【作者】 肖良红；

【导师】 苏旭平；

【作者基本信息】 湘潭大学 ， 材料学， 2012， 博士

【摘要】 GA汽车镀锌钢板具有优异的抗腐蚀性能、好的喷涂性和可焊接性，已广泛应用于汽车外壳覆盖件生产，但在冲压成形时GA钢板锌镀层容易产生粉化缺陷。本研究课题从影响GA钢板锌镀层粉化的因素（镀层组织、残余应力、界面结合强度、冲压成形时镀层所处应力状态）入手，研究了GA钢板合金化锌镀层粉化的机理。对上海宝钢提供的H220BD型GI镀锌钢板在不同合金化退火工艺参数下进行了退火处理、测量分析了各种合金化组织的残余应力、与基底的界面结合强度，利用这些测量参数对GA钢板冲压成形过程中的应力状态进行了有限元模拟计算；结合粉化检测实验结果，对比各种合金化工艺参数所得组织的性能，确定了GA钢板最佳合金化工艺参数。首先将H220BD型GI钢板在盐浴炉中、按温度分别为450℃、500℃、550℃，每个温度下的退火时间为5秒、10秒、30秒、120秒进行合金化退火获得GA钢板，各种GA钢板锌镀层中主要组织均为δ相；随着退火时间的延长，δ相的厚度增加。当退火时间足够长时，镀层表面也会出现较多的δ相。通过粉化检测发现：500°C下的合金化退火所得锌镀层在相同变形条件下的粉化量比450°C下和550°C下合金化退火所得组织的都要小。在相同合金化退火温度下，GA钢板锌镀层的粉化量首先随合金化退火保温时间的延长而增加，随后又随退火时间的增加而略有减少；在较长退火时间（>30s）下、温度越高，粉化越严重。用纳米压痕方法测量分析了不同合金化工艺参数下所获得的GA钢板锌镀层中的残余应力。当扩散退火时间大于5s时，镀层中存在110MPa-240MPa之间的残余拉应力；GA钢板锌镀层中的残余拉应力首先随退火时间的延长而迅速增大，后来又随时间的增加而减少，而残余拉应力一般随温度的升高而减小。其中500℃和550℃下保温退火10s和120s所得组织的残余应力较小，它们的粉化量都较小；而450℃和550℃下保温退火30s所得组织的残余应力都较大，它们的粉化量都较大。新开发了一种普适的镀层/基底界面强度测试方法，并用这种新型鼓泡法测量得到GA钢板锌镀层与钢基底的界面强度在3.8995J/m2-174.888J/m2之间，发现界面强度首先随退火时间的延长而迅速降低；退火到一定时间后，又随时间的延长而稍有增加，但基本趋于稳定。当退火时间较短时，界面结合强度随退火温度的升高而降低，当退火时间延长到一定时间后，温度对界面结合强度的影响很小。其中450℃和550℃下保温退火30s所得合金化锌镀层与基底的界面强度都较低，它们的粉化量都较大；在500℃下退火所得试样的界面强度都较大（>9J/m2），它们的粉化量都较小。通过粉化检测实验，对比各种镀层的界面结合强度和残余应力得到：δ相太薄或太厚粉化都较严重；残余应力越大、界面强度越低，粉化量越大。GA钢板最佳合金化工艺参数为500°C下保温120秒左右。通过以上研究工作为解决GA钢板锌镀层在冲压成形时出现的粉化问题提供理论依据，对提高我国GA钢板质量，促进连续镀锌工业的发展有所贡献。更多还原

【Abstract】 For its high corrosion resistance, excellent formability, weldability and paintabilityperformance, the galvannealed (GA) steel sheet produced by continuous hot-dip zinccoating on steel sheet is widely used in automotive industry. However, the powderingdefect of zinc coating on galvannealed (GA) sheet steel usually occurs during stampingforming. From the influence factors, such as microstructure, residual stress, interfacestrength and the stress state of coating, the powdering mechanisms of the zinc coatingsystem on galvannealed (GA) sheet steel has been studied. First, the H220BD galvanized(GI) sheet steel provided by Shanghai Bao Steel Co. Ltd. were galvannealed usingdifferent parameters. Then the residual stress of the galvannealed zinc coating and theinterface strength between zinc coating and substrate steel were measured choosingappropriate methods, and the stress states of zinc coating during the stamping forming ofGA sheet steel were calculated and simulated by FEM using these measured results.Finally, the optimal galvannealing parameters were confirmed by checking the powderingmass lose and comparing the properties of the galvannealed zinc coatings.First, the H220BD galvanized sheet steels were galvannealed in a salt bath furnaceunder450℃,500℃,550℃and holding5s,10s,30s,120s for each temperature to obtain12kinds of zinc-coated GA steel sheets. It is found that the main component of the zinccoatings on all GA steel sheets is the δ phase, and the thickness of δ phase increases withgalvannealing time, and large amount of δ phase may appear on the surface of coating aftergalvannealing an enough long time. It is also found that the powdering mass lose of thezinc coating galvannealed under500℃is less then those obtained under450℃or550℃.The powdering mass lose of the galvannealed zinc coating first increases then slightlyreduces with the prolongation of the galvannealing time, and the powdering defectbecomes more serious under a higher galvannealing temperature holding for a samerelatively long time (>30s).The residual stress of the zinc coating on galvannealed (GA) steel sheets, obtainedusing different galvannealing parameters, was measured by nano-indentation. It is foundthat a110Mpa-240Mpa tensile residual stress exists in the zinc coating galvannealed for arelatively long time (>5s), and the residual stress first increases then decreases with theprolongation of galvannealing time, but generally reduces as the galvannealingtemperature increases. The residual stress and the powdering mass lose of the zinc coatings galvannealed under500℃and550℃both holding10s and120s are small. On thecontrary, the residual stress and the powdering mass lose of those coatings galvannealedunder450℃and550℃both holding30s are larger.A novel universal blister test method was developed to measure the interface strengthof coating/substrate material system, and the interface strength between galvannealed zinccoating and steel substrate obtained using this new method is3.8995J/m2-174.888J/m2. Itis found that the interface strength decreases rapidly first with the prolongation ofgalvannealing time, after a certain time, then slightly increases with galvannealing time butkeeps stabilized finally. During a short galvannealing time, the interface strength decreaseswith the increase of temperature, after galvannealing a certain long time, the effect oftemperature on the interface strength bacomes little. Moreover, the interface strength of thezinc coatings galvannealed under450℃and550℃both holding30s is small but theirpowdering mass lose is large. On the contrary, the interface strength of those coatingsgalvannealed under500℃is larger (>9J/m2) and their powdering is slighter.After checking the powdering mass lose and comparing the interface strength andresidual stress, the conclusions can be obtained as: Either the δ phase is too thin or toothick, the powdering of the galvannealed zinc coating is serious. The larger the residualstress or the lower the interface strength, the powdering mass lose is greater. The optimalparameters are galvannealing under500°C holding about120s.The above research work can provide theoretical basis to solve the powderingproblem of the zinc coating on GA steel sheet, and contributes to improve the quality ofour GA steel plate and to promote the development of the continuous galvanizing industry.更多还原