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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
• • 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
  • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • 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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B17/00—Drilling rods or pipes; Flexible drill strings; Kellies; Drill collars; Sucker rods; Cables; Casings; Tubings
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B17/00—Drilling rods or pipes; Flexible drill strings; Kellies; Drill collars; Sucker rods; Cables; Casings; Tubings
  • E21B17/01—Risers
• • 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
  • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints

Definitions

• the present invention relates to a low alloy steel.
• a steel pipe called a riser, flowline, or trunkline is used for transmission of crude oil or natural gas between an oil well or gas well located at the bottom of the sea and a platform on the sea or between the platform and a refinery station on the land.
• a riser, flowline, or trunkline is used for transmission of crude oil or natural gas between an oil well or gas well located at the bottom of the sea and a platform on the sea or between the platform and a refinery station on the land.
• a steel pipe for transmitting crude oil or natural gas exploited from oil fields containing such a corrosive gas is sometimes broken by embrittlement attributable to hydrogen formed from a corrosion reaction called hydrogen induced cracking (hereinafter, referred to as “HIC”) and sulfide stress cracking (hereinafter, referred to as “SSC”).
• HIC hydrogen induced cracking
• SSC sulfide stress cracking
• Patent Document 1 JP5-255746A proposes a steel provided with excellent HIC resistance by defining the heat history and heat treatment conditions at the production time without substantially containing Ni, Cu and Ca.
• Patent Document 2 JP6-336639A proposes a steel provided with HIC resistance and SSC resistance by essentially adding Cr, Ni and Cu.
• Patent Document 3 JP2002-60894A proposes a steel in which the HIC resistance and SSC resistance are enhanced by defining the specific ranges of amounts of C, Ti, N, V and 0.
• Patent Document 4 JP2010-24504A
• a high-strength steel in which, by reducing the amounts of C and Mn and by adding 0.5% or more of Mo, the hardening of weld heat affected zone is restrained, and both of HIC resistance and SSC resistance of base metal and HAZ are achieved.
• An objective of the present invention is to provide a low alloy steel in which a HAZ has excellent hydrogen embrittlement resistance in wet hydrogen sulfide environments or the like without requiring much cost.
• HZ Heat Affected Zone
• the reason why the HAZ is highly susceptible to hydrogen embrittlement is as follows.
• hydrogen intrudes into the steel on account of corrosion reaction.
• This hydrogen can move freely in the crystal lattice of the steel.
• This hydrogen is so-called diffusible hydrogen.
• This hydrogen accumulates in a dislocation or a vacancy, which is one kind of defects in the crystal lattice to embrittle the steel.
• the HAZ is an as-quenched structure being heated to a high temperature by the heat history of welding, and cooled rapidly. Therefore, in the HAZ, the dislocations and vacancies in which hydrogen is trapped exist densely as compared with a thermally refined base metal. As a result, it is considered that the HAZ is highly susceptible to hydrogen embrittlement as compared with the base metal.
• the present invention has been made based on the above-described findings, and the gist thereof is low alloy steels described in the following items (1) to (5).
• a low alloy steel containing, by mass percent, C: 0.01 to 0.15%, Si: 3% or less, Mn: 3% or less, B: 0.005 to 0.050%, and Al: 0.08% or less, and the balance being Fe and impurities, wherein in the impurities, N: 0.01% or less, P: 0.05% or less, S: 0.03% or less, and O: 0.03% or less.
• Hv in the formula means the maximum value of Vickers hardness of HAZ
• B means the content of B (mass %)
• a low alloy steel in which a HAZ has excellent resistance to embrittlement attributable to hydrogen such as stress corrosion cracking in wet hydrogen sulfide environments.
• This low alloy steel is best suitable as a starting material of a steel pipe for the transmission of crude oil or natural gas.
• C carbon
• C is an element effective in enhancing the hardenability of steel and increasing the strength thereof. In order to achieve these effects, 0.01% or more of C must be contained. However, if the content of C exceeds 0.15%, the hardness in the quenched state increases too much, and the HAZ is hardened, so that the hydrogen embrittlement susceptibility of HAZ is enhanced. Therefore, the C content is set to 0.01 to 0.15%.
• the lower limit of the C content is preferably 0.02%, further preferably 0.03%.
• the C content is preferably 0.12% or less, further preferably less than 0.10%.
• Si silicon is an element effective for deoxidation, but brings about a decrease in toughness if being contained excessively. Therefore, the Si content is set to 3% or less.
• the Si content is preferably 2% or less.
• the lower limit of the Si content is not particularly defined; however, even if the Si content is decreased, the deoxidizing effect decreases, the cleanliness of steel is deteriorated, and an excessive decrease in the Si content leads to an increase in production cost. Therefore, the Si content is preferably 0.01% or more.
• Mn manganese
• Mn manganese
• the Mn content is set to 3% or less.
• the lower limit of the Mn content is not particularly defined; however, in order to achieve the strength increasing effect of Mn, 0.2% or more of Mn is preferably contained.
• the lower limit thereof is further preferably 0.4%, and the preferable upper limit thereof is 2.8%.
• B (boron) is an element that constitutes the findings, which are the basis of the present invention.
• B occupies the accumulation site of hydrogen, such as the dislocation or vacancy in the HAZ. Therefore, B is an element effective in enhancing the hydrogen embrittlement resistance.
• B segregates at grain boundaries, thereby enhancing the hardenability indirectly, and contributes to the improvement in strength. In order to achieve these effects, 0.005% or more of B must be contained.
• the B content is set to 0.005 to 0.050%.
• the lower limit of the B content is preferably 0.006%, further preferably 0.008%.
• the upper limit thereof is preferably 0.045%, further preferably 0.040%.
• the lower limit of the B content be controlled according to the highest hardness of HAZ. That is to say, in order to attain sufficient hydrogen embrittlement resistance, the B content is preferably in the range satisfying Formula (1) in the relationship with the maximum value of Vickers hardness of HAZ:
• Hv in the formula means the maximum value of Vickers hardness of HAZ
• B means the content of B (mass %).
• the maximum value of Vickers hardness of HAZ is a value that is determined by a Vickers test in which the test force is 98.07N in conformity to JIS Z2244. Al: 0.08% or less
• Al is an element effective for deoxidation, but if being contained excessively, the effect is saturated, and also the toughness is decreased. Therefore, the Al content is set to 0.08% or less.
• the Al content is preferably 0.06% or less.
• the lower limit of the Al content is not particularly defined; however, an excessive decrease in the Al content does not sufficiently achieve the deoxidizing effect, deteriorates the cleanliness of steel, and also increases the production cost. Therefore, 0.001% or more of Al is preferably contained.
• the Al content in the present invention means the content of acid soluble Al (so-called “sol.Al”).
• the low alloy steel in accordance with the present invention contains the above-described elements, and the balance consists of Fe and impurities.
• the “impurities” mean components that are mixed on account of various factors including raw materials such as ore or scrap when a steel material is produced on an industrial scale. Of the impurities, concerning the elements described below, the content thereof must be restricted stringently.
• N nitrogen
• the N content must be restricted to 0.01% or less.
• the N content is preferably 0.008% or less.
• the lower limit of the N content is not particularly defined; however, an excessive decrease in the N content leads to a remarkable increase in production cost. Therefore, the lower limit of the N content is preferably 0.0001%.
• P phosphorus
• HAZ tungsten carbide
• the P content is restricted to 0.05% or less.
• the lower limit of the P content is not particularly defined; however, an excessive decrease in the P content leads to a remarkable increase in production cost. Therefore, the lower limit of the P content is preferably 0.001%.
• S sulfur
• S sulfur
• the lower limit of the S content is not particularly defined; however, an excessive decrease in the S content leads to a remarkable increase in production cost. Therefore, the lower limit of the S content is preferably 0.0001%.
• the 0 content must be set to 0.03% or less.
• the 0 content is preferably 0.025% or less.
• the lower limit of the 0 content need not particularly be defined; however, an excessive decrease in the 0 content leads to a remarkable increase in production cost. Therefore, the 0 content is preferably 0.0005% or more.
• the low alloy steel in accordance with the present invention may contain the elements described below in lieu of a part of Fe.
• One or more elements selected from Cr (chromium), Mo (molybdenum), Ni (nickel) and Cu (copper) may be contained because these elements enhance the hardenability and contribute to the improvement in strength. However, if the contents thereof are excessively high, the HAZ is hardened remarkably, and therefore the hydrogen embrittlement susceptibility may be enhanced. Therefore, if one or more elements of these elements are contained, the contents thereof are set to 1.5% or less in total.
• the lower limit of the contents of these elements is preferably 0.02%, further preferably 0.05%.
• the upper limit thereof is preferably 1.2%.
• One or more elements selected from Ti (titanium), V (vanadium) and Nb (niobium) may be contained because these elements are elements that form fine carbo-nitrides and contribute to the improvement in strength, and also stably supplement diffusible hydrogen, and bring about a considerable effect of reducing the hydrogen embrittlement susceptibility.
• the contents thereof are excessively high, the formation of carbo-nitrides becomes excessive, and therefore the toughness may be decreased. Therefore, if one or more elements of these elements are contained, the contents thereof are set to 0.2% or less in total.
• the lower limit of the contents of these elements is preferably 0.001%, further preferably 0.003%.
• the upper limit thereof is preferably 0.15%.
• At least one of Ca (calcium) and Mg (magnesium) may be contained because these elements improve the hot workability of steel. However, if the contents thereof are excessively high, these elements combine with oxygen to remarkably decrease the cleanliness, so that the hot workability may rather be deteriorated. Therefore, if at least one kind of these elements is contained, the contents thereof are set to 0.05% or less in total.
• the lower limit of the contents of Ca and/or Mg is preferably 0.0005%, further preferably 0.001%.
• the upper limit thereof is preferably 0.03%.
• test material was prepared by machining a 12 mm-thick low alloy steel plate having the chemical composition given in Table 1 into a 12 mm square and a 100 mm length. This test material was subjected to HAZ-simulated thermal cycle in which the test material was heated to a temperature of 1350° C., at which the hardening of HAZ was remarkable, for 3 seconds by high-frequency induction heating, and thereafter was rapidly cooled. By using this test material, the tests described below were conducted.
• test specimen having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was sampled from the obtained test material, and the SCC resistance was evaluated by a four-point bending test in conformity to EFC16 specified by the European Federation of Corrosion.
• the test specimen was immersed in a 5% common salt+0.5% acetic acid aqueous solution of normal temperature (24° C.), in which 1 atm hydrogen sulfide gas is saturated, for 336 hours, whereby the presence of occurrence of SSC was examined.
• a low alloy steel in which a HAZ has excellent resistance to embrittlement attributable to hydrogen such as stress corrosion cracking in wet hydrogen sulfide environments.
• This low alloy steel is best suitable as a starting material of a steel pipe for the transmission of crude oil or natural gas.

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