JP2011246744A - Galvannealed cold rolled steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvannealed cold rolled steel sheet having a uniform galvannealed layer which has a good surface property using as a substrate a cold rolled steel sheet containing an easily oxidizable element in a large amount, and to provide a method for producing the same.SOLUTION: The cold rolled steel sheet includes a chemical composition which contains, in mass%, 0.02% or more and 0.25% or less C, 0.01% or more and 2.5% or less Si, 0.5% or more and 3.5% or less Mn, 0.1% or less P, 0.01% or less S, 1.0% or less sol. Al, and 0.01% or less N. In the galvannealed layer, the mass per unit area is 10 g/mor more and 80 g/mor less; the alloying degree is 7 mass% or more and 16 mass% or less; and the distribution of plating thickness satisfies the formula: d/d≤2.5, wherein dis the maximum plating thickness; and dis the average plating thickness.

Description

  The present invention relates to an alloyed hot-dip galvanized cold-rolled steel sheet and a method for producing the same. The present invention has good surface properties even when the cold-rolled steel sheet, which is a base material for galvannealed alloy, contains a large amount of oxidizable elements such as Si, Mn, Al and Cr. The present invention relates to an alloyed hot-dip galvanized cold-rolled steel sheet having a uniform alloyed hot-dip galvanized layer and a method for producing the same.

  In recent years, there has been a demand for improving the fuel efficiency of automobiles to protect the global environment. To reduce the weight of the vehicle body and ensure the safety of passengers, high-strength steel sheets, particularly high-strength hot-dip zinc, have been developed for rust-proof materials. There is a growing need for plated steel sheets.

  A steel plate used for a member for an automobile is not sufficient only with high strength, and must satisfy various performances such as press formability and corrosion resistance. However, an alloyed hot-dip galvanized cold-rolled steel sheet based on a cold-rolled steel sheet containing a large amount of easily oxidizable elements such as Si, Mn, Al and Cr for the purpose of increasing the strength is During the annealing process, an oxide film of the easily oxidizable element is formed on the surface of the steel sheet, and zinc wettability is reduced during hot dip galvanization, and non-plating is likely to occur. Further, even when non-plating does not occur, the oxide film becomes a barrier for diffusion of iron from the steel sheet to the hot dip galvanized layer during the alloying process, making the alloying process extremely difficult. Furthermore, when the oxide film is unevenly formed on the surface of the steel sheet, the alloying speed differs between the part where the oxide film is formed and the part where the oxide film is not formed. Harmful properties.

  For this reason, in alloyed hot-dip galvanized cold-rolled steel sheets based on cold-rolled steel sheets containing a large amount of oxidizable elements such as Si, Mn, Al and Cr, uniform alloying and melting with excellent surface properties It was very difficult to obtain a galvanized film.

  In this regard, several methods for forming an alloyed hot-dip galvanized film have been proposed for high-strength steel sheets containing a large amount of oxidizable elements such as Si, Mn, and Al. For example, Patent Document 1 discloses a method for producing an alloyed hot-dip galvanized high-tensile cold-rolled steel sheet in which an alloyed hot-dip galvanizing line is passed after Fe-based pre-plating. Patent Document 2 also discloses a technique in which an easily oxidizable element is concentrated on the surface by annealing and then pickled, and after removing the oxide, hot dip galvanizing is performed. Furthermore, Patent Document 3 discloses a technique for suppressing uneven alloying by applying a sulfur compound to the surface of a steel sheet before annealing and then passing it through an alloying hot dip galvanizing line.

  On the other hand, as a method for alloying a steel sheet containing a large amount of an easily oxidizable element, for example, Patent Document 4 discloses that an easily oxidizable element is proximate to the steel sheet surface by increasing the coiling temperature during hot rolling. A technique aimed at improving the adhesion of hot dip galvanizing by internally oxidizing the steel is disclosed.

By the way, the present inventors disclosed in Patent Document 5 that, in a hot-rolled steel sheet containing a large amount of Si, Mn, Al, and Cr, Fe ₂ SiO ₄ melts the temperature of a rough bar obtained by rough rolling a slab. It has proposed about the method of manufacturing the hot rolled steel plate excellent in surface property by raising to the temperature which changes, and promoting scale removal.

JP-A-5-331537 Japanese Patent Laid-Open No. 3-243751 Japanese Patent Laid-Open No. 11-50220 JP 9-310163 A JP 2005-342770 A

  Each of the methods disclosed in Patent Documents 1 to 3 is a method that is a practically preferable method because it adds a special processing step before the alloying hot dip galvanizing treatment and causes an increase in manufacturing cost. Absent.

  Further, according to the study by the present inventors, it is difficult to stably form a uniform internal oxide layer in the vicinity of the surface layer by the method disclosed in Patent Document 4, and therefore, a uniform surface having excellent surface properties. It was difficult to obtain an alloyed hot-dip galvanized film. In addition, because it actively oxidizes the steel plate surface, grain boundary oxidation on the steel plate surface is remarkable, and zinc penetration into the crystal grain boundary becomes noticeable during hot dip galvanizing, making the grain boundary brittle. Become. As a result, at the time of processing, not only the interface between the steel sheet and the plating layer as the plating base material, but also the crystal grains of the surface layer portion of the steel sheet as the plating base material are peeled off along with the plating layer along the crystal grain boundary where zinc has penetrated. As a result, plating peeling is likely to occur, and it has been extremely difficult to ensure good powdering properties.

  As described above, the present invention is an alloying and melting process based on a cold-rolled steel sheet containing a large amount of easily oxidizable elements such as Si, Mn, Al and Cr, which has been difficult to manufacture by the conventional technology. An object of the present invention is to provide an alloyed hot-dip galvanized cold-rolled steel sheet having a uniform alloyed hot-dip galvanized layer having good surface properties and a method for producing the same.

  The inventors of the present invention have found that the alloyed hot-dip galvanized cold-rolled steel sheet based on a cold-rolled steel sheet containing a large amount of easily oxidizable elements such as Si, Mn, Al and Cr has a uniform surface property. In order to establish a practical method that makes it possible to obtain an alloyed hot-dip galvanized cold-rolled steel sheet having an alloyed hot-dip galvanized layer, intensive studies were conducted.

  First, when the surface properties of the galvannealed cold-rolled steel sheet were investigated, the galvannealed cold-rolled steel sheet had a linear plating defect extending in the rolling direction as a characteristic plating defect. (Hereinafter referred to as “linear defects”) may occur.

  Then, when this linear defect was investigated in detail, as for the part (linear defect part) where the linear defect generate | occur | produced, the plating thickness was locally thick compared with the normal part. From this, the part where the plating thickness was locally thickened after the alloying process is the other part (normal part) when the plating layer is pushed into the steel plate as the base material by skin pass rolling after the alloying process. It became clear that it became white and smooth in appearance, and was manifested as a linear plating defect.

  Since the linear defect portion is generated in the form of a line extending in the rolling direction, it was estimated that the defect was extended in the rolling direction by hot rolling and / or cold rolling. Therefore, the process was traced back to investigate in detail the cold-rolled steel sheet before annealing, the pickled steel sheet before cold rolling, and the hot-rolled steel sheet before pickling.

  As a result, in cold-rolled steel sheets before annealing and pickled steel sheets before cold rolling, many traces of crystal grains falling on the surface of the steel sheet were observed at the sites where linear defects occurred in alloyed hot-dip galvanized cold-rolled steel sheets. It was confirmed that the intergranular corrosion progressed locally. In addition, the scale was removed appropriately in the pickled steel sheet.

In the hot-rolled steel sheet before pickling, a thick scale mainly composed of Fe ₂ SiO ₄ is formed at a site where linear defects occur in the galvannealed cold-rolled steel sheet, and the interface between the steel sheet and the scale is Exhibited severe irregularities. In addition, it was confirmed that easily oxidizable elements such as Si, Mn, and Al and elements such as Cu and Ni that are less easily oxidized than Fe are concentrated.

  From the above, starting from the scale that could not be removed before the finish hot rolling, linear defects were generated at various stages of the hot rolling process under various temperature conditions. It was found to be formed by growing while being pushed. Hereinafter, the scale pushed into the steel starting from the scale that could not be removed before the finish hot rolling is referred to as “residual scale” in order to distinguish it from the scale formed on the surface of the steel.

  That is, when a residual scale is formed by the above process, an easily oxidizable element such as Si, Mn, and Al, Cu, Ni, etc. that are less oxidizable than Fe are concentrated around the residual scale, and the steel and scale in the steel sheet In the vicinity of the interface, it becomes complicated and uneven. When pickling treatment is performed on such a hot-rolled steel sheet, the scale is removed by pickling treatment, but in the vicinity of the unevenness or in the vicinity thereof, the form of corrosion is different from the other part (normal part). It will change. For this reason, a part where the intergranular corrosion locally progresses in the unevenness or the vicinity thereof, and a phenomenon such as a drop of crystal grains on the steel sheet surface occurs in this part. When such a pickled steel sheet is cold-rolled and galvannealed, the intergranular corrosion locally progresses and the crystal grains on the steel sheet surface fall off. Since the area of the reaction interface with the steel plate as the substrate is larger than that of other normal parts, alloying proceeds abnormally and the plating thickness locally increases. And, the plating layer is pushed into the steel plate as the base material by skin pass rolling after the alloying treatment, so that the part where the plating thickness is locally thick after the alloying treatment is compared with the other part (normal part). As a result, a white and smooth appearance is exhibited, and is manifested as a linear plating defect (linear defect).

  Therefore, it became clear for the first time that it is necessary to promote scale removal in the hot rolling stage in order to suppress linear defects in the galvannealed steel sheet.

  Conventionally, in an alloyed hot-dip galvanized cold-rolled steel sheet that uses a cold-rolled steel sheet as the plating base, the remaining scale in the hot-rolled steel sheet stage before cold rolling of the cold-rolled steel sheet that is the plated base material is alloyed and melted. It was thought that the surface properties of the galvanized cold rolled steel sheet were not affected. This is because the steel sheet surface is significantly larger than the irregularities formed on the surface of the steel sheet due to the remaining scale, and is cold-rolled. This is because it was thought that the difference between the remaining scale portion and the other portions disappeared by hot rolling.

Next, the present inventors further examined the promotion of scale removal in the hot rolling stage in order to suppress linear defects in the alloyed hot-dip galvanized steel sheet.
Regarding the scale removal of steel plates containing a large amount of oxidizable elements such as Si, Mn, Al and Cr, the scale removal is promoted by raising the coarse bar temperature to the temperature at which Fe ₂ SiO ₄ melts. Have been proposed in Patent Document 5 for removing scale in the hot rolling stage for suppressing linear defects, not only at a predetermined temperature of the rough bar, but also at the predetermined temperature. This study newly revealed that it is important to manage the holding time.

FIG. 1 shows the time during which the surface temperature of the coarse bar is maintained at 1000 ° C. or higher and 1300 ° C. or lower when the coarse bar is heated by induction heating, and a scale (red scale or island shape) mainly composed of Fe ₂ SiO ₄ (firelight). It is a figure which shows the relationship with the scale residual area rate in the time of a hot-rolled steel sheet of a scale). The removal rate of the scale was improved by increasing the time during which the surface temperature of the coarse bar was maintained at 1000 ° C. or more and 1300 ° C. or less, and the scale remaining area ratio was 0 by setting it to 50 seconds or more. Since the surface temperature of the coarse bar after induction heating is set to the same condition, the fluctuation in the removal rate of the scale is caused by changing the time for holding the coarse bar at 1000 ° C. or higher and 1300 ° C. or lower. Therefore, in order to promote scale removal in the hot rolling stage, it is important to set the time for maintaining the surface temperature of the rough bar at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or longer.

  By the way, as a means for holding the surface temperature of the rough bar at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or longer, when applying induction heating, high frequency heating, electric heating, etc., the accuracy of temperature control in finish hot rolling is increased. In order to increase, it is preferable to arrange these heating devices close to the finishing hot rolling device. In this case, the above-mentioned holding is performed by passing a rough bar through a heating device having a predetermined length, and thereafter, descaling and finish hot rolling are performed promptly. For this reason, it may occur that the leading end portion of the coarse bar is supplied to the finishing rolling device while the trailing end portion of the coarse bar stays in the heating device. At this time, since the transfer speed of the entire coarse bar is the transfer speed based on the finishing rolling device, the stay time in the heating device at the rear end portion of the coarse bar may be different from the stay time at the front end portion of the coarse bar. In particular, when finishing hot rolling is completed in a short time, the staying time in the heating device at the rear end of the coarse bar is shorter than the staying time at the leading end of the coarse bar. As described above, the time for staying in the heating device at the rear end portion of the coarse bar is limited by the finish hot rolling, and it may be difficult to secure the holding time. Even in such a case, one of the simplest methods for securing the holding time is to perform finish hot rolling at a low speed, but this method is not preferable because it causes a reduction in productivity.

  Therefore, as a practical method for ensuring the above holding time also for the rear end portion of the rough bar, the time required for the rough bar to pass through the finish rolling device is increased by increasing the rolling reduction of finish hot rolling. Then, the idea of newly securing the staying time in the heating device at the rear end of the coarse bar was studied.

  FIG. 2 shows the relationship between the thickness of the hot-rolled steel sheet when the thickness of the coarse bar is 40 mm and the scale remaining area ratio at the time of the hot-rolled steel sheet in the latter half of the rolling of the hot-rolled steel sheet. By reducing the thickness of the hot-rolled steel sheet, that is, by increasing the reduction ratio of finish hot rolling, the scale removal rate is improved, and by reducing the thickness of the hot-rolled steel sheet to 3.0 mm or less, the scale remaining area ratio is 0. It became. This is because as a result of increasing the reduction ratio of the finish hot rolling, it is possible to secure time for staying in the induction heating device at the rear end portion of the coarse bar.

  In addition, the scale remaining area ratio evaluated the remaining ratio in a full length by an area ratio by observing the surface of the stationary part except the 10 m of rolling direction front and rear ends and 100 mm of width direction both ends in a longitudinal direction.

  As described above, in order to suppress linear defects in the galvannealed steel sheet, it is necessary to promote scale removal in the hot rolling stage, and the hot rolling stage for suppressing linear defects. It was newly found out that it is important not only to set the coarse bar to a predetermined temperature but also to manage the time for holding the rough bar at the predetermined temperature for descaling.

The present invention has been made on the basis of the above new findings, and the gist thereof is as follows.
(1) In an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the cold-rolled steel sheet, the cold-rolled steel sheet is in mass%, C: 0.02% to 0.25%, Si: 0.01% to 2.5%, Mn: 0.5% to 3.5%, P: 0.1% or less, S: 0.01% or less, sol. The alloyed hot-dip plated layer has a chemical composition containing Al: 1.0% or less and N: 0.01% or less, and the basis weight is 10 g / m ² or more and 80 g / m ² or less, and the degree of alloying is Alloyed hot-dip galvanized cold-rolled steel sheet, characterized in that it is 7% by mass or more and 16% by mass or less and the distribution of plating thickness satisfies the following formula (i):
d _max / d ₀ ≦ 2.5 (i)
Here, d _max is the maximum plating thickness, and d ₀ is the average plating thickness.

  (2) The chemical composition further includes one or more selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less in terms of mass%. The alloyed hot-dip galvanized cold-rolled steel sheet according to (1) above, which is contained.

  (3) The above (1) or the above, wherein the chemical composition further contains one or two kinds selected from the group consisting of Cr: 2% or less and Mo: 2% or less in terms of mass% The alloyed hot-dip galvanized cold-rolled steel sheet according to (2).

  (4) The chemical composition further comprises one or two selected from the group consisting of Cu: 1% or less and Ni: 1% or less in terms of mass%. The alloyed hot-dip galvanized cold-rolled steel sheet according to any one of (3).

(5) The alloyed hot-dip galvanized cold-rolled as described in any one of (1) to (4) above, wherein the chemical composition further contains, by mass%, B: 0.01% or less steel sheet.
(6) The chemical composition was selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less in terms of mass%. The alloyed hot-dip galvanized cold-rolled steel sheet according to any one of (1) to (5) above, further containing one or more kinds.

  (7) The chemical composition further includes one or more selected from the group consisting of Bi: 0.05% or less, Sb: 0.05% or less, and Sn: 0.05% or less in terms of mass%. The alloyed hot-dip galvanized cold-rolled steel sheet according to any one of (1) to (6) above, which is contained.

(8) A method for producing an alloyed hot-dip galvanized cold-rolled steel sheet comprising the following steps (A) to (F):
(A) A slab surface temperature adjusting step of adjusting the surface temperature of the slab having the chemical composition according to any one of (1) to (7) above to 1200 ° C or higher and 1350 ° C or lower;
(B) The slab obtained by the slab surface temperature adjusting step is subjected to descaling within 10 seconds after the slab surface temperature is adjusted, and then subjected to rough hot rolling to obtain a rough bar. Process;
(C) The surface temperature of the rough bar is kept at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or longer, and the coarse bar is descaled within 10 seconds after the holding, and then the finishing temperature: Ar ₃ points or higher and 960 Finishing hot rolling step in which hot rolling is performed by finishing hot rolling at 400 ° C. or lower and winding temperature: 400 ° C. or higher and 750 ° C. or lower;
(D) Pickling step of pickling the hot-rolled steel sheet to obtain a pickled steel sheet;
(E) a cold rolling process in which the pickled steel sheet is cold-rolled to form a cold-rolled steel sheet; and (F) continuous hot-dip galvanized steel that is subjected to recrystallization annealing, hot-dip galvanizing treatment and alloying treatment. A recrystallization annealing temperature in the recrystallization annealing is Ac ₁ point or more and 950 ° C. or less, an alloying treatment temperature in the alloying treatment is 650 ° C. or less, and an atmosphere in the continuous hot dip galvanizing process is 650 ° C. or more. The dew point in the temperature range of 950 ° C. or lower is −25 ° C. or higher, the dew point in the temperature range of 550 ° C. or lower before the hot dip galvanizing treatment is −25 ° C. or lower, and the basis weight of the alloyed hot-dip plating is 10 g. / ^{m 2} or more 80 g / ^{m 2} or less is continuous galvanizing process.

  (9) The alloyed hot-dip galvanized cold-rolled steel sheet according to (8) above, wherein the thickness of the coarse bar is 35 mm or more, and the thickness of the hot-rolled steel sheet is 3.0 mm or less. Production method.

  According to the present invention, an alloyed hot-dip galvanized cold-rolled steel sheet based on a cold-rolled steel sheet containing a large amount of an easily oxidizable element is an alloy having a uniform alloyed hot-dip galvanized layer having good surface properties. It becomes possible to obtain a hot-dip galvanized cold-rolled steel sheet. The alloyed hot-dip galvanized cold-rolled steel sheet according to the present invention is suitable for press forming such as the body of an automobile, and among them, applications that require complicated forming.

When the rough bar is heated by induction heating, the surface temperature of the coarse bar is maintained at 1000 ° C. or higher and 1300 ° C. or lower, and the scale (red scale or island scale) mainly composed of Fe ₂ SiO ₄ at the time of hot-rolled steel sheet. It is a figure which shows the relationship with a scale residual area ratio. It is a figure which shows the relationship between the thickness of a hot-rolled steel plate when the thickness of a rough bar is 40 mm, and the scale residual area ratio at the time of the hot-rolled steel plate in the latter half of the rolling of the hot-rolled steel plate.

  Hereinafter, regarding the galvannealed cold-rolled steel sheet according to the present invention, the chemical composition of the cold-rolled steel sheet as the plating base, the basis weight of the alloyed hot-dip plated layer, the degree of alloying and the distribution of the plating thickness, and the alloying and melting A preferred method for producing a galvanized cold-rolled steel sheet will be described in detail. In the following description, “%” defining the chemical composition is “% by mass” unless otherwise specified.

1. Chemical composition of cold-rolled steel sheet as plating base (C: 0.02% to 0.25%)
C has the effect | action which raises the intensity | strength of a steel plate. When the C content is less than 0.02%, it is difficult to sufficiently obtain the effect by the above action. Therefore, the C content is 0.02% or more. Preferably it is 0.06% or more. On the other hand, if the C content exceeds 0.25%, the toughness and weldability deteriorate significantly. Therefore, the C content is 0.25% or less. Preferably it is 0.20% or less.

(Si: 0.01% to 2.5%)
Si has the effect of increasing the strength of the steel sheet. Moreover, it has an effect | action in strengthening a ferrite, homogenizing a steel structure, and improving workability. If the Si content is less than 0.01%, it is difficult to obtain the effect by the above action. Therefore, the Si content is 0.01% or more. Preferably it is 0.2% or more, More preferably, it is 0.5% or more. On the other hand, if the Si content exceeds 2.5%, an oxide film of Si is formed on the surface of the steel sheet during the annealing process after cold rolling, and the wettability of zinc is reduced during hot dip galvanization, and non-plating is likely to occur. . Even if no plating occurs, the oxide film becomes a barrier for iron diffusion from the steel sheet to the hot dip galvanized layer during the alloying process, making the alloying process extremely difficult. Furthermore, when the oxide film is formed unevenly on the surface of the steel sheet, portions having different alloying speeds are unevenly present, and the surface properties are impaired as alloying treatment unevenness. Therefore, the Si content is 2.5% or less. Preferably it is 2.0% or less.

(Mn: 0.5% to 3.5%)
Mn has the effect of promoting transformation strengthening and increasing the strength of the steel sheet. If the Mn content is less than 0.5%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 0.5% or more. Preferably it is 1.0% or more, More preferably, it is 1.5% or more. On the other hand, if the Mn content exceeds 3.5%, an oxide film of Mn is formed on the surface of the steel sheet in the annealing process after cold rolling, and the wettability of zinc is reduced during hot dip galvanization, and non-plating is likely to occur. . Even if no plating occurs, the oxide film becomes a barrier for iron diffusion from the steel sheet to the hot dip galvanized layer during the alloying process, making the alloying process extremely difficult. Furthermore, when the oxide film is formed unevenly on the surface of the steel sheet, portions having different alloying speeds are unevenly present, and the surface properties are impaired as alloying treatment unevenness. Therefore, the Mn content is 3.5% or less. Preferably it is 3.0% or less.

(P: 0.1% or less)
P is contained as an impurity and deteriorates toughness and weldability. Therefore, the P content is 0.1% or less. Preferably it is 0.05% or less, More preferably, it is 0.02% or less. The lower the P content, the better. Therefore, it is not necessary to limit the lower limit, but from the viewpoint of the refining cost, it is preferably 0.004% or more.

(S: 0.01% or less)
S is contained as an impurity, and forms MnS in the steel to deteriorate the bendability and hole expandability. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less. The lower the S content, the better. Therefore, it is not necessary to limit the lower limit, but from the viewpoint of the refining cost, it is preferably 0.0003% or more.

(Sol.Al: 1.0% or less)
Al has the effect | action which reduces the oxygen amount in steel and makes a steel plate healthy. Al added to the molten steel in the steel making process and used to reduce the amount of oxygen in the steel becomes an oxide, and the surplus Al is sol. It remains in the steel as Al. Therefore, sol. Al content is over 0%. Preferably it is 0.0005% or more. On the other hand, sol. If the Al content exceeds 1.0%, an Al oxide film is formed on the surface of the steel sheet in the annealing process after cold rolling, and the zinc wettability is lowered during hot dip galvanization, and non-plating is likely to occur. Even if no plating occurs, the oxide film becomes a barrier for iron diffusion from the steel sheet to the hot dip galvanized layer during the alloying process, making the alloying process extremely difficult. Furthermore, when the oxide film is formed unevenly on the surface of the steel sheet, portions having different alloying speeds are unevenly present, and the surface properties are impaired as alloying treatment unevenness. Therefore, sol. Al content shall be 1.0% or less. Preferably it is less than 0.050%, more preferably less than 0.010%.

(N: 0.01% or less)
N is contained as an impurity, and induces slab cracking by forming nitrides in the steel during the continuous casting process. Therefore, the N content is 0.01% or less. More preferably, it is 0.005% or less. The lower the N content, the better. Therefore, it is not necessary to limit the lower limit, but from the viewpoint of the refining cost, it is preferable to set it to 0.0005% or more.

(Ti: 0.1% or less, Nb: 0.1% or less, and V: one or more selected from the group consisting of 0.1% or less)
Ti, Nb, and V have the effect of improving the workability of the steel sheet by delaying recrystallization and refining crystal grains. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if the content of any element exceeds 0.1%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the contents of Ti, Nb and V are each 0.1% or less. Preferably it is 0.05% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.003% or more.

(One or two selected from the group consisting of Cr: 2% or less and Mo: 2% or less)
Cr and Mo have the function of promoting transformation strengthening by stabilizing austenite like Mn, and have the effect of increasing the strength of the steel sheet. Therefore, you may contain 1 type or 2 types of these elements. However, if the Cr content is more than 1% or the Mo content is more than 1%, the workability deteriorates remarkably. Therefore, the contents of Cr and Mo are both 1% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.001% or more.

(Cu: 1% or less selected from the group consisting of 1% or less and Ni: 1% or less)
Cu and Ni have a corrosion-inhibiting effect and have the effect of concentrating on the surface to suppress the penetration of hydrogen and to suppress delayed fracture. Therefore, you may contain 1 type or 2 types of these elements. However, even if the Cu content is more than 1% and the Ni content is more than 1%, the effect by the above action is saturated and disadvantageous in cost. Therefore, the contents of Cu and Ni are both 1% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.01% or more.

(B: 0.01% or less)
B has the effect of increasing the strength of the steel sheet by suppressing nucleation from the grain boundaries and enhancing the hardenability. Therefore, you may make it contain. However, even if the B content exceeds 0.01%, the effect of the above action is saturated and disadvantageous in cost. Therefore, the B content is 0.01% or less. In order to more reliably obtain the effect of the above action, the content is preferably 0.0002% or more.

(Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: one or more selected from the group consisting of 0.01% or less)
Ca, Mg, REM, and Zr have the effect of improving bendability by finely dispersing inclusions in the steel. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if the content of any element exceeds 0.01%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the contents of Ca, Mg, REM, and Zr are each 0.01% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.0001% or more.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

(One or more selected from the group consisting of Bi: 0.05% or less, Sb: 0.05% or less, and Sn: 0.05% or less)
Since Bi, Sb, and Sn have a melting point lower than that of zinc, they tend to be dissolved in a hot dip galvanizing bath in the hot dip galvanizing process, thereby improving the wettability of hot dip galvanizing with respect to a steel sheet. Furthermore, since it concentrates in a solidification interface in a continuous casting process and narrows a dendrite space | interval and makes solidification segregation small, it has the effect | action which improves the bendability of the steel plate which is a plating base material. Furthermore, as a result of improving the bendability of the steel sheet that is the plating base material, cracking of the steel sheet that is the plating base material during processing is suppressed, and plating peeling starting from the cracking of the steel sheet is suppressed, so alloyed molten zinc The powdering resistance of the plated steel sheet is improved. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when the content of any element exceeds 0.05%, grain boundary embrittlement due to melting of each element present at the crystal grain boundary in the hot dip galvanizing step becomes significant. Therefore, the contents of Bi, Sb, and Sn are each 0.05% or less. Preferably it is 0.01% or less, More preferably, it is 0.005% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain any element in an amount of 0.0001% or more. More preferably, any element is contained by 0.0003% or more.

2. Plating layer (1) Weight per unit area The amount per unit area is 10 g / m ² or more and 80 g / m ² or less.

If the basis weight is more than 80 g / m ^2, it is necessary to increase the amount of Fe diffusion in the alloying treatment in order to secure the degree of alloying described later. For this reason, a brittle iron-zinc alloy layer ( Many Γ phases are formed. Moreover, since it is necessary to increase the alloying treatment temperature in order to avoid a decrease in productivity, a more brittle iron-zinc alloy layer (Γ ₁ layer) may be formed. As a result, it may be difficult to ensure good powdering resistance. Therefore, the basis weight is 80 g / m ² or less. Preferably, it is 60 g / m ² or less.
On the other hand, if the basis weight is less than 10 g / m ² , it may be difficult to ensure sufficient corrosion resistance. Therefore, the basis weight is 10 g / m ² or more.

(2) Degree of alloying The degree of alloying, that is, the Fe concentration in the plating layer is 7 mass% or more and 16 mass% or less.

  When the cold-rolled steel sheet, which is the base material for alloying hot dip galvanizing, contains a large amount of oxidizable elements such as Si, Mn, Al and Cr, these oxidizable elements in the annealing process of the continuous hot dip galvanizing process Concentrates on the surface of the cold-rolled steel sheet. For this reason, if the degree of alloying is less than 7% by mass, a region where alloying is not completed partially occurs, and alloying treatment unevenness may occur. Therefore, the alloying degree is 7% by mass or more. On the other hand, if the degree of alloying exceeds 16% by mass, many brittle iron-zinc layers are formed, and it may be difficult to ensure good powdering resistance. Therefore, the degree of alloying is 16% by mass or less.

(3) Plating thickness distribution The plating thickness distribution satisfies the following formula (1).
d _max / d ₀ ≦ 2.5 (1)
Here, d _max is the maximum plating thickness, and d ₀ is the average plating thickness.

  In order to suppress the above-mentioned linear defects, it is necessary to prevent a portion where the plating thickness is locally increased after the alloying treatment. If the plating thickness distribution does not satisfy the above formula (1), the plating layer is pushed into the steel plate as the base material by skin pass rolling after the alloying treatment, so that the plating thickness is locally increased after the alloying treatment. The exposed part becomes whiter and smoother than the other part (normal part), and is manifested as a linear defect. Therefore, the plating thickness distribution satisfies the above formula (1). The right side of the formula (1) is preferably 2.0, and more preferably 1.5.

  Here, the distribution of the plating thickness is in the steady portion of the galvannealed cold-rolled steel sheet. For example, in the case of a steel strip, the range of 50 m and the edge portion of the rolling front end portion and the rolling rear end portion, respectively. The cross section in the width direction of the steel sheet (the surface having the rolling direction as the normal line) is mirror-polished except for the unsteady part, and the plating thickness is measured with an optical microscope in the range of 1/8 to 7/8 of the plate width. By seeking.

3. Manufacturing method Hereinafter, a suitable manufacturing method of the galvannealed cold-rolled steel sheet will be described in detail.
(1) Slab surface temperature adjustment process The surface temperature of the slab having the above chemical composition is set to 1200 ° C or higher and 1350 ° C or lower.

The surface temperature of the slab subjected to the rough hot rolling process greatly affects the peelability of the scale by the descaling process in the rough hot rolling process.
By setting the surface temperature of the slab to 1200 ° C. or higher, it is referred to as an interface between the base material and the scale (hereinafter referred to as the base material / scale interface). Promotes melting of Fe ₂ SiO ₄ in), it is possible to increase the scale peelability by descaling processing in the rough hot rolling step described later. If the surface temperature of the slab is less than 1200 ° C., the scale peelability may not be sufficient. On the other hand, when the surface temperature of the slab exceeds 1350 ° C., the scale thickness becomes too thick, and the peelability of the scale may be significantly reduced. Therefore, the surface temperature of the slab used for the rough hot rolling step is set to 1200 ° C. or higher and 1350 ° C. or lower. The time for maintaining the surface temperature of the slab in the temperature range of 1200 ° C. to 1350 ° C. is preferably 1 hour to 5 hours.

  In addition, the slab used for the rough hot rolling process may be a slab in a high temperature state after continuous casting or partial rolling, or may be a slab once cooled after continuous casting or in partial rolling. The means for adjusting the surface temperature of the slab may be one that heats the slab by inserting it into a heating furnace or the like, or may be one that retains the temperature with a heat retaining cover or the like.

(2) Rough hot rolling step The slab obtained by the above slab surface temperature adjusting step is subjected to descaling within 10 seconds after adjusting the surface temperature of the slab, and then subjected to rough hot rolling to obtain a rough bar. To do.

If the time from the adjustment of the surface temperature of the slab to the start of the descaling process is less than 10 seconds, the Fe ₂ SiO _{4 at} the base material / scale interface melted in the slab surface temperature adjustment process solidifies again due to the temperature decrease, The scale peelability may not be sufficient. Therefore, the time from the adjustment of the surface temperature of the slab to the start of the descaling process is 10 seconds or less. Here, “after adjusting the surface temperature of the slab” means after adjusting the surface temperature of the slab. For example, when the slab is charged in a heating furnace and heated, the slab is heated. It means after extraction from the furnace.
Rough hot rolling may be performed by a conventional method.

(3) Finishing hot rolling step The surface temperature of the rough bar obtained by the rough hot rolling step is kept at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or more and descaled to the rough bar within 10 seconds after the holding. Next, finish hot rolling of finishing temperature: Ar ₃ points to 960 ° C. and winding temperature: 400 ° C. to 750 ° C. is made into a hot-rolled steel sheet.

This step is a particularly important step in suppressing the plating defects.
The surface temperature of the rough bar is kept at 1000 ° C or higher and 1300 ° C or lower for 50 seconds or more, and the scale before finishing hot rolling is more reliably removed by applying descaling treatment to the rough bar within 10 seconds after the holding. can do.

That is, by maintaining the surface temperature of the coarse bar at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or more, the melting of Fe ₂ SiO _{4 at} the substrate / scale interface is promoted, and the scale peelability by the subsequent descaling process is increased. Can be increased. The time for which the surface temperature of the coarse bar is 1000 ° C. or higher and 1300 ° C. or lower is preferably 70 seconds or longer. If the surface temperature of the coarse bar is less than 1000 ° C., or if the time for 1000 ° C. or more is less than 50 seconds, the peelability of the scale may not be sufficient. On the other hand, when the surface temperature of the coarse bar exceeds 1300 ° C., the scale thickness becomes too thick, and the peelability of the scale may be significantly reduced. Therefore, the surface temperature of the coarse bar is set to 1000 ° C. or higher and 1300 ° C. or lower and held for 50 seconds or longer. The upper limit of the time during which the surface temperature of the rough bar is 1000 ° C. or higher and 1300 ° C. or lower is not particularly limited, but is preferably 150 seconds or shorter from the viewpoint of productivity. In the present invention, “Fe ₂ SiO ₄ ” means stoichiometrically Fe ₂ SiO ₄ , the basic component is Fe ₂ SiO ₄ , but other elements such as P, Al, and Cr are combined. Including those that are. The means for setting the surface temperature of the coarse bar to 1000 ° C. or higher and 1300 ° C. or lower is not particularly limited. The coarse bar may be charged in a heating furnace and heated. It is preferable to apply induction heating, high frequency heating, electric heating or the like because the heating time of the coarse bar can be shortened, and it is particularly preferable to apply induction heating.

The time from the completion of the holding to the descaling process being 10 seconds or less. If the time is less than 10 seconds, the Fe ₂ SiO _{4 at} the substrate / scale interface melted by holding the surface temperature of the coarse bar at 1000 ° C. or more and 1300 ° C. or less for 50 seconds or more is solidified again due to the temperature drop. As a result, the peelability of the scale may not be sufficient. Therefore, the time from the completion of the holding to the descaling process being 10 seconds or less. Since it is preferable that the time from the completion of the holding to the descaling process be as short as possible, no lower limit is defined.

The finishing temperature is Ar ₃ or higher and 960 ° C. or lower. If the finishing temperature is less than Ar ₃ points, the deformation resistance during hot rolling increases, and operation may become difficult. On the other hand, if the finishing temperature exceeds 960 ° C., the scale generated after finishing hot rolling becomes thick, and it may be difficult to ensure good surface properties. Therefore, the finishing temperature is set to Ar ₃ or higher and 960 ° C. or lower.

  The coiling temperature is 400 ° C. or higher and 750 ° C. or lower. When the coiling temperature is less than 400 ° C., hard bainite and martensite are excessively generated, and the subsequent cold rolling may be difficult. On the other hand, if the coiling temperature exceeds 750 ° C., oxidation of the steel sheet surface proceeds excessively, and it may be difficult to ensure good surface properties. Therefore, the coiling temperature is 400 ° C. or higher and 750 ° C. or lower.

  As described above, it is preferable to apply induction heating, high-frequency heating, electric heating, or the like as means for holding the surface temperature of the coarse bar at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or longer. And when applying induction heating, high frequency heating, electric heating, etc., it is preferable to arrange these heating devices close to the finishing hot rolling device in order to increase the accuracy of temperature control in finishing hot rolling. . In this case, the above-mentioned holding is performed by passing a rough bar through a heating device of a predetermined length, and thereafter, descaling and finish hot rolling are performed immediately. There may be a case where the tip of the coarse bar is supplied to the finishing mill while the end is staying. For this reason, the time which stays in a heating apparatus about the rear-end part of a rough bar receives restrictions by finishing hot rolling, and it may become difficult to ensure the said holding time.

  Therefore, in order to easily maintain the surface temperature of the rough bar at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or more, it is preferable to increase the rolling reduction in finish hot rolling. Specifically, the thickness of the coarse bar is preferably 35 mm or more, and the thickness of the hot-rolled steel plate is preferably 3.0 mm or less. The upper limit of the thickness of the rough bar and the lower limit of the thickness of the hot-rolled steel sheet do not need to be specified, but if the difference between the thickness of the rough bar and the thickness of the hot-rolled steel sheet is large, the reduction rate in finish hot rolling becomes excessive. Therefore, the thickness of the coarse bar is preferably 60 mm or less, and the thickness of the hot-rolled steel plate is preferably 1.6 mm or more.

(4) Pickling process The hot-rolled steel sheet obtained by the finishing hot rolling process is pickled to obtain a pickled steel sheet.
Pickling may be performed by a conventional method, for example, by dipping in a hydrochloric acid bath or a sulfuric acid bath. It should be noted that skin pass rolling may be performed for flattening before and after pickling. In particular, skin pass rolling prior to pickling also has an effect of enhancing the scale peelability in pickling, and thus is preferable for ensuring better surface properties.

(5) Cold rolling process Cold-rolled steel sheet is obtained by subjecting the pickled steel sheet obtained by the above pickling process to cold rolling.
Cold rolling may be performed by a conventional method.

(6) Continuous hot-dip galvanizing step The cold-rolled steel plate is subjected to recrystallization annealing, hot-dip galvanizing treatment and alloying treatment to obtain an alloyed hot-dip galvanized steel plate.

An annealing temperature shall be Ac ₁ point or more. When the annealing temperature is less than Ac ₁ point, a large amount of non-recrystallized grains remain or the composite structure becomes insufficient, and sufficient workability cannot be obtained. Accordingly, the annealing temperature is set to Ac ₁ point or higher. On the other hand, when the annealing temperature exceeds 950 ° C., the grain boundary oxidation becomes remarkable as will be described later, which may deteriorate the surface properties of the steel sheet. Accordingly, the annealing temperature is set to 950 ° C. or lower.

The alloying temperature is 650 ° C. or lower. When the alloying treatment temperature is higher than 650 ° C., the hard Γ ₁ phase is formed thick, and the powdering resistance may deteriorate. In addition, since the alloying reaction is remarkably fast, the influence of alloying unevenness generation factors such as residual scale in the hot rolling stage is promoted, alloying unevenness is likely to occur, and it is difficult to suppress linear defects. There is a case. Therefore, the alloying temperature is 650 ° C. or lower. The lower limit of the alloying treatment temperature does not need to be specified in particular, and may be within a range in which the degree of alloying of the present invention can be ensured. In order to ensure the mechanical properties of the high-strength steel plate, the lower the alloying treatment temperature, the better.

  In the prior art, when a steel sheet containing a large amount of an easily oxidizable element is used as a plating base material, the alloying treatment speed has to be lowered, so that the alloying treatment temperature has to be set high. However, in the present invention, by optimizing the dew point, which will be described later, it is possible to stably ensure good cleanliness for the steel sheet surface during immersion in the hot dip galvanizing bath, so that the alloying treatment temperature can be lowered than in the prior art. This makes it possible to secure stable powdering resistance more stably.

  The atmosphere in the continuous hot dip galvanizing process is such that the dew point in the temperature range of 650 ° C. or higher and 950 ° C. or lower is −25 ° C. or higher, and the dew point in the temperature range of 550 ° C. or lower before the hot dip galvanizing treatment is −25 ° C. or lower. And

  When a steel sheet containing a large amount of easily oxidizable elements such as Si, Mn, Al and Cr is used as the plating base, the plating property is inferior because the oxide film of the easily oxidizable elements is formed on the steel sheet surface during the reduction annealing process. Due to the formation.

  However, when the atmosphere in the high temperature region of the reduction annealing process is set to a high dew point according to the study by the present inventors, specifically, the dew point in the temperature range of 650 ° C. to 950 ° C. is set to −25 ° C. When the dew point in a temperature range of 550 ° C. or lower before galvanizing is -25 ° C. or lower, good plating properties are ensured even when a steel sheet containing a large amount of the easily oxidizable element is used as a plating base. It became clear that it was possible.

  The reason for this is not clear, but the oxidizing power is increased by making the atmosphere in the high temperature region of the reduction annealing process a high dew point, and the oxidizing action against the above oxidizable elements becomes stronger. It is presumed that the easily oxidizable element is oxidized inside the steel plate, and as a result, the formation of an oxide film of the easily oxidizable element on the surface of the steel plate is suppressed and the plating property is improved. Also, it is estimated that reducing power is increased by lowering the atmosphere in the low temperature range before the hot dip galvanizing treatment, the reducing action on iron becomes stronger, the steel plate surface becomes even cleaner, and the plating property is improved. .

  Here, the dew point in the temperature range of 650 ° C. or more and 950 ° C. or less is defined as the high temperature range even if the dew point is −25 ° C. or more in the temperature range of less than 650 ° C. Is not sufficient, and does not contribute to the improvement of the plating property. Further, if the dew point is set to -25 ° C. or higher in a temperature range exceeding 950 ° C., the grain boundary oxidation becomes remarkable and the surface properties of the steel sheet may be deteriorated. In addition, the upper limit of the dew point in the temperature range of 650 ° C. or more and 950 ° C. or less is not particularly required from the viewpoint of plating properties. However, if the dew point is too high, the formation of oxide is excessively promoted, and the oxide is likely to be deposited on a roll such as a hearth roll in an annealing furnace. It may cause a decrease. Accordingly, the dew point is preferably + 20 ° C. or lower. More preferably, it is 0 degreeC or less.

Incidentally, the dew point of high purity industrial _N 2 -H ₂ mixture gas is usually -60 ° C. or less. For this reason, adjustment of the dew point in the reduction annealing furnace is performed by increasing the dew point of the mixed gas supplied into the reduction annealing furnace in advance or by directly blowing water vapor into the reduction annealing furnace. According to the former method, the atmosphere in the reduction annealing furnace can be made more uniform, which is preferable from the viewpoint of obtaining stable surface properties. The gas blowing is preferably performed from the vicinity of the end of the high temperature region.

  Further, from the viewpoint of improving the cleanliness of the steel sheet before plating, it is preferable to quickly reduce the dew point in the atmosphere when the control for setting the atmosphere to a high dew point in the above temperature range is completed. However, since the furnace in the continuous hot dip galvanizing facility is continuous, it is difficult to make the high dew point atmosphere and the low dew point atmosphere continuous. Therefore, when the dew point in the temperature range of 650 ° C. or higher is set to −25 ° C. or higher as described above, the ambient temperature that is the lower limit of the temperature range is from a region sufficiently separated from the region of 650 ° C. Without it, it is difficult to stably realize an atmosphere with a dew point of −25 ° C. or lower. From this point of view, in the present invention, the atmospheric temperature in the region where the control for setting the atmosphere to a low dew point is started is reduced to 550 ° C., which is 100 ° C. lower than the lower limit of the temperature range where the atmosphere is set to a high dew point. It is set.

  The lower limit of the dew point in the temperature range of 550 ° C. or lower before plating is not particularly specified. From the viewpoint of improving the cleanliness of the steel sheet, the lower the dew point, the better. However, excessively reducing the dew point increases the equipment burden, so that it is preferable to set the temperature to -50 ° C or higher industrially.

  Thus, according to the present invention, it is possible to provide an alloyed hot-dip galvanized steel sheet having excellent surface properties with suppressed plating defects and a method for producing the same, which were difficult to manufacture with conventional techniques. Become.

The present invention will be described more specifically with reference to examples.
Steel having the chemical composition shown in Table 1 was melted in a converter to produce a 270 mm thick slab. The obtained slab was reheated and then manufactured under the conditions shown in Table 2. The obtained hot-rolled steel sheet was scale-removed by pickling and then cold-rolled to a thickness of 1.6 mm. This cold-rolled steel sheet was annealed, subjected to hot dip galvanizing at a bath temperature of 460 ° C. during the cooling after annealing, and heated to a temperature shown in Table 2 after the plating to perform an alloying treatment. Thereafter, skin pass rolling was performed at 0.4% to obtain an alloyed hot-dip galvanized steel sheet.

The surface of the obtained galvannealed steel sheet was observed. In addition, the cross section in the width direction of the steel sheet (the surface with the rolling direction as the normal line) is mirror-polished, and the plating thickness is measured at a pitch of 5 mm using an optical microscope in the range of 1/8 to 7/8 of the plate width. For the maximum plating thickness and d _max, the average plating thickness d ₀ was determined.

  The evaluation results are shown in Table 2. The underlined numerical values in Tables 1 and 2 indicate that the content, conditions, or mechanical properties indicated by the numerical values are outside the scope of the present invention.

In the present invention example in which the chemical composition and production conditions of the cold rolled steel sheet are within the scope of the present invention, the alloying processability is good, the plating thickness distribution d _max / d ₀ ≦ 2.5 can be achieved, and there is no occurrence of linear defects. An alloyed hot-dip galvanized steel sheet having good surface properties could be produced.

On the other hand, even if the chemical composition of the cold rolled steel sheet is within the scope of the present invention, No. No. 3 has a low slab surface temperature. 4 and 7 have a short time for maintaining the surface temperature of the coarse bar at 1000 ° C. or higher and 1300 ° C. or lower, so that Fe ₂ SiO ₄ cannot be sufficiently melted, the scale removal effect is reduced, and linear defects are generated. did.

  No. No. 8 has a large thickness of the hot-rolled steel sheet, so that the induction heating device staying time in the latter half of the rolling of the hot-rolled steel sheet cannot be sufficiently secured, and the time for maintaining the bar surface temperature at 1000 ° C. or higher and 1300 ° C. or lower. Shortened and linear defects occurred. Furthermore, since the dew point in the temperature range of 550 ° C. or lower was more than −25 ° C., non-plating occurred.

No. No. 10 had a dew point of less than −25 ° C. in a temperature range of 650 ° C. or higher, so that the alloying treatment was not properly performed and non-plating occurred.
No. Since the alloying treatment temperature of No. 11 was too high, the alloying proceeded excessively, and slight reaction unevenness in the cold-rolled steel sheet, which was not originally a problem, was promoted, and linear defects were generated.

  No. In Nos. 20 and 21, non-plating occurred because Si and Mn were high in the chemical composition of the cold-rolled steel sheet.

Claims (9)

In an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of a cold-rolled steel sheet,
The cold-rolled steel sheet is in mass%, C: 0.02% to 0.25%, Si: 0.01% to 2.5%, Mn: 0.5% to 3.5%, P : 0.1% or less, S: 0.01% or less, sol. Having a chemical composition containing Al: 1.0% or less and N: 0.01% or less;
The alloyed hot-plated layer has a basis weight of 10 g / m ² or more and 80 g / m ² or less, an alloying degree of 7% by mass or more and 16% by mass or less, and the plating thickness distribution is represented by the following formula (1). An alloyed hot-dip galvanized cold-rolled steel sheet characterized by satisfaction.
d _max / d ₀ ≦ 2.5 (1)
Here, d _max is the maximum plating thickness, and d ₀ is the average plating thickness.   The chemical composition further contains, in mass%, one or more selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less. The alloyed hot-dip galvanized cold-rolled steel sheet according to claim 1.   3. The chemical composition according to claim 1, wherein the chemical composition further contains one or two selected from the group consisting of Cr: 2% or less and Mo: 2% or less in mass%. Alloyed hot-dip galvanized cold-rolled steel sheet.   The chemical composition further comprises one or two selected from the group consisting of Cu: 1% or less and Ni: 1% or less by mass%. The alloyed hot-dip galvanized cold-rolled steel sheet according to claim 1.   5. The alloyed hot-dip galvanized cold-rolled steel sheet according to claim 1, wherein the chemical composition further contains B: 0.01% or less in terms of mass%.   The chemical composition is, in mass%, one selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less The alloyed hot-dip galvanized cold-rolled steel sheet according to any one of claims 1 to 5, further comprising two or more kinds.   The chemical composition further contains one or more selected from the group consisting of Bi: 0.05% or less, Sb: 0.05% or less, and Sn: 0.05% or less in mass%. The alloyed hot-dip galvanized cold-rolled steel sheet according to any one of claims 1 to 6. A method for producing a galvannealed cold-rolled steel sheet characterized by having the following steps (A) to (F):
(A) A slab surface temperature adjusting step of adjusting the surface temperature of the slab having the chemical composition according to any one of claims 1 to 7 to 1200 ° C or higher and 1350 ° C or lower;
(B) The slab obtained by the slab surface temperature adjusting step is subjected to descaling within 10 seconds after the slab surface temperature is adjusted, and then subjected to rough hot rolling to obtain a rough bar. Process;
(C) The surface temperature of the rough bar is kept at 1000 ° C. or higher and 1300 ° C. or lower for 50 seconds or longer, and the coarse bar is descaled within 10 seconds after the holding, and then the finishing temperature: Ar ₃ points or higher and 960 Finishing hot rolling step in which hot rolling is performed by finishing hot rolling at 400 ° C. or lower and winding temperature: 400 ° C. or higher and 750 ° C. or lower;
(D) Pickling step of pickling the hot-rolled steel sheet to obtain a pickled steel sheet;
(E) a cold rolling process in which the pickled steel sheet is cold-rolled to form a cold-rolled steel sheet; and (F) continuous hot-dip galvanized steel that is subjected to recrystallization annealing, hot-dip galvanizing treatment and alloying treatment. A recrystallization annealing temperature in the recrystallization annealing is Ac ₁ point or more and 950 ° C. or less, an alloying treatment temperature in the alloying treatment is 650 ° C. or less, and an atmosphere in the continuous hot dip galvanizing process is 650 ° C. or more. The dew point in the temperature range of 950 ° C. or lower is −25 ° C. or higher, the dew point in the temperature range of 550 ° C. or lower before the hot dip galvanizing treatment is −25 ° C. or lower, and the basis weight of the alloyed hot-dip plating is 10 g. / ^{m 2} or more 80 g / ^{m 2} or less is continuous galvanizing process.   The method for producing a galvannealed cold-rolled steel sheet according to claim 8, wherein the thickness of the rough bar is 35 mm or more, and the thickness of the hot-rolled steel sheet is 3.0 mm or less.

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