摘要近年来，包晶钢的应用越来越广泛。
然而，表面裂纹是包晶钢的主要缺陷之一，对其表面质量影响很大，严重影响产品的合格率。
因此，观察裂纹形态及组织并分析其产生原因是非常有意义的。
本文的实验材料为15CrMo包晶钢连铸坯，剥皮2mm，无探伤裂纹，然后进行连轧。
选取连轧10道次后的直径为100mm棒材，编号为1#；选取12道次后的直径为75mm 的棒材，编号为2#。
本文采用金相检验，扫描电子显微镜和化学成分分析等方法对棒材的表面裂纹进行了观察分析，了解了表面裂纹的形态及组织特征，并根据实验结果对裂纹形成原因进行了分析。
结果表明，1# 试样裂纹内C、O含量高，Ca、Si和Al的氧化物夹渣较多，还有少量Mg的化合物，而且裂纹周围有大量的Si和MnS等夹杂物；2# 试样裂纹内C含量较高，主要为Al、Si和Ca的氧化物夹渣富集，并且有大量Mg的化合物，裂纹内还有少量的As、Cu、Ni等元素。
裂纹周围有Si夹渣，大量的Si、Ca及Al的氧化物等夹杂物和MnS夹杂。
结合实验结果分析可知：1#试样表面裂纹产生的主要原因是由于浇注过程中结晶器保护渣的流动性恶化，结晶器内钢水液面波动大，钢水的流动不合理，浸入式水口插入结晶器深度不够等因素使保护渣卷入到坯壳中形成Al和Si氧化物夹渣富集于钢坯表层下面。
夹渣形成以后，在连轧过程中，夹杂物逐渐露出钢材表面，在应力作用下沿夹渣富集处形成裂纹。
2#试样表面裂纹周围存在脱碳现象，说明表面裂纹主要是在均热炉中均热时因为温度不均匀导致局部过热而产生的，并且在随后的轧制过程中不断扩展。
针对表面裂纹的产生原因，提出了以下减少表面裂纹的措施：选用合适的保护渣、采用电磁制动技术、控制钢水的过热度、选择大容量的中间包等。
关键字：表面裂纹；结晶器；保护渣；夹杂物 (000000000000000000000000000000000000000000)AbstrctIn recent years, application of peritectic steel has been more extensive. However, surface crack is one of the major defect, which has great impact on the quality of the surface and seriously affects the passing rate of products seriously. So it’s very meaningful to observe the morphologies and the mirco-structure of the surface cracks and investigate in causes of the surface cracks.The test materials are two peritectic steel bars (15CrMo), which are after rolling in this paper, both of the length are 250mm, diameters are 100 mm and 75 mm. The former is numbered 1# and the latter is numbered 2# .The present paper has analyzed the surface cracks of the bars by mircroscopic test, election diffraction spectrometer (EDS) and chemical composition analysis of the inclusions, the morphologies and the mirco-structure characters of the surface cracks have been known. According to the results of cracks, the causes of the surface cracks are analyzed. The results shows that there are high content of C, O; more oxide inclusions of Ca, Si and Al and a small amount of Mg compounds in the crack of the first sample (1#) .Besides, around the crack there are a large number of other inclusions such as Si and MnS. There are high content of C in the crack of the second sample (2#). Furthermore, oxide slags of Al, Si and Ca enrich in the crack and a large number of Mg compounds in the crack, a small number of As, Cu, Ni and other elements are also found in the crack. There are inclusion of Si, a large number more oxide slags of Ca, Si and inclusions of Al and MnS around the crack.Combining the experimental results, the paper shows that the causes of the surface cracks of the first sample(1#) is that the mold wall is involved in the blank shell, forms oxide slags of Al, Si and Ca and enrich below the surface billet because of some factors, which are the deterioration of the mobility of mold powder in the process of casting, the large fluctuations of the surface of molten steel in mold flux , the unreasonable flow of molten steel, the inappropriate depth of submerged nozzle etc. After the formation of slag, inclusions are gradually exposed in the surface of steel during the process of rolling, and form cracks alongthe slag enrichment under the stress. The decarbonization phenomenon is found around the surface crack of the second sample (2#). This shows that surface cracks are mainly formed when the bars are heated uniformly in uniform-heating furnace.For the causes of surface cracks, some measures are proposed as the following so as to reduce the surface cracks: select the appropriate mold flux, use the technology of electromagnetic braking, control the overheating of molten steel and select the large tank middle-capacity etc.Key words：Surface crack；Mold wall；Mold flux；Inclusions目录1 文献综述 (1)1.1 本课题的主要研究意义及研究内容 (1)1.1.1 本课题的研究意义 (1)1.1.2 本课题的研究内容 (1)1.2 包晶钢的简介 (1)1.2.1 包晶钢的定义 (1)1.2.2 包晶钢的凝固特点 (2)1.3 棒材缺陷的研究现状 (3)1.3.1 棒材缺陷的种类 (3)1.3.2 棒材缺陷形成的根源 (3)1.4 包晶钢棒材表面裂纹的研究 (4)1.4.1 表面裂纹的形成机理 (4)1.4.2 表面裂纹的影响因素 (4)1.4.3 减少铸坯表面微裂纹的措施 (8)2 实验研究方法 (9)2.1 实验材料 (9)2.2 实验设备 (9)2.3 实验过程 (10)2.3.1 宏观检验 (10)2.3.2 微观检验 (10)2.3.3 现场工艺了解 (11)3 实验结果与讨论 (12)3.1 宏观形貌特征 (12)3.2 微观形貌特征 (12)3.2.1 1# 试样裂纹SEM分析 (12)3.2.2 2# 试样裂纹SEM分析 (17)3.3 试样的金相组织 (20)3.4 讨论与分析 (21)4 结论 (24)参考文献 (25)致谢 (27)1 文献综述1.1 本课题的主要研究意义及研究内容1.1.1 本课题的研究意义近年来，包晶钢的应用越来越广泛。
