Study On Effect Of On-line Accelerated Cooling On Microstructure And Properties Of 27SiMn Seamless Steel Pipe

The influence of the on-line accelerated cooling process on the microstructure and properties of 27 SiMn seamless steel tubes was studied.The continuous cooling transformation curve of 27 SiMn steel was measured,and an on-line accelerated cooling experimental platform was designed.The effect of final cooling temperature with specific cooling rate and tempering temperature and holding time on the microstructure and properties of 27 SiMn seamless steel tubes were investigated.The main research contents and conclusions are as follows:(1)The Formast-F full automatic phase change instrument was used to measure the expansion temperature curve of 27 SiMn steel at different cooling rates,and the phase transition point was measured by the tangent line method,and the CCT curve of 27 SiMn steel is finally measured.(2)Heated to austenitizing temperature 930 ? with 30 min preservation,and at the cooling rate of 22~ 26 ? /s,27 SiMn steel was cooled to 315~675 ?.With the decrease of final cooling temperature,the strength of 27 SiMn steel increased gradually,and the elongation,cross section shrinkage and impact energy first increased and then decreased.The microstructure of 27 SiMn steel is mainly ferrite and pearlite at the final cooling temperature of 600~675?,and the microstructure appears bainite at the end cooling temperature of 600?;the final cooling temperature between 465~530 ?,the microstructure is mainly bainite;the final cooling temperature between 315~385 ?,the microstructure is mainly martensite.The experimental results show that,the experimental steel has the best comprehensive properties at the final cooling temperature 465 ?,with tensile strength 1070 MPa,yield strength 860 MPa,elongation 18%,the section shrinkage rate 42%,and the impact power 48 J.(3)The final cooling temperature of a set of experimental steels was controlled at 465?,and tempering experiments was conducted after natural cooling.The tempering temperature at 250~600?,with the increase of tempering temperature,the strength first increased and then decreased,the elongation and the section shrinkage first decreased and then increased,and tempering temperature at 400~600?,the impact work gradually increased with the tempering temperature rising.At tempering temperature 350?,the maximum tensile strength is 1110 MPa,the elongation reached the minimum value of 12%,and section shrinkage reached the minimum value of 27%.The one-step temper brittleness is most likely to occur in 27 SiMn steel at tempering temperature of 300~350?.Tempering at 450?,holding time between 30~110min,holding time has no obvious effect on mechanical properties and microstructure.The experimental results show that,with the final cooling temperature 465? and then tempered at 450? for 30 min,the experimental steel has the best comprehensive properties,with tensile strength 877 MPa,yield strength 753 MPa,elongation 15%,the section shrinkage rate 60%,and the impact power 62 J.