TiN析出相对中碳Cr-Mo耐磨钢凝固组织的影响

EFFECT OF TiN PRECIPITATES ON SOLIDIFICATION MICROSTRUCTURE OF MEDIUM CARBON Cr-Mo WEAR RESISTANT STEEL

借助热力学理论计算,设计了固液两相区保温凝固和连续冷却凝固实验,采用OM,SEM,EDS和EPMA等方法,研究了中碳Cr-Mo耐磨钢中Ti N的析出行为及其对凝固组织的影响.结果表明,随着Ti,N含量的增加,Ti N在固液两相区的析出温度逐渐升高.当钢中Ti的质量分数为0.090%,N为0.014%时,Ti N直接在液相区析出.在液固两相区内不同温度保温并水淬凝固后,Ti N主要分布于凝固组织的粗大枝晶间、枝晶前沿和剩余液相区域的等轴晶晶界处.此外,少量Ti N分布在粗大枝晶和等轴晶内.在连续冷却凝固过程中,Ti N析出相的形成温度是影响凝固组织粗细的主要因素.随着Ti含量的增加,Ti N析出相的形成温度升高,钢液实际凝固温度增高,凝固区间增大,局部凝固时间延长,凝固组织的二次枝晶臂间距不断增大;当Ti的质量分数超过0.066%后,Ti N析出相的形成温度与液相线接近或高于液相线,钢液实际凝固温度变化不大,二次枝晶臂间距趋于稳定.

As an important type of wear-resistant material, the low-alloyed medium carbon wear resistant steel has been widely used in mining, power and metallurgical industries due to its low cost and excellent mechanical properties. However, the coarse as-cast microstructure tends to form in large wear resistant castings because of the long solidification time. As a result, spalling wear resulting from the preferential initiation and propagation of cracks along interdendrite will occur during service process, which severely degrades the wear resistance and service life. In this work, Ti is added to improve the mechanical properties of medium carbon Cr-Mo wear resistant steel. The precipitation behavior of TiN in the solidification process and its effect on the solidification microstructure were investigated by thermodynamic calculation, constant temperature solidification experiment at solid-liquid two phase region and continuous cooling solidification experiment by using OM, SEM, EDS and EPMA. The results show that TiN precipitation temperature gradually increases at solid-liquid two-phase region with the increase of contents of Ti and N. TiN precipitates directly in the liquid region when Ti and N contents （mass fraction） are 0.090% and 0.014%, respectively. Holding at different temperatures of solid-liquid two-phase region, a very small amount of TiN precipitates are present within the dendritic arm, and a large number of TiN precipitates are present at the interdendritic positions and frontiers of dendrites. After quenching, in the remaining liquid most of TiN are present at the boundaries of equiaxed grain and a little amount of TiN stay within the equiaxed grain. During the continuous cooling solidification, TiN precipitation temperature is the main factor affecting the refinement of solidification microstructure. With the increase of Ti content, TiN precipitation temperature increases. At the same time, the actual solidification temperature of liquid steel rises, the solidification temperature range broadens and the local solidification time extends, which results in the increase of secondary dendrite arm spacing. When Ti content exceeds 0.066%, TiN precipitation temperature is near or above the liquidus line. The actual solidification temperature of liquid steel remains unchanged. Therefore, the secondary dendrite arm spacing becomes stable.