Torsion bar part

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

Technical Field

[0001] The present disclosure relates to a torsion bar component, and more particularly to a torsion bar component for use in a seat belt retractor.

Background Art

[0002] A seat belt retractor equipped in an automobile restrains an occupant with a seat belt when the vehicle body rapidly decelerates due to an impact such as a collision or sudden braking. Thereby, the seat belt retractor prevents the occupant from jumping forward due to inertia and protects the occupant.

[0003] When the vehicle body rapidly decelerates, an excessive load is applied to the body of the occupant restrained by the seat belt from the seat belt. Therefore, the seat belt retractor not only restrains the occupant but also has a mechanism for relaxing the load applied to the occupant. As a mechanism for mitigating the impact, the seat belt retractor includes a torsion bar component. When an excessive load acts on the occupant via the seat belt, the torsion bar component mounted on the seat belt retractor is torsionally deformed. As a result, the force for restraining the occupant by the seat belt is limited. Therefore, excellent torsion characteristics are required for the steel material used for the torsion bar component. Here, excellent torsion characteristics mean that when the steel material is twisted, the twist angle until breakage is large.

[0004] Techniques for enhancing the torsion characteristics of the steel material used for the torsion bar component have been proposed in Japanese Unexamined Patent Application Publication No. 2009-120906 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2009-120907 (Patent Document 2), and Japanese Unexamined Patent Application Publication 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%, and 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. The ferrite area ratio is 99% or more, the ferrite grain size number (A) on the outermost surface is 3.0 to less than 7.0, the ferrite grain size number (B) at the D / 4 part (D: diameter of the wire rod or bar) is 3.0 to less than 7.0, and the difference between the ferrite grain size numbers (A) and (B) is within 0.5.

[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%, and P: 0.001 to 0.020%, S: 0.020% or less, and N: 0.01% or less, with the balance being iron and inevitable impurities. The microstructure is a structure having spherical carbides throughout the ferrite matrix, and the degree of spheroidization of the spherical carbides is No. 1 to 2 defined in JIS G 3507-2.

[0007] Patent Documents 1 and 2 describe that in all of these steel materials, by making the structure as uniform as possible, the number of twists until the steel material breaks can be increased when a twist test is carried out at -40°C.

[0008] The steel material disclosed in Patent Document 3 contains, by mass%, C: 0.03 to 0.25%, Si: more than 0 to 0.35%, Mn: more than 0 to 0.6%, P: more than 0 to 0.05%, S: more than 0 to 0.05%, and Al: 0.005 to 0.2%, with the balance being Fe and unavoidable impurities. The metallographic structure contains ferrite and the balance consists of at least one of pearlite and spherical carbide. The crystal grain size number of ferrite is 8.5 or more, and the half-value width of the X-ray diffraction peak on the (211) plane of ferrite is 0.90° or less. In this steel material, ferrite is the main metallographic structure, and the torsional characteristics are enhanced by reducing the strain in the steel material. Further, the ferrite crystal grain size is refined to increase the initial torque during torsion. According to Patent Document 3, this enables sufficient absorption of impact energy.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0011] In Patent Document 1 and Patent Document 2, the problem is to improve the twisting characteristics by increasing the number of twists until breakage by a twisting test. In Patent Document 3, in addition to improving the twisting characteristics, the problem is to increase the initial torque during twisting.

[0012] As described above, when the vehicle body decelerates rapidly, the torsion bar component twists in the circumferential direction. At this time, as the twist angle (number of twists) increases, the torque required for the torsion bar component to be torsionally deformed also increases. At this time, as in Patent Document 3, when the initial torque increases, the impact during vehicle deceleration is absorbed. However, after the increase in the initial torque, a further increase in torque correlates with an increase in the load on the occupant. That is, if the torque continues to increase, the load on the occupant restrained by the seat belt also increases. Therefore, it is preferable that the torsion bar component can suppress the increase in torque associated with twisting.

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

Means for Solving the Problems

[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 consists of 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 component and the maximum value of the internal friction measured in the range of -50 to 150 °C (Q -1 ) max are used such that Fn defined by formula (1) is 1.0×10 -4 ~15.0×10 -4 . Fn = β × (Q -1 ) max (1)

Advantages of the Invention

[0015] In the torsion bar component of the present disclosure, an increase in torque associated with twisting can be suppressed.

Brief Description of the Drawings

[0016]

Figure 1

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MODE FOR CARRYING OUT THE INVENTION

[0017] The present inventors have studied a torsion bar component capable of suppressing an increase in torque accompanying twisting.

[0018] The inventors first examined the torsion bar parts from the perspective of chemical composition. As a result, in terms of mass%, if the chemical composition 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 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 consists of Fe and impurities, the inventors considered that it would be suitable for the torsion bar parts.

[0019] The inventors further examined means for suppressing an increase in torque associated with twisting in the torsion bar parts having the above chemical composition. Specifically, using steel materials having the above chemical composition, torsion bar parts were manufactured by various manufacturing methods. Then, a twisting test was performed on the torsion bar parts. As a result, the inventors found the following matters.

[0020] FIG. 1 is a graph of the twist angle (°) and torque (N·m) obtained by performing a twisting test using a torsion bar part. Referring to FIG. 1, the graph S1 shown by the broken line is the result of a twisting test using a conventional torsion bar part. The graph S2 shown by the solid line is the result of performing a twisting test using a torsion bar part different from S1.

[0021] In the case of the torsion bar component shown in Graph S1, as the torsional angle increases (i.e., with torsion), the torque increases. In this case, with torsion, the load applied to the occupant restrained by the seat belt also increases. On the other hand, in the case of the torsion bar component shown in Graph S2, although the torque increases until the torsion bar component yields, after the torsion bar component yields, even when the torsional angle increases, the torque is almost constant, and the increase in torque is suppressed. In this case, even as the torsion progresses, the load applied to the occupant restrained by the seat belt can be suppressed.

[0022] Here, the inventors observed the appearances of the torsion bar component of Graph S1 after the torsion test and the torsion bar component of Graph S2 after the torsion test. As a result, in the torsion bar component of Graph S1, the entire shaft portion of the torsion bar component was uniformly twisted, and uniform torsion occurred. On the other hand, in the torsion bar component of Graph S2, the shaft portion was locally twisted, and local torsion occurred at the end of the shaft portion. From the above results, the inventors considered that local torsion is related to the suppression of the increase in torque.

[0023] Based on the above results, the inventors considered as follows. When the torsion bar component is twisted, local torsion is generated in the shaft portion of the torsion bar component. Before the plastic deformation in the circumferential direction of the shaft portion at the location becomes excessive due to the local torsion (i.e., before the local torsion in the circumferential direction becomes excessive), the local torsion is propagated in the axial direction of the torsion bar component (i.e., to the axially adjacent position at the location). If such plastic deformation behavior can be realized, the increase in torque can be suppressed while the local torsion is propagated in the axial direction of the torsion bar component. That is, the increase in torque associated with torsion can be suppressed.

[0024] Based on the above findings, the inventors further studied means for generating local torsion during twisting while propagating the local torsion in the axial direction. Here, the inventors focused on the dissolved C and dissolved N in the torsion bar component. Dissolved C and dissolved N are interstitial solid solution elements and strongly interact with dislocations. Therefore, the amounts of dissolved C and dissolved N are related to dislocation movement. Local torsion is generated by the rapid growth of dislocations. Therefore, by adjusting the amounts of dissolved C and dissolved N, the growth behavior of dislocations can be controlled. As a result, there is a possibility of controlling the behavior of local torsion.

[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 affects the behavior of dislocations together with the amounts of dissolved C and dissolved N. Therefore, there is a possibility that the amount of strain and the amounts of dissolved C and dissolved N interact to adjust the rapid growth of dislocations.

[0026] Based on the above considerations, the inventors further investigated and studied the amount of strain in the torsion bar component, the amounts of dissolved C and dissolved N, and the behavior of local torsion. As a result, taking the half-value width of the X-ray diffraction peak corresponding to the (211) plane of α-Fe in the shaft portion of the torsion bar component as β (°) and the maximum value of the internal friction measured in the range of -50 to 150 °C as (Q -1 ) max when, it was found that if Fn defined by Equation (1) is 1.0×10 -4 ~15.0×10 -4 , the increase in torque accompanying twisting can be sufficiently suppressed. Fn = β × (Q -1 ) max (1)

[0027] The torsion bar component of this embodiment is completed based on the above technical idea 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 0.0050%, 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 consisting of Fe and impurities, a ferrite area ratio of 75% or more, and the half-value 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 the internal friction (Q -1 ) max Using these, Fn defined by Equation (1) is 1.0×10 -4 ~15.0×10 -4 . Fn = β × (Q -1 ) max (1)

[0029] The torsion bar component of the second configuration is the torsion bar component of the first configuration and contains at least one selected from the group consisting 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 0.0050%, 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% in mass%.

[0030] Hereinafter, the torsion bar component of the present embodiment will be described in detail. Note that, unless otherwise specified, "%" regarding an element means mass%.

[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 at both ends of the shaft portion 20. The shaft portion 20 has a cylindrical shape, and the cross section perpendicular to the axial direction (longitudinal direction) is circular. The pair of flange portions 30 are coaxially disposed at both ends of the shaft portion 20. Specifically, among 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 from the axial direction, the flange portion 30 has a polygonal shape. In FIG. 2, the flange portion 30 when the torsion bar component is viewed from the axial direction is hexagonal. However, the shape of the flange portion 30 is not limited to hexagonal. When the torsion bar component is viewed from the axial direction, the width of the flange portion 30 is larger than the diameter of the shaft portion 20.

[0033] The torsion bar component having the above configuration is housed in a seat belt retractor. Inside 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 operates, the locking base is fixed to the frame of the seat belt retractor via an engaging member. Therefore, one of the flange portions 30 becomes a fixed end when the load limiter mechanism operates. The other flange portion 30 is connected to a spool. When the load limiter mechanism operates and the torsion bar component twists, the other flange portion 30 becomes a driving end that rotates in conjunction with the rotation of the spool. As described above, when the load limiter mechanism operates, one flange portion 30 rotates relative to the other flange portion 30. As a result, torsional deformation occurs in the shaft portion 20 of the torsion bar component. Note that depending on the structure of the seat belt retractor, the flange portion 30 may be provided with a connecting shaft portion for connecting to other components 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 is, by 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%, 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 consists of Fe and impurities. (Feature 2) The ferrite area ratio is 75% or more. (Feature 3) The half-value 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 the internal friction (Q -1 ) max measured in the range of -50 to 150 °C are such that Fn defined by Equation (1) is 1.0×10 -4 to 15.0×10 -4 . Fn = β × (Q -1 ) max (1) Hereinafter, Features 1 to 3 will be described.

[0035] [(Feature 1) Regarding the chemical composition] The torsion bar component of this embodiment contains the following elements.

[0036] C: 0.003 to 0.200% Carbon (C) is dissolved in the steel material which is the material of the torsion bar component, and when the shaft portion 20 of the torsion bar component is twisted in the circumferential direction, local torsion is generated, and the generated local torsion is propagated in the axial direction. Thereby, an increase in torque accompanying the twisting is suppressed. If the C content is less than 0.003%, the above effect cannot be obtained sufficiently. On the other hand, if the C content exceeds 0.200%, the amount of dissolved C in the torsion bar component becomes excessively large. In this case, although local torsion occurs when the torsion bar component is twisted, it breaks halfway before the local torsion sufficiently propagates in the axial direction of the torsion bar component. Therefore, the C content is 0.003 to 0.200%. The preferable lower limit of the C content is 0.005%, more preferably 0.008%, and still more preferably 0.010%. The preferable upper limit of the C content is 0.190%, more preferably 0.175%, and still more preferably 0.165%.

[0037] Mn: 0.10 to 0.80% Manganese (Mn) increases the strength of the torsion bar component by solid solution strengthening. Mn further fixes S in the steel material which is the material of the torsion bar component, and improves the hot workability of the steel material. If the Mn content is less than 0.10%, the above effect cannot be obtained sufficiently. 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 preferable lower limit of the Mn content is 0.11%, more preferably 0.13%, and still more preferably 0.15%. The preferable upper limit of the Mn content is 0.78%, more preferably 0.75%, and still more preferably 0.70%.

[0038] Al: 0.005 to 0.080% Aluminum (Al) deoxidizes the steel. Al further combines with N in the steel material, which is the material of the torsion bar part, to form AlN. Due to the pinning effect, AlN refines the crystal grains. As a result, the cold workability (wire drawing workability and cold forging workability) of the steel material is enhanced. If the Al content is less than 0.005%, the above effects cannot be obtained sufficiently. On the other hand, if the Al content exceeds 0.080%, the above effects saturate. If the Al content exceeds 0.080%, furthermore, oxide-based inclusions are excessively generated. In this case, the steel material is likely to develop defects during hot rolling. Therefore, the Al content is 0.005 to 0.080%. The preferable lower limit of the Al content is 0.007%, more preferably 0.010%, and even more preferably 0.015%. The preferable upper limit of the Al content is 0.078%, more preferably 0.075%, and even more preferably 0.070%.

[0039] Si: 0.50% or less Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si enhances the strength of the torsion bar part by solid solution strengthening. If Si is contained even slightly, the above effects can be obtained to a certain extent. On the other hand, if the Si content exceeds 0.50%, the strength of the steel material, which is the material of the torsion bar part, increases excessively. In this case, the cold workability (wire drawing workability and cold forging workability) of the steel material decreases. Therefore, the Si content is 0.50% or less. The preferable lower limit of the Si content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable 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 inevitable impurity. That is, the P content is more than 0%. P segregates at grain boundaries and reduces the cold workability of the steel material which is the material of the torsion bar parts. Therefore, the P content is 0.030% or less. It is preferable that the P content is as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable 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. That is, the S content is more than 0%. S forms MnS in the steel material which is the material of the torsion bar parts and improves the machinability of the steel material. If even a little S is contained, the above effect can be obtained to a certain extent. However, if the S content exceeds 0.050%, the MnS in the steel material coarsens and the cold workability of the steel material decreases. Therefore, the S content is 0.050% or less. The preferable lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable 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 inevitably contained. That is, the N content is more than 0%. Similar to C, N dissolves in the steel material which is the material of the torsion bar parts and suppresses the increase in torque accompanying the twisting of the torsion bar parts. If even a little N is contained, the above effect can be obtained to a certain extent. On the other hand, if the N content exceeds 0.0120%, the dissolved N in the torsion bar parts becomes excessive. In this case, local torsion during twisting breaks before it sufficiently propagates in the axial direction of the torsion bar parts. Therefore, the N content is 0.0120% or less. The preferable lower limit of the N content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferable 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 impurity inevitably contained. That is, the O content is more than 0%. O forms oxides and reduces the cold workability of the steel material which is the material of the torsion bar part. Therefore, the O content is 0.0100% or less. It is preferable that the O content is as low as possible. However, excessive reduction of the O content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable 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 balance of the chemical composition of the torsion bar part of the present embodiment consists of Fe and impurities. Here, the impurities are those mixed from ore, scrap, or the manufacturing environment, etc. as raw materials when industrially manufacturing the steel material which is the material of the torsion bar part, and are those allowed within a range not adversely affecting the torsion bar part of the present embodiment.

[0045] [Regarding Optional Elements] The chemical composition of the torsion bar component of this embodiment may further contain one or more 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%. Hereinafter, these optional elements will be described.

[0046] [Regarding the first group (Ti, Nb, Cr, Mo, and V)] The chemical composition of the torsion bar component of this embodiment may contain one or more selected from the group consisting of Ti, Nb, Cr, Mo, and V in place of a part of Fe. All of these elements form precipitates (carbides, nitrides, or carbonitrides) and increase the strength of the torsion bar component.

[0047] Ti: 0 to 0.040% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti forms precipitates that are carbides, nitrides, or carbonitrides and increase the strength of the torsion bar component. Even if a little Ti is contained, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.040%, excessive precipitates are generated. In this case, the cold workability of the steel material that is the material of the torsion bar component decreases. Therefore, the Ti content is 0 to 0.040%. The preferable lower limit of the Ti content is 0.001%, more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.010%. The preferable upper limit of the Ti content is 0.038%, more preferably 0.035%, and still more preferably 0.030%.

[0048] Nb: 0 to 0.040% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms precipitates that are carbides, nitrides, or carbonitrides, enhancing the strength of the torsion bar component. Even if a small amount of Nb is contained, the above effects can be obtained to a certain extent. However, if the Nb content exceeds 0.040%, excessive precipitates are generated. In this case, the cold workability of the steel material, which is the material of the torsion bar component, deteriorates. Therefore, the Nb content is 0 to 0.040%. The preferable lower limit of the Nb content is 0.001%, more preferably 0.003%, still more preferably 0.005%, and even more preferably 0.010%. The preferable upper limit of the Nb content is 0.038%, more preferably 0.035%, and still more preferably 0.030%.

[0049] Cr: 0 to 0.30% Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, Cr forms precipitates, enhancing the strength of the torsion bar component. Even if a small amount of Cr is contained, the above effects can be obtained to a certain extent. However, if the Cr content exceeds 0.30%, excessive precipitates are generated. In this case, the cold workability of the steel material, which is the material of the torsion bar component, deteriorates. Therefore, the Cr content is 0 to 0.30%. The preferable lower limit of the Cr content is 0.01%, more preferably 0.02%, and still more preferably 0.03%. The preferable upper limit of the Cr content is 0.28%, more preferably 0.25%, and still more preferably 0.20%.

[0050] Mo: 0 to 0.20% Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, Mo forms precipitates and increases the strength of the torsion bar part. If even a small amount of Mo is contained, the above effect can be obtained to a certain extent. However, if the Mo content exceeds 0.20%, excessive precipitates are generated. In this case, the cold workability of the steel material, which is the material of the torsion bar part, decreases. Therefore, the Mo content is 0 to 0.20%. The preferable lower limit of the Mo content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable 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 may not be contained. That is, the V content may be 0%. When contained, V forms precipitates and increases the strength of the torsion bar part. If even a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content exceeds 0.20%, excessive precipitates are generated. In this case, the cold workability of the steel material, which is the material of the torsion bar part, decreases. Therefore, the V content is 0 to 0.20%. The preferable lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable 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 part of the present embodiment may contain one or more selected from the group consisting of Ca and Mg in place of a part of Fe. All of these spheroidize MnS and improve the hot workability of the steel material, which is the material of the torsion bar part.

[0053] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca spheroidizes MnS and improves the hot workability of the steel material which is the material of the torsion bar part. If even a little Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse oxide inclusions are generated. In this case, the cold workability of the steel material deteriorates. Therefore, the Ca content is 0 to 0.0050%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferable 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 may not be contained. That is, the Mg content may be 0%. When contained, Mg spheroidizes MnS and improves the hot workability of the steel material which is the material of the torsion bar part. If even a little Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, coarse oxide inclusions are generated. In this case, the cold workability of the steel material deteriorates. Therefore, the Mg content is 0 to 0.0050%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0055] [Regarding Group 3 (Cu, Ni, and Sn)] The chemical composition of the torsion bar component of this embodiment may contain one or more selected from the group consisting of Cu, Ni, and Sn instead of a part of Fe. All of these enhance the corrosion resistance of the torsion bar component.

[0056] Cu: 0 to 0.40% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the corrosion resistance of the torsion bar component. Even if a little 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, which is the material of the torsion bar component, will decrease. Therefore, the Cu content is 0 to 0.40%. The preferable lower limit of the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable 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 may not be contained. That is, the Ni content may be 0%. When contained, Ni enhances the corrosion resistance of the torsion bar component. Even if a little Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.40%, the above effect will saturate. If the Ni content exceeds 0.40%, furthermore, the raw material cost will increase and the manufacturability will also decrease. Therefore, the Ni content is 0 to 0.40%. The preferable lower limit of the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable 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 may not be contained. That is, the Sn content may be 0%. When contained, Sn enhances the corrosion resistance of the steel material. Even if 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 will decrease. Therefore, the Sn content is 0 to 0.100%. The preferable lower limit of the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the Sn content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0059] [Regarding Group 4 (B)] The chemical composition of the torsion bar component of this embodiment may contain B in place of a part of Fe.

[0060] B: 0 to 0.0050% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B strengthens the grain boundaries and enhances the cold workability of the steel material which is the material of the torsion bar component. Even if 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 will generate excessive nitrides. In this case, the cold workability of the steel material will rather decrease. Therefore, the B content is 0 to 0.0050%. The preferable lower limit of the B content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferable 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 selected from the group consisting of Bi, Se, Te, and Pb instead of a part of Fe. All of these elements enhance the machinability of the steel material that serves as the material for the torsion bar component.

[0062] Bi: 0 to 0.100% Bismuth (Bi) is an optional element and may not be contained. That is, the Bi content may be 0%. When contained, Bi enhances the machinability of the steel material that is the material of the torsion bar component. If even a little 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 deteriorates. Therefore, the Bi content is 0 to 0.100%. The preferable lower limit of the Bi content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable 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 may not be contained. That is, the Se content may be 0%. When contained, Se enhances the machinability of the steel material that is the material of the torsion bar component. If even a little Se is contained, the above effect can be obtained to some extent. However, if the Se content exceeds 0.100%, the above effect saturates and the raw material cost increases. Therefore, the Se content is 0 to 0.100%. The preferable lower limit of the Se content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable 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 may not be contained. That is, the Te content may be 0%. When contained, Te improves the machinability of the steel material which is the material of the torsion bar part. If even a little 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 deteriorates. Therefore, the Te content is 0 to 0.050%. The preferable lower limit of the Te content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable 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 may not be contained. That is, the Pb content may be 0%. When contained, Pb improves the machinability of the steel material which is the material of the torsion bar part. If even a little 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 deteriorates. Therefore, the Pb content is 0 to 0.090%. The preferable lower limit of the Pb content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the Pb content is 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0066] [(Feature 2) Regarding ferrite area ratio] In the torsion bar part of this embodiment, furthermore, the ferrite area ratio is 75% or more.

[0067] If the ferrite area ratio is 75% or more, sufficient torsion occurs during twisting. The 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, and is, 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 the torsion bar component is determined by the following method. Referring to FIG. 3, in the shaft portion 20 of the torsion bar component, a test piece including a cross section perpendicular to the axial direction of the shaft portion 20 (the cross-sectional shape is circular) is taken. The cross section in the test piece is defined as the observation surface 25. The observation surface 25 is etched with a 3 volume% nital solution to reveal the metal structure. Referring to FIG. 4, when the diameter of the observation surface 25 is D, at a depth position of D / 4 in the radial direction from the surface corresponding to the outer peripheral surface of the shaft portion 20 on the observation surface 25 of the test piece, four locations with a 90° pitch around the central axis of the observation surface 25 are defined as the observation fields of view 26. The rectangular observation fields of view 26 are observed using an optical microscope at a magnification of 500 times. The short side of each observation field of view 26 is 174 μm in the radial direction, and the long side is 232 μm in a direction perpendicular to the short side.

[0069] In each observation field of view, ferrite can be easily identified by contrast. In an optical micrograph, ferrite is observed as a region with high brightness (white) where no substructure is confirmed. Pearlite is observed as a striped region. Cementite is observed as a region where no substructure is observed and has a lower brightness than ferrite. Precipitates and inclusions are observed as regions with a lower brightness than ferrite.

[0070] In the four observation fields of view 26, ferrite is specified. Based on the total area of the four observation fields of view and the total area of the specified ferrite, the ferrite area ratio (%) is determined. The ferrite area ratio is a value obtained by rounding off the numerical value of the first decimal place of the obtained value (i.e., an integer value).

[0071] [(Characteristic 3) Regarding Fn] In the torsion bar component of the present embodiment, further, 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 (Q -1 ) max of the internal friction measured in the range of -50 to 150 °C are used, and Fn defined by Equation (1) is 1.0×10 -4 to 15.0×10 -4 . Fn = β × (Q -1 ) max (1)

[0072] The half-width β (°) correlates with the amount of strain in the torsion bar component. That is, the half-width β is an index of the amount of strain in the torsion bar component. The maximum value (Q -1 ) max of the internal friction means the maximum value of the Snoek Peak among the internal frictions 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 (Q -1 ) max of the internal friction correlates with the total amount of dissolved C and dissolved N in the torsion bar component. That is, the maximum value (Q -1 ) max of the internal friction is an index of the total amount of dissolved C and dissolved N in the torsion bar component. In the following description, the maximum value (Q -1 ) max will be simply referred to as (Q -1 ) max as well.

[0073] Fn is composed of the product of the half-width β (°) and (Q -1 ) max and correlates with dislocation movement. Therefore, Fn correlates with the occurrence and axial propagation degree of local torsion during twisting. If Fn is less than 1.0×10 -4 , uniform torsion occurs without local torsion occurring in the shaft portion 20 during twisting. As a result, the increase in torque accompanying twisting cannot be sufficiently suppressed. On the other hand, if Fn is 15.0×10 -4If it exceeds this value, excessive local torsion in the circumferential direction will occur in the shaft portion 20. As a result, during twisting, the torsion bar component will break before the local torsion propagates in the axial direction of the shaft portion 20 of the torsion bar component.

[0074] If Fn is 1.0×10 -4 ~15.0×10 -4 then the amount of strain and the total amount of dissolved C and dissolved N are appropriate. Therefore, local torsion occurs in the shaft portion 20 during twisting, and the local torsion propagates in the axial direction of the shaft portion 20. As a result, an increase in torque associated with twisting can be sufficiently suppressed.

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

[0076] Note that Fn is a value calculated using the half-width β (°) with two significant figures after the decimal point and (Q -1 ) max with five significant figures after the decimal point, and rounding off the sixth digit after the decimal point (that is, the significant figures are five digits after the decimal point).

[0077] [Measurement method of half-width β] The half-width β (°) of the torsion bar component of the present embodiment is measured by the X-ray diffraction method (XRD) in the following manner.

[0078] Referring to FIG. 5, in the shaft portion 20 of the torsion bar component, a test piece 10 is taken with an observation surface 27 being a cross-section that includes the central axis of the shaft portion 20 and is parallel to the central axis. The observation surface 27 of the test piece 10 is wet-polished with emery paper (#400 - #1500, SiC waterproof abrasive paper). Further, the observation surface 27 of the test piece 10 is polished with a polishing cloth impregnated with a diamond suspension to finish the observation surface to a mirror surface. The diamond suspension is a liquid in which diamond powder with a particle size of 1 - 6 μm is dispersed in a diluent such as alcohol or pure water. After polishing with the diamond suspension, the observation surface 27 is further polished using colloidal silica. Thereby, the processed and metamorphic layer on the surface layer of the observation surface 27 of the test piece 10 is removed.

[0079] As shown in FIG. 5, among the polished observation surfaces 27, at a depth position of D / 4 (where D is the diameter of the shaft portion 20) from one surface corresponding to the outer peripheral surface of the shaft portion 20 of the torsion bar component, and parallel to the central axis of the shaft portion 20, nine measurement points α arranged at a pitch of 1 mm are selected. Further, the nine positions that are line-symmetrical with respect to the nine selected measurement points α and with the central axis of the shaft portion 20 as the axis of symmetry are also selected as the measurement points α. That is, 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 method (XRD). The half-width of the X-ray diffraction peak corresponding to the (211) plane of α-Fe is measured from the obtained X-ray diffraction peak. Here, the half-width means the full width at half maximum (FWHM). The X-ray diffraction peak corresponding to the (211) plane of α-Fe appears in the range of 150 - 170° when the measurement by the X-ray diffraction method is performed under the following conditions.

[0081] The diffraction profile obtained by X-ray diffraction measurement is pre-processed according to the following procedure.

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

[0083] Subsequently, using the intensity at the scanning start angle and the intensity at the scanning end angle of the X-ray diffraction measurement, a straight line is drawn between the intensity at the measurement start angle and the intensity at the measurement end angle. Then, with this straight line as the background, it is subtracted from the smoothed diffraction profile.

[0084] For the diffraction profile from which the background has been subtracted, the Rachinger method described in Non-Patent Document 2 is applied to remove the X-ray diffraction peak corresponding to the Kα2 line 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 line to the X-ray diffraction peak corresponding to the Kα2 line is set to 2:1. From the diffraction profile obtained by the above procedure, the FWHM is determined. The determined 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 above 18-point measurement is regarded as one X-ray diffraction measurement. At the same measurement position, the above measurement is repeated three times. The arithmetic mean value of the half-widths obtained by the three measurements is taken as the half-width β (°). For the X-ray diffractometer, for example, the product name: AutoMATE manufactured by Rigaku Corporation is used. The measurement conditions for X-ray diffraction are as follows. (Measurement conditions for X-ray diffraction) X-ray source: CrKα Accelerating voltage: 40 kV Accelerating current: 40 mA Collimator diameter: φ1 mm Counting time at each measurement point α: 90 seconds X-ray scanning range (scanning start angle ~ scanning end angle): 146° ~ 165°

[0086] [Measurement method of the maximum value of internal friction (Q -1 ) max The (Q -1 ) max of this embodiment is measured by the following method. Figure 6A shows the maximum value of internal friction (Q -1 ) max ​It is a perspective view of a torsion bar component for explaining the sampling position of a test piece for measurement. FIG. 6B is a cross-sectional view perpendicular to the axial direction (longitudinal direction) of the shaft portion of the torsion bar component shown in FIG. 6A. Referring to FIGS. 6A and 6B, in a cross-section (circular shape) perpendicular to the axial direction of the shaft portion 20 of the torsion bar component, the test piece 28 is sampled from a position D / 4 in the radial direction from the surface of the shaft portion 20 (where D is the diameter of the shaft portion 20). The size of the test piece 28 is a plate shape with a length of 105 mm × a width of 5 mm × a thickness of 0.7 mm. The longitudinal direction of the test piece 28 is parallel to the axial direction of the shaft portion 20. Referring to FIG. 6B, in a cross-section perpendicular to the axial direction of the shaft portion 20, the test piece 28 is sampled such that the normal line N28 at the center of the width of the surface 28A with a width of 5 mm and extending in the axial direction of the shaft portion 20 is in the radial direction of the cross-section. Note that if the cross-sectional area of the test piece 28 is constant (width 5 mm × thickness 0.7 mm), the length of the test piece 28 may be shortened. The length of the test piece 28 may be the same as the length of the torsion bar component.

[0087] Using a low-frequency torsional pendulum type internal friction measurement device, the (Q -1 ) max is obtained by the free vibration decay method. Specifically, the test piece 28 is attached to a low-frequency torsional pendulum type internal friction measurement device. Next, torsion is applied to the test piece 28 by an excitation coil, and the test piece 28 is self-excited 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. Then, it is shifted to free decay vibration and the vibration is recorded. The logarithmic decrement δ is obtained from the recorded vibration and converted to the internal friction Q -1 . The internal friction Q -1 is converted from the logarithmic decrement δ by the following formula. Q -1 = δ / π Here, π represents the circumference ratio.

[0088] The above operations are performed while raising the temperature at 1.0 °C / min in the range of -50 to 150 °C, and a graph of the temperature and the internal friction Q -1 is obtained. The maximum value (that is, the snake peak) is read from the obtained graph, and the value obtained by subtracting the background value is (Q -1 ) maxLet's assume. Specifically, as shown in FIG. 7A, a straight line connecting a pair of bottoms BT of the obtained graph (peak waveform graph) SP is defined as the baseline (background numerical value) BL. The internal friction Q of the baseline BL -1 is subtracted, and the graph SP is corrected as shown in FIG. 7B. From the corrected graph SP, the maximum value (snake peak) X1 is read, and the obtained maximum value X1 is (Q -1 ) max Let's assume. In addition, for example, IFM-1500L manufactured by Vacuum Riko Co., Ltd. (currently Advance Riko Co., Ltd.) can be used as the low-frequency torsional pendulum type internal friction measuring device.

[0089] [Regarding the effects of the torsion bar parts of this embodiment] The torsion bar parts of this embodiment satisfy Features 1 to 3. Therefore, in the torsion bar parts of this embodiment, the increase in torque accompanying twisting can be sufficiently suppressed.

[0090] [Regarding the size and application of the torsion bar parts of this embodiment] The size of the torsion bar parts of this embodiment is not particularly limited. The diameter of the shaft portion 20 of the torsion bar parts 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 (axial length of the torsion bar parts) is, for example, 2 to 10 mm. The torsion bar parts of this embodiment are suitable for use in seat belt retractors.

[0091] [Manufacturing method of the torsion bar parts of this embodiment] Hereinafter, the manufacturing method of the torsion bar parts of this embodiment will be described. The torsion bar parts satisfying Features 1 to 3 may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferred example of the manufacturing method of the torsion bar parts 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 (Process 3) Wire Drawing Process (Process 4) Cold Forging Process In Process 1 and Process 2, a steel material that serves as the raw material for the torsion bar component is manufactured. In Process 3 and Process 4, the torsion bar component is manufactured. Hereinafter, each process will be described.

[0093] [(Process 1) Raw Material Manufacturing Process] In the raw material manufacturing process, first, molten steel having a chemical composition that satisfies Feature 1 is manufactured. Using the manufactured molten steel, an ingot or bloom is manufactured by a well-known casting method. For example, an ingot is manufactured by the ingot casting method, or a slab (bloom) is manufactured by the continuous casting method. Through the above processes, the raw material (ingot or bloom) is manufactured.

[0094] [(Process 2) Hot Working Process] In the hot working process, hot working is performed on the manufactured raw material to manufacture a steel material. The steel material is, for example, a bar or a wire rod. The hot working process includes a cogging rolling process, a finishing rolling process, and a cooling process.

[0095] [Cogging Rolling Process] In the cogging rolling process, using a cogging rolling mill, the heated raw material is hot rolled (cogging rolled) to manufacture a billet. When a continuous rolling mill is installed downstream of the cogging rolling mill, the billet after cogging rolling may be further hot rolled using the continuous rolling mill to manufacture an even smaller-sized billet. The heating temperature in the heating furnace in the cogging rolling process is not particularly limited, but is, for example, 1000 to 1300 °C.

[0096] [Finishing Rolling Process] In the finishing rolling process, using a continuous rolling mill, the billet after cogging rolling is hot rolled to manufacture a steel material. The heating temperature in the heating furnace in the finishing rolling process 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 a predetermined cooling stop temperature (°C). Specifically, boiling water cooling is carried out by immersing the steel material exiting the rolling stand where the final reduction is carried out in finish rolling into boiling water at 100°C. The temperature of the steel material when it 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: 500~200°C The cooling stop temperature can be obtained, for example, by measuring the temperature of the steel material immediately after it is pulled out of the boiling water with a thermometer. The thermometer is, for example, a thermograph, a radiation thermometer, etc.

[0098] The steel material is pulled out of the boiling water at the cooling stop temperature and cooled to room temperature. By boiling water cooling, C and N in the steel material are in a supersaturated solid solution state. In this state, boiling water cooling is stopped and the steel material is cooled. In this case, reheating occurs in the steel material and the steel material temperature rises. As a result, a part of the dissolved C and dissolved N in the steel material combines with Ti, Nb, and Fe to form precipitates (carbides, nitrides, carbonitrides). By carrying out boiling water cooling and switching from boiling water cooling to air cooling at the cooling stop temperature in the above temperature range, the amount of dissolved C and the amount of dissolved N in the steel material can be adjusted to an appropriate range.

[0099] If the cooling stop temperature (°C) is less than 200°C, sufficient reheating during air cooling cannot be obtained. In this case, precipitates are not sufficiently formed during air cooling. As a result, (Q -1 ) max becomes excessively large. On the other hand, if the cooling stop temperature (°C) exceeds 500°C, the supersaturation of dissolved C and dissolved N in the steel material during air cooling is insufficient. In this case as well, precipitates are not sufficiently formed during air cooling. As a result, (Q -1 ) max becomes excessively large.

[0100] Through the above manufacturing process, the steel material that is the raw material of the torsion bar part of the present embodiment is manufactured. In the manufacturing process of the steel material, after the hot working process, a cold rolling process using work rolls is not carried out.

[0101] [(Process 3) Wire drawing process] In the wire drawing process, wire drawing is performed on a steel material to produce a steel wire. The area reduction rate RR in wire drawing is 3 to 20%. Here, the area reduction rate RR (%) is defined by the following formula. Area reduction rate RR = (1 - (area of the cross-section perpendicular to the axial direction of the steel wire after wire drawing / area of the cross-section perpendicular to the axial direction of the steel material before wire drawing)) × 100 When wire drawing is performed multiple times, the area reduction rate RR is the cumulative area reduction rate (%) in the multiple wire drawing processes.

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

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

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

[0105] Through the above steps, the torsion bar parts of this embodiment are manufactured.

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

Example

[0107] The effects of the torsion bar parts of this embodiment will be further specifically described by way of examples. The conditions in the following examples are one set of conditions adopted to confirm the feasibility and effects of the torsion bar parts of this embodiment. Therefore, the torsion bar parts of this embodiment are not limited to this one set of conditions.

[0108] Steel materials having the chemical compositions shown in Table 1A and Table 1B were manufactured. In the following description, Table 1A and Table 1B are also collectively referred to as "Table 1". The "-" in Table 1 indicates that the corresponding element content is below the impurity level.

[0109]

Table 1A

[0110]

Table 1B

[0111] Specifically, blooms were manufactured by continuous casting using molten steel. A blooming process was performed on the manufactured blooms to produce billets. Specifically, the blooms were heated to 1200 °C using a heating furnace. The heated blooms were hot-rolled using a blooming mill and a continuous rolling mill to produce billets. The billets produced in the blooming process were allowed to cool to room temperature.

[0112] For the manufactured billets, a finishing rolling process was carried out. Specifically, the billets were heated to 1100 °C. For the heated billets, finishing rolling (continuous rolling) was carried out using a continuous rolling mill to produce steel materials (wire rods).

[0113] For the steel materials after finishing rolling, a cooling process was carried out. Specifically, for the steel materials immediately after finishing rolling, boiling water cooling by immersing them in boiling water at 100 °C was carried out. The steel materials were pulled out from the boiling water at the cooling stop temperature (°C) shown in Table 2 and then air-cooled. In Tests No. 33 and 34, boiling water cooling was not carried out, and the steel materials after finishing rolling were air-cooled (indicated by "-" in the "Cooling stop temperature (°C)" column in Table 2). Through the above manufacturing process, the steel materials of each test number were manufactured.

[0114]

Table 2

[0115] Using the manufactured steel materials, torsion bar parts were manufactured. For the steel materials of each test number shown in Table 1, wire drawing was carried out at the reduction ratio RR (%) described in Table 2 to produce steel wires. Further, for the steel wires, upsetting was carried out at the cross-sectional increase rate DR (%) shown in Table 2 to manufacture torsion bar parts with the shape shown in Figure 2. The FA of each test number based on the reduction ratio RR (%) and the cross-sectional increase rate DR (%) is shown in Table 2. In Tests No. 37 and 38, the wire drawing process and the cold forging process were not carried out, and the torsion bar parts were manufactured by cutting. Through the above manufacturing process, torsion bar parts with the shape shown in Figure 2 were manufactured. The torsion bar parts 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 shape of the flange portion 30 as viewed from the axial direction of the torsion bar part was hexagonal, and the width W was 10.4 mm. The diameter of the shaft portion 20 was 9.25 mm. Three torsion bar parts of each test number were manufactured.

[0116] For the torsion bar parts of each test number, the ferrite area ratio was measured 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 Test] The following evaluation tests were conducted on the torsion bar parts of each test number. (Test 1) Measurement Test of Half-Width β (Test 2) Maximum Value of Internal Friction (Q -1 ) max Measurement Test (Test 3) Evaluation Test of Torque Increase Amount Associated with Twisting The following describes Tests 1 to 3.

[0118] [(Test 1) Evaluation Test of Half-Width β] Based on the method described in the above-mentioned [Method for Measuring Half-Width β], the half-width β (°) of the torsion bar parts of each test number was determined. As the X-ray diffractometer, the product name: AutoMATE manufactured by Rigaku Corporation was used. For the calculation of the half-width, the software attached to the X-ray diffractometer was used. The obtained half-width β (°) is shown in Table 2.

[0119] [(Test 2) Measurement Test of Maximum Value of Internal Friction (Q -1 ) max Measurement Test The (Q -1 ) max of the torsion bar parts of each test number was evaluated by the following method. Based on the method described in the above-mentioned [Method for Measuring Maximum Value of Internal Friction (Q -1 ) max the (Q -1 ) max of each test number was determined. For the measurement, IFM-1500L manufactured by Vacuum Riko Co., Ltd. (currently Advance Riko Co., Ltd.) was used. Also, the frequency of torsional vibration was approximately 1.3 Hz. The obtained (Q -1 ) max is shown in the column of "(Q -1 ) max (×10 -4 )" in Table 2, and Fn (= β × (Q -1 )max ) is shown in the column of "Fn(×10 -4 )" in Table 2.

[0120] [(Test 3) Torque Increase Amount Evaluation Test with Twisting] The torque increase amount associated with the twisting of the torsion bar parts for each test number was evaluated by the following twisting test. Three torsion bar parts for each test number were manufactured by the above manufacturing method.

[0121] Using the torsion bar parts for each test number, the following twisting test was conducted for evaluation. Specifically, as shown in Fig. 8, the flange portion 30 at one end of the torsion bar part was fixed with a fixing jig 40, and the flange portion 30 at the other end was fixed to a rotating jig 50. Then, the other end fixed to the rotating jig 50 was rotated at 5000 revolutions per minute. At this time, the torque (N·m) and the twist angle (°) were measured. The test temperature was room temperature (20 ± 5°C). Based on the measurement results, a graph (torque-twist angle diagram) with the vertical axis representing torque (N·m) and the horizontal axis representing twist angle (°) was created. For the torque data, an analog filter was applied to remove high-frequency components. As the analog filter, a Bessel-type low-pass filter (cutoff frequency: 300 Hz, attenuation slope: -48 dB / oct) was used.

[0122] Based on the created graph, the yield torque T y (N·m) was obtained. The yield torque T y was obtained by applying mutatis mutandis the method for obtaining the yield strength described in "13 Yield Strength (Offset Method) Rp" of JIS Z 2241:2022 to the twisting test. Specifically, as shown in Fig. 9, the yield torque T y was obtained by drawing a line parallel to the straight part of the torque-twist angle diagram at a distance equal to the specified plastic twist angle. The intersection of this parallel line and the torque-twist angle diagram was taken as the yield torque T y to be obtained. The specified plastic twist angle was 45°.

[0123] Furthermore, from the created graph, the torque T 1260 (N·m) at a twist angle of 1260° was obtained. The obtained yield torque T yand torque T 1260 Based on this, the torque increase amount ΔT was obtained by the following formula: ΔT = T y / T 1260

[0124] When ΔT was 0.85 or more, it was determined that the increase in torque associated with twisting was sufficiently suppressed (denoted as "E" (Excellent) in Table 2). On the other hand, when ΔT was less than 0.85, it was determined that the increase in torque associated with twisting could not be sufficiently suppressed (denoted as "NA" (Not Accepted) in Table 2). In addition, when local torsion did not propagate and broke, and torque T 1260 could not be measured, since the 1260° twist itself could not be obtained in the first place, it was also determined in this case that the increase in torque associated with twisting could not be sufficiently suppressed. The evaluation results are shown in the "ΔT" column of Table 2.

[0125] [Evaluation Results] Referring to Tables 1 to 2, the torsion bar parts with test numbers 1 to 28 satisfied Features 1 to 3. Therefore, in the torsion bar parts with these test numbers, the increase in torque associated with twisting could be sufficiently suppressed.

[0126] On the other hand, in test numbers 29 and 30, the cooling stop temperature was too high. Therefore, Fn exceeded 15.0×10 -4 . As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so torque T 1260 could not be obtained. As a result, the increase in torque associated with twisting could not be sufficiently suppressed.

[0127] In test numbers 31 and 32, the cooling stop temperature was too low. Therefore, Fn exceeded 15.0×10 -4 . As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so torque T 1260 could not be obtained. As a result, the increase in torque associated with twisting could not be sufficiently suppressed.

[0128] In Tests Nos. 33 and 34, the cooling process was not carried out. Therefore, Fn exceeded 15.0×10 -4 . As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so the torque T 1260 could not be obtained. Consequently, the increase in torque accompanying the twisting could not be sufficiently suppressed.

[0129] In Tests Nos. 35 and 36, the area reduction rate RR(%) was too high. Therefore, Fn exceeded 15.0×10 -4 . As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so the torque T 1260 could not be obtained. Consequently, the increase in torque accompanying the twisting could not be sufficiently suppressed.

[0130] In Tests Nos. 37 and 38, the wire drawing process and the cold forging process were not carried out. Therefore, Fn was less than 1.0×10 -4 . As a result, the increase in torque accompanying the twisting could not be sufficiently suppressed.

[0131] In Tests Nos. 39 and 40, the area increase rate DR(%) was too high. Therefore, Fn exceeded 15.0×10 -4 . As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so the torque T 1260 could not be obtained. Consequently, the increase in torque accompanying the twisting could not be sufficiently suppressed.

[0132] In Tests Nos. 41 and 42, FA was too high. Therefore, Fn exceeded 15.0×10 -4 . As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so the torque T 1260 could not be obtained. Consequently, the increase in torque accompanying the 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 carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A torsion bar component, whose chemical composition is by 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%, 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 consists of Fe and impurities, the ferrite area ratio is 75% or more, The half-value 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 the internal friction measured in the range of -50 to 150 °C (Q －１ ) ｍａｘ Using these, Fn defined by Equation (1) is 1.0×10 －４ to 15.0×10 －４ is the torsion bar component. Fn = β × (Q －１ ) ｍａｘ (1)

2. The torsion bar component according to Claim 1, whose chemical composition is by mass%, 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 0.0050%, 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%, and contains one or more selected from the group consisting of, the torsion bar component.

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