Evenness of solidification structure and solute distribution in the continuous casting billet of gear steel
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摘要: 分析了20CrMnTi齿轮钢200 mm×200 mm连铸坯凝固组织和溶质分布的均匀性，提出了不同过热度时粗大等轴晶和点状偏析的共生机理，制定了齿轮钢连铸均质性控制策略。结果表明，齿轮钢连铸坯凝固组织受到钢水过热度的直接影响，提高过热度会减小等轴晶面积，降低对中性，减小等轴晶尺寸；齿轮钢连铸坯凝固组织直接影响C元素中心偏析和断面分布均匀性，提高等轴晶率有利于降低中心偏析，但会增大等轴晶区溶质波动范围；齿轮钢连铸坯凝固组织直接影响半宏观尺度的点状偏析，提高等轴晶率会增加大尺寸点状偏析数量，Cr、Mn溶质偏聚程度也会增加。为了同时提高齿轮钢组织和成分均匀性，比较理想的连铸低倍结构特征是等轴晶率低、等轴晶尺寸小而均匀、对中性良好。Abstract: The evenness of solidification structure and solute distribution in the continuous casting billet of 20CrMnTi gear steel was experimentally investigated, and the symbiotic relationship and formation mechanism of coarse equiaxed dendrites and spot segregation were proposed. The control strategy on the homogeneity of continuous casting bloom for gear steel was developed. The results indicate that the solidification structure of the bloom is closely affected by superheat during casting. As the superheat increases, the ratio of equiaxed area decreases, the sizes of the equiaxed dendrites reduce, and the alignment of center equiaxed zone worsens. The central segregation and solute distribution evenness are related to solidification structure. The central segregation ratio decreases with the increase in equiaxed ratio, while the solute fluctuation range increases. In addition, the spot segregation is also relevant to the solidification structure. The number of large size spot segregation increases, and the concentration of Cr and Mn in the spot segregation increases, as the equiaxed ratio increases. In order to improve the evenness of microstructure and composition in gear steel, the target solidification structure of continuous casting bloom should possess low equiaxed ratio, small and uniform equiaxed dendrites, and good alignment of equiaxed zones.
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Key words:
- gear steel /
- solidification structure /
- central segregation /
- carbon content range /
- spot segregation
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图 1 连铸坯试样C含量（实心圆）、枝晶和点状偏析（A-D）的取样位置
Figure 1. Sampling position for C content(solid circle), dendrite grain and spot segregation(A-D) in the billet
图 2 1#试样低倍组织
(a)宏观凝固结构；(b) E区放大；(c) F区放大；(d) G区放大
Figure 2. Macrostructure of 1# billet
图 3 2#试样的低倍组织
(a)宏观凝固结构；(b) H区放大；(c) I区放大；(d) J区放大
Figure 3. Macrostructure of 2# billet
图 4 齿轮钢连铸坯C含量分布
(a)、 (b)：1#；(c)、 (d)：2#；其中，横坐标负值代表左侧和内侧、正值代表右侧和外侧
Figure 4. Carbon content distribution in the billets of the gear steel
图 5 1#铸坯凝固组织演变
(a)试样A；(b)试样B；(c)试样C；(d)试样D
Figure 5. Evolution of solidification structure in 1# billet
图 6 1#试样距表层不同位置的SDAS
Figure 6. SDAS in 1# billet at different locations from the surface
图 7 2#试样凝固组织演变
(a)试样A；(b)试样B；(c)试样C；(d)试样D
Figure 7. Evolution of solidification structure in 2# billet
图 8 2#试样距表层不同位置的SDAS
Figure 8. SDAS in 2# billet at different locations from the surface
图 9 齿轮钢连铸坯等轴晶区枝晶胞块尺寸
Figure 9. Sizes of dendrite cell in equiaxed zone in the billets of gear steel
图 10 齿轮钢连铸坯点状偏析尺寸分布
Figure 10. Spot segregation size distribution in the billets of gear steel
图 11 1#试样等轴晶区点状偏析
Figure 11. Spot segregation in the equiaxed zone of 1# billet
(a)OM；(b)Cr-EPMA；(c)Mn-EPMA
图 12 2#试样等轴晶区的点状偏析
Figure 12. Spot segregation in the equiaxed zone of 2# billet
(a)OM；(b)Cr-EPMA；(c)Mn-EPMA
图 13 齿轮钢连铸坯粗大等轴晶和点状偏析的共生机理
Figure 13. Symbiotic relationship and formation mechanism of coarse equiaxed dendrite and spot segregation
表 1 20CrMnTi钢种主要化学成分
Table 1. Main chemical compositions of 20CrMnTi steel
% 编号 C Mn Si P S Cr Ti 标准范围 0.17~0.23 0.80~1.20 0.17~0.37 ≤0.030 ≤0.030 1.00~1.30 0.04~0.10 1# 0.21 0.92 1.07 0.013 0.030 1.16 0.061 2# 0.21 0.90 1.02 0.013 0.024 1.10 0.056 
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表 2 连铸机生产工艺参数
Table 2. Parameters of the continuous caster
试验号 拉速/
(m·min−1)过热
度/ ℃二冷比水量/
(L·kg−1)M-EMS F-EMS 1# 1.3 27 0.515 √ √ 2# 1.3 38 0.505 √ √ 注：M-EMS参数为100 A/3 Hz；F-EMS参数为50 A/5 Hz。
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表 3 C元素偏析比、标准差和极差
Table 3. Segregation ratio, standard deviation and polar deviation of C contents
编号 碳偏析 正偏析 负偏析 标准差 极差 1# C 1.13 0.90 0.009 0.05 C（去除中心点） 1.04 0.90 0.008 0.03 2# C 1.32 0.94 0.014 0.08 C（去除中心点） 1.05 0.95 0.007 0.02
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