A microsegregation model in the two-phase region of an ND steel continuous casting billet
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摘要:
通过构建ND钢连铸坯凝固两相区内溶质的微观偏析模型, 不仅研究了C、S和P元素对固液两相区内钢的高温力学参数以及溶质再分配的影响, 还对P元素偏析比随冷却速率(CR) 的变化规律进行了探究.通过分析模型结果表明: 初始C的质量分数在0.075%~0.125%之间时, 随着初始C含量的增加, P、S元素的偏析加剧, 凝固末端温度下降幅度变大, 导致脆性温度区间增大; 增加P和S元素的初始含量, P、S元素的偏析比降低, 但会加剧其在枝晶间残余液相中的富集, 直接导致零塑性温度(ZDT) 下降; ND钢中的Cu含量低于显著提高裂纹敏感性的临界含量, 且凝固过程中Cu元素的偏析比较低, 因此在ND钢凝固过程中Cu元素不能主导裂纹的诱发; 在一定的冷却速率波动范围内, P元素的偏析比随着冷却速率(CR)的提高略有下降.
Abstract:ND steel is a low alloy steel that resists the dew point corrosion of sulfuric acid.To improve the special performance of ND steel, the chemical composition of ND steel not only contains conventional elements but also adds corrosion-resistant elements, such as Cu, Cr, and Ni.During the solidification process, the molten steel will undergo a phase change reaction.Owing to the differences in the distribution coefficients and diffusion coefficients of solute elements in different phases, solute elements will be redistributed in the solid-liquid two-phase region during solidification, which will lead to microsegregation of solute elements.The microsegregation of solute element makes the zero strength temperature and zero plasticity temperature (ZDT) of steel decrease, which makes the temperature range of brittleness expand and deteriorates the mechanical property of high temperature of the continuous casting billet, and finally increases the probability of inducing surface cracks.This paper takes the microsegregation of solute elements as the research background.Herein, a microsegregation model for the solute in the solidified two-phase region of an ND steel continuous casting billet was established.In the model, the effects of elements C, S, and P on high-temperature mechanical parameters and solute redistribution of steel in its solid-liquid two-phase region were studied, and the variation law of the segregation ratio of elemental P with cooling rate (CR) was also explored.According to the analysis of the model results, when the initial C content was between 0.075%and 0.125%, with an increase in the initial C content, segregation of P and S elements intensified, and the temperature drop at the solidification end became larger, leading to the increase in the brittle temperature range.According to the analysis of the model results, increasing the initial content of P and S will decrease the segregation ratio of P and S elements but will increase the enrichment content of P and S elements in the residual liquid phase between dendrites, directly leading to the decline of ZDT.Analysis of the model results shows that the Cu content in ND steel is lower than the critical content that significantly increases the crack sensitivity, and the segregation ratio of Cu element is at a low level during solidification.Therefore, elemental Cu cannot dominate the induced crack in ND steel during solidification.Finally, within a certain range of cooling rate fluctuation, the segregation ratio of P will decrease slightly with increasing CR.
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图 1 枝晶间液相中P质量分数随固相率的变化
Figure 1. Change of P mass fraction in the intercrystalline liquid phase with the solid phase ratio
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图 2 ND钢枝晶间残余液相凝固点温度随固相率的变化
Figure 2. Changes in temperature of the residual liquid phase solidifica-tion point of ND steel dendrite with solid phase ratio
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图 3 凝固过程中枝晶间各元素偏析比变化
Figure 3. Segregation ratio of each element of the dendrites in the solid-ification process
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图 4 C质量分数对溶质偏析比的影响. (a) f=0.75; (b) f=0.99
Figure 4. Effect of C mass fraction on solute segregation ratio: (a) f=0.75; (b) f=0.99
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图 5 C质量分数对零强度温度(ZST) 和零塑性温度(ZDT) 的影响
Figure 5. Influence of C mass fraction on zero-strength temperature (ZST) and zero-plastic temperature (ZDT)
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图 6 S质量分数对枝晶间S元素偏析的影响. (a) f=0.75; (b) f=0.99
Figure 6. Influence of S mass fraction on the segregation of S elements in dendrites: (a) f=0.75; (b) f=0.99
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图 7 S质量分数对零强度温度(ZST) 和零塑性温度(ZDT) 的影响
Figure 7. Influence of S mass fraction on zero-strength temperature (ZST) and zero-plastic temperature (ZDT)
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图 8 P质量分数对枝晶间P元素偏析的影响. (a) f=0.75; (b) f=0.99
Figure 8. Influence of P mass fraction on the segregation of P elements in dendrites: (a) f=0.75; (b) f=0.99
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图 9 P质量分数对零强度温度(ZST) 和零塑性温度(ZDT) 的影响
Figure 9. Influence of P mass fraction on zero-strength temperature (ZST) and zero-plastic temperature (ZDT)
表 1 1873K时各元素活度相互作用系数
Table 1 Activity interaction coefficient of each element at 1873K
eij C Si Mn P S Cu Ni Cr Mn -0.07 - - -0.0035 -0.048 - - - S 0.11 0.063 -0.026 0.029 -0.028 -0.0084 0 -0.011 
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表 2 溶质元素在各相中的凝固参数
Table 2 Solidification parameters of the solute elements in each phase
元素 kiδ/L kiγ/L Diδ/(cm2·s－1) Diγ/(cm2·s－1) mi C 0.19 0.34 5.08×10-5 8.26×10-6 78.0 Si 0.77 0.52 3.70×10-7 1.17×10-8 7.6 Mn 0.76 0.78 1.86×10-7 2.47×10-9 4.9 P 0.23 0.13 4.81×10-7 4.10×10-8 34.4 S 0.05 0.035 2.16×10-6 6.27×10-7 38.0 Cu 0.53 0.88 2.21×10-7 2.63×10-9 5.32 Ni 0.83 0.95 1.36×10-7 1.63×10-10 4.69 Cr 0.95 0.86 2.239×10-7 4.236×10-10 1.04
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表 3 实验钢种化学成分(质量分数)
Table 3 Chemical composition of experimental steel
% C Si Mn P S 0.13 0.35 1.52 0.016 0.002
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表 4 ND钢化学成分(质量分数)
Table 4 ND steel element composition
% C Si Mn P S Cu Ni Cr 0.07~0.125 0.20~0.40 0.40~0.60 ＜0.025 ＜0.010 0.25~0.45 0.10~0.20 0.75~1.00
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