Electroplated steel sheet with excellent surface appearance and its manufacturing method

Abstract

The present invention relates to a zinc-nickel alloy electroplated steel sheet, comprising a base steel sheet and a zinc-nickel plating layer formed on at least one side of the base steel sheet and having a hairline pattern, wherein the surface of the base steel sheet forming the interface with the zinc-nickel plating layer has a surface roughness of 0.7 to 1.0 μm based on the centerline average roughness (Ra).The present invention provides a zinc-nickel electroplated steel sheet and a method for producing the same, which not only has a beautiful surface appearance after hairline processing, but is also cost-competitive with existing stainless steel or vinyl-laminated steel sheets and ensures high productivity in a coil polishing line.

Description

[Technical field]

[0001] The present invention relates to an electroplated steel sheet having excellent surface appearance and a manufacturing method thereof, and more particularly to a zinc-nickel alloy electroplated steel sheet having excellent surface appearance and image clarity and a manufacturing method thereof. [Background technology]

[0002] In terms of crystal structure, pure galvanized steel sheets show a morphology in which thin hexagonal plate-shaped crystal structures with eta-shaped hexagonal close packing are assembled, whereas zinc-nickel plated steel sheets show a surface structure in which the plate-shaped crystals have disappeared and granular crystal structures are formed.

[0003] In addition, zinc-nickel plated steel sheets have excellent corrosion resistance and high microhardness of the plating layer compared to pure zinc plated steel sheets because nickel, an electrochemically stable and light element, is added. The excellent corrosion resistance of zinc-nickel plated steel sheets is due to the fact that selective corrosion of zinc occurs first during the corrosion process, forming a corrosion product of zinc hydroxide on the surface and a nickel-enriched layer in the lower layer, which suppresses the generation of red rust due to dissolution of the base iron. The increase in microhardness with an increase in the nickel content of the plating layer of zinc-nickel plated steel sheets is not linear, but shows nonlinear behavior with a sharp increase in the gamma phase alone region.

[0004] Recently, steel sheets with a surface appearance similar to that of stainless steel, which are made by applying a hairline pattern to the surface of a carbon steel-based plated steel sheet, have been gaining attention as a material for video home appliances. This hairline pattern steel sheet, which has a polished plated layer, has the advantage of being able to achieve a surface appearance equal to or better than that of existing stainless steel or vinyl coated metal at a lower manufacturing cost.

[0005] In order to ensure a clear hairline pattern during hairline processing of plated steel sheets and to ensure good polishing efficiency, the plated layer must have high hardness and excellent corrosion resistance. Electrogalvanized steel sheets (EG) and hot-dip galvanized steel sheets (GI), which are attracting attention as competing materials to zinc-nickel plated steel sheets, have a plated layer hardness of 60-70Hv, which is very low compared to zinc-nickel plated steel sheets, and have the disadvantage of deteriorating surface polishing efficiency and surface appearance after polishing.

[0006] In addition, electrolytic galvanized steel sheets and hot-dip galvanized steel sheets are susceptible to the generation of white rust, and therefore, corrosion products in the form of white rust formed on the surface of the steel sheet during transportation and storage remain on the steel sheet surface even after polishing, impairing surface quality. In addition, there is a drawback in that deterioration of surface appearance occurs due to rapid progress of corrosion under conditions in which the remaining amount of plating after polishing is small compared to before polishing.

[0007] Although various studies have been conducted to improve the surface appearance such as image clarity and gloss after hairline processing of zinc-nickel plated steel sheets, there has been insufficient consideration of the correlation taking into account the changes in the physical properties of the plating layer before and after hairline processing. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been devised in view of the above-mentioned circumstances, and provides a zinc-nickel alloy electroplated steel sheet having a beautiful surface appearance after hairline processing and excellent image clarity, and a manufacturing method thereof. [Means for solving the problem]

[0009] According to one aspect of the present invention, there is provided a zinc-nickel alloy electroplated steel sheet comprising: a base steel sheet; and a zinc-nickel plating layer located on at least one side of the base steel sheet and having a hairline pattern formed thereon, the base steel sheet surface forming an interface with the zinc-nickel plating layer having a surface roughness of 0.7 to 1.0 μm based on the center line average roughness (Ra).

[0010] The zinc-nickel plating layer is gamma (Ni5Zn 21 ) may consist of a single phase.

[0011] The zinc-nickel plating layer may have a hardness of 250 to 400 Hv.

[0012] According to another aspect of the present invention, there is provided a method for producing a zinc-nickel alloy electroplated steel sheet, the method including the steps of: temper rolling a base steel sheet; immersing the temper rolled base steel sheet in a sulfuric acid bath containing nickel sulfate hydrate and zinc sulfate hydrate to form a zinc-nickel plating layer on the base steel sheet; and polishing and hairline patterning the zinc-nickel plating layer.

[0013] The temper rolling step can be performed using an electric discharge machined roll in capacity (-) mode.

[0014] The surface of the roll may be coated with chromium.

[0015] The temper rolling step may be performed at a drawing rate of 0.3 to 1.2%.

[0016] The surface roughness of the temper-rolled base steel sheet surface may be 0.7 to 1.0 μm based on the center line average roughness (Ra).

[0017] The zinc-nickel plating layer formed on the above-mentioned base steel sheet is gamma (Ni5Zn 21 ) may consist of a single phase.

[0018] The zinc-nickel plating layer formed on the base steel sheet may have a hardness of 250 to 400 Hv.

[0019] The change in center line average roughness of the zinc-nickel plating layer after the polishing and hairline patterning may be −1.00 to −0.35 μm.

[0020] The rate of change in surface roughness of the zinc-nickel plating layer by the polishing can be 60 to 85%, and the rate of change in surface roughness of the zinc-nickel plating layer by the hairline patterning can be 15 to 40%.

[0021] The thickness of the plating layer after the above-mentioned polishing and hairline patterning can be 0.2 to 0.75 times the thickness of the plating layer before the polishing and hairline patterning. Effect of the Invention

[0022] According to the present invention, there is provided a zinc-nickel electroplated steel sheet and a manufacturing method thereof, which not only has a beautiful surface appearance after hairline processing, but also has excellent price competitiveness compared to existing stainless steel or vinyl-laminated steel sheets and ensures high productivity in a coil polishing line. [Brief description of the drawings]

[0023] [Figure 1] 1 shows an image of a steel sheet surface after polishing and hairline processing according to the present invention and an analysis result at 2,000 times magnification using a scanning electron microscope (SEM), where (a) shows Example 1 and (b) shows Comparative Example 1. [Diagram 2] 1 shows the results of analyzing the roughness profile in the direction perpendicular to the rolling direction of a steel plate before and after polishing and hairline processing according to Example 1 of the present invention, where (a) shows the state before processing and (b) shows the state after processing. [Diagram 3] 1 shows the results of analyzing the roughness profile in the direction perpendicular to the rolling direction of a steel sheet before and after polishing and hairline processing according to Comparative Example 1 of the present invention, where (a) shows the state before processing and (b) shows the state after processing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, preferred embodiments of the present invention will be described with reference to various examples. However, the present invention can be modified into several other forms, and the scope of the present invention is not limited to the following embodiments.

[0025] The present invention relates to an electroplated steel sheet having excellent surface appearance and a manufacturing method thereof, and more particularly to a zinc-nickel alloy electroplated steel sheet having excellent surface appearance and image clarity and a manufacturing method thereof.

[0026] The zinc-nickel alloy electroplated steel sheet having the hairline pattern formed thereon is generally subjected to the processes of polishing, hairline polishing, brushing, surface cleaning, and hot air drying in a coil grinding line, followed by painting in a subsequent color coating line to complete the final product. The present inventors, during various studies on improving the surface appearance such as image clarity and glossiness after hairline processing of zinc-nickel plated steel sheet, have confirmed that the hardness of the zinc-nickel plating layer and the surface roughness of the raw material before polishing and hairline processing are the main factors affecting the productivity during coil grinding and the surface appearance such as glossiness and image clarity after processing, and have thus completed the present invention.

[0027] According to one aspect of the present invention, there is provided a zinc-nickel alloy electroplated steel sheet comprising: a base steel sheet; and a zinc-nickel plating layer located on at least one side of the base steel sheet and having a hairline pattern formed thereon, the base steel sheet surface forming an interface with the zinc-nickel plating layer having a surface roughness of 0.7 to 1.0 μm based on the center line average roughness (Ra).

[0028] The type of the base steel sheet is not particularly limited, and may be any type that can be used in the technical field to which the present invention pertains, such as a hot-rolled steel sheet or a cold-rolled steel sheet. However, in the present invention, since the base steel sheet affects the electrodeposition characteristics of zinc or nickel particles during zinc-nickel electroplating and determines the surface roughness of the plated steel sheet, it is necessary to set a preferred rolling method in the temper rolling process that determines the final surface roughness of the base steel sheet, as will be described later, and the associated surface roughness of the base steel sheet.

[0029] At least one surface of the base steel sheet is formed with a zinc-nickel plating layer formed by electroplating, and the zinc-nickel plating layer may have a hairline pattern.

[0030] The surface roughness of the base steel sheet surface forming the interface with the zinc-nickel plating layer is preferably 0.7 to 1.0 μm based on the center line average roughness (Ra). If it is less than 0.7 μm, the surface roughness is uniform during plating, but there is a drawback that plating adhesion deteriorates. In contrast, if the surface roughness exceeds 1.0 μm, there is a drawback that the surface roughness of the plated material increases due to the influence of the roughness characteristics of the base steel sheet, and as a result, the surface appearance after hairline processing deteriorates. However, in consideration of ensuring a beautiful surface appearance after hairline processing and the increase in surface roughness relative to the base steel sheet after zinc-nickel electroplating, the surface roughness of the base steel sheet is more preferably 0.7 to 0.9 μm.

[0031] The zinc-nickel plating layer formed on the above-mentioned base steel sheet is gamma (Ni5Zn 21) single phase. In order to ensure good corrosion resistance in electroplating of zinc-based alloys, it is necessary to reduce the corrosion rate of zinc, which is highly electrochemically reactive (active) in a corrosive environment, and maintain sacrificial corrosion protection for a long time. In this respect, when the gamma single phase is used, not only is the gamma phase electrochemically stable, but there is no galvanic corrosion caused by the potential difference between crystalline phases in a mixed phase such as eta + gamma, so good corrosion resistance can be ensured. In addition, in the gamma single phase region, the hardness of the plating layer increases rapidly with the increase in nickel content in the plating layer, making it easy to ensure high plating layer hardness.

[0032] In particular, as described below, the acid bath applied to form a zinc-nickel plating layer on a steel substrate in the present invention has a non-uniform nickel distribution in the thickness direction compared to an alkaline bath, and exhibits a distribution behavior in which the nickel content gradually increases from the surface toward the interface between the plating layer and the steel substrate, and the nickel depletion region in the surface region decreases as the nickel content in the plating layer increases, thereby achieving substantially high plating layer hardness and an excellent surface appearance in the surface region where hairline polishing is performed.

[0033] In addition, the hardness of the zinc-nickel plating layer formed on the base steel sheet of the present invention is preferably 250 to 400 Hv. If the hardness of the zinc-nickel plating layer is less than 250 Hv, there is a problem that a part of the plating layer is peeled off during hairline pattern processing, which hinders the securing of a uniform hairline pattern in the rolling direction, and increases the rolling force on the material, resulting in the generation of fine craters in the surface structure phase during operation. It can be said that the higher the hardness of the plating layer, the easier the surface polishing is and the higher the efficiency is. However, if the hardness of the zinc-nickel plating layer exceeds 400 Hv, the further improvement in the ease of surface polishing is slight, while the increase in the content of nickel, which is relatively expensive compared to zinc in the plating layer, increases the residual stress of the zinc-nickel plating layer, which causes fine cracks to occur on the surface and deteriorates price competitiveness, which is not preferable.

[0034] According to another aspect of the present invention, there is provided a method for producing a zinc-nickel alloy electroplated steel sheet, the method including the steps of: temper rolling a base steel sheet; immersing the temper rolled base steel sheet in a sulfuric acid bath containing nickel sulfate hydrate and zinc sulfate hydrate to form a zinc-nickel plating layer on the base steel sheet; and polishing and hairline patterning the zinc-nickel plating layer.

[0035] First, a base steel sheet is prepared. The surface of the base steel sheet may be cleaned by a pretreatment process such as degreasing or pickling, and the pretreatment conditions are not particularly limited in the present invention.

[0036] Since the above-mentioned base steel sheet affects the electrodeposition characteristics of zinc or nickel particles during zinc-nickel electroplating and determines the surface roughness of the plated steel sheet, it is necessary to set a preferred rolling method in the temper rolling process, which determines the final surface roughness of the base steel sheet, and the associated surface roughness of the base steel sheet.

[0037] Accordingly, the step of temper rolling the base steel sheet is preferably performed using an electric discharge machining roll in a capacity (-) mode. In the processing method using a temper rolling roll, shot blast texturing is a physical method of projecting metal grit onto the roll surface to impart roughness, whereas electric discharge machining is an electrical method of removing particles on the roll surface by electrical sparks between the roll and an electrode in an insulating liquid to impart roughness. Therefore, a roll manufactured using an electric discharge machining method has the advantages of increased roughness uniformity and reduced roughness deviation, and excellent image clarity, compared to a roll manufactured through shot blasting.

[0038] Furthermore, even in the electric discharge machining modes, the capacity (+) and impulse (-) modes have inferior roughness reproducibility during operation compared to other modes, and the impulse (+) mode has excellent workability but has the disadvantage of having the greatest number of roughness peaks per unit length, which inhibits uniform electrodeposition during zinc-nickel electroplating. Therefore, it is preferable to use an electric discharge machining roll in capacity (-) mode to temper roll the base steel sheet.

[0039] At this time, it is more preferable to coat the surface of the roll with chromium in order to extend the service life of the roll, flatten the roll roughness over time, and reduce the deviation in surface roughness between working materials.

[0040] Meanwhile, in consideration of the surface roughness and strength of the steel sheet, the temper rolling step is preferably performed at an elongation rate of 0.3 to 1.2%. More specifically, if the elongation rate during temper rolling is less than 0.3%, the transfer rate decreases, resulting in insufficient adjustment of the surface roughness of the base steel sheet. On the other hand, if the elongation rate during temper rolling exceeds 1.2%, the surface roughness of the base steel sheet increases and work hardening occurs due to potential multiplication, which increases the yield strength and causes processing cracks during press forming of the final product.

[0041] As described above, the surface roughness of the temper-rolled base steel sheet surface is preferably 0.7 to 1.0 μm based on the center line average roughness (Ra). If it is less than 0.7 μm, the surface roughness during plating is uniform, but there is a drawback in that plating adhesion is deteriorated. On the other hand, if the surface roughness exceeds 1.0 μm, the plated material is affected by the roughness characteristics of the base steel sheet, and the surface roughness increases, resulting in a drawback in that the surface appearance after hairline processing is deteriorated. However, in consideration of ensuring a beautiful surface appearance after hairline processing and the increase in surface roughness relative to the base steel sheet after zinc-nickel electroplating, the surface roughness of the base steel sheet is more preferably 0.7 to 0.9 μm.

[0042] The temper rolled base steel sheet is immersed in a sulfuric acid bath containing nickel sulfate hydrate and zinc sulfate hydrate to form a zinc-nickel plating layer on the base steel sheet. The method of forming the zinc-nickel plating layer is not particularly limited, but for example, a method can be used in which the base steel sheet is placed at the negative electrode of a vertical plating cell type electroplating simulator, and then a sulfuric acid bath plating solution is circulated to form a zinc-nickel plating layer on one side, and a zinc-nickel plating layer is formed on the other side in the same manner as above.

[0043] The sulfuric acid bath is preferably an acidic bath with a pH of 0.5 to 3.5. In general, an acidic bath has a high current efficiency due to its high electrical conductivity compared to an alkaline bath, and shows a non-uniform distribution of alloy elements in the thickness direction in a normal current density range. That is, a zinc-nickel alloy electroplated steel sheet manufactured under acidic bath conditions shows a distribution in which the nickel content increases from the base steel sheet to the interface of the zinc-nickel plating layer on the surface of the plating layer. In a zinc-nickel alloy electroplated steel sheet, a nickel-enriched layer is formed as corrosion progresses, which can suppress corrosion. In the case of an acidic bath, the nickel concentration at the interface between the base steel sheet and the plating layer is relatively high, and the nickel-enriched layer, which is a mechanism for strengthening the corrosion resistance of a zinc-nickel plated steel sheet, is strengthened, which contributes to ensuring stable corrosion resistance even after polishing and hairline polishing.

[0044] In addition, since the dissolution rate of nickel sulfate or nickel carbonate, which is the nickel source for the zinc plating bath, is fast under acidic bath conditions, an acidic bath is advantageous over an alkaline bath in terms of preventing surface defects such as dents caused by undissolved particles when raw materials are added to replenish nickel from a commercial production perspective.

[0045] The sulfuric acid bath preferably contains nickel sulfate hydrate of 40 to 60 g / L and zinc sulfate hydrate of 60 to 90 g / L. When the nickel sulfate hydrate is less than 40 g / L and the zinc sulfate hydrate is more than 90 g / L, the hardness of the formed zinc-nickel plating layer may be less than 250 Hv, which may cause a problem that a part of the plating layer peels off during hairline pattern processing, preventing the securing of a uniform hairline pattern in the rolling direction, and increases the rolling force on the material, causing fine craters to occur in the surface structure phase during operation. On the other hand, when the nickel sulfate hydrate exceeds 60 g / L and the zinc sulfate hydrate is less than 60 g / L, the hardness of the formed zinc-nickel plating layer may exceed 400 Hv, which may result in only a slight improvement in the ease of surface polishing, while the increase in the content of nickel, which is relatively expensive compared to zinc in the plating layer, increases the residual stress of the zinc-nickel plating layer, causing fine cracks to occur on the surface, and causing a deterioration in price competitiveness, which is not preferable.

[0046] The hardness of the zinc-nickel plating layer thus formed is preferably 250 to 400 Hv. If the hardness of the zinc-nickel plating layer is less than 250 Hv, a part of the plating layer peels off during hairline pattern processing, preventing the securing of a uniform hairline pattern in the rolling direction, and increasing the rolling force on the material, resulting in the generation of fine craters in the surface structure phase during operation. It can be said that the higher the hardness of the plating layer, the easier and more efficient the surface polishing is. However, if the hardness of the zinc-nickel plating layer exceeds 400 Hv, the further improvement in the ease of surface polishing is slight, while the increase in the content of nickel, which is relatively expensive compared to zinc in the plating layer, increases the residual stress of the zinc-nickel plating layer, causing fine cracks on the surface and deteriorating price competitiveness, which is not preferable.

[0047] In addition, the zinc-nickel plating layer is gamma (Ni5Zn 21) single phase. In order to ensure good corrosion resistance in electroplating of zinc-based alloys, it is necessary to reduce the corrosion rate of zinc, which is highly electrochemically reactive (active) in a corrosive environment, and maintain sacrificial corrosion protection for a long time. In this respect, when the gamma single phase is used, not only is the gamma phase electrochemically stable, but there is no galvanic corrosion caused by the potential difference between crystalline phases in a mixed phase such as eta + gamma, so good corrosion resistance can be ensured. In addition, in the gamma single phase region, the hardness of the plating layer increases rapidly with the increase in nickel content in the plating layer, making it easy to ensure high plating layer hardness.

[0048] Next, the zinc-nickel plating layer is polished and processed into a hairline pattern. The polishing and hairline patterning may be performed using a commonly used abrasive belt method, but is not limited thereto.

[0049] Polishing serves to flatten the material before hairline processing, increase the gloss of the material after hairline processing, and determine the amount of plating to be polished off by adjusting the degree of pressing force, i.e., the amount of plating remaining after processing. In addition, in the case of hairline processing, it serves to impart a hair-like pattern to the surface of the zinc-nickel plated steel sheet that has been flattened through polishing.

[0050] More specifically, from the standpoint of removing the surface plating layer and ensuring the flattening effect, it is preferable that the rotation speed of the polishing abrasive belt is faster than the rotation speed of the hairline abrasive belt and that the surface roughness of the polishing abrasive belt is greater than the surface roughness of the hairline abrasive belt.

[0051] Accordingly, when the total change in surface roughness after polishing and hairline processing is taken as 100%, the contribution rate of the polishing to the change in surface roughness is preferably 60 to 85%, and the contribution rate of the hairline pattern processing to the change in surface roughness is preferably 15 to 40%. If the contribution rate of the polishing to the change in surface roughness is less than 60%, the degree of roughness flattening is insufficient, making it difficult to ensure a beautiful surface appearance after processing, and if it exceeds 85%, the loss of the plating layer is excessive, the remaining plating amount is not reached, and damage such as crater shape may occur on the surface of the plating layer. On the other hand, if the contribution rate of the hairline pattern processing to the change in surface roughness is less than 15%, the hairline pattern is not easily recognized by the naked eye on the steel sheet surface, and if it exceeds 40%, there is a problem that the surface appearance is deteriorated due to the unevenness of the hairline pattern profile.

[0052] The change in surface roughness after polishing and hairline processing compared to before can be calculated using the following formula. Contribution rate of change in polished surface roughness = (Ra before processing - Ra after polishing) x 100 / (Ra before processing - Ra after processing) Contribution rate of change in hairline surface roughness = (Ra after polishing - Ra after hairline) x 100 / (Ra before processing - Ra after processing)

[0053] Meanwhile, it is preferable that the change in center line average roughness of the zinc-nickel plating layer after the polishing and hairline pattern processing is -1.00 to -0.35 μm. If it is less than -1.00 μm, there is a problem that fine cracks or craters are generated in the plating layer due to an increase in the rolling force for surface flattening. On the other hand, if it exceeds -0.35 μm, there is a problem that the effect of surface flattening is insufficient or the depth of peaks and valleys on the roughness profile in the direction perpendicular to the rolling becomes large, resulting in deterioration of surface quality. Furthermore, it is preferable that the roughness profile in the direction perpendicular to the rolling of the material before and after processing is a regular and uniform sine curve.

[0054] The thickness of the plating layer after the polishing and hairline patterning is preferably 0.2 to 0.75 of the thickness of the plating layer before the polishing and hairline patterning. That is, the ratio of the thickness of the plating layer after the polishing and hairline patterning to the thickness of the plating layer before the polishing and hairline patterning is preferably 0.2 to 0.75. If it is less than 0.2, the plating thickness of the raw material before the polishing and hairline patterning is thick, or the plating thickness of the processed material is thin, resulting in an increase in manufacturing cost, and the amount of plating that needs to be removed through polishing increases, increasing the load during coil polishing, and the corrosion resistance of the processed material is deteriorated. On the other hand, if it exceeds 0.75, there is a disadvantage that the polishing effect is small and it is difficult to ensure a beautiful surface appearance.

[0055] The zinc-nickel electroplated steel sheet of the present invention as described above has excellent gloss and image clarity after polishing and hairline pattern processing, and is not only cost-competitive with stainless steel or vinyl-laminated steel sheet in terms of realizing a hairline pattern surface appearance, but also ensures high productivity through high-speed operation in a coil polishing line. EXAMPLES

[0056] (Example) The present invention will be described in more detail with reference to the following examples. The following examples are provided to explain the present invention in more detail, but are not intended to limit the present invention.

[0057] Using a temper rolling test machine, the temper rolling roll processing method, processing mode and elongation rate were controlled to produce base steel sheets (ultra-low carbon steel) with different surface roughnesses of 0.6 mm, 140 mm wide and 250 mm long.

[0058] Thereafter, zinc sulfate heptahydrate and nickel sulfate hexahydrate that had been degreased and pickled were added, and a zinc-nickel plating layer was formed on the base steel sheet through electroplating. At this time, the base steel sheet was placed at the negative electrode of a vertical plating cell type electroplating simulator, and a plating solution was circulated to form a zinc-nickel plating layer on one side, and then a zinc-nickel plating layer was formed on the other side in the same manner as above, thereby controlling the plating coverage on both sides of the base steel sheet.

[0059] When the plating layer was formed, the pH of the plating bath was 1.5 to 2.5, the current density was 100 A / dm2, and the flow rate was 1.5 m / s. 30 g / L of sodium sulfate was added to the sulfuric acid bath to correct the conductivity of the plating bath.

[0060] Thereafter, the base steel sheet on which the zinc-nickel plating layer was formed was passed through a polishing and hairline processing device to finally produce a zinc-nickel plated steel sheet with a hairline pattern. At this time, polishing belts of No. 120 to 180 and No. 240 to 320 were used for polishing and hairline, respectively. The manufacturing conditions for each test piece are summarized and shown in Tables 1 and 2 below.

[0061] [Table 1]

[0062] [Table 2]

[0063] The zinc-nickel plated steel sheets manufactured as described above were analyzed for the microhardness and crystalline phase of the plating layer to confirm in advance the physical properties of the steel sheet that affect the surface appearance when forming the hairline pattern. The hardness of the plating layer was measured by applying a load of 5gf using an ultra-microhardness tester (Simadzu, DUH-W201S). Meanwhile, the plating layer crystalline phase was measured by irradiating the test piece with CuKα radiation at an accelerating voltage of 40kV using an X-ray diffraction analyzer (Rigaku, D / MAX 2500V / PC), and analyzing the obtained peaks according to JCPDS.

[0064] In addition, to investigate the effect of the elongation rate of temper rolling on the yield strength of the steel sheet, some test pieces were processed into KS 13B and then subjected to an Instron tensile test at a rate of 10 MPa / s up to the yield zone and a deformation rate of 0.007 s after yielding. -1 The yield strength was evaluated.

[0065] The plating layer of the zinc-nickel plated steel sheets and polished and hairline treated materials prepared as described above was dissolved using a diluted hydrochloric acid solution, and the weight difference before and after was analyzed to confirm the change in plating amount.

[0066] In addition, the surface structure of some of the test specimens that had undergone polishing and hairline processing was analyzed using a JEOL JSM-7001F field emission scanning electron microscope (FE-SEM) to analyze the causes of differences in surface appearance, such as image clarity.

[0067] Furthermore, the roughness of an area of ​​2.4 mm wide and 1.8 mm long was measured using a 3D non-contact roughness measuring device from Veeco Instruments for the bare steel sheets before plating, zinc-nickel plated steel sheets, polished only materials, and hairline processed materials after polishing, which were manufactured as described above, to confirm the change in the surface roughness of the steel sheets due to the process.

[0068] In addition to the analysis items for each process step described above, corrosion resistance, image clarity, and press formability were measured, and the results are shown in the following Table 3. The evaluation methods for corrosion resistance, image clarity, and press formability are as follows.

[0069] 1.Corrosion resistance Corrosion resistance (red rust generation rate) was evaluated by cutting a test piece into a size of 75 x 150 mm, masking the edges with Teflon tape (Nitto Denko Corp. NITOFLON, No. 903UL), placing the test piece in a salt spray tester STP-200 (SUGA Test Instruments, Japan), and leaving it in accordance with JIS (Japan Industrial Standards) Z 2371 (5% sodium chloride, 1-2 ml spray amount per hour, chamber temperature 35°C). The results were evaluated as excellent when the result was 0-10%, excellent when it was over 10% to 40%, normal when it was over 40% to 70%, and insufficient when it was over 70%.

[0070] 2. Imageability The image clarity was evaluated by placing an image clarity measuring device manufactured by Rhopoint Instruments on the final test piece that had been polished and hairline processed, and measuring the distinctness of image (DOI) value.

[0071] DOI values ​​over 30 were rated as excellent, over 20-30 as excellent, 10-20 as average, and less than 10 as insufficient.

[0072] 3. Press formability Press formability was confirmed by painting some test pieces of the hairline and polished material, applying MVP 840TW water-insoluble plastic processing oil from VS&Ein Chemical Co., Ltd. to the final material, and then pressing the steel plate with a SIMPAC 250-ton servo press (SV1P-250) under a load of 13 tons. The length at which paint peeling occurred on the side of the bead or drawing was divided by the entire circumference to determine the incidence of paint damage. In this case, if peeling occurred on either the bead or drawing, it was also determined to be paint peeling.

[0073] The evaluation method was as follows: if the incidence of coating damage was less than 10%, it was rated as excellent; if it was between 10% and 20%, it was rated as average; and if it was more than 20%, it was rated as insufficient.

[0074] [Table 3]

[0075] With reference to Tables 1 to 3, it can be seen that Examples 1 to 5, which satisfy the conditions proposed by the present invention, have excellent corrosion resistance, image clarity, and press workability. However, Comparative Examples 1 to 16 do not satisfy the conditions of temper rolling, zinc-nickel electroplating, polishing, and hairline polishing proposed by the present invention, and therefore do not ensure corrosion resistance, image clarity, and press workability.

[0076] Specifically, in the cases of Comparative Examples 1 to 5 and 13, the temper rolling conditions proposed by the present invention (rolling method and mode, temper rolling elongation rate, and the associated steel sheet roughness) were not satisfied, and therefore it was found that the corrosion resistance, image clarity, and press formability were not simultaneously secured at excellent levels. In particular, it was confirmed that it is important to control the temper rolling elongation rate at an appropriate level in that, although the yield strength of the base steel sheet also increases when the temper rolling elongation rate increases, the zinc-nickel plating layer has a much higher hardness than a typical plating layer of pure zinc composition, and the plating layer restricts the deformation of the base steel sheet, which can be a major factor in deteriorating the formability of the final product.

[0077] In the cases of Comparative Examples 6 to 12, the zinc-nickel electroplating conditions proposed by the present invention (amounts of zinc sulfate and nickel sulfate hydrate, plating time (plating amount), hardness of the plating layer, and crystal phase) were not satisfied, and therefore it is clear that excellent levels of corrosion resistance and image clarity could not be simultaneously ensured.

[0078] In addition, in the case of Comparative Examples 14 to 16, the contribution rates of the change in surface roughness in each of the polishing and hairline polishing processes do not satisfy the conditions proposed by the present invention, and it can be confirmed that excellent levels of corrosion resistance and image clarity cannot be secured simultaneously.

[0079] Meanwhile, Figure 1 shows images of the surface of steel sheets after polishing and hairline processing according to the present invention and the results of analysis using a scanning electron microscope (SEM) at a magnification of 2,000 times, where (a) shows Example 1 and (b) shows Comparative Example 1. Referring to Figure 1, it can be seen that the steel sheet manufactured according to Example 1 has a clear hairline pattern in the rolling direction on the surface and microstructure and has excellent surface quality, whereas the steel sheet manufactured according to Comparative Example 1 has an unclear hairline pattern on the surface and microstructure and does not have excellent surface quality.

[0080] FIG. 2 shows the results of an analysis of the roughness profile in the direction perpendicular to the rolling direction of a steel sheet before and after polishing and hairline processing in accordance with Example 1 of the present invention, where (a) shows the state before processing and (b) shows the state after processing. FIG. 3 shows the results of an analysis of the roughness profile in the direction perpendicular to the rolling direction of a steel sheet before and after polishing and hairline processing in accordance with Comparative Example 1 of the present invention, where (a) shows the state before processing and (b) shows the state after processing.

[0081] 2 and 3, it can be seen that the steel sheet manufactured according to Example 1 has small peaks and valleys and a regular and uniform curve shape before and after processing. On the other hand, it can be seen that the steel sheet manufactured according to Comparative Example 1 has relatively large peaks and valleys compared to Example 1, and the period of the curve is also irregular.

[0082] Although the embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that the scope of the present invention is not limited thereto, and that various modifications and variations are possible without departing from the technical idea of ​​the present invention described in the claims.

Claims

1. Base steel sheet; and a zinc-nickel plating layer located on at least one surface of the base steel sheet and having a hairline pattern formed thereon; the surface roughness of the base steel sheet surface forming the interface with the zinc-nickel plating layer is 0.7 μm or more and less than 1.0 μm based on the center line average roughness (Ra); The zinc-nickel plating layer is gamma (Ni ５ Zinc ２１ ) A zinc-nickel alloy electroplated steel sheet, characterized in that it consists of a single phase.

2. 2. The zinc-nickel alloy electroplated steel sheet according to claim 1, wherein the zinc-nickel plating layer has a hardness of 250 to 400 Hv.

3. A step of temper rolling the base steel sheet; immersing the temper-rolled base steel sheet in a sulfuric acid bath containing nickel sulfate hydrate and zinc sulfate hydrate to form a zinc-nickel plating layer on the base steel sheet; and polishing and hairline patterning the zinc-nickel plating layer; The surface roughness of the temper-rolled base steel sheet surface is 0.7 μm or more and less than 1.0 μm based on the center line average roughness (Ra), The zinc-nickel plating layer is gamma (Ni ５ Zinc ２１ ) a single phase.

4. The method for producing a zinc-nickel alloy electroplated steel sheet according to claim 3, wherein the temper rolling is performed using an electric discharge roll in a capacity (-) mode.

5. 5. The method for producing a zinc-nickel alloy electroplated steel sheet according to claim 4, wherein the surface of the roll is coated with chromium.

6. The method for producing a zinc-nickel alloy electroplated steel sheet according to any one of claims 3 to 5, wherein the temper rolling is performed at a drawing rate of 0.3 to 1.2%.

7. The method for producing a zinc-nickel alloy electroplated steel sheet according to any one of claims 3 to 6, characterized in that the hardness of the zinc-nickel plating layer formed on the base steel sheet is 250 to 400 Hv.

8. The method for producing a zinc-nickel alloy electroplated steel sheet according to any one of claims 3 to 7, characterized in that a change in center line average roughness of the zinc-nickel plating layer after the polishing and hairline patterning is -1.00 to -0.35 μm.

9. The method for producing a zinc-nickel alloy electroplated steel sheet according to any one of claims 3 to 8, characterized in that a change rate of surface roughness of the zinc-nickel plating layer by the polishing is 60 to 85%, and a change rate of surface roughness of the zinc-nickel plating layer by the hairline patterning is 15 to 40%.

10. The method for producing a zinc-nickel alloy electroplated steel sheet according to any one of claims 3 to 9, characterized in that the thickness of the plating layer after the polishing and hairline patterning is 0.2 to 0.75 times the thickness of the plating layer before the polishing and hairline patterning.

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