Low carbon alloy steel tube having ultra high strength and excellent toughness at low temperature and method of manufacturing the same

Abstract

A low carbon alloy steel tube and a method of manufacturing the same, especially for a stored gas inflator pressure vessel, in which the steel tube consists essentially of, by weight: about 0.06% to about 0.18% carbon, about 0.3% to about 1.5% manganese, about 0.05% to about 0.5% silicon, up to about 0.015% sulfur, up to about 0.025% phosphorous, and at least one of the following elements: up to about 0.30% vanadium, upto t about 0.10% aluminum, up to about 0.06% niobium, up to about 1% chromium, up to about 0.70% nickel, up to about 0.70% molybdenum, up to about 0.35% copper, up to about 0.15% residual elements, and the balance iron and incidental impurities. After a high heating rate of about 100° C. per second; rapidly and fully quenching the steel tubing in a water-based quenching solution at a cooling rate of about 100° C. per second. The steel has a tensile strength of at least about 145 ksi and as high as 220 ksi and exhibits ductile behavior at temperatures as low as -100° C.

Description

RELATED APPLICATION [0001] This application is a Continuation-in-part of U.S. Nonprovisional patent application Ser. No. 10 / 957,605, filed on Oct. 5, 2004.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to low carbon alloy steel tubes having ultra high strength and excellent toughness at low temperature and also to a method of manufacturing such a steel tube. The steel tube is particularly suitable for making components for containers for automotive restraint systems, an example of which is an automotive airbag inflator. [0004] In addition, alternative steel compositions in the low carbon, low alloy category and different heat treatment processes were developed and tested in order to decrease the manufacturing cost. [0005] 2. Brief Description of the Prior Art [0006] Japanese Publication No. 10-140249 [Application date Nov. 5, 1996] and Japanese Publication No. 10-140283 [Application date Nov. 12, 1996] illustrate in general terms st...

AI Technical Summary

Benefits of technology

[0020] First, the present invention first relates to certain novel low carbon alloy steels suitable for cold forming having more than high tensile strength (UTS 145 ksi minimum) and preferably ultra high tensile strength (UTS 160 ksi minimum and possibly 175 ksi or 220 ksi), and, consequently, a very high burst pressure. Moreover, the steel has excellent toughness at low temperature, with guaranteed ductile behavior at −60° C., i.e., a ductile-to-brittle transition temperature (DBTT) below 60° C., and possibly as low as −100° C.

[0021] Second, the present invention also relates to a process of manufacturing such a steel tube which essentially comprises a novel rapid induction austenizing / high speed quench / no temper technique. In a preferred method, there is an extremely rapid induction austenizing with an ultra fast water quenching step that eliminates any tempering step, so as to create a low carbon alloy steel tube that also is suitable for cold forming having ultra high tensile strength (UTS 145 ksi minimum and up to 220 ksi), and, consequently, a very high burst pressure. Moreover, the steel has excellent toughness at low temperature, with guaranteed ductile behavior at −60° C., i.e., a ductile-to-brittle transition temperature (DBTT) that is below −60° C., and possibly even as low as −100° C.

Problems solved by technology

It is believed that most of the examples shown in these documents that are alleged to be ductile after a DW test, would in fact not show ductile behavior at low temperature in a burst test and, therefore, would not qualify for certain airbag inflator applications due to a lack of compliance with governmental regulations (e.g. US DOT).

Various examples are given for a quench and temper material, but mechanical properties obtained are relatively low.

In the past, air bag systems were of the type employing 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 made of a steel tube filled with argon gas or the like, and this type is increasingly being used.

These welds are highly critical and as such require considerable labor, and in certain instances testing to assure weld integrity throughout the pressure vessel and airbag deployment.

It has been observed that these welds can crack or fail, thus, risking the integrity of the accumulator, and possibly the operation of the airbag.

This is a destructive-type test in which a canister is subjected to internal pressures significantly higher than those expected during normal operational use, i.e., airbag deployment.

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