WO2011151908A1 - エアバッグ用鋼管とその製造方法 - Google Patents
エアバッグ用鋼管とその製造方法 Download PDFInfo
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- WO2011151908A1 WO2011151908A1 PCT/JP2010/059417 JP2010059417W WO2011151908A1 WO 2011151908 A1 WO2011151908 A1 WO 2011151908A1 JP 2010059417 W JP2010059417 W JP 2010059417W WO 2011151908 A1 WO2011151908 A1 WO 2011151908A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D7/00—Modifying the physical properties of iron or steel by deformation
- C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
- C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60R—VEHICLES, VEHICLE FITTINGS, OR VEHICLE PARTS, NOT OTHERWISE PROVIDED FOR
- B60R21/00—Arrangements or fittings on vehicles for protecting or preventing injuries to occupants or pedestrians in case of accidents or other traffic risks
- B60R21/02—Occupant safety arrangements or fittings, e.g. crash pads
- B60R21/16—Inflatable occupant restraints or confinements designed to inflate upon impact or impending impact, e.g. air bags
- B60R21/26—Inflatable occupant restraints or confinements designed to inflate upon impact or impending impact, e.g. air bags characterised by the inflation fluid source or means to control inflation fluid flow
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- C21—METALLURGY OF IRON
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
- C21D9/085—Cooling or quenching
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/32—Soft annealing, e.g. spheroidising
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/34—Methods of heating
- C21D1/42—Induction heating
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/25—Process efficiency
Definitions
- the present invention relates to a steel pipe for an airbag having a high strength of a tensile strength of 1000 MPa or more and a high toughness of vTrs100 of ⁇ 80 ° C. or less, and a method for producing the same. More specifically, the present invention can be manufactured by a relatively inexpensive and simple manufacturing process, and can be applied with a short-time quenching heat treatment using rapid heating means such as high-frequency induction heating. It is related with a steel pipe and its manufacturing method.
- an airbag system has been developed and mounted. This is a system that reduces the injury by absorbing the kinetic energy of the occupant by deploying an air bag with gas or the like between the occupant and the occupant before the occupant collides with the steering wheel or the instrument panel in the event of a vehicle collision. It is. Conventionally, a system using explosive chemicals has been adopted as an air bag system. However, in recent years, a system using a high-pressure filling gas has been developed and its application is spreading.
- the above-mentioned system using high-pressure filling gas is to keep the gas etc. constantly at a high pressure and to inject the high-pressure gas into the air bag at the time of collision. Therefore, the steel pipe used for the accumulator of high-pressure gas is extremely Stress is applied at a large strain rate in a short time. For this reason, the steel pipe is required to have high dimensional accuracy, workability and weldability, unlike a simple structure such as a conventional pressure cylinder or line pipe, and also requires high strength and excellent burst resistance. Is done.
- an ultra-high-strength seamless steel pipe having a tensile strength exceeding 1000 MPa has been used as an accumulator in an airbag system.
- the burst pressure is about 100 MPa at a TS of 800 MPa, whereas the burst pressure increases to 130 MPa when the TS is 1000 MPa.
- the thickness can be reduced by about 20%.
- the accumulator needs to have excellent low temperature toughness so that the accumulator is not brittlely broken and causes a secondary disaster at the time of collision.
- a seamless steel pipe for an accumulator has been given high strength and high toughness by quenching and tempering.
- the accumulator is required to have sufficient low temperature toughness even in a temperature range of ⁇ 60 ° C. or lower in a state after undergoing diameter reduction processing as described later.
- an accumulator for an air bag is cut into a predetermined length by cutting a seamless steel pipe, which is a raw pipe, into a short pipe, and at least one end thereof is reduced in diameter by pressing or spatula drawing (this) Is called bottle processing), and finally processed into a shape necessary for mounting an initiator or the like. Therefore, in order to guarantee the operation as an accumulator for an air bag, there are cases in which only the toughness of the seamless steel pipe as a material is incomplete. This is because the toughness of the bottle portion is reduced by the diameter reduction processing, which is the final processing, and cracks may occur during high-pressure loading. Therefore, in consideration of such a decrease in toughness, the seamless steel pipe used for the airbag accumulator needs to have toughness lower than the operating environment temperature of the accumulator.
- the jointed steel pipe constituting the accumulator has an elongation of 10% or more, a tensile strength of 1000 MPa or more, and has a ductile fracture surface in a Charpy impact test at ⁇ 80 ° C., preferably ⁇ 100 ° C.
- Low temperature toughness that is, low temperature toughness where vTrs100 is ⁇ 80 ° C. or lower, preferably ⁇ 100 ° C. or lower) is required.
- Patent Document 1 discloses a conventional technique related to a seamless steel pipe for an airbag system having a high strength and high toughness with a tensile strength of 1000 MPa or more.
- Patent Document 1 after making a seamless steel pipe by using a steel material having a chemical composition in a predetermined range, the seamless steel pipe is subjected to cold drawing to obtain a steel pipe having a predetermined dimension, and the Ac3 transformation point.
- a method for producing a seamless steel pipe for an air bag is proposed in which the steel is heated to a temperature in the range of 1050 ° C. or less, then quenched, and then tempered at a temperature in the range of 450 ° C. or more and the Ac1 transformation point or less. .
- this method it is excellent in workability and weldability at the time of manufacturing an inflator for an airbag. Further, as an inflator, it has a tensile strength of 900 MPa or more and a high ductility in a drop test at ⁇ 60 ° C. against a halved steel pipe. It is said that a seamless steel pipe having toughness can be obtained. However, this method is expensive because it needs to contain a large amount of Cr as a steel composition in order to obtain strength and toughness.
- Patent Document 2 shows that when high-frequency heating and quenching is employed, a high-strength and high-toughness seamless steel pipe for an airbag system having a tensile strength exceeding 1000 MPa can be produced by fine graining by rapid heating.
- a steel material having a chemical composition in a predetermined range is formed into a seamless steel pipe, and then the seamless steel pipe is subjected to a cold drawing process to obtain a steel pipe having a predetermined size, and 10 ° C./second.
- a cold drawing process After heating to 900 to 1000 ° C. at the above heating rate, quenching is performed, and then tempering is performed at a temperature not higher than the Ac1 transformation point. It is intended to obtain a high toughness that exhibits ductility even in a burst test at ⁇ 80 ° C. or lower.
- Patent Document 2 a specific example in which heating for quenching is performed at 20 ° C./second is shown.
- Patent Document 3 also shows an example of using induction heating and quenching, but as shown in Table 3 of Examples of the same document, only short-time heating in the range of 900 to 1000 ° C. is assumed. There are similar problems.
- Patent Document 4 induction heating and quenching is used, but as shown in the examples, this is a result of heating in the range of 920 to 940 ° C. and has the same problem as Patent Document 2.
- this burst resistance performance has a 100% ductile fracture lower limit temperature (vTrs100) in a Charpy impact test of ⁇ 80 ° C. or lower, preferably ⁇ 100 ° C. or lower.
- Patent Document 1 since a large amount of Cr is contained, cold drawing workability is not sufficient, and a large degree of workability is ensured at the cold drawing stage. For this purpose, there are problems that intermediate softening annealing is required in the middle and the manufacturing cost increases. In addition, exhibiting ductility in a drop weight test at ⁇ 60 ° C. does not necessarily mean exhibiting ductility in a burst test at ⁇ 60 ° C.
- the manufacturing process is simple.
- most steel compositions have a sum of Cr and Mo contents in the range of more than 0.6%.
- the strength of the seamless steel pipe is increased after hot pipe forming, and the cold drawing process becomes difficult due to the increased strength.
- Softening annealing is required before processing, which complicates the process and increases costs.
- the present invention is suitable for quenching by low-frequency mass-production high-frequency induction heating, which has the performance required today as an accumulator for airbags, that is, sufficient performance for increasing accumulator pressure and thinning of steel pipes.
- An object of the present invention is to provide a high steel pipe for an air bag and a method for manufacturing the same.
- the final product has a tensile strength of 1000 MPa or more, preferably 1050 MPa or more, and is excellent in low-temperature toughness that exhibits ductile fracture even in a burst test at ⁇ 60 ° C., particularly 100% ductile fracture lower limit temperature
- the steel pipe described in the above (I) is designed to be an alloy that can be manufactured in a state where stable characteristics can be obtained even by quenching heat treatment by mass-production high-frequency heating, and more specifically Specifically, there is a need for a method of manufacturing a steel pipe for an air bag that can be provided as a tough product having a certain level or higher even when the heating temperature for heating and quenching exceeds 1000 ° C. (eg, 1020 to 1040 ° C.). It is called Technical Issue (IV).
- the present inventors investigated the relationship among alloy elements, strength and low temperature toughness in a seamless steel pipe for an airbag system that is quenched and tempered after cold working.
- the outstanding balance of strength and toughness required today includes Mn at a low level, even if Mo is not contained, and has been added in large quantities so far. It was found that the steel was obtained by adding a small amount of Cr, which had been used, and steel containing appropriate amounts of Cu, Ni, Ti, and B.
- the present inventors examined the influence of alloying elements on the strength and toughness of a seamless steel pipe for an airbag system manufactured by quenching and tempering after cold working and having a TS exceeding 1000 MPa. As a result, the following knowledge was obtained and the present invention was completed.
- the present invention has been completed based on the above findings and further studies.
- the gist of the present invention is as follows. (1) C: 0.05 to 0.20%, Si: 0.10 to 0.50%, Mn: 0.10 to 1.00%, P: 0.025% or less, S: 0.005%
- the steel pipe for an air bag according to (1) characterized in that the Ti content is in mass% and has a steel composition of more than 0.020% and not more than 0.050%.
- Mo represents (Cr + Mo), and the element symbol means a numerical value when the content of these elements is expressed in mass%. When the Mo content is 0, 0 (zero) is substituted for Mo in the formula (1).
- the seamless steel pipe for an airbag system according to any one of (1) to (3), further having a steel composition containing V: 0.02 to 0.20% (5)
- a seamless steel pipe made by hot pipe making using a billet having the steel composition described in any one of 1) to (4) is cooled to a cold working degree of 40% or more.
- a steel pipe of a predetermined size is formed by performing inter-working, and after correction, if necessary, quenching is performed by heating to a temperature not lower than the Ac3 transformation point by high-frequency heating and quenching, and then heating to a temperature not higher than the Ac1 transformation point.
- a method for producing a seamless steel pipe for an air bag characterized by performing tempering.
- a seamless steel pipe having extremely high strength and excellent low-temperature burst performance can be provided, and the pressure of an air bag accumulator whose diameter is reduced at the end (pressure of 145 MPa or more) / thin wall weight reduction ( (Example: Seamless steel pipe having a wall thickness of 3.6 mm to 1.7 mm and a diameter of 60.3 mm to 25.0 mm).
- FIG. 1 is a graph showing the relationship between Cr + Mo and Cu + Ni in a preferred embodiment of the present invention. It is explanatory drawing of the test piece used for the characteristic evaluation in the Example of this invention.
- C 0.05-0.20% C is an element effective for increasing the strength of steel at low cost, but if its content is less than 0.05%, it is difficult to obtain a desired tensile strength of 1000 MPa or more, and if it exceeds 0.20%, it is processed. And weldability are reduced. Therefore, the C content is set to 0.05 to 0.20%. A preferred range for the C content is 0.07 to 0.17%.
- Si 0.10 to 0.50%
- Si is an element that improves the hardenability of the steel and improves the strength, and a content of 0.10% or more is necessary.
- the Si content is set to 0.10 to 0.50%.
- a preferable range of the Si content is 0.20 to 0.50%.
- Mn 0.10 to 1.00%
- Mn has a deoxidizing action and is an effective element for improving 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 is stretched during hot rolling, and toughness. Decreases. In the present invention, even if Mn is suppressed to 1.00% or less, it is necessary to ensure the intended tensile strength of 1000 MPa or more and excellent low-temperature burst performance. In order to improve the hardenability. Therefore, the Mn content is set to 0.10 to 1.00%. The Mn content is preferably 0.40 to 0.90% from the viewpoint of balance between strength and toughness.
- P 0.025% or less P causes a decrease in toughness due to grain boundary segregation. In particular, when the content exceeds 0.025%, the decrease in toughness becomes significant. Therefore, the content of P is set to 0.025% or less.
- the P content is preferably 0.020% or less, and more preferably 0.015% or less.
- S decreases the toughness particularly in the steel pipe T direction, that is, in the direction orthogonal to the rolling direction (longitudinal direction) of the steel pipe.
- the content of S is set to 0.005% or less.
- the S content is preferably 0.003% or less.
- Al 0.005% or more and 0.10% or less
- Al is an element having a deoxidizing action and effective in improving toughness and workability. However, if the content exceeds 0.10%, the generation of ground becomes remarkable. Therefore, the Al content is set to 0.10% or less. In addition, in order to acquire such an effect of Al, it is necessary to make it contain 0.005% or more.
- the Al content in the present invention refers to the content of acid-soluble Al (so-called “sol.Al”).
- Ca 0.0005 to 0.0050%
- Ca has an effect of fixing S present as an inevitable impurity in the steel as a sulfide, improving anisotropy of toughness, and increasing the T-direction toughness of the steel pipe, thereby increasing the burst resistance. This effect is manifested at a content of 0.0003% or more, particularly 0.0005% or more. However, if the content exceeds 0.0050%, inclusions increase and the toughness decreases. Therefore, the Ca content is set to 0.0005 to 0.0050%.
- Nb 0.005 to 0.050%
- Nb is finely dispersed as carbide in the steel and has an effect of strongly pinning the crystal grain boundary. Thereby, it has the effect of making the crystal grains finer and improving the toughness of the steel. In order to acquire the effect, it contains 0.005% or more, but if it contains more than 0.050%, the carbide is coarsened and the toughness is lowered. Therefore, the Nb content is set to 0.005 to 0.050%.
- Ti 0.005 to 0.050%
- Ti has the effect of fixing N in steel and improving toughness.
- the finely dispersed Ti nitride has the effect of strongly pinning the crystal grain boundaries, making the crystal grains finer, and improving the toughness of the steel.
- fixing N in steel is also important for bringing out the effect of B described later. Therefore, in order to obtain these effects, 0.005% or more is contained, but if it exceeds 0.050%, the nitride is coarsened and the toughness is lowered. Therefore, the Ti content is set to 0.005 to 0.050%.
- the preferable content is more than 0.020% to 0.035%.
- B 0.0005 to 0.0050% B segregates at the grain boundaries in the steel, remarkably improves the hardenability of the steel, and contributes to improved toughness.
- the effect is expressed by containing 0.0005% or more.
- the B content is set to 0.0005 to 0.0050%. Preferably, it is 0.0030% or less.
- blending B improves strength by improving hardenability. If B is not in a solid solution state, it does not segregate at the grain boundaries. 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, the B content preferably satisfies the relationship of the following formula (2) or (3) from the stoichiometric ratio of B, Ti, and N.
- N-Ti / 3.4 ⁇ 0 B ⁇ 0.0005 When N-Ti / 3.4> 0, B- (N-Ti / 3.4) ⁇ (10.8 / 14) ⁇ 0.0005 (3)
- B, N, and Ti in the formula (2) are numerical values when the content of each element is expressed in mass%.
- the upper limit of the N content is set to 0.010% or less.
- a preferable range of the N content is 0.002 to 0.008%.
- Cu 0.01 to 0.50% Cu has the effect of increasing the hardenability of the steel and improving the strength and toughness. The effect is manifested when the content is 0.01% or more, preferably 0.03% or more. However, the content exceeding 0.50% causes a significant increase in alloy cost. Therefore, the Cu content is set to 0.01 to 0.50%.
- the preferable content is 0.03% or more, particularly 0.05% or more, and more preferably 0.15% or more.
- Ni 0.01 to 0.50% Ni has the effect of increasing the hardenability of steel and thus improving strength and toughness. The effect is manifested when the content is 0.01% or more, preferably 0.03% or more. However, if the content exceeds 0.50%, the alloy cost will be exceeded. Therefore, the Ni content is set to 0.01 to 0.50%.
- the preferable content is 0.03% or more, particularly 0.05% or more, and more preferably 0.15% or more.
- Cr 0.01 to 0.50% Cr has the effect of increasing the hardenability of steel and increasing the resistance to temper softening and improving the strength and toughness. The effect is manifested if each element contains 0.01% or more. However, the content exceeding 0.50% is unsuitable because it causes excessive strength during cold drawing and deteriorates workability. Therefore, the Cr content is set to 0.01 to 0.50%. Preferably, the content is 0.18 to 0.40%.
- the containing balance is limited as follows. Limited by the formula of Cu, Ni, Cr, Mo content: When Mo is added in the present invention, it is preferable to satisfy the following relationship among the contents of Cu, Ni, Cr, and Mo.
- Cr and Mo prevent spheroidization of cementite that precipitates during tempering, and in steels containing B as in the present invention, it is easy to form a compound of B (boride) at grain boundaries, High strength materials tend to reduce toughness. Therefore, higher strength by inclusion of Cu and Ni is more targeted by the present invention than higher strength by Cr content (even when Mo is added, limiting Mo to a very small amount). And it is suitable as a high toughness airbag steel pipe. Specifically, it is important to satisfy the following formula (1) for the balance of Cr, Mo, Cu, and Ni.
- M in the formula (1) represents “Cr + Mo”, and the element symbol is a numerical value when the content of each element is expressed in mass%. When Mo is not contained, 0 is substituted for Mo of “Cr + Mo”.
- Mo Less than 0.10%
- Mo is not contained in principle, but a very small amount may be contained if desired.
- Mo has the effect of improving the hardenability of steel and increasing the resistance to temper softening and improving the strength and toughness. The effect can be observed even in a trace amount, but in order to obtain a certain effect, it is preferable to contain 0.01% or more. However, if 0.10% or more is contained, the alloy cost is exceeded.
- Mo content is high, the strength tends to be high even in the air cooling after the hot pipe making of the seamless steel pipe, and a softening heat treatment is required before the cold drawing process, resulting in an increase in manufacturing cost. Therefore, even when Mo is added, the Mo content is less than 0.10%.
- V 0.02 to 0.20%
- V has an effect of increasing strength by precipitation strengthening.
- the content of V is 0.02% or more, the effect is exhibited, but when it exceeds 0.20%, the toughness is lowered. Therefore, when V is added, the content of V is preferably 0.02 to 0.20%.
- a preferable range of the V content is 0.03 to 0.10%.
- a cast piece cast by a continuous casting machine having a cylindrical mold may be used, or a cast piece cast into a rectangular mold and then formed into a cylindrical shape by hot forging.
- the steel according to the present invention suppresses the addition of ferrite stabilizing elements such as Cr (when adding Mo, Cr and Mo) and adds austenite stabilizing elements such as Cu and Ni. Even when continuous casting is performed in a round shape, the effect of preventing center cracking is great, and it can be used as a round CC billet.
- the steel composition of the present invention is suitable for a seamless steel pipe for an airbag accumulator.
- a steel pipe may be manufactured using the slab described in (B) whose chemical composition is adjusted as described above as a raw material, and the pipe making method of the steel pipe is particularly limited. is not. For example, the Mannesmann-Mandrel method is adopted.
- the steel pipe produced as a seamless steel pipe as described above is cold worked under conditions that provide predetermined dimensional accuracy and surface properties.
- the cold working only needs to obtain predetermined dimensional accuracy and surface properties.
- the cold working is not particularly limited to specific methods such as cold drawing and cold rolling.
- the degree of processing is preferably 3% or more in terms of area reduction (section reduction rate).
- it exceeds 50% the development of wrinkles on the inner surface is generally remarkable, so it should be less than 50%. preferable.
- cold working is performed multiple times to obtain the final product dimensions.
- the thickness that is, as the degree of work increases, the strength of the steel pipe increases due to work hardening.
- the steel according to the present invention can achieve a degree of work reduction of 40% or more, preferably over 40%, without performing preheating softening treatment or intermediate softening annealing even during cold working.
- the area reduction rate is synonymous with the cross-section reduction rate and is defined by the following equation.
- Area reduction ratio (%) (S 0 ⁇ S f ) ⁇ 100 / S 0
- S 0 Cross-sectional area of the steel pipe before cold working
- S f Cross-sectional area of the steel pipe after completion of cold working
- the total area reduction rate is also treated as the above area reduction rate.
- the present invention does not exclude cold working in which soft annealing is performed in the middle.
- cold processing eg, cold drawing
- This straightening process is not particularly limited, but, for example, two roll-type rolling mills are provided in about four rows, the center positions of the roll gaps in each row are staggered (that is, offset), and the roll gap is further increased.
- a method of adjusting and bending and unbending by passing a steel pipe between them is preferable.
- the offset amount is 1% or more of the outer diameter of the steel pipe and the roll gap amount is 1% or less of the outer diameter of the steel pipe.
- the offset amount is 50% or less of the outer diameter of the steel pipe and the roll gap amount is 5% or more of the outer diameter of the steel pipe, there is no problem such as generation of wrinkles on the inner surface of the steel pipe.
- the heating rate at this time can be adjusted with the feed rate of the steel pipe which passes along a high frequency coil etc., it is preferable to set it as about 25 degreeC / second or more. More preferably, it is 50 degreeC / second or more, More preferably, it is 100 degreeC / second or more.
- the cooling after heating to a temperature of at least the Ac3 transformation point is rapid cooling in order to obtain a desired tensile strength of 1000 MPa or more stably and reliably, but it is preferable to perform a rapid cooling treatment such as water quenching.
- a rapid cooling treatment such as water quenching.
- the cooling rate between 800 ° C. and 500 ° C. during the rapid cooling treatment is preferably 50 ° C./second or more. More preferably, it is 125 ° C./second or more.
- the steel pipe that has been quenched and cooled to near normal temperature is tempered at a temperature not higher than the Ac1 transformation point in order to impart desired tensile strength of 1000 MPa or more and burst resistance.
- the bending may be corrected with a straightener or the like as appropriate by the method described in (E). Even if the steel pipe having the steel composition described in (A) is used, the strength and toughness intended by the present invention cannot be stably secured if the heating rate in the quenching stage or the cooling rate is insufficient. There is a case.
- the seamless steel pipe tempered in this way is cut into a predetermined length by cutting it into a predetermined length, and at least one end thereof is reduced in diameter by press working or spatula drawing, etc. It is finally processed into a shape necessary for mounting, and used as an airbag accumulator.
- Example 1 This example was conducted in order to investigate the relationship between the steel composition and the low temperature toughness of a material manufactured by simulating the manufacturing conditions of a seamless steel pipe.
- Steels having the chemical compositions of the six steel types shown in Table 1 were melted by vacuum melting and cold-rolled after hot rolling to obtain 5 mm-thick plate (working degree: 40%). Then, it heated to 920 degreeC with the average temperature increase rate of 300 degrees C / sec by high frequency heating, and after holding
- Table 2 shows the tensile strength (TS) of each steel obtained by the tensile test and the 100% ductile fracture surface lower limit temperature (vTrs100) obtained by the Charpy impact test.
- TS tensile strength
- vTrs100 100% ductile fracture surface lower limit temperature
- FIG. 1 the total Cr and Mo contents and the total Cu and Ni contents of each steel of this example are plotted, and vTrs100 is ⁇ 80 ° C. or higher, ⁇ , and ⁇ 100 ° C. or higher. Is indicated by a circle. From FIG. 1, it is understood that excellent low temperature toughness can be ensured by satisfying the relationship of the formula (1) with respect to the total content of Cr and Mo with respect to the total content of Cu and Ni.
- Example 2 Steel having the chemical composition shown in Table 3 was melted in a converter, and a cylindrical billet having an outer diameter of 191 mm was manufactured by continuous casting.
- the round CC billet was cut to a desired length, heated to 1250 ° C., and finished to a diameter of 70 mm and a wall thickness of 4.0 mm by piercing and rolling by a normal Mannesmann-mandrel mill method.
- the obtained seamless steel pipe was subjected to cold drawing (cold drawing) by an ordinary method as a raw pipe to finish the outer diameter to 60.3 mm and the wall thickness to 3.6 mm. This is steel pipe size 1. After correcting these cold drawn steel pipes with a straightener, they were heated to 920 ° C.
- the straightening by this steel pipe size 1 straightener is provided with 3 rows of 2 roll type rolling mills, and the center position of the roll gap of the 2nd row is shifted 20 mm above the center position of the 1st row, Further, the center position of the roll gap in the third row is shifted 3 mm above the center position in the first row, and the roll gaps in the second row and the third row are each 58.8 mm (outer diameter -1.5 mm). ) And 57.3 mm (outer diameter -3.0 mm), and bending and unbending were performed by passing a steel pipe between them.
- a seamless steel pipe having an outer diameter of 51.0 mm and a wall thickness of 3.0 mm finished by drilling and rolling is cold-drawn (cold drawing) by an ordinary method, and the outer diameter is 40.degree.
- the thickness was 0 mm and the wall thickness was 2.6 mm.
- This is steel pipe size 2.
- the straightening with the steel pipe size 2 straightener is performed in the same manner as in the steel pipe size 1, and the center position of the roll gap in the second row is shifted 10 mm above the center position in the first row, and three rows.
- the center position of the eye roll gap is shifted 3 mm above the center position of the first row, and the roll gaps of the second and third rows are 39.5 mm (outer diameter -0.5 mm) and 39, respectively.
- the thickness was adjusted to .2 mm (outer diameter -0.8 mm), and bending and unbending were performed by passing a steel pipe between them.
- a tensile test having the shape shown in FIG. 2 was taken and the strength characteristics were investigated.
- the numbers in the figure indicate dimensions (unit: mm).
- six steel pipes subjected to induction hardening and tempering were each cut into 300 mm lengths, and both pipe ends were subjected to press working so that the ratio of the diameter of the reduced diameter portion / the diameter of the unreduced diameter portion was 0.6.
- a reduced diameter portion having a length of 25 mm was provided to form an accumulator bottle portion. Thereafter, one end was welded and sealed, and the other end was welded with a closing member penetrating the high-pressure hose.
- This test specimen was immersed in ethanol in a chamber cooled to ⁇ 60 ° C., ethanol was injected into the pipe from a high-pressure hose to increase the internal pressure, and the pipe was ruptured.
- all six of the steel pipe size 1 and 2 specimens using steel A to steel B have a brittle fracture surface area ratio of less than 5% in the opening and satisfy sufficient burst performance.
- 3 of each of 6 steel pipe size 1 and 2 specimens using steel C were prematurely broken from the reduced diameter portion, and the burst pressure was significantly reduced.
- all the six test bodies using the steel D and the steel E had the brittle fracture surface area ratio of the opening part of 5% or more, and did not satisfy performance.
- Steels A to B in Table 4 are steels whose components satisfy the conditions specified in the present invention.
- Steels C to E are steels that do not satisfy the relational expression (1) of Cu, Ni, Cr, and Mo contents that are satisfactory in the present invention, or steels that do not satisfy the range of other components.
- Example 3 Steel having the chemical composition shown in Table 5 was melted in a converter, and a cylindrical billet having an outer diameter of 191 mm was manufactured by continuous casting. This round CC billet was cut to a desired length, heated to 1250 ° C., and then subjected to piercing and rolling by a normal Mannesmann-Mandrel mill method to produce a seamless steel pipe. Thereafter, cold drawing (cold drawing) was performed by a normal method to finish various product steel pipe sizes. Table 6 shows the steel pipe size at the time of hot pipe making and the steel pipe size after the subsequent cold drawing.
- These cold drawn steel pipes were straightened with a straightener and then heated using a high-frequency induction heating device at an average temperature increase rate of 300 ° C./second under the conditions shown in Table 6, followed by water quenching. .
- This water quenching was performed by spraying water in a spray form from a nozzle arranged in a ring shape, passing the steel pipe through the ring, and cooling the outer surface of the steel pipe.
- the high-frequency induction heating apparatus and the water quenching equipment described above are connected, and the cooling speed was changed by changing the passing speed of the steel pipe.
- a soaking treatment for 30 minutes for tempering was performed in a normal walking beam furnace, and the tensile strength was adjusted to 1000 MPa or more.
- a fixed length was cut out from each steel pipe subjected to quenching and tempering in this way, and a tensile test was performed in accordance with the metal material tensile test method specified in JISZ 2241 using the collected No. 11 test piece in JISZ 2201. .
- fixed length was cut out from each steel pipe, it was cut
- a Charpy impact test was carried out using a test piece in which a 2 mm V notch was introduced into a rectangular material having a length of 55 mm, a height of 10 mm, and a width of the original thickness of the steel pipe taken from the expanded direction from the developed pipe.
- Table 6 shows the relationship between the lower limit temperature (denoted by vTrs100 in Table 6) at which the ductile fracture surface ratio 100% obtained at this time can be secured and the tensile test results.
- each of the steel pipes subjected to induction hardening and tempering was cut into 300 mm lengths, and both pipe ends were subjected to press working so that the ratio of the diameter of the reduced diameter portion / the diameter of the unreduced diameter portion was 0.6
- a reduced diameter portion having a length of 25 mm was provided to obtain the shape of the bottle portion of the accumulator. Thereafter, one end was welded and sealed, and the other end was welded with a closing member penetrating the high-pressure hose.
- This test specimen was immersed in ethanol in a chamber cooled to ⁇ 60 ° C., ethanol was injected into the pipe from a high-pressure hose to increase the internal pressure, and the pipe was ruptured. If the brittle fracture surface area ratio of the opening is less than 5%, it is listed in Table 6 as acceptable (indicated by ⁇ in Table 6) and rejected if not less than 5% (indicated by x in Table 6). .
- test no. No. 22 was subjected to softening heat treatment soaking at 620 ° C. for 20 minutes before cold drawing.
- Test No. 21 no. Except for No. 22, the cold drawing process could be finished in one time even without softening heat treatment after hot pipe making.
- a high strength seamless steel pipe for an airbag system having excellent low temperature burst performance as an airbag accumulator component including a reduced diameter portion by performing induction quenching and tempering using steel of chemical composition according to the present invention. It is clear that can be manufactured inexpensively and with high efficiency.
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Abstract
Description
より具体的には、本発明は、比較的安価かつ簡素な製造工程で製造可能であり、かつ高周波誘導加熱等の急速加熱手段を利用する短時間の焼入熱処理が適用可能な、エアバッグ用鋼管とその製造方法に関する。
このような観点から、アキュムレータ用の継目無鋼管は、焼入れ焼戻しを行うことで高強度と高靭性が付与されるようになってきた。具体的には、アキュムレータとして、後述のような縮径加工を受けた後の状態において、-60℃以下の温度域においても十分な低温靭性を備えていることが求められる。
特許文献2においては、20℃/秒で焼入れのための加熱を行う具体例が示されているが、工業的な生産性を考慮すれば、より短時間で急速加熱すること、また到達温度での保持時間も短時間であることが望ましい。短時間での急速加熱、短い保持時間で熱処理する場合は、加熱温度のばらつきを考慮すれば局部的に到達温度がAc3を下回ることもあるため、可能であれば高目の加熱温度に設定することが望ましい。しかし、高周波加熱の場合、急速加熱となるため設定温度を超えて加熱されるオーバーシュートの問題もあり、高周波加熱焼入れでは到達温度が1000℃を超える場合も考慮する必要があるが、量産化に際してのかかる問題に関しては特許文献2は何ら述べていない。むしろ、到達温度が1000℃を超える場合、γ粒の粗粒化を招来して靭性が低下することが述べられている。
(I)最終製品として、1000MPa以上、望ましくは1050MPa以上の引張強度を有し、-60℃でのバースト試験においても延性破壊を呈するような低温靭性にも優れ、特に100%延性破面下限温度(vTrs100)が-80℃以下、好ましくは-100℃以下であるエアバッグ用鋼管が求められる。技術課題(I)という。
(i)C、Si、Mn、Al、Ca、Nb、Ti、およびBを適正量含有させ、これまで高強度化のためにむしろ多量に添加されてきたCrを0.5%以下、およびMoを含有させず(必要により0.1%未満の少量含有は許容される)、それに代えてCuおよびNiをそれぞれ0.01-0.50%、かつCuとNiの含有量の合計量を、{(Cr+Mo)2+0.3}以上とすることが上記課題(I)、(II)、(III)を同時に達成する上で有効であることが判明した。
(ii)さらに、急速加熱焼入れ時の加熱温度が、例えば1040℃となって、目標とする狙い範囲の上限(1000℃)よりも高くなった場合、即ち、上記課題(IV)については、Ti:0.02%超0.05%以下を含有させることで、前述の目標の強度と靭性を同時に達成できることを知った。
ここに、本発明の要旨とするところは、以下の通りである。
(1)C:0.05~0.20%、Si:0.10~0.50%、Mn:0.10~1.00%、P:0.025%以下、S:0.005%以下、Al:0.005~0.10%、Ca:0.0005~0.0050%、Nb:0.005~0.050%、Ti:0.005~0.050%、Cu:0.01~0.50%、Ni:0.01~0.50%、Cr:0.01~0.50%、B:0.0005~0.0050%、N :0.002~0.010%、を含み、残部がFeおよび不可避不純物からなり、かつ下記式(1)を満足する鋼組成を有し、引張強度が1000MPa以上であり、vTrs100が-80℃以下の高靭性とを有することを特徴とするエアバッグシステム用継目無鋼管。
Cu+Ni≧(M)2+0.3 ・・・ (1)
なお、式(1)の“M”は、Crを表し、元素記号は、それらの元素の含有量を質量%で示したときの数値を意味する。
(3)質量%で、さらにMo:0.10%未満を含有し、且つ下式(1)を満たす鋼組成を有することを特徴とする、(1)または(2)に記載のエアバッグ用継目無鋼管。
Cu+Ni≧(M)2+0.3 ・・・ (1)
なお、式(1)の“M”は、(Cr+Mo)を表し、元素記号は、それらの元素の含有量を質量%で示したときの数値を意味する。Mo含有量が0の場合は、式(1)のMoに0(ゼロ)を代入する。
(5)(1)~(4)のいずれかに記載の鋼組成を有するビレットを用いて熱間製管によって製造された継目無鋼管素管に、1回の冷間加工の加工度が40%以上の冷間加工を施して所定寸法の鋼管とし、所望により、矯正を行なった後に、高周波加熱によってAc3変態点以上の温度に加熱して急冷する焼き入れを行い、次いでAc1変態点以下の温度に加熱して焼き戻しを行うことを特徴とする、エアバッグ用継目無鋼管の製造方法。
(A)鋼の化学組成
本明細書において「%」は、特段の説明が無い限り、「質量%」を意味する。
Cは、安価に鋼の強度を高めるのに有効な元素であるが、その含有量が0.05%未満では所望の1000MPa以上の引張強度が得難く、又、0.20%を超えると加工性及び溶接性が低下する。したがって、Cの含有量を、0.05~0.20%とした。なお、C含有量の好ましい範囲は、0.07~0.17%である。
Siは、脱酸作用を有するほか、鋼の焼入れ性を高めて強度を向上させる元素であり、0.10%以上の含有量が必要である。しかし、その含有量が0.50%を超えると靱性が低下するため、Siの含有量を0.10~0.50%とした。なお、Si含有量の好ましい範囲は0.20~0.50%である。
Mnは、脱酸作用があり,又,鋼の焼入れ性を高めて強度と靱性を向上させるのに有効な元素である。しかし,その含有量が0.10%未満では十分な強度と靱性が得られず,一方,1.00%を超えるとMnSの粗大化が生じて,それが熱間圧延時に展伸し,靱性が低下する。本発明にあっては、Mnを1.00%以下に抑えても、目的とする1000MPa以上の引張強度と優れた低温バースト性能を確保する必要があるため、後述するようにBを配合することで焼き入れ性の改善を行なっている。このため,Mnの含有量を0.10~1.00%とした。Mnの含有量は0.40~0.90%とすることが強度と靭性のバランスの点で好ましい。
Pは、粒界偏析に起因する靱性低下をもたらし、特に、その含有量が0.025%を超えると靱性の低下が著しくなる。したがって、Pの含有量を0.025%以下とした。なお、Pの含有量は0.020%以下とするのが好ましく、0.015%以下であれば一層好ましい。
Sは、特に鋼管T方向、すなわち、鋼管の圧延方向(長手方向)に直交する方向の靱性を低下させてしまう。特に、その含有量が0.005%を超えると鋼管T方向の靱性低下が著しくなる。したがって、Sの含有量を0.005%以下とした。なお、Sの含有量は0.003%以下とするのが好ましい。
Alは、脱酸作用を有し、靱性及び加工性を高めるのに有効な元素である。しかし、0.10%を超えて含有させると、地疵の発生が著しくなる。したがって、Alの含有量を0.10%以下とした。なお、このようなAlの効果を得るためには、0.005%以上含有させることが必要である。なお、本発明にいうAl含有量とは、酸可溶Al(所謂「sol.Al」)の含有量を指す。
Caは、鋼中に不可避不純物として存在するSを硫化物として固定し、靱性の異方性を改善して、鋼管のT方向靱性を高め、これによって耐バースト性を高める作用を有する。この効果は0.0003%以上、特に0.0005%以上の含有量で発現する。しかし、0.0050%を超えて含有させると、介在物が増加して、かえって靭性が低下する。したがって、Caの含有量を0.0005~0.0050%とした。
Nbは、鋼中で炭化物として微細に分散し、結晶粒界を強くピン止めする効果がある。それにより、結晶粒を細粒化せしめ、鋼の靭性を向上させる効果を有する。その効果を得るためには、0.005%以上含有させるが、0.050%を越えて含有させると、炭化物が粗大化し、かえって靭性が低下する。従って、Nbの含有量を0.005~0.050%とした。
Tiは、鋼中でNを固定し、靭性を向上させる効果を有する。また、微細に分散したTi窒化物は、それにより、結晶粒界を強くピン止めし、結晶粒を細粒化せしめ、鋼の靭性を向上させる効果を有する。また、鋼中のNの固定は、後述のBの効果を引き出す上でも重要である。従って、それらの効果を得るために、0.005%以上含有させるが、0.050%を越えて含有させると、窒化物が粗大化し、かえって靭性が低下する。従って、Tiの含有量を0.005~0.050%とした。特に、急速加熱による焼入れを実施する際は、加熱温度超過によって結晶粒径が粗大化しやすく、靭性が低下しやすいため、Ti窒化物による結晶粒界ピン止め効果を十分に活用するのが好適である。その意味で、好ましい含有量は0.020%超~0.035%である。
Bは、鋼中で粒界偏析し、鋼の焼き入れ性を著しく向上させ、靭性向上に寄与する。その効果は、0.0005%以上含有させることで発現する。一方、0.0050%超含有させると、結晶粒界に硼化物が粗大に析出するため、かえって靭性が低下する。従って、Bの含有量を0.0005~0.0050%とした。好ましくは、0.0030%以下である。
Bは固溶状態で無いと、結晶粒界に偏析しない。従って、Bと化合物を造りやすいNは、Tiによって固定されていることが好ましく、Bは、Nによって固定される量以上に含有されていることが好ましい。その意味で、B含有量は、B、Ti、Nの化学両論比から、下記の式(2)または(3)の関係を満たしていると好適である。
N-Ti/3.4≦0のとき
B≧0.0005 ・・・ (2)
N-Ti/3.4>0のとき
B-(N-Ti/3.4)×(10.8/14)≧0.0005 ・・・ (3)
式(2)中のB、N、Tiはそれぞれの元素の含有量を質量%で表したときの数値である。
N:0.002~0.010%、
Nは、鋼中に不可避的に存在する不純物である。しかしながら、本発明ではTiとの窒化物形成により、その分散を活用し、結晶粒界ピン止め効果を十分に活用するため、Nの含有量を制御することが肝要である。その効果を発揮させるためには、Nの含有量を0.002%以上とする。一方、Nが過剰に含有すると、固溶Nの増加や、Bと化合物を形成し、固溶B量の低下を招く。従って、Nの含有量の上限を0.010%以下とする。好ましいNの含有量の範囲は、0.002~0.008%である。
Cuは、鋼の焼き入れ性を高め、強度と靭性を向上させる効果がある。その効果は、0.01%以上、好ましくは0.03%以上含有されていれば発現する。しかしながら、0.50%を越えて含有させるのは合金コストの著しい増加を招く。従って、Cuの含有量を、0.01~0.50%とした。好ましい含有量は0.03%以上、特に0.05%以上であり、より好ましくは0.15%以上である。
Niは、鋼の焼き入れ性を高め、以て強度と靭性を向上させる効果がある。その効果は、0.01%以上、好ましくは0.03%以上含有されていれば発現する。しかしながら、0.50%を越えて含有させるのは合金コストの超過を招く。従って、Niの含有量を、0.01~0.50%とした。好ましい含有量は0.03%以上、特に0.05%以上であり、より好ましくは0.15%以上である。
Crは、鋼の焼き入れ性を高め、また、焼き戻し軟化抵抗を高めて、強度と靭性を向上させる効果がある。その効果は、各々の元素が0.01%以上含有されていれば発現する。しかしながら、0.50%を越えて含有させるのは冷間引抜の際の強度超過を招き、加工性が低下するため不適当である。従って、Crの含有量を、0.01~0.50%とした。好ましくは0.18~0.40%である。
Cu、Ni、Cr、Mo含有量の式による限定:
本発明においてMoを添加したときには、Cu、Ni、Cr、Moの各含有量の間に下記のような関係を満足させるようにするのが好ましい。
式(1)の“M”は、“Cr+Mo”を表し、元素記号は、それぞれの元素の含有量を質量%で表したときの数値である。なお、Moを含有しない場合は、“Cr+Mo”のMoには0が代入される。
Mo:0.10%未満
本発明において、Moは原則的には含有されないが、所望により極少量は含有させてもよい。Moは、鋼の焼き入れ性を高め、また、焼き戻し軟化抵抗を高めて、強度と靭性を向上させる効果がある。その効果は、微量でも認めら得るが、確実な効果を得るには、0.01%以上含有されることが好ましい。しかしながら、0.10%以上を含有させるのは合金コストの超過を招く。また、Mo含有量が高いと、継目無鋼管の熱間製管後の空冷においても、強度が高くなる傾向があり、冷間引抜加工前に軟化熱処理が必要となり、製造コストの上昇を招く。従って、Moを添加するときでも、Moの含有量を、0.10%未満とした。
Vは、析出強化により強度を高める作用がある。これらVの作用は0.02%以上含有させると効果を発揮するが,0.20%を超えると靭性が低下する。したがって,添加する場合のVの含有量は0.02~0.20%とするのがよい。V含有量の好ましい範囲は,0.03~0.10%である。
(B)素材
本発明においては、鋼管の素材となる鋼塊について、特に限定しない。円柱型の鋳型を有する連続鋳造機にて鋳込まれた鋳片でも良いし、矩形型に鋳込んだ後に、熱間鍛造により円柱状に成形した物でも良い。本発明に係る鋼は、Cr(Moを添加するときは、CrおよびMo)といったフェライト安定化元素の添加を抑制し、CuおよびNiといったオーステナイト安定化元素を添加しているため、ラウンドCCビレットとして丸形状に連続鋳造鋳込を行った場合にも中心割れが防止できる効果が大きく、ラウンドCCビレットとして使用できる。
ラウンドCCビレット中心部の割れが多い場合、継目無鋼管素管を冷間加工、特に冷間引抜を行った後、矯正加工を施すと、割れが拡張し、高周波焼入れ焼戻しを施して、最終的に縮径加工したところ、内面から割れが生じることがある。従って、特にラウンドCCビレットを素材とする場合、本発明の鋼組成は、エアバッグアキュムレータ用継目無し鋼管に好適である。
本発明においては、上記のように化学組成を調整した(B)に記載の鋳片を素材として、鋼管を製造すればよく、鋼管の製管法は特に限定するものではない。例えば、マンネスマン-マンドレル法が採用される。
上記のようにして継目無鋼管として製管された鋼管は、所定の寸法精度、表面性状が得られる条件下で冷間加工される。冷間加工は、所定の寸法精度と表面性状が得られればよい。その意味で、冷間加工について、冷間引抜、冷間圧延等の具体的方法に、特に限定しなくてもよい。加工度は減面率(断面減少率)で3%以上とするのが好ましいが、一方、50%を越えると、一般的に内面しわ疵の発達が著しいことから、50%未満とすることが好ましい。なお、冷間加工は複数回実施して最終製品の寸法とする場合もあるが、薄肉化すればするほど、すなわち加工度が高くなると、鋼管の強度が加工硬化により上昇するため、加工中に割れが発生しやすくなるので、途中で軟化させるための中間熱処理が必要となる。ところが(A)で限定した化学成分の鋼を採用することにより、素管の強度は低いので、冷間加工によっても強度が過剰に高くならず、冷間加工前の軟化熱処理が省略でき、好適である。本発明に係る鋼は、冷間加工に際しても、予熱軟化処理や中間の軟化焼鈍を行うことなく、減面率40%以上、好ましくは40%超の加工度を達成することができる。
減面率(%)=(S0-Sf)×100/S0
但し
S0:冷間加工前の鋼管の断面積
Sf:冷間加工完了後の鋼管の断面積
また、中間に軟化焼鈍を介在させることなく行うものであれば、複数回の冷間加工の総減面率も上記減面率として扱う。もちろん、本発明において中間に軟化焼鈍を行う冷間加工を排除するものではない。
本発明の対象は、引張強度が1000MPaを越え、エアバッグシステム用として必要な寸法精度、表面性状および低温靭性を具備した継目無鋼管であることから、冷間引抜後、強度が従来鋼よりも高くなる傾向があり、スプリングバックなどで鋼管に曲がりが生じる可能性がある。鋼管に曲がりがあると、下記の高周波加熱による焼き入れ時に、高周波コイルに真直に鋼管が通過しない問題が懸念される。従って、好ましい態様においては、高周波加熱による焼き入れのために、冷間加工(例:冷間引抜)後に矯正加工を行う。
上記(E)の矯正加工の後、鋼管には所要の引張強度を確保するとともに、T方向靱性を高めて耐バースト性をも確保するための熱処理が施される。鋼管に引張強度で1000MPa以上の高強度と、耐バースト性とを具備させるためには、少なくともAc3変態点以上の温度に加熱してから急冷し、次いで、Ac1 変態点以下の温度で焼戻しする処理を行う。
急冷されて常温近傍まで冷却された鋼管は、所望の1000MPa以上の引張強度と耐バースト性を付与するためにAc1変態点以下の温度で焼戻しをする。焼戻しの温度がAc1変態点を越えると上記特性を安定、且つ、確実に得ることが困難になる。
焼戻しの後、(E)で述べたような方法で、適宜ストレートナー等で曲がりを矯正してもよい。(A)に記載の鋼組成の素管を用いても、焼入段階の加熱速度や、冷却速度が不十分であると本発明の目的とする強度や靭性を安定して確保することができない場合がある。
このようにして焼き戻された継目無鋼管は、すでに述べたように、所定長さに切断して短管とした後に、少なくとも一端をプレス加工やへら絞り加工などで縮径加工し、イニシエータ等の装着に必要な形状に最終加工され、エアバッグアキュムレータとして使用される。
本例は、継目無鋼管の製造条件をシミュレートして製造した材料について鋼組成と低温靭性との関係を調べるために行った。
表1に示す6鋼種の化学組成の鋼を真空溶解にて溶製し、熱間圧延後に冷間圧延を施して5mm厚の板材(加工度は40%)とした。その後、高周波加熱により、平均昇温速度300℃/秒にて920℃まで加熱し、920℃×5秒の保持後、水冷で焼き入れ処理を行い、次いで、焼き戻しを実施した。
この熱処理を施した板材から、JIS Z2201の14A号の引張試験片(平行部径4mm、平行部長20mm)を圧延方向に垂直に採取し、JIS Z2241に準拠して引張試験を行った。また、同様に圧延方向に垂直に、JIS Z2242に準拠して、2.5mm幅のサブサイズのVノッチシャルピー試験片を採取し、シャルピー衝撃試験を行った。
鋼1~3については、焼戻し温度を調整することにより、TSを1000MPa付近に調整したが、式(1)を満足しないため、vTrs100が-80℃よりも高温になり、このままの材料としては十分な靭性を有するものの、ボトル加工により靭性が低下すると、十分な低温バースト性能が得られない可能性がある。
なお、式(1)の元素記号は、それらの元素の含有量を質量%で示したときの数値を意味する。Moが含有されない鋼の場合には、Mo=0とする。
表3に示す化学組成を有する鋼を転炉にて溶製し、連続鋳造によって外径191mmの円柱状ビレットを製造した。このラウンドCCビレットを所望の長さに切断し、1250℃に加熱した後、通常のマンネスマン-マンドレルミル方式による穿孔、圧延により外径が70mmで肉厚が4.0mmに仕上げた。
得られた継目無鋼管を、素管として、通常の方法で冷間引抜加工(冷間引抜き加工)し、外径を60.3mm、肉厚を3.6mmに仕上げた。これを鋼管サイズ1とする。
これら冷間引抜加工を施した鋼管を、ストレートナーによって矯正した後、高周波誘導加熱装置を用いて平均昇温速度300℃/秒にて920℃まで加熱し、920℃×5秒の保持後、水焼入れを行ない、次いで通常のウォーキングビーム炉で焼き戻しの為の30分の均熱処理を行った。
冷間引抜加工を施した鋼管を、ストレートナーによって矯正した後、高周波誘導加熱装置を用いて920℃まで加熱し5秒保持した後、水焼入れし、次いで通常のウォーキングビーム炉で焼き戻しの為の30分の均熱処理を行った。
また、高周波焼入れと焼戻しを行った鋼管を、300mm長さにそれぞれ6本切断し、両管端にプレス加工を施して、縮径部の直径/未縮径部の直径の比が0.6になるような縮径部を25mm長さで設けて、アキュムレータボトル部の形状とした。その後、片端を溶接して封じ、もう一方の端部を高圧ホースが貫通する閉鎖部材を溶接した。
その結果、鋼A~鋼Bを用いた鋼管サイズ1および2の試験体の各6本中全てが、開口部の脆性破面面積率が5%未満であり、十分なバースト性能を満足することが確認された。
一方、鋼Cを用いた鋼管サイズ1および2の試験体の各6本中の、各3本が縮径部から早期破壊し、バースト圧が著しく低下した。また、鋼Dおよび鋼Eを用いた試験体6本全てが、開口部の脆性破面面積率が5%以上であり、性能を満足しなかった。
表4における鋼A~鋼Bは、成分が本発明で規定する条件を満たす鋼である。鋼C~鋼Eは、本発明で満足すべき、Cu、Ni、Cr、Mo含有量の関係式(1)を満たさない鋼、または、それ以外の成分の範囲を満たさない鋼である。
表5に示す化学組成を有する鋼を転炉にて溶製し、連続鋳造によって外径191mmの円柱状ビレットを製造した。このラウンドCCビレットを所望の長さに切断し、1250℃に加熱した後、通常のマンネスマン-マンドレルミル方式による穿孔、圧延による熱間製管を実施して継目無鋼管を作成した。
その後、通常の方法で冷間引抜加工(冷間引抜き加工)し、種々の製品鋼管サイズに仕上げた。この際の熱間製管時の鋼管サイズおよびその後の冷間引抜加工後の鋼管サイズを表6に示す。
これら冷間引抜加工を施した鋼管を、ストレートナーによって矯正した後、高周波誘導加熱装置を用いて平均昇温速度300℃/秒にて表6に示す条件で加熱し、その後水焼入れを行なった。
なお、この水焼入れは、リング状に配置されたノズルからスプレー状に水を噴射し、そのリング内部に鋼管を通過させ、鋼管外表面を冷却することで行った。前述の高周波誘導加熱装置と水焼入れ設備は連結しており、鋼管の通過速度を変化させることで、冷却速度を変化させた。次いで通常のウォーキングビーム炉で焼き戻しの為の30分の均熱処理を行い、引張り強度が1000MPa以上となるように調整した。
また、各鋼管から一定長さを切り出し、それを室温で管の長さ方向に切断して展開した。展開した管からそのT方向から採取した長さ55mm、高さ10mm、幅を鋼管の元肉厚とした矩形材に2mmVノッチを導入した試験片を用いて、シャルピー衝撃試験を実施した。このとき得られた延性破面率100%を確保できる下限温度(表6中のvTrs100で表記)と、引張試験結果との関係を表6に示す。
この試験体を、-60℃に冷却されたチャンバー内でエタノール中に浸漬し、高圧ホースからエタノールを管内に注入して内圧を高めて管を破裂させ、破壊形態を観察した。開口部の脆性破面面積率が5%未満であれば合格(表6中の○で表記)、5%以上であれば不合格(表6中の×で表記)として、表6に記載した。
一方、本発明の鋼組成範囲から、Tiのみが外れる鋼Hについては、1000℃を超えて加熱すると著しく結晶粒が粗大化し、それに伴ってvTrs100の上昇およびバースト性能の低下が見られた。
また、本発明の鋼組成範囲から、Bの含有量のみが外れる鋼Iについては、1000℃を超えて加熱されるとvTrs100の上昇およびバースト性能の低下が見られた。
試験No.21、No.22以外は、熱間製管後に軟化熱処理を施さなくても、冷間引抜加工が1回で仕上げることが出来た。
Claims (6)
- 質量%で、
C:0.05~0.20%,
Si:0.10~0.50%,
Mn:0.10~1.00%,
P:0.025%以下,
S:0.005%以下,
Al:0.005~0.10%,
Ca:0.0005~0.0050%,
Nb:0.005~0.050%,
Ti:0.005~0.050%,
Cu:0.01~0.50%,
Ni:0.01~0.50%,
Cr:0.01~0.50%
B:0.0005~0.0050%、
N :0.002~0.010%、
残部がFeおよび不可避不純物からなり、かつ下記式(1)を満足する鋼組成を有し、引張強度が1000MPa以上であり、vTrs100が-80℃以下の高靭性とを有することを特徴とするエアバッグ用継目無鋼管。
Cu+Ni≧(M)2+0.3 ・・・ (1)
なお、式(1)の“M”は、Crを表し、元素記号は、それらの元素の含有量を質量%で示したときの数値を意味する。 - 前記Ti含有量が、質量%で、0.020%超0.050%以下であることを特徴とする、請求項1に記載のエアバッグ用鋼管。
- 質量%で、さらにMo:0.10%未満を含有し、且つ下式(1)を満たす鋼組成を有することを特徴とする、請求項1または2に記載のエアバッグ用継目無鋼管。
Cu+Ni≧(M)2+0.3 ・・・ (1)
なお、式(1)の“M”は、(Cr+Mo)を表し、元素記号は、それらの元素の含有量を質量%で示したときの数値を意味する。 - 質量%で、さらに、V:0.02~0.20%を含有する鋼組成を有することを特徴とする請求項1または2に記載のエアバッグ用継目無鋼管。
- 質量%で、さらに、V:0.02~0.20%を含有する鋼組成を有することを特徴とする請求項3に記載のエアバッグ用継目無鋼管。
- 請求項1~5のいずれかに記載の鋼組成を有するビレットを用いて熱間製管によって製造された継目無鋼管素管に、1回の冷間加工の加工度が40%以上の冷間加工を施して所定寸法の鋼管とし、矯正を行なった後に、高周波加熱によってAc3変態点以上に加熱して急冷する焼き入れを行い、次いでAc1変態点以下の温度に加熱して焼き戻しを行うことを特徴とする、エアバッグ用継目無鋼管の製造方法。
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Also Published As
Publication number | Publication date |
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ES2726767T3 (es) | 2019-10-09 |
CA2776984C (en) | 2015-11-17 |
US9080223B2 (en) | 2015-07-14 |
MX2012005710A (es) | 2012-06-12 |
EP2484793A1 (en) | 2012-08-08 |
CN102741438A (zh) | 2012-10-17 |
CA2776984A1 (en) | 2011-12-08 |
KR101425738B1 (ko) | 2014-07-31 |
CN102741438B (zh) | 2014-11-05 |
IN2012DN03019A (ja) | 2015-07-31 |
KR20120056890A (ko) | 2012-06-04 |
US20120205016A1 (en) | 2012-08-16 |
PL2484793T3 (pl) | 2019-09-30 |
EP2484793B1 (en) | 2019-03-13 |
EP2484793A4 (en) | 2016-01-13 |
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