预相变马氏体对超级贝氏体钢组织热稳定性的影响

为明确超级贝氏体组织失稳机制以及探索提高超级贝氏体钢中残余奥氏体热稳定性的方法,通过预相变马氏体工艺,即在等温贝氏体相变前引入预相变马氏体,制备了中碳超级贝氏体钢。对比分析了回火前后中碳超级贝氏体钢显微组织和力学性能的变化,研究了预相变马氏体对中碳超级贝氏体钢中贝氏体组织及残余奥氏体热稳定性的影响。结果表明:预相变马氏体的存在能够细化贝氏体铁素体板条,提高残余奥氏体含量和热稳定性。预相变马氏体的引入及其对超级贝氏体组织的细化作用使得试验钢的屈服强度超过1 000 MPa,伸长率大于20%;300~600 ℃回火1 h后,高碳薄膜状残余奥氏体首先发生分解,形成细小的碳化物,然后贝氏体铁素体板条发生回复和再结晶,形成沿原板条方向的铁素体晶粒;600 ℃回火后试验钢的屈服强度仍与回火前相当,主要是预相变马氏体周围的薄膜状残余奥氏体未发生明显分解,能够抑制相邻贝氏体铁素体板条的回复。

Effect of prior martensite on thermal stability of microstructure of super banitic steel

In order to clarify the instability mechanism of super bainite microstructure and explore the method to improve the thermal stability of retained austenite in super bainitic steel, medium-carbon super bainitic steel was prepared though prior martensite process, i.e. prior martensite was formed before isothermal bainite transformation.The microstructure and mechanical properties of medium-carbon super bainitic steel before and after tempering were compared and analyzed, and the effect of prior martensite on the bainite microstructure and thermal stability of retained austenite in medium-carbon super bainitic steel was studied.The results show that the presence of prior martensite can refine the bainitic ferrite laths, and improve the content and thermal stability of retained austenite.The introduction of the hard prior martensite and its refinement of super bainite microstructure make the yield strength and elongation of the tested steel exceed 1 000 MPa and 20%, respectively.After tempering at 300-600 ℃ about 1 h, the film-like retained austenite with high carbon content decomposes into fine carbides firstly, and then the bainitic ferrite laths recover and recrystallize, and form new ferrite grains, which along the original lath direction.The yield strength of the tested steel after tempering at 600 ℃ is still comparable to that before tempering, which can be attributed to the fact that the film-like retained austenite around the prior martensite does not decompose significantly, which can inhibit the coarsening of adjacent bainitic ferrite laths.