ZN-NI ELECTROPLATED STEEL SHEET HAVING EXCELLENT CORROSION RESISTANCE AND FORMABILITY AND MANUFACTURING METHOD THEREFOR

Abstract

Provided is a ZnNi electroplated steel sheet having excellent corrosion resistance and formability and a manufacturing method therefor. The steel sheet includes: abase steel sheet; and a ZnNi plated layer formed on the base steel sheet, wherein a Ni alloying degree within the ZnNi plated layer is 7.5-14%, and the Ni amount is 40-50 atom % in a region within 1 pm toward the ZnNi plated layer from an interface of the base steel sheet and the ZnNi plated layer. Provided is a ZnNi electroplated steel sheet, which has superior corrosion resistance and formability despite having a relatively low nickel alloying degree, has excellent price competitiveness compared with that of a conventional ZnNi plated steel sheet and two-layer plated steel sheet, and can ensure high productivity.

Claims

1. A ZnNi electroplated steel sheet having excellent corrosion resistance and workability, the ZnNi electroplated steel sheet comprising: a base steel sheet; and a ZnNi plated layer formed on the base steel sheet, wherein a Ni alloying degree in the ZnNi plated layer is 7.5 to 14%, and a content of Ni is 40 to 50 atom % in a region within 1 pm from a boundary of the base steel sheet and the ZnNi plated layer toward the ZnNi plated layer.

2. The electroplated steel sheet of claim 1, wherein a coating weight of the ZnNi plated layer, formed on one surface of the base steel sheet, is 20 to 50g/m.sup.2, and a coating weight of the ZnNi plated layer, formed on the other surface of the base steel sheet, is 0.25 to 0.8 times that of the coating weight of the ZnNi plated layer formed on one surface of the base steel sheet.

3. A method of manufacturing a ZnNi electroplated steel sheet having excellent corrosion resistance and workability, the method comprising: preparing a base steel sheet; and dipping the base steel sheet in a sulfuric acid bath, having a pH of 1 to 5, including a zinc sulfate heptaheptahydrate and a zinc sulfate heptahydrate, to form a ZnNi plated layer on the base steel sheet, wherein a content of the zinc sulfate heptaheptahydrate is 45 to 93 g/L, and a ratio of the zinc sulfate heptahydrate and the zinc sulfate heptaheptahydrate is 0.4 to 1.4.

4. The method of claim 3, wherein in the forming the ZnNi plated layer, a coating weight of the ZnNi plated layer, coated on one surface of the base steel sheet, is 20 to 50g/m.sup.2, and a coating weight of the ZnNi plated layer, coated on the other surface of the base steel sheet, is 0.25 to 0.8 times that of the coating weight of the ZnNi plated layer coated on one surface of the base steel sheet.

5. The method of claim 3, after the forming of the plated layer, further comprising: heating the steel sheet to 190 to 260 C. on the basis of a peak metal temperature (PMT).

6. The method of claim 5, wherein the heating is performed using induction hardening or hot-air hardening.

Description

MODE FOR INVENTION

[0024] In the description below, an example embodiment of the present disclosure will be described in greater detail. It should be noted that the example embodiment is provided to describe the present disclosure in greater detail, and to not limit the present disclosure. The scope of right of the present disclosure may be determined based on the matters recited in the claims and the matters rationally inferred therefrom.

Embodiment

[0025] After degreasing and pickling a base steel sheet (extremely low carbon steel) having a thickness of 0.8 mm, a width of 140 mm and a length of 250 mm, the base steel was electroplated and thermally treated under conditions, listed in Table 1, to form a ZnNi plated layer on the steel sheet. In this case, a ZnNi plated layer was formed on one surface of the base steel sheet by placing the base steel sheet on a cathode of a vertical plating cell type electroplating simulator and circulating a plating solution. Then, a ZnNi plated layer was formed on the other surface of the base steel sheet in the same manner as described above. Thus, coating weights on both surfaces of the base steel sheet were controlled to be different from each other. When the plated layer was formed, current density was 100 A/dm.sup.2, a flow rate was 1.5 m/s, and a sulfuric acid bath included 30 g/L of sodium sulfate. The heating treatment was performed using an induction hardening apparatus.

TABLE-US-00001 TABLE 1 Plating Conditions of Sulfuric Acid Bath Conditions Zinc Sulfate Nickel Sulfate Zinc Sulfate Plating time Plating time Heating Heptaheptahydrate Hexaheptahydrate Heptaheptahydrate/Nickel of One of the Other Temperature on the classification pH (g/L) (g/L) Sulfate Hexaheptahydrate Surface (sec) Surface (sec) basis of PMT ( C.) CE1 3 75.0 20.0 3.75 10 8.8 CE2 3 65.0 45.0 1.44 10 8.8 CE3 3 64.2 43.2 1.49 10 8.8 IE1 1 62.4 46.8 1.33 10 8.8 IE2 3 62.4 46.8 1.33 10 8.8 IE3 5 62.4 46.8 1.33 10 8.8 CE4 7 62.4 46.8 1.33 10 8.8 CE5 9 62.4 46.8 1.33 10 8.8 IE4 1 52.0 66.0 0.79 10 8.8 IE5 3 52.0 66.0 0.79 10 8.8 IE6 5 52.0 66.0 0.79 10 8.8 CE6 7 52.0 66.0 0.79 10 8.8 CE7 9 52.0 66.0 0.79 10 8.8 IE7 1 46.1 78.6 0.59 10 8.8 IE8 3 46.1 78.6 0.59 10 8.8 IE9 5 46.1 78.6 0.59 10 8.8 CE8 7 46.1 78.6 0.59 10 8.8 CE9 9 46.1 78.6 0.59 10 8.8 IE10 1 38.9 92.7 0.42 10 8.8 IE11 3 38.9 92.7 0.42 10 8.8 IE12 5 38.9 92.7 0.42 10 8.8 CE10 7 38.9 92.7 0.42 10 8.8 CE11 9 38.9 92.7 0.42 10 8.8 CE12 3 35.2 99.8 0.35 10 8.8 CE13 3 28.0 114.0 0.25 10 8.8 CE14 3 52.0 66.0 0.79 10 8.8 180 IE13 3 52.0 66.0 0.79 10 8.8 190 IE14 3 52.0 66.0 0.79 10 8.8 200 IE15 3 52.0 66.0 0.79 10 8.8 230 IE16 3 52.0 66.0 0.79 10 8.8 260 CE15 3 52.0 66.0 0.79 10 8.8 270 CE16 3 52.0 66.0 0.79 10 8.8 290 CE17 3 52.0 66.0 0.79 4.0 10.0 IE17 3 52.0 66.0 0.79 7.5 5.8 IE18 3 52.0 66.0 0.79 10.0 8.8 IE19 3 52.0 66.0 0.79 13.9 7.5 IE20 3 52.0 66.0 0.79 17.0 5.0 CE18 3 52.0 66.0 0.79 20.8 15.5 CE19 3 52.0 66.0 0.79 10.0 2.0 IE21 3 52.0 66.0 0.79 10.0 2.8 IE22 3 52.0 66.0 0.79 10.0 4.0 IE23 3 52.0 66.0 0.79 10.0 5.8 IE24 3 52.0 66.0 0.79 10.0 7.5 IE25 3 52.0 66.0 0.79 10.0 8.8 CE20 3 52.0 66.0 0.79 10.0 9.5 IE: Inventive Example, CE: Comparative Example

[0026] For the ZnNi electroplated steel sheet manufactured as described above, an alloying degree of the ZnNi plated layer, a content of NI in a region within 1 m from a boundary between base steel sheet and ZnNi plated layer toward the ZnNi plated layer, a coating weight, corrosion resistance, and workability were measured, and results thereof are listed in Table 2.

[0027] In the corrosion resistance, a fraction of red rust occurring was measured by cutting a sample into a size of 75150 mm, masking edges of the cut sample with a Teflon tape (Nitto Denko Corp., NITOFLON, No. 903UL), and putting the sample in a salt spraying tester STP-200 (SUGA Test Instruments Co., Ltd., Japan) to be left based on Japan Industrial Standards (JIS) Z 2371 (5% of sodium chloride, 1 to 2 ml of spraying time per hour, and a chamber temperature of 35 C.). The fraction was evaluated to be (significantly excellent) when 0 to 10%, to be o (excellent) when more than 10% to 40%, to be (normal) when more than 40% to 70%, and to be (poor) when more than 70%.

[0028] In the corrosion resistance, sacrificial corrosion protection was measured by cutting a sample into a size of 2020 mm and dipping the cut sample in 3.5% of a sodium chloride at a temperature of 30 C. to evaluate corrosion potential using a potentiostat. 273A model (EG & G Princeton Applied Research, US), equipped with 352 SoftCorr III used as corrosion measurement software, was used a potentiostat apparatus. In this case, a saturated calomel electrode (SCE) and a high-density carbon electrode were used as a reference electrode and a counter electrode, respectively. A scanning rate was 0.333 mV/s, and the corrosion potential was derived by means of Tafel extrapolation. Electrochemical stability, for example, sacrificial corrosion protection was compared by comparing corrosion potentials under respective conditions based on a point of time at which 72 hours have passed since the dipping time. The sacrificial corrosion protection was evaluated to be (excellent) when the corrosion potential was lower than that of the base steel, to be (normal) when the corrosion potential was equal to that of the base steel, and to be (poor) when the corrosion potential was higher than that of the base steel.

[0029] In the workability, powdering resistance was measured by cutting a sample into a size of 50100 mm, attaching a Nichiban tape, Nichiban CT-24, to a surface of the sample, placing the sample on a V block and pressing a center of the sample using a pressing device such that a load is applied thereto to have a bending angle of 60 degrees, and removing the tape from the sample, after the test was finished, to determine whether the plated layer was peeled off. The powdering was evaluated to be (excellent) when the plated layer was not peeled off and to be (poor) when the plated layer was peeled off.

[0030] In the workability, the tensile r-value was calculated in an elongation section of 10 to 15% by working a sample based on JIS 13B standard and testing the sample using a tensile tester, Instron 5582 (Instron Inc., USA), at a room temperature and at a tensile rate of 10 mm per minute. In the case in which a heating treatment was performed, the tensile r-value was evaluated to be (significantly excellent) when the calculated tensile r-value was improved by more than 60% as compared to a tensile r-value of an unheated sample, a reference sample, to be (excellent) when the calculated tensile r-value was improved by more than 30% to 60% as compared to the tensile r-value of the reference sample, and to be (poor) when the calculated tensile r-value was improved by less than 0% as compared to the tensile r-value of the reference sample (poor as compared to the reference sample). In the case in which coating weights on both surfaces of a steel sheet were different from each other, the tensile r-value was evaluated to be (significantly excellent) when the calculated tensile r-value was improved by more than 20% as compared to a tensile r-value of a sample, in which coating weights on both surfaces had the same value of 30 g/m.sup.2, a reference sample, to be (excellent) when the calculated tensile r-value was improved by more than 5% to 20% as compared to the tensile r-value of the reference sample, to be (normal) when the calculated tensile r-value was improved by 0% to 5% as compared to the tensile r-value of the reference sample, and to be (poor) when the calculated tensile r-value was improved by less than 0% (poor as compared to the sample in which coating weights on both surfaces had the same value of 30 g/m.sup.2).

TABLE-US-00002 TABLE 2 Content of Ni (at %) in region Corrosion within 1 m from Coating Coating Weight Resistance boundary between base Weight of of the Sacrificial Workability Alloying steel sheet and ZnNi plated One Surface Other Surface Fraction of Red Corrosion Tensile Powdering Classification Degree (%) layer toward ZnNi plated layer (g/m.sup.2) (g/m.sup.2) Rust Occurring Protection r-Value Resistance CE1 4.3 34.4 30 24 X X CE2 6.4 36.3 30 24 CE3 7.0 37.2 30 24 IE1 7.5 41.2 30 24 IE2 7.5 41.7 30 24 IE3 7.5 41.9 30 24 CE4 7.5 39.5 30 24 CE5 7.5 39.4 30 24 IE4 10.5 44.4 30 24 IE5 10.5 44.3 30 24 IE6 10.5 44.1 30 24 CE6 10.5 35.7 30 24 CE7 10.5 36.2 30 24 IE7 12.0 46.0 30 24 IE8 12.0 45.9 30 24 IE9 12.0 46.2 30 24 CE8 12.0 36.0 30 24 X CE9 12.0 36.3 30 24 X IE10 14.0 48.8 30 24 IE11 14.0 48.9 30 24 IE12 14.0 49.1 30 24 CE10 14.0 38.9 30 24 X X CE11 14.0 38.9 30 24 X X CE12 15.0 50.5 30 24 X CE13 17.0 52.1 30 24 X X CE14 10.5 44.3 30 24 IE13 10.5 44.3 30 24 IE14 10.5 44.3 30 24 IE15 10.5 44.3 30 24 IE16 10.5 44.3 30 24 CE15 10.5 44.3 30 24 CE16 10.5 44.3 30 24 X CE17 10.5 44.3 10 2 X X IE17 10.5 44.3 20 15 IE18 10.5 44.3 30 24 IE19 10.5 44.3 40 20 IE20 10.5 44.3 50 12.5 CE18 10.5 44.3 60 30 X X X CE19 10.5 44.3 30 4 IE21 10.5 44.3 30 7.5 IE22 10.5 44.3 30 10 IE23 10.5 44.3 30 15 IE24 10.5 44.3 30 20 IE25 10.5 44.3 30 24 CE20 10.5 44.3 30 26 IE: Inventive Example, CE: Comparative Example

[0031] As can be seen from Tables 1 and 2, Inventive Examples 1 to 25, satisfying the conditions proposed by the present disclosure, had excellent corrosion resistance (fraction of red rust occurring and sacrificial corrosion protection) and excellent workability (tensile r-value and powdering resistance).

[0032] Since Comparative Examples 1 to 13 did not satisfy the sulfuric acid bath conditions proposed by the present disclosure, they did not satisfy the condition such as a Ni alloying degree in an ZnNi plated layer, or a content of Ni in a region within 1 m from a boundary between a base steel sheet and the ZnNi plated layer toward the ZnNi plated layer, and thus, did not secure excellent levels of corrosion resistance and workability.

[0033] Since Comparative Examples 14 to 16 did not the heating conditions proposed by the present disclosure, they did not simultaneously secure excellent levels of corrosion resistance and workability.

[0034] Since Comparative Examples 17 to 20 did not satisfy coating weight conditions proposed by the present disclosure, they did not secure excellent levels of corrosion resistance and workability.