JP2014227573A - High strength steel having excellent high-pressure hydrogen environment embrittlement resistance properties and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel having excellent hydrogen embrittlement properties in high-pressure hydrogen environment like 70 MPa or more, and a production method therefor.SOLUTION: A high strength steel having high strength and also excellent hydrogen embrittlement properties in high-pressure hydrogen environment is obtained with a composition where C: 0.20 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.10 to 1.0%, P: 0.02% or less, S: 0.02% or less, Ni: 1.0 to 5.0%, Cr: 0.5 to 2.5%, Mo: 0.1 to 1.5%, and V: 0.005 to 0.3%, based on mass%, and if desired, also one or two of Nb: 0.1% or less and Cu: 0.5% or less are contained, the rest consists of Fe and inevitable impurity, and Ceq represented by the following formula is 0.75 or more. Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14, where each element symbol in the formula represents a numerical value of mass% for each element.

Description

  The present invention relates to a high-strength steel excellent in high-pressure hydrogen environment embrittlement resistance used for a high-pressure hydrogen storage pressure accumulator and the like, and a method for producing the same.

The spread of hydrogen stations that store and supply high-pressure hydrogen is important as a hydrogen infrastructure for building a hydrogen society. A hydrogen pressure accumulator is a main device constituting the hydrogen station, and development of an FRP (Fiber Reinforced Plastics) pressure accumulator, a combined FRP and liner pressure accumulator, and a steel pressure accumulator is underway. Among these, steel accumulators have already been proven to be highly safe, and technological developments aimed at lowering costs are advancing, so they are expected to be used in hydrogen stations to be constructed in the future.
In recent years, it has been desired to increase the pressure and capacity of accumulators for the spread of hydrogen stations. In particular, it is necessary to increase the charging pressure to extend the cruising range of fuel cell vehicles. It is necessary to store high-pressure hydrogen gas of 70 MPa or more in the vessel. However, it is known that low-alloy high-strength steel used in conventional steel accumulators is hydrogen embrittled in a high-pressure hydrogen gas environment. SUS316L, which is an austenitic stainless steel, can be cited as a material that is not easily embrittled even in such a high-pressure hydrogen gas environment. However, since a significant increase in weight and cost due to an increase in the thickness of the pressure accumulator is inevitable, application to the pressure accumulator is not realistic.
Patent Documents 1 to 4 report steel pressure accumulator materials that suppress hydrogen embrittlement sensitivity that can be used even in a high-pressure hydrogen environment. Patent Document 1 and Patent Document 2 show a component system that is less susceptible to hydrogen embrittlement than a conventional high-strength low-alloy steel in a range of tensile strength in the atmosphere of 900 to 950 MPa. In Patent Document 3, it is possible to produce a steel material with low hydrogen embrittlement susceptibility by controlling the substructure, and in Patent Document 4 by controlling the fine carbide form, both of which are high strength steels having an atmospheric tensile strength of 800 MPa or more. It is proposed that there is.

JP 2009-46737 A JP 2009-275249 A JP 2012-107333 A JP 2009-74122 A

In order to increase the pressure and capacity of the accumulator, it is necessary to make it thicker and larger than the conventional shape. Ensuring hardenability is a problem when increasing the size. Large materials with large mass effects have a lower internal cooling rate during quenching, so in the case of component systems with poor hardenability, the lower cooling rate inside becomes the upper bainite structure or ferrite / pearlite structure, so tensile properties and toughness Cause a decline. Therefore, a steel type to which an alloy element that improves hardenability is added is used for large materials, and the carbon equivalent (Ceq) shown in Formula (1) is used as an index of hardenability.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 Formula (1) However, the element symbol in the formula represents a numerical value of mass% of each element.
By providing a restriction on the carbon equivalent, it is possible to obtain a target structure even inside a large material.

In the compositions shown in Patent Documents 1 to 3, when the product size is small, a high-strength steel excellent in water-resistant material embrittlement susceptibility can be produced. However, since Ceq is low, the size that can be produced is limited. Patent Document 4 is only a result of a plate thickness of 12 mm, and the effect of the cooling rate assuming a large material has not been studied, and only evaluation in a 45 MPa hydrogen environment has been conducted. It has not been studied whether characteristics equivalent to those in a 45 MPa hydrogen environment can be obtained even under an environment.
Another problem with large materials is the sensitivity to hydrogen embrittlement of the tempered martensite structure. Generally, hardenability means the ease of forming a martensite structure, and a tempered steel with improved hardenability has a tempered martensite structure. The tempering conditions at this time are selected according to the target tensile strength, but depending on the tempering conditions, it is known that coarse cementite precipitated by tempering becomes an accumulation site of hydrogen and causes an increase in hydrogen embrittlement sensitivity. Yes. It is also known that hydrogen embrittlement susceptibility increases as the strength of steel increases.

  The present invention has been made to solve the above-described problems of the conventional ones, and enhances Ceq to ensure hardenability, and is a material having high strength and excellent hydrogen embrittlement susceptibility while being high strength, and An object is to provide a manufacturing method.

That is, among the high-strength steels excellent in high-pressure hydrogen environment embrittlement resistance according to the present invention, the first present invention is mass%, C: 0.20 to 0.50%, Si: 0.01 to 0 .40%, Mn: 0.10 to 1.0%, P: 0.02% or less, S: 0.02% or less, Ni: 1.0 to 5.0%, Cr: 0.5 to 2. 5%, Mo: 0.1 to 1.5%, V: 0.005 to 0.3%, with the balance being composed of Fe and inevitable impurities, and Ceq represented by the following formula: It is characterized by being 0.75 or more.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, the element symbol in a formula shows the numerical value of the mass% of each element.

  The high-strength steel excellent in the high-pressure hydrogen environment embrittlement resistance according to the second aspect of the present invention is the composition according to the first aspect of the present invention, further comprising, in mass%, Nb: 0.1% or less, Cu: 0.00. 1 type or 2 types of 5% or less are contained, It is characterized by the above-mentioned.

  The high-strength steel excellent in high-pressure hydrogen environment embrittlement resistance according to the third aspect of the present invention is characterized in that, in the first or second aspect of the present invention, the tensile strength in the atmosphere after tempering is 800 MPa to 1100 MPa. To do.

  The high-strength steel excellent in the high-pressure hydrogen environment embrittlement resistance according to the fourth aspect of the present invention is measured by a comparison method of JIS G 0552 (steel ferrite grain size test method) in any of the first to third aspects of the present invention. The grain size number after tempering is 6 or more.

  The high-strength steel excellent in the high-pressure hydrogen environment embrittlement resistance according to the fifth aspect of the present invention is the hydrogen embrittlement index (TS ratio in notch tensile test) in any of the first to fourth aspects of the present invention. : Value obtained by dividing TS in hydrogen by TS in air) is 0.6 or more.

  The high-strength steel excellent in high-pressure hydrogen environment embrittlement resistance according to the sixth aspect of the present invention is characterized by being used in a high-pressure hydrogen environment of 70 MPa or more in any of the first to fifth aspects of the present invention. .

  The high-strength steel excellent in high-pressure hydrogen environment embrittlement resistance according to the seventh aspect of the present invention is the martensite structure, any of the lower bainite structure or the martensite structure in any of the first to sixth aspects of the present invention. It consists of a mixed structure of a lower bainite structure.

  The manufacturing method of the high strength steel excellent in the high pressure hydrogen environment embrittlement resistance according to the eighth aspect of the present invention normalizes the high strength steel having the composition according to the first or second aspect of the present invention, and then 800 It is characterized by tempering at a temperature in the range of 560 to 680 ° C. after quenching at a temperature not lower than C.

Below, the effect | action of the alloy element in this invention and the reason for limitation are demonstrated.
C: 0.20 to 0.50%
Since C is an element necessary for improving the hardenability and ensuring the strength, the lower limit is set to 0.2%. However, if it is excessively contained, it becomes a factor for forming coarse carbides during tempering, so the upper limit is made 0.5%. For the same reason, the desirable lower limit is 0.25% and the desirable upper limit is 0.45%.

Si: 0.01-0.40%
Si is an element necessary for securing strength, deoxidation during steel making, and the lower limit for obtaining the effect is 0.01%. However, excessive content causes toughness reduction and temper embrittlement due to inclusions, so the upper limit is made 0.40%. For the same reason, the desirable lower limit is 0.03% and the desirable upper limit is 0.30%.

Mn: 0.10 to 1.0%
Mn is an element necessary for ensuring strength, and the lower limit for obtaining the effect is 0.10%. However, excessive content causes a decrease in toughness due to inclusions, so the upper limit is made 1.0%. For the same reason, the desirable lower limit is 0.20% and the desirable upper limit is 0.90%.

P: 0.02% or less P has an upper limit of 0.02%, which can be industrially sufficiently realized.
S: 0.02% or less S has an upper limit of 0.02%, which can be industrially sufficiently realized.

Ni: 1.0-5.0%
Since Ni is an element necessary for improving the hardenability and ensuring the strength and toughness, the lower limit for obtaining the effect is set to 1.0%. However, excessive content causes an increase in production cost, so the upper limit is set to 5.0%. For the same reason, the desirable lower limit is 1.5% and the desirable upper limit is 4.0%.

Cr: 0.5 to 2.5%
Cr is an element necessary for improving the strength, and the lower limit for obtaining the effect is 0.5%. However, excessive content causes formation of coarse carbides during tempering, so the upper limit is set to 2.5%. For the same reason, the desirable lower limit is 1.0% and the desirable upper limit is 2.0%.

Mo: 0.1 to 1.5%
Mo is an element necessary for improving the strength, and the lower limit for obtaining the effect is 0.1%. However, excessive content causes an increase in production cost, so the upper limit is set to 1.5%. For the same reason, the desirable lower limit is 0.2% and the desirable upper limit is 1.0%.

V: 0.005-0.3%
V is an element necessary for improving the strength, and the lower limit for obtaining the effect is 0.005%. However, excessive content causes a decrease in toughness, so the upper limit is made 0.3%. For the same reason, the desirable lower limit is 0.03% and the desirable upper limit is 0.2%.

Ceq (shown by the following formula) is 0.75 or more Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, the element symbol in a formula shows the numerical value of the mass% of each element.
Ceq is an index representing the hardenability of steel. If Ceq is 0.75 or more, the cooling rate is the smallest in the range assumed in the production of a member (for example, a pressure accumulator) used in an actual high pressure resistant hydrogen environment. Even if it becomes, the target organization can be obtained to the inside.

Nb: 0.1% or less Nb is an element effective for improving the strength and reducing the crystal grain size, and is contained as desired. However, an excessive content causes an increase in manufacturing cost and a deterioration in forgeability, so the upper limit is set to 0.1%. Desirably, the upper limit is 0.08%. In order to obtain the above effect sufficiently, it is desirable to set the lower limit to 0.01%.

Cu: 0.5% or less Cu is an element effective for improving the strength, and is contained as desired. However, excessive content causes deterioration of forgeability, so the upper limit is made 0.5%. Desirably, the upper limit is 0.4%. In order to obtain the above effect sufficiently, it is desirable to set the lower limit to 0.01%.

Target strength: room temperature strength after tempering is 800 MPa or more In the high strength steel of the present invention, the room temperature strength after tempering is desirably 800 MPa or more, and more desirably 850 MPa or more.
However, since the hydrogen embrittlement susceptibility in a hydrogen atmosphere increases as the strength increases, the upper limit of room temperature strength is preferably 1100 MPa, and more preferably 1050 MPa. The room temperature strength can be adjusted by selecting the composition and setting the tempering conditions.

Crystal grain size: grain size number 6 or more (JISG 0552)
The high-strength steel of the present invention preferably has a grain size number of 6 or more after tempering measured by a comparative method of JISG 0552 (steel ferrite grain size test method). When the crystal grain size number is 6 or more, hydrogen embrittlement resistance that is exceptionally superior to that of conventional steel can be exhibited. There are various methods for obtaining fine particles, such as processing, heat treatment, and combinations thereof, but in the present invention, there is no effect on the effect obtained by using any method, so a method for obtaining fine particles It does not specifically limit about.

Hydrogen embrittlement index (TS ratio in notch tensile test: value obtained by dividing TS in hydrogen by TS in air): 0.6 or more The hydrogen embrittlement index (TS ratio in notch tensile test: value obtained by dividing TS in hydrogen by TS in air) is defined as an index for evaluating hydrogen embrittlement. It is desirable that the brittleness index is 0.6 or more. As a general tendency, the hydrogen embrittlement index decreases as the smooth tensile strength in the atmosphere increases. If the hydrogen embrittlement resistance index is 0.6 or more, it is judged that the hydrogen embrittlement resistance is particularly excellent.

Production Method The high strength steel of the present invention is subjected to normalization to remove strain during forging, and 800 ° C. or more in order to obtain an optimum crystal grain size, for the low alloy steel having the above composition. It is desirable to perform quenching, and after quenching, it is desirable to perform tempering at 560 to 680 ° C. in order to adjust to the optimum strength range.

  As described above, according to the present invention, a uniform structure can be obtained even in a large material, and a steel material having high strength and excellent hydrogen embrittlement resistance even under a high-pressure hydrogen atmosphere can be obtained.

It is a graph which shows the relationship between the smooth tensile strength in air | atmosphere and the hydrogen embrittlement resistance index in the notch tensile test in an Example.

Hereinafter, embodiments of the present invention will be described.
In mass%, C: 0.20 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.10 to 1.0%, P: 0.02% or less, S: 0.02 %: Ni: 1.0-5.0%, Cr: 0.5-2.5%, Mo: 0.1-1.5%, V: 0.005-0.3%, Further, if desired, Ceq containing one or two of Nb: 0.1% or less and Cu: 0.5% or less, the balance being composed of Fe and unavoidable impurities, and represented by the following formula A steel having a composition of 0.75 or more is melted.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, the element symbol in a formula shows the numerical value of the mass% of each element.
The melting method is not particularly limited and can be performed according to a conventional method, for example, by vacuum induction melting.
Examples of the composition include those based mainly on steel types provided by ASME SA723M.

Although the obtained material can also be made into a predetermined shape by casting, it can be made into a predetermined shape through hot working such as hot forging. After hot working, normalization can be performed to remove distortion during forging. Normalization can be performed, for example, under conditions of 800 to 1000 ° C.
Further, the above material can be tempered at, for example, 800 ° C. or higher, and tempered within a range of 560 to 680 ° C. and 2 to 10 hours after quenching. The tempering time is not limited to the above, and can be changed according to the material mass. By quenching, the structure can be either a martensite structure, a lower bainite structure, or a mixed structure of a martensite structure and a lower bainite structure. If these structures are obtained, practically sufficient strength and toughness can be obtained. Moreover, the optimal crystal grain size in which the grain size number by JISG0552 becomes 6 or more can be obtained by quenching, and exceptionally excellent hydrogen embrittlement resistance can be expressed.
In tempering, the strength can be adjusted, and the tensile strength in the air can be suitably set to 800 MPa to 1100 MPa.

  Moreover, the said material can be used for the pressure accumulator etc. which are used in a high pressure hydrogen environment, and is suitable for what is used in a 70 MPa or more high pressure hydrogen environment. In a high-pressure hydrogen environment, hydrogen embrittlement resistance must be excellent, and as one of the indicators, the hydrogen embrittlement index (TS ratio in notch tensile test: TS in hydrogen is It is desirable that the value divided by TS is 0.6 or more.

Test materials having the composition shown in Table 1 (the balance being Fe and inevitable impurities) were melted using a vacuum induction melting furnace, 50 kg steel ingots were obtained for each test material, and forging with a thickness of 35 mm by hot forging. Processed into a plate.
Each test material was subjected to normalization under conditions of 900 ° C. × 2 hours after hot forging, and further quenched at 850 ° C. × 8 hours. In this example, thick materials were assumed, and the average cooling rate during quenching was adjusted to 40 ° C./min for all test materials. This cooling rate corresponds to the cooling rate at the position of half the plate thickness when a steel material having a plate thickness of about 200 mm is water-cooled. Thereafter, tempering was performed between 560 ° C. and 680 ° C. in order to adjust the tensile strength. Tempering retention time was 6 hours.

  Sample 14 smooth test piece (diameter 8mm, gauge distance 40mm) specified in JIS Z 2201 is taken from the test material after quenching and tempering, and an annular V-shaped notch is formed in the center of the parallel part. Introduced. The notch bottom was 4 mm in diameter. The tensile test was carried out using a high-pressure hydrogen environmental fatigue tester under a 99 MPa hydrogen environment and an atmospheric atmosphere. The deformation rate in the tensile test was 0.0028 mm / s. The test temperature was 20 ° C.

Further, the structure was observed before the tensile test. In Examples 1 to 5, the upper bainite structure and the ferrite structure that decrease the strength and toughness were not recognized. Further, it was confirmed that fine grains having a crystal grain size number of 6 or more were obtained.
Next, a 45-degree V-shaped groove (V notch) was formed at a depth of 2 mm at the center of a specimen material having a length of 55 mm and a square bar having a side of 10 mm, and a Charpy impact test at 20 ° C. was performed. The measured absorbed energy is shown in Table 1. As a result, each of Examples 1 to 5 showed higher absorbed energy than Comparative Examples 1 to 3, and was found to be superior in toughness compared to Comparative Examples 1 to 3. Comparative Examples 1 to 3 are structures having upper bainite, which is considered to have caused a decrease in toughness.
As described above, in the inventive material, the above-mentioned structure and crystal grain size are obtained even at a thickness of 200 mm or more, and it can be seen that the hardenability is excellent. However, the thickness of the material of the present invention is not particularly limited, and can be suitably used as a material for a pressure accumulator having a thickness of 50 mm or more, for example.

In FIG. S. The relationship between the ratio and the smooth tensile strength in the air is shown. In Examples 1 to 5, the smooth tensile strength in the air is in the range of 800 MPa to 1100 MPa, the hydrogen embrittlement index is 0.6 or more, and it is confirmed that the high pressure hydrogen environment embrittlement resistance is superior to the comparative example. It was done. Some of the comparative examples show good hydrogen embrittlement resistance on the low strength side (less than 800 to 950 MPa), but even the material has poor hydrogen embrittlement resistance on the high strength side (950 MPa to 1100 MPa). I understand that From these points, the present invention is particularly noted in that it has excellent hydrogen embrittlement resistance even on the high strength side.
The effect of grain refinement is that the grain boundary length increases, the hydrogen concentration per grain boundary unit length decreases, and the substructures such as blocks and packets become finer as the grain refines. It is speculated that it contributes to the improvement of the resistance to.

Claims (8)

In mass%, C: 0.20 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.10 to 1.0%, P: 0.02% or less, S: 0.02 %: Ni: 1.0-5.0%, Cr: 0.5-2.5%, Mo: 0.1-1.5%, V: 0.005-0.3%, A high-strength steel excellent in high-pressure hydrogen environment embrittlement resistance, characterized in that the balance is composed of Fe and inevitable impurities, and Ceq represented by the following formula is 0.75 or more.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, the element symbol in a formula shows the numerical value of the mass% of each element.   2. The high-pressure hydrogen environment embrittlement resistance according to claim 1, wherein the composition further contains one or two of Nb: 0.1% or less and Cu: 0.5% or less in mass%. Excellent high strength steel.   The high-strength steel excellent in high-pressure hydrogen environment embrittlement resistance according to claim 1 or 2, wherein the tensile strength in the atmosphere after tempering is 800 MPa to 1100 MPa.   The resistance to resistance according to any one of claims 1 to 3, wherein the grain size number after tempering is 6 or more, as measured by a comparison method of JIS G 0552 (steel ferrite grain size test method). High-strength steel with excellent high-pressure hydrogen environment embrittlement characteristics.   The hydrogen embrittlement index (TS ratio in notch tensile test: value obtained by dividing TS in hydrogen by TS in air) is 0.6 or more. High strength steel excellent in the high pressure hydrogen environment embrittlement resistance of any one of 1-4.   The high-strength steel excellent in high-pressure hydrogen environment embrittlement resistance according to any one of claims 1 to 5, which is used in a high-pressure hydrogen environment of 70 MPa or more.   The high-pressure hydrogen environment embrittlement resistance according to any one of claims 1 to 6, characterized by comprising either a martensite structure, a lower bainite structure, or a mixed structure of a martensite structure and a lower bainite structure. High strength steel.   The high-strength steel having the composition according to claim 1 is normalized, then quenched at 800 ° C. or higher, and then tempered in a range of 560 to 680 ° C. Of high-strength steel with excellent heat treatment characteristics.

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