HIGH STRENGTH GALVANIZED STEEL SHEET HAVING EXCELLENT BENDABILITY AND WELDABILITY, AND METHOD OF MANUFACTURING THE SAME

1 . A high strength galvanized steel sheet having excellent bendability and weldability, comprising by mass %:

C: equal to or more than 0.05% and less than 0.12%;

P: 0.001 to 0.040%; and

S: equal to or less than 0.0050%,

wherein a steel sheet surface layer, constituting a portion of the steel sheet up to a depth of 10 μm measured from each surface of the steel sheet, has a structure containing more than 70% of ferrite phase by a volume fraction,

a steel sheet inner layer portion, on an inner side than the depth of 10 μm measured from each surface, has a structure containing 20 to 70% by a volume fraction of ferrite phase with an average crystal grain size equal to or smaller than 5 μm,

the steel sheet has a tensile strength equal to or larger than 980 MPa, and

the steel sheet has a galvanized layer on a surface thereof.

2 . The steel sheet of claim 1 , wherein the structure of the steel sheet inner layer portion contains:

20 to 70% of ferrite phase by a volume fraction, with an average crystal grain size equal to or smaller than 5 μm;

30 to 80% of bainite phase and/or martensitic phase by a volume fraction, with an average crystal grain size of equal to or smaller than 5 μm; and

residual austenite phase and/or pearlite phase equal to or less than 5% (inclusive of 0%) by a volume fraction as the remainder.

3 . The steel sheet of claim 1 , wherein the steel sheet, further comprises by mass %:

C: equal to or more than 0.05% and less than 0.12%;

P: 0.001 to 0.040%;

S: equal to or less than 0.0050%;

Si: 0.01 to 1.6%;

Mn: 2.0 to 3.5%;

Al: 0.005 to 0.1%;

N: equal to or less than 0.0060%; and

the remainder as Fe and incidental impurities.

4 . The steel sheet of claim 3 , further comprising by mass %:

at least one element selected from the group consisting of Cr: more than 0.5% and equal to or less than 2.0%, Mo: 0.01 to 0.50%, and B: 0.0001 to 0.0030% and

the remainder as Fe and incidental impurities.

5 . The steel sheet of claim 3 , further comprising by mass %:

at least one element selected from the group consisting of Ti: 0.010 to 0.080% and Nb: 0.010 to 0.080%; and

the remainder as Fe and incidental impurities.

6 . The steel sheet of claim 5 , comprising by mass %:

C: equal to or more than 0.05% and less than 0.12%;

P: 0.001 to 0.040%;

S: equal to or less than 0.0050%;

Si: 0.01 to 1.6%;

Mn: 2.0 to 3.5%;

Al: 0.005 to 0.1%;

N: equal to or less than 0.0060%;

Cr: more than 0.5% and equal to or less than 2.0%;

Mo: 0.01 to 0.50%;

Ti: 0.010 to 0.080%;

Nb: 0.010 to 0.080%;

B: 0.0001 to 0.0030%; and

the remainder as Fe and incidental impurities.

7 . A method of manufacturing a galvanized steel sheet comprising:

subjecting a steel slab having a composition according to claim 1 to heating at a temperature of 1150° C. to 1300° C. and then hot rolling the slab at a hot finishing rolling temperature of 850 to 950° C. to form a steel sheet;

cooling the steel sheet from the hot finishing rolling temperature to (the hot finishing rolling temperature minus 100° C.) at an average cooling rate of 5 to 200° C./second;

winding up the steel sheet into a coil at a temperature of 400 to 650° C.;

subjecting the steel sheet to pickling and cold rolling; and

subjecting the steel sheet to annealing including a two-stage temperature raising processes,

wherein the annealing step includes:

primary heating the steel sheet from 200° C. to an intermediate temperature of 500 to 800° C. at a primary average heating rate of 5 to 50° C./second at an excess air ratio of 1.10 to 1.20 maintained up to the intermediate temperature;

secondary heating the steel sheet from the intermediate temperature to an annealing temperature of 730 to 900° C. at a secondary average heating rate of 0.1 to 10° C./second at an excess air ratio of less than 1.10 maintained up to the annealing temperature;

holding the steel sheet in a range of the annealing temperature for 10 to 500 seconds;

cooling the steel sheet to a temperature of 450 to 550° C. at an average cooling rate of 1 to 30° C./second; and

subjecting the steel sheet to a galvanizing process and optionally, an alloying process.

8 . The steel sheet of claim 2 , wherein the steel sheet, further comprises by mass %:

C: equal to or more than 0.05% and less than 0.12%;

P: 0.001 to 0.040%;

S: equal to or less than 0.0050%;

Si: 0.01 to 1.6%;

Mn: 2.0 to 3.5%;

Al: 0.005 to 0.1%;

N: equal to or less than 0.0060%; and

the remainder as Fe and incidental impurities.

9 . The steel sheet of claim 8 , further comprising by mass %:

at least one element selected from the group consisting of Cr: more than 0.5% and equal to or less than 2.0%, Mo: 0.01 to 0.50%, and B: 0.0001 to 0.0030% and

the remainder as Fe and incidental impurities.

10 . The steel sheet of claim 4 , further comprising by mass %:

at least one element selected from the group consisting of Ti: 0.010 to 0.080% and Nb: 0.010 to 0.080%; and

the remainder as Fe and incidental impurities.

11 . The steel sheet of claim 9 , further comprising by mass %:

at least one element selected from the group consisting of Ti: 0.010 to 0.080% and Nb:

0.010 to 0.080%; and

the remainder as Fe and incidental impurities.