Electrogalvanized hot-rolled steel sheet and its manufacturing method

Abstract

To provide an electro-galvanized hot rolled steel sheet that has superior appearance uniformity, glossiness and whiteness, and a method of manufacturing the same.SOLUTION: The present invention relates to a hot rolled steel sheet which has an electro-galvanization layer on a top surface of a hot rolled steel sheet, wherein the surface of the electro-galvanization layer has a maximum height roughness Rz of 4.0 to 20.0 μm, and a smooth part of the electro-galvanization layer has a number density of 10 or less per sectional length of 2.5 mm of the electro-galvanization layer. Here, the smooth part is an area of 1.0 μm or less in maximum height roughness Rz when a roughness curve of the surface of the electro-galvanization layer is divided into areas of 0.02 mm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrogalvanized hot-rolled steel sheet, and more particularly to an electrogalvanized hot-rolled steel sheet having excellent surface appearance uniformity and gloss and whiteness suitable for home appliance applications, and a method for producing the same. [Background technology]

[0002] Electrogalvanized steel sheets are used in a wide range of applications, including home appliances, automobiles, and building materials. In particular, electrogalvanized steel sheets for home appliances, which are used unpainted, are required to have excellent surface appearance.

[0003] Electrogalvanized steel sheets are characterized by a white appearance with little gloss, but are also required to have excellent uniformity in appearance, i.e., no surface defects such as unevenness, and little difference in gloss depending on the position when viewed from various angles. In other words, the surface appearance of electrogalvanized steel sheets is required to have excellent uniformity in appearance, a moderate gloss, and high whiteness.

[0004] Since the electrogalvanized layer is formed to conform to the surface irregularities of the substrate steel sheet, the surface appearance of the electrogalvanized steel sheet to which the electrogalvanized layer is applied is largely dependent on the surface properties of the substrate steel sheet. For this reason, a cold-rolled steel sheet with minimal surface irregularities is usually used as the substrate steel sheet for the electrogalvanized steel sheet.

[0005] On the other hand, it is known that hot-rolled steel sheets, which are manufactured through a hot-rolling process and a pickling process for removing oxide scale formed on the surface, have greater surface irregularities than cold-rolled steel sheets. Therefore, when electrogalvanizing is performed using a hot-rolled steel sheet as a substrate steel, differences in gloss occur depending on the position due to differences in local surface irregularities, resulting in a problem of inferior appearance uniformity compared to electrogalvanized steel sheets using a cold-rolled steel sheet as a substrate steel.

[0006] As a method for improving the appearance uniformity of an electrogalvanized steel sheet using a hot-rolled steel sheet as a base steel sheet, Patent Document 1 discloses that an electrogalvanized steel sheet with excellent appearance can be produced by pickling the hot-rolled steel sheet and then etching it with an aqueous ferric salt solution. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 11-131264 Summary of the Invention [Problem to be solved by the invention]

[0008] In recent years, the surface appearance of electro-galvanized hot-rolled steel sheets using hot-rolled steel sheets as the base steel sheet has come to be required to have extremely high surface appearance properties, such as excellent appearance uniformity, moderate gloss, and high whiteness, similar to that of electro-galvanized steel sheets using cold-rolled steel sheets as the base steel sheet.

[0009] However, the method using the etching treatment disclosed in Patent Document 1 could not fully satisfy the extremely high level of surface appearance properties that have recently been required of electro-galvanized hot-rolled steel sheets. In particular, when the surface unevenness of the base hot-rolled steel sheet varies greatly depending on the position, the difference in surface unevenness depending on the position remains even after the etching treatment, causing a difference in gloss depending on the position on the electro-galvanized hot-rolled steel sheet, making it impossible to obtain an electro-galvanized hot-rolled steel sheet with excellent uniformity in appearance.

[0010] The present invention has been made to solve the above problems and has an object to provide an electro-galvanized hot-rolled steel sheet having excellent surface appearance, and in particular, an object to provide an electro-galvanized hot-rolled steel sheet having excellent appearance uniformity, appropriate gloss, and high whiteness. [Means for solving the problem]

[0011] In order to solve the above problems, the present inventors have conducted extensive research into electrogalvanized hot-rolled steel sheets with excellent surface appearance.

[0012] As a result, it was revealed that by subjecting the surface of a hot-rolled steel sheet to a blasting treatment under specified conditions and then performing an electrogalvanized layer formation treatment, an electrogalvanized hot-rolled steel sheet having a specified surface roughness can be obtained, and that this electrogalvanized hot-rolled steel sheet has excellent appearance uniformity, as well as moderate gloss and high whiteness.

[0013] The present invention was made based on these findings, and the gist of the present invention is as follows. [1] An electrogalvanized hot-rolled steel sheet having an electrogalvanized layer on the surface of the hot-rolled steel sheet, An electrogalvanized hot-rolled steel sheet, wherein the maximum height roughness Rz of the surface of the electrogalvanized layer is 4.0 μm or more and 20.0 μm or less, and the number density of smooth portions on the surface of the electrogalvanized layer is 10 or less per 2.5 mm of cross-sectional length of the electrogalvanized layer. Here, the smooth portion refers to a region where the maximum height roughness Rz is 1.0 μm or less when the roughness curve of the surface of the electrogalvanized layer is divided into regions of 0.02 mm each. [2] A method for producing an electro-galvanized hot-rolled steel sheet according to [1], characterized in that the surface of the hot-rolled steel sheet is subjected to a blast treatment with particles having an average particle size of 60 μm or more and 300 μm or less, and then an electro-galvanized layer is formed.

[0014] Here, the maximum height roughness Rz of the surface and the roughness curve are based on the specifications of JIS B0601-2001. [Effects of the Invention]

[0015] According to the present invention, it is possible to obtain an electrogalvanized hot-rolled steel sheet having excellent surface appearance, in particular, an electrogalvanized hot-rolled steel sheet having excellent appearance uniformity, appropriate gloss, and high whiteness. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0017] The electrogalvanized hot-rolled steel sheet of the present invention is an electrogalvanized hot-rolled steel sheet having an electrogalvanized layer on its surface, wherein the surface of the electrogalvanized layer has a maximum roughness height Rz of 4.0 μm to 20.0 μm, and the number density of smooth portions on the surface of the electrogalvanized layer is 10 or less per 2.5 mm cross-sectional length of the electrogalvanized layer. Here, the smooth portions refer to regions having a maximum roughness height Rz of 1.0 μm or less when the roughness curve of the surface of the electrogalvanized layer is divided into regions every 0.02 mm.

[0018] The electrogalvanized hot rolled steel sheet of the present invention that satisfies the above properties has excellent uniformity in appearance, as well as appropriate gloss and high whiteness.

[0019] The appearance uniformity is determined by measuring the gloss of the electro-galvanized hot-rolled steel sheet using a gloss meter and determining the difference in gloss between the lowest and highest gloss points in the measured areas. The electro-galvanized hot-rolled steel sheet is determined to be excellent when there is little difference in surface gloss between different parts.

[0020] The appropriate gloss level varies depending on the application and location of the electro-galvanized hot-rolled steel sheet, but if the gloss level is too low, the glossiness is insufficient, and if the gloss level is too high, the reflected light is often deemed to be too strong. In typical applications, a gloss level of 5 to 40 at a measurement angle of 60° is required, with a gloss level of 7 to 30 being more preferred in many cases.

[0021] Higher whiteness is often required for electrogalvanized steel sheets, and is usually evaluated by brightness (L value), with an L value of 75 or higher being desirable.

[0022] The type of hot-rolled steel sheet used in the present invention is not particularly limited. Various steel sheets can be used, such as low-carbon steel sheets, ultra-low-carbon steel sheets, IF steel sheets, and high-tensile steel sheets containing various alloying elements. The thickness of the steel sheet is not particularly limited, but is usually 1.2 to 3.2 mm.

[0023] In the present invention, the term "hot-rolled steel sheet" refers to a hot-rolled steel sheet after pickling, which is produced through a hot-rolling process and a pickling process for removing oxide scale formed on the surface. The pickling method used in the pickling process is not particularly limited, and any method can be used as long as it can remove oxide scale from the steel sheet surface after the hot-rolling process, such as immersion in an aqueous solution of hydrochloric acid or sulfuric acid.

[0024] The surface of a hot-rolled steel sheet has large-pitch irregularities with a width of about 0.1 to 0.2 mm. The present invention is characterized in that, while maintaining the large-pitch irregularities with a width of about 0.1 to 0.2 mm, a blasting treatment is further performed to intentionally form small-pitch irregularities, thereby imparting appropriate irregularities to the surface of the hot-rolled steel sheet. When the hot-rolled steel sheet obtained in this manner is subjected to an electrogalvanized layer formation treatment, an electrogalvanized hot-rolled steel sheet with excellent appearance uniformity and little difference in gloss depending on the position when viewed from different angles can be obtained.

[0025] The maximum height roughness Rz of the electrogalvanized layer surface of the electrogalvanized hot-rolled steel sheet of the present invention is set to 4.0 μm or more and 20.0 μm or less. If Rz is less than 4.0 μm, the surface will be too smooth, resulting in high gloss and low whiteness, and the desired surface appearance will not be obtained. On the other hand, if Rz exceeds 20.0 μm, the unevenness will be too large, resulting in low gloss and whiteness, and the desired surface appearance will not be obtained. The lower limit of the maximum height roughness Rz is preferably 5.0 μm, more preferably 7.0 μm. On the other hand, the upper limit of the maximum height roughness Rz is preferably 15.0 μm, more preferably 13.0 μm.

[0026] In the present invention, the number density of smooth portions on the surface of the electrogalvanized layer is 10 or less per 2.5 mm of cross-sectional length of the electrogalvanized layer, where the smooth portions refer to regions having a maximum roughness height Rz of 1.0 μm or less when the roughness curve of the surface of the electrogalvanized layer is divided into regions every 0.02 mm.

[0027] The surface of a hot-rolled steel sheet has smooth areas with little localized unevenness, and these smooth areas may remain even after blasting. If too many of these smooth areas remain, local differences in gloss will be noticeable even after the subsequent electrogalvanized layer formation process, resulting in insufficient uniformity in appearance. Therefore, the number density of the smooth areas is set to 10 or less per 2.5 mm cross-sectional length of the electrogalvanized layer. The upper limit of the number density of the smooth areas is preferably 6 or less, more preferably 3 or less, per 2.5 mm cross-sectional length of the electrogalvanized layer.

[0028] The coating weight of the electrogalvanized layer in the electrogalvanized hot-rolled steel sheet of the present invention is not particularly limited, but is preferably 4 to 30 g / m 2 4g / m is preferred. 2 If the coating weight is less than 30 g / m, the corrosion resistance improving effect inherent to the electrogalvanized layer will be insufficient. 2 If it exceeds this, the cost will be high.

[0029] In the electrogalvanized hot-rolled steel sheet of the present invention, an inorganic coating, an organic coating, or a composite coating of these may be applied as an upper layer to the electrogalvanized layer, depending on the purpose.

[0030] Next, a method for producing an electrogalvanized hot-rolled steel sheet according to the present invention will be described.

[0031] In the method for producing an electrogalvanized hot-rolled steel sheet according to the present invention, the hot-rolled steel sheet is subjected to a blast treatment, followed by an electrogalvanized layer formation treatment. The blast treatment provides appropriate irregularities to the surface of the hot-rolled steel sheet serving as the base steel sheet, and the subsequent electrogalvanized layer formation treatment can achieve a maximum height roughness Rz of 4.0 μm or more on the surface of the electrogalvanized layer on the electrogalvanized hot-rolled steel sheet.

[0032] The average particle size of the particles used in the blasting treatment is 60 μm or more and 300 μm or less. If the average particle size is less than 60 μm, many smooth portions will be formed, and the number density of smooth portions on the surface of the electrogalvanized layer will exceed 10. From the above viewpoint, it is more preferable that the average particle size is 100 μm or more. On the other hand, if the average particle size exceeds 300 μm, the surface roughness of the electrogalvanized hot-rolled steel sheet will be too high, and the maximum height roughness Rz of the surface of the electrogalvanized layer will exceed 20.0 μm. From the above viewpoint, it is more preferable that the average particle size is 250 μm or less.

[0033] The shape of the particles used in the blasting treatment is not particularly limited, and the shape of the particles may be any shape, such as spherical, oval, or polygonal, as long as the average particle size satisfies the above range.

[0034] The material of the particles used in the blasting treatment is not particularly limited, and examples thereof include metal particles such as iron, stainless steel, zinc, aluminum, and copper, ceramic particles such as alumina and zirconia, glass particles such as soda-lime glass, and resin particles such as nylon resin, urea resin, and melamine resin.

[0035] It is more preferable that the particles used in the blasting treatment are metal particles containing 50% by mass or more of Fe. Examples of such particles include iron powder and stainless steel particles. These particles are relatively inexpensive and easily available.

[0036] In the present invention, the blasting method is not particularly limited. For example, shot blasting in which particles are projected using a centrifugal blaster, sand blasting in which particles are projected using compressed air, wet blasting in which particles and water are projected using compressed air, etc. can be used.

[0037] In the present invention, the number of times of blasting is not particularly limited. The number of times of blasting may be one, or two or more times. From the viewpoint of reliably forming unevenness by the blasting, the number of times of blasting is preferably two or more. On the other hand, from the viewpoint of processing costs, the number of times of blasting is preferably 20 or less.

[0038] In the present invention, the surface of a hot-rolled steel sheet is subjected to a particle blasting treatment, and then an electrogalvanized layer is formed. The method for forming the electrogalvanized layer is not particularly limited, and all known electrogalvanizing methods are applicable. Prior to forming the electrogalvanized layer, electrolytic degreasing or pickling may be performed as necessary. Furthermore, when an inorganic coating, an organic coating, or a composite coating thereof is applied on top of the electrogalvanized layer, the method for forming these coatings is not particularly limited, and any of coating methods such as coating, reaction, and electrolytic methods may be applied. [Example]

[0039] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0040] A hot-rolled steel sheet having a thickness of 2.6 mm was used as a base steel sheet for producing an electrogalvanized hot-rolled steel sheet.

[0041] Next, the hot-rolled steel sheets were subjected to blasting under the conditions shown in Table 1. Wet blasting involved projecting particles mixed with water at a compressed air pressure of 0.2 MPa. Shot blasting was performed at an impeller rotation speed of 1000 rpm. Sand blasting was performed at a compressed air pressure of 0.2 MPa. All wet blasting, shot blasting, and sand blasting were performed at a processing speed of 100 mm / s.

[0042] The particles used for blasting were stainless steel, iron, alumina, zirconia, and melamine resin. The particles used for blasting were spherical particles with an average circularity of 0.9, and polygonal particles with an average circularity of 0.9 or less. Here, circularity indicates the degree of unevenness of the particle surface, and refers to Wadell's circularity, which can be calculated using the following formula.

[0043] Circularity = perimeter of a circle with the same projected area / perimeter of the particle Here, "perimeter of a circle with equal projected area" refers to the length of the contour of a circle equal to the area of ​​the particle's shadow projected onto a flat surface when the particle is observed from directly above. "Particle perimeter" refers to the length of the contour of the particle's shadow projected onto a flat surface when the particle is observed from directly above. The smaller the degree of roughness of the particle surface, the closer the circularity is to 1.

[0044] For comparison, some test pieces were prepared without blasting or by surface grinding using a Hotani D-100 grinding brush roll with a reduction of 3 mm.

[0045] Hot-rolled steel sheets that had been blasted under the conditions shown in Table 1, hot-rolled steel sheets that had not been blasted, and hot-rolled steel sheets that had been surface-ground under the above conditions were subjected to alkaline electrolytic degreasing and pickling, and then to an electrogalvanized layer formation process. The electrogalvanized layer formation process involved placing the steel sheets as the cathode in an electrogalvanized bath containing 440 g / L of zinc sulfate heptahydrate, pH 1.5, and a bath temperature of 50°C, at 50 A / dm 2 The electrolysis time was varied to form a predetermined amount of electrolytic zinc plating layer.

[0046] The test pieces obtained as described above were evaluated as follows. The evaluation results are shown in Table 1. (1) Zinc coating thickness The coating weight of the electrogalvanized layer was measured by the following method. The plating layer of a 50 mm x 50 mm test piece was dissolved in hydrochloric acid, and the amount of Zn contained in the plating layer was measured using an ICP emission spectrometer, and the plating adhesion weight was calculated. (2) Maximum height roughness Rz of the electrogalvanized layer surface Each test piece was measured using a stylus-type surface roughness meter with a cone shape, a tip radius of 2 μm, and a tip angle of 60°, with a measurement length of 2.5 mm, according to JIS B0601-2001. The obtained cross-sectional curve was used to obtain a roughness curve with a cutoff wavelength of 0.8 mm. The maximum height roughness Rz was calculated from this roughness curve. The measurement direction using the surface roughness meter was the rolling direction of the steel sheet. (3) Number density of smooth areas on the surface of the electrogalvanized layer The roughness curve obtained in (2) above was divided into regions every 0.02 mm, the maximum height roughness Rz of each region was calculated, and the number of regions with an Rz of 1.0 μm or less was determined. This number of regions was taken as the number density of smooth portions per 2.5 mm of cross-sectional length. (4) Appearance uniformity The gloss level was measured at 10 randomly selected points on each 150mm x 70mm test piece using a Konica Minolta GM-60S gloss meter with a measurement diameter of 2mm x 4mm and a measurement angle of 60°, and the appearance uniformity was evaluated based on the difference in gloss level between the lowest and highest gloss levels. Appearance uniformity was judged according to the following criteria, with ◎ or ○ representing pass and × representing fail. ◎: Gloss difference ≦ 2 ○: 2<gloss difference≦5 ×: 5<Difference in glossiness (5) Glossiness The gloss of each test piece was measured using a Nippon Denshoku PG-1M with a measurement diameter of 10.5 mm x 60 mm and a measurement angle of 60°. The gloss was judged according to the following criteria, with ◎ or ○ representing pass and × representing fail. ◎: 7≦glossiness≦30 ○: 5≦glossiness<7 or 30<glossiness≦40 ×: Glossiness < 5 or 40 < glossiness (6) Whiteness For each test piece, the L value was measured using a Konica Minolta CM-2600d spectrophotometer in SCE (specular reflection excluded) mode. The whiteness was judged according to the following criteria, with ⊚ or ○ representing pass and × representing fail. ◎: 80≦L value 〇: 75≦L value<80 ×: L value < 75

[0047] [Table 1]

[0048] As is clear from Table 1, all of the electrogalvanized hot-rolled steel sheets of the present invention have excellent uniformity in appearance, as well as appropriate gloss and high whiteness. [Industrial Applicability]

[0049] The electrogalvanized hot-rolled steel sheets of the present invention have excellent uniformity in appearance, as well as moderate gloss and high whiteness, and are therefore applicable in a wide range of fields such as home appliances, automobiles, building materials, etc. In particular, they are suitable for use in home appliances where they are used without coating.

Claims

1. An electrogalvanized hot-rolled steel sheet having an electrogalvanized layer on a surface of the hot-rolled steel sheet, an electrogalvanized hot-rolled steel sheet in which the surface roughness in maximum height Rz of the electrogalvanized layer is 4.0 μm or more and 20.0 μm or less, and the number density of smooth portions on the surface of the electrogalvanized layer is 10 or less per 2.5 mm cross-sectional length of the electrogalvanized layer. Here, the smooth portion refers to a region where the maximum height roughness Rz is 1.0 μm or less when the roughness curve of the surface of the electrogalvanized layer is divided into regions of 0.02 mm each.

2. 2. The method for producing an electro-galvanized hot-rolled steel sheet according to claim 1, wherein the surface of the hot-rolled steel sheet is subjected to a blast treatment with particles having an average particle size of 60 μm or more and 300 μm or less, and then an electro-galvanized layer is formed.

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