Analysis on the formation of V-shape segregation in rail steel bloom
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摘要: 针对重轨钢连铸大方坯V型偏析，检测了V型偏析的宏观形貌特征和凝固组织，计算对比了理论收缩体积与实际V型体积。通过凝固末端压下工艺对比、钢种对比及凝固组织调控试验，探究了V型偏析形成的影响因素。结果表明，重轨钢大方坯V型偏析分两步形成，首先是凝固前沿捕捉的晶核充分长大成粗大球化等轴晶组织后发生搭接，搭接产生封闭的富集溶质微区而形成初始V型偏析，进一步在钢液静压力和凝固收缩的负压抽吸等综合作用下发生组织滑移产生中心凹陷，导致V型偏析的宏观形貌进一步改变（减小V型角）而形成最终的V型偏析。建立了协同改善重轨钢大方坯V型偏析和中心偏析的关键工艺，实现了连铸坯V型偏析和中心偏析协同改善。Abstract: In order to improve V-shape segregation in rail steel bloom, the feature of macro-profile and microstructure were tested and theoretical shrinkage value and actual volume of V-shape was compared. The influencing factors including dynamic soft reduction processes, grade of steel and technology of micro-structure control were experimentally investigated. Result shows the V-shape segregation of rail steel bloom tends to form from two steps. The first step is ‘initial V-shape segregation formed’ where the crystal nucleus caught by solidification front of bloom fully developes into coarse connected equiaxed crystals, this results into the closed micro-regions, the molten steel in micro-regions tend to be solute rich during the solidification process. And then the second step is V-shape changed, which is formed from the microstructure subside and slip by the effect of the static pressure and the negative pressure suction due to shrinkage. Based the results the key processes has been established to improve both V-shape segregation and central segregation of rail steel bloom.
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Key words:
- rail steel /
- solidification structure /
- V-shape segregation /
- central segregation
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图 1 V型偏析在大方坯上的横纵向宏观形貌关系
Figure 1. The macro-profile relationship between horizontal and vertical of V-shape segregation in bloom
图 2 V型偏析检验取样位置
Figure 2. The diagram of sampling position for V-shape segregation testing
图 3 V型偏析所处晶区
Figure 3. The solidification structure zone of V-shape segregation appeared
图 4 V型偏析典型局部凝固组织形貌
Figure 4. Solidification structure morphology of V-shape segregation
图 5 V型偏析的V型单元体积测算
Figure 5. Diagram of measurement and calculation for the V-shape segregation element volume
图 6 铸坯凝固收缩模拟计算与实测分布对比
Figure 6. Comparison between the numerical simulation and actual measure results for shrinkage porosity
图 7 V型偏析单元实际体积测算与收缩单元理论体积计算示意
Figure 7. Diagram for the measurement and calculation of actual V-shape segregation and shrinkage porosity volume
图 8 V型偏析区域对应连铸过程的凝固区间
Figure 8. The corresponding zone in solidification process of V-shape segregation
图 11 典型钢种关键物性参数对比
Figure 11. Comparison of the key physical property parameters of representative grade steel
图 12 钢种典型纵向中心低倍照片
Figure 12. Representative macro-photograph of bloom center in lengthways
图 13 铸坯横纵向实物低倍照片
Figure 13. Macro-photograph of bloom for the cross section and lengthways center
图 14 铸坯中心C偏析度检测结果
Figure 14. Testing result of C segregation degree of bloom central line
图 15 V型偏析形成过程示意
Figure 15. Schematic diagram of formation process for V-shape segregation
表 1 试验铸机关键装备技术条件
Table 1. The key equipment and technical parameters of continuous casting machine for experiment
连铸机机型 铸机流数 铸坯规格/mm 基本半径/m 冶金长度/m 结晶器电磁搅拌 轻压下 凝固末端电搅 全弧形 6机6流 280×380 12 35.60 有 有 有 
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表 2 凝固组织调控试验的关键工艺
Table 2. The key processes of experiment for solidification structure control
项目 电磁搅拌 二冷（弱冷）/（L·kg−1） 拉速/（m·min−1） M-EMS F-EMS 电流/A 频率/Hz 电流/A 频率/Hz 工艺一 200～300 2.0～2.5 0.25～0.28 0.67 工艺二 50～100 2.0～2.5 300～400 5.5～7.0 0.30～0.34 0.65
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表 3 铸坯中心C偏析度统计
Table 3. Statistical result of C segregation degree for bloom center
项目 原工艺（n=30） 试验工艺（n=30） MAX 1.34 1.12 MIN 0.95 0.99 平均 1.15 1.06
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