Study On The Normalizing And Quenching And Tempering Process Of 20# Hot-rolled Seamless Steel Pipe

This article is the research on the normalizing and quenching and tempering process of 20# hot-rolled seamless steel pipe.The research is mainly divided into three parts: The first part is a thermal simulation experiment on 20# steel to determine the TTT curve and CCT curve of 20# steel.Understand the transformation law of its supercooled austenite under isothermal cooling and continuous cooling;the second part is the normalizing experiment on 20# steel,through setting different holding time and different cooling speed to explore the actual production due to thick wall The reasons for the uneven heat dissipation on the inner wall of the seamless steel pipe that affect its performance,and the impact of the increase in cooling speed on the mechanical properties of the seamless steel pipe;the third part is the quenching and tempering process,setting different quenching and tempering processes for 20# steel and setting medium and low temperature The tempered samples were compared to explore the changes in mechanical properties of 20# steel after quenching and tempering,which provided a reference for the performance enhancement of thick-walled pipes with poor performance.(1)TTT curve measurement: The critical point of 20# steel is measured by the expansion method A1=718℃,A3=820℃,Ms=451℃,Mf=60℃;supercooled austenite to pearlite and supercooled austenite to shell The demarcation temperature of the intensification is about 600 ℃,and the bainite transformation occurs during the isothermal transformation at 600℃;there are two nose tips in the TTT curve,which are about 650℃ and 550℃ respectively.(2)CCT curve measurement: The phase transition point of 20# steel measured by the expansion method is Ac1=721℃,Ac3=826℃,Ar1=701℃,Ar3=749℃;the cooling rate range is 5℃/s～20℃/s,The microstructure of 20# experimental steel is ferrite and pearlite;in the cooling rate range of 25 ℃/s～40 ℃/s,the microstructure of 20#experimental steel is ferrite,bainite and martensite,20 #Experimental steel martensite transformation start line is 460℃.(3)Normalizing process: Coarse pearlite and ferrite structures are easily formed under slow cooling speed,and lamellar cementite is formed at the grain boundaries,which in turn surrounds the ferrite,resulting in poor mechanical properties of 20# steel;As the cooling rate increases,uniform and fine pearlite and ferrite structures are obtained,and the distance between the pearlite sheets is reduced,and the mechanical properties are improved.(4)Quenching and tempering process: It is found in the experiment that the strength of quenched samples at 1050°C is higher after quenching and tempering,but the plastic toughness is slightly insufficient.The strength of quenched samples at 950°C is lower than that of quenched samples at 1050°C after quenching,but the plastic toughness is better;The comprehensive mechanical properties of tempered samples at medium and low temperature are poor.In the experiments in this chapter,the comprehensive mechanical properties of 20# experimental steel under the quenching and tempering process of 950°C water quenching and 500°C tempering are the best.The structure after quenching and tempering is tempered sorbite and ferrite,and the tensile strength is 670 MPa,the upper yield strength is 635 MPa,the lower yield strength is 630 MPa,the elongation after fracture is 28%,and the reduction of area is 66.79%.