JP7743824B2 - Alloying treatment device and alloying treatment method for hot-dip galvanized steel sheet - Google Patents

Description

The present invention relates to an alloying treatment device and alloying method for hot-dip galvanized steel sheets.

Among hot-dip galvanized steel sheets, there is alloyed hot-dip galvanized steel sheet, which is produced by hot-dip galvanizing a steel sheet and then alloying the entire or part of the coating layer to an Fe-Zn alloy.

Generally, galvannealed steel sheets are manufactured using the following process. First, the steel sheet is annealed and then cooled. Next, the steel sheet is immersed in a coating bath filled with molten zinc and then pulled vertically upward. After the steel sheet is pulled out of the coating bath, pressurized gas is sprayed from a wiping nozzle onto the surface of the steel sheet to squeeze out excess molten zinc, ensuring that the molten zinc adhering to the surface of the steel sheet is uniform in both the width and length directions. The steel sheet is then heated in a heating zone located directly above the wiping nozzle, and is then kept at a temperature in a holding zone to diffuse iron into the zinc layer, thereby completing the required alloying process.

If the alloying process is not performed properly, i.e., if the alloying is over-alloyed or under-alloyed, the quality characteristics will be impaired, so the degree of alloying must be controlled with high precision. The following technologies have been disclosed as ways to control the degree of alloying.

Patent Document 1 discloses a method for controlling the degree of alloying by irradiating X-rays onto a steel sheet after alloying treatment and calculating the degree of alloying from the X-ray diffraction intensity.

Patent Document 2 also discloses a method of controlling the degree of alloying by arranging radiation thermometers at multiple positions in the strip temperature holding zone within a hot-dip galvanizing furnace, comparing the radiation energy with a representative strip temperature measurement value to determine the emissivity of the steel strip at each position, determining the position where the emissivity is in the range of 0.4 to 0.7 as the alloying position, and manipulating the fuel flow rate and strip passing speed in the alloying furnace so that this alloying position remains constant.

Furthermore, Patent Document 3 discloses a method for controlling the degree of alloying in an alloying treatment device for hot-dip galvanizing, in which a temperature measurement means that is not affected by the surface emissivity of the steel sheet is provided downstream of the alloying zone, a radiation thermometer that can measure temperature at three or more points in the width direction of the steel sheet is provided upstream or downstream of the temperature measurement means that is not affected by the surface emissivity of the steel sheet, and the degree of alloying is controlled by partially adjusting the amount of heat in the width direction within the alloying zone to correspond to the temperature measurement positions of the radiation thermometer in the width direction of the steel sheet.

Japanese Patent Application Publication No. 1-301155 Japanese Unexamined Patent Publication No. 7-150328 Japanese Patent Application Publication No. 13-117049

However, the conventional method is feedback control in which the degree of alloying or temperature distribution of the steel sheet is measured downstream of the alloying zone and then the heating amount is adjusted, so it takes time from obtaining the measurement results to improving the alloying unevenness.
Furthermore, the parameters that determine the degree of alloying are the amount of heat in the heating device and the temperature of the holding zone. However, in actual operation, the amount of heat brought into the holding zone from the steel plate varies depending on the amount of heat, steel plate size, and plate running speed. As a result, a sudden change in the amount of heat brought in can temporarily change the temperature of the holding zone, making it difficult to maintain a constant temperature in the holding zone, resulting in uneven alloying.

The present invention was developed in response to the above-mentioned issues, and aims to provide an alloying treatment device and alloying method for hot-dip galvanized steel sheets that can reduce the occurrence of alloying unevenness.

One aspect of the present invention provides an alloying treatment apparatus for hot-dip galvanized steel sheet that performs an alloying treatment on hot-dip galvanized steel sheet, the alloying treatment apparatus comprising: a heating zone that heats the hot-dip galvanized steel sheet, which is a steel sheet that has been subjected to hot-dip galvanizing treatment; a retention zone that retains the heat of the hot-dip galvanized steel sheet heated in the heating zone; a thermometer that measures the retention zone temperature, which is the temperature of the retention zone; and a control unit that adjusts the amount of heat in the heating zone depending on the retention zone temperature.

One aspect of the present invention provides an alloying treatment method for hot-dip galvanized steel sheet, which performs an alloying treatment on hot-dip galvanized steel sheet, comprising: a heating step of heating the hot-dip galvanized steel sheet, which is a steel sheet that has been subjected to hot-dip galvanizing treatment, in a heating zone; and a heat retention step of retaining the heat of the hot-dip galvanized steel sheet heated in the heating zone in a heat retention zone, wherein the heating step adjusts the heat amount of the heating zone depending on the temperature of the heat retention zone, which is the temperature of the heat retention zone.

According to one aspect of the present invention, an alloying treatment device and alloying method for hot-dip galvanized steel sheet can reduce the occurrence of alloying unevenness.

1 is a configuration diagram showing a plating treatment facility according to an embodiment of the present invention. 1 is a graph showing the relationship between the holding zone temperature _Tk and the heat amount Q in steel type A of Example 1. 1 is a graph showing the relationship between the holding zone temperature _Tk and the heat amount Q in steel type B of Example 1. 1 is a graph showing the relationship between the holding zone temperature _Tk and the heat amount Q in steel type C of Example 1. 1 is a graph showing the relationship between the holding zone temperature _Tk and the heat amount Q in steel type D of Example 1.

In the following detailed description, embodiments of the present invention will be described with reference to the drawings. In the description of the drawings, identical or similar parts will be designated by identical or similar reference numerals, and redundant explanations will be omitted. The drawings are schematic and may differ from the actual product. Furthermore, the embodiments shown below are examples of devices and methods that embody the technical concept of the present invention, and the technical concept of the present invention does not limit the materials, structure, arrangement, etc. of component parts to those described below. The technical concept of the present invention may be modified in various ways within the technical scope defined by the claims.

<Alloying treatment equipment>

1 is a side view showing an alloying treatment device 1 according to one embodiment of the present invention. The alloying treatment device 4 is provided in a plating treatment facility 1 that performs hot-dip galvanizing and alloying treatment on a steel sheet 6. The plating treatment facility 1 includes a plating bath 2, a wiping nozzle 3, the alloying treatment device 4, and a top roll 5.
The coating bath 2 is a coating tank containing molten zinc. In the hot-dip galvanizing process, the steel sheet 6 is immersed in the coating bath 2, whereby the surface of the steel sheet 6 is plated with zinc.

The wiping nozzle 3 is installed above the coating bath 2 and sprays pressurized gas. At least one pair of wiping nozzles 3 is installed facing each other in the thickness direction of the steel sheet 6 that has been pulled vertically upward from the coating bath 2. By spraying pressurized gas onto both the front and back surfaces of the steel sheet 6, the molten zinc adhering to the surface of the steel sheet 6 is squeezed out to a predetermined thickness.

In the plating processing equipment 1, the steel sheet 6 is pulled up from the plating bath 2 and transported (threaded) vertically upward until it passes through the top rolls 5 installed above the plating bath 2. After passing through the top rolls 5, the steel sheet 6 changes threading direction and is transported to the next process.

The alloying treatment device 4 is a device that performs alloying treatment on the steel plate S that has been hot-dip galvanized in the coating bath 2, and has a heating zone 41, a holding zone 42, a cooling zone 43, a thermometer 44, and a control unit 45. The heating zone 41 and the holding zone 42 are collectively referred to as the alloying zone.

The heating zone 41 is located above the wiping nozzle 3 and is a mechanism for heating the steel sheet 6 that has been subjected to hot-dip galvanizing and from which the molten zinc has been squeezed out by the wiping nozzle 3. The heating zone 41 may be any heating method as long as it can adjust the amount of heat. Gas combustion methods, induction heating methods, etc. can be used. The heating zone 41 heats the steel sheet 6 at a heat amount Q set by the control unit 45. The heat amount Q can be defined as the output or heat amount of the heating mechanism of the heating zone 41. For example, if the heating zone 41 uses electromagnetic induction heating (IH: Induction Heating), the heat amount Q can be defined as the IH output (kW).

The maintaining zone 42 is provided above the heating zone 41 , that is, downstream in the sheet running direction, and is equipment for maintaining the temperature of the steel sheet 6 heated in the heating zone 41 .
The cooling zone 43 is provided above the retaining zone 42 , that is, downstream in the sheet running direction, and is equipment for cooling the steel sheet 6 that has passed through the retaining zone 42 .

The thermometer 44 is a device for measuring the temperature of the retention zone 42. There are no particular limitations on the method for measuring the temperature of the thermometer 44, but it is preferable to measure using a thermocouple. It is also preferable that the thermometer 44 measures the temperature at the center of the retention zone 42. The temperature measured in the retention zone 42 is also called the retention zone temperature T _k (°C).

The control unit 45 is a device that controls the heat amount Q of the heating zone 41 in accordance with the correlation between the holding zone temperature _Tk , the heat amount Q, and the degree of alloying, and the measured holding zone temperature _Tk . The method of controlling the heat amount Q by the control unit 45 will be described later.

<Alloying treatment method>
The alloying treatment method according to this embodiment will be described below. In the alloying treatment method according to this embodiment, first, a steel sheet 6 is immersed in a coating bath 2 filled with molten zinc, and then the steel sheet 6 is pulled vertically upward, and pressurized gas is sprayed from a wiping nozzle 3 onto the surface of the steel sheet 6 to squeeze out excess molten zinc.

Next, the steel sheet 6 is heated in the heating zone 41 (heating step). In the heating step, the control unit 45 controls the heating amount in the heating zone 41 according to the holding zone temperature _Tk measured by the thermometer 44 so that the heating amount Q is set for each steel type (type of base metal) of the steel sheet 6. Details of the set heating amount Q will be described later. Furthermore, the steel type is defined as a type of steel that does not have any overlap in composition.

After the heating step, the steel plate 6 heated in the heating zone 41 is kept at a constant temperature in the heat retention zone 42 (heat retention step).
After the heat retention step, the steel sheet 6 is cooled in a cooling zone 43 (cooling step). The steel sheet 6 cooled in the cooling step has its passing direction changed by the top rolls 5 and is transported to the next step.
The heating step, the heat-retaining step, and the cooling step are performed continuously on the transported steel plate 6, as shown in Fig. 1. Moreover, since the steel plate 6 is a steel strip having a long longitudinal length, the heating step, the heat-retaining step, and the cooling step are performed in parallel on different longitudinal positions of the steel plate 6.

During the heating process and the heat retention process, the temperature T _k of the heat retention zone is measured by the thermometer 44. The measured temperature T _k of the heat retention zone is transmitted to the control unit 45 and used to control the amount of heat Q in the heating process.

Here, the inventors investigated the relationship between the holding zone temperature _Tk measured by the thermometer 44, the heat amount Q, and the occurrence of alloying unevenness, and confirmed that under conditions where alloying unevenness did not occur, there was a correlation between the holding zone temperature _Tk and the IH output (heat amount Q), and that this relationship could be expressed by a regression equation. In other words, it was found that the holding zone temperature _Tk and the heat amount Q were correlated with the occurrence of alloying unevenness, and that by setting the heat amount Q according to the holding zone temperature _Tk , it was possible to manufacture alloyed hot-dip galvanized steel sheets within an allowable range where alloying unevenness does not occur. It was also found that the correlation between the holding zone temperature _Tk , the heat amount Q, and the degree of alloying differs depending on the type of base material (steel type).

The alloying treatment method according to this embodiment is based on the above findings. That is, in this embodiment, the relationship between the holding zone temperature T _k measured by the thermometer 44, the heat amount Q applied in the heating zone 41, and the degree of alloying is determined in advance for each base material type. Specifically, the relationship between the holding zone temperature T _k and the heat amount Q is determined in advance for each steel type from the values of the holding zone temperature T _k and the heat amount Q in actual operation performance data (past performance data) in which no alloying unevenness (under-alloying or over-alloying) occurred. Then, during the alloying treatment, the holding zone temperature T _k is measured using the thermometer 44, and the heat amount Q is adjusted according to the measured holding zone temperature T _k . By controlling alloying as described above, it is possible to reduce the occurrence of alloying unevenness and shorten the length of the portion where alloying unevenness occurs.

In addition, when considering the relationship between the holding zone temperature _Tk and the heat amount Q, manufacturing conditions such as the size (thickness, width) of the steel sheet 6, the line speed (transport speed of the steel sheet 6), the coating weight, and the degree of alloying may be further taken into consideration. In other words, these manufacturing conditions may be taken into consideration in the regression equation calculated from the holding zone temperature _Tk and the heat amount Q under conditions where no alloying unevenness occurs. For example, when calculating the regression equation from actual operational performance data, a regression equation for each manufacturing condition may be calculated using not only the steel type but also performance data with similar manufacturing conditions, and the regression equation for the performance data with similar manufacturing conditions may be used to set the heat amount Q. Furthermore, when determining the regression equation, the manufacturing conditions may be reflected in the regression equation as a coefficient. If the actually applied heat amount Q is outside the upper limit of the regression equation (i.e., the heat amount Q is excessive) relative to the heat amount Q calculated from the regression equation, the steel sheet is likely to become over-alloyed with a high Fe % (degree of alloying), which makes it more likely to cause alloying unevenness. This also increases the risk of zinc peeling. When the actually applied heat amount Q falls below the lower end of the regression equation (the heat amount Q is too low) relative to the heat amount Q calculated from the regression equation, insufficient alloying occurs, and uneven alloying is likely to occur. Note that over-alloying is likely to become apparent when the Fe% exceeds 15%, while insufficient alloying is likely to become apparent when the Fe% is 8% or less. From the viewpoint of suppressing uneven alloying, it is desirable to control the heat amount Q of the heating zone during production using the heat amount Q of the heating zone calculated from the regression equation as a target value, and as for the actual control results of the heat amount Q of the heating zone during production, it is preferable to control the actual value of the heat amount Q of the heating zone during production within a range of ±10% of the heat amount Q of the heating zone calculated from the regression equation, i.e., within a range of 0.9 to 1.1 x the heat amount Q of the heating zone calculated from the regression equation.

Furthermore, in the heating process, the constants in the following formula (1) may be determined by performing regression on the relationship between the holding zone temperature _Tk and the heat amount Q under conditions where no alloying defects occurred for each steel type, and the heat amount Q (kW) may be controlled to be the value calculated from formula (1) depending on the size and operating conditions of the steel sheet 6. In formula (1), t is the thickness (mm) of the steel sheet 6, W is the width (mm) of the steel sheet 6, L _S is the line speed (mpm), C _W is the coating weight (g/m ² ) per side of the steel sheet 6, Fe% is the alloying degree (%) of the steel sheet 6, and a0 to a6 are constants depending on the steel type. The line speed LS and coating weight CW can be set to target values or actual values. The coating weight CW is estimated from the target value or actual value of the line speed LS and the target value or actual value of the manufacturing conditions of the wiping nozzle 3. The manufacturing conditions of the wiping nozzle 3 refer to the nozzle height, the nozzle gas pressure, and the distance between the nozzle and the steel sheet 6. A target value can be set for the alloying degree Fe%. Furthermore, the constants a0 to a6 can be determined for each steel type from past performance data in which no alloying unevenness occurred.
Q=a0 x t ^a1 x W ^a2 x L _S ^a3 x C _W ^a4 x Fe% ^a5 x T _k ^a6 ...(1)

In other words, according to the alloying treatment apparatus and alloying treatment method of the present embodiment, it is possible to produce a galvannealed steel sheet with little alloying unevenness over the entire length and width, even if the holding zone temperature _Tk varies depending on the heating amount, steel sheet size, and sheet passing speed. Furthermore, since it is possible to adjust the heating amount Q in a shorter time than with conventional alloying treatment methods, it is possible to reduce the amount of alloying unevenness, which leads to an improvement in yield.

Furthermore, in the alloying treatment method according to this embodiment, it is preferable to record the values of the holding zone temperature _Tk and the heat amount Q at the longitudinal position of the steel sheet 6 so that they can be known. By doing so, even if a location occurs where the heat amount Q is outside the predetermined control range, the longitudinal position of the steel strip can be identified from the data on the holding zone temperature _Tk and the heat amount Q, and therefore, by removing only that location on a separate inspection line such as a recoil line, yield loss can be minimized.

The alloying treatment method according to this embodiment can be applied to any galvannealed steel sheet, and the steel sheet composition is not limited. For example, the steel sheet composition can be as follows. Unless otherwise specified, the percentages used for components refer to mass percent.
C: 0.3% or less If the C content exceeds 0.3%, weldability deteriorates, so the C content is set to 0.3% or less.

Si: 2.5% or less Si is an effective element for strengthening steel and achieving good material properties. However, it is known that Si-containing steel inhibits oxidation reactions during oxidation treatment. Therefore, if the Si content exceeds 2.5%, the formation of an oxide film during oxidation treatment is inhibited. Furthermore, the alloying temperature also increases, making it difficult to obtain the desired mechanical properties. Therefore, the Si content is set to 2.5% or less.

Mn: 3.5% or less Mn is an element effective in increasing the strength of steel. However, if the Mn content exceeds 3.0%, it may be difficult to ensure the balance between weldability, plating adhesion, strength, and ductility. Therefore, the Mn content is set to 3.5% or less.

P: 0.100% or less P is an element effective in strengthening steel. However, if the P content exceeds 0.100%, grain boundary segregation can cause embrittlement and deteriorate impact resistance. Therefore, the P content is set to 0.100% or less.

S: 0.0100% or less S forms inclusions such as MnS, which can cause deterioration in impact resistance and cracks along the metal flow of welds. For this reason, the S content should be as low as possible. Therefore, the S content is set to 0.0100% or less.

The balance of the composition is Fe and unavoidable impurities.
In order to control the balance between strength and ductility, the following elements are added: Al: 0.01% to 0.1%, Mo: 0.01% to 1.0%, Nb: 0.005% to 0.1%, Ti: 0.005% to 0.2%, Cu: 0.02% to 1.0%, Ni: 0.05% to 1.0%, Cr: 0.01% to 0.8%, B: 0.0001% to 0.005%, Sb: 0.0 The steel sheet may contain one or more elements selected from the group consisting of Cr: 0.01% to 0.10%, Sn: 0.001% to 0.10%, Ca: 0.0001% to 0.005%, Mg: 0.0001% to 0.005%, REM: 0.0001% to 0.005%, W: 0.005% to 0.1%, and Zr: 0.005% to 0.1%, as required.

Since Al is thermodynamically the easiest to oxidize, it oxidizes before Si and Mn, inhibiting the oxidation of Si and Mn on the steel sheet surface and promoting oxidation inside the steel sheet. This effect is achieved at 0.01% or more. On the other hand, an Al content exceeding 0.1% increases costs. Therefore, if Al is contained, the Al content should preferably be between 0.01% and 0.1%.

If the Mo content is less than 0.01%, it is difficult to obtain the strength adjustment effect or the effect of improving plating adhesion when added in combination with Nb, Ni, or Cu. On the other hand, if the Mo content exceeds 1.0%, costs will increase. Therefore, if Mo is contained, the Mo content should preferably be 0.01% or more and 1.0% or less.

If the Nb content is less than 0.005%, it is difficult to obtain the strength adjustment effect or the effect of improving plating adhesion when added in combination with Mo. On the other hand, if the Nb content exceeds 0.1%, it will result in an increase in costs. Therefore, if Nb is contained, the Nb content should preferably be 0.005% or more and 0.1% or less.

If the Ti content is less than 0.005%, it is difficult to obtain the strength adjustment effect, and if it exceeds 0.2%, it will cause a deterioration in plating adhesion. Therefore, if Ti is contained, the Ti content should preferably be 0.005% or more and 0.2% or less.

If the Cu content is less than 0.02%, it is difficult to achieve the effect of promoting the formation of residual gamma phase or the effect of improving plating adhesion when added in combination with Ni or Mo. On the other hand, if the Cu content exceeds 1.0%, costs will increase. Therefore, if Cu is contained, the Cu content should preferably be 0.02% or more and 1.0% or less.

If the Ni content is less than 0.05%, it is difficult to achieve the effect of promoting the formation of residual gamma phase or the effect of improving plating adhesion when added in combination with Ni or Mo. On the other hand, if the Nb content exceeds 1.0%, costs will increase. Therefore, if Nb is contained, the Ni content should preferably be 0.05% or more and 1.0% or less.

If the Cr content is less than 0.01%, it may be difficult to obtain good hardenability and the balance between strength and ductility may deteriorate. On the other hand, if the Cr content exceeds 0.8%, costs will increase. Therefore, if Cr is contained, the Cr content should preferably be 0.01% or more and 0.8% or less.

B is an element that is effective in improving the hardenability of steel. If the B content is less than 0.0001%, it is difficult to obtain the desired hardening effect, and if it exceeds 0.005%, it has the effect of promoting oxidation of Si on the outermost surface of the steel sheet, resulting in a deterioration of plating adhesion. Therefore, if B is contained, the B content should preferably be 0.0001% or more and 0.005% or less.

Sb and Sn are elements that effectively prevent denitrification, deboronization, etc., and thus prevent a decrease in the strength of steel. To achieve this effect, it is preferable that each content be 0.001% or more. However, if the Sb and Sn contents exceed 0.10%, impact resistance will deteriorate. Therefore, if Sb or Sn is contained, it is preferable that the Sb content or Sn content be 0.001% or more and 0.10% or less, respectively.

Ca is an element that is effective in reducing the number of inclusions and improving bendability and delayed fracture resistance. If the Ca content is less than 0.0001%, it is difficult to obtain the desired effect of improving bendability and delayed fracture resistance, while if it exceeds 0.005%, the number of coarse inclusions increases too much, deteriorating bendability and delayed fracture resistance. Therefore, the Ca content is preferably 0.0001% or more and 0.005% or less.

Mg is an element that is effective in reducing the number of inclusions and improving bendability and delayed fracture resistance. If the Mg content is less than 0.0001%, it is difficult to obtain the desired effect of improving bendability and delayed fracture resistance, while if it exceeds 0.005%, the number of coarse inclusions increases too much, deteriorating bendability and delayed fracture resistance. Therefore, the Mg content is preferably 0.0001% or more and 0.005% or less.

REM (Rare-Earth-Metal) is an element that is effective in reducing the number of inclusions and improving bendability and delayed fracture resistance. If the REM content is less than 0.0001%, it is difficult to achieve improvements in bendability and delayed fracture resistance, and if it exceeds 0.005%, the number of coarse inclusions increases too much, deteriorating bendability and delayed fracture resistance. Therefore, the REM content is preferably 0.0001% or more and 0.005% or less.

W is an element that is effective in refining the structure and increasing the strength of steel. If the W content is less than 0.005%, it is difficult to achieve a high-strength effect, and if it exceeds 0.1%, the strength-increasing effect saturates, while the number of coarse inclusions increases, deteriorating bendability and delayed fracture resistance. Therefore, the W content is preferably 0.005% or more and 0.1% or less.

Zr is an element that is effective in refining the structure and increasing the strength of steel. If the Zr content is less than 0.005%, it is difficult to achieve a high-strength effect, and if it exceeds 0.1%, the strength-increasing effect saturates, while the number of coarse inclusions increases, deteriorating bendability and delayed fracture resistance. Therefore, the Zr content is preferably 0.005% or more and 0.1% or less.

Such steel plate 6 can be manufactured by any known or arbitrary method. For example, a slab having the above-mentioned chemical composition can be heated and hot-rolled to form a hot-rolled steel plate, which can then be pickled and, if necessary, cold-rolled to form a cold-rolled steel plate.

While the present invention has been described above with reference to specific embodiments, it is not intended that the invention be limited by these descriptions. By reading the description of the present invention, those skilled in the art will recognize other embodiments of the present invention that incorporate various modifications in addition to the disclosed embodiments. Therefore, it should be understood that the embodiments of the invention set forth in the claims also encompass embodiments that incorporate these modifications described herein, either alone or in combination.

In Example 1, steel sheets S of steel types A to D shown in Table 1 were subjected to alloying treatment using an apparatus similar to the alloying treatment apparatus 4 shown in Figure 1, and the holding zone temperature _Tk , heat amount Q, and the occurrence of alloying unevenness were investigated. The manufacturing conditions other than the steel type and holding zone temperature _Tk were similar. The results of this investigation are shown in Figures 2 to 5. In Figures 2 to 5, plots of "○" indicate conditions under which alloying unevenness did not occur, and plots of "×" indicate conditions under which alloying unevenness occurred.

As shown in Figures 2 to 5, under conditions where no alloying unevenness occurred, there was a correlation between the holding zone temperature _Tk and the IH output (heat amount Q), and it was confirmed that this relationship can be expressed by the regression equation shown in the graph. Furthermore, as shown in Figures 2 and 3, it was confirmed that alloying unevenness occurred under conditions that deviated from this regression equation. In other words, it was found that the holding zone temperature _Tk and the heat amount Q are correlated with the occurrence of alloying unevenness, and that by setting the heat amount _Q according to the holding zone temperature Tk, it is possible to produce galvannealed steel sheets within an allowable range where alloying unevenness does not occur. It was also found that the correlation between the holding zone temperature _Tk , the heat amount Q, and the degree of alloying differs depending on the type of base material (steel type).

Next, in Example 2, the following production test of a galvannealed steel sheet was carried out using the alloying treatment method according to the above embodiment. The production conditions for the hot-dip galvanized steel sheet were 0.5 mm to 1.6 mm thick x 800 mm to 1880 mm wide, and a coating weight of 48 g/ ^m2 on one side. The same steel type was used in the example and comparative example, and the steel composition is shown in Table 1.

In Example 2, the alloying treatment apparatus 4 shown in Fig. 1 was used, and an induction heating type heating device was installed in the heating zone 41. A thermometer 44 was installed in the holding zone 42 to measure the holding zone temperature _Tk . Then, the heat amount Q was adjusted according to the measured holding zone temperature _Tk using a table or a calculation formula prepared in advance for each steel type.

Furthermore, in the comparative example, the degree of alloying at the center of the strip width was detected using the alloying degree meter described in Patent Document 1, and the heating of the alloying zone was controlled so that the specified degree of alloying was achieved. If uneven alloying was observed through visual inspection, the heating of the alloying zone was manually controlled to suppress the uneven alloying. The visual inspection was performed by checking the color unevenness of the surface at the center and edge of the steel strip.

In Example 2, the coating layer of the manufactured galvannealed steel sheet was inspected for uneven alloying. The occurrence rate of uneven alloying (insufficient alloying, excessive alloying) (the ratio of the weight of coils determined to have uneven alloying to the weight of charged coils) is shown in Table 2. The inventive example represents data in which induction heating output was controlled based on the temperature of the holding zone furnace, while the comparative example represents data in which feedback control was performed based on the degree of alloying. The data is also the combined data for multiple steel types (steel types A to D).

As shown in Table 2, it was confirmed that the alloying treatment device 4 and alloying treatment method according to the above embodiment significantly reduced the amount of alloying unevenness. In other words, the alloying treatment device 4 and alloying treatment method according to the above embodiment made it possible to control the alloying degree to a predetermined level and produce steel with as little alloying unevenness as possible. Furthermore, since the temperature history during alloying is important even for steel types not included in this example, controlling the amount of heat depending on the holding temperature, which affects the alloying temperature history, is an effective method for suppressing alloying unevenness.

REFERENCE SIGNS LIST 1 Plating treatment equipment 2 Plating bath 3 Wiping nozzle 4 Alloying treatment device 41 Heating zone 42 Holding zone 43 Cooling zone 44 Thermometer 45 Control unit 5 Top roll 6 Steel sheet

Claims (4)

An alloy treatment apparatus for a hot-dip galvanized steel sheet, which performs an alloying treatment on a hot-dip galvanized steel sheet,
a heating zone for heating the hot-dip galvanized steel sheet, which is a steel sheet that has been subjected to hot-dip galvanizing treatment;
a heat retention zone for retaining the heat of the hot-dip galvanized steel sheet heated in the heating zone;
a thermometer for measuring the temperature of the heat retention zone, which is the temperature of the heat retention zone;
a control unit that adjusts the heating amount so that the heating amount is set according to the measured temperature of the holding zone based on a relationship between the temperature of the holding zone and the heating amount of the heating zone, which is set in advance for each steel type of the hot-dip galvanized steel sheet;
Equipped with
The relationship between the holding zone temperature and the heating amount is determined from values of the holding zone temperature and the heating amount in past performance data in which no alloying unevenness occurred . An alloy treatment apparatus for a hot-dip galvanized steel sheet, which performs an alloying treatment on a hot-dip galvanized steel sheet,
a heating zone for heating the hot-dip galvanized steel sheet, which is a steel sheet that has been subjected to hot-dip galvanizing treatment;
a heat retention zone for retaining the heat of the hot-dip galvanized steel sheet heated in the heating zone;
a thermometer for measuring the temperature of the heat retention zone, which is the temperature of the heat retention zone;
a control unit that adjusts the heating amount so that the heating amount is set according to the measured temperature of the holding zone based on a relationship between the temperature of the holding zone and the heating amount of the heating zone, which is set in advance for each steel type of the hot-dip galvanized steel sheet;
Equipped with
The control unit calculates the heating amount from the measured temperature of the holding zone and equation (1), and adjusts the heating amount of the heating zone so as to achieve the calculated heating amount .
Q=a0 x t ^a1 x W ^a2 x L _S ^a3 x C _W ^a4 x Fe% ^a5 x T _k ^a6 ...(1)
Q: Heating amount (kW)
t: thickness of steel plate (mm)
W: width of steel plate (mm)
L _S : Line speed (mpm)
C _W : plating coverage per side (g/m ² )
Fe%: Degree of alloying (%)
_Tk : Temperature of storage zone (°C)
a0 to a6: Constants according to the steel type A method for alloying a hot-dip galvanized steel sheet, comprising:
a heating step of heating the hot-dip galvanized steel sheet, which is a steel sheet that has been subjected to hot-dip galvanizing treatment, in a heating zone;
a heat retention step of retaining the heat of the hot-dip galvanized steel sheet heated in the heating zone in a heat retention zone;
Equipped with
In the heating step, the heating amount is adjusted based on a relationship between a holding zone temperature, which is a temperature of the holding zone that is preset for each steel type of the hot-dip galvanized steel sheet, and a heating amount of the heating zone, so that the heating amount is set according to a holding zone temperature, which is a measured temperature of the holding zone ;
The alloying treatment method for a hot-dip galvanized steel sheet , wherein the relationship between the holding zone temperature and the heating amount is determined from values of the holding zone temperature and the heating amount in past performance data in which no alloying unevenness occurred . A method for alloying a hot-dip galvanized steel sheet, comprising:
a heating step of heating the hot-dip galvanized steel sheet, which is a steel sheet that has been subjected to hot-dip galvanizing treatment, in a heating zone;
a heat retention step of retaining the heat of the hot-dip galvanized steel sheet heated in the heating zone in a heat retention zone;
Equipped with
In the heating step, a heat amount of the heating zone is adjusted based on a relationship between a heat amount of the heating zone and a temperature of the heating zone, the temperature of the heating zone being preset for each steel type of the hot-dip galvanized steel sheet, so that the heat amount is set according to a measured temperature of the heating zone,
When adjusting the heating amount, the heating amount is calculated from the measured temperature of the holding zone and formula (1), and the heating amount of the heating zone is adjusted so as to achieve the calculated heating amount.
Q = a0 × t ^a1 ×W ^a2 ×L _S ^a3 ×C _W ^a4 ×Fe% ^a5 ×T _k ^a6 ...(1)
Q: Heating amount (kW)
t: thickness of steel plate (mm)
W: width of steel plate (mm)
L _S : Line speed (mpm)
C _W : plating adhesion amount per side (g/m ² )
Fe%: Degree of alloying (%)
T _k : Heat retention temperature (℃)
a0 to a6: Constants according to the steel type