Bolt

Abstract

The bolt has a prescribed chemical composition, and 95% or more of the metal structure at the D / 4 position is a hard structure, where D is the diameter of the shaft portion. Width at half maximum [beta] C of x-ray diffraction peak corresponding to (211) plane of ferrite phase at shaft center of connection part between bolt head and shaft part, and (211) of ferrite phase in region from surface of connection part to depth of 500 [mu] m toward central axis of shaft part the ratio [beta] MAX / [beta] C of the maximum value [beta] MAX of the full width at half maximum of the X-ray diffraction peak corresponding to the plane is 1.50 or less, and the tensile strength is 800-1700 MPa.

Description

Technical Field

[0001] The present invention relates to a bolt, and particularly to a non-quenched and tempered bolt.

[0002] This application claims priority based on Japanese Patent Application No. 2023-041287 filed in Japan on March 15, 2023, the contents of which are incorporated herein by reference. Background Art

[0003] High-strength bolts with a tensile strength of 800 MPa or higher are used in various machinery, including vehicles, and in civil engineering and construction, for reasons of weight reduction, miniaturization, and cost reduction. However, high-strength bolts are known to be susceptible to hydrogen embrittlement. To achieve higher bolt strength, it is necessary to improve hydrogen embrittlement resistance in addition to strength.

[0004] As a method for improving the hydrogen embrittlement resistance of high-strength parts such as high-strength bolts, there is a known method of strengthening the structure by forming a pearlite structure or a bainite structure through wire drawing, and many proposals have been made to date (for example, Patent Documents 1 to 5).

[0005] However, in order to further achieve space efficiency, it is desired to further increase the strength of bolts. In particular, in order to cope with the increase in strength of non-heat-treated bolts, the existing technology alone may not be sufficient.

[0006] One method for producing high-strength bolts with a tensile strength of 800 MPa or greater is to form a steel wire made of alloy steel to which alloying elements such as Cr, Mo, and V are added into a predetermined shape and then quench and temper the wire. However, this method is disadvantageous in terms of manufacturing costs due to the high heat treatment costs.

[0007] On the other hand, to reduce manufacturing costs, a method is known in which post-forming quenching and tempering is omitted and wire drawing is performed on a wire rod whose strength has been increased through rapid cooling, precipitation strengthening, etc., thereby producing high-strength bolts with a specified strength. Bolts produced using this method are called non-quenched and tempered bolts.

[0008] For example, in non-quenched and tempered bolts manufactured by cold working a steel wire strengthened by drawing pearlite or bainite, hydrogen is captured at the interface between the cementite phase and the ferrite phase within the pearlite or bainite structure. This is believed to suppress the intrusion of hydrogen into the steel material, improving hydrogen embrittlement resistance. However, this technique alone is not sufficient to achieve a sufficient improvement in hydrogen embrittlement resistance, and further improvement is desired.

[0009] Patent Document 6 describes a method for manufacturing non-quenched and tempered bolts, describing a bolt forging method in which a rod-shaped metal material is preformed to form a preformed bolt material with a bulging pressing side. The preformed bolt material is then formed to have a larger shaft volume than the finished bolt material. However, even bolts manufactured using the method in Patent Document 6 exhibit insufficient hydrogen embrittlement resistance.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 11-315348

[0013] Patent Document 2: Japanese Patent Application Laid-Open No. 2001-348618

[0014] Patent Document 3: Japanese Patent Application Laid-Open No. 2005-281860

[0015] Patent Document 4: International Publication No. 2016 / 121820

[0016] Patent Document 5: International Publication No. 2017 / 122830

[0017] Patent Document 6: Japanese Patent Application Laid-Open No. 2007-136460 Summary of the Invention

[0018] Technical problem to be solved by the invention

[0019] The present invention has been made in view of the above circumstances, and provides a bolt having a tensile strength of 800 MPa to 1700 MPa and excellent hydrogen embrittlement resistance.

[0020] Means for solving technical problems

[0021] The gist of the present invention is as follows.

[0022] [1] A bolt, the chemical composition of which, in terms of mass %, comprises:

[0023] C: 0.18~0.80%,

[0024] Si: 0.01~1.50%,

[0025] Mn: 0.50~2.00%,

[0026] Al: 0.005~0.080%,

[0027] P: 0.030% or less,

[0028] S: 0.030% or less,

[0029] Ti: 0.005~0.100%,

[0030] B: 0.0003~0.0050%,

[0031] N: 0.0150% or less,

[0032] O: 0.0100% or less,

[0033] The rest is composed of Fe and impurities.

[0034] In this bolt,

[0035] The shaft diameter is set to D, and more than 95% of the metal structure at the D / 4 position is hard structure.

[0036] The half-maximum width β of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase at the center of the shaft portion of the connection between the bolt head and the shaft portion C , the maximum value β of the half-maximum width of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase in the region from the surface of the connecting portion to the central axis of the shaft portion at a depth of 500 μm MAX Ratio β MAX / β C The tantalum index is below 1.50, and the tensile strength is 800~1700MPa.

[0037] [2] A bolt, the chemical composition of which, in terms of mass %, comprises:

[0038] C: 0.18~0.80%,

[0039] Si: 0.01~1.50%,

[0040] Mn: 0.50~2.00%,

[0041] Al: 0.005~0.080%,

[0042] P: 0.030% or less,

[0043] S: 0.030% or less,

[0044] Ti: 0.005~0.100%,

[0045] B: 0.0003~0.0050%,

[0046] N: 0.0150% or less,

[0047] O: 0.0100% or less,

[0048] One or more selected from the following groups A, B and C,

[0049] The rest is composed of Fe and impurities.

[0050] In this bolt,

[0051] The shaft diameter is set to D, and more than 95% of the metal structure at the D / 4 position is hard structure.

[0052] The half-maximum width β of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase at the center of the shaft portion of the connection between the bolt head and the shaft portion C , the maximum value β of the half-maximum width of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase in the region from the surface of the connecting portion to the central axis of the shaft portion at a depth of 500 μm MAX Ratio β MAX / β C Below 1.50,

[0053] The tensile strength is 800~1700MPa.

[0054] [Group A] One or more selected from the group consisting of Cr: 1.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.20% or less, and W: 0.20% or less.

[0055] [Group B] One or two selected from Cu: 0.50% or less and Ni: 0.50% or less.

[0056] [Group C] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.0100% or less, Ce: 0.020% or less, and Sn: 0.0400% or less.

[0057] [3] The bolt according to [2], having a chemical composition containing the group A in mass %.

[0058] [4] The bolt according to [2], having a chemical composition containing the group B in mass %.

[0059] [5] The bolt according to [2], having a chemical composition containing the C group in mass %.

[0060] Effects of the Invention

[0061] According to the present invention, a bolt having a tensile strength of 800 MPa to 1700 MPa and excellent hydrogen embrittlement resistance can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 This is a schematic cross-sectional view of a shaft portion for explaining the measurement position of the area ratio of the hard structure of steel in the bolt according to the embodiment of the present invention.

[0063] Figure 2 β represents a modification 1 of the bolt according to the embodiment of the present invention. MAX / β C The diagram showing the measurement position is a schematic cross-sectional view of the central axis of the shaft including the vicinity of the connection between the bolt head and the shaft.

[0064] Figure 3 β represents the bolt as an embodiment of the present invention. MAX / β C The diagram showing the measurement position is a schematic cross-sectional view of the central axis of the shaft including the vicinity of the connection between the bolt head and the shaft.

[0065] Figure 4 β represents a second modification of the bolt according to the embodiment of the present invention. MAX / β C The diagram showing the measurement position is a schematic cross-sectional view of the central axis of the shaft including the vicinity of the connection between the bolt head and the shaft.

[0066] Figure 5 This is a process diagram for explaining step 1 of the method for manufacturing a bolt according to the embodiment of the present invention.

[0067] Figure 6 This is a process diagram for explaining step 2 of the method for manufacturing a bolt according to the embodiment of the present invention.

[0068] Figure 7 This is a process diagram for explaining step 3 of the method for manufacturing a bolt according to the embodiment of the present invention. DETAILED DESCRIPTION

[0069] Hereinafter, the bolt according to the embodiment of the present invention will be described in detail.

[0070] The chemical composition of the bolt according to the embodiment of the present invention contains, in mass %, C: 0.18-0.80%, Si: 0.01-1.50%, Mn: 0.50-2.00%, Al: 0.005-0.080%, P: 0.030% or less, S: 0.030% or less, Ti: 0.005-0.100%, B: 0.0003-0.0050%, N: 0.0150% or less, O: 0.0100% or less, and the remainder is Fe and impurities. In this bolt, when the shaft diameter is D, 95% or more of the metal structure at the position of D / 4 is a hard structure, and the half-value width β of the X-ray diffraction peak corresponding to the (211) plane of ferrite at the center of the shaft at the connection between the bolt head and the shaft is C, the maximum value β of the half-maximum width of the X-ray diffraction peak corresponding to the (211) plane of ferrite in the region from the surface of the connection portion to the central axis of the shaft portion at a depth of 500 μm MAX Ratio β MAX / β C The tantalum index is below 1.50, and the tensile strength is 800~1700MPa.

[0071] In the case of a heat-treated bolt, the half-width of the X-ray diffraction peak corresponding to the (211) plane of ferrite remains constant at any position around the bolt head and the connection between the bolt head and the shaft (however, in the case of a bolt in which the threaded portion is formed by rolling after heat treatment, the half-width of the X-ray diffraction peak corresponding to the (211) plane of ferrite around the threaded portion may vary). Due to the influence of processing, the half-width of the X-ray diffraction peak corresponding to the (211) plane of ferrite in the bolt disclosed herein varies depending on the measurement position.

[0072] Chemical Composition

[0073] The chemical composition of the steel used in the bolts according to the embodiments of the present invention is described below. In the following description, "%" for the content of each element in the chemical composition represents "mass %." Furthermore, numerical ranges expressed using "~" indicate a range that includes the numerical values ​​listed before and after the "~" as the lower and upper limits. Furthermore, numerical ranges that include "exceed" or "below" in addition to the numerical values ​​listed before and after the "~" indicate a range that excludes these numerical values ​​as the lower and upper limits.

[0074] [C: 0.18%~0.80%]

[0075] C is an essential element required to ensure tensile strength. When the C content is less than 0.18%, it is difficult to obtain the desired tensile strength. Therefore, the lower limit of the C content is set to 0.18% or more. Preferably, it is 0.20% or more. In addition, when the C content exceeds 0.80%, cold workability deteriorates. Therefore, the upper limit of the C content is set to 0.80% or less. Preferably, it is 0.75% or less.

[0076] [Si: 0.01~1.50%]

[0077] Si (silicon) is a deoxidizing element and an element that improves tensile strength by solid solution strengthening. When the Si content is less than 0.010%, the effect of Si inclusion will not be fully demonstrated. Therefore, the lower limit of the Si content is set to more than 0.01%. Preferably, it is more than 0.05%. In addition, when the Si content exceeds 1.50%, the effect of Si inclusion is saturated, and the ductility during hot rolling deteriorates and surface defects are easily generated. Due to these surface defects, cold workability is sometimes reduced. Therefore, the upper limit of the Si content is set to less than 1.50%. Preferably, it is less than 1.00%.

[0078] [Mn: 0.50~2.00%]

[0079] Mn (manganese) is an element that promotes phase transformation (bainite transformation) to hard structure and improves the tensile strength of steel. When Mn content is lower than 0.50%, the containing effect of Mn will not fully show. Therefore, the lower limit of Mn content is set to more than 0.50%. Preferably, more than 0.60%. In addition, when Mn content exceeds 2.00%, martensitic structure is sometimes generated locally, and cold workability can deteriorate. Therefore, the upper limit of Mn content is set to less than 2.00%. Preferably, less than 1.50%.

[0080] [Al: 0.005~0.080%]

[0081] Al (aluminum) is a deoxidizing element and an element that forms AlN that functions as pinned particles. AlN refines the grains, thereby improving cold workability. In addition, Al is an element that has the effect of reducing the solid solution N to suppress dynamic strain aging and improving the hydrogen embrittlement resistance. Therefore, the Al content is set to 0.005% or more. Preferably, it is 0.010% or more. In addition, when the Al content exceeds 0.080%, the above effect is saturated, and coarse oxides such as Al2O3 are formed, which sometimes become the cause of fatigue fracture. Therefore, the upper limit of the Al content is set to 0.080% or less. Preferably, it is 0.060% or less.

[0082] [P: 0.030% or less]

[0083] P (phosphorus) is an impurity element that is inevitably mixed into steel. It is an element that segregates at grain boundaries, deteriorates hydrogen embrittlement resistance, and deteriorates cold workability. The bolts involved in this embodiment do not need to contain P, so the lower limit of the P content is 0%. However, from the perspective of reducing manufacturing costs (dephosphorization costs), the P content can exceed 0%, or it can be more than 0.002%, or it can be more than 0.005%. When the P content exceeds 0.030%, the deterioration of hydrogen embrittlement resistance and the deterioration of cold workability become significant. Therefore, the P content is limited to less than 0.030%. Preferably, it is less than 0.015%.

[0084] [S: 0.030% or less]

[0085] S (sulfur), like P, is an impurity element that is inevitably mixed into steel. It is an element that segregates at grain boundaries, deteriorating hydrogen embrittlement resistance and cold workability. Therefore, the bolts involved in this embodiment do not need to contain S, so the lower limit of the S content is 0%. However, from the perspective of reducing manufacturing costs (desulfurization costs), the S content can exceed 0%, or it can be more than 0.002%, or it can be more than 0.005%. When the S content exceeds 0.030%, the deterioration of hydrogen embrittlement resistance and the deterioration of cold workability become significant. Therefore, the S content is limited to 0.030% or less. It is preferably 0.015% or less, and more preferably 0.010% or less.

[0086] [Ti: 0.005~0.100%]

[0087] Ti (titanium) is a deoxidizing element and an element that forms TiN that functions as pin-punched particles. In addition, Ti is an element that has the effect of reducing solid-solution N to suppress dynamic strain aging and the effect of improving hydrogen embrittlement resistance. In addition, it has the effect of suppressing the generation of BN in steel. When the Ti content is less than 0.005%, deterioration of cold workability caused by dynamic strain aging, deterioration of hydrogen embrittlement resistance, and excessive generation of BN in steel may sometimes occur. Therefore, the lower limit of the Ti content is set to 0.005% or more. Preferably, it is 0.007% or more, and more preferably, it is 0.0010% or more. On the other hand, when the Ti content exceeds 0.100%, the above effect is saturated, and it becomes easy to produce defects during hot rolling. Therefore, the Ti content is set to 0.100% or less. Preferably, it is 0.080% or less, and more preferably, it is 0.040% or less.

[0088] [B: 0.0003~0.0050%]

[0089] B (boron) is an element that promotes phase transformation to hard structure (bainite transformation) and improves tensile strength. When the B content is less than 0.0003%, the phase transformation to hard structure (bainite transformation) is not promoted, and during the isothermal phase transformation treatment, excessive proeutectoid ferrite structure and pearlite structure are sometimes generated. Therefore, the lower limit of the B content is set to 0.0003% or more. It is preferably 0.0005% or more, and more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.0050%, BN and Fe are generated in the steel. 23 (BC) 6 etc. may deteriorate cold workability. Therefore, the B content is set to 0.0050% or less, preferably 0.0030% or less.

[0090] [N: 0.0150% or less]

[0091] N (nitrogen) is an element that deteriorates cold workability due to dynamic strain aging. The bolts involved in this embodiment do not need to contain N, so the lower limit of the N content is 0%. However, from the viewpoint of reducing manufacturing costs (denitrification costs), the N content can exceed 0%, or it can be more than 0.0002%, or it can be more than 0.0005%. When the N content exceeds 0.0150%, the deterioration of the cold workability caused by dynamic strain aging is significant. Therefore, the N content is limited to less than 0.0150%. Preferably, it is less than 0.0040%.

[0092] [O: 0.0100% or less]

[0093] O (oxygen) is an impurity element that inevitably mixes into steel and exists in the steel as oxides such as Al and Ti. The bolts involved in this embodiment do not need to contain O, so the lower limit of the O content is 0%. However, from the perspective of reducing manufacturing costs (deoxidation costs), the O content can exceed 0%, and can also be 0.0002% or more, or even 0.0005% or more. When the O content exceeds 0.0100%, coarse oxides are generated in the steel, which is prone to fatigue fracture. Therefore, the O content is set to 0.0100% or less. Preferably, it is 0.0050% or less.

[0094] [Remainder: Fe and impurities]

[0095] In the chemical composition of the steel used in the bolts according to this embodiment, the remainder besides the aforementioned elements is Fe and impurities. Here, impurities refer to components contained in the raw materials or components mixed in during the manufacturing process, and are not intentionally added to the steel.

[0096] In addition, the chemical composition of the steel of the bolt according to the present embodiment may contain one or more elements selected from the following group A, group B, and group C in place of a portion of Fe.

[0097] [Group A] One or more selected from the group consisting of Cr: 1.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.20% or less, and W: 0.20% or less.

[0098] [Group B] One or two selected from Cu: 0.50% or less and Ni: 0.50% or less.

[0099] [Group C] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.0100% or less, Ce: 0.020% or less, and Sn: 0.0400% or less.

[0100] About [Group A]

[0101] The chemical composition of the steel of the bolt according to this embodiment may contain, by mass%, one or more of Cr, Mo, Nb, V, and W in place of a portion of Fe. These elements promote transformation to a hard structure (bainite transformation) and increase tensile strength.

[0102] [Cr: below 1.50%]

[0103] Cr (chromium) is an optional element that promotes phase transformation to hard structure (bainite transformation) and improves the tensile strength of steel. In order to show the effect of Cr content, it is more preferably 0.02% or more, more preferably 0.03% or more, more preferably 0.05% or more, and particularly preferably 0.10% or more. On the other hand, when the Cr content exceeds 1.50%, martensite structure is locally generated, and cold workability may sometimes deteriorate. Therefore, the Cr content is set to 1.50% or less. It is preferably 1.30% or less, and more preferably 1.00% or less.

[0104] [Mo: 0.50% or less]

[0105] Mo (molybdenum) is an optional element. Like Cr, it is an element that promotes phase transformation (bainite transformation) to hard structure and improves the tensile strength of steel. In order to show the effect of Mo, it is more preferably 0.02% or more, more preferably 0.03% or more, more preferably 0.05% or more, and particularly preferably 0.10% or more. On the other hand, when the Mo content exceeds 0.50%, martensite is locally generated, and sometimes cold workability deteriorates. Therefore, the Mo content is set to 0.50% or less. It is preferably 0.40% or less, more preferably 0.35% or less.

[0106] [Nb: 0.050% or less]

[0107] Nb (niobium) is an optional element that promotes phase transformation to hard structure (bainite phase transformation) and improves tensile strength. In order to show the effect of Nb content, it is preferably 0.002% or more. On the other hand, when the Nb content exceeds 0.050%, the hot rolling ductility of the steel decreases, and surface defects become more likely to occur during wire rolling. Due to these surface defects, cold workability is sometimes reduced. Therefore, the Nb content is set to 0.050% or less. Preferably, it is 0.040% or less.

[0108] [V: 0.20% or less]

[0109] V (vanadium) is an optional element that promotes phase transformation (bainite transformation) to hard structure and improves tensile strength. From the viewpoint of obtaining these effects, the V content is preferably more than 0% or more than 0.02%, more preferably more than 0.04%. On the other hand, when the V content exceeds 0.20%, the hot rolling ductility of the steel decreases, and it becomes easy to produce surface defects during wire rolling. Due to the surface defects, cold workability is sometimes reduced. Therefore, the V content is set to less than 0.20%. Preferably, it is less than 0.15%.

[0110] [W: 0.20% or less]

[0111] W (tungsten) is an optional element, which is an element that promotes phase transformation (bainite transformation) to hard structure and improves tensile strength. From the viewpoint of obtaining these effects, the content of W is preferably more than 0% or more than 0.02%, more preferably more than 0.04%. On the other hand, when the content of W exceeds 0.20%, the hot rolling ductility of steel decreases, and surface flaws become easy to produce during wire rolling. Due to the surface flaws, cold workability is sometimes reduced. Therefore, the content of W is set to less than 0.20%. Preferably, less than 0.15%.

[0112] About [Group B]

[0113] Furthermore, the chemical composition of the steel of the bolt according to the present embodiment may contain one or both of Cu and Ni in place of a portion of Fe.

[0114] [Cu: 0.50% or less]

[0115] Cu (copper) is an optional element and may also be contained when it is desired to improve hydrogen embrittlement resistance. Cu may exceed 0%, or may be 0.02% or more. On the other hand, if the Cu content exceeds 0.50%, the hot rolling ductility of the steel decreases, and surface flaws are easily generated during wire rolling. Due to these surface flaws, cold workability may sometimes decrease. Therefore, the Cu content is set to 0.50% or less. Preferably, it is 0.35% or less.

[0116] [Ni: 0.50% or less]

[0117] Ni (nickel) is an optional element. When containing Cu, Ni may also be contained to suppress the reduction in hot rolling ductility of the steel caused by Cu. Ni may exceed 0%, or may be 0.02% or more. On the other hand, if the Ni content exceeds 0.50%, the hot rolling ductility of the steel decreases, and surface defects become more likely to occur during wire rolling. Due to these surface defects, cold workability may sometimes decrease. Therefore, the Ni content is set to 0.50% or less. Preferably, it is 0.35% or less.

[0118] About [Group C]

[0119] Furthermore, the chemical composition of the steel of the bolt according to the present embodiment may contain one or two or more of Ca, Mg, and Sn in place of a portion of Fe.

[0120] [Ca: 0.0100% or less]

[0121] Ca (calcium) is an optional element. Ca is a deoxidizing element, has the shape of the MnS in the steel spheroidized, improves the effect of cold workability, machinability, therefore, when wanting to improve these characteristics, can also contain. Ca can exceed 0%, can also be more than 0.0002%, can also be more than 0.0005%. On the other hand, if Ca content exceeds 0.0100%, then Ca-type inclusions easily sneak into the steel, and sometimes cold workability can reduce. Therefore, Ca content is made below 0.0100%. Preferably, below 0.0050%.

[0122] [Mg: 0.0100% or less]

[0123] Mg (magnesium) is an optional element. Mg is a deoxidizing element that has the effect of spheroidizing the shape of MnS in the steel and improving cold workability and machinability. Therefore, it can be added when it is desired to improve these characteristics. Mg can exceed 0%, or it can be more than 0.0002%, or it can be more than 0.0005%. On the other hand, if the Mg content exceeds 0.0100%, Mg-based inclusions are easily mixed into the steel, and sometimes cold workability can be reduced. Therefore, the Mg content is set to below 0.0100%. It is preferably below 0.0050%.

[0124] [Ce: 0.020% or less]

[0125] Ce (cerium) is an optional element. Ce is a deoxidizing element that has the effect of spheroidizing the shape of MnS in the steel and improving cold workability and machinability. Therefore, it can be added when it is desired to improve these characteristics. Ce can exceed 0%, or it can be more than 0.002%, or it can be more than 0.005%. On the other hand, if the Ce content exceeds 0.020%, Ce-type inclusions are easily mixed into the steel, and cold workability is sometimes reduced. Therefore, the Ce content is set to below 0.020%. Preferably, it is below 0.015%.

[0126] [Sn: 0.0400% or less]

[0127] Sn (tin) is an optional element. Sn has the effect of improving corrosion resistance, so it can also be contained when it is desired to improve corrosion resistance. Sn can exceed 0%, or it can be more than 0.0002%, or it can be more than 0.0005%. On the other hand, if the Sn content exceeds 0.0400%, the hot rolling ductility of the steel decreases, and surface defects are easily generated during wire rolling. Due to these surface defects, cold workability is sometimes reduced. Therefore, the Sn content is set to 0.0400% or less. Preferably, it is 0.0200% or less.

[0128] Metal Organization

[0129] Next, the metal structure of the steel of the bolt according to this embodiment is described. In cross-section C of the shaft (a cross-section perpendicular to the central axis of the shaft), assuming the shaft diameter of the bolt is D, at a depth of D / 4 from the shaft surface, 95% or more of the metal structure is hard structure.

[0130] Furthermore, regarding the bolt according to this embodiment, the half-value width β of the X-ray diffraction peak corresponding to the (211) plane of ferrite at the center of the shaft is C The maximum value of the half-height width of the X-ray diffraction peak corresponding to the (211) plane of ferrite in the region 500 μm from the surface of the shaft is β MAX Ratio β MAX / β C Below 1.50.

[0131] (Hard tissue)

[0132] Hard structure is a supercooled phase transformation structure, a structure obtained through isothermal phase transformation. When observed with a scanning electron microscope, hard structure is observed as a structure containing a ferrite phase dispersed with a carbide phase. Hard structure is sometimes also referred to as bainite structure. Hard structure does not have pearlite structure, which is a layered structure of carbide phases and ferrite phases. In addition, in the present invention, the hard structure does not include martensite structure.

[0133] It should be noted that in the microstructure of the steel material of this embodiment, when the structure other than the martensite structure, the proeutectoid ferrite structure, and the pearlite structure is defined as a "hard structure", if the area ratio of the hard structure in the microstructure is 95% or more and the tensile strength of the steel of the bolt is 800 to 1700 MPa, then the microstructure of the steel of the bolt is substantially a hard structure.

[0134] The hard structure has high strength and excellent workability. As a result, the strength and cold workability of the steel of the bolt are improved. The metal structure of the steel of the bolt involved in this embodiment is continuously cooled after hot rolling and then undergoes isothermal phase transformation treatment. Therefore, in addition to the hard structure, it sometimes also contains pro-eutectoid ferrite structure and pearlite structure. Through observation under a scanning electron microscope, the pro-eutectoid ferrite structure and pearlite structure are clearly identified as hard structures. When the area ratio of the pro-eutectoid ferrite structure and pearlite structure exceeds 5%, the metal structure becomes uneven, so cracks are easily generated when forged into the shape of a bolt. Therefore, it is desirable that the pro-eutectoid ferrite structure and pearlite structure are 5% or less, more preferably 1% or less, and can also be 0%. In addition, when martensite structure is included in the metal structure, cracks are easily generated when forged into the shape of a bolt. Therefore, martensite structure should not be contained in the metal structure. Under a scanning electron microscope, martensite is observed as a white structure and is clearly identified as a hard structure.

[0135] (Method for measuring the area ratio of hard tissue)

[0136] In this specification, the area ratio (%) of the hard structure refers to a value obtained by the following procedure.

[0137] Using picroethanol (a mixture of 4g of picric acid in 100ml of ethanol), etch the C-section (a cross-section perpendicular to the bolt axis) of the bolt shaft for 10 seconds to reveal the metal structure. If the structure is difficult to discern, an additional etching of up to 10 seconds is possible.

[0138] Then, if Figure 1 As shown, four observation positions are selected at a depth of D / 4 from the shaft surface of the C-section of the etched shaft (i.e., a circumferential position) along the circumferential direction every 90°, and for each observation position, a FE-SEM (field emission scanning electron microscope) is used to take a SEM photograph with a magnification of 1000 times.

[0139] In the four SEM photos obtained, the structures other than the hard structure (proeutectoid ferrite structure and pearlite structure) were visually marked, and the area ratio (%) of the structures other than the hard structure relative to the entire metal structure was calculated by image analysis. By subtracting the obtained area ratio (%) of the structures other than the hard structure from 100%, the area ratio (%) of the hard structure can be obtained. It should be noted that the position of the C-section of the shaft of the bolt is set to the C-section of the shaft at a position 7 mm away from the bolt support surface along the bolt axis. If no structures other than the hard structure are found, the area ratio of the hard structure can be set to 100%.

[0140] (β MAX / β C )

[0141] In the bolt according to the present embodiment, the half-width β of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase at the center of the shaft portion of the connection portion between the bolt head and the shaft portion is C , the maximum value β of the half-maximum width of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase in the region from the surface of the connection portion to the central axis of the shaft portion at a depth of 500 μm MAX Ratio β MAX / β C The inventors of the present invention have conducted in-depth research to improve the hydrogen embrittlement resistance of bolts. As a result, they found that the half-width of the X-ray diffraction peak of the ferrite phase is related to the hydrogen embrittlement resistance. More specifically, they found that when β MAX / β C When it is 1.50 or less, the bolt has excellent hydrogen embrittlement resistance.

[0142] In β MAX / β C When β exceeds 1.50, the hydrogen embrittlement resistance of the bolt is insufficient and fracture caused by hydrogen embrittlement is likely to occur. It is preferably 1.40 or less, and more preferably 1.35 or less. MAX / β C The lower limit of β is not particularly specified, but is preferably 0.50 or more from the perspective of manufacturability. MAX / β C The lower limit of may be 0.70 or more, or 0.80 or more.

[0143] (Measurement position and measurement method of half-width of X-ray diffraction peak)

[0144] As β MAX / β C The half-height width of the denominator β C The measurement is carried out at the center of the shaft where the bolt head and the shaft meet. MAX / β C The molecular β MAXThe half-width of the X-ray diffraction peak is measured at multiple positions in the area from the surface of the connection between the bolt head and the shaft to a depth of 500μm toward the central axis of the shaft, and the maximum value is taken. The bolt involved in this embodiment is a so-called non-tempered bolt, and the strain introduced during the cold working for finishing into the bolt shape remains on the bolt. Therefore, the half-width of the X-ray diffraction peak may vary greatly depending on the measurement position. The reason for measuring the half-width ratio of the X-ray diffraction peak at the connection part is because it was found that the connection part between the bolt head and the shaft part is a position that is easily subjected to stress concentration and is easily the starting point of delayed fracture caused by hydrogen embrittlement. By controlling the half-width ratio of the X-ray diffraction peak at the connection part, the hydrogen embrittlement resistance can be improved. The measurement position is explained below with reference to the accompanying drawings.

[0145] Figure 2 is the half-height width β MAX and half-width β C A cross-sectional diagram illustrating the measurement position of Figure 2 , an L-section (a section parallel to the bolt axis) is shown, which is a section near the connection between the bolt head and the shaft and includes the central axis of the shaft.

[0146] First, in Figure 2 In the figure, symbol 1 is the outline of the supporting surface of the bolt in the L-section, symbol 2 is the outline of the cylindrical portion in the L-section, and symbol 3 is the outline of the surface of the connection portion between the bolt head and the shaft. It should be noted that the outline of the cylindrical portion in the case of a fully threaded bolt is the outline of an incomplete threaded portion. The outline 1 of the lower surface of the bolt head and the outline 2 of the surface of the shaft are oriented in mutually intersecting directions. Here, the so-called supporting surface of the bolt refers to the part of the surface that is directly subjected to force when tightening the threaded component (excluding the side (flank) surface of the thread). The cylindrical portion refers to the cylindrical portion located between the head and the threaded portion of the external threaded component. In Figure 2 In the example, contour line 1 is substantially perpendicular to contour line 2 on the shaft surface. However, as will be shown in another example later, this does not necessarily have to be perpendicular. Furthermore, because the connection between the lower surface of the bolt head and the shaft surface is slightly rounded, contour line 3 on the surface of the connection can be approximated as a concave curve by connecting contour lines 1 and 2 extending in the intersecting direction.

[0147] Here, the extension line of the contour line 1 of the lower surface of the bolt head is defined as l flg , let the extension line of the contour line 2 of the surface of the shaft be l axis , extend the line l flg With extension cord axis The intersection point is set to A.

[0148] Next, if Figure 2As shown, a line segment is drawn from the intersection point A to the contour line 3 of the surface of the connection portion, and the intersection point of the line segment and the contour line 3 is set as O. The line segment drawn from the intersection point A to the contour line 3 is connected to the extension line l flg A line segment that makes an angle of 45°.

[0149] Then, if Figure 2 As shown, the line extending from the intersection O to the central axis 4 of the shaft portion and the extension line l flg Parallel extension line l measure . Extend the line measure The intersection point with the central axis 4 is set as C.

[0150] Then, if Figure 2 As shown, the intersection point C is set to the half-height width β C In addition, if Figure 2 As shown, from the intersection O along the extension line l measure The area D up to a depth of 500 μm is defined as the half-height width β MAX measurement area.

[0151] like Figure 2 As shown in FIG. 1 , the L-section is mirror-finished as follows. The L-section is a section near the connection between the bolt head and the shaft, and is an L-section of the connection including the central axis of the bolt. Figure 2 X-ray diffraction measurement is performed at the intersection C of the ferrite phase, and the half-height width (full width at half height means FWHM: Full Width Half Maximum) of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase is measured at the intersection C. The X-ray diffraction measurement is performed 5 times. The average value of the 5 half-height widths obtained by the 5 measurements is set as the half-height width β C .

[0152] In addition, if Figure 2 As shown, from along the extension line l measure Five measurement positions are set in the area D up to a depth of 500 μm. For example, five points at a distance of 100 μm, 200 μm, 300 μm, 400 μm, and 500 μm from the intersection O are set as measurement positions, and at each measurement position, the half-height width (half-height width means FWHM: Full Width Half Maximum) of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase is measured. The X-ray diffraction measurement is performed 5 times at each measurement point, and the average of the 5 times is set as the half-height width at each measurement position. In addition, the maximum half-height width among the half-height widths obtained at the 5 measurement positions is set as the half-height width β MAX .

[0153] Then, find the half-height width β C The maximum value of the half-height width β MAXRatio β MAX / β C .

[0154] The ferrite phase to be measured refers primarily to α-Fe with a body-centered cubic structure contained in the hard structure. More specifically, it refers to the portion of the hard structure other than the carbides (sometimes also referred to as bainitic ferrite). Furthermore, the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase is an X-ray diffraction peak with a peak position within the range of 150 to 170° when X-ray diffraction measurement is performed under the following conditions.

[0155] The diffraction curve obtained by X-ray diffraction measurement is preprocessed according to the following procedure. The following is based on the premise that the measurement interval of the X-ray diffraction measurement does not include any X-ray diffraction peak other than the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase.

[0156] The obtained diffraction curve was smoothed by the Savitsky-Golay method (A. Savitzky and MJE Golay, Anal. Chem., 36 (1964), pp. 1627-1639.) The number of smoothing points (equivalent to the value 2m+1 in Savitzky and Golay (1964)) was set to 7.

[0157] Next, using the intensities at the scan start and end angles of the X-ray diffraction measurement, a straight line is drawn between the measurement start and end angles, and this straight line is used as the background to subtract from the smoothed diffraction curve. The background-subtracted diffraction curve is subjected to Rachinger's method (Rachinger, WA, J. Sci. Instrum., 25 (1948), pp. 254-255.), removing the X-ray diffraction peak corresponding to the Kα2 line from the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase in the background-subtracted diffraction curve. The intensity ratio of the X-ray diffraction peak corresponding to the Kα1 line to the X-ray diffraction peak corresponding to the Kα2 line is set to 2:1. The full width at half maximum (FWHM) is calculated from the diffraction curve obtained using the above steps and is used as the full width at half maximum (FWHM) of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase at the measurement position.

[0158] It should be noted that it suffices to satisfy the following measurement conditions except for the diffraction apparatus, and therefore an X-ray diffraction apparatus equivalent to that manufactured by Rigaku may be used.

[0159] (X-ray diffraction measurement conditions)

[0160] X-ray diffraction apparatus: Rigaku AutoMATE

[0161] X-ray target: CrKα

[0162] Accelerating voltage: 40 kV

[0163] Acceleration current: 40mA

[0164] Collimator diameter: φ150μm

[0165] X-ray scanning range (scanning start angle ~ scanning end angle): 146°~165°

[0166] In addition, when performing X-ray diffraction measurement, the L-section of the shaft portion of the bolt can be processed as described below and then measured.

[0167] Specifically, the L-section of the bolt shaft is first wet-ground using sandpaper (silicon carbide water-resistant abrasive paper) with a grit size of #400 to #1500. The test piece is then polished to a mirror finish using a polishing cloth infused with a diamond suspension (a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water). The surface of the test piece is then polished with colloidal silica to remove the surface deterioration layer. The full width at half maximum of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase is then measured in an area ranging from 500 μm from the center of the shaft and the surface of the connection (e.g., 5 points at 100 μm, 200 μm, 300 μm, 400 μm, and 500 μm from the surface).

[0168] If the bolt shaft has a coating such as electroplated zinc on its surface, the measurement starting point is not the surface of the coated area, but the surface of the steel base. Furthermore, the measurement starting point is set so that the X-ray irradiation area (a circular area with the same diameter as the collimator) at the measurement starting point does not include areas other than the steel base. In other words, the measurement starting point is set so that the X-ray irradiation area at the measurement starting point circumscribes the surface of the steel base.

[0169] Furthermore, as another example of a bolt, for example, Figure 3 As shown, there is a bolt with a tapered end portion on the head side of the shaft. Figure 3 In the figure, reference 1 is the outline of the bolt's bearing surface in the L-section, reference 2 is the outline of the cylindrical portion in the L-section, and reference 3 is the outline of the surface connecting the bolt head and the shaft. Furthermore, in this example, between outlines 2 and 3 is a tapered outline 2a provided on the shaft.

[0170] The contour line 1 of the lower surface of the bolt head and the contour line 2a of the cone are oriented in directions that intersect each other. Figure 3In the figure, the contour line 2a of the tapered portion of the shaft is tilted relative to the contour line 2 by the amount of the tapered portion and is not perpendicular to the contour line 1. Furthermore, the connection between the lower surface of the bolt head and the surface of the shaft is slightly rounded, so the contour line 3 of the surface of the connection is approximately represented by a concave curve, connecting the contour lines 1 and 2a extending in the intersecting direction.

[0171] Thus, in the case of a bolt having a tapered shape, such as Figure 3 As shown, the extension line of the contour line 1 of the lower surface of the bolt head is set as l flg , the extension line of the contour line 2a of the tapered part of the shaft is set as l axis , extend the line l flg With extension cord axis The intersection of the line segment and the contour line 3 is set as A. Next, a line segment is drawn from the intersection A to the contour line 3, and the intersection of the line segment and the contour line 3 is set as O. The line segment drawn from the intersection A to the contour line 3 is the intersection of the line segment and the extension line l flg Then, from the intersection point O to the central axis 4 of the shaft, the extension line l flg Parallel extension line l measure . Extend the line measure The intersection point with the central axis 4 is defined as C. Then, the intersection point C is defined as the half-height width β C In addition, from the intersection O to the extension line l measure The area D up to a depth of 500 μm is defined as the half-height width β MAX measurement area.

[0172] Furthermore, if Figure 4 As another example of a bolt, there is a bolt in which the lower surface of the head is undercut at the connection portion between the shaft and the head. Figure 4 In the figure, symbol 1 is the contour line of the support surface of the bolt in the L-section, symbol 2 is the contour line of the surface of the cylindrical portion in the L-section, and symbol 3a is the contour line of the surface of the connection portion between the bolt head and the shaft. Contour line 3a is a contour line of a cut-off shape. Point B, which is the connection point between contour line 2 of the shaft and contour line 3a of the cut-off shape, is located at a position lower than the lower surface of the head in the figure. In addition, contour line 1 of the lower surface of the head and contour line 2a of the shaft are oriented in directions perpendicular to each other. Contour line 3a of the surface of the connection portion is in the shape of a cut-off shape and is approximately represented as a concave curve by connecting contour lines 1 and 2a extending in the intersecting direction.

[0173] Thus, in the case of a bolt with a cut bottom, e.g. Figure 4 As shown, the extension line of the contour line 1 of the lower surface of the bolt head is set as l flg , let the extension line of the axis contour line 2 be l axis , extend the line lflg With extension cord axis The intersection of the line segment and the contour line 3a is defined as A. Next, a line segment is drawn from the intersection A to the contour line 3a, and the intersection of the line segment and the contour line 3a is defined as O. The line segment drawn from the intersection A to the contour line 3a is the line segment that intersects the extension line l flg Then, from the intersection point O to the central axis 4 of the shaft, the extension line l flg Parallel extension line l measure . Extend the line measure The intersection point with the central axis 4 is defined as C. Then, the intersection point C is defined as the half-height width β C In addition, from the intersection O to the extension line l measure The area D up to a depth of 500 μm is defined as the half-height width β MAX measurement area.

[0174] Tensile Strength

[0175] The tensile strength of the bolts according to this embodiment is set in the range of 800 to 1700 MPa. Despite having a tensile strength of 800 MPa or greater, the bolts according to this embodiment exhibit excellent hydrogen embrittlement resistance. Furthermore, since the tensile strength is 1700 MPa or less, the bolts have excellent manufacturing adaptability.

[0176] In this specification, the tensile strength of the steel of the bolt means a value measured based on the test method described in JIS B 1051:2014.

[0177] Next, a method for manufacturing a bolt according to this embodiment will be described. The bolt according to this embodiment includes a step of manufacturing a wire rod and a steel wire, and a step of manufacturing a bolt from the obtained steel wire.

[0178] <Manufacturing of Wire Rods and Steel Wires>

[0179] A steel sheet having the same chemical composition as the bolt according to the embodiment of the present invention is heated and hot rolled to a finishing temperature exceeding 900°C. After hot rolling, the sheet is cooled from the coiling end temperature to 500°C at an average cooling rate of 10°C / s or higher (the time-based arithmetic mean of the cooling rates in the temperature range from the coiling end temperature to 500°C). The sheet is then held at a constant temperature (isothermal transformation treatment) to obtain a wire rod.

[0180] Specifically, after the coiling after hot rolling is completed, the wire rod is immediately immersed in a molten salt bath at 350-500°C and directly maintained at a constant temperature. From the perspective of sufficient temperature maintenance and productivity of the wire rod, the immersion time in the molten salt bath is set to 5-150 seconds. It should be noted that the cooling after keeping it in the molten salt bath for a predetermined time can be water cooling or natural cooling. It should be noted that the same effect can be obtained even if a lead bath, fluidized bed or other equipment is used as the immersion bath instead of the molten salt bath.

[0181] The steel wire is then produced by wire drawing, which is performed in a single or multiple passes to achieve a total cross-sectional reduction of 15 to 65%.

[0182] Whether drawing is done in a single or multiple passes, the total reduction of area should be set at 15% or higher. If the total reduction of area is too low, hydrogen embrittlement resistance may be reduced. Furthermore, sufficient tensile strength may not be achieved.

[0183] Regardless of whether wire drawing is performed in a single pass or multiple passes, if the total cross-sectional reduction ratio is too high, processing cracks are likely to occur when bolts are made from the steel wire (cold forging). Therefore, the total cross-sectional reduction ratio is set to 65% or less.

[0184] It should be noted that the diameter of the steel wire to be manufactured is not particularly limited, as long as an appropriate diameter is selected according to the size of the bolt involved in the embodiment of the present invention, for example, it can be set to 10.0 mm or less. The diameter of the shaft of the bolt can also be set to 10.0 mm or less.

[0185] Next, the obtained steel wire is cold worked (cold forging) to be processed into the shape of a bolt. The cold forging process includes a shearing process, a front extrusion process, a bolt head pre-forming process, and a bolt head finishing process. Figures 5 to 7 Each process is described.

[0186] The bolts involved in this embodiment are not limited to those manufactured by the following manufacturing method. Even if they are manufactured by a manufacturing method other than the following manufacturing method, as long as the chemical composition, metal structure and the ratio of the half-height width of the X-ray diffraction peak (ratio β MAX / β C ) meets the scope of the present invention and is included in the bolt of the present invention.

[0187] In the shearing process, the obtained steel wire is cut and processed into sheets of the required length.

[0188] In the front extrusion process, such as Figure 5As shown, sheet material 1 is subjected to forward extrusion processing, and a portion of sheet material 1 is reduced in diameter to produce extruded material 1a. The reduced diameter portion 2 (forward extrusion portion) of extruded material 1a includes a portion corresponding to the lower portion of the neck when finished into the shape of a flanged hexagonal bolt. Subsequently, the bolt head is formed through a bolt head preforming step and a bolt head finishing step (step 1).

[0189] The cross-sectional shrinkage ratio R in the front extrusion process can be set to 0.10 or more, for example. When the cross-sectional area of ​​the large diameter portion 3 of the extruded material 1a is A0 and the cross-sectional area of ​​the reduced diameter portion 2 of the extruded material 1a is A1, the cross-sectional shrinkage ratio R is defined as (A0-A1) / A0. The front extrusion process can adjust the strain distribution of the bolt, and the ratio of the half-height width of the X-ray diffraction peak at the connection between the bolt head and the shaft (ratio β) can be adjusted. MAX / β C ) is set to 1.50 or less.

[0190] In the bolt head preforming process, such as Figure 6 As shown, a preform 1b having an upset-forged large-diameter portion 13 is formed by upsetting the large-diameter portion 3 (the portion not subjected to the upsetting process) of an extruded material 1a that has undergone a forward extrusion process. The outer diameter of the large-diameter portion 13 of the preform 1b is increased relative to the large-diameter portion 3 of the extruded material 1a, and the height of the large-diameter portion 13 is decreased (step 2).

[0191] In the bolt head finishing process, such as Figure 7 As shown, the preformed material 1b after the preforming process of the bolt head is inserted into the hole 21 of the finishing forming die, and is upset by the punch 20 to be processed into the final shape of the bolt. At this time, the preformed material 1b is positioned by arranging a ejector 22 below the preformed material 1b. When the preformed material 1b is inserted into the hole 21 of the finishing forming die, it is configured so that the lower end 13a of the large diameter portion 13 (upsetting portion) of the preformed material 1b does not contact the upper end 21a of the hole 21 of the finishing forming die (process 3). It should be noted that an appropriate die hole corresponding to the shape of the formed bolt head can also be formed on the punch 20. For example, when forming a hexagonal bolt with a flange, a punch with a hexagonal die hole is used.

[0192] The distance between the lower end 13a of the large diameter portion 13 (upset portion) and the upper end 21a of the finishing die pass 21 is preferably (ΔL) mm or more and (ΔL+r) mm or less. ΔL can be calculated using the following formula.

[0193] ΔL=L2×[(S2 / S1)-1]

[0194] In the above formula, S1 is the cross-sectional area of ​​the shaft of the preform material 1b, S2 is the cross-sectional area of ​​the shaft after the bolt head finishing process, L2 is the shaft length after the bolt head finishing process, and r is the shaft radius after the bolt head finishing process.

[0195] When the distance between the lower end 13a and the upper end 21a is less than (ΔL) mm, the half-height width ratio β of the X-ray diffraction peak MAX / β C It may exceed 1.50. If it exceeds (ΔL+r) mm, buckling may occur during the bolt head finishing process. Therefore, the distance between the lower end 13a and the upper end 21a is preferably greater than (ΔL) mm and less than (ΔL+r) mm.

[0196] In this way, by involving the bolt head preforming step and the bolt head finishing step, the half-height width ratio β of the X-ray diffraction peak at the connection portion between the bolt head and the shaft can be reduced to MAX / β C Set to 1.50 or less.

[0197] After the cold forging process, a threaded portion is formed on the shaft of the bolt by rolling.

[0198] The bolts according to this embodiment are high-strength even when left as they are. However, to improve other mechanical properties required of a bolt, such as yield strength, yield strength ratio, and ductility, they may be held at 200-600°C for 10-300 minutes after being finished into a bolt shape, followed by cooling. It should be noted that this heat treatment does not correspond to heat treatment for quenching and tempering.

[0199] Furthermore, coating treatments such as electrogalvanizing may be applied for rust prevention. Furthermore, if the rust-proof coating treatment causes hydrogen intrusion into the bolt, a heat treatment to release hydrogen to the outside of the bolt may be performed by maintaining the bolt at 150-250°C for 60-480 minutes followed by cooling. This heat treatment does not constitute heat treatment for quenching and tempering.

[0200] Thus, in this embodiment, the steel wire having the chemical composition of the present invention is formed into the shape of a bolt by cold forging according to the shearing step, the front extrusion step, the bolt head preforming step, and the bolt head finishing step, so that the half-height width ratio β of the X-ray diffraction peak at the connection portion between the bolt head and the shaft can be made MAX / β C When the ratio is 1.50 or less, delayed fracture starting from the connection portion due to hydrogen embrittlement can be prevented.

[0201] Example

[0202] <Manufacturing of Bolts>

[0203] First, steel wire was produced using steel sheets with the chemical compositions shown in Tables 1A to 2B according to the following steps. In the chemical compositions of the various steel grades in Tables 1A to 2B, the remainder, other than the elements listed in Tables 1A to 2B, consists of Fe and impurities. It should be noted that the chemical composition of the steel wire can be considered identical to that of the steel sheet. This is because the subsequent steps, including hot rolling, isothermal transformation treatment, water cooling, air cooling, and wire drawing, do not affect the chemical composition of the steel wire.

[0204] [Table 1A]

[0205]

[0206] [Table 1B]

[0207]

[0208] [Table 2A]

[0209]

[0210] [Table 2B]

[0211]

[0212] At levels 1 to 63 and 65 to 68, the steel sheets were hot rolled under the conditions shown in Table 2. Cooling was then performed at an average cooling rate of at least 10°C / s from the coiling end temperature to 500°C, yielding wires with the diameters shown in Table 2. Subsequently, the steel sheets were subjected to a sequential process of isothermal transformation treatment, water cooling, and wire drawing under the conditions shown in Tables 3A and 3B to yield steel wires. All levels represent single-pass drawing.

[0213] The steel sheets were hot rolled at level 64 under the conditions shown in Table 3B, and naturally cooled without isothermal transformation treatment. Steel wires were then obtained by wire drawing under the conditions shown in Table 3B.

[0214] Next, the steel wires of each level were cold worked (cold forged) under the conditions shown in Tables 3A and 3B to form flanged hexagonal bolts with a nominal diameter of 8.0 mm or 4.0 mm (M8 or M4 flanged hexagonal bolts specified in JIS B1189:2015).

[0215] "Forging 1" in Table 3A and Table 3B is the condition for cold working described below. That is, the shearing process, the front extrusion process, the bolt head preforming process and the bolt head finishing process are performed. In the shearing process, the steel wire is cut into a sheet material. In the front extrusion process, as shown in FIG. Figure 5As shown in FIG. 1 , the sheet material 1 is subjected to forward extrusion processing to obtain an extruded material 1a having a reduced diameter portion 2 and a large diameter portion 3. The cross-sectional shrinkage ratio is set to 0.12. In the bolt head preforming process, as shown in FIG. Figure 6 As shown in FIG. 1 , by upsetting the large diameter portion 3 of the extruded material 1a, a preformed material 1b having an upsetting large diameter portion 13 is formed. Figure 7 As shown, the preformed material 1b is inserted into the finishing die die 21 and upset-forged by a punch 20 formed with a hexagonal die hole to form the final shape of a flanged hexagonal bolt. During processing, the preformed material 1b is configured so that the lower end 13a of the large diameter portion 13 does not contact the upper end 21a of the finishing die die 21. The distance between the lower end 13a of the large diameter portion 13 (upset portion) and the upper end 21a of the finishing die die 21 is set to 0.45mm. It should be noted that in "Forging 1", ΔL = 0.30mm. ΔL + r exceeds 0.45mm.

[0216] In "Forging 2" in Tables 3A and 3B, the preform 1b was positioned so that the lower end 13a of the large-diameter portion 13 contacted the upper end 21a of the finishing die pass 21 during the bolt head finishing process. Specifically, the distance between the lower end 13a of the large-diameter portion 13 (the upset portion) and the upper end 21a of the finishing die pass 21 was 0 mm, less than ΔL. Other conditions were the same as for "Forging 1."

[0217] "Forging 3" in Tables 3A and 3B was cold worked identically to "Forging 1," except that the cross-sectional reduction ratio in the forward extrusion step was set to 0.11, and the distance between the lower end 13a of the large-diameter portion 13 (upset portion) and the upper end 21a of the finishing die pass 21 in the bolt head finishing step was set to 0.42 mm. Note that in "Forging 3," ΔL = 0.41 mm. ΔL + r exceeded 0.42 mm.

[0218] It should be noted that the chemical composition of the steel used in the bolts is considered to be the same as that of the steel wire. This is because the cold working (cold forging) and heat treatment described above do not affect the chemical composition of the steel used in the bolts. In other words, the chemical composition of the steel used in the bolts is considered to be the same as that of the steel sheet.

[0219] [Table 3A]

[0220]

[0221] [Table 3B]

[0222]

[0223] <Measurement in Bolts>

[0224] For bolts of each level, the area ratio of the hard tissue (bainite structure) at the D / 4 position, the confirmation of the remaining part other than the hard tissue (bainite structure) at the D / 4 position, the measurement of the full width at half maximum of the X-ray diffraction peak, and the measurement of the tensile strength are respectively carried out by the above method.

[0225] <Measurement of the full width at half maximum of the X-ray diffraction peak>

[0226] For bolts of each level, the full width at half maximum and the maximum value β of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase are measured by the said measurement method. MAX Ratio β MAX / β C . In the calculation of the full width at half maximum, the software attached to the X-ray diffraction device (Rigaku AutoMATE) is used.

[0227] <Measurement of the tensile strength of the bolt>

[0228] For bolts of each level, the tensile strength is measured by the said measurement method. The crosshead displacement speed during the tensile test is set to 3.0 mm / min. [[ID=2I]]

[0229] <Evaluation of the hydrogen embrittlement resistance characteristics of the bolt>

[0230] For the obtained bolts, the hydrogen embrittlement resistance characteristics are measured by the following method. First, by electrolytic hydrogen charging of the bolts, the bolts are made to contain 0.5 ppm of diffusible hydrogen. The method of electrolytic hydrogen charging is based on ISO 16573. Then, Cd plating is applied to the samples to prevent hydrogen from being released from the mechanical components to the air during the test. Then, in the air, a load of 90% of the maximum tensile load of the bolt is applied to the bolt, and it is kept in this state for more than 100 hours.

[0231] As a result, the case where no fracture occurs after 100 hours is judged to have good hydrogen embrittlement resistance characteristics, and the case where fracture occurs after 100 hours is judged to have poor hydrogen embrittlement resistance characteristics.

[0232] The above results are shown in Table 4A and Table 4B.

[0233] [[ID=3I]] [Table 4A]

[0234]

[0235] [Table 4B]

[0236]

[0237] The bolts of levels 1 to 41 as the examples of the present invention all have good hydrogen embrittlement resistance characteristics.

[0238] Bolts of standards 42 to 63 do not have the chemical composition of the present invention. Therefore, as shown below, bolts of standards 42 to 63 do not meet the requirements of tensile strength, cold workability, and hydrogen embrittlement resistance.

[0239] The C content of the bolts of level 42 deviates from the chemical composition of the present invention, and therefore deviates from the tensile strength of the bolts targeted by the present invention.

[0240] The C content of the bolt of level 43 deviated from the chemical composition of the present invention, so cracks occurred during forging, making it impossible to evaluate the tensile strength and hydrogen embrittlement resistance.

[0241] The contents of Si, Ti, B, N, O, Nb, V, Ce, Cu, Ca, Mg, Sn, and W in bolts of levels 44, 48, 49, 51, 52, 53, and 56 to 63 deviated from the chemical composition of the present invention. Therefore, cracks occurred during forging, making it impossible to evaluate tensile strength and hydrogen embrittlement resistance.

[0242] The B content in the Level 50 bolt deviated from the chemical composition of the present invention. Therefore, the area ratio of the hard structure at a depth of D / 4 from the shaft surface was less than 95%, resulting in an uneven structure with ferrite mixed in the hard structure. This caused cracks to form during forging, making it impossible to evaluate tensile strength and hydrogen embrittlement resistance.

[0243] The contents of Mn, Cr, and Mo in bolts of levels 45, 54, and 55 deviated from the chemical composition of the present invention. Therefore, locally generated martensite caused cracks during forging, making it impossible to evaluate tensile strength and hydrogen embrittlement resistance.

[0244] The P and S contents of the bolts of levels 46 and 47 deviate from the chemical composition of the present invention, and therefore do not have sufficient hydrogen embrittlement resistance.

[0245] Level 64 bolts are not subjected to isothermal phase transformation treatment but are naturally cooled. Therefore, the area ratio of the hard structure at a depth of D / 4 from the shaft surface is less than 95%, resulting in cracks during forging, making it impossible to evaluate tensile strength and hydrogen embrittlement resistance.

[0246] Level 65 bolts were not immersed in the molten salt bath for a sufficient time during the isothermal transformation treatment. As a result, martensite formed during the water cooling treatment after the isothermal transformation treatment, causing cracks during forging. This made it impossible to evaluate tensile strength and hydrogen embrittlement resistance.

[0247] When the bolts of level 66 and 67 are inserted into the die hole of the finishing mold, a cold forging process is performed in which the lower end of the expansion portion of the preformed bolt contacts the upper end of the die hole of the finishing mold. Therefore, the half-height width ratio β of the X-ray diffraction peak is MAX / β CWhen the ratio exceeds 1.50, the hydrogen embrittlement resistance is insufficient.

[0248] The wire drawing conditions of the level 68 bolts are not optimal, and therefore deviate from the tensile strength of the present invention.

[0249] Industrial Applicability

[0250] The bolt disclosed herein has a tensile strength of 800 MPa to 1700 MPa and excellent hydrogen embrittlement resistance, and therefore has high industrial applicability.

Claims

1. A bolt, the chemical composition of which, by mass%, comprises: C：0.18~0.80%、 Si: 0.01~1.50%, Mn: 0.50~2.00%, Al：0.005~0.080%、 P: 0.030% or less, S: 0.030% or less, Ti: 0.005~0.100%, B：0.0003~0.0050%、 N: 0.0150% or less, O: 0.0100% or less, The rest is composed of Fe and impurities. In this bolt, The shaft diameter is set to D, and more than 95% of the metal structure at the D / 4 position is hard structure. The half-maximum width β of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase at the center of the shaft portion of the connection between the bolt head and the shaft portion C , the maximum value β of the half-maximum width of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase in the region from the surface of the connecting portion to the central axis of the shaft portion at a depth of 500 μm MAX Ratio β MAX / β C Below 1.50, The tensile strength is 800~1700MPa.

2. A bolt, the chemical composition of which, by mass%, comprises: C：0.18~0.80%、 Si: 0.01~1.50%, Mn: 0.50~2.00%, Al：0.005~0.080%、 P: 0.030% or less, S: 0.030% or less, Ti: 0.005~0.100%, B：0.0003~0.0050%、 N: 0.0150% or less, O: 0.0100% or less, One or more selected from the following groups A, B and C, The rest is composed of Fe and impurities. In this bolt, The shaft diameter is set to D, and more than 95% of the metal structure at the D / 4 position is hard structure. The half-maximum width β of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase at the center of the shaft portion of the connection between the bolt head and the shaft portion C , the maximum value β of the half-maximum width of the X-ray diffraction peak corresponding to the (211) plane of the ferrite phase in the region from the surface of the connecting portion to the central axis of the shaft portion at a depth of 500 μm MAX Ratio β MAX / β C Below 1.50, Tensile strength is 800~1700MPa, [Group A] One or more selected from the group consisting of Cr: 1.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.20% or less, and W: 0.20% or less, [Group B] One or two selected from Cu: 0.50% or less, Ni: 0.50% or less, [Group C] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.0100% or less, Ce: 0.020% or less, and Sn: 0.0400% or less. The bolt according to claim 2 , having a chemical composition containing the group A by mass. The bolt according to claim 2 , having a chemical composition containing the B group by mass. The bolt according to claim 2 , having a chemical composition containing the C group by mass.

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