High strength galvanized steel sheet having excellent formability and method for manufacturing the same
A high strength galvanized steel sheet includes a steel sheet which is a base sheet and a galvanized layer on a surface of the steel sheet, the steel sheet has a composition including, by mass %, 0.08 to 0.15% of C, 0.5 to 1.5% of Si, 0.5 to 1.5% of Mn, 0.1% or less of P, 0.01% or less of S, 0.01 to 0.1% of Al, 0.005% or less of N, and the balance Fe with inevitable impurities, and a microstructure composed of 75 to 90% of a ferrite phase as a main phase and 10 to 25% of a second phase including at least pearlite in terms of an area fraction with respect to the entire microstructure; an average grain size of the pearlite is 5 μm or smaller; and the pearlite has an area fraction of 70% or greater with respect to the total area of the second phase.
1. A high strength galvanized steel sheet having excellent formability, comprising a steel sheet which is a base sheet and a galvanized layer on a surface of the steel sheet, wherein the steel sheet is a high strength steel sheet which has a composition comprising, by mass %,
0.08 to 0.15% of C, 0.5 to 1.5% of Si,
0.5 to 1.5% of Mn, 0.1% or less of P,
0.01% or less of S, 0.01 to 0.1% of Al,
0.005% or less of N, and
the balance Fe with inevitable impurities, and a microstructure composed of 75 to 90% of a ferrite phase as a main phase and 10 to 25% of a second phase including at least pearlite in terms of an area fraction with respect to the entire microstructure; an average grain size of the pearlite is 5 μm or smaller; and the pearlite has an area fraction of 70% or greater with respect to the total area of the second phase.
2. The high strength galvanized steel sheet according to claim 1 , wherein the composition further comprises, by mass %, one or more selected from the group consisting of 0.05 to 0.5% of Cr, 0.005 to 0.2% of V, and 0.005 to 0.2% of Mo.
3. The high strength galvanized steel sheet according to claim 1 , wherein the composition further comprises, by mass %, one or two selected from the group consisting of 0.01 to 0.1% of Ti and 0.01 to 0.1% of Nb.
4. The high strength galvanized steel sheet according to claim 1 , wherein the composition further comprises, by mass %, 0.0003 to 0.0050% of B.
5. The high strength galvanized steel sheet according to claim 1 , wherein the composition further comprises, by mass %, one or two selected from the group consisting of 0.05 to 0.5% of Ni and 0.05 to 0.5% of Cu.
6. The high strength galvanized steel sheet according to claim 1 , wherein the composition further comprises, by mass %, one or two selected from the group consisting of 0.001 to 0.005% of Ca and 0.001 to 0.005% of total REM.
7. The high strength galvanized steel sheet according to claim 1 , wherein the galvanized layer is a galvannealed layer.
8. A method of manufacturing the high strength galvanized steel sheet having excellent formability according to claim 1 comprising:
a hot rolling step of subjecting a steel to hot rolling to form a hot rolled sheet;
pickling of the hot rolled sheet;
a continuous annealing step of subsequently subjecting the hot rolled sheet to annealing and subsequent cooling to a temperature for entry into a galvanizing bath; and,
after the continuous annealing step, a galvanizing step of performing galvanizing to form a galvanized layer on a surface of the hot rolled sheet by dipping the hot rolled sheet in the galvanizing bath,
the continuous annealing step and the galvanizing step being continuously performed with a continuous galvanizing line to manufacture a galvanized steel sheet having the galvanized layer on the surface thereof, wherein the steel has a composition comprising, by mass %,
0.08 to 0.15% of C, 0.5 to 1.5% of Si,
0.5 to 1.5% of Mn, 0.1% or less of P,
0.01% or less of S, 0.01 to 0.1% of Al,
0.005% or less of N, and
the balance Fe with inevitable impurities;
the annealing is performed by holding the sheet for 5 to 400 s in a first temperature region of an A c1 transformation point to an A c3 transformation point; and,
in cooling after the annealing, the sheet is cooled from a first temperature region to 700° C. at an average cooling rate of 5° C./s or higher, and a residence time in a second temperature region of 700° C. to the temperature for entry into the galvanizing bath is 15 to 400 s.
9. The method according to claim 8 , wherein the hot rolling step comprises heating the steel at a temperature of 1100° C. to 1280° C.; subsequently subjecting the steel to hot rolling with a finish hot rolling temperature of 870° C. to 950° C. to form a hot rolled sheet; and, after the hot rolling, coiling the hot rolled sheet at a coiling temperature of 350° C. to 720° C.
10. The method according to claim 8 , wherein a cooling time in a temperature region of 700° C. to 550° C. in the second temperature region is 10 s or longer.
11. The method according to claim 8 , wherein an alloying step of alloying the galvanized layer is performed subsequent to the galvanizing step.
12. The method according to claim 8 , wherein the composition further comprises, by mass %, one or more selected from the group consisting of 0.05 to 0.5% of Cr, 0.005 to 0.2% of V, and 0.005 to 0.2% of Mo.
13. The method according to claim 8 , wherein the composition further comprises, by mass %, one or two selected from the group consisting of 0.01 to 0.1% of Ti and 0.01 to 0.1% of Nb.
14. The method according to claim 8 , wherein the composition further comprises, by mass %, 0.0003 to 0.0050% of B.
15. The method according to claim 8 , wherein the composition further comprises, by mass %, one or two selected from the group consisting of 0.05 to 0.5% of Ni and 0.05 to 0.5% of Cu.
16. The method according to claim 8 , wherein the composition further comprises, by mass %, one or two selected from the group consisting of 0.001 to 0.005% of Ca and 0.001 to 0.005% of total REM.