Technical Field
[0001] The present invention relates to an apparatus for continuously annealing a steel
strip after cold rolling, wherein the term "steel strip" refers to steel products
ultimately applicable to production of cans, steel furniture, automobiles, etc. after
continuous annealing, and, if necessary, tin plating, zinc plating, etc.
Background Art
(Prior Art)
[0002] Generally, the higher the tension of a steel strip in the furnace in an apparatus
for continuous annealing, the better from the viewpoint of preventing walk or fluttering
on one hand, and the lower the tension, the better from the viewpoint of preventing
heat buckle.
[0003] To satisfy such contracting requirements, it is an ordinary operating practice to
decrease the tension of steel strip in a heating furnace or a soaking furnace where
the steel strip is at a high temperature and increase the tension in a cooling furnace
or an overaging furnace where the steel strip is at a relative low temperature.
[0004] Such tension distribution can be obtained by finely adjusting and controlling a hearth
roll speed, and when it is desired to change the tension radically at a position in
the furnace, it is an ordinary practice to provide a bridle roll in the furnace.
[0005] Fig. 1 shows one example of that practice, where sets of hearth rolls 3 are so arranged
in the individual furnaces as to guide a steel strip 1 into the individual furnaces
from the inlet side to the outlet side and a set of bridle rolls 2 is arranged just
before a rapid cooling furnace 6 in which the steel strip 1 that has passed through
a heating furnace 4 and a soaking pit 5 is to be exposed to a gas jet stream of a
high speed, and the tension of the steel strip 1 becomes largest after passing through
the set of bridle rolls 2. Then, the steel strip 1 passes through the successive quenching
furnace 6, an overaging furnace 7 and a final cooling furnace 8 under the largest
tension.
(Problems to be solved by the Invention)
[0006] A set of bridle rolls 2 and sets of hearth rolls 3 provided for these purposes must
surely restrict the steel strip 1 so as to prevent any slip. Actually, the bridle
rolls 2 and hearth rolls 3 have often slipped, particularly when the thickness of
steel strip becomes smaller, and have failed to perform their proper functions.
[0007] An object of the present invention is to solve these problems and to provide an apparatus
for continuously annealing a steel strip, capable of performing functions of bridle
rolls and heath rolls so satisfactorily as to attain the stable operation of the apparatus
for annealing.
Disclosure of the Invention
[0008] To solve the above-mentioned problems, the present invention provides apparatuses
given in the following items (1) to (5):
(1) An apparatus for continuously annealing a steel strip, which comprises furnaces
and sets of rolls so arranged in the furnaces as to guide a steel strip fed into the
furnaces from the inlet side to the outlet side, at least one roll of the sets of
rolls having such a diameter D as to make a surface pressure p defined by the following
equation (I) 10 kPa (kilopascal) or more:

wherein the unit tension of the steel strip at the inlet side to the roll is σ1, the
unit tension at the outlet side to the roll is σ2, the thickness of the steel strip
is h and the diameter of the roll is D.
(2) An apparatus as described in the above-mentioned item (1), wherein at least one
roll of the sets of rolls is a bridle roll.
(3) An apparatus as described in the above-mentioned item (1), wherein at least one
roll of the sets of rolls is a hearth roll.
(4) An apparatus for continuously annealing a steel strip, which comprises furnaces
and sets of rolls so arranged in the furnaces as to guide a steel strip fed into the
furnaces from the inlet side to the outlet side, at least one roll of the sets of
rolls being a bridle roll having such a diameter D as to make a surface pressure p
defined by the following equation (I) 10 kPa (kilopascal) or more:

wherein the unit tension of the steel strip at the inlet side to the each bridle roll
is σ1, the unit tension at the outlet side to the each bridle roll is σ2, the thickness
of the steel strip is h and the diameter of the bridle roll is D.
(5) An apparatus for continuously annealing a steel strip, which comprises furnaces
and sets of rolls so arranged in the furnaces as to guide a steel strip fed into the
furnaces from the inlet side to the outlet side, at least one roll of the sets of
rolls being a hearth roll having such a diameter D as to make a surface pressure p
defined by the following equation (I) 10 kPa (kilopascal) or more:

wherein the unit tension of the steel strip at the inlet side to the hearth roll is
σ1, the unit tension at the outlet side to the hearth roll is σ2, the thickness of
the steel strip is h and the diameter of the hearth roll is D.
(Function)
[0009] A maximum tension T2 that cannot generate any slip technologically can be given by
the following equation (II):
wherein the tension at the inlet side to bridle rolls 2 is T1, the tension at the
outlet side is T2, winding angles of a steel strip 1 around bridle rolls 2a, 2b and
2c are ϑ1, ϑ2 and ϑ3, respectively, and sum total (ϑ1 + ϑ2 + ϑ3) is ϑ, and the coefficient
of friction is µ, as shown in Fig. 2.
[0010] In other words, the tension ratio T2/T1 depends solely upon parameter µϑ and is independent
on the absolute values of the tensions. However, the actual bridle rolls in the apparatus
are liable to undergo slip when the absolute values of the tensions become smaller
even in the same tension ratios. That is, it seems that the coefficient of friction
µ is susceptible to tensions.
[0011] Number of the bridle rolls is not limited, and only one bridle roll will do. Number
of the bridle rolls can be determined from ϑ derived by inverse operation of the equation
(II).
[0012] As a result of measuring the coefficient of friction at slip generation limits of
hearth rolls and bridle rolls having different diameters in an actual apparatus for
continuous annealing, it has been found that the coefficient of friction is a function
of surface pressure p given by the following equation (III).
wherein the diameter of a bridle roll is D and the width of a steel strip is W, as
shown in Fig. 3.
[0013] An important fact is, as is apparent from Fig. 3, that the critical point of the
coefficient of friction is at a surface pressure p of about 10 kPa (the surface pressure
at this critical point will be hereinafter called a critical surface pressure pc).
[0014] In a region whose surface pressure is at or above pc, the bridle rolls and the hearth
rolls are in a state of normally restricting a steel strip, whereas in a region whose
surface pressure is below pc they are in a slip state, that is, the friction is in
a state of kinetic friction. In order to make the bridle rolls and hearth rolls normally
function, their diameters must be so set that the surface pressure can be at or above
pc.
[0015] Generally, the object of hearth rolls is to convey a steel strip without changing
tensions on the steel strip. The present inventor has found that the friction is in
a state of kinetic friction below a surface pressure of 10 kPa, as is apparent from
Fig. 3, and a slip occurs even due a slight difference in the tension between the
inlet side and the outlet side to a hearth roll. The slip phenomenon, which appears
even if there is no substantial difference in the tension, seems surprising, but since
the control unit to motors driving the hearth rolls generally has no means for detecting
a true travelling speed of steel strip, and the roll peripheral speed is thus controlled
to an instructed value (same as the peripheral speed of a bridle roll), and thus such
a phenomenon seems to appear. Furthermore, the roll peripheral speed is not controlled
as instructed, because the hearth roll diameter thermally expanded in furnaces at
an elevated temperature cannot be detected exactly, and thus this fact also promotes
appearance of such a phenomenon. The slip presence leads to generation of defects
on the steel strip, or buildups (fixation of iron component in the form of small spherical
projections that also lead to defects of steel strip) on the surface of hearth roll
due to continued slip. These problems can be solved by selecting appropriate roll
diameters so as to give a critical surface pressure or more according to the present
invention.
[0016] Normal operation is so carried out that the unit tension σ of a steel strip can be
kept constant, and thus the equation (III) can be given as the following equation
(IV), when a correlation of tension

is taken into the equation (III):

Brief Description of the Drawings
[0017] Fig. 1 is a schematic side view showing an arrangement of bridle rolls and hearth
rolls in the furnaces according to the present invention.
[0018] Fig. 2 is a diagram illustrating the general relation between the winding angles
and tensions of a steel strip around bridle rolls.
[0019] Fig. 3 is a diagram showing actual measurements of relations between the surface
pressure and coefficient of friction of bridle rolls and hearth rolls as basic data
for the present invention.
Modes for carrying out the Invention
[0020] Examples are given below.
Example 1
[0021] One design example of bridle rolls based on the above-mentioned findings will be
given below.
[0022] Under the following conditions, i.e. unit tension of a steel strip at the inlet side
to the bridle roll σ1 = 0.7 kg/mm², unit tension at the outlet side σ2 = 1.2 kg/mm²,
thickness of steel strip h = 0.25 mm, number of rolls = 3, winding angle of steel
strip around the respective rolls = 180° and tension ratio at the respective rolls
is constant (= σ2/σ1 = 1.2), a critical surface pressure pc = 10 kPa is inserted into
the equation (IV) to obtain D1, D2 and D3. The results are D1 = 390 mm, D2 = 460 mm
and D3 = 550 mm.
[0023] Actual differences in the roll diameter give a complicatedness to their manufacturing
and maintenance, and all roll diameters are set to constant 390 mm or less in view
of safety.
[0024] Data shown in Fig. 3 relate to smoothly finished bridle rolls and hearth rolls, each
made of steel, and are those obtained at a line speed of 350 to 600 mpm.
Example 2
[0025] One design example of hearth rolls based on the above-mentioned findings will be
given below.
[0026] Tensions of a steel strip at the inlet side and the outlet side to the hearth rolls
are generally constant (σ1 = σ2), and the equation (IV) can be applied to the hearth
rolls as conditions for preventing slip occurrence in the same manner as in the case
of bridle rolls.
[0027] Under conditions of σ1 = σ2 = 0.7 kg/mm² and thickness of steel strip h = 0.25 mm,
roll diameters D will be 350 mm or less.
[0028] When the surface finishing roughness of bridle rolls or hearth rolls or the line
speed considerably differs, there would be a possibility for a change in the critical
surface pressure pc defined by Fig. 3, but there is no change in the basic concept
of setting the bridle roll diameter or the hearth roll diameter to give the critical
surface pressure or more.
[0029] The concept of selecting roll diameters so that the surface pressure p can be at
least such a critical value as not to cause any slip can be applied not only to the
bridle rolls, but also to all the hearth rolls, as mentioned above.
[0030] When the hearth roll diameter cannot be reduced due to the dimensional limit of the
apparatus, as in a heating furnace provided with radiant tubes, care must be paid
to the design and adjustment of the hearth roll control system so as not to give a
driving force in excess of the necessary torque to the conveying of a steel strip
by the hearth rolls. For example, speed instructions common to the furnaces must be
limited so as not to cause an error in the speed over a predetermined value (a few
mpm). It is also another technical knowledge suggested by the present invention to
give an appropriate suspending characteristic, etc.
Industrial Utility of the Invention
[0031] As described above, bridle rolls and hearth rolls arranged in the apparatus for continuous
annealing can give a sure tension-amplifying action to a steel strip as their proper
function according to the present invention, thereby attaining a high level, stable
operation in the apparatus for continuously annealing the steel strip, and also preventing
serious defects such as occurrence of defects (flaws) due to a slip between the steel
strip and the rolls.
1. An apparatus for continuously annealing steel strip, which comprises furnaces and
sets of rolls so arranged in the furnaces as to guide a steel strip fed into the furnaces
from the inlet side to the outlet side, at least one roll of the sets of rolls having
such a diameter D as to make a surface pressure p defined by the following equation
(I) 10 kPa (kilopascal) or more:
wherein the unit tension of the steel strip at the inlet side to the roll is σ1, the
unit tension at the outlet side to the roll is σ2, the thickness of the steel strip
is h and the diameter of the roll is D.
2. An apparatus according to Claim 1, wherein at least one roll of the sets of rolls
is a bridle roll.
3. An apparatus according to Claim 1, wherein at least one roll of the sets of rolls
is a hearth roll.
4. An apparatus for continuously annealing a steel strip, which comprises furnaces and
sets of rolls so arranged in the furnaces as to guide a steel strip fed into the furnaces
from the inlet side to the outlet side, at least one roll of the sets of rolls being
a bridle roll having such a diameter D as to make a surface pressure p defined by
the following equation (I) 10 kPa (kilopascal) or more:
wherein the unit tension of the steel strip at the inlet side to the bridle roll is
σ1, the unit tension at the outlet side to the bridle roll is σ2, the thickness of
the steel strip is h and the diameter of the bridle roll is D.
5. An apparatus for continuously annealing a steel strip, which comprises furnaces and
sets of rolls so arranged in the furnaces as to guide a steel strip fed into the furnaces
from the inlet side to the outlet side, at least one roll of the sets of rolls being
a hearth roll having such a diameter D as to make a surface pressure p defined by
the following equation (I) 10 kPa (kilopascal) or more:
wherein the unit tension of the steel strip at the inlet side to the hearth roll is
σ1, the unit tension at the outlet side to the hearth roll is σ2, the thickness of
the steel strip is h and the diameter of the hearth roll is D.