高钢级X100管线钢中的M-A岛

M-A islands in high grade X100 pipeline steel

通过显微组织分析方法,研究了冷却开始温度及冷却速度对高钢级X100管线钢热模拟试样中M-A岛的体积分数和尺寸的影响.结果表明降低冷却开始温度和提高冷却速度都可以细化组织,导致M-A岛的体积分数降低,使M-A岛由尺寸较大的块状、条状转变成尺寸较小且弥散分布的块状、条状.利用透射电镜观察了热轧后以不同冷却速度冷却所得试样组织中M-A岛的形貌.发现在低的冷却速度下,M-A岛由残余奥氏体、尺寸及取向均不同的马氏体板条组成.在马氏体板条中存在孪晶,证明了碳的扩散,也说明M-A岛中M是孪晶马氏体.在高的冷却速度下,转变为针状、薄膜状的M-A岛弱化了铁素体板条的界面,降低了管线钢的韧性.因此只有控制冷速在一定范围内,才能获得尺寸细小、弥散分布的M-A岛.

The effects of cooling temperature and cooling rate on the size and volume fraction of M-A islands in thermal simulation samples of high grade X100 pipeline steel were investigated by the microstructure analysis method. The results show that reducing the cooling temperature and increasing the cooling rate can refine the microstructure, decrease the volume fraction of M-A islands, and make the M-A islands from larger blocks and strips into smaller and dispersed ones. The morphology of M-A islands in X100 pipeline steel with different cooling rates after hot rolling was observed by transmission electron microscopy （TEM）. It is found that the M-A islands consist of retained austenite and different sizes and orientations of martensite plates at low cooling rate. Microtwins in the martensite plates prove carbon diffusion and show that martensite in the M-A islands is twinned martensite. At high cooling speed, the M-A islands, which change into the acicular and flaky, weaken the ferrite lath interface and reduce the toughness of the pipeline steel. So in order to obtain small size and dispersive M-A islands, the cooling speed must be controlled in a certain range.