本文選題：微合金鋼 + 表面橫裂紋��； 參考：《重慶大學》2015年碩士論文

【摘要】：微合金鋼是鋼鐵工業(yè)中的重要產(chǎn)品之一,但其裂紋敏感性較強,在連鑄過程中容易形成表面缺陷,尤其是表面橫裂紋問題。研究發(fā)現(xiàn),微裂紋的產(chǎn)生與鑄坯的表層組織狀態(tài)相關(guān)。當鑄坯表層組織奧氏體晶粒粗大、沿奧氏體晶界有大量膜狀鐵素體以及第二相析出時,則會在彎曲或矯直時沿奧氏體晶界產(chǎn)生表面橫裂紋。但同時,第二相的析出也可以從釘扎晶界細化奧氏體、促進鐵素體形核從而消除膜狀鐵素體等方面對表層組織產(chǎn)生影響,所以研究第二相與表層組織之間的關(guān)系,對從控制鑄坯組織的角度來解決微合金鋼表面橫裂紋問題具有重大意義。本文以多個微合金元素含量不同的鋼種為研究對象,分析雙相變過程中冷卻速度、回熱條件對第二相析出-回溶行為的影響,以及第二相析出行為對原始奧氏體長大過程、先共析鐵素體分布以及回熱過程奧氏體細化的影響,以期通過控制第二相的析出行為,獲得均勻細小的奧氏體晶粒,為雙相變工藝參數(shù)的選擇提供理論和實驗依據(jù)。本文的研究結(jié)果可簡要概括如下:①通過微合金鋼第二相析出的熱力學動力學分析可知:第二相的析出順序為Ti CN、Nb CN、VCN;連鑄過程中第二相形核的位置主要在奧氏體晶界,其次在晶內(nèi)均勻形核。②通過高溫激光共聚焦顯微鏡模擬微合金鋼凝固冷卻過程,以及第二相析出的熱力學動力學溫度分析,發(fā)現(xiàn)樣品表面的細小浮凸與第二相的析出有關(guān);并提出了通過原位觀察表面細小浮凸的產(chǎn)生,間接觀察表征連續(xù)冷卻過程中第二相析出行為的方法。③當Ti元素的理論析出量達到0.008%時,能起到有效釘扎晶界的作用,抑制原始奧氏體長大;理論析出量超過0.008%,多析出的Ti N對釘扎作用的加強效果有限,原始奧氏體晶粒尺寸不能得到進一步細化。④Nb CN具有促進先共析鐵素體形核的作用;以冷卻速度為5.0℃?s-1進行冷卻,可以獲得在奧氏體晶內(nèi)彌散分布的第二相與先共析鐵素體,該微觀組織結(jié)構(gòu)有利于后續(xù)回熱過程奧氏體晶粒的細化。⑤VCN會延長鐵素體向奧氏體轉(zhuǎn)變的孕育時間,抑制回熱過程中奧氏體的形核;回熱溫度為1000℃、回熱速度為3.0℃?s-1,可以獲得均勻細小的奧氏體晶粒。
[Abstract]:Microalloyed steel is one of the important products in iron and steel industry, but its crack sensitivity is relatively strong, and it is easy to form surface defects in continuous casting process, especially the problem of surface transverse cracks. It is found that the formation of microcracks is related to the surface microstructure of the billet. When the austenite grain in the surface layer of the billet is coarse, a large number of film ferrite and the second phase precipitate along the austenite grain boundary, the surface transverse crack will occur along the austenitic grain boundary when bending or straightening. But at the same time, the precipitation of the second phase can also refine austenite from grain boundary, promote ferrite nucleation and eliminate the influence of membrane ferrite on the surface structure, so the relationship between the second phase and the surface structure is studied. It is of great significance to solve the problem of transverse crack on the surface of microalloyed steel from the point of view of controlling the structure of billet. In this paper, a number of steel grades with different contents of microalloyed elements are studied, and the effects of cooling rate, regenerative conditions on the precipitation-resolution behavior of the second phase and the growth process of the original austenite during the double phase transformation are analyzed. The effect of preeutectoid ferrite distribution and austenite refinement during the regenerative process was studied in order to obtain uniform fine austenite grains by controlling the precipitation behavior of the second phase and to provide theoretical and experimental basis for the selection of technological parameters of double phase transformation. The results of the present study can be summarized as follows: the thermodynamic kinetic analysis of the second phase precipitation of microalloyed steel shows that the precipitation sequence of the second phase is TiCNNb CNCN-VCN.The nucleation of the second phase is mainly located at the austenite grain boundary during continuous casting. Secondly, the homogeneous nucleation in the crystal was simulated by high temperature laser confocal microscope to simulate the solidification and cooling process of microalloyed steel, and the thermodynamic kinetic temperature analysis of the precipitation of the second phase. It was found that the fine convexity on the surface of the sample was related to the precipitation of the second phase. A method for indirectly observing the precipitation behavior of the second phase in continuous cooling process by observing in situ the formation of fine convexity on the surface is proposed. 3. When the theoretical precipitation amount of Ti reaches 0.008, it can effectively pinpoint the grain boundary. The primary austenite growth was inhibited, the theoretical precipitation amount was more than 0.008, the strengthening effect of the multiple precipitated tin on pinning was limited, and the grain size of the original austenite could not be further refined. 4Nb CN could promote the nucleation of proeutectoid ferrite. The second phase and proeutectoid ferrite dispersed in austenitic crystals can be obtained by cooling at a cooling rate of 5.0 鈩，

本文編號：1910763