A galvannealed steel sheet includes: a steel sheet; a coating layer on a surface of the steel sheet; and a mixed layer formed between the steel sheet and the coating layer, in which the mixed layer includes a base iron portion having fine grains having a size of greater than 0 μm and equal to or smaller than 2 μm, a Zn—Fe alloy phase, and oxides containing one or more types of Mn, Si, Al, and Cr, and in the mixed layer, the oxides and the Zn—Fe alloy phase are present in grain boundaries that form the fine grains and the Zn—Fe alloy phase is tangled with the base iron portion.
[Mn]+[Si]+[Al]+[Cr]≧0.4  (Expression 1)

A galvannealed steel sheet includes: a steel sheet; a coating layer on a surface of the steel sheet; and a mixed layer formed between the steel sheet and the coating layer, in which the mixed layer includes a base iron portion having fine grains having a size of greater than 0 μm and equal to or smaller than 2 μm, a Zn—Fe alloy phase, and oxides containing one or more types of Mn, Si, Al, and Cr, and in the mixed layer, the oxides and the Zn—Fe alloy phase are present in grain boundaries that form the fine grains and the Zn—Fe alloy phase is tangled with the base iron portion.
[Mn]+[Si]+[Al]+[Cr]≧0.4  (Expression 1)

A gradient steel material with a high plastic surface layer and a high strength inner layer, and a manufacturing method are provided. Weight percentages of the components of the gradient steel material are: C≤0.15%, Si≤1%, Mn≤1.5%, and the balance of Fe and inevitable impurities, the surface layer of the steel material being ferrite, and the inner layer being ferrite+bainite. The manufacturing method therefor comprises: smelting, casting, rolling, and a heat treatment, wherein in the heat treatment step, a steel material is heated to an austenite temperature Ac3 or more and kept at said temperature for more than 3 min; thereafter, the material is cooled to a temperature range between Ar3 and Ar1 in a two-phase zone at a cooling rate of less than 0.5° C./s, and is then cooled to room temperature at a cooling rate of greater than 5° C./s. The present steel material does not need to be obtained by means of the compound preparation of different materials as only a single material is processed. At the same time, the composition of the steel material is simple. Although the internal and external microstructures are different, the difference is a gradual process, and the strength at the interface is good.

A gradient steel material with a high plastic surface layer and a high strength inner layer, and a manufacturing method are provided. Weight percentages of the components of the gradient steel material are: C≤0.15%, Si≤1%, Mn≤1.5%, and the balance of Fe and inevitable impurities, the surface layer of the steel material being ferrite, and the inner layer being ferrite+bainite. The manufacturing method therefor comprises: smelting, casting, rolling, and a heat treatment, wherein in the heat treatment step, a steel material is heated to an austenite temperature Ac3 or more and kept at said temperature for more than 3 min; thereafter, the material is cooled to a temperature range between Ar3 and Ar1 in a two-phase zone at a cooling rate of less than 0.5° C./s, and is then cooled to room temperature at a cooling rate of greater than 5° C./s. The present steel material does not need to be obtained by means of the compound preparation of different materials as only a single material is processed. At the same time, the composition of the steel material is simple. Although the internal and external microstructures are different, the difference is a gradual process, and the strength at the interface is good.

Provided are a non-quenched and tempered wire rod having excellent drawability and impact toughness suitable for materials for automobiles or mechanical parts and a method of manufacturing the same. According to an embodiment of the present disclosure, the non-quenched and tempered wire rod includes, in percent by weight (wt %), 0.05 to 0.35% of carbon (C), 0.05 to 0.5% of silicon (Si), 0.5 to 2.0% of manganese (Mn), 1.0% or less of chromium (Cr), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), 0.01 to 0.07% of soluble aluminum (sol.Al), 0.01% or less of nitrogen (N), at least one of 0.1% or less of niobium (Nb), 0.5% or less of vanadium (V), and 0.1% or less of titanium (Ti), and the remainder of iron (Fe) and inevitable impurities, and includes a ferrite-pearlite layered structure, as a microstructure, in a rolling direction.

This hot-rolled steel sheet has a predetermined chemical composition. The metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite. The average grain size of the metallographic structure excluding the residual austenite is 7.0 μm or less. The C concentration in the residual austenite is 0.5 mass % or more. The number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10.sup.6 carbides/mm.sup.2 or more.

An example track shoe, cutting edge, or other component of a machine is formed in a heated process, such as hot-rolling followed by air-hardening. The air-hardening process involves cooling the component by flowing air over the component (e.g., air cooling), such that the component is cooled at a controlled rate. During the air-cooling process, such as in the range of about 250° C. to about 1100° C., the component may be machined, such as by shearing, punching, drilling, etc. The machining may form the final shape of the component. As the air-hardening process is completed, and the component approaches room temperature, the component may have at least 5% bainitic crystal composition, and as high as greater than 80% bainitic crystal composition, resulting in relatively high hardness and fracture toughness. The final track shoe may have a hardness between about 40 HRC and 55 HRC.

An example track shoe, cutting edge, or other component of a machine is formed in a heated process, such as hot-rolling followed by air-hardening. The air-hardening process involves cooling the component by flowing air over the component (e.g., air cooling), such that the component is cooled at a controlled rate. During the air-cooling process, such as in the range of about 250° C. to about 1100° C., the component may be machined, such as by shearing, punching, drilling, etc. The machining may form the final shape of the component. As the air-hardening process is completed, and the component approaches room temperature, the component may have at least 5% bainitic crystal composition, and as high as greater than 80% bainitic crystal composition, resulting in relatively high hardness and fracture toughness. The final track shoe may have a hardness between about 40 HRC and 55 HRC.

In this cast austenitic stainless steel, in a cross section when heated at 1000° C., an average number Nc per unit area of carbides having an equivalent circle diameter of 500 nm or larger in a center portion of an austenite crystal grain is 6.0×10.sup.−2 particles/μm.sup.2 or more, and, when an average number per unit area of the carbides having an equivalent circle diameter of 500 nm or larger in a vicinity of a grain boundary in an austenite crystal grain is represented as Ngb, Ngb/Nc is 1.3 or less.

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is less than 1.020, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.