对5Cr2NiMoVSi钢进行常规油淬和模拟300×300mm截面模具油淬时心部的冷速进行淬火处理,利用透射电镜等手段分析了它们的淬、回火组织.结果表明,淬态下,常规处理试样以板条马氏体为主;模拟处理试样主要是B_I型贝氏体.后者与低碳低合金钢中的B_I型贝氏体不一样,表现出较高的回火稳定性.在高温回火后,大量残余奥氏体转变,生成粗大链状分布的M_3C,是大截面模具回火脆化的主要原因. The quenching and tempering microstructure of 5Cr2NiMoVSi steel was studied by conventional oil quenching and simulated 300 × 300mm section die quenching and tempering at the time of oil quenching.The results showed that under the condition of quenching, The sample is mainly lath martensite, and the sample is mainly B_I type bainite, which is different from B_I type bainite in low carbon and low alloy steels and shows high tempering stability After tempering at high temperature, a large amount of residual austenite is transformed into M_3C with coarse chain distribution, which is the main reason of tempering embrittlement of large cross section mold.