Method of heat treatment of a metal strip

Abstract

 A gas emitted from float nozzle units (21) keeps a metal strip (A) in a floating condition while a heated gas emitted from heat transfer nozzle units (22) is blown onto and heats the metal strip for a heat treatment. The gas flow rate through the float nozzle units is not reduced beyond a certain level such that the metal strip can be kept in the floating condition while the gas flow rate through the heat transfer nozzle units is controlled independently of the flow rate through the float nozzle units and may be reduced to zero.

Description

[0001] This invention relates to a method of heat treatment of a metal strip. A floating-type heat treatment furnace is used to carry out a heat treatment, such as annealing and solution heat treatment, of a metal strip such as an aluminum plate, a stainless steel plate, a copper plate, a plate of a copper alloy or a plate of an iron-nickel alloy, while it is being transported inside the furnace in a floating condition. This invention relates to an improvement in such heat treatment method of a metal strip.

[0002] When a floating-type heat treatment furnace is used to carry out a heat treatment of a metal strip, a heated infurnace environmental gas is guided to nozzles by means of a gas-circulating fan and emitted out therethrough to heat the metal strip while causing it to float and be transported horizontally inside the furnace at the same time. Two kinds of nozzles are usually provided for this purpose, the socalled float nozzles and heat transfer nozzles. The float nozzles are those which are so structured and positioned as to provide a static pressure for causing the metal strip to float, and the heat transfer nozzles are those which are so structured and positioned as to provide a dynamic pressure for heating it convectively. It is not that the float nozzles do not contribute at all to the heating of the metal strip, or that the heat transfer nozzles do not contribute at all to the flotation of the metal strip, but the float nozzles, because they are primarily for causing the metal strip to float, must be operated such that the gas flow rate therethrough should be sufficiently large to be able to keep the metal strip in the floating condition.

[0003] There have been many methods proposed for the control of the gas flow rate for such a floating-type heat treatment furnace, as disclosed, for example, in Japanese Patent Publications Tokkai 5-255761 and 5-255763. These prior art methods are disadvantageous, however, in that the extent of control is limited because the gas flow rate through the float nozzles and that through the heat transfer nozzles are under a single control such that if the flow rate through the heat transfer nozzles is reduced by 10%, for example, the flow rate through the float nozzles is also reduced by 10%. In order to keep the metal strip in a floating condition, the flow rate of the environmental gas through the float nozzles cannot be reduced beyond a certain limit. When it is desired to prevent wrinkles from being formed on the metal strip, furthermore, float nozzles are sometimes provided both above and below the metal strip, serving as wave-generating nozzles for forming waves on the metal strip. In such a situation, the gas flow rate through the float nozzles cannot be reduced too much because of the requirement that waves must be formed on the metal strip, and the control on the gas flow rate from the heat transfer nozzles becomes even more severely limited. If the extent of control on the gas flow rate is limited, there arise many problems, as will be described below.

[0004] When a metal strip is subjected to a heat treatment so as to make a thin plate into a thicker plate, it is necessary to preliminarily increase the furnace temperature to a considerable degree and accordingly to raise the temperature of the environmental gas to be emitted through the heat transfer nozzles such that the same transport speed and temperature of the material can be maintained between both the thin and thick parts. If the gas flow rate through the heat transfer nozzles is kept the same while its temperature is raised, however, not only does the temperature increase at the thin part of the metal strip, before it is made thicker, but also the metal strip will eventually break off. When the furnace temperature is raised, therefore, it is necessary to reduce the gas flow rate through the heat transfer nozzles according to the rise in the temperature. According to the prior art control whereby the gas flow rates through the float nozzles and the heat transfer nozzles are under a single control, however, the flow rate of the gas through the heat transfer nozzles cannot be controlled over a sufficiently wide range, and the furnace temperature can be raised only to the extent allowed by such a limited control. If the desired change in the thickness of a metal strip is relatively small such that the required increase in the furnace temperature is correspondingly small, a furnace operating using this prior art technology may not give rise to any problem. If the desired change in thickness is relatively large and the furnace temperature must be increased beyond the allowed range, however, the prior art method is no longer applicable. In such a situation, the heat treatment will have to be discontinued until the furnace temperature reaches a required level. It now goes without saying that this affects the productivity efficiency adversely in a serious manner.

[0005] One encounters a similar difficulty when the thickness of a metal strip is reduced by a heat treatment or when the transport speed of the metal strip is changed while its thickness is kept the same. When the transport speed of the metal strip is changed inside the furnace, the limited range of control over the flow rate of the gas from the heat transfer nozzles, as described above, can be compensated for by providing accumulators in front of and at the back of the furnace for heat treatment but a large accumulator is necessary to serve as a buffer zone.

[0006] It is therefore an object of this invention to eliminate the problems of prior art methods of uniquely controlling both the flow rate of the gas through float nozzles and that of the gas through heat transfer nozzles, as described above.

[0007] A method of heat treatment of a metal strip according to this invention, with which the above and other objects can be accomplished, may be characterized as separately controlling the gas flow rates through float nozzles and through heat transfer nozzles such that the gas flow rate through the float nozzles is kept sufficiently large, even when it is reduced, such that the metal strip can be kept in the floating condition even if the gas flow rate through the heat transfer nozzles is reduced to zero.

[0008] A wind tunnel with partitioning guide plates is connected to the float nozzles, and a gas-circulating fan for directing the environmental gas towards the float nozzles is provided on the upstream side of the wind tunnel. Similarly, another wind tunnel with partitioning guide plates is connected to the heat transfer nozzles, and another gas-circulating fan for directing the environmental gas towards the heat transfer nozzles is provided on the upstream side of this wind tunnel.

[0009] Any known method may be used for controlling the flow rates of the gas flowing through the float nozzles and the heat transfer nozzles. The flow rate through the heat transfer nozzles, for example, may be controlled by adjusting the number of rotation of the gas-circulating fan or the opening of the dampers in the wind tunnel, or both, by using as control parameters the measured nozzle pressure value or the measured flow rate value inside the wind tunnel. More in detail, the number of rotation of the motor for driving the gas-circulating fan may be controlled through a rotation control means by inputting the measured nozzle pressure value into a calculating device and using a signal outputted therefrom according to the difference between a pre-set value and the measured value.

[0010] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:

Fig. 1 is a schematic sectional side view of a furnace using a method according to this invention taken transversely to the direction of motion of a metal strip;

Fig. 2 is a schematic sectional longitudinal view of the furnace of Fig. 1 taken along line 2-2; and

Fig. 3 is a schematic sectional side view of another furnace using a method according to this invention taken transversely to the direction of motion of a metal strip.

[0011] Figs. 1 and 2 show an example of heat treatment fumace 11 adapted to use a method according to this invention, provided with a plurality of float nozzle units 21 and a plurality of heat transfer nozzle units 22. The float nozzle units 21, each provided with a plurality of float nozzles, are primarily for causing flotation of the metal strip A by providing static pressure, and the heat transfer nozzle units 22, each provided with a plurality of heat transfer nozzles, are primarily for heating the metal strip A by providing dynamic pressure, as explained above. Since the metal strip A must be kept in a floating condition while it is being transported horizontally inside the furnace 11, as shown by a horizontal arrow in Fig. 1, the float nozzle units 21 are characterized wherein the flow rate of gas therethrough is not reduced to zero but only to the extent that the metal strip A can still be kept in a floating condition even if the gas flow rate through the heat transfer nozzles is reduced to zero. Since the heat transfer nozzles are not expected to significantly contribute to the flotation of the metal strip A, by contrast, the gas flow rate therethrough may be reduced by 100%, or to zero, depending on the kind of heat treatment being carried out.

[0012] As shown in Fig. 1, the plurality of float nozzle units 21 may be arranged all on the underside of the metal strip A at uniform intervals longitudinally in the direction of motion of the metal strip A, and the heat transfer nozzle units 23 are provided both on the upper side and the underside of the metal strip A, some of them being provided on the underside between mutually adjacent pairs of the float nozzle units 21, the others of the heat transfer nozzle units 22 being above the metal strip A, each opposite one of the float nozzle units 21 or the heat transfer nozzle units 22 disposed on the underside of the metal strip A.

[0013] As shown in Fig. 2, the float nozzle units 21 are each connected through a header 21a to a wind tunnel 31a which is formed by guide boards 31 and has a gas-circulating fan 41 on the upstream side of the wind tunnel 31a. Similarly, the heat transfer nozzle units 22 are each connected through another header 22a to another wind tunnel 32a which is formed by guide boards 32 and has another gas-circulating fan 42 on the upstream side of the wind tunnel 32a. The gas-circulating fan 41 serves to circulate the environmental gas heated by a heater 51 to the float nozzle units 21 through the wind tunnel 31a and the headers 21a, causing it to flow upward out through the float nozzle units 21 toward the metal strip A and thereby keeping the metal strip A in a floating condition. The gas-circulating fan 42 serves to circulate the environmental gas heated by the heater 51 to the heat transfer nozzle units 22 through the wind tunnel 32a and the headers 22a, causing it to flow out upward or downward through the heat transfer nozzle units 22 to thereby heat the metal strip A.

[0014] Motors 61 and 62 are connected to the gas-circulating fans 41 and 42, respectively. Rotation control devices 71 and 72 are connected to the motors 61 and 62, respectively. Calculating devices 81 and 82 are connected to the rotation control devices 71 and 72, respectively. Although not shown, devices for measuring gas pressure at nozzle positions are provided, and a measured nozzle pressure value at the float nozzles 21 is inputted to the calculating device 81 and measured nozzle pressure value at the heat transfer nozzle units 22 is inputted to the calculating device 82. The calculating devices 81 and 82 compare these measured values with a pre-set value and output signals, according to which the rotary motion of the motors 61 and 62 is controlled independently through the rotation control devices 71 and 72.

[0015] As a test, a thin, belt-like elongated aluminum strip of thickness 2.5mm and width 1500mm was subjected to an annealing process. In this test experiment, the furnace temperature was 580°C, the temperature of the metal strip was 500°C and the transfer speed of the metal strip was 50m/minute. In preparation for a subsequent annealing process for another aluminum strip which is thicker, the furnace temperature was increased to 650°C at a rate of 100°C/hour while the flow rate through the float nozzle units was reduced to 90% and the flow rate through the heat transfer nozzle units was reduced to 30% at a rate of about 1.7%/minute. The furnace temperature rose to 650°C in about 42 minutes after the control of the flow rates was started. No problem was encountered regarding the quality of the heat-treated strip.

[0016] For the purpose of comparison, another thin, belt-like elongated aluminum strip was subjected to an annealing process under the same conditions as described above, except it was done according to the prior art technology by placing both flow rates through the float nozzle and heat transfer nozzle units under a same control. In this comparison experiment, too, it was attempted to raise the furnace temperature to 650°C, but the furnace temperature could be raised only to 600°C because the flow rates could be reduced only to 90% in order to keep the metal strip in a floating condition. Thus, the heat treatment had to be discontinued while the furnace temperature was raised to the desired level of 650°C. In other words, it was proved by this comparison experiment that the prior art technology adversely affects the productivity.

[0017] Explained in more general terms, the furnace temperature is made higher for the heat treatment of a thicker metal strip than for the heat treatment of a thinner metal strip. The temperature of the metal strip is affected by the furnace temperature (that is, the temperature of the environmental gas inside the furnace) through radiative heating, but it is more strongly influenced by the flow rate of the gas convectively transferring heat. If a thicker metal strip being subjected to a heat treatment is replaced by a thinner metal strip with the same gas flow rate through the heat transfer nozzles kept the same, for example, the thin metal strip will undergo a thermal deformation and may even break because the furnace temperature is still too high when the thinner metal strip is first introduced into the furnace. In other words, since the furnace temperature itself is high enough to start with in order to heat the metal strip to a desired temperature level, the flow rate of the gas through the heat transfer nozzle units may have to be reduced to zero. Since this thinner metal strip must also be kept in a floating condition, however, the flow rate of the gas through the float nozzle units cannot be reduced to zero. In other words, if the flow rate through the float nozzle units is to be varied between a maximum value and a minimum value, this minimum value cannot be zero but depends on the weight of the metal strip to be kept in the floating condition. If the flow rate through the heat transfer nozzle units is reduced to zero, as described above, the furnace temperature becomes lower gradually. When it becomes impossible to keep the metal strip at a desired temperature merely through the environmental gas, the flow rate through the heat transfer nozzle units is gradually increased to maintain the temperature of the metal strip at the desired level.

[0018] Although the invention has been described above with reference to only one example, this is not intended to limit the scope of the invention. Many modifications and variations are possible within the scope of the invention. For example, the arrangement of the float nozzle and heat transfer nozzle units is not required to be as described above with reference to Figs. 1 and 2, although the float nozzle units are generally disposed on the underside of the metal strip. Fig. 3 shows another furnace using a method according to this invention, similar to the one described above with reference to Figs. 1 and 2 but different therefrom in that both float nozzle units 23 and 23' and heat transfer nozzle units 24 are disposed not only on the underside of the metal strip B but also thereabove. As shown in Fig. 3, float nozzle units 23 and heat transfer nozzle units 24 are alternately disposed on the underside of the metal strip B in the direction of motion of the metal strip B, and float nozzle units 23' and heat transfer nozzle units 24 are also alternately disposed above the metal strip B in the direction of motion of the metal strip B such that the float nozzle units 23' above the metal strip B are exactly above different ones of the float nozzle units below the metal strip B and those of the heat transfer nozzle units 24 above the metal strip B are exactly above different ones of the heat transfer nozzle units 24 below the metal strip B. As also shown schematically in Fig. 3, those of the float nozzle units 23' above the metal strip B are smaller than those of the float nozzles 23 below the metal strip B. A furnace with float nozzle units arranged thus is particularly useful for the heat treatment of a thinner metal strip because thin metal strips are more strongly influenced by the upward force from the float nozzle units 23, tending be vibrate if there were no float nozzles above the metal strip B to stabilize its horizontal motion.

Claims

1. A method of subjecting a metal strip (A) to a heat treatment while keeping said metal strip in a floating condition, said method comprising the steps of:

emitting a float gas through float nozzles (21) towards said metal strip (A) at a first flow rate; and

emitting a heating gas through heat transfer nozzles (22) towards said metal strip at a second flow rate, said first flow rate and said second flow rate being independently controlled, said first float rate being sufficiently large to keep said metal strip in said floating condition even when said second flow rate is reduced to zero.

Drawing