Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-11-12
2001-08-28
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S330000, C148S331000, C148S332000, C148S333000, C148S334000, C148S335000
Reexamination Certificate
active
06280538
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a hot rolled high strength steel sheet of excellent formability which can be used suitably, for example, as steel sheet used for the materials of automobiles.
BACKGROUND ART
In recent years, with a view point of improvement for the safety and reduction of fuel cost of automobiles, demands for increasing the strength and reducing the thickness of steel sheets used for automobiles have been increased more and more. Most of thin steel sheets used as the material sheets for automobiles are fabricated by press forming into predetermined shapes. Since the formability usually tends to be worsened as th e strength increases, various studies have been made in order to obtain steel sheets of high strength and also excellent in the formability.
Press forming includes various fabrication modes and required working characteristics vary depending on the members to be pressed in which elongation (E1) and stretch frangibility (&lgr;) are considered most important as the index for the formability required for hot rolled high strength steel sheets.
As steel sheets of excellent elongation, steel sheets that utilize TRIP (Transformation Induction Plasticity) of retained austenite have been developed as disclosed, for example, in Japanese Published Unexamined Patent Application No. Hei 7-252592, which attains 20,000 N/mm
2
·% or more in strength-elongation balance (TS×E1).
On the other hand, as steel sheets having elongation above a certain level, stretch frangibility and low yield ratio characteristics, so-called tri-phase steel sheets comprising three phase structure of ferrite-bainite-martensite have been disclosed, for example, in Japanese Published Unexamined Patent Application No. Sho 57-70257.
However, while the steel sheet of utilizing the TRIP phenomenon of the retained austenite is comparatively satisfactory in the strength-elongation balance but it shows only about 40% of the stretch frangibility (&lgr;) which does not reach a level capable of satisfying user's demand. In addition, since this steel sheet also has a relatively high yield ratio, it is poor in a shape freezing property and lacks in adaptability to the application use requiring the shape freezability.
The shape freezability means an extent of dimensional change when a work is released from a forming die upon fabrication and a material having a small amount of dimesional change is called as “material of satisfactory shape freezability”. That is, when a steel sheet is manufactured by press forming into a product, it is important to ensure the dimensional accuracy required for the final product. In this case, a steel sheet pressed by a pressing machine and is in intimate contact with a forming die shows a shape conforming the forming die but changes the shape upon deforming due to the release of residual stresses. That is, any steel sheet causes considerable change of dimension upon detaching from a pressing die and it is desirable that the dimensional change is small, that is, the shape freezability is satisfactory in order to enhance the dimensional accuracy of a product. It is said that the yield strength of a material steel sheet gives most significant effect on the shape freezability and it is considered that the yield strength increases as the strength of the material steel sheet increases and, correspondingly, the shape freezability is worsened. Accordingly, it has been demanded for the development of a material having excellent shape freezability while ensuring high strength.
The &lgr; described above is a measure for expressing the stretch frangibility, which is a value determined by the following relation:
&lgr;=(
D−D
0
)×100
/D
0
(%)
in which D represents a hole diameter in a case where a conical punch with a 60° apex is forced into an initial hole of a diameter (D
0
) to enlarge the hole and when a crack penetrates the sheet along the thickness.
Further, the tri-phase steel sheet is excellent in the stretch frangibility and the low yield ratio characteristic compared with a steel sheet based on the retained austenite but it has no sufficient strength-elongation balance.
As a means for obtaining a sheet having a retained austenite and still having an excellent stretch frangibility, a method of incorporating Al by about 0. 10% or more in a steel has been proposed as described, for example, in Japanese Published Unexamined Patent Application No. Hei 5-105986. However, if a great amount of Al is incorporated in the steel, the amount of oxide series inclusions increases, which results in a problem of surface defect or degradation of ductility and also gives undesired effects on the yield and the productivity.
The present invention has been accomplished taking notice on the foregoing problem and it is an object thereof to provide a hot rolled high strength steel sheet having an excellent strength-elongation balance compared with existent steel sheets and also having an excellent stretch frangibility.
DISCLOSURE OF THE INVENTION
The foregoing object can be attained in accordance with the present invention by a hot rolled high strength steel sheet of excellent formability comprising, on the basis of mass %,
C
:0.05 ~ 0.15%
Si
:0.5 ~ 2.0%
Mn
:0.5 ~ 2.0%
P
:0.05% or less
S
:0.010% or less
Al
:0.005 ~ less than 0.10%,
and the balance of Fe and inevitable impurities, in which the micro-structure mainly comprises three phases of ferrite, retained austenite and bainite, the amount of the ferrite [V(F)] is 60~95 area %, the amount of the retained austenite [V(&ggr;)] is 3~15 vol %, the amount of the bainite [V(B)] in the area ratio is 1.5 times or more of the amount of the retained austenite [V(&ggr;)] as the area ratio and the average hardness of the bainite is 240-400 Hv.
In the hot rolled steel sheet according to the present invention, the average grain size of the ferrite is particularly preferably 5.0 &mgr;m or more and it is effective to incorporate an appropriate amount of Cr, or incorporate an appropriate amount of Ca and/or REM or, further, incorporate at least one element selected from the group consisting of Nb, Ti, V, Ni, Mo, B and Cu to further improve the physical properties.
Further, the hot rolled steel sheet according to the present invention has excellent characteristics compared with existent hot rolled steel sheets in that it has a value of tensile strength (TS) x elongation (E1) as high as 22,000 N/mm
2
·% or more.
REFERENCES:
patent: 5505796 (1996-04-01), Kawano et al.
patent: 0 586 704 (1994-03-01), None
patent: 0881 308 (1998-12-01), None
patent: 57-70257 (1982-04-01), None
patent: 4-32512 (1992-02-01), None
patent: 5-105986 (1993-04-01), None
patent: 5-171345 (1993-07-01), None
patent: 6-306537 (1994-11-01), None
patent: 7-252592 (1995-10-01), None
patent: 2609732 (1997-02-01), None
patent: 9-104947 (1997-04-01), None
patent: 9-137249 (1997-05-01), None
patent: 9-241792 (1997-09-01), None
Mukai Yoichi
Nakaya Michiharu
Soshiroda Tetsuo
Kabushiki Kaisha Kobe Seiko Sho
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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