Method and apparatus for cooling steel strip

In a cooling zone of a continuous annealing line engineered to continuously process longitudinally travelling steel strip, the strip is first slowly cooled to a desired temperature at a rate of not higher than 20.degree. C./sec. by means of a jet stream of cooling gas ejected against the surface of the strip and subsequently quenched to a desired temperature at a rate of not lower than 70.degree. C./sec. Thinner strip is quenched by high-speed gas-jet cooling that is effected by ejecting a jet stream of cooling gas against its surface, while heavier strip is quenched by bringing it into contact with the perimeter of cooling rolls cooled by a coolant. Namely, quenching is achieved either by high-speed gas-jet cooling or roll cooling depending on the thickness of the strip being processed. A cooling apparatus for implementing the cooling method described above comprises a high-speed gas-jet cooling zone where the strip is cooled by means of a jet stream of cooling gas ejected against its surface and a roll cooling zone following the high-speed gas-jet cooling zone where the strip is brought into direct contact with a plurality of cooling rolls. Means to exert longitudinal tension on the strip travelling through the roll cooling zone are provided at the entry and exit ends thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method and apparatus for cooling steel strip, 
and more particularly to a method and apparatus for cooling steel strip of 
various thicknesses optimum for use in a cooling zone of a continuous 
annealing line. 
2. Description of the Prior Art 
Gas-jet cooling, cooled-rolls contact cooling, combinations of the above 
two (such as a method disclosed in Japanese Provisional Patent Publication 
No. 41321-1981) and several other methods have been proposed as means of 
cooling steel strip in continuous annealing equipment. Gas-jet cooling 
comprises shooting forth a jet stream of furnace atmosphere gas cooled in 
the cooling zone of a continuous annealing furnace against the surface of 
steel strip. Cooling by contact with cooled rolls is effected by bringing 
steel strip into contact with rolls cooled with a coolant passed along the 
inside perimeter of the roll body. 
Using a furnace atmosphere gas and keeping the strip out of physical 
contact, gas jet cooling has a merit of high operational efficiency. On 
the other hand, cooling by contact with cooled rolls has recently 
attracted increasing attention because of a high cooling rate it achieves 
by bringing strip into direct contact with rolls. 
But these methods are not without problems. With gas-jet cooling, high 
cooling rate becomes difficult to attain as strip thickness increases. 
When a thinner and wider strip is cooled with cooled rolls, a slight 
temperature variation across the strip width can lead to undesirable 
fracture or drawing on account of the low rigidity instrinsic to thinner 
strip. Setting aside such fracture and drawing, uniform distribution of 
tension across the strip width may be impaired, whereupon it becomes 
difficult to keep strip in uniform contact with the surface of cooled 
rolls widthwise, with ensuring uneven cooling. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a method and equipment for 
consistently cooling steel strip of whatever size with a desired cooling 
rate, solving the problems with the conventional gas-jet and cooled-roll 
strip cooling technologies. 
Another object of this invention is to provide a method and apparatus 
ensuring uniform widthwise cooling of steel strip of whatever thickness. 
In order to achieve the above objects, a strip cooling method according to 
this invention comprises slowly cooling longitudinally travelling steel 
strip to a desired temperature at a cooling rate of not higher than 
20.degree. C./sec with a coolign gas ejected against the surface of the 
strip in a cooling zone of a continuous annealing furnace in which the 
running strip is continuously processed. Then, the strip is quenched to a 
desired temperature with a cooling rate of not lower than 70.degree. 
C./sec. The desired quenching is achieved with thinner materials that can 
be quenched at a rate of not lower than 70.degree. C./sec with a high-seed 
gas jet that is ejected against the strip surface. On the other hand, 
heavier materials may not be quenched at a rate of not lower than 
70.degree. C./sec by high-speed gas jet cooling. Quenching of such heavier 
materials is then achieved by bringing the strip in contact with the 
surface of coolant-cooled rolls. For achieving the desired quenching, in 
other words, high-speed gas-jet cooling or cooled-roll cooling is chosen 
depending on the thickness of the strip processed. The rates of slow 
cooling and quenching depend on the metallurgical requirements of finished 
product. Water or heat-transfer mediums having high boiling points (such 
as diphenyl-based ones having a boiling point of 250.degree. C. to 
300.degree. C.) are used as a roll-cooling coolant. 
An apparatus to implement the above cooling method comprises a high-speed 
gas-jet cooling zone where steel strip is cooled by a jet stream of 
cooling gas ejected against the surface thereof and a roll cooling zone 
disposed immediately downstream of the high-speed gas-get cooling zone 
where the strip is cooled by direct contact with cooling rolls. Means to 
exert a high tension on the strip running through the roll-cooling zone 
are provided at the entry and exit end of the roll-cooling zone. 
As mentioned above, the technology of this invention is characterized by 
the combination of high-speed gas-jet cooling and roll cooling. When 
thinner strip is passed, cooling is effected by only high-speed gas-jet 
cooling, with the cooling rolls retracted away from the strip. With 
heavier strip, uniform cooling throughout the stock is achieved by 
applying gas-jet cooling preliminary to roll cooling. 
According to the technology of this invention, steel strip in a wide 
thickness range (such as from 0.3 mm to 2.0 mm) can be continuously 
annealed on a single line. Attainable benefits are efficient continuous 
annealing and remarkably increased productivity. A choice of a cooling 
method or apparatus suited for a specific strip thickness ensures 
achievement of uniform cooling and production of good-quality strip. 
Conventionally, widthwise temperature variations have often occurred 
(i.e., between the strip edge and center) upstream of the cooling zone 
particularly on heavier materials (such as those having a thickness of 0.7 
mm or above). According to this invention, such temperature inequalities 
can be eliminated in the high-speed gas-jet cooling zone prior to the 
application or roll cooling. 
Exertion of tension on the strip causes the surface of the strip to adhere 
uniformly to the perimeter of cooling rolls, thereby insuring uniform 
cooling across the width of the strip.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to accompanying drawings, a cooling apparatus embodying the 
principle of this invention will be described. 
FIG. 1 schematically shows a continuous annealing line incorporating a 
cooling apparatus according to this invention. As seen in the diagram, the 
continuous annealing line comprises a heating furnace 1, a soaking furnace 
2, a primary cooling furnace 3, an overaging furnace 5 and a secondary 
cooling furnace 6 which are sequentially disposed, with provision made to 
continuously pass steel strip S therethrough over a number of hearth rolls 
8 or other appropriate means. The hearth rolls 8 are driven by a drive 
unit (not shown) comprising a motor, speed reducer and so on. Passed over 
the hearth rolls 8, the strip S travels up and down through each furnace. 
The continuous annealing line is preceded and followed by such ordinary 
equipment as a payoff reel, welder, electrolytic cleaner, entry-side 
looper, exit-side looper, skinpass mill, shear and tension reel (not 
shown). 
The cooling apparatus of this invention is characterized by the primary 
cooling furnace 3 which comprises a high-speed gas-jet cooling zone 11 and 
a subsequent roll cooling zone 31. 
A concrete example of the high-speed gas-jet cooling zone 11 is shown in 
FIGS. 2 and 3. The high-speed gas-jet cooling zone 11 has a vertical 
furnace chamber 12, in which pairs of gas ejection boxes 15 are disposed 
on both sides of the vertical pass line of the strip S and mounted on 
furnace walls 13. A large number of nozzles 17 to eject a cooling gas 
against the strip S are provided on that surface 16 of the gas ejection 
box 15 which faces the strip S. A circulating fan 19 is provided outside 
the furnace chamber 12 and driven by a motor 20. The circulating fan 19 
has an intake pipe 21 whose end opens into the furnace chamber 12 and a 
discharge pipe 22 connected to the gas ejection box 15. A cooling heat 
exchanger 23 is provided midway on the intake pipe 21. The heat exchanger 
23 has a large number of fin tubes 26 extended across a chamber 24 
therein. Both ends of the fin tube 26 are fastened to headers 25 fitted on 
the side walls of the chamber 24, with cooling water supplied to the 
headers 25 form a cooling water pipe 27. The furnace atmosphere gas 
admitted into the intake pipe 21 is cooled to a desired temperature on 
comining in contact with the fin tubes 26 in the cooling heat exchanger 
23, with the pressure thereof boosted by the circulation fan 19. The 
boosted cooling gas is ejected in jet streams toward the surface of the 
strip S to be cooled from the nozzles 17 on the gas ejection box 15. 
Support rolls 29 are provided at appropriate intervals to suppress the 
fluttering of the strip S between the vertically adjoining gas ejection 
boxes 15. The support rolls 29 are driven synchronously with the 
travelling strip S and equipped with a mechanism (not shown) to retract 
away from the strip S when the line is stopped or on some other occasions. 
As shown in FIG. 1, the roll cooling zone 31 has a horizontally extending 
furnace chamber 32 through which the strip S travels horizontally. Bridle 
rolls 34 to exert tension on the strip S are provided at the entry and 
exit ends of the furnace chamber 32. Between the bridle rolls 34, there 
are provided water-cooled rolls 36 that are offset with respect to each 
other on both sides of the pass line of the strip S. Cooling water to cool 
the roll surface is passed each water-cooled roll 36. The cooling roll 36 
is driven and pushed up and down by a drive unit 38, thereby coming in and 
out of contact with the strip S. 
FIG. 4 shows details of the drive unit 38. As seen the upper water-cooled 
roll 36 is rotatably supported at both ends thereof by elevatable bearing 
boxes 41 and 42 and rotated by a motor 43 connected to one end of the 
shaft thereof extending through the bearing box 41 by way of a coupling. 
The water-cooled roll 36 is driven in such a manner that the peripheral 
speed thereof is equal to the travel speed of the strip S. Through a 
support 44 for each of the bearing boxes 41 and 42 is vertically passed a 
screw shaft 45 whose lower end is supported by a bearing 46. The support 
44 for each of the bearing boxes 41 and 42 contains a screw block (not 
shown) with which the screw shaft 45 is engaged. Miter gear boxes 48 and 
49 facing each are provided above the supports 44 of the bearing boxes 41 
and 42. A motor 50 to drive a bevel gear is fitted to the miter gear box 
48, with an output shaft 51 extending downward therefrom connected to said 
screw shaft 45 through a coupling 52. Another output shaft 54 horizontally 
extending from the miter gear box 48 is connected to an input shaft 58 of 
the opposite miter gear box 49 through a coupling 55 and an intermediate 
shaft 57. An output shaft 51 of the miter gear box 49 is also connected to 
the screw shaft 45. As the motor 50 drives the screw shaft 45, the 
water-cooled roll 36 moves up and down through the supports 44 of the 
heating boxes 41 and 42. 
Similarly, the lower water-cooled roll 36 is rotatably supported at both 
ends thereof by elevatable bearing boxes 61 and 62 and driven by a motor 
63 that is fitted via a coupling to one end of the shaft thereof extending 
through the bearing box 61. A rod 65 of a hydraulic cylinder 64 is 
connected to the bottom of each of the bearing boxes 61 and 62 so that the 
bearing boxes 61 and 62 or the water-cooled roll 36 is moved up and down 
by the motion of the hydraulic cylinder 64. 
Water to cool the water-cooled rolls is supplied through rotary joints (not 
shown) connected to those shaft ends thereof passing through the bearing 
boxes 42 and 62. Thus, the water-cooled rolls 36 are cooled by the water 
circulated therethrough by way of the cooling water pipe. 
Next, the reason why the high-speed gas-jet cooling zone 11 and the 
roll-cooling zone 31 are provided side by side will be discussed in terms 
of the relationship with the cooling rate. First, the influence of the 
cooling rate on the mechanical properties of the continuously annealed 
strip is as shown in FIG. 5; minimum yield point and maximum elongation 
are obtained when the cooling rate is between not lower than 70.degree. 
C./sec and not higher than 200.degree. C./sec. In other words, cooling 
rates within this range is desirable from the viewpoint of mechanical 
properties. 
FIG. 6 shows the relationship between the strip thickness and cooling rate 
studied in relation to high-speed gas-jet cooling and roll cooling. In the 
figure, curves G and G' exhibit examples of high-speed gas-jet cooling 
while curve B shows an example of roll cooling. As indicated by curve R, 
the cooling rate of roll cooling exceeds the aforementioned upper limit of 
200.degree. C./sec when strip thickness is under 0.5 mm. Hence, it is 
desirable to cool strip thinner than 0.5 mm by high-speed gas-jet cooling. 
Curve G shows an example in which 0.5 mm thick strip is cooled by a 
high-speed gas-jet at a cooling rate of 70.degree. C./sec. Curve G' shows 
an example in which high-speed gas-jet cooling is achieved at practically 
the highest cooling rate allowable in view of equipment cost. (Cooling 
facilities operating at higher cooling rates might be prohibitive from an 
economical viewpoint.) In order to achieve a cooling rate within the 
desirable range (70.degree. C./sec to 200.degree. C./sec) mentioned 
before, therefore, a choice must be made between high-speed gas-jet 
cooling and roll cooling within the range in which curves G and G' of 
high-speed gas-jet cooling and curve R of roll cooling are indicated by 
solid lines. While high-speed gas-jet cooling is preferable for strip 
thinner than 0.5 mm, the desired cooling rate is difficult to obtain with 
strip heavier than 0.7 mm without employing roll cooling. For strip 
ranging between not less than 0.5 mm and not more than 0.7 mm in 
thickness, either high-speed gas-jet cooling or roll cooling may be chosen 
so far as the cooling rate of 70.degree. C./sec minimum is attainable. The 
choice between the two methods depends on the capacity of the cooling 
equipment involved. The aforementioned range of strip thickness can vary 
to some extent depending on the capacity of the cooling equipment employed 
and the temperature variations demanded of the steel strip processed. The 
range given before is practical example to which this invention is by no 
means limited. 
To achieve effective high-speed gas-jet cooling, it is preferable to keep 
the distance "d" between the tip of the gas nozzle 17 and the surface of 
the strip S at 100 mm or under as shown in FIG. 7. Bringing the gas nozzle 
17 close to the strip permits greater heat transfer, reduces the power 
requirement of the circulation fan 19, and ensures a closely controlled 
change in the temperature distribution throughout the strip that is 
conductive to the entry-side temperature distribution control for the 
subsequent roll cooling operation. The power of the circulation fan 19 
tends to increase sharply when the distance "d" exceeds 100 mm, although 
the tendency varies with nozzle specifications. The distance "d" should 
preferably be not smaller than 30 mm since the surface of the strip S 
might come in contact with the tip of the gas nozzle 17 when fluttering if 
the distance "d" is too small. 
The nozzles 17 should preferably be of the projected type as shown in FIG. 
7 or FIG. 8. If a jet stream of gas hitting the surface of the strip 
hovers thereover, gases ejected thereafter may be prevented from reaching 
the strip surface, with a resulting drop in cooling efficiency. The 
nozzles 17 of the projected type leave enough space for the escape of 
cooling gases between the strip and the facing side of the gas ejection 
box. As a consequence, a jet gas stream "a" ejected against the strip S 
flows away to permit the following gas streams to flow smoothly, whereby 
the strip S is at all times exposed to freshly supplied cooling gas and 
greater heat transfer ensues. 
It is also preferable to divide the gas ejection box 15 widthwise into 
multiple zones as indicated by dot-dash lines in FIG. 8 to permit a 
widthwise adjustment of strip temperature through the control of the gas 
ejection rate in each zone 15s. This provision assures uniform widthwise 
temperature distribution in thinner strip (such as not more than 0.7 mm). 
Also, the widthwise temperature distribution in materials heavier than 0.7 
mm in thickness can be controlled with the shape and temperature 
distribution after roll cooling in mind. 
The cooling rate of gas-jet slow cooling is controlled by ajdusting the 
volume of cooling gas supplied and the length of the strip over which 
cooling gas is ejected (i.e., the number of gas ejection boxes turned on). 
To lower the cooling rate, for example, either the rotational speed of the 
cooling gas circulation fan is lowered or the damper opening is throttled 
and more gas ejection boxes are set to work. 
Furthermore, the gas-jet cooling zone 11 may be divided into a slow cooling 
zone 15a (see FIG. 2) for thinner materials outfitted with common gas 
ejecting means on the soaking furnace side and a high-efficiency cooling 
zone 15b equipped with high-speed gas ejecting means on the roll cooling 
zone side. 
The following paragraphs describe a continuous annealing operation 
implemented by use of the cooling apparatus just described. FIG. 9 shows 
examples of heat cycles for a thinner stock (indicated by a solid line) 
and a heavier stock (indicated by a broken line) and a heavier stock 
(indicated by a broken line) processed on the cooling apparatus according 
to this invention. Strip passes through a heating and soaking zone, 
high-speed gas-jet cooling zone, roll cooling zone and overaging zone in 
that order. The thinner strip (0.5 mm thick in the illustrated example) is 
slowly cooled (at a cooling rate of 10.degree. C./sec in the illustrated 
example) in the high-speed gas-jet cooling zone until quenching begins, 
and then quenched at a cooling rate of not lower than 70.degree. C./sec 
(the cooling rate being 100.degree. C./sec in the illustrated example). 
After that, the thinner stock is subjected to overaging without being 
cooled in the roll cooling zone. Therefore, the water-cooled rolls 36 in 
the roll-cooling zone are retracted away from the strip S as shown at (a) 
in FIG. 10. On the other hand, the heat cycle for the heavier stock (1.0 
mm thick in the illustrated example) comprises preliminary slow cooling at 
a temperature between the soaking temperature and a temperature not higher 
than the A.sub.1 transformation point effected in the gas-jet cooling zone 
(the cooling rate being 10.degree. C./sec in the illustrated example), 
quenching at a cooling rate of 100.degree. C./sec in the roll-cooling 
zone, and subsequent overaging, as indicated by a broken line in FIG. 9. 
In this mode of operation, the water-cooled rolls 36 are kept in contact 
with the strip S as shown at (b) in FIG. 10. 
Slow cooling is started at the soaking temperature that varies with the 
type and grade of steel and product, generally ranging between 700.degree. 
C. and 850.degree. C. for cold-rolled strip. The temperature at which 
quenching is started should metallurgically be not higher than the A.sub.1 
transformation point. Because of the need to keep the strip in good shape, 
the quenching starting temperature is generally set between 650.degree. C. 
and 700.degree. C. With the exception of some special materials, quenching 
is usually completed at a temperature of about 400.degree. C. whence 
overaging is started. 
This invention is by no means limited to the preferred embodiment described 
above. For example, the furnace chamber in the roll-cooling zone may of a 
vertical design. A vertical furnace chamber in the roll-cooling zone 
permits reducing the line length, although its maintainability is lower 
than that of a horizontal chamber. Also, the water-cooled rolls may not be 
driven. While the upper water-cooled rolls may be moved up and down by 
hydraulic cylinders, the lower water-cooled rolls may be moved up and down 
by screw shafts. Or both upper and lower water-cooled rolls may be moved 
up and down either by screw shafts or hydraulic cylinders.