EP4578565A1 - Steel tube exhibiting excellent fatigue characteristics against hydrogen and production method therefor, and steel material and production method therefor - Google Patents

Steel tube exhibiting excellent fatigue characteristics against hydrogen and production method therefor, and steel material and production method therefor Download PDF

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Publication number
EP4578565A1
EP4578565A1 EP23872574.1A EP23872574A EP4578565A1 EP 4578565 A1 EP4578565 A1 EP 4578565A1 EP 23872574 A EP23872574 A EP 23872574A EP 4578565 A1 EP4578565 A1 EP 4578565A1
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EP
European Patent Office
Prior art keywords
less
steel
temperature
steel pipe
hydrogen
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EP23872574.1A
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German (de)
English (en)
French (fr)
Inventor
Naho INOUE
Hiroshi Okano
Yoshihiro Nishihara
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JFE Steel Corp
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JFE Steel Corp
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Publication of EP4578565A1 publication Critical patent/EP4578565A1/en
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    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B17/00—Tube-rolling by rollers of which the axes are arranged essentially perpendicular to the axis of the work, e.g. "axial" tube-rolling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
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    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • 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
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    • 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
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    • 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
    • C21D9/085—Cooling or quenching
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • 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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a steel pipe with a good fatigue property in hydrogen, a method for producing the steel pipe, a steel material, and a method for producing the steel material.
  • An austenitic stainless steel such as SUS 316L, which is more resistant to hydrogen embrittlement than low-alloy steels, has been used for a steel structure used in a high-pressure hydrogen gas environment.
  • an austenitic stainless steel such as SUS 316L
  • SUS 316L is high in steel material cost and has low strength, and when designed to withstand a high hydrogen pressure, has a large wall thickness and results in an increased price of a structure for hydrogen itself.
  • a steel for a high-pressure hydrogen environment described in Patent Literature 1 is a steel used in a high-pressure hydrogen environment, in which Ca/S is less than 1.5 or 11 or more to reduce the diffusible hydrogen concentration ratio and suppress embrittlement due to diffusible hydrogen.
  • a low-alloy high-strength steel described in Patent Literature 3 is a Cr-Mo high-strength low-alloy steel with good elongation and reduction in area characteristics even in a 45-MPa hydrogen atmosphere and with excellent high-pressure hydrogen environment embrittlement resistance provided by tempering at a relatively high temperature of 560°C to 580°C to adjust the grain size number after tempering to 8.4 or more and the tensile strength in a very narrow range of 900 to 950 MPa.
  • Patent Literature 4 proposes a low-alloy steel for a high-pressure hydrogen gas environment.
  • adding V increasing the Mo content as compared with existing steels, increasing the tempering temperature, and utilizing a V-Mo carbide improve the carbide form at a grain boundary and greatly improve hydrogen environment embrittlement resistance.
  • NPL 1 Matsunaga et al., Int J Hydrogen Energy, Vol. 40 (2015), pp. 5739-5748
  • Non Patent Literature 1 it is known that the fatigue life of a material decreases in a high-pressure hydrogen environment. This means that the service life of a line pipe material decreases when the line pipe material is designed based on a conventional natural gas line pipe.
  • the related art described above can suppress the occurrence of hydrogen-induced cracking in a sour environment but cannot sufficiently increase fatigue strength in hydrogen gas, that is, there is a problem in that it is difficult to achieve both the suppression of the occurrence of hydrogen-induced cracking in a sour environment and high fatigue strength in hydrogen gas, which more easily affects the service life
  • a steel pipe with a good fatigue property in hydrogen in a high-pressure hydrogen gas environment which is suitable for a steel structure used in a high-pressure hydrogen gas environment, such as a line pipe for 100% hydrogen gas or a natural gas containing hydrogen at a hydrogen partial pressure of 1 MPa or more (natural gas is a gas containing hydrocarbons, such as methane and ethane, as main components), a method for producing the steel pipe, a steel material, and a method for producing the steel material.
  • the present invention has been further studied based on such new findings, and the gist of the present invention is as follows:
  • the present invention can provide a steel pipe and a steel material with a very good fatigue property in a high-pressure hydrogen gas environment and is industrially very useful.
  • An implementation method for a steel pipe is more specifically described as a first embodiment, and then an implementation method for a steel material is more specifically described as a second embodiment.
  • the C content is an element necessary to increase strength. The effect is insufficient at less than 0.10%.
  • the C content is 0.10% or more.
  • the C content is preferably 0.13% or more.
  • the C content is more preferably 0.15% or more, still more preferably 0.18% or more.
  • more than 0.45% may result in a quenching crack at the time of quenching, causes the formation of a coarse carbide, and results in a degradation of the fatigue property in hydrogen.
  • the C content is 0.45% or less.
  • the C content is preferably 0.43% or less.
  • the C content is more preferably 0.40% or less, still more preferably 0.38% or less.
  • Si is contained as a deoxidizer in steelmaking and as an element for ensuring hardenability, but the effects are insufficient at less than 0.01%, so that the Si content is 0.01% or more.
  • the Si content is preferably 0.1% or more.
  • the Si content is more preferably 0.15% or more.
  • more than 2.0% results in an embrittled grain boundary, a decrease in the low-temperature toughness, and a degradation of the fatigue property in hydrogen.
  • the Si content is 2.0% or less.
  • the Si content is preferably 1.5% or less.
  • the Si content is preferably 1.0% or less, more preferably 0.8% or less.
  • B is an element for ensuring hardenability
  • the B content when B is contained, the B content may be 0% or more, but the above effect is difficult to ensure at less than 0.0005%, so that the B content is preferably 0.0005% or more.
  • more than 0.005% results in lower toughness.
  • the B content when B is contained, the B content is 0.005% or less.
  • the B content is preferably 0.004% or less.
  • the B content is more preferably 0.003% or less, still more preferably 0.002% or less.
  • Austenite remaining in a steel pipe may increase the amount of hydrogen in the steel and increase hydrogen embrittlement sensitivity. Furthermore, when austenite is transformed into martensite by stress loading during use, hydrogen cracking is likely to occur because martensite is very hard, and cracking may occur from the martensite portion.
  • retained austenite is 3% or less to reduce the fatigue crack growth rate. Retained austenite is preferably 2% or less, more preferably 1% or less. The retained austenite may be 0%.
  • an electric-resistance-welded pipe or a UOE steel pipe can be produced by performing the treatment so as to have the same thermal history.
  • a steel pipe according to the present invention can be produced by sequentially performing the following steps (1) to (3).
  • the temperature in the following description is the temperature at the center of the plate thickness of a steel raw material or a steel pipe.
  • the average cooling rate means the temperature at a quarter thickness position from the inner surface of a steel pipe.
  • the temperature at the center of the plate thickness and the temperature at the quarter thickness position from the inner surface of a steel pipe are estimated from the surface temperature of the steel pipe measured with a radiation thermometer using heat-transfer calculation or the like in consideration of the heat transfer coefficient of the steel material.
  • the average cooling rate from 550°C to 50°C is preferably 12°C/s or less, more preferably 10°C/s or less. Although the lower limit is not particularly limited, the average cooling rate from 550°C to 50°C is preferably 1°C/s or more.
  • the cooling method is not particularly limited, and an arbitrary method, such as water cooling, oil cooling, or air cooling, can be used alone or in combination, but water cooling or oil cooling is preferred from 800°C to 550°C, and air cooling is preferred from 550°C to 50°C.
  • Group B cooling to 50°C or less at an average cooling rate of 10°C/s or more from 800°C to 300°C and at an average cooling rate of 5°C/s or less from 300°C to 50°C at the quarter thickness position from the inner surface of a steel pipe.
  • an average cooling rate of less than 10°C/s from 800°C to 300°C at the quarter thickness position from the inner surface of a steel pipe the predetermined carbide density cannot be achieved, and the fatigue property deteriorates.
  • the cooling stop temperature is more than 50°C, a desired carbide density cannot be achieved, the transformation is not completed, and a desired steel microstructure cannot be formed after tempering. Thus, quenching is performed to a temperature of 50°C or less.
  • the cooling stop temperature is preferably 45°C or less, more preferably 40°C or less. Although the lower limit is not particularly limited, the cooling stop temperature is preferably 25°C or more.
  • Tempering temperature 400°C or more and Ac 1 temperature or lower
  • Heating at an average heating rate of 0.01°C/s or more and a tempering temperature of 400°C or more can result in a decrease in austenite, a decrease in hydrogen in the steel, and a predetermined carbide density.
  • the tempering temperature is preferably 450°C or more, more preferably 500°C or more.
  • heating to a temperature higher than the Ac 1 temperature may result in an increase in austenite and hydrogen in the steel.
  • the tempering temperature is the Ac 1 temperature or lower, preferably (Ac 1 temperature - 30) °C or lower.
  • the upper limit of the average heating rate during tempering is preferably, but not limited to, 1°C/s or less.
  • tempering time is less than 60 minutes.
  • the tempering time is preferably 50 minutes or less.
  • An excessively short tempering time results in no decrease in austenite and the amount of hydrogen in a steel material, so that the tempering time is preferably 10 minutes or more, more preferably 20 minutes or more.
  • Each element symbol in the formula represents the element content (% by mass) of the steel and is 0 for an element not contained.
  • dehydrogenation treatment for removing hydrogen from steel materials
  • the holding time R (s) is preferably determined from the plate thickness or the wall thickness t (mm) of a steel material or a steel pipe and the hydrogen diffusion coefficient D (rmn 2 ⁇ s -1 ) in the steel at room temperature using the following formula (A).
  • the hydrogen diffusion coefficient varies depending on a component contained and the metallic microstructure and a value within a range from, for example, 1 x 10 -5 to 5 x 10 -3 mm 2 /s, more preferably 5 x 10 -4 mm 2 /s or less may be adopted.
  • the dehydrogenation treatment step is performed before pipe production or welding for connecting steel pipes.
  • the dehydrogenation treatment is preferably performed at a high temperature because the hydrogen diffusion coefficient D at a high temperature is small and hydrogen is released quickly.
  • the calculation may be performed using a diffusion coefficient D' (diffusion coefficient at corresponding temperature) at the holding temperature for the value of D in the formula (A).
  • the dehydrogenation treatment temperature is preferably 550°C or less.
  • the dehydrogenation treatment temperature T is more preferably 500°C or less.
  • the dehydrogenation treatment temperature T is still more preferably 400°C or less, most preferably 300°C or less.
  • the dehydrogenation treatment temperature T is preferably room temperature or higher for the reason that the dehydrogenation treatment at a temperature lower than room temperature increases the treatment time and cost.
  • the dehydrogenation treatment temperature T is more preferably 50°C or more.
  • the dehydrogenation treatment temperature T is still more preferably 100°C or more, most preferably 150°C or more.
  • the dehydrogenation treatment temperature T herein is the temperature of the atmosphere in the dehydrogenation treatment step.
  • the room temperature refers to 20°C ⁇ 10°C.
  • heating if conducted, takes time for the temperature Tc at the center of the plate thickness of a steel material or a steel pipe to reach the temperature of the atmosphere in the dehydrogenation treatment step (dehydrogenation treatment temperature T), so even if the holding time R (s) is satisfied at the ambient temperature, the dehydrogenation treatment may be insufficient if the dehydrogenation treatment temperature T (ambient temperature) has not been reached at the center of the plate thickness.
  • At least the former can appropriately control the amount of hydrogen in the steel material in the surface layer portion of the steel material or the steel pipe, and when the latter is also performed, the amount of hydrogen in the steel material from the surface layer portion to the center of the plate thickness of the steel material or the steel pipe can be appropriately controlled.
  • the temperature Tc at the center of the plate thickness may be actually measured with a thermocouple or the like or may be predicted using a finite element method or the like.
  • the scale on the steel surface inhibits dehydrogenation and is therefore preferably removed before dehydrogenation treatment.
  • the scale removal method may be, for example, but is not limited to, physical cleaning by high-pressure cleaning or a chemical method using a scale remover.
  • the thickness of scale to be removed is not particularly limited, the scale removal effect can be obtained when the scale is removed by approximately 100 ⁇ m.
  • a steel material according to the present invention is more specifically described below.
  • the chemical composition, metallic microstructure, and crack growth rate of the steel material are the same as those described for the steel pipe, and the steps other than the rolling step and the cooling step (the casting step, the heating step, the reheating and quenching step, the tempering step, and the dehydrogenation treatment step) in the production method are performed in the same manner as described for the steel pipe.
  • the rolling step and the cooling step are performed as described below.
  • a steel raw material heated in the heating step as described in the method for producing a steel pipe is hot-rolled using a hot-rolling mill under the following conditions.
  • Finish rolling temperature 820°C or more
  • a finish rolling temperature of less than 820°C results in excessively large rolling force and a higher risk of occurrence of rolling trouble.
  • the finish rolling temperature is 820°C or more.
  • the finish rolling temperature is preferably 850°C or more, more preferably 900°C or more.
  • the upper limit of the finish rolling temperature is not particularly limited, an excessively high temperature tends to result in a nonuniform metallic microstructure, so that the finish rolling temperature is preferably 1200°C or less.
  • the finish rolling temperature is more preferably 1150°C or less, still more preferably 1100°C or less.
  • a steel material with the chemical composition described above is hot-rolled, then heated, and held at a temperature of the Ac 3 temperature or higher and 1000°C or less, and is cooled under the cooling conditions of the following Group A or Group B.
  • the temperature is preferably held for 10 minutes or more, more preferably 15 minutes or more, still more preferably 20 minutes or more.
  • the upper limit is not particularly limited, the temperature is preferably held for 60 minutes or less, more preferably 45 minutes or less.
  • Heating temperature after hot rolling Ac 3 temperature or higher and 1000°C or less
  • a heating temperature lower than the Ac 3 temperature in the cooling step results in ferrite remaining in the steel after cooling, a decrease in the strength of a steel material, and a degradation of the fatigue property.
  • the heating temperature is the Ac 3 temperature or higher.
  • the heating temperature is preferably the Ac 3 temperature + 30°C or more, more preferably the Ac 3 temperature + 50°C or more.
  • the Ac 3 temperature + 30°C or more or the Ac 3 temperature + 50°C or more is not applied to a composition system in which the Ac 3 temperature + 30°C or the Ac 3 temperature + 50°C exceeds 1000°C.
  • a heating temperature of more than 1000°C may result in coarse austenite grains and a decrease in the impact absorbed energy and toughness of the material after heat treatment.
  • Steel materials with a good fatigue property in hydrogen gas have the above chemical composition and include various types, such as a sheet, a plate, and a steel pipe, with high fatigue crack growth resistance in hydrogen gas, or may be steel materials for a hydrogen pipeline formed into a predetermined shape.
  • the steel pipes were heated and held at 950°C for steel pipes with a Ac 3 temperature of 950°C or less or at 1000°C for steel pipes with a Ac 3 temperature of more than 950°C, were then water-cooled under the conditions shown in Table 2-3, and were then tempered under the conditions shown in Table 2-3.
  • the metallic microstructure and mechanical properties were evaluated. The evaluation method is described below.
  • the tempering temperature was arbitrarily adjusted so that the materials had a tensile strength in the range of 520 MPa to 700 MPa.
  • the ambient temperature that is, the dehydrogenation treatment temperature T, was kept at 50°C for 3 hours, followed by natural cooling.
  • test specimen taken from a steel material with plate thickness of 10 mm or less was ground from the surface by 0.5 mm resulting in test specimen thickness of 2 mm, 5 mm, 8 mm, or 9 mm, respectively, and for a test specimen taken from a steel material with plate thickness other than these, a test specimen with a thickness of 10 mm was taken from a position of t/2 (t: sheet thickness), and the front and back sides of a crack growth portion were mirror-polished.
  • Tables 2-1, 2-2, and 2-3 show the results.
  • a carbide measurement method for a steel material is described below.
  • a test specimen was cut out from a cross section parallel to the thickness direction from the center position of the plate thickness of a steel material and was subjected to nital etching, and a carbide was observed by SEM.
  • Ten fields were randomly selected and observed at an acceleration voltage of 15 kV and a magnification of 20000 times.
  • Tables 2-1, 2-2, and 2-3 show the average value of the 10 fields as the number of carbides, with Y indicating that the number of carbides with a diameter of 200 nm or more is 20 pieces/10 ⁇ m 2 or less, and N indicating that the number of carbides with a diameter of 200 nm or more is more than 20 pieces/10 ⁇ m 2 .
  • a method for measuring the amount of austenite in a steel material is described below.
  • a sample for metallic microstructure observation was taken from the center of the sheet width in the center in the longitudinal direction of each of the steel materials and the steel pipes thus produced, a cross section parallel to the longitudinal direction was buffed as an observation surface, the surface layer was then removed by chemical polishing using picric acid etching, and X-ray diffractometry was performed. More specifically, a Co-K ⁇ radiation source was used for an incident X-ray, and the area fraction of retained austenite was calculated from the intensity ratios of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.
  • the amount of hydrogen remaining in the steel was measured by thermal desorption analysis method using a low-temperature programmed hydrogen analyzer ⁇ gas chromatograph type> (JTF-20AL).
  • the thermal desorption analysis was performed in the temperature range of room temperature to 400°C at a heating rate of 200°C/h, and the sum total thereof was taken as the amount of hydrogen.
  • the specimen has a cylindrical shape with 30 mm in length and 7 ⁇ in diameter in the longitudinal direction of the steel pipe at the quarter thickness position of the steel plate and at the quarter thickness position from the inner surface of the steel pipe.
  • the amount of hydrogen is the amount of H shown in Tables 1-1 and 1-2, and the amount before being subjected to a high-pressure hydrogen fatigue test explained in aging described later.
  • the examples of the present invention all satisfied the condition that the fatigue crack growth rate in hydrogen gas was 1.0 x 10 -6 m/cycle or less.
  • Table 2-1 Steel material No. Steel pipe No.
  • Example 2 All satisfied the condition that the crack growth rate da/dN in hydrogen gas was 1.0 x 10 -6 m/cycle or less. Among them, the crack propagation characteristics were better when the reheating and quenching steps were performed under more suitable conditions.
  • Table 3 Billet No. Steel pipe No.
  • Example 14 14, 43, and 97 in Example 1 was performed at a dehydrogenation treatment temperature T (ambient temperature) of 50°C for a holding time of 3 hours, in the present example, the dehydrogenation treatment of the steel pipes Nos. 14D, 43D, and 97D was performed at a dehydrogenation treatment temperature T (ambient temperature) of 50°C so that the holding time tc after the temperature Tc at the center of the plate thickness reached 50°C satisfied the formula (A).
  • T ambient temperature
  • the dehydrogenation treatment temperature T (ambient temperature) was 50°C, and the holding time tc satisfied the formula (A) at a dehydrogenation treatment temperature T of 50°C, but the holding time tc after the temperature Tc at the center of the plate thickness reached 50°C did not satisfy the formula (A).
  • the dehydrogenation treatment temperature T (ambient temperature) was 50°C, but neither the holding time t at the ambient temperature nor the holding time tc after the temperature Tc at the center of the plate thickness reached 50°C did not satisfied the formula (A).
  • Dehydrogenation holding time t is Y
  • Dehydrogenation holding time t is 50°C and the holding time t satisfies the formula (A)
  • “Dehydrogenation holding time t is N” means that the dehydrogenation treatment temperature T (ambient temperature) is 50°C, but the holding time t does not satisfy the formula (A).
  • Holding time tc at center temperature of steel plate thickness Tc is Y
  • the holding time tc after the temperature Tc at the center of the plate thickness reaches 50°C satisfies the formula (A)
  • “Holding time tc at steel material center temperature Tc is N” means that the temperature Tc at the center of the plate thickness reaches 50°C, but the holding time tc after Tc reaches 50°C does not satisfy the formula (A).
  • the examples of the present invention all satisfied the condition that the crack growth rate da/dN in hydrogen gas was 1.0 x 10 -6 m/cycle or less. Among them, a steel pipe subjected to the dehydrogenation treatment under more suitable conditions had better crack propagation characteristics. [Table 4] Billet No. Steel pipe No.

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EP23872574.1A 2022-09-29 2023-09-28 Steel tube exhibiting excellent fatigue characteristics against hydrogen and production method therefor, and steel material and production method therefor Pending EP4578565A1 (en)

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