JP5533000B2 - Method for producing galvannealed steel sheet - Google Patents

Description

  The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet. In particular, the present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet capable of producing an alloyed hot-dip galvanized steel sheet having improved surface properties by reducing unevenness of (local) surface irregularities. .

  In recent years, galvanized steel sheets have been used in large quantities in home appliances, building materials, and the automotive industry from the viewpoint of rust prevention, and in particular, alloyed galvanized steel sheets are used from the viewpoints of economy, paintability, weldability, etc. Widely applied. In particular, after the development of ultra-low carbon steel sheets with excellent workability, ultra-low carbon alloyed hot-dip galvanized steel sheets have been widely applied to panels such as automobile fenders and doors.

  By the way, automotive panels are required to have a more beautiful appearance than their commercial characteristics, but extremely low carbon alloyed hot-dip galvanized steel sheets have uneven surface irregularities due to differences in local alloying reactions. Exhibited a streak pattern defect that developed like a streak, which remained as irregularities even after panel painting, and had a problem of impairing the surface appearance. In particular, this problem was significant when alloyed hot dip galvanizing was performed on a Ti-containing ultra-low carbon steel sheet using Ti to fix solid solution C that adversely affects workability.

The difference in the diffusion of iron into the plating layer affects the shape of the irregularities. For example, in Patent Document 1, hot rolling is performed on a low carbon steel slab to form a hot-rolled sheet, followed by cold rolling and annealing, and then melting. In the method for producing an alloyed hot-dip galvanized steel sheet, which is subjected to galvanizing and heat alloying treatment to obtain an alloyed hot-dip galvanized steel sheet, the hot rolling finish rolling temperature is determined by the sheet width direction and the surface temperature in the rolling direction It is disclosed that the _Ar3 transformation point + 30 ° C. or higher, the finish rolling temperature of hot rolling is the surface temperature in the rolling direction and the plate width direction of the steel plate surface, and the deviation is within 30 ° C.

JP-A-10-18011

  In patent document 1, it reports that generation | occurrence | production of a streak pattern defect can be prevented by controlling the finishing temperature of hot rolling in the narrow range with the said technique. However, the technique disclosed in Patent Document 1 has a problem that it is difficult to satisfy a requirement for a very beautiful appearance in an automotive exterior panel (for example, a door panel, a side outer panel, a hood, etc.). .

  Accordingly, the present invention provides a method for producing an alloyed hot-dip galvanized steel sheet capable of producing an alloyed hot-dip galvanized steel sheet having excellent surface properties that can withstand the use of automotive exterior panels. Is an issue. In addition, when the present invention uses a Ti-containing ultra-low carbon steel sheet as a base material, particularly, even when a large amount of Ti that is likely to cause streak defects is contained, the surface texture is stably secured. It is an object of the present invention to provide a method for producing an alloyed hot-dip galvanized steel sheet.

  The streak pattern defect is generated when the alloying reaction of the plating layer proceeds locally non-uniformly during the alloying process performed after the hot dip galvanization. This non-uniformity in the alloying reaction is caused by a difference in texture on the surface of the base material as described in Patent Document 1 or Japanese Patent Laid-Open No. 2001-172744 (hereinafter sometimes referred to as “Patent Document 2”). There is a view that this is caused by the remaining unrecrystallized structure on the surface of the base material as described in the above. Recently, as described in Japanese Patent Application Laid-Open No. 2008-106321 (hereinafter, sometimes referred to as “Patent Document 3”), an aggregate of fine crystal grains on the surface of a base material is referred to as a grain boundary. There is also the view that the aggregate of fine grains on the surface of the base metal is related to the occurrence of streak defects, which is caused by the difference in the alloying rate (difference in iron diffusion rate) with the crystal grains. That is, the portion where fine crystal grain aggregates are present in stripes has a higher grain boundary density on the surface than the portion where the aggregates are not present in stripes, so that the alloying reaction proceeds. This is a view that the portion where the alloying reaction has progressed is locally convex, resulting in a streak defect. The formation of fine crystal grains is affected by Ti oxides. If the amount of Ti is relatively increased, Ti oxides are locally unevenly distributed during hot rolling, resulting in the occurrence of streak defects. . Therefore, for example, the technique disclosed in Patent Document 3 deals with the component rule of suppressing the Ti amount.

  The techniques disclosed in Patent Document 2 and Patent Document 3 focus on the generation and control of Ti oxide during hot rolling or at the slab formation stage. Patent Literature 2 and Patent Literature 3 propose component regulations such as suppression of Ti amount and hot rolling conditions, and reduce the occurrence of streak defects by optimizing the conditions in the hot dip galvanizing line. This idea is not disclosed.

  Actually, in ultra-low carbon steel, in order to improve the workability, it is necessary to fix solute C, and Ti is very effective as an element for fixing solute C. As an element replacing Ti, for example, there is Nb, and these may be added in combination. However, addition of a large amount of Nb has a limit because it reduces workability. In particular, in order to improve the workability, it is essential to add Ti having a solid solution C amount or more. Although the excessively added Ti is consumed by becoming an oxide or a nitride, when an excessive amount of Ti is added, the amount of Ti dissolved in the base material steel plate (the amount of solid solution Ti) increases. Further, N is an element that renders Ti harmless by producing Ti nitride. However, since N is an element contained in the air, it is difficult to control the addition amount of N very strictly. Therefore, in practice, even if the same amount of Ti is added in the steelmaking stage, the actual amount of the solute Ti contained in the steel plate varies greatly. Thus, even if only the components of the base steel plate are defined in advance, it is difficult to control the final composition of the steel plate and it is difficult to reduce the occurrence of streak defects.

  On the other hand, as a method for canceling the influence of the base steel plate within the continuous hot dip galvanizing, for example, in JP 2009-13447 A (hereinafter sometimes referred to as “Patent Document 4”), the steel plate surface before annealing is used. A technique has been disclosed in which an abnormal layer on the surface of the base material is removed by grinding almost uniformly with a brush roll over the entire width, and the surface of the base material is homogenized for imparting uniform strain. Japanese Patent Laid-Open No. 11-50222 (hereinafter sometimes referred to as “Patent Document 5”) discloses that one type selected from a carbon compound, a nitrogen compound, and a boron compound on the surface of a base material before annealing. Alternatively, two or more types (hereinafter referred to as “chemical solutions”) are attached, and then annealed in a non-oxidizing atmosphere, and then immersed in a molten zinc bath to perform plating, thereby improving film uniformity and adhesion. A technique for producing an improved hot dip galvanized steel sheet is disclosed.

  However, in order to completely remove the abnormal layer on the surface of the base material with the technique disclosed in Patent Document 4, it is necessary to increase the amount of grinding. If such a grinding facility is provided, there is a problem in terms of cost. is there. Further, when the grinding amount is increased, the surface defects of the base material due to the brush roll do not disappear even after plating, so that there is a quality problem that a surface appearance abnormality such as a brush residue is caused and the surface properties are impaired.

  Further, as in the technique disclosed in Patent Document 5, when a chemical solution is applied, the chemical solution on the surface of the steel sheet (actually, a dry product solidified during annealing) is wound around a hearth roll as a transport roll, It becomes the cause of the pit-like defect called 疵. When a steel sheet is transported using a transport roll on which a hearth roll is formed, a defect is formed on the surface of the steel sheet, resulting in a fatal defect in a steel sheet that requires a very high surface quality such as an automotive exterior panel. There is a problem of quality that damages. Moreover, there is a problem in terms of cost if a facility for applying a chemical solution is newly provided.

  In this way, the technology that counteracts the influence of the base steel sheet by treating the base steel sheet before annealing to the alloyed hot-dip galvanized steel sheet used in automotive exterior panels that require extremely high surface properties. There are problems in terms of quality and cost in applying.

  Based on the above, in order to reduce the occurrence of streak pattern defects, the present inventors are not limited to only the components of the steel sheet, but it is effective to optimize the conditions after annealing in the hot dip galvanizing line. I thought it was. More specifically, it is ideal to reduce the occurrence of streak defects by optimizing the operating conditions of the hot dip galvanizing line. In particular, the surface state changes in various processes, and the surface state is hot dip zinc. In alloyed hot-dip galvanized steel sheets that change the appearance by affecting the alloying reactivity (iron diffusion) during alloying after plating, it is most preferable to deal with the operating conditions within continuous hot-dip galvanizing Thought.

Based on this idea, the present inventors have examined measures that can be taken in the annealing furnace. First, for the alloyed hot-dip galvanized steel sheet using various ultra-low carbon steels as a base steel sheet, the inventors of the present invention have a solid solution Ti amount Ti * represented by the following formula (1) and occurrence of streak defects. The relationship was investigated. The streak defect was rated according to the following criteria. The results are shown in FIG. The vertical axis in FIG. 1 is the rating value, and the horizontal axis is the solid solution Ti amount Ti * represented by the following formula (1).
Ti * = Ti−48 × (N / 14 + S / 32 + C / 12) Formula (1)
In formula (1), Ti is the titanium content (mass%) of the base steel sheet, N is the nitrogen content (mass%) of the base steel sheet, S is the sulfur content (mass%) of the base steel sheet, and C is It is carbon content (mass%) of a base material steel plate.

<Rating criteria>
5: Remarkable streak defects occur over the entire length in the coil longitudinal direction (failed)
4: Remarkable streak defects occur in 1/4 or less of the coil longitudinal direction (fail)
3: A light streak defect occurs in 1/4 or less of the coil longitudinal direction and / or a significant streak defect occurs in 1/16 or less of the coil longitudinal direction (fail)
2: Light streak defect occurs in 1/16 or less of the coil longitudinal direction (pass)
1: No streak defect occurs over the entire length in the coil longitudinal direction (pass)

  As shown in FIG. 1, the amount of solid solution Ti expressed by the formula (1) has a great influence on the occurrence of a streak pattern defect. When Ti * ≦ 0.00, the streak pattern defect is a quality problem. It was found that substantially no occurrence occurred. On the other hand, as described above, in practice, even if only the components of the base steel plate are specified from the degree of workability required for the alloyed hot-dip galvanized steel plate and the variation in N in the steel, It is difficult to keep the occurrence of this at an acceptable level. Accordingly, the present inventors have examined operating conditions for hot dip galvanizing that can maintain the occurrence of streak defects at an acceptable level even when Ti * is greater than 0.00.

  From the results shown in FIG. 1, the present inventors considered that a streak defect occurs because solute Ti in steel is locally unevenly distributed. The reason why solute Ti is unevenly distributed locally is unknown, but due to the occurrence of streaks, non-uniform thickness of scale during hot rolling (not only unevenness of rolling temperature and unevenness of deskeling, but also during rolling Including mechanical scale damage such as rolls). That is, when a material in which solute Ti is present in steel is hot-rolled, Ti-based precipitates such as Ti oxide are sufficiently generated in the surface layer, so that the surface layer has relatively less solute Ti. On the other hand, in a site (bulk site) other than the surface layer, Ti-based precipitates are less likely to be generated than in the surface layer, so that the bulk site has a relatively large amount of solid solution Ti. If the scale thickness becomes non-uniform during hot rolling, it is considered that the local uneven distribution of the solid solution Ti occurs because the bulk portion where the solid solution Ti is relatively large is exposed only in the portion where the scale thickness is thin. .

  The portion where a large amount of solute Ti is present on the surface of the steel sheet is high in the cleanliness of the base steel sheet. Therefore, the diffusion of iron from the base metal is promoted during the alloying treatment by heating after hot dip galvanizing, and the alloy It is thought that chemical growth will be faster. That is, when the solid solution Ti is locally unevenly distributed for some reason during hot rolling, the unevenly distributed portion is stretched during the subsequent cold rolling, and the stretched portion is further subjected to alloying treatment after hot dipping. It was speculated that as a result of the alloying being promoted at times, a streaky pattern emerged.

  As a result of intensive studies, the inventors of the present invention reduce the amount of solid solution Ti by positively precipitating solid solution Ti locally distributed on the steel sheet surface as Ti-based inclusions during annealing. I thought that it would be possible to prevent the occurrence of streak defects if it was possible. Therefore, the present inventors use materials with different amounts of solid solution Ti, increase the dew point in the annealing furnace, and precipitate solid solution Ti as a Ti oxide (which may contain some Mn), Before plating, it was examined whether or not the amount of solute Ti on the steel sheet surface layer could be reduced.

  The present invention is mainly intended for ultra-low carbon steel, has high iron purity, and does not contain many oxidizable elements. For this reason, it was considered indispensable to maintain an extremely strict reducing atmosphere, and it was investigated whether or not a Ti-based oxide would precipitate when the dew point in the annealing furnace was changed. As a result, the dew point in the annealing furnace is appropriately controlled according to the amount of solute Ti in the steel. More specifically, when the amount of solute Ti is large, the rear zone of the heating zone in the annealing furnace. It was found that Ti in steel can be precipitated as a Ti-based oxide by operating in a high dew point atmosphere.

  Below, the investigation result which confirmed that it becomes possible to precipitate Ti in steel as Ti-type oxide by control of a dew point is illustrated.

  The amount of element added (mass%. In the following, mass% may be simply referred to as “%”) is the amount shown in Table 1 below, with the balance being Fe and unavoidable impurities in the base steel plate (striped pattern) When a steel plate in which defects can occur is fixed in a reducing atmosphere (reducing atmosphere for iron, the same applies hereinafter) with a hydrogen concentration of 10% in a laboratory plating simulator, the dew point and the steel plate An annealed plate whose temperature was changed under two conditions was prepared, and the surface state of the annealed plate was observed with FE-SEM (SUPRA55VP, acceleration voltage: 5 kV, WD; 6.6 mm, image; AsB image) manufactured by Carl Zeiss. did. FIG. 2 shows the control mode of the dew point of the reducing atmosphere and the steel sheet temperature, and FIG. 3 shows the observation result (photograph) by FE-SEM. FIG. 2A is a diagram for explaining a control mode of the dew point. In FIG. 2A, the vertical axis represents the dew point [° C.] of the reducing atmosphere, and the horizontal axis represents time [s]. Under conditions 1 and 2, the dew point is kept constant until the steel plate temperature reaches 850 ° C., and when the steel plate temperature reaches 850 ° C., the dew point is changed, and after the dew point becomes −60 ° C. The dew point was maintained at -60 ° C. FIG. 2B is a diagram for explaining a control mode of the steel sheet temperature, where the vertical axis in FIG. 2B is the steel sheet temperature [° C.] and the horizontal axis is time [s]. A zone where the steel plate temperature is up to 850 ° C. corresponds to a heating zone of a continuous hot dip galvanizing line (hereinafter sometimes referred to as “CGL”), and then the steel plate temperature is kept above the recrystallization temperature for a certain period of time. This zone corresponds to a so-called tropical zone of CGL, and in this study, it was kept at 850 ° C. for 30 seconds. Thereafter, the zone where the steel plate temperature is lowered from 850 ° C. to 500 ° C. corresponds to the CGL cooling zone, and the zone where the steel plate temperature is maintained at 500 ° C. corresponds to the CGL low temperature holding zone. In this survey, in order to confirm how the precipitates on the steel sheet surface change when the annealing dew point is changed, the control mode of the steel sheet temperature is the same in both conditions 1 and 2, so in FIG. Only one type of heat pattern is shown. 3A is a view showing the FE-SEM observation result (photograph) of the steel sheet annealed under condition 1, and FIG. 3B is the FE-SEM observation of the steel sheet annealed under condition 2. It is a figure which shows a result (photograph). Note that Ti * in Table 1 below is the amount of solid solution Ti represented by the above formula (1).

  The elements contained in the precipitate shown in Fig. 3 (a) and the elements contained in the precipitate shown in Fig. 3 (b) are respectively EDX (XFlash 4010 manufactured by Bruker). ). The results (at%) are shown in Table 2.

  As shown in FIGS. 3A and 3B and Table 2, the dew point of the reducing atmosphere when the steel plate temperature is 780 ° C. or lower (hereinafter sometimes referred to as “annealing dew point”). When changed, the morphology and composition of the precipitates clearly change. In condition 2 where the annealing dew point at the initial stage of the reduction furnace corresponding to the heating zone in CGL is set to −40 ° C., fine precipitates appear in a bead shape at the grain boundaries as shown in FIG. Thus, Ti contained in the precipitate was 0.83 at%, and Al was 1.99 at%. That is, when the annealing was performed under the condition 2, the substance precipitated at the crystal grain boundary was an Al-based oxide. On the other hand, in condition 1 where the annealing dew point in the initial stage of the reduction furnace corresponding to the heating zone in CGL was set to −10 ° C., large precipitates appeared at the grain boundaries as shown in FIG. Thus, Ti contained in the precipitate was 13.30 at%, and Al was 0.29 at%. That is, the material that precipitated at the grain boundaries when annealed under Condition 1 was a Ti-based oxide. From the above, it can be estimated that when the annealing dew point is increased, the precipitate changes from the Al-based oxide to the Ti-based oxide.

  That is, even if there is a portion where solute Ti is locally distributed, by increasing the annealing dew point, it becomes possible to fix the solute Ti as a precipitate of Ti-based oxide, and as a result, It is considered that the composition where the solid solution Ti is unevenly distributed approaches that of pure iron. When a steel plate whose composition is close to pure iron is immersed in a hot dip galvanizing bath, a uniform Fe-Al layer is formed, and as a result, the alloying reaction easily proceeds uniformly during the subsequent alloying treatment, resulting in streak defects. It is estimated that the occurrence of is suppressed.

  As described above, the present inventors can reduce the solid solution Ti of the surface layer by homogenizing the surface layer by controlling the dew point when annealing the steel sheet containing the solid solution Ti, and thereafter It has been found that the occurrence of streak defects can be reduced in the hot dip galvanizing process and alloying process. Further, as shown in FIG. 3 (a), even under the condition 1 where the annealing dew point is increased, a spongy reduced iron layer is not formed by scale reduction or subsequent scale reduction in a strongly reducing atmosphere. Therefore, it is considered that the problem of the hearth roll is hardly caused in the reducing furnace atmosphere of condition 1.

  Based on this knowledge, the present inventors added a base material steel plate (steel plate capable of generating streak pattern defects) in which the elements (mass%) shown in Table 3 below are added and the balance is Fe and inevitable impurities. Using the dew point of the reducing atmosphere in which the hydrogen concentration is 10% in the lab plating simulator, and performing annealing with the steel plate temperature controlled, and changing the annealing dew point between -45 ° C and 0 ° C, We examined in detail how the surface of the steel sheet after annealing changes, that is, under what conditions the Ti-based oxide is generated. Specifically, the steel plate surface after annealing was observed with FE-SEM (SUPRA55VP manufactured by Carl Zweis, acceleration voltage: 5 kV, WD; 6.6 mm, image: AsB image), and the size was within a visual field of 10 μm × 10 μm. After specifying the place where the precipitate exists, the precipitate was analyzed by EDX to investigate whether the precipitate was a Ti-based oxide. The dew point of the reducing atmosphere is set to five annealing dew points: -45 ° C, -35 ° C, -20 ° C, -5 ° C, and 0 ° C, and the dew point is changed when the steel plate temperature reaches 850 ° C. It started and it controlled by maintaining a dew point at -60 degreeC after a dew point became -60 degreeC. The steel plate temperature was controlled in the form shown in FIG. FIG. 4 shows the results of investigating the Ti-based oxide on the surface of the steel sheet annealed under such conditions. The vertical axis in FIG. 4 is the dew point [° C.], and the horizontal axis is the solute Ti amount Ti *. In FIG. 4, the case where a Ti-based oxide was confirmed was marked with “◯”, and the case where a Ti-based oxide was not confirmed was marked with “X”. Even if the Ti-based oxide is confirmed, if a spongy reduced iron layer is observed on the surface, there is a concern about the occurrence of a hearth roll soot, and the surface appearance of the steel sheet may be impaired. Therefore, in FIG. 4, when the sponge-like reduced iron layer was observed on the surface, it was set as “x”. Note that Ti * in Table 3 below is the amount of solid solution Ti represented by the above formula (1).

  From FIG. 4, when Ti *> 0.00, the Ti-based oxide could be generated by setting the annealing dew point to −35 ° C. or higher. However, when the annealing dew point was set to 0 ° C., a sponge-like reduced iron layer accompanying the scale formation was formed on the surface. Therefore, it is considered that by setting the annealing dew point to −35 ° C. or more and −5 ° C. or less, it is possible to produce an alloyed hot-dip galvanized steel sheet having excellent surface properties that can withstand the use of exterior panels for automobiles. It is done.

  The present invention has been completed based on the above findings.

  The present invention will be described below. In order to facilitate understanding of the present invention, reference numerals in the accompanying drawings are appended in parentheses, but the present invention is not limited to the illustrated embodiments.

The present invention includes an annealing step (S4) for treating a steel plate in a reduction furnace (3), a hot dip galvanizing step (S5) for galvanizing the annealed steel plate, and alloying the galvanized steel plate. having a processing performed alloying step (S6), a method for manufacturing a galvannealed steel sheet, the reducing furnace, the heating zone (3a, 3b), soaking (3c), and a cooling zone (3d), and an analysis step (S1) for analyzing the composition of the steel plate to be annealed in order in the moving direction of the steel plate moving in the reduction furnace as necessary, and a low temperature holding zone is analyzed in the analysis step A judgment step (S2) for judging whether or not the solid solution Ti amount Ti * represented by the following formula (1) of the obtained steel sheet is Ti *> 0.00, and Ti *> in the judgment step When it is determined that the temperature is 0.00, the heating zone is reduced before the steel plate reaches the heating zone. Kutomo 600 ° C. or more regions dew point control step (S3) for the dew point of the reducing atmosphere to -35 ° C. or higher -5 ° C. or less in (3b) is provided before the annealing step (S4), the temperature of the steel sheet The step (S41) of heating the steel plate for 3 seconds or more in the heating zone (3b) in which the dew point of the reducing atmosphere is −35 ° C. or more and −5 ° C. or less when the recrystallization temperature is at least 600 ° C. or more is the annealing step. It is the manufacturing method of the galvannealed steel plate characterized by being contained .
Ti * = Ti−48 × (N / 14 + S / 32 + C / 12) Formula (1)
However, in Formula (1), Ti is titanium content (mass%) of a steel plate, N is nitrogen content (mass%) of a steel plate, S is sulfur content (mass%) of a steel plate, C is carbon content of a steel plate. Amount (% by mass).

Here, the “reducing furnace (3)” means an annealing furnace in which the atmosphere in the furnace is maintained in a reducing atmosphere (however, it is a reducing atmosphere for iron and is oxidized for each component in the steel sheet. May be a sex atmosphere) . The “recrystallization temperature” varies depending on the composition of the base steel plate. In the present invention, the recrystallization temperature of the ultra-low carbon steel having a carbon content of 0.01% or less is, for example, 820 ° C. Further, “the temperature of the steel sheet is at least 600 ° C. or more and the recrystallization temperature or less” means that the steel sheet temperature when the steel sheet is heated in the region (3b) where the dew point of the reducing atmosphere is −35 ° C. or more and −5 ° C. or less. While the upper limit value is the recrystallization temperature, the lower limit value of the steel sheet temperature can be varied from 600 ° C. to the lower limit. That is, in the heating step (S41) in the present invention, for example, when the steel sheet temperature is 780 ° C. or more and the recrystallization temperature or less, the dew point of the reducing atmosphere is −35 ° C. or more and −5 ° C. or less (3b ) In which the steel plate is heated for 3 seconds or more.

Moreover, in the said invention , it is preferable to control the dew point of the reducing atmosphere in a reduction furnace (3) by flowing in humidified nitrogen after a heating zone (3b) and before the end of soaking (3c).

Moreover, in the said invention , it is preferable that the hydrogen concentration of the reducing atmosphere in a reduction furnace (3) is 1 to 60 mass%.
Moreover, in the said invention, a steel plate is the mass%, C: 0.01% or less, S: 0.02% or less, N: 0.0050% or less, and Ti: 0.01% or more and 0.10 % Or less is preferable.

  In this invention, when the temperature of a steel plate is at least 600 degreeC or more and below a recrystallization temperature, the process of heating a steel plate with the heating zone (3b) whose dew point of a reducing atmosphere is -35 degreeC or more and -5 degrees C or less (S41) Is included. Therefore, according to the present invention, even when an alloyed hot-dip galvanized steel sheet is manufactured using a Ti-containing ultra-low carbon steel, which is likely to generate streak defects, as a base steel sheet, it is possible to suppress the occurrence of streak defects. Thus, it is possible to provide a method for producing an alloyed hot-dip galvanized steel sheet capable of producing an alloyed hot-dip galvanized steel sheet having excellent surface properties that can withstand the use of automotive exterior panels. Moreover, in this invention, generation | occurrence | production of a streak pattern defect is suppressed by changing the operating conditions in CGL, without accompanying a large installation of an installation. Therefore, according to the present invention, it is possible to prevent an increase in manufacturing cost.

It is a figure which shows the relationship between the amount of solid solution Ti, and a stripe pattern defect generation | occurrence | production aspect. It is a figure which shows the control form of the dew point of reducing atmosphere, and steel plate temperature. Fig.2 (a) is a figure which shows the control form of the dew point of a reducing atmosphere, FIG.2 (b) is a figure which shows the control form of steel plate temperature. It is a photograph which shows the observation result by FE-SEM. FIG. 3A is a photograph observing the surface state of the steel sheet annealed under condition 1, and FIG. 3B is a photograph observing the surface state of the steel sheet annealed under condition 2. It is a figure which shows the investigation result of the steel plate surface. It is a figure which shows the example of a form of the continuous hot-dip galvanizing line to which this invention is applied. It is a flowchart which shows the example of a form of this invention. It is a figure which shows the determination result of a streak pattern defect. It is a figure which shows the determination result of a streak pattern defect.

  Embodiments of the present invention will be described below.

  FIG. 5 is a simplified view showing an example of a continuous hot dip galvanizing line (CGL) to which the present invention is applied, and only a part of the CGL is extracted and shown. The arrow of FIG. 5 is the moving direction of a steel plate. As shown in FIG. 5, a continuous hot dip galvanizing line 10 (hereinafter sometimes referred to as “CGL10”) to which the present invention is applied includes a pretropical zone 2, a reduction furnace 3, a snout 4, and zinc. A pot 5 and an alloying furnace 6 are provided. The CGL 10 is provided with an inlet looper, a pre-processing machine (both not shown) and the like on the upstream side in the moving direction of the steel plate 1 relative to the pre-tropical zone 2, and downstream of the alloying furnace 6 in the moving direction of the steel plate 1. Are provided with a cooling device (not shown). The steel sheet 1 treated with the CGL 10 begins to be heated in the pre-tropical zone 2 and is rapidly heated to a recrystallization temperature or lower (for example, about 600 to 780 ° C.) in the heating zone 3a maintained in a reducing atmosphere. The heating zone 3b is heated to the recrystallization temperature or higher. Next, after heating to a higher temperature (for example, about 850 ° C.) in the soaking zone 3c maintained in a reducing atmosphere and completing the recrystallization completely, in the cooling zone 3d maintained in the reducing atmosphere. It is cooled to about 500 ° C. The steel sheet cooled in the cooling zone 3d is then led to the zinc pot 5 through the snout 4 maintained in a reducing atmosphere, and hot dip galvanized. The steel plate immersed in the zinc pot 5 and subjected to hot dip galvanization is pulled up from the zinc pot 5 and heated in the alloying furnace 6 to be subjected to alloying treatment. In the reduction furnace 3, the heating zone 3 a corresponds to, for example, a DFF furnace that rapidly heats the steel sheet 1 by applying a reducing flame to the steel sheet 1 with a direct fire burner (hereinafter referred to as “DFF”). In the all radiant tube method (hereinafter, sometimes referred to as “all radiant method”), it is difficult to specify the heating zone 3a. 3a. Note that when the steel plate temperature is less than 600 ° C., Ti oxide is not generated. Therefore, in this invention, when a steel plate temperature is 600 degreeC or more, a Ti oxide is effectively produced | generated by making a dew point high.

  In CGL 10, the reduction furnace 3 is maintained in a reducing atmosphere in order to maintain the cleanliness of the steel sheet. When a scale is generated on the steel plate in this region, when forming hot dip galvanizing by subsequent immersion in the zinc pot 5, plating is repelled, resulting in poor appearance such as non-plating, and adhesion of the plating is lowered. Therefore, in the CGL 10, a strong reducing atmosphere is maintained by flowing hydrogen gas into the reduction furnace 3 and keeping the dew point after the cooling zone 3c low. In addition, when excessive scale is generated, a pit-shaped wrinkle called a hearth roll wrinkle is generated by wrapping around a hearth roll that is a transport roll of the steel plate in the reduction furnace 3, which causes the surface appearance of the steel plate to deteriorate. Therefore, it is not preferable.

  The CGL 10 to which the present invention is applied does not have a non-oxidation furnace. In a non-oxidizing furnace, the furnace is heated with a burner, so the amount of scale generated on the surface of the steel sheet increases, and the scale is then reduced in the soaking zone and cooling zone maintained in a reducing atmosphere. Occur. When spongy reduced iron is generated, it wraps around a hearth roll or the like and a hearth roll is generated, which causes the surface appearance of the steel sheet to deteriorate. For this reason, the CGL 10 to which the present invention is applied is intended for one that does not have a non-oxidation furnace. As the CGL 10 that is the subject of the present invention, an all-radiant system in which the reduction furnace 3 is all heated by a radiant tube is effective. As another heating method of the steel plate, CGL adopting a direct heating method in which a frame burner is directly applied to the steel plate is also effective and applicable. This is because, in the direct heating method, when the burner is directly applied to the steel plate, it is set so that the reducing flame hits the steel plate, so that excessive scale is not generated even when heated.

  FIG. 6 is a flowchart showing an embodiment of the present invention. As shown in FIG. 6, the manufacturing method of the galvannealed steel sheet of the present invention includes an analysis step (S1), a determination step (S2), a dew point control step (S3), an annealing step (S4), It has a hot dip galvanizing step (S5) and an alloying step (S6), and the annealing step (S4) has a heating zone step (S41) and a soaking zone step (S42).

  The analysis step (hereinafter referred to as “S1”) is a step of analyzing the composition of the steel sheet 1 to be annealed, and the analysis result in S1 is used in the determination step. S1 can be made into the form which analyzes the composition of the steel plate 1 using the ladle value at the time of steelmaking, for example.

  The determination step (hereinafter referred to as “S2”) is a step of determining whether or not the solid solution Ti amount Ti * represented by the above formula (1) is Ti *> 0.00. If a negative determination is made in S2, the steel sheet 1 is considered to have a composition that is substantially free of solute Ti. Therefore, if a negative determination is made in S2, for example, a gas with a high dew point is allowed to flow into the heating zone 3b without performing dew point control for the purpose of suppressing streak defects caused by the presence of solid solution Ti. Without proceeding to the annealing step (S4). By not allowing the high dew point gas (humidified gas) to flow into the heating zone 3b, it is possible to suppress the generation of the hearth roll soot and to reduce the manufacturing cost. On the other hand, when an affirmative determination is made in S2, the steel sheet 1 is considered to have a composition in which solute Ti is present, and therefore the dew point control step is subsequently performed.

  The dew point control step (hereinafter referred to as “S3”) is a step of setting the dew point of the reducing atmosphere of the heating zone 3b to −35 ° C. or more and −5 ° C. or less before the steel sheet reaches the heating zone 3b. From the viewpoint of making the surface quality of the alloyed hot-dip galvanized steel sheet easy to improve, etc., S3 sets the dew point of the reducing atmosphere in the heating zone to −35 ° C. or more and −5 ° C. before the steel plate reaches the heating zone 3b. Preferably, the dew point after the cooling zone 3d is set to −35 ° C. or lower until the steel sheet reaches the cooling zone 3d.

  A predetermined time (for example, about 5 minutes) from the start of the process for setting the dew point to −35 ° C. or more and −5 ° C. or less (hereinafter referred to as “dew point control process”) until the target dew point is reached. Cost. Therefore, instead of waiting the steel plate immediately before the reduction furnace 3 until the dew point control processing is completed, the steel plate is allowed to enter the reduction furnace 3 that has been previously set to the target dew point in S3 with zero waiting time (or By rapidly performing the dew point control process at the same time as the steel sheet enters the heating zone 3a), it becomes possible to improve the productivity of the alloyed hot-dip galvanized steel sheet with suppressed generation of streak defects. In addition, production of an alloyed hot-dip galvanized steel sheet in which generation of streak defects is suppressed by continuously flowing a steel sheet having a Ti * value greater than 0.00 to the reduction furnace 3 that has been previously set to the target dew point in S3. It becomes easy to improve the property.

  In the present invention, the form of the dew point control process in S3 is not particularly limited, but it is preferable to control the dew point by flowing a humidified gas such as humidified nitrogen from the heating zone 3b. As described above, in order to reduce the occurrence of streak pattern defects, it is effective to quickly precipitate solute Ti on the surface of the steel sheet as a Ti-based oxide. For this reason, at least the reducing atmosphere in the heating zone 3b It is preferable to adjust the dew point, preferably the dew point of the reducing atmosphere in the heating zone 3a, the heating zone 3b, the soaking zone 3c, and the cooling zone 3d. Here, in the CGL 10, the gas in the reduction furnace 3 flows in a direction from the snout 5 toward the heating zone 3a due to the furnace pressure. Therefore, behind the zone where the dew point needs to be adjusted (for example, behind the heating zone 3b (specifically downstream in the moving direction of the steel plate, the same applies hereinafter)), or behind the heating zone 3b and the cooling zone. It is possible to easily adjust the dew point of the heating zone 3b and the like by flowing the humidified gas from behind 3d and the like. When S3 is a form which controls dew points, such as heating zone 3b, by letting humid gas flow in, S3 throws in humidification gas to heating zone 3b at a time, for example, when a steel plate moves to heating zone 3a. If the dew point of the heating zone 3b is monitored to confirm that the dew point of the heating zone 3b has reached the target dew point, the amount of humidified gas input is reduced to maintain the dew point of the heating zone 3b. it can.

  In the present invention, the reducing atmosphere in the reduction furnace 3 may contain hydrogen along with nitrogen. From the viewpoint of making it possible to easily maintain a reducing atmosphere, the hydrogen content is preferably 1% by mass or more. By maintaining the reducing atmosphere, it is possible to suppress oxidation of the steel sheet surface. When the oxidation of the steel sheet surface is suppressed, the generation of hearth rolls due to the formation of a sponge-like reduced iron layer due to scale formation is suppressed, so the surface properties of the galvannealed steel sheet are reduced. It becomes easy to improve. Note that generation of a scale is not preferable because it causes non-plating due to repelling of hot dip galvanizing in the hot dip galvanizing process and poor plating adhesion after the alloying process. Moreover, it is preferable that the hydrogen to contain is 60 mass% or less from a viewpoint of suppressing manufacturing cost.

  The annealing step (hereinafter referred to as “S4”) is a step of processing the steel sheet in the reduction furnace 3. S4 is a heating zone 3b in which the dew point of the reducing atmosphere is set to −35 ° C. or more and −5 ° C. or less to heat the steel plate to the recrystallization temperature or less (for example, 780 ° C.) for 3 seconds or more and 240 seconds or less. It is adjusted so that the dew point of the tropical process (hereinafter referred to as “S41”) and the reducing atmosphere has a lower dew point than the reducing atmosphere in the heating zone 3b (preferably −35 ° C. or lower). The soaking zone 3c heats and holds the steel sheet (hereinafter referred to as “S42”). In addition, S4 is a cooling zone 3d that is adjusted so that the dew point of the reducing atmosphere is lower than the dew point of the reducing atmosphere in the soaking zone 3c (for example, -60 ° C. or higher and −50 ° C. or lower). And a step (not shown) of cooling the steel plate.

  The reason why the lower limit of the dew point in S41 is −35 ° C. is that if it is less than −35 ° C., it is difficult to precipitate the solid solution Ti as a Ti-based oxide. Moreover, the upper limit of the dew point in S41 is set to −5 ° C. When the dew point becomes higher than −5 ° C., a sponge-like reduced iron layer accompanying the scale formation is formed on the steel sheet surface, and the hearth roll is generated. This is because the surface appearance of the steel sheet may be impaired. The reason for heating the steel sheet in S41 is 3 seconds or more because the minimum time required for precipitating solute Ti as an oxide-based precipitate is 3 seconds. On the other hand, the upper limit of the heating time is not limited. Although depending on the equipment length and heating capacity, it is needless to say that a shorter one is preferable from the viewpoint of productivity, and about 240 seconds is sufficient.

  In addition, the upper limit of the steel plate temperature in S41 is defined as the recrystallization temperature because recrystallization starts at a temperature higher than that, and Ti-based precipitates appear after recrystallization. This is because it becomes easy to align in one direction locally, and as a result, a streak defect is likely to occur after the alloying process. Moreover, when the steel sheet is exposed to an environment having a temperature higher than the recrystallization temperature and a high dew point for a long time, a sponge-like reduced iron layer accompanying the scale formation is formed on the steel sheet surface, and the hearth roll There is a possibility that wrinkles occur and the surface appearance of the steel sheet is impaired. Therefore, in this invention, the steel plate temperature in S41 was made into the recrystallization temperature or less. From the same viewpoint, it is preferable that S42 in which the steel plate temperature becomes higher than the recrystallization temperature is a step of heating and holding the steel plate in the soaking zone 3c in which the dew point of the reducing atmosphere is −35 ° C. or less. The time for heating and holding the steel sheet in S42 can be, for example, 1 second or more and 120 seconds or less. In addition, although the recrystallization temperature varies depending on the components in the steel, in the present invention, an ultra-low carbon steel having a C content of 0.01% by mass is assumed as the base steel plate. It is 820 degrees C or less. In addition, the lower limit value of the steel plate temperature in S41 is set to at least 600 ° C. from the viewpoint of making it possible to suppress the occurrence of streak pattern defects by precipitating solute Ti present in the surface layer as a Ti-based oxide.

  In the hot dip galvanizing step (hereinafter referred to as “S5”), after annealing in S4, the steel plate led through the snout 4 is immersed in the zinc pot 5 to perform galvanization (hot galvanizing). It is a process to apply. The steel plate that has been galvanized in S5 is then lifted from the zinc pot 5 and subjected to alloying treatment in the alloying furnace 6 (alloying step (hereinafter referred to as “S6”)). In the present invention, the forms of S5 and S6 are not particularly limited and may be known forms.

  As described above, the method for producing an alloyed hot-dip galvanized steel sheet according to the present invention has, in particular, S4 for processing the steel sheet in the reduction furnace 3 in which the dew point is controlled. Therefore, even if it is a composition in which solute Ti can exist in the steel sheet, it becomes possible to precipitate the solute Ti as a Ti-based oxide in the annealing furnace and suppress the generation of streak pattern defects. It becomes possible to improve the surface properties of the galvannealed steel sheet.

  Next, a steel plate to which the present invention is applied will be described.

<C: 0.01% or less>
From the viewpoint of producing an alloyed hot-dip galvanized steel sheet having good workability, it is preferable to lower the carbon (C) content. When a large amount of carbon is contained, it is necessary to add a large amount of Ti in order to fix the solute C. If a large amount of Ti is added, streak pattern defects are likely to occur, which is not preferable. Therefore, in the present invention, C: 0.01% or less is used from the viewpoint of making it easy to suppress generation of streak pattern defects.

<S: 0.02% or less, N: 0.0050% or less>
Sulfur (S) and nitrogen (N) are elements that fix solute Ti as precipitates, and greatly affect fluctuations in the amount of solute Ti. On the other hand, from the viewpoint of producing an alloyed hot-dip galvanized steel sheet having good workability, it is preferable to lower the S content and the N content. Therefore, in the present invention, S: 0.02% or less, N: 0.0050% or less.

<Ti: 0.01% or more and 0.10% or less>
Titanium (Ti) is an element that affects the amount of dissolved Ti. Ti is an element required to increase the ratio (hereinafter referred to as “r value”) of the strain in the plate width direction of the steel plate and the strain in the plate thickness direction of the steel plate, which occurs when the steel plate is strained. The content should be such that C and N in the steel can be sufficiently fixed. Therefore, it is 0.01% or more. On the other hand, if excessive Ti is added, it will exist as solute Ti, and the effect of fixing C and N by Ti will be saturated, and the amount of solute Ti will increase, which will be disadvantageous for muscle generation. Therefore, the upper limit is made 0.10%.

<Ti * = Ti−48 × (N / 14 + S / 32 + C / 12)> 0.00 Formula (1)>
In this invention, the said Formula (1) was employ | adopted as a parameter | index showing the amount of solid solution Ti which becomes the cause of generation | occurrence | production of the streak pattern defect of an galvannealed steel plate. In Ti * ≦ 0.00, since solid solution Ti does not substantially exist, it is considered that uneven distribution of solid solution Ti that becomes a starting point of a streak defect after the alloying treatment does not occur. That is, in order to prevent the occurrence of streak defects by applying the present invention, it is a steel plate having a composition in which solute Ti can exist, in other words, Ti * represented by the above formula (1). Must be greater than 0.00. On the other hand, when Ti * ≦ 0.00, it is not necessary to control the dew point for the purpose of preventing the occurrence of streak defects. However, from the standpoint of making the form capable of preventing the occurrence of hearth roll soot, when Ti * ≦ 0.00, at least the dew point after the cooling zone is lowered (for example, the dew point is set to less than −35 ° C.). .) Is preferred.

  The present invention aims to prevent and suppress the occurrence of streak defects when a steel plate that can contain solute Ti is used as a base material, and has excellent surface properties that can withstand the use of exterior panels for automobiles. It aims at manufacturing the galvannealed steel plate which has this. Therefore, as long as it has various properties such as workability required for use in such applications, elements other than those described above may be contained in the base steel plate. In addition to the above elements, P, Si, Mn, Nb, and Al may further be contained in view of making it possible to produce an alloyed hot-dip galvanized steel sheet having excellent mechanical properties. Hereinafter, these elements will be described.

<P: 0.04% or less>
Phosphorus (P) is an element contained as an impurity, but has the effect of increasing the strength of the steel sheet by solid solution strengthening while suppressing a decrease in the r value, and can therefore be contained for the purpose of improving the strength. However, if the P content exceeds 0.04%, the alloying processability may be lowered to lower the plating adhesion, or the plating surface may have a streak pattern due to P segregation. For this reason, the P content is set to 0.04% or less. Although it does not specifically limit about a minimum, Since it will become a cost increase factor when trying to reduce content significantly, it is preferable to set it as 0.005% or more.

<Si: 0.05% or less>
Silicon (Si) is an element contained as an impurity, and has an effect of increasing the strength of the steel sheet by solid solution strengthening. However, if the Si content exceeds 0.05%, the wettability with the plating is insufficient, and non-plating or the like may occur, leading to a reduction in quality. For this reason, Si content shall be 0.05% or less. When not aiming at solid solution strengthening by Si, it is preferable to make it 0.02% or less.

<Mn: 0.5% or less>
Mn has the effect of increasing the strength of the steel sheet by solid solution strengthening. However, if the Mn content exceeds 0.5%, the yield stress increases, the elongation deteriorates, and cracks and cracks are likely to occur during processing. For this reason, Mn content shall be 0.5% or less. In order to further improve the moldability, the Mn content is preferably 0.3% or less.

<Nb: 0.005% or more and 0.06% or less>
Nb combines with C in the same way as Ti to produce fine NbC precipitates, and improves the mechanical properties, particularly the r value. When this effect is expected, the Nb content is preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.06%, Nb becomes excessive as compared with C. Therefore, due to the solid solution Nb, the yield stress increases and the elongation decreases and wrinkles are likely to occur during processing. Moreover, since the r value in the 45 ° direction increases, anisotropy increases. For this reason, Nb content shall be 0.005% or more and 0.06% or less.

<Al: 0.01% or more and 0.08% or less>
Al is added for deoxidation. It is preferable to contain 0.01% or more. On the other hand, the deoxidation effect by Al is saturated at a content of 0.08%, and inclusion defects increase when exceeding that. Therefore, the Al content is 0.01% or more and 0.08% or less.

  The present invention will be further described with reference to examples.

1. Experiment 1
To the CGL having a DFF furnace as the pre-stage heating furnace, controlled so that the steel plate temperature on the DFF furnace exit side matches the steel plate temperature in the heating zone 3a, and having a low temperature holding zone downstream of the cooling zone, An alloyed hot-dip galvanized steel sheet was produced by passing a steel sheet having the composition shown in Table 4. And according to the following rating standard, the condition of the streaks pattern defect of the manufactured galvannealed steel sheet was evaluated as a pass (◯) for a rating of 2 or less and a reject (x) for a rating of 3 or more. The results are shown in FIG. The vertical axis in FIG. 7 is the dew point of the heating zone 3b (dew point of the reducing atmosphere in the heating zone 3b) [° C.], and the horizontal axis is the steel plate temperature [° C.] on the exit side of the heating zone 3a. Note that Ti * in Table 4 below is the amount of solid solution Ti represented by the above formula (1).
<Rating criteria>
5: Remarkable streak defects occur over the entire length in the coil longitudinal direction (failed)
4: Remarkable streak defects occur in 1/4 or less of the coil longitudinal direction (fail)
3: A light streak defect occurs in 1/4 or less of the coil longitudinal direction and / or a significant streak defect occurs in 1/16 or less of the coil longitudinal direction (fail)
2: Light streak defect occurs in 1/16 or less of the coil longitudinal direction (pass)
1: No streak defect occurs over the entire length in the coil longitudinal direction (pass)

The operating conditions of CGL10 in this experiment were as follows.
<Operating conditions>
Steel plate temperature on the exit side of the heating zone 3a: 500 ° C, 600 ° C, 700 ° C, 820 ° C, 860 ° C
Dew point of reducing atmosphere in heating zone 3b: -45 ° C, -35 ° C, -15 ° C
Steel plate temperature on the soaking zone 3c: 860 ° C
Dew point of reducing atmosphere on the exit side of the soaking zone 3c: -40 ° C
Dew point of reducing atmosphere on the exit side of the cooling zone 3d: −50 ° C.
Time for holding steel plate in heating zone 3a: 30 seconds Time for holding steel plate in heating zone 3b: 60 seconds Time for holding steel plate in soaking zone 3c: 30 seconds Time for holding steel plate in cooling zone 3d: 30 seconds Time held in the low temperature holding zone: 30 seconds Hydrogen content in the reducing atmosphere of the reducing furnace 3: 3 mass% or more and 15 mass% or less Steel sheet temperature when the zinc pot 5 enters: 463 ° C.
Hot dip galvanizing bath temperature of zinc pot 5: 460 ° C
Al content of hot dip galvanizing bath of zinc pot 5: 0.14% by mass
Steel plate temperature on the outlet side of the alloying furnace 6: 550 ° C

  As shown in FIG. 7, the steel plate temperature on the exit side of the heating zone 3a and the dew point of the reducing atmosphere in the heating zone 3b affected the occurrence of streak defects. In this experiment, the dew point of the reducing atmosphere in the heating zone 3b is -35 ° C or -15 ° C where the Ti-based oxide is deposited, and the steel plate temperature on the exit side of the heating zone 3a is 600 ° C or higher and 820 ° C or lower. By doing so, it was possible to suppress the occurrence of a streak pattern defect and to ensure a surface property at an acceptable level. On the other hand, when the dew point of the reducing atmosphere in the heating zone 3b is set to −45 ° C. at which the Ti-based oxide does not precipitate, generation of streak pattern defects cannot be suppressed, and an acceptable level surface property is ensured. I couldn't. Moreover, when the steel plate temperature on the heating zone 3a exit side is too low at 500 ° C., the dew point is lowered before the solid solution Ti of the steel plate surface layer changes to the Ti-based oxide, so the solid solution Ti is present on the steel plate surface layer. As a result of remaining, it is considered that a streak defect occurs after the alloying process. In addition, when the steel plate temperature on the exit side of the heating zone 3a is set to 860 ° C. which exceeds the recrystallization temperature, the streak defect does not occur, so the evaluation is ○, but the steel plate is held for 60 seconds in such a temperature environment. As a result, a spongy reduced iron layer accompanying the scale formation was formed on the surface of the steel sheet, and there was a case in which a hearth roll was generated.

2. Experiment 2
Since the generation of the streak pattern defect is greatly influenced by the surface state of the base steel plate, it may be difficult to accurately specify the conditions for suppressing the generation of the streak pattern defect only by experiments of several coils. Therefore, the effect of the present invention was confirmed by flowing 3575 coils through the same CGL as that used in Experiment 1. In this experiment, the rating of the streak pattern defect was performed based on the same criteria as in Experiment 1 above. Based on the rating, the state of the streak defect is evaluated for each of the front and back surfaces of the steel sheet, and a coil having an average rating of 2.0 or less on the front and back surfaces passes (○), and the coil exceeds 2.0. Was rejected (●). The results are shown in FIG. Table 5 shows the composition range of the coils used in this experiment. In Table 5, Ti * is the amount of solid solution Ti represented by the above formula (1).

The operating conditions of CGL in this experiment were as follows. The operating conditions not described below were set the same as in Experiment 1 above.
<Operating conditions>
Steel plate temperature on heating zone 3a exit side: 600 ° C. or higher and 820 ° C. or lower Dew point of reducing atmosphere in heating zone 3b: −53 ° C. or higher and −16 ° C. or lower Hydrogen content in reducing atmosphere of reducing furnace 3: 1% by mass or higher 20 Mass% or less

  As shown in FIG. 8, depending on the surface state of the base steel plate, even when the base steel plate satisfies Ti *> 0.00, the dew point of the reducing atmosphere in the heating zone 3b may be set to less than −35 ° C. Although a streak pattern defect may not occur, at least when the base material steel plate satisfies Ti *> 0.00, the dew point of the reducing atmosphere in the heating zone 3b is −35 ° C. or higher (in this experiment, −35 ° C. or higher). -16 ° C or lower), it is possible to keep the state of the streak defect on the steel sheet surface that has undergone the alloying process at an acceptable level regardless of the surface condition of the base steel sheet that satisfies Ti *> 0.00. there were.

  The present invention can be used, for example, when producing an alloyed hot-dip galvanized steel sheet used as an automotive exterior panel or the like.

S1 ... Analysis step S2 ... Judgment step S3 ... Dew point control step S4 ... Annealing step S41 ... Heating zone step S42 ... Soaking step S5 ... Hot dip galvanizing step S6 ... Alloying step 1 ... Steel plate 2 ... Pre-tropical zone 3 ... Reduction furnace 3a 3b ... Heating zone 3c ... Soaking zone 3d ... Cooling zone 4 ... Snout 5 ... Zinc pot 6 ... Alloying furnace 10 ... Continuous hot dip galvanizing line (continuous hot dip galvanizing equipment)

Claims (4)

An alloying process comprising: an annealing process for treating a steel sheet in a reduction furnace; a hot-dip galvanizing process for galvanizing the annealed steel sheet; and an alloying process for alloying the galvanized steel sheet . A method for producing a hot dip galvanized steel sheet,
In the reduction furnace, a heating zone, a soaking zone, and a cooling zone, and if necessary, a low temperature holding zone, are provided in order in the moving direction of the steel plate moving in the reduction furnace,
An analysis step for analyzing the composition of the steel sheet to be annealed;
The determination step of determining whether or not the solid solution Ti amount Ti * represented by the following formula (1) of the steel sheet whose composition has been analyzed in the analysis step is Ti *> 0.00, and
When it is determined that Ti *> 0.00 in the determination step, the dew point of the reducing atmosphere in the region of at least 600 ° C. or higher of the heating zone is −35 ° C. before the steel sheet reaches the heating zone. A dew point control step of −5 ° C. or less is provided before the annealing step,
The step of heating the steel plate for 3 seconds or more in the heating zone in which the dew point of the reducing atmosphere is −35 ° C. or more and −5 ° C. or less when the temperature of the steel plate is at least 600 ° C. or more and the recrystallization temperature or less is the annealing. A method for producing an alloyed hot-dip galvanized steel sheet, which is included in the process.
Ti * = Ti−48 × (N / 14 + S / 32 + C / 12) Formula (1)
However, in said Formula (1), Ti is titanium content (mass%) of the said steel plate, N is nitrogen content (mass%) of the said steel plate, S is sulfur content (mass%) of the said steel plate, C is It is carbon content (mass%) of the said steel plate. The alloyed hot-dip galvanized steel sheet according to claim 1 , wherein the dew point of the reducing atmosphere in the reduction furnace is controlled by flowing humidified nitrogen after the heating zone and before the end of the soaking zone. Manufacturing method. The method for producing an galvannealed steel sheet according to claim 1 or 2, wherein the reducing atmosphere in the reducing furnace has a hydrogen concentration of 1% by mass or more and 60% by mass or less . The steel sheet contains, by mass%, C: 0.01% or less, S: 0.02% or less, N: 0.0050% or less, and Ti: 0.01% or more and 0.10% or less. The manufacturing method of the galvannealed steel plate of any one of Claims 1-3 characterized by these.