- 期刊
高強度汽車用熱軋鋼材性質改善
Improvement in Mechanical Properties of a High-Strength Hot-Rolled Automobile Steel
摘要
45公斤級熱軋汽車用鋼（SAPH-440）由於同時要求嚴格的抗拉強度及延伸率。在量產時容易發生機械性質不合格的現象。本研究探討合金成分與製程條件的效應以改善之。鋼中添加錳主要會造成晶粒細化，其次造成波來鐵體積增加；而添加碳的效果剛好相反。晶粒細可同時提高降伏及抗拉強度，且對延性危害較小，而增加波來鐵對提高抗拉強度較顯著，且明顯降低延性，故需控制適當的錳、碳比例方能得到優良的強度延性組合。其次，探討其相變態行為時發現，粗大的沃斯田鐵晶粒在慢速冷卻時容易變態成粗大的多角形肥粒鐵，在快速冷卻容易變態成變韌鐵，在高溫盤捲時，容易變態成粗大的惠得曼肥粒鐵，這三種顯微組織是造成機械性質不良的主因。細小的沃斯田鐵晶粒在較廣的冷卻速率及盤捲溫度範圍內，可以變態成細小的肥粒鐵加波來鐵組織，而獲得優良的機械性質。根據上述結果，本研究提出適當的成分設計及製程條件，使SAPH-440的合格率大幅改善。
並列摘要
Mass production of 45-kg grade hot-rolled automobile steel SAPH 440 suffered from high rejecion ratio. This study investigated the effects of chemical compositions and process parameters in order to improve the mechanical properties of SAPH 440 steel. The addition of Mn results in significant refinement of ferrite grain size and slight increases in the volume fraction of pearlite; while the addition of C results in the reverse trend. Grain refinement increases both yield and tensile strengths without marked sacrifice of elongation. Increasing pearlite volume fraction deteriorates ductility although it increases tensile strength. Coarse austenite is transformed to coarse polygonal ferrite, bainite, and Widmanstatten ferrite when it is cooled with a slow colling rate, high cooling rate, and coiled at a high temperature, respectively. Those three microstructures are found to be harmful to ductility. Fine austenite is transformed to fine polygonal ferrite plus pearlite in a wide range of cooling rate and coiling temperature and results in good combination of strength and ductility. Based on the above observations, the adequate compositions and rolling conditions have been proposed and implemented in mill and the production yeild of SAPH 440 has been greatly improved.