Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1994-03-04
1996-08-06
Ip, Sikyin
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148654, 148506, C22C 3806, C22C 3858, C21D 946
Patent
active
055429963
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance and a method for manufacturing same.
BACKGROUND OF THE INVENTION
For the purpose of reducing the weight of an automobile or ensuring the safety of passengers, cold-rolled steel sheets having such a high tensile strength as to permit achievement of a higher strength and reduction of the weight of various structural members, are widely used as materials for protective components of an automobile such as a bumper reinforcement and a door guard bar. As a cold-rolled steel sheet having such a high tensile strength, ultra-high-strength cold-rolled steel sheets having a tensile strength of over 100 kgf/mm.sup.2 are proposed as follows: Patent Provisional Publication No. 61-3,843 published on Jan. 9, 1986, which consists essentially of:
carbon (C): from 0.02 to 0.30 wt. %,
silicon (Si): from 0.01 to 2.5 wt. %,
manganese (Mn): from 0.5 to 2.5 wt. %,
and
the balance being iron (Fe) and incidental impurities
(hereinafter referred to as the "prior art 1"). Patent Provisional Publication No. 61-217,529 published on Sep. 27, 1986, which consists essentially of:
carbon (C): from 0.12 to 0.70 wt. %,
silicon (Si): from 0.4 to 1.0 wt. %,
manganese (Mn): from 0.2 to 2.5 wt. %,
soluble aluminum (Sol.Al): from 0.01 to 0.07 wt. %,
nitrogen (total N): up to 0.02 wt. %,
and
the balance being iron (Fe) and incidental impurities
(hereinafter referred to as the "prior art 2").
However, the prior arts 1 and 2 described above have the following problems:
It is true that the cold-rolled steel sheets of the prior arts 1 and 2 are excellent in workability and have a high tensile strength of over 100 kgf/mm.sup.2. An ultra-high-strength cold-rolled steel sheet having a tensile strength of over 100 kgf/mm.sup.2 is usually formed through the bending. In the cold-rolled steel sheets of the prior arts 1 and 2, however, when the tensile strength of the steel sheet becomes higher over 100 kgf/mm.sup.2, a fracture phenomenon (hereinafter referred to as the "delayed fracture") is suddenly caused by hydrogen penetrating into the interior of the steel sheet under the effect of a corrosion reaction taking place along with the lapse of time at a portion formed by the above-mentioned bending of the cold-rolled steel sheet. Therefore, even with a high tensile strength, a cold-rolled steel sheet susceptible to the delayed fracture, has a fatal defect as a material for protective components of an automobile, for example.
Under such circumstances, there is a strong demand for the development of an ultra-high-strength cold-rolled steel sheet excellent in the property inhibiting the occurrence of delayed fracture (hereinafter referred to as "delayed fracture resistance") and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same, but such an ultra-high-strength cold-rolled steel sheet and a method for manufacturing same have not as yet been proposed.
An object of the present invention is therefore to provide an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same.
DISCLOSURE OF THE INVENTION
In accordance with one of the features of the present invention, there is provided an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance, which consists essentially of:
carbon (C): from 0.1 to 0.25 wt. %,
silicon (Si): up to 1 wt. %,
manganese (Mn): from 1 to 2.5 wt. %,
phosphorus (P): up to 0.020 wt. %,
sulfur (S): up to 0.005 wt. %,
soluble aluminum (Sol.Al): from 0.01 to 0.05 wt. %,
nitrogen (N): from 0.0010 to 0.0050 wt. %,
and
the balance being iron (Fe) and incidental impurities; and
said cold-rolled steel sheet satisfying the following formulae (1) and (2):
in said formula (1):
Ceq=C+(Si/24)+(Mn/6);
and
in said formula (2):
where, in said formulae (1) and (2):
P.sub.DF : delayed fracture resistance index,
TS: tensile st
REFERENCES:
patent: 3573898 (1971-04-01), Murai et al.
patent: 3738874 (1973-06-01), Allten et al.
patent: 4472208 (1984-09-01), Kunishige
Hosoya Yoshihiro
Kanetoh Shuzi
Nagataki Yasunobu
Okita Tomoyoshi
Takada Yasuyuki
Ip Sikyin
NKK Corporation
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