Zinc-coated steel sheet

Abstract

This invention relates to a zinc-coated steel sheet, comprising a steel sheet with a thickness of 0.10–0.95 mm and a zinc-coated layer on the steel sheet. The steel sheet contains C: 0.05–0.30%, Si: 0.01–3.00%, Mn: 0.80–3.00%, and Al: 0.010–2.000%. The tensile strength of the zinc-coated steel sheet is 550–1500 MPa. When the thickness of the internal oxide layer on each side of the steel sheet is set to A (μm), the thickness of the decarburized layer on each side of the steel sheet is set to B (μm), and the thickness of the steel sheet is set to t (μm), the ratio of A / B is 0.01–0.50, and the ratio of B / t is 0.001–0.200.

Description

Technical Field

[0001] This invention relates to zinc-coated steel sheets. More specifically, this invention relates to thin and high-strength zinc-coated steel sheets with excellent appearance properties. Background Technology

[0002] In recent years, the use of high-strength steel sheets in various fields such as automobiles, home appliances, and building materials has been trending towards higher strength. For example, in the automotive industry, the use of high-strength steel sheets is increasing to improve fuel efficiency and reduce vehicle weight. Such high-strength steel sheets typically contain elements such as C, Si, and Mn to enhance the steel's strength.

[0003] Automotive components such as doors, hoods, and roofs have a much larger surface area compared to skeletal components like pillars. Therefore, from a lightweighting perspective, the high-strength steel sheets used in these components are generally thinner and much wider. Furthermore, since these components constitute a large portion of the car's exterior, they require not only high strength but also an excellent appearance.

[0004] On the other hand, especially when used in automotive components, high-strength steel sheets are coated with hot-dip galvanized or alloyed hot-dip galvanized layers to improve corrosion resistance. Generally, before such coatings are formed, the steel sheets (typically cold-rolled steel sheets) are annealed at a certain temperature to reduce strain and / or improve workability.

[0005] Relatedly, hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets containing C, Si, and Mn as steel sheet components, primarily suitable for use as automotive components, and methods for manufacturing the same have been disclosed (e.g., Patent Documents 1-4). Furthermore, Patent Documents 1-4 teach that, in order to manufacture the various coated steel sheets described in Patent Documents 1-4, an annealing process may be performed before the coating process, or an annealing process may also be performed.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2014 / 054141

[0009] Patent Document 2: Japanese Patent Application Publication No. 2014-058741

[0010] Patent Document 3: International Publication No. 2013 / 157222

[0011] Patent Document 4: Japanese Patent Application Publication No. 2010-065269 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] Elements typically found in high-strength steel sheets, such as Si and Mn, which have a strong affinity for oxygen, may combine with oxygen in the atmosphere during the annealing process to form an oxide layer near the surface of the steel sheet. Examples of such layers include: external oxidation, where Si and Mn oxides form as a film on the outer surface of the steel sheet; and internal oxidation, where oxides form on the inner surface of the steel sheet.

[0014] When a zinc-based coating (e.g., hot-dip galvanized layer or alloyed hot-dip galvanized layer) is formed on the surface of a steel sheet with an external oxide layer, the oxides, existing as a film on the surface of the steel sheet, hinder the interdiffusion of the steel component (e.g., Fe) and the coating component (e.g., Zn). As a result, the reaction between the steel and the coating may be adversely affected, leading to insufficient coating (e.g., an increase in uncoated areas). This can deteriorate the appearance and rust resistance of the coated steel sheet. Furthermore, these oxides can also affect the Fe-Zn reaction rate during alloying of the coating, potentially resulting in insufficient coating (e.g., uneven alloying). This can also deteriorate the appearance of the coated steel sheet. Therefore, from the viewpoint of obtaining a coated steel sheet with improved coating properties and thus a good appearance, a steel sheet with an internal oxide layer containing oxides is preferred over a steel sheet with an external oxide layer.

[0015] On the other hand, annealing conditions that lead to the formation of an internal oxide layer not only result in an internal oxide layer but also cause decarburization near the surface of the steel sheet. Decarburization refers to the process where carbon near the surface of the steel sheet combines with oxygen in the annealing atmosphere to form CO2, which is released into the system. The area lacking carbon is called the decarburized layer. Since carbon in steel contributes to its strength, a thicker decarburized layer reduces the strength near the steel sheet surface. The reduction in surface strength caused by this decarburized layer is relatively minor for thick zinc-coated steel sheets and is not particularly problematic. However, for very thin zinc-coated steel sheets, the strength reduction caused by decarburization becomes significant.

[0016] In view of the actual situation, the present invention aims to provide a zinc-coated steel sheet that can balance sufficient strength and coating properties in zinc-coated steel sheets of varying thicknesses.

[0017] Methods for solving problems

[0018] The inventors have discovered that in zinc-plated steel sheets with a thickness of 0.10 to 0.95 mm, by strictly controlling the ratio of the thickness of the internal oxide layer to the thickness of the decarburized layer on the surface of the steel sheet, as well as the ratio of the thickness of the decarburized layer to the thickness of the steel sheet, it is possible to achieve both high strength and high plating performance.

[0019] This invention is based on the above understanding, and its main points are as follows.

[0020] (1) A zinc-coated steel sheet comprising a steel sheet with a thickness of 0.10 to 0.95 mm and a zinc coating formed on at least one side of the steel sheet, the steel sheet having the following composition:

[0021] It contains, by mass%:

[0022] C: 0.05~0.30%

[0023] Si: 0.01~3.00%

[0024] Mn: 0.80~3.00%

[0025] Al: 0.010~2.000%

[0026] P: below 0.100%

[0027] S: Below 0.100%

[0028] N: below 0.0300%

[0029] O: below 0.010%

[0030] B: 0~0.0100%

[0031] Ti: 0~0.100%

[0032] Nb: 0~0.100%

[0033] V: 0~0.10%

[0034] Cr: 0–1.00%

[0035] Ni: 0-0.10%

[0036] Cu: 0–0.10%

[0037] Mo: 0–0.50%

[0038] W: 0-0.50%

[0039] Ca: 0–0.100%

[0040] Mg: 0–0.100%

[0041] Zr: 0~0.100%

[0042] Hf: 0~0.100%, and

[0043] REM: 0~0.100%,

[0044] The remaining part consists of Fe and impurities.

[0045] The zinc-coated steel sheet has a tensile strength of 550–1500 MPa.

[0046] When the thickness of the internal oxide layer on each side of the steel plate is set to A (μm), the thickness of the decarburized layer on each side of the steel plate is set to B (μm), and the plate thickness of the steel plate is set to t (μm), A / B: 0.01~0.50, B / t: 0.001~0.200.

[0047] (2) According to the zinc-coated steel sheet described in (1), wherein the width of the steel sheet is 1500 to 3000 mm, and the standard deviation of the width direction in the thickness of the internal oxide layer is set to a (μm) and the standard deviation of the width direction in the thickness of the decarburized layer is set to b (μm), A: 1.0 μm or more, B: 10 μm or more, a / A: 0.15 or less, b / B: 0.20 or less.

[0048] (3) The zinc-coated steel sheet according to (1) or (2), wherein, by mass %, Si+Mn: less than 5.00% and Si / Mn: less than 1.50.

[0049] (4) The zinc-coated steel sheet according to any one of (1) to (3), wherein the steel sheet has a composition containing one or more of the following elements in a mass percentage:

[0050] B: 0.0001~0.0100%

[0051] Ti: 0.001~0.100%

[0052] Nb: 0.001~0.100%

[0053] Cr: 0.01~1.00%

[0054] Mo: 0.01~0.50%, and

[0055] W: 0.01–0.50%.

[0056] (5) The zinc-coated steel sheet according to any one of (1) to (4), wherein the steel sheet has a composition containing one or more of the following elements in a mass percentage:

[0057] V: 0.01~0.10%

[0058] Ni: 0.01~0.10%

[0059] Cu: 0.01–0.10%

[0060] Ca: 0.001~0.100%

[0061] Mg: 0.001~0.100%

[0062] Zr: 0.001~0.100%

[0063] Hf: 0.001~0.100%, and

[0064] REM: 0.001~0.100%.

[0065] (6) The zinc-coated steel sheet according to any one of (1) to (5), wherein the zinc-coated steel sheet is an alloyed hot-dip galvanized steel sheet, the zinc coating comprises Fe: 5-15% and Al: 0.01-1%, the remainder being Zn and impurities, and the amount of the zinc coating on each single side is 10-100 g / m². 2 .

[0066] Invention Effects

[0067] According to the present invention, it is possible to provide zinc-coated steel sheets that can balance sufficient strength and coating properties in zinc-coated steel sheets with a thickness of 0.10 to 0.95 mm, and thin and high-strength zinc-coated steel sheets with excellent appearance properties can be obtained. Attached Figure Description

[0068] Figure 1 This is a diagram illustrating the method for measuring the thickness of the internal oxide layer in this invention.

[0069] Figure 2 This is a diagram illustrating the method for measuring the thickness of the decarburized layer in this invention. Detailed Implementation

[0070] For example, zinc-coated steel sheets used in automotive components are generally manufactured as follows: steel billets with a specified composition of C, Si, and Mn are hot-rolled and cold-rolled, then annealed, zinc-coated, and subsequently alloyed as needed. During the annealing prior to zinc-coating, in order to form a zinc coating without uncoated areas and / or to reduce alloying inhomogeneity (inhomogeneity in the appearance after alloying), it is preferable to appropriately control the annealing conditions to form an internal oxide layer containing oxides such as Si, Mn, and Al on the surface (i.e., the interior) of the steel sheet. By forming this oxide inside the steel sheet, the interdiffusion of the steel component (e.g., Fe) and the coating component (e.g., Zn) is well induced during zinc-coating, resulting in a well-formed zinc coating without uncoated areas. This reduces alloying inhomogeneity during alloying and improves coating properties. As a result, zinc-coated steel sheets with excellent appearance properties can be obtained. Therefore, in zinc-coated steel sheets, controlling the annealing conditions to form an internal oxide layer on the surface of the steel sheet is effective in ensuring sufficient coating properties.

[0071] On the other hand, if annealing is performed under conditions that form an internal oxide layer, a decarburized layer usually forms near the surface of the steel sheet. Regarding the decarburized layer, since there is insufficient carbon formation, which greatly contributes to the strength of the steel sheet, the presence of the decarburized layer leads to a reduction in strength near the surface of the steel sheet. When zinc-coated steel sheets are used in thicker components, the strength reduction caused by the decarburized layer is not significant. However, for thinner components, such as automotive components like doors, engine hoods, or roofs, the strength reduction caused by the decarburized layer is significant, making the control of the decarburized layer thickness extremely important. Therefore, in zinc-coated steel sheets with a thickness of 0.10 to 0.95 mm (less than 1 mm), it is preferable to balance the following: forming an internal oxide layer from the viewpoint of obtaining good plating properties (appearance); and reducing the thickness of the decarburized layer from the viewpoint of ensuring strength.

[0072] Therefore, the inventors conducted various studies to achieve both high strength and high plating performance in zinc-coated steel sheets with a thickness of 0.10 to 0.95 mm. They found that the following were effective: adding a specified amount or more of C, Mn, Si, and Al to the steel, and then closely controlling the ratio of the thickness of the internal oxide layer to the thickness of the decarburized layer and the ratio of the thickness of the decarburized layer to the thickness of the steel sheet. More specifically, the inventors found that when the thickness of the internal oxide layer on each side of the steel sheet is set to A (μm), the thickness of the decarburized layer on each side of the steel sheet is set to B (μm), and the thickness of the steel sheet is set to t (μm), setting A / B in the range of 0.01 to 0.50 and B / t in the range of 0.001 to 0.200 is important for ensuring both plating performance and strength.

[0073] Zinc-coated steel sheet

[0074] The zinc-coated steel sheet (hereinafter sometimes simply referred to as "coated steel sheet") of the present invention will be described in detail below.

[0075] The zinc-coated steel sheet of the present invention comprises a steel sheet and a zinc-coated layer formed on at least one side of the steel sheet. Therefore, the zinc-coated layer can be formed on one side or both sides of the steel sheet. Furthermore, in the present invention, the zinc-coated layer only needs to be formed on the steel sheet, but other coatings can also be provided between the steel sheet and the zinc-coated layer. As described below, the zinc-coated layer refers to a coating containing zinc, such as a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.

[0076] [Steel plate]

[0077] (Composition)

[0078] The composition of the steel plate in this invention will be described. Unless otherwise specified, the "%" for element content refers to "mass %". In the numerical range of the composition, the range indicated by "~" unless otherwise specified refers to the range including the values ​​before and after "~" as the lower and upper limits.

[0079] (C: 0.05~0.30%)

[0080] Carbon (C) is an important element in ensuring the strength of zinc-coated steel sheets. Insufficient C content may result in inadequate strength. Therefore, the C content is 0.05% or more, preferably 0.07% or more, more preferably 0.08% or more, and even more preferably 0.10% or more. On the other hand, excessive C content may lead to excessively high strength and reduced weldability. Therefore, the C content is 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

[0081] (Si: 0.01~3.00%)

[0082] Silicon (Si) is an effective element for improving the strength of steel sheets. Furthermore, Si is one of the elements that combines with oxygen during annealing to form internal oxides. On the other hand, Si also affects plating properties and alloying speed. If the Si content is insufficient, internal oxide formation may not be adequate. Therefore, the Si content is 0.01% or more, preferably 0.03% or more or 0.05% or more, more preferably 0.10% or more or 0.30% or more. On the other hand, if the Si content is excessive, it may cause deterioration of surface properties, resulting in poor appearance. Furthermore, a Si-based oxide film (external oxide layer) may form on the surface of the steel sheet. Therefore, the Si content is 3.00% or less, preferably 2.50% or less, more preferably 2.00% or less.

[0083] (Mn: 0.80~3.00%)

[0084] Manganese (Mn) is an effective element for improving the strength of steel sheets by obtaining a hard microstructure. Furthermore, Mn is also one of the elements that combines with oxygen during annealing to form internal oxides. If the Mn content is insufficient, adequate strength may not be guaranteed. Therefore, the Mn content is 0.80% or more, preferably 1.00% or more, and more preferably 1.20% or more. On the other hand, if excessive Mn is added, the metal microstructure may become uneven due to Mn segregation, reducing workability. Therefore, the Mn content is 3.00% or less, preferably 2.80% or less, and more preferably 2.50% or less.

[0085] (Si+Mn: less than 5.00% and Si / Mn: less than 1.50)

[0086] In the composition of the steel sheet of the present invention, from the viewpoint of further suppressing the formation of the external oxide layer and thus promoting the formation of the internal oxide layer, it is preferable to set the sum of the Si content and the Mn content (Si+Mn) to 5.00% or less, more preferably to 4.80% or less, and even more preferably to 4.50% or less. The lower limit of Si+Mn is not particularly limited, but for example, Si+Mn may also be 0.81% or more, 0.90% or more, 1.00% or more, or 1.20% or more. Furthermore, from the viewpoint of improving the appearance of the zinc-coated steel sheet, it is preferable to set the ratio of Si content to Mn content (Si / Mn) to 1.50 or less, more preferably to 1.20 or less. Si and Mn contribute to improving the strength of the steel; on the other hand, if they are present in excessive amounts, they may deteriorate the appearance. Moreover, regarding this effect on appearance, Si has a more significant impact than Mn. Therefore, from the viewpoint of strength and the formation of internal oxides, by containing a specified amount of Si and Mn and reducing the Si / Mn ratio, it is possible to maintain the appearance properties better. There is no particular limitation on the lower limit of the Si / Mn ratio, but for example, it can be 0.01 or more, 0.03 or more, or 0.10 or more. Al is an element that can replace Si, but when Al is added as a substitute for Si, the Al content does not need to be considered in the above sums and ratios.

[0087] (Al: 0.010~2.000%)

[0088] Al (aluminum) is an element that functions as a deoxidizer. If the Al content is below 0.010%, the deoxidation effect may not be sufficient. Therefore, the Al content is 0.010% or more, preferably 0.100% or more, and more preferably 0.200% or more. On the other hand, if Al is present in excess, it may cause a decrease in processability and a deterioration in surface properties. Therefore, the Al content is 2.000% or less, preferably 1.500% or less, and more preferably 1.000% or less.

[0089] (P: below 0.100%)

[0090] Phosphorus (P) is an impurity commonly found in steel. When the P content exceeds 0.100%, weldability may decrease. Therefore, the P content is preferably 0.100% or less, more preferably 0.080% or less, more preferably 0.050% or less, and even more preferably 0.020% or less. While there is no particular limitation on the lower limit of the P content, from a manufacturing cost perspective, the P content can also be greater than 0% or 0.001%.

[0091] (S: less than 0.100%)

[0092] Sulfur (S) is an impurity commonly found in steel. When the S content exceeds 0.100%, weldability may decrease, leading to increased MnS precipitation and reduced workability such as bendability. Therefore, the S content is preferably 0.100% or less, more preferably 0.080% or less, more preferably 0.050% or less, and even more preferably 0.020% or less. While there is no particular limitation on the lower limit of the S content, from the viewpoint of desulfurization cost, the S content can also be greater than 0% or 0.001%.

[0093] (N: below 0.0300%)

[0094] Nitrogen (N) is an impurity commonly found in steel. When the N content exceeds 0.0300%, weldability may decrease. Therefore, the N content is preferably 0.0300% or less, more preferably 0.0200% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. While there is no particular limitation on the lower limit of the N content, from a manufacturing cost perspective, the N content can also be more than 0% or more than 0.0010%.

[0095] (O: less than 0.010%)

[0096] Oxygen (O) is an impurity commonly found in steel. When the O content exceeds 0.010%, it can potentially lead to a deterioration in ductility. Therefore, the O content is preferably 0.010% or less, more preferably 0.008% or less, more preferably 0.006% or less, and even more preferably 0.005% or less. While there is no particular limitation on the lower limit of the O content, from a manufacturing cost perspective, the O content can also be greater than 0% or more than 0.001%.

[0097] In addition to the elements described above, the steel plate of this invention may also contain optional elements as described below, as needed.

[0098] (B: 0~0.0100%)

[0099] Boron (B) is an element that improves hardenability, thereby contributing to increased strength, and also strengthens grain boundaries through segregation at grain boundaries, thus improving toughness. Therefore, it can be included as needed. Thus, the B content is 0% or more, preferably 0.0001% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the B content is 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less.

[0100] (Ti: 0~0.100%)

[0101] Titanium (Ti) is an element that precipitates during the cooling of steel in the form of TiC and contributes to increased strength; therefore, it can be included as needed. Thus, the Ti content is 0% or more, preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if Ti is present in excess, coarse TiN may form, impairing toughness; therefore, the Ti content is 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.

[0102] (Nb: 0~0.100%)

[0103] Niobium (Nb) has the effect of forming NbC in steel and refining grain size, thus contributing to increased strength. Therefore, it can be included as needed. Consequently, the Nb content is 0% or more, preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the Nb content is 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.

[0104] (V: 0~0.10%)

[0105] Vanadium (V) is an element that contributes to strength by forming vitamin C, and therefore can be included as needed. Therefore, the V content is 0% or more, preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the V content is 0.10% or less, preferably 0.09% or less, and more preferably 0.08% or less.

[0106] (Cr: 0-1.00%)

[0107] Chromium (Cr) is an element that contributes to the strength and corrosion resistance of steel, and therefore can be included as needed. Thus, the Cr content is 0% or more, preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, if Cr is present in excess, Cr carbides may form in large quantities, which can impair hardenability. Therefore, the Cr content is 1.00% or less, preferably 0.90% or less, more preferably 0.80% or less, and even more preferably 0.60% or less.

[0108] (Ni: 0-0.10%)

[0109] Nickel (Ni) is an element that contributes to the strength and corrosion resistance of steel, and therefore can be included as needed. Thus, the Ni content is 0% or more, preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, from the viewpoint of manufacturing cost, the Ni content is 0.10% or less, preferably 0.08% or less.

[0110] (Cu: 0-0.10%)

[0111] Cu (copper) is an element that contributes to the strength and corrosion resistance of steel, and therefore can be included as needed. Therefore, the Cu content is 0% or more, preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, from the viewpoint of suppressing reduced toughness, cracking of the slab after casting, and reduced weldability, the Cu content is 0.10% or less, preferably 0.08% or less.

[0112] (Mo: 0-0.50%)

[0113] Mo (molybdenum) is an element that contributes to the strength and corrosion resistance of steel, and therefore can be included as needed. Thus, the Mo content is 0% or more, preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the Mo content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0114] (W: 0-0.50%)

[0115] Tungsten (W) is effective in improving the strength of steel, and therefore can be included as needed. Thus, the W content is 0% or more, preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, from the viewpoint of suppressing the reduction in toughness and weldability, the W content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0116] (Ca: 0~0.100%)

[0117] Ca (calcium) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Ca content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, even more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if Ca is present in excess, the surface properties may deteriorate significantly; therefore, the Ca content is 0.100% or less, preferably 0.080% or less, and even more preferably 0.050% or less.

[0118] (Mg: 0-0.100%)

[0119] Magnesium (Mg) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Mg content is 0% or more, preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.010% or more. On the other hand, if Mg is present in excess, the deterioration of surface properties may become more pronounced; therefore, the Mg content is 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.

[0120] (Zr: 0~0.100%)

[0121] Zirconium (Zr) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Zr content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if Zr is present in excess, the deterioration of surface properties may become more pronounced; therefore, the Zr content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0122] (Hf: 0~0.100%)

[0123] Hafnium (Hf) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Hf content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if Hf is present in excess, the deterioration of surface properties may become more pronounced; therefore, the Hf content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0124] (REM: 0~0.100%)

[0125] Rare earth elements (REMs) are elements that help control inclusions, especially the fine dispersion of inclusions, and improve toughness; therefore, they can be included as needed. Thus, the REM content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if excessive REM is present, the deterioration of surface properties may become more pronounced; therefore, the REM content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less. It should be noted that REM is an abbreviation for Rare Earth Metal, referring to elements belonging to the lanthanide series. REM is usually added in the form of mixed rare earth alloys.

[0126] In this invention, the steel plate may also contain one or more of the optional elements listed above: B: 0.0001–0.0100%, Ti: 0.001–0.100%, Nb: 0.001–0.100%, Cr: 0.01–1.00%, Mo: 0.01–0.50%, and W: 0.01–0.50%. The steel plate may also alternatively or additionally contain one or more of the optional elements listed above: V: 0.01–0.10%, Ni: 0.01–0.10%, Cu: 0.01–0.10%, Ca: 0.001–0.100%, Mg: 0.001–0.100%, Zr: 0.001–0.100%, Hf: 0.001–0.100%, and REM: 0.001–0.100%.

[0127] In the steel sheet of this invention, the remaining portion other than the above-mentioned components consists of Fe and impurities. Here, impurities refer to components that are mixed in during the industrial manufacturing of steel sheets due to various factors in the manufacturing process, such as raw materials like ores and waste, and are permitted to be present within a range that does not adversely affect the characteristics of the zinc-plated steel sheet of this invention.

[0128] The compositional analysis of the steel sheet can be performed using any chemical analysis known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). Specifically, C and S can be determined using combustion-infrared absorption spectrometry, and N can be determined using inert gas melting-thermal conductivity analysis. These analyses are preferably performed on samples of the steel sheet collected according to the methods of JIS G0417:1999.

[0129] (Thickness and width of the steel plate)

[0130] This invention focuses on zinc-coated steel sheets using steel plates with a thickness of 0.10 to 0.95 mm. More specifically, for example, it focuses on zinc-coated steel sheets using steel plates with a thickness of 0.10 to 0.95 mm that are suitable for use in automotive components such as doors, hoods, or roofs. From the viewpoint of ensuring strength, the thickness of the steel sheet is preferably 0.20 mm or more, more preferably 0.30 mm or more. From the perspective of lightweighting, the thickness is preferably thinner, preferably 0.70 mm or less, more preferably 0.60 mm or less, and even more preferably 0.50 mm or less. Furthermore, since the zinc-coated steel sheet of this invention is suitable for use, particularly in automotive components such as doors, hoods, or roofs, the width of the zinc-coated steel sheet (i.e., the steel plate) is typically 1000 mm or more, and depending on the case, 1500 mm or more. The thickness of the steel sheet can be determined using a micrometer or similar tool. When measuring the thickness of a steel sheet after removing the coating, the coating can be removed and the thickness determined by measuring with a micrometer or by observing the cross-section. The width of the steel sheet can be directly measured using a scale or measuring tape.

[0131] (Internal oxide layer)

[0132] The zinc-coated steel sheet of the present invention has an internal oxide layer containing oxides on the surface (inside) of the steel sheet. This internal oxide layer contains one or more elements, besides oxygen, that are present in the steel sheet. Typically, it has a composition containing Si, O, Fe, and Mn, and further, depending on the situation, Al. Typically, the oxide has a composition of Mn₂SiO₄ or SiO₂, MnO, etc. In the zinc-coated steel sheet of the present invention, instead of forming an external oxide layer, i.e., a layer of oxides formed in a film on the surface of the steel sheet, an internal oxide layer, i.e., a layer containing oxides inside the steel sheet, is formed. Therefore, when a coating is formed, good interdiffusion of the steel component (e.g., Fe) and the coating component (e.g., Zn) is induced, and a zinc-coated coating is well formed on the steel sheet, resulting in a zinc-coated steel sheet with excellent appearance properties, free of uncoated portions. Furthermore, when alloying treatment is performed, uneven alloying can be suppressed, resulting in a zinc-coated steel sheet with excellent appearance properties. It should be noted that, to ensure plating performance, only a certain amount of internal oxide layer needs to be formed. Conversely, if the internal oxide layer is formed too thickly, the decarburized layer, which is a factor in reducing strength, will also become thicker. Therefore, it is not preferable to form an excessive amount of internal oxide layer. For example, the thickness of the internal oxide layer on each side can be about 0.01 to 0.20 times the thickness of the steel plate. The thickness A (μm) of the internal oxide layer on each side of the steel plate in this invention is not particularly limited, but it can be, for example, 1.0 μm or more, 2.0 μm or more, 3.0 μm or more, or 5.0 μm or more. The upper limit of A is not particularly limited, but from the viewpoint of suppressing the excessive formation of the decarburized layer, it can be, for example, 50.0 μm or less, 40.0 μm or less, 30.0 μm or less, or 20.0 μm or less.

[0133] (Decarburized layer)

[0134] The zinc-coated steel sheet of the present invention has a decarburized layer near the surface of the steel sheet. The decarburized layer has a lower carbon concentration than the main part of the steel sheet (e.g., the center of the sheet thickness). In the present invention, the decarburized layer refers to a region existing on the steel sheet side from the interface between the steel sheet and the zinc-coated layer, and having a lower carbon concentration compared to the base material. Furthermore, in the zinc-coated steel sheet of the present invention, since the thickness of the decarburized layer of the steel sheet is controlled relative to the sheet thickness and the thickness of the internal oxide layer as described below, the strength reduction of the zinc-coated steel sheet caused by the presence of the decarburized layer can be suppressed, and high strength can be achieved while ensuring coating performance. From the viewpoint of ensuring higher strength, the thickness B (μm) of the decarburized layer in the present invention is preferably small. However, since the decarburized layer is formed together with the internal oxide layer, there is no particular limitation, but B is substantially 10 μm or more, for example, it can also be 15 μm or more, 20 μm or more, 25 μm or more, 30 μm or more, or 40 μm or more. From the viewpoint of ensuring higher strength, especially higher and more uniform strength, the upper limit of B is preferably 200 μm or less, more preferably 150 μm or less or 120 μm or less, and even more preferably 100 μm or less or 90 μm or less.

[0135] (A / B: 0.01~0.50)

[0136] In the zinc-coated steel sheet of the present invention, to obtain high plating performance, it is necessary to promote the formation of the internal oxide layer; on the other hand, to obtain high strength, it is necessary to suppress the formation of the decarburized layer. Therefore, it is preferable to ensure a certain thickness A (μm) of the internal oxide layer per side while reducing the thickness B (μm) of the decarburized layer per side. Thus, in the present invention, the lower limit of A / B is set to 0.01. By having A / B of 0.01 or higher, both high strength and high plating performance can be achieved. If A / B becomes lower than 0.01, the formation of the internal oxide layer may be insufficient, resulting in reduced plating performance, and / or the decarburized layer may become thicker, resulting in insufficient strength. The lower limit of A / B is preferably 0.03 or 0.05, more preferably 0.10 or 0.15, and even more preferably 0.20. On the other hand, generally, under conditions that cause internal oxidation, the thickness B of the decarburized layer becomes thicker than the thickness A of the internal oxide layer, so the upper limit of A / B is substantially 0.50. The upper limit of A / B can also be lower than 0.50, 0.45, 0.40, or 0.30. It should be noted that, from the viewpoint of ensuring strength, the decarburized layer is preferably thinner, but to ensure plating properties, some is formed during the formation of the internal oxide layer; therefore, in this invention, the thickness B of the decarburized layer per single side will not be 0. It should be noted that A refers to the thickness of the internal oxide layer per single side of the steel plate.

[0137] (B / t: 0.001~0.200)

[0138] Furthermore, in the zinc-coated steel sheet of the present invention, in order to suppress the formation of a decarburized layer and achieve high strength, it is preferable to reduce the thickness B (μm) of the decarburized layer per side relative to the thickness t (μm) of the steel sheet. In particular, since the thickness of the steel sheet of the present invention is very thin, ranging from 0.10 to 0.95 mm (100 to 950 μm), such control becomes extremely important. Therefore, in the present invention, the upper limit of B / t is set to 0.200. By keeping B / t below 0.200, the effect of strength reduction caused by the presence of the decarburized layer can be suppressed, and a zinc-coated steel sheet with sufficient strength can be obtained. If B / t exceeds 0.200, the decarburized layer may become thicker relative to the thickness of the steel sheet, resulting in insufficient strength. The upper limit of B / t can also be 0.190, 0.180, 0.160, 0.140, 0.120, or 0.100. On the other hand, in this invention, to ensure plating properties, an internal oxide layer of a certain thickness needs to be formed, and therefore a decarburized layer is also formed. Thus, the lower limit of B / t is substantially 0.001. The lower limit of B / t can also be 0.010, 0.020, 0.030, 0.040, or 0.050. It should be noted that B refers to the thickness of the decarburized layer on each single side of the steel plate. That is, when B affects the strength of the steel plate, both sides of the steel plate are affected.

[0139] (Preferred embodiment when the plate width is 1500-3000 mm)

[0140] When zinc-coated steel sheets are used for automotive components such as doors, hoods, or roofs, these components have very large areas, so the zinc-coated steel sheets typically have a width of 1500 mm or more. According to a preferred embodiment of the invention, by controlling not only the ratio of the thickness of the internal oxide layer to the thickness of the decarburized layer and the ratio of the thickness of the decarburized layer to the thickness of the steel sheet surface within specified ranges, as described above, but also by strictly controlling the deviations in the thickness of the internal oxide layer and the decarburized layer in the width direction of the steel sheet, good properties can be reliably achieved throughout the steel sheet, even in such wide and therefore large-area zinc-coated steel sheets. More specifically, in zinc-coated steel sheets having a thickness of 0.10 to 0.95 mm and a wide width of 1500 to 3000 mm, when the thickness of the internal oxide layer on each side of the steel sheet is set to A (μm), the thickness of the decarburized layer on each side of the steel sheet is set to B (μm), the standard deviation of the thickness of the internal oxide layer in the width direction is set to a (μm), and the standard deviation of the thickness of the decarburized layer in the width direction is set to b (μm), by controlling A: 1.0 μm or more, B: 10 μm or more, a / A: 0.15 or less, and b / B: 0.20 or less, both high strength and high coating performance can be reliably achieved in the entire steel sheet. In particular, according to a preferred embodiment of the present invention, with the above configuration, high coating performance and improved appearance can be maintained not only before the steel sheet is formed but also even after the form is formed and strain is applied. Hereinafter, the preferred embodiment of the present invention will be described in more detail.

[0141] (Width of the steel plate)

[0142] The width of the steel plate can be 1500-3000mm, for example, it can also be 1600mm or more, 1800mm or more, or 2000mm or more. As described above, the width of the steel plate can be directly measured using a scale, measuring tape, etc.

[0143] (A: ≥1.0μm and a / A: ≤0.15)

[0144] In a preferred embodiment of the present invention, when the thickness of the internal oxide layer on each single side of the steel sheet is set to A (μm) and the standard deviation of the thickness of the internal oxide layer in the width direction is set to a (μm), A is 1.0 μm or more, and a / A is 0.15 or less. By setting a / A to 0.15 or less, deviations in the thickness of the internal oxide layer in the width direction of the steel sheet having a width of 1500 to 3000 mm can be suppressed, and the thickness of the internal oxide layer can be controlled sufficiently uniformly throughout the steel sheet. As a result, the zinc-plated steel sheet of the present invention becomes able to have high and uniform plating properties as a whole. On the other hand, if the value of a / A is too large, the thickness of the internal oxide layer throughout the steel sheet may not be sufficiently uniform, and the plating properties in the width direction may not become uniform, making it impossible for the zinc-plated steel sheet to have high and uniform plating properties as a whole. a / A is preferably 0.14 or less or 0.12 or less, more preferably 0.10 or less, and even more preferably 0.08 or less. There is no specific lower limit for a / A, but since manufacturing may cause some deviations in the thickness of the internal oxide layer, a / A can also be greater than 0 or 0.01.

[0145] (B: above 10μm and b / B: below 0.20)

[0146] In a preferred embodiment of the present invention, when the thickness of the decarburized layer on each single side of the steel sheet is set to B (μm) and the standard deviation of the thickness of the decarburized layer in the width direction is set to b (μm), B is 10 μm or more, and b / B is 0.20 or less. By setting b / B to 0.20 or less, deviations in the thickness of the decarburized layer in the width direction of a steel sheet with a width of 1500 to 3000 mm can be suppressed. The uniformity of the decarburized layer in the width direction greatly affects the appearance of the outer panel when it is subjected to slight strain. That is, when local strength unevenness occurs under conditions such as thin steel sheet, high steel sheet strength, and relatively thick decarburized layer, strain imposed from the outside may concentrate in the areas with thick decarburized layer and low strength, causing local unevenness on the surface of the steel sheet. In such cases, even if there is a good appearance before forming, appearance unevenness in the L direction (rolling direction) may occur after strain is imposed. By reducing the b / B ratio, localized material inhomogeneity can be suppressed, maintaining high plating quality even after forming, thus ensuring a uniform appearance. The b / B ratio is preferably 0.15 or less, more preferably 0.12 or less, and even more preferably 0.10 or less. While the lower limit of the b / B ratio is not particularly limited, it can also be greater than 0 or 0.01, as it can cause some thickness variations in the decarburized layer during manufacturing.

[0147] (Determination of the thickness A (μm) of the internal oxide layer and calculation of its thickness deviation a (μm))

[0148] The thickness A (μm) of the internal oxide layer and the calculation of its deviation a (μm) are performed as follows. The zinc coating of the zinc-plated steel sheet to be measured is chemically dissolved and removed using a solution containing an inhibitor such as hydrochloric acid. The composition along the thickness direction of the steel sheet is analyzed using a glow discharge surface analyzer (GDS) to evaluate the thickness of the internal oxide layer. Specifically, the sputtering depth after sputtering for a certain period from the steel sheet surface towards the thickness direction is measured using a roughness gauge, laser microscope, etc., and the sputtering velocity per unit time is calculated. Here, if internal oxidation of Mn occurs, the Mn concentration profile temporarily decreases compared to the Mn signal intensity of the steel substrate, then slowly increases until it reaches the Mn signal intensity of the steel substrate. In this invention, the position where the Mn signal intensity reaches 0.9 times the Mn signal intensity of the steel substrate is defined as the internal oxidation position. The sputtering time to reach this internal oxidation position is converted into thickness based on the sputtering velocity, and this thickness is set as the thickness of the internal oxide layer. That is, in this invention, the internal oxide layer refers to the region where the Mn concentration is less than 0.9 times that of the steel substrate in the measurement using a glow discharge surface analyzer (GDS). Figure 1 An example of a GDS curve for Mn, etc., is shown. The above operation is performed at three or more locations, and the thickness of the internal oxide layer obtained at each location is averaged to obtain the thickness A (μm) of the internal oxide layer per side in this invention. In particular, in the case of a steel plate with a width of 1500–3000 mm, a total of 18 locations are used in the width direction of the steel plate: three locations at one end, the center, and the other end opposite to one end; three locations at equal intervals between one end and the center; and three locations at equal intervals between the center and the other end. The thickness A (μm) of the internal oxide layer per side in this invention is obtained by averaging the thickness of the internal oxide layer obtained at each location. Furthermore, the standard deviation a (μm) of the thickness of the internal oxide layer in the width direction in this invention is set to the standard deviation of the thickness of the internal oxide layer at the 18 locations obtained as described above. A high-frequency type of GDS is used. In this specification, the "one end" and "the other end" as the measurement location are respectively set to 100mm inside from the actual one end and the other end of the steel plate.

[0149] (Determination of the thickness B (μm) of the decarburized layer and calculation of its thickness deviation b (μm))

[0150] The thickness B (μm) of the decarburized layer and the calculation of its deviation b (μm) are performed as follows. After chemically dissolving and removing the zinc coating of the zinc-plated steel sheet to be tested in a solution containing an inhibitor such as hydrochloric acid, the thickness of the decarburized layer is evaluated by measuring the cross-sectional hardness distribution of the steel sheet. Specifically, the steel sheet is etched with 2% nitric acid ethanol to observe the steel sheet microstructure and estimate the thickness of the decarburized layer. Then, the Vickers hardness of the cross-section is measured every 5 μm from the surface (the interface between the steel sheet and the coating) towards the thickness direction. A measurement length of approximately 100–150 μm is usually sufficient for judgment. The thickness of the decarburized layer is defined as the distance from the surface of the steel sheet to the hardness of the steel sheet, based on Vickers hardness. It should be noted that the "hardness of the steel sheet" in this embodiment refers to the average hardness measured at any 10 locations in the center of the thickness direction of the steel sheet cross-section. An example of the actual measurement of the thickness of the decarburized layer is shown below. Figure 2 The Vickers hardness load is not specifically specified, but since measurements are taken every 5 μm, a load that is not too large is preferred. Figure 2 The graph shows measurements taken under a load of 25 gf. The above operations were performed at three or more locations, and the thickness of the decarburized layer at each location was averaged to obtain the thickness B (μm) of the decarburized layer per side in this invention. Specifically, in the case of steel plates with a width of 1500–3000 mm, the thickness B (μm) of the decarburized layer per side in this invention was obtained from nine locations in the width direction of the steel plate: three locations at one end, the center, and the opposite end; three locations at equal intervals between one end and the center; and three locations at equal intervals between the center and the other end. This was performed on both the front and back sides, totaling 18 locations. The thickness of the decarburized layer at each location was averaged to obtain the thickness B (μm) of the decarburized layer per side in this invention. Furthermore, the standard deviation b (μm) of the thickness of the decarburized layer in the width direction in this invention was set to the standard deviation of the thickness of the decarburized layer at the 18 locations as described above.

[0151] (tensile strength)

[0152] The zinc-coated steel sheet of the present invention has a tensile strength of 550 to 1500 MPa. With a tensile strength of 550 MPa or more, sufficient strength can be ensured, and for example, in the case of automotive components, weight reduction due to thinner walls can be achieved. The tensile strength is preferably 580 MPa or more, or 600 MPa or more, more preferably 620 MPa or more, further preferably 650 MPa or more, and most preferably 700 MPa or more. On the other hand, if the tensile strength is too high, the workability may decrease; therefore, the tensile strength is set to 1500 MPa or less, or possibly 1400 MPa or less, or 1300 MPa or less. The tensile test was performed using the method specified in JIS-Z2241:2011 for JIS No. 5 tensile test piece, and the crosshead test speed was set to 30 mm / min. In particular, in the case of steel plates with a width of 1500 to 3000 mm, the tensile test is performed in the width direction of the steel plate at a total of nine locations: one end, the central part, and the other end opposite to the one end; three locations at equal intervals between one end and the central part; and three locations at equal intervals between the central part and the other end. The average value of the tensile strength obtained at each location is taken as the tensile strength of the present invention.

[0153] [Zinc-based coating]

[0154] The zinc-based coating of this invention is formed on at least one side of a steel sheet. "Zinc-based coating" typically refers to a coating whose main component (i.e., more than 50%) is Zn, but it also includes coatings where, after plating, heat treatment (e.g., alloying) causes the components in the steel to diffuse, resulting in a Zn content of less than 50%. Zinc-based coatings can be formed by various methods, but hot-dip galvanizing is preferred. Furthermore, from the viewpoint of improving weldability and / or paintability, alloying after hot-dip galvanizing is more preferred. Therefore, the zinc-based coated steel sheet of this invention is preferably a hot-dip galvanized steel sheet, and more preferably an alloyed hot-dip galvanized steel sheet. In this invention, since an internal oxide layer is formed on the surface of the steel sheet instead of an external oxide layer, a zinc-based coating can be formed, for example, with the uncoated portion suppressed, and in the case of alloying treatment, a zinc-based coating can be formed with alloying unevenness suppressed.

[0155] (Composition of zinc-based coatings)

[0156] The preferred composition of the zinc-based coating in this invention will be described, but any zinc-based coating containing Zn is acceptable, and the composition is not particularly limited. Typically, the zinc-based coating contains 50% by mass or more of Zn. Hereinafter, unless otherwise specified, "%" in relation to element content refers to "mass %". In the numerical ranges of the composition, the range indicated by "~" unless otherwise specified refers to the range including the values ​​stated before and after "~" as the lower and upper limits.

[0157] (Fe: 0-15%)

[0158] When Fe is heat-treated after forming a Zn-containing coating on a steel sheet, it can be incorporated into the coating through diffusion from the steel sheet. Therefore, the lower limit of Fe content can also be 0%. When alloying treatment is performed to form an alloyed hot-dip galvanized layer, Fe diffuses from the steel into the coating; therefore, the lower limit of Fe content can also be 1%, preferably 3%, and more preferably 5%. On the other hand, the upper limit of Fe content is preferably 15%, preferably 12%, and more preferably 10%. Thus, for example, when the zinc-based coating is an alloyed hot-dip galvanized layer, the Fe content in the coating can also be 5% to 15%.

[0159] (A1: 0-30%)

[0160] Al is an element that improves the corrosion resistance of a coating by being included together with Zn or by alloying, and therefore can be included as needed. Thus, for example, in the case of forming a coating by electroplating zinc, the Al content can be 0%. For forming a coating containing both Zn and Al, an Al content of 0.01% or more is preferred, for example, 0.1% or more or 0.5% or more. On the other hand, when Al exceeds 30%, the effect of improving corrosion resistance saturates, so an Al content of 30% or less is preferred, for example, 20% or less, 10% or less, 5% or less, or 1% or less. Even when Al is not present in the plating bath, Al in the steel diffuses into the coating during alloying treatment to form an alloyed hot-dip galvanized layer. Therefore, in this case, for example, the Al content can be 0.01% to 1%.

[0161] The basic composition of zinc-based coatings is as described above. Furthermore, zinc-based coatings, especially alloyed hot-dip galvanized coatings, may optionally contain elements found in the steel or other elements. These optional elements are not particularly limited, but from the viewpoint of fully utilizing the role and function of the basic components constituting the coating, it is preferable to set the total content to 5% or less, more preferably to 2% or less.

[0162] In the zinc-based coating of this invention, the remaining portion besides the aforementioned components consists of Zn and impurities. Here, impurities in the zinc-based coating refer to components that are introduced during the manufacturing process of the coating through various factors, represented by the raw materials, and are not intentionally added to the coating. As impurities, the coating may also contain trace amounts of elements other than the basic components and optional additives described above, within a range that does not impair the effects of this invention.

[0163] (Amount of adhesive applied per side)

[0164] The amount of zinc-based coating applied in this invention is not particularly limited, but can be, for example, 10 to 100 g / m². 2 The amount of coating applied to each single side has a significant impact on corrosion resistance. From a corrosion resistance perspective, the lower limit of the coating amount is preferably 15 g / m². 2 More preferably 20g / m 2 More preferably 30g / m 2 Ideally, from the viewpoints of formability, weldability, and economy, the upper limit of the adhesion amount per single side is preferably 90 g / m. 2 More preferably 70g / m 2 Further preferred is 60g / m 2 It is advisable.

[0165] The composition and adhesion amount of the zinc-based coating can be determined by inductively coupled plasma (ICP) luminescence spectrophotometry. Specifically, the composition and adhesion amount of the zinc-based coating can be determined by dissolving only the coating from a zinc-coated steel sheet and performing ICP analysis on the resulting solution. It should be noted that the coating adhesion amount in this invention is the amount per single side; therefore, when a zinc-based coating is formed on both sides of the steel sheet, the coating adhesion amount on both sides is calculated as being the same.

[0166] <Manufacturing Method of Zinc-Coated Steel Sheet>

[0167] Hereinafter, a preferred method for manufacturing the zinc-coated steel sheet of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the zinc-coated steel sheet of the present invention, and is not intended to limit the zinc-coated steel sheet to zinc-coated steel sheets manufactured by the method described below.

[0168] [Steel Plate Fabrication]

[0169] The steel plate of the present invention can be obtained by performing the following processes: a casting process in which molten steel with adjusted composition is cast to form a steel billet; a hot rolling process in which the steel billet is hot rolled to obtain a hot-rolled steel plate; a pickling process in which the surface oxides (oxide scale) formed in the hot rolling process are removed, and preferably the internal oxide layer is removed; a cold rolling process in which the cold-rolled steel plate is cold-rolled to obtain a cold-rolled steel plate; a grinding process in which the cold-rolled steel plate is ground; and an annealing process in which the cold-rolled steel plate is annealed.

[0170] (Casting process)

[0171] There are no particular limitations on the conditions of the casting process. For example, after smelting using a blast furnace or electric furnace, various secondary refining processes can be carried out, followed by casting using conventional continuous casting, ingot casting, or slab casting methods. Scrap materials can also be used in the raw materials, but the amount of scrap used should be adjusted to ensure that the content of each element in the obtained steel plate meets the aforementioned ranges.

[0172] (Hot rolling process)

[0173] Hot-rolled steel sheets can be obtained by hot rolling the cast steel billet as described above. The hot rolling process is performed by directly hot rolling the cast steel billet or by temporarily cooling and then reheating and hot rolling it. In the case of reheating, the heating temperature of the steel billet can be, for example, 1100°C to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction rate of each rolling process can be appropriately determined according to the desired metal structure and plate thickness. For example, the finish rolling temperature can be set to 800°C to 1050°C and the finish rolling reduction rate can be set to 50% to 80%. In this invention, since the plate thickness is ultimately reduced to 0.10 to 0.95 mm, the load in the cold rolling process is large. Therefore, it is preferable to set a relatively high hot rolling reduction rate and minimize the plate thickness as much as possible during hot rolling. Specifically, it is preferable to set the plate thickness after hot rolling to be 2.5 mm or less. However, the thickness of the internal oxide layer and decarburized layer formed near the surface of the steel sheet during hot rolling is basically constant. Therefore, if the thickness of the hot-rolled sheet is reduced, the impact of internal oxidation and decarburization during hot rolling becomes relatively larger. On the other hand, if the internal oxide layer and decarburized layer formed in the hot rolling process are not sufficiently removed by subsequent pickling and grinding processes and remain in considerable quantities, the internal oxide layer and decarburized layer in the annealing process will not form well or will form unevenly, which is undesirable. Therefore, in order to suppress the formation of internal oxide layer and decarburized layer in the hot rolling process and reduce the impact of internal oxidation and decarburization during hot rolling, it is preferable to set the coiling temperature to 600°C or below, and more preferably to 550°C or below. In particular, when manufacturing steel plates with a width of 1500 to 3000 mm, it is preferable that, in addition to the steps described above, the hot-rolling lubricant is used as the hot-rolling lubricant, and the hot-rolled and coiled hot-rolled coils are stored in a horizontal position with the holes in the inner diameter of the hot-rolled coils in a horizontal manner, thereby homogenizing the formation of the internal oxide layer in the hot-rolling process.

[0174] (Hot-rolled lubricating oil)

[0175] If the hot rolling lubrication conditions are unsuitable, the strain distribution on the surface of the steel sheet during hot rolling may become uneven. In such cases, the internal oxidation unevenness in the width direction during hot rolling increases, which is detrimental to the formation of a uniform internal oxide layer and decarburized layer in the subsequent annealing process. Therefore, as the hot rolling lubricant for hot rolling rolls, a hot rolling lubricant with excellent lubricity is preferred, for example, a lubricant containing calcium sulfonate is preferred. By using such a hot rolling lubricant, the strain distribution on the surface of the steel sheet during hot rolling can be homogenized. Therefore, it becomes possible to significantly reduce the internal oxidation unevenness in the width direction during hot rolling.

[0176] (Horizontal storage of hot-rolled coils)

[0177] Hot-rolled and coiled hot-rolled coils are temporarily stored until they are supplied to subsequent pickling and cold rolling processes. Generally, they are stored with the holes in the inner diameter of the hot-rolled coil upright (vertical) or horizontally (layout). However, if the hot-rolled coil is stored vertically, the temperature deviation along the width of the sheet may increase, resulting in uneven internal oxidation along the width of the sheet. This is detrimental to the formation of a uniform internal oxide layer and decarburized layer in the subsequent annealing process. Therefore, to suppress or reduce such uneven internal oxidation, it is preferable to store the hot-rolled coil horizontally.

[0178] (Pickling process)

[0179] To remove the surface oxide scale formed on the surface of the hot-rolled steel sheet during the hot rolling process, a pickling process is performed. Hydrochloric acid-based solutions are typically used in the pickling process, and the same conditions can be applied in this invention. Furthermore, it is preferable to remove the oxide layer by pickling until the internal oxide layer formed during the hot rolling process is reached.

[0180] (Cold rolling process)

[0181] Cold-rolled steel sheets can be obtained by cold rolling hot-rolled steel sheets. The reduction rate of cold rolling can be appropriately determined according to the desired metal structure and sheet thickness, for example, 30% to 90%. In this invention, the desired sheet thickness can be obtained by appropriately adjusting the reduction rates of the hot rolling and cold rolling processes. However, since the sheet thickness in this invention is as thin as less than 1 mm, it is advisable to make the reduction rate of hot rolling greater than that of cold rolling to prevent breakage during the cold rolling process.

[0182] (Grinding process)

[0183] To remove foreign matter remaining on the surface of the steel sheet, and further remove the oxide scale and internal oxide layer formed on the surface and outer layer of the steel sheet during hot rolling, and to homogenize the surface properties and promote the formation of the internal oxide layer in the subsequent annealing process, a grinding process is preferably performed. More specifically, by performing a grinding process to fully remove the internal oxide layer formed during hot rolling, and further applying strain to the surface of the steel sheet, the diffusion of oxygen in the annealing atmosphere from the surface of the steel sheet to the interior is promoted, and the reaction with the oxygen is also activated, thus promoting the formation of the internal oxide layer in the annealing process. The grinding process is not particularly limited, but it can be performed, for example, by grinding the surface of the steel sheet using a powerful grinding brush. The desired surface roughness and grinding amount can be achieved by appropriately selecting the number of powerful grinding brushes, the rotation speed, and the material. Especially when manufacturing wide steel plates with a width of 1500–3000 mm, by implementing a grinding process using multiple high-power grinding brushes, the surface roughness can be homogenized throughout the steel plate in the width direction, and strain can be uniformly imparted throughout the steel plate. Therefore, even for wide steel plates, the reaction with oxygen in the annealing atmosphere is activated at all locations on the steel plate, making it possible to uniformly control the thickness deviation of the internal oxide layer and decarburized layer of the steel plate. In particular, when manufacturing steel plates with a width of 1500–3000 mm, the number of high-power grinding brushes arranged on the upper and lower parts of the steel plate is preferably 3 or more (3 pairs). This is because using more high-power grinding brushes to perform grinding bit by bit is advantageous for homogenizing the surface roughness and uniformly imparting strain throughout the steel plate in the width direction. The grinding amount is preferably 2 g / m² per single side, with the total amount of all high-power grinding brushes. 2 The grinding process can also be performed before the annealing and plating production line described later.

[0184] (Annealing process)

[0185] Next, the obtained cold-rolled steel sheet is annealed. Annealing is preferably carried out at a temperature of 750°C or higher, more preferably at a temperature of 780°C or higher. From the viewpoint of suppressing the formation of the external oxide layer, the upper limit of the annealing temperature is preferably 920°C or lower. The heating rate up to the annealing temperature is not particularly limited, for example, it can be carried out at 1 to 10°C / second. In addition, from the viewpoint of fully forming the internal oxide layer and suppressing the formation of the external oxide layer, the holding time at this annealing temperature is about 5 to 300 seconds, preferably set to 50 to 100 seconds. If the annealing time is too long, the decarburized layer may be generated excessively relative to the internal oxide layer, resulting in a high B / t value and insufficient strength. The atmosphere in the annealing process becomes an important factor in controlling the internal oxide layer and the decarburized layer. The oxygen potential can be set to high at the beginning of annealing and then reduced, or the oxygen potential can be increased throughout the annealing process. The oxygen potential can be expressed as the partial pressure of water vapor P in the annealing furnace.H2O Divide by the hydrogen partial pressure P H2 The commonly used logarithm of the obtained value is log(P). H2O / P H2 To obtain the internal oxide layer and decarburized layer of this invention, the value obtained from the above formula can be controlled to be, for example, -3.0 to -1.0. Methods to increase the oxygen potential include using a combustion atmosphere, adding a small amount of oxygen, raising the dew point, or combining these methods. When controlling the dew point, a dew point of -20°C or higher is preferable, and more preferably -10°C or higher. If the dew point is too low, an external oxide layer may form on the surface of the steel plate. No specific upper limit is set for the dew point, but efforts are needed to suppress condensation in order to keep it above room temperature. The annealing atmosphere is preferably a reducing atmosphere, more preferably a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere with hydrogen content of 5% or less (e.g., 5% hydrogen and 95% nitrogen). It should be noted that the annealing process can be performed continuously with the formation of the zinc-based coating described later.

[0186] (Control of internal oxide layer thickness and decarburized layer thickness and their deviations)

[0187] The formation of the internal oxide layer and decarburized layer, as described above, mainly depends on the conditions in the hot rolling, grinding, and annealing processes. However, in addition to these factors, the thickness of the internal oxide layer also depends on the Si content in the steel, and the thickness of the decarburized layer depends on the Si, Mn, and C content. Increasing the Si content in the steel increases both the thickness of the internal oxide layer and the thickness of the decarburized layer, while increasing the Mn content decreases the thickness of the decarburized layer. Similarly, even increasing the C content in the steel decreases the thickness of the decarburized layer. Therefore, increasing the C and Mn content is effective in increasing the A / B ratio. Furthermore, as mentioned above, when the thickness of the hot-rolled plate is reduced to below 2.5 mm, the effects of internal oxidation and decarburization during hot rolling become relatively greater. Therefore, it is important to suppress internal oxidation and decarburization during hot rolling and reduce their impact by coiling the steel plate at a relatively low temperature during the hot rolling process. In addition, it is important to thoroughly remove the internal oxide layer formed during hot rolling through pickling and grinding processes before the annealing process. Furthermore, the grinding process not only removes the internal oxide layer formed during hot rolling but also applies strain to the steel plate. Applying strain to the steel plate promotes the formation of the internal oxide layer in the subsequent annealing process, making the grinding process a crucial step. Moreover, especially when manufacturing steel plates with widths exceeding 1500 mm, it becomes necessary to appropriately select the hot-rolling lubricant, the storage method of the hot-rolled coil, and the conditions of the grinding process, from the perspective of controlling the deviations a / A and b / B of the internal oxide layer thickness and decarburized layer thickness in the width direction. Therefore, in order to set the values ​​of A, B, A / B, B / t, a / A, and / or b / B to appropriate values, simply controlling the formation of the internal oxide layer and decarburized layer during the annealing process is insufficient; it is necessary not only to appropriately control the Si, Mn, and C content in the steel but also to appropriately control the conditions of the hot rolling and grinding processes. Because the interactions of these conditions are complex, generalizations are not possible. A, B, A / B, B / t, a / A, and b / B can be appropriately controlled by fixing each condition to a certain extent. However, simply combining the steel plate composition, hot rolling conditions, grinding conditions, and annealing conditions alone cannot satisfy the characteristics of the internal oxide layer and decarburized layer of this invention. Therefore, it is important to appropriately combine the steel plate composition, hot rolling conditions, grinding conditions, and annealing conditions to control these characteristics within the desired range.

[0188] [Formation of Zinc-Based Coatings]

[0189] Zinc-based coatings can be formed by either electroplating or hot-dip galvanizing, but hot-dip galvanizing is preferred. In hot-dip galvanizing, the coating conditions can be appropriately set considering the desired coating composition, thickness, and adhesion amount. For example, after annealing at 430–500°C and subsequent cooling, the cold-rolled steel sheet can be immersed in a molten zinc bath for 1–5 seconds. The coating adhesion amount is, for example, 10–100 g / m². 2 That's it.

[0190] After the zinc-based coating is formed, alloying is preferably performed to improve weldability and / or paintability. The alloying conditions can be set within the normal range, for example, alloying can be performed at a temperature of 450 to 600°C.

[0191] Example

[0192] The present invention will now be described in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0193] (Example A: Manufacturing of zinc-coated steel sheets)

[0194] In this example, zinc-coated steel sheets with a thickness of 0.10–0.95 mm (100–950 μm) and a width of 1000 mm were manufactured. First, molten steel with adjusted composition was cast to form a billet. The billet was temporarily cooled and then reheated and hot-rolled at a reduction rate of 50% or more. The resulting hot-rolled steel sheet was then coiled. In all examples, the reheating temperature of the billet was set to 1200°C, and the finishing rolling temperature was set to 950°C. The thickness and coiling temperature after hot rolling are shown in Table 1. Next, the surface oxide scale was removed using hydrochloric acid until it was below 0.2 μm. After pickling, cold rolling was performed at the cold rolling reduction rate shown in Table 1. Samples were collected from each cold-rolled steel sheet according to JIS G0417:1999, and the composition of the steel sheet was analyzed. In addition, the thickness of the cold-rolled steel sheet was measured at any five points using a micrometer, and the average of these measurements was used to calculate the sheet thickness t. The composition and thickness of the steel sheets for each example are shown in Tables 1 and 2.

[0195] Next, the surfaces of each cold-rolled steel sheet were ground using a high-powered grinding brush, and then annealed and hot-dip galvanized via a continuous plating production line. All but No. 26 underwent further alloying treatment. In Table 1, those that underwent alloying treatment after hot-dip galvanizing are indicated as "alloyed," and those that did not undergo alloying treatment after hot-dip galvanizing are indicated as "hot-dip galvanized." The temperature, dew point, and holding time during annealing were set as described in Table 1, and the heating rate up to the annealing temperature was set to 10°C / second. Furthermore, regarding the annealing atmosphere, for Nos. 1–19, 21, 22, and 24–27, a reducing atmosphere of 5% hydrogen and 95% nitrogen was used, according to the formula: log(P H2O / P H2 The oxygen potential is determined to be between -3.0 and -1.0, and the dew point is controlled accordingly. For No. 20 and No. 23, annealing is performed with a low oxygen potential while controlling the dew point. For hot-dip galvanizing, after stopping cooling at 480°C, the cold-rolled steel sheet is immersed in a molten zinc bath for 3 seconds, aiming to achieve a coating density of 50 g / m² per single side. 2 The coating adhesion amount is adjusted by moving left and right. For Examples No. 101–125, 127, and 128, alloying was performed at 550°C after hot-dip galvanizing. It should be noted that in all examples, the coating contains 5–15% Fe and 0.01–1% Al.

[0196] (Determination of the thickness A of the internal oxide layer)

[0197] After chemically dissolving and removing the coatings in each example using hydrochloric acid solution with added inhibitors, the thickness composition of the steel plate in the thickness direction was analyzed using a high-frequency type GDS to evaluate the thickness of the internal oxide layer. Specifically, the sputtering depth was measured after sputtering for a certain period of time from the steel plate surface toward the thickness direction using a laser microscope, and the sputtering velocity per unit time was calculated. The location where the Mn signal intensity reaches 0.9 times the Mn signal intensity of the steel substrate was determined, and the sputtering time to reach this location was converted into depth based on the sputtering velocity. This depth was set as the thickness of the internal oxide layer. The above operation was performed at three locations, and the thickness A (μm) of the internal oxide layer (per single side) was obtained by averaging the thicknesses calculated at each location.

[0198] (Determination of the thickness B of the decarburized layer)

[0199] After chemically dissolving and removing the coatings in each example using a hydrochloric acid solution containing an inhibitor, the thickness of the decarburized layer was evaluated by measuring the cross-sectional hardness distribution of the steel plate. Specifically, the steel plate microstructure was observed by etching it with 2% nitric acid ethanol to estimate the thickness of the decarburized layer. Then, Vickers hardness was measured at 5 μm intervals from the steel plate surface (interface between the steel plate and the coating) towards the plate thickness. The measurement length was set up to 100 μm. The location where the hardness distribution reached the steel plate hardness was determined, and this location was considered the thickness of the decarburized layer. The above measurements were performed at three locations, and the thickness of the decarburized layer was determined from these three data points, obtaining the thickness B (μm) of the decarburized layer in this invention (per single side). The load for the Vickers hardness measurement was set to 25 gf.

[0200] (Evaluation of tensile strength)

[0201] The tensile tests were conducted using the method specified in JIS-Z2241:2011 for JIS No. 5 tensile test specimens. The crosshead test speed for the tensile tests was set to 30 mm / min. The results are shown in Table 2.

[0202] (Evaluation of plating performance)

[0203] The evaluation of coating performance was conducted by assessing the appearance of the zinc-coated steel sheets in each example. Specifically, for the alloyed hot-dip galvanized steel sheets (“Alloyed” in Table 1), the appearance after alloying was visually observed. Those with no visible alloying inconsistencies were marked as ◎, those with minimal alloying inconsistencies as ○, and those with visible alloying inconsistencies or uncoated areas as ×. For the hot-dip galvanized steel sheets (“Hot-dip Galvanized” in Table 1), the appearance after coating was visually observed. Those with no visible uncoated areas were marked as ◎, and those with visible uncoated areas as ×. The results are shown in Table 2.

[0204] In this example, zinc-coated steel sheets with a tensile strength of 550–1500 MPa and a coating quality rating of ◎ or 0 are evaluated as having sufficient strength and coating quality.

[0205]

[0206] Table 2

[0207]

[0208] Referring to Tables 1 and 2, in Example No. 104, the excessive C content resulted in excessively high steel plate strength, making cold rolling impossible and preventing the attainment of a suitable plate thickness. In Example No. 109, the low C content led to reduced tensile strength. In Example No. 110, the excessive Si content caused surface deterioration, resulting in reduced plating properties. In Example No. 113, the low Mn content resulted in reduced tensile strength. In Example No. 119, the excessive Al content caused surface deterioration, resulting in reduced plating properties. In Examples No. 120 and 123, the low dew point during annealing prevented the formation of an internal oxide layer and decarburized layer, resulting in insufficient plating properties. In Example No. 127, the long annealing time led to an excessive formation of the decarburized layer relative to the internal oxide layer, resulting in a high B / t ratio and reduced tensile strength. In Example No. 128, the increased thickness after hot rolling leads to a greater load during the cold rolling process, making it impossible to achieve a suitable plate thickness. In contrast, in all embodiments of the present invention, by appropriately controlling the composition of the steel plate, the ratio of the thickness of the internal oxide layer to the thickness of the decarburized layer (A / B) on the surface of the steel plate, and the ratio of the thickness of the decarburized layer to the plate thickness (B / t), sufficient strength and plating properties can be achieved. In particular, for Examples No. 101–103, 105–108, 112, 114–118, 121, 122, and 124–126, where the sum of the Si and Mn contents (Si+Mn) is controlled to 5.00% or less and the ratio of Si content to Mn content (Si / Mn) is controlled to 1.50 or less, extremely high plating properties can be achieved.

[0209] (Example B: Manufacturing of wide zinc-coated steel sheets)

[0210] In this example, zinc-coated steel sheets with a thickness of 0.10–0.95 mm (100–950 μm) and a width of 1500–3000 mm were manufactured. First, molten steel with adjusted composition was cast to form a billet. The billet was temporarily cooled and then reheated. A calcium sulfonate-containing lubricant was used as the hot-rolling lubricant, and hot-rolling was performed at a reduction rate of 50% or more. The resulting hot-rolled steel sheet was coiled, and the hot-rolled coil was stored according to the storage method shown in Table 3. In all examples, the reheating temperature of the billet was set to 1200°C, and the finishing rolling temperature was set to 850°C. The presence or absence of the calcium sulfonate-containing lubricant, the thickness of the hot-rolled sheet, and the coiling temperature are as described in Table 3. Afterward, the hot-rolled coil was pickled with hydrochloric acid to remove the surface oxide scale until it was below 0.2 μm. After pickling, cold rolling was performed using the cold rolling reduction rates listed in Table 3. Samples were collected from each cold-rolled steel sheet according to JIS G0417:1999, and the composition of the steel sheets was analyzed. Furthermore, the thickness of the steel sheet was measured at five random locations using a micrometer, and the average of these measurements was used to calculate the sheet thickness. The width was further measured using a measuring tape. The composition, thickness, and width of the steel sheets for each example are shown in Tables 3 and 4.

[0211] Next, for each cold-rolled steel sheet, in the grinding area preceding the continuous coating production line, the surface is ground with the number of high-powered grinding brushes shown in Table 3 (the number of brushes when a pair of high-powered grinding brushes positioned above and below the steel sheet is considered as one brush) at a total of 2 g / m² per single side using all the high-powered grinding brushes. 2 After grinding, annealing and hot-dip galvanizing were performed, and alloying treatment was further applied except for No. 21. In Table 3, those that underwent alloying treatment after hot-dip galvanizing are indicated as "alloyed," and those that did not undergo alloying treatment after hot-dip galvanizing are indicated as "hot-dip galvanized." The temperature, dew point, and holding time during annealing were set as described in Table 3, and the heating rate up to the annealing temperature was set to 10°C / second. Furthermore, regarding the annealing atmosphere, in a reducing atmosphere of 3% hydrogen and 97% nitrogen, according to the formula: log(P) H2O / P H2 The oxygen potential is calculated to be -3.0 to -1.0 to control the dew point. Regarding hot-dip galvanizing, after stopping annealing and cooling at 480°C, the cold-rolled steel sheet is immersed in a molten zinc bath for 3 seconds, aiming to achieve a coating density of 50 g / m² per single side. 2 The coating adhesion amount is adjusted by moving left and right. For examples Nos. 201–220 and 222–225, alloying was performed at 550°C after hot-dip galvanizing. It should be noted that in all examples, the coating contains 5–15% Fe and 0.01–1% Al.

[0212] (Determination of the thickness A of the internal oxide layer and calculation of its thickness deviation a (μm))

[0213] After chemically dissolving and removing the coatings in each example using hydrochloric acid solution with added inhibitors, the thickness of the internal oxide layer was evaluated by analyzing the composition along the thickness of the steel plate using a high-frequency type GDS. Specifically, the sputtering depth was measured after sputtering for a certain period of time from the steel plate surface toward the thickness of the steel plate using a laser microscope, and the sputtering velocity per unit time was calculated. The position where the Mn signal intensity reaches 0.9 times the Mn signal intensity of the steel substrate was determined, and the sputtering time to reach this position was converted into depth based on the sputtering velocity. This depth was set as the thickness of the internal oxide layer. The above operation was performed at 9 points in the width direction of the steel plate (center, 100 mm from both edges, and 3 points each dividing the edges and center into 4 sections), for a total of 18 locations on both sides. The thickness A (μm) of the internal oxide layer (per single side) was obtained by averaging the thicknesses obtained at each location. In addition, the standard deviation a (μm) of the thickness of the internal oxide layer in the width direction was obtained by calculating the standard deviation.

[0214] (Determination of the thickness B of the decarburized layer and calculation of its thickness deviation b (μm))

[0215] After chemically dissolving and removing the coatings in each example using hydrochloric acid solution with added inhibitors, the thickness of the decarburized layer was evaluated by measuring the cross-sectional hardness distribution of the steel plate. Specifically, the steel plate microstructure was observed by etching it with 2% nitric acid ethanol to estimate the thickness of the decarburized layer. Then, Vickers hardness was measured at 5 μm intervals from the steel plate surface (interface between the steel plate and the coating) towards the plate thickness. The measurement length was set up to 100 μm. The location where the hardness distribution reached the steel plate hardness was determined, and this location was considered the thickness of the decarburized layer. Similar to the internal oxide layer, measurements were performed at 9 points on both sides, totaling 18 locations, to calculate the average value and standard deviation, thus obtaining the thickness B (μm) of the decarburized layer in this invention (per single side) and the standard deviation b (μm) of the width direction of the decarburized layer thickness. The load for the Vickers hardness measurement was set to 25 gf.

[0216] (Evaluation of tensile strength)

[0217] The tensile test was conducted using the method specified in JIS-Z2241:2011 for JIS No. 5 tensile test specimens. The crosshead test speed for the tensile test was set to 30 mm / min. The results are shown in Table 4. Furthermore, the tensile test was performed at 9 points, similar to the internal oxide layer test, and the average value was taken as the tensile strength of the present invention.

[0218] (Evaluation of plating performance)

[0219] The evaluation of plating performance is conducted using the same criteria as in Example A.

[0220] (Evaluation of coating properties after 5% strain)

[0221] For steel plates rated ◎ or ○ for coating properties, further specimens were collected at the same locations as those used for tensile testing, and the appearance after being subjected to a 5% strain through tension was evaluated. Even if no appearance non-uniformity was identified before tensioning, it is possible for non-uniformity to occur after tensioning. Specimens without appearance non-uniformity were designated as ○, and those with appearance non-uniformity were designated as ×.

[0222] In this example, similar to Example A, the case with a tensile strength of 550-1500 MPa and a coating performance evaluation (except for the coating performance evaluation after 5% strain assignment) of ◎ or 0 is evaluated as a zinc-coated steel sheet with sufficient strength and coating performance.

[0223]

[0224]

[0225] Referring to Tables 3 and 4, in Example No. 201, the low C content resulted in decreased tensile strength. In Example No. 207, the high Si content led to deterioration of surface properties, resulting in reduced plating properties. In Example No. 208, the low Mn content resulted in decreased tensile strength. In Example No. 211, the high Al content led to deterioration of surface properties, resulting in reduced plating properties. In Example No. 225, the high coiling temperature prevented sufficient suppression of decarburization layer formation during the hot rolling process, resulting in a higher B / t ratio and decreased tensile strength.

[0226] In contrast, in all embodiments of the present invention, sufficient strength and plating properties can be achieved by appropriately controlling the composition of the steel sheet, the ratio of the thickness of the internal oxide layer to the thickness of the decarburized layer (A / B) on the surface of the steel sheet, and the ratio of the thickness of the decarburized layer to the thickness of the steel sheet (B / t). In particular, in Examples No. 202-206, 209, 210, 212, and 215-222, in addition to the composition of the steel sheet, A / B, and B / t, the thickness of the internal oxide layer (A), the thickness of the decarburized layer (B), and their width-direction deviations (a / A and b / B) on the surface of the steel sheet are appropriately controlled, no appearance unevenness occurs even after a 5% strain is applied, thus achieving sufficient and uniform strength and plating properties throughout the steel sheet. In other embodiments, namely Examples No. 213, 214, 223, and 224, although sufficient strength and plating properties are achieved, appearance unevenness is observed after a 5% strain is applied. Specifically, in Examples No. 213 and 214, it is believed that due to the small number of high-strength grinding brushes, the uniformity of surface roughness and strain in the steel sheet throughout the width direction was insufficient. As a result, the desired a / A and / or b / B were not obtained, resulting in uneven appearance after applying 5% strain. In Example No. 223, due to the lack of suitable hot-rolling lubricant used in the hot-rolling process, a / A and b / B became high, resulting in uneven appearance after applying 5% strain. In Example No. 224, due to inappropriate storage methods for the hot-rolled and coiled hot-rolled coils, b / B became high, resulting in uneven appearance after applying 5% strain.

[0227] Industrial availability

[0228] The zinc-coated steel sheet of the present invention, due to its excellent coating properties and strength, provides a high-strength zinc-coated steel sheet with superior appearance characteristics. This zinc-coated steel sheet is suitable for use in automobiles, home appliances, building materials, and especially in automobiles. Therefore, the present invention can be considered an invention of extremely high industrial value.

Claims

1. A zinc-coated steel sheet comprising a steel sheet with a thickness of 0.10 to 0.95 mm and a zinc-coated layer formed on at least one side of the steel sheet, the steel sheet having the following composition: It contains, by mass%: C：0.05～0.30％、 Si: 0.01~3.00% Mn: 0.80~3.00% Al：0.010～2.000％、 P: below 0.100% S: Below 0.100% N: below 0.0300% O: below 0.010% B：0～0.0100％、 Ti: 0~0.100% Nb: 0~0.100% V：0～0.10％、 Cr：0～1.00％、 Ni: 0-0.10% Cu: 0–0.10% Mo: 0–0.50% W：0～0.50％、 Ca: 0–0.100% Mg: 0–0.100% Zr：0～0.100％、 Hf: 0~0.100%, and REM: 0~0.100%, The remaining part consists of Fe and impurities. The zinc-coated steel sheet has a tensile strength of 550–1500 MPa. When the thickness of the internal oxide layer on each side of the steel plate is set to A (in μm), the thickness of the decarburized layer on each side of the steel plate is set to B (in μm), and the thickness of the steel plate is set to t (in μm), then A / B: 0.01~0.50, B / t: 0.001~0.

200. The steel plate has a width of 1500-3000 mm. When the standard deviation of the width direction in the thickness of the internal oxide layer is set to a in μm and the standard deviation of the width direction in the thickness of the decarburized layer is set to b in μm, A: 1.0 μm or more, B: 10 μm or more, a / A: 0.15 or less, b / B: 0.20 or less.

2. The zinc-coated steel sheet according to claim 1, wherein, Si+Mn content (in mass%): less than 5.00%, Si / Mn ratio: less than 1.

50.

3. The zinc-coated steel sheet according to claim 1 or 2, wherein, The steel plate has a composition containing one or more of the following elements by mass percent: B：0.0001～0.0100％、 Ti: 0.001~0.100% Nb: 0.001~0.100% Cr：0.01～1.00％、 Mo: 0.01~0.50%, and W：0.01～0.50％。 4. The zinc-coated steel sheet according to claim 1 or 2, wherein, The steel plate contains one or more of the following elements by mass percentage. composition: V：0.01～0.10％、 Ni: 0.01~0.10% Cu: 0.01–0.10% Ca: 0.001~0.100% Mg: 0.001~0.100% Zr：0.001～0.100％、 Hf: 0.001~0.100%, and REM: 0.001~0.100%.

5. The zinc-coated steel sheet according to claim 1 or 2, wherein, The zinc-coated steel sheet is an alloyed hot-dip galvanized steel sheet. The zinc coating consists of 5-15% Fe and 0.01-1% Al, with the remainder being Zn and impurities. The coating thickness on each side is 10-100 g / m². 2 .

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