Method for producing steel with retained austenite

Abstract

The present invention relates to a process for producing steel with retained austenite. In one embodiment, the process comprises the steps of heating a steel alloy to produce austenite, quenching the steel to produce martensite, and carbon partitioning to transfer carbon from the martensite to the austenite.

Description

FIELD OF THE INVENTION [0001] The invention is directed to a method for producing steels with microstructures containing retained austenite. BACKGROUND OF THE INVENTION [0002] The difficulty that this invention seeks to address is that of creating a microstructure in steel, (e.g., but not limited to, low carbon sheet steel) that contains austenite at the ambient temperature at which the steel is to be used in some application, typically room temperature. Without substantial additions of expensive alloying elements such as nickel, austenite usually must be enriched with carbon (or sometimes nitrogen) in order to stabilize it to room temperature. (Actually, it is metastable, and undergoes subsequent transformation to martensite during deformation, a key component of its utility in service.) The problem is to enrich some austenite in the microstructure with carbon, by controlling microstructure evolution and carbon partitioning, without having to use a much higher carbon-containing ste...

AI Technical Summary

Benefits of technology

[0005] The present invention recognizes that the carbon partitioning and growth of the body-centered phase are decoupled in martensite transformations and that this decoupling provides a mechanism for controlling the austenite fraction and its carbon concentration (the kinetics of carbon partitioning are separate from the mechanisms of ferrite formation), since the extent of martensite transformation is controlled primarily by temperature only, rather than both time and temperature. This new concept provides additional flexibility for implementing more convenient or less costly processing strategies or methodologies for producing steel with retained austenite.

Problems solved by technology

The problem is to enrich some austenite in the microstructure with carbon, by controlling microstructure evolution and carbon partitioning, without having to use a much higher carbon-containing steel, which is usually undesired for reasons such as reduced weldability.

A limitation of this approach is the processing constraint that is needed to control the bainite transformation.

The time / temperature / alloying approaches are quite challenging, especially to match with the thermal characteristics of commercial processing facilities.

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