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%, a standard deviation SD of Mn intensity of 0.40 or less as calculated by prescribed condition 1 using EPMA, and an interval I of Mn concentration portions of 50 μm or less as calculated by prescribed condition 2 using EPMA. 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 an exterior component in automotive 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 the coating appearance deteriorates, the final appearance after painting 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 of the coating on alloyed hot-dip galvanized steel sheets, they have not been entirely effective. Therefore, there is still room for improvement in suppressing the ribbed pattern and improving 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 of alloyed hot-dip galvanized steel sheets, the unalloyed coating sometimes adheres to the mold, impairing formability or productivity. Additionally, 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] The standard deviation SD obtained under the following condition 1 is below 0.40.

[0045] The interval I obtained by the following condition 2 is less than 50 μm.

[0046] Condition 1: Using an electron beam microanalyzer (EPMA), a 2mm square rectangular area on the surface of the base steel plate is divided into multiple 400×400 areas for elemental mapping, specifying the Mn intensity in each of these areas. The standard deviation (SD) is calculated from the distribution of the number of detection points of the Mn intensity in the rectangular areas.

[0047] Condition 2: Using an electron beam microanalyzer (EPMA), a 2mm square rectangular region on the surface of the base steel plate was divided into multiple 400×400 segments for elemental mapping, and the Mn intensity in each of these segments was determined. The average Mn intensity MI in the specified rectangular region was then calculated. ave The rectangular region described above is divided into 400 segments along a direction orthogonal to the rolling direction, defined as 400 segmented regions. The average value MI in each of these segmented regions is then considered. ave Regions exhibiting Mn intensity exceeding 34% occupying more than 50% of the area are designated as Region 1. Regions with Mn intensity exceeding 34% are further divided relative to the aforementioned average Mn content. aveThe region where the Mn intensity is lower than 34% accounts for more than 50% is designated as the second region. The interval I between the first region and the second region is determined.

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

[0049] Mn: 0.01–1.30% or 1.70–2.00%.

[0050] In the alloyed hot-dip galvanized steel sheet of this disclosure, the above-mentioned chemical composition may also contain at least one element selected from the group consisting of the following elements, expressed in mass percent:

[0051] Nb: 0.001~0.100%

[0052] Al: 0.001~1.000%

[0053] Cu: 0.001~1.000%

[0054] Cr: 0.001~2.000%

[0055] Ni: 0.001~0.500%

[0056] Mo: 0.001~3.000%

[0057] W: 0.001~0.100%

[0058] V: 0.001~1.000%

[0059] O: 0.001~0.020%

[0060] Ta: 0.001~0.100%

[0061] Co: 0.001~3.000%

[0062] Sn: 0.001~1.000%

[0063] Sb: 0.001~0.500%

[0064] As: 0.001~0.050%

[0065] Mg: 0.001~0.050%

[0066] Zr: 0.001~0.050%

[0067] Ca: 0.0001~0.0500%

[0068] REM: 0.0001~0.0500%.

[0069] 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:

[0070] Ferrite: 94-100%

[0071] The total percentage of martensite and bainite: 0–4%, and

[0072] Retained austenite: 0–2%.

[0073] 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:

[0074] Fe: 5-25%

[0075] Al: 0-1.0%

[0076] Si: 0-1.0%

[0077] Mg: 0-1.0%

[0078] Mn: 0~1.0%

[0079] Ni: 0-1.0%

[0080] Sb: 0–1.0%, and

[0081] Remaining components: Zn and impurities.

[0082] Invention Effects

[0083] 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

[0084] Figure 1 It is a schematic diagram showing the morphology of the Mn-concentrated portion on the surface of the base steel plate.

[0085] Figure 2 This is a schematic diagram used to illustrate the mechanism of rib-like patterns in alloyed hot-dip galvanized layers.

[0086] Figure 3 This is a schematic diagram used to illustrate condition 1.

[0087] Figure 4 This is a schematic diagram used to illustrate condition 2. 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 sheet originates from uneven Mn concentration on the base metal surface caused by microsegregation of Mn during slab solidification. For example... Figure 1 As shown, the Mn-concentrated portion on the base metal surface of the slab extends along the rolling direction during rolling, forming a rib-like pattern. That is, when a steel sheet is obtained by rolling a slab, the Mn-concentrated portion on the surface of the steel sheet extends in a rib-like pattern 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 oxide (e.g., Mn2SiO4) is generated in the surface layer of the steel plate, and the pinning effect of this Mn oxide can inhibit the growth of surface grains. Figure 2 (B) That is, in the Mn-rich portion of the base steel sheet, the grain boundary density tends to be higher compared to the portion outside the Mn-rich portion. Subsequently, if a zinc coating is formed relative to the base steel sheet and further alloying is performed, alloying is promoted in the portion with the higher grain boundary density. Figure 2 (C)) The coating grows thicker, thus creating rib-like patterns within 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 concentration of Mn on the surface of the base steel sheet. However, according to the inventors' novel insights, the degree of prominence of the rib-like pattern in the coating varies depending on the degree of this uneven Mn concentration. That is, even if uneven Mn concentration occurs, if the intervals between these uneven concentrations are sufficiently narrow or sufficiently wide, rib-like patterns that impair the appearance are less likely to occur. Therefore, by controlling the Mn distribution 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 in the coating does not depend solely on the "amount" of concentrated Mn on the surface of the base steel sheet as disclosed in conventional art (e.g., Patent Document 3), but more precisely, on the "distribution morphology" of Mn.

[0091] The distribution morphology of Mn on the surface of the base steel plate can be quantified by methods such as the standard deviation of Mn intensity obtained using electron beam microanalysis (EPMA). Furthermore, the distribution morphology of Mn on the surface of the base steel plate can be controlled, for example, by adjusting the heating temperature of the slab before hot rolling (i.e., allowing Mn to diffuse at high temperatures).

[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. Furthermore, the standard deviation SD, determined under condition 1 below, is 0.40 or less, and the interval I, determined under condition 2 below, is 50 μm or less.

[0121] Condition 1: Using an electron beam microanalyzer (EPMA), a 2mm square rectangular area on the surface of the base steel plate is divided into multiple 400×400 segments for elemental mapping, and the Mn intensity in each of these segments is specified. The standard deviation SD is calculated from the distribution of the number of detection points of the Mn intensity in the rectangular area.

[0122] Condition 2: Using an electron beam microanalyzer (EPMA), a 2mm square rectangular region on the surface of the base steel plate was divided into multiple 400×400 segments for elemental mapping, and the Mn intensity in each of these segments was determined. The average Mn intensity MI in the specified rectangular region was then calculated. ave The rectangular region described above is divided into 400 segments along a direction orthogonal to the rolling direction, defined as 400 segmented regions. The average value MI in each of these segmented regions is then considered. ave Regions exhibiting Mn intensity exceeding 34% occupying more than 50% of the area are designated as Region 1. Regions with Mn intensity exceeding 34% are further divided relative to the aforementioned average Mn content. ave The region where the Mn intensity is lower than 34% accounts for more than 50% is designated as the second region. The interval I between the first region and the second region is determined.

[0123] 1.1 Chemical composition of the base steel plate

[0124] 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.

[0125] (C: 0.0005~0.0100%)

[0126] 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.

[0127] (Si: 0.01~0.50%)

[0128] 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.

[0129] (Mn: 0.01~2.00%)

[0130] 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 above 1.00%, or below 1.20%. Alternatively, the Mn content can be above 1.75%, or below 1.95%.

[0131] (P: below 0.100%)

[0132] 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.

[0133] (S: below 0.0100%)

[0134] 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.

[0135] (N: below 0.0200%)

[0136] 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.

[0137] (Ti: 0.040~0.180%)

[0138] Ti is an element that fixes carbon (C) to improve the mechanical properties of the base steel sheet, such as tensile strength 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.

[0139] (Nb: 0~0.100%)

[0140] 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.

[0141] (B: 0.0005~0.0100%)

[0142] 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.

[0143] (A1: 0-1.000%)

[0144] 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.

[0145] 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 omitted, therefore their content is limited to 0%. These elements will not substantially affect the problem-solving mechanism in this embodiment.

[0146] (Cu: 0~1.000%)

[0147] 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.

[0148] (Cr: 0~2.000%)

[0149] 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.

[0150] (Ni: 0~0.500%)

[0151] 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.

[0152] (Mo: 0~3.000%)

[0153] 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.

[0154] (W: 0~0.100%)

[0155] 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.

[0156] (V: 0~1.000%)

[0157] 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.

[0158] (O: 0~0.020%)

[0159] 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.

[0160] (Ta: 0~0.100%)

[0161] 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 than 0.001%, 0.005%, 0.010%, or 0.020%, or less than 0.100%, or less than 0.080%, 0.060%, or 0.040%.

[0162] (Co: 0-3.000%)

[0163] 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.

[0164] (Sn: 0~1.000%)

[0165] 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.

[0166] (Sb: 0~0.500%)

[0167] 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.

[0168] (As: 0~0.050%)

[0169] 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.

[0170] (Mg: 0-0.050%)

[0171] 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.

[0172] (Zr: 0~0.050%)

[0173] 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.

[0174] (Ca: 0~0.0500%)

[0175] 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.

[0176] (REM: 0~0.0500%)

[0177] 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).

[0178] 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 ores and waste.

[0179] 1.2 Condition 1

[0180] In this embodiment, regarding the base steel plate, the standard deviation SD obtained under condition 1 above is 0.40 or less. That is, when analyzing the distribution of Mn on the surface of the base steel plate using EPMA mapping, the non-uniformity of Mn strength is small. Figure 3As shown, under condition 1 above, using EPMA, an EPMA mapping for Mn strength is obtained for a 2mm square rectangular area on the surface of the base steel plate. Here, the rectangular area is divided into multiple 400×400 segments for element mapping, specifying the Mn strength in each segment. The Mn strength in each specified segment is summarized in a histogram, and the standard deviation SD can be calculated from the distribution of the number of detection points of Mn strength in the rectangular area. According to the inventors, if the standard deviation SD calculated in this way is too large, the uneven distribution of Mn on the surface of the base steel plate is too large, and when an alloyed hot-dip galvanized layer is formed relative to the base steel plate, rib-like patterns are more likely to occur in the coating. In contrast, in this embodiment, by making the standard deviation SD calculated in this way 0.40 or less, uneven alloying speed can be suppressed during the alloying of the hot-dip galvanized layer, and the aforementioned rib-like patterns are less likely to occur. The standard deviation SD can also be 0.39 or less or 0.38 or less. The lower limit of the standard deviation (SD) is not specifically limited and can be 0, but in reality it often exceeds 0. The standard deviation (SD) can also be above 0.20 or above 0.30.

[0181] 1.3 Condition 2

[0182] In this embodiment, regarding the base steel plate, the spacing I determined by condition 2 above is 50 μm or less. That is, when analyzing the Mn distribution on the surface of the base steel plate using EPMA mapping, the spacing between regions with relatively high Mn intensity (region 1) is narrow in the direction orthogonal to the rolling direction. In other words, for the surface of the base steel plate, the narrow distance between the Mn-concentrated regions, which is the factor contributing to the rib-like pattern of the coating in the direction orthogonal to the rolling direction, is... Figure 4 As shown, under condition 2 above, using EPMA, an EPMA mapping for Mn strength is obtained for a 2mm square rectangular region on the surface of the base steel plate. Here, the rectangular region is divided into multiple 400×400 segments for element mapping, specifying the Mn strength in each segment. The average value of the Mn strength in each specified segment is obtained, i.e., the average Mn strength MI in the specific rectangular region. ave ( Figure 4 (A)). On the other hand, for the same rectangular area, it is divided into 400 sections in a direction orthogonal to the rolling direction, and these sections are set as 400 sections. The average value MI is then used to define these sections. ave Regions exhibiting Mn intensity exceeding 34% occupying more than 50% of the area are designated as Region 1. Furthermore, in these segmented regions, Mn intensity is compared to the average value... aveRegion 2 is defined as the area where the Mn intensity is more than 34% lower, accounting for more than 50% of the total area. The interval I between the first region and the second region within the rectangular region is calculated. Figure 4 (B) According to the inventors' understanding, if the interval I calculated in this way is 50 μm or less, the distance between the Mn-concentrated portions, which are the cause of the rib-like pattern in the coating, becomes sufficiently narrow, and rib-like patterns are less likely to occur in the coating when an alloyed hot-dip galvanized layer is formed relative to the base steel sheet. The interval I can also be 45 μm or less or 40 μm or less. There is no particular limitation on the lower limit of the interval I. The interval I can also be 0 μm, 5 μm or more, 10 μm or more, 15 μm or more, 20 μm or more, 25 μm or more, or 30 μm or more. It should be noted that in this embodiment, in an EPMA mapping, when the first region and the second region are repeatedly present and multiple intervals I are specified, it is sufficient that the largest interval among the multiple intervals I is 50 μm or less. Furthermore, in the EPMA mapping, if the first region or the second region does not exist, the interval I is considered to be 0 μm.

[0183] It should be noted that the "rolling direction" of the base steel plate in this application can be specified by the direction 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.

[0184] 1.4 Metal Structure

[0185] 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 may be specified, for example, as follows.

[0186] 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.

[0187] 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 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.

[0188] 1.5 coating

[0189] 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%, 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 adhesion is not particularly limited, and a general adhesion amount is preferred.

[0190] 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 added inhibitors (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.

[0191] 1.6 Other properties of alloyed hot-dip galvanized steel sheets

[0192] 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 meet the above-mentioned standard deviation SD and interval I. Furthermore, the alloyed hot-dip galvanized steel sheet in this embodiment may also have the following characteristics.

[0193] (Tensile strength TS)

[0194] 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.

[0195] (Total elongation EL)

[0196] 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%.

[0197] (Yield point YP)

[0198] 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.

[0199] (Methods for determining tensile strength, total elongation, and yield point)

[0200] 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 that is parallel to the rolling right angle of the steel strip.

[0201] (plate thickness)

[0202] 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, but it can be greater than 0.2 mm or less than 6.0 mm.

[0203] 1.7 Effect

[0204] As described above, the alloyed hot-dip galvanized steel sheet of this embodiment controls the distribution morphology of Mn (the distribution morphology of Mn concentration areas) on the surface of the base steel sheet to a predetermined state. Therefore, by... Figure 1 and 2 The mechanism shown promotes alloying of the coating in the Mn-concentrated areas. Even when a thicker coating is formed, the spacing between the thicker portions is sufficiently small and dense, making it less likely to be perceived as a ribbed pattern. As a result, the alloyed hot-dip galvanized steel sheet of this embodiment exhibits excellent coating appearance and post-coating appearance.

[0205] 2. Manufacturing method of alloyed hot-dip galvanized steel sheet

[0206] The alloyed hot-dip galvanized steel sheet of this embodiment can be manufactured through a consistent process of continuous casting, hot rolling, cold rolling, and annealing. In particular, it is important to allow Mn to diffuse and disperse concentrated Mn portions on the surface of the slab by holding the continuously cast slab at a high temperature. Hereinafter, an example of a method for manufacturing the alloyed hot-dip galvanized steel sheet will be described, but the method for manufacturing the alloyed hot-dip galvanized steel sheet is not limited to this example. For example, the method for manufacturing the alloyed hot-dip galvanized steel sheet of this embodiment is characterized by comprising:

[0207] A steel slab with the above chemical composition is obtained by continuous casting;

[0208] The above-mentioned steel slab is heated and then hot-rolled to obtain a hot-rolled plate;

[0209] The hot-rolled sheet is coiled up;

[0210] The above-mentioned hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet; and

[0211] The above-mentioned cold-rolled sheet is annealed.

[0212] Furthermore, the manufacturing method includes:

[0213] When cooling the steel slab after continuous casting, at least the surface of the steel slab in the width direction (the surface that contacts the rolls during rolling) is held at a temperature of 600°C or higher for at least 240 minutes.

[0214] During the heating of the steel slab before hot rolling, at least the surface of the steel slab in the width direction (the surface that contacts the rolls during rolling) is held at a temperature of 1150°C or higher and 1300°C or lower for 120 minutes or more and 300 minutes or less. Hereinafter, each process will be described in detail, focusing on the key points of this embodiment.

[0215] 2.1 Cooling process of slab after continuous casting

[0216] As described above, when cooling the continuously cast steel slab, by maintaining at least the width-direction surface of the slab (the surface in contact with the rolls during rolling) at a temperature of 600°C or higher for at least 240 minutes, Mn diffuses into the surface of the slab, and the concentrated Mn areas become easier to disperse. As a result, when the rolled base steel sheet undergoes alloying hot-dip galvanizing, the ribbed pattern of the coating becomes less visible. There are no particular limitations on the method used to control the temperature of the continuously cast steel slab as described above; it is sufficient to control the slab temperature from the continuous casting machine end to the heating furnace before hot rolling. For example, setting the secondary cooling during continuous casting to slow cooling, adjusting the position of the trolley used to transport the slab, and using an insulation cover to keep the slab warm are all effective methods. As an example, in the case of continuously casting slabs by a continuous casting machine (CC) and stacking them in multiple sections to form a slab stack, then covering the slab stack with an insulation cover, and then transporting the slabs by a trolley, the above-mentioned temperature control during the slab cooling process can be achieved by controlling the cooling rate of the slab surface in at least the width direction to a low speed of 30°C / h or less.

[0217] 2.2 Heating temperature and time in the furnace

[0218] As described above, during the heating of the steel slab before hot rolling, by maintaining the surface of the steel slab, at least in the width direction (the surface in contact with the rolls during rolling), at a temperature of 1150°C or higher and 1300°C or lower for 120 minutes or more and 300 minutes or less, Mn diffuses into the surface of the steel slab, and the concentrated Mn areas become easier to disperse. As a result, when the rolled base steel sheet undergoes alloying hot-dip galvanizing treatment, the ribbed pattern of the coating becomes less visible. There are no particular limitations on the form used to control the temperature of the steel slab before hot rolling as described above; the temperature of the slab can be controlled simply by placing the steel slab into the heating furnace and controlling the temperature within the furnace.

[0219] 2.3 Other processes

[0220] In the manufacturing method of this embodiment, as described above, the temperature of the steel slab is controlled from continuous casting to hot rolling, and hot rolling, coiling, cold rolling, annealing, and plating are performed on this basis. There are no particular limitations on the hot rolling conditions, coiling conditions, cold rolling conditions, annealing conditions, and plating conditions. An example of each process is shown below.

[0221] (Finishing temperature of hot rolling)

[0222] When hot-rolling a heated slab, the roughing conditions can be set to general conditions. Similarly, the finishing conditions can also be set to general conditions. However, since the finishing temperature 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 can be above 900°C or below 950°C.

[0223] (Roll-up)

[0224] 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.

[0225] (Reduction rate in cold rolling)

[0226] 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.

[0227] (annealing atmosphere)

[0228] Annealing can be carried out in a high dew point atmosphere or a low dew point atmosphere. For example, the dew point in the annealing atmosphere can be above -60°C or below 30°C.

[0229] (Hold the annealing temperature)

[0230] 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.

[0231] (Annealing holding time)

[0232] 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.

[0233] (Cooling rate after annealing)

[0234] There are no particular restrictions on the cooling conditions after the above annealing.

[0235] (Cooling stop temperature after annealing and reheating)

[0236] 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.

[0237] (Retention temperature)

[0238] 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.

[0239] (Duration of stay)

[0240] To achieve the desired effect, the dwell time should preferably be set to be between 30 and 300 seconds.

[0241] (Tempering)

[0242] 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 (where Ms is below).

[0243] (plating)

[0244] 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.

[0245] (Composition of the plating bath)

[0246] 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.

[0247] (Temperature of the steel plate after immersion in the plating bath)

[0248] 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, resulting in 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 formation of the internal oxide layer; 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 the area where the temperature of the steel sheet exiting the alloying furnace is kept warm in a way that prevents a rapid drop.

[0249] (Post-processing)

[0250] 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.

[0251] (Surface smoothing ratio)

[0252] Furthermore, skin rolling can also be performed with the aim of improving ductility through the correction of the steel sheet shape or the introduction of movable dislocations. The reduction rate of skin rolling 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 set as the lower limit. If it exceeds 1.5%, productivity decreases significantly, therefore this is set as the upper limit. Skin rolling can be performed online or offline. Furthermore, skin rolling with the target reduction rate can be performed in one go or in multiple stages.

[0253] Example

[0254] 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.

[0255] 1. Preparation of base steel plates

[0256] Steel slabs with various chemical compositions are smelted and continuously cast to produce steel billets. During the cooling process after continuous casting, a portion of the steel slabs are stacked in multiple stages, and the temperature of the slabs is maintained using an insulation hood. Furthermore, when the slabs are transported by trolley, the cooling rate is controlled at a low speed of less than 30°C / h. It should be noted that a temperature sensor is installed on the surface of the steel slab in the width direction to monitor the temperature change of the surface in the width direction and to measure the holding time above 600°C during the cooling process. The cooled steel slabs are then inserted into a heating furnace and subjected to homogenization treatment by holding them at a specified temperature for a specified time before being removed and exposed to the atmosphere. Next, hot rolling is performed to obtain a hot-rolled sheet with a thickness of 3.2 mm, which is then coiled at a specified temperature. The oxide scale on the hot-rolled sheet is then removed by pickling, followed by cold rolling (75% reduction) to a cold-rolled sheet with a thickness of 0.8 mm. Finally, the cold-rolled sheet is annealed. For all steel plates, the holding temperature during annealing was set to 800°C, and the holding time was set to 60 seconds. Immediately after annealing, plating and alloying treatment was performed, followed by surface smoothing. 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 components other than those shown in Tables 1 and 2 are Fe and impurities.

[0257] Table 1

[0258]

[0259] Table 2

[0260]

[0261] 2. Plating

[0262] 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.

[0263] 3. Evaluation

[0264] 3.1 Properties of the base steel plate

[0265] In alloyed hot-dip galvanized steel sheets, the coating is dissolved and removed using 15% hydrochloric acid with added inhibitors to expose the surface of the base steel sheet. Then, EPMA is used to perform mapping on Mn, and the standard deviation SD and interval I are calculated as follows: conditions 1 and 2 below.

[0266] Condition 1: such as Figure 3 As shown, using EPMA, a 2mm square rectangular area on the surface of the base steel plate is divided into multiple 400×400 segments for element mapping, specifying the Mn intensity in each segment. The standard deviation SD is calculated from the distribution of the number of detection points of Mn intensity in the rectangular area.

[0267] Condition 2: such as Figure 4 As shown, using EPMA, a 2mm square rectangular area on the surface of the base steel plate is divided into multiple 400×400 segments for element mapping, specifying the Mn intensity in each segment. The average Mn intensity MI in that specific rectangular area is... ave Furthermore, the rectangular region is divided into 400 segments along a direction orthogonal to the rolling direction, and these segments are defined as 400 segments. The average value MI is then defined within each segment. ave Region 1 is defined as the area where segments exhibiting Mn intensity exceeding 34% account for more than 50% of the total area. Regions with Mn intensity exceeding 34% are then divided relative to the average Mn content. ave The region exhibiting a Mn intensity lower than 34% that occupies more than 50% of the area is designated as Region 2. The interval I between Region 1 and Region 2 within this rectangular region is determined.

[0268] 3.2 Mechanical properties

[0269] 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.

[0270] 3.3 Coating properties

[0271] 3.3.1 Wettability

[0272] The wettability of the coating relative to the base steel sheet during plating is evaluated according to the following criteria.

[0273] ○: Good wettability (100% confirmation of coating on the surface of the base steel plate was obtained through visual inspection)

[0274] ×: Poor wettability (visual inspection revealed areas on the surface of the base steel plate that were not coated).

[0275] 3.3.2 Presence or absence of ribbed patterns in the coating

[0276] 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.

[0277] ◎: The ribbed pattern of the plating cannot be seen.

[0278] 〇: The ribbed pattern of the coating is not visible at a distance of 50cm from the steel plate.

[0279] △: 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.

[0280] ×: The ribbed pattern of the coating can also be seen at a distance of 100cm from the steel plate.

[0281] 3.4 Formability and processability

[0282] 3.4.1 Whether the coating adheres to the mold

[0283] Alloyed hot-dip galvanized steel sheets are pressed (condition: deep drawing of cylinders), and the presence or absence of coating adhesion on the mold is evaluated according to the following criteria.

[0284] ○: There is virtually no coating adhering to the mold.

[0285] ×: The coating adheres poorly to the mold.

[0286] 3.4.2 Embrittlement during secondary processing

[0287] 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.

[0288] ○: No secondary processing causing embrittlement (crack length after pressing is less than 10mm)

[0289] ×: Secondary processing has caused embrittlement (the crack length after pressing is more than 10mm).

[0290] 4. Results

[0291] Table 3 shows the evaluation results of the manufacturing conditions, properties, mechanical characteristics, coating properties, and formability and processability of the base steel plate.

[0292] Table 3

[0293]

[0294] It should be noted that the alloyed hot-dip galvanized steel sheets No. 1 to 39 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 39 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 being Zn and impurities.

[0295] The following information can be obtained from the results shown in Table 3.

[0296] Regarding No. 27, because the holding time above 600°C is too short when cooling the slab, Mn cannot be sufficiently diffused and dispersed on the slab surface. As a result, the uneven distribution (standard deviation SD) of Mn on the surface of the rolled and annealed base steel plate increases, producing distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0297] Regarding No. 28, because the heating temperature before hot rolling the slab is too low, Mn cannot be sufficiently diffused and dispersed on the surface of the slab. Therefore, the gaps I of the Mn concentration areas cannot be sufficiently narrowed on the surface of the base steel plate after rolling and annealing, resulting in distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0298] Regarding No. 29, due to the excessively low heating temperature before hot rolling of the slab and the excessively short heating holding time, Mn cannot be sufficiently diffused and dispersed on the surface of the slab. As a result, the uneven distribution (standard deviation SD) of Mn on the surface of the base steel plate after rolling and annealing becomes larger, and the intervals I of the Mn concentration areas cannot be sufficiently narrowed, resulting in distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0299] Regarding No. 30, due to the short heating and holding time before hot rolling the slab, Mn cannot be sufficiently diffused and dispersed on the slab surface. As a result, the uneven distribution (standard deviation SD) of Mn on the surface of the base steel plate after rolling and annealing becomes larger, and the interval I of the Mn concentration area cannot be sufficiently narrowed, resulting in a distinct rib-like pattern in the alloyed hot-dip galvanized layer.

[0300] Regarding No. 31, 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 or Nb. As a result, it hinders the alloying of the coating, creating unalloyed coating portions in the coated steel sheet, which easily leads to coating adhesion on the mold.

[0301] Regarding No. 32, 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 believed 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.

[0302] Regarding No. 33, due to the excessive Mn content in the base steel plate, there is a tendency for the elongation to decrease.

[0303] Regarding No. 34, 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.

[0304] Regarding No. 35, it is believed that 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.

[0305] Regarding No. 36, 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 through C solution. As a result, unalloyed coating portions are formed in the coated steel sheet, making it prone to adhesion to the mold.

[0306] Regarding No. 37, due to insufficient B content in the base steel plate, it is unable to adequately compensate for the function of C removed from the grain boundaries, resulting in secondary processing embrittlement.

[0307] Regarding No. 38, it is believed that the excessive boron (B) 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 die, impairing formability and productivity. Furthermore, due to the insufficient holding time above 600°C during slab cooling, manganese (Mn) cannot fully diffuse and disperse on the slab surface. This results in greater uneven distribution (standard deviation SD) of Mn on the surface of the rolled and annealed base steel sheet, producing distinct rib-like patterns in the alloyed hot-dip galvanized layer.

[0308] Regarding No. 39, due to the excessive Ti content in the base steel sheet, the balance between strength and ductility of the coated steel sheet becomes poor.

[0309] In contrast, No. 1 to No. 26 can suppress the formation of rib-like patterns in the coating of alloyed hot-dip galvanized steel sheets. In addition, they can also meet the mechanical properties, formability and other characteristics that are easily required for alloyed hot-dip galvanized steel sheets.

[0310] Summarizing the above results, the alloyed hot-dip galvanized steel sheets exhibit excellent appearance, with barely visible ribbed patterns in the coating. Furthermore, they also meet the mechanical properties and formability requirements typically associated with alloyed hot-dip galvanized steel sheets.

[0311] An alloyed hot-dip galvanized steel sheet, comprising a base steel sheet and a coating,

[0312] The aforementioned base steel plate has the following chemical composition: containing, by mass %:

[0313] C: 0.0005~0.0100%

[0314] Si: 0.01~0.50%

[0315] Mn: 0.01~2.00%

[0316] P: below 0.100%

[0317] S: below 0.0100%

[0318] N: below 0.0200%

[0319] Ti: 0.040~0.180%

[0320] Nb: 0~0.100%

[0321] B: 0.0005~0.0100%

[0322] Al: 0~1.000%

[0323] Cu: 0~1.000%

[0324] Cr: 0–2.000%

[0325] Ni: 0~0.500%

[0326] Mo: 0–3.000%

[0327] W: 0~0.100%

[0328] V: 0~1.000%

[0329] O: 0~0.020%

[0330] Ta: 0~0.100%

[0331] Co: 0-3.000%

[0332] Sn: 0~1.000%

[0333] Sb: 0~0.500%

[0334] As: 0-0.050%

[0335] Mg: 0–0.050%

[0336] Zr: 0~0.050%

[0337] Ca: 0–0.0500%, and

[0338] REM: 0~0.0500%,

[0339] The remaining part consists of Fe and impurities.

[0340] The standard deviation SD obtained by condition 1 above is less than 0.40, and the interval I obtained by condition 2 above is less than 50 μm.

Claims

1. An alloyed hot-dip galvanized steel sheet which is an alloyed hot-dip galvanized steel sheet having a base steel sheet and a plated layer, the base steel sheet having a chemical composition containing, in mass%: C：0.0005～0.0100％、 Si: 0.01 to 0.50%, Mn: 0.42 to 2.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, Ti: 0.040 to 0.180%, Nb: 0 to 0.100%, B：0.0005～0.0100％、 Al：0～1.000％、 Cu: 0 to 1.000%, Cr：0～2.000％、 Ni: 0 to 0.500%, Mo: 0 to 3.000%, W：0～0.100％、 V：0～1.000％、 O：0～0.020％、 Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr：0～0.050％、 Ca: 0 to 0.0500%, and REM: 0 to 0.0500%, the remainder consisting of Fe and impurities, a standard deviation SD calculated by Condition 1 below is 0.40 or less, an interval I calculated by Condition 2 below is 50 μm or less, Condition 1: using an electron probe micro-analyzer (EPMA), an element map is performed by dividing a rectangular region of 2 mm square in the surface of the base steel sheet into a plurality of sections of 400 x 400, the Mn intensity in each of the sections is specified, and a standard deviation SD is calculated from the number distribution of detected points of the Mn intensity in the rectangular region; Condition 2: A rectangular region of 2 mm square in the surface of the base steel sheet is divided into a plurality of sections of 400 x 400 using an electron probe micro analyzer (EPMA), the intensity of Mn in each of the sections is specified, and the average value MI of the intensity of Mn in the rectangular region is specified ave The rectangular region is divided into 400 in a direction orthogonal to the rolling direction, and the 400 divided regions are set. The interval I of the first regions adjacent to the second regions sandwiching the second region in the rectangular region is found. ave The first regions in which the sections showing a high intensity of Mn of 34% or more account for 50% or more of the regions are set as first regions, and the interval I of the first regions adjacent to the second regions sandwiching the second region in the rectangular region is found. ave The second regions in which the sections showing a low intensity of Mn of 34% or more account for 50% or more of the regions are set as second regions, and the interval I of the first regions adjacent to the second regions sandwiching the second region in the rectangular region is found.

2. The galvannealed steel sheet according to claim 1, wherein, the chemical composition contains, in mass%: Mn: 0.42 to 1.30% or 1.70 to 2.00%.

3. Galvannealed steel sheet according to claim 1 or 2, wherein, the chemical composition contains, in mass%, at least one selected from the group consisting of: Nb: 0.001 to 0.100%, Al：0.001～1.000％、 Cu: 0.001 to 1.000%, Cr：0.001～2.000％、 Ni: 0.001 to 0.500%, Mo: 0.001 to 3.000%, W：0.001～0.100％、 V：0.001～1.000％、 O：0.001～0.020％、 Ta: 0.001 to 0.100%, Co: 0.001 to 3.000%, Sn: 0.001 to 1.000%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, Mg: 0.001 to 0.050%, Zr：0.001～0.050％、 Ca: 0.0001 to 0.0500%, REM: 0.0001 to 0.0500%.

4. The galvannealed steel sheet according to claim 1 or 2, wherein, the metal structure of the base steel sheet is, in area%: ferrite: 94 to 100%, the total of martensite and bainite: 0 to 4%, and retained austenite: 0 to 2%.

5. The galvannealed steel sheet according to claim 1 or 2, wherein, the chemical composition of the plated layer is, in mass%: Fe: 5 to 25%, Al：0～1.0％、 Si: 0 to 1.0%, Mg: 0 to 1.0%, Mn: 0 to 1.0%, Ni: 0 to 1.0%, Sb: 0 to 1.0%, and the remainder: Zn and impurities.

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