TWI913050B - Surface treated steel - Google Patents

Abstract

The surface-treated steel of this case comprises: steel and a plating layer, the plating layer being formed on at least a portion of the surface of the steel and containing Zn; when the portion of the surface of the steel in which the plating layer is not formed is defined as a non-plated portion, at least a portion of the non-plated portion contains a compound containing Zn and Mg; the central portion of the first boundary and the second boundary in the thickness direction is a first region central portion, in which the atomic ratio of Mg to Zn, Mg/Zn, is 0.090 or more; the central portion of the aforementioned second boundary in the thickness direction and the aforementioned surface of the compound is a second region central portion, in which the atomic ratio of Mg to Zn, Mg/Zn, is less than 0.090.

Description

Surface treated steel

Field of Invention This disclosure relates to surface-treated steel. This application is based on a priority claim made in Japan Patent Application No. 2024-080155 filed on May 16, 2024, the contents of which are incorporated herein by reference.

Background of the Invention Among surface-treated steels with good corrosion resistance, zinc (Zn)-based coated steel sheets are the most commonly used. These zinc-based coated steel sheets are used in various manufacturing industries, including automotive, home appliances, and building materials. For example, in the building materials sector, research on improving the corrosion resistance of zinc-based coated steel sheets has been ongoing to address the need for longer lifespan. Among these efforts, there has been ongoing research exploring the use of Al and Mg in zinc-based coatings to enhance corrosion resistance. For example, Patents 1-3 disclose a coated steel that achieves high corrosion resistance by containing a certain amount of Al and Mg.

[Previous Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent Application Publication No. 2006-193791 [Patent Document 2] International Publication No. 2011/001662 [Patent Document 3] Japanese Patent Application Publication No. 2021-172878

Summary of the Invention Problem to be Solved by the Invention The zinc-plated steel (zinc-based coated steel sheet) disclosed in the aforementioned patents 1-3 exhibits excellent long-term corrosion resistance on flat surfaces. However, during use, coated steel is sometimes cut to predetermined dimensions. In this case, the cut surface (cut end face) will not have a coating. Furthermore, even on coated surfaces, the coating may peel off due to uncoated areas or damage, or cracks may appear in the coating due to cutting, punching, bending, drawing, etc., resulting in areas where no coating has formed (exposed steel sheet). The inventors of this case, through their review, have learned that while the coated steel in patents 1-3 exhibits excellent corrosion resistance in the coated portion, red rust sometimes forms in the early stages of corrosion in areas such as the cut surfaces, uncoated portions, and/or exposed portions of the steel sheet due to damage or processing after coating formation (collectively referred to as non-coated portions). Therefore, they have been seeking a technology to suppress the formation of red rust in these non-coated portions.

Considering the above background, this disclosure is based on surface-treated steel, represented by surface-treated steel sheet, which has a Zn-containing plating layer (Zn-based plating layer); and the problem of this disclosure is to provide a surface-treated steel that can suppress the formation of red rust in the non-plated part.

Means for Solving the Problem The inventors of this case reviewed a method for suppressing red rust formation in non-plated areas. They found that red rust formation can be suppressed by forming a compound in the non-plated area, which consists of two regions: a region with high Mg concentration and a region with high Zn concentration.

This disclosure is based on the above-mentioned views. The key points of this disclosure are as follows: [1] This disclosure discloses a surface-treated steel material, comprising: steel and a plating layer, the plating layer being formed on at least a portion of the surface of the steel and containing Zn; When the portion of the surface of the steel where the plating layer is not formed is defined as a non-plated portion, at least a portion of the non-plated portion contains a compound containing Zn and Mg; Using an FE-TEM with added energy dispersive X-ray diffraction, the concentration distribution of O, Mg, Al, Fe, and Zn was continuously measured linearly along the thickness direction of the compound from the surface of the aforementioned unplated steel towards the surface of the aforementioned surface-treated steel. The location where the Mg concentration first reaches 0.5 atomic percent or more was defined as the first boundary. The location where the Mg concentration first becomes less than 0.5 atomic percent closer to the surface of the surface-treated steel than the first boundary was defined as the second boundary. At this point, the area between the first and second boundaries constitutes the first region of the compound, and the area between the second boundary and the aforementioned surface of the compound constitutes the second region of the compound. The central portion of the first and second boundaries along the thickness direction is the central portion of the first region. In the central portion of this first region, the atomic ratio of Mg to Zn (Mg/Zn) is 0.090 or more. The central portion of the aforementioned second boundary in the thickness direction and the aforementioned surface of the aforementioned compound constitutes the central portion of the second region, in which the atomic ratio of Mg to Zn, Mg/Zn, is less than 0.090. [2] As described in [1], the surface-treated steel may also contain, by mass %: Zn: greater than 50.0%, Al: greater than 0.2% to less than 40.0%, Mg: greater than 0.2% to less than 12.5%. [3] As described in [1] or [2], the aforementioned first region may also exhibit a halo pattern in an electron beam diffraction pattern obtained by a transmission electron microscope. [4] The surface-treated steel as described in any of [1] to [3], wherein the aforementioned compound may also be present in 50% or more of the surface of the aforementioned steel in the non-plated portion. [5] The surface-treated steel as described in any of [1] to [4], wherein the thickness of the aforementioned first region may also be 0.05 μm or more. [6] The surface-treated steel as described in any of [1] to [5], wherein the thickness of the aforementioned second region may also be 0.05 μm or more.

Invention Effects According to the above-described embodiments disclosed herein, a surface-treated steel capable of suppressing red rust formation in non-plated areas can be provided.

Embodiments of the Invention The form in which the invention is implemented A description is given regarding a surface-treated steel of one embodiment of the present invention (the surface-treated steel of this embodiment).

As shown in Figure 1, the surface-treated steel 1 of this embodiment includes: steel 11 and a plating layer 12, the plating layer 12 being formed on at least a portion of the surface of the steel 11 and containing Zn. Furthermore, regarding the surface-treated steel 1 of this embodiment, if the portion of the surface of the steel 11 where the plating layer 12 is not formed is defined as the non-plated portion 41, at least a portion of the non-plated portion 41 contains a compound 31 containing Zn and Mg. In Figure 1, among the surface 101, the surface that comes into contact with the plating bath and has the plating layer formed is the plating surface 103; the surface exposed when the steel is lifted from the plating bath and cut to a predetermined size is the end face 102. The end face 102 is perpendicular to the plating surface 103, but in most cases it is slightly perpendicular to the plating surface 103.

<Steel Material> The surface-treated steel material 1 of this embodiment has significant characteristics in terms of compound 31. Therefore, the steel material 11 is not particularly limited. The steel material 11 can be determined according to the product to which it is applied or the required strength, plate thickness, etc. For example, hot-rolled steel plates (hot-rolled steel plates) as described in JIS G 3131:2018, JIS G 3113:2018, etc., or cold-rolled steel plates (cold-rolled steel plates) as described in JIS G 3141:2021, JIS G 3135:2018, etc., can be used. As mentioned above, steel materials other than steel plates, such as steel pipes, steel wires, and various steel components, can also be used. The following explanation uses a surface-treated steel sheet (where the steel material is a steel sheet) as an example to illustrate the surface-treated steel material of this embodiment.

<Polyplating Layer> The surface-treated steel (surface-treated steel sheet) 1 of this embodiment has a plating layer 12, which is formed on at least a portion of the surface of the steel 11 and contains Zn. In this embodiment, the Zn-based plating layer is a plating layer with a Zn concentration (content) greater than 50.0% by mass. Although the plating layer 12 can be formed on the entire plating surface 103 of the steel sheet 11 (both sides (or, for cut plating sheets, mostly the surface other than the end face)) (100% by area), there may also be unplated, damaged, or other areas that peel off and are not formed (non-plated portion 41). The area ratio of the non-plated portion should preferably be less than 10% (or even 0%) of the total area of the plating surface. The plating layer 12 can also be formed on a portion of the surface 101 other than the plating surface 103 (end face 102 in Figure 1). However, if the steel sheet is a surface-treated sheet cut to a predetermined size after plating, the end face (cut end face) will mostly not have a plating layer formed.

In the surface-treated steel sheet 1 of this embodiment, in addition to making the plating layer 12 a Zn-based plating layer, a specific treatment is performed to suppress red rust formation even in the non-plated areas 41 (improving red rust resistance). The reason for this is unclear, but it can be assumed that through the specific treatment described later, the Zn contained in the Zn-based plating layer and the Mg contained in the treatment solution will form a predetermined compound in the non-plated areas 41. If the plating layer 12 is not a Zn-based plating layer, the effect of forming compound 31 cannot be fully obtained. Furthermore, even if steel plate 11 contains Mg, without specific treatment, the amount of Mg in the steel plate is trace, and the elution of Mg from the steel plate is minimal, thus failing to achieve the same effect. Also, even if the Zn-based plating layer contains Mg, without specific treatment, the two predetermined regional structures will not form, thus failing to obtain a sufficient effect.

In coating layer 12 (Zn-based coating layer), the concentration (content) of elements other than Zn is not limited. However, the chemical composition of the coating layer, in mass % should preferably include: Zn: greater than 50.0%, Al: greater than 0.2% and less than 40.0%, Mg: greater than 0.2% and less than 12.5%.

Furthermore, the chemical composition of the coating may, depending on the requirements, contain one or more of the following: Sn, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb, and B. That is, the chemical composition of the coating, by mass percent, may also include: Al: greater than 0.2% and less than 40.0%, Mg: greater than 0.2% and less than 12.5%, Sn: 0% and less than 20.0%, Bi: 0% and less than 5.0%, In: 0% and less than 2.0%, Ca: 0% and less than 3.0%, Y: 0% and less than 0.50%, La: 0% and less than 0.50%, Ce: 0% and less than 0.50%, Si: 0% and less than 2.50%, Cr: 0% and less than 0.25%, Ti: 0% and more The content of the following components is specified: 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% or more and less than 5.00%; Sr: 0% or more and less than 0.50%; Sb: 0% or more and less than 0.50%; Pb: 0% or more and less than 0.50%; B: 0% or more and less than 0.50%; and the remainder being greater than 50.0% Zn and impurities. In this case, including the portion with the coating, the surface-treated steel sheet can achieve excellent corrosion resistance and is therefore suitable.

The rationale for the suitable chemical composition of coating 12 is explained. Unless otherwise stated, the percentages (%) of the element concentrations (contents) in the chemical composition of the coating are by mass.

[Al: Greater than 0.2% and less than 40.0%] Al is an effective element used in zinc-based coatings to improve corrosion resistance. Therefore, it may be included. To fully obtain the above effect, the Al concentration should preferably be greater than 0.2%. On the other hand, when the Al concentration is above 40.0%, the sacrificial corrosion protection of the coating will decrease. Therefore, the Al concentration should preferably be less than 40.0%. The Al concentration should be even less than 25.0%.

[Mg: Greater than 0.2% and less than 12.5%] Mg is an element that improves the corrosion resistance of plating coatings. To achieve this improved corrosion resistance, the Mg concentration should ideally be greater than 0.2%. A concentration of 1.0% or higher is preferable, and even more ideally, 3.0% or higher. On the other hand, if the Mg concentration is 12.5% or higher, the effect of improving corrosion resistance becomes saturated. Furthermore, the processability of the plating coating may sometimes decrease. Manufacturing problems such as increased slag formation in the plating bath may also occur. Therefore, the Mg concentration should ideally be less than 12.5%.

[Sn: 0% or more and less than 20.0%] [Bi: 0% or more and less than 5.0%] [In: 0% or more and less than 2.0%] These elements contribute to improved corrosion resistance at the expense of corrosion protection. Therefore, more than one of them may be included. To achieve the above effects, it is advisable to set the concentration to 0.05% or more, and more preferably 0.1% or more. Among these, Sn is a low-melting-point metal and can be easily included without compromising the properties of the plating bath, making it suitable. On the other hand, if the Sn concentration is greater than 20.0%, the Bi concentration is 5.0% or more, or the In concentration is 2.0% or more, the corrosion resistance will decrease. Therefore, it is advisable to set the Sn concentration to below 20.0%, the Bi concentration to below 5.0%, and the In concentration to below 2.0%, respectively.

[Ca: 0% to 3.0%] Ca reduces the amount of slag that easily forms during the plating process, thus improving the manufacturability of the coating. Therefore, it can be included. To achieve this effect, the Ca concentration should be 0.1% or higher. On the other hand, if the Ca concentration is high, the corrosion resistance of the planar portion of the plating layer tends to deteriorate, and the corrosion resistance around the weld area may also deteriorate. Therefore, the Ca concentration should be 3.0% or lower.

[Y: 0% or more and less than 0.50%] [La: 0% or more and less than 0.50%] [Ce: 0% or more and less than 0.50%] Y, La, and Ce are elements that help improve corrosion resistance. To achieve this effect, each of these elements should ideally be present at a concentration of 0.05% or more, preferably 0.10% or more. On the other hand, if the concentration of these elements is excessive, the viscosity of the plating bath will increase, making the construction of the plating bath itself difficult, potentially preventing the production of steel with good plating properties. Therefore, the concentration of Y should ideally be less than 0.50%, the concentration of La should ideally be less than 0.50%, and the concentration of Ce should ideally be less than 0.50%.

[Si: 0% or more and less than 2.50%] Si is an element that helps improve corrosion resistance. Si also helps prevent the formation of an excessively thick alloy layer between the steel sheet and the coating when forming a plating layer on a steel sheet, thus improving the adhesion between the steel sheet and the coating. To achieve this effect, the Si concentration should preferably be 0.10% or more, and more preferably 0.20% or more. On the other hand, if the Si concentration reaches 2.50% or more, excessive Si will precipitate in the coating, reducing both corrosion resistance and the workability of the coating. Therefore, the Si concentration should preferably be less than 2.50%, and more preferably less than 1.50%.

[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%] These elements contribute to improved corrosion resistance. To achieve this effect, the concentration of one or more of these elements should ideally be 0.05% or more. On the other hand, if the concentration of these elements is excessive, the viscosity of the plating bath will increase, making the construction of the plating bath itself difficult, potentially preventing the production of steel with good plating properties. Therefore, the concentration of each element should be set to less than 0.25%.

[Fe: 0% to 5.00%] Fe can be incorporated into the plating layer during its manufacture. Sometimes it may contain up to approximately 5.00%, but within this range, the adverse effects on the surface-treated steel sheet of this embodiment are minimal. Therefore, the Fe concentration should ideally be set below 5.00%.

[Sr: 0% or more and less than 0.50%] [Sb: 0% or more and less than 0.50%] [Pb: 0% or more and less than 0.50%] If Sr, Sb, and Pb are included in the plating layer, the appearance of the plating layer will change and zinc flowers will form, confirming an improvement in metallic luster. To achieve this effect, the concentration of one or more of Sr, Sb, and Pb should be set at 0.05% or more, and preferably at 0.10% or more. On the other hand, if the concentration of these elements is excessive, the viscosity of the plating bath will increase, making the construction of the plating bath itself difficult, and it may be impossible to produce steel with good plating properties. Therefore, the concentration of each element should be set to less than 0.50%.

[B: 0% or more and less than 0.50%] If the element B is contained in the plating layer, it will combine with Zn, Al, Mg, etc. to produce various intermetallic compounds. This intermetallic compound has the effect of improving LME resistance. To obtain this effect, the B concentration should be set to 0.05% or more, and more preferably 0.10% or more. On the other hand, if the B concentration becomes excessive, the melting point of plating will significantly increase, the plating workability will deteriorate, and a surface-treated steel sheet with good plating properties may not be obtained. Therefore, the B concentration should be set to less than 0.50%.

[Remaining Portion: Zn and Impurities] The chemical composition of the plating layer, excluding the elements mentioned above, consists of Zn and impurities. The Zn concentration in plating layer 12 should preferably be greater than 50.0%, more preferably above 70.0%, and even more preferably above 85.0%. Impurities are elements introduced during the manufacturing process. The total concentration of impurities is usually below 0.5%, but preferably below 0.1%.

The amount of coating 12 is not limited; however, to improve corrosion resistance, it should be 10 g/ ^m² or more per side. On the other hand, even if the corrosion resistance is saturated even with an adhesion amount greater than 250 g/ ^m² per side, it is not economically viable. Therefore, the adhesion amount per side should preferably be 250 g/ ^m² or less.

The chemical composition of the coating can be determined by the following method. First, the coating is peeled and dissolved using an acid containing an inhibitor that inhibits corrosion of the base iron (steel sheet) (e.g., an acid obtained by adding 1% by mass of HIBIRON (A-6) (manufactured by Sugimura Chemical Co., Ltd.) to 10% by mass hydrochloric acid). Then, the obtained acid solution is analyzed by ICP analysis, thereby obtaining the chemical composition of the coating 12.

Furthermore, the adhesion amount of the coating can be determined by the following method: A 30mm × 30mm sample is taken from the surface-treated steel sheet. The coating is peeled off and dissolved from this sample using an acid containing an inhibitor that suppresses corrosion of the base iron (steel) (e.g., an acid obtained by adding 1% by mass of HIBIRON (A-6) (manufactured by Sugimura Chemical Co., Ltd.) to 10% by mass hydrochloric acid). The weight change of the coated steel sheet after peeling and dissolution is measured, and the adhesion amount is calculated from the result.

<Compound> In this embodiment, the surface-treated steel sheet 1 contains a Mg-containing compound 31 in at least a portion of the non-plated portion 41 of the surface 101 (plated surface 103, end face 102) of the steel sheet 11. In this embodiment, the compound 31 is composed of a first region R1 and a second region R2. Here, using an FE-TEM with additional energy dispersive X-ray diffraction, the concentration distribution of O, Mg, Al, Fe, and Zn is continuously measured by line analysis along the thickness direction of the compound (usually the same as the thickness direction of the steel plate) from the surface of the steel plate towards the surface of the surface-treated steel plate (the surface of the compound can also be considered the surface of the compound if no other layers are formed thereafter). The location where the Mg concentration first becomes 0.5 atomic% or higher is defined as the first boundary IF1. The location where the Mg concentration first becomes less than 0.5 atomic% on the surface side of the surface-treated steel plate 1 (the upper side in Figure 2), which is closer to the first boundary IF1, is defined as the second boundary IF2. The location where the Zn concentration first becomes less than 0.05 atomic% on the surface side of the surface-treated steel plate 1 (the upper side in Figure 2), which is closer to the second boundary IF2, is defined as the surface SFC of the compound. At this point, the area between the first boundary IF1 and the second boundary IF2 is the first region R1, and the area between the second boundary IF2 and the surface SFC of the compound is the second region R2. As will be explained later, it can be considered that the mechanisms for improving corrosion resistance differ between the first region R1 and the second region R2. Furthermore, by creating two different region structures, a more effective suppression of red rust formation can be achieved through a synergistic effect than in either region alone. These will be explained separately.

[First Region] In the first region, the central portion of the first boundary and the second boundary along the thickness direction is the central portion of the first region. In this central portion, the atomic ratio of Mg to Zn (Mg/Zn) is 0.090 or higher. Because Mg-based corrosion products dissolve in ambient moisture, the pH near the steel surface increases. This region with higher Mg concentration has a passive effect on the steel surface (inhibiting anodic reactions). Therefore, having this region below the compound region (the region on the steel plate side of the compound) provides excellent inhibition of red rust formation. When the Mg/Zn ratio is less than 0.090, the Mg concentration is too low to achieve a sufficient effect. Ideally, the Mg/Zn ratio should be 0.090 or higher, and the Mg concentration should be 2.0–50.0 atomic%. If the Mg concentration is low, the effect may be insufficient. On the other hand, even if the Mg concentration is greater than 50.0 atomic%, the effect of compound formation is still saturated. Furthermore, although there is no upper limit to the Mg/Zn ratio, the effect of Mg concentration will saturate, therefore the Mg/Zn ratio can also be below 50.00.

The thickness of the first region should be 0.05 μm or more. When it is less than 0.05 μm, the effect of inhibiting red rust formation will be smaller. Although there is no upper limit to the thickness, the possibility of peeling will increase with the increase of thickness, so it can also be less than 30.00 μm.

The first region should exhibit a halo pattern in the electron beam diffraction pattern of a transmission electron microscope. At this time, since it is amorphous, the anisotropy of the compound will disappear and the corrosion starting point will be reduced, thereby further improving the effect of inhibiting the formation of red rust. As shown in Figure 3, the so-called halo pattern in the electron beam diffraction pattern of a transmission electron microscope means that a halo-like (fuzzy ring-shaped) contrast pattern can be observed during electron diffraction. The halo pattern also includes the point-like diffraction pattern shown in the photo on the right side of Figure 3.

[Second Region] In the first region, the central portion of the second region is formed by the second boundary in the thickness direction and the central portion of the compound surface. In this central portion of the second region, the atomic ratio of Mg to Zn (Mg/Zn) is less than 0.090. This region with a higher Zn concentration has the following functions: insulation, prevention of Mg outflow from the Mg-rich lower region (Mg has high solubility and easily flows into water), and physical protection (inhibiting corrosion agents from reaching the steel surface). Therefore, having this region as the upper region of the compound region (the region on the surface side of the compound) provides excellent inhibition of red rust formation. If the atomic ratio of Mg to Zn (Mg/Zn) is 0.090 or higher, the Zn concentration is too low to achieve a sufficient effect. Ideally, the Mg/Zn ratio should be less than 0.090 and the Zn concentration should be between 2.0 and 50.0 atomic%. If the Zn concentration is less than 2.0 atomic%, the effect may be insufficient. On the other hand, even if the Zn concentration is greater than 50.0 atomic%, the effect of compound formation is still saturated.

The thickness of the aforementioned second region should preferably be 0.05 μm or more. When it is less than 0.05 μm, the effect of inhibiting red rust formation will be smaller. Although there is no upper limit to the thickness, the possibility of peeling will increase with the increase of the thickness, so it can also be less than 30.00 μm.

The Mg concentration and Mg/Zn ratio in the central part of the first region, and the Zn concentration and Mg/Zn ratio in the central part of the second region, can be obtained using the following method: Test pieces are cut from the surface-treated steel plate at the locations where compounds have formed using a low-temperature FIB (Focused Ion Beam) method, allowing for observation of the cross-section along the thickness direction. While the cutting location is determined by SEM observation during the FIB cutting process, Au is deposited as an electron irradiation protective film before SEM observation to form a carbon-based protective film. The cutting is then performed using an accelerating voltage of 40-5 kV. A Cu sieve is used as the sample support screen. The locations of compound formation are determined using micro-XRD (X-ray microdiffraction). In other words, the following locations are defined as sites where compounds have formed: These locations show the presence of Fe by micro-partial XRD, but no metallic monomeric phases or intermetallic compound phases containing Zn, Al, or Mg, and no phases outside the metal layer are detected. Micro-partial XRD is performed under the following conditions: the X-ray source is a Cr tube bulb, the irradiation diameter is Φ300μm, the applied voltage is 35kV, the applied current is 25mA, and the scanning range is 20~120°. The cross-sectional structure of the cut sample is observed using a transmission electron microscope (TEM) at, for example, 10000x magnification. If the entire thickness of the compound cannot be captured in the field of view, analysis can be performed using multiple fields of view. Using FE-TEM with added energy dispersive X-ray diffraction, the concentration distribution of O, Mg, Al, Fe, and Zn was continuously analyzed in a linear fashion from the surface of the test piece towards the surface of the surface-treated steel plate. The analysis results defined the location where the Mg concentration first exceeds 0.5 atomic percent as the first boundary IF1, and the location where the Mg concentration first falls below 0.5 atomic percent closer to the surface of the surface-treated steel plate than from the first boundary IF1 as the second boundary IF2. Using TEM-EDS, quantitative analysis of Zn and Mg was performed at at least five points in the central part of each region (CP1, CP2). O, Mg, Al, Fe, and Zn were the analyte elements. The average Zn and Mg concentrations at each point in each region were used as the Zn and Mg concentrations for each region. Observation and EDS analysis were performed at an accelerating voltage of 200 kV and a probe diameter of 1 nmΦ. For example, a transmission electron microscope (TEM) such as the JEM-2100F (manufactured by NEC Corporation) was used, an EDS measurement apparatus such as the JED-2300T (manufactured by NEC Corporation) was used, and a fiducial-in-the-blank (FIB) apparatus such as the NB5000 (manufactured by Hitachi High-Technologies) was used.

Whether a halo pattern is observed in the electron beam diffraction pattern of the first region using a transmission electron microscope (TEM) is determined by performing TEM electron diffraction. TEM electron diffraction is performed using FIB-TEM under the following conditions: For the test piece used in the quantitative analysis described above, the probe diameter is set to approximately 10 nm for electron diffraction, and the diffraction pattern is observed. The presence of any pattern as shown in Figure 3 is considered a halo pattern.

The thicknesses of the first and second regions are determined by measuring the distance between the first and second boundaries, or the distance between the second boundary and the compound surface. The distances are measured at three locations and the average of these measurements is taken as the thickness of the first and second regions.

While the presence of the aforementioned compound in the non-plated areas improves corrosion resistance (red rust resistance), for the overall surface treatment of the steel sheet to achieve a sufficient effect, at least 50% of the non-plated areas (within 50% coverage) should be coated with the compound (coverage rate of 50% or more). The coverage rate can also be 100%.

The coating percentage can be determined using μ-XRF and the following method. The following areas are defined as non-coated areas: areas where Fe is detected by micro-section XRD but no metallic monomeric or intermetallic compound phases containing Zn, Al, or Mg are detected. Micro-section XRD is performed under the following conditions: the X-ray source is a Cr tube bulb, the irradiation diameter is Φ300μm, the applied voltage is 35kV, the applied current is 25mA, and the scanning range is 20~120°. For the unplated area, the analytical elements were defined as Mg, Al, Fe, and Zn. Mg distribution was analyzed using μ-XRF, and the intensity of the μ-XRF spectrum was measured. The area ratio of the region with a Mg concentration of 0.5 atomic% or higher relative to the area of the unplated area was defined as the "compound coverage". The μ-XRF measurement conditions were as follows: Measurement gas environment: Vacuum tube voltage: 15kV; Tube current: 50μA; Tube bulb: Rh; Tube scanning speed: 4.00 mm ^/s; X-ray spot size: 30μm.

<Manufacturing Method> Regarding the surface-treated steel sheet of this embodiment, regardless of the manufacturing method, the above-mentioned characteristics will achieve its effect. However, it can be manufactured using a method including the following steps: (I) Plating step: Forming a Zn-containing plating layer on the surface of the steel sheet (base steel sheet); (II) Processing step: Cutting and/or punching the steel sheet with the plating layer (plated steel sheet) to form an arbitrary shape; and (III) Compound formation step: Forming a predetermined compound on the non-plated areas. Suitable conditions are explained for each step.

[Plate Coating Step] In the plate coating step, steel materials such as steel sheets are immersed in a Zn-containing plating bath or electroplated, thereby forming a Zn-containing plating layer on the surface. The conditions for forming the plating layer are not particularly limited. Sufficient plating adhesion can be obtained by conventional methods. Furthermore, the steel materials used in the plate coating step and their manufacturing methods are not limited. For surface-treated steel sheets, hot-rolled steel sheets as described in JIS G 3131:2018 and JIS G 3113:2018, or cold-rolled steel sheets as described in JIS G 3141:2021 and JIS G 3135:2018, can be used as the steel sheets to be immersed in the plating bath. Steel materials other than steel sheets, such as steel pipes, steel wires, and various steel components, can also be used. The composition of the plating bath is adjusted according to the desired chemical composition of the plating layer. After the steel is removed from the plating bath, the amount of plating can be adjusted by wiping as needed.

[Processing Steps] In the processing steps, cutting and/or punching are performed to shape the plated steel sheet into any shape. Once cutting or punching is performed, an end face without a coating is formed at the cut section. An end face is also formed at the punched section. In the processing steps, bending, drawing, and other processes can also be performed to change the shape. At this time, non-coated areas may sometimes be generated on the coated surface.

[Compound Formation Step] In the compound formation step, a predetermined compound is formed on the non-plated portion. The compound formation step includes the following first treatment and second treatment. (First Treatment) The non-plated portion of the steel sheet after the processing step is exposed to the following solution for 1.0 to 20.0 hours; the solution contains ^Cl- : 1.0 to 100.0 mM, SO4 _2- ^: 0.1 to 10.0 mM, Mg ²⁺ : 1.0 to 100.0 mM, CO ₃ ^2- : 1.0 to 100.0 mM, has a pH of 4.5 to 7.0, and a liquid temperature of 25 to 60°C. (Second Treatment) The steel plate after the first treatment is exposed to the following solution for 1.0–20.0 hours; the solution contains ^Cl⁻ : ^1.0–100.0 mM, _SO₄²⁻ : 0.1–10.0 mM, ^Na⁺ : 1.0–100.0 mM, _CO₃²⁻ : 1.0–100.0 mM, with a pH of 4.5–7.0 ^and a temperature of 25–60°C. In both the first and second treatments, the plated areas may also come into contact with the solution as long as the non-plated areas are in contact with it.

In the first treatment, the solution contains ^Mg²⁺ , thereby forming a first region with a higher Mg concentration. While the Mg concentration in this first region depends on the Mg content in the solution, if the plating contains Mg, the Mg concentration in the first region will be higher, resulting in improved corrosion resistance at the end face. As the Mg concentration in the plating increases, the Mg concentration in the first region will also increase. Sufficient results cannot be obtained if the concentrations of various ions or the pH of the solution are outside the aforementioned ranges. Furthermore, if the liquid temperature is below 25°C or the contact time is less than 1.0 hour, a sufficient reaction will not occur, and sufficient results cannot be obtained. On the other hand, when the liquid temperature is above 60°C, the compound formation rate tends to decrease. Furthermore, if the contact time exceeds 20.0 hours, in addition to saturation, productivity will decrease. Regarding fluorine compounds, since the Mg concentration of compounds formed on non-plated parts will decrease, they should not be included in the solution used in the first treatment.

In the second treatment, the solution is brought into contact with a ^Mg²⁺- free solution to create a second region with a high Zn concentration. Sufficient effectiveness cannot be obtained if the concentrations of any ions or the pH of the solution are outside the aforementioned ranges. Furthermore, if the liquid temperature is below 25°C or the contact time is less than 1.0 hour, a sufficient reaction will not occur, and sufficient effectiveness cannot be obtained. On the other hand, when the liquid temperature is above 60°C, the compound formation rate tends to decrease. Also, if the contact time is greater than 20.0 hours, the effect becomes saturated, but productivity decreases.

Various chemical transformation treatments and coating treatments can be performed after the plating step and before the compound formation step. Furthermore, the uneven texture of the plating surface can be used to form plating layers such as Cr, Ni, and Au, and then further coating can be applied to give it a design.

The surface-treated steel of this embodiment can also form a film on the plating layer. One or more films can be formed. The types of films directly above the plating layer include, for example, chromate films, phosphate films, and non-chromate films. The chromate treatment, phosphate treatment, and non-chromate treatment used to form these films can be carried out by known methods. Regarding chromate treatment, the following methods exist: electrolytic chromate treatment, which forms a chromate film through electrolysis; reactive chromate treatment, which forms a film by reacting with the substrate and then rinsing off the remaining treatment solution; and coating-type chromate treatment, which forms a film by applying the treatment solution to the substrate and drying it without rinsing. Any of these methods can be used. Electrolytic chromate treatment can be exemplified by the use of the following components: chromic acid, silica sol, resins (phosphate, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene-butadiene latex, diisopropanolamine-modified epoxy resin, etc.), and hard silica. Phosphate treatment can be exemplified by zinc phosphate treatment, zinc-calcium phosphate treatment, and manganese phosphate treatment. Chromate-free treatment is particularly suitable due to its environmental friendliness. Regarding chromate-free treatment, the following methods exist: electrolytic chromate-free treatment, which forms a chromate-free film through electrolysis; reactive chromate-free treatment, which forms a film by reacting with the substrate and then rinsing off the remaining treatment solution; and coating-type chromate-free treatment, which forms a film by applying the treatment solution to the substrate and drying it without rinsing. Any of these methods can be used.

Furthermore, one or more organic resin films may be present on the film directly above the coating layer. Organic resins are not limited to specific types and may include, for example, polyester resins, polyurethane resins, epoxy resins, acrylic resins, polyolefin resins, or modified forms of these resins. Modified forms refer to resins obtained by reacting reactive functional groups contained in the resin structure with other compounds (monomers or crosslinking agents, etc.), wherein the other compounds contain functional groups in their structure that can react with the reactive functional groups.

Regarding this organic resin, one or more organic resins (unmodified) may be used, or one or more of the following organic resins may be used: resins obtained by modifying at least one other organic resin in the presence of at least one organic resin. Furthermore, the organic resin film may also contain any coloring pigments or rust-preventing pigments. Aqueous materials that are dissolved or dispersed in water may also be used. [Example]

The following hot-rolled steel sheet is prepared as the steel material, which meets JIS G 3131:2018 and has a thickness of 4.5 mm. The steel sheet is subjected to melt plating to form a Zn-based plating layer with the chemical composition recorded in Tables 1 and 2. The total impurity concentration (content) in the plating layer is 0.1% by mass or less. Furthermore, the adhesion amount of the plating layer is set at 135 g/ ^m² for both sides of the plating surface. The obtained plating steel sheet (surface-treated steel sheet) is cut using an electric shearing machine to form end faces, and the end faces have: a portion with a plating layer and a portion without a plating layer (exposed steel sheet). No unplated portion is formed on the plated surface. The end face of the plated steel sheet undergoes a first treatment, bringing it into contact with the solution under the conditions ^described in Tables 3 and 4. However, the solution contacted by sample number 2-23 (solution 2-23) is, as described below, a solution containing fluoride: ^Cl⁻ : 10.0 mM, _SO₄²⁻ : 5.0 mM, ^Mg²⁺ : 20.0 mM, fluoride: 1.0 mg/L. A second treatment is then performed, bringing the end face into contact with the solution under the conditions described in Tables 3 and 4. This forms a compound.

For the obtained surface-treated steel sheet, the following items were measured according to the above points: the coverage of the non-plated compound, the thickness of the first and second regions where the compound is formed, the Mg concentration and Mg/Zn ratio in the central part of the first region, the Zn concentration and Mg/Zn ratio in the central part of the second region, and the presence or absence of a halo pattern in the first region. However, for examples where neither of the two regional structures is formed, the coverage and the presence or absence of a halo pattern in the first region are not evaluated. Furthermore, for examples where the Mg/Zn ratio in the central part of the first or second region is outside the scope of this disclosure, the presence or absence of a halo in the first region is also not evaluated. The results are shown in Tables 3 to 6.

Furthermore, an exposure test was conducted on the surface-treated steel sheet, and the area of red rust on the end face after 90 days was determined. The exposure conditions were as follows: The steel sheet sample was tilted at a 30° angle from the horizontal, facing south, with the treated cut end face as the top, and the atmospheric exposure test was conducted. After exposure, the ratio of the area of red rust formed to the area without a coating was evaluated as follows: S, AA, A, or B indicates good red rust resistance; S, AA, or A indicates excellent red rust resistance. The following conditions define an area where red rust formation exceeds 100%: not only the areas without a coating, but also the surrounding areas develop red rust. S: Greater than 60% and less than 70% AA: Greater than 70% and less than 80% A: Greater than 80% and less than 90% B: Greater than 90% and less than 100% C: Greater than 100% The results are shown in Tables 5 and 6.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

As shown in Tables 1-6, steel plates with a surface treatment containing a predetermined compound exhibit excellent resistance to red rust. Conversely, steel plates without a predetermined compound on their end faces exhibit poor resistance to red rust.

Industrial Applicability According to this disclosure, a surface-treated steel that can suppress the formation of red rust on non-plated areas can be provided. Therefore, it has high industrial applicability.

1: Surface-treated steel plate (surface-treated steel material) 11: Steel plate 12: Plating layer (Zn-based plating layer) 31: Compound 41: Non-plated area 101: Surface 102: End face 103: Plating surface IF1: First boundary IF2: Second boundary R1: First region R2: Second region SFC: Surface of compound CP1: Central part of the first region CP2: Central part of the second region

Figure 1 is a schematic diagram illustrating one form of the surface-treated steel of this embodiment, namely, a surface-treated steel sheet. Figure 2 is a schematic diagram illustrating an example of a cross-section of the surface-treated steel of this embodiment at the location where the compound is formed on the non-plated surface. Figure 3 is a diagram illustrating an example of a halo pattern from FIB-TEM electron diffraction.

11: Steel Plate

31: Compounds

IF1: First Boundary

IF2: Second Boundary

R1: First Region

R2: Second Region

SFC: Surface of a Compound

CP1: Central Section of Region 1

CP2: Central Section of the Second District

Claims (11)

A surface-treated steel material, characterized by comprising: steel and a plating layer, the plating layer being formed on at least a portion of the surface of the steel and comprising Zn; when the portion of the surface of the steel where the plating layer is not formed is defined as a non-plated portion, at least a portion of the non-plated portion contains a compound containing Zn and Mg; Using an FE-TEM with added energy dispersive X-ray diffraction, the concentration distribution of O, Mg, Al, Fe, and Zn was continuously measured linearly along the thickness direction of the compound from the surface of the aforementioned unplated steel towards the surface of the aforementioned surface-treated steel. The location where the Mg concentration first reaches 0.5 atomic percent or more was defined as the first boundary. The location where the Mg concentration first becomes less than 0.5 atomic percent closer to the surface of the surface-treated steel than the first boundary was defined as the second boundary. At this point, the area between the first and second boundaries constitutes the first region of the compound, and the area between the second boundary and the aforementioned surface of the compound constitutes the second region of the compound. The central portion of the first and second boundaries along the thickness direction is the central portion of the first region. In the central portion of this first region, the atomic ratio of Mg to Zn (Mg/Zn) is 0.090 or more. The aforementioned thickness direction, specifically the second boundary and the central portion of the aforementioned compound's surface, constitutes the second central region, in which the atomic ratio of Mg to Zn is less than 0.090. For example, in the surface-treated steel of claim 1, the chemical composition of the aforementioned coating, by mass%, comprises: Zn: greater than 50.0%, Al: greater than 0.2% to less than 40.0%, Mg: greater than 0.2% to less than 12.5%. For example, the surface-treated steel of claim 1, wherein the aforementioned first region displays a halo pattern in an electron beam diffraction pattern obtained by passing through an electron microscope. For example, the surface-treated steel of claim 2, wherein the aforementioned first region displays a halo pattern in an electron beam diffraction pattern obtained by passing through an electron microscope. For any of the surface-treated steel items 1 to 4, wherein the aforementioned compound is present in more than 50% by area on the surface of the aforementioned non-plated portion of the aforementioned steel. The surface-treated steel as described in any of claims 1 to 4, wherein the thickness of the aforementioned first region is 0.05 μm or more. For example, in the surface-treated steel of claim 5, the thickness of the aforementioned first region is 0.05 μm or more. For surface-treated steel as described in any of claims 1 to 4, wherein the thickness of the aforementioned second region is 0.05 μm or more. For example, in the surface-treated steel of claim 5, the thickness of the aforementioned second region is 0.05 μm or more. For example, in the surface-treated steel of claim 6, the thickness of the aforementioned second region is 0.05 μm or more. For example, in the surface-treated steel of claim 7, the thickness of the aforementioned second region is 0.05 μm or more.