=2.31×105 f1/2·D-1·lnD.A correlation between evolvement rule and variation in hardness of different tempering temperatures is investigated by statistical characterization analysis. It found that the number per unit area control to 41-46/287μm2, the area ratio of carbides is 3.5% or more, and the average size of carbonitride is between 0.47μm and 0.52μm. The hardness of tested steels could reached 58HRC or even higher.