- 1. 北京科技大学
- 2. 北京科技大学冶金与生态工程学院
Characteristic of the As-cast High-carbon Micro-alloyed Continuous Casting Bloom Steel for Expansion-break Connecting Rods
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Abstract: Expansion-break connecting rod is a high-end product for automotive precision transmission. It needs to have high strength, high toughness and brittle cleavage fracture characteristics during cracking processing. Continuous casting production of non-quenched and tempered steel for high-carbon, sulfur-containing, easy-cutting and expanding connecting rods is the current development direction. Based on the typical bloom continuous casting production process and the analysis of the as-cast structure and composition uniformity, the common inconsistent fracture morphology due to as-cast hereditary factors were studied. Taking the typical German C70S6 steel as an example, a 250mm×280mm section arc continuous caster was adopted to study the as-cast low-magnification structure and dendrite morphology of the bloom casting with popular mold electromagnetic stirring, and the composition distribution at different crystal regions was studied as well. The results show that the common center shrinkage defects of high-carbon steel continuous casting blooms are under control, which is beneficial to improve the defect detection qualification rate of subsequent hot-rolled bars. However, it is found that there are obvious negative segregation white bands of C and S and the deflection of columnar crystals in the solidification front of the initial solidified shell. Both the image analysis of the metallographic sample and the solidification simulation through phase field method show that this columnar crystal has the characteristics of countercurrent growth, and its deflection angle is the result of the primary dendrite tip growing in the opposite direction to the swirling direction; 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 the differences for different solute elements. Finally, the heredity of this as-cast structure and composition segregation on the structure of subsequent hot-rolled bars and connecting rods, as well as the influence of the fracture inconsistency of its expansion and fracture processing, is discussed. It is pointed out that the as-cast quality control from the very beginning of casting has special significance to meet the requirement of both processing and service properties of these high grade alloy steels.