JP7758007B2 - Electrogalvanized hot-rolled steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to electrogalvanized hot-rolled steel sheet, and in particular to electrogalvanized hot-rolled steel sheet with excellent surface appearance, excellent uniformity, and gloss and whiteness suitable for home appliance applications, and a method for producing the same.

Electro-galvanized steel sheets are used in a wide range of applications, including home appliances, automobiles, and building materials. In particular, electro-galvanized steel sheets for home appliances, which are used unpainted, are required to have excellent surface appearance.

Electro-galvanized steel sheets are characterized by a white appearance with little gloss, but they are also required to have excellent uniformity in appearance, with no surface defects such as unevenness, and little variation in gloss depending on the position when viewed from various angles. In other words, the surface appearance of electro-galvanized steel sheets is required to have excellent uniformity in appearance, a moderate level of gloss, and high whiteness.

However, since the electrogalvanized layer is formed to conform to the surface irregularities of the base steel sheet, the surface appearance of the electrogalvanized steel sheet to which the electrogalvanized layer is applied depends greatly on the surface properties of the base steel sheet. For this reason, cold-rolled steel sheets with minimal surface irregularities are usually used as base steel sheets for electrogalvanized steel sheets.

On the other hand, hot-rolled steel sheets, which are manufactured through a hot-rolling process and a pickling process to remove oxide scale formed on the surface, are known to have greater surface irregularities than cold-rolled steel sheets. For this reason, when electrogalvanizing is performed using hot-rolled steel sheets as the base 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 cold-rolled steel sheets as the base steel.

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

Japanese Patent Application Publication No. 11-131264

In recent years, the surface appearance of electro-galvanized hot-rolled steel sheets, which use hot-rolled steel sheets as the base steel sheet, has come to require extremely high-level surface appearance characteristics, such as excellent appearance uniformity, moderate gloss, and high whiteness, just like electro-galvanized steel sheets, which use cold-rolled steel sheets as the base steel sheet.

However, the method using etching disclosed in Patent Document 1 was unable to fully satisfy the extremely high level of surface appearance required of electro-galvanized hot-rolled steel sheets in recent years. In particular, when there is significant variation in surface unevenness depending on the position on the base hot-rolled steel sheet, the variation in surface unevenness depending on the position remains even after etching, causing differences in gloss depending on the position on the electro-galvanized hot-rolled steel sheet, making it impossible to obtain electro-galvanized hot-rolled steel sheets with excellent uniformity in appearance.

The present invention, which was made to solve the above problems, aims to provide an electro-galvanized hot-rolled steel sheet with excellent surface appearance, and in particular, to provide an electro-galvanized hot-rolled steel sheet with excellent appearance uniformity, moderate gloss, and high whiteness.

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

As a result, it was revealed that by blasting the surface of a hot-rolled steel sheet under specified conditions and then performing an electrogalvanized layer formation process, it is possible to obtain an electrogalvanized hot-rolled steel sheet with a specified surface roughness, and that this electrogalvanized hot-rolled steel sheet has excellent appearance uniformity, as well as a moderate gloss and high whiteness.

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 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] 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 diameter of 60 μm or more and 300 μm or less, and then an electro-galvanized layer is formed.

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

The present invention makes it possible to obtain electrogalvanized hot-rolled steel sheets with excellent surface appearance. In particular, it is possible to obtain electrogalvanized hot-rolled steel sheets with excellent appearance uniformity, moderate gloss, and high whiteness.

The following describes an embodiment of the present invention. Note that the present invention is not limited to the following embodiment.

The electrogalvanized hot-rolled steel sheet of the present invention is an electrogalvanized hot-rolled steel sheet having an electrogalvanized layer on the surface of the 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 cross-sectional length of the electrogalvanized layer. Here, the smooth portions refer to regions 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 every 0.02 mm.

The electrogalvanized hot-rolled steel sheet of the present invention, which satisfies the above properties, has excellent appearance uniformity, as well as moderate gloss and high whiteness.

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 among the measured areas. Electro-galvanized hot-rolled steel sheet is determined to be excellent when there is little difference in surface gloss between different areas.

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, it will not be shiny enough, and if the gloss level is too high, it will often be considered that the reflected light is 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 desirable.

Higher whiteness is often required for electro-galvanized steel sheets, and is usually evaluated by brightness (L value), with an L value of 75 or higher being desirable.

The type of hot-rolled steel sheet used in the present invention is not particularly limited. Various steel sheets can be used, including 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 typically 1.2 to 3.2 mm.

In the present invention, hot-rolled steel sheet refers to a hot-rolled steel sheet that has been produced through a hot-rolling process and a pickling process to remove oxide scale formed on the surface. The pickling method used in the pickling process is not particularly limited, and any method that can remove oxide scale from the steel sheet surface after the hot-rolling process can be used, such as immersion in an aqueous solution of hydrochloric acid or sulfuric acid.

The surface of the hot-rolled steel sheet is formed with large-pitch irregularities with a width of approximately 0.1 to 0.2 mm. The present invention is characterized by imparting appropriate irregularities to the surface of the hot-rolled steel sheet by maintaining the large-pitch irregularities with a width of approximately 0.1 to 0.2 mm and then intentionally forming small-pitch irregularities through a blasting process. When the hot-rolled steel sheet obtained in this way is subjected to an electrogalvanized layer formation process, an electrogalvanized hot-rolled steel sheet with excellent appearance uniformity and minimal difference in gloss depending on the position when viewed from different angles can be obtained.

The maximum height roughness Rz of the electrogalvanized layer surface of the electrogalvanized hot-rolled steel sheet of the present invention is 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 achieved. 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 achieved. The lower limit of maximum height roughness Rz is preferably 5.0 μm, more preferably 7.0 μm. On the other hand, the upper limit of maximum height roughness Rz is preferably 15.0 μm, more preferably 13.0 μm.

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. Here, the smooth portions refer to regions 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 every 0.02 mm.

The surface of hot-rolled steel sheet contains smooth areas with minimal localized irregularities, and these smooth areas may remain even after blasting. If too many of these smooth areas remain, localized 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 smooth areas is set to 10 or less per 2.5 mm of cross-sectional length of the electrogalvanized layer. The upper limit of the number density of smooth areas is preferably 6 or less per 2.5 mm of cross-sectional length of the electrogalvanized layer, and more preferably 3 or less.

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/ ^m2 . If it is less than 4 g/ ^m2 , the corrosion resistance-improving effect inherent to the electrogalvanized layer will be insufficient. On the other hand, if it exceeds 30 g/ ^m2 , the cost will be high.

The electrogalvanized hot-rolled steel sheet of the present invention may have an inorganic coating, an organic coating, or a composite coating of these applied to the top layer of the electrogalvanized layer, depending on the purpose.

Next, we will explain the manufacturing method of the electrogalvanized hot-rolled steel sheet of the present invention.

In the manufacturing method of electrogalvanized hot-rolled steel sheet of the present invention, the hot-rolled steel sheet is subjected to a blast treatment, followed by an electrogalvanized layer formation treatment. The blast treatment imparts appropriate irregularities to the surface of the hot-rolled steel sheet, which serves 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 electrogalvanized layer surface of the electrogalvanized hot-rolled steel sheet.

The average particle size of the particles used in the blasting treatment should be 60 μm or more and 300 μm or less. If the average particle size is less than 60 μm, many smooth areas will be formed, and the number density of smooth areas on the surface of the electrogalvanized layer will exceed 10. From this perspective, an average particle size of 100 μm or more is more preferable. 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 electrogalvanized layer surface will exceed 20.0 μm. From this perspective, an average particle size of 250 μm or less is more preferable.

There are no particular restrictions on the shape of the particles used in the blasting process. As long as the average particle size falls within the above range, the particle shape may be any shape, such as spherical, oval, or polygonal.

There are no particular restrictions on the material of the particles used in the blasting process. For example, 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 can be used.

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

In the present invention, there are no particular limitations on the blasting method. For example, shot blasting, in which particles are projected using a centrifugal blaster, sand blasting, in which particles are projected using compressed air, and wet blasting, in which particles are projected together with water using compressed air, can be used.

In the present invention, there is no particular limitation on the number of times the blast treatment is performed. The blast treatment may be performed once, or two or more times. From the perspective of reliably forming unevenness through the blast treatment, it is preferable that the blast treatment be performed two or more times. On the other hand, from the perspective of treatment costs, it is preferable that the blast treatment be performed 20 times or less.

In the present invention, the surface of a hot-rolled steel sheet is blasted with particles before forming an electrogalvanized layer. There are no particular limitations on the method for forming the electrogalvanized layer, and all known electrogalvanizing methods are applicable. Prior to forming the electrogalvanized layer, electrolytic degreasing and pickling can be performed as needed. Furthermore, when an inorganic coating, organic coating, or composite coating of these is applied on top of the electrogalvanized layer, there are no particular limitations on the method for forming these coatings, and any method, such as coating, reaction, or electrolytic, can be applied.

The present invention will be described below using examples. Note that the present invention is not limited to the following examples.

A 2.6 mm thick hot-rolled steel sheet was used as the base steel sheet for producing the electro-galvanized hot-rolled steel sheet.

The hot-rolled steel sheets were then blasted 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.

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 on the particle surface, and refers to Wadell's circularity, which can be calculated using the following formula:

Circularity = perimeter of a circle with equal projected area / perimeter of a 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 shadow of a particle projected onto a flat surface below when the particle is observed from directly above. "Perimeter of a particle" refers to the length of the contour of the shadow of a particle projected onto a flat surface below when the particle is observed from directly above. The smaller the degree of unevenness of the particle surface, the closer the circularity is to 1.

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

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, followed by an electrogalvanized layer formation process. The electrogalvanized layer formation process involved electrolysis at 50 A/ ^dm2 using the steel sheet as the cathode in an electrogalvanized bath containing 440 g/L of zinc sulfate heptahydrate, at a pH of 1.5, and at a bath temperature of 50°C, and the electrolysis time was varied to form an electrogalvanized layer with a predetermined coating weight.

The test pieces obtained as described above were subjected to the following evaluations. The evaluation results are shown in Table 1.
(1) Coating Weight of Zinc Plating Layer The coating weight of the electrolytic zinc plating layer was measured by the following method.
The plating layer of the test piece having a size of 50 mm x 50 mm was dissolved with 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
For each test piece, a stylus-type surface roughness meter with a cone shape, a tip radius of 2 μm, and a tip angle of 60° was used to measure a measurement length of 2.5 mm in accordance with JIS B0601-2001, and a roughness curve was obtained using the obtained cross-sectional curve with a cutoff wavelength of 0.8 mm. From this roughness curve, the maximum height roughness Rz was calculated. The measurement direction using the surface roughness meter was the rolling direction of the steel sheet.
(3) Number density of smooth portions 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 cross-sectional length.
(4) Appearance Uniformity The gloss was measured at any 10 points on each 150 mm × 70 mm test piece using a Konica Minolta gloss meter GM-60S under conditions of a measurement diameter of 2 mm × 4 mm and a measurement angle of 60°, and the appearance uniformity was evaluated based on the difference in gloss between the point with the lowest gloss and the point with the highest gloss. The pass/fail of the appearance uniformity was judged according to the following criteria, with ◎ or ○ being pass and × being fail.
◎: Glossiness difference ≦2
○: 2<gloss difference≦5
×: 5<difference in glossiness (5) Glossiness For each test piece, the glossiness was measured using a PG-1M manufactured by Nippon Denshoku Corporation under the conditions of a measurement diameter of 10.5 mm × 60 mm and a measurement angle of 60°. The pass/fail of the glossiness was judged according to the following criteria, with ◎ or ○ being pass and × being fail.
◎: 7≦glossiness≦30
Good: 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 ◯: 75≦L value<80
×: L value < 75

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 moderate gloss and high whiteness.

The electrogalvanized hot-rolled steel sheets of the present invention have excellent appearance uniformity, as well as moderate gloss and high whiteness, making them suitable for a wide range of applications, including home appliances, automobiles, and building materials. They are particularly suitable for use in home appliances where they are used unpainted.

Claims (2)

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. The method for manufacturing electrogalvanized hot-rolled steel sheet according to claim 1, characterized in that the surface of the hot-rolled steel sheet is blasted with particles having an average particle size of 60 μm or more and 300 μm or less, and then an electrogalvanized layer is formed.