The chemical composition of X70QS seamless steel tube was designed according to the API 5L standard. CCT curve of test steel was measured using thermal expansion instrument

the results show pearlite disappeared and bainite content was more when the cooling rate reached 20 龙/s

the appropriate cooling rate of quenching is more than 20 °C/s. Mass production of X70QS seamless steel tube was carried out using the process of “ 100 t EAF—LF + VD—CCM― ACCU- ROLL machine-quenching and tempering". In the production process

controlled S content

non-metallic inclusion

casting billet segregation. Also solved the problem of sticking steel in piercing. The results show the microstructure of X70QS seamless steel tube is ferrite and tempering bainite

grain size is not smaller than 8. 5

the total grade of all kinds of inclusion is not bigger than 2 ； the mechanical performa

nce and H

2

S resistance performance meet with the requirement of API 5L standard ； the ductile-brittle transition temperature is below -60 ℃ .