含鋁/矽低碳鋼之合金碳化物析出研究

Abstract

據文獻顯示[1]，若將含有(Ti,Mo,V)C之熱軋鋼板先行冷軋再於700°C退火1小時，其碳化物明顯粗化至10-50 nm之尺寸，而沒有冷軋之鋼板則得以繼續維持其3-5 nm之超細尺寸，由此可知若要於冷軋鋼板中設計奈米尺寸之碳化物，無法採取過去熱軋鋼板設計之概念，因而衍生了本研究之合金設計思維。 本研究目標在於控制極低碳鋼之微合金碳化物的固溶與析出行為以應用於析出強化型冷軋鋼捲之開發，首先運用熱力學平衡相圖計算法配合溶度積法進行可適合金之探索與製程參數之選擇，最後添加高鋁及高矽合金含量以形成肥粒鐵單相材料，熱軋盤捲後再以實際冷軋暨退火達成目標顯微結構，並利用顯微結構分析與機械性能量測來驗證其碳化物偏聚強化之效應，其成果可應用於IF鋼、HSLA鋼、電磁鋼片之產品設計。 經實驗發現，含高鋁及高矽合金含量的(Nb,V)鋼，在冷軋80%後於980°C、1050°C等較高的第一階段退火溫度有較為均勻的晶粒分布，同時對比於IF鋼，(Nb,V)鋼因NbC碳化物的高溫析出而擁有顯著的晶粒細化作用，若以預軋8%導入差排，再於700°C短時間時效5分鐘，便能以差排強化與析出強化將降伏強度提升至約450 MPa，延伸率由6.6%回復至14.6%，同時在高解析TEM影像中也觀察到差排附近有許多奈米等級的(V,Nb)C型碳化物以B-N方位關係析出於肥粒鐵基材，成功實踐微合金添加所預期的析出強化目標。 為追求再提升目標鋼材的延伸率，同時能以Ashby-Orowan equation來進行析出強化貢獻的評估，將製程修正為預拉5%導入差排，經烘烤170°C_20min將碳原子先一步擴散到差排附近後，再於700°C短時間時效5分鐘，最終得到降伏強度為429.1 MPa，延伸率則達20.2%，強度與傳統IF鋼的超細晶材料(~500 nm)相當，塑性與延展性則更為優異，若與BH鋼相比，兩者抗拉強度與延伸率相近，但短時間時效減少了應變時效的影響，利於二次加工，同樣在高解析TEM影像中也可觀察到諸多奈米級(V,Nb)C型碳化物以B-N方位關係析出於肥粒鐵基材。

According to the literature[1], if the hot-rolled steel plate containing (Ti,Mo,V)C is cold-rolled first and then annealed at 700°C for 1 hour, the carbides will be significantly coarsened to a size of 10-50 nm. Conversely, the steel plate annealing directly without cold rolling can maintain its ultra-fine size of 3-5 nm. It can be seen that the concept of hot-rolled steel plate design in the past is not feasible for the cold-rolled steel plate. So the goal of this research is designing a cold-rolled steel plate which can precipitate nano-sized carbides. The goal of this research is to control the solid solution and precipitation behavior of micro-alloyed carbides of ultra low carbon steel for the development of precipitation-strengthened cold-rolled steel coils. First, the thermodynamic equilibrium phase diagram calculation method combined with the solubility product method is used to find the suitable alloy and decide the process parameters. Subsequently, alloy with high aluminum and high silicon content form a single-phase ferritic material. After hot-rolling and coiling, cold rolling and annealing are used to achieve the target microstructure. The microstructure analysis and mechanical performance test are both used to verify the effect of carbide strengthening. The results can be applied to the product design of IF steel, HSLA steel, and electromagnetic steel sheet. Experiments have found that (Nb,V) steel with high aluminum and silicon alloy content has a more uniform grain size distribution at the higher 1st stage annealing temperature of 980°C, 1050°C, etc. after 80% cold rolling. At the same time, compared with IF steel, (Nb,V) steel has a significant grain refinement effect due to the high temperature precipitation of NbC carbides. Pre-rolling 8% is introduced to produce dislocations, then short-time aging at 700°C for 5 minutes. The yield strength can be increased to about 450 MPa with dislocation strengthening and precipitation strengthening, and the elongation restored from 6.6% to 14.6%. At the same time, it is also observed in the high-resolution TEM image that there are many nanoscale (V,Nb)C-type carbides obey the B-N orientation relationship with the ferrite matrix. Consequently, it successfully achieve the goal of precipitation strengthening by the microalloying elements adding. In order to further improve the elongation of the target steel, and at the same time, hoping that the Ashby-Orowan equation can be used to evaluate the contribution of precipitation strengthening. The process is modified to pre-stretch 5% to produce dislocations. After baking at 170°C for 20 minutes, the carbon atoms diffuse to dislocations first, then short time aging at 700°C for 5 minutes, the yield strength is 429.1 MPa and the elongation is 20.2%. The strength is equivalent to that of the ultra-fine-grained material (~500 nm) of traditional IF steel, and the ductility is even better. If compared with the BH steel, the tensile strength and elongation of the two are similar, but the short time aging reduce the effect of strain aging, which is beneficial to secondary processing. Also, it is observed in the high-resolution TEM image that many nanoscale (V,Nb)C-type carbides obey the B-N orientation relationship with the ferrite matrix.