JP2008231447A - Galvannealed steel sheet superior in appearance quality, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet superior in appearance quality, which shows a more improved effect of the appearance quality than a conventionally proposed method for improving the appearance quality, and to provide a manufacturing method therefor which can be applied even to a high-strength steel sheet including P or Mn that tends to cause a poor appearance without lowering the productivity. <P>SOLUTION: The galvannealed steel sheet superior in the appearance quality has a coating film of an iron-zinc-based alloy containing Zn of 85% or more on the surface of a steel sheet. The base steel sheet has a structure in the surface, in which ferrite grains having a ratio of a length of a major axis in a rolling direction of the steel sheet to a length of a minor axis in a width direction of the steel sheet in an amount of 5 or more occupy 90% or more of the whole area, and a structure between the surface and a 100 μm or more deep position from the surface, in which the ferrite grains having the ratio of the length of the major axis in the rolling direction of the steel sheet to the length of the minor axis in the width direction of the steel sheet in an amount of 5 or more occupy 10% or less of the whole area. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an galvannealed steel sheet excellent in appearance quality and a method for producing the same. More specifically, mainly in an alloyed hot-dip galvanized steel sheet based on a high-strength steel sheet, the appearance after plating is more uniform and beautiful than the conventional alloyed hot-dip galvanized steel sheet. The present invention relates to an alloyed hot-dip galvanized steel sheet excellent in appearance quality that can be used for automobiles and the like, and a method for producing the same.

  Alloyed hot dip galvanized steel sheets are widely used in automobiles, home appliances, building materials and the like because of their excellent weldability, paintability, and post-paint corrosion resistance. This alloyed hot-dip galvanized steel sheet is a hot-dip galvanized steel sheet, and then heat-treated to diffuse Fe in the steel and Zn in the plating to cause an alloying reaction, thereby causing an iron-zinc alloy layer on the steel surface. Is formed.

  This alloying reaction is said to occur preferentially from the grain boundaries of the steel, but when the surface crystal grain size of the steel sheet is coarse, the total grain boundary length of the crystal is larger than when the grain size is fine. Therefore, there is a difference in the alloying reaction rate of the steel sheet as a whole. That is, the alloying of the steel plate having coarse crystals is slower in alloying and the plating becomes thinner than the steel plate having fine crystals. Therefore, in a steel plate in which coarse grains and fine grains are mixed, unevenness of plating is generated due to a local alloying speed difference, resulting in poor appearance.

  Moreover, when many elements which are easy to segregate are contained in grain boundaries, such as P, it has the function which further promotes the alloying reaction rate difference, and the external appearance defect of plating worsens further.

  For this reason, various measures for improving the appearance of plating by making the crystal grain size of the surface layer of the steel sheet uniform have been proposed. For example, in Patent Document 1, the ratio R = d1 / d2 between the average ferrite grain diameter d1 on the outermost surface at a certain point on the steel sheet and the average ferrite grain diameter d2 on the outermost surface at a point 10 mm or more away from that point is 0. It is said that the appearance quality is improved by setting the ratio to 0.9 or more and 1.1 or less. However, even if R is 0.9 or more and 1.1 or less, in recent ultra-low carbon steels with an emphasis on workability, the alloying reaction of Fe and Zn has a slow base metal crystal orientation. Therefore, when there is a local P concentration difference on the steel sheet surface, the difference in alloying speed is promoted and the appearance quality is not sufficiently improved. In addition, in the P-added steel, the alloying speed is further reduced, and the productivity is lowered.

  In Patent Document 2, the surface appearance of the hot-rolled sheet serving as a plating original sheet is made 70% or less in terms of area ratio in a field of view of 400 μm of ferrite crystals of 15 μm or less, thereby improving the plating appearance. However, even if ferrite crystal grains of 15 μm or less occupy an area ratio of 70% or less, if the distribution is uneven, the appearance of plating is inferior.

  In Patent Document 3, the finish rolling finish temperature of hot rolling is set to 1000 ° C. or (Ar3 transformation point + 20 ° C.), and then taken in a range of 700 to 820 ° C., and then cold rolled, annealed, and melted. It is said that a hot-dip galvanized steel sheet with excellent appearance quality can be manufactured by galvanizing. However, if the finish rolling temperature is in the above range, the appearance of plating due to the presence of non-recrystallized parts can be suppressed, but the appearance of plating due to non-uniform structure (mixed grain structure of coarse and fine grains) can be suppressed. It is difficult to eliminate defects. Further, in recent ultra-low carbon steels with emphasis on workability, there is a local iron crystal orientation in which the alloying reaction of Fe and Zn is slow, so there is a local P concentration difference on the steel sheet surface. However, the difference in alloying speed is promoted, and the appearance quality is not sufficiently improved. In P-added steel, the alloying speed is further reduced, which lowers productivity and is not a good measure.

  Furthermore, in patent document 4, it is supposed that the plating external appearance defect resulting from a fine structure can be suppressed by making the ferrite particle diameter of the steel surface in Ti containing steel into 8 micrometers or more. However, even if the surface ferrite grain size is 8 μm or more, the plating appearance does not improve if the grain size varies, such as when there are locally coarse crystals.

JP-A-7-228944 JP 2001-316663 A JP 2001-342522 A JP-A-2-38550

  As described above, various types of alloyed hot-dip galvanized steel sheets with excellent appearance quality and their manufacturing methods have been proposed, but all of them are sufficient to meet the demands for severe appearance quality from recent customers. It cannot be answered. In addition, when a high-strength steel sheet containing P, Mn, or the like is used as a base, the appearance is more likely to deteriorate. However, the effect of improvement is small with the conventionally proposed methods for these steel sheets, and the productivity is low. There is also a thing which lowers, and it cannot be said that it is an industrial good measure.

  Therefore, the present invention has a greater improvement effect than the conventionally proposed methods, and is likely to be defective in appearance, and can be applied to high-strength steel plates containing P, Mn, etc. without reducing productivity. An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet excellent in quality and a method for producing the same.

  In order to solve the above-mentioned problems, the present inventors made an alloyed hot-dip galvanized steel sheet based on a steel sheet containing 0.005% to 0.2% P and made only the structure of the surface layer of the base metal coarse. Thus, the present inventors have found that the appearance quality is very beautiful and that the alloying rate of Fe and Zn can be increased, that is, the productivity can be improved as compared with steel of the same component. That is, the gist of the present invention is as follows.

(1) In mass%,
C: 0.001% to 0.01%,
Si: 0.001% or more and 0.2% or less,
Mn: 0.01% or more and 3% or less,
P: 0.005% or more and 0.2% or less,
S; 0.001% or more and 0.03% or less,
Al; 0.005% or more and 0.1% or less,
In addition,
Ti: 0.001% or more and 0.05% or less,
Nb: 0.001% or more and 0.05% or less,
B; 0.0001% to 0.005%,
N; including 0.0001% or more and 0.05% or less of one or more,
An alloyed hot-dip galvanized steel sheet having an iron-zinc alloy coating containing 85% or more of Zn on the surface of the steel sheet, the balance of which is made of Fe and inevitable impurities. The ferrite grain having a length of the minor axis in the width direction of the steel sheet of 5 or more is composed of 90% or more in area ratio, and the structure in the range of 100 μm or more in depth from the surface of the base iron is the length of the major axis in the steel sheet rolling direction. An alloyed hot-dip galvanized steel sheet excellent in appearance quality, characterized in that ferrite grains having a widthwise minor axis length of 5 or more are 10% or less in area ratio.

(2) Manufacture of alloyed hot-dip galvanized steel sheet that is subjected to pickling, cold rolling, annealing, hot-dip galvanizing, and heat alloying after hot rolling the low-carbon steel slab comprising the components described in (1) In the method, at the time of hot finish rolling, different circumferential speed rolling defined by the following formula (A) is performed at least one pass, and the Ar3 temperature is represented by the following formula (C). The finish temperature of hot finish rolling is (Ar3 point-30 ° C.) or higher and Ar3 point or lower in any of the steel plate width direction and the steel plate rolling direction. Method for producing a galvannealed steel sheet.
Different peripheral speed ratio = High speed side roll peripheral speed / Low speed side roll peripheral speed (A)
Ar3 = 900−500 × (C%) − 50 × (Mn%) + 200 × (P%) (C)
C%: C mass% of the steel sheet to be hot rolled, Mn%: Mn mass% of the steel sheet to be hot rolled,
P%: P mass% of the steel sheet to be hot rolled

(3) Manufacture of an alloyed hot-dip galvanized steel sheet that is hot-rolled with a low-carbon steel slab comprising the components described in (1) and then subjected to pickling, cold rolling, annealing, hot-dip galvanizing, and heat alloying treatment. In the method, at the time of hot finish rolling, at least one pass of different diameter roll rolling in which the ratio of the diameters of the upper and lower rolling rolls defined by the following formula (B) is 1.2 or more and 1.5 or less is applied, and the Ar3 temperature is set. When defined by the following formula (C), the hot finish rolling end temperature is (Ar3 point −30 ° C.) or more and Ar3 point or less in both the steel plate width direction and the steel plate rolling direction. A method for producing a galvannealed steel sheet having excellent appearance quality.
Ratio of diameter of upper and lower rolling rolls in each stand = larger diameter side roll diameter in upper and lower directions / smaller diameter side roll diameter in upper and lower sides (B)
Ar3 = 900−500 × (C%) − 50 × (Mn%) + 200 × (P%) (C)

(4) In the method for producing an alloyed hot-dip galvanized steel sheet according to (2) or (3), the amount of rolling oil sprayed during cold rolling is 0. method for manufacturing a galvannealed steel sheet excellent in appearance quality, characterized in that a 1 mg / ^{m 2} or more 5 mg / ^{m 2} or less.

  The alloyed hot-dip galvanized steel sheet of the present invention and the alloyed hot-dip galvanized steel sheet that has undergone the production method of the present invention are excellent in appearance quality, and excellent in slidability, adhesion, and productivity. For this reason, it can be used for automobiles, home appliances, building materials and the like, and its industrial value is extremely large.

  Hereinafter, the present invention will be described in detail.

  First, the reasons for limiting the steel components of the galvannealed steel sheet of the present invention will be described.

C: 0.001% to 0.01% C is an element necessary for strengthening steel. If the C content is less than 0.001%, the strength is insufficient, and if it exceeds 0.01%, embrittlement and r value decrease are caused. For this reason, the amount of C is made 0.001% or more and 0.01% or less.

Si: 0.001% or more and 0.2% or less Si is an element having the effect of strengthening and deoxidizing steel. If Si is less than 0.001%, the effect is insufficient. On the other hand, if it is added excessively, it not only becomes brittle, but also impairs the wettability of plating at the time of hot dip galvanizing and deteriorates the plating adhesion. For this reason, Si is made 0.001% or more and 0.2% or less.

Mn: 0.01% or more and 3% or less Mn is an element having an effect of strengthening and deoxidizing steel. If Mn is less than 0.01%, the effect is insufficient. On the other hand, excessive addition causes embrittlement and a decrease in r value. For this reason, Mn is made 0.01% or more and 3% or less.

P: 0.005% or more and 0.2% or less P is an element necessary for strengthening steel. When P is less than 0.005%, the effect is insufficient. On the other hand, when P is added excessively, it becomes brittle. For this reason, P is made 0.005% or more and 0.2% or less.

S: 0.001% or more and 0.03% or less S is an impurity, and it is preferable to reduce the S content because it degrades workability and hot brittleness. However, excessive reduction causes an increase in manufacturing cost. For this reason, S is made 0.001% or more and 0.03% or less.

Al: 0.005% or more and 0.1% or less Al has a deoxidizing effect. Moreover, the affinity with N in steel is strong, and there exists an effect which fixes the solid solution N as a precipitate and improves workability. However, if added excessively, the workability is deteriorated. For this reason, Al is made 0.005% or more and 0.1% or less.
Further, one or more of Ti, Nb, B, and N can be arbitrarily added as selective elements, and the range is defined below.

Ti: 0.001% or more and 0.05% or less Ti has an effect of fixing C and N in the steel as precipitates and improving workability. If Ti is less than 0.001%, the effect is poor, and even if added over 0.05%, the effect is no longer saturated and the cost is simply increased. For this reason, Ti is preferably 0.001% or more and 0.05% or less.

Nb: 0.001% or more and 0.05% or less Nb has an effect of fixing C in the steel as a precipitate and improving workability. If Nb is less than 0.001%, the effect is poor, and even if added over 0.05%, the effect is no longer saturated and the cost is simply increased. Therefore, Nb is preferably 0.001% or more and 0.05% or less.

B: 0.0001% or more and 0.005% or less B has the effect of preferentially concentrating on the crystal grain boundaries, lowering the grain boundary energy, and suppressing embrittlement. If B is less than 0.0001%, the effect is insufficient. On the other hand, if it exceeds 0.005%, the workability decreases. For this reason, B is preferably 0.0001% or more and 0.005% or less.

N: 0.0001% or more and 0.05% or less N is more preferably as little as possible because N deteriorates workability when it exceeds 0.05%. However, excessive reduction only increases the cost, so 0.0001% is set as the lower limit. Therefore, N is preferably 0.0001% or more and 0.05% or less.

  In the steel of the present invention, besides these components, Cu, Cr, Co, Ni, Mo, W, Mg, V, Ce, La, Nd, Pr, Sm, etc. may be added as necessary. it can.

Next, the iron-zinc alloy coating will be described. The Zn content in the iron-zinc alloy coating is 85% or more. If it is less than 85%, it is inferior to coating property and weldability, and corrosion resistance after coating. The adhesion amount of the iron-zinc alloy coating is not particularly specified, but is preferably ²⁰ to 100 g / m ² from the viewpoint of corrosion resistance and workability.

  Next, the steel structure will be described. FIG. 1 is a view showing the structure of the ground iron of the galvannealed steel sheet, and FIG. 2 is a view for explaining the length of the long axis in the steel sheet rolling direction and the length of the short axis in the steel sheet rolling width direction. As shown in FIGS. 1 and 2, the structure of the ground surface of the galvannealed steel sheet of the present invention is such that the length of the long axis 1 in the steel sheet rolling direction / the length of the short axis 2 in the steel sheet width direction is 5 or more. Ferrite grains are 90% or more in area ratio. Since the stretched ferrite grains having such a size have excessive strain in the crystal grains, the alloying reaction of Fe and Zn is not affected by the influence of the crystal orientation of the iron alloy, because the plating alloying speed is not significantly affected. Since it occurs suddenly, the plating grows uniformly and the appearance is beautiful. Furthermore, since the alloying reaction is fast, it is possible to reduce the alloying heating temperature and increase the sheet passing speed, and there is an advantage that productivity is increased. On the other hand, if the length of the major axis in the rolling direction of the steel sheet / the length of the minor axis in the width direction of the steel sheet is less than 5, the amount of strain in the crystal is insufficient, and the rate of plating alloying depends on the influence of the crystal orientation of the steel Therefore, the plated alloy layer 5 grown on the base iron 4 made of the crystal grains 3 having various crystal orientations has severe irregularities, becomes an alloyed hot dip galvanized with a poor appearance, and is inferior in productivity. The schematic diagram is shown in FIG. Moreover, FIG. 4 is a diagram showing the relationship between the plating thickness at an arbitrary position in a 500 μm × 500 μm visual field enlarged to an arbitrary 200 times that of the alloyed hot-dip galvanized steel sheet and the plating base structure corresponding to each position. It is. The length of the major axis in the rolling direction of the steel sheet / the length of the minor axis in the width direction of the steel sheet is less than 5, and the plating thickness generated on the ground crystal has a large variation of about 1 to 9 μm. This is considered to be a result of the difference in the plating alloying speed due to the difference in the thickness. On the other hand, the plating thickness produced on the base iron crystal whose length in the steel plate rolling direction major axis / steel plate width direction minor axis is 5 or more is about 5 to 8 μm, and the variation is smaller than the former. This is considered to be a result of the alloying of the plating progressing at an approximately equal speed on each crystal. Thus, it is important that the structure of the surface of the ground iron is composed of ferrite grains in which the length of the long axis in the steel plate rolling direction / the length of the short axis in the steel plate width direction is 5 or more.

  However, it is not necessary that the surface structure of the ground iron is completely covered with ferrite grains in which the length of the major axis in the rolling direction of the steel sheet / the length of the minor axis in the width direction of the steel plate is 5 or more. That's fine. When evaluated in three stages of ×, ○, ◎ according to the degree of appearance irregularity by visual observation, if it occupies 90% or more as shown in FIG. 98% or more. It should be noted that the structure of the surface of the base iron as defined herein means that an arbitrary part of the alloyed hot-dip galvanized steel sheet is immersed in fuming nitric acid and an arbitrary part of the surface of the base iron where the grain boundaries are etched is 500 μm × 500 μm It is a structure of a ground iron observed when observed with an optical microscope or a scanning electron microscope in a visual field.

  Moreover, there is no problem even if such a stretched structure is present inside the base iron in appearance. However, a stretched structure up to the center of the plate thickness impairs workability and is not suitable for practical use. Therefore, the structure in the range of depth of 100 μm or more from the surface of the ground iron needs to have an area ratio of 10% or less of ferrite grains in which the length of the major axis in the steel sheet rolling direction / the length of the minor axis in the steel sheet width direction is 5 or more. is there. When a tensile test was carried out with a JIS13B tensile test piece, when the elongation was less than 20%, x was evaluated when the elongation was less than 20% and less than 40%, and ◎ when the elongation was 40% or more. When the area ratio is more than 10%, the processability is inferior as shown in FIG. In addition, the structure in the range of depth of 100 μm or more from the surface of the steel defined here is a 500 × 500 μm field of view of the steel plate cross section, which is enlarged 200 times from the surface (front and back) to a depth of 100 μm or more. It is a structure of the iron base observed when observed with an optical microscope or a scanning electron microscope.

  The alloyed hot-dip galvanized steel sheet of the present invention is manufactured by hot rolling a low carbon steel slab, pickling, and further performing cold rolling, annealing, hot-dip galvanizing, and heat alloying treatment. The slab heating condition is not particularly specified, and there is no problem as long as it is a condition for producing a general steel plate. However, during the hot finish rolling, the different peripheral speed rate is 1% to 20%. 1 pass or more and / or rolling of different diameter rolls in which the ratio of the diameters of the upper and lower rolling rolls is 1.2 or more and 1.5 or less, and the finish rolling finish temperature is the sheet width and heat in the rolling direction. The finish rolling temperature at the time of hot rolling needs to be (Ar3 point−30 ° C.) or more and Ar3 point or less in any of the sheet width and the rolling direction.

The reason for setting the different peripheral speed ratio to 1.05 or more and 1.50 or less is that if it is less than 1.05, the ground surface structure of the galvannealed steel sheet cannot be the stretched ferrite structure defined in the present invention. If it exceeds .50, it will no longer have any impact on the organization and will only make it difficult to operate. A preferred range is from 1.20 to 1.40. Here, the different peripheral speed ratio of the present invention is the ratio of the peripheral speeds of the upper and lower rolls of each stand, that is,
Different peripheral speed ratio = High speed side roll peripheral speed / Low speed side roll peripheral speed (A)
It is represented by The number of rolling passes is one, but there is no problem even if more passes are applied. Further, the stand for carrying out the different peripheral speed rolling is not particularly specified, but it is preferable that the stand is used at the final stand because the maximum effect can be obtained.

Moreover, if the reason why the ratio of the diameters of the upper and lower rolling rolls is in the range of 1.2 to 1.5 is less than 1.2, the ground surface structure of the galvannealed steel sheet is defined in the present invention. This is because, when the stretched ferrite structure cannot be obtained and exceeds 1.5, the structure becomes an abnormal structure up to the inside of the base iron. A preferable range is 1.25 or more and less than 1.35. Here, the ratio of the diameter of the upper and lower rolling rolls is
Ratio of diameter of upper and lower rolling rolls in each stand = larger diameter side roll diameter in upper and lower directions / smaller diameter side roll diameter in upper and lower sides (B)
It is represented by The number of rolling passes is one, but there is no problem even if more passes are applied. Further, a stand for rolling different diameter rolls is not particularly specified, but it is desirable that the stand can be used at the final stand because the maximum effect can be obtained.

  The reason for setting the finish rolling temperature to (Ar3 point −30 ° C.) or more and Ar3 point or less is that, in the temperature range of Ar3 point −30 ° C., the stretched ferrite structure defined in the present invention reaches the inside of the galvannealed steel sheet. It is because it produces | generates and is inferior to workability. On the other hand, at a temperature exceeding the Ar3 point, the stretched ferrite structure defined in the present invention is not generated on the surface of the ground iron of the galvannealed steel sheet. A preferable range is (Ar3 point−20 ° C.) or more and (Ar3 point−10 ° C.) or less.

  Even if only the finish rolling temperature is set to (Ar3 point-30 ° C.) or more and Ar3 point or less, the alloyed hot-dip galvanized steel sheet of the present invention cannot be obtained. The alloyed hot-dip galvanized steel sheet of the present invention can be realized by a synergistic effect by combining with roll rolling. There is no problem even if both the different speed rolling and the different diameter roll rolling are combined.

The Ar3 point referred to here is the γ → α transformation temperature in the cooling process and is obtained from the following equation.
Ar3 = 900−500 × (C%) − 50 × (Mn%) + 200 × (P%) (C)
Here, C% is C mass% of the steel sheet to be hot rolled, Mn% is Mn mass% of the steel sheet to be hot rolled, and P% is P mass% of the steel sheet to be hot rolled.

  As for the rolling roll used for different peripheral speed rolling or different diameter roll rolling, it is desirable that the surface roughness Ra is 0.5 μm or more and 1.5 μm or less. By using a rolling roll having a roughness within this range, it is possible to introduce an appropriate strain only to the steel sheet surface layer, and to effectively form a stretched structure on the surface of the ground iron after galvannealing. it can.

After hot rolling, pickling is performed, and the scale generated during hot rolling is removed. What is necessary is just to implement the pickling conditions by the method currently performed conventionally, for example, a steel plate is immersed in 50 degreeC or more hydrochloric acid.
Although it cold-rolls after pickling, it is preferable that the rolling reduction shall be 70% or more and 90% or less. If it is less than 70%, an unrecrystallized structure tends to remain up to the center of the sheet thickness after annealing, and the workability is inferior. If the rolling reduction ratio exceeds 90%, a recrystallized microstructure is easily formed up to the steel sheet surface. Is difficult to obtain. Moreover, it is desirable that the rolling oil adhesion amount on the steel sheet after cold rolling is 0.1 mg / m ² or more and 5 mg / m ² or less. By reducing the amount of rolling oil sprayed during cold rolling, the amount of rolling oil attached is within this range, but as a result, an appropriate frictional force is generated between the roll and the steel sheet, effectively extending the stretch structure of the present invention. Can be formed. The rolling oil adhesion amount is the amount of oil adhering to the surface of the steel sheet after cold rolling. For example, the fixed oil surface of the cold rolled steel sheet surface can be measured with a wiping medium such as cloth. Wipe off, immerse the wiping medium in normal-hexane, carbon tetrachloride, carbon tetrachloride substitute (S-316), etc. to extract oil, extract, filter, etc. It can be obtained by measuring with an oil concentration meter (for example, OCMA-300 manufactured by Horiba, Ltd.) or an infrared spectrophotometer.

  After the cold rolling, it is preferable to heat and anneal at a soaking temperature of 750 ° C. or higher and 850 ° C. or lower for 30 seconds or more and 150 seconds or less. When below this range, the unrecrystallized structure tends to remain up to the center of the plate thickness and is inferior in workability. When exceeding this range, the surface of the steel sheet tends to become a recrystallized microstructure, and it is difficult to obtain the surface structure of the present invention. After annealing, hot dip galvanization and heat alloying treatment are performed. The temperature of the galvanizing bath is preferably 440 ° C to 500 ° C, and the heating alloying temperature is preferably 480 to 560 ° C. By producing under the above conditions, a hot dip galvanized steel sheet excellent in appearance quality of the present invention can be realized.

Steels having the compositions shown in Table 1 were melted and continuously cast into slabs. The slab was heated at 1200 ° C. for 1 hour and then hot-rolled to obtain a hot-rolled steel plate having a thickness of 4 mm. Thereafter, cold rolling, annealing, hot dip galvanizing, and heat alloying treatment were performed to produce an alloyed hot dip galvanized steel sheet. Details of the hot rolling conditions and the cold rolling conditions are shown in Tables 2 and 3. In addition, the ratio of the diameter of the upper and lower rolling rolls used at the time of hot rolling is 1.0, that is, the same diameter roll. All of the annealing after the cold rolling was heated at 800 ° C. for 120 seconds, and the hot dip galvanizing was immersed in a Zn-0.13% Al plating bath with a bath temperature of 460 ° C. for 3 seconds, and the adhesion amount was 45 g / m by wiping. It adjusted so that it might be set to ^2, and it heat-alloyed at the temperature of 480-580 degreeC after that. The produced galvannealed steel sheet was evaluated as follows.

(1) Appearance Visually observed and evaluated in three stages of x, o, and ◎ according to the degree of unevenness in appearance.

(2) Observation of ferrite structure at a depth of 100 μm or more from the surface iron surface ferrite and the surface of the iron surface Dissolve the plating with 10% hydrochloric acid and observe the surface of the iron surface in a 500 × 500 μm field of view magnified 200 times with an optical microscope. The area ratio of ferrite grains in which the length of the major axis in the rolling direction of the steel sheet / the length of the minor axis in the width direction of the steel sheet was 5 or more was determined. The ferrite having a depth of 100 μm or more from the surface of the ground iron was observed at the center of the plate thickness.

(3) Workability A tensile test was carried out with a JIS 13B tensile test piece. The case where the elongation was less than 20% was evaluated as x, the case where it was 20% or more and less than 40% was evaluated as ◯, and the case where it was 40% or more was evaluated as ◎. The results are shown in Table 2 and Table 3.

  Table 2 shows the results for Steel A. A1 to A3 are cases where the finish rolling temperature is changed within the scope of the present invention. Both are good in appearance and workability, but A2 in the range of (Ar3 point−20 ° C.) to (Ar3 point−10 ° C.) is particularly excellent. A4 is a case where the different peripheral speed ratio is reduced from that of A2, and is an acceptable level although the appearance quality is slightly lowered. A5 is a case where the number of rolling passes is increased from A2, but the appearance and workability are not particularly changed. A6 is a case where the roughness of the rolling roll is reduced from that of A2, and is an acceptable level although the appearance quality is slightly lowered. A7 is a case where the roughness of the rolling roll is increased from that of A2, and the appearance is improved but the workability is slightly lowered. A8 and A9 are cases where the rolling reduction during cold rolling is reduced or increased from A2, and the overall performance of the appearance and workability is slightly inferior to A2, but both are acceptable levels. A10 is a case where the amount of rolling oil at the time of cold rolling is increased from that of A2, and the lubricity is improved and the amount of strain in the steel is reduced. On the other hand, A11 is a case where the different peripheral speed ratio is outside the range of the present invention, and no stretched structure is observed on the surface of the ground iron, and the appearance is inferior. A12 is a case where the finish rolling temperature exceeds the range of the present invention, and the entire base iron has a recrystallized structure, so that the appearance is inferior.

  Table 3 shows the results for Steel B. B1 to B3 are cases where the finish rolling temperature is changed within the scope of the present invention. Both are good in appearance and workability, but B2 in the range of (Ar3 point−20 ° C.) or more and (Ar3 point−10 ° C.) or less is particularly excellent. B4 is a case where the different peripheral speed ratio is reduced as compared with B2, and is an acceptable level although the appearance quality is slightly lowered. B5 is a case where the number of rolling passes is increased from B2, but the appearance and workability are not particularly changed. B6 is a case where the roughness of the rolling roll is reduced from B2, and is an acceptable level although the appearance quality is slightly lowered. B7 is the case where the roughness of the rolling roll was increased from B2, and the appearance was improved but the workability was slightly reduced. B8 and B9 are cases where the rolling reduction during cold rolling is reduced or increased as compared with B2, and both are inferior to B2 in overall performance of appearance and workability, but both are acceptable levels. B10 is a case where the amount of rolling oil at the time of cold rolling is increased from A2, and the lubricity is improved and the amount of strain in the steel is reduced, so that the appearance quality is slightly lowered, but it is a pass level. On the other hand, B11 is a case where the different peripheral speed ratio is outside the range of the present invention, and no stretched structure is recognized on the surface of the ground iron, and the appearance is inferior. B12 is a case where the finish rolling temperature exceeds the range of the present invention, and since the entire base iron has a recrystallized structure, the appearance is inferior.

A steel having the composition shown in A of Table 1 was melted and continuously cast into a slab. The slab was heated at 1200 ° C. for 1 hour and then hot-rolled to obtain a hot-rolled steel plate having a thickness of 4 mm. Thereafter, cold rolling, annealing, hot dip galvanizing, and heat alloying treatment were performed to produce an alloyed hot dip galvanized steel sheet. Details of the hot rolling conditions and the cold rolling conditions are shown in Table 4. In addition, the hot rolling was rolled in all 6 stands, and the irregular roll rolling was performed in the subsequent 4 to 6 stands. In any stand, the peripheral speed ratio is 1.0. All of the annealing after the cold rolling was heated at 800 ° C. for 120 seconds, and the hot dip galvanizing was immersed in a Zn-0.13% Al plating bath having a bath temperature of 460 ° C. for 3 seconds, and the amount of adhesion was 45 g / m ² by wiping. After that, a heat alloying treatment was performed at a temperature of 480 to 580 ° C. The produced galvannealed steel sheet was evaluated in the same manner as in Example 1. The results are shown in Table 4.

  A13 to A15 are cases where the finish rolling temperature is changed within the scope of the present invention. Both are good in appearance and workability, but A14 in the range of (Ar3 point−20 ° C.) or more and (Ar3 point−10 ° C.) or less is particularly excellent. A16 is a case where the ratio of the diameters of the upper and lower rolls from the 4th stand to the 6th stand is increased from that of A14, which is a pass level although the appearance quality is slightly lowered. A17 was the case where the roughness of the rolling roll was increased from that of A14. The appearance was improved, but the workability was slightly lowered. A18 is a case where the rolling reduction at the time of cold rolling is increased from A14, and although it is somewhat inferior in overall performance of appearance and workability than A14, both are acceptable levels. A19 is a case where the amount of rolling oil at the time of cold rolling is increased from that of A14. Since the lubricity is improved and the amount of strain in the steel is reduced, the appearance quality is slightly lowered, but it is an acceptable level. On the other hand, A20 is a case where the ratio of the upper and lower roll diameters is lower than the range of the present invention, and no stretched structure is observed on the surface of the ground iron and the appearance is inferior. A21 is a case where the ratio of the upper and lower roll diameters exceeds the range of the present invention, and becomes an abnormal structure up to the inside of the base iron and is inferior in workability. A22 is a case where the finish rolling temperature exceeds the range of the present invention, and the entire base iron has a recrystallized structure, so that the appearance is inferior.

It is a figure which shows the structure | tissue of the base iron of a galvannealed steel plate. It is a figure for demonstrating the length of a steel plate rolling direction long axis, and the length of a steel plate rolling width direction short axis. It is a schematic diagram of the plating alloy layer which has the severe unevenness | corrugation which grew on the base iron which consists of a crystal | crystallization which has a various crystal orientation. It is the figure which showed the relationship between the plating thickness of the arbitrary locations in the 500 micrometers x 500 micrometers visual field expanded 200 times as much as the alloyed hot-dip galvanized steel sheet, and the plating foundation structure | tissue corresponding to each location. It is a figure which shows the relationship between the ratio which the ferrite grain which the length of a steel plate rolling direction major axis / steel plate width direction minor axis length is 5 or more occupies on the surface of a ground iron, and the appearance quality. It is a figure which shows the relationship between the ratio for which the ferrite grain which the length of a steel plate rolling direction major axis / steel plate width direction minor axis length is 5 or more occupies the area | region of 100 micrometers or more from a surface iron surface, and workability.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate rolling direction long axis 2 Steel plate width direction short axis 3 Crystal grain 4 Base iron 5 Plating alloy layer

Claims (4)

% By mass
C: 0.001% to 0.01%,
Si: 0.001% or more and 0.2% or less,
Mn: 0.01% or more and 3% or less,
P: 0.005% or more and 0.2% or less,
S; 0.001% or more and 0.03% or less,
Al; 0.005% or more and 0.1% or less,
In addition,
Ti: 0.001% or more and 0.05% or less,
Nb: 0.001% or more and 0.05% or less,
B; 0.0001% to 0.005%,
N; including 0.0001% or more and 0.05% or less of one or more,
An alloyed hot-dip galvanized steel sheet having an iron-zinc alloy coating containing 85% or more of Zn on the surface of the steel sheet, the balance of which is made of Fe and inevitable impurities. The ferrite grain having a length of the minor axis in the width direction of the steel sheet of 5 or more is composed of 90% or more in area ratio, and the structure in the range of 100 μm or more in depth from the surface of the base iron is the length of the major axis in the steel sheet rolling direction. An alloyed hot-dip galvanized steel sheet excellent in appearance quality, characterized in that ferrite grains having a widthwise minor axis length of 5 or more are 10% or less in area ratio. In the method for producing an alloyed hot-dip galvanized steel sheet, which is hot-rolled with a low carbon steel slab comprising the components of claim 1 and then subjected to pickling, cold rolling, annealing, hot dip galvanizing, and heat alloying treatment. At the time of hot finish rolling, different peripheral speed rolling defined by the following formula (A) is performed at least one pass, and the Ar3 temperature is defined by the following formula (C). When the hot finish rolling finish temperature is (Ar3 point-30 ° C.) or higher and Ar3 point or lower in any of the steel plate width direction and the steel plate rolling direction, the alloyed molten zinc excellent in appearance quality Manufacturing method of plated steel sheet.
Different peripheral speed ratio = High speed side roll peripheral speed / Low speed side roll peripheral speed (A)
Ar3 = 900−500 × (C%) − 50 × (Mn%) + 200 × (P%) (C)
C%: C mass% of the steel sheet to be hot rolled, Mn%: Mn mass% of the steel sheet to be hot rolled,
P%: P mass% of the steel sheet to be hot rolled In the method for producing an alloyed hot-dip galvanized steel sheet, which is hot-rolled with a low carbon steel slab comprising the components of claim 1 and then subjected to pickling, cold rolling, annealing, hot dip galvanizing, and heat alloying treatment. At the time of hot finish rolling, rolling of different diameter rolls in which the ratio of the diameters of the upper and lower rolling rolls defined by the following formula (B) is 1.2 or more and 1.5 or less is performed, and the Ar3 temperature is set to the following (C ) When the hot finish rolling finish temperature is defined by the formula, the appearance quality is characterized by being (Ar3 point-30 ° C.) or more and Ar3 point or less in both the steel plate width direction and the steel plate rolling direction. A method for producing an excellent galvannealed steel sheet.
Ratio of diameter of upper and lower rolling rolls in each stand = larger diameter side roll diameter in upper and lower directions / smaller diameter side roll diameter in upper and lower sides (B)
Ar3 = 900−500 × (C%) − 50 × (Mn%) + 200 × (P%) (C) In the manufacturing method of the galvannealed steel plate of Claim 2 or 3, the amount of rolling oil spraying at the time of cold rolling is 0.1 mg / m < ² > in the amount of rolling oil adhesion on the steel plate after cold rolling. The manufacturing method of the galvannealed steel plate excellent in the external appearance quality characterized by being 5 mg / m < ² > or less above.

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