Effect of Mn Partitioning during Intercritical Annealing on Following γ→α Transformation and Resultant Mechanical Properties of Cold-rolled Dual Phase Steels

• Published in: ISIJ International Vol. 51; no. 5; pp. 818 - 825
• Main Authors: Toji, Yuki, Yamashita, Takako, Nakajima, Katsumi, Okuda, Kaneharu, Matsuda, Hiroshi, Hasegawa, Kohei, Seto, Kazuhiro
• Format: Journal Article
• Language: English
• Published: Tokyo The Iron and Steel Institute of Japan 01.01.2011; Iron and Steel Institute of Japan
• Subjects: Applied sciences; dual phase steel; Exact sciences and technology; Forming; intercritical annealing; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Production techniques; Rolling; α→γ transformation; γ→α transformation; Cold rolling; Annealing; Heat treatment; Mechanical properties; Forming; Cold rolled sheet; α→γ transformation; γ→ α transformation; intercritical annealing; Dual phase steel
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.51.818

Sequential transformation phenomena of α→γ→α during intercritical annealing and subsequent cooling were investigated to achieve a more advanced control of mechanical properties in a low-carbon cold-rolled DP steel sheet with a chemical composition of 0.13mass%C-1.4mass%Si-2.0mass%Mn. The steel was intercritically annealed at 1073 K for 0–1000 s, then air-cooled to 873–1073 K (quenching start temperature: Tq), followed by water-quenching. The tensile strength increased with an extension of the annealing time, especially at the low Tq, corresponding to the increase in the volume fraction of martensite. This means the γ→α transformation during air-cooling was delayed by extending the annealing time. Microstructural observation and elemental analysis by EPMA indicated that the volume fraction of γ during annealing was almost saturated after annealing for 250 s, whereas the Mn content in γ was still increasing at that time. These results suggest that the retardation of the γ→α transformation during air-cooling by extending the annealing time results from the chemical stabilization of γ by the enrichment of Mn during intercritical annealing. In order to obtain the less scattering of mechanical properties in cold-rolled DP steel sheets, precise microstructural control considering the partitioning of substitutional alloying elements during intercritical annealing is quite important.