常化后冷却工艺对1600MPa级超高强钢组织性能的影响

Effect of cooling process after normalizing on the microstructure and properties of 1600 MPa ultra-high strength steel

为改善高强度钢的塑性和韧性，对中碳低合金马氏体高强度钢分别采用常化后空冷＋回火和常化后控冷＋回火工艺，研究常化后冷却工艺对钢中残余奥氏体及力学性能的影响.采用扫描电镜获得钢的组织形态，利用X射线衍射和电子背散射衍射技术分析钢中残余奥氏体的体积分数、形貌和分布.发现两种工艺下均得到板条马氏体＋残余奥氏体组织，残余奥氏体均匀分布在板条之间，随工艺参数不同，其体积分数在3%~10%变化.常化后加速冷却能显著细化马氏体板条，提高钢的屈服强度和抗拉强度100 MPa以上，冲击功下降4 J.残余奥氏体的体积分数随常化控冷终冷温度的升高呈现先升高后降低的变化。

The effects of two different processes, normalizing with air cooling ＋ tempering and normalizing with controlled cooling＋ tempering, on the retained austenite and mechanical properties of medium-carbon low-alloy martensitic ultra-high strength steel were investigated to improve its plasticity and toughness. The microstructure of the steel was observed by scanning electron microscopy （ SEM） . The volume fraction, shape and distribution of retained austenite in the steel were measured by X-ray diffraction （ XRD） and electron back-scattering diffraction （EBSD）. The results show that lath martensite and retained austenite （M＋RA） are obtained by the two processes and a uniform distribution of retained austenite is located between lath martensites. The volume fraction of retained austenite varies from 3% to 10% with treatment parameters. Normalizing with controlled cooling can significantly refine lath martensite while the yield strength and tensile strength increase more than 100 MPa and the impact energy decreases by 4 J. When the quenching temperature rises, the volume fraction of retained martensite increases firstly and then decreases. Normalizing with controlled cooling can also act as a method for improving the microstructure and properties of martensitic steel.