ELECTROGALVANIZED STEEL SHEET HAVING SUPERB WHITENESS AND METHOD FOR MANUFACTURING SAME

Abstract

The present invention provides an electrogalvanized steel sheet and a method manufacturing same, the electrogalvanized steel sheet having superb whiteness, and an attractive exterior surface due to reduction in surface scale.

Claims

1. An electrogalvanized steel sheet having excellent whiteness, comprising: a base steel sheet having a grain size of 10 to 20 μm in an internal structure; a nickel coating layer having an adhesion amount of 50 to 300 mg/m.sup.2 provided on the base steel sheet; and a galvanized layer provided on the nickel coating layer, wherein a whiteness L value is 86.5 or more.

2. The electrogalvanized steel sheet having excellent whiteness of claim 1, wherein the electrogalvanized steel sheet comprises a single or a plurality of resin layers provided on the galvanized layer.

3. A method for manufacturing an electrogalvanized steel sheet having excellent whiteness, comprising operations of: preparing a base steel sheet having a grain size of 10 to 20 μm in an internal structure; forming a nickel coating layer having an adhesion amount of 50 to 300 mg/m.sup.2 on the base steel sheet by electroplating; and forming a galvanized layer on the nickel coating layer by electroplating, wherein the galvanized layer is formed by using a galvanizing bath containing Fe ions in a concentration of less than 500 ppm; and Na, Ca, and Mg ions at a combined concentration of 50 to 150 ppm.

4. The method for manufacturing an electrogalvanized steel sheet having excellent whiteness of claim 3, further comprising: forming a single layer or a plurality of resin layers on the galvanized layer.

5. The method for forming an electrogalvanized steel sheet having excellent whiteness of claim 3, wherein the electroplating is performed using a sulfuric acid bath.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1 is a result of analyzing a surface of an electrogalvanized steel sheet according to an embodiment of the present disclosure using a scanning electron microscope (SEM) at a magnification of 10,000 times, and (a) in FIG. 1 is a photograph in Inventive Example 2, and (b) in FIG. 1 is a photograph in Comparative Example 10.

BEAT MODE FOR INVENTION

[0023] Hereinafter, preferred embodiments of the present disclosure will be described. The present disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0024] Hereinafter, the present disclosure will be described in detail.

[0025] Hereinafter, a method for manufacturing a steel sheet of the present disclosure will be described in detail.

[0026] The method for manufacturing a steel sheet according to another aspect of the present disclosure may include operations of: preparing a base steel sheet; electroplating a nickel coating layer on the base steel sheet; and electroplating a galvanized layer on the plated nickel coating layer.

[0027] Preparing a Base Steel Sheet

[0028] Depending on the purpose of use of a final plated steel sheet, a base steel sheet provided with appropriate physical properties may be prepared. The base steel sheet of the present disclosure is not limited to a specific steel type, but a base steel sheet having a grain size of 10 to 20 μm in an internal structure may be preferable.

[0029] Since surface scale, a prominent surface defect in an electrogalvanized steel sheet, affects a subsequent metallic nickel coating amount, or the like, depending on a degree of occurrence, it is necessary to set an optimal characteristic of the base steel sheet. A grain size of the base steel sheet affects grain boundary permeation behavior of a pickling solution during pickling, thereby affecting a degree of removal of surface scales. That is, when the grain size of the base steel sheet is fine, a grain boundary permeation area of the pickling solution is increased based on the same pickling conditions (acid concentration, temperature, reaction time, or the like), thereby improving corrosion efficiency and removing the scale formed on the surface of the steel sheet becomes easy. If the acid concentration is increased or the reaction time is increased to remove a scale from the steel sheet, scale removal efficiency may be improved, but there may be a problem in that manufacturing costs may be increased or an environmental load may be increased during waste liquid treatment. Therefore, as described above, a method of improving corrosion efficiency through an increase in the reaction area between the steel sheet and the pickling solution and facilitating scale removal therethrough is preferred.

[0030] When the grain size of the internal structure of the base steel sheet is less than 10 μm, a degree of grain refinement and corrosion efficiency improvement due thereto is insignificant, but there may be a problem in that manufacturing costs may be increased due to an expensive steel-type additive element for grain refinement. However, when the grain size thereof exceeds 20 μm, there is a disadvantage in that the corrosion efficiency is deteriorated because a permeation area of the pickling solution is reduced due to grain coarsening.

[0031] Therefore, it is preferable that the grain size of the steel sheet of the present disclosure is 10 to 20 μm, and it is more preferable that the grain size is 13 to 15 μm in consideration of variables such as the content of the steel-type additive element and an effect on manufacturing cost thereof, and the pickling efficiency of the steel sheet.

[0032] The base steel sheet may be manufactured differently by changing the steel composition and content thereof, and the composition and fraction of the microstructure are not particularly limited. The base steel sheet may secure cleanliness of the surface through the pretreatment process, but in the present disclosure, the pretreatment conditions (hot rolling, pickling, cold rolling, annealing) are not particularly limited.

[0033] Forming a Nickel Coating Layer

[0034] A nickel coating layer having an adhesion amount of 50 to 300 mg/m.sup.2 may be formed on the base steel sheet.

[0035] The nickel coating layer formed on the base steel sheet contributes to securing an attractive exterior surface appearance after subsequent galvanizing by hiding surface scale. As a result of analyzing the whiteness of the steel sheet according to the adhesion amount of the nickel coating layer, the surface of the steel sheet is smoothed due to an effect of providing nucleation sites of fine nickel particles as the adhesion amount thereof increases, and a size of electrodeposited particles becomes uniform and fine, so whiteness and glossiness may increase. However, when the adhesion amount thereof is excessive, it was confirmed that even if the adhesion amount thereof is increased, a degree of increase in whiteness is insignificant or, on the contrary, whiteness is reduced. In addition, in terms of crystal orientation of a zinc layer after subsequent electrogalvanizing, a degree of orientation of a base plane decreases, while a degree of orientation of a pyramid plane rapidly increases, which adversely affects other physical properties, such as corrosion resistance of the steel sheet, or the like.

[0036] When the adhesion amount of the nickel coating layer is less than 50 mg/m.sup.2, there is a problem in that an effect of hiding surface scale and an effect of smoothing the surface is insufficient. On the other hand, when the adhesion amount exceeds 300 mg/m.sup.2, while manufacturing costs may be increased, there is a problem in that the degree of increase in whiteness becomes insignificant or, on the contrary, whiteness decreases.

[0037] In the present disclosure, in order to form the nickel coating layer, it may be performed under normal electroplating bath conditions. A nickel coating layer is formed on the base steel plate by reacting the base steel plate with a sulfate-based nickel coating plating bath. A method of forming a nickel coating layer on one surface by circulating a plating solution after placing the base steel sheet on a negative electrode of an electroplating simulator of a vertical plating cell type may be used.

[0038] Forming Galvanized Layer

[0039] When a galvanized layer is formed, a galvanizing bath containing Fe ions in a concentration of less than 500 ppm and Na, Ca, and Mg ions at a combined concentration of 50 to 150 ppm may be used.

[0040] Meanwhile, as the galvanizing bath, a sulfuric acid-based galvanizing bath may be used.

[0041] However, in the case of the sulfuric acid bath electroplating, high concentration (98%) sulfuric acid was mainly used in the past, but in recent years, the sulfuric acid concentration has been gradually lowered due to workplace hazards, equipment corrosion, and the like. To this end, a process of diluting the high-concentration sulfuric acid, a raw material, is required, and a risk of inclusion of various ions in the plating solution increases depending on a degree of inclusion of impurities during dilution.

[0042] In the composition of the electroplating solution, it was confirmed that zinc is an element contributing to the improvement of whiteness, whereas Fe or cationic impurities such as Na, Ca, and Mg are components decreasing whiteness when the content in the plating solution increases. Other cationic impurities such as Al and K are also present in the plating solution, but the content thereof is relatively small. Therefore, in the present disclosure, impurities Na, Ca, and Mg, which have a large effect on whiteness, are controlled.

[0043] A vacancy of Fe ions, the most important factor in decreasing whiteness, is mainly affected by current density and a concentration of Fe ions in a plating solution. When the concentration of Fe ions in the plating solution is 500 ppm or more, Fe ions, present as impurities in the solution are easily precipitated due to the property of having a rare precipitation potential compared to Zn, and co-deposited in the steel sheet at the same time as zinc, resulting in significantly inferior whiteness and surface quality. In particular, since a Fe vacancy rate increases as the current density increases, a high Fe ion concentration in the plating solution acts as an obstacle during a high current density operation to ensure high productivity.

[0044] In addition, when the concentration of Na, Ca, and Mg ions is less than 50 ppm, conductivity of the plating solution decreases, making it difficult to secure high current density. When the concentration of Na, Ca, and Mg ions exceeds 150 ppm, Fe ion vacancies are promoted, so that the surface quality may be deteriorated, such as a decrease in whiteness.

[0045] In addition, when a content of zinc in the plating solution is large, the zinc content is also very important since zinc interferes with Fe vacancy. However, the zinc content is not particularly limited in the present disclosure.

[0046] In the present disclosure, in order to form the galvanized layer, it may be performed under normal electroplating bath conditions. The steel sheet on which the nickel coating layer is formed reacts with a sulfuric acid-based galvanizing bath to form a galvanized layer. A method of forming a galvanized layer on one surface by circulating a plating solution after placing a steel sheet on a cathode of an electroplating simulator of a vertical plating cell type may be used.

[0047] After forming the galvanized layer, a single resin layer or a plurality of resin layers may be formed as needed.

[0048] The steel sheet manufactured by the above-described manufacturing method may include: a base steel sheet having a grain size of 10 to 20 μm in an internal structure; a nickel coating layer having an adhesion amount of 50 to 300 mg/m.sup.2 provided on the base steel sheet; and a galvanized layer provided on the nickel coating layer.

[0049] In the electrogalvanized steel sheet manufactured as described above, when a degree of occurrence of surface scale is visually confirmed, it is not observed, and a whiteness L value of 86.5 or more may be secured. An attractive exterior appearance, a high degree of whiteness, and high productivity may be secured through a high-speed operation in a process line.

[0050] Hereinafter, the present disclosure will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure.

MODE FOR INVENTION

Example

[0051] Base steel sheets (ultra-low carbon steel) having different grain sizes, with a thickness of 0.6 mm, a width of 140 mm, and a length of 250 mm, to which the same pre-treatment (hot rolling, pickling, cold rolling, annealing) conditions were applied, were manufactured. Thereafter, a nickel coating layer and a galvanized layer were sequentially formed on the base steel sheet through electroplating after degreasing and pickling treatment. In this case, a nickel coating layer and a galvanized layer were formed on one surface by circulating a sulfuric acid-based plating solution after placing the base steel sheet on a negative electrode of a vertical plating cell-type electroplating simulator. In this case, an adhesion amount of the nickel coating layer was controlled by varying an energization time under the same current density (10 A/dm.sup.2) and an electrolytic flow rate (1.5 m/s) according to a degree of occurrence of surface scale. In the case of zinc plating formed on the nickel coating layer, the same current density (100 A/dm.sup.2), electrolytic flow rate (1.5 m/s), and energization time (7 seconds) were applied to secure 20 g/m.sup.2, a target adhesion amount. In addition, a degree of influence of major components such as Fe, Na, Ca, Mg, and the like, in the plating solution during zinc plating was simultaneously confirmed. The adhesion amount of the nickel coating layer and the galvanized layer was confirmed by an X-ray fluorescence analyzer (XRF), each of which having a calibration curve input, and by dissolving a plating layer using a wetting method, and then measuring a difference in weight before and after using an ultra-precision scale. The manufacturing conditions of each specimen are illustrated in Table 1 below.

TABLE-US-00001 TABLE 1 Zn plating Base solution steel component sheet Ni coating Zn plating Na + Grain Current Flow Energization Adhesion Current Flow Energization Adhesion Ca + Specimen size density rate time amount density rate time amount Zn Fe Mg No. (μm) (A/dm.sup.2) (m/s) (second) (g/m.sup.2) (A/dm.sup.2) (m/s) (second) (g/m.sup.2) (g/l) (ppm) (ppm)  1 12 10 1.5 2 94.9 100 1.5 7 19.8 62 235 118  2 15 10 1.5 2 94.9 100 1.5 7 19.8 62 268 123  3 19 10 1.5 2 94.9 100 1.5 7 19.8 62 292 109  4 15 10 1.5 1.5 58.0 100 1.5 7 19.8 62 268 123  5 15 10 1.5 4 224.7 100 1.5 7 19.8 62 268 123  6 15 10 1.5 5 285.0 100 1.5 7 19.8 62 268 123  7 15 10 1.5 2 94.9 100 1.5 7 19.8 62 359 123  8 15 10 1.5 2 94.9 100 1.5 7 19.8 62 478 123  9 15 10 1.5 2 94.9 100 1.5 7 19.8 62 387  68 10 15 10 1.5 2 94.9 100 1.5 7 19.8 62 387  92 11 15 10 1.5 2 94.9 100 1.5 7 19.8 62 387 141 12 23 10 1.5 2 94.9 100 1.5 7 19.8 62 275 115 13 25 10 1.5 2 94.9 100 1.5 7 19.8 62 258 105 14 30 10 1.5 2 94.9 100 1.5 7 19.8 62 302 125 15 23 10 1.5 2 285.0 100 1.5 7 19.8 62 275 115 16 25 10 1.5 2 285.0 100 1.5 7 19.8 62 258 105 17 30 10 1.5 2 285.0 100 1.5 7 19.8 62 302 125 18 15 10 1.5 1 31.8 100 1.5 7 19.8 62 268 123 19 15 10 1.5 6 340.1 100 1.5 7 19.8 62 268 123 20 15 10 1.5 2 94.9 100 1.5 7 19.8 62 523 123 21 15 10 1.5 2 94.9 100 1.5 7 19.8 62 387 167

[0052] For the electrogalvanized steel sheet prepared as described above, a degree of occurrence of surface scale on a surface of the steel sheet was visually confirmed, and whiteness was measured with a Minolta CR-400 colorimeter, and results thereof were shown in Table 2 below. Meanwhile, for some specimens confirmed to have inferior whiteness, a peak obtained after irradiating the specimen with an acceleration voltage of 40 kV using Cu Kα radiation with an X-ray diffraction analyzer (Rigaku, D/MAX 2500V/PC) to determine the cause was analyzed, and a plating structure was analyzed with a JEOL's JSM-7001F field emission scanning electron microscope (FE-SEM).

TABLE-US-00002 TABLE 2 Whether Specimen or not surface No. scale is observed Whiteness (L) Classification 1 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 1 2 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 2 3 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 3 4 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 4 5 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 5 6 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 6 7 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 7 8 Not occur Very good (86.5 or more Inventive and less than 88.0) Example 8 9 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 9 10 Not occur Excellent (88.0 or more Inventive and less than 89.5) Example 10 11 Not occur Very good (86.5 or more Inventive and less than 88.0) Example 11 12 Occur Very good (86.5 or more Comparative and less than 88.0) Example 10 13 Occur Very good (86.5 or more Comparative and less than 88.0) Example 10 14 Occur Normal (85.0 or more Comparative and less than 86.5) Example 10 15 Occur Normal (85.0 or more Comparative and less than 86.5) Example 10 16 Occur Normal (85.0 or more Comparative and less than 86.5) Example 10 17 Occur Insufficient Comparative (less than 85.0) Example 10 18 Occur Excellent (88.0 or more Comparative and less than 89.5) Example 10 19 Not occur Normal (85.0 or more Comparative and less than 86.5) Example 10 20 Not occur Insufficient Comparative (less than 85.0) Example 10 21 Not occur Insufficient Comparative (less than 85.0) Example 10

[0053] As can be seen from Tables 1 and 2, it can be confirmed, in Inventive Examples 1 to 11 satisfying the conditions disclosed in the present disclosure, the electrogalvanized steel sheet has excellent surface quality and whiteness. However, in the case of Comparative Examples 1 to 10, it can be confirmed that an excellent level of surface quality and whiteness may not be secured since the conditions disclosed by the present disclosure are not satisfied. In the present disclosure, if a whiteness L value is 88.0 or more and less than 89.5, it was described as “excellent”, if the whiteness L value was 86.5 or more and less than 88.0, it was described as “very good”, if the whiteness L value was 85.0 or more and less than 86.5, it was described as “normal”, and if the whiteness L value is less than 85.0, it was described as “insufficient”.

[0054] FIG. 1 is a result of analyzing a surface of an electrogalvanized steel sheet according to an embodiment of the present disclosure using a scanning electron microscope (SEM) at a magnification of 10,000 times, and (a) in FIG. 1 is a photograph of Inventive Example 2, and (b) in FIG. 1 is a photograph of Comparative Example 10. As illustrated in FIG. 1, it can be seen that, in Inventive Example (a), a grain and orientation of a plating structure are very uniform. On the other hand, in the case of Comparative Example (b), it can be seen that a size of the grain is relatively large and irregular, and a plate-like structure is greatly developed, so that an incident light absorption area is large. In addition, in terms of crystal orientation, it can be confirmed that surface quality is not excellent because a pyramid plane orientation fraction is high compared to base plane orientation.

[0055] In the case of Comparative Examples 1 to 6, it can be seen that removal efficiency of a hot-rolled scale formed on a surface of the steel sheet during pickling is lowered due to a coarse grain size as a grain size of the base steel sheet proposed by the present disclosure is not satisfied, so that surface scale is observed on a surface of the steel sheet after galvanizing. For this reason, excellent surface quality could not be secured. In particular, in the case of Comparative Example 6, the grain size was very coarse, so it can be seen that a width of a decrease in whiteness due to an excessive increase in the adhesion amount of the nickel coating is relatively large.

[0056] In the case of Comparative Examples 7 and 8, since a nickel coating adhesion amount suggested by the present invention was not satisfied, excellent surface quality and whiteness were not secured at the same time. In particular, in the case of Comparative Example 7 in which the nickel coating adhesion amount is very small, the surface quality is inferior due to an insufficient hiding effect of the surface scale, and in the case of Comparative Example 8 in which the nickel coating adhesion amount is excessive, the surface scale was not observed, but it was confirmed that whiteness was decreased.

[0057] In Comparative Examples 9 and 10, it can be seen that the electrogalvanized steel sheet may not secure excellent whiteness because the conditions for a sum of Fe ions and Na, Ca, and Mg concentrations in the plating solution proposed by the present disclosure were not satisfied. In particular, in the case of Comparative Example 9, in which the Fe ion concentration itself is high, and in Comparative Example 10, in which cationic impurities such as Na, Ca, and Mg are present in an excess amount even at an appropriate Fe concentration, it can be seen that Fe vacancies in the plating layer are promoted, so that whiteness is insufficient to an insufficient level.

[0058] While example 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 present disclosure as defined by the appended claims.