WO2020129337A1 - 電縫鋼管 - Google Patents
電縫鋼管 Download PDFInfo
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- WO2020129337A1 WO2020129337A1 PCT/JP2019/036550 JP2019036550W WO2020129337A1 WO 2020129337 A1 WO2020129337 A1 WO 2020129337A1 JP 2019036550 W JP2019036550 W JP 2019036550W WO 2020129337 A1 WO2020129337 A1 WO 2020129337A1
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- C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
- C21D3/02—Extraction of non-metals
- C21D3/04—Decarburising
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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- 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
- C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
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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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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
- C21D9/085—Cooling or quenching
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- 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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- 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
- C21D9/505—Cooling thereof
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/25—Process efficiency
Definitions
- the present invention relates to an electric resistance welded steel pipe (electric resistance welded steel pipe, electric resistance welded steel pipe or tube), and in particular, even if the carbon content is 0.40 mass% or more, quench cracking does not occur.
- ERW steel pipe electric resistance welded steel pipe, electric resistance welded steel pipe or tube
- Rod bars have been used for automobile parts such as drive shafts and steering shafts that require high rigidity.
- a steel pipe has been used instead of the steel bar.
- the steel pipe is quenched and tempered to ensure the required strength. If the strength of the steel pipe can be increased by quenching, the required strength can be secured with a steel pipe having a thinner wall thickness, which is advantageous in terms of weight reduction. From the viewpoint of improving the strength of the steel pipe after quenching, it is desirable that the carbon content in the steel is high. Therefore, a steel pipe having a high carbon content tends to be used.
- quench cracks occur when steel materials are quenched. That is, when the steel material is rapidly cooled from the red hot state, cracks do not occur at first because the compressive residual stress is generated in the surface layer of the steel material due to the thermal stress.
- Ms martensitic transformation start
- volume expansion occurs due to martensitic transformation
- tensile stress is generated in the surface layer of the steel material.
- quench cracking occurs on the surface. Since the tensile stress due to this martensitic transformation increases as the carbon content increases, quench cracking is more likely to occur in steel materials with a higher carbon content. Particularly, when the carbon content is 0.40 mass% or more, the occurrence of quench cracking becomes remarkable. Quenching cracks have a markedly adverse effect on the static strength and fatigue strength required for the parts, so quenching must be avoided.
- Patent Document 1 proposes a technique of increasing the temper softening resistance by adding Si in excess of 2 mass% to the induction hardened steel used for automobile parts and the like.
- the C content can be suppressed to 0.60 mass% or less, and therefore occurrence of quench cracks and deterioration of workability can be prevented. ..
- Patent Document 2 proposes a technique in which a steel material is processed into a component shape, and immediately before quenching, induction quenching and forced cooling of a portion where quench cracking is likely to occur.
- forced cooling is stopped when the temperature of the portion reaches Ar1 point to (Ar1-50)° C., and the portion is reheated to the Ac1 point or higher.
- coarsening of the former austenite grain size of the quenched structure is suppressed, and the quench cracking resistance is improved.
- Patent Document 1 the technology proposed in Patent Document 1 is intended for round bars, not for ERW steel pipes.
- Si exceeding 2 mass% is added to the material for the electric resistance welded pipe as proposed in the cited document 1, the electric resistance weldability is significantly impaired, and it becomes difficult to secure the welding quality.
- Patent Document 2 the technology proposed in Patent Document 2 is intended for steel bars, not for electric resistance welded steel pipes. Further, the above-mentioned technology requires a quenching equipment capable of highly precise controlled cooling in order to secure a proper prior austenite grain size.
- the present invention has been made in view of the above circumstances, and provides an electric resistance welded steel pipe that does not cause quench cracking and has excellent fatigue strength even though the carbon content is 0.40 mass% or more.
- the purpose is to do.
- the inventors obtained the following findings as a result of repeated studies to solve the above problems.
- ferrite decarburized layer also referred to as a complete decarburization layer
- quench cracking can be prevented. That is, during quenching, the interior undergoes martensitic transformation and tensile stress is generated in the surface layer.
- the surface deferred ferrite decarburized layer remains ferrite even after quenching, and since ferrite has a soft and easy-to-stretch structure, quench cracking does not occur.
- a ferrite decarburized layer of optimum thickness is formed by a heating/rolling schedule that keeps the steel pipe as long as possible in the austenite-ferrite ( ⁇ - ⁇ ) two-phase region during hot-diameter rolling. can do.
- an electric resistance welded steel pipe (outer diameter 40 mm, wall thickness 4 mm) having a C content of 0.41 mass% and different ferrite decarburized layer depths was prepared.
- the electric resistance welded steel pipe was subjected to induction water quenching at a cooling rate of 50° C./s, and the occurrence of quench cracks was evaluated.
- the evaluation results are shown in Table 1.
- the electric resistance welded steel pipe after induction hardening was tempered at 400°C for 20 minutes. Then, a torsion fatigue test was performed to evaluate the fatigue strength of the electric resistance welded steel pipe after the tempering.
- the inner surface and the outer surface of the electric resistance welded steel pipe after tempering are ground by 0.5 mm in the depth direction to completely remove the influence of surface microcracks and surface decarburization.
- the prepared reference sample was prepared, and the sample having a fatigue strength of 80% or less of the reference sample was defined as insufficient fatigue strength. The evaluation results are also shown in Table 1.
- Fig. 1 is a graph showing the relationship between the residence time in the two-phase region during the production process of ERW steel pipe accompanied by hot reduction rolling and the depth of the ferrite decarburized layer on the surface of the obtained ERW pipe.
- the “two-phase region residence time” refers to the time during which the steel pipe as a raw material stays in the two-phase region of austenite-ferrite ( ⁇ - ⁇ ) during the hot-diameter rolling and the subsequent cooling process.
- FIG. 1 shows the experimental results under four conditions in which the depth of the preliminary decarburized layer before the hot-diameter rolling is 0 ⁇ m, 3 ⁇ m, 5 ⁇ m, and 10 ⁇ m. The lowermost line in FIG.
- the "preliminary decarburized layer” refers to the total decarburized layer formed by heating before the diameter-reducing rolling.
- the depth of the preliminary decarburized layer can be measured by rapidly cooling the steel pipe after heating and before the diameter-reducing rolling so that decarburization does not proceed any further.
- the preliminary decarburized layer depth is less than 5 ⁇ m, a ferrite decarburized layer with a depth of 20 ⁇ m or more cannot be obtained unless the residence time in the two-phase region is 10 minutes or more.
- the ferrite decarburized layer depth can be set to 20 to 50 ⁇ m by setting the two-phase region residence time to 1 to 5 minutes.
- the heat treatment for holding in the two-phase region is performed off-line, it is possible to secure a sufficient residence time in the two-phase region and set the ferrite decarburized layer depth to 20 to 50 ⁇ m regardless of the depth of the preliminary decarburized layer. ..
- off-line heat treatment is not desirable because it causes a decrease in productivity and an increase in cost. Therefore, by forming a preliminary decarburized layer with an appropriate depth in advance before entering the two-phase region in the cooling process after the diameter-reduction rolling, it is necessary even if the two-phase region residence time is within 5 minutes. The ferrite decarburized layer depth can be obtained.
- the temperature range in which ferrite decarburization is likely to proceed is wide in the two-phase range. That is, although ferrite decarburization proceeds only in the two-phase region, it tends to proceed particularly in the high temperature region among the two-phase region.
- the preliminary decarburized layer is formed on the surface, the amount of C is small in that portion, so that the upper limit temperature of the two-phase region rises and becomes wide on the high temperature side.
- the presence of the preliminary decarburized layer allows the subsequent ferrite decarburization to proceed advantageously, and the desired ferrite decarburized layer depth can be obtained in a short time. With such a short two-phase region residence time, online production is possible. Therefore, the electric resistance welded steel pipe of the present invention can be efficiently manufactured online.
- the present invention has been made based on the above findings, and its gist structure is as follows.
- composition of the components is% by mass,
- composition of the components is% by mass, Nb: 0.05% or less, W: 0.5% or less, V: 0.50% or less, and Mo: 2.0% or less, one or more selected from the group consisting of 2 or more, the electric resistance welded steel pipe according to 1 or 2 above.
- composition of the components is% by mass
- REM The electric resistance welded steel pipe according to any one of 1 to 3 above, containing 0.020% or less.
- the present invention it is possible to provide an electric resistance welded steel pipe in which quench cracking does not start even though the carbon content is 0.40% or more. Further, the electric resistance welded steel pipe of the present invention is excellent in productivity.
- the electric resistance welded steel pipe of the present invention can be very suitably used, for example, for manufacturing automobile parts.
- the electric resistance welded steel pipe of the present invention has the component composition described above.
- each component contained in the component composition will be described.
- "%" as a unit of the content of components means “% by mass” unless otherwise specified.
- the C content 0.40 to 0.55% If the C content is less than 0.40%, sufficient hardness cannot be obtained even by quenching, and the required fatigue resistance cannot be obtained. Therefore, the C content is set to 0.40% or more, preferably 0.41% or more. On the other hand, if the C content exceeds 0.55%, the weldability deteriorates, and stable electric resistance welding quality cannot be obtained. Therefore, the C content is 0.55% or less, preferably 0.50% or less.
- Si 0.10 to 1.0% Si may be added for deoxidation, and if it is less than 0.10%, a sufficient deoxidation effect cannot be obtained. At the same time, Si is also a solid solution strengthening element, and it is necessary to add 0.10% or more to obtain the effect. Therefore, the Si content is set to 0.10% or more. On the other hand, if the Si content exceeds 1.0%, the hardenability of the steel pipe deteriorates. Therefore, the Si content is set to 1.0% or less, preferably 0.4% or less.
- Mn 0.10-2.0%
- Mn is an element that improves hardenability, and it is necessary to add 0.10% or more to obtain the effect. Therefore, the Mn content is set to 0.10% or more, preferably 0.20% or more, and more preferably 1.0% or more. On the other hand, when the Mn content exceeds 2.0%, the electric resistance welding quality deteriorates. Therefore, the Mn content is set to 2.0% or less, preferably 1.8% or less.
- P 0.10% or less
- P is an element contained as an impurity, segregates at grain boundaries and the like, and adversely affects weld crackability and toughness. Therefore, the P content is reduced to 0.10% or less.
- the P content is preferably 0.05% or less.
- the lower limit of the P content is not limited, but since P is unavoidably contained in the steel, the P content may be 0.001% or more.
- S 0.010% or less
- S is an element that exists in steel as a sulfide-based inclusion and reduces hot workability, toughness, and fatigue resistance. Therefore, it is necessary to reduce the S content to 0.010% or less.
- the S content is preferably 0.005% or less.
- the lower limit of the S content is not limited, but since S is unavoidably contained in the steel, the S content may be 0.001% or more.
- Al 0.010-0.100%
- Al is an element effective for deoxidation. Further, Al has the effect of ensuring the strength after quenching by suppressing the growth of austenite grains during quenching.
- the Al content is set to 0.010% or more, preferably 0.030% or more.
- the Al content is set to 0.100% or less, preferably 0.080% or less.
- Cr 0.05-0.30%
- Cr is an element having an effect of improving hardenability.
- the Cr content is set to 0.05% or more, preferably 0.10% or more.
- the Cr content is set to 0.30% or less, preferably 0.25% or less.
- Ti 0.010 to 0.050%, Ti has a function of fixing N in steel as TiN. However, if the Ti content is less than 0.010%, the ability to fix N is not sufficiently exhibited. Therefore, the Ti content is set to 0.010% or more. On the other hand, if the Ti content exceeds 0.050%, the workability and toughness of steel deteriorate. Therefore, the Ti content is 0.050% or less, preferably 0.040% or less.
- B 0.0005 to 0.0030%
- B is an element that improves hardenability. If the B content is less than 0.0005%, the effect of improving hardenability is not sufficiently exhibited. Therefore, the B content is set to 0.0005% or more, preferably 0.0010% or more. On the other hand, when the B content exceeds 0.0030%, the effect is saturated, and in addition, B segregates at the grain boundaries to promote grain boundary fracture and deteriorate toughness. Therefore, the B content is set to 0.0030% or less, preferably 0.0025% or less.
- Ca 0.0001 to 0.0050%
- Ca is an element effective in reducing the crack starting point at the time of fatigue fracture in a use environment where a nonmetallic inclusion has a spherical shape and is subjected to repeated stress.
- the Ca content is set to 0.0001% or more, preferably 0.0010% or more.
- the Ca content is set to 0.0050% or less, preferably 0.0040% or less.
- N 0.0005 to 0.005%
- N is an element that combines with Al and has an effect of refining crystal grains.
- the N content is set to 0.0005% or more, preferably 0.0010% or more.
- the N content exceeds 0.0050%, the amount of free B is reduced by combining with B to form BN, and as a result, the hardenability improving effect of B is impaired. Therefore, the N content is set to 0.0050% or less, preferably 0.0040% or less.
- the component composition in one embodiment of the present invention may be such that the above elements and the balance are Fe and unavoidable impurities.
- the component composition may further optionally contain one or both of Cu and Ni in the amounts described below.
- Cu 1.0% or less
- Cu is an element that improves hardenability and is effective in improving the strength and fatigue strength of steel.
- the Cu content exceeds 1.0%, the workability is significantly reduced. Therefore, when Cu is added, the Cu content is 1.0% or less, preferably 0.5% or less.
- the lower limit of the Cu content is not particularly limited, the Cu content is preferably 0.001% or more from the viewpoint of sufficiently obtaining the effect of adding Cu.
- Ni 1.0% or less
- Ni is an element that improves the hardenability and is effective in improving the strength of steel.
- the Ni content exceeds 1.0%, the workability is significantly reduced. Therefore, when Ni is added, the Ni content is 1.0% or less, preferably 0.5% or less.
- the lower limit of the Ni content is not particularly limited, but it is preferable that the Ni content is 0.1% or more from the viewpoint of sufficiently obtaining the effect of adding Ni.
- the component composition may further optionally contain one or more selected from the group consisting of Nb, W, V, and Mo in the amounts described below. You can
- Nb 0.05% or less
- Nb is an element that improves hardenability, and also forms carbides to contribute to an increase in strength.
- the Nb content exceeds 0.05%, the effect is saturated and the workability is further deteriorated. Therefore, when Nb is added, the Nb content is 0.05% or less, preferably 0.04% or less.
- the lower limit of the Nb content is not particularly limited, but from the viewpoint of obtaining a sufficient Nb addition effect, the Nb content is preferably 0.001% or more, and more preferably 0.002% or more. More preferable.
- W 0.5% or less W is an element having the effect of improving the strength of steel by forming a carbide. However, if the W content exceeds 0.5%, unnecessary carbides are precipitated and the fatigue resistance and workability deteriorate. Therefore, when W is added, the W content is 0.5% or less, preferably 0.4% or less. On the other hand, the lower limit of the W content is not particularly limited, but it is preferable that the W content is 0.01% or more from the viewpoint of sufficiently obtaining the effect of adding W.
- V 0.50% or less
- V is an element that forms a carbide and has the effect of increasing the strength of steel.
- V is also an element having an effect of improving temper softening resistance.
- the V content is 0.50% or less, preferably 0.40% or less.
- the lower limit of the V content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of adding V, the V content is preferably 0.001% or more, and more preferably 0.002% or more. More preferable.
- Mo 2.0% or less Mo is an element that improves hardenability and is effective in improving the strength and fatigue strength of steel. However, if the Mo content exceeds 2.0%, the workability is significantly reduced. Therefore, when Mo is added, the Mo content is 2.0% or less, preferably 0.5% or less. On the other hand, the lower limit of the Mo content is not limited, but from the viewpoint of sufficiently obtaining the effect of adding Mo, the Mo content is preferably 0.001% or more, and more preferably 0.002% or more. preferable.
- the above component composition may further optionally contain REM (rare earth metal) in an amount described below.
- REM 0.020% or less REM is an element having a non-metallic inclusion in a spherical shape and having an effect of reducing a crack starting point at the time of fatigue fracture in a use environment where repeated stress is applied.
- the REM content exceeds 0.020%, the amount of inclusions becomes too large and the cleanliness decreases. Therefore, when REM is added, the REM content is 0.020% or less.
- the lower limit of the REM content is not particularly limited, it is preferable that the REM content be 0.0020% or more from the viewpoint of sufficiently obtaining the effect of adding REM.
- the electric resistance welded steel sheet in one embodiment of the present invention is mass%, C: 0.40 to 0.55%, Si: 0.10 to 1.0%, Mn: 0.10 to 2.0%, P: 0.10% or less, S: 0.010% or less, Al: 0.010-0.100%, Cr: 0.05-0.30%, Ti: 0.010 to 0.050%, B: 0.0005 to 0.0030%, Ca: 0.0001 to 0.0050%, N: 0.0005 to 0.0050%, Either or both of Cu: 1.0% or less and Ni: 1.0% or less, optionally; 1 or 2 or more selected from the group consisting of Nb: 0.05% or less, W: 0.5% or less, V: 0.50% or less, and Mo: 2.0% or less, Optionally, it can have a REM: 0.020% or less, and a constituent composition of the balance Fe and inevitable impurities.
- the electric resistance welded steel pipe of the present invention has a ferrite decarburized layer having a depth of 20 to 50 ⁇ m on the surface.
- the ferrite decarburized layer depth is set to 20 ⁇ m or more.
- the ferrite decarburized layer depth exceeds 50 ⁇ m, quenching cracks do not occur, but due to insufficient quenching hardness of the surface layer portion, strength and fatigue strength as a part cannot be secured. There is also a means of cutting the surface decarburized portion, but this will result in a significant cost increase. Therefore, the depth of the ferrite decarburized layer is 50 ⁇ m or less, preferably 40 ⁇ m or less.
- the size of the electric resistance welded steel pipe of the present invention is not particularly limited and may be any size, but the ratio of the wall thickness t (mm) to the outer diameter D (mm) of the steel pipe, t/D, is 10 to. It is preferably 35%.
- the present invention is based on the idea of preventing quench cracking by providing a ferrite decarburized layer of a specific depth on the surface layer of a steel pipe, it is not particularly limited, and an electric resistance welded steel pipe having an arbitrary microstructure is provided. Can be applied to.
- the microstructure of the electric resistance welded steel pipe preferably has, for example, a microstructure composed of ferrite and pearlite, or a microstructure composed of ferrite, pearlite, and bainite.
- the electric resistance welded steel pipe in one embodiment of the present invention can have a microstructure of ferrite, pearlite, and optionally bainite.
- the electric resistance welded steel pipe of the present invention is used after being quenched and tempered.
- the Vickers hardness after quenching and tempering is not particularly limited, but in the case of a steel pipe used as an automobile part or the like, it is preferably 350 HV or more.
- the Vickers hardness after quenching and tempering is preferably 700 HV or less. Since the hardness of the extreme surface layer does not increase due to quenching, if it affects the fatigue strength, it can be removed by cutting or the like without any problem.
- the electric resistance welded steel pipe is not particularly limited, but can be manufactured, for example, by sequentially performing the following steps (1) to (5).
- a steel strip having the above composition is continuously roll-formed into a substantially cylindrical shaped body,
- a steel pipe (base pipe) is obtained by butt-welding the circumferential ends of the molded body with each other and performing electric resistance welding.
- both hot rolled steel strip and cold rolled steel strip can be used.
- the roll forming, electric resistance welding, and heating are not particularly limited and can be performed by any method.
- the electric resistance welding is preferably performed by a high frequency electric resistance welding method.
- the hot reduction rolling and the subsequent cooling may be performed by any method without particular limitation.
- An example of suitable manufacturing conditions is described below.
- the heating temperature of the shell before the diameter-reduction rolling is Ac3 point or higher.
- the heating temperature is preferably 1000° C. or lower. If the heating temperature exceeds 1000° C., the surface properties of the product deteriorate.
- the reduction rolling end temperature is preferably set to more than 700°C.
- the diameter reduction rolling end temperature is 700° C. or lower, ductility due to processing strain decreases.
- the reduction rolling end temperature is preferably 950° C. or lower. If the temperature reduction finish temperature exceeds 950° C., the surface properties of the steel pipe deteriorate and the productivity decreases.
- the cumulative diameter reduction ratio in the diameter reduction rolling is preferably 80% or less. If the cumulative diameter reduction ratio exceeds 80%, the work hardening of the entire material increases, the ductility decreases, and the productivity decreases.
- the residence time in the two-phase region refers to the time during which the steel pipe stays in the two-phase region of austenite-ferrite ( ⁇ - ⁇ ) during the hot-diameter rolling and the subsequent cooling process, as described above. ..
- the "preliminary decarburized layer depth” refers to the decarburized layer depth formed by heating before the diameter-reduction rolling, as described above.
- the depth of the decarburized layer can be adjusted by controlling the heating temperature and the heating time.
- the heating temperature in the heating before the diameter-reduction rolling is 860° C. or higher.
- the heating temperature is higher than 1000° C.
- the ferrite decarburized layer in the finally obtained electric resistance welded steel pipe becomes too thick, resulting in insufficient quenching. Therefore, the heating temperature is preferably 1000° C. or lower.
- a hot-rolled steel strip (plate thickness: 4.3 mm) having the composition shown in Table 2 was continuously cold formed by a plurality of rolls to form a substantially cylindrical open pipe. Then, the circumferential ends of the open pipe were abutted against each other, pressed against each other, and electro-welded using a high frequency electric resistance welding method to obtain a steel pipe (outer diameter 89.1 mm ⁇ wall thickness 4.3 mm).
- the obtained steel pipe was heated to 930°C with an induction heater. At that time, in order to change the depth of the preliminary decarburized layer, the heating temperature was changed between 900 and 950°C. After that, the steel pipe was hot reduced to a diameter of 40 m ⁇ and a wall thickness of 4.0 mm with a stretch reducer. At that time, the residence time in the two-phase region was intentionally changed by changing the passage speed in order to change the depth of the ferrite decarburized layer. The steel pipe after hot-diameter rolling was air-cooled to about 250° C., then dropped into a water tank and cooled to room temperature.
- the ferrite decarburized layer depth on the outer surface and inner surface of the electric resistance welded steel pipe obtained by the above procedure was measured according to JIS G 0558. The measurement was carried out at four points at 90° intervals in the circumferential direction starting from the welded portion, and the average value of the measured values at these four points was adopted. The measurement results are shown in Table 3.
- quenching was performed under the following conditions. First, the electric resistance welded steel pipe was held in a vacuum furnace at 950° C. for 20 minutes, and then immediately immersed in a water tank in a sufficiently stirred state and quenched. The cooling rate during quenching was measured by attaching a thermocouple to an ERW steel pipe as a sample. The cooling rate between 900 and 200°C during water quenching was 50°C/s or more.
- the electric resistance welded steel pipe that did not suffer from quench cracking was tempered at 400°C for 20 minutes.
- a reference sample was prepared by grinding the outer surface and the inner surface of the tube to a thickness of 1.0 mm and removing the affected portion of the surface decarburized layer. Then, a torsional fatigue test was performed to measure the fatigue strength of both the as-tempered ERW steel pipe without surface grinding and the reference sample. The reduction rate of the fatigue strength of the as-tempered electric resistance welded steel pipe with respect to the fatigue strength of the reference sample was calculated and shown in Table 3 as the fatigue strength reduction rate.
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Abstract
Description
C :0.40~0.55%、
Si:0.10~1.0%、
Mn:0.10~2.0%、
P :0.10%以下、
S :0.010%以下、
Al:0.010~0.100%、
Cr:0.05~0.30%、
Ti:0.010~0.050%、
B :0.0005~0.0030%、
Ca:0.0001~0.0050%、および
N :0.0005~0.0050%を含み、
残部Feおよび不可避的不純物からなる成分組成を有し、
外表面および内表面に、表面からの深さが20~50μmであるフェライト脱炭層を有する、電縫鋼管。
Cu:1.0%以下および
Ni:1.0%以下のいずれか一方または両方を含む、上記1に記載の電縫鋼管。
Nb:0.05%以下、
W :0.5%以下、
V :0.50%以下、および
Mo:2.0%以下からなる群より選択される1または2以上を含む、上記1または2に記載の電縫鋼管。
REM:0.020%以下を含む、上記1~3のいずれか一項に記載の電縫鋼管。
本発明の電縫鋼管は、上述した成分組成を有する。以下、前記成分組成に含まれる各成分について説明する。なお、特に断らない限り、本明細書において成分の含有量の単位としての「%」は「質量%」を意味する。
C含有量が0.40%未満では、焼入れしても十分な硬度が得られず、要求される耐疲労特性が得られない。そのため、C含有量を0.40%以上、好ましくは0.41%以上とする。一方、C含有量が0.55%を超えると、溶接性が悪くなるため、安定した電縫溶接品質が得られない。そのため、C含有量を0.55%以下、好ましくは0.50%以下とする。
Siは脱酸のために添加する場合もあり、0.10%未満では十分な脱酸効果が得られない。同時に、Siは固溶強化元素でもあり、その効果を得るためには0.10%以上の添加が必要である。そのため、Si含有量を0.10%以上とする。一方、Si含有量が1.0%を超えると、鋼管の焼入れ性が低下する。そのため、Si含有量を1.0%以下、好ましくは0.4%以下とする。
Mnは焼入れ性を向上させる元素であり、その効果を得るには0.10%以上の添加が必要である。そのため、Mn含有量を0.10%以上、好ましくは0.20%以上、より好ましくは1.0%以上とする。一方、Mn含有量が2.0%を超えると電縫溶接品質が低下する。そのため、Mn含有量を2.0%以下、好ましくは1.8%以下とする。
Pは、不純物として含有される元素であり、粒界等に偏析し、溶接割れ性および靭性に悪影響を及ぼす。そのため、P含有量を0.10%以下に低減する。なお、P含有量は、0.05%以下とすることが好ましい。一方、P含有量の下限は限定されないが、Pは鋼中に不可避的に含有されるため、P含有量は0.001%以上であってよい。
Sは、鋼中では硫化物系介在物として存在し、熱間加工性、靭性、耐疲労特性を低下させる元素である。そのため、S含有量を0.010%以下に低減する必要がある。なお、S含有量は0.005%以下とすることが好ましい。一方、S含有量の下限は限定されないが、Sは鋼中に不可避的に含有されるため、S含有量は0.001%以上であってよい。
Alは脱酸に有効な元素である。また、Alは焼入れ時のオーステナイト粒の成長を抑制することで焼入れ後の強度を確保する効果を有する。前記効果を得るために、Al含有量を0.010%以上、好ましくは0.030%以上とする。一方、Al含有量が0.100%を超えるとその効果は飽和するだけでなく、Al系の介在物が増え、疲労強度が低下する。そのため、Al含有量を0.100%以下、好ましくは0.080%以下とする。
Crは焼入れ性を向上させる効果を有する元素である。前記効果を得るために、Cr含有量を0.05%以上、好ましくは0.10%以上とする。一方、Cr含有量が0.30%を超えると、酸化物が形成されやすくなり、電縫溶接部にCr酸化物が残存して電縫溶接品質が低下する。そのため、Cr含有量を0.30%以下、好ましくは0.25%以下とする。
Tiは鋼中のNをTiNとして固定する作用を有する。しかし、Ti含有量が0.010%未満ではNを固定する能力が十分に発揮されない。そのため、Ti含有量を0.010%以上とする。一方、Ti含有量が0.050%を超えると鋼の加工性、靭性が低下する。そのため、Ti含有量を0.050%以下、好ましくは0.040%以下とする。
Bは焼入れ性を向上させる元素である。B含有量が0.0005%未満では焼入れ性向上効果が十分に発揮されない。そのため、B含有量を0.0005%以上、好ましくは0.0010%以上とする。一方、B含有量が0.0030%を超えると、その効果は飽和することに加え、Bが粒界に偏析して粒界破壊を促進し、靱性を劣化させる。そのため、B含有量を0.0030%以下、好ましくは0.0025%以下とする。
Caは、非金属介在物の形態を球状とし、繰り返し応力が付与されるような使用環境下での疲労破壊時の割れ起点の低減に有効な元素である。前記効果を得るために、Ca含有量を0.0001%以上、好ましくは0.0010%以上とする。一方、Ca含有量が0.0050%を超えると、介在物量が多くなりすぎて清浄度が低下する。そのためCa含有量を0.0050%以下、好ましくは0.0040%以下とする。
Nは、Alと結合し、結晶粒を微細化する効果を有する元素である。前記効果を得るために、N含有量を0.0005%以上、好ましくは0.0010%以上とする。一方、N含有量が0.0050%を超えると、Bと結合しBNを形成することでフリーB量が低下し、その結果、Bによる焼入れ性向上効果が阻害される。そのため、N含有量は0.0050%以下、好ましくは0.0040%以下とする。
Cuは焼入れ性を向上させる元素であり、鋼の強度および疲労強度の向上に有効である。しかし、Cu含有量が1.0%を超えると加工性が著しく低下する。そのため、Cuを添加する場合、Cu含有量は1.0%以下、好ましくは0.5%以下とする。一方、Cu含有量の下限は特に限定されないが、Cuの添加効果を十分に得るという観点からは、Cu含有量を0.001%以上とすることが好ましい。
Niは焼入れ性を向上させる元素であり、鋼の強度向上に有効である。しかし、Ni含有量が1.0%を超えると加工性が著しく低下する。そのため、Niを添加する場合、Ni含有量は1.0%以下、好ましくは0.5%以下とする。一方、Ni含有量の下限は特に限定されないが、Niの添加効果を十分に得るという観点からは、Ni含有量を0.1%以上とすることが好ましい。
Nbは焼入れ性を向上させる元素であり、また、炭化物を形成して強度上昇に寄与する。しかし、Nb含有量が0.05%を超えると、その効果が飽和することに加え、さらに加工性が低下する。そのため、Nbを添加する場合、Nb含有量は0.05%以下、好ましくは0.04%以下とする。一方、Nb含有量の下限は特に限定されないが、Nbの添加効果を十分に得るという観点からは、Nb含有量を0.001%以上とすることが好ましく、0.002%以上とすることがより好ましい。
Wは炭化物を形成することで鋼の強度を向上させる効果を有する元素である。しかし、W含有量が0.5%を超えると不必要な炭化物が析出し、耐疲労特性および加工性が低下する。そのため、Wを添加する場合、W含有量は0.5%以下、好ましくは0.4%以下とする。一方、W含有量の下限は特に限定されないが、Wの添加効果を十分に得るという観点からは、W含有量を0.01%以上とすることが好ましい。
Vは炭化物を形成し、鋼の強度を上昇させる効果を有する元素である。また、Vは焼戻し軟化抵抗を向上させる効果を有する元素でもある。しかし、V含有量が0.50%を超えると効果が飽和するとともに、かえって加工性が低下する。そのため、Vを添加する場合、V含有量は0.50%以下、好ましくは0.40%以下とする。一方、V含有量の下限は特に限定されないが、Vの添加効果を十分に得るという観点からは、V含有量を0.001%以上とすることが好ましく、0.002%以上とすることがより好ましい。
Moは焼入れ性を向上させる元素であり、鋼の強度および疲労強度の向上に有効である。しかし、Mo含有量が2.0%を超えると加工性が著しく低下する。そのため、Moを添加する場合、Mo含有量は2.0%以下、好ましくは0.5%以下とする。一方、Mo含有量の下限は限定されないが、Moの添加効果を十分に得るという観点からは、Mo含有量を0.001%以上とすることが好ましく、0.002%以上とすることがより好ましい。
REMは、非金属介在物の形態を球状とし、繰り返し応力が付与されるような使用環境下での疲労破壊時の割れ起点を低減する効果を有する元素である。しかし、REM含有量が0.020%を超えると、介在物量が多くなりすぎて清浄度が低下する。そのため、REMを添加する場合、REM含有量は0.020%以下とする。一方、REM含有量の下限は特に限定されないが、REMの添加効果を十分に得るという観点からは、REM含有量を0.0020%以上とすることが好ましい。
C :0.40~0.55%、
Si:0.10~1.0%、
Mn:0.10~2.0%、
P :0.10%以下、
S :0.010%以下、
Al:0.010~0.100%、
Cr:0.05~0.30%、
Ti:0.010~0.050%、
B :0.0005~0.0030%、
Ca:0.0001~0.0050%、
N :0.0005~0.0050%、
任意に(optionally)、Cu:1.0%以下およびNi:1.0%以下のいずれか一方または両方、
任意に、Nb:0.05%以下、W:0.5%以下、V:0.50%以下、およびMo:2.0%以下からなる群より選択される1または2以上、
任意に、REM:0.020%以下、並びに
残部Feおよび不可避的不純物からなる(consisting of)成分組成を有することができる。
本発明の電縫鋼管は、表面に深さ20~50μmのフェライト脱炭層を有する。上述したように、フェライト脱炭層深さが20μm未満であると、焼入れの際に焼割れが発生する。焼割れの発生を防止するために、フェライト脱炭層深さを20μm以上とする。一方、フェライト脱炭層深さが50μmを超える場合には、焼割れは発生しないが、表層部の焼入れ硬さ不足により、部品としての強度、疲労強度が確保できなくなる。表層脱炭部を切削する手段もあるが、大幅なコストアップとなる。そのため、フェライト脱炭層深さを50μm以下、好ましくは40μm以下とする。
本発明の電縫鋼管の寸法は、とくに限定されることなく任意の寸法とすることができるが、鋼管の外径D(mm)に対する肉厚t(mm)の比、t/Dを10~35%とすることが好ましい。
本発明は、鋼管の表層に特定の深さのフェライト脱炭層を設けることによって焼割れを防止するという考え方に基づくものであるため、とくに限定されることなく、任意のミクロ組織を有する電縫鋼管に適用することができる。電縫鋼管のミクロ組織は、例えば、フェライトおよびパーライトからなるミクロ組織、またはフェライト、パーライト、およびベイナイトからなるミクロ組織を有することが好ましい。言い換えると、本発明の一実施形態における電縫鋼管は、フェライト、パーライト、および任意にベイナイトからなるミクロ組織を有することができる。
本発明の電縫鋼管は、焼入れ焼戻しを施して使用される。焼入れ焼戻し後のビッカース硬さは、とくに限定されないが、自動車部品などとして使用される鋼管の場合、350HV以上とすることが好ましい。一方、靱性の劣化や遅れ破壊の発生を抑制するという観点からは、焼入れ焼戻し後のビッカース硬さを700HV以下とすることが好ましい。なお、極表層部分は焼入れにより硬度上昇はしないため、これが疲労強度に影響する場合には、これを切削などにより除去すれば問題ない。
上記電縫鋼管は、特に限定されないが、例えば、下記(1)~(5)の工程を順次行うことにより製造することができる。
(1)上記成分組成を有する鋼帯を連続的にロール成形して略円筒状の成形体とし、
(2)前記成形体の円周方向端部同士を突き合わせ電縫溶接して鋼管(素管)とし、
(3)前記鋼管を加熱し、
(4)加熱された前記鋼管に熱間縮径圧延を施し、
(5)前記熱間縮径圧延後の鋼管を冷却する。
縮径圧延前の素管の加熱温度は、Ac3点以上とすることが好ましい。前記加熱温度がAc3点未満の場合、電縫溶接部の靭性が低下することに加え、白色層におけるC量の均一化の進行が遅くなる。一方、前記加熱温度は、1000℃以下とすることが好ましい。前記加熱温度が1000℃を超えると、製品の表面性状が劣化する。
縮径圧延終了温度は700℃超とすることが好ましい。縮径圧延終了温度が700℃以下の場合、加工歪みによる延性が低下する。一方、縮径圧延終了温度は950℃以下とすることが好ましい。縮径圧延終了温度が950℃を超えると、鋼管の表面性状が劣化することに加え、生産性が低下する。
縮径圧延における累積縮径率は、80%以下とすることが好ましい。累積縮径率が80%を超えると、素材全体の加工硬化が大きくなり、延性が低下することに加え、生産性が低下する。
Claims (4)
- 質量%で、
C :0.40~0.55%、
Si:0.10~1.0%、
Mn:0.10~2.0%、
P :0.10%以下、
S :0.010%以下、
Al:0.010~0.100%、
Cr:0.05~0.30%、
Ti:0.010~0.050%、
B :0.0005~0.0030%、
Ca:0.0001~0.0050%、および
N :0.0005~0.0050%を含み、
残部Feおよび不可避的不純物からなる成分組成を有し、
外表面および内表面に、表面からの深さが20~50μmであるフェライト脱炭層を有する、電縫鋼管。 - 前記成分組成が、さらに、質量%で、
Cu:1.0%以下および
Ni:1.0%以下のいずれか一方または両方を含む、請求項1に記載の電縫鋼管。 - 前記成分組成が、さらに、質量%で、
Nb:0.05%以下、
W :0.5%以下、
V :0.50%以下、および
Mo:2.0%以下からなる群より選択される1または2以上を含む、請求項1または2に記載の電縫鋼管。 - 前記成分組成が、さらに、質量%で、
REM:0.020%以下を含む、請求項1~3のいずれか一項に記載の電縫鋼管。
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US17/415,730 US20210310091A1 (en) | 2018-12-19 | 2019-09-18 | Electric resistance welded steel pipe or tube |
CA3123534A CA3123534C (en) | 2018-12-19 | 2019-09-18 | Electric resistance welded steel pipe or tube |
EP19900949.9A EP3901301B1 (en) | 2018-12-19 | 2019-09-18 | Electric resistance welded steel pipe |
MX2021007336A MX2021007336A (es) | 2018-12-19 | 2019-09-18 | Tuberia o tubo de acero soldada por resistencia electrica. |
CN201980083936.6A CN113227423B (zh) | 2018-12-19 | 2019-09-18 | 电阻焊钢管 |
JP2020510141A JP6747623B1 (ja) | 2018-12-19 | 2019-09-18 | 電縫鋼管 |
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WO2023157975A1 (ja) * | 2022-02-21 | 2023-08-24 | 日本製鉄株式会社 | 鋼管、車両用部品、鋼管の製造方法及び車両用部品の製造方法 |
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CA3123534C (en) | 2023-05-23 |
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ZA202104330B (en) | 2023-11-29 |
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