Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-09-26
2003-08-26
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S530000, C148S531000, C148S533000, C427S398100, C427S398400, C427S398500, C427S402000, C427S405000, C427S406000, C427S409000, C427S419100, C427S419200, C427S433000, C427S436000, C428S621000, C428S622000, C428S623000, C428S626000, C428S628000, C428S629000, C428S632000, C428S659000, C428S457000, C428S472100, C428S472200, C428S933000, C428S939000
Reexamination Certificate
active
06610422
ABSTRACT:
1. Field of the Invention
The present invention relates to a passivated and/or painted steel sheet using a coated steel sheet having a hot-dip Al—Zn base coating layer containing 20 to 95 mass % Al, and a method for manufacturing the same.
2. Description of the Related Arts
Steel sheets coated by a hot-dip Al—Zn base coating layer containing 20 to 90 mass % Al give superior corrosion resistance, as described in JP-B-46-7161, (the term “JP-B” referred herein signifies the “Examined Japanese patent publication”), to hot-dip galvanized steel sheets. Owing to the advantageous property, the coated steel sheets increase in demand in recent years centering on the building materials.
For manufacturing that type of coated steel sheet, a hot-rolled steel sheet is pickled and descaled, or further is cold-rolled to prepare a substrate steel sheet. Thus prepared substrate steel sheet is introduced to a continuous hot-dip coating apparatus, where the following-given treatment is applied thereto.
First, the substrate steel sheet enters an annealing furnace which is kept to a reducing atmosphere, where the steel sheet is heated to a specified temperature to undergo annealing treatment. During the course of annealing, rolling oil or the like attached to the surface of the substrate steel sheet is removed, and oxide film formed thereon is reduced and removed. After that, the substrate steel sheet passes through a snout immersed at the bottom thereof in a coating bath, and enters a hot-dip galvanizing bath containing a specified amount of aluminum. The substrate steel sheet which is immersed in the coating bath is then pulled up therefrom via a sink roll, and the coating weight on the substrate steel sheet is adjusted by injecting a pressurized gas from gas-wiping nozzles, arranged above the coating bath, against the surface of the coated steel sheet. Then, the coated steel sheet is cooled in a cooling unit to obtain the hot-dip Al—Zn base coated steel sheet having a specified coating layer thereon.
For assuring specified quality and material properties of coating layer, the continuous hot-dip coating apparatus is precisely controlled within a predetermined control range, in terms of heat treatment condition and atmospheric condition of the annealing furnace, and operating conditions such as composition of coating bath liquor and cooling rate after coated.
The coating layer of thus prepared coated steel sheet has a portion of dendrites of Al which contains mainly Zn to a supersaturation level and a balance portion of gaps between dendrites, which dendrites layer in the direction of coating layer thickness. Owing to the characteristic film structure, the hot-dip Al—Zn base coated steel sheet gives excellent corrosion resistance.
The coating bath normally contains Si to about 1.5 mass %. The Si functions to suppress the growth of alloy phase at interface between the coating layer and the substrate steel sheet, thus the hot-dip Al—Zn base coated steel sheet has around 1 to 2 &igr;m of the alloy phase thickness. Since thinner alloy phase gives more increased portion of the characteristic film structure which provides excellent corrosion resistance, the suppression of growth of the alloy phase contributes to the improved corrosion resistance. The alloy phase is harder than the coating layer and functions as the origin of crack generation during working, so the suppression of growth of alloy phase reduces the crack generation and improves the workability. Since the cracked portion has low resistance to corrosion because of the exposure of substrate steel sheet, the reduction in crack generation also improves the corrosion resistance at worked portion.
The coating bath generally contains inevitable impurities, Fe eluted from steel sheet and from equipment in the bath, and Si added to the bath for suppressing the growth of alloy phase. Other elements than those given above may be added to the coating bath. Those elements exist in the alloy phase and in the coating layer in a form of alloy or single elements.
Practical applications of the hot-dip Al—Zn base coated steel sheets in as-coated state are very rare. These steel sheets are normally treated before use further by passivation or painting on the surface thereof to prepare surface-treated steel sheets.
When a hot-dip Al—Zn base coated steel sheet is worked by bending or the like, it may generate cracks on the coating layer at the worked portion depending on the degree of the working. On this type of coated steel sheet, the alloy phase having about 1 &igr;m to about 2 &igr;m thickness, existing at the interface between the coating layer and the substrate steel sheet, becomes the origin of the cracks, and the dendrite gaps in the coating layer provide the crack propagation route. Accordingly, compared with a hot-dip galvanized steel sheet having the same coating layer thickness and being subjected to the same degree of working, the hot-dip Al—Zn base coated steel sheet likely gives relatively large crack opening. As a result, there occurs a problem of visible crack generation to degrade the appearance of the steel sheet, depending on the degree of working. Although the hot-dip Al—Zn base coated steel sheet has superior corrosion resistance to the hot-dip galvanized steel sheet having the same coating layer thickness therewith, as described above, the hot-dip Al—Zn base coated steel sheet has a drawback in significant reduction of corrosion resistance at the crack-generated portion, where the substrate steel sheet exposes, compared with the portion of no crack generation.
Countermeasures to these problems have been proposed. For example, JP-B-61-28748 discloses a method for improving the ductility of a coated steel sheet by applying a specific heat treatment to a hot-dip Al—Zn base coated steel sheet. Solely that kind of heat treatment in related art, however, fails to sufficiently improve the ductility of the coating layer.
As described above, the hot-dip Al—Zn base coated steel sheets are normally used as the passivation-treated steel sheets which are prepared by applying passivation treatment to the surface thereof or as the coated steel sheets which are prepared by applying coating thereon. From the point of preventing the crack generation at a portion of work such as bending, sole improvement in the ductility of the coating layer to some extent, as disclosed in related art, not necessarily improves directly the performance of the products, or the performance of workability and the corrosion resistance of the worked portion on the surface-treated steel sheet after subjected to passivation treatment or coating, to a practically applicable level.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a coated steel sheet having excellent workability and corrosion resistance at worked portion, and to provide a method for manufacturing same.
To attain the object, the present invention provides a method for manufacturing a coated steel sheet, comprising the steps of: forming an Al—Zn base coating layer containing 20 to 95 mass % Al on a steel sheet by immersing thereof in a hot-dip coating bath; and forming a passivated layer on the coating layer. The method for manufacturing the coated steel sheet includes the step of applying a thermal history to the coating layer.
The step of applying thermal history has the steps of: applying a first thermal history of less than 11° C./sec of average cooling rate during the first 10 seconds after the steel sheet left the hot-dip coating bath; and applying a second thermal history of 0.5×(T−100)(° C./hr) or less of average cooling rate thereto in a temperature range of from T(° C.) between 130° C. and 300° C. to 100° C.
The step of applying the second thermal history is preferably in the following:
(1) The average cooling rate of the coating layer in a temperature range of from T(° C.) between 130° C. and 300° C. to 100° C., after solidification of a hot-dip coated metal, is 0.5×(T−100)(° C./hr) or less.
(2) After solidification of the hot-dip coated metal, the heating i
Fukushima Yuichi
Hori Shinji
Inagaki Jun-ichi
Inoue Norio
Ishida Nobuyuki
Koehler Robert R.
NKK Corporation
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