JP2019167576A - Method for manufacturing hot-dip galvanized steel sheet - Google Patents

Abstract

To provide a method for manufacturing a hot-dip galvanized steel sheet, capable of preventing alloying unevenness from occurring even in a situation that an edge center difference occurs in the thickness of a grain boundary oxidation layer to obtain excellent appearance.SOLUTION: The method for manufacturing a hot-dip galvanized steel sheet sequentially includes the following (a)-(d) steps using a steel sheet having a Si content of 1.0 mass% or more as a plating original sheet, (a) the step of hot-rolling a steel sheet having a Si content of 1.0 mass% or more at a winding temperature of 600°C or more; (b) the step of acid-cleaning the hot rolled steel sheet to remove a surface oxide layer and cold-rolling the steel sheet; (c) the step of subjecting the cold-rolled steel sheet to annealing and hot-dip galvanizing; and (d) the step of subjecting the hot-dip galvanized layer to alloying by an induction type heating furnace. The thickness of a grain boundary oxidation layer in the cold-rolled steel sheet obtained in the following (b) step is 4 μm or more in a central portion of the cold-rolled steel sheet in the width direction and is less than 4 μm in a region to 50 mm from the end of the cold-rolled steel sheet in the width direction.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the original plating plate.

  Steel sheets used for structural members of automobiles are required to have higher strength in order to improve fuel efficiency. In addition, when applied to structural members of automobiles, an alloyed hot dip galvanized layer is also formed on the surface of the plating original plate from the viewpoint of improving the durability of the automobile body.

  In general, a high-strength steel sheet having an increased strength by increasing the content of alloy elements such as Si is used for the plating original sheet on which the alloyed hot-dip galvanized layer is formed. However, when the content of Si, which is an oxide-forming element, is increased to 1.0% by mass or more, an oxide layer (Si-containing oxide layer) is easily formed on the surface of the steel sheet when the cold-rolled steel sheet is annealed. Since the physical layer inhibits the plating wettability, “non-plating” and “alloying unevenness” occur when an alloyed hot-dip galvanized steel sheet is produced.

  On the other hand, it is also known that a grain boundary oxide layer is formed in the vicinity of the steel sheet surface when the hot rolling coiling temperature is 600 ° C. or higher for a steel sheet having a Si content of 1.0 mass% or higher. Yes. This grain boundary oxide layer is formed by oxidizing Si inside the steel plate. When the grain boundary oxide layer is formed, Si forms an oxide along the grain boundary inside the steel sheet during annealing of the cold-rolled steel sheet after cold rolling, and the formation of the oxide layer on the steel sheet surface It is suppressed. As a result, the occurrence of “non-plating” and “alloying unevenness” due to the presence of the oxide layer is suppressed.

  The grain boundary oxide layer generated at the time of hot rolling is a region from the end of the width direction of the steel sheet (direction perpendicular to the rolling direction; the same applies hereinafter) to 50 mm (hereinafter referred to as “the vicinity of the width direction edge portion”). There is a difference in thickness between the width direction center portion of the steel sheet (hereinafter sometimes referred to as “width direction center portion”). The thickness of the grain boundary oxide layer (depth from the steel sheet surface: the same applies hereinafter) gradually decreases from the width direction center part to the width direction both end parts. Even in a cold-rolled steel sheet after pickling and cold rolling, it is usually 4 μm or more at the center part in the width direction of the steel sheet, and it remains below 4 μm near the edge part in the width direction of the steel sheet. Hereinafter, the difference in the thickness of the grain boundary oxide layer generated between the width direction center portion of the steel plate and the vicinity of the width direction edge portion of the steel plate may be referred to as “edge / center difference”.

  The above phenomenon occurs because the cooling rate at the time of hot rolling is different between the vicinity of the width direction edge portion and the width direction center portion, and this difference in cooling rate is the edge center difference as described above. It is estimated that it appears.

  If there is an edge / center difference in the thickness of the grain boundary oxide layer, after alloying the plating layer, the degree of progress of alloying differs between the vicinity of the edge in the width direction of the steel sheet and the center part in the width direction of the steel sheet, When viewed in the entire width direction of the steel sheet, uneven alloying occurs. In particular, alloying unevenness is likely to occur in a region where the thickness of the grain boundary oxide layer is less than 4 μm.

  For these reasons, it becomes difficult to perform proper alloying in the entire width direction of the steel sheet. For example, when the center portion of the steel plate is appropriately alloyed, the vicinity of the edge portion in the width direction of the steel plate becomes insufficiently alloyed (Fe content in the plating layer becomes low), and uneven alloying occurs. Moreover, when the width direction edge part of a steel plate is appropriately alloyed, the width direction center part of a steel plate is excessively alloyed (the Γ phase formation amount in a plating layer increases and Fe content becomes high). There arises a problem that the powdering resistance of the alloyed hot-dip galvanized layer is lowered.

  As a method for correcting the inconvenience due to the difference between the edge and the center as described above, there is a method of cutting the vicinity of the edge in the width direction in which alloying unevenness is likely to occur at the stage after hot rolling winding. The yield of steel sheet products decreases.

  As a method for suppressing the formation of the grain boundary oxide layer, it is conceivable to set the coiling temperature in hot rolling to less than 600 ° C. However, in a steel sheet having a Si content of 1.0 mass% or more, it is not effective during plating. Plating, uneven alloying, etc. occur, leading to poor appearance of the galvannealed steel sheet. In addition, the strength increases at the entire length of the hot-rolled steel sheet coil, and the strength becomes very high particularly at the leading end of the coil. Therefore, the leading end is required to be cut off, resulting in a decrease in yield.

  Under these circumstances, when a steel sheet with an Si content of 1.0% by mass or more and an edge-center difference in the thickness of the grain boundary oxide layer is used as the plating base plate, proper alloying without uneven alloying is achieved. It is difficult to process.

Various techniques have been proposed so far for performing appropriate alloying processing without unevenness in alloying. As such a technique, for example, Patent Document 1 describes, “Hot-rolling slab heating temperature satisfying a predetermined component composition: 1170 ° C. or less, hot-rolling temperature: 600 ° C. or less, and cold-cooling as necessary. After the hot rolling, in the continuous hot dip galvanizing line, annealing is performed at a temperature not lower than the Ac ₁ transformation point and not higher than the Ac ₃ transformation point, and then plating is performed. Proposed method of manufacturing a high-strength galvannealed steel sheet that is alloyed to a temperature of 300 ° C and cooled to a temperature of 300 ° C at a cooling rate of 10 ° C / sec or more after the surface molten zinc layer disappears " Has been.

  In this technology, the heating during the alloying process is performed by induction heating, and the transformation of austenite to pearlite during the cooling after annealing in the continuous hot dip galvanizing line is performed using the local heating of the surface layer of the plating plate. In addition to minimizing as much as possible, a film with excellent powdering resistance and homogeneity of the alloy phase is obtained. And by manufacturing on the above conditions, the high intensity | strength galvannealed steel plate excellent in the strength-ductility balance and the film | membrane characteristic is obtained.

  On the other hand, Patent Document 2 defines “the hot slab heating temperature Ts in relation to the Si content [% Si] as Ts (° C.) ≦ 1190−67 [% Si] and the hot rolling coiling temperature. After hot rolling under conditions of 600 ° C. or less, pickling the hot-rolled steel sheet, and cold rolling as necessary, in a continuous hot-dip galvanizing line, a zinc bath having an Al content in the bath of 0.15 wt% or less In the induction heating type alloying furnace, the alloying heat treatment is performed so that the furnace outlet side plate temperature becomes 450 to 550 ° C., and the surface of the molten zinc layer disappears until the temperature reaches 300 ° C. up to 10 ° C. A method for producing a high-strength galvannealed steel sheet that is cooled at a cooling rate of at least / sec has been proposed.

  This technology makes it possible to produce high-strength alloyed hot-dip galvanized steel sheets with excellent strength-ductility balance, excellent surface appearance and powdering resistance using existing continuous hot-dip galvanizing line equipment It was made from the viewpoint. And the desired effect is acquired by manufacturing on the above conditions.

Japanese Patent No. 2565038 Japanese Patent No. 3097232

  In the induction heating type alloying furnace, both of the techniques of Patent Documents 1 and 2 are assumed to use a steel plate having a Si content of less than 1.0% by mass as a plating base plate, and further, grain boundary oxidation. It is manufactured at a coiling temperature of 600 ° C. or less where the layer is difficult to form. Therefore, in these techniques, it is not assumed that a steel sheet having a Si content of 1.0 mass% or more is used as a plating original sheet, and an edge-center difference in the grain boundary oxide layer thickness may occur. There is no technology.

  For this reason, even when a steel plate having a Si content of 1.0% by mass or more is used as a plating original plate and an edge / center difference occurs in the thickness of the grain boundary oxide layer, uneven alloying occurs. In fact, it is desirable to establish a technology that can suppress the occurrence of this.

  The present invention has been made in view of the above circumstances, and its purpose is to use a steel plate having a Si content of 1.0% by mass or more as a plating original plate, and to reduce the thickness of the grain boundary oxide layer to An object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet capable of preventing the occurrence of alloying unevenness and obtaining a good appearance even under a situation where a center difference occurs.

The production method of the present invention that has solved the above problems is a method of producing an alloyed hot-dip galvanized steel sheet,
Grain boundary oxidation in a cold-rolled steel sheet obtained by using the steel sheet having a Si content of 1.0% by mass or more as the plating original sheet and sequentially including the following steps (a) to (d) and obtained in the following step (b): The thickness of the layer is 4 μm or more at the center in the width direction of the cold-rolled steel sheet, and is less than 4 μm in the region from the end in the width direction of the cold-rolled steel sheet to 50 mm.
(A) a step of hot rolling a steel sheet having a Si content of 1.0% by mass or more at a coiling temperature of 600 ° C. or more;
(B) a step of cold rolling after pickling the hot-rolled steel sheet to remove the surface oxide layer,
(C) A step of performing annealing and hot dip galvanizing treatment on the cold-rolled steel sheet,
(D) A step of alloying the hot-dip galvanized layer in an induction heating type heating furnace.

  As an embodiment of the present invention, it is preferable that the annealing and hot dipping treatment of (c) and the alloying treatment of (d) are performed in a continuous hot dipping galvanizing line.

  Moreover, in the manufacturing method of this invention, it is preferable that the surface temperature of the said cold-rolled steel sheet when implementing the alloying process of said (d) shall be less than 450 degreeC.

  The present invention is configured as described above, and uses a steel sheet having a Si content of 1.0% by mass or more, under a situation where an edge-center difference occurs in the thickness of the grain boundary oxide layer. In addition, an alloyed hot-dip galvanized steel sheet capable of preventing the occurrence of uneven alloying and obtaining a good appearance can be produced.

  In a steel sheet having a Si content of 1.0% by mass or more, a grain boundary oxide layer is formed when hot rolling is performed so that the coiling temperature is 600 ° C. or more. The edge-center difference as described above tends to occur. In order to prevent such an edge-center difference from occurring, another process is required, and there are restrictions on equipment, resulting in an increase in manufacturing cost.

  The present inventors use a steel plate having a Si content of 1.0 mass% or more as a plating original plate, and even in a situation where an edge-center difference occurs in the thickness of the grain boundary oxide layer, alloying is performed. Manufacturing conditions for obtaining an alloyed hot-dip galvanized steel sheet capable of preventing occurrence of unevenness and obtaining a good appearance were examined from various angles.

As a result, if an alloyed hot-dip galvanized steel sheet is manufactured by sequentially including the following steps (a) to (d), alloyed hot-dip galvanizing that can prevent occurrence of unevenness of alloying and obtain a good appearance It discovered that a steel plate was obtained and completed this invention.
(A) a step of hot rolling a steel sheet having a Si content of 1.0% by mass or more at a coiling temperature of 600 ° C. or more;
(B) a step of cold rolling after pickling the hot-rolled steel sheet to remove the surface oxide layer,
(C) A step of performing annealing and hot dip galvanizing treatment on the cold-rolled steel sheet,
(D) A step of alloying the hot-dip galvanized layer in an induction heating type heating furnace.

  The respective steps (a) to (d) will be described.

[Step (a)]
In the step (a), a steel sheet having a Si content of 1.0% by mass or more is hot-rolled at a coiling temperature of 600 ° C. or more. If a steel sheet having a Si content of 1.0% by mass or more is hot-rolled at a coiling temperature of 600 ° C. or higher, a grain boundary oxide layer is formed, and the thickness of the grain boundary oxide layer is set to the above-described thickness. Such edge-center difference is likely to occur.

  That is, in the step (a), hot rolling is performed in a situation where the edge-center difference is likely to occur. For that purpose, it is necessary to perform hot rolling at a coiling temperature of 600 ° C. or higher. However, if the coiling temperature becomes too high, the equipment life is expected to be shortened, so the upper limit is below the hot rolling temperature. is there.

[Step (b)]
In the step (b), the hot-rolled steel sheet obtained in the step (a) is pickled to remove the surface oxide layer, and then cold-rolled. The thickness of the grain boundary oxide layer formed in the step (a) is 4 μm at the center in the width direction of the cold-rolled steel plate at the cold-rolled steel plate stage obtained by cold rolling in the step (b). As described above, in the vicinity of the edge portion in the width direction of the cold-rolled steel sheet, it is less than 4 μm, and an edge-center difference is generated.

[Step (c)]
In the step (c), annealing and hot dip galvanizing are performed on the cold-rolled steel sheet having the edge-center difference as described above. When the cold-rolled steel sheet is annealed, due to the presence of the grain boundary oxide layer, Si forms an oxide along the crystal grain boundary inside the steel sheet, and the formation of the oxide layer on the steel sheet surface is suppressed. As a result, the occurrence of “non-plating” and “alloying unevenness” due to the presence of the oxide layer is suppressed. However, in the subsequent alloying of the hot dip galvanized layer, if it is heated in a normal heating furnace, “alloying unevenness” due to the edge-center difference will occur.

[Step (d)]
In the step (d), the galvanized layer is alloyed in an induction heating type heating furnace. When alloying the hot dip galvanized layer, heating by an induction heating furnace can prevent occurrence of “alloying unevenness” due to edge-center difference. Hereinafter, the induction heating furnace may be referred to as an “IH (Induction Heating) heater”.

  Here, “alloying unevenness” means that the difference in Fe concentration in the plating layer is greater than 3.0 mass% in the vicinity of the width direction center portion and the width direction edge portion. As described above, the vicinity in the width direction edge portion means a region from the end in the width direction (direction perpendicular to the rolling direction) of the steel plate to 50 mm. It is as follows. That is, the region from the end portion to 50 mm is a region where alloying unevenness is likely to occur (that is, the region where the thickness of the grain boundary oxide layer is less than 4 μm), and the end portion is more than the position 50 mm from the end portion. The grain boundary oxide layer becomes thinner (ie, the “edge / center difference” becomes larger). By comparing this area with the center in the width direction, the occurrence of unevenness in alloying occurs. It is because it can grasp.

  Moreover, the width direction center part means the area | region to 200 mm from the center of the said steel plate width direction toward both edge parts, respectively. Alternatively, when the plate width is narrow, it may be a region from the center in the steel plate width direction to the 40% position of the plate width toward both edge portions. In short, the width direction edge part vicinity and the width direction center part should just be in the state which does not overlap in a region within the range of the width of a steel plate.

  In the present invention, if an IH heater is used for alloying the hot dip galvanized layer [step (d)], not all of the reasons why the above alloying unevenness is eliminated are clarified. Perhaps I could think of it as follows. That is, by heating with an IH heater, the interface between the plating original plate (cold rolled steel plate) and the hot dip galvanized layer is efficiently heated, and the temperature of the hot dip galvanized layer becomes higher than the surface temperature of the steel plate. It is presumed that the alloying effectively proceeds even in the vicinity of the edge portion where the alloying is unlikely to proceed.

  On the other hand, when alloying the hot dip galvanized layer, when using an infrared heating furnace (hereinafter sometimes referred to as “infrared heater”), the center portion in the width direction of the steel sheet is excessively alloyed. As a result, the plating quality deteriorates. In the infrared heater, alloying proceeds by radiation from the plating surface, and particularly, the alloying proceeds excessively at the center portion in the width direction where the grain boundary oxide layer is easily heated.

  In the step (d), the surface temperature of the cold-rolled steel sheet when performing the alloying treatment of the hot-dip galvanized layer (hereinafter, sometimes referred to as “plate temperature”) is not particularly limited. From the viewpoint of suppressing excessive alloying when the grain boundary oxide layer is particularly thick, the plate temperature is preferably less than 450 ° C. More preferably, it is 430 degrees C or less, More preferably, it is 410 degrees C or less.

  When the plate temperature is 450 ° C. or higher, when the grain boundary oxide layer is thick, the alloying progresses between the IH heater and the subsequent cooling zone due to the latent heat during the alloying process, so the Fe concentration in the plating layer May become too high and the powdering resistance may deteriorate. However, if the plate temperature at the time of alloying is too low, the progress of alloying slows down and the productivity is lowered.

  Although the steel plate used as a plating original plate by this invention should just be a normal high-tensile steel plate, Si content needs to be 1.0 mass% or more as a steel plate which achieves higher intensity | strength. The upper limit of Si content is approximately 2.7% by mass or less. Moreover, about components, such as C and Mn, it does not limit at all, Usually, C: About 0.05-0.5 mass% and Mn: About 1.6-4.0 mass% should just contain. . It is also acceptable to contain inevitable impurities such as P, S, N, and O.

About the kind of hot dip galvanization formed in the plating original plate surface, an alloy element may be included in a plating layer. The galvanized layer is coated on one side or both sides of the steel plate. The adhesion amount of hot dip galvanizing is, for example, about 30 to 120 g / m ² per side.

  As described above, in the present invention, a steel plate having a Si content of 1.0% by mass or more is used as a plating original plate, and the thickness of the grain boundary oxide layer in the cold-rolled steel plate is 4 μm at the center in the width direction of the cold-rolled steel plate. A steel sheet having a Si content of 1.0% by mass or more is used at a coiling temperature of 600 ° C. using a steel sheet that is less than 4 μm in the region from the edge in the width direction of the cold-rolled steel sheet to 50 mm. Step of hot rolling as described above [Step of (a)], Step of pickling hot rolled steel sheet to remove surface oxide layer and then cold rolling [Step of (b)], Cold rolled steel plate In contrast, a step of performing annealing and hot dip galvanizing treatment [the step of (c) above], a step of alloying a hot dip galvanized layer in a heating furnace of an induction heating method [step of the above (d)], Edge center caused by the thickness of grain boundary oxide layer by including sequentially -The inconvenience due to the difference is corrected, and an alloyed hot-dip galvanized steel sheet with reduced unevenness in alloying is obtained.

  In the present invention, the annealing and hot dipping treatment of (c) and the alloying treatment of (d) may be performed in separate independent processes, but these treatments are performed in a continuous hot dip galvanizing line. It is preferable to carry out continuously in a series of steps. By continuously performing annealing, hot dipping treatment and alloying treatment in a continuous hot dip galvanizing line, productivity can be improved and manufacturing costs can be reduced.

In any case, the annealing to the cold-rolled steel sheet is, for example, a reducing atmosphere containing about 1 to 30% hydrogen (the balance is nitrogen), the temperature range from the Ac ₁ transformation point of the steel sheet to about 970 ° C., 10 It is preferable to hold for about 54000 seconds.

  Hereinafter, based on the examples, the effects of the present invention will be described more specifically, but the following examples are not of a nature that limits the present invention, and the design change in accordance with the gist of the above and the following description, Both are included in the technical scope of the present invention.

(1) Test material The cold-rolled steel sheet shown below was used as a plating original sheet.
Component composition of steel sheet: C 0.22% by mass, Si 1.15% by mass, Mn 2.23% by mass (the balance: iron and inevitable impurities such as P, S, N, O)
Shape of steel plate: plate thickness 1.4 mm, plate width 70 mm, length 150 mm (from a coil having a plate width of 1000 mm, steel plate for width direction center portion: cut out in the shape centered on the width center. Steel plate for width direction edge portion. : Cut out in the above shape from the edge.)
Hot rolling coiling temperature: 650 ° C
Thickness of the grain boundary oxide layer: 11.0 μm at the center in the width direction of the cold-rolled steel sheet, 1.1 μm at the position 50 mm from the edge in the width direction of the cold-rolled steel sheet (in the photograph taken using the SEM of the steel plate cross section) Measured)

(2) Experimental procedure The steel plate was set in a hot dip galvanizing simulator and subjected to reduction annealing, hot dip galvanizing treatment and alloying treatment. The conditions are as shown below.

[Reduction annealing conditions]
Annealing atmosphere: N ₂ -5% H ₂
Annealing temperature: 630 ° C
Holding time: 200 seconds

[Hot galvanizing conditions]
Al concentration in the bath: 0.13 mass%
Bath temperature: 460 ° C
Intrusion board temperature: 460 ° C
Immersion time: 4 seconds Amount of hot dip galvanized coating (only on one side): 70 g / m ²

[Alloying conditions]
Atmosphere: Air atmosphere Alloying temperature (plate temperature): (A) 400-460 ° C. when heated by IH heater, (B) 670-700 ° C. when heated by infrared heater
Alloying time: 14 seconds

(3) Evaluation method The alloyed plating layer was dissolved in hydrochloric acid to which hexamethylenetetramine (inhibitor) specified in JIS K 8847 was added, and before and after dissolution based on high frequency inductively coupled plasma (ICP). From the mass change, the plating adhesion amount and the Fe adhesion amount were determined, and the Fe concentration (mass%) in the plating layer was determined from the mass ratio.

  In measuring the Fe concentration (mass%) in the vicinity of the width direction edge portion, in order to improve the measurement accuracy, the region from the width direction edge portion steel plate cut out above to a region of 50 mm from the width direction end portion is included. Thus, the test piece of width 50mm x length 50mm was further cut out and used. Moreover, when measuring the Fe concentration (mass%) in the width direction center part, the test piece of width 50mm x length 50mm was further cut out and used from the steel plate for width direction center parts cut out above. Therefore, the Fe concentration (mass%) in the plating layer determined as described above is an average value for each test piece.

  In the Fe concentration in the plating layer, the difference (mass% difference) between the width direction center portion and the width direction edge portion is 3.0% or less, and the Fe concentration (mass%) is 5% or more and 15% or less. A case was evaluated as “◯”. Moreover, the case where the difference in Fe concentration between the width direction center portion and the width direction edge portion was 3.0% or less and the Fe concentration was more than 15% and 20% or less was evaluated as “Δ”.

  In any case, when the difference in Fe concentration between the width direction center portion and the width direction edge portion was larger than 3.0%, it was evaluated as “x”. If the difference in Fe concentration between the width direction center portion and the width direction edge portion is larger than 3.0%, alloying unevenness occurs only by that.

  On the other hand, the Fe concentration in the plating layer is most preferably in the range of 5 to 15% in terms of plating appearance. If the Fe concentration in the plating layer is less than 5%, alloying is insufficient and the corrosion resistance is poor. When the Fe concentration in the plating layer is more than 15% and 20% or less, the powdering resistance is slightly lowered, but the difference in Fe concentration between the width direction center portion and the width direction edge portion is 3%. If it is 0.0% or less, uneven alloying does not occur, so the product is acceptable (evaluated as “Δ”). On the other hand, when the Fe concentration in the plating layer is more than 20%, the amount of Γ phase formed due to excessive alloying increases and the powdering resistance is remarkably deteriorated, which is not suitable as a product.

  The results are shown in Table 1 (Test Nos. 1 to 5 in Table 1).

  From this result, it can be considered as follows. First, by alloying with an IH heater (Test Nos. 1 to 3), the difference in Fe concentration between the center in the width direction and the edge in the width direction can be reduced to 3.0% or less, and unevenness in alloying can be prevented. It is clear that it can be suppressed (Examples 1 to 3). Among these, especially in the example (test Nos. 2 and 3) in which the temperature during alloying is less than 450 ° C., the Fe concentration in the plating layer is in the range of 5 to 15%, which is most preferable in terms of plating appearance. (Examples 2 and 3).

  On the other hand, in the examples (test Nos. 4 and 5) in which the alloying treatment was performed with the infrared heater, the difference in Fe concentration between the width direction center portion and the width direction edge portion could not be 3.0% or less. It is clear that uneven formation has not been suppressed (Comparative Examples 1 and 2).

Claims (3)

A method for producing an alloyed hot-dip galvanized steel sheet,
Grain boundary oxidation in a cold-rolled steel sheet obtained by using the steel sheet having a Si content of 1.0% by mass or more as the plating original sheet and sequentially including the following steps (a) to (d) and obtained in the following step (b): The thickness of the layer is 4 μm or more at the center in the width direction of the cold-rolled steel sheet, and less than 4 μm in the region from the end in the width direction of the cold-rolled steel sheet to 50 mm. Production method.
(A) a step of hot rolling a steel sheet having a Si content of 1.0% by mass or more at a coiling temperature of 600 ° C. or more;
(B) a step of cold rolling after pickling the hot-rolled steel sheet to remove the surface oxide layer,
(C) A step of performing annealing and hot dip galvanizing treatment on the cold-rolled steel sheet,
(D) A step of alloying the hot-dip galvanized layer in an induction heating type heating furnace.   The method for producing an alloyed hot-dip galvanized steel sheet according to claim 1, wherein the annealing and hot-dipping treatment of (c) and the alloying treatment of (d) are performed in a continuous hot-dip galvanizing line.   The manufacturing method of the galvannealed steel plate of Claim 1 or 2 which makes the surface temperature of the said cold-rolled steel plate when implementing the alloying process of said (d) less than 450 degreeC.