Effect Of Niobium On Microstructure And Mechanical Properties Of Low Carbon Martensitic Steel In Heat Treatment Process

In this study,three kinds of low-carbon steel(C-Mn steel,Nb-free steel and Nb-alloyed steel)were investigated.The effect of Nb on transformation behavior of the experimental steel under static continuous cooling conditions was studied;the effect of Nb on the austenite grain growth of the experimental steel at different austenization temperatures was analyzed;and the effect of Nb on microstructure and mechanical properties of the experimental steel under the condition of quenching-tempering process was studied,in order to provide reliable experimental data for industrial production.Main research contents and results are listed as follows:(1)The effect of Nb on transformation behavior of experimental steel was studied with dilate metor experiments.The results showed that Nb increased the critical temperature(Ac1,Ac3)of the low carbon martensitic steel.According to the continuous cooling transformation curves,Nb expanded the range of cooling rates of both ferrite and pearlite transformation.while it decreased the transformation starting temperature and finishing temperature of the high-temperature transformation product.Under the condition of 0.2?0.5?/s,the transformation of bainite was inhibitioned by Nb.Besides,the temperature range of martensite transformation was expanded,and the critical cooling rate for martensite formation and its transformation temperature are increased.(2)The effect of Nb on austenite grain growth of experimental steel was studied with high temperature confocal laser scanning microscope.The results showed that,with Nb addition,when the austenization temperature was above 900?,the austenite grain sizes of the steel were significantly reduced,the coarsening of the austenite grain was delayed,but the temperature for generating mixed crystal was increased.(3)The effect of Nb on microstructure and mechanical properties of experimental steel was studied with hot rolling and off-line heat treatment process.The results showed that,with Nb addition,when the tempering temperature was equal to 200? and 300?,the tensile strength of the experimental steel was increased by about 30MPa,and its yield strength,low temperature impact toughness and elongation at break were all increased.The tempering temperature was equal to 400? and 500?,the yield strength of the experimental steel was increased by about 30MPa,and its low temperature impact toughness and elongation at break were all increased.The tempering temperature was equal to 600?,the elongation at break of the experimental steel was increased,and the impact energy of a quarter of the sample was decreased by about 1.5 J at-20?.(4)The effect of Nb on microstructure and mechanical properties of experimental steel was studied with hot rolling and on-line heat treatment process.The results showed that,with Nb addition,when the tempering temperature was equal to 200?,the impact energy of a quarter of the sample was increased by about 2J at-20?.The tempering temperature was equal to 300?,the yield strength of the experimental steel was increased by about 60MPa,the impact energy at-20? was increased.when the tempering temperature was 400? and 500?,the tensile strength of the experimental steel was increased by about 25MPa,the yield strength of the experimental steel was increased by about 20MPa,the impact energy at-20? was increased,Specifically speaking,when the tempering temperature was 500?,the elongation at break of the experimental steel was increased by about 2 percent.when the tempering temperature was 600?,the yield strength of the experimental steel was decreased by about 110MPa,the elongation at break of the experimental steel was increased,and the impact energy of a quarter of the sample was decreased by about 2J at-20?.