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페라이트계 스테인리스강의 성형성에 미치는 응고조직 미세화의 영향

Title
페라이트계 스테인리스강의 성형성에 미치는 응고조직 미세화의 영향
Other Titles
Effect of Grain Refinement of Solidification Structure on the Formability of Ferritic Stainless Steel
Author
이문형
Alternative Author(s)
LEE MUN HYUNG
Advisor(s)
박주현
Issue Date
2017-08
Publisher
한양대학교
Degree
Master
Abstract
The effect of nitrogen content on the formation of an equiaxed solidification structure of Fe-16Cr steel was investigated by controlling TiN particles. Moreover, alumina and magnesia refractories were employed to control the type of oxide inclusion. When the melting was carried out in the alumina refractory, the grain size continuously decreased with increasing nitrogen content. However, a minimum grain size was observed at a specific nitrogen content (approx. 150 ppm) when the steel was melted in the magnesia refractory. Most of the single TiN particles had a cuboidal shape and fine irregularly shaped particles were located along the grain boundary. The type of TiN-oxide hybrid inclusion was strongly affected by the refractory material where Al2O3-TiN and MgAl2O4-TiN hybrid-type inclusions were obtained in the alumina and magnesia refractory experiments, respectively. When the nitrogen content increased, the number density of TiN linearly increased in the alumina refractory experiments. However, the number of TiN exhibits a maximum at about [N]=150 ppm, at which a minimum grain size was obtained in the magnesia refractory experiments. Therefore, the larger the number density of TiN, the smaller the primary grain size after solidification. The number density of TiN in the steel melted in the magnesia refractory was greater than that in the steel melted in the alumina refractory at given Ti and N contents, which was due to the lower planar lattice disregistry of MgAl2O4-TiN interface rather than that of Al2O3-TiN interface. When ∆TTiN (= difference between the TiN precipitation temperature and the liquidus of the steel) was 20-40 K, the number density of effective TiN was maximized and thus, the grain size was minimized after solidification. The growth of TiN particles during melting and solidification was well predicted by the combinatorial simulation of ‘Ostwald ripening model’ in conjunction with ‘Diffusion controlled model’. Also, in order to to evaluate the influence of the equiaxed grain formation in the cast samples and that of grain growth on the degree of ridging of final products and to identify which is the more dominant factor affecting the ridging phenomenon. Hence, we analyzed the Zener pinning effect of TiN particles on grain boundaries and discussed its influence on grain growth during isothermal heat treatment at 1473 K (1200 oC). Finally, we confirmed the relationship between grain size of solidified samples and degree of ridging. The tendency of ridging height corresponded to the grain size of solidified sample. The sheet composed ND//{001} and ND//{111} orientation colonies undergoes more severe ridging than random texture. Therefore, ridging is resulted from difference of anisotropic plastic between matrix and colony. Consequently, in order to reduce the ridging of ferritic stainless steel during the forming process is to form the random texture by enhancing the fine equiaxed grain during the solidification process.
URI
http://dcollection.hanyang.ac.kr/jsp/common/DcLoOrgPer.jsp?sItemId=000000102604http://hdl.handle.net/20.500.11754/33566
Appears in Collections:
GRADUATE SCHOOL[S](대학원) > MATERIALS ENGINEERING(재료공학과) > Theses (Ph.D.)
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