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

a high-strength steel sheet and process technology, applied in the field of high-strength steel sheets, can solve the problems of reducing the stretch-flanging performance of steel sheets, preventing ensuring stretch-flanging performance, and disadvantageous ductility of martensite single-phase structure steel sheets, etc., to achieve excellent press formability, excellent elongation and stretch-flanging performance, and high strength

Inactive Publication Date: 2009-11-12
KOBE STEEL LTD
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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

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 per

Method used

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Examples

Experimental program
Comparison scheme
Effect test

example 1

[0073]Steel slabs having chemical compositions shown in Table 1 below were melted, and the steel slabs were heated to about 1000 to 1100° C. The steel slabs were subjected to hot rolling or further preliminary annealing under the conditions described in Table 2 below, producing material steel sheets. The average cooling rate after the hot rolling was 50° C. / sec. test pieces for observing structures were collected from the material steel sheets, and the space factors of bainite were determined by observing structure constitutions with a microscope and subjecting microscope structure pictures after being corroded with nital to image analysis. The values of Ac3 point and Ms point calculated from the components by a known equation are also shown in Table 1 for reference. Moreover, the results of structure observation are also shown in Table 2. The obtained material steel sheets were subjected to final annealing (two-phase region annealing) and tempering under the conditions shown in Tab...

example 2

[0107]The actions and effects of the high strength steel sheet of this embodiment and method for manufacturing the same will be described below with reference to Examples.

[0108]First, a method for preparing test steel sheets tested in these Examples will be described. In these Examples, Steel slabs having the compositions of constituents represented by steel symbols A to Y having the compositions of constituents shown in Table 5 were tested. As shown in Table 6 and 7, 56 types of test steel sheets were prepared under different hot rolling conditions, preliminary annealing conditions, and in different annealing steps and tempering steps from the steel slabs having the compositions of constituents of these A to Y, and the tensile strength, ductility, stretch-flanging performance and other characteristics of the test steel sheets were determined. Among the steel slabs having the compositions of constituents of A to Y, B, C, E, F, I, J, L, N to Y are the steel slabs having the compositi...

example 3

[0181]Now, the present invention will be described more specifically by referring to Examples. The present invention is not restricted in itself by the following Examples. Therefore, it is possible to carry out the invention by properly modifying the Examples within the above described or later describe spirit of the invention, and such modifications are all to be included in the technical scope of the present invention.

[0182]steel slabs having compositions of chemical constituents shown in Tables 10 and 11 below were prepared, and material steel sheets were prepared from the steel slabs under the hot rolling conditions and preliminary annealing conditions shown in Tables 12 and 13 below. Tables 10 and 11 also show the point (Ac3 transformation point) and martensite transformation start temperature, Ms point, for each steel type determined by equations (1) and (2).

AC3(° C.)=910−203·√[C]−15.2·[Ni]+44.7·[Si]+104·[V]+31.5·[Mo]+13.1·[W]−330·[Mn]+11·[Cr]+20·[Cu]−720·[P]−400[Al]−120·[As]−...

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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...

Claims

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Application Information

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IPC IPC(8): C21D6/00C22C38/02C22C38/04C22C38/00C22C38/12C22C38/08C22C38/16C22C38/18C22C38/14
CPCC21D1/25C21D6/005C21D9/46C21D2211/005C22C38/14C22C38/02C22C38/04C22C38/06C22C38/12C21D2211/008
Inventor SAITO, KENJIMASUDA, TOMOKAZUMIURA, MASAAKIMUKAI, YOICHIIKEDA, SHUSHI
Owner KOBE STEEL LTD
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