Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-03-29
2003-04-29
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S651000
Reexamination Certificate
active
06554925
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing cold-rolled steel sheet.
2. Description of the Related Arts
Cold-rolled steel sheets are widely used as basic materials for exterior sheets of automobiles and other equipment. Since the major form of the cold-rolled steel sheets for automobiles is press-formed members, various kinds of workability characteristics are required responding to the shapes of the members. In particular, automobile-use requests the cold-rolled steel sheets for press-forming having excellent deep-drawing performance suitable for exterior sheets for automobiles. Recently, the request of automobile manufacturers relating to rationalization becomes severer than ever, particularly in the request for cost reduction of base materials and for improvement in production yield. To cope with these requirements, the material manufacturing faces serious issues of rationalization of manufacturing method, improvement of material quality, and homogeneity of material.
Based on the above-described background, and in view of rationalization of manufacturing method and improvement of material quality, JP-B-60-45692, (the term “JP-B-” referred to herein signifies the “Examined Japanese Patent Publication”), discloses a technology for improving the surface properties and the deep-drawing performance of a steel sheet using a process of continuous casting and direct feeding to rolling by hot-rolling a very low carbon steel slab containing not more than 0.015% C, wherein the hot rolling is begun in a range of temperature of the surface at center of the slab width from 600° C. to less than 900° C., and applying soaking within a period of 30 minutes during the hot-rolling step.
From the point of improvement of material quality, JP-A-5-112831, (the term “JP-A-” referred to herein signifies the “Unexamined Japanese Patent Publication”), discloses a technology for improving the r value by applying a final reduction in thickness during the hot-rolling to 30% or more, and by beginning rapid cooling immediately after the completion of hot-rolling, thus reducing the grain size in the hot-rolled steel sheet.
The above-described prior arts, however, leave a problem on the uniformity of mechanical properties within a coil, though the surface properties and the deep drawing performance of the cold-rolled steel sheet are improved to a relatively favorable level. That is, the technology of JP-B-60-45692 adopts the heating temperature in the hot-rolling step to a low level, or to the ferritic domain. Accordingly, the congregation texture of the steel sheet after the hot-rolling differs in the width direction thereof owing to the temperature distribution in the material width direction during the rolling, (temperature reduction is significant at edges and peripheral zone thereof). As a result, the mechanical properties of the steel sheet in the coil width direction induce dispersion after cold-rolling and annealing.
If the structure and the mechanical properties of the steel sheet in the coil width direction generated dispersion, the workability within a plane of the material becomes non-homogeneous. Particularly when superior deep drawing performance is requested for the exterior sheets of automobiles and other uses, the quality of press-formed steel sheets have variations (such as cracks and wrinkles). Consequently, the automobile manufacturers have to apply blank layout in a coil under a low yield condition, (or to apply blank layout in a non-reasonable direction such as 45 degrees, or the product is not cut from nearby zone to coil edges).
Also in the technology of JP-A-5-112831, the dispersion of material quality can not necessarily be reduced to a satisfactory level. That is, with the range of cooling speed that is a feature of the technology, (according to the examples given in JP-A-5-112831, the average cooling speed in a period of one second from the start of cooling ranges from 90 to 105° C./sec, and the average cooling speed in a period of 3 seconds after the start of cooling ranges from 65 to 80° C./sec), the time until the start of cooling becomes long under the commercial hot-rolling conditions because particularly the cooling speed at top section of the rolling is slow, which allows the enhancement of coarse grain formation owing to the austenitic grain growth. Consequently, it was found that these sections are not necessarily able to prepare fine grains in the hot-rolled steel sheet.
In addition, the cooling immediately after the hot-rolling, which is a feature of the technology, is difficult to be actualized on commercial facilities because of the structural limitation thereof. That is, instruments have to be installed so that the cooling unit cannot be positioned directly next to the exit of the final stand of the finish rolling mill. Therefore, to bring the time to start cooling after completed the hot-rolling to 0.1 second or less is substantially difficult. Furthermore, since the technology adopts a large reduction in thickness, 30% or more, at the final stand of the finish rolling mill, the travel of steel sheet becomes unsteady and likely induces bad sheet shapes. With the bad shapes of hot-rolled coil sheet, users have a problem of unable to perform press-forming at a high yield.
As described above, practical application of the technology of JP-A-5-112831 has many issues yet to be solved.
In this regard, an object of the present invention is to provide a method for manufacturing cold-rolled steel sheet for deep drawing, which method solves the above-described problems of prior art, and allows to manufacture cold-rolled steel sheets suitable for the uses as exterior sheets for automobiles and other uses, giving superior press-formability with less variations in press-formability within a coil, on an industrially stable basis.
Another object of the present invention is to provide a method for manufacturing cold-rolled steel sheet for deep drawing, which method allows to manufacture cold-rolled steel sheets having superior sheet shape adding to the advantages described above, on an industrially stable basis.
As for the cold-rolled steel sheet and the surface-treated steel sheet, which are required to have good workability, they need to have mechanical properties of superior elongation and deep drawing performance, and less anisotropic property. The shape of steel sheet and the transferability of the hot-rolled steel strip during manufacturing process are also important variables to manufacture that kind of steel sheet.
According to prior art, mildness and high ductility are gained in very low carbon and nitrogen base compositions by adding elements to form carbide and elements to form nitride, such as Ti and Nb. The concept is based on that the interstitial elements such as carbon and nitrogen are eliminated as far as possible during the steel making stage, and that the interstitial elements at a level being left non-eliminated or the interstitial element at a level that cannot be eliminated on an economical basis are fixed as precipitates, thus rejecting the presence of interstitial elements in the steel.
With the increasing severity in requirements for workability, however, sole composition adjustment cannot anymore provide steel sheets that satisfy the requirements, and the manufacturing process is requested to contribute to further improvement of the material quality. It is known that, in concept, the effective use of the cooling technology improves the mechanical properties of steel sheets after cooling and annealing by reducing the grain size in the hot-rolled steel sheets. The procedure is to simultaneously apply the following-given two steps to reduce the grin size in the hot-rolled steel sheets: (1) to shorten the time between the completion of the hot-rolling and the start of the cooling step, (hereinafter referred to as the “time to start cooling”), and (2) to increase the cooling speed as far as possible.
The basis of the technology is the following. For the step (1), since the strain which is ind
Imada Sadanori
Inazumi Toru
Inoue Tadashi
Ishiguro Yasuhide
Motoyashiki Yoichi
Frishauf, Holts, Goodman & Chick, P.C.
NKK Corporation
Yee Deborah
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