WO2010090349A1 - 耐座屈性能及び溶接熱影響部靭性に優れた低温用高強度鋼管およびその製造方法 - Google Patents
耐座屈性能及び溶接熱影響部靭性に優れた低温用高強度鋼管およびその製造方法 Download PDFInfo
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
- B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
- B21C37/08—Making tubes with welded or soldered seams
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
- B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
- B23K35/24—Selection of soldering or welding materials proper
- B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
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- B23K9/00—Arc welding or cutting
- B23K9/02—Seam welding; Backing means; Inserts
- B23K9/025—Seam welding; Backing means; Inserts for rectilinear seams
- B23K9/0253—Seam welding; Backing means; Inserts for rectilinear seams for the longitudinal seam of tubes
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- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K9/00—Arc welding or cutting
- B23K9/18—Submerged-arc welding
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K9/00—Arc welding or cutting
- B23K9/23—Arc welding or cutting taking account of the properties of the materials to be welded
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- 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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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- 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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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
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- C—CHEMISTRY; METALLURGY
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/50—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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
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- 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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/04—Tubular or hollow articles
- B23K2101/06—Tubes
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2103/00—Materials to be soldered, welded or cut
- B23K2103/02—Iron or ferrous alloys
- B23K2103/04—Steel or steel alloys
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- C—CHEMISTRY; METALLURGY
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
- Y10T428/12958—Next to Fe-base component
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
- Y10T428/12958—Next to Fe-base component
- Y10T428/12965—Both containing 0.01-1.7% carbon [i.e., steel]
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12861—Group VIII or IB metal-base component
- Y10T428/12951—Fe-base component
- Y10T428/12972—Containing 0.01-1.7% carbon [i.e., steel]
Definitions
- the present invention relates to a high strength steel pipe having a strength of API (American Petroleum Institute) X100 grade, and in particular, an earthquake zone with a ground thickness of about 20 mm to 40 mm and a severe ground deformation. ) And buckling resistance suitable for steel pipes for transporting natural gas and crude oil used in the permafrost region and welded heat affected zone toughness It is related with the thing excellent in (toughness).
- the high-strength steel pipes according to the present invention satisfy all the standards of APIX 100 class, and also have the tensile strength of APIX 100 class and other characteristics adjusted outside the API standard range. included.
- Non-Patent Document 3 and Patent Document 1 include a seam weld heat-affected zone near a fusion line due to diffusion of B contained in a weld metal into a base metal in a seam weld zone of a steel pipe. It has also been shown to improve toughness.
- island martensite which is harmful to toughness even when the prior austenite grain size, which is slightly away from the melting line, is as small as 150 ⁇ m or less.
- Upper bainite structure containing a large amount of Martensite-Austenite Constituent, which may lead to a decrease in toughness.
- Patent documents 2 to 5 relate to a high-strength welded steel pipe and a method for producing the same, and in all cases, when adding B to the base metal component, an appropriate amount is added in consideration of the toughness of the weld heat affected zone. ing. Further, Patent Documents 4 and 5 propose that different parameter formulas (parameter formulas) are used depending on whether or not B is added when the alloy amount of the base material is appropriate.
- Patent Document 2 discloses a technique for improving the toughness of a weld heat affected zone, but does not discuss the deformation performance of the base material.
- Patent Document 3 is intended for the X80 class, and the target strength level is different from the present invention.
- Patent Documents 4 and 5 stipulate uniform elongation in a tensile test in the pipe axis direction of the base metal part, but have a buckling resistance as described later. In order to improve, it is important to control the ratio of 0.5% proof strength (yield ratio (YR)) with respect to the tensile strength to a low level.
- welded steel pipes such as UOE steel pipes and ERW steel pipes used in line pipes are formed by cold forming the steel sheet into a tubular shape, and welding the butt portion.
- the present invention clarifies the effect of B addition on the weldability and the toughness of the heat affected zone of the base steel plate used for the APIX100 grade thick steel pipe, and the tensile strength is 760 MPa or more and 930 MPa or less, Welding at ⁇ 30 ° C. while having a uniform elongation of 5% or more and a base material performance of yield ratio (YR: Yield ratio) of 85% or less with respect to tensile strength
- Yield ratio yield ratio
- the present invention further aims to provide a high-strength welded steel pipe having strength characteristics and deformation performance equivalent to those described above even in a steel pipe after coating treatment in consideration of buckling resistance performance after coating treatment. To do.
- HAZ Heat Affected Zone
- LBZ Local Brittle Zone
- HAZ Coarse-grain HAZ
- ICHGHAZ Inter-critically Coarse-Grain-Grain
- the root portion refers to the vicinity of the meeting portion where the inner surface weld metal and the outer surface weld metal cross.
- the CGHAZ microstructure is divided into the lower bainite structure, the upper bainite containing a large amount of hard phase MA, and high-strength martensite. Toughness is improved by making the structure mainly composed of lower bainite below the rate. In particular, when the lower bainite has a structure in which at least 50% of the area fraction is secured, the toughness is most improved and the Charpy absorbed energy at ⁇ 30 ° C. is greatly improved.
- the P CM is .19 to .25% of the component composition APIX100 class base material strength can be ensured, a suitable B amount in the range is 5 ⁇ 15 ppm.
- the ratio of the 0.5% yield strength to the tensile strength is 85% or less, and the uniform elongation is 5% or more.
- Coarse island martensite structure promotes the generation and propagation of fracture, and in order to ensure the desired low temperature toughness, the structure size of island martensite and tempered martensite is controlled with high precision (control). It is important to.
- DWTT Drop Weight Tear Test
- the ductile fracture surface ratio at 20 ° C is correlated with the size of island martensite, and the Charpy absorbed energy of the base material is the bainitic nature of island martensite and base metal. Correlation is observed with the size of the ferrite (bainitic ferrite). 8).
- the strain aging resistance is improved, and excellent buckling resistance can be ensured even after the coating treatment. For this purpose, it is important to control the area fraction of island martensite with high accuracy.
- the present invention has been made by further study based on the above knowledge, that is, the present invention, 1.
- the component composition of the base material is mass%, C: more than 0.03%, 0.08% or less, Si: 0.01 to 0.5%, Mn: 1.5 to 3.0%, P: 0 .015% or less, S: 0.003% or less, Al: 0.01 to 0.08%, Nb: 0.005 to 0.025%, Ti: 0.005 to 0.025%, N: 0.0.
- the component composition of the weld metal of seam welding is mass%, C: 0.03-0.10%, Si: 0.5% or less, Mn: 1.5-3.0%, P: 0.015%
- the balance consists of weld metal parts with Fe and inevitable impurities, The microstructure of the weld heat affected zone where the prior austenite grain size is 50 ⁇ m or more
- the main axis is a bainite structure containing island martensite having an area ratio of 4% or more and 12% or less.
- the chemical composition of the base metal part and / or the weld metal part is, by mass%, Ca: 0.0005 to 0.01%, REM: 0.0005 to 0.02%, Zr: 0.0005 to 0.00.
- the steel having the base material component described in 7.1 or 5 is heated to a temperature of 1000 to 1300 ° C., and the cumulative rolling reduction exceeding 950 ° C. is 10% or more and the accumulation is 750 ° C. or less.
- accelerated cooling is performed to a temperature of 450 ° C. or higher and lower than 650 ° C. at a cooling rate of 10 ° C./s or higher.
- Buckling resistance and welding characterized by reheating to 500 to 750 ° C.
- the steel sheet obtained by the manufacturing method described in 9.7 or 8 is formed into a cylindrical shape, and the welding heat input of each of the inner and outer surfaces when the butt portion is welded layer by layer from the inner and outer surfaces is 80 kJ / cm or less, Production of high strength welded steel pipe for low temperature excellent in buckling resistance and toughness of weld heat affected zone, characterized in that heat input balance on outer surface side and inner surface side satisfies following formula (3) Method. Inner surface heat input ⁇ Outer surface heat input (3) 10. 10.
- the composition of the base metal constituting the steel pipe, the base metal microstructure and the tensile strength characteristics, the constituent composition of the weld metal in the seam weld of the steel pipe, and the old austenite near the fusion line in the longitudinal seam weld of the steel pipe The microstructure of the region where the particle size is 50 ⁇ m or more is defined.
- C 0.03% or more and 0.08% or less C is a solid solution in a supersaturation in a low-temperature transformation structure such as a martensite structure or an island martensite structure in a second phase ( (Solid solution) contributes to an increase in strength. In order to obtain this effect, it is necessary to add more than 0.03%. However, if added over 0.08%, the hardness of the circumferential welded portion of the steel pipe is remarkably increased and cold cracking is likely to occur. Therefore, the upper limit is made 0.08%. In order to secure the amount of island martensite that is a hard phase necessary for controlling the yield ratio to be low, 0.05% or more is preferably added.
- Si 0.01 to 0.5% Si is an element that acts as a deoxidizing agent and increases the strength of the steel by solid solution strengthening. However, if it is less than 0.01%, there is no effect, and if added over 0.5%, toughness Is significantly reduced, so the upper limit is made 0.5%. More preferably, it is 0.01 to 0.2%. By suppressing to 0.2% or less, it becomes possible to suppress the formation of island martensite (MA) contained in the upper bainite structure in the CGHAZ structure of the steel pipe seam welded portion, and to improve the joint HAZ toughness Can do.
- MA island martensite
- an upper limit shall be 0.2%.
- Mn acts as a hardenability improving element.
- the effect can be obtained by addition of 1.5% or more, but in the continuous casting process, the concentration rises at the center segregation part, and if it exceeds 3.0%, it causes delayed fracture at the center segregation part. Therefore, the upper limit is made 3.0%. More preferably, it is 1.6 to 2.5%.
- Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained with addition of 0.01% or more, but if added over 0.08%, the cleanliness in the steel is lowered and the toughness is deteriorated, so the upper limit is 0.08%. And More preferably, it is 0.02 to 0.06%.
- Nb 0.005 to 0.025%
- Nb has an effect of expanding the austenite non-recrystallized region during hot rolling, and 0.005% or more is added to make the non-recrystallized region 950 ° C. or less.
- the Charpy absorbed energy is significantly impaired among the toughness of HAZ and the toughness of the base material, so the upper limit is made 0.025%. More preferably, it is 0.010 to 0.025%.
- Ti forms nitrides and is effective in reducing the amount of solute N in the steel.
- the precipitated TiN suppresses the coarsening of austenite grains by the pinning effect, and contributes to the improvement of the toughness of the base material and HAZ. Addition of 0.005% or more is necessary to obtain the pinning effect, but if added over 0.025%, carbides are formed, and the toughness is significantly deteriorated by precipitation hardening. Is 0.025%. More preferably, it is 0.008 to 0.020%.
- N 0.001 to 0.010%
- N usually exists as an inevitable impurity in steel, but TiN is formed by addition of Ti.
- TiN is formed by addition of Ti.
- it is necessary to be present in the steel in an amount of 0.001% or more, but if it exceeds 0.010%, 1450 in the vicinity of the weld, particularly in the vicinity of the weld bond. Since TiN decomposes in a region heated to a temperature higher than or equal to 0 ° C. and the adverse effect of solute N is significant, the upper limit is made 0.010%. More preferably, it is 0.002 to 0.005%.
- B 0.0003 to 0.0020%
- B is an element that plays an important role in the present invention. Since the steel according to the present invention contains B, generation of polygonal ferrite is suppressed. For this reason, compared to steel not containing B, it becomes possible to carry out austenite zone rolling even at a lower temperature range, and as a result, the toughness evaluated in the DWTT test or the like is improved. In addition, B segregates at the austenite grain boundary in the heat affected zone, has the effect of improving hardenability, suppresses the formation of upper bainite containing MA harmful to toughness, and facilitates the formation of lower bainite or martensite. To do.
- This effect is significant when added in an amount of 0.0003% or more and 0.0020% or less, and if added over 0.0020%, the toughness of both the base metal and the weld heat affected zone decreases due to precipitation of B-based carbides.
- the upper limit is made 0.0020%.
- a preferable range is 0.0005% or more and 0.0015% or less. More preferably, it is 0.0007 to 0.0012%.
- Cu, Ni, Cr, Mo, V, one or more of Cu, Ni, Cr, Mo, V all act as a hardenability improving element, so for the purpose of increasing the strength, one of these elements, Or two or more of them are added.
- Cu 0.01 to 1% Cu contributes to the hardenability improvement of steel by adding 0.01% or more. However, since addition of 1% or more causes toughness deterioration, the upper limit is set to 1%, and when Cu is added, 0.01 to 1%. More preferably, it is 0.1 to 0.5%.
- Ni 0.01 to 1% Ni contributes to improving the hardenability of steel by adding 0.01% or more. In particular, even if added in a large amount, it does not cause toughness deterioration, so it is effective for toughening. However, since it is an expensive element, when Ni is added, the upper limit is 1%, and when Ni is added, it is 0. .01 to 1%. More preferably, it is 0.1 to 0.5%.
- Cr 0.01 to 1% Cr also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if added over 1%, the toughness deteriorates, so the upper limit is made 1%, and when adding Cr, the content is made 0.01 to 1%. More preferably, it is 0.1 to 0.5%.
- Mo 0.01 to 1% Mo also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if adding over 1%, the toughness deteriorates, so the upper limit is made 1%, and when adding Mo, 0.01 to 1%. More preferably, it is 0.1 to 0.5%.
- V 0.01 to 0.1%
- V forms precipitation strengthening by forming carbonitride, and contributes especially to the softening prevention of a weld heat affected zone. This effect can be obtained by addition of 0.01% or more, but if added over 0.1%, precipitation strengthening remarkably reduces toughness, so the upper limit is made 0.1% and 0 is added when V is added. .01 to 0.1%. More preferably, it is 0.01 to 0.05%.
- O 0.005% or less
- P 0.015% or less
- S 0.003% or less
- O, P, and S are inevitable impurities and define the upper limit of the content.
- O is 0.005% or less in order to suppress the formation of inclusions that are coarse and adversely affect toughness. If the P content is large, the central segregation is remarkable and the base material toughness is deteriorated. If the content of S is large, the amount of MnS produced increases remarkably and the toughness of the base material deteriorates. More preferably, they are O: 0.003% or less, P: 0.01% or less, and S: 0.001% or less.
- the present invention 760 MPa or more in tensile strength of the base material, and, in order to achieve the above 760 MPa in joint strength, the P CM was 0.19% or more, 0.25% or less in terms of the circumferential weld ensuring the To do. More preferably, it is 0.23% or less.
- the above is the basic component composition of the base material part of the steel pipe according to the present invention.
- one or more of Ca, REM, Zr, and Mg can be added.
- Ca, REM, Zr, Mg Ca, REM, Zr, and Mg form oxysulfides or carbonitrides in steel, mainly for the purpose of suppressing the austenite grain coarsening in the weld heat affected zone by the pinning effect and improving toughness. Can be added.
- Ca 0.0005 to 0.01%
- the lower limit of Ca is 0.0005%.
- the Ca addition amount exceeds 0.01%, coarse CaO is likely to be generated, and the toughness including the base material is reduced, and the ladle nozzle blockage is caused.
- the upper limit is made 0.01%, and when added, 0.0005 to 0.01%. More preferably, it is 0.001 to 0.005%.
- REM 0.0005 to 0.02% REM forms an oxysulfide in steel, and when added in an amount of 0.0005% or more, provides a pinning effect that prevents the weld heat affected zone from becoming coarse.
- the upper limit is made 0.02%, and when added, it is made 0.0005 to 0.02%. More preferably, it is 0.001 to 0.005%.
- Zr forms carbonitrides in the steel and brings about a pinning effect that suppresses the austenite grain coarsening particularly in the weld heat affected zone.
- addition of 0.0005% or more is necessary.
- the upper limit is made 0.03%, and when added, 0.0005 to 0.03%. More preferably, it is 0.001 to 0.01%.
- Mg 0.0005 to 0.01%
- Mg is produced as fine oxides in the steel during the steelmaking process, and in particular, has a pinning effect that suppresses the coarsening of austenite grains in the weld heat affected zone.
- addition of 0.0005% or more is necessary, but if added over 0.01%, the cleanliness in the steel is lowered and the toughness is lowered.
- the upper limit is 0.01%, and when added, the content is 0.0005 to 0.01%. More preferably, it is 0.001 to 0.005%.
- composition composition of weld metal In the description of [Composition composition of weld metal],% means mass%.
- C 0.03-0.10%
- the weld metal part also needs to have a tensile strength of 760 MPa or more, and in order to obtain this strength, it is necessary to contain 0.03% or more. .
- the upper limit was made 0.10%. More preferably, it is 0.05 to 0.08%.
- Si 0.5% or less Si is useful for deoxidizing the weld metal and ensuring good workability. However, if it exceeds 0.5%, the weld workability is deteriorated. 5%. More preferably, it is 0.3% or less.
- Mn 1.5 to 3.0% Mn is an important element for increasing the strength of the weld metal. In particular, in order to make the tensile strength 760 MPa or more, it is necessary to contain 1.5% or more. However, if it exceeds 3.0%, the weldability deteriorates, so the upper limit was made 3.0%. More preferably, it is 1.6 to 2.5%.
- P 0.015% or less
- S 0.005% or less
- P and S are segregated at grain boundaries in the weld metal and deteriorate their toughness, so the upper limits were made 0.015% and 0.005%, respectively. . More preferably, they are 0.01% or less and 0.003% or less, respectively.
- Al acts as a deoxidizing element.
- the upper limit is set to 0.05%. More preferably, it is 0.03% or less.
- Nb 0.005 to 0.05%
- Nb is an element effective for increasing the strength of the weld metal.
- it is necessary to contain 0.005% or more, but if it exceeds 0.05%, the toughness deteriorates, so the upper limit was made 0.05%. More preferably, it is 0.005 to 0.04%, and still more preferably 0.005 to 0.03%.
- Ti acts as a deoxidizing element in the weld metal and is effective in reducing oxygen in the weld metal. To obtain this effect, 0.005% or more must be contained, but when it exceeds 0.03%, the excess Ti forms carbides and degrades the toughness of the weld metal. Was 0.03%. More preferably, it is 0.005 to 0.02%.
- Reduction of the solid solution N in the weld metal also has an effect of improving toughness, and the upper limit is made 0.010% because it is remarkably improved especially by setting it to 0.010% or less. More preferably, it is 0.008% or less.
- Reduction of the amount of oxygen in the weld metal has an effect of improving toughness, and the upper limit is made 0.045% because it is remarkably improved especially by setting it to 0.045% or less.
- the amount of oxygen in the weld metal is less than 0.015%, the amount of oxide effective for refining the structure of the weld metal decreases, and conversely the toughness of the weld metal deteriorates, so the lower limit was made 0.015%. . More preferably, it is 0.015 to 0.035%.
- B 0.0003 to 0.0050%
- addition of B is effective in order to make the microstructure of the weld metal a fine bainite main structure. Addition of 0003% or more and 0.0050% or less is necessary.
- a more preferable range is 0.0005 to 0.0050%, and a more preferable range is 0.0005 to 0.0030% or less. Even more preferably, it is 0.0007 to 0.0020%.
- Cu, Ni, Cr, Mo, V one or more of Cu, Ni, Cr, Mo, V, when adding one or more of Cu, Ni, Cr, Mo, V, Cu: 0.01 to 1.0%, Ni: 0.0. 01 to 2.5%, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.5%.
- V 0.1% or less
- An appropriate amount of V addition is an effective element because it increases strength without degrading toughness and weldability, and in order to exert this effect, it must contain 0.01% or more. preferable. On the other hand, if it exceeds 0.1%, the toughness of the reheated portion of the weld metal deteriorates significantly, so the upper limit was made 0.1%. More preferably, it is 0.05% or less.
- the above is the basic component composition of the weld metal part of the steel pipe according to the present invention, but when further improving the toughness of the weld metal part, one or more of Ca, REM, Zr, and Mg can be added.
- Ca, REM, Zr, Mg Ca, REM, Zr, and Mg can be added for the purpose of forming an oxysulfide or carbonitride in the steel, suppressing the austenite grain coarsening in the weld metal part by a pinning effect, and improving toughness.
- Ca 0.0005 to 0.01%
- the lower limit of Ca is 0.0005%.
- the upper limit is 0.01%, and when added, 0.0005 to 0.01% And More preferably, it is 0.001 to 0.005%.
- REM 0.0005 to 0.02% REM forms an oxysulfide in steel, and adding 0.0005% or more brings about a pinning effect that prevents coarsening of austenite grains in the weld metal part.
- the upper limit is made 0.02%, and when added, it is made 0.0005 to 0.02%. More preferably, it is 0.001 to 0.01%.
- Zr forms carbonitride in steel and brings about a pinning effect that suppresses coarsening of austenite grains in the weld metal part.
- addition of 0.0005% or more is necessary, but if added over 0.03%, the cleanliness of the weld metal part is remarkably lowered and the toughness is lowered. Therefore, the upper limit is made 0.03%, and when added, 0.0005 to 0.03%. More preferably, it is 0.001 to 0.01%.
- Mg 0.0005 to 0.01%
- Mg is formed as a fine oxide and has a pinning effect that suppresses the coarsening of austenite grains in the weld metal part.
- addition of 0.0005% or more is necessary.
- the upper limit is made 0.01%, and when added, 0.0005 to 0.01%. More preferably, it is 0.001 to 0.005%.
- Microstructure of base material In the present invention, excellent buckling resistance and a target of the Charpy impact test at ⁇ 40 ° C., the absorbed energy of 210 J or more when the plate thickness is less than 25 mm, and 150 J or more when the plate thickness is 25 mm or more are achieved. Therefore, in order to obtain excellent strain aging characteristics, it is preferable to define the microstructure of the base material. By defining the microstructure of the base material, it is possible to achieve the target ductile fracture surface ratio of 85% or more at ⁇ 20 ° C. in the DWTT test.
- the tensile strength characteristic of the base material is a round house type and an SS curve (curve) having a high work hardening coefficient (n value).
- the yield ratio (0.5% yield strength / tensile strength) is an index equivalent to the n value, and a soft phase and a hard phase are combined in order to achieve a low yield ratio of 85% or less. Organize in two phases.
- bainite is used as the soft phase
- island martensite is used as the hard phase.
- the area ratio of island martensite is preferably 4% or more.
- the bainite of the microstructure of the base material refers to bainitic ferrite in a narrow sense.
- the long axis diameter of the island martensite exceeds 2 ⁇ m, it becomes difficult to achieve a ductile fracture surface ratio of 85% or more in the DWTT test (test temperature: ⁇ 20 ° C.).
- the major axis diameter of island martensite exceeds 2 ⁇ m and the major axis diameter of bainitic ferrite surrounded by a boundary having a misorientation angle of 15 ° or more exceeds 20 ⁇ m, the thickness is less than 25 mm.
- the Charpy absorption energy at ⁇ 40 ° C. is 210 J or more and the plate thickness is 25 mm or more, it becomes difficult to achieve Charpy absorption energy at ⁇ 40 ° C. of 150 J or more.
- the area ratio of island martensite exceeds 12%, it is difficult to achieve the above-mentioned base material toughness due to refinement of the microstructure. If the area ratio of the island-shaped martensite is in the range of 4 to 12%, a yield ratio of 85% or less can be achieved.
- the area ratio of the island-like martensite is in the range of 4 to 12% means that the microstructure includes the bainite and the island-like martensite as well as the remaining structure within the allowable range as described later. This means that the area ratio of island martensite to the whole is in the range of 4 to 12%.
- the microstructure of the base steel sheet mainly composed of a bainite structure containing island martensite having an area ratio of 4% or more and 12% or less excellent strain aging characteristics can be obtained as described later.
- C is concentrated to an untransformed austenite phase by bainite transformation that occurs during accelerated cooling and subsequent reheating, and the untransformed austenite that is enriched in C becomes island-like martensite. Therefore, the amount of solute C in the bainite phase is smaller than in the case of steel of the prior art.
- the yield stress (YS) rises due to strain aging and is accompanied by a high temperature and a long thermal history in a general steel pipe coating process at 250 ° C. for 30 minutes.
- the yield ratio and the decrease in uniform elongation can be suppressed, and even if the steel according to the present invention is subjected to a thermal history that deteriorates the characteristics due to strain aging, the present invention steel has a uniform elongation of 5% or more. And, yield ratio: 85% or less can be secured.
- the microstructure of the base steel sheet is mainly composed of a bainite structure containing island martensite having an area ratio of 4% or more and 12% or less, and the major axis diameter of the island martensite contained is 2 ⁇ m or less.
- the major axis diameter of bainitic ferrite surrounded by a boundary having a misorientation angle of 15 ° or more is specified to be 20 ⁇ m or less.
- mainly a bainite structure including island-shaped martensite means that 95% or more of the entire structure is the structure, and that the remainder includes pearlite or martensite.
- the area ratio of island martensite is identified by observing at least 10 fields of view randomly with a scanning electron microscope (magnification ratio 2000 times) at the center of the plate thickness.
- the lower bainite structure in which fine cementite is precipitated in the lath is excellent in toughness while maintaining high strength, and this structure can be obtained by increasing the hardenability.
- a method by adding a component such as B or a method by increasing the cooling rate in the ⁇ - ⁇ transformation section of the weld heat affected zone by a decrease in welding heat input can be considered.
- the ratio of the most brittle structure (LBZ) is 50 ⁇ m near the melting line.
- the prior austenite grain size is 50 ⁇ m near the melting line.
- the steel having the above-described component composition is heated to a temperature of 1000 to 1300 ° C., and the cumulative reduction rate at over 950 ° C. is 10% or more, and the cumulative reduction rate at 750 ° C. or less is 75% or more.
- accelerated cooling to a temperature of 450 ° C. or higher and lower than 650 ° C. is performed at a cooling rate of 10 ° C./s or higher. Reheating to 500 to 750 ° C. above the accelerated cooling stop temperature at a speed to produce a base steel plate.
- the heating temperature, rolling end temperature, cooling end temperature, reheating temperature, and other temperatures are the average temperature of the steel sheet.
- the average temperature is obtained by calculation based on the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity.
- the cooling rate is an average cooling rate obtained by dividing the temperature difference necessary for cooling to the cooling end temperature (less than 450 to 650 ° C.) by the time required for the cooling after the hot rolling is completed.
- the heating rate is an average temperature increase rate obtained by dividing the temperature difference required for reheating up to the reheating temperature (500 to 750 ° C.) by the time required for reheating after cooling.
- each manufacturing condition will be described in detail.
- Heating temperature 1000-1300 °C
- the lower limit temperature for complete austenite is 1000 ° C.
- the upper limit was set to 1300 ° C. More preferably, it is 1000 to 1150 ° C.
- Cumulative rolling reduction above 950 ° C . Rolling in the austenite recrystallization region for 10% or more suppresses mixing of grains such as formation of coarse austenite grains. Since the effect cannot be expected when the cumulative rolling reduction is less than 10%, the cumulative rolling reduction at over 950 ° C. is set to 10% or more.
- Cumulative rolling reduction at 750 ° C. and 950 ° C. or less 20% or more Rolling on the relatively high temperature side of the austenite non-recrystallized region suppresses mixing of grains such as formation of coarse austenite grains. If the cumulative rolling reduction at 750 ° C. and 950 ° C. corresponding to this temperature range is less than 20%, the effect is small. Therefore, the cumulative rolling reduction at 750 ° C. and 950 ° C. or less is preferably 20% or more.
- Cumulative rolling reduction at 750 ° C. or lower: 75% or more A bay in which austenite grains are expanded by accumulating large pressure in this temperature region on the low temperature side of the austenite non-recrystallized region, and then transformed by accelerated cooling. Nitic ferrite and island martensite are refined and toughness is greatly improved.
- the cumulative rolling reduction at 750 ° C. or less is set to 75% or more. More preferably, it is 80% or more. It is a feature of the present invention that cumulative large pressure reduction is performed in this temperature region on the low temperature side of the austenite non-recrystallized region. As described above, since the steel according to the present invention contains B, generation of polygonal ferrite is suppressed.
- austenite non-recrystallized region spreads to a lower temperature region than steel not containing B. For this reason, even if it is simply austenite non-recrystallized zone rolling, it becomes possible to carry out austenite non-recrystallized zone rolling in a temperature range lower than that of conventional steel, so the effect of improving toughness through refinement of the structure is remarkable. It will be.
- Rolling end temperature 650 ° C. or more
- the hot rolling end temperature is less than 650 ° C.
- proeutectoid ferrite is generated from the austenite grain boundaries in the subsequent air cooling process, which causes a decrease in the base material strength.
- the lower limit temperature was set to 650 ° C. More preferably, it is 650 to 700 ° C.
- Cooling rate of accelerated cooling 10 ° C./s or more
- the microstructure needs to be a bainite-based structure. For this reason, accelerated cooling is performed after hot rolling.
- the cooling rate of accelerated cooling is set to 10 ° C./s or more. More preferably, it is 12 to 50 ° C./s.
- Cooling stop temperature of accelerated cooling less than 450 to 650 ° C. This process is an important production condition in the present invention.
- accelerated cooling is completed during the bainite transformation, that is, in a temperature range where untransformed austenite exists. Immediately after that, reheating is performed, and transformation from untransformed austenite to bainite occurs.
- the bainitic ferrite in bainite that is formed at such a relatively high temperature, the amount of C solid solution is small, so that C Discharged into untransformed austenite. Therefore, as the bainite transformation proceeds during reheating, the amount of C in the untransformed austenite increases.
- the cooling stop temperature is less than 450 ° C, it is difficult to secure sufficient untransformed austenite, and sufficient island martensite cannot be obtained during air cooling after reheating, and a low yield ratio of 85% or less is achieved. Can not.
- the cooling stop temperature is 650 ° C. or higher, C is consumed in the pearlite that precipitates during cooling and no island-like martensite is generated, so the upper limit was made less than 650 ° C.
- the temperature is more preferably 500 to 550 ° C.
- Reheating rate after stopping cooling 0.5 ° C / s or more Concentrating C in untransformed austenite by reheating immediately after accelerated cooling, and generating island martensite in the subsequent air cooling process Can do.
- reheating immediately after accelerated cooling means starting reheating at a temperature rising rate of 0.5 ° C./s or more within 3 minutes after stopping accelerated cooling.
- the rate of temperature increase is less than 0.5 ° C./s, cementite in the bainite becomes coarse and the base material toughness decreases, so the rate of temperature increase is 0.5 ° C./s or more. More preferably, it is 1.0 to 10 ° C./s.
- Reheating temperature after stopping cooling 500 ⁇ 750 °C
- the reheating temperature is less than 500 ° C., C concentration to austenite does not occur sufficiently, and the required island-like martensite area ratio cannot be ensured.
- the reheating temperature is specified to be 750 ° C. or less. Preferably, it is 700 degrees C or less. From the viewpoint of balance between strength and toughness, the temperature is more preferably 580 to 680 ° C. There is no need to set the temperature holding time at the reheating temperature.
- the cooling after reheating is preferably basically air cooling.
- reheating after accelerated cooling is performed with a radio-frequency heating apparatus arranged on the same line (inline) as the accelerated cooling apparatus, it is possible to heat immediately after the accelerated cooling. Is preferable.
- the area ratio and particle size of island martensite are controlled, the tensile strength is 760 MPa or more and 930 MPa or less, the uniform elongation is 5% or more, and the ratio of 0.5% proof stress to the tensile strength
- the ductile fracture surface ratio is 85% or more
- the Charpy absorbed energy at -40 ° C is less than 25 mm
- the plate is 210 J or more.
- the thickness is 25 mm or more, a steel sheet having a high toughness of 150 J or more can be obtained.
- the high-temperature steel pipe for low temperature excellent in buckling resistance performance and toughness of the heat affected zone according to the present invention is obtained by using a U-press and an O-press in accordance with a conventional method using a base steel plate having the above-described tensile strength characteristics.
- O-press is made into a cylindrical shape, and then manufactured by seam welding.
- Seam welding is performed by submerged arc welding on the inner and outer surfaces one after the other after tack welding, and the flux used for submerged arc welding is not particularly limited. (Fused flux) or calcined flux (baked flux) may be used. Moreover, preheating before welding or heat treatment after welding (post weld treatment: abbreviated as PWHT) is performed as necessary.
- the plate thickness P CM is from 0.19 to 0.25% of the base material steel plate in the above-mentioned composition of ingredients at about 20 mm ⁇ 40 mm, the following heat input 80 kJ / cm
- Such a structure is effective in improving the low-temperature toughness of LBZ (Local Brittle Zone) where the toughness is most deteriorated in the joint HAZ shown in FIG.
- FIG. 1A shows a Charpy test piece 1 having an outer surface FL notch
- FIG. 1B shows a Charpy test piece 3 having a root-FL notch
- the most brittle structure 4 (LBZ) at the notch position means a HAZ coarse grain 8 (CGHAZ) structure (former austenite grain size of 50 ⁇ m or more) in the vicinity of the bond 7 in the outer surface side welding, and the HAZ on the inner surface in the root part of the inner surface side welding.
- the heat input balance between the outer surface side welding and the inner surface side welding satisfies the following formula (3), the ⁇ grain coarsening of the HAZ coarse grain (CGHAZ) portion on the inner surface side can be suppressed, and the outer surface
- the joint HAZ toughness sampled from the FL (Fusion line) position on the side and the root side can be stably achieved.
- “Securing stably” means that the cumulative failure probability when the joint HAZ Charpy test is performed 100 times or more at a test temperature of ⁇ 30 ° C. or less is 1% or less.
- the lower bainite structure refers to the precipitation of carbides mainly composed of cementite in the lath of bainitic ferrite having a lath width of 1 ⁇ m or less, and the upper bainite is an island martensite (MA) and / or between the laths. Or it contains cementite.
- the hardness is 250 ⁇ HV (98N) ⁇ 350, and the joint HAZ Charpy is tested 100 times or more at a test temperature of ⁇ 30 ° C.
- Excellent weld heat-affected zone toughness with a cumulative failure probability of 1% or less when the test is carried out is achieved.
- tube expansion is performed at a tube expansion rate of 0.4% or more and 2.0% or less according to the required roundness.
- the tube expansion ratio is less than 0.4%, it is difficult to achieve the usually required roundness particularly when the plate thickness is 20 mm or more.
- the pipe expansion rate is more than 2.0%, there is a concern that the strain concentration at the bond portion at the boundary between the weld metal and the weld heat affected zone is excessively increased and the pipe expansion cracks.
- the joint characteristics may deteriorate due to excessive strain introduction. From the viewpoint of improving the roundness and securing the joint strength and toughness, it is preferably 0.5 to 1.5%.
- the microstructure of the HAZ coarse grain (CGHAZ) in the weld heat affected zone where the prior austenite grain size is 50 ⁇ m or more near the melting line is randomly located at a position 6 mm from the outer surface with a scanning electron microscope (5000 times magnification). Observe over 10 fields of view to identify.
- Steels having various chemical compositions shown in Table 1 are melted in a converter and made into slabs having a thickness of 170 to 250 mm by continuous casting, followed by hot rolling and accelerated cooling shown in Table 2.
- Steel plates 1 to 10 were produced under reheating conditions. The reheating was performed using an induction heating type heating device installed on the same line as the accelerated cooling facility.
- these steel sheets were formed by U-press and O-press, and then subjected to internal seam welding by submerged arc welding and then external seam welding. Thereafter, the tube was expanded at a tube expansion ratio of 0.6 to 1.2% to obtain a steel tube having an outer diameter of 400 to 1626 mm.
- Tables 3-1 and 3-2 show chemical compositions of the weld metal parts 6 and 5 of the inner surface seam welding and the outer surface seam welding of the steel pipes 1-1 to 10.
- a full thickness tensile test piece based on API-5L was taken from the pipe axis direction for the base metal part and from the pipe circumferential direction for the seam welded part, and subjected to a tensile test. Carried out.
- V-notch Charpy impact test specimens 1 and 3 were collected from two positions of outer surface FL and Root-FL shown in FIGS. A Charpy impact test was performed at a test temperature of ⁇ 30 ° C. The notch position 2 was a position where HAZ and weld metal exist at a ratio of 1: 1.
- HAZ toughness The test results of the hardness of HAZ coarse particles (CGHAZ) and the toughness of HAZ coarse particles (CGHAZ) (hereinafter referred to as HAZ toughness) are collectively shown in Tables 4-1 and 4-2.
- V-notch Charpy impact test piece of JIS Z2202 (1980) was collected from the center position of the thickness of the base material portion of the steel pipe, and the Charpy impact test was performed at a test temperature of ⁇ 40 ° C.
- a DWTT test piece compliant with API-5L was taken from the steel pipe and tested at a test temperature of ⁇ 20 ° C. to obtain an SA value (Share Area: ductile fracture surface ratio).
- the tensile strength of the base steel sheet is 760 MPa or more and 930 MPa or less, has a uniform elongation of 5% or more, and the ratio of 0.5% proof stress to the tensile strength is 85% or less and the test temperature in the base material is ⁇ 40 ° C. If the Charpy absorbed energy is less than 25 mm, 210 J or more, if the thickness is 25 mm or more, 150 J or more, DWTSA-20 ° C. is 85% or more, and the strength of the seam welded joint of the steel pipe is 760 MPa or more and 930 MPa or less. Charpy absorbed energy at a test temperature of ⁇ 30 ° C. in the grains (CGHAZ) of 100 J or more is within the target range of the present invention.
- Test No. 1, 2, and 3 are examples of the invention in which the base material and the welded portion satisfy the provisions of claims 1 and 4, and the desired strength, yield ratio, uniform elongation, toughness of the base material part and the seam welded part It exhibits high HAZ toughness, and in the microstructure of the base material part, it mainly comprises a bainite structure containing island martensite with an area ratio of 4% or more and 12% or less, and the major axis diameter of the contained island martensite is 2 ⁇ m or less. In addition, the major axis diameter of bainitic ferrite surrounded by a boundary having a misorientation angle of 15 ° or more was 20 ⁇ m or less.
- test no. 4, 5 and 6 have base material components within the scope of the invention described in claim 1, but the rolling reduction of 750 ° C. or less in the rolling of the steel sheet was less than 75% (see Table 2). Toughness decreased. The microstructure of the welded portion satisfies the provisions of claim 1 and good toughness is obtained.
- Test No. 7, 8, and 9 have base metal components within the scope of the invention described in claim 1, but have high welding heat input, and the lower bainite fraction in the microstructure of the HAZ coarse grain (CGHAZ) portion of the joint is described in claim 1. Since the fraction of the upper bainite structure was higher than the prescribed lower limit, the HAZ toughness was reduced in both the outer surface side and the inner surface side Root portion.
- CGHAZ coarse grain
- Test No. No. 10 was a B-free system, and because the fraction of the upper bainite structure was high, the HAZ toughness was reduced in both the outer surface side and the inner surface side Root portion.
- P CM is below the lower limit of the present invention, the tensile strength and the tensile strength of the joint of the base metal is less than 760 MPa,
- the HAZ coarse grain (CGHAZ) structure became the upper bainite structure, and the HAZ toughness decreased in both the outer surface side and the inner surface side Root portion.
- HAZ coarse (CGHAZ) structure becomes martensite, the outer surface side, HAZ toughness on the inner surface side Root portion both decreased.
- Test No. 13 is a welding heat input of 80 kJ / cm or less on both the inner surface side and the outer surface side, but the welding heat input on the inner surface side is higher than the welding heat input on the outer surface side, and the austenite grain size is large in the microstructure of the Root part. Therefore, a coarse upper bainite structure was obtained, and the HAZ toughness on the root side was lowered. [Example 2]
- Steels having various chemical compositions shown in Table 5 were melted in a converter and made into slabs having a thickness of 160 to 250 mm by continuous casting, followed by hot rolling and accelerated cooling shown in Table 6.
- Steel plates 11 to 24 were produced under reheating conditions. The reheating was performed using an induction heating type heating device installed on the same line as the accelerated cooling facility.
- these steel sheets were formed by U-press and O-press, and then subjected to internal seam welding by submerged arc welding and then external seam welding. Thereafter, the tube was expanded at a tube expansion ratio of 0.6 to 1.2% to obtain a steel tube having an outer diameter of 400 to 1626 mm.
- Tables 7-1 and 7-2 show the chemical compositions of the weld metal parts of the inner surface seam welding and outer surface seam welding of the steel pipes 11-1 to 11-24.
- a full thickness tensile test piece based on API-5L was taken from the pipe axis direction for the base metal part and from the pipe circumferential direction for the seam welded part, and subjected to a tensile test. Carried out.
- V-notch Charpy impact test specimens 1 and 3 were collected from two positions of outer surface FL and Root-FL shown in FIGS. A Charpy impact test was performed at a test temperature of ⁇ 30 ° C. The notch position 2 was a position where HAZ and weld metal exist at a ratio of 1: 1.
- HAZ toughness The test results of the hardness of HAZ coarse particles (CGHAZ) and the toughness of HAZ coarse particles (CGHAZ) (hereinafter referred to as HAZ toughness) are collectively shown in Tables 8-1 and 8-3.
- V-notch Charpy impact test piece of JIS Z2202 (1980) was collected from the center position of the thickness of the base material portion of the steel pipe, and the Charpy impact test was performed at a test temperature of ⁇ 40 ° C.
- a DWTT test piece compliant with API-5L was taken from the steel pipe and tested at a test temperature of ⁇ 20 ° C. to obtain an SA value (Share Area: ductile fracture surface ratio).
- the tensile strength of the base steel sheet is 760 MPa or more and 930 MPa or less, has a uniform elongation of 5% or more, and the ratio of 0.5% proof stress to the tensile strength is 85% or less and the test temperature in the base material is ⁇ 40 ° C. If the Charpy absorbed energy is less than 25 mm, 210 J or more, if the thickness is 25 mm or more, 150 J or more, DWTSA-20 ° C. is 85% or more, and the strength of the seam welded joint of the steel pipe is 760 MPa or more and 930 MPa or less. Charpy absorbed energy at a test temperature of ⁇ 30 ° C.
- the tensile test of the base material, the Charpy test, and the Charpy test of the welding heat affected zone (HAZ) were similarly implemented and evaluated.
- the evaluation criteria after the strain aging treatment were determined based on the same criteria as the evaluation criteria before the strain aging treatment described above.
- Test No. 14, 15, 16, 17, and 18 are examples of the invention in which the base material and the welded portion satisfy the provisions of claims 1 and 4, and the desired base material strength, yield ratio, uniform elongation, toughness, and The high HAZ toughness of seam welds is shown, and in the microstructure of the base metal part, the major axis diameter of the island-like martensite is mainly composed of a bainite structure containing island-like martensite with an area ratio of 4% to 12%.
- the major axis diameter of bainitic ferrite which is 2 ⁇ m or less and surrounded by a boundary having an orientation difference angle of 15 ° or more was 20 ⁇ m or less.
- test no. 19, 20, 21, and 22 the base material component is within the scope of the invention of claim 1, but the rolling reduction of 750 ° C. or less was less than 75% in rolling the steel sheet (see Table 6).
- Base material toughness decreased. The microstructure of the welded portion satisfies the provisions of claim 1 and good toughness is obtained.
- Test No. Nos. 23, 24, 25, and 26 have the base material component within the scope of the invention of claim 1, but have high welding heat input, and the lower bainite fraction is claimed in the microstructure of the HAZ coarse grain (CGHAZ) portion of the joint. Since the upper limit of the upper bainite structure was higher than the prescribed lower limit of 1 described above, the HAZ toughness was reduced in both the outer surface side and the inner surface side Root portion.
- Test No. No. 27 was a B-free system, and the HAB toughness was reduced in both the outer surface side and the inner surface side Root portion because the fraction of the upper bainite structure was increased.
- P CM is below the lower limit of the present invention, the tensile strength and the tensile strength of the joint of the base metal is less than 760 MPa,
- the HAZ coarse grain (CGHAZ) structure became the upper bainite structure, and the HAZ toughness decreased in both the outer surface side and the inner surface side Root portion.
- HAZ coarse (CGHAZ) structure becomes martensite, the outer surface side, HAZ toughness on the inner surface side Root portion both decreased.
- the base material toughness also deteriorated.
- Test No. No. 30 is a welding heat input of 80 kJ / cm or less on both the inner surface side and the outer surface side, but the welding heat input on the inner surface side is higher than the welding heat input on the outer surface side, and the austenite grain size is large in the microstructure of the root part. Therefore, a coarse upper bainite structure was obtained, and the HAZ toughness on the root side was lowered.
- Test No. 14 to 18 the results of the tensile test and Charpy test of the base metal, the Charpy test of the weld heat affected zone (HAZ), etc. after the strain aging treatment of holding at 250 ° C. for 30 minutes are the same as those before strain aging. It was an excellent one.
- test no. In the comparative example of 31, since the cooling stop temperature at the time of steel plate production was too low, the necessary MA fraction could not be secured, and before and after the strain aging treatment of holding at 250 ° C. for 30 minutes, The evaluation criteria for yield ratio were not met.
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Abstract
Description
一方、ラインパイプに用いられるUOE鋼管やERW鋼管のような溶接鋼管は、鋼板を冷間で管状に成形して、突合せ部を溶接後、通常防食等の観点から鋼管外面にコーティング処理が施されるため、製管時の加工歪みとコーティング処理時の加熱により歪時効が生じ、0.5%耐力が上昇し、コーティング処理後の鋼管における降伏比は鋼板における降伏比よりも大きくなってしまうという問題がある。しかしながら、特許文献1~5に記載の技術では、この点について解決されていない。このため、コーティング処理後も低降伏比を有し、その結果、高い耐座屈性能を有する高強度溶接鋼管が求められていた。
1.鋼管のシーム溶接部の溶接熱影響部(HAZ:Heat Affected Zone)において靭性が最も低下する部位(最脆化組織LBZ:Local Brittle Zoneと称す)は、外面側ではボンド近傍のHAZ粗粒(以下、CGHAZ(Coarse−grain HAZ)と称す)組織であり、内面側のRoot部では内面のCGHAZ組織が2相域(Ac1~Ac3点)に再加熱されるICCGHAZ(Inter−critically Coarse−grain HAZ)組織であり、いずれもHAZ粗粒域(溶融線近傍の旧オーステナイト粒径が50μm以上となる領域:Coarse−grain HAZ、CGHAZ)が起因となる。なお、Root部とは内面溶接金属と外面溶接金属がクロスする会合部近傍を指す。
2.母材のPCM値と、溶接後の冷却においてγ(austenite)−α(ferrite)相変態する800℃から500℃の温度域の冷却速度(cooling rate)を調整することによって、外面側や内面側によらず、CGHAZのミクロ組織を、下部ベイナイト組織(lower bainite structure)あるいは、硬質相(hard phase)のMAを大量に含む上部ベイナイトや、強度の高いマルテンサイト(martensite)を一定の面積分率以下とした下部ベイナイト主体の組織とすることで靭性が向上する。特に、下部ベイナイトを少なくとも面積分率(area fraction)で50%以上確保した組織とすると最も靭性が向上し、−30℃におけるシャルピー吸収エネルギーが大幅に向上する。
8.母材組織を島状マルテンサイトを有するベイナイト組織とすることにより、耐歪時効性が向上し、コーティング処理後も優れた耐座屈性を確保できる。このためには、島状マルテンサイトの面積分率を高精度にコントロールすることが重要である。
1.母材の成分組成が、質量%で、C:0.03%超え、0.08%以下、Si:0.01~0.5%、Mn:1.5~3.0%、P:0.015%以下、S:0.003%以下、Al:0.01~0.08%、Nb:0.005~0.025%、Ti:0.005~0.025%、N:0.001~0.010%、O:0.005%以下、B:0.0003~0.0020%を含有し、
更に、Cu:0.01~1%、Ni:0.01~1%、Cr:0.01~1%、Mo:0.01~1%、V:0.01~0.1%の一種または二種以上を含有し、
下記式(1)で計算されるPCM値(単位は%)が0.19≦PCM≦0.25を満足し、残部がFeおよび不可避的不純物であり、
母材の引張特性が、760MPa以上930MPa以下の引張強度と5%以上の一様伸びで、降伏比が85%以下であり、かつ試験温度−40℃でのシャルピー吸収エネルギーが、板厚25mm未満の場合には210J以上であり、板厚25mm以上の場合には150J以上である母材部と、
シーム溶接の溶接金属の成分組成が、質量%で、C:0.03~0.10%、Si:0.5%以下、Mn:1.5~3.0%、P:0.015%以下、S:0.005%以下、Al:0.05%以下、Nb:0.005~0.05%、Ti:0.005~0.03%、N:0.010%以下、O:0.015~0.045%、B:0.0003~0.0050%を含有し、
更に、Cu:0.01~1%、Ni:0.01~2.5%、Cr:0.01~1%、Mo:0.01~1.5%、V:0.1%以下の一種または二種以上を含有し、
残部がFe及び不可避的不純物である溶接金属部からなり、
鋼管のシーム溶接部における溶融線近傍で旧オーステナイト粒径が50μm以上となる溶接熱影響部のミクロ組織が、下部ベイナイト、または、面積率で少なくとも50%以上の下部ベイナイトと、上部ベイナイトおよび/またはマルテンサイトを備えた混合組織であることを特徴とする耐座屈性能および溶接熱影響部靭性に優れた低温用高強度鋼管。
PCM(%)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B…(1)
但し、各元素は含有量(質量%)を示す。
2.鋼管の長手方向に内外面から1層ずつ溶接した鋼管のシーム溶接部において、外面側の溶融線近傍の溶接熱影響部硬さが下記式(2)を満たすことを特徴とする1に記載の耐座屈性能および溶接熱影響部靭性に優れた低温用高強度鋼管。
250≦HV(98N)≦350 …(2)
但し、HV(98N):10kgfで測定したビッカース硬度を示す。
3.鋼管のシーム溶接部の継手強度が760MPa以上930MPa以下であることを特徴とする1または2に記載の耐座屈性能および溶接熱影響部靭性に優れた低温用高強度鋼管。
4.鋼管の母材部のミクロ組織において、面積率4%以上12%以下の島状マルテンサイトを含むベイナイト組織を主体とし、含有する島状マルテンサイトの長軸径が2μm以下であり、かつ、方位差角15°以上の境界で囲まれるベイニティックフェライトの長軸径(long axes size)が20μm以下であることを特徴とする、1乃至3のいずれか一つに記載の耐座屈性能および溶接熱影響部の靭性に優れた低温用高強度鋼管。
5.更に、母材部及び/または溶接金属部の化学成分に、質量%で、Ca:0.0005~0.01%、REM:0.0005~0.02%、Zr:0.0005~0.03%、Mg:0.0005~0.01%の一種または二種以上を含有することを特徴とする1乃至4のいずれか一つに記載の耐座屈性能および溶接熱影響部靭性に優れた低温用高強度鋼管。
6.さらに250℃以下の温度で30分以下の歪時効処理を施した後においても一様伸びが5%以上、降伏比が85%以下であることを特徴とする4または5に記載の耐座屈性能および溶接熱影響部靭性に優れた低温用高強度鋼管。
7.1または5に記載の母材成分を有する鋼を、1000~1300℃の温度に加熱し、950℃超えでの累積圧下率(cumulative rolling reduction)が10%以上、750℃以下での累積圧下率が75%以上となるように650℃以上の圧延終了温度で熱間圧延した後、10℃/s以上の冷却速度で450℃以上650℃未満の温度まで加速冷却し、その後ただちに0.5℃/s以上の昇温速度(heating rate)で加速冷却(accelerated cooling)の停止温度(stopping temperature)以上の500~750℃まで再加熱を行うことを特徴とする、耐座屈性能および溶接熱影響部靭性に優れた低温用高強度鋼管用鋼板の製造方法。
8.さらに、前記熱間圧延において750℃超え950℃以下での累積圧下率が20%以上であることを特徴とする、7に記載の耐座屈性能および溶接熱影響部靭性に優れた低温用高強度鋼管用鋼板の製造方法。
9.7または8に記載の製造方法により得られる鋼板を筒状に成形し、その突合せ部を内外面から1層ずつ溶接する際の内外面それぞれの溶接入熱が80kJ/cm以下であり、外面側および内面側の入熱バランス(heat input balance)が下記式(3)を満たすことを特徴とする、耐座屈性能および溶接熱影響部の靭性に優れた低温用高強度溶接鋼管の製造方法。
内面入熱≦外面入熱 …(3)
10.鋼管の長手方向に内外面から1層ずつ溶接した後、0.4%以上、2.0%以下の拡管率(expansion ratio)にて拡管(pipe expansion)することを特徴とする9記載の低温用高強度溶接鋼管の製造方法。
C:0.03%超え、0.08%以下
Cはマルテンサイト組織等の低温変態組織(low−temperature transformation structure)や第2相の島状マルテンサイト組織においては過飽和(supersaturation)に固溶(solid solution)することで強度上昇に寄与する。この効果を得るためには0.03%を超える添加が必要であるが、0.08%を超えて添加すると、鋼管の円周溶接部の硬度上昇が著しくなり、溶接低温割れが発生しやすくなるため、上限を0.08%とする。なお、降伏比を低く制御する上で必要な硬質相である島状マルテンサイトの量を確保するためには、好ましくは0.05%以上添加する。
Siは脱酸材(deoxidizing agent)として作用し、さらに固溶強化により鋼材の強度を増加させる元素であるが、0.01%未満ではその効果がなく、0.5%を超えて添加すると靱性が著しく低下するため上限を0.5%とする。より好ましくは、0.01~0.2%である。0.2%以下に抑制することで、鋼管シーム溶接部のCGHAZ組織中の上部ベイナイト組織に含まれる島状マルテンサイト(MA)の生成を抑制することが可能となり、継手HAZ靱性を向上させることができる。また、0.2%以下に抑制することで、鋼管母材部ミクロ組織中の島状マルテンサイトの過剰な生成を抑制し、母材靱性を向上させることができる。このため、好ましくは、上限を0.2%とする。
Mnは焼入性向上元素として作用する。1.5%以上の添加によりその効果が得られるが、連続鋳造プロセスでは中心偏析部での濃度上昇が著しく、3.0%を超える添加を行うと、中心偏析部での遅れ破壊の原因となるため、上限を3.0%とする。より好ましくは、1.6~2.5%である。
Alは脱酸元素として作用する。0.01%以上の添加で十分な脱酸効果が得られるが、0.08%を超えて添加すると鋼中の清浄度が低下し、靱性劣化の原因となるため、上限を0.08%とする。より好ましくは、0.02~0.06%である。
Nbは熱間圧延時のオーステナイト未再結晶領域を拡大する効果があり、950℃以下を未再結晶領域とするため、0.005%以上添加する。一方、0.025%を超えて添加すると、HAZの靱性および母材の靱性のうち特にシャルピー吸収エネルギーを著しく損ねることから上限を0.025%とする。より好ましくは、0.010~0.025%である。
Tiは窒化物を形成し、鋼中の固溶N量低減に有効で、析出したTiNはピンニング効果でオーステナイト粒の粗大化を抑制して、母材、HAZの靱性向上に寄与する。当該ピンニング効果を得るためには0.005%以上の添加が必要であるが、0.025%を超えて添加すると炭化物を形成するようになり、その析出硬化で靱性が著しく劣化するため、上限を0.025%とする。より好ましくは、0.008~0.020%である。
Nは通常鋼中の不可避不純物として存在するが、Ti添加により、TiNを形成する。TiNによるピンニング効果で、オーステナイト粒の粗大化を抑制するために0.001%以上鋼中に存在することが必要であるが、0.010%を超える場合、溶接部、特に溶接ボンド近傍で1450℃以上に加熱された領域でTiNが分解し、固溶Nの悪影響が著しいため、上限を0.010%とする。より好ましくは、0.002~0.005%である。
Bは、本発明において重要な役割を果たす元素である。本発明にかかる鋼はBを含有するので、ポリゴナルフェライトの生成が抑制される。このため、Bを含有しない鋼に比べて、より低温域でも、オーステナイト域圧延を実施することが可能となり、その結果、DWTT試験などで評価される靱性が向上する。また、Bは溶接熱影響部においてオーステナイト粒界に偏析し、焼入性を高める効果があり、靭性に有害なMAを含む上部ベイナイトの生成を抑制し、下部ベイナイトあるいはマルテンサイトの生成を容易にする。
Cu、Ni、Cr、Mo、Vはいずれも焼入性向上元素として作用するため、高強度化を目的に、これらの元素の一種、または二種以上を添加する。
Cuは、0.01%以上添加することで鋼の焼入性向上に寄与する。しかし、1%以上の添加を行うと、靱性劣化が生じるため、上限を1%とし、Cuを添加する場合は、0.01~1%とする。より好ましくは、0.1~0.5%である。
Niは、0.01%以上添加することで鋼の焼入性向上に寄与する。特に、多量に添加しても靭性劣化を生じないため、強靱化に有効であるが、高価な元素であるため、Niを添加する場合は、上限を1%とし、Niを添加する場合は0.01~1%とする。より好ましくは、0.1~0.5%である。
Crもまた0.01%以上添加することで鋼の焼入性向上に寄与する。一方、1%を超えて添加すると、靱性が劣化するため、上限を1%とし、Crを添加する場合は0.01~1%とする。より好ましくは、0.1~0.5%である。
Moもまた0.01%以上添加することで鋼の焼入性向上に寄与する。一方、1%を超えて添加すると、靱性が劣化するため、上限を1%とし、Moを添加する場合は0.01~1%とする。より好ましくは、0.1~0.5%である。
Vは炭窒化物を形成することで析出強化し、特に溶接熱影響部の軟化防止に寄与する。0.01%以上の添加によりこの効果が得られるが、0.1%を超えて添加すると、析出強化が著しく靱性が低下するため、上限を0.1%とし、Vを添加する場合は0.01~0.1%とする。より好ましくは、0.01~0.05%である。
本発明でO、P、Sは不可避的不純物であり含有量の上限を規定する。Oは、粗大で靱性に悪影響を及ぼす介在物生成を抑制するため、0.005%以下とする。Pは、含有量が多いと中央偏析が著しく、母材靭性が劣化するため、0.015%以下とする。Sは、含有量が多いとMnSの生成量が著しく増加し、母材の靭性が劣化するため、0.003%以下とする。より好ましくは、O:0.003%以下、P:0.01%以下、S:0.001%以下である。
PCMはC+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×Bで表す溶接割れ感受性指数で、各元素は含有量(質量%)とし、含有しない元素は0とする。
Ca、REM、Zr、Mg
Ca、REM、Zr、Mgは鋼中で酸硫化物あるいは炭窒化物を形成し、主に溶接熱影響部におけるオーステナイト粒粗大化をピンニング効果(pinning effect)で抑制し、靱性を向上させる目的で添加することができる。
製鋼プロセスにおいて、Ca添加量が0.0005%未満の場合、脱酸反応支配でCaSの確保が難しく靱性改善効果が得られないので、Caを添加する場合にはCaの下限を0.0005%とする。
REMは鋼中で酸硫化物(oxysulphide)を形成し、0.0005%以上添加することで溶接熱影響部の粗大化を防止するピンニング効果をもたらす。しかし、高価な元素であり、かつ0.02%を超えて添加しても効果が飽和するため、上限を0.02%とし、添加する場合は、0.0005~0.02%とする。より好ましくは、0.001~0.005%である。
Zrは鋼中で炭窒化物(carbonitride)を形成し、とくに溶接熱影響部においてオーステナイト粒の粗大化を抑制するピンニング効果をもたらす。十分なピンニング効果を得るためには、0.0005%以上の添加が必要であるが、0.03%を超えて添加すると、鋼中の清浄度が著しく低下し、靱性が低下するようになるため、上限を0.03%とし、添加する場合は、0.0005~0.03%とする。より好ましくは、0.001~0.01%である。
Mgは製鋼過程で鋼中に微細な酸化物として生成し、特に、溶接熱影響部においてオーステナイト粒の粗大化を抑制するピンニング効果をもたらす。十分なピンニング効果を得るためには、0.0005%以上の添加が必要であるが、0.01%を超えて添加すると、鋼中の清浄度が低下し、靱性が低下するようになるため、上限を0.01%とし、添加する場合は、0.0005~0.01%とする。より好ましくは、0.001~0.005%である。
溶接金属においてもCは鋼の強化元素として重要な元素である。特に、継手部のオーバーマッチング(over matching)を達成するため、溶接金属部においても引張強度を760MPa以上とする必要があり、この強度を得るために0.03%以上含有している必要がある。一方、0.10%を超えていると、溶接金属の高温割れが発生しやすくなるため、上限を0.10%とした。より好ましくは、0.05~0.08%である。
Siは溶接金属の脱酸ならびに良好な作業性を確保するために有用であるが、0.5%を超えると、溶接作業性の劣化を引き起こすため、上限を0.5%とした。より好ましくは、0.3%以下である。
Mnは溶接金属の高強度化に重要な元素である。特に、引張強度を760MPa以上とするためには1.5%以上含有させる必要があるが、3.0%を超えると溶接性が劣化するため、上限を3.0%とした。より好ましくは、1.6~2.5%である。
P,Sは溶接金属中では粒界に偏析し、その靱性を劣化させるため、上限をそれぞれ0.015%,0.005%とした。より好ましくは、それぞれ0.01%以下,0.003%以下である。
Alは脱酸元素として作用するが、溶接金属部においてはむしろTiによる脱酸の方が靱性改善効果が大きく、かつAl酸化物系の介在物が多くなると溶接金属シャルピーの吸収エネルギーの低下が起こるため、積極的には添加せず、その上限を0.05%とする。より好ましくは、0.03%以下である。
Nbは溶接金属の高強度化に有効な元素である。特に、引張強度を760MPa以上とするためには0.005%以上含有させる必要があるが、0.05%を超えると靭性が劣化するため、上限を0.05%とした。より好ましくは、0.005~0.04%であり、さらに好ましくは、0.005~0.03%である。
Tiは溶接金属中では脱酸元素として働き、溶接金属中の酸素の低減に有効である.この効果を得るためには0.005%以上の含有が必要であるが、0.03%を超えた場合、余剰となったTiが炭化物を形成し、溶接金属の靱性を劣化させるため、上限を0.03%とした。より好ましくは、0.005~0.02%である。
溶接金属中の固溶Nの低減もまた靱性改善効果があり、特に0.010%以下とすることで著しく改善されるため、上限を0.010%とした。より好ましくは、0.008%以下である。
溶接金属中の酸素量の低減は靱性改善効果があり、特に0.045%以下とすることで著しく改善されるため、上限を0.045%とした。一方、溶接金属中の酸素量を0.015%未満とすると溶接金属の組織微細化に有効な酸化物量が低下し、逆に溶接金属の靭性が劣化するため、下限を0.015%とした。より好ましくは、0.015~0.035%である。
強度グレードが760MPa以上930MPa以下のラインパイプ用溶接管においては、溶接金属のミクロ組織を微細なベイナイト主体組織とするために、B添加が有効であり、このような効果を得るためには0.0003%以上、0.0050%以下の添加が必要である。なお、より好ましい範囲は、0.0005~0.0050%であり、さらに好適な範囲は0.0005~0.0030%以下である。より一層好ましくは、0.0007~0.0020%である。
Cu、Ni、Cr、Mo、Vの一種または二種以上を添加する場合、Cu:0.01~1.0%、Ni:0.01~2.5%、Cr:0.01~1.0%、Mo:0.01~1.5%とする。
適量のV添加は靱性および溶接性を劣化させずに強度を高めることから有効な元素であり、この効果を発揮させるためには0.01%以上を含有することが好ましい。一方、0.1%を超えると溶接金属の再熱部の靱性が著しく劣化するため、上限を0.1%とした。より好ましくは、0.05%以下である。
Ca、REM、Zr、Mgは鋼中で酸硫化物あるいは炭窒化物を形成し、溶接金属部におけるオーステナイト粒粗大化をピンニング効果で抑制し、靱性を向上させる目的で添加することができる。
製鋼プロセスにおいて、Ca添加量が0.0005%未満の場合、脱酸反応支配でCaSの確保が難しく靱性改善効果が得られないので、Caを添加する場合にはCaの下限を0.0005%とする。
REMは鋼中で酸硫化物を形成し、0.0005%以上添加することで溶接金属部のオーステナイト粒の粗大化を防止するピンニング効果をもたらす。しかし、高価な元素であり、かつ0.02%を超えて添加しても効果が飽和するため、上限を0.02%とし、添加する場合は、0.0005~0.02%とする。より好ましくは、0.001~0.01%である。
Zrは鋼中で炭窒化物を形成し、溶接金属部においてオーステナイト粒の粗大化を抑制するピンニング効果をもたらす。十分なピンニング効果を得るためには、0.0005%以上の添加が必要であるが、0.03%を超えて添加すると、溶接金属部の清浄度が著しく低下し、靱性が低下するようになるため、上限を0.03%とし、添加する場合は、0.0005~0.03%とする。より好ましくは、0.001~0.01%である。
Mgは微細な酸化物として生成し、溶接金属部においてオーステナイト粒の粗大化を抑制するピンニング効果をもたらす。十分なピンニング効果を得るためには、0.0005%以上の添加が必要であるが、0.01%を超えて添加すると、溶接金属中の清浄度が低下し、靱性が低下するようになるため、上限を0.01%とし、添加する場合は、0.0005~0.01%とする。より好ましくは、0.001~0.005%である。
本発明では、優れた耐座屈性能と、−40℃でのシャルピー衝撃試験で目標の、板厚25mm未満の場合には210J以上、板厚25mm以上の場合には150J以上の吸収エネルギーを達成するため、そして、優れた耐歪時効特性を得るため、母材のミクロ組織を規定することが好ましい。母材のミクロ組織を規定することにより、DWTT試験で目標の−20℃での延性破面率85%以上を達成することもできる。
また、母材鋼板のミクロ組織を、面積率4%以上12%以下の島状マルテンサイトを含むベイナイト組織を主体とすることにより、後述するように、優れた耐歪時効特性を得ることができる。
これは、後述の製造プロセスにおいて、加速冷却時、および、その後の再加熱時に生じるベイナイト変態によって、Cが未変態オーステナイト相に濃化し、このCが濃化した未変態オーステナイトが島状マルテンサイトになるため、ベイナイト相の固溶C量が従来技術の鋼の場合に比べ少なくなるためである。
その結果、本発明においては、250℃で30分という、一般的な鋼管のコーティング工程では高温かつ長時間に相当する熱履歴を経ても、歪時効による降伏応力(YS)上昇や、これに伴う降伏比の上昇や一様伸びの低下を抑制することができ、従来鋼であれば歪時効により特性劣化するような熱履歴を受けても、本発明鋼では、一様伸び:5%以上、および、降伏比:85%以下を確保することができる。
鋼管の高強度化に伴い、従来の溶接入熱では溶接熱影響部のミクロ組織として粗大な島状マルテンサイトを含む上部ベイナイトを形成しやすく、低温靱性が劣化する。そこで粗大な島状マルテンサイトを含む上部ベイナイトを一定面積率以下に抑制することが必要となる。
本発明では、上述した成分組成を有する鋼を、1000~1300℃の温度に加熱し、950℃超えでの累積圧下率が10%以上、750℃以下での累積圧下率が75%以上となるように650℃以上の圧延終了温度で熱間圧延した後、10℃/s以上の冷却速度で450℃以上650℃未満の温度まで加速冷却し、その後ただちに0.5℃/s以上の昇温速度で加速冷却停止温度以上の500~750℃まで再加熱を行い、母材鋼板を製造する。
なお、本発明において、加熱温度、圧延終了温度、冷却終了温度および、再加熱温度等の温度は鋼板の平均温度とする。平均温度は、スラブもしくは鋼板の表面温度より、板厚、熱伝導率等のパラメータを考慮して、計算により求めたものである。また、冷却速度は、熱間圧延終了後、冷却終了温度(450~650℃未満)まで冷却するのに必要な温度差をその冷却を行うのに要した時間で割った平均冷却速度である。
また、加熱速度は、冷却後、再加熱温度(500~750℃)までの再加熱に必要な温度差を再加熱するのに要した時間で割った平均昇温速度である。以下、各製造条件について詳しく説明する。
熱間圧延を行うにあたり、完全にオーステナイト化するための下限温度は1000℃である。一方、1300℃を超える温度まで鋼片を加熱すると、TiNピンニングを行っていても、オーステナイト粒成長が著しく、母材靱性が劣化するため、上限を1300℃とした。より好ましくは、1000~1150℃である。
オーステナイト再結晶域で圧延を行うことで、粗大オーステナイト粒の生成等の混粒化が抑制される。累積圧下率が10%未満では効果が期待できないため、950℃超えでの累積圧下率を10%以上とした。
オーステナイト未再結晶域の比較的高温側で圧延を行うことで、粗大オーステナイト粒の生成等の混粒化が抑制される。この温度域に相当する750℃超え950℃以下での累積圧下率が20%未満では効果が小さいため、750℃超え950℃以下での累積圧下率を20%以上とすることが好ましい。
オーステナイト未再結晶域の低温側のこの温度域にて累積で大圧下を行うことにより、オーステナイト粒が伸展し、その後の加速冷却で変態生成するベイニティックフェライトおよび島状マルテンサイトが微細化し、靱性が大幅に向上する。
なお、オーステナイト未再結晶域の低温側のこの温度域にて累積で大圧下を行うのは、本発明の特徴である。前述のように、本発明に係る鋼はBを含有するので、ポリゴナルフェライトの生成が抑制される。すなわち、Bを含有しない鋼に比べて、オーステナイト未再結晶域が、より低温域に広がる。このため、単にオーステナイト未再結晶域圧延といっても、従来鋼よりも低い温度域でオーステナイト未再結晶域圧延を実施することが可能となるので、組織の微細化を通じた靭性向上効果が顕著になるものである。
熱間圧延終了温度が650℃未満では、その後の空冷過程においてオーステナイト粒界から初析フェライトが生成し、母材強度低下の原因となることから、初析フェライト生成を抑制するため、下限温度を650℃とした。より好ましくは、650~700℃である。
引張強度760MPa以上の高強度を達成するため,ミクロ組織をベイナイト主体の組織にする必要がある。このため,熱間圧延後加速冷却を実施する。冷却速度が10℃/s未満の場合、比較的高温でベイナイト変態が開始するため、十分な強度を得ることができない。よって、加速冷却の冷却速度を10℃/s以上とした。より好ましくは、12~50℃/sである。
このプロセスは本発明において、重要な製造条件である。本発明では、まず、加速冷却をベイナイト変態途中すなわち未変態オーステナイトが存在する温度域で終了する。その後ただちに再加熱を行い、未変態オーステナイトからベイナイトへの変態が生じるが、このように比較的高温で生成するベイナイト中のベイニティックフェライトでは、そのC固溶量が少ないため、Cが周囲の未変態オーステナイトへ排出される。そのため、再加熱時のベイナイト変態の進行に伴い、未変態オーステナイト中のC量が増加する。このとき、オーステナイト安定化元素である、Mn、Si等が一定以上含有されていると、再加熱終了時でもCが濃縮した未変態オーステナイトが残存する。そして、再加熱後の冷却過程(空冷)でMAへと変態する。こうして、最終的に、母材組織は島状マルテンサイトを含むベイナイト組織となる。
加速冷却後ただちに再加熱することで、未変態オーステナイトにCを濃縮させ、その後の空冷過程で島状マルテンサイトを生成させることができる。なお、ここで、加速冷却後ただちに再加熱するとは、加速冷却停止後、3分以内に0.5℃/s以上の昇温速度での再加熱を開始することをいう。
昇温速度が0.5℃/s未満の場合、ベイナイト中のセメンタイトが粗大化し、母材靱性が低下するため、昇温速度は0.5℃/s以上とする。より好ましくは、1.0~10℃/sである。
再加熱温度が500℃未満では、十分にオーステナイトへのC濃化が起こらず、必要とする島状マルテンサイト面積率を確保することができない。
本発明に係る耐座屈性能および溶接熱影響部の靭性に優れた低温用高強度鋼管は上述した引張強度特性を備えた母材鋼板を常法に従い、Uプレス(U−press)、Oプレス(O−press)で円筒形(pipe shape)とした後、シーム溶接(seam welding)を行って製造する。
内面入熱≦外面入熱 …(3)
ここで、下部ベイナイト組織は、ラス幅が1μm以下のベイニティックフェライトのラス内にセメンタイトを主体とする炭化物が析出したものを指し、上部ベイナイトはラス間に島状マルテンサイト(MA)および/またはセメンタイトを含むものである。外面側のシーム溶接で得られる溶融線近傍の溶接熱影響部が上記ミクロ組織の場合、その硬度は250≦HV(98N)≦350となり、−30℃の試験温度で100回以上の継手HAZシャルピー試験を実施したときの累積破損確率が1%以下という優れた溶接熱影響部靱性が達成される。
得られた鋼管の継手強度を評価するため、API−5Lに準拠した全厚引張試験片を母材部については管軸方向から、シーム溶接部については管の円周方向より採取し、引張試験を実施した。
HAZ粗粒(CGHAZ)の硬度、HAZ粗粒(CGHAZ)の靱性(以下HAZ靭性)の試験結果をまとめて表4−1および表4−2に示す。
[実施例2]
得られた鋼管の継手強度を評価するため、API−5Lに準拠した全厚引張試験片を母材部については管軸方向から、シーム溶接部については管の円周方向より採取し、引張試験を実施した。
HAZ粗粒(CGHAZ)の硬度、HAZ粗粒(CGHAZ)の靱性(以下HAZ靭性)の試験結果をまとめて表8−1および表8−3に示す。
なお、製造した鋼板を250℃にて30分間保持して、歪時効処理した後、母材の引張試験およびシャルピー試験、溶接熱影響部(HAZ)のシャルピー試験を同様に実施し、評価した。なお、歪時効処理後の評価基準は、上述した歪時効処理前の評価基準と同一の基準で判定した。
なお、試験No.14~18の本発明例では、250℃にて30分間保持の歪時効処理後も、母材の引張試験およびシャルピー試験、溶接熱影響部(HAZ)のシャルピー試験などの結果は、歪時効前と同等の優れたものであった。これに対して、試験No.31の比較例においては、鋼板製造時の冷却停止温度が低すぎたため、必要なMA分率を確保できず、250℃にて30分間保持の歪時効処理の前も後も、鋼管母材の降伏比の評価基準を満たさなかった。
2:シャルピー試験片のノッチ位置
3:Root−FLノッチのシャルピー試験片
4:ノッチ位置における最脆化組織
5:外面溶接金属
6:内面溶接金属
7:溶融線
8:溶融線近傍の旧オーステナイト粒径50μm以上となるHAZ粗粒領域(CGHAZ)
9:Ac3点に加熱された位置
10:Ac1点に加熱された位置
11:内面のHAZ粗粒組織が、2相域(Ac1点(10)~Ac3点(9))に加熱された領域(ICGHAZ)
Claims (10)
- 母材の成分組成が、質量%で、
C:0.03%超え、0.08%以下、
Si:0.01~0.5%、
Mn:1.5~3.0%、
P:0.015%以下、
S:0.003%以下、
Al:0.01~0.08%、
Nb:0.005~0.025%、
Ti:0.005~0.025%、
N:0.001~0.010%、
O:0.005%以下、
B:0.0003~0.0020%
を含有し、更に、
Cu:0.01~1%、
Ni:0.01~1%、
Cr:0.01~1%、
Mo:0.01~1%、
V:0.01~0.1%
の一種または二種以上を含有し、
下記式(1)で計算されるPCM値(単位は%)が0.19≦PCM≦0.25を満足し、残部がFeおよび不可避的不純物であり、
母材の引張特性が、760MPa以上930MPa以下の引張強度と5%以上の一様伸びで、降伏比が85%以下であり、かつ試験温度−40℃でのシャルピー吸収エネルギーが、板厚25mm未満の場合には210J以上であり、板厚25mm以上の場合には150J以上である母材部と、
シーム溶接の溶接金属の成分組成が、質量%で、
C:0.03~0.10%、
Si:0.5%以下、
Mn:1.5~3.0%、
P:0.015%以下、
S:0.005%以下、
Al:0.05%以下、
Nb:0.005~0.05%、
Ti:0.005~0.03%、
N:0.010%以下、
O:0.015~0.045%、
B:0.0003~0.0050%
を含有し、更に、
Cu:0.01~1%、
Ni:0.01~2.5%、
Cr:0.01~1%、
Mo:0.01~1.5%、
V:0.1%以下
の一種または二種以上を含有し、
残部がFe及び不可避的不純物である溶接金属部からなり、
鋼管のシーム溶接部における溶融線近傍で旧オーステナイト粒径が50μm以上となる溶接熱影響部のミクロ組織が、下部ベイナイト、または、面積率で少なくとも50%以上の下部ベイナイトと、上部ベイナイトおよび/またはマルテンサイトを備えた混合組織である低温用高強度鋼管。
PCM(%)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B…(1)
但し、各元素は含有量(質量%)を示す。 - 鋼管の長手方向に内外面から1層ずつ溶接した鋼管のシーム溶接部において、外面側の溶融線近傍の溶接熱影響部硬さが下記式(2)を満たす請求項1に記載の低温用高強度鋼管。
250≦HV(98N)≦350 …(2)
但し、HV(98N):10kgfで測定したビッカース硬度を示す。 - 鋼管のシーム溶接部の継手強度が760MPa以上930MPa以下である請求項1または2に記載の低温用高強度鋼管。
- 鋼管の母材部のミクロ組織において、面積率4%以上12%以下の島状マルテンサイトを含むベイナイト組織を主体とし、含有する島状マルテンサイトの長軸径が2μm以下であり、かつ、方位差角15°以上の境界で囲まれるベイニティックフェライトの長軸径が20μm以下である請求項1乃至3のいずれか一つに記載の低温用高強度鋼管。
- 更に、母材部及び/または溶接金属部の化学成分に、質量%で、
Ca:0.0005~0.01%、
REM:0.0005~0.02%、
Zr:0.0005~0.03%、
Mg:0.0005~0.01%
の一種または二種以上を含有する請求項1乃至4のいずれか一つに記載の低温用高強度鋼管。 - さらに250℃以下の温度で30分以下の歪時効処理を施した後においても一様伸びが5%以上、降伏比が85%以下であることを特徴とする4または5に記載の低温用高強度鋼管。
- 請求項1または5に記載の母材成分を有する鋼を、1000~1300℃の温度に加熱し、950℃超えでの累積圧下率が10%以上、750℃以下での累積圧下率が75%以上となるように650℃以上の圧延終了温度で熱間圧延した後、10℃/s以上の冷却速度で450℃以上650℃未満の温度まで加速冷却し、その後ただちに0.5℃/s以上の昇温速度で加速冷却停止温度以上の500~750℃まで再加熱を行う低温用高強度鋼管用鋼板の製造方法。
- さらに、前記熱間圧延において750℃超え950℃以下での累積圧下率が20%以上であることを特徴とする、請求項7に記載の低温用高強度鋼管用鋼板の製造方法。
- 請求項7または8に記載の製造方法により得られる鋼板を筒状に成形し、その突合せ部を内外面から1層ずつ溶接する際の内外面それぞれの溶接入熱が80kJ/cm以下であり、外面側および内面側の入熱バランスが下記式(3)を満たす低温用高強度溶接鋼管の製造方法。
内面入熱≦外面入熱 …(3) - 鋼管の長手方向に内外面から1層ずつ溶接した後、0.4%以上2.0%以下の拡管率にて拡管する請求項9記載の低温用高強度溶接鋼管の製造方法。
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CN102308013B (zh) | 2013-07-03 |
CA2751705C (en) | 2018-10-09 |
EP2395122A4 (en) | 2014-11-12 |
US8765269B2 (en) | 2014-07-01 |
RU2011136852A (ru) | 2013-03-20 |
JP2010202976A (ja) | 2010-09-16 |
JP4853575B2 (ja) | 2012-01-11 |
EP2395122B1 (en) | 2016-10-05 |
CN103334053A (zh) | 2013-10-02 |
CA2751705A1 (en) | 2010-08-12 |
CN102308013A (zh) | 2012-01-04 |
KR20110100317A (ko) | 2011-09-09 |
EP2395122A1 (en) | 2011-12-14 |
US20120018028A1 (en) | 2012-01-26 |
CN103334053B (zh) | 2014-12-31 |
KR101231270B1 (ko) | 2013-02-07 |
RU2493286C2 (ru) | 2013-09-20 |
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