NON-COATED AUSTENITIC STEEL SHEET WITH IMPROVED CORROSION RESISTANCE IN ALKALINE ENVIRONMENT AND METHOD FOR MANUFACTURING SAME

Abstract

A non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to an embodiment comprises, by weight %, C: more than 0% but not more than 0.04%, Si: more than 0% but not more than 0.4%, Mn: more than 0% but not more than 0.5%, Cr: more than 0% but not more than 2.0%, Ni: 33-40%, Co: more than 0% but not more than 4.0%, the balance being Fe and other inevitable impurities, and the value of equation (1) below may be 0.83 or less. Equation (1): 9.0-0.2495×Ni+0.9×Cr−0.005×Co, wherein Ni, Cr, and Co refer to the content (wt %) of each element.

Description

TECHNICAL FIELD

The present invention relates to a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment and a method of manufacturing the same.

BACKGROUND ART

In recent years, in order to prevent global warming, efforts have been made to utilize hydrogen, an eco-friendly fuel. In particular, hydrogen production is being explored through electrolysis methods of using electrolysis of water in conjunction with renewable energy. Electrolysis methods include high-temperature electrolysis, proton exchange membrane electrolysis, and alkaline electrolysis, among which the alkaline electrolysis is a method that has been most widely commercialized.

An alkaline electrolysis device includes an electrolyte, an anode, a cathode, a separator, bipolar plates, and the like, and the electrolyte may include a strong alkaline solution such as a 25 to 30% KOH solution. Therefore, the components included in the alkaline electrolysis devices need to employ materials that are highly corrosion-resistant in strong alkaline solutions.

Conventionally, pure Ni metal or Ni-coated stainless steel has been used for bipolar plates in alkaline electrolysis devices.

However, the conventional technologies have issues such as low price competitiveness due to the use of expensive Ni, and reduced overall cell performance caused by defects in a nickel coating layer. In addition, since the conventional technologies employ large-area bipolar plates, it is difficult to achieve proper coating adhesion, and the better the corrosion resistance of the stainless steel, the more challenging it become to achieve proper coating.

DISCLOSURE

Technical Problem

To resolve the above-described issues, the present invention is directed to providing a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment by controlling the alloy composition and manufacturing process.

Technical Solution

A non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment may include, in percent by weight (wt %), more than 0% and 0.04% or less of C, more than 0% and 0.4% or less of Si, more than 0% and 0.5% or less of Mn, more than 0% and 2.0% or less of Cr, 33 to 40% of Ni, more than 0% and 4.0% or less of Co, the balance being Fe and other inevitable impurities, and have a value of Expression (1) of 0.83 or less.

$\begin{array}{cc}9.-0.2495\times \mathrm{Ni}+0.9\times \mathrm{Cr}-0.005\times \mathrm{Co},& \mathrm{Expression}\left(1\right)\end{array}$

In Expression (1), Ni, Cr and Co represent the content (wt %) of each element.

The non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment may have a current density ratio expressed by Expression (2) in a range of 1.9 or less.

$\begin{array}{cc}\mathrm{current}\mathrm{density}\mathrm{of}\mathrm{the}\mathrm{non}-\mathrm{coated}\mathrm{austenitic}\mathrm{steel}\mathrm{sheet}\mathrm{having}\mathrm{improved}\mathrm{corrosion}\mathrm{resistance}\mathrm{in}\mathrm{an}\mathrm{alkaline}\mathrm{enivronment}/\mathrm{current}\mathrm{density}\mathrm{of}\mathrm{pure}\mathrm{Ni}\mathrm{metal}& \mathrm{Equation}\left(2\right)\end{array}$

In the non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment, the alkaline environment may have a concentration of [OH—] ions of 0.3 mol to 7.5 mol based on a molar concentration.

A method of manufacturing a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment, may include: preparing an ingot comprising, in percent by weight (wt %), more than 0% and 0.04% or less of C, more than 0% and 0.4% or less of Si, more than 0% and 0.5% or less of Mn, more than 0% and 2.0% or less of Cr, 33 to 40% of Ni, more than 0% and 4.0% or less of Co, the balance being Fe and other inevitable impurities; reheating the ingot and hot-rolling the reheated ingot to produce a hot-rolled steel sheet; and subjecting the hot-rolled steel sheet to solution-heat-treating.

In the method of manufacturing a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment, the slab may have a value of Expression (1) of 0.83 or less.

$\begin{array}{cc}9.-0.2495\times \mathrm{Ni}+0.9\times \mathrm{Cr}-0.005\times \mathrm{Co}.& \mathrm{Expression}\left(1\right)\end{array}$

In Expression (1), Ni, Cr and Co represent the content (wt %) of each element.

In the method of manufacturing a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment, the reheating may be performed at a temperature of 1150° C. to 1350° C.

In the method according to an embodiment, the solution heat treatment may be performed at a temperature of 800° C. to 900° C.

Advantageous Effects

According to an embodiment of the disclosed invention, a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment may be provided by controlling the alloy composition and the manufacturing process.

MODES OF THE INVENTION

Hereinafter, examples of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the examples shown herein but may be embodied in other forms. In order to make the description of the present invention clear, unrelated parts are not shown and, the sizes of components are exaggerated for clarity.

Throughout the specification, when a part is referred to as “including”, “comprising” and/or “having” a certain element, it is understood that, unless expressed otherwise, the description does not preclude the presence or addition of one or more elements.

The singular form of a noun corresponding to an item may include one or a plurality of the items unless clearly indicated otherwise in a related context.

Hereinafter, the reason for numerically limiting the alloy element contents in the embodiments of the present invention will be described. Unless otherwise specified, the units thereof are expressed in weight percent (wt %).

A non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment may include, in percent by weight (wt %), more than 0% and 0.04% or less of C, more than 0% and 0.4% or less of Si, more than 0% and 0.5% or less of Mn, more than 0% and 2.0% or less of Cr, 33 to 40% of Ni, more than 0% and 4.0% or less of Co, the balance being Fe and other inevitable impurities.

The content of C (carbon) may be more than 0% and 0.04% or less.

C is an element that forms carbides and lowers the quality of steel, and may be controlled to an extremely low level through various decarburization processes. However, performing an extreme decarburization process may increase processing costs. Considering this, the content of C may be more than 0% and 0.04% or less. Preferably, the content of C may be 0.01% to 0.02%.

The content of Si (silicon) may be more than 0% and 0.4% or less.

Si is an essential element added for deoxidation during alloy refining. Therefore, the addition of Si may facilitate deoxidation during refining, to thereby reduce the oxygen content and enable more favorable control of inclusions. However, excessive addition of Si may degrade the quality of the steel due to inclusions. Considering this, the content of Si may be more than 0% and 0.4% or less. Preferably, the content of Si may be 0.10% to 0.2%.

The content of Mn (manganese) may be more than 0% and 0.5% or less.

Mn is an element effective for solid solution strengthening and improving hot workability. In particular, Mn may be used as a deoxidizer together with Si during alloy refining. However, when the content of Mn is excessive, sulfides such as MnS may be formed, resulting in poor corrosion resistance. In consideration of this, the content of Mn may be more than 0% and 0.5% or less. Preferably, the content of Mn may be 0.15% to 0.35%.

The content of Cr (chromium) may be more than 0% and 2.0% or less.

Cr may be dissolved at a level of 0 V relative to the hydrogen electrode in an alkaline environment. Therefore, when Cr is contained, corrosion resistance may be reduced in an alkaline environment. However, since Cr is an essential element added in the stainless steel manufacturing process, additional costs may be required for removing Cr introduced from the scrap, etc. In consideration of this, the content of Cr may be more than 0% and 2.0% or less. Preferably, the content of Cr may be 0.02% to 2.0%.

The content of Ni (nickel) may be 33% to 40%.

Ni is an essential element for ensuring corrosion resistance in an alkaline environment. In consideration of this, the content of Ni may be 33% or more. However, Ni is an expensive element, and excessive addition of Ni may decrease the price competitiveness. Considering this, the upper limit of the Ni content may be limited to 40%.

The content of Co (cobalt) may be more than 0% and 4.0% or less.

Co, along with Ni, is a highly stable element in an alkaline environment, and is effective in improving corrosion resistance. However, Co is an expensive element, and excessive addition of Co may decrease the price competitiveness. Considering this, the content of Co may be more than 0% and 4.0% or less. Preferably, the content of Co may be more than 0.01% and 4.0% or less, and more preferably 0.01% or more and 2.0% or less.

The remaining component(s) of the disclosed invention is iron (Fe). However, unintended impurities may inevitably be introduced from raw materials or the surrounding environment in a typical manufacturing process, and thus cannot be excluded. Since such impurities may be well known to those skilled in the art of conventional manufacturing processes, details thereof are not described in this specification.

A non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment may have a value of Expression (1) in a range of 0.83 or less.

$\begin{array}{cc}9.-0.2495\times \mathrm{Ni}+0.9\times \mathrm{Cr}-0.005\times \mathrm{Co},& \mathrm{Expression}\left(1\right)\end{array}$

In Expression (1), Ni, Cr and Co represent the content (wt %) of each element.

Ni, Cr, and Co are elements that may directly affect corrosion resistance in an alkaline environment. Therefore, the disclosed invention aims to provide an austenitic steel sheet having improved corrosion resistance in an alkaline environment by controlling the value of Expression (1) including Ni, Cr, and Co in a range of 0.83 or less. Specifically, the non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment according to an embodiment may have a value of Expression (1) in a range of −1 to 0.83, more specifically 0.001 to 0.83, and even more specifically 0.003 to 0.8. Within the range, the non-coated austenitic steel sheet may realize a current density equivalent to or improved than that of pure Ni metal, while further improving the alkaline resistance and corrosion resistance.

The non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to an embodiment may have a current density ratio expressed by Expression (2) in a range of 1.9 or less.

$\begin{array}{cc}\mathrm{current}\mathrm{density}\mathrm{of}\mathrm{the}\mathrm{non}-\mathrm{coated}\mathrm{austenitic}\mathrm{steel}\mathrm{sheet}\mathrm{having}\mathrm{improved}\mathrm{corrosion}\mathrm{resistance}\mathrm{in}\mathrm{an}\mathrm{alkaline}\mathrm{enivronment}/\mathrm{current}\mathrm{density}\mathrm{of}\mathrm{pure}\mathrm{Ni}\mathrm{metal}& \mathrm{Expression}\left(2\right)\end{array}$

The current density ratio refers to the value obtained by dividing the current density of the non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment by the current density of the pure Ni metal. Therefore, when the current density ratio is 1.0, it may be evaluated that the steel sheet has a corrosion resistance equivalent to that of pure Ni metal.

On the other hand, the current density may tend to decrease as the corrosion resistance in an alkaline environment improves. In addition, when the current density ratio is lower than 1.0, it may be evaluated that the steel sheet has better corrosion resistance than pure Ni metal.

In this respect, the lower limit of the current density ratio expressed by Expression (2) may be 0.001, specifically 0.01, more specifically 0.1, and even more specifically 0.5. Within the range, the non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to an embodiment of the present invention may implement more advantageous properties for use as a material for electrolysis batteries such as high-temperature electrolysis, proton exchange membrane electrolysis, and alkaline electrolysis. In addition, the effect of improving productivity and cost reduction in achieving the desired properties may be further increased.

The non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to an embodiment may have a current density ratio expressed by Expression (2) in a range of 1.9 or less. Preferably, the current density ratio may be 0.001 to 1.9, more preferably 0.1 to 1.5, more preferably 0.5 to 1.07, and even more preferably 0.8 to 1.07.

The alkaline environment may have a concentration of [OH—] ions of 0.3 mol to 7.5 mol based on the molar concentration.

Meanwhile, since the non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to an example of the disclosed invention does not undergo a separate coating, cell performance degradation due to a defective coating layer may not occur. In addition, since the non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to an example of the disclosed invention does not undergo a large-area coating process, excellent price competitiveness may be ensured.

Next, a method of manufacturing a non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to another aspect of the disclosed invention is described.

According to an embodiment, a method of manufacturing an non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment may include: preparing an ingot including, in percent by weight (wt %), more than 0% and 0.04% or less of C, more than 0% and 0.4% or less of Si, more than 0% and 0.5% or less of Mn, more than 0% and 2.0% or less of Cr, 33 to 40% of Ni, more than 0% and 4.0% or less of Co, the balance being Fe and other inevitable impurities; reheating the ingot and hot-rolling the reheated ingot to produce a hot-rolled steel sheet; and subjecting the hot-rolled steel sheet to solution-heat-treating.

The slab may have a value of Expression (1) of 0.83 or less.

$\begin{array}{cc}9.-0.2495\times \mathrm{Ni}+0.9\times \mathrm{Cr}-0.005\times \mathrm{Co}.& \mathrm{Expression}\left(1\right)\end{array}$

In Expression (1), Ni, Cr and Co represent the content (wt %) of each element.

Specifically, the value of Expression (1) may be in a range of −1 to 0.83, more specifically 0.001 to 0.83, and even more specifically 0.003 to 0.8.

The reasons for numerically limiting the component range of each alloy composition and the value of Expression (1) are as described above, and the following provides details of manufacturing operations.

After preparing a slab satisfying the alloy composition and Expression (1), a series of processes including reheating, hot rolling, and solution heat treating may be performed.

First, the slab may be reheated at a temperature of 1150 to 1350° C. and then hot rolled.

By reheating at a temperature of 1150 to 1350° C. and hot rolling, coarse precipitates generated during the slab preparation may be re-decomposed and internal grains may be controlled to an appropriate size.

After the hot-rolling to produce a hot-rolled steel sheet, the hot-rolled steel sheet is subject to a solution heat treatment at 800 to 900° C.

Meanwhile, the method of manufacturing a non-coated austenitic steel sheet with improved corrosion resistance in an alkaline environment according to an example of the disclosed invention does not perform a separate Ni coating process, and thus has excellent productivity and superior price competitiveness.

Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present invention is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

EXAMPLES

For the various alloy composition ranges shown in Table 1 below, ingots measuring 150×150×280 mm were manufactured by melting in a vacuum melting furnace. The ingots were reheated to 1240° C. and then hot-rolled to a thickness of 3 mm to produce hot-rolled steel sheets. The hot-rolled steel sheets were subjected to solution heat treatment at 850° C. to produce specimens.

	TABLE 1
	Alloy composition (wt %)
Class.	C	Si	Mn	Cr	Ni	Co
Example 1	0.01	0.1	0.15	0.02	33	0.02
Example 2	0.01	0.1	0.15	0.12	34.7	0.01
Example 3	0.011	0.1	0.15	0.1	36	0.01
Example 4	0.011	0.15	0.2	0.49	36.2	0.01
Example 5	0.015	0.15	0.3	0.1	36.1	0.3
Example 6	0.02	0.16	0.35	0.1	36.3	1.0
Example 7	0.017	0.2	0.3	0.1	36	2.0
Example 8	0.02	0.2	0.3	2.0	40	4.0
Example 9	0.02	0.2	0.3	2.0	40	2.0
Comparative	0.015	0.1	0.15	18	10	0.01
Example 1
Comparative	0.018	0.15	0.2	0.5	33	1.0
Example 2
Comparative	0.02	0.17	0.2	1.5	38	2.0
Example 3
Comparative	0.03	0.2	0.18	0.97	36.2	0.01
Example 4
Comparative	0.025	0.23	0.3	1.98	36	0.01
Example 5
Comparative	0.013	0.3	0.35	4.0	36	0.01
Example 6

Table 2 below shows the values of Expression (1) and the current density ratios. The values of Expression (1) were calculated by calculating Expression (1) below. Expression (1):

9.0−0.2495×Ni+0.9×Cr−0.005×Co

In Expression (1), Ni, Cr, and Co represent the content (wt %) of each element.

The current density ratio was calculated and expressed as follows.

$\mathrm{Current}\mathrm{density}\mathrm{ratio}=\left(\mathrm{current}\mathrm{density}\mathrm{of}\mathrm{non}-\mathrm{coated}\mathrm{austenitic}\mathrm{steel}\mathrm{sheet}\mathrm{having}\mathrm{improved}\mathrm{corrosion}\mathrm{resistance}\mathrm{in}\mathrm{an}\mathrm{alkaline}\mathrm{environment}/\mathrm{current}\mathrm{density}\mathrm{of}\mathrm{pure}\mathrm{Ni}\mathrm{metal}\right)$

The current density was measured by applying a voltage of 0.4 V versus the standard hydrogen electrode to the specimen and pure Ni metal in a 30% KOH solution for 6 hours.

On the other hand, the current density may tend to decrease as the corrosion resistance in an alkaline environment improves. In addition, when the current density ratio is 1.0, it may be evaluated that the steel sheet has corrosion resistance equivalent to that of pure Ni metal, and when the current density ratio is lower than 1.0, it may be evaluated that the steel sheet has corrosion resistance better than that of pure Ni metal.

	TABLE 2
	Class.	Expression (1)	Current Density Ratio
	Example 1	0.78	1.04
	Example 2	0.45	0.97
	Example 3	0.11	0.98
	Example 4	0.41	0.99
	Example 5	0.08	1.0
	Example 6	0.03	0.95
	Example 7	0.1	0.84
	Example 8	0.8	0.99
	Example 9	0.81	1.07
	Comparative	22.7	48.17
	Example 1
	Comparative	1.21	1.95
	Example 2
	Comparative	0.86	1.97
	Example 3
	Comparative	0.84	2.02
	Example 4
	Comparative	1.8	2.11
	Example 5
	Comparative	3.62	3.8
	Example 6

Referring to Table 2, Examples 1 to 9 satisfied the alloy composition, Expression (1), and manufacturing method presented in the disclosed invention. Therefore, Examples 1 to 9 satisfied a current density ratio of 1.9 or less. That is, Examples 1 to 9 may be evaluated as having improved corrosion resistance in an alkaline environment. However, Comparative Examples 1 and 6 had excessively high Cr content, and Comparative Example 1 had excessively low Ni content. Therefore, Comparative Examples 1 and 6 did not satisfy the value of Expression (1) of 0.83 or less. Therefore, Comparative Examples 1 and 6 did not satisfy the current density ratio of 1.9 or less. That is, Comparative Examples 1 and 6 had significant inferior corrosion resistance in an alkaline environment. Comparative Examples 2 to 5 satisfied the alloy composition, but did not satisfy the value of Expression (1) of 0.83 or less. Therefore, Comparative Examples 2 to 5 did not satisfy the current density ratio of 1.9 or less. That is, Comparative Examples 2 to 5 had inferior corrosion resistance in an alkaline environment.

According to an embodiment of the disclosed invention, by controlling the alloy composition and the manufacturing method, a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment may be provided.

In addition, according to an example of the disclosed invention, a non-coated austenitic steel sheet may be manufactured with a reduced amount of expensive Ni element added and without introducing a separate coating process. Therefore, according to an example of the disclosed invention, a non-coated austenitic steel sheet having excellent price competitiveness may be manufactured.

Claims

1. A non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment, comprising in percent by weight (wt %), more than 0% and 0.04% or less of C, more than 0% and 0.4% or less of Si, more than 0% and 0.5% or less of Mn, more than 0% and 2.0% or less of Cr, 33 to 40% of Ni, more than 0% and 4.0% or less of Co, the balance being Fe and other inevitable impurities, and 9. - 0.2495 × Ni + 0.9 × Cr - 0.005 × Co, Expression ⁢ ( 1 )

having a value of Expression (1) of 0.83 or less,

wherein in Expression (1), Ni, Cr and Co represent the content (wt %) of each element.

2. The non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment of claim 1, wherein a current density ratio expressed by Expression (2) is 1.9 or less, current ⁢ density ⁢ of ⁢ the ⁢ non - coated ⁢ austenitic ⁢ steel ⁢ sheet ⁢ having ⁢ improved ⁢ corrosion ⁢ resistance ⁢ in ⁢ an ⁢ alkaline ⁢ enivronment / current ⁢ density ⁢ of ⁢ pure ⁢ Ni ⁢ metal Expression ⁢ ( 2 )

3. The non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment of claim 1, wherein the alkaline environment has a concentration of [OH—] ions of 0.3 mol to 7.5 mol based on a molar concentration.

4. A method of manufacturing a non-coated austenitic steel sheet having improved corrosion resistance in an alkaline environment, comprising

preparing an ingot comprising, in percent by weight (wt %), more than 0% and 0.04% or less of C, more than 0% and 0.4% or less of Si, more than 0% and 0.5% or less of Mn, more than 0% and 2.0% or less of Cr, 33 to 40% of Ni, more than 0% and 4.0% or less of Co, the balance being Fe and other inevitable impurities;

reheating the ingot and hot-rolling the reheated ingot to produce a hot-rolled steel sheet; and

subjecting the hot-rolled steel sheet to solution-heat-treating.

5. The method of claim 4, wherein the slab has a value of Expression (1) of 0.83 or less, 9. - 0.2495 × Ni + 0.9 × Cr - 0.005 × Co, Expression ⁢ ( 1 )

wherein in Expression (1), Ni, Cr and Co represent the content (wt %) of each element.

6. The method of claim 4, wherein the reheating is performed at a temperature of 1150° C. to 1350° C.

7. The method of claim 4, wherein the solution heat treatment is performed at a temperature of 800° C. to 900° C.