冷却速率对变形与未变形X80管线钢组织的影响

EFFECT OF COOLING RATE ON MICROSTRUCTURE OF DEFORMED AND UNDEFORMED X80 PIPELINE STEELS

通过相同温度、不同冷却速率下不变形和变形量为0.7的热模拟压缩实验,研究了冷却速率对变形和未变形X80管线钢组织的影响.结果表明:与未变形试样比较,真应变0.7的变形使X80管线钢的贝氏体转变开始温度提高了30—80℃,且冷却速率越大,相变点越低,相变经历的时间越短.无变形试样连续冷却时得到的组织主要为贝氏体,且存在原始奥氏体晶界.真应变为0.7时,在1—40℃/s范围内,随冷却速率增加,X80管线钢的组织依次为:多边形铁素体、准多边形铁素体、块状铁素体、粒状贝氏体、针状铁素体(+粒状贝氏体)和板条贝氏体.淬火组织为马氏体.X80管线钢在真应变为0.7的条件下获得针状铁素体(贝氏体铁素体)的冷却速率范围为10—20℃/s.在该范围内,冷却速率越大,大角度晶界比例越高,晶粒越细小.

The effects of cooling rate and deformation on microstructures of an X80 pipeline steel were investigated by thermo-mechanically simulated tests heated up to the same temperature and cooled at different rates under 0 and 0.7 true strain deformation.The results reveal that the start temperature for bainite transformation will be increased by 30—80℃higher under 0.7 true strain than under no deformation.But the increase of cooling rate depresses the start temperature for bainite transformation and accelerates the progress of its transformation.It is found that the microstructure of the cooled samples without deformation is all composed of bainite with prior austenite grain boundaries （PAGB）,but it is complicated for the cooled samples under 0.7 true strain.In the range of 1—40℃/s, with the increase of cooling rate,microstructures appeare successively such as polygonal ferrite,quasipolygonal ferrite,massive ferrite,granular bainite,acicular ferrite（＋granular bainite） and lathy bainite.Full martensite steel can be achieved by quenching.Under current experiment conditions, acicular ferrite can be obtained in the steel at cooling rates of 10—20℃/s.In this cooling rate range, the fraction of high angle grain boundaries（HAGBs） increases and the grain size decreases with the increase of cooling rate.