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研究了X70管线钢模拟热影响区的局部脆化现象和临界再热粗晶区粒状贝氏体的形 成及其对韧性的影响规律. 结果表明, 二次峰值温度为780℃时, $M$--$A$组元在原奥氏体晶界成链状分布; 二次峰值温度为840℃时, 原粗晶区晶界位置生成了细晶贝氏体带, 及大颗粒的$M$--$A$组元; 二次峰值温度为900℃时, $M$--$A$组元弥散分布，原粗晶区的奥氏体晶界消失. 在二次加热过程中奥氏体主要在原粗晶区晶界形核长大. X70钢的粗晶区出现再加热脆化现象, 它的脆化温度区间是一个比$A_{\rm c1}$--$A_{\rm c3}$更窄的温度区间, 主要在$T^{2}_{\rm P}$=$A_{\rm c1}$--840℃温度范围内. 临界再热粗晶区脆化的主要原因是在晶界上有链状分布的$M$--$A$组元.

The local brittle zone phenomena of pipeline X70 steel were studied by means of simulated heat affected zone with the emphasis on the forming of granular bainitic in the intercritical coarse grained region and its influence on the toughness. The experimental results showed that $M$--$A$ constituents mainly distributed at prior austenite grain boundaries ~at ~second ~peak ~temperature of 780℃, large particle $M$--$A$ constituent and fine grain bainitic zone were formed at prior austenite grain boundaries at 840℃, and $M$--$A$ constituent distributed uniformity at 900℃ with the disappear of prior austenite grain boundaries which suggested that the austenite is mainly nucleared and grows at CGHAZ grain boundaries. Re--heat brittle phenomena was found in CGHAZ of X70 pipeline steel at an second peak temperature of $A_{\rm c1}$--840℃ which is narrower than $A_{\rm c1}$--$A_{\rm c3}$ and is mainly caused by $M$--$A$ constituents distributed in chain at prior austenite grain boundaries.