厚度规格和热轧工艺对热轧X70M管线钢DWTT的影响

INFLUENCE OF THICKNESS AND HOT ROLLING ON DWTT OF HOT ROLLED PIPELINE STEEL X70M

热轧X70M及以上管线钢具有更高的强度和优异的低温韧性,是长距离输送油气的主要钢种。落锤撕裂试验采用全壁厚试样,比夏比冲击试验更能反映钢管的断裂情况,被广泛用于评价管线钢的抗脆性断裂能力。本文介绍了DWTT的检测评价方法,分析了厚度规格和热轧工艺对X70M管线钢DWTT的影响。研究表明:在相同工艺条件下,随热轧厚度增加,DWTT剪切面积比呈降低趋势,韧脆转变温度升高,对于剪切面积比≥85%的临界温度,10.8 mm厚度规格约在-38℃;X70M组织主要由针状铁素体、少量多边形铁素体、贝氏体组成,可以有效阻止裂纹扩展,有利于落锤撕裂韧性能;晶粒度一般为11级以上;在相同成分体系条件下,降低卷取温度可以细化晶粒,提高韧性。对于15.5 mm厚度规格开展不同的热轧对比工艺对比试验,其中-10℃试验条件下,低温卷取(480~510℃)DWTT剪切面积比平均97%,优于高温卷取(540~570℃)DWTT剪切面积80.5%。

Pipeline steel grade of X70M or more than X70M is mainly used for oil and natural gas transportation because of larger strength and better low temperature toughness.The drop weight tear test,which adopts full wall thickness samples,can reflect the fracture of steel pipe better than Charpy impact test,and is widely used to evaluate the brittle-fracture resistance of pipeline steel.This paper introduces test methods of DWTT,and analysis the effect of thickness and hot rolling on DWTT.Under the same process conditions,DWTT shear area ratio decreases with the increase of hot rolling thickness,and the tough-brittle transition temperature increases.For critical temperatures with shear area ratio≥85%,the 10.8 mm thickness specification is about-38℃.The X70M structure is mainly composed of acicular ferrite,a small amount of polygonal ferrite and bainite,which can effectively prevent crack propagation and is conducive to the tear toughness of the drop hammer.The grain size is generally above grade 11.Under the condition of the same composition system,reducing the coiling temperature can refine the grain and improve the toughness.For 15.5 mm thickness specifications,different hot rolling comparative process comparative tests were carried out.Under-10℃test conditions,the DWTT shear area ratio of low temperature cooling(480~510℃)was 97%on average,which was better than that of high temperature cooling(540~570℃)DWTT shear area of 80.5%.