A new type of low cost Mn-Cr-Mo-La air-cooled bainitic rail steel with high strength and toughness was specifically designed by taking advantage of the complementary role of rare earth and alloy elements in this research.The phase transformation behaviors and microstructures were studied through the determination of the continuous cooling transformation (CCT) curve.The strengthening and toughening mechanism and microstructure refinement mechanism were studied through TEM observation of fine substructures after hot rolling simulation tests based on the continuous rolling process BD1-BD2-CCS of field rails.The results show that a large number of fine bainite plates can be obtained during the cooling rate range from 0.8 to 2 ℃/s by means of adding RE which can greatly reinforce the hardenability of Mn, Cr, and Mo elements.The microstructure of Mn-Cr-Mo-La bainitic steel after hot rolling simulation tests is composed of refined carbide-free bainitic plates with stable interlath retained austenite (RA).The widths of sub-units are about 39~80 nm and bulk ultra fine sub-subunits 20 nm×32 nm.Both ultra-fine micro-twins with various sizes of 2~20 nm and high density dislocation can be found inside the bainitic plates, resulting in 1 260 MPa tensile strength of test steel and up to 95 J of impact energy at room temperature.This kind of nanoscale bainite contributes to the prevention of the rail cracking by hindering crack propagation.%利用稀土元素与合金元素的优势互补作用,设计一种低成本高强韧钢轨专用的Mn-Cr-Mo-La系贝氏体钢,通过测定其动态连续冷却转变(CCT)曲线,研究相变行为及组织特征;基于现场钢轨连轧工艺BD1-BD2-CCS进行热连轧模拟试验,采用透射电镜(TEM)观察精细亚结构,分析试验钢的组织细化和强韧化机理.结果表明:由于添加稀土元素可加强Mn、Cr、Mo的淬透性作用,试验钢在冷速范围0.8~2 ℃/s内可获得大量细小板条贝氏体;其热连轧模拟后的精细组织为板条间带有稳定残余奥氏体膜的无碳化物贝氏体,亚单元宽度细化至39~80 nm,块状超细亚单元尺寸为20 nm×32 nm,板条内发现2~20 nm的微细孪晶和高密度位错,使试验钢抗拉强度达1 260 MPa,室温下冲击功达95 J,该纳米级贝氏体组织有利于阻碍裂纹扩展,防止贝氏体钢轨轨裂.
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