The present invention relates to a zinc plated steel sheet having excellent surface quality and spot weldability, and a manufacturing method therefore. A zinc plated steel sheet according to one aspect of the present invention comprises a base steel sheet and a zinc-based plating layer formed on the surface of the base steel sheet, wherein the GDOES profile of oxygen, which is measured in the depth direction from the surface of the base steel sheet, has a form in which a local minimum point and a local maximum point alternately appear in the depth direction from the surface, and the difference (a local maximum value—a local minimum value) between the oxygen concentration (a local minimum value) at the local minimum point and the oxygen concentration (a local maximum value) at the local maximum point can be 0.1 wt % or more.

The present invention relates to a zinc plated steel sheet having excellent surface quality and spot weldability, and a manufacturing method therefore. A zinc plated steel sheet according to one aspect of the present invention comprises a base steel sheet and a zinc-based plating layer formed on the surface of the base steel sheet, wherein the GDOES profile of oxygen, which is measured in the depth direction from the surface of the base steel sheet, has a form in which a local minimum point and a local maximum point alternately appear in the depth direction from the surface, and the difference (a local maximum value—a local minimum value) between the oxygen concentration (a local minimum value) at the local minimum point and the oxygen concentration (a local maximum value) at the local maximum point can be 0.1 wt % or more.

A cold-rolled sheet according to an example of the present invention comprises at most 0.004 wt % (exclusive of 0 wt %) of C, at most 0.02 wt % (exclusive of 0 wt %) of Si, 0.1 to 0.3 wt % of Mn, at most 0.05 wt % (exclusive of 0 wt %) of Al, at most 0.02 wt % (exclusive of 0 wt %) of P, at most 0.001 wt % (exclusive of 0 wt %) of S, at most 0.004 wt % (exclusive of 0 wt %) of N, 0.015 to 0.035 wt % of Ti, and 0.001 to 0.003 wt % of B, with the balance being Fe and other inevitable impurities, and has a microstructure in which the crystal grain aspect ratio defined by the following equation 1 is 1.4 to 4.0.

Crystal grain aspect ratio=average crystal grain diameter in the rolling direction/average crystal grain diameter in the thickness direction   [Equation 1]

A non-oriented electrical steel sheet according to an embodiment of the present invention includes, in wt %, Si: 1.5% or less, C: 0.01% or less (excluding 0%), Mn: 0.03 to 3%, P: 0.01 to 0.2%, S: 0.001 to 0.02%, Al: 0.01% or less (excluding 0%), N: 0.005% or less (excluding 0%), Cu: 0.02 to 0.3%, 0.0001 to 0.005 wt % of Ca and Mg either alone or in total, 0.001 to 0.2 wt % of Sb and Sn either alone or in total, and a balance of Fe and inevitable impurities.

A hot-rolled steel sheet for non-oriented electrical steel sheets includes, by mass %: C: 0.0010% to 0.0050%; Si: 1.90% to 3.50%; Al: 0.10% to 3.00%; Mn: 0.05% to 2.00%; P: 0.100% or less; S: 0.005% or less; N: 0.0040% or less; B: 0.0060% or less; Sn: 0% to 0.50%; Sb: 0% to 0.50%; Cu: 0% to 0.50%; REM: 0% to 0.0400%; Ca: 0% to 0.0400%; Mg: 0% to 0.0400%; and a remainder including Fe and impurities, in which a hardness H.sub.D of a deformed structure of a thickness middle portion (½t position) in a sheet width direction end portion of the hot-rolled steel sheet for non-oriented electrical steel sheets is Hv 220 or less.

A hot-rolled steel sheet for non-oriented electrical steel sheets includes, by mass %: C: 0.0010% to 0.0050%; Si: 1.90% to 3.50%; Al: 0.10% to 3.00%; Mn: 0.05% to 2.00%; P: 0.100% or less; S: 0.005% or less; N: 0.0040% or less; B: 0.0060% or less; Sn: 0% to 0.50%; Sb: 0% to 0.50%; Cu: 0% to 0.50%; REM: 0% to 0.0400%; Ca: 0% to 0.0400%; Mg: 0% to 0.0400%; and a remainder including Fe and impurities, in which a hardness H.sub.D of a deformed structure of a thickness middle portion (½t position) in a sheet width direction end portion of the hot-rolled steel sheet for non-oriented electrical steel sheets is Hv 220 or less.

A steel sheet including a chemical composition satisfying an equivalent carbon content of 0.60% or more and less than 0.85%, and a steel microstructure with an area fraction of ferrite: less than 40%, tempered martensite and bainite: 40% or more in total, retained austenite: 3% to 15%, and ferrite, tempered martensite, bainite, and retained austenite: 93% or more in total. A 90-degree bending at a curvature radius/thickness ratio of 4.2 in a rolling (L) direction with respect to an axis extending in a width (C) direction causes a change of 0.40 or more in (a grain size in a thickness direction)/(a grain size in a direction perpendicular to the thickness) of the tempered martensite in an L cross section in a 0- to 50-μm region from a surface of the steel sheet on a compression side. The steel sheet has a tensile strength of 980 MPa or more.

A steel sheet including a chemical composition satisfying an equivalent carbon content of 0.60% or more and less than 0.85%, and a steel microstructure with an area fraction of ferrite: less than 40%, tempered martensite and bainite: 40% or more in total, retained austenite: 3% to 15%, and ferrite, tempered martensite, bainite, and retained austenite: 93% or more in total. A 90-degree bending at a curvature radius/thickness ratio of 4.2 in a rolling (L) direction with respect to an axis extending in a width (C) direction causes a change of 0.40 or more in (a grain size in a thickness direction)/(a grain size in a direction perpendicular to the thickness) of the tempered martensite in an L cross section in a 0- to 50-μm region from a surface of the steel sheet on a compression side. The steel sheet has a tensile strength of 980 MPa or more.

An essentially lead free steel having improved machinability while reducing or eliminating lead (except for trace impurities) and without detriment of the material properties of the steel. The properties of the lead free steel are dependent on both the composition and method of manufacture. The improved lead free steel has, in percent by weight (wt-%): Carbon: 0.39-0.43%; Manganese: 0.75-1.00%; Silicon: 0.15-0.35%; Chromium: 0.80-1.05%; Molybdenum: 0.15-0.25%; at least one of Tellurium: 0.003-0.090 wt-%, Selenium: 0.080-0.2 wt-%, Sulfur: 0.065-0.09% wt-%, and Bismuth: 0.03-0.1 wt-%; and the balance being Fe and normally occurring scrap steel impurities. The hot-rolled lead-free steel product is subjected to a heat treatment at a first temperature for a first duration, at a second temperature for a second duration that is less than the first temperature, at a third temperature for a third time period that is greater than the second temperature, and subsequently cooling the steel product.

An essentially lead free steel having improved machinability while reducing or eliminating lead (except for trace impurities) and without detriment of the material properties of the steel. The properties of the lead free steel are dependent on both the composition and method of manufacture. The improved lead free steel has, in percent by weight (wt-%): Carbon: 0.39-0.43%; Manganese: 0.75-1.00%; Silicon: 0.15-0.35%; Chromium: 0.80-1.05%; Molybdenum: 0.15-0.25%; at least one of Tellurium: 0.003-0.090 wt-%, Selenium: 0.080-0.2 wt-%, Sulfur: 0.065-0.09% wt-%, and Bismuth: 0.03-0.1 wt-%; and the balance being Fe and normally occurring scrap steel impurities. The hot-rolled lead-free steel product is subjected to a heat treatment at a first temperature for a first duration, at a second temperature for a second duration that is less than the first temperature, at a third temperature for a third time period that is greater than the second temperature, and subsequently cooling the steel product.