Method of rolling sheet and rolling machine

Details Method of rolling sheet and rolling machine Method of rolling sheet and rolling machine

2002-09-25

2004-11-23

Larson, Lowell A. (Department: 3725)

Metal deforming

By use of roller or roller-like tool-element

With carrier for roller-couple or tool-couple

Reexamination Certificate

active

06820453

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a rolling mill provided with a pair of work rolls having different diameters, and a sheet rolling method employing the same rolling mill.
2. Description of Related Art
A conventional rolling mill is provided with upper and lower work rolls respectively having different diameters and supported by upper and lower backup rolls. In this type of rolling mill, the larger work roll, i.e., the work roll having a larger diameter, is driven by a motor or the like to roll a sheet. A rolling mill provided with work rolls having different diameters, sometimes called a differential rolling mill (as compared with ordinary rolling mills provided with work rolls of the same diameter) is able to roll a sheet at a high draft by a low rolling force, which is advantageous in manufacturing steel sheets by rolling. Since only a small rolling force is necessary, edge drop resulting from the flattening of the rolls can be suppressed and hence steel sheets having a small thickness deviation can be manufactured.
Generally, as shown in
FIG. 8
, working rolls
11
′ and
12
′ included in most rolling mills are shifted downstream by an offset e with respect to backup rolls
13
′ and
14
′. The work rolls are thus shifted downstream with respect to the backup rolls because a rolling mill in which work rolls are shifted downstream with respect to backup rolls is able to stabilize loading conditions for loading a rolled sheet more effectively than a rolling mill in which work rolls are shifted upstream with respect to backup rolls.
A related art is disclosed in JP-B No. 47421/1976.
Recently, hot rolling techniques for hot-rolling sheets are required to be capable of rolling sheets in a greater rolling width, i.e., the width of the rolled sheet, and in smaller thickness, and of rolling sheets at higher drafts. However, the diameter of the smaller work roll of the differential rolling mill is smaller and the mechanical strength of the smaller work roll is insufficient to meet the foregoing requirements. More specifically, a high stress is induced in necks, including stepped parts, at the joints of the body, which is used for rolling, of the smaller work roller and the journals, supported in bearings, of the smaller work roll.
Thus, the upper limit of the rolling width of steel sheets hot-rolled by differential rolling mills has been 4 ft (about 1200 mm). Even the differential rolling mill that needs a relatively low rolling force requires a high rolling force exceeding 3000 tons (3000 tf=2.94×10
7
N) when the rolling width exceeds 4 ft. An excessively high stress unbearable by the mechanical strength of the smaller work roll is thus induced in the necks of the smaller work roll.
The present invention is intended to meet the foregoing requirements required of rolling mills for hot-rolling sheets, including capability of rolling sheets in an increased width exceeding 4 ft by reducing mechanical load on work rolls.
SUMMARY OF THE INVENTION
A sheet rolling method according to a first aspect of the present invention includes: disposing a pair of work rolls respectively having different diameters between upper and lower backup rolls; and driving only the large-diameter work roll having the greater diameter for rolling to produce a sheet; wherein the small-diameter work roll having the smaller diameter is disposed so that a rotational axis of the small-diameter work roll is positioned on a mill center or a downstream side with respect to the mill center in a rolling direction, and the large-diameter work roll is disposed so that a rotational axis of the large-diameter work roll is positioned on a downstream side with respect to the rotational axis of the small-diameter work roll in the rolling direction.
Since the sheet rolling method does not shift both the two working rolls on the upstream side of the mill center plane including the center axes of the backup rolls with respect to the rolling direction, loading conditions for rolling the sheet is stabilized, and the sheet can be smoothly and continuously rolled.
This sheet rolling method is characterized in reducing mechanical load on the work rolls even when a high rolling force is necessary for rolling a wide sheet.
The ability of the sheet rolling method to reduce the mechanical load on the work rolls can be reasoned as follows.
When a rolling mill provided with two work rolls respectively having different diameters operates for rolling to produce a sheet, the following forces a) to c) are exerted on the journals, supported in bearings, of the smaller work roll having a smaller diameter:
a) A horizontal force acting downstream with respect to the rolling direction resulting from driving only the large-diameter work roll having the greater diameter and exerted on the small-diameter work roll by a sheet being rolled (Force SR
1
in FIG.
3
);
b) A roll bender force acting on the work roll in a plane (vertical plane) perpendicular to the rolling direction (Force P
B
, not shown); and
c) A horizontal force equal to the difference between the horizontal components of vertical forces exerted on the small-diameter work roll by the backup roll and the large-diameter work roll (SB
1
and SD
1
shown in
FIG. 2
) (Force P
mt
shown in FIG.
2
).
These forces are exerted on the journals supported in the bearings to induce stresses in the necks of the small-diameter work roll.
Although all those forces are produced necessarily during the rolling operation, the magnitude (and the direction, in some cases) of the horizontal force (P
mt
) produced by the forces (SB
1
and SD
1
) is dependent on the dispositions of the large and the small-diameter work rolls relative to the backup rolls, represented by offsets.
According to an exemplary embodiment of the invention, the small-diameter and the large-diameter work rolls are disposed such that the offset of the axis of the large-diameter work roll with respect to the mill center plane is greater than the offset which could be zero in some cases of the small-diameter work roll with respect to the mill center plane. This arrangement provides for the horizontal component (SB
1
) of the vertical force exerted by the large-diameter work roll on the small-diameter work roll and used to determine the horizontal force (P
mt
, the force c)) to be directed upstream with respect to the rolling direction. Consequently, the horizontal force (P
mt
, the force c)) is reduced. Since the direction of the horizontal component (SB
1
) is opposite to the rolling direction, the horizontal force that acts on the small-diameter work roll, i.e., the resultant force acting on the small-diameter work roll, i.e., the sum of the horizontal force (SR
1
, the force a)) and the horizontal component (SB
1
), is reduced. When the horizontal force is reduced, the mechanical load on the small-diameter work roll is reduced accordingly even if the vertical force, such as the force b), does not change. Consequently, a sheet having a big width and a small thickness can be produced and draft at which the sheet can be rolled by one rolling mill can be increased.
In the sheet rolling method according to the first aspect of the present invention, it is preferable that an offset e
1
by which the rotational axis of the small-diameter work roll is shifted from the mill center plane, and an offset e
2
by which the rotational axis of the large-diameter work roll is shifted from the rotational axis of the small-diameter work roll (refer to
FIG. 1
for e
1
and e
2
) meet inequalities:
0 mm≦
e
1
and 0 mm<
e
2
<7 mm
when the small-diameter work roll has necks of a diameter of about 270 mm or below, and a rolling force of 3000 tons (3000 tf=2.94×10
7
N, 1 ton=1 tf=9800 N) or above is used for rolling work.
The diameter of the body of the small-diameter work roll having the necks of a diameter of 270 mm or below is limited by the relation of the small-diameter work roll with support means including bearings and is considerabl

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