JP4770428B2 - High strength hot dip galvanized steel sheet manufacturing method and hot dip galvanized steel sheet manufacturing equipment - Google Patents

Description

  The present invention relates to a method for manufacturing a high-strength hot-dip galvanized steel sheet that is manufactured by performing hot-dip galvanization after heat treatment in an annealing furnace having a direct-fired heating zone and a vertical reduction zone. Moreover, this invention relates to the manufacturing equipment of the hot dip galvanized steel plate suitable for manufacture of a high strength steel plate.

  Since steel plates are inexpensive metal materials, they are widely used in fields such as automobiles, home appliances, and building materials. In recent years, in the automobile industry, in addition to improving durability, the weight of automobiles has been reduced from the viewpoint of improving fuel efficiency and reducing exhaust gas, and the use of high-strength hot-dip galvanized steel sheets has increased rapidly. A high-strength hot-dip galvanized steel sheet is usually manufactured using continuous hot-dip galvanizing equipment, with Si, Mn, etc. added as elements in the steel. When a hot dip galvanizing is performed after annealing a steel sheet to which a large amount of Si and Mn are added using a continuous hot dip galvanizing facility, there is a problem in that plating defects occur due to Si and Mn oxides on the steel sheet surface.

  When hot-dip galvanizing is applied to high-strength steel sheets containing a large amount of Si or Mn, an appropriate Fe oxide film is once formed on the steel sheet surface, and then the Fe oxide film thickness is reduced to a level that does not hinder plating. Then, it is effective in preventing the occurrence of defective plating.

  For example, in Patent Document 1, as a method of reducing the thickness of the Fe oxide film formed after forming an appropriate Fe oxide film on the steel sheet surface, a continuous hot dip galvanizing facility having no oxidation furnace is used, and a reduction furnace is provided. Divide into two or more zones to adjust the dew point of each zone. That is, in the first zone, water vapor is introduced into the furnace to adjust the dew point to form an appropriate Fe oxide film on the steel plate surface, and then in the second zone A method is described in which the dew point is adjusted to reduce the oxide film.

  Further, Patent Document 2 describes that a method using direct flame reduction is effective as a method for generating an oxide film. Further, Patent Document 3 shows that hot-dip galvanization can be performed on a high-strength steel sheet to which Si or Mn is added in a larger amount than Patent Document 1 if direct fire reduction is used. Here, the direct fire reduction is a method of reducing the steel plate surface after oxidizing the steel plate surface using a direct fire heating type direct flame. This divides the direct fire zone into multiple zones (zones), burns the zone through which the steel plate first passes at a high air ratio to oxidize the surface once, and lowers the air ratio in the rear zone to have reducibility This is done by reducing the steel sheet surface while heating with a flame.

  FIG. 1 shows an important point of a hot-dip galvanized steel sheet manufacturing facility in which an oxide film is generated using direct-fire reduction in a direct-fire heating type direct-fire zone, and then this oxide film is reduced and then galvanized. It is a schematic side view which shows the example of a part structure.

  In FIG. 1, 1 is a steel plate, 2 is a direct-fired heating zone (hereinafter also simply referred to as “direct-fired heating zone”), and 3 is a vertical reduction zone (hereinafter simply referred to as “reducing zone”). 4) is a cooling zone, 5 is a snout, 6 is a hot dipping bath, and 7 is a gas wiping device. The direct fire heating zone 2 and the vertical reduction zone 3 are connected. The hot dip plating tank 6 holds hot galvanized metal.

  The direct fire heating zone is divided into a plurality of heating zones, and a direct fire heating burner 103 is disposed in each heating zone, and fuel gas is supplied from the fuel supply system 101 and combustion air is supplied from the air supply system 102. The fuel gas flow rate, the combustion air flow rate, and the flow rate ratio of each heating zone can be controlled independently.

  The reduction zone 3 is provided with a plurality of in-furnace rolls arranged at a predetermined height at the upper and lower portions in the furnace. The steel plate 1 traveling in the reduction zone 3 is supported by the upper furnace roll 9a and the lower furnace roll 9b and travels in the vertical direction. This steel plate traveling path in the vertical direction is referred to as a vertical path. There are a plurality of longitudinal paths, and the traveling directions of the steel plates are opposite in adjacent longitudinal paths. A radiant tube burner 8 is disposed between the vertical passes of the reduction zone 3 so as to face the steel plate.

A low dew point reducing gas containing H ₂ and N ₂ is supplied from the gas supply pipe 105 to a plurality of locations in the cooling zone 4, and the supplied gas flows to the inlet side of the reducing zone 3, and the direct fire heating zone 2 and the reducing zone 3 flows out from the connecting part 16 connecting the 3 to the direct fire heating zone 2. By this gas, the atmosphere of the reduction zone 3 is maintained in a reducing property.

  A steel sheet fed from a steel sheet feeding device (not shown) is directly heated by using a fuel gas in the front stage of the direct fire heating zone 2 to remove the rolling oil on the surface of the steel sheet and to remove Fe oxide ( Oxide film). The heating rate is high because of direct flame heating, and even if the steel plate temperature is low, the combustion gas temperature is high, and the radicals in the combustion gas reach the steel plate and participate in the reaction with the steel plate, so the reaction rate is fast and oxidation There is a feature that the film is formed quickly. Due to this feature, in the case of heating by direct fire, oxidizable elements such as Si and Mn are preferentially oxidized in the case of heating other than direct fire, and compared with diffusion in the steel sheet and concentration in the steel sheet surface layer. It is advantageous that Fe oxide is generated quickly and the concentration of the easy oxide is small.

  In the latter stage of the direct heating heating zone, reduction is carried out by adjusting the combustion air ratio, so that the reduced pure iron layer is formed on the surface while leaving the easy oxide as it is, thereby facilitating plating.

  Next, the steel plate is passed through the reduction zone 3. The steel plate is heated and annealed at a predetermined temperature for a predetermined time by the high-temperature radiant tube 8 while the reduction zone 3 is passed through, and at the same time, the oxide film remaining on the surface of the steel plate is reduced. The reduction is performed by introducing a reducing gas into the furnace. Usually, a reducing reaction is performed using a mixed gas of hydrogen and nitrogen having a hydrogen concentration of several percent to several tens of percent (vol%) as the reducing gas. The progress of this is determined by the temperature pattern in the furnace, the plate passing speed, the hydrogen concentration of the furnace gas and the supply amount. Appropriate conditions are selected so that the reduction of the steel sheet is completed in the reduction zone.

The steel plate 1 with the oxide film reduced is adjusted to a steel plate temperature suitable for being immersed in the hot dipping bath 6 in the cooling zone 4, and then dipped in the hot dipping bath 6, pulled up from the hot dipping bath 6, and gas. The wiping device 7 is adjusted to a required plating adhesion amount, further subjected to spangle adjustment or alloying treatment, and then cooled, or cooled without being subjected to the above treatment, to obtain a required hot dip galvanized steel sheet.
Japanese Patent No. 3014529 Japanese Patent Laid-Open No. 5-195084 Japanese Patent No. 2530939

  When hot-dip galvanizing a steel sheet with high Si or Mn concentration, if the Fe-based oxide film formed in the direct flame heating zone is thickened and the formed Fe-based oxide film is sufficiently reduced in the next reduction step However, it should be possible to plate steel sheets with higher Si and Mn concentrations, but if the oxide film is thick, the reduction of the Fe-based oxide film will be insufficient during the reduction process, leaving an unreduced oxide layer. Therefore, there is a problem in that good and stable plating cannot be obtained.

  An object of the present invention is to provide a high-strength hot-dip galvanized plate that can prevent the occurrence of poor plating properties when a high-strength hot-dip galvanized steel plate is manufactured by heat-annealing a steel plate having a high Si concentration and Mn concentration. It is providing the manufacturing method of a steel plate.

  Moreover, the subject of this invention is providing the manufacturing equipment of the hot dip galvanized steel plate suitable for manufacture of the high intensity | strength hot dip galvanized steel plate with high Si density | concentration and Mn density | concentration.

  The gist of the present invention for solving the above problems is as follows.

  (1) A steel sheet containing one or more of Si: 0.2 to 3% and Mn: 1 to 3% by mass% is heated in a direct flame heating type direct flame, and further a vertical reduction zone In the method of manufacturing a high-strength hot-dip galvanized steel sheet in which the surface is reduced and annealed in a reducing atmosphere and then immersed in a hot dip galvanizing bath and galvanized, in the vertical reduction zone, the atmosphere is at least in the entry region A method for producing a high-strength hot-dip galvanized steel sheet, characterized by causing a gas to flow in the direction opposite to the steel sheet traveling direction.

  (2) Measuring the dew point of the atmospheric gas in the reduction zone near the connection between the direct fire zone and the reduction zone, and performing combustion control in the direct flame zone from the measured value, A manufacturing method of high strength hot-dip galvanized steel sheet.

  (3) Measure the dew point of the atmospheric gas in the reducing zone near the connection between the direct-fired zone and the reducing zone and the dew point of the atmospheric gas on the exit side of the reducing zone, and adjust the flow rate of the atmospheric gas supplied to the reducing zone from the relationship The manufacturing method of the high intensity | strength hot-dip galvanized steel plate as described in (1) or (2) characterized by the above-mentioned.

  (4) In an annealing furnace having a direct-fire heating type direct-fired zone and a vertical reduction zone, and a hot-dip galvanized steel sheet manufacturing apparatus equipped with a hot-dip galvanizing apparatus downstream thereof, at least in the entrance region of the vertical reduction zone Production of hot-dip galvanized steel sheet, characterized in that a partition is provided between the vertical passes of adjacent steel strips to regulate the direction of the atmosphere gas flow between the vertical passes in the direction opposite to the steel plate traveling direction. Facility.

  (5) The facility for manufacturing a hot-dip galvanized steel sheet according to (4), wherein an opening area ratio after partition installation is 10% or less.

  (6) The vertical reduction zone is provided with a dew point meter that measures the dew point of the atmospheric gas in the vertical reduction zone in the vicinity of the connection between the direct heating zone of the direct fire heating method and the vertical reduction zone. Equipment for producing the hot-dip galvanized steel sheet according to 4) or (5).

  (7) The hot-dip galvanized steel sheet manufacturing equipment according to (6), further comprising a dew point meter for measuring the dew point of the atmospheric gas on the reduction zone outlet side in the vertical reduction zone.

  According to the present invention, the flow path in which the flow direction of the reducing gas in the reduction zone is opposite to the traveling direction of the steel plate is configured, so that the direct heating heating zone and the reduction zone are accompanied by the steel plate. It has the effect of suppressing the gas flow that flows through. Further, in the reduction zone, the high dew point gas generated by the reduction of the oxide film flows from the generation position of the high dew point gas to the direct fire heating zone side in the direction opposite to the steel plate traveling direction. Since the high dew point gas does not flow in the traveling direction of the steel plate after the reduction, the reduced steel plate is not oxidized again by the high dew point gas. The gas with a high dew point generated in the reduction zone accumulates in the direction opposite to the direction of travel of the steel sheet, ensuring an atmosphere distribution that gradually reduces the reducibility from the reduction zone inlet to the reduction zone outlet. The maximum reduction power can be secured near the reduction zone exit. As a result, the reduction reaction of the oxide film is stable, and even with a steel sheet having a high Si concentration and Mn concentration, a good reduction action is stably produced in the reduction zone. Expressed.

  The inventors manufactured a high-strength hot-dip galvanized steel sheet by heat-treating a steel sheet with a high Si concentration and Mn concentration in a heat treatment furnace equipped with a direct-fire heating type direct-fired zone and a vertical reduction zone, followed by hot-dip galvanizing. In doing so, various methods for stably obtaining good plating properties were studied. As a result, it has been found that by forming a flow path in which the flow direction of the atmospheric gas in the reduction zone is opposite to the steel plate traveling direction, good plating properties can be obtained stably. The present invention has been made based on this finding.

  The present invention will be described below in comparison with the prior art.

  The structure of the prior art reduction zone 3 and the flow of atmospheric gas in the hot dip galvanized steel sheet manufacturing facility of FIG. 1 will be described with reference to FIG. In FIG. 2, the low dew point reducing gas that has flowed from the cooling zone 4 to the reduction zone 3 flows from the outlet side of the reducing zone 3 toward the inlet side, and further flows out to the direct fire heating zone 2. In the reduction zone 3, the steel plates are heated by a large number of radiant tube burners 8 arranged between the vertical passes of the steel plates, but the furnace wall has a structure like a large box in which they are stored. The gas in the belt 3 flows freely. Therefore, there are the following problems.

  The high dew point gas generated by the reduction of the oxide film diffuses into the furnace as indicated by the arrows in FIG. 2, and the high dew point gas cannot be efficiently discharged to the direct heating zone 2.

  Further, since the atmosphere in the direct fire heating zone 2 is a combustion component of fuel gas, it contains a large amount of moisture and has a high dew point. However, the high dew point gas in the direct fire heating zone 2 is reduced along with the steel plate 1. By entering the zone 3 and diffusing into the reduction zone 3, the reducing ability of the atmospheric gas in the reduction zone 3 becomes unstable.

  Further, when the oxide film formed on the surface of the steel sheet is thickened in the direct heating zone 2, the dew point in the reduction zone 3 is increased by reducing the oxide film in the reduction zone 3, but the atmosphere with the increased dew point is reduced in the reducing furnace 3. It diffuses inside and inhibits the reducing ability.

  Further, when a high-strength hot-dip galvanized steel sheet containing a large amount of Si and Mn is produced in the course of manufacturing a normal hot-dip galvanized steel sheet that does not contain a large amount of Si and Mn, the reducing ability of the reduction zone 3 is reduced over time. The steel plate becomes unstable and the reduction of the steel plate becomes unstable, and stable plating cannot be performed.

  As described above, the conventional apparatus of FIG. 2 has a problem that the reducing action of the atmospheric gas in the reduction zone 3 is lowered. Therefore, in the steel plate having a high Si concentration and Mn concentration, the reducing ability is lowered or becomes unstable. Therefore, it was not possible to reliably prevent the occurrence of poor plating properties.

  FIG. 3 is a schematic side view showing a configuration example of the reduction zone of the annealing furnace of the hot dip galvanized steel sheet manufacturing facility according to the embodiment of the present invention. In the apparatus of FIG. 3, a partition 11 is added between adjacent vertical paths of the reduction zone 3, and dew point meters 13 and 14 are connected to the direct heating zone 2 and the reduction zone 3, respectively. It is added to the nearby reduction zone side and the reduction zone exit side (reduction zone 3 near the connection between the reduction zone 3 and the cooling zone 4). FIG. 4 is a schematic diagram for explaining the flow of the atmospheric gas in the reduction furnace 3 of FIG.

  In FIG. 3, since the partition 11 is provided between adjacent vertical paths so as to face the steel strip, the flow of atmospheric gas in the direction orthogonal to the steel strip path surface is restricted. As a result, a flow path is formed in which the flow path of the gas having a high reducing action flowing from the cooling zone into the reduction zone is in the direction opposite to the steel plate passing direction. This flow in the direction opposite to the steel sheet passing direction has an effect of suppressing the gas flow from the direct heating heating zone to the reduction zone accompanying the steel plate.

  In the reduction zone, the dew point of the atmospheric gas increases due to the reduction of the oxide film. Since the atmospheric gas having an increased dew point flows toward the reduction zone in the direction opposite to the traveling direction of the steel sheet, as indicated by the arrow in FIG. 4, the steel sheet whose surface oxide film has been reduced is oxidized again. There is no fear. Even if the oxide film formed thick in the direct-fired heating zone is reduced in the reduction zone and the dew point rises, the gas with the increased dew point flows toward the reduction zone entry side, so the dew point downstream of the steel plate travel direction is raised. There is nothing.

  Also, the gas with a high dew point in the reduction zone accumulates on the inlet side of the reduction zone in the direction opposite to the steel plate traveling direction, and the reduction action gradually increases from the reduction zone inlet toward the reduction zone outlet. The distribution of the atmospheric gas is secured, and the maximum reducing power can be secured near the reduction zone outlet. Therefore, the reduction zone can maintain a good reducing power.

  In the reduction zone having the structure shown in FIG. 3, the reduction force distribution as described above is obtained, so that a good reduction action is stably expressed in the reduction zone, and even a steel plate having a high Si concentration and Mn concentration is stable. As a result, good plating properties are exhibited.

  In order to improve the reducibility in the reduction zone by defining the gas flow direction in the reduction zone 3, it is preferable to provide a partition 11 between adjacent vertical paths in the entire region of the reduction zone 3. If there is no partition, the partition 11 may be provided only in a partial region of the reduction zone 3. In this case, the oxide is the most, the steel plate temperature rises, and the reduction proceeds most in the entrance region of the reduction zone 3 (region corresponding to the upstream side from the reduction zone center). It is preferable to provide at least between the vertical paths of the entrance side region of the reduction zone 3.

  The partition 11 does not have a structure that completely separates adjacent vertical paths, and may have a partial opening. As shown in FIG. 3, the normal saddle-type reduction zone has a structure in which a radiant tube burner 8 is installed between vertical paths of a steel plate. In such a case, as shown in FIG. 3, the partition 11 includes a furnace wall between adjacent vertical paths and a roll lower end on the side where the steel plate of the in-furnace roll facing the furnace wall is not wound. Between the upper end portion of the roll and the upper end portion of the lower furnace wall 3b and the upper in-furnace roll 9a, and the upper end portion of the upper furnace wall 3a and the lower in-furnace roll 9b. The gas flow direction can be regulated in the direction opposite to the steel plate traveling direction. The partition 11 is preferably provided so that one end thereof is in contact with the lower furnace wall 3b or the upper furnace wall 3a of the reduction zone, and the other end is provided close to the in-furnace roll 9a or 9b. Then, it may be provided with a slight gap between the radiant tube burner 8.

  If the part (opening part) in which the partition 11 is not provided becomes large, the gas moves through the opening part, and the action of regulating the gas flow direction in the direction opposite to the steel plate traveling direction is reduced. If the opening area ratio of the partition installation portion is 25% or less, the effect of improving the reduction by the action of restricting the gas flow direction to the direction opposite to the steel plate traveling direction has been confirmed. In order to exhibit the effect of regulating in the direction opposite to the traveling direction more reliably, it is more preferable that the partition plate 11 is provided so that the opening area ratio after the partition installation is 10% or less.

  Here, the opening area ratio of the partition installation part means that there is no thing that regulates the gas flow in the direction perpendicular to the cross section in the cross section with respect to the area in the furnace of the vertical furnace cross section in the partition installation part. It is the ratio of the area of the part. In other words, it is the ratio that the space between the opposing steel plates cannot be separated.

  As shown in FIG. 3, when dividing the space between the steel plate 1 and the steel plate 1, which are adjacent to each other in the opposite direction, by the partition 11 and the radiant tube burner 8 so as to be substantially divided into two, the vertical direction of the portion When the sectional area in the furnace is S0, the area of the partition 11 is S1, and the sectional area of the radiant tube burner 8 is S2, the opening area ratio S = (S0− (S1 + S2)) / S0 × 100 (%).

  Any partition may be used as long as it has heat resistance in the temperature range to be used and strength that does not break during operation, and can substantially block gas. For example, a known heat-resistant material such as a ceramic board is used.

  The present invention is directed to a steel sheet containing 0.2 to 3% Si and 1 to 3% Mn by mass%. The reason for limiting the steel component composition will be described.

Si: 0.2-3 mass%
If Si is less than 0.2%, the effect of improving the strength is small, and if it exceeds 3%, good plating properties cannot be ensured even by the method of the present invention.

Mn: 1-3 mass%
If Mn is less than 1%, the effect of improving the strength is small, and if it exceeds 3%, good plating properties cannot be ensured even by the method of the present invention.

  Steel components other than Si and Mn are not particularly limited. The thing of the component composition used for manufacture of a high intensity | strength thin steel plate can be used.

  In the facility shown in FIG. 1, a high-strength hot-dip galvanized steel sheet is manufactured as follows using a hot-dip galvanized steel sheet manufacturing apparatus in which the reduction zone 3 has the structure shown in FIG.

  A steel plate having a component composition defined by the present invention fed from a steel plate feeding device not shown in the drawing is heated to a predetermined temperature along a heat pattern set in accordance with the steel plate component composition and material standards in the direct flame heating zone 2. The temperature is raised by heating, and at the same time, the rolling oil on the surface of the steel sheet is removed, and an oxide film is formed on the surface of the steel sheet. In the direct-fired heating zone, the oxide film is generated in an amount that can sufficiently ensure the plating property by subsequent reduction. In the steel plate targeted by the present invention, the oxide film is preferably formed so that its thickness is 0.01 to 1 μm. In addition, the larger the amount of Si and Mn, the better the oxide film thickness. In direct flame heating, selective oxidation of easily oxidizable elements such as Si and Mn is suppressed by early generation of Fe oxide. The operating conditions in the direct-fired heating zone are as follows: the steel plate temperature in the direct-fired heating zone outlet side zone is set higher than that for steel plates with low Si and Mn, and the air ratio used during combustion is set higher than the stoichiometric air-fuel ratio. Combustion control is performed so that excess oxygen in the gas increases. If there is room for direct flame reduction after the above-mentioned oxide film thickness is secured, reduction may be performed, but with the length of a normal direct flame heating zone, the formed oxide film may be completely reduced. Since it is not possible, it passes through a direct flame heating zone with insufficient reduction.

  Next, the steel plate is passed through the reduction zone 3. While the steel sheet is passed through the reduction zone 3, the steel sheet is heat-annealed at a predetermined temperature and at a predetermined time by a high-temperature radiant tube burner 8 according to a heat pattern set in accordance with the steel sheet component composition and material standards. The oxide film on the surface of the steel sheet is reduced by the reducing atmosphere gas flowing in the direction opposite to the traveling direction of the steel sheet. The reduction is performed until the sufficiently generated Fe oxide is reduced as Fe. The reduction rate depends on the temperature of the reduction zone and the amount of hydrogen. Although the temperature of the reduction zone is determined by the annealing conditions, the annealing temperature cannot be changed greatly. Therefore, the adjustment of reduction is mainly the adjustment of the amount of hydrogen, specifically the flow rate of reducing gas supplied into the furnace and / or hydrogen gas. The concentration is adjusted to an appropriate condition.

  The steel plate 1 with the oxide film reduced is adjusted to a steel plate temperature suitable for immersing in the hot dipping bath 6 in the cooling zone 4 according to a conventional method, and is then dip plated in the hot dipping bath 6, from the hot dipping bath 6. The steel sheet is pulled up and adjusted to the required coating amount by the gas wiping device 7 and further cooled after being subjected to spangle adjustment or alloying treatment, or cooled without being subjected to the above-described treatment, to obtain a required hot-dip galvanized steel sheet. .

  The hot-dip galvanized steel sheet manufactured as described above, in which excessive oxidation of easily oxidizable elements such as Si and Mn is suppressed in the direct heating zone 2 and the Fe oxide is sufficiently reduced in the reduction zone 3 Then, stable and good plating properties are exhibited.

  In the reduction zone having the structure shown in FIG. 3, the atmospheric gas flows from the reduction zone exit side to the entry side in the direction opposite to the sheet passing direction, so that the dew point at each location in the reduction zone is stabilized. Therefore, by measuring the dew point, the oxidation state of the direct flame heating zone and the reduction state of the reduction zone can be evaluated, and these can be easily controlled.

  If the thickness of the oxide film in the direct heating zone is thick, the reduction reaction load in the reduction zone increases, and the dew point of the atmospheric gas in the reduction zone near the connection with the reduction zone increases. Therefore, by measuring the dew point of the atmospheric gas in the reduction zone near the connection between the direct heating zone and the reduction zone, the degree of oxidation (oxide film thickness) in the direct flame heating zone can be evaluated. Moreover, the thickness of the oxide film formed in the direct fire heating zone can be controlled by controlling the combustion conditions of the gas for directly heating the steel plate in the direct fire heating zone. Therefore, based on the measurement results of the dew point of the atmospheric gas in the reduction zone near the connection between the direct heating zone and the reduction zone, the combustion control of the gas in the direct heating zone, especially the air-fuel ratio control, is performed, and the direct flame heating zone is formed. It is possible to control the oxide film thickness.

  Moreover, the reduction state in the reduction zone of the oxide film can be evaluated from the dew point of the atmosphere gas on the reduction zone exit side. The degree of reduction of the oxide film in the reduction zone can be adjusted by adjusting the hydrogen amount (gas supply amount or hydrogen concentration) of the reducing gas supplied into the furnace. Therefore, it is possible to control the degree of reduction of the oxide film in the reduction zone by adjusting the hydrogen amount of the reducing gas supplied into the furnace based on the measurement result of the dew point of the atmospheric gas on the reduction zone exit side.

  For example, an appropriate oxide film thickness is formed in the direct fire heating zone, and this oxide film is sufficiently reduced in the reduction zone to obtain good plating properties. In the vicinity of the connection portion between the direct fire heating zone and the reduction zone When the dew point of the atmospheric gas in the reduction zone (DP1) is DP1m and the dew point of the atmospheric gas on the exit side of the reduction zone (DP2) is DP2m, the thickness of the oxide film formed in the direct flame heating zone, Table 1 shows the degree of reduction, the plating property, and the dew point DP1 of the atmosphere gas in the reduction zone near the connection between the direct heating zone and the reduction zone, and the dew point DP2 of the atmosphere gas in the vicinity of the reduction zone exit side. There is a corresponding relationship.

  For example, A is a case where the oxide film thickness is excessive with respect to the appropriate condition C, but a good plating is obtained as a result of sufficient reduction of the oxide film. In this case, the dew point DP1 is higher than the dew point DP1m of the appropriate condition C because the amount of reduction of the oxide film in the reduction zone increases. In addition, since the oxide film is sufficiently reduced in the reduction zone passage plate, the reduction reaction does not proceed near the reduction zone exit side, so that the dew point DP2 is lower than the dew point DP2m of the appropriate condition C. Therefore, by performing the control as described in the control method of Table 1 based on the measurement results of each dew point, more appropriate oxide film formation can be achieved in the direct flame heating zone, and more appropriate reduction can be achieved in the reduction zone. This makes it possible to obtain better plating properties.

  Further, for example, as in the case of A described above, when the dew point DP1 is high and the dew point DP2 is low, it is determined that the oxide film is sufficiently reduced although the oxide film is excessively formed. Combustion control is performed to reduce the oxide film thickness in the fire heating zone, combustion control is performed to reduce the oxide film thickness, and in response to this, control is performed to reduce the reducing property in the reduction zone. Efficient operation is possible while securing plating.

  In Table 1, “↓” (downward arrow) is changed to a condition for loosening the degree of oxidation or reduction, “↑” (upward arrow) is changed to a condition for increasing the degree of oxidation or reduction, “→ "(Horizontal arrow) indicates that the degree of oxidation or reduction is not changed.

  In addition, dew point DP1 corresponding to the proper oxidation state in the direct fire heating zone, dew point DP2 corresponding to the proper reduction state in the reduction zone, component composition that does not know the proper degeneration point, plating failure due to scale residue occurs in steel sheets of material standards Even if it does, change the combustion conditions to increase the degree of oxidation in the direct-fired heating zone (or weaken the degree of oxidation) and increase the degree of reduction (or reduce the degree of reduction) in the reduction zone. Guidelines for changing to conditions where good plating can be obtained by changing the atmospheric gas supply conditions and grasping the changing tendency of each dew point DP1, DP2 to decrease or increase with respect to each dew point before the condition change Can be obtained.

  For example, if the dew points DP1 and DP2 are both high, the oxide film becomes thicker, so that it is determined that the dew point DP2 has increased, and the combustion control in the direct heating heating zone is changed to a direction in which the oxide film thickness is reduced. That's fine. And what is necessary is just to grasp | ascertain again the change tendency of each dew point after changing to this condition, and to change into more suitable conditions.

  The reducing gas supplied into the furnace may be supplied not only to the cooling zone but also to the reduction zone. In this case, the same control as described above can be performed.

Steel composed of the composition shown in Table 2, the balance Fe and inevitable impurities was hot-rolled, pickled, and cold-rolled to produce a plating original plate having a thickness of 1 mm and a width of 1 m. This steel sheet is annealed in an annealing furnace equipped with a direct-fired heating zone, a reduction zone (a vertical reduction zone), and a cooling zone, followed by hot dip galvanization in a hot dip galvanizing tank, and a coating amount of 40 g / per side on a gas wiping device. It was adjusted to m ^2. The direct fire heating zone includes a direct fire pre-tropical zone, a direct fire oxidation zone, and a direct fire reduction zone. As shown in FIG. 3, the reduction zone was provided with a partition plate made of ceramic board between all the longitudinal paths of the reduction zone, and an experiment was conducted to change the area ratio of the opening portion of the partition plate. The partition plate was structured in a wing shape as a whole in consideration of rectification.

The manufacturing conditions are as follows.
・ Line speed: 80 mpm
・ Direct fire heating zone:
After preheating in the direct fire pre-tropical zone, the steel plate was directly heated and oxidized in the direct fire oxidation zone, and then directly reduced in the direct fire reduction zone so that the steel plate temperature was 600-630 ° C. after direct fire reduction. Table 3 shows the air ratio in the direct-fire oxidation zone and the air ratio in the direct-fire reduction zone.
・ Reduction zone:
Annealing conditions: Steel plate heating temperature was 800 ° C.
・ Hot galvanizing:
Bath temperature: 470 ° C
Atmosphere _{Gas: H 2: 8vol% -N 2} : 92vol% ( dew point: about -50 ° C.), flow rate 1000 Nm ³ / H
The appearance of the hot-dip galvanized steel sheet thus produced was visually observed and the presence or absence of non-plating was evaluated. The evaluation results are shown in Table 3.

  As shown in Table 3, good plating properties were exhibited when the opening area ratio of the partition was 10% or less. In the conventional apparatus, the radiant tube itself does not allow gas to pass, so 70% of the area between the steel plates is blocked (open area ratio = 30%), but the flow is disturbed against the gas and flows between the steel plates. It is considered that non-plating had occurred because of the shape that caused In the conventional apparatus, improvement of non-plating is not recognized even when the air ratio in the direct-fired heating zone is increased, whereas an example of the present invention apparatus (No. 4) in which the partition plate is provided to reduce the opening area ratio by 5%. ) Improved non-plating. The open area ratio was reduced and non-plating was improved, and there was a remarkable effect especially when the open area ratio was 10% or less.

  The method for producing a high-strength hot-dip galvanized steel sheet according to the present invention is a method for producing a high-strength hot-dip galvanized steel sheet containing Si 0.2 to 3% and Mn 1 to 3% by mass% without causing poor plating properties. Can be used.

  The equipment for producing a hot-dip galvanized steel sheet of the present invention can be used as an apparatus for producing a high-strength hot-dip galvanized steel sheet containing 0.2 to 3% Si and 1 to 3% Mn by mass%.

It is a schematic side view which shows the principal part structural example of the manufacturing equipment of a hot dip galvanized steel plate. It is a schematic diagram explaining the schematic structure of the reduction zone in the conventional galvanized steel sheet manufacturing equipment and the flow of the atmospheric gas in the reduction furnace. It is a schematic side view which shows the structural example of the reduction zone of the annealing furnace of the manufacturing equipment of the hot dip galvanized steel plate which concerns on embodiment of this invention. It is a schematic diagram explaining the flow of the atmospheric gas in the reduction zone of FIG.

Explanation of symbols

1 Steel plate 2 Direct flame heating type (direct flame heating zone)
3 Vertical type reduction zone (reduction zone)
4 Cooling zone 5 Snout 6 Hot dipping bath 7 Gas wiping device 8 Radiant tube burner 9a, 9b In-furnace roll 11 Partition 13, 14 Dew point meter 16 Direct flame heating zone and reduction zone connection 101 Fuel supply system 102 Air supply system 103 Direct fire heating burner 105 Gas supply piping

Claims (7)

A steel sheet containing at least one of Si: 0.2 to 3% and Mn: 1 to 3% by mass% is heated in a direct-fired heating zone and further reduced in a vertical reduction zone. In the method of manufacturing a high-strength hot-dip galvanized steel sheet that is subjected to surface reduction and annealing in the hot-dip galvanizing bath and then galvanized, in the vertical reduction zone, the atmosphere gas is removed at least in the entry region. A method for producing a high-strength hot-dip galvanized steel sheet characterized by flowing in a direction opposite to the traveling direction. The high-strength molten zinc according to claim 1, wherein the dew point of the atmospheric gas in the reduction zone near the connection between the direct-fire zone and the reduction zone is measured, and the combustion control in the direct-fire zone is performed from the measured value. Manufacturing method of plated steel sheet. Measure the dew point of the atmospheric gas in the reducing zone near the connection between the direct fire zone and the reducing zone and the dew point of the atmospheric gas on the outlet side of the reducing zone, and adjust the flow rate of the atmospheric gas supplied to the reducing zone from the relationship The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1 or 2. In an annealing furnace having a direct flame heating zone and a vertical reduction zone and a hot dip galvanized steel sheet equipped with a hot dip galvanizing device downstream of the direct heating system, steel adjacent to at least the entrance region of the vertical reduction zone A facility for manufacturing a hot dip galvanized steel sheet, characterized in that a partition is provided between the vertical passes of the belt to restrict the direction of the flow of the atmospheric gas between the vertical passes in the direction opposite to the traveling direction of the steel plate. The facility for manufacturing a hot dip galvanized steel sheet according to claim 4, wherein an opening area ratio after partition installation is 10% or less. The vertical reduction zone is provided with a dew point meter that measures the dew point of the atmospheric gas in the vertical reduction zone in the vicinity of the connection between the direct heating zone and the vertical reduction zone of the direct fire heating system. 5. Manufacturing equipment for hot-dip galvanized steel sheet according to 5. The hot-dip galvanized steel sheet manufacturing equipment according to claim 6, further comprising a dew point meter for measuring the dew point of the atmospheric gas on the reduction zone outlet side in the vertical reduction zone.

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