Effect Of Accelerating Cooling Process After Sizing On Microstructure And Properties Of 20# Seamless Steel Tube

In this paper,the effect of accelerated cooling process on the microstructures and properties of 20#seamless steel pipe was studied.The main study ideas were divided into three parts.The first part was to preliminarily understand the transition law of undercooled austenite continuous cooling of experimental steel through thermal simulation experiment of 20#steel.The second part was the transformation rule of using 20#steel supercooled austenite,and borrow the accelerated cooling platform in the laboratory to study the influence of cooling speed,final cooling temperature and tempering temperature on the microstructure and properties of 20#steel.This paper studies whether the comprehensive properties of the experimental steel can reach the level of Q355 steel by using the online accelerated cooling process or the online accelerated cooling+tempering process.In the third part,the production data show that it was reasonable to replace the traditional heat treatment process of seamless steel tube with the on-line accelerated cooling process.The main conclusions of this paper are as follows:（1）The critical phase transition point temperature of 20#steel was determined by thermal simulation experiment as follows:Ac₁=717℃,Ac₃=839℃,Ar₁=696℃,Ar₃=758℃.Draw the CCT curve of 20#steel by expansion method,different final cooling temperature curve.（2）When the cooling rate of the experimental steel was controlled at 25℃/s and the final cooling temperature was between 455℃and 615℃.The microstructure of20#steel was that austenite was completely transformed into ferrite and bainite.When the temperature was lowered again,martensite microstructure would be formed.Under this process,the tensile strength,upper and lower yield strength of 20#steel will increase with the decrease of the final cooling temperature.The section shrinkage and elongation of 20#steel decrease with the decrease of final cooling temperature.（3）The final cooling temperature was controlled at 510℃.When the cooling rate was 10℃/s～16℃/s,the microstructure of 20#steel was completely transformed into ferrite and pearlite by austenite.When the cooling rate was 20℃/s～30℃/s,the microstructure of 20#steel was the complete transformation of austenite into ferrite and bainite structure.Under this process,the tensile strength,the upper and lower yield strength of 20#steel increase with the increase of cooling rate.The section shrinkage and elongation of 20#steel decrease with the increase of cooling rate.（4）Combining the first two sets of data,it can be seen that 20#steel has the best comprehensive performance when the cooling rate was 25℃/s and the final cooling temperature was 510℃.The microstructure of 20#steel under its process was acicular ferrite+lamellar Bainite,the mechanical properties show that the upper yield strength was 355 MPa,the lower yield strength was 345 MPa,the tensile strength was535 MPa,the elongation was 21%,and the section shrinkage was 59%.（5）The tempering temperature was controlled at 400～600℃in the tempering process.At a tempering temperature of 450℃,the comprehensive performance of 20#seamless steel pipe was the best.Its tempering microstructure was recrystallized ferrite and a large number of original bainite structures,and its mechanical properties are tensile strength of 660 MPa,upper yield strength of 470 MPa,lower yield strength of 465 MPa,elongation of 21%,and section shrinkage of 56%.（6）On-line accelerated cooling technology can be directly used in factory production,and the 20#seamless steel tube can meet the practical requirements without subsequent tempering treatment.