Characteristics of the as-cast high-carbon microalloyed continuous casting bloom steel for expansion-break connecting rods

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.