Galvanized steel sheet and method of manufacturing the same

Patent metadata

Field	Value
Publication number	US-9970092-B2
Application number	US-201214347120-A
Country	US
Kind code	B2
Filing date	Sep 28, 2012
Priority date	Sep 30, 2011
Publication date	May 15, 2018
Grant date	May 15, 2018

Abstract

A galvanized steel sheet includes a steel sheet and a plating layer on the surface of the steel sheet, in which the steel sheet includes, as a steel chemical composition, by mass %, C: 0.05 to 0.40%, Si: 0.5 to 3.0% and Mn: 1.5 to 3.0%, a microstructure of the steel sheet includes ferrite, bainite, by volume fraction, 30% or more of a tempered martensite, and 8% or more of an austenite, and tensile strength of the steel sheet is 980 MPa or more, and the plating layer includes an oxide including at least one chemical element selected from Si, Mn and Al, and when seen at a cross section including the steel sheet and the plating layer in a plate thickness direction, a projection area fraction of the oxide is 10% or more.

First claim

The invention claimed is:

1. A galvanized steel sheet comprising:
a steel sheet; and
a plating layer on a surface of the steel sheet,
wherein the steel sheet includes, as a steel chemical composition, by mass %,
C: 0.05 to 0.40%,
Si: 0.5 to 3.0%,
Mn: 1.5 to 3.0%,
P: limited to 0.04% or less,
S: limited to 0.01% or less,
N: limited to 0.01% or less,
Al: limited to 2.0% or less,
O: limited to 0.01% or less, and
the balance of Fe and unavoidable impurities,

wherein a microstructure of the steel sheet includes a ferrite, a bainite, and by volume fraction, 30% or more of a tempered martensite, 8% or more of an austenite, and limited to 10% or less of a pearlite,
wherein a total volume fraction of the tempered martensite and the bainite is 40% or more, and an area fraction of grains having a grain size of more than 35 μm occupied per unit area of the microstructure is 10% or less,
wherein a tensile strength of the steel sheet is 980 MPa or more,
and wherein a plating metal in the plating layer includes, as a plating chemical composition, limited to 15 mass % or less of Fe, limited to 2 mass % or less of Al, and the balance of Zn and unavoidable impurities,
the plating layer includes oxides including at least one chemical element selected from Si, Mn, and Al, and
when seen at a cross section including the steel sheet and the plating layer in a thickness direction, a projection area fraction obtained by dividing a length that the oxides are projected onto an interface between the plating layer and the steel sheet by a length of the interface between the plating layer and the steel sheet is 10% to 67% and a coverage of the plating layer to the steel sheet is 99% or more, and
wherein the projection area fraction is an average of values measured at five visual fields at a magnification of 10000 times.

2. The galvanized steel sheet according to claim 1 ,
wherein the steel further includes, as the steel chemical composition, by mass %, at least one selected from
Mo: 0.01 to 1.0%,
Cr: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cu: 0.05 to 1.0%,
Nb: 0.005 to 0.3%,
Ti: 0.005 to 0.3%,
V: 0.005 to 0.5%,
B: 0.0001 to 0.01%, and
a total of at least one of elements selected from Ca, Mg, and REM: 0.0005 to 0.04%.

3. The galvanized steel sheet according to claim 1 ,
wherein the plating layer is a hot dip galvanized layer.

4. The galvanized steel sheet according to claim 1 ,
wherein the plating layer is a galvannealed layer.

5. The galvanized steel sheet according to claim 1 ,
wherein an amount of Fe is limited to less than 7 mass % in the plating chemical composition.

6. The galvanized steel sheet according to claim 1 ,
wherein the plating chemical composition includes 7 mass % to 15 mass % of Fe.

7. The galvanized steel sheet according to claim 1 ,
wherein the plating chemical composition includes more than 0 mass % and 2 mass % or less of Al.

8. A method of manufacturing the galvanized steel sheet of claim 1 , the method comprising:
a first process of casting a steel which includes, as a steel chemical composition, by mass %,
C: 0.05 to 0.40%,
Si: 0.5 to 3.0%,
Mn: 1.5 to 3.0%,
P: limited to 0.04% or less,
S: limited to 0.01% or less,
N: limited to 0.01% or less,
Al: limited to 2.0% or less,
O: limited to 0.01% or less, and
the balance of Fe and unavoidable impurities;

a second process of heating the steel directly or after once cooled;
a third process of hot rolling the steel so that the hot rolling is completed at a temperature of an Ar 3 transformation point or higher;
a fourth process of coiling the steel at 300° C. to 700° C.;
a fifth process of pickling the steel;
a sixth process of cold rolling the steel by a cold rolling mill having a work roll with a roll size of 1,400 mm or less with a cumulative rolling reduction of 30% or more and less than 100%;
a seventh process of heating the steel and retaining the steel at 550° C. to 750° C. for 20 seconds or more;
an eighth process of annealing the steel at 750° C. to 900° C.;
a ninth process of cooling the steel to an intermediate cooling temperature in a temperature range of 500° C. or higher and lower than 750° C. at a first average cooling rate of 0.1° C./s to 30° C./s and cooling the steel from the intermediate cooling temperature to a cooling stop temperature of 100° C. or higher and lower than 350° C. at a second average cooling rate which is equal to or higher than the first average cooling rate;
a tenth process of controlling a temperature of the steel within a temperature range of a temperature, which is lower than a plating bath temperature by 40° C., or higher and a temperature, which is higher than the plating bath temperature by 40° C., or lower;
an eleventh process of galvanizing by immersing the steel into a hot dip galvanizing bath flowing at a flow rate of 10 m/min to 50 m/min; and
a twelfth process of cooling the steel to a temperature of lower than 100° C.;
wherein the second average cooling rate is 1° C./s to 100° C./s, and
a time when the temperature of the steel is within a temperature range of 350° C. to 500° C. is 20 seconds or more in the processes after the ninth process.

9. The method of manufacturing the galvanized steel sheet according to claim 8 ,
wherein the steel further includes, as the steel chemical composition, by mass %, at least one selected from
Mo: 0.01 to 1.0%,
Cr: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cu: 0.05 to 1.0%,
Nb: 0.005 to 0.3%,
Ti: 0.005 to 0.3%,
V: 0.005 to 0.5%,
B: 0.0001 to 0.01%, and
a total of at least one of elements selected from Ca, Mg, and REM: 0.0005 to 0.04%.

10. The method of manufacturing the galvanized steel sheet according to claim 8 ,
wherein, in the ninth process, when the first average cooling rate is equal to the second average cooling rate, the first average cooling rate is 1° C./s or more and 30° C./s or less.

11. The method of manufacturing the galvanized steel sheet according to claim 8 , further comprising:
a process of reheating and holding the steel in the temperature range of 350° C. to 500° C. after the tenth process.

12. The method of manufacturing the galvanized steel sheet according to claim 8 , further comprising:
a process of heating the steel to 460° C. to 600° C. so as to perform alloy treatment after the twelfth process.

13. The galvanized steel sheet according to claim 2 ,
wherein the plating layer is a hot dip galvanized layer.

14. The galvanized steel sheet according to claim 2 ,
wherein the plating layer is a galvannealed layer.

15. The galvanized steel sheet according to claim 2 ,
wherein an amount of Fe is limited to less than 7 mass % in the plating chemical composition.

16. The galvanized steel sheet according to claim 2 ,
wherein the plating chemical composition includes 7 mass % to 15 mass % of Fe.

17. The galvanized steel sheet according to claim 2 ,
wherein the plating chemical composition includes more than 0 mass % and 2 mass % or less of Al.

18. The method of manufacturing the galvanized steel sheet according to claim 9 ,
wherein, in the ninth process, when the first average cooling rate is equal to the second average cooling rate, the first average cooling rate is 1° C./s or more and 30° C./s or less.

19. The method of manufacturing the galvanized steel sheet according to claim 9 , further comprising:
a process of reheating and holding the steel in the temperature range of 350° C. to 500° C. after the tenth process.

20. The method of manufacturing the galv

Assignees

• Nippon Steel & Sumitomo Metal Corp

Inventors

• Nozaki Takayuki
• Takahashi Manabu
• Fujita Nobuhiro
• Azuma Masafumi
• Wakabayashi Chisato

Classifications

• C21D8/02

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

• C23C2/28

  Thermal after-treatment, e.g. treatment in oil bath · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• Y10T428/12799

  Next to Fe-base component [e.g., galvanized] · CPC title

• C22C38/04

  containing manganese · CPC title