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

doi:10.13251/j.issn.0254-6051.2021.12.020

Abstract

Abstract: 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.

Key words: medium-low carbon bainitic steel, Nb, carbides, retained austenite, properties

CLC Number:

TG335.11

Gao Shi, Pan Xuefu. Effect of Nb on microstructure and properties of medium-low carbon super-bainitic steel[J]. Heat Treatment of Metals, 2021, 46(12): 124-129.

References

[1]Caballero F G, Bhadeshia H, Mawella K J A, et al. Design of novel high strength bainitic steels: Part 1[J]. Materials Science and Technology, 2001, 17(5): 512-516.
[2]Caballero F G, Bhadeshia H. Very strong bainite[J]. Current Opinion in Solid State and Materials Science, 2004, 8(3): 251-257.
[3]Caballero F G, Bhadeshia H, Mawella K J A, et al. Design of novel high strength bainitic steels: Part 2[J]. Materials Science and Technology, 2001, 17(5): 517-522.
[4]Sharma S, Sangal S, Mondal K. Development of new high-strength carbide-free bainitic steels[J]. Metallurgical and Materials Transactions A, 2011, 42(13): 3921-3933.
[5]Sherif M. Characterisation and development of nanostructured, ultrahigh strength, and ductile bainitic steels[D]. Cambridge: University of Cambridge, 2009.
[6]Wang X L, Wu K M, Hu F, et al. Multi-step isothermal bainitic transformation in medium-carbon steel[J]. Scripta Materialia, 2014, 74: 56-59.
[7]徐光, 操龙飞, 补丛华, 等.超级贝氏体钢的现状和进展[J]. 特殊钢, 2012, 33(1): 18-21.
Xu Guang, Cao Longfei, Bu Conghua, et al. Status and progress of super-bainitic steels[J]. Special Steel, 2012, 33(1): 18-21.
[8]王泽民, 朱晓勇, 刘文庆. Mo对Nb-Mo-V微合金化钢中碳化物析出的影响[J]. 材料研究学报, 2010, 24(2): 217-222.
Wang Zemin, Zhu Xiaoyong, Liu Wenqing. Influence of Mo on tempering precipitation in Nb-Mo-V microalloyed steels[J]. Chinese Journal of Materials Research, 2010, 24(2): 217-222.
[9]雍岐龙. 钢铁材料中的第二相[M]. 北京: 冶金工业出版社, 2006.
[10]王有铭, 李曼云, 韦光. 钢材的控制轧制和控制冷却[M]. 2版. 北京: 冶金工业出版社, 2009.
[11]高石, 蔡庆伍, 潘学福. 含Nb无碳化物贝氏体/马氏体钢的组织与性能[J]. 金属热处理, 2015, 40(6): 70-74.
Gao Shi, Cai Qingwu, Pan Xuefu. Microstructure and mechanical properties of carbide free bainite/martensite Nb-doped steel[J]. Heat Treatment of Metals, 2015, 40(6): 70-74.
[12]葛心慧. 钢中残余奥氏体的作用[J]. 南通职业大学学报, 1995(1): 26-28.
Ge Xinhui. Effect of residual austenite in steel[J]. Journal of Nantong Vocational University, 1995(1): 26-28.
[13]方鸿生, 王家军, 杨志刚, 等. 贝氏体相变[M]. 北京: 科学出版社, 1999.
[14]徐祖耀. 低碳钢中的残余奥氏体[J]. 上海金属, 1995, 17(1): 1-6.
Xu Zuyao. Retained austenite in low carbon steels[J]. Shanghai Metals, 1995, 17(1): 1-6.
[15]高绪涛, 赵爱民, 赵征志, 等. 热轧TRIP钢残余奥氏体及其稳定性研究[J]. 材料工程, 2011(11): 39-43.
Gao Xutao, Zhao Aimin, Zhao Zhengzhi, et al. Investigation of retained austenite and its stability in hot rolled TRIP steels[J]. Journal of Materials Engineering, 2011(11): 39-43.
[16]周玉, 武高辉. 材料分析测试技术: 材料X射线衍射与电子显微分析[M]. 哈尔滨: 哈尔滨工业大学出版社, 1998.