特厚板慢速凝固过程组织结构形成及夹杂物控制

Along with the high speed development of the high-rise building sturctural steel, muclear power engineering steel and military use boat steel, etc., quantity demand scale of thick slab grows year by year, production technology of more than 400mm's thick continuous casting slab is received more attention increasingly in world wide. At present, thick slab's quality problem mainly are poor weld ability, whichi is easily appear lamellar tearing when welding; uneven cheimical composition, which is often have delamination and segregation phenomenon; "Z"performance of the casting blank is poor. These problems are closely related with the shape and quantity of second phase particles formed in steel solidification process. According to those mentioned problems as well as the slow solidification speed of the super thick slab, the hot simulation methods were adopted to research the microstructure and elements segregation behavior of super thick slab at slow solidification rate, distribution and comoposition of inclusion(chdeposition) were also researched, the CSLM were adopted to observe and research the formation of different sorts of inclusion in situ. To clarify the elements's macro and micro segregation mechanism, and conditions and location distribution of extra thick plate, reveal tissue formation and evlution principle ofextra thick plate under slow solidification cinditions, provide theoretical foundation for the extra thick plate's casting

随着高层建筑结构钢、核电工程用钢和军工船板钢等的迅速发展，厚板坯需求规模逐年扩大，大于400mm 的特厚连铸板坯生产技术在世界范围内日益受到重视。目前，厚板质量问题主要表现在可焊接性差，焊接时易出现层状撕裂；化学成分不均匀，常有分层、偏析等现象发生，铸坯的"Z"向性能较差。这些问题与钢液凝固过程中产生的第二相粒子的形状和数量密切相关。本项目针对特厚板连铸凝固速度缓慢及铸坯存在的焊接性差、成分偏析、 "Z"向性能不好等问题，采用实验研究和数值模拟相结合的方法对特厚铸坯慢速凝固速率下的凝固组织结构及元素偏析行为进行研究，采用CSLM方法对各种夹杂物的形成进行原位观察，研究凝固过程夹杂物（析出物）分布规律及组成形式，阐明特厚板各元素宏观、微观偏析产生机理、产生条件及位置分布规律，揭示特厚铸坯慢速凝固条件下组织形成和演变规律，为特厚板铸坯生产提供理论基础。

全面了解特厚板坯慢速凝固过程是实现特厚板坯质量控制的重要前提。鉴于实际生产中结晶器无法承受长时间的高温侵蚀，特依据相似原理设计符合冶金准则且能实现慢速凝固的试验设备及方法，利用数值模拟的手段对特厚铸坯慢速凝固过程中的温度场，坯壳形成规律及固液两相区的变化进行数值模拟，通过热态试验研究特厚板坯慢速凝固过程并分析所形成特厚铸坯的组织结构以及元素分布及夹杂物特征。本研究旨在揭示特厚铸坯慢速凝固过程及铸坯质量特性，为控制特厚板坯慢速凝固过程和改善特厚板坯质量提供理论依据。.数学模拟研究显示，慢速凝固条件下，105mm特厚铸坯初生坯壳为约为10mm时，能满足钢液静压力的要求；钢液在慢速凝固过程中，温度梯度先变大后变小，温度梯度减小到一定状态会保持一个平衡趋势，使得完全凝固后的铸坯降温呈现均匀化，同时凝固坯壳厚度也越来越均匀化；在本试验条件下，铸坯在距离弯月面3.51m处固液两相区区间值为最大，固液两相区中，高温区（1479℃-1488℃）变化速率快于低温区（1425℃-1457℃）变化速率，中温区（1457℃-1479℃）变化速率较为均匀。.热态实验研究发现，室温为20℃~30℃时，平均冷却速率为11.95℃/min的慢速凝固过程中，铸坯的表面无横纵裂纹，所得到的铸坯的晶粒尺寸比常速下的晶粒尺寸小；利用高温共聚焦实验验证不同冷速下试样的晶体大小演变规律，发现在10℃/min~30℃/min的慢速凝固过程中，晶体粒度呈现细小致密状态，与热态试验结论一致。同时，在热态试验所产生的特厚板坯的微观组织中，珠光体自边缘向中心由带状变为点状，自铸坯边缘向中心方向上，铁素体的方向性减弱，与珠光体互相缠绕，形成均匀致密的微观组织，均匀分布消除了碳偏析，利于特厚铸坯质量提高。