High strength thin steel sheet having high hydrogen embrittlement resisting property and high workability

Abstract

The present invention provides a high strength thin steel sheet that has high hydrogen embrittlement resisting property and high workability. The high strength thin steel sheet having high hydrogen embrittlement resisting property has a metallurgical structure after stretch forming process to elongate 3% comprises: (i) 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 9% or less (may be 0%) in total of ferrite and pearlite in terms of proportion of area to the entire structure, wherein the mean axis ratio (major axis / minor axis) of the residual austenite grains is 5 or higher, or (ii) 1% or more residual austenite in terms of proportion of area to the entire structure; mean axis ratio (major axis / minor axis) of the residual austenite grains is 5 or higher; mean length of minor axes of the residual austenite grains is 1 μm or less; minimum distance between the residual austenite grains is 1 μm or less; and the steel has tensile strength of 1180 MPa or higher.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a high strength thin steel sheet that has high hydrogen embrittlement resisting property (particularly the hydrogen embrittlement resisting property after being subjected to forming process) and high workability, especially to a high strength thin steel sheet that has high resistance against fractures due to hydrogen embrittlement such as season crack and delayed fracture that pose serious problems for steel sheets having tensile strength of 1180 MPa or higher, and has high workability. [0003] 2. Description of the Related Art [0004] There are increasing demands for the steel sheet, that is pressed or bent into a form of a high-strength component of automobile or industrial machine, to have both high strength and high ductility at the same time. In recent years, there are increasing needs for high strength steel sheets having strength of 1180 MPa or higher, as the automobiles are bei...

AI Technical Summary

Benefits of technology

[0017] The present invention has been made with the background described above, and has an object of providing a high strength thin steel sheet that shows high hydrogen embrittlement resisting property in a harsh operating environment over a long period of time after the process of forming the steel sheet into a part, and has improved workability and tensile strength of 1180 MPa or higher.

[0022] When the metal structure is controlled as described above, hydrogen embrittlement resisting property of the high strength thin steel sheet can be sufficiently improved without adding much alloy elements. The phrase “after the forming process” means the state of the steel sheet after being stretched with an elongation ratio of 3%. Specifically, the steel sheet is subjected to uniaxial stretching of 3% at the room temperature (the stretching process of 3% elongation may hereinafter be referred to simply as “processing”).

[0024] According to the present invention, it is made possible to manufacture, with a high level of productivity, a high strength thin steel sheet having tensile strength of 1180 MPa or higher that neutralizes hydrogen that infiltrates from the outside after the steel sheet has been formed into a part thereby to maintain satisfactory hydrogen embrittlement resisting property, and demonstrates high workability during the forming process. Use of the high strength thin steel sheet makes it possible to manufacture high strength parts that hardly experience delayed fracture, such as bumper, impact beam and other reinforcement members and other automobile parts such as seat rail, pillar, etc.

Problems solved by technology

In the realm of high strength of 1180 MPa upward, however, the TRIP steel sheet is known to suffer a newly emerging problem of delayed fracture caused by hydrogen embrittlement, similarly to the conventional high strength steel.

Delayed fracture refers to the failure of high-strength steel under stress, that occurs as hydrogen originating in corrosive environment or the atmosphere infiltrates and diffuses in microstructural defects such as dislocation, void and grain boundary, and makes the steel brittle.

This results in decreases in ductility and toughness of the metallic material.

Also hydrogen embrittlement has rarely been regarded as a problem for thin steel sheets where hydrogen that has infiltrated therein is immediately released due to the small thickness.

For these reasons, much efforts have not been dedicated to counter the hydrogen embrittlement.

However, in the case of “New Development in Elucidation of Delayed Fracture” (published by The Iron and Steel Institute of Japan in January, 1997), for example, 0.4% or higher of C content and much alloy elements are contained, and therefore application of this technology to a thin steel sheet compromises the workability required of the thin steel sheet.

The technology also has a drawback related to the manufacturing process, since it takes several hours or longer period of heat treatment to cause the alloy carbide to precipitate.

However, improvement of the workability without variability essentially requires it to provide a tempering process which makes it necessary to strictly control the temperature and duration of the process.

This also sometimes increases the possibility of tempering embrittlement and makes it difficult to reliably improve workability.

Although there is a steel of composite structure of martensite and ferrite or the like developed to improve ductility, such a steel has a high notch sensitivity due to mixed presence of hard phase and soft phase, thus making it difficult to achieve sufficient improvement of hydrogen embrittlement resisting property.

However, even when a large number of carbide grains or the like are diffused as the trap site for hydrogen, there is a limitation to the hydrogen trapping capability and delayed fracture attributable to hydrogen cannot be fully suppressed.

However, this technology is intended for thick steel sheets and, although consideration is given to delayed fracture after welding with a large input heat, no consideration is given to the environment (for example, corrosive environment, etc.) in which automobile parts manufactured by using thin steel sheets are used.

However, it is difficult to improve the hydrogen embrittlement resisting property in such an environment as hydrogen is generated through corrosion of the steel sheet simply through the trapping effect achieved by controlling the form of precipitate.

However, the TBF steels reported in the documents described above show delayed fracture characteristic of about 1000 seconds at the most in terms of the time before crack occurrence measured in cathode charging test, indicating that these steels are not meant to endure the harsh operating environment such as that of automobile parts over a long period of time.

Moreover, since the heat treatment conditions reported in the documents described above involve heating temperature being set higher, there are such problems as low efficiency of practical manufacturing process.

Also there has been such a problem that press forming operation leads to lower hydrogen embrittlement resisting property.

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