Characteristics of the as-cast high-carbon microalloyed continuous casting bloom steel for expansion-break connecting rods
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摘要: 脹斷連桿是汽車精密傳動用高端產品，需具高強高韌和裂解加工脆性解理斷裂特性。連鑄化生產高碳易切削脹斷連桿用微合金非調質鋼是當前的發展方向。基于大方坯連鑄生產典型工藝及其鑄態組織、成分均勻性分析，研究了脹斷連桿加工過程常見斷口形貌不合的鋼坯遺傳性因素。以常用德系C70S6鋼為例，采用250 mm×280 mm斷面弧形連鑄機，解析其在一定結晶器電磁攪拌條件下所澆鑄大方坯的鑄態低倍結構和枝晶形貌，并分析其不同晶區的成分分布特點。結果表明，當前連鑄條件下大方坯中心縮孔和后續熱軋棒材探傷合格率可控，但鑄坯初凝坯殼凝固前沿發生明顯的C、S負偏析白亮帶區及其柱狀晶偏轉現象。金相試樣圖像分析和相場法凝固模擬表明，鑄坯中柱狀晶具有逆流生長特征，其偏轉角是一次枝晶尖端向旋流方向逆向生長的結果。自鑄坯角部至寬、窄面中心，實測柱狀晶區的一次枝晶偏轉角約在?7°到27°之間。利用X射線能譜分析（EDS）進一步檢測了鋼中主要合金元素Si、Mn、Mo在鑄坯不同晶區的分布，揭示了其鑄態偏析特征與差異性。據此，探討了這種鑄態組織和成分偏析對后續熱軋棒材和連桿成品組織的遺傳性，以及對其脹斷加工斷口不合的影響，可為源頭鑄態質量的控制提供依據。Abstract: Expansion-break connecting rods are high-end products for automotive precision transmission. They need to have high strength, high toughness, and brittle cleavage fracture characteristics during cracking processing. Continuous casting production of nonquenched and tempered steel for high-carbon, sulfur-containing, free-cutting, and expanding connecting rods is the current trend for efficient production. Based on the typical bloom continuous casting process and analysis of the as-cast structure and composition uniformity, the common unqualified fracture morphology due to as-cast hereditary factors were studied. Taking the typical German C70S6 steel as an example, a 250 mm × 280 mm section-curved continuous caster was adopted to study the as-cast macrostructure and dendrite morphology of the bloom casting with popular mold electromagnetic stirring. Moreover, the chemical distribution at different crystal regions was studied. Results show that the common center shrinkage defects of high-carbon steel continuous casting blooms are under control, which are beneficial to improve the qualified rate of internal flaw detection for their subsequent hot-rolled bars. However, it is found that there are obvious negative segregation white bands of carbon and sulfur and the deflection of columnar crystals in the solidification front of the initial solidified shell. Both the image analysis of the metallographic sample and solidification simulation by a phase-field method show that this columnar crystal has countercurrent growth characteristics. In addition, its deflection angle is the result of the primary dendrite tip growing in the opposite direction to the swirling fluid flow. In the center of the narrow surface, the measured primary dendrite deflection angle of the columnar crystal region is between ?7° and 27°. EDS was used to further detect the distribution of the main alloying elements Si, Mn, and Mo in the different crystal regions of the bloom casting, revealing the segregation characteristics of the as-cast product and differences of each solute element. Finally, the heredity of this as-cast structure and composition segregation on the structure of subsequent hot-rolled bars and connecting rods was discussed. Moreover, the influence of the fracture inconsistency of its expansion and fracture processing were explored. It is pointed out that the as-cast quality control from the very beginning of casting has special significance in meeting the requirement of both processing and service properties of these high-grade high strength low alloy steels.
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圖 1 鑄坯橫截面晶區特征及鉆屑與金相試樣切取示意圖
Figure 1. Schematic of the as-cast bloom structure and sampling for chemistry and metallographic analysis
圖 2 大方坯低倍試樣。（a）全斷面低倍形貌；（b）柱狀晶偏轉；（c）皮下白亮帶
Figure 2. Low-magnification specimens of blooms: (a) full-section morphology; (b) columnar crystal deflection; (c) subsurface white bright band
圖 3 鑄坯斷面自表及里不同晶區鑄態枝晶形貌（1#, 3#, 6#, 12#）
Figure 3. Morphology of as-cast dendrites in different crystal regions from surface inward (1#, 3#, 6#, 12#)
圖 4 相場模擬橫向流動下的枝晶逆流偏轉及其溶質分布特征。（a）0 m·s?1；（b）0.04 m·s?1；（c）0.06 m·s?1
Figure 4. Phase-field simulation of dendrite countercurrent deflection and its solute distribution characteristics: (a) the velocity is 0 m·s?1; (b) the velocity is 0.04 m·s?1; (c) the velocity is 0.06 m·s?1
圖 5 鑄坯橫斷面中柱狀晶偏轉生長形貌
Figure 5. Morphology of the columnar crystal deflective growth in bloom casting
圖 6 鑄坯寬窄面柱狀晶偏轉角曲線。（a）窄面柱狀晶偏轉角分布；（b）寬面柱狀晶偏轉角分布
Figure 6. Deflection angle curve of the wide and narrow side columnar crystals: (a) narrow side of the bloom casting; (b) wide side of the bloom casting
圖 7 鑄坯縱剖低倍及其不同區域碳偏析特征
Figure 7. As-cast macrostructure at the longitudinal section of the bloom casting and its cross-sectional carbon segregation
圖 8 不同晶區合金元素EDS分析值（1#激冷層、3#白亮帶區域、12#等軸晶區）
Figure 8. EDS values of the solute elements in different crystal regions (1# quench layer, 3# white bright band area, 12# equiaxed crystal region)
表 1 大方坯連鑄機及生產工藝基本參數
Table 1. Bloom continuous casting machine and its basic production parameters
Continuous casting machine parameters Value Number of castings three streams Cross section 250 mm × 280 mm Radius of continuous caster 11 m Mould length 780 mm Maximum metallurgical length 23 m Nozzle type Immersion straight-through Middle- electromagnetic stirring（M-EMS） 500 A, 3 Hz First-electromagnetic stirring（F-EMS） 530 A, 8 Hz Water flow rate at the mold 2750 L·min?1 Superheat 11 K–24 K Casting speed 0.75 m·min?1 Second cooling specific water 0.224 L·kg?1 
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表 2 鑄坯斷面鉆屑點碳硫成分（質量分數）
Table 2. Carbon and sulfur concentration in the cross section of the bloom casting
% Numbering C S Numbering C S 1-1 0.728 0.0657 2-1 0.698 0.0565 1-2 0.690 0.0625 2-2 0.763 0.0700 1-3 0.689 0.0624 2-3 0.754 0.0705 1-4 0.719 0.0650 2-4 0.763 0.0727 1-5 0.750 0.0678 2-5 0.736 0.0692 1-6 0.751 0.0696 2-6 0.700 0.0628 1-7 0.764 0.0745 2-7 0.690 0.0669 1-8 0.729 0.0604 2-8 0.706 0.0673 0-0 0.891 0.0824
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表 3 鑄坯不同位置合金元素含量差異度比值
Table 3. Distribution ratio of the solute elements in different crystal regions
Element 1#/3# 12#/3# 12#/1# Mn 5.5 8 1.45 Mo 6.43 11.43 1.78 Si 2.2 3.78 1.89
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