Influence of the soft reduction process on the sensitivity of the inner crack in heavy rail steel bloom
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摘要: 為研究壓下對連鑄坯內部裂紋產生的影響，利用ABAQUS有限元軟件建立了230 mm×280 mm斷面大方坯壓下數學模型。通過壓下模型對重軌鋼連鑄坯壓下過程進行熱力耦合模擬計算，對壓下過程中產生的內部裂紋進行了預測。首先，對連鑄坯不同中心固相率為0.3～0.7的溫度場進行計算；然后，利用壓下模型計算了連鑄坯中心固相率0.3～0.7時凝固前沿的等效塑性應變。研究結果表明，在連鑄坯中心固相率為0.3～0.7的位置處分別施加7 mm壓下量進行壓下，連鑄坯凝固前沿等效塑性應變未超過臨界等效塑性應變（0.4%），連鑄坯未出現內裂紋；同時，對連鑄坯在中心固相率為0.6位置處進行了不同壓下量的研究，研究結果表明，當連鑄坯壓下量超過7 mm時，凝固前沿的等效塑性應變超過臨界塑性應變（0.4%），連鑄坯出現內裂紋，并且壓下量越大，連鑄坯內裂紋越嚴重。同時，工業試驗結果與模型計算結果基本吻合，驗證了模型計算的準確性。Abstract: Continuous casting technology has greatly improved production efficiency; however, in the continuous casting process of heavy rail steel, problems of center segregation, center porosity, and shrinkage cavity of the bloom occur, which notably affect the billet quality of heavy rail steel. The soft reduction technique can effectively improve these problems, but internal cracks will appear, and the quality of the bloom will deteriorate if the parameters are not properly set. To examine the inner crack induced by the soft reduction of continuous casting bloom, this study established a mathematical model for the soft reduction of a 230 mm×280 mm section bloom by the ABAQUS finite-element software. The thermal-stress coupling model was used to predict the inner crack of the heavy rail steel bloom, using the soft reduction model during the soft reduction process. First, the temperature at different central solidification fractions of 0.3–0.7 in the bloom was calculated. The equivalent plastic strain that was located at the solidification front at different central solidification fractions was then calculated by the soft reduction model. Results indicated that the equivalent plastic strain at the solidification front in the bloom did not exceed the critical strain of 0.4% at the central solidification fractions with a reduction amount of 7 mm. Moreover, the inner crack did not occur at the solidification front. Simultaneously, the soft reduction model calculation with a different reduction amount was conducted at the central solidification fraction of 0.6. Results showed that the equivalent plastic strain at the solidification front exceeded the critical plastic strain of 0.4% when the reduction amount was more than 7 mm, which resulted in an inner crack. The greater the reduction amount, the more serious is the inner crack. Concurrently, industrial experimental results had a good agreement with the model calculation results.
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Key words:
- bloom /
- reduction /
- inner crack /
- numerical simulation /
- equivalent plastic strain
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圖 2 U71Mn重軌鋼物性參數。（a）熱物性參數；（b）流變應力（應變速率為0.001 s?1）；（c）固相分數和液相分數
Figure 2. Parameters of U71Mn steel: (a) physical parameters; (b) flow-stress (strain rate is 0.001 s?1); (c) solid and liquid fraction
圖 4 碳當量與臨界應變的關系
Figure 4. Relationship between the carbon equivalent and critical strain
圖 5 不同網格數溫度場計算結果
Figure 5. Temperature field calculation results of different grid numbers
圖 6 大方坯溫度分布。（a）鑄坯中心及表面溫度；（b）鑄坯液芯溫度分布
Figure 6. Temperature distribution in the bloom: (a) center and surface temperature of bloom; (b) temperature distribution of the bloom liquid core
圖 7 不同鑄坯中心固相率鑄坯溫度場分布
Figure 7. Temperature distribution at different central solidification fractions
圖 8 鑄坯橫截面等效塑性應變分布
Figure 8. Equivalent plastic strain distribution in the cross section of the bloom
圖 9 鑄坯等效塑性應變。（a）不同固相率下等效塑性應變；（b）凝固前沿等效塑性應變
Figure 9. Equivalent plastic strain of the bloom at: (a) different solid fractions; (b) solidification front
圖 10 鑄坯等效塑性應變。（a）不同壓下量等效塑性應變；（b）凝固前沿等效塑性應變
Figure 10. Equivalent plastic strain of the bloom (a) at different reduction amounts and (b) at the solidification front
圖 11 試樣縱剖酸侵低倍照片。（a）取樣示意圖；（b）壓下5 mm；（c）壓下7 mm；（d）壓下8 mm；（e）壓下10 mm；（f）壓下12 mm
Figure 11. Acid erosion pictures of the longitudinal section sample: (a) sampling diagram; (b) reduction of 5 mm; (c) reduction of 7 mm; (d) reduction of 8 mm; (e) reduction of 10 mm; (f) reduction of 12 mm
表 1 U71Mn鋼種的化學成分（質量分數）
Table 1. Chemical composition of U71Mn steel (mass fraction)
% C Si Mn P S Cr 0.7 0.25 1.15 0.005 0.008 0.08 
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