Surface-treated steel sheet

Abstract

The surface-treated steel sheet has: a steel sheet; a zinc-based plated layer formed on the surface of the steel sheet; and a chemical conversion treatment layer formed on the surface of the zinc-based plated layer, the chemical conversion treatment layer containing Si, C, O, and P, the C concentration in the chemical conversion treatment layer being 20.0 mass% or more, the O concentration being 15.0 mass% or more, the Si concentration being 10.0 mass% or more, the P concentration being 0.10 mass% or more, in the case where the thickness of the chemical conversion treatment layer is set to t, in the case where a range from the surface of the chemical conversion treatment layer as a starting point to a position at a distance of t / 10 from the surface of the chemical conversion treatment layer in the thickness direction as an ending point is set to a surface layer region, a range from a position at a distance of 9t / 10 from the surface of the chemical conversion treatment layer in the thickness direction as a starting point to the interface between the chemical conversion treatment layer and the zinc-based plated layer as an ending point is set to an interface side region, and a region sandwiched by the surface layer region and the interface side region is set to an intermediate region, the maximum value of the P concentration in the surface layer region is 1.5 times to 5.0 times the average P concentration in the intermediate region.

Description

Technical Field

[0001] This invention relates to surface-treated steel sheets.

[0002] This application claims priority based on Japanese Patent Application No. 2021-055665, filed in Japan on March 29, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] Traditionally, zinc-coated steel sheets, with a zinc-based coating on their surface, have been widely used in automobiles, building materials, and home appliances. This zinc coating imparts excellent corrosion resistance to the steel sheet.

[0004] In addition, zinc-coated steel sheets have been used in recent years as a design material due to the unique appearance of the coating, which changes from silvery-white to a dull grayish-brown through oxidation. Among them, zinc-coated steel sheets containing Sb in the coating produce a significant flower-like crystalline pattern (zinc flower pattern) caused by zinc crystallization, and are therefore used as a material with even greater design appeal.

[0005] However, zinc-plated steel sheets can develop zinc oxides, or white rust, due to corrosion, which can damage their appearance.

[0006] As a technique to suppress the formation of such white rust, conventionally, a chemical conversion treatment based on resin is generally performed on the zinc-based plating, as shown in Patent Documents 1 to 4.

[0007] However, in chemical conversion treatments involving resins, as disclosed in Patent Documents 1-4, the use of organic resin components results in insufficient resistance to black slag during processing. Resistance to black slag during processing refers to the ability to withstand the following: during metal pressing or other processing, the surface of the metal material is subjected to strong sliding through a pressing mold, causing black slag-like substances to form on the coating of the metal material surface, which then adhere and accumulate, thereby damaging the appearance. Black slag-like substances can sometimes be generated due to the organic resin components in the chemical conversion treatment.

[0008] As a chemical conversion treatment capable of suppressing the formation of such black slag, a chemical conversion treatment based on an organosilicon compound having cyclic siloxane bonds, as shown in Patent Document 5, is known. Patent Document 5 describes a chemical conversion treatment based on an organosilicon compound having cyclic siloxane bonds that, due to its very low organic resin content, can suppress the formation of black slag even under harsh processing conditions, and also exhibits high resistance to fingerprints and white rust.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 2020-7606

[0012] Patent Document 2: International Publication No. 2014 / 084371

[0013] Patent Document 3: International Publication No. 2018 / 083784

[0014] Patent Document 4: International Publication No. 2016 / 162422

[0015] Patent Document 5: International Publication No. 2012 / 147860 Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] However, the chemical conversion treatment based on organosilicon compounds with cyclic siloxane bonds, as shown in Patent Document 5, is developed with the intention of developing applications other than those of existing zinc-based plating, i.e., applications other than design applications. On the other hand, as mentioned above, zinc-plated steel sheets have been used as design materials in recent years. In design applications, a high degree of fingerprint resistance is required compared to existing applications, but the chemical conversion treatment based on organosilicon compounds with cyclic siloxane bonds, as shown in Patent Document 5, cannot adequately meet the requirements for design applications.

[0018] Given the above background, the present invention is based on a surface-treated steel sheet having a zinc-based coating and a chemical conversion treatment layer formed on the surface of the zinc-based coating. The objective is to provide a surface-treated steel sheet with sufficient corrosion resistance (resistance to white rust) and superior fingerprint resistance (which is also sufficient for appearance design purposes) compared to the past.

[0019] means for solving problems

[0020] The inventors have investigated a method for improving fingerprint resistance in surface-treated steel sheets having a zinc-based coating and a chemical conversion coating. However, from the viewpoint of resistance to black slag, the chemical conversion coating was studied on the premise that it does not contain organic resin.

[0021] As a result, the following understanding was obtained.

[0022] (a) By adjusting the surface free energy through P segregation in the surface portion of the chemical conversion treatment layer, a surface that is not easily adhered to by fingerprints can be formed.

[0023] (b) Furthermore, by causing F segregation near the interface between the chemical conversion treatment layer and the coating, not only fingerprint resistance but also corrosion resistance (white rust resistance) can be improved.

[0024] (c) Furthermore, the presence of Sb near the interface between the chemical conversion treatment layer and the coating can also improve the resistance to blackening.

[0025] This invention is based on the above-mentioned understanding. The main points of this invention are as follows.

[0026] [1] One aspect of the surface-treated steel sheet of the present invention comprises: a steel sheet; a zinc-based coating formed on the surface of the steel sheet; and a chemical conversion treatment layer formed on the surface of the zinc-based coating, wherein the chemical conversion treatment layer comprises Si, C, O and P, wherein the concentration of C in the chemical conversion treatment layer is 20.0% by mass or more, the concentration of O is 15.0% by mass or more, the concentration of Si is 10.0% by mass or more, and the concentration of P is 0.10% by mass or more, and when the thickness of the chemical conversion treatment layer is set to t, the surface of the chemical conversion treatment layer is taken as the starting point. The area extending from the surface of the chemical conversion layer at a distance of t / 10 in the thickness direction is defined as the surface region. The area extending from the surface of the chemical conversion layer at a distance of 9t / 10 in the thickness direction to the interface between the chemical conversion layer and the zinc-based plating layer is defined as the interface side region. The area sandwiched between the surface region and the interface side region is defined as the intermediate region. The maximum P concentration in the surface region is 1.5 to 5.0 times the average P concentration in the intermediate region.

[0027] [2] According to the surface-treated steel plate described in [1], Al may also be present in the interface side region, and the F content in the interface side region is more than 20% of the total F content of the chemical conversion treatment layer.

[0028] [3] The surface-treated steel sheet according to [1] or [2] may also have Sb in the interface side region described above.

[0029] [4] The surface-treated steel sheet according to any one of [1] to [3], wherein the surface of the zinc-based coating can also be conventionally zinc spangled.

[0030] Invention Effects

[0031] According to the above-described solution of the present invention, a surface-treated steel sheet with excellent fingerprint resistance can be provided.

[0032] In addition, according to a preferred embodiment of the present invention, it is possible to provide a surface-treated steel sheet that not only has excellent fingerprint resistance, but also excellent corrosion resistance and / or blackening resistance. Attached Figure Description

[0033] Figure 1 This is a diagram showing an example of a cross-section of the surface-treated steel plate according to this embodiment.

[0034] Figure 2 This figure illustrates the chemical conversion treatment layer of the surface-treated steel sheet according to this embodiment.

[0035] Figure 3 This is a figure illustrating an example of the results obtained by performing continuous point analysis on the chemical conversion treatment layer of the surface-treated steel sheet of this embodiment using EDS. Detailed Implementation

[0036] Hereinafter, a surface-treated steel sheet according to one embodiment of the present invention (the surface-treated steel sheet of this embodiment) will be described.

[0037] like Figure 1 As shown, the surface-treated steel plate 1 of this embodiment includes: a steel plate 11, a zinc-based plating layer 12 formed on the surface of the steel plate 11, and a chemical conversion treatment layer 13 formed on the surface of the zinc-based plating layer 12.

[0038] Furthermore, the chemical conversion treatment layer 13 comprises Si, C, O, and P, wherein the C concentration is 20.0% by mass or more, the O concentration is 15.0% by mass or more, the Si concentration is 10.0% by mass or more, and the P concentration is 0.10% by mass or more. Additionally, as... Figure 2 As shown, when the thickness of the chemical conversion treatment layer 13 is set to t, the area starting from the surface of the chemical conversion treatment layer 13 and ending at a position t / 10 away from the surface of the chemical conversion treatment layer 13 in the thickness direction is defined as the surface region 101, the area starting from a position 9t / 10 away from the surface of the chemical conversion treatment layer 13 in the thickness direction and ending at the interface between the chemical conversion treatment layer 13 and the zinc-based plating layer 12 (a position t away from the surface in the thickness direction) is defined as the interface side region 102, and the area sandwiched between the surface region 101 and the interface side region 102 is defined as the intermediate region 103. The maximum value of the P concentration in the surface region 101 is 1.5 to 5.0 times the average P concentration in the intermediate region 103.

[0039] exist Figure 1 In the process, the zinc-based coating 12 and the chemical conversion treatment layer 13 (surface area 101 + intermediate area 103 + interface side area 102) are formed on one side of the steel plate 11, but can also be formed on both sides.

[0040] <Steel Plate (Base Steel Plate)>

[0041] The surface-treated steel sheet 1 of this embodiment has characteristics in the zinc-based coating 12 and the chemical conversion treatment layer 13. Therefore, there is no particular limitation on the steel sheet 11. The steel sheet 11 can be determined according to the product to which it is applied, the required strength, the sheet thickness, etc., for example, hot-rolled steel sheet as described in JIS G3193:2019 or cold-rolled steel sheet as described in JIS G3141:2017 can be used.

[0042] <Zinc-based coating>

[0043] The zinc-based coating 12 of the surface-treated steel plate 1 in this embodiment is a zinc-based coating formed on the surface of the steel plate 11 and containing zinc.

[0044] [Chemical Composition]

[0045] Zinc-based coatings 12 can be any coating with zinc as the main component, and there are no restrictions on the chemical composition.

[0046] However, a chemical composition with the following composition is preferred, as it exhibits a more significant improvement in corrosion resistance: Al: 4.0% or more and less than 25.0%, Mg: 0% or more and less than 12.5%, Sn: 0% to 20%, Bi: 0% or more and less than 5.0%, In: 0% or more and less than 2.0%, Ca: 0% to 3.0%, Y: 0% to 0.5%, La: 0% or more and less than 0.5%, Ce: 0% or more and less than 0.5%, Si: 0% or more and less than 2.5%, Cr: 0% or more and less than 0. 25%, Ti: 0% or more and less than 0.25%, Ni: 0% or more and less than 0.25%, Co: 0% or more and less than 0.25%, V: 0% or more and less than 0.25%, Nb: 0% or more and less than 0.25%, Cu: 0% or more and less than 0.25%, Mn: 0% or more and less than 0.25%, Fe: 0% to 5.0%, Sb: 0% or more and less than 0.5%, Sr: 0% or more and less than 0.5%, Pb: 0% or more and less than 0.5%, B: 0% or more and less than 0.5%, and the remainder includes Zn and impurities.

[0047] The reasons for the preferred chemical composition of the zinc-based coating (hereinafter, sometimes simply referred to as the coating) 12 will be explained. Hereinafter, the numerical range indicated by “~” includes the values ​​at both ends as the lower limit and the upper limit as the upper limit. That is, for example, 0% to 20% means 0% or more and 20% or less.

[0048] In addition, unless otherwise specified, the "%" related to the chemical composition of zinc-based coatings is "mass %".

[0049] [Al: 4.0% or more but less than 25.0%]

[0050] Al is an effective element for improving corrosion resistance in zinc-based coatings. Furthermore, when the chemical conversion treatment layer contains Al, it is preferable that the coating contains Al. To fully obtain the above-mentioned effects, it is preferable to set the Al concentration to 4.0% or more.

[0051] On the other hand, when the Al concentration is 25.0% or higher, the corrosion resistance of the cut end face of the zinc-based coating decreases. Therefore, an Al concentration of less than 25.0% is preferred.

[0052] The zinc-based coating 12 may contain Al, with the remainder containing Zn and impurities. However, it may also contain the following elements.

[0053] [Mg: 0% or more and less than 12.5%]

[0054] Mg is an element that improves the corrosion resistance of coatings. To fully achieve the above-mentioned effect, it is preferable to set the Mg concentration to more than 1.0%.

[0055] On the other hand, when the Mg concentration is 12.5% ​​or higher, the effect of improving corrosion resistance becomes saturated, and the processability of the coating decreases. Furthermore, it leads to manufacturing problems such as increased dross formation in the plating bath. Therefore, it is preferable to set the Mg concentration to less than 12.5%.

[0056] [Sn: 0%~20.0%]

[0057] [Bi: 0% or more and less than 5.0%]

[0058] [In: 0% or more and less than 2.0%]

[0059] The aforementioned elements contribute to improved corrosion resistance and sacrificial corrosion protection (alternative corrosion protection). Therefore, one or more of these elements may be included. To achieve the aforementioned effects, it is preferable to set the concentration to 0.05% or more or 0.1% or more, respectively.

[0060] Among these, Sn is preferred because it is a low-melting-point metal that can be easily contained without impairing the properties of the plating bath.

[0061] On the other hand, corrosion resistance decreases when the Sn concentration exceeds 20.0%, the Bi concentration is 5.0% or more, or the In concentration is 2.0% or more. Therefore, it is preferable to set the Sn concentration to 20.0% or less, the Bi concentration to less than 5.0%, and the In concentration to less than 2.0%, respectively.

[0062] [Ca: 0%–3.0%]

[0063] Ca is an element that reduces the amount of scum that easily forms during operation and helps improve the manufacturability of plating. Therefore, it can contain Ca. To achieve this effect, it is preferable to set the Ca concentration to 0.1% or more.

[0064] On the other hand, when the Ca concentration is high, the corrosion resistance of the planar portion of the coating tends to deteriorate, and sometimes the corrosion resistance around the welded portion also deteriorates. Therefore, the Ca concentration is preferably 3.0% or less.

[0065] [Y: 0%~0.5%]

[0066] [La: 0% or more and less than 0.5%]

[0067] [Ce: 0% or more and less than 0.5%]

[0068] Y, La, and Ce are elements that contribute to improved corrosion resistance. To achieve this effect, it is preferable to contain at least one of these elements at a concentration of 0.05% or more, or 0.1% or more, respectively.

[0069] On the other hand, when the concentrations of these elements are excessive, the viscosity of the plating bath increases, making bath preparation itself difficult and raising concerns about the inability to produce steel with good plating properties. Therefore, it is preferable to set the Y concentration to 0.5% or less, the La concentration to less than 0.5%, and the Ce concentration to less than 0.5%.

[0070] [Si: 0% or more and less than 2.5%]

[0071] Si is an element that contributes to improved corrosion resistance. Furthermore, Si also helps to prevent the formation of an excessively thick alloy layer between the steel plate surface and the coating when a coating is formed on a steel plate, thus improving the adhesion between the steel plate and the coating. To achieve these effects, it is preferable to set the Si concentration to 0.1% or more. More preferably, it is 0.2% or more.

[0072] On the other hand, when the Si concentration reaches 2.5% or higher, excessive Si precipitation in the coating not only reduces corrosion resistance but also decreases the processability of the coating. Therefore, it is preferable to set the Si concentration to less than 2.5%. More preferably, the Si concentration is 1.5% or less.

[0073] [Cr: 0% or more and less than 0.25%]

[0074] [Ti: 0% or more and less than 0.25%]

[0075] [Ni: 0% or more and less than 0.25%]

[0076] [Co: 0% or more and less than 0.25%]

[0077] [V: 0% or more and less than 0.25%]

[0078] [Nb: 0% or more and less than 0.25%]

[0079] [Cu: 0% or more and less than 0.25%]

[0080] [Mn: 0% or more and less than 0.25%]

[0081] The aforementioned elements contribute to improved corrosion resistance. To achieve this effect, it is preferable to set the concentration of one or more of these elements at 0.05% or higher.

[0082] On the other hand, when the concentration of these elements is excessive, the viscosity of the plating bath increases, making the preparation of the plating bath itself difficult, raising concerns that it may be impossible to produce steel with good plating properties. Therefore, it is preferable to set the concentration of each element to less than 0.25%.

[0083] [Fe: 0%–5.0%]

[0084] Fe is introduced into the coating as an impurity during the coating process. Sometimes it can be present at around 5.0%, but if it is within this range, the adverse effect on the surface-treated steel sheet of this embodiment is minimal. Therefore, it is preferable to set the Fe concentration to 5.0% or less.

[0085] [B: 0% or more and less than 0.5%]

[0086] Bo (B) is an element that, when present in the coating, combines with elements such as Zn, Al, and Mg to form various intermetallic compounds. These intermetallic compounds have the effect of improving the intermetallic metal content (LME). To achieve this effect, it is preferable to set the B concentration to 0.05% or higher.

[0087] On the other hand, if the B concentration is excessive, the melting point of the coating will increase significantly, raising concerns that the coating operability will deteriorate and a surface-treated steel sheet with good coating properties may not be obtained. Therefore, it is preferable to set the B concentration to less than 0.5%.

[0088] [Sb: 0% or more and less than 0.5%]

[0089] If the coating contains sulfur (Sb), the appearance of the coating changes, forming zinc flowers, and an improvement in metallic luster can be observed. To achieve this effect, it is preferable to set the Sb concentration to 0.03% or higher.

[0090] On the other hand, if the Sb concentration is excessive, the viscosity of the plating bath increases, making bath preparation itself difficult and raising concerns about the inability to produce steel with good plating properties. Therefore, it is preferable to set the Sb concentration to less than 0.5%.

[0091] [Sr: 0% or more and less than 0.5%]

[0092] [Pb: 0% or more and less than 0.5%]

[0093] Sr and Pb are elements that contribute to the formation of zinc flowers. To achieve this effect, it is preferable to set the concentration of one or more of Sr and Pb to 0.05% or more.

[0094] On the other hand, when the concentration of these elements is excessive, the viscosity of the plating bath increases, making the preparation of the plating bath itself difficult, raising concerns that it may be impossible to produce steel with good plating properties. Therefore, it is preferable to set the concentration of each element to less than 0.5%.

[0095] The thickness of the zinc-based coating 12 is not limited, but to improve corrosion resistance, it is preferred to be 10 g / m² per single side. 2 That's all. On the other hand, even if the thickness exceeds 200g / m... 2 However, the corrosion resistance also becomes saturated, and it becomes economically unfavorable. Therefore, the preferred thickness is 200 g / m. 2 the following.

[0096] [Zinc flower]

[0097] From an aesthetic design perspective, the surface-treated steel sheet 1 of this embodiment preferably has a zinc flower pattern formed on the surface of the zinc-based coating 12.

[0098] As for zinc spangle patterns, there are generally conventional zinc spangles and small zinc spangles, but from the point of view of appearance design, conventional zinc spangles are preferred. That is, in this embodiment, the surface of the surface-treated steel sheet 1 with the zinc-based coating 12 is preferably finished with conventional zinc spangle.

[0099] <Chemical Conversion Treatment Layer>

[0100] The chemical conversion treatment layer 13 of the surface-treated steel plate 1 of this embodiment comprises Si, C, O, and P, with a C concentration of 20.0% by mass or more, an O concentration of 15.0% by mass or more, and a Si concentration of 10.0% by mass or more. That is, the chemical conversion treatment layer 13 exhibits excellent resistance to black slag by using inorganic components as the main components. While there is no upper limit specified for the C, O, and Si concentrations, when the C concentration exceeds 40.0% by mass, it is possible that the main components will become organic components. Therefore, the C concentration is preferably 40.0% by mass or less. Furthermore, from the viewpoint of reducing the pulverization of the chemical conversion treatment layer 13, an O concentration of 40.0% by mass or less and a Si concentration of 25.0% by mass or less are preferred.

[0101] The chemical conversion treatment layer 13 of the surface-treated steel sheet of this embodiment is obtained by applying a treatment solution containing organosilicon compounds such as silane coupling agents and P compounds such as phosphates onto a zinc-containing plating layer under specified conditions and then drying it. Therefore, the chemical conversion treatment layer 13 of the surface-treated steel sheet of this embodiment contains Si, C, O and P.

[0102] Without at least 0.10% by mass of phosphorus (P), the required corrosion resistance cannot be obtained. Therefore, the P concentration is set to 0.10% by mass or more. On the other hand, excessive P concentration makes the material prone to pulverization, which is undesirable. Furthermore, a high P concentration in the treatment solution results in a higher concentration of soluble components, which tends to reduce corrosion resistance. Therefore, the P concentration in the chemical conversion treatment layer is preferably 5.0% by mass or less.

[0103] The chemical conversion treatment layer 13 of the surface-treated steel sheet of this embodiment may further contain F, Zr, and / or V from these compounds by including F compound (fluorine compound), Zr compound (zirconium compound), and / or V compound (vanadium compound) in the treatment solution. Additionally, the chemical conversion treatment layer 13 may also contain Al, Zn, Sb, etc., dissolved from zinc-based coatings.

[0104] In the chemical conversion treatment layer 13, the solid content mass ratio [(Ws) / (Vs)] of Si from the organosilicon compound (V) to P from the phosphorus compound (W) is preferably 0.15 to 0.31. When the solid content mass ratio [(Ws) / (Vs)] is less than 0.15, the effect of the phosphorus compound (W) as a dissolution inhibitor cannot be obtained, and therefore it is not preferred.

[0105] On the other hand, if the solid content mass ratio [(Ws) / (Vs)] exceeds 0.31, the water solubility of the chemical conversion treatment layer becomes significant, which is therefore undesirable. The solid content mass ratio [(Ws) / (Vs)] is more preferably 0.16 to 0.28, and even more preferably 0.18 to 0.25.

[0106] Furthermore, in the chemical conversion treatment layer 13 of the surface-treated steel plate 1 in this embodiment, when the thickness of the chemical conversion treatment layer is set to t, the range from the surface of the chemical conversion treatment layer 13 as the starting point to a position t / 10 away from the surface of the chemical conversion treatment layer 13 in the thickness direction as the ending point is defined as the surface region 101, the range from a position 9t / 10 away from the surface of the chemical conversion treatment layer 13 in the thickness direction as the starting point to the interface between the chemical conversion treatment layer 13 and the zinc-based plating layer 12 as the ending point is defined as the interface side region 102, and the region sandwiched between the surface region and the interface side region is defined as the intermediate region 103, the maximum value of the P concentration in the surface region 101 is 1.5 times to 5.0 times the average P concentration in the intermediate region 103.

[0107] By segregating phosphorus (P) in the surface region of the chemically converted layer 13, the surface free energy is adjusted, thereby creating a surface that is less prone to fingerprint adhesion. (Improved fingerprint resistance.)

[0108] When the maximum P concentration in the surface region 101 is less than 1.5 times the average P concentration in the intermediate region 103, fingerprint resistance becomes insufficient. Preferably, the maximum P concentration in the surface region 101 is 2.0 times or more the average P concentration in the intermediate region 103. In this case, fingerprint resistance can be further improved.

[0109] On the other hand, when the ratio exceeds 5.0, brittle phosphorus compounds are formed on the surface, which cause pulverization during pressing, making it undesirable as a steel sheet containing a zinc-based coating.

[0110] Furthermore, in the chemical conversion treatment layer 13 of the surface-treated steel plate 1 in this embodiment, Al is present in the interface side region 102, and the F content (concentration) of the interface side region 102 is preferably 20% or more of the F content (concentration) of the entire chemical conversion treatment layer 13.

[0111] The presence of Al in the interfacial region 102 of the chemical conversion treatment layer 13, with F concentrated within the aforementioned range, indicates that Al and F have undergone complex chlorination. This complex salt is poorly soluble and does not readily allow external corrosive agents to penetrate. Therefore, the formation of such a complex salt improves the corrosion resistance of the surface-treated steel sheet and inhibits the formation of white rust.

[0112] When Al is absent in the interface region 102, or when the F content in the interface region 102 is less than 20% of the total F content in the chemical conversion treatment layer 13, the aforementioned effect cannot be fully obtained. Although there is no upper limit, even if the F content in the interface region 102 is increased by a certain amount, if F becomes excessive relative to Al, the amount of F that is not co-chlorinated with Al increases, the effect saturates, and the economy is poor. Therefore, for example, it can be set to 60% or less.

[0113] Furthermore, in the chemical conversion treatment layer 13 of the surface-treated steel sheet 1 in this embodiment, Sb (containing Sb) is preferably present in the interface side region 102. In this case, the blackening of the surface-treated steel sheet can be suppressed (blackening resistance is improved). The mechanism is not yet clear, but it may be similar to how flash treatment with Co or the like helps prevent blackening of the surface-treated steel sheet.

[0114] Furthermore, in the surface-treated steel sheet of this embodiment, the presence or absence of elements such as Si, C, O, P, and F in the chemical conversion treatment layer, the ratio of the maximum P concentration in the surface region to the average P concentration in the middle region, the ratio of the F content in the surface region to the overall F content of the chemical conversion treatment layer, and the presence or absence of Al and Sb in the interface side region are determined by linear analysis using EDS.

[0115] Specifically, a test piece was cut from a zinc-plated steel sheet with a chemically converted layer using the cryogenic FIB (Focused Ion Beam) method. The cross-sectional structure of the cut test piece was observed using a transmission electron microscope (TEM) at high magnification, bringing the entire chemically converted layer into the field of view. To identify the constituent elements of each layer, TEM-EDS (Energy Dispersive X-ray Spectroscopy) was used for linear analysis along the thickness direction, and quantitative analysis of the chemical composition was performed at various locations. The method of linear analysis was not particularly limited, but it could be continuous point analysis at intervals of several nanometers, or it could be the determination of an elemental distribution map in any region, with the elemental thickness distribution determined by averaging along the surface direction.

[0116] The C, O, Si, and P concentrations in the chemical conversion treatment layer were set to values ​​obtained by averaging the linear analysis results of the entire chemical conversion treatment layer.

[0117] In addition, the maximum value of P concentration in the surface region is adopted from the maximum value of the concentration obtained by linear analysis of the surface region along the thickness direction.

[0118] The F content in the interface region and the F content of the entire chemical conversion treatment layer are respectively defined as "the average F concentration of the target region (layer)" × "the volume of the region (layer)". (The volume of the interface region is 1 / 10 of the total volume of the chemical conversion treatment layer.)

[0119] In addition, in the interface region, linear analysis is performed along the thickness direction. If the average Al concentration is 0.10% by mass or more, Al is considered to be present. If the maximum Sb concentration is 0.01% by mass or more, Sb is considered to be present.

[0120] There are no particular limitations on the apparatus used in the analysis; for example, a TEM (JEM-2100F electrolytic emission transmission electron microscope manufactured by NEC) or an EDS (JED-2300T manufactured by NEC) can be used.

[0121] The thickness of the chemical conversion treatment layer 13 on the surface-treated steel sheet 1 of this embodiment is preferably 10 to 2000 nm. When the thickness is less than 10 nm, it is not possible to cover the surface of the steel, and sometimes sufficient corrosion resistance cannot be obtained, so it is not preferred. On the other hand, when the thickness is greater than 2000 nm, the resistance to black slag during processing is reduced, so it is not preferred. A thickness of 200 to 800 nm is more preferred.

[0122] The thickness of the chemical conversion treatment layer can be determined by cross-sectional observation using TEM.

[0123] The interface between the coating and the chemical conversion treatment layer is determined by TEM cross-section observation, and the thickness of the chemical conversion treatment layer is defined as the distance from this interface to the surface of the chemical conversion treatment layer.

[0124] <Manufacturing Method>

[0125] Next, a preferred manufacturing method for the surface-treated steel sheet of this embodiment will be described.

[0126] Regardless of the manufacturing method, the surface-treated steel sheet of this embodiment can achieve the desired effect as long as it possesses the aforementioned characteristics. However, the manufacturing method described below allows for stable manufacturing and is therefore preferred.

[0127] That is, the surface-treated steel sheet of this embodiment can be manufactured by a manufacturing method including the following steps.

[0128] (I) The plating process involves immersing steel (steel plate) in a plating bath containing Zn to form a zinc-based plating layer on the surface;

[0129] (II) The process of applying a surface treatment metal agent (treatment solution) to steel with a zinc-based coating;

[0130] (III) A heating process, in which a steel plate coated with a surface treatment metallizing agent is heated to form a chemical conversion treatment layer containing Si, C, O, and P; and

[0131] (IV) Cooling process, which cools the steel plate after the heating process.

[0132] The optimal conditions for each process are explained below.

[0133] [Plating Process]

[0134] There are no particular restrictions on the plating process. As long as it is carried out using the usual hot-dip galvanizing method to achieve sufficient coating adhesion, it is acceptable.

[0135] Furthermore, there are no restrictions on the manufacturing method of the steel used in the plating process.

[0136] For example, it could be the manufacturing method of galvanized steel sheet specified in JIS G3302:2019, or the manufacturing method of clad steel sheet specified in JIS G3323:2019.

[0137] The composition of the plating bath is almost identical to that of the coating, so the composition of the plating bath can be adjusted according to the desired composition of the zinc-based coating. When the surface of the coating is set to conventional zinc spangle finishing, it is preferable to contain 0.03 to 0.15% by mass of Sb in the plating bath.

[0138] [Coating Process]

[0139] In the coating process, a surface treatment metal agent (treatment solution) is applied to the steel sheet (steel sheet with zinc coating) after the plating process using a roller coater or similar equipment.

[0140] The surface treatment metallizing agent includes an organosilicon compound (V) as a film-forming component, which is a compound containing Si, C, and O. The organosilicon compound is not particularly limited, and for example, it is a compound obtained by combining a silane coupling agent (A) containing one amino group in the molecule and a silane coupling agent (B) containing one glycidyl group in the molecule in a solid component mass ratio of 0.5 to 1.7 [(A) / (B)].

[0141] The preferred ratio of silane coupling agent (A) to silane coupling agent (B) is 0.5 to 1.7, expressed as a solid content ratio [(A) / (B)]. When the solid content ratio [(A) / (B)] is less than 0.5, bath stability and resistance to black slag are significantly reduced, which is therefore undesirable. On the other hand, when the solid content ratio [(A) / (B)] exceeds 1.7, water resistance is significantly reduced, which is also undesirable.

[0142] The surface treatment metallizing agent contains a phosphorus compound (W) as an inhibitory component. The phosphorus compound (W) is not particularly limited, and examples include phosphoric acid, ammonium phosphate, potassium phosphate, and sodium phosphate.

[0143] Regarding the amount of phosphorus compound (W), the preferred solid content mass ratio of Si from organosilicon compound (V) to P from phosphorus compound (W) [(Ws) / (Vs)] is 0.15 to 0.31. When the solid content mass ratio of Si from organosilicon compound (V) to P from phosphorus compound (W) [(Ws) / (Vs)] is less than 0.15, good corrosion resistance cannot be obtained due to insufficient inhibitor components, and therefore it is not preferred. On the other hand, when the solid content mass ratio [(Ws) / (Vs)] exceeds 0.31, water solubility of the coating (chemical conversion treatment layer) becomes significant, and good corrosion resistance cannot be obtained, and therefore it is not preferred.

[0144] The solid content of the surface-treated metallurgical agent is set at 3.0–15.0% by mass. When the solid content of the surface-treated metallurgical agent is less than 3.0% by mass, the corrosion resistance decreases. The reason for this is not yet clear, but it is presumed to be because residual water in the chemical conversion treatment layer (coating) reduces the barrier properties of the coating. On the other hand, when the solid content of the surface-treated metallurgical agent exceeds 15.0% by mass, it is difficult for phosphorus (P) to segregate on the surface in the surface region. The reason for this is not yet clear, but it is presumed to be because the compounds containing Si, C, and O in the surface-treated metallurgical agent inhibit the migration of P within the agent.

[0145] The surface-treated metallizing agent is preferably used within 72 hours after mixing an organosilicon compound (V), which is a compound containing Si, C, and O, with a phosphorus compound (W) (after preparing the surface-treated metallizing agent). If the time exceeds 72 hours, the effect of causing P segregation in the surface region decreases. The reason for this is not yet clear, but it is presumed that the reaction between the organosilicon compound (V) and the phosphorus compound (W) in the surface-treated metallizing agent inhibits the movement of P within the agent.

[0146] When controlling the process to form a composite salt of Al and F in the interfacial region of the chemical conversion treatment layer, it is preferable to contain Al in the zinc-based coating and a fluorine compound (X) in the surface treatment metallizer. By containing the fluorine compound, the Al on the coated surface dissolves, and simultaneously, Al reacts with F to form an insoluble salt in the interfacial region of the chemical conversion treatment layer.

[0147] Examples of fluorine compounds include, for instance, hydrogen fluoride.

[0148] Regarding the amount of fluorine compound (X), it is preferable to set the concentration of F from the fluorine compound (X) contained in the surface treatment metallurgy to be 0.03 to 4.50% by mass. When it is less than 0.03% by mass, the amount of Al dissolved on the coated surface is insufficient, and the insoluble salt obtained through the reaction of Al and F will not form, making it difficult to achieve the effect of improving corrosion resistance. When it exceeds 4.50% by mass, the dissolution of Al on the coated surface becomes excessive, resulting in a deterioration in appearance.

[0149] Furthermore, regarding the amount of fluorine compound (X), the mass ratio of the solid components Si from organosilicon compound (V) to F from fluorine compound (X) [(Xs) / (Vs)] is preferably 0.01 to 0.30. When the mass ratio of the solid components Si from organosilicon compound (V) to F from fluorine compound (X) [(Xs) / (Vs)] is less than 0.01, the effect of improving corrosion resistance cannot be sufficiently obtained. On the other hand, when the mass ratio of the solid components [(Xs) / (Vs)] exceeds 0.30, the water solubility of the chemical conversion treatment layer becomes significant, which is therefore undesirable.

[0150] The surface treatment metal agent may contain a Zr compound (Y). There are no particular limitations on the Zr compound (Y), but examples include ammonium zirconium carbonate, zirconium hydrogen hexafluoride, and ammonium zirconium hexafluoride.

[0151] Regarding the amount of Zr compound (Y), the mass ratio of the solid components of Si from organosilicon compound (V) to Zr from Zr compound (Y) [(Ys) / (Vs)] is preferably 0.06 to 0.15. When the mass ratio of the solid components of Si from organosilicon compound (V) to Zr from Zr compound (Y) [(Ys) / (Vs)] is less than 0.06, the effect of improving corrosion resistance becomes insufficient. On the other hand, when the mass ratio of the solid components [(Ys) / (Vs)] exceeds 0.15, the effect of improving corrosion resistance becomes saturated.

[0152] The surface-treated metal agent may contain a V compound (Z). There are no particular limitations on the V compound (Z), and examples include vanadium pentoxide (V₂O₅), metavanadate (HVO₃), ammonium metavanadate, sodium metavanadate, vanadium oxychloride (VOCl₃), vanadium trioxide (V₂O₃), vanadium dioxide (VO₂), vanadium oxysulfate (VOSO₄), vanadium oxyacetylacetonate (VO(OC(=CH₂)CH₂COCH₃))₂, vanadium acetylacetonate (V(OC(=CH₂)CH₂COCH₃))₃), vanadium trichloride (VCl₃), and vanadium phosphomolybdic acid. Alternatively, a V compound obtained by reducing a pentavalent vanadium compound to a tetravalent to divalent vanadium compound using an organic compound having at least one functional group selected from hydroxyl, carbonyl, carboxyl, primary to tertiary amino, amide, phosphate, and phosphonic acid groups may also be used.

[0153] Regarding the amount of compound V (Z), the mass ratio of the solid components of Si from organosilicon compound (V) to V from compound V (Z) [(Zs) / (Vs)] is preferably 0.05 to 0.17. When the mass ratio of the solid components of Si from organosilicon compound (V) to V from compound V (Z) [(Zs) / (Vs)] is less than 0.05, the effect of improving corrosion resistance cannot be sufficiently obtained. On the other hand, when the mass ratio of the solid components [(Zs) / (Vs)] exceeds 0.17, the bath stability decreases, and therefore it is not preferred.

[0154] When controlling the presence of Sb in the interfacial region of the chemical conversion treatment layer, it is preferable to include Sb in the zinc-based coating and hydrogen fluoride in the surface treatment metallurgical, with the pH of the surface treatment metallurgical set to 1-5. By including hydrogen fluoride in the surface treatment metallurgical and setting the pH of the surface treatment metallurgical to 1-5, Sb in the zinc-based coating dissolves in the surface treatment metallurgical and migrates to the interfacial region of the chemical conversion treatment layer.

[0155] When the pH is set to 1–5, a pH adjuster may be included in the surface treatment metal agent. There are no particular limitations on the pH adjuster; organic acids such as formic acid, acetic acid, and lactic acid, ammonium salts, and amines can be used.

[0156] [Heating process / Cooling process]

[0157] In the heating process, a steel plate coated with a surface treatment metal agent is heated and dried using a drying furnace or similar device, thereby forming a chemical conversion treatment layer on the surface.

[0158] In the heating process, by heating the surface treatment metal agent at an appropriate heating rate, phosphorus (P) can be segregated in the surface region of the chemical conversion layer. The reason for this is not yet clear, but it is presumed to be that during the formation of the chemical conversion layer, the poor fusion between the film containing Si, O, and C and P causes P to migrate to the unformed parts of the chemical conversion layer.

[0159] To induce phosphorus segregation, after coating at room temperature, the heating rate should be set to 10–150 °C / second, up to the temperature of 55 °C (the point at which the surface treatment metallizer dries and forms the chemical conversion layer). When the heating rate exceeds 150 °C / second, phosphorus migration becomes insufficient, failing to adequately increase the phosphorus concentration in the surface region. Conversely, when the heating rate is less than 10 °C / second, phosphorus migration increases, resulting in an excessive phosphorus concentration in the surface region.

[0160] In the heating process, when the surface treatment metal agent contains hydrogen fluoride, by controlling the time from the moment the surface treatment metal agent is applied until the chemical conversion treatment layer is formed, F can be concentrated in the interface region between the chemical conversion treatment layer and the coating layer, forming a layer containing a composite salt of Al and F.

[0161] The time from the application of the surface treatment metallizing agent to the formation of the chemical conversion layer is preferably set to 1.0 to 10.0 seconds. If the time until the formation of the chemical conversion layer is less than 1.0 seconds, almost no composite chlorination of Al and F is generated, and an F-concentrated layer sufficient to improve corrosion resistance cannot be obtained. On the other hand, if the time until the formation of the chemical conversion layer exceeds 10.0 seconds, the etching of the coating surface using hydrogen fluoride becomes excessive, and the appearance of the chemically converted steel sheet deteriorates.

[0162] Example

[0163] Cold-rolled steel sheets with a thickness of 0.8 mm, conforming to JIS G3141:2017, were immersed in a plating bath with the composition shown in Table 1, resulting in coated steel sheets with the thicknesses shown in Table 8. In Table 1, for example, Zn-0.2%Al indicates a composition containing 0.2% by mass of Al, with the remainder consisting of Zn and impurities. Furthermore, hot-dip galvanizing was applied for a to g, and electroplating was applied for h.

[0164] In addition, a water-based surface treatment metal agent was prepared, which was obtained by mixing silicon compounds (silane coupling agents), phosphorus compounds (P compounds), fluorine compounds (F compounds), zirconium compounds (Zr compounds), and vanadium compounds (V compounds) shown in Tables 2 to 6 in the proportions shown in Table 7.

[0165] The surface treatment metal agent is applied to the plated steel sheet using a roller coater and then heated to 55°C to dry, forming a chemical conversion treatment layer. The combination of the plated steel sheet, the surface treatment metal agent, and the coating and drying conditions is set as shown in Tables 10-1 to 10-12.

[0166] As a result, surface-treated steel sheets No. 1 to 171 were manufactured.

[0167] The surface-treated steel sheets were evaluated for fingerprint resistance, chalking resistance, corrosion resistance, appearance, and blackening resistance according to the following criteria.

[0168] <Fingerprint Resistance>

[0169] Vaseline (registered trademark) was applied to the surface of a flat test piece. After 10 minutes, the Vaseline was removed, and the color difference (ΔE) before and after Vaseline application was measured using a spectrophotometer (SUGA Testing Machine Co., Ltd. SC-T45). The evaluation criteria are as follows: a result of ◎+, ◎, or 〇 indicates excellent fingerprint resistance.

[0170] ◎+:ΔE≤0.5

[0171] ◎: 0.5<ΔE≤1

[0172] ○: 1 < ΔE ≤ 2

[0173] ×:ΔE>2

[0174] <Resistance to chalking>

[0175] Flat test pieces were prepared, and a tight bending test was performed based on JIS Z 2248:2014. A transparent tape peeling test was then conducted on the bent portion. The peeled portion was then observed using a scanning electron microscope to evaluate the residual condition of the chemically converted coating. A value of ○ indicates excellent powdering resistance, while a value of △ indicates no problems in practical use.

[0176] <Evaluation Criteria>

[0177] 〇: No cracks or peeling were observed in the chemically converted film.

[0178] △: Cracks exist in the coating after chemical conversion treatment, but no peeling occurs.

[0179] ×: Observation of film peeling after chemical conversion treatment

[0180] <Corrosion Resistance (White Rust Resistance)>

[0181] Flat plate test pieces were prepared, and each test piece was subjected to a salt spray test according to JIS Z 2371:2015. The formation of white rust on the surface after 144 hours (the proportion of the test piece area with white rust) was evaluated. If △, it was judged to have sufficient corrosion resistance in practical use; if 〇, it was judged to have excellent corrosion resistance; and if ◎, it was judged to have even better corrosion resistance.

[0182] <Evaluation Criteria>

[0183] ◎ = Rust is less than 5% of the total area.

[0184] ○ = Rust covers more than 5% but less than 10% of the total area.

[0185] △ = Rust covers an area of ​​10% or more but less than 30% of the total area.

[0186] × = Rust accounts for more than 30% of the total area.

[0187] <Appearance>

[0188] The appearance of the flat test pieces was evaluated visually according to the following criteria. A score of 0 indicates excellent appearance.

[0189] <Evaluation Criteria>

[0190] 〇: No whitening was observed

[0191] △: Localized whitening was observed.

[0192] <Blackening Resistance>

[0193] After the test panels were kept in a humidification chamber at 70°C and 80% relative humidity for 6 days, they were taken out and the blackening condition of the test panels was judged by visual inspection.

[0194] The evaluation criteria are set as follows: if it is 0, it is judged as qualified; if it is ◎, it is judged as having particularly excellent resistance to blackening.

[0195] ◎: The area of ​​the blackened region is less than 1%.

[0196] 〇: The area of ​​blackened regions is more than 1% but less than 25%.

[0197] △: The area of ​​blackened region is more than 25% but less than 50%.

[0198] ×: The area of ​​blackened region exceeds 50%.

[0199] [Table 1]

[0200] Plating composition zinc flower a Zn-0.2%Al small zinc flower b Zn - 0.2% Al - 0.08% Sb Conventional zinc flower c Zn-6.0% Al-3.0% Mg none d Zn-11.0%Al-3.0%Mg-0.2%Si none e Zn-16.0%Al-6.0%Mg-0.2%Si none f Zn-19.0%Al-6.0%Mg-1.5%Sn-0.5%Ca-0.2%Si none g Zn-24.0%Al-12.0%Mg-0.5%Ca-1.2%Si none h Zn none

[0201] [Table 2]

[0202] name A1 3-Aminopropyltrimethoxysilane A2 3-Aminopropyltriethoxysilane B1 3-Epoxypropoxypropyltrimethoxysilane B2 3-Epoxypropoxypropyl=ethoxysilane

[0203] [Table 3]

[0204] name W1 Phosphoric acid W2 ammonium phosphate

[0205] [Table 4]

[0206] name X1 hydrogen fluoride

[0207] [Table 5]

[0208] name Y1 ammonium zirconium carbonate Y2 Zirconium hexafluoride

[0209] [Table 6]

[0210] name Z1 <![CDATA[Vanadyl sulfate VOSO4]]> Z2 <![CDATA[Vanadyl acetylacetonate VO(OC(=CH2)CH2COCH3)]]>

[0211] [Table 7]

[0212]

[0213] [Table 8]

[0214]

[0215] [Table 9]

[0216]

[0217] [Table 10-1]

[0218]

[0219] [Table 10-2]

[0220]

[0221] [Table 10-3]

[0222]

[0223] [Table 10-4]

[0224]

[0225] [Table 10-5]

[0226]

[0227] [Table 10-6]

[0228]

[0229] [Table 10-7]

[0230]

[0231] [Table 10-8]

[0232]

[0233] [Table 10-9]

[0234]

[0235] [Table 10-10]

[0236]

[0237] [Table 10-11]

[0238]

[0239] [Table 10-12]

[0240]

[0241] As can be seen from Tables 1 to 10-12, for Invention Examples No. 1 to 16, 33 to 122, 129 to 136, 149 to 164, in which the chemical conversion treatment layer contains a specified amount of Si, C, O and P, and the maximum P concentration in the surface region is 1.5 to 5.0 times that of the average P concentration in the intermediate region, it has sufficient resistance to chalking and corrosion, and at the same time excellent resistance to fingerprints.

[0242] On the other hand, the comparative examples No. 17-32, 137-148, and 165-171 are poor in one of the following: fingerprint resistance, chalking resistance, and corrosion resistance.

[0243] Furthermore, in the invention examples, when Al is present in the interface side region and the F content in the interface side region is more than 20% of the total F content of the chemical conversion treatment layer, the corrosion resistance is even better.

[0244] In addition, the resistance to blackening is even better when Sb is present in the interface side area.

[0245] Symbol Explanation

[0246] 1 Surface-treated steel plate

[0247] 11 steel plate

[0248] 12 coatings

[0249] 13 Chemical Conversion Treatment Layer

[0250] 101 Surface Area

[0251] 102 Interface side area

[0252] 103 Middle Area

Claims

1. A surface-treated steel plate, characterized in that, have: Steel plate; A zinc-based coating formed on the surface of the steel plate; and A chemical conversion treatment layer formed on the surface of the zinc-based coating, The chemical conversion treatment layer contains Si, C, O, and P. The chemical conversion treatment layer has a C concentration of 20.0% by mass or more, an O concentration of 15.0% by mass or more, a Si concentration of 10.0% by mass or more, and a P concentration of 0.10% by mass or more. When the thickness of the chemical conversion layer is set to t, the area starting from the surface of the chemical conversion layer and ending at a position t / 10 of the thickness direction from the surface of the chemical conversion layer is defined as the surface region. The area starting from a position 9t / 10 of the thickness direction from the surface of the chemical conversion layer and ending at the interface between the chemical conversion layer and the zinc-based plating layer is defined as the interface side region. The area sandwiched between the surface region and the interface side region is defined as the intermediate region. The maximum P concentration in the surface region is 1.5 to 5.0 times that of the average P concentration in the intermediate region. Sb is present in the interface side region.

2. The surface-treated steel plate according to claim 1, characterized in that, Al exists in the interface side region. The F content in the interface side region is more than 20% of the total F content of the chemical conversion treatment layer.

3. The surface-treated steel plate according to claim 1 or 2, characterized in that, The surface of the zinc-based coating is finished with conventional zinc spangle.

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