Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/02—Pretreatment of the material to be coated
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
  • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
  • B21C37/15—Making tubes of special shape; Making tube fittings
  • B21C37/20—Making helical or similar guides in or on tubes without removing material, e.g. by drawing same over mandrels, by pushing same through dies ; Making tubes with angled walls, ribbed tubes and tubes with decorated walls
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
  • C10G75/00—Inhibiting corrosion or fouling in apparatus for treatment or conversion of hydrocarbon oils, in general
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
  • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
  • C21D7/06—Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
  • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
  • C22C19/052—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 40%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
  • C22C19/053—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 30% but less than 40%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
  • C22C19/055—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 20% but less than 30%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/058—Alloys based on nickel or cobalt based on nickel with chromium without Mo and W
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/10—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of nickel or cobalt or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/10—Oxidising

Definitions

• the present invention relates to a method for manufacturing a metal pipe having a scale layer at least on the inner surface thereof. More particularly, the present invention relates to a method for manufacturing a metal pipe excellent in high-temperature strength and corrosion resistance.
• the metal pipe obtained by the present invention is suitably used as a pipe used in a carburizing gas atmosphere containing hydrocarbon gas, CO gas, and the like, such as a pyrolytic furnace pipe, a reforming furnace pipe, a heating furnace pipe, and a heat exchanger pipe in an oil refining plant, a petrochemical plant, and the like.
• a metal pipe containing 20 to 35 mass % of Cr and 20 to 70 mass % of Ni has been used as a pyrolytic furnace pipe, a reforming furnace pipe, a heating furnace pipe, a heat exchanger pipe, and the like used in a carburizing gas atmosphere containing hydrocarbon gas, CO gas, and the like in, for example, an oil refining plant or a petrochemical plantmass %mass %.
• This metal pipe is excellent in high-temperature strength and corrosion resistance.
• an oxide scale layer consisting mainly of Cr is preferably formed on the inner surface of the metal pipe in order to prevent carburization.
• the oxide scale layer consisting mainly of Cr is highly dense, and has an effect of shielding the intrusion of carbon into the metal pipe.
• the oxide scale layer consisting mainly of Cr has a weak catalytic action against coking (deposit of carbon). Therefore, the oxide scale layer consisting mainly of Cr also has an effect of restraining coking on the surface of metal pipe. As a result, the thermal conductivity to a fluid introduced into the metal pipe can be kept for a long period of time.
• Patent Document 1 discloses a method in which when a stainless steel pipe containing 12 to 20 mass % of Cr and 40 mass % or less of Ni is used in a high temperature and pressure water environment, the steel pipe is subjected to heat treatment of being heated to 800 to 1100°C under an inert gas atmosphere containing 0.01 to 0.5 vol% of oxygen and being held at that temperature for 2 to 20 minutes to form a scale layer on the surface of the steel pipe in order to prevent the Ni release from the steel pipe.
• Patent Document 2 discloses an invention in which an austenitic stainless steel containing 14 mass % or less of Cr is heat treated at a temperature not lower than 1100°C while the CO concentration in a barrel furnace is controlled to at least 150 ppm to prevent unevenness of scale caused by abnormal oxidation of the steel surface.
• Patent Document 3 discloses an invention relating to a stainless steel used in a carburizing gas atmosphere, the stainless steel having an oxide scale layer consisting mainly of Cr, in which the Cr concentration in a Cr depleted zone is at least 10% by mass, on the surface of a base metal containing 20 to 55 mass % of Cr, and further having an oxide scale layer consisting mainly of Cr, in which the Cr content is at least 50% by mass, on the outside thereof.
• Patent Document 4 relates to a method for manufacturing an ethylene pyrolytic furnace pipe excellent in coking resistance, and discloses an invention in which a pipe containing 15 to 30 mass % of Cr and 15 to 50 mass % of Ni is subjected to cold working of at least 50 ⁇ m depth from the surface, and then the pipe is heated to a temperature not lower than 1100°C in an atmosphere containing less than 5 vol% of oxygen and at least 20 vol% of nitrogen.
• the stainless steel having the oxide scale layer as described in Patent Document 3 is excellent in carburization resistance and coking resistance. However, in actual manufacturing, it is difficult to uniformly form the oxide scale layer consisting mainly of Cr over the entire inner surface, of pipe.
• Patent Document 4 describes that a fine-grained layer of not less than No. 7 in the austenitic crystal grain size can be created to a depth of at least 30 ⁇ m from the surface with cold working and nitrogen permeating heat treatment, so that the stability of Cr 2 O 3 oxide film produced during the use under an actual operation condition of 750 to 1100°C can be improved.
• the oxide scale produced in the nitrogen permeating heat treatment is removed, and a stable Cr 2 O 3 oxide film is formed on the fine-grained layer in the actual operation.
• the formation of oxide film during the actual operation requires a long period of time. In this method, therefore, carburization or coking may occur before the stable oxide film is formed.
• the present invention has been made to solve the above problems with prior arts, and accordingly an objective thereof is to provide a method for manufacturing a metal pipe having excellent resistance to carburization or coking caused by a carburizing gas by forming a uniform oxide scale layer consisting mainly of Cr on the inner surface of metal pipe.
• the present inventors earnestly conducted studies on the method for uniformly forming the oxide scale layer consisting mainly of Cr having carburization resistance and coking resistance over the entire inner surface of metal pipe, and resultantly obtained the findings concerning the cause for the formation of nonuniform scale and the method for preventing the formation of nonuniform scale, as described below.
• the present invention was completed on the basis of the above-described findings, and the gist thereof is methods for manufacturing a metal pipe given in the items (1) to (4) listed below.
• a metal pipe having an oxide scale layer consisting mainly of Cr formed uniformly on the inner surface of the metal pipe can be manufactured.
• the metal pipe obtained by the manufacturing method of the present invention is excellent in carburization resistance and coking resistance in a carburizing gas environment.
• the present invention provides a method for manufacturing a metal pipe according to claim 1, wherein the inner surface of a metal pipe having a predetermined chemical composition is subjected to mechanical treatment; the metal pipe is subjected to heat treatment such as to be held in a temperature range of 1050 to 1270°C for 0.5 to 60 minutes; and thereby an oxide scale layer consisting mainly of Cr is formed on at least the inner surface of the metal pipe.
• the chemical composition of the metal pipe obtained by the manufacturing method of the present invention, and the mechanical treatment and heat treatment to which the metal pipe is subjected are described.
• “%" relating to the content of each element means “mass %".
• the metal pipe obtained by the manufacturing method of the present invention must contain 20 to 55% of Cr and 20 to 70% of Ni.
• the upper limit of Cr content is set to 55%. To ensure the workability and to prevent the structural stability from deteriorating, the upper limit of Cr content is preferably set to 35%. The further preferable range of Cr content is 22 to 33%.
• Ni is an element necessary for obtaining a stable austenitic structure. Ni should be contained in an appropriate amount depending on the Cr content. Ni has an effect of reducing the intrusion rate of carbon into the metal material. This effect is achieved in the case where the Ni content is set to at least 20%. However, even if Ni is added excessively, the effect saturates, and the manufacturing cost is increased. Excessive Ni makes the manufacture of pipe difficult. Therefore, the Ni content is set to 20 to 70%. The lower limit of Ni content is preferably set to 23%, and the upper limit thereof is preferably set to 60%, further preferably 50%.
• the starting material for a metal pipe for manufacturing ethylene preferably contains Cr: 20 to 35% and Ni: 20 to 60%.
• the metal pipe obtained by the manufacturing method of the present invention has the above-described chemical composition, and other components are not limited.
• the metal pipe preferably has a chemical composition consisting of C: 0.01 to 0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr: 20 to 55%, Ni: 20 to 70%, N: 0.001 to 0.25%, O (oxygen): 0.02% or less, the balance being Fe and impurities.
• the reasons for restricting the content of each element are described.
• the impurities are components that mixedly enter from raw ore, scrap, and the like when the metal pipe is manufactured on an industrial basis, and are permitted as far as the content range does not adversely affect the present invention.
• C Carbon is an element effective in ensuring the high-temperature strength. This effect is remarkable when at least 0.01% of C is contained. If the C content exceeds 0.6%, the toughness may be deteriorated extremely. Therefore, the C content is preferably set to 0.01 to 0.6%.
• the lower limit of C content is further preferably set to 0.02%, and the upper limit thereof is further preferably set to 0.45%, still further preferably 0.3%.
• Si (Silicon) has an effect of assisting the uniform formation of the oxide scale layer consisting mainly of Cr because the affinity of Si for oxygen is high. This effect is remarkable when at least 0.1% of Si is contained. However, if the Si content exceeds 5%, the weldability is deteriorated, and the micro-structure may become unstable. Therefore, the Si content is preferably set to 0.1 to 5%.
• the upper limit of Si content is preferably set to 3%, further preferably 2%, and the lower limit thereof is preferably set to 0.3%.
• Mn Manganese
• Mn is an element effective for deoxidation and in improving the workability. Also, because Mn is an austenite producing element, some of Ni can be replaced with Mn. To achieve these effects, at least 0.1 % of Mn is preferably contained. However, if Mn is contained excessively, the formation of the oxide scale layer consisting mainly of Cr may be hindered. Therefore, the Mn content is preferably set to 0.1 to 10%. The upper limit of Mn content is preferably set to 5%, further preferably 2%.
• P (Phosphorus) and S (Sulfur) are preferably reduced in amount as far as possible because these elements segregate at the crystal grain boundary and deteriorate the hot workability.
• the P content is preferably 0.08% or less, and the S content is preferably 0.05% or less.
• the P content is further preferably set to 0.05% or less, and the S content is further preferably set to 0.03% or less.
• the P content is still further preferably set to 0.04% or less, and the S content is still further preferably set to 0.015% or less.
• N (Nitrogen) is an element effective in improving the high-temperature strength. This effect is remarkable when at least 0.001% of N is contained. However, the excessive addition of N may hinder the workability greatly. Therefore, the N content is preferably set to 0.001 to 0.25%. The upper limit of N content is preferably set to 0.2%.
• O Oxygen
• Oxygen is an element existing as an impurity. If the O content exceeds 0.02%, the oxide-base inclusions in the metal material precipitate in large amounts, which decrease the workability, so that the inclusions are a cause for the surface defects of pipe. Therefore, the O content is preferably set to 0.02% or less.
• the above-described metal pipe may further contain one or more elements selected from the elements given in the items (a) to (g) listed below.
• Cu is an element for stabilizing the austenitic phase.
• Cu is also an element effective in improving the high-temperature strength. Therefore, Cu may be contained in the above-described metal pipe. However, if the Cu content is excessive, the hot workability may be decreased. Therefore, if Cu is contained, the content thereof is preferably set to 5% or less. The upper limit of the Cu content is further preferably set to 3%. The above-described effects are remarkable when 0.1% or less of Cu is contained.
• Co is an element for stabilizing the austenitic phase. If Co is contained, some of Ni can be replaced with Co. Therefore, Co may be contained in the above-described metal pipe. However, if the Co content is excessive, the hot workability may be decreased. Therefore, if Co is contained, the content thereof is preferably set to 5% or less. The upper limit of the Co content is further preferably set to 3%. The above-described effect is remarkable when 0.1 % or less of Co is contained.
• Mo Mo
• W Tin
• Ta tantalum
• Mo Mo
• W Tin
• Ta tantalum
• Mo Mo
• the Mo content is preferably set to 3% or less
• the W and Ta contents each are preferably set to 6% or less.
• the upper limit of each of these elements is further preferably set to 2.5%, still further preferably 2% or less.
• the upper limit of the total amount is preferably set to 10%.
• Ti (Titanium) and Nb (Niobium) have great effects of improving the high-temperature strength, ductility, and toughness even if minute amounts of them are contained. Therefore, one or two selected from these elements may be contained in the above-described metal pipe. However, if the contents of these elements are excessive, the workability and weldability may be deteriorated. Therefore, if one or two of these elements are contained, the Ti content is preferably set to 1% or less, and the Nb content is preferably set to 2% or less. For each of these elements, the above-described effects are remarkable when at least 0.01 % of each of these elements is contained. When these elements are contained compositely, the upper limit of the total amount is preferably set to 2%.
• B (Boron), Zr (Zirconium) and Hf (Hafnium) are elements effective in strengthening the grain boundary and improving the hot workability and high-temperature strength. Therefore, at least one selected from these elements may be contained in the above-described metal pipe. However, if the contents of these elements are excessive, the weldability may be deteriorated. Therefore, if at least one of these elements is contained, the B and Zr contents each are preferably set to 0.1% or less, and the Hf content is preferably set to 0.5% or less. For each of these elements, the above-described effects are remarkable when at least 0.001% of each of these elements is contained. When these elements are contained compositely, the upper limit of the total amount is preferably set to 0.3%.
• Mg (Magnesium), Ca (Calcium) and Al (Aluminum) are elements effective in improving the hot workability. Therefore, at least one selected from these elements may be contained in the above-described metal pipe. However, if the contents of these elements are excessive, the weldability may be deteriorated. Therefore, if at least one of these elements is contained, the Mg content is preferably set to 0.1% or less, the Ca content is preferably set to 0.1% or less, and the Al content is preferably set to 1% or less.
• the upper limits of the Mg content and the Ca content each are further preferably set to 0.05%, and the upper limit of the Al content is further preferably set to 0.6%.
• the above-described effect is remarkable when at least 0.001% of each of Mg and Ca is contained and when at least 0.01% of A1 is contained.
• the lower limits of the Mg content and the Ca content each are preferably set to 0.002%.
• the upper limit of the total amount is preferably set to 0.5%.
• Y (Yttrium) and Ln (Lanthanide) group elements are elements effective in improving the oxidation resistance. Therefore, at least one selected from these elements may be contained in the above-described metal pipe. However, if the contents of these elements are excessive, the workability is deteriorated. Therefore, if at least one of these elements is contained, the content of each element is preferably set to 0.15% or less. The above-described effect is remarkable when at least 0.0005% of each of these elements is contained.
• the upper limit of the content of each of these elements is further preferably set to 0.10%. When these elements are contained compositely, the upper limit of the total amount is preferably set to 0.15%.
• the Ln group elements are elements of La, which is element number 57, through Lu, which is element number 71. Among the Ln group elements, at least one of La, Ce and Nd is preferably used.
• a lubricant is used to reduce the friction between the metal pipe and a working tool.
• the lubricant is usually removed by degreasing and cleaning after working.
• some of the lubricant remains on the inner surface of pipe.
• the lubricant remaining on the surface of the metal pipe hinders the formation of the oxide scale layer consisting mainly of Cr.
• mechanical treatment is performed to remove the remaining lubricant.
• the oxide scale produced at the time of hot pipe-making, dirt, and the like are adhered and remain on the surface of the metal pipe. Such remainder is preferably removed because it hinders the uniform formation of the oxide scale layer consisting mainly of Cr.
• the mechanical treatment is treatment for enhancing the cleanliness of the inner surface of the metal pipe by physically removing deposits such as the lubricant remaining on the surface of the metal pipe, dirt, and oxide scale.
• the mechanical treatment includes, for example, blasting treatment, grinding treatment (or friction treatment) for removing the deposits by bringing an abrasive into direct contact with the inner surface of metal pipe and rubbing the inner surface thereof with the abrasive, and a process for removing the deposits by spraying high-pressure water without the use of abrasive.
• blasting treatment for example, there are available air-blasting in which blast media are propelled by compressed air, sandblasting (one kind of air-blasting) in which sand is used as the blast media, shotblasting in which blast media are propelled by the centrifugal force of an impeller made of an abrasion-resistant alloy, shotpeening (one kind of shotblasting) mainly used for giving strain to the metal surface, wet blasting, and the like.
• shots on the surface can be removed simultaneously with the giving of strain.
• Wet blasting in which blast media are propelled together with high-pressure water can also be applied.
• a nonmetal such as silica sand (SiO 2 ), alumina (Al 2 O 3 ), zirconia (ZrO 2 ), boron nitride (BN), or silicon carbide (SiC), a mixture of these nonmetals or an abrasive containing these nonmetals as principal components is suitably used.
• an abrasive consisting of a metal such as cast steel, stainless steel, metallic glass (amorphous), or Cr may be used.
• a nonwoven fabric or the like to which the abrasive is stuck may also be used.
• the shape of the abrasive is not limited, and the abrasive can take any shape such as a granular shape, a grit shape, or a powder shape.
• the size of the abrasive is not limited. However, in the case where the surface roughness is restrained to enhance the coking resistance, the average grain size (the average of the major axis and the minor axis) is preferably 300 ⁇ m or less, further preferably 150 ⁇ m or less.
• the abrasive may be shot blasted from one end or both ends of the metal pipe, or may be shot blasted while a blasting nozzle is inserted into the metal pipe and is moved in the pipe.
• the abrasive or the nonwoven fabric to which the abrasive is stuck may be brought into direct contact with the inner surface of metal pipe in a state of being dry or being wetted by a liquid and may be moved while rubbing the inner surface of metal pipe.
• the metal pipe is subjected to mechanical treatment, subsequently heat treatment, and thereby an oxide scale layer consisting mainly of Cr is formed on the inner surface of the metal pipe.
• the heat treatment temperature is lower than 1050°C, the oxide scale layer formed on the surface of metal pipe is thin, so that the shielding property against the intrusion of carbon into metal material is insufficient.
• the heat treatment temperature exceeds 1270°C, pores or cracks are occurred in the oxide scale layer, and the denseness is decreased, which results in a decrease in carburization resistance. Therefore, the heat treatment is performed in the temperature range of 1050 to 1270°C.
• the lower limit of heat treatment temperature is preferably 1120°C, further preferably 1160°C.
• the holding time in the above-described temperature range is set to 0.5 to 60 minutes.
• the lower limit of the holding time is preferably set to 2 minutes, further preferably 5 minutes.
• the upper limit of the holding time is preferably set to 30 minutes, further preferably 15 minutes.
• the gas atmosphere in the heat treatment may be any atmosphere in which the oxide scale layer consisting mainly of Cr can be formed.
• the atmosphere of atmospheric gas or a gas obtained by burning a hydrocarbon fuel (LNG, butane, etc.) and air may be used.
• the atmosphere of DX gas, NX gas, RX gas, COG (C gas), or hydrogen gas whose dew point is controlled may be used.
• the atmosphere of a gas obtained by mixing these gases in an arbitrary ratio may also be used.
• Oxide scale layer consisting mainly of Cr
• the oxide scale layer consisting mainly of Cr is very important from the viewpoints of carburization resistance and coking resistance.
• the oxide scale layer containing at least 50% of Cr has a high denseness and is excellent in shielding property against the intrusion of carbon into metal material.
• the oxide scale layer consisting mainly of Cr restrains coking on the surface of metal material because the catalytic action thereof against coking is weak. As a result, the thermal conductivity to a fluid in the pipe is kept for a long period of time. For example, in the case where the metal pipe is used as a decomposition reaction tube, the yield of a reaction product such as olefin is stabilized.
• the Cr content in the oxide scale layer is preferably at least 80%.
• the oxide scale layer having a high Cr content is denser and achieves a great effect of shielding the intrusion of carbon into the metal material.
• the content of element in the oxide scale layer can be measured by EDX. The measurement should be made from the surface of the oxide scale layer. The determination of element is made by the fraction of the detected element excluding C (carbon), O (oxygen), and the like.
• the present invention is especially useful in manufacture of a metal pipe having a rib-shaped protrusion on the inner surface thereof.
• a metal pipe having a rib-shaped protrusion on the inner surface thereof it is thought that the metal pipe is liable to be attacked by carburizing gas, and the oxide scale is liable to peel off.
• a metal pipe having high carburization resistance on the inner surface of the pipe and high repairability of the film can be obtained.
• a pipe having a protrusion on the inner surface thereof, a pipe having fins, and the like are cited as the pipe having the rib-shaped protrusion.
• the protrusion, the fin, and the like may be formed integrally with the pipe itself or may be formed by welding or the like means.
• the present invention is explained below more specifically by way of example.
• the present invention is not limited to the example.
• Metal materials having the chemical composition given in Table 1 were melted by using an electric furnace or a vacuum furnace to form billets.
• the obtained billets were hot forged and cold rolled to produce metal pipes having an outside diameter of 56 mm and a wall thickness of 6 mm.
• the metal pipes of specimen Nos. 1 to 10 were subjected to mechanical treatment of the conditions given in Table 2. For some metal pipes, the mechanical treatment was omitted. Then, the metal pipes were subjected to heat treatment under the conditions given in Table 2 to form oxide scale. Some metal pipes were subjected to alumina blasting as the mechanical treatment, and were not subjected to heat treatment.
• each of the metal pipes was cut at a total of five places at a 2-m pitch along the pipe longitudinal direction to sample ring-shaped specimens each having a width of 50 mm and specimens for observation of oxide scale (20 x 20 mm square), described later. TABLE 1 Specimen No.
• Base metal chemical composition (mass %) C Si Mn P S Cr Ni N O Others 1 0,21 0,36 0,42 0,020 ⁇ 0.001 25,8 24,5 0,04 0,01 0.5Ti 2 0,11 1,67 0,28 0,017 ⁇ 0.001 25,3 38,3 0,02 0,01 1.2Mo, 0.5Ti 3 0,08 0,35 1,20 0,025 ⁇ 0.001 20,7 30,5 0,02 0,01 - 4 0,11 1,85 3,20 0,022 0,001 28,9 42,5 0,05 ⁇ 0.01 1.3Nb, 0.003Ca 5 0,01 0,12 0,15 0,018 ⁇ 0.001 31,2 60,8 0,01 0,01 0.004B, 0.029La 6 0,12 1,54 0,32 0,022 ⁇ 0.001 25,6 38,3 0,02 0,01 1.0W, 1.1Cu, 0.01Y 7 0,11 1,48 0,36 0,020 ⁇ 0.001 24,8 38,5 0,02 0,01 0.004Mg, 0.55Co 8 0,12 1,61 0,34 0,017 ⁇ 0.001 25,7 38,5 0,02
• EDX analysis was made from the surface of the specimen for observation, and the Cr content (mass %) in the oxide scale layer produced on the metal pipe was determined from the average of three measurements.
• carburization and coking tests were conducted by holding the ring-shaped specimen at 1000°C for 300 hours in the gas atmosphere of 15%CH 4 -3%CO 2 -82%H 2 by a volume ratio.
• Concerning the coking resistance the mass of specimen was measured before and after the test to determine the increase amount due to coke deposit, and deposited coke amount per unit area (mg/cm 2 ) was determined.
• Concerning the carburization resistance the amount of C intruding into the base metal was evaluated.
• metal chips were sampled at a 0.5-mm pitch in the depth direction from the surface of the specimen having been tested, and the amount of C (mass %) at a depth of 0.5 to 1.0 mm and the amount of C (mass %) at a depth of 1.0 to 1.5 mm were measured by chemical analysis. After the amount of C (mass %) of base metal before testing was subtracted, the average value of both the amounts of C was defined as the amount of intruding C (mass %) at a depth of 1 mm.
• Table 2 denotes the number of specimens meeting the criteria of the above items (1), (2) and (3) per five specimens. For example, 3/5 denotes that three of five are acceptable.
• the present invention aims at excellent carburization resistance and coking resistance throughout the overall length of the inner surface of metal pipe. Therefore, it was determined that each of the criteria of the present invention is met when all of the five specimens are acceptable.
• a metal pipe having an oxide scale layer consisting mainly of Cr formed uniformly on the inner surface of the metal pipe can be manufactured, so that the metal pipe is excellent in carburization resistance and coking resistance in a carburizing gas environment.
• the metal pipe obtained by the present invention is suitably used especially as a pipe used in a carburizing gas atmosphere containing hydrocarbon gas, CO gas, and the like, such as a pyrolytic furnace pipe, a reforming furnace pipe, a heating furnace pipe, and a heat exchanger pipe in an oil refining plant, a petrochemical plant, and the like.

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