KR20180064497A - Manufacturing method of hot-dip galvanized steel sheet - Google Patents

Abstract

A method of manufacturing a hot-dip galvanized steel sheet capable of obtaining a galvanized steel sheet with high plating adhesion and good plating appearance even when hot-dip galvanizing is applied to steel strips containing 0.2 mass% or more of Si.
In the present invention, the steel strip is conveyed in the order of the heating zone, the crack zone, and the cooling zone in the annealing furnace, annealing the steel strip, and then hot dip galvanizing the steel strip discharged from the cooling zone. The reducing gas or non-oxidizing gas supplied to the cracks is a humidifying gas and a drying gas. The fluctuation range of the amount of water supplied to the cracks by the humidifying gas is controlled to be 20 [mu] m by controlling the flow rate of the drying gas by suppressing the fluctuation of the pressure in the annealing furnace while the width of the steel strip passing through the cracks, %.

Description

Manufacturing method of hot-dip galvanized steel sheet

The present invention relates to an annealing furnace comprising an annealing furnace in which a heating zone, a soaking zone and a cooling zone are arranged in parallel in this order, and a continuous hot-dip galvanizing apparatus having a hot- To a method of manufacturing a hot-dip galvanized steel sheet.

In recent years, in the field of automobiles, household appliances, building materials, etc., demand for high tensile strength steel plates (high-tensile steel plates) contributing to light weight of structures is increasing. As the Heiden's steel, for example, it is known that a steel sheet having good hole expandability by containing Si in steel, or a steel sheet having easy ductility due to containing Si or Al and easily forming residual gamma can be produced have.

However, in the case of producing a galvannealed steel sheet using a high-tensile steel sheet containing a large amount of Si (particularly 0.2 mass% or more) as a base material, the following problems arise. The galvannealed hot-dip galvanized steel sheet is produced by annealing a steel sheet of a base material at a temperature of about 600 to 900 DEG C in a reducing atmosphere or a non-oxidizing atmosphere, subjecting the steel sheet to hot-dip galvanization, .

Here, Si in the steel is an easily oxidizing element and selectively oxidized in a reducing atmosphere or a non-oxidizing atmosphere generally used, and is concentrated on the surface of the steel sheet to form an oxide. This oxide lowers the wettability with molten zinc during the plating treatment, and causes the plating to occur. As a result, the Si concentration in the steel is increased and the wettability is rapidly lowered, resulting in a large amount of unplated plating. Further, there is a problem in that the plating adhesion is inferior even when the plating does not reach the plating. Further, when Si in the steel is selectively oxidized to be concentrated on the surface of the steel sheet, there arises a problem that remarkable alloying delay occurs in the alloying process after hot dip galvanizing, thereby remarkably inhibiting the productivity.

In view of such a problem, Patent Document 1 discloses a method of oxidizing the inside of a steel sheet by annealing a steel sheet under a reducing atmosphere after once oxidizing the surface of the steel sheet by using a direct-fired type heating furnace (DFF) Discloses a method of suppressing the concentration of Si on the surface to improve the wettability and adhesion of hot dip galvanizing. The reduction annealing after heating is described as good in the normal method (dew point -30 to -40 占 폚).

Patent Document 2 proposes a continuous annealing hot-dip coating method using an annealing furnace and a hot-dip coating bath in which a hot-plate front end, a heating end rear end, a hot-tip zone and a cooling zone are sequentially provided and a hot- The atmosphere in the furnace of each zone is made to be an atmosphere composed of 1 to 10 vol% of hydrogen and the balance of nitrogen and inevitable impurities and the steel sheet reaching temperature during heating at the heating stage front end is set to 550 Deg.] C to not more than 750 [deg.] C and the dew point is set to less than -25 [deg.] C, To thereby oxidize Si internally to inhibit Si from concentrating on the surface of the steel sheet. It is also described that a mixed gas of nitrogen and hydrogen is introduced into the heating stage rear end and / or the heating stage by wetting.

Patent Document 3 discloses a method in which the dew point of the gas in the reducing furnace is changed from -30 deg. C to 0 deg. C or lower by changing the position of supply and discharge of the furnace gas while measuring the dew point of furnace gas, So as to suppress the concentration of Si on the surface of the steel sheet. In the heating furnace, any of DFF (direct heating furnace), NOF (without oxidation furnace) and radiant tube type may be used, but it is preferable because the radiant tube type can remarkably exhibit the inventive effect. have.

Japanese Laid-Open Patent Publication No. 2010-202959 WO2007 / 043273 Japanese Laid-Open Patent Publication No. 2009-209397

However, in the method described in Patent Document 1, the internal oxidation amount of Si tends to be insufficient and the alloying temperature becomes higher by 30 to 50 캜 than usual due to the influence of Si in the steel, though the plating adhesion after reduction is good. Resulting in a problem that the tensile strength of the resulting steel sheet is lowered. If the oxidation amount is increased to secure a sufficient internal oxidation amount, an oxide scale is adhered to the roll in the annealing furnace, and a pressing scratch or a so-called pickup defect occurs on the steel sheet. For this reason, means for simply increasing the amount of oxidation can not be taken.

In the method described in Patent Document 2, since the heating and heating of the heating stage front end, the rear end of the heating stage, and the heating stage are indirect heating, oxidation of the surface of the steel sheet as in the case of the direct heating of Patent Document 1 is unlikely to occur, The internal oxidation of Si is insufficient even when compared with this, and the problem that the alloying temperature becomes high is more remarkable. Further, in addition to the change in the amount of water carried in the furnace depending on the outside air temperature fluctuation and the kind of the steel sheet, the gas mixture dew point is also likely to fluctuate in accordance with the outside-air temperature fluctuation, and it is difficult to stably control the optimum dew point range. As the dew-point fluctuation is large in this way, surface defects such as plated coating are generated even in the above-described dew point range and temperature range, and it is difficult to produce a stable product.

In the method described in Patent Document 3, oxidation of the surface of the steel sheet can occur if a DFF is used in the heating furnace. However, since the humidifying gas is not positively supplied to the annealing furnace, Lt; 0 > C. In addition, when the dew point is raised, the dew point at the upper portion of the furnace tends to be higher. When the temperature is 0 DEG C at the lower portion of the furnace, there may be a high dew point atmosphere above + 10 DEG C in the furnace. .

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and it is an object of the present invention to provide a method for manufacturing a hot-dip galvanized steel sheet having a high plating adhesion and a good plating appearance even when hot dip galvanizing is applied to a steel strip containing Si in an amount of 0.2 mass% And a method thereof.

The structure of the present invention is as follows.

[1] A method for producing a hot-dip galvanized steel sheet using a continuous hot-dip galvanizing apparatus having an annealing furnace in which a heating stand, a crack stand, and a cooling stand are arranged in parallel in this order and a hot-

A step of conveying the steel strip inside the annealing furnace in the order of the heating stand, the crack stand, and the cooling stand, and annealing the steel stand,

A step of performing hot dip galvanizing on the steel strip discharged from the cooling furnace by using the hot dip galvanizing facility

Lt; / RTI &

The reducing gas or non-oxidizing gas supplied to the cracking zone is a humidifying gas humidified by a humidifying device and a drying gas not humidified by the humidifying device,

The fluctuation of the pressure in the annealing furnace is suppressed by controlling the flow rate of the drying gas while the width and the sheet passing speed of the steel strip passing through the cracks are constant, Wherein the fluctuation range of the amount of water supplied to the steel strip is 20% or less.

[2] The method according to the above [1], wherein the flow rate and the dew point of the humidifying gas are set so that the moisture content (M) (g / min) supplied to the crack zone by the humidifying gas satisfies the following formula (1) Wherein the hot dip galvanized steel sheet is produced by a method comprising the steps of:

40 + Vf (W-0.9) (S + 4) / 90 <M <60 + Vf (W-

Here, Vf is the volume (m 3) of the crack zone, W is the width (m) of the steel strip passing through the crack zone, and S is the steel plate passing speed (㎧).

[3] The flow rate and the dew point of the humidifying gas are adjusted so that the water content (M) (g / min) satisfies the above formula (1) when at least one of the width and the sheet passing speed of the steel strip passing through the cracks fluctuates. Of the hot-dip galvanized steel sheet according to [2].

[4] In the upper half region in the height direction of the crack band, at a position separated from the position of the supply port of the humidifying gas formed in the crack band by 1 m or more, the inner wall position of the crack band Described in any one of [1] to [3] above, wherein the dew point in the crack zone measured in the dew point measuring section formed in the cracking zone is controlled to be not less than -25 ° C and not more than 0 ° C A method of manufacturing a hot dip galvanized steel sheet.

[5] The hot plate includes a flame-type heating furnace, wherein the continuous hot-dip galvanizing apparatus has an alloying facility adjacent to the hot-dip galvanizing facility,

The method for producing a hot-dip galvanized steel sheet according to any one of the above [1] to [4], further comprising a step of heat-alloying the zinc plating applied to the steel strip using the alloying facility.

According to the method of producing a hot-dip galvanized steel sheet of the present invention, even when hot-dip galvanizing is carried out on steel strips containing 0.2 mass% or more of Si, the plating adherence is high and good plating appearance can be obtained.

1 is a schematic view showing a configuration of a continuous hot dip galvanizing apparatus 100 according to an embodiment of the present invention.
Fig. 2 is a schematic view showing a supply system of humidified gas and dry gas to the crack base 12 in Fig. 1. Fig.

(Mode for carrying out the invention)

First, the configuration of a continuous hot dip galvanizing apparatus 100 used in a method of manufacturing a hot-dip galvanized steel sheet according to an embodiment of the present invention will be described with reference to FIG. The continuous hot dip galvanizing apparatus 100 includes an annealing furnace 20 in which a heating base 10, a crack base 12 and cooling bases 14 and 16 are arranged in parallel in this order, A hot-dip galvanizing bath 22 as a zinc-plating facility, and an alloying facility 23 adjacent to the hot-dip galvanizing bath 22. In the present embodiment, the heating stand 10 includes a first heating stand 10A (front end of heating stand) and a second heating stand 10B (rear end of heating stand). The cooling stage includes a first cooling stage 14 (a rapid cooling stage) and a second cooling stage 16 (a cooling stage). The Snart 18 connected to the second cooling base 16 has its tip immersed in the hot dip galvanizing bath 22 so that the annealing furnace 20 and the hot dip galvanizing bath 22 are connected.

The pulley P is introduced into the first heating stand 10A from the lower inlet of the first heating stand 10A. In each zone 10, 12, 14, 16, one or more hearth rolls are disposed at the top and bottom. When the cord P is folded back 180 degrees from the hose roll as a base point, the steel strips P are conveyed in the vertical direction in a predetermined band of the annealing furnace 20 to form multiple passes. 1 shows an example of 10 passes in the crack bed 12, 2 passes in the first cooling bed 14, and 2 passes in the second cooling bed 16, the number of passes is not limited to this, It can be set appropriately according to the conditions. Further, in some hustling rolls, the pulleys P are turned at right angles without being folded back, and the pulleys P are moved as follows. In this manner, the steel strip P is conveyed in the order of the heating table 10, the crack plate 12, and the cooling plates 14 and 16 in the annealing furnace 20 to anneal the steel plate P .

In the annealing furnace 20, adjoining bays communicate with each other through communicating portions connecting the upper portions of the bands or the lower portions. In the present embodiment, the first heating stand 10A and the second heating stand 10B communicate with each other through throttling (restriction portion) connecting the upper portions of the respective stands. The second heating stand 10B and the crack base 12 communicate with each other through a throat connecting the lower portions of the respective stands. The crack base 12 and the first cooling bands 14 communicate with each other through throats connecting the lower portions of the bands. The first cooling bands 14 and the second cooling bands 16 communicate with each other through throats connecting the lower portions of the bands. The height of each throat may be suitably set, but it is preferable that the height of each throat is as low as possible from the standpoint of enhancing the independence of each atmosphere. The gas in the annealing furnace 20 flows from the downstream side of the furnace to the upstream side and is discharged from the furnace inlet of the lower portion of the first heating stand 10A.

(Heating table)

In the present embodiment, the second heating stand 10B is a flame-type heating furnace (DFF). A known DFF can be used. Although not shown in Fig. 1, a plurality of burners are dispersedly arranged on the inner wall of the flame type heating furnace in the second heating stand 10B so as to oppose the steel strip P. Fig. It is preferable that the plurality of burners are divided into a plurality of groups, and the fuel ratio and the air ratio can be controlled independently for each group. The combustion exhaust gas from the second heating stand 10B is supplied to the inside of the first heating stand 10A and preheats the steel strip P with the heat.

The combustion rate is a value obtained by dividing the amount of the fuel gas actually introduced into the burner by the amount of the fuel gas of the burner at the maximum combustion load. Burning rate is 100% when burner is burned with maximum burning load. When the burning load is low, the burner can not obtain a stable combustion state. Therefore, the burning rate is preferably 30% or more.

The air ratio is a value obtained by dividing the amount of air actually introduced into the burner by the amount of air required to completely burn the fuel gas. In this embodiment, the heating burners of the second heating stand 10B are divided into four groups # 1 to # 4, and the three groups (# 1 to # 3) , And the final zone (# 4) were used as reduction burners, and the air ratio of the oxidizing burner and the reducing burner was individually controllable. In the burner for oxidation, it is preferable to set the air ratio to 0.95 or more and 1.5 or less. In the reducing burner, the air ratio is preferably 0.5 or more and less than 0.95. In addition, the temperature inside the second heating table 10B is preferably 800 to 1200 占 폚.

(Cracked band)

In the present embodiment, in the crack base 12, the steel strip P can be indirectly heated by using a radiant tube RT (not shown) as a heating means. The average temperature Tr (占 폚) inside the crack base 12 is measured by inserting a thermocouple into the crack base, but it is preferably 700 to 900 占 폚.

A reducing gas or a non-oxidizing gas is supplied to the crack base (12). As the reducing gas, a mixed gas of H ₂ -N _{2 is} usually used, and a gas (dew point: about -60 ° C) having a composition of, for example, H ₂ : 1 to 20% by volume and the balance N ₂ and inevitable impurities . As the non-oxidizing gas, a gas having a composition consisting of N ₂ and inevitable impurities (dew point: about -60 ° C) can be mentioned.

In the present embodiment, the reducing gas or the non-oxidizing gas supplied to the crack base 12 is in the form of a humidifying gas and a drying gas. Here, &quot; dry gas &quot; means that the dew point is the reducing gas or the non-oxidizing gas at about -60 DEG C to -50 DEG C and is not humidified by the humidifier. On the other hand, "humidifying gas" means a gas whose dew point is humidified at 0 to 30 ° C by a humidifier.

For example, in manufacturing a high-tensile steel sheet having a component composition containing Si of 0.2 mass% or more, it is preferable to supply a humidifying gas to the crack bed 12 in addition to the dry gas in order to raise the dew point in the crack base . On the other hand, for example, when producing a normal steel plate (with a tensile strength of about 270 MPa), it is preferable that only the dry gas is supplied to the crack base 12 and the mixed gas is not supplied.

2 is a schematic view showing a supply system of a humidifying gas and a drying gas to the crack base 12. As shown in Fig. The humidifying gas is supplied to the three systems of the humidifying gas supply ports 42A, 42B and 42C, the humidifying gas supply ports 44A, 44B and 44C and the humidifying gas supply ports 46A, 46B and 46C. 2, the reducing gas or the non-oxidizing gas (dry gas) is sent to the humidifying device 26 by the gas distributing device 24, and the remaining portion is supplied to the drying gas piping 32 And is supplied into the crack base 12 through the dry gas supply ports 48A, 48B, 48C and 48D.

The position and the number of the dry gas supply ports are not particularly limited and may be suitably determined in consideration of various conditions. However, it is preferable that a plurality of the dry gas supply ports are disposed at the same height position, and it is preferable that the dry gas supply ports are evenly arranged in the direction of advancement of the steel strip.

The humidified gas is distributed to the three systems in the humidifying gas distributing device 30 and is supplied to the humidifying gas supply ports 42A, 42B, and 42C via the respective humidifying gas pipes 36, And is supplied into the crack base 12 through the humidifying gas supply ports 44A, 44B and 44C and the humidifying gas supply ports 46A, 46B and 46C.

The position and the number of the humidifying gas supply ports are not particularly limited and may be suitably determined in consideration of various conditions. However, it is preferable that the humidifying gas supply port is formed at one or more positions in each of four zones, that is, two in the vertical direction of the crack base 12 and two in the entry / exit direction. This is because it is possible to uniformly control the dew point of the entire crack base 12. Reference numeral 38 denotes a flow meter for humidifying gas, and 40 denotes a dew point system for humidifying gas. The dew point of the humidifying gas may be changed by slight condensation or the like in the piping 34 and 36 for humidifying gas so that the dew point system 40 is installed just in front of the humidifying gas supply ports 42, .

In the humidifying device 26, there is a humidifying module having a fluorine-based or polyimide-based hollow fiber membrane or a flat membrane, and a drying gas is made to flow inside the membrane, and a circulating water bath Circulate pure water. A fluorine-based or polyimide-based hollow fiber membrane or flat membrane is one kind of ion exchange membrane having an affinity for water molecules. When there is a difference in moisture concentration between the inside and the outside of the hollow fiber membrane, a force is generated to equalize the difference in concentration, and the moisture moves through the membrane toward the low water concentration as a driving force. The drying gas temperature varies depending on seasonal or day-to-day temperature changes. However, in this humidifying device, heat exchange can be performed by sufficiently taking in contact area of gas and water through the water vapor permeable membrane. The dry gas is humidified to the dew point equal to the set water temperature, thereby enabling high-precision dew point control. The dew point of the humidifying gas can be arbitrarily controlled within the range of 5 to 50 캜. If the dew point of the humidifying gas is higher than the piping temperature, the dew condensation occurs in the piping, and the condensed water may directly flood into the pipeline. Therefore, the piping for the humidifying gas is heated and heated to a temperature higher than the humidifying gas dew point .

The pressure in the annealing furnace fluctuates occasionally depending on the combustion condition of the heating stand 10 and the operation condition of the cooling fan in the cooling stands 14 and 16 regardless of whether or not the humidifying gas is supplied to the crack base 12 do. If the withstand pressure is too high, excessive force may be applied to the furnace wall, thereby damaging the annealing furnace. Conversely, if the furnace inner pressure is excessively low, oxygen other than the annealing furnace may be mixed into the crack base 12, The combustion gas of the burner 10 flows into the burner and adversely affects the quality of the steel plate. Therefore, in general, control is performed to increase or decrease the flow rate of the gas supplied to the crack base 12 so as to suppress the fluctuation of the internal pressure, preferably, to keep the internal pressure constant. Therefore, when performing the operation of supplying both the humidified gas and the dry gas to the crack base 12, in the conventional control method, not only the flow rate of the dry gas but also the flow rate of the humidified gas are varied, The amount of water supplied to the cracks also fluctuated.

However, it is necessary to always supply the necessary amount of water to the crack base 12 in view of internal oxidation of Si and Mn in the steel strip. The amount of water supplied to the crack base 12 is insufficient and the dew point in the crack base 12 falls below the lower limit of the proper range. As a result, So that the plating appearance may deteriorate in some cases. Further, in the operation of performing the alloying treatment, the alloying temperature is elevated, and as a result, the desired tensile strength is not obtained in some cases. If the flow rate of the humidifying gas is increased to suppress the fluctuation of the internal pressure, the amount of water supplied to the crack base 12 becomes excessive, resulting in roll pick-up, scratches due to roll pick- Resulting in deterioration of the plating appearance.

Thus, in the present embodiment, while the width of the steel strip passing through the crack base 12 and the passing plate speed are constant (hereinafter also referred to as "under the same operating condition"), the fluctuation of the pressure in the annealing furnace It is very important to make the amount of moisture supplied to the crack base 12 by the humidifying gas as maximum as possible, specifically, to make the variation range of the moisture amount 20% or less. As a result, a good plating appearance can be obtained, and in the operation of performing the alloying treatment, lowering of the alloying temperature can suppress the decrease of the tensile strength. Here, the "fluctuation of the amount of water" is supplied to the crack is large, when the maximum amount of water under the same operating conditions as M _max, a minimum of M _min, is defined as _{(M max -M min) / M} max. The water content can be calculated by the following formula (2).

The form of suppressing the fluctuation range of the water content to 20% or less is not particularly limited. In one form, the dew point of the humidifying gas is made constant, and the fluctuation range of the flow rate is controlled to 20% or less. In the case where a plurality of humidifying gas supply ports are formed as in the present embodiment, it is preferable that the flow rate of the humidifying gas from each of the supply ports and the total humidifying gas flow rate is made as maximum as possible (specifically, 20% or less) .

The amount of water M (g / min) put into the crack base 12 by the humidifying gas needs to be adjusted by the volume of the crack zone, the width of the steel strip P passing through the crack zone 12, . The inventors of the present invention have found that the flow rate and the dew point of the humidifying gas are set so that the water amount M (g / min) supplied to the crack base 12 by the humidifying gas satisfies the following formula (1) , And found to be effective for obtaining a good plating appearance.

40 + Vf (W-0.9) (S + 4) / 90 <M <60 + Vf (W-

Here, Vf is the volume (m3) of the crack base 12, W is the width m of the steel strip P passing through the crack base 12, and S is the passing speed of the steel strip P.

When the water content M (g / min) satisfies the formula (1) when at least one of the width W of the steel strip P passing through the crack base 12 and the passing speed S fluctuates , It is effective to change the flow rate and dew point of the humidifying gas.

The volume Vf of the crack base 12 is substantially constant. When the width W of the belt P passing through the crack base 12 and the passing speed S are increased or when one of the width W and the passing speed S is constant and the other is increased , The amount of water by the humidifying gas is increased based on the equation (1) because the area of the steel belt contacting the gas in the crack base 12 increases per unit time. When the width W and the passing speed S of the belt P passing through the crack base 12 decrease or when one of the width W and the passing speed S is constant and the other is decreased On the contrary, it is necessary to reduce the amount of moisture by the humidifying gas based on the formula (1). The amount of moisture by the humidifying gas is adjusted based on the equation (1) even when one of the width W and the passing speed S increases and the other decreases. Either way, it is preferable to adjust the flow rate and the dew point of the humidifying gas so as to satisfy the equation (1) without waiting for the change of the dew point in the crack bed 12 accompanied by the change of the operating condition.

The water content M (g / min) can be calculated from the dew point Tw (° C) of the humidifying gas and the total flow rate Vm (Nm 3 / hr).

The flow rate Vm of the humidifying gas supplied into the crack base 12 is not particularly limited as long as it is controlled as described above, but it is maintained within a range of approximately 100 to 400 (Nm3 / hr). The flow rate of the drying gas supplied into the crack base 12 is not particularly limited, but is maintained within a range of approximately 10 to 300 (Nm3 / hr).

In the crack base 12, the water vapor is easier to collect at the upper part because the specific gravity is smaller than that of the nitrogen gas. Therefore, the dew point measuring instrument 50 is placed in the upper half region of the crack base 12 in the height direction. In addition, the vicinity of the humidifying gas supply port is an area which is not suitable for dew point measurement because it locally becomes a dead point. Therefore, it is preferable that the dew-point measuring instrument 50 is disposed at a position separated by 1 m or more from the position of each humidifying gas supply port and at a position separated by 1 m or more from the inner wall position of the crack band facing each supply port. It is preferable to control the flow rate of the humidifying gas so that the dew point in the crack base 12 measured in the dew point measuring instrument 50 is kept at -25 캜 or more and 0 캜 or less. As a result, a good plating appearance can be obtained, and in the operation of performing the alloying treatment, lowering of the alloying temperature can suppress the decrease of the tensile strength.

(Cooling base)

In the present embodiment, the cooling zones 14 and 16 cools the steel strip P. The steel strip P is cooled to about 480 to 530 캜 in the first cooling zone 14 and cooled to about 470 to 500 캜 in the second cooling zone 16.

The reducing gas or non-oxidizing gas is also supplied to the cooling bands 14 and 16, but only dry gas is supplied here. The supply of the drying gas to the cooling bands 14 and 16 is not particularly limited, but it is preferable that the drying gas is supplied from two or more places in the height direction and two or more places in the longitudinal direction so as to be evenly charged into the cooling bath. The total gas flow rate Qcd of the dry gas to be supplied to the cooling bands 14 and 16 is measured by a gas flow meter (not shown) installed in the pipe, and is not particularly limited, but is 200 to 1000 (Nm3 / hr) . The fluctuation of the pressure in the annealing furnace may be suppressed by adjusting only the flow rate of the dry gas supplied to the crack base, but it is also preferable to adjust the flow rate of the dry gas supplied to the cooling base.

(Hot-dip galvanizing bath)

Hot dip galvanizing can be performed on the steel strip P discharged from the second cooling table 16 by using the hot dip galvanizing bath 22. [ Hot dip galvanizing may be carried out according to a regular method.

(Alloying facility)

The galvanizing applied to the steel strip P can be heat-alloyed using the galvannealing facility 23. The alloying treatment may be carried out according to a regular method. According to this embodiment, since the alloying temperature does not become high, deterioration of the tensile strength of the produced galvannealed galvanized steel sheet can be suppressed. However, in the present invention, the alloying facility 23 and the subsequent alloying treatment are not essential. This is because the effect of obtaining a good plating appearance can be obtained even when the alloying treatment is not performed.

The steel strip P to be subjected to the annealing and hot dip galvanizing treatment is not particularly limited, but the effect of the present invention can be advantageously obtained in the case of a steel strip having a component composition containing 0.2 mass% or more of Si, that is, a high tensile steel.

Example

(Experimental conditions)

Using the continuous hot-dip galvanizing apparatus shown in Figs. 1 and 2, the strips of the component compositions shown in Table 1 were annealed under various annealing conditions shown in Table 2, and then subjected to hot dip galvanizing and alloying treatment. Steel A and Steel B are all high tensile strength steels. The "time" shown in Table 2 means the elapsed time from the start of the operation, and the elapsed time from the start of the operation to the elapsed time of the operation of the continuous hot dip galvanizing apparatus Changed.

The second heating stage was a DFF. The heating burners were divided into four groups # 1 to # 4, the three groups (# 1 to # 3) on the upstream side in the steel sheet moving direction were oxidizing burners, and the final zone (# 4) , And the air ratio of the oxidizing burner and the reducing burner was set to the values shown in Table 2. The length of each group in the steel sheet conveyance direction is 4 m.

The crack zone was set to RT with a volume (Vf) of 700 m 3. The average temperature (Tr) of the inside of the cracks was set as shown in Table 2. As the drying gas, 15 vol.% And H ₂ gas (dew point: -50 ℃) having a composition comprising the balance of N ₂ and inevitable impurities was used. A part of the dry gas was humidified by a humidifying device having ten hollow fiber membrane type humidifying modules to prepare a humidifying gas. A maximum of 500 L / min of dry gas and a maximum of 20 L / min of circulating water flow through each module. The circulating constant temperature water tank is common to each module and can supply pure water of 200 L / min in total. The dry gas supply port and the humidifying gas supply port were arranged at the positions shown in Fig.

As shown in Table 2, eight types of steel strips having different steel types, plate thicknesses, and plate widths were shipped. The first half (from 0:00 to 0:55) is a comparative example, and the second half (time 0:55 to 1:50) is a demonstration. That is, in the first half, the flow rate of the drying gas supplied to the cracks, the flow rate of the humidifying gas, and the flow rate of the drying gas supplied to the cooling bed were varied as shown in Table 2, and the furnace internal pressure was kept constant. As shown in Table 2, the dew point of the humidifying gas is kept constant while the type, width, and passing speed of the strip passing through the cracks are constant, and the fluctuation range of the flow rate of the humidifying gas is 20% or less did. Then, by controlling the flow rate of the drying gas supplied to the crack stand and the cooling stand, the furnace internal pressure was kept constant.

In the column of "dew point" of the cracked portion in Table 2, the dew point in the measured cracks measured at the position of the dew point measuring portion 50 of FIG. 2 was shown. The dew point in the vicinity of the humidifying gas supply port was measured at a position 80 cm away from the humidifying gas supply port 40B in Fig. The &quot; humidifying gas dew point &quot; indicates the dew point measured by the dew point system 40 for humidifying gas shown in Fig.

The above-mentioned dry gas (dew point: -50 ° C) was supplied to the first cooling zone and the second cooling zone from the lowermost part of each zone at the flow rates shown in Table 2.

The plating bath temperature was 460 ° C, the Al concentration in the plating bath was 0.130%, and the deposition amount was adjusted to 50 g / m 2 per side by gas wiping. Further, the line speed was set to 1.0 to 2.0 kN with the change of the plate thickness. After the hot-dip galvanizing, alloying treatment was conducted in an induction heating type alloy furnace such that the degree of film alloying (Fe content) was 10 to 13%. The alloying temperatures at that time are shown in Table 2.

(Assessment Methods)

The evaluation of the appearance of the plating was carried out by means of an optical surface defects system (detection of scratches caused by uneven plating defects or roll pick-ups of at least 0.5) and alloying irregularity judging by visual inspection. If there is any unevenness in alloying, &quot; C &quot; The results are shown in Table 2.

Further, the tensile strength of the galvannealed steel sheet produced under various conditions was measured. Steel grade A of high tensile strength steel was over 590 MPa, and high grade steel grade B was over 980 MPa. The results are shown in Table 2.

(Evaluation results)

In the comparative example, when the dew point in the crack base was lower than -25 占 폚, the outer appearance of the plating deteriorated due to the partial unplating, and the tensile strength failed due to the increase in the alloying temperature. In addition, when the crack-to-dew point exceeded 0 占 폚, roll pickup occurred, and scratches caused by roll pickup occurred on the surface of the steel strip, resulting in deterioration of the plating appearance. In the time zones 0:20, 0:35, and 0:45, the water content also satisfies the formula (1), but the fluctuation in the water content with respect to the time zone before and after is large and the dew point falls within the range of -25 to 0 캜 Because there was no, the alloying irregularity of the hardness was seen.

On the other hand, in the case of the present invention, even if the gas flow rate of the entirety of the cracked bed changes, a predetermined amount of moisture can be stably supplied, so that a good surface appearance can be obtained over the entire width of the entire coil length, and desired tensile properties can be obtained. The tensile strength and the surface appearance were stabilized at a particularly high position in the time zone of 1:20 to 2:00 where the variation of the water content was controlled within 20% and the dew point was controlled at -25 to 0 캜 by satisfying the formula (1) .

[Industrial Availability]

According to the method of producing a hot-dip galvanized steel sheet of the present invention, even when hot-dip galvanizing is carried out on steel strips containing 0.2 mass% or more of Si, the plating adherence is high and good plating appearance can be obtained. Further, according to the method for producing hot-dip galvanized steel of the present invention, since the alloying temperature does not become high during the further alloying treatment, the lowering of the tensile strength of the produced galvannealed steel sheet can be suppressed.

100: continuous hot dip galvanizing apparatus
10: Heating table
10A: First heating zone (front end)
10B: Second heating zone (rear end, firing type heating furnace)
12:
14: First cooling zone (fast cooling zone)
16: Second cooling base (cold base)
18: Snout
20: annealing furnace
22: Hot-dip galvanizing bath
23: Alloying equipment
24: Dry gas distribution device
26: Humidifying device
28: Circulating constant temperature bath
30: Humidifying gas distribution device
32: Piping for dry gas
34, 36: Piping for humidifying gas
38: Flowmeter for humidifying gas
40: Dew point system for humidifying gas
42A, 42B, 42C: humidifying gas supply port
44A, 44B, 44C: a humidifying gas supply port
46A, 46B, 46C: humidifying gas supply port
48A, 48B, 48C, 48D: dry gas supply port
50: Dew point measuring section
52A: Upper Hus Roll
52B: Lower Hus Roll
P: Coil

Claims (5)

A method for producing a hot-dip galvanized steel sheet using a continuous hot-dip galvanizing apparatus having an annealing furnace in which a heating stand, a crack stand, and a cooling stand are arranged in parallel in this order, and a hot-
A step of conveying the steel strip inside the annealing furnace in the order of the heating stand, the crack stand, and the cooling stand, and annealing the steel stand,
A step of performing hot dip galvanizing on the steel strip discharged from the cooling furnace by using the hot dip galvanizing facility
Lt; / RTI &
The reducing gas or non-oxidizing gas supplied to the cracking zone is a humidifying gas humidified by a humidifying device and a drying gas not humidified by the humidifying device,
The fluctuation of the pressure in the annealing furnace is suppressed by controlling the flow rate of the drying gas while the width and the sheet passing speed of the steel strip passing through the cracks are constant, Wherein the fluctuation range of the amount of water supplied to the steel strip is 20% or less. The method according to claim 1,
Wherein the flow rate and the dew point of the humidifying gas are set so that the amount of water (M) (g / min) supplied to the cracks by the humidifying gas satisfies the following formula (1) .
40 + Vf (W-0.9) (S + 4) / 90 <M <60 + Vf (W-
Here, Vf is the volume (m 3) of the crack zone, W is the width (m) of the steel strip passing through the crack zone, and S is the steel plate passing speed (㎧). 3. The method of claim 2,
The flow rate and dew point of the humidifying gas are changed so that the water content (M) (g / min) satisfies the above-mentioned formula (1) when at least one of the width of the steel strip passing through the cracks and the passing speed changes , And a method for producing a hot-dip galvanized steel sheet. 4. The method according to any one of claims 1 to 3,
Wherein a distance of 1 m or more from the position of the supply port of the humidifying gas formed in the cracking zone and a distance of 1 m or more from the position of the inner wall of the cracking zone opposed to the supply port in the upper half region in the height direction of the cracking band Wherein the dew point in the crack zone measured in a dew point measuring section formed in the cracking zone is controlled to be not less than -25 DEG C and not more than 0 DEG C at a position away from the crack zone. 5. The method according to any one of claims 1 to 4,
Wherein the heating stand comprises a flame type heating furnace, the continuous hot dip galvanizing system having an alloying facility adjacent to the hot dip galvanizing facility,
A method of manufacturing a hot-dip galvanized steel sheet, further comprising the step of heat-alloying galvanizing applied to the steel strip using the alloying facility.

Images