Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-01-24
2002-07-30
King, Roy (Department: 1742)
Metal treatment
Stock
Ferrous
C420S128000
Reexamination Certificate
active
06425963
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to hot-rolled steel sheet suitable for use in structural components, suspension components, etc. for automobiles, and more particularly to hot-rolled steel sheet having improved bake hardenability and fatigue resistance, crash resistance, and resistance to room temperature aging. The expression “improvement in bake hardenability” refers to increase in yield strength as well as in tensile strength after forming and paint baking.
2. Description of the Related Art
For automobiles increase in strength per unit weight has been required in order to increase gas mileage by reducing weight. However, the increase in strength of steel sheet makes it difficult to perform press forming. For passenger safety, improvement in crash resistance, that is evaluated by the amount of absorbed energy at high strain rates, such as at a time of collision, has also been desired.
In order to increase strength while preventing deterioration in press formability, techniques utilizing so-called “bake hardenability” (hereinafter referred to as “BH”) have been known, in which the strength is relatively low during forming so that working is easily performed and the strength is increased by paint baking, for example, as disclosed in Japanese Unexamined Patent Publication Nos. 6-73498 and 7-268544. The techniques have been widely used for cold-rolled steel sheets. However, with respect to the improvement in bake hardenability obtained by the above techniques, only yield strength is increased and tensile strength is not increased. Thus, although the dent resistance in outer panel for automobiles is effectively improved, the fatigue resistance and crash resistance required for inter panels are not improved.
On the other hand, Japanese Unexamined Patent Publication No. 1-180917 discloses a method for producing a hot-rolled steel sheet having excellent workability and bake hardenability, in which a steel containing 0.030% to 0.100% by weight of C, 0.0015% to 0.0150% by weight of N, and 0.025% to 0.100% by weight of Al is heated to 1,200° C. or less, finish-rolling is performed at temperatures from (Ar
3
+30° C.) to 950° C., and quenching is performed at a cooling rate of 30° C./s or more to 500° C. or less within 3 seconds after rolling, followed by coiling at 400 to 500° C. In the technique disclosed in Japanese Unexamined Patent Publication No. 1-180917, quenching is performed after rolling so that the amount of C and N dissolved in the steel sheet is increased, thus improving the BH.
Japanese Unexamined Patent Publication No. 4-74824 discloses a method for producing a hot-rolled steel sheet having excellent bake hardenability and workability, in which a steel containing 0.02% to 0.13% by weight of C, 0.0080% to 0.0250% by weight of N, and 0.10% or less of sol. Al is re-heated to 1,100° C. or more, hot rolling that finishes at temperatures of 850 to 950° C. is performed, and cooling is performed to 350° C. or less at a cooling rate of 15° C./second or more, with or without air cooling being included, followed by coiling.
Japanese Unexamined Patent Publication No. 63-96248 discloses a bake hardenable hot-rolled steel sheet, in which a steel containing 0.010% to 0.025% by weight of C, 0.0015% to 0.0030% by weight of N, 0.01% to 0.05% of Nb, and 0.008% or less of sol. Al, is used, and appropriate amounts of solute C and solute N remain by controlling the coiling temperature after hot rolling. According to the disclosure, the fatigue limit increases after forming and paint baking.
Japanese Unexamined Patent Publication No. 10-183301 discloses a technique with respect to a steel containing 0.01% to 0.12% by weight of C and 0.0001% to 0.01% by weight of N, in which the BH (increase in yield strength by baking treatment) is improved by controlling the cooling rate after hot rolling and the coiling temperature.
However, with respect to hot-rolled steel sheets produced using the technique disclosed in Japanese Unexamined Patent Publication No. 1-180917, the resistance to room temperature aging is deteriorated, which is disadvantageous. Additionally, although yield strength after paint baking is increased, an increase in tensile strength is not achieved at the same time, and thus significant improvements in fatigue resistance and crash resistance are not expected.
Hot-rolled steel sheets produced using the technique disclosed in Japanese Unexamined Patent Publication No. 4-74824 have a multi-phase structure mainly composed of ferrite and martensite, and although tensile strength after forming and paint baking is increased, an improvement in resistance to room temperature aging is not taken into consideration, and the resistance to room temperature aging is deteriorated, which is disadvantageous.
With respect to steel sheets disclosed in Japanese Unexamined Patent Publication No. 63-96248, in comparison with an increase in yield strength, the fatigue limit is not greatly increased, to approximately 25 MPa at most, and fatigue resistance is not substantially increased.
With respect to hot-rolled steel sheets produced using the technique disclosed in Japanese Unexamined Patent Publication No. 10-183301, although yield strength after forming and paint baking is increased, an increase in tensile strength is not achieved. Therefore, fatigue resistance and crash resistance are not substantially improved.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the disadvantages associated with the conventional techniques described above. Specifically, it is an object of the present invention to provide a high tensile strength hot-rolled steel sheet having a tensile strength exceeding about 370 MPa suitable for use in interior materials for automobiles and a method for producing the same, in which bake hardenability, fatigue resistance, crash resistance, and resistance to room temperature aging are improved without excessive addition of dissolved elements.
In one aspect, a high tensile strength hot-rolled steel sheet having excellent bake hardenability, fatigue resistance, crash resistance, and resistance to room temperature aging, in accordance with the present invention, contains about 0.01% to 0.12% by weight of C, 2.0% by weight or less of Si, 0.01% to 3.0% by weight of Mn, 0.2% by weight or less of P, 0.001% to 0.1% by weight of Al, 0.003% to 0.02% by weight of N, and the balance being Fe and incidental impurities. The hot-rolled steel sheet has a structure including a ferrite having an average grain diameter of about 8 &mgr;m or less, or preferably about 6 &mgr;M or less, as a primary phase, and further contains about 0.003% to 0.01% by weight, or preferably about 0.005% to 0.01% by weight of solute N. The ratio of an average concentration Ngb of solute N within a range of ±5 nm from the ferrite grain boundary to an average concentration Ng of solute N in grains, namely, Ngb/Ng, ranges from about 100 to 10,000.
The high tensile strength hot-rolled steel sheet having excellent bake hardenability, fatigue resistance, crash resistance, and resistance to room temperature aging may further contain at least one of about 0.001% to 0.1% by weight of Ti and about 0.001% to 0.1% by weight of Nb and/or at least one element selected from the group consisting of about 0.1% to 1.5% by weight of Ni, about 0.1% to 1.5% by weight of Cr, and about 0.1% to 1.5% by weight of Mo.
In the high tensile strength hot-rolled steel sheet having excellent bake hardenability, fatigue resistance, crash resistance, and resistance to room temperature aging, the structure may be selected from the group consisting of pearlite, bainite, martensite, and retained austenite, or combinations, as a secondary phase.
In the high tensile strength hot-rolled steel sheet having excellent bake hardenability, fatigue resistance, crash resistance, and resistance to room temperature aging, a plated layer may be formed on the surface thereof.
In another aspect, a method for producing a high tensile strength hot-rolled steel sheet having excellent bake harden
Furukimi Osamu
Kaneko Shinjiro
Shimizu Tetsuo
Kawasaki Steel Corporation
King Roy
Schnader Harrison Segal & Lewis LLP
Wilkins, III Harry D.
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