GAS GENERATOR PIPE FOR AIRBAG MODULE, AND METHOD FOR MANUFACTURING THE GAS GENERATOR PIPE

Abstract

The present invention relates to a gas generator pipe of an airbag module, the gas generator pipe consisting of a steel alloy with a martensitic matrix. The gas generator pipe is characterized in that the gas generator pipe has a tensile strength, Rm, of at least 1,100 MPa, and the steel alloy has the following alloying elements apart from iron and melt-related impurities in mass percent (Ma %): C 0.05-0.18% Si 0.4-2.6% Mn 0.2-1.4 % Cr 2.0-4.0% Mo 0.05-1.0% N <0.015% and
at least one of the alloying elements Nb, V, Al and Ti in total at least 0.01%,
the gas generator pipe has been subjected to a quenching and partitioning heat treatment and
the gas generator pipe has a microstructure of martensite and austenite and the amount of austenite in the microstructure is at least 5%.

Furthermore, the invention relates to a method of manufacturing such a gas generator pipe.

Claims

1. Gas generator pipe of an airbag module, the gas generator pipe consisting of a steel alloy with a martensitic matrix, characterized in that the gas generator pipe has a tensile strength, Rm, of at least 1100 MPa, and the steel alloy has in mass percent (Ma %) the following alloying elements apart from iron and melt-related impurities: C 0.05-0.18% Si 0.4-2.6% Mn 0.2-1.4% Cr 2.0-4.0% Mo 0.05-1.0% N <0.015% and at least one of the alloying elements Nb, V, Al and Ti, in total at least 0.01 Ma %, the gas generator pipe has been subjected to quenching and partitioning heat treatment and the gas generator pipe has a microstructure of martensite and austenite and the amount of austenite in the microstructure is at least 5%.

2. The gas generator pipe according to claim 1, characterized in that the carbon content is less than 0.15 Ma %, for example 0.14 Ma %, or less than 0.12 Ma %, in particular in the range of 0.06 to 0.13 Ma %, and more preferably is 0.10 Ma %.

3. The gas generator pipe according to claim 1, characterized in that the silicon content is in the range of 1.0-2.6 Ma %, preferably in the range of 1.4-2.6 Ma %, preferably in the range of 1.7-2.4 Ma % and more preferably is 2 Ma %.

4. The gas generator pipe according to claim 1, characterized in that the chromium content is in the range of 2.1-3.8 Ma %, in particular in the range of 2.2-3.6 Ma %, preferably in the range of 2.5-3.5 Ma % and further preferably is 3 Ma %.

5. The gas generator pipe according to claim 1, characterized in that the manganese content is in the range of 0.3-0.9 Ma %.

6. The gas generator pipe according to claim 1, characterized in that the nitrogen content is in the range of 0.006-0.012 Ma %.

7. The gas generator pipe according to claim 1, characterized in that the alloy comprises boron in an amount in the range of 0.001-0.004 Ma %.

8. The gas generator pipe according claim 1, characterized in that at least one of the following alloying elements is present in the steel alloy in the indicated amounts in mass percent: Nb 0.015-0.1% V 0.025-0.5% Ti 3.8*N-5.5*N.

9. The gas generator pipe according to claim 1, characterized in that the steel alloy comprises nickel, Ni, in an amount of at most 3 Ma %, preferably up to 0.5 Ma % and most preferably up to 0.1 Ma %.

10. The gas generator pipe according to claim 1, characterized in that the gas generator pipe has a microstructure of martensite and austenite and the amount of austenite in the microstructure is preferably in the range of 5 to 20%, in particular in the range of 5 to 15%.

11. The gas generator pipe according to claim 10, characterized in that the amount of austenite in the microstructure, determined at 1 mm depth measured from the outer surface of the pipe, is more than 5%.

12. The gas generator pipe according to claim 10, characterized in that the microstructure comprises bainite, ferrite and/or pearlite in a total amount of less than 10%, preferably less than 5%.

13. The gas generator pipe according to claim 1, characterized in that the gas generator pipe has an energy absorption capacity, expressed by the product of tensile strength, Rm, and elongation at break, A, of 18,000 MPa %, determined on a round sample with an elongation measurement length of 20 mm.

14. The gas generator pipe according to claim 1, characterized in that the steel alloy has a transition temperature of −40° C. and preferably −60° C.

15. A method for manufacturing a gas generator pipe for an airbag module, the method comprising: providing a gas generator pipe of an airbag module, the gas generator pipe consisting of a steel alloy with a martensitic matrix, characterized in that the gas generator pipe has a tensile strength, Rm, of at least 1100 MPa, and the steel alloy has in mass percent (Ma %) the following alloying elements apart from iron and melt-related impurities: C 0.05-0.18% Si 0.4-2.6% Mn 0.2-1.4% Cr 2.0-4.0% Mo 0.05-1.0% N <0.015% and at least one of the alloying elements Nb, V, Al and Ti, in total at least 0.01 Ma %, the gas generator pipe has been subjected to quenching and partitioning heat treatment and the gas generator pipe has a microstructure of martensite and austenite and the amount of austenite in the microstructure is at least 5%; characterized in that the method comprises a quenching step and a partitioning step, the quenching step comprising an active cooling phase and optionally a subsequent passive cooling phase.

16. The method according to claim 15, characterized in that in the active cooling phase the gas generator pipe is cooled at a cooling rate greater than the critical cooling rate to a temperature T1 which is between martensite start temperature +/−100° C., and in a second passive cooling step in air to a temperature T2 which is preferably greater than 150° C. and less than the martensite start temperature.

17. The method according to claim 15, characterized in that in the active cooling phase the gas generator pipe is cooled at a cooling rate greater than the critical cooling rate to a temperature T1 which is between martensite start temperature and martensite start temperature minus 150° C.

18. The method according to claim 15, characterized in that in the partitioning step the gas generator pipe is heated to a temperature T3 which is greater than the martensite start temperature and less than or equal to 500° C. and is held at his temperature.

19. The method according to claim 15, characterized in that the method comprises a step of cold forming, in particular cold drawing, of at least part of the gas generator pipe after the partitioning step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] An embodiment of the invention is explained in more detail in the following description of the figures, wherein:

[0056] FIG. 1: shows a schematic representation of an embodiment of a gas generator pipe for an airbag module;

[0057] FIG. 2: shows a schematic representation of heat treatment according to a first embodiment of the invention;

[0058] FIG. 3: shows a schematic representation of heat treatment according to a second embodiment of the invention; and

[0059] FIG. 4: shows a pipe wall section of a gas generator pipe according to two embodiments of the invention with associated diagram of the austenite content in the pipe wall.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] FIG. 1 shows an example of a gas generator 1 for an airbag module (not shown). Gas generator 1 comprises a gas generator pipe 10 according to the invention. In the embodiment shown in FIG. 1, the pipe ends 101 are tapered or drawn in. The taper of the pipe ends 101 can be produced by cold forming. In the embodiment shown in FIG. 1, the pipe ends 101 each have a diameter D1 which is smaller than the diameter D0 of the pipe element 10 in its middle area 102. The diameters of the pipe ends 101 can also be different. In the embodiment shown in FIG. 1, gas generator 1 has a combustion chamber 14, in which an igniter 12 and the other pyrotechnical components are provided. The combustion chamber 14 is closed at one pipe end 101 by a welded-on disc 17. The cold gas storage 15 adjoins the combustion chamber 14. The cold gas storage 15 is separated from the combustion chamber 14 by the membrane 11, which can also be referred to as a bursting disc. The cold gas storage 15 is located in the middle area 102 of the pipe element 10, which has the larger diameter D0. The cold gas storage 15 is connected to the diffuser 13. FIG. 1 shows a filling hole 16 in the area of the diffuser 13. The pipe end 101 of the diffuser 13 is welded to a disk 17, i.e. closed by it.

[0061] In FIG. 2 it is shown that the gas generator pipe, which in this embodiment can be present in the form of a bloom during heat treatment, is heated in a first step to a temperature higher than the Ac3 temperature of the material of the gas generator pipe. In a first quenching step, the gas generator pipe is cooled at a high cooling rate to a temperature T1 which, in the embodiment shown, is above the martensite start temperature, Ms. In this way, the quenching temperature can be reliably reached. In a second cooling step, the gas generator pipe is cooled down to a temperature T2, which is below the Ms temperature, by passive cooling, for example by transporting the gas generator pipe during production. In the partitioning step, the gas generator pipe is then heated to a temperature T3, which is above the Ms temperature, and held at this temperature.

[0062] The method according to FIG. 3 differs from the first embodiment according to fig-ure 2 in that in the second embodiment in FIG. 3 the quenching step only includes one active cooling step. In this case, the gas generator pipe is cooled in the active cooling phase at a cooling rate greater than the critical cooling rate to a temperature T1, which lies between the martensite start temperature and the martensite start temperature—150° C. A passive cooling step is not performed. Instead, the gas generator pipe is heated directly from temperature T1 to a temperature T3 which is higher than the martensite start temperature, and preferably less than or equal to 500° C.

[0063] FIG. 4 shows a pipe wall section of a gas generator pipe with two-phase cooling according to the invention. The associated diagram shows on the horizontal axis the distance D or measuring point, measured from the outside of the pipe 103, and on the vertical axis the austenite content A. Curve K1 shows a degressively increasing austenite content A1.1 over the pipe wall from the outside to the inside of the pipe 104 and a pronounced almost constant austenite content A1.2 already at less than half of the pipe wall thickness WD. In comparison, curve K2 shows a gas generator pipe with only one active cooling. Both a comparatively low austenite content on the outside of the pipe and a significantly flatter increase are visible.

[0064] For example, in the cold gas storage 15 there can be a pressure of 580 bar. In the combustion chamber 14, for example, the pressure can increase from 580 bar to 1,200 bar, when the igniter is ignited. Due to its properties, the gas generator pipe, can reliably withstand this pressure without fear of brittle fracture or expansion of a brittle crack.

REFERENCE NUMBERS

[0065] 1 Gas generator [0066] 10 Gas generator pipe [0067] 101 Pipe end [0068] 102 middle area [0069] 103 pipe outside [0070] 104 pipe inside [0071] 11 membrane [0072] 12 igniter [0073] 13 diffuser [0074] 14 combustion chamber [0075] 15 cold gas storage [0076] 16 fill hole [0077] 17 disc [0078] A austenite portion [0079] D distance [0080] WD wall thickness