The thermomechanical process of multi-forging for grain refinement -medium carbon steel 1045 for example

本研究所發展的多道次鍛壓製程，藉塑性形的變累積應變方式達到晶粒細化的效果，提供熱機製程晶粒細化的另一選擇。本研究之多道次鍛壓晶粒細化熱機製程，藉由將中碳鋼AISI-1045材料經反覆地疊合、鍛壓以及切斷的鍛壓材料方式，產生數種再結晶的晶粒細化現象，實現多道次鍛壓的晶粒細化的可能性。本研究藉由應用於金屬成形模擬的商用有限元素軟體DEFORM 2D，成功結合多道次鍛壓實驗與模擬，並採所建立最佳鍛壓參數之探討流程，有效控制材料鍛壓過程中的顯微結構發展，以晶粒尺寸均勻分散以及最小晶粒尺寸為本研究的品質目標。由於鍛壓後的材料已產生部分細化現象，因此有必要針對後續道次鍛壓前的材料加熱情形探討。首次的鍛壓採材料加溫1000℃並加熱15min的設定，而材料持續的高溫將使晶粒成長過劇，因此第二道次鍛壓必須將材料加溫降至900℃及加熱12min的設定，而第三道次則須再降低溫度至850℃並加溫15min的設定。以24mm厚及初始晶粒100mm中碳鋼AISI-1045為例，三個道次的鍛壓速度保持固定在7.2mm/min，可達到7.6mm的細化效果。本研究所得的每道次的鍛壓參數，將可提供相關人員取得多道次鍛壓晶粒細化製程條件參考。This study is proposed to develop a multi-forging process, which can be served as an alternative thermomechanical process by accumulating strain to achieve the effect of grain refinement. In this study a medium carbon steel AISI-1045 is used as a sample material to demonstrate the potential of the multi-forging process, in which the medium steel is porcessed repeatedly by stacking, forging, cutting, and heat treating. A commercial finite element code DEFORM 2D is applied to simulate the grain refinement process of multi-forging in this study. The experiments performed in this study have verified the simulation results. A procedure is successfully established in this study to optimize the process parameters in order to achieve a uniform grain distribution and a minimum size in the multi-forged workpiece. For the first forging step, the material is heated in a environment of 1000℃ for 15 min of material. However, the grain growth in material is so severe at such high temperature, that the heating temperature and the heating time on the material for the subsequent forging step must be regulated. Therefore, the material must be heated in a 900℃ environment for 12 min before the second forging step, and in 850℃ for 15 min before the third forging step. As a result, after three forging steps under a constant forging rate 7.2 mm/min, an average grain size of 7.6 μm can be found uniformly distributed in the forged medium carbon steel, which has initially 24 mm in thickness and an average grain size of 100 μm. The results will be served as a reference to apply the multi-forging process for grain refinement.摘要 I Abstract II 目錄 III 圖目錄 V 表目錄 IX 符號說明 X 1 前言 1 1.1文獻回顧 2 1.1.1材料機械性質強化 2 1.1.2晶粒細化後的材料性質改善 3 1.1.3超塑性成形特性 4 1.1.4塑性加工後的材料退火機制 7 1.1.5塑性加工下的晶粒演化模型 12 1.1.5.1動態再結晶 13 1.1.5.2介動態再結晶 14 1.1.5.3靜態再結晶 15 1.1.5.4 晶粒成長 16 1.1.6晶粒細化製程技術 16 1.1.6.1往復式擠製 17 1.1.6.2 等通道角擠形技術 18 1.1.6.3累積輥軋接合技術 20 1.1.6.4高壓扭轉技術 21 1.1.6.5限制凹槽壓形技術 21 1.1.6.6摩擦攪拌製程技術 22 1.1.6.7摩擦熱機製程技術 23 1.1.6.8循環式熱處理技術 24 1.1.6.9快速凝固粉燒結擠製技術 24 1.1.6.10其它晶粒細化技術 25 1.2研究目的與方法 27 2 多道次鍛壓製程實驗 28 2.1鍛壓製程 28 2.2實驗設備 29 2.3實驗設定 30 2.4顯微組織觀察 33 3 有限元素模擬 34 3.1有限元素法 34 3.2有限元素模型建立 37 3.3有限元素前處理 37 3.4多道次鍛壓模擬 39 4 結果與討論 42 4.1.首道次鍛壓 42 4.2第二道次鍛壓 47 4.3鍛壓材料組織 52 4.4多道次鍛壓模擬之最佳參數 52 5 結論與未來展望 66 6 參考文獻 67 附錄A、晶粒尺寸量測 71 附錄B、鍛壓速度於晶粒尺寸與應變 72 附錄C、第二道次鍛壓之變異數分析 74 附錄D、局部錘鍛之有線元素模擬 75 誌謝 7