FIELD OF THE INVENTION
[0001] The present invention relates to a process for continuously annealing a cold rolled
steel strip. More particularly, the present invention relates to a process for continuously
annealing a cold rolled steel strip, which process is capable of completing the annealing
operation within a short time and, also, capable of obtaining a cold rolled steel
strip having an excellent workability, especially formability and an excellent surface
quality, at a low cost.
BACKGROUND OF THE INVENTION
[0002] It is known that a cold rolled steel strip having a deep drawing quality can be produced
by tightly or loosely coiling a cold rolled steel strip and, then, by annealing it
batchwise in a box type annealing furnace. This type of method needs several days
to complete the entire process thereof and, therefore, is extremely inefficient. In
order to avoid the above-mentioned disadvantage, various attempts have been made to
continuously carry out the annealing process, and some of the attempts have been practically
used in steel industry.
[0003] The continuous annealing method can exhibit an extremely high efficiency in comparison
with the conventional batch type annealing method. However, it is strongly desired
to increase the efficiency of the continuous annealing method to such an extent that
the continuous annealing operation is completed within a few minutes.
[0004] In order to accelerate the continuous annealing operation, it has been attempted
to rapidly heat the steel strip by using a direct fired furnace or to rapidly cool
the heated steel strip with water or gas in the primary stage of the cooling operation.
However, both rapid heating operation and rapid cooling operation in the above-mentioned
processes cause an oxide layer to be formed on the peripheral surface of the steel
strip. Therefore, it is necessary to eliminate the oxide layer from the annealed steel
strip. Examples of the accelerated continuous annealing processes are as follows.
(1) Japanese Patent Application Laying-open (Kokai) No. 52-14431 (1977) discloses
an annealing process in which a steel strip is rapidly heated to a predetermined temperature
in a direct fired furnace and, then, rapidly cooled with water, reheated, overaged
and, finally, subjected to a pickling operation to remove an oxide layer formed on
the peripheral surface of the steel strip.
(2) Japanese Patent Application Laying-open (Kokai) No. 53-17518 (1978) discloses
a process wherein a steel strip is rapidly heated to a predetermined temperature and
maintained at the temperature in the direct fired furnace, rapidly cooled with water
and, reheated, overaged while the oxide layer on the peripheral surface thereof is
removed by reducting it.
[0005] Especially, in the above-mentioned process (I), the heating and cooling operations
result in the formation of a considerably large thickness of the oxide layer, and
this large thickness causes the time necessary for completing the elimination of the
oxide layer to be undesirably long. In addition, the sheet surface is coarsened due
to the pickling for removing the oxide layer.
[0006] In the above-mentioned process (2), the elimination of the oxide layer from the steel
strip is carried out during the reheating overaging operation at a relatively low
temperature. Therefore, in order to effectively attain the elimination of the oxide
layer, the reducing operation should be carried out by using a strictly controlled
reducing atmosphere having a special concentration of hydrogen and dew point. Also,
in the processes (1) and (2), in order to overage the steel strip after the rapid
cooling, it is necessary to reheat the steel strip to an overaging temperature thereof.
[0007] When the electrolytic cleaning is omitted in the processing line of the cold rolled
steel strip including the direct fired furnace, iron powder, which is adhered on the
surface of the cold rolled steel strip and remains on the steel strip conveyed into
the direct fired furnace, is oxidized in this furnace. When the quantity of so oxidized
iron powder is large, the oxide layer is partly peeled off from the sheet surface
as the steel travels through a direct fired holding furnace. The peeled oxide layer
is picked up by and firmly bonded around the hearth rollers of the holding furnace
due to sintering phenomenon thereof, which causes generation of flaws on the high-temperature
steel strips conveyed by the hearth rollers.
[0008] As is well known, the combustion air ratio is adopted as a parameter of the combustion
condition in the direct fired furnace. The oxidation tendency of the sheet surface
is dependent upon the combustion air ratio.
SUMMARY OF THE INVENTION
[0009] It is an object of the present invention to provide a continuously annealing process
of a cold rolled steel strip, capable of eliminating the problems of a surface-quality
caused by conventional processes.
[0010] It is another object of the present invention to provide a continuously annealing
process of a cold rolled steel strip, capable of eliminating the surface-quality problems
caused by conventional processes and simultaneously omitting the reheating step for
overaging of the steel strip.
[0011] The present invention involves a concept of combining the following:
using instead of a direct fired furnace a radiation heating furnace, thereby preventing
the formation of an appreciable oxide layer during the stages for heating to and holding
within the requisite annealing temperature range;
cooling the annealed steel strip by a cooling medium consisting of a mixture of gas
and liquid, and;
removing the oxide film formed during the cooling stage, by means of pickling at the
final step.
[0012] In accordance with the objects of the present invention, there is provided a process,
comprising the steps of:
introducing a cold rolled steel strip into at least one radiation-heating furnace,
in which the steel strip is heated to a temperature range of from the recrystallization
temperature to the Ac3 point of said steel strip and held within said temperature range over a period of
at least 10 seconds;
cooling said steel strip by a cooling medium consisting of a mixture of gas and liquid,
and;
pickling said steel strip.
[0013] In accordance with the objects of the present invention, there is provided a process,
comprising the steps of:
introducing a cold rolled steel strip into at least one radiation-heating furnace,
in which the steel strip is heated within a temperature range of from the recrystallization
temperature to the Ac3 point of the steel strip and held within said temperature range over a period of
at least 10 seconds;
cooling the steel strip by using a cooling medium consisting of a mixture of gas and
liquid over a primary- cooling temperature range of from 600°C at the lowest to a
temperature near to an overaging treatment temperature;
overaging the primary cooled steel strip;
secondary-cooling said overaged steel strip, and;
pickling the secondarily cooled steel strip.
DETAILED DESCRIPTION OF THE INVETNION
[0014] The process without the overaging step of the present invention can be applied to
cold-rolled non-aging low carbon steel strips, for example, cold-rolled, extremely
low carbon aluminum killed steel strips, and cold-rolled non- or retarded-aging extremely
low carbon steel strips containing a small amount of Ti, Nb, V or B, which are capable
of forming carbonitrides. In other words, the process of the present invention can
be applied to various cold-rolled low carbon steel strips which include the usual
type of cold-rolled low carbon steel strips having a drawing quality and a deep drawing
quality, for example, bodies of automobiles, high tensile strength cold-rolled low
carbon steel strips and other types of cold-rolled low- carbon steel strips suitable
for various surface-treating processes, for example, metal plating and coating processes.
Since, there is no necessity of overaging in the non- or retarded-aging steels, the
steel strips are directly pickled after cooling, so as to remove the oxide layer.
[0015] In the process with the overaging step of the present invention, the primary cooling
operation is followed by an overaging operation. In the cases of usual cold rolled
low carbon steel strips having an aging property, the overaging operation is usually
applied to them. In this case, the cooling operation must be terminated when the temperature
of the steel strip reaches a level near an overaging temperature of the steel strip,
the cooled steel strip may be overaged and, then, the overaged steel strip may be
additionally cooled to a desired temperature.
[0016] The present invention is hereinafter explained in detail in the sequence of the production
steps.
[0017] Steel is processed in a conventional manner until the cold rolling step. Before applying
the processes of the present invention, the peripheral surface of the cold-rolled
steel strip may be cleaned to remove grease or rolling oil therefrom by a conventional
surface-cleaning method. Otherwise, the process of the present invention may be applied
to the cold rolled steel strip without surface-cleaning it.
[0018] In the process of the present invention, radiation heating means, such as a radiant-tube
heating and other appropriate heating means, are employed for heating the steel strip
to a temperature within the range of from the recrystallization temperature to the
A
C3 point of the steel strip and also for holding the steel strip within the temperature
range of from the recrystallization temperature to the Ac
3 point over a period of at least 10 seconds, preferably from 10 to 60 seconds. Although
a radiation heating furnace is well known the present invention is characterized as
compared with the prior arts in the combination use of the radiation heating furnace
for heating steel strip to and holding it within the requisite annealing temperature
range, cooling the steel strip by a cooling medium consisting of a mixture of gas
and liquid, a pickling removal of the oxide layer formed by the cooling and overaging
without reheating, if overaging is necessary.
[0019] The holding temperature depends on the composition and the other properties of the
steel strip, as well as the objective properties of the final product. The holding
temperature is not always constant but may be varied in the direction of the holding
zone so as to adapt the heat cycle for recrystallization and growth of crystal grains.
[0020] In the processes of the present invention, the heating rate is lower than that of
the direct fired heating, but the surface deterioration due to the direct contact
of flame with the sheet surface is prevented. As a result, the pickling time in the
final step is shortened as compared with that of steel strips heated in the direct
fired furnace, and also a surface quality problem involved in the pickling is mitigated.
When a non-oxidizing or reducing atmosphere inert to the steel is employed for the
furnace atmosphere and is maintained during the heating and holding zones, the oxidation
of the sheet surface in these zones is prevented. As a result, the pickling at the
final step is carried out at an extent sufficient to remove only a thin oxide layer
formed at this cooling step or the primary cooling step. The fact that no oxide layer
is formed during the heating step advantageously leads to the prevention of the oxide
pick up on the hearth rollers. The reducting atmosphere preferably comprises a mixture
of 4% by volume or more of hydrogen gas, with the balance consisting of nitrogen gas,
and preferably exhibits a dew point of 10°C or less.
[0021] The steel strip, which is subjected to the holding in a radiation heating furnace,
is then rapidly cooled at a cooling rate from 10 to 300°C/second which is preferable
from the view points of the metallurgical properties of the annealed steel strip.
In the process without the overaging step of the present invention, the steel strip
may be directly rapid-cooled from the holding temperature, or may be slow-cooled from
the holding temperature to a temperature of preferably 600°C at the lowest and then
rapid-cooled, if necessary. In the process with the overaging step of the present
invention, the rapid cooling must start at a temperature between the above holding
temperature and 600°C from the view points of shortening the overaging time period.
[0022] Although the steel strip can be rapidly cooled by immersing it into water, e.g. boiled
water, the spraying of a cooling medium consisting of a mixture of gas and liquid
hereinafter referred to as a gas-liquid cooling medium not only realizes the rapid
cooling but also provides such advantages as, good shape of the cooled steel strip,
and easiness in controlling the cooling rate. The term "gas-liquid cooling medium"
herein is preferably a fluid which is produced through such a process that a high
speed gas stream and a liquid stream of a predetermined pressure are injected from
their respective nozzles as jet streams and these streams are then mixed with each
other by intersecting with each other so that the liquid (e.g., water) reduces itself
to fine particles mixed in the gas in the form of a mist, or in a form almost equivalent
to spray.
[0023] The liquid is preferably water and the gas is usually selected from inert gases,
such as nitrogen gas, and mixtures of nitrogen and hydrogen. In a preferable example,
the gas-liquid cooling medium consists of a mixture of nitrogen gas with water. One
of the advantages of the gas-liquid cooling medium is to control the cooling rate
in the range of 10 to 300°C/second in such a manner as to control the temeprature
near to the overaging temperature and then to overage without reheating. That is,
when the present invention is applied to a cold-rolled low carbon steel strip having
an aging property, the primary cooling operation can be advantageously terminated
when the temperature of the steel strip reaches a level near the overaging temperature
of the steel strip, the cooled steel strip is overaged and, then, additionally cooled
to a desired temperature.
[0024] The overaging operation is carried out for the purpose of precipitating carbon in
the steel matrix which has been super-saturated with a solid solution carbon. The
overaging operation is preferably carried out in a temperature range of from 300 to
550°C, more preferably, from 350 to 450°C, for 3 minutes or less, more preferably
2 minutes or less. It is not always necessary that the steel strip be maintained at
a constant temperature throughout the overaging operation. That is, the overaging
temperature in an initial stage of the overaging operation may be higher than that
in a final stage of the overaging operation. The temperature profile from the entry
to withdrawal ends of the overaging equipment may be gradual or may decrease stepwise.
[0025] When the gas-liquid cooling medium contains water, the peripheral surface portion
of the steel strip cannot be prevented from oxidation. That is, the resultant layer
of oxides causes the appearance of the steel strip surface to be unsatisfactory, and
the surface property of the steel strip to be unsuitable to the surface treatments.
Therefore, it is necessary to eliminate the layer of oxides from the peripheral surface
of the steel strip.
[0026] The elimination of the oxide layer is effected by any conventional pickling methods
effective for eliminating various oxides. For example, the oxide layer can be removed
by treating the peripheral surface of the steel strip with, for example, an acid aqueous
solution of an inorganic acid, such as hydrochloric acid, sulfuric acid or phosphoric
acid, or of an organic acid, such as formic acid or oxalic acid. The treatment may
be effected by immersing the steel strip in an acid aqueous solution, by spraying
the acid aqueous solution onto a peripheral surface of the steel strip, or by subjecting
the steel strip to an electrolytic pickling with an acid aqueous solution.
[0027] In the processes of the present invention, the oxide layer formed in the cooling
and, optionally, overaging operations, is very thin. Therefore, the oxide layer can
be readily eliminated by the above-mentioned methods. After the pickling operation
is completed, the pickled steel strip is linsed with water. However, since the peripheral
surface of the acid-cleaned steel strip is reactive to oxygen and easily rusts, it
is preferable that the linsed steel strip be neutralized with a diluted alkali aqueous
solution. This neutralization is effective for preventing rust and discoloration of
the peripheral surface of the steel strip.
[0028] Usually, the cold rolled steel strip, for example, to be used for producing an automobile
body, is coated before the working process. In this case, the surface of steel strip
is treated with zinc phosphate. The quality of the zinc phosphate film formed on the
surface of the steel strip can be improved by applying the following treatment to
the steel strip after the pickling operation.
[0029] That is, as a surface pre-treatment an aqueous suspension containing water-insoluble
phosphate, for example, Zn
3(P0
4)
2 , is sprayed onto the surface of the pickled steel strip, or a thin film of Ni, Zn
or Mn is flash-coated on the pickled steel strip surface by means of electroplating.
Thereafter, as a pre-coating operation the surface of steel strip is treated with
the zinc phosphate. The above-mentioned surface pre-treatment is effective for promoting
the formation of crystal nuclei of the zinc phosphate and for providing a dense film
of the zinc phosphate. Therefore, the above-mentioned surface pre-treatment is very
effective for enhancing the bonding strength of the zinc phosphate layer to the coating
layer and for increasing the resistance of the coating layer to corrosion.
[0030] The process of the present invention can exhibit the following advantages.
(1) By utilizing the cooling operation with a mixture of a gas and a liquid, the cooling
rate of the steel strip can be easily controlled. For example, the steel strip can
be easily cooled to a temperature near to the overaging temperature of the steel strip.
Therefore, the overaging operation can be directly applied to the cooled steel strip
without heating the cooled steel strip to the overaging temperature. A reheating step
for overaging treatment is not necessary, if such treatment is required for having
an aging property.
(2) Since no appreciable oxide layer is formed in the heating and holding stages,
the oxide layer to be removed by the pickling is merely that formed at the cooling
step, especially the primary cooling step. The pickling operation can therefore be
simplified.
(3) It is possible to prevent the oxide adhesion on the hearth rollers.
(4) Surface quality of the final product is excellent.
[0031] The following specific examples are presented for the purpose of clarifying the present
invention. However, it should be understood that these examples are intended only
to illustrate the present invention and are not intended to limit the scope of the
present invention in any way.
[0032] Examples 1, 2 and 3 and Comparative Examples I, 2 and 3
[0033] In these examples, 0.8 mm thick cold rolled steel strips were processed under the
conditions given in Table 1. The oxide film thickness in the comparative examples
1 and 2 was determined by preliminary experiments, in which the steel strips were
heated under the same condition as in these examples by a direct fired furnace followed
by a rapid cooling by means of blowing nitrogen gas on the strips in the next zone.
Samples were cut from the strips and pickled in a 5% hydrochloric acid aqueous solution.
The weight decrease of samples due to pickling was reduced to the thickness of the
oxide layer of FeO.
Examples 4, 5 and 6 and Control Examples 4, 5 and 6
[0034] The 0.8 mm thick cold rolled steel strips were processed under the conditions given
in Table 2. The oxide layer thickness was determined by the same procedure as described
above.
1. A process for continuously annealing a cold rolled steel strip, comprising the
steps of:
introducing a cold rolled steel strip into at least one radiation heating furnace,
in which said steel strip is heated to a temperature within the range of from the
recrystallization temperature to the Ac3 point of said steel strip and held within said temperature range over a period of
at least 10 seconds;
cooling said steel strip by a cooling medium consisting of a mixture of gas and liquid,
and;
pickling said steel strip.
2. A process for continuously annealing a cold rolled steel strip, comprising the
steps of:
introducing a cold rolled steel strip into at least one radiation heating furnace,
in which said steel strip is heated to a temperature within the range of from the
recrystallization temperature to the Ac3 point of said steel strip and held at said temperature range over a period of at
least 10 seconds;
cooling said steel strip by a cooling medium consisting of a mixture of gas and liquid
over a primary- cooling temperature range of from 600°C at the lowest to a temperature
near to an overaging treatment temperature;
overaging said primary cooled steel strip;
secondary-cooling said overaged steel strip, and;
pickling said secondary cooled steel strip.
3. A process according to claim 1 or 2, wherein the cooling rate by using said cooling
medium is in the range of from 10 to 300°C/second.
4. A process according to claim 1 or 2, wherein the cooling by using said cooling
medium is carried out by spraying the mixture of gas and liquid.
5. A process according to claim 4, wherein said liquid is water.
6. A process according to claim 4, wherein said gas is inert gas.
7. A process according to claim 6, wherein said inert gas is nitrogen.
8. A process according to claim 4, wherein a high speed gas stream and a liquid stream
of a predetermined pressure are injected from their respective nozzles as jet streams
and these streams are then mixed with each other by being intersected with each other,
thereby producing the sprayed cooling medium.
9. A process according to claim 1 or 2, wherein the holding time is from 10 to 60
seconds.
10. A process according to claim 1 or 2, wherein a non oxidizing atmosphere is maintained
at the heating and holding zones of said at least one radiation-heating furnace.
11. A process according to claim 1 or 2, wherein a reducing atmosphere is maintained
at the heating and holding zones of said at least one radiation-heating furnace.
12. A process according to claim 2, wherein said overaging operation is carried out
in a temperature range of from 300 to 550°C for 3 minutes or less.
13. A process according to claim 12, wherein said overaging operation is carried out
in a temperature range of from 350 to 450°C.
14. A process according to claim 1 or 2 wherein said pickling is carried out with
an aqueous solution containing at least one acid selected from the group consisting
of hydrochloric acid, sulfuric acid, phosphoric acid, formic acid and oxalic acid.
15. A process according to claim 1 or 2, wherein said pickling step includes neutralizing
of the acid-cleaned steel strip.
16. A process according to claim 2, wherein an overaging temperature in an initial
stage of the overaging operation is higher than that in a final stage of the overaging
operation.