热镀锌双相钢钢板表面橘皮缺陷研究

Investigation of Orange Peel-like Defects on Galvanizied DP Steel Sheets

目的研究一种热镀锌双相钢表面橘皮状缺陷的特征及形成机理。方法采用数字显微镜、扫描电镜和辉光光谱仪对正常和缺陷位置的镀锌态、去锌层态、冷轧态和热轧态的形貌和表层成分分布开展对比分析研究。结果橘皮状缺陷特征是由于锌晶粒异常长大至350μm,为正常晶粒的4倍。镀锌态缺陷区较正常区Mn含量高26%,纵截面观察基板表层有约2~4μm深的微裂纹,界面处检测出Al、O、Mn峰,存在Mn表面富集。去锌层态和冷轧态基板表层呈冷轧轧裂特征,存在大量垂直于轧制方向的10~30μm的横向微裂纹。卷取温度为650℃时,热轧态边、中位置氧化层厚度分别为6.5、5.8μm,较550℃时增加25%和23%。GDS检测表明,对应缺陷区热轧态发生共析转变,相构成以低氧含量的Fe_(3)O_(4)、FeO、Fe为主。实践证实,卷取温度下调至560℃以下时,可有效消除该缺陷。结论缺陷形成机理是,热轧钢卷在570℃以上温度卷取,经缓慢冷却至室温时,形成纯铁包覆Fe_(3)O_(4)的共析转变氧化铁皮,卷取温度高也使氧化层增厚,导致酸洗难度增大,氧化铁皮无法有效清除。酸洗后残存的氧化层和Mn富集导致表层协同变形能力弱,冷轧形成大量10μm级横向微裂纹。微裂纹处的凹凸表面诱发Fe-Zn相爆发生长,导致锌晶粒异常长大。厚带钢具有更多的内部热量,出锌锅冷速慢,也为镀后锌晶粒长大创造条件。

The work aims to investigate the characteristics and formation mechanism of orange peel-like defects on the surface of a hot-dip galvanized dual phase steel.A comparative analysis of the surface morphology and surface composition distribution of the galvanized,dezincified,cold rolled and hot rolled states in normal and defective zones was carried out with digital microscopy(DM),scanning electron microscopy(SEM)and glow discharge spectroscopy(GDS).The orange peel-like defect was characterized by an abnormal growth of the spangle to about 350μm,which was about four times the normal.The peak Mn values of GDS at the interface between the zinc layer and the substrate were 2.4 wt.%and 1.9 wt.%for the defective and normal zones in the galvanized state,respectively,which indicated the relatively pronounced surface enrichment in the defective zone.The longitudinal cross-sectional sample was observed to have a large number of microcracks with a depth of about 2-4μm on the surface of the substrate.The line scanning results at the interface between the zinc layer and the substrate showed the presence of aluminum,oxygen and manganese peaks,which meant the presence of oxides at the interface.The surface of the substrates in the dezincified and cold-rolled states exhibited cold-rolled cracking characteristics,with a significant number of transverse microcracks of 10-30μm perpendicular to the rolling direction.The thickness of the oxide layer at the side and center zones of the hot-rolled state at a coiling temperature(CT)of 650℃was 6.5μm and 5.8μm,respectively,which was 25%and 23%higher than that at 550℃.GDS tests revealed a trend towards a lower oxygen content and a slower oxygen content variation in the hot rolled state corresponding to the defective zone.This is an indication of a eutectic transformation in this zone,where the phase composition was dominated by low oxygen content of Fe_(3)O_(4),FeO and Fe.Practices demonstrated that the defect can be effectively eliminated when the coiling temperature decreased to below 560℃.Here is the formation mechanism of defect.When the hot rolled coil is coiled at above 570℃and slowly cooled to room temperature,the surface generates pure iron wrapped with ferroferric oxide of eutectic transformation,while the high CT also leads to a thicker hot rolled oxide layer.These factors make pickling more difficult and the surface oxide layer cannot be effectively removed.Based on the theory of inhomogeneous deformation,the residual oxide layer and Mn enrichment will lead to a weaker external cooperative deformation ability than the interior,and a large number of transverse microcracks of 10μm are formed on the surface of the cold-rolled state.The uneven morphological characteristics of the surface caused by microcracks induce outburst growthof Fe-Zn compounds,resulting in abnormally grown spangles.In addition,thick strips have more core heat coming out from the zinc pot,which also creates conditions for spangle growth after galvanizing.