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 %, in a cross-sectional view of the base material steel sheet, the existence ratio ER of an prescribed internal oxide in line with a prescribed condition 1 is 40% or less, and the interval I between the internal oxides is 300 μm or more. 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 existence ratio ER of the internal oxide specified by the following condition 1 is 40% or less, The spacing I of the internal oxides specified by the following condition 2 is 300 μm or more. Galvannealed steel sheet Disclose.

[0009] Condition 1: A cross section of the base steel sheet perpendicular to the rolling direction is observed, and grain boundary-type internal oxides contained in the cross section are identified. Here, the field of observation is within a range of 2 mm along the surface direction of the base steel sheet. The internal oxides are those located within a range of 0.5 μm to 5.0 μm in the depth direction from the surface of the base steel sheet. The identified internal oxides are projected onto the surface of the base steel sheet. The proportion of the internal oxides projected onto the surface of the base steel sheet in the surface of the base steel sheet is identified as the abundance ratio ER of the internal oxides.

[0010] Condition 2: A cross section of the base steel sheet perpendicular to the rolling direction is observed, and grain boundary-type internal oxides contained in the cross section are identified. Here, the field of observation is within a range of 2 mm along the surface direction of the base steel sheet. The internal oxides are those located within a range of 0.5 μm to 5.0 μm in the depth direction from the surface of the base steel sheet. The identified internal oxides are projected onto the surface of the base steel sheet. On the surface of the base steel sheet, the spacing I between adjacent internal oxides is identified based on the positions of the internal oxides projected onto the surface of the base steel sheet.

[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 abundance ratio ER of the inner oxide may be 5% or more.

[0013] 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:

[0014] 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.

[0015] 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]

[0016] 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]

[0017] [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] 1 is a schematic diagram for explaining the flow when determining the abundance ratio ER and the interval I under conditions 1 and 2. For ease of understanding, the diagram is enlarged in the thickness direction and the plating layer is omitted. DETAILED DESCRIPTION OF THE INVENTION

[0018] 1. Galvannealed steel sheet According to the inventor's new findings, the streak pattern in galvannealed steel sheets is caused by uneven internal oxidation in the surface layer of the base steel sheet. As shown in FIG. 1, the Mn-enriched portions on the surface of the base steel of a cast 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 pattern. 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 as internal oxides by selective oxidation in the Mn-enriched portions on the steel sheet surface, 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 tends to be high in the portions of the base steel sheet where internal oxides exist. 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 high grain boundary density (Fig. 2(C)), and the plating layer grows thicker, resulting in the appearance of streaks in the plating layer (Fig. 2(D)).

[0019] As described above, the plating streak pattern in galvannealed steel sheets can be said to be caused by internal oxidation unevenness in the surface layer of the base steel sheet. However, according to the inventors' new findings, the level of visibility of the plating streak pattern varies depending on the degree of the internal oxidation unevenness. That is, even if internal oxides are formed in the base steel sheet, plating streak patterns that impair appearance are unlikely to occur if the spacing between the internal oxides is sufficiently narrow or sufficiently wide. In this regard, the plating streak pattern in galvannealed steel sheets can be improved by controlling the distribution of internal oxides 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 in the surface layer of the base steel sheet, as disclosed in the prior art (e.g., Patent Document 3), but more precisely, it depends on the "distribution form" of Mn oxides as internal oxides.

[0020] The distribution of Mn oxides as internal oxides in the surface layer of a base steel sheet can be quantified by observing the cross section of the base steel sheet. Furthermore, the distribution of the internal oxides in the surface layer of a base steel sheet can be controlled, for example, by subjecting the hot-rolled sheet to super-pickling after hot rolling, dissolving the Mn concentrated in the surface layer (the interface between the base steel sheet and the scale) and reducing the starting points for the formation of internal oxides. In this case, the spacing between the internal oxides becomes sufficiently wide, making it difficult for plating streaks that impair appearance to occur. In other words, while Mn is thought to easily concentrate in the surface layer of a hot-rolled steel sheet, super-pickling, which dissolves and removes the entire surface layer of the steel sheet, is thought to reduce the Mn-enriched areas on the newly exposed steel sheet surface after pickling. As a result, the number of internal oxidation starting points decreases, the pitch of the internal oxides increases, and the occurrence of the above-mentioned plating streaks is thought to be suppressed.

[0021] 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.

[0022] 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 balance is Fe and impurities. In the galvannealed steel sheet according to this embodiment, the abundance ratio ER of inner oxides specified by the following condition 1 is 40% or less, and the spacing I of inner oxides specified by the following condition 2 is 300 μm or more.

[0023] Condition 1: A cross section of the base steel sheet perpendicular to the rolling direction is observed, and grain boundary-type internal oxides contained in the cross section are identified. Here, the field of observation is within a range of 2 mm along the surface direction of the base steel sheet. The internal oxides are those located within a range of 0.5 μm to 5.0 μm in the depth direction from the surface of the base steel sheet. The identified internal oxides are projected onto the surface of the base steel sheet. The proportion of the internal oxides projected onto the surface of the base steel sheet in the surface of the base steel sheet is identified as the abundance ratio ER of the internal oxides.

[0024] Condition 2: A cross section of the base steel sheet perpendicular to the rolling direction is observed, and grain boundary-type internal oxides contained in the cross section are identified. Here, the field of observation is within a range of 2 mm along the surface direction of the base steel sheet. The internal oxides are those located within a range of 0.5 μm to 5.0 μm in the depth direction from the surface of the base steel sheet. The identified internal oxides are projected onto the surface of the base steel sheet. On the surface of the base steel sheet, the spacing I between adjacent internal oxides is identified based on the positions of the internal oxides projected onto the surface of the base steel sheet.

[0025] 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.

[0026] (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.

[0027] (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.

[0028] (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 and 1.20% or less. Alternatively, the Mn content may be 1.75% or more and 1.95% or less.

[0029] (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.

[0030] (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.

[0031] (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.

[0032] (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.

[0033] (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.

[0034] (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.

[0035] (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.

[0036] 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.

[0037] (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.

[0038] (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.

[0039] (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.

[0040] (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.

[0041] (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.

[0042] (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.

[0043] (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.

[0044] (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 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 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0045] (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.

[0046] (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.

[0047] (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.

[0048] (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.

[0049] (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.

[0050] (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.

[0051] (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.

[0052] (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.

[0053] 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.

[0054] 1.2 Condition 1 In the galvannealed steel sheet according to this embodiment, the abundance ratio ER of inner oxides specified by the above condition 1 is 40% or less.

[0055] As shown in FIG. 3, under the above-mentioned condition 1, a cross section 10a of the base steel sheet 10 perpendicular to the rolling direction (the direction into the paper in FIG. 3) is observed, and grain boundary-type internal oxides 15 contained in the cross section 10a are identified. Here, the field of observation is set to a range of 2 mm along the surface direction of the base steel sheet 10. More specifically, the observation is performed using a field emission scanning electron microscope (FE-SEM). The measurement was performed using a microscope with a magnification of 1000x for each image. Multiple images were acquired along the surface direction of the base steel sheet 10, and the multiple images were stitched together to identify the cross-sectional state within a 2 mm range along the surface direction of the base steel sheet 10. The grain boundary internal oxides 15 are those located within a depth direction of 0.5 μm to 5.0 μm from the surface 10 ax of the base steel sheet 10. That is, in this application, the "internal oxides" are considered to be those located at a depth of 0.5 μm or more from the surface 10 ax, as opposed to the "external oxides" (oxide scale, etc.) present at the surface 10 ax of the base steel sheet 10. Furthermore, since the grain boundary internal oxides 15 do not usually form at a depth greater than 5.0 μm from the surface 10 ax, it is sufficient to measure the internal oxides located at a depth of 5.0 μm or less from the surface 10 ax.

[0056] "Intergranular internal oxides" refer to oxides that precipitate and form along the grain boundaries of the base steel sheet. As mentioned above, all intergranular internal oxides can suppress the growth of crystal grains in the surface layer through a pinning effect. In other words, in areas where intergranular internal oxides exist, the grain boundary density tends to increase and the alloying of the coating layer tends to be promoted.

[0057] The grain boundary type internal oxide 15 may contain, for example, Mn, or may contain Si in addition to Mn. The internal oxide may contain, for example, Mn2SiO4, or may contain an oxide having a composition other than this. The position, size, and shape of the grain boundary type internal oxide 15 included in the observation field can be easily identified, for example, through elemental analysis. Observation may be performed using a known electron microscope or the like.

[0058] As shown in FIG. 3 , under the above-mentioned condition 1, the grain boundary-type internal oxides 15 identified by the above-mentioned cross-sectional observation are projected onto the surface 10ax of the base steel sheet 10, and then the presence ratio ER is determined. Specifically, the proportion of the internal oxides 15 projected onto the surface 10ax of the base steel sheet 10 in the surface 10ax of the base steel sheet 10 ([(total length L of the internal oxides projected onto the surface (= L1 + L2 + )) / (total length L0 of the entire surface (= 2 mm))] × 100) is determined as the presence ratio ER of the internal oxides 15. Note that, as shown as L1 in FIG. 3 , when multiple internal oxides 15 overlap in the depth direction, they are counted as one internal oxide after being projected onto the surface 10ax of the base steel sheet 10. The same applies to the case of determining the interval I described later.

[0059] When the abundance ratio ER specified in this manner is 40% or less, the amount of internal oxide 15 in the surface layer of the base steel sheet 10 is small, and unevenness of the internal oxide 15 is less likely to occur. The abundance ratio ER may be 38% or less or 35% or less. The lower limit of the abundance ratio ER is not particularly limited and may be 0%, but when annealing at a high dew point is assumed as described later, it may be more than 0%, 5% or more, 8% or more, or 10% or more.

[0060] 1.3 Condition 2 In the galvannealed steel sheet according to this embodiment, the spacing I between the inner oxides specified by the above condition 2 is 300 μm or more.

[0061] As shown in FIG. 3 , under the above-mentioned condition 2, a cross section 10a of the base steel sheet 10 perpendicular to the rolling direction is observed, and the grain boundary-type internal oxides 15 contained in the cross section 10a are identified. Here, the observation field is within a range of 2 mm along the surface direction of the base steel sheet 10. More specifically, as in the above-mentioned condition 1, the observation is performed using a field emission scanning electron microscope (FE-SEM), with the observation field per image being 1000x, multiple images are acquired along the surface direction of the base steel sheet 10, and the multiple images are stitched together to identify the state of the cross section within a range of 2 mm along the surface direction of the base steel sheet 10. The grain boundary-type internal oxides 15 are those located within a range of 0.5 μm to 5.0 μm in the depth direction from the surface 10ax of the base steel sheet 10. The grain boundary-type internal oxides 15 are identified in the same manner as in the above-mentioned condition 1.

[0062] 3, under the above-mentioned condition 2, the grain boundary-type internal oxides 15 identified by the above-mentioned cross-sectional observation are projected onto the surface 10ax of the base steel sheet 10, and then the interval I is identified. Specifically, on the surface of the base steel sheet 10, the interval I between adjacent internal oxides 15 is identified based on the positions of the internal oxides 15 projected onto the surface of the base steel sheet 10.

[0063] When the interval I thus determined is 300 μm or greater, the intervals between internal oxides and between high-grain-boundary-density regions are sufficiently wide in the surface layer of the base steel sheet 10, and as a result, even when a galvannealed layer is formed, plating streaks that impair the appearance are unlikely to occur. The interval I may be 350 μm or greater, or 400 μm or greater. The upper limit of the interval I is not particularly limited. As described above, the observation field under condition 2 is within a range of 2 mm along the surface direction of the base steel sheet 10, but the interval I may be greater than 2 mm, 2 mm or less, 1 mm or less, 800 μm or less, or 600 μm or less. When multiple intervals I are determined in one field of view, the smallest interval I may be 300 μm or greater. For example, as shown in FIG. 3, when intervals I1 and I2 are determined as intervals I, the shorter of these intervals, I1, may be 300 μm or greater.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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-described chemical composition and satisfies the above-described requirements for the abundance ratio ER and interval I. Furthermore, the galvannealed steel sheet in this embodiment may further have the following properties.

[0071] (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.

[0072] (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.

[0073] (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.

[0074] (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.

[0075] (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.

[0076] 1.7 Effects As described above, in the galvannealed steel sheet according to this embodiment, the distribution morphology of the internal oxides in the surface layer of the base steel sheet is controlled to a predetermined state. As a result, even if alloying of the coating is promoted in areas where internal oxides are present (areas with high grain boundary density) through the mechanism shown in Figures 1 and 2, and a thick coating layer is formed, the intervals between the areas where the coating layer is thick are sufficiently wide, making it difficult to visually recognize streaks in the coating. As a result, the galvannealed steel sheet according to this embodiment has excellent coating appearance and appearance after painting.

[0077] 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 dissolve the surface layer of the steel sheet by using super-pickling as the pickling condition after hot rolling. This reduces the starting points for the formation of internal oxides, making it easier to satisfy the above-mentioned abundance ratio ER and interval I. Alternatively, the formation of internal oxides may be suppressed by setting the dew point during annealing to a low dew point. 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 may 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; pickling the hot-rolled sheet by immersing it in an acid treatment solution; Cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; and Annealing the cold-rolled sheet; Including, the acid treatment solution contains hydrochloric acid, the concentration of hydrochloric acid in the acid treatment solution is 7% by mass or more, the temperature of the acid treatment solution is 80°C or more, and the immersion time in the acid treatment solution is 30 seconds or more; The dew point in the atmosphere during the annealing is -35°C or higher and 30°C or lower. Each step will be described in detail below, focusing on the key points of this embodiment.

[0078] 2.1 Pickling conditions In the manufacturing method of a galvannealed steel sheet according to this embodiment, the conditions for pickling the hot-rolled sheet by immersing it in an acid treatment solution are important. That is, by using conditions that allow not only the scale on the sheet surface but also the Mn in the surface layer of the sheet to dissolve (conditions that result in over-pickling, more than usual), the Mn concentrated in the surface layer of the sheet can be dissolved, reducing the number of starting points for internal oxide formation. As a result, the above-mentioned abundance ratio ER and interval I are more likely to be satisfied. Specifically, in this embodiment, the acid treatment solution preferably contains hydrochloric acid, and the concentration of hydrochloric acid in the acid treatment solution is preferably 7% by mass or more. Using an acid treatment solution containing hydrochloric acid at a high concentration in this way makes it easier to dissolve not only the scale on the surface of the hot-rolled sheet but also the Mn in the surface layer of the hot-rolled sheet. The concentration of hydrochloric acid in the acid treatment solution may be 9% by mass or more or 15% by mass or less. In addition, the temperature of the acid treatment solution is preferably 80°C or higher. By using an acid treatment solution with a high temperature in this way, it is easier to dissolve not only the scale on the surface of the hot-rolled sheet but also the Mn in the surface layer of the hot-rolled sheet. The temperature of the acid treatment solution may be 85°C or higher, or 90°C or lower. Furthermore, the immersion time in the acid treatment solution is preferably 30 seconds or longer. By extending the immersion time in the acid treatment solution, it becomes easier to dissolve not only the scale on the surface of the hot-rolled sheet, but also the Mn in the surface layer of the hot-rolled sheet. The immersion time may be 35 seconds or longer, or 60 seconds or shorter.

[0079] 2.2 Annealing conditions When forming a galvannealed layer, it is often preferable to generate internal oxides in the surface layer of the sheet from the viewpoint of increasing the alloying rate, etc. Thus, in order to generate internal oxides in a portion of the surface layer of the sheet within the above-mentioned range of abundance ratio ER while satisfying the above-mentioned interval I, as described above, it is preferable to perform pickling under conditions that result in overpickling to dissolve scale and Mn on the sheet surface and reduce the starting points for internal oxide generation, followed by cold rolling, and annealing in a high dew-point atmosphere after cold rolling to generate internal oxides in a portion of the surface layer of the sheet, and form a galvannealed layer during or after annealing. For example, the dew point of the annealing atmosphere may be −35°C or higher, −30°C or higher, −25°C or higher, or −20°C or higher, or may be 30°C or lower, 25°C or lower, 20°C or lower, 15°C or lower, or 0°C or lower. Note that by using a low dew-point atmosphere during annealing, it is possible to suppress the generation of internal oxides in the surface layer of the sheet and achieve the above-mentioned abundance ratio ER. However, when the annealing atmosphere is a low dew point atmosphere of less than −35° C., outer oxides are likely to form on the outermost surface of the base steel sheet. Specifically, outer oxides of Si and Mn are formed on the outermost surface of the base steel sheet, and these outer oxides may repel the plating, resulting in bare spots, or inhibit the diffusion of Fe, slowing down alloying, leading to concerns that the desired galvannealed steel sheet may not be obtained.

[0080] 2.3 Other processes In the manufacturing method according to this embodiment, continuous casting, hot rolling, coiling, pickling, cold rolling, annealing, and the like may be performed as described above. The pickling conditions and the atmosphere during annealing are as described above. The hot rolling conditions, coiling conditions, cold rolling conditions, and annealing conditions excluding the atmosphere are not particularly limited. Furthermore, the plating treatment conditions are not particularly limited. An example of each step is shown below.

[0081] (Hot rolling conditions) When hot rolling a slab, the slab may be heated before hot rolling. The heating temperature is not particularly limited and may be, for example, 1100°C or higher and 1300°C or lower. The heating time is also not particularly limited and may be, for example, 30 minutes or higher and 300 minutes or lower. When hot rolling the heated slab, general conditions may be adopted for the rough rolling conditions and finish rolling conditions. However, since the finish rolling temperature in hot rolling is a factor that has an effect on controlling the texture of the base steel sheet, it is preferable to control it within a predetermined temperature range. For example, the finish rolling temperature in hot rolling may be 900°C or higher and 950°C or lower.

[0082] (winding conditions) 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.

[0083] (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.

[0084] (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.

[0085] (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.

[0086] (Cooling rate after annealing) There are no particular restrictions on the cooling conditions after the annealing.

[0087] (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.

[0088] (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.

[0089] (residence time) The retention time is preferably 30 seconds or more and 300 seconds or less in order to obtain the desired effect.

[0090] (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).

[0091] (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.

[0092] (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.

[0093] (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 oxide 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.

[0094] (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.

[0095] (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]

[0096] 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.

[0097] 1. Preparation of base steel plate Steels with various chemical compositions were melted and continuously cast into steel slabs. The cooled steel slabs were placed in a furnace heated to 1200°C and held there for 90 minutes for homogenization before being removed into the atmosphere. They were then hot-rolled to obtain hot-rolled sheets with a thickness of 3.2 mm and coiled at the specified temperature. The oxide scale on these hot-rolled sheets was then removed by pickling. The pickling conditions were varied to adjust the amount of dissolved and removed scale and concentrated Mn from the surface layer. After pickling, the sheets were cold-rolled (75% reduction) to a thickness of 0.8 mm. These cold-rolled sheets were then annealed. The holding temperature during annealing for all steel sheets was 800°C for 60 seconds. The atmosphere during annealing was N2-4% H2 with a dew point of 0°C or -40°C. Immediately after annealing, the sheets were plated and alloyed, followed by skin-pass rolling. The chemical compositions of the base steel plates obtained by analyzing samples taken from each of the obtained steel plates are shown in the following Tables 1 and 2. The balance other than the components shown in Tables 1 and 2 is Fe and impurities.

[0098] [Table 1]

[0099] [Table 2]

[0100] 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.

[0101] 3. Evaluation 3.1 Observation of grain boundary-type internal oxides in base steel sheet In the galvannealed steel sheet, a cross section in a direction perpendicular to the rolling direction was observed, and as shown in FIG. 3 , the abundance ratio ER of inner oxides specified by the following condition 1 and the spacing I of inner oxides specified by the following condition 2 were determined.

[0102] Condition 1: As shown in FIG. 3, a cross section of the base steel sheet perpendicular to the rolling direction was observed, and grain boundary-type internal oxides contained in the cross section were identified. Here, the observation field was within a 2 mm range along the surface direction of the base steel sheet. More specifically, the observation was performed using a field emission scanning electron microscope (FE-SEM), with the observation field per image being 1000x. Multiple images were acquired along the surface direction of the base steel sheet 10, and the multiple images were stitched together to identify the state of the cross section within a 2 mm range along the surface direction of the base steel sheet 10. The internal oxides targeted were those located within a depth range of 0.5 μm to 5.0 μm from the surface of the base steel sheet. The identified internal oxides were projected onto the surface of the base steel sheet. The proportion of the internal oxides projected onto the surface of the base steel sheet on the surface of the base steel sheet was identified as the abundance ratio ER of the internal oxides.

[0103] Condition 2: As shown in FIG. 3, a cross section of the base steel sheet perpendicular to the rolling direction was observed, and grain boundary-type internal oxides contained in the cross section were identified. Here, the observation field was within a 2 mm range along the surface direction of the base steel sheet. More specifically, as in Condition 1, observation was performed using a field emission scanning electron microscope (FE-SEM), with the observation field per image being 1000x. Multiple images were acquired along the surface direction of the base steel sheet 10, and the multiple images were stitched together to identify the state of the cross section within a 2 mm range along the surface direction of the base steel sheet 10. The internal oxides targeted were those located within a depth direction of 0.5 μm to 5.0 μm from the surface of the base steel sheet. The identified internal oxides were projected onto the surface of the base steel sheet. On the surface of the base steel sheet, the spacing I between adjacent internal oxides was identified based on the positions of the internal oxides projected onto the surface of the base steel sheet.

[0104] 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.

[0105] 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)

[0106] 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.

[0107] 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.

[0108] 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)

[0109] 4.Results Table 3 shows the manufacturing conditions of the base steel sheet, the properties of the galvannealed steel sheet, and the evaluation results of the mechanical properties, galvanizability, and formability / workability.

[0110] [Table 3]

[0111] The galvannealed steel sheets according to Nos. 1 to 41 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 41 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.

[0112] The results shown in Table 3 reveal the following:

[0113] For No. 27, the temperature of the pickling solution was low and the immersion time during pickling was too short, so the surface layer of the base steel sheet could not be properly dissolved. As a result, the ratio ER of internal oxides in the base steel sheet after annealing increased, and clear streaks appeared in the galvannealed layer.

[0114] In the case of No. 28, the temperature of the pickling solution was too low to properly dissolve the surface layer of the base steel sheet. As a result, the internal oxide content ER in the base steel sheet after annealing increased, and clear streaks appeared in the galvannealed layer.

[0115] For No. 29, the immersion time during pickling was too short, so the surface layer of the base steel sheet could not be dissolved properly. As a result, the internal oxide content ER increased in the base steel sheet after annealing, and clear streaks appeared in the galvannealed layer.

[0116] For No. 30, the immersion time during pickling was even shorter than for No. 28, and therefore the surface layer of the base steel sheet could not be properly dissolved. As a result, the presence ratio ER of the internal oxides in the base steel sheet after annealing increased, and the spacing I of the internal oxides decreased, resulting in the formation of clear streaks in the galvannealed layer.

[0117] In the case of No. 31, the hydrochloric acid concentration in the pickling solution was too low, so the surface layer of the base steel sheet could not be properly dissolved during pickling. As a result, the presence ratio ER of the internal oxides in the base steel sheet after annealing increased, and the spacing I of the internal oxides decreased, resulting in the formation of clear streaks in the galvannealed layer.

[0118] For No. 32, the dew point during annealing was too low, so an outer oxide layer formed on the surface of the base steel sheet, which caused the coating to be repelled during the plating process, resulting in bare areas, and the desired galvannealed steel sheet could not be obtained.

[0119] In the case of No. 33, 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.

[0120] In the case of No. 34, 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, it is thought that Si inhibited the 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.

[0121] In the case of No. 35, the Mn content in the base steel sheet was too high, so the elongation tended to decrease.

[0122] In the case of No. 36, 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.

[0123] In No. 37, 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.

[0124] In the case of No. 38, 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.

[0125] In No. 39, 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.

[0126] In No. 40, the B content in the base steel sheet was too high, which is thought to have inhibited alloying of the coating, resulting in a large number of unalloyed coated areas in the coated steel sheet, and the unalloyed coating adhered to the mold during press working, impairing formability and productivity. In addition, the temperature of the pickling solution was low and the immersion time during pickling was too short, so the surface layer of the base steel sheet could not be properly dissolved. As a result, the presence ratio ER of internal oxides in the base steel sheet after annealing increased and the spacing I of internal oxides became smaller, resulting in the formation of clear streaks in the galvannealed hot-dip coating layer.

[0127] In No. 41, 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.

[0128] To summarize the above results, it can be said that the following galvannealed steel sheets have excellent appearance, with plating streaks that are difficult to see.

[0129] 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 existence ratio ER of the internal oxide specified by the above condition 1 is 40% or less, The spacing I of the internal oxides specified by the above condition 2 is 300 μm or more. 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: 0.43-2.00%, 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 existence ratio ER of the inner oxide specified by the following condition 1 is 40% or less, The interval I between the internal oxides specified by the following condition 2 is 300 μm or more. Galvannealed steel sheet. Condition 1: A cross section of the base steel sheet perpendicular to the rolling direction is observed, and grain boundary-type internal oxides contained in the cross section are identified, with the field of observation being within a range of 2 mm along the surface direction of the base steel sheet. The inner oxides are those within a range of 0.5 μm to 5.0 μm in the depth direction from the surface of the base steel sheet. The identified inner oxides are projected onto the surface of the base steel sheet. The proportion of the inner oxides projected onto the surface of the base steel sheet is determined as the abundance ratio ER of the inner oxides. Condition 2: A cross section of the base steel sheet perpendicular to the rolling direction is observed, and grain boundary-type internal oxides contained in the cross section are identified, with the field of observation being within a range of 2 mm along the surface direction of the base steel sheet. The inner oxides are those within a range of 0.5 μm to 5.0 μm in the depth direction from the surface of the base steel sheet. The identified inner oxides are projected onto the surface of the base steel sheet. On the surface of the base steel plate, the interval I between adjacent inner oxides is determined based on the positions of the inner oxides projected onto the surface of the base steel plate.

2. The chemical composition is, in mass %, Mn: 0.43 to 1.30% or 1.70 to 2.00% and The galvannealed steel sheet according to claim 1.

3. The existence ratio ER of the inner oxide is 5% or more. The galvannealed steel sheet according to claim 1.

4. 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 any one of claims 1 to 3.

5. 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 any one of claims 1 to 3, wherein

6. 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 any one of claims 1 to 3, wherein

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