【摘要】：Mn18Cr18N高氮奧氏體不銹鋼因其優(yōu)良的耐蝕性、塑韌性被廣泛用于發(fā)電機護環(huán)生產。由于冶金技術的制約,該鋼種的傳統(tǒng)鑄造組織晶粒粗大且缺陷嚴重,電渣重熔工藝的運用極大提高了鑄坯的組織質量。雖然組織缺陷大大減少,但是晶粒組織基本由粗大的柱狀晶組成。Mn18Cr18N鋼是非相變鋼,無法通過熱處理手段優(yōu)化組織性能。所以,細化晶粒成為改善組織性能的關鍵技術手段。護環(huán)加工所用坯料的組織狀態(tài)大部是分鍛態(tài)的,由于鍛造過程中加工工序繁多,工藝復雜,研究電渣重熔Mn18Cr18N鋼的鑄態(tài)組織的熱變形行為,對優(yōu)化護環(huán)生產工藝具有重要的現實意義。本文研究內容如下:首先,針對電渣重熔Mn18Cr18N奧氏體不銹鋼進行單向熱壓縮實驗,獲得了不同變形條件下的應力-應變曲線,分析了該鋼種的熱變形規(guī)律。結果表明,該鋼種對應變速率非常敏感,具有明顯的正應變速率敏感性。其次,研究了Mn18Cr18N奧氏體不銹鋼鑄態(tài)組織在不同變形條件下的動態(tài)再結晶規(guī)律。分析獲得了溫度、應變、應變速率對動態(tài)再結晶軟化行為的影響,并構建了該鋼種的本構方程及動力學模型。并借助金相顯微鏡(OM)的組織觀測,獲得了變形組織的動態(tài)再結晶百分數,通過origin數據擬合軟件,建立了動態(tài)再結晶百分數模型和晶粒尺寸模型。研究結果表明:動態(tài)再結晶晶粒主要以“項鏈”狀模式從原始晶界處產生。隨著溫度的升高、應變速率的降低,動態(tài)再結晶百分數逐漸增大。對比鍛態(tài)組織發(fā)現,鑄態(tài)組織變形激活能較高,而動態(tài)再結晶激活能較低,說明了鑄態(tài)組織較難發(fā)生塑性變形,卻更易發(fā)生動態(tài)再結晶。然后,分析了Mn18Cr18N奧氏體不銹鋼微觀變形組織在動態(tài)再結晶機制下的能量耗散規(guī)律,基于動態(tài)材料模型,建立了該鋼種在不同應變下的熱加工圖。研究發(fā)現,增大應變可以提高該鋼的熱加工性能。熱加工圖的建立有助于分析熱加工工藝參數對微觀組織的影響,為優(yōu)化護環(huán)加工工藝提供理論依據。最后,針對Mn18Cr18N奧氏體不銹鋼在Gleeble-1500D熱模擬實驗機雙道次熱壓縮試驗,研究了不同溫度、保溫時間、應變對該鋼種靜態(tài)再結晶規(guī)律的影響。研究結果表明,溫度越高,道次間隔保溫時間越長,靜態(tài)再結晶百分數越大。
[Abstract]:Mn18Cr18N high-nitrogen austenitic stainless steel is widely used in generator ring production because of its excellent corrosion resistance and ductility. Due to the restriction of metallurgical technology, the traditional casting microstructure of the steel is coarse in grain size and serious in defect, and the application of electroslag remelting technology greatly improves the microstructure quality of the cast billet. Although the microstructure defects are greatly reduced, the grain structure is basically composed of coarse columnar crystals. Mn18Cr18N steel is not a phase change steel and can not be optimized by heat treatment. Therefore, grain refinement has become a key technology to improve microstructure and properties. The microstructure state of the blanks used in the ring protection machining is mostly in the split forging state. Due to the numerous working procedures and the complex process during the forging process, the hot deformation behavior of the as-cast structure of the Mn18Cr18N steel remelted by electroslag is studied. It has important practical significance to optimize the production process of protective ring. The main contents of this paper are as follows: firstly, unidirectional thermal compression experiments were carried out for electroslag remelted Mn18Cr18N austenitic stainless steel. The stress-strain curves under different deformation conditions were obtained, and the hot deformation law of the steel was analyzed. The results show that the steel is very sensitive to strain rate and has obvious positive strain rate sensitivity. Secondly, the dynamic recrystallization of Mn18Cr18N austenitic stainless steel under different deformation conditions was studied. The effects of temperature and strain rate on the dynamic recrystallization softening behavior were analyzed and the constitutive equation and kinetic model of the steel were established. The dynamic recrystallization percentage of deformed microstructure was obtained by means of microstructure observation of metallographic microscope (OM). The dynamic recrystallization percentage model and grain size model were established by origin data fitting software. The results show that the dynamic recrystallization grains are mainly formed in the "necklace" pattern from the original grain boundary. With the increase of temperature and the decrease of strain rate, the percentage of dynamic recrystallization increases gradually. Compared with the forged microstructure, the deformation activation energy of as-cast microstructure is higher, but the dynamic recrystallization activation energy is lower, which indicates that the as-cast microstructure is more difficult to produce plastic deformation, but more prone to dynamic recrystallization. Then, the energy dissipation law of the microstructure of Mn18Cr18N austenitic stainless steel under dynamic recrystallization mechanism was analyzed. Based on the dynamic material model, the hot working diagram of the steel was established under different strain. It is found that the hot working properties of the steel can be improved by increasing the strain. The establishment of the hot working diagram is helpful to analyze the influence of the process parameters on the microstructure and to provide the theoretical basis for optimizing the processing technology of the protective ring. Finally, the effects of different temperature, holding time and strain on the static recrystallization of Mn18Cr18N austenitic stainless steel in Gleeble-1500D thermal simulation machine were studied. The results show that the higher the temperature, the longer the holding time and the higher the percentage of static recrystallization.
【學位授予單位】：太原科技大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG142.1

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