High-strength hot-dipped galvanized steel sheet having excellent surface quality and spot weldability, and manufacturing method therefor

Patent metadata

Field	Value
Publication number	US-12331405-B2
Application number	US-202118268149-A
Country	US
Kind code	B2
Filing date	Dec 7, 2021
Priority date	Dec 21, 2020
Publication date	Jun 17, 2025
Grant date	Jun 17, 2025

Abstract

A galvanized steel sheet including a base steel sheet, and a zinc-based plating layer on the surface of the base steel sheet. The base steel sheet may include a first surface layer region corresponding to a depth of 25 μm from an interface between the base steel sheet and the zinc-based plating layer in a thickness direction of the base steel sheet, and a second surface layer region adjacent to the first surface layer region and corresponding to a depth of 25 μm to 50 μm in the thickness direction of the base steel sheet. A fraction of ferrite contained in the first surface layer region is 55 area % or more, an average grain size of the ferrite contained in the first surface layer region is 2 to 10 μm, and a fraction of ferrite contained in the second surface layer region is 30 area % or more.

First claim

The invention claimed is:

1. A galvanized steel sheet, comprising:
a base steel sheet; and
a zinc-based plating layer provided on a surface of the base steel sheet, wherein the base steel sheet includes a first surface layer region corresponding to a depth of 25 μm from an interface between the base steel sheet and the zinc-based plating layer in a thickness direction of the base steel sheet and a second surface layer region adjacent to the first surface layer region and corresponding to a depth of 25 μm to 50 μm in the thickness direction of the base steel sheet, a fraction of ferrite contained in the first surface layer region is 55 area % or more, an average grain size of the ferrite contained in the first surface layer region is 2 to 10 μm, a fraction of ferrite contained in the second surface layer region is 30 area % or more, and an average grain size of ferrite contained in the second surface layer region is 1.35 to 7 μm, an average depth (a) of an internal oxidation layer formed on the base steel sheet is 2 μm or more, a difference (b−c) between an average depth (b) of the internal oxidation layer at an edge portion of a plated steel sheet in a width direction and an average depth (c) of the internal oxidation layer at a center portion of the plated steel sheet in the width direction exceeds zero,
the average depth (b) of the internal oxidation layer at an edge portion side is an average value of a depth of the internal oxidation layer measured at a point 0.5 cm apart from an edge of the plated steel sheet in a width direction toward a central portion of the plated steel sheet in the width direction of the plated steel sheet and a point 1.0 cm apart from the edge of the plated steel sheet in the width direction toward the central portion of the plated steel sheet in the width direction of the plated steel sheet,
the average depth (c) of the internal oxidation layer at the central portion is an average value of a depth of the internal oxidation layer measured at a point 15 cm apart from the edge of the plated steel sheet in the width direction toward the central portion of the plated steel sheet in the width direction of the plated steel sheet and a point 30 cm apart from the edge of the plated steel sheet in the width direction toward the central portion of the plated steel sheet in the width direction of the plated steel sheet, and a depth of the internal oxidation layer measured at the center of the plated steel sheet in the width direction, and
the average depth (a) of the internal oxidation layer formed on the base steel sheet is the average value of the average depth (b) of the internal oxidation layer at the edge portion side and the average depth (c) of the internal oxidation layer at the central portion.

2. The galvanized steel sheet of claim 1 , wherein the fraction and average grain size of the ferrite contained in the first surface layer region and the second surface layer region satisfy the following relational expressions 1 and 2,
F 2*100/ F 1≥65(%)  [Relational Expression 1]

in relational expression 1, F1 denotes the fraction (area %) of the ferrite contained in the first surface layer region, and F2 denotes the fraction (area %) of the ferrite contained in the second surface layer region,
( S 1− S 2)*100/ S 2≤17(%)  [Relational Expression 2]

in relational expression 2, S1 denotes the average grain size (μm) of the ferrite contained in the first surface layer region, and S2 denotes the average grain size (μm) of the ferrite contained in the second surface layer region.

3. The galvanized steel sheet of claim 1 , wherein a ratio of an average hardness of the first surface layer region to an average hardness of a central portion of the base steel sheet is 90% or less, and
a ratio of an average hardness of the second surface layer region to the average hardness of the central portion of the base steel sheet is 95% or less.

4. The galvanized steel sheet of claim 1 , wherein a plating adhesion amount of the zinc-based plating layer is in a range of from 30 to 70 g/m 2 .

5. The galvanized steel sheet of claim 1 , wherein the base steel sheet contains a composition containing, by wt %, C: 0.05 to 1.5%, Si: 2.5% or less, Mn: 1.5 to 20.0%, S—Al (acid-soluble aluminum): 3.0% or less, Cr: 2.5% or less, Mo: 1.0% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, Sb+Sn+Bi: 0.1% or less, N: 0.01% or less, and balance being Fe and unavoidable impurities.

6. The galvanized steel sheet of claim 5 , wherein a tensile strength of the galvanized steel sheet is 900 MPa or more.

7. The galvanized steel sheet of claim 5 , wherein a surface layer portion of the base steel sheet contains oxide containing at least one of Si, Mn, Al, and Fe.

8. The galvanized steel sheet of claim 1 , wherein a thickness of the base steel sheet is 1.0 to 2.0 mm.

9. A method for manufacturing the galvanized steel sheet of claim 1 , comprising:
reheating a steel slab to a temperature range of 950 to 1300° C.;
providing a hot-rolled steel sheet by hot rolling the reheated slab at a finish rolling start temperature of 900 to 1150° C. and a finish rolling end temperature of 850 to 1050° C.;
coiling the hot-rolled steel sheet in a temperature range of 590 to 750° C.;
heating both edges of the coiled hot-rolled coil for 5 to 24 hours by raising the temperature to a temperature range of 600 to 800° C. at a heating rate of 10° C./s higher;
heating the hot-rolled steel sheet in a heating zone at a heating rate of 1.3 to 4.3° C./s;
annealing the hot-rolled steel sheet in a soaking zone having a dew point temperature of −10 to +30° C., an atmosphere gas of N 2 -5 to 10% H 2 , and a temperature range of 650 to 900° C.;
slowly cooling the annealed hot-rolled steel sheet in a slow cooling zone in a temperature range of 550 to 700° C.;
quenching the slowly cooled hot-rolled steel sheet in a quenching zone in a temperature range of 270 to 550° C.;
forming the zinc-based plating layer by reheating the quenched hot-rolled steel sheet and then immersing the reheated quenched hot-rolled steel sheet in a zinc-based plating bath at a lead in temperature of 420 to 550° C.; and
optionally alloying the steel sheet, on which the zinc-based plating layer is formed, by heating the steel sheet to a temperature range of 480 to 560° C.

10. The method of claim 9 , wherein a threading speed is 40 to 130 mpm during the annealing.

11. The method of claim 9 , wherein the steel slab contains a composition containing, by wt %, C: 0.05 to 0.30%, Si: 2.5% or less, Mn: 1.5 to 10.0%, S—Al (acid-soluble aluminum): 1.0% or less, Cr: 2.0% or less, Mo: 0.2% or less, B: 0.005% or less, Nb: 0.1% or less, Ti: 0.1% or less, Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, and balance being Fe and unavoidable impurities.

Assignees

• Posco Co Ltd

Inventors

• Kang Ki-Cheol
• Park Jong-Chan
• So Seul-Gi
• Kim Myung-Soo

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D9/46

  for sheet metals · CPC title

• C21D2211/005

  Ferrite · CPC title

• C21D8/0294

  involving a localised treatment · CPC title

• C21D8/0273

  Final recrystallisation annealing · CPC title