Hot-dip galvanized steel sheet and manufacturing method thereof

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 350° C.;

retaining the steel in the thirteenth temperature range;

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 a fourteenth 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.;

performing an alloying treatment to the steel at a fifteenth temperature range of equal to or lower than 600° C.; and

cooling the steel to a temperature equal to or lower than 40° C.

8. The manufacturing method of the hot-dip galvanized steel sheet according to claim 6 , wherein the annealing is performed at a temperature lower than 840° C.

9. The manufacturing method of the hot-dip galvanized steel sheet according to claim 6 , wherein the annealing is performed at a temperature equal to or higher than 840° C.

10. The manufacturing method of the hot-dip galvanized steel sheet according to claim 6 ,

wherein the molten steel 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 10%,

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%, and

REM: 0.0005% to 0.04%.

11. The galvannealed steel sheet according to claim 2 ,

wherein the microstructure contains, by volume fraction, 40% to 80% of ferrite.

12. The galvannealed steel sheet according to claim 2 ,

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.

13. The galvannealed steel sheet according to claim 2 , 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%, and

REM: 0.0005% to 0.04%.

14. The manufacturing method of the galvannealed steel sheet according to claim 7 , wherein the annealing is performed at a temperature lower than 840° C.

15. The manufacturing method of the galvannealed steel sheet according to claim 7 , wherein the annealing is performed at a temperature equal to or higher than 840° C.

16. The manufacturing method of the galvannealed steel sheet according to claim 7 , wherein the molten steel 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%, and

REM: 0.0005% to 0.04%.