重轨钢铸坯中MnS夹杂物的析出与长大

Precipitation and growth of MnS inclusions in a heavy rail steel

铸坯中MnS夹杂物的形貌及尺寸对钢的性能影响显著,因此了解并调控其析出长大过程具有重要意义。采用连铸坯枝晶生长热模拟试验,观测了U78CrV重轨钢铸坯凝固过程中MnS夹杂的形貌及尺寸变化规律,结合热力学和动力学计算,分析了重轨钢铸坯中MnS夹杂的析出长大行为。热模拟试验表明,重轨钢铸坯中MnS主要在凝固末期析出,多分布于枝晶间隙。其中,柱状晶区中MnS主要呈球形、椭圆形及短棒状,平均等效半径为2.42μm,最大等效半径为4.19μm;等轴晶区中MnS多呈不规则形状,平均等效半径为4.01μm,最大等效半径为7.58μm。热力学计算表明,柱状晶区MnS析出凝固分数为0.97,析出温度为1663 K,高于固相线9 K;等轴晶区MnS析出凝固分数为0.95,析出温度为1623 K,高于固相线20 K。动力学分析表明,柱状晶区MnS理论长大半径为2.74μm,等轴晶区MnS理论长大半径为5.98μm,计算结果与试验结果较为吻合。通过比较柱状晶区与等轴晶区MnS的析出时间,讨论了等轴晶区MnS尺寸明显大于柱状晶区的原因。通过降低初始硫含量、减轻铸坯芯部硫元素的偏析以及提高冷却速率,可以有效降低芯部等轴晶区的MnS夹杂物的尺寸,减小MnS对钢材性能的危害。

It is of great significance to understand and control the process of precipitation and growth of MnS inclusion,result from its morphology and size in slab have a significant effect on the properties of steel.The morphology and size variation of MnS inclusions in the solidification process of U78CrV heavy rail steel bloom were observed by thermal simulator for dendrite growth of continuous casting slab.Combined with thermodynamic and kinetic calculation,the precipitation and growth behavior of MnS inclusions in steel was analyzed.The thermal simulation experiments show that MnS in steel is mainly precipitated at the end of solidification and is mostly distributed in interdendritic region.The MnS in columnar zone is mainly spherical,elliptical and short rod-shaped,with an average equivalent radius of 2.42μm and a maximum equivalent radius of 4.19μm.The MnS in equiaxed zone is mostly irregular,with an average equivalent radius of 4.01μm and a maximum equivalent radius of 7.58μm.The thermodynamic calculation shows that the solidification fraction of MnS precipitation in columnar zone is 0.97,and the corresponding temperature is 1663 K,which is 9 K higher than the solidus.Whereas,the solidification fraction and the corresponding temperature in equiaxed zone is 0.95 and 1623 K,which is 20 K higher than the solidus.The kinetic analysis shows that the theoretical growth radius of MnS in columnar zone is 2.74μm and that in equiaxed zone is 5.98μm,which is in good agreement with the experimental results.By comparing the difference of MnS growth time between the columnar zone and equiaxed zone,the reason why the size of MnS in equiaxed zone is obviously larger than that in columnar zone is discussed.Meanwhile,by reducing S content,reducing the segregation of S element in the center of the bloom and increasing the cooling rate,the size of MnS inclusions in equiaxed zone can be effectively reduced and the harm of MnS to the properties of steel can be reduced.