Posco Co., Ltd

Jong-Chan Park

Min-Seo Koo

Jae-Hoon Lee

Young-Roc Im

Jong-Kweon Kim

Myung-Soo Kim

Ki-Cheol Kang

Jai-Hyun Kwak

Man-Young Park

Tae-Oh Lee

Eul-Yong Choi

Tae-Kyo Han

Il-Hyun Kim

Seul-Gi So

Provided are a high-strength steel sheet and a method for manufacturing same, the high-strength steel sheet including: an alloy system having C, Si, Mn, Cr, Al, Nb, Ti, B, P, S, N, and the remainder of Fe and other inevitable impurities. The contents of C, Si, and Al satisfy equation (1) below. The microstructure includes, by an area fraction, greater than 50% to 70% or less of tempered martensite, and the remainder of residual austenite, fresh martensite, ferrite, and bainite, in which a cementite phase as a second phase is precipitated and distributed in an area fraction of 1-3% between bainite laths, or at a lath on the tempered martensite or grain boundaries. [Equation (1)][C]+([Si]+[Al])/5; 0.35 wt. % (wherein [C], [Si], and [Al] denote wt % of C, Si, and Al, respectively.)

High-strength steel sheet and manufacturing method thereof

A galvanized steel sheet including a base steel sheet, and a zinc-based plating layer on the surface of the base steel sheet. The base steel sheet may include a first surface layer region corresponding to a depth of 25 μm from an interface between the base steel sheet and the zinc-based plating layer in a thickness direction of the base steel sheet, and a second surface layer region adjacent to the first surface layer region and corresponding to a depth of 25 μm to 50 μm in the thickness direction of the base steel sheet. A fraction of ferrite contained in the first surface layer region is 55 area % or more, an average grain size of the ferrite contained in the first surface layer region is 2 to 10 μm, and a fraction of ferrite contained in the second surface layer region is 30 area % or more.

High-strength hot-dipped galvanized steel sheet having excellent surface quality and spot weldability, and manufacturing method therefor

Provided is a high-strength steel sheet including: 0.12% to less than 0.17% of carbon (C), 0.3% to 0.8% of silicon (Si), 2.5% to 3.0% of manganese (Mn), 0.4% to 1.1% of chromium (Cr), 0.01% to 0.3% of aluminum (Al), 0.01% to 0.03% of niobium (Nb), 0.01% to 0.03% of titanium (Ti), 0.001% to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.01% or less of sulfur(S): 0.01% or less of nitrogen (N), and a balance of iron (Fe) and inevitable impurities. The contents of C, Si, and Al satisfy: [C]+([Si]+[Al])/5≤0.35 wt %. A microstructure includes more than 1% to 4% or less of retained austenite, more than 10% to 20% or less of fresh martensite, 5% or less (excluding 0%) of ferrite, more than 50% to 70% or less of tempered martensite, and a balance of bainite.

Provided is a high-strength cold rolled steel sheet, a high-strength hot-dip galvanized steel sheet manufactured using the cold rolled steel sheet, and manufacturing methods therefor, the high-strength cold rolled steel sheet comprising, by wt %, 0.17-0.21% of carbon (C), 0.3-0.8% of silicon (Si), 2.7-3.3% of manganese (Mn), 0.3-0.7% of chromium (Cr), 0.01-0.3% of aluminum (Al), 0.01-0.03% of titanium (Ti), 0.001-0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen (N) and the balance of iron (Fe) and other inevitable impurities, wherein the amounts of carbon (C), silicon (Si) and aluminum (Al) satisfy the following mathematical relation (1). [Mathematical relation (1)] [C]+([Si]+[Al])/5&#x2264;0.35% (wherein [C], [Si] and [Al] respectively mean the wt % of C, Si and Al.)

High-strength cold rolled steel sheet having high hole expansion ratio, highstrength hot-dip galvanized steel sheet, and manufacturing methods therefor

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Gwangyang-si, South Korea

Jong-Chan Park