Process for manufacturing a steel tube for air bags

Abstract

A seamless steel tube for air bag use is formed from a steel comprising, in mass percent, C: 0.04-0.20%, Si: 0.10-0.50%, Mn: 0.10-1.00%, P: at most 0.025%, S: at most 0.005%, Al: at most 0.10%, Cr: 0.01-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, a remainder of Fe and unavoidable impurities. The tube is cold drawn at least one time with a working ratio so that reduction in area is greater than 40% to obtain predetermined dimensions. The tube is then quench hardened by heating to at least the Ac3 point at a temperature rate increase of at least 50° C. per second followed by cooling at a cooling rate of at least 50° C. per second at least in a temperature range of 850-500° C. Tempering is done at a temperature of at most the Ac1 point.

Description

TECHNICAL FIELD[0001]This invention relates to a process for inexpensively manufacturing a seamless steel tube which is suitable as a steel tube for air bags (air bag systems) and of which are required a high strength as expressed by a tensile strength of at least 900 MPa and a high level of toughness as expressed by a value of vTrs100 (the lowest Charpy fracture appearance transition temperature at which the percent ductile fracture is 100%) of −60° C. or below.BACKGROUND ART[0002]In recent years, the automotive industry has actively promoted the introduction of safety equipment. One example of such equipment which has been developed is an air bag system, which has been installed in many automobiles. At the time of a collision, an air bag system inflates an air bag with a gas or the like between a passenger and the steering wheel, the instrument panel, or the like before the passenger impacts these objects and reduces injuries of the passenger by absorbing the kinetic energy thereo...

AI Technical Summary

Benefits of technology

The present invention provides a process for manufacturing a high-strength and high-toughness steel tube for air bags using a reduced amount of expensive alloy elements, such as Cr and Mo. Instead, the steel tube contains a combination of Cu and Ni, which helps with the cold drawing process. The resulting steel tube has a high tensile strength of at least 900 MPa and excellent low-temperature toughness of at least -30 °C. The manufacturing process is simpler and less expensive than previous methods, and can still provide the necessary strength and toughness for air bag applications.

Problems solved by technology

However, that technique not only increases the alloy cost but also makes it difficult to carry out cold drawing after tube forming.

In this case, the steel tube is finished to a product with desired dimensions while carrying out softening between successive times of cold drawing, so the overall manufacturing costs increase.

The present inventors noted that as a result of relying on strengthening by Cr and Mo in a conventional high-strength steel tube for air bags, the strength after the completion of hot tube forming becomes high, thereby leading to a decrease in productivity during cold drawing, and the alloy cost increases.