JP2017222900A - Production method of alloyed galvanized steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of an alloyed galvanized steel plate capable of suppressing uneven cooling in the plate width direction in cooling after alloying and suppressing generation of defects caused by the uneven cooling without impairing improvement of productivity of the steel plate.SOLUTION: In a production method of an alloyed galvanized steel plate, an annealed steel plate is galvanized and heated to alloy the zinc plating applied thereon and then sprayed with fine droplets of a cooling liquid to be cooled to a cooling-stop temperature T (°C). The liquid droplets have dissolved therein carbon dioxide of 1.0% or more of saturated solubility of carbon dioxide at 20°C and 1 atm of carbon dioxide. The average diameter D (μm) of the liquid droplets is determined so that the liquid droplets vaporize before coming into contact with the surface of the steel plate and the thus formed vapor film is kept during the cooling period from the beginning of cooling to the end of cooling when the cooling-stop temperature T (°C) is attained.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet using a continuous alloying hot-dip galvanizing facility, and particularly relates to cooling performed after alloying.

  In general, an alloyed hot-dip galvanized steel sheet is manufactured as follows using a continuous alloying hot-dip galvanizing equipment 100 as shown in FIG. First, a steel plate (steel strip) P annealed in a continuous annealing furnace (not shown) is continuously introduced into a hot dip galvanizing bath 20 where hot dip galvanization is applied to the steel plate P. The steel plate P is moved upward by the sink roll 21 in the hot dip galvanizing bath 20. After the steel sheet P is pulled up above the hot dip galvanizing bath 20, the plating adhesion amount is adjusted by the gas wiping device 22. Thereafter, the zinc plating applied to the steel sheet P in the alloying furnace 23 is heated and alloyed. Then, the droplet group which refined the cooling liquid with the cooling device 10 is sprayed toward the steel plate P, and the steel plate P is cooled. Thereafter, the steel plate temperature is measured by the radiation thermometer 24 in the vicinity of the top roll 25. In addition, the arrow in FIG. 1 is the advancing direction of the steel plate P.

  As a cooling method after alloying, cooling (gas cooling) for injecting a gas such as air and cooling (mist cooling) for injecting a droplet group in which the cooling liquid is made fine like the cooling device 10 in FIG. There is. Since mist cooling has higher cooling efficiency than gas cooling, cooling can be performed at a high cooling rate with a limited cooling facility length, and is used to improve productivity.

  However, in mist cooling after alloying, uneven cooling occurs in the width direction of the steel sheet, which is known to cause shape defects such as wrinkles on the steel sheet surface or deformation of the steel sheet. ing. In order to prevent this uneven cooling, various measures have been studied.

  In Patent Literature 1, there is a method of dividing the cooling device into a plurality of zones in the steel plate traveling direction and determining which one of mist cooling and gas cooling is used for each zone according to the steel plate temperature on the exit side of each zone. Have been described. Further, Patent Document 2 describes a method in which the cooling device is divided into two, and the latter cooling is performed more slowly than the preceding cooling.

JP 2000-256818 A JP 2006-111945 A

  However, in the methods described in Patent Documents 1 and 2, the cooling rate in the latter half zone of the cooling facility is largely restricted. Therefore, if the cooling facility length is not extended, it is necessary to regulate the plate passing speed of the steel plate, which becomes an impediment to productivity improvement.

  Therefore, in view of the above problems, the present invention can suppress uneven cooling in the width direction of the steel sheet in cooling after alloying without inhibiting improvement in the productivity of the steel sheet, and can suppress the occurrence of defects resulting therefrom. It aims at providing the manufacturing method of a galvannealed steel plate.

  The present inventor has intensively studied to solve the above problems. Due to the droplets ejected from the cooling nozzle adhering to the steel plate surface, the cooling unevenness in the steel plate width direction is enlarged. That is, from the start to the end of cooling, the droplet group does not contact the surface of the steel sheet, and the droplet group is evaporated to form a vapor film (hereinafter also referred to as “film boiling state”). As a result of cooling, uneven cooling in the width direction of the steel sheet can be suppressed. Then, the inventor sets the average droplet diameter D (μm) of the droplet group to less than a predetermined value in relation to the cooling stop temperature T (° C.), thereby boiling the film from the start to the end of the cooling. It has been found that the state can be maintained, and as a result, uneven cooling in the width direction of the steel sheet can be suppressed.

This invention is completed by said knowledge, The summary structure is as follows.
[1] Hot-dip galvanizing is performed on the annealed steel sheet, and then the galvanization applied to the steel sheet is heat-alloyed, and then a droplet group in which a coolant is refined is sprayed toward the steel sheet to thereby inject the steel sheet. Is a method for producing an alloyed hot-dip galvanized steel sheet that is cooled to a cooling stop temperature T (° C.),
In the droplet group, carbon dioxide of 1.0% or more of the saturated dissolution amount of carbon dioxide at 20 ° C. and 1 atmosphere of carbon dioxide is dissolved, and the average droplet diameter D (μm) of the droplet group is dissolved. ) Is a state in which the droplet group does not contact the surface of the steel plate from the start of the cooling to the end of cooling at the cooling stop temperature T (° C.), and the droplet group evaporates to form a vapor film. A method for producing an alloyed hot-dip galvanized steel sheet, characterized in that the size is maintained.
[2] The alloy according to [1], wherein an average droplet diameter D (μm) of the droplet group satisfies the following formula (1) in relation to the cooling stop temperature T (° C.): Method for producing a galvannealed steel sheet.
D <5.0 × 10 ⁻³ × T ² −2.0 × T + 2.2 × 10 ² (1)
[3] The average droplet diameter D (μm) of the droplet group further satisfies the following formula (2) in relation to the cooling stop temperature T (° C.): [2] Method for producing an alloyed hot-dip galvanized steel sheet.
D ≧ 3.0 × 10 ⁻³ × T ² −1.2 × T + 1.3 × 10 ² (2)
[4] Hot-dip galvanizing is performed on the annealed steel sheet, and then the galvanization applied to the steel sheet is heat-alloyed, and then a droplet group in which a coolant is refined is sprayed toward the steel sheet to form the steel sheet. Of the galvannealed steel sheet is cooled to a cooling stop temperature T (° C.), and the droplet group has a saturated dissolution amount of carbon dioxide in the case of 20 ° C. and 1 atmosphere of carbon dioxide. From the start of cooling until the end of cooling at the cooling stop temperature T (° C.), the droplet group does not contact the surface of the steel plate, and the droplet group is dissolved. A relational expression between the average droplet diameter D (μm) of the droplet group and the cooling stop temperature T (° C.), which maintains the state where the vapor film is formed by evaporation, is satisfied in advance. Alloying characterized by cooling with an average droplet diameter D (μm) Manufacturing method of hot dip galvanized steel sheet.

  According to the method for producing an alloyed hot-dip galvanized steel sheet of the present invention, the unevenness of cooling in the width direction of the steel sheet in the cooling after alloying is suppressed without hindering the productivity improvement of the steel sheet, and the occurrence of defects due to it is suppressed. It is possible to suppress.

FIG. 1 is a schematic view of a continuous alloying hot dip galvanizing facility. 2A is a schematic diagram of the cooling device, and FIG. 2B is a schematic diagram of an air header of the cooling device. FIG. 3 is a conceptual diagram showing the relationship between the steel plate temperature and the cooling capacity in mist cooling after alloying. FIG. 4 is a graph showing the relationship between the average droplet diameter D of the droplet group and the transition temperature T ₁ in one embodiment of the present invention. FIG. 5 is a graph showing the relationship between the cooling stop temperature T and the average droplet diameter D of the droplet group in one embodiment of the present invention.

  As shown in FIG. 1, a continuous alloying hot dip galvanizing equipment 100 used in the method for producing an alloyed hot dip galvanized steel sheet in one embodiment of the present invention includes an annealing furnace (not shown), a hot dip galvanizing bath 20, a sink. It has a roll 21, a gas wiping device 22, an alloying furnace 23, a cooling device 10, a scanning radiation thermometer 24, and a top roll 25.

  The steel sheet P annealed in a continuous annealing furnace (not shown) is continuously introduced into the hot dip galvanizing bath 20 where the hot dip galvanizing is applied to the steel sheet P. The steel plate P is moved upward by the sink roll 21 in the hot dip galvanizing bath 20. After the steel sheet P is pulled up above the hot dip galvanizing bath 20, the plating adhesion amount is adjusted by the gas wiping device 22. Thereafter, the zinc plating applied to the steel sheet P in the alloying furnace 23 is heated and alloyed. Then, the droplet group which refined the cooling liquid with the cooling device 10 is sprayed toward the steel plate P, and the steel plate P is cooled. Thereafter, the steel plate temperature is measured by the radiation thermometer 24 in the vicinity of the top roll 25.

  The configuration of the cooling device 10 will be described with reference to FIG. As shown in FIG. 2A, the main part of the cooling device 10 is an air header 12 and a nozzle 14 attached thereto. Inside the air header 12 is a coolant header (not shown). The air header 12 is supplied with air pressurized to a predetermined pressure, and the coolant header is supplied with coolant pressurized to a predetermined pressure. The air and the cooling liquid are mixed inside the nozzle 14, and as a result, the cooling liquid is miniaturized, and a droplet group is sprayed toward the steel plate P from the opening of the nozzle 14. In this cooling liquid, carbon dioxide is dissolved in advance. As shown in FIG. 2B, one air header 12 is provided with a plurality of nozzles 14 at predetermined intervals in the extending direction. Since the air header 12 is installed so that the extending direction thereof coincides with the width direction of the steel plate P, the steel plate P can be cooled in the width direction. As shown in FIG. 2 (A), a plurality of air headers 12 are arranged in the traveling direction of the steel sheet P in accordance with the cooling equipment length. Furthermore, since the air header 12 is disposed on both surfaces of the steel plate P, the front and back surfaces of the steel plate P can be cooled. The coolant is not particularly limited, but is preferably water. The structure of the cooling device 10 is not limited to that described above as long as it is a device capable of spraying a droplet group.

  In addition to the case where water and air are mixed and jetted as described above, a method of spraying droplets by pressurizing only water may be used.

  With reference to FIG. 3, the cooling mechanism of the steel plate in the mist cooling after alloying will be described. First, at the stage where the temperature of the steel sheet is high immediately after the start of cooling, the cooling capacity is relatively low, and the change in the cooling capacity is small with respect to the temperature change of the steel sheet (area A). In this region A stage, the droplet group does not contact the surface of the steel sheet, and the droplet group evaporates and a film is formed in the vicinity of the steel sheet surface (between the steel sheet surface and the droplet group). In the boiling state, the steel sheet is cooled. This region A is a state where a stable heat flow rate can be obtained.

When the temperature of the steel sheet drops below the predetermined temperature T _1, along with the reduction of the temperature of the steel strip cooling capacity is increased (region B). In this region B, some droplets evaporate without contacting the surface of the steel sheet to form a vapor film, but the remaining droplets do not evaporate and collide with the surface of the steel sheet (partial). In the film boiling state), the steel sheet is cooled. As the steel plate temperature decreases, the proportion of droplets that directly collide with the surface of the steel plate increases. This region B is a state in which only an unstable heat flow rate can be obtained, and the cooling unevenness in the width direction of the steel sheet increases. That is, the steel plate temperature after alloying and before cooling varies somewhat in the width direction, or the injection amount inevitably varies by a few percent between the plurality of nozzles 14 shown in FIG. As a cause, in the region B, the portion where the steel plate temperature is lowered is cooled with a higher cooling capacity, so that the cooling unevenness in the steel plate width direction is expanded. Incidentally, the steel sheet temperature T ₁ of switching from the region A to the region B, is referred to herein as a "transition temperature".

  When the steel plate temperature further decreases, the cooling capacity also decreases as the steel plate temperature decreases, and a stable cooling state is obtained (region C). In the region C, the steel sheet is cooled in a state where all droplets collide with the surface of the steel sheet.

That is, if the film boiling state in the region A can be maintained from the start to the end of cooling, uneven cooling in the steel sheet width direction can be suppressed. The present inventors, to expand the region A on the low temperature side to shift the curve of FIG. 3 in the left direction, that was investigated means to a temperature lower than the transition temperature T ₁ of the cooling stop temperature T (° C.). As a result, it was found that the region A spreads to the low temperature side as the average droplet diameter D (μm) of the droplet group near the steel plate surface is smaller.

  In the present invention, a coolant in which carbon dioxide is dissolved is used. As a result, since carbon dioxide is dissolved in the droplet group, when the cooling liquid droplets collide with the steel sheet and take heat away from the steel sheet, the gas cannot be dissolved as the temperature of the cooling liquid rises. Foam. Due to the foaming, a vapor film is easily formed, and the film boiling state of the region A can be maintained even at a low temperature.

  When the amount of carbon dioxide dissolved in the cooling liquid is small, the foaming effect is not sufficiently exhibited. Therefore, in the present invention, the carbon dioxide having a saturation dissolution amount of 1.0% or more of carbon dioxide at 20 ° C. and 1 atmosphere of carbon dioxide is used. A cooling solution in which carbon is dissolved is used. In the case of 20 ° C. and 1 atm of air, the saturated dissolution amount of carbon dioxide in water is 0.61 mg / L. On the other hand, in the case of 20 ° C. and 1 atmosphere of carbon dioxide, the saturated dissolution amount of carbon dioxide in water is 1700 mg / L.

  The cooling liquid in which carbon dioxide is dissolved is supplied from the cooling header by, for example, bubbling carbon dioxide at a predetermined pressure into a container containing the cooling liquid (bubbling). What is necessary is just to mix with air inside the nozzle 14.

  In the present invention, carbon dioxide is dissolved in the cooling liquid in terms of industrial availability. However, similar to carbon dioxide, the same effect can be obtained if it is a gas that dissolves in water.

Therefore, the relationship between the average droplet diameter D (μm) of the droplet group and the transition temperature T ₁ (° C.) was examined using the continuous alloying galvanizing equipment shown in FIG. Water was used as the cooling liquid. Moreover, it was set as the cooling liquid which melt | dissolved the carbon dioxide 10% or more of saturation amount. The average droplet diameter D (μm) of the droplet group was changed by changing the flow ratio of air to water. The other experimental conditions are as follows.
Steel plate: plate thickness 1.0mm, plate width 1000mm, tensile strength 450MPa
Plate speed: 150 mpm
Hot dip galvanizing bath temperature: 460 ° C
Steel plate temperature immediately after alloying: 500 ° C
FIG. 4 shows the relationship between the average droplet diameter D (μm) of the droplet group and the transition temperature T ₁ (° C.) in the mist cooling after alloying. As shown in FIG. 4, as the transition temperatures T ₁ mean droplet diameter D of the droplet group ([mu] m) is small becomes lower.

Therefore, based on FIG. 4, the average droplet diameter D (μm) of the droplet group and the cooling stop temperature T (° C.) for making the transition temperature T ₁ (° C.) less than the cooling stop temperature T (° C.) Sought the relationship. As a result, if the average droplet diameter D (μm) of the droplet group satisfies the following formula (1) in relation to the cooling stop temperature T (° C.), the transition temperature T ₁ (° C.) is changed to the cooling stop temperature T. It was found that the temperature could be less than (° C.).
D <5.0 × 10 ⁻³ × T ² −2.0 × T + 2.2 × 10 ² (1)
The cooling stop temperature T (° C.) can usually be in the range of 250 to 450 ° C. Therefore, in one embodiment of the present invention, once the cooling stop temperature T (° C.) is determined, the value is substituted into the right side of Equation (1), and the average droplet diameter D (μm) of the droplet group is the value on the right side. The steel sheet may be cooled by controlling to less than the above. The average droplet diameter D (μm) of the droplet group in the present invention is defined by the Sauter average particle size, and can be measured by an immersion method or a laser method of irradiating a droplet with laser light. it can.

  The average droplet diameter D (μm) of the droplet group can be controlled by the structure of the nozzle, the pressure ratio of air and cooling liquid before mixing, the flow rate ratio of air and cooling liquid, and the like. For example, if the air pressure is increased with respect to the coolant pressure, the average droplet diameter D (μm) of the droplet group decreases. Further, if the air flow rate is increased with respect to the coolant flow rate, the average droplet diameter D (μm) of the droplet group is reduced. If the flow rate is the same, the smaller the nozzle diameter, the smaller the average droplet diameter D (μm) of the droplet group.

However, in order to reduce the average droplet diameter D (μm) of the droplet group, it is necessary to increase the air injection pressure, for example, as described above. In that case, additional costs such as increased cost of compressed air production equipment such as blowers and compressors and increased running costs are required. Therefore, it is not possible to reduce the average droplet diameter D (μm) of the droplet group more than necessary. It is not preferable. Therefore, the inventor investigated the lower limit of the average droplet diameter D (μm) of the droplet group at which the effect of reducing the cooling unevenness in the steel plate width direction is saturated with respect to the cooling stop temperature T (° C.). The lower limit of the average droplet diameter D (μm) of the droplet group that saturates the cooling unevenness in the width direction of the steel sheet is the limit temperature at which partial film boiling (transition boiling) occurs. It has been found that the average droplet diameter D (μm) of the above satisfies the following formula (2) further in relation to the cooling stop temperature T (° C.).
D ≧ 3.0 × 10 ⁻³ × T ² −1.2 × T + 1.3 × 10 ² (2)
FIG. 5 shows the relationship between the cooling stop temperature T (° C.) according to the equations (1) and (2) and the average droplet diameter D (μm) of the droplet group. A broken line is Formula (1) and a continuous line is Formula (2).

  The relational expression between the average droplet diameter D (μm) of the droplet group and the cooling stop temperature T (° C.) may be different from the formula (1) and the formula (2) depending on the operation conditions, particularly the cooling conditions. There is. For this reason, the present invention is not limited to these equations, and the average droplet diameter D (μm) of the droplet group is related to the cooling stop temperature T (° C.) from the start of cooling to the cooling stop temperature T ( It is important that the film boiling state be maintained until the cooling is completed. Thereby, it is possible to suppress cooling unevenness in the steel plate width direction in cooling after alloying, and to suppress generation of defects due to the cooling unevenness. In the present invention, cooling is performed in a film boiling state having a relatively low cooling capacity, but sufficient cooling capacity can be obtained by appropriately controlling the droplet spray amount. Therefore, the productivity is not hindered as in Patent Document 1 including gas cooling and Patent Document 2 in which the latter cooling is performed more slowly than the preceding cooling.

  In another embodiment of the present invention, in the operating conditions to which the present invention is applied, the droplet group does not contact the surface of the steel plate from the start of cooling to the end of cooling at the cooling stop temperature T (° C.), and the film is boiling. Is maintained in advance, a relational expression between the average droplet diameter D (μm) of the droplet group and the cooling stop temperature T (° C.) is obtained in advance, and the average droplet of the droplet group is satisfied so as to satisfy the obtained relational expression. The steel sheet after alloying may be cooled by controlling the diameter D (μm). Thereby, it is possible to suppress cooling unevenness in the steel plate width direction in cooling after alloying, and to suppress generation of defects due to the cooling unevenness.

  Using the continuous alloying hot-dip galvanizing equipment shown in FIG. 1, an alloyed hot-dip galvanized steel sheet was manufactured under the same operating conditions as the experiments for obtaining the above-described equations (1) and (2). The cooling stop temperature T (° C.) is set to three levels of 300 ° C., 350 ° C., and 400 ° C., and the average droplet diameter D (μm) of the droplet group and the ratio (%) of the dissolution amount to the saturated dissolution amount of carbon dioxide are shown. As shown in FIG. The temperature difference (maximum temperature-minimum temperature) of the steel sheet after cooling was measured with a radiation thermometer, and the results are shown in Table 1.

  In addition, an image obtained by photographing the vicinity of the surface of the steel sheet being cooled with a camera was visually observed, and the state of the droplet group being cooled was examined.

  Moreover, about the defect of a steel plate, the obtained steel plate was observed visually and the thing without a wrinkle-like pattern was set as the pass.

  As shown in Table 1, in the comparative example not satisfying the formula (1), the temperature difference in the steel plate width direction was increased, and a defect such that a wrinkled pattern was generated on the steel plate surface occurred. On the other hand, in the example of the present invention satisfying the formula (1), the temperature difference in the width direction of the steel sheet was very small, and there was no defect that a wrinkled pattern was generated on the surface of the steel sheet. Moreover, as a result of observation, in the comparative example, the initial stage of cooling was in a film boiling state, but in the latter half it was in a partial film boiling state, whereas in the inventive example, the film boiling state was from the initial stage to the end of cooling. It was maintained.

  According to the method for producing an alloyed hot-dip galvanized steel sheet of the present invention, the unevenness of cooling in the width direction of the steel sheet in the cooling after alloying is suppressed without hindering the productivity improvement of the steel sheet, and the occurrence of defects due to it is suppressed. It is possible to suppress.

DESCRIPTION OF SYMBOLS 100 Continuous alloying hot dip galvanization equipment 10 Cooling device 12 Air header 14 Nozzle 20 Hot dip galvanizing bath 21 Sink roll 22 Gas wiping device 23 Alloying furnace 24 (Scanning type) Radiation thermometer 25 Top roll P Steel plate

Claims (4)

Hot-dip galvanizing is applied to the annealed steel sheet, and then the galvanization applied to the steel sheet is heat-alloyed, and then cooling of the steel sheet is stopped by spraying a group of droplets with a refined coolant toward the steel sheet. A method for producing an alloyed hot-dip galvanized steel sheet that is cooled to a temperature T (° C),
In the droplet group, carbon dioxide of 1.0% or more of the saturated dissolution amount of carbon dioxide at 20 ° C. and 1 atmosphere of carbon dioxide is dissolved,
The average droplet diameter D (μm) of the droplet group is such that the droplet group does not contact the surface of the steel plate from the start of cooling until the end of cooling at the cooling stop temperature T (° C.). A method for producing an alloyed hot-dip galvanized steel sheet, characterized in that the droplet group has a size capable of maintaining a state in which a vapor film is formed by evaporation. 2. The alloyed molten zinc according to claim 1, wherein an average droplet diameter D (μm) of the droplet group satisfies the following formula (1) in relation to the cooling stop temperature T (° C.). Manufacturing method of plated steel sheet.
D <5.0 × 10 ⁻³ × T ² −2.0 × T + 2.2 × 10 ² (1) 3. The alloying according to claim 2, wherein an average droplet diameter D (μm) of the droplet group further satisfies the following formula (2) in relation to the cooling stop temperature T (° C.). Manufacturing method of hot dip galvanized steel sheet.
D ≧ 3.0 × 10 ⁻³ × T ² −1.2 × T + 1.3 × 10 ² (2) Hot-dip galvanizing is applied to the annealed steel sheet, and then the galvanization applied to the steel sheet is heat-alloyed, and then cooling of the steel sheet is stopped by spraying a group of droplets with a refined coolant toward the steel sheet. A method for producing an alloyed hot-dip galvanized steel sheet that is cooled to a temperature T (° C),
In the droplet group, carbon dioxide of 1.0% or more of the saturated dissolution amount of carbon dioxide at 20 ° C. and 1 atmosphere of carbon dioxide is dissolved,
From the start of the cooling to the end of cooling at the cooling stop temperature T (° C.), the droplet group does not contact the surface of the steel sheet, and the droplet group evaporates to form a vapor film. A relational expression between the average droplet diameter D (μm) of the droplet group and the cooling stop temperature T (° C.) is obtained in advance,
A method for producing an alloyed hot-dip galvanized steel sheet, wherein cooling is performed with an average droplet diameter D (μm) satisfying the relational expression.

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