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利用热膨胀仪对低碳含铌钢(0.028%C-0.25%Si-1.82%Mn-0.085%Nb)进行热处理模拟,即950℃正火后快速冷却到中间温度350～550℃,随后进行不同加热速率、保温温度及保温时间的回火处理.采用光学显微镜、扫描电镜和图像分析方法,分析了不同回火条件下组织中的马氏体--奥氏体(MA)形貌、尺寸及分布.结果表明:回火前的终冷温度在贝氏体相变温度区间及提高回火升温速率,会增加回火组织中MA的体积分数,MA体积分数最高达到7.9%.提高回火温度和延长回火时间,MA的体积分数会出现峰值.回火后,MA平均尺寸在0.77～1.48μm.提高终冷温度、升温速率、回火温度和延长回火时间,会使回火后的MA粗大,并呈多边形化.MA的体积分数和平均尺寸主要受中间冷却过程结束时未转变奥氏体量、回火过程中铁素体向残余奥氏体碳扩散程度以及回火后残余奥氏体稳定性的影响.
The heat treatment simulations of low-carbon, niobium-bearing steel (0.028% C-0.25% Si-1.82% Mn-0.085% Nb) were performed using a thermal dilatometer, ie, normalizing at 950 ° C followed by rapid cooling to an intermediate temperature of 350-550 ° C followed by different heating Rate, holding temperature and holding time.The microstructure, size and distribution of martensite-austenite (MA) in different tempering conditions were analyzed by optical microscopy, scanning electron microscopy and image analysis The results show that the final cooling temperature before tempering in the bainite transformation temperature range and increasing the tempering heating rate will increase the volume fraction of MA in the tempering, MA volume fraction up to 7.9% .Thermalizing temperature and When the tempering time was extended, the volume fraction of MA peaked, and the average size of MA after tempering was between 0.77 and 1.48 μm. Increasing the final cooling temperature, the heating rate, the tempering temperature and the lengthening of the tempering time will make the MA Coarse and polygonal. The volume fraction and the average size of MA are mainly affected by the amount of un-transformed austenite at the end of the intercooling process, the extent of ferrite to retained austenite during tempering, and the residual austenite after tempering The impact of stability.