EP3372702B1 - Steel member and steel plate and manufacturing method for them - Google Patents
Steel member and steel plate and manufacturing method for them Download PDFInfo
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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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- 229910000831 Steel Inorganic materials 0.000 title claims description 143
- 239000010959 steel Substances 0.000 title claims description 143
- 238000004519 manufacturing process Methods 0.000 title claims description 32
- 238000010438 heat treatment Methods 0.000 claims description 67
- 238000010791 quenching Methods 0.000 claims description 39
- 230000000171 quenching effect Effects 0.000 claims description 39
- 239000013078 crystal Substances 0.000 claims description 34
- 238000005496 tempering Methods 0.000 claims description 31
- 238000000034 method Methods 0.000 claims description 27
- 239000000203 mixture Substances 0.000 claims description 24
- 229910001563 bainite Inorganic materials 0.000 claims description 17
- 238000003466 welding Methods 0.000 claims description 17
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 15
- 229910000734 martensite Inorganic materials 0.000 claims description 14
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 12
- 239000012535 impurity Substances 0.000 claims description 9
- 229910052742 iron Inorganic materials 0.000 claims description 8
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- 229910052802 copper Inorganic materials 0.000 claims description 5
- 229910052804 chromium Inorganic materials 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 238000005098 hot rolling Methods 0.000 claims description 2
- 229910052748 manganese Inorganic materials 0.000 claims 2
- 230000015556 catabolic process Effects 0.000 description 27
- 238000006731 degradation reaction Methods 0.000 description 27
- 229910001566 austenite Inorganic materials 0.000 description 15
- 238000001000 micrograph Methods 0.000 description 15
- 230000000694 effects Effects 0.000 description 14
- 238000005498 polishing Methods 0.000 description 12
- 230000001965 increasing effect Effects 0.000 description 10
- 150000002910 rare earth metals Chemical class 0.000 description 10
- 238000005096 rolling process Methods 0.000 description 9
- 239000002344 surface layer Substances 0.000 description 8
- 238000001816 cooling Methods 0.000 description 7
- 239000000463 material Substances 0.000 description 7
- 238000005259 measurement Methods 0.000 description 7
- 229910001568 polygonal ferrite Inorganic materials 0.000 description 7
- 230000035945 sensitivity Effects 0.000 description 7
- 229910045601 alloy Inorganic materials 0.000 description 6
- 239000000956 alloy Substances 0.000 description 6
- 230000007423 decrease Effects 0.000 description 6
- 238000012360 testing method Methods 0.000 description 6
- 239000000126 substance Substances 0.000 description 5
- 230000002411 adverse Effects 0.000 description 4
- 238000011156 evaluation Methods 0.000 description 4
- 229910052758 niobium Inorganic materials 0.000 description 4
- 239000002436 steel type Substances 0.000 description 4
- 229910003460 diamond Inorganic materials 0.000 description 3
- 239000010432 diamond Substances 0.000 description 3
- 229910001651 emery Inorganic materials 0.000 description 3
- 230000002708 enhancing effect Effects 0.000 description 3
- 238000009863 impact test Methods 0.000 description 3
- 230000006872 improvement Effects 0.000 description 3
- 229910052751 metal Inorganic materials 0.000 description 3
- 239000002184 metal Substances 0.000 description 3
- 238000007517 polishing process Methods 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 239000002002 slurry Substances 0.000 description 3
- 230000001629 suppression Effects 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- 238000000137 annealing Methods 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 229910052799 carbon Inorganic materials 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 229910001562 pearlite Inorganic materials 0.000 description 2
- 238000005504 petroleum refining Methods 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 230000000630 rising effect Effects 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- 229910000859 α-Fe Inorganic materials 0.000 description 2
- 229910000967 As alloy Inorganic materials 0.000 description 1
- 238000007546 Brinell hardness test Methods 0.000 description 1
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- SIXSYDAISGFNSX-UHFFFAOYSA-N scandium atom Chemical compound [Sc] SIXSYDAISGFNSX-UHFFFAOYSA-N 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
Images
Classifications
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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
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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
- 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
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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
- 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
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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
- 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
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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
- C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C21D2201/00—Treatment for obtaining particular effects
- C21D2201/05—Grain orientation
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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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
- 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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Description
- 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. Hereinafter, low-temperature toughness is sometimes simply referred to as "toughness".
- There is a tendency for middle temperature/high temperature pressure vessels used in chemical industries including petroleum refining to be required to achieve higher resistance to high temperature and high pressure for the purpose of achieving high efficiency of operations. Therefore, 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.
- To achieve higher strength, the steel plate is subjected to normalizing and quenching. However, when 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. By the way, 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. However, the steel member subjected to PWHT for a long time has a problem that the low-temperature toughness is degraded.
- 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.
- There has been proposed, against the foregoing problems, the technique in which high strength and high toughness are achieved by properly adjusting chemical components while suppressing the amount of the alloy element. For example, 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). However, when this technique is applied to a thick steel plate obtained mainly by casting using an ingot casting method, 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. However, 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.
-
- Patent Document 1:
JP Hei-06-279919 A - Patent Document 2:
JP 2000-345281 A - Further,
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. 080%; Cu: 0.05 to 0.40%; Ni: 0.05 to 0.40%; Cr: 1.25 to 1.50%; Mo: 0.45 to 0.65%; N: 0.0030 to 0.0060%; and B: 0.0003 to 0.0010%, with the remainder consisting of Fe and inevitable impurities, wherein 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 µm 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 µm 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:
- C: 0.110% (% is by mass, the same shall apply hereinafter with respect to chemical components) or more and 0.15% or less,
- Si: 0.50% or more and 0.80% or less,
- Mn: 0.40% or more and 0.65% or less,
- P: exceeding 0% and 0.0060% or less,
- S: exceeding 0% and 0.0070% or less,
- Al: 0.030% or more and 0.080% or less,
- Cu: 0.05% or more and 0.20% or less,
- Ni: 0.05% or more and 0.30% or less,
- Cr: 1.05% or more 1.50% or less,
- Mo: 0.45% or more and 0.65% or less,
- N: 0.0030% or more and 0.0070% or less,
- B: 0.0003% or more and 0.0010% or less, and
- V: 0% or more and 0.030% or less,
- a thickness is 100 mm or less, and
- a structure in a thicknesswise central portion of the steel member satisfies the following (a) and (b), and Charpy absorbed energy at -38°C is 100 J or more:
- (a) the structure is at least one of tempered bainite and tempered martensite, and
- (b) 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.
- 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:
- C: 0.110% or more and 0.15% or less,
- Si: 0.50% or more and 0.80% or less,
- Mn: 0.40% or more and 0.65% or less,
- P: exceeding 0% and 0.0060% or less,
- S: exceeding 0% and 0.0070% or less,
- Al: 0.030% or more and 0.080% or less,
- Cu: 0.05% or more and 0.20% or less,
- Ni: 0.05% or more and 0.30% or less,
- Cr: 1.05% or more 1.50% or less,
- Mo: 0.45% or more and 0.65% or less,
- N: 0.0030% or more and 0.0070% or less,
- B: 0.0003% or more and 0.0010% or less, and
- V: 0% or more and 0.030% or less,
- A method for manufacturing the steel plate of the present invention, which could solve the foregoing problems, includes hot-rolling a steel slab having the above composition; performing quenching under a condition of a heating temperature of 910°C or higher and 940°C or lower and a holding time at the heating temperature of 25 minutes or more and 55 minutes or less; and, after the quenching, performing tempering at a heating temperature of 620°C or higher and an Ac1 point or lower under a condition of the heating temperature and a heating time such that a PT value represented by the following equation (1) is 19.2 or more and 20.6 or less:
- A method for manufacturing the steel member is also included in the present invention. The method for manufacturing the steel member includes welding using the above steel plate; and performing a post weld heat treatment under a condition of a heating temperature and a heating time such that a PPWHT value represented by the following equation (2) is 20 or more:
- 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.
- Furthermore, 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.
- As a result, it has been found that, to obtain the steel member whose thicknesswise central portion has high toughness, a fine microstructure is formed, and refining of grain boundary carbide, which easily undergoes coarsening and serves as a fracture origin, is performed. In detail, (a) a structure is made to be at least one of tempered bainite and tempered martensite, and (b) 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. Hereinafter, 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".
- First, the above-mentioned (a) and (b) with respect to a microstructure of the thicknesswise central portion of the steel member of the present invention will be described.
- In the following description, "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) 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 %. Particularly, when the polygonal ferrite exists, the structure is mainly an upper bainite structure having a large crystal grain size, thus failing to ensure satisfactory toughness.
- (b) 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.
- When 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. However, in the present invention, the above-mentioned (b) is defined so as to obtain high toughness by reliable refining of the structure.
- In the case of the structure of tempered bainite and tempered martensite, so-called large angle grain boundaries in which misorientation (crystal misorientation) between two adjacent crystal grains is 15° or more generally exhibit large misorientation between two adjacent crystal grains. Therefore, the progress of brittle fracture is curved and the fracture surface unit of brittle fracture decreases, thus contributing to an improvement in toughness. Meanwhile, 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. When the Cr-Mo steel constituting the steel member is subjected to PWHT, grain boundary carbide M23C6 is generally formed. When 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.
- In the present invention, it has been found that, if a value represented by 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.
- In the present invention, 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 µm or less, 35 µm or less, 30 µm or less, 25 µm or less, and 15 µm or less. The lower limit of the average equivalent circle diameter D of large angle grain boundaries is approximately 10 µm from a manufacturing point of view. The maximum diameter d of the grain boundary carbide can be set at, for example, 0.8 µm or less. The maximum diameter d of the grain boundary carbide can also be set at 0.70 µm or less, and 0.60 µm or less. The lower limit of the maximum diameter d of the grain boundary carbide is approximately 0.20 µm in the range of the composition and manufacturing conditions defined in the present invention.
- In the present invention, there is a need to control the structure of the thicknesswise central portion as mentioned above, and there is no particular limitation on the structure of other parts, for example, a thickness surface layer part. In the part which exists at the surface layer side from the thicknesswise central portion, 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.
- In the thicknesswise central portion, to obtain a fine structure of the above-mentioned (a) and (b), there is a need to particularly make the composition of a steel plate used in the manufacture of the steel member composition to be mentioned below. To decrease the average equivalent circle diameter D such that the above-mentioned D/d is 54 or less, there is a need to enhance the hardenability by including B in the below-mentioned amount leading to the existence as free B (solid-soluted B). It is important that N capable of easily combining with B to form BN is fixed as AlN by adding Al in the below-mentioned amount, in order to ensure free B. This AlN is useful for suppressing coarsening of prior austenite (γ) grains during quenching to obtain a fine structure.
- To decrease the average equivalent circle diameter D, it is effective to improve the hardenability by adding the alloy element, as mentioned above. Excessive C, excessive Cu, and excessive Ni increase the strength more than necessary, causing degradation of the toughness. Therefore, there is a need to set the upper limit of C, Cu, and Ni from the viewpoint of ensuring toughness.
- In the present invention, 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.
- As mentioned in detail below, it is important to properly control, as manufacturing conditions, conditions of quenching and tempering during the manufacture of a steel plate to be subjected to welding.
- First, the composition of a steel plate and a steel member required to ensure the structure and properties will be described.
- 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. To sufficiently exert these effects, 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. However, 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 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. To exert these effects, 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. However, 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 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. To exert these effects, 0.40% or more of Mn is contained. The Mn content is preferably 0.45% or more, and more preferably 0.46% or more. However, 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.
- P as an inevitable impurity exerts an adverse influence on the toughness of a base material and the weld zone, and is particularly segregated on grain boundaries of a steel member, thus causing intergranular cracks, leading to degradation of the toughness. To prevent these disadvantages, the P content is suppressed to 0.0060% or less. The P content is preferably 0.0050% or less.
- S is an element which easily forms MnS, causing weld cracks during welding of a steel plate. Therefore, 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 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 (γ) 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. To sufficiently exert this effect, 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 is an important element in the present invention, along with Al. By forming AlN to fix N during quenching, the hardenability improving effect due to free B can be maximized. AlN is useful for suppressing coarsening of prior austenite (γ) grains during quenching to obtain a fine structure. If the N content is less than 0.0030%, coarsening of austenite (γ) 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. Therefore, the hardenability improving effect due to free B is suppressed, causing coarsening of the structure, leading to degradation of the toughness. Therefore, 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.
- As mentioned above, the existence of 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. To obtain such effect, 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. Meanwhile, if B is contained excessively, the hardenability may be degraded, or weld cracks may occur, so that the upper limit of the B content is set at 0.0010%. The B content is preferably 0.0009% or less, and more preferably 0.0008% 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. To obtain these effects, 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.
- In the present invention, Nb content is suppressed to 0.005% or less, Ti content is suppressed to 0.001% or less, and the total content of Ca, Mg, rare earth metal (REM), and Zr is suppressed to 0.0010% or less. As mentioned above, Nb and Ti refine prior austenite (γ) grains during quenching, leading to degradation of the hardenability. As a result, 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. Furthermore, Ca, Mg, REM, and Zr increase inclusions, leading to degradation of the toughness. As is apparent from the above, the contents of these elements are preferably suppressed as small as possible, and the content of any element may be zero. In the present invention, 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.
- Next, a method for manufacturing the steel plate and the steel member of the present invention will be described. First, a method for manufacturing the steel plate will be described.
- 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. 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
- By setting the heating temperature of quenching at 910 to 940°C and setting the heat holding time at 25 minutes or more, prior austenite (γ) grains can be allowed to grow to some extent, thus improving the hardenability to obtain a fine structure.
- If the heating temperature of quenching is lower than 910°C, prior austenite (γ) grains during quenching keep fine grain size. Therefore, a fine structure cannot be obtained in the portion with a low cooling rate, such as the thicknesswise central portion of the steel plate, thus failing to ensure excellent toughness. Therefore, the heating temperature of quenching is set at 910°C or higher. The heating temperature is preferably 920°C or higher. Meanwhile, if 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.
- Even when the heating temperature during quenching is in the above range, prior austenite (γ) grains keep fine grain size if the holding time at the heating temperature (heat holding time) is shorter than 25 minutes. Therefore, sufficient hardenability cannot be obtained even when a predetermined amount of B is contained, thus causing coarsening of the structure, leading to degradation of the toughness. Therefore, 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.
- It is preferred that the prior austenite (γ) grain diameter is adjusted in a range of about 50 to 100 µm by controlling the conditions during quenching as mentioned above, because it is easy to obtain a fine structure.
- Subsequently to the above quenching, tempering is performed at a temperature of 620°C or higher and an Ac1 point or lower under a condition of the heating temperature and a heating time such that a PT value represented by the following equation (1) is 19.2 or more and 20.6 or less:
- In the above quenching, because of a high cooling rate in the vicinity of a surface layer regardless of the thickness, a hardness of the surface layer easily increases. Therefore, after quenching, 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 Ac1 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. Meanwhile, if the tempering temperature exceeds the Ac1 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 Ac1 point or lower. The tempering temperature is preferably 750°C or lower. The Ac1 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 PT value represented by the defined equation (1) is within the above ranges. If the PT value is less than 19.2, the hardness increases excessively, causing defects such as degradation of the workability. Therefore, the PT value is 19.2 or more, preferably 19.3 or more, and more preferably 19.4 or more. Meanwhile, if the PT value exceeds 20.6, coarsening of carbide occurs, causing degradation of properties such as toughness. Therefore, the PT 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.
- 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 PPWHT 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 PPWHT value is 20.3). In the present invention, even after subjected to a heat treatment under such severe condition at high temperature for a long time, a steel member having sufficiently excellent toughness can be obtained. The upper limit of the PPWHT value is approximately 21. The condition of PWHT includes, for example, the condition of a heating temperature of 600 to 690°C and heating time of 5 hours to 22 hours:
- 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.
- The present disclosure will be more specifically described below by way of Examples but is not limited to the following Examples. Various modifications can be made to these examples as long as they are adaptable to the above-mentioned and below-mentioned concepts and are included within the technical scope of the present disclosure.
- 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 Ac1 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.
- Furthermore, using a truck-type electric furnace in an air atmosphere, 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. In this case, the PPWHT 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.
- When the steel member is manufactured, the steel plate is subjected to PWHT after welding. After multilayer welding was performed as the 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.
- Using the specimen thus obtained, evaluation of metal structure, a tensile test, and a Charpy impact test were carried in accordance with the following procedures. To evaluate the workability of the steel plate, which is the property required in the manufacturing process of a steel member, surface layer hardness was measured using a steel plate before subjecting to PWHT.
- The metal structure was observed in the following manner.
- (1) To enable observation of a thickness cross section including front and rear surfaces of a steel plate, which is parallel to the rolling direction and is perpendicular to a surface of the steel plate, samples were taken from the steel plate.
- (2) Using a method of polishing such as polishing with a wet emery polishing paper (#150 to #1000) or polishing with an abrasive such as diamond slurry having the same function as that of the above polishing, mirror finishing of an observation surface was performed.
- (3) The polished sample was etched with a 3% nital solution, thereby allowing crystal grain boundaries to appear.
- (4) In the t (thickness)/2 portion, the structure appeared was photographed at a magnification of 400 times. In the present example, photographing was performed as a micrograph in size of 6 cm × 8 cm. In the thus obtained micrograph, the region where polygonal ferrite is formed on prior austenite (γ) grain boundaries was discriminated, followed by filling with black color. Next, the micrograph was captured in an image analyzer. In the case of a magnification of 400 times, the region of the micrograph corresponds to 150 µm × 200 µm. In the case of any magnification, the micrograph was captured in an image analyzer such that the total of the region is 1 mm × 1 mm or more. In the case of 400 times, at least 35 micrographs were captured. (5) In the image analyzer, a black area ratio was calculated every micrograph and an average of all micrographs was regarded as a polygonal ferrite (F) fraction, and the remainder after deducting the polygonal ferrite (F) fraction from the whole area ratio was regarded as a fraction of at least one of tempered bainite and tempered martensite (B+M).
- 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.
- Using an EBSP method, 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.
- (1) To enable observation of a thickness cross section including front and rear surfaces of a steel plate, which is parallel to the rolling direction and is perpendicular to a surface of the steel plate, samples were taken from the steel plate.
- (2) Using a method of polishing with a wet emery polishing paper (#150 to #1000) or polishing having the same function as that of the above polishing (polishing with an abrasive such as diamond slurry), mirror finishing of an observation surface was performed.
- (3) Using an EBSP apparatus manufactured by TexSEM Laboratories Inc., the size of crystal grains (large angle grains) surrounded by crystal grain boundaries was measured under the assumption of regarding, as a crystal grain boundary, boundary in which the crystal misorientation is 15° or more in the t (thickness)/2 portion of the thickness direction within a measuring range of 200 × 200 µm and 0.5 µm pitch. Data of measurement points having a confidence index, indicating the reliability of measurement orientation, of less than 0.1 were excluded from objects to be analyzed.
- (4) An average of the thus obtained size of crystal grains surrounded by large angle grain boundaries was calculated and regarded as 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" in the present invention. Crystal grains having grain sizes of 1.0 µm or less surrounded by large angle grain boundaries were determined as measurement noise and were excluded from the calculation of average grain size.
- The size of grain boundary carbide was measured as follows.
- (1) To enable observation of a thickness cross section including front and rear surfaces of a steel plate, which is parallel to the rolling direction and is perpendicular to a surface of the steel plate, samples were taken from the steel plate.
- (2) Using a method of polishing with a wet emery polishing paper (#150 to #1000) or polishing having the same function as that of the above polishing (polishing with an abrasive such as diamond slurry), mirror finishing of an observation surface was performed.
- (3) The polished sample was etched with a 3% nital solution, thereby allowing crystal grain boundaries to appear.
- (4) In the t (thickness)/2 portion, the structure appeared was photographed at a magnification of 1,000 times. In the present example, photographing was performed as a micrograph in size of 6 cm × 8 cm. Next, the micrograph was captured in an image analyzer. In the case of a magnification of 1,000 times, the region of the micrograph corresponds to 60 µm × 80 µm. The micrograph was captured in an image analyzer such that the total of the region is 0.4 mm × 0.4 mm or more. In the case of 1,000 times, at least 35 micrographs were captured.
- (5) In the image analyzer, the short axis length was calculated as the size of grain boundary carbide every micrograph, and a maximum value of the grain boundary carbide size of all micrographs was calculated.
- 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.
- Full-sized V-notched specimens were taken in the direction perpendicular to the rolling direction from the t (thickness) /2 portion and a Charpy impact test was performed at a test temperature of -38°C in accordance with the procedure of ASTMA370, and then the Charpy absorbed energy was measured. An average of Charpy absorbed energy of three specimens was employed. The case where the Charpy absorbed energy at -38°C, vE-38, is 100 J or more was rated as excellent toughness, i.e. excellent impact properties.
- To evaluate the workability of a steel plate, using the steel plate before subjecting to PWHT, a Brinell hardness test was performed in a position at a depth of 1 mm from a surface in accordance with the procedure of ASTM A370. The case where the average of HBW is 200 or less was rated as excellent workability, while the case where the average of HBW exceeds 200 was rated as ordinary workability.
- These results are shown in Table 3-1 and Table 3-2. The following Nos. indicate test Nos. of Table 2-1, Table 2-2, Table 3-1, and Table 3-2.
[Table 1-1] Symbol of steel type Composition (% by mass) Balance being iron and inevitable impurities C Si Mn P s Al Cu Ni Cr Mo V Nb Ti B Total of Ca, Mg, REM, and Zr N A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A1 0.139 0.57 0.48 0.0035 0.0005 0.061 0.17 0.21 1.43 0.63 0.009 0 0 0.0007 0 0.0031 A2 0.135 0.50 0.46 0.0015 0.0005 0.058 0.15 0.19 1.40 0.58 0.003 0 0 0.0007 0 0.0047 A3 0.139 0.55 0.47 0.0015 0.0005 0.057 0.17 0.22 1.45 0.61 0.007 0 0 0.0006 0 0.0040 A4 0.143 0.59 0.47 0.0015 0.0005 0.056 0.18 0.24 1.50 0.64 0.007 0 0 0.0007 0 0.0047 A5 0.139 0.55 0.47 0.0050 0.0021 0.058 0.15 0.29 1.40 0.60 0.027 0 0 0.0008 0 0.0047 A6 0.140 0.75 0.61 0.0050 0.0021 0.058 0.15 0.26 1.40 0.60 0.027 0 0 0.0007 0 0.0049 A7 0.140 0.55 0.46 0.0050 0.0022 0.057 0.15 0.21 1.41 0.60 0 0 0 0.0006 0 0.0049 A8 0.137 0.54 0.46 0.0050 0.0005 0.057 0.10 0.15 1.39 0.59 0 0 0 0.0007 0 0.0048 A9 0.140 0.55 0.47 0.0050 0.0033 0.056 0.10 0.16 1.40 0.60 0 0 0 0.0006 0 0.0046 A10 0.141 0.80 0.46 0.0050 0.0010 0.057 0.15 0.25 1.40 0.60 0.027 0 0 0.0008 0 0.0047 A11 0.134 0.67 0.59 0.0050 0.0010 0.055 0.10 0.16 1.40 0.45 0 0 0 0.0005 0 0.0050 A12 0.141 0.55 0.63 0.0050 0.0014 0.055 0.20 0.29 1.05 0.60 0 0 0 0.0005 0 0.0054 A13 0.140 0.54 0.46 0.0015 0.0020 0.057 0.15 0.25 1.39 0.60 0.027 0 0 0.0006 0 0.0051 A14 0.139 0.55 0.46 0.0050 0.0021 0.068 0.10 0.16 1.40 0.60 0 0 0 0.0007 0 0.0048 A15 0.114 0.55 0.46 0.0015 0.0005 0.058 0.17 0.22 1.45 0.61 0.006 0 0 0.0006 0 0.0049 A16 0.134 0.55 0.46 0.0015 0.0005 0.058 0.17 0.22 1.43 0.62 0.005 0 0 0.0006 0 0.0048 A17 0.149 0.55 0.47 0.0015 0.0005 0.058 0.17 0.22 1.44 0.62 0.006 0 0 0.0006 0 0.0048 A18 0.137 0.54 0.47 0.0050 0.0005 0.060 0.17 0.21 1.44 0.60 0.005 0 0 0.0007 0 0.0047 [Table 1-2] Symbol of steel type Composition (% by mass) Balance being iron and inevitable impurities C Si Mn P S Al Cu Ni Cr Mo V Nb Ti B Total of Ca, Mg, REM, and Zr N A21 0.139 0.55 0.45 0.0015 1 0.0005 0.040 0.17 0.21 1.44 0.62 0.006 0 0 0.00031 0 0.0055 A22 0.138 0.55 0.41 0.0015 0.0005 0.048 0.17 0.21 1.44 0.62 0.006 0 0 0.0007 0 0.0055 A23 0.141 0.55 0.45 0.0015 0.005 0.070 0.17 0.21 1.44 0.62 0.006 0 0 0.0007 0 0.0055 A24 0.143 0.55 0.45 0.0015 0.0005 0.078 0.17 0.21 1.44 0.62 0.006 0 0 0.0007 0 0.0055 A25 0.140 0.55 0.47 0.0050 0.0018 0.056 0.08 0.16 1.39 0.60 0 0 0 0.0007 0 0.0043 A26 0.141 0.56 0.47 0.0050 0.0020 0.052 0.10 0.16 1.40 0 0 0 0 0.0008 0 0.0045 B1 0.157 0.56 0.47 0.0050 0.0019 0.055 0.10 0.16 1.40 0.60 0 0 0 0.0008 0 0.0044 B2 0.139 0.55 0.46 0.0050 0.0026 0.056 0.15 0.26 1.40 0.60 0.027 0 0 0 0 0.0047 B3 0.111 0.56 0.47 0.0050 0.0018 0.060 0.15 0.21 1.40 0.60 0.028 0.011 0 0.0007 0 0.0048 B4 0.109 0.55 0.46 0.0050 0.0016 0.057 0.15 0.21 1.40 0.60 0.028 0 0 0.0007 0 0.0047 B5 0.080 0.55 0.47 0.0050 0.0014 0.054 0.10 0.16 1.40 0.60 0 0 0 0.0007 0 0.0050 B6 0.140 0.74 0.61 0.0050 0.0019 0.057 0.15 0.26 1.39 0.60 0.027 0 0 0 0 0.0046 B7 0.109 0.54 0.47 0.0050 0.0019 0.054 0.10 0.16 1.39 0.59 0 0 0 0.0006 0 0.0049 B8 0.077 0.55 0.46 0.0050 0.0013 0.055 0.09 0.16 1.40 0.59 0 0 0 0 0 0.0040. B9 0.137 0.55 0.46 0.0050 0.0018 0.057 0.10 0.26 1.40 0.60 0 0 0.014 0.0006 0 0.0046 B10 0.138 0.55 0.47 0.0050 0.0017 0.056 0.10 0.16 1.41 0.60 0 0.010 0 0.0006 0 0.0048 B11 0.141 0.55 0.47 0.0150 0.0013 0.056 0.10 0.16 1.40 0.60 0 0 0 0.007 0 0.0043 B12 0.138 0.55 0.46 0.0150 0.0019 0.058 0.10 0.16 1.40 0.60 0 0 0 0 0 0.0048 B13 0.139 0.55 0.46 0.0015 0.0018 0.055 0.10 0.15 1.40 0.60 0 0 0 0 0 0.0044 B14 0.138 0.55 0.46 0.0050 0.0019 0.041 0.10 0.16 1.40 0.59 0 0 0 0.0002 0 0.0053 B15 0.080 0.55 0.46 0.0050 0.0025 0.059 0.15 0.21 1. 40 0.60 0.027 0 0 0.0009 0 0.0044 B16 0.160 0.55 0.46 0.0050 0.0012 0.059 0.40 0.40 1.40 0.60 0.027 0 0 0.0006 0 0.0047 [Table 2-1] Test No. Symbol of steel type Ac1 point Thickness (mm) Quenching Tempering Temperature (°C) Time (min.) Temperature (°C) Time (min.) PT value 1 A1 774 35 930 25 760 25 20.27 2 A1 774 63 930 25 760 50 20.58 3 A1 774 63 930 30 730 30 19.76 4 A1 774 94 930 35 724 12 19.24 5 A1 774 94 930 35 760 20 20.17 6 A1 774 94 930 35 710 20 19.19 7 A1 774 94 915 55 730 25 19.68 8 A1 774 94 930 25 730 25 19.68 9 A1 774 94 930 55 730 25 19.68 10 A1 774 94 930 5 730 25 19.68 11 A1 774 94 930 35 760 70 20.73 12 A2 772 93 930 30 730 25 19.68 13 A3 774 93 930 30 730 25 19.68 14 A4 776 93 930 30 730 25 19.68 15 A5 772 93 930 30 730 25 19.68 16 A6 775 93 930 30 730 25 19.68 17 A7 773 93 930 30 730 25 19.68 18 A8 774 93 930 30 730 25 19.68 19 A9 774 93 930 30 730 25 19.68 20 A10 778 93 930 30 730 25 19.68 21 A11 775 93 930 30 730 25 19.68 22 A12 767 93 930 30 730 25 19.68 23 A13 772 93 930 30 730 25 19.68 24 A14 774 93 930 30 730 25 19.68 25 A15 774 93 930 30 730 35 19.83 26 A16 774 93 930 30 730 25 19.68 [Table 2-2] Test No. Symbol of steel type Ac1 point Thickness (mm) Quenching Tempering Temperature (°C) Time (min.) Temperature (°C) Time (min.) PT value 27 A17 774 93 930 30 730 25 19.68 28 A18 774 93 930 30 730 25 19.68 31 A21 774 93 930 30 730 25 19.68 32 A22 775 93 930 30 730 25 19.68 33 A23 774 93 930 30 730 25 19.68 34 A24 774 93 930 30 730 25 19.68 35 A25 774 93 930 30 730 25 19.68 36 A26 774 93 930 30 760 25 20.27 37 B1 774 93 930 30 730 25 19.68 38 B2 773 93 930 30 730 25 19.68 39 B3 773 93 930 30 730 25 19.68 40 B4 773 93 930 30 730 25 19.68 41 B5 774 93 930 30 730 25 19.68 42 B6 774 93 930 30 730 25 19.68 43 B7 773 93 930 30 730 25 19.68 44 B8 774 93 930 30 730 25 19.68 45 B9 773 93 930 30 730 25 19.68 46 B10 774 93 930 30 730 25 19.68 47 B11 774 93 930 30 730 25 19.68 48 B12 774 93 930 30 730 25 19.68 49 B13 774 93 930 30 730 25 19.68 50 B14 774 93 930 30 730 25 19.68 51 B15 773 93 930 30 730 25 19.68 52 B16 771 93 930 30 730 25 19.68 [Table 3-1] Test No. Structure Effective grain size D (µm) Maximum carbide size d (µm) D/d HBW Ave YS (MPa) TS (MPa) vE-38 (J) B+M (area %) F (area %) 1 100 0 12.0 0.50 24.00 190 414 552 439 2 100 0 12.0 0.70 17.14 178 398 545 120 3 100 0 12.0 0.50 24.00 185 370 554 422 4 100 0 15.0 0.50 30.00 197 412 573 188 5 100 0 16.0 0.60 26.67 180 384 561 171 6 100 0 14.0 0.40 35.00 216 412 575 209 7 100 0 16.0 0.50 32.00 185 398 566 149 8 100 0 16.0 0.50 32.00 190 418 564 377 9 100 0 16.0 0.60 26.67 189 395 561 251 10 90 10 35.0 0.50 70.00 180 386 545 56 11 100 0 16.0 0.85 18.82 168 306 510 22 12 100 0 26.0 0.50 52.00 181 381 546 255 13 100 0 21.6 0.50 43.20 192 386 555 269 14 100 0 19.0 0.50 38.00 188 389 564 320 15 100 0 18.0 0.50 36.00 190 396 558 420 16 100 0 15.3 0.42 36.43 199 408 579 178 17 100 0 17.8 0.50 35.60 186 409 570 215 18 100 0 24.0 0.50 48.00 175 405 562 337 19 100 0 23.5 0.50 47.00 181 384 548 128 20 100 0 18.0 0.50 36.00 192 407 584 202 21 100 0 26.0 0.50 52.00 186 396 558 168 22 100 0 24.0 0.50 48.00 182 387 564 165 23 100 0 20.0 0.50 40.00 186 393 557 277 24 100 0 23.0 0.50 46.00 181 391 548 165 25 100 0 24.0 0.45 53.33 184 369 537 256 26 100 0 21.0 0.50 42.00 188 374 546 348 [Table 3-2] Test No. Structure Effective grain size D (µm) Maximum carbide size d (µm) D/d HBW Ave YS (MPa) TS (MPa) vE-38 (J) B+M (area %) F (area %) 27 100 0 18.5 0.50 37.00 192 390 566 259 28 100 0 21.0 0.50 42.00 182 370 548 277 31 100 0 18.0 0.50 36.00 181 374 548 346 32 100 0 20.0 0.50 40.00 182 380 553 337 33 100 0 21.0 0.50 42.00 181 380 554 410 34 100 0 19.0 0.50 38.00 188 397 565 356 35 100 0 22.6 0.50 45.20 189 389 555 146 36 100 0 22.0 0.60 36.67 173 364 535 109 37 100 0 35.0 0.60 58.33 201 395 564 86 38 100 0 37.9 0.50 75.80 185 380 544 8 39 100 0 40.9 0.50 81.80 185 424 560 9 40 100 0 31.6 0.50 63.20 182 391 534 28 41 70 30 43.8 0.40 109.50 172 347 497 34 42 100 0 40.2 0.50 80.44 193 389 567 13 43 100 0 45.0 0.50 90.00 185 368 521 14 44 100 0 45.0 0.50 90.00 171 327 487 56 45 100 0 42.0 0.50 84.00 199 397 554 11 46 80 20 43.0 0.50 86.00 211 413 567 12 47 100 0 20.0 0.50 40.00 181 391 556 56 48 100 0 42.0 0.50 84.00 182 373 545 9 49 100 0 45.0 0.50 90.00 177 367 538 8 50 100 0 30.0 0.50 60.00 180 380 539 25 51 100 0 25.0 0.40 62.50 170 372 512 11 52 100 0 14.0 0.60 23.33 198 429 595 91 - 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.
- Meanwhile, regarding examples other than the above, either 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 PT 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 PT 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 (γ) 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 (γ) 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 inFig. 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.
Claims (4)
- A steel member having a composition comprising, in % by mass:C: 0.110% or more and 0.15% or less,Si: 0.50% or more and 0.80% or less,Mn: 0.40% or more and 0.65% or less,P: exceeding 0% and 0.0060% or less,S: exceeding 0% and 0.0070% or less,Al: 0.030% or more and 0.080% or less,Cu: 0.05% or more and 0.20% or less,Ni: 0.05% or more and 0.30% or less,Cr: 1.05% or more 1.50% or less,Mo: 0.45% or more and 0.65% or less,N: 0.0030% or more and 0.0070% or less,B: 0.0003% or more and 0.0010% or less, andV: 0% or more and 0.030% or less,wherein Nb content is suppressed to 0.005% or less, Ti content is suppressed to 0.001% or less, and the total content of Ca, Mg, REM, and Zr is suppressed to 0.0010% or less, the remainder consists of iron and inevitable impurities,a thickness is 100 mm or less, anda structure in a thicknesswise central portion of the steel member satisfies the following (a) and (b), and Charpy absorbed energy at -38°C is 100 J or more:(a) the structure is at least one of tempered bainite and tempered martensite, and(b) 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.
- A steel plate used for manufacturing the steel member according to claim 1, which has a composition comprising, in % by mass,C: 0.110% or more and 0.15% or less,Si: 0.50% or more and 0.80% or less,Mn: 0.40% or more and 0.65% or less,P: exceeding 0% and 0.0060% or less,S: exceeding 0% and 0.0070% or less,Al: 0.030% or more and 0.080% or less,Cu: 0.05% or more and 0.20% or less,Ni: 0.05% or more and 0.30% or less,Cr: 1.05% or more 1.50% or less,Mo: 0.45% or more and 0.65% or less,N: 0.0030% or more and 0.0070% or less,B: 0.0003% or more and 0.0010% or less, andV: 0% or more and 0.030% or less,wherein Nb content is suppressed to 0.005% or less, Ti content is suppressed to 0.001% or less, the total content of Ca, Mg, REM, and Zr is suppressed to 0.0010% or less, and the remainder consists of iron and inevitable impurities, and wherein a thickness of the steel plate is 100 mm or less.
- A method for manufacturing the steel plate according to claim 2, which comprises hot-rolling a steel slab having the composition according to claim 2; performing quenching under a condition of a heating temperature of 910°C or higher and 940°C or lower and a holding time at the heating temperature of 25 minutes or more and 55 minutes or less; and, after the quenching, performing tempering at a heating temperature of 620°C or higher and an Ac1 point or lower under a condition of the heating temperature and a heating time such that a PT value represented by the following equation (1) is 19.2 or more and 20.6 or less:
- A method for manufacturing the steel member according to claim 1, which comprises welding using the steel plate according to claim 2; and performing a post weld heat treatment under a condition of a heating temperature and a heating time such that a PPWHT value represented by the following equation (2) is 20 or more:
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