Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2003-06-18
2004-09-28
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S687000, C428S632000, C427S433000, C427S349000
Reexamination Certificate
active
06797411
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a galvanized steel sheet, a method for manufacturing the same, and a method for press-formed product.
BACKGROUND OF THE INVENTION
Demand of galvanized steel sheets having superior rust-preventive performance increases as thin steel sheets for automobiles, household electric appliances, and building materials. Galvanized steel sheets used for press-forming are required to have an adequate level of surface roughness, or of microscopic roughness profile on the surface thereof because the microscopic roughness increases the retainability of lubrication oil between the work (galvanized steel sheet) and a press-mold, decreases the sliding resistance of the work, and prevents the occurrence of die-galling.
Generally a mean roughness Ra defined in JIS B0601 is adopted as an index of the texture of microscopic roughness on the surface of steel sheet. For galvanized steel sheets used in press-forming, generally the oil-retainability between the work and the mold during the press-forming is assured by regulating the mean roughness Ra within a specified range.
Other indexes, however, such as the maximum height of roughness profile, Rmax, and the ten-point height of roughness profile may also be applied. Alternatively, JP-A-7-136701, (the term “JP-A” referred herein signifies the “Unexamined Japanese patent publication”), defines the sum of the volumes of profile valley portions per unit are as the index, and gives evaluation of excellent press-formability for the index larger than a specified value thereof. In any case, the press-formability cannot be assured unless the surface of target galvanized steel sheet has a certain level of microscopic roughness profile.
Particularly for galvanized steel sheets that have a coating film consisting mainly of &eegr; phase, the film is soft and has low melting point compared with the surface of alloyed hot-dip galvanized steel sheets, so they likely induce adhesion to the press-mold and may degrade the press-formability. Consequently, that type of galvanized steel sheets has to assure increased oil-retainability. With these reasons, that type of galvanized steel sheets are often requested to have relatively large values of height of roughness profile on the surface, or mean roughness Ra, necessary to assure the press-formability compared with alloyed hot-dip galvanized steel sheets.
On the other hand, the galvanized steel sheets used in exterior plates of automobiles and the like are requested to have press-formability and also excellent image clarity after painting. Therefore, to improve only the image clarity after coating, the surface of galvanized steel sheet is finished to a bright face. However, improvement in the press-formability needs to establish a certain level of surface roughness. The two requirements conflict with each other.
The relation between the image sharpness after painting and the microscopic texture of surface of steel sheet is described in, for example, JP-B-6-75728, (the term “JP-B” referred herein signifies the “Examined Japanese patent publication”). According to the disclosure, since the coating film itself acts as a low pass filter to the microscopic roughness profile on the surface of steel sheet, the short-period roughness profile is covered by the coating film, thus the short-period roughness profile does not give influence on the image sharpness after coating. On the other hand, the long-period roughness profile portions having wavelengths of several hundreds of micrometers or larger are not covered by the coating, thus degrading the image sharpness.
A countermeasures to the phenomenon is to regulate the filtered centerline waviness Wca which is an index for expressing the microscopic roughness profile on the surface of steel sheet before coating to not exceed a certain level, thus improving the image sharpness after coating. The term “filtered centerline waviness Wca” is a parameter that is defined by JIS B0610, and represents the mean height of roughness profile on the surface after treated by high-pass cut-off.
Other than the filtered centerline waviness Wca, peak count PPI is applied as an index of influence on the image sharpness after coating. As specified in SAE 911 Standard, the peak count PPI is the number of peaks of roughness profile per one inch length. Large peak count means the large number of short-period roughness profile in the microscopic roughness profile on the surface, or, when compared on the same mean roughness Ra, the long-period wave length components are relatively decreased. That is, if the mean roughness Ra is the same, larger peak count PPI should give superior image sharpness after coating.
Consequently, the galvanized steel sheets for press-forming use need to have a surface roughness with a certain level of microscopic roughness profile, and, when the image sharpness after coating is required, the long-period components are necessary to be decreased. In particular, different from the alloyed hot-dip galvanized steel sheets that form microscopic roughness profile on the surface thereof during alloying stage, the galvanized steel sheets that have a coating film consisting mainly of &eegr; phase give smooth surface thereof after coating, so there is a strong need of giving surface roughness by some means.
Temper rolling is applied as a means to give microscopic roughness profile on the surface of galvanized steel sheets used for press-forming. The temper rolling is a means that uses a rolling roll having microscopic roughness profile on the surface thereof, and that applies a plastic extension in an approximate range of from 0.5 to 2.0% to the steel sheet, thus inducing a pressure on the roll byte to transfer the roughness profile on the surface of the rolling roll to the surface of the steel sheet. Therefore, the texture of microscopic roughness profile formed on the surface of galvanized steel sheet depends on the texture of roughness profile on the surface of the rolling roll.
The applicable method to form microscopic roughness profile on the surface of temper rolling roll includes shot-blasting, electrical discharge machining, laser beam machining, and electron beam machining. For example, JP-A-7-136701 and JP-B-6-75728 disclose a method that uses a temper rolling roll finished by laser dull treatment, and JP-A-11-302816 discloses a method that uses a temper rolling roll finished on the surface thereof by electron beam machining.
Zinmnik et al. (Stahl und Eisen, Vol.118, No.3, pp.75-80, 1998) reports a method to increase the peak count PPI on the surface of steel sheet using a temper rolling roll, which method is called the “Pretex process”. According to the report, hard metallic chromium is electrically deposited to form microscopic roughness profile on the surface of rolling roll. Zinmnik et al. describe that the method can create short pitch and dense roughness profile compared with the rolling-roll surface machining by shot-blasting.
According to the report, a rolling roll with shot-blast finish creates around 120 of peak count PPI on the surface of steel sheet, and the Pretex process can increase the peak count PPI to around 230. The threshold of the peak count PPI given in the report is ±0.5 &mgr;m, (in contrast, the threshold of the peak count PPI referred in the descriptions is ±0.635 &mgr;m).
The related art applying temper rolling, which is used as a method to provide a certain level of surface roughness on the surface of galvanized steel sheet for press-forming, has problems described below.
First, the degree of transferring the microscopic roughness profile on the surface of a rolling roll by the temper rolling onto the surface of a galvanized steel sheet has a limitation. Thus, even when the surface of rolling roll has fine roughness profile, all of the profile cannot be transferred onto the surface of steel sheet, and the peak count PPI on the surface of the steel sheet cannot be increased.
The temper rolling transfers the microscopic roughness profile on the surface of rolling roll onto the surface of
Ando Satoru
Inagaki Jun-ichi
Kimura Yukio
Noro Hisato
Sato Kaoru
Frishauf Holtz Goodman & Chick P.C.
NKK Corporation
Zimmerman John J.
LandOfFree
Galvanized steel sheet, method for manufacturing the same,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Galvanized steel sheet, method for manufacturing the same,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Galvanized steel sheet, method for manufacturing the same,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3227848