Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-04-04
2004-03-16
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S533000, C148S602000, C148S648000, C148S650000, C148S651000, C148S654000, C148S661000, C148S320000, C420S008000, C420S128000, C428S939000
Reexamination Certificate
active
06706419
ABSTRACT:
TECHNICAL FIELD
This invention relates to a steel sheet having painting bake hardenability (BH), anti aging property at room temperature and formability at the same time, and a method of producing the steel sheet.
The letters BH are an abbreviation of bake hardenability or bake hardening and it means a simplified evaluation, by means of a tensile test, of the increase in the mechanical strength of a steel sheet resulting from the baking of a painting after press forming in car manufacturing. BH is measured as follows: first, the flow stress of a steel sheet is measured under a 2% tensile deformation imposed at a tensile test; then, after a prescribed heat treatment (usually, at 170° C. for 20 min., but heat treatments at 150° C. and 160° C. are also included in the present invention), the upper yield stress of the steel sheet is measured in another tensile test; suppose the flow stress at the first tensile test under the 2% tensile deformation is &sgr;1 and the upper yield stress at the second tensile test is &sgr;2, the amount of EH is given as &sgr;2-&sgr;1. Note that, when there is no upper yield point, the 0.2% proof stress of the steel sheet is used.
A steel sheet according to the present invention is used for cars, home electric appliances, buildings, etc. and it includes both a cold-rolled or hot-rolled steel sheet in the narrow sense of the word without surface treatment and a cold-rolled or hot-rolled steel sheet in the broad sense of the word with surface treatment such as alloying hot dip galvanizing, electrolytic plating, etc. as an anti-corrosion measure.
BACKGROUND ART
The production of ultra low carbon steels has been made easier thanks to the latest technical advancement of the vacuum degassing treatment of molten steel, and the demands for ultra low carbon steels having excellent workability has been increasing. Among this kind of product, the ultra low carbon steel sheets containing Ti and Nb added in combination disclosed in Japanese Unexamined Patent Publication No. S59-31827 and the like, for example, have painting bake hardenability (BH) as well as extremely good workability, and are excellent also in hot dip galvanizing property. For this reason, these steel sheets have come to claim a significant position in the market.
The amount of BH of the steel sheets, however, is not beyond the level of those of conventional BH steel sheets, and they have a shortcoming that, when it is attempted to increase the amount of BH of the steel sheets, it becomes impossible to maintain their anti aging property at room temperature.
A steel sheet having an enhanced BH is excellent in workability thanks to its low strength at the stage of press forming, and is also excellent especially in dent resistance owing to the fact that it becomes hard after it is finally formed into the shape of a product component. Generally speaking, when the amount of solute C or solute N in steel is increased, the amount of BH is increased but, on the other hand, anti aging property at room temperature poses a problem.
As an example of the technology related to a steel sheet having both high bake hardenability and anti aging property at room temperature, Japanese Examined Patent Publication No. H3-2224 proposes a technology to obtain a cold-rolled steel sheet having a high r-value, high bake hardenability, good ductility and anti aging property at room temperature at the same time, by adding a large amount of Nb, B and Ti, together, to an ultra low carbon steel so as to make the annealed structure of the steel a composite structure consisting of a ferrite phase and a phase formed through low temperature transformation.
It has been made clear, however, that the proposed technology has problems related to actual production operation as described in 1) and 2) below.
1) In a steel having a composition comprising a large amount of Nb, B and Ti, the transformation point where the steel transforms from &agr; phase to &ggr; phase does not fall and thus annealing at a very high temperature is required for obtaining a composite structure, which in turn causes troubles such as strip breakage during continuous annealing.
2) Since the temperature range where the steel has an &agr;+&ggr; phase is very narrow, there arises the case that the structure varies along the width of the steel sheet resulting in the large dispersion of product quality or that the structure may or may not become a composite structure depending on the fluctuation of annealing temperature by several degrees Celsius. Therefore, the production tends to be very unstable.
As another example, Japanese Unexamined Patent Publication No. H7-300623 teaches that it is possible to obtain both a high BE value and anti aging property at room temperature, by increasing the carbon concentration at crystal grain boundaries of an ultra low carbon cold-rolled steel sheet containing Nb through controlling the cooling rate after annealing. The technology disclosed therein, however, does not realize a high BH value and anti aging property at room temperature in a sufficiently well-balanced manner.
There is another problem in conventional BH steel sheets that, whereas a prescribed amount of BH is obtained as far as the heat treatment of BH is conducted under a condition of 170° C. for 20 min., the amount of BH is lowered under a heat treatment condition of 160° C. for 10 min. or 150° C. for 10 min.
As described above, conventional BH steel sheets have shortcomings that stable production is difficult and that anti aging property at room temperature is lost when the amount of BH is increased Further, they have another problem in that a sufficient amount of BH is not obtained when the temperature at the baking of a painting is lowered from currently adopted 170° C. to 160 or 150° C.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide a steel sheet having both high bake hardenability and anti aging property at room temperature and capable of maintaining a sufficient amount of BH even under a low BH temperature, and a method of producing the steel sheet.
As a result of assiduous studies for achieving the above object, the present inventors obtained the following finding which was hitherto unknown.
That is to say, the present inventors discovered that it was possible, by adding Cr, MO, V and so forth to a steel retaining solute N, to obtain both a high BH value and anti aging property at room temperature and maintain high bake hardenability even when the baking of a painting was conducted at a lower temperature for a shorter period of time.
The present invention is a totally new steel sheet which was hitherto unknown to the market, worked out on the basis of the philosophy and findings described above, and a method of producing the steel sheet. The gist, therefore, is as follows:
(1) A cold-rolled steel sheet excellent in painting bake hardenability and anti aging property at room temperature characterized by: containing, in mass, 0.0001 to 0.20% of C, 2.0% or less of Si, 3.0% or less of Mn, 0.15% or less of P, 0.015% or less of S and, in addition, 0.10% or less of Al and 0.001 to 0.10% of N so as to satisfy the expression 0.52Al/N<5 and, further, one or more of 2.5% or less of Cr, 1.0% or less of Mo and 0.1% or less of V so as to satisfy the expression (Cr+3.5Mo +39V)≧0.1, with the balance consisting of Fe and unavoidable impurities; having the value of BE170, which is evaluated after applying a 2% tensile deformation and then a heat treatment at 170° C. for 20 min., equal to or more than 45 MPa, and any of the value of BH160, which is evaluated after applying a 2% tensile deformation and then a heat treatment at 160° C. for 10 min., and the value of BH150, which is evaluated after applying a 2% tensile deformation and then a heat treatment at 150° C. for 10 min., equal to or more than 35 MPa; and having the yield point elongation at a tensile test after applying a heat treatment at 100° C. for 1 h. equal to or less than 0.6%.
(2) A hot-rolled steel sheet excellent in painting bake hardenability and anti aging proper
Miyasaka Akihiro
Sugiura Natsuko
Sugiyama Masaaki
Takahashi Manabu
Yoshinaga Naoki
Kenyon & Kenyon
Koehler Robert R.
Nippon Steel Corporation
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