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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S148/00—Metal treatment
  • Y10S148/902—Metal treatment having portions of differing metallurgical properties or characteristics
  • Y10S148/909—Tube

Definitions

• the present invention relates to a low-alloy steel in which so-called SCC (Stress Corrosion Cracking), which is generated on a steel pipe line under cathodic protection buried in soil, is less likely to occur. It can be widely used as structural steel used under similar conditions, including line pipes for transporting natural gas. Background art
• An object of the present invention is to provide steel having excellent SCC resistance on the outer surface of a pipeline without impairing the basic requirements as a pipeline.
• the present inventors conducted a test to reproduce the outer surface SCC of the pi-line on steel having a composition having the required strength, low-temperature toughness, weldability, and the like required for the line pipe.
• the material requirements to improve the SCC.
• the steel surface must be smooth on average and the local irregularities must be smaller than a certain level.
• shot blasting to satisfy a certain level of smoothness further improved the external SCC resistance.
• the outer surface of the pipeline, SCC is considered to be a crack that occurs when magnetite that is thinly formed on the surface cracks due to stress fluctuations and at this time iron elutes. Therefore, if micro plastic deformation is suppressed to suppress cracking of magnetite, external SCC will be less likely to occur.
• the mouth tissue is homogeneous, the characteristics are further improved.
• the present invention has been made based on the above findings.
• Outer surface of pi-line characterized in that the surface is adjusted to 50 ⁇ m smoothness.
• the Sblast resistance of the outer surface of the pipeline is characterized by adjusting the smoothness of the surface to the center line average roughness, Ra ⁇ 7 m, and the maximum height Rmax ⁇ 50 m by the shot blast. Excellent steel, and in mass%,
• Nb 0.005 to 0.1% Ti 0.005 to 0.1%, V 0.00 0.1%, Mo 0.03 to 0.5%, Cr 0.06%, Ni 0. 1 to 0.8%, Cu 0.8 to 0.8%, B 0.0003 to 0.003%, Ca 0.00 to 0.011%
• the main structure is steel with excellent SCC resistance on the outer surface of the pipeline, which is an aquifer, a bainetic ferrite or a bainite.
• the surface roughness is represented according to J1S B0601.
• Ra represents the average line roughness
• Rmax represents the maximum height.
• the surface smoothness was defined as Ram and Rmax ⁇ 50 m.
• Ra ⁇ 5 m and Rmax 35 m In order to further improve the external SCC resistance, it is particularly desirable that Ra ⁇ 5 m and Rmax 35 m.
• shot blast is particularly preferred as a method of adjusting the surface.
• the SCC resistance of the outer surface can be improved.
• the SCC characteristics of the outer surface can be further improved by setting the chemical composition to a specific range.
• C is limited to 0.03 to 0.16%. C is extremely effective in improving the strength of steel, and at least 0.03% is required to obtain strength as structural steel. However, as the amount of C increases, the non-uniformity of the structure increases, and the outer SCC resistance decreases, so the upper limit was set to 0.16%. However, if the content exceeds 0.10%, it becomes difficult to obtain an appropriate strength without forming a ferrite-perlite structure, so that the upper limit is desirably limited to 0.10%. Good.
• Si is an element added for deoxidation and strength improvement, and is not directly related to the outer surface SCC resistance.However, adding too much impairs the basic performance of the line pipe such as HAZ toughness and on-site weldability.
• the upper limit was set to 0.5%. However, steel can be deoxidized with other elements such as A1, and Si need not always be added.
• Mn achieves high strength while maintaining low C content with good SCC characteristics on the outer surface It is an element necessary for If it is less than 0.5%, the effect is too small, and if it exceeds 2.0%, the deflection becomes large and a hardened phase which is harmful to the outer surface SCC characteristics appears and becomes chewy. It also deteriorates the on-site weldability. Therefore, the addition amount of Mn was set to 0.5 to 2.0%.
• P which is an impurity
• the main reason for this is that not only the low-temperature toughness of the base material and HAZ is further improved, but also the reduction of P has the effect of improving the SCC characteristics of the outer surface of the pipeline that progresses in the form of intergranular cracking.
• A1 is an element that is usually contained in steel as a deoxidizer, and is also effective in refining the structure.
• A1 exceeds 0.10%, A1 type nonmetal oxide is added and low-temperature toughness is deteriorated. Therefore, the upper limit is set to 0.10%.
• deoxidation is possible with other elements such as Si, and A1 does not necessarily need to be added.
• N is also an element that is difficult to remove from steel, but in some cases it has the effect of forming A1N, TiN, etc. to refine the structure. However, if the content is too large, deterioration of low-temperature toughness and strain aging embrittlement due to solid solution N occur, so the upper limit was set to 0.1%.
• the purpose of adding Nb, Ti, V, Mo, Cr, Ni, Cu, B, and Ca is explained.
• the main purpose of adding these elements to the basic components is to further improve the outer surface SCC resistance and expand the applicable range without impairing the excellent characteristics of the steel of the present invention.
• These elements themselves do not directly affect the external SCC resistance.
• the aim is to achieve high strength while maintaining low C with good SCC characteristics on the outer surface, and to achieve microstructural refinement to suppress non-uniform micro-strain. In this way, cracking of the magnetite is suppressed, and as a result, the SCC characteristics of the outer surface are further improved. Therefore, it is not always necessary to contain the above-mentioned elements, and the amount of addition is of a nature that should be naturally restricted.
• the lower limit of the amount of the element added is defined as an amount at which the effect of the addition is not remarkable.
• Nb and Ti have the effect of suppressing coarsening of austenite grains and reducing the structure during hot working or heat treatment.
• the addition of Nb and Ti exceeding 0.1% adversely affects HAZ toughness and on-site weldability, so the upper limit was set to 0.1%. Since Ti and Nb have a great effect on the refinement of the structure, it is desirable to add 0.005% or more.
• V, Mo, Cr, ⁇ , and Cu are added to improve the hardenability of steel and to achieve high strength through the formation of precipitates.
• B is added in an amount of 0.0003% or more, it contributes to high strength through improvement of hardenability, but if it exceeds 0.003%, the low-temperature toughness deteriorates, so the upper limit was made 0.003%.
• the sulfide morphology is controlled to improve low-temperature toughness.
• the content is less than 0.001%, there is practically no effect. If the content exceeds 0.01%, large inclusions are formed, which adversely affects the low-temperature toughness. Therefore, the upper limit was set to 0.01%.
• the outer surface SCC of the pie ply is caused by cracking of the magnet due to non-uniform micro plastic deformation, so that if the structure is uniform, the difference in micro deformation is small. And external SCC is less likely to occur. If there is a large soft polygonal light generated at high temperature in the tissue, microscopic deformation will be reduced. Therefore, such a light is generated Not limited to organizations that are primarily focused on lighter, vanity, or pay nights. In other words, even with steel of the same chemical composition, the structure can be changed from ferrite perlite to ashingle perlite by increasing the cooling rate, etc.
• ⁇ th is 70% or more of the actual yield strength.
• Table 2 the steel of any chemical composition adjusted to have the smoothness of the surface shown in the present invention shows a value of 70% or more, and furthermore has a short shot. It is clear that higher values are obtained by toblasting or adjusting the chemical composition.
• a steel having excellent SCC resistance on the outer surface of a pipeline that does not depend on the soundness of the coating without impairing the low-temperature toughness and on-site weldability and without significantly increasing the cost. it can. As a result, the safety of the pipeline is significantly improved.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
• Rigid Pipes And Flexible Pipes (AREA)

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• 1996
  • 1996-06-28 JP JP8170004A patent/JPH1017986A/ja not_active Withdrawn
• 1997
  • 1997-06-26 WO PCT/JP1997/002220 patent/WO1998000569A1/ja active IP Right Grant
  • 1997-06-26 CA CA002259241A patent/CA2259241C/en not_active Expired - Fee Related
  • 1997-06-26 KR KR1019980710743A patent/KR100311345B1/ko not_active Expired - Fee Related
  • 1997-06-26 US US09/202,989 patent/US6517643B1/en not_active Expired - Fee Related
  • 1997-06-26 AU AU32752/97A patent/AU721205C/en not_active Withdrawn - After Issue
  • 1997-06-26 EP EP97928484A patent/EP0949340B1/en not_active Expired - Lifetime
  • 1997-06-26 DE DE69730739T patent/DE69730739T2/de not_active Expired - Fee Related

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