JP3149801B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet mainly used for automobiles, which is excellent in plating peeling resistance during processing and sharpness after coating, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, galvanized steel sheets have been used in large quantities in the fields of home appliances, building materials, and automobiles. Particularly, alloyed hot-dip galvanized steel sheets are widely used in terms of economy, rust prevention function, and performance after painting.

[0003] Normally, a hot-dip galvanized steel sheet is manufactured by subjecting a base material to appropriate degreasing and washing, annealing in a reducing atmosphere composed of hydrogen and nitrogen, cooling to a temperature near the plating temperature, and immersing it in a hot-dip zinc bath. You. For galvannealed steel sheet, hot-dip galvanized steel sheet should be 3 ~ 500 ~ 600 ℃.
Heat for 60 seconds to diffuse the base material Fe into the galvanized layer,
It is manufactured by forming a Fe—Zn alloy layer. Generally, the average Fe content of the plating layer is 8 to 12% by weight, and the coating weight is 20 to 70 g / m ² per side. If the amount is less than this range, it is difficult to manufacture by ordinary means, and if the amount exceeds this range, it is difficult to ensure the powdering resistance of the plating layer, so that it is not generally supplied.

[0004] The alloyed hot-dip galvanized steel sheet manufactured as described above is subjected to press working, processed into a predetermined shape, and painted. In order to prevent powdering, flaking and low-temperature chipping which are likely to occur during use after press working, the alloyed hot-dip galvanized steel sheet is required to have an adhesion of a plating film.

It is also important to consider that the finished appearance of a painted product is good, and there is also a demand for a steel sheet having good clarity after painting, which is an indicator of the appearance.

[0006] Powdering is a phenomenon in which a plating film becomes powdery and peels off in a region where a steel sheet undergoes compression deformation during press working. When powdering occurs, the corrosion resistance of the portion is deteriorated, and the generated Zn powder adheres to the mold, thereby impairing the appearance quality of the molded product. It is said that powdering becomes remarkable when the Fe—Zn alloy develops too much during the alloying process and an excessive hard Fe—Zn alloy layer such as a Г phase is generated excessively. Powdering has been improved by controlling the amount of plating and the degree of alloying.

[0007] Low-temperature chipping is a phenomenon in which a plating film is peeled off from an interface with a base material together with a coating film when small stones collide with a car body at a high speed in a low temperature range of 0 ° C or less. This is a problem of adhesion between the plating film and the surface of the base material, and a method of adjusting the roughness of the surface of the base material in contact with the plating film has been proposed in order to improve the low-temperature chipping property.

[0008] During the press working, the steel plate slides on the surface of the mold. At this time, a part of the plating film may be flaked and peeled off. This phenomenon is called flaking. The occurrence of flaking not only impairs the appearance quality and corrosion resistance of the molded product, but also fluctuates the wrinkle holding pressure during press working, which may cause press breakage and poor shape. The cause of the flaking is also due to the brittleness of the plating film and poor adhesion between the plating film and the base material surface, but the biggest effect is caused by the steel sheet and the mold that inhibits the inflow of the steel sheet. Is the frictional resistance between

As a method for preventing flaking, it is conceivable to increase the surface roughness of the plating film to improve the retention of the lubricant during press working. However, this method reduces the surface roughness of the plating film. The problem is that the surface becomes rough and the sharpness after painting, which is an important quality for exterior applications of automobiles, is impaired.

As a measure against the flaking, for example, as described in Japanese Patent Application Laid-Open No. 05-236984, a Fe thin film is electroplated on an alloyed hot-dip galvanized film to improve the slipperiness of a plated steel sheet. There is a way to do that. However, in this method, the electroplating must be further performed on the hot-dip coated steel sheet, so that the economic efficiency is impaired.

[0011]

The problem to be solved by the present invention relates to an alloyed hot-dip galvanized steel sheet mainly used for automobiles, and it is necessary to improve the sharpness after coating and the flaking resistance of the plating film. To provide an alloyed hot-dip galvanized steel sheet excellent in quality and a method for producing the same.

[0012]

The gist of the present invention is to provide an alloyed hot-dip galvanized steel sheet having excellent paint clarity and anti-flaking properties described in (1) below, and its production described in (2). In the way.

(1) C: 0.004% or less by weight, Si:
0.03 to 0.13%, Mn: 0.05 to 0.4%, P: 0.007 to 0.02
%, Ti: 0.018 to 0.05%, Nb: 0 to 0.02%, Al:
0.005 to 0.1%, B: 0 to 0.005%, the balance being a steel made of Fe and unavoidable impurities, containing at least 0.18% to 0.5% of Al on at least one side, and the surface roughness is arithmetic average roughness. (Ra) 1 μm or less, number of valley peaks: 320
An alloyed hot-dip galvanized steel sheet having an Fe—Zn alloy plating film having a thickness of 600 to 25.4 mm and having excellent paint clarity and flaking resistance.

(2) In the step of heating the base material before plating, in a reducing atmosphere consisting of hydrogen and nitrogen having a dew point in the range of -20 to -40 ° C., at a temperature range of 600 to 500 ° C. for 20 seconds or more. (1) The method according to (1), further comprising a step of retaining the base material for 60 seconds or less, and performing an alloying treatment after galvanizing using a plating bath containing 0.11% by weight or more and 0.14% by weight or less of Al. 3. The method for producing an alloyed hot-dip galvanized steel sheet according to item 1.

The present inventors have completed the present invention based on the following findings regarding various characteristics of the plating film.

On the surface of the plating film, there are plateau-shaped or intermittent bank-shaped protrusions and minute recesses having a diameter of about 2 to 20 μm. Many of these projections are formed around the crystal grain boundaries on the surface of the base material, and many depressions are found at positions corresponding to the inside of the crystal grains on the surface of the base material. In many cases, the convex portion has a flat top due to the influence of skin pass rolling, leveling, etc. performed after alloying. On the other hand, crystal grains of alloy plating were observed in the recessed portion, and it seems that the surface after the alloying reaction was maintained. Hereinafter, the minute concave portion on the plating surface is referred to as “crater”.

The projections or craters on the surface of the plating film are caused by the fact that the reactivity of Fe—Zn alloying between the base material and the plating film is different between the crystal grains of the base material and the grain boundaries. Since the crystal grain boundary has higher reactivity than the inside of the crystal grain, the Fe—Zn alloying reaction proceeds quickly at the grain boundary, and the plating film corresponding to the upper part of the grain boundary becomes convex. The craters increase in a portion corresponding to the inside of the crystal grain where the alloying reaction is relatively slow.

The frequency of occurrence of craters on the plating film surface is determined by the number of valley peaks (here measured during a measurement length of 25.4 mm, the depth of the valley being the deepest) measured by measuring the cross-sectional curve of the surface roughness. The number of valleys exceeds 50% of the depth). As the number of craters increases, the number of valley peaks increases, and the larger the number of valley peaks, the lower the coefficient of friction,
Flaking hardly occurs.

Ti or Nb is added to an extremely low C steel to fix C in the steel as a precipitate, and further, a steel having a limited P or B content is used as a base metal, and a zinc plating bath having a high Al content is used. By performing alloying after hot-dip plating, the number of craters can be increased and the number of valley peaks can be increased. This is presumed to be because when the Al content of the plating film is high, the difference in the alloying reaction rate between the crystal grain boundaries on the surface of the base material and the inside of the crystal grains is increased.

When there are many craters on the surface of the plating film as described above, generally, the surface roughness becomes rough, and the sharpness after coating deteriorates, which is not preferable for exterior use.
However, if the base material is made to contain an appropriate amount of Si, and in the heating step before plating, the recrystallized base material is kept at a low temperature in a specific reducing atmosphere, and then plated and alloyed, the plating surface becomes And the clarity after painting can be improved.

The reason is presumed as follows. F
The e-Zn alloying reactivity is affected by the crystal texture of the base material. For this reason, crystals having high reactivity in alloying and crystals having low reactivity exist on the surface of the base material due to the difference in texture. Highly reactive crystals, for example, # 10
In a crystal having a {100} texture in which the {0} planes are accumulated in parallel with the surface of the base material, craters are less likely to occur because the difference in reactivity between the grain boundary and the inside of the grain is small. On the other hand, in a crystal having a low reactivity, for example, a crystal having a {111} texture in which {111} faces of the crystal are accumulated in parallel with the surface of the base material, the difference in reactivity between the grain boundary and the inside of the grain is reduced. Craters appear more clearly because of the large

In the steel to which Si is added, Si oxide is concentrated in the outermost layer of the base material during reduction annealing, and the alloying reactivity on the crystal surface is reduced. At this time, the crystal of the texture having higher reactivity has a much lower reactivity. Although the reactivity of the grain boundaries tends to decrease due to the concentration of the Si oxide, the reactivity in the crystal grains decreases more greatly. For this reason, the difference in reactivity between the grain boundaries and the inside of the grains is increased, and more craters are recognized than in steel not containing Si. Furthermore, since the reactivity of the crystal of the highly textured texture is particularly greatly reduced, the difference in reactivity between the crystals is reduced, the difference in the crater depth is reduced, and the arithmetic average roughness of the plating surface is reduced. It is thought to decrease.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting each factor and condition in implementing the present invention will be described below. In addition,% described below
The indication means% by weight.

(1) Chemical composition of steel sheet C: Since the workability of the steel sheet is impaired, the C content is preferably as small as possible. Further, C dissolved in steel segregates at the crystal grain boundary of the base metal, delays the Fe—Zn alloy reaction at the crystal grain boundary, and has the effect of suppressing the formation of craters.
In the present invention, in order to secure the workability of the steel sheet and the number of valley peaks on the surface of the plating film, C is fixed and rendered harmless by adding Ti or Nb. When the C content increases, Ti and Nb must be added in a large amount, which is not economical, and also excessively increases carbides and impairs the workability of the steel sheet. Therefore, C
The upper limit of the content is 0.004%. Preferably it is 0.003% or less.

Si: When a proper amount of Si is contained, Si oxides are generated on the surface of the base material during reduction annealing of the base material, thereby increasing the crater on the surface of the plating film. This improves the retention of the lubricant used during processing and prevents flaking. At the same time, it also has the effect of reducing the surface roughness of the film surface and improving the sharpness after painting. In order to obtain these effects, the lower limit is made 0.03%. S on base metal surface
When the amount of i-oxide is excessive, the difference in alloying reactivity between the inside of the crystal grain and the grain boundary is reduced, and the number of valley peaks is excessively reduced. Therefore, the upper limit is set to 0.13%.

Mn: More than 0.05% is contained in order to suppress hot brittleness caused by S mixed into steel as an unavoidable impurity. If added in excess, the workability of the steel sheet is impaired, so the upper limit is made 0.4%.

P: An element suitable for increasing the strength of a steel sheet without impairing workability. In the present invention, in order to increase the number of craters and secure the number of valley peaks, an extremely low C IF steel (Inter
stitial Free steel). However, since a steel plate for an automobile body requires a certain level of strength, P is contained to maintain the strength. Therefore, the lower limit of the P content
0.007%. If P is excessively added, segregation occurs at the crystal grain boundaries of the base material, and the alloying reaction at the grain boundaries is suppressed to reduce the number of valley peaks. Therefore, the upper limit of the P content is set to 0.02%.

Ti: The solid solution C of the ultra-low C steel is fixed and contained in an amount of 0.018% or more in order to increase the alloying reactivity at the crystal grain boundaries. Excessive addition impairs economics.
0.05%.

Nb: It is not necessary to add Nb, but it has the effect of fixing solid solution C similarly to Ti, and reduces the crystal grain size of the steel sheet after hot rolling, and deep drawing after the subsequent cold rolling and annealing. It is added as necessary because it has the effect of enhancing the properties. When added, the content is desirably 0.003% or more. Excessive addition hinders crystal grain growth during annealing and deteriorates workability. Therefore, the upper limit is set to 0.02%.

Al: Used for deoxidizing steel and for fixing N, which is an unavoidable impurity, to render it harmless. If the content is less than 0.005%, there is no effect, and if it exceeds 0.1%, not only the effect is saturated, but also an oxide is formed on the surface of the base material during reduction annealing, and the wettability of Zn is impaired during hot-dip plating. Therefore, the range of the Al content is 0.005% to
0.1%.

B: It is not necessary to add. However, B has an effect of suppressing brittle fracture after press working, which is likely to occur when processing extremely low C IF steel. In order to obtain the effect, it is desirable to contain 0.0005% or more. Even if it is contained excessively, not only the effect is saturated, but also the generation of craters is suppressed,
Since the workability of the steel sheet is also impaired, the upper limit is made 0.005%.

In addition, it is better that the number of elements mixed as unavoidable impurities, for example, S and N, is small, and S is 0.015%
Hereinafter, it is desirable that N is not more than 0.004%.

(2) Plating film Al content: There is an effect of increasing craters on the plating film surface and increasing the number of valley peaks. In order to obtain the required number of valley peaks, the Al content in the plating film should be 0.18% or more. When Al is contained in the plating film, a Fe—Zn alloy layer is formed at the interface with the base material, and the alloying reactivity in the crystal grains is reduced. This makes craters more likely to occur. When the Al content in the plating film is excessive, the number of valley peaks increases remarkably, and the sharpness after coating is impaired. Also,
When an extremely low C IF steel is used as the base material, if the Al content in the plating film is excessive, alloying is excessively delayed, and a long time is required for alloying. Therefore, the upper limit is 0.
50%.

Fe content: The Fe content in the plating film is not specified in the present invention, but is controlled within the range of 7 to 18% in order to maintain good powdering resistance and chipping resistance. Is preferred.

Arithmetic mean roughness (Ra) of plating film surface:
Using a three-dimensional roughness measuring instrument, the undulation component of 0.8 mm or more is excluded by a filter treatment, and the plating surface of 10 mm square is measured by 10 or more scanning lines to obtain the value. When Ra exceeds 1 μm, the sharpness after painting deteriorates and is not suitable as a steel plate for automobile exteriors. Therefore, Ra of the steel sheet of the present invention is 1 μm.
The following is assumed.

The number of valley peaks on the surface of the plating film: The number of valley peaks is calculated by using a three-dimensional roughness measuring device in the same manner as described above, and excluding the undulation component of 0.8 mm or more by a filter treatment and the cross-sectional curve of the surface. The counting is performed using the roughness average line and a reference line provided at a position 50% of the maximum depth from the roughness average line. The valley is counted as one valley when the cross-sectional curve rises to the positive side from the roughness average line and then exceeds the negative reference line further to the negative side. The total number of valleys per measurement length 25.4 mm is the number of valley peaks. And As the number of valley peaks of the plated steel sheet increases, the sliding friction resistance decreases, and flaking hardly occurs.
The lower limit of the number of valley peaks is defined as the limit where flaking does not occur.
320 / 25.4mm. On the other hand, if the number of valley peaks is too large, the sharpness after painting is impaired. Therefore, the upper limit is set to 600 / 25.4 mm.

(3) Manufacturing Method As the base material of the steel sheet of the present invention, a cold-rolled steel sheet having the chemical composition described in the above (1) is preferable. However, a steel sheet obtained by annealing this cold-rolled sheet or a hot-rolled steel sheet from which an oxide film has been removed may be used as a base material. The equipment for applying the hot-dip galvanizing may be a commonly used equipment. After the base material is degreased by alkali degreasing or the like, reduction annealing is performed and hot-dip plating is performed. Then, an alloying process is performed to obtain a product. The preferred production conditions are shown below.

For pretreatment of the base material before hot-dip galvanizing, general equipment such as alkali cleaning is sufficient. The temperature of the pretreated base material is raised to 600 ° C. or more in a reducing atmosphere. When recrystallization annealing is required, reduction annealing is performed to complete recrystallization by heating to a temperature higher than the recrystallization temperature. Thereafter, in the course of cooling, the sample is held in a reducing atmosphere for 20 to 60 seconds in a temperature range of 600 to 500 ° C., and then subjected to hot-dip plating.
Holding at 600 to 500 ° C is referred to as “low temperature holding”). Thereby, the effect of increasing the number of craters and improving the sharpness after painting becomes remarkable.

The atmosphere at the time of maintaining the low temperature has a hydrogen concentration of 6 to
A dew point of −20 to −40 ° C. in an atmosphere of 12% by volume and residual nitrogen is particularly preferred. Further, it is preferable that the base material is retained in the above atmosphere at a temperature range of 600 to 500 ° C. for 20 to 60 seconds. If the hydrogen concentration in the atmosphere exceeds 12% by volume, the dew point is lower than -40 ° C, or if the holding temperature is lower than 500 ° C or if the holding time is shorter than 20 seconds, crater formation may not occur. The number of valley peaks is insufficient and the flaking resistance is not improved, and the sharpness after coating is not improved. On the other hand, when the hydrogen concentration in the atmosphere is less than 6% by volume, when the dew point is higher than -20 ° C, or when the holding temperature exceeds 600 ° C or the holding time exceeds 60 seconds,
The amount of oxide on the surface increases, and the base material transfer roll in the furnace is contaminated with the oxide, which causes surface flaws such as abrasions. If the holding time exceeds 60 seconds, operability deteriorates and economic efficiency is impaired.

Next, after cooling to a temperature near the plating bath, it is immersed in the plating bath, hot-dip plated, and subjected to alloying treatment. In order to contain 0.18 to 0.5% of Al in the plating film, it is preferable to perform plating using a plating bath containing 0.11 to 0.14% of Al. In addition, A in the plating bath here
The l content does not include Al in the dross.

The coating weight is generally ²⁰ to 70 g / m ² , and the present invention may have a similar coating weight. The temperature of the plated steel sheet at the time of alloying is preferably 480 to 600 ° C.
It is particularly preferable to carry out the reaction at 520 ° C. or higher where the ζ phase (FeZn13), which is one of the Fe—Zn alloy phases, disappears. This is because, in order to make flaking less likely to occur, it is better for the surface layer of the plating film to have less ζ phase. It is preferable that the Fe content of the plating film is in the range of 7 to 18% as an average value of the entire plating film.

In order to correct or adjust the mechanical properties, flatness and shape of the steel sheet, it is desirable to perform light rolling (skin pass rolling) after the alloying treatment. This can also be expected to improve the sharpness. Skin pass rolling may be lubricated rolling (dry rolling) or non-lubricated rolling (wet rolling). Further, a normal leveler passing process or the like may be performed.

[0043]

【Example】

(Example 1) Steel having the chemical composition shown in Table 1 was melted and subjected to hot rolling and cold rolling to obtain a coiled steel sheet having a thickness of 0.8 mm and a width of 200 mm.

[0044]

[Table 1]

This was passed through a washing line and degreased with a 10% aqueous sodium hydroxide solution. Further, using an experimental hot-dip plating equipment having a direct-fired reduction furnace, reduction annealing was performed at 850 ° C. for 60 seconds in an atmosphere of 10% by volume of hydrogen, 90% by volume of nitrogen and a dew point of −30 ° C., and 600 to 500 After cooling to a temperature in the range of 10.degree. C. and staying in this range for 10 to 60 seconds, the solution was cooled to 440.degree. To 520.degree. The coating weight was adjusted to 40 ± 5 g / m ² per side by blowing high-pressure gas. The Al content of the plating bath is 0.12-0.15%, and the temperature of the plating bath is 460.
° C. In addition, the temperature reached by the steel sheet reaches 480-
The coating was heated to 580 ° C. and maintained at this temperature range for 20 seconds to adjust the Fe content of the plating film to a range of 8 to 15%.

Measurement of the chemical composition of the plating film: The chemical composition in the film is 10% by weight containing 0.1% by weight of the inhibitor.
The plating film was dissolved in an aqueous solution of hydrochloric acid, and the solution was analyzed.

Measurement of surface roughness: Arithmetic average roughness (Ra) using a three-dimensional roughness meter [Surfcom manufactured by Tokyo Seimitsu Co., Ltd.]
Was measured. 1μm contact needle, scanning speed 0.06mm / sec,
The number of scans was 10, and the measurement area was 10 mm × 10 mm.

The number of valley peaks was determined from the cross-sectional curve of a measurement length of 25.4 mm from which a waviness component of 0.8 mm or more was removed using the same measuring instrument.

Measurement of coefficient of friction: The coefficient of friction (μ) was measured using a slidability evaluation apparatus shown in FIG. Width 30mm, length 27
A test piece 1 of 0 mm is placed on a die 2 of a slidability evaluation device and a radius of 5 mm.
And presses the punch 3 by applying a wrinkle holding load (P) to form the test piece 1 into a U-shape. The wrinkle holding load is 750, 1000,
The conditions were 1250 and 1500 kgf. In each case, the maximum value (F) of the press-in force of the punch 3 was determined, and the friction coefficient was determined from the following equation from the increment (dP) of the wrinkle holding load and the increase (dF) of the maximum value of the punch pressure input.

The surfaces of the die 2, the wrinkle holding bead 4 and the punch 3 were polished with # 600 abrasive paper. 2.5 g / m ^{2 of a} rust-preventive oil was applied as a lubricant to both surfaces of the test piece 1, and the press-in speed of the punch 3 was 60 mm / min.

Μ = dF / 2dP ---- μ: 0.24 or less: ◎ More than 0.24 or less: 0.2 More than 0.28 or less: 0.32 or less: Δ More than 0.32: × Investigation of anti-flaking property: using the slidability evaluation apparatus shown in FIG. And evaluated. A test piece 1 having a width of 30 mm and a length of 270 mm is held between a die 2 and a wrinkle holding bead 4, and a punch 3 is press-fitted with a wrinkle holding load of 1000 kgf to form the test piece 1 into a U-shape. 2 g / m ^{2 of} rust-preventive oil was applied as a lubricant on both sides of the test piece 1, and the press-in speed of the punch 3 was
It was 60 mm / min. Adhere an adhesive tape on the surface of the side of the test piece molded into a U-shape that was pressed by the beads,
The state of the stripped piece of the plating film adhered to the tape was visually determined, and evaluated according to the following categories.

No flaking: ○ Slightly present: Δ Flaking: X
A three-coat coating (total thickness: about 100 μm) of 40 μm and a top coat of 35 to 40 μm was applied. The mapping sharpness of this test piece was measured as an NSIC value using a mapping sharpness measuring device [NSIC type manufactured by Suga Test Instruments Co., Ltd.], and evaluated in the following categories. The NSIC value is 100 for blackboard polished glass.
The closer the NSIC value is to 100, the better the sharpness.

[0053] Table 2 shows various performance evaluation results.

[0054]

[Table 2]

As shown in Table 2, the samples Nos. 1 to 20 in which both the chemical composition of the base material and the composition of the plating film satisfy the conditions specified by the present invention have neither flaking resistance nor sharpness after coating. Both are good.

Test Nos. 21 and 23 using steels J and K whose Si and Mn contents are outside the conditions specified by the present invention are inferior in sharpness after coating. Low Si content, A in plating film
l Sample No. 22 whose content is outside the range specified by the present invention
The number of valley peaks is less than the value specified in the present invention, and the flaking resistance is not preferred. Sample No. 24 in which Mn is excessively contained and Sample No. 25 in which P is excessively contained are:
The displacement is also unfavorable in the flaking resistance. In Test No. 26 using steel N having an insufficient Ti content, solid solution C in the steel segregates at the grain boundary and the number of valley peaks is insufficient, so that the flaking resistance is not preferable.

In Test No. 27, the chemical composition of the base material was within the condition range specified by the present invention, but the Al content in the plating film exceeded the upper limit of the range specified by the present invention. Is inferior in clarity. In Test No. 28 as well, the chemical composition of the base material is within the range specified by the present invention, but the flaking resistance is inferior because the Al content in the plating film is too small. Trial No. 29
Is insufficient in flaking resistance because of insufficient time for holding at low temperature.

[0058]

The steel sheet of the present invention hardly suffers from flaking defects during press working, and has excellent sharpness when painted. If this is used as a material for an exterior part of an automobile, the yield and workability in the processing step are excellent, and the appearance of the automobile is also excellent. Further, this steel sheet can be economically and rationally manufactured according to the method specified by the present invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross section of an apparatus for evaluating the slidability of a plated steel sheet.

[Brief description of reference numerals]

 DESCRIPTION OF SYMBOLS 1 Test piece 2 Dice 3 Punch 4 Wrinkle holding bead

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁷ identification code FI C23C 2/40 C23C 2/40 (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] (1) C: 0.004% or less by weight%, Si: 0.03 to
0.13%, Mn: 0.05-0.4%, P: 0.007-0.02%, T
i: 0.018 to 0.05%, Nb: 0 to 0.02%, Al: 0.005
-0.1%, B: 0-0.005%, and the balance is made of steel composed of Fe and unavoidable impurities.
l of 0.18% or more and 0.5% or less, the surface roughness of which is 1 μm or less in arithmetic average roughness (Ra), and the number of valley peaks: 320 to 600
An alloyed hot-dip galvanized steel sheet with excellent paint clarity and flaking resistance, characterized by having a Fe-Zn alloy plating film of /25.4 mm. 2. The step of heating the base material before plating,
Including a step of retaining the base material in a reducing atmosphere of hydrogen and nitrogen having a dew point of −20 to −40 ° C. in a temperature range of 600 to 500 ° C. for 20 seconds to 60 seconds, and 0.11% by weight 2. The method for producing a galvannealed steel sheet according to claim 1, wherein galvanizing is performed after using a plating bath containing 0.14% by weight or less of Al and then subjected to alloying treatment.