316H不锈钢铁素体的形成与控制

 为了提高316H不锈钢中厚板在钠冷快堆高温、强中子辐射环境下的质量稳定性,要求其残余铁素体面积分数不大于1%,而316H不锈钢中厚板的残余铁素体面积分数在常规工艺下为2%~8%。为了满足该要求,首先根据相图分析优化了316H不锈钢中C、Cr、Ni、Mo、N等元素含量,将其铬镍当量比控制到1.3以下,使其平衡态组织下铁素体面积分数小于7%。此外,根据相变过程,在凝固初期应快速冷却使钢水快速通过δ相区,尽可能少析出δ铁素体;在钢水快速通过δ相区后,应减缓冷却速度,使析出的铁素体尽可能多地通过包晶反应和高温扩散转变为奥氏体。因此,为了在316H连铸过程中实现这一目标,将钢水过热度从(45±5) ℃降低到(35±5)℃,铁素体面积分数最高由15%以上降低至10%附近;结晶器冷却水强度由2 700 L/min提高至3 000 L/min可以继续使铁素体面积分数从10%降至7%;最后再将二冷水比水量由0.75 L/kg降低至0.55 L/kg,整个连铸坯断面铁素体面积分数可全部降低至7%以下。通过连铸生产过热度、结晶器冷却强度、二冷水配水量3个工艺参数分步骤调整后得到的较低铁素体含量,连铸坯轧制成的钢板依然不能满足技术要求,需要将铸坯进行均质化处理,通过不同保温温度与保温时间的交叉试验,获得了最佳的均质化工艺,即1 250 ℃保温24 h,基本消除铸坯内残余铁素体组织,实现了残余铁素体面积分数不大于0.1%的316H中厚板的高效生产。

Ferrite formation and control in 316H stainless steel

In order to improve the quality stability of 316H stainless steel under the condition of sodium cold fast reactor high temperature and strong neutron radiation environment,the ferrite area percent of 316H stainless steel must be lower than 1%,which is much lower than 2%-8% in 316H stainless steel plates produced by conventional production process. The purpose is to reduce the ferrite area percent by the following method. The ratio of equivalent Cr and Ni was controlled below 1.3 and the ferrite content was controlled below 7% according to the phase diagram calculation combined with analysis of C,Cr,Ni,Mo and N in 316H stainless steel. The molten steel should be cooled rapidly at the initial stage of solidification to make the molten steel pass quickly δ-Fe phase region according to the phase transformation process. The cooling rate should be slowed down after the temperature of molten steel lower than δ-Fe phase zone,so that the δ-Fe can be transformed into austenite through peritectic reaction and high-temperature diffusion as much as possible. According to the above analysis,the highest ferrite content was decreased from more than 15% to about 10% by reducing the superheat of molten steel from (45±5) ℃ to (35±5) ℃,the increasing of cooling water intensity from 2 700 L/min to 3 000 L/min can reduce the ferrite area percent from 10% to 7% continually. Finally,the amount of secondary cooling water is reduced from 0.75 L/kg to 0.55 L/kg,and the ferrite area percent of whole continuous casting slab section can be reduced to less than 7%. The steel plate rolled with low ferrite content slab obtained by adjusting the three process parameters of continuous casting production superheat,mold cooling intensity and secondary cooling water distribution step by step still cannot meet the technical requirements. For eliminating residual ferrite thoroughly,the cross test of different holding temperature and holding time were test,the optimal homogenization technology for eliminating residual ferrite holding was that 1 250 ℃ holding for 24 h. Ultimately,the 316H stainless steel containing residual ferrite less than 0.1% was obtained by the above measures.