JP5234226B2 - Manufacturing method of steel pipe for airbag - Google Patents

Abstract

In a process for manufacturing a steel tube for air bags having a high strength and high toughness which can simplify cold drawing and reduce the alloy cost, a seamless steel tube is formed from a steel comprising, in mass percent, C: 0.04 - 0.20%, Si: 0.10 - 0.50%, Mn: 0.10 - 1.00%, P: at most 0.025%, S: at most 0.005%, Al: at most 0.10%, Cr: 0.01 - 0.50%, Cu: 0.01 - 0.50%, Ni: 0.01 - 0.50%, and a remainder of Fe and unavoidable impurities, and the seamless steel tube is subjected to cold drawing at least one time with a working ratio such that the reduction in area is greater than 40% to obtain predetermined dimensions, then to quench hardening by heating to a temperature of at least the Ac 3 point at a rate of temperature increase of at least 50° C per second followed by cooling at a cooling rate of at least 50° C per second at least in a temperature range of 850 - 500° C, and to tempering at a temperature of at most the Ac 1 point.

Description

  The present invention requires high toughness that is suitable for airbag steel pipes and has a high tensile strength of 900 MPa or more, and vTrs100 (the lowest Charpy fracture surface transition temperature at which the ductile fracture surface ratio is 100%) is −60 ° C. or less. The present invention relates to an inexpensive method for manufacturing a seamless steel pipe.

  In recent years, in the automobile industry, introduction of devices pursuing safety has been actively promoted. As one of such devices, an air bag system has been developed and installed in many automobiles. The airbag system is a system that reduces the injury by absorbing the kinetic energy of the occupant by deploying the airbag with gas etc. between the occupant and the occupant before the occupant collides with the steering wheel or instrument panel at the time of collision. It is. As an air bag system, a method using explosive chemicals was initially adopted, but in recent years, a method using a high-pressure filling gas has been developed and its application is expanding.

  In an airbag system that uses high-pressure filling gas, a deployment gas such as an inert gas (eg, argon) that blows into the airbag at the time of a collision is always stored in the pressure accumulator (accumulator) connected to the airbag. The pressure is maintained at a high pressure, and in the event of a collision, gas is ejected from the accumulator at a stretch into the airbag to deploy the airbag. An accumulator is generally manufactured by welding a lid to both ends after subjecting a steel pipe cut to an appropriate length to diameter reduction processing as necessary.

  Therefore, stress is applied to a steel pipe used for an accumulator of an air bag system (hereinafter referred to as an air bag accumulator or simply an accumulator) at a large strain rate in a very short time. For this reason, unlike conventional structures such as pressure cylinders and line pipes, this type of steel pipe requires high dimensional accuracy, workability and weldability, and also requires high strength and excellent burst resistance. Is done.

  Recently, there is an increasing demand for weight reduction of automobiles. From this point of view, there is a demand for thinner and lighter air bag steel pipes for vehicles. To ensure a high burst pressure even when thin, high tensile strength of 900 MPa or more, and even 1000 MPa or more is required. Accumulators made from seamless steel pipes have been used in airbag systems. For example, in the case of an accumulator made of a seamless steel pipe having an outer diameter of 60 mm and a wall thickness of 3.55 mm, the burst pressure is about 100 MPa at a TS of 800 MPa, whereas when the TS is 1000 MPa, the burst pressure is up to 130 MPa. improves. At the same time, when the outer diameter of the airbag accumulator and the required burst pressure are constant, the thickness can be reduced by about 20%.

Furthermore, even in a cold region, for example, the accumulator needs to have excellent low temperature toughness so that the accumulator is not brittlely broken at the time of a collision and causes a secondary disaster.
From such a point of view, seamless steel pipes for accumulators have come to realize high strength and high toughness by quenching and tempering. Specifically, for an accumulator, a low temperature toughness in which a fracture surface exhibits ductility in a Charpy impact test at −60 ° C. (that is, vTrs100 is −60 ° C. or less), preferably a Charpy impact at −80 ° C. The low temperature toughness that the fracture surface exhibits ductility in the test (vTrs100 is −80 ° C. or lower) is required.

With regard to a seamless steel pipe for an air bag system having a high strength and high toughness, for example, Patent Document 1 discloses that a seamless steel pipe is hot-formed using a steel material having a chemical composition within a predetermined range. The steel pipe is subjected to cold drawing to obtain a steel pipe of a predetermined size, and then heated to a temperature in the range of Ac ₃ points or more and 1050 ° C. or less and then quenched, and then in the range of 450 ° C. or more and Ac ₁ point or less. There has been proposed a method for producing a seamless steel pipe for an air bag, characterized by performing quenching and tempering treatment by tempering at a temperature.

  By this method, it is excellent in workability and weldability at the time of manufacturing an airbag inflator, and further, as an inflator, it has a tensile strength of 900 MPa or more and a high toughness that exhibits ductility in a drop test at -60 ° C. against a halved steel pipe. It is said that a seamless steel pipe is obtained. However, exhibiting ductility in a drop weight test at -60 ° C does not necessarily mean exhibiting ductility in a burst test at -60 ° C.

Patent Document 2 proposes a method of manufacturing a steel pipe for an airbag system having a tensile strength exceeding 1000 MPa by high-frequency induction heating and quenching and fine graining by rapid heating. For example, when a seamless steel pipe is used as a raw pipe, a seamless steel pipe is made hot using a steel material having a chemical composition in a specific range, and the seamless steel pipe is subjected to cold drawing to obtain a steel pipe having a predetermined size. To do. The steel pipe is quenched after heating and then tempered at a temperature below the Ac ₁ transformation point. By performing a tempering treatment after quenching, it is desirable to obtain high toughness that exhibits ductility even in a burst test at −80 ° C. or lower.

  However, in the methods disclosed in Patent Documents 1 and 2, as shown in specific examples, in order to obtain a steel pipe having a tensile strength of 1000 MPa or more and a high toughness, a large amount of expensive alloys such as Cr and Mo are used. It was necessary to make it contain. In the case of Patent Document 1, Cr + Mo is 1.0 to 2.5% by mass, and in Patent Document 2, a steel material of Cr + Mo: 0.92% by mass is often adopted. When containing a large amount of Cr and Mo, in addition to high raw material costs due to expensive Mo, the strength of the steel pipe is likely to increase after the hot steel pipe is manufactured, and subsequent cold drawing is difficult. become. Therefore, soft annealing is required before cold drawing, which complicates the process and increases the manufacturing cost.

Even in Patent Document 3 that uses steel of Cr + Mo: 1.0 to 1.18% by mass, there are the same problems as in Patent Documents 1 and 2.
Patent Document 4 discloses a steel composition containing Cr, Mo, Cu, and Ni with respect to a seamless steel pipe excellent in burst resistance. The characteristics of the steel composition are evaluated as Cr + Mo: 0. It is a seamless steel pipe of .76% by mass or more, and the tensile strength at that time is at most 947 MPa.

JP-A-2004-76034 WO 2004 / 104255A1 US 2005/0076975 A1 WO 2002 / 079526A1

  Conventional steel pipes for airbags have been reinforced by the addition of Cr and Mo in order to ensure high strength and high toughness. However, this method increases the alloy cost and makes cold drawing after pipe making difficult. Therefore, if the difference between the size of the seamless steel pipe and the size of the air pipe steel pipe which is the final product is large, it is necessary to repeat the cold drawing process many times in the cold drawing process. In that case, since it finishes in a desired product dimension, performing intermediate | middle softening annealing for every cold drawing process, when it sees synthetically, manufacturing cost will increase.

  By simplifying the cold drawing process or reducing alloy costs, the present invention produces a steel pipe for an air bag having high strength and toughness that is cheaper than the conventional method by using cheaper means. It aims to provide a way to do.

  From another aspect, an object of the present invention is to provide a method of manufacturing a steel pipe for an air bag having a small diameter and a thickness equal to or smaller than that of a conventional product by using a material / manufacturing method that is lower in cost than in the past. To do.

  As a result of the conventional steel pipe for high-strength airbag relying on strengthening with Cr and Mo, the present inventors have high strength after the end of hot pipe making, and the reduction in productivity and the alloy cost in cold drawing. An alloy composition and production capable of securing a high strength with a tensile strength of 900 MPa or more and an excellent low temperature toughness with a vTrs100 of −60 ° C. or less while suppressing the use of these alloy elements as much as possible. The method was examined.

As a result, the following knowledge was obtained and the present invention was reached.
(A) In the manufacture of a steel pipe for an air bag that is quenched and tempered after cold drawing, if the heating conditions and cooling conditions at the time of quenching are set well, not necessarily containing a large amount of Cr and Mo, High strength and low temperature toughness can be secured. In particular, it is effective to contain Cu and Ni instead of Cr and Mo.

  (B) Steel with reduced Cr and Mo, and containing Cu or Ni instead, is easy to be cold drawn after hot pipe making, and in one cold drawing process in the cold drawing process. It becomes possible to increase the degree of processing (area reduction), and it is possible to simplify the cold drawing process.

In the present invention, by 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%, the balance A pipe making process for making a seamless steel pipe from steel composed of Fe and inevitable impurities, and the obtained seamless steel pipe has a surface reduction rate of more than 40% and less than 50% in one cold drawing process. performing cold drawing process at least once to be a more cold drawing engineering to obtain the steel pipe having a predetermined size, the cold drawing steel tube, heated to a temperature above Ac3 point or more of the heating rate 50 ° C. / s After that, quenching is performed 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 the temperature is less than the Ac 1 point temperature. Characterized in that it comprises a heat treatment step of subjecting the tempered at a temperature, and a tensile strength of at least 900 MPa, VTrs100 is method for producing a steel pipe for an air bag having low temperature toughness of -60 ° C. or less.

The preferred embodiments of the method for producing a steel pipe for an air bag according to the present invention are listed as follows:
The steel may optionally further contain one or more of the following elements:
Mo: less than 0.10%
-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: 0.0003% or less.

The concentrations of Cu, Ni, Cr, and Mo in the steel preferably satisfy the following formula (1):
Cu + Ni ≧ (Cr + Mo) ² +0.3 (1)
The element symbol of the formula (1) means a numerical value when the content of these elements is expressed by mass%. However, when Mo is not contained, Mo = 0 (zero).

The wall thickness of the steel pipe is preferably 2.0 mm or less after the cold drawing process is completed. The cold drawing step is preferably performed by one cold drawing.
The heating for quenching in the heat treatment step is preferably performed by high frequency induction heating. In this case, it is preferable to correct the steel pipe obtained in the cold drawing step before the heating for quenching.

  According to the present invention, it is possible to manufacture a steel pipe for an air bag having high tensile strength of 900 MPa or more and excellent low temperature toughness of vTrs100 of −60 ° C. or less while suppressing the amount of expensive Mo to 0 or a small amount. It becomes. In addition, since the strength of the seamless steel pipe obtained by hot pipe making is not too high, the processing rate in the subsequent cold drawing process can be increased as compared with the conventional one, and the intermediate softening annealing is necessary. The number of inter-drawings 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 case.

Below, 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, "%" related to the chemical composition of steel means "mass%". The balance of the chemical composition of the steel excluding the elements described below is Fe and inevitable impurities.

C: 0.04 to 0.20%
C is an element effective for increasing the strength of steel at a low cost. If its content is less than 0.04%, it is difficult to obtain high strength (tensile strength), and if it exceeds 0.20%, workability and weldability deteriorate. Therefore, the C content is set to 0.04% or more and 0.20% or less. A preferable range of the C content is 0.07% or more and 0.20% or less, and a more preferable range is 0.12% or more and 0.17% or less. In order to target a tensile strength of 1000 MPa or more, it is desirable to contain 0.06% or more of C.

Si: 0.10 to 0.50%
In addition to having a deoxidizing action, Si is an element that improves the hardenability of the steel and improves the strength. For this purpose, the Si content is set to 0.10% or more. However, if the content exceeds 0.50%, the toughness decreases, so the Si content is set to 0.50% or less. A preferable range of the Si content is 0.20% or more and 0.45% or less.

Mn: 0.10 to 1.00%
In addition to having a deoxidizing action, Mn is an element effective for enhancing the hardenability of steel and improving the strength and toughness. However, if its content is less than 0.10%, sufficient strength and toughness cannot be obtained. On the other hand, if it exceeds 1.00%, MnS coarsens, which expands during hot rolling and decreases toughness. To do. For this reason, content of Mn shall be 0.10% or more and 1.00% or less. A preferable Mn content is 0.30% or more and 0.80% or less.

P: 0.025% or less P is contained as an impurity in the steel and causes toughness reduction due to grain boundary segregation. In particular, when the P content exceeds 0.025%, the toughness is significantly lowered. Therefore, the content of P is set to 0.025% or less. The P content is preferably 0.020% or less, more preferably 0.015% or less.

S: 0.005% or less S is also contained in the steel as an impurity, and particularly reduces the toughness in the steel pipe T direction (direction perpendicular to the rolling direction (longitudinal direction) of the steel pipe). When the S content exceeds 0.005%, the toughness in the steel pipe T direction decreases significantly, so the S content is set to 0.005% or less. A preferable S content is 0.003% or less.

Al: 0.10% or less Al is an element that has a deoxidizing action and is effective in enhancing the toughness and workability of steel. However, when an amount of Al exceeding 0.10% is contained, the generation of ground becomes remarkable. Therefore, the Al content is set to 0.10% or less. Since the Al content may be at the impurity level, the lower limit is not particularly defined, but is preferably 0.005% or more. The Al content referred to in the present invention refers to the 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 steel by increasing the hardenability and temper softening resistance of the steel. The effect is manifested when Cr is contained in an amount of 0.01% or more. However, Cr as a hardenability improving element causes hardening of the steel in the cooling process after hot pipe making and restricts the degree of work in one cold drawing, so softening annealing is sandwiched between them. The necessity of performing a plurality of cold drawing processes in the cold drawing process is increased. Furthermore, an increase in the Cr content also leads to an increase in alloy costs. For these reasons, the Cr content is set to 0.01% or more and 0.50% or less. The preferable content of Cr is 0.15% or more and 0.45% or less, and the more preferable content is 0.18% or more and 0.35% or less.

Mo: 0 to less than 0.10% Mo has the effect of improving the strength and toughness of steel by increasing the hardenability and temper softening resistance of the steel. However, in the present invention, the necessary strength and toughness are ensured by Ni and Cu, so the 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%. When the Mo content is high, the strength of the seamless steel pipe tends to be too high even if the seamless steel pipe obtained by hot pipe making is air-cooled. As a result, in the next cold drawing process, it is necessary to perform soft annealing before processing, and the degree of cold drawing (reduction rate) is limited, so that a steel pipe of a predetermined size can be obtained. The number of necessary cold drawing processes and previous soft annealing increases. This tendency becomes remarkable when Mo becomes 0.10% or more. Further, since Mo is a very expensive metal, an increase in the Mo content leads to a significant increase in alloy costs. That is, 0.10% or more of Mo is harmful in achieving the object of the present invention. Therefore, when Mo is contained, the Mo content is less than 0.10%, but 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 manifested if it contains 0.01% or more, preferably 0.03% or more of Cu. However, if Cu is contained in excess of 0.50%, the alloy cost increases. Therefore, the Cu content is set to 0.01% or more and 0.50% or less. A preferable Cu content is 0.03% or more, particularly 0.05% or more, and more preferably 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 increasing the hardenability of the steel and thereby improving the strength and toughness. The effect is manifested if it contains 0.01% or more, preferably 0.03% or more of Ni. However, if Ni is contained exceeding 0.50%, the alloy cost increases. Therefore, the Ni content is set to 0.01% or more and 0.50% or less. A preferable Ni content is 0.03% or more, particularly 0.05% or more, and more preferably 0.15% or more. The upper limit of the Ni content is preferably 0.40%, more preferably 0.35%.

(Cu + Ni), which is the sum of the contents of Cu and Ni, is preferably 0.20% or more and 0.65% or less, and more preferably 0.28% or more and 0.60% or less.
In the preferable aspect of this invention, Cu, Ni, Cr, and Mo content in steel are adjusted so that the following formula (1) may be satisfied.

Cu + Ni ≧ (Cr + Mo) ² +0.3 (1)
The element symbol of Formula (1) is a numerical value when the content of each element is expressed in mass%. When Mo is not contained, Mo is zero.

  Cr and Mo prevent spheroidization of cementite that precipitates during tempering, and in particular, steel containing B easily forms B and a compound (boride) at the grain boundary. Is prone to decline. By suppressing Cr and Mo so as to satisfy the formula (1) and containing Cu and Ni, it is easy to manufacture a high strength and high toughness airbag steel pipe.

In a preferred embodiment of the present invention, at least one element selected from one or both of the following two groups (i) and (ii) can be further contained.
(I) Nb, Ti, V
(Ii) Ca, B
Nb: 0.050% or less Nb has the effect of finely dispersing as carbides in steel and strongly pinning the grain boundaries. As a result, the crystal grains are refined and the toughness of the steel is improved. However, when Nb is contained in a larger amount than 0.050%, the carbides are coarsened and the toughness is lowered. Therefore, the content of Nb when added is set to 0.050% or less. The effect of Nb is recognized even in a very small amount, but in order to obtain the effect sufficiently, it is desirable to contain 0.005% or more.

Ti: 0.050% or less Ti has an effect of fixing N in steel and improving toughness. The finely dispersed Ti nitride strongly pins the crystal grain boundaries, refines the crystal grains, and improves the toughness of the steel. However, when Ti is contained in a larger amount than 0.050%, the nitride is coarsened and the toughness is lowered. Therefore, when Ti is added, the content of Ti is set to 0.050% or less. Although the effect of Ti is recognized even in a trace amount, in order to obtain the effect sufficiently, it is desirable to contain 0.005% or more. A preferable content of Ti is 0.008 to 0.035%.

V: 0.20% or less V has the effect of securing toughness and increasing the strength by precipitation strengthening, but if the V content exceeds 0.20%, the toughness is reduced. Therefore, when V is added, the content of V is set to 0.20% or less. Although the action of V can be observed even in a trace amount, in order to obtain a sufficient effect, it is desirable to contain 0.02% or more. A preferable range of the V content is 0.03 to 0.10%.

Ca: 0.005% or less Ca fixes S present as an inevitable impurity in steel as a sulfide, improves anisotropy of toughness, and increases the T-direction toughness of a steel pipe, thereby improving burst resistance. Has an enhancing effect. However, if Ca is contained in excess of 0.005%, inclusions increase and the toughness decreases. Therefore, when Ca is added, the content of Ca is set to 0.005% or less. Although the effect of Ca is recognized even in a very small amount, it is desirable to contain 0.0005% or more in order to obtain a sufficient effect.

B: 0.0003% or less B is segregated at grain boundaries in steel when added in a small amount, and remarkably improves the hardenability of the steel. However, when 0.0003% or more of B is contained, a boride precipitates coarsely at the grain boundaries, and a tendency to lower toughness is recognized. Accordingly, the B content when added is 0.0003% or less. Although the effect of B is recognized even in a minute amount, it is desirable to contain 0.0005% or more in order to ensure a sufficient effect.

In the present invention, when aiming at a tensile strength of 1000 MPa or more, it is desirable to improve strength by improving hardenability by blending B.
Note that B does not segregate at grain boundaries unless it is contained in a solid solution state. Therefore, it is preferable that N which can easily form a compound with B is fixed by Ti, and B is preferably contained in an amount more than the amount fixed by N. In that sense, it is preferable that the B content satisfies the relationship of the following formula (2) from the stoichiometric ratio of B, Ti, and N.

B- (N-Ti / 3.4) × (10.8 / 14) ≧ 0.0001 (2)
B, N, and Ti in the formula (2) are numerical values when the content of each element is expressed in mass%.

(B) Pipe-making process A seamless steel pipe is obtained by hot pipe-making using a steel ingot made of steel with a chemical composition adjusted as described in (A) above as a raw material.

  The form and production method of the steel ingot used as the raw material of a hot steel pipe are not specifically limited. For example, a cast piece (round CC billet) cast by a continuous casting machine having a cylindrical mold may be used, or a steel ingot formed into a cylindrical shape by hot forging after casting into a rectangular mold may be used. . The steel used in the present invention suppresses the addition of ferrite stabilizing elements such as Cr and Mo, and from the relationship of adding an austenite stabilizing element such as Cu and Ni, continuous casting casting into a round shape as a round CC billet. When performed, the effect of preventing the center crack is great, and the compatibility with the round CC is sufficiently high. Thereby, the processing process to a round billet by the partial rolling etc. required when casting in a rectangular shape can be omitted.

  The hot pipe making method for making seamless steel pipe is not particularly limited. For example, the mandrel-Mannesmann method is adopted. As for cooling after hot pipe making, it is preferable that the cooling rate such as cooling is small because cold drawing becomes easy. The shape of the obtained seamless steel pipe is not particularly limited, and may be, for example, a diameter of 32 to 50 mm and a thickness of about 2.5 to 3.0 mm.

(C) Cold drawing process The seamless steel pipe obtained by hot pipe making generally has a large thickness and diameter and insufficient dimensional accuracy. In order to obtain predetermined dimensions (outer diameter and thickness of the steel pipe) and surface properties, the seamless steel pipe is used as a base pipe and subjected to cold drawing. In the present invention, in order to take advantage 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 set to more than 40%. If the degree of work in one cold drawing exceeds 50%, the occurrence of internal wrinkles and cracks is likely to occur, so the preferred degree of work is 42 to 48%, more preferably 43 to 46%. When the cold drawing process is performed twice or more in the cold drawing process, the degree of processing in at least one cold drawing may be 40% or more, and cold drawing having a degree of processing of less than 40% is used in combination. It is permissible.

The degree of work in cold drawing is synonymous with the area reduction rate (section reduction rate) defined by the following equation.
Area reduction ratio (%) = (S ₀ −S _f ) × 100 / S ₀
However, S ₀ : Cross section of steel pipe before cold drawing process S _f : Cross section of steel pipe after completion of cold drawing process “Cross section area of steel pipe” It is an area.

  The “degree of work (or reduction in area) of one cold drawing” means that the total degree of work in a plurality of cold drawing operations is “one time” as long as it is performed without interposing soft annealing. Treated as “Cold drawing degree”. By using the steel according to the present invention, the degree of processing of one cold drawing can be over 40%. Therefore, if the finished dimension of the seamless steel pipe obtained by hot pipe making is appropriately selected, a predetermined value can be obtained. It becomes possible to manufacture a thin steel pipe having a dimension by only one cold drawing. This can greatly simplify the production of thin-walled steel pipes that conventionally required two cold drawing steps and required softening annealing in the middle.

  The cold drawing method is well known and may be carried out according to a conventional method. For example, a seamless steel pipe produced by the mandrel-Mannesmann method as described above is used as a raw pipe, and this is allowed to cool to room temperature, and then drawn with a die and a plug to reduce the diameter and reduce the thickness. The steel pipe for airbag is preferably, for example, 30 mm in diameter or less and 2 mm in thickness or less. There is no particular limitation on the processing method as long as cold drawing from a seamless steel pipe to a steel pipe having a required size can be realized, but the above-described drawing process is preferable.

  The steel used in the present invention can be processed, for example, with a reduction in area of 46% by one cold drawing. Therefore, if the final dimension of the steel pipe for airbag is 1.7 mm thick and the outer diameter is 25 mm, the dimension of the raw pipe subjected to cold drawing is, for example, 31.8 mm outer diameter and 2.5 mm thick. For example, a product having a predetermined size can be obtained by one cold drawing.

(D) Straightening Since the steel pipe for airbags produced in the present invention has a tensile strength of 900 MPa or more and a surface reduction rate of cold drawing of 40% or more, the strength after cold drawing is conventional steel. In some cases, the steel pipe after the cold drawing process may be bent due to a springback or the like.

As will be described later, in order to ensure high strength and high toughness, a steel pipe having a predetermined size by cold drawing is heated to a temperature higher than the Ac ₃ transformation point by rapid heating for quenching. Rapid heating is typically 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 straight through the high-frequency coil used for high-frequency induction heating. Therefore, in a preferred embodiment, straightening is performed after cold drawing to eliminate bending of the steel pipe.

  The correction method is not particularly limited, and may be performed by a conventional method. For example, by arranging about 4 rows of 2 roll type stands, staggering the center position of the roll gap in each row (ie, offsetting), adjusting the amount of roll gap, and passing the steel pipe between the rolls, bending And a method of adding bending processing is preferable. The higher the degree of bending and bending back at this time, the higher the correction effect. From that point of view, the offset amount (the amount of deviation of the roll axis between adjacent roll pairs) should be 1% or more of the outer diameter of the steel pipe, and less than the roll gap amount that is 1% smaller than the outer diameter of the steel pipe. preferable. On the other hand, in order to avoid problems such as cracks in 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 a roll gap amount that is 5% smaller than the outer diameter of the steel pipe.

(E) Heat treatment After performing the straightening process of (D) as necessary, the steel pipe is subjected to a heat treatment in order to impart the required tensile strength to the steel pipe and to increase the T-direction toughness and ensure the burst resistance. Apply. In order to provide a steel pipe with high tensile strength of 900 MPa or more and excellent low-temperature toughness or burst resistance, it is quenched by heating to a temperature above the Ac ₃ (transformation) point, and then Ac ₁ ( Tempering is performed at a temperature below the transformation point.

If the heating temperature before quenching is lower than the Ac ₃ point at which the austenite single phase is obtained, good T direction toughness (and hence good burst resistance) cannot be ensured. On the other hand, if the heating temperature is too high, austenite grains begin to grow rapidly, become coarse grains, and toughness decreases. More desirably, it is 1000 ° C. or lower.

Heating to a temperature of Ac ₃ point or higher at the time of quenching is performed by rapid heating at a heating rate of 50 ° C./s or higher. As the heating rate, a value of an average heating rate in a temperature range of 200 ° C. or more and the heating temperature can be adopted. If the heating rate is less than 50 ° C./s, the austenite grain size cannot be refined, and the tensile properties and the low temperature toughness or burst resistance performance deteriorate. In order to obtain a steel pipe having a tensile strength of 1000 MPa or more and a 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 a temperature of Ac ₃ point or higher by rapid heating is held at a temperature of Ac ₃ point or higher for a short time, and then rapidly cooled for quenching. This holding time is preferably in the range of 0.5 to 8 seconds. More desirably, 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, the austenite grain size tends to be coarsened, especially when the holding temperature is high. Reducing the particle size is necessary to ensure extremely high toughness.

  The cooling rate for quenching is controlled so that the cooling rate in the temperature range of at least 850 to 500 ° C. is 50 ° C./s or more. This cooling rate is desirably 100 ° C./s or more. In order to set the tensile strength to 1000 MPa or more and vTrs100 to −80 ° C. or less, it is desirable to set the cooling rate to 150 ° C./s or more. If the cooling rate is too low, quenching becomes incomplete, the martensite ratio decreases, and sufficient tensile strength cannot be obtained.

The steel pipe that has been quenched and cooled to near normal temperature is tempered at a temperature of Ac ₁ point or less in order to give a tensile strength of 900 MPa or more and sufficient burst resistance. When the tempering temperature exceeds the Ac ₁ point, it becomes difficult to stably and surely obtain the intended tensile strength and low temperature toughness.

  The method of tempering is not particularly limited. For example, the tempering method may be carried out by cooling after soaking with a heat treatment furnace such as a hearth roller type continuous furnace, high-frequency induction heating or the like. Preferable soaking 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 corrected as appropriate with a straightener or the like by the method described in (D).

  In order to process an air bag accumulator from an air bag steel pipe manufactured in this way, the steel pipe is cut to a predetermined length to make a short pipe, and if necessary, at least one end thereof is subjected to press working or The diameter may be reduced by spatula drawing or the like (referred to as bottle processing) and finally processed into a shape necessary for mounting an initiator or the like. Therefore, the predetermined dimensions and dimensional accuracy of the steel pipe for an air bag referred to in this specification means dimensions and dimensional accuracy related to the tube thickness and diameter. Finally, lids are attached to both ends of the steel pipe by welding.

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 melted in a converter and is continuously cast (round CC). A cylindrical billet having an outer diameter of 191 mm was manufactured. The round CC billet is cut to a desired length, heated to 1250 ° C., and then drilled and rolled by a normal Mannesmann Piercer mandrel mill method to produce a first blank tube having an outer diameter of 31.8 mm and a wall thickness of 2.5 mm. And a second element tube having an outer diameter of 42.7 mm and a wall thickness of 2.7 mm.

  The two types of raw pipes thus obtained are subjected to a drawing process using a die and a plug in a usual manner, and after one or two cold drawing processes (cold drawing processes), an outer diameter of 25 The steel pipe was finished with a thickness of 0.0 mm and a thickness of 1.7 mm. Regarding the comparative steels G and H in Table 1, when a first pipe having an outer diameter of 31.8 mm and a wall thickness of 2.5 mm was used to produce a steel pipe having the above-mentioned shape by a single drawing, the fracture occurred. Generated and could not be manufactured.

  In Comparative Examples 9 and 10, the second raw pipe was used to obtain a steel pipe having an outer diameter of 32.0 mm and a wall thickness of 2.2 mm by the first drawing, and then the second time through softening annealing at 630 ° C. for 20 minutes. The outer diameter was 25.0 mm and the wall thickness was 1.7 mm.

  After correcting the cold drawn steel pipe with a straightener, up to 920 ° C. at an average temperature increase rate of 300 ° C./s (average value in a temperature range of 200 to 900 ° C.) using a high frequency induction heating device. After heating and holding at 920 ° C for 2 seconds, water quenching was performed by water cooling (average cooling rate of 150 ° C / s in the temperature range of 850 to 500 ° C). Subsequently, in order to temper the steel pipe, a soaking treatment was performed at 350 to 500 ° C. for 30 minutes in a bright annealing furnace, and the steel pipe was cooled to room temperature by natural cooling and cooling in the furnace to obtain a steel pipe for an airbag.

  A tube having a certain length was cut out from each of the obtained steel tubes, and the tube was cut and developed in the length direction of the tube at room temperature. Charpy impact test was conducted at various temperatures below -40 ° C using test pieces with a 2 mm V notch introduced into a rectangular material 55 mm long, 10 mm high and 1.7 mm wide taken from the T direction from the expanded tube. did. By this test, the lower limit temperature (vTrs100) at which the ductile fracture surface ratio becomes 100% was determined.

  Moreover, the tension test was done according to the metal material tensile test method prescribed | regulated to JISZ2241, using the No. 11 test piece prescribed | regulated to JISZ2201 extract | collected from the L direction of the steel pipe. The above test results are shown together in Table 2 together with the steel pipe production conditions.

  As is apparent from Table 2, when steels A to F having the chemical composition of the steel according to the present invention were used, the alloy contained no expensive Mo or only a small amount of less than 0.10%, and an alloy. Despite the low cost, it is possible to process into a predetermined product size by one cold drawing even at a processing rate of 46% area reduction. By performing cooling, a high level of product performance can be achieved as a steel pipe for an airbag. In particular, when steels A to C, E, and F having a composition satisfying the above-described formula (1) are used, vTrs100 is −100 ° C. or less, low temperature toughness is extremely high, and excellent resistance in a low temperature environment. It is clear that burst performance can be expected.

  On the other hand, the steels F and G of the comparative examples contain a large amount of Mo, so the alloy cost is high. In addition, cracks occurred when cold drawing with a reduction in area of 40% or more was performed. Therefore, it is necessary to perform the cold drawing process twice or more with a surface area reduction rate of less than 40%, and an intermediate softening annealing is required, which increases the manufacturing cost of the steel pipe for airbag.

Claims (8)

By 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 %: Al: 0.10% or less, Cr: 0.01-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, the balance being Fe and inevitable impurities A pipe making process for performing hot pipe making of seamless steel pipe from steel comprising:
A cold drawing process in which the obtained seamless steel pipe is subjected to a cold drawing process in which the area reduction rate of one cold drawing process is more than 40% and 50% or less to obtain a steel pipe having a predetermined size; In addition, after the cold drawn steel pipe is heated to a temperature of Ac3 point or higher at a temperature increase rate of 50 ° C / s or higher, the cooling rate in the temperature range of at least 850 to 500 ° C is 50 ° C / s or higher. A heat treatment step in which quenching is performed by cooling, and then tempering is performed at a temperature equal to or lower than the Ac1 point temperature;
The manufacturing method of the steel pipe for airbags which has the tensile strength of 900 Mpa or more characterized by containing vMPs100 and the low temperature toughness whose vTrs100 is -60 degrees C or less.   The manufacturing method of the steel pipe for airbags of Claim 1 whose said steel is less than Mo: 0.10%.   The steel is at least one selected from Nb: not more than 0.050%, Ti: not more than 0.050% and V: not more than 0.20%, and Ca: not more than 0.005% and B: 0.0030% The manufacturing method of the steel pipe for airbags of Claim 1 or 2 containing the at least 1 sort (s) chosen from the following. The manufacturing method of the steel pipe for airbags in any one of Claims 1-3 with which the density | concentration of Cu, Ni, Cr, and Mo of the said steel satisfies the following (1) Formula.
Cu + Ni ≧ (Cr + Mo) 2 +0.3 (1)
The element symbol in Formula (1) means a numerical value when the content of these elements is expressed in mass%. However, when Mo is not contained, Mo = 0.   The manufacturing method of the steel pipe for airbags in any one of Claims 1-4 whose wall thickness of the steel pipe after the said cold drawing process completion is 2.0 mm or less.   The manufacturing method of the steel pipe for airbags of Claim 5 with which the said cold drawing process is performed by one cold drawing. Performed by high-frequency induction heating to heat for quenching in the heat treatment step, the manufacturing method for an air bag steel according to any one of claims 1 to 6.   The manufacturing method of the steel pipe for airbags of Claim 7 which corrects the steel pipe obtained at the cold drawing process before the heating for the said quenching.