JP2003171738A - Steel tube with high strength and high toughness for air bag, and accumulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel tube for air bag which functions with certainty even in cold districts and causes no brittle fracture even under high pressure at ≤-40°C. <P>SOLUTION: L/T, as the ratio of the X-ray integrated intensity of ä110} plane in the plane perpendicular to the axial direction L of the steel tube to that in the plane perpendicular to the circumferential direction T of the steel tube, is made to ≤4. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength and high-toughness airbag steel tube suitable for an airbag component, and particularly for a pressure accumulator thereof. Furthermore, the present invention relates to a pressure accumulator for an airbag, which is composed of such a steel pipe.

[0002]

2. Description of the Related Art In recent years, in the automobile industry, introduction of devices pursuing safety has been actively promoted. One of them is the airbag system. This is installed in an automobile, and before the occupant collides with the steering wheel or instrument panel at the time of a collision, deploys an airbag between the occupant and the occupant with gas to absorb kinetic energy of the occupant and reduce obstacles. It is a system that aims at

In the air bag system, a system using explosive chemicals has been conventionally used, but in recent years, due to environmental problems and recycling problems, a pressure accumulator made of a steel pipe filled with argon gas has been used in recent years. The method to do was developed.

Steel pipes used for accumulators filled with high-pressure inert gas such as argon gas (hereinafter, steel pipes for airbags) are
Normally, after maintaining a high pressure of an inert gas or the like, at the time of collision, a small amount of explosive powder is ignited to expel the gas at once, so stress is applied at a high strain rate in an extremely short time.
Therefore, the steel pipe for an air bag is required to have high strength and high toughness as well as high workability and weldability, unlike conventional structures such as a pressure cylinder and a line pipe. Further, considering use in cold regions, it is required to have sufficient toughness even at about -40 ° C. In recent years, it has become even more demanding to secure safety in cold regions, and there is a strong demand for this low temperature toughness.

The steel tube used as the material for the pressure accumulator is generally cut to a required length with an outer diameter of 25 to 100 mm and a wall thickness of 2 to 5 mm.

[0006]

Even in the prior art, for example, Japanese Patent Laid-Open Nos. 8-325641 and 10-140249 have already been used.
In Japanese Patent Laid-Open No. 10-140250 and Japanese Patent Laid-Open No. 10-140283, various types of high-strength and high-toughness steel pipes for airbags in which the components and the processing method are defined are proposed.

However, any of the proposals requires further improvement in order to sufficiently meet the above-mentioned demands of today.
Therefore, the present inventors have paid attention to such a texture of the steel pipe, and have conceived to improve the structure itself which affects the toughness when a better toughness is required.

A first object of the present invention is to provide a steel pipe suitable for an airbag requiring high toughness. In particular, one of the performances required for airbags is that they operate reliably even in cold regions, so it is safe that the steel pipes for airbags, that is, the accumulators, do not brittlely break even under low temperature and high pressure. Requested above.

Therefore, a second object of the present invention is to provide a pressure accumulator for an air bag, which is made of a steel pipe and can assure this low temperature toughness.

[0010]

The low temperature toughness required for a steel pipe for an air bag is evaluated by a burst test by internal pressure in a low temperature atmosphere. This burst test is 30 cm long
A lid is welded to both ends of a steel pipe of a certain size, and a test body having a structure in which a pressure medium can be injected from one end is prepared, and the test is performed in an atmosphere temperature controller whose circumference is covered with a steel plate for safety. The breaking pressure at this time is, for example, outer diameter 40 mm, wall thickness 2.6 mm, length 30
Even if the steel pipe of 0 mm has sufficient strength at 150 MPa or more even at -40 ° C, it may break brittlely and the opening may run over the entire length of the test piece.

In the present invention, such a phenomenon is prevented by ductile fracture in order to secure further safety in the automobile. Therefore, the present inventors have made efforts to clarify the factors that improve the low temperature toughness in order to solve the above problems. In particular, as a result of intensive studies on the influence of the texture on the low temperature toughness, the anisotropy rather than the composition of the texture has a great influence, and the anisotropy of the specific plane orientation improves the low temperature toughness. Knowing that they are closely related, the present invention has been completed.

The present invention is as follows. (1) In the plane perpendicular to the axial direction L and the circumferential direction T of the steel pipe, {11
A steel pipe for an air bag, wherein the L / T ratio of the X-ray integrated intensity ratio of the 0} plane is 4 or less.

(2) It is characterized in that it is composed of a steel pipe in which the L / T ratio of the X-ray integrated intensity ratio of the {110} plane in the plane perpendicular to the axial direction L and the circumferential direction T of the steel pipe is 4 or less. Accumulator for airbags.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 1 is a schematic view of a steel pipe 10,
The L-plane is shown as a vertical hatch and the T-plane is shown as a diagonal hatch. Here, in the prior art, "brittle fracture and the opening runs over the entire length of the test piece" means that brittle fracture occurs along the T-plane in the figure.

Therefore, first, in order to evaluate the texture of the steel pipe according to the present invention, the L and T planes shown in FIG.
The X-ray integrated intensity ratio of the 0}, {200}, {211}, and {222} planes was measured.

Here, L and T mean surfaces perpendicular to the axial and circumferential stress axes, respectively. The X-ray integrated intensity ratio is a value obtained by dividing the peak area of a specific plane orientation obtained by X-ray spectroscopy by the peak area of the same plane orientation obtained from a crystallographically disordered sample such as a powder sample. is there.

In the present invention, in order to further evaluate the anisotropy of the texture, the X-ray integrated intensity ratio of the L plane having a specific plane orientation is further divided by the X-ray integrated intensity ratio of the T plane for comparison. As a result, the value on the {110} plane changed the most when the manufacturing conditions were changed. Therefore, the texture was evaluated by the L / T ratio (displayed as L / T110) of the X-ray integrated intensity ratio of the {110} plane. I found that I can do it.

Generally, the cleaved surface of iron is said to be {110}, and therefore, as long as such an idea is followed, L / T11
If 0 is greater than 1, for example 10 or more,
The {110} planes do not accumulate on the T-plane, which is considered to be rather advantageous for preventing brittle fracture during internal pressure burst fracture. Contrary to expectations, however, L / T110 should be closer to 1 (eg, 4 or less) and the {110} planes should be equally integrated on the L and T planes in Fig. 1 to prevent brittle fracture during internal pressure burst fracture. Turned out to be valid.

That is, FIG. 2 shows the {110} plane, {200} of the conventional steel pipe for comparison (comparative) and the steel pipe of the present invention (invention).
It is a graph which shows the result when L / T was measured about the plane, {211} plane, and {222}.
An overwhelming anisotropy of the 0} plane to the L plane was observed. This means that {110} planes are not accumulated on the T plane.

FIG. 3 shows the test result at this time. This is an investigation of the texture and internal pressure burst characteristics of steel pipes manufactured under various conditions. In the figure, A and B indicate the steel types in Table 1 described later, which are indicated by the symbols ◯ and Δ, respectively.
The texture is changed by changing the heat treatment conditions. The white symbols indicate the cases outside the scope of the present invention, and the black symbols indicate the cases within the scope of the present invention.

Based on such knowledge, the fracture surface after the rupture test from 0 ° C. to −80 ° C. at intervals of 20 ° C. was examined, and the internal pressure burst property was determined at the critical temperature (Tb) showing a ductile fracture surface. evaluated. As a result, it was found that in order to improve the internal pressure burst performance, the anisotropy of the texture of {110} plane orientation in the T and L planes should be reduced, that is, L / T110 should be close to 1. did.

However, the burst test also has a problem that the test is troublesome and expensive. In order to solve this, the results of the burst test and the general-purpose Charpy impact test were also compared and evaluated.

Regarding toughness, as shown in FIG.
V notch on plate 32 cut out from the circumferential direction of
An impact test piece containing 34 was used. This is because the crack propagates in the same axial direction of the steel pipe as in the burst test. In the Charpy impact test, the ductility fracture rate was evaluated at the lower limit temperature (vTrs100) that can ensure 100%.

From these results, as shown in FIG. 5, the lower limit temperature (Tb) showing a ductile fracture surface in the internal pressure burst test is vTrs10.
It was found that it almost matches 0. That is, this means that the internal pressure burst rupture property can be estimated by evaluating vTrs100 even in the Charpy impact test.

FIG. 7 collectively showing the results of Examples described later.
The effect of L / T110 on this vTrs100 is also shown, but the same tendency as in FIG. 3 is recognized, and the effectiveness of the present invention is clear by the evaluation of this vTrs100.

In the present invention, the steel composition is not particularly limited as long as the above-mentioned predetermined texture can be obtained, but in a preferred embodiment, so-called carbon steel or low alloy steel is used, which has the following composition, for example. Is.

% By mass, C: 0.01 to 0.30%, Si: 0.50%
Below, Mn: 0.20 to 2.0%, P: 0.025% or less, S: 0.02
% Or less, Al: 0.1% or less. More preferably, one or more of the following components may be added if necessary in order to improve heat treatment characteristics and strength / toughness.

Cu: 0 to 0.5%, Cr: 0 to 1.0%, Ni: 0
~ 1.5%, Mo: 0 ~ 0.5%, Ti: 0 ~ 0.1%, V: 0 ~
0.2%, Nb: 0-0.1%, B: 0-0.005%, Ca: 0-
0.005%, Mg: 0 to 0.005%.

Examples of the method for manufacturing a steel pipe according to the present invention include the following. Hot working: Seamless steel pipe is manufactured by performing hot piercing and rolling such as extrusion and Mannesmann method. When this is subjected to heat treatment such as quenching and tempering, the anisotropy is further improved. However, if higher dimensional accuracy is required for steel pipes for airbags, the following cold working is necessary.

Cold working: A seamless steel pipe produced by hot piercing / rolling is subjected to cold drawing or rolling with a hole-type roll, followed by heat treatment such as quenching and tempering. Is effective in reducing the anisotropy of the texture.
If particularly high dimensional accuracy is required, L / T can be obtained by performing light cold working with a reduction in area of 10% or less after heat treatment.
If 110 can be set to 4 or less, the burst rupture property required for a steel pipe for an airbag can be maintained.

Performing the quenching treatment with a high-frequency heating device is effective in suppressing generation and deformation of thick oxide scale during heat treatment. FIG. 6 is a schematic sectional view showing a pressure accumulator for an airbag according to the present invention. In the figure, a pressure accumulator 1 according to the present invention is a tube body 5 obtained by cutting a steel pipe and then drawing both ends.
And a lid 3 welded to one end of the pipe body 5. The airbag actuating device 2 is attached to one end of the tubular body 5. When the airbag actuating device 2 senses a shock and operates, the high pressure gas filled in the pressure accumulator 1 is ejected to activate the airbag 4 shown by a broken line in the figure.

Next, the function and effect of the present invention will be described with reference to the embodiments.

[0033]

[Examples] Billets of steel A and steel B having the chemical compositions shown in Table 1 were used for perforation and rolling by the Mannesmann-mandrel mill system, and then an outer diameter of 70 mm was obtained by a reducer.
A seamless steel pipe finished to a wall thickness of 4.0 mm was subjected to cold drawing, quenching and tempering, and the anisotropy of the obtained texture was measured and the toughness was evaluated.

Cold finish dimensions are 60 mm outer diameter, 3.5 mm wall thickness
Based on quenching at 930 ° C and tempering at 550 ° C, L / T110 was changed by changing the cold workability and heat treatment temperature.
The heat treatment conditions are as follows: tensile strength is 700 for steels A and B,
Selected to be 800MPa or more.

As to toughness, as shown in FIG. 4, an impact test piece according to JIS Z 2202 (10 mm x 2.5 mm-
Charpy impact test using 2V notch)
It was evaluated at the lower limit temperature (vTrs100) that can ensure a ductile fracture surface ratio of 100%.

The results are summarized in Table 2 and FIG. In the figure, "A" and "B" indicate the steel types shown in Table 1, respectively, and
This is indicated by the symbol Δ. The texture is changed by changing the heat treatment conditions. The white symbols indicate the cases outside the scope of the present invention, and the black symbols indicate the cases within the scope of the present invention.

As can be seen from these results, L / T
It was shown that when 110 was 4 or less, Tb and vTrs100 were -40 ° C or less, and sufficient low temperature toughness was obtained as a steel pipe for an air bag.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

As described above, according to the present invention, a high-strength steel pipe having sufficient toughness even at a low temperature of -40 ° C can be obtained, and a product of excellent quality which has never been obtained as a steel pipe for an airbag can be obtained. As a result, the practical significance of the present invention is great.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a test surface for obtaining an X-ray integrated intensity ratio of a steel pipe according to the present invention.

FIG. 2 is a graph showing the anisotropy of tissues in the example of the present invention and the comparative example.

FIG. 3 is a graph showing the relationship between texture and internal pressure burst characteristics.

FIG. 4 is an explanatory view of collecting test pieces in a toughness test of a steel pipe according to the present invention.

FIG. 5 is a graph showing the results of the Charpy impact test and the results of the burst test in comparison.

FIG. 6 is a schematic sectional view of a pressure accumulator for an airbag according to the present invention.

FIG. 7 is a graph showing a summary of the results of Examples.

Claims (2)

[Claims] 1. A steel pipe for an air bag, wherein the L / T ratio of the X-ray integrated intensity ratio of the {110} plane in a plane perpendicular to the axial direction L and the circumferential direction T of the steel pipe is 4 or less. 2. An air comprising a steel pipe having an L / T ratio of X-ray integrated intensity ratio of {110} plane in a plane perpendicular to the axial direction L and the circumferential direction T of the steel pipe is 4 or less. Accumulator for bag.