JP4506357B2 - Galvanized steel sheet and its manufacturing method - Google Patents

Description

  The present invention relates to a hot-dip galvanized steel sheet and a method for producing the same, for example, by press-forming a plated steel sheet and then coating and using the plated steel sheet.

  In general, Zn hot-dip galvanized steel sheets, that is, hot-dip galvanized steel sheets, are alloyed hot-dip galvanized steel sheets (hereinafter simply referred to as “GA steel sheets”) that have been alloyed immediately after plating, and hot-dip galvanized sheets that have not been alloyed. Although it is roughly classified into a steel plate (hereinafter simply referred to as “GI steel plate”), in this specification, the term “hot dip galvanized steel plate” refers to the latter “GI steel plate”.

  GA steel sheets have been widely used as automotive steel sheets because of their excellent weldability and formability. On the other hand, recently, it has been often required to improve the corrosion resistance of steel sheets, so that it is easy to use thickening (a large amount of coating) and use GI steel sheets with low production costs as steel sheets for automobiles. It is being considered.

  When a plated steel sheet is used for automobiles, first, the plated steel sheet is press-formed into the shape of an automobile part, and then painted. However, sharpness is required as a characteristic of the coated steel sheet at that time. In general, the sharpness of the steel sheet after painting is inferior when using the GI steel sheet than when using the GA steel sheet.

  By the way, the sharpness after painting is also a value judgment about the appearance of the steel plate that is judged visually, and both of them often define the surface properties of the plated steel plate itself, There is confusion. In addition, even in the past, several inventions have been proposed that define the surface properties of hot-dip galvanized steel sheets with excellent appearance. Unless it is clearly distinguished from sex, the present invention cannot be properly understood, and the relationship between the present invention and the prior art cannot be clarified. Therefore, in the following, the differences between the appearance characteristics of the plated steel sheet and the sharpness characteristics of the coated steel sheet will be clarified.

  In Patent Document 1, hot dip galvanization is applied to a cold-rolled steel sheet having a JIS ten-point average roughness (Rz) of 1 to 5 μm and a centerline average roughness (Ra) of less than 0.1 to 1.5 μm. GI steel sheets, GA steel sheets subjected to alloying treatment if necessary, and methods for producing them are disclosed.

Patent Documents 2 and 3 disclose GI steel sheets or GA steel sheets that define the texture of cold-rolled steel sheets that are base steel sheets.
However, in Cited Documents 1 to 3, “the plating film is excellent in appearance” is determined by whether or not white spots or streaks are found in the plating film itself by visual observation. This is a characteristic, and this is not directly related to the sharpness after painting, and it is impossible to estimate the sharpness after painting.

  Here, the sharpness is a characteristic representing the sharpness of an image when an object is projected on the paint surface and the degree of distortion of the image. Specifically, for example, the characteristic is evaluated with an NSIC value by a sharpness evaluation apparatus described in an embodiment described later.

  Patent Document 4 defines the contents of Fe, Pb, Sb and Al in the hot-dip galvanized film, and further has an average line center roughness Ra of 1.0 μm or more and a length of 1 inch in the average line direction of the roughness curve. A hot dip galvanized steel sheet excellent in plating adhesion and continuous spot weldability, in which the number of ridges ppi of 80 to 250 is specified, and a method for producing the same are disclosed.

  In Patent Document 4, the surface properties of the hot-dip galvanized steel sheet are defined by the Ra and ppi of the plating film. The properties required for the plating film at that time are plating adhesion, press formability, and spot weldability. However, there is nothing related to the sharpness after painting. In paragraph 0020, “beautiful appearance” is obtained, but the content only means that “the spangle pattern cannot be identified” as shown in the embodiment. Obviously, this has nothing to do with the sharpness after painting. In other words, nothing is revealed about Ra and ppi of the plating film and the sharpness after painting.

  Patent Document 5 describes the properties of the steel sheet surface after temper rolling, the center line waviness height Wca of the cutoff value of 800 μm of the waviness curve curve is 0.65 μm or less, and Wca × PC is 6 or less (PC: surface filtration) A sharp-defining steel sheet is disclosed which is defined as a number of crests having a cut-off value of 800 μm and a height of 1.0 μm or more on a wave undulation curve.

In Patent Document 5, the surface properties of the steel sheet after temper rolling are as follows. A steel plate having excellent image clarity after painting is disclosed, in which the number PC ₁ per inch is 3 or less, and the number PC ₂ per inch in a valley of 1.0 μm or more is defined as 3 or less.

  However, Patent Documents 5 and 6 are both related to GA steel plates and electroplated steel plates, and the high cost is inevitable, and it is not clear whether GI steel plates can be evaluated for sharpness by the same index, Furthermore, there is no suggestion on how to improve the sharpness in the case of GI steel sheet, as it cannot be done.

Japanese Patent Laid-Open No. 11-158597 JP-A-11-323521 JP-A-11-323522 JP 2001-247951 A JP-A-8-132103 JP-A-8-174007

  The subject of this invention is providing the hot-dip galvanized steel plate excellent in the clarity after coating, and its manufacturing method.

The inventors of the present invention have investigated the factors that adversely affect the sharpness of the GI steel sheet in order to improve the sharpness after painting, and as a result of various experiments, the following knowledge has been obtained.
a) Temper rolling after hot dip galvanization has a great influence on the sharpness after painting. Even if temper rolling is performed on GI steel sheet under the same conditions as temper rolling of GA steel sheet, good sharpness is achieved. Sex cannot be obtained stably. In other words, the sharpness of the GI steel sheet cannot be evaluated by the indexes of the above cited references 5 and 6. It is necessary to evaluate with GI steel plate's own index.

  b) It is because the surface state of the plating film is different between the GA steel plate and the GI steel plate. That is, the surface state of the plating film is different on the surface of the plating film of the GI steel plate. That is, on the surface of the plating film of GI steel sheet, there are crystal grain boundaries that occur during the solidification of the plating, and irregular irregular shapes generally called sagging and sazanami due to the uneven solidification state. This is because such irregular shapes do not occur in steel plates and electroplated steel plates.

c) Therefore, the temper rolling of the hot dip galvanized steel sheet needs to be carried out under conditions that can minimize the influence of the surface state as plated.
d) Further, when the surface state of the GI steel sheet having good visibility was investigated, it was found that there was a good range in the shape of the transferred dull eye.

  e) Moreover, in the case of GI steel sheet, it is preferable that a deep dull eye is attached. Although the so-called spangle pattern may be observed even after coating due to the influence of the crystal grain boundaries at the time of solidification described above, it is considered that the influence of the crystal grain boundaries can be suppressed by applying deep dull eyes.

f) Furthermore, it was also found that the sharpness was better when the diameter of the transferred dull eye was not so large. That is, it is preferable that the shape of the dull eye is fine and deep on average.
g) In order to realize such a surface state of the GI steel sheet, at the time of temper rolling, it is necessary to use a work roll having a surface roughness of a predetermined value or more and to roll with a temper rolling oil with as much load as possible. Good. At this time, it is preferable to use a work roll which has been subjected to electric discharge dull processing.

The present invention has been completed based on the above-described knowledge. Specifically, when temper rolling is performed on a hot dip galvanized steel sheet, a roll having a surface subjected to discharge dull processing is used, and an organic Or, using an inorganic temper rolling oil and rolling with a rolling load of 1.1 kN / mm or more at a linear load, the plated surface after temper rolling meets the following conditions It is a manufacturing method of a galvanized steel sheet.

Filter centerline swell height: Wca ≦ 0.6μm
Area ratio of parts not subjected to temper rolling: M ≦ 60%
Number of convex peaks per 25.4 mm of roughness curve: ppi ≧ 200
Average depth of dull eye: d ≧ 1μm
Assuming that the shape of the round eye is a circle, the average diameter per piece (μm): 40 μm ≦ L <50 μm

  According to the present invention, a hot-dip galvanized steel sheet excellent in sharpness after painting can be obtained stably, and is particularly useful in applications such as automotive outer panels that require corrosion resistance due to thickening and corrosion resistance after painting. is there.

The hot dip galvanized steel sheet and the production method of the present invention will be described in detail.
1) Base material of plated steel sheet:
The steel type of the base material of the plated steel sheet is not particularly limited. What is necessary is just to select suitably according to a use. For example, the steel types when assumed to be used as automobile steel plates include extremely low carbon steel, low carbon steel, and alloy elements such as Si, Mn, P, Al, Cr, Ni, Cu and Mo. Such as carbon steel. From the viewpoint of mechanical properties, steel types such as those for general use, (deep) drawing, and high strength, such as cold-rolled steel plates and hot-rolled steel plates can be used.

  Although the surface shape of the base steel plate is not particularly limited, it is preferable that the filtered centerline waviness height Wca is as small as possible. This is because if the Wca on the surface of the base material is large, the Wca after plating tends to be large, which tends to adversely affect the sharpness after painting. The preferred base material surface Wca is 0.8 μm or less.

2) Plating film:
In the present invention, the plating method itself may be a conventional hot dip galvanizing method. Although the plating film obtained at that time may also be a plating film of a normal GI steel sheet, preferred forms are as follows.

  In order to obtain a hot-dip galvanized steel sheet with excellent sharpness after painting, it is preferable that the surface of the plating film has as little defects as possible, such as solidification unevenness called sagging and sazanami, non-plating, and dross. These defects are greatly affected by the amount of Al in the plating bath. Specifically, the amount of Al in the bath is preferably about 0.13 to 0.3%, and at this time, the Al content of the plating film is about 0.15 to 0.5%.

  Moreover, it is better that Pb and Sb are not contained in the plating film as much as possible. When Pb or Sb is included, a large spangle pattern is likely to occur on the plating surface, which often adversely affects the sharpness after painting. Pb adversely affects the performance of the GI steel sheet (for example, corrosion resistance and resistance to peeling over time). The preferable content in the plating film is 50 ppm or less in terms of the total amount of Pb and Sb.

  The weight per unit area of the plating film may be appropriately selected depending on the application, but is preferably 90 to 120 g / m <2> per side when a certain level of high corrosion resistance and a balance between weldability and workability are required.

3) Surface properties of the plating film after temper rolling By applying temper rolling after plating to the specified surface properties described below on the surface of the plating film, a GI steel sheet with excellent sharpness after coating is obtained. It is done.

(i) Wave height of filtered center line: Wca ≦ 0.6μm
FIG. 1 explains the outline of how to obtain Wca. Wca is based on JIS B0601, except that the cut-off value of the waviness curve is 0.8 mm. According to the waviness curve f (x) [(JIS B0601 (1994))]. However, in the present invention, the cutoff value of the filtered waviness curve is set to 0.8 mm])), and Wca is obtained by the following equation.

L: reference length,
Sn: The distance from the nth crossing point to the center line (L _{1 in} Fig. 1) from the peak to the valley of the wavy undulation curve to the (n + 1) th crossing point Wca is the workpiece during temper rolling The surface shape of the roll, elongation, and temper rolling conditions such as rolling oil can be changed.

  The smaller the Wca, the better the clarity after painting. Preferably it is 0.6 μm or less. The lower limit is not particularly limited, but when temper rolling with a dulled work roll, it is difficult to make it 0.2 μm or less.

(ii) Area ratio of the part not subjected to temper rolling: M ≦ 60%
As described above, since the surface of the as-plated GI steel sheet has irregularities due to crystal grain boundaries and the like in many cases, it is not adversely affected by irregularities on the surface of the as-plated film by temper rolling. Such a surface state can be obtained.

  FIG. 2 is a microphotograph (magnification 100 times) of the surface of the GI steel sheet after temper rolling. As shown in the figure, the GI surface after temper rolling using a work roll whose surface has been dulled is a part subjected to temper rolling (skin pass processed part: black part) and a part not subjected to temper rolling (skin pass processed part: black part) Unprocessed plating part: white part). At this time, the smaller the area ratio of the portion that has not undergone temper rolling, the better the sharpness.

If S ₀ is the area of the observation field, and S ₁ is the area of the portion subjected to temper rolling, the area ratio M of the unprocessed plated portion determined by the following formula needs to be 60% or less.
M = {(S ₀ −S ₁ ) / S ₀ } × 100
When M exceeds 60%, the sharpness deteriorates even if the other conditions defined in the present invention are satisfied. Preferably it is 40% or less, More preferably, it is 35% or less. In addition, when it is going to obtain the steel plate with M too small, what is called a winding phenomenon that a plating film (zinc) is picked up by a temper rolling roll tends to occur. Therefore, about 20% is practical as the lower limit of M, and the effect of improving the sharpness after painting is sufficiently exhibited when M = 20%.

Here, M can be changed by rolling as much as possible or temper rolling using a work roll whose surface is appropriately dulled.
(Iii) Number of convex peaks per 25.4 mm of roughness curve: ppi ≧ 200
FIG. 3 explains the outline of how to obtain ppi. In the present invention, ppi is the center line (L1 in FIG. 3) of the roughness curve of the plating surface (however, in the present invention, the cutoff value of the filtering waviness curve is 0.8 mm) according to the US SAE911 standard. In the roughness curve of the convex plating surface of the roughness curve per 25.4mm (according to JIS B0601 (1994)), the height from the center line of the roughness curve per 25.4mm (1 inch) is 0.5μm or more This is a count of the number of peaks (that is, peaks higher than L2 in FIG. 3).

  The more ppi, the better the clarity after painting. Moreover, when there is too little ppi, since the holding | maintenance property of the press oil at the time of press molding is inferior, it becomes easy to generate | occur | produce a crack and a mold galling in the case of molding. The upper limit of ppi is not particularly limited, but it is not realistic to obtain a GI steel plate surface exceeding 300 because the work roll surface is costly to process.

(Iv) Average depth of dull eye d ≧ 1μm
As described above, it is preferable that the plating surface has a deep rounded surface. The so-called spangle pattern may be observed even after painting due to the influence of the above-mentioned crystal grain boundaries during solidification, but it is considered that this grain boundary can be suppressed by applying a deep dull eye. . Specifically, the average depth d of the dull eye is set to 1 μm or more. Note that the dull eye depth can be measured by a depth of focus method using an optical microscope. Specifically, based on the focal length of the portion of the GI steel sheet surface that has not undergone temper rolling, the focal length of the center portion of the dull eye to be measured is defined as the depth of the dull eye. Further, the same measurement is performed for about 3 to 5 dulls for one sample, and the average value is defined as the average depth d of the dull. The upper limit of the dull eye depth is not particularly specified, but about 5 μm is realistic. If you want to make a deep dull, it is necessary to increase the surface roughness of the work roll or increase the rolling load. In the former case, it is necessary to change the work roll frequently, and in the latter case In addition to the fact that the zinc wrapping phenomenon is likely to occur, it is considered that in some cases, the degree of work of temper rolling increases and adversely affects the mechanical properties.

(V) Average diameter per dull eye L <50μm
As described above, the sharpness tends to be better when the diameter of the transferred dull eye is not so large. If each of the above conditions (i) to (iv) is within the specified range, the sharpness after painting is good, but more specifically, if the shape of the dull eye is assumed to be a circle, The average diameter is preferably less than 50 μm.

  Note that M in (ii) and L in (v) are obtained by photographing the surface of the GI steel sheet after temper rolling at a magnification of 100 to 300 times with an optical microscope, and performing a dull eye portion (part subjected to tempering processing) by image processing And M can be calculated from the respective areas, and L can be calculated from the number and area of the dull eye portions.

(Vi) Centerline average roughness Ra:
If each condition relating to the above-mentioned surface property is within the specified range, Ra does not significantly affect the sharpness. However, in order to improve the press formability, Ra should be in the range of 0.8 to 1.5 μm.

4) Manufacturing method As described above, the GI steel sheet having the surface properties described above can be obtained by adjusting the composition of the plating bath in the hot dip galvanizing process, and by changing the composition of the hot dip galvanized steel sheet thus obtained. It can be obtained by temper rolling.

  Here, although the manufacturing method concerning this invention is demonstrated in detail, the hot dip galvanization process itself is already well-known, and what is necessary is just to utilize it also in this invention, and there is no restriction | limiting in particular. Therefore, temper rolling will be described below.

(a) Work roll used for temper rolling As the work roll that comes into contact with the steel sheet, it is preferable to use a roll whose surface is subjected to discharge dull processing. Since a roll subjected to electric discharge dull processing has a wide adjustable range of high ppi compared to a roll subjected to shot blasting, it is easy to obtain a GI steel sheet of the present invention in which high ppi and low Wca are arranged side by side. In order to keep the Ra of the steel sheet within the above-mentioned range, it is appropriate that the Ra of the work roll surface is about 2.0 to 3.5 μm although there is an influence by the electric discharge machining state and other rolling conditions.

(b) Tempered rolling oil In order to obtain the GI steel sheet of the present invention, the temper rolling is performed with a large rolling load as described later, and therefore, the work roll is likely to be wound with plating (zinc). Therefore, in order to suppress this winding, it is preferable to perform temper rolling in a wet state. Moreover, since the effect of suppressing wrapping is poor with mere water, it is preferable to use an organic or inorganic tempered rolling liquid in order to enhance the lubricating effect. For example, the temper rolling may be performed while spraying a steel sheet with an organic tempered oil diluted to 1% or more after dilution with water.

(c) Reduction load in temper rolling:
In order to deepen the area ratio M of the portion not subjected to the temper rolling process, it is preferable to sufficiently apply the rolling load. A preferable load is 0.8 kN / mm or more, more preferably 1 kN / mm, in terms of a line load in the steel strip width direction. The upper limit is not particularly specified, but about 3.0 kN / mm is realistic in terms of winding and mechanical properties.

  Since the continuous hot dip galvanizing line usually has temper rolling equipment, rolling may be performed in-line. However, in some cases, temper rolling can be performed on a separate line. Moreover, although mild rolling may be performed for the shape correction (mainly correction of flatness) of a steel plate after temper rolling, the surface state of this invention should just be obtained finally.

  In addition, the adhesion amount control immediately after plating is usually controlled by gas wiping or the like. At this time, it is preferable to use an inert gas such as nitrogen gas rather than the atmosphere because a surface with less surface sagging and sazanami can be easily obtained.

  Moreover, it may be rapidly cooled by mist spray or the like during plating solidification. Since such rapid cooling has the effect of suppressing the generation and growth of spangle patterns, it may be preferable to use it. On the other hand, as described above, when there are few impurities such as Pb and Sb in the plating film, the spangle does not increase originally, and in this case, it is not always necessary to cool rapidly. Moreover, since chemical | medical solution mist may reduce corrosion resistance by the adhesion, it may be better not to use in that case.

  Next, the function and effect of the present invention will be described more specifically with reference to examples.

An ultra-low carbon-Ti-added cold-rolled steel strip with a thickness of 0.8 mm having the chemical composition shown in Table 1 is used as the base material, and the Al concentration in the plating bath is 0.15%, Pb, Sb in the continuous hot dipping line. Using a total of 50 ppm or less of a plating bath, hot dip galvanizing with a plating adhesion amount per side of 80 g / m ² was performed.

  Wiping is performed using nitrogen gas, and mist spray is not used. Subsequent to the plating step, the plated steel strip was subjected to temper rolling in the line while changing conditions such as skin pass elongation and rolling load (line load). The conditions for temper rolling are as follows.

Work roll surface: EDM finish. Ra having a level of 2.5 μm and ppi having three levels of 200, 250, and 300 were used.
Tempered rolling oil: Quark J263 Kwar J263 diluted with water to 5% was used.

Rolling load: 0.1-1.1kN / mm
Elongation rate: 0 to 1.0%
The surface properties of the GI steel plate thus obtained were measured as follows.

M, L: As described above, observation was performed at 100 to 300 times with an optical microscope, and calculation was performed by image processing.
d: When observing with an optical microscope at a magnification of 100 to 300, the focal depth of the surface as plated was used as a reference, and the difference from the focal depth at the center of the dull eye was defined as the dull eye depth. For each sample, the depth of the dull eyes was measured for 3 to 5 dull eyes, and the average value was defined as d.

Wca, ppi: Measured using a two-dimensional surface roughness measuring device.
Next, zinc phosphate treatment, electrodeposition coating (film thickness 15 mm), intermediate coating (film thickness 20 mm), top coating (film thickness 20 μm), and clear coating (20 μm) are applied in this order to the obtained GI steel sheet. did.

Regarding the sharpness of the obtained coated plate, the NSIC value was measured with a sharpness evaluation device (HA-NSIC manufactured by Suga Test Instruments Co., Ltd.). The evaluation criteria were as follows, and 70 or more were accepted.
◎: NSIC value 80 or more ○: Same 70 or more and less than 80 △: Same 50 or more and less than 70 ×: Same 50 or less

The measurement results were as shown in Table 2.
As can be seen from Table 2, in the comparative example in which even one condition is out of the prescribed range among the conditions prescribed in the present invention, excellent sharpness was not obtained. On the other hand, in the examples of the present invention satisfying all the prescribed conditions, excellent sharpness was obtained for all.

It is a schematic diagram for demonstrating how to obtain | require Wca. It is an example of the surface microphotograph of the hot dip galvanized steel plate after temper rolling. It is a schematic diagram for demonstrating how to obtain | require ppi.

Claims (1)

When temper rolling is performed on hot-dip galvanized steel sheets, rolls with surface discharge galling are used, and organic or inorganic temper rolling oil is used, and rolling is performed at a rolling load of 1.1 kN / mm or more with a linear load. A method for producing a hot-dip galvanized steel sheet, in which the plated surface after temper rolling satisfies the following conditions :
Wca ≦ 0.4μm
M ≦ 50%
ppi ≧ 220
d ≧ 4μm
40μm ≦ L <50μm
here,
Wca: The height of the waviness centerline waviness (μm) when the cut-off value of the surface waviness curve is 0.8mm
M: Area ratio of the part not subjected to temper rolling (%)
Here, M is a value obtained by the following equation , where S ₀ is the area of the observation field, and S ₁ is the area of the portion subjected to temper rolling.
M = {(S ₀ −S ₁ ) / S ₀ } × 100
ppi: Peak of the convex part with a height of 0.5μm or more from the center line of the roughness curve per 25.4mm length in the average line direction of the roughness curve when the cutoff value of the roughness curve is 0.8mm number
d: Average depth of the eyes (μm)
L: Average diameter (μm) per piece assuming a round shape

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