# US 12,006,562 B2 · Hot dip galvanized steel sheet and method for producing same

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- Number: US 12,006,562 B2
- Document ID: US12006562B2
- Status: Granted
- Kind: B2
- Application number: 17/425,234
- Filed: 2020-02-06
- Published: 2022-03-24
- Granted: 2024-06-11
- Earliest claimed: 2019-02-06
- Assignees: Nippon Steel Corporation
- Applicants: Nippon Steel Corporation
- Inventors: Takafumi Yokoyama; Hiroyuki Kawata; Kunio Hayashi; Yuji Yamaguchi; Satoshi Uchida
- Web page: https://patentgrep.com/patent/US12006562B2

## CPC

- C22C 38/58: with more than 1.5% by weight of manganese
- C23C 2/12: Aluminium or alloys based thereon
- C21D 1/185: {from an intercritical temperature}
- 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 1/26: Methods of annealing
- C21D 1/76: Adjusting the composition of the atmosphere
- C21D 8/00: Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment (hardening articles or materials formed by forging or rolling with no further heating beyond that required for the formation C21D1/02)
- C21D 8/021: involving particular fabrication steps or treatments of ingots or slabs
- C21D 8/0226: {Hot rolling}
- C21D 8/0236: {Cold rolling}
- C21D 8/0436: {Cold rolling}
- C21D 9/46: for sheet metals
- 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 38/005: {containing rare earths, i.e. Sc, Y, Lanthanides}
- C22C 38/008: {containing tin}
- 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 38/34: with more than 1.5% by weight of silicon
- C22C 38/38: with more than 1.5% by weight of manganese
- 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 38/52: with cobalt
- C22C 38/54: with boron
- C22C 38/60: containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
- 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 2/29: {Cooling or quenching}
- 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 2211/009: Pearlite
- 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)
- 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)

## Abstract

Provided are a hot dip galvanized steel sheet comprising a base steel sheet wherein the base steel sheet has a predetermined composition and contains ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, remaining structures consist of bainite, when defining a region having a hardness of 90% or less of the hardness at a position of ¼ thickness to the base steel sheet side from an interface of the base steel sheet and a hot dip galvanized layer as a “soft layer”, there is a soft layer having a thickness of 10 μm or more at the base steel sheet side from the interface, the soft layer contains tempered martensite, and an increase rate in a thickness direction of an area % of tempered martensite from the interface to the inside of the base steel sheet inside the soft layer is 5.0%/μm or less, and a method for producing the same.

## Claims

### Claim 1

A hot dip galvanized steel sheet comprising a base steel sheet and a hot dip galvanized layer on at least one surface of the base steel sheet, wherein the base steel sheet has a chemical composition comprising, by mass %,
C: 0.050% to 0.350%,
Si: 0.10% to 2.50%,
Mn: 1.00% to 3.50%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001% to 1.500%,
N: 0.0100% or less,
O: 0.0100% or less,
Ti: 0% to 0.200%,
B: 0% to 0.0100%,
V: 0% to 1.00%,
Nb: 0% to 0.100%,
Cr: 0% to 2.00%,
Ni: 0% to 1.00%,
Cu: 0% to 1.00%,
Co: 0% to 1.00%,
Mo: 0% to 1.00%,
W: 0% to 1.00%,
Sn: 0% to 1.00%,
Sb: 0% to 1.00%,
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
Ce: 0% to 0.0100%,
Zr: 0% to 0.0100%,
La: 0% to 0.0100%,
Hf: 0% to 0.0100%,
Bi: 0% to 0.0100%,
REM other than Ce and La: 0% to 0.0100% and
a balance of Fe and impurities,
a steel microstructure at a range of ⅛ thickness to ⅜ thickness centered about a position of ¼ thickness from a surface of the base steel sheet contains, by area %,
ferrite: 0% to 50%,
retained austenite: 0% to 30%,
tempered martensite: 5% or more,
fresh martensite: 0% to 10%, and
pearlite and cementite in total: 0% to 5%,
when there are remaining structures, the remaining structures consist of bainite,
when defining a region having a hardness of 90% or less of the hardness at a position of ¼ thickness to the base steel sheet side from an interface of the base steel sheet and the hot dip galvanized layer as a “soft layer”, there is a soft layer having a thickness of 10 μm or more at the base steel sheet side from the interface,
the soft layer contains tempered martensite, and
an increase rate in a thickness direction of an area % of tempered martensite from the interface to the inside of the base steel sheet inside the soft layer is 5.0%/μm or less.

### Claim 2 (depends on 1)

The hot dip galvanized steel sheet according to claim 1, wherein the steel microstructure further contains, by area %, retained austenite: 6% to 30%.

### Claim 3 (depends on 1)

A method for producing the hot dip galvanized steel sheet according to claim 1, comprising:
a hot rolling step of hot rolling a slab having the chemical composition of the base steel sheet according to claim 1 to obtain a hot rolled steel sheet;
a cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; and
a hot dip galvanizing step of hot dip galvanizing the cold rolled steel sheet to obtain the hot dip galvanized steel sheet;
wherein:
(A) the cold rolling step satisfies the conditions of the following (A1) and (A2):
(A1) a rolling line load satisfies the following formula (1) and cold rolling with a rolling reduction of 6% or more is performed one time or more:
13≤A/B≤35 formula (1)
wherein in formula (1), A is the rolling line load in kgf/mm and B is the tensile strength of the hot rolled steel sheet in kgf/mm2;
(A2) a total cold rolling reduction is 30 to 80%; and
(B) the hot dip galvanizing step comprises: subjecting the cold rolled steel sheet to a first soaking treatment followed by a first cooling to obtain a first cooled steel sheet; subjecting the first cooled steel sheet to a second soaking treatment to obtain a second soaked steel sheet; dipping the second soaked steel sheet in a hot dip galvanizing bath to obtain a coated steel sheet; a second cooling of the coated steel sheet to obtain a second cooled steel sheet; and subjecting the second cooled steel sheet to a third soaking treatment; and wherein the hot dip galvanizing step further satisfies the following conditions (B1) to (B6):
(B1) in the first soaking treatment, the cold rolled steel sheet is heated to a maximum heating temperature in an atmosphere satisfying the following formulas (2) and (3), wherein the maximum heating temperature is at least Ac1+30° C. or more and 950° C. or less, and an average heating rate from 650° C. to the maximum heating temperature is 0.5° C./s to 10.0° C./s;
(B2) in the first soaking treatment, the cold rolled steel sheet which has been heated to the maximum heating temperature is held at the maximum heating temperature for 1 second to 1000 seconds;
(B3) in the first cooling, an average cooling rate in a temperature range of 700° C. to 600° C. is 10° C./s to 100° C./s;
(B4) in the second soaking treatment, the first cooled steel sheet is held in a range of 300° C. to 600° C. for 80 seconds to 500 seconds in an atmosphere satisfying the following formulas (4) and (5);
(B5) in the second cooling, cooling the coated steel sheet to a temperature of Ms-50° C. or less wherein Ms represents martensite transformation start temperature of the base steel sheet; and
(B6) in the third soaking treatment, the second cooled steel sheet is heated to a temperature region of 200° C. to 420° C., then held in the temperature region for 5 seconds to 500 seconds;
−1.10≤log(PH2O/PH2)≤−0.07 formula (2)
0.010≤PH2≤0.150 formula (3)
log(PH2O/PH2)<−1.10 formula (4)
0.0010≤PH2≤0.1500 formula (5)
wherein PH2O and PH2 represent partial pressures of water vapor and hydrogen respectively of the respective atmosphere of the conditions (B1) and (B4).