# US 12,180,571 B2 · High-strength cold rolled steel sheet and galvannealed steel sheet having excellent burring property, and manufacturing method therefor

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- Number: US 12,180,571 B2
- Document ID: US12180571B2
- Status: Granted
- Kind: B2
- Application number: 17/312,753
- Filed: 2019-12-19
- Published: 2022-02-24
- Granted: 2024-12-31
- Earliest claimed: 2018-12-19
- Assignees: Posco
- Applicants: Posco
- Inventors: Hang-Sik Cho; Jai-Hyun Kwak
- Web page: https://patentgrep.com/patent/US12180571B2

## CPC

- C21D 9/48: deep-drawing sheets
- C22C 38/38: with more than 1.5% by weight of manganese
- C21D 1/19: by interrupted quenching
- C21D 1/20: Isothermal quenching, e.g. bainitic hardening
- C21D 1/25: Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
- C21D 1/84: Controlled slow cooling (cooling-beds for metal rolling B21B43/00)
- C21D 6/002: {containing Cr (C21D6/004 takes precedence)}
- C21D 6/005: {containing Mn}
- C21D 6/008: {containing Si}
- C21D 8/0236: {Cold rolling}
- C21D 8/0436: {Cold rolling}
- C21D 8/0447: {characterised by the heat treatment}
- C21D 9/46: for sheet metals
- C22C 38/02: containing silicon
- C22C 38/06: containing aluminium
- C22C 38/22: with molybdenum or tungsten
- C22C 38/28: with titanium or zirconium
- C22C 38/32: with boron
- C22C 38/34: with more than 1.5% by weight of silicon
- C23C 2/06: Zinc or cadmium or alloys based thereon
- C21D 2211/001: Austenite
- C21D 2211/002: Bainite
- C21D 2211/005: Ferrite
- C21D 2211/008: Martensite
- 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)
- C23C: COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL (making metal-coated products by extrusion B21C23/22; covering with metal by connecting pre-existing layers to articles, see the relevant places, e.g. B21D39/00, B23K; metallising of glass C03C; metallising mortars, concrete, artificial stone, ceramics or natural stone C04B41/00; enamelling of, or applying a vitreous layer to, metals C23D; treating metal surfaces or coating of metals by electrolysis or electrophoresis C25D; single-crystal film growth C30B; by metallising textiles D06M11/83; decorating textiles by locally metallising D06Q1/04)

## Abstract

A high strength cold rolled steel sheet having excellent burring properties includes: by weight %, 0.13-0.25% of carbon (C), 1.0-2.0% of silicon (Si), 1.5-3.0% of manganese (Mn), 0.08-1.5% of aluminum (Al)+chromium (Cr)+molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur(S), 0.01% or less of nitrogen (N), and the balance of Fe and inevitable impurities; and, by area fraction, 3-25% of ferrite, 20-40% of martensite, 5-20% of residual austenite. The ferrite has an average grain size of 2 μm or less at the reference point of t/4 (wherein t refers to a steel sheet thickness), with the average ratio between lengths in the thickness direction and in the rolling direction being 1.5 or less.

## Claims

### Claim 1

A cold rolled steel sheet comprising:
by weight %, 0.13-0.25% of carbon (C), 1.0-2.0% of silicon (Si), 1.5-3.0% of manganese (Mn), 0.08-1.5% of aluminum (Al)+chromium (Cr)+molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur(S), 0.01% or less of nitrogen (N), and a balance of Fe, and unavoidable impurities; and
by area fraction, 8-25% of ferrite, 20-40% of martensite, and 5-20% of retained austenite, wherein, based on a t/4 point where t is a steel sheet thickness, the ferrite has an average grain size of 2 μm or less, and an average value of a ratio of a ferrite length in a rolling direction of a steel sheet with respect to a length of the ferrite in a thickness direction of the steel sheet is 1.5 or less.

### Claim 2 (depends on 1)

The cold rolled steel sheet of claim 1, further comprising: 15 to 50% of bainite in an area fraction.

### Claim 3 (depends on 1)

The cold rolled steel sheet of claim 1, wherein the martensite is composed of tempered martensite and fresh martensite,
wherein a proportion of the tempered martensite in the martensite exceeds 50 area %.

### Claim 4 (depends on 1)

The cold rolled steel sheet of claim 1, wherein the ferrite is contained in an amount of 3 to 15% by area fraction.

### Claim 5 (depends on 1)

The cold rolled steel sheet of claim 1, wherein the average value of a ratio is 0.5 or more.

### Claim 6 (depends on 1)

The cold rolled steel sheet of claim 1, further comprising, by weight %, at least one of boron (B): 0.001-0.005% and titanium (Ti): 0.005-0.04%.

### Claim 7 (depends on 1)

The cold rolled steel sheet of claim 1, wherein the aluminum (Al) is contained in an amount of 0.01 to 0.09% by weight.

### Claim 8 (depends on 1)

The cold rolled steel sheet of claim 1, wherein the chromium (Cr) is contained in an amount of 0.01 to 0.7% by weight.

### Claim 9 (depends on 8)

The cold rolled steel sheet of claim 8, wherein the chromium (Cr) is contained in an amount of 0.2 to 0.6% by weight.

### Claim 10 (depends on 1)

The cold rolled steel sheet of claim 1, wherein the molybdenum (Mo) is contained in an amount of 0.02 to 0.08% by weight.

### Claim 11 (depends on 1)

The cold rolled steel sheet of claim 1, further comprising: a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion ratio (HER) of 25% or more.

### Claim 12 (depends on 11)

The cold rolled steel sheet of claim 11, wherein the hole expansion ratio (HER) is 30% or more.

### Claim 13 (depends on 1)

The cold rolled steel sheet of claim 1, further comprising: an alloyed hot-dip galvanized layer disposed on a surface of the cold rolled steel sheet.

### Claim 14 (depends on 1)

A method of manufacturing the cold rolled steel sheet of claim 1, the method comprising:
cold rolling a steel material, and then, heating the steel material until the steel material is completely transformed into austenite, the steel material including, by weight %, carbon (C): 0.13 to 0.25%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 3.0%, aluminum (Al)+chromium (Cr)+molybdenum (Mo): 0.08 to 1.5%, phosphorus (P): 0.1% or less, sulfur(S): 0.01% or less, nitrogen (N): 0.01% or less, and a balance of Fe and unavoidable impurities,
slowly cooling the heated steel material at a cooling rate of 5 to 12° C./s to a slow cooling stop temperature of 630 to 670° C., and then maintaining the slow cooling stop temperature for 10 to 90 seconds,
rapidly cooling the slow-cooled steel material at a cooling rate of 7 to 30° C./s to a temperature range of a martensitic transformation end temperature (Mf) or more and a martensitic transformation start temperature (Ms) or less, and
partitioning treating by maintaining the rapidly-cooled steel material for 300 to 600 seconds at a temperature exceeding the martensitic transformation start temperature (Ms) and a bainite transformation start temperature (Bs) or less.

### Claim 15 (depends on 14)

The method of claim 14, wherein the steel material further comprises, by weight %, at least one of boron (B): 0.001-0.005% and titanium (Ti): 0.005-0.04%.

### Claim 16 (depends on 14)

The method of claim 14, wherein the aluminum (Al) is contained in the steel material in an amount of 0.01 to 0.09% by weight.

### Claim 17 (depends on 14)

The method of claim 14, wherein the chromium (Cr) is contained in the steel material in an amount of 0.01 to 0.7% by weight.

### Claim 18 (depends on 14)

The method of claim 14, wherein the chromium (Cr) is contained in the steel material in an amount of 0.2% to 0.6% by weight.

### Claim 19 (depends on 14)

The method of claim 14, wherein the molybdenum (Mo) is contained in the steel material in an amount of 0.02 to 0.08% by weight.

### Claim 20 (depends on 14)

The method of claim 14, further comprising:
forming a hot-dip galvanized layer on a surface of the cold rolled steel sheet and performing alloy processing thereon.