JP2005060796A - Method for producing high strength and high toughness welded steel tube for air bag bottle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high strength and high toughness welded steel tube for air bag bottle and the high strength and high toughness welded steel tube for air bag bottle having ≥900 MPa tensile strength and the high toughness showing ductility in a drop hammer test at -60°C to semi-cut steel tube. <P>SOLUTION: To the welded steel tube having the composition composed by mass% of 0.02-0.2% C, ≤1% Si, 1.5-4% Mn, ≤0.1% P, ≤0.01% S, ≤0.1% Al, ≤0.01% N, ≤0.1% Ti, ≤0.1% Nb, ≤0.01% B, a reducing-rolling at ≥700°C rolling-finishing temperature and ≤35% accumulated shrinkage diameter ratio is applied and the obtained steel tube is used as a blank steel tube, and a cold-drawing treatment is applied to this blank steel tube to form the steel tube having a prescribed size. Then, after cold-drawing treatment, an annealing treatment can be applied. Further, one or more kinds among Cu, Ni, Cr, Mo and/or one or two kinds of Ca and REM can be contained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength welded steel pipe, and more particularly to a high-strength welded steel pipe excellent in toughness and workability, which is suitable for an airbag.

  In recent years, there has been a strong desire to improve the collision safety of automobiles, and in particular, the introduction of safety devices that protect passengers in the event of a collision has been actively promoted. In particular, in the event of a collision, the mounting of an airbag that deploys between the occupant and the steering wheel or instrument panel to absorb the kinetic energy of the occupant and reduce occupant damage is becoming common. In particular, driver seat airbags loaded in the steering wheel and passenger seat airbags loaded in the instrument panel are becoming standard equipment. Furthermore, recently, in addition to these, in order to protect the occupant in the case of a side collision, there are an increasing number of automobiles equipped with a side airbag or a curtain airbag covering the side window on the seat. The airbag inflator includes a bottle which is a container part.

  Conventionally, many methods have been employed for airbags, such as a method of generating gas using explosives stored in an inflator and a method using thermal expansion of a reaction gas stored in an inflator. Recently, in consideration of recyclability and environmental considerations, instead of using explosives, a method of filling an inflator with an inert gas such as argon at a high pressure has been adopted.

  In general, airbag bottles are manufactured by processing steel pipes. The steel pipe is cold drawn to a predetermined size and cut to a predetermined length, and then both ends of the pipe are processed by pressing or the like and a sealing plate is welded to form a bottle. For this reason, as a steel pipe for airbag bottles, a steel pipe having sufficient strength and toughness, excellent workability, and excellent weldability is desired.

  In response to such a request, for example, in Patent Document 1, C: 0.01 to 0.20%, Si: 0.50% or less, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.10 Or less: 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 : One or more of 0.005% or less, steel made of the balance Fe and inevitable impurities, after pipe making, cold work, or after cold work, annealing, normalizing, or A method of manufacturing a steel pipe for a high-strength, high-toughness air bag that has been quenched and tempered has been proposed.

  Patent Document 2 discloses that a steel having the same composition as that described in Patent Document 1 is piped and then normalized at 850 to 1000 ° C. and then cold-worked to a predetermined dimension, or A method of manufacturing a steel pipe for a high-strength, high-toughness air bag that is subjected to stress relief annealing, normalizing, or quenching and tempering has been proposed.

Further, in Patent Document 3, a steel having the same composition as that described in Patent Document 1 was piped and then quenched at 850 to 1000 ° C. or further tempered at 450 ° C. or more and less than the Ac1 transformation point. After that, a method of manufacturing a steel pipe for a high-strength, high-toughness air bag is proposed in which a cold working is performed to a predetermined dimension, or an annealing treatment is performed after the cold working.
Japanese Patent Laid-Open No. 10-140283 Japanese Patent Laid-Open No. 10-140249 Japanese Patent Laid-Open No. 10-140250

  Recently, the airbag system has been required to be smaller and lighter, and as a steel pipe for an airbag bottle, higher strength has been demanded. In order to satisfy such requirements, there is a demand for a steel pipe that can obtain a tensile strength of 900 MPa or more as an air bag bottle after cold drawing or heat treatment.

  The technologies described in Patent Document 1, Patent Document 2, and Patent Document 3 are aimed at producing 590 MPa class high-strength seamless steel pipes, which are desired as steel pipes for airbag bottles as described above. There is a problem that requests cannot be handled.

  In addition, the airbag bottle is subjected to a burst test by hydraulic pressure, and the strength and ductility in the pipe circumferential direction are evaluated. In this burst test, the crack during the test is required to be a ductile crack. Usually, the crack at the time of the test is rejected if it is a brittle crack. In order to prevent brittle cracking in the burst test, it is necessary to perform heat treatment, but for the purpose of securing strength, it is common to perform quenching and tempering treatment.

  However, performing quenching and tempering has problems that the process becomes complicated and the manufacturing period becomes long, and the inflator manufacturing cost increases. For this reason, there is a demand for a steel pipe for an airbag bottle that can satisfy the required characteristics without requiring a heat treatment or simply by performing a simple heat treatment.

  In an air bag filled with an inert gas, the inert gas is filled in the inflator at a high pressure, and it is necessary to keep the inflator constantly at a high pressure. Therefore, from the viewpoint of seam reliability, the steel pipe for the airbag is used. As such, seamless steel pipes are used exclusively. However, seamless steel pipes are expensive, and application of welded steel pipes is being studied to meet the demand for further reduction in manufacturing costs.

  The present invention advantageously solves the above-described problems of the prior art, and halves the tensile strength of 900 MPa or more as an air bag bottle without requiring heat treatment or simply performing heat treatment. To propose a manufacturing method for high strength and high toughness welded steel pipes for airbag bottles and high strength and toughness welded steel pipes for airbag bottles that have high toughness that does not or does not break in a drop weight test at -60 ° C for steel pipes. With the goal.

  In order to achieve the above-described problems, the present inventors have intensively studied various factors affecting the strength and toughness of a welded steel pipe. As a result, after adjusting C, Si, Mn, N, Ti, Nb, and B to an appropriate range, the structure is bainite or bainitic ferrite structure, or the bainite phase or bainitic ferrite phase is the main phase, A welded steel pipe with a structure containing martensite of area% or less as the second phase is used as a raw steel pipe, and the steel pipe is subjected to a cold drawing process, so that it is optimal for an airbag bottle without quenching and tempering treatment. It has been found that a welded steel pipe having strength, toughness and workability can be obtained.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.02 to 0.2%, Si: 1% or less, Mn: 1.5 to 4%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less Ti: 0.1% or less, Nb: 0.1% or less, B: 0.01% or less, the composition comprising the balance Fe and inevitable impurities, and the main phase of bainite or bainitic ferrite structure, or bainite or bainitic ferrite The material steel pipe is a welded steel pipe having a structure containing a martensite phase of 15% or less in area ratio as a second phase, and the raw steel pipe is subjected to cold drawing to obtain a steel pipe having a predetermined size. Manufacturing method of high strength and high toughness welded steel pipe for airbag bottles.
(2) In (1), in addition to the above composition, the following group A or group B
Group A: mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less selected from the group B: mass%, Ca : High-strength, high-toughness welded steel pipe for airbag bottles containing one or two groups selected from one or two selected from 0.02% or less and REM: 0.02% or less Manufacturing method.
(3) By mass%, C: 0.02 to 0.2%, Si: 1% or less, Mn: 1.5 to 4%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less , Ti: 0.1% or less, Nb: 0.1% or less, B: 0.01% or less, and heat treatment or soaking treatment is performed on the welded steel pipe having the composition composed of the remaining Fe and inevitable impurities, and then the rolling end temperature is 700 The air is characterized in that it is drawn at a temperature of not less than ℃ and the cumulative diameter reduction ratio is 35% or less, and the obtained steel pipe is used as a raw steel pipe, and the raw steel pipe is subjected to cold drawing to obtain a steel pipe having a predetermined size. Manufacturing method of high strength and high toughness welded steel pipe for bag bottles.
(4) The airbag bottle according to (3), wherein the drawing rolling includes an intermediate heat treatment in which reheating is performed to 800 ° C. or more, and an accumulated diameter reduction ratio after the intermediate heat treatment is set to 35% or less. Of manufacturing high strength and high toughness welded steel pipes.
(5) In addition to the above composition in (3) or (4), the following group A or group B
Group A: mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less selected from the group B: mass%, Ca : High-strength, high-toughness welded steel pipe for airbag bottles containing one or two groups selected from one or two selected from 0.02% or less and REM: 0.02% or less Manufacturing method.
(6) A welded steel pipe that has been cold-drawn or is subjected to cold-drawing and subsequent normalization, in mass%, C: 0.02 to 0.2%, Si: 1% or less, Mn : 1.5-4%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, Ti: 0.1% or less, Nb: 0.1% or less, B: 0.01% or less, It has a composition comprising the remaining Fe and inevitable impurities, and a bainite or bainitic ferrite structure, or a structure containing bainite or bainitic ferrite as the main phase and a martensite phase with an area ratio of 15% or less as the second phase. Tensile strength: High strength of 900MPa or more, and high strength and high strength for airbag bottles, characterized by ductile fracture or no toughness in the drop test at -60 ° C for halved steel pipes Tough welded steel pipe.

  According to the present invention, a welded steel pipe having appropriate strength, toughness, and workability as a steel pipe for an air bag bottle can be stably manufactured with high dimensional accuracy without performing quenching and tempering treatment, and the industrially remarkable effect can be obtained. Play. Moreover, the welded steel pipe which becomes this invention is a welded steel pipe suitable for uses, such as a member for motor vehicles, a member for construction, besides the use of an airbag bottle. In addition, it is possible to apply to uses, such as a member for motor vehicles, and a member for construction, without performing a cold drawing process.

  In the present invention, by mass, C: 0.02 to 0.2%, Si: 1% or less, Mn: 1.5 to 4%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% In the following, Ti: 0.1% or less, Nb: 0.1% or less, B: 0.01% or less, the composition comprising the balance Fe and unavoidable impurities, bainite or bainitic ferrite structure, or bainite or bainitic ferrite A welded steel pipe having a structure including a martensite phase with an area ratio of 15% or less as a second phase as a main phase is used as a raw steel pipe, and the raw steel pipe is subjected to cold drawing to obtain a steel pipe having a predetermined size.

  First, the reasons for limiting the composition of the steel pipe used will be explained. Hereinafter, the mass% in the composition is simply referred to as%.

C: 0.02-0.2%
C is an element that contributes to increasing the strength of steel, and in the present invention, it is necessary to contain 0.02% or more in order to ensure a desired structure and strength. On the other hand, if over 0.2%, the workability and toughness are lowered. If the content is less than 0.02%, the crystal grains tend to become coarse during welding, causing insufficient strength and uneven deformation. For this reason, in this invention, C was limited to 0.02 to 0.2% of range. In addition, Preferably it is 0.02 to 0.1%. In order to obtain a desired structure, it is necessary to balance the C content and the Mn content.

Si: 1% or less
Si is an element that increases the strength of steel, and it is preferable to contain 0.01% or more in order to obtain such an effect. However, excessive inclusion significantly reduces ERW weldability. For this reason, in this invention, it limited to 1% or less. In addition, Preferably it is 0.15-0.5%.

Mn: 1.5-4%
Mn is an element that improves the strength. In order to secure a desired strength, Mn is required to be contained in an amount of 1.5% or more in the present invention. On the other hand, if the content exceeds 4%, the risk of occurrence of defects due to center segregation increases, and cracks occur during welding, and the electro-weldability is significantly reduced. In the present invention, in order to improve the toughness, in order to obtain an optimum structure, Mn is contained in an appropriate amount in balance with the C amount. However, if Mn is less than 1.5%, the target strength can be combined with any C amount. And the target toughness cannot be secured at the same time. On the other hand, if Mn is less than 1.5%, a ferrite + pearlite structure cannot be obtained and the target strength cannot be secured. For these reasons, Mn was limited to a range of 1.5 to 4%. In addition, Preferably it is 1.5 to 3.0%.

P: 0.1% or less P is an element useful for increasing the strength, and the effect becomes remarkable when the content is 0.01% or more. However, if the content exceeds 0.1%, the weldability is remarkably deteriorated. For this reason, P was limited to 0.1% or less. In addition, when the reinforcement | strengthening by P is not so required, or when weldability becomes a problem, it is preferable to set it as 0.05% or less.

S: 0.01% or less S exists as non-metallic inclusions in steel, but when stress such as bending or pipe expansion is applied, cracks are generated starting from these non-metallic inclusions. The steel pipe may break. For this reason, it is preferable to reduce S as much as possible. If the content is reduced to 0.01% or less, the above-described adverse effects are reduced. Therefore, in the present invention, S is limited to 0.01% or less. In addition, Preferably it is 0.005% or less, More preferably, it is 0.0010% or less.

Al: 0.1% or less
Al is an element that acts as a deoxidizer and has the effect of improving the ductility and toughness by suppressing the coarsening of crystal grains, and is preferably contained in an amount of 0.005% or more. However, if it exceeds 0.1%, the amount of oxide inclusions increases and the cleanliness decreases. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.05% or less.

N: 0.01% or less N is an element that combines with Al to refine crystal grains. For this purpose, N is preferably contained in an amount of 0.001% or more, but if contained over 0.01%, ductility is reduced. . For this reason, N was limited to 0.01% or less.

Ti: 0.1% or less
Ti combines with C and precipitates in the steel as TiC, and has the effect of suppressing the strengthening of the steel and further the softening of the weld heat affected zone. Such an effect becomes remarkable when the content is 0.005% or more. However, when the content exceeds 0.1%, the electric resistance weldability is lowered and the ductility is lowered. For this reason, Ti was limited to 0.1% or less.

Nb: 0.1% or less
Nb, like Ti, binds to C and precipitates as NbC in the steel, and has the effect of strengthening the steel and further suppressing the softening of the weld heat affected zone. Such an effect becomes remarkable when the content is 0.005% or more. However, when the content exceeds 0.1%, the electric resistance weldability is lowered and the ductility is lowered. For this reason, Nb was limited to 0.1% or less.

B: 0.01% or less B has an action of suppressing ferrite transformation in continuous cooling and shifting to a long time side to easily obtain a bainite structure. In the present invention, B is an important element for securing a desired structure. One. By suppressing the ferrite transformation by B, the fraction of martensite phase that adversely affects toughness can be kept low. Such an effect is remarkably recognized when the content is 0.0005% or more, but even if the content exceeds 0.01%, the above-described effect is saturated and the toughness deteriorates. For this reason, B was limited to 0.01% or less. In addition, Preferably it is 0.0005 to 0.0030%.

  In addition to the basic components described above, one or two groups selected from the following group A or group B can be further contained as required.

Group A: Cu: 1% or less Ni: 1% or less, Cr: 1% or less, Mo: 1% or less selected from 1% or less A group of Cu, Ni, Cr, Mo is any Is a useful element that can increase the strength without impairing the ductility and toughness, and can be selected and contained as necessary in the present invention. Such an effect is remarkably recognized when Cu, Ni, Cr and Mo are contained in amounts of 0.01% or more. On the other hand, when Cu, Ni, Cr, and Mo each contain more than 1%, ductility and weldability deteriorate, and hot workability and cold workability deteriorate. In addition, these elements are expensive, and a large amount is economically disadvantageous. For this reason, in group A, it is preferable to limit to Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, and Mo: 1% or less.

Group B: 1 or 2 types selected from Ca: 0.02% or less, REM: 0.02% or less Group B Ca and REM are both spherical in shape of non-metallic inclusions, and have ductility and workability. It is an element that acts effectively for improvement, and can be selected and contained as necessary in the present invention. Such an effect is recognized when Ca: 0.001% or more and REM: 0.001% or more. On the other hand, if both Ca and REM are contained in excess of 0.02%, the amount of inclusions becomes excessive and the cleanliness is lowered. For this reason, in Group B, it is preferable to limit to Ca: 0.02% or less and REM: 0.02% or less. When Ca and REM are used in combination, the total content is preferably 0.03% or less.

Remaining Fe and Inevitable Impurities The remainder other than the above components is Fe and inevitable impurities.

  Next, the structure limitation of the material steel pipe will be described.

  The structure of the material steel pipe used in the present invention is preferably a bainite or bainitic ferrite structure in order to ensure predetermined strength and toughness. The structure of the raw steel pipe may be a structure containing bainite or bainitic ferrite as the main phase and a martensite phase with an area ratio of 15% or less as the second phase. In addition, the main phase as used in the field of this invention shall mean the phase which occupies 50% or more by an area ratio. In order to further improve toughness by using bainite or bainitic ferrite as the main phase, it is preferable to limit the martensite phase to 15% or less in terms of area ratio as the second phase. Thereby, desired toughness can be ensured. The second phase may contain a ferrite phase, a pearlite phase, a cementite phase, and a carbide in addition to the martensite phase.

  Next, a preferred method for manufacturing a steel pipe will be described.

  It is preferable that the molten steel having the above composition is melted by a known melting method such as a converter or an electric furnace and used as a rolling material such as a slab by a known casting method such as a continuous casting method or an ingot-making method. Next, the rolled material such as slab is made into a hot rolled steel plate (strip steel) by hot rolling, or cold rolled and annealed on the hot rolled steel plate (steel strip) to obtain a cold rolled steel plate (steel strip). It is preferable to use a steel pipe material. Then, these steel pipe materials (strip steel) are used to make a welded steel pipe. The method for manufacturing a welded steel pipe is, for example, an open pipe formed by roll forming or bending, cold, warm or hot, and both edges of the open pipe are heated above the melting point using induction heating and squeezed with a squeeze roll. It is preferable to use an electric resistance welding method in which contact welding is performed, or a solid-phase welding method in which both edges of an open pipe are heated to below the melting point using induction heating and a pressure welding is performed with a squeeze roll, or a forging method. However, the present invention is not limited to these methods.

  In the present invention, the welded steel pipe having the above composition is used as a raw material, and after subjecting the raw material to heat treatment or soaking, the rolling finish temperature is 700 ° C. or more, and the drawing reduction is 35% or less. A material steel pipe is preferable. Thereby, a raw steel pipe having a bainite or bainitic ferrite structure, or a structure containing bainite or bainitic ferrite as a main phase and a martensite phase with an area ratio of 15% or less as a second phase can be easily obtained. . In addition, in this invention, the manufacturing method of a raw material steel pipe is not limited to this.

  The conditions for the heat treatment are not particularly limited as long as the rolling end temperature can be set to 700 ° C. or higher, and it is preferably performed in the range of 700 to 1000 ° C. If the welded steel pipe is manufactured warmly or hotly and the welded steel pipe has a temperature at which it can be drawn and rolled, a soaking treatment is sufficient for soaking the pipe temperature. . Needless to say, when the temperature of the welded steel pipe used as a raw material is too low, the above heat treatment is performed.

  The rolling end temperature of the drawing rolling is preferably 700 ° C. or higher. When the rolling end temperature is less than 700 ° C., ferrite transformation nuclei introduced by processing increase, ferrite transformation is promoted, and the ferrite fraction increases. For this reason, desired strength cannot be obtained, and as a result, the martensite fraction increases and the toughness deteriorates. In addition, it is preferable that rolling end temperature shall be 900 degrees C or less from a viewpoint of ensuring productivity and prevention of deterioration of surface skin. Moreover, it is preferable that the cumulative diameter reduction ratio of drawing rolling is 35% or less. If the cumulative diameter reduction exceeds 35%, the ferrite transformation nuclei increase and the desired structure cannot be secured, and the desired toughness cannot be secured.

  Further, in the present invention, intermediate heat treatment may be performed in which rolling is temporarily interrupted during the drawing and reheated to a temperature of 800 ° C. or higher. In this case, since the processing strain before the intermediate heat treatment is released, the cumulative diameter reduction after the intermediate heat treatment is preferably 35% or less.

  In addition, it cannot be overemphasized that the manufacturing method of a raw material steel pipe is not limited to an above-described method.

  The obtained material steel pipe is then subjected to a cold drawing process to obtain a steel pipe (product) having a predetermined size.

  The cold drawing process does not require a special device and can be performed using a generally known cold drawing device. The cold drawing conditions are not particularly limited as long as the steel pipe can have a predetermined size, but the diameter reduction rate can be adjusted to 5 to 25% and the thickness reduction rate to 10 to 30% within the proper range. It is preferable from the viewpoint of ensuring dimensional accuracy.

  In the present invention, the properties such as strength and toughness required as a desired steel pipe for an airbag can be sufficiently satisfied even with the cold drawing treatment described above, but the normalizing treatment is performed after the cold drawing treatment. It is preferable to apply. As a result, particularly excellent pipe circumferential direction toughness can be secured stably.

  The normalizing process is preferably a process of heating to a temperature in the range of 850 to 1000 ° C. and air cooling. If the normalizing temperature is less than 850 ° C., the austenite grains cannot be sufficiently uniformed. On the other hand, if the normalizing temperature is higher than 1000 ° C., the crystal grains become coarse and it becomes difficult to ensure the desired toughness. For this reason, it is preferable to limit the normalizing temperature to 850-1000 degreeC. In addition, Preferably it is 850-950 degreeC.

  The welded steel pipe that has been cold drawn or further subjected to normalization is preferably subjected to descaling by pickling and bending correction as necessary to obtain a product pipe (steel pipe). The

  The welded steel pipe manufactured by the above-described manufacturing method has a high tensile strength: 900 MPa or more and a high toughness that does not break or does not break in a drop weight test at -60 ° C. against a halved steel pipe. It becomes a steel pipe excellent in workability and weldability, and is a steel pipe suitable for an airbag bottle.

  In the present invention, since the target strength can be sufficiently secured even if the cooling after the normalizing treatment is air cooling, a steel pipe having excellent characteristics can be stably manufactured at a low cost without requiring special equipment. There is an advantage.

  The steel pipe material (thickness: 2.6 mm hot rolled steel sheet or 2.6 mm thick annealed cold rolled steel sheet) having the composition shown in Table 1 is roll formed into an open pipe, and both ends of the open pipe are melted by induction heating. It heated above and collided with the squeeze roll, and it was set as the welded steel pipe (outer diameter: 146mmphi x wall thickness: 2.6mm). These welded steel pipes were further subjected to drawing rolling under the conditions shown in Table 2 to obtain raw steel pipes (outer diameter: 70.0 mmφ × wall thickness: 2.6 mm). Test specimens are collected from the obtained steel tube, the structure is observed and imaged using an optical microscope and a scanning electron microscope, and the types and fractions of the main phase and the second phase are obtained using an image analyzer. It was. In addition, 10 visual fields or more were observed with each test piece, the average value of each structure fraction was obtained, and it was set as the structure fraction of each raw material steel pipe.

  These steel pipes were cold-drawn with a reduction ratio of 4%, a reduction ratio of 19%, or a reduction ratio of 9% and a reduction ratio of 15%, and an outer diameter of 67.0 mmφ x wall thickness of 2.1 mm Or a steel pipe having an outer diameter of 63.5 mmφ × thickness of 2.2 mm. As shown in Table 2, some steel pipes were subjected to a normalizing treatment at 900 ° C. for 15 minutes after cold drawing. Subsequently, these steel pipes were straightened to remove the bends, thereby obtaining product pipes.

  Tensile test pieces (JIS 12A test pieces) were collected from the obtained welded steel pipes (product pipes) in accordance with the provisions of JIS Z 2201, were subjected to tensile tests, and the tensile properties in the length direction were investigated. The tensile test was performed in accordance with the provisions of JIS Z 2241. Furthermore, a water pressure burst test was conducted, and the tensile strength TS in the circumferential direction was converted from the burst pressure.

  Moreover, about the obtained welded steel pipe (product pipe), the drop weight test in -60 degreeC was implemented, and the toughness of the circumferential direction was investigated. The drop weight test at -60 ° C is a test in which a welded steel pipe (product pipe) is divided into half-circles and arranged so as to be convex upward, and a weight (5 kg) is dropped on it from a height of 2 m. Was carried out at −60 ° C. A 0.5 mm V notch was introduced in the pipe length direction at the upper part on the inner surface side of the welded steel pipe to be tested (semicircular half). The V notch was 45 ° and the tip radius was R0.25 mm. After the test, the fracture surface was observed to investigate the presence of brittle fracture. The test was repeated three times, and the case where no brittle fracture occurred in the three tests was marked with ◯, the case where all brittle fracture occurred was marked with ×, and the others were marked with Δ.

  Further, the obtained welded steel pipe (product pipe) was subjected to spatula drawing to reduce the pipe end to an outer diameter of 45 mm, and a crack in the processed part was observed to evaluate workability. The case where there was no cracking was marked with ◯, and the case where cracking occurred was marked with ×.

  Further, after reducing the diameter of the tube end to 45 mm by spatula drawing, a sealing plate was welded to the tube end, and the presence or absence of cracking after welding was examined visually and under a microscope to evaluate the weldability. The case where no crack was generated after welding was marked with ◯, and the case where crack was generated was marked with x.

  In addition, a test piece is collected from the obtained product tube, the structure is observed and imaged using an optical microscope and a scanning electron microscope, and the type and fraction of the main phase and second phase are measured using an image analyzer. Asked. In addition, 10 or more visual fields were observed with each test piece, and the average value of each visual field was obtained to obtain the tissue fraction of each product tube.

  The obtained results are shown in Table 3.

  Each of the examples of the present invention is a welded steel pipe having a tensile strength of 900 MPa or more and high toughness, excellent workability, and excellent weldability. On the other hand, the comparative example out of the scope of the present invention has a tensile strength of less than 900 MPa, has reduced toughness, or has deteriorated workability and weldability, and is sufficient as a steel pipe for an air bag bottle. Characteristics are not obtained.

Claims (6)

% By mass
C: 0.02 to 0.2%, Si: 1% or less,
Mn: 1.5-4%, P: 0.1% or less,
S: 0.01% or less, Al: 0.1% or less,
N: 0.01% or less, Ti: 0.1% or less,
Nb: 0.1% or less, B: 0.01% or less, the composition comprising the balance Fe and unavoidable impurities, bainite or bainitic ferrite structure, or bainite or bainitic ferrite as the main phase, 15% in area ratio High strength for airbag bottles, characterized in that a welded steel pipe having a structure containing the following martensite phase as a second phase is a raw steel pipe, and the raw steel pipe is subjected to cold drawing to obtain a steel pipe of a predetermined size. Manufacturing method of high toughness welded steel pipe. In addition to the said composition, 1 group or 2 groups chosen from following A group or B group is contained, The manufacture of the high strength high toughness welded steel pipe for airbag bottles of Claim 1 characterized by the above-mentioned. Method.
Record
Group A: mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less selected from the group B: mass%, Ca : One or two selected from 0.02% or less, REM: 0.02% or less % By mass
C: 0.02 to 0.2%, Si: 1% or less,
Mn: 1.5-4%, P: 0.1% or less,
S: 0.01% or less, Al: 0.1% or less,
N: 0.01% or less, Ti: 0.1% or less,
Nb: 0.1% or less, B: 0.01% or less, heat treatment or soaking treatment is applied to a welded steel pipe having a composition composed of the balance Fe and inevitable impurities, and then the rolling end temperature is 700 ° C. or more and the cumulative diameter reduction rate High strength and toughness for airbag bottles, characterized by subjecting the obtained steel pipe to a raw steel pipe and subjecting the raw steel pipe to a cold drawing treatment to obtain a steel pipe of a predetermined size. Manufacturing method of welded steel pipe. 4. The air bag bottle height according to claim 3, wherein the drawing rolling includes an intermediate heat treatment for reheating to 800 ° C. or more, and a cumulative diameter reduction ratio after the intermediate heat treatment is set to 35% or less. 5. Manufacturing method of high strength and toughness welded steel pipe. The high strength high toughness welded steel pipe for airbag bottles according to claim 3 or 4, further comprising one group or two groups selected from the following group A or group B in addition to the composition: Manufacturing method.
Record
Group A: mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less selected from the group B: mass%, Ca : One or two selected from 0.02% or less, REM: 0.02% or less A welded steel pipe that has been subjected to cold drawing treatment or cold drawing treatment and subsequent normalization treatment in mass%.
C: 0.02 to 0.2%, Si: 1% or less,
Mn: 1.5-4%, P: 0.1% or less,
S: 0.01% or less, Al: 0.1% or less,
N: 0.01% or less, Ti: 0.1% or less,
Nb: 0.1% or less, B: 0.01% or less, the composition comprising the balance Fe and unavoidable impurities, bainite or bainitic ferrite structure, or bainite or bainitic ferrite as the main phase, 15% in area ratio It has a structure containing the following martensite phase as the second phase, high tensile strength: 900 MPa or more, and high toughness that does not break or does not break in a drop weight test at -60 ° C on a halved steel pipe A high-strength, high-toughness welded steel pipe for airbag bottles.