JP7339123B2 - High hardness hydrogen embrittlement resistant steel - Google Patents

Description

The present invention is applicable to hydrogen production plants, storage facilities, various hydrogen transportation means (ships, trailers, etc.), and hydrogen energy utilization equipment such as hydrogen stations and fuel cell vehicles (valves, piping, joints, nozzles, compressors, pressure accumulators, etc.). instruments, measuring instruments, etc.), and hot dies and rolls that have water-cooled holes and are exposed to high temperatures. ), it relates to a steel material made of a high-hardness hydrogen embrittlement-resistant steel that can be suitably used.

As a material with high hardness and excellent resistance to hydrogen embrittlement, by adjusting the amount of each alloying element and its balance, the austenite structure is stabilized, and it is inexpensive and has excellent resistance to hydrogen embrittlement. A non-magnetic steel having high hardness and excellent corrosion resistance has been proposed (see Patent Document 1).
However, in the proposed steel, the amount of N added is set to 0.1000% or less in consideration of the deterioration of corrosion resistance due to the formation of coarse carbonitrides due to the addition of N. Therefore, there is no mention or suggestion of the point of view or effect of increasing the γ stability at low cost to increase the strength and improving the corrosion resistance by adding a larger amount of N than this. In addition, this proposal does not mention or suggest suppression of smooth fracture, and is highly susceptible to hydrogen embrittlement.

In addition, as a material with high hardness and excellent hydrogen embrittlement resistance, a steel with controlled stacking fault energy (hereinafter also referred to as "SFE") and further suppressed hydrogen embrittlement susceptibility has been proposed (Patent Reference 2).
However, even in this proposal, considering the deterioration of corrosion resistance due to the formation of coarse carbonitrides due to the addition of N, the content of N is specified as low as 0.1000% or less, and a large amount of N exceeding this is added. There is no mention or suggestion of the effect of increasing the γ stability at low cost, increasing the strength, and improving the corrosion resistance.

Also, a material with high strength and excellent resistance to sulfide stress corrosion cracking has been proposed (see Patent Document 3). However, the material of this proposal is characterized by the addition of Mn and V, with no addition of Mo or Al, and Ni is not essential. In addition, there is no mention or suggestion regarding suppression of occurrence of smooth fracture by adjusting the stacking fault energy. And ductility, corrosion resistance, and resistance to severe hydrogen penetration environment are unknown.

JP 2016-183372 A JP 2019-49036 A JP-A-9-249940

Steel materials used for nozzles of equipment using high-pressure hydrogen gas and molds with water-cooling holes are required to have durability even when exposed to environments where hydrogen easily penetrates. If there is a steel material that can be used as a member applicable to such situations, it will be possible to promote the construction of a society that utilizes hydrogen energy. Therefore, there is a demand for a steel material that has excellent hydrogen embrittlement resistance, high hardness, high strength, high weather resistance, and is relatively inexpensive.

The strength here is a high strength that far exceeds SUS316 and ASTM XM19 steel materials that are approved for use in a high-pressure hydrogen environment, and is equal to or higher than the age-hardening material of expensive SUH660.
However, steel such as SUH660, which has high austenite stability but high strength, has high susceptibility to hydrogen embrittlement. In order to suppress such hydrogen embrittlement susceptibility, it is considered effective to homogenize the worked structure and improve the stacking fault energy (SFE) while maintaining high austenite stability. This is because a high SFE requires energy to introduce stacking faults.

Materials for hydrogen utilization applications are desired to meet the requirements of low cost, high strength, and excellent corrosion resistance while having sufficient resistance to hydrogen embrittlement. , it is difficult to meet all demands.

Therefore, the problems to be solved by the present invention are to reduce the cost by replacing Ni as a γ-stabilizing element by adding Mn and N, and to increase the hardness by V(C,N) precipitates. By understanding the effects of the addition of alloying elements on tensile properties, cost, corrosion resistance, and the susceptibility to hydrogen embrittlement (development of smooth fracture surface) peculiar to high-strength materials, we can adjust the amount of addition and the composition balance to realize low-cost and excellent steel. Another object of the present invention is to provide a material having hydrogen embrittlement resistance, tensile properties and corrosion resistance.

Now, the means for solving the above problems in order to provide a steel material that is inexpensive and has excellent hydrogen embrittlement resistance, tensile properties and corrosion resistance is
In the first means, in mass%, C: more than 0.10 to 0.60%, Si: 0.05 to 0.80%, Mn: 2.0 to 10.0%, P: 0.050 % or less, S: 0.050% or less, Ni: 8.0-18.0%, Cr: 8.0-18.0%, Mo: 0.01-0.50%, Cu: 0.05- 1.00%, V: more than 0.50 to 3.00%, Al: 0.001 to 0.100%, N: more than 0.100 to 0.250%, and the balance consists of Fe and unavoidable impurities ,
The values of the following formulas 1, 2, and 3 satisfy formula 1: 0.2 to 1.5, formula 2: -100 or less, and formula 3: 4.0 or more. High hardness hydrogen embrittlement resistant steel.
Formula 1: V/{4([C]+[N])}
Formula 2: 551-462 ([C] + [N] - 0.07 [V]) - 9.2 [Si] - 8.1 [Mn] - 13.7 [Cr] - 29 ([Ni] + [Cu])-18.5 [Mo]
Formula 3: 2.3 [Ni] + 3.0 [Mo] + 5.0 [Al] + 5.6 [Cu] - [Cr] - [Si] - 1.2 [Mn]
In addition, the mass % value of the corresponding component composition is substituted for the [element symbol] in the formula.

In the second means, in addition to the ingredients described in the first means,
Furthermore, at least one of B: 0.010% or less, Ca: 0.050% or less, and Mg: 0.050% or less in terms of mass%,
The remainder consists of Fe and unavoidable impurities,
The values of the following formulas 1, 2, and 3 satisfy formula 1: 0.2 to 1.5, formula 2: -100 or less, and formula 3: 4.0 or more. High hardness hydrogen embrittlement resistant steel.
Formula 1: V/{4([C]+[N])}
Formula 2: 551-462 ([C] + [N] - 0.07 [V]) - 9.2 [Si] - 8.1 [Mn] - 13.7 [Cr] - 29 ([Ni] + [Cu])-18.5 [Mo]
Formula 3: 2.3 [Ni] + 3.0 [Mo] + 5.0 [Al] + 5.6 [Cu] - [Cr] - [Si] - 1.2 [Mn]
In addition, the mass % value of the corresponding component composition is substituted for the [element symbol] in the formula.

According to the above means of the present application, by defining the value of Formula 1 to be 0.2 to 1.5, V, C and N can be effectively used for precipitation hardening, and the precipitates of V (C, N) Since it is possible to obtain steel with high hardness at low cost, it is possible to obtain steel with high hardness and excellent tensile properties at low cost. In the invention of the present application, in addition to Mn, by adding a large amount of N to more than 0.100%, the γ stability is increased at low cost, the strength is increased, and the corrosion resistance is improved. Down is also worn.

In addition, by specifying the value of formula 2 to −100 or less, it is possible to obtain a steel exhibiting a stable γ structure that is excellent in resistance to hydrogen embrittlement. Brittleness can be obtained.

In addition, by specifying the value of Formula 3 to be 4.0 or more, a high SFE can be secured and the occurrence of local slip can be suppressed. It can improve ductility at normal temperature, normal pressure, and atmospheric conditions, and can suppress the increase in hydrogen embrittlement susceptibility seen in high-strength materials.

Therefore, by adjusting the additive components of each alloy element so that the values of formulas 1 to 3 are within the specified range, it is possible to obtain a material that is inexpensive and has excellent hydrogen embrittlement resistance, tensile properties, and corrosion resistance.

Prior to the description of the mode for carrying out the invention, the reasons for defining the component composition, the formulas 1 to 3, and the values in the means according to the invention of the present application will be sequentially explained below. In addition, % in a chemical component is the mass %.

C: more than 0.10 to 0.60%
C is an element that combines with V and N to form V(C, N) [vanadium carbonitride] in steel to strengthen precipitation. For this purpose, C needs to be greater than 0.10%.
On the other hand, if the C content exceeds 0.60%, it forms coarse vanadium carbonitrides, degrading the corrosion resistance of the steel.
Therefore, C should be more than 0.10 to 0.60%.

Si: 0.05-0.80%
Si is an element added as a deoxidizing agent in the steelmaking stage. Therefore, Si is contained in steel in an amount of 0.05% or more. On the other hand, if the Si content in the steel exceeds 0.80%, the ductility of the steel deteriorates, and the hydrogen embrittlement resistance deteriorates due to ferrite formation. Therefore, Si should be 0.05 to 0.80%.

Mn: 2.0-10.0%
Mn is an element that stabilizes the γ structure and has excellent resistance to hydrogen embrittlement. Therefore, Mn must be 2.0% or more. On the other hand, if the Mn content exceeds 10.0%, the stacking fault energy (SFE) is lowered, thereby promoting the generation of smooth fracture surfaces. Therefore, Mn is set to 2.0 to 10.0%.

P: 0.050% or less P is an impurity element and is an element contained in normal refining. However, when the P content exceeds 0.050%, the ductility, toughness and hot workability of the obtained steel deteriorate. Therefore, P is set to 0.050% or less.

S: 0.050% or less S is an impurity element and is an element contained in normal refining. However, when the S content exceeds 0.050%, the resulting steel deteriorates in ductility, toughness and hot workability. Therefore, S is set to 0.050% or less.

Ni: 8.0-18.0%
Ni is an element that stabilizes austenite, increases SFE, and provides excellent resistance to hydrogen embrittlement. Therefore, 8.0% or more of Ni is contained. However, if the Ni content exceeds 18.0%, the effect tends to saturate, and since Ni is an expensive element, the cost becomes high. Therefore, Ni should be 8.0 to 18.0%.

Cr: 8.0-18.0%
Cr is an element that improves corrosion resistance. Therefore, Cr must be 8.0% or more. However, when the Cr content exceeds 18.0%, the effect of improving the corrosion resistance of Cr reaches saturation, and the hydrogen embrittlement resistance deteriorates due to the generation of ferrite. Therefore, Cr is set to 8.0 to 18.0%.

Mo: 0.01-0.50%
Mo is an element that improves corrosion resistance, raises SFE, and provides excellent resistance to hydrogen embrittlement. Therefore, Mo should be 0.01% or more. However, when Mo is contained in excess of 0.50%, it is a high element, so the cost increases. Therefore, Mo is set to 0.01 to 0.50%.

Cu: 0.05-1.00%
Cu is an element that stabilizes austenite, increases SFE, and provides excellent resistance to hydrogen embrittlement. Therefore, Cu should be 0.05% or more. However, when the Cu content exceeds 1.00%, the hot workability deteriorates. Therefore, Cu should be 0.05 to 1.00%.

V: more than 0.50 to 3.00%
V is an element that precipitates and strengthens steel by forming vanadium carbonitrides [V(C,N)]. Therefore, V shall contain more than 0.50%. However, if the V content exceeds 3.00%, coarse carbonitrides are formed and the corrosion resistance deteriorates. Moreover, since V is an expensive element, the cost is increased. Therefore, V should be more than 0.50 to 3.00%.

Al: 0.001-0.100%
Al is an element that deoxidizes during refining and increases the SFE of steel. Therefore, Al should be 0.001% or more. However, when the Al content exceeds 0.100%, the ductility is lowered and the hydrogen embrittlement resistance is deteriorated due to the generation of ferrite. Therefore, Al is set to 0.001 to 0.100%.

N: more than 0.100 to 0.250%
N is a strong austenite stabilizing element. In addition, those in the state of solid solution in the matrix strengthen the matrix by solid solution and improve corrosion resistance. Furthermore, it is an element that generates V(C, N) by performing aging heat treatment and imparts precipitation strengthening.
Therefore, the content of N shall be more than 0.100%. By adding a large amount of N in this way, it is intended to increase the γ stability at a low cost as a substitute for Ni, which is expensive, and to increase the strength of the steel material by solid solution strengthening and precipitation strengthening, and to improve the corrosion resistance. are doing.
However, when the N content exceeds 0.250%, the corrosion resistance is deteriorated due to the formation of coarse carbonitrides, and the formation of nitrides reduces the ductility and increases the manufacturing cost. There is a need. Therefore, N should be more than 0.100 to 0.250%.

Further, optional additional components will be explained. Any one or more of B, Ca and Mg may optionally be added in the present invention.

B: 0.010% or less B is an element that improves hot workability and can be added as necessary. However, if the B content exceeds 0.010%, the effect of improving hot workability is saturated, and the hot workability is rather deteriorated. Therefore, B is set to 0.010% or less.

Ca: 0.050% or less Ca is an element that improves hot workability and can be added as necessary. However, if the Ca content exceeds 0.050%, the effect of improving the hot workability is saturated, and the hot workability is rather deteriorated. Therefore, Ca should be 0.050% or less.

Mg: 0.050% or less Mg is an element that improves hot workability and can be added as necessary. However, if the content of Mg exceeds 0.050%, the effect of improving the hot workability is saturated and the hot workability is rather deteriorated. Therefore, Mg should be 0.050% or less.

Formula 1: 0.2 ≤ V / {4 ([C] + [N])} ≤ 1.5
By setting the value of V/{4([C]+[N])} in Equation 1 in the range of 0.2 to 1.5, V, C, and N can be effectively utilized for precipitation hardening. If the value of Formula 1 is less than the lower limit of 0.2, the excessive C and N degrade corrosion resistance. On the other hand, when the value of Equation 1 exceeds the upper limit of 1.5, V is excessive, resulting in high cost.
Therefore, V/{4([C]+[N])} is set to 0.2 to 1.5.

Formula 2: 551-462 ([C] + [N] - 0.07 [V]) - 9.2 [Si] - 8.1 [Mn] - 13.7 [Cr] - 29 ([Ni] + [Cu]) -18.5 [Mo] ≤ -100
Formula 2 is an index representing the stability of a γ structure that is excellent in resistance to hydrogen embrittlement. The lower the value of Equation 2, the more stable the γ structure. If the value is -100 or less, the γ structure is stable.
Therefore, 551-462 ([C] + [N]-0.07 [V])-9.2 [Si]-8.1 [Mn]-13.7 [Cr]-29 ([Ni ]+[Cu])−18.5[Mo] is −100 or less.

Formula 3: 2.3 [Ni] + 3.0 [Mo] + 5.0 [Al] + 5.6 [Cu] - [Cr] - [Si] - 1.2 [Mn] ≥ 4.0
Formula 3 is a formula showing how to ensure the stability of the processed structure. When the value of this formula is 4.0 or more, a high SFE is ensured, the occurrence of local slip is suppressed, and as a result, the development of a smooth fracture surface is prevented.
Therefore, the value of 2.3 [Ni] + 3.0 [Mo] + 5.0 [Al] + 5.6 [Cu] - [Cr] - [Si] - 1.2 [Mn] in formula 3 is 4.0 That's it.

Embodiments of the present invention will be described in sequence.
No. 1, which is an example of the invention of the present application, contains the chemical components shown in Table 1, and the balance is Fe and unavoidable impurities. 1 to 17 and No. 1 which is a comparative example thereof. Steel ingots of test materials Nos. 18 to 36 were melted as shown below.

First, each No. shown in Table 1. 100 kg of steel ingots composed of the chemical compositions of Invention Examples and Comparative Examples were melted in a vacuum induction melting furnace (VIM), and after heating these steel ingots to 1150 ° C., these steel ingots were forged and drawn to each have a diameter of 15 mm. of steel bars were produced.
Further, these steel bars were heated at 1000 to 1250° C. for 10 minutes or more and then cooled with water to obtain steel materials subjected to solution heat treatment.
Next, each of these solution heat-treated steel materials was heated to 600 to 900° C. for 30 minutes or more, and then air-cooled for aging treatment.
Each test piece was produced from these aged steel materials.

(Hot workability)
Among the above-mentioned manufacturing processes of test pieces used for various investigations, those that could be processed without problems in the forging of steel bars with a diameter of 15 mm are indicated by "○" as good workability in the column of "φ15 forging" in Table 2. did. On the other hand, those which had poor hot workability and were unable to continue working due to frequent cracking were marked with "x" in the column of "φ15 steel bar" in Table 2. For the comparative examples marked with x, the test pieces were not appropriately obtained, and thus other characteristics were not evaluated.

(Aging hardness)
The evaluation of "aging hardness (HRC)" in Table 2 is evaluated by measuring the Rockwell hardness of the test piece, and the actual hardness of 39 HRC or more is evaluated as good, and "○" is displayed. A hardness of 35 to 38 HRC was indicated as "Δ", and a hardness of less than 35 HRC was evaluated as insufficient hardness and indicated as "x".

(Tensile property evaluation)
As the evaluation of tensile properties in Table 2, first, for tensile strength, various test pieces were further processed into rod-shaped tensile test pieces of φ6 mm × 30 mm L of parallel parts, and then in the atmosphere at a stroke speed of 1.0 mm / min. A tensile test was performed at
In Table 2, those with a tensile strength of 1200 MPa or more are evaluated as good and indicated as "○", those less than 1200 MPa and 1100 MPa or more are indicated as "△", and those less than 1100 MPa are evaluated as inferior and indicated as "×". ” was displayed.

Next, the "restriction of area" of the tensile properties in Table 2 was evaluated. In the above tensile test, when the initial cross-sectional area is S ₀ and the minimum cross-sectional area after breaking is S, the value obtained by 100 (S ₀ - S) / S ₀ is the "restriction" (%) be.
If the actual value of the measured reduction is 30% or more, it is evaluated as good tensile properties and displayed as "○", and if the reduction is 25 to 29%, it is indicated as "△", and if it is less than 25%, the tensile properties are inferior. It was evaluated as a thing and displayed as "x".

(Smooth fracture rate)
The evaluation of the "smooth fracture rate" in Table 2 was carried out by observing and photographing the central portion of the cross section of the fractured sample after the tensile property evaluation described above with a scanning electron microscope (SEM). A portion where no dimples were observed in ^the image obtained by SEM was defined as a smooth fractured surface.
In Table 2, "smooth fracture rate" of 15% or less is evaluated as good and indicated as "○", "smooth fracture rate" of 16 to 20% is indicated as "△", and 21% or more is inferior. It was set as "x" as a thing.

(corrosion resistance)
As an evaluation of "corrosion resistance" in Table 2, after processing the above various 15 mm test pieces into φ12 × 21 mmL rod-shaped corrosion test pieces, a salt spray test (spraying 50 ppm diluted salt water at 35 ° C. for 16 hours) was carried out. After that, the surface of the test piece was observed, and if the number of punctate rusts with a major diameter of 1 mm or more was 15 or less, it was evaluated as good, and "○" was displayed in the "corrosion resistance evaluation" column, and 16 or more. The product was evaluated as inferior and marked with "X".

(Hydrogen embrittlement resistance)
As the evaluation of "hydrogen embrittlement resistance" in Table 2, (1) a hydrogen-charged test piece was used for evaluation. First, a Ni wire was electrically welded to the end of a test piece similar to that used for the evaluation of the tensile properties described above, and the portion other than the parallel portion was covered with a resin film to block hydrogen penetration. This test piece was immersed in a solution of 0.01 N sulfuric acid and 0.5 g/l ammonium thiocyanate, and charged with hydrogen by a cathode charging method under conditions of 68 A/mm ² , 30° C., 24 hours. went.

(2) Hydrogen embrittlement resistance evaluation: Immediately after charging with hydrogen, the same test as the evaluation of tensile properties was conducted to evaluate the reduction of area. The results are shown in % in the column of "Restriction (%) after charging with hydrogen" in Table 2.
Furthermore, the change in the reduction value with and without hydrogen charging is indicated by the relative reduction ratio (RRA), which is the ratio with and without hydrogen charging. RRA is a value of [restriction of material with hydrogen charging]/[restriction of material without hydrogen charging].
In the "RRA" column of Table 2, those with an RRA of 0.80 or more are indicated as "○" as having excellent resistance to hydrogen embrittlement, and those with an RRA of less than 0.80 are indicated as "×" as being inferior in resistance to hydrogen embrittlement. was displayed.

No. 6, which is the invention steel of the present invention. 1 to 17, as shown in Table 2, all of hot workability, aging hardness, tensile strength, reduction of area, smooth fracture rate, corrosion resistance evaluation, and RRA are good evaluations of ○ or △, A balance was ensured for all evaluation items. Therefore, a material that is inexpensive but has excellent workability, tensile properties, resistance to hydrogen embrittlement, and resistance to corrosion has been obtained.

These evaluation tests were performed in order of hot workability (φ15 mm forging), aging hardness, tensile properties (tensile strength and reduction of area), smooth fracture rate, corrosion resistance evaluation, and reduction of area (RRA) of hydrogen-charged material. With respect to the comparative steels, when the desired properties were not obtained, it was judged that they were not worthy of subsequent evaluation tests, and the evaluation was discontinued.

First, comparative steel No. Nos. 22, 23, 26, 34 to 36 had poor hot workability and cracks occurred during forging of φ15, so there was no need to evaluate the properties thereafter.
Also, the comparative steel No. Nos. 18, 28, 29 and 31 lacked aging hardness and were inferior in properties, so that there was no need to evaluate the properties in subsequent tests.
Comparative steel no. In No. 20, the results of drawing in the tensile evaluation properties were unsatisfactory, and the subsequent properties were not evaluated.
Comparative steel no. Nos. 21 and 33 had a high smooth fracture surface ratio, so there was no need to evaluate the subsequent characteristics.
Comparative steel no. Nos. 19, 24 and 30 were inferior in corrosion resistance to salt spray. There was no need to evaluate the subsequent characteristics.
Comparative example no. Nos. 25, 27 and 32 had a small relative drawing ratio (RRA) before and after hydrogen charging, and were greatly affected by hydrogen.

Claims (2)

% by mass, C: more than 0.10 to 0.60%, Si: 0.05 to 0.80%, Mn: 2.0 to 10.0%, P: 0.050% or less, S: 0. 050% or less, Ni: 8.0 to 18.0%, Cr: 8.0 to 18.0%, Mo: 0.01 to 0.50%, Cu: 0.05 to 1.00%, V: Contains more than 0.50 to 3.00%, Al: 0.001 to 0.100%, N: more than 0.100 to 0.250%, and the balance consists of Fe and inevitable impurities,
The values of the following formulas 1, 2, and 3 satisfy formula 1: 0.2 to 1.5, formula 2: -100 or less, and formula 3: 4.0 or more. High hardness hydrogen embrittlement resistant steel.
Formula 1: V/{4([C]+[N])},
Formula 2: 551-462 ([C] + [N] - 0.07 [V]) - 9.2 [Si] - 8.1 [Mn] - 13.7 [Cr] - 29 ([Ni] + [Cu]) -18.5 [Mo],
Formula 3: 2.3 [Ni] + 3.0 [Mo] + 5.0 [Al] + 5.6 [Cu] - [Cr] - [Si] - 1.2 [Mn].
In addition, the mass % value of the corresponding component composition is substituted for the [element symbol] in the formula. In addition to the steel components described in claim 1, at least one of B: 0.010% or less, Ca: 0.050% or less, and Mg: 0.050% or less in mass% containing, the remainder consisting of Fe and inevitable impurities,
The values of the following formulas 1, 2, and 3 satisfy formula 1: 0.2 to 1.5, formula 2: -100 or less, and formula 3: 4.0 or more. High hardness hydrogen embrittlement resistant steel.
Formula 1: V/{4([C]+[N])},
Formula 2: 551-462 ([C] + [N] - 0.07 [V]) - 9.2 [Si] - 8.1 [Mn] - 13.7 [Cr] - 29 ([Ni] + [Cu]) -18.5 [Mo],
Formula 3: 2.3 [Ni] + 3.0 [Mo] + 5.0 [Al] + 5.6 [Cu] - [Cr] - [Si] - 1.2 [Mn].
In addition, the mass % value of the corresponding component composition is substituted for the [element symbol] in the formula.