Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-06-06
2004-11-16
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S320000, C148S602000, C148S654000, C148S661000
Reexamination Certificate
active
06818074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates mainly to steel sheets for automobiles, and more particularly, to high-ductility steel sheets having very high strain age hardenability and excellent press formability such as ductility, stretch-flanging formability, and drawability, in which the tensile strength increases remarkably through a heat treatment after press forming, and to methods for manufacturing the same. The term “steel sheets” as herein used shall include hot-rolled steel sheets, cold-rolled steel sheets, and hot-dip galvanized steel sheets. The term “steel sheets” as herein used shall also include steel sheets and steel strips.
2. Description of the Related Art
In recent years, weight reduction in automobile bodies has become a very important issue in relation to emission gas control for the purpose of preserving global environments. More recently, efforts are made to achieve higher strength of automotive steel sheets and to reduce steel sheet thickness in order to reduce the weights of automobile bodies.
Because most of the body parts of automobiles made of steel sheets are formed by press working, steel sheets used must have excellent press formability. In order to achieve excellent press formability, it is necessary to ensure high ductility. Stretch flanging is frequently applied, so that the steel sheets to be used must have a high hole-expanding ratio. In general, however, a higher strength of steel sheet tends to result in a lower ductility and a lower hole-expanding ratio, thus leading to poor press formability. As a result, there has conventionally been an increasing demand for high-strength steel sheets having high ductility and excellent press formability.
Importance is now placed on safety of an automobile body to protect a driver and passengers upon collision, and for this purpose, steel sheets must have improved impact resistance as a standard of safety upon collision. For the purpose of improving the crashworthiness, a higher strength in a completed automobile is more favorable. There has therefore been the strongest demand for steel sheets having low strength, high ductility, and excellent press formability upon forming automobile parts, and having high strength and excellent crashworthiness in completed products.
To satisfy such a demand, a steel sheet high both in press formability and strength was developed. This is a bake hardenable type steel sheet of which the yield stress increases by applying a bake treatment including holding at a high temperature of 100 to 200° C. after press forming. In this steel sheet, the C content remaining finally in a solid solution state (solute C content) is controlled within an appropriate range so as to keep the softness, shape fixability, and ductility during press forming. In a bake treatment performed after the press forming of this steel sheet, the solute C is fixed to a dislocation introduced during the press forming and inhibits the movement of the dislocation, thus resulting in an increase in yield stress. In this bake hardenable type automotive steel sheet, the yield stress can be increased, but the tensile strength cannot be increased.
Japanese Examined Patent Application Publication No. 5-24979 discloses a bake hardenable high-strength cold-rolled steel sheet having a composition comprising C: 0.08 to 0.20%, Mn: 1.5 to 3.5% and the balance Fe and incidental impurities, and having a structure composed of uniform bainite containing not more than 5% of ferrite or composed of bainite partially containing martensite. The cold-rolled steel sheet disclosed in Japanese Examined Patent Publication No. 5-24979 is manufactured by rapidly cooling the steel sheet to a temperature in the range of 400 to 200° C. in the cooling step after continuous annealing and then slowly cooling the same. A high degree of baking hardening conventionally unavailable is thereby achieved through conversion from the conventional structure mainly comprising ferrite to a structure mainly comprising bainite in the steel sheet.
In the steel sheet disclosed in Japanese Examined Patent Application Publication No. 5-24979, a high degree of baking hardening conventionally unavailable is obtained through an increase in yield strength after bake treatment. Even in this steel sheet, however, it is yet difficult to increase tensile strength after the bake treatment, and an improvement in crashworthiness cannot still be achieved.
On the other hand, some hot-rolled steel sheets are proposed with a view to increasing not only yield stress but also tensile strength by applying a heat treatment after press forming.
For example, Japanese Examined Patent Application Publication No. 8-23048 proposes a method for manufacturing a hot-rolled steel sheet comprising the steps of reheating a steel containing C: 0.02 to 0.13%, Si: not more than 2.0%, Mn: 0.6 to 2.5%, sol. Al: not more than 0.10%, and N: 0.0080 to 0.0250% to a temperature of not less than 1,100° C. and applying hot finish rolling at a temperature of 850 to 950° C. The method also comprising the steps of cooling the hot-rolled steel sheet at a cooling rate of not less than 15° C./second to a temperature of less than 150° C., and coiling the same, thereby forming a composite structure mainly comprising ferrite and martensite. In the steel sheet manufactured by the technique disclosed in Japanese Examined Patent Application Publication No. 8-23048, the tensile strength and the yield stress increase by strain age hardening; however, a serious problem is posed in that coiling of the steel sheet at a very low coiling temperature as less than 150° C. results in large variations in mechanical properties. Another problem includes a large variation in increment of yield stress after press forming and bake treatments, as well as poor press formability due to a low hole-expanding ratio (&lgr;) and decreased stretch-flanging workability.
Japanese Unexamined Patent Application Publication No. 11-199975 proposes a hot-rolled steel sheet for working excellent in fatigue characteristics, containing C: 0.03 to 0.20%, appropriate amounts of Si, Mn, P, S and Al, Cu: 0.2 to 2.0%, and B: 0.0002 to 0.002%, of which the microstructure is a composite structure comprising ferrite as a primary phase and martensite as a second phase, and the ferrite phase contains Cu in a solid-solution and/or precipitation state of not more than 2 nm. The steel sheet disclosed in Japanese Unexamined Patent Application Publication No. 11-199975 has an object based on the fact that the fatigue limit ratio is remarkably improved only when Cu and B are added in combination, and Cu is present in an ultra fine state not more than 2 nm. For this purpose, it is essential to complete hot finish rolling at a temperature above the A
r3
transformation point, air-cool the sheet within the temperature region of A
r3
to A
r1
for 1 to 10 seconds, cool the sheet at a cooling rate of not less than 20° C./second, and coil the cooled sheet at a temperature of not more than 350° C. A low coiling temperature of not more than 350° C. causes serious deformation of the shape of the hot-rolled steel sheet, thus inhibiting industrially stable manufacture.
On the other hand, some automobile parts must have high corrosion resistance. A hot-dip galvanized steel sheet is suitable as a material applied to portions requiring high corrosion resistance. For this reason, a particular demand exists for hot-dip galvanized steel sheets excellent in press formability during forming, and is considerably hardened by a heat treatment after the forming.
To respond to such a demand, for example, Japanese Patent Publication No. 2802513 proposes a method for manufacturing a hot-dip galvanized steel sheet using a hot-rolled steel sheet as a black plate. The method comprises the steps of hot-rolling a steel slab containing C: not more than 0.05%, Mn: 0.05 to 0.5%, Al: not more than 0.1% and Cu: 0.8 to 2.0% at a coiling temperature of not more than 530° C. The method further comprising the subsequent steps of reducing the steel sheet surface by
Furukimi Osamu
Matsuoka Saiji
Sakata Kei
Shimizu Tetsuo
JFE Steel Corporation
Piper Rudnick LLP
Yee Deborah
LandOfFree
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