High strength galvanized steel sheet having excellent bendability and weldability, and method of manufacturing the same

Abstract

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.

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.