[Object] What is provided is a manufacturing method of a galvannealed steel sheet capable of further promoting alloying of zinc plating with the steel sheet. [Resolution Means] A manufacturing method of a galvannealed steel sheet, including: forming on a surface of a steel sheet after hot rolling and pickling which contains, by mass %, C: 0.001% to 0.350%, Si: 0.001% to 2.500% or P: 0.001% to 0.100%, or combination thereof, Mn: 0.10% to 3.00%, S: 0.001% to 0.010%, N: 0.0010% to 0.0065%, and sol. Al: 0.001% to 0.800% with a remainder being Fe and impurities, grooves having an opening surface width of 10 μm to 25 μm and a depth of 10 μm to 30 μm at intervals of 20 μm to 500 μm; cold rolling the steel sheet at a rolling reduction of 30% or more; reduction annealing the steel sheet after the cold rolling; immersing the steel sheet in a hot-dip galvanizing bath containing 0.10 mass % to 0.20 mass % of Al with a remainder consisting of Zn and optional components, and adhering a hot-dip galvanized layer to the surface of the steel sheet; and heating the steel sheet to which the hot-dip galvanized layer is adhered, and alloying the steel sheet with the hot-dip galvanized layer.

[Object] What is provided is a manufacturing method of a galvannealed steel sheet capable of further promoting alloying of zinc plating with the steel sheet. [Resolution Means] A manufacturing method of a galvannealed steel sheet, including: forming on a surface of a steel sheet after hot rolling and pickling which contains, by mass %, C: 0.001% to 0.350%, Si: 0.001% to 2.500% or P: 0.001% to 0.100%, or combination thereof, Mn: 0.10% to 3.00%, S: 0.001% to 0.010%, N: 0.0010% to 0.0065%, and sol. Al: 0.001% to 0.800% with a remainder being Fe and impurities, grooves having an opening surface width of 10 μm to 25 μm and a depth of 10 μm to 30 μm at intervals of 20 μm to 500 μm; cold rolling the steel sheet at a rolling reduction of 30% or more; reduction annealing the steel sheet after the cold rolling; immersing the steel sheet in a hot-dip galvanizing bath containing 0.10 mass % to 0.20 mass % of Al with a remainder consisting of Zn and optional components, and adhering a hot-dip galvanized layer to the surface of the steel sheet; and heating the steel sheet to which the hot-dip galvanized layer is adhered, and alloying the steel sheet with the hot-dip galvanized layer.

Provided is a steel sheet used as a material for an automotive exterior panel, etc., and a method for manufacturing the same. More particularly, provided is a cold-rolled steel sheet and a hot-dip galvanized steel sheet, which have excellent bake hardening properties, corrosion resistance, and anti-aging properties, and a method for manufacturing the same.

Provided is a steel sheet used as a material for an automotive exterior panel, etc., and a method for manufacturing the same. More particularly, provided is a cold-rolled steel sheet and a hot-dip galvanized steel sheet, which have excellent bake hardening properties, corrosion resistance, and anti-aging properties, and a method for manufacturing the same.

A hot-rolled coated steel sheet including: in wt %, C: 0.05-0.14%, Si: 0.1-1.0%, Mn: 1.0-2.0%, P: 0.001-0.05%, S: 0.001-0.01%, AI: 0.01-0.1%, Cr: 0.005-1.0%, Ti: 0.005-0.13%, Nb: 0.005-0.03%, N: 0.001-0.01%, Fe residues, and other inevitable impurities; a mixed structure of ferrite and bainite as a main phase; and as a remaining structure, one or more selected from the group consisting of martensite, austenite, and phase martensite (MA), wherein a fraction of the ferrite and bainite is 95-99 area % and Equation 1 is satisfied. [Equation 1] FCO.sub.{110}<112>+FCO.sub.{112}<111>≥10 where, FCO.sub.{110}<112> and FCO.sub.{112}<111>, each representing an area fraction occupied by a structure having ac crystal orientation of {110}<112> and {112}<111>.

A hot-rolled coated steel sheet including: in wt %, C: 0.05-0.14%, Si: 0.1-1.0%, Mn: 1.0-2.0%, P: 0.001-0.05%, S: 0.001-0.01%, AI: 0.01-0.1%, Cr: 0.005-1.0%, Ti: 0.005-0.13%, Nb: 0.005-0.03%, N: 0.001-0.01%, Fe residues, and other inevitable impurities; a mixed structure of ferrite and bainite as a main phase; and as a remaining structure, one or more selected from the group consisting of martensite, austenite, and phase martensite (MA), wherein a fraction of the ferrite and bainite is 95-99 area % and Equation 1 is satisfied. [Equation 1] FCO.sub.{110}<112>+FCO.sub.{112}<111>≥10 where, FCO.sub.{110}<112> and FCO.sub.{112}<111>, each representing an area fraction occupied by a structure having ac crystal orientation of {110}<112> and {112}<111>.

An Al—Zn—Si—Mg alloy coated strip that has Mg.sub.2Si particles in the coating microstructure is disclosed. The distribution of Mg.sub.2Si particles is such that the surface of the coating has only a small proportion of Mg.sub.2Si particles or is at least substantially free of any Mg.sub.2Si particles.

An Al—Zn—Si—Mg alloy coated strip that has Mg.sub.2Si particles in the coating microstructure is disclosed. The distribution of Mg.sub.2Si particles is such that the surface of the coating has only a small proportion of Mg.sub.2Si particles or is at least substantially free of any Mg.sub.2Si particles.

A method for manufacturing a hot-dip galvanized steel sheet according to one aspect of the present invention includes an annealing step of reduction-annealing a steel sheet having a Si content of 1.0% by mass or more and 3.0% by mass or less at a temperature equal to or higher than an A3 point of the steel sheet in an atmosphere having a dew point of −20° C. or higher; and a hot-dip galvanizing step of making the annealed steel sheet enter into a galvanizing bath to form a galvanized layer on a surface of the steel sheet, wherein a temperature of the steel sheet entering into the galvanizing bath is set to 390° C. or lower.

Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.