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| Bainite microstructure and its tempering stability of 70Si3MnCrMo steel |
| ZHAO Jia-li1，ZHANG Fu-cheng1,2，YU Bao-dong3，LIU Hui4 |
（1. State Key Laboratory of Metastable Materials Science and Technology， Yanshan University， Qinhuangdao 066004， Hebei， China
2. National Engineering Research Center for Equipment and Technology of Cold Strip Rolling， Yanshan University， Qinhuangdao 066004， Hebei， China
3. Switch Design Institute， China Railway Shanhaiguan Bridge Group Co.， Ltd.， Qinhuangdao 066205， Hebei， China
4. Company Office， China Railway Shanhaiguan Bridge Group Co.， Ltd.， Qinhuangdao 066205， Hebei， China） |
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Abstract The 70Si3MnCrMo steel was treated by austempering and continuous cooling treatment to obtain bainite microstructure. Mechanical properties of the steel were detected via tensile test and impact test experiments. The phase composition and microstructure morphology of the steel were analyzed by XRD， SEM and TEM. Results show that for austempering treatment， the best comprehensive mechanical properties of the steel were obtained when the tempering temperature was 200 ℃， with a product of strength and ductility of 26.4 GPa·%. For continuous cooling treatment， the best comprehensive mechanical properties were obtained when the tempering temperature was 300 ℃， with a product of strength and ductility of 28.6 GPa·%. A carbide-free bainite microstructure composed of bainite ferrite and retained austenite formed in the steel at a relatively low tempering temperature. The ultra-fine bainitic ferrite plate had a ultra-high strength， and the stable residual austenite guaranteed the high plasticity and toughness. Observations show that there were ultra-fine subunits in the bainitic ferrite plate in the steel treated by continuous cooling process. Retained austenites were in shape of film and block possessing different stability， and distributed among the bainitic ferrite plate. The retained austenite gradually transformed to martensite during the tensile process， which made a good combination of strength and toughness， and presented an excellent comprehensive properties.
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Received: 23 May 2016
Published: 13 January 2017
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2017, Vol. 52 
