奈米级析出物强化热轧汽车用钢开发
黄庆渊1 ■颜鸿威2 ■潘永村1 ■杨哲人2
1. 中国钢铁股份有限公司钢铁铝品研究发展处
2. 国立台湾大学材料科学与工程学研究所
为了节省能源消耗与兼顾乘车安全性，汽车用钢材使用的强度等级愈来愈高。过去高强度钢通常采用麻田散铁或变韧铁为主要显微组织的相变态强化设计，然而此类钢材加工成形性不够好。
为了得到优良强度与加工性组合，本研究采用高钛(>0.08wt%)复合添加钒之合金设计配合热轧轧延参数之精致控制，利用肥粒铁晶粒细化及奈米析出复合强化机构来产制高强度热轧汽车用钢。其冶金机制是利用高温时非常稳定之TiN来抑制热轧制程再加热时沃斯田铁晶粒成长，并于热轧制程上控制完轧温度尽可能接近Ar3来大幅增加肥粒铁成核位置，配合采用快速冷却模式来防止于高温产生肥粒铁相变化，以避免肥粒铁晶粒粗化。另外再藉由热轧盘卷温度的适当控制，使奈米尺寸的(Ti,V)C界面析出物(<10nm)持续于肥粒铁相变化过程中析出，而得到大量均匀散布于肥粒铁晶粒中之析出物，充分发挥良好的析出强化效果。本研究藉由微米级(<3μm)细晶肥粒铁中奈米级析出物之大量产生，使得热轧后之钢材不需再经过任何热处理，其抗拉强度可以稳定达到>780MPa的超高强度，而且伸长率还可达到18%的水平。此钢材超高强度与良好加工成形性，已经被广泛应用于卡车大梁及汽车之防撞钢梁、安全汽囊零件等汽车部件上。
To save the energy and increase the passenger safety, the required strength level of automobile steels has become higher and higher. Traditionally, the microstructure of the high strength steels is mainly composed of martensite or bainite for getting the transformation strengthening effect, but these steels usually exhibit poor formability. To reach the demand of high strength and formability balance, the steels with composition of high Ti content (>0.08wt%) and minor V addition undergoing careful control of hot rolling parameters are used to get the grain refinement and nanoprecipitation coupled strengthening mechanisms for the production of high strength automobile steel. The stable TiN precipitates inhibit the grain growth during reheating treatment. In addition, the proper control of the finish rolling temperature nearby the Ar3 temperature promotes a lot of ferrite nucleation sites on the prior austenite grain boundaries. During the subsequent cooling stage, fast cooling is applied to further prevent the occurrence of ferrite transformation and corresponding coarsening of ferrite grain size. Additionally, by controlling the coiling temperature of the strip, lots of nano-sized (<10nm) (Ti,V)C interface precipitates are consecutively appeared during the slow cooling period. The ultra fine grained refinement and nano-precipitation technologies ensure the strength of steel higher than 780 MPa, and the elongation of the steel better than 18%, sparing the common strengthening by heat treatment. This steel has been applied in several parts of automobile, such as truck frame, door impact beam, and container of safety steam capsule etc.
壹、前言
随着地球暖化问题日益严重，降低车体重量以减少汽车行驶当中的二氧化碳排放，已经是刻不容缓的核心议题。汽车车体重量约有70% 为钢铁材料所构成，所以高强度汽车用钢的开发与运用是降低车体重量最有效的方法，同时也已被证实对汽车之节能减碳有相当之帮助。由于汽车用钢材在使用的过程中，均可能比一般高强度钢材经过更繁复的成形加工，因此汽车用钢除了强度的要求外，其加工性要求亦较严格，所以高强度汽车用钢材在开发的过程中，均将比其它高强度钢材面临更大的挑战，全球各大先进钢厂也均将热轧汽车用钢产品之可生产等级，做为评鉴各钢厂技术能力的重要指标。过去国内超过60 公斤级以上的热轧汽车用钢均由国外进口，包括HT620 (SAE J1392
80XLF, SAE J2340 080XF, ASTM1011/1018 Gr.80)、HT690 与HT780(JSH780) 三种等级之钢材，其中
