Study On Alloy System And Quenching And Tempering Heat Treatment Process Of Seamless Steel Pipe For BJ890 Boom

In this paper,Nb-V and Nb-V-Ti microalloying with heat treatment processes were studied for BJ890 seamless steel pipe for boom.Through microalloying and heat treatment,recrystallization of ferrite and spheroidization of cementite are controlled to obtain excellent comprehensive mechanical properties,which provides guidance for production practice.The main results are as follows:(1)For the Nb-V microalloyed BJ890 steel quenched at 920 ℃,with the increase of tempering temperature(450～620 ℃),the lath ferrite recrystallizes,and the cementite shortens and spheroidizes..Among them,620 ℃ tempering has the best comprehensive mechanical properties,with tensile strength of 1022 MPa,yield strength of 982 MPa,elongation of 19.8%,impact energy of 93 J at-20 ℃,and impact energy of 56 J at-40 ℃,meeting the requirements of relevant standards.(2)With the increase of tempering time,the lath ferrite in the microstructure of NbV microalloyed BJ890 steel at the optimum tempering temperature recrystallizes,the dispersed fine grained cementite increases and short rod cementite spheroidizes;the tensile strength and yield strength are reduced,and low-temperature impact performance is improved.Under the condition of 60 min tempering,the comprehensive mechanical properties are the best,with tensile strength of 1025 MPa,yield strength of 983 MPa,elongation of 19.6%,impact energy of 127 J at-20 ℃,impact energy of 67 J at-40 ℃,ETT ductile-brittle transition temperature of-30 ℃,FATT ductile-brittle transition temperature of-33 ℃,showing good mechanical properties.(3)With the increase of tempering time,the lath ferrite in the microstructure of NbV-Ti microalloyed BJ890 steel at 620 ℃ recrystallizes,the dispersed fine grained cementite increases and the short rod cementite spheroidizes,The optimal heat treatment process is 920 ℃ for 14 min quenching,620 ℃ for 60 min tempering.The tensile strength is 1046 MPa,the yield strength is 996 MPa,the elongation is 19.20%,the impact energy at-20 ℃ is 155 J,and the impact energy at-40 ℃ is 89 J.(4)Compared with Nb-V microalloying,the carbide precipitation of Nb-V-Ti microalloyed BJ890 steel after tempering heat treatment is relatively small,and recrystallization of large-sized lath ferrite is hindered.Under the same tempering process,Nb-V-Ti microalloyed BJ890 steel has higher tensile and yield strength,but lower elongation and low-temperature impact properties.The large size lath ferrite needs to be eliminated by extending the tempering time to obtain the best comprehensive mechanical properties...