JP7656191B2 - Torsion bar steel and torsion bar parts - Google Patents

Description

This disclosure relates to torsion bar steel and torsion bar parts used in torsion bars used as load limiters in seat belt retractors.

The seat belt device installed in automobiles protects occupants by restraining them with the seat belt when the vehicle body is subjected to a large deceleration due to an impact, etc. However, even if the occupant is restrained by the seat belt, the occupant tends to move forward due to a large inertia, so a large load is applied to the seat belt and the occupant is subjected to a large impact from the seat belt.
Therefore, there is a need for a mechanism that can absorb the collision energy acting on the occupant and limit the load applied to the seat belt during a collision.

For example, Patent Document 1 discloses a seat belt retractor equipped with a load limiter mechanism. This seat belt retractor is equipped with a torsion bar, and when the belt is prevented from being withdrawn during a collision, a tension force is applied to the seat belt due to the inertial movement of the occupant forward, so that the seat belt retractor shaft rotates while twisting the twisted portion of the torsion bar, absorbing the impact energy of the occupant. When the torsion bar is twisted to a certain extent, the rotation of the seat belt retractor shaft is prevented, preventing the withdrawal of the seat belt and protecting the occupant, and also preventing the twisting of the torsion bar, preventing the breakage of the torsion bar.
As described above, the torsion bar functions as an energy absorbing member of the seat belt retractor, and is therefore required not to break due to torsional plastic deformation until it has completely absorbed the impact energy of the occupant.

Torsion bars are generally manufactured by annealing cold heading carbon steel of JIS G 3507 and then cold forging it. In contrast, the following steel materials for torsion bars have been disclosed.
Patent Document 2 discloses a steel for torsion bars having excellent low-temperature ductility, characterized in that solute N is fixed by Al and Nb.
Patent Document 3 discloses a steel for torsion bars that can be used to manufacture torsion bar parts by cold forging a hot-rolled wire rod without annealing it.
Patent Document 4 discloses a steel for torsion bars that can exhibit high twisting characteristics and high initial torque even if it has a common chemical composition.
Patent Document 5 discloses a steel for torsion bars that has a uniform ferrite-based structure and is therefore capable of exhibiting excellent twisting characteristics.
Patent Document 6 discloses a steel for torsion bars having excellent ductility during high-speed deformation, which contains 0.0001 to 0.1% C, 1% or less Si, 0.05 to 1% Mn, and 0.1% or less Al, has a metal structure consisting of ferrite or ferrite-pearlite, and has a ratio of the total length of grain boundaries where cementite has precipitated to the total length of the ferrite grain boundaries of 30% or less.

Japanese Utility Model Publication No. 61-11085 JP 2003-313626 A JP 2011-225943 A JP 2019-173068 A JP 2009-120906 A JP 2006-022397 A

In the conventional techniques disclosed in Patent Documents 1 to 6, the decrease in the twist angle due to strain aging, that is, the specific twist angle at the time of torsional fracture, is not considered.
When manufacturing a torsion bar, strain is introduced during the wire drawing and heading processes, and as time passes after manufacturing, strain aging occurs. Strain aging hardens the bar and reduces uniform elongation, making it more likely to transition to localized deformation early, so strain aging causes a decrease in the twist angle of the torsion bar.
However, from the viewpoint of exerting a sufficient energy absorbing ability, it is desirable to suppress the decrease in the twist angle even after the lapse of time after manufacture.

In addition, torsion bar parts may be subjected to, for example, electroplating after heading in order to impart rust resistance. In this case, hydrogen penetrates into the steel material during the electroplating process. Furthermore, not only in the case of electroplating, but also in the case of the steel material of the torsion bar parts, hydrogen penetration into the steel material reduces ductility. However, the conventional techniques disclosed in Patent Documents 1 to 6 do not consider the reduction in ductility due to hydrogen.
However, from the viewpoint of exerting a sufficient energy absorption capacity, it is desirable for the torsion bar steel material and torsion bar components to be less susceptible to hydrogen penetration.

Therefore, the problem that this disclosure aims to solve is to provide a steel material for torsion bars that has a sufficient twist angle even when strain aging occurs, and that has a sufficient twist angle by suppressing hydrogen penetration into the steel material, and a torsion bar part that has a sufficient twist angle even when strain aging occurs, is capable of effectively absorbing collision energy, and further has a sufficient twist angle by suppressing hydrogen penetration into the steel material.

Means for solving the above problems include the following aspects.
<1> In mass%,
Si: 0.02% to 0.60%,
Mn: 0.30% to 2.00%,
C: 0.0400% or less,
P: 0.020 or less,
S: 0.020% or less,
Al: 0.005% to 0.100%,
Cr: 0.010% to 0.300%,
N: 0.0100% or less; and O: 0.0150% or less;
Further, one or both of Ti and Nb are contained in the range of Ti: 0.500% or less and Nb: 0.500% or less, and when the content of element X is [X] (unit: mass%), [Ti*] and [Nb*] are expressed by the following formula, [Ti*] + [Nb*] is 1.00 or more,
The balance has a chemical composition consisting of Fe and impurities,
A steel material for torsion bars, having a ferrite area ratio of 98.0% or more in a metal structure in a cross section perpendicular to the longitudinal direction, and having a tensile strength of 280 to 550 MPa.
[Ti*]=[Ti]/{(48/12)[C]+(48/14)[N]}
[Nb*]=[Nb]/{(93/12)[C]+(93/14)[N]}
<2> In mass%,
Si: 0.02% to 0.60%,
Mn: 0.30% to 2.00%,
C: 0.0400% or less,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.005% to 0.100%,
Cr: 0.010% to 0.300%,
N: 0.0100% or less; and O: 0.0150% or less;
Further, one or both of Ti and Nb are contained in the range of Ti: 0.500% or less and Nb: 0.500% or less, and when the content of element X is [X] (unit: mass%), [Ti*] and [Nb*] are expressed by the following formula, [Ti*] + [Nb*] is 1.00 or more,
The balance includes a steel material having a chemical composition consisting of Fe and impurities,
At the center of the shaft portion in the longitudinal direction, the ferrite area ratio of the metal structure in a cross section perpendicular to the longitudinal direction is 98.0% or more,
At the center of the longitudinal direction of the shaft portion, a Vickers hardness in a cross section perpendicular to the longitudinal direction is 90 to 200 HV10;
After aging treatment at 150°C for 1 hour, the specific torsion angle at torsion fracture at room temperature and -40°C is 0.60 rad/mm or more,
The cumulative amount of hydrogen released from the steel material from room temperature to 300° C., as measured by a hydrogen thermal desorption method at a heating rate of 100° C./hr, is 0.50 ppm by mass or less.
Torsion bar parts.
[Ti*]=[Ti]/{(48/12)[C]+(48/14)[N]}
[Nb*]=[Nb]/{(93/12)[C]+(93/14)[N]}
<3> The torsion bar part according to <2>, wherein a plating layer is formed on a surface of the steel material.

According to the present disclosure, there is provided a steel material for a torsion bar that has a sufficient twist angle even when strain aging occurs, and that has a sufficient twist angle by suppressing hydrogen penetration into the steel material, and a torsion bar part that has a sufficient twist angle even when strain aging occurs, is capable of effectively absorbing collision energy, and further has a sufficient twist angle by suppressing hydrogen penetration into the steel material.

FIG. 2 is a diagram showing positions at which Vickers hardness is measured in a cross section perpendicular to the longitudinal direction at the center of a shaft portion of a torsion bar component. FIG. 1 is a diagram for explaining a twist angle evaluation test in an embodiment, in which (a) shows a steel wire obtained after wire drawing of a steel material (wire material) for a torsion bar, (b) shows a test piece obtained by cold forging the steel wire of (a), and (c) shows a twist angle evaluation test method.

An embodiment that is an example of the present disclosure will be described.
In this specification, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. However, when the numerical values before and after "to" are followed by "more than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.
In the present specification, the upper limit of a numerical range described in stages may be replaced with the upper limit of another numerical range described in stages, and the lower limit of a numerical range described in stages may be replaced with the lower limit of another numerical range described in stages. Also, the upper limit or lower limit may be replaced with a value shown in the present disclosure.
With regard to the contents of elements in chemical compositions, "%" means "mass %".

[Steel material for torsion bars]
The torsion bar steel material according to the present disclosure is
In mass percent,
Si: 0.02% to 0.60%,
Mn: 0.30% to 2.00%,
C: 0.0400% or less,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.005% to 0.100%,
Cr: 0.010% to 0.300%,
N: 0.0100% or less; and O: 0.0150% or less;
Further, one or both of Ti and Nb are contained in the range of Ti: 0.500% or less and Nb: 0.500% or less, and when the content of element X is [X] (unit: mass%), [Ti*] and [Nb*] are expressed by the following formula, [Ti*] + [Nb*] is 1.00 or more,
The balance has a chemical composition consisting of Fe and impurities,
A steel material for torsion bars, having a ferrite area ratio of 98.0% or more in a metal structure in a cross section perpendicular to the longitudinal direction, and having a tensile strength of 280 to 550 MPa.
[Ti*]=[Ti]/{(48/12)[C]+(48/14)[N]}
[Nb*]=[Nb]/{(93/12)[C]+(93/14)[N]}

<Chemical composition>
C: 0.0400% or less Carbon (carbon) is an element that generates carbides and carbonitrides and reduces the twist angle. When C exists in the form of a solid solution in ferrite, it reduces the twist angle due to strain aging.
The C content in the torsion bar steel material according to the present disclosure is 0.0400% or less. If the C content exceeds 0.0400%, the desired microstructure (ferrite fraction) cannot be satisfied, and the twist angle at room temperature and low temperature decreases with an increase in carbides (NbC, TiC) or carbonitrides (NbCN, TiCN) precipitated in the formed cementite or ferrite. The C content may be 0.0200% or less, or 0.0100% or less.
Although there is no lower limit for the C content, in consideration of manufacturability (manufacturing costs), the C content may be 0.0015% or more.

Si: 0.02% to 0.60%
Silicon (Si) is a deoxidizing element and also has the effect of solid solution strengthening. The Si content in the steel material for torsion bars according to the present disclosure is 0.02% to 0.60%.
If the Si content is less than 0.02%, the desired room temperature strength (tensile strength) cannot be satisfied. The Si content may be 0.05% or more, or may be 0.15% or more.
On the other hand, if the Si content exceeds 0.60%, the amount of solid solution strengthening of ferrite by Si increases too much, so that the deformability at room temperature and low temperatures decreases, and the twist angle at room temperature and low temperatures decreases. The Si content may be 0.50% or less.

Mn: 0.30% to 2.00%
Mn (manganese) is an element that has the effect of solid solution strengthening. By precipitating S (sulfur) contained as an impurity as MnS, the deterioration of hot brittleness due to S is suppressed. The Mn content in the torsion bar steel material according to the present disclosure is 0.30% to 2.00%.
If the Mn content is less than 0.30%, the desired room temperature strength (tensile strength) cannot be satisfied. The Mn content may be 0.80% or more.
On the other hand, if the Mn content exceeds 2.00%, the amount of solid solution strengthening of ferrite by Mn increases too much, so that the deformability at low temperatures decreases and the twist angle at low temperatures decreases. The Mn content may be 1.50% or less.

P: 0.020% or less P (phosphorus) is an element that has the effect of solid solution strengthening, but on the other hand, it is an element that segregates at grain boundaries and reduces cold workability and twist angle. It also has the effect of promoting hydrogen penetration into steel material, which deteriorates the ductility of the steel material. The P content in the steel material for torsion bars according to the present disclosure is 0.020% or less. If the P content exceeds 0.020%, the effect of promoting hydrogen penetration into the steel material becomes significant, and the twist angle decreases. The P content may be 0.015% or less.
The lower limit of the P content is not limited, but in consideration of manufacturability (manufacturing costs), the lower limit of the P content may be 0.001% or 0.005%.

S: 0.020% or less S (sulfur), like P, is an element that segregates to grain boundaries and reduces cold workability and twist angle. The S content in the steel material for torsion bars according to the present disclosure is 0.020% or less. If the S content exceeds 0.020%, the grain boundary segregation caused by S reduces the deformability at low temperatures and reduces the twist angle at low temperatures. The S content may be 0.015% or less, or 0.010% or less.
On the other hand, S is an element that forms MnS together with Mn to improve machinability. From the viewpoint of improving machinability by forming MnS and suppressing an increase in manufacturing costs (desulfurization costs), the lower limit of the S content may be 0.001%, 0.002%, or 0.005%.

Al: 0.005% to 0.100%
Al (aluminum) is a deoxidizing element. In addition, AlN, which is a nitride of Al, refines crystal grains, thereby improving cold workability. The Al content in the torsion bar steel material according to the present disclosure is 0.005% to 0.100%.
If the Al content exceeds 0.100%, the effect of refining grains becomes saturated and defects become more likely to occur during hot rolling. The Al content may be 0.050% or less.
On the other hand, if the Al content is less than 0.005%, the deoxidizing effect cannot be obtained.The Al content is preferably 0.010% or more.

Cr: 0.010% to 0.300%
Cr (chromium) is a solid solution strengthening element and an element that improves hardenability. It also has the effect of suppressing hydrogen penetration into the steel material, and suppresses the decrease in the twist angle caused by hydrogen. The Cr content in the torsion bar steel material according to the present disclosure is 0.010% to 0.300%.
If the Cr content exceeds 0.300%, the effect of suppressing hydrogen penetration into the steel material becomes saturated, and defects are likely to occur during hot rolling. The Cr content may be 0.200% or less.
On the other hand, if the Cr content is less than 0.010%, the effect of suppressing hydrogen penetration into the steel material cannot be obtained.The Cr content is preferably 0.020% or more.

N: 0.0100% or less N (nitrogen) is an element that exists in a solid solution state in ferrite and reduces the twist angle by strain aging. The solid solution N in ferrite is precipitated as nitrides (AlN, TiN, NbN) or carbonitrides (TiCN, NbCN), suppressing the reduction in the twist angle.
The N content in the torsion bar steel material according to the present disclosure is 0.0100% or less. If the N content exceeds 0.0100%, the twist angle at room temperature and low temperature decreases with an increase in nitrides (AlN, TiN, NbN) or carbonitrides (TiCN, NbCN) precipitated in the ferrite. The upper limit of the N content may be 0.0070%.
The lower limit of the N content is not limited, but in consideration of manufacturability (increase in denitrification costs), the lower limit of the N content may be 0.0010%.

Ti: 0.500% or less Nb: 0.500% or less [Ti*] + [Nb*]: 1.00 or more Ti (titanium) and Nb (niobium) are optional elements, and the steel material for torsion bars according to the present disclosure contains one or two of these elements. Ti and Nb are elements that form carbides such as TiC and NbC, nitrides such as TiN and NbN, or carbonitrides such as TiCN and NbCN, and have the effect of suppressing strain aging due to solute C and solute N.
In the steel material for torsion bars according to the present disclosure, when the content of element X is [X] (unit: mass%), Ti is contained within the range of 0.500% or less, Nb is contained within the range of 0.500% or less, and when [Ti*] and [Nb*] are expressed by the following formula, [Ti*] + [Nb*] is contained within the range of 1.00 or more.
[Ti*]=[Ti]/{(48/12)[C]+(48/14)[N]}
[Nb*]=[Nb]/{(93/12)[C]+(93/14)[N]}

The above formula represents the atomic weight composition ratio of Ti and Nb to the sum of the atomic weight compositions of C and N. When [Ti*] + [Nb*] is 1.00, the dissolved C and N bond with Ti and Nb of the same atomic weight composition to form TiC, NbC, or TiN, NbN.
When [Ti*]+[Nb*] is less than 1.00, C and N are dissolved in the ferrite, so that the twist angle at room temperature and low temperatures decreases. [Ti*]+[Nb*] may be 2.00 or more.
On the other hand, the upper limit of [Ti*] + [Nb*] is not limited as long as the contents of Ti, Nb, C, and N are within the above-mentioned ranges, but is preferably 8.00 in consideration of manufacturability (manufacturing costs).
In addition, the Ti content may be 0.300% or less, and the Nb content may be 0.300% or less.

O: 0.0150% or less O (oxygen) is an element that combines with Al, Ti, etc. to form oxide-based inclusions, which deteriorates cold workability and twist angle. The O content in the steel material for torsion bars according to the present disclosure is 0.0150% or less.
If the O content exceeds 0.0150%, the twist angle at room temperature and low temperatures decreases. The O content may be 0.0100% or less.
Although there is no restriction on the lower limit of the O content, in consideration of manufacturability (increase in deoxidization costs), the lower limit of the O content may be 0.0010%.

The balance of the chemical composition of the steel material for torsion bars according to the present disclosure is Fe and impurities. The term "impurities" refers to substances that are unintentionally mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of steel materials.

<Metal structure>
The torsion bar steel material according to the present disclosure has a ferrite area ratio of 98.0% or more in the metal structure in a cross section perpendicular to the longitudinal direction. This increases the twist angle of the torsion bar component. If the ferrite area ratio is below 98.0%, the twist angle at room temperature and low temperature decreases. The ferrite area ratio is preferably 99.0% or more, and may be 100%.
The structure other than ferrite (the remaining structure) is not particularly limited, but is usually a structure mainly composed of carbides and nitrides. Specifically, it is cementite, TiC, NbC, TiN, NbN, and MnS. It is also considered that the remaining structure includes AlN, TiCN, NbCN, TiS, Ti ₄ C ₂ S ₂ , FeTiP, and the like.

(Method of measuring ferrite area ratio)
The ferrite area ratio of the torsion bar steel material according to the present disclosure is measured as follows.
A cross section perpendicular to the longitudinal direction of the steel material for torsion bars (hereinafter simply referred to as C-section) is etched with a 3% Nital solution at room temperature for 10 seconds to reveal the metal structure. If the metal structure is not sufficiently revealed, further etching may be performed for up to 5 seconds.
Next, when the shaft diameter of the steel material for torsion bar is D, four observation positions are selected at 90° intervals in the circumferential direction from a position (i.e., a circumferential position) at a depth of 0.25D from the shaft surface (outer peripheral surface) in the C cross section after etching, and an optical microscope photograph is taken at a magnification of 500 times for each observation position using an optical microscope. In the four obtained optical microscope photographs, structures other than ferrite (cementite, etc.) are visually marked, and the area ratio (area %) of the structure other than ferrite to the entire metal structure is obtained by image analysis. The area ratio (area %) of the structure other than ferrite obtained is subtracted from 100% to obtain the area ratio (%) of ferrite.

<Tensile strength>
The torsion bar is required to have a tensile strength within an appropriate range in order to absorb the collision energy acting on the occupant, prevent the occupant from being ejected from the vehicle or from a secondary collision, and reduce the load on the occupant during a collision. The tensile strength of the torsion bar steel material according to the present disclosure is 280 to 550 MPa.
If the tensile strength is less than 280 MPa, the torsion bar will not be able to absorb enough impact energy, which may increase the load on the occupant's body during a collision.
On the other hand, if the tensile strength exceeds 550 MPa, the twist angle of the torsion bar at room temperature and low temperature decreases, and there is a possibility that the torsion bar breaks while absorbing the impact energy. The tensile strength may be 500 MPa or less.
The tensile strength of the torsion bar steel material according to the present disclosure is a value measured using a JIS Z2241:2011 No. 14A test piece in accordance with the test method described in JIS Z2241:2011.

[Torsion bar parts]
The torsion bar component according to the present disclosure comprises, in mass %,
Si: 0.02% to 0.60%,
Mn: 0.30% to 2.00%,
C: 0.0400% or less,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.005% to 0.100%,
Cr: 0.010% to 0.300%,
N: 0.0100% or less; and O: 0.0150% or less;
Further, one or both of Ti and Nb are contained in the range of Ti: 0.500% or less and Nb: 0.500% or less, and when the content of element X is [X] (unit: mass%), [Ti*] and [Nb*] are expressed by the following formula, [Ti*] + [Nb*] is 1.00 or more,
The balance includes a steel material having a chemical composition consisting of Fe and impurities,
At the center of the shaft portion in the longitudinal direction, the ferrite area ratio of the metal structure in a cross section perpendicular to the longitudinal direction is 98.0% or more,
At the center of the longitudinal direction of the shaft portion, a Vickers hardness in a cross section perpendicular to the longitudinal direction is 90 to 200 HV10;
After aging treatment at 150°C for 1 hour, the specific torsion angle at torsion fracture at room temperature and -40°C is 0.60 rad/mm or more,
The cumulative amount of hydrogen released from the steel material from room temperature to 300° C., as measured at a temperature rise rate of 100° C./hr by hydrogen thermal desorption spectrometry, is 0.50 mass ppm or less.
[Ti*]=[Ti]/{(48/12)[C]+(48/14)[N]}
[Nb*]=[Nb]/{(93/12)[C]+(93/14)[N]}

The chemical composition and metal structure of the steel material that is the base material for the torsion bar components according to the present disclosure are the same as those of the steel material for torsion bars according to the present disclosure described above, and therefore will not be described here.

<Vickers hardness>
The torsion bar is required to have a Vickers hardness within an appropriate range in order to absorb the collision energy acting on the occupant, prevent the occupant from being ejected from the vehicle or from a secondary collision, and reduce the load on the occupant during a collision. The Vickers hardness of the C cross section at the center of the shaft of the torsion bar part according to the present disclosure is 90 to 200 HV10.
If the Vickers hardness falls below 90HV10, the torsion bar will not be able to absorb enough impact energy, which may increase the load on the occupant's body during a collision.
On the other hand, if the Vickers hardness exceeds 200 HV10, the twist angle of the torsion bar at room temperature and low temperatures decreases, and there is a possibility that the torsion bar may break while absorbing the impact energy. The Vickers hardness may be 190 HV10 or less.

The Vickers hardness of the torsion bar component according to the present disclosure is measured as follows.
When the shaft diameter of the torsion bar part is D, as shown in Fig. 1, four positions were selected at 90° intervals in the circumferential direction from a position (i.e., a circumferential position) at a depth of 0.25D from the shaft surface (outer peripheral surface) in a cross section (cross section C) perpendicular to the longitudinal direction of the torsion bar part and including the center of the shaft, and the Vickers hardness was measured at each position, and the average value of the Vickers hardness at the four positions was taken as the Vickers hardness of the torsion bar part. The Vickers hardness at each position was measured in accordance with the test method described in JIS Z2244:2009.
The metal structure of the torsion bar component is also measured at the same positions as for the Vickers hardness, i.e., four positions are selected at 90° intervals in the circumferential direction at a depth of 0.25D from the surface of the shaft on the C cross section at the center of the shaft, and the metal structure is measured and the average value is calculated.

<Specific torsion angle at torsional fracture at room temperature and -40°C after aging treatment>
The torsion bar part according to the present disclosure has a specific torsion angle of 0.60 rad/mm or more at room temperature (25° C.) and at −40° C. when it fractures due to torsion after aging treatment in which it is held at 150° C. for 1 hour. In other words, the torsion bar part according to the present disclosure has a sufficient twist angle even when strain aging occurs, and can exhibit a sufficient energy absorption capacity.
The specific torsion angles at torsional break at room temperature (25° C.) and at −40° C. are preferably 0.65 rad/mm or more, and more preferably 0.70 rad/mm or more.

<Hydrogen temperature programmed desorption analysis>
The torsion bar part according to the present disclosure has a cumulative amount of hydrogen released from the steel material from room temperature to 300°C, measured by a hydrogen thermal desorption method at a heating rate of 100°C/hr, of 0.50 mass ppm or less. That is, the torsion bar part according to the present disclosure has a sufficient twist angle and can exhibit sufficient energy absorption capacity because the amount of hydrogen penetrating into the steel material of the torsion bar part is suppressed. The cumulative amount of hydrogen released from the steel material from room temperature to 300°C is preferably 0.40 mass ppm or less. In addition, when a plating layer is formed on the surface of the steel material that is the base material of the torsion bar part, the plating layer is removed to expose the surface of the steel material, and then the cumulative amount of hydrogen released from the steel material from room temperature (25°C) to 300°C is measured by a hydrogen thermal desorption method.

[Method of manufacturing steel material for torsion bars and torsion bar parts]
The method for manufacturing the torsion bar steel material and torsion bar component according to the present disclosure is not particularly limited, but they can be manufactured, for example, by the following method.
A steel slab having the chemical composition of the steel material for torsion bars according to the present disclosure is produced. The obtained steel slab is heated and then hot-rolled to produce a wire material (steel material for torsion bars). In order to stabilize the precipitated carbides, nitrides, and carbonitrides, a heat treatment may be performed after the hot rolling. The obtained wire material is then drawn to produce a steel wire. The obtained steel wire is then cold-forged to be processed into the shape of a torsion bar part. In addition, cutting may be used when processing into the shape of a torsion bar part.
In order to provide the torsion bar part with rust resistance, the torsion bar part according to the present disclosure may have a plating layer formed on the surface of the steel material thereof, for example, an electroplating process may be performed after processing into the shape of the torsion bar part.

The torsion bar part according to the embodiment of the present disclosure has at least a shaft portion, but is not particularly limited in size, shape, etc. For example, in the examples described below, a steel wire having a diameter of 9.0 mm is manufactured and formed into a torsion bar part, but the diameter of the manufactured steel wire is not limited to 9.0 mm, and an appropriate diameter may be selected according to the size of the torsion bar part according to the embodiment of the present disclosure.
Moreover, the wire drawing does not have to be performed in a single pass, but may be performed in an appropriate number of passes depending on the wire drawing area reduction rate.
Furthermore, the shape of the shaft portion of the torsion bar component is not limited to having the same diameter over the entire shaft portion, and may have, for example, a tapered shape in which the diameter at the center of the shaft portion is different from the diameter at the end portions.

The torsion bar steel material and torsion bar parts disclosed herein will be explained in more detail below with reference to examples. However, these examples do not limit the torsion bar steel material and torsion bar parts disclosed herein.

<Manufacturing of steel materials for torsion bars>
A 150 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace. In the chemical composition of each steel type in Table 1, the balance other than the elements shown in Table 1 is Fe and impurities. "-" indicates that the component is not contained, and underlined items indicate that they are outside the scope of the present disclosure.

The steel ingot was then heated to 1250°C, and hot forged into a round bar with a diameter of 20 mm. The outer periphery of the obtained round bar was cut to produce a wire (steel material for torsion bars) with a diameter of 9.2 to 12.0 mm. The chemical composition of the wire is considered to be the same as that of the steel ingot. This is because neither hot forging nor cutting affects the chemical composition of the wire. The diameter of the wire to be produced is not limited to the above range, and an appropriate diameter may be selected depending on the dimensions of the torsion bar component according to the embodiment of the present disclosure.
Table 1 shows the area ratio of ferrite in the metal structure of the wire rod measured by the above-mentioned method (ferrite area ratio), and the tensile strength evaluated by preparing No. 14A test pieces (JIS Z2241:2011) from the wire rod.

<Manufacturing of torsion bar parts>
The wire thus obtained was then annealed by being held at 650° C. for 5 hours and then cooled in a furnace, after which it was drawn into a steel wire having a diameter of 9.0 mm.
The steel wire thus obtained was then cold forged into the shape of a torsion bar part (shaft length: 45 mm) shown in Fig. 2(b), and the surface was then electroplated with zinc.
Seven torsion bar parts of each steel type were prepared, and one was used for measuring the ferrite area ratio. The ferrite area in the steel material of the torsion bar parts was measured by the method described above.

<Evaluation of the twist angle of torsion bar parts>
The twist angle of the obtained torsion bar component was evaluated as follows.
The manufactured torsion bar part was subjected to aging treatment by holding at 150° C. for 1 hour, and the twist angle of the torsion bar part was evaluated. The twist angle evaluation test of the torsion bar part was carried out in the following procedure.
Of the seven torsion bar parts of each steel type, six were prepared, excluding one used for measuring the ferrite area ratio, and the above-mentioned aging treatment was performed. Of the six, three were subjected to a room temperature (25°C) twist angle evaluation test, and three were subjected to a low temperature (-40°C) twist angle evaluation test. One side of the torsion bar part was fixed with a jig, and the other side was rotated with a jig at 50 revolutions per second to break it. Thereafter, the specific twist angle at the time of torsional fracture, i.e., the twist angle, was evaluated. That is, the average value of the value (= specific twist angle at torsional fracture) obtained by dividing the torsional rotation number (unit: rad) of the three pieces by the shaft length of the torsion bar part was taken as the specific twist angle at the time of torsional fracture, and this was taken as the twist angle of the torsion bar part.
The pass/fail judgment of the torsion angle evaluation test was made as follows: when the specific torsion angle at torsion break at room temperature and at -40°C was 0.60 rad/mm or more, it was judged as pass (◯). When the above conditions were not satisfied, it was judged as fail (×).

<Hydrogen temperature programmed desorption analysis>
The amount of hydrogen in the obtained torsion bar parts was evaluated as follows.
The electrolytic zinc-plated torsion bar part was subjected to reverse electrolysis to remove the zinc plating on the surface, and the torsion bar part was immediately placed in a heating furnace and heated from room temperature to 600° C. at a heating rate of 100° C./hr, and the cumulative amount of hydrogen from room temperature to 300° C. was measured by a gas chromatograph using a thermal conductivity detector. Ar was used as the carrier gas.
Table 2 shows the steel types, measurement results, and evaluation results.

In levels 1 to 20 in which the chemical composition, ferrite area ratio, and accumulated hydrogen amount were within the ranges of the present disclosure, the Vickers hardness was 90 to 200 HV10, and the specific torsion angle at torsional fracture at room temperature and -40°C was both 0.60 rad/mm or more.
On the other hand, in levels 21 to 32, at least one of the chemical composition, ferrite area ratio, and cumulative hydrogen amount was outside the range of the present disclosure, the Vickers hardness was less than 90 HV10 or more than 200 HV10, or the specific torsion angle at torsion fracture at room temperature and -40°C was less than 0.60 rad/mm. The torsion angle was insufficient due to the chemical composition, etc., except for levels 22 and 24. In particular, level 26 had a low Cr content that suppresses hydrogen penetration into the steel material, and level 27 had a high P content that promotes hydrogen penetration into the steel material, so that all of the levels had a large cumulative hydrogen amount and an insufficient torsion angle. Levels 22 and 24 had good torsion angles but insufficient hardness.
In level 33, the chemical composition, ferrite area ratio, and accumulated hydrogen amount were all within the ranges disclosed in the present disclosure, but the hardness was excessive and the twist angle was reduced.
Furthermore, in the cases of Levels 34 and 35, the area reduction rate during wire drawing was low, and therefore the hardness was insufficient.
In addition, in standard 36, the area reduction rate during wire drawing was too high, and therefore the hardness was excessive.

Claims (3)

In mass percent,
Si: 0.02% to 0.60%,
Mn: 0.30% to 2.00%,
C: 0.0400% or less,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.005% to 0.100%,
Cr: 0.010% to 0.300%,
N: 0.0100% or less; and O: 0.0150% or less;
Further, one or both of Ti and Nb are contained in the range of Ti: 0.500% or less and Nb: 0.500% or less, and when the content of element X is [X] (unit: mass%), [Ti*] and [Nb*] are expressed by the following formula, [Ti*] + [Nb*] is 1.00 or more,
The balance has a chemical composition consisting of Fe and impurities,
A steel material for torsion bars, having a ferrite area ratio of 98.0% or more in a metal structure in a cross section perpendicular to the longitudinal direction, and having a tensile strength of 280 to 550 MPa.
[Ti*]=[Ti]/{(48/12)[C]+(48/14)[N]}
[Nb*]=[Nb]/{(93/12)[C]+(93/14)[N]} In mass percent,
Si: 0.02% to 0.60%,
Mn: 0.30% to 2.00%,
C: 0.0400% or less,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.005% to 0.100%,
Cr: 0.010% to 0.300%,
N: 0.0100% or less; and O: 0.0150% or less;
Further, one or both of Ti and Nb are contained in the range of Ti: 0.500% or less and Nb: 0.500% or less, and when the content of element X is [X] (unit: mass%), [Ti*] and [Nb*] are expressed by the following formula, [Ti*] + [Nb*] is 1.00 or more,
The balance includes a steel material having a chemical composition consisting of Fe and impurities,
At the center of the shaft portion in the longitudinal direction, the ferrite area ratio of the metal structure in a cross section perpendicular to the longitudinal direction is 98.0% or more,
At the center of the longitudinal direction of the shaft portion, a Vickers hardness in a cross section perpendicular to the longitudinal direction is 90 to 200 HV10;
After aging treatment at 150°C for 1 hour, the specific torsion angle at torsion fracture at room temperature and -40°C is 0.60 rad/mm or more,
The cumulative amount of hydrogen released from the steel material from room temperature to 300° C., as measured by a hydrogen thermal desorption method at a heating rate of 100° C./hr, is 0.50 ppm by mass or less.
Torsion bar parts.
[Ti*]=[Ti]/{(48/12)[C]+(48/14)[N]}
[Nb*]=[Nb]/{(93/12)[C]+(93/14)[N]} The torsion bar part according to claim 2, in which a plating layer is formed on the surface of the steel material.