冷速对低合金高强钢厚板截面组织性能影响

Effect of cooling rate on the microstructure and properties of HSLA steel thick plates

针对低合金高强钢的低淬透性问题,研究了C质量分数为0.04%的材料在不同冷却介质和冷却速度下的微观组织差异性变化情况。当冷速16.2~40.5℃/s获得板条贝氏体组织,冷速在8.1~16.2℃/s获得板条贝氏体和粒状贝氏体混合组织。粒状贝氏体的体积分数随着冷速的降低而不断增加。当提升冷却速度时,微观组织中的大角度晶界密度显著增加,而小角度晶界密度略有增加,有效晶粒尺寸降低。通过多尺度仿真计算方法,建立了厚板冷却过程的温度场和组织场计算模型,计算了20~35 mm厚板在不同换热系数下的马氏体、板条贝氏体、粒状贝氏体体积分数分布情况。结果显示,当换热系数为2000 W/(m2K)时,35 mm厚板心部粒状贝氏体的体积分数大约为73%,而20 mm、28 mm厚板心部基本上能够获得100%板条贝氏体组织。通过提升冷却强度,可降低35 mm厚板心部的粒状贝氏体体积分数,改善材料的强韧性。

For the low hardenability problem of low alloy high-strength steel,the microstructure variation of tested steel with C content of 0.04wt%were investigated,under different cooling medias and cooling rates.When the cooling rate is between 16.2-40.5℃/s,the microstructure lath bainite is obtained,and when the cooling rate is between 8.1-16.2℃/s,the mixed microstructure of lath bainite and granular bainite is obtained.The volume fraction of granular bainite increases continuously with the decrease of cooling rate.When the cooling rate is increased,the density of high angle grain boundaries in the microstructure significantly increases,while the density of low angle grain boundaries slightly increases,and the effective grain size decreases.Through the multi-scale simulation method,the simulation model of cooling process of thick plate is established,in order to calculate the temperature and microstrucre.With this simulation model,the volume fractions distribution of martensite,lath bainite and granular bainite in 20-35 mm thick plates under different heat transfer coefficients have been calculated.The results show that when the heat transfer coefficient is set as 2000 W/(m~2K),the volume fraction of granular bainite in the center of 35 mm thick plate can reach about 73%,and 100%lath bainite can be obtained in the center of 20 mm and 28 mm thick plates.By improving the cooling intensity,the volume fraction of granular bainite in the center of a 35 mm thick plate can be reduced,which can improve the strength and toughness of the steel.