Enhancing ductility of the Al-Si coating on hot stamping steel by controlling the Fe-Al phase transformation during austenitization
Enhancing ductility of the Al-Si coating on hot stamping steel by controlling the Fe-Al phase transformation during austenitization
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DOI：
10.1007/s11431-014-5576-3
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发表时间：
2014-09-01
影响因子：
4.6
通讯作者：
Zhang YiSheng
中科院分区：
文献类型：
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作者：
Gui ZhongXiang;Liang WeiKang;Zhang YiSheng
在本研究中，首先通过配备能量色散 X 射线分析 (EDAX) 的环境扫描电子显微镜 (ESEM) 研究了 Al-10% Si 涂层硼钢热冲压前的奥氏体化热处理。使用 Gleeble 3500（一种高温下单轴塑性变形的热机械模拟器）评估涂层的开裂行为。通过光学显微镜仔细评估涂层中裂纹的程度和数量。热浸镀条件下，从镀层表面到钢基体，镀层由Al-Si共晶基体、Fe2Al7Si、Fe3Al2Si3和Fe2Al5组成。涂层保持致密、连续和光滑。在奥氏体化过程中，在 Fe 扩散过程的促进下，涂层中富铝的 Fe-Al 金属间化合物转变为更多富铁的金属间化合物。涂层最终显示出两种类型的Fe-Al金属间化合物的共存，即FeAl2和FeAl。微裂纹和柯肯德尔空洞分别出现在涂层和扩散区。在热拉伸测试中，涂层严重破裂并破碎成碎片。暴露在涂层各部分之间的裸钢被氧化并覆盖一层薄薄的 FeO (x) 层。氧化物的出现降低了Al-Si涂层的附着力。研究发现，韧性 FeAl 比脆性 FeAl2 更适合作为涂层微观结构。因此，通过控制奥氏体化过程中延展性富铁金属间相变，可以提高热冲压硼钢Al-Si涂层的延展性。实验表明，较高的奥氏体化温度或较长的停留时间有利于富铁金属间化合物的转变，增加了FeAl的体积分数。这种相变还有助于降低裂纹密度和深度。
In this study, austenitizing heat treatment before hot stamping of Al-10% Si coated boron steel is first investigated through environment scanning electron microscopy (ESEM) equipped with energy dispersive x-ray analysis (EDAX). The cracking behavior of the coating was evaluated using Gleeble 3500, a thermo-mechanical simulator under uniaxial plastic deformation at elevated temperatures. The extent and number of cracks developed in the coating were carefully assessed through an optical microscope. The coating layer under hot-dipped condition consists of an Al-Si eutectic matrix, Fe2Al7Si, Fe3Al2Si3 and Fe2Al5, from the coating surface to the steel substrate. The coating layer remains dense, continuous and smooth. During austenitization, the Al-rich Fe-Al intermetallics in the coating transform to more Fe-rich intermetallics, promoted by the Fe diffusion process. The coating finally shows the coexistence of two types of Fe-Al intermetallics, namely, FeAl2 and FeAl. Microcracks and Kirkendall voids occur in the coating layer and diffusion zone, respectively. The coating is heavily cracked and broken into segments during the hot tensile tests. Bare steel exposed between the separate segments of the coating is oxidized and covered with a thin FeO (x) layer. The appearance of the oxide decreases the adhesion of the Al-Si coating. It is found that the ductile FeAl is preferred as a coating microstructure instead of the brittle FeAl2. Therefore, the ductility of the Al-Si coating on hot stamping boron steel could be enhanced by controlling the ductile Fe-rich intermetallic phase transformations within it during austenitization. Experiments indicate that a higher austenitizing temperature or longer dwell time facilitate the Fe-rich intermetallics transformation, increasing the volume fraction of FeAl. This phase transformation also contributes to reducing the crack density and depth.