连铸坯低压缩比轧制制造特厚板的机理研究

With the development of China's high-end equipment manufacturing, new energy industry and other industries, it is necessary to a large amount of high-grade heavy plates. Domestically, heavy-grade plates, with high strength, high low temperature toughness, high lamination cracks resitant and other high level properties, are made by large single heavy ingots, mostly, with large compression ratio,, it is usually not less than 4.0. However, a rolling process of heavy plates with low compression ratio has been reported abroad. The ratio could be controlled less than 2.5 and it has many advantages such as combined process, energy-saved system, high effective, etc. Currently, the rolling process of heavy plates with low compression ratio is perplexed with ultrason flaw using slabs at home. In order to control the stubborn defect, a new process has been proposed, which is combined the low compression radio rolling with heavy reduction at the end of continuous casting slab solidification. In this project, solidification structure, segregation and porosities will be studied corresponding to slow rate of solidification process of slabs. The effects of heavy reduction at the end of continuous casting solidification on solidification structure and macro- or micro-defects will be clarified and the effects of heavy reduction at the end of continuous casting solidification on microstructure, inclusions, precipitates, structural inheritance and hydrogen embrittlement in the course of pre-heating and rolling will be explored. The necessary condition of the rolling processs on void closure will be studied also. Based on the studies in the project, the manufacturing mechanisms of more than 60 mm thickness heavy pates using continuous casting slabs with low compression ratio, not greater than 2.0, will be clarified, which satisfy the strict requirements such as ultrasonic flaw, lamellar tearing resistance, low temperature impact toughness and so on.

随着我国高端装备制造业、新能源产业的发展，需要大量高档次特厚板。国内生产高强、高韧、高抗层撕裂等高级别的特厚板，大多采用大单重钢锭进行大压缩比轧制工艺，通常压缩比控制不小于4.0。国外已有报道利用连铸坯低压缩比轧制特厚板的工艺，压缩比控制不大于2.5，具有流程短、系统节能、成材率高等优点。目前，国内利用连铸坯低压缩比轧制制造特厚板一直受探伤不合问题困扰，为了解决这一问题，本研究提出铸坯凝固末端大压下结合低压缩比轧制工艺。针对特厚板用连铸坯慢速凝固条件下凝固组织、偏析、疏松和缩孔等进行研究，弄清连铸坯凝固末端大压下对凝固组织、宏观和微观缺陷的影响，探明凝固末端大压下对加热、轧制工艺中板材显微组织、夹杂物、析出物、组织遗传性和氢脆敏感性的影响，研究低压缩比轧制条件下压合细微空隙的必要条件。弄清控制压缩比不大于2.0制造厚度超60mm、满足探伤、“Z”向性能及低温冲击性等要求的特厚板制造机理。

低压缩比轧制技术生产特厚板一直受探伤不合的限制，要解决这一问题，需要首先改善铸坯内部质量。因此，本研究对铸坯宏观偏析、疏松和缩孔的分布及末端重压下的改善情况进行了分析，且对重压下及低压缩比轧制下铸坯凝固组织、夹杂物、析出物、裂纹及氢扩散开展了相关研究，并且首次以实验手段开展了铸坯压下效率的研究。建立了CET判据G^1.8/V和疏松模型G∙T ^(-0.895)，并研究了连铸参数对枝晶转变和缩孔（疏松）的变化规律的影响；溶质分布模拟结果表明，对82B高碳钢，C偏析指数最大值为1.36，而Mn最大值为1.10，并且，微观偏析模型的选择对宏观偏析预测数值有一定影响，Voller-Beckermann模型更为合适。凝固末端重压下实验结果表明，在合适的压下参数时可以有效的抑制铸坯中心偏析和中心疏松，改善宏观偏析压下位置最好在中心固相率0.7之前，且压下量不宜过大，而中心疏松的改善则需在中心固相率0.7以后施加较大的压下量（10mm以上）。重压下技术的应用可使铸坯心部铁素体尺寸及奥氏体晶粒尺寸减小，且使轧板内部带状组织分布更为均匀，并且重压下条件下轧板综合力学性能得到了一定提高。轧制过程中，对于球形缩孔，直径小于6mm的在轧制过程中都能够焊合，而铸坯内部小于20μm的内部裂纹通过加热可以愈合，尺寸较大的裂纹在轧制过程中也会逐渐愈合。针对Q345钢种，析出物尺寸在20-40nm之间，析出物多为(Nb,Ti)(C,N)析出。重压下条件下铸坯近表层处析出物的数量明显增多，且随着轧制道次的增加，析出物尺寸明显减小。钢中难变形夹杂物在轧制过程中变形情况不大，并且在轧制后期夹杂物周边会产生微小裂纹，裂纹沿轧制方向延伸；易变形夹杂物在轧制过程中容易变形，且沿轧制方向延伸。并且，轧制过程中内部夹杂物距轧件上表面的相对位置会产生变化。压下效率研究结果表明，示踪剂实验所得压下效率与模拟计算所得结果较为一致，且压下位置对压下效率影响较大。氢扩散模拟结果表明，钢板厚度越大，温度越低，氢扩散进程越慢，因此，厚板生产除堆冷去氢外，还需进行去氢退火。