0.1%C-2%Si-5%Mn超微細フェライト+オーステナイト鋼の短時間組織形成と力学的特性に及ぼす二相域焼鈍前組織の影響

• Published in: 鉄と鋼 Vol. 105; no. 2; pp. 197 - 206
• Main Authors: 安達, 節展, 鳥塚, 史郎, 足立, 大樹, 伊東, 篤志
• Format: Journal Article
• Language: Japanese
• Published: 一般社団法人 日本鉄鋼協会 2019
• Subjects: austenite; ductility; ferrite; ferrite + cementite; intercritical annealing; martensite; Mn; prior structure; short time formation; strength; TRIP
• ISSN: 0021-1575; 1883-2954
• DOI: 10.2355/tetsutohagane.TETSU-2018-121

Ultrafine ferrite + austenite steels with the chemical composition of 0.1%C-2%Si-5wt%Mn show excellent strength (TS=1200 MPa) and high ductility (TEl=25%) balance, compared to conventional TRIP steels. This steel is expected as the third generation AHSS. This steel can be produced by a simple intercritical annealing, however, longer annealing time is necessary to obtain appropriate ferrite + austenite structure. It is difficult to produce this steel by continuous annealing process. If the annealing time can be drastically reduced, this new TRIP steels can be commercialized. We focused on the effect of the prior microstructures before annealing on the formation of ferrite + austenite structure. The effect of the prior structure is not clear. Therefore, in this study, two kind of prior structures, ultrafine grained ferrite + cementite and martensite were used in 0.1%C-2%Si-5wt%Mn steels. It was found that the prior structure of ferrite + cementite can form large amount (20%) of austenite in a very short time (600 s). This is because cementite finely dispersed in the structure effectively acts as a preferential nucleation site of reverse transformed austenite and C and Mn are concentrated in cementite to enable a short time formation of austenite. Excellent strength-ductility balance (32000 MPa%) which is superior to conventional TRIP steels is also obtained.