AUSTENITIC STAINLESS STEEL WITH IMPROVED HYDROGEN EMBRITTLEMENT RESISTANCE AND LOW-TEMPERATURE IMPACT TOUGHNESS, AND MANUFACTURING METHOD THEREFOR

Abstract

An austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness, according to an embodiment, may comprise, by wt %, C in an amount greater than 0% and less than or equal to 0.1%, Si in an amount greater than 0% and less than or equal to 1.5%, 12-23% of Cr, 1-12% of Ni, 10-25% of Mn, Cu in an amount greater than 0% and less than or equal to 1.2%, 0.1-0.3% of N and the balance of Fe and impurities, wherein the value of formula (1) may be 1.0-12.3. Formula (1): In formula (1) (Ni+15 X N)×(0.03 X Mn), Ni, N and Mn mean the amounts (wt %) of respective elements.

Description

TECHNICAL FIELD

The present invention relates to an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness and a method of manufacturing the same.

BACKGROUND ART

With the development and increasing distribution of fuel cell vehicles using hydrogen as fuel, there is a growing demand for the development of hydrogen storage containers and parts. Hydrogen storage containers may be classified into liquefied hydrogen storage containers and gaseous hydrogen storage containers depending on the form of hydrogen, and the operating temperature varies depending on the form of hydrogen. Therefore, the materials used for hydrogen storage containers need to have less degradation in the mechanical property of the steel due to hydrogen at various temperatures.

Meanwhile, the liquefied hydrogen storage method has higher storage efficiency compared to the gaseous storage method, and is therefore expected to be used for various fields. Accordingly, materials used for hydrogen storage containers needs to be selected considering property degradation not only at room temperature but also at extremely low temperatures.

In addition, since hydrogen storage containers are often exposed to hydrogen in a gaseous form, there is a risk of hydrogen embrittlement. Therefore, materials used for hydrogen storage containers need to be selected considering hydrogen embrittlement caused by gaseous hydrogen.

In other words, materials used for hydrogen containers need to be selected with simultaneous consideration of property degradation caused by temperature and property degradation caused by hydrogen. However, it is not easy to achieve the characteristic of preventing property degradation caused by temperature and the characteristic of preventing property degradation caused by hydrogen.

DISCLOSURE

Technical Problem

To resolve the above-described issues, the present invention is directed to providing an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness and a method of manufacturing the same by controlling an alloy composition and a manufacturing method.

Technical Solution

An austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment comprises, in percent by weight (wt %), more than 0% and 0, 1% or less of C, more than 0% and 1.5% or less of Si, 12 to 23% of Cr, 1 to 12% of Ni, 10 to 25% of Mn, more than 0% and 1.2% or less of Cu, 0.1 to 0.3% of N, and the remainder of Fe and impurities, and a value of Expression (1) below is 1.0 to 12.3.

$\begin{array}{cc}\left(\mathrm{Ni}+15\times N\right)\times \left(0.03\times \mathrm{Mn}\right)& \mathrm{Expression}\left(1\right)\end{array}$

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

The austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment may have a −196° C. Charpy impact toughness value of 50 J or more.

The austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment may have a relative notch tensile strength (RNTS) of 0.8 to 1.0.

A method of manufacturing an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness the method comprising: preparing a slab comprising, in percent by weight (wt %), more than 0% and 0, 1% or less of C, more than 0% and 1.5% or less of Si, 12 to 23% of Cr, 1 to 12% of Ni, 10 to 25% of Mn, more than 0% and 1.2% or less of Cu, 0.1 to 0.3% of N, and the remainder of Fe and impurities; hot-rolling the slab to produce a hot-rolled steel sheet; and annealing the hot-rolled steel sheet, and the slab has a value of Expression (1) below in a range of 1.0 to 12.3.

$\begin{array}{cc}\left(\mathrm{Ni}+15\times N\right)\times \left(0.03\times \mathrm{Mn}\right)& \mathrm{Expression}\left(1\right)\end{array}$

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

In the method of manufacturing an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment, the annealing may be performed at 900° C. to 1200° C.

Advantageous Effects

According to an embodiment of the disclosed invention, by controlling an alloy composition and a manufacturing method, an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness and a method of manufacturing the same can be provided.

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 %).

An austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment may comprises, in percent by weight (wt %), more than 0% and less than or equal to 0, 1% of C, more than 0% and less than or equal to 1.5% of Si, 12 to 23% of Cr, 1 to 12% of Ni, 10 to 25% of Mn, more than 0% and less than or equal to 1.2% of Cu, 0.1 to 0.3% of N, and the remainder of Fe and impurities.

The content of C (carbon) may be more than 0% and less than or equal to 0, 1%.

C is an element effective in stabilizing austenite, suppressing 8-ferrite, and increasing strength by solid solution strengthening. However, when the content of C is excessive, C may easily combine with carbide forming elements (Cr, Ti, Nb, etc.) to reduce the corrosion resistance, ductility, and toughness of the base material. Considering this, the content of C may be 0, 1% or less. Preferably, the content of C may be 0.02 to 0, 1%.

The content of Si (silicon) may be more than 0% and less than or equal to 1.5%.

Si is an element effective in improving corrosion resistance and solid solution strengthening. However, when the content of Si is excessive, the stability of the ferrite phase increases, which may lead to the formation of intermetallic compounds due to a sigma phase and the like, resulting in reduced ductility and toughness of the base material. Considering this, the content of Si may be more than 0% and less than or equal to 1.5%. Preferably, the content of Si may be 0.4 to 1.5%, and more preferably, 0.4 to 0.5%.

The content of Cr (chromium) may be 12 to 23%.

Cr is an element that needs to be added to improve corrosion resistance in a stainless steel. Considering this, Cr may be added in an amount of 12% or more. However, when the content of Cr is excessive, excessive 8-ferrite may remain in the steel, which may reduce hot workability. In addition, when the content of Cr is excessive, the austenite phase in the steel becomes unstable, which requires the addition of a large amount of Ni to maintain phase stability, thereby increasing the production costs. Preferably, the content of Cr may be 12 to 21.4%.

The content of Ni (nickel) may be 1 to 12%.

Ni is a strong austenite-stabilizing element along with Mn and N. In addition, Ni is an important element that directly affects hydrogen embrittlement and low-temperature toughness. In addition, Ni is an element effective in suppressing the formation of 8-ferrite. Considering this, Ni may be added in an amount of 1% or more. However, when the content of Ni is excessive, the probability of surface defects occurring increases, and price competitiveness may decrease. Considering this, the upper limit of the Ni content may be limited to 12%. Preferably, Ni may be 1.0 to 12%, and more preferably, Ni may be 1.3 to 5.0%.

The content of Mn (manganese) may be 10 to 25%.

Mn is a strong austenite-stabilizing element along with Ni and N. In addition, Mn is an element that may replace expensive Ni and is therefore essential for cost reduction. In addition, Mn is an element effective in suppressing degradation of mechanical properties in a hydrogen environment by increasing the stability of the austenite phase. Considering this, Mn may be added in an amount of 10% or more. However, when the content of Mn is excessive, corrosion resistance may be rapidly reduced due to the formation of MnS inclusions. In addition, when the content of Mn is excessive, issues related to increased Mn may arise. Considering this, the upper limit of the Mn content may be limited to 25%. Preferably, the content of Mn may be 10.6 to 25%, and more preferably, 10.6 to 19.3%.

The content of Cu (copper) may be more than 0% and less than or equal to 1.2%.

33 Cu is an element useful for stabilizing the austenite phase and may be utilized as a substitute for expensive Ni. However, when the content of Cu is excessive, a low melting point phase may be formed, which may reduce hot workability and degrade surface quality. Considering this, the content of Cu may be more than 0% and less than or equal to 1.2%, and preferably from 0.7% to 1.2%, and more preferably from 0.7 to 0.9%.

The content of N (nitrogen) may be 0.1 to 0.3%.

N is an austenite stabilizing element and is effective in improving strength through solid solution strengthening. Considering this, N may be added in an amount of 0, 1% or more. However, when the content of N is excessive, surface quality may be lowered due to pore formation. Considering this, the upper limit of the N content may be limited to 0.3%. Preferably, the content of N may be 0.1 to 0.2%.

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.

One of the main causes of embrittlement in steel materials is the hydrogen environment and the temperature to which the material is exposed. Therefore, in order to review the use of steel materials in an extremely low-temperature environment such as liquefied hydrogen, there is a need to evaluate the toughness at extremely low temperatures and the resistance to hydrogen embrittlement caused by hydrogen.

Steels exposed to a hydrogen environment are likely to be exposed not only to the hydrogen environment but also to various temperature ranges. In addition, steel materials tend to exhibit reduced toughness and increased brittleness as the temperature decreases. Therefore, even when general steels appear to have no issues at room temperature, the steels tend to exhibit degradation in material properties as the temperature decreases.

In general, it is known that austenitic structures are favorable for low-temperature toughness, and martensitic structures and ferrite structures are relatively less favorable for low-temperature toughness.

However, general-purpose austenitic stainless steels, despite the relatively excellent low-temperature toughness, may suffer from severe hydrogen embrittlement when exposed to a hydrogen environment, thereby causing issues with the long-term durability of the material.

According to an example of the disclosed invention, by controlling the value of Expression (1) composed of Ni, N, and Mn, which are austenite stabilizing elements, in a range of 1.0 to 12.3, an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness may be provided.

$\begin{array}{cc}\left(\mathrm{Ni}+15\times N\right)\times \left(0.03\times \mathrm{Mn}\right)& \mathrm{Expression}\left(1\right)\end{array}$

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

Expression (1) is a hydrogen-related property relationship equation, composed of Ni, N, and Mn, which may directly affect hydrogen-related properties.

Meanwhile, Mn is a cost-effective element that may replace Ni. Therefore, in order to improve hydrogen-related properties while ensuring price competitiveness, it is also required to adjust the content of Mn and Ni.

Therefore, when the value of Expression (1) is in the range of 1.0 to 12.3, the hydrogen embrittlement resistance and low-temperature impact toughness of the austenitic stainless steel may be improved while the price competitiveness may be ensured.

The value of Expression (1) may specifically be from 1.0 to 10, more specifically from 1.0 to 5, and even more specifically from 1, 2 to 3.6. Within the above range, the austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment of the present invention may further improve the balance between the characteristic of preventing the degradation of properties due to temperature and the characteristic of preventing the degradation of properties due to hydrogen, thereby further enhancing the effect of simultaneously improving hydrogen embrittlement resistance and low-temperature impact toughness.

The austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment may have a −196° C. Charpy impact toughness value of 50 J or more, specifically 60 J or more, and more specifically 100 J or more by controlling the alloy composition and the manufacturing method. That is, according to one example of the disclosed invention, the steel may exhibit excellent low-temperature impact toughness, making it applicable not only at room temperature but also in a low-temperature environment.

The austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to an embodiment may have a relative notch tensile strength (RNTS) of 0.8 to 1.0.

The RNTS may be expressed as a ratio of a notch tensile strength in a hydrogen environment (NTS_H) and a notch tensile strength in air (NTS_air).

$\mathrm{RNTS}=\frac{{\mathrm{NTS}}_H}{{\mathrm{NTS}}_{\mathrm{air}}}$

A RNTS value closer to 1.0 may be interpreted as indicating less hydrogen embrittlement caused by hydrogen.

According to an example of the disclosed invention, the RNTS value is 0.8 to 1.0, specifically 0.81 to 0.99, more specifically 0.81 to 0.93, indicating very excellent hydrogen embrittlement resistance. In addition, within the above range, the austenitic stainless steel according to the present invention has both the characteristic of preventing the degradation of properties due to temperature and the characteristic of preventing the degradation of properties due to hydrogen, further improving the hydrogen embrittlement resistance and the low-temperature impact toughness.

Next, a method of manufacturing an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness according to another aspect of the disclosed invention will be described.

A method of manufacturing an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness, the method comprising: preparing a slab comprising, in percent by weight (wt %), more than 0% and 0, 1% or less of C, more than 0% and 1.5% or less of Si, 12 to 23% of Cr, 1 to 12% of Ni, 10 to 25% of Mn, more than 0% and 1.2% or less of Cu, 0.1 to 0.3% of N, and the remainder of Fe and impurities; hot-rolling the slab to produce a hot-rolled steel sheet; and annealing the hot-rolled steel sheet, and the slab has a value of Expression (1) in a range of 1.0 to 12.3,

$\begin{array}{cc}\left(\mathrm{Ni}+15\times N\right)\times \left(0.03\times \mathrm{Mn}\right),& \mathrm{Expression}\left(1\right)\end{array}$

In Expression (1), Ni, N, and Mn represent the content (weight %) of each element.

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 manufacturing a slab satisfying the alloy composition and Expression (1), a series of processes including hot-rolling and annealing may be performed.

The annealing may be performed at 900° C. to 1200° C.

In the annealing process after the hot rolling, the annealing temperature may significantly affect the relief of residual stress and the microstructure.

When the annealing temperature is below 900° C., coarse carbides may be generated, leading to the structure uneven, or Cr23 C6 precipitates may be formed around the grain boundaries, causing intergranular corrosion. However, when the annealing temperature exceeds 1200° C., the grains may become extremely coarsened.

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.

EXAMPLE

For the various alloy composition ranges shown in Table 1 below, slabs were prepared by melting in a vacuum melting furnace. The slabs were hot rolled to produce hot-rolled steel sheets, and then the hot-rolled steel sheets were annealed at a temperature of 1050° C. to produce specimens.

	TABLE 1
	Alloy composition (wt %)
Classification	C	Si	Cr	Ni	Mn	Cu	N
Embodiment	0.06	0.4	14.0	2.4	16.1	0.7	0.15
1
Embodiment	0.02	0.4	16.1	2.5	14.2	0.8	0.20
2
Embodiment	0.10	0.4	15.9	2.4	18.0	0.8	0.20
3
Embodiment	0.10	0.4	16.1	2.5	14.0	0.8	0.10
4
Embodiment	0.02	0.4	12.0	2.6	14.1	0.9	0.10
5
Embodiment	0.02	0.4	12.1	2.4	17.9	0.8	0.20
6
Embodiment	0.02	0.4	16.0	2.5	17.9	0.8	0.10
7
Embodiment	0.10	0.4	12.2	2.4	14.1	0.7	0.20
8
Embodiment	0.10	0.4	12.0	2.5	18.1	0.8	0.10
9
Embodiment	0.03	0.4	16.1	2.0	11.0	0.8	0.11
10
Embodiment	0.02	0.4	17.3	2.4	10.6	0.7	0.13
11
Embodiment	0.03	0.4	15.6	1.3	19.3	0.8	0.15
12
Embodiment	0.03	0.5	16.2	3.5	12.5	0.8	0.15
13
Embodiment	0.03	0.4	15.9	2.9	15.7	0.8	0.13
14
Embodiment	0.03	0.4	16.8	4.7	11.4	0.8	0.14
15
Embodiment	0.03	0.4	19.3	3.8	13.5	0.8	0.16
16
Embodiment	0.02	0.4	21.4	4.6	14.1	0.8	0.18
17
Embodiment	0.02	0.4	20.9	4.1	16.9	0.9	0.20
18
Embodiment	0.02	0.4	18.4	3.1	13.1	0.8	0.17
19
Embodiment	0.02	0.4	16.7	2.8	11.5	0.8	0.16
20
Comparative	0.02	0.6	18.1	8.0	1.2	0.3	0.05
Example 1
Comparative	0.02	0.5	16.7	9.8	1.5	0.4	0.06
Example 2
Comparative	0.02	0.4	16.4	2.3	5.8	0.5	0.10
Example 3
Comparative	0.02	0.4	17.6	2.6	6.5	0.4	0.08
Example 4
Comparative	0.03	0.4	16.3	0.8	8.3	0.3	0.07
Example 5

Table 2 below shows the values of Expression (1), Charpy impact toughness, and RNTS. The values of Expression (1) were calculated by Expression (1) below.

$\begin{array}{cc}\left(\mathrm{Ni}+15\times N\right)\times \left(0.03\times \mathrm{Mn}\right)& \mathrm{Expression}\left(1\right)\end{array}$

In Expression (1), Ni, N, and Mn represent the content (weight %) of each element.

The Charpy impact toughness was measured by performing an impact test at a temperature of −196° C. using the ASTM E23 type A specimen standard.

The RNTS was obtained by introducing hydrogen into the steel using an electrochemical method, followed by a slow strain rate tensile (SSRT) test to measure the RNTS. In this case, the RNTS specimens were prepared according to the ASTM E8 specimen standard, and the SSRT test was performed according to the ASTM G129. The RNTS value was calculated using the following Equation.

$\mathrm{RNTS}=\frac{{\mathrm{NTS}}_H}{{\mathrm{NTS}}_{\mathrm{air}}}$

A RNTS value closer to 1.0 may be interpreted as indicating less hydrogen embrittlement caused by hydrogen.

TABLE 2
		−196° C. Charpy
Classification	Expression (1)	impact toughness (J)	RNTS
Embodiment 1	2.2	50.6	0.86
Embodiment 2	2.3	60.3	0.88
Embodiment 3	2.9	56.6	0.90
Embodiment 4	1.7	71.0	0.81
Embodiment 5	1.7	100.4	0.81
Embodiment 6	2.9	51.0	0.87
Embodiment 7	2.1	57.1	0.80
Embodiment 8	2.3	59.6	0.86
Embodiment 9	2.2	68.5	0.88
Embodiment 10	1.2	63.7	0.87
Embodiment 11	1.4	64.3	0.88
Embodiment 12	2.1	52.1	0.81
Embodiment 13	2.2	68.9	0.87
Embodiment 14	2.3	64.1	0.82
Embodiment 15	2.3	72.6	0.91
Embodiment 16	2.5	65.7	0.92
Embodiment 17	3.1	60.2	0.93
Embodiment 18	3.6	57.3	0.90
Embodiment 19	2.2	58.5	0.86
Embodiment 20	1.8	60.8	0.84
Comparative Example 1	0.3	110.3	0.75
Comparative Example 2	0.5	135.4	0.77
Comparative Example 3	0.7	108.6	0.65
Comparative Example 4	0.7	113.8	0.70
Comparative Example 5	0.5	80.7	0.61

Referring to Table 2, Examples 1 to 20 satisfied the alloy composition, Expression (1), and the manufacturing method presented in the disclosed invention. Therefore, Examples 1 to 20 satisfied the −196° C. Charpy impact toughness value of 50 J or more and the RNTS value of 0.8 to 1.0. That is, Examples 1 to 20 exhibited excellent hydrogen embrittlement resistance and low-temperature impact toughness while achieving excellent price competitiveness due to low Ni content. However, Comparative Examples 1 to 5 did not satisfy the value of Expression (1) of 1.0 to 12.3. Therefore, Comparative Examples 1 to 5 did not satisfy the RNTS value of 0.8 to 1.0. That is, Comparative Examples 1 to 5 exhibited inferior hydrogen embrittlement resistance. In addition, Comparative Examples 1 and 2 had higher Ni contents compared to Examples 1 to 20 and Comparative Examples 3 to 5. Therefore, Comparative Examples 1 and 2 showed lower price competitiveness.

In addition, Comparative Example 5 had a Ni content of less than 1% and a Mn content of less than 10%, which were both low. In addition, Comparative Example 5 had the lowest RNTS value. In other words, Comparative Examples 1 to 5 had the lowest hydrogen embrittlement resistance.

According to an example of the disclosed invention, by controlling the alloy composition and the manufacturing method, an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness and a method of manufacturing the same may be provided.

In addition, according to an example of the disclosed invention, by reducing the amount of expensive Ni element added, an austenitic stainless steel with excellent price competitiveness may be provided.

Claims

1. An austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness, comprising, in percent by weight (wt %), ( Ni + 15 × N ) × ( 0.03 × Mn ) Expression ⁢ ( 1 )

more than 0% and 0, 1% or less of C, more than 0% and 1.5% or less of Si, 12 to 23% of Cr, 1 to 12% of Ni, 10 to 25% of Mn, more than 0% and 1.2% or less of Cu, 0.1 to 0.3% of N, and the remainder of Fe and impurities, and

wherein a value of Expression (1) below is 1.0 to 12.3,

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

2. The austenitic stainless steel of claim 1, wherein a Charpy impact toughness value at −196° C. is 50 J or more.

3. The austenitic stainless steel of claim 1, wherein a relative notch tensile strength (RNTS) is from 0.8 to 1.0.

4. A method of manufacturing an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness, the method comprising: ( Ni + 15 × N ) × ( 0.03 × Mn ) Expression ⁢ ( 1 )

preparing a slab comprising, in percent by weight (wt %), more than 0% and 0, 1% or less of C, more than 0% and 1.5% or less of Si, 12 to 23% of Cr, 1 to 12% of Ni, 10 to 25% of Mn, more than 0% and 1.2% or less of Cu, 0.1 to 0.3% of N, and the remainder of Fe and impurities;

hot-rolling the slab to produce a hot-rolled steel sheet; and

annealing the hot-rolled steel sheet,

wherein the slab has a value of Expression (1) below in a range of 1.0 to 12.3,

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

5. The method of claim 4, wherein the annealing is performed at 900° C. to 1200° C.