微合金钢第二相析出行为对铸坯表层组织影响及控制

发布时间：2018-05-19 16:15

  本文选题：微合金钢 + 表面横裂纹　； 参考：《重庆大学》2015年硕士论文

【摘要】：微合金钢是钢铁工业中的重要产品之一,但其裂纹敏感性较强,在连铸过程中容易形成表面缺陷,尤其是表面横裂纹问题。研究发现,微裂纹的产生与铸坯的表层组织状态相关。当铸坯表层组织奥氏体晶粒粗大、沿奥氏体晶界有大量膜状铁素体以及第二相析出时,则会在弯曲或矫直时沿奥氏体晶界产生表面横裂纹。但同时,第二相的析出也可以从钉扎晶界细化奥氏体、促进铁素体形核从而消除膜状铁素体等方面对表层组织产生影响,所以研究第二相与表层组织之间的关系,对从控制铸坯组织的角度来解决微合金钢表面横裂纹问题具有重大意义。本文以多个微合金元素含量不同的钢种为研究对象,分析双相变过程中冷却速度、回热条件对第二相析出-回溶行为的影响,以及第二相析出行为对原始奥氏体长大过程、先共析铁素体分布以及回热过程奥氏体细化的影响,以期通过控制第二相的析出行为,获得均匀细小的奥氏体晶粒,为双相变工艺参数的选择提供理论和实验依据。本文的研究结果可简要概括如下:①通过微合金钢第二相析出的热力学动力学分析可知:第二相的析出顺序为Ti CN、Nb CN、VCN;连铸过程中第二相形核的位置主要在奥氏体晶界,其次在晶内均匀形核。②通过高温激光共聚焦显微镜模拟微合金钢凝固冷却过程,以及第二相析出的热力学动力学温度分析,发现样品表面的细小浮凸与第二相的析出有关;并提出了通过原位观察表面细小浮凸的产生,间接观察表征连续冷却过程中第二相析出行为的方法。③当Ti元素的理论析出量达到0.008%时,能起到有效钉扎晶界的作用,抑制原始奥氏体长大;理论析出量超过0.008%,多析出的Ti N对钉扎作用的加强效果有限,原始奥氏体晶粒尺寸不能得到进一步细化。④Nb CN具有促进先共析铁素体形核的作用;以冷却速度为5.0℃?s-1进行冷却,可以获得在奥氏体晶内弥散分布的第二相与先共析铁素体,该微观组织结构有利于后续回热过程奥氏体晶粒的细化。⑤VCN会延长铁素体向奥氏体转变的孕育时间,抑制回热过程中奥氏体的形核;回热温度为1000℃、回热速度为3.0℃?s-1,可以获得均匀细小的奥氏体晶粒。
[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，

本文编号：1910763