1. Field of Invention
The present invention relates to can steel sheet and can steel strip and, particularly, to a can steel sheet and can steel strip having uniform material quality in both the width and length directions even in extremely thin and wide steel sheet and steel strip. The present invention also relates to a method of producing the can steel sheet and steel strip.
In the present invention, the can steel sheet and steel strip include surface-treated plates, such as by Sn plating, Ni plating, Cr plating and the like.
2. Description of the Related Art
A surface-treated steel sheet for cans is produced by the surface treatment of a plate by Sn, Ni or Cr plating or the like as a tin plate having a Sn deposit of 2.8 g/m.sup.2 or more, or a lightly tin coated steel sheet having a Sn deposit of 2.8 g/m.sup.2 or less, and is used for drink cans, food cans, etc.
Such can steel sheets are classified by their temper grade, which is represented by a target value of Rockwell T hardness (HR30T), so that single-rolled products are divided into T1 to T6, and double-rolled products are divided into DR8 to DR10.
In recent years, a further improvement in productivity of steel-fabricating process has been considered as a main object of can makers with increases in the consumption of drink cans. At the same time, activities for resources saving and cost reduction have also be continued. Therefore, it has recently been greatly demanded to provide can steel sheets satisfying these requirements of the can makers. Namely, a measure for improving productivity is an increase in the speed of the steel-fabricating work, and thus a steel sheet that causes no problems in high-speed steel fabrication is demanded.
Such a steel sheet must have hardness precision, dimensional precision of the steel sheet size including thickness, flatness, lateral bending precision, etc., all of which must be controlled more strictly than steel sheets for other use such as automobile steel sheets. For example, printing shift is affected by the flatness of a steel sheet, and the flatness is significantly affected by nonuniformity of material quality.
A rational steel-fabrication method has recently been established, in which a steel sheet is used over its entire width except for several millimeters of its ends in the width direction. From this point, it is necessary for a can steel strip to have uniform material quality and thickness over a whole coil.
In addition to the use of the steel sheet over its entire width, as a measure for resources saving and cost reduction, the weight of a can is decreased. Cans such as three-piece cans and two-piece cans can also be produced by using a thin steel sheet due to the recent progress in steel-fabrication technology, thereby tending to decrease the weight of a can.
With a thin steel sheet, the strength of a can is inevitably decreased. Therefore, the shape of a can is changed by necking in, and the strength of a can is improved by applying deep drawing, ironing, stretching, bulging, dome forming of the bottom, or the like after coating and baking. Recently, there has been a demand for a can steel thin sheet having excellent steel-fabrication workability and deep drawability.
Of course, it is demanded that these workabilities are uniform over a whole coil.
In order to improve the productivity of the steel-fabrication process with the recent progress in steel-fabrication technology, the width of a can steel strip, and the weight of a coil are increased, leading to production and supply of a steel strip having a width of 4 feet (about 1220 mm) or more, or a steel strip coil having a weight of 10 tons or more.
As described above, from the viewpoints of productivity, resources saving and cost reduction, it is necessary to supply a raw material used as a can steel sheet in the form of a steel strip coil having a small thickness, a large width and a heavy weight. It is also necessary that the material have high workability and uniformity in material quality in the width and length directions.
However, by conventional techniques, it is difficult to produce a thin and wide steel strip having uniform material quality over the entire width of a steel sheet, and the dimensions of a steel strip that can be produced practically include a thickness and a width both of which are limited to about 0.20 mm and 950 mm, respectively, from the viewpoint of passing ability of continuous annealing.
Even in the production of a steel strip having a width larger than 950 mm, it is difficult to obtain substantially uniform thickness and material quality over at least 95% of the whole width.
In order to comply with these requirements, Japanese Unexamined Patent Publication No. 9-327702 proposes a technique for producing a thin steel sheet by hot rolling, including cross-direction edge heating of a sheet bar using an edge heater, and pair cross rolling.
However, the method disclosed in the above Japanese Unexamined Patent Publication No. 9-327702 achieves uniform hardness in a steel strip and improves thickness precision and flatness, but causes the phenomenon that .DELTA.r representing planar anisotropy of r value is high at both ends of the steel strip in the length direction, thereby causing the problem of reducing yield of the front and rear ends of the steel strip.
This .DELTA.r is an important index for application to, particularly, two-piece cans.
Namely, in general, pressing of a tin plate does not require a high r value because a surface tin layer has a lubricating function during pressing. However, high planar anisotropy .DELTA.r causes significant earring, and thus a necessary can height cannot be obtained, thereby causing the need to increase the disk diameter of the plate to be pressed. This is uneconomical due to deterioration in yield. Also, a can body has nonuniformity in thickness, causing damage to the wall surface of the can body due to galling, deterioration in precision of the can diameter, deterioration in can strength, etc.
Furthermore, a high .DELTA.r value readily causes wrinkles in the upper portion of the can body, and readily causes wrinkles due to circumferential buckling in necking in. Therefore, coating adhesion and film adhesion deteriorate, and thus a rate of necking in cannot be increased, causing difficulties in decreasing the diameter of a can cover, and increasing the can strength. Also, the ear becomes a knife edge under high pressure in drawing, and the resultant iron pieces adhere to the mold and cause the problem of damaging the can surface, and various other problems. Although the progress in two-piece can steel-fabrication technology permits the use of a high-strength thin steel sheet, a portion with high .DELTA.r cannot be used, and thus conventionally must be cut off and removed. Therefore, a can steel sheet having low .DELTA.r and causing no earring is greatly demanded.
Japanese Unexamined Patent Publication No. 9-176744 proposes a method of improving uniformity in r values within a steel strip. Although this method comprises regulating the coiling temperature in the direction of the coil length, it is not necessarily an effective method because dynamic control of the coiling temperature in the coil causes defects in the shape of the coil, defects in pickling due to variations in pickling property, etc.
General factors which affect the above-described r value and .DELTA.r include (1) hot rolling conditions such as the finisher delivery temperature (FDT), the coiling temperature (CT), and the like, (2) the draft of cold rolling, (3) annealing conditions, etc., which must be optimized.
From these viewpoints, unlike an automobile steel sheet, the thickness of a hot-rolled finished can steel sheet is as small as 2 to 3 mm even if the reduction of cold rolling is set to a value of as high as about 90% of the upper limit ability of the rolling mill used because the product has a small thickness. Therefore, the hot rolling time is necessarily increased, and temperature decreases, particularly temperature decreases at the front and rear ends of the steel strip in the length direction and the ends in the width direction, are increased, thereby increasing nonuniformity in temperature within the coil. The nonuniformity in temperature decreases the r value, and increases .DELTA.r, increasing nonuniformity in these values in the steel strip. This makes production of a can steel strip very difficult.
In the future, this problem will be accompanied with the problem that as a coil of a can steel sheet, i.e., a can steel strip, is increased in weight, strength and width, and decreased in thickness to increase the need for a hot-rolled thin steel strip for decreasing a rolling load of cold rolling, a temperature difference in the steel strip during hot rolling, i.e., nonuniformity in material quality, further increases.
As described above, a thin and wide can steel strip having excellent quality and uniformity in properties is greatly demanded from the viewpoints that the production cost of the can body is decreased by decreasing the can weight, and that productivity is improved by widening the coil, i.e., the steel strip. However, the conventional technique of producing such a steel strip causes an increase in .DELTA.r at the ends of the steel strip in the width direction and at the ends in the length direction, and thus causes insufficient uniformity in .DELTA.r. This also causes a decrease in the r value, thereby making steel-fabrication press impossible. Therefore, in some applications of cans, the ends of a steel sheet in the length direction and width direction must be cut off and removed by trimming or the like, inevitably decreasing the yield.
In recent years, a so-called continuous hot-rolling technique has been brought into practical use, in which after rough rolling, sheet bars are successively joined to each other before finish rolling. Although, in this method, all ends in the length direction are expected to become stationary portions except the front end of the first sheet bar to be joined and the rear end of the last sheet bar to be joined, nonuniformity in material quality caused by the lower temperatures of the ends of the sheet bars than the centers is not completely eliminated under present conditions.