Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-12-12
2004-02-17
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S330000, C148S332000, C148S333000, C148S336000, C148S603000, C148S652000
Reexamination Certificate
active
06692584
ABSTRACT:
TECHNICAL FIELD
The present invention relates to high tensile cold-rolled steel sheets which have superior workability and are steel sheets suitably and primarily used for automobile bodies. In particular the present invention relates to a high tensile cold-rolled steel sheet having a tensile strength (TS) of 440 MPa or more, and superior ductility and strain age-hardening characteristics, and relates to a manufacturing method therefor. The high tensile cold-rolled steel sheet of the present invention is suitably used for various applications from relatively light fabrications, such as simple bending or pipe formation by roll forming, to relatively complicated drawing. In the present invention, the steel sheet includes a steel strip in coil form.
In addition, in the present invention, “superior strain age-hardening characteristics” mean that when aging treatment is performed under the conditions of a temperature of 170° C. and a holding time of 20 minutes after a predeformation of a 5%-tensile strain, an increased amount (hereinafter referred to as “BH amount”; BH amount=yield stress after aging treatment−predeformation stress before aging treatment) of deformation stress before and after this aging treatment is 80 MPa or more, and that an increase amount (hereinafter referred to as “&Dgr;TS”; &Dgr;TS=tensile strength after ageing treatment−tensile strength before predeformation) of tensile strength before and after strain aging treatment (the predeformation+the aging treatment described above) is 50 MPa or more.
BACKGROUND ART
Due to recent emission gas restrictions associated with global environmental conservation measures, reduction in body weight of automobiles has become a very important subject. In order to reduce the body weight of automobiles, it is effective to increase the strength of steel sheets which are used in a large quantity, that is, it is effective to decrease the thickness of the steel sheets by using high tensile steel sheets.
However, automobile parts which are formed of thin high tensile steel sheets must fully satisfy its performance imposed thereon in accordance with its role. As the performances mentioned above, for example, static strengths against bending or resilient deformation, fatigue resistance, or crash resistance properties may be mentioned. Accordingly, high tensile steel sheets used for automobile parts must also have superior properties after press forming and fabrication.
In addition, press forming is performed for steel sheets in a process for manufacturing automobile parts, and when press forming is performed for a steel sheet having an excessively high strength, a problem may arise in that,
(1) shape freezing properties are degraded, or
(2) defects such as cracking or necking occur during press forming due to a decrease in ductility.
As a result, high tensile steel sheets have not been widely used for automobile bodies.
In order to overcome the problems described above, for example, as a cold-rolled steel sheet for forming exterior panels, a steel sheet which is formed of an extremely low carbon steel sheet and which finally contains carbon in a solid-solution state at a concentration in an appropriately controlled range has been well known. This type of steel sheet maintains its softness during press forming and also ensures the shape-freezing properties and ductility, and in addition, this steel sheet is formed to ensure dent resistance by an increase in yield stress using a strain age-hardening phenomenon which occurs during a paint baking step performed at approximately 170° C. for 20 minutes after press forming. This type of steel sheet has the softness since carbon is solute therein during press forming, and during a paint baking step performed after the press forming, the solute carbon is fixed at dislocations formed during the press forming, whereby the yield stress is increased.
However, in this type of steel sheet, in order to prevent the generation of stretcher strain which may form surface defects, the increase of yield stress caused by strain age-hardening is suppressed to a lower level. Accordingly, the reduction in weight of parts is not significant in practice.
That is, the reduction in weight of parts cannot be sufficiently performed only by the increase in yield stress caused by strain ageing, and an increase in strength properties is necessary when deformation further occurs. In other words, an increase in tensile strength after strain aging is necessary.
On the other hand, in applications in which the appearances are not so important, a steel sheet containing a further increased bake hardening amount using solute N or a steel sheet having further improved bake hardening property by forming a composite structure composed of ferrite and martensite has been proposed.
For example, Japanese Unexamined Patent Application Publication No. 60-52528 disclosed a method for manufacturing a high-tensile thin steel sheet having superior ductility and spot weldability by the steps of performing hot rolling of a steel containing 0.02% to 0.15% of C, 0.8% to 3.5% of Mn, 0.02% to 0.15% of P, 0.10% or less of Al, and 0.005% to 0.025% of N at a temperature of 550° C. or less, and performing annealing after cold rolling by controlled cooling heat treatment. A steel sheet manufactured by a method disclosed in Japanese Unexamined Patent Application Publication No. 60-52528 is a steel sheet having a composite structure formed of cold-transformed product phases primarily composed of ferrite and martensite and having superior ductility, and in addition, the steel sheet is formed to obtain a high strength by using strain aging during paint baking caused by N which is intentionally added.
According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 60-52528, the increase in the yield stress (YS) by strain age-hardening is large; however, the increase in the tensile strength (TS) is small. In addition, since the mechanical properties considerably vary, for example, since the increase in the yield stress (YS) considerably varies, the thickness of the steel sheet cannot be reduced to a level at which the current requirement of weight reduction can be satisfied.
In addition, a so-called transformation induced plasticity type steel sheet having a composite structure composed of ferrite, bainite, and residual austenite and having significantly improved ductility has also been proposed.
For example, Japanese Unexamined Patent Application Publication No. 61-217529 disclosed a method for manufacturing a high tensile steel sheet having superior ductility by annealing a steel sheet composed of 0.12% to 0.70% of C, 0.4% to 1.8% of Si, 0.2% to 2.5% of Mn, 0.01% to 0.07% of Al, 0.02% or less of N, and the balance of Fe and unavoidable impurities under controlled continuous annealing conditions. However, the steel sheet produced by the technique described in Japanese Unexamined Patent Application Publication No. 61-217529 improves its ductility by precipitating N in the form of AlN using Al and does not substantially contain an interstitial element such as C or N. Accordingly, the strength is not substantially improved by paint baking treatment which is performed after press forming. Consequently, since the strength of the finished product is extremely low, there has been a problem in that the steel sheet described above cannot be used for an application in which crash resistance properties are strongly required. In addition, the steel sheet produced by the technique described in Japanese Unexamined Patent Application Publication No. 61-217529 contains Si, Mn, or the like at a higher concentration compared to a steel sheet having the same strength, and hence, the paintability and weldability are inferior.
In view of improving safety for passengers, a steel sheet having superior workability and crash resistance properties has been desired. That is, a steel which is soft and has superior workability in press forming and which has yield stress and tensile strength, both increased by heat treatmen
Furukimi Osamu
Sakata Kei
Tosaka Akio
JFE Steel Corporation
Piper Rudnick LLP
Yee Deborah
LandOfFree
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