KR20130020811A - Process for producing steel pipe for air bag - Google Patents

Abstract

The manufacturing method of the high strength and high toughness airbag steel pipe which can simplify the cold drawing process and reduce the alloy cost is mass%, C: 0.04 to 0.20%, Si: 0.10 to 0.50%, Mn: 0.10 to 1.00%, P: 0.025% or less, S: 0.005% or less, Al: 0.10% or less, Cr: 0.01% to 0.50%, Cu: 0.01% to 0.50%, Ni: 0.01% to 0.50%, and the balance is seamless from steel composed of Fe and unavoidable impurities After steelmaking without steel pipes, cold drawing is carried out to the seamless steel pipe at a processing rate of 40% or more at least once to obtain a predetermined dimension, and heated to a temperature of at least Ac ₃ at a temperature rising rate of 50 ° C / s or more, and then at least It cools so that cooling rate of temperature range of 850-500 degreeC may be 50 degree-C / s or more, and harden to said steel pipe, and then temper to the temperature below Ac ₁ point temperature.

Description

Manufacturing method of steel pipe for airbag {PROCESS FOR PRODUCING STEEL PIPE FOR AIR BAG}

The present invention has a high strength of 900 MPa or more, suitable for an air bag, and has a high toughness of vTrs100 (minimum Charpy wave transition temperature at which the ductility is 100%) of -60 ° C. The present invention relates to an inexpensive method for producing a steel pipe.

In recent years, introduction of the device which pursued safety was actively promoted in the automobile industry. As one such device, an airbag system has been developed and mounted on many automobiles. The airbag system is a system in which an airbag is deployed with gas or the like between them and the rider before the rider collides with a handle, an instrument panel, or the like at the time of collision, thereby absorbing the kinetic energy of the rider and reducing injury. As an airbag system, a method of using an explosive chemical was originally employed, but in recent years, a method of using a high-pressure filling gas has been developed, and its application has been expanded.

In an airbag system using a high-pressure filling gas, a gas for deployment, such as an inert gas (eg, argon) that is blown into the airbag at the time of a collision, is kept at a high pressure at all times in a accumulator (accumulator) connected to the airbag, and The air bag is blown out from the accumulator to the air bag at once to deploy the air bag. Accumulators are generally manufactured by subjecting a steel pipe cut to a suitable length to shaft diameter processing as necessary, and then welding the lids at both ends.

Therefore, the steel pipe used for the accumulator (henceforth an airbag accumulator or simply an accumulator) of an airbag system is loaded with stress at a large distortion speed in a very short time. For this reason, this type of steel pipe requires high dimensional accuracy, workability and weldability, unlike conventional structures such as pressure cylinders and line pipes, and also requires high strength and excellent burst resistance.

In recent years, weight reduction of automobiles is strongly demanded. In view of this, thinner and lighter weights are required for airbag steel pipes for automobiles. Accumulators made of high strength seamless steel pipes with a tensile strength of 900 MPa or more and 1000 MPa or more are used for the air bag system in order to secure a high burst pressure even in thin thickness. It became. For example, in the case of an accumulator made of a seamless steel pipe having an outer diameter of 60 mm and a thickness of 3.55 mm, the burst pressure is only about 100 MPa at TS of 800 MPa, whereas when TS is 1000 MPa, the burst pressure is improved to 130 MPa. At the same time, if the outer diameter and the required burst pressure of the airbag accumulator are constant, a thickness of about 20% can be reduced.

For example, even in a cold region, the accumulator needs excellent low-temperature toughness so that the accumulator is not brittle and destroyed in the event of a collision.

In view of this, the seamless steel pipe for the accumulator has been realized by high strength and toughness by quenching and tempering. Specifically, for the accumulator, low-temperature toughness, in which the wavefront is ductile in the Charpy impact test at -60 ° C (that is, vTrs100 is -60 ° C or less), preferably the wavefront in the Charpy impact test at -80 ° C Low temperature toughness is required (vTrs100 is -80 ° C. or less), indicating ductility.

For seamless steel pipes for high strength and high toughness airbag systems, for example, Patent Document 1 discloses a seamless steel pipe using steel materials having a chemical composition in a predetermined range, and cold drawing the seamless steel pipes. Iii) processing to form a steel pipe of a predetermined dimension, and then quenched after heating to a temperature within the range of Ac ₃ or more and 1050 ° C or less, and then quenched at a temperature within the range of 450 ° C or more and Ac ₁ or less. A method for producing a seamless steel pipe for an air bag, which is characterized by performing a quenching tempering treatment, has been proposed.

By this method, it is excellent in workability and weldability at the time of manufacturing airbag inflator, and as inflator, it has tensile strength of 900 MPa or more and high toughness which shows ductility in drop test at -60 ° C on inverse steel pipe. We are trying to get a seamless steel pipe. However, showing ductility in a drop test at -60 ° C does not necessarily mean showing ductility in a burst test at -60 ° C.

Patent Document 2 proposes a method of producing a steel pipe for an airbag system having a high tensile strength exceeding 1000 MPa by performing high frequency induction heating quenching and fine granulation by rapid heating. For example, when a seamless steel pipe is used as a primary pipe, a seamless steel pipe is hot-rolled using a steel material of a specific range of chemical composition, and cold drawing is performed on the seamless steel pipe to obtain a steel pipe having a predetermined dimension. The steel pipe is quenched after heating, and then tempered at a temperature below Ac ₁ transformation temperature. By tempering after quenching, high toughness which shows ductility is obtained also preferably in the burst test of -80 degrees C or less.

However, in the methods disclosed in Patent Documents 1 and 2, as shown in the specific examples, in order to obtain a steel pipe having a tensile strength of 1000 MPa or more and high toughness, it was necessary to contain a large amount of expensive alloys such as Cr and Mo. . In the case of patent document 1, it becomes Cr + Mo: 1.0-2.5 mass%, and in patent document 2, the steel material of Cr + Mo: 0.92 mass% is adopted in most cases. When a large amount of Cr and Mo are contained, in addition to the high raw material cost caused by expensive Mo, in particular, the strength of the steel pipe tends to be high after hot pipemaking of a seamless steel pipe, and subsequent cold drawing processing becomes difficult. Thereby, softening annealing is required before cold drawing processing, and the process becomes complicated and manufacturing cost increases.

Patent document 3 using Cr + Mo: 1.0-1.18 mass% steel also has the same problem as that of patent document 1, 2.

Patent document 4 discloses a steel composition containing Cr, Mo, Cu, and Ni for a seamless steel pipe having excellent burst resistance, and its characteristics are evaluated by Cr + Mo: 0.76 mass% or more. It is a steel pipe, and the tensile strength at this time is also 947 MPa at most.

Japanese Patent Publication 2004-76034 WO 2004 / 104255A1 US 2005 / 0076975A1 WO 2002 / 079526A1

In the conventional steel pipe for airbags, in order to ensure high strength and high toughness, reinforcement was aimed at by adding Cr and Mo. However, this method not only increases the alloy cost, but also makes it difficult to cold draw after the steelmaking. For this reason, if the difference between the size of the seamless pipe of a primary pipe | tube and the size of the steel pipe for airbags which are final products is large, it is necessary to repeat cold drawing process several times in a cold drawing process. In that case, each time the cold drawing process is performed, because the intermediate softening annealing is performed to the desired product size, the manufacturing cost increases.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing an inexpensive, high strength and highly tough airbag steel pipe for conventional airbags by a means that is cheaper than conventional methods by simplifying a cold drawing process or reducing an alloy cost. do.

On the other hand, an object of this invention is to provide the method of manufacturing the steel pipe for thin and small diameter airbags equivalent to or more than the conventional one, using the raw material and manufacturing method of lower cost than the conventional one.

The present inventors have found that the steel pipe for high-strength airbags of the prior art relies on reinforcement by Cr and Mo, resulting in high strength after the end of hot tubing, resulting in a decrease in productivity in cold drawing and an increase in alloy cost. In view of the fact that the use of these alloying elements is suppressed as much as possible, an alloy composition and a manufacturing method capable of securing high strength of tensile strength of 900 MPa or more and excellent low-temperature toughness of vTrs100 of -60 ° C or less have been studied.

As a result, the following findings were obtained and the present invention was reached.

(a) In the production of steel bags for airbags which are quenched and tempered after cold drawing, if the heating and cooling conditions are well set during quenching, high strength and low temperature toughness can be secured without necessarily containing a large amount of Cr and Mo. Can be. In particular, it is effective to contain Cu and Ni in place of Cr or Mo.

(b) The steel which reduced Cr and Mo, and instead contained Cu and Ni, is easy to cold-draw after hot piping, and the workability in one cold drawing process in a cold drawing process (reduction rate) I)) can be enlarged, and the cold drawing process can be simplified.

The present invention is, in mass%, C: 0.04 to 0.20%, Si: 0.10 to 0.50%, Mn: 0.10 to 1.00%, P: 0.025% or less, S: 0.005% or less, Al: 0.10% or less, Cr: 0.01 -0.50%, Cu: 0.01-1.50%, Ni: 0.01-1.50%, the remainder being cold-drawn once in the manufacturing process which performs hot piping of seamless steel pipe from the steel which consists of Fe and an unavoidable impurity, and the obtained seamless steel pipe The cold drawing process of performing the cold drawing process more than 40% of the reduction | deduction rate of processing at least once, and obtaining a steel pipe of a predetermined dimension, and heating the cold drawn steel pipe to the temperature of Ac ₃ or more with the temperature increase rate of 50 degree-C / s or more, And a heat treatment step of performing quenching by cooling so that the cooling rate in the temperature range of at least 850 to 500 ° C. is 50 ° C./s or more, and then tempering to a temperature equal to or lower than the Ac ₁ point temperature. It is a manufacturing method of steel pipe for airbags.

When enumerating the preferable aspect of the manufacturing method of the steel pipe for airbags which concerns on this invention, it is as follows:

The said steel may further contain the following 1 type, or 2 or more types of elements as needed:

Mo: less than 0.10%

At least one of Nb: 0.050% or less, Ti: 0.050% or less, and V: 0.20% or less;

At least one of Ca: 0.005% or less and B: 0.0030% or less.

It is preferable that the concentrations of Cu, Ni, Cr, and Mo in the steel satisfy the following formula (1):

Cu + Ni ≧ (Cr + Mo) ² +0.3... (One)

The element symbol of Formula (1) means the numerical value at the time of expressing content of these elements by the mass%. However, when Mo is not contained, let Mo = 0 (zero).

The thickness of the steel pipe after the end of the cold drawing process is preferably 2.0 mm or less. The cold drawing step is preferably performed by one cold drawing.

In the heat treatment step, heating for hardening is preferably performed by high frequency induction heating, in which case, it is preferable to correct the steel pipe obtained by the cold drawing step before heating for hardening.

According to the present invention, it is possible to suppress the amount of expensive Mo to 0 or a small amount, and to produce a steel pipe for an air bag having a high strength of 900 MPa or more and an excellent low-temperature toughness of vTrs100 of -60 ° C or less. In addition, since the strength of the seamless steel pipe obtained by hot canning is too high, the processing rate in the subsequent cold drawing process can be increased compared with the conventional one, and the number of cold drawing requiring intermediate soft annealing in between can be reduced. . Therefore, according to the present invention, both the alloy cost and the manufacturing cost of the steel pipe for airbag can be reduced as compared with the conventional one.

Hereinafter, the chemical composition and manufacturing process of the steel pipe for airbags of this invention are demonstrated more concretely.

(A) chemical composition of steel

In this specification, "%" regarding the chemical composition of steel means "mass%." Remainder of the chemical composition of steel except the element described below is Fe and an unavoidable impurity.

C: 0.04 to 0.20%

C is an element effective in increasing the strength of steel at low cost. If the content is less than 0.04%, it is difficult to obtain high strength (tensile strength). If the content exceeds 0.20%, workability and weldability decrease. Therefore, content of C is made into 0.04% or more and 0.20% or less. The preferable range of C content is 0.07% or more and 0.20% or less, and more preferable ranges are 0.12% or more and 0.17% or less. When aiming at the tensile strength of 1000 MPa or more, it is preferable to contain C 0.06% or more.

Si: 0.10 to 0.50%

Si is an element which not only has a deoxidation effect but improves hardenability of steel and improves strength. The content of Si is made 0.10% or more for this purpose. However, when the content exceeds 0.50%, the toughness decreases, so the content of Si is made 0.50% or less. The preferable range of Si content is 0.20% or more and 0.45% or less.

Mn ： 0.10 ~ 1.00%

Mn is an element which is effective in improving the hardenability of steel and improving strength and toughness in addition to having a deoxidizing action. However, if the content is less than 0.10%, sufficient strength and toughness cannot be obtained. On the other hand, if the content exceeds 1.00%, coarsening of MnS occurs, which causes the whole body to expand during hot rolling, resulting in a decrease in toughness. Thereby, content of Mn is made into 0.10% or more and 1.00% or less. Preferable content of Mn is 0.30% or more and 0.80% or less.

P: not more than 0.025%

P is contained as an impurity in steel and brings about the fall of toughness resulting from grain boundary segregation. In particular, when the content of P exceeds 0.025%, the decrease in toughness becomes remarkable. Therefore, content of P is made into 0.025% or less. The content of P is preferably 0.020% or less, and more preferably 0.015% or less.

S ： 0.005% or less

S is also contained as an impurity in steel, and especially toughness of steel pipe T direction (direction orthogonal to the rolling direction (length direction) of a steel pipe) falls. When content of S exceeds 0.005%, since the toughness fall of the steel pipe T direction will become remarkable, content of S shall be 0.005% or less. Preferable content of S is 0.003% or less.

Al: 0.10% or less

Al is an element which is effective in deoxidizing and improving toughness and workability of steel. However, when Al is contained in an amount exceeding 0.10%, occurrence of minor scratches becomes remarkable. Therefore, content of Al is made into 0.10% or less. Since Al content may be an impurity level, the minimum is not specifically determined, It is preferable to set it as 0.005% or more. Al content referred to in this invention refers to content of acid soluble Al (so-called "sol.Al").

Cr: 0.01% to 0.50%

Cr has the effect of improving the strength and toughness of the steel by increasing the hardenability and temper softening resistance of the steel. The effect is expressed when Cr is contained in an amount of 0.01% or more. However, Cr as a hardenability improving element causes hardening of steel in the cooling process after hot canning, and restrict | limits the workability to one cold drawing, Therefore, it cold-processes several times of cold drawing processing with softening annealing. The necessity to perform in a drawing process becomes high. In addition, an increase in Cr content also leads to an increase in alloy cost. For the same reason as above, the content of Cr is made 0.01% or more and 0.50% or less. Preferable content of Cr is 0.15% or more and 0.45% or less, and more preferable content is 0.18% or more and 0.35% or less.

Mo: Less than 0 to 0.10%

Mo has the effect of improving the strength and toughness of the steel by increasing the hardenability and temper softening resistance of the steel. The effect is expressed when it contains 0.01% or more. However, in the present invention, since necessary strength and toughness are ensured by Ni and Cu, addition of Mo is not essential. That is, Mo may be 0%.

Even when Mo is added, the content is made less than 0.10%. If there is much Mo content, even if it air-cools the seamless steel pipe obtained by hot manufacturing, there exists a tendency for the strength of a seamless steel pipe to become high too much. As a result, in the following cold drawing process, it is necessary to perform soft annealing before processing, and also the workability (reduction reduction rate) of cold drawing process is limited, and the cold drawing process and softening annealing required before to make steel pipe of predetermined dimension are performed. The number of times increases. This tendency becomes remarkable when Mo becomes 0.10% or more. In addition, since Mo is a very expensive metal, an increase in Mo content leads to a remarkable increase in alloy cost. In other words, 0.10% or more of Mo is harmful in achieving the object of the present invention. Therefore, Mo content in the case of containing Mo is made less than 0.10%, and the preferred content is 0.01% or more and 0.05% or less.

Cu: 0.01% to 0.50%

Cu has the effect of improving strength and toughness by increasing the hardenability of steel. The effect is expressed if it contains 0.01% or more, preferably 0.03% or more of Cu. However, containing Cu in excess of 0.50% causes an increase in alloy cost. Therefore, content of Cu is made into 0.01% or more and 0.50% or less. Preferable Cu content is 0.03% or more, especially 0.05% or more, More preferably, it is 0.15% or more. The upper limit of the Cu content is preferably 0.40%, more preferably 0.35%.

Ni: 0.01% to 0.50%

Ni has the effect of improving the hardenability of steel and thereby improving strength and toughness. The effect is expressed when it contains 0.01% or more, preferably 0.03% or more of Ni. However, containing Ni above 0.50% results in an increase in alloy cost. Therefore, content of Ni is made into 0.01% or more and 0.50% or less. Preferable Ni content is 0.03% or more, especially 0.05% or more, More preferably, it is 0.15% or more. The upper limit of the Ni content is preferably 0.40%, more preferably 0.35%.

It is preferable that it is 0.20% or more and 0.65% or less, and, as for (Cu + Ni) which is a sum of content of Cu and Ni, it is more preferable that it is 0.28% or more and 0.60% or less.

In a preferred embodiment of the present invention, the Cu, Ni, Cr, and Mo contents in the steel are adjusted to satisfy the following formula (1).

Cu + Ni ≧ (Cr + Mo) ² +0.3... (One)

The element symbol of Formula (1) is a numerical value at the time of expressing content of each element by the mass%. When Mo is not contained, Mo is called zero.

Cr and Mo interfere with spheroidization of cementite precipitated during tempering, and especially in steels containing B, it is easy to form B and compounds (borides) at grain boundaries, and therefore toughness is particularly low in high strength materials. easy to do. By suppressing Cr and Mo and containing Cu and Ni so as to satisfy Formula (1), manufacture of an airbag steel pipe with high strength and high toughness becomes easy.

In a preferable embodiment in the present invention, at least one element selected from one or both of the following two groups of (i) and (ii) can be further contained.

(i) Nb, Ti, V

(ii) Ca, B

Nb ： 0.050% or less

Nb is finely dispersed as carbide in steel and has the effect of strongly fixing a grain boundary. As a result, grains are refined and the toughness of steel is improved. However, when Nb is contained in an amount greater than 0.050%, carbides coarsen and the toughness is rather deteriorated. Therefore, content of Nb at the time of adding is made into 0.050% or less. In addition, although the said effect of Nb appears also in the trace amount, it is preferable to contain 0.005% or more in order to fully acquire the effect.

Ti: 0.050% or less

Ti has the effect of fixing N in steel and improving toughness. The finely dispersed Ti nitride strongly fixes the grain boundaries and refines the grains, thereby improving the toughness of the steel. However, when Ti is contained in a larger amount than 0.050%, the nitride coarsens and the toughness decreases. Therefore, content of Ti when adding is made into 0.050% or less. Although the effect of Ti appears in trace amount, it is preferable to contain 0.005% or more in order to acquire the effect sufficiently. Preferable content of Ti is 0.008 to 0.035%.

V: 0.20% or less

Although V has the effect of securing the toughness and increasing the strength by precipitation strengthening, when the content of V exceeds 0.20%, the toughness is reduced. Therefore, content of V in the case of adding is made into 0.20% or less. Although the action of V appears in a small amount, it is preferable to contain 0.02% or more in order to obtain a sufficient effect. The range with preferable V content is 0.03 to 0.10%.

Ca: 0.005% or less

Ca has the effect of fixing S, which is an unavoidable impurity in steel, as a sulfide, improving the anisotropy of toughness, and increasing the T-direction toughness of the steel pipe to increase the burst resistance. However, when Ca is contained exceeding 0.005%, an interference | inclusion increases and rather toughness falls. Therefore, content of Ca at the time of addition is made into 0.005% or less. Although the effect of said Ca appears in a very small amount, it is preferable to contain 0.0005% or more in order to acquire sufficient effect.

B ： 0.0030% or less

B is grain boundary segregation in steel by adding a trace amount, and the hardenability of steel is improved remarkably. However, when 0.0030% or more of B is contained, borides precipitate coarsened at grain boundaries, and a tendency for the toughness to decrease is observed. Therefore, content of B in the case of adding is made into 0.0030% or less. Although the effect of B appears also in a trace amount, it is preferable to contain 0.0005% or more in order to ensure sufficient effect.

In the present invention, when aiming at a tensile strength of 1000 MPa or more, it is preferable to improve the strength by improving the hardenability by blending B.

In addition, B does not segregate at grain boundaries unless it is contained in a solid solution state. Therefore, it is preferable that N which is easy to make B and a compound is fixed by Ti, and it is preferable that B contains more than the quantity fixed by N. In that sense, the B content is preferably satisfied from the stoichiometric ratios of B, Ti, and N in the following formula (2).

B- (N-Ti / 3.4) x (10.8 / 14)> 0.0001... (2)

B, N, and Ti in Formula (2) are numerical values when content of each element is shown by the mass%.

(B) manufacturing process

As described in the above (A), a seamless steel pipe is obtained by hot forming using a steel ingot made of steel having a chemical composition adjusted as a material.

The form and manufacturing method of the steel ingot which becomes a raw material of a hot steel pipe are not specifically limited. For example, it may be a cast piece (round CC billet) injected in a continuous casting machine having a columnar mold, or may be a steel ingot formed into a columnar shape by hot forging after being injected into a rectangular shape. In the steel used in the present invention, continuous casting is carried out in a circular form as a round CC billet from a relationship in which ferrite stabilizing elements such as Cr and Mo are suppressed and austenite stabilizing elements such as Cu and Ni are added. Even in this case, the effect of preventing the center crack is great, and the suitability for round CC is also high enough. Thereby, the process to a round billet by the crushing rolling etc. which are necessary when injected into a rectangular shape can be skipped.

The hot production method for making a seamless steel pipe is not particularly limited. For example, the mandrel-Mannesman method is employed. Cooling after hot tubing is preferable because cooling rate, such as cooling, becomes small, since cold drawing becomes easy. The shape of the obtained seamless steel pipe is not particularly limited, but may be, for example, about 32 to 50 mm in diameter and about 2.5 to 3.0 mm in thickness.

(C) cold drawing process

Seamless steel pipes obtained by hot canning generally have large thicknesses and diameters and insufficient dimensional accuracy. In order to obtain a predetermined dimension (outer diameter and thickness of the steel pipe) and surface properties, the seamless steel pipe is used as the elementary pipe, and cold drawing is performed thereon. In the present invention, in order to make use of the characteristics of the steel to be used, the workability (reduction rate) of at least one cold drawing process performed in the cold drawing process is over 40%. Since the inner surface wrinkles and a crack generate | occur | produce easily when the workability of one cold drawing exceeds 50%, the preferable workability is 42 to 48%, More preferably, it is 43 to 46%. In the cold drawing process, when performing cold drawing twice or more, the workability in at least one cold drawing may be 40% or more, and it is permissible to use cold drawing which has a workability of less than 40% together.

The degree of workability in cold drawing is synonymous with the reduction rate (section reduction rate) defined by the following formula.

Reduction rate (%) = (S ₀ -S _f ) × 100 / S ₀

only

S ₀ : Cross section of steel pipe before cold drawing process

S _f : Cross section of steel pipe after cold drawing process is completed

"The cross-sectional area of a steel pipe" is the cross-sectional area only of the pipe wall part except the hollow part in a pipe cross section.

When "processing degree (or reduction rate) of one cold drawing" is performed without intervening softening annealing in between, we treat the total processing degree in plural cold drawing operations as "processing degree of one cold drawing", too. . By using the steel according to the present invention, the workability of one cold drawing can be exceeded by 40%. Therefore, if a finishing dimension of a seamless steel pipe obtained by hot canning is appropriately selected, a thin steel pipe of a predetermined size can be cold drawn once. It becomes possible to manufacture only. Thereby, the manufacture of the thin steel pipe which required two cold drawing processes conventionally and softening annealing in the middle can be greatly simplified.

The processing method of cold drawing is well-known, and what is necessary is just to carry out according to a commercial method. For example, a seamless steel pipe produced by the mandrel-Mannesman method as described above is used as the elementary pipe, and after being cooled to room temperature, drawing is performed by a die and a plug to reduce the diameter and thickness. It is preferable that the steel pipe for air bags is 30 mm or less in diameter, and 2 mm or less in thickness, for example. If cold drawing from the seamless steel pipe of an element pipe to a steel pipe of the required dimension can be realized, there will be no restriction | limiting in particular in a processing method, However, drawing process of the said system is preferable.

In the steel used in the present invention, it is possible to process at a reduction ratio of 46% by one cold drawing. Therefore, when the final dimension of the steel pipe for airbags is 1.7 mm thick and the outer diameter is 25 mm, if the dimensions of the element pipe subjected to cold drawing are, for example, an outer diameter of 31.8 mm and a thickness of 2.5 mm, the cold pipe may be of a predetermined size by one cold drawing. You can get the product.

The steel pipe for airbags produced by the present invention tends to have a higher strength after cold drawing than conventional steel in that the tensile strength is 900 MPa or more, and the cold drawing reduction rate is 40% or more. There may be a bend in the steel pipe after the PS step.

As described below, in order to secure high strength and high toughness, the steel pipe having a predetermined dimension is cold-heated to be heated above the Ac ₃ transformation temperature by rapid heating in order to quench. Furnace is performed by high frequency induction heating. If the steel pipe to be quenched is bent, there is a concern that the steel pipe does not pass through the high frequency coil used for high frequency induction heating. Therefore, in a preferable aspect, after cold drawing, a straightening process is performed and the bending of a steel pipe is eliminated.

This calibration method is not specifically limited, What is necessary is just to implement it according to a conventional method. For example, by installing four rows of two-roll stands, shifting the center positions of the roll gaps in each row differently (ie, offsetting), adjusting the roll gap amount, and passing the steel pipe between the rolls. , A method of adding bending and backing is preferred. The higher the degree of workability of bending and returning at this time, the higher the effect of calibration. From that point of view, it is preferable that the offset amount (deviation amount of the roll axis between adjacent roll pairs) is 1% or more of the outer diameter of the steel pipe, and is set to be less than the roll gap amount which is 1% smaller than the outer diameter of the steel pipe. On the other hand, in order to avoid problems such as cracking of the steel pipe, it is preferable to set the offset amount to 50% or less of the outer diameter of the steel pipe, and to make the roll gap amount smaller than 5% smaller than the outer diameter of the steel pipe.

(E) heat treatment

If necessary, after the calibration process (D) is performed, the steel pipe is heat-treated to provide the tensile strength required for the steel pipe and to increase the T-direction toughness to secure the burst resistance. In order to provide the steel pipe with a tensile strength of 900 MPa or more and excellent low temperature toughness or burst resistance, it is quenched by heating to a temperature of Ac ₃ (transformation) point or higher, and then at a temperature of Ac ₁ (transformation) point or lower. Temper.

If the heating temperature before quenching is lower than Ac ₃ point which becomes an austenite single phase, good T direction toughness (and therefore good burst resistance) cannot be ensured. On the other hand, when the above-mentioned heating temperature is too high, the austenite grains start to grow rapidly, become granulated, and the toughness decreases. More preferably, it is 1000 degrees C or less.

Heating to the temperature of Ac ₃ or more at the time of quenching is performed by rapid heating of 50 degreeC / s or more of heating rate. As for this heating rate, the value of the average heating rate in the temperature range to 200 degreeC or more and a heating temperature can be employ | adopted. If the heating rate is less than 50 ° C / s, the austenite grain size cannot be miniaturized, and the tensile properties, low temperature toughness or burst resistance deteriorate. In order to obtain a steel pipe having a tensile strength of 1000 MPa or more and vTrs100 of -80 ° C or less, the heating rate is preferably 80 ° C / s or more, more preferably 100 ° C / s or more. Such rapid heating can be achieved by high frequency induction heating. In this case, the heating rate can be adjusted by the feed rate of the steel pipe passing through the high frequency coil.

The steel pipe heated to the temperature of Ac ₃ or more by rapid heating is held at a temperature of Ac ₃ or more for a short time, and then rapidly cooled for quenching. The holding time is preferably in the range of 0.5 to 8 seconds. More preferably, it is 1 to 4 seconds. If the holding time is too short, the uniformity of mechanical properties may be inferior. If the holding time is too long, particularly when the holding temperature is high, coarsening of the austenite grain size is likely to occur. Refining the grain size is necessary to ensure very high toughness.

The cooling rate for quenching is controlled so that the cooling rate in the temperature range of at least 850-500 degreeC becomes 50 degreeC / s or more. This cooling rate becomes like this. Preferably it is 100 degreeC / s or more. In order to make vTrs100 of -80 degreeC or less and tensile strength 1000 Mpa or more, it is preferable to make cooling rate into 150 degreeC / s or more. If the cooling rate is too slow, the quenching becomes incomplete, the ratio of martensite falls, and sufficient tensile strength cannot be obtained.

The steel pipe quenched and cooled to near normal temperature is tempered at a temperature of Ac ₁ or less in order to impart a tensile strength of 900 MPa or more and sufficient burst resistance. When the temperature of tempering exceeds Ac ₁ point, it becomes difficult to reliably and reliably obtain the target tensile strength and low-temperature toughness.

Although the method of tempering is not specifically limited, For example, what is necessary is just to carry out by cooling after a crack heating by heat processing furnaces, such as a hearth roller type continuous furnace, high frequency induction heating, etc. Preferred cracking conditions in the heat treatment furnace are a temperature of 350 to 500 ° C. and a holding time of 20 to 30 minutes. After tempering, the bending may be appropriately corrected with a straightener or the like as described in (D).

In order to process the airbag accumulator from the airbag steel pipe manufactured as described above, the steel pipe is cut into a predetermined length to form a single pipe, and then at least one end thereof is subjected to a shaft diameter processing by press working, spinning, or the like (bottle) What is necessary is just to final process into the shape required for attachment of an initiator, etc.). Therefore, the predetermined dimension and the dimensional accuracy as the steel pipe for airbags referred to in the present specification mean the dimensions and the dimensional precision regarding the tube thickness and the diameter. At the end, the cover is welded on both ends of the steel pipe.

Example

Steel having the chemical composition shown in Table 1 (Ac ₁ point is in the range of 720 to 735 ° C., Ac ₃ point is in the range of 835 to 860 ° C.) is dissolved in a converter, and is continuously cast (round CC). A cylindrical billet having an outer diameter of 191 mm was prepared. The round CC billet was cut to a desired length, heated to 1250 ° C., and then punched and rolled by a conventional Mannesmann pian mandrel mill method to obtain a primary tube having an outer diameter of 31.8 mm and a thickness of 2.5 mm. A second element pipe having an outer diameter of 42.7 mm and a thickness of 2.7 mm was obtained.

Thus, two kinds of element pipes obtained in this way are drawn by using dies and plugs, and are subjected to cold drawing processing (cold drawing) once or twice, with a diameter of 25.0 mm and a thickness of 1.7 mm. Finished with. With respect to Comparative steels G and H of Table 1, when the first element pipe having an outer diameter of 31.8 mm and a thickness of 2.5 mm was used, attempts were made to produce the steel pipe of the above-described shape in one round, and no fracture occurred.

In Comparative Examples 9 and 10, the second PS was used to form a steel pipe having an outer diameter of 32.0 mm and a thickness of 2.2 mm by the first PS, and the second PS through the soft annealing at 630 ° C. for 20 minutes. 25.0mm thickness 1.7mm finished.

After the steel pipes subjected to this cold drawing process were calibrated with a straightener, they were heated to 920 ° C at an average temperature increase rate of 300 ° C / s (average of the temperature range of 200 to 900 ° C) using a high frequency induction heating apparatus, and 920 After holding at 2 ° C. for 2 seconds, water cooling (average cooling rate of 150 ° C./s at a temperature range of 850 to 500 ° C.) was performed to quench water. Subsequently, in order to temper the steel pipe, it was cracked at 350 to 500 ° C. for 30 minutes in a bright annealing furnace, and cooled to room temperature by natural cooling and cooling in the furnace to obtain a steel pipe for an air bag.

The tube of constant length was cut out from each obtained steel pipe, it cut | disconnected and developed in the longitudinal direction of the tube at room temperature. The Charpy impact test was performed at various temperatures of -40 degrees C or less using the test piece which introduce | transduced the 2 mmV notch into the rectangular material of length 55mm, height 10mm, and width 1.7mm collected from the T direction from the expanded tube. By this test, the lower limit temperature (vTrs100) at which the ductile wavefront was 100% was obtained.

In addition, a tensile test was conducted in accordance with the metal material tensile test method specified in JIS Z 2241 using the No. 11 test piece specified in JIS Z 2201 taken in the L direction of the steel pipe. The above test result was put together in Table 2 with the manufacturing conditions of a steel pipe, and was shown.

As can be seen from Table 2, in the case of using steels A to F having the chemical composition of the steel according to the present invention, even though it does not contain expensive Mo at all, or contains only a small amount of less than 0.10%, the alloy cost is low Even with a machining rate of 46% reduction, it can be processed to a predetermined product dimension by one cold drawing, and high-level product performance can be achieved as a steel pipe for airbags by rapid heating and rapid cooling in the subsequent quenching process. have. In particular, when steels A to C, E, and F having a composition satisfying the above formula (1) are used, vTrs100 is -100 ° C or lower, and low-temperature toughness is extremely high, and excellent burst resistance performance is expected in a low-temperature environment. Turned out to be.

On the other hand, since steels F and G of the comparative example contain a large amount of Mo, the alloy cost is large. Furthermore, when cold reduction processing of 40% or more of reduction of reduction rate was performed, a crack generate | occur | produced. For this reason, it is necessary to perform cold drawing process twice or more with the reduction rate of less than 40%, the intermediate soft annealing is needed, and the manufacturing cost of the steel pipe for airbags also increases.

Claims (9)

In mass%, C: 0.04 to 0.20%, Si: 0.10 to 0.50%, Mn: 0.10 to 1.00%, P: 0.025% or less, S: 0.005% or less, Al: 0.10% or less, Cr: 0.01 to 0.50%, A pipe forming step of performing hot pipe forming of a seamless steel pipe from a steel consisting of Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, and the balance of Fe and unavoidable impurities;
A cold drawing step of obtaining at least one cold drawing process in which a reduction ratio of one cold drawing process is more than 40% to the obtained seamless steel pipe to obtain a steel pipe having a predetermined dimension; and
The cold drawn steel pipe was quenched by heating to a temperature of at least ₃ Ac at a heating rate of 50 ° C./s or more, and then cooling to a cooling rate of at least 850 to 500 ° C. to be 50 ° C./s or more, Subsequently, the manufacturing method of the steel pipe for airbags characterized by including the heat processing process of tempering to the temperature below Ac ₁ point temperature. The method according to claim 1,
The said steel is a manufacturing method of the steel pipe for airbags which further contains Mo: less than 0.10%. The method according to claim 1 or 2,
A method for producing a steel pipe for an airbag, wherein the steel contains at least one selected from Nb: 0.050% or less, Ti: 0.050% or less, and V: 0.20% or less. The method according to any one of claims 1 to 3,
A method for producing a steel pipe for an airbag, wherein the steel contains at least one selected from Ca: 0.005% or less and B: 0.0030% or less. The method according to any one of claims 1 to 4,
The manufacturing method of the steel pipe for airbags whose density | concentration of Cu, Ni, Cr, and Mo of the said steel satisfy | fills following (1) formula.
Cu + Ni ≧ (Cr + Mo) ² +0.3... (One)
The element symbol in Formula (1) means the numerical value at the time of expressing content of these elements by the mass%. However, when Mo is not contained, let Mo = 0. The method according to any one of claims 1 to 5,
The manufacturing method of the steel pipe for airbags whose thickness of the steel pipe after completion of the said cold drawing process is 2.0 mm or less. The method of claim 6,
The said cold drawing process is a manufacturing method of the steel pipe for airbags performed by one cold drawing. The method according to any one of claims 1 to 7,
The manufacturing method of the steel pipe for airbags which heats for hardening by high frequency induction heating in the said heat processing process. The method according to claim 8,
Method for producing a steel pipe for airbags, before the heating for the quenching, to correct the steel pipe obtained by the cold drawing process