Microstructure, properties and surface quality of hot-rolled galvanized dual phase steel
ZOU Ying1, HAN Yun2, LIU Hua-sai1, LI Yan1, QIU Mu-sheng1, WANG Chao-bin3
1. Jingtang Technology Center, Shougang Research Institute of Technology, Beijing 100043, China; 2. Sheet Metal Research Institute, Shougang Research Institute of Technology, Beijing 100043, China; 3. Manufacturing Department, Shougang Jingtang United Iron and Steel Co., Ltd., Tangshan 063210, Hebei, China
Abstract:In order to solve the problem of insufficient corrosion resistance in hot rolled and pickled dual phase steel for automobile,a hot-rolled galvanized ferrite-bainite dual phase steel was developed. The effects of process parameters on microstructure and properties of ferrite-bainite dual phase steel were studied by using scanning electron microscope (SEM),transmission electron microscope (TEM),tensile testing machine,forming testing machine,and so on. Moreover,the surface quality of dual phase steel under the optimal annealing process was evaluated. The results showed that the microstructure of as-hot-rolled steel was mainly composed of ferrite and bainite, and the volume percent of bainite was 13.3%. The bainite decomposed into cementite when soaking at 620-700 ℃,and the volume percent of bainite decreased from 10.1% to 6.6% with the soaking temperature increase from 620 ℃ to 700 ℃. When soaking at 740 ℃ or above,the cementite disappeared and the volume percent of bainite decreased from 17.2% to 10.7% with the increase of soaking temperature from 740 ℃ to 820 ℃. During cooling and galvanizing,nano-sized NbC or (Nb,Ti)C was precipitated in the steel. With the increase of soaking temperature,the yield and tensile strength of the test steel first increased and then decreased,while the total elongation and hole expansion rate first decreased and then increased. The effects of soaking temperature on mechanical properties and hole expansion rate were mainly based on its effects on cementite formation,volume percent of bainite and hardness difference of microstructure. Excellent mechanical properties with yield strength of 520 MPa,tensile strength of 606 MPa and total elongation of 22% were achieved,when soaking at 780 ℃. The local formability of the test steel was also outstanding,with an average hole expansion rate as high as 107%. Besides,the minimum relative bending radius was 0 under transverse and longitudinal bending test with a bending angle of 180°. The platability of the test steel was improved by the combination of pre-oxidation and furnace humidification. The control of small convexity at hot rolling and large skin pass rolling force at pickling and zinc plating finishing was helpful to obtain higher coating thickness uniformity.
邹英, 韩赟, 刘华赛, 李研, 邱木生, 王朝斌. 热基镀锌双相钢的组织性能与表面质量[J]. 钢铁, 2021, 56(11): 104-111.
ZOU Ying, HAN Yun, LIU Hua-sai, LI Yan, QIU Mu-sheng, WANG Chao-bin. Microstructure, properties and surface quality of hot-rolled galvanized dual phase steel[J]. Iron and Steel, 2021, 56(11): 104-111.
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