High-strength steel sheets and processes for production of the same

Abstract

A high strength steel sheet with both excellent elongation and stretch-flanging performance is provided.The high strength steel sheet of the present invention comprises, in percent by mass, C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1%, and Fe and inevitable impurities as the remainder, and has a structure mainly composed of tempered martensite and annealed bainite. The space factor of the tempered martensite is 50 to 95%, the space factor of the annealed bainite is 5 to 30%, and the mean grain size of the tempered martensite is 10 μm or smaller in terms of the equivalent of a circle diameter. The steel sheet has a tensile strength of 590 MPa or higher.The high strength steel sheet of the present invention has a space factor of the martensite phase which is a main component of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter; in the martensite phase, the space factor of the martensite phase having a grain size of 10 μm or larger in terms of the equivalent of a circle diameter is 15% or lower; and the space factor of the retained austenite phase in the metal structure is 3% or lower.The high strength steel sheet of the present invention is a dual phase steel sheet mainly composed of a ferrite phase and martensite, and the space factor of the ferrite phase is 5 to 30%, and the space factor of the martensite phase is 50 to 95%. Moreover, the ferrite phase is annealed martensite.

Description

TECHNICAL FIELD[0001]The present invention relates to a high strength steel sheet for which high press formability is required, typically including steel sheets for automobiles, particularly to a high strength steel sheet with both elongation and stretch-flanging performance and a method for manufacturing the same.BACKGROUND ART[0002]High strength steel sheets, which are generally used by being press-molded, are used in industrial product such as automobiles, electric devices and industrial machines. Since high strength steel sheets are used for the purpose of lightening industrial products, they need not only have high strength, but also have the ability to form various configurations of the products. Accordingly, it is required for high strength steel sheets to have excellent press formability. To meet this requirement, high-strength steel sheets having excellent elongation and stretch-flanging performance, which are necessary for improving press formability, are required.[0003]Ex...

AI Technical Summary

Benefits of technology

[0037]In the present invention, a structure which is mainly composed of especially tempered martensite and finely dispersed annealed bainite is provided, wherein the space factors thereof are defined to have predetermined amounts, and the mean grain size of tempered martensite is defined 10 μm or smaller. Accordingly, a high strength steel sheet which has strength as high as 590 MPa or higher, excellent elongation and stretch-flanging performance, and thus excellent press formability can be provided.

[0038]According to the present invention, it is also possible to provide a high strength steel sheet in which the space factor of the retained austenite phase is 3% or lower and the space factor of the fine martensite phase is 80% or higher by a relatively simple heat treatment step. Since this high strength steel sheet has a tensile strength of 780 MPa or higher, and also has excellent elongation and stretch-flanging performance, it is excellent in press formability.

[0039]According to the present invention, it is also possible to achieve a high strength steel sheet which has excellent elongation and stretch-flanging performance at the same time by designing the steel sheet especially for a dual phase steel sheet mainly composed of a ferrite phase and martensite, ensuring high strength of the steel sheet as a whole, and appropriately controlling the space factors of especially the ferrite phase and martensite and the mean grain sizes of the same.

Problems solved by technology

However, because of the coexistence of soft ferrite and rigid martensite, distortion (stress) is concentrated at the interface of the two phases when deformed, and therefore the interface is likely to serve as the starting point of rupture, thereby disadvantageously preventing ensuring stretch-flanging performance (local elongation).

However, since martensite which has been transformed from retained austenite in the TRIP steel is extremely hard, it likely serves as the starting point of rupture, lowering the stretch-flanging performance of the steel sheet.

However, the martensite single-phase structure steel sheet disadvantageously has low ductility, and insufficient elongation.

However, since the steel sheet produced by the steelmaking method disclosed in Patent document 4 has a relatively high cooling stop temperature in the steelmaking process, a large amount of bainite is deposited, while a large amount of retained austenite also remains therein, and therefore the steel sheet has excellent ductility, but has insufficient stretch-flanging performance.