JP2002194501A - High strength and high toughness steel pipe for airbag and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material excellent in workability, excellent in workability and weldability, and excellent in workability suitable for a steel pipe for an airbag which requires high strength and high toughness, and a method for producing the same. SOLUTION: In mass%, C: 0.05 to 0.20%, Si: 0.50%
%, Mn: 0.50-2.00%, P: 0.020% or less, S:
0.020% or less, sol.Al: 0.10% or less, Mo: 0.10 to 0.50
%, Cr: 0.10 to 1.00%, Cu: 0.10 to 0.50%, Ti: 0.005
~ 0.050%, and, if necessary, Ni: 0.10 ~ 0.
One or more of 50% and Nb: 0.010 to 0.050% are contained, and the austenite grain size after quenching and tempering is 11.0 or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength high-toughness steel pipe which is excellent in workability and weldability, and is suitable for airbags requiring high strength and high toughness of 780 N / mm ² or more, and a method for producing the same About. Especially when the internal pressure burst test is performed at -40 ° C,
The present invention relates to a high-strength and toughness steel tube for an airbag, which does not exhibit a brittle fracture surface, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the automobile industry has been actively introducing devices that pursue safety. Among them, before an occupant collides with a steering wheel, an instrument panel, or the like at the time of a collision, the occupant is required to carry out the same. An airbag system that deploys an airbag between the occupant with gas or the like and absorbs the kinetic energy of the occupant to reduce injuries has been developed and installed.

[0003] As an airbag system, a system using explosive chemicals has conventionally been adopted, but it is expensive.
In addition, a system using an accumulator made of a steel pipe filled with argon gas has recently been developed due to environmental problems and recycling problems. The steel pipe used for the accumulator such as the argon gas keeps the inert gas and the like blown into the airbag at the time of the collision at a high pressure, and adds a small amount of the gas for the ignition of the explosive at the time of the collision, thereby ejecting the gas at once. Since stress is applied at a large strain rate in an extremely short time, unlike a simple structure such as a conventional pressure cylinder or line pipe, high strength, high toughness, high workability and weldability are required. In particular, considering use in cold regions, it is required to have sufficient toughness even at about −40 ° C.

For this purpose, the following proposals have already been made in the prior art. Japanese Patent Application Laid-Open No. 8-3255641 discloses that C: 0.01 to 0.15% and Mo: 0.05 to 0.1%.
50%, V: 0.02 to 0.10%, Ni: 0.05 to 0.50%, Cr: 0.05
1.00%, Cu: 0.05-0.50%, Ti: 0.02-0.10%, Nb:
It is excellent in workability to form steel containing at least one of 0.02 to 0.10% and B: 0.0005 to 0.005%, after performing quenching and tempering treatment, then performing cold drawing and annealing. A method for producing a high-strength, high-toughness steel pipe has been proposed.

[0005] JP-A-10-140238 discloses that C: 0.01 to
0.20%, Mo: 0.50% or less, V: 0.10% or less, Ni: 0.50% or less, Cr: 1.00% or less, Cu: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: A steel pipe for high-strength and toughness airbags in which steel containing at least one of 0.005% or less is hot-formed and then subjected to stress relief annealing, annealing, normalizing, quenching, or quenching and tempering. Manufacturing methods have been proposed.

[0006] JP-A-10-140249 discloses that C: 0.01 to
0.20%, Mo: 0.50% or less, V: 0.10% or less, Ni: 0.50% or less, Cr: 1.00% or less, Cu: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: After producing steel containing at least one of 0.005% or less, 850-1000 ℃
A method for producing a steel pipe for a high-strength and high-toughness airbag, which is subjected to cold working to a predetermined size or subjected to stress relief annealing, normalizing or quenching and tempering after normalizing at a predetermined temperature.

[0007] JP-A-10-140250 discloses that C: 0.01 to
0.20%, Mo: 0.50% or less, V: 0.10% or less, Ni: 0.50% or less, Cr: 1.00% or less, Cu: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: After producing steel containing at least one of 0.005% or less, 850-1000 ℃
A method for producing a steel pipe for a high-strength and toughness airbag, which is subjected to an annealing treatment after being subjected to quenching at a predetermined size while cold working or after cold working.

In JP-A-10-140283, C:
0.01 to 0.20%, Mo: 0.50% or less, V: 0.
10% or less, Ni: 0.50% or less, Cr: 1.00% or less, Cu: 0.50
%, Ti: 0.10% or less, Nb: 0.10% or less, B: 0.005%
% Of steel containing at least one selected from the group consisting of steel of at least one of the following types: high-density by cold-working, or by annealing, normalizing, or quenching and tempering after cold-working to predetermined dimensions. 2. Description of the Related Art Steel pipes for airbags that require high precision, high strength and high toughness have been proposed.

[0009]

However, none of the proposals can satisfy both price and performance as accumulator steel pipes used for airbag systems demanded today.

An object of the present invention is to provide excellent workability and weldability,
Another object of the present invention is to provide a high-strength high-toughness steel pipe excellent in workability suitable for an airbag component requiring high strength and high toughness, and a method for producing the same.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies in order to solve the above-mentioned problems, and as a result, the conventional one has no specified heat treatment conditions and grain size, and has a burst fracture surface characteristic. Is also unknown. When better toughness was required, it was conceived that the size of austenite crystal grains that affect toughness had to be defined.

[0012] In this regard, "High strength ERW-OCTG AP15AC for low temperature"
"Development of L80" (Iron and Steel, Vol. 72, No. 5, p. S462 1986) proposes a method of improving toughness by performing induction hardening to refine austenite grains. However, this is for oil country tubular goods, and the crystal grain size obtained here is 10.8, and sufficient toughness has not been obtained for use as a steel pipe for airbags.

Here, the present inventors have found a specific component suitable for a steel pipe for an accumulator of an airbag system.
Further, after the steel having such a chemical composition is formed, cold working is performed in order to obtain a predetermined dimensional accuracy, and then a predetermined heat treatment is performed, whereby an austenite grain size number of 11.0 or more can be easily realized. It has been found that a high strength and high toughness steel pipe can be obtained.

That is, as a result of examining the conditions necessary for improving the burst fracture surface characteristics required to be improved as a steel pipe for an airbag, as shown in FIG. In a Charpy impact test, it was found that the temperature was lower than the lower limit temperature (vTrs100) at which a ductile fracture rate of 100% could be secured. In particular, the performance required for an airbag is that it does not show a brittle fracture in an internal pressure burst test at −40 ° C., but a material with vTrs100 = −40 ° C. satisfies this performance.

FIG. 1 is a graph showing the results of an internal pressure burst test performed while preparing a material having each vTrs100 including a material used in an embodiment described later and changing the temperature.

In the present invention, the toughness is evaluated by vTrs100, but this is an evaluation in consideration of a sufficient safety factor.
Since the relationship between the austenitic crystal grain size and vTrs100 of the material used in the internal pressure burst test is as shown in FIG. 2, it has been found that the austenitic crystal grain size should be increased in order to improve the toughness.

Here, the present invention is as follows. (1) In mass%, C: 0.05 to 0.20%, Si: 0.50% or less, M
n: 0.50 to 2.00%, P: 0.020% or less, S: 0.020%
Below, sol.Al: 0.10% or less, Mo: 0.10-0.50%, Cr: 0.
10 to 1.00%, Cu: 0.10 to 0.50%, Ti: 0.005 to 0.050
%, The balance being a steel composition consisting of Fe and unavoidable impurities, and austenite grain size after quenching and tempering
High-strength, high-toughness steel pipe for airbags, having a diameter of 11.0 or more.

(2) The steel composition further comprises, by mass%,
The steel pipe for a high-strength and high-toughness airbag according to claim 1, containing at least one of i: 0.10 to 0.50% and Nb: 0.010 to 0.050%.

(3) In mass%, C: 0.05 to 0.20%, Si: 0.5
0% or less, Mn: 0.50 to 2.00%, P: 0.020% or less,
S: 0.020% or less, sol.Al: 0.10% or less, Mo: 0.10-0.
50%, Cr: 0.10-1.00%, Cu: 0.10-0.50%, Ti: 0.00
5 to 0.050%, and if necessary, Ni: 0.10 to
0.50% and Nb: 0.010 to 0.050%, at least one of which is made of a steel material having a steel composition composed of Fe and unavoidable impurities, and then cold-worked the obtained tube material to a predetermined size. A method for producing a steel pipe for a high-strength, high-toughness airbag, comprising: furnace quenching at 900 to 960 ° C .;

(4) In mass%, C: 0.05 to 0.20%, Si: 0.5
0% or less, Mn: 0.50 to 2.00%, P: 0.020% or less,
S: 0.020% or less, sol.Al: 0.10% or less, Mo: 0.10-0.
50%, Cr: 0.10-1.00%, Cu: 0.10-0.50%, Ti: 0.00
5 to 0.050%, and if necessary, Ni: 0.10 to
After 0.50% and Nb: 0.010 to 0.050%, at least one of the remaining portions is made of a steel material having a steel composition composed of Fe and unavoidable impurities, and then the obtained tube material is cold-worked to a predetermined size. A method for producing a steel pipe for a high-strength and tough airbag, wherein the austenite grain size is made 11.0 or more by induction hardening and tempering at 900 to 1000 ° C.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of a steel material used in the present invention, that is, the steel composition as described above, are as follows. In this specification, "%" indicating the steel composition is "% by mass" unless otherwise specified.

C: 0.05 to 0.20% C is an element added to obtain the necessary strength of steel at low cost, but if it is less than 0.05%, sufficient strength cannot be obtained.
The variation in strength increases depending on the cooling rate during induction hardening. On the other hand, if the content exceeds 0.20%, workability and weldability deteriorate, and toughness decreases.
%. A preferred upper limit is 0.15% and a lower limit is 0.05%.

Si: 0.50% or less Si is an element that inhibits the cold workability of steel. If the content exceeds 0.50%, the workability deteriorates. Preferably, it is at most 0.30%.

Mn: 0.50 to 2.00% Mn is an effective element for improving the strength and toughness of steel. However, if it is less than 0.5%, sufficient strength and toughness cannot be obtained. 0.50-2.
00%. The preferred lower limit is 0.80% and the upper limit is 1.60%.

P: 0.020% or less P causes a decrease in toughness due to segregation at the P grain boundary.
% Or less. S: 0.020% or less S is combined with Mn in steel to form inclusions due to MnS, thereby deteriorating workability and lowering toughness.

Al: 0.10% or less Al is an effective element for improving workability.
%, The effect is reduced, so the content is set to 0.10% or less.

By limiting the chemical composition ratio in the steel as in the above steel composition, sufficient strength, toughness, high workability and weldability as a steel pipe for an accumulator of an airbag can be obtained. , Ni, Cr, Cu, Ti, Nb
Higher strength and toughness can be obtained by adding more than one kind. Each of these additives has the effect of improving the toughness, but the reasons for limiting the content are as follows.

Mo: 0.10 to 0.50% Mo has the effect of increasing the strength by solid solution strengthening and improving the quenchability to improve the toughness. However, if it is less than 0.10%, there is no effect, and if it exceeds 0.50%, the welding is effected. Since the part is hardened and the toughness is rather reduced, the content is set to 0.10 to 0.50%.

Ni: 0.10 to 0.50% Ni is an element effective for improving hardenability and toughness. However, if it is less than 0.10%, there is no effect.
Although it has the effect even if it exceeds, it is expensive, so 0.10 ~ 0.
50%.

Cr: 0.10 to 1.00% Cr is an effective element for improving the strength, toughness and corrosion resistance of steel. However, if it is less than 0.10%, there is no effect, and if it exceeds 1.00%, the workability and the toughness of the welded portion are increased. 0.
10 to 1.00%.

Cu: 0.10 to 0.50% Cu is an effective element for improving the corrosion resistance and toughness of steel. However, if it is less than 0.10%, there is no effect, and if it exceeds 0.50%, the hot workability deteriorates. 0.10 to 0.50%.

Ti: 0.005 to 0.05% Ti is effective in improving the toughness by making the structure finer, but when it is less than 0.005%, there is no effect, and when it exceeds 0.05%, the toughness is adversely deteriorated. 0.05%.

Nb: 0.01 to 0.05% Nb is effective in improving toughness by refining the structure like Ti, but is ineffective at less than 0.01% and conversely deteriorates toughness at more than 0.05%. Therefore, it was set to 0.01 to 0.05%.

In addition, it is permissible for V to be contained in a total amount of about 0.01% or less. The balance is Fe except for inevitable impurities. According to the present invention, the steel pipe having such a steel composition has an austenite grain size of 11.0 or more after quenching / tempering. If the crystal grain size is less than 11.0, sufficient strength and toughness cannot be obtained. Preferably it is 12.0 or more. The austenite grain size can be adjusted by changing the quenching and tempering conditions.

As mentioned earlier, the austenite grain size is a grain size determined by the temperature and the holding time when steel is heated above the transformation point. There are two types of austenitic stainless steel, one in which the austenitic crystal grain size is revealed by heat treatment of carbon steel or the like, and the other in which austenitic stainless steel in which actual crystal grains are already generated. The case of the steel of the present invention corresponds to the former. Originally, the austenite crystal grains in the former case should be referred to as “old austenite crystal grains” because no austenite structure exists in the temperature range below the A ₁ transformation point. Therefore, in the present invention, it is also called "austenite crystal grain".

Further, the steel of the present invention has a fine martensitic bainite structure because it is subjected to quenching and tempering heat treatment. Therefore, Bech described in JIS G 0551
Austenitic crystal grains were revealed by et-Beaujard method by corroding steel with a saturated aqueous solution of biculic acid without heat treatment.

In the present invention, a pipe is made from a steel material whose chemical composition has been adjusted as described above. The steel pipe manufactured in this manner can secure sufficient strength, toughness, high workability and weldability for an accumulator of an airbag.

In order to obtain high strength and high toughness in a steel pipe made of a steel material whose chemical composition has been adjusted as described above, it is desirable to perform quenching and tempering after cold working. In general, there are two types of quenching: furnace quenching in which heating is appropriately performed in a heating furnace, and induction heating quenching (induction quenching). In the case of quenching at 900 to 960 ° C., steel having the steel composition according to the present invention is hardened. In this case, the austenite grain size can be 11.0 or more, and the toughness required for a steel pipe for an airbag can be obtained.

In the case of furnace quenching at a temperature lower than 900 ° C., sufficient strength cannot be obtained because of insufficient quenching. On the other hand, in the case of furnace quenching at a temperature higher than 960 ° C., austenite crystal grains become coarser. High toughness cannot be obtained.

In order to obtain higher toughness, induction hardening is effective. After induction hardening,
Due to the effects of rapid heating and rapid cooling, finer austenite grains can be obtained than in the case of quenching in a furnace.

Therefore, the holding time at 900 to 1000 ° C. is 10 seconds.
Sufficient high strength and high toughness can be obtained by performing the following induction hardening and tempering treatment to make the austenite crystal grain size 11.0 or more. At a quenching temperature of less than 900 ° C., sufficient strength cannot be obtained because quenching cannot be performed sufficiently. In the case of induction quenching, the required toughness can be obtained even at a quenching temperature of 960 ° C. or more because of short-time heating, but if it exceeds 1000 ° C., the austenite crystal grains are coarsened and the required toughness cannot be obtained.

The holding time in normal induction hardening is 10
Although it is less than sec, the shorter the holding time, the finer the crystal grains and the better the toughness. Since the change in dimension due to induction hardening has reproducibility, higher dimensional accuracy can be obtained by determining the dimension of the cold working in advance in consideration of the dimensional change.

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

[0044]

EXAMPLE A billet of the steel of the present invention having the chemical composition shown in Table 1 and a comparative steel were drilled and rolled by a Mannesmann-mandrel mill method, and then an outer diameter of 76.2 was reduced with a reducer.
mm, a seamless steel pipe finished to a thickness of 4.0 mm, cold drawn, finished to an outer diameter of 60.33 mm and a thickness of 3.35 mm, quenched,
After the tempering treatment, various properties of the obtained steel material were evaluated.

The properties were evaluated for strength, toughness, and austenite grain size. For the strength, a tensile test was performed using a JIS No. 11 test piece. As for the toughness, as shown in FIG. 3, a V-notch is placed in a plate 2 cut out from the circumferential direction of the steel pipe 1 and developed and processed, and an impact test piece 3 according to JIS Z 2202 (10 mm × 2.5 mm-2V notch) A Charpy impact test was carried out using the sample and evaluated at a lower limit temperature (vTrs100) at which a ductile fracture ratio of 100% could be secured. About the austenite grain size, the circumferential section of the steel pipe was measured by a method specified in JIS G 0551.

The results are summarized in Tables 2 and 3. As shown in Table 2, in steel Nos. 1 to 13 of test symbols A to M, 90
By performing quenching and tempering at 0 to 960 ° C., the austenite crystal grain size becomes 11.0 or more, and good strength and toughness can be obtained.

On the other hand, steel Nos. 1, 5 and
In Nos. 8 and 11, quenching from a temperature of less than 900 ° C. or more than 960 ° C. does not provide sufficient strength and toughness. Further, steel Nos. 13 to 20 having test symbols R to Z did not have sufficient strength and toughness despite being quenched in the range of 900 to 960 ° C.

As shown in Table 3, steels N with test symbols AA to AM
o. Austenite grain size of 11.0 was obtained by induction hardening at 900-1000 ° C and holding time of 3 sec.
As described above, good strength and toughness can be obtained.

On the other hand, steel Nos. 1, 5 and
In the case of 8 and 11, since the induction hardening was performed at 900 ° C or lower or 1000 ° C or higher, sufficient strength and toughness could not be obtained. Steel Nos. 14 to 22 indicated by test symbols AR to AZ did not have sufficient strength toughness despite induction hardening at 900 to 1000 ° C. and a holding time of 3 seconds or less.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

As described above, according to the present invention,
It can be seen that by defining the steel composition and the austenite grain size after quenching and tempering, a desired low-temperature toughness can be ensured and a steel pipe useful as a steel pipe for an air bag can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of an internal pressure burst test.

FIG. 2 is a graph showing the relationship between austenite grain size and toughness value.

FIG. 3 is a schematic explanatory view showing how a test piece is cut out in an example.

 ──────────────────────────────────────────────────の Continued on the front page F term (reference) 4K042 AA06 BA01 BA02 BA05 BA11 CA05 CA06 CA08 CA09 CA10 CA12 DA01 DA02 DB01 DB07 DC02 DC03

Claims (4)

[Claims] 1. Mass%: C: 0.05 to 0.20%, Si: 0.50% or less, Mn: 0.50 to 2.00
%, P: 0.020% or less, S: 0.020% or less, sol.
10% or less, Mo: 0.10 to 0.50%, Cr: 0.10 to 1.00%, C
u: 0.10 to 0.50%, Ti: 0.005 to 0.050%, the balance having a steel composition consisting of Fe and unavoidable impurities,
Austenite grain size after quenching and tempering is 11.0
A steel pipe for a high-strength, high-toughness airbag characterized by the above. 2. The steel composition according to claim 1, further comprising:
The steel pipe for a high-strength and high-toughness airbag according to claim 1, which contains at least one of: 0.10 to 0.50% and Nb: 0.010 to 0.050%. 3. Mass%: C: 0.05 to 0.20%, Si: 0.50% or less, Mn: 0.50 to 2.00
%, P: 0.020% or less, S: 0.020% or less, sol.
10% or less, Mo: 0.10 to 0.50%, Cr: 0.10 to 1.00%, Cu:
0.10 to 0.50%, Ti: 0.005 to 0.050%, Ni: 0.10 to 0.50% and Nb: 0.010%
After the pipe is made from a steel material having a steel composition comprising at least one of 0.055% and Fe and unavoidable impurities, the obtained pipe material is cold-worked to a predetermined size, and then 900-96%.
A method for producing a steel pipe for a high-strength, high-toughness airbag, wherein austenite crystal grain size is 11.0 or more by furnace quenching and tempering at 0 ° C. 4. Mass%: C: 0.05 to 0.20%, Si: 0.50% or less, Mn: 0.50 to 2.00
%, P: 0.020% or less, S: 0.020% or less, sol.
10% or less, Mo: 0.10 to 0.50%, Cr: 0.10 to 1.00%, Cu:
0.10 to 0.50%, Ti: 0.005 to 0.050%, Ni: 0.10 to 0.50% and Nb: 0.010%
After the pipe is made from a steel material having a steel composition consisting of at least one of 0.055% and Fe and unavoidable impurities, the obtained pipe material is cold-worked to a predetermined size, and then 900 to 10%.
A method for producing a steel pipe for a high-strength, high-toughness airbag, wherein an austenite crystal grain size is 11.0 or more by induction hardening and tempering at 00 ° C.