Method for manufacturing high-strength galvanized steel sheet

Patent metadata

Field	Value
Publication number	US-10544477-B2
Application number	US-201515327489-A
Country	US
Kind code	B2
Filing date	Jun 9, 2015
Priority date	Jul 25, 2014
Publication date	Jan 28, 2020
Grant date	Jan 28, 2020

Abstract

A method for manufacturing a high-strength galvanized steel sheet includes performing hot rolling, cold rolling, first annealing, pickling, and second annealing. The first annealing is performed to obtain a steel sheet having a steel microstructure including ferrite in an amount of 10% or more and 60% or less in terms of area ratio, and martensite, bainite, and retained austenite in a total amount of 40% or more and 90% or less in terms of area ratio. The second annealing includes heating to an annealing temperature of 750° C. or higher and 850° C. or lower, holding at the annealing temperature for 10 seconds or more and 500 seconds or less, cooling at an average cooling rate of 1° C./s or more and 15° C./s or less, performing a galvanizing treatment, and cooling to a temperature of 150° C. or lower at an average cooling rate of 5° C./s or more and 100° C./s or less.

First claim

The invention claimed is:

1. A method for manufacturing a high-strength galvanized steel sheet, the method comprising performing hot rolling on a steel slab having a chemical composition containing, by mass %, C:0.120% or more and 0.180% or less, Si:0.01% or more and 1.00% or less, Mn:2.20% or more and 3.50% or less, P:0.001% or more and 0.050% or less, S:0.010% or less, sol.Al:0.005% or more and 0.100% or less, N:0.0001% or more and 0.0060% or less, Nb:0.010% or more and 0.100% or less, Ti:0.010% or more and 0.100% or less, and the balance being Fe and inevitable impurities in order to obtain a hot-rolled steel sheet, performing cold rolling on the hot-rolled steel sheet in order to obtain a cold-rolled steel sheet, then performing first annealing on the cold-rolled steel sheet, performing pickling on the annealed steel sheet, and then performing second annealing on the pickled steel sheet in order to obtain a galvanized steel sheet, wherein the first annealing includes performing heating to an annealing temperature of 780° C. or higher and 850° C. or lower at an average heating rate of 1° C./s or less in a temperature range from 700° C. to the annealing temperature, holding the heated steel sheet at an annealing temperature of 780° C. or higher and 850° C. or lower for 10 seconds or more and 500 seconds or less, and cooling the held steel sheet from the annealing temperature to a cooling stop temperature of 500° C. or lower at an average cooling rate of 5° C./s or more in order to obtain a steel sheet having a steel intermediate microstructure including ferrite in an amount of 10% or more and 60% or less in terms of area ratio, and martensite, bainite, and retained austenite in a total amount of 40% or more and 90% or less in terms of area ratio, wherein the pickling is performed so that the amount of decrease in the weight of the steel sheet due to pickling is 0.05 g/m 2 or more and 5 g/m 2 or less in terms of Fe, and wherein the second annealing includes heating the pickled steel sheet to an annealing temperature of 750° C. or higher and 850° C. or lower, holding the heated steel sheet at an annealing temperature of 750° C. or higher and 850° C. or lower for 10 seconds or more and 500 seconds or less, cooling the held steel sheet from the annealing temperature at an average cooling rate of 1° C./s or more and 15° C./s or less, performing a galvanizing treatment including dipping the steel sheet in a galvanizing bath, cooling the galvanized steel sheet to a temperature of 150° C. or lower at an average cooling rate of 5° C./s or more and 100° C./s or less in order to obtain a steel sheet having a steel microstructure including, in terms of area ratio, 10% or more and 60% or less of ferrite and, in terms of area ratio, 40% or more and 90% or less of martensite.

2. The method for manufacturing a high-strength galvanized steel sheet according to claim 1 , wherein an alloying treatment is further performed on the galvanized steel sheet before cooling is performed at an average cooling rate of 5° C./s or more and 100° C./s or less.

3. The method for manufacturing a high-strength galvanized steel sheet according to claim 1 , wherein the steel slab has the chemical composition further containing, by mass %, one or more selected from among Mo:0.05% or more and 1.00% or less, V:0.02% or more and 0.50% or less, Cr:0.05% or more and 1.00% or less, and B:0.0001% or more and 0.0030% or less.

4. The method for manufacturing a high-strength galvanized steel sheet according to claim 1 , wherein the hot rolling includes starting cooling within 3 seconds after hot finish rolling has been performed, cooling the hot-rolled steel sheet at an average cooling rate of 5° C./s or more and 200° C./s or less in a temperature range from the finishing delivery temperature of the hot rolling to a temperature of (the finishing delivery temperature of the hot rolling-100° C.), coiling the cooled steel sheet at a coiling temperature of 450° C. or higher and 650° C. or lower, and wherein the cold rolling is performed with a rolling reduction of 40% or more.

5. A method for manufacturing a high-strength galvanized steel sheet, the method comprising performing first annealing on the cold-rolled steel sheet which is hot-rolled and cold-rolled and having a chemical composition containing, by mass %, C: 0.120% or more and 0.180% or less, Si:0.01% or more and 1.00% or less, Mn:2.20% or more and 3.50% or less, P:0.001% or more and 0.050% or less, S:0.010% or less, sol.Al: 0.005% or more and 0.100% or less, N:0.0001% or more and 0.0060% or less, Nb:0.010% or more and 0.100% or less, Ti:0.010% or more and 0.100% or less, and the balance being Fe and inevitable impurities, performing pickling on the annealed steel sheet, and then performing second annealing on the pickled steel sheet in order to obtain a galvanized steel sheet, wherein the first annealing includes performing heating to an annealing temperature of 780° C. or higher and 850° C. or lower at an average heating rate of 1° C./s or less in a temperature range from 700° C. to the annealing temperature, holding the heated steel sheet at an annealing temperature of 780° C. or higher and 850° C. or lower for 10 seconds or more and 500 seconds or less, and cooling the held steel sheet from the annealing temperature to a cooling stop temperature of 500° C. or lower at an average cooling rate of 5° C./s or more in order to obtain a steel sheet having a steel intermediate microstructure including ferrite in an amount of 10% or more and 60% or less in terms of area ratio, and martensite, bainite, and retained austenite in a total amount of 40% or more and 90% or less in terms of area ratio, wherein the pickling is performed so that the amount of decrease in the weight of the steel sheet due to pickling is 0.05 g/m 2 or more and 5 g/m 2 or less in terms of Fe, and wherein the second annealing includes heating the pickled steel sheet to an annealing temperature of 750° C. or higher and 850° C. or lower, holding the heated steel sheet at an annealing temperature of 750° C. or higher and 850° C. or lower for 10 seconds or more and 500 seconds or less, cooling the held steel sheet from the annealing temperature at an average cooling rate of 1° C./s or more and 15° C./s or less, performing a galvanizing treatment including dipping the steel sheet in a galvanizing bath, cooling the galvanized steel sheet to a temperature of 150° C. or lower at an average cooling rate of 5° C./s or more and 100° C./s or less in order to obtain a steel sheet having a steel microstructure including, in terms of area ratio, 10% or more and 60% or less of ferrite and, in terms of area ratio, 40% or more and 90% or less of martensite.

6. The method for manufacturing a high-strength galvanized steel sheet according to claim 5 , wherein an alloying treatment is further performed on the galvanized steel sheet before cooling is performed at an average cooling rate of 5° C./s or more and 100° C./s or less.

7. The method for manufacturing a high-strength galvanized steel sheet according to claim 5 , wherein the cold-rolled steel sheet has the chemical composition further containing, by mass %, one or more selected from among Mo:0.05% or more and 1.00% or less, V:0.02% or more and 0.50% or less, Cr:0.05% or more and 1.00% or less, and B:0.0001% or more and 0.0030% or less.

8. The method for manufacturing a high-strength galvanized steel sheet according to claim 2 , wherein the steel slab has the chemical composition further containing, by mass %, one or more selected from among Mo:0.05% or more and 1.00% or less, V:0.02% or more and 0.50% or less, Cr:0.05% or more and 1.00% or less, and B:0.0001% or more and 0.0030% or less.

9. The method for manufacturing a high-strength galvanized steel shee

Assignees

• Jfe Steel Corp

Inventors

• Kimura Hideyuki
• Fujita Koichiro
• Hasegawa Hiroshi
• Aoyama Mai

Classifications

• C22C38/04

  containing manganese · CPC title

• C22C38/02

  containing silicon · CPC title

• C22C38/12

  containing tungsten, tantalum, molybdenum, vanadium, or niobium · CPC title

• C21D9/46 Primary

  for sheet metals · CPC title

• C22C38/24

  with vanadium · CPC title