Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
TITLE OF INVENTION
METHOD FOR HEAT TREATING LONGER-LENGTH PRODUCT, METHOD
FOR MANUFACTURING LONGER-LENGTH PRODUCT, AND HEAT
TREATMENT FURNACE USED FOR SAID METHODS
TECHNICAL FIELD
[0001]
The present invention relates to a method for heat treating longer-length
products, with which products that have longer-length than before can be heat
treated,
a method for manufacturing longer-length products using this heat treating
method,
and a heat treatment furnace used when the heat treating method and the
manufacturing method are carried out.
[0002]
Unless otherwise described, the terms in this description are defined as
follows:
"Longer-length product": Defined as a steel bar and any other longer-length
product, including a longer-length metal tube of a small diameter.
"Effective furnace length": Defined as a furnace length corresponding to a
maximum length of a product that can be heat treated at a uniform temperature
in a
heat treatment furnace.
"Gradient heating": Defined as heating performed by incorporating
differences of temperatures in varying outputs of heat sources disposed in
divisions
of endmost heating zones of a long and cylindrical batch-type heat treatment
furnace
with opposite ends thereof being enclosed when a product to be heat treated is
heated
by using the heat treatment furnace, the heat treatment furnace being
configured so
that the furnace is divided into a plurality of heating zones along a
longitudinal
direction, including endmost zones; the endmost zones each being further
subdivided
into a plurality of divisions; and the heat source is disposed in each
division of the
endmost heating zones.
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BACKGROUND ART
[0003]
In general, the batch-type heat treatment furnace is used for the heat
treatment
of longer-length products such as metal tubes and steel bars.
FIG. 1 is schematic views showing a general configuration example of a
conventional heat treatment furnace for longer-length products, FIG. 1 (a)
being a
transverse sectional view, and FIG. 1 (b) being a longitudinal sectional view.
As
shown in FIG. 1, the heat treatment furnace is a cylindrical vessel with
opposite ends
thereof being enclosed and with the inner space thereof being divided into a
plurality
of heating zones along a longitudinal direction. The vessel wall of the
furnace is of
a double construction consisting of a water-cooled wall 2 and a heat-
insulating wall 3,
and each of both end walls thereof is also of a double construction consisting
of a
water-cooled wall 7 and a heat-insulating wall 8. On the inner peripheral
surface of
the peripheral wall of furnace, an electric heater 1 is disposed as a heat
source for
each heating zone. On both end walls of the furnace, the heater 1 is not
provided.
[0004]
A longer-length product is heat treated by charging a product to be heat
treated 5 as being held on a trailer 4 into a space surrounded by the electric
heater 1,
that is, into the heating zones in the heat treatment furnace, and by heating
the
product to be heat treated 5 by using the heaters 1. The heat treatment
temperature
is controlled by individually controlling the outputs of the heaters 1 based
on the in-
furnace temperature measurement results obtained by thermometers provided in
the
furnace.
[0005]
As shown in FIG. 1, in the conventional heat treatment furnace, although a
plurality of heat sources (electric heaters) are provided on the peripheral
wall of
furnace, no heat source is provided on any of end walls of furnace. Therefore,
heat
is dissipated from the end portions of the longer-length product, so that the
temperatures of the end potions thereof decrease significantly as compared
with the
temperature of the central portion thereof For the conventional heat treatment
furnace, on account of the occurrence of this temperature decrease, the
permissible
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length that could be subjected to heat-treatment for a longer-length product
is
restricted, and thus the effective furnace length is shortened.
[0006]
Accordingly, the conventional heat treatment furnace is designed so that the
length of in-furnace space, that is, the overall length of the portion in
which the
heaters are disposed is much longer than the length of the product to be heat
treated,
and the effective furnace length is way longer than the length of the product
to be
heat treated.
[0007]
For the conventional heat treatment furnace, unfortunately, if the length of
the
heat treatment furnace is increased to extend the effective furnace length for
the
purpose of heat treatment of a product that is longer than before, the
equipment
refurbishment cost increases accordingly. Also, in the case where the furnace
length is not increased, the length of product to be heat treated must be
inevitably
decreased, so that the user's requirement cannot be met flexibly. The
conventional
heat treatment furnace poses such problems.
[0008]
As a method for controlling the temperature of the product to be heat treated,
the methods described below have been proposed.
For example, Patent Literature 1 discloses a method for controlling the in-
furnace temperature to a predetermined temperature. With this method, in the
batch-type heat treatment furnace for heat treating a workpiece held in the
furnace by
a plurality of burners, the temperatures of the combustion regions of burners
are
feedback controlled so as to reach the predetermined value and become
constant, and
on the other hand, the target temperature of the combustion region is
restricted to be
a preset value when the in-furnace temperature reaches within a specific
temperature
range lower than the target temperature, whereby the in-furnace temperature is
controlled to the predetermined constant temperature.
[0009]
Patent Literature 2 discloses a method for controlling the temperature. With
this method, in a vacuum furnace having openings for charging and discharging
an
article to be treated, the openings being provided on the front and rear ends
of
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furnace, the calorific value of a lower heater and the calorific values of the
front and
rear zones and the intermediate zone of an upper heater are controlled
independently
of each other. In the vacuum furnace, an article to be treated is heated
primarily by
radiation from a heater, and it is difficult to uniformly heat the article to
be treated
because convective heating scarcely occurs. Therefore, defective quality may
be
brought about on account of this nonuniform heating. This method is used to
overcome this problem.
[0010]
Unfortunately, these temperature controlling methods are to control individual
heat sources (burners or heaters) independently based on the detected in-
furnace
temperature to keep the in-furnace temperature at a proper temperature and to
prevent variations in in-furnace temperature by causing the in-furnace
temperature to
follow up a change in temperature, and these methods do not take into account
the
kind of product to be heat treated and the mutual interference of heat
sources, so that
these methods are insufficient as a method for controlling the temperature in
the
whole of furnace. Accordingly, as a method taking into account these factors,
the
methods described below have been proposed.
[0011]
Patent Literature 3 discloses a method for controlling the temperature of a
heating furnace. With this method, in the heating furnace configured so that
temperature control is carried out independently for heaters arranged in
heating zones
as being subdivided into plural divisions, the deviation of the measured
temperature
of each heater from the set temperature that is assigned to each heater is
multiplied
by a correction value as a function of time only or of time and temperature,
the
correction value giving heater output distribution peculiar to the furnace in
a soaking
region, and the resultant value is given as a heater output control value.
[0012]
Patent Literature 4 discloses a control method used for a vacuum furnace in
which a plurality of heaters for heating a product to be treated are provided,
a
calorific value regulator is provided individually in an electric power supply
path for
each heater, and between the calorific value regulator and a temperature
detector
provided in the furnace, an individual deviation setting instrument is
provided so as
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to be capable of giving deviation to the calorific values of the plurality of
heaters.
In the control method disclosed in Patent Literature 4, in heating, the
predetermined
setting value of each deviation setting instrument is changed according to the
kind
(size, shape, etc.) of a product to be treated and the pressure and
temperature of
atmospheric gas, whereby by the detected value sent from one temperature
detector,
the calorific values of the plurality of heaters are controlled in the state
in which
deviation is given to the calorific values. Thereby, as in the case where the
plurality
of heaters are controlled individually, the product to be treated can be
heated in the
state in which the temperature of the whole of the product to be treated is
uniformized.
[0013]
These methods are to homogenize the in-furnace temperature distribution of
the whole of the product to be heat treated by setting a deviation value for
each
heating zone in addition to the conventional temperature control in which
individual
burners or heaters are controlled independently. Unfortunately, these methods
are
insufficient as a method for homogenizing the treatment temperature of a
longer-
length workpiece to be heat treated such as a longer-length product because in
the
heat treatment furnace in which, as shown in FIG. 1, the heat sources are
disposed on
the peripheral wall so as to correspond to the heating zones, and on the other
hand,
no heat source is provided on both end walls, a remarkable decrease in
temperature
occurs in the end portions of the longer-length product as compared with the
central
portion thereof.
CITATION LIST
PATENT LITERATURE
[0014]
Patent Literature 1: Japanese Patent Application Publication No. S62-4828
Patent Literature 2: Japanese Patent Application Publication No. H05-271751
Patent Literature 3: Japanese Patent Application Publication No. S62-112726
Patent Literature 4: Japanese Patent Application Publication No. H04-52215
SUMMARY OF INVENTION
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TECHNICAL PROBLEM
[0015]
An objective of the present invention is to provide a method for heat treating
longer-length products, in which in a cylindrical batch-type heat treatment
furnace
with opposite ends thereof being enclosed and being provided with heat sources
on
the peripheral wall thereof, and on the other hand, is not provided with heat
sources
on both end walls thereof, the effective furnace length capable of heating the
overall
length of a product to be heat treated to a uniform temperature (for example,
10 C
or less with respect to the target temperature) is extended, and even if the
space
length in the furnace is the same, a product to be heat treated that is longer
in length
than before can be heat treated, and to provide a method for manufacturing
longer-
length products, which uses the above-described heat treating method. Another
objective of the present invention is to provide a heat treatment furnace used
when
the heat treating method and the manufacturing method are carried out.
SOLUTION TO PROBLEM
[0016]
To achieve the above objectives, the present inventors first examined the
temperature distribution especially in both end portions of a product to be
heat
treated (longer-length product), when heated by using a conventional heat
treatment
furnace having no heat source on both end walls thereof.
[0017]
FIG. 2 is a diagram showing one example of temperature distribution in the
longitudinal direction of a product to be heat treated in a conventional heat
treatment
furnace for longer-length products. FIG. 2 shows a measurement result obtained
by
using the conventional heat treatment furnace shown in FIG. 1, showing a
measurement result in the heating zone in the endmost portion of the furnace
and in
the heating zone next to this heating zone in the endmost portion. In FIG. 2,
the
product temperature on the ordinate is represented by a temperature difference
with
respect to the target temperature, which is the basis.
[0018]
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As shown in FIG. 2, the temperatures of the product at positions in the range
from the vicinity of the middle of the endmost heating zone to the adjacent
heating
zone to the endmost heating zone were equal to the target temperature or close
to the
target temperature. However, the temperatures of the product at positions in
the
range from the middle of the endmost heating zone to a portion close to the
end wall
of furnace did not reach the target temperature, and the temperature of the
product at
the position close to the end wall of furnace was about 80 C lower than the
target
temperature. Although not shown in FIG. 2, the same tendency was also
recognized in the heating zone in the endmost portion on the opposite side of
the heat
treatment furnace. That is, in the conventional heat treatment furnace shown
in FIG.
1, uniform heating cannot be performed throughout the overall length of a
product to
be heat treated.
[0019]
In the actual operation, as shown in FIG. 2, the position of the end portion
of
the product to be heat treated (longer-length product) is located in the
vicinity of the
middle of the endmost heating zone of furnace (in this example, at a position
of 1.4
m away from the end wall of the heat treatment furnace), and is decided so as
to be
within the range of the effective furnace length.
[0020]
To prevent this decrease in temperature in the end portion of the product to
be
heat treated, the whole of the endmost heating zone of the heat treatment
furnace was
heated to a temperature higher than the temperature of central heating zones
(heating
zones other than the endmost heating zone), and the product temperature in
this case
was measured.
[0021]
FIG. 3 is a diagram showing temperature distribution in a longitudinal
direction of a product to be heat treated in the conventional heat treatment
furnace
for longer-length products, showing the case where the furnace endmost zone is
heated to a temperature higher than the temperature of central heating zones
(herein,
referring to the heating zones lying between two endmost heating zones). FIG.
3
shows a measurement result in the endmost heating zone of the furnace and in
the
adjacent heating zone to this endmost heating zone.
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[0022]
At the time of testing, the temperature of the product to be heat treated in
the
central heating zones was set at the target temperature, and the temperature
of the
product to be heat treated in the endmost heating zones was set so as to be 20
C or
40 C higher than the temperature of the product to be heat treated in the
adjacent
heating zone to the endmost heating zone. For comparison, the measurement was
also made in the case where the temperature of the product to be heat treated
in the
endmost eating zone was set at the target temperature that is the same as the
target
temperature for product to be heat treated in the central heating zones.
[0023]
As shown in FIG. 3, in the vicinity of the middle of the endmost heating zone,
the product to be heat treated overheats exceeding the target temperature, and
on the
other hand, in the end portion of the product to be heat treated close to the
end wall
of furnace, the decrease in temperature still remains. Heating is not
performed
uniformly throughout the overall length of the product to be heat treated.
[0024]
Successively, the present inventors studied a method in which the heat
dissipation from the end portion of product is restricted by heat-insulating
plates
installed on the outside of the end face of the product to be heat treated,
and the
quantity of heat is gradiently given to the endmost heating zone of furnace
and the
adjacent heating zone to the endmost heating zone of furnace. The reason for
our
study is that, with this method, it can be anticipated that the product to be
heat treated
will be prevented from overheating remarkably in the vicinity of the middle
portion
of the endmost heating zone, and the decrease in temperature of the product to
be
heat treated will be suppressed.
[0025]
FIG. 4 is schematic views showing a general configuration of a heat treatment
furnace provided with heat-insulating plates, FIG. 4 (a) being a transverse
sectional
view, and FIG. 4 (b) being a longitudinal sectional view. In the heat
treatment
furnace shown in FIG. 4, twelve heat-insulating plates 9 made of SUS304 are
installed in a lapped manner on the outside of the end face of the product to
be heat
treated, which is placed on a trailer 4. By using this heat treatment furnace,
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measurement was made of the product temperature in the case where the heating
quantity of the endmost heating zone was made somewhat higher than that of the
adjacent heating zone to the endmost heating zone.
[0026]
FIG. 5 is a diagram showing a measurement result of temperature distribution
of a product to be heat treated (longer-length product) in the endmost heating
zone
and the adjacent heating zone to the endmost heating zone, the measurement
result
being obtained by using the above-described heat treatment furnace provided
with
the heat-insulating plates. For comparison, the temperature distribution in
the case
where the heating quantity of the adjacent heating zone to the endmost heating
zone
is made equal to that of the endmost heating zone is shown.
[0027]
From the result shown in FIG. 5, it can be seen that in the case where the
allowable range of temperature distribution throughout the overall length of
the
product to be heat treated is made 10 C or less with respect to the target
temperature, by somewhat increasing the heat input in the endmost heating zone
relative to that in the adjacent heating zone to the endmost heating zone, the
product
to be heat treated can be heated uniformly by restraining the overheat of the
product
to be heat treated to the upper limit or less of the allowable range in the
vicinity of
the middle portion of the endmost heating zone and by avoiding the decrease in
temperature in the end portion of the product to be heat treated.
[0028]
Unfortunately, this heat treatment furnace has a problem that the product to
be
heat treated is cooled by cooling gas introduced from the end wall of furnace
after
treatment; however, the cooling rate decreases because the gas is blocked by
the
heat-insulating plates 9, and the time period required for cooling increases
about two
to three times the conventional cooling time period. Also, it is necessary to
study
the number of and installation locations of the heat-insulating plates
depending on
the number of and sizes of the products loaded on the trailer, so that it
cannot be said
that this configuration is practical.
[0029]
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Accordingly, the present inventors made an attempt to make a temperature
distribution homogeneous over the product to be heat treated positioned in the
endmost heating zone of furnace by subdividing the endmost heating zone such
that
the length of division thereof is shorter than the length of a central heating
zone, by
disposing a heat source in each division, and by applying weighted heating
(gradient
heating) only in endmost heating zones. That is, the present inventors studied
the
application of a method for controlling the temperature of the end portion of
the
product to be heat treated by varying heating outputs of heat sources of
divisions of
the endmost heating zones (hereinafter, referred to as a "division gradient
heating
control method").
[0030]
The present invention has been made to provide a method for heat treating
longer-length products of item (1) described below, a method for manufacturing
longer-length products of item (2), and a heat treatment furnace of item (3)
used for
the heat treating method and the manufacturing method.
[0031]
(1) A method for heat treating longer-length products, in which a cylindrical
batch-type heat treatment furnace opposite ends of which are enclosed and the
inside
space of which is divided into a plurality of heating zones along a
longitudinal
direction is used, a longer-length product to be heat treated is charged into
the heat
treatment furnace, and heat treatment is performed, wherein
the heat treatment furnace is configured so that, among the heating zones, an
endmost heating zone thereof is subdivided into a plurality of divisions each
having a
length shorter than the length of a heating zone other than said endmost
heating
zones; and a heat source is disposed in each division of the endmost heating
zones,
and the heat treating method includes a series of steps of:
(Step 1) in advance, determining the heating output pattern of each heat
source in each division in the endmost heating zone based on the measurement
result
of actual temperature in the end portion of the product to be heat treated at
the time
of heating; and
(Step 2) controlling a heating output of an individual heat source, at the
time
of heat treatment for the product to be heat treated, based on the heating
output
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pattern of each heat source determined in step 1 and further the measurement
result
of in-furnace temperatures of each division of the endmost heating zones and
of each
of heating zones other than the endmost heating zones.
[0032]
In the method for heat treating longer-length products of the present
invention,
if, in step 2, the heating output of each heat source in each division of the
endmost
heating zones is adjusted, at the time of heat treatment, based on the
measurement
result of actual temperature in the end portion of the product to be heat
treated, the
temperature control of the product to be heat treated can be carried out with
higher
accuracy.
[0033]
In the method for heat treating longer-length products of the present
invention,
if, in steps 1 and 2, an electric heater is used as the heat source, the
heating output
pattern is adjusted easily, and if the heating of the product to be heat
treated is by
way of radiative heating, accurate temperature control is carried out easily.
[0034]
(2) A method for manufacturing longer-length products, wherein heat
treatment is performed by using the method for heat treating longer-length
products
of the above item (1).
[0035]
(3) A heat treatment furnace for longer-length products, which is a batch-type
heat treatment furnace having a cylindrical shape opposite ends of which are
enclosed and the inside space of which is divided into a plurality of heating
zones
along a longitudinal direction and which is used to heat-treat a longer-length
product
to be heat treated which is charged into the heat treatment furnace,
wherein the heat treatment furnace is configured so that, among the heating
zones, an endmost heating zone thereof is subdivided into a plurality of
divisions
each having a length shorter than the length of each of heating zones other
than the
endmost heating zones; and a heat source is disposed in each division of the
endmost
heating zones,
and wherein the furnace at least includes:
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means for determining the heating output pattern of each heat source in each
division of the endmost heating zones;
means for measuring the in-furnace temperature of each division of the
endmost heating zones and of heating zones other than the endmost heating
zones;
and
means for controlling the heating output of individual heat source for each
division of the endmost heating zones.
[0036]
If the heat treatment furnace for longer-length products of the present
invention further includes means for measuring the actual temperature in the
end
portion of the product to be heated, the temperature control of the product to
be heat
treated can be carried out with higher accuracy.
[0037]
In the treatment furnace for longer-length products of the present invention,
if
the heat source is an electric heater, the heating output pattern is adjusted
easily, and
if the heating of the product to be heat treated is by way of radiative
heating, accurate
temperature control is carried out easily.
ADVANTAGEOUS EFFECTS OF INVENTION
[0038]
According to the method for heat treating longer-length products of the
present invention, when heat treatment is performed by using the cylindrical
batch-
type heat treatment furnace opposite ends of which are enclosed and which is
provided with the heat sources on the peripheral wall thereof, while no heat
source
being disposed on either end wall thereof, even if the space length in the
furnace is
the same, the effective furnace length can be made longer, and the longer-
length
product to be heat treated can be heated throughout the overall length thereof
to a
uniform temperature with high accuracy. Thereby, the equipment refurbishment
cost for a furnace body can be reduced significantly.
[0039]
A method for manufacturing longer-length products of the present invention
is the one using the above-described heat treating method, and by using this
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,
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manufacturing method, a longer-length product having no variations in quality
characteristics can be manufactured.
If the heat treatment furnace for longer-length products of the present
invention is used, the heat treating method and method for manufacturing
longer-
length products of the present invention can be carried out easily.
BRIEF DESCRIPTION OF DRAWINGS
[0040]
FIG. 1 is schematic views showing a general configuration example of a
conventional heat treatment furnace for longer-length products, FIG. 1 (a)
being a
transverse sectional view, and FIG. 1 (b) being a longitudinal sectional view.
FIG. 2 is a diagram showing one example of temperature distribution in a
longitudinal direction of a product to be heat treated in a conventional heat
treatment
furnace for longer-length products.
FIG. 3 is a diagram showing one example of temperature distribution in a
longitudinal direction of a product to be heat treated in the case where an
endmost
heating zone is heated to a temperature higher than that in central heating
zones by
using the conventional heat treatment furnace for longer-length products.
FIG. 4 is schematic views showing a general configuration of a heat treatment
furnace provided with heat-insulating plates, FIG. 4 (a) being a transverse
sectional
view, and FIG. 4 (b) being a longitudinal sectional view.
FIG. 5 is a diagram showing a measurement result of temperature distribution
of a
product to be heat treated in an endmost heating zone and the adjacent heating
zone
thereto, the measurement result being obtained by using the heat treatment
furnace
provided with the heat-insulating plates.
FIG. 6 is a diagram exemplarily showing a measurement result of temperature
distribution of a product to be heat treated in the m-zone and the adjacent
heating
zone thereto in the case where a division gradient heating control method is
applied.
FIG. 7 is a diagram showing a study result obtained by heat transfer
simulation,
showing the relationship between the intensification amount of heat input in
the m-1
zone in the case where a division gradient heating control method is applied
and a
temperature deviation range.
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FIG. 8 is schematic views exemplarily showing a general configuration of a
heat
treatment furnace used for a method for heat treating longer-length products
of the
present invention, FIG. 8 (a) being a transverse sectional view, and FIG. 8
(b) being a
longitudinal sectional view.
FIG. 9 is a schematic view showing the installation positions of thermocouples
on a
product to be heat treated charged into a heat treatment furnace in Example.
FIG. 10 is a diagram showing one example of a measurement result of
temperature in
the end portion of a product to be heat treated at the time of heating, which
was
obtained in Example.
DESCRIPTION OF EMBODIMENTS
[0041]
1. Method for heat treating longer-length products
A method for heat treating longer-length products of the present invention is
the one that uses a cylindrical batch-type heat treatment furnace opposite
ends of
which are enclosed and the inside space of which is divided into a plurality
of
heating zones in a longitudinal direction, wherein each of endmost heating
zones of
the heat treatment furnace is subdivided into a plurality of divisions each
having a
length shorter than the length of each of heating zones other than the endmost
heating zones, a heat source is disposed in each division, and the heat
treating
method includes the following steps 1 and 2:
(Step 1) in advance, determining the heating output pattern of each heat
source in each division of endmost heating zones based on the measurement
result of
actual temperature in the end portion of a product to be heat treated at the
time of
heating.
(Step 2) controlling the heating output of individual heat source, at the time
of
heat treatment for the product to be heat treated, based on the heating output
pattern
of each heat source determined in step 1 and further the measurement result of
in-
furnace temperature of each division of endmost heating zones and of each of
heating
zones other than the endmost heating zones.
[0042]
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Since the number of divisions in endmost heating zones does not exert an
influence on the solution of problems of the present invention, hereunder, for
convenience of explanation, the endmost heating zone is also represented by "m-
zone", and the endmost heating zone on the opposite side "n-zone", wherein as
the
m-zone and n-zone is equivalent, the m-zone is referred hereinafter.
[0043]
FIG. 6 is a diagram exemplarily showing a measurement result of temperature
distribution of a product to be heat treated in the m-zone and the adjacent
heating
zone to the m-zone in the case where the division gradient heating control
method is
applied. In this example, the m-zone as being the endmost heating zone is
equally
subdivided into three divisions as being m-1 zone, m-2 zone, and m-3 zone,
numbering from the end, and gradient heating in which the amount of heat (heat
input) was increased in the m-1 zone only was performed.
[0044]
In FIG. 6, "after refurbishment" means the case where gradient heating is
performed by using the heat treatment furnace in which the m-zone is
subdivided
into three divisions, and "before refurbishment" means the case of the
conventional
heating system where the same heat treatment furnace with the m-zone being
subdivided into three is used, the amount of heat in each of m-1 zone, m-2
zone, and
m-3 zone is constant, and thereby the gradient heating is not applied. The
intensification ratio of the amount of heat in the m-1 zone in the case of
after
refurbishment was set to +35% by referring to the study result obtained by the
after-
described heat transfer simulation shown in FIG. 7.
[0045]
As being evident from the result shown in FIG. 6, by applying the division
gradient heating control method in endmost heating zones of furnace, the whole
of
the product to be heat treated can be heated uniformly by suppressing the
overheat of
the end portion of the product to be heat treated within the tolerable range
and by
avoiding the decrease in temperature in the end portion thereof.
[0046]
FIG. 7 is a diagram showing a study result obtained by heat transfer
simulation, showing the relationship between the intensification amount of
heat input
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in the m-1 zone and a temperature deviation range in the case where the
division
gradient heating control method is applied. In this description, the
"temperature
deviation range" is a difference between the highest value and the lowest one
in the
temperature distribution of the product to be heat treated along the m-zone
and the
adjacent heating zone to the m-zone. As shown in FIG. 7, in the case where the
intensification amount of heat input in the m-1 zone is zero (that is, the
conventional
heating system in which gradient heating is not performed), the temperature
deviation range is about 80 C, whereas the temperature deviation range is
decreased
by the gradient heating, and indicates a minimum value of 10 C when the
intensification amount of heat input is +35%.
[0047]
The above is an explanation of the case where the m-zone is subdivided into
three, and the gradient heating is performed. In general, it has been revealed
that
the overall length of the product to be heat treated including the end
portions can be
heated to a uniform temperature by subdividing each of endmost heating zones
(the
m-zone and the n-zone) of furnace such that the length of division thereof is
shorter
than the length of a central heating zone, and by controlling the heating
outputs of
divisions of the endmost heating zones. Thereby, even if the space length in
the
furnace is the same, the effective furnace length can be enlarged
significantly.
[0048]
The heating outputs of divisions of the endmost heating zones that are
obtained by subdividing the endmost heating zones of furnace as described
above
can be controlled by determining, in advance, the ratio of output of the heat
source in
the division of endmost heating zones (hereinafter, referred to as a "heating
output
pattern") based on the actual temperature measurement result in the end
portion of
the product to be heat treated at the time of heating so that the temperature
distribution in the end portion of the product to be heat treated becomes
homogeneous in the entire region of the endmost heating zones of the furnace.
[0049]
CA 02790579 2012-08-20
- 17 -
FIG. 8 is schematic views exemplarily showing a general configuration of a
heat treatment furnace used for a method for heat treating longer-length
products of
the present invention, FIG. 8 (a) being a transverse sectional view, and FIG.
8 (b)
being a longitudinal sectional view. In this heat treatment furnace, among a
plurality of heating zones shown in FIG. 1, the endmost heating zone, that is,
the m-
zone is subdivided into three divisions as being m-1 zone, m-2 zone, and m-3
zone,
and similarly, the other endmost heating zone on the opposite side, that is,
the n-zone
is subdivided into three as being n-1 zone, n-2 zone, and n-3 zone.
[0050]
As shown in FIG. 8, this heat treatment furnace is a cylindrical vessel
opposite ends of which are enclosed, and the peripheral wall of the furnace is
of a
double construction consisting of a water-cooled wall 2 and a heat-insulating
wall 3,
and each of both end walls thereof is also of a double construction consisting
of a
water-cooled wall 7 and a heat-insulating wall 8. On the inner peripheral
surface of
the peripheral wall of furnace, an electric heater 6 is disposed as a heat
source for
each division and each of other heating zones. On both end walls of the
furnace,
the heater 6 is not provided.
[0051]
The reason why in the heat treatment furnace used for the heat treating
method of the present invention, endmost heating zones (the m-zone and the n-
zone)
each is subdivided into a plurality of divisions (m-1 zone, m-2 zone, and m-3
zone;
n-1 zone, n-2 zone, and n-3 zone) each division having a length shorter than
the
length of a central heating zone is as follows. For example, as shown in FIG.
3,
when a longer-length product is heat treated, if the whole of the endmost
heating
zone is heated to a higher temperature than the target, the product to be heat
treated
overheats in the vicinity of the central portion of the endmost heating zone,
while the
temperature decreases in the end portion of the product to be heat treated
close to the
end wall of furnace, and heating cannot be performed uniformly.
[0052]
That is, the reason why the endmost heating zone of the heat treatment
furnace of the present invention is subdivided into a plurality of divisions
is that the
output ratio of each heat source in each division can be adjusted by
subdividing the
CA 02790579 2012-08-20
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endmost heating zone. Although, in the heat treatment furnace exemplarily
shown
in FIG. 8, the number of divisions of the endmost heating zone of furnace is
three
both in the m-zone and n-zone, the number of divisions of the endmost heating
zone
of furnace may be determined appropriately based on the status quo of
temperature
decrease in the endmost heating zone, which has been grasped in advance.
[0053]
The reason why the heating output pattern of each heat source in each division
of the endmost heating zone is determined in advance based on the measurement
result of actual temperature of the end portion of the product to be heat
treated at the
time of heating is that both end portions of the longer-length product should
be
heated uniformly.
[0054]
The measurement of actual temperature of the product to be heat treated
corresponds to the temperature measurement in individual division constituting
the
endmost heating zone of furnace, and the temperature measurement can be made
by
installing thermocouples in predetermined locations in the end portion of the
product
to be heat treated. By this measurement of actual temperature, the result of
temperature measurement in the end portion (the portion receiving heat mainly
from
each heat source of each division) of the product to be heat treated is
determined in
advance so as to correspond to each of subdivided divisions of the endmost
heating
zones, and based on this result, the heating output pattern of each heat
source (the
output ratio of individual heat source) in the individual division of the
endmost
heating zones is determined.
[0055]
As a desirable example of the heating output pattern, there is available a
pattern in which the m-zone in the endmost portion of furnace is subdivided
into
three divisions, and the intensification amount of heat input in the m-1 zone
of these
divisions is set to +35% as shown in FIGS. 6 and 7. With this pattern, it can
be
expected that the temperature deviation range of the product to be heat
treated in the
m-zone and the adjacent heating zone thereto will be suppressed to about 10 C
or
less.
[0056]
CA 02790579 2012-08-20
- 19 -
When the longer-length product is heat treated, the heating output of
individual heat source is controlled based on the heating output pattern of
each heat
source determined as described above and further the measurement result of in-
furnace temperature of the endmost heating zones of furnace and of central
heating
zones (heating zones other than the endmost heating zones). This is for the
purpose
of heating the overall length of the product to be heat treated to a uniform
temperature with high accuracy.
[0057]
That is, in the endmost heating zones of furnace, the product to be heat
treated
is heated with the preset heating output pattern predetermined as described
above,
and further, considering the measurement result of in-furnace temperature of
the
endmost heating zones of furnace and of heating zones other than the endmost
heating zones, the heating output of individual heat source is controlled.
Thereby,
variations in furnace temperature caused by the changes of the number of
products to
be heat treated and/or of the position thereof in the furnace can be
suppressed, and
the accuracy of uniform heating of the overall length of the product to be
heat treated
including end portions can be improved.
[0058]
In each division of the endmost heating zones of furnace, the heating output
pattern has been set in advance. However, since the heating output is changed
by
the heating output control based on the measurement result of in-furnace
temperature
at the time of actual heat treatment, the heating output pattern sometimes
departs
from the preset pattern.
[0059]
It can also be said that the method for homogenizing a temperature
distribution in a product to be treated described in Patent Literature 4
shares
similarity with the heat treating method of the present invention with respect
to the
fact that the temperature is controlled after the predetermined setting value
of each
deviation setting instrument has been changed according to the kind of a
product to
be treated before heating.
[0060]
CA 02790579 2012-08-20
- 20 -
The main purpose of the method described in Patent Literature 4 is to carry
out the temperature control of a plurality of heating zones with the detection
value of
single thermometer by using the deviation setting instrument provided in each
zone.
In contrast, the heat treating method of the present invention is a method in
which, to
prevent the decrease in temperature in the end portion of the longer-length
product,
the heating output pattern is set in advance by further subdividing only the
endmost
heating zones, and the heating output of heat source in the individual
division is
controlled considering the measurement result of in-furnace temperature of
each
division. Therefore, the heat treating method of the present invention
obviously
differs from the method described in Patent Literature 4.
[0061]
Also, in the method described in Patent Literature 4, the control is carried
out
with a constant setting value, whereas, in the present invention, the heating
output
pattern sometimes shifts from the preset pattern. Therefore, the heat treating
method of the present invention differs from the method described in Patent
Literature 4 in this respect as well.
[0062]
In the method for heat treating longer-length products of the present
invention,
an embodiment mode can be used in which the heating output of each heat source
in
each division of endmost heating zones of furnace can be adjusted based on the
measurement result of actual temperature of the end portion of the product to
be heat
treated at the time of heat treatment. The actual temperature of the end
portion of
the product to be heat treated is measured in the actual heat treatment by
thermocouples installed in the end portion of the product to be heat treated.
[0063]
In the actual heat treatment, since the number of and the position of products
to be heat treated in the furnace are different, after the heat treatment has
been started
with the preset heating output pattern, the heating output of individual heat
source is
controlled based on the measurement result of in-furnace temperatures of the
endmost heating zones of furnace and of heating zones other than the endmost
heating zones.
[0064]
CA 02790579 2012-08-20
- 21 -
In the method for heat treating longer-length products of the present
invention,
an embodiment mode can be used in which the heating output of heat source in
each
division can be adjusted based on the measurement result of actual temperature
in the
end portion of the product to be heat treated. In the case where heating
outputs of
heat sources in all of subdivided divisions of endmost heating zones are
adjusted, this
adjustment can be performed in various modes according to the temperature
measurement result, such as a mode in which the heating outputs for all
divisions are
adjusted, or a mode in which the heating output of heat source in only one of
divisions is adjusted.
[0065]
By using such a mode of embodiment, the variations in temperature in the end
portion of the product to be heat treated is further decreased, and more
accurate
temperature control of the product to be heat treated can be carried out.
[0066]
In the method for heat treating longer-length products of the present
invention,
as the heat source for heating the product to be heat treated, an electric
heater is
preferably used. Although a burner, a radiant tube, or the like can be used as
the
heat source, the electric heater is suitably used because the heating output
pattern is
adjusted easily.
[0067]
Also, in the method for heat treating longer-length products of the present
invention, if the heating of the product to be heat treated is by way of
radiative
heating, accurate temperature control can be carried out easily. However, in
the
case where the temperature of only the end portion of the product to be heat
treated is
controlled with the predetermined heating output pattern as in the heat
treating
method of the present invention, if convection occurs in the furnace, even if
the
heating output pattern is determined, the heating is not performed with this
heating
output pattern, and accurate control is difficult to carry out.
[0068]
Therefore, it is desirable to use the heat treating method of the present
invention to carry out the control in a vacuum heat treatment furnace or the
control in
a heat treatment furnace in which radiative heating is performed in a gas
atmosphere
CA 02790579 2012-08-20
- 22 -
having a low heat capacity, such as hydrogen gas. It can be said that, among
them,
the method of the present invention is preferably used for the control in the
vacuum
heat treatment furnace because of the difficulty in control of gas in case of
hydrogen
atmosphere.
[0069]
According to the above-described method for heat treating longer-length
products of the present invention, in the case where used is the cylindrical
batch-type
heat treatment furnace opposite ends of which are enclosed and which is
provided
with the heat sources on the peripheral wall thereof, while no heat source
being
disposed on both end walls thereof, even if the space length in the furnace is
the
same, the effective furnace length can be made longer, and a product to be
heat
treated that is longer than before can be heated throughout the overall length
thereof
to a uniform temperature with high accuracy.
[0070]
In the method for heat treating longer-length products of the present
invention,
either one of the endmost heating zones of furnace may be divided, and the
above-
described effect can be achieved on the side that is divided.
[0071]
2. Method for manufacturing longer-length products
A method for manufacturing longer-length products of the present invention
is the one that heat treatment is performed by using the above-described
method for
heat treating longer-length products of the present invention.
[0072]
In the general practice of manufacturing of longer-length products, only the
heat treatment process is carried out by the heat treating method of the
present
invention, and the processes other than the heat treatment process are carried
out by
conformity to a customary method.
[0073]
According to the method for manufacturing longer-length products of the
present invention, in the heat treatment process, since a product to be heat
treated
that is longer than before can be heated throughout the overall length thereof
to a
uniform temperature, a longer-length product having no variations in quality
CA 02790579 2012-08-20
- 23 -
characteristics, such as mechanical properties and corrosion resistance, can
be
manufactured.
[0074]
3. Heat treatment furnace for longer-length products
A heat treatment furnace for longer-length products of the present invention
is
a batch-type heat treatment furnace for longer-length products, which has a
cylindrical shape opposite ends of which are enclosed and the inside space of
which
is divided into a plurality of heating zones in a longitudinal direction, and
into which
a longer-length product to be heat treated is charged to be heat treated. This
heat
treatment furnace has a feature of having a configuration described below. The
heat
treatment furnace of the present invention is configured so that, among the
heating
zones, an endmost heating zone is subdivided into a plurality of divisions
each
having a length shorter than the length of each of heating zones other than
the
endmost heating zones, and the heat source is disposed in each of the
divisions.
The heat treatment furnace of the present invention at least includes means
for
determining the heating output pattern of each heat source in each division of
the
endmost heating zones, means for measuring the in-furnace temperature of each
division and of each of heating zones other than the endmost heating zones,
and
means for controlling the heating output of the individual heat source for
each
division and each of other heating zones.
[0075]
The heat treatment furnace for longer-length products of the present invention
has a general configuration exemplarily shown in FIG. 8. In the configuration
exemplarily shown in FIG. 8, each of endmost heating zones (m-zone and n-zone)
in
of furnace is subdivided. However, the heat treatment furnace may be
configured
so that either one of the endmost heating zones is subdivided.
[0076]
The reason why an endmost heating zone is subdivided such that the length of
division be shorter than the length of a central heating zone in the heat
treatment
furnace of the present invention is that, as described in the heat treating
method of
the present invention, the endmost heating zone is subdivided, and the output
ratio of
CA 02790579 2012-08-20
- 24 -
the heat source disposed in the individual division of the endmost heating
zones is
adjusted.
[0077]
The reason why the heat treatment furnace of the present invention at least
includes a means for determining the heating output pattern of each heat
source in
each division in the endmost heating zones is that both end portions of the
longer-
length product are to be heated uniformly. The phrase "at least" means that
the heat
treatment furnace may have a means for determining the heating output pattern
of
each heat source in heating zones other than the subdivided divisions of the
endmost
heating zones.
[0078]
In determining the heating output pattern, as a procedure, first, it is
necessary
that the measurement result of temperature in the end portion of the product
to be
heat treated has been determined in advance so as to correspond to each
division
constituting the endmost heating zones of the furnace. This measurement result
of
actual temperature can be obtained in advance by measuring the temperature by
installing thermocouples in the end portion of the product to be heat treated.
Next,
based on the measurement result of actual temperature, the heating output
pattern of
each heat source in each division of endmost heating zones of furnace (the
output
ratio of individual heat source) is determined. This heating output pattern
can be set
by the operator based on the measurement result of actual temperature.
[0079]
Also, a system can be adopted in which, for example, an output pattern setting
instrument is installed in the heat treatment furnace, the relationship
between the
measurement result of actual temperature and the heating output pattern to be
set is
inputted in advance into the output pattern setting instrument, the output
pattern
setting instrument selects, on receipt of signals of the temperature
measurement
result, a proper heating output pattern based on the individual temperature
measurement results, and an output indication is given to each heat source in
each
division of the endmost heating zones.
[0080]
CA 02790579 2012-08-20
- 25 -
Therefore, as the means for determining the heating output pattern, available
are various methods such as a method for determining the heating output
pattern
artificially based on the thermocouples installed on the product to be heat
treated and
the measurement result of actual temperature, a method for determining the
heating
output pattern automatically by using the output pattern setting instrument,
or the like.
[0081]
The reason why the heat treatment furnace of the present invention has the
means for measuring the in-furnace temperatures of each division of the
endmost
heat zones and of each of heating zones other than the endmost heating zones
and the
means for controlling the heating output of each heat source for each division
and for
each of other heating zones is that by controlling the heating output of the
heat
source in each division and other heating zones, fluctuation in furnace
temperature
during heat treatment is suppressed, and the accuracy of uniform heating
throughout
the overall length of the product to be heat treated including the end
portions is
improved.
[0082]
As the means for measuring the furnace temperature of each division and each
of other heating zones, a temperature detector that has conventionally been
used to
measure the in-furnace temperature may be used. For example, a thermocouple
can
be used.
[0083]
As the means for controlling the heating output, an output controller or the
like can be used. The output controller is configured so that the target value
of in-
furnace temperature is inputted therein in advance, signals transmitted from
the
temperature detector are compared with this target temperature, and a control
signal
is sent to the heat source while PID control is carried out.
[0084]
The heat treatment furnace for longer-length products of the present invention
that further has means for measuring the actual temperature in the end portion
of the
product to be heat treated enables more accurate temperature control of the
product
to be heat treated.
[0085]
CA 02790579 2012-08-20
- 26 -
As the above-described temperature measuring means, available is a
thermocouple that is installed in the end portion of the product to be heat
treated at
the time of actual heat treatment and can measure the temperature.
[0086]
If the above-described heat treatment furnace for longer-length products of
the present invention is used, the heat treating method and method for
manufacturing
longer-length products of the present invention can be carried out easily.
[0087]
The mode of practical heat treatment operation using the heat treating method
of the present invention is as follows: as the heat treatment furnace, a
vacuum heat
treatment furnace provided with electric heaters as the heat sources is used.
(1) In performing heat treatment, first, the in-furnace temperatures in
central
heating zones excluding endmost heating zones of furnace are collectively
controlled
until the in-furnace temperature reaches an initial target temperature (a
temperature
lower than the product temperature required finally for the product to be heat
treated).
(2) After the initial target temperature has been reached, all of divisions
and
other heating zones are shifted toward an individual control system, and the
in-
furnace temperature of each of divisions and other heating zones is
individually
controlled until the in-furnace temperature reaches an ultimate target
temperature
(the product temperature required finally for the product to be heat treated).
(3) Regarding the endmost heating zones of furnace, gradient heating is
controlled with the preset heating output pattern by using the division
gradient
heating control method.
(4) The in-furnace temperature of each of divisions and other heating zones is
finely adjusted so that the temperature of the product to be heat treated
becomes the
control value.
EXAMPLE
[0088]
The temperature distribution in the end portion of a product to be heat
treated
was examined by using the heat treatment furnace of the present invention
having the
configuration shown in FIG. 8, by charging a metal tube into the furnace as
the
CA 02790579 2012-08-20
- 27 -
longer-length product to be heat treated (longer-length product), and by
heating the
metal tube by applying the heat treating method of the present invention. For
comparison, the similar examination was made of the case where the
conventional
heat treating method was applied.
[0089]
The heat treatment furnace used had a construction such that the inside space
thereof is divided into a plurality of heating zones including a m-zone and a
n-zone,
and each of the heating zones has a length of 3 m. The m-zone as being an
endmost
heating zone is subdivided into three divisions as being m-1 zone, m-2 zone,
and m-3,
numbering in that order from the end, while the n-zone as being the other
endmost
heating zone on the opposite end is subdivided into three divisions as being n-
1 zone,
n-2 zone, and n-3 zone, numbering in that order from the end. Each of the
subdivided divisions constituting the m-zone and n-zone as the endmost heat
zones
has a length of 1 m.
[0090]
The product to be heat treated was placed in the heat treatment furnace so
that
the tube end to be heat treated is positioned in the m-1 zone in the endmost
heating
zone as being the m-zone, and a total of five thermocouples were installed on
the
product to be heat treated. These thermocouples were installed at four
positions
where first position is 600 mm away from the end wall of furnace and other
three
intermediate positions are equally spaced with 200 mm interval from the first
position and from each other, and the last position is 1500 mm away from the
end
wall of furnace.
[0091]
FIG. 9 is a schematic view showing the installation positions of
thermocouples. In the n-zone as well, thermocouples were installed at a total
of
five positions on the product to be heat treated in the same way as in the m-
zone.
The encircled numerals of 1 to 5 and 6 to 10 in FIG. 9 indicate the
installation
positions of thermocouples.
[0092]
Heating was started by equalizing the output of each heat source in each of
three-divisions of the endmost heating zone (m-1 zone, m-2 zone, and m-3 zone)
of
CA 02790579 2012-08-20
- 28 -
the m-zone to the output of central heating zones, that is, by making the
output ratio
100% with respect to the heat source in the central heating zone. Thereafter,
heating was performed by varying the output ratio of m-1 zone to 142% and the
output ratio of m-2 zone to 85% (the output ratio of m-3 zone was not changed,
still
being 100%).
[0093]
That is, in the test of Examples, the division gradient heating control method
used in the heat treating method of the present invention was applied. The
output
ratio in this test was determined and set based on the measurement result of
actual
temperature of the product to be heat treated obtained in advance. In the n-
zone as
well, heating was performed by varying the output ratio in the same way in
process
of heating.
[0094]
FIG. 10 shows one example of a measurement result of temperatures in the
end portion of the product to be heat treated. FIG. 10 is charts on which the
measurement result of temperatures of the product to be heat treated is
recorded
automatically. The encircled numerals 1 to 10 in FIG. 10 indicate the product
temperatures measured by using thermocouples at the installation positions
shown in
FIG.9.
[0095]
As shown in FIG. 10, in the time period from the start of heating (temperature
rise) to output change (gradient heating start), among the temperatures
measured by
individual thermocouples, the temperatures of the product to be heat treated
especially measured at positions close to the end portion (encircled numerals
of 1, 2,
6 and 7) deviated far from target temperature 10 C, and the difference over
the
target temperature was about 50 C at a maximum.
[0096]
It can be seen from the charts that after the outputs given to the heat
sources
in the m-1 zone and the m-2 zone had been changed as described above and the
gradient heating had been started in process of heating, as marked up by oval
marks
(broken lines) in FIG. 10, the temperatures of the product to be heat treated
were kept
CA 02790579 2012-08-20
- 29 -
within 10 C with respect to the target temperature at all temperature
measurement
positions.
[0097]
FIG. 6, explained before, is a diagram obtained by summarizing one example of
the
measurement result of temperatures in the end portion of the product to be
heat
treated, the measurement being made as described above. In the case where the
output ratios of all of the three-divisions (the m-1 zone, m-2 zone, and m-3
zone) of
the m-zone were made 100% (indicated as "before refurbishment" represented by
outlined square marks (0)), at a position close to the end portion of the
product to be
heat treated (in the vicinity of the middle of the m-1 zone), there was a
temperature
difference of about 45 C over that of the adjacent heating zone to the m-zone.
In
contrast, in the case where the heat treating method of the present invention
was
applied (indicated as "after refurbishment" represented by solid circle marks
(8 )),
the temperature difference over that of the adjacent heating zone to the m-
zone
reduced significantly to within 7 C.
[0098]
From the results shown in FIGS. 10 and 6, it could be confirmed that by
applying the heat treating method of the present invention, the effective
furnace
length of the cylindrical batch-type heat treatment furnace having no heat
source on
both end walls thereof can be extended, and the temperature control for
controlling
the temperatures throughout the overall length including the end portions of
the
longer-length product to be heat treated to within 10 C with respect to the
target
temperature can be carried out sufficiently.
INDUSTRIAL APPLICABILITY
[0099]
According to the method for heat treating longer-length products of the
present invention, the effective furnace length can be extended, and the
longer-length
product to be heat treated can be heated throughout the overall length thereof
to a
uniform temperature with high accuracy.
According to the method for manufacturing longer-length products of the
present invention that uses the above-described heat treating method, a
product
CA 02790579 2012-08-20
- 30 -
having no variations in quality characteristics, such as mechanical properties
and
corrosion resistance, can be manufactured. Also, if the heat treatment furnace
for
longer-length products of the present invention is used, the heat treating
method and
manufacturing method of the present invention can be carried out easily.
Therefore, the heat treating method of the present invention, the method for
manufacturing longer-length products to which this heat treating method is
applied,
and the heat treatment furnace of the present invention can be utilized
effectively for
the heat treatment and manufacture of longer-length products.
REFERENCE SIGNS LIST
[0100]
1: electric heater, 2: water-cooled wall, 3: heat-insulating wall, 4: trailer,
5: product to be heat treated, 6: electric heater, 7: water-cooled wall,
8: heat-insulating wall, 9: heat-insulating plate
