# US 10,370,736 B2 · Ultrahigh-strength steel for welding structure with excellent toughness in welding heat-affected zones thereof, and method for manufacturing same

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- Number: US 10,370,736 B2
- Document ID: US10370736B2
- Status: Granted
- Kind: B2
- Application number: 15/104,020
- Filed: 2014-12-22
- Published: 2017-01-05
- Granted: 2019-08-06
- Earliest claimed: 2013-12-24
- Assignees: Posco
- Applicants: Posco
- Inventors: Hong-Chul Jeong; Ho-Soo Kim
- Web page: https://patentgrep.com/patent/US10370736B2

## CPC

- C21D 9/46: for sheet metals
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 8/0226: {Hot rolling}
- C22C 38/00: Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00)
- C22C 38/001: {containing N}
- C22C 38/002: {containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60}
- C22C 38/02: containing silicon
- C22C 38/06: containing aluminium
- C22C 38/14: containing titanium or zirconium
- C22C 38/42: with copper
- C22C 38/44: with molybdenum or tungsten
- C22C 38/46: with vanadium
- C22C 38/48: with niobium or tantalum
- C22C 38/50: with titanium or zirconium
- C22C 38/54: with boron
- C22C 38/58: with more than 1.5% by weight of manganese
- C21D 2211/002: Bainite
- C21D 2211/005: Ferrite
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)
- C22C: ALLOYS (treatment of alloys C21D, C22F)

## Abstract

Provided is a ultrahigh strength steel for a welded structure having superior toughness in a weld heat-affected zone (HAZ) comprising: by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.1% to 0.6%, manganese (Mn): 1.5% to 3.0%, nickel (Ni): 0.1% to 0.5%, molybdenum (Mo): 0.1% to 0.5%, chromium (Cr): 0.1% to 1.0%, copper (Cu): 0.1% to 0.4%, titanium (Ti): 0.005% to 0.1%, niobium (Nb): 0.01% to 0.03%, boron (B): 0.0003% to 0.004%, aluminum (Al): 0.005% to 0.1%, nitrogen (N): 0.001% to 0.006%, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, iron (Fe) as a residual component thereof, and inevitable impurities.

## Claims

### Claim 1

A steel sheet for a welded structure having superior toughness in a weld heat-affected zone (HAZ), the steel sheet comprising:
by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.1% to 0.6%, manganese (Mn): 1.5% to 3.0%, nickel (Ni): 0.1% to 0.5%, molybdenum (Mo): 0.1% to 0.5%, chromium (Cr): 0.1% to 1.0%, copper (Cu): 0.1% to 0.4%, titanium (Ti): 0.005% to 0.1%, niobium (Nb): 0.01% to 0.03%, boron (B): 0.0003% to 0.004%, aluminum (Al): 0.005% to 0.1%, nitrogen (N): 0.001% to 0.006%, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, iron (Fe) as a residual component thereof, and inevitable impurities,
wherein the Ti and N component contents satisfy Formula 1 below, the N and B component contents satisfy Formula 2 below, and the Mn, Cr, Mo, Ni, and Nb component contents satisfy Formula 3 below; and
wherein the steel sheet comprises: a microstructure including, by area fraction, acicular ferrite in an amount of 30% to 40% and bainite in an amount of 60% to 70%, and the microstructure comprises: TiN precipitates having a size of 0.01 μm to 0.05 μm and a density of 1.0×103/mm2 or more, and being dispersed at an interval of 50 μm or less,
3.5≤Ti/N≤7.0 [Formula 1]
1.5≤N/B≤4.0 [Formula 2]
4.0≤2Mn+Cr+Mo+Ni+3Nb≤7.0 [Formula 3]
wherein in the Formulas 1 to 3, respective component units are wt %.

### Claim 2 (depends on 1)

The steel sheet of claim 1, wherein the steel sheet further comprises, by wt %, one or more elements among vanadium (V): 0.005% to 0.2%, calcium (Ca): 0.0005% to 0.005%, and rare earth elements (REM): 0.005% to 0.05%.

### Claim 3 (depends on 1)

The steel sheet of claim 1, further comprising: a weld HAZ formed in the steel sheet, wherein the weld HAZ comprises: a microstructure including, by area fraction, acicular ferrite in an amount of 30% to 40% and bainite in an amount of 60% to 70%, and an austenite crystal grain having a size of less than 200 μm.

### Claim 4

A method of manufacturing a steel sheet for a welded structure having a weld heat-affected zone (HAZ), the method comprising:
preparing a slab comprising, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.1% to 0.6%, manganese (Mn): 1.5% to 3.0%, nickel (Ni): 0.1% to 0.5%, molybdenum (Mo): 0.1% to 0.5%, chromium (Cr): 0.1% to 1.0%, copper (Cu): 0.1% to 0.4%, titanium (Ti): 0.005% to 0.1%, niobium (Nb): 0.01% to 0.03%, boron (B): 0.0003% to 0.004%, aluminum (Al): 0.005% to 0.1%, nitrogen (N): 0.001% to 0.006%, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, iron (Fe) as a residual component thereof, and inevitable impurities, wherein the Ti and N component contents satisfy Formula 1 below, the N and B component contents satisfy Formula 2 below, and the Mn, Cr, Mo, Ni, and Nb component contents satisfy Formula 3 below;
heating the slab to a temperature of 1100° C. to 1200° C.;
hot finish rolling the heated slab at a temperature of 870° C. to 900° C. to form a hot finish rolled steel sheet; and
cooling the hot finish rolled steel sheet to a temperature of 420° C. to 450° C. at a cooling speed of 4° C./s to 10° C./s to form a hot rolled steel sheet, wherein the hot-rolled steel sheet has a microstructure including, by area fraction, acicular ferrite in an amount of 30% to 40% and bainite in an amount of 60% to 70%, and the microstructure further comprises TiN precipitates having a size of 0.01 μm to 0.05 μm and a density of 1.0×103/mm2 or more, and being dispersed at an interval of 50 μm or less,
3.5≤Ti/N≤7.0 [Formula 1]
1.5≤N/B≤4.0 [Formula 2]
4.0≤2Mn+Cr+Mo+Ni+3Nb≤7.0 [Formula 3].

### Claim 5 (depends on 4)

The method of claim 4, wherein the slab further comprises, by wt %, one or more elements among V vanadium (V): 0.005% to 0.2%, calcium (Ca): 0.0005% to 0.005%, and rare earth elements (REM): 0.005% to 0.05%.

### Claim 6 (depends on 4)

The method of claim 4, further comprising; forming a weld HAZ by welding the hot-rolled steel sheet, wherein the weld HAZ has a microstructure including, by area fraction, acicular ferrite in an amount of 30% to 40% and bainite in an amount of 60% to 70%, as a microstructure, and an austenite crystal grain having a size of less than 200 μm.