EP3372702B1 - Élément en acier et tôle d'acier, et leurs procédés de production - Google Patents

Élément en acier et tôle d'acier, et leurs procédés de production Download PDF

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EP3372702B1
EP3372702B1 EP16862027.6A EP16862027A EP3372702B1 EP 3372702 B1 EP3372702 B1 EP 3372702B1 EP 16862027 A EP16862027 A EP 16862027A EP 3372702 B1 EP3372702 B1 EP 3372702B1
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pwht
steel plate
steel
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EP3372702A4 (fr
EP3372702A1 (fr
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Ryota Miyata
Yoshitake Kobayashi
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • 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
    • 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
    • C22C38/18—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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • 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
    • C21D2211/002—Bainite
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a steel member and a steel plate and manufacturing method for them. More particularly, the present invention relates to a steel member obtained by subjecting a steel plate to welding and a post weld heat treatment (hereinafter sometimes referred to as "PWHT”), especially a steel member which is excellent in strength and low-temperature toughness of the thicknesswise central portion of the steel member even when PWHT is performed at high temperature for a long time, a steel plate used for manufacturing the steel member, and a manufacturing method for them.
  • PWHT post weld heat treatment
  • low-temperature toughness is sometimes simply referred to as "toughness”.
  • steel plates used in steel members such as pressure vessels are required to achieve higher strength. From the viewpoint of the safety, the steel members are also required to have high-level low-temperature toughness.
  • the steel plate is subjected to normalizing and quenching.
  • the steel plate has a large thickness, because of a low cooling rate of the inside, especially the thicknesswise central portion, of the steel plate during normalizing or quenching, there arises a problem that high strength is hardly obtained.
  • the steel member such as the pressure vessel is obtained by welding the steel plate, followed by subjecting to stress relief annealing for removing strain, i.e., PWHT. To remove strain, PWHT is performed for a long time.
  • PWHT stress relief annealing for removing strain
  • the method of ensuring high toughness includes an increase in the amount of an alloy element.
  • a Cr-Mo steel containing Cr and Mo as alloy elements is used in the steel member such as the pressure vessel. It has been known that, when using, as the Cr-Mo steel, for example, a 2.25Cr-1.0Mo steel, satisfactory toughness is obtained even in a thicknesswise central portion of a thick steel plate which hardly ensures the toughness. However, intentions towards energy saving and cost reduction have recently increased. Therefore, under the assumption of use of a Cr-Mo steel containing an alloy element in the suppressed amount compared to the 2.25Cr-1.0Mo steel, it is strongly required to realize a steel member which is excellent in strength and toughness of the thicknesswise central portion of the steel member.
  • Patent Documents 1 and 2 disclose a technique in which the low-temperature toughness is improved with respect to steels having a composition of 1.25Cr-0.5Mo level, which hardly ensure toughness.
  • Patent Document 1 discloses a technique in which the addition of Nb and Ca ensures the hardenability and suppresses degradation of properties during stress relief (SR) (stress relief annealing).
  • SR stress relief annealing
  • Ca may form coarse inclusions, thus exerting an adverse influence on the toughness. Therefore, it is considered to be difficult to stably ensure the toughness of the thicknesswise central portion of the thick steel member.
  • Patent Document 2 discloses a technique in which the austenite grains are refined by performing controlled rolling, or controlled rolling and accelerated cooling before quenching in a manufacturing process, thus ensuring low-temperature toughness.
  • the controlled rolling in the technique sometimes cause degradation of a productivity of a rolling line, so that it is hardly to say that this technique is suitable for practical use.
  • EP 2 918 694 A1 discloses a steel member, which comprises (by mass percent) C: 0.12 to 0.18%; Si: 0.50 to 0.80%; Mn: 0.40 to 0.70%; P: 0.015% or less (not including 0%); S: 0.005% or less (not including 0%) ; Al: 0.040 to 0.
  • a microstructure of the thicknesswise central portion of the steel member satisfies all of the following (a) to (d): (a) the microstructure is at least one of tempered bainite and tempered martensite, (b) the mean equivalent circle diameter of grains is 20 ⁇ m or less, each grain being surrounded by a large-angle grain boundary having a crystal misorientation of 15° or more between two adjacent grains, (c) the maximum size of grain boundary carbide is 0.8 ⁇ m or less, and (d) a fraction of the grain boundary carbide is 1.0 area% or more.
  • the present invention has been made in view of the foregoing circumstances, and it is an object of the present invention to provide a steel member in which inside the steel material exhibits high strength and high low-temperature toughness even when PWHT is performed for a long time, especially at high temperature for a long time after welding in a manufacturing process of the steel member, a steel plate which is useful for the manufacture of the steel member, and a manufacturing method for them.
  • the "inside the steel material” particularly means the "thicknesswise central portion". The same shall apply hereinafter.
  • a steel member of the present invention which could solve the foregoing problems, has a composition including:
  • a steel plate of the present invention which could solve the foregoing problems, is a steel plate used for manufacturing the above steel member, which has a composition including:
  • a method for manufacturing the steel member is also included in the present invention.
  • the steel plate of the present invention When the steel plate of the present invention is used in the manufacture of a steel member, it is possible to obtain a steel member in which inside the steel material exhibits high strength and sufficiently excellent toughness even when PWHT is performed for a long time, especially at high temperature for a long time after welding in the manufacturing process of the steel member. As a result, it is possible to provide middle temperature/high temperature pressure vessels which exhibit high strength and high toughness.
  • the steel member of the present invention contributes to energy saving and cost reduction since the alloy element amount is suppressed.
  • Fig. 1 is a graph showing the relationship between D/d and Charpy absorbed energy at -38°C in Examples.
  • the inventors have intensively studied so as to obtain a steel member which is excellent in low-temperature toughness and strength of the thicknesswise central portion of the steel member even when the steel member is manufactured by subjecting a steel plate to PWHT especially for a long time under the assumption of use of the steel plate formed of a Cr-Mo steel whose alloy element amount is suppressed compared to the 2.25Cr-1.0Mo steel.
  • a fine microstructure is formed, and refining of grain boundary carbide, which easily undergoes coarsening and serves as a fracture origin, is performed.
  • a structure is made to be at least one of tempered bainite and tempered martensite, and
  • a value represented by D/d is set at 54 or less, where D is an average equivalent circle diameter of crystal grains surrounded by large angle grain boundaries with crystal misorientation of 15° or more between two adjacent crystal grains, and d is a maximum diameter of grain boundary carbide; and temper embrittlement sensitivity is suppressed, specifically, the below-mentioned composition is made to be satisfied.
  • the "average equivalent circle diameter of crystal grains surrounded by large angle grain boundaries with crystal misorientation of 15° or more between two adjacent crystal grains” is sometimes simply referred to as “large angle grain boundary size”.
  • the “suppression of temper embrittlement sensitivity” is also referred to as “suppression of temper embrittlement” or “suppression of intergranular cracks”.
  • structure of thicknesswise central portion is simply referred to as “structure”.
  • the below-mentioned properties, i.e., strength and low-temperature toughness mean properties of at least thicknesswise central portion of the steel member, i.e., after subjecting a steel plate to welding and PWHT.
  • a structure is at least one of tempered bainite and tempered martensite.
  • the tempered bainite and tempered martensite are fine structures and are structures which are particularly effective in ensuring the strength and toughness of the thicknesswise central portion.
  • the steel member of the present invention has a structure which is at least one of tempered bainite and tempered martensite.
  • Examples of the other structure, which can be inevitably included, include polygonal ferrite, retained austenite, pearlite, and the like.
  • the total area % of these structures is suppressed to 5 area % or less, and most preferably 0 area %.
  • the structure is mainly an upper bainite structure having a large crystal grain size, thus failing to ensure satisfactory toughness.
  • a value represented by D/d is 54 or less, where D is an average equivalent circle diameter of crystal grains surrounded by large angle grain boundaries with crystal misorientation of 15° or more between two adjacent crystal grains, and d is a maximum diameter of grain boundary carbide.
  • the structure of the thicknesswise central portion is made to be at least one of tempered bainite and tempered martensite, as mentioned above, the structure can be refined.
  • the above-mentioned (b) is defined so as to obtain high toughness by reliable refining of the structure.
  • the steel member of the present invention is subjected to PWHT, especially PWHT for a long time, more especially PWHT at high temperature for a long time, as mentioned above.
  • PWHT grain boundary carbide M 23 C 6 is generally formed.
  • the condition of the PWHT becomes severe condition at high temperature for a long time, the grain boundary carbide easily undergoes coarsening and serves as a fracture origin, causing degradation of the toughness.
  • D/d satisfies 54 or less, as mentioned in the above (b), with respect to the relationship between an average equivalent circle diameter D in terms of the large angle grain boundary size and a maximum diameter d of the grain boundary carbide, it is possible to ensure sufficiently excellent toughness even after PWHT.
  • the above-mentioned D/d is preferably 50 or less, and more preferably 48 or less. Considering the composition, manufacturing conditions and the like defined in the present invention, the lower limit value of D/d is about 12.
  • D/d is 54 or less, and there is no particular limitation on each value of the average equivalent circle diameter D of large angle grain boundaries and the maximum diameter d of the grain boundary carbide.
  • the average equivalent circle diameter D of large angle grain boundaries can be set at, for example, 45 ⁇ m or less, 35 ⁇ m or less, 30 ⁇ m or less, 25 ⁇ m or less, and 15 ⁇ m or less.
  • the lower limit of the average equivalent circle diameter D of large angle grain boundaries is approximately 10 ⁇ m from a manufacturing point of view.
  • the maximum diameter d of the grain boundary carbide can be set at, for example, 0.8 ⁇ m or less.
  • the maximum diameter d of the grain boundary carbide can also be set at 0.70 ⁇ m or less, and 0.60 ⁇ m or less.
  • the lower limit of the maximum diameter d of the grain boundary carbide is approximately 0.20 ⁇ m in the range of the composition and manufacturing conditions defined in the present invention.
  • a cooling rate during quenching is generally higher than that in the thicknesswise central portion, so that it is easy to obtain a fine structure compared to the thicknesswise central portion, and both strength and toughness tend to be more improved compared to the thicknesswise central portion.
  • each content of Nb and Ti is suppressed.
  • the reason is that inclusion of a large amount of these elements makes it difficult to achieve D/d in the above range. These elements also increase the strength more than necessary, causing degradation of workability. Furthermore, a total content of Ca, Mg, REM, and Zr is also suppressed. The reason is that these elements increase inclusions, causing degradation of the toughness.
  • To control the size of the grain boundary carbide there is also a need to control the content of Cr, in addition to C.
  • To ensure the toughness by suppressing temper embrittlement sensitivity there is also a need to control the content of Si.
  • the C is an element which is required to obtain at least one of tempered bainite and tempered martensite during quenching of a steel plate even in the thicknesswise central portion with a low cooling rate, and to decrease the average grain size D by enhancing hardenability thereby setting D/d in the above range. It is also an element which is required to obtain sufficient base material strength by ensuring the grain boundary carbide.
  • the C content is set at 0.110% or more.
  • the C content is preferably 0.120% or more, and more preferably 0.130% or more.
  • excessive C content causes coarsening of grain boundary carbide after PWHT for a long time, leading to degradation of the toughness. During welding of a steel plate, weld cracks easily occur. Therefore, the C content is set at 0.15% or less.
  • the C content is preferably 0.145% or less.
  • Si 0.50% or more and 0.80% or less
  • Si is an element which is effective in improving the base material strength of a steel member, i.e., the strength of the thicknesswise central portion. It is also an element to be used as a deoxidizing agent. It is also an element useful for suppressing temper embrittlement sensitivity, thereby ensuring the toughness.
  • the Si content is set at 0.50% or more.
  • the Si content is preferably 0.55% or more, and more preferably 0.60% or more.
  • excessive Si content enhances temper embrittlement sensitivity, leading to degradation of the toughness, so that the Si content is set at 0.80% or less.
  • the Si content is preferably 0.75% or less, and more preferably 0.70% or less.
  • Mn 0.40% or more and 0.65% or less
  • Mn is an element which is effective in improving the hardenability by stabilizing austenite and achieving lowering of the transformation temperature to obtain a fine structure, thus ensuring the strength and toughness.
  • 0.40% or more of Mn is contained.
  • the Mn content is preferably 0.45% or more, and more preferably 0.46% or more.
  • excessive Mn content enhances temper embrittlement sensitivity, leading to degradation of the toughness. Therefore, the Mn content is 0.65% or less, preferably 0.60% or less, more preferably 0.55% or less, and still more preferably 0.50% or less.
  • the P content is suppressed to 0.0060% or less.
  • the P content is preferably 0.0050% or less.
  • the S content is preferably as small as possible, and the S content is suppressed to 0.0070% or less, preferably 0.0050% or less, and more preferably 0.0030% or less.
  • Al 0.030% or more and 0.080% or less
  • Al is a very important element in the present invention, as mentioned above, and is an element required to fix N as AlN during quenching and to ensure the hardenability due to free B. AlN is useful for suppressing coarsening of prior austenite ( ⁇ ) grains during quenching to obtain a fine structure. Al is also an element required for deoxidation. To exert these effects, the Al content is set at 0. 030% or more. The Al content is preferably 0.040% or more, more preferably 0.045% or more, and still more preferably 0.050% or more. Meanwhile, excessive Al content enables formation of alumina-based coarse inclusions, causing degradation of the toughness. Therefore, the Al content is set at 0.080% or less. The Al content is preferably 0.075% or less, and more preferably 0.071% or less.
  • Cu and Ni are elements which are effective in increasing the strength without significantly impairing the toughness.
  • 0.05% or more, preferably 0.10% or more, and more preferably 0.11% or more of Cu is contained, and 0.05% or more, preferably 0.10% or more, more preferably 0.15% or more, and still more preferably 0.16% or more of Ni is contained.
  • the addition of a large amount of these elements increases the strength more than necessary, as mentioned above, causing degradation of the toughness. Therefore, the upper limit of the Cu content is set at 0.20% or less, and the upper limit of the Ni content is set at 0.30% or less.
  • the Cu content is preferably 0.18% or less, and more preferably 0.17% or less.
  • the Ni content is preferably 0.28% or less, and more preferably 0.26% or less.
  • Cr is an element which is effective in suppressing coarsening of carbide due to PWHT, thereby ensuring the toughness of a steel member. It is also an element which is effective in ensuring the strength in middle temperature/high temperature region and improving the corrosion resistance. To exert these effects, 1.05% or more of Cr is contained.
  • the Cr content is preferably 1.10% or more, and more preferably 1.20% or more. Meanwhile, if Cr is contained excessively, temper embrittlement sensitivity is easily enhanced after PWHT, causing intergranular cracks, leading to exert an adverse influence on the toughness. Excessive Cr causes degradation of the workability and weldability, and an increase in manufacturing cost. Therefore, the Cr content is set at 1.50% or less.
  • the Cr content is preferably 1.45% or less, and more preferably 1.40% or less.
  • Mo is an element which is effective in enhancing the hardenability and suppressing temper embrittlement. To obtain these effects, there is a need to contain 0.45% or more of Mo.
  • the Mo content is preferably 0.50% or more, and more preferably 0.55% or more. Meanwhile, the effect is scarcely improved even when the Mo content exceeds 0.65%, leading to an increase in manufacturing cost, so that the upper limit of the Mo content is set at 0. 65% or less.
  • the Mo content is preferably 0.62% or less.
  • N 0.0030% or more and 0.0070% or less
  • N is an important element in the present invention, along with Al.
  • AlN is useful for suppressing coarsening of prior austenite ( ⁇ ) grains during quenching to obtain a fine structure. If the N content is less than 0.0030%, coarsening of austenite ( ⁇ ) grain occurs due to lack of AlN, thus failing to obtain a fine structure, leading to degradation of the toughness . Therefore, the N content is set at 0.0030% or more.
  • the N content is preferably 0.0035% or more, and more preferably 0.0040% or more. Meanwhile, if the N content exceeds 0.0070%, the N-fixing effect due to Al cannot be obtained and BN is formed.
  • the N content is set at 0.0070% or less.
  • the N content is preferably 0.0060% or less, more preferably 0.0055% or less, and still more preferably 0.0050% or less.
  • B as free B (solid-soluted B) enhances the hardenability, thus making it possible to decrease the average grain size D even in the thicknesswise central portion of a thick steel plate which is cooled at a low cooling rate during quenching. As a result, it is possible to ensure excellent toughness even in the thicknesswise central portion.
  • 0.0003% or more of B is required even under the assumption of controlling the above-mentioned contents of Al and N, and quenching conditions .
  • the B content is preferably 0.0005% or more, and more preferably 0.0007% or more.
  • the B content is preferably 0.0009% or less, and more preferably 0.0008% or less.
  • V 0% or more and 0.030% or less
  • V is an element which is effective in forming carbide and nitride, thereby contributing to an improvement in strength, and enhancing the hardenability to obtain a fine structure.
  • 0.003% or more of V may be preferably contained.
  • the V content is more preferably 0.005% or more. Meanwhile, excessive addition of V causes an increase in cost, so that the upper limit is set at 0.030% or less.
  • the V content is preferably 0.027% or less, more preferably 0.020% or less, and still more preferably 0.010% or less.
  • Nb content is 0.005% or less
  • Ti content is 0.001% or less
  • the total content of Ca, Mg, REM, and Zr is 0.0010% or less
  • Nb content is suppressed to 0.005% or less
  • Ti content is suppressed to 0.001% or less
  • the total content of Ca, Mg, rare earth metal (REM), and Zr is suppressed to 0.0010% or less.
  • Nb and Ti refine prior austenite ( ⁇ ) grains during quenching, leading to degradation of the hardenability.
  • the large angle grain boundary size increases, i.e., the average equivalent circle diameter D increases, so that D/d exceeds a defined range.
  • Nb and Ti are elements which increase the strength more than necessary, leading to degradation of the workability.
  • Ca, Mg, REM, and Zr increase inclusions, leading to degradation of the toughness.
  • REM means that lanthanoid elements, i.e., fifteen elements from La to Lu, and scandium and yttrium are included.
  • the steel plate and the steel member of the present invention include the above-mentioned chemical components, with the balance being iron and inevitable impurities.
  • a steel slab having the above-mentioned composition is hot-rolled by a conventional method to obtain a steel plate, and then the steel plate is subjected to quenching and tempering.
  • quenching and tempering To obtain a fine structure defined in the above-mentioned (a) and (b) of a steel member, there is a need to perform quenching and tempering in the manufacturing process of a steel plate under the following conditions.
  • Heating temperature of quenching 910°C or higher and 940°C or lower, and holding time at the heating temperature: 25 minutes or more and 55 minutes or less
  • prior austenite ( ⁇ ) grains can be allowed to grow to some extent, thus improving the hardenability to obtain a fine structure.
  • the heating temperature of quenching is set at 910°C or higher.
  • the heating temperature is preferably 920°C or higher.
  • the heating temperature exceeds 940°C, N fixed as AlN is partially solid-soluted and combined with B to form BN, thus failing to obtain the hardenability improving effect due to free B. As a result, a fine structure cannot be obtained, leading to degradation of the toughness. Therefore, the heating temperature of quenching is set at 940°C or lower.
  • the heating temperature is preferably 935°C or lower.
  • the heat holding time is set at 25 minutes or more.
  • the heat holding time is preferably 30 minutes or more. From the viewpoint of the productivity, the upper limit of the heat holding time is 55 minutes or less.
  • the prior austenite ( ⁇ ) grain diameter is adjusted in a range of about 50 to 100 ⁇ m by controlling the conditions during quenching as mentioned above, because it is easy to obtain a fine structure.
  • tempering temperature 620°C or higher and an Ac 1 point or lower
  • tempering is performed, thus enabling an improvement in workability such as bending of the steel plate. Therefore, in the manufacturing process of a steel member, tempering is performed to reduce the hardness of the surface layer from the viewpoint of improving the workability of the steel plate. Tempering is performed under the condition of the tempering temperature of 620°C or higher and an Ac 1 point or lower. By setting the tempering temperature at 620°C or higher, the hardness of the surface layer is sufficiently reduced, thus making it possible to ensure satisfactory workability.
  • the tempering temperature is preferably 700°C or higher.
  • the tempering temperature exceeds the Ac 1 point, the structure partially undergoes reverse transformation and then air-cooled, leading to intermixing of polygonal ferrite. As a result, at least one of tempered bainite and tempered martensite as a desired structure cannot be obtained, leading to a decrease in strength, and degradation of the toughness because the reversely transformed portion has a coarse structure. Therefore, the upper limit of the tempering temperature is set at the Ac 1 point or lower.
  • the tempering temperature is preferably 750°C or lower.
  • the Ac 1 point can be determined by the method in the below-mentioned Examples.
  • Tempering is performed under the condition of the heating temperature and the heating time such that a P T value represented by the defined equation (1) is within the above ranges. If the P T value is less than 19.2, the hardness increases excessively, causing defects such as degradation of the workability. Therefore, the P T value is 19.2 or more, preferably 19.3 or more, and more preferably 19.4 or more. Meanwhile, if the P T value exceeds 20.6, coarsening of carbide occurs, causing degradation of properties such as toughness. Therefore, the P T value is 20.6 or less, preferably 20.3 or less, and more preferably 2 0 . 0 or less .
  • the steel plate of the present invention has a thickness of 100 mm or less.
  • the lower limit of the thickness is 6 mm or more, and 10 mm or more.
  • the steel member obtained by using the steel plate also has the same thickness as that of the steel plate.
  • the steel member of the present invention is obtained by welding the steel plate obtained by subjecting to the quenching and tempering, using a common method, followed by subjecting to the post weld heat treatment (PWHT) for removal of strain as mentioned above.
  • PWHT post weld heat treatment
  • the method for manufacturing the steel member of the present invention is characterized in that the post weld heat treatment is performed under a condition of a heating temperature and a heating time such that a P PWHT value represented by the following equation (2) is 20 or more.
  • the condition indicates severe condition at high temperature for a long time (e.g., when the temperature is 680°C or higher and the heating time is 20 hours or more, a P PWHT value is 20.3).
  • the upper limit of the P PWHT value is approximately 21.
  • the steel member of the present invention can be used, for example, as middle temperature/high temperature pressure vessels used in chemical industries including petroleum refining.
  • Each of steel slabs satisfying the compositions shown in Table 1-1 and Table 1-2 was hot-rolled by a conventional method and then subj ected to quenching and tempering under the conditions shown in Table 2-1 and Table 2-2 to obtain steel plates each having a thickness shown in Table 2-1 and Table 2-2.
  • the thickness is also the thickness of a specimen that simulates a steel member.
  • Each of Ac 1 points shown in Table 2-1 and Table 2-2 was determined by analyzing a change in expansion rate when heated at a temperature rising rate of 0.5°C/sec using steel plates each having the composition shown in Table 1-1 and Table 1-2.
  • the heating temperature of quenching and tempering is the temperature in the thicknesswise central portion of the steel plate, and is the temperature obtained by calculating from the furnace atmospheric temperature and the in-furnace time of a heat treatment furnace using a difference method, or the temperature measured by inserting a thermocouple into a dummy material having the same thickness when using an experimental furnace.
  • a heat treatment was performed by simulating PWHT after welding under the condition of the heating temperature of 690°C and the heat holding time of 22 hours to obtain a specimen that simulates a steel member.
  • the condition is the most severe conditions among conditions that are currently being carried out.
  • the P PWHT value is 20.6. Both a temperature rising rate from room temperature to the heating temperature, and a temperature falling rate from the heating temperature to room temperature were set at 55°C/hr or less.
  • the steel plate is subjected to PWHT after welding.
  • the welding scarcely exerts an adverse influence on properties, especially toughness, of a steel member including the welded heat affected zone, so that a specimen was fabricated without subjecting to a heat treatment with respect to welding in the present example.
  • the metal structure was observed in the following manner.
  • the tempered bainite as used herein refers to a structure in which upper bainite, lower bainite, bainitic ferrite or the like is tempered. It is generally difficult to select these structures including tempered martensite, and the structure is sufficiently tempered after PWHT. Therefore, the structure other than polygonal ferrite was regarded as at least one of tempered bainite and tempered martensite (B+M). It was also confirmed that pearlite structure is not contained in any specimen used in the present example.
  • an average equivalent circle diameter (large angle grain boundary size) of crystal grains surrounded by large angle grain boundaries with misorientation (crystal misorientation) of 15° or more between two adjacent crystal grains was determined.
  • the measurement procedure was as follows.
  • the size of grain boundary carbide was measured as follows.
  • Round bar tensile specimens were taken in the direction perpendicular to the rolling direction from the t (thickness) /2 portion and a tensile test was performed in accordance with the procedure of ASTM A370, and then the yield strength and the tensile strength were measured. The case where YS as the yield strength is 310 MPa or more and TS as the tensile strength is 515 MPa or more was rated as high strength.
  • Table 1-1, Table 1-2, Table 2-1, Table 2-2, Table 3-1, and Table 3-2 will reveal the followings.
  • Samples Nos. 1 to 5, 7 to 9, and 12 to 36 are produced under defined conditions using steels satisfying the composition defined in the present invention. Therefore, the thus obtained steel plates exhibited excellent workability and the thus obtained steel member had desired structure and exhibited excellent strength and toughness in the thicknesswise central portion.
  • composition or manufacturing conditions deviate(s) from the defined range or condition, thus failing to ensure the workability of the steel plate, or either tensile properties or impact properties in the thicknesswise central portion are inferior.
  • Sample No. 6 satisfies the composition but is not sufficiently tempered because of excessively low P T value during tempering, leading to high Brinell hardness, i.e., inferior workability. Meanwhile, sample No. 11 satisfies the composition but causes coarsening of carbide because of excessively high P T value during tempering, leading to degradation of properties.
  • Sample No. 10 satisfies the composition but is not sufficiently quenched because of too short heating time of quenching, so that D/d exceeds the upper limit, leading to inferior toughness.
  • Sample No. 37 contains excessive C, thereby causing degradation of the toughness, leading to high Brinell hardness and inferior workability.
  • Samples Nos. 38, 42, and 49 do not contain B, thereby increasing D/d, leading to inferior toughness.
  • Sample No. 48 does not contain B, thereby increasing D/d, and contain excessive P, leading to inferior in toughness.
  • Samples No. 39 and No. 46 contain excessive Nb, thereby refining prior austenite ( ⁇ ) grains during quenching, thus failing to obtain sufficient hardenability, leading to increased D/d and inferior toughness. In sample No. 46, the workability was also degraded.
  • Samples Nos. 40 and 43 are lacking in C content, thus failing to ensure sufficient hardenability, leading to increased D/d and inferior toughness.
  • Sample No. 41 is lacking in C content, thus failing to ensure desired strength due to formation of a large amount of ferrite, leading to increased D/d and inferior toughness .
  • Sample No. 44 is lacking in C content and does not contain B, thus failing to ensure sufficient hardenability, leading to low strength, increased D/d, and degraded toughness.
  • Sample No. 51 is lacking in C content, thus failing to ensure desired toughness due to small carbide size and increased D/d.
  • Sample No. 45 contains excessive Ti, thus failing to obtain sufficient hardenability due to refined prior austenite ( ⁇ ) grains during quenching, leading to increased D/d and inferior toughness.
  • Sample No. 47 was inferior in toughness because of excessive P content.
  • Sample No. 50 is lacking in B content, thus failing to achieve sufficient hardenability, leading to degradation of the toughness.
  • Sample No. 52 contains excessive Cu and Ni and contains excessive C, causing degradation of the toughness.
  • Fig. 1 is a graph showing the relationship between D/d and Charpy absorbed energy at -38°C using data in Table 2-1, Table 2-2, Table 3-1, and Table 3-2. As is apparent from this graph, the adjustment of D/d to 54 or less enables ensuring sufficiently excellent toughness. As mentioned above, samples No. 47 and 52 in Fig. 1 are examples in which the toughness was degraded because of deviation of the composition from the defined range, although D/d satisfies the scope of the present invention.

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Claims (4)

  1. Élément d'acier ayant une composition comprenant, en % en masse :
    C : 0,110 % ou plus et 0,15 % ou moins,
    Si : 0,50 % ou plus et 0,80 % ou moins,
    Mn : 0,40 % ou plus et 0,65 % ou moins,
    P : en excès de 0 % et 0,0060 % ou moins,
    S : en excès de 0 % et 0,0070 % ou moins,
    Al : 0,030 % ou plus et 0,080 % ou moins,
    Cu : 0,05 % ou plus et 0,20 % ou moins,
    Ni : 0,05 % ou plus et 0,30 % ou moins,
    Cr : 1,05 % ou plus et 1,50 % ou moins,
    Mo : 0,45 % ou plus et 0,65 % ou moins,
    N : 0,0030 % ou plus et 0,0070 % ou moins,
    B : 0,0003 % ou plus et 0,0010 % ou moins, et
    V : 0 % ou plus et 0,030 % ou moins,
    dans lequel la teneur en Nb est réduite à 0,005 % ou moins, la teneur en Ti est réduite à 0,001% ou moins, et la teneur totale en Ca, Mg, REM et Zr est réduite à 0,0010 % ou moins, le reste est constitué de fer et d'impuretés inévitables,
    une épaisseur est de 100 mm ou moins, et
    une structure dans une portion centrale dans le sens de l'épaisseur de l'élément d'acier satisfait les points (a) et (b) suivants, et l'énergie absorbée de Charpy à - 38 °C est de 100 J ou plus :
    (a) la structure est au moins une de bainite revenue et de martensite revenue, et
    (b) une valeur représentée par D/d est de 54 ou moins, où D est un diamètre de cercle équivalent moyen de grains cristallins entourés par des joints de grain à grand angle avec une désorientation cristalline de 15° ou plus entre deux grains cristallins adjacents, et d est un diamètre maximal de carbure de joint de grain.
  2. Plaque d'acier utilisée pour la fabrication de l'élément d'acier selon la revendication 1, qui a une composition comprenant, en % en masse,
    C : 0,110 % ou plus et 0,15 % ou moins,
    Si : 0,50 % ou plus et 0,80 % ou moins,
    Mn : 0,40 % ou plus et 0,65 % ou moins,
    P : en excès de 0 % et 0,0060 % ou moins,
    S : en excès de 0 % et 0,0070 % ou moins,
    A1: 0,030 % ou plus et 0,080 % ou moins,
    Cu : 0,05 % ou plus et 0,20 % ou moins,
    Ni : 0,05 % ou plus et 0,30 % ou moins,
    Cr : 1,05 % ou plus et 1,50 % ou moins,
    Mo : 0,45 % ou plus et 0,65 % ou moins,
    N : 0,0030 % ou plus et 0,0070 % ou moins,
    B : 0,0003 % ou plus et 0,0010 % ou moins, et
    V : 0 % ou plus et 0,030 % ou moins,
    dans laquelle la teneur en Nb est réduite à 0,005 % ou moins, la teneur en Ti est réduite à 0,001 % ou moins, et la teneur totale en Ca, Mg, REM et Zr est réduite à 0,0010 % ou moins, le reste est constitué de fer et d'impuretés inévitables, et dans laquelle une épaisseur de la plaque d'acier est de 100 mm ou moins.
  3. Procédé de fabrication de la plaque d'acier selon la revendication 2, qui comprend laminer à chaud une ébauche d'acier ayant la composition selon la revendication 2 ; réaliser une trempe selon une condition d'une température de chauffage de 910°C ou supérieure et 940°C ou inférieure et une durée de maintien à la température de chauffage de 25 minutes ou plus et 55 minutes ou moins ; et, après la trempe, réaliser un revenu à une température de chauffage de 620 °C ou supérieure et un point AC1 ou inférieure selon une condition de la température de chauffage et d'une durée de chauffage telle qu'une valeur PT représentée par l'équation suivante (1) est de 19,2 ou plus et 20,6 ou moins : valeur P T = T T × 20 + logt T × 10 − 3
    Figure imgb0009
     où TT désigne la température de chauffage (K) de revenu, et tT désigne la durée de chauffage (h) de revenu.
  4. Procédé de fabrication de l'élément d'acier selon la revendication 1, qui comprend souder en utilisant la plaque d'acier selon la revendication 2; et réaliser un traitement thermique post-soudage selon une condition d'une température de chauffage et d'une durée de chauffage telle qu'une valeur PPWHT représentée par l'équation suivante (2) est de 20 ou plus : valeur P PWHT = T PWHT × 20 + logt PWHT × 10 − 3
    Figure imgb0010
     où TPWHT désigne la température de chauffage (K) de traitement thermique post-soudage, et tPWHT désigne la durée de chauffage (h) de traitement thermique post-soudage.
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