JP7842349B2 - Zinc-plated steel sheet with excellent press formability, chemical treatment properties, and appearance quality, and method for manufacturing the same. - Google Patents

Description

This invention relates to a zinc-plated steel sheet with excellent press-formability, chemical conversion treatment properties, and appearance quality, and to a method for producing the same.

Zinc-plated steel sheets are used in automobile bodies and other components to improve corrosion resistance and appearance. In the automobile manufacturing process, automobile manufacturers produce automobile bodies by press-forming steel sheets. Therefore, zinc-plated steel sheets, which are formed into complex shapes for automobile bodies, require excellent press-formability. To date, coating technologies have been developed to impart high lubricity to the steel sheet surface.

Among these, phosphate-based coatings have been widely used as lubricating coatings because they exhibit excellent lubricity and can relatively easily form a uniform, well-adhering coating by utilizing the chemical reaction between the plated surface and the treatment solution. For example, Patent Document 1 shows a galvanized steel sheet having a pre-phosphate coating mainly composed of zinc phosphate. In this process, surface preparation using a titanium colloid-containing aqueous solution or zinc phosphate colloid-containing water is necessary as a pretreatment to form a dense coating.

On the other hand, manganese phosphate coatings are known to be hard among phosphate coatings and have a high effect in suppressing galling of the plating onto the mold even when formed under high surface pressure, thus providing a high lubricating effect even in thin films. For example, Patent Documents 2 to 7 disclose a technology for improving the press formability of steel sheets by applying a phosphate-based inorganic lubricating coating, mainly composed of manganese phosphate, to the plated surface.

Plated steel sheets are shipped with rust-preventive oil applied after manufacturing, and then painted after press forming and joining by automobile manufacturers. In the painting process, alkaline degreasing is first performed to remove oil from the steel sheet surface. However, if the steel sheet has been stored for a long period with the oil coating still on, causing strong adhesion between the rust-preventive oil and the steel sheet, or if the degreasing solution has been used repeatedly and has become contaminated with oil and metal ions, weakening its degreasing power, then the degreasing may not be sufficient, potentially negatively impacting subsequent chemical conversion treatments and the appearance after painting.

Specifically, if oil remains on the steel plate surface even after degreasing, the formation reaction of the chemical conversion coating is inhibited in those areas, resulting in an uneven coating and, consequently, an uneven appearance after painting. This problem is particularly common in steel plates coated with lubricating coatings, including the aforementioned phosphate-based coatings, which are intended to improve press formability. Because the coating itself may have the effect of enhancing lubricant retention, this issue is more likely to occur, and improvement technologies have been needed.

Patent No. 4100358 Patent No. 2131655 Patent No. 2691797 Patent No. 3199980 Patent No. 4786769 Patent No. 5146607 Patent No. 5168417

This invention overcomes the above problems and provides a zinc-plated steel sheet and a method for manufacturing the same, which exhibits excellent press formability, chemical treatment properties, and appearance quality.

The gist of this invention is as follows:
(1) A zinc-plated steel sheet having excellent press formability, chemical treatment properties, and appearance quality, characterized in that an inorganic film is applied to the zinc-plated surface of a hot-dip galvanized steel sheet, comprising manganese phosphate, one or more salts from chlorides, sulfates, fluorides, and bromides , and the inorganic film is characterized in that Mn: 1.0 mg/ ^m² to 100 mg/ ^m² , P: 10.0 mg/ ^m² to 100 mg/ ^m² , and the total number of moles [A] of one or more of Cl, S, F, and Br derived from chlorides, sulfates, fluorides, and bromides is 1.0 × ^10⁻⁵ mol/ ^m² to 5.0 × ^10⁻³ mol/ ^m² .
(2) The zinc-plated steel sheet described in (1), characterized in that, in the inorganic film, the ratio [A]/[P] of the sum of the number of moles [A] in the range from the outermost layer to 2 nm as measured by an X-ray photoelectron spectroscopy analyzer to the number of moles of P [P] is 0.01 or more and 5.0 or less.
(3) A method for producing a zinc-plated steel sheet with excellent press formability, chemical treatment properties, and appearance quality, characterized by containing an aqueous solution containing manganese ions or permanganate ions in a concentration of 0.01 mol/L or more and 2.5 mol/L or less, phosphate ions in a concentration of 0.6 mol/L or more and 3.0 mol _/L or less, and one or more of chloride ions, sulfate ions, fluoride ions, and bromide ions added to the solution not as an acid but as an alkali metal salt, ammonium salt, or salt of a zinc compound, such that the ratio of the molar concentration of the sum of these ions [ _AL ] to the molar concentration of phosphate ions [ _PL ] [ _AL ] is 0.2 or more and 1.5 or less, and containing residual unavoidable impurities, in contact with the plated surface for a time of 0.1 seconds or more and 10 seconds or less, followed by drying, to obtain the zinc-plated steel sheet described in (1) or (2).

This invention provides a zinc-plated steel sheet with excellent press formability, chemical treatment properties, and appearance quality.

This diagram illustrates the relationship between [A] in the coating, chemical treatment properties, and sliding properties. This diagram illustrates the relationship between the [ _AS ]/[ _PS ] ratio in the coating, chemical treatment properties, and sliding properties.

The details of this invention are described below.

As a result of diligent research to solve the aforementioned problems, we discovered that by incorporating an appropriate amount of compounds containing halogen ions such as fluoride ions, chloride ions, and bromide ions, and oxoacid ions such as sulfate ions (hereinafter referred to as Ion Group A) into an inorganic lubricating film mainly composed of manganese phosphate, as described in Patent Document 2, etc., the chemical conversion treatment properties can be improved without impairing the sliding properties.

The metal salts composed of the aforementioned ion group A have higher solubility in alkaline aqueous solutions compared to manganese phosphate, which is the main component of the inorganic film. Therefore, it is believed that good chemical conversion treatment properties are obtained when a portion of the film, along with the rust-preventive oil, dissolves and detaches during alkaline degreasing treatment.

Furthermore, a detailed investigation into the relationship between the surface condition of the inorganic coating, chemical conversion treatment properties, and surface appearance after painting revealed that the higher the relative abundance of ion group A relative to phosphate ions on the surface, the more uniform the crystal size of the formed chemical conversion coating becomes, resulting in a more beautiful surface appearance after painting. This is thought to be because the presence of ion group A on the surface accelerates the desorption reaction of rust-preventive oil. Even if there are areas where the degree of adsorption of rust-preventive oil between the steel plate and the rust-preventive oil is strong locally, the degreasing solution from the surrounding area penetrates further, allowing the rust-preventive oil to be removed from the entire surface, thus increasing the uniformity of degreasing and chemical conversion treatment properties.

Furthermore, since the degreasing process is performed after the steel plates are pressed and assembled, the removal of a portion of the inorganic lubricating film during the degreasing process does not impair the inherent property of the lubricating film, which is to improve the sliding properties of the steel plates during pressing. The embodiments of the present invention will be described in detail below.

The inorganic film on the base hot-dip galvanized surface contains manganese in the range of 1.0 mg/ ^m² to 100 mg/ ^m² . If the manganese content is less than 1.0 mg/ ^m² , adhesion of the plating to the mold cannot be sufficiently suppressed, and sufficient moldability cannot be ensured. If the manganese content exceeds 100 mg/ ^m² , the effect of suppressing plating adhesion saturates, and the chemical conversion treatment properties also deteriorate in this invention.

Furthermore, the film contains phosphorus in a range of 10.0 mg/ ^m² to 100 mg/ ^m² . If the phosphorus content is less than 10.0 mg/ ^m² , sufficient compatibility with the rust-preventive oil during processing and sliding cannot be achieved, and adequate moldability cannot be ensured. If the phosphorus content exceeds 100 mg/ ^m² , the compatibility effect with the rust-preventive oil becomes saturated, and the chemical conversion treatment properties also deteriorate in this invention.

The appropriate amount of ion group A to be contained in the coating is determined by the sum of the moles of ion group A contained in the coating per unit area, as shown in Figure 1. When [A] is between 1.0 × ^10⁻⁵ mol/ ^m² and 5.0 × ^10⁻³ mol/ ^m² , good sliding properties are ensured while improving the chemical conversion treatment properties. If the content of [A] is less than 1.0 × ^10⁻⁵ mol/ ^m² , the improvement in chemical conversion treatment properties is not obtained, and conversely, if it exceeds this amount, it becomes difficult to continuously form a hard manganese phosphate coating that ensures sliding properties, and the press formability deteriorates.

Furthermore, regarding the relative abundance of ion group A in the outermost layer of the inorganic film, as shown in Figure 2, increasing this ratio is preferable because it increases the uniformity of the degreasing and chemical conversion treatment reactions, as described above, and allows for more uniform size of the chemical conversion film crystals. To achieve this, the ratio [ _AS ]/[ _PS ] of the total number of moles of one or more ion group A present in the outermost layer of the inorganic film _to the number of moles of P present in the outermost layer [ _PS ] should be 0.01 or higher. On the other hand, if [ _AS ]/[ _PS ] becomes too large, the sliding properties tend to be somewhat inferior, so the upper limit should be 5.0 or less.

Here, the outermost layer of the inorganic film is defined as the average value obtained by removing impurities and dirt attached to the surface by Ar sputtering using an X-ray photoelectron spectroscopy analyzer (e.g., ULVAC-PHI PHI5800), and measuring the atomic concentration at three equally spaced points from the point where the atomic concentration of carbon (C) is 10% or less, to 2 nm in depth, at 1 nm intervals toward the steel plate. Atomic concentration is a value obtained by dividing the area intensity, obtained by removing the background from the peaks of each element detected from the measurement site using an X-ray photoelectron spectroscopy analyzer, by the relative sensitivity coefficient set for each instrument, and can be calculated using analysis software.

Furthermore, even if the above-mentioned coating contains unavoidable impurities in addition to the components contained in the hot-dip galvanizing process, it does not affect the properties of the steel sheet, such as sliding properties or chemical conversion treatment properties. Examples of unavoidable impurities include Li, Be, C, F, Na, Mg, Al, Si, Cl, K, Ca, Ni, Mo, V, W, Ti, Fe, Rb, Sr, Y, Zn, Nb, Cs, Ba, Cr, Cd, Pb, Sn, As, and lanthanides.

The hot-dip galvanized steel sheet that forms an inorganic film can be any hot-rolled or cold-rolled steel sheet with a metal film containing 50% or more Zn by mass on its surface. The plating composition may not only contain pure Zn, but also Zn-Fe, Zn-Ni, Zn-Al, Zn-Mn, Zn-Cr, etc., where Zn is the main component and one or more alloying elements and impurity elements such as Fe, Ni, Co, Al, Pb, Sn, Sb, Cu, Ti, Si, B, P, N, S, O, etc. In particular, a so-called alloyed hot-dip galvanized steel sheet, obtained by immersing the steel sheet in a Zn plating bath containing a small amount of Al and then heat-treating it to incorporate approximately 5-20% Fe in the plating phase relative to the total mass of the plating, exhibits excellent corrosion resistance after painting and spot weldability, making it suitable as an automotive steel sheet for use in this invention.

While there are no particular restrictions on the amount of plating applied, considering the corrosion resistance required for automotive steel sheets, it is preferable that the total amount of all components constituting the plating be 20 g/ ^m² or more. If the amount of plating is too high, there will be a lot of peeling during steel sheet processing, which may remain in the mold and cause indentations, so it is preferable that the upper limit be 100 g/ ^m² . Furthermore, this invention is not affected in any way by the properties of the steel sheet that serves as the base material for hot-dip galvanizing, and there are no restrictions on strength class, composition, or microstructure. In addition, there are no restrictions on the raw materials used in manufacturing the steel sheet or the manufacturing method of the steel sheet.

The method for producing this coating involves contacting a solution containing manganese ions or permanganate ions and phosphoric acid, and also containing one or more of the following ions—chloride ions, sulfate ions, fluoride ions, and bromide ions—added to the aqueous solution as a salt (not an acid, but an alkali metal salt, ammonium salt, or zinc compound), with the plated surface, and then drying it.

The oxoate ions and halogen ions mentioned above are strong acids or acids with very strong etching power. If they are mixed into the treatment solution in their acidic form, the dissolution reaction on the steel sheet surface may proceed excessively upon contact between the solution and the steel sheet, potentially causing surface defects. Therefore, they must be added to the treatment solution as salts.

Preferably, the salts used are alkali metal salts, ammonium salts, and zinc compounds. The cations and ion group A compounds contained in these salts have high solubility in aqueous solutions, thus not adversely affecting the chemical conversion treatment improvement effect, nor do they adversely affect other steel sheet properties such as sliding properties and corrosion resistance.

The composition of the processing solution used can be adjusted to achieve an appropriate film state by considering the method and contact time of contacting the plated surface with a solution containing manganese ions or permanganate ions, phosphoric acid, and ions from group A. Preferably, the molar concentration of phosphorus [ _PL ] is 0.6 mol/L or more and 3.0 mol/L or less, the ratio of the molar concentration of phosphorus to the total molar concentration of one or more ions from group A [ _AL ] [ _AL ] / [ _PL ] is 0.2 or more and 1.5 or less, and it is desirable to contact this solution with the plated surface for a time of 0.1 seconds or more and 10 seconds or less, and then dry it.

In this invention, the inorganic film mainly composed of manganese phosphate formed on the plated surface is formed by a mechanism in which sparingly soluble phosphate compounds precipitate, utilizing the increase in pH that occurs when the plated surface is dissolved by contact with an acidic solution containing phosphoric acid. On the other hand, since ion group A cannot form sparingly soluble compounds on its own in response to pH fluctuations, it is necessary to incorporate ion group A along with the formation of phosphate compounds. Therefore, in order to ensure that ion group A exists in an appropriate state within the inorganic film, it is necessary to control the formation rate of the phosphate compounds and the concentration of ion group A in the solution.

If the [ _PL ] is less than 0.6 mol/L, the film formation rate is slow, and as the phosphate compound forms over time, group A ions are released, preventing sufficient incorporation of group A ions into the film. If it exceeds 3.0 mol/L, the amount of film formed becomes too large, resulting in an excessive amount of phosphate film in typical film formation processes, making it impossible to guarantee the effectiveness of the chemical conversion treatment.

Furthermore, if the ratio [ _AL ]/[ _PL ] is less than 0.2, the required amount of ion group A cannot be incorporated into the film, and a sufficient improvement in chemical conversion treatment performance cannot be obtained. Conversely, if it exceeds 1.5, sludge adheres to the plated surface due to overdissolution by ion group A, causing discoloration of the surface.

The time from when the aforementioned treatment solution comes into contact with the steel plate surface until it dries should be between 0.1 seconds and 10 seconds. Less than 0.1 seconds will not allow for sufficient phosphate film formation, and sliding properties cannot be guaranteed. On the other hand, if it exceeds 10 seconds, over-dissolution of the plating surface by ion group A will cause sludge to adhere to the plating surface, resulting in discoloration.

Furthermore, methods for bringing the steel plate into contact with the above-mentioned treatment liquid include methods such as immersing the steel plate in the treatment liquid and then homogenizing the amount of liquid applied using a ringer roll or gas wiping, or roll coating.

Furthermore, rinsing with water is not always necessary after contacting the steel plate with the treatment solution. If rinsing is performed, the method is not specifically defined, but it can be done by immersion or spraying. In this case, excessive rinsing may reduce the [ _AS ]/[ _PS ] ratio of the inorganic coating surface, so it is important to adjust the rinsing time, water temperature, and water pressure as appropriate.

The drying of the processing liquid or moisture is performed using a blower, hot air furnace, induction heater, etc. It is sufficient that the moisture is removed within a specified time; there are no specific requirements for the drying temperature, but from an economic standpoint, it is preferable to keep it below 200°C.

The content of each element and ion in the coating can be measured using ICP emission spectrometry or similar methods by dissolving the coating in chromic acid. For oxoacid ions, the mol concentration in the coating can be obtained by measuring the content of chlorine, sulfur, and other halogen ions, which are the elements constituting the oxoacid, using the above method. Alternatively, the amount of elements present on the coating surface may be quantitatively evaluated using X-ray fluorescence with a calibration curve created using ICP emission spectrometry or similar methods. The type of oxoacid can be identified by the presence of chlorine and sulfur on the coating surface and within the coating using photoelectron spectroscopy (XPS) or similar methods. Before performing the above analysis, the sample should be ultrasonically cleaned using hexane or ethanol to remove oil and dirt from the surface.

The present invention will be described non-limitingly using examples.
For the galvanized steel sheets to which the inorganic film was applied, alloyed hot-dip galvanized steel sheets (AS, Fe: 10 mass%, Al: 0.2 mass%, remainder Zn) and hot-dip galvanized steel sheets (GI) were used. The treatment solution under the conditions shown in Table 1 was applied using a roll coater, and then dried in a hot-air drying oven at 50°C to form the film.

<Chemical treatment properties>
The sample used was a plated steel sheet coated with the above-mentioned inorganic lubricating film, to which rust-preventive oil (Noxrust 550HN, manufactured by Nippon Parkerizing Co., Ltd.) was applied at a rate of 2.0 g/ ^m² on one side. To degrade the degreasing ability, the sample was immersed for 90 seconds in Fine Cleaner 4380 (40°C, manufactured by Nippon Parkerizing Co., Ltd.), a degreasing solution containing 5 g/L of the above-mentioned rust-preventive oil and 100 ppm each of iron and zinc. Subsequently, it was immersed for 30 seconds in the surface conditioning agent Percolene Z (manufactured by Nippon Parkerizing Co., Ltd.), and then immersed for 90 seconds in PB-3080 (manufactured by Nippon Parkerizing Co., Ltd., liquid temperature 43°C). The chemical conversion treatment film was evaluated by SEM observation (secondary electron beam imaging), with ◎ and ○ according to the evaluation criteria below being considered as passing.
Evaluation criteria:
◎: A dense chemical conversion film with a crystal size of 1 to 10 μm is formed.
○: Although there is some variation in crystal size, a dense and opaque chemical conversion film of 1 to 10 μm is formed.
△: The chemical conversion film is formed without transparency, but there are crystals larger than 10 μm in size. ×: The chemical conversion film is not formed in some areas.

<Sliding properties>
After the coating treatment, samples were cut to a width of 17 mm and a length of 300 mm. After applying 1 g/ ^m² of Knoxrust 550HN (manufactured by Nippon Parkerizing Co., Ltd.), a draw bead test was performed at a tensile speed of 500 mm/min. The pull-out load was measured while varying the clamping load from 200 to 800 kgf (1.96 × 10³ to 7.84 × 10³ N). The slope was determined from a plot with the clamping load on the horizontal axis, and this was multiplied by 1/2 to obtain the coefficient of friction. ◎ and ○ according to the evaluation criteria below were considered passing grades.
Evaluation criteria:
◎: Friction coefficient less than 0.15 ○: Friction coefficient less than 0.20 △: Friction coefficient 0.20 or more and less than 0.30 ×: Friction coefficient greater than 0.30

<Exterior Evaluation>
The appearance of galvanized steel sheets after coating was visually evaluated for unevenness. The observation area was 210 mm x 300 mm, and the evaluation was carried out according to the following criteria, with a circle (○) indicating a pass.
Evaluation criteria:
○: No visible inconsistencies are present.
△: Clearly visible irregularities cover less than 50% of the area.
×: Clearly visible irregularities cover 50% or more of the area.

As shown in Table 1, for samples (levels 1 to 31) manufactured under conditions within the scope of the present invention, we were able to obtain galvanized steel sheets with excellent chemical conversion treatment properties, superior sliding properties, and a good appearance.

Of these, while levels 8 and 17 met the acceptable level for chemical conversion treatment, the [ _AS ]/[ _PS ] ratio on the inorganic film surface was slightly low, resulting in some unevenness in the crystal size of the chemical conversion film.

On the other hand, in comparative examples levels 32 to 34, the total amount of element A in the coating was less than the range of the present invention, and sufficient chemical conversion treatment properties could not be obtained. Furthermore, in level 35, the total amount of element A in the coating was more than the range of the present invention, and sufficient sliding properties could not be obtained.

At levels 36 and 37, the amount of P or Mn in the coating was less than the range of the present invention, and sufficient sliding properties could not be obtained. At levels 38 and 39, the addition of strong acids, sulfuric acid and hydrochloric acid, to the treatment solution resulted in uneven appearance due to uneven treatment.

Claims (3)

A zinc-plated steel sheet exhibiting excellent press formability, chemical treatment properties, and appearance quality, characterized in that an inorganic film is applied to the zinc-plated surface of a hot-dip galvanized steel sheet, comprising manganese phosphate, one or more salts from chlorides, sulfates, fluorides, and bromides, and residual unavoidable impurities, wherein the inorganic film has ^Mn : 1.0 mg/ ^m² to 100 mg/ ^m² , P: 10.0 mg/ ^m² to 100 mg/ ^m² , and furthermore, the total number of moles [A] of one or more of Cl, S, F, and Br derived from chlorides, sulfates, fluorides, and bromides is 1.0 × ^10⁻⁵ mol/m² to 5.0 × ^10⁻³ mol/ ^m² . The zinc-plated steel sheet according to claim 1, characterized in that, in the inorganic coating, the ratio [A]/[P] of the total number of moles [A] in the range from the outermost layer to 2 nm, as measured by an X-ray photoelectron spectroscopy analyzer, to the number of moles of P [P] is 0.01 or more and 5.0 or less. A method for producing a zinc-plated steel sheet with excellent press formability, chemical conversion treatmentability, and appearance quality, characterized by containing an aqueous solution containing manganese ions or permanganate ions in a concentration of 0.01 mol/L or more and 2.5 mol/ _L or less, phosphate ions in a concentration of 0.6 mol/L or more and 3.0 mol/L or less, and one or more of chloride ions, sulfate ions, fluoride ions, and bromide ions added to the solution not as an acid but as an alkali metal salt, _ammonium salt, or salt of a zinc compound, such that the ratio of the molar concentration of the sum of these ions [ _AL ] to the molar concentration of phosphate ions [ _PL ] is 0.2 or more and 1.5 or less, and containing residual unavoidable impurities, in contact with the plated surface for a time of 0.1 seconds or more and 10 seconds or less, followed by drying, to obtain the zinc-plated steel sheet according to claim 1 or 2.