Other

Posco

Yeon Sang Ahn

Il Hiun Kuk

Won Pyo Hong

Jun Wha Hong

Joo Hyun Ryu

Thak Sang Byun

Chang Hyo Seo

Kyoo Young Lee

Hong Deuk Kim

Provided is a cold-rolled steel sheet, a galvanized steel sheet and a galvannealed steel sheet, comprising, by wt %, 0.06-0.15% of C, 1.2% or less (excluding 0%) of Si, 1.7-2.7% of Mn, 0.15% or less (excluding 0%) of Mo, 1.0% or less (excluding 0%) of Cr, 0.1% or less (excluding 0%) of P, 0.01% or less (excluding 0%) of S, 0.001-0.04% of Ti, 0.001-0.04% of Nb, 0.01% or less (excluding 0%) of N, 0.01% or less (excluding 0%) of B, and a remainder of Fe and unavoidable impurities, wherein the Si, C, Mn, Mo and Cr contents satisfy Relationship 1, and wherein a microstructure comprising, in area %, 10-70% of ferrite, 10-50% of the sum of bainite and retained austenite, and a remainder of fresh martensite, wherein a ratio (Mb/Mt) is 60% or more, and methods of manufacturing these steel sheets.([Si]+[C]&#xd7;3)/([Mn]+[Mo]+[Cr])&#x2265;0.18.&#x2003;&#x2003;[Relationship 1]

Cold-rolled steel sheet with excellent formability, galvanized steel sheet, and manufacturing method thereof

The present invention relates to the heat treatment processes for manufacturing high toughness SA 508 Gr.3 steels including the intercritical heat treatment step in addition to the conventional heat treatment process, wherein the intercritical heat treatment(IHT) is added between the quenching step and the tempering step and is performed at 680&deg; C. to 750&deg; C., which is the ferrite/austenite two phase region, for 1 hour to 8 hours. When compared with the conventional heat treatment process the room temperature impact energy and the upper shelf energy of the steels manufactured by the invention increase significantly and the ductile-to-brittle transition temperature decreases. Also the present invention relates to the modified tempering processes for manufacturing high toughness SA 508 Gr.3 steels, wherein the tempering after the intercritical heat treatment is performed at the temperature lower than 635&deg; C. The modified tempering is performed to compensate the decrease of strength due to the intercritical heat treatment and to additionally increase the toughness of the intercritically heat treated SA 508 Gr.3 steels.

Intercritical heat treatment process for toughness improvement of SA 508 GR.3 steel

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Gwangyang-si, South Korea

Yeon Sang Ahn