JP5011953B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to an galvannealed steel sheet and a method for producing the same. In particular, the present invention relates to an alloyed hot-dip galvanized steel sheet having excellent surface properties and formability and a method for producing the same.

  In recent years, hot-dip galvanized steel sheets have been used in large quantities in the industrial fields of home appliances, building materials, and automobiles from the viewpoint of rust prevention, and in particular, alloyed hot-dip galvanizing has been used from the viewpoints of economy, paintability, and weldability. Widely applied. In particular, since ultra-low carbon steel sheets with excellent workability have been developed, ultra-low carbon alloyed hot-dip galvanized steel sheets have been widely used for panels such as automobile fenders and doors.

  By the way, automotive panels are required to have a beautiful appearance due to their commercial characteristics, but the extremely low carbon alloyed hot-dip galvanized steel sheet has uneven stripes on the surface due to differences in alloying reaction. There was a problem that the appearance of the streak pattern defect developed and the appearance deteriorated. This problem is particularly noticeable when galvannealed alloy is applied to a Ti-containing ultra-low carbon steel plate that uses Ti to fix solute C.

  For example, Patent Document 1 discloses an invention that pays attention to the difference in texture because the difference in the diffusion of iron into the plating layer affects the shape of the unevenness. According to the invention, it has been reported that the streak defect is improved by controlling the finishing temperature of the hot rolling in a narrow range, but it is not sufficient to satisfy the requirement of a very beautiful appearance like an automotive panel. It was enough.

  Patent Document 2 discloses a technique for controlling the amount and shape of precipitates, assuming that streak defects occur when non-recrystallization occurs in the surface layer. However, recrystallization up to the surface layer did not satisfy the requirement for a very beautiful appearance like an automotive panel, as described above.

Specific examples of the “automobile panel” include a door panel, a side panel, and a hood.
JP-A-10-18011 JP 2001-172744 A

  The present invention provides an alloyed hot-dip galvanized steel sheet having excellent surface properties and formability that can withstand the use of automotive panels, which can advantageously solve the above problems industrially, and a method for producing the same. With the goal.

  Specifically, the present invention provides an alloyed hot-dip galvanized steel sheet using a Ti-containing ultra-low carbon steel sheet as a base material and improved surface properties and formability as described above, and a method for producing the same.

  The streak defect is caused by the non-uniform progress of the alloying reaction of the plating layer during the alloying process performed after hot dip galvanization. However, this non-uniformity of the alloying reaction is caused by the difference in texture on the surface of the base plate as described in Patent Document 1 or the non-recrystallized grains on the surface of the base plate as described in Patent Document 2. It has been clarified for the first time by detailed examinations by the present inventors that it is caused by the distribution state of the crystal grain size in the surface layer portion of the steel plate which is the plating substrate, rather than by the remaining of the plating. That is, because the alloying reactivity of the grain boundary part of the steel plate that is the plating substrate is higher than the intragranular part, the plating surface of the grain boundary part has a convex shape, and the plating surface of the intragranular part has a concave shape, The plating surface is rough and has a blackish appearance in the portion where the unevenness is severe, and the whitish appearance is conversely shown in the portion where the unevenness is small, thereby forming a streak defect.

  Further investigation based on this new knowledge revealed that the surface of the steel sheet has a lot of fine precipitates, and the unevenness is severe, the plating surface has a dark appearance, and on the contrary, coarse precipitates are generated and fine precipitates are formed. It was found that there were few irregularities in the portion where there was a little, and the plating surface had a whitish appearance. The reason for this is not necessarily clear, but it is considered that the portion where there are many fine precipitates is because the crystal grains in the surface layer portion of the steel sheet are uniform and fine.

The inventors have further studied and found out that Ti precipitates on the surface layer portion of the steel sheet are particularly likely to cause non-uniformity in size.
That is, in the case of a Ti-containing ultra-low carbon steel sheet, Ti nitride is first precipitated in the slab stage of continuous casting and the subsequent cooling process. Then, Ti sulfide precipitates in the slab heating stage used for hot rolling, and the remaining solid solution Ti precipitates as Ti carbide after winding after hot rolling. In addition, during hot rolling, Ti nitrides and Ti sulfides that have already precipitated may change to Ti oxides, or residual solid solution Ti may newly precipitate as oxides. Indicates complex precipitation behavior.

  Among the Ti precipitates, the oxides and nitrides of Ti are coarse, so the action of refining the crystal grain size of the steel sheet surface layer portion is small, and relatively coarse sulfides are crystal grains of the steel plate surface layer part. Although the diameter is slightly refined, it has been found that fine carbide has a strong effect of refining the crystal grain size of the steel sheet surface layer.

  Therefore, the portion where a large amount of Ti oxide is formed during hot rolling results in a decrease in fine Ti carbide, and the plated surface in the final product has a whitish appearance, while the formation of Ti oxide is small. As a result, the fine Ti carbide increases as a result, and the plated surface in the final product has a dark appearance.

  This non-uniformity in the amount of Ti oxide produced is caused by non-uniformity in the scale state formed on the surface of the steel sheet in the hot rolling process (for example, blister generation), and then the steel sheet is rolled. As a result, the non-uniform scale is stretched in the rolling direction to form a streak. Therefore, if the scale formed on the surface of the steel sheet in the hot rolling process can be made uniform, the generation of Ti oxide can be made uniform and the streak defect can be suppressed. Have difficulty.

  As described above, the uneven distribution of Ti carbide cannot be avoided as long as the uneven distribution of Ti oxide on the surface layer of the steel plate is inevitable. Therefore, the present inventors have newly conceived of suppressing the streak pattern defect by reducing the Ti content within a range that does not adversely affect the formability and suppressing the formation of Ti carbide itself as much as possible. This is the first point of the present invention.

  In this regard, even if the Ti nitride in the steel sheet changes to Ti oxide, there is no difference in the effect of grain refinement, but when Ti sulfide changes to Ti oxide, the crystal grains become slightly coarser and Since the portion becomes slightly whitish, it is preferable to suppress the formation of Ti sulfide.

  Specifically, the Ti content is set to 0.014% or less, or Ti ≦ (48/14) × N + (48/32) × S, preferably Ti ≦ (48/14) × N. To. Here, the former suppresses the formation of Ti carbide by reducing the absolute amount of Ti content. On the other hand, in the latter, when the contents of N and S are large, the production of Ti carbide is suppressed correspondingly, and when the content of N is large, the production of Ti carbide and Ti sulfide is suppressed correspondingly. Therefore, these contributions are taken into consideration.

  However, if the Ti content is simply reduced, the solid solution C remains in the steel, which causes a new problem of normal temperature aging deterioration of the steel sheet during storage. Therefore, Nb is contained in order to fix the solute C. Nb carbide, as well as Ti carbide, leads to the refinement of crystals in the steel plate surface layer portion. Therefore, when the precipitation state of Nb carbide in the steel plate surface layer portion is non-uniform, the above-described mechanism causes streak pattern defects. Arise.

  As a result of intensive studies on this point, the present inventors have newly found out that Nb, unlike Ti, can effectively suppress streak pattern defects when contained in a large amount. This is the second point of the present invention.

  That is, Nb hardly precipitates as nitrides and sulfides, and the ability to form oxides is not as high as that of Ti. Therefore, the influence of scale in the hot rolling process is small, and Nb that forms carbides is not present in the steel sheet surface layer. Uniformity is suppressed. However, the Nb carbide produced in the hot rolling process is coarse and unfavorable because it causes non-uniform crystal grain growth in the annealing process after cold rolling. For this reason, it is necessary to suppress the production | generation of the coarse Nb carbide | carbonized_material in a hot rolling process as much as possible, and it is so preferable that coiling temperature is low. In this case, Nb carbide is precipitated in the annealing process, but Nb carbide is once finely precipitated in the temperature rising process of the annealing process and then re-dissolved, so that the absolute amount of finely precipitated Nb carbide is Even when the amount is small, non-uniform crystal grain growth is caused, and in the portion where the Nb carbide is reduced, the crystal grains become coarse, resulting in a whitish appearance. In order to prevent this, the inventors have newly found that it is necessary to secure a sufficient amount of Nb carbide precipitation and to ensure unmelted Nb carbide during annealing. Specifically, the Nb content is set to 0.021% or more.

  Further, when the Ti content is reduced, like the solid solution C, the solid solution N also remains in the steel, resulting in a new problem of normal temperature aging deterioration of the steel sheet. Therefore, this problem can be solved by fixing a solid solution N by containing a large amount of Al which is usually contained only for deoxidation in extremely low carbon steel.

  Further, by reducing the Ti content, S which is usually fixed by Ti in the ultra-low carbon steel is not fixed, and a new problem of material defects due to hot brittleness due to S also arises. Therefore, this problem is solved by strictly regulating the Mn content and fixing S.

  The steel sheet according to the present invention has a uniform black appearance on the entire surface because fine Nb carbide is generated in the surface layer portion of the steel sheet as the plating substrate. The black appearance is attributed to the fact that the plating surface has many irregularities, and has a center line average roughness (Ra) of 0.5 μm or more. In order to obtain a more uniform surface property as a whole, it is preferably 0.7 μm or more, more preferably 0.9 μm or more.

The present invention made based on the above-described new findings is as follows.
(1) An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, wherein the chemical composition in mass% of the steel sheet is
C: 0.0038% or less, Si: 0.20% or less, Mn: 0.03~ 0.16%,
P: 0.03% or less, S: 0.03% or less, Al: 0.010 to 0.23%,
N: 0.0040% or less, Ti: 0.003~0.014%, and Nb: 0.021~ 0.035%,
Contain, together with the balance being iron and impurities, and satisfying any of the following (1) and (2) (each element symbol in the formula means the content by mass percent of the element) An alloyed hot-dip galvanized steel sheet having a center line average roughness (Ra) of the plated surface of the alloyed hot-dip galvanized steel sheet of 0.5 μm or more .

Mn / {S- (32/48) × Ti *} ≧ 14 (1)
S- (32/48) × Ti * ≦ 0 (2)
However, Ti * = Ti− (48/14) × N, and when Ti * ≦ 0, Ti * = 0.

(2) An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, wherein the chemical composition in mass% of the steel sheet is
C: 0.0025% or less, Si: 0.20% or less, Mn: 0.03~ 0.16%,
P: 0.03% or less, S: 0.03% or less, Al: 0.010 to 0.23%,
N: 0.0040% or less, Ti: 0.003% or more, and Nb: 0.026 to 0.035 %
And the balance is composed of iron and impurities, and satisfies the following formulas (1) and (3) (each element symbol in the formula means a content in mass% of the element), and the alloy An alloyed hot-dip galvanized steel sheet, characterized in that the center line average roughness (Ra) of the plated surface of the hot-dip galvanized steel sheet is 0.5 μm or more .

Mn / {S- (32/48) × Ti *} ≧ 14 (1)
Ti ≦ (48/14) × N + (48/32) × S (3)
However, Ti * = Ti− (48/14) × N, and when Ti * ≦ 0, Ti * = 0.

(3) The galvannealed steel sheet according to (2), wherein the chemical composition satisfies the following formula (4):
Ti ≦ (48/14) × N (4)
(4) The alloyed hot-dip galvanized steel sheet according to any one of (1) to (3), wherein the chemical composition contains B: 0.000020% or less in place of part of iron.

  (5) The galvannealed steel sheet according to any one of the above (1) to (4), wherein the r value in the rolling 45 ° direction is 1.6 or more and the bake hardening amount in the rolling direction is 15 MPa or less.

(6) A method for producing an alloyed hot-dip galvanized steel sheet comprising the following steps:
(A) The steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 is hot-rolled to complete the rolling at 860 to 1000 ° C and wound up in a temperature range of 640 ° C or lower. Hot rolling process for hot rolled steel sheet;
(B) A pickling process in which the hot-rolled steel sheet is pickled to form a pickled steel sheet;
(C) a cold rolling step in which the pickled steel sheet is cold-rolled at a rolling reduction of 70% or more to obtain a cold-rolled steel sheet;
(D) An annealing step in which the cold-rolled steel sheet is annealed at 750 to 900 ° C. to obtain an annealed steel sheet; and (E) the hot-dip galvanized steel and alloying treatment are sequentially applied to the annealed steel sheet. Alloying hot dip galvanizing process.

  In the present invention, the coating bake hardening amount in the rolling direction is obtained by applying heat treatment at 170 ° C. for 20 minutes after applying 2% pre-strain to simulate standard coating conditions as shown in the examples. This is the required value.

  ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of a streak pattern defect is suppressed in the galvannealed steel plate which uses a Ti containing ultra-low carbon steel plate as a base material, and supply of galvannealed with the beautiful appearance is attained. Moreover, this alloyed hot-dip galvanized steel sheet retains workability advantageous for application to automotive panel applications possessed by ultra-low carbon steel sheets. Specifically, it has excellent workability as indicated by a high r value in the 45 ° direction and has a relatively small amount of paint baking and curing, so that the deterioration of strain aging is slow even when stored at room temperature. Therefore, the galvannealed steel sheet of the present invention is optimal for automotive panel applications including side frames.

The present invention will be described in detail below.
In the following description, “%” defining the chemical composition is “% by mass” unless otherwise specified.

[Chemical composition of base steel sheet]
C: 0.40038% or less C is an impurity element that lowers the r value of the steel sheet. In the present invention, in order to obtain the target processability, the C content is set to 0.0043% or less. The C content is preferably as low as possible, more preferably 0.0025% or less, and even more preferably 0.000020% or less. However, if the C content is less than 0.0005%, a significant increase in cost is incurred, so the C content is preferably set to 0.0005% or more.

Si: 0.20% or less Si has the effect of increasing the adhesion of the plating, but easily oxidizes, and therefore causes repelling and insufficient alloying in the hot dip galvanizing process. Therefore, the Si content is 0.20% or less. When applied to an outer panel of an automobile, the Si content is preferably 0.02% or less.

Mn: 0.03 to 0.2%
Mn combines with S to form MnS and has a function of preventing hot brittleness due to S. Therefore, the Mn content is set to 0.03% or more, preferably 0.05% or more. However, if it is contained in a large amount, the formability of the steel sheet deteriorates. Therefore, the Mn content is 0.2% or less, preferably 0.16% or less.

  S in steel causes hot brittleness to generate scale and the like, thereby causing deterioration of surface properties and flaws. Therefore, it is necessary to fix S as Ti sulfide or MnS. In the present invention, in order to limit the Ti content, it is necessary to fix S mainly by Mn. For this purpose, it is necessary to satisfy the following formula (1).

Mn / {S- (32/48) × Ti *} ≧ 14 (1)
However, Ti * = Ti− (48/14) × N, and when Ti * ≦ 0, Ti * = 0.
As will be described later, when the Ti content is limited to 0.014% or less, it is not always necessary that the Mn content satisfies the above formula (1).

P: 0.03% or less P has a function of lowering the alloying rate, and a large amount makes it difficult to obtain an appropriate degree of alloying. Therefore, the P content is set to 0.03% or less. Preferably it is 0.025% or less.

S: 0.03% or less S has a function of precipitating excess Ti after forming Ti nitride as Ti sulfide, and suppressing precipitation of Ti carbide that greatly affects the streak pattern. However, since Ti sulfide is also a fine precipitate, there is no influence on the streak pattern even if it is not as fine as Ti carbide, which is a fine precipitate. Moreover, when S exists excessively, it will cause the flaw by hot brittleness. Therefore, the S content is 0.03% or less. The S content is preferably 0.008% or less, more preferably 0.005% or less.

N: 0.0040% or less N has a function of generating coarse Ti nitride and detoxifying Ti. However, when N is contained excessively, it causes aging deterioration at room temperature. Therefore, the N content is set to 0.0040% or less. Preferably it is 0.0033% or less.

Al: 0.010 to 0.23%
In addition to deoxidation during steel making, Al is an element necessary for suppressing normal temperature aging deterioration by fixing nitrogen in steel as AlN. For this reason, Al content shall be 0.010% or more. In this invention, in order to suppress the production | generation of Ti carbide | carbonized_material and Ti sulfide which influence a streak pattern, the upper limit of Ti content is restrict | limited strictly. For this reason, fixation of nitrogen in steel with Al is more important than conventional ultra-low carbon steel. Therefore, the Al content is preferably 0.031% or more, more preferably 0.061% or more. On the other hand, even if Al is contained in a large amount, the effect is saturated, so the Al content is 0.23% or less, preferably 0.10% or less.

Ti: 0.003% or more Ti has the effect of fixing the N in the steel, thereby suppressing aging deterioration due to the solute N and improving the r value of the steel sheet. In order to obtain the effects of these actions, the Ti content is set to 0.003% or more. Preferably it is 0.005% or more.

However, as described above, Ti affects the generation of streak pattern defects by various changes in the form of precipitation. Therefore, for the upper limit of Ti content,
(A) Strictly limit the absolute amount as 0.014% or less, or
(B) Depending on the N and S content,
Ti ≦ (48/14) × N + (48/32) × S (3)
To satisfy. In this case, in order to suppress the formation of Ti sulfide,
Ti ≦ (48/14) × N (4)
It is preferable to satisfy.

When the Ti content is limited to 0.014% or less as in (A) above, the Mn content does not necessarily satisfy the above formula (1) with respect to the Ti and S contents. Instead, Ti content depends on S content,
S- (32/48) × Ti * ≦ 0 (2)
(The meaning of Ti * in the formula is the same as above) may be satisfied. In other words, this is a case where the value of (32/48) × Ti * is set to low S so as to be higher than the S content. In this way, the occurrence of streak defects due to Ti is suppressed, and at the same time, the excess S is completely fixed by Ti, the deterioration of surface properties and the generation of wrinkles due to S can be suppressed, and the Mn content is Ti and S content. Regardless of, it can be adjusted within the above range.

Nb: 0.021 to 0.05%
Nb has the effect of fixing C in steel as a carbide and suppressing normal temperature aging deterioration. In the present invention, in order to strictly limit the upper limit of the Ti content, it is necessary to fix C in the steel with Nb. However, if the Nb carbide is not uniformly and finely deposited on the surface layer of the steel plate, a streak defect occurs. In order to precipitate Nb carbide uniformly and finely, the Nb content is set to 0.021% or more. More preferably, the content is 0.026% or more. On the other hand, since a large amount of Nb causes a decrease in moldability, the Nb content is 0.05% or less, preferably 0.035% or less.

B: 0.000020% or less in some cases B has an effect of improving weldability and secondary work brittleness resistance, and can be contained as necessary. In order to surely obtain the effect by the above action, the B content is preferably set to 0.0002% or more. However, if a large amount of B is contained, the formability of the steel sheet is deteriorated. Therefore, when B is contained, the B content is set to 0.0010% or less. Preferably it is 0.0009% or less.

  Other than the above are iron and impurities. Typical impurities that do not greatly affect the streak pattern defects and surface properties are 0.5% or less of Cr, Mo, Ni, Cu, V, W, 0.01% or less of Ca, Sn, As, respectively. , Sb and the like.

[Hot rolling conditions]
Hot rolling is preferably performed under the following conditions.
A steel ingot obtained by continuous casting or a steel slab obtained by partial rolling is hot-rolled to complete the rolling at 860 to 1000 ° C. and wound up in a temperature range of 640 ° C. or less to hot-rolled steel plate And

  The steel ingot or steel piece to be subjected to hot rolling may be in a high temperature state after continuous casting or after partial rolling, and once cooled, it is heated or held at 1300 ° C. or lower. There may be. The temperature for heating or holding is preferably lower from the viewpoint of suppressing the formation of Ti oxide, and more preferably 1250 ° C. or lower.

  When the hot rolling process consists of a rough hot rolling process and a finishing hot rolling process, the rough rolled material after completion of rough rolling may be directly subjected to finishing hot rolling, or after the completion of rough rolling. The rough rolled material may be subjected to finish hot rolling after heating or holding.

When the hot rolling completion temperature is less than 860 ° C., the reduction below the Ar ₃ transformation point increases, and the formability deteriorates. On the other hand, when the hot rolling completion temperature exceeds 1000 ° C., the problem of scale wrinkles occurs. Therefore, the hot rolling completion temperature is 860 to 1000 ° C, and more preferably 880 to 940 ° C.

  The average cooling rate until winding after completion of hot rolling is preferably 5 ° C./s or more from the viewpoint of suppressing the precipitation of Nb carbide in the hot rolling step. It is preferable to increase the cooling rate in the high temperature region, and it is more preferable to rapidly cool the average cooling rate to 10 ° C./s or higher up to 750 ° C.

  The coiling temperature is set to 640 ° C. or lower because it is preferable to suppress the precipitation in the hot rolling stage in order to precipitate Nb carbide uniformly and finely. 580 ° C. or lower is preferable, and 550 ° C. or lower is more preferable.

The obtained hot rolled steel sheet is then scaled by pickling.
[Cold rolling conditions and annealing / galvanizing conditions]
The hot-rolled steel sheet is subjected to cold rolling after pickling. If the rolling reduction in the cold rolling process is high, the r value of the final product is improved. Therefore, the rolling reduction is preferably 70% or more, particularly 80% or more.

  After the cold-rolled steel sheet is annealed, hot dip galvanizing is performed, and then alloying treatment of the plating is performed. In the present invention, except for the annealing temperature, these treatment conditions are not particularly limited, and may be the same as the conventional one.

  The higher the annealing temperature before hot dip galvanizing, the softer the cold-rolled steel sheet and the better the formability, so 750 ° C. or higher, preferably 810 ° C. or higher. On the other hand, if the annealing temperature exceeds 900 ° C., an unfavorable texture develops due to transformation and the r value of the steel sheet decreases, so the temperature is set to 900 ° C. or less.

  The alloying treatment is preferably performed so that the iron concentration during plating is 5 to 13%. If the iron concentration is less than 5%, the plating is soft and inferior in slidability, and if it exceeds 13%, the plating becomes brittle and peels off. Thereafter, as necessary, a skin pass is applied to correct the flatness and adjust the surface roughness.

There is no problem even if the plating surface layer is subjected to post-treatment such as lubrication or Fe plating after the alloying treatment.
[Characteristics of tensile test]
The alloyed hot-dip galvanized steel sheet according to the present invention is assumed to be mainly used for automotive panel applications such as side panels.

  Therefore, the tensile strength in the rolling direction is preferably 285 MPa or more as the strength of the vertical wall portion that can withstand the inflow resistance of the flange. However, if the tensile strength is too high, shape defects due to elastic recovery occur. Accordingly, the tensile strength in the rolling direction is preferably 325 MPa or less.

  Regarding the workability (deep drawability), the r value in the 45 ° direction is preferably 1.6 or more. If the r value in the 45 ° direction is less than 1.6, cracking during pressing becomes a problem. Moreover, it is preferable that the bake hardening amount (BH) in the rolling direction is 15 MPa or less so that normal temperature aging deterioration does not become a problem. When BH exceeds 15 MPa, special management is required for the storage period and location so that stretcher strain does not occur during pressing. More preferably, the r value in the 45 ° direction is 1.7 or more and BH is 9 MPa or less.

  In addition, the coating bake hardening amount is prescribed | regulated to JISG3313 about an electrogalvanized steel plate, and it is based on it in this invention. This hardening amount (BH amount) has a correlation with the amount of solute C, and if the amount of solute C is large, aging deterioration during storage at room temperature is brought about. Therefore, when the amount of BH is high, aging deterioration at room temperature tends to occur.

  The “striated pattern defect” referred to in the present invention is as described above. In the present invention, the evaluation is performed by visual four-stage evaluation, and particularly when it is used as a panel for an automobile, it is required that the streak is hardly seen. In addition, in the evaluation in the Example mentioned later, all the conventional products corresponded to "x" in which a clear streak is seen over the entire length in the visual evaluation.

Steels having chemical compositions shown in Table 1 were melted in a converter to produce 250 mm thick slabs. After the obtained slab was reheated to 1220 ° C., it was hot-rolled to a thickness of 4.4 mm under the conditions shown in Table 2. The obtained hot-rolled steel sheet was descaled by hydrochloric acid pickling and then cold-rolled to a thickness of 0.7 mm. The cold rolled steel sheet was then annealed at the annealing temperatures shown in Table 2, and then hot dip galvanized with a bath temperature of 460 ° C. was applied during cooling, followed by heating and heating for alloying. The plating adhesion amount was 45 g / m ² , and the alloying treatment was performed so that the iron concentration during plating was 9.5% by mass. Thereafter, a skin pass of 0.6% elongation was performed, and the surface of the obtained galvannealed steel sheet was visually observed. The evaluation criteria for visual observation were as follows:
◎: There are almost no streaks,
○: Some thin light muscles are seen,
Δ: Mild streaks are seen over the entire length,
X: Streaks are clearly seen over the entire length.

  Further, from this galvannealed steel sheet, a JIS No. 5 tensile test piece was taken in the rolling direction at a quarter width position, and a tensile test was conducted to determine the tensile strength (TS) and yield strength (YS) in the rolling direction. ) And elongation (EL). However, for the r value, r values (r0, r45, r90, respectively) when pulled in three directions of 0 °, 45 °, and 90 ° with respect to the rolling direction are obtained, and an average r that is an average value thereof is obtained. Values were also calculated.

Further, a sample of the alloyed hot-dip galvanized steel sheet was subjected to heat treatment at 170 ° C. for 20 minutes after imposing 2% prestrain, imitating baking at the time of coating, and the coating bake hardening amount (BH) was obtained.
The surface observation results and the tensile test results are summarized in Table 3.

In the test No.1~17, reduction of muscle pattern of the surface was observed. However, tests No. 3 and No. 5 with a high winding temperature, and tests No. 8, No. 15 and No. 16 with a large amount of Ti sulfide generation showed a difference in the degree of streak pattern improvement. About test No. 3, the streak pattern generate | occur | produced only about the part corresponded to the inner peripheral part after winding of hot rolling. Since the said part is corresponded to the part with the slow cooling rate after winding of hot rolling, the said stripe pattern is considered to originate in the coarsening of Nb carbide | carbonized_material. Test No. 7 is an example in which the Ti content of 0.000014% or less does not satisfy the formula (1) but satisfies the formula (2), and a slight streak pattern was observed.

  When a comparative example is seen, in test No. 18 with low Al content, the amount of BH was high. In Test No. 19 having a remarkably low Nb content, the surface properties were good because there was almost no precipitate of Nb affecting the state of the crystal grains in the surface layer, but the BH amount was high. In these, there is a concern that due to aging deterioration at room temperature, there may be deterioration of formability such as elongation deterioration and deterioration of surface properties due to the occurrence of stretcher strain.

  Tests No. 20 and No. 23 with high Ti content have a striking pattern and are not suitable for automotive panel applications. Nb is added to such an extent that the amount of BH is low and aging deterioration at normal temperature is not a problem. In Test No. 21, where the Nb content is lower than the range of the present invention, the precipitation state of the fine precipitates on the surface layer becomes uneven. Therefore, the streak pattern is remarkable and is not suitable for automotive panel applications. Moreover, in the test No. 22 which did not satisfy | fill (1) Formula, the material flaw considered to be due to hot brittleness occurred.

Claims (6)

An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, wherein the chemical composition in mass% of the steel sheet,
C: 0.0038% or less, Si: 0.20% or less, Mn: 0.03~ 0.16%,
P: 0.03% or less, S: 0.03% or less, Al: 0.010 to 0.23%,
N: 0.0040% or less, Ti: 0.003~0.014%, and Nb: 0.021~ 0.035%,
Contain, together with the balance being iron and impurities, and satisfying any of the following (1) and (2) (each element symbol in the formula means the content by mass percent of the element) An alloyed hot-dip galvanized steel sheet having a center line average roughness (Ra) of the plated surface of the alloyed hot-dip galvanized steel sheet of 0.5 μm or more .
Mn / {S- (32/48) × Ti *} ≧ 14 (1)
S- (32/48) × Ti * ≦ 0 (2)
However, Ti * = Ti− (48/14) × N, and when Ti * ≦ 0, Ti * = 0. An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, wherein the chemical composition in mass% of the steel sheet,
C: 0.0025% or less, Si: 0.20% or less, Mn: 0.03~ 0.16%,
P: 0.03% or less, S: 0.03% or less, Al: 0.010 to 0.23%,
N: 0.0040% or less, Ti: 0.003% or more, and Nb: 0.026 to 0.035 %
And the balance is composed of iron and impurities, and satisfies the following formulas (1) and (3) (each element symbol in the formula means a content in mass% of the element), and the alloy An alloyed hot-dip galvanized steel sheet, characterized in that the center line average roughness (Ra) of the plated surface of the hot-dip galvanized steel sheet is 0.5 μm or more .
Mn / {S- (32/48) × Ti *} ≧ 14 (1)
Ti ≦ (48/14) × N + (48/32) × S (3)
However, Ti * = Ti− (48/14) × N, and when Ti * ≦ 0, Ti * = 0. The alloyed hot-dip galvanized steel sheet according to claim 2, wherein the chemical composition satisfies the following formula (4).
Ti ≦ (48/14) × N (4)   The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the chemical composition contains B: 0.0020% or less instead of part of iron.   The galvannealed steel sheet according to any one of claims 1 to 4, wherein an r value in a rolling 45 ° direction is 1.6 or more and a coating bake hardening amount in the rolling direction is 15 MPa or less. A method for producing an galvannealed steel sheet characterized by including the following steps:
(A) The steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 is hot-rolled to complete the rolling at 860 to 1000 ° C and wound up in a temperature range of 640 ° C or lower. Hot rolling process for hot rolled steel sheet;
(B) A pickling process in which the hot-rolled steel sheet is pickled to form a pickled steel sheet;
(C) a cold rolling step in which the pickled steel sheet is cold-rolled at a rolling reduction of 70% or more to obtain a cold-rolled steel sheet;
(D) An annealing step in which the cold-rolled steel sheet is annealed at 750 to 900 ° C. to obtain an annealed steel sheet; and (E) the hot-dip galvanized steel and alloying treatment are sequentially applied to the annealed steel sheet. Alloying hot dip galvanizing process.