Hot-dip galvanized steel sheet and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-10407760-B2
Application number	US-201214347067-A
Country	US
Kind code	B2
Filing date	Sep 28, 2012
Priority date	Sep 30, 2011
Publication date	Sep 10, 2019
Grant date	Sep 10, 2019

Abstract

A hot-dip galvanized steel sheet includes: a steel sheet; and a plated layer on a surface of the steel sheet, a microstructure contains, by volume fraction, equal to or more than 20% and equal to or less than 99% in total of one or two of martensite and bainite, a residual structure contains one or two of ferrite, residual austenite of less than 8% by volume fraction, and pearlite of equal to or less than 10% by volume fraction, tensile strength is equal to or greater than 980 MPa, the plated layer is a hot-dip galvanized layer which contains oxides including one or two or more of Si, Mn, and Al, contains equal to or less than 15 mass % of Fe, and a remainder including Zn, Al, and unavoidable impurities, and when a cross section including the steel sheet and the hot-dip galvanized layer is seen in a sheet thickness direction, a projected area ratio is equal to or more than 10% and equal to or less than 90%.

First claim

The invention claimed is:

1. A hot-dip galvanized steel sheet comprising:
a steel sheet; and
a plated layer on a surface of the steel sheet,
wherein the steel sheet contains, by mass %,
C: equal to or more than 0.05% and less than 0.40%,
Si: 0.5% to 3.0%,
Mn: 1.5% to 3.0%,
O: limited to 0.006% or less,
P: limited to 0.4% or less,
S: limited to 0.01% or less,
Al: limited to 2.0% or less,
N: limited to 0.01% or less, and
the remainder including Fe and unavoidable impurities,
wherein a microstructure of the steel sheet contains,
by volume fraction, equal to or more than 20% and equal to or less than 99% in total of one or two of a martensite and a bainite, and
a residual structure including a ferrite, and one or two of a residual austenite of less than 8% by volume fraction, and a pearlite of equal to or less than 10% by volume fraction,
wherein a tensile strength of the steel sheet is equal to or greater than 980 MPa,
wherein the plated layer is a hot-dip galvanized layer which contains oxides including one or two or more of Si, Mn, and Al, contains equal to or less than 15 mass % of Fe, and the remainder including Zn, Al, and unavoidable impurities, and
wherein when a cross section including the steel sheet and the hot-dip galvanized layer is seen in a sheet thickness direction, a projected area ratio which is an area ratio obtained by dividing a length of the oxides projected to an interface between the hot-dip galvanized layer and the steel sheet by a length of the interface between the hot-dip galvanized layer and the steel sheet, is equal to or more than 19% and equal to or less than 62%.

2. A galvannealed steel sheet comprising:
a steel sheet; and
a plated layer on a surface of the steel sheet,
wherein the steel sheet contains, by mass %,
C: equal to or more than 0.05% and less than 0.40%,
Si: 0.5% to 3.0%,
Mn: 1.5% to 3.0%,
O: limited to 0.006% or less,
P: limited to 0.04% or less,
S: limited to 0.01% or less,
Al: limited to 2.0% or less,
N: limited to 0.01% or less, and
the remainder including Fe and unavoidable impurities,
wherein a microstructure of the steel sheet contains,
by volume fraction, equal to or more than 20% and equal to or less than 99% in total of one or two of a martensite and a bainite, and
a residual structure including a ferrite, and one or two of a residual austenite of less than 8% by volume fraction, and a pearlite of equal to or less than 10% by volume fraction,
wherein a tensile strength of the steel sheet is equal to or greater than 980 MPa,
wherein the plated layer is a galvannealed layer which contains oxides including one or two or more of Si, Mn, and Al, contains equal to or more than 7 mass % and equal to or less than 15 mass % of Fe, and the remainder including Zn, Al, and unavoidable impurities, and
wherein when a cross section including the steel sheet and the galvannealed layer is seen in a sheet thickness direction, a projected area ratio which is an area ratio obtained by dividing a length of the oxides projected to an interface between the galvannealed layer and the steel sheet by a length of the interface between the galvannealed layer and the steel sheet, is equal to or more than 19% and equal to or less than 62%.

3. The hot-dip galvanized steel sheet according to claim 1 ,
wherein the microstructure contains, by volume fraction, 40% to 80% of ferrite.

4. The hot-dip galvanized steel sheet according to claim 1 ,
wherein the microstructure contains, by volume fraction, more than 60% and equal to or less than 99% of one or two of martensite and bainite.

5. The hot-dip galvanized steel sheet according to claim 1 ,
wherein the steel sheet further contains, by mass %, one or two or more of
Cr: 0.05% to 1.0%,
Mo: 0.01% 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%,
Ca: 0.0005% to 0.04%,
Mg: 0.0005% to 0.04%,
REM: 0.0005% to 0.04%.

6. A manufacturing method of the hot-dip galvanized steel sheet of claim 1 , the method comprising:
casting a molten steel having the composition of the steel sheet to obtain a steel;
heating the steel to a first temperature range of 1100° C. to lower than 1300° C., directly or after cooling once;
completing a hot rolling of the steel at a temperature equal to or higher than an Ar3 transformation point;
coiling the steel in a second temperature range of 300° C. to 700° C.;
pickling the steel;
performing cold rolling of the steel with a cumulative rolling reduction of 40% to 80% using a cold rolling mill including a work roll having a roll diameter of 200 mm to 1400 mm;
retaining the steel in a third temperature range of 550° C. to 750° C. for 20 seconds to 2000 seconds during heating the steel to an annealing temperature, when the steel passes through a continuous galvanizing line;
maintaining the steel in a fourth temperature range of 750° C. to 900° C. for 10 seconds to 1000 seconds, in an N 2 atmosphere in which an H 2 concentration is equal to or less than 20% and a dew point is equal to or higher than −20° C., while performing an annealing;
performing a first cooling of cooling the steel to a fifth temperature range of 500° C. to 750° C. at an average cooling rate of 1° C./sec to 200° C./sec;
performing a second cooling of cooling the steel to a sixth temperature range between a temperature which is lower than a hot dip galvanizing bath temperature by 40° C. and a temperature which is higher than the hot dip galvanizing bath temperature by 50° C., at an average cooling rate which is 1° C./sec to 200° C./sec and is faster than the average cooling rate of the first cooling;
galvanizing the steel by immersing the steel in a hot dip galvanizing bath which flows at a flow velocity of 10 m/min to 50 m/min after setting a plating bath immersion sheet temperature which is a temperature when immersing the steel in the hot dip galvanizing bath, as the sixth temperature range; and
cooling the steel to a temperature equal to or lower than 40° C.

7. A manufacturing method of the galvannealed steel sheet of claim 2 , the method comprising:
casting a molten steel having the composition of the steel sheet to obtain a steel;
heating the steel to a seventh temperature range of 1100° C. to lower than 1300° C., directly or after cooling once;
completing a hot rolling of the steel at a temperature equal to or higher than an Ar3 transformation point;
coiling the steel in an eighth temperature range of 300° C. to 700° C.;
pickling the steel;
performing cold rolling of the steel with a cumulative rolling reduction of 40% to 80% using a cold rolling mill including a work roll having a roll diameter of 200 mm to 1400 mm;
retaining the steel in a ninth temperature range of 550° C. to 750° C. for 20 seconds to 2000 seconds during heating the steel to an annealing temperature, when the steel passes through a continuous galvanizing line;
maintaining the steel in a tenth temperature range of 750° C. to 900° C. for 10 seconds to 1000 seconds, in an N 2 atmosphere in which an H 2 concentration is equal to or less than 20% and a dew point is equal to or higher than −20° C., while performing an annealing;
performing a third cooling of cooling the steel to an eleventh temperature range of 500° C. to 750° C. at an average cooling rate of 1° C./sec to 200° C./sec;
performing a fourth cooling of cooling the steel to a twelfth temperature range of 500° C. to 25° C., at an average cooling rate which is 1° C./sec to 200° C./sec and is faster than the average cooling rate of the third cooling;
heating the steel again to a thirteenth temperature range of 350° C. to 500° C., in a case where a cooling stop temperature of the fourth cooling is lower than 35

Assignees

• Nippon Steel & Sumitomo Metal Corp
• Nippon Steel Corp

Inventors

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

Classifications

• C22C18/00

  Alloys based on zinc · CPC title

• C21D8/0278

  involving a particular surface treatment  (C21D8/0294 takes precedence) · CPC title

• C21D8/0273

  Final recrystallisation annealing · CPC title

• C22C38/12

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

• C22C38/02

  containing silicon · CPC title