Alloyed hot-dip galvanized steel sheet

Abstract

The alloyed hot-dip galvanized steel sheet of the present disclosure has a base steel sheet having a prescribed chemical composition in mass%, and a plated layer, and the existence ratio ER of a specific internal oxide is 40% or less and the interval I of the internal oxide is 300 μm or more when a cross section of the base steel sheet is observed under prescribed conditions. The alloyed hot-dip galvanized steel sheet of the present disclosure is excellent in appearance and the like.

Description

Technical Field

[0001] This application discloses alloyed hot-dip galvanized steel sheets. Background Technology

[0002] Due to its excellent paintability, alloyed hot-dip galvanized steel sheet is frequently used as exterior components in applications such as automobile bodies, household appliances, and building materials. However, alloyed hot-dip galvanized steel sheet is prone to developing rib-like patterns (also known as rib designs) in the coating. These rib-like patterns persist even after painting, sometimes failing to meet the aesthetic requirements for exterior components. In other words, if rib-like patterns form in the coating and deteriorate its appearance, the final painted appearance is also likely to be poor.

[0003] Various techniques have been proposed to counteract the rib-like patterns in the coating of alloyed hot-dip galvanized steel sheets. For example, Patent Document 1 discloses a technique that suppresses the generation of uneven coating on the steel sheet surface by optimizing the balance of Ti and Nb contained in the base steel sheet made of IF steel to suppress the addition of Mn, P, and Si. Furthermore, Patent Document 2 discloses a technique that, in the base steel sheet made of IF steel, suppresses the generation of rib-like patterns in the coating caused by surface defects of the steel sheet by setting the intensity of {100} plane X-rays in the direction parallel to the surface of the steel sheet to 0.8 or less in a random intensity ratio and setting the proportion of non-recrystallized grains to 0.10% or less. Moreover, Patent Document 3 discloses a technique that suppresses the generation of rib-like patterns in the coating by setting the Mn concentration of the annealed surface layer to below a predetermined upper limit in the base steel sheet made of BH steel.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2006-291272

[0007] Patent Document 2: Japanese Patent Application Publication No. 2016-132801

[0008] Patent Document 3: International Publication No. 2016 / 170794 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] As mentioned above, although solutions have been proposed from various perspectives regarding the ribbed pattern in the coating of alloyed hot-dip galvanized steel sheets, they have not been entirely effective. Therefore, there is still room for improvement in suppressing the ribbed pattern in the coating and enhancing the appearance of alloyed hot-dip galvanized steel sheets.

[0011] Furthermore, the alloyed hot-dip galvanized steel sheets used in the aforementioned applications are mostly pressed and formed. In conventional techniques, during the pressing process of alloyed hot-dip galvanized steel sheets, the unalloyed coating sometimes adheres to the mold, impairing formability and productivity. In addition, cracking sometimes occurs due to embrittlement during secondary processing.

[0012] Furthermore, alloyed hot-dip galvanized steel sheets used in the aforementioned applications preferably exhibit a high balance between strength and ductility. In the past, there has been insufficient research on alloyed hot-dip galvanized steel sheets with an excellent balance between strength and ductility.

[0013] Methods for solving problems

[0014] This application discloses an alloyed hot-dip galvanized steel sheet as one of the means to solve the above-mentioned problems.

[0015] It is an alloyed hot-dip galvanized steel sheet, consisting of a base steel sheet and a coating.

[0016] The aforementioned base steel plate has the following chemical composition: containing, by mass %:

[0017] C: 0.0005~0.0100%

[0018] Si: 0.01~0.50%

[0019] Mn: 0.01~2.00%

[0020] P: below 0.100%

[0021] S: below 0.0100%

[0022] N: below 0.0200%

[0023] Ti: 0.040~0.180%

[0024] Nb: 0~0.100%

[0025] B: 0.0005~0.0100%

[0026] Al: 0~1.000%

[0027] Cu: 0~1.000%

[0028] Cr: 0–2.000%

[0029] Ni: 0~0.500%

[0030] Mo: 0–3.000%

[0031] W: 0~0.100%

[0032] V: 0~1.000%

[0033] O: 0~0.020%

[0034] Ta: 0~0.100%

[0035] Co: 0-3.000%

[0036] Sn: 0~1.000%

[0037] Sb: 0~0.500%

[0038] As: 0-0.050%

[0039] Mg: 0–0.050%

[0040] Zr: 0~0.050%

[0041] Ca: 0–0.0500%, and

[0042] REM: 0~0.0500%,

[0043] The remaining part consists of Fe and impurities.

[0044] Under the following condition 1, the presence ratio of specific internal oxides ER is less than 40%.

[0045] The spacing I of the specific internal oxides is 300 μm or more under the following condition 2.

[0046] Condition 1: Observe a cross-section of the aforementioned base steel plate perpendicular to the rolling direction, and specifically identify the grain boundary type internal oxides contained in the aforementioned cross-section, wherein the field of view for observation is set to a range of 2 mm along the surface direction of the aforementioned base steel plate. The aforementioned internal oxides are defined as oxides in the depth direction ranging from 0.5 μm to 5.0 μm from the surface of the aforementioned base steel plate. Project the specific internal oxides onto the surface of the aforementioned base steel plate. The proportion of the internal oxides projected onto the surface of the aforementioned base steel plate is defined as the aforementioned presence ratio ER of the internal oxides.

[0047] Condition 2: Observe a cross-section of the aforementioned base steel plate perpendicular to the rolling direction, and identify the grain boundary type internal oxides contained in the cross-section. The field of view for observation is set to a range of 2 mm along the surface direction of the base steel plate. The internal oxides are defined as oxides extending from the surface of the base steel plate in a depth direction of 0.5 μm or more and 5.0 μm or less. Project the identified internal oxides onto the surface of the base steel plate. Based on the position of the internal oxides projected onto the surface of the base steel plate, determine the spacing I between adjacent internal oxides.

[0048] In the alloyed hot-dip galvanized steel sheet disclosed herein, the above chemical composition may also contain, in weight percent:

[0049] Mn: 0.01–1.30% or 1.70–2.00%.

[0050] In the alloyed hot-dip galvanized steel sheet disclosed herein, the aforementioned presence ratio ER of the internal oxides may also be 5% or more.

[0051] In the alloyed hot-dip galvanized steel sheet of this disclosure, the above chemical composition may also contain, by mass percent, at least one element selected from the group consisting of the following elements:

[0052] Nb: 0.001~0.100%

[0053] Al: 0.001~1.000%

[0054] Cu: 0.001~1.000%

[0055] Cr: 0.001~2.000%

[0056] Ni: 0.001~0.500%

[0057] Mo: 0.001~3.000%

[0058] W: 0.001~0.100%

[0059] V: 0.001~1.000%

[0060] O: 0.001~0.020%

[0061] Ta: 0.001~0.100%

[0062] Co: 0.001~3.000%

[0063] Sn: 0.001~1.000%

[0064] Sb: 0.001~0.500%

[0065] As: 0.001~0.050%

[0066] Mg: 0.001~0.050%

[0067] Zr: 0.001~0.050%

[0068] Ca: 0.0001~0.0500%

[0069] REM: 0.0001~0.0500%.

[0070] In the alloyed hot-dip galvanized steel sheet disclosed herein, the microstructure of the aforementioned base steel sheet can also be expressed as an area ratio:

[0071] Ferrite: 94-100%

[0072] The total percentage of martensite and bainite: 0–4%, and

[0073] Retained austenite: 0–2%.

[0074] In the alloyed hot-dip galvanized steel sheet disclosed herein, the chemical composition of the coating can also be expressed in mass percent as follows:

[0075] Fe: 5-25%

[0076] Al: 0-1.0%

[0077] Si: 0-1.0%

[0078] Mg: 0-1.0%

[0079] Mn: 0~1.0%

[0080] Ni: 0-1.0%

[0081] Sb: 0–1.0%, and

[0082] Remaining components: Zn and impurities.

[0083] Invention Effects

[0084] The alloyed hot-dip galvanized steel sheet disclosed herein exhibits less visible rib-like patterns in the coating, resulting in excellent coating appearance and post-coating appearance. Furthermore, the coating of the alloyed hot-dip galvanized steel sheet disclosed herein has excellent wettability relative to the base steel sheet, which can suppress the adhesion of unalloyed coating to the mold during pressing and also easily suppress embrittlement during secondary processing. Attached Figure Description

[0085] Figure 1It is a schematic diagram showing the morphology of the Mn-concentrated portion on the surface of the base steel plate.

[0086] Figure 2 This is a schematic diagram used to illustrate the mechanism of rib-like patterns in alloyed hot-dip galvanized layers.

[0087] Figure 3 This is a schematic diagram illustrating the process when a specific ratio ER and interval I are present under conditions 1 and 2. For ease of understanding, it is shown enlarged along the plate thickness direction, and the plating is omitted. Detailed Implementation

[0088] 1. Alloyed hot-dip galvanized steel sheet

[0089] According to the inventor's novel insight, the rib-like pattern in the coating of alloyed hot-dip galvanized steel sheets originates from uneven internal oxidation within the surface layer of the base steel sheet. For example... Figure 1 As shown, the Mn-concentrated portion on the base metal surface of the cast slab extends along the rolling direction due to rolling, thus forming a rib-like pattern. That is, when a slab is rolled to obtain a steel sheet, the Mn-concentrated portion on the surface of the steel sheet becomes a rib-like structure extending along the rolling direction. Figure 2 As shown, for steel plates in such a state ( Figure 2 (A)), for example, in the case of annealing, Mn oxides (e.g., Mn2SiO4) are generated as internal oxides in the Mn-concentrated portions on the surface of the steel plate through selective oxidation. The pinning effect of these Mn oxides can suppress the grain growth of the surface layer. Figure 2 (B) That is, the grain boundary density tends to be higher in the portions of the base steel sheet where internal oxides exist. Subsequently, if a zinc coating is formed relative to the base steel sheet and further alloying is performed, alloying is promoted in the portions with high grain boundary density. Figure 2 (C)) The coating grows thicker, thus producing rib-like patterns in the coating. Figure 2 (D)).

[0090] As described above, the rib-like pattern in the coating of alloyed hot-dip galvanized steel sheets can be attributed to uneven internal oxidation in the surface layer of the base steel sheet. However, according to the inventors' novel insights, the degree of prominence of the rib-like pattern varies depending on the extent of this uneven internal oxidation. That is, even if internal oxides are generated in the base steel sheet, rib-like patterns that impair appearance are less likely to occur if the spacing between these internal oxides is sufficiently narrow or sufficiently wide. Therefore, by controlling the distribution of internal oxides on the surface of the base steel sheet, the rib-like pattern in the coating of alloyed hot-dip galvanized steel sheets can be improved. In this way, the inventors have discovered that the degree of prominence of the rib-like pattern does not depend solely on the "amount" of concentrated Mn on the surface of the base steel sheet as disclosed in conventional techniques (e.g., Patent Document 3), but more precisely, on the "distribution morphology" of the Mn oxides as internal oxides.

[0091] The distribution morphology of Mn oxides, which are internal oxides, in the surface layer of the base steel sheet can be quantified by observing the cross-section of the base steel sheet. Furthermore, the distribution morphology of these internal oxides in the surface layer of the base steel sheet can be controlled, for example, by setting the pickling conditions of the hot-rolled sheet to over-pickling, dissolving the Mn concentrated in the surface layer (the interface between the base steel sheet and the oxide scale), thereby reducing the starting point for the formation of internal oxides. In this case, the spacing between the internal oxides becomes sufficiently wide, making it less likely to produce coating rib patterns that impair the appearance. That is, while Mn is considered to easily accumulate in the surface layer of the hot-rolled steel sheet, by setting the pickling conditions to over-pickling to dissolve and remove the entire surface layer of the steel sheet, the concentration of Mn in the newly exposed steel sheet surface after pickling is reduced. As a result, the number of internal oxidation starting points is reduced, the spacing between the internal oxides is widened, and the formation of the aforementioned coating rib patterns can be suppressed.

[0092] This invention is based on the aforementioned new insights. The embodiments of this invention will now be described. It should be noted that these descriptions are merely illustrative of embodiments of the invention, and the invention is not limited to the following embodiments.

[0093] The alloyed hot-dip galvanized steel sheet of this embodiment has a base steel sheet and a coating. The base steel sheet has the following chemical composition: containing, by mass %,:

[0094] C: 0.0005~0.0100%

[0095] Si: 0.01~0.50%

[0096] Mn: 0.01~2.00%

[0097] P: below 0.100%

[0098] S: below 0.0100%

[0099] N: below 0.0200%

[0100] Ti: 0.040~0.180%

[0101] Nb: 0~0.100%

[0102] B: 0.0005~0.0100%

[0103] Al: 0~1.000%

[0104] Cu: 0~1.000%

[0105] Cr: 0–2.000%

[0106] Ni: 0~0.500%

[0107] Mo: 0–3.000%

[0108] W: 0~0.100%

[0109] V: 0~1.000%

[0110] O: 0~0.020%

[0111] Ta: 0~0.100%

[0112] Co: 0-3.000%

[0113] Sn: 0~1.000%

[0114] Sb: 0~0.500%

[0115] As: 0-0.050%

[0116] Mg: 0–0.050%

[0117] Zr: 0~0.050%

[0118] Ca: 0–0.0500%, and

[0119] REM: 0~0.0500%,

[0120] The remainder consists of Fe and impurities.

[0121] In the alloyed hot-dip galvanized steel sheet of this embodiment, the presence ratio ER of the internal oxides is 40% or less under the following condition 1, and the spacing I of the internal oxides is 300 μm or more under the following condition 2.

[0122] Condition 1: Observe a cross-section of the aforementioned base steel sheet perpendicular to the rolling direction, and identify the grain boundary type internal oxides contained in the cross-section. The field of view for observation is set to a range of 2 mm along the surface direction of the base steel sheet. The internal oxides are defined as oxides extending from the surface of the base steel sheet in a depth direction of 0.5 μm or more and 5.0 μm or less. Project the identified internal oxides onto the surface of the base steel sheet. The proportion of the internal oxides projected onto the surface of the base steel sheet is defined as the presence ratio ER of the internal oxides.

[0123] Condition 2: Observe a cross-section of the aforementioned base steel plate perpendicular to the rolling direction, and identify the grain boundary type internal oxides contained in the cross-section. The field of view for observation is set to a range of 2 mm along the surface direction of the base steel plate. The internal oxides are defined as oxides extending from the surface of the base steel plate in a depth direction of 0.5 μm or more and 5.0 μm or less. Project the identified internal oxides onto the surface of the base steel plate. Based on the position of the internal oxides projected onto the surface of the base steel plate, determine the spacing I between adjacent internal oxides.

[0124] 1.1 Chemical composition of the base steel plate

[0125] First, the reasons for limiting the chemical composition of the base steel plate will be explained. Here, the "%" in the composition refers to mass percentage. Furthermore, in this application, the "~" indicating a numerical range is used, unless otherwise specified, to mean the lower limit and upper limit of the value described before and after it.

[0126] (C: 0.0005~0.0100%)

[0127] The lower the carbon content in the base steel sheet, the higher the mechanical properties such as elongation or r-value. In this embodiment, carbon can be fixed by Ti and Nb (described later). However, if TiC and NbC are excessively increased, the annealing temperature dependence of the mechanical properties of the base steel sheet may become higher, narrowing the range of annealing conditions for obtaining the desired mechanical properties. Furthermore, if there is a large amount of dissolved carbon remaining in the base steel sheet that is not fixed by Ti and Nb, it may sometimes hinder the alloying of the coating. For this reason, the carbon content is 0.0100% or less, or it can be 0.0080%, 0.0060%, or 0.0040% or less. It should be noted that, from the viewpoint of suppressing excessive increases in steelmaking costs, the carbon content is 0.0005% or more, or it can be 0.0010%, 0.0015%, or 0.0020% or more.

[0128] (Si: 0.01~0.50%)

[0129] Si is an element that improves the strength of the base steel sheet. On the other hand, if the base steel sheet contains excessive Si, the wettability of the coating relative to the base steel sheet may sometimes deteriorate. Furthermore, if the base steel sheet contains excessive Si, the alloying of the coating slows down, and in alloyed hot-dip galvanized steel sheets, there may sometimes be more unalloyed coatings. For this reason, the Si content is 0.50% or less, and can also be 0.48% or less, or 0.46% or less. Furthermore, the Si content is 0.01% or more, and can also be 0.05% or more, 0.10% or more, or 0.20% or more.

[0130] (Mn: 0.01~2.00%)

[0131] Mn is an element that improves the strength of the base steel sheet. On the other hand, if the Mn content is excessively high, the tensile strength of the steel sheet may decrease. For this reason, the Mn content is 2.00% or less, or even 1.95% or less. Furthermore, the Mn content is 0.01% or more, or even 0.05%, 0.10%, or 0.20% or more. Moreover, according to the inventors' new insights, when the Mn content in the base steel sheet is within a specific range, it becomes less likely to produce coating rib patterns that impair appearance. That is, if the Mn content is low, it is less likely to produce uneven Mn concentration that causes the aforementioned undesirable rib pattern appearance. Furthermore, if the Mn content is high, the Mn concentration increases throughout the surface of the base steel sheet, and the aforementioned uneven Mn concentration is easily eliminated. In view of the above, in this embodiment, from the viewpoint of improving the appearance of the alloyed hot-dip galvanized steel sheet and ensuring tensile strength, the Mn content may also be 0.01 to 1.30% or less, or 1.70 to 2.00%. The Mn content can be above 0.50% or below 1.20%. Alternatively, the Mn content can be above 1.75% or below 1.95%.

[0132] (P: below 0.100%)

[0133] Phosphorus (P) is an element that increases the strength of the base steel sheet and can be added arbitrarily. On the other hand, if the base steel sheet contains excessive P, the alloying of the coating slows down, and sometimes there is an increase in unalloyed coatings in alloyed hot-dip galvanized steel sheets. For this reason, the P content is 0.100% or less, and can also be 0.090% or less. There is no particular limitation on the lower limit of the P content. The P content can be 0% or more, and can also be 0.001% or more.

[0134] (S: below 0.0100%)

[0135] Sulfur (S) is an element that segregates at the grain boundaries of the base steel sheet, causing secondary processing embrittlement. Furthermore, it is an element that forms non-metallic inclusions such as MnS in the steel, leading to a decrease in the ductility of the base steel sheet. Lower levels of S are preferred. The S content can be 0% or more, or 0.0005% or more, 0.0010% or more, or 0.0020% or more; it can also be 0.0100% or less, or 0.0090% or less, or 0.0080% or less.

[0136] (N: below 0.0200%)

[0137] Nitrogen (N) is an element that forms coarse nitrides in the base steel sheet, reducing its workability. Furthermore, N contributes to porosity during welding. Excessive N content can also combine with Ti to form TiN, reducing the effective amount of Ti used for C immobilization. Therefore, the N content is typically 0.0200% or less, but can also be 0.0150%, 0.0100%, 0.0080%, or 0.0060% or less. It should be noted that while the N content is 0% or more, from the viewpoint of preventing excessive increases in steelmaking costs, the N content can also be 0.0001% or more, or 0.0010% or more.

[0138] (Ti: 0.040~0.180%)

[0139] Ti is an element that fixes carbon (C) to improve the mechanical properties of the base steel sheet, such as elongation or γ-value. This effect is easily achieved when the Ti content is 0.040% or higher. The Ti content can also be 0.045% or higher, or 0.050% or higher. On the other hand, if the Ti content is too high, the balance between the strength and ductility of the base steel sheet may deteriorate. This problem is easily avoided when the Ti content is 0.180% or lower. The Ti content can also be 0.150% or lower, 0.120% or lower, or 0.100% or lower.

[0140] (Nb: 0~0.100%)

[0141] Like Ti, Nb is an element that immobilizes carbon (C) to improve the mechanical properties of the base steel sheet, such as elongation or r-value. However, this effect is weaker compared to Ti. As mentioned above, the C immobilization effect can be achieved with Ti, therefore the Nb content can be 0%. The Nb content can be 0% or more, or it can be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Nb content is too high, the annealing temperature dependence of the mechanical properties of the base steel sheet may become higher, and the range of annealing conditions for obtaining the desired mechanical properties may become narrower. For this reason, the Nb content can be 0.100% or less, or it can be 0.060% or less, or 0.040% or less.

[0142] (B: 0.0005~0.0100%)

[0143] Boron (B) is an element used to prevent secondary processing embrittlement of the base steel sheet. In this embodiment, as described above, although C at grain boundaries can be removed by immobilizing C with Ti and Nb, secondary processing embrittlement becomes more likely after the removal of C at grain boundaries. In this embodiment, this problem is easily avoided by including B in the base steel sheet instead of C. In particular, this problem is further avoided when the B content is 0.0005% or more. The B content can also be 0.0007% or more or 0.0009% or more. On the other hand, if the base steel sheet contains excessive B, the alloying of the coating slows down, and sometimes there is more unalloyed coating in alloyed hot-dip galvanized steel sheets. For this reason, the B content is 0.0100% or less, or 0.0050% or less or 0.0020% or less.

[0144] (A1: 0-1.000%)

[0145] Al is an element that acts as a deoxidizer in steel and is added as needed. On the other hand, excessive Al content can sometimes worsen the balance between strength and ductility. The Al content can be 0% or more, or 0.001% or more, 0.005% or more, 0.010% or more, or 0.015% or more; and it can be 1.000% or less, or 0.700% or less, 0.500% or less, 0.300% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0146] The basic chemical composition of the base steel plate in this embodiment is as described above. Furthermore, the base steel plate in this embodiment may, as needed, contain at least one of the following elements. These elements may also be absent, therefore their content is limited to 0%. These elements will not substantially affect the problem-solving mechanism in this embodiment.

[0147] (Cu: 0~1.000%)

[0148] Cu is an element that can help improve at least one of strength and corrosion resistance. On the other hand, excessive Cu content may lead to a deterioration in toughness. The Cu content can be 0% or more, or 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more, and can also be 1.000% or less, or 0.800% or less, 0.600% or less, 0.400% or less, 0.250% or less, or 0.150% or less.

[0149] (Cr: 0~2.000%)

[0150] Cr is an element that improves the hardenability of steel and can contribute to the improvement of at least one of strength and corrosion resistance. On the other hand, if the Cr content is excessive, it can sometimes reduce toughness in addition to increasing the cost of the alloy. The Cr content can be 0% or more, or 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more, and can also be 2.000% or less, or 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, or 0.150% or less.

[0151] (Ni: 0~0.500%)

[0152] Ni is an element that improves the hardenability of steel and can contribute to the improvement of at least one of strength and heat resistance. On the other hand, if the Ni content is excessive, the effect may become saturated, leading to an increase in manufacturing costs. The Ni content can be 0% or more, or 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more, and can also be 0.500% or less, or 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[0153] (Mo: 0~3.000%)

[0154] Mo is an element that improves the hardenability of steel and can contribute to the improvement of at least one of strength and corrosion resistance. On the other hand, if the Mo content is excessive, the deformation resistance during processing may increase. The Mo content is 0% or more, or it can be 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more, and it can be 3.000% or less, or it can be 2.000% or less, 1.000% or less, 0.500% or less, or 0.100% or less.

[0155] (W: 0~0.100%)

[0156] W is an element that improves the hardenability of steel and can contribute to increased strength. On the other hand, excessive W content may lead to the formation of large inclusions. The W content can be 0% or more, or 0.001% or more, 0.005% or more, or 0.010% or more, or 0.100% or less, or 0.080% or less, 0.050% or less, or 0.030% or less.

[0157] (V: 0~1.000%)

[0158] V is an element that can help improve strength through precipitation strengthening, etc. On the other hand, if V is present in excess, a large amount of precipitates may be generated, which can reduce toughness. The V content is 0% or more, but can also be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more. In addition, it is 1.000% or less, but can also be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.070% or less.

[0159] (O: 0~0.020%)

[0160] O is an element that can be introduced during the manufacturing process. Refining to reduce the O content to its limit requires time, leading to decreased productivity. On the other hand, excessive O content can sometimes form large inclusions, reducing the toughness of the steel. The O content can be 0% or more, or 0.001% or more, 0.002% or more, or 0.003% or more; it can also be 0.020% or less, or 0.015% or less, 0.010% or less, or 0.005% or less.

[0161] (Ta: 0~0.100%)

[0162] Ta is an element that helps control the morphology of carbides and increase their strength. On the other hand, if there is an excessive amount of Ta, many fine Ta carbides may precipitate, reducing toughness. The Ta content can be 0% or more, or 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more, and can also be 0.100% or less, or 0.080% or less, 0.060% or less, or 0.040% or less.

[0163] (Co: 0-3.000%)

[0164] Co is an element that can help improve at least one of hardenability and heat resistance. On the other hand, excessive Co content may reduce processability and increase raw material costs. The Co content can be 0% or more, or 0.001% or more, 0.010% or more, 0.020% or more, or 0.050% or more, and can be 3.000% or less, or 2.000% or less, 1.000% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

[0165] (Sn: 0~1.000%)

[0166] Sn is an element that can help improve corrosion resistance. On the other hand, excessive Sn content may lead to a decrease in toughness. The Sn content can be 0% or more, or 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more, or 1.000% or less, or 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.050% or less.

[0167] (Sb: 0~0.500%)

[0168] Sb is an element that can help improve corrosion resistance. On the other hand, excessive Sb content may lead to a decrease in toughness. The Sb content can be 0% or more, or 0.001% or more, 0.005% or more, or 0.010% or more, or 0.500% or less, or 0.300% or less, 0.100% or less, or 0.050% or less.

[0169] (As: 0~0.050%)

[0170] As is an element that can help improve the machinability of steel. On the other hand, if there is an excessive amount of As, the machinability may be reduced. The As content can be 0% or more, or 0.001% or more, 0.005% or more, or 0.010% or more, or 0.050% or less, or 0.040% or less, 0.030% or less, or 0.020% or less.

[0171] (Mg: 0-0.050%)

[0172] Mg is an element that helps control the form of sulfides. On the other hand, excessive Mg content may reduce toughness. The Mg content can be 0% or more, or 0.001% or more, 0.003% or more, or 0.005% or more, or 0.050% or less, or 0.030% or less, 0.020% or less, or 0.015% or less.

[0173] (Zr: 0~0.050%)

[0174] Zr is an element that helps control the morphology of sulfides. On the other hand, excessive Zr content may lead to saturation and increase manufacturing costs. Zr content can be 0% or more, or 0.001% or more, 0.003% or more, 0.005% or more, or 0.010% or more, and can also be 0.050% or less, or 0.040% or less, 0.030% or less, or 0.020% or less.

[0175] (Ca: 0~0.0500%)

[0176] Ca is an element whose form of sulfides can be controlled by adding trace amounts. On the other hand, if the Ca content is excessive, the effect may become saturated, leading to an increase in manufacturing costs. The Ca content can be 0% or more, or 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more; and it can be 0.0500% or less, or 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less.

[0177] (REM: 0~0.0500%)

[0178] Like Ca, REM is an element whose sulfide form can be controlled by adding trace amounts. On the other hand, excessive REM content may lead to the formation of large inclusions. REM content can be 0% or more, or 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more; and can also be 0.0500% or less, or 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less. It should be noted that REM in this specification refers to the collective content of 17 elements: scandium (Sc) atomic number 21, yttrium (Y) atomic number 39, and lanthanum (La) atomic number 57 to lutetium (Lu) atomic number 71 (all lanthanides).

[0179] In this embodiment, regarding the chemical composition of the base steel plate, the remaining components other than those described above are Fe and impurities. Impurities refer to components introduced during the industrial manufacturing of the base steel plate of this embodiment due to various factors in the manufacturing process, such as raw materials like ore and scrap iron.

[0180] 1.2 Condition 1

[0181] In the alloyed hot-dip galvanized steel sheet of this embodiment, the proportion of internal oxides ER, as specified in condition 1 above, is 40% or less.

[0182] like Figure 3 As shown, under condition 1 above, the base steel plate 10 is subjected to changes in its direction relative to the rolling direction ( Figure 3The observation is performed on a section 10a perpendicular to the front and back of the paper, specifically examining the grain boundary type internal oxides 15 contained within this section 10a. Here, the field of view is set to a range of 2 mm along the surface direction of the base steel plate 10. More specifically, the observation is performed using a field emission scanning electron microscope (FE-SEM), with the magnification set to 1000x in a single image. Multiple images are acquired along the surface direction of the base steel plate 10, and these multiple images are linked together to specifically examine the state of the section within a range of 2 mm along the surface direction of the base steel plate 10. Furthermore, the grain boundary type internal oxides 15 are defined as oxides within a depth range of 0.5 μm or more and 5.0 μm or less from the surface 10ax of the base steel plate 10. That is, in this application, oxides at a depth of 0.5 μm or more from the surface 10ax are considered "internal oxides" relative to "external oxides" (such as oxide scale) present in the surface 10ax of the base steel plate 10. Furthermore, grain boundary type internal oxide 15 typically does not form at depths exceeding 5.0 μm from the surface 10ax, so it is sufficient to use internal oxides at depths below 5.0 μm from the surface 10ax as the measurement target.

[0183] "Grain boundary internal oxides" refer to oxides that precipitate and form along the grain boundaries of the base steel sheet. As mentioned above, grain boundary internal oxides can inhibit grain growth on the surface through the pinning effect. That is, in the regions where grain boundary internal oxides are present, the grain boundary density tends to increase, which also promotes the alloying of the coating.

[0184] The grain boundary type internal oxide 15 may contain, for example, Mn, or it may contain Si in addition to Mn. This internal oxide may contain, for example, Mn₂SiO₄, or it may contain oxides with other compositions. The position, size, and shape of the grain boundary type internal oxide 15 within the field of view can be easily determined, for example, through elemental analysis. Observation can be performed using a known electron microscope or the like.

[0185] like Figure 3 As shown, under condition 1 above, the specific grain boundary type internal oxide 15 observed through the cross-section is projected onto the surface 10ax of the base steel plate 10, and a specific proportion ER is defined thereon. Specifically, the proportion of internal oxide 15 projected onto the surface 10ax of the base steel plate 10 is defined as the proportion ER of the presence of internal oxide 15 ([(total length of internal oxides projected onto the surface L (=L1+L2+····)) / (total length of the surface L0 (=2mm))]×100). It should be noted that, as Figure 3 As indicated by L1, in the case of multiple overlaps of internal oxide 15 in the depth direction, after being projected onto the surface 10ax of the parent steel plate 10, it is converted into one internal oxide. The same applies in the case of a specific interval I described later.

[0186] When the specific presence ratio ER is 40% or less, the surface of the base steel plate 10 has less internal oxide 15, and it becomes less prone to unevenness of internal oxide 15. The presence ratio ER can also be 38% or less or 35% or less. There is no particular limitation on the lower limit of the presence ratio ER; it can be 0%, but in the case of annealing at a high dew point as described below, it can be more than 0%, more than 5%, more than 8%, or more than 10%.

[0187] 1.3 Condition 2

[0188] In the alloyed hot-dip galvanized steel sheet of this embodiment, the spacing I of the internal oxides specified by condition 2 above is 300 μm or more.

[0189] like Figure 3 As shown, under condition 2 above, a section 10a perpendicular to the rolling direction of the base steel plate 10 is observed to specifically examine the grain boundary type internal oxides 15 contained in this section 10a. Here, the field of view is set to a range of 2 mm along the surface direction of the base steel plate 10. More specifically, similar to condition 1 above, observation is performed using a field emission scanning electron microscope (FE-SEM). The field of view in one image is set to 1000x, and multiple images are acquired along the surface direction of the base steel plate 10. These multiple images are then linked together to specifically examine the state of the section within a range of 2 mm along the surface direction of the base steel plate 10. The grain boundary type internal oxides 15 are defined as oxides extending from the surface 10ax of the base steel plate 10 in a depth direction of 0.5 μm or more and 5.0 μm or less. The identification of the grain boundary type internal oxides 15 is the same as under condition 1 above.

[0190] like Figure 3 As shown, under condition 2 above, the specific grain boundary type internal oxide 15 is projected onto the surface 10ax of the base steel plate 10 through the cross-section described above, with a specific interval I. Specifically, the interval I between adjacent internal oxides 15 is specified on the surface of the base steel plate 10 based on the position of the internal oxides 15 projected onto the surface of the base steel plate 10.

[0191] When the specific spacing I is 300 μm or more, the spacing between internal oxides in the surface layer of the base steel plate 10 is sufficiently wide, and the spacing between regions with high grain boundary density is also sufficiently wide. As a result, even when an alloyed hot-dip galvanized layer is formed, it is not easy to produce coating rib patterns that would impair the appearance. The spacing I can also be 350 μm or more, or 400 μm or more. There is no particular upper limit to the spacing I. As mentioned above, the field of view under condition 2 is within 2 mm along the surface direction of the base steel plate 10, but this spacing I can also exceed 2 mm, or be less than 2 mm, less than 1 mm, less than 800 μm, or less than 600 μm. It should be noted that when there are multiple specific spacings I in a field of view, it is acceptable as long as the smallest spacing I is 300 μm or more. For example, as Figure 3 As shown, when I1 and I2 are specified as interval I, it is sufficient as long as the shorter of them, I1, is 300 μm or more.

[0192] It should be noted that the "rolling direction" of the base steel plate in this application can be specified by the orientation of the ribs formed in the base steel plate. For example, as Figure 1 As shown, the rolling direction can be specified based on the elongation direction of the Mn-rich portion. The elongation direction of the Mn-rich portion in the base steel sheet can also be specified, for example, by element mapping using EPMA as described above.

[0193] 1.4 Metal Structure

[0194] The microstructure of the base steel plate is not particularly limited and can be adjusted according to the required properties of the base steel plate. In this embodiment, the microstructure of the base steel plate, in terms of area ratio, may be, for example, ferrite: 94-100%, martensite and bainite combined: 0-4%, and retained austenite: 0-2%. The area ratio of each phase and microstructure can be specified, for example, as follows.

[0195] The area ratio of retained austenite can be evaluated using high-resolution crystal structure analysis via EBSD (electron backscatter diffraction). Specifically, samples are collected using a section parallel to the rolling and thickness directions of the base steel sheet as the observation plane, and the observation plane is ground to a mirror finish. Furthermore, to remove the surface processing layer, electrolytic polishing or mechanical polishing using colloidal silica is performed. Next, at the 1 / 4 thickness position, each field of view is set to 10000 μm. 2The above describes the crystal structure analysis using the EBSD method across five fields of view. The step interval was set to 0.01–0.20 μm. The data obtained via EBSD were analyzed using analysis software (e.g., TSL's "OIMAnalysys"). Based on observations at each location, regions identified as FCC iron were determined to be retained austenite, and the area ratio of the retained austenite was calculated.

[0196] The area fractions of ferrite, martensite, bainite, and pearlite were measured as follows. First, samples were collected from a section parallel to the rolling and thickness directions of the base steel sheet. The observation surface was then ground and etched with nitric acid and ethanol. Next, within a range of 1 / 8 to 3 / 8 of the thickness, centered at a point 1 / 4 of the thickness from the surface, a field of view was set at 1200 μm. 2 The above observations were performed using a field emission scanning electron microscope (FE-SEM) across a total of five fields of view. The area fractions of ferrite, bainite, martensite, and pearlite were then measured. The identification of each microstructure was performed as follows: regions where cementite precipitated in lamellar form were identified as pearlite. Regions with relatively low brightness were identified as ferrite. The area fractions were calculated using a point counting method. The total area fraction of martensite and bainite was then calculated by subtracting the area fraction of retained austenite obtained previously using the EBSD method from the area fractions of the other regions.

[0197] 1.5 coating

[0198] The alloyed hot-dip galvanized steel sheet of this embodiment has the aforementioned base steel sheet and coating. The coating is formed on at least one surface of the base steel sheet. The coating is preferably an alloyed hot-dip galvanized layer having a composition known to those skilled in the art. For example, the coating may contain additive elements such as Al in addition to Zn. In the alloyed hot-dip galvanized steel sheet of this embodiment, the chemical composition of the coating may, for example, be Fe: 5-25% by mass, Al: 0-1.0%, Si: 0-1.0%, Mg: 0-1.0%, Mn: 0-1.0%, Ni: 0-1.0%, Sb: 0-1.0%, and the remainder: Zn and impurities. The Al content in the coating may also be greater than 0% by mass or more than 0.1% by mass. The amount of coating is not particularly limited, and a general amount is preferred.

[0199] The chemical composition of the coating can be specified, for example, by the following steps: After removing the surface coating with a non-corrosive coating stripper (e.g., NEOREVER SP-751 manufactured by Sansai Chemical Co., Ltd.), the coating is dissolved in hydrochloric acid with an inhibitor (e.g., HIBIRON manufactured by Sugimura Chemical Industrial Co., Ltd.), and the resulting solution is subjected to inductively coupled plasma (ICP) emission spectroscopy analysis, thereby determining the chemical composition of the coating.

[0200] 1.6 Other properties of alloyed hot-dip galvanized steel sheets

[0201] In this embodiment, the alloyed hot-dip galvanized steel sheet only needs to have the above-mentioned chemical composition of the base steel sheet and satisfy the above-mentioned requirements for the presence ratio ER and interval I. Furthermore, the alloyed hot-dip galvanized steel sheet in this embodiment may also have the following characteristics.

[0202] (Tensile strength TS)

[0203] To improve the lightweight nature of structures using steel as a raw material and enhance their resistance to plastic deformation, it is preferable that the steel raw material possesses high work hardening capacity and exhibits maximum strength. On the other hand, if the tensile strength of the steel sheet is too high, it may sometimes become prone to fracture with low energy during plastic deformation, resulting in reduced formability. The tensile strength of alloyed hot-dip galvanized steel sheets can be, for example, 430 MPa or more, 450 MPa or more, 500 MPa or more, or 520 MPa or more; alternatively, it can be 600 MPa or less, 580 MPa or less, 550 MPa or less, or 500 MPa or less.

[0204] (Total elongation EL)

[0205] When cold-forming steel sheets to manufacture structures, the sheets need to be stretched in order to be finished into complex shapes. If the total elongation of the steel sheet is too low, the raw material may crack during cold forming. There is no particular limitation on the total elongation of the steel sheet, but it can be, for example, more than 25% or more than 30%, or less than 40% or less than 35%.

[0206] (Yield point YP)

[0207] In this embodiment, the yield point of the steel plate is not particularly limited, but it can be, for example, 195 MPa or more, 200 MPa or more, 210 MPa or more, 250 MPa or more, 280 MPa or more, or 300 MPa or more. Alternatively, it can be 420 MPa or less, 400 MPa or less, 380 MPa or less, 350 MPa or less, 340 MPa or less, or 320 MPa or less.

[0208] (Methods for determining tensile strength, total elongation, and yield point)

[0209] The tensile test for determining tensile strength, total elongation and yield point is conducted according to JIS Z 2241. JIS No. 5 test pieces are collected from the length of the test piece in a direction parallel to the rolling right angle of the steel strip.

[0210] (plate thickness)

[0211] The thickness of the base steel sheet is a factor affecting the rigidity of the formed steel component; the greater the thickness, the higher the rigidity of the component. If the thickness is too small, it can sometimes lead to a decrease in rigidity, and the compressibility can be reduced due to the unavoidable non-ferrous inclusions present within the steel sheet. On the other hand, if the thickness is too large, the compressive load can sometimes increase, leading to die wear and reduced productivity. There is no particular limitation on the thickness of the base steel sheet; it can be 0.2 mm or more, or 6.0 mm or less.

[0212] 1.7 Effects

[0213] As described above, the alloyed hot-dip galvanized steel sheet of this embodiment controls the distribution morphology of internal oxides in the surface layer of the base steel sheet to a predetermined state. Therefore, through... Figure 1 and 2 The mechanism shown promotes alloying of the coating in the portions containing internal oxides (the portions with high grain boundary density). Even when a thicker coating is formed, the sufficiently wide spacing between the thicker coated portions makes it less likely to be perceived as a ribbed pattern of the coating. As a result, the alloyed hot-dip galvanized steel sheet of this embodiment exhibits excellent coating appearance and post-coating appearance.

[0214] 2. Manufacturing method of alloyed hot-dip galvanized steel sheet

[0215] The alloyed hot-dip galvanized steel sheet of this embodiment can be manufactured through continuous management of casting, hot rolling, cold rolling, and annealing. It is particularly important to set the pickling conditions after hot rolling to over-pickling to dissolve the surface layer of the steel sheet. This lowers the starting point for the formation of internal oxides, making it easier to meet the aforementioned presence ratio ER and interval I. Alternatively, the formation of internal oxides can be suppressed by setting the dew point during annealing to a low dew point. Hereinafter, an example of a method for manufacturing alloyed hot-dip galvanized steel sheet will be described, but the method for manufacturing alloyed hot-dip galvanized steel sheet is not limited to this example. For example, the method for manufacturing alloyed hot-dip galvanized steel sheet of this embodiment is characterized by comprising:

[0216] Steel slabs with the above chemical composition are obtained by continuous casting;

[0217] The above-mentioned steel slab is heated and then hot-rolled to obtain a hot-rolled plate;

[0218] The hot-rolled sheet is coiled up;

[0219] The hot-rolled plate was immersed in an acid treatment solution for pickling.

[0220] The above-mentioned hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet; and

[0221] The above-mentioned cold-rolled sheet is annealed.

[0222] The aforementioned acid treatment solution contains hydrochloric acid, the concentration of hydrochloric acid in the acid treatment solution is 7% by mass or more, the temperature of the acid treatment solution is 80°C or more, and the immersion time in the acid treatment solution is 30 seconds or more.

[0223] The dew point of the atmosphere during the annealing process is above -35°C and below 30°C. Hereinafter, each step will be described in detail, focusing on its crucial components as described in this embodiment.

[0224] 2.1 Pickling conditions

[0225] In the manufacturing method of alloyed hot-dip galvanized steel sheet according to this embodiment, the conditions for pickling the hot-rolled sheet by immersing it in an acid treatment solution are important. Specifically, by setting conditions that can dissolve not only the oxide scale on the sheet surface but also the Mn in the surface layer (compared to normal conditions, this is called over-pickling), the Mn concentrated in the surface layer of the sheet can be dissolved, thereby lowering the starting point for the formation of internal oxides. As a result, it becomes easier to meet the aforementioned presence ratio ER and interval I. Specifically, in this embodiment, the acid treatment solution contains hydrochloric acid, and the concentration of hydrochloric acid in the acid treatment solution is preferably 7% by mass or more. By using an acid treatment solution containing hydrochloric acid at such a high concentration, not only the oxide scale on the surface of the hot-rolled sheet becomes easier to dissolve, but also the Mn in the surface layer of the hot-rolled sheet becomes easier to dissolve. The concentration of hydrochloric acid in the acid treatment solution can also be 9% by mass or more, or 15% by mass or less. Furthermore, the temperature of the acid treatment solution is preferably 80°C or higher. By setting the temperature of the acid treatment solution to a high temperature, not only does the oxide scale on the surface of the hot-rolled sheet become easier to dissolve, but the Mn layer on the surface of the hot-rolled sheet also becomes easier to dissolve. The temperature of the acid treatment solution can be above 85°C or below 90°C. Furthermore, the immersion time in the acid treatment solution is preferably 30 seconds or more. By setting the immersion time in the acid treatment solution to a long time, not only does the oxide scale on the surface of the hot-rolled sheet become easier to dissolve, but the Mn layer on the surface of the hot-rolled sheet also becomes easier to dissolve. The immersion time can be 35 seconds or more or 60 seconds or less.

[0226] 2.2 Annealing conditions

[0227] When forming an alloyed hot-dip galvanized layer, it is often desirable to generate internal oxides in the surface layer of the sheet from the viewpoint of increasing the alloying speed. In order to generate internal oxides in a portion of the sheet surface layer within the aforementioned presence ratio ER and to satisfy the aforementioned interval I, as described above, pickling is performed under conditions equivalent to over-pickling to dissolve the oxide scale and Mn on the sheet surface, thereby lowering the starting point for internal oxide formation. Following this, cold rolling is performed, and then annealing is carried out in a high dew point atmosphere to generate internal oxides in a portion of the sheet surface layer. An alloyed hot-dip galvanized layer is then formed during or after annealing. For example, the dew point in the annealing atmosphere can be above -35°C, above -30°C, above -25°C, or above -20°C, or it can be below 30°C, below 25°C, below 20°C, below 15°C, or below 0°C. It should be noted that sometimes the aforementioned presence ratio ER is achieved by setting the annealing atmosphere to a low dew point atmosphere to suppress the formation of internal oxides in the sheet surface layer. However, when the annealing atmosphere is a low dew point atmosphere below -35°C, external oxides are easily formed on the outermost surface of the base steel sheet. Specifically, there is concern that Si- or Mn-based external oxides may form on the outermost surface of the base steel sheet, which could resist plating and result in no plating, or hinder Fe diffusion and slow down alloying, thus failing to obtain the desired alloyed hot-dip galvanized steel sheet.

[0228] 2.3 Other processes

[0229] In the manufacturing method of this embodiment, as described above, continuous casting, hot rolling, coiling, pickling, cold rolling, and annealing are preferably performed. The pickling conditions and the atmosphere during annealing are as described above. There are no particular limitations on the hot rolling conditions, coiling conditions, cold rolling conditions, and annealing conditions other than the atmosphere. Furthermore, there are no particular limitations on the plating treatment conditions. An example of each process is shown below.

[0230] (Hot rolling conditions)

[0231] When hot rolling a slab, it can be preheated before hot rolling. There are no particular limitations on the heating temperature; for example, it can be above 1100°C and below 1300°C. There are also no particular limitations on the heating time; for example, it can be above 30 minutes and below 300 minutes. Furthermore, when hot rolling the heated slab, general conditions can be used for both roughing and finishing. However, since the finishing temperature of hot rolling is a factor that affects the control of the texture of the base steel sheet, it is advisable to control it within a specified temperature range. For example, the finishing temperature of hot rolling can be above 900°C or below 950°C.

[0232] (Crawling conditions)

[0233] There is no particular limitation on the coiling temperature of hot-rolled steel sheets. For example, a temperature above 500°C and below 800°C is preferable.

[0234] (Reduction rate in cold rolling)

[0235] The reduction rate during cold rolling is important from the viewpoint of obtaining a texture with excellent r-value. For example, the total reduction rate during cold rolling is preferably 70% or more, and more preferably 90% or less. Annealing can also be performed at a temperature of 700°C or less to reduce the cold rolling load before cold rolling.

[0236] (Hold the annealing temperature)

[0237] If the heating temperature during annealing is too low, the ductility of the steel sheet is easily reduced. On the other hand, excessive heating not only leads to increased costs, but can also cause problems such as poor sheet shape during high-temperature plate passing or reduced roll life. Based on the above considerations, the maximum heating temperature (annealing holding temperature) during annealing is preferably 750°C or higher, and more preferably 900°C or lower.

[0238] (Annealing holding time)

[0239] During annealing, it is preferable to hold the heating at the aforementioned temperature for at least 5 seconds. If the holding time is too short, the decrease in strength may become significant. Furthermore, uneven hardness may also increase. From these points of view, a holding time of at least 10 seconds is more preferable. More preferably, it is at least 20 seconds.

[0240] (Cooling rate after annealing)

[0241] There are no particular restrictions on the cooling conditions after the above annealing.

[0242] (Cooling stop temperature after annealing and reheating)

[0243] Furthermore, after the aforementioned annealing and cooling, if the cooling stop temperature is lower than the plating bath temperature, the temperature can be reheated to a range of 350°C to 600°C and held there. It should be noted that if the cooling stop temperature is too low, not only will a significant investment in equipment be required, but the effect will also be saturated.

[0244] (Retention temperature)

[0245] Furthermore, the steel sheet can be held in a temperature range of 350–600°C after reheating and before immersion in the plating bath. Holding in this temperature range suppresses temperature unevenness in the width direction of the sheet, improving the appearance after plating. It should be noted that when the cooling stop temperature after annealing is 350–600°C, holding can be performed without reheating.

[0246] (Duration of stay)

[0247] To achieve the desired effect, the dwell time should preferably be set to be between 30 and 300 seconds.

[0248] (Tempering)

[0249] In a series of annealing processes, the cold-rolled sheet or the steel sheet obtained by coating the cold-rolled sheet can also be cooled to room temperature, or reheated during the cooling process (below Ms).

[0250] (plating)

[0251] A hot-dip galvanized layer is formed on the surface of the steel sheet through a hot-dip galvanizing process. In this case, the corrosion resistance of the cold-rolled steel sheet is improved, which is therefore preferable. It should be noted that in the manufacturing method of this embodiment, the coating may also be formed on the surface of the sheet during annealing. Alternatively, the coating may be formed on the surface of the sheet after annealing.

[0252] (Composition of the plating bath)

[0253] The composition of the plating bath only needs to be Zn-based, and the chemical composition of the alloyed coating should fall within the target range. Preferably, the effective Al content (the value obtained by subtracting the total Fe content from the total Al content in the plating bath) is 0.050 to 0.250% by mass. If the effective Al content in the plating bath is too low, Fe may excessively penetrate into the coating, reducing the coating adhesion. On the other hand, if the effective Al content in the plating bath is too high, Al-based oxides that hinder the movement of Fe and Zn atoms may form at the boundary between the steel plate and the coating, resulting in insufficient alloying. More preferably, the effective Al content in the plating bath is 0.065% by mass or more, and more preferably 0.180% by mass or less.

[0254] (Temperature of the steel plate after immersion in the plating bath)

[0255] To alloy the hot-dip galvanized layer, the steel sheet with the hot-dip galvanized layer should preferably be heated to a temperature range of 450–600°C. If the alloying temperature is too low, alloying may not be sufficient. On the other hand, if the alloying temperature is too high, over-alloying may occur, leading to excessive Fe concentration in the coating due to the formation of the Γ phase, thus reducing the coating adhesion. The alloying temperature can also be above 470°C or below 580°C. The alloying temperature needs to be adjusted according to the surface condition of the steel sheet, such as its composition and the degree of internal oxide formation; therefore, it can be set while checking the Fe concentration in the coating. The alloying of the hot-dip galvanized layer is carried out, for example, in an alloying furnace and a heat-insulating zone. The residence time in the alloying furnace and heat-insulating zone should preferably be approximately 30 seconds, for example, about 10 seconds in the alloying furnace and about 20 seconds in the heat-insulating zone. It should be noted that the heat-insulating zone is a section where the temperature of the steel sheet exiting the alloying furnace is kept warm in a way that prevents a rapid drop in temperature.

[0256] (Post-processing)

[0257] On the surface of alloyed hot-dip galvanized steel sheets, for the purpose of improving paintability and weldability, an upper coating or various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment can also be applied.

[0258] (Surface smoothing ratio)

[0259] Furthermore, surface finishing can also be implemented to improve ductility by correcting the shape of the steel sheet and introducing movable dislocations. The reduction rate of surface finishing after heat treatment is preferably in the range of 0.1% to 1.5%. Below 0.1%, the effect is small and control is difficult, therefore this is the lower limit. If it exceeds 1.5%, productivity decreases significantly, therefore this is set as the upper limit. Surface finishing can be performed online or offline. Furthermore, surface finishing with the target reduction rate can be performed in one go or in multiple stages.

[0260] Example

[0261] The following illustrates an embodiment of the invention. The invention is not limited to this single example. Various conditions can be employed to achieve the invention's purpose without departing from its spirit and intent.

[0262] 1. Preparation of base steel plates

[0263] Steel slabs with various chemical compositions are smelted and continuously cast to produce steel billets. The cooled steel billets are then inserted into a furnace heated to 1200°C and held for 90 minutes for homogenization before being exposed to the atmosphere. Next, they are hot-rolled to obtain a 3.2 mm thick hot-rolled sheet, which is then coiled at a specified temperature. The oxide scale on the hot-rolled sheet is then removed by pickling. The dissolution and removal of oxide scale and concentrated Mn on the surface of the sheet are adjusted by changing the pickling conditions. After pickling, cold rolling (75% reduction) is performed to finish-roll the sheet to a thickness of 0.8 mm. This cold-rolled sheet is then annealed. For all steel sheets, the holding temperature during annealing is set to 800°C, and the holding time is set to 60 seconds. The atmosphere during annealing is N2-4%H2, and the dew point is set to 0°C or -40°C. Immediately after annealing, a plating and alloying treatment is performed, followed by surface finishing rolling. The chemical composition of the base steel plates obtained by analyzing samples collected from each steel plate is shown in Tables 1 and 2 below. It should be noted that the remaining components other than those shown in Tables 1 and 2 are Fe and impurities.

[0264] Table 1

[0265]

[0266] Table 2

[0267]

[0268] 2. Plating

[0269] Each steel sheet is hot-dip galvanized, followed by alloying treatment. In the hot-dip galvanizing process, the steel sheet is immersed in a 450°C galvanizing bath for 5 seconds. Afterwards, alloying treatment is performed at 590°C, followed by cooling to room temperature to obtain alloyed hot-dip galvanized steel sheet. It should be noted that the coating treatment of the base steel sheet can be carried out using the same equipment and production line as the annealing process.

[0270] 3. Evaluation

[0271] 3.1 Observation of grain boundary-type internal oxides in the base steel plate

[0272] In alloyed hot-dip galvanized steel sheets, the cross-section in the direction orthogonal to the rolling direction is observed, such as... Figure 3 As shown, the specific proportion of internal oxides ER is determined by condition 1 below and the interval I of internal oxides is determined by condition 2 below.

[0273] Condition 1: such as Figure 3As shown, a section perpendicular to the rolling direction of the aforementioned base steel plate is observed to specifically identify the grain boundary type internal oxides contained in that section. Here, the field of view is set to a range of 2 mm along the surface direction of the aforementioned base steel plate. More specifically, observation is performed using a field emission scanning electron microscope (FE-SEM), with the field of view in one image set to 1000x. Multiple images are acquired along the surface direction of the base steel plate 10, and these multiple images are linked together to specifically identify the state of the section within a 2 mm range along the surface direction of the base steel plate 10. The aforementioned internal oxides are defined as oxides with a depth range of 0.5 μm or more and 5.0 μm or less in the depth direction from the surface of the aforementioned base steel plate. The specific internal oxides are projected onto the surface of the aforementioned base steel plate. The proportion of the internal oxides projected onto the surface of the aforementioned base steel plate is defined as the aforementioned presence ratio ER of the internal oxides.

[0274] Condition 2: such as Figure 3 As shown, a section perpendicular to the rolling direction of the aforementioned base steel plate is observed to specifically identify the grain boundary type internal oxides contained in the section. Here, the field of view is set to a range of 2 mm along the surface direction of the aforementioned base steel plate. More specifically, similar to condition 1, observation is performed using a field emission scanning electron microscope (FE-SEM, with the field of view set to 1000x in a single image). Multiple images are acquired along the surface direction of the base steel plate 10, and these multiple images are linked together to specifically identify the state of the section within a 2 mm range along the surface direction of the base steel plate 10. The aforementioned internal oxides are defined as oxides with a depth range of 0.5 μm or more and 5.0 μm or less in the depth direction from the surface of the aforementioned base steel plate. The specific internal oxides are projected onto the surface of the aforementioned base steel plate. Based on the position of the internal oxides projected onto the surface of the aforementioned base steel plate, the spacing I of adjacent internal oxides is specified.

[0275] 3.2 Mechanical properties

[0276] The yield point (YP), tensile strength (TS), total elongation (EL), and average r-value of alloyed hot-dip galvanized steel sheets were determined. The measurement conditions are as described above.

[0277] 3.3 Coating properties

[0278] 3.3.1 Wettability

[0279] The wettability of the coating relative to the base steel sheet is evaluated according to the following criteria.

[0280] ○: Good wettability (100% of the surface of the base steel plate was confirmed to have been coated by visual inspection)

[0281] ×: Poor wettability (visual inspection revealed areas on the surface of the base steel plate that were not coated).

[0282] 3.3.2 Presence or absence of rib-like patterns in the coating

[0283] The appearance of alloyed hot-dip galvanized steel sheets is visually observed, and the degree of clarity of the coating rib pattern is evaluated according to the following evaluation criteria.

[0284] ◎: The ribbed pattern of the plating cannot be seen.

[0285] 〇: The ribbed pattern of the coating is not visible at a distance of 50cm from the steel plate.

[0286] △: At a distance of 50cm from the steel plate, the ribbed pattern of the coating can be seen, but at a distance of 100cm, the ribbed pattern of the coating cannot be seen.

[0287] ×: The ribbed pattern of the coating can also be seen at a distance of 100cm from the steel plate.

[0288] 3.4 Formability and processability

[0289] 3.4.1 Whether the coating adheres to the mold

[0290] Alloyed hot-dip galvanized steel sheets are pressed (condition: deep drawing of cylinder), and the presence or absence of coating adhesion on the mold is evaluated according to the following criteria.

[0291] ○: There is virtually no coating adhering to the mold.

[0292] ×: The coating adheres poorly to the mold.

[0293] 3.4.2 Embrittlement during secondary processing

[0294] Alloyed hot-dip galvanized steel sheets are deep-drawn into cylinders and cooled at 0°C for more than 5 minutes. They are then crushed by pressing. The presence or absence of secondary processing embrittlement is evaluated according to the following criteria.

[0295] ○: No secondary processing causing embrittlement (crack length after pressing is less than 10mm)

[0296] ×: Secondary processing has caused embrittlement (the crack length after pressing is more than 10mm).

[0297] 4. Results

[0298] Table 3 shows the evaluation results of the manufacturing conditions of the base steel sheet, the properties of the alloyed hot-dip galvanized steel sheet, mechanical properties, coating properties, and formability and processability.

[0299]

[0300] It should be noted that the alloyed hot-dip galvanized steel sheets No. 1 to 41 are all steel sheets whose metal microstructure, by area ratio, meets the following requirements: ferrite: 94-100%, martensite and bainite combined: 0-4%, and retained austenite: 0-2%. Furthermore, the alloyed hot-dip galvanized steel sheets No. 1 to 41 are all steel sheets whose coating chemical composition, by mass %, meets the following requirements: Fe: 5-25%, Al: 0-1.0%, Si: 0-1.0%, Mg: 0-1.0%, Mn: 0-1.0%, Ni: 0-1.0%, Sb: 0-1.0%, and the remainder: Zn and impurities.

[0301] The following information can be obtained from the results shown in Table 3.

[0302] Regarding No. 27, due to the low temperature of the pickling solution and the excessively short immersion time during pickling, the surface layer of the base steel sheet could not be properly dissolved. As a result, the proportion of internal oxides ER in the annealed base steel sheet increased, producing distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0303] Regarding No. 28, due to the excessively low temperature of the pickling solution, the surface layer of the base steel sheet could not be properly dissolved. As a result, the proportion of internal oxides (ER) in the annealed base steel sheet increased, producing distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0304] Regarding No. 29, due to the insufficient immersion time during pickling, the surface layer of the base steel sheet could not be properly dissolved. As a result, the proportion of internal oxides ER in the annealed base steel sheet increased, producing distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0305] Regarding No. 30, compared to No. 28, the shorter immersion time during pickling prevents the surface layer of the base steel sheet from dissolving adequately. As a result, the proportion of internal oxides ER in the annealed base steel sheet increases, and the spacing I of the internal oxides decreases, producing a distinct rib-like pattern in the alloyed hot-dip galvanized layer.

[0306] Regarding No. 31, due to the low concentration of hydrochloric acid in the acid treatment solution, the surface layer of the base steel plate could not be properly dissolved during pickling. As a result, the proportion of internal oxides ER in the annealed base steel plate increased, and the spacing I of the internal oxides decreased, resulting in distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0307] Regarding No. 32, due to the excessively low dew point during annealing, an external oxide layer is formed on the surface of the base steel sheet, which resists plating during the plating process, resulting in unplated steel sheet and failing to obtain the alloyed hot-dip galvanized steel sheet that was intended to be obtained.

[0308] Regarding No. 33, it is believed that due to the excessive carbon content in the base steel sheet, a large amount of dissolved carbon remains in the base steel sheet that is not fixed by Ti and Nb. As a result, alloying of the coating is hindered, resulting in unalloyed coating portions in the coated steel sheet, which easily leads to coating adhesion to the mold.

[0309] Regarding No. 34, it is believed that the excessive Si content in the base steel sheet deteriorates the wettability of the coating relative to the base steel sheet. Furthermore, it is argued that Si hinders the alloying of the coating, resulting in an increase in the amount of unalloyed coating in the coated steel sheet. During pressing, this unalloyed coating adheres to the die, impairing formability and productivity.

[0310] Regarding No. 35, due to the excessive Mn content in the base steel plate, there is a tendency for the elongation to decrease.

[0311] Regarding No. 36, it is believed that the excessive phosphorus content in the base steel sheet hinders the alloying of the coating. As a result, there is an increase in the amount of unalloyed coating in the coated steel sheet. During pressing, this unalloyed coating adheres to the mold, impairing formability and productivity.

[0312] Regarding No. 37, due to the excessive sulfur content in the base steel plate, secondary processing embrittlement occurs due to segregation at the grain boundaries of the base steel plate.

[0313] Regarding No. 38, it is believed that due to the insufficient Ti content in the base steel sheet, the immobilization of C in the base steel sheet becomes inadequate, hindering the alloying of the coating due to C solid solution. As a result, unalloyed coating portions are formed in the coated steel sheet, making it prone to adhesion to the mold.

[0314] Regarding No. 39, due to insufficient B content in the base steel plate, it is unable to fully compensate for the function of C removed from the grain boundaries, resulting in secondary processing embrittlement.

[0315] Regarding No. 40, it is believed that the excessive boron content in the base steel sheet hinders the alloying of the coating. As a result, the amount of unalloyed coating in the coated steel sheet increases. During pressing, this unalloyed coating adheres to the mold, impairing formability and productivity. Furthermore, the low temperature of the pickling solution and the insufficient immersion time during pickling prevent adequate dissolution of the surface layer of the base steel sheet. Consequently, the proportion of internal oxides (ER) in the annealed base steel sheet increases, and the spacing (I) of internal oxides decreases, resulting in distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0316] Regarding No. 41, due to the excessive Ti content in the base steel sheet, the balance between strength and ductility of the coated steel sheet becomes poor.

[0317] If the above results are summarized, the following alloyed hot-dip galvanized steel sheets are not easily visible in terms of the rib-like pattern of the coating, and have excellent appearance.

[0318] An alloyed hot-dip galvanized steel sheet, comprising a base steel sheet and a coating,

[0319] The aforementioned base steel plate has the following chemical composition: containing, by mass %:

[0320] C: 0.0005~0.0100%

[0321] Si: 0.01~0.50%

[0322] Mn: 0.01~2.00%

[0323] P: below 0.100%

[0324] S: below 0.0100%

[0325] N: below 0.0200%

[0326] Ti: 0.040~0.180%

[0327] Nb: 0~0.100%

[0328] B: 0.0005~0.0100%

[0329] Al: 0~1.000%

[0330] Cu: 0~1.000%

[0331] Cr: 0–2.000%

[0332] Ni: 0~0.500%

[0333] Mo: 0–3.000%

[0334] W: 0~0.100%

[0335] V: 0~1.000%

[0336] O: 0~0.020%

[0337] Ta: 0~0.100%

[0338] Co: 0-3.000%

[0339] Sn: 0~1.000%

[0340] Sb: 0~0.500%

[0341] As: 0-0.050%

[0342] Mg: 0–0.050%

[0343] Zr: 0~0.050%

[0344] Ca: 0–0.0500%, and

[0345] REM: 0~0.0500%,

[0346] The remaining part consists of Fe and impurities.

[0347] The presence ratio of the specific internal oxide ER under condition 1 is less than 40%, and the spacing I of the specific internal oxide under condition 2 is more than 300 μm.

Claims

1. An alloyed hot-dip galvanized steel sheet, comprising a base steel sheet and a coating, The base steel plate has the following chemical composition: containing, by mass %: C：0.0005～0.0100％、 Si: 0.01~0.50% Mn: 0.43~2.00% P: below 0.100% S: below 0.0100% N: below 0.0200% Ti: 0.040~0.180% Nb: 0~0.100% B：0.0005～0.0100％、 Al：0～1.000％、 Cu: 0~1.000% Cr：0～2.000％、 Ni: 0~0.500% Mo: 0–3.000% W：0～0.100％、 V：0～1.000％、 O：0～0.020％、 Ta: 0~0.100% Co: 0-3.000% Sn: 0~1.000% Sb: 0~0.500% As: 0-0.050% Mg: 0–0.050% Zr：0～0.050％、 Ca: 0–0.0500%, and REM: 0~0.0500%, The remaining part consists of Fe and impurities. Under the following condition 1, the presence ratio of specific internal oxides ER is less than 40%. Under the following condition 2, the spacing I of specific internal oxides is 300 μm or more. Condition 1: Observe a cross-section of the base steel plate perpendicular to the rolling direction, and identify the grain boundary-type internal oxides contained in that cross-section, wherein... The field of view is set to a range of 2 mm along the surface direction of the base steel plate. The internal oxides are defined as oxides in the range of 0.5 μm and 5.0 μm or more in the depth direction from the surface of the base steel plate. The specific internal oxides are projected onto the surface of the base steel plate. The proportion of the internal oxides projected onto the surface of the base steel plate is defined as the presence ratio ER of the internal oxides. Condition 2: Observe a section of the base steel plate perpendicular to the rolling direction, and identify the grain boundary type internal oxides contained in the section. The field of view is set to a range of 2 mm along the surface direction of the base steel plate. The internal oxides are defined as oxides in the depth direction of 0.5 μm and 5.0 μm from the surface of the base steel plate. Project the identified internal oxides onto the surface of the base steel plate. Based on the position of the internal oxides projected onto the surface of the base steel plate, identify the spacing I of adjacent internal oxides.

2. The alloyed hot-dip galvanized steel sheet according to claim 1, wherein, The chemical composition, expressed in mass percent, contains: Mn: 0.43–1.30% or 1.70–2.00%.

3. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The presence ratio of the internal oxide is ER of 5% or more.

4. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The chemical composition, expressed as a percentage by mass, contains at least one element selected from the group consisting of: Nb: 0.001~0.100% Al：0.001～1.000％、 Cu: 0.001~1.000% Cr：0.001～2.000％、 Ni: 0.001~0.500% Mo: 0.001~3.000% W：0.001～0.100％、 V：0.001～1.000％、 O：0.001～0.020％、 Ta: 0.001~0.100% Co: 0.001~3.000% Sn: 0.001~1.000% Sb: 0.001~0.500% As: 0.001~0.050% Mg: 0.001~0.050% Zr：0.001～0.050％、 Ca: 0.0001~0.0500% REM: 0.0001~0.0500%.

5. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The microstructure of the base steel plate, expressed as an area ratio, is as follows: Ferrite: 94-100% The total percentage of martensite and bainite: 0–4%, and Retained austenite: 0–2%.

6. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The chemical composition of the coating, expressed in mass percent, is: Fe: 5-25% Al：0～1.0％、 Si: 0-1.0% Mg: 0-1.0% Mn: 0~1.0% Ni: 0-1.0%, Sb: 0-1.0%, and the remainder: Zn and impurities.

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