JP2019019394A - Heat treatment method and heat treatment apparatus for steel sheet - Google Patents

Abstract

To suppress variation in mechanical properties in the longitudinal direction of a thick steel plate without decreasing the productivity.SOLUTION: In the heat treatment method, a steel plate is subjected to a heat treatment while going through the heat treatment apparatus. At least one of the fuel supply amount and a combustion gas flow rate to a heating source in each zone is adjusted in the heat treatment furnace partitioned into a plurality of zones to control the ambient temperature for each zone. In each of the zones, the temperature is measured at a plurality of points apart from each other, and at least one of the fuel supply amount and the combustion gas flow rate to the heating source in each zone is adjusted based on the measured temperature at the plurality of points.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a heat treatment method and a heat treatment apparatus for a steel plate, particularly a thick steel plate having a thickness of about 6 to 200 mm.

  Typical heat treatments for steel sheets include quenching, normalizing, annealing, and tempering. In the heat treatment of these steel plates, since the processing temperature of the steel plate directly affects the quality of the final product, the temperature control of the heat treatment furnace for performing the heat treatment of the steel plate is extremely important in the manufacturing process of the steel plate.

  2. Description of the Related Art Conventionally, a continuous heating furnace, which is a typical furnace for heat-treating steel sheets, is divided into a plurality of zones, and the temperature is adjusted for each zone. Specifically, the temperature of each zone is adjusted by controlling the flow rate of fuel and combustion air supplied to the heating device (for example, a burner or a radiant tube) of each zone.

  Regarding the temperature adjustment of each zone, for example, in Patent Document 1, the surface temperature of the target position of the plate material is measured with a radiation thermometer installed in each zone in the furnace, and the surface state of the plate material in the continuous furnace is monitored. How to do is described.

Japanese Patent Laid-Open No. 5-142170

  By the way, the conveyance method of the steel plate in a heat treatment furnace changes with plate | board thickness of the steel plate to heat-process. If the steel plate is relatively thin, the time required for the heat treatment is shortened. Therefore, as described in Patent Document 1, the plate is continuously passed in one direction from the entrance to the exit of the heat treatment furnace.

  On the other hand, in the case of a thick steel plate having a thickness of about 6 to 200 mm, the time required for the heat treatment is longer than that of the thin plate. Therefore, with thick steel plates, after transporting for a predetermined time from the inlet side to the outlet side of the heat treatment furnace with a transport roll, the transport direction is reversed, transported for a predetermined time toward the inlet side, and then again toward the outlet side. In general, so-called oscillation (stagnation) is performed a predetermined number of times according to heat treatment conditions.

  However, when heat treatment including oscillation in the heat treatment furnace as described above is performed, mechanical properties such as tensile strength and toughness may vary in the longitudinal direction of the thick steel plate after heat treatment. This was a problem specific to heat treatment.

  As a result of intensive studies on such problems, the temperature of each zone of the heat treatment furnace is adjusted based on the temperature measured by the furnace thermometer installed in each zone. Depending on the relative positional relationship with the gauge, the temperature inside the furnace may not be controlled so that the entire thick steel plate is at the desired uniform temperature, and a new high temperature part and low temperature part may occur in the thick steel plate. I found out.

  That is, as shown in FIG. 4, in the zone 2 on the right side from the center and the front where the thick steel plate P exists immediately below the furnace thermometer 4, the measured temperature decreases (from the set furnace temperature). And the combustion air is increased to increase the temperature in the zone. However, the thick steel plate P does not exist directly below the furnace thermometer 4 as in the left zone 2 from the front, but when the thick steel plate P partially exists in the zone 2, Since the measurement temperature is not particularly lowered, the operation for raising the temperature in the zone is not performed. As a result, the portion of the thick steel plate entering the zone 2 on the left side is not sufficiently heated, causing a temperature deviation in the thick steel plate P described above.

  As a method for reducing the temperature deviation in the thick steel plate, a method of increasing the residence time of the thick steel plate in the furnace by increasing the oscillation distance to achieve uniform heating can be considered. However, when such an oscillation with a long distance is performed, the occupation ratio (cover ratio) of the steel plate in the heat treatment furnace decreases, leading to a decrease in productivity.

  This invention is made | formed in view of said situation, and it aims at reducing the dispersion | variation in the mechanical characteristic in the longitudinal direction of a thick steel plate, without reducing productivity.

  The inventors have investigated a method for solving the problem of temperature deviation of the thick steel plate due to the relative position of the furnace thermometer and the thick steel plate in each zone of the heat treatment furnace. Based on these measured temperatures measured, by adjusting at least one of the fuel supply amount to the heating source and the combustion gas flow rate in each zone, without changing the oscillation conditions, The inventors have newly found that the variation in mechanical properties in the longitudinal direction of the thick steel plate can be reduced, and the present invention has been completed.

The present invention is based on the above-described novel findings, and the gist of the present invention is as follows.
1. A heat treatment furnace is divided into a plurality of zones, and the atmosphere temperature is controlled for each zone by adjusting at least one of the fuel supply amount to the heating source and the combustion gas flow rate in each zone. In performing heat treatment on the steel sheet,
In each zone, temperature measurement is performed at a plurality of points separated from each other, and at least one of the fuel supply amount to the heating source and the combustion gas flow rate in each zone is adjusted based on the measurement temperatures at the plurality of points. Heat treatment method for steel sheet.

2. The steel sheet according to 1 above, wherein at least one of a fuel supply amount to a heating source and a combustion gas flow rate in each zone is adjusted based on an average temperature obtained by averaging the measurement temperatures at the plurality of points. Heat treatment method.

3. The temperature measured at the plurality of points is averaged by weighted averaging according to the length of the steel plate occupying each zone or by a constant α, and based on the average temperature, the amount of fuel supplied to the heating source in each zone and The heat treatment method for a steel sheet according to the above 1, wherein at least one of the combustion gas flow rates is adjusted.

4). A heat treatment apparatus in which the inside of a heat treatment furnace is partitioned into a plurality of zones,
Each of the plurality of zones is provided with a heating source and has a thermometer for measuring a temperature in each zone at a plurality of positions spaced apart from each other.

5. When the length in the conveyance direction of each steel plate of the plurality of zones is L,
One of the thermometers is disposed at a position of 0.28L to 0.38L in the transport direction from the entrance side of each zone,
5. The heat treatment apparatus for thick steel plates according to 4, wherein another one of the thermometers is disposed at a position of 0.62L to 0.72L in the transport direction from the entrance side of each zone.

  According to the present invention, variations in mechanical properties in the longitudinal direction of a thick steel plate can be reduced without reducing productivity.

It is the schematic of the heat processing furnace which concerns on one Embodiment of this invention. It is a schematic diagram of one zone among a plurality of zones of a heat treatment furnace. It is a graph which shows the relationship between a thick steel plate length and the temperature difference in a board. It is the schematic which shows the conventional heat processing method.

Hereinafter, the detail of the heat processing method and heat processing apparatus of the steel plate concerning this invention is demonstrated.
In one embodiment of the present invention, as shown in FIG. 1, the interior of the heat treatment furnace 1 (heat treatment apparatus) is partitioned into a plurality of zones 2. Each of the plurality of zones 2 is provided with a burner 3 (heating source) for heating, and a plurality of furnace thermometers 4 (thermometers) in the illustrated example are separated from each other for measuring the temperature in the zone 2. Are arranged. The thick steel plate P is carried into the heat treatment furnace 1 from the heat treatment furnace inlet 6 by the transport roll 5. The thick steel plate P passing through the heat treatment furnace 1 is subjected to heat treatment while causing oscillation Q by reciprocating between the entrance side and the exit side of the furnace as indicated by arrows in the figure. The thick steel plate P that has been subjected to the heat treatment is transported from the heat treatment furnace outlet 7 to the outside of the heat treatment furnace 1.

  In the heat treatment furnace 1, the ambient temperature is controlled for each zone 2 by adjusting at least one of the fuel supply amount to the burner 3 and the combustion gas flow rate. The combustion gas includes pure oxygen in addition to air. The length of the heat treatment furnace 1 in the conveyance direction is generally 50 to 100 m.

  Each of the zones 2 preferably has a length in the transport direction of 3 to 10 m. More preferably, it is 3-5 m. In FIG. 1, only three zones 2 are sectioned, but FIG. 1 is only a schematic view of the heat treatment furnace 1, and it is preferable that five to ten zones 2 are formed.

The burner 3 is a heating source for controlling the ambient temperature for each zone. In the present embodiment, the burner 3 is disposed on the ceiling of each zone 2, and the ambient temperature in the zone 2 is increased by burning fuel such as natural gas or by-product gas using air or oxygen. Let Instead of disposing the burner 3 on the ceiling of each zone 2, it is also possible to dispose the burner 3 on the side wall of the furnace at the heights of the upper and lower parts of the thick steel plate.
In the present embodiment, a burner is used as a heating source, but instead, a radiant tube or the like may be used as a heating source.

Each of the furnace thermometers 4 is arranged in each zone 2 so as to be separated in the conveying direction of the thick steel plate P. If the length of the transport direction of the zone 2 is L, one of the thermometers 4 is arranged at a position of 0.28L to 0.38L in the transport direction from the end of the zone 2 on the heat treatment furnace inlet 6 side. One furnace thermometer 4 is preferably disposed at a position of 0.62 L to 0.72 L in the transport direction from the end of the zone 2 on the heat treatment furnace inlet 6 side. By arranging in this way, the furnace temperature can be detected in a wider range by two furnace thermometers.
More preferably, one of the thermometers 4 is disposed at a position of 0.33 L in the conveying direction from the end of the zone 2 on the heat treatment furnace inlet 6 side, and the other furnace thermometer 4 is disposed at the heat treatment furnace inlet 6. It is arranged at a position of 0.67 L in the transport direction from the end of the zone 2 on the side.

The distance between the steel plate side tip of the furnace thermometer 4 and the surface of the thick steel plate P conveyed in the zone 2 is preferably 150 to 300 mm. When it is longer than 300 mm, it is impossible to detect the furnace temperature drop due to the steel plate, and when it is shorter than 150 mm, the range in which the furnace thermometer can detect the furnace temperature drop due to the steel plate becomes narrow.
In this embodiment, two furnace thermometers 4 are arranged in each zone 2, but instead of this, three or more furnace thermometers 4 are arranged apart from each other in the conveying direction of the thick steel plate P. It is good to do.

  The conveyance roll 5 conveys the thick steel plate P by rotating the roll. After transporting the thick steel plate P from the heat treatment furnace inlet 6 side to the heat treatment furnace outlet 7 side for a predetermined time, reverse the rotation direction of the roll, and transport it from the heat treatment furnace outlet 7 side to the heat treatment furnace inlet 6 side for a predetermined time, Then, it conveys again from the heat treatment furnace inlet 6 side to the heat treatment furnace outlet 7 side. After such an oscillation is performed a predetermined number of times according to the heat treatment conditions, the heat-treated thick steel plate P is carried out from the heat treatment furnace outlet 7 side.

A heat treatment method using such a heat treatment apparatus will be described below.
In each zone 2 in the heat treatment furnace 1, the heat treatment temperature of the thick steel plate P is controlled by controlling the atmospheric temperature in each zone 2 based on the temperature measured at a plurality of points measured by the furnace thermometer 4. . Control of the atmospheric temperature in each zone 2 is performed by adjusting at least one of the fuel supply amount to the burner 3 and the combustion gas flow rate as described above.

In addition, the measurement temperature at a plurality of points measured by the furnace thermometer 4 is an index for controlling the ambient temperature in each of the zones 2.
In one embodiment, an average value of measured temperatures at a plurality of points measured in one zone 2 is used as an index for controlling the ambient temperature in the zone 2. For example, if two furnace thermometers 4 are installed in one zone 2 and the measured temperatures are 620 ° C. and 610 ° C., the average value of these measured temperatures is set to 615 ° C. It is used as an index for controlling the ambient temperature inside. By doing so, it is possible to suppress the occurrence of over-baking and under-baking of the steel plate due to the variation in the measurement temperature due to the positional relationship between the steel plate and the measurement point, and the atmospheric temperature being not properly controlled.

In another embodiment, the zone 2 is divided into a front stage and a rear stage, and the temperature obtained by performing a weighted average of the measured temperatures of the front stage and the rear stage according to the occupation length in the transport direction of the front and rear steel plates P is the ambient temperature. It is used as an index of control.
Specifically, as shown in FIG. 2, among each zone 2, the first stage (heat treatment furnace inlet 6 side) is the first zone 2A, and the second stage (heat treatment furnace outlet 7 side) is the second zone 2B. The weighted average is performed by the following formula (1), and the value obtained by the formula (1) is used as an index for controlling the ambient temperature in the zone 2. By doing in this way, since combustion control is appropriately performed based on the furnace temperature of the place where the steel sheet exists, the atmosphere temperature in the furnace is controlled to the target temperature, and the steel sheet is at a desired temperature. Heat treatment can be performed, and overheating of a portion where there is no steel plate and where there is no steel plate can be prevented.
[A (° C.) × X / (X + Y)] + [B (° C.) × Y / (X + Y)] (1)
here,
A is the measured temperature of the first zone 2A,
B is the measured temperature of the second zone 2B,
X is the length in the transport direction in which the steel plate occupies the first zone 2A,
Y is the length in the transport direction in which the steel plate occupies the second zone 2B.

The measured temperature A of the first zone 2A may be an average measured temperature of all the furnace thermometers 4 installed in the first zone 2A, or is arranged in the first zone 2A. It is good also as employ | adopting as the measurement temperature A only the temperature of the furnace thermometer 4 which is measuring the part which the thick steel plate P occupies among the furnace thermometers 4 which are present. Or it is good also as employ | adopting as the measurement temperature A the temperature of the furnace thermometer 4 arrange | positioned nearest to the 2nd zone 2B side among the furnace thermometers 4 installed in the 1st zone 2A. Alternatively, the temperature of the furnace thermometer 4 arranged farthest on the second zone 2B side may be adopted as the measurement temperature A.
The measurement temperature B of the second zone 2B may be an average measurement temperature of all the furnace thermometers 4 installed in the second zone 2B, or is arranged in the second zone 2B. It is good also as employ | adopting only the temperature of the furnace thermometer 4 which is measuring the part which the thick steel plate P occupies among the furnace thermometers 4 currently used as the measurement temperature B. Or it is good also as employ | adopting as the measured temperature B the temperature of the furnace thermometer 4 arrange | positioned nearest to the 1st zone 2A side among the furnace thermometers 4 installed in the 2nd zone 2B. Alternatively, the temperature of the furnace thermometer 4 arranged farthest on the first zone 2A side may be adopted as the measurement temperature B.

In the above embodiment, the zone 2 is divided into the former stage and the latter stage, and the measured temperatures of the former stage and the latter stage are weighted and averaged according to the occupation length in the conveying direction of the former stage and the latter stage thick steel plates P. Instead of performing the weighted average according to the length, the weighted average may be performed using the constant α. Specifically, among each zone 2, the first stage is the first zone 2A and the second stage is the second zone 2B. The weighted average is performed by the following formula (2), and the value obtained by the formula (2) is calculated. It is used as an index for controlling the atmospheric temperature in the zone 2.
[A (° C.) × α] + [B (° C.) × (1−α)] (2)
here,
α is a constant,
A is the measured temperature of the first zone 2A,
B is the measured temperature of the second zone 2B.

The constant α is determined according to the positional relationship between the thick steel plate P and the furnace thermometer 4 and is an arbitrary value from 0 to 1. For example, three consecutive zones 2 are designated as zone 2-1, zone 2-2, and zone 2-3 from the heat treatment furnace inlet 6 side, respectively, and the front stage (heat treatment furnace inlet 6 side) and rear stage ( At least one furnace thermometer 4 is disposed on each of the heat treatment furnace outlet 7 side, and when the thick steel plate P is oscillated across these three zones 2-1 to 2-3, the bottom end of the thick steel plate P enters. For the zone 2-1 that goes out, the furnace thermometer 4 arranged in the rear stage in the zone 2-1 has a higher frequency than that of the furnace thermometer 4 arranged in the former stage. Therefore, it is preferable that α is small (the temperature measured by the subsequent thermometer 4 is regarded as important). Therefore, in the zone 2-1, it is preferable that α is more than 0 and 0.4 or less.
Regarding the zone 2-3 in which the top end of the thick steel plate P enters and exits, the furnace thermometer 4 arranged in the former stage is thicker than the furnace thermometer 4 arranged in the latter stage. Since the steel plates P are close to each other, it is better that α is larger (the measurement temperature of the preceding furnace thermometer 4 is emphasized). Therefore, in zone 2-3, α is preferably 0.6 or more and less than 1.
Regarding the central zone 2-2, the frequency with which the thick steel plate P approaches is not significantly changed between the former stage and the latter stage. Therefore, α is preferably set to 0.5.
In addition, when the thick steel plate P does not enter to the front | former stage of the zone 2-1, it is preferable to set (alpha) of the zone 2-1 to 0 (it controls only by the measurement temperature of the latter-stage furnace thermometer 4). Further, when the thick steel plate P does not enter the subsequent stage of the zone 2-3, it is preferable to set the α in the zone 2-3 to 1 (controlled only by the measured temperature of the furnace thermometer 4 in the previous stage).

  By controlling the atmospheric temperature in the zone 2 based on the above index, the entire thick steel plate P can be obtained at a desired uniform temperature in the longitudinal direction regardless of the length of the oscillated steel plate. It can be heat treated.

Example 1
Under the following conditions, heat treatment of various steel plates P having a thickness of 120 mm and a length of 5.4 to 12.0 m was performed in a heat treatment furnace having the following specifications according to the structure shown in FIG. At that time, the atmospheric temperature of each zone 2 in the heat treatment furnace 1 was controlled.
Setting furnace temperature: 625 ℃
Furnace time: 120 minutes Zone length: 4.8m
Number of zones: 3
Oscillation speed: 1.5m / min
In the conventional example, regardless of the position of the thick steel plate P, the measured temperature of the furnace thermometer 4 is simply used as the ambient temperature of the zone 2 as it is. In the present invention example, the measured temperature of the furnace thermometer 4 in each zone 2 is used. Was calculated, and the ambient temperature of zone 2 was controlled based on this value.

  The in-plate temperature difference of the thick steel plate P heat-treated by controlling the atmospheric temperature in the zone 2 according to the conventional example and the example of the present invention was measured. The in-plate temperature was calculated by measuring the thickness 1 / 2t (60 mm from the surface of the steel plate) of the thick steel plate P at a pitch of 50 mm in the longitudinal direction and calculating the heat transfer difference in the thickness direction. Further, the temperature difference in the plate was calculated as the difference between the maximum temperature of the measured points (excluding the portion of the steel plate end 100 mm) and the minimum temperature.

  The result is shown in FIG. When the length of the thick steel plate P is up to 7 m, there is almost no difference in the temperature deviation in the plate due to the difference in the atmospheric temperature control method in the zone 2 between the conventional example and the present invention example. However, when the length of the thick steel plate P exceeds 7 m, the temperature difference gradually increases in the conventional example, and when the length of the thick steel plate P is 12 m, the temperature difference within the plate becomes 60 ° C. at the maximum. On the other hand, in the example of the present invention, the temperature difference in the plate can be suppressed to about 40 ° C. at the maximum. Thus, according to the present invention, the in-plate temperature difference can be reduced without changing the oscillation distance. As a result, variations in the mechanical properties in the length direction of the plate could be suppressed.

1 Heat treatment furnace (heat treatment equipment)
2 Zone 2A 1st zone 2B 2nd zone 3 Burner (heating source)
4 Furnace thermometer (thermometer)
5 Transport roll 6 Heat treatment furnace inlet 7 Heat treatment furnace outlet P Thick steel plate

Claims (5)

A heat treatment furnace is divided into a plurality of zones, and the atmosphere temperature is controlled for each zone by adjusting at least one of the fuel supply amount to the heating source and the combustion gas flow rate in each zone. In performing heat treatment on the steel sheet,
A steel plate that measures temperature at a plurality of points separated from each other in the zone and adjusts at least one of a fuel supply amount to a heating source and a combustion gas flow rate in each zone based on the measured temperatures at the plurality of points. Heat treatment method.   The steel plate according to claim 1, wherein at least one of a fuel supply amount to a heating source and a combustion gas flow rate in each zone is adjusted based on an average temperature obtained by averaging the measured temperatures at the plurality of points. Heat treatment method.   The temperature measured at the plurality of points is averaged by weighted averaging according to the length of the steel plate occupying each zone or by a constant α, and based on the average temperature, the amount of fuel supplied to the heating source in each zone and The heat treatment method for a steel sheet according to claim 1, wherein at least one of the combustion gas flow rates is adjusted. A heat treatment apparatus in which the inside of a heat treatment furnace is partitioned into a plurality of zones,
Each of the plurality of zones is provided with a heating source and has a thermometer for measuring a temperature in each zone at a plurality of positions spaced apart from each other. When the length in the conveyance direction of each steel plate of the plurality of zones is L,
One of the thermometers is disposed at a position of 0.28L to 0.38L in the transport direction from the entrance side of each zone,
The another one of the said thermometers is a heat processing apparatus of the thick steel plate of Claim 4 arrange | positioned in the position of 0.62L-0.72L in the said conveyance direction from the entrance side of each zone.

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