JP3978073B2 - Steel for torsion bars with excellent cold ductility - Google Patents

Description

【００２６】
これらの鋼線を１１．６φに伸線後、１３０ｍｍの長さに切断し、ねじり試験機にセットした。氷点下４５℃の温度まで試験片を冷却した後、この試験片を一方向に２．５ｒｐｍの回転速度で連続的にねじり回転させた。破断までのねじり回転数の結果を表２に示す。
【００２７】
【表２】

【００２８】
試番１〜３は本発明例であり、いずれも氷点下４５℃でのねじり回転数が６回を上回り、十分な延性がある。試番８〜１０は比較例である。試番８は、鋼のＰ量が高く、かつＢを添加していないため、十分な延性が得られなかった例である。試番９、１０はＢの添加量が少ないために十分な延性が得られなかった例である。
【００２９】
【発明の効果】
以上の結果から、本発明によれば鋼の成分を最適にすることにより、氷点下温度のような低温域においても、ねじり変形の延性が優れたトーションバー用に適した鋼を得ることができる。従って、本発明は産業上の効果が極めて顕著な発明であるといえる。
【図面の簡単な説明】
【図１】氷点下４５℃まで冷却した棒状試験片のねじり試験での破断までの回転数とＰ量、Ｂ量との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to steel used for a torsion bar used as a load limiter in a seat belt retractor.
[0002]
[Prior art]
A seat belt device installed in an automobile protects the occupant by preventing the occupant from jumping out of the seat by restraining the occupant with the seat belt when a large deceleration speed is applied to the vehicle body due to an impact or the like. Yes.
[0003]
By the way, in the seat belt retractor, when the seat belt restrains the occupant at the time of a vehicle collision, a large vehicle deceleration occurs, so the occupant tries to move forward due to a large inertia. For this reason, a large load is applied to the seat belt, and the occupant receives a large impact from the seat belt.
[0004]
Therefore, an energy absorption mechanism for absorbing the collision energy acting on the passenger is required. Japanese Laid-Open Patent Application No. 61-11085 discloses a seat belt retractor that limits the load applied to the seat belt at the time of a collision in a belt-mounted state. This seat belt retractor is equipped with a torsion bar, and if the belt is to be prevented from being pulled out in the event of a collision, a tensile force is applied to the seat belt due to inertial movement of the occupant forward. It rotates while twisting the torsion of the bar to absorb the impact energy of the passenger. When the torsion bar is twisted by a predetermined amount, the rotation of the seat belt take-up shaft is prevented to prevent the seat belt from being pulled out and the occupant is protected, and the torsion bar is prevented from being twisted and the torsion bar is broken. Is prevented.
[0005]
Japanese Patent Laid-Open No. 2001-163178 is an invention that is an improvement of the Japanese Utility Model Publication No. 61-11085. This is a method of increasing the energy absorption capacity by producing the hardness of the surface portion lower than the hardness of the central portion in the cross section of the steel used for the torsion bar. Further, Japanese Patent Application Laid-Open No. 2001-122077 discloses an improved steel for torsion bars. Conventionally, the steel forming the torsion bar absorbs energy by plastic deformation of torsion, and therefore has to be ductile and spheroidized. In Japanese Patent Laid-Open No. 2001-122077, the C content of steel is set to 0.04% by weight or less, and Ti or Nb can be added to enable sufficient torsional deformation without spheroidizing annealing. . Thus, the steel for torsion bars has been improved several times and has achieved certain results.
[0006]
[Problems to be solved by the invention]
However, in the conventional technology, the energy absorption amount in a low temperature region such as a sub-freezing temperature is not always satisfactory. In other words, since automobiles are used even in cold regions such as 45 ° C below freezing, it is necessary for the torsion bar to absorb sufficient energy even at such temperatures. Therefore, the necessity of developing new steel materials is desired. It was. That is, an object is to provide a steel for a torsion bar having sufficient ductility even at a low temperature.
[0007]
[Means for Solving the Problems]
As a result of detailed studies to solve such problems, the present inventors have found that a steel having sufficient torsional deformability can be produced even in a low temperature region by optimally adjusting the components of the steel. The present invention has been reached.
That is, the gist of the present invention is as follows.
[0009]
( 1 ) In mass%,
C: 0.002 to 0.05%,
Si: 0.3% or less,
Mn: 0.05 to 1%
P: 0.011 to 0.02%,
S: 0.02% or less (including 0%),
N: 0.002 to 0.007%
Further,
Sol. Al: 3 × N content (%) to 0.1%,
Nb: 2 × N content (%) to 0.08%,
B: (0.1 × P content (%) − 0.0007) to 0.003%
A torsion bar steel excellent in low-temperature ductility, characterized in that the balance is made of Fe and inevitable impurities.
[0010]
( 2 ) The steel for torsion bars excellent in low temperature ductility as described in the above item (1), which is used in a seat belt retractor.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
First, the reason for limiting the chemical composition of steel will be described.
[0012]
C: 0.002 to 0.05%
When the torsion bar is torsionally plastically deformed, ductility is necessary in order to have sufficient energy absorption capability. If C exceeds 0.05%, the pearlite fraction increases and the ductility deteriorates, so the upper limit is made 0.05%. On the other hand, even if the deformation resistance is too small, the absorbed energy is small, so a minimum strength is necessary. When C is less than 0.002%, precipitation hardening does not work. Therefore, the lower limit of the C amount is 0.002%.
[0013]
Si: 0.3% or less Si is an element having a high work hardening ability, but also an element inhibiting ductility. If it exceeds 0.3%, sufficient ductility cannot be obtained, so the upper limit is made 0.3%.
[0014]
Mn: 0.05 to 1%
First, Mn has an action of preventing hot brittleness due to S contained as an impurity. If it is less than 0.05%, hot brittleness cannot be prevented, so Mn is made 0.05% or more. On the other hand, Mn is an element that causes deterioration of ductility. If it exceeds 1%, sufficient ductility cannot be obtained, so the upper limit is made 1%.
[0015]
S: 0.02% or less (including 0%)
S does not bring about a positive effect in the steel of the present invention. From the viewpoint of preventing hot brittleness and reducing MnS which causes a decrease in ductility, it is better to be as small as possible. If over 0.02%, ductility deteriorates, so the upper limit of S is made 0.02%.
[0016]
Sol. Al: 3 × N content (%) to 0.1%
Solid solution N inhibits ductility both at room temperature and at low temperatures. In order to reduce the solid solution N, Sol. Since the amount of Al is more than three times that of N, Sol. Let the lower limit of Al be 3 times the amount of N. On the other hand, if Al exceeds 0.1%, it is sufficient for reducing the solid solution N, and nozzle clogging in continuous casting becomes severe, so the upper limit is made 0.1%.
[0017]
Nb: 2 × N content (%) to 0.08%
In the present invention, Nb has two meanings. One is to sufficiently reduce solid solution N harmful to ductility, and the other is to use precipitation hardening of Nb (C, N) in order to give an appropriate strength to steel. If Nb is less than twice N, solid solution N increases and ductility decreases, and precipitation hardening cannot be obtained. Therefore, the lower limit of Nb is made twice the amount of N. On the other hand, if Nb exceeds 0.08%, Nb (C, N) precipitates hinder dislocation migration and deteriorate ductility, so the upper limit of Nb is 0.08%.
[0018]
N: 0.002 to 0.007%
If N exceeds 0.007%, solid solution N remains and deteriorates ductility, so the upper limit is made 0.007%. If it is less than 0.002%, the precipitation effect of Nb (C, N) will not be effective. Therefore, the lower limit of N is set to 0.002%.
[0019]
Next, the reason why the component ranges of P and B are as follows will be described.
P: 0.011 to 0.02% and B: (0.1 × P content (%) − 0.0007) to 0.003%
Since the addition amounts of P and B are the most important in the present invention, they will be described in detail. C: 0.02%, Si: 0.1%, Mn: 0.3%, S: 0.009%, Sol. In a steel composed of Al: 0.03% and Nb: 0.03%, a steel slab covered with an addition amount in the range of 0 to 0.03% of P and 0 to 0.0035% of B is 1120 ° C. And then hot rolled, finished at 950 ° C., and then air-cooled to produce various steel wires of 12.1 mmφ. This was drawn to 11.6φ, cut to a length of 130 mm, set in a torsion tester, and the specimen was cooled to a temperature of 45 ° C. below freezing. The test piece was continuously twisted and rotated in one direction at a rotation speed of 2.5 rpm. The result of the torsional rotational speed until the breakage is shown in FIG. Here, a sample that has been broken after being rotated six times (that is, 2160 degrees) or more is indicated by a mark ◯, and a sample that has been broken by a rotation less than six times (that is, 2160 degrees) is indicated by a mark X.
[0020]
From FIG. 1, when the P amount is 0.007% or less, the B addition amount is 0 to 0.003%, and it does not break unless it rotates 6 times or more, but the P amount exceeds 0.007%. Then, if B is not added, it will not be able to withstand more than 6 rotations, and if the amount of P exceeds 0.02%, no matter how much B is added, it will break in less than 6 times, and B will be 0.003%. When it exceeded, it turned out that it fractures | ruptures by less than 6 rotations. Therefore, based on this result, the case where the amount of P is as high as 0.011% or more is targeted.
P: 0.011 to 0.02% and B: (0.1 × P content (%) − 0.0007) to 0.003%
Is within the scope of the present invention.
[0021]
In steels with a large amount of C, such as structural steels, it is known that P deteriorates toughness, but P has an adverse effect on torsional deformation at 45 ° C below freezing of ultra-low carbon steel, and B It was found for the first time to have an improving effect. Although this mechanism is not yet clear, it is presumed that P is segregated at the grain boundary to deteriorate ductility at low temperatures even in a very small amount, and B has a role of preventing grain boundary segregation of P. Further, it is presumed that the early fracture when the amount of B is large is caused by deterioration of ductility due to precipitation of Fe ₂₃ (C, B) ₆ .
[0022]
Regarding the production conditions of the steel of the present invention, in order to obtain ductility, in order to prevent the dissolution of AlN and Nb (C, N) as much as possible, the heating temperature before hot rolling is limited to 1150 ° C. or less and introduced by rolling. In order to avoid remaining dislocations, it is preferable to set the finishing temperature to 880 ° C. or higher, and further to air cooling after finish rolling.
[0023]
The present invention will be described in more detail with reference to the following examples. However, these examples are not intended to limit the present invention, and any of the above-described changes in design in accordance with the gist of the invention will be described below. It is included in the scope.
[0024]
【Example】
The steel pieces of 6 kinds of chemical composition shown in Table 1 were melted, and the steel pieces that were rolled in pieces were heated to 1100 ° C and hot-rolled, finished at 940 ° C and air-cooled to produce 12.1 mmφ steel wire did.
[0025]
[Table 1]

[0026]
These steel wires were drawn to 11.6φ, cut to a length of 130 mm, and set in a torsion tester. After the test piece was cooled to a temperature of 45 ° C. below freezing, the test piece was continuously twisted and rotated in one direction at a rotation speed of 2.5 rpm. Table 2 shows the results of the torsional rotational speed until breakage.
[0027]
[Table 2]

[0028]
Trial numbers 1 to 3 are examples of the present invention, and all of them have a sufficient ductility because the number of torsional rotations at 45 ° C. below freezing exceeds 6 times. Test numbers 8 to 10 are comparative examples. Test No. 8 is an example in which sufficient ductility was not obtained because the amount of P in steel was high and B was not added. Test Nos. 9 and 10 are examples in which sufficient ductility was not obtained due to the small amount of B added.
[0029]
【The invention's effect】
From the above results, according to the present invention, by optimizing the components of the steel, it is possible to obtain a steel suitable for a torsion bar having excellent ductility in torsional deformation even in a low temperature range such as a sub-freezing temperature. Therefore, it can be said that the present invention is an invention with extremely remarkable industrial effects.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the number of revolutions until fracture in a torsion test of a rod-shaped test piece cooled to 45 ° C. below freezing point, the P amount and the B amount.

Claims (2)

% By mass
C: 0.002 to 0.05%,
Si: 0.3% or less,
Mn: 0.05 to 1%
P: 0.011 to 0.02%,
S: 0.02% or less (including 0%),
N: 0.002 to 0.007%
Further,
Sol. Al: 3 × N content (%) to 0.1%
Nb: 2 × N content (%) to 0.08%
B: (0.1 × P content (%) − 0.0007) to 0.003%
A torsion bar steel excellent in low-temperature ductility, characterized in that the balance is made of Fe and inevitable impurities. Torsion bar steel with excellent low temperature ductility according to claim 1, characterized in that for use in the seat belt retractor.

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