Galvanized steel sheets have been widely used in automotive industries. However, it is still a challenge for steel makers to produce defect free hot dip galvanized steel sheets especially for outer panel applications in automotive body. In this study, the root cause of uncommon pinhole defects on painted hot dip galvanized steel panel was investigated. Optical microscopy (OM), scanning electron microscope (SEM) coupled with energy dispersive spectrometry (EDS), and auger electron spectroscopy (AES) were used to characterize these pinhole defects from top surface to base steel layer by layer. A reannealing experiment was carried out to reproduce the key features of the corresponding defect on the base steel. It was found that the pinhole defects were composed of a series of connected voids in the paint layers and started from the dents on the base steel prior to hot dip galvanizing. Based on the results of coarse grains around the dents and residual boron oxides at the dent bottom, further cause of the dents was attributed to the boron containing buildups on furnace rolls in the high temperature section of hot dip galvanizing line. As the surface of furnace rolls was cleaned up, the dent defects on the base steel, the bare spot defects in the zinc coating and the pinhole defects on the painted panel were eliminated successfully.

镀锌钢板已广泛用于汽车工业。然而，对于钢铁制造商而言，生产无缺陷的热浸镀锌钢板，尤其是汽车车身的外板应用，仍然是一个挑战。在这项研究中，研究了涂漆的热浸镀锌钢板上常见的针孔缺陷的根本原因。使用光学显微镜（OM），扫描电子显微镜（SEM）结合能量色散光谱（EDS）和俄歇电子能谱（AES）逐层表征这些针孔缺陷，从上表面到基础钢层。进行了一次再退火实验，以重现母材上相应缺陷的关键特征。发现针孔缺陷由涂料层中的一系列连接的空隙组成，并且是从热浸镀锌之前的基础钢上的凹痕开始的。根据凹痕周围的粗大晶粒和凹痕底部残留的氧化硼的结果，造成凹痕的进一步原因是热浸镀锌生产线的高温段中炉辊上的含硼堆积物。通过清理炉辊表面，可以成功消除基体钢上的凹痕缺陷，镀锌层上的裸点缺陷和涂漆板上的针孔缺陷。凹痕的进一步原因归因于热浸镀锌线高温段炉辊上的含硼堆积物。通过清理炉辊表面，可以成功消除基体钢上的凹痕缺陷，镀锌层上的裸点缺陷和涂漆板上的针孔缺陷。凹痕的进一步原因归因于热浸镀锌线高温段炉辊上的含硼堆积物。通过清理炉辊表面，可以成功消除基体钢上的凹痕缺陷，镀锌层上的裸点缺陷和涂漆板上的针孔缺陷。