Microstructural transformation and plasticity mechanism of 2 GPa medium-carbon medium-manganese nano-bainitic steel
LING Yu1,2, HU Feng1,2,3, YAN Heng1,2, ZHOU Wen1,2, ZHANG Zhi-cheng4, WU Kai-ming2,5
1. Collaborative Innovation Center for Advanced Steels, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China; 2. International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China; 3. School of Electromechanical Engineering, Guangzhou University of Technology, Guangzhou 510006, Guangdong, China; 4. Hubei Province Key Laboratory of High Performance Special Steel, Daye Special Steel Co., Ltd., Huangshi 435001, Hubei, China; 5. Metals Valley and Band (Foshan) Metallic Composite Co.,Ltd., Foshan 528000, Guangdong, China
Abstract:High C(0.78%-0.98%)(mass percent) and high Si (about 1.5%) steels adopt low temperature bainite (usually 150-250 ℃) to obtain 2.0 GPa ultra-high strength,but with low plasticity (usually ≤8.0%).At the same time,a very long bainite transformation time (usually≥4 d) is required. The composition design of reducing the C content (Fe-0.30C-1.5Si-1.5Ni) can significantly accelerate the bainite transformation (isothermal at 300 ℃ for 0.5 d) and obtain excellent strength (tensile strength for (1 138±6)MPa) and plasticity (elongation for 18.5%±1.5 %) matching performance,but it is difficult to achieve ultra-high strength (≥1 500 MPa) level. Referring to the alloy design,microstructure and mechanical properties of high/medium C bainitic steel,the alloy composition of "medium C,replacing Si with Al,and replacing Ni with Mn"(Fe-0.30C-1.2Al-5.0Mn) was adopted,isothermal transformation near Ms temperature can obtain high-plastic nano-bainitic steel with strength of 2.0 GPa ((2 029±9) MPa) and elongation of more than 10.0 %,and the bainite transformation time is moderate (300 ℃isothermal for 2 days),and the fabrication cost of the alloy is low(about 0.5% Ni mass percent).The ultra-high strength of Fe-0.30C-1.2Al-5.0Mn steel is mainly to the total volume percent of hard-phase bainitic ferrite and martensit is 85.1%,which the width of bainitic-ferrite lath is (85±30) nm. The high plasticity is mainly to the volume percent of the soft-phase retained austenite of 14.9%,which the C mass percent of 1.12%,and the retained austenite film between in the bainitic-ferrite laths is (30±15)nm. At the same time,the pair of C and Mn elements can increase the stability of retained austenite. Especially compared with low Mn content,the 5% Mn element has a more obvious stabilization effect on retained austenite,making it more stable under low stress and the phase transformation is not prone to occur,but the TRIP effect persists to improve plasticity during high stress processes.
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