结合实验研究和热力学计算对不同间隙元素含量下Ti和Nb微合金化对超纯11%Cr铁素体不锈钢铸态、轧态和焊接热影响区(HAZ)的影响进行了研究和分析. 研究结果表明: 钢中间隙元素含量在0.0163%时Ti和Nb微合金化效果较好, 在相同的轧制工艺下可获得更好的轧态组织,相同焊接工艺条件下HAZ晶粒尺寸也较细小. 分析表明: 在相同的凝固条件下,随着C和N含量的提高及Ti和Nb的加入, 固/液两相区温度 区间增大,有助于提高柱状晶前沿过冷度, 提高非均质形核几率,进而使得铸态组织中等轴晶比例提高, 晶粒尺寸减小. 另一方面,随着Ti的加入和N含量的提高, TiN生成区域从固相区提高到固/液两相区,有助于促进高温铁素体的非均质形核.
Ultra–purified ferrite stainless steels (UP–FSS) are widely used in the fields of automobile, household appliances etc. Much better performance than conventional ferrite stainless steels is obtained by minimizing the interstitial elements level in UP–FSS. However, some negative effects, such as degradation of the formability and ridging–resistance, are also brought by purifying the steels, which lead to abnormally growth of the columnar grains. Moreover, the grains in heat affected zone (HAZ) are apt to coarsen during welding process. One of the effective ways to resolve such problems is to increase the equiaxial grain ratio in as–cast microstructure and refine the grain size. Micro–alloying of steels with strong carbide and nitride former, such as Ti and Nb, is a way to do that. In this work, the effects of Ti and Nb micro–alloying on the as–cast, as–rolled, and HAZ microstructures of the ultrapurified 11%Cr ferrite stainless steels with different interstitial element levels were investigated by both experimental research and thermodynamic calculation. The results indicated that the effect of Ti and Nb micro–alloying was better when the content of C and N in the steel was 0.0163%, which showed the minimum grain size in as–cast, as–rolled, and HAZ microstructures, and the equiaxial grain ratio in as–cast microstructure was evidently improved as well. It was found by calculation that, with increasing of C and N content and addition of Ti and Nb to the steels, the solid–liquid two–phase region was broadened. This was beneficial to increase the undercooling in front of the liquid–solid interface and the probability of heterogeneous nucleation, and consequently, increase the equiaxial grain ratio and decrease te mean grain size in as–cast microstructure. On the other hand, with addition of Ti and increase of N conent, TiN type particles might precipitate in olid–liquid two–phase regio, which was helpful for promoting the heterogeneous nucleation of δ ferrite.
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