Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the present invention relates to a metal material that has excellent high-temperature strength and superior corrosion resistance, and in particular is used in a carburizing gas atmosphere containing hydrocarbon gas and CO gas. More particularly, it relates to a metal material having excellent weldability and metal dusting resistance, which is suitable as a raw material for cracking furnaces, reforming furnaces, heating furnaces, heat exchangers, etc. in petroleum refining, petrochemical plants, and the like.
• a synthetic gas reformed in the above-described reactors that is, a gas containing H 2 , CO, CO, H 2 O, and hydrocarbon such as methane comes into contact with the metal material of a reaction tube and the like at a temperature of about 1000°C or higher.
• a temperature range on the surface of the metal material, elements such as Cr and Si, which have higher oxidation tendency than Fe or Ni or the like, are oxidized selectively, and a dense film of chromium oxide or silicon oxide or the like is formed, by which corrosion is restrained.
• a portion such as a heat exchange part in which the temperature is relatively low, however, the diffusion of element from the inside to the surface of metal material is insufficient.
• oxide film which achieves a corrosion restraining effect, delays, and additionally, such a gas having a composition containing hydrocarbon comes to have carburizing properties, so that carbon intrudes into the metal material through the surface thereof, and carburization occurs.
• a heating furnace tube and the like for a catalytic cracking furnace for increasing the octane value of naphtha obtained by distillation of crude oil as well a heavily carburizing environment consisting of hydrocarbon and hydrogen is created, so that carburization and metal dusting occur.
• Patent Document 1 proposes an Fe-based alloy or a Ni-based alloy containing 11 to 60% (mass%, the same shall apply hereinafter) of Cr concerning the metal dusting resistance in an atmospheric gas of 400 to 700°C containing H 2 , CO, CO 2 and H 2 O.
• the invention of an Fe-based alloy containing 24% or more of Cr and 35% or more of Ni, a Ni-based alloy containing 20% or more of Cr and 60% or more of Ni, and an alloy material in which Nb is further added to these alloys is excellent.
• a Cr or Ni content in the Fe-based alloy or the Ni-based alloy is merely increased, a sufficient carburization restraining effect cannot be achieved, so that a metal material having higher metal dusting resistance has been demanded.
• Patent Document 2 to prevent corrosion caused by metal dusting of a high-temperature alloy containing iron, nickel, and chromium, one or more kinds of metals of the VIII group, the IB group, the IV group, and the V group of the element periodic table and a mixture thereof are adhered to the surface by the ordinary physical or chemical means, and the alloy is annealed in an inert atmosphere to form a thin layer having a thickness of 0.01 to 10 ⁇ m, by which the alloy surface is protected.
• Sn, Pb, Bi, and the like are especially effective.
• this method may lose effectiveness in that the thin layer is exfoliated in long-term use.
• Patent Document 3 relates to the metal dusting resistance of a metal material in an atmospheric gas of 400 to 700°C containing H 2 , CO, CO 2 and H 2 O.
• Patent Document 3 discloses that the addition of an element producing stable carbide in the metal material, such as Ti, Nb, V and Mo, or the alloying element in which the interaction co-factor ⁇ represents a positive value, such as Si, Al, Ni, Cu and Co is effective in restraining metal dusting in addition to enhancing the protecting properties of oxide film.
• an element producing stable carbide in the metal material such as Ti, Nb, V and Mo, or the alloying element in which the interaction co-factor ⁇ represents a positive value, such as Si, Al, Ni, Cu and Co is effective in restraining metal dusting in addition to enhancing the protecting properties of oxide film.
• the increase of Si, Al and the like sometimes leads to the decrease in hot workability and weldability. Therefore, considering the manufacturing stability and plant working, this metal material leaves room for improvement.
• Patent Document 4 and Patent Document 5 disclose a method for pre-oxidizing a low Si-based 25Cr-20Ni (HK40) heat resistant steel or a low Si-based 25Cr-35Ni heat-resisting steel at a temperature near 1000°C for 100 hours or longer in the air.
• Patent Document 6 discloses a method for pre-oxidizing an austenitic heat-resisting steel containing 20 to 35% of Cr in the air.
• Patent Document 7 proposes a method for improving the carburization resistance by heating a high Ni-Cr alloy in a vacuum and by forming a scale film.
• Patent Document 8 proposes an austenitic alloy whose contents of Si, Cr and Ni satisfy the formula of Si ⁇ (Cr + 0.15Ni - 18)/10; thereby a Cr-based oxide film having high adhesiveness even in an environment, in which the alloy is subjected to a heating/cooling cycle, is formed to provide the alloy with excellent carburization resistance even in an environment in which the alloy is exposed to a corrosive gas at high temperatures.
• Patent Document 9 proposes an austenitic stainless steel having excellent scale exfoliation resistance even in an environment in which the steel is subjected to a heating/cooling cycle, which is produced by containing Cu and a rare earth element (Y and Ln group) therein and thereby forming a uniform oxide film having high Cr concentration in the film.
• Patent Document 10 proposes a method for improving the carburization resistance by forming a concentrated layer of Si or Cr by performing surface treatment.
• all of these prior arts require special heat treatment or surface treatment, and therefore they are inferior in economy.
• scale restoration scale recycling
• Patent Document 11 proposes a stainless steel pipe having excellent carburization resistance and containing 20 to 55% of Cr, which is produced by forming a Cr-deficient layer, which has a Cr concentration of 10% or higher and lower than the Cr concentration of the base material, on the surface of steel pipe.
• a Cr-deficient layer which has a Cr concentration of 10% or higher and lower than the Cr concentration of the base material, on the surface of steel pipe.
• Patent Document 12 and Patent Document 13 propose a metal material in which the gas dissociative adsorption (gas/metal surface reaction) is restrained by containing a proper amount of one kind or two or more kinds of P, S, Sb and Bi. Since these elements segregate on the metal surface, even if the elements are not added excessively, the elements can restrain carburization and metal dusting corrosion significantly. However, since these elements segregate not only on the metal surface but also at the grain boundary of metal grainy, a problem associated with hot workability and weldability remains to be solved.
• Patent Document 14 describes a technique for enhancing corrosion resistance by containing Cu, and on the other hand, for increasing the hot workability improving effect due to B by reducing S and O as far as possible.
• Patent Document 15 describes a technique for improving corrosion resistance and crevice corrosion resistance excellent in sulfuric acid and sulfate environments by setting the G.I. value (General Corrosion Index) represented by "-Cr + 3.6Ni + 4.7Mo + 11.5Cu,” at 60 to 90 and by setting the C.I. value (Crevice Corrosion Index) represented by "Cr + 0.4Ni + 2.7Mo + Cu + 18.7N" at 35 to 50.
• G.I. value General Corrosion Index
• Patent Document 16 describes a technique for improving hot workability by adding B exceeding 0.0015% while increasing a Cu content and by keeping an oxygen content low.
• the upper limit of a C content is restricted to a low level to avoid the decrease in corrosion resistance. Therefore, the solid-solution strengthening of C cannot be anticipated, and a sufficient high-temperature strength cannot be obtained. For this reason, these techniques are unsuitable for a metal material used at high temperatures.
• the present invention has been made in view of the present situation, and accordingly an object thereof is to provide a metal material that has metal dusting resistance, carburization resistance, and coking resistance, and further has improved weldability due to the restraint of reaction between carburizing gas and the metal surface in an ethylene plant cracking furnace tube, a heating furnace tube of catalytic reforming furnace, a synthetic gas reforming furnace tube, and the like.
• the inventors analyzed a phenomenon that carbon intrudes into a metal in a molecular state, and revealed that this phenomenon progresses in an elementary process consisting of the following items (a) to (c).
• Oxide scale is formed positively on the metal surface during the use of metal material, by which the contact with the metal of the gas molecules consisting of C compounds is broken off.
• oxide scale consisting of Cr and Si acts effectively.
• a carburizing gas environment such as an ethylene plant cracking furnace tube, a heating furnace tube of catalytic reforming furnace, and a synthetic gas reforming furnace tube
• the partial pressure of oxygen in gas is low. Therefore, it was revealed that oxide scale consisting mainly of Cr can be formed on the gas side and oxide scale consisting mainly of Si can be formed on the metal side by containing proper amounts of Cr and Si.
• the inventors studied various methods capable of restraining HAZ cracks at the time of welding while improving the corrosion resistance by adding a considerable amount of Si or Cu. As a result, the inventors came to obtain a knowledge that the HAZ cracks can be restrained by the following methods (f) and (g).
• the present invention has been completed based on the above-described knowledge, and the gists of the present invention are as described in the following items (1) to (3).
• the gists are called invention (1) to invention (3), and are sometimes generally named the present invention.
• a carburization resistant metal material characterized by consisting of, by mass%, C: 0.08 to 0.4%, Si: 0.6 to 2.0%, Mn: 0.05 to 2.5%, P: 0.04% or less, S: 0.015% or less, Cr: 22 to 30%, Ni: 20% or higher and less than 30%, Cu: 0.5 to 10.0%, Al: 0.01 to 1%, Ti: 0.01 to 1%, N: 0.15% or less, and O (oxygen): 0.02% or less , the balance being Fe and impurities, and satisfying Expression (1).
• a carburization resistant metal material characterized by consisting of, by mass%, C: 0.08 to 0.4%, Si: 0.6 to 2.0%, Mn: 0.05 to 2.5%, P: 0.04% or less, S: 0.015% or less, Cr: 18 to 30%, Ni: 20% or higher and less than 30%, Cu: 0.5 to 10.0%, Al: 0.01 to 1%, Ti: 0.01 to 1%, N: 0.15% or less, and O (oxygen): 0.02% or less , the balance being Fe and impurities, and satisfying Expression (1).
• the metal material in accordance with the present invention has an effect of restraining reaction between carburizing gas and the metal surface, and has excellent metal dusting resistance, carburization resistance, and coking resistance. Further, since the weldability is improved, the metal material can be used for welded structure members of cracking furnaces, reforming furnaces, heating furnaces, heat exchangers, etc. in petroleum refining, petrochemical plants, and the like, and can significantly improve the durability and operation efficiency of apparatus.
• the metal material in accordance with the present invention is suitable as a metal material used for reaction tubes and heat exchangers used for heat exchange in a temperature range (400 to 800°C) lower than the conventional temperature range, so that metal dusting, which poses a problem in this temperature range, can be restrained effectively.
• composition range of metal material is restricted according to the invention.
• the "%" representation of the content of each element means “mass%”.
• C is one of important elements in the present invention. Carbon not only enhances the strength at high temperatures but also achieves an effect of improving the weldability in combination with chromium to form carbides. In particular, the effect is remarkable in the metal material in accordance with the present invention that has high Si and Cu contents. To sufficiently achieve this effect, 0.08% or more of C must be contained. However, if C content exceeds 0.4%, the toughness of alloy lowers extremely, so that the upper limit of C content is set at 0.4%.
• the C content is preferably in the range of higher than 0.1% and 0.35% or less, further preferably in the range of higher than 0.15% and 0.25% or less.
• Si is one of important elements in the present invention. Since silicon has a strong affinity with oxygen, it forms Si-based oxide scale in the lower layer of a protective oxide scale layer such as Cr 2 O 3 , and isolates carburizing gas. This action is brought about when the Si content is 0.6% or higher. However, if the Si content exceeds 2.0%, the weldability decreases remarkably, so that the upper limit of Si content is set at 2.0%.
• the Si content is preferably in the range of 0.7 to 2.0%, further preferably in the range of 0.8 to 1.5%.
• Mn manganese
• Mn manganese
• Mn has deoxidizing ability and also improves the workability and weldability, so that 0.05% or more of Mn is added.
• manganese is an austenite-generating element, some of Ni can be replaced with Mn.
• excessive addition of Mn harms the carburizing gas isolating properties of protective oxide scale layer, so that the upper limit of Mn content is set at 2.5%.
• the Mn content is preferably in the range of 0.1 to 2.0%, further preferably in the range of 0.7 to 1.6%.
• P phosphorus decreases the hot workability and weldability, so that the upper limit of P content is set at 0.04%. In particular, when the Si and Cu contents are high, this effect is important.
• the upper limit of P content is preferably 0.03%, further preferably 0.025%. However, since phosphorus acts to restrain the dissociative adsorption reaction on the metal surface of carburizing gas, it may be contained when the decrease in weldability can be permitted.
• S sulfur decreases the hot workability and weldability like phosphorus, so that the upper limit of S content is set at 0.015%. In particular, when the Si and Cu contents are high, this effect is important.
• the upper limit of S content is preferably 0.01 %, further preferably 0.003%.
• sulfur acts to restrain the dissociative adsorption reaction on the metal surface of carburizing gas, it may be contained when the decrease in weldability can be permitted.
• Cr forms oxide scale such as Cr 2 O 3 stably, and has an effect of isolating carburizing gas. Therefore, even in a severe carburizing gas environment, chromium provides sufficient carburization resistance, metal dusting resistance, and coking resistance. Also, chromium improves the weldability because it combines with carbon to form carbides. In particular, when the Si and Cu contents are high, this effect is important. To sufficiently achieve this effect, 18% or more of Cr must be contained. However, since excessive addition decreases not only the workability but also the structural stability, the upper limit of Cr is set at 30%. The lower limit of Cr content is preferably 19%, further preferably 22%, and still further preferably 23%. Also, the upper limit of Cr content is preferably 28%, further preferably 27%.
• Ni nickel is an element necessary for obtaining a stable austenitic micro-structure according to the Cr content, and therefore 20% or more of Ni must be contained. Also, when carbon intrudes into the metal material, nickel has a function of reducing the intrusion rate. Further, nickel acts to secure the high-temperature strength of the metal micro-structure. However, the nickel content higher than necessary may lead to cost increase and manufacturing difficulties, and may also accelerate coking and metal dusting especially in a gas environment that contains hydrocarbon. Therefore, the upper limit of Ni content is restricted to less than 30%. Preferably, the content of Ni is 22.5% or higher and less than 30%. Further preferably, the content of Ni is higher than 25% and 29.5% or less.
• Cu copper is one of important elements in the present invention. Copper restrains reaction between carburizing gas and the metal surface, and greatly improves the metal dusting resistance and the like. Also, since copper is an austenite-generating element, some of Ni can be replaced with Cu. To achieve the metal dusting resistance improving effect, 0.5% or more of Cu must be contained. However, if Cu exceeding 10.0% is contained, the weldability decreases, so that the upper limit of Cu content is set at 10.0%.
• the Cu content is preferably 1.0 to 6.0%, further preferably 2.1 to 4.0%.
• Al is an element effective in improving the high-temperature strength. Also, aluminum has an effect as a deoxidizer because it has a high affinity with oxygen. In addition, aluminum serves as one of the constituent elements of oxide scale, and enhances the gas isolating properties. This effect can be anticipated especially in an environment in which the carburizing properties are strong. To achieve this effect, it is effective to contain 0.01% or more of Al. On the other hand, if the Al content exceeds 1%, the weldability is impaired. Therefore, the Al content is set in the range of 0.01 to 1%. The Al content is preferably in the range of 0.12 to 0.8%, further preferably in the range of 0.2 to 0.6%.
• Ti titanium is an element effective in improving the high-temperature strength. Also, since titanium has an affinity with oxygen, it serves as one of the constituent elements of oxide scale, and enhances the gas isolating properties. This effect can be anticipated especially in an environment in which the carburizing properties are strong. Therefore, titanium is contained positively. To achieve this effect, it is effective to contain 0.01% or more of Ti. However, if titanium is contained excessively, the workability and weldability decrease, so that the upper limit of Ti content is set at 1%.
• the Ti content is preferably in the range of 0.12 to 0.8%, further preferably in the range of 0.2 to 0.6%.
• N nitrogen
• the upper limit of N content is set at 0.15%.
• the preferred upper limit thereof is 0.05%.
• O oxygen
• Oxgen is an impurity element mingled from a raw material or the like when the metal material is melted . If the O content exceeds 0.02%, large amounts of oxide inclusions exist in the metal material, so that the workability decreases, and also a flaw may occur on the surface of metal material. Therefore, the upper limit of O content is set at 0.02%.
• invention (3) relating to a metal material whose strength, ductility, and toughness are improved is explained.
• Invention (3) relates to a carburization resistant metal material characterized by further containing, by mass%, at least one kind of a component selected from at least one group of the first group to the fifth group described below in a metal material specified in invention (1) or (2):
• Co acts to stabilize the austenite phase, so that it can replace some of Ni component. Therefore, cobalt may be contained as necessary. However, if the Co content exceeds 10%, cobalt deteriorates the hot workability. Therefore, when cobalt is contained, the content is 10% or less. From the viewpoint of hot workability, the Co content is preferably in the range of 0.01 to 5%, further preferably in the range of 0.01 to 3%.
• Mo (molybdenum) and W (tungsten) are solid-solution strengthening elements, so that either one or both of them may be contained as necessary.
• molybdenum molybdenum deteriorates the workability and impairs the structural stability if the content exceeds 2.5%. Therefore, when molybdenum is contained, the content is 2.5% or less.
• the Mo content is preferably 0.01 to 2.3%.
• tungsten deteriorates the workability and impairs the structural stability if the content exceeds 5%. Therefore, when tungsten is contained, the content is 5% or less.
• the W content is preferably 0.01 to 2.3%.
• B boron
• V vanadium
• Zr zirconium
• Nb niobium
• Hf hafnium
• zirconium When zirconium is contained, zirconium deteriorates the weldability if the content exceeds 0.1%. Therefore, when zirconium is contained, the content is 0.1% or less.
• the Zr content is preferably 0.001 to 0.05%.
• niobium When niobium is contained, niobium deteriorates the weldability if the content exceeds 2%. Therefore, when niobium is contained, the content is 2% or less.
• the Nb content is preferably 0.001 to 0.1%.
• hafnium hafnium deteriorates the weldability if the content exceeds 0.5%. Therefore, when hafnium is contained, the content is 0.5% or less.
• the Hf content is preferably 0.001 to 0.1%.
• Mg (magnesium) and Ca (calcium) have an effect of improving the hot workability, so that one kind or two or more kinds of these elements may be contained as necessary.
• magnesium deteriorates the weldability if the content exceeds 0.1%. Therefore, when magnesium is contained, the content is 0.1% or less.
• the Mg content is preferably 0.0005 to 0.1%.
• calcium deteriorates the weldability if the content exceeds 0.1%. Therefore, when calcium is contained, the content is 0.1 % or less.
• the Ca content is preferably 0.0005 to 0.1 %.
• Y (yttrium), La (lanthanum), Ce (cerium) and Nd (neodymium) have an effect of improving the oxidation resistance, so that one kind or two or more kinds of these elements may be contained as necessary.
• these elements deteriorate the workability if the content of any one element thereof exceeds 0.15%. Therefore, when these elements are contained, the content of any one element thereof is 0.15% or less.
• the content is preferably 0.0005 to 0.15%.
• the metal material in accordance with the present invention having a function of restraining the reaction between carburizing gas and the metal surface has only to satisfy the requirements specified in the above-described items (f) and (g) because the metal material has problems of metal dusting, carburization, and coking.
• the metal material in accordance with the present invention may be formed into a required shape such as a thick plate, sheet, seamless tube, welded tube, forged product, and wire rod by means of melting, casting, hot working, cold rolling, welding, and the like. Also, the metal material may be formed into a required shape by means of powder metallurgy, centrifugal casting, and the like.
• the surface of the metal material having been subjected to final heat treatment may be subjected to surface treatment such as pickling, shotblasting, shotpeening, mechanical cutting, grinding, and electropolishing. Also, on the surface of the metal material in accordance with the present invention, one or two or more irregular shapes such as protruding shapes may be formed.
• the metal material in accordance with the present invention may be combined with various kinds of carbon steels, stainless steels, Ni-based alloys, Co-based alloys, Cu-based alloys, and the like to be formed into a required shape.
• the joining method of the metal material in accordance with the present invention to the various kinds of steels and alloys is not subject to any restriction. For example, mechanical joining such as pressure welding and "staking" and thermal joining such as welding and diffusion treatment can be performed.
• a metal material having a chemical composition given in Table 1 was melted by using a high-frequency heating vacuum furnace, and a metal plate having a plate thickness of 6 mm was manufactured by hot forging and hot rolling.
• the metal plate was subjected to solid-solution heat treatment at 1160 to 1230°C for 5 minutes, a part of which was cut to produce a test piece .
• test piece measuring 15 mm wide and 20 mm long was cut. This test piece was isothermally maintained at 650°C in a 45%CO-42.5%H 2 -6.5%CO 2 -6%H 2 O (percent by volume) gas atmosphere. The test piece was taken out after 200 hours had elapsed, and the presence of a pit formed on the surface of test piece was examined by visual observation and by optical microscope observation. The results are summarized in Table 2.
• Table 2 indicates that, among Nos. 24 to 33 metal materials in which the chemical composition deviates from the condition specified in the present invention, Nos. 24, 26, 28, 32 and 33 metal materials had a pit formed after 200 hours had elapsed. Therefore, the metal dusting resistance is poor in a synthetic gas environment containing CO. On the other hand, in all of the metal materials specified in the present invention, no pit is formed, and therefore, these metal materials have excellent metal dusting resistance.
• a metal material having a chemical composition given in Table 1 was melted by using a high-frequency heating vacuum furnace, and a metal plate having a plate thickness of 6 mm was manufactured by hot forging and hot rolling.
• the metal plate was subjected to solid-solution heat treatment at 1160 to 1230°C for 5 minutes, a part of which was cut to produce a test piece.
• a test piece measuring 15 mm wide and 20 mm long was cut. This test piece was isothermally maintained at 650°C in a 30%C 3 H 8 -70%H 2 (percent by volume) gas atmosphere.
• the test piece was taken out after 10 hours had elapsed, and the presence of a pit formed on the surface of test piece was examined by visual observation and by optical microscope observation. The results are summarized in Table 2.
• Table 2 indicates that, among Nos. 24 to 33 metal materials in which the chemical composition deviates from the condition specified in the present invention, Nos. 24, 26 to 28, and 31 to 33 metal materials had a pit formed in the 10-hour test. Therefore, the metal dusting resistance is poor in a hydrocarbon gas environment. On the other hand, in all of the metal materials specified in the present invention, no pit is formed, and therefore, these metal materials have excellent metal dusting resistance.
• a metal material having a chemical composition given in Table 1 was melted by using a high-frequency heating vacuum furnace, and two metal plates each having a plate thickness of 12 mm, a width of 50 mm, and a length of 100 mm was manufactured from each metal material by hot forging and hot rolling.
• the metal plates were subjected to solid-solution heat treatment at 1200°C for 5 minutes, a part of which was cut to produce a test piece.
• a V-type edge having an angle of 30 degrees and a root thickness of 1.0 mm was prepared on one side in the lengthwise direction of the test piece. Thereafter, the periphery of the test pieces was restraint-welded onto a commercially available metal plate of SM400C specified in JIS G3106(2004) measuring 25 mm thick, 150 mm wide, and 150 mm long by using a covered electrode of DNiCrMo-3 specified in JIS Z3224(1999). Subsequently, multi-layer welds were made by TIG welding under a condition of heat input of 6 kJ/cm by using a TIG welding wire of YNiCrMo-3 specified in JIS Z3334(1999).
• Table 2 indicates that in No. 24 metal material in which the C content deviates from the condition specified in the present invention and No. 25 metal material that does not satisfy Expression (1), HAZ cracks are recognized. Also, it is indicated that for Nos. 28 to 31 metal materials in which the Cu, Si and S contents deviate from the condition specified in the present invention though the C content meets the specified condition, the HAZ crack restraining effect is small. On the other hand, in all of the metal materials specified in the present invention, HAZ cracks are not generated. Therefore, the weldability thereof is excellent.
• a metal material that has an effect of restraining reaction between carburizing gas and the metal surface, has excellent metal dusting resistance, carburization resistance, and coking resistance, and further has improved weldability.
• This metal material can be used for welded structure members of cracking furnaces, reforming furnaces, heating furnaces, heat exchangers, etc. in petroleum refining, petrochemical plants, and the like, and can significantly improve the durability and operation efficiency of apparatus.

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