WO2024127490A1 - Tige d'acier précontraint (pc) et boulon à haute résistance ayant une excellente résistance à la fracture retardée et ayant une structure pour empêcher une chute de vis, et procédé pour la production de ceux-ci - Google Patents

Tige d'acier précontraint (pc) et boulon à haute résistance ayant une excellente résistance à la fracture retardée et ayant une structure pour empêcher une chute de vis, et procédé pour la production de ceux-ci Download PDF

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WO2024127490A1
WO2024127490A1 PCT/JP2022/045798 JP2022045798W WO2024127490A1 WO 2024127490 A1 WO2024127490 A1 WO 2024127490A1 JP 2022045798 W JP2022045798 W JP 2022045798W WO 2024127490 A1 WO2024127490 A1 WO 2024127490A1
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hardness
thread
decarburized
steel
rolling
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PCT/JP2022/045798
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Japanese (ja)
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重信 濱中
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濱中ナット株式会社
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  • the present invention relates to high-strength bolts and PC steel bars that are given delayed fracture resistance through surface decarburization treatment, have a screw-out prevention structure formed at the shear start point due to a strength structure that increases from the bottom of the thread toward the middle of the thread, and have an introduced axial force of at least F11T level with a product strength of at least 930 MPa, and a method for manufacturing the same.
  • a decarburized layer is formed by surface decarburization to make the surface harder than the core, and then the decarburized layer is rolled to reduce the delayed fracture susceptibility (improve delayed fracture resistance) of the threaded portion (Patent Documents 1 to 4). That is, in high-strength bolts, a partially decarburized or ferritic decarburized layer 3 is typically formed on the non-decarburized metal 4 by rolling after surface decarburization, and a completely decarburized layer is formed in the surface layer 1, as shown in Fig. 1(a).
  • tempering in a temperature range where the tempering characteristics are highly effective may be unavoidable, and slight changes in tempering temperature have a large effect on strength, so that insufficient heat treatment is likely to occur, and damage is likely to occur in the neck area (a) in Figure 8.
  • Non-Patent Document 1 High-strength bolts made of low-carbon steel with C of 0.2% or less have been proposed, but there is a new report that low-carbon steel with C of 0.2% or less is more likely to cause damage in the incomplete thread area (c) in Figure 8 (Non-Patent Document 1), and sufficient axial force of F11T or more cannot be obtained. Therefore, it is considered difficult to impart delayed fracture resistance while preventing thread stripping in high-strength bolts and PC steel bars that can be applied with an axial force of F11T or more. In order to impart delayed fracture resistance while preventing thread stripping, it is considered necessary to further devise the addition of alloying elements and to devise the thread structure.
  • Japanese Patent Application Laid-Open No. 58-1020 Japanese Patent Application Laid-Open No. 62-267420 Japanese Patent Application Laid-Open No. 6-57367 Japanese Patent Application Laid-Open No. 7-54441 JP 2002-276637 A JP 2013-139872 A
  • the inventors have found that, in considering why it is difficult to impart delayed fracture resistance and thread backout prevention properties to high strength bolts of F11T or more, one of the reasons is that the delayed fracture resistance due to hydrogen embrittlement caused by surface decarburization and the shear strength properties of threads having a decarburized layer are in an opposing relationship, and strengthening one makes the other weak, making it difficult to impart these opposing properties at the same time.
  • the part where this shear starts is called the thread shear start point K. Therefore, in the manufacture of high-strength bolts and PC steel bars that impart delayed fracture resistance, it is desired to improve the hardness of the thread shear start point, and it was found that it is necessary to provide a thread structure that increases the hardness from the thread root to the middle of the thread.
  • a decarburized layer remains on the surface of the thread from the thread root to the thread, maintaining delayed fracture resistance, while forming a structure in which the hardness increases from the thread root toward the thread, improving the hardness at the shear start point K and ensuring the strength of the shear progression path, thereby imparting a screw-loosening prevention structure to high-strength bolts and PC steel bars (second perspective).
  • the present invention aims to provide a high-strength bolt and PC steel bar of F11T or more that adopts quenching and tempering treatment to obtain sufficient axial force, is not affected by the tempering characteristics when the carbon concentration is high, and has excellent delayed fracture resistance and excellent screw removal resistance, based on the first and second findings above.
  • a decarburization process is performed to soften the surface hardness below the core hardness, forming a decarburized layer, which is then rolled.
  • the high-strength bolt and PC steel bar are made of a billet containing, by mass, 0.2% to 0.5% C, at least 0.15% to 0.80% Si, and 0.10% to 2.0% Mn, and are subjected to a rolling process after final quenching and tempering to give the bolt product strength of 930 MPa or more in yield strength, while the plastic processing by rolling gives the core of the thread a hardness of at least HV400, while increasing the hardness from the decarburized part at the bottom of the thread toward the thread, and have a screw-out prevention structure with a hardness of Hv360 or more, preferably 380 or more, at the thread shear starting point K.
  • C is set to 0.20% or more and 0.5% or less
  • the core has a strength of 400 Hv or more, and a product strength of 930 Hv or more required for F11T or more is maintained, while a decarburized layer is secured on the surface layer of the thread, and the protruding height of the non-decarburized part of the thread is increased by plastic deformation due to final rolling, so that the hardness at the shear starting point K can be increased to Hv360 or more, preferably Hv380 or more. Therefore, a high strength bolt of F11T or more can be given an excellent delayed fracture resistance as well as an excellent structure to prevent backing out.
  • the present invention uses the widely used alloy element B added, so-called B steel, or CrMo steel added with Cr and Mo, and can provide high-strength bolts of F11T or more with an excellent delayed fracture resistance as well as an excellent structure to prevent backout, without adopting measures such as selecting materials to which other alloy elements are added or selecting a thread shape that relieves stress concentration on the thread.
  • a product strength of 930 MPa or more is obtained, 2) delayed fracture of the neck portion and incomplete thread portion of a hydrogen embrittlement high strength bolt due to the formation of a decarburized layer is prevented, and 3) the final plastic deformation with respect to the preferred decarburization gradient before rolling ensures an Hv strength of 360 or more, preferably 380 or more, at the screw-out shear starting point K, preventing screw-out, resulting in a structure suitable for high strength bolts and PC steel bars that require a high tensile strength of F11T or more.
  • the thread is covered with a partially decarburized layer 2 on the non-decarburized raw metal 4, and a completely decarburized layer 1 remains on the surface, but this structure changes due to the plastic deformation caused by the final rolling of the present invention. That is, according to the method of the present invention, the upper part of the decarburized layer having a carbon concentration gradient moves by plastic flow so as to cover the adjacent decarburized layer by rolling to form a thread (FIG. 7(a) ⁇ (b)). As shown in FIG.
  • the carbon concentration gradient of the decarburized layer leaves a decarburized layer with a high carbon concentration at the thread root, while the incompletely decarburized layer moves toward the thread crest side by plastic deformation, and the non-decarburized raw metal structure rises more, and the height E of the non-decarburized part of the thread tends to increase.
  • this is due to the increase in hardness at the shear start point K of screw-out, where the female thread crest on the nut side presses and contacts the thread part on the bolt side.
  • the conventional method of providing delayed fracture resistance by surface decarburization FIG.
  • a high-strength bolt that imparts a hardness of Hv 400 or more to the core can ensure a hardness of Hv 360 or more, preferably 380 or more, at the shear starting point K, and a high-strength bolt that has delayed fracture resistance to an axial force of 930 MPa or more required for F11T or more can ensure a high screw-out prevention structure that can withstand high axial forces.
  • the high strength bolt of the present invention it is important to decarburize the surface of the billet to form a decarburized layer in which the carbon concentration drops at a constant gradient from the non-decarburized layer.
  • the decarburization process is suitably carried out by heating at a temperature of 1250°C or higher and 1350°C or lower for 3 hours or longer in an atmosphere with an oxygen concentration of 3% or higher. Thereafter, the billet having a decarburized layer on its surface is subjected to normal wire rolling and, if necessary, normal low-temperature annealing (softening annealing), after which it is lightly stretched with an area reduction rate of 10% or less, and formed into a bolt blank in a cold rolling machine.
  • normal low-temperature annealing softening annealing
  • the bolt blank is rolled and then quenched and tempered to produce a bolt product, but in the present invention, the bolt blank is rolled after quenching and tempering.
  • the quenching and tempering conditions may be the same in the conventional method and the method of the present invention. That is, B steel is preferably heated to 830°C to 930°C, preferably 850°C to 900°C, then quenched (oil or water quenched), and then tempered at 400°C to 500°C and quenched.
  • the hardness distribution before rolling should be Hv 400 or more at the core and Hv 360 or more, preferably 380 or more, at a depth of 0.6 mm, as shown in Figure 4. In other words, it is necessary to ensure a hardness gradient of Hv 40/0.6 mm or more (corresponding decarburization gradient).
  • the quenched and tempered bolt blank is then subjected to a normal rolling process.
  • FIG. 2 is a comparative diagram of the cross-sectional structure distribution of the thread portion of a high-strength bolt manufactured by a conventional method (a) and a high-strength bolt manufactured by the present invention (b).
  • FIG. 1 shows a cross-sectional view of the thread structure distribution in a high-strength bolt manufactured by a conventional method (a) and a cross-sectional view illustrating the shear stress caused by screw back (b).
  • 1 is a graph showing the relationship between the hardness distribution before rolling of samples A, B and C in standard CrMo steel.
  • 1 is a table showing the relationship between the types of samples A, B, and C after rolling and the hardness (HV0.3) at specified locations of the threads.
  • FIG. 1 is a table showing the tensile test results of PC steel bars with M27 rolled threads for samples A, B, and C.
  • Photograph 4 shows the fracture morphology of sample C and its schematic diagram.
  • FIG. 2 is an explanatory diagram showing the change before (a) and after (b) plastic deformation by rolling according to the present invention.
  • FIG. 4 is an explanatory diagram showing a fractured portion of a bolt.
  • C is an important element that greatly affects the strength of the high-strength bolt of the present invention, which is mainly composed of a tempered martensite structure or a tempered bainite structure.
  • the high-strength bolt of the present invention which is mainly composed of a tempered martensite structure or a tempered bainite structure.
  • the upper limit is set to 0.5%, preferably 0.4%.
  • Si has the effect of suppressing the coarsening of cementite when the martensite structure is subjected to tempering heat treatment, improving the strength-toughness balance. If the Si content is 0.15% or less, the cementite in the martensite structure tends to coarsen, so the amount of Si added should be 0.15% or more. On the other hand, if too much Si is added, there is a risk that it may actually impair toughness, so the upper limit is set at 0.80%.
  • Mn is an element that is effective in improving hardenability and strength, and should be added at 0.20% or more. However, if it exceeds 2.00%, it can form MnS, which promotes segregation, and impair workability, so the upper limit for addition in CrMo steel is 2.00%. In B steel, a small amount of B improves hardenability, so the upper limit for Mn is preferably 1.00% or less.
  • the remainder contains Fe and unavoidable impurities.
  • P and S are unavoidable impurities and are harmful elements that reduce toughness. Therefore, it is recommended that the content of both P and S be kept below 0.040%.
  • N generates BN to reduce free B
  • Ti is added for the purpose of fixing N as TiN.
  • the upper limit is set to 0.030%. Therefore, in the present invention, low-medium carbon B steel with a C content of 0.2% to 0.5% in mass % is used as the B steel, and the billet composition includes at least Si of 0.15% to 0.80%, Mn of 0.10% to 2.0%, B of 0.0003% to 0.0030, and Ti of 0.005% to 0.030.
  • the composition of the CrMo steel billet uses low-medium carbon CrMo steel with C of 0.2% to 0.5%, and contains at least Si of 0.15% to 0.80%, Mn of 0.10% to 1.0%, Cr of 0.5% to 1.2%, Mo of 0.1% to 0.5%, and P and S of 0.04% or less.
  • Cr improves hardenability and strength, so it is preferable to add 0.5% or more, but in consideration of economic efficiency, it is preferable to add 1.2% or less.
  • Mo improves hardenability with a relatively small amount and improves the strength of the tempered martensite structure or tempered bainite structure by its precipitation effect, so it is preferable to add 0.10% or more, but in consideration of economic efficiency, it is preferable to add 0.6% or less.
  • Nb generates fine carbides during rolling, widens the recrystallization temperature range, and enhances the controlled rolling effect. It is effective in improving the strength of the tempered martensite structure or tempered bainite structure by its precipitation strengthening effect, so it is recommended to add 0.01% or more, but it is recommended to add 0.04% or less in consideration of economy.
  • Al is recommended to add 0.01% or more as a deoxidizer, but excessive addition reduces toughness, so the upper limit is recommended to be 0.1%.
  • Cu improves strength without reducing toughness, so it may be added 0.01% or more. For economy, the upper limit is recommended to be 0.5% or less.
  • Ni improves hardenability and toughness, so it may be added 0.01% or more, but it is recommended to add 0.25% or less in consideration of economy.
  • V also improves hardenability and improves strength in the tempered martensite structure or tempered bainite structure by its precipitation strengthening effect, so it may be added 0.005% or more, but it is recommended to add 0.08% or less in consideration of economy.
  • a billet containing 0.20-0.5 wt.% C is produced, 1) this is subjected to a long-term decarburization process, 2) the bolt blank is drawn and rolled in the usual way, 3) a bolt blank is produced by the usual cold heading, 4) the bolt is quenched and tempered in the usual way, and finally, 5) the bolt is rolled to produce the desired high-strength bolt.
  • the bolt is finally quenched and tempered after the rolling process, but when quenched and tempered after rolling, a phenomenon is observed in which the hardness decreases from the thread root to the middle of the thread.
  • an improvement in hardness is observed in the thread portion from the thread root to the middle of the thread ( Figure 4).
  • a billet having the above-mentioned steel composition is heated in an atmosphere with an oxygen concentration of 3% or more at a temperature of 1250°C or more and 1350°C or less for 3 hours or more, preferably 5 hours or less. This is to form a relatively thick decarburized layer on the billet surface in order to form a softened layer on the surface. If the oxygen concentration is less than 3% and the heating temperature is less than 1250°C, a sufficient thickness of the decarburized layer cannot be obtained on the threaded portion of the high-strength bolt.
  • the heating temperature exceeds 1350°C, the decarburized layer becomes too thick and the strength of the tensile test may decrease, so the upper limit of the heating temperature is preferably 1350°C. If the heating time is excessively long, the decarburized layer may become too thick and the average hardness of the entire thickness or the strength of the tensile test may decrease, so the heating time should not exceed 5 hours.
  • the decarburized billet material is rolled into wire rod using VH rolling rolls to produce wire rods of specified dimensions.
  • the rolling is performed after quenching and tempering.
  • the quenching and tempering conditions may be the same in the conventional method and the method of the present invention, as described above.
  • (B steel composition) Low-medium carbon steel with a C content of 0.2% or more and 0.35% or less by mass, and with at least the following composition: Si is 0.15% or more and 0.80% or less, Mn is 0.10% or more and 2.0% or less, B is 0.0003% or more and 0.0030% or less, Ti is 0.005% or more and 0.030% or less, and P and S are 0.04% or less.
  • (CrMo steel composition) Low-medium carbon steel with a C content of 0.2% or more and 0.5% or less by mass, Si content of 0.15% or more and 0.80% or less, Mn content of 0.10% or more and 1.0% or less, Cr and Mo used instead of B and Ti, and P and S content of 0.04% or less, with the following composition in particular: Composition C: 0.41 Si: 0.30 Mn: 0.77 P+S: 0.015 Cr: 1,05 Mo: 0.16
  • High strength bolts for friction joints Manufacture of SCM435 as specified in JIS BG4053 and 15B24 as specified in SAE J1268.
  • High-strength bolts for friction joints are 0.5 to 1 mm larger than the product dimensions, and are rolled into wire rod, drawn into steel wire, and cold heading formed into bolt blanks. When hot rolling into wire rod, the heating atmosphere of the billet is adjusted to obtain a material with a decarburized layer 1.2 mm deep remaining on the surface of the steel wire after drawing.
  • high-strength bolts for friction joints are formed to a specified thread size by cold heading, the threads are formed by rolling, and then quenched and tempered to a specified hardness.
  • the bolts are formed to a specified thread size by cold heading, quenched and tempered to a specified hardness, and then the threads are formed by rolling, utilizing the work hardening caused by plastic processing during thread forming to compensate for the decrease in quenched hardness of the threaded portion due to decarburization.
  • high-strength PC steel bars with threads at the ends are also formed to a specified thread size by cold drawing, the threads are formed by rolling, and then quenched and tempered to a specified hardness.
  • the bolts are formed to a specified thread size by cold drawing, quenched and tempered to a specified hardness, and then the threads are formed by rolling, utilizing the work hardening caused by plastic processing during thread forming to compensate for the decrease in quenched hardness of the threaded portion due to decarburization.
  • the surface layer of the present invention is a decarburized layer, which is soft, and the core has a hardness to obtain a specified strength, and in high-strength bolts for friction joints as applied mutatis mutandis in JIS B1186 and high-strength PC steel bars with threads at the end as specified in JIS G3109, the strength is increased by plastic deformation of the threads, preventing shear fracture of the threads, and the stress concentration area at the bottom of the threads is softened by decarburization to prevent delayed fracture.
  • a bolt blank is formed by cold drawing a wire that has been annealed at a low temperature that does not affect the decarburization depth, and cold heading. The bolt blank is quenched and tempered to improve the yield strength of the bolt product to 930 MPa or more.
  • the bolt blank which still has a decarburized layer, is rolled, and the threads are work-hardened due to plastic deformation of the threads, so that the difference in hardness between the position at half the height of the thread and the center of the thread root is controlled to HV30 or less.
  • the following billets were used: Composition (mass%) C: 0.41 Si: 0.30 Mn: 0.77 P: 0.011 S: 0.004 Cr: 1.05 Mo: 0.16
  • the atmosphere in the heating furnace is adjusted to an oxygen concentration of 3% or more, and the material is heated at a temperature of 1250°C to 1350°C for 3 hours to 5 hours.
  • Sample A is decarburized at 1250°C for 3 hours, Sample B at 1300°C for 5 hours, and Sample C at 1350°C for 6 hours.
  • the material is then subjected to wire rolling, low-temperature annealing, light elongation, and cold heading. Quenching and tempering are performed by heating and water cooling from 855°C, and then heating and water cooling from 540°C.
  • the threaded portion formed by heading is then rolled into the specified thread shape.
  • FIG. 3 shows the hardness distribution from the outermost surface of a PC steel bar in a quenched state after cold drawing (before rolling) to a depth of 1.2 mm.
  • Samples A and B have a hardness distribution that changes linearly with distance from the surface, with a surface hardness of HV360 and a hardness of HV400 1.2 mm inside from 340.
  • sample C is an example of excessive decarburization, where the linear change in hardness with distance from the surface is lost, and even 1.2 mm inside the sample, the hardness is only HV360.
  • FIG. 4 shows the measurement positions for the internal hardness of M27 2 mm pitch threads after rolling.
  • Figure 5 shows the results of hardness measurements inside the screw.
  • the inside (position 2 in Figure 2), which is strongly subjected to plastic deformation, is hardened by the plastic deformation during thread rolling, and has increased strength compared to the bottom of the thread (position 1 in Figure 2), where the degree of plastic deformation of the thread is less.
  • Figure 5 shows the tensile test results for samples without decarburization, sample A, sample B, and sample C.
  • Tests were conducted according to the method specified in JIS B1051 (2014 edition), Section 9.2, Tensile test for determining the tensile strength Rm of male threaded parts.
  • the test machine used was a universal testing machine with a maximum test capacity of 1000KN manufactured by Tokyo Test Machinery Co., Ltd.
  • Photo 1 in Figure 6A shows the fracture mode of a screw without decarburization. There was no abnormality in the fitting part with the nut, and after the test, the nut could be removed by hand, and breakage occurred from the loose thread part.
  • Photo 2 in Figure 6B shows the fracture mode of the thread of sample A of the present invention. There was no abnormality in the fitting part with the nut, but after the test the nut could be removed by hand and broke from the loose thread part.
  • Photo 3 in Figure 6C shows the fracture mode of the thread of sample B of the present invention. There was no abnormality in the fitting part with the nut, but after the test the nut could be removed by hand and broke from the loose thread part.
  • Photo 4 in Figure 6D shows the fracture morphology of the threads of sample C, which had been decarburized too far.
  • the male threads which had become soft due to excessive decarburization on the surface, lacked strength and were unable to withstand the threads of the nut, causing the threads to shear off and resulting in unscrewing.
  • the hardness gradient (decarburization gradient) from the surface to the core shown in Figure 3 before rolling is important, and that this affects the hardness inside the thread shown in Figure 4 after rolling.
  • the hardness gradient (decarburization gradient) from the surface to the core and the plastic deformation caused by rolling give the core shown in Figure 1 (b) a hardness of Hv 400 or more, imparting a product strength of 930 MPa or more while imparting a hardness of Hv 360 or more, preferably 380 or more, to the shear starting point K, thereby providing a screw-loosening prevention structure to high-strength bolts and PC steel bars of F11T or more with product strength of 930 MPa or more.
  • a gradient decarburized layer forms the thread due to the plastic flow caused by plastic deformation during rolling, and due to work hardening caused by plastic processing, a decarburized layer remains on the thread surface from the bottom of the thread to the surface of the thread, as shown in Figure 1 (b), improving delayed fracture resistance, while increasing the core bulge H, which is the non-decarburized part of the thread, improving the hardness of the shear starting point K and providing a screw backout prevention structure that ensures the strength of the shear progression path.

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Abstract

Le boulon à haute résistance et la tige d'acier précontraint (PC) selon la présente invention sont produits par un traitement de décarburation hautement précis dans une décarburation de longue durée et un traitement impliquant un laminage après trempe et revenu finaux. Selon l'invention, une billette d'acier à faible-moyenne teneur en carbone contenant du C à une proportion de 0,2 % à 0,5 % en masse est utilisée et soumise à une décarburation hautement précise pendant une longue durée, et le degré de décarburation de la couche décarburée est ajusté de telle sorte qu'un gradient de concentration de carbone constant est obtenu, d'une partie noyau vers une couche de surface; et une étape de laminage est réalisée après une étape de trempe et de revenu finaux. La hauteur de la partie de non-décarburation est augmentée tout en maintenant la couche décarburée sur une surface d'un filetage de vis, et la dureté du noyau de vis est maintenue à 400 Hv ou plus. Même lorsqu'une force axiale de F11T ou plus, correspondant à 930 MPa ou plus, est appliquée, il est possible de maintenir une résistance à la fracture retardée élevée; et une structure pour empêcher une chute de la vis est ménagée au point de départ K de cisaillement de filetage de vis.
PCT/JP2022/045798 2022-12-13 2022-12-13 Tige d'acier précontraint (pc) et boulon à haute résistance ayant une excellente résistance à la fracture retardée et ayant une structure pour empêcher une chute de vis, et procédé pour la production de ceux-ci WO2024127490A1 (fr)

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PCT/JP2022/045798 WO2024127490A1 (fr) 2022-12-13 2022-12-13 Tige d'acier précontraint (pc) et boulon à haute résistance ayant une excellente résistance à la fracture retardée et ayant une structure pour empêcher une chute de vis, et procédé pour la production de ceux-ci

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PCT/JP2022/045798 WO2024127490A1 (fr) 2022-12-13 2022-12-13 Tige d'acier précontraint (pc) et boulon à haute résistance ayant une excellente résistance à la fracture retardée et ayant une structure pour empêcher une chute de vis, et procédé pour la production de ceux-ci

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