JP3907656B2 - Hot dip galvanizing method - Google Patents

Description

The present invention belongs to the technical field concerning the galvanizing method, in particular, Si content: 1.8 mass% or more Si-containing steel sheet, after having improved coatability by a redox method, galvanized those belonging to the technical field concerning the galvanized how to.

  Steel materials that contain elements (metals) that are more easily oxidized than iron, such as Si, are concentrated on the steel sheet surface during the annealing process (reduction furnace) before plating, and the wettability with molten zinc is poor. Therefore, there is a problem that non-plating occurs.

  To prevent this non-plating, a redox method has been proposed in which a thick oxide film of 400 to 10,000 mm is formed on the surface of a steel sheet in a non-oxidizing furnace and then annealed in a reducing furnace (Japanese Patent Laid-Open No. 55-122865). Issue gazette). However, this method has a problem that the effect is not stable because a non-oxidizing furnace is used under oxidizing conditions.

  Therefore, as an advancement of this technology, Japanese Patent Laid-Open Nos. H4-202630, H4-202631, H4-202632, H4-202633, H4-254531, H4-254532, Many improved techniques have been proposed in Japanese Laid-Open Patent Publication No. 6-306561. These techniques take measures such as improving alloying characteristics. In other words, it is a method for improving alloying characteristics by growing and reducing a relatively thin oxide film to form an iron layer on the surface.

  In addition to these, for example, there is a technique described in JP-A-7-34210, and there are many methods for stabilizing the effect. However, in this case as well, the concentration of the atmospheric gas is controlled to control the oxide film thickness under the condition that the oxide film is thin.

As for the method for controlling the thickness of the oxide film, there are many controls for setting the air ratio of the burner and the concentration of gas in the atmosphere.
Japanese Unexamined Patent Publication No. 55-122865 Japanese Laid-Open Patent Publication No. H4-202630 JP-A-4-02631 JP-A-4-02632 Japanese Patent Laid-Open No. 4-202633 JP-A-4-54531 Japanese Patent Laid-Open No. 4-25532 JP-A-6-306561 JP 7-34210 A

  In a Si-containing steel sheet, an oxide film is grown on the surface using a redox method and reduced to form an iron layer, thereby ensuring plating properties. However, in the case of a Si-containing steel sheet having a high Si content, Fe is difficult to oxidize, and the concentration of Si becomes severe during reduction. For this reason, it is necessary to increase the thickness of the oxide film formed by oxidation by the oxidation-reduction method. This tendency and the necessity of increasing the oxide film thickness are particularly remarkable in the case of a Si-containing steel sheet having a Si content of 1.2% by mass or more, and further in the case of a Si-containing steel sheet having a Si content of 1.8% by mass or more. It is remarkable. In addition to a steel sheet containing Si, a steel sheet containing an element that is more easily oxidized than Fe needs to have such a tendency and a thick oxide film.

  In order to increase the thickness of the oxide film formed by oxidation in the oxidation-reduction method, a method of increasing the residence time of the oxidation zone by reducing the line speed can be considered, but at the same time, the residence time of the reduction zone also increases and The concentration of silicon becomes intense, and an iron layer obtained by reducing an appropriate oxide film cannot be formed.

  Therefore, in order to perform the reduction corresponding to the thin oxide film thickness, the reduction ability in the reduction furnace is reduced. For this purpose, it is necessary to adjust the atmospheric gas concentration in the reduction furnace. However, adjustment of the atmospheric gas concentration requires replacement of the atmospheric gas, which takes several tens of minutes, so it is not practical for a line through which different types of steel plates are passed.

  If the length of the oxidation furnace is increased, the oxidation time becomes longer, and the thickness of the oxide film can be increased without reducing the line speed. However, it is necessary to pass a steel plate that does not contain silicon in the line, and the balance of oxidation and reduction is determined by the type of steel plate. Therefore, if the oxidation furnace is lengthened to oxidize the plate containing silicon, the oxidation capacity is large. Therefore, it is necessary to perform an operation in which the steel sheet is not easily oxidized in the oxidation furnace, and there is a problem that the equipment becomes redundant.

The present invention was made in view of such circumstances, and an object, Si content: Upon 1.8 after oxidizing reduced by the mass% or more Si-containing steel sheet oxidation-reduction method, hot dip galvanized, An object of the present invention is to provide a hot dip galvanizing method capable of increasing the thickness of an oxide film formed by oxidation in the oxidation-reduction method without decreasing the line speed and without increasing the length of the oxidation furnace. It is. Further, in order to increase the oxide film growth rate and increase the thickness of the oxide film, it is necessary to control the oxide film, and a method for controlling the oxide film thickness by adding plate temperature or oxygen water vapor is also proposed.

  As a result of intensive studies to achieve the above object, the present inventors have completed the present invention. According to the present invention, the above object can be achieved.

  The present invention thus completed and capable of achieving the above object relates to a hot dip galvanizing method, which comprises the hot dip galvanizing method according to claims 1 to 3 (the hot dip galvanizing according to the first to third inventions). Method), which has the following configuration.

That is, the hot dip galvanizing method according to claim 1 is a hot dip galvanizing method in which a Si-containing steel plate having a Si content of 1.8% by mass or more is improved in plating property by an oxidation-reduction method and then hot dip galvanized. after the 700 ° C. temperature above without oxidation state temperature of the carried out by flame irradiation oxidation in the oxidation-reduction method, this time is passed through the steel sheet oxidation region of the flame, the oxide film on the steel sheet surface 560 to A hot dip galvanizing method characterized by growing at an oxide film growth rate of 2000 kg / s [first invention].

  In the hot dip galvanizing method according to claim 2, the flame irradiation is performed by flame irradiation with a burner. At this time, oxygen and / or water vapor is used as combustion air of the burner, and oxygen is more than 0% by volume with respect to the amount of combustion air. 2. The hot dip galvanizing method according to claim 1, wherein the flow rate is less than volume% and the water vapor is charged at a flow rate exceeding 0 volume% and not more than 40 volume% with respect to the combustion air amount.

  In the hot dip galvanizing method according to claim 3, the annealing line of the hot dip galvanizing equipment is configured in the order of a non-oxidation zone or a reduction zone, an oxidation zone, and a reduction zone, and oxidation is performed by the oxidation-reduction method in this oxidation zone. 3. The hot dip galvanizing method according to claim 1 or 2 [third invention].

According to the hot dip galvanizing method of the present invention, the Si content: 1.8% by mass or more of the Si-containing steel plate is oxidized and reduced by the oxidation-reduction method, and then hot-dip galvanized without reducing the line speed. Thus, it is possible to increase the thickness of the oxide film formed by the oxidation / reduction method without increasing the furnace length.

As described above, the hot dip galvanizing method according to the present invention is a hot dip galvanizing method in which a Si-containing steel plate having a Si content of 1.8% by mass or more is improved in platability by a redox method and then hot dip galvanized. After the temperature of the steel sheet is set to 700 ° C. or higher in a non-oxidized state, oxidation by the oxidation-reduction method is performed by flame irradiation. At this time, the steel sheet is passed through the oxidation region of the flame, and an oxide film is formed on the steel sheet surface. The hot dip galvanizing method is characterized by growing at an oxide film growth rate of 560 to 2000 Å / s.

As described above, after the temperature of the steel sheet is set to 700 ° C. or higher in a non-oxidized state, oxidation by the oxidation-reduction method is performed by flame irradiation, and at this time, if the steel sheet is passed through the oxidation region of the flame, the oxidation is performed. The film growth rate increases, and as a result, the thickness of the oxide film can be increased. Therefore, the thickness of the oxide film can be increased without decreasing the line speed and without increasing the length of the oxidation furnace. At this time, the growth rate of the oxide film can be set to 560 to 2000 Å / s, and by doing so, a sufficiently thick oxide film can be formed.

Therefore, according to the hot dip galvanizing method according to the present invention, after oxidation reduction of the Si-containing steel sheet having a Si content of 1.8% by mass or more by the oxidation reduction method, without reducing the line speed, Without increasing the length of the oxidation furnace, the thickness of the oxide film formed by oxidation in the oxidation-reduction method can be increased.

Oxide film growth rate during oxidation by the oxidation-reduction method: 560 to 2000 Å / s is extremely high and rapid compared to the oxide film growth rate in the case of the prior art (for example, about 30 to 50 Å / s). The oxide film growth rate is high. That is, in the case of the hot dip galvanizing method according to the present invention, an oxide film is rapidly grown on the surface of the steel sheet during oxidation by the oxidation-reduction method. The oxide film growth rate is the rate of formation of the oxide film in the thickness direction. For example, when the oxide film growth rate is 2000 Å / s, the thickness of the oxide film is formed at a rate of 2000 Å / s (seconds). These velocities change during the formation of the oxide film due to changes in the plate temperature, the difference in the position of the flame irradiation, etc., and are not constant. Therefore, the average value during the rapid formation of the oxide film is mainly used. .

In the hot dip galvanizing method according to the present invention, an oxide film is rapidly grown on the steel sheet surface at an oxide film growth rate of 560 to 2000 liters / s during oxidation by the redox method as described above. It is possible to grow a thick oxide film at a line speed that achieves the desired residence time.

The oxide film growth rate on the steel sheet surface is set to 560 to 2000 mm / s because if it is less than 560 m / s, a sufficient oxide film thickness cannot be achieved, and if it exceeds 2000 mm / s, the oxide film thickness can be controlled. This is because it becomes difficult to reduce the accuracy of the thickness of the oxide film, or the oxide film thickness becomes too thick to be reduced in the reduction furnace.

By making the temperature of the steel sheet 700 ° C. or more before the flame irradiation and then performing the flame irradiation, the oxide film growth rate can be more reliably set to 560 to 2000 mm / s, As a result, a sufficiently thick oxide film can be formed.

The flame irradiation is performed by flame irradiation with a burner. At this time, oxygen and / or water vapor are supplied to the combustion air of the burner, oxygen is a flow rate of more than 0% by volume and less than 20% by volume, and water vapor is the amount of combustion air. If the flow rate is more than 0% by volume and 40% by volume or less, the growth rate of the oxide film can be easily increased to a high level of 560 to 2000 liters / s. As a result, a sufficiently thick oxide film can be formed. It can be formed easily [second invention].

  FIG. 5 shows the relationship between the plate temperature and the oxide film thickness. It can be seen that the higher the plate temperature, the thicker the oxide film grows. Therefore, it is important to keep the plate at a high temperature in order to rapidly grow the oxide film. Although it is desirable to increase the plate temperature from the viewpoint of such rapid growth of the oxide film, it is desirable that the plate temperature be about 850 ° C. or less in order to ensure the tension of the plate in the continuous line.

  In FIG. 6, when there is no flame irradiation, when the flame is irradiated, when oxygen is added to the combustion air of the burner during the flame irradiation by the burner (in the case of oxygen enrichment), when the flame is irradiated by the burner The growth ratio (oxide film ratio) of the oxide film thickness when steam is added to the combustion air of the burner and when oxygen and steam are added to the combustion air of the burner during flame irradiation by the burner is shown. This ratio indicates that the larger the ratio, the higher the oxide film growth rate. Compared to the case without flame irradiation, the oxide film growth rate is higher when the flame irradiation is performed, and compared with that when the oxygen is enriched, the oxide film growth rate is higher. The growth rate of oxide film is highest when oxygen and water vapor are added.

  FIG. 7 shows the relationship between the oxygen addition amount and the water vapor addition amount and the oxide film thickness ratio. This ratio indicates that the larger the ratio, the higher the oxide film growth rate. It can be seen that the oxide film thickness grows by the addition of oxygen (oxygen enrichment) or the addition of water vapor, but the effect reaches its peak when a certain amount is added. Since the addition of oxygen and water vapor involves utility costs, it is more effective to use a range that is less than the peak flow rate.

  From this point of view, it is desirable that the amount of oxygen added (flow rate) is more than 0% by volume and not more than 20% by volume with respect to the amount of combustion air of the burner, as in the case of the second invention described above. It can be seen that the volume% is desirable, and the amount of water vapor added is desirably greater than 0 volume% and not greater than 40 volume% with respect to the amount of combustion air in the burner as in the case of the second invention described above. . In addition, if only oxygen is added, the flame temperature may increase or the flame length may be shortened. The plate temperature changes due to the amount of heat transfer to the plate, and the growth rate of the oxide film changes. . Further, since the flame temperature is lowered by adding only water vapor, the decrease in the oxide film growth rate due to the decrease in the plate temperature may be offset by the increase in the oxide film growth rate due to the water vapor addition. Therefore, when oxygen and water vapor are added at a constant mixing ratio, it is possible to grow an oxide film under conditions where the flame temperature and flame length are substantially constant, and it becomes possible to operate with a stable plate temperature. . Therefore, the oxide film growth rate increases at a substantially constant rate with respect to the added amounts of oxygen and water vapor, and therefore, the oxide film thickness can be easily controlled. In other words, after setting a plate temperature at which a constant oxide film thickness can be secured with a certain amount of oxygen and water vapor added, an operation for controlling a predetermined oxide film thickness by increasing or decreasing the flow rate of oxygen water vapor becomes possible.

  FIG. 8 shows the growth rate of the oxide film when it is rapidly oxidized from the state where there is no oxide film, and when the oxide film is rapidly oxidized after being grown 3,000 mm. It can be seen that the growth rate of the oxide film decreases because the growth rate decreases as the film thickness increases.

Since the oxide film grows faster when the plate temperature in the oxidation zone is higher, the annealing line of the hot dip galvanizing equipment is configured in the order of non-oxidation zone or reduction zone, oxidation zone, reduction zone. When oxidation is performed by the oxidation-reduction method, it is possible to reliably grow an oxide film on the surface of the steel plate at an oxide film growth rate of 560 to 2000 mm / s, and to facilitate such rapid growth. Furthermore, the oxide film growth rate can be easily increased [third invention]. At this time, if the plate temperature is kept as high as possible in the non-oxidation zone or the reduction zone in the non-oxidation state and is oxidized rapidly in the oxidation zone to form an oxide film thickness, the oxide film growth rate is easily increased.

  When the non-oxidation zone is used as the oxidation zone, the oxide film grows gradually and the diffusion of oxygen is hindered. Therefore, it is better not to oxidize at a low temperature but to generate an oxide film rapidly at a high temperature. It is possible to make it. Similarly, in order to grow the oxide film rapidly, it is preferable to use a method of enriching oxygen or adding water vapor to the combustion air of the burner as described above.

  When enriching oxygen or adding water vapor to the combustion air of the burner, the oxide film thickness can be controlled by changing the addition concentration of oxygen or water vapor while keeping the amount of combustion of the burner constant.

  In a hot dip galvanizing facility having an annealing line and a hot dip galvanizing apparatus, the annealing line is configured in the order of a non-oxidation zone or a reduction zone, an oxidation zone, and a reduction zone, and oxidation is performed by the oxidation-reduction method in this oxidation zone. According to the hot dip galvanizing facility characterized by this, the hot dip galvanizing method according to the present invention as described above can be reliably performed.

  In the present invention, when oxidation in the oxidation-reduction method is performed by flame irradiation with a burner, by installing a plurality of burners and changing the number of burning burners, the flame irradiation width can be changed, thereby The oxide film thickness can be controlled by changing the flame irradiation time. When the burner combustion amount is reduced, the flame length is shortened, so that the flame does not irradiate the steel sheet, and the oxide film growth rate decreases rapidly. Therefore, it is possible to stably form an oxide film by preparing a plurality of burners and arranging the burners so as to surely irradiate the steel plate with a flame even if the burner burns down. In this case, the burner combustion amount is continuously reduced to such an extent that the flame irradiation effect does not decrease, and when it becomes smaller than the set value, a method of extinguishing a part of the plurality of burners is adopted to continuously It becomes possible to grow an oxide film thickness.

  In the oxidation by the oxidation-reduction method, the plate temperature affects the oxide film thickness as described above (FIG. 5). As can be seen, the oxide film thickness can be controlled by controlling the plate temperature. Such control of the plate temperature can be performed, for example, as follows when the plate has a non-oxidation zone, a reduction zone, an oxidation zone, or a reduction zone as in the above-described hot dip galvanizing equipment.

  That is, the plate temperature can be controlled by controlling the burner combustion amount in the oxidation zone with the furnace temperature in the oxidation zone. In this case, when the burner combustion amount is reduced, the flame length is shortened, and the rate at which the flame irradiates the steel sheet is also reduced. Therefore, the effect of reducing the oxide film growth rate is great as the plate temperature is lowered. In order to reduce the effect and improve the controllability, the following method can be considered. In order to control the plate temperature during oxidation, the combustion amount of the oxidation zone burner is kept constant, or the furnace temperature of the non-oxidation zone or reduction zone before the oxidation zone is used, and the front zone of the oxidation zone (non-oxidation zone) Alternatively, the heating ability of the reduction zone) can be controlled to control the plate temperature during oxidation. Alternatively, the heating temperature of the front zone (non-oxidation zone or reduction zone) of the oxidation zone is controlled by the exit zone temperature of the oxidation zone or the inlet side plate temperature of the rear zone (reduction zone) of the oxidation zone. This can be done by controlling. These methods can be combined to control the plate temperature during oxidation.

  In the oxidation by the oxidation-reduction method so far, the non-oxidation zone is only operated as the oxidation zone, and the air ratio has been controlled to control the growth rate of the oxide film. The amount of combustion is controlled in order to achieve the annealing conditions of the plate, and the plate temperature during oxidation cannot be controlled, and the oxide film thickness is controlled by the air ratio. The effect was not stable because of a large change. In contrast, in the present invention, by providing an oxidation zone downstream of the non-oxidation zone, the combustion amount of the burner is made substantially constant, and the intrusion plate temperature to the oxidation zone is adjusted by the combustion amount of the front zone. It was found that when the film was controlled, the thickness of the oxide film was stabilized because the oxide film thickness was controlled at the plate temperature with the flame being in a constant state.

  In order to increase the thickness of the oxide film, a method of lengthening the furnace length to increase the oxidation time is conceivable. However, the length of the furnace cannot be increased due to equipment limitations. Moreover, even when there is no equipment restriction and the furnace length is lengthened, it is necessary to lower the oxidation capacity when passing another easily oxidizable steel sheet, and the equipment becomes large. On the other hand, since an oxide film is rapidly grown in the present invention, a thick oxide film can be formed without increasing the furnace length and without decreasing the line speed.

  For example, a hot dip galvanizing equipment as shown in FIG. 3 may be used as a hot dip galvanizing equipment for rapidly growing an oxide film upon oxidation by the oxidation-reduction method as in the present invention. In this hot dip galvanizing equipment, the annealing line is configured in the order of a non-oxidation zone, an oxidation zone, and a reduction zone. That is, an oxidation zone is provided at the rear of the non-oxidation zone.

  An example of a hot dip galvanizing facility not provided with such an oxidation zone is shown in FIG.

  When using the hot dip galvanizing equipment shown in FIG. 2 and oxidizing the non-oxidizing zone (non-oxidizing furnace) under oxidizing conditions (normal oxidation), and using the hot dip galvanizing equipment shown in FIG. FIG. 4 shows the distribution of the oxide film thickness in the length direction of the furnace when rapid oxidation is performed in the oxidation zone (during rapid oxidation). In FIG. 4, the traveling direction of the steel sheet is from the left to the right in the figure, and the right arrow among the two arrows indicating the roll position indicates the position of the roll in the furnace.

  As can be seen from FIG. 4, when oxidizing using the former non-oxidizing furnace under oxidizing conditions (normally during oxidation), the oxide film grows gradually, so that the oxide film thickness is becoming thicker, When the oxide film thickness increases, the oxide film comes into contact with the roll in the furnace. On the other hand, when rapid oxidation is performed in the latter oxidation zone (during rapid oxidation), since the oxidation rate is fast, it is possible to grow an oxide film without contact with the roll. For this reason, in the latter case, it is considered that the oxide film is difficult to peel off. That is, in the latter case, the temperature of the steel plate is raised in the non-oxidizing state in a non-oxidizing furnace (without oxidizing or hardly oxidizing the steel plate), thereby rapidly oxidizing in the oxidation zone. Since the oxide film thickness increases rapidly, it comes into contact with the roll in front of the center roll (left) before the oxide film is formed or when the oxide film is slightly formed (very thin). However, there is little contact with the roll when the oxide film thickness is increasing in the oxidation zone or when the oxide film thickness is increased. For this reason, it is considered that peeling of the oxide film hardly occurs. Moreover, there is a roll at the entrance of the reduction furnace, and the steel sheet may come into contact with the roll and the oxide film may be peeled off. In any case, in the latter case (shown in FIG. 3), the oxide film thickness can be increased by rapid oxidation, and the number of contact with the roll is reduced. For this reason, it is considered possible to reduce the frequency of occurrence of scratches due to the peeling of the oxide film due to the roll and the adhesion of the oxide film peeled off to the roll.

  The equipment shown in FIG. 2 has a horizontal line configuration. Here, a vertical line configuration is shown in FIG. In the case of the vertical line shown in FIG. 1, since the curvature of the plate at the roll is large, it is considered that the oxide film peels more easily than in the case of the horizontal line shown in FIG.

As described above, in the case of a Si-containing steel sheet with a high Si content, Fe is difficult to oxidize under oxidizing conditions, and Si concentration is intense during reduction. It is necessary to increase the thickness, and this tendency and the necessity to increase the oxide film thickness are particularly remarkable in the case of a Si-containing steel sheet having a Si content of 1.8% by mass or more . As described above, the hot dip galvanizing method according to the present invention increases the thickness of the oxide film formed by oxidation in the oxidation-reduction method without reducing the line speed, without increasing the furnace length of the oxidation furnace. can do. Therefore, the hot dip galvanizing method according to the present invention is useful and valuable particularly in the case of a Si-containing steel sheet having a Si content of 1.8% by mass or more .

  Examples of the present invention and comparative examples will be described below. The present invention is not limited to this embodiment, and can be implemented with appropriate modifications within a range that can be adapted to the gist of the present invention, all of which are within the technical scope of the present invention. include.

[Example 1]
A hot dip galvanizing facility having an annealing line having a preheating chamber, a non-oxidation zone, an oxidation zone, and a reduction zone in this order, a hot dip galvanizing apparatus having a hot dip galvanizing bath and air wiping means, and a roll for transferring steel plates. Using a horizontal line, a hot dip galvanized steel sheet was obtained as follows.

  C: 0.1% by mass, Si: 1.8% by mass, Mn: 1.5% by mass, a high-tensile steel plate having a steel component consisting of Fe and inevitable impurities in the balance is preheated to 400 ° C. in a preheating chamber, Heat to 700 ° C. Thereafter, the steel plate is heated to 850 ° C. using a burner that irradiates the steel plate with flame in an oxidation furnace. At this time, the air ratio in the burner combustion air is 1.2. Thereby, an oxide film is grown and formed on the steel plate surface. The growth rate of the oxide film is 560 ｓ / s, and the thickness of the formed oxide film is 5600 Å.

The steel sheet on which the oxide film has been formed is put into a reduction furnace in a hydrogen atmosphere (air mixture of air and hydrogen, and the hydrogen concentration in the gas is 15% by volume), and the oxide film is reduced in this reduction furnace Then, hot dip galvanizing was performed through a hot dip galvanizing bath, and then the plating amount was adjusted to 50 g / m ² by air wiping to obtain a hot dip galvanized steel sheet (No. 1). The temperature of the steel sheet entering the reduction furnace is 850 ° C. The furnace temperature of the reduction furnace is 900 ° C.

  As can be seen from the above, heating the steel sheet to a high temperature in a non-oxidizing furnace and then heating it further to a higher temperature by means of flame irradiation with a burner in an oxidizing furnace can greatly increase the oxide film growth rate. it can.

[Example 2]
In the flame irradiation by the burner in the oxidation furnace, steam is introduced into the burner combustion air at a flow rate of 10% by volume with respect to the amount of combustion air. Except for this point, the hot dip galvanized steel sheet is prepared in the same manner as in Example 1. Obtained (No. 2). The oxide film growth rate by oxidation in this oxidation furnace is 770 Å / s, and the thickness of the oxide film formed by this oxidation is 7700 Å.

[Example 3]
When irradiating the flame with the burner in the oxidation furnace, oxygen was introduced into the burner combustion air at a flow rate of 5% by volume with respect to the amount of combustion air. Except for this point, the hot dip galvanized steel sheet was prepared in the same manner as in Example 1. Obtained (No. 3). The oxide film growth rate by oxidation in this oxidation furnace is 620 Å / s, and the thickness of the oxide film formed by this oxidation is 6200 Å.

[Example 4]
When igniting the flame with the burner in the oxidation furnace, oxygen was introduced into the burner combustion air at a flow rate of 5% by volume and water vapor was introduced at a flow rate of 10% by volume. A hot dip galvanized steel sheet was obtained (No. 4). The oxide film growth rate by oxidation in this oxidation furnace is 850 Å / s, and the thickness of the oxide film formed by this oxidation is 8500 Å.

[Example 5]
The non-oxidizing furnace was heated to 600 ° C, and the oxidizing furnace was heated to 750 ° C. During the flame irradiation by the burner in the oxidation furnace, oxygen was introduced into the combustion air of the burner at a flow rate of 5% by volume and water vapor was introduced at a flow rate of 10% by volume. Except for these points, a hot-dip galvanized steel sheet was obtained by the same method as in Example 1 (No. 5). The oxide film growth rate by oxidation in this oxidation furnace is 180 Å / s, and the thickness of the oxide film formed by this oxidation is 1800 Å. The temperature of the steel sheet entering the reduction furnace is 750 ° C. The furnace temperature of this reduction furnace is 800 ° C. (different from the case of Example 1).

[Example 6]
A steel plate similar to that in Example 1 is preheated to 400 ° C in a preheating chamber, and then heated to 700 ° C in a non-oxidizing furnace. Thereafter, the steel sheet is heated to 850 ° C. by a method called atmospheric oxidation, instead of irradiating the steel sheet with a flame in an oxidation furnace. Thereby, an oxide film is grown and formed on the steel plate surface. The growth rate of the oxide film is 50 Å / s, and the thickness of the formed oxide film is 500 Å.

  After the formation of the oxide film, the amount of plating by reduction treatment, hot dip galvanizing, and air wiping was adjusted in the same manner as in Example 1 to obtain a hot dip galvanized steel sheet (No. 6).

[Example 7]
A steel plate similar to that in Example 1 is preheated to 400 ° C. in a preheating chamber, and then a non-oxidizing furnace is operated by oxidation and heated to 700 ° C. However, the atmosphere was such that the burner air ratio of this non-oxidizing furnace was 1.2. For this reason, when heating in the non-oxidizing furnace, the steel sheet is oxidized to form an oxide film. The thickness of this oxide film is 2000 mm. The oxide film growth rate is 100 Å / s.

  Thereafter, in the same manner as in Example 1, heating up to 850 ° C. in an oxidation furnace, reduction treatment, hot dip galvanizing, and adjustment of the plating amount by air wiping were performed to obtain a hot dip galvanized steel sheet (No. 7).

  At this time, the growth rate of the oxide film by oxidation in the oxidation furnace is 180 Å / s, and the thickness of the oxide film formed by this oxidation is 1800 Å.

  The sum of the thickness of the oxide film formed in the non-oxidation furnace and the thickness of the oxide film formed in the oxidation furnace is 3800 mm. This total thickness is important from the viewpoint of improving the plating property by the oxidation-reduction method. The oxide film growth rate in the non-oxidation furnace and the oxidation furnace is 130 Å / s. From the standpoint of preventing peeling of the oxide film due to contact with the roll, this oxide film peeling also affects the oxide film growth rate in this non-oxidation furnace and oxidation furnace, but when the oxide film is relatively thick Since it becomes a problem, the oxide film growth rate in the oxidation furnace is more important.

〔result〕
The galvanized steel sheet thus obtained was examined for plating characteristics. The results are shown in Table 1.

  In the case of Nos. 1 to 4, the oxide film growth rate is 560 to 850 Å / s, and the thickness of the formed oxide film is 5600 to 8500 Å, which is thick. For this reason, the galvanized steel plate with a favorable plating external appearance was obtained.

In the case of No. 5, the oxide film growth rate is as low as 180 Å / s, and the thickness of the formed oxide film is as thin as 1800 Å. For this reason, dot-like non-plating generate | occur | produced and the favorable galvanized steel plate was not obtained.

In the case of No. 6, the oxide film growth rate is as low as 50 cm / s, and the thickness of the formed oxide film is as thin as 500 mm. For this reason, dot-like non-plating generate | occur | produced and the favorable galvanized steel plate was not obtained.

In the case of No. 7, the oxide film growth rate in the oxidation furnace is as low as 130 Å / s, and the thickness of the formed oxide film is as thin as 3800 Å. For this reason, dot-like non-plating generate | occur | produced and the favorable galvanized steel plate was not obtained.

According to the hot dip galvanizing method of the present invention, the Si content: 1.8% by mass or more of the Si-containing steel plate is oxidized and reduced by the oxidation-reduction method, and then hot-dip galvanized without reducing the line speed. Since the thickness of the oxide film formed by oxidation by the oxidation-reduction method can be increased without lengthening the furnace length of Si , the Si content: 1.8% by mass or more of the Si-containing steel plate as a base material is not plated. When it is going to manufacture a galvanized steel plate without alloy or a galvannealed steel plate, it can use suitably .

It is a schematic diagram which shows the hot dip galvanization equipment which an annealing line consists of a vertical type line. It is a schematic diagram which shows the hot dip galvanization equipment which an annealing line consists of a horizontal line. It is a schematic diagram which shows the hot dip galvanization equipment which has a pre-tropical zone, a non-oxidation zone, an oxidation zone, a reduction zone, and a cooling zone as an annealing line. It is a figure which shows distribution of the length direction of the furnace of the oxide film thickness at the time of normal oxidation and rapid oxidation. It is a figure which shows the relationship between plate temperature and an oxide film thickness. It is a figure which shows the oxide film ratio in the case of no flame, with flame, oxygen enrichment, H ₂ O (water vapor) addition, oxygen enrichment + H ₂ O (water vapor) addition. It is a figure which shows the relationship between oxygen and water vapor | steam addition ratio and an oxide film thickness ratio. It is a figure which shows the relationship between a residence time and an oxide film growth rate.

Claims (3)

Si content: 1.8% by mass or more Si-containing steel sheet After improving the plating properties by oxidation-reduction method, hot-dip galvanizing is performed, and the temperature of the steel sheet is set to 700 ° C or higher in a non-oxidized state. After that, oxidation by the oxidation-reduction method is performed by flame irradiation. At this time, the steel plate is passed through the oxidation region of the flame, and an oxide film is grown on the steel plate surface at an oxide film growth rate of 560 to 2000 mm / s. A hot dip galvanizing method characterized.   The flame irradiation is performed by flame irradiation with a burner. At this time, oxygen and / or water vapor are supplied to the combustion air of the burner, oxygen is a flow rate of more than 0% by volume and less than 20% by volume, and water vapor is the amount of combustion air. The hot dip galvanizing method according to claim 1, wherein the hot dip galvanizing is performed at a flow rate of more than 0 vol% and 40 vol% or less.   The hot dip galvanizing method according to claim 1 or 2, wherein the annealing line of the hot dip galvanizing equipment is constructed in the order of a non-oxidation zone or a reduction zone, an oxidation zone, and a reduction zone, and oxidation is carried out by the redox method in this oxidation zone. .

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