JP6326265B2 - Austenitic stainless steel excellent in hot workability and hydrogen embrittlement resistance and its production method - Google Patents

Description

  The present invention relates to an austenitic stainless steel that is used in a high-pressure hydrogen gas environment and has excellent hot workability and excellent resistance to hydrogen embrittlement, and a method for producing the same.

In recent years, from the viewpoint of preventing global warming, technology development using hydrogen as energy is progressing in order to suppress the emission of greenhouse gases (CO ₂ , NO _x , SO _x ). For this reason, development of metal materials used for hydrogen storage and transport is expected.

  Conventionally, hydrogen gas up to a pressure of about 40 MPa is filled and stored as a high-pressure gas in a thick Cr-Mo steel cylinder. Moreover, JIS standard SUS316 austenitic stainless steel (hereinafter referred to as “SUS316 steel”) is used as a piping material or a high-pressure hydrogen gas tank liner of a fuel cell vehicle (see Non-Patent Document 1). SUS316 steel has hydrogen embrittlement resistance under high-pressure hydrogen gas environment, for example, carbon steel containing the above Cr-Mo steel, JIS 304 SUS304 austenitic stainless steel (hereinafter referred to as “SUS304 steel”). It is good compared with.

  In recent years, prior to the general sale of fuel cell vehicles, public trials and demonstration experiments of hydrogen stations are in progress. For example, a hydrogen station that can store a large amount of hydrogen as liquid hydrogen, pressurize the liquid hydrogen, and supply it as high-pressure hydrogen gas of 70 MPa or more is in the demonstration stage. Also, a technology called precooling has been put into practical use in which hydrogen filled in a fuel cell vehicle tank is precooled to a low temperature of about −40 ° C. in a hydrogen station. Therefore, it is assumed that the metal material used for the liquid hydrogen container and hydrogen gas piping associated with the dispenser of the hydrogen station is exposed to high pressure of 70 MPa and low temperature of −40 ° C. Furthermore, at the hydrogen station, supplying higher pressure hydrogen and precooling to a lower temperature are now being studied. For this reason, the metal material used for said use is requested | required of the more excellent hydrogen embrittlement resistance compared with the past.

  However, for example, even the above SUS316 austenitic stainless steel may be hydrogen embrittled in a low temperature / high pressure hydrogen gas environment as described in Patent Document 1. This is said to be caused by segregation of Ni, which is present in the material of SUS316 series austenitic stainless steel and is caused by component distribution during solidification.

  Patent Document 2 discloses an austenitic high-Mn stainless steel having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment and a liquid hydrogen environment exceeding 40 MPa. This stainless steel does not consider the influence of solidification segregation during production.

On the other hand, in austenitic stainless steel, impurities that cannot be dissolved in the austenite phase during solidification are concentrated in the final solidified portion, and cracks occur during hot rolling. For this reason, hot workability is also important.
For example, Patent Document 3 discloses an austenitic stainless steel excellent in hot workability. In this stainless steel, hot workability is enhanced by utilizing the δ ferrite phase. However, the δ ferrite phase deteriorates the hydrogen embrittlement resistance unless it is properly manufactured.

  Patent Document 4 discloses a method for producing a high Ni stainless steel sheet. The manufacturing method described in Patent Document 4 targets a stainless steel plate having an Ni content of 15 to 45%, and deviates from the Ni content range of a general austenitic stainless steel.

  As described above, the austenitic stainless steel that has both the hydrogen embrittlement resistance in a high temperature hydrogen gas environment at a low temperature of more than 40 MPa and excellent hot workability has not yet appeared.

JP 2007-126688 A International Publication No. 2012-043877 Japanese Patent Laid-Open No. 11-293212 JP-A-4-110419

"Influence of Ni and Cr content on high-pressure hydrogen environment embrittlement characteristics of 316 stainless steel" Pressure Technology, Vol. 47, No. 2, p.

  An object of the present invention is to provide an austenitic stainless steel that has both hydrogen embrittlement resistance and excellent hot workability in a high-temperature hydrogen gas environment at a low temperature and over 40 MPa.

  In order to solve the above-mentioned problems, the inventors of the present invention have disclosed an alloy component composition of austenitic stainless steel composed of Cr, Mn, Ni, Mo and trace elements as main elements, metal structure, and high-pressure hydrogen gas. We have conducted intensive research on the relationship between hydrogen embrittlement resistance and hot workability in the environment. As a result, the following new knowledge was obtained and the present invention was completed.

(A) Ni is an element that controls the deformation structure of austenitic stainless steel into a form that is less susceptible to hydrogen embrittlement. For this reason, securing the amount of Ni is the most important for improving the hydrogen embrittlement resistance. However, the concentration of Ni varies in the steel due to solidification segregation of the austenitic stainless steel. In the tensile test in high-pressure hydrogen gas, the sensitivity to hydrogen embrittlement increases in a region where the Ni concentration is low. Therefore, the crack generated on the surface layer of the test piece preferentially propagates the region where the Ni concentration is low. As a result, ductility decreases in high-pressure hydrogen gas.

(B) The variation in Ni concentration increases as the amount of Cr, Mo, Si, and Nb that are ferrite forming elements increases. On the other hand, the variation in Ni concentration becomes smaller as the amount of Ni, C, and Mn that are austenite-generating elements increases. The variation of the Ni concentration is such that the ratio (Creq / Nieq) between Creq and Nieq calculated by the following formulas (1) and (2) considering the contribution of each element to the variation of the Ni concentration is 1.57 or less. By doing so, it is significantly suppressed.
Creq = Cr + Mo + 1.5Si + 0.5Nb (1)
Nieq = Ni + 30C + 0.5Mn (2)
However, the element symbol in Formula (1) and Formula (2) represents content in the mass% of the element.

(C) About the test piece which carried out the tension fracture in the high pressure hydrogen gas, the propagation path of the crack was analyzed with the scanning electron microscope and the electron beam microanalyzer. As a result, it was found that Cr in addition to Ni has an influence on the hydrogen embrittlement resistance. Furthermore, the magnitude of the contribution of Ni and Cr to improving the hydrogen embrittlement resistance was examined. In order to exhibit good hydrogen embrittlement resistance in a temperature range from room temperature to −70 ° C., [Ni] +0.37 [Cr] (elements in the formula) The symbol represents the content in mass% of the element.) Is preferably 18.0% or more, more preferably 19.0% or more.

(D) In the austenitic stainless steel, cracks may occur because S segregated at the austenite grain boundaries lowers the bonding strength of the grain boundaries during hot working. Conventionally, there has been a general method for improving hot workability by intentionally containing a δ ferrite phase in an austenitic stainless steel and dissolving S in the δ ferrite phase. However, when the knowledge (b) and (c) is utilized from the viewpoint of improving hydrogen embrittlement resistance, such a δ ferrite phase is not generated. For this reason, it is difficult to apply the conventional knowledge.

In order to improve hot workability in an austenitic stainless steel that secures hydrogen embrittlement resistance by using the component composition based on the findings of (b) and (c) above, S is added by Al deoxidation and Ca addition. It is effective to reduce the amount as much as possible. Specifically, excellent hot workability can be obtained by controlling the P value calculated by the equation (3) considering the S amount reduction function by Ca to -5 or less. Furthermore, in order to improve hot workability, it is also effective to increase the grain boundary strength by adding a small amount of B.
P value = (S + O−0.8Ca) × 10000−30 (3)
However, the element symbol in Formula (3) represents content in the mass% of the element.

(E) When the knowledge of (d) is applied, Al ₂ O ₃ .CaO inclusions are produced in the steel. In the component composition in which Nb, Ti, V is added alone or in combination to the component composition using the knowledge of (d) above, (Nb, Ti, V) (C, N) with Al ₂ O ₃ .CaO inclusions as nuclei The composite carbonitride of This carbonitride has a coherent precipitation relationship with the austenite matrix, and a matched strain field is formed at the interface between the carbonitride and the matrix. This matched strain field acts as a trap site for hydrogen atoms. Therefore, the hydrogen embrittlement resistance of the austenitic stainless steel at a low temperature is further improved. In order to form a matching strain field, it is necessary to control the T value calculated by Equation (4) to be 1.2 or more.
T value = (Nb + Ti + V) / (C + N) (4)
However, the element symbol in Formula (4) represents content in the mass% of the element.

(F) In a normal stainless steel manufacturing process, a steel piece (semi-finished product) such as a slab becomes a target size product by hot working after heating for a certain time. In the hot rolling process, by performing preliminary rolling and heat treatment on the steel slab and then performing the final hot rolling, Ni diffusion in the steel can be promoted, and variation in Ni concentration can be further suppressed. The present inventors diligently studied pre-rolling and heat treatment conditions before final hot rolling. As a result, after pre-rolling the steel slab at a rolling reduction of 25 to 60%, if heat treatment is performed at 1100 to 1250 ° C. for 60 minutes or more, the hydrogen embrittlement resistance is further improved. became.

This invention is made | formed based on the knowledge of said (a)-(f), The summary is as follows.
(1) By mass%, C: 0.1% or less, Si: 0.2-1.2%, Mn: 0.5-2.5%, P: 0.06% or less, S: 0.008 % or less, Ni: 10.0~15.0%, Cr: 16.0~20.0%, Mo: 2~ 3.5%, Cu: 0.08~0.5%, N: 0.01 -0.1%, Al: 0.01-0.3 % , Ca: 0.01% or less, O: 0.015% or less, B: 0.0001-0.008 % , and further Ti : 0.50% or less, Nb: 0.50% or less, and V: 0.50% or less, and the balance is composed of Fe and inevitable impurities, the formula (1) and the formula The ratio of Creq to Nieq calculated in (2) (Creq / Nieq) is 1.56 or less, and the P value calculated in Equation (3) is -5 or less. Excellent austenitic stainless steel processability and resistance to hydrogen embrittlement.

Creq = Cr + Mo + 1.5Si + 0.5Nb (1)
Nieq = Ni + 30C + 0.5Mn (2)
P value = (S + O−0.8Ca) × 10000−30 (3)
However, the element symbol in Formula (1), Formula (2), and Formula (3) represents content in the mass% of the element.

(2) The above component (18) in which the component calculated by [Ni] +0.37 [Cr] in the negative segregation part (the element symbol in the formula represents the content in mass% of the element) is 18% or more ( An austenitic stainless steel excellent in hot workability and hydrogen embrittlement resistance as described in 1).

(3) The austenitic stainless steel excellent in hot workability and hydrogen embrittlement resistance according to (1) or (2), wherein the T value calculated by formula (4) is 1.2 or more.
T value = (Nb + Ti + V) / (C + N) (4)
However, the element symbol in Formula (4) represents content in the mass% of the element.

(4) It has a hot rolling process which heat-rolls and heat-rolls the steel slab which consists of a component composition in any one of (1)-(3), In the said hot rolling process, On the other hand, hot workability and hydrogen embrittlement resistance are characterized by pre-rolling at a rolling reduction rate of 25 to 60% and performing a final hot rolling after heat treatment at 1100 to 1250 ° C. for 60 minutes or more. Method for producing austenitic stainless steel with excellent resistance.

  ADVANTAGE OF THE INVENTION According to this invention, the austenitic stainless steel which has the hydrogen embrittlement resistance in the low pressure and high pressure hydrogen gas environment of more than 40 Mpa, and the outstanding hot workability, and its manufacturing method can be provided.

  Hereinafter, the present invention will be described in detail. First, the component composition of the austenitic stainless steel of the present invention will be described. In the following description, “%” display of the content of each element means “mass%”.

C: 0.1% or less C is an element effective for stabilizing the austenite phase. In order to improve hydrogen embrittlement resistance by stabilizing the austenite phase and suppressing variation in Ni concentration, the C content is preferably 0.01% or more. On the other hand, excessive addition of C causes a decrease in the ductility of the austenite phase due to accelerated precipitation of Cr-based carbides, resulting in a decrease in hydrogen embrittlement resistance. For this reason, it is necessary to make the upper limit of C content 0.1%. A more preferable upper limit of the C content is 0.07%.

Si: 0.2-1.2%
Si is an element effective for stabilizing the austenite phase. In order to improve the hydrogen embrittlement resistance by stabilizing the austenite phase, the Si content needs to be 0.2% or more. The Si content is preferably 0.3% or more. On the other hand, the excessive addition of Si promotes the formation of a δ ferrite phase that is the starting point of crack generation due to hydrogen embrittlement. Excessive Si addition also promotes the formation of intermetallic compounds such as a sigma phase, leading to a decrease in hot workability and toughness. For this reason, the upper limit of Si content needs to be 1.2%. The Si content is more preferably 1.0% or less.

Mn: 0.5 to 2.5%
Mn is an element effective for stabilizing the austenite phase. In order to improve the hydrogen embrittlement resistance by suppressing the formation of the work-induced martensite phase by stabilizing the austenite phase and suppressing the variation in Ni concentration, the Mn content needs to be 0.5% or more. The Mn content is preferably 0.7% or more. On the other hand, the excessive addition of Mn promotes the formation of the δ ferrite phase, which is the starting point of cracking due to hydrogen embrittlement, so the upper limit needs to be 2.5%. The Mn content is more preferably 2.1% or less.

P: 0.06% or less P is contained as an impurity in the austenitic stainless steel of the present invention. Since P is an element that decreases hot workability, it is preferably reduced as much as possible. Specifically, the P content is preferably 0.06% or less, and more preferably 0.05% or less. However, since extreme reduction of the P content leads to an increase in steelmaking costs, the P content is preferably 0.008% or more.

S: 0.008% or less S is an element that segregates at the austenite grain boundary during hot working and induces cracking during hot working by weakening the bonding force of the grain boundary. Therefore, the upper limit of the S content needs to be 0.008%. The upper limit with preferable S content is 0.005%. Since it is preferable to reduce the S content as much as possible, there is no particular lower limit. However, extreme reduction leads to an increase in steelmaking costs. For this reason, it is preferable that S content is 0.0001% or more.

Ni: 10.0-15.0%
Ni is an element having the greatest effect of improving the hydrogen embrittlement resistance of austenitic stainless steel. In order to sufficiently obtain this effect, the Ni content needs to be 10.0% or more. The Ni content is preferably 12.0% or more. On the other hand, excessive Ni addition causes an increase in material cost, so the upper limit of Ni content is 15.0%. The Ni content is preferably 14.0% or less.

Cr: 16.0 to 20.0%
Cr is an element indispensable for obtaining the corrosion resistance required for stainless steel. In addition, Cr is an element that contributes to an increase in strength of austenitic stainless steel. In order to secure strength comparable to that of existing SUS316 steel, the Cr content needs to be 16.0% or more. The Cr content is preferably 17.0% or more. On the other hand, excessive addition of Cr leads to a variation in Ni concentration and a reduction in the ductility of the austenite phase due to the promotion of Cr-based precipitates, thereby reducing the hydrogen embrittlement resistance. For this reason, it is necessary to make the upper limit of Cr content 20.0%. The Cr content is preferably 18.5% or less.

Mo: 3.5% or less Mo is an element contributing to an increase in strength and corrosion resistance of austenitic stainless steel. In order to ensure the same strength and corrosion resistance as existing SUS316 steel, the Mo content is preferably 2% or more. On the other hand, excessive addition of Mo promotes variation in Ni concentration and leads to an increase in material cost. For this reason, the upper limit of Mo content needs to be 3.5%. A more preferable upper limit of the Mo content is 2.5%.

Cu: 0.08 to 0.5%
Cu is an element effective for stabilizing the austenite phase. In order to improve the hydrogen embrittlement resistance by stabilizing the austenite phase, the Cu content needs to be 0.08% or more. The Cu content is preferably 0.10% or more. On the other hand, excessive addition of Cu leads to a decrease in strength and the hot workability is also impaired, so the upper limit of the Cu content needs to be 0.5%. The Cu content is more preferably 0.4% or less.

N: 0.01 to 0.1%
N is an element effective for stabilizing the austenite phase, increasing the strength, and further improving the corrosion resistance. In order to obtain these effects, the N content is preferably 0.01% or more. The N content is preferably 0.03% or more. On the other hand, excessive N addition promotes the formation of Cr-based precipitates, and reduces the corrosion resistance and toughness of the austenite phase. For this reason, the upper limit of N content needs to be 0.1%. The N content is more preferably 0.08% or less.

Al: 0.3% or less Al is an element effective for deoxidation and improvement of hot workability. On the other hand, excessive addition of Al causes a significant increase in manufacturing cost. For this reason, it is necessary to make the upper limit of Al content 0.3%. The upper limit with preferable Al content is 0.18%. The lower limit of the Al content is not particularly required, but the Al content is preferably 0.01% or more in order to obtain a sufficient deoxidation effect.

Ca: 0.01% or less Ca is an element effective for deoxidation and improving hot workability. In order to obtain this effect, the Ca content is preferably 0.001% or more. On the other hand, excessive addition of Ca causes a significant increase in manufacturing cost. For this reason, it is necessary to make the upper limit of Ca content 0.01%. The upper limit with preferable Ca content is 0.008%.

O: 0.015% or less O reduces the hot workability and toughness of the austenite phase by forming an oxide in the steel. For this reason, it is necessary to make the upper limit of O (oxygen) content 0.015% or less. The O content is preferably 0.010% or less. The O (oxygen) content is preferably reduced as much as possible, but the extreme reduction leads to an increase in steelmaking costs. Therefore, the O (oxygen) content is preferably 0.001% or more.

B: 0.005% or less B is an element that improves the hot workability by increasing the bonding strength of the austenite grain boundaries. In order to obtain this effect, the B content is preferably 0.0001% or more. On the other hand, excessive addition of B causes a significant increase in manufacturing cost. For this reason, the upper limit of B content needs to be 0.008%. The upper limit with preferable B content is 0.005%.

  In addition to the above elements, the austenitic stainless steel of the present invention further contains one or more of Ti: 0.50% or less, Nb: 0.50% or less, V: 0.50% or less. Including.

Ti: 0.50% or less Ti is an element necessary for the production of carbonitrides having inclusions of Al ₂ O ₃ .CaO as a nucleus, so 0.001% or more is preferably added. On the other hand, excessive addition of Ti causes an increase in alloy cost and a decrease in the toughness of the austenite phase, so the upper limit of the Ti content needs to be 0.50%. The upper limit with preferable Ti content is 0.30%.

Nb: 0.50% or less Since Nb is an element necessary for the production of carbonitrides including inclusions of Al ₂ O ₃ .CaO, 0.001% or more is preferably added. On the other hand, excessive Nb addition causes an increase in alloy cost and a decrease in the toughness of the austenite phase, so the upper limit of the Nb content needs to be 0.50%. The upper limit with preferable Nb content is 0.30%.

V: 0.50% or less Since Ti is an element necessary for the production of carbonitrides with inclusions of Al ₂ O ₃ .CaO as a nucleus, it is preferable to add 0.001% or more. On the other hand, excessive addition of Ti causes an increase in alloy cost and a decrease in the toughness of the austenite phase, so the upper limit of the Ti content needs to be 0.50%. The upper limit with preferable Ti content is 0.30%.

  In the component composition of the austenitic stainless steel of the present invention, in addition to the above, the ratio (Creq / Nieq) of Creq and Nieq calculated by the formulas (1) and (2) is 1.56 or less. As a result, variation in Ni concentration due to solidification segregation is remarkably suppressed. A more preferable ratio of Creq to Nieq (Creq / Nieq) is 1.50 or less.

Creq = Cr + Mo + 1.5Si + 0.5Nb (1)
Nieq = Ni + 30C + 0.5Mn (2)
However, the element symbol in Formula (1) and Formula (2) represents content in the mass% of the element.

Moreover, in the component composition of the austenitic stainless steel of this invention, P value calculated by Formula (3) is controlled to -5 or less. As a result, the influence of S that contributes to a reduction in the bonding strength of the austenite grain boundaries is remarkably reduced, and the hot workability is improved. The P value is preferably −8 or less. Further, the P value is preferably reduced as much as possible, but an excessive decrease in the P value leads to an increase in steelmaking cost, and thus is preferably −28 or more.
P value = (S + O−0.8Ca) × 10000−30 (3)
However, the element symbol in Formula (3) represents content in the mass% of the element.

Furthermore, when the above component ranges are satisfied, Al ₂ O ₃ .CaO inclusions are generated in the steel. In the above component composition, Nb, Ti, and V are added alone or in combination. Therefore, a composite carbonitride of (Nb, Ti, V) (C, N) with Al ₂ O ₃ · CaO inclusions as the nucleus. Precipitates. This carbonitride has a coherent precipitation relationship with the austenite matrix, and a coherent strain field is formed at the interface between the precipitate and the matrix. This matched strain field acts as a trap site for hydrogen atoms. Therefore, the hydrogen embrittlement resistance of the austenitic stainless steel at a low temperature is further improved.

In order to form a matching strain field, it is necessary to control the T value calculated by Equation (4) to be 1.2 or more. When the T value is 1.2 or more, since the carbonitride of (Nb, Ti, V) (C, N) is sufficiently precipitated, the trap effect of hydrogen atoms can be sufficiently obtained. The T value is more preferably 1.25 or more in order to further improve the trapping effect of hydrogen atoms. Further, an excessive increase in T value causes an increase in alloy cost and a decrease in ductility of the austenite phase. For this reason, it is preferable that T value is 5.5 or less.
T value = (Nb + Ti + V) / (C + N) (4)
However, the element symbol in Formula (4) represents content in the mass% of the element.

Moreover, in the negative segregation part contained in steel, [Ni] +0.37 [Cr] (the element symbol in the formula represents the content in mass% of the element) is 18.0% or more. This tends to improve the hydrogen embrittlement resistance. When a tensile test piece is performed in high-pressure hydrogen gas, a brittle fracture surface accompanied by a crack is observed in a test piece in which a decrease in ductility is confirmed. The amount of Ni and the amount of Cr are related to the crack generation / propagation path. In the region where [Ni] +0.37 [Cr] is less than 18.0%, the hydrogen embrittlement resistance of the austenite phase is lower than that of the surrounding area, and therefore, it becomes a preferential crack generation / propagation path. In the region where [Ni] +0.37 [Cr] is 19.0% or more, such generation of cracks is not recognized. Therefore, [Ni] +0.37 [Cr] is preferably 19.0% or more.
The concentration distribution of Ni and Cr in steel can be measured, for example, by line analysis using an electron beam probe microanalyzer.

"Production method"
Next, the manufacturing method of the austenitic stainless steel of this invention is demonstrated. In order to produce the austenitic stainless steel of the present invention, first, a stainless steel having the above component composition is melted to produce a steel piece such as a slab. Next, the steel slab is heated to a predetermined temperature to perform hot rolling (hot rolling process). In the hot rolling step, it is preferable to perform preliminary rolling (intermediate hot rolling) and heat treatment before the final hot rolling in order to improve the hydrogen embrittlement resistance of the austenitic stainless steel. The austenitic stainless steel of the present invention is not limited to a steel plate. Therefore, the steel slab is not limited to the slab, and it can be achieved by selecting a steel slab (billette, bloom, etc.) having a preferable shape with respect to the shape of the target product (bar, pipe, etc.). Needless to say.

Hereinafter, the intermediate hot rolling and heat treatment conditions before final hot rolling will be described in detail.
Intermediate hot rolling is performed at a rolling reduction of 25 to 60% with respect to the steel slab. The strain introduced by the intermediate hot rolling promotes the diffusion of the alloy elements during the heat treatment and reduces Ni segregation in the austenitic stainless steel. In order to obtain this effect sufficiently, a rolling reduction of 25% or more is necessary. A more preferable rolling reduction is 40% or more. Moreover, excessive rolling in intermediate hot rolling leads to insufficient reduction in the final hot rolling, leading to a reduction in strength and a deterioration in hydrogen gas embrittlement resistance due to coarsening of the final product. For this reason, it is preferable that the upper limit of the rolling reduction is 60%. A more preferable rolling reduction is 50% or less.

  After the intermediate hot rolling, it is preferable to perform a heat treatment at 1100 to 1250 ° C. for 60 minutes or more. When the heat treatment temperature is 1100 or higher, the diffusion of alloy elements can be further promoted in the austenitic stainless steel after intermediate hot rolling. As the heat treatment temperature increases, the diffusion of the alloy elements is promoted. A more preferable heat treatment temperature is 1150 ° C. or higher. However, if the heat treatment temperature is too high, production costs may increase due to the addition of a grinding process due to abnormal oxidation of the steel material. For this reason, it is preferable that the upper limit of heat processing temperature shall be 1250 degreeC. A more preferable heat treatment temperature is 1200 ° C. or less.

  When the heat treatment temperature is 1100 to 1250 ° C., negative heat segregation due to diffusion of alloy elements in the austenitic stainless steel can be reduced by setting the heat treatment time to 60 minutes or more. The effect of performing the heat treatment is saturated after the heat treatment time of 60 minutes, so there is no need to provide an upper limit.

  Stainless steels having chemical components shown in Tables 1 to 3 were melted to produce slabs. Thereafter, the slab was heated and hot rolled to produce a hot-rolled annealed plate having a thickness of 15 mm. In addition, as hot rolling, under the conditions shown in Table 3, intermediate hot rolling, intermediate heat treatment, final hot rolling up to a thickness of 15 mm, heat treatment temperature of 1050 ° C. and heat treatment time of 4 minutes are performed. I went in this order. Moreover, test pieces D1 to D7 shown in Tables 1 to 3 are hot-rolled annealed plates produced by changing the conditions of intermediate hot rolling and intermediate heat treatment.

In Table 2, (-) indicates that it was not intentionally added.
In Table 3, “Creq / Nieq” is the ratio (Creq / Nieq) of Creq and Nieq calculated by the above formulas (1) and (2), and “P value” is calculated by the above formula (3). The “T value” is a value calculated by the above equation (4).

  “[Ni] +0.37 [Cr]” shown in Table 3 is a component calculated by the above formula in the negative segregation part in the steel, and is a value obtained by the following method. A test piece was cut out from a hot-rolled annealed plate having a thickness of 15 mm produced by the above method so that the surface perpendicular to the rolling direction was the observation surface. Then, the test piece was embedded in resin and mirror polishing was performed. Then, using an electron beam probe microanalyzer (EPMA), a line analysis of the test piece embedded in the resin was performed, and the concentration distribution of the Ni amount and the Cr amount in the center portion from the plate surface layer was investigated. The analysis of the concentration distribution of the amount of Ni and the amount of Cr was carried out for each steel type with three fields of view, and the most [Ni] +0.37 [Cr] (the element symbol in the formula is the content in mass% of the element) Represents a value (mass%) of [Ni] +0.37 [Cr] in the negative segregation part of the steel type.

Next, the hydrogen embrittlement resistance and hot workability of the hot-rolled annealed plates of each steel type were evaluated by the following methods.
"Hydrogen embrittlement resistance"
A round bar tensile test piece having a parallel part with an outer diameter of 7 mm and a length of 35 mm and a distance between gauge points of 25 mm was collected from the longitudinal direction / center part of the hot-rolled annealed plate. Using this round bar tensile test piece, (1) an atmospheric tension test and (2) a high-pressure hydrogen gas tensile test were performed.

The atmospheric tensile test (1) was performed under the conditions of test temperature: normal temperature, −50 ° C., −70 ° C., test environment: air, and crosshead speed: 0.028 mm / sec.
The tensile test in high-pressure hydrogen gas (2) was performed in the same manner as the atmospheric tensile test (1) except that the test environment was 105 MPa hydrogen.
Then, the value of “(squeezing in high-pressure hydrogen gas / squeezing in the atmosphere) × 100 (%)” is calculated, and when this value is 80% or more, hydrogen embrittlement resistance in high-pressure hydrogen gas is calculated. Evaluated as passing. The results are shown in Table 4.

"Hot workability"
Hot workability was evaluated at 1200 ° C. and 1100 ° C. by a thermoresta test. A test piece having a diameter of 8 mm and a length of 110 mm was sampled from the hot rolled annealed plate from the longitudinal direction / center of the plate. The test piece was heated from room temperature to 1200 ° C. over 60 seconds and held for 60 seconds. When evaluating at 1200 degreeC, the tension test was done after 60-second holding | maintenance at 1200 degreeC. In the case of evaluation at 1100 ° C., after holding at 1200 ° C. for 60 seconds, it was cooled to 1100 ° C. at 20 ° C./second, and then held for 60 seconds, and then a tensile test was performed. At any test temperature, the crosshead speed in the tensile test was 20 mm / second. Then, at each test temperature, those having a drawing value of 60% or more were evaluated as passing hot workability. The results are shown in Table 4.

  As shown in Table 4, test pieces A1 to A6, B1 to B5, C1 to C4, and D1 to D7, which are examples of the present invention, are resistant to hydrogen at any temperature of normal temperature, −50 ° C., and −70 ° C. The evaluation of the embrittlement characteristic was 80% or more, and the evaluation of hot workability at 1200 ° C. and 1100 ° C. was 60% or more, which was acceptable.

The test pieces A1 to A6 have a T value within a preferable range. The test pieces A1 to A6 have an evaluation of hydrogen embrittlement resistance at -70 ° C of 85% or more, and it has been confirmed that they have excellent hot workability while having excellent hydrogen embrittlement resistance. .
In the test pieces B1 to B5, [Ni] +0.37 [Cr] is 19% or more, and the T value is within a preferable range. The test pieces B1 to B5 had an evaluation of hydrogen embrittlement resistance of 98% or higher at any temperature of normal temperature, −50 ° C., and −70 ° C., and had excellent hydrogen embrittlement resistance while being excellent. It was confirmed that it showed hot workability.

  The test piece C3 is an example in which [Ni] +0.37 [Cr] is less than 18% and the intermediate heat treatment temperature is less than 1100 ° C. The test piece C4 is an example in which [Ni] +0.37 [Cr] is less than 18% and the intermediate heat treatment time is less than 60 minutes. It was confirmed that the test pieces C3 and C4 also had better hydrogen embrittlement resistance than the test pieces E1 to E9.

  Test pieces D3 and D7 were subjected to intermediate hot rolling and intermediate heat treatment under preferable conditions. For this reason, in the test pieces D3 and D7, the evaluation of the hydrogen embrittlement resistance at −70 ° C. was 98% or more, which was very good. In addition, the test pieces D3 and D7 had a better evaluation of the hydrogen embrittlement resistance compared to the test piece D1 that was not subjected to intermediate hot rolling and intermediate heat treatment. In addition, from the results of test pieces D1, D2, and D4 to D6, no effect is found in D2 and D4 to D6 that have been subjected to intermediate hot rolling and intermediate heat treatment outside the preferred conditions. It was.

In the test pieces E1 and E2, “Creq / Nieq” does not satisfy the scope of the present invention. Therefore, in the test pieces E1 and E2, the Ni concentration fluctuation in the steel becomes large, the amount of [Ni] +0.37 [Cr] necessary for maintaining the hydrogen embrittlement resistance cannot be secured, and the hydrogen embrittlement resistance It is presumed that the evaluation of characteristics has been lowered.
The test pieces E3 and E4 have P values that do not satisfy the scope of the present invention. As a result, the evaluation of hot workability was lowered. Furthermore, in the test pieces E3 and E4, it is presumed that the defect considered to be generated during hot rolling when producing the hot-rolled annealed sheet was the starting point, and the hydrogen embrittlement resistance at -70 ° C. was lowered.

In the test piece E5, the Ni content does not satisfy the scope of the present invention. As a result, it is estimated that the amount of [Ni] +0.37 [Cr] necessary for maintaining the hydrogen embrittlement resistance could not be secured, and the evaluation of the hydrogen embrittlement resistance was lowered.
In the test piece E6, the Cr amount does not satisfy the scope of the present invention. As a result, it is presumed that the ductility of the austenite phase was lowered and the evaluation of the hydrogen embrittlement resistance at −50 ° C. and −70 ° C. was lowered.

In the test piece E7, the Mo amount does not satisfy the scope of the present invention. As a result, the Ni concentration fluctuation in the steel becomes large, the amount of [Ni] +0.37 [Cr] necessary for maintaining the hydrogen embrittlement resistance cannot be secured, and the evaluation of the hydrogen embrittlement resistance becomes low. It is estimated that
In the test pieces E8 and E9, the amount of Mn does not satisfy the scope of the present invention. In the case of the test piece E9 with a small amount of Mn, the stability of the austenite phase is insufficient, and in the case of the test piece E8 with a large amount of Mn, it is estimated that the evaluation of the hydrogen embrittlement resistance is lowered due to the formation of the δ ferrite phase .

  The austenitic stainless steel of the present invention has extremely excellent hydrogen embrittlement resistance in a hydrogen gas at a low temperature up to -70 ° C. and a high pressure of over 40 MPa. For this reason, the austenitic stainless steel of the present invention is applicable as a material for a high-pressure hydrogen gas tank, a high-pressure hydrogen gas tank liner, a high-pressure hydrogen gas, and a liquid hydrogen pipe that store hydrogen gas having a pressure exceeding 40 MPa.

Claims (4)

In mass%, C: 0.1% or less, Si: 0.2-1.2%, Mn: 0.5-2.5%, P: 0.06% or less, S: 0.008% or less, Ni: 10.0~15.0%, Cr: 16.0~20.0 %, Mo: 2~ 3.5%, Cu: 0.08~0.5%, N: 0.01~0. 1%, Al: 0.01 to 0.3 % , Ca: 0.01% or less, O: 0.015% or less, B: 0.0001 to 0.008 % ,
Further, Ti includes 0.50% or less, Nb: 0.50% or less, and V: 0.50% or less, and the balance includes Fe and inevitable impurities,
The ratio (Creq / Nieq) between Creq and Nieq calculated by formula (1) and formula (2) is 1.56 or less,
An austenitic stainless steel excellent in hot workability and hydrogen embrittlement resistance, wherein the P value calculated by formula (3) is −5 or less.
Creq = Cr + Mo + 1.5Si + 0.5Nb (1)
Nieq = Ni + 30C + 0.5Mn (2)
P value = (S + O−0.8Ca) × 10000−30 (3)
However, the element symbol in Formula (1), Formula (2), and Formula (3) represents content in the mass% of the element.   The component calculated by [Ni] +0.37 [Cr] in the negative segregation part (the element symbol in the formula represents the content in mass% of the element) is 18% or more. Austenitic stainless steel with excellent hot workability and hydrogen embrittlement resistance. 3. The austenitic stainless steel excellent in hot workability and hydrogen embrittlement resistance according to claim 1 or 2, wherein the T value calculated by the formula (4) is 1.2 or more.
T value = (Nb + Ti + V) / (C + N) (4)
However, the element symbol in Formula (4) represents content in the mass% of the element.   It has the hot rolling process which heats the steel slab which consists of a component composition as described in any one of Claims 1-3, and performs hot rolling, In the said hot rolling process, with respect to the said steel slab The hot workability and hydrogen embrittlement characteristics are characterized by pre-rolling at a rolling reduction of 25 to 60% and performing a final hot rolling after heat treatment at 1100 to 1250 ° C. for 60 minutes or more. An excellent method for producing austenitic stainless steel.