# US 2020/0399729 A1 · High strength steel sheet having excellent formability and manufacturing method thereof

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- Number: US 2020/0399729 A1
- Document ID: US20200399729A1
- Status: Published application
- Kind: A1
- Application number: 16/324,290
- Filed: 2017-08-04
- Published: 2020-12-24
- Earliest claimed: 2016-08-12
- Assignees: Posco
- Applicants: Posco
- Inventors: Sang-Ho HAN; Je-Woong LEE
- Web page: https://patentgrep.com/patent/US20200399729A1

## CPC

- C21D 9/46: for sheet metals
- C21D 6/005: {containing Mn}
- C21D 6/008: {containing Si}
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 8/0226: {Hot rolling}
- C21D 8/0236: {Cold rolling}
- C21D 8/04: to produce plates or strips for drawing, e.g. for deep-drawing
- C21D 8/0426: {Hot rolling}
- C21D 8/0436: {Cold rolling}
- C21D 8/0473: {Final recrystallisation annealing}
- C21D 9/48: deep-drawing sheets
- 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 38/004: {Very low carbon steels, i.e. having a carbon content of less than 0,01%}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/12: containing tungsten, tantalum, molybdenum, vanadium, or niobium
- C22C 38/14: containing titanium or zirconium
- 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 2/06: 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 2211/004: Dispersions; Precipitations
- 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 steel sheet including, by weight, C: 0.001% to 0.004%, Si: 0.5% or less (excluding 0%), Mn: 1.2% or less (excluding 0%), P: 0.005% to 0.12%, S: 0.01% or less, N: 0.01% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01% to 0.04%, a remainder of Fe, and unavoidable impurities, wherein the contents of Ti, N, and S satisfy the following Relationship 1, a ratio (b/a) of an average random intensity ratio (b) of an orientation group of (111) [1-10] to (111) [−1-12] to an average random intensity ratio (a) of an orientation group of (001) [1-10] to (110) [1-10] at a point t/4 (where t is a thickness of the steel sheet) in a thickness direction of the steel sheet is 2.3 or more, and a bake hardenability (BH) of the steel sheet is 4 MPa or more.

## Claims

### Claim 1

A high strength steel sheet comprising, by weight, C: 0.001% to 0.004%, Si: 0.5% or less (excluding 0%), Mn: 1.2% or less (excluding 0%), P: 0.005% to 0.12%, S: 0.01% or less, N: 0.01% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01% to 0.04%, a remainder of Fe, and unavoidable impurities,
wherein the contents of Ti, N, and S satisfy the following Relationship 1,
a ratio (b/a) of an average random intensity ratio (b) of an orientation group of (111) [1-10] to (111) [−1-12] to an average random intensity ratio (a) of an orientation group of (001) [1-10] to (110) [1-10] at a point t/4 (where t is a thickness of the steel sheet) in a thickness direction of the steel sheet is 2.3 or more, and
a bake hardenability (BH) of the steel sheet is 4 MPa or more.
−0.02≤[Ti]−(24/7)[N]−(3/2)[S]≤0.025 [Relationship 1]
where each of [Ti], [N], and [S] refers to the content (weight %) of the corresponding element.

### Claim 2 (depends on 1)

The high strength steel sheet according to claim 1, further comprising one or more selected from the group consisting of, by weight, Nb: 0.005% to 0.04%, and B: 0.002% or less (excluding 0%).

### Claim 3 (depends on 1)

The high strength steel sheet according to claim 1, wherein a pin as defined by the following Formula 1 is 80% or more.
Pin(%)={Nin/(Nin+Ngb)}×100 [Formula 1]
(where Nin refers to the number of carbides having an equivalent circular diameter of 20 nm or less present in crystal grains, and Ngb refers to the number of carbides having an equivalent circular diameter of 20 nm or less present in grain boundaries)

### Claim 4 (depends on 1)

The high strength steel sheet according to claim 1, comprising FeTiP precipitates of 0.2/μm2 or less.

### Claim 5 (depends on 1)

The high strength steel sheet according to claim 1, wherein a product of yield strength (YS) and average plastic anisotropy coefficient (Lankford value, r-value) is 290 MPa or more.

### Claim 6

A manufacturing method of a high strength steel sheet, comprising:
hot rolling a steel slab comprising, by weight, C: 0.001% to 0.004%, Si: 0.5% or less (excluding 0%), Mn: 1.2% or less (excluding 0%), P: 0.005% to 0.12%, S: 0.01% or less, N: 0.01% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01% to 0.04%, a remainder of Fe, and unavoidable impurities, to obtain a hot-rolled steel sheet;
coiling the hot-rolled steel sheet at a temperature within a range of 450° C. to 750° C.;
cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 75% or more to obtain a cold-rolled steel sheet;
heating the cold-rolled steel sheet to an annealing temperature within a range of 830° C. to 880° C., and continuous annealing the heated cold-rolled steel sheet at the annealing temperature for an annealing time of 30 to 80 seconds;
cooling the continuous annealed cold-rolled steel sheet to a temperature within a range of 650° C. or lower at a rate of 2 to 10° C./sec; and
temper rolling the cooled cold-rolled steel sheet at a reduction ratio of 0.3% to 1.6%, and
wherein an average heating rate from a temperature (recrystallization start temperature+20° C.) to the annealing temperature at a time of heating the cold-rolled steel sheet is 5° C./sec or lower.

### Claim 7 (depends on 6)

The manufacturing method according to claim 6, further comprising one or more selected from the group consisting of, by weight, Nb: 0.005% to 0.04%, and B: 0.002% or less (excluding 0%).

### Claim 8 (depends on 6)

The manufacturing method according to claim 6, wherein a finish rolling temperature during the hot rolling operation is Ar3 (° C.) or higher.

### Claim 9 (depends on 8)

The manufacturing method according to claim 8, wherein an average cooling rate from the finish rolling temperature to the coiling temperature is 10 to 200° C./sec.

### Claim 10 (depends on 6)

The manufacturing method according to claim 6, wherein the annealing temperature (T,° C.) and the annealing time (t, sec) during the continuous annealing operation satisfy the following Relationship 2.
30≤0.001*T*t≤70 [Relationship 2]

### Claim 11 (depends on 6)

The manufacturing method according to claim 6, further comprising hot-dip galvanizing a surface of the temper rolled cold-rolled steel sheet.

### Claim 12 (depends on 11)

The manufacturing method according to claim 11, further comprising performing an alloy heat treatment at a temperature within a range of 450° C. to 600° C. after the hot-dip galvanizing operation.