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
• • 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
  • 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
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
• • 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
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
• • 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
  • C22C38/08—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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/24—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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

• the present invention relates to hot rolled wire rods used for manufacturing steel wires. More specifically, the present invention relates to a hot rolled wire rod used to manufacture steel wires for use in parts or a reinforcing member for a flexible riser and the like to be applied under a sour environment containing hydrogen sulfide.
• a flexible riser is used to draw crude oil.
• the flexible riser is manufactured by using a resin pipe and a steel wire.
• the steel wire is used as a reinforcing material for the resin pipe.
• Oilfields are located in sour environments containing hydrogen sulfide, and thus the above-mentioned steel wire is required to have a high strength and to suppress sulfide stress corrosion cracking (sulfide stress cracking: SSC) (hereinafter sometimes referred to as an "SSC resistance").
• SSC sulfide stress cracking
• the hot rolled wire rod used as material for such a steel wire is also required to have high strength and excellent SSC resistance.
• Patent Document 1 A technique proposed in Patent Document 1 is known to provide a high-strength steel material with excellent SSC resistance.
• the steel material disclosed in this patent document has a composition including, in percent by mass, C: 0.25 to 0.35%, Si: 0.10 to 0.30%, Mn: 0.8% or less, P: 0.010% or less, S: 0.003% or less, Al: 0.003 to 0.1%, N: 0.0040% or less, Cr: 0.5 to 0.7%, Mo: 0.5 to 1.0%, Cu: 0.05 to 0.8%, Ti: 0.015 to 0.030%, Nb: 0.005 to 0.025%, V: 0.05 to 0.10%, and B: 0.0005 to 0.0015%, wherein P, Ti and N are adjusted to satisfy a relationship of P / effective Ti content ⁇ 1.6, with the balance being Fe and inevitable impurities, and has a microstructure including a tempered martensite phase in which an average grain size of a prior austenite grain is 12 ⁇ m or less and a Mo
• the above-mentioned patent document describes a manufacturing method for the high-strength steel material.
• a steel material satisfying the aforesaid composition is subjected to a high-temperature heating process which involves holding the steel material at a heating temperature of more than 1,200°C to less than 1,270°C for 30 minutes or less.
• the heated steel material is then hot-rolled into a hot rolled steel material, which is thereafter subjected to a quenching process twice or more, followed by a tempering process.
• the quenching process is performed by holding the hot rolled steel material at a heating temperature ranging from 850 to 920°C for 5 to 10 minutes, followed by quenching to room temperature at an average cooling rate of 30°C/s or higher. Then, the tempering process is performed by holding the steel material at a temperature ranging from 600 to 680°C for 15 to 30 minutes.
• Patent Document 1 JP 2013-227611 A
• the steel material disclosed in the above-mentioned Patent Document 1 is designed to uniformize the distribution of alloy elements such as C, Cr, Mo and Nb to thereby reduce macro-segregation, and further to completely suppress coarse inclusions.
• the steel material attains the high strength with a yield strength (hereinafter sometimes referred to as a YS, which is an abbreviation of yield strength) of over-120 ksi (827 MPa), while maintaining the SSC resistance.
• a yield strength hereinafter sometimes referred to as a YS, which is an abbreviation of yield strength
• YS yield strength
• the requirements for the performance of steel materials have become stricter, and especially, higher strength and improved SSC resistance are required.
• the present invention has been made in view of the foregoing circumstances, and it is an object of the present invention to provide a hot rolled wire rod having high strength and excellent SSC resistance.
• the inventors have diligently studied to further improve the SSC resistance of a hot rolled wire rod while enhancing its strength.
• the suppression of segregation of sulfur (S) that would be generated in the wire rod can improve the SSC resistance while ensuring the strength of the wire rod. That is, sulfur (S) tends to be segregated at crystal grain boundaries. Such segregation of sulfur (S) reduces the strength of the grain boundary.
• the embrittlement of the crystals due to hydrogen proceeds, the grain boundary rupture is more likely to occur. Consequently, the SSC resistance is supposed to be degraded.
• a hot rolled wire rod according to the present invention that can solve the above-mentioned problems includes, in percent by mass, C: 0.20 to 0.5%, Si: 0.05 to 0.3%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.1%, P: exceeding 0% and 0.01% or less, and S: exceeding 0% and 0.01% or less, with the balance being iron and inevitable impurities.
• the sulfur (S) content in the hot rolled wire rod is measured at 300 sites or more at intervals of 200 ⁇ m using an electron beam microanalyzer.
• a segregation ratio (S max /S ave ) is defined as a ratio of the maximum sulfur (S) content S max (% by mass) to an average sulfur (S) content S ave (% by mass), the requirement that the segregation ratio is 30 or less is satisfied.
• the above-mentioned hot rolled wire rod may further include, as other elements, in percent by mass,
• the present invention appropriately controls the composition of the hot rolled wire rod and additionally suppresses the segregation of sulfur (S) generated in the wire rod, thereby making it possible to produce the hot rolled wire rod having the high strength and excellent SSC resistance.
• the segregation ratio of sulfur (S) is 30 or less, preferably 28 or less, and more preferably 27 or less. The smaller segregation ratio, the better the hot rolled wire rod becomes.
• the term "segregation ratio" as used herein means a ratio of the maximum sulfur (S) content S max to an average sulfur (S) content S ave when the sulfur (S) content in the hot rolled wire rod is measured at 300 sites or more at intervals of 200 ⁇ m using an electron beam microanalyzer, an average S content (% by mass) is described as S ave , and the maximum S content (% by mass) is described as S max .
• S ave the average S content S ave (% by mass) is equal to the maximum S content S max (% by mass), and thereby the segregation ratio (S max /S ave ) is 1.
• the sulfur (S) content in the hot rolled wire rod should be measured by element mapping from an area including a center toward a surface layer of the hot rolled wire rod so that the mapping area is not biased toward the center or the surface layer.
• the hot rolled wire rod of the present invention needs to appropriately control the composition, while satisfying the segregation ratio of 30 or less. That is, the hot rolled wire rod of the present invention includes, in percent by mass, C: 0.20 to 0.5%, Si: 0.05 to 0.3%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.1%, P: exceeding 0% and 0.01% or less; and S: exceeding 0% and 0.01% or less.
• Carbon (C) is an element required to ensure the strength of the wire rod.
• the C content is 0.20% or more.
• the C content is preferably 0.22% or more, and more preferably 0.23% or more.
• the C content is set at 0.5% or less, preferably 0.48% or less, and more preferably 0.47% or less.
• Silicon (Si) is an element required for deoxidation and solid-solution strengthening.
• the Si content is set at 0.05% or more.
• the Si content is preferably 0.06% or more, and more preferably 0.07% or more.
• sulfur (S) is more likely to be segregated, causing the hydrogen embrittlement, thereby reducing the SSC resistance. Therefore, the Si content is set at 0.3% or less, preferably 0.27% or less, and more preferably 0.25% or less.
• Manganese (Mn) is an element that improves the hardenability and enhances the strength of the wire rod.
• the Mn content needs to be 0.3% or more.
• the Mn content is preferably 0.4% or more, and more preferably 0.45% or more.
• any excessive Mn content facilitates the segregation of impurity elements, especially sulfur (S).
• S sulfur
• the Mn content is set at 1.5% or less, preferably 1.40% or less, and more preferably 1.30% or less.
• Aluminum (Al), like Si, is an element to be added for deoxidation.
• the Al content is set at 0.001% or more.
• the Al content is preferably 0.003% or more, and more preferably 0.005% or more.
• the Al content is set at 0.1% or less.
• the Al content is preferably 0.09% or less, and more preferably 0.08% or less.
• Phosphorus (P) is an element that tends to be segregated at the crystal grain boundaries, decreasing the strength of the grain boundary, thereby easily causing grain boundary rupture due to hydrogen. Therefore, the P content is set at 0.01% or less.
• the P content is preferably 0.009% or less, and more preferably 0.008% or less.
• the P content is preferably reduced as much as possible. Decreasing the P content to less than 0.0001% increases a cost. Thus, the P content is preferably set at 0.0001% or more.
• S Sulfur
• S is an element that tends to be segregated at the crystal grain boundaries and the center of the wire rod, decreasing the strength of the grain boundary, thereby easily causing grain boundary rupture due to hydrogen.
• the S content is set at 0.01% or less.
• the S content is preferably 0.009% or less, and more preferably 0.008% or less.
• the S content is preferably reduced as much as possible. Decreasing the S content to less than 0.0001% increases a cost.
• the S content is preferably set at 0.0001% or more.
• the composition of the hot rolled wire rod in the present invention has been mentioned above, with the balance being iron and inevitable impurities.
• the wire rod may include, as other elements, in percent by mass,
• the method for manufacturing the hot rolled wire rod in the present invention is not particularly limited and can be carried out by ordinary methods, which involve smelting and bloom rolling a steel satisfying the above-mentioned composition to produce a steel slab, and then heating and hot-rolling the steel slab.
• the heating temperature of the steel slab is preferably in a range of, e.g. 700 to 1,000°C. In this temperature range, the hot-rolling should be performed.
• the hot-rolling process may be performed using mills which include roughing mills, intermediate mills and finishing mills, supported by multiple stands.
• the total rolling distortion through initial three passes of the roughing mills is recommended to be 0.3 or more.
• the initial three passes mean initial three mills in the roughing mills.
• the total rolling distortion By setting the total rolling distortion through the initial three passes to 0. 3 or more, dynamic recrystallization can occur. Consequently, sulfur (S) can be uniformly dispersed to reduce the segregation of sulfur (S), thereby improving the SSC resistance.
• S sulfur
• the total rolling distortion is preferably 0.4 or more, and more preferably 0.5 or more.
• the upper limit of the rolling distortion is not particularly limited, but is normally 2.0 or less due to the limitation of facilities.
• the wire rod obtained by the hot rolling is preferably subjected to a heat treatment including quenching and tempering, to thereby have its metal microstructure converted to martensite.
• the quenching may involve heating the wire rod, for example, to 850 to 1,000°C, and then cooling it to the room temperature at an average cooling rate of 30°C/s or more.
• the upper limit of the average cooling rate is, for example, 100°C/s.
• the tempering may be performed by heating, for example, to 400 to 650°C.
• the above-mentioned heat treatment should be applied once.
• the productivity can be improved, compared to the technique of Patent Document 1 mentioned above in which the quenching is performed twice or more.
• the hot rolled wire rod obtained through such a heat treatment can be used as material for manufacturing steel wires that require the SSC resistance, such as the parts or reinforcing members of flexible risers, which are applied under sour environments containing hydrogen sulfide.
• a steel with the composition shown in Table 1 below was smelted, and the smelted steel obtained was casted to fabricate a steel slab.
• the balance of the steel was made of iron and inevitable impurities.
• the thus-obtained steel slab was subjected to bloom rolling to form a billet, and the obtained billet was then hot-rolled into a wire rod.
• the billet before the hot rolling was a square block of 155 mm x 155 mm, and then hot-rolled into the wire rod having a wire diameter of 11 to 16 mm.
• the hot rolling was controlled such that the total rolling distortion ( ⁇ ) through the initial three passes was shown in Table 2 below.
• the obtained steel wire was subjected to quenching and tempering to produce a test piece.
• the wire rod was heated to 850 to 1,000°C, held in this temperature range for 5 to 15 minutes, and then cooled to the room temperature at an average cooling rate of 30°C/s or higher.
• the tempering the quenched wire rod was held at a temperature in a range of 400 to 650°C for 50 to 70 minutes.
• a specimen for observation of the metal microstructure was taken from the above-mentioned test piece, and then embedded in a mount.
• the metal microstructure of the specimen was observed with an optical microscope at a magnification of 400x.
• the metal microstructure of each test piece was found to be formed of martensite.
• Element mapping for sulfur (S) as a measurement element was executed by an electron probe microanalyzer (Electron Probe Microanalyzer; EPMA). Element mapping was carried out from an area including center of the test piece toward its surface since segregation of sulfur usually tends to occur at the center. The measurement was performed at 300 sites or more of the test piece, including its center and its surface layer, at intervals of 200 ⁇ m. The sulfur (S) content (% by mass) was calculated based on a characteristic X-ray spectrum intensity to thereby determine the average value S ave (% by mass) and the maximum value S max (% by mass).
• the segregation ratio (S max /S ave ) is defined as the ratio of the maximum sulfur (S) content S max (% by mass) to the average sulfur (S) content S ave (% by mass), and the results in the test pieces are shown in Table 2 below.
• the SSC resistance of the obtained test piece was evaluated in the following procedure.
• a specimen for Method A defined by NACE TM0177 was taken from each obtained test piece, and the SSC resistance of the specimen was evaluated by the Method A.
• the obtained specimen was immersed in Solution A containing 5.0% by mass of NaCl and 0.5% by mass of CH 3 COOH, and the above solution was saturated with H 2 S gas.
• 80% of stress in the yield strength measured was applied to the specimen immersed in the solution, and a time to rupture of each specimen was measured.
• the measurement result is shown in Table 2 below.
• specimens having the time-to-rupture of 720 hours or more were rated as pass and evaluated to have excellent SSC resistance.
• Specimens Nos. 2 to 4, 11 to 15, 17 and 19 to 23 are the examples satisfying the requirements specified by the present invention.
• the composition and the segregation ratio of sulfur (S) were controlled appropriately, whereby the high yield strength of 900 MPa or higher was achieved, and the SSC resistance was improved.
• specimens Nos. 1, 5 to 10, 16 and 18 are comparative examples which do not satisfy the requirements specified by the present invention.
• the C content was too small, whereby the yield strength of the wire rod became less than 900 MPa.
• the rolling distortion induced through the initial three passes was less than 0.3, leading to the segregation of sulfur (S).
• the segregation ratio of sulfur (S) exceeded 30, thereby failing to improve the SSC resistance of the wire rod.
• Specimen No. 6 contained the excessive amount of Si, and the segregation ratio of sulfur (S) exceeded 30, thereby failing to improve the SSC resistance of the wire rod.

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• Chemical & Material Sciences (AREA)
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• Mechanical Engineering (AREA)
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• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)

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• 2015
  • 2015-03-18 JP JP2015054674A patent/JP2015212412A/ja not_active Ceased
  • 2015-03-23 KR KR1020187028245A patent/KR20180112868A/ko active Application Filing
  • 2015-03-23 WO PCT/JP2015/058696 patent/WO2015159650A1/fr active Application Filing
  • 2015-03-23 CN CN201580019443.8A patent/CN106164316B/zh not_active Expired - Fee Related
  • 2015-03-23 KR KR1020167028162A patent/KR20160131106A/ko active IP Right Grant
  • 2015-03-23 EP EP15780257.0A patent/EP3133182A4/fr not_active Withdrawn
  • 2015-03-23 BR BR112016024110A patent/BR112016024110A2/pt not_active Application Discontinuation

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