Nb对中低碳超级贝氏体钢组织与性能的影响

针对无Nb和0.05wt%Nb两种中低碳钢,研究了Nb对0.25wt%C超级贝氏体钢组织与性能的影响。结果表明,对两种试验钢采用轧后先快冷后缓冷的等温工艺,均可获得贝氏体组织。300 ℃等温8 h,含Nb钢贝氏体含量达到80%,屈服强度提高12% (109 MPa),冲击吸收能量达到52 J。通过Thermal-Calc软件计算并结合TEM观察发现,含Nb钢中Nb元素与Mo等元素形成复杂碳化物,析出的细小碳化物钉扎板条边界,细化贝氏体板条,抑制板条合并与粗化,提高板条的稳定性。等温8 h后,含Nb钢的贝氏体铁素体板条尺寸在150~200 nm之间。利用背散射电子和EBSD分析发现,Nb元素通过促进碳化物的析出,降低过冷奥氏体稳定性,促进贝氏体转变,抑制马氏体转变,细化残留奥氏体,提高了组织的均匀性和稳定性,是性能提高的主要机制。

Effect of Nb on microstructure and properties of medium-low carbon super-bainitic steel

Based on non-Nb and 0.05wt%Nb steels, effect of Nb on microstructure and properties of 0.25wt%C super-bainitic steel was studied. The test results show that bainite structure can be obtained for the both steels by rolling, rapid cooling and then slow cooling isothermal process. When isothermal treated at 300 ℃ for 8 h, bainite content in Nb containing steel is more than 80%, yield strength is increased by 12% (109 MPa), impact absorbed energy reaches 52 J. The result of calculation by Thermal-Calc software and observation by TEM indicates that tiny carbides formed by Nb-Mo and other elements are precipitated, that pin lath boundary, inhibit lath merging and coarsening, and improve the stability of the lath. Size of bainitic ferrite lath in Nb containing steel is in the range of 150-200 nm by holding for 8 h. By using BSE and EBSD analysis, because of Nb element promoting the precipitation of carbides, reducing the stability of undercooled austenite, promoting bainite transformation, inhibiting martensite transformation, refining residual austenite, the uniformity and stability of microstructure is improved, that is the main mechanism to improve properties.