WO2018174270A1 - 線材、及び平鋼線 - Google Patents
線材、及び平鋼線 Download PDFInfo
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- WO2018174270A1 WO2018174270A1 PCT/JP2018/011862 JP2018011862W WO2018174270A1 WO 2018174270 A1 WO2018174270 A1 WO 2018174270A1 JP 2018011862 W JP2018011862 W JP 2018011862W WO 2018174270 A1 WO2018174270 A1 WO 2018174270A1
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
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- C21D2211/002—Bainite
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- C21D2211/00—Microstructure comprising significant phases
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a wire rod and a flat steel wire.
- a flat steel wire is used as a reinforcing material for flexible pipes used for transporting natural gas, crude oil, and the like mined on the seabed at high pressure.
- This type of flat steel wire is formed by subjecting a hot-rolled wire to 40-80% flat-pressure processing, and is used in a processed structure or by quenching and tempering.
- mining sites have become deep seas, and the transport distance of mined materials has become longer, so the demand for higher strength is increasing for flexible pipes and flat steel wires that are their reinforcements.
- the flat steel wire needs to have a characteristic that hydrogen induced cracking (HIC) does not easily occur, that is, a hydrogen induced crack resistance.
- HIC hydrogen induced cracking
- Patent Document 1 has been proposed as a technique for providing a high-strength material used in such a sour environment.
- Patent Document 1 describes a technique for obtaining a high-strength flat steel wire excellent in hydrogen embrittlement resistance by performing cold working on high-carbon steel having a pearlite structure and performing tempering for a short time.
- Patent Document 1 discloses a flat steel wire having a tensile strength of 1300 MPa or more and having excellent resistance to hydrogen-induced cracking in an environment of pH 5.6 or higher. This flat steel wire has a pH of 5 In the HIC test of .5 or less, cracks are formed even when the tensile strength is 1100 MPa. The present inventors thought that the strong decrease in hydrogen embrittlement resistance due to inclusion peeling was the cause of the lack of hydrogen-induced cracking resistance of the flat steel wire of Patent Document 1.
- the present invention has been made in view of the above situation, and an object of the present invention is to provide a wire rod having a tensile strength of 1100 MPa or more and capable of obtaining a flat steel wire excellent in resistance to hydrogen-induced cracking.
- the gist of the present invention is as follows.
- the wire according to one embodiment of the present invention has a chemical composition of mass%, C: 0.15 to 0.85%, Si: 0.10 to 2.00%, Mn: 0.30 to 1 50%, Al: 0.001 to 0.080%, Ca: 0.0002 to 0.0050%, N: 0.0020 to 0.0080%, P: 0.020% or less, S: 0.020 %: O: 0.0050% or less, Cr: 0 to 1.00%, V: 0 to 0.15%, Ti: 0 to 0.050%, Nb: 0 to 0.050%, Cu: 0 -1.00%, Ni: 0-1.50%, Mo: 0-1.00%, B: 0-0.0100%, REM: 0-0.0100%, Zr: 0-0.1000% the balance: a wire is Fe and impurities, and a CaO and Al 2 O 3, and oxides satisfying the formula a and formula B are defined as composite oxides, The composition ratio of the composite oxide defined by the formula C, measured
- the chemical composition may contain Cr: 0.05 to 1.00% by mass.
- the chemical composition is mass%, V: 0.02 to 0.15%, Ti: 0.002 to 0.050%, and Nb: One or more of 0.002 to 0.050% may be contained.
- the chemical composition is Cu: 0.01 to 1.00%, Ni: 0.01 to 1. One or more of 50%, Mo: 0.01 to 1.00% and B: 0.0002 to 0.0100% may be contained.
- the chemical composition is% by mass, REM: 0.0002 to 0.0100%, and Zr: 0.0002 to 0.00. You may contain 1 type (s) or 2 or more types of 1000%.
- the tensile strength may be 600 to 1400 MPa.
- the flat steel wire according to another aspect of the present invention has a chemical composition of mass%, C: 0.15 to 0.85%, Si: 0.10 to 2.00%, Mn: 0.00.
- the average equivalent circle diameter of the composite oxide is 3.0 ⁇ m or less.
- the structure in the central portion may include 98% by area or more of tempered martensite.
- the structure in the central portion may include 20 to 60 area% ferrite and 40 to 60 area% bainite.
- the tensile strength may be 1100 to 1500 MPa.
- the chemical composition may contain Cr: 0.05 to 1.00% by mass.
- the chemical composition is, in mass%, V: 0.02 to 0.15%, Ti: 0.002 to One or more of 0.050% and Nb: 0.002 to 0.050% may be contained.
- the chemical composition is, by mass, Cu: 0.01 to 1.00%, Ni: 0.01 to One or more of 1.50%, Mo: 0.01 to 1.00% and B: 0.0002 to 0.0100% may be contained.
- the chemical composition is% by mass, REM: 0.0002 to 0.0100%, and Zr: 0.0002 to You may contain 0.1000% of 1 type, or 2 or more types.
- the wire rod of the present invention can produce a flat steel wire according to this embodiment having a tensile strength of 1100 MPa or more and excellent hydrogen-induced cracking resistance in a severe sour environment having a pH of 5.5 or less. Since the flat steel wire according to this embodiment has a tensile strength of 1100 MPa or more and excellent resistance to hydrogen-induced cracking, it can be used, for example, as a flat steel wire for tension reinforcement of flexible pipes used in severe sour environments.
- the present inventors conducted various studies in order to solve the above-described problems. Conventionally, sulfide has been rendered harmless by the addition of Ca or the like, but even in this case, HIC sometimes occurs in the vicinity of the central axis of the wire rod and the flat steel wire.
- the present inventors have found that the presence of a composite oxide containing Al 2 O 3 and CaO has an influence on HIC in a cracked portion near the central axis. Then, the present inventors have found that HIC can be effectively prevented by controlling the composition and size of the composite oxide containing Al 2 O 3 and CaO in the vicinity of the central axis. . That is, the following findings (a) to (d) were obtained.
- C 0.15-0.85%
- C is an element that strengthens steel. To obtain this effect, C must be contained by 0.15% or more. On the other hand, if the C content exceeds 0.85%, the strength increases excessively, so that cracks are formed inside during flat pressure processing, and the hydrogen-induced crack resistance is deteriorated. Therefore, a suitable C content is 0.15 to 0.85%. Further, from the viewpoint of suppressing crack formation, the C content is preferably 0.20% or more, more preferably 0.30% or more, 0.35% or more, or 0.40% or more. .
- the C content is preferably 0.75% or less, and further 0.65% or less, 0.60% or less, or to improve hydrogen-induced cracking resistance, It is desirable that the content be 0.50% or less.
- Si 0.10 to 2.00%
- Si is an element that dissolves in the matrix and improves the strength of the flat steel wire. In order to obtain this effect, 0.10% or more of Si must be contained. However, when Si is contained exceeding 2.00%, cracks occur in the wire during flat pressure processing. Therefore, the Si content is 0.10% to 2.00%. In order to increase the strength, Si should be contained in an amount of 0.30% or more, and 0.50% or more, 0.55% or more, 0.60% or more, or 0.70 or more. preferable. When it is desired to suppress the cracking of the wire rod when processing into a flat steel wire, the Si content is preferably less than 2.00%, 1.80% or less, 1.70% or less, or 1.50%. More preferably, it is as follows.
- Mn 0.30 to 1.50%
- Mn has an effect of enhancing the hardenability of steel and is an element necessary for increasing the strength of a flat steel wire after heat treatment. In order to obtain this effect, 0.30% or more of Mn must be contained. However, when the content of Mn exceeds 1.50%, the strength of the wire becomes too high, and there arises a problem that the wire is cracked when the wire is processed into a flat steel wire. Therefore, the Mn content in the wire according to this embodiment is 0.30 to 1.50%. In order to increase the hardenability of the flat steel wire and increase the strength, the Mn content is preferably 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more.
- the Mn content is preferably 1.30% or less, 1.10% or less, 1.05% or less, or 1.00%. The following is even more preferable.
- S 0.020% or less S is an impurity. If the S content exceeds 0.020%, MnS becomes a stretched form, and the resistance to hydrogen-induced cracking is reduced. In order to improve the hydrogen-induced cracking resistance, the upper limit of S to be contained must be controlled in consideration of the balance with elements such as Ca that can easily form sulfides by combining with S. Therefore, the upper limit of the S content is 0.020% or less. From the viewpoint of improving the resistance to hydrogen-induced cracking, the S content is preferably less than 0.010%, more preferably less than 0.008% or less than 0.005%. From the viewpoint of steelmaking cost, the S content may be 0.001% or more, 0.003% or more, or 0.005% or more.
- P 0.020% or less
- P is an impurity. If the P content exceeds 0.020%, hydrogen-induced cracking is likely to occur, and the flat steel wire cannot suppress hydrogen-induced cracking in a severe sour environment at pH 5.5 or lower. Therefore, the P content is 0.020% or less. From the viewpoint of improving the resistance to hydrogen-induced cracking, the P content is preferably 0.015% or less, more preferably less than 0.013% or less than 0.010%, and less than 0.008%. Even more preferred. From the viewpoint of steelmaking cost, the P content may be 0.003% or more, or 0.005% or more.
- Al 0.001 to 0.080%
- Al is an element having a deoxidizing action, and is necessary for reducing the amount of oxygen in the wire. In order to obtain this effect, 0.001% or more of Al must be contained.
- a preferable Al content is 0.002 or more, or 0.005% or more, more preferably 0.015% or more, and further preferably 0.020% or more, or 0.025% or more.
- the Al content is set to 0.080% or less.
- the Al content is preferably 0.060% or less, more preferably 0.050% or less, 0.045% or less, or 0.040% or less. Even more preferred.
- N 0.0020 to 0.0080% N is dissolved in the ferrite phase and has the effect of improving the strength of the flat steel wire. Furthermore, N has the effect of forming nitrides and carbonitrides by combining with Al and Ti, etc., and has the effect of refining austenite grains during hot rolling, improving the resistance to hydrogen-induced cracking of flat steel wires. effective. In order to obtain these effects, N must be contained in an amount of 0.0020% or more, preferably 0.0030% or more, 0.0035% or more, or 0.0040% or more.
- the N content is preferably 0.0060% or less, and more preferably 0.0055% or less, 0.0050% or less, 0.0045% or less, or 0.0040% or less.
- O is an impurity. O forms a coarse oxide and reduces the hydrogen-induced cracking resistance of the steel. Accordingly, a lower O content is preferable.
- the O content is 0.0050% or less.
- the preferred O content is less than 0.0050%, more preferably less than 0.0040%, and even more preferably less than 0.0035%. From the viewpoint of steelmaking cost, the O content may be 0.0007% or more, or 0.0010% or more.
- Ca 0.0002 to 0.0050% Ca is effective in finely dispersing MnS by being contained in MnS. By finely dispersing MnS, hydrogen-induced cracking caused by MnS can be suppressed. In order to obtain the effect of suppressing hydrogen-induced cracking due to Ca, Ca should be contained in an amount of 0.0002% or more. To obtain a higher effect, 0.0005% or more, 0.0008% or more, 0.005% or more. What is necessary is just to contain 0010% or more or 0.0015% or more. However, if the Ca content exceeds 0.0050%, the effect is saturated, and the oxide produced by reacting with oxygen in the steel together with Al becomes coarse, and the oxide is pulverized during flat pressure processing.
- the appropriate Ca content is 0.0050% or less.
- the Ca content is preferably 0.0040% or less, more preferably 0.0030% or less, 0.0025% or less, or 0.0020% or less.
- the operational effects of the optional elements Cr, Ti, Nb, V, Cu, Ni, Mo, B, REM, and Zr and the reasons for limiting the content will be described.
- % For optional ingredients is% by weight.
- Cr 0 to 1.00%
- Cr may be contained in the wire in an amount of 0.05% or more in order to enhance the hardenability of the steel and increase the strength of the flat steel wire.
- the appropriate Cr content in the wire according to this embodiment is 1.00% or less.
- the Cr content is preferably 0.80% or less, and more preferably 0.60% or less.
- Ti 0 to 0.050% Ti combines with N and C to form carbides, nitrides or carbonitrides, and has the effect of refining austenite grains during hot rolling due to their pinning effect. Since there exists an effect which improves a characteristic, you may make it contain. In order to acquire this effect, it is preferable to contain Ti 0.002% or more. From the viewpoint of improving the resistance to hydrogen-induced cracking, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.050%, not only the effect is saturated, but a large amount of coarse TiN is generated, which causes cracks to be formed during flat pressure processing, and has resistance to hydrogen-induced cracking. There is a possibility of deteriorating. Therefore, the Ti content is 0.050% or less, and more preferably 0.035% or less.
- Nb 0 to 0.050% Nb combines with N and C to form carbides, nitrides or carbonitrides, and has the effect of refining austenite grains during hot rolling due to their pinning effect, and resistance to hydrogen-induced cracking of flat steel wires Since there exists an effect which improves a characteristic, you may make it contain.
- Nb is preferably contained in an amount of 0.002% or more. From the viewpoint of improving the resistance to hydrogen-induced cracking, the Nb content is preferably 0.005% or more, and more preferably 0.010% or more.
- the Nb content exceeds 0.050%, not only the effect is saturated, but also the steel productivity such as cracking in the steel slab in the step of rolling the steel ingot or slab into the steel slab. Adversely affect. Therefore, the Nb content is 0.050% or less, preferably 0.035% or less, and more preferably 0.030% or less.
- V 0 to 0.15%
- V combines with C and N to form carbides, nitrides or carbonitrides, and can increase the strength of the flat steel wire. In order to acquire this effect, it is preferable to contain V 0.02% or more.
- the content of V exceeds 0.15%, the strength of the flat steel wire increases due to the precipitated carbide or carbonitride, and cracks may be formed during flat pressure processing. Therefore, the V content is 0.15% or less. From the viewpoint of suppressing cracking during flat pressure processing, the V content is more preferably 0.10% or less, and even more preferably 0.08% or less. In order to stably obtain the effect of V described above, the lower limit of the V content is more preferably 0.03% or more.
- Cu 0 to 1.00%
- Cu is an element that enhances the hardenability of steel and may be contained. In order to obtain the effect of improving hardenability, it is preferable to contain Cu by 0.01% or more. However, if the Cu content exceeds 1.00%, the strength of the wire becomes excessively high, and problems such as the occurrence of cracks in the wire occur when processing into a flat steel wire. Accordingly, the Cu content when contained is 1.00% or less. From the viewpoint of improving hardenability, the Cu content is preferably 0.10% or more, and more preferably 0.30% or more. In consideration of workability to a flat steel wire, the Cu content when contained is preferably 0.80% or less, and more preferably 0.50% or less.
- Ni is an element that enhances the hardenability of steel and may be contained. In order to obtain the effect of improving hardenability, it is preferable to contain Ni by 0.01% or more. However, if the Ni content exceeds 1.50%, the strength of the wire becomes excessively high, and problems such as the occurrence of cracks in the wire occur when processing into a flat steel wire. Therefore, when Ni is contained, the content of Ni is 1.50% or less. From the viewpoint of improving hardenability, the Ni content when contained is preferably 0.10% or more, and more preferably 0.30% or more. In consideration of workability into a flat steel wire, the Ni content is preferably 1.00% or less, and more preferably 0.60% or less.
- Mo 0 to 1.00%
- Mo is an element that enhances the hardenability of steel and may be contained. In order to obtain the effect of improving hardenability, it is preferable to contain 0.01% or more of Mo. However, if the Mo content exceeds 1.00%, the strength of the wire becomes too high, and problems such as the occurrence of cracks in the wire occur when processing into a flat steel wire. Therefore, the Mo content when contained is 1.00% or less. From the viewpoint of improving hardenability, the Mo content is preferably 0.02% or more, and more preferably 0.05% or more. In consideration of workability to a flat steel wire, the Mo content when contained is preferably 0.50% or less, and more preferably 0.30% or less.
- B 0 to 0.0100% B is effective in increasing the hardenability of steel by adding a trace amount, and 0.0002% or more may be contained in order to obtain this effect.
- the content of B when contained is 0.0100% or less.
- the B content may be 0.0010% or more, and more preferably 0.0020% or more.
- the B content is preferably 0.0050% or less, and more preferably 0.0030% or less.
- REM 0 to 0.0100% REM is a general term for rare earth metals, and has the effect of finely dispersing MnS by being contained in MnS as in Ca. Since the resistance to hydrogen-induced cracking can be improved by finely dispersing MnS, REM may be contained. In order to obtain the effect of suppressing hydrogen-induced cracking, REM should be contained in an amount of 0.0002% or more, and in the case of obtaining a higher effect, 0.0005% or more may be contained. However, even if the content of REM exceeds 0.0100%, the effect is saturated, and the oxide generated by reacting with oxygen in the steel becomes coarse, causing cracks during flat pressure processing.
- the content of REM in the case of containing is 0.0100% or less.
- the content of REM is preferably 0.0050% or less, and more preferably 0.0030% or less.
- REM rare earth element
- Y yttrium
- 15 elements lanthanoid
- La lanthanum
- Lu lutetium
- the content of REM means the total value of the contents of these 17 elements.
- Zr 0 to 0.1000% Zr reacts with O to produce an oxide, and if added in a small amount, Zr has an effect of finely dispersing the oxide and suppressing hydrogen-induced cracking, and may be contained when it is desired to obtain the effect. .
- Zr may be contained in an amount of 0.0002% or more, and in the case of obtaining a higher effect, 0.0010% or more may be contained.
- the content of Zr exceeds 0.1000%, the effect is saturated and reacts with N and S in the steel to produce coarse nitrides and sulfides. It causes a decrease in cracking characteristics. Therefore, the content of Zr when contained is 0.1000% or less.
- the Zr content is preferably 0.0800% or less, and more preferably 0.0500% or less.
- the balance of the chemical composition of the wire contains Fe and impurities.
- Impurity is a mixture of ore, scrap, or production environment as a raw material when industrially producing steel materials, and has a substantial effect on the properties of the wire according to this embodiment. It refers to what is not given.
- the wire having the above components contains an oxide containing a predetermined amount or more of CaO and Al 2 O 3 .
- the inventors of the present invention have found that voids generated around the oxide during the flat pressure processing of the wire promote hydrogen-induced cracking.
- the composition ratio and size of the oxide within an appropriate range, it becomes possible to finely pulverize it during flat pressure processing, and the oxide moves following the base material during pulverization.
- the present inventors have found that the adhesion between the base material and the oxide is also improved, and the resistance to hydrogen-induced cracking after flat pressure processing is improved. In order to obtain this effect, it is necessary to strictly control the composition ratio and size of the oxide.
- the oxide that affects the resistance to hydrogen-induced cracking of the wire and the flat steel wire is an oxide containing CaO and Al 2 O 3 and satisfying the formulas A and B.
- an oxide containing CaO and Al 2 O 3 and satisfying Formula A and Formula B may be abbreviated as “composite oxide”.
- Formula A (Content of oxide-forming elements other than Ca and Al in oxide in unit mol%) ⁇ (1/3) ⁇ (Ca content in oxide in unit mol% or Al content is large) Way): Formula A (O content in oxide in mol%) ⁇ (S content in oxide in mol%): Formula B “Oxide forming elements other than Ca and Al” described in Formula A above are Si, Mg, and Mn in the chemical composition of the wire according to this embodiment.
- the composite oxide causes hydrogen-induced cracking, and is targeted for improvement in the wire according to the present embodiment. Therefore, in the wire according to this embodiment, the composition and size of the complex oxide are limited.
- various inclusions other than the composite oxide do not substantially affect the hydrogen-induced cracking.
- various inclusions other than the complex oxide are not particularly limited.
- oxides other than CaO and Al 2 O 3 are in trace amounts due to the chemical composition of the wire. Therefore, oxides other than CaO and Al 2 O 3 do not affect hydrogen-induced cracking.
- Si, Mg, and content of the other oxides forming elements, such as Mn (mol%) is Ca content (mol%) or Al content It is considered that the complex oxide which is 1/3 or more of (mol%) does not exist at the crack starting point in the evaluation test of the wire rod and the flat steel wire and does not affect the hydrogen induced cracking.
- the complex oxide to be controlled is limited to those containing CaO and Al 2 O 3 and satisfying the following formulas A and B.
- a composite oxide which is a control object in a wire according to the present embodiment may be substantially limited to the oxide of CaO and Al 2 O 3 Prefecture.
- the evaluation of the composite oxide is performed at the center of the C cross section of the wire, that is, at the center of the cross section perpendicular to the rolling direction of the wire.
- the central portion 11 of the C cross section of the wire 1 is a range of 1/10 of the diameter d of the wire from the center of the C cross section of the wire 1 when the C cross section of the wire 1 is substantially circular.
- the central portion 11 of the C cross section of the substantially circular wire 1 is a region within a concentric circle of the cross section of the wire 1 having a diameter of 1 / 5d (2 / 10d).
- the C cross section of the wire 1 is not substantially circular
- an area where the C cross section of the wire 1 has a similarity ratio of 1/5 and the geometrical center of the C cross section of the wire 1 coincides with the region of the wire 1 described above. It is regarded as the central part 11 of the C cross section.
- the complex oxide tends to collect at the center of the slab, and therefore the complex oxide tends to collect at the center even in a wire obtained by rolling the slab.
- the composition of the complex oxide in the central part is substantially the same as that in the peripheral part, and if the precipitation of coarse complex oxide is suppressed in the central part, the composition is also suppressed in the peripheral part. It is thought that there is.
- the composite oxide is evaluated at the center of the C cross section of the wire.
- composition ratio ⁇ of CaO and Al 2 O 3 calculated by the following formula C is 0.00 ⁇ ⁇ ⁇ 3.00. Since it becomes brittle, it can be pulverized during flat pressure processing.
- Composition ratio ⁇ (CaO concentration in the oxide in unit mass%) / (Al 2 O 3 concentration in the oxide in unit mass%): Formula C Specifically, when ⁇ is close to 0.00, that is, when the component of the composite oxide is mainly Al 2 O 3, or when ⁇ is smaller than 3.00, the composite oxide is pulverized during flat pressure processing. Is done.
- controlling the average value of the composite oxide composition ratio ⁇ at the center of the wire within the above range improves the resistance to hydrogen-induced cracking of the wire after flat pressure processing (ie, flat steel wire). Needed.
- the average value of the composition ratio ⁇ of the composite oxide at the center of the wire is 3.00 or more, the component of the composite oxide is mainly CaO, and hydrogen resistance induction after flat pressure processing is performed regardless of the size control. Cracking characteristics deteriorate.
- the average value of the composite oxide composition ratio ⁇ at the center of the wire is defined as 0 or more and 3.00 or less.
- the upper limit of the average value of the composition ratio ⁇ of the composite oxide at the center of the wire is preferably 1.00 or less, more preferably 0.60 or less. Further, the lower limit of the average value of the composition ratio ⁇ of the complex oxide at the center of the wire may be 0.02, 0.05, 0.10, 0.15, or 0.20.
- the average value of the equivalent circle diameter of the composite oxide is set to 6.0 ⁇ m or less.
- the lower limit of the average value of the equivalent circle diameter of the composite oxide is not particularly limited, but may be defined as 2.0 ⁇ m, 2.5 ⁇ m, 3.0 ⁇ m, 3.5 ⁇ m, or 4.0 ⁇ m.
- (D-1) Chemical Composition of Composite Oxide The chemical composition of the composite oxide is considered to be substantially uniform in one wire regardless of its size. Therefore, the observation is made in 10 visual fields in the C cross section of the central portion of the wire, and the chemical composition analysis is performed only with respect to the composite oxide having the largest equivalent circle diameter in each visual field (the composite oxide with the easiest chemical composition analysis).
- the composition ratio ⁇ is calculated, and the value obtained by averaging the composite oxide composition ratio ⁇ in the 10 fields of view is regarded as the average value of the composite oxide composition ratio ⁇ measured at the center of the wire. Can do. If this value satisfies the requirements of the wire according to this embodiment described above, the wire is considered to satisfy the requirements of the wire according to this embodiment.
- a specific method for analyzing the chemical composition of the composite oxide of the wire will be described below.
- the inclusion such as complex oxide was observed at a magnification of 1000 times using a field emission scanning electron microscope (FE-SEM). Observe 10 places on the backscattered electron image and take a picture.
- the area per field of view is 8000 ⁇ m 2 (vertical 100 ⁇ m, horizontal 80 ⁇ m) or more.
- the chemical composition of each inclusion is measured using EDS together, and it is determined whether or not the inclusion is a complex oxide to be controlled in the wire according to the present embodiment.
- elemental analysis is performed by obtaining a characteristic X-ray spectrum for the maximum size composite oxide in each photograph using an energy dispersive X-ray analyzer (EDS).
- EDS energy dispersive X-ray analyzer
- the elements contained in the complex oxide are identified from the peak energy of the obtained characteristic X-ray spectrum, and the content (mol%) of these elements is quantified from the peak height.
- the CaO / Al 2 O 3 having a mass ratio of the composite oxide is calculated.
- the composition ratio ⁇ of the maximum size composite oxide is determined. Then, by averaging these composition ratios ⁇ in 10 fields of view, the average value of the composition ratios ⁇ of the composite oxide measured at the center of the wire is calculated.
- the content (mol%) of other oxide-forming elements such as Si, Mg, and Mn is 1/3 or more of the larger one of Ca content (mol%) and Al content (mol%).
- the oxide (that is, the oxide not satisfying the above formula A) is, for example, an oxide containing CaO, Al 2 O 3 and SiO 2 , and is determined not to be controlled in the wire according to the present embodiment.
- inclusions whose O content (mol%) is smaller than S content (mol%) that is, inclusions not satisfying the above formula B) are sulfide inclusions, and in the wire according to this embodiment Judged not to be controlled. Such inclusions are ignored in confirming the state of the composite oxide.
- the chemical composition of each inclusion is measured using EDS together, and it is determined whether or not the inclusion is a complex oxide to be controlled in the wire according to the present embodiment.
- the area of the maximum size complex oxide is measured from each photograph by ordinary image analysis, and the equivalent circle diameter obtained from the area is calculated.
- the photograph for measuring the equivalent circle diameter is preferably a reflected electron image.
- the average value of the equivalent circle diameters of the composite oxides measured at the center of the C cross-section of the wire is obtained by obtaining the average value of the equivalent circle diameters of the largest composite oxides in 10 photographs obtained by the above method. can get.
- the metallographic structure of the wire does not substantially affect the hydrogen-resistant cracking characteristics of the flat steel wire. This is because, as described above, the state of the sulfide and the composite oxide is dominant with respect to the hydrogen-induced cracking resistance characteristics of the flat steel wire. Therefore, the metal structure of the wire is not limited. However, when workability is taken into consideration, it is preferable to control the metal structure of the wire to a pearlite structure, a ferrite structure, or a bainite structure.
- the metal structure of the wire may be defined as including a pearlite structure, a ferrite structure, and a bainite structure having a total area of 99 area% or more.
- the diameter of the wire is not particularly limited. Since the diameter of the wire rod for flat steel wire currently distributed in the market is usually 7 to 16 mm, the diameter of the wire rod according to this embodiment may be defined as 7 to 16 mm.
- the tensile strength of the wire is not particularly limited. In view of the chemical composition of the wire, the tensile strength of the wire is often about 600 to 1400 MPa. Therefore, you may prescribe
- the effect of the wire according to the present embodiment can be obtained regardless of the method of manufacturing the wire.
- the wire may be manufactured by the following manufacturing method.
- the following manufacturing process is an example, and even when the wire composition whose chemical composition and other requirements are within the range of the wire material according to the present embodiment is obtained by a process other than the following, the wire material is the present embodiment. Needless to say, it is included in the wire.
- the molten steel components are adjusted in a converter, and after adding the Ca alloy to the molten steel, a steel piece is obtained using continuous casting. Thereafter, the steel slab is reheated and product is rolled hot to finish a steel material having a predetermined diameter.
- Desulfurizing agent is added to the hot metal discharged from the blast furnace, and the mixture is stirred and desulfurized by the KR (Kanbara Reactor) method for removing sulfur, followed by dephosphorization and decarburization in a converter. Then, when the molten steel is discharged from the converter to the molten steel pan, the alloy of the molten steel is adjusted by adding an alloy such as metal Al for elements other than Ca, REM, and Zr in the target chemical composition. Subsequently, the molten steel is degassed by RH (Ruhrstahl-Heraeus), and a Ca alloy is added to the molten steel.
- the composition of the Ca alloy is, for example, Ca: 40% by mass and Si: 60% by mass.
- the Ca alloy is added by, for example, a powder injection method in which Ca alloy powder is blown into steel together with an inert gas.
- the timing for adding the Ca alloy is 30 minutes or more and 60 minutes or less from the addition of the metal Al.
- the addition of the Ca alloy is less than 30 minutes from the addition of the metal Al, a part of the Ca added in the steel is consumed by reacting with the coarse Al 2 O 3 floating in the steel. Therefore, the sulfide detoxification effect by Ca cannot be obtained.
- the addition of the Ca alloy is made less than 30 minutes after the addition of the metal Al, coarse Al 2 O 3 remains, so that the equivalent circle diameter of the oxide in the rolled wire does not become 6.0 ⁇ m or less.
- the composition ratio ⁇ (CaO mass% / Al 2 O 3 mass%) of the composite oxide is 0.00 ⁇ ⁇ ⁇ 3.00.
- This molten steel is made into a steel piece using a continuous casting method.
- the casting speed when making a steel slab is preferably 0.6 m / min to 1.4 m / min. At the time of casting, some of the inclusions float and do not remain in the steel slab, but other inclusions descend and remain in the steel slab.
- inclusions that have been levitated once fall again, and coarse inclusions may increase in the slab.
- the inclusions that descend are increased, so that coarse inclusions may increase in the steel slab.
- the obtained steel slab is hot-rolled to produce a wire rod.
- Hot rolling is performed by heating the steel slab to 1020 ° C or higher.
- the final finishing temperature of hot rolling is 800 to 960 ° C.
- the cross-sectional area ratio of the hot-rolled wire and the steel slab before hot rolling is 100.0 or more. Hot rolling is performed. If the rolling temperature in the final finish rolling is less than 800 ° C.
- the size and composition ratio of the composite oxide can be controlled.
- the flat steel wire according to the present embodiment is obtained by rolling the wire according to the present embodiment.
- the shape of the flat steel wire 2 is not particularly limited, but the shape of the C cross section is preferably a shape obtained by crushing a circle as illustrated in FIG.
- the short diameter of the C cross section is referred to as the thickness t of the flat steel wire 2
- the long diameter of the C cross section is referred to as the width w of the flat steel wire 2.
- the L cross section of the flat steel wire 2 mentioned later means a cross section that is parallel to the rolling direction and the minor axis direction of the flat steel wire and substantially includes the central axis of the flat steel wire.
- the central axis of the flat steel wire is an axis that passes through the center of the C cross section and is parallel to the rolling direction.
- the minor axis direction of the flat steel wire is the minor axis direction of the cross section perpendicular to the rolling direction of the flat steel wire.
- the central portion 21 in the L cross section of the flat steel wire 2 is 1 / of the short diameter of the flat steel wire 2 (thickness t of the flat steel wire 2) from the central axis of the flat steel wire 2. It means an area within a range of 7 or less.
- the central portion 21 in the L cross section of the flat steel wire is a region having a depth of 5 / 14t or more from the surface of the flat steel wire in the L cross section.
- central portion 21 in the L cross section of the flat steel wire 2 may be simply referred to as “central portion”.
- central portion an arbitrary axis passing through the center of the C cross section of the flat steel wire and an axis perpendicular thereto are used. It can be regarded as a major axis and a minor axis.
- the chemical composition of the flat steel wire is, by mass, C: 0.15 to 0.85%, Si: 0.10 to 2.00%, Mn: 0.30 to 1.50%, Al: 0.001 To 0.080%, Ca: 0.0002 to 0.0050%, N: 0.0020 to 0.0080%, P: 0.020% or less, S: 0.020% or less, O: 0.0050%
- Cr 0 to 1.00%
- V 0 to 0.15%
- Ti 0 to 0.050%
- Nb 0 to 0.050%
- Cu 0 to 1.00%
- REM 0 ⁇ 0.0100%
- Zr 0 ⁇ 0.1000%
- the balance containing Fe and impurities including.
- the chemical composition of the flat steel wire according to this embodiment matches the chemical composition of the wire according to this embodiment.
- the preferable upper limit mentioned above regarding the each element in the chemical composition of a wire, and a preferable lower limit are naturally applicable also in the chemical composition of a flat steel wire.
- the form of an oxide (composite oxide) containing CaO and Al 2 O 3 is defined.
- the definition of the complex oxide of the flat steel wire and its composition ratio ⁇ is the same as the definition of the complex oxide of the wire and its composition ratio ⁇ .
- the average value of the composition ratio ⁇ of the complex oxide observed in the center portion of the flat steel wire is 0.00 ⁇ ⁇ ⁇ 3.00. Since the flat steel wire is obtained by rolling a wire, the chemical composition of the composite oxide of the flat steel wire according to this embodiment matches the chemical composition of the composite oxide of the wire according to this embodiment.
- the average value of the equivalent circle diameter of the complex oxide observed at the center of the flat steel wire is 3.0 ⁇ m or less.
- the average equivalent circle diameter of the composite oxide observed at the center of the flat steel wire is more than 3.0 ⁇ m, the hydrogen-induced cracking resistance of the flat steel wire is impaired by voids generated around the composite oxide. .
- the metal structure of a flat steel wire does not significantly affect the hydrogen-induced cracking resistance characteristics of a flat steel wire, as does the metal structure of a wire. Therefore, the metal structure of the flat steel wire is not particularly limited.
- the structure in the center portion of the flat steel wire contains tempered martensite of 98 area% or more, it is preferable because the tensile strength of the flat steel wire can be further improved.
- the structure in the central portion of the flat steel wire contains 20 to 60 area% ferrite and 40 to 60 area% bainite, it is preferable because the toughness of the flat steel wire can be improved.
- the width w and thickness t of the flat steel wire are not particularly limited. Since the width of a flat steel wire currently distributed in the market is usually 13 to 16 mm and the thickness t is usually 2 to 7 mm, the width and thickness of the flat steel wire according to the present embodiment are as described above. You may prescribe to.
- the tensile strength of the flat steel wire is not particularly limited. In view of the use of the flat steel wire, the tensile strength of the flat steel wire is desirably about 1100 to 1500 MPa, and this can be achieved by appropriately adjusting the heat treatment conditions of the flat steel wire.
- the shape of the sulfide inclusions in the flat steel wire need not be limited for the same reason as the sulfide inclusions in the wire rod.
- the evaluation method of the complex oxide of flat steel wire is basically the same as the evaluation method of the complex oxide of wire. However, the evaluation of the composite oxide of the wire is performed at the center of the C cross section of the wire, but only in the point that the evaluation of the composite oxide of the flat steel wire is performed at the center of the L cross section of the flat steel wire.
- the evaluation method of the complex oxide differs between the flat steel wire and the wire rod.
- the L cross section of the flat steel wire in this embodiment is a cross section including the central axis of the flat steel wire, when evaluating the composite oxide, the cross section slightly separated from the central axis of the flat steel wire is measured.
- the composite oxide may be evaluated using the surface.
- an axis parallel to the rolling direction of the measurement surface is regarded as the central axis of the flat steel wire, and the central portion on the measurement surface may be specified. Even if there is a slight gap between the measurement surface and the central axis of the flat steel wire, the evaluation result of the composite oxide is not substantially affected.
- the manufacturing method of the flat steel wire according to the present embodiment includes a step of flat-pressure processing the wire according to the present embodiment.
- the area reduction rate in flat pressure machining is 40% or more.
- the complex oxide in the wire is not sufficiently pulverized, so it is difficult to set the maximum equivalent circle diameter of the complex oxide of the flat steel wire to 3.0 ⁇ m or less. .
- the wire material before flat pressure processing or the flat steel wire after flat pressure processing may be appropriately heat treated. This is because the form of the composite oxide and sulfide does not change significantly at the heat treatment temperature for ordinary steel.
- Steels A and B having chemical compositions shown in Table 1 were produced by the following method.
- the hot metal discharged from the blast furnace was desulfurized using the KR method, and dephosphorized and decarburized in the converter. Thereafter, metal Al or the like was added to the molten steel in order to adjust elements other than Ca, REM, and Zr in the chemical composition.
- Samples for analysis were taken from the molten steel, component analysis was performed, and chemical compositions other than Ca, REM, and Zr were adjusted. Thereafter, the molten steel was degassed with RH, and a CaSi alloy was added to the molten steel.
- the composition of the CaSi alloy was Ca: 40% by mass and Si: 60% by mass.
- the CaSi alloy was added by a powder injection method in which CaSi alloy powder was blown into the steel together with an inert gas.
- test numbers A1, A4, A5 and B1 the addition of the CaSi alloy was performed 40 minutes after the addition of the metal Al.
- test numbers A2 and B2 the addition of the CaSi alloy was performed 25 minutes after the addition of the metal Al.
- test numbers A3 and B3 the addition timing of the CaSi alloy was performed 70 minutes after the addition of metal Al.
- the molten steel thus obtained was cast into a steel ingot.
- the casting speed was 0.9 m / min.
- this steel ingot was reheated at 1250 ° C. for 12 hours, and then rolled into 122 mm square steel pieces to obtain a rolling material.
- the rolling material was heated to 1050 ° C. and rolled into a wire with a diameter of 12 mm.
- A4 was heated to 1250 ° C. and then hot-rolled to a diameter of 16 mm, cut to a length of 1500 mm, and ground to a diameter of 12 mm.
- the surface of the wire was lubricated, and then primary wire drawing was performed to obtain a wire having a diameter of 11 mm. Then, about all the steel materials, the wire-drawn wire rod was flat-rolled (flat pressure processing) with a cold rolling mill and formed into a flat steel wire having a width of 15 mm and a thickness of 3 mm.
- Test Nos. A1 to A4 and Test Nos. B1 to B3 are flat steel wires, heated at 900 ° C. for 15 min, immersed in cold oil and quenched, and tempered at 450 ° C. for 60 min. Processed. On the other hand, in test number A5, annealing treatment was performed at 450 ° C. for 60 minutes after flat rolling (flat pressure processing).
- steels a to au (test numbers 1 to 47 in Table 4-1 and Table 4-2) having chemical compositions shown in Table 2-1 and Table 2-2 were melted in the same manner as Steel A1, and obtained.
- the steel ingot was heated at 1250 ° C. for 12 hours, and then the steel slab that was rolled into 122 mm square steel slabs was used as the material for rolling.
- the rolling material was heated at 1050 ° C. and flat rolled (flat pressure processing) into a wire having a diameter of 12 mm. Thereafter, the surface of the wire was lubricated and primary wire drawing was performed so as to obtain a wire having a diameter of 11 mm.
- the drawn wire was rolled with a cold rolling mill and formed into a flat steel wire having a width of 15 mm and a thickness of 3 mm.
- the formed flat steel wire was heated at 900 ° C. for 15 minutes after cold rolling, then immersed in cold oil for quenching treatment, and heat treatment at 450 ° C. for 60 minutes.
- the test and evaluation were stopped for the sample in which the flat steel wire was cracked when it was cold-rolled into the flat steel wire without performing the steps after the heat treatment. In the evaluation result column of such a sample, a symbol “-” is described.
- Table 2-1 and Table 2-2 indicate that the component composition is outside the scope of the present invention.
- Table 3-1 values outside of the preferable production conditions are underlined.
- Tables 3-1 and 4-2 values outside the range of the present invention are also underlined.
- “composite oxide composition ratio ⁇ ” indicates the average value of the composite oxide composition ratio ⁇ at the center of the wire
- “average equivalent circle diameter” indicates the center of the wire or the center of the flat steel wire.
- the average value of the equivalent circle diameter of the composite oxide in the part is shown.
- the average value of the composition ratio ⁇ of the composite oxide in the central portion of the flat steel wire is not measured because it substantially coincides with that in the central portion of the wire.
- the average value of the composite oxide composition ratio ⁇ at the center of the wire and the average value of the equivalent circle diameter of the composite oxide at the center of the wire or the center of the flat steel wire were investigated by the above-described method.
- the tensile strength of the wire, the structure of the flat steel wire, the tensile strength, and the resistance to hydrogen-induced cracking were investigated by the methods described below.
- the area ratios of the ferrite structure and the bainite structure in the flat steel wire were calculated by averaging the area ratios of the five ferrite structures and the bainite structure.
- the value obtained by subtracting the area ratio of the ferrite structure and bainite structure from 100% The average area ratio was considered.
- Tensile strength of flat steel wire A flat steel wire was cut into a length of 400 mm, and a tensile test was performed with 100 mm above and below fixed with a hydraulic chuck. The tensile stress was calculated by dividing the maximum load obtained by the cross-sectional area of the flat steel wire. Since the tensile strength is preferably 1100 MPa or more, the tensile strength of 1100 MPa or more was evaluated as an acceptable product.
- Af total area (mm 2 ) of crack generation parts measured by UST
- w width of flat steel wire (mm)
- L length of flat steel wire (mm).
- test numbers A2 and B2 since the time from the addition of metal Al to the addition of the CaSi alloy was 25 minutes, the composite oxide of the wire became coarse, and hydrogen-induced cracking occurred in the flat steel wire. Has occurred.
- test numbers A3 and B3 since the time from the addition of metal Al to the addition of CaSi alloy was 70 minutes, the average value of the composition ratio ⁇ of the composite oxide of the wire became 3.00 or more, and flat steel Hydrogen-induced cracking has occurred in the wire.
- Test No. A4 has a small reduction in cross-sectional area during hot rolling, and pulverization of the oxide during hot rolling was insufficient, so that the maximum size of the composite oxide of the wire became out of the scope of the present invention. Hydrogen-induced cracking has occurred in the wire.
- Test Nos. 16, 17, 20, and 46 had a chemical composition outside the scope of the present invention, and cracking occurred in the flat steel wire when it was cold-rolled into a flat steel wire (at the time of flat pressure processing). The test was stopped without performing the above step.
- Test No. 16 has Mn and Ca contents outside the scope of the present invention, and cracks occur during flat pressure processing.
- Test No. 17 has a C content outside the range of the present invention, and cracking occurs during flat pressure processing.
- Test No. 20 has a Si content outside the range of the present invention, and cracking occurs during flat pressure processing.
- Test No. 18 has an S content outside the scope of the present invention, and hydrogen-induced cracking occurs.
- Test No. 21 has C and N contents outside the scope of the present invention, and the tensile strength does not reach 1100 MPa.
- Test number 22 indicates that the Ca content is outside the scope of the present invention, the composition ratio of the composite oxide is outside the scope of the present invention, and hydrogen-induced cracking occurs.
- Test No. 42 has a Si content outside the range of the present invention, and the tensile strength does not reach 1100 MPa.
- Test No. 43 has a Mn content outside the range of the present invention, and the tensile strength has not reached 1100 MPa.
- test number 45 the Al content is outside the range of the present invention, the maximum size of the composite oxide is outside the range of the present invention, and hydrogen-induced cracking occurs.
- Test No. 46 has an N content outside the range of the present invention, and cracking occurs during flat pressure processing.
- the Ca content is outside the range of the present invention, MnS is not refined, the maximum size of the composite oxide is outside the range of the present invention, and hydrogen-induced cracking occurs.
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Abstract
Description
本願は、2017年3月24日に、日本に出願された特願2017-059111号に基づき優先権を主張し、その内容をここに援用する。
しかし、平鋼線を高強度化した場合には、平鋼線の水素感受性が高まり水素誘起割れが助長される。さらに、線材に40~80%の平圧加工を施すと線材中に存在する硫化物が伸長することで、鉄と硫化物との間に剥離が生じ、剥離によって生じた空隙に水素が集まることで水素誘起割れが一層発生しやすくなる。このため、サワー環境で使用されるフレキシブルパイプに適用する、高強度かつ耐水素誘起割れ特性に優れた平鋼線を製造できる熱間圧延線材の開発が望まれている。これまで、このようなサワー環境で使用される高強度材を提供する技術として、特許文献1が提案されている。
(1)本発明の一態様に係る線材は、化学組成が、質量%で、C:0.15~0.85%、Si:0.10~2.00%、Mn:0.30~1.50%、Al:0.001~0.080%、Ca:0.0002~0.0050%、N:0.0020~0.0080%、P:0.020%以下、S:0.020%以下、O:0.0050%以下、Cr:0~1.00%、V:0~0.15%、Ti:0~0.050%、Nb:0~0.050%、Cu:0~1.00%、Ni:0~1.50%、Mo:0~1.00%、B:0~0.0100%、REM:0~0.0100%、Zr:0~0.1000%、残部:Fe及び不純物である線材であって、CaOとAl2O3とを含み、且つ式A及び式Bを満たす酸化物が複合酸化物と定義され、前記線材の圧延方向と垂直な断面における、前記線材の中心軸から前記線材の直径の1/10の範囲内である中心部において測定される、式Cによって定義される前記複合酸化物の組成比εの平均値が0.00≦ε<3.00であり、前記断面の前記中心部において測定される前記複合酸化物の円相当径の平均値が6.0μm以下である。
(酸化物中のCa、Al以外の酸化物形成元素の単位mol%での含有量)<(1/3)×(酸化物中の単位mol%でのCa含有量又はAl含有量のうち多い方):式A
(単位mol%での酸化物中のO含有量)≧(単位mol%での酸化物中のS含有量):式B
(組成比ε)=(単位質量%での前記複合酸化物中のCaO濃度)/(単位質量%での前記複合酸化物中のAl2O3濃度):式C
(2)上記(1)に記載の線材では、前記化学組成が、質量%で、Cr:0.05~1.00%を含有してもよい。
(3)上記(1)又は(2)に記載の線材では、前記化学組成が、質量%で、V:0.02~0.15%、Ti:0.002~0.050%及びNb:0.002~0.050%の1種又は2種以上を含有してもよい。
(4)上記(1)~(3)の何れか一項に記載の線材では、前記化学組成が、質量%で、Cu:0.01~1.00%、Ni:0.01~1.50%、Mo:0.01~1.00%及びB:0.0002~0.0100%の1種又は2種以上を含有してもよい。
(5)上記(1)~(4)の何れか一項に記載の線材では、前記化学組成が、質量%で、REM:0.0002~0.0100%及びZr:0.0002~0.1000%の1種又は2種以上を含有してもよい。
(6)上記(1)~(5)の何れか一項に記載の線材では、引張強さが600~1400MPaであってもよい。
(7)本発明の別の態様に係る平鋼線は、化学組成が、質量%で、C:0.15~0.85%、Si:0.10~2.00%、Mn:0.30~1.50%、Al:0.001~0.080%、Ca:0.0002~0.0050%、N:0.0020~0.0080%、P:0.020%以下、S:0.020%以下、O:0.0050%以下、Cr:0~1.00%、V:0~0.15%、Ti:0~0.050%、Nb:0~0.050%、Cu:0~1.00%、Ni:0~1.50%、Mo:0~1.00%、B:0~0.0100%、REM:0~0.0100%、及びZr:0~0.1000%、残部:Fe及び不純物である平鋼線であって、CaOとAl2O3とを含み、且つ式A及び式Bを満たす酸化物が複合酸化物と定義され、前記平鋼線の圧延方向及び短径方向に平行であって、且つ前記平鋼線の中央軸を含む断面における、前記中央軸から前記平鋼線の短径の1/7以内の範囲内である中央部において測定される、下記式Cによって定義される前記複合酸化物の組成比εの平均値が0.00≦ε<3.00であり、前記断面の前記中央部において測定される前記複合酸化物の円相当径の平均値が3.0μm以下である。
(酸化物中のCa、Al以外の酸化物形成元素の単位mol%での含有量)<(1/3)×(酸化物中の単位mol%でのCa含有量又はAl含有量のうち多い方):式A
(単位mol%での酸化物中のO含有量)≧(単位mol%での酸化物中のS含有量):式B
(組成比ε)=(単位質量%での前記複合酸化物中のCaO濃度)/(単位質量%での前記複合酸化物中のAl2O3濃度):式C
(8)上記(7)に記載の平鋼線では、前記中央部における組織が、98面積%以上の焼戻しマルテンサイトを含んでもよい。
(9)上記(7)に記載の平鋼線では、前記中央部における組織が、20~60面積%のフェライトと40~60面積%のベイナイトとを含んでもよい。
(10)上記(7)~(9)の何れか一項に記載の平鋼線では、引張強さが1100~1500MPaであってもよい。
(11)上記(7)~(10)の何れか一項に記載の平鋼線では、前記化学組成が、質量%で、Cr:0.05~1.00%を含有してもよい。
(12)上記(7)~(11)の何れか一項に記載の平鋼線では、前記化学組成が、質量%で、V:0.02~0.15%、Ti:0.002~0.050%及びNb:0.002~0.050%の1種又は2種以上を含有してもよい。
(13)上記(7)~(12)の何れか一項に記載の平鋼線では、前記化学組成が、質量%で、Cu:0.01~1.00%、Ni:0.01~1.50%、Mo:0.01~1.00%及びB:0.0002~0.0100%の1種又は2種以上を含有してもよい。
(14)上記(7)~(13)の何れか一項に記載の平鋼線では、前記化学組成が、質量%で、REM:0.0002~0.0100%及びZr:0.0002~0.1000%の1種又は2種以上を含有してもよい。
(a)平鋼線の水素誘起割れは、鋼中の粗大な硫化物を起点に発生する。特に、MnSなどの硫化物が粗大である場合、熱間圧延した線材を平圧加工する際に、硫化物の周囲に空隙が生じ、pH5.5以下の厳しいサワー環境において水素誘起割れを促進する要因となる。
(b)そのため、線材に含有される硫化物をなるべく微細化する必要がある。硫化物を微細化するためには、CaあるいはMgを微量添加し、CaやMgを一部含むMnS、またはCaSやMgSとすることが効果的である。
(c)Caを添加すると、CaOとAl2O3とを含む酸化物(以下、複合酸化物と称する場合がある)が生じる場合があり、平圧加工時には、これら複合酸化物の周囲に空隙が生じ、pH5.5以下の厳しいサワー環境において水素誘起割れを促進する要因となる。
(d)よって、平圧加工時に複合酸化物周囲の空隙形成を抑制する必要がある。そのためには、複合酸化物のサイズおよび組成比を制御して、平圧加工時に複合酸化物が粉砕されるようにすることが効果的である。粉砕した複合酸化物は母材と良く密着しているため、高強度かつpH5.5以下の厳しいサワー環境における水素誘起割れの発生が大きく低減される。
本実施形態に係る線材、及びこれを圧延して得られる本実施形態に係る平鋼線は、上記の知見に基づいて完成されたものである。以下に、まず本実施形態に係る線材について説明する。
(A)化学組成について:
以下、本実施形態に係る線材の化学組成について詳細に説明する。なお、化学組成の含有量は質量%である。
Cは、鋼を強化する元素である。この効果を得るにはCを0.15%以上含有させなくてはならない。一方、Cの含有量が0.85%を超えると、強度が上がりすぎてしまうために平圧加工時に内部に割れが形成され、耐水素誘起割れ特性が劣化する。したがって、適切なCの含有量は0.15~0.85%である。さらに、き裂形成抑制の観点からCの含有量を0.20%以上とすることが好ましく、さらには0.30%以上、0.35%以上、又は0.40%以上とすることが好ましい。一方、炭化物の割れ抑制の観点から、Cの含有量を0.75%以下とすることが好ましく、さらに耐水素誘起割れ特性を向上させるには0.65%以下、0.60%以下、又は0.50%以下とすることが望ましい。
Siはマトリックスに固溶し、平鋼線の強度を向上させる元素である。この効果を得るには0.10%以上のSiを含有させなくてはならない。しかし、2.00%を超えてSiを含有させると平圧加工の際、線材に割れが生じる。よって、Siの含有量は0.10%~2.00%である。より強度を高めたい場合には、Siは0.30%以上含有させればよく、0.50%以上、0.55%以上、0.60%以上、又は0.70以上含有させれば一層好ましい。平鋼線へ加工する際に線材の割れを抑制したい場合には、Si含有量を2.00%未満とすることが好ましく、1.80%以下、1.70%以下、又は1.50%以下とすることがより好ましい。
Mnは、鋼の焼入れ性を高める効果があり、熱処理後の平鋼線の高強度化に必要な元素である。この効果を得るには0.30%以上のMnを含有させなくてはならない。しかし、Mnの含有量が1.50%を超えると、線材の強度が高くなりすぎてしまい、線材を平鋼線へ加工する際に線材に割れが発生するなどの問題が生じる。そのため、本実施形態に係る線材におけるMnの含有量は0.30~1.50%である。なお、平鋼線の焼入れ性を上げて高強度化するには、Mn含有量は、好ましくは0.40%以上であり、0.50%以上、0.60%以上、0.70%以上、0.80%以上、又は0.90%以上含有させることが一層好ましい。平鋼線へ加工する際に線材の割れを抑制したい場合には、Mn含有量は1.30%以下とすることが好ましく、1.10%以下、1.05%以下、又は1.00%以下であればより一層好ましい。
Sは、不純物である。Sの含有量が0.020%を超えると、MnSは延伸された形態となり、耐水素誘起割れ特性を低下させる。耐水素誘起割れ特性を改善するには、CaなどのSと結合して硫化物を生成しやすい元素とのバランスを考えて、含有するSの上限を制御しなければならない。よって、Sの含有量の上限は0.020%以下とする。耐水素誘起割れ特性を改善する観点からSの含有量は0.010%未満であれば好ましく、0.008%未満、又は0.005%未満であればより一層好ましい。製鋼コストの観点から、S含有量を0.001%以上、0.003%以上、又は0.005%以上としてもよい。
Pは、不純物である。Pの含有量が0.020%を超えると、水素誘起割れが発生しやすくなり、平鋼線では、pH5.5以下の厳しいサワー環境で水素誘起割れを抑制することができない。よって、Pの含有量は0.020%以下とする。耐水素誘起割れ特性を改善する観点から、Pの含有量は0.015%以下が好ましく、0.013%未満、又は0.010%未満であれば更に好ましく、0.008%未満であればより一層好ましい。製鋼コストの観点からP含有量を0.003%以上、又は0.005%以上としてもよい。
Alは脱酸作用を有する元素であり、線材中の酸素量低減のために必要である。この効果を得るためには、0.001%以上のAlを含有させなくてはならない。好ましいAl含有量は0.002以上、又は0.005%以上であり、より好ましくは0.015%以上であり、さらに好ましくは0.020%以上、又は0.025%以上である。一方、Alの含有量が0.080%を超えると、粗大な酸化物が生成し、耐水素誘起割れ特性が劣化する。よって、Al含有量を、0.080%以下とする。粗大な酸化物の生成を抑制するため、Al含有量は0.060%以下であることが好ましく、さらには0.050%以下、0.045%以下、又は0.040%以下であることが一層好ましい。
Nはフェライト相に固溶し、平鋼線の強度を向上する効果がある。さらにNには、Al及びTiなどと結合して窒化物や炭窒化物を生成し、熱間圧延時のオーステナイト粒を微細化する効果があり、平鋼線の耐水素誘起割れ特性を改善する効果がある。これらの効果を得るために、Nは0.0020%以上含有させなければならず、0.0030%以上、0.0035%以上、又は0.0040%以上含有させることが好ましい。しかし、Nを線材に過剰に含有させると、窒化物や炭窒化物が粗大化して延性を低下、平圧延加工時に内部割れが生じることから、Nの含有量を0.0080%以下とする必要がある。N含有量は好ましくは0.0060%以下であり、0.0055%以下、0.0050%以下、0.0045%以下、又は0.0040%以下、とするのがより一層好ましい。
Oは不純物である。Oは粗大な酸化物を形成し、鋼の耐水素誘起割れ特性を低下させる。したがって、O含有量は低い方が好ましい。O含有量は0.0050%以下である。好ましいO含有量は0.0050%未満であり、より好ましくは0.0040%未満であり、さらに好ましくは、0.0035%未満である。製鋼コストの観点から、O含有量を0.0007%以上、又は0.0010%以上としてもよい。
Caは、MnS中に含まれることで、MnSを微細に分散する効果がある。MnSを微細に分散することで、MnSに起因にした水素誘起割れを抑制できる。Caによる水素誘起割れ抑制効果を得るためには、Caは0.0002%以上含有させればよく、より高い効果を得たい場合には、0.0005%以上、0.0008%以上、0.0010%以上、又は0.0015%以上を含有させれば良い。しかし、Caの含有量が0.0050%を超えると、その効果は飽和するし、Alとともに鋼中の酸素と反応して生成する酸化物が粗大となるとともに、平圧加工時に酸化物の粉砕が困難になるため、かえって耐水素誘起割れ特性の低下を招く。したがって、含有させる場合の適正なCaの含有量は、0.0050%以下である。耐水素誘起割れ特性を向上させる観点から、Caの含有量は0.0040%以下であることが好ましく、0.0030%以下、0.0025%以下、又は0.0020%以下であれば一層好ましい。
本実施形態に係る線材には、後述する残部のFeの一部に代えて、必要に応じて、Cr、Ti、Nb、V、Cu、Ni、Mo、B、REM、Zrから選択される少なくとも1種または2種以上の元素を含有させてもよい。ただし、これら任意元素を含有することなく本実施形態に係る線材はその課題を解決することができるので、任意元素の含有量の下限値は0%である。以下、任意元素であるCr、Ti、Nb、V、Cu、Ni、Mo、B、REM、Zrの作用効果と、含有量の限定理由について説明する。任意成分についての%は質量%である。
Crは、Mnと同様に、鋼の焼入れ性を高めて、平鋼線を高強度化するために、線材に0.05%以上含有させても良い。一方、Crの含有量が1.00%を超えると、線材の強度が高くなりすぎてしまい、平鋼線へ加工する際に線材に割れが発生する問題が生じる。そのため、本実施形態に係る線材における適正なCrの含有量は1.00%以下である。なお、平鋼線の焼入れ性を上げる場合、Crは0.10%以上含有させるのが好ましく、0.20%以上含有させれば一層好ましい。平鋼線へ冷間加工する際に線材の割れを抑制したい場合には、Cr含有量を0.80%以下とすることが好ましく、0.60%以下であればより一層好ましい。
Tiは、NやCと結合して、炭化物、窒化物又は炭窒化物を形成し、それらのピンニング効果によって熱間圧延時にオーステナイト粒を微細化する効果があり、平鋼線の耐水素誘起割れ特性を改善する効果があるため、含有させても良い。この効果を得るために、Tiは0.002%以上含有させることが好ましい。耐水素誘起割れ特性を改善する観点から、Tiの含有量を0.005%以上とするのが好ましく、0.010%以上とすることが一層好ましい。一方、Tiの含有量が0.050%を超えると、その効果が飽和するだけでなく、粗大なTiNが多数生成し、平圧加工時に割れが形成される原因となり、耐水素誘起割れ特性を劣化させる可能性がある。よって、Tiの含有量は0.050%以下とし、0.035%以下であることが一層好ましい。
Nbは、NやCと結合して、炭化物、窒化物又は炭窒化物を形成し、それらのピンニング効果によって熱間圧延時にオーステナイト粒を微細化する効果があり、平鋼線の耐水素誘起割れ特性を改善する効果があるため、含有させても良い。この効果を得るために、Nbは0.002%以上含有させることが好ましい。耐水素誘起割れ特性を改善する観点から、Nbの含有量を0.005%以上とするのが好ましく、0.010%以上とすることが一層好ましい。一方、Nbの含有量が0.050%を超えると、その効果が飽和するだけでなく、鋼塊や鋳片を鋼片に分塊圧延する工程で鋼片に割れが生じるなど鋼の製造性に悪影響を及ぼす。よって、Nbの含有量は0.050%以下とし、0.035%以下であることが好ましく、0.030%以下であることが一層好ましい。
VはC及びNと結合して、炭化物、窒化物又は炭窒化物を形成し、平鋼線の強度を高めることが出来る。この効果を得るためにはVを、0.02%以上含有させることが好ましい。一方、Vの含有量が0.15%を超えると、析出する炭化物や炭窒化物によって平鋼線の強度が増大し、平圧加工時に割れが形成される場合がある。よって、Vの含有量は0.15%以下とする。平圧加工時の割れを抑制する観点から、Vの含有量は、0.10%以下であることがより好ましく、0.08%以下であれば一層好ましい。なお、前述したVの効果を安定して得るためには、Vの含有量の下限は0.03%以上であることが一層好ましい。
Cuは、鋼の焼入れ性を高める元素であり、含有させても良い。焼入れ性を高める効果を得るためには、Cuを0.01%以上含有させることが好ましい。しかし、Cuの含有量が1.00%を超えると、線材の強度が高くなりすぎてしまい、平鋼線へ加工する際に線材に割れが発生するなどの問題が生じる。したがって、含有させる場合のCuの含有量は1.00%以下である。焼入れ性を向上させる観点から含有させる場合のCuの含有量は0.10%以上であることが好ましく、0.30%以上含有させれば一層好ましい。なお、平鋼線への加工性を考慮して、含有させる場合のCuの含有量は0.80%以下とすることが好ましく、0.50%以下であればより一層好ましい。
Niは、鋼の焼入れ性を高める元素であり、含有させても良い。焼入れ性を高める効果を得るためには、Niを0.01%以上含有させることが好ましい。しかし、Niの含有量が1.50%を超えると、線材の強度が高くなりすぎてしまい、平鋼線へ加工する際に線材に割れが発生するなどの問題が生じる。したがって、含有させる場合のNiの含有量は1.50%以下である。焼入れ性を向上させる観点から、含有させる場合のNiの含有量は0.10%以上であることが好ましく、0.30%以上含有させれば一層好ましい。なお、平鋼線への加工性を考慮して、含有させる場合のNiの含有量は1.00%以下とすることが好ましく、0.60%以下であればより一層好ましい。
Moは、鋼の焼入れ性を高める元素であり、含有させても良い。焼入れ性を高める効果を得るためには、Moを0.01%以上含有させることが好ましい。しかし、Moの含有量が1.00%を超えると、線材の強度が高くなりすぎてしまい、平鋼線へ加工する際に線材に割れが発生するなどの問題が生じる。したがって、含有させる場合のMoの含有量は1.00%以下である。焼入れ性を向上させる観点から含有させる場合のMoの含有量は0.02%以上であることが好ましく、0.05%以上含有させればより一層好ましい。なお、平鋼線への加工性を考慮して、含有させる場合のMoの含有量は0.50%以下とすることが好ましく、0.30%以下であればより一層好ましい。
Bは、微量添加することで鋼の焼入れ性を高めるのに有効であり、この効果を得たい場合には0.0002%以上含有させても良い。一方、0.0100%を超えてBを含有させても効果が飽和するだけでなく、粗大な窒化物が生成するので、水素誘起割れが発生しやすくなる。したがって、含有させる場合のBの含有量は0.0100%以下である。さらに焼入れ性を高めたい場合には、Bの含有量を0.0010%以上とすればよく、0.0020%以上であればより一層好ましい。なお、耐水素誘起割れ特性を向上させるためには、Bの含有量は0.0050%以下とすることが好ましく、0.0030%以下であればより一層好ましい。
REMは希土類金属の総称であり、Caと同じようにMnS中に含まれることで、MnSを微細に分散する効果がある。MnSを微細に分散することで、耐水素誘起割れ特性を改善することが出来るため、REMを含有させてもよい。水素誘起割れを抑制する効果を得るためには、REMは0.0002%以上含有させればよく、より高い効果を得たい場合には、0.0005%以上を含有させれば良い。しかし、REMの含有量が0.0100%を超えても、その効果は飽和するし、鋼中の酸素と反応して生成する酸化物が粗大となり、平圧加工時の割れの原因となる。したがって、含有させる場合の、REMの含有量は、0.0100%以下である。平鋼線への加工性の観点から、REMの含有量は0.0050%以下であることが好ましく、0.0030%以下であれば一層好ましい。
なお、REM(希土類元素)は、スカンジウム(Sc)、イットリウム(Y)の2元素と、ランタン(La)からルテチウム(Lu)までの15元素(ランタノイド)の総称を指す。これらの元素を添加する場合、単独で添加しても良く、混合物として添加しても良い。REMの含有量とは、これら17元素の含有量の合計値を意味する。
Zrは、Oと反応して酸化物を生成し、微量に添加すれば酸化物を微細に分散させて、水素誘起割れを抑制する効果があり、その効果を得たい場合に含有させても良い。水素誘起割れを抑制する効果を得るためには、Zrは0.0002%以上含有させればよく、より高い効果を得たい場合には、0.0010%以上を含有させれば良い。しかし、Zrの含有量が0.1000%を超えた場合、その効果は飽和するし、鋼中のNやSと反応し、粗大な窒化物や硫化物を生成するため、逆に耐水素誘起割れ特性の低下を招く。したがって、含有させる場合のZrの含有量は、0.1000%以下である。耐水素誘起割れ特性に悪影響を与える酸化物を低減させる観点から、Zrの含有量は0.0800%以下であることが好ましく、0.0500%以下であれば一層好ましい。
上述の成分を有する線材は、CaOとAl2O3とを所定量以上含む酸化物を含有するものとなる。本発明者らは、線材を平圧加工する際にこの酸化物の周囲に生じる空隙が、水素誘起割れを促進することを知見した。また、この酸化物の組成比とサイズとを適当な範囲に制御することで、平圧加工時にこれを微細に粉砕することが可能となるとともに、粉砕時には酸化物が母材に追従して移動することで母材と酸化物との間の密着性も良好となり、平圧加工後の耐水素誘起割れ特性が向上することを本発明者らは知見した。この効果を得るためには酸化物の組成比やサイズを厳正に制御する必要がある。
(単位mol%での酸化物中のCa、Al以外の酸化物形成元素の含有量)<(1/3)×(単位mol%での酸化物中のCa含有量又はAl含有量のうち多い方):式A
(単位mol%での酸化物中のO含有量)≧(単位mol%での酸化物中のS含有量):式B
上記式Aに記載の「Ca、Al以外の酸化物形成元素」とは、上述の本実施形態に係る線材の化学組成の中ではSi、Mg、及びMnである。
複合酸化物は、水素誘起割れを生じさせるものであり、本実施形態に係る線材において改善の対象とされる。従って本実施形態に係る線材においては、複合酸化物の組成及びサイズが限定される。
一方、本発明者らが確認した限りでは、複合酸化物以外の種々の介在物は水素誘起割れに実質的に影響しない。従って、本実施形態に係る線材において、複合酸化物以外の種々の介在物は特に限定されない。
例えば本実施形態に係る線材において、CaO及びAl2O3以外の酸化物は、線材の化学組成に起因して微量である。従ってCaO及びAl2O3以外の酸化物は、水素誘起割れに影響しない。また、CaOとAl2O3とを含む酸化物であっても、Si、Mg、及びMnなどのその他酸化物形成元素の含有量(mol%)がCa含有量(mol%)又はAl含有量(mol%)の1/3以上である複合酸化物は、線材及び平鋼線の評価試験において割れの起点に存在せず、水素誘起割れに影響しないと考えられる。同様の理由により、O含有量(mol%)がS含有量(mol%)よりも少ない介在物、即ち式Bを満たさない介在物も、耐水素誘起割れ特性に影響しないと考えられる。
以上の事項に鑑みて、本実施形態に係る線材においては、制御対象とされる複合酸化物を、CaOとAl2O3とを含み、且つ以下の式A及びBを満たすものに限定される。
また、本実施形態に係る線材において制御対象とされる複合酸化物を、実質的にCaOとAl2O3とからなる酸化物に限定してもよい。
(組成比ε)=(単位質量%での前記酸化物中のCaO濃度)/(単位質量%での前記酸化物中のAl2O3濃度):式C
具体的には、εが0.00に近い場合、すなわち複合酸化物の成分がAl2O3主体となる場合や、εが3.00よりも小さい場合、複合酸化物は平圧加工時に粉砕される。従って、線材の中心部における複合酸化物の組成比εの平均値を上記範囲内に制御することが、平圧加工後の線材(即ち平鋼線)の耐水素誘起割れ特性を向上させるために必要とされる。一方、線材の中心部における複合酸化物の組成比εの平均値が3.00以上の場合には、複合酸化物の成分がCaO主体となり、サイズ制御に関係なく平圧加工後の耐水素誘起割れ特性が劣化する。以上の理由により、線材の中心部における複合酸化物の組成比εの平均値は0以上3.00以下と規定される。より安定な耐水素誘起割れ特性を得るためには、線材の中心部の複合酸化物の組成比εの平均値の上限は1.00以下が好ましく、より好ましくは0.60以下である。また、線材の中心部の複合酸化物の組成比εの平均値の下限を0.02、0.05、0.10、0.15、又は0.20としてもよい。
次に、本実施形態に係る線材の複合酸化物の評価方法を説明する。なお、線材が含む複合酸化物はクラスター状に存在することがあるが、この場合であっても、クラスターを構成する各複合酸化物を独立したものとして取り扱う。クラスター全体を1つの複合酸化物とみなすことはしない。
複合酸化物の化学組成は、そのサイズによらず、一の線材において実質的に一様であると考えられる。従って、線材の中心部のC断面における10視野において観察を行い、各視野において最大の円相当径を有する複合酸化物(最も化学組成の分析が容易な複合酸化物)に関してのみ化学組成の分析及び組成比εの算出を行い、この10視野における複合酸化物の組成比εの平均することにより求められる値を、線材の中心部において測定される複合酸化物の組成比εの平均値とみなすことができる。この値が上述した本実施形態に係る線材の要件を満たせば、その線材は本実施形態に係る線材の要件を満たすものとみなされる。線材の複合酸化物の化学組成の具体的な分析方法に関して以下に説明する。
線材の中心部に含まれる全ての複合酸化物の円相当径を測定することは現実的ではない。以下に説明する方法によって得られる値を、線材のC断面の中心部において測定される複合酸化物の円相当径の平均値であるとみなすことができる。
線材のC断面(すなわち圧延方向に垂直な切断面)を鏡面研磨した後、電界放射型走査型電子顕微鏡(FE-SEM)を用いて倍率1000倍で介在物を観察できるように10箇所を観察し、写真撮影する。1視野あたりの面積は、8000μm2(縦100μm、横80μm)以上とする。この際、合わせてEDSを用いて各介在物の化学組成を測定し、これにより介在物が本実施形態にかかる線材において制御対象とされる複合酸化物であるか否かを判断する。次いで、得られた各写真について、通常の画像解析により最大サイズの複合酸化物の面積をそれぞれの写真から測定し、面積から求められる円相当径を算出する。円相当径を測定するための写真は、反射電子像とすることがよい。上記方法により求めた、10箇所の写真の最大の複合酸化物の円相当径の平均値を求めることにより、線材のC断面の中心部において測定される複合酸化物の円相当径の平均値が得られる。
線材の径についても特に限定されない。現在市場に流通する平鋼線用の線材の径は7~16mmとされることが通常であるので、本実施形態に係る線材の径を7~16mmと規定しても良い。
本実施形態にかかる線材の製造方法では、水素誘起割れを抑制するために、溶鋼段階でCaを添加してCaO及びAl2O3を含む酸化物の組成比εを適正化し、さらにこの複合酸化物のサイズを制御する。
本実施形態にかかる平鋼線2においては、C断面の短径を平鋼線2の厚さtと称し、C断面の長径を平鋼線2の幅wと称する。
また、後述する平鋼線2のL断面とは、平鋼線の圧延方向及び短径方向に平行であって、且つ平鋼線の中央軸を実質的に含む断面を意味する。平鋼線の中央軸とはC断面の中央を通り圧延方向に平行な軸である。平鋼線の短径方向とは、平鋼線の圧延方向に垂直な断面の短径方向である。
平鋼線2のL断面における中央部21とは、図3に示されるように、平鋼線2の中央軸から平鋼線2の短径(平鋼線2の厚さt)の1/7以内の範囲内の領域を意味する。換言すると、平鋼線のL断面における中央部21は、L断面における、平鋼線の表面から深さ5/14t以上の領域である。
以下、平鋼線2のL断面における中央部21を、単に「中央部」と称する場合がある。平鋼線2のC断面が実質的に円形であり、そのC断面の短軸及び長軸を特定できない場合、平鋼線のC断面の中心を通る任意の軸、及びこれに垂直な軸を長軸及び短軸とみなせば良い。
例えば平鋼線の金属組織は、線材の金属組織と同様に、平鋼線の耐水素誘起割れ特性に大きく影響しない。従って平鋼線の金属組織は特に限定されない。例えば、平鋼線の中央部における組織が98面積%以上の焼戻しマルテンサイトを含む場合、平鋼線の引張強さを一層向上させることができるので好ましい。一方、例えば平鋼線の中央部における組織が20~60面積%のフェライトと40~60面積%のベイナイトとを含む場合、平鋼線の靭性などを向上させられるので好ましい。
平鋼線の幅w及び厚さtについても特に限定されない。現在市場に流通する平鋼線の幅は13~16mmとされ、厚さtは2~7mmとされることが通常であるので、本実施形態に係る平鋼線の幅及び厚さをこのように規定しても良い。
平鋼線の引張強さも特に限定されない。平鋼線の用途に鑑みると、平鋼線の引張強さは1100~1500MPa程度とされることが望ましく、これは平鋼線の熱処理条件を適宜調整することで達成可能である。
平鋼線の硫化物系介在物も、線材の硫化物系介在物と同じ理由で、その形態などを限定する必要はない。
具体的には、表1、表2-1、及び表2-2に示す化学組成の鋼を溶製し、以下の方法で線材及び平鋼線を作製した。なお、これら表中の「-」の表記は、当該元素の含有量が不純物レベルであり、実質的に含有されていないと判断できることを示す。
線材を340mm長さに切断し、上下70mmを油圧チャックで固定し引張試験を行った。得られた最大荷重を線材の断面積で除することで引張強さを算出した。引張強さが600MPa以上であることが好ましいため、引張強さ600MPa以上を合格品と評価した。
平鋼線のL断面を鏡面研磨した後、ピクラールで腐食し、FE-SEMを用いて倍率2000倍でL断面の中央部においてそれぞれ任意の5箇所を観察し、5枚の写真を撮影した。得られた各写真にOHPシートを重ね、各透明シートにおけるフェライト組織、ベイナイト組織と重なる領域に色を塗った。次いで、各透明シートにおける「色を塗った領域」の面積率を画像解析ソフトにより求めることにより、上記5箇所それぞれにおけるフェライト組織及びベイナイト組織の面積率を求めた。そして、5箇所のフェライト組織及びベイナイト組織の面積率を平均することで、平鋼線におけるフェライト組織及びベイナイト組織の面積率を算出した。また、フェライト、ベイナイト、及びマルテンサイト以外の組織がいずれの平鋼線においても実質的に確認されなかったので、フェライト組織及びベイナイト組織の面積率を100%から差し引いた値を、マルテンサイト組織の面積率の平均値とみなした。
平鋼線を400mm長さに切断し、上下100mmを油圧チャックで固定し引張試験を行った。得られた最大荷重を平鋼線の断面積で除することで引張応力を算出した。引張強さが1100MPa以上であることが好ましいため、引張強さ1100MPa以上を合格品と評価した。
150mm長さに切断した平鋼線を用いて耐水素誘起割れ特性を評価した。試験片への水素供給のための溶液は、5%NaCl+CH3COONa水溶液を用いてpH5.0に調整したものを用いた。窒素ガスで脱気後、硫化水素(H2S)+二酸化炭素(CO2)混合ガスを導入し、溶液中に平鋼線を浸積して割れの発生を調査した。このとき、硫化水素の分圧は0.1MPa、試験温度は25℃であり、試験時間は96時間であった。試験後、平鋼線の厚み方向に対して周波数10kHzの超音波探傷試験(UST:Ultra sonic Test)によって割れ発生の有無を確認した。超音波探傷試験によって割れが生じたと判定される割れ発生部の面積の合計を画像解析によって求め、下記式Dを用いて水素誘起割れ発生率(χ(%))を求めた。水素誘起割れが生じた平鋼線を、耐水素誘起割れ特性に関して不合格と判定した。
試験番号A3およびB3は、金属Alを添加してからCaSi合金を添加するまでの時間が70分であったため、線材の複合酸化物の組成比εの平均値が3.00以上となり、平鋼線に水素誘起割れが発生している。
試験番号A4は熱間圧延時の断面積減少量が小さく、熱間圧延時の酸化物の粉砕が不十分であったため、線材の複合酸化物の最大サイズが本発明の範囲外となり、平鋼線に水素誘起割れが発生している。
11 中心部
2 平鋼線
21 中央部
Claims (14)
- 化学組成が、質量%で、
C:0.15~0.85%、
Si:0.10~2.00%、
Mn:0.30~1.50%、
Al:0.001~0.080%、
Ca:0.0002~0.0050%、
N:0.0020~0.0080%、
P:0.020%以下、
S:0.020%以下、
O:0.0050%以下、
Cr:0~1.00%、
V:0~0.15%、
Ti:0~0.050%、
Nb:0~0.050%、
Cu:0~1.00%、
Ni:0~1.50%、
Mo:0~1.00%、
B:0~0.0100%、
REM:0~0.0100%、
Zr:0~0.1000%、
残部:Fe及び不純物である線材であって、
CaOとAl2O3とを含み、且つ式A及び式Bを満たす酸化物が複合酸化物と定義され、
前記線材の圧延方向と垂直な断面における、前記線材の中心軸から前記線材の直径の1/10の範囲内である中心部において測定される、式Cによって定義される前記複合酸化物の組成比εの平均値が0.00≦ε<3.00であり、
前記断面の前記中心部において測定される前記複合酸化物の円相当径の平均値が6.0μm以下である
ことを特徴とする線材。
(酸化物中のCa、Al以外の酸化物形成元素の単位mol%での含有量)<(1/3)×(酸化物中の単位mol%でのCa含有量又はAl含有量のうち多い方):式A
(単位mol%での酸化物中のO含有量)≧(単位mol%での酸化物中のS含有量):式B
(組成比ε)=(単位質量%での前記複合酸化物中のCaO濃度)/(単位質量%での前記複合酸化物中のAl2O3濃度):式C - 前記化学組成が、質量%で、
Cr:0.05~1.00%
を含有することを特徴とする請求項1に記載の線材。 - 前記化学組成が、質量%で、
V:0.02~0.15%、
Ti:0.002~0.050%及び
Nb:0.002~0.050%
の1種又は2種以上を含有することを特徴とする請求項1又は請求項2に記載の線材。 - 前記化学組成が、質量%で、
Cu:0.01~1.00%、
Ni:0.01~1.50%、
Mo:0.01~1.00%及び
B:0.0002~0.0100%
の1種又は2種以上を含有することを特徴とする請求項1~請求項3の何れか一項に記載の線材。 - 前記化学組成が、質量%で、
REM:0.0002~0.0100%及び
Zr:0.0002~0.1000%
の1種又は2種以上を含有することを特徴とする請求項1~請求項4の何れか一項に記載の線材。 - 引張強さが600~1400MPaであることを特徴とする請求項1~請求項5の何れか一項に記載の線材。
- 化学組成が、質量%で、
C:0.15~0.85%、
Si:0.10~2.00%、
Mn:0.30~1.50%、
Al:0.001~0.080%、
Ca:0.0002~0.0050%、
N:0.0020~0.0080%、
P:0.020%以下、
S:0.020%以下、
O:0.0050%以下、
Cr:0~1.00%、
V:0~0.15%、
Ti:0~0.050%、
Nb:0~0.050%、
Cu:0~1.00%、
Ni:0~1.50%、
Mo:0~1.00%、
B:0~0.0100%、
REM:0~0.0100%、及び
Zr:0~0.1000%、
残部:Fe及び不純物である平鋼線であって、
CaOとAl2O3とを含み、且つ式A及び式Bを満たす酸化物が複合酸化物と定義され、
前記平鋼線の圧延方向及び短径方向に平行であって、且つ前記平鋼線の中央軸を含む断面における、前記中央軸から前記平鋼線の短径の1/7以内の範囲内である中央部において測定される、下記式Cによって定義される前記複合酸化物の組成比εの平均値が0.00≦ε<3.00であり、
前記断面の前記中央部において測定される前記複合酸化物の円相当径の平均値が3.0μm以下である
ことを特徴とする平鋼線。
(酸化物中のCa、Al以外の酸化物形成元素の単位mol%での含有量)<(1/3)×(酸化物中の単位mol%でのCa含有量又はAl含有量のうち多い方):式A
(単位mol%での酸化物中のO含有量)≧(単位mol%での酸化物中のS含有量):式B
(組成比ε)=(単位質量%での前記複合酸化物中のCaO濃度)/(単位質量%での前記複合酸化物中のAl2O3濃度):式C - 前記中央部における組織が、98面積%以上の焼戻しマルテンサイトを含むことを特徴とする請求項7に記載の平鋼線。
- 前記中央部における組織が、20~60面積%のフェライトと40~60面積%のベイナイトとを含むことを特徴とする請求項7に記載の平鋼線。
- 引張強さが1100~1500MPaであることを特徴とする請求項7~9の何れか一項に記載の平鋼線。
- 前記化学組成が、質量%で、
Cr:0.05~1.00%
を含有することを特徴とする請求項7~10の何れか一項に記載の平鋼線。 - 前記化学組成が、質量%で、
V:0.02~0.15%、
Ti:0.002~0.050%及び
Nb:0.002~0.050%
の1種又は2種以上を含有することを特徴とする請求項7~11の何れか一項に記載の平鋼線。 - 前記化学組成が、質量%で、
Cu:0.01~1.00%、
Ni:0.01~1.50%、
Mo:0.01~1.00%及び
B:0.0002~0.0100%
の1種又は2種以上を含有することを特徴とする請求項7~12の何れか一項に記載の平鋼線。 - 前記化学組成が、質量%で、
REM:0.0002~0.0100%及び
Zr:0.0002~0.1000%
の1種又は2種以上を含有することを特徴とする請求項7~請求項13の何れか一項に記載の平鋼線。
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