MXPA06003933A - Low carbon alloy steel tube having ultra high strength and excellent toughnes at low temperature and method of manufacturing the same - Google Patents

Abstract

A low carbon alloy steel tube and a method of manufacturing the same, in which the steel tube consists essentially of, by weight:about 0.06%to about 0.18%carbon;about 0.5%to about 1.5%manganese;about 0.1%to about 0.5%silicon;up to about 0.015%sulfur;up to about 0.025%phosphorous;up to about 0.50%nickel;about 0.1%to about 1.0%chromium;about 0.1%to about 1.0%molybdenum;about 0.01%to about 0.10%vanadium;about 0.01%to about 0.10%titanium;about 0.05%to about 0.35%copper;about 0.010%to about 0.050%aluminum;up to about 0.05%niobium;up to about 0.15%residual elements;and the balance iron and incidental impurities. The steel has a tensile strength of at least about 145 ksi and exhibits ductile behavior at temperatures as low as -60°C.

Description

STEEL ALLOY TUBE (? ON LOW CONTENT OF CARBON THAT HAS ULTRA HIGH RESISTANCE AND EXCELLENT TENACITY AT LOW TEMPERATURE AND METHOD FOR YOUR MANUFACTURING RELATED REQUESTS This application claims the benefit of United States Provisional Patent Application No. 60 / 509,806 filed October 10, 2003, and of the United States Non-Provisional Patent Application No. filed October 5, 2004.

BACKGROUND OF THE INVENTION The present invention relates to a low carbon steel alloy tube having ultra high strength and excellent toughness at low temperature, and also to a method for manufacturing said steel tube. The steel tube is particularly suitable for the manufacture of components for containers which in turn are components of containment systems for automotive, an example of which is an airbag inflator (airbag) for automotive. Air bag inflators for containment systems for vehicle occupants must comply with strict structural and functional standards. Therefore, in the manufacturing process strict procedures and tolerances are imposed. While experience in the field indicates that the industry has been successful in meeting the structural and functional standards of the past, new and / or improved properties are needed to meet the evolving requirements, while at the same time it is also important a continuous reduction of manufacturing costs. Airbags or supplemental restraint systems are an important safety feature in many current vehicles. In the past, airbag systems were of the type that used explosive chemicals, but they are expensive, and due to environmental and recycling problems, in recent years a new type of inflator has been developed using an accumulator developed from a steel tube that is filled with argon gas or another similar gas, this type of use being increasing. The aforementioned accumulator is a container that in normal times maintains the gas or similar at high pressure, which expands inside the airbag at the time of the collision of a car, by means of a single-stage expansion or in stages multiple Therefore, a steel tube used as an accumulator will receive a voltage with a high rate of effort in an extremely short period of time. Therefore, in comparison with a simple structure, such as an ordinary pressure cylinder, the steel tube described above is required to have greater dimensional accuracy, malleability and weldability, and must also have high strength, toughness and excellent resistance to blowouts. Dimensional accuracy is important to ensure a very accurate volume of gas that expands the airbag. In the tubular members used to make accumulators, the cold forming properties are very important, since these are brought to their final shape after the seamless tube is already manufactured. Depending on the configuration of the container, different shapes are obtained by cold forming. It is crucial that after cold forming pressure vessels are obtained without cracks or surface defects. Moreover, it is vital that after cold forming a very good tenacity can be obtained even at low temperatures. The developed steel has very good weldability, and for this application it is not necessary to preheat before welding, or a thermal treatment after welding. The carbon equivalent, as defined by the formula: Ceq =% C +% Mn / 6 + (% Cr +% Mo +% V) / 5 + (% Ni +% Cu) / 15 it must be less than about 0.63%. In the preferred embodiment of the present invention, to better guarantee weldability, the carbon equivalent, defined above, should be less than about 0.60%. To produce a gas vessel, a cold drawn tube made in accordance with the present invention is cut to size and then cold formed using different known technologies (such as fastening, stuffing or the like) to obtain the desired shape. In an alternative form, a welded tube can be used. Then, to produce the accumulator, at each end of the container a diffuser and cover are welded by means of any suitable technology, such as friction welding, tungsten arc welding in inert gas or laser welding. These welds are highly critical and as such require considerable manpower and, in certain cases, require a test to ensure the integrity of the weld in the pressure vessel and the deployment of the airbag. It has been observed that these welds can crack or fail, thus putting at risk the integrity of the accumulator, and possibly the operation of the airbag. The inflators are tested to ensure that they maintain their structural integrity during the deployment of the airbag. One of those trials is the so-called bursting test. This is a destructive type test in which a container is subjected to internal pressures significantly greater than expected during its normal operational use, ie in the deployment of the air bag. In this test, the inflator is subjected to increasing internal pressures until rupture occurs. In reviewing the results of the burst test and studying the container specimens from these trials, it has been found that the fracture occurs through different alternative paths: ductile fracture, fragility fracture and sometimes by a combination of both. It has been observed that in the ductile fracture an outward break occurs, exemplified by an open protrusion (such as would be the case of an exploding bubble). The surface where the rupture occurs is inclined approximately 45 degrees with respect to the external surface of the tube and is located within a certain area. In the fragility fracture, on the other hand, an unrestrained longitudinal crack is exhibited along the length of the inflator, which is indicative of a fragile area of the material. In this case the surface of the fracture is normal to the external surface of the tube. These two fracture modes have different surfaces when viewed under a scanning electron microscope - the dimples are characteristic of the ductile fracture, while a crack is indicative of fragility. Sometimes a combination of these two types of fracture can be observed, and fragility fractures can be propagated from an area with ductile rupture. Since the entire system, including the air bag inflator, can be used in vehicles operating in very different climates, it is crucial that the material exhibits a ductile behavior over a wide range of temperatures, from very cold to very hot environments .

BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a tube of low carbon steel alloy, suitable for cold forming which has an ultra high strength (UTS of 999.7 MPa (145 ksi) minimum), and consequently a very high burst pressure. . Moreover, said steel has excellent tenacity at low temperature, with a ductile behavior guaranteed at -60 ° C, ie a transition temperature from ductile to brittle (DBTT) below -60 ° C and possibly as low as -100. ° C. The present invention is also related to a manufacturing process of said steel tube. The material of the present invention is designed to manufacture components for containers, used in turn in components of automotive containment systems, an example of which is an inflator for automotive air bags.

DESCRIPTION OF THE PREFERRED MODALITIES Although the present invention is capable of being performed in different ways, it will be described hereinafter as a currently preferred embodiment, with the proviso that the present description be considered as an exemplification of the invention which is not intended to limit the same to the specific modality illustrated.

The present invention relates to a steel tube for use in pressure vessels for stored gas inflators. More particularly, the present invention relates to steel classified as low carbon and with ultra high strength for application in seamless pressure vessels, with a ductile behavior guaranteed at -60 ° C, ie having a transition temperature from ductile to brittle below -60 ° C. More particularly, the present invention relates to a chemical composition and a manufacturing process for obtaining a seamless steel tube for use in the manufacture of an inflator. A schematic illustration of a method for producing the seamless steel tube of low ultra high strength carbon alloy can be as follows: 1. Steel fabrication. 2. Casting of steel. 3. Hot rolling of the tube. 4. Operations of completion of the hollow section of the hot rolled piece. 5. Cold drawing. 6. Heat treatment. 7. Cold drawn tube termination operations. One of the main objectives of the steelmaking process is to refine the iron to separate carbon, silicon, sulfur, phosphorus and manganese. In particular, sulfur and phosphorus are harmful to steel because they impair the mechanical properties of the material. Before or after the basic manufacturing process, the ladle metallurgy process is carried out to carry out specific purification steps that allow faster processing in the basic steelmaking operation. The steelmaking process is carried out under extreme purity practices to obtain a low content of sulfur and phosphorus, which, in turn, is crucial to obtain the high tenacity required by the product. Accordingly, the objective of an inclusion level of 2 or less -simple series-, and a level of 1 or less -thick series- has been imposed under the recommendations of Standard ASTM E45: Method of the worst field (Method A) . In the preferred embodiment of the present invention, the maximum microinclusion content measured according to the aforementioned standard should be: In addition, the practice of extreme cleaning allows obtaining an oversized inclusion content with a size of 30 μm or less. This content of inclusions is obtained by limiting the total oxygen content to 20 ppm. The practice of extreme cleaning in secondary metallurgy is carried out by bubbling inert gases in the ladle furnace to force inclusion and impurities to float. The production of a fluid slag capable of absorbing impurities and the modification of the shape and size of the inclusions by means of the addition of SiCa to the liquid steel, produces a high quality steel with a low content of inclusions. The chemical composition of the steel obtained is as follows (in each case "%" refers to "percentage of mass"): CARBON (C) Carbon is an element that economically increases the strength of steel, but if its content is less than 0.06% it is difficult to obtain the desired strength. On the other hand, if the steel has a C content greater than 0.18%, then the capacity to be cold worked, the weldability and the tenacity decrease. Therefore, the range of carbon content comprises between 0.06% and 0.18%. A preferred range of carbon content comprises between 0.07% and 0.12% and even more preferred is the range between 0.08% and 0.11%.

MANGANESE (Mn) Mn is an effective element to increase the hardenability of steel, and therefore increases its strength and its tenacity. If its content is less than 0.5% it is difficult to obtain the desired strength, while if it exceeds 1.5% the stratified structures become marked and the toughness is reduced. In concordance, the manganese content is between 0.5% and 1.5%. However, a preferred range for manganese comprises between 1.00% and 1.40% and a more preferred range comprises between 1.03% and 1.18%.

SILICON (SI) Silicon is an element that has a deoxidizing effect during the steelmaking process, and also increases the strength of the steel. If the Si content is less than 0.10% steel is susceptible to oxidation, on the other hand, if the Si exceeds 0.50%, the tenacity and workability decrease. Therefore, the content of Si is between 0.1% and 0.5%. A preferred range of Si content comprises between 0.15% and 0.35%.

SULFUR (S) Sulfur is an element that reduces the tenacity of steel. In concordance, the content of S is limited to a maximum of 0.015%. A preferred maximum value is 0.010%, and a maximum preferred value is 0.003%.

FOSFORO (P) The P is an element that reduces the tenacity of steel. In concordance, the content of P is limited to a maximum of 0.025%. A preferred maximum value is 0.015%, and a maximum preferred value is 0.012%.

NICKEL (Nl) Ni is an element that increases the strength and tenacity of steel, but it is very expensive, therefore, the Ni content is limited to a maximum of 0.50%. A preferred maximum value is 0.20%, and a maximum preferred value is 0. 10% CHROME (Cr) Cr is an effective element to increase the strength, toughness, and corrosion resistance of steel. If its content is less than 0.10% it is difficult to obtain the desired strength, while if it exceeds 1.0% the toughness in the welding zones is markedly reduced. In concordance, the content of Cr is between 0.1% and 1.0%. However, a preferred range for Cr comprises between 0.55% and 0.80%, and a higher preference range between 0.63% and 0.73%.

MOLYBDENUM (Mo) The Mo is an effective element to increase the strength of the steel and helps slow the softening during tempering. If its content is less than 0.10% it is difficult to obtain the desired strength, while if it exceeds 1.0% the toughness in the welding zones is markedly reduced. In concordance, the content of Mo is between 0.1% and 1.0%. However, this ferroalloy is expensive and it is necessary to reduce the maximum content. Therefore, a preferred content range of Mo is between 0.30% and 0.50%, and a higher preference range between 0.40% and 0.45%.

VANADIO (V) Vanadium is an effective element to increase the strength of steel, still added in small amounts, and allows the delay of softening during tempering. The content of V considered optimal comprises between 0.01% and 0.10%. However, this ferroalloy is expensive and it is necessary to reduce the maximum content. Therefore, a preferred V content range comprises between 0.01% and 0.07%, and a higher preference range between 0.03% and 0.05%.

TITANIUM (Ti) Ti is an effective element to increase the strength of steel, still added in small quantities. The content of Ti considered optimal comprises between 0.01% and 0.10%. However, this ferroalloy is expensive, and it is necessary to reduce the maximum content. Therefore, a range of preferred Ti content comprises between 0.01% and 0.05%, and a higher preference range between 0.025% and 0.035%.

COPPER (Cu) This element improves the resistance to corrosion of the tube, therefore the content of Cu is in the range between 0.05% and 0.35%, and a preferred range between 0.15% and 0.30%.

ALUMINUM (Al) This element is added to the steel during the manufacturing process to reduce the content of inclusions and to refine the steel grain. A preferred range of Al content comprises between 0.010% and 0.050%. The preferred ranges for other elements not listed above are as follows: Element% by weight Niobium 0.05% max Sn 0.05% max Sb 0.05% max Pb 0.05% max As 0.05% max The residual elements in a single steel spoon used to produce tubes or chambers will be: Sn + Sb + Pb + As < 0.15% max, and S + P < 0.025 The next step is the casting of the steel to produce a solid steel bar capable of being perforated and laminated to form a seamless steel tube. The steel is molded in the factory in a solid cylindrical íocho of uniform diameter along its axis. The solid cylindrical billet of ultra high purity steel was heated at an temperature between approximately 1200 ° C and 1300 ° C, and at this point it undergoes the rolling process. Preferably, the billet is heated to a temperature between about 1250 ° C and then passed through the rolling epoch.

The billet is perforated, preferably using the known Manessmann process, and subsequently the external diameter and wall thickness are substantially reduced while its length increases substantially during hot rolling. For example, a solid bar of 148 mm diametre exíerno is rolled in hot to form a hot rolled tube of 48.3 mm exíerno diameíro with a wall thickness of 3.25 mm. The reduction of the area of the transverse section, measured as the ratio of the cross-sectional area of the solid billet to the cross-sectional area of the hot-rolled tube, is important to obtain a refined microstructure necessary to obtain the desired mechanical properties. Therefore, the minimum reduction of the cross-sectional area is 15: 1. the preferred and most preferred cross-sectional area reductions being 20: 1 and 25: 1 respectively. The hot rolled seamless tube thus manufactured is cooled to room temperature. The tube without hot-rolled stock of ultra high purity steel thus manufactured, has an approximately uniform wall thickness, tan in its circumference around the tube and in its length along the axis of the tube. The hot-rolled tube is then subjected to different finishing steps, for example, it is cut into 2 to 4 pieces of determined length, it is blunt-ended, if necessary it is subjected to grinding in a rotary recirculation equipment, and it is subjected to to non-destructive tests by means of one or more different techniques, known as electro-magnetic test techniques or by ultrasound. Then, the surface of each piece of hot rolled tubing is conditioned appropriately for cold drawing. This conditioning includes pickling by immersion in acid solution and the application of an appropriate layer of lubricants, such as the known combination of zinc phosphate and sodium stearate, or a reactive oil. After conditioning the surface, the tube is cold drawn, passing it through an external matrix with a smaller diameter than the external diameter of the drawn tube. In most cases, the inner surface of the tube is supported by an internal mandrel fixed at the end of a bar, such that the mandrel remains close to the die during drawing, the drawing operation is performed without the need for pre-heat the tube above the ambient temperature. The seamless tube is cold drawn at least once, each step reduces the external diameter and wall thickness of the tube. The cold drawn steel tube thus manufactured has a uniform external diameter in the direction of the tube axis, and a uniform wall thickness both in its circumference and in its length along the axis of the tube. The cold drawn tube has an outer diameter preferably between 10 and 70 mm, and a wall thickness of 1 to 4 mm. The cold-drawn tube is then thermally treated in an unattended oven at least at the same rate as the upper austenitizing femperafure, or Ac3 (which for the specific chemical composition described here is approximately 880 ° C), but preferably above approximately 920 ° C and below approximately 1050 ° C. The maximum austenitizing temperature is imposed to prevent grain thickening. The process can be carried out in a combusible furnace as in an induction furnace, but preferably it is carried out in this last type of furnace. The transit time in the furnace depends strongly on the type of furnace used. It has been found that the high surface quality required by this application is best achieved by using an induction type furnace. This is due to the nature of the induction process in which very short transit times are needed, thus avoiding oxidation. Preferably, the heating rate in the austenitizing is at least about 100 ° C per second, but more preferably at least about 200 ° C per second. The velocity of heat exmere- ated above, and consequently the very cold calendar temperatures, are important to obtain a very fine grain microstructure, which in turn guarantees the required mechanical properties. In addition, an appropriate fill factor, defined as the ratio between the circular area defined by the outer diameter of the tube and the circular area defined by the inner diameter of the coil of the induction furnace, is important to obtain the high heating rates required . The minimum fill factor is approximately 0.36. In the zone of exit of the oven or in the neighborhoods of the same the tube abruptly cools by means of a fluid of suitable for rapid cooling. The rapid cooling fluid is preferably water, or a cooling solution based on water. The temperature of the tube drops rapidly at room temperature, preferably at a rate of at least about 100 ° C per second, more preferably at a rate of at least about 200 ° C per second. This extremely high cooling rate is crucial to obtain a complete microstructure transformation. The steel tube is then subjected to tempering at an appropriate time and cycle time, at a temperature below the Ac1. Preferably, the tempering temperaure is about 400 ° C and 600 ° C, and more preferably between about 450 ° C and 550 ° C. The stabilization time must be long enough to guarantee a very good homogeneity in the temperature, but if it is too long, the desired mechanical properties are not obtained. Therefore, stabilization times of between about 2 and 30 minutes, preferably between about 4 and 20 minutes have been used. The tempering process is preferably carried out in a reducing or neutral protective atmosphere to avoid decarburization and oxidation of the tube. The ultra high strength steel tube thus manufactured is processed through different finishing steps, rectification in the known rotary grinding equipment, and subjected to non-destructive testing by one or more different known techniques. Preferably, for this class of applications, the tubes must be tested by means of both known techniques, ultrasound and electromagnetic. After the thermal transfer, the tube can be subjected to a chemical process to obtain an object with a desirable appearance and a very low surface roughness. For example, the tube can be stripped in a solution that contains sulfuric acid and hydrochloric acid, it can be phosphated using zinc phosphate, and it can be oiled using a peiroleum based oil, a water based oil, or a mineral oil. The steel tube obtained by the method described must have the following mechanical properties to meet the requirements established for the present invention: Elasticity limit approximately 862 MPa (125 ksi) minimum, more preferably 930 MPa (135 ksi) minimum tensile strength approximately 1000 MPa (145 ksi) minimum Elongation upon breaking approximately 9% minimum Hardness approximately 40 HRC maximum, most preferred 37 HRC maximum.

Tests to determine the yield strength, tensile strength, elongation, and hardness should be performed according to the procedures described in ASTM E8 and ASTM A370 Standards. For the tensile test, a full part of normal size is preferred to evaluate the entire tubular section. The test of demolition shall conform to the requirements of Specification 39 DOT of CFR 49, paragraph 178.65. Also, a pipe section should not crack when it is crushed with a V-shaped tool at an angle of 60 degrees until the opposite sides are 6 times the wall thickness of the pipe. The developed steel fully complies with this test. To obtain a good balance between strength and idleness, the preceding austenitic grain size (sometimes referred to as a matrix) should preferably be 7 or thinner, and 9 or finer in denomination according to ASTM E-112. . This is achieved thanks to the extremely short heating cycle during austenitization. The steel tube obtained by the disclosed method should have the properties to meet the requirements established for the present invention. The demand of the industry is continuously decreasing the required roughness values. The present invention has a good visual appearance, for example, with a surface finish for the finished tube of 3.2 microns at most on both internal and external surfaces. This requirement is obtained through cold drawing, short ausienizing times, tempering in neutral or reducing atmosphere, and adequate chemical conditioning of the surface in the different steps of the process. A burst test by hydraulic pressure should be performed by sealing the ends of the tube section, for example by welding flat steel plates to the ends of the tube. It is important that a 300 mm tube section remains free of restraints in such a way that the total burst voltage can develop. Pressurization of the pipe section should be done by pumping oil, water, alcohol, or a mixture of them. The pressure required for the spray test depends on the size of the tube. When bursting is tested, the ultra high strength seamless steel tube has a ductile garanized behavior at -60 ° C. The tests carried out on the samples produced show that this grade has a ductile behavior guaranteed at -60 ° C, with a transition temperature from ductile to brittle below -60 ° C. The inventors have found that a most representative validation test is the burst test performed at both ambient temperature and low temperature, instead of the Charpy impact test (according to ASTM E23). This is due to the fact that in these products a relatively thin wall thickness and a small external diameter are used, therefore, it is not possible to machine an ASTM standard sample for the Charpy impaction test from the tube in the transverse direction. In addition, to obtain this Charpy impact probe with a smaller size, an elastic deformation should be applied to a curve probe of the tube. Esío has a sensitive effect on the mechanical properties of steel, in particular on impact resistance. Therefore, with this procedure, a representative impact test can not be obtained.

Claims (39)

1. A steel alloy tube with a low carbon content that essentially contains, by weight, enlightens approximately 0.06% and approximately 0.18% carbon; range approximately 0.5% and approximately 1.5% manganese; between about 0.1% and about 0.5% silicon; up to approximately 0.015% sulfur; up to approximately 0.025% phosphorus; up to about 0.50% nickel; between about 0.1% and about 1.0% chromium; between about 0.1% and about 1.0% molybdenum; between about 0.01% and about 0.1% vanadium; between approximately 0.01% and approximately 0.1% of the organism; it is approximately 0.05% and approximately 0.35% copper; between approximately 0.010% and approximately 0.050% aluminum; it had approximately 0.05% niobium; up to about 0.15% residual elements, the rest being iron and incidental impurities, and further characterized because the steel tube has a tensile strength of at least about 999.7 MPa (145 ksi) and a transition temperature from ductile to brittle less than -60 ° C.

2. The low carbon steel alloy tube of claim 1, further characterized in that the steel tube contains essentially, in weight percentages, between about 0.07% and about 0.12% carbon; between about 1.00% and about 1.40% manganese; between about 0.15% and about 0.35% silicon; up to approximately 0.010% sulfur; up to approximately 0.015% phosphorus; up to about 0.20% nickel; between about 0.55% and about 0.80% chromium; between about 0.30% and about 0.50% molybdenum; between about 0.01% and about 0.07% vanadium; range approximately 0.01% and approximately 0.05% titanium; between approximately 0.15% and approximately 0.30% copper; between about 0.010% and about 0.050% aluminum; up to approximately 0.05% of niobium; up to approximately 0.15% residual elements, the rest being iron and incidental impurities.

3. The low carbon steel alloy tube of claim 1, further characterized in that the steel tube contains essentially, in percentages by weight, between about 0.08% and about 0.11% carbon; between about 1.03% and about 1.18% manganese; between about 0.15% and about 0.35% silicon; to approximately 0.003% sulfur; up to approximately 0.012% phosphorus; up to about 0.10% nickel; between about 0.63% and about 0.73% chromium; between about 0.40% and about 0.45% molybdenum; between about 0.03% and about 0.05% vanadium; between approximately 0.025% and approximately 0.035% of thifanium; enire approximately 0.15% and approximately 0.30% copper; between about 0.010% and about 0.050% aluminum; up to approximately 0.05% of niobium; up to approximately 0.15% residual elements, the rest being iron and incidental impurities.

4. The low carbon steel alloy tube of claim 1, further characterized in that the steel tube has an elasticity limit of at least about 861.8 MPa (125 ksi).

5. The low carbon steel alloy tube of claim 1 further characterized in that the steel tube has a yield strength of at least about 930.7 MPa (135 ksi).

6. The low carbon steel alloy tube of claim 1 further characterized in that the steel tube has an elongation upon breaking of at least about 9%.

7. The low carbon steel alloy tube of claim 1 further characterized in that the steel tube has a hardness no greater than about 40 HRC.

8. The low carbon steel alloy tube of claim 1 further characterized in that the steel tube has a hardness no greater than about 37 HRC.

9. The low carbon steel alloy tube of claim 1 further characterized in that the steel tube has a carbon equivalent of less than about 0.63%, being the carbon equivalent determined according to the formula: Ceq =% C +% Mn / 6 + (% Cr +% Mo +% V) / 5 + (% Ni +% Cu) / 15.

10. The low carbon steel alloy tube of claim 9 further characterized in that the steel tube has a carboa equivalent of less than about 0.60%.

11. The low carbon steel alloy tube of claim 9, further characterized in that the steel tube has a carbon equivalent of less than about 0.56%.

12. The low carbon steel alloy tube of claim 1, further characterized in that the steel tube has a maximum microinclusion content of 2 or less - fine series - and a level of 1 or less - coarse series - Measured according to the ASTM E45 Standard - Method of the worst field (Method A).

The low carbon steel alloy tube of claim 1, further characterized in that the steel tube has a maximum microinclusion content measured in accordance with ASTM E45 - Worst Field Method (Method A) as follows :

14. The low carbon steel alloy tube of claim 13, further characterized by obtaining an oversized inclusion content whose size is 30 μm or smaller.

15. The carbon-alloy steel alloy tube of claim 14, further characterized in that the total oxygen content is limited to 20 ppm.

16. The low carbon steel alloy tube of claim 1, further characterized in that the tube has a seamless tube configuration.

17. A pressure vessel for stored gas inflator comprising the low carbon steel alloy tube of claim 1.

18. An air bag inflator for automobiles comprising the steel alloy tube with low conn of carbon of claim 1.

19. A steel alloy tube with low carbon content that essentially contains, in percentages by weight, between about 0.08% and about 0.11% carbon; between about 1.03% and about 1.18% manganese; between approximately 0.15% and approximately 0.35% silicon; to approximately 0.003% sulfur; up to approximately 0.012% phosphorus; up to about 0.10% nickel; between about 0.63% and about 0.73% chromium; between about 0.40% and about 0.45% molybdenum; between about 0.03% and about 0.05% vanadium; range approximately 0.025% and approximately 0.035% Tiian; enire approximately 0.15% and approximately 0.30% copper; approximately 0.010% and approximately 0.050% aluminum; hadfa approximately 0.05% niobium; up to about 0.15% residual elements, the rest being iron and incidental impurities, further characterized in that the steel tube has a yield strength of at least about 930.7 MPa (135 ksi), a tensile strength of at least about 999.7 MPa (145 ksi), an elongation upon breaking of at least about 9%, a hardness not exceeding 37 HRC, and a transition temperature from ductile to brittle less than -60 ° C.

20. The low carbon steel alloy tube of claim 19, further characterized in that the tube has a seamless tube configuration.

21. A pressure vessel for stored gas inflator comprising the low carbon steel alloy tube of claim 19.

22. An air bag inflator for auomotors comprising the steel alloy tube with low content carbon of claim 19.

23. A method for manufacturing a length of steel tube to make a pressure vessel used in a stored gas inflator comprising the following steps: producing a tube length from a steel material which essentially comprises, in percentages by weight, between about 0.06% and about 0.18% carbon; between about 0.5% and about 1.5% manganese; between about 0.1% and about 0.5% silicon; up to approximately 0.015% sulfur; it had approximately 0.025% phosphorus; it had approximately 0.50% nickel; between about 0.1% and about 1.0% chromium; between about 0.1% and about 1.0% molybdenum; between about 0.01% and about 0.1% vanadium; enire approximately 0.01% and approximately 0.1% titanium; between approximately 0.05% and approximately 0.35% copper; between about 0.010% and about 0.050% aluminum; up to approximately 0.05% of niobium; up to approximately 0.15% residual elements, the rest being iron and incidental impurities; submitting the steel tube to a cold drawing process to obtain the desired dimensions; heat-austenizing the cold drawn steel tube in an austenitizing furnace of the induction type at an temperature at least equal to Ac3, at a heating rate of at least about 100 ° C per second; after the heating step, quickly cool the steel tube in a rapid cooling fluid until the tube reaches approximately the ambient temperature at a cooling rate of at least about 100 ° C per second; and after the rapid cooling step, subject the steel tube to tempering for approximately 2 to 30 minutes at a lower temperature of Ac1.

The method of claim 23 further characterized in that the steel tube produced comprises essentially, in weight percentages, between about 0.07% and about 0.12% carbon; between about 1.00% and about 1.40% manganese; between approximately 0.15% and approximately 0.35% silicon; it had approximately 0.010% sulfur; up to approximately 0.015% phosphorus; up to about 0.20% nickel; between about 0.55% and about 0.80% chromium; between approximately 0.30% and approximately 0.50% molybdenum; range approximately 0.01% and approximately 0.07% vanadium; between about 0.01% and about 0.05% titanium; between approximately 0.15% and approximately 0.30% copper; between about 0.010% and about 0.050% aluminum; it had approximately 0.05% niobium; to approximately 0.15% residual elements, the rest being iron and incidental impurities.

The method of claim 23 further characterized in that the steel tube produced comprises essentially, in weight percentages, between about 0.08% and about 0.11% carbon; between about 1.03% and about 1.18% manganese; between about 0.15% and about 0.35% silicon; to approximately 0.003% sulfur; up to approximately 0.012% phosphorus; up to about 0.10% nickel; between about 0.63% and about 0.73% chromium; between about 0.40% and about 0.45% molybdenum; between about 0.03% and about 0.05% vanadium; between approximately 0.025% and approximately 0.035% iitanium; between approximately 0.15% and approximately 0.30% copper; between about 0.010% and about 0.050% aluminum; up to approximately 0.05% of niobium; up to approximately 0.15% residual elements, the rest being iron and incidental impurities.

26. The method of claim 23 further characterized in that the finished steel tube has a yield strength of at least about 861.8 MPa (125 ksi).

27. The method of claim 23 further characterized in that the finished steel tube has an elastic limit of at least about 930.7 MPa (135 ksi).

The method of claim 23 further characterized in that the finished steel tube has a tensile strength of at least about 999.7 MPa (145 ksi).

29. The method of claim 23 further characterized in that the finished steel tube has an elongation upon breaking of at least about 9%.

30. The method of claim 23 further characterized in that the finished steel tube has a hardness no greater than about 40 HRC.

31. The method of claim 23 further characterized in that the finished steel tube has a hardness no greater than about 37 HRC.

32. The method of claim 23 further characterized in that the finished steel tube has a temperature of ductile to brittle transition from -60 ° C.

33. The method of claim 23 further characterized in that in the heating step during austenitizing the steel tube is heated to a temperature between about 920 ° C and 1050 ° C.

34. The method of claim 33 further characterized in that in the heating step during austenitizing the steel tube is heated at a rate of at least about 200 ° C per second.

35. The method of claim 23 further characterized in that in the rapid cooling step the steel tube is cooled at a rate of at least about 200 ° C per second.

36. The method of claim 23 further characterized in that in the tempering step the steel tube is treated at a temperature between 400 ° C and 600 ° C.

37. The method of claim 36 further characterized in that in the tempering step the steel tube is treated for about 4 to 20 minutes.

38. The method of claim 23 further comprising a step of termination further characterized in that the tempered steel tube is pickled, phosphatized and oiled.

39. A method for manufacturing a length of steel tube to make a pressure vessel used in a stored gas inflator comprising the following steps: producing a tube length from a steel material essentially comprising, by weight, between approximately 0.08% and approximately 0.11% carbon; between about 1.03% and about 1.18% manganese; between about 0.15% and about 0.35% silicon; to approximately 0.003% sulfur; up to approximately 0.012% phosphorus; up to about 0.10% nickel; between about 0.63% and about 0.73% chromium; between about 0.4% and about 0.45% molybdenum; between about 0.03% and about 0.05% vanadium; between about 0.025% and about 0.035% titanium; between approximately 0.15% and approximately 0.30% copper; between about 0.010% and about 0.050% aluminum; up to approximately 0.05% of niobium; up to approximately 0.15% residual elements, the rest being iron and incidental impurities; submit the steel tube to a cold drawing process to obtain the desired dimensions; heat austenizing the cold-drawn steel tube in an austenitizing furnace of the induction type at a temperature between about 920 ° C and 1050 ° C, at a heating rate of at least about 200 ° C per second; after the heating step, suddenly cooling the steel tube in a fast water-based cooling solution would have the tube reach approximately room temperature at a cooling rate of at least about 200 ° C per second; and after the rapid cooling step, submit the steel tube to temper for approximately 4 to 20 minutes at a temperature between about 450 ° C and 550 ° C; subjecting the steel tube to a finishing step where the tempered steel tube is pickled, phosphatized and oiled; and further characterized in that the finished steel tube has a yield strength of at least about 930.7 MPa (135 ksi), a tensile strength of at least about 999.7 MPa (145 ksi), an elongation upon breakage of at least less about 9%, a hardness no greater than about 37 HRC, a transition temperature from ductile to brittle less than -60 ° C, and a good appearance on its surface.