胀断连杆用高碳微合金钢连铸大方坯的铸态组织

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

胀断连杆是汽车精密传动用高端产品,需具高强高韧和裂解加工脆性解理断裂特性.连铸化生产高碳易切削胀断连杆用微合金非调质钢是当前的发展方向.基于大方坯连铸生产典型工艺及其铸态组织、成分均匀性分析,研究了胀断连杆加工过程常见断口形貌不合的钢坯遗传性因素.以常用德系C70S6钢为例,采用250 mm×280 mm断面弧形连铸机,解析其在一定结晶器电磁搅拌条件下所浇铸大方坯的铸态低倍结构和枝晶形貌,并分析其不同晶区的成分分布特点.结果表明,当前连铸条件下大方坯中心缩孔和后续热轧棒材探伤合格率可控,但铸坯初凝坯壳凝固前沿发生明显的C、S负偏析白亮带区及其柱状晶偏转现象.金相试样图像分析和相场法凝固模拟表明,铸坯中柱状晶具有逆流生长特征,其偏转角是一次枝晶尖端向旋流方向逆向生长的结果.自铸坯角部至宽、窄面中心,实测柱状晶区的一次枝晶偏转角约在−7°到27°之间.利用X射线能谱分析(EDS)进一步检测了钢中主要合金元素Si、Mn、Mo在铸坯不同晶区的分布,揭示了其铸态偏析特征与差异性.据此,探讨了这种铸态组织和成分偏析对后续热轧棒材和连杆成品组织的遗传性,以及对其胀断加工断口不合的影响,可为源头铸态质量的控制提供依据.

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.