Effect Of Mo And Ni On Bainite Transformation And Microstructure And Properties In Low Carbon Boron Bearing Steel

With the rapid development of the economy,all fields have higher requirements for the quality of steel.Improving the performance and reducing the cost of steel have become one of the hot key points of steel research.In this paper,the microstructures and properties of three kinds of low carbon steels(C-Mn-B steel、C-Mn-B-Mo steel and C-Mn-B-Ni steel)under continuous cooling transformation,isothermal transformation and heat treatment processes were studied.The main results are as follows:(1)The phase transformation behavior of the experimental steel under continuous cooling conditions was studied.The results show that the microstructure gradually transformed from ferrite+pearlite+bainite to full bainite and the microhardness increased gradually with the increase of cooling rates.The microstructure of the experimental steel transformed from ferrite+bainite to full bainite when Mo or Ni element was added.The starting temperature of transformation of the experimental steel was increased after deformation.Ferrite transformation was promoted and grain structure was refined under deformation condition.(2)The isothermal transformation behavior under deformed and undeformed conditions was studied.The results showed that the microstructure gradually transformed from lath bainite to granular bainite plus a small amount of martensite,and the microhardness gradually decreased with the increase of isothermal holding temperature.At the same isothermal temperature,laths of bainite are refined and the microhardness of the experimental steel was effectively improved when Mo or Ni element was added.The amount of bainite decreased,the amount of ferrite increased and the microhardness decreased gradually with the increase of isothermal holding temperature under deformation condition.(3)The effect of quenching and tempering processes on the microstructure and properties of the experimental steel was studied.The results showed that the mechanical properties of the experimental steel increased first and then decreased with the increase of quenching temperature.The yield strength of the experimental steel increased by 206MPa and 109MPa,and the tensile strength increased by 207MPa and 92MPa for Mo-bearing steel and Ni-bearing steel,respectively.The mechanical properties of the Ni-bearing steel at 900℃ quenching and 400℃ tempering were good,the yield strength was 709MPa and the tensile strength was 797MPa,the elongation was 19.5%,and the impact energy at-20℃ was 204J.The on-line quenching and tempering experiments showed that the yield strength of the experimental steel increased by 87MPa to 139MPa,the tensile strength increased by 113MPa to 138MPa,and the impact energy decreased slightly under the same tempering temperature,and the on-line quenching process significantly improved the mechanical properties of the experimental steels.