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High strength thin steel sheet having high hydrogen embrittlement resisting property

Inactive Publication Date: 2006-06-29
KOBE STEEL LTD
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0012] The present invention has been made with the background described above, and the object of the present invention is to make available a high strength thin steel sheet that shows high hydrogen embrittlement resisting property with workability improved under the tensile strength of 1180 MPa or higher.

Problems solved by technology

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.
Because in the case of the single phase structure of martensite, a carbide (for example, film-like cementite) is likely to precipitate in the grain boundaries, thus making intergranular fracture likely to occur.

Method used

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  • High strength thin steel sheet having high hydrogen embrittlement resisting property
  • High strength thin steel sheet having high hydrogen embrittlement resisting property
  • High strength thin steel sheet having high hydrogen embrittlement resisting property

Examples

Experimental program
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Effect test

first embodiment

[0050] The first high strength thin steel sheet according to the present invention comprises higher than 0.10 and up to 0.25% of C (contents of components given in terms of percentage in this patent application all refer to percentage by weight), 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, 0.15% or less P, 0.02% or less S, 1.5% or less (higher than 0%) of Al, 1.0% or less (higher than 0%) of Mo and 0.1% or less (higher than 0%) of Nb, with balance of iron and inevitable impurities, and

[0051] the metal structure comprises:

[0052] 1% or more residual austenite;

[0053] 80% or more in total of bainitic ferrite and martensite; and

[0054] 9% or less (may be 0%) in total of ferrite and pearlite in terms of the proportion of area to the entire structure, and

[0055] the mean axis ratio (major axis / minor axis) of the residual austenite grains is 5 or higher.

(1% or more residual austenite in the area proportion to the entire structure)

[0056] It is necessary that the metal contains 1% or more res...

second embodiment

[0110] The second high strength thin steel sheet according to the present invention comprises:

[0111] C: higher than 0.10 up to 0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities, wherein the metal structure comprises:

residual austenite; 1% by area or more in proportion to the entire structure;

while the mean axis ratio (major axis / minor axis) of said residual austenite grains is 5 or higher;

mean length of minor axes of said residual austenite grains is 1 μm or less; and

[0112] a minimum distance between said residual austenite grains is 1 μm or less; and the steel has tensile strength of 1180 MPa or higher. The structure may further contain 80% or more bainitic ferrite and martensite in total and / or 9% or less (may be 0%) ferrite and pearlite in total, and the structure may not contain them.

[0113] The reason why the above requirement...

example 1

[0118] Sample steels A-1 through Y-1 having the compositions described in Table 1 were melt-refined in vacuum to make test slabs. The slabs were processed in the following procedure (hot rolling→cold rolling→continuous annealing) thereby to obtain hot-rolled steel plates measuring 3.2 mm in thickness. The steel plates were pickled to remove scales from the surface and then cold rolled so as to reduce the thickness to 1.2 mm.

Starting temperature (SRT): Held at a temperature between 1150 and 1250° C. for 30 minutes.

Finishing temperature (FDT): 850° C.

Cooling rate: 40° C. / s

Winding-up temperature: 550° C.

Rolling ratio: 50%

[0119] Each steel specimen was kept at a temperature of A3 point+30° C. for 120 seconds, then cooled in air at a mean cooling rate of 20° C. / s to temperature T0 shown in Table 2, and was kept at T0 for 240 seconds, followed by air-assisted water cooling to the room temperature.

[0120] No. 116 shown in Table 2 was made by heating a cold-rolled steel sheet to ...

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Abstract

The purpose of the present invention is to provide a high strength thin steel sheet that has high hydrogen embrittlement resisting property. In order to achieve the above purpose, a high strength thin steel sheet having high hydrogen embrittlement resisting property comprises: C: 0.10 to 0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities; and the metal structure comprises: residual austenite; 1% by area or more in proportion to the entire structure; bainitic ferrite and martensite: 80% or more in total; and ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis / minor axis) of said residual austenite grains is 5 or higher, 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...

Claims

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Application Information

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IPC IPC(8): C22C38/06
CPCC21D2211/002C22C38/02C22C38/04C22C38/06C22C38/08C22C38/12C22C38/16
Inventor YUSE, FUMIOIKEDA, SHUSHIMUKAI, YOICHIAKAMIZU, HIROSHIKINUGASA, JUNICHIROSAITO, KENJISUGIMOTO, KOICHIHOJO, TOMOHIKO
Owner KOBE STEEL LTD
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