Steel material, steel wire, and torsion bar component

Abstract

Provided is a steel material that suppresses an increase in torque associated with twisting. A steel material according to the present disclosure contains 0.006-0.050% of C, 0.10-0.80% of Mn, 0.005-0.080% of Al, at most 0.50% of Si, at most 0.030% of P, at most 0.050% of S, at most 0.0120% of N, at most 0.0100% of O, 0.003-0.040% of Ti, and 0.003-0.040% of Nb, with the balance consisting of Fe and impurities, wherein the ferrite area ratio is at least 90%, and [Ti*] of expression (1) and [Nb*] of expression (2) satisfy expression (3), and the maximum value (Q-1)max of internal friction as measured in the range of -50 to 150 °C is 1.2×10-4 to 10.0×10-4.　(1): [Ti*]=Ti / {(48 / 12)C+(48 / 14)N} (2): [Nb*]=Nb / {(93 / 12)C+(93 / 14)N} (3): [Ti*]+[Nb*]<1.00

Description

Steel materials, steel wire and torsion bar parts

[0001] The present disclosure relates to a steel material, a steel wire, and a torsion bar part, and more particularly to a steel material, a steel wire, and a torsion bar part suitable as materials for a torsion bar part for a seat belt retractor.

[0002] Seatbelt retractors installed in automobiles restrain occupants with seatbelts when the vehicle suddenly decelerates due to an impact such as a collision or sudden braking. This prevents the occupant from being thrown forward due to inertia, protecting the occupant.

[0003] When a vehicle suddenly decelerates, an occupant restrained by a seat belt is subjected to an excessive load from the seat belt. Therefore, a seat belt retractor not only restrains the occupant but also has a mechanism for absorbing the load applied to the occupant. The seat belt retractor includes a torsion bar component as a mechanism for absorbing the impact. When an excessive load acts on the occupant via the seat belt, the torsion bar component mounted on the seat belt retractor is twisted and deformed. As a result, the force with which the seat belt restrains the occupant is limited. Therefore, the steel material used for the torsion bar component must have excellent torsional properties. Here, excellent torsional properties mean that the steel can twist to a large angle before breaking.

[0004] Techniques for improving the torsional characteristics of steel materials used to make torsion bar components are proposed in Japanese Patent Laid-Open No. 2009-120906 (Patent Document 1), Japanese Patent Laid-Open No. 2009-120907 (Patent Document 2), and Japanese Patent Laid-Open No. 2019-173068 (Patent Document 3).

[0005] The steel material disclosed in Patent Document 1 contains C: 0.002 to 0.02%, Si: 0.3% or less, Mn: 0.1 to 0.5%, P: 0.001 to 0.020%, S: 0.020% or less, Al: 0.02% or less, and N: 0.01% or less, with the balance being iron and inevitable impurities, has a ferrite area ratio of 99% or more, has a ferrite grain size number (A) of the outermost surface of 3.0 to less than 7.0, has a ferrite grain size number (B) at a D / 4 part (D: diameter of wire rod or steel bar) of 3.0 to less than 7.0, and has a difference between the ferrite grain size numbers (A) and (B) of 0.5 or less.

[0006] The steel material disclosed in Patent Document 2 contains C: 0.02 to 0.10%, Si: 0.3% or less, Mn: 0.1 to 0.5%, Al: 0.01 to 0.06%, P: 0.001 to 0.020%, S: 0.020% or less, and N: 0.01% or less, with the balance being iron and unavoidable impurities, and has a microstructure in which spheroidized carbides are present over the entire surface of a ferrite matrix, and the degree of spheroidization of the spheroidized carbides is No. 1 to 2 as specified in JIS G 3507-2.

[0007] Patent Documents 1 and 2 state that, in any of these steel materials, by making the structure as uniform as possible, it is possible to increase the number of twists until the steel material breaks when a torsion test is carried out at −40° C.

[0008] The steel material disclosed in Patent Document 3 contains, by mass%, 0.03 to 0.25% C, more than 0 to 0.35% Si, more than 0 to 0.6% Mn, more than 0 to 0.05% P, more than 0 to 0.05% S, and 0.005 to 0.2% Al, with the balance being Fe and unavoidable impurities. The metal structure includes ferrite with the balance being at least one of pearlite and spheroidal carbides, the ferrite having a grain size number of 8.5 or greater, and the full width at half maximum of the X-ray diffraction peak in the (211) plane of ferrite being 0.90° or less. This steel material has ferrite as the primary metal structure, and by reducing strain in the steel material, the torsional properties are improved. Furthermore, the ferrite grain size is reduced to increase the initial torque during torsion. Patent Document 3 states that this allows for sufficient absorption of impact energy.

[0009] JP 2009-120906 A JP 2009-120907 A JP 2019-173068 A

[0010] In Patent Documents 1 and 2, the object is to improve the twisting characteristics by increasing the number of twists until breakage in a twisting test. In Patent Document 3, the object is to increase the initial torque during twisting in addition to improving the twisting characteristics.

[0011] As described above, when a vehicle body suddenly decelerates, the torsion bar component twists in the circumferential direction. At this time, as the twisting angle (number of twists) increases, the torque required for torsional deformation of the torsion bar component also increases. At this time, as described in Patent Document 3, an increase in the initial torque absorbs the impact during vehicle deceleration. However, after the increase in the initial torque, a further increase in torque correlates with an increase in the load on the occupant. In other words, if the torque continues to increase, the load on the occupant restrained by the seat belt also increases. Therefore, it is preferable for the torsion bar component to be able to suppress the increase in torque associated with twisting.

[0012] An object of the present disclosure is to provide a steel material, a steel wire, and a torsion bar component that can suppress an increase in torque associated with twisting.

[0013] The steel material of the present disclosure has a chemical composition, in mass%, of C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, Sn: 0 to 0.100%, and B: 0 to 0.0 ... 50%, Bi: 0 to 0.100%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.090%, and further contains one or two selected from the group consisting of Ti: 0.003 to 0.040% and Nb: 0.003 to 0.040%, the balance consisting of Fe and impurities, the ferrite area ratio is 90% or more, [Ti*] defined by formula (1) and [Nb*] defined by formula (2) satisfy formula (3), and the maximum value of internal friction (Q －１ ) ｍａｘ is 1.2 x 10 －４ ~10.0 x 10 －４ [Ti*] = Ti / {(48 / 12)C + (48 / 14)N} (1) [Nb*] = Nb / {(93 / 12)C + (93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in the chemical composition in mass %.

[0014] The steel wire and torsion bar components of the present disclosure are made of the above-described steel material.

[0015] The steel material, steel wire, and torsion bar component of the present disclosure can suppress an increase in torque associated with twisting.

[0016] FIG. 1 is a graph of the twist angle (°) and torque (N·m) obtained by performing a twist test using a steel material for use in a torsion bar part. FIG. 2 is a perspective view of a torsion bar part for explaining the observation surface of the ferrite area ratio from the steel material. FIG. 3 is a schematic diagram for explaining the position of the observation field on the observation surface shown in FIG. 2. FIG. 4A is a graph of the maximum value of internal friction (Q －１ ) ｍａｘFig. 4B is a cross-sectional view perpendicular to the axial direction (longitudinal direction) of the steel material shown in Fig. 4A. Fig. 5A is a graph showing the relationship between temperature and internal friction Q obtained by the automatic vibration damping method. －１ FIG. 5B is a diagram showing an example of a graph showing the relationship between the torque and the twisting angle. FIG. 5B is a diagram showing an example of a graph after baseline correction has been performed on the graph of FIG. 5A. FIG. 6 is a side view of a torsion bar component. FIG. 7 is a schematic diagram for explaining a test for evaluating torque increase due to twisting in the examples. FIG. 8 is a schematic diagram of a torque-twisting angle diagram obtained by a test for evaluating torque increase due to twisting in the examples.

[0017] The present inventors have conducted research into steel materials that can suppress an increase in torque due to twisting.

[0018] The present inventors first investigated the chemical composition of steel suitable for torsion bar parts, and found that the composition was, in mass%, C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, Ti: 0 to 0.040%, Nb: 0 to 0.040%, Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca ... The inventors considered that a steel sheet having a chemical composition consisting of: 0 to 0.0050%, Mg: 0 to 0.0050%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, Sn: 0 to 0.100%, B: 0 to 0.0050%, Bi: 0 to 0.100%, Se: 0 to 0.100%, Te: 0 to 0.050%, Pb: 0 to 0.090%, and the balance being Fe and impurities, would be suitable for use in torsion bar parts.

[0019] The present inventors further investigated means for suppressing the increase in torque due to twisting in a steel material having the above chemical composition. Specifically, steel materials were manufactured by various manufacturing methods using materials having the above chemical composition. Then, twisting tests were carried out on the steel materials. As a result, the present inventors discovered the following.

[0020] 1 is a graph showing the relationship between the twist angle (°) and the torque (N·m) obtained by performing a twist test using a steel material for torsion bar components. Referring to FIG. 1, the dashed line in graph S1 shows the results of a twist test using a conventional steel material for torsion bar components. The solid line in graph S2 shows the results of a twist test using a steel material different from that used in graph S1.

[0021] In the case of the steel material shown in graph S1, the torque increases as the twist angle increases (i.e., as the vehicle twists). In this case, the load on the occupant restrained by the seat belt also increases as the vehicle twists. On the other hand, in the case of the steel material shown in graph S2, the torque increases until the steel material yields, but after the steel material yields, the torque remains almost constant even if the twist angle increases, and the increase in torque is suppressed. In this case, the load on the occupant restrained by the seat belt can be suppressed even if the vehicle twists further.

[0022] Here, the inventors observed the appearance of the steel material of graph S1 after the torsion test and the steel material of graph S2 after the torsion test. As a result, the steel material of graph S1 was twisted uniformly throughout, with uniform torsion occurring. On the other hand, the steel material of graph S2 was twisted locally, with local torsion occurring at the end of the steel material. From these results, the inventors considered that local torsion is related to the suppression of torque increase.

[0023] Based on the above results, the inventors of the present invention came up with the following idea: When a steel material is twisted, a local twist is generated at a certain location in the steel material. Before the local twist causes excessive plastic deformation in the circumferential direction of the steel material at that location (i.e., before the local twist in the circumferential direction becomes excessive), the local twist is propagated in the axial direction of the steel material (i.e., to a position axially adjacent to that location). If such plastic deformation behavior can be achieved, it is possible to suppress an increase in torque while the local twist propagates in the axial direction (longitudinal direction) of the steel material. In other words, it is possible to suppress an increase in torque due to twisting.

[0024] Based on the above findings, the inventors further investigated means for generating a local twist during twisting and propagating the local twist in the axial direction of a steel material. Here, the inventors focused on solute C and solute N in a steel material. Solute C and solute N are interstitial solute elements and interact strongly with dislocations. Therefore, the amounts of solute C and solute N are related to dislocation motion. Local twist occurs due to the rapid multiplication of dislocations. Therefore, by adjusting the amounts of solute C and solute N, it is possible to control the multiplication behavior of dislocations. As a result, it may be possible to control the behavior of the local twist.

[0025] Based on the above considerations, the inventors of the present invention have concluded the following. In the above chemical composition, Ti and Nb combine with C and / or N to form carbides, nitrides, or carbonitrides. Therefore, if Ti and Nb form appropriate amounts of Ti precipitates and Nb precipitates (carbides, nitrides, and carbonitrides), appropriate amounts of solute C and solute N can be adjusted. In this case, when the steel material is twisted, an appropriate local twist can be generated in the steel material, and the local twist can be propagated in the axial direction of the steel material. As a result, the increase in torque associated with twisting can be suppressed.

[0026] Based on the above findings, the inventors have investigated the relationship between the Ti content, Nb content, C content, and N content in the above chemical composition, and further investigated the total amount of solute C and solute N required to achieve the propagation of the above-mentioned local twist in the axial direction of the steel material. As a result, in the above chemical composition, one or more of Ti and Nb are made essential elements, and further, [Ti*] defined by formula (1) and [Nb*] defined by formula (2) are adjusted to satisfy formula (3), and further, the maximum value of internal friction (Q －１ ) ｍａｘ to 1.2 x 10 －４ ~10.0 x 10 －４It was found that the increase in torque due to twisting can be sufficiently suppressed by: [Ti*] = Ti / {(48 / 12) C + (48 / 14) N} (1) [Nb*] = Nb / {(93 / 12) C + (93 / 14) N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in the chemical composition in mass %.

[0027] The steel material, steel wire, and torsion bar component of this embodiment have been completed based on the above technical concept, and have the following configuration.

[0028] The steel material of the first configuration has a chemical composition, in mass %, of C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, Sn: 0 to 0.100%, B: 0 to 0.0 0.050%, Bi: 0 to 0.100%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.090%, and further contains one or two selected from the group consisting of Ti: 0.003 to 0.040% and Nb: 0.003 to 0.040%, with the balance consisting of Fe and impurities, the ferrite area ratio is 90% or more, [Ti*] defined by formula (1) and [Nb*] defined by formula (2) satisfy formula (3), and the maximum value of internal friction (Q －１ ) ｍａｘ is 1.2 x 10 －４ ~10.0 x 10 －４ [Ti*] = Ti / {(48 / 12)C + (48 / 14)N} (1) [Nb*] = Nb / {(93 / 12)C + (93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in the chemical composition in mass %.

[0029] The steel material of the second configuration is the steel material of the first configuration, and the chemical composition contains, in mass%, one or more elements selected from the group consisting of Cr: 0.01 to 0.30%, Mo: 0.01 to 0.20%, V: 0.01 to 0.20%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.40%, Sn: 0.001 to 0.100%, B: 0.0001 to 0.0050%, Bi: 0.001 to 0.100%, Se: 0.001 to 0.100%, Te: 0.001 to 0.050%, and Pb: 0.001 to 0.090%.

[0030] The steel wire of this embodiment is made of a steel material having the first or second configuration. Here, the steel wire in this embodiment means a rod-shaped or wire-shaped steel material obtained by wiredrawing a steel material.

[0031] The torsion bar component of this embodiment is made of a steel material having the first or second configuration.

[0032] The steel material of this embodiment will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0033] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition is, in mass %, C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Cu: 0.0 ... : 0-0.40%, Ni: 0-0.40%, Sn: 0-0.100%, B: 0-0.0050%, Bi: 0-0.100%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.090%, and further contains one or two selected from the group consisting of Ti: 0.003-0.040% and Nb: 0.003-0.040%, with the balance consisting of Fe and impurities. (Feature 2) The ferrite area fraction is 90% or more. (Feature 3) [Ti*] defined by formula (1) and [Nb*] defined by formula (2) satisfy formula (3). [Ti*] = Ti / {(48 / 12)C + (48 / 14)N} (1) [Nb*] = Nb / {(93 / 12)C + (93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, the element symbols in formulas (1) and (2) are substituted with the mass% content of the corresponding element in the chemical composition. (Feature 4) The maximum value of internal friction (Q －１ ) ｍａｘ is 1.2 x 10 －４ ~10.0 x 10 －４ Features 1 to 4 will be explained below.

[0034] [(Feature 1) Chemical Composition] The steel material of this embodiment contains the following elements.

[0035] C: 0.006 to 0.050% Carbon (C) dissolves in steel, generating local torsion when the steel is twisted and propagating the local torsion in the axial direction. This suppresses the increase in torque associated with twisting. If the C content is less than 0.006%, the above effect cannot be fully achieved. On the other hand, if the C content exceeds 0.050%, the amount of solute C in the steel becomes excessive. In this case, the steel fractures before the local torsion during twisting fully propagates in the axial direction. Therefore, the C content is 0.006 to 0.050%. The preferred lower limit of the C content is 0.007%, more preferably 0.008%, and even more preferably 0.010%. The preferred upper limit of the C content is 0.048%, more preferably 0.044%, and even more preferably 0.039%.

[0036] Mn: 0.10 to 0.80% Manganese (Mn) increases the strength of steel through solid solution strengthening. Mn also fixes S in the steel, improving the hot workability of the steel. If the Mn content is less than 0.10%, the above effect cannot be fully achieved. On the other hand, if the Mn content exceeds 0.80%, the strength of the steel becomes excessively high. In this case, the deformability during twisting decreases. Therefore, the Mn content is 0.10 to 0.80%. The preferred lower limit of the Mn content is 0.11%, more preferably 0.13%, and even more preferably 0.15%. The preferred upper limit of the Mn content is 0.78%, more preferably 0.75%, and even more preferably 0.70%.

[0037] Al: 0.005 to 0.080% Aluminum (Al) deoxidizes steel. Furthermore, Al bonds with N in the steel to form AlN. AlN refines the crystal grains through its pinning effect. As a result, the cold workability (wire drawing workability and cold forging ability) of the steel is improved. If the Al content is less than 0.005%, the above effect is not fully achieved. On the other hand, if the Al content exceeds 0.080%, the above effect saturates. If the Al content exceeds 0.080%, oxide-based inclusions are generated in excess, making the steel more susceptible to defects during hot rolling. Therefore, the Al content is 0.005 to 0.080%. The preferred lower limit of the Al content is 0.007%, more preferably 0.010%, and even more preferably 0.015%. The upper limit of the Al content is preferably 0.078%, more preferably 0.075%, and even more preferably 0.070%.

[0038] Si: 0.50% or less Silicon (Si) is inevitably contained. In other words, the Si content is greater than 0%. Si increases the strength of steel materials through solid solution strengthening. Even if even a small amount of Si is contained, the above effect can be obtained to some extent. However, if the Si content exceeds 0.50%, the strength of the steel material increases excessively, and the cold workability (wire drawing workability and cold forging ability) decreases. Therefore, the Si content is 0.50% or less. The preferred lower limit of the Si content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Si content is 0.44%, more preferably 0.38%, and even more preferably 0.30%.

[0039] P: 0.030% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. P segregates at grain boundaries and reduces the cold workability of steel. Therefore, the P content is 0.030% or less. The lower the P content, the better. However, excessive reduction in the P content increases manufacturing costs. Therefore, taking into account normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the P content is 0.028%, more preferably 0.024%, and even more preferably 0.019%.

[0040] S: 0.050% or less Sulfur (S) is unavoidably contained. In other words, the S content is greater than 0%. S forms MnS in steel, improving the machinability of the steel. Even if even a small amount of S is contained, the above effect can be obtained to some extent. However, if the S content exceeds 0.050%, the MnS in the steel becomes coarse, reducing the cold workability of the steel. Therefore, the S content is 0.050% or less. The preferred lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the S content is 0.045%, more preferably 0.038%, and even more preferably 0.028%.

[0041] N: 0.0120% or less Nitrogen (N) is unavoidably contained. In other words, the N content is greater than 0%. Like C, N dissolves in the steel material and suppresses the increase in torque associated with twisting. Even if even a small amount of N is contained, the above effect can be achieved to some extent. However, if the N content exceeds 0.0120%, the amount of dissolved N in the steel material becomes excessive. In this case, the steel material fractures before the local twist during twisting is sufficiently propagated in the axial direction. Therefore, the N content is 0.0120% or less. The preferred lower limit of the N content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the N content is 0.0100%, more preferably 0.0078%, and even more preferably 0.0048%.

[0042] O: 0.0100% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O forms oxides and reduces the cold workability of steel. Therefore, the O content is 0.0100% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The upper limit of the O content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0068%, and even more preferably 0.0048%.

[0043] One or two selected from the group consisting of Ti: 0.003 to 0.040% and Nb: 0.003 to 0.040%. Titanium (Ti) and niobium (Nb) both form carbides, nitrides, or carbonitrides, and adjust the amounts of dissolved C and N in the steel material. This suppresses the increase in torque associated with twisting. If the Ti content is less than 0.003% or the Nb content is less than 0.003%, the above effect cannot be sufficiently obtained. On the other hand, if the Ti content exceeds 0.040% or the Nb content exceeds 0.040%, excessive carbides, nitrides, or carbonitrides of Ti and / or Nb are formed. In this case, the maximum value of internal friction (Q ) measured in the range of -50 to 150°C is －１ ) ｍａｘbecomes excessively small. As a result, the increase in torque due to twisting cannot be sufficiently suppressed. Therefore, the chemical composition of the steel material of this embodiment contains one or two elements selected from the group consisting of a Ti content of 0.003 to 0.040% and a Nb content of 0.003 to 0.040%. The chemical composition of the steel material of this embodiment may contain Ti within the above-mentioned range and may not contain Nb. It may not contain Ti but may contain Nb within the above-mentioned range. It may contain Ti within the above-mentioned range and Nb within the above-mentioned range. The lower limit of the Ti content is preferably 0.005%, more preferably 0.008%, and even more preferably 0.010%. The upper limit of the Ti content is preferably 0.038%, more preferably 0.035%, and even more preferably 0.030%. The lower limit of the Nb content is preferably 0.005%, more preferably 0.008%, and even more preferably 0.010%. The upper limit of the Nb content is preferably 0.038%, more preferably 0.035%, and even more preferably 0.030%.

[0044] The balance of the chemical composition of the steel material of this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material of this embodiment.

[0045] [Regarding optional elements] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, Sn: 0 to 0.100%, B: 0 to 0.0050%, Bi: 0 to 0.100%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.090%. These optional elements will be described below.

[0046] [Regarding Group 1 (Cr, Mo, and V)] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cr, Mo, and V. All of these elements form precipitates (carbides, nitrides, or carbonitrides) and increase the strength of the steel material.

[0047] Cr: 0 to 0.30% Chromium (Cr) is an optional element and does not necessarily need to be contained. In other words, the Cr content may be 0%. When contained, Cr forms precipitates and increases the strength of the steel. Even if even a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content exceeds 0.30%, excessive precipitates are formed. In this case, the cold workability of the steel deteriorates. Therefore, the Cr content is 0 to 0.30%. The preferred lower limit of the Cr content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Cr content is 0.28%, more preferably 0.25%, and even more preferably 0.20%.

[0048] Mo: 0 to 0.20% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. In other words, the Mo content may be 0%. When contained, Mo forms precipitates and increases the strength of the steel. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.20%, excessive precipitates are formed. In this case, the cold workability of the steel deteriorates. Therefore, the Mo content is 0 to 0.20%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Mo content is 0.18%, more preferably 0.16%, and even more preferably 0.12%.

[0049] V: 0 to 0.20% Vanadium (V) is an optional element and does not necessarily need to be contained. In other words, the V content may be 0%. When contained, V forms precipitates and increases the strength of the steel. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.20%, excessive precipitates are formed. In this case, the cold workability of the steel deteriorates. Therefore, the V content is 0 to 0.20%. The preferred lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the V content is 0.18%, more preferably 0.16%, and even more preferably 0.12%.

[0050] [Regarding Group 2 (Ca and Mg)] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg, instead of a portion of Fe. Both of these elements spheroidize MnS and improve the hot workability of the steel material.

[0051] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, Ca spheroidizes MnS, improving the hot workability of the steel. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse oxide-based inclusions are formed. In this case, the cold workability of the steel deteriorates. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the Ca content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0052] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When contained, Mg spheroidizes MnS, improving the hot workability of the steel. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, coarse oxide-based inclusions are formed. In this case, the cold workability of the steel deteriorates. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the Mg content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0053] [Regarding Group 3 (Cu, Ni, and Sn)] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, and Sn, instead of a portion of Fe. All of these elements improve the corrosion resistance of the steel material.

[0054] Cu: 0 to 0.40% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When contained, Cu enhances the corrosion resistance of the steel material. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.40%, the steel material becomes embrittled. As a result, the hot workability and cold workability of the steel material decrease. Therefore, the Cu content is 0 to 0.40%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Cu content is 0.38%, more preferably 0.35%, and even more preferably 0.30%.

[0055] Ni: 0 to 0.40% Nickel (Ni) is an optional element and does not need to be contained. In other words, the Ni content may be 0%. When contained, Ni enhances the corrosion resistance of the steel. Even if even a small amount of Ni is contained, the above effect can be obtained to a certain extent. However, if the Ni content exceeds 0.40%, the above effect saturates. If the Ni content exceeds 0.40%, raw material costs will increase and manufacturability will also decrease. Therefore, the Ni content is 0 to 0.40%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Ni content is 0.38%, more preferably 0.35%, and even more preferably 0.30%.

[0056] Sn: 0 to 0.100% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. When contained, Sn improves the corrosion resistance of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.100%, the hot workability and cold workability of the steel material deteriorate. Therefore, the Sn content is 0 to 0.100%. The preferred lower limit of the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Sn content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0057] [Regarding Fourth Group (B)] The chemical composition of the steel material of this embodiment may further contain B instead of a portion of Fe.

[0058] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, B strengthens grain boundaries and improves the cold workability of steel. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content exceeds 0.0050%, B forms excessive nitrides. In this case, the cold workability of the steel actually deteriorates. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0030%, even more preferably 0.0018%, and even more preferably 0.0015%.

[0059] [Regarding Group 5 (Bi, Se, Te, and Pb)] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Bi, Se, Te, and Pb in place of a portion of Fe. All of these elements improve the machinability of the steel material.

[0060] Bi: 0 to 0.100% Bismuth (Bi) is an optional element and does not need to be contained. In other words, the Bi content may be 0%. When contained, Bi improves the machinability of the steel. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.100%, the hot workability of the steel decreases. Therefore, the Bi content is 0 to 0.100%. The preferred lower limit of the Bi content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Bi content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0061] Se: 0 to 0.100% Selenium (Se) is an optional element and does not necessarily need to be contained. In other words, the Se content may be 0%. When contained, Se improves the machinability of steel. Even if even a small amount of Se is contained, the above effect can be obtained to a certain extent. However, if the Se content exceeds 0.100%, the above effect saturates and raw material costs increase. Therefore, the Se content is 0 to 0.100%. The preferred lower limit of the Se content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Se content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0062] Te: 0 to 0.050% Tellurium (Te) is an optional element and does not necessarily need to be contained. In other words, the Te content may be 0%. When contained, Te improves the machinability of the steel. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.050%, the hot workability of the steel deteriorates. Therefore, the Te content is 0 to 0.050%. The preferred lower limit of the Te content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Te content is 0.045%, more preferably 0.040%, and even more preferably 0.030%.

[0063] Pb: 0 to 0.090% Lead (Pb) is an optional element and does not necessarily need to be contained. In other words, the Pb content may be 0%. When contained, Pb improves the machinability of the steel. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.090%, the hot workability of the steel decreases. Therefore, the Pb content is 0 to 0.090%. The preferred lower limit of the Pb content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Pb content is 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0064] [(Feature 2) Ferrite Area Ratio] Furthermore, the steel material of this embodiment has a ferrite area ratio of 90% or more.

[0065] If the ferrite area ratio is 90% or more, sufficient torsion occurs during twisting. A preferable lower limit of the ferrite area ratio is 95%. In the metal structure of the steel material, the remainder other than ferrite is not particularly limited, but may be, for example, one or more selected from the group consisting of pearlite, cementite, precipitates, and inclusions.

[0066] [Method for Measuring Ferrite Area Ratio] The ferrite area ratio in the metallographic structure of a steel material is determined by the following method. Referring to FIG. 2, a test piece including a cross section (having a circular cross section) perpendicular to the axial direction of the steel material is taken. This cross section of the test piece is designated as the observation surface 25. The observation surface 25 is etched with a 3 volume % nital solution to reveal the metallographic structure. Referring to FIG. 3, when the diameter of the observation surface 25 is designated as D, four observation fields 26 are located at a depth of D / 4 in the radial direction from the surface corresponding to the outer circumferential surface of the steel material on the observation surface 25 of the test piece, and are spaced at 90° intervals around the central axis of the observation surface 25. The rectangular observation fields 26 are observed at a magnification of 500 times using an optical microscope. The short side of each observation field 26 is 174 μm in the radial direction, and the long side is 232 μm in the direction perpendicular to the short side.

[0067] In each observation field, ferrite is easily distinguishable by its contrast. In optical micrographs, ferrite is observed as a bright (white) area where no substructure is discernible. Pearlite is observed as a striped area. Cementite is observed as an area with no discernible substructure and is darker than ferrite. Precipitates and inclusions are observed as areas darker than ferrite.

[0068] Ferrite is identified in the four observation fields 26. The ferrite area ratio (%) is calculated based on the total area of ​​the four observation fields and the total area of ​​the identified ferrite. The ferrite area ratio is calculated by rounding the obtained value to one decimal place (i.e., an integer).

[0069] [(Feature 3) Regarding Formulas (1) to (3)] In the steel material of this embodiment, [Ti*] defined by Formula (1) and [Nb*] defined by Formula (2) further satisfy Formula (3). [Ti*] = Ti / {(48 / 12)C + (48 / 14)N} (1) [Nb*] = Nb / {(93 / 12)C + (93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, the element symbols in Formulas (1) and (2) are substituted with the content, in mass %, of the corresponding element in the chemical composition. Feature 3 will be described below.

[0070] [Ti*] is an index of the amount of Ti precipitates in a steel material. Here, Ti precipitates are Ti carbides, Ti nitrides, or Ti carbonitrides. [Ti*] is expressed as the ratio of the atomic % of Ti to the sum of the atomic % of C and the atomic % of N. [Ti*] is a value obtained by rounding off the calculated value to one decimal place. [Nb*] is an index of the amount of Nb precipitates in a steel material. Here, Nb precipitates are Nb carbides, Nb nitrides, or Nb carbonitrides. [Nb*] is expressed as the ratio of the atomic % of Nb to the sum of the atomic % of C and the atomic % of N. [Nb*] is a value obtained by rounding off the calculated value to one decimal place.

[0071] Fn is defined as [Ti*] + [Nb*]. Fn is an index relating to the amount of solute C and N in a steel material. When Fn is 1.00 or more, all C and N in the steel material combine with Ti and Nb to form precipitates. In this case, an appropriate amount of solute C and N is not present in the steel material. Therefore, uniform torsion occurs without local torsion during twisting. As a result, the increase in torque associated with twisting cannot be sufficiently suppressed. On the other hand, when Fn is less than 1.00, some of the C and N do not combine with Ti and Nb and exist in the steel material as solute C and N. In this case, local torsion occurs during twisting, and the local torsion propagates in the axial direction. As a result, the increase in torque associated with twisting can be sufficiently suppressed. Therefore, Fn is less than 1.00.

[0072] The upper limit of Fn is preferably 0.90, more preferably 0.80, and even more preferably 0.60. The lower limit of Fn is not particularly limited. However, when the chemical composition of the steel satisfies Feature 1, the lower limit of Fn is, for example, 0.03.

[0073] [(Feature 4) Maximum value of internal friction of steel (Q －１ ) ｍａｘ Regarding the steel material of this embodiment, the maximum value of internal friction (Q －１ ) ｍａｘ is 1.2 x 10 －４ ~10.0 x 10 －４ is.

[0074] Here, the maximum value of internal friction (Q －１ ) ｍａｘ is the internal friction Q measured in the range of -50 to 150°C －１ The maximum value of the Snoek Peak is usually observed in the range of 0 to 100°C, and the maximum value of the Snoek Peak is observed in the range of 35 to 55°C. －１ ) ｍａｘ is correlated with the total amount of solute C and solute N in the steel. －１ ) ｍａｘ is an index of the total amount of solute C and solute N in the steel material. In the following explanation, the maximum value of internal friction (Q －１ ) ｍａｘ Simply (Q －１ ) ｍａｘ It is also called.

[0075] (Q －１ ) ｍａｘ is 1.2 x 10 －４ If the ratio is less than 1 / 2, the total amount of solute C and solute N in the steel is too small. In this case, localized twisting does not occur during twisting, but uniform twisting occurs. As a result, the increase in torque due to twisting cannot be sufficiently suppressed. －１ ) ｍａｘ is 10.0 x 10 －４If the total amount of solute C and solute N in the steel exceeds this value, the total amount of solute C and solute N in the steel is too large. In this case, excessive local twisting occurs in the circumferential direction of the steel. As a result, when twisted, the steel breaks before the local twisting propagates in the axial direction of the steel. (Q －１ ) ｍａｘ is 1.2 x 10 －４ ~10.0 x 10 －４ If the total amount of solute C and solute N is appropriate, then during twisting, local torsion occurs in the circumferential direction of the steel material, and the local torsion propagates in the axial direction of the steel material without excessive local torsion. As a result, the increase in torque due to twisting can be sufficiently suppressed.

[0076] (Q －１ ) ｍａｘ The preferred lower limit is 1.3 × 10 －４ and more preferably 1.6 × 10 －４ and more preferably 1.9 × 10 －４ and more preferably 2.1 × 10 －４ (Q －１ ) ｍａｘ The preferred upper limit is 9.8 × 10 －４ and more preferably 9.4 × 10 －４ and more preferably 8.8 × 10 －４ is.

[0077] [Maximum value of internal friction (Q －１ ) ｍａｘ Measurement method of (Q －１ ) ｍａｘ can be measured by the following method. －１ ) ｍａｘ4B is a perspective view of a steel material illustrating the location of test pieces for measuring . FIG. 4B is a cross-sectional view perpendicular to the axial direction (longitudinal direction) of the shank of the steel material shown in FIG. 4A . Referring to FIGS. 4A and 4B , a test piece 28 is taken from a circular cross-section perpendicular to the axial direction of the steel material, at a depth D / 4 (D is the diameter of the steel material) in the radial direction from the surface corresponding to the outer circumferential surface of the steel material. The size of the test piece 28 is a plate-like shape with a length of 105 mm, a width of 5 mm, and a thickness of 0.7 mm. The longitudinal direction of the test piece 28 is parallel to the axial direction (longitudinal direction) of the steel material. Referring to FIG. 4B , the test piece 28 is taken from a cross-section perpendicular to the axial direction of the shank 20, with a width of 5 mm, so that a normal N28 at the width center of the surface 28A extending in the axial direction of the shank 20 is aligned radially in the cross-section. The size of the test piece 28 may be shortened as long as the cross-sectional area is constant (width 5 mm × thickness 0.7 mm). For example, when the test piece is taken from a torsion bar component made of steel, the length of the test piece may be the same as the length of the torsion bar component.

[0078] Using a low-frequency torsion pendulum-type internal friction measurement device, the free vibration damping method was used to measure the internal friction (Q －１ ) ｍａｘ Specifically, the test piece 28 is attached to a low-frequency torsion pendulum-type internal friction measuring device. Next, a torsion is applied to the test piece 28 by an excitation coil, and the test piece 28 is made to self-oscillate until the amplitude of the torsional vibration reaches a predetermined value. At this time, the frequency of the torsional vibration is set to 1.0 to 2.0 Hz. Thereafter, the test piece is made to transition to free damping vibration, and the vibration is recorded. The logarithmic damping factor δ is calculated from the recorded vibration, and the internal friction Q －１ Convert to: Internal friction Q －１ is converted from the logarithmic decay rate δ using the following formula: －１ = δ / π where π represents the constant of the circumference of a circle.

[0079] The above operation was carried out in the range of -50 to 150°C while increasing the temperature at a rate of 1.0°C / min, and the relationship between the temperature and the internal friction Q －１ The maximum value (i.e., the snake peak) is read from the graph obtained, and the value obtained by subtracting the background value is calculated as (Q －１ ) ｍａｘSpecifically, as shown in FIG. 5A, the line connecting the pair of bottoms BT of the obtained graph (peak waveform diagram) SP is taken as the baseline (background value) BL. The internal friction Q of the baseline BL is －１ The graph SP is corrected as shown in FIG. 5B by subtracting the maximum value (snake peak) X1 from the corrected graph SP. The obtained maximum value X1 is then calculated as (Q －１ ) ｍａｘ The low-frequency torsion pendulum type internal friction measuring device may be, for example, IFM-1500L manufactured by Shinku Riko Co., Ltd. (currently Advance Riko Co., Ltd.).

[0080] [Effects of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies Features 1 to 4. Therefore, with the steel material of the present embodiment, the increase in torque due to twisting can be sufficiently suppressed.

[0081] [Uses and Shape of Steel Material of the Present Embodiment] The steel material of the present embodiment is widely applicable to applications requiring suppression of torque increase due to twisting. The steel material of the present embodiment is particularly suitable as a material for torsion bar components included in seat belt retractors. The steel material of the present embodiment is a steel bar or wire rod having a circular cross section perpendicular to the axial direction (longitudinal direction) of the steel material. The diameter of the steel material is not particularly limited, but is, for example, 5 to 20 mm.

[0082] [Method for manufacturing steel material according to this embodiment] A method for manufacturing steel material according to this embodiment will be described below. Steel material satisfying Features 1 to 4 may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing steel material according to this embodiment.

[0083] An example of the manufacturing method of this embodiment includes the following steps: (Step 1) Material manufacturing step (Step 2) Hot working step Each step will be described below.

[0084] [(Step 1) Material Production Step] In the material production step, a material for the steel material of this embodiment is prepared. Specifically, molten steel having a chemical composition that satisfies Feature 1 is produced. The produced molten steel is used to produce an ingot or bloom by a well-known casting method. For example, an ingot is produced by an ingot casting method. Alternatively, a cast piece (bloom) is produced by a continuous casting method. Through the above steps, a material (ingot or bloom) is produced.

[0085] [(Step 2) Hot Working Step] In the hot working step, the produced raw material is subjected to hot working to produce a steel material. The steel material is, for example, a steel bar or a wire rod. The hot working step includes a blooming step, a finish rolling step, and a cooling step.

[0086] [Blooming Process] In the blooming process, the heated material is hot-rolled (blooming) using a blooming mill to produce billets. If a continuous rolling mill is installed downstream of the blooming mill, the billet after blooming may be further hot-rolled using the continuous rolling mill to produce even smaller billets. The heating temperature in the heating furnace in the blooming process is not particularly limited, but is, for example, 1000 to 1300°C.

[0087] [Finish Rolling Step] In the finish rolling step, the billet obtained after the blooming step is hot-rolled using a continuous rolling mill to produce a steel material. The heating temperature in the heating furnace in the finish rolling step is not particularly limited, but is, for example, 1000 to 1300°C.

[0088] [Cooling process] In the cooling process, the steel material after finish rolling is subjected to boiling water cooling until it reaches a predetermined cooling stop temperature (°C). Specifically, the steel material that has left the rolling stand where the final reduction was performed in finish rolling is subjected to boiling water cooling by immersing it in boiling water at 100°C. The steel material temperature at which the steel material is pulled out of the boiling water is defined as the cooling stop temperature (°C). The cooling stop temperature in boiling water cooling is as follows: Cooling stop temperature: 500 to 200°C The cooling stop temperature can be obtained, for example, by measuring the temperature of the steel material with a thermometer immediately after it is pulled out of the boiling water. The thermometer may be, for example, a thermograph, a radiation thermometer, or the like.

[0089] The steel is pulled out of boiling water at the cooling stop temperature and allowed to cool to room temperature. Boiling water cooling puts the C and N in the steel into a supersaturated solid solution state. In this state, boiling water cooling is stopped and the steel is allowed to cool naturally. In this case, reheating occurs in the steel, causing the steel temperature to rise. As a result, some of the solute C and N in the steel combine with Ti, Nb, and Fe to form precipitates (carbides, nitrides, carbonitrides). By performing boiling water cooling and then switching from boiling water cooling to natural cooling at a cooling stop temperature within the above temperature range, the amounts of solute C and N in the steel can be adjusted to appropriate ranges.

[0090] If the cooling stop temperature (°C) is less than 200°C, sufficient heat recovery cannot be obtained during cooling. In this case, precipitates are not sufficiently formed during cooling. As a result, (Q －１ ) ｍａｘ On the other hand, if the cooling stop temperature (°C) exceeds 500°C, the degree of supersaturation of solute C and solute N in the steel material during natural cooling is insufficient. In this case, too, precipitates are not sufficiently formed during natural cooling. As a result, (Q －１ ) ｍａｘ becomes excessively large.

[0091] The steel material according to this embodiment is manufactured by the above manufacturing process. Note that in the manufacturing process of the steel material according to this embodiment, a cold rolling process using work rolls is not carried out after the hot working process.

[0092] [Regarding the steel wire of this embodiment] The steel wire of this embodiment is made of the steel material of this embodiment described above. Specifically, the steel wire of this embodiment is obtained by wiredrawing the steel material of this embodiment. The steel wire of this embodiment satisfies the above-described features 1 to 4. Therefore, it is possible to suppress an increase in torque due to twisting of the steel material. The steel wire of this embodiment is a steel bar or wire rod having a circular cross section in the axial direction. The diameter of the steel wire is not particularly limited, but is, for example, 4 to 19 mm.

[0093] [Method for manufacturing steel wire according to this embodiment] An example of a method for manufacturing a steel wire according to this embodiment is as follows. The method for manufacturing a steel wire according to this embodiment includes a wiredrawing process. In the wiredrawing process, the steel material according to this embodiment is drawn under well-known conditions to manufacture the steel wire according to this embodiment. The area reduction rate RR in the wiredrawing process is 3 to 20%. Here, the area reduction rate RR (%) is defined by the following formula: Area reduction rate RR = (1 - (area of ​​cross section perpendicular to the axial direction of the steel wire after wiredrawing / area of ​​cross section perpendicular to the axial direction of the steel material before wiredrawing)) × 100 When wiredrawing is performed multiple times, the area reduction rate RR is the cumulative area reduction rate (%) in the multiple wiredrawing processes.

[0094] [Regarding the Torsion Bar Component of the Present Embodiment] The steel wire and torsion bar component of the present embodiment are made of the steel material of the present embodiment described above. FIG. 6 is a side view of the torsion bar component of the present embodiment. Referring to FIG. 6, the torsion bar component of the present embodiment includes a shaft portion 20 and a pair of flange portions 30 disposed on both ends of the shaft portion 20. The shaft portion 20 is cylindrical, and a cross section perpendicular to the axial direction (longitudinal direction) is circular. The pair of flange portions 30 are disposed coaxially with the shaft portion 20 at both ends of the shaft portion 20. Specifically, of the pair of flange portions 30, the first flange portion 30L is disposed at the first end 20L of the shaft portion 20, and the second flange portion 30R is disposed at the second end 20R of the shaft portion 20.

[0095] When the torsion bar component is viewed in the axial direction, the flange portion 30 has a polygonal shape. In Fig. 6, the flange portion 30 is hexagonal when viewed in the axial direction. However, the shape of the flange portion 30 is not limited to a hexagon. When the torsion bar component is viewed in the axial direction, the width of the flange portion 30 is greater than the diameter of the shaft portion 20.

[0096] The torsion bar component having the above-described configuration is housed in a seat belt retractor. In the seat belt retractor, one of the pair of flange portions 30 (30L or 30R) is connected to a locking base. When the load limiter mechanism of the seat belt retractor is activated, the locking base is fixed to the frame of the seat belt retractor via an engaging member. Therefore, one flange portion 30 becomes the fixed end when the load limiter mechanism is activated. The other flange portion 30 is connected to the spool. When the load limiter mechanism is activated and the torsion bar component twists, the other flange portion 30 becomes the driving end that rotates in conjunction with the rotation of the spool. As described above, when the load limiter mechanism is activated, the other flange portion 30 rotates relative to the one flange portion 30. This causes torsional deformation in the shaft portion 20 of the torsion bar component. Depending on the structure of the seat belt retractor, the flange portion 30 may include a connecting shaft portion for connection to another component of the seat belt retractor.

[0097] The torsion bar component of this embodiment is made of the steel material of this embodiment described above. That is, the torsion bar component of this embodiment satisfies the above-described features 1 to 4. Therefore, it is possible to suppress an increase in torque due to twisting of the steel material.

[0098] The size of the torsion bar component of this embodiment is not particularly limited. The diameter of the shaft portion 20 of the torsion bar component is, for example, 4 to 15 mm, and the length of the shaft portion 20 is, for example, 10 to 55 mm. The thickness of the flange portion 30 (the axial length of the torsion bar component) is, for example, 2 to 10 mm.

[0099] [Method for Manufacturing the Torsion Bar Component of the Present Embodiment] An example of a method for manufacturing the torsion bar component of the present embodiment is as follows: The method for manufacturing the torsion bar component of the present embodiment includes a cold working step or a cutting step.

[0100] In the cold working process, the steel wire after the wire drawing process is cold worked under known conditions to form a torsion bar part. The cold working is, for example, heading. After the cold working process, cutting may be further performed to form the torsion bar part with the above-mentioned connecting shaft part. Furthermore, instead of the cold working process, cutting may be performed on the steel wire after the wire drawing process to form the torsion bar part. The torsion bar part is manufactured by the above manufacturing process.

[0101] In the manufacturing process of the torsion bar component, a plating process may be performed after the cold working process and / or the cutting process, if necessary. In the plating process, a known plating treatment is performed on the intermediate product after the cold working and / or the cutting process to form a plating layer on the surface. The composition of the plating layer is not particularly limited.

[0102] The effects of the steel material of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0103] Steel materials having the chemical compositions shown in Tables 1A and 1B were manufactured. In the following description, Tables 1A and 1B will be collectively referred to as "Table 1." In Table 1, "-" indicates that the content of the corresponding element is below the impurity level.

[0104]

[0105]

[0106] Specifically, a bloom was produced by continuous casting using molten steel. The produced bloom was subjected to a blooming process to produce a billet. Specifically, the bloom was heated to 1200°C using a heating furnace. The heated bloom was hot rolled using a blooming mill and a continuous rolling mill to produce a billet. The billet produced in the blooming process was allowed to cool to room temperature.

[0107] The produced billet was subjected to a finish rolling process. Specifically, the billet was heated to 1100° C. The heated billet was subjected to hot rolling (finish rolling) using a continuous rolling mill to produce a steel material (wire material).

[0108] A cooling step was carried out on the steel material after finish rolling. Specifically, boiling water cooling was carried out on the steel material immediately after finish rolling by immersing it in boiling water at 100°C. The steel material was pulled out of the boiling water at the cooling stop temperature (°C) shown in Table 2, and then allowed to cool naturally. Note that in test numbers 41 and 42, boiling water cooling was not carried out, and the steel material after finish rolling was allowed to cool naturally (indicated by "-" in the "Cooling stop temperature (°C)" column in Table 2). Steel materials of each test number were produced by the above production steps.

[0109]

[0110] The ferrite area ratio was measured for the steel material of each test number based on the above-mentioned [Method for measuring ferrite area ratio]. As a result, the ferrite area ratio was 90% or more for all test numbers.

[0111] [Evaluation Tests] The following evaluation tests were carried out on the steel materials with each test number. (Test 1) Maximum value of internal friction (Q －１ ) ｍａｘ (Test 2) Evaluation test of torque increase amount due to twisting Test 1 and Test 2 will be explained below.

[0112] [(Test 1) Maximum value of internal friction of steel (Q －１ ) ｍａｘ Measurement test of the (Q －１ ) ｍａｘ The maximum value of internal friction (Q －１ ) ｍａｘ Based on the method described in the measurement method for each test number, (Q －１ ) ｍａｘ The frequency of the torsional vibration was about 1.3 Hz. The measurement was performed using an IFM-1500L manufactured by Shinku Riko Co., Ltd. (currently Advance Riko Co., Ltd.). The obtained (Q －１ ) ｍａｘ In Table 2, "(Q－１ ) ｍａｘ (x10 －４ ) column.

[0113] [(Test 2) Evaluation Test of Torque Increase Due to Twisting] A torsion bar component was manufactured using the steel material of each test number, and the torque increase due to twisting of the torsion bar component was evaluated. Specifically, the torsion bar component of each test number was manufactured by the following steps.

[0114] The steel material of each test number was subjected to wire drawing to produce a steel wire. The wire drawing conditions were the same for all test numbers. The steel wire was subjected to cold working (heading) to produce a torsion bar part having the shape shown in Figure 6. In the torsion bar part, the axial length L2 of the shaft portion 20 was 50 mm, and the axial length L3 of the flange portion 30 was 5 mm. The shape of the flange portion 30 as viewed from the axial direction of the torsion bar part was hexagonal, with a width W of 10.4 mm. The diameter of the shaft portion 20 was 9.25 mm. Three torsion bar parts of each test number were produced.

[0115] The torque increase associated with the twisting of the torsion bar component of each test number was evaluated using the following twisting test. Specifically, as shown in FIG. 7 , the flange portion 30 at one end of the torsion bar component was fixed with a fixing jig 40, and the flange portion 30 at the other end was fixed with a rotating jig 50. The other end fixed to the rotating jig 50 was then rotated at 5,000 revolutions per minute. The torque (N m) and twist angle (°) were measured. The test temperature was room temperature (20±5°C). Based on the measurement results, a graph (torque-twist angle diagram) was created with torque (N m) on the vertical axis and twist angle (°) on the horizontal axis. An analog filter was applied to the torque data to remove high-frequency components. A Bessel-type low-pass filter (cutoff frequency: 300 Hz, attenuation slope: -48 dB / oct) was used as the analog filter.

[0116] Based on the graph created, the yield torque T ｙ (N m) was calculated. ｙ The yield torque T was calculated using the following method. ｙThe yield torque T was determined by applying the method for determining the yield strength described in "13. Yield strength (offset method) Rp" of JIS Z 2241:2022 to the torsion test. Specifically, as shown in FIG. ｙ was determined by drawing a parallel line at a distance equal to the specified plastic twist angle from the straight line portion of the torque-torsion angle diagram. Specifically, the intersection of this parallel line and the torque-torsion angle diagram was determined as the yield torque T ｙ The specified plastic twist angle was 45°.

[0117] Furthermore, from the created graph, the torque T at a twist angle of 1260° １２６０ The yield torque T (N m) was calculated. ｙ , and torque T １２６０ Based on this, the torque increase amount ΔT was calculated using the following formula: ΔT = T ｙ / T １２６０ When ΔT was 0.85 or more, it was determined that the increase in torque due to twisting was sufficiently suppressed (represented by "E" (Excellent) in Table 2). On the other hand, when ΔT was less than 0.85, it was determined that the increase in torque due to twisting was not sufficiently suppressed (represented by "NA" (Not Accepted) in Table 2). Note that if the local twist did not propagate and the specimen broke, the torque T １２６０ When the torque increase due to the twisting was not measured, it was determined that the torque increase due to the twisting could not be sufficiently suppressed because the twisting was not achieved to begin with. The evaluation results are shown in the "ΔT" column of Table 2.

[0118] [Evaluation Results] Referring to Tables 1 and 2, the steel materials with test numbers 1 to 29 satisfied features 1 to 4. Therefore, the steel materials with these test numbers were able to sufficiently suppress the increase in torque associated with twisting.

[0119] On the other hand, in test numbers 30 and 31, the Ti content was too high. －１ ) ｍａｘ is 1.2 x 10 －４ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0120] In test numbers 32 and 33, the Nb content was too high.－１ ) ｍａｘ is 1.2 x 10 －４ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0121] In test numbers 34 and 35, the Ti content and Nb content were too low. －１ ) ｍａｘ is 10.0 x 10 －４ Therefore, in the torque increase evaluation test, the local twist did not propagate and the specimen broke midway. １２６０ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0122] In test numbers 36 and 37, Fn was too high. －１ ) ｍａｘ is 1.2 x 10 －４ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0123] In test numbers 38 and 39, the cooling stop temperature was too high. －１ ) ｍａｘ is 10.0 x 10 －４ Therefore, in the torque increase evaluation test, the local twist did not propagate and the specimen broke midway. １２６０ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0124] In test numbers 40 and 41, the cooling stop temperature was too low. －１ ) ｍａｘ is 10.0 x 10 －４ Therefore, in the torque increase evaluation test, the local twist did not propagate and the specimen broke midway. １２６０ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0125] In Test Nos. 42 and 43, boiling water cooling was not performed. －１ ) ｍａｘ is 10.0 x 10 －４Therefore, in the torque increase evaluation test, the local twist did not propagate and the specimen broke midway. １２６０ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0126] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. The chemical composition, in mass%, is: C: 0.006-0.050%, Mn: 0.10-0.80%, Al: 0.005-0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, one or two elements selected from the group consisting of Ti: 0.003-0.040% and Nb: 0.003-0.040%, Cr: 0-0.30%, Mo: 0-0.20%, V: 0-0.20%, Ca: 0-0.0050%, Mg: 0-0.0050%, Cu: 0-0.40%, Ni: 0-0.40%, Sn: 0-0.100%, B: 0-0.0050%, Bi: 0-0.100%, Se: 0-0.100%, Te: 0-0.050%, Pb: 0-0.090%, and the balance being Fe and impurities, the ferrite area ratio is 90% or more, [Ti*] defined by formula (1) and [Nb*] defined by formula (2) satisfy formula (3), and the maximum value of internal friction (Q －１ ) ｍａｘ is 1.2 x 10 －４ ~10.0 x 10 －４ A steel material in which: [Ti*]=Ti / {(48 / 12)C+(48 / 14)N} (1) [Nb*]=Nb / {(93 / 12)C+(93 / 14)N} (2) [Ti*]+[Nb*]<1.00 (3) Here, the element symbols in formulas (1) and (2) are substituted with the content in mass% of the corresponding element in the chemical composition.

2. A steel material according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of Cr: 0.01-0.30%, Mo: 0.01-0.20%, V: 0.01-0.20%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, Cu: 0.01-0.40%, Ni: 0.01-0.40%, Sn: 0.001-0.100%, B: 0.0001-0.0050%, Bi: 0.001-0.100%, Se: 0.001-0.100%, Te: 0.001-0.050%, and Pb: 0.001-0.090%. Steel material.

3. A steel wire made of the steel material according to claim 1 or 2.

4. A torsion bar part comprising a shaft portion and the steel material according to claim 1 or 2.