EP3354359B1

EP3354359B1 - Method for manufacturing plated steel sheet having excellent clarity of image after coating

Info

Publication number: EP3354359B1
Application number: EP16848998.7A
Authority: EP
Inventors: Sun-Ho Jeon
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2015-09-23
Filing date: 2016-09-23
Publication date: 2020-12-16
Anticipated expiration: 2036-09-23
Also published as: EP3354359A4; EP3354359A1; KR101746944B1; KR20170036187A; WO2017052273A8; US20180257121A1; CN108136455A; WO2017052273A1

Description

[Technical Field]

The present disclosure relates to a plated steel sheet used in automotive outer panel, or the like, and in more detail, to a method of manufacturing a plated steel sheet having excellent image clarity after coating.

[Background Art]

Since plated steel sheets plated with zinc (Zn), aluminum (Al), or the like, have excellent corrosion resistance, weldability, and paintability, plated steel sheets have commonly been used as steel sheets for automotive outer panels. As such, plated steel sheets have been used in automotive outer panels, so that not only press formability, but also excellent image clarity after coating is required.
Image clarity after coating refers to the degree to which an image reflected on a painted surface is clearly viewable without being distorted, since a painted surface is smooth. In the related art, image clarity after coating has generally been treated as a task of painting technique (Patent Document 1), and a method of painting and the improvement of paint have commonly been reviewed. As a result, due to advancements in painting and paint technology, the quality of painting has been significantly improved.
Recently, in order to reduce costs, automakers have aggressively pursued the application of a 2-coat painting system omitted a primer process, in which a surfacer or a primer coating process is omitted from a 3-coat painting system including a primer or an electrodeposition coating process of the related art, a surfacer or a primer coating process, and a sealer or base coating process and clear coating process.
The object of a primer coating process is to adjust surface unevenness of a concave-convex portion, an orange peel defect, or the like, on a surface of a plated steel sheet to be painted. Since a primer process is omitted, an overall thickness of a painting system may be significantly reduced, and a function of a primer coating process described above may not be performed. Thus, unevenness of a plated steel sheet may be exposed after painting.
Therefore, as a primer process omitted 2-coat painting system is applied, a relatively high quality of a surface of a plated steel sheet, a material to be painted, is required.
According to a trend described above, in research into a plated steel sheet to improve image clarity after coating to date, the effects of surface roughness of a temper rolling roll, form control of surface roughness, or the like, have mainly been reviewed. However, from the viewpoint of waviness, surface form profile of a plated steel sheet, a factor affecting image clarity after coating, a surface roughness (Ra) and a peak number per unit length (RPc) of a micro concave-convex portion are provided as short-period (short-wavelength) waviness covered with a film of paint and are known for insignificantly affecting improvement of image clarity after coating.
Furthermore, research into waviness of a plated steel sheet is limited to controlling the average waviness (Wca) of a plated steel sheet by adjusting a roll form of temper rolling after plating and operation conditions of temper rolling.
US 2015/209848 A1 discloses a flat product made of a metal material has been provided with deterministic surface texture which has a plurality of depressions which have a depth in the range of from 2 to 14 µm.
O. Deutscher et.al. characterized the paintability of hot dip galvanized steel sheets based on a new uniform measuring procedure and uniform characterizing parameters in "Characterising the surface waviness of hot dip galvanised steel sheets for optical high-quality paintability (Carsteel)" in [EUR / EUROPÄISCHE ATOMGEMEINSCHAFT, EURATOM, ISSN 1018-5593 ; 23854], 9 March 2009 (2009-03-09), pages 1-133.
Basjan Berhout discloses in the article titled "PREMIUM PAINT APPEARANCE: SERICA, THE HOT-DIP GALVANISED SURFACE FINISH FOR EXPOSED AUTOMOTIVE PANELS" (in INTERNATIONAL PAINT&COATING MAGAZINE, no. 34, 31 August 2015 (2015-08-31), pages 92-96), information about a commercial steel product named Serica.
In an exemplary embodiment, in Patent Document 2, a method of improving press formability and image clarity after coating in such a manner that solid particles are projected onto a surface of a galvanized steel sheet to adjust average surface roughness of a galvanized steel sheet, that is, an average surface roughness (Ra), to be 0.3 µm to 3.0 µm, a peak number per inch (PPI) of a surface of a steel sheet to be 250 or more (98 or more in the case of a peak number per centimeter (RPc)), and an average waviness (Wca) of a surface of a steel sheet to be 0.8 µm, respectively, is proposed.
In addition, a method of controlling an average surface roughness (Ra), a peak number per unit length (RPc), and average waviness (Wca) of a plated steel sheet having been temper rolled by adjusting a roughness and waviness of a roll after a temper rolling roll is processed using a laser process (LT) in Patent Documents 3 and 4 and an electronic beam process (EBT) in Patent Document 5.
However, there is a problem in which, even in the case in which a form profile of a surface of a plating layer is removed by controlling an average waviness (Wca) of a plated steel sheet having been temper rolled, a form profile of a base plate remaining in a plating layer and a base metal in a fabrication process of automobile components before painting appears and is maintained, even after painting.

Patent Document 1: Korean Patent Application No. 2010-0112186
Patent Document 2: Korean Patent Application No. 2002-0068525
Patent Document 3: Japanese Patent Application No. Hei7-136701
Patent Document 4: Japanese Patent Application No. Hei6-075728
Patent Document 5: Japanese Patent Application No. Hei11-302816

[Disclosure]

[Technical Problem]

An aspect of the present disclosure provides a method of improving image clarity after coating of a plated steel sheet having no defect, even during painting, while avoiding an orange peel defect, a long wavelength waviness defect, occurring during a process of painting automobile components, as well as a plated steel sheet having excellent image clarity after coating, manufactured thereby.

[Technical Solution]

According to the present disclosure, a method of manufacturing a plated steel sheet having excellent image clarity after coating is provided in claim 1. The method includes providing a cold rolled steel sheet by cold rolling; plating the cold rolled steel sheet to manufacture a plated steel sheet; temper rolling the plated steel sheet by inserting the plated steel sheet into a temper rolling mill, wherein a long-wavelength waviness (Wsa1-5) of a final stand rolling roll is 0.4 µm or less during the cold rolling, wherein the plating of the cold rolled steel sheet is performed to allow a plating amount on a single surface to be 45 g/m2 or greater, wherein, in the temper rolling the plated steel sheet, a roll having a long-wavelength waviness (Wsal-5) of 0.4 µm or less (not including 0) is used, wherein the cold rolled steel sheet has a long-wavelength waviness (Wsal-5) of 0.2 µm or less (not including 0).
A plated steel sheet having excellent image clarity after coating is manufactured using a method described above.

[Advantageous Effects]

According to the present disclosure, a method of manufacturing a plated steel sheet having excellent image clarity after coating without a surface defect, such as an orange peel defect, even after painting by controlling a long-wavelength waviness of a cold rolled steel sheet, a plating material, is provided.
In detail, in the present disclosure, since there is no change in a roughness value, such as an average roughness (Ra) and a peak number per unit length (RPc) of a plated steel sheet, image clarity after coating may be improved, while press formability of the plated steel sheet is not degraded.
In addition, the plated steel sheet of an exemplary embodiment has excellent image clarity after coating, thereby being applied to an automotive outer panels.

[Description of Drawings]

FIG. 1 is a schematic diagram of a phenomenon in which a form profile of a surface of a base metal is reproduced after a process of a plated steel sheet.
FIG. 2 is a graph illustrating a relationship between a long-wavelength waviness (Wsa1-5) of the plated steel sheet and image clarity after coating of a painted steel sheet, the plated steel sheet having been painted.

[Best Mode for Invention]

A method of appropriately adjusting a rough value, such as an average roughness (Ra) and a peak number per unit length (RPc) of a plated steel sheet and a form profile of an average waviness (Wca) by performing temper rolling after the plated steel sheet for automotive outer panels is painted and plated using a method of preventing a painting defect, such as an orange peel defect, is proposed. However, since the roughness value is a short-period wavelength and is filtered by a painting layer (a film of paint) of a painted steel sheet, it is known that an effect on image clarity after coating is insignificant.
Accordingly, in order to prevent a painting defect, such as the orange peel defect, after the plated steel sheet for the automotive outer panels is painted, the inventors of the present disclosure have examined an effect of long-wavelength waviness of a cold rolled steel sheet, an original plate (a plating material) of the plated steel sheet on the plated steel sheet and the painted steel sheet, in detail.
As a result, it was confirmed that, as illustrated in FIG. 1, even in the case in which the form profile of a surface of the plating layer is removed by temper rolling the long-wavelength waviness, or the like, to be an optimized temper rolling roll to control the long-wavelength waviness of the plated steel sheet, the form profile of the base metal (an original plate) remaining between the plating layer and a base steel sheet appear on a surface of the plating layer again due to fabrication during a process of manufacturing automobile components before painting and is maintained even after painting, thereby causing a problem in which image clarity after coating is degraded. As such, it can be confirmed that image clarity after coating is determined not only by the long-wavelength waviness of the plated steel sheet, but also by a combination of the long-wavelength waviness of the cold rolled steel sheet, the plating material.
Between long-wavelength wavinesses described above, the long-wavelength waviness occurring in a plating process may be controlled by optimizing the long-wavelength waviness of a roll used during temper rolling. However, it was confirmed that, since the long-wavelength waviness of the cold rolled steel sheet, the plating material, is exposed after the plated steel sheet is processed, image clarity after coating may be improved by controlling the long-wavelength waviness of the cold rolled steel sheet itself.
As the long-wavelength waviness, Wsa1-5 is used, rather than a filtered center line waviness of the related art Wca according to JIS B0651 specification, since according to SEP 1941 specification, Wsa1-5 is illustrated by filtering only the long-wavelength waviness, the form profile of 1 mm to 5 mm, most visible to the naked eye in the form profile including unevenness, a dent, or the like, having a size equal to or less than a measured size (50 mm).
In other words, numbers 1 and 5 denote a lower limit value and an upper limit value of a cut-off value. The number 1 refers to a roughness cut-off value removing a roughness component having a wavelength of 1 mm or less in the form profile, while the number 5 refers to a coarse form cut-off value removing a coarse form having a wavelength of 5 mm or greater in the form profile. Therefore, since the roughness component and a form covered by painting in the form profile is coarse, only a pure long-wavelength waviness in which the coarse form not observed due to an orange peel defect, or the like, is removed is illustrated.
On the other hand, Wca meaning the filtered center line waviness includes has the toughness component of 0.8 or greater, in which the cut-off value is 0.8 mm to 8.0 mm, and the coarse form component of 5 mm to 8 mm, the long-wavelength waviness observed as an orange peel defect may not be accurately illustrated. Thus, Wca does not perfectly match the form profile observed with the naked eye.
Accordingly, in an exemplary embodiment, a method in which a surface defect, such as the orange peel defect, does not appear even in the case in which a component is processed after plating, and the long-wavelength waviness (Wsa1-5) is maintained even after the painting to secure excellent image clarity after coating by controlling the long-wavelength waviness (Wsa1-5) of the cold rolled steel sheet, the plating material, in order to prevent the painting defect, such as the orange peel defect, after the plated steel sheet for the automotive outer panels is painted, is proposed.
Hereinafter, an exemplary embodiment will be described in detail.
According to the present disclosure, a method of manufacturing the plated steel sheet having excellent image clarity after coating includes providing the cold rolled steel sheet by cold rolling and manufacturing the plated steel sheet by plating the cold rolled steel sheet.
In a case in which the plated steel sheet is manufactured according to the invention, the cold rolled steel sheet, the plating material, may be provided, while the cold rolled steel sheet having a long-wavelength waviness (Wsa1-5) of 0.2 µm or less.
When the long-wavelength waviness (Wsa1-5) of the cold rolled steel sheet is 0.2 µm or less (not including 0), the surface defect, such as the orange peel defect, does not appear, even in the case in which the components are not process after plating. Thus, excellent image clarity after coating is secured even after plating.
However, in a case in which the long-wavelength waviness (Wsa1-5) of the cold rolled steel sheet, the plating material, is greater than 0.2 µm, the surface defect, such as the orange peel defect, appear when the components are processed after plating and is maintained even after plating, causing image clarity after coating to be degraded.
As described above, in order to obtain the cold rolled steel sheet having the long-wavelength waviness (Wsa1-5) of 0.2 µm or less, the long-wavelength waviness (Wsa1-5) of the rolling roll used in cold rolling is used. In detail, the long-wavelength waviness (Wsa1-5) of a final stand rolled roll is controlled to be 0.4 µm or less (not including 0).
In a case in which the long-wavelength waviness (Wsa1-5) of the final stand rolled roll is greater than 0.4 µm during cold rolling, the long-wavelength waviness (Wsa1-5) of the cold rolled steel sheet to be manufactured is greater than 0.2 µm, so that a defect, such as the orange peel defect, occurs when the component is processed after plating and remains even after painting, causing a problem in which image clarity after coating is degraded.
In the meantime, any steel sheet of which the plated steel sheet of the related art is manufactured may be used as the cold rolled steel sheet.
For example, the cold rolled steel sheet may be provided as a low carbon steel sheet. For example, the low carbon steel sheet is required to include, by wt%, carbon (C): 0.002% or less (not including 0%), manganese (Mn): 0.08% or less (not including 0%), silicon (Si): 0.003% or less (not including 0%), aluminum (Al): 0.028% or less (not including 0%), or the like. In order to improve mechanical properties of the plated steel sheet, the low carbon steel sheet may further include copper (Cu), nickel (Ni), molybdenum (Mo), Niobium (Nb), vanadium (V), boron (B), or the like and may include Fe as a residual component thereof and inevitable impurities.
However, the cold rolled steel sheet according to an exemplary embodiment is not limited to the low carbon steel sheet.
Subsequently, the cold rolled steel sheet, the long-wavelength waviness (Wsa1-5) of which is controlled, is plated to manufacture the plated steel sheet. In this case, plating is performed so that an amount of plating on a surface thereof may be 45 g/m² or greater. In more detail, plating may be performed so that an amount of plating on a surface thereof may be within a range of 45 g/m² to 80 g/m².
When the amount of plating is less than 45 g/m² during plating, even in the case in which the cold rolled steel sheet having the long-wavelength waviness (Wsa1-5) desired in an exemplary embodiment is used as the plating material, the surface defect, such as the orange peel defect, when the components are processed after plating and is maintained even after painting, thereby causing a problem in which image clarity after coating is degraded.
The present disclosure is not specifically limited to a plating method described above, but in an exemplary embodiment, is not particularly limited, and for example, hot-dip galvanizing, electroplating, or the like, may be used.
The plated steel sheet in the case of using the hot-dip plating is, for example, a hot-dip galvanized steel sheet, a hot-dip aluminized steel sheet, and an aluminum alloy plated steel sheet and is a hot-dip galvannealed steel sheet alloyed with the plated steel sheet.
In addition, the plated steel sheet in the case of using the electroplating may be, for example, an electro-galvanized steel sheet or an alloy electro-galvanized steel sheet.
The plated steel sheet manufactured, as described above, is inserted into the temper rolling mill of the related art, thereby performing temper rolling. In this case, temper rolling is performed using a temper rolling roll having a long wavelength waviness (Wsa1-5) of 0.4 µm or less (not including 0) at a roll force of 150 tons or greater.
In a case in which a roll having a long-wavelength waviness (Wsa1-5) of 0.4 µm or less is used during temper rolling, a plated steel sheet having a long-wavelength waviness (Wsa1-5) of 0.28 µm or less (not including 0) may finally be obtained.
The long-wavelength waviness (Wsa1-5) of the plated steel sheet before processing is determined by the long-wavelength waviness of the cold rolled steel sheet, the plating material and the long-wavelength waviness after plating. The long-wavelength waviness (Wsa1-5) of the plated steel sheet affects a long-wavelength waviness value after processing. Therefore, in order to control the long wavelength waviness of the plated steel sheet after processing to be less than 0.35 µm, the long wavelength waviness of the plated steel sheet before processing is controlled to be 0.28 µm or less.
Hereinafter, the plated steel sheet manufactured by the inventive method will be described in detail.
The plated steel sheet includes a cold rolled steel sheet and a plating layer formed on the cold rolled steel sheet. The cold rolled steel sheet has a long-wavelength waviness (Wsa1-5) of 0.2 µm or less.
In this case, the plating layer may be a galvanized layer, an aluminized layer, a zinc alloy plating layer, or an aluminum alloy plating layer, but is not limited thereto.
The plated steel sheet is manufactured using a manufacturing method according to an exemplary embodiment, described above. In detail, a rolling roll having a long-wavelength waviness (Wsa1-5) of 0.4 µm or less during cold rolling is used as a final stand rolling roll, so that a long-wavelength waviness (Wsa1-5) of the cold rolled steel sheet satisfies 0.2 µm or less, and the plated steel sheet having excellent image clarity after coating may be provided.
In the meantime, the inventors evaluated image clarity after coating of a painted steel sheet as illustrated in FIG. 2 to draw a relationship therebetween to determine an effect of the long-wavelength waviness (Wsa1-5) of the plated steel sheet on image clarity after coating of the painted steel sheet. In general, image clarity after coating of a steel sheet of an automobile is quantitatively assessed using wave-scan DOI (distinctness of image) by BYK-Gardner. A DOI value of the wave-scan DOI is continuously expressed as a structure spectrum within a range of a short-period wavelength to a long-period wavelength. A value of each wavelength band is expressed as a value of dimensionless numbers 0 to 100.
As a result of an evaluation, it can be confirmed that, as illustrated in FIG. 2, the long-period wavelength of the plated steel plate (not painted) worked using a cup process (5% of an upper surface is processed) is highly correlated with the long-period wavelength of the plated steel sheet painted using a 2-coat painting system.
In more detail, in the long-period wavelength of 0.35 µm of the plated steel sheet having been processed, an orange peel defect in which the plated steel sheet has a long-period wavelength of 5 µm or greater is formed, thereby causing a problem in which image clarity after coating is degraded.
As a result, formation of the orange peel defect and degradation of image clarity after coating after painting may be estimated by measuring the long-wavelength waviness (Wsa1-5) after 5% of the plated steel sheet is processed. Thus, in an exemplary embodiment, the long-wavelength waviness may be measured after 5% of the plated steel sheet is processed, thereby evaluating image clarity after coating.
Therefore, the plated steel sheet according to an exemplary embodiment may have a long-wavelength waviness of less than 0.35 µm (not including 0) after processing at a strain rate of 5%.

[Industrial Applicability]

Hereinafter, embodiments of the present inventive concept will be described with reference to schematic views illustrating embodiments of the present inventive concept. In the drawings, for example, due to manufacturing techniques and/or tolerances, modifications of the shape shown may be estimated. Thus, embodiments of the present inventive concept should not be construed as being limited to the particular shapes of regions shown herein, for example, to include a change in shape results in manufacturing. The following embodiments may also be constituted alone, in combination or in partial combination.

(Example Embodiment)

After a pickled hot-rolled steel sheet of a low carbon steel of the related art was provided, the pickled hot-rolled steel sheet was cold rolled to manufacture a cold rolled steel sheet. In this case, a surface concave-convex portion of a rolling roll in a final process of a cold rolling process was provided using shot blasting and electric discharge machining, thereby adjusting an average roughness (Ra) and a peak number per unit length (RPc) to adjust a long-wavelength waviness (Wsa1-5) to be within a range of 0.18 µm to 0.66 µm. Subsequently, the pickled hot-rolled steel sheet was cold rolled at a cold rolling reduction ratio of 77% to 83% using the rolling roll. As a result, a cold rolled steel sheet having an average roughness (Ra) of 0.61 µm to 0.85 µm, a peak number per unit length (RPc) of 34/cm to 56/cm, and a long-wavelength waviness of 0.18 µm to 0.28 µm was manufactured. Here, a roughness values and a long-wavelength waviness value, such as the average roughness (Ra), the peak number per unit length (RPc), and the long wavelength waviness (Wsal-5) of each roll and the cold rolled steel sheet, were measured using a contact type two-dimensional roughness gauge by Hosakawa Micron Corp. according to the provisions SEP 1941.
After annealing heat treatment was performed, under conditions of the related art, to each cold-rolled steel sheet having been manufactured, as described above, the cold-rolled steel sheet was immersed in a hot dip galvanizing bath to be coated to have a coating amount of 40 g/m² to 80 g/m² on a surface thereof using an air knife. Subsequently, the cold rolled steel sheet plated was processed through electric discharge machining and temper rolled at a roll force of 150 tons using a temper rolling work roll having an average roughness (Ra) of 2.7 µm and a long-wavelength waviness (Wsa1-5) of 0.3 µm, thereby manufacturing, a hot-dip galvanized steel sheet having a long-wavelength waviness (Wsa1-5) of 0.17 µm to 0.46 µm.
Each hot-dip galvanized steel sheet having been manufactured was processed through a cupping test using a forming tester so that a strain rate of a flat portion, a surface to be evaluated, may be 5%, and then the long-wavelength waviness (Wsa1-5) was measured. A determination was made as to whether an orange peel defect was present with the naked eye, and a result was illustrated in Table 1 below.

In this case, the cupping test was performed such that a diameter of the flat portion of a cup, having been deformed to be flat, was at least 100 mm, and the strain rate was adjusted depending on a forming height.

[Table 1]

Steel Grade	Final Stand Rolling Roll		Cold rolled steel sheet			Plating Amount (g/m²)	Temper rolling roll			Plated Steel Sheet			Evaluation of Image clarity		Remark
Steel Grade	Average Roughness (µm)	Wsa 1-5	Average Roughness (µm)	RPc (/cm)	Wsa 1-5	Plating Amount (g/m²)	Average Rughness (µm)	Wsa 1-5	Roll Force (ton)	Average Roghness (µm)	RPc (/cm )	Wsa 1-5 Before Processing	Wsa 1-5 After forming	Defect	Remark
1	2.1	0.3 6	0.85	56	0.18	60	2.7	0.3	150	1.15	78	0.27	0.28	None	Inventive Example 1
2	2.1	0.2 6	0.81	43	0.18	60	2.7	0.3	150	1.21	76	0.26	0.28	None	Inventive Example 2
3	2.1	0.2 6	0.75	36	0.18	60	2.7	0.3	150	1.09	77	0.26	0.27	None	Inventive Example 3
4	2.1	0.28	0.72	34	0.19	60	2.7	0.3	150	1.15	75	0.23	0.26	None	Inventive Example 4
5	2.1	0.25	0.66	39	0.18	60	2.7	0.3	150	1.12	77	0.24	0.25	None	Inventive Example 5
6	2.1	0.23	0.61	35	0.18	60	2.7	0.3	150	1.24	75	0.23	0.25	None	Inventive Example 6
7	2.1	0.66	0.7	35	0.28	60	2.7	0.3	150	1.06	78	0.46	0.54	Occurrence	Comparative Example 1
8	2.1	0.49	0.7	35	0.26	60	2.7	0.3	150	1.11	77	0.35	0.42	Occurrence	Comparative Example 2
9	2.1	0.42	0.7	35	0.26	60	2.7	0.3	150	1.07	77	0.33	0.38	Occurrence	Comparative Example 3
10	2.1	0.38	0.7	35	0.2	60	2.7	0.3	150	1.1	75	0.28	0.3	None	Inventive Example 7
11	2.1	0.34	0.7	35	0.2	60	2.7	0.3	150	1.08	78	0.27	0.2 7	None	Inventive Example 8
12	2.1	0.26	0.7	35	0.2	60	2.7	0.3	150	1.08	78	0.23	0.27	None	Inventive Example 9
13	2.1	0.18	0.7	35	0.19	60	2.7	0.3	150	1.12	78	0.17	0.24	None	Inventive Example 10
14	2.1	0.34	0.7	35	0.2	40	2.7	0.3	150	1.08	78	0.32	0.37	Occurrence	Comparative Example 4
15	2.1	0.34	0.7	35	0.2	45	2.7	0.3	150	1.16	76	0.27	0.32	None	Inventive Example 11
16	2.1	0.34	0.7	35	0.2	50	2.7	0.3	150	1.21	78	0.26	0.3	None	Inventive Example 12
17	2.1	0.34	0.7	35	0.2	75	2.7	0.3	150	1.11	80	0.27	0.29	None	Inventive Example 13
18	2.1	0.34	0.7	35	0.2	80	2.7	0.3	150	1.19	79	0.26	0.28	None	Invent ive Example 14
19	2.1	0.42	0.7	35	0.26	45	2.7	0.3	150	1.12	80	0.37	0.47	Occurence	Comparative Example 5

(In Table 1, 'Wsa1-5 After Processing' refers to a Wsa1-5 value after 5% processing, while 'Defect' refers to whether an orange peel defect was formed.)

As illustrated in Table 1, in the case of using a rolling roll having a long-wavelength waviness (Wsa1-5) of 0.4 µm or less (Inventive Examples 1 to 14), during a final stand rolling of a cold rolling process, a cold rolled steel sheet having a long-wavelength waviness (Wsa1-5) of 0.2 µm or less may be obtained. In a case in which a hot-dip galvanizing process is performed using the cold rolled steel sheet as a plating material at a plating amount of 45 g/m² or greater on a surface thereof, a long-wavelength waviness (Wsa1-5) after a 5% cupping process of a hot-dip galvanized steel sheet, the painting material, is less than 0.35 µm. In addition, even when being observed with the naked eye, a long-wavelength defect, such as the orange peel defect, did not occur, so that image clarity after coating was excellent.
In the meantime, in the case of using a rolling roll having a long-wavelength waviness (Wsa1-5) of greater than 0.4 µm (Comparative Examples 1 to 3 and 5), during a final pass of the cold rolling process, a cold rolled steel sheet having a long-wavelength waviness (Wsa1-5) of greater than 0.2 µm may be obtained. In a case in which a hot-dip galvanizing process is performed using the cold rolled steel sheet as the plating material at the plating amount of 45 g/m² or greater on a surface thereof, the long-wavelength waviness (Wsa1-5) after the 5% cupping process of the hot-dip galvanized steel sheet, the painting material, is 0.35 µm or greater, so that the long-wavelength defect, such as the orange peel defect, was confirmed with the naked eye.
In addition, even in the case in which a roll having the long-wavelength waviness (Wsa1-5) of 0.4 µm or less is used, in a case in which the plating amount on a single surface is less than 45 g/m² during plating (Comparative Example 4), the long-wavelength waviness (Wsa1-5) after the 5% cupping process is 0.35 µm or greater, so that the long-wavelength defect, such as the orange peel defect, was confirmed with the naked eye.
While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the appended claims.

Claims

A method of manufacturing a plated steel sheet having excellent image clarity after coating, comprising:
providing a cold rolled steel sheet by cold rolling;

plating the cold rolled steel sheet to manufacture a plated steel sheet; and

temper rolling the plated steel sheet by inserting the plated steel sheet into a temper rolling mill,

wherein a long-wavelength waviness (Wsa1-5) of a final stand rolling roll is 0.4 µm or less (not including 0) during the cold rolling,

wherein the plating of the cold rolled steel sheet is performed to allow a plating amount on a single surface to be 45 g/m² or greater,

wherein, in the temper rolling the plated steel sheet, a roll having a long-wavelength waviness (Wsa1-5) of 0.4 µm or less (not including 0) is used,

wherein the cold rolled steel sheet has a long-wavelength waviness (Wsa1-5) of 0.2 µm or less (not including 0).
The method of claim 1, wherein the plated steel sheet is provided as one of a hot-dip galvanized steel sheet, a hot-dip aluminized steel sheet, an aluminum alloy plated steel sheet, an electro-galvanized steel sheet, an alloy electro-galvanized steel sheet, and a hot-dip galvannealed steel sheet.