HK1097577B - Heat-resistant cast steel for reaction tube for hydrogen production being excellent in aging ductility and creep rupture strength - Google Patents

Description

Heat-resistant cast steel for hydrogen production reaction tube having excellent aged ductility and creep rupture strength

Technical Field

The present invention relates to a heat-resistant cast steel excellent in aged ductility and creep rupture strength, which is a pipe material of a reaction tube for hydrogen production that uses hydrocarbons such as petroleum fuels and natural gas as raw materials and generates hydrogen or a synthesis gas containing hydrogen as a main component by a steam reforming reaction.

Background

In a steam reforming furnace as a hydrogen production apparatus of a petroleum refinery, a mixed gas of a petroleum fuel (naphtha, crude gasoline, etc.) and steam is fed into a reaction tube, and a reforming reaction [ CmHn + mH + is carried out by a catalyst under high temperature and pressure (temperature: about 700 to 900 ℃ C., pressure: about 1 to 3MPa)₂O→(n/2+m)H₂+mCO]A synthesis gas containing hydrogen as a main component is produced. The reforming reaction tube is required to have high-temperature strength and high-temperature creep strength so as to be able to withstand long-term continuous operation under high-temperature and high-pressure conditions. Conventionally, as such a pipe material, high-carbon, high-Cr — Ni heat-resistant cast steel, which is a precipitation-strengthening alloy, has been used. Specifically, as the 1 st generation material, SCH22(0.4C-25Cr-20Ni-Fe) was used, and 1N519(0.3C-24Cr-24Ni-1.5Nb-Fe) was used as the 2 nd generation material, and HP-Nb and Ti materials (0.5C-25Cr-35Ni-Nb and Ti-Fe) were developed, which were obtained by alloying a small amount of Nb, Ti, etc) And the like as a 3 rd generation material until now.

Patent document 1: japanese patent publication No. 55-47105

Patent document 2: japanese patent publication No. 57-40900

Patent document 3: japanese unexamined patent publication No. 5-239599

In recent years, there has been a demand for clean energy as a measure against environmental pollution, and a fuel cell using hydrogen as a fuel has been drawing attention, and has been expected to be a power source for automobiles and the like, and has been developed as a small-sized distributed power source and the like, and has been partially put into practical use. Accordingly, as a hydrogen production apparatus for supplying hydrogen to a fuel cell, there have been developed small-sized hydrogen production apparatuses and on-site hydrogen production apparatuses (so-called "hydrogen stations") using city gas (LNG), hydrocarbons such as alcohols, kerosene, and diesel oil as raw materials, in addition to naphtha and Liquefied Petroleum Gas (LPG).

The steam reforming reaction of the hydrogen production apparatus for a fuel cell is performed at a relatively low temperature and a low pressure (temperature: about 750 to 800 ℃ C., pressure: about 1MPa or less) as compared with the operating conditions of a large-sized apparatus in a petroleum refinery, but the power demand of the fuel cell greatly fluctuates between daytime and nighttime, and therefore the operation of the hydrogen production apparatus requires repeated load fluctuations of the reforming reaction tube in accordance with the power demand. If such load fluctuations are repeated every day, creep and fatigue are accumulated in the reaction tube in an overlapping manner, which causes fatigue failure. Therefore, the reforming reaction tube of the hydrogen production apparatus for a fuel cell is required to have excellent fatigue properties, such as high-temperature strength and high-temperature creep rupture strength. The precipitation hardening type high-C high-Cr — Ni heat-resistant cast steel used in large-scale equipment of petroleum refining plants is a material having high-temperature characteristics (high-temperature strength and creep rupture strength) required for continuous operation at high temperature and high pressure, but has a problem in aged ductility and fatigue fracture resistance required for a load-variable hydrogen production system, and cannot be used stably for a long period of time. Further, when HK40 material or the like is used in a long-term use environment at a temperature range of about 800 ℃, the above-described embrittlement phenomenon by the precipitation of the σ phase also becomes a problem.

Disclosure of Invention

The present invention has been made to solve the above-mentioned problems associated with the materials for the reforming reaction tube for hydrogen production, and an object of the present invention is to provide a heat-resistant cast steel which has improved aged ductility and fatigue characteristics for improving durability stability of a reaction tube subject to repeated load fluctuations, such as a hydrogen production system for a fuel cell, and which is economically superior, while maintaining heat resistance and high-temperature creep rupture strength required for a steam reforming reaction tube in a high-temperature pressurized use environment.

The heat-resistant cast steel for hydrogen production reaction tubes is characterized by comprising, by mass: 0.1 to 0.5%, Si: 2.5% or less, Mn: 2.5% or less, Cr: 15-26%, Ni: 8-23%, Nb: 0.1 to 1.2%, Ti: 0.01-1.0%, Ce: 0.001-0.15%, N: less than 0.06%, the balance being substantially Fe, and the parameter value P represented by the following formula [1] being 20 to 45.

P＝89.3-78.4C+0.1Si-5.7Mn-1.7Cr

+0.01Ni+2Nb+5.3Ti-36.5N-50.8Ce......[1]

Further, each element symbol in the formula [1] represents the content (%) of the element.

The heat-resistant cast steel of the present invention may have a composition containing any one of the following combination elements (1) to (3) as required:

(1) selected from B: 0.001 to 0.05%, Zr: 0.01-0.5%, La: 0.001-0.15% of 1 or more species or more than 2 species.

(2)Al：0.01～0.3％

(3) Selected from B: 0.001 to 0.05%, Zr: 0.01-0.5%, La: 0.001-0.15% of 1 or more species and 2 or more species of Al: 0.01 to 0.3 percent.

The heat-resistant cast steel of the present invention may be limited to C: 0.1-0.3%, and for Cr and Ni, adjusting the ratio of Cr: 15-20%, Ni: 8-18% of the composition.

The heat-resistant cast steel of the present invention having the above chemical composition has chromium carbide (Cr) in the matrix of the austenite phase₂₃C₆) And the like, and therefore, the metal structure has heat resistance and high-temperature creep rupture strength necessary for the steam reformer reaction of the hydrogen production apparatus under a high-temperature and high-pressure environment, and the precipitation of secondary carbides is suppressed under a high-temperature and long-term aging, and moreover, there is no embrittlement due to the sigma phase precipitation, which has been a problem in the conventional HK40 material, and as these effects, the high-level elongation can be stably maintained during long-term use. As an effect of improving the aged ductility, improved fatigue characteristics required for reformer reaction tubes in which thermal fatigue cycles are repeated due to load fluctuations, such as those in hydrogen production systems for fuel cells, can be secured, and the service life can be improved.

Detailed Description

The heat-resistant cast steel of the present invention is adjusted to the following composition in order to ensure heat resistance and high-temperature strength against a high-temperature and high-pressure environment in which hydrogen generation is performed by steam reforming, and to ensure aged ductility and fatigue characteristics required for a load-shifting type use environment. The contents of the components are all mass%.

C：0.1～0.5％

C is bonded to Nb at grain boundaries during casting and solidification of molten steel to form NbC crystals and precipitate, and C dissolved in a matrix of an austenite phase is bonded to Cr to precipitate and form fine chromium carbides (Cr) when the reaction tube is used at high temperatures₂₃C₆). As a result of the precipitation strengthening action, creep rupture strength can be improved. The amount of C required for obtaining creep rupture strength which can withstand a use environment of 1000 ℃ is 0.1% or more as a reformer reaction tube to be installed in a large-scale apparatus of a petroleum refinery. Increasing the amount of C increases the creep rupture strength, but above 0.5%, withSecondary carbide (Cr) precipitated in the course of long-term high-temp. use₂₃C₆) The increase in the accumulation causes the ductility to decrease and the fatigue characteristics to deteriorate. Therefore, it is required to limit the amount of C to 0.5% or less. For applications where it is desired to maintain fatigue characteristics at a higher level, it is desirable to limit the material of the reaction tube to a range of 0.1% to 0.3% for in-situ devices such as hydrogen generation devices for fuel cells, where load fluctuations are repeated.

Si: 2.5% or less

Si is an element added for deoxidation of molten steel and imparting fluidity to molten steel during casting. If the content of the modifier is 2.5% or more, and more than 2.5% is sufficient to obtain such an effect, the time-dependent ductility is lowered, and the weldability required for piping construction of the reformer constituting the hydrogen production system is lowered. Therefore, it is preferably 0.3 to 1.0%.

Mn: 2.5% or less

Mn is an element that improves weldability required for piping construction of a reformer by deoxidation of molten steel and fixation of S in molten steel (formation of MnS), and contributes to improvement of ductility. For this effect, the Mn content exceeds 2.5% and is approximately saturated, so 2.5% is the upper limit. Preferably 0.4 to 1%.

Cr：15～26％

Cr is an element necessary for securing high-temperature strength and oxidation resistance. In order to ensure creep rupture strength capable of withstanding a high temperature environment of 1000 ℃ required for a reaction tube of a large hydrogen production system in a petroleum refinery, it is required to contain at least Cr 15%. Although the high-temperature strength and the oxidation resistance are improved as the amount of Cr increases, if it exceeds 26%, the oxidation resistance is improved, but the aged ductility is lowered and the fatigue characteristics are lowered. This reduction in fatigue characteristics is accompanied by chromium carbides (Cr) precipitated during long-term use₂₃C₆) Accumulation increases the resulting phenomenon. Therefore, the upper limit of the Cr content is 26%. Further, like the reformer reaction tube of the in-situ hydrogen production system for fuel cells, the demand for the reformer reaction tube is subject to load fluctuationsThe fatigue characteristics are desirably limited to 15 to 20% in terms of the environment in which the fatigue characteristics are maintained at a higher level. On the other hand, as in a large-scale hydrogen production system of a petroleum refinery, it is advantageous to set the Cr content to a high range of 20 to 26% in a use environment where the system is continuously operated in a high temperature region.

Ni：8～23％

Ni is an element necessary for ensuring oxidation resistance and metal structure stability. If the content is less than 8%, it becomes difficult to secure the high-temperature creep rupture strength required for the reformer reaction tube, and the ductility after aging is also reduced. Therefore, the Ni content is required to be 8% or more. However, the increase in the amount of Ni decreases the amount of C solid solution in the matrix, which promotes secondary carbides (mainly Cr) during the practical use of the reactor tube₂₃C₆) Precipitation and increase of the metal oxide cause a decrease in aged ductility and deterioration in fatigue characteristics. Therefore, the Ni content cannot exceed 23%. In addition, a range of 8 to 18% is desirable for use environments where fatigue characteristics according to load fluctuations are required to be maintained at a higher level, such as reformer reaction tubes incorporated in-situ hydrogen generation apparatuses for fuel cells. On the other hand, a high range of 18 to 23% is advantageous for use in continuous operation in a high temperature region, as in a large-scale hydrogen production apparatus of an oil refinery.

Nb：0.1～1.2％

Nb combines with C to form NbC, increasing creep rupture strength and contributing to increased age ductility. This effect can be obtained by including 0.1% or more of Nb. However, since too much increase leads to decrease in oxidation resistance, the upper limit is 1.2%.

Ti：0.01～1.0％

Ti has a strong deoxidizing effect, and when it is dissolved in a matrix, it bonds with C to precipitate and form fine (Nb, Ti) C double carbides, thereby playing a role in improving creep rupture strength. To obtain this effect, at least 0.01% is necessary. However, if the amount is too large, the cleanliness of the steel is impaired and the quality is deteriorated as the amount of titanium oxide produced increases, so that the upper limit is 1.0%.

Ce：0.001～0.15％

Ce contributes to improvement of high-temperature oxidation resistance after being solid-dissolved in the matrix. In order to obtain such an effect, 0.001% or more is required. Preferably 0.01% or more. The effect increases as the amount of Ce increases, but when the amount increases too much, a large amount of cerium oxide is produced, and the cleanliness is impaired, resulting in a decrease in quality. Therefore, the upper limit is 0.15%.

N: less than 0.06%

N is a interstitial solid solution type element, and has the effect of stabilizing the austenite phase of the matrix and improving the high-temperature tensile strength. However, too much increase in N results in a decrease in aged ductility in the temperature region of around 800 ℃. In order to suppress the reduction in ductility, the upper limit is 0.06%. Preferably 0.01 to 0.05.

B：0.001～0.05％

B precipitates in grain boundaries to improve grain boundary ductility and suppresses chromium carbide (Cr)₂₃C₆) The grain growth (coarsening) of (b) contributes to the improvement of creep rupture strength. This effect is obtained by containing 0.001% or more of B. However, if the amount exceeds 0.05%, the sensitivity to weld cracking increases, and weldability necessary for piping construction of the reforming reaction tube is impaired, so that 0.05% is set as the upper limit.

Zr：0.01～0.5％

The Zr precipitates to form MC type carbide, and the creep rupture strength is improved. This effect is obtained by containing 0.01% or more of Zr. An increase in the amount increases the effect, but when the amount exceeds 0.5%, the amount of the formed zirconium oxide increases to lower the cleanliness and hence the ductility, so 0.5% is defined as the upper limit.

La：0.001～0.15％

La is solid-dissolved in the matrix to improve high-temperature oxidation resistance. This effect is obtained by containing 0.001% or more of La. The effect increases with the amount of La, but if the amount is too large, the amount of lanthanum oxide produced is large, which leads to a decrease in cleanliness and a decrease in ductility, so that the upper limit of 0.15% is defined. Preferably 0.01% to 0.1%.

Al：0.01～0.3％

Al is an element added as a deoxidizer and having an effect of improving high-temperature oxidation resistance. This effect is obtained by containing 0.01% or more of Al. However, if the content of the aluminum oxide is more than 0.3%, the cleanliness of the steel may be deteriorated due to an increase in the amount of the aluminum oxide formed, and the ductility may be deteriorated, so that the upper limit of 0.3% is set.

The chemical composition of the heat-resistant cast steel of the present invention requires that the parameter value [ P ] of the formula [1] be adjusted to satisfy the composition balance of P20 to 45, in addition to the above-mentioned specifications for the respective constituent elements, such that P is 89.3 to 78.4C +0.1Si-5.7Mn-1.7Cr +0.01Ni +2Nb +5.3Ti-36.5N-50.8 Ce. This formula is experimentally obtained from an evaluation test of aged ductility [ measurement of elongation at break after aging treatment at 800 ℃ for 3000 hours ], and the parameter value P (20 to 45) is a value obtained as a condition for maintaining high-temperature creep rupture strength and securing high ductility such as elongation at break after aging of not less than 20%. As an effect of remarkably improving the aged ductility by the adjustment of the component balance, improved fatigue characteristics required for a load-swing reformer reaction tube in which fatigue failure is a problem, such as in-situ hydrogen generation systems, can be ensured.

The reformer reaction tube made of a heat-resistant cast tube according to the present invention is manufactured as a cast tube by centrifugal casting, and therefore, is significantly advantageous in terms of cost compared to the tube manufacturing process by thermoplastic processing, and the manufactured cast tube body can be assembled as a reformer constituent tube material by welding after being subjected to fine machining.

Examples

Melting the molten steel in an Ar gas environment of a high-frequency induction melting furnace to obtain a cast steel solution with a given composition, and casting the cast steel solution by using a metal mold centrifugal force to obtain a test tube. Tube size (after machining): outer diameter 137X wall thickness 20X length 260 (mm). The test pieces cut out from the respective test materials were subjected to a tensile rupture test, a creep rupture test, a fatigue life test, and microscopic observation of a metal structure. The creep rupture test was conducted in the as-cast condition, and the tests other than the creep rupture test were conducted after the aging treatment in an electric furnace.

The steel compositions of the test materials are shown in table 1, and the test results are shown in table 2.

< I > age tensile ductility

The rectangular test piece was subjected to aging treatment (800 ℃ C. times.3000 hours), and then a tensile test piece was prepared, and the elongation at break was measured by a tensile test according to JIS-Z2241.

Test piece shape: diameter of the parallel part is 8.75mm-4D

Test temperature: at room temperature

The symbol in the column entitled "fracture ductility after aging" in Table 2 is as follows.

Elongation at break of 20% or more

An elongation at break of 20%

< II > creep characteristics

Test pieces were prepared from the respective test materials, and the fracture life (hours) was measured by a tensile creep rupture test in accordance with JIS-Z2272.

Test piece shape: the diameter of the parallel part is 6mm, and the distance between the marked points is 30mm

Test temperature: 800 deg.C

Tensile stress: 80MPa

< III > fatigue characteristics

The test materials were subjected to aging treatment (800 ℃ C. times.1000 hours) to prepare test pieces, and the number of repetitions of breakage Nf (the number of repetitions until the stress range reached 75% of the maximum stress) was measured as the fatigue life by the following fatigue test specified in JIS-Z2273.

The symbols in the column "fatigue characteristics" in table 2 are shown below.

Repeat the process more than 1000 times

Repeat less than 1000 times

Test piece shape: solid round bar (diameter 10mm)

Test temperature: 800 deg.C

Total deformation range (ε t): plus or minus 0.3 percent

Deformation speed: 10^-1% per second (C-C type recovery triangle wave)

Punctuation distance (g.l.): 15mm

(IV) Observation of metallic Structure

The test piece after the aging treatment (800 ℃ C. times.3000 hours) was ground, subjected to electrolytic corrosion (corrosive solution: 10N aqueous potassium hydroxide solution), and then examined by microscopic observation for the presence or absence of the precipitation of the sigma phase.

In comparative examples (No.21-No.26) in Table 1 and Table 2, No.21 is a material corresponding to SCH13(JIS-G5122), No.22 is a material corresponding to SCH22(JIS-G5122), No.23 is SCH13+ Nb, No.24 is SCH22+ Nb, Ti, No.25 is a high-N material, and No.26 is a low-C Ti austenitic steel.

The inventive examples (nos. 1 to 12) did not precipitate the σ phase even after being aged at high temperature for a long time, had excellent structure stability, high elongation at break and creep rupture life after aging, and also had good fatigue characteristics, and had various characteristics promising as a material for a reformer reaction tube for hydrogen production, particularly a reformer reaction tube for a load operation type apparatus that repeats a heat cycle.

On the other hand, in comparative examples (Nos. 21 to 26), 21(SCH13) and 22(SCH22), the aged fracture elongation and creep rupture life were low, and the aged fatigue life was also low.

No.23 shows a slight improvement in creep rupture life after aging as a result of addition of Nb based on SCH13, but the elongation at break and fatigue characteristics after aging were low. In No.24, as an effect of co-adding Nb and Ti based on SCH22, although the creep rupture life after aging was significantly improved, the ductility and fatigue characteristics after aging were also reduced with the precipitation of the σ phase, and the suitability as a material for a reaction tube of a load fluctuation type reformer was poor.

No.25 has high structural stability of the matrix and no precipitation of the sigma phase as an effect of high N content, but has low ductility and creep rupture life after aging and low fatigue characteristics. No.26 was excellent in the elongation at break and fatigue characteristics after aging, but the creep rupture strength was low because the parameter value P deviated from the upper limit specified in the present invention, and the suitability for high-temperature and high-pressure applications of the steam reforming reaction tube was poor.

TABLE 2

*1: aging treatment: 800 ℃ x 3000 hours, no σ phase deposition, and x

*2: aging treatment: elongation at break of not less than 20% at 800 ℃ for 3000 hours, and elongation at break of not less than 20%

*3: creep test at 800 deg.C and 80MPa load

*4: aging treatment: fatigue life is more than or equal to 10 ℃ multiplied by 1000 hours at 800 DEG C³Fatigue life < 10³Next time

Industrial applicability

The heat-resistant cast steel of the present invention has high levels of ductility and creep rupture life even after long-term aging at high temperatures, and also has improved fatigue characteristics. Therefore, the material is particularly excellent in fatigue characteristics, and therefore, is suitable as a material for a reaction tube of a steam reformer of a large-sized hydrogen production apparatus or a fuel cell hydrogen production apparatus in a petroleum refinery operating under high-temperature and high-pressure conditions, and is suitable as a material for a reaction tube of a load-shifting fuel cell hydrogen production apparatus, such as an on-site hydrogen production apparatus (hydrogen station or the like), which repeats thermal cycles due to fluctuations in operating load during the daytime and at night, and can alleviate and eliminate the problem of cracks occurring with repeated thermal cycles, thereby enabling stable operation over a long period of time.

The heat-resistant cast steel of the present invention is advantageous in terms of cost because the amount of expensive Ni is reduced. The reaction tube is manufactured by centrifugal casting, is economically more advantageous than tube-making processing by plastic processing, and is a material having excellent practical value. The heat-resistant cast steel of the present invention is also suitable as a heat treatment furnace roll in steel production.

Claims (3)

1. A heat-resistant cast steel for hydrogen production reaction tubes excellent in aged ductility and creep rupture strength, characterized by containing, in mass%, C: 0.18 to 0.5%, Si: 2.5% or less, Mn: 2.5% or less, Cr: 15-26%, Ni: 8-23%, Nb: 0.1 to 1.2%, Ti: 0.01-1.0%, Ce: 0.001-0.15%, N: 0.06% or less, and the balance being substantially Fe, and a parameter value P represented by the following formula is 20 to 45.

P＝89.3-78.4C+0.1Si-5.7Mn-1.7Cr

+0.01Ni+2Nb+5.3Ti-36.5N-50.8Ce

2. The heat-resistant cast steel for hydrogen production reaction tubes according to claim 1, which further comprises a component selected from the group consisting of B: 0.001 to 0.05%, Zr: 0.01-0.5%, La: 0.001-0.15% of 1 or more species or more than 2 species.

3. The heat-resistant cast steel for hydrogen production reaction tubes according to claim 1 or 2, further comprising Al: 0.01 to 0.3 percent.