冷却强度对超高硫中碳钢方坯凝固过程MnS生长的影响

 中碳超高硫易切削钢SAE144是兼具力学性能与切削性能的结构钢,用于制造汽车发动机密封阀件等,产品多采用转炉/电炉→LF精炼→连铸小方坯→线棒材热轧→冷拉及机加工成型流程生产,近年来市场热度稳步提升。若钢中MnS尺寸过大,零件加工使用过程易发生探伤不合、切削性能差、带状组织严重、力学性能各相异性显著,甚至拉拔加工断裂等问题。MnS夹杂物多在铸坯凝固后期形成,随着轧制与钢基体同步变形,控制该类钢种铸坯内MnS原始尺寸成为控制热轧材中MnS夹杂物形态及尺寸的最关键环节。为控制热轧超高硫中碳钢盘条中MnS夹杂物,利用钢坯凝固数值模拟、第二相析出理论、Ostwald熟化理论计算分析了160 mm2钢坯中硫元素偏析及MnS的生成、长大和熟化过程。计算结果表明,当固相分数f_s为0.446、硫微观偏析比达到2.19时,铸坯在凝固末期生成MnS。凝固过程中MnS的生长过程决定了钢坯中MnS颗粒的直径。理论计算表明,当连铸二次冷却水量固定为0.6L/kg时,拉速为1.6、2.1和2.6 m/min时,160 mm2方坯中心的MnS分别增长到30.6、32.2和34.6 μm,与实际测试结果一致。控制该类钢种线材中MnS尺寸的关键是提高二冷区的冷却强度,降低连铸拉速。基于该系列计算方法,提出了160 mm2钢坯中与MnS直径控制目标相匹配的连铸工艺参数控制范围。

Effects of cooling intensity on MnS growth during solidification in ultra-high sulfur medium carbon steel billet

Medium-carbon ultra-high sulfur free-cutting steel SAE144 is a structural steel with both mechanical properties and machinability,thus used as automotive engine sealing valves, etc. The production process is mostly converter/electric furnace→LF refining→continuous casting→wire rod hot rolling→cold drawing and machining,the market popularity has steadily increased in recent years. If the MnS inclusions in the steel are not properly controlled, problems such as failure in flaw detection, poor machinability, severe band structure, significant dissimilarity in mechanical properties, and even drawing fractures are likely to occur. MnS inclusions are mostly formed in the late stage of billet solidification and are deformed synchronously with the steel matrix during rolling. Controlling the original size of MnS in the billet has become the most critical step to control the morphology and size of MnS inclusions in hot-rolled products.In order to control MnS inclusions in hot-rolled ultra-high sulfur medium-carbon steel wire rod, the segregation of S element and generation, growth and ripening of MnS was analyzed by the billet solidification numerical simulation calculation, second phase precipitation theory and Ostwald ripening theory in 160 mm2 billet. The calculation results show that MnS is generated at the end of the solidification in cast billet when solid phase fraction f_s=0.446 with the segregation ratio of S reaches 2.19. The growth process of MnS during solidification determines the diameter of MnS particles in the billet. The theoretical calculation shows that when the secondary cooling water volume is fixed to 0.6 L/kg, the MnS in the 160 mm2 billet center grows up to 30.6, 32.2 and 34.6 μm at the casting speed of 1.6, 2.1 and 2.6 m/min respectively, which is consistent with the actual test results. The key to controll the size of MnS in wire rod is to increase the cooling intensity of the secondary cooling zone and reduce the casting speed of the billet. Based on the calculation method in this paper, control range of continuous casting process parameters matching the MnS diameter in 160 mm2 billet is presented.