Galvannealed steel sheet

Abstract

An alloyed hot-dip galvanized steel sheet according to the present disclosure has a base material steel sheet and a coating layer, wherein the base material steel sheet has a prescribed chemical composition in mass %, the standard deviation SD of the Mn strength in line with a prescribed condition 1 - using an EPMA - is 0.40 or less, and the distance I between Mn-concentrated portions in line with a prescribed condition 2 - using an EPMA - is 50 μm or less. The alloyed hot-dip galvanized steel sheet according to the present disclosure is excellent in appearance and the like.

Description

[Technical Field]

[0001] The present application discloses a galvannealed steel sheet.

[0002] Due to their excellent paintability, galvannealed steel sheets are widely used for automobile body applications as well as for exterior components such as home appliances and building materials. However, galvannealed steel sheets are prone to developing streaks in the coating. These streaks may remain even after painting, and may not satisfy the appearance required for exterior components. In other words, if streaks develop in the coating, deteriorating the appearance of the coating is likely to result in a poor appearance after painting as well.

[0003] Various techniques have been proposed to address streaks in the coating layer of galvannealed steel sheets. For example, Patent Document 1 discloses a technique for suppressing the occurrence of coating unevenness on the steel sheet surface by optimizing the balance between Ti and Nb contained in a base steel sheet made of IF steel and reducing the amounts of Mn, P, and Si added. Patent Document 2 also discloses a technique for suppressing the occurrence of streaks in the coating due to surface defects in an IF steel base steel sheet by setting the {100} plane X-ray intensity in a direction parallel to the steel sheet surface to a random intensity ratio of 0.8 or less and setting the proportion of unrecrystallized grains to 0.10% or less. Patent Document 3 also discloses a technique for suppressing the occurrence of streaks in the coating by setting the Mn concentration in the surface layer after annealing to a predetermined upper limit or less in a base steel sheet made of BH steel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-291272 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-132801 [Patent Document 3] International Publication No. 2016 / 170794 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, solutions have been proposed from various perspectives regarding the streaks in the coating of galvannealed steel sheets, but it is difficult to say that sufficient effects have been achieved. In this regard, there is still room for improvement in improving the appearance of galvannealed steel sheets while suppressing the streaks in the coating.

[0006] Furthermore, the galvannealed steel sheets used for the above-mentioned applications are often subjected to press forming before use. In conventional techniques, during press forming of the galvannealed steel sheets, the unalloyed coating may adhere to the mold, impairing formability and productivity. Furthermore, cracks may occur due to embrittlement during secondary work.

[0007] Furthermore, it is preferable that the galvannealed steel sheet used for the above applications has a good balance between strength and ductility. In the prior art, sufficient studies have not been conducted on galvannealed steel sheets having a good balance between strength and ductility. [Means for solving the problem]

[0008] As one of the means for solving the above problems, the present application provides: A galvannealed steel sheet having a base steel sheet and a coating layer, The base steel plate is, in mass%, C: 0.0005 to 0.0100%, Si: 0.01 to 0.50%, Mn: 0.01 to 2.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, Ti: 0.040~0.180% Nb: 0 to 0.100%, B: 0.0005~0.0100%, Al: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0~2.000%, Ni: 0 to 0.500% Mo: 0 to 3.000%, W: 0 to 0.100%, V: 0 to 1.000%, O: 0 to 0.020%, Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500% As: 0~0.050%, Mg: 0 to 0.050% Zr: 0 to 0.050%, Ca: 0 to 0.0500%, and REM: 0~0.0500%, and the balance being Fe and impurities, The standard deviation SD calculated by the following condition 1 is 0.40 or less. The interval I determined by the following condition 2 is 50 μm or less. Galvannealed steel sheet Disclose.

[0009] Condition 1: Using an electron probe microanalyzer (EPMA), a 2 mm square region on the surface of the base steel sheet is divided into multiple 400 x 400 areas, elemental mapping is performed, and the Mn intensity in each of the areas is identified. The standard deviation SD is calculated from the distribution of the number of detected points of the Mn intensity in the rectangular region.

[0010] Condition 2: Using an electron probe microanalyzer (EPMA), a 2 mm square region on the surface of the base steel sheet is divided into a plurality of 400 × 400 areas, element mapping is performed, and the Mn intensity in each of the areas is identified. The average Mn intensity in the rectangular regions is calculated as MI ave The rectangular region is divided into 400 regions in a direction perpendicular to the rolling direction, and the average value MI aveThe first region is defined as a region in which 50% or more of the area shows a Mn intensity that is 34% or more higher than the average Mn intensity. ave The second region is defined as a region in which 50% or more of the area has an Mn intensity that is 34% or more lower than that of the first region. The distance I between the adjacent first regions sandwiching the second region in the rectangular region is determined.

[0011] In the galvannealed steel sheet of the present disclosure, the chemical composition is, in mass%, Mn: 0.01 to 1.30% or 1.70 to 2.00% It may also contain:

[0012] In the galvannealed steel sheet of the present disclosure, the chemical composition is, in mass%, Nb: 0.001 to 0.100%, Al: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001 to 2.000%, Ni: 0.001 to 0.500%, Mo: 0.001 to 3.000%, W: 0.001 to 0.100%, V: 0.001 to 1.000%, O: 0.001 to 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 to 0.050%, Ca: 0.0001 to 0.0500%, REM: 0.0001 to 0.0500% It may contain at least one selected from the group consisting of:

[0013] In the galvannealed steel sheet of the present disclosure, the metal structure of the base steel sheet has, in terms of area ratio, Ferrite: 94~100%, Martensite and bainite combined: 0 to 4%, and Retained austenite: 0-2% may be.

[0014] In the galvannealed steel sheet of the present disclosure, the chemical composition of the coating layer is, in mass%, Fe: 5-25% Al: 0-1.0%, Si: 0 to 1.0% Mg: 0-1.0% Mn: 0 to 1.0% Ni: 0 to 1.0% Sb: 0 to 1.0%, and Remainder: Zn and impurities may be. [Effects of the Invention]

[0015] The galvannealed steel sheet of the present disclosure has hardly visible streaks in the coating and is excellent in coating appearance and appearance after painting. Furthermore, the galvannealed steel sheet of the present disclosure has excellent wettability of the coating to the base steel sheet, which inhibits adhesion of the unalloyed coating to a mold during press working and also tends to inhibit secondary work embrittlement. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a schematic view showing the morphology of a Mn-enriched portion on the surface of a base steel sheet. [Figure 2] FIG. 2 is a schematic diagram for explaining the mechanism by which streaks occur in a galvannealed layer. [Figure 3] FIG. 10 is a schematic diagram for explaining condition 1. [Figure 4] FIG. 10 is a schematic diagram for explaining condition 2. DETAILED DESCRIPTION OF THE INVENTION

[0017] 1. Galvannealed steel sheet According to the inventors' new findings, the streak pattern in galvannealed steel sheets is caused by uneven Mn enrichment that occurs on the surface of the base steel due to microsegregation of Mn during slab solidification. As shown in FIG. 1, the Mn-enriched portions on the surface of the base steel of a slab are elongated in the rolling direction by rolling, resulting in the streak pattern. That is, when a steel sheet is obtained by rolling a slab, the Mn-enriched portions on the surface of the steel sheet extend in the rolling direction in a streak-like manner. As shown in FIG. 2, when a steel sheet in this state (FIG. 2(A)) is annealed, for example, Mn oxides (e.g., Mn2SiO4) are formed in the surface layer of the steel sheet, and the pinning effect of these Mn oxides can suppress grain growth in the surface layer (FIG. 2(B)). That is, the grain boundary density is likely to be higher in the Mn-enriched portions of the base steel sheet compared to portions other than the Mn-enriched portions. After that, a zinc plating layer is formed on the base steel sheet, and then an alloying process is performed. This promotes alloying in areas with higher grain boundary density (Fig. 2(C)), causing the plating layer to grow thicker, resulting in the appearance of streaks in the plating layer (Fig. 2(D)).

[0018] As described above, the plating streak pattern in galvannealed steel sheets can be said to be caused by uneven Mn concentration on the surface of the base steel sheet. Meanwhile, the inventors' new findings indicate that the level of visibility of the plating streak pattern varies depending on the degree of uneven Mn concentration. That is, even if Mn concentration unevenness occurs, plating streak patterns that impair appearance are unlikely to occur if the intervals between the concentration unevenness are sufficiently narrow or sufficiently wide. In this regard, the plating streak pattern in galvannealed steel sheets can be improved by controlling the Mn distribution on the surface of the base steel sheet. Thus, the inventors have discovered that the visibility level of the plating streak pattern does not depend solely on the "amount" of concentrated Mn on the surface layer of the base steel sheet, as disclosed in the prior art (e.g., Patent Document 3), but more precisely, depends on the "distribution form" of Mn.

[0019] The distribution of Mn on the surface of the base steel sheet can be quantified by the standard deviation of Mn intensity obtained using an electron probe microanalyzer (EPMA), etc. The distribution of Mn on the surface of the base steel sheet can also be controlled by, for example, the heating temperature of the slab before hot rolling (to diffuse Mn at a high temperature), etc.

[0020] The present invention has been completed based on the above-mentioned new findings. Hereinafter, embodiments of the present invention will be described. Note that these descriptions are intended to be merely examples of embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0021] The galvannealed steel sheet according to this embodiment has a base steel sheet and a coating layer. The base steel sheet contains, in mass %, C: 0.0005 to 0.0100%, Si: 0.01 to 0.50%, Mn: 0.01 to 2.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, Ti: 0.040~0.180% Nb: 0 to 0.100%, B: 0.0005~0.0100%, Al: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0~2.000%, Ni: 0 to 0.500% Mo: 0 to 3.000%, W: 0 to 0.100%, V: 0 to 1.000%, O: 0 to 0.020%, Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500% As: 0~0.050%, Mg: 0 to 0.050% Zr: 0 to 0.050%, Ca: 0 to 0.0500%, and REM: 0~0.0500%, The standard deviation SD determined by the following condition 1 is 0.40 or less, and the interval I determined by the following condition 2 is 50 μm or less.

[0022] Condition 1: Using an electron probe microanalyzer (EPMA), a 2 mm square region on the surface of the base steel sheet is divided into multiple 400 x 400 areas, elemental mapping is performed, and the Mn intensity in each of the areas is identified. The standard deviation SD is calculated from the distribution of the number of detected points of the Mn intensity in the rectangular region.

[0023] Condition 2: Using an electron probe microanalyzer (EPMA), a 2 mm square region on the surface of the base steel sheet is divided into a plurality of 400 × 400 areas, element mapping is performed, and the Mn intensity in each of the areas is identified. The average Mn intensity in the rectangular regions is calculated as MI ave The rectangular region is divided into 400 regions in a direction perpendicular to the rolling direction, and the average value MI ave The first region is defined as a region in which 50% or more of the area shows a Mn intensity that is 34% or more higher than the average Mn intensity. ave The second region is defined as a region in which 50% or more of the area has an Mn intensity that is 34% or more lower than that of the first region. The distance I between the adjacent first regions sandwiching the second region in the rectangular region is determined.

[0024] 1.1 Chemical composition of base steel plate First, the reasons for limiting the chemical composition of the base steel sheet will be explained. Here, "%" for components means mass %. Furthermore, in this application, unless otherwise specified, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0025] (C: 0.0005 to 0.0100%) The less C there is in the base steel sheet, the more improved the mechanical properties, such as elongation and r-value. In this embodiment, C can be fixed by Ti or Nb, as described below. However, if the amount of TiC or NbC increases too much, the mechanical properties of the base steel sheet become more dependent on the annealing temperature, and the range of annealing conditions for obtaining desired mechanical properties may become narrower. Furthermore, if there is a large amount of solute C remaining in the base steel sheet without being fixed by Ti or Nb, it may hinder alloying of the plating. In this regard, the C content is 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. Note that, from the viewpoint of preventing an excessive increase in steelmaking costs, the C content is 0.0005% or more, and may be 0.0010% or more, 0.0015% or more, or 0.0020% or more.

[0026] (Si: 0.01 to 0.50%) Si is an element that improves the strength of a base steel sheet. On the other hand, if the base steel sheet contains excessive Si, the wettability of the plating to the base steel sheet may deteriorate. Furthermore, if the base steel sheet contains excessive Si, the alloying of the plating may be delayed, and the amount of unalloyed plating in the galvannealed steel sheet may increase. In this regard, the Si content may be 0.50% or less, 0.48% or less, or 0.46% or less. Furthermore, the Si content may be 0.01% or more, 0.05% or more, 0.10% or more, or 0.20% or more.

[0027] (Mn: 0.01 to 2.00%) Mn is an element that improves the strength of a base steel sheet. On the other hand, an excessively high Mn content may reduce the elongation of the steel sheet. In this regard, the Mn content is 2.00% or less, and may be 1.95% or less. The Mn content may be 0.01% or more, 0.05% or more, 0.10% or more, or 0.20% or more. Furthermore, according to the inventor's new findings, when the Mn content in a base steel sheet is within a specific range, plating streaks that impair appearance are less likely to occur. That is, if the Mn content is low, Mn concentration unevenness that causes the above-mentioned streaky appearance defect is less likely to occur. Furthermore, if the Mn content is high, the Mn concentration increases over the entire surface of the base steel sheet, and the above-mentioned Mn enrichment unevenness is more likely to be eliminated. In view of the above, in this embodiment, from the viewpoint of improving the appearance of the galvannealed steel sheet and ensuring elongation, the Mn content may be 0.01 to 1.30% or 1.70 to 2.00%. The Mn content may be 0.50% or more, or 1.00% or more, and may be 1.20% or less. Alternatively, the Mn content may be 1.75% or more and 1.95% or less.

[0028] (P:0.100% or less) P is an element that improves the strength of the base steel sheet and may be added as desired. On the other hand, if the base steel sheet contains an excessive amount of P, the alloying of the coating may be delayed, resulting in a large amount of unalloyed coating in the galvannealed steel sheet. In this regard, the P content is 0.100% or less, and may be 0.090% or less. There is no particular lower limit for the P content. The P content is 0% or more, and may be 0.001% or more.

[0029] (S:0.0100% or less) S is an element that segregates at grain boundaries in a base steel sheet, causing secondary work embrittlement, and also generates nonmetallic inclusions such as MnS in the steel, resulting in a decrease in the ductility of the base steel sheet, so the lower the S content, the better. The S content is 0% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0020% or more, or may be 0.0100% or less, 0.0090% or less, or 0.0080% or less.

[0030] (N:0.0200% or less) N is an element that forms coarse nitrides in the base steel sheet and reduces the workability of the steel sheet. N is also an element that causes blowholes during welding. Furthermore, excessive N content may combine with Ti to form TiN, reducing the effective amount of Ti for immobilizing C. In this regard, the N content is 0.0200% or less, and may be 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. Although the N content is 0% or more, from the viewpoint of preventing an excessive increase in steelmaking costs, the N content may be 0.0001% or more or 0.0010% or more.

[0031] (Ti: 0.040 to 0.180%) Ti is an element that fixes C and improves the mechanical properties of the base steel sheet, such as the elongation and r-value. When the Ti content is 0.040% or more, this effect is easily achieved. The Ti content may be 0.045% or more, or 0.050% or more. On the other hand, if the Ti content is too high, the balance between the strength and ductility of the base steel sheet may be deteriorated. When the Ti content is 0.180% or less, this problem is easily avoided. The Ti content may be 0.150% or less, 0.120% or less, or 0.100% or less.

[0032] (Nb: 0 to 0.100%) Like Ti, Nb is an element that fixes C and improves the mechanical properties of the base steel sheet, such as the elongation and r-value. However, this effect is weaker than that of Ti. As described above, since Ti has the effect of fixing C, the Nb content may be 0%. The Nb content is 0% or more, and may 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 dependency of the mechanical properties of the base steel sheet increases, and the range of annealing conditions for obtaining desired mechanical properties may become narrow. In this regard, the Nb content may be 0.100% or less, 0.060% or less, or 0.040% or less.

[0033] (B: 0.0005 to 0.0100%) B is an element for preventing secondary work embrittlement of the base steel sheet. As described above, in this embodiment, C can be immobilized by Ti and Nb and removed from the grain boundaries, but removing C from the grain boundaries makes secondary work embrittlement more likely to occur. In this embodiment, by including B in place of C in the base steel sheet, this problem can be easily avoided. In particular, when the B content is 0.0005% or more, this problem can be more easily avoided. The B content may 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 may be delayed, resulting in a large amount of unalloyed coating in the galvannealed steel sheet. In this regard, the B content may be 0.0100% or less, 0.0050% or less, or 0.0020% or less.

[0034] (Al: 0 to 1.000%) Al is an element that acts as a deoxidizing agent for steel and is added as needed. However, excessive Al content may impair the balance between strength and ductility. The Al content is 0% or more, and may be 0.001% or more, 0.005% or more, 0.010% or more, or 0.015% or more, or may be 1.000% or less, 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.

[0035] The basic chemical composition of the base steel sheet in this embodiment is as described above. Furthermore, the base steel sheet in this embodiment may contain at least one of the following elements as necessary. These elements do not necessarily need to be contained, so the lower limit of their content is 0%. These elements do not substantially affect the problem-solving mechanism in this embodiment.

[0036] (Cu: 0 to 1.000%) Cu is an element that can contribute to improving at least one of strength and corrosion resistance. On the other hand, excessive Cu content may cause deterioration of toughness. The Cu content is 0% or more, and may be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more, or may be 1.000% or less, 0.800% or less, 0.600% or less, 0.400% or less, 0.250% or less, or 0.150% or less.

[0037] (Cr:0~2.000%) Cr is an element that can improve the hardenability of steel and contribute to improving at least one of strength and corrosion resistance. On the other hand, excessive Cr content may increase alloy costs and reduce toughness. The Cr content is 0% or more, and may be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more, or may be 2.000% or less, 1.500% or less, 1.000% or less, 0.500% or less, 0.300% or less, or 0.150% or less.

[0038] (Ni: 0 to 0.500%) Ni is an element that can improve the hardenability of steel and contribute to improving at least one of strength and heat resistance. However, if Ni is contained in an excessive amount, the effect saturates and there is a risk of increasing manufacturing costs. The Ni content is 0% or more, and may be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more, or may be 0.500% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[0039] (Mo: 0 to 3.000%) Mo is an element that can improve the hardenability of steel and contribute to improving at least one of strength and corrosion resistance. On the other hand, excessive Mo content may increase deformation resistance during processing. The Mo content is 0% or more, and may be 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more, or may be 3.000% or less, 2.000% or less, 1.000% or less, 0.500% or less, or 0.100% or less.

[0040] (W:0~0.100%) W is an element that can improve the hardenability of steel and contribute to improving its strength. However, excessive W content can cause the formation of coarse inclusions. The W content is 0% or more, and may be 0.001% or more, 0.005% or more, or 0.010% or more, or may be 0.100% or less, 0.080% or less, 0.050% or less, or 0.030% or less.

[0041] (V:0~1.000%) V is an element that can contribute to improving strength through precipitation strengthening, etc. On the other hand, excessive V content may cause the formation of a large amount of precipitates, which may reduce toughness. The V content is 0% or more, and may be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or more, or may be 1.000% or less, 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.070% or less.

[0042] (O:0~0.020%) O is an element that can be mixed in during the manufacturing process. Reducing the O content to the minimum requires a long refining time, resulting in reduced productivity. On the other hand, excessive O content can cause the formation of coarse inclusions, which can reduce the toughness of the steel. The O content is 0% or more, and may be 0.001% or more, 0.002% or more, or 0.003% or more, or may be 0.020% or less, 0.015% or less, 0.010% or less, or 0.005% or less.

[0043] (Ta: 0 to 0.100%) Ta is an element that can contribute to controlling the morphology of carbides and increasing strength. On the other hand, excessive Ta content may cause the precipitation of many fine Ta carbides, resulting in a decrease in toughness. The Ta content may be 0%, 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more, or may be 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0044] (Co: 0-3.000%) Co is an element that can contribute to improving at least one of hardenability and heat resistance. On the other hand, excessive Co content may reduce workability and increase raw material costs. The Co content is 0% or more, and may be 0.001% or more, 0.010% or more, 0.020% or more, or 0.050% or more, or may be 3.000% or less, 2.000% or less, 1.000% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

[0045] (Sn: 0 to 1.000%) Sn is an element that can contribute to improving corrosion resistance. On the other hand, excessive Sn content may result in a decrease in toughness. The Sn content is 0% or more, and may be 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more, or may be 1.000% or less, 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.050% or less.

[0046] (Sb: 0 to 0.500%) Sb is an element that can contribute to improving corrosion resistance. However, excessive Sb content may result in a decrease in toughness. The Sb content is 0% or more, and may be 0.001% or more, 0.005% or more, or 0.010% or more, or may be 0.500% or less, 0.300% or less, 0.100% or less, or 0.050% or less.

[0047] (As: 0 to 0.050%) As is an element that can contribute to improving the machinability of steel. However, excessive As content may result in a decrease in workability. The As content is 0% or more, and may be 0.001% or more, 0.005% or more, or 0.010% or more, or may be 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less.

[0048] (Mg: 0 to 0.050%) Mg is an element that can contribute to controlling the morphology of sulfides. However, excessive Mg content may result in a decrease in toughness. The Mg content is 0% or more, and may be 0.001% or more, 0.003% or more, or 0.005% or more, or may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less.

[0049] (Zr: 0 to 0.050%) Zr is an element that can contribute to controlling the morphology of sulfides. However, if Zr is contained in an excessive amount, the effect saturates and production costs may increase. The Zr content is 0% or more, and may be 0.001% or more, 0.003% or more, 0.005% or more, or 0.010% or more, or may be 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less.

[0050] (Ca: 0 to 0.0500%) Ca is an element that can control the morphology of sulfides by adding a small amount. However, if an excessive amount of Ca is added, the effect saturates and there is a risk of an increase in production costs. The Ca content is 0% or more, and may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more, or may be 0.0500% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less.

[0051] (REM: 0 to 0.0500%) Like Ca, REM is an element whose addition in trace amounts can control the morphology of sulfides. However, excessive REM content may result in the formation of coarse inclusions. The REM content is 0% or more, and may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more, or may be 0.0500% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0052] In this embodiment, the chemical composition of the base steel sheet is such that the balance other than the above-mentioned components is Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel sheet according to this embodiment is industrially manufactured.

[0053] 1.2 Condition 1 In this embodiment, the standard deviation SD of the base steel sheet obtained under the above condition 1 is 0.40 or less. That is, when the distribution of Mn on the surface of the base steel sheet is analyzed by EPMA mapping, the variation in Mn intensity is small. As shown in FIG. 3 , under the above condition 1, an EPMA is used to obtain EPMA mapping of Mn intensity for a 2 mm square rectangular region on the surface of the base steel sheet. The rectangular region is then divided into a plurality of 400 × 400 areas, element mapping is performed, and the Mn intensity in each area is identified. The Mn intensity in each identified area is compiled into a histogram, and the standard deviation SD can be obtained from the distribution of the number of detected Mn intensity points in the rectangular region. According to the inventor's findings, if the standard deviation SD obtained in this way is too large, the variation in Mn distribution on the surface of the base steel sheet is too large, and streaks are likely to occur in the coating when a galvannealed layer is formed on the base steel sheet. In contrast, in this embodiment, by setting the standard deviation SD thus determined to be 0.40 or less, it is possible to suppress variations in the alloying rate during alloying of the hot-dip galvanized layer, and the above-mentioned streaks are less likely to occur. The standard deviation SD may be 0.39 or less or 0.38 or less. The lower limit of the standard deviation SD is not particularly limited and may be 0, but in reality it will be greater than 0. The standard deviation SD may be 0.20 or more or 0.30 or more.

[0054] 1.3 Condition 2 In this embodiment, the interval I of the base steel sheet obtained under the above condition 2 is 50 μm or less. That is, when the distribution of Mn on the surface of the base steel sheet is analyzed by EPMA mapping, the interval between regions (first regions) where the Mn intensity is relatively high is narrow in the direction perpendicular to the rolling direction. In other words, on the surface of the base steel sheet, the distance between Mn-enriched regions, which are a cause of the generation of streaks in the plating, is narrow in the direction perpendicular to the rolling direction. As shown in FIG. 4 , under the above condition 2, an EPMA is used to obtain EPMA mapping of Mn intensity for a 2 mm square rectangular region on the surface of the base steel sheet. Here, the rectangular region is divided into a plurality of 400 × 400 areas, and element mapping is performed to identify the Mn intensity in each area. The Mn intensity in each identified area is averaged, i.e., the average Mn intensity in the rectangular region, MI ave (Fig. 4(A)). On the other hand, the same rectangular area is divided into 400 areas in the direction perpendicular to the rolling direction. Among these divided areas, the average value MI ave The first region is defined as the area where 50% or more of the area shows a Mn intensity 34% or higher than that of the first region. aveThe second region is defined as a region in which 50% or more of the area exhibits a Mn intensity 34% or more lower than that of the first region. The distance I between adjacent first regions sandwiching the second region in the rectangular region is determined ( FIG. 4(B) ). According to the inventor's findings, if the distance I determined in this manner is 50 μm or less, the distance between Mn-enriched regions, which is a cause of streaks in the coating, is sufficiently narrow, and streaks are unlikely to occur in the coating when a galvannealed layer is formed on the base steel sheet. The distance I may be 45 μm or less or 40 μm or less. The lower limit of the distance I is not particularly limited. The distance I may 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. In this embodiment, when first regions and second regions are repeatedly present in one EPMA mapping and multiple distances I are identified, the largest of the multiple distances I may be 50 μm or less. In addition, in EPMA mapping, if the first region or the second region does not exist, the interval I is considered to be 0 μm.

[0055] In the present application, the "rolling direction" of a base steel sheet can be identified by the direction of streaks formed in the base steel sheet. For example, as shown in Fig. 1, the rolling direction can be identified based on the elongation direction of Mn-enriched portions. The elongation direction of Mn-enriched portions in the base steel sheet may be identified by, for example, element mapping using the EPMA as described above.

[0056] 1.4 Metal structure The metal structure of the base steel sheet is not particularly limited and can be adjusted depending on the performance required of the base steel sheet. In this embodiment, the metal structure of the base steel sheet may have, in area percentage, for example, ferrite: 94 to 100%, the sum of martensite and bainite: 0 to 4%, and retained austenite: 0 to 2%. The area percentage of each phase and structure can be specified, for example, as follows.

[0057] The area ratio of retained austenite is evaluated by high-resolution crystal structure analysis using the EBSD method (electron backscatter diffraction). Specifically, a sample is taken from a cross section of the base steel sheet parallel to the rolling direction and thickness direction, and the observation surface is polished to a mirror finish. Furthermore, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed surface layer. Next, at the 1 / 4 thickness position, a 10,000 μm field of view is measured. 2 Based on the above, crystal structure analysis is performed using the EBSD method for five fields of view. The step distance between graded points is set to 0.01 to 0.20 μm. The data obtained by the EBSD method is analyzed using analysis software (for example, "OIM Analysys" manufactured by TSL). From the observation results at each position, the area determined to be FCC iron is determined to be retained austenite, and the area ratio of retained austenite is calculated.

[0058] The area ratios of ferrite, martensite, bainite, and pearlite are measured as follows: First, a sample is taken from a cross section of the base steel sheet parallel to the rolling direction and thickness direction, with the observation surface being polished and etched with nital. Next, a 1200 μm field of view is measured in the range of 1 / 8 to 3 / 8 thickness, centered at the 1 / 4 thickness position from the surface. 2 A total of five fields of view are observed using a field emission scanning electron microscope (FE-SEM). The area fractions of ferrite, bainite, martensite, and pearlite are then measured. Each structure is identified as follows: Areas where cementite precipitates in a lamellar form are judged to be pearlite. Areas with relatively low brightness are judged to be ferrite. The area fraction of each is calculated using the point counting method. The area fraction of the remaining areas is then divided by the area fraction of retained austenite obtained previously using the EBSD method, and this is taken as the total area fraction of martensite and bainite.

[0059] 1.5 Plating layer The galvannealed steel sheet according to this embodiment includes the above-described base steel sheet and a coating layer. The coating layer is formed on at least one surface of the base steel sheet. The coating layer may be a galvannealed steel sheet having a composition known to those skilled in the art. For example, the coating layer may contain additional elements such as Al in addition to Zn. In the galvannealed steel sheet according to this embodiment, the chemical composition of the coating layer may be, for example, in mass %, 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 balance: Zn and impurities. The Al content in the coating layer may be greater than 0 mass % or 0.1 mass % or greater. The coating weight of the coating layer is not particularly limited and may be a general coating weight.

[0060] The chemical composition of the plating layer can be determined, for example, by the following procedure: After removing the surface coating with a coating remover that does not corrode the plating (e.g., Neo River SP-751 manufactured by Sansai Kako Co., Ltd.), the plating layer is dissolved with hydrochloric acid containing an inhibitor (e.g., Hibilon manufactured by Sugimura Chemical Industry Co., Ltd.), and the resulting solution is subjected to inductively coupled plasma (ICP) atomic emission spectrometry to determine the chemical composition of the plating layer.

[0061] 1.6 Other properties of galvannealed steel sheets The galvannealed steel sheet in this embodiment may be any steel sheet as long as the base steel sheet has the above-mentioned chemical composition and satisfies the above-mentioned standard deviation SD and interval I. Furthermore, the galvannealed steel sheet in this embodiment may further have the following properties.

[0062] (Tensile strength TS) In order to reduce the weight of structures using steel as a material and improve the resistance of the structures to plastic deformation, it is preferable that the steel material have a large work hardening capacity and exhibit maximum strength. On the other hand, if the tensile strength of the steel sheet is too high, it may be prone to fracture with low energy during plastic deformation, resulting in reduced formability. The tensile strength of the galvannealed steel sheet may be, for example, 430 MPa or more, 450 MPa or more, 500 MPa or more, or 520 MPa or more, or may be 600 MPa or less, 580 MPa or less, 550 MPa or less, or 500 MPa or less.

[0063] (Full stretch EL) When steel sheets are cold-formed to manufacture structures, the steel sheets need to have elongation in order to be finished into complex shapes. If the total elongation of the steel sheet is too low, the material may crack during cold forming. The total elongation of the steel sheet is not particularly limited, but may be, for example, 25% or more, or 30% or more, or 40% or less, or 35% or less.

[0064] (Yield point YP) In this embodiment, the yield point of the steel plate is not particularly limited, but may 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, or may 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.

[0065] (Methods for measuring tensile strength, total elongation, and yield point) The tensile test for measuring the tensile strength, total elongation and yield point is conducted in accordance with JIS Z 2241, using a JIS No. 5 test piece taken in such a way that the longitudinal direction of the test piece is parallel to the direction perpendicular to the rolling direction of the steel strip.

[0066] (plate thickness) The thickness of the base steel plate is a factor that affects the rigidity of the steel part after forming; the greater the plate thickness, the higher the rigidity of the part. If the plate thickness is too small, the rigidity will decrease and press formability may be reduced due to the influence of unavoidable non-ferrous inclusions present inside the steel plate. On the other hand, if the plate thickness is too large, the press forming load will increase, which may result in mold wear and reduced productivity. The thickness of the base steel plate is not particularly limited, but may be 0.2 mm or more and 6.0 mm or less.

[0067] 1.7 Effects As described above, in the galvannealed steel sheet according to this embodiment, the distribution morphology of Mn (distribution morphology of Mn-enriched portions) on the surface of the base steel sheet is controlled to a predetermined state. As a result, even if alloying of the coating in the Mn-enriched portions is promoted by the mechanism shown in Figures 1 and 2 and a thick coating layer is formed, the intervals between the portions where the coating layer is formed thick are sufficiently small and dense, making it difficult to visually recognize the streaks in the coating. As a result, the galvannealed steel sheet according to this embodiment has excellent coating appearance and appearance after painting.

[0068] 2. Manufacturing method of galvannealed steel sheet The galvannealed steel sheet according to this embodiment can be manufactured by consistently managing continuous casting, hot rolling, cold rolling, and annealing. In particular, it is important to maintain the slab after continuous casting at a high temperature to diffuse Mn on the surface of the slab and disperse Mn-enriched portions. Hereinafter, an example of a method for manufacturing a galvannealed steel sheet will be described, but the method for manufacturing a galvannealed steel sheet is not limited to the following example. For example, the method for manufacturing a galvannealed steel sheet according to this embodiment can be obtaining a steel slab having said chemical composition by continuous casting; The steel slab is heated and then hot-rolled to obtain a hot-rolled sheet. coiling the hot-rolled sheet; Cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; and Annealing the cold-rolled sheet; Including, When cooling the steel slab after the continuous casting, at least the surface in the width direction (the surface that comes into contact with the rolling roll during rolling) of the surface of the steel slab is held at a temperature of 600°C or higher for 240 minutes or more; When heating the steel slab before hot rolling, at least the surface in the width direction of the surface of the steel slab (the surface that comes into contact with the rolling roll during rolling) is held at a temperature of 1150°C or higher and 1300°C or lower for 120 minutes or higher and 300 minutes or lower; Each step will be described in detail below, focusing on the key points of this embodiment.

[0069] 2.1 Cooling process of slab after continuous casting As described above, when cooling a steel slab after continuous casting, at least the widthwise surface (the surface that comes into contact with the rolling rolls during rolling) of the steel slab is maintained at a temperature of 600°C or higher for 240 minutes or longer. This allows Mn to diffuse on the surface of the steel slab, making it easier to disperse Mn-enriched portions. As a result, when the base steel sheet after rolling is subjected to a galvannealed hot-dip galvanizing process, the streaks of the coating become less visible. The method for controlling the temperature of the steel slab after continuous casting as described above is not particularly limited, and the temperature of the slab may be controlled between the end of the continuous casting machine and the heating furnace before hot rolling. For example, it is effective to perform slow cooling during secondary cooling during continuous casting, adjust the installation position of a cart for transporting the slab, or keep the slab warm using a heat-insulating cover. As an example, when slabs are continuously cast by a continuous casting machine (CC) and transported by transport rolls, they are stacked in multiple layers to form a slab stack, and the slab stack is then covered with an insulating cover. Furthermore, when the slabs are transported by a cart, the above-mentioned temperature control during the slab cooling process can be achieved by controlling the cooling rate of at least the width direction surface of the slab during transport to a low rate of 30°C / h or less.

[0070] 2.2 Heating temperature and time in the heating furnace As described above, when heating a steel slab before hot rolling, at least the widthwise surface of the steel slab (the surface that comes into contact with the rolling rolls during rolling) is held at a temperature of 1150°C to 1300°C for 120 minutes to 300 minutes, thereby diffusing Mn on the surface of the steel slab and facilitating the dispersion of Mn-enriched portions. As a result, when the base steel sheet after rolling is subjected to a galvannealed hot-dip galvanizing treatment, streaks in the coating become less visible. The method for controlling the temperature of the steel slab before hot rolling as described above is not particularly limited; the steel slab may be loaded into a heating furnace and the temperature of the slab may be controlled by controlling the temperature in the heating furnace.

[0071] 2.3 Other processes In the manufacturing method according to this embodiment, as described above, the temperature of the steel slab may be controlled from continuous casting to hot rolling, and then hot rolling, coiling, cold rolling, annealing, plating, etc. may be performed. The hot rolling conditions, coiling conditions, cold rolling conditions, annealing conditions, and plating conditions are not particularly limited. An example of each step will be shown below.

[0072] (Hot rolling finishing temperature) When the heated slab is hot-rolled, general conditions may be adopted as the rough rolling conditions in the hot rolling. General conditions may also be adopted as the finish rolling conditions in the hot rolling. However, since the finish rolling temperature in the hot rolling is a factor that has an effect on controlling the texture of the base steel sheet, it is advisable to control it within a predetermined temperature range. For example, the finish rolling temperature in the hot rolling may be 900°C or higher or 950°C or lower.

[0073] (winding) The coiling temperature of the hot-rolled sheet is not particularly limited and may be, for example, 500°C or higher and 800°C or lower.

[0074] (reduction rate in cold rolling) The reduction rate in cold rolling is important from the viewpoint of obtaining a texture with an excellent r-value. For example, the total reduction rate in cold rolling is preferably 70% or more and 90% or less. Annealing at a temperature of 700°C or less may be performed before cold rolling in order to reduce the cold rolling load.

[0075] (Annealing atmosphere) Annealing may be performed in a high dew point atmosphere or a low dew point atmosphere. For example, the dew point of the annealing atmosphere may be −60° C. or higher or 30° C. or lower.

[0076] (Annealing holding temperature) If the heating temperature during annealing is too low, the ductility of the steel sheet is likely to decrease. On the other hand, excessively high heating temperatures not only increase costs but also cause problems such as poor sheet shape during high-temperature sheet passing and reduced roll life. From these perspectives, the maximum heating temperature during annealing (annealing holding temperature) is preferably 750°C or higher and 900°C or lower.

[0077] (Annealing holding time) During annealing, it is preferable to hold the above heating temperature for 5 seconds or more. If the holding time is too short, the strength may decrease significantly. Also, the hardness may tend to vary widely. From these viewpoints, the holding time is more preferably 10 seconds or more, and even more preferably 20 seconds or more.

[0078] (Cooling rate after annealing) There are no particular restrictions on the cooling conditions after the annealing.

[0079] (Cooling stop temperature after annealing and reheating) Furthermore, after the cooling after the annealing, if the cooling stop temperature is lower than the coating bath temperature, the steel sheet may be reheated and retained in a temperature range of 350°C to 600°C. If the cooling stop temperature is too low, not only will a significant capital investment be required, but the effect will also saturate.

[0080] (residence temperature) Furthermore, after reheating and before immersion in the coating bath, the steel sheet may be retained in a temperature range of 350 to 600°C. Retention in this temperature range suppresses temperature unevenness in the width direction of the sheet and improves the appearance after coating. Note that, if the cooling stop temperature after the above-mentioned annealing is 350 to 600°C, retention can be performed without reheating.

[0081] (residence time) The retention time is preferably 30 seconds or more and 300 seconds or less in order to obtain the desired effect.

[0082] (tempering) In the series of annealing steps, the cold-rolled sheet or the steel sheet that has been plated on the cold-rolled sheet may be reheated after being cooled to room temperature or during the cooling to room temperature (but at or below Ms).

[0083] (plating) The hot-dip galvanizing step forms a hot-dip galvanized layer on the surface of the steel sheet. This is preferable because it improves the corrosion resistance of the cold-rolled steel sheet. In the manufacturing method according to this embodiment, a galvanized layer may be formed on the surface of the sheet during annealing. Alternatively, the galvanized layer may be formed on the surface of the sheet after annealing.

[0084] (Plating bath composition) The composition of the plating bath may be such that it is primarily composed of Zn and that the chemical composition of the plating layer after alloying falls within the desired range. The plating bath preferably has an effective Al content (the total Al content minus the total Fe content in the plating bath) of 0.050 to 0.250 mass%. If the effective Al content in the plating bath is too low, excessive penetration of Fe into the plating layer may occur, resulting in reduced plating adhesion. On the other hand, if the effective Al content in the plating bath is too high, Al-based oxides that inhibit the movement of Fe and Zn atoms may form at the boundary between the steel sheet and the plating layer, preventing sufficient alloying. The effective Al content in the plating bath is more preferably 0.065 mass% or more, and more preferably 0.180 mass% or less.

[0085] (Steel sheet temperature after immersion in the plating bath) To perform alloying treatment on the hot-dip galvanized layer, the steel sheet bearing the hot-dip galvanized layer is heated to a temperature range of 450 to 600°C. If the alloying temperature is too low, the alloying process may not proceed sufficiently. On the other hand, if the alloying temperature is too high, the alloying process may proceed too far, resulting in the formation of a Γ phase, which may increase the Fe concentration in the coating layer and reduce coating adhesion. The alloying temperature may be 470°C or higher or 580°C or lower. The alloying temperature must be adjusted depending on the surface condition of the steel sheet, such as the composition and the degree of internal oxidation layer formation, and can be set while checking the Fe concentration in the coating layer. The hot-dip galvanized layer is alloyed, for example, in an alloying furnace and a holding zone. The residence time in the alloying furnace and the holding zone may be, for example, approximately 10 seconds in the alloying furnace and approximately 20 seconds in the holding zone, for a total of approximately 30 seconds. The holding zone is a section that maintains the heat of the steel sheet after it leaves the alloying furnace to prevent a sudden drop in temperature.

[0086] (Post-processing) For the purpose of improving paintability and weldability, the surface of the galvannealed steel sheet may be coated with an upper layer of plating or subjected to various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc.

[0087] (skin pass rolling ratio) Furthermore, skin-pass rolling may be performed for the purpose of correcting the steel sheet shape or introducing mobile dislocations to improve ductility. The reduction ratio of the skin-pass rolling after heat treatment is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. If it exceeds 1.5%, productivity drops significantly, so this is the upper limit. Skin-pass may be performed inline or offline. Furthermore, skin-pass with the desired reduction ratio may be performed all at once, or it may be performed in several steps. [Example]

[0088] An example of the present invention will be described below. The present invention is not limited to this example of conditions. The present invention allows various conditions to be adopted as long as they do not deviate from the gist of the invention and the object is achieved.

[0089] 1. Preparation of base steel plate Steels with various chemical compositions were melted and continuously cast to produce steel slabs. During the cooling process after continuous casting, some slabs were stacked and the slab temperature was maintained using a thermal insulation cover. Furthermore, when the slabs were transported on a cart, the cooling rate during transport was controlled to a low rate of 30°C / h or less. A temperature sensor was installed on the surface at the center of the width of the slab to monitor the temperature change on the widthwise surface of the slab and measure the holding time at 600°C or higher during the slab cooling process. The cooled steel slabs were inserted into a heating furnace and subjected to a homogenization treatment by holding them at a predetermined temperature for a predetermined time, after which they were removed into the atmosphere. They were then hot-rolled to obtain a hot-rolled sheet with a thickness of 3.2 mm, which was then coiled at the predetermined temperature. The oxide scale of this hot-rolled sheet was then removed by pickling, and it was cold-rolled (75% reduction) to a thickness of 0.8 mm. The cold-rolled sheet was then annealed. For all steel sheets, the holding temperature during annealing was 800°C, and the holding time was 60 seconds. Immediately after annealing, the steel sheets were plated and alloyed, and then skin-pass rolled. Samples taken from each of the resulting steel sheets were analyzed to find the chemical compositions of the base steel sheets, as shown in Tables 1 and 2 below. The balance other than the components shown in Tables 1 and 2 is Fe and impurities.

[0090] [Table 1]

[0091] [Table 2]

[0092] 2. Plating Each steel sheet was subjected to hot-dip galvanization and then to alloying treatment. In the hot-dip galvanization process, the steel sheet was immersed in a hot-dip galvanization bath at 450°C for 5 seconds. Then, alloying treatment was performed at 590°C, and the steel sheet was cooled from the alloying treatment temperature to room temperature to obtain a hot-dip galvannealed steel sheet. The plating treatment for the base steel sheet can be performed using the same equipment and line as the annealing treatment.

[0093] 3. Evaluation 3.1 Properties of base steel sheet For galvannealed steel sheets, the coating layer was dissolved and removed using 15 mass% hydrochloric acid containing an inhibitor to expose the surface of the base steel sheet, and Mn was then mapped using an EPMA, and the standard deviation SD and interval I were determined under the following conditions 1 and 2.

[0094] Condition 1: As shown in Figure 3, an EPMA was used to divide a 2 mm square region on the surface of the base steel sheet into multiple 400 x 400 areas for elemental mapping, and the Mn intensity in each area was identified. The standard deviation SD was calculated from the distribution of the number of detected points for the Mn intensity in the rectangular region.

[0095] Condition 2: As shown in Figure 4, an EPMA was used to divide a 2 mm square area on the surface of the base steel sheet into multiple 400 x 400 areas, and element mapping was performed to identify the Mn intensity in each area. The average Mn intensity in the rectangular area was calculated as MI. ave In addition, the rectangular region was divided into 400 regions in a direction perpendicular to the rolling direction, and the average value MI ave The first region is the area where 50% or more of the area shows a Mn intensity 34% or higher than that of the first region. ave The area in which 50% or more of the area showed a Mn intensity 34% or more lower than that of the first region was taken as the second region. In the rectangular region, the distance I between the first regions adjacent to each other with the second region sandwiched therebetween was determined.

[0096] 3.2 Mechanical properties The yield point (YP), tensile strength (TS), total elongation (EL) and average r-value of the galvannealed steel sheets were measured under the same conditions as described above.

[0097] 3.3 Plating 3.3.1 Wettability The wettability of the plating on the base steel sheet during plating was evaluated according to the following criteria. ○: Good wettability (visual observation indicates that 100% of the surface of the base steel sheet is plated) ×: Poor wettability (visual observation reveals unplated areas on the surface of the base steel sheet)

[0098] 3.3.2 Presence or absence of plating streak patterns The appearance of the galvannealed steel sheet was visually observed, and the degree of appearance of the plating streak pattern was evaluated according to the following evaluation criteria. ◎: No plating streak pattern is visible. ◯: No plating streak pattern is visible at a distance of 50 cm from the steel plate. △: The plating streak pattern is visible at a position 50 cm away from the steel sheet, but is not visible at a position 100 cm away. ×: The plating streak pattern is visible even at a distance of 100 cm from the steel sheet.

[0099] 3.4 Molding / processability 3.4.1 Presence or absence of plating adhesion to the mold The galvannealed steel sheets were press-formed (conditions: cylindrical deep drawing), and the presence or absence of adhesion of the plating to the mold was evaluated according to the following criteria. ○: Almost no adhesion of plating to the mold ×: There is a lot of adhesion of plating to the mold.

[0100] 3.4.2 Secondary work embrittlement The galvannealed steel sheets were deep-drawn into cylindrical shapes, cooled at 0°C for 5 minutes or more, and then crushed in a press, and the presence or absence of secondary work embrittlement was evaluated according to the following criteria. ○: No secondary processing embrittlement (crack length after pressing is less than 10 mm) ×: Secondary processing embrittlement occurred (crack length after pressing is 10 mm or more)

[0101] 4.Results Table 3 shows the manufacturing conditions of the base steel sheet, the properties of the base steel sheet, the mechanical properties, the platability, and the evaluation results of the formability and workability.

[0102] [Table 3]

[0103] The galvannealed steel sheets according to Nos. 1 to 39 all had base steel sheet metallurgical structures that satisfied, in terms of area percentage, ferrite: 94 to 100%, the sum of martensite and bainite: 0 to 4%, and retained austenite: 0 to 2%. The galvannealed steel sheets according to Nos. 1 to 39 all had coating layer chemical compositions that satisfied, in terms of mass percentage, Fe: 5 to 25%, Al: 0 to 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 balance: Zn and impurities.

[0104] The results shown in Table 3 reveal the following:

[0105] In the case of No. 27, the time for which the slab was held at 600°C or above during cooling was too short, which prevented the Mn from being sufficiently diffused and dispersed on the slab surface. As a result, the variation in the distribution of Mn (standard deviation SD) on the surface of the base steel sheet after rolling and annealing became large, and clear streaks appeared in the galvannealed layer.

[0106] In the case of No. 28, the heating temperature before hot rolling the slab was too low, which prevented sufficient diffusion and dispersion of Mn on the slab surface. As a result, the spacing I between Mn-enriched areas on the surface of the base steel sheet after rolling and annealing could not be narrowed sufficiently, resulting in clear streaks in the galvannealed layer.

[0107] In the case of No. 29, the heating temperature before hot rolling the slab was too low and the heating holding time was too short, which prevented sufficient diffusion and dispersion of Mn on the slab surface. As a result, the variation in Mn distribution (standard deviation SD) on the surface of the base steel sheet after rolling and annealing became large, and the spacing I between Mn-enriched areas could not be made sufficiently narrow, resulting in the formation of clear streaks in the galvannealed layer.

[0108] In the case of No. 30, the heating and holding time before hot rolling the slab was too short, which prevented sufficient diffusion and dispersion of Mn on the slab surface. As a result, the variation in Mn distribution (standard deviation SD) on the surface of the base steel sheet after rolling and annealing became large, and the spacing I between Mn-enriched areas could not be narrowed sufficiently, resulting in clear streaks in the galvannealed layer.

[0109] In the case of No. 31, the C content in the base steel sheet was too high, which is thought to have resulted in a large amount of solute C remaining in the base steel sheet without being fixed by Ti or Nb. As a result, alloying of the coating was inhibited, unalloyed coated areas were formed in the coated steel sheet, and the coating was prone to adhere to the mold.

[0110] In the case of No. 32, the base steel sheet contained too much Si, which is thought to have deteriorated the wettability of the plating to the base steel sheet. In addition, Si is thought to have inhibited alloying of the plating, resulting in a large amount of unalloyed plating on the plated steel sheet. During plastic forming, this unalloyed plating adhered to the mold, impairing formability and productivity.

[0111] In the case of No. 33, the Mn content in the base steel sheet was too high, so the elongation tended to decrease.

[0112] In the case of No. 34, the P content in the base steel sheet was too high, which is thought to have inhibited the alloying of the plating. As a result, the plated steel sheet had a large amount of unalloyed plating, and during press forming, the unalloyed plating adhered to the mold, impairing formability and productivity.

[0113] In No. 35, the S content in the base steel sheet was too high, which caused secondary work embrittlement due to segregation at the grain boundaries of the base steel sheet.

[0114] In the case of No. 36, the Ti content in the base steel sheet was too low, which resulted in insufficient fixation of C in the base steel sheet, and it is believed that the alloying of the plating was inhibited by the solute C. As a result, unalloyed plated areas were generated in the plated steel sheet, and the plating was prone to adhere to the mold.

[0115] In No. 37, the B content in the base steel sheet was too low, so the function of the C removed from the grain boundaries could not be fully compensated for, resulting in secondary work embrittlement.

[0116] In the case of No. 38, the B content in the base steel sheet was too high, which is thought to have inhibited alloying of the coating, resulting in many unalloyed coated areas in the coated steel sheet, and the unalloyed coating adhering to the die during press forming, impairing formability and productivity. In addition, the holding time at 600°C or above when cooling the slab was too short, which prevented sufficient diffusion and dispersion of Mn on the slab surface. This resulted in large variations in the distribution of Mn (standard deviation SD) on the surface of the base steel sheet after rolling and annealing, and clear streaks in the galvannealed layer.

[0117] In No. 39, the Ti content in the base steel sheet was too high, resulting in a poor balance between strength and ductility of the plated steel sheet.

[0118] In contrast, Nos. 1 to 26 were able to suppress the occurrence of streaks in the coating of the galvannealed steel sheet, and also satisfied the properties that are likely to be required of a galvannealed steel sheet, such as mechanical properties and formability.

[0119] To summarize the above results, the following galvannealed steel sheets have excellent appearance with hardly visible streaks in the coating, and also satisfy the properties that are often required of galvannealed steel sheets, such as mechanical properties and formability.

[0120] A galvannealed steel sheet having a base steel sheet and a coating layer, The base steel plate is, in mass%, C: 0.0005 to 0.0100%, Si: 0.01 to 0.50%, Mn: 0.01 to 2.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, Ti: 0.040~0.180% Nb: 0 to 0.100%, B: 0.0005~0.0100%, Al: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0~2.000%, Ni: 0 to 0.500% Mo: 0 to 3.000%, W: 0 to 0.100%, V: 0 to 1.000%, O: 0 to 0.020%, Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500% As: 0~0.050%, Mg: 0 to 0.050% Zr: 0 to 0.050%, Ca: 0 to 0.0500%, and REM: 0~0.0500%, and the balance being Fe and impurities, The standard deviation SD obtained by the above condition 1 is 0.40 or less, The interval I determined by the above condition 2 is 50 μm or less. Galvannealed steel sheet.

Claims

1. A galvannealed steel sheet having a base steel sheet and a coating layer, The base steel plate is, 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: 0-3.000%, W: 0-0.100%, V: 0-1.000%, O: 0 to 0.020%, Ta: 0-0.100%, Co: 0-3.000%, Sn: 0-1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0500%, and REM: 0 to 0.0500%, and the balance being Fe and impurities, The standard deviation SD obtained by the following condition 1 is 0.40 or less, The interval I determined by the following condition 2 is 50 μm or less. Galvannealed steel sheet. Condition 1: Using an electron probe microanalyzer (EPMA), a 2 mm square region on the surface of the base steel sheet is divided into a plurality of 400 × 400 areas, element mapping is performed, and the Mn intensity in each of the areas is identified. The standard deviation SD is calculated from the distribution of the number of detection points of the Mn intensity in the rectangular region. Condition 2: Using an electron probe microanalyzer (EPMA), a rectangular region of 2 mm square on the surface of the base steel sheet is divided into a plurality of 400 × 400 areas, and element mapping is performed to identify the Mn intensity in each of the areas. ａｖｅ The rectangular region is divided into 400 regions in a direction perpendicular to the rolling direction, and the average value MI of the divided regions is determined. ａｖｅ The first region is defined as a region in which 50% or more of the area shows a Mn intensity that is 34% or higher than the average Mn intensity MI. ａｖｅ The area in which 50% or more of the area shows a Mn intensity 34% or more lower than that of the first area is defined as a second area. The distance I between the first areas adjacent to each other across the second area in the rectangular area is determined.

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

3. The chemical composition is, in mass %, 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: 0.001-3.000%, 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: 0.001-0.050%, Zr: 0.001 to 0.050%, Ca: 0.0001-0.0500%, REM: 0.0001-0.0500% It contains at least one selected from the group consisting of The galvannealed steel sheet according to claim 1 or 2.

4. The metal structure of the base steel plate is, in terms of area ratio, Ferrite: 94-100%, The sum of martensite and bainite: 0 to 4%; and Retained austenite: 0 to 2% The galvannealed steel sheet according to claim 1 or 2, wherein

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

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