Study on Countermeasures to Improve Heat Treatment Process as Causing Standard-exceeding Strength of X65Q Steel Linepipe

Described in the article is the heat on-line partitioning technology(HOPtechnology)for the linepipe steel which makes it possible to get the bainite+martensite/austenite complex structure.Though the X65Q steel linepipe is pro- duced by a certain domestic company with the HOP technology,but under the as-tempered & quenched furnishing condition its strength exceeds the specific API value.The reason for the excessive strength is investigated,serial studies on the heat treatment processes as applied to the pipe body are conducted,and the effect by the tempering temperature on the pipe body microstructure property is analyzed.The analysis result reveals that the excessive strength of the X65Q steel linepipe is caused by the rather low tempering temperature or the insufficient soaking time.Therefore it is possible to get the X65 steel linepipe having the strength up to relevant API requirement by means of adopting the process that involves 650~680℃tempering temperature,60 min.soaking time,and air-cooling of the pipe after running out of the furnace at 400℃.As a result,the problem of defective excessive strength is fixed.