Steel materials, steel wires and torsion bar parts

Abstract

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

Description

Technical Field

[0001] The present disclosure relates to steel materials, steel wires, and torsion bar parts, and more particularly, to steel materials, steel wires, and torsion bar parts suitable as materials for torsion bar parts for seat belt retractors.

Background Art

[0002] A seat belt retractor installed in a vehicle 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 flying 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 relaxing the impact, the seat belt retractor includes a torsion bar part. When an excessive load acts on the occupant through the seat belt, the torsion bar part 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 as the material of the torsion bar part. Here, excellent torsion characteristics mean that when the steel material is twisted, the torsion angle until it breaks is large.

[0004] Techniques for improving the torsion characteristics of the steel material used as the material of the torsion bar part have been proposed in Japanese Patent Application Laid-Open No. 2009-120906 (Patent Document 1), Japanese Patent Application Laid-Open No. 2009-120907 (Patent Document 2), and Japanese Patent Application 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%, 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 unavoidable 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 unavoidable impurities, and the microstructure is a structure having spherical carbides throughout the ferrite matrix, and the degree of the spheroidized structure 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, when a torsion test is carried out at -40°C, the number of torsion cycles until the steel material breaks can be increased.

[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. Furthermore, the ferrite crystal grain size is refined to increase the initial torque during torsion. Patent Document 3 describes that this can sufficiently absorb impact energy.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] In Patent Documents 1 and 2, the problem is to enhance the torsional characteristics by increasing the number of torsions until fracture in the torsion test. In Patent Document 3, in addition to enhancing the torsional characteristics, the problem is to increase the initial torque during torsion.

[0011] As described above, when the vehicle body decelerates rapidly, the torsion bar component twists circumferentially. At this time, as the twist angle (number of twists) increases, the torque required for the torsion bar component to undergo torsional deformation 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 for the torsion bar component to be able to suppress the increase in torque associated with twisting.

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

Means for Solving the Problems

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

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

Effects of the Invention

[0015] In the steel material, steel wire, and torsion bar parts of the present disclosure, an increase in torque associated with the number of twists can be suppressed.

Brief Description of the Drawings

[0016]

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MODE FOR CARRYING OUT THE INVENTION

[0017] The present inventors have studied steel materials capable of suppressing an increase in torque accompanying twisting.

[0018] First, the present inventors examined steel materials suitable for torsion bar part applications from the viewpoint of chemical composition. As a result, in terms of mass%, C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, Ti: 0 to 0.040%, Nb: 0 to 0.040%, Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca: 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 present inventors considered that it was suitable for torsion bar part applications.

[0019] Furthermore, the present inventors examined means capable of suppressing an increase in torque accompanying twisting in the steel materials having the above chemical composition. Specifically, using the materials having the above chemical composition, steel materials were manufactured by various manufacturing methods. Then, a twisting test was performed on the steel materials. As a result, the present inventors found the following matters.

[0020] FIG. 1 is a graph of the torsional angle (°) and torque (N·m) obtained by performing a torsion test using a steel material for torsion bar parts. Referring to FIG. 1, the graph S1 indicated by the dashed line is the result of a torsion test using a conventional steel material for torsion bar parts. The graph S2 indicated by the solid line is the result of performing a torsion test using a steel material different from that of graph S1.

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

[0022] Here, the present inventors observed the appearance of the steel material of graph S1 after the torsion test and the steel material of graph S2 after the torsion test. As a result, in the steel material of graph S1, the entire steel material was uniformly twisted, and uniform torsion occurred. On the other hand, in the steel material of graph S2, it was locally twisted, and local torsion occurred at the ends of the steel material. From the above results, the present inventors considered that local torsion is related to the suppression of the increase in torque.

[0023] Based on the above results, the present inventors considered as follows. When the steel material is twisted, local torsion is generated at a certain location of the steel material. Before the plastic deformation in the circumferential direction of the steel material at the said 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 steel material (i.e., to the axially adjacent position of the said location). If such plastic deformation behavior can be realized, the increase in torque can be suppressed while the local torsion propagates in the axial direction (longitudinal direction) of the steel material. That is, the increase in torque accompanying torsion can be suppressed.

[0024] Based on the above findings, the inventors further studied means for generating local torsion during twisting while causing the local torsion to propagate in the axial direction of the steel material. Here, the inventors focused on the dissolved C and dissolved N in the steel material. 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, it may be possible to control the behavior of local torsion.

[0025] Based on the above considerations, the inventors thought as follows. Among the above chemical compositions, Ti and Nb combine with C and / or N to form carbides, nitrides or carbonitrides. Therefore, if an appropriate amount of Ti precipitates and Nb precipitates (carbides, nitrides and carbonitrides) are formed by Ti and Nb, the amounts of dissolved C and dissolved N can be adjusted to an appropriate level. In this case, when the steel material is twisted, appropriate local torsion can be generated in the steel material, and further, the local torsion can be propagated in the axial direction of the steel material. As a result, an increase in torque associated with twisting can be suppressed.

[0026] Based on the above findings, the inventors studied the relationships among the Ti content, Nb content, C content and N content in the above chemical composition, and further studied the total amounts of dissolved C and dissolved N for realizing the propagation of the above local torsion in the axial direction of the steel material. As a result, in the above chemical composition, further, at least one of Ti and Nb is an essential element, and further, [Ti*] defined by formula (1) and [Nb*] defined by formula (2) are adjusted to satisfy formula (3), and further, the maximum value of the internal friction (Q -1 ) max measured in the range of -50 to 150 °C is made to be 1.2×10 -4 ~10.0×10 -4 , and it was found that an increase in torque associated with twisting can be sufficiently suppressed. [Ti*]=Ti / {(48 / 12)C+(48 / 14)N} (1) [Nb*]=Nb / {(93 / 12)C+(93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, in each of the element symbols in formulas (1) and (2), the content in mass % of the corresponding element in the chemical composition is substituted.

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

[0028] The steel material of the first configuration has a chemical composition in mass % of C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, Sn: 0 to 0.100%, B: 0 to 0.0050%, Bi: 0 to 0.100%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.090%. Further, it contains one or two selected from the group consisting of Ti: 0.003 to 0.040% and Nb: 0.003 to 0.040%. The balance consists of Fe and impurities. The ferrite area ratio is 90% or more, and [Ti*] defined by formula (1) and [Nb*] defined by formula (2) satisfy formula (3). The maximum value (Q -1 ) max of the internal friction measured in the range of -50 to 150 °C is 1.2×10 -4 ~10.0×10 -4 is. [Ti*] = Ti / {(48 / 12)C + (48 / 14)N} (1) [Nb*] = Nb / {(93 / 12)C + (93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, in each of the element symbols in formulas (1) and (2), the content in mass % of the corresponding element in the chemical composition is substituted.

[0029] The steel material of the second configuration is the steel material of the first configuration, and its chemical composition is, by mass%, 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 these.

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

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

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

[0033] [Features of the steel material of this embodiment] The steel material of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition, by mass%, contains C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, Cr: 0 to 0.30%, Mo: 0 to 0.20%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, Sn: 0 to 0.100%, B: 0 to 0.0050%, Bi: 0 to 0.100%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.090%, and further contains one or two selected from the group consisting of Ti: 0.003 to 0.040% and Nb: 0.003 to 0.040%, and the balance consists of Fe and impurities. (Characteristic 2) The ferrite area ratio is 90% or more. (Characteristic 3) [Ti*] defined by formula (1) and [Nb*] defined by formula (2) satisfy formula (3). [Ti*]=Ti / {(48 / 12)C+(48 / 14)N} (1) [Nb*]=Nb / {(93 / 12)C+(93 / 14)N} (2) [Ti*]+[Nb*]<1.00 (3) Here, for each element symbol in formula (1) and formula (2), the content in mass% of the corresponding element in the chemical composition is substituted. (Characteristic 4) The maximum value (Q -1 ) max of the internal friction measured in the range of -50 to 150 °C is 1.2×10 -4 ~10.0×10 -4 . The following describes Characteristics 1 to 4.

[0034] [(Characteristic 1) Regarding the chemical composition] The steel material of this embodiment contains the following elements.

[0035] C: 0.006 to 0.050% Carbon (C) dissolves in the steel material, causing local torsion to occur when the steel material is twisted and propagating the local torsion in the axial direction. This suppresses the increase in torque associated with the twist. If the C content is less than 0.006%, the above effect cannot be obtained sufficiently. On the other hand, if the C content exceeds 0.050%, the dissolved C in the steel material becomes excessive. In this case, the local torsion during twisting breaks before it can propagate sufficiently in the axial direction of the steel material. Therefore, the C content is 0.006 - 0.050%. The preferable lower limit of the C content is 0.007%, more preferably 0.008%, and even more preferably 0.010%. The preferable upper limit of the C content is 0.048%, more preferably 0.044%, and even more preferably 0.039%.

[0036] Mn: 0.10 - 0.80% Manganese (Mn) increases the strength of the steel material by solid solution strengthening. Mn further fixes S in the steel material, enhancing 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 steel material becomes excessively high. In this case, the deformability during twisting decreases. Therefore, the Mn content is 0.10 - 0.80%. The preferable lower limit of the Mn content is 0.11%, more preferably 0.13%, and even more preferably 0.15%. The preferable upper limit of the Mn content is 0.78%, more preferably 0.75%, and even more preferably 0.70%.

[0037] Al: 0.005 - 0.080% Aluminum (Al) deoxidizes the steel. Al further combines with N in the steel material to form AlN. AlN refines the crystal grains due to the pinning effect. 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 effect cannot be obtained sufficiently. On the one hand, if the Al content exceeds 0.080%, the above effects will saturate. Moreover, if the Al content exceeds 0.080%, oxide inclusions will be excessively generated, and defects are likely to occur in the steel material during hot rolling. Therefore, the Al content is 0.005 - 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%.

[0038] Si: 0.50% or less Silicon (Si) is inevitably contained. That is, the Si content exceeds 0%. Si increases the strength of the steel material by solid solution strengthening. If Si is contained even slightly, the above effects can be obtained to a certain extent. However, if the Si content exceeds 0.50%, the strength of the steel material will increase excessively, and the cold workability (wire drawing workability and cold forging workability) will decrease. 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%.

[0039] P: 0.030% or less Phosphorus (P) is an impurity inevitably contained. That is, the P content exceeds 0%. P segregates at the grain boundaries and reduces the cold workability of the steel material. Therefore, the P content is 0.030% or less. The lower the P content, the better. 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%.

[0040] 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 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%.

[0041] 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 and suppresses the increase in torque accompanying twisting. If even a little N is contained, the above effect can be obtained to a certain extent. However, if the N content exceeds 0.0120%, the dissolved N in the steel material becomes excessively high. In this case, local torsion during twisting breaks before sufficiently propagating in the steel material axial direction. 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%.

[0042] O: 0.0100% or less Oxygen (O) is an inevitable impurity. That is, the O content is more than 0%. O forms oxides and reduces the cold workability of steel materials. 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%.

[0043] One or two selected from the group consisting of Ti: 0.003 to 0.040% and Nb: 0.003 to 0.040% Both titanium (Ti) and niobium (Nb) form carbides, nitrides or carbonitrides and adjust the amount of dissolved C and dissolved N in the steel material. Thereby, an increase in torque accompanying twisting is suppressed. When the Ti content is less than 0.003% or the Nb content is less than 0.003%, the above effects cannot be sufficiently obtained. On the other hand, when the Ti content exceeds 0.040% or the Nb content exceeds 0.040%, carbides, nitrides or carbonitrides of Ti and / or Nb are excessively generated. In this case, the maximum value of the internal friction (Q -1 ) max becomes excessively small. As a result, an increase in torque accompanying twisting cannot be sufficiently suppressed. Therefore, the chemical composition of the steel material of the present embodiment contains one or two selected from the group consisting of a Ti content: 0.003 to 0.040% and an Nb content: 0.003 to 0.040%. The chemical composition of the steel material of the present embodiment may contain Ti within the above range and may not contain Nb. It may not contain Ti and may contain Nb within the above range. It may contain Ti within the above range and Nb within the above range. The preferable lower limit of the Ti content is 0.005%, more preferably 0.008%, 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%. The preferable lower limit of the Nb content is 0.005%, more preferably 0.008%, and still 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%.

[0044] The balance of the chemical composition of the steel material of the present embodiment consists of Fe and impurities. Here, the impurities mean those mixed from ores, scraps as raw materials, or the manufacturing environment, etc. during the industrial production of the steel material, and are allowed within a range that does not adversely affect the steel material of the present embodiment.

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

[0046] [Regarding the First Group (Cr, Mo, and V)] The chemical composition of the steel material of the present embodiment may further contain one or more selected from the group consisting of 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 steel material.

[0047] 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 and increases the strength of the steel. If even 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 decreases. 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 even more preferably 0.03%. The preferable upper limit of the Cr content is 0.28%, more preferably 0.25%, and even more preferably 0.20%.

[0048] Mo: 0 to 0.20% Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, Mo forms precipitates and increases the strength of the steel. If even a small amount of Mo is contained, the above effects 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 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%.

[0049] 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 steel. If even a small amount of V is contained, the above effects can be obtained to a certain extent. However, if the V content exceeds 0.20%, excessive precipitates will form. In this case, the cold workability of the steel material will deteriorate. 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%.

[0050] [Regarding the second group (Ca and Mg)] The chemical composition of the steel material of the present embodiment may further 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.

[0051] 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. Even if a little Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse oxide inclusions will form. In this case, the cold workability of the steel material will deteriorate. 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%.

[0052] 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. Even if a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, coarse oxide inclusions are formed. 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%.

[0053] [Regarding the third group (Cu, Ni, and Sn)] The chemical composition of the steel material of the present embodiment may further contain one or more selected from the group consisting of Cu, Ni, and Sn in place of a part of Fe. All of these enhance the corrosion resistance of the steel material.

[0054] 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 steel material. Even if a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.40%, the steel material becomes brittle. Therefore, the hot workability and cold workability of the steel material deteriorate. 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%.

[0055] 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 steel material. Even if 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%, furthermore, the raw material cost increases and the manufacturability also decreases. 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%.

[0056] 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 a certain extent. However, if the Sn content exceeds 0.100%, the hot workability and cold workability of the steel material 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%.

[0057] [Regarding Group 4 (B)] The chemical composition of the steel material of the present embodiment may further contain B in place of a part of Fe.

[0058] 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 improves the cold workability of the steel material. Even if a small amount of B is contained, the above effect can be obtained to a certain extent. However, if the B content exceeds 0.0050%, B will excessively generate 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%.

[0059] [Regarding Group 5 (Bi, Se, Te, and Pb)] The chemical composition of the steel material of the present embodiment may further 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 improve the machinability of the steel material.

[0060] 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 improves the machinability of the steel material. Even if a small amount of Bi is contained, the above effect can be obtained to a certain extent. However, if the Bi content exceeds 0.100%, the hot workability of the steel material will decrease. 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%.

[0061] 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 improves the machinability of the steel material. Even if 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 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%.

[0062] 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. Even if a small amount of Te is contained, the above effect can be obtained to a certain 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%.

[0063] 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. Even if a small amount of Pb is contained, the above effect can be obtained to a certain extent. However, if the Pb content exceeds 0.090%, the hot workability of the steel material deteriorates. Therefore, the Pb content is from 0 to 0.090%. The preferable lower limit of the Pb content is 0.001%, more preferably 0.002%, and still more preferably 0.003%. The preferable upper limit of the Pb content is 0.080%, more preferably 0.060%, and still more preferably 0.040%.

[0064] [(Feature 2) Regarding the ferrite area ratio] In the steel material of this embodiment, furthermore, the ferrite area ratio is 90% or more.

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

[0066] [Method for measuring the ferrite area ratio] The ferrite area ratio in the metal structure of the steel material is determined by the following method. Referring to FIG. 2, a test piece including a cross-section perpendicular to the axial direction of the steel material (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. 3, 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 steel material on the observation surface 25 of the test piece, and at four locations with a 90° pitch around the central axis of the observation surface 25, they are defined as the observation fields of view 26. Using an optical microscope at a magnification of 500 times, the rectangular observation fields of view 26 are observed. 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 the direction perpendicular to the short side.

[0067] In each observation field, 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 lower brightness than ferrite. Precipitates and inclusions are observed as regions with lower brightness than ferrite.

[0068] In four observation fields 26, ferrite is identified. Based on the total area of the four observation fields and the total area of the identified ferrite, the ferrite area ratio (%) is determined. Note that the ferrite area ratio is a value obtained by rounding the numerical value of the first decimal place of the obtained value (i.e., an integer value).

[0069] [(Feature 3) Regarding Equations (1) to (3)] The steel material of this embodiment further satisfies [Ti*] defined by Equation (1) and [Nb*] defined by Equation (2) for Equation (3). [Ti*] = Ti / {(48 / 12)C + (48 / 14)N} (1) [Nb*] = Nb / {(93 / 12)C + (93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, for each element symbol in Equation (1) and Equation (2), the content in mass % of the corresponding element in the chemical composition is substituted. Hereinafter, Feature 3 will be described.

[0070] [Ti*] is an index of the amount of Ti precipitates in the steel material. Here, the Ti precipitates are Ti carbides, Ti nitrides, or Ti carbonitrides. [Ti*] is represented by the ratio of the atomic % of Ti to the total of the atomic % of C and the atomic % of N. [Ti*] is a value of the second decimal place obtained by rounding the third decimal place of the calculated value. [Nb*] is an indicator of the amount of Nb precipitates in the steel material. Here, the Nb precipitates are Nb carbides, Nb nitrides, or Nb carbonitrides. [Nb*] is represented by the ratio of the atomic percentage of Nb to the total of the atomic percentages of C and N. [Nb*] is a value rounded to the second decimal place obtained by rounding the third decimal place of the calculated value.

[0071] Define Fn = [Ti*] + [Nb*]. Fn is an indicator related to the amounts of dissolved C and dissolved N in the steel material. When Fn is 1.00 or more, all of the C and N in the steel material combine with Ti and Nb to form precipitates. In this case, an appropriate amount of dissolved C and dissolved N does not exist in the steel material. Therefore, uniform torsion occurs without local torsion during twisting. As a result, the increase in torque accompanying twisting cannot be sufficiently suppressed. On the other hand, if Fn is less than 1.00, a part of C and N does not combine with Ti and Nb and exists in the steel material as dissolved C and dissolved N. In this case, local torsion occurs during twisting, and the local torsion propagates in the axial direction. As a result, the increase in torque accompanying twisting can be sufficiently suppressed. Therefore, Fn is less than 1.00.

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

[0073] [(Feature 4) Regarding the maximum value (Q -1 ) max of the internal friction of the steel material] In the steel material of the present embodiment, furthermore, the maximum value (Q -1 ) max of the internal friction measured in the range of -50 to 150 °C is 1.2×10 -4 to 10.0×10 -4 .

[0074] Here, the maximum value (Q -1 ) max of the internal friction is the internal friction Q -1 measured in the range of -50 to 150 °C.Among them, it means the maximum value of the Snoek Peak. 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 dissolved C and dissolved N in the steel material. That is, the maximum value of the internal friction (Q -1 ) max is an index of the total amount of dissolved C and dissolved N in the steel material. In the following description, the maximum value of the internal friction (Q -1 ) max will simply be referred to as (Q -1 ) max as well.

[0075] (Q -1 ) max is less than 1.2×10 -4 , then the total amount of dissolved C and dissolved N in the steel material is too small. In this case, uniform torsion occurs without local torsion during twisting. As a result, the increase in torque associated with twisting cannot be sufficiently suppressed. On the other hand, if (Q -1 ) max exceeds 10.0×10 -4 , then the total amount of dissolved C and dissolved N in the steel material is too large. In this case, excessive local torsion occurs in the circumferential direction of the steel material. As a result, during twisting, the steel material breaks before the local torsion propagates in the axial direction of the steel material. (Q -1 ) max is 1.2×10 -4 ~10.0×10 -4 , then the total amount of dissolved C and dissolved N is appropriate. Therefore, during twisting, local torsion occurs in the circumferential direction of the steel material, and without excessive local torsion occurring, the local torsion propagates in the axial direction of the steel material. As a result, the increase in torque associated with twisting can be sufficiently suppressed.

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

[0077] [Measurement method of the maximum value of internal friction (Q -1 ) max The (Q -1 ) max of this embodiment can be measured by the following method. FIG. 4A is a perspective view of a steel material for explaining the sampling position of a test piece for measuring the maximum value of internal friction (Q -1 ) max from the steel material. FIG. 4B is a cross-sectional view perpendicular to the axial direction (longitudinal direction) of the shaft portion of the steel material shown in FIG. 4A. Referring to FIGS. 4A and 4B, in a cross-section (circular shape) perpendicular to the axial direction of the steel material, the test piece 28 is sampled from a position D / 4 in the radial direction from the surface corresponding to the outer peripheral surface of the steel material (D is the diameter of the steel material). 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 (longitudinal direction) of the steel material. Referring to FIG. 4B, in a cross-section perpendicular to the axial direction of the shaft portion 20, among the test pieces 28, the test piece 28 is sampled so that the normal line N28 at the center position in the width direction of the surface 28A having a width of 5 mm and extending in the axial direction of the shaft portion 20 becomes 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. For example, when sampling a test piece from a torsion bar component made of a steel material, the length of the test piece may be the length of the torsion bar component.

[0078] Using a low-frequency torsional pendulum type internal friction measuring device, by the free vibration decay method, (Q -1 ) max ​To obtain it. Specifically, the test piece 28 is attached to a low-frequency torsional pendulum type internal friction measuring device. Next, torsion is applied to the test piece 28 by an exciting 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 it is 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 pi.

[0079] The above operations are performed while increasing 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 ) max is used. Specifically, as shown in Fig. 5A, the straight line connecting a pair of bottoms BT of the obtained graph (peak waveform diagram) SP is defined as the baseline (background value) BL. The internal friction Q -1 of the baseline BL is subtracted to correct the graph SP as shown in Fig. 5B. The maximum value (snake peak) X1 is read from the corrected graph SP, and the obtained maximum value X1 is (Q -1 ) max is used. Note that, 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.

[0080] [Regarding the effect of the steel material of this embodiment] The steel material of this embodiment satisfies Features 1 to 4. Therefore, in the steel material of this embodiment, the increase in torque accompanying the number of twists can be sufficiently suppressed.

[0081] [Regarding the use and shape of the steel material of this embodiment] The steel material of this embodiment can be widely applied to applications where suppression of torque increase associated with the number of twists is required. The steel material of this embodiment is particularly suitable as a material for the torsion bar component included in a seat belt retractor. Note that the steel material of this embodiment is a bar steel or wire rod having a circular cross-section perpendicular to the axial direction (longitudinal direction) of the steel material. The diameter of the steel material is not particularly limited, but for example, it is 5 to 20 mm.

[0082] [Manufacturing method of the steel material of this embodiment] Hereinafter, the manufacturing method of the steel material of this embodiment will be described. The steel material satisfying Features 1 to 4 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 steel material of this embodiment.

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

[0084] [(Step 1) Material manufacturing step] In the material manufacturing step, the material of the steel material of this embodiment is prepared. Specifically, molten steel having a chemical composition satisfying 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 steps, a material (ingot or bloom) is manufactured.

[0085] [(Step 2) Hot working step] In the hot working step, hot working is performed on the manufactured material to manufacture the steel material. The steel material is, for example, a bar steel or a wire rod. The hot working step includes a cogging rolling step, a finishing rolling step, and a cooling step.

[0086] [Cogging rolling step] In the block rolling process, a billet is manufactured by hot rolling (block rolling) the heated material using a block rolling mill. When a continuous rolling mill is installed downstream of the block rolling mill, hot rolling may be further performed on the billet after block rolling using the continuous rolling mill to manufacture a billet with an even smaller size. The heating temperature in the heating furnace in the block rolling process is not particularly limited, but for example, it is 1000 - 1300°C.

[0087] [Finishing Rolling Process] In the finishing rolling process, hot rolling is performed on the billet after the block rolling process using a continuous rolling mill to manufacture steel. The heating temperature in the heating furnace in the finishing rolling process is not particularly limited, but for example, it is 1000 - 1300°C.

[0088] [Cooling Process] In the cooling process, boiling water cooling is performed on the steel after finishing rolling until a predetermined cooling stop temperature (°C). Specifically, boiling water cooling is performed by immersing the steel that has exited the rolling stand where the final reduction was performed in the finishing rolling in boiling water at 100°C. The temperature of the steel 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 immediately after it is pulled out of the boiling water using a thermometer. The thermometer is, for example, a thermograph, a radiation thermometer, etc.

[0089] The steel is pulled out of the boiling water at the cooling stop temperature and air-cooled to room temperature. By boiling water cooling, C and N in the steel become supersaturated and dissolve. In this state, boiling water cooling is stopped and the steel is air-cooled. In this case, reheating occurs in the steel and the steel temperature rises. As a result, a part of the dissolved C and dissolved N in the steel combines with Ti, Nb, and Fe to form precipitates (carbides, nitrides, carbonitrides). By performing boiling water cooling and switching from boiling water cooling to air-cooling at the cooling stop temperature within the above temperature range, the amounts of dissolved C and dissolved N in the steel can be adjusted to an appropriate range.

[0090] If the cooling stop temperature (°C) is less than 200°C, sufficient reheating during slow cooling cannot be obtained. In this case, sufficient precipitates are not formed during slow 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 solid-solved C and solid-solved N in the steel material during slow cooling is insufficient. Also in this case, sufficient precipitates are not formed during slow cooling. As a result, (Q -1 ) max becomes excessively large.

[0091] Through the above manufacturing process, the steel material according to the present embodiment is manufactured. In the manufacturing process of the steel material of the present embodiment, a cold rolling process using work rolls is not performed after the hot working process.

[0092] [Regarding the steel wire of the present embodiment] The steel wire of the present embodiment is made of the steel material of the above-described present embodiment. Specifically, the steel wire of the present embodiment is obtained by performing wire drawing on the steel material of the present embodiment. The steel wire of the present embodiment satisfies the above-described Features 1 to 4. Therefore, an increase in torque associated with the number of twists of the steel material can be suppressed. The steel wire of the present embodiment is a bar steel or wire rod having a circular cross-section with respect to the axial direction. The diameter of the steel wire is not particularly limited, but is, for example, 4 to 19 mm.

[0093] [Manufacturing method of the steel wire of the present embodiment] An example of the manufacturing method of the steel wire of the present embodiment is as follows. The manufacturing method of the steel wire of the present embodiment includes a wire drawing process. In the wire drawing process, wire drawing is performed on the steel material of the present embodiment under known conditions to manufacture the steel wire of the present embodiment. 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 a plurality of times, the area reduction rate RR is the cumulative area reduction rate (%) in the plurality of wire drawing processes.

[0094] [Regarding the torsion bar component of the present embodiment] The steel wire and the torsion bar component of the present embodiment are made of the steel material of the above-described present embodiment. FIG. 6 is a side view of the torsion bar component of the present embodiment. Referring to FIG. 6, the torsion bar component of the present embodiment includes a shaft portion 20 and a pair of flange portions 30 disposed at both ends of the shaft portion 20. The shaft portion 20 has a columnar shape, and a 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.

[0095] When the torsion bar component is viewed from the axial direction, the flange portion 30 has a polygonal shape. In FIG. 6, 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 a hexagon. 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.

[0096] The torsion bar component having the above 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 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 connection shaft portion for connecting to other components of the seat belt retractor.

[0097] The torsion bar component of the present embodiment is made of the steel material of the above-described present embodiment. That is, the torsion bar component of the present embodiment satisfies the above-described Features 1 to 4. Therefore, an increase in torque associated with the twisting of the steel material can be suppressed.

[0098] 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 (axial length of the torsion bar component) is, for example, 2 to 10 mm.

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

[0100] In the cold working process, cold working is performed on the steel wire after the wire drawing process under well-known conditions to form the torsion bar component. The cold working is, for example, forging. Note that after the cold working process, cutting may be further performed to form the torsion bar component with the above-described connection shaft portion. Further, instead of the cold working process, cutting may be performed on the steel wire after the wire drawing process to form the torsion bar component. Through the above manufacturing processes, the torsion bar component is manufactured.

[0101] In the manufacturing process of the torsion bar parts, a plating process may be carried out after the cold working process and / or the cutting process as required. In the plating process, a well-known plating treatment is carried out on the intermediate product after cold working and / or cutting to form a plating layer on the surface. The composition of the plating layer is not particularly limited.

Example

[0102] The effects of the steel material 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 steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one set of conditions.

[0103] 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.

[0104]

Table 1A

[0105]

Table 1B

[0106] Specifically, blooms were manufactured by continuous casting using molten steel. A blooming rolling process was carried out 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 rolling process were allowed to cool to room temperature.

[0107] A finishing rolling process was carried out on the manufactured billets. Specifically, the billets were heated to 1100 °C. The heated billets were hot-rolled (finishing rolling) using a continuous rolling mill to produce steel materials (wire rods).

[0108] A cooling process was carried out on the steel material after finish rolling. Specifically, boiling water cooling was carried out by immersing the steel material immediately after finish rolling in boiling water at 100 °C. The steel material was pulled out from the boiling water at the cooling stop temperature (°C) shown in Table 2, and then allowed to cool. In Test Nos. 41 and 42, boiling water cooling was not carried out, and the steel material after finish rolling was allowed to cool. (Indicated by "-" in the column of "Cooling stop temperature (°C)" in Table 2). Through the above manufacturing process, steel materials of each test number were manufactured.

[0109] [Table 2]

[0110] For the steel materials of each test number, the ferrite area ratio was measured based on the above [Method for measuring ferrite area ratio]. As a result, in any test number, the ferrite area ratio was 90% or more.

[0111] [Evaluation test] The following evaluation tests were carried out on the steel materials of each test number. (Test 1) Measurement test of the maximum value of internal friction (Q -1 ) max of the steel material (Test 2) Torque increase amount evaluation test accompanying twisting The following describes Test 1 and Test 2.

[0112] [(Test 1) Measurement test of the maximum value of internal friction (Q -1 ) max of the steel material] For (Q -1 ) max of the steel materials of each test number, it was evaluated by the following method. Based on the method described in the above [Method for measuring the maximum value of internal friction (Q -1 ) max , (Q -1 ) max of each test number was obtained. Also, the frequency of torsional vibration was about 1.3 Hz. For the measurement, IFM-1500L manufactured by Vacuum Riko Co., Ltd. (currently Advance Riko Co., Ltd.) was used. The obtained (Q -1 )max is shown in the column of “(Q -1 ) max (×10 -4 )” in Table 2.

[0113] [(Test 2) Torque Increase Amount Evaluation Test with Twisting] Torsion bar parts were manufactured using the steel materials of each test number, and the increase amount of torque during twisting of the torsion bar parts was evaluated. Specifically, the torsion bar parts of each test number were manufactured through the following steps.

[0114] Wire drawing was performed on the steel materials of each test number to manufacture steel wires. The wire drawing conditions were the same for all test numbers. Cold working (forging) was performed on the steel wires to manufacture torsion bar parts with the shape shown in Fig. 6. For the torsion bar parts, the axial length L2 of the shaft portion 20 was 50 mm, and the axial length L3 of the flange portion 30 was 5 mm. The shape of the flange portion 30 as viewed from the axial direction of the torsion bar part was hexagonal, 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.

[0115] The increase amount of torque accompanying the twisting of the torsion bar parts of each test number was evaluated by the following twisting test. Specifically, as shown in Fig. 7, one end flange portion 30 of the torsion bar part was fixed with a fixing jig 40, and the other end flange portion 30 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 being the torque (N·m) and the horizontal axis being the 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.

[0116] Based on the created graph, the yield torque T y (N·m) was obtained. The yield torque T ywas obtained by the following method. The yield torque T y was obtained by applying the method for obtaining the yield strength described in "13 Yield Strength (Offset Method) Rp" of JIS Z 2241:2022 to the torsion test. Specifically, as shown in Fig. 8, that is, the yield torque T y was obtained by drawing a line parallel to the straight line portion of the torque-twist angle diagram at a distance equal to the specified plastic twist angle. Specifically, the intersection point of this parallel line and the torque-twist angle diagram was defined as the yield torque T y to be obtained. The specified plastic twist angle was set to 45°.

[0117] Furthermore, from the graph created, the torque T 1260 (N·m) at a twist angle of 1260° was obtained. Based on the obtained yield torque T y , and the torque T 1260 , the torque increase amount ΔT was obtained by the following equation. ΔT = T y / T 1260 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 fracture occurred, and the torque T 1260 could not be measured, since the twist of 1260° 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.

[0118] [Evaluation Results] Referring to Tables 1 to 2, the steel materials with test numbers 1 to 29 satisfied Features 1 to 4. Therefore, for these steel materials with test numbers, the increase in torque associated with twisting could be sufficiently suppressed.

[0119] On the other hand, in test numbers 30 and 31, the Ti content was too high. Therefore, (Q -1 ) max was 1.2×10 -4It was less than that. As a result, the increase in torque associated with twisting could not be sufficiently suppressed.

[0120] In Test Nos. 32 and 33, the Nb content was too high. Therefore, (Q -1 ) max was less than 1.2×10 -4 As a result, the increase in torque associated with twisting could not be sufficiently suppressed.

[0121] In Test Nos. 34 and 35, the Ti content and the Nb content were too low. Therefore, (Q -1 ) max exceeded 10.0×10 -4 As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so T 1260 could not be obtained. As a result, the increase in torque associated with twisting could not be sufficiently suppressed.

[0122] In Test Nos. 36 and 37, Fn was too high. Therefore, (Q -1 ) max was less than 1.2×10 -4 As a result, the increase in torque associated with twisting could not be sufficiently suppressed.

[0123] In Test Nos. 38 and 39, the cooling stop temperature was too high. Therefore, (Q -1 ) max exceeded 10.0×10 -4 As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so T 1260 could not be obtained. As a result, the increase in torque associated with twisting could not be sufficiently suppressed.

[0124] In Test Nos. 40 and 41, the cooling stop temperature was too low. Therefore, (Q -1 ) max exceeded 10.0×10 -4 As a result, in the torque increase amount evaluation test, local torsion did not propagate and it broke halfway, so T 1260could not be obtained. As a result, the increase in torque associated with the twisting could not be sufficiently suppressed.

[0125] In Test Nos. 42 and 43, boiling water cooling was not performed. Therefore, (Q -1 ) max exceeded 10.0×10 -4 . As a result, in the torque increase amount evaluation test, local torsional strain did not propagate and it broke halfway, so T 1260 could not be obtained. As a result, the increase in torque associated with the twisting could not be sufficiently suppressed.

[0126] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for 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. The chemical composition is, by mass%, C: 0.006 to 0.050%, Mn: 0.10 to 0.80%, Al: 0.005 to 0.080%, Si: 0.50% or less, P: 0.030% or less, S: 0.050% or less, N: 0.0120% or less, O: 0.0100% or less, One or two selected from the group consisting of Ti: 0.003 to 0.040% and Nb: 0.003 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 90% or more, [Ti*] defined by formula (1) and [Nb*] defined by formula (2) satisfy formula (3), - The maximum value of the internal friction measured in the range of -50 to 150 °C (Q －１ ) ｍａｘ is 1.2 × 10 －４ to 10.0 × 10 －４ is as follows. Steel material. [Ti*] = Ti / {(48 / 12)C + (48 / 14)N} (1) [Nb*] = Nb / {(93 / 12)C + (93 / 14)N} (2) [Ti*] + [Nb*] < 1.00 (3) Here, for each element symbol in formula (1) and formula (2), the content in mass% of the corresponding element in the chemical composition is substituted.

2. The steel material according to Claim 1, wherein the chemical composition is, by mass%, 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%, contains one or more selected from the group consisting of, Steel material.

3. Consisting of the steel material according to Claim 1 or Claim 2, Steel wire.

4. A shaft part, and consisting of the steel material according to Claim 1 or Claim 2, A torsion bar component.

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