重轨钢铸坯和钢轨加热过程中MnS夹杂转变机制

Effects of heating and holding process on the MnS inclusions in the bloom and rail of heavy rail steels

主要论述了不同加热保温过程对重轨钢中MnS夹杂物的影响。试验结果表明钢轨中大尺寸长条状MnS夹杂物分裂成小尺寸短粗状的粒子,当均热时间超过3.5 h,长度大于80μm的MnS夹杂物数量减少,尺寸小于5μm的MnS夹杂物数量增加;而在铸坯中的变化不明显。其行为遵循Ostwald熟化机制,保温时间越长,MnS颗粒就越大。温度在850℃时S为扩散控制性元素,MnS易分裂。铸坯内MnS夹杂物三维形貌有板片状、长条状和不规则状;在铸坯轧制成钢轨的过程中,MnS的形貌随着钢基体沿着加工方向延伸成条带状。采用Scheil模型计算出凝固分率为0.94时,才有较纯的MnS夹杂物开始析出,其形貌易受到晶界的影响。MnS夹杂物形成的限制性因素是[%Mn]·[%S]浓度积和温度。在[Mn]含量不变条件下,只有降低[S]含量,且采用强冷,才能降低Mn和S在凝固前沿的聚集,从而进一步降低MnS夹杂物析出数量和减小尺寸。

The effects of different heating and holding process on the MnS inclusions in the bloom and rail of heavy rail steels was investigated. It was found that the large elongated MnS inclusions in rail would split into small particles after heating and holding process. The amount of MnS inclusions larger than 80μm decreased, while the amount of MnS particles smaller than 5μm increased when the soaking time exceeded 3.5 hours. However, the phenomenon was not obvious for the MnS inclusions in bloom. The behavior of MnS inclusions followed the Ostwald ripening mechanism, that longer soaking time lead to larger MnS particles. The MnS inclusions were more likely to split at 850℃, at which sulfur was the diffusion controlled element. The three-dimensional morphologies of MnS inclusions in bloom included plate-like, strip-like and irregular, which were then elongated to strip- like along the rolling direction during the subsequent rolling process of bloom into rail. Pure MnS inclusions would start to precipitate only when the solid fraction reached 0.94 on the basis of calculation by Scheil model, the morphology was influenced by the pre-existed grain boundaries. The restrictive factors of the formation of MnS were concentration product of [% Mn]. [%S] and temperature. To reduce the accumulation of Mn and S in the front of solidification and further to reduce the precipitating amount and size of MnS inclusions, decreasing sulphur content and increasing cooling rate were proposed under the condition of constant Mn content in molten steel.