# US 12,180,557 B2 · Cold rolled steel sheet and method for producing same

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- Number: US 12,180,557 B2
- Document ID: US12180557B2
- Status: Granted
- Kind: B2
- Application number: 17/434,986
- Filed: 2020-03-31
- Published: 2022-05-12
- Granted: 2024-12-31
- Earliest claimed: 2019-04-08
- Assignees: Nippon Steel Corporation
- Applicants: Nippon Steel Corporation
- Inventors: Shohei Yabu; Koutarou Hayashi; Akihiro Uenishi
- Web page: https://patentgrep.com/patent/US12180557B2

## CPC

- C21D 8/0236: {Cold rolling}
- C21D 1/25: Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
- C21D 6/005: {containing Mn}
- C21D 6/008: {containing Si}
- C21D 8/0226: {Hot rolling}
- C21D 8/0247: characterised by the heat treatment
- C21D 8/0263: {following hot rolling}
- C21D 8/0273: {Final recrystallisation annealing}
- C21D 9/46: for sheet metals
- 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 38/005: {containing rare earths, i.e. Sc, Y, Lanthanides}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/08: containing nickel {(C22C38/105 takes precedence)}
- C22C 38/10: containing cobalt
- C22C 38/12: containing tungsten, tantalum, molybdenum, vanadium, or niobium
- C22C 38/14: containing titanium or zirconium
- C22C 38/16: containing copper
- C22C 38/32: with boron
- C22C 38/34: with more than 1.5% by weight of silicon
- C22C 38/38: with more than 1.5% by weight of manganese
- C23C 2/02: Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence)
- C23C 2/0224: {Two or more thermal pretreatments}
- C23C 2/024: {by cleaning or etching}
- C23C 2/06: Zinc or cadmium or alloys based thereon
- C23C 2/28: Thermal after-treatment, e.g. treatment in oil bath
- C23C 2/40: Plates; Strips
- C21D 2211/001: Austenite
- C21D 2211/002: Bainite
- C21D 2211/003: Cementite
- 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

The present invention relates to a cold rolled steel sheet containing C: 0.15% or more and 0.40% or less, Si: 0.50% or more and 4.00% or less, Mn: 1.00% or more and 4.00% or less, and sol. Al: 0.001% or more and 2.000% or less, having a metallic structure consisting of 35 to 65 area % of ferrite phases, 35 to 65 area % of hard second phases, and 0 to 5 area % of remaining phases, wherein 60% or more of the ferrite phases are recrystallized ferrite phases, an average crystal grain size defined by 15° grain boundaries is 5.0 μm or less, a maximum connecting rate of the hard second phases is 10% or more, and a two-dimensional isoperimetric constant of the hard second phases is 0.20 or less.

## Claims

### Claim 1

A cold rolled steel sheet having a chemical composition comprising, by mass %,
C: 0.15% or more and 0.40% or less,
Si: 0.50% or more and 4.00% or less,
Mn: 1.00% or more and 4.00% or less,
sol. Al: 0.001% or more and 2.000% or less,
P: 0.020% or less,
S: 0.020% or less,
N: 0.010% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more and 0.010% or less,
V: 0% or more and 1.00% or less,
Cr: 0% or more and 1.00% or less,
Mo: 0% or more and 1.00% or less,
Cu: 0% or more and 1.00% or less,
Co: 0% or more and 1.00% or less,
W: 0% or more and 1.00% or less,
Ni: 0% or more and 1.00% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less,
Zr: 0% or more and 0.010% or less, and
balance: iron and impurities, and
a metallic structure consisting of ferrite phases, hard second phases consisting of martensite phases and retained austenite phases, and remaining phases consisting of cementite phases and bainite phases, wherein
an area ratio of the ferrite phases is 35% or more and 65% or less,
an area ratio of the hard second phases is 35% or more and 65% or less,
an area ratio of the remaining phases is 0% or more and 5% or less,
60% or more of the ferrite phases are recrystallized ferrite phases,
an average crystal grain size defined by boundaries with differences in crystal orientations of 15 degrees or more is 5.0 μm or less,
a maximum connecting rate of the hard second phases is 10% or more, and
a two-dimensional isoperimetric constant of the hard second phases is 0.20 or less.

### Claim 2 (depends on 1)

The cold rolled steel sheet according to claim 1, wherein the chemical composition comprises, by mass %, one or more selected from the group consisting of
Ti: 0.001% or more and 0.200% or less,
Nb: 0.001% or more and 0.200% or less,
B: 0.0005% or more and 0.010% or less,
V: 0.005% or more and 1.00% or less,
Cr: 0.005% or more and 1.00% or less,
Mo: 0.005% or more and 1.00% or less,
Cu: 0.005% or more and 1.00% or less,
Co: 0.005% or more and 1.00% or less,
W: 0.005% or more and 1.00% or less,
Ni: 0.005% or more and 1.00% or less,
Ca: 0.0003% or more and 0.010% or less,
Mg: 0.0003% or more and 0.010% or less,
REM: 0.0003% or more and 0.010% or less, and
Zr: 0.0003% or more and 0.010% or less.

### Claim 3 (depends on 1)

The cold rolled steel sheet according to claim 1, having a hot dip galvanized layer on the surface thereof.

### Claim 4 (depends on 1)

The cold rolled steel sheet according to claim 1, having a hot dip galvannealed layer on the surface thereof.

### Claim 5 (depends on 1)

A method for producing the cold rolled steel sheet according to claim 1, comprising:
a hot rolling step of rough rolling a slab having the chemical composition according to claim 1, then finish rolling it wherein a rolling reduction of a final stage of the finish rolling is 15% or more and 50% or less and an end temperature of the finish rolling is Ar3° C. or more and 950° C. or less, cooling it down to a coiling temperature of less than 400° C. by an average cooling rate of 50° C./s or more, and coiling it at the coiling temperature,
a tempering step of tempering the hot rolled steel sheet in a temperature region of 450° C. or more and less than 600° C. under conditions of a tempering parameter ξ defined by following Formula 1 of 14000 to 18000,
a cold rolling step of pickling the tempered steel sheet, then cold rolling it by a rolling reduction of 30% or more, and
an annealing step of heating the cold rolled steel sheet in a temperature region of 500° C. to Ac1° C. by an average heating rate of 5.0° C./s or less up to a maximum heating temperature of (Ac1+10)° C. or more and (Ac3−10)° C. or less, holding it at the maximum heating temperature for 60 seconds or more, then cooling it in a temperature region of (Ac1−50)° C. to a cooling stop temperature by an average cooling rate of 20° C./s or more down to the cooling stop temperature of Ms° C. or less:
ξ=(T+273)·[log10(t/3600)+20] Formula 1:
T [° C.]: tempering temperature, t [s]: tempering time
Ac1 [° C.]=751−16×[% C]+35×[% Si]−28×[% Mn]
Ac3 [° C.]=881−353×[% C]+65×[% Si]−24×[% Mn]
Ar3 [° C.]=910−203×[% C]+44.7×[% Si]−24×[% Mn]−50×[% Ni]
Ms [° C.]=521−353×[% C]−22×[% Si]−24×[% Mn]
where % C, % Si, % Mn, and % Ni are contents [mass %] of C, Si, Mn, and Ni.

### Claim 6 (depends on 5)

The method for producing the cold rolled steel sheet according to claim 5, further comprising cooling down to the cooling stop temperature of Ms° C. or less, then holding at a temperature of 200° C. or more and 450° C. or less for 60 seconds or more and 600 seconds or less.

### Claim 7 (depends on 5)

The method for producing the cold rolled steel sheet according to claim 5, further comprising hot dip galvanization at a temperature of 430° C. or more after the annealing step to produce a cold rolled steel sheet having a hot dip galvanized layer on the surface thereof.

### Claim 8 (depends on 5)

The method for producing the cold rolled steel sheet according to claim 5, further comprising hot dip galvanization at a temperature of 430° C. or more after the annealing step, then alloying treatment at 400° C. or more and 600° C. or less to produce a cold rolled steel sheet having a hot dip galvannealed layer on the surface thereof.

### Claim 9 (depends on 6)

The method for producing the cold rolled steel sheet according to claim 6, further comprising hot dip galvanization at a temperature of 430° C. or more after the annealing step to produce a cold rolled steel sheet having a hot dip galvanized layer on the surface thereof.

### Claim 10 (depends on 6)

The method for producing the cold rolled steel sheet according to claim 6, further comprising hot dip galvanization at a temperature of 430° C. or more after the annealing step, then alloying treatment at 400° C. or more and 600° C. or less to produce a cold rolled steel sheet having a hot dip galvannealed layer on the surface thereof.