PRODUCTION OF THE HIGH STRENGTH AUSTENITIC STAINLESS STEEL SHEET HAVING ULTRAFINE GRAIN

Investigators have succeeded in super grain refining to 0.5mum of metastable austeniticstainless steels based of Fe-Cr-Ni system by martensite austenite reversion. The purpose of this现在的工作应该被找到三个条件，以满足超级复习：1 . Austenite transforms to martensite almost all，(2) The martensite perfectly reverses to austenite by annealing at relatively low(3) Ms temperature of the reversed austenite exist under the room temperature. Nextpurpose is determine the suitable chemical composition for the high strength steel containingmolybdenum and nitrogen. Lastly, using 370kg ingot，thermo-mechanical treatments carried out on the pilot line in steel works. And then，tensile properties and press formabilities were examined. Results obtained were as follows；1. Typical composition of ultrafine grain steel in Fe-Cr-Ni system was Fe-15.5%Cr-10%Ni. A specimen90% cold rolling prior to annealing of 600 C-10min. has the ultrafine grain with the mean sizeof 0.5mum,giving 0.2% proof stress of about 70kgf/mm 2.2. The strength of 13%Cr-10%Ni-2.5%Mo-0.12%Nsteel increased in comparison with 15.5%Cr-10%Ni steel. It is attributed to refining of austenitegrain rather than solid soution hardening by molybdenum and nitrogen. After reversion treatment，this high strength steel has an austenite grain of 0.2mum, giving 0.2% proof stress of 100kgf/mma tensile strength of 120kgf/mm^2 and an elongation of 35% respectively.3. The texture of this highstrength steel was of {110} type. A deep drawability not ough，because both values of n and r were relatively lower than 18-8 austenitic steel. Therefore,thissteel will be able to used for the parts not required the severe forming, for example，austenitic stainless for railroad vihicles。

Investigators have already succeeded in super grain refining to 0.5mum of metastable austenitic stainless steels based of Fe-Cr-Ni system by martensite austenite reversion. The purpose of this present work is to found that three conditions should be satisfied for the grain super refining : (1) Austenite transforms to martensite almost all, by cold rolling. (2) The martensite perfectly reverses to austenite by annealing at relatively low temperature. (3) Ms temperature of the reversed austenite exist under the room temperature. Next purpose is to determine the suitable chemical composition for the high strength steel containing molybdenum and nitrogen. Lastly, using 370kg ingot, the thermo-mechanical treatments were carried out on the pilot line in steel works. And then, the tensile properties and press formabilities were examined. Results obtained were as follows;1. Typical composition of ultrafine grain steel in Fe-Cr-Ni system was Fe-15.5%Cr-10%Ni. A specimen with 90% cold rolling prior to annealing of 600゜C-10min. has the ultrafine grain with the mean size of 0.5mum,giving 0.2% proof stress of about 70kgf/mm^2.2. The strength of 13%Cr-10%Ni-2.5%Mo-0.12%N steel increased in comparison with 15.5%Cr-10%Ni steel. It is attributed to refining of austenite grain rather than solid soution hardening by molybdenum and nitrogen. After reversion treatment, this high strength steel has an austenite grain of 0.2mum, giving 0.2% proof stress of 100kgf/mm^2, a tensile strength of 120kgf/mm^2 and an elongation of 35% respectively.3. The texture of this high strength steel was of {110} type. A deep drawability was not enough, because both values of n and r were relatively lower than 18-8 austenitic steel. Therefore,this steel will be able to used for the parts not required the severe forming, for example, austenitic stainless for railroad vihicles.