热基镀锌双相钢的组织性能与表面质量

 为了解决汽车用热轧和酸洗双相钢存在的耐蚀性不足问题,开发了一种热基镀锌铁素体贝氏体双相钢。利用扫描电镜、透射电镜、拉伸试验机和成形试验机等设备,研究了工艺参数对铁素体贝氏体双相钢组织性能的影响,并评价了最优退火工艺下双相钢的表面质量。结果表明,热轧钢板的显微组织主要由铁素体和贝氏体组成,贝氏体体积分数为13.3%。当在620~700 ℃之间均热时,贝氏体分解形成渗碳体,随均热温度由620升高至700 ℃,贝氏体体积分数由10.1%下降至6.6%;均热温度在740 ℃及以上时,渗碳体消失,随均热温度由740 ℃升高至820 ℃,贝氏体体积分数由17.2%下降至10.7%。在冷却及镀锌过程中,试验钢中析出纳米级NbC或(Nb,Ti)C复合粒子。随均热温度升高,试验钢屈服和抗拉强度先升高后降低,断后伸长率和扩孔率先降低后升高。均热温度对力学性能和扩孔率的影响,主要是基于对渗碳体形成、贝氏体体积分数及组织硬度差的影响。在780 ℃均热时,可获得屈服强度520 MPa、抗拉强度606 MPa、断后伸长率22%的优异力学性能。此外,试验钢的局部成形性能同样优异,扩孔率达到107%,180°横纵向折弯的最小相对弯曲半径为0。采用预氧化和炉内加湿相结合工艺,提高了钢板可镀性。进行热轧小凸度、酸洗及镀锌光整大轧制力控制,有助于获得较高镀层厚度均匀性。

Microstructure,properties and surface quality of hot-rolled galvanized dual phase steel

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.