-45- 鑛冶 53/4 奈米級析出物強化熱軋汽車用鋼開發 The Development of Nano-Precipitation Strengthened Hot-Rolled Automobile Steels 黃 慶 淵 1 ■ 顏 鴻 威 2 ■ 潘 永 村 1 ■ 楊 哲 人 2 C.Y. Huang, H.W. Yen, Y. T. Pan, J.R. Yang 為了節省能源消耗與兼顧乘車安全 性,汽車用鋼材使用的強度等級愈來愈 高。過去高強度鋼通常採用麻田散鐵或 變韌鐵為主要顯微組織的相變態強化設 計,然而此類鋼材加工成形性不夠好。 為了得到優良強度與加工性組合,本研 究採用高鈦(>0.08wt%)複合添加釩之合 金設計配合熱軋軋延參數之精緻控制, 利用肥粒鐵晶粒細化及奈米析出複合強 化機構來產製高強度熱軋汽車用鋼。其 冶金機制是利用高溫時非常穩定之TiN 來抑制熱軋製程再加熱時沃斯田鐵晶粒 成長,並於熱軋製程上控制完軋溫度盡 可能接近Ar3來大幅增加肥粒鐵成核位 置,配合採用快速冷卻模式來防止於高 溫產生肥粒鐵相變化,以避免肥粒鐵晶 粒粗化。另外再藉由熱軋盤捲溫度的適 當控制,使奈米尺寸的(Ti,V)C界面析 出物(<10nm)持續於肥粒鐵相變化過程 中析出,而得到大量均勻散布於肥粒鐵 晶粒中之析出物,充分發揮良好的析出 強化效果。本研究藉由微米級(<3μm) 細晶肥粒鐵中奈米級析出物之大量產 生,使得熱軋後之鋼材不需再經過任 何熱處理,其抗拉強度可以穩定達到 >780MPa的超高強度,而且伸長率還可 達到18%的水準。此鋼材超高強度與良 好加工成形性,已經被廣泛應用於卡車 大樑及汽車之防撞鋼樑、安全汽囊零件 等汽車部件上。 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 nano- precipitation 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 (<10 nm) (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 隨 著地球暖化問題日益嚴重,降低車體重量以減少汽車行駛當中的二氧化碳排放,已經是 刻不容緩的核心議題。汽車車體重量約有 70% 為鋼鐵材料所構成,所以高強度汽車用鋼 的開發與運用是降低車體重量最有效的方法,同時也已被證實對汽車之節能減碳有相當之助 1. 中國鋼鐵股份有限公司鋼鐵鋁品研究發展處 2. 國立台灣大學材料科學與工程學研究所
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-45-鑛冶 53/4
奈米級析出物強化熱軋汽車用鋼開發The Development of Nano-Precipitation Strengthened Hot-Rolled Automobile Steels
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 nano-precipitation 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 (<10 nm) (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.
* The measured values of specimen thickness are ranged from 100nm to 200nm. The reasonably estimated value of Inter-particle spacing must be under the thickness of the specimen.
界面析出是利用γ→α的相變態過程中,肥粒鐵含過飽和的 C 排放至界面(Cγ→α )與合金元素形成 MX 碳化物 (如圖-3 所示),肥粒鐵的變態溫度低,界面會聚集較高的碳(Cγ→α),而且於低溫相變化時,以擴散機構產生相變化的肥粒鐵,因擴散速率較低,γ→α界面移動速率較
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