Torsion bar component

Abstract

Provided is a torsion bar component that can suppress an increase in torque associated with twisting. A torsion bar component according to the present disclosure contains, in mass%, 0.003-0.200% of C, 0.10-0.80% of Mn, 0.005-0.080% of Al, 0.50% or less of Si, 0.030% or less of P, 0.050% or less of S, 0.0120% or less of N, and 0.0100% or less of O, the remaining portion being Fe and impurities, and has a ferrite area percentage of 75% or more. In a shaft portion of the torsion bar component, Fn defined by formula (1) by using a half-value breadth β of a peak obtained through X-ray diffraction corresponding to the (211) plane of α-Fe and the maximum value (Q-1)max of the internal friction measured at -50 to 150°C, is 1.0×10-4 to 15.0×10-4.　(1): Fn=β×(Q-1)max

Description

Torsion bar parts

[0001] The present disclosure relates to torsion bar components, and more particularly to torsion bar components for seat belt retractor applications.

[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] A. Savitzky and M. J. E. Golay, “Smoothing and Differentiation of Data by Simplified Least Squares Procedures”, Anal. Chem. , 36 (1964), p. 1627-1639. W. A. Rachinger, A correction for the α1α2 doublet in the measurement of widths of X-ray diffraction lines, J. Sci. Instrument. 25 (1948), p. 254-255.

[0011] 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.

[0012] 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.

[0013] An object of the present disclosure is to provide a torsion bar component that can suppress an increase in torque associated with twisting.

[0014] The torsion bar component of the present disclosure has a chemical composition, in mass %, of C: 0.003 to 0.200%, 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: 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, the ferrite area ratio is 75% or more, and the half width β (°) of the X-ray diffraction peak corresponding to the (211) plane of α-Fe in the shaft portion of the torsion bar part and the maximum value of internal friction (Q －１ ) ｍａｘ and Fn defined by formula (1) is 1.0 × 10 －４ ~15.0 x 10 －４ Fn = β × (Q －１ ) ｍａｘ (1)

[0015] The torsion bar component of the present disclosure can suppress an increase in torque that accompanies twisting.

[0016] FIG. 1 is a graph of the torsion angle (°) and torque (N·m) obtained by performing a torsion test using a torsion bar component. FIG. 2 is a side view of the torsion bar component. FIG. 3 is a perspective view of the torsion bar component for explaining the observation surface of the torsion bar component for the ferrite area ratio. FIG. 4 is a schematic diagram for explaining the position of the observation field on the observation surface shown in FIG. 3. FIG. 5 is a vertical cross-sectional view of the torsion bar component for explaining the method for measuring the half-value width β. FIG. 6A is a graph of the maximum value of internal friction (Q －１ ) ｍａｘ Fig. 6B is a cross-sectional view perpendicular to the axial direction (longitudinal direction) of the shaft portion of the torsion bar part shown in Fig. 6A. Fig. 7A is a graph showing the relationship between temperature and internal friction Q obtained by the automatic vibration damping method.－１ 7B is a diagram showing an example of a graph showing the relationship between the above and the torque-torsion angle. FIG. 7B is a diagram showing an example of a graph after baseline correction has been performed on the graph of FIG. 7A. FIG. 8 is a schematic diagram for explaining a test for evaluating torque increase due to twisting in the examples. FIG. 9 is a schematic diagram of a torque-torsion angle diagram obtained by a test for evaluating torque increase due to twisting in the examples.

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

[0018] The present inventors first studied the torsion bar component from the viewpoint of chemical composition, and as a result, the composition was found to be, in mass %, C: 0.003 to 0.200%, 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 ... The inventors considered that a chemical composition consisting of a: 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 torsion bar parts.

[0019] The present inventors further investigated means for suppressing an increase in torque due to twisting in a torsion bar component having the above chemical composition. Specifically, torsion bar components were manufactured by various manufacturing methods using steel having the above chemical composition. Then, torsion bar components were subjected to twisting tests. 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 torsion bar component. Referring to FIG. 1, the dashed line in graph S1 represents the results of the twist test using a conventional torsion bar component. The solid line in graph S2 represents the results of the twist test using a torsion bar component different from that in graph S1.

[0021] In the case of the torsion bar component shown in graph S1, the torque increases as the twisting 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 torsion bar component shown in graph S2, the torque increases until the torsion bar component yields, but after the torsion bar component yields, the torque remains almost constant and the increase in torque is suppressed even if the twisting angle increases. 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 torsion bar component shown in graph S1 after the torsion test and the torsion bar component shown in graph S2 after the torsion test. As a result, in the torsion bar component shown in graph S1, the entire shaft of the torsion bar component was twisted uniformly, and uniform torsion occurred. On the other hand, in the torsion bar component shown in graph S2, the shaft was twisted locally, and local torsion occurred at the end of the shaft. From these results, the inventors concluded that local torsion is related to the suppression of torque increase.

[0023] Based on the above results, the inventors came up with the following idea: When the torsion bar component is twisted, a local twist is generated in the shaft portion of the torsion bar component. Before the local twist causes excessive plastic deformation in the circumferential direction of the shaft portion 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 torsion bar component (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 of the torsion bar component. 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. Here, the inventors focused on solute C and solute N in the torsion bar component. 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 local twist.

[0025] Furthermore, the inventors also focused on the amount of strain in the torsion bar component. The amount of strain in the torsion bar component, along with the amounts of solute C and N, affects the behavior of dislocations. Therefore, the amount of strain interacts with the amounts of solute C and N, potentially adjusting the rapid multiplication of dislocations.

[0026] Based on the above considerations, the inventors further investigated and examined the amount of strain in the torsion bar component, the amount of dissolved C and N, and the behavior of local torsion. As a result, the half-width of the X-ray diffraction peak corresponding to the (211) plane of α-Fe in the shaft portion of the torsion bar component was defined as β (°), and the maximum value of internal friction measured in the range of -50 to 150°C was defined as (Q －１ ) ｍａｘ When Fn defined by formula (1) is 1.0 × 10 －４ ~15.0 x 10 －４ It was found that if Fn=β×(Q －１ ) ｍａｘ (1)

[0027] The torsion bar component of this embodiment has been completed based on the above technical concept and has the following configuration.

[0028] The torsion bar component of the first configuration has a chemical composition, in mass %, of C: 0.003 to 0.200%, 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: 0 to 0.0050%, Mg: 0 to 0.005 0%, 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, the ferrite area ratio is 75% or more, and the half width β (°) of the X-ray diffraction peak corresponding to the (211) plane of α-Fe in the shaft portion of the torsion bar part and the maximum value of internal friction (Q －１ ) ｍａｘ and Fn defined by formula (1) is 1.0 × 10 －４ ~15.0 x 10 －４ Fn = β × (Q －１ ) ｍａｘ (1)

[0029] The torsion bar part of the second configuration is the torsion bar part of the first configuration, and has a chemical composition, in mass %, of Ti: 0.001 to 0.040%, Nb: 0.001 to 0.040%, 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 0.0001. 50%, 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 torsion bar component of this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0031] [Regarding the Torsion Bar Component of the Present Embodiment] Fig. 2 is a side view of the torsion bar component of the present embodiment. Referring to Fig. 2, 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.

[0032] When the torsion bar component is viewed in the axial direction, the flange portion 30 has a polygonal shape. In FIG. 2 , 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.

[0033] 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.

[0034] [Features of the Torsion Bar Component of the Present Embodiment] The torsion bar component of the present embodiment satisfies the following features 1 to 3. (Feature 1) The chemical composition, in mass %, is: C: 0.003 to 0.200%, 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: 0-0.20%, 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 consisting of Fe and impurities. (Feature 2) The ferrite area ratio is 75% or more. (Feature 3) The half-width β (°) of the X-ray diffraction peak corresponding to the (211) plane of α-Fe in the shaft portion of the torsion bar component and the maximum value of internal friction (Q －１ ) ｍａｘ and Fn defined by formula (1) is 1.0 × 10 －４ ~15.0 x 10 －４ Fn = β × (Q －１ ) ｍａｘ (1) Features 1 to 3 will be explained below.

[0035] [(Feature 1) Chemical Composition] The torsion bar component of this embodiment contains the following elements.

[0036] C: 0.003 to 0.200% Carbon (C) dissolves in the steel material from which the torsion bar component is made. This generates localized torsion when the shaft portion 20 of the torsion bar component is twisted in the circumferential direction, and propagates the localized torsion in the axial direction. This suppresses the increase in torque associated with twisting. If the C content is less than 0.003%, the above effect cannot be fully achieved. On the other hand, if the C content exceeds 0.200%, the amount of dissolved C in the torsion bar component becomes excessive. In this case, localized torsion occurs when the torsion bar component is twisted, but the torsion bar component breaks before the localized torsion occurs and propagates axially. Therefore, the C content is 0.003 to 0.200%. The preferred lower limit of the C content is 0.005%, more preferably 0.008%, and even more preferably 0.010%. The upper limit of the C content is preferably 0.190%, more preferably 0.175%, and even more preferably 0.165%.

[0037] Mn: 0.10 to 0.80% Manganese (Mn) increases the strength of the torsion bar component through solid solution strengthening. Mn also fixes S in the steel material from which the torsion bar component is made, improving the hot workability of the steel material. If the Mn content is less than 0.10%, the above effect cannot be sufficiently achieved. On the other hand, if the Mn content exceeds 0.80%, the strength of the torsion bar component 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%.

[0038] Al: 0.005 to 0.080% Aluminum (Al) deoxidizes steel. Furthermore, Al bonds with N in the steel material used to make torsion bar components to form AlN. AlN refines crystal grains through its pinning effect. As a result, the cold workability (wire drawing workability and cold forging ability) of the steel material 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 further excessively formed. In this case, defects are more likely to occur in the steel material 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%.

[0039] 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 the torsion bar component through solid solution strengthening. Even if even a small amount of Si is contained, the above effect can be obtained to some extent. On the other hand, if the Si content exceeds 0.50%, the strength of the steel material from which the torsion bar component is made increases excessively. In this case, the cold workability (wire drawing workability and cold forging ability) of the steel material 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%.

[0040] 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 the steel material used to make torsion bar components. 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, considering 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%.

[0041] S: 0.050% or less Sulfur (S) is inevitably contained. In other words, the S content is greater than 0%. S forms MnS in the steel material that is the raw material for torsion bar components, improving the machinability of the steel material. 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 material becomes coarse, and the cold workability of the steel material decreases. 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%.

[0042] 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 from which the torsion bar component is made, suppressing the increase in torque associated with twisting of the torsion bar component. Even if even a small amount of N is contained, the above effect can be achieved to some extent. On the other hand, if the N content exceeds 0.0120%, the amount of dissolved N in the torsion bar component becomes excessive. In this case, the torsion bar component breaks before 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%.

[0043] 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 the steel material used to make torsion bar components. Therefore, the O content is 0.0100% or less. The O content is preferably as low as possible. However, excessive reduction in 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%.

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

[0045] [Regarding optional elements] The chemical composition of the torsion bar component of this embodiment may further include one or more elements selected from the group consisting of 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: 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 the first group (Ti, Nb, Cr, Mo, and V)] The chemical composition of the torsion bar part of the present embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, Nb, Cr, Mo, and V. Any of these elements forms precipitates (carbides, nitrides, or carbonitrides) and increases the strength of the torsion bar part.

[0047] Ti: 0 to 0.040% Titanium (Ti) is an optional element and does not necessarily need to be contained. In other words, the Ti content may be 0%. When contained, Ti forms precipitates such as carbides, nitrides, or carbonitrides, thereby increasing the strength of the torsion bar component. Even if even a small amount of Ti is contained, the above effect can be achieved to some extent. However, if the Ti content exceeds 0.040%, excessive precipitates are formed. In this case, the cold workability of the steel material from which the torsion bar component is made deteriorates. Therefore, the Ti content is 0 to 0.040%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Ti content is 0.038%, even more preferably 0.035%, and even more preferably 0.030%.

[0048] Nb: 0 to 0.040% Niobium (Nb) is an optional element and does not necessarily need to be contained. In other words, the Nb content may be 0%. When contained, Nb forms precipitates such as carbides, nitrides, or carbonitrides, thereby increasing the strength of the torsion bar component. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.040%, excessive precipitates are formed. In this case, the cold workability of the steel material from which the torsion bar component is made deteriorates. Therefore, the Nb content is 0 to 0.040%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Nb content is 0.038%, even more preferably 0.035%, and even more preferably 0.030%.

[0049] 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 torsion bar component. 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 material from which the torsion bar component is made 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%.

[0050] 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 torsion bar component. 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 material from which the torsion bar component is made 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%.

[0051] 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 torsion bar component. 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 material from which the torsion bar component is made 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%.

[0052] [Regarding the Second Group (Ca and Mg)] The chemical composition of the torsion bar component of this embodiment may contain one or more elements selected from the group consisting of Ca and Mg, instead of a portion of Fe. Any of these elements spheroidizes MnS and improves the hot workability of the steel material that is the raw material for the torsion bar component.

[0053] 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 material used to make torsion bar components. Even if even a small amount of Ca is contained, the above effect can be achieved 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 material is reduced. 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%.

[0054] 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 material used to make torsion bar components. Even if even a small amount of Mg is contained, the above effect can be achieved 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 material is reduced. 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%.

[0055] [Regarding the third group (Cu, Ni, and Sn)] The chemical composition of the torsion bar component of the present embodiment may contain one or more elements selected from the group consisting of Cu, Ni, and Sn, instead of a portion of Fe. Any of these elements enhances the corrosion resistance of the torsion bar component.

[0056] 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 improves the corrosion resistance of the torsion bar component. 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 hot workability and cold workability of the steel material from which the torsion bar component is made deteriorate. 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%.

[0057] Ni: 0 to 0.40% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When contained, Ni improves the corrosion resistance of the torsion bar component. 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%.

[0058] 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%.

[0059] [Regarding the Fourth Group (B)] The chemical composition of the torsion bar component of this embodiment may contain B instead of a portion of Fe.

[0060] 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 the steel material used to make torsion bar components. Even if even a small amount of B is contained, the above effect can be achieved to some extent. However, if the B content exceeds 0.0050%, B generates excessive nitrides. In this case, the cold workability of the steel material is actually reduced. 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%.

[0061] [Regarding Group 5 (Bi, Se, Te, and Pb)] The chemical composition of the torsion bar component of this embodiment may contain one or more elements selected from the group consisting of Bi, Se, Te, and Pb, instead of a portion of Fe. All of these elements improve the machinability of the steel material used to make the torsion bar component.

[0062] Bi: 0 to 0.100% Bismuth (Bi) is an optional element and does not necessarily need to be contained. In other words, the Bi content may be 0%. When contained, Bi improves the machinability of the steel material from which the torsion bar component is made. 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 material 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%.

[0063] 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 the steel material from which the torsion bar components are made. 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%.

[0064] 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 material from which the torsion bar component is made. 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 material decreases. 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%.

[0065] 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 material from which the torsion bar components are made. 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 material 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%.

[0066] [(Feature 2) Ferrite Area Ratio] Furthermore, the torsion bar component of the present embodiment has a ferrite area ratio of 75% or more.

[0067] If the ferrite area ratio is 75% or more, sufficient torsion occurs during twisting. A preferable lower limit of the ferrite area ratio is 85%. In the metal structure of the torsion bar component, 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.

[0068] [Method for Measuring Ferrite Area Ratio] The ferrite area ratio in the metal structure of a torsion bar component is determined by the following method. Referring to FIG. 3, a test piece including a cross section (having a circular cross section) perpendicular to the axial direction of the shank 20 of the torsion bar component is taken from the shank 20. This cross section of the test piece is designated as the observation surface 25. The observation surface 25 is etched with a 3% by volume Nital solution to reveal the metal structure. Referring to FIG. 4, when the diameter of the observation surface 25 is D, four observation fields 26 are defined as positions at a depth of D / 4 in the radial direction from the surface corresponding to the outer circumferential surface of the shank 20 on the observation surface 25 of the test piece, spaced at 90° intervals around the central axis of the observation surface 25. The rectangular observation fields 26 are observed at a magnification of 500x 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.

[0069] 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.

[0070] 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).

[0071] [(Feature 3) Regarding Fn] In the torsion bar component of this embodiment, the half-width β (°) of the X-ray diffraction peak corresponding to the (211) plane of α-Fe (ferrite, body-centered cubic lattice) in the shaft portion 20 and the maximum value of internal friction (Q －１ ) ｍａｘ and Fn defined by formula (1) is 1.0 × 10 －４ ~15.0 x 10 －４ Fn = β × (Q －１ ) ｍａｘ(1)

[0072] The half-value width β (°) correlates with the amount of strain in the torsion bar component. In other words, the half-value width β is an index of the amount of strain in the torsion bar component. －１ ) ｍａｘ means the maximum value of the Snoek Peak among the internal friction values ​​measured in the range of -50 to 150°C. Usually, the Snoek Peak appears in the range of 0 to 100°C, and the maximum value of the Snoek Peak appears in the range of 35 to 55°C. The maximum value of the internal friction (Q －１ ) ｍａｘ is correlated with the total amount of solute C and solute N in the torsion bar component. －１ ) ｍａｘ is an index of the total amount of solute C and solute N in the torsion bar component. In the following explanation, the maximum value of internal friction (Q －１ ) ｍａｘ Simply (Q －１ ) ｍａｘ It is also called.

[0073] Fn is the half-width β (°) and (Q －１ ) ｍａｘ and Fn, and correlates with dislocation motion. Therefore, Fn correlates with the occurrence of local twisting during twisting and the degree of propagation in the axial direction. －４ If Fn is less than 15.0×10, no localized twist occurs in the shaft portion 20 during twisting, and uniform twist occurs. As a result, the increase in torque due to twisting cannot be sufficiently suppressed. －４ If the bending moment exceeds this value, excessive local torsion in the circumferential direction occurs in the shaft portion 20. As a result, during twisting, the torsion bar component breaks before the local torsion propagates in the axial direction of the shaft portion 20 of the torsion bar component.

[0074] Fn is 1.0 × 10 －４ ~15.0 x 10 －４ If so, the amount of strain and the total amount of solute C and solute N are appropriate. Therefore, a local twist occurs in the shank 20 during twisting, and the local twist propagates in the axial direction of the shank 20. As a result, an increase in torque due to twisting can be sufficiently suppressed.

[0075] The preferred lower limit of Fn is 1.1 × 10 －４ and more preferably 1.2 × 10 －４ and more preferably 1.5 × 10 －４ and more preferably 2.0 × 10 －４ The preferred upper limit of Fn is 14.0 × 10 －４ and more preferably 12.0 × 10 －４ and more preferably 10.0 × 10 －４ and more preferably 9.0 × 10 －４ and more preferably 8.0 × 10 －４ and more preferably 7.5 × 10 －４ is.

[0076] Fn is the half-width β (°) with two significant figures to the second decimal place and the half-width Q (°) with five significant figures to the fifth decimal place. －１ ) ｍａｘ The calculation is performed using the formula and the value is rounded to the sixth decimal place (i.e., the significant figures are the fifth decimal place).

[0077] [Method for Measuring Half-Width Width β] The half-width width β (°) of the torsion bar component of this embodiment is measured by the X-ray diffraction method (XRD) in the following manner.

[0078] Referring to FIG. 5 , a test piece 10 is taken from the shaft portion 20 of a torsion bar component, with the observation surface 27 being a cross section including and parallel to the central axis of the shaft portion 20. The observation surface 27 of the test piece 10 is wet-polished with #400 to #1500 emery paper (water-resistant SiC abrasive paper). The observation surface 27 of the test piece 10 is then polished with a polishing cloth impregnated with a diamond suspension to achieve a mirror finish. The diamond suspension is a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. After polishing with the diamond suspension, the observation surface 27 is further polished using colloidal silica. This removes the surface damage layer on the surface of the observation surface 27 of the test piece 10.

[0079] 5, on the observation surface 27 after polishing, nine measurement points α are selected at a depth of D / 4 (D is the diameter of the shank 20) ​​from one surface corresponding to the outer circumferential surface of the shank 20 of the torsion bar component, and are arranged at 1 mm intervals parallel to the central axis of the shank 20. In addition, nine positions that are symmetrical to the selected nine measurement points α with respect to the central axis of the shank 20 are also selected as measurement points α. In other words, 18 measurement points α are selected on the observation surface.

[0080] For each measurement point α, an X-ray diffraction peak is obtained by X-ray diffraction (XRD). From the obtained X-ray diffraction peak, the half width of the X-ray diffraction peak corresponding to the (211) plane of α-Fe is measured. Here, the half width means the full width at half maximum (FWHM). When the X-ray diffraction measurement is performed under the following conditions, the X-ray diffraction peak corresponding to the (211) plane of α-Fe appears in the range of 150 to 170°.

[0081] The diffraction profile obtained by X-ray diffraction measurement is pretreated in the following procedure.

[0082] The obtained diffraction profile is smoothed by the Savitzky-Golay method described in Non-Patent Document 1. The number of smoothing points (corresponding to the value of 2m+1 in the Savitzky-Golay method) is set to 7.

[0083] Next, a straight line is drawn between the intensities at the start and end angles of the X-ray diffraction measurement, and this line is then subtracted from the smoothed diffraction profile as a background.

[0084] The Rachinger method described in Non-Patent Document 2 is applied to the diffraction profile from which the background has been subtracted, and the X-ray diffraction peak corresponding to the Kα2 ray is removed from the X-ray diffraction peak corresponding to the (211) plane of α-Fe in the diffraction profile from which the background has been subtracted. The intensity ratio of the X-ray diffraction peak corresponding to the Kα1 ray to the X-ray diffraction peak corresponding to the Kα2 ray is set to 2:1. The FWHM is calculated from the diffraction profile obtained by the above procedure. The calculated FWHM is taken as the half-width of the X-ray diffraction peak corresponding to the (211) plane of α-Fe at the measurement position.

[0085] The measurement of the above 18 points constitutes one X-ray diffraction measurement. The above measurement is repeated three times at the same measurement position. The arithmetic mean of the half-widths obtained from the three measurements is taken as the half-width β (°). For example, an X-ray diffraction device manufactured by Rigaku Corporation under the trade name AutoMATE is used. The X-ray diffraction measurement conditions are as follows: (X-ray diffraction measurement conditions) Radiation source: CrKα Acceleration voltage: 40 kV Acceleration current: 40 mA Collimator diameter: φ1 mm Counting time at each measurement point α: 90 seconds X-ray scanning range (scan start angle to scan end angle): 146° to 165°

[0086] [Maximum value of internal friction (Q －１ ) ｍａｘ Measurement method of (Q －１ ) ｍａｘ is measured in the following way: Figure 6A shows the maximum value of internal friction (Q －１ ) ｍａｘFIG. 6B is a perspective view of a torsion bar component illustrating the location of a test piece for measuring the strain. FIG. 6B is a cross-sectional view perpendicular to the axial direction (longitudinal direction) of the shank of the torsion bar component shown in FIG. 6A. Referring to FIGS. 6A and 6B, a test piece 28 is taken from a cross-section (circular shape) perpendicular to the axial direction of the shank 20 of the torsion bar component, at a depth D / 4 (D is the diameter of the shank 20) ​​in the radial direction from the surface of the shank 20. 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 of the shank 20. Referring to FIG. 6B, the test piece 28 is taken from a cross-section perpendicular to the axial direction of the shank 20 such that a normal N28 at the width center of a surface 28A of the test piece 28, which is 5 mm wide and extends 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). The length of the test piece 28 may be the same as the length of the torsion bar component.

[0087] 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.

[0088] 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. 7A, the straight line connecting the pair of bottoms BT of the obtained graph (peak waveform diagram) SP is defined 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. 7B 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.).

[0089] [Effects of the Torsion Bar Component of the Present Embodiment] The torsion bar component of the present embodiment satisfies Features 1 to 3. Therefore, the torsion bar component of the present embodiment can sufficiently suppress an increase in torque due to twisting.

[0090] [Regarding the Size and Use of the Torsion Bar Component of the Present Embodiment] The size of the torsion bar component of the present 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. The torsion bar component of the present embodiment is suitable for use in a seat belt retractor.

[0091] [Manufacturing Method of Torsion Bar Component of This Embodiment] A manufacturing method of the torsion bar component of this embodiment will be described below. A torsion bar component satisfying Features 1 to 3 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 manufacturing method of the torsion bar component of this embodiment.

[0092] An example of the manufacturing method of this embodiment includes the following steps: (Step 1) Material manufacturing step (Step 2) Hot working step (Step 3) Wire drawing step (Step 4) Cold forging step Steps 1 and 2 manufacture a steel material that will be used as a material for the torsion bar component. Steps 3 and 4 manufacture the torsion bar component. Each step will be described below.

[0093] [(Step 1) Material Production Step] In the material production step, first, 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, or a cast piece (bloom) is produced by a continuous casting method. Through the above steps, a material (ingot or bloom) is produced.

[0094] [(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.

[0095] [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.

[0096] [Finish Rolling Step] In the finish rolling step, the billet after blooming 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.

[0097] [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.

[0098] 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.

[0099] 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.

[0100] The steel material that is the raw material for the torsion bar component of this embodiment is manufactured by the above manufacturing process. Note that in the manufacturing process of the steel material, a cold rolling process using work rolls is not performed after the hot working process.

[0101] [(Step 3) Wiredrawing Step] In the wiredrawing step, a steel material is drawn to produce a steel wire. The area reduction rate RR in the wiredrawing step 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)) x 100 When wiredrawing is performed multiple times, the area reduction rate RR is the cumulative area reduction rate (%) in the multiple wiredrawing steps.

[0102] [(Step 4) Cold Forging Step] In the cold forging step, the steel wire after wire drawing is subjected to forging (upset forging) to manufacture a torsion bar part including the shaft portion 20 and the pair of flange portions 30 (30L and 30R) shown in FIG. 2 . Here, the cross-sectional area increase rate DR of the shaft portion 20 due to forging is 1 to 10%. Here, the cross-sectional area increase rate DR of the shaft portion 20 is defined by the following formula: Cross-sectional area increase rate DR = ((cross-sectional area perpendicular to the axial direction of the shaft portion 20 of the torsion bar part after forging / cross-sectional area perpendicular to the axial direction of the steel wire before forging) - 1) x 100

[0103] [Manufacturing conditions in the wire drawing process and the cold forging process] In the wire drawing process and the cold forging process, the area reduction rate RR (%) in the wire drawing process and the area increase rate DR (%) in the cold forging process further satisfy the following formula (A): 4≦RR+DR≦20 (A).

[0104] [Regarding formula (A)] FA = RR + DR. If FA is less than 4, the amount of strain contained in the torsion bar component is too small. In this case, Fn is 1.0 × 10 －４ On the other hand, if FA exceeds 20, the amount of strain contained in the torsion bar component is too large. In this case, Fn is 15.0 × 10 －４ If FA is 4 to 20, the amount of strain of the torsion bar component falls within an appropriate range, and the full width at half maximum β falls within an appropriate range.

[0105] The torsion bar component of this embodiment is manufactured through the above steps.

[0106] In the manufacturing process of the torsion bar part, a plating process may be performed after the cold forging process, if necessary. In the plating process, a known plating treatment is performed on the intermediate product after the cold forging to form a plating layer on the surface. The composition of the plating layer is not particularly limited.

[0107] The effects of the torsion bar component of this embodiment will be described more specifically with reference to examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the torsion bar component of this embodiment. Therefore, the torsion bar component of this embodiment is not limited to this one example of conditions.

[0108] 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.

[0109]

[0110]

[0111] 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.

[0112] The produced billet was subjected to a finish rolling process. Specifically, the billet was heated to 1100° C. The heated billet was subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce a steel material (wire material).

[0113] 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 33 and 34, 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.

[0114]

[0115] Torsion bar parts were manufactured using the produced steel materials. Steel wires were produced by wiredrawing the steel materials with each test number shown in Table 1 at the area reduction rate RR (%) shown in Table 2. Furthermore, the steel wires were forged at the cross-sectional area increase rate DR (%) shown in Table 2 to produce torsion bar parts with the shape shown in Figure 2. The FA for each test number based on the area reduction rate RR (%) and the cross-sectional area increase rate DR (%) is shown in Table 2. Note that for test numbers 37 and 38, the torsion bar parts were manufactured by cutting without performing the wiredrawing process or the cold forging process. The torsion bar part with the shape shown in Figure 2 was manufactured using the above manufacturing process. The torsion bar part had an axial length L2 of the shaft portion 20 of 50 mm and an axial length L3 of the flange portion 30 of 5 mm. The flange portion 30 had a hexagonal shape when viewed from the axial direction of the torsion bar part, with a width W of 10.4 mm. The diameter of the shaft portion 20 was 9.25 mm. Three torsion bar parts were manufactured for each test number.

[0116] The ferrite area ratio was measured for the torsion bar parts of each test number based on the above-mentioned [Method for Measuring Ferrite Area Ratio]. As a result, the ferrite area ratio was 75% or more for all test numbers.

[0117] [Evaluation Tests] The following evaluation tests were carried out on the torsion bar parts with each test number. (Test 1) Measurement test of half-value width β (Test 2) Measurement test of maximum value of internal friction (Q －１ ) ｍａｘ (Test 3) Evaluation test of torque increase amount due to twisting Tests 1 to 3 will be explained below.

[0118] [(Test 1) Evaluation Test of Half-Width β] Based on the method described in the above [Method for Measuring Half-Width β], the half-width β (°) of the torsion bar component of each test number was determined. An AutoMATE product manufactured by Rigaku Corporation was used as the X-ray diffraction device. The software attached to the X-ray diffraction device was used to calculate the half-width. The obtained half-width β (°) is shown in Table 2.

[0119] [(Test 2) Maximum value of internal friction (Q －１ ) ｍａｘ Measurement test of the torsion bar parts of each test number (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 measurement was carried out using an IFM-1500L manufactured by Shinku Riko Co., Ltd. (currently Advance Riko Co., Ltd.). The frequency of the torsional vibration was approximately 1.3 Hz. The obtained (Q －１ ) ｍａｘ In Table 2, "(Q －１ ) ｍａｘ (x10 －４ " column, and Fn (= β × (Q －１ ) ｍａｘ ) in Table 2 －４ ) column.

[0120] [(Test 3) Evaluation Test for Torque Increase Amount Due to Twisting] The torque increase amount due to twisting of the torsion bar component of each test number was evaluated by the following twisting test. Three torsion bar components of each test number were manufactured by the above-mentioned manufacturing method.

[0121] The torsion bar components with each test number were evaluated using the following torsion test. Specifically, as shown in FIG. 8 , 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.

[0122] Based on the graph created, the yield torque T ｙ (N m) was calculated. 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. ｙ was obtained 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. The intersection of this parallel line and the torque-torsion angle diagram is the desired yield torque T ｙ The specified plastic twist angle was 45°.

[0123] 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

[0124] 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.

[0125] [Evaluation Results] Referring to Tables 1 and 2, the torsion bar parts with test numbers 1 to 28 satisfied features 1 to 3. Therefore, the torsion bar parts with these test numbers were able to sufficiently suppress the increase in torque caused by twisting.

[0126] On the other hand, in test numbers 29 and 30, the cooling stop temperature was too high. Therefore, Fn was 15.0 × 10 －４ Therefore, in the torque increase evaluation test, the local twist did not propagate and the specimen broke midway. １２６０As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0127] In test numbers 31 and 32, the cooling stop temperature was too low. Therefore, Fn was 15.0 × 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.

[0128] In test numbers 33 and 34, the cooling step was not performed. Therefore, Fn was 15.0 × 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.

[0129] In test numbers 35 and 36, the area reduction rate RR (%) was too high. Therefore, Fn was 15.0 × 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.

[0130] In test numbers 37 and 38, the wire drawing process and the cold forging process were not performed. －４ As a result, the increase in torque due to twisting could not be sufficiently suppressed.

[0131] In test numbers 39 and 40, the area increase rate DR (%) was too high. Therefore, Fn was 15.0 × 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.

[0132] In test numbers 41 and 42, FA was too high. Therefore, Fn was 15.0 × 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.

[0133] 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. A torsion bar part having a chemical composition, in mass%, of C: 0.003-0.200%, 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, Ti: 0-0.040%, Nb: 0-0.040%, 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%, The torsion bar component has a half-width β(°) of an X-ray diffraction peak corresponding to the (211) plane of α-Fe in a shaft portion of the torsion bar component and a maximum value of internal friction (Q －１ ) ｍａｘ Using the above, Fn defined by formula (1) is 1.0 × 10 －４ ~15.0 x 10 －４ Torsion bar parts. Fn = β × (Q －１ ) ｍａｘ (1) 2. A torsion bar part according to claim 1, having a chemical composition, in mass%, of Ti: 0.001-0.040%, Nb: 0.001-0.040%, 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%, A torsion bar component comprising one or more elements selected from the group consisting of Te: 0.001 to 0.050% and Pb: 0.001 to 0.090%.