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
  • C22C38/001—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
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten

Definitions

• the present invention relates to a martensitic stainless steel suitable for pipelines or the like which are used under environments containing both moist carbon dioxide gas and moist hydrogen sulfide.
• JP-A-6-100943 the term "JP-A-” referred to herein signifies "Unexamined Japanese Patent Publication"
• JP-A-4-266018 the term "JP-A-8-100235
• JP-A-8-100236 the term "Unexamined Japanese Patent Publication”
• An object of the present invention is to provide a martensitic stainless steel applicable under environments containing both moist carbon dioxide gas and moist hydrogen sulfide, and having excellent field welding performance.
• the inventors of the present invention carried out various investigations on the components of martensitic stainless steel, and obtained the following-given findings.
• the inventors of the present invention have developed a martensitic stainless steel consisting essentially of 0.02% or less C, 0.02% or less N, 0.1 to 0.3% Si, 0.1 to 0.3% Mn, 10 to 13% Cr, 5 to 8% Ni, 1.5 to 3% Mo, by weight, and balance of Fe and inevitable impurities, and satisfying 0.02 to 0.04% (C + N) by weight.
• the object of the present invention can be achieved also by a martensitic stainless steel consisting essentially of 0.02% or less C, 0.02% or less N, 0.1 to 0.3% Si, 0.1 to 0.3% Mn, 10 to 13% Cr, 5 to 8% Ni, 1.5 to 3% Mo, further one or both of 0.1 to 3% Wand 0.1 to 3% Cu, by weight, and balance of Fe and inevitable impurities, and satisfying 0.02 to 0.04% (C + N) by weight, a martensitic stainless steel consisting essentially of 0.02% or less C, 0.02% or less N, 0.1 to 0.3% Si, 0.1 to 0.3% Mn, 10 to 13% Cr, 5 to 8% Ni, 1.5 to 3% Mo, further one or both of 0.01 to 0.1% Ti and Nb, by weight, and balance of Fe and inevitable impurities, and satisfying 0.02 to 0.04% (C + N) by weight, or a martensitic stainless steel consisting essentially of 0.02% or less C, 0.02% or less N,
• Carbon is an element to form a carbide combining with Cr, thus strengthening the steel. Carbon, however, reduces the amount of chromium which is effective in corrosion resistance and increases the hardness at a weld heat-affected zone (HAZ), thus requiring heat treatment after welding. Accordingly, the C content is specified to 0.02 wt.% or less.
• Nitrogen combines with Cr to form a compound, thus reducing the amount of Cr which is effective in corrosion resistance, and increases the hardness at the HAZ. Consequently, the N content is specified to 0.02 wt.% or less.
• Silicon is added as a deoxidizer.
• the Si content of not more than 0.1 wt.% gives no effect of deoxidization.
• the Si content of more than 0.3 wt.% induces crystallization of delta ferrite, then an additional Ni amount are needed to maintain the phase balance. Therefore, the Si content is specified to a range of from 0.1 to 0.3 wt.%.
• Manganese is added as a desulfurizer.
• the Mn content of not more than 0.1 wt.% gives no effect of desulfurization, and degrades hot workability.
• the Mn content of more than 0.3 wt.% degrades the corrosion resistance under an environment containing carbon dioxide and hydrogen sulfide. Accordingly, the Mn content is specified to a range of from 0.1 to 0.3 wt.%.
• Chromium is an element which is effective to improve the corrosion resistance under an environment containing moist carbon dioxide gas.
• less than 10 wt.% of Cr content cannot attain the effect.
• the corrosion resistance increases.
• Cr is a powerful element to produce ferrite, if the Cr content exceeds 13 wt.%, surplus addition of Ni which is an expensive element to produce austenite is required. Consequently, the Cr content is specified to a range of from 10 to 13 wt.%.
• Ni is an element necessary to form a martensitic structure, less than 5 wt.% of Ni content degrades toughness and corrosion resistance owing to generating a large quantity of ferritic phase. If the Ni content exceeds 8 wt.%, the economy degrades. Therefore, the Ni content is specified to a range of from 5 to 8 wt.%.
• Molybdenum is an effective element to attain corrosion resistance. However, less than 1.5 wt.% of Mo content gives insufficient effect. If Mo is added over 3 wt.%, addition of expensive Ni is required because Mo is an element to generate ferrite.
• the amount of (C + N) is 0.02 wt.% or more to attain an aimed strength, and is not more than 0.04 wt.% to control the hardness at the HAZ.
• W and Cu Furthermore, one or both of W and Cu, one or both of Ti and Nb, or one or both of W and Cu and one or both of Ti and Nb may be added. In those cases, however, the amount of W, Cu, Ti, and Nb is requested to be limited as follows.
• Each of W and Cu is an element effective to attain strength and corrosion resistance. Addition of W or Cu to less than 0.1 wt.% does not attain sufficient effect, and, to over 3 wt.% degrades the hot workability. Accordingly, the content of W and Cu is specified to a range of from 0.1 to 3 wt.%.
• Each of Ti and Nb forms a carbide with C in steel, and refines grains to improve the strength and toughness. Addition of Ti or Nb to less than 0.01 wt.% does not attain sufficient effect, and, to over 0.1 wt.% saturates the effect. Consequently, the content of Ti and Nb is specified to a range of from 0.01 to 0.1 wt.%.
• the steels with the components adjusted as described above according to the present invention are stable in their mechanical characteristics against variations of production conditions such as heat treatment.
• the steels according to the present invention may be prepared by melting using adequate methods such as converter, electric furnace, or combination of them, if only the components thereof are adjusted to a specified range. After prepared by melting, the steels are formed in billets and slabs by a continuous casting machine or a mold, then are worked into a specified shape such as steel pipes and steel plates by hot-rolling, followed by applying heat treatment to attain an aimed strength. After established a martensitic structure by a heat treatment, the steels are preferred to be subjected to a tempering to adjust the strength thereof.
• Steels A through Q having respective chemical compositions given in Table 1 were prepared by melting in a vacuum melting furnace. Each of the steels was hot-rolled to a steel plate having 12 mm in thickness. The steel plate was quenched by water from 9.00°C ⁇ 10°C. and then tempered at 640°C ⁇ 5°C to obtain aimed proof stresses of from 600 to 700 MPa. For each of thus prepared steel plates, the corrosion resistance and the field welding performance described below were tested.
• the corrosion resistance to a moist carbon dioxide gas was evaluated in terms of plate thickness loss by immersing a steel plate in a solution of 5%NaCl-30atmCO 2 at 180°C for 96 hours. If the corrosion rate converted to one-year value is not more than 0.3 mm/y, no practical application problem occurs.
• the corrosion resistance to a moist hydrogen sulfide was evaluated in terms of presence/absence of fracture on the steel plate by the stress corrosion crack test for a sulfide, (Resistant SSC test) of TM0177 specified by NACE. That is, a steel plate was immersed in an aqueous solution of 5%NaCl+0.5%acetic acid saturated with latmH 2 S for 720 hours while applying a load of 60% of the proof stress. If no fracture occurs under the test, no practical application problem occurs.
• the field welding performance was evaluated by the hardness at a reproduced HAZ section. If the hardness is not more than 350 Hv, no preheating and postheating treatment are required.
• the steels A through J which are the Example Steels according to the present invention, gave 600 to 700 MPa of proof stress, 0.3 mm/y or less of corrosion rate in a moist carbon dioxide gas, and 350 Hv or less of hardness, giving no fracture in a moist hydrogen sulfide, being applicable in an environment containing both a moist carbon dioxide gas and a moist hydrogen sulfide, giving excellent field welding performance, thus showing adaptability to pipelines.
• the Comparative Steel K contained less amount of Cr content and showed no sufficient corrosion resistance to a moist carbon dioxide.
• the Comparative Steel L contained large amount of Si which is a deoxidizer
• the Comparative Steel M contained large amount of Mn as a desulfurizer
• the Comparative Steel N contained less amount of Mo, so that these comparative steels were inferior in corrosion resistance to a moist hydrogen sulfide.
• the Comparative Steel O contained less amount of Ni, so a delta ferrite deposited, which degraded the corrosion resistance to a moist carbon dioxide gas.
• the Comparative Steel P contained less amount of (C + N), and failed to attain satisfactory strength.
• the Comparative Example Q contained large amount of C and N, so that the strength was high and that the field welding performance was inferior.

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• 1998
  • 1998-12-18 JP JP36049398A patent/JP3620319B2/ja not_active Expired - Lifetime
• 1999
  • 1999-12-16 DE DE69928696T patent/DE69928696T2/de not_active Expired - Lifetime
  • 1999-12-16 EP EP99959849A patent/EP1143024B1/de not_active Expired - Lifetime
  • 1999-12-16 WO PCT/JP1999/007067 patent/WO2000037700A1/ja active IP Right Grant
• 2001
  • 2001-06-15 NO NO20012962A patent/NO20012962L/no not_active Application Discontinuation

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Non-Patent Citations (6)

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