微量合金元素对11wt.%Cr铁素体不锈钢组织和性能的影响

IMR OpenIR

	微量合金元素对11wt.%Cr铁素体不锈钢组织和性能的影响
	刘静
学位类型	硕士
导师	罗兴宏
	2011
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	超纯铁素体不锈钢微合金化凝固组织间隙元素
摘要	"超纯铁素体不锈钢(UP-FSS)的性能较普通铁素体不锈钢有了很大提高，但超纯化也带来凝固组织柱状晶异常，焊接热影响区(HAZ)晶粒易粗化等缺陷，影响其成形性和可焊性。本文通过向11wt.%Cr UP-FSS添加Ti、Nb、B、Al等微合金化元素，力求改善其铸态、冷轧态及HAZ组织，揭示微合金化作用机制及间隙元素对UP-FSS组织和性能的影响。首先，采用Thermo-Calc软件研究了Ti、Nb、C、N元素含量对11wt.%Cr UP-FSS平衡相图的影响。结果表明，添加Ti、Nb后，钢中出现了TiN、MX等析出相，其中TiN在Ti、N含量达到一定条件时在固-液两相区析出，MX和其他析出相则在固相中析出。此外，Ti、Nb、C、N含量的增加还使得固-液两相区温度区间增大。在此基础上制备了13种不同成分的FSS，分析了其铸态组织及析出相形貌。实验结果表明，向钢中添加Ti、Nb后，等轴晶比例和晶粒球化率提高，晶粒尺寸和伸长率降低。复合添加Ti和Nb的钢比单独添加Ti的钢具有更小的晶粒尺寸和更高的等轴晶比例，且当钢中间隙元素含量较高时，这一效果更好。而B、Al的加入则使得铸态组织等轴晶比例降低，晶粒粗大。钢中的析出相随加入元素总量的增多由氧化物过度到氧化物和碳氮化物，最后转变为(Ti,Nb)(C,N)及少量共生析出相，且碳氮化物数量逐步增多。分析表明，TiN在固-液两相区析出和结晶温度范围的增大是改善铸态晶粒尺寸和形貌的主要因素。对实验钢冷轧态板材分析结果表明，复合添加Ti和Nb元素后，冷轧态组织细小均匀。相对于超纯钢，间隙元素含量较高的钢中Ti和Nb复合添加对组织改善的效果更好。实验条件下各个成分的实验钢的平均塑性应变比 12r' type="#_x0000_t75">均大于1，表明板材具有良好的冲压成形性能。随着微合金化元素的加入，Δr值降低，复合添加Ti和Nb的样品Δr最低。板材经TIG焊接后，焊缝成形状况良好。微合金化元素的加入，对焊接热影响区晶粒有一定细化作用，复合添加Ti和Nb并提高N含量的样品热影响区晶粒尺寸最低。"
其他摘要	"Ultra-purified ferrite stainless steel (UP-FSS) has better performance than conventional FSS. However, the abnormal growth of columnar grains during solidification and the grain coarsening in the heat affected zone (HAZ) during welding process brought by purifying the steels cause the degradation of the formability and ridging-resistance. The purpose of present research is to attempt to improve the as-cast, as-rolled, and HAZ structures by adding micro-alloying elements Ti, Nb, B, and Al into steels. The mechanisms of micro-alloying elements and effects of interstitial elements on structures and properties will be explored. The effects of Ti, Nb, C, and N contents on equilibrium phase diagrams of 11wt.%Cr UP-FSS were investigated by employing the Thermo-Calc software. The results illustrate that when Ti and Nb are added into the steel, TiN will precipitate in the solid-liquid two-phase region when Ti and N reach some certain contents; MX and other precipitates will form in the solid region. Additionally, the solidification interval will be widened by increasing the content of Ti, Nb, C or N. Based on calculated results, 13 steels with different chemical compositions were designed and prepared; the as-cast structures and the morphology of precipitates are observed. It is found that the addition of Ti, Nb to the melt enhances the equiaxed grain proportion and globularity factor, and reduces grain sizes and elongation factor. The steels with both Ti and Nb addition show higher equiaxed grain proportion and smaller grain size than the steels with only Ti addition, and better structures are obtained when interstitial element content is a little higher than ultra purity standard. The additions of B or Al get lower equiaxed grain proportion and coarse grains in this experiment. As the amount of micro-alloying elements added into the steels increases，the precipitates convert from oxides to oxides and carbonitrides, (Ti, Nb)(C, N) and finally some complex particles, meanwhile, the number of carbonitrides increases accordingly. The analysis shows that the precipitation of TiN in the solid-liquid two-phase region and the enlargement of solidification interval are the two main factors which contribute to solidification structure improvement. As-rolled structures of experimental steels were also investigated. It is found that the structures of the as-rolled sheets with both Ti and Nb addition consist of homogeneous and small grains, especially when interstitial element content is a little higher than the ultra-purity standard. The average plastic strain ratio 12r' type="#_x0000_t75"> is larger than 1 in all experimental steels in the present work, which reveals good formability. With the addition of micro-alloying elements, Δr decreases, and the smallest Δr value is found in samples with both Ti and Nb addition. The welding joints were observed after the TIG welding experiment. The addition of micro-alloying elements is beneficial to refine the HAZ structures, and the best effect was observed in samples with both Ti and Nb addition and high N content."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64387
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	刘静. 微量合金元素对11wt.%Cr铁素体不锈钢组织和性能的影响[D]. 北京. 中国科学院金属研究所,2011.