WO2017111489A1

WO2017111489A1 - Austenitic steel material having excellent hydrogen-embrittlement resistance

Info

Publication number: WO2017111489A1
Application number: PCT/KR2016/015085
Authority: WO
Inventors: 이순기; 김성규; 강상덕
Original assignee: 주식회사 포스코
Priority date: 2015-12-22
Filing date: 2016-12-22
Publication date: 2017-06-29
Also published as: CN108431275A; CA3009463A1; JP6703608B2; EP3395989A1; KR20180085797A; US20190010590A1; EP3395989A4; EP3395989B1; JP2019505675A; CA3009463C

Abstract

Disclosed is an austenitic material having excellent hydrogen-embrittlement resistance, comprising, by weight, 0.1-0.5% of C, 5% or less (0% exclusive) of Cu, 1% or less (0% exclusive) of N, a content of Mn satisfying Mn≥-10.7C+24.5, 10% or less of Cr, 5% or less of Ni, 5% or less of Mo, 4% or less of Si, 5% or less of Al, and a balance amount of Fe and inevitable impurities, with a T-El₂/T-El₁ ratio of 0.5 or higher, wherein T-El₁ is an elongation at break according to a tensile test at 25ºC under an atmospheric condition of 1 atm and T-El₂is an elongation at break according to a tensile test at 25 ºC under a hydrogen condition of 70 MPa.

Description

Austenitic steels with excellent hydrogen embrittlement resistance

The present invention relates to an austenitic steel having excellent hydrogen embrittlement resistance, and more particularly, to an austenitic steel having excellent hydrogen embrittlement resistance, which can be preferably applied to a high pressure hydrogen gas storage tank, a pipe and a transportation facility. .

In terms of global warming and pollution prevention, many efforts have been made to reduce environmental pollutants and greenhouse gas emissions, and technology using hydrogen as an energy source has recently made great progress. Hydrogen is attracting attention as a new energy source of the future as it is the most environmentally friendly energy source, unlike fossil fuels such as coal and petroleum.

Hydrogen vehicles have been most commonly used in the form of compressing hydrogen at high pressure and storing it in a high-pressure gas, and such a container has to withstand high pressure, and therefore high strength and especially to minimize hydrogen loss due to hydrogen permeation. To this end, the hydrogen permeability must be low and the hydrogen embrittlement resistance must be excellent to suppress embrittlement by hydrogen infiltration.

Basically, hydrogen storage vessels and facilities have a basic goal of reducing the storage loss due to the permeation of hydrogen, and therefore, a material of a face centered cubic structure (FCC) having a high permeability of hydrogen may be suitable. Typical FCC-based materials used for this purpose are austenitic stainless steels of Cr-Ni. These austenitic stainless steels are used in high pressure gas storage containers or liners and piping materials because of their excellent hydrogen embrittlement resistance under high pressure hydrogen gas environments.

However, in recent years, the pressure of hydrogen gas is increased to tens or hundreds of MPa for long-distance operation and mass storage by one-time charging of hydrogen. Therefore, in the case of austenitic stainless steel with low strength, the material thickness is increased to withstand the load under high pressure. As a result, it is difficult to avoid the increase in the weight and size of the container or equipment, which is a limitation in commercialization.

As a technique to solve this problem, Japanese Laid-Open Patent Publication No. 5-98391 and International Publication No. 2014-111285 disclose a technique of increasing the strength of austenitic stainless steel by cold working, but by cold working. The increase is not suitable as a container for storing hydrogen because the increase in ductility and toughness and the stability of austenite can be reduced to generate an organic martensite. On the other hand, Korean Patent Publication No. 10-2006-0018250 discloses a technique for securing the stability of austenite by performing two cold working in different directions, but to increase the stability of austenite Since a large amount of Cr, Ni is added as an element, this is disadvantageous in terms of cost.

Meanwhile, in Korean Unexamined Patent Publication No. 10-2011-0004491 and Korean Unexamined Patent Publication No. 10-2013-0045931, Ni, an expensive alloy element of the conventional austenitic stainless steel, is replaced with a low-cost alloy element Mn. In order to secure stable austenite and improve hydrogen embrittlement resistance, a technique is still contained, which also contains a large amount of expensive alloying elements, which is an obstacle to commercialization.

One of several objects of the present invention is to provide an austenitic steel having excellent hydrogen embrittlement resistance without the addition of expensive alloying elements.

In order to achieve the above object, one aspect of the present invention, by weight, C: 0.1 ~ 0.5%, Cu: 5% or less (excluding 0%), N: 1% or less (excluding 0%) Mn content satisfies Mn≥-10.7C + 24.5, Cr: 10% or less, Ni: 5% or less, Mo: 5% or less, Si: 4% or less, Al: 5% or less, balance Fe And elongation at break (T-El ₂ ) according to tensile test under high pressure hydrogen conditions of 25 ° C. and 70 MPa to breakdown elongation (T-El ₁ ) according to tensile test under atmospheric conditions of 25 ° C. and 1 atm, including unavoidable impurities. It provides an austenitic steel having excellent hydrogen embrittlement resistance of the ratio (T-El ₂ / T-El ₁ ) of 0.5 or more.

As one of various effects of the present invention, the austenitic steel of the present invention has an advantage of having excellent hydrogen embrittlement resistance without the addition of expensive alloying elements.

1 is a graph showing the composition range of carbon and manganese of the present invention.

Figure 2 is a photograph of the fracture surface after the room temperature tensile test for the specimen according to Inventive Example 1.

Containers for storing or transporting hydrogen should basically have a low transmittance of hydrogen. Therefore, in the case of steel materials, it is essential to secure an FCC structure with a low transmittance of hydrogen. The FCC structure needs to be stably maintained even for external deformation such as plastic deformation.

On the other hand, in order to overcome the disadvantages of the economics of the austenitic stainless steel, which is a conventional steel with excellent hydrogen embrittlement resistance, recently replaced expensive nickel with low-cost manganese, and by adding carbon to stabilize austenite at room temperature Attempts have been made. However, such high carbon and high manganese austenitic steels have partial stacking potentials due to low stacking defect energy, thereby making it easier to planarize slips. Becomes high. In addition, the addition of carbon for austenite stabilization causes dynamic strain aging to greatly improve the work hardening of steel. Accordingly, such high carbon and high manganese austenitic steels are not suitable for applications requiring hydrogen embrittlement resistance.

Accordingly, the present inventors attempted to improve the hydrogen embrittlement resistance of steel materials by appropriately controlling the relationship between carbon and manganese content while relatively lowering the carbon content, and as a result, the present invention has been derived.

Hereinafter, an austenitic steel having excellent hydrogen embrittlement resistance, which is an aspect of the present invention, will be described in detail.

First, the alloy component and the preferred content range of the austenitic steels will be described in detail. It is noted that the content of each component described below is based on weight unless otherwise specified.

Carbon (C): 0.1-0.5%

Carbon is an element that stabilizes austenite and improves the strength of steel, and in particular, serves to lower Ms and Md, which are transformation points of austenite into epsilon or alpha martensite by cooling or processing. If the carbon content is insufficient, the austenite stability may be insufficient, and the hydrogen embrittlement resistance may be drastically deteriorated because it is not easy to maintain the FCC structure due to the process organic transformation into epsilon or alpha martensite due to external stress. Therefore, in the present invention, the carbon content is preferably controlled to 0.1% or more, more preferably 0.15% or more, and even more preferably 0.2% or more. However, if the content is excessive, dynamic strain aging with dislocations increases the work hardening of the steel to deteriorate the hydrogen embrittlement resistance, and carbides may easily precipitate and deteriorate ductility or toughness. Therefore, in the present invention, the carbon content is preferably controlled to 0.5% or less, more preferably 0.45% or less.

Manganese (Mn): [Mn] ≥-10.7 [C] +24.5 (where [Mn] and [C] refer to the weight percent of the element)

In the present invention, the content of manganese is preferably determined by paying attention to the relationship between the carbon and other elements added together, and after the room temperature tensile test to ensure a stable austenitic or epsilon martensite with a low permeability of hydrogen stable hydrogen Manganese content range that can improve the chemical conversion is shown in FIG. The graph of FIG. 1 is a result derived by various experiments of the present inventors.

That is, in order to secure a microstructure capable of obtaining excellent hydrogen embrittlement resistance both before and after the tensile test, it is controlled in the range of -10.7 [C] + 24.5 (%) or more under the premise that other components satisfy the range defined by the present invention. It is preferable to. If the amount of manganese added is less than -10.7 [C] + 24.5 (%), the stability of austenite decreases, thereby forming a BCC-based microstructure by deformation, thereby degrading hydrogen embrittlement resistance.

Copper (Cu): 5% or less (except 0%)

Copper is an element that stabilizes the austenite structure for obtaining hydrogen embrittlement resistance and promotes slippage by increasing stacking defect energy. On the other hand, when high carbon is added, copper has a very low solid solubility in carbides and a slow diffusion in austenite, concentrating at the interface between austenite and nucleated carbides. It is effectively slowed down and consequently serves to suppress the production of carbides. The suppression of the formation of carbides reduces the diffusion sites of carbon to improve hydrogen embrittlement resistance, and also to improve the ductility and toughness of steel materials. When more than 0.5% of copper is added in the present invention, such carbide production inhibitory effect can be sufficiently obtained. However, if the content is excessive, hot workability of the steel may be degraded. Therefore, in the present invention, the copper content is preferably controlled to 5% or less, and more preferably to 3.5% or less.

Nitrogen (N): 1% or less (except 0%)

In addition to carbon, nitrogen is an element that stabilizes austenite to improve the toughness of steel. In particular, nitrogen is a very advantageous element for enhancing the strength of steel through solid solution strengthening, such as carbon. In particular, as can be seen through Equation 1 to be described later, it is well known as an element that promotes the slip effectively by increasing the stacking defect energy. However, in the present invention, even if nitrogen is not added, there is no major problem in terms of securing physical properties. On the other hand, if the content is excessive, since the surface quality and physical properties of the steel may be degraded by forming coarse nitride, in the present invention, it is preferable to control the content of nitrogen to 1% or less, 0.5% or less This is more preferable.

In addition to the elements described above, the austenitic steels of the present invention may include Cr, Ni, Mo, Si, and Al.

Chromium (Cr): 10% or less

Chromium stabilizes austenite up to the range of the proper addition amount, thereby increasing hydrogen embrittlement resistance, and is dissolved in austenite to increase the strength of steel. In addition, chromium is also an element for improving the corrosion resistance of steel materials. However, in the present invention, even if chromium is not added, there is no major problem in terms of securing physical properties. On the other hand, chromium is a carbide forming element, when the content thereof is excessive, it forms a carbide at the austenite grain boundary to provide a place for easy hydrogen diffusion and deteriorates the toughness of the steel. Therefore, in the present invention, the content of chromium is preferably controlled to 10% or less, more preferably 8% or less.

Nickel (Ni): 5% or less

Nike is a very effective austenite stabilizing element and serves to lower Ms and Md, which are transformation points of austenite into epsilon or alpha martensite by cooling or processing. In particular, as can be seen through Equation 1 to be described later, it is well known as an element that promotes the slip effectively by increasing the stacking defect energy. However, in the present invention, even if nickel is not added, there is no major problem in terms of securing physical properties. On the other hand, nickel is an expensive alloy element, when the content is excessive, there is a problem that the economic efficiency is lowered. Therefore, in the present invention, it is preferable to control the content of nickel to 5% or less.

Molybdenum (Mo): 5% or less

Molybdenum stabilizes austenite in an appropriate amount range, and serves to improve hydrogen embrittlement resistance of steel by lowering Ms and Md, which are transformation points of austenite into epsilon or alpha martensite by cooling or processing. In addition, it is dissolved in the steel to increase the strength of the steel, it is organized in the austenite grain boundary to increase the stability of the grain boundary to reduce the energy, thereby acting to suppress the grain boundary precipitation of carbonitride. In particular, as can be seen through Equation 1 to be described later, it is well known as an element that promotes the slip effectively by increasing the stacking defect energy. However, in the present invention, even if molybdenum is not added, there is no major problem in terms of securing physical properties. On the other hand, molybdenum is an expensive alloying element, and when its content is excessive, there is a problem that the economical efficiency is lowered. Therefore, in the present invention, the content of molybdenum is preferably controlled to 5% or less, more preferably 4% or less.

Silicon (Si): 4% or less

Silicon is an element that improves the castability of molten steel and, in particular, when added to an austenitic steel, is dissolved in the steel to effectively increase the strength of the steel. However, in the present invention, even if silicon is not added, there is no major problem in terms of securing physical properties. On the other hand, if the content is excessive, the stacking defect energy is reduced to encourage the generation of partial potential and cause stress concentration, thereby reducing the hydrogen embrittlement resistance of the steel. Therefore, in the present invention, it is preferable to control the content of silicon to 4% or less.

Aluminum (Al): 5% or less

Aluminum stabilizes austenite in an appropriate amount range, and serves to improve hydrogen embrittlement resistance of steel by lowering Ms and Md, which are transformation points of austenite into epsilon or alpha martensite by cooling or processing. In addition, it is employed in the steel material to increase the strength of the steel, affect the activity of the carbon in the steel to effectively suppress the formation of carbides, and serves to increase the toughness of the steel. In addition, it is an element that greatly increases the stacking defect energy of steel to induce cross-slip, suppress partial stress generation, reduce stress concentration, and increase hydrogen embrittlement resistance. However, in the present invention, even if aluminum is not added, there is no major problem in terms of securing physical properties. However, in order to improve the hydrogen embrittlement resistance more, it is more preferable to add 0.2% or more. On the other hand, if the content is excessive, since the castability and surface quality of the steel may be degraded by forming oxides and nitrides, in the present invention, it is preferable to control the content of aluminum to 5% or less.

In addition to the above composition, the rest is Fe. However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, not all of them are specifically mentioned in the present specification. On the other hand, addition of an effective component other than the said composition is not excluded.

According to one embodiment, the austenitic steel of the present invention may have a stacking defect energy (SFE) of 30mJ / m ^{2 or} more, which is defined by Equation 1 below.

[Equation 1]

^{SFE (mJ / m 2) =} 1.6 [Ni] - 1.3 [Mn] + 0.06 [Mn] 2 - 1.7 [Cr] + 0.01 [Cr] 2 + 15 [Mo] - 5.6 [Si] + 1.6 [Cu] + 5.5 [Al]-60 ([C] + 1.2 [N]) ^1/2 + 26.3 ([C] + 1.2 [N]) ([Cr] + [Mn] + [Mo]) ^1/2 + 0.6 { [Ni] ([Cr] + [Mn])} ^1/2

(Where [Ni], [Mn], [Cr], [Mo], [Si], [Cu], [Al], [C], and [N] each represent the content (% by weight) of the corresponding element. box)

In general, in the case of high manganese steel having a high content of manganese as in the present invention, when compared with conventional carbon steels, the lamination defect energy is relatively low, so that a partial potential is relatively easily generated, and the slip of the partial potential is limited to a specific slip surface. Accumulation and stress concentration are likely to be caused. However, such concentration of stress facilitates the diffusion of hydrogen. In high-manganese steels such as the present invention, there is a possibility that the fracture strength of the material decreases due to the diffusion of hydrogen, that is, embrittlement by hydrogen may occur. Very high. Therefore, in the present invention, it is necessary to specifically control the deformation behavior of the steel through the control of the stacking fault energy through the control of the alloy component and the composition range, and as a result of the present inventors, the stacking fault energy (SFE) defined by Equation 1 below. ), It was found that the possibility of embrittlement caused by hydrogen can be significantly suppressed when controlling to 30 mJ / m ² or more.

On the other hand, the degree of work hardening by stress concentration of the steel can be measured by the work hardening rate according to the tensile test. According to one embodiment, the austenitic steel of the present invention may have a strain hardening rate of 14000 N / mm ² or less according to a tensile test under atmospheric conditions of 25 ° C. and 1 atm. This work hardening rate can be calculated from true strain and true stress. If the maximum value of the work hardening rate by the tensile test exceeds 14000 N / mm ² , the stress concentration due to the electric potential becomes too large to facilitate the diffusion and accumulation of hydrogen, which may cause hydrogen embrittlement.

According to one embodiment, the austenitic steel of the present invention may be a tensile strength of 800MPa or less according to the tensile test under the atmospheric conditions of 25 ℃ and 1 atm. If the tensile strength exceeds 800 MPa, hydrogen embrittlement resistance may be inferior due to high work hardening caused by stress concentration.

According to one embodiment, the austenitic steel of the present invention may include an austenite structure of 95 area% or more as its microstructure. If the area fraction of the austenitic structure is less than 95%, there is a fear that the target hydrogen embrittlement resistance may not be secured.

According to one embodiment, the austenitic steels of the present invention may be made of austenite tissue or epsilon martensite tissue and austenite tissue after the tensile test under the atmospheric conditions of 25 ℃ and 1 atm. If the microstructure includes a ferrite structure after the tensile test, there is a fear that the target hydrogen embrittlement resistance may not be secured.

The austenitic steels of the present invention can be manufactured according to a conventional method for producing steel using steel slabs satisfying the above-described component system, and for example, reheating steel slabs satisfying the above-described component system and rough rolling And after finishing rolling, it can manufacture by cooling.

In this case, however, the finishing rolling finish temperature needs to be controlled to a temperature exceeding the unrecrystallized temperature. When finishing finishing at a temperature below the unrecrystallized temperature, the strength of the steel is excessively increased due to the generation and accumulation of excessive dislocations, thereby facilitating stress concentration and breakdown by hydrogen, and also causing fermentation of hydrogen during tensile deformation. Can be generated early, which in turn makes it difficult to achieve the desired hydrogen embrittlement resistance.

In addition, after the end of the rolling steel needs to be accelerated cooling to suppress the formation of carbides, which reduces the elongation of the steel when carbide is formed, in particular hydrogen accumulates at the interface between the carbide and austenite, ultimately hydrogen embrittlement resistance Because it makes you inferior. Since carbon, chromium, molybdenum, etc. are the major carbide forming elements, whether or not accelerated cooling and cooling rate are given as follows according to the amount of addition of these elements.

[Equation 2]

Cooling rate (℃ / s) ≥ 15 [C] + [Cr] + [Mo]

(Where [C], [Cr] and [Mo] refer to the content (% by weight) of the corresponding element)

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the description of these examples is only for illustrating the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

To prepare a slab having a composition shown in Table 1, after the hot rolling and cooling to prepare a rolled material. In this case, all the process conditions except the finishing rolling finishing temperature and cooling rate in all examples were controlled in the same way, the finishing rolling finish temperature and cooling rate are shown in Table 2 below. For reference, in the case of Comparative Example 5 in which the cooling rate is not described in Table 2, it means that simple air cooling was performed.

Then, the microstructure of the rolled material was observed, and the austenite fraction was measured. Thereafter, the rolled material was subjected to a tensile test under atmospheric conditions of 25 and 1 atm, and then tensile strength, work hardening rate, and elongation at break (T-El ₁ ) were measured, and the ferrite fraction was measured. In addition, the tensile elongation (T-El ₂ ) was measured after a tensile test was performed on the rolled material under high pressure hydrogen conditions of 25 and 70 MPa. The results are shown in Table 3 below.

Table 1

TABLE 2

TABLE 3

Looking at Table 3, Inventive Examples 1 to 5 satisfying the component range of the present invention is obtained a stable austenite in which no ferrite is produced after the tensile deformation at room temperature, low work hardening rate and tensile strength is controlled, in particular finishing finishing The temperature is rolled beyond the recrystallization temperature to suppress the generation and scale of dislocations, and the cooling rate satisfies the range controlled by the present invention, thereby effectively suppressing carbide formation, resulting in excellent hydrogen embrittlement resistance with a very high ratio of break elongation. It shows that austenitic steels can be obtained.

On the other hand, in Comparative Example 1, the content of carbon and manganese did not satisfy the range controlled by the present invention, and it was found that the ratio of fracture elongation was low, that is, the hydrogen embrittlement resistance was inferior due to the high content of carbon. Can be.

In particular, it can be seen that in Comparative Example 2 the content of manganese does not satisfy the range controlled by the present invention, austenite is unstable and ferrite sensitive to hydrogen embrittlement after tensile deformation is inferior to hydrogen embrittlement resistance.

In Comparative Example 3, the content of carbon and manganese and the stacking defect energy satisfy the range controlled by the present invention, but the copper content exceeds the range controlled by the present invention.

In Comparative Example 4, it is found that the addition of carbon in excess of the range controlled by the present invention results in a high work hardening rate and inferior hydrogen embrittlement resistance due to excessive precipitation of carbides at the austenite grain boundary.

In addition, Comparative Example 5 can be seen that the hydrogen embrittlement resistance is inferior because the content of manganese does not fall within the range controlled by the present invention does not obtain the target microstructure.

On the other hand, Figure 2 is a photograph observing the fracture surface after the room temperature tensile test for the specimen according to Inventive Example 1, the fracture form appeared dimples typical of ductile failure.

While the exemplary embodiments of the present invention have been shown and described as described above, various modifications and other embodiments may be made by those skilled in the art. Such modifications and other embodiments are all considered and included in the appended claims, without departing from the true spirit and scope of the invention.

Claims

By weight, C: 0.1-0.5%, Cu: 5% or less (except 0%), N: 1% or less (except 0%), and the content of Mn is [Mn] ≥-10.7 [C] Satisfies +24.5 (where [Mn] and [C] mean the weight% of the corresponding element), Cr: 10% or less, Ni: 5% or less, Mo: 5% or less, Si: 4% or less, Al: 5% or less, including residual Fe and unavoidable impurities,

Ratio of elongation at break (T-El 2 ) according to tensile test under 25 ° C. and 70 MPa hydrogen conditions to tensile elongation (T-El 1 ) according to tensile test under atmospheric conditions of 25 ° C. and 1 atm (T-El 2 / Austenitic steel with excellent hydrogen embrittlement resistance of T-El 1 ) of 0.5 or more.
The method of claim 1,

An austenitic steel having excellent hydrogen embrittlement resistance, wherein a stacking fault energy (SFE) of 30 mJ / m 2 or more is defined by Equation 1 below.

[Equation 1]

SFE (mJ / m 2) = 1.6 [Ni] - 1.3 [Mn] + 0.06 [Mn] 2 - 1.7 [Cr] + 0.01 [Cr] 2 + 15 [Mo] - 5.6 [Si] + 1.6 [Cu] + 5.5 [Al]-60 ([C] + 1.2 [N]) 1/2 + 26.3 ([C] + 1.2 [N]) ([Cr] + [Mn] + [Mo]) 1/2 + 0.6 { [Ni] ([Cr] + [Mn])} 1/2

(Where [Ni], [Mn], [Cr], [Mo], [Si], [Cu], [Al], [C], and [N] each represent the content (% by weight) of the corresponding element. box)
The method of claim 1,

Austenitic steel with excellent hydrogen embrittlement resistance at a strain hardening rate of 14000 N / mm 2 or less according to a tensile test at 25 ° C. and 1 atm atmosphere.
The method of claim 1,

Austenitic steel with excellent hydrogen embrittlement resistance with tensile strength of 800 MPa or less according to tensile test at 25 ° C and 1 atm atmosphere.
The method of claim 1,

Austenitic steels containing austenitic structure of 95 or more% (including 100 area%) as the microstructure.
The method of claim 1,

Austenitic steels having a microstructure consisting of austenite structure or epsilon martensite structure and austenite structure after a tensile test at 25 ° C. and 1 atm atmosphere.