JPH04218654A - Manufacture of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To hold the alloying degree of a galvannealed steel sheet to the properly. CONSTITUTION:Three pieces or above of radiation type sheet temp. gauges 6, 7 and 8 are arranged in an alloying furnace 1 in the furnace direction to measure the sheet temp., and the difference in the sheet temp. by the adjacent sheet temp. gages (the sheet temp. gage 7-the sheet temp. gage and the sheet temp. 8-the sheet temp. system 7) are found one by one from the lower direction of the furnace, and the indicated value of the sheet temp. system in the lower direction between the first adjacent sheet temp. systems in which the above difference is regulated to the predefined value or below is obtd. to control the flow rate of fuel in the alloying furnace so that the indicated value of a sheet temp. system for controlling will coincide with the above indicated value.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an alloyed hot-dip galvanized steel sheet.

0002

BACKGROUND OF THE INVENTION Conventionally, hot-dip galvanized steel sheets have been known that have been subjected to an alloying treatment so that part or all of the plating layer is an Fe--Zn alloy layer. In this alloying process, as shown in FIG. 2, an alloying furnace 2 is placed directly above the hot-dip galvanizing tank 4, and the zinc on the surface of the steel sheet 1 pulled up from the galvanizing tank 4 is removed by a squeezing device 3.
After that, the steel sheet is immediately heated in the alloying furnace 2 to diffuse iron into the zinc layer. Since insufficient alloying impairs its quality characteristics, it is necessary to control the alloying treatment conditions with high precision.

Conventionally, such alloying treatment has had the following difficulties. (1) There are many factors that affect alloying processing, such as sheet temperature, amount of zinc deposited, aluminum concentration in the plating bath, etc., and it is difficult to clarify in advance the appropriate heat treatment conditions in the alloying furnace. be. (2) As shown in Figure 3, the emissivity of the surface of a galvanized steel sheet changes depending on the sheet temperature, and changes suddenly during the process of alloying. Therefore, it is difficult to measure the true plate temperature using the emissivity setting type plate thermometer (radiation thermometer) that is normally used. (3) In alloying furnaces, steel sheets are strongly affected by radiation from flames, so it has been difficult to maintain constant heat treatment conditions by controlling the furnace temperature.

As a conventional technique, the radiant energy of a steel plate is measured by taking advantage of the fact that alloying progresses when the emissivity of the galvanized surface suddenly changes as shown in FIG. 3, and the emissivity is calculated by a calculator. A method of estimating the degree of alloying from the ratio and controlling the degree of alloying by controlling the furnace temperature (Japanese Patent Application Laid-Open No. 50-67730
). In addition, there is a method to control the degree of alloying by measuring the radiant energy and controlling its absolute value (
JP-A-57-185966).

However, with these techniques, (a) it is difficult to accurately calculate emissivity from radiant energy values; (b) Since the radiant energy of the steel sheet is affected by factors other than the degree of alloying, such as the Al concentration in the bath and the amount of zinc deposited, it is difficult to accurately control the degree of alloying by controlling only the radiant energy. There were drawbacks such as.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to overcome the drawbacks of the prior art described above and to provide a method for controlling the degree of alloying with high precision.

[0007]

[Means for Solving the Problems] The present invention takes the following measures to ensure a proper degree of alloying of a steel plate after alloying treatment. ■ Taking advantage of the fact that the readings of three or more radiation thermometers placed sequentially in the furnace height direction in the alloying furnace change due to the decrease in emissivity due to alloying of the plated steel sheets, Accurately determine the alloying completion position in the plating bath by reducing the difference between the average readings of the thermometer. ■ Control the fuel flow rate of the alloying furnace so that the average reading of the control plate thermometer located at the uppermost position in the furnace matches the average reading of the plate thermometer corresponding to the position where alloying of the plating layer is completed.

[0008]

[Operation] The principle of the present invention is shown below. The zinc on the surface of the steel sheet entering the alloying furnace is remelted and forms an alloy with the iron in the steel sheet at the interface with the steel sheet, and this alloy layer grows until the entire surface is covered with an alloy layer (solid phase). Therefore, as shown in FIG. 3, a sudden change in emissivity ε occurs.

FIG. 4 shows the relationship between the true plate temperature and the indicated value of the plate thermometer when the set emissivity ε of the plate thermometer is 0.5. As shown in Figure 3, the emissivity of the steel plate surface changes rapidly during the process of alloying, so in a type of plate thermometer that sets the emissivity in advance, the true emissivity and the set radiation for the plate thermometer are As the deviation from the rate increases, the true plate temperature and the plate thermometer indication value deviate greatly, as shown in part A shown in FIG. In the part A of FIG. 4, the difference between the true emissivity and the set emissivity is large, and therefore the difference between the true plate temperature and the plate thermometer reading is large. If the true plate temperature fluctuates by 10°C in this part A, the plate temperature indication value will fluctuate by about 100°C. Further, when the alloying process is completed, the emissivity of the steel plate becomes stable, so that it becomes as shown in, for example, part B in FIG. 4. In part B of Figure 4, when the true plate temperature fluctuates by 10°C, the plate thermometer reading is 10°C.
There will be some variation.

The present invention takes advantage of this phenomenon and completes alloying by performing an operation at three or more locations in the furnace height direction to determine the average of the plate thermometer readings from the plate thermometer readings that have a certain amplitude. This is to find the position. In order to prove the above principle, FIG. 5 shows the relationship between the average value of the plate temperature indication, the amplitude of the indication value fluctuation, and the Fe concentration in the plating layer. In the case of Fig. 5, the indicated value fluctuation is small (approximately 5°C) when the average plate temperature indication is in the range of 545°C to 555°C, and at that time the Fe concentration in the plating layer is 1.
The alloying degree is 2 to 13%, which is approximately the optimum degree of alloying. In other words,
The average sheet temperature indication when the fluctuation range of the indication value is small corresponds to the degree of alloying (Fe concentration in the plating layer), and the degree of alloying can be controlled by controlling the average sheet temperature indication. becomes.

A method for determining the alloying completion position from the average reading of each plate thermometer will be described below. As shown in FIG. 6, five plate thermometers 11, 12, 13 are installed in the alloying furnace 2.
, 14, 15 are installed in sequence in the direction of the furnace height, and the control plate thermometer 2
An example of finding the alloying completion position will be explained using the case in which 1 is installed as an example. (2) Set the average plate temperature standard value Ts (°C) for each plate thermometer and the standard value ΔTs (°C) for the difference between the average values of adjacent plate thermometers. ■ The average reading of each plate thermometer and the average standard value of plate temperature Ts (
℃), and the difference between the average values of each adjacent plate thermometer and the reference value ΔTs (℃) of the difference between the average values.

The average readings of the plate thermometers 11, 12, 13, 14, and 15 are T1, T2, T3, and T4.
, T5. In addition, the difference between the average readings of adjacent plate thermometers is determined as follows. ΔTA = T2 - T1, ΔTB = T3 - T
2, ΔTC = T4 - T3, ΔTD = T5
-T4, Hereinafter, each of the following cases will be judged.

Case 1: (FIG. 7a) When T1 to T5 are all higher than Ts, and the difference between the average readings of adjacent plate thermometers is all lower than ΔTs: It is judged that overalloying (poor powdering) is occurring, and the control plate is Lower the set value for the thermometer 21 by ΔT1 (°C). Here, ΔT1 is a set value determined from empirical values obtained in actual operation.

Case 2: (FIG. 7b) When T1 to T5 are all lower than Ts, and the differences between the average values of adjacent plate thermometers are all lower than ΔTs: It is determined that alloying is insufficient (uneven baking), and the control plate temperature is Increase the set value for Total 21 by ΔT2 (°C). Here, ΔT2 is a set value determined based on empirical values obtained in actual operation.

Case 3: If a bending point is found in the line connecting the average plate temperature values, the alloying completion position is determined by comparing the difference between the average values of adjacent plate thermometers and its reference value. Specifically, the procedure is as follows. o If ΔTA = T2 - T1 ≦ΔTs, it is determined that alloying is completed at the position of plate thermometer 11, and control plate thermometer 2
The set value for 1 is the average indicated value of the plate thermometer 11.

o ΔTB = T3 - T2 ≦ΔTs
If so, it is determined that alloying is completed at the position of the plate thermometer 12,
The setting value for the control plate thermometer 21 is the average of the indicated values of the plate thermometer 12. o If ΔTC = T4 - T3 ≦ΔTs (
7C), it is determined that alloying is completed at the position of the plate thermometer 13, and the set value for the control plate thermometer 21 is set to the average of the indicated values of the plate thermometer 13.

o ΔTD = T5 - T4 ≦ΔTs
If so, it is determined that alloying is completed at the position of the plate thermometer 14, and the set value for the control plate thermometer 21 is set to the average of the indicated values of the plate thermometer 14. Case 4: When none of the above occurs (Fig. 7d): It is determined that alloying is insufficient (uneven burnt), and the setting value for the control plate thermometer 21 is set to Δ.
Raise T (℃).

If there are three plate thermometers, the difference between the average readings will be 2.
Although it is possible to achieve the object of the present invention by obtaining four or more, it is possible to grasp the state of alloying in the furnace more precisely by arranging four or more.

[0019]

[Example 1] Fig. 1 shows an outline of the apparatus used in the example. The total length of the alloying furnace 2 is 40 m, and the sheet passing speed range varies depending on the target area weight and sheet thickness, and ranges from 60 to 120 m.
m/min.

5 m, 10 m and 1 m from the exit side of the alloying furnace 2
Plate thermometers 8, 7, and 6, each of which is a radiation thermometer, were placed at a distance of 5 m, and the average indicated temperature of the plated steel sheet 1 was determined at the center position of the steel sheet in the sheet width direction. Initially, the alloying target temperature was set on the plate thermometer 6, and the fuel gas flow rate of the alloying furnace was controlled. After that, the difference between the average readings of each plate thermometer 6, 7, and 8 is compared with the reference value of the difference, and the alloying completion position is determined from the rate of change.
Using the plate thermometer 8 as a control plate thermometer, the fuel gas flow rate of the alloying furnace was controlled to match the average indicated value of the plate thermometer corresponding to the alloying completion position, and to reach the alloying target temperature.

When 200 samples of 50 coils thus manufactured were evaluated, the results shown in Table 1 were obtained.

[0022]

[Table 1] ──────────────────────────
──────────
Burning unevenness occurrence rate Powdering defect rate (by visual inspection)
(Based on 180° bending test)
Conventional method 6/200 (3%) 4
1/200 (20.5%) Invention method 2
/200 (1%) 3/200 (1.5%)
──────────────────────────
────────── Conventionally, the occurrence rate of uneven burning was low because operators observed the plate surface on the exit side of the alloying furnace, but this invention has further reduced the incidence. . Powdering, which in the past was not controlled at all because there was no means to check it, has been drastically reduced by the present invention.

[Example 2] Using the equipment used in Example 1, a line that was normally operating at a line speed of 120 m/min was changed to a line speed of 90 m/min due to operational circumstances.
An example of operation according to the invention during a transient period when down to m/min is described with reference to Table 2.

[0024]

[Table 2]

Table 2 shows data on changes over time from state 1 to state 5. In condition 1, the line speed is 120 m/
min, and the furnace was operated in steady state at a set temperature of 485°C. The standard value of this furnace is based on past operation data, average standard value Ts of each plate thermometer reading: 482℃
The reference value ΔTs of the difference between the average values of adjacent plate thermometers was set at 10°C. At this time, the average readings of plate thermometers 6, 7, and 8 are 4 each.
They were 70°C, 481°C, and 485°C. The difference between the average values is 1
The temperatures were 1°C and 4°C, and alloying of the steel plate was completed at the position of the plate thermometer 8.

[0026] Line speed is from 120m/min to 9
When the speed decreased to 0 m/min, the plate temperature on the exit side of the alloying furnace increased, and the average readings of plate thermometers 6, 7, and 8 were 486, respectively.
℃, 492℃, and 496℃ (state 2). The average indicated value of each plate thermometer is larger than the average indicated value reference value Ts (482°C), and the reference value ΔT of the difference between the averages of adjacent plate thermometers.
Based on experience, the set temperature of the furnace was lowered by ΔT (3°C) to 482°C. Therefore, the amount of fuel gas (coke oven gas) is automatically 400Nm3.
/hr. The fuel gas amount was lowered and we waited approximately 10 seconds for the furnace condition to stabilize.

When the furnace condition is becoming stable, each plate thermometer 6, 7
, 8, the average readings were 475°C, 480°C, 485°C, and the difference between the averages of adjacent plate thermometers was 5°C, 5°C (condition 3).
). The average reading of each plate thermometer is the average reference value Ts (482
C), and the difference between the averages of adjacent plate thermometers was rapidly decreasing. Focusing on plate thermometer 7, the alloying completion position was determined to be the position of plate thermometer 7. At this time, the average reading of the plate thermometer 7 was 480°C, so it was judged appropriate to set the temperature in the furnace to 480°C.

Therefore, the set temperature of the furnace is further increased by ΔT(2
℃) to 480℃, and the fuel gas amount is automatically reduced to 320℃.
We lowered the rate to Nm3/hr and waited for the furnace condition to stabilize. The average readings of plate thermometers 6, 7, and 8 are 470°C and 47°C, respectively.
5℃, 480℃, the average reading of each plate thermometer is smaller than the average reading reference value Ts (480℃), and the difference between the average readings of adjacent plate thermometers (5℃, 5℃) is the average value. It was determined that alloying of the steel plate was completed at the position of the plate thermometer 8 where the difference was smaller than the standard value ΔTs (10° C.) (state 4).

After several more minutes, the furnace condition became completely stable, and the furnace could continue to operate at a line speed of 90 m/min. The alloying completion position was also the position of plate thermometer 8, which was the same as when the line speed was 120 m/min. As described above, even if the line speed changed, it was possible to control the temperature of the furnace immediately, and the time required to change from state 1 to state 5 in Table 2 was approximately 2 minutes. During this time, the steel plate traveled 90 x 2 = 180 m, but only 1.5 m had powdering defects.

[0030]

[Effects of the Invention] According to the present invention, by grasping and controlling the alloying completion position in the alloying furnace, the occurrence rate of uneven burning and powdering is much lower than that of the conventional method, and the alloying of steel sheets is improved. You can keep the temperature at an appropriate level. In addition, as the plate thermometer used in the present invention, a generally commercially available profile type plate thermometer can also be used.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an alloying furnace used in an example of the present invention.

FIG. 2 is an explanatory diagram of a conventional alloying furnace.

FIG. 3 is a graph showing the relationship between sheet temperature and emissivity of a hot-dip galvanized steel sheet.

FIG. 4 is a graph showing the relationship between true plate temperature and plate thermometer reading.

FIG. 5 is a graph showing the relationship between the plate temperature indicated average value, the indicated value fluctuation range, and the Fe concentration in the plating layer.

FIG. 6 is an explanatory diagram showing the arrangement of plate thermometers in an example of the alloying furnace used in the present invention.

FIG. 7 is an explanatory diagram of various cases in which the alloying completion position is found.

[Explanation of symbols]

1 Plated steel plate 2 Alloying furnace 3 Squeezing device 4 Plating tank 5. Think Roll 6 Plate thermometer 7 Plate thermometer 8 Plate thermometer 11 Plate thermometer 12 Plate thermometer 13 Plate thermometer 14 Plate thermometer 15 Plate thermometer 21 Control plate thermometer

Claims (1)

[Claims] Claim 1: When a steel plate is immersed in a hot-dip galvanizing tank and the plated steel plate is alloyed using an alloying furnace for hot-dip galvanizing, three plate thermometers consisting of radiation type thermometers are installed in the alloying furnace. The difference between the average readings of adjacent plate thermometers is sequentially determined from the bottom of the furnace. The position of the lower plate thermometer is grasped as the alloying completion position, and the alloying furnace is adjusted so that the average reading of the control plate thermometer matches the supporting average value of the plate thermometer at the alloying completion position. A method for manufacturing an alloyed hot-dip galvanized steel sheet, characterized by controlling fuel flow rate.