低成本900MPa级工程机械用钢连续冷却转变行为的研究

Study on continuous cooling transformation behavior of low cost 900 MPa construction machinery steel

针对当前不含Mo低成本900 MPa级工程机械用钢的生产,采用Formastor-FⅡ相变仪,研究了900 MPa级工程机械用钢的连续冷却相变行为,分析了试验钢在连续冷却条件下的显微组织、显微硬度变化规律和贝氏体相变过程;结合热膨胀法和金相-硬度法绘制了试验钢的连续冷却转变曲线。结果表明:当冷却速率为0.25~0.5℃/s时,试验钢组织主要为铁素体和粒状贝氏体;冷却速率为1~2℃/s时,试验钢组织由粒状贝氏体和板条贝氏体组成;冷却速率为5~20℃/s时,试验钢组织为板条贝氏体和互锁状贝氏体,随着冷却速率的提高,板条贝氏体相变温度区间变窄,互锁状贝氏体相变温度区间变宽。冷却速率为5℃/s时,以板条贝氏体相变为主导,晶界形核速率高于晶内形核速率;冷却速率为10~20℃/s时,以互锁状贝氏体相变为主导,晶内形核速率高于晶界形核速率。冷却速率为0.25~2℃/s时,试验钢显微硬度随着冷却速率的增加而增加,硬度值从188HV升高到239HV;冷却速率为2~5℃/s时,出现硬度平台;冷却速率为5~20℃/s时,试验钢显微硬度随冷却速率的增加而增加,硬度值从240HV升高到270HV。

In view of the current production of low cost 900 MPa construction machinery steel without Mo, the continuous cooling transformation of 900 MPa construction machinery steel was studied by using Formaster-FⅡ phase transformation instrument, and the microstructure, microhardness and bainite transformation process of the test steel under continuous cooling were analyzed.The continuous cooling transformation curve of the test steel was drawn by thermal expansion method and metallographic-hardness method.The results show that when the cooling rate is 0.25-0.5 ℃/s, the microstructure of the test steel is mainly ferrite and granular bainite, and when the cooling rate is 1-2 ℃/s, the microstructure of the test steel is composed of granular bainite and lath bainite.When the cooling rate is 5-20 ℃/s, the microstructure of the test steel is lath bainite and interlocking bainite.With the increase of cooling rate, the lath bainite transformation temperature range becomes narrower and the interlocking bainite transformation temperature range becomes wider.When the cooling rate is 5 ℃/s, the lath bainite phase becomes dominant, and the grain boundary nucleation rate is higher than the intragranular nucleation rate;when the cooling rate is 10-20 ℃/s, the interlocking bainite phase becomes dominant, and the intragranular nucleation rate is higher than the grain boundary nucleation rate.When the cooling rate is 0.25-2 ℃/s, the microhardness of the test steel increases with the increase of the cooling rate, the hardness value increases from 188 HV to 239 HV, the hardness platform appears when the cooling rate is 2-5 ℃/s, and when the cooling rate is 5-20 ℃/s, the microhardness of the test steel increases with the increase of the cooling rate, and the hardness increases from 240 HV to 270 HV.