通过金相、扫描电镜,相变临界点和过冷奥氏体等温转变的分析,研究了Nb对共析钢（质量分数：C 0.75%,C 0.78%）珠光体相变微观组织和等温转变动力学的影响。结果表明：微量Nb（质量分数为0.04%、0.064%）的加入使钢中先共析铁素体的量较未含铌钢有所增加,这很可能是Nb的加入使得共析碳含量明显升高的结果,同时使珠光体相变产物的形貌发生明显变化,珠光体中渗碳体的展弦比显著降低。此外Nb提高了先共析铁素体和珠光体的开始转变温度,即Nb的加入在一定程度上降低了过冷奥氏体的热力学稳定性;相变动力学表明加入Nb 0.04%可使珠光体相变的鼻子点温度升高约50℃,且最快开始相变时间比不含铌钢推迟一个数量级以上,即Nb的加入推迟了鼻子点和较低温度下的珠光体相变,提高了钢的淬透性。
Effect of Nb on the microstructure and isothermal transformation kinetics of pearlite in eutectoid steels（C 0.75%,C 0.78%） was studied by metallographic,SEM and critical temperature of transformation and TTT（time temperature transformation）.The results show that the amount of pro-eutectoid ferrite in the steels with micro-addition Nb（0.04%,0.064%） increases through comparing with steel without Nb addition.That is probably because that the carbon content at eutectoid point increases with addition of Nb,meanwhile the morphology of transformed pearlite changes obviously,namely the aspect ratio of cementite reduces markedly.In addition,the transformation temperatures of both pro-eutectoid ferrite and pearlite have been raised by addition of Nb,namely thermodynamic stability of overcooling austenite is reduced by Nb.The analysis of transformation kinetics shows that Nb 0.04% could raise the nose temperature of TTT graph by about 50 ℃ and retard the eutectoid transformation start time for about one order of magnitude,which imply that addition of Nb retard the nose temperature and the pearlite transformation at lower temperature,then the hardenability is improved.
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