High strength hot dip galvanized steel sheet and high strength galvannealed steel sheet excellent in shapeability and plateability and methods of production and apparatuses for production of the same

A hot dip galvanized steel sheet excellent in shapeability and plateability, comprising

• at least one high strength steel sheet portion containing approximately, by mass %, C:
• 0.05 to 0.25%, Si:
• 0.3 to 2.5%, Mn:
• 1.5 to 2.8%, P:
• 0.03% or less, S:
• 0.02% or less, Al:
• 0.005 to 0.5%, N:
• 0.0060% or less, and the balance of Fe and unavoidable impurities,
• wherein the at least one high strength steel sheet portion includes (i) a galvanized layer thereon which contains Al:
• 0.05 to about 10 mass % and Fe:
• about 0.05 to 3 mass % and the balance of Zn and unavoidable impurities, (ii) oxides containing Si in an average content of about 0.6 to 10 mass % at crystal grain boundaries and in crystal grains at a side of the at least one high strength steel sheet portion of about 5 μm or less from an interface between the at least one high strength steel sheet portion and a plating layer and (iii) Fe—Zn alloy with an average grain size of about 0.5 to 3 μm at a side of the plating layer.

A hot dip galvanized steel sheet excellent in shapeability and plateability, comprising:

• at least one high strength steel sheet containing approximately, by mass %, C:
• 0.05 to 0.25%, Si:
• 0.3 to 2.5%, Mn:
• 1.5 to 2.8%, P:
• 0.03% or less, S:
• 0.02% or less, Al:
• 0.005 to 0.5%, N:
• 0.0060% or less, and the balance of Fe and unavoidable impurities,
• wherein the at least one high strength steel sheet portion includes (i) a galvanized layer thereon containing Al:
• about 0.05 to 10 mass % and Fe:
• about 0.05 to 3 mass % and the balance of Zn and unavoidable impurities, and (ii) oxides containing Si in an average content of about 0.6 to 10 mass % at the crystal grain boundaries and in crystal grains at a side of the at least one high strength steel sheet portion of about 5 μm or less from an interface between the high strength steel sheet and a plating layer, and (iii) Fe—Zn alloy with an average grain size of about 0.5 to 3 μm at a side of the plating layer in a ratio of about 1 grain/500 μm or more.

A hot dip galvanized steel sheet excellent in shapeability and plateability, comprising:

• at least one high strength steel sheet containing approximately, by mass %, C:
• 0.05 to 0.25%, Si:
• 0.3 to 2.5%, Mn:
• 1.5 to 2.8%, P:
• 0.03% or less, S:
• 0.02% or less, Al:
• 0.005 to 0.5%, N:
• 0.0060% or less, and the balance of Fe and unavoidable impurities,
• wherein the at least one high strength steel sheet portion includes (i) a galvanized layer thereon containing Al:
• about 0.05 to 10 mass % and Mg:
• about 0.01 to 5 mass % and the balance of Zn and unavoidable impurities, and (ii) oxides containing Si in an average content of about 0.6 to 10 mass % at the crystal grain boundaries and in the crystal grains at a side of the at least one high strength steel sheet portion of about 5 μm or less from an interface between the at least one high strength steel sheet portion and the plating layer.

A hot dip galvanized steel sheet excellent in shapeability and plateability, comprising:

• at least one high strength steel sheet containing approximately, by mass %, C:
• 0.05 to 0.25%, Si:
• 0.3 to 2.5%, Mn:
• 1.5 to 2.8%, P:
• 0.03% or less, S:
• 0.02% or less, Al:
• 0.005 to 0.5%, N:
• 0.0060% or less, and the balance of Fe and unavoidable impurities,
• wherein the at least one high strength steel sheet portion includes (i) a galvanized layer thereon containing Al:
• about 4 to 20 mass %, Mg:
• about 2 to 5 mass %, and Si:
• about 0 to 0.5 mass % and the balance of Zn and unavoidable impurities, and (ii) oxides containing Si in an average content of about 0.6 to 10 mass % at crystal grain boundaries and in crystal grains at a side of the at least one high strength steel sheet portion of about 5 μm or less from the interface between the at least one high strength steel sheet portion and a plating layer.

A galvannealed steel sheet excellent in shapeability and plateability, comprising:

• at least one high strength steel sheet portion containing approximately, by mass %, C:
• 0.05 to 0.25%, Si:
• 0.3 to 2.5%, Mn:
• 1.5 to 2.8%, P:
• 0.03% or less, S:
• 0.02% or less, Al:
• 0.005 to 0.5%, N:
• 0.0060% or less, and the balance of Fe and unavoidable impurities,
• wherein the at least one high strength steel sheet portion includes (i) a zinc alloy plating layer containing Fe and the balance of Zn and unavoidable impurities, (ii) first oxides containing Si in an average content of about 0.6 to 10 mass % at crystal grain boundaries and in crystal grains at a sheet steel side of about 5 μm or less from an interface between the at least one high strength steel sheet portion and a plating layer, and (iii) second oxides containing Si in an average content of about 0.5 to 1.5 mass % in the plating layer.

A method for producing at least one high strength galvannealed steel sheet excellent in shapeability and plateability, comprising:

• a) continuously hot dip galvanizing the at least one high strength steel sheet containing Si and Mn;
• b) during procedure (a), causing (i) a presence of at least one type of Si oxides selected from FeSiO3, Fe2SiO4, MnSiO3, and Mn2SiO4 at a surface of the at least one high strength steel sheet or an interface of the at least one high strength steel sheet and plating and (ii) a presence of oxides of SiO2 at an inside surface side of the at least one high strength steel sheet so as to form a galvanized layer on the at least one high strength steel sheet;
• and
• c) galvannealing the at least one high strength steel sheet.

A method for producing at least one high strength galvannealed steel sheet excellent in shapeability and plateability, comprising:

• a) continuously hot dip galvanizing the at least one high strength steel sheet containing C, Si and Mn;
• b) during procedure (a), causing (i) a presence of at least one type of Si oxides selected from FeSiO3, Fe2SiO4, MnSiO3, and Mn2SiO4 at a surface of the at least one high strength steel sheet or an interface of the at least one high strength steel sheet and plating and (ii) a presence of oxides of SiO2 at an inside surface side of the at least one high strength steel sheet so as to form a galvanized layer on the at least one high strength steel sheet;
• and
• c) galvannealing the at least one high strength steel sheet.

A method for producing at least one sheet which is at least one of (i) at least one high strength hot dip galvanized steel sheet or (ii) at least one high strength galvannealed steel sheet excellent in shapeability and plateability, comprising:

• a) continuously hot dip galvanizing the at least one sheet containing C, Si, and Mn;
• b) during procedure (a), reducing the at least one sheet in a reduction zone with an atmosphere comprised of H2 in about 1 to 60 vol % and the balance of one or more of N2, H2O, CO2, CO2, and CO and unavoidable impurities and controlled to a log PO2 of oxygen partial pressure in the atmosphere of:
• −0.000034T2+0.105T−0.2 [Si %]2+2.1 [Si %]−98.8≦log PO2≦−0.000038T2+0.107T−90.4 (equation 1)
• 923≦T≦1173 (equation 2)
• where,
• T:
• maximum peak temperature (K) of steel sheet
• [Si %]:
• Si content in steel sheet (mass %).

A method for producing at least one high strength hot dip galvanized steel sheet excellent in shapeability and plateability, comprising:

• a) final rolling a steel slab containing C, Si, and Mn at an Ar3 point or more in temperature;
• b) cold rolling the at least one sheet by about 50 to 85%;
• c) hot dip galvanizing the at least one sheet;
• d) during step (c), using a continuous hot dip galvanization facility having a reduction zone with an atmosphere comprised of H2 in about 1 to 60 vol % and the balance of one or more of N2, H2O, O2, CO2, and CO and unavoidable impurities and controlled to a log PO2 of oxygen partial pressure in the atmosphere of:
• −0.000034T2+0.105T−0.2 [Si %]2+2.1 [Si %]−98.8≦log PO2≦0.000038T2+0.107T−90.4 (equation 1)
• 923≦T≦1173 (equation 2)
• where,
• T:
• maximum peak temperature (K) of steel sheet
• [Si %]:
• Si content in steel sheet (mass %);
• e) annealing the at least one sheet in a dual-phase coexisting temperature range of ferrite and austeniste of about 1023K to 1153K;
• f) cooling the at least one sheet from a maximum peak temperature to about 923K by an average cooling rate of about 0.5 to 10 degree/second;
• g) further cooling the at least one sheet from about 923K to 773K by an average cooling rate of about 3 degree/second or more and further from about 773K by an average cooling rate of about 0.5 degree/second or more for hot dip galvanization so as to thereby form a hot dip galvanized layer on the surface of a cooled-rolled steel sheet;
• and
• h) controlling a cooling time from about 773K to the 623K after plating to about 25 seconds to 240 seconds.

A method for producing at least one high strength galvannealed steel sheet excellent in shapeability and plateability, comprising:

• a) final rolling a steel slab containing C, Si, and Mn at an Ar3 point or more in temperature;
• b) cold rolling the at least one sheet by about 50 to 85%;
• c) hot dip galvanizing the at least one sheet;
• d) during step (c), using a continuous hot dip galvanization facility having a reduction zone with an atmosphere comprised of H2 in about 1 to 60 vol % and the balance of one or more of N2, H2O, O2, CO2, and CO and unavoidable impurities and controlled to a log PO2 of oxygen partial pressure in the atmosphere of:
• −0.000034T2+0.105T−0.2 [Si %]2+2.1 [Si %]−98.8≦log PO2≦−0.000038T2+0.107T−90.4 (equation 1)
• 923≦T≦1173 (equation 2)
• where,
• T:
• maximum peak temperature (K) of steel sheet
• [Si %]:
• Si content in steel sheet (mass %),
• e) annealing the at least one sheet in a dual-phase coexisting temperature range of ferrite and austeniste of about 1023K to 1153K;
• f) cooling the at least one sheet from a maximum peak temperature to about 923K by an average cooling rate of about 0.5 to 10 degree/second;
• g) further cooling the at least one sheet from about 923K to 773K by an average cooling rate of about 3 degree/second or more and further from about 773K by an average cooling rate of 0.5 degree/second or more down to about 693K to 733K and maintaining the cooling rate from 773K to a plating bath for about 25 seconds to 240 seconds;
• h) hot dip galvanizing the at least one sheet so as to form a hot dip galvanized layer on a surface of a cold rolled steel sheet;
• and
• i) alloying the at least one sheet on which the hot dip galvanized layer is formed so as to form a zinc alloy plating layer on the surface of at least one sheet,
• wherein procedure (h) is performed in a hot dip galvanization bath of a composition comprised of an effective bath Al concentration of about 0.07 to 0.105 wt % and the balance of Zn and unavoidable impurities, and
• wherein procedure (c) is performed at a temperature T (K) satisfying the following
• 720≦T≦690×exp(1.35×[Al %])
• wherein, [Al %]:
• effective bath Al concentration in galvanization bath (wt %).