JP2000219949A - Method for producing Zn-Al-Si alloy-plated steel sheet excellent in design - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] A continuous hot-dip plating facility equipped with an in-line annealing furnace. Excellent melting
Manufactures Zn-Al-Si alloy plated steel sheets. SOLUTION: Annealing is performed under the condition that the dew point and the hydrogen concentration of the atmosphere gas in the annealing furnace satisfy the following equations (1) to (3). (1) + 10 ° C ≧ (dew point) ≧ −40 ° C. (2) 2% ≦ (hydrogen concentration) ≦ 30%, remaining nitrogen (3) (dew point) ≧ [(hydrogen concentration) × 2− (annealing temperature) / 10 + 10 ]
° C

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip Al-Zn-based plated steel sheet having high corrosion resistance and excellent design properties, which is suitable for building materials and home appliances.

[0002]

2. Description of the Related Art Galvanized steel sheets have been used for building materials and home appliances.
The amount of hot-dip Al-Zn plated steel sheets having high durability has been increasing. Among them, about 55 wt% Al in the plating film,
A hot-dip Zn-Al-Si alloy-plated steel sheet containing 1 to 2% by weight of Si has a beautiful spangle pattern on its surface, and thus has one of the features of design.

[0003] However, when this hot-dip Zn-Al-Si alloy-plated steel sheet is manufactured by a continuous hot-dip-coating facility, the spangle pattern on the surface shows the longitudinal direction and even between the coils for each production chance, and even within the same coil. Variations in size are often seen in the width direction. For example, when the base material coil is changed, the spangle size is often different even when plated under the same plating conditions. In particular, a very fine spangle may be formed, and such a product is inferior in design, so that it cannot be usually used for an application in which appearance is regarded as important.

The following methods have been proposed as techniques for suppressing the variation of the spangle pattern of hot-dip Zn-Al-Si alloy plating. First, there are the following proposals for the base steel sheet: (1) Japanese Patent Application Laid-Open No. 9-235661: Grinding the steel sheet surface by 0.05 μm or more, (2) Japanese Patent Application Laid-Open Nos. 10-18009 and 10-18013 : Controls the roughness and undulation of the steel sheet surface. (3)
Nos. 10-18010 and 10-18012: Control the texture and grain size of the steel sheet surface.

Regarding the plating conditions, there are the following proposals: (4) Japanese Patent Application Laid-Open No. 9-241814: The plating bath immersion time is set to 2 seconds or more. (5) Japanese Patent Application Laid-Open No. 9-25550: Control the dew point, hydrogen concentration and bath temperature in the snout.

[0006]

However, each of the above methods has the following problems. First, (1), (2),
The method (3) imposes restrictions on the steps before the continuous plating equipment, such as the rolling step, and makes it impossible to use a steel sheet that does not meet the conditions as a plating base sheet.

The method (4) is advantageous in that the spangle pattern can be controlled only by the plating conditions, but it is necessary to newly provide equipment such as a device for raising and lowering a roll in a bath, which is disadvantageous in cost. On the other hand, if the immersion time is too long, the spangle becomes finer as a whole, and the original design of the hot-dip Zn—Al—Si alloy-plated steel sheet is impaired.

The method (5) is to suppress the generation of zinc vapor from the plating bath by controlling the atmosphere in the snout. Zinc vapor evaporated from the plating bath is condensed and adheres on the wall surface of the snout, and falls and adheres to the surface of the steel sheet and the plating bath, causing plating defects or uneven spangles. Therefore, this method only does not worsen the variation of the spangle pattern generally seen due to, for example, the plating base material (eg, change of the coil), and the variation due to the plating base material itself. Cannot be suppressed.

SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous hot-dip plating apparatus which does not have the above-mentioned problems and has a small variation in spangle pattern, and in particular, a molten Zn-Al alloy capable of preventing the spangle pattern from becoming fine. -To provide a method for producing a Si alloy plated steel sheet.

[0010]

Means for Solving the Problems The present inventors have proposed molten Zn-
After examining the operating conditions to prevent the spangle pattern of Al-Si alloy-plated steel sheets from becoming finer, a reducing atmosphere, which is performed before plating in order to promote the alloying reaction with the plating metal necessary for ensuring the adhesion of the plating, In the activation treatment of the steel sheet surface by annealing below, it has been found that, when the reactivity of the steel sheet surface is suppressed to some extent, the spangle pattern becomes larger as a whole, and fine spangles are hardly generated.

Conventional typical annealing conditions are as follows: dew point in an annealing furnace: -40 ° C. or less, atmosphere gas: hydrogen + nitrogen (hydrogen concentration: 5 to 75%), annealing temperature: about 600 ° C. Over 800 ° C (depending on material and mechanical properties).

However, when annealing is performed under such conditions, the surface of the steel sheet before plating is highly activated and the reactivity becomes extremely high. Is too fast, the spangle becomes fine, and it is considered that the spangle becomes non-uniform.

Conversely, for example, if a steel sheet is annealed in an oxidizing air atmosphere and then immersed in a hot-dip plating bath, a beautiful spangle can be stably obtained. But,
In this case, the reactivity of the steel sheet surface is greatly impaired, and the steel sheet surface and the plating bath hardly react with each other, so that no plating adhesion is obtained and non-plating is liable to occur.

Therefore, as a result of studying from the viewpoint of suppressing the variation of the spangle pattern and miniaturization by appropriately suppressing the reactivity of the surface of the steel sheet within a range that ensures plating adhesion and does not generate non-plating, By controlling the dew point and hydrogen concentration of the reducing gas atmosphere in the annealing furnace,
It turns out that this is possible.

According to the present invention, Al: 30 to 70% by weight, Si: 0.05 to
2.0% by weight, balance: Reduction in the annealing furnace when manufacturing hot-dip Zn-Al-Si alloy-coated steel sheets having a plating composition consisting of Zn and unavoidable impurities in a continuous hot-dip plating facility equipped with an in-line annealing furnace. Gas atmosphere and dew point are as follows (1) ~
A method for producing a hot-dip Zn-Al-Si alloy-plated steel sheet having excellent design properties, characterized by performing annealing under conditions satisfying the expression (3), wherein spangles on a plating surface are sublimated without being refined. .

(1) + 10 ° C. ≧ (dew point) ≧ −40 ° C. (2) 2% ≦ (hydrogen concentration) ≦ 30%, balance nitrogen (3) (dew point) ≧ [(hydrogen concentration) × 2- (annealing temperature ) / 10 + 10]
° C.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The molten Zn-Al-Si of the present invention is described below.
The method for producing the alloy-plated steel sheet will be described in detail.

[0018] The type of base steel sheet to be subjected to hot-dip Zn-Al-Si alloy plating according to the method of the present invention is not particularly limited. For example, Al-killed steel, ultra-low carbon steel to which Ti, Nb, etc. are added, and high-strength steel of 410 N class or more to which C, P, Si, Mn or the like is added can be used. The base steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet, and the rolling conditions thereof are not particularly limited.

The base steel sheet is subjected to a pretreatment such as alkali degreasing if necessary, and then subjected to Zn-Al-Si alloy plating using a continuous hot-dip plating apparatus equipped with an in-line annealing apparatus.
The continuous hot-dip plating equipment to be used may be any equipment provided with an in-line annealing furnace. This type of continuous hot-dip galvanizing equipment was developed for hot-dip galvanizing, and its main equipment is a steel sheet surface cleaning device.
(Oxidizing furnace, non-oxidizing furnace, etc.), an annealing furnace, a hot-dip plating apparatus, a cooling apparatus, and a chemical conversion treatment apparatus.

In a general plating equipment of this kind,
First, the steel sheet is heated in an oxidation furnace or a non-oxidation furnace to remove oil adhering to the surface and clean the steel sheet surface. Since the surface of the steel sheet is oxidized or weakly oxidized by this heating, annealing is performed in a subsequent annealing furnace in a reducing atmosphere. Due to the annealing in the reducing atmosphere, the iron oxide on the surface of the steel sheet is reduced and the surface is activated in addition to the original function of annealing to remove strain. This facilitates the alloying reaction between the steel sheet surface and the plated metal, and secures the plating adhesion. Therefore, annealing in a reducing atmosphere is a necessary process for obtaining plating adhesion.

The reducing atmosphere in the annealing furnace is generally maintained by supplying dry hydrogen gas and dry nitrogen gas to the annealing furnace. Conventionally, in order to activate the steel sheet surface as much as possible, only dry hydrogen gas and dry nitrogen gas were supplied to the annealing furnace, and the dew point of the reducing atmosphere gas in the annealing furnace was controlled to be as low as possible.

However, activating the steel sheet surface by annealing under such conditions to increase the reactivity may cause spangles to be fine and non-uniform when the steel sheet is immersed in a hot-dip plating bath. The present inventors have found. The details of the spangle refinement mechanism are unknown, but the present inventors presumed that the progress of the reduction of the steel sheet surface caused the reactivity with the plating bath to be too high. Therefore, in the present invention, for the purpose of greatly sublimating spangles, the reactivity of the steel sheet surface is suppressed within a range where no plating or a decrease in plating adhesion occurs.

That is, annealing is performed while controlling the gas atmosphere in the annealing furnace maintained in the reducing atmosphere so as to satisfy the following equations (1) to (3): (1) + 10 ° C. ≧ (dew point) ≧ −40 ℃ (2) 2% ≦ (hydrogen concentration) ≦ 30%, balance nitrogen (3) (dew point) ≧ [(hydrogen concentration) × 2- (annealing temperature) / 10 + 10]
° C.

If the dew point of the gas atmosphere in the annealing furnace is too high or the hydrogen concentration is too low, activation by reduction of the steel sheet surface becomes insufficient, and the alloy layer growing on the steel sheet-plating interface becomes sufficient or uniform. Since it does not grow, the adhesion of the plating is poor, and in severe cases, non-plating or plating peeling occurs. On the other hand, if the dew point is too low or the hydrogen concentration is too high, the reactivity of the steel sheet surface is too high,
Spangles tend to be finer, and the spangles may vary widely. Further, the annealing temperature also affects the sublimation state of the spangle. That is, as defined in the equation (3), as the annealing temperature becomes lower and the hydrogen concentration becomes higher, unless the dew point is controlled to be higher, the spangles are more likely to be miniaturized. In other words, the lower limit of the dew point increases,
The control range of dew point is reduced.

In the present invention, the annealing temperature means the maximum steel sheet temperature in the annealing furnace. The hydrogen concentration is
It is the volume ratio of hydrogen at normal temperature. Considering operating conditions and measurement variations, the gas atmosphere in the annealing furnace has a dew point of -35 ° C.
It is preferable to perform annealing at a temperature of + 5 ° C. and a hydrogen concentration of 5 to 15%, and under the conditions not deviating from the above equation (3) in relation to the annealing temperature.

In the present invention, since the dew point in the annealing furnace is controlled to be relatively high, a desired dew point cannot often be obtained by supplying only the dry hydrogen gas and the dry nitrogen gas to the annealing furnace as in the related art. . In that case, it is necessary to supply moisture to the annealing furnace in the form of water and / or steam so as to increase the dew point in the annealing furnace. This moisture can also be supplied by using an undried gas for one or both of hydrogen gas and nitrogen gas, but it is difficult to control the dew point strictly. Therefore, dry hydrogen gas and dry nitrogen gas are supplied to the annealing furnace. It is preferable that a piping system capable of supplying a controlled amount of water and / or steam be provided separately from the piping system described above, and that the amount of water necessary for dew point management be supplied in a controlled amount while measuring the dew point. .

The annealing temperature is set mainly as a temperature necessary for securing the mechanical properties of the steel sheet, but is usually about 600 ° C. to 800 ° C.
It is in the range of over ℃. The temperature profile in the annealing furnace is not particularly limited. When the annealing temperature is the same as the temperature of the steel sheet at the time of entering the plating bath, the inside of the annealing furnace is maintained at a constant temperature. If the annealing temperature becomes higher than that, temperature changes such as heating and cooling are observed in the annealing furnace, so the furnace has a cooling zone, or a heating zone and a cooling zone, or a heating zone and a soot and a cooling zone. Are classified. After the cooling zone,
In some cases, a low-temperature holding zone for maintaining the temperature at the plating bath intrusion is provided.

In the present invention, "inside the annealing furnace" means an area on the inlet side of the annealing furnace or between the heating zone and the cooling zone. Therefore, the low temperature holding zone is not included in the annealing furnace. The residence time of the steel sheet in the annealing furnace (that is, the annealing time) is usually about 3 to 5 minutes. The highest plate temperature is usually around one minute.

The dew point and hydrogen concentration in the annealing furnace were
It is not necessary to control the entire zone in the "inside the annealing furnace" to the above range, but since the dew point and the hydrogen concentration do not fluctuate so much in the furnace, the entire zone is preferably managed as such if possible. When the dew point or the hydrogen concentration fluctuates, it is preferable that the dew point and the hydrogen concentration fall within the range specified in the present invention at least near the maximum temperature of the plate in the annealing furnace.

The steel sheet whose surface has been activated by being subjected to annealing in a reducing atmosphere in an annealing furnace is cooled, if necessary, in a cooling zone to lower the temperature to a predetermined plating bath intrusion temperature, or Usually, the steel sheet is immersed in a hot-dip plating bath via a snout provided for the purpose of shielding the steel sheet from the surrounding atmosphere, and is subjected to Zn-Al-Si alloy plating. The plating bath penetration temperature is usually about ± 30 ° C
Temperature range.

As described in JP-A-9-25550, the dew point of a snout affects plating quality (plating unevenness, plating adhesion). However, since the inside of the snout is kept in an inert gas atmosphere and the residence time of the steel sheet is short, the influence on the reactivity of the steel sheet surface, and therefore on the spangle, is small. That is, the spalling status of the spangle is determined in the annealing furnace before the snout.

The composition of the hot-dip plating bath is Al: 30 to 70% by weight,
Si: 0.05 to 2.0% by weight, balance: Zn and unavoidable impurities. If the Al content is higher or lower than this, a beautiful spangle pattern does not appear on the surface. If the Si content is too low, a spangle pattern will not be exhibited, and if the Si content is too high, non-plating surface defects are likely to occur. The preferred range is A
l: 45 to 65% by weight, Si: 1.2 to 1.8% by weight, more preferably Al: 50 to 58% by weight.

The plating conditions other than those described above are not particularly limited as long as they do not affect the surface quality, performance, and operation, and may be the same as those conventionally used for hot-dip Zn—Al—Si alloy plating. The plating weight is usually in the range of 30 to 150 g / m ² per side. The plating is usually applied to both sides of the steel sheet, but it is also possible to use single-sided plating by known means.

The steel sheet plated with the hot-dip Zn—Al—Si alloy by the method of the present invention can exhibit high corrosion resistance even when used without chemical conversion treatment. However, a suitable chemical conversion treatment may be performed in a chemical conversion treatment apparatus generally installed in a continuous hot-dip plating facility, whereby the corrosion resistance is further improved. The chemical conversion treatment can be a zinc phosphate treatment, but a chromate treatment is preferable because it is more effective in improving corrosion resistance.

The chromate treatment may be any of a coating type, a reaction type, and an electrolytic type, and the composition of the processing solution and the processing method are not particularly limited, and may be appropriately selected from conventional ones. A preferred chromate treatment method is a coating type chromate treatment which has a large effect of improving corrosion resistance in a short treatment time. The adhesion amount of the chromate film formed by the chromate treatment may be in the range of 3 to 150 mg / m ^{2 in} terms of Cr metal.

The hot-dip Zn-Al-Si alloy-plated steel sheet may be subjected to a chromate treatment as described above, or may be subjected to a thin-film resin coating without performing the chromate treatment. Suitable resin types for coating include polyester, polyurethane, acrylic, epoxy, vinyl butyral, and the like. The resin coating may be a solvent-based resin coating, but is preferably performed using a water-based resin coating in which a resin is dispersed or dissolved in water. The coating thickness of the resin is preferably 5 μm or less. If the resin coating is thicker than this, in addition to an increase in cost, resin scum is generated at the time of molding, and the appearance is deteriorated. A more preferred coating thickness is in the range of 0.5 to 3.0 μm.

[0037]

EXAMPLE A cold-rolled steel strip (thickness: 0.6 mm × 920 mm) of three types (A, B, C) of low-carbon Al-killed steel was used as a plating base material. As will be described later, the sizes of spangles obtained are different even if the plating conditions are the same.

These three types of steel strips were subjected to Zn-Al-deposition under the conditions shown in Table 1 (detailed annealing conditions are shown in Table 3) by using a continuous hot-dip coating equipment equipped with a non-oxidizing furnace and an annealing furnace. Si alloy plated. In the annealing furnace, piping systems for each gas of H ₂ , N ₂ , and steam were independently installed in the heating zone. The gas atmosphere and dew point in the furnace were controlled by adjusting the flow rate of each gas while measuring them with a hydrogen concentration meter and a dew point meter installed at a position where the plate temperature in the furnace reached the maximum temperature.

[0039]

[Table 1]

The spangles and plating adhesion were evaluated in the following manner using the obtained test pieces of the hot-dip Zn-Al-Si alloy-plated steel strip. Table 3 (annealing temperature 600 ° C), Table 4 (680 ° C), Table 5 (750 ° C),
The results are summarized in Table 4 (850 ° C).

Evaluation method of spangle First, seven standard samples having clearly different spangle patterns were prepared, and these samples were changed from 0 (very fine) to 6 (very coarse) depending on the size of the spangle. Classified. This numerical value is used as a spangle code.

On the other hand, an enlarged photograph of the surface of each standard sample was taken, and on the photograph, the number of spangle boundary lines crossing a line segment corresponding to an actual length of 30 mm was counted, and the spangle diameter was calculated by the following equation. The spangle diameter was 30 mm / the number of boundary lines. The measurement of the spangle diameter was repeated five times per standard sample, and the average value was defined as “spangle diameter”. Table 2 shows the correspondence between the spangle diameter and the spangle cord.

[0043]

[Table 2]

The spangle size (spangle diameter) of each hot-dip coated steel strip obtained in the example was visually compared with the above standard sample, and the spangle code of the standard sample having the closest spangle size was recorded. It was done. When all of the samples of the three types of base materials A, B, and C had a code of 2 or more, the samples were judged as acceptable (O).

Evaluation Method of Adhesion of Plating The adhesion of plating was determined by measuring the entire width (920 mm) of the resulting plated steel strip.
Was subjected to close contact bending using a rock former. Only those that did not cause peeling over the entire width in the plate width direction were judged as acceptable (O), and the rest were judged as unacceptable (X). When all of the spangle and plating adhesion (A, B, C) were ○, the overall evaluation was passed (合格).

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

[Table 5]

[0049]

[Table 6]

Each of the annealing temperatures (600 ° C., 68
Figure 1 shows the relationship between annealing conditions (dew point, hydrogen concentration) at 0 ° C, 750 ° C, and 850 ° C), spangles and adhesion. In this figure, the region surrounded by the solid line is the range of the annealing conditions in which both spangle and adhesion are good.

As can be seen from Tables 3 to 6, even if the annealing conditions and the plating conditions are the same, the spangle size may vary considerably when the base material changes. However, as shown in FIG. 1 and Tables 3 to 6, especially when the annealing temperature is low, the material is annealed under the conditions specified in the present invention and then hot-dip, regardless of the type of the base material. Without spoiling the adhesion, it is possible to obtain a beautiful spangle in which spangles do not become finer and are almost uniformly opened. On the other hand, if annealing is performed outside the conditions specified in the present invention to a high dew point or a high hydrogen concentration side, plating adhesion will be poor. vice versa,
If annealing is performed out of the conditions specified in the present invention to a low dew point or a low hydrogen concentration side, spangles become finer. Fine spangles also increase the variation in spangle diameter.

[0052]

According to the present invention, variations in spangles between coils or within a coil can be achieved without impairing the plating adhesion.
(Especially, spangle refining) is suppressed, and a hot-dip Zn—Al—Si plated steel sheet having excellent design properties can be stably manufactured.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the results of the examples based on the annealing conditions (annealing temperature,
It is shown as the relationship between hydrogen concentration), spangle and adhesion. The part surrounded by the thick line has good spangle and adhesion.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Yasuo Tsujimoto 1850 Minato, Wakayama Sumitomo Metal Industries, Ltd. Inside Wakayama Steel Works (72) Inventor Hiroto Masumoto 1850 Minato, Wakayama City Sumitomo Metal Industries, Ltd.F inside Wakayama Steel Works Terms (reference) 4K027 AA02 AA22 AB15 AB44 AB48 AC82 AE03 AE12 AE25 AE33 AE34 4K044 AA02 AB02 BA10 BA15 BA19 BA21 BB03 BB04 BC09 CA11 CA16 CA53 CA62

Claims (3)

[Claims] 1. A hot-dip Zn—Al—Si alloy plated steel sheet having a plating composition comprising 30 to 70% by weight of Al, 0.05 to 2.0% by weight of Si, and the balance of Zn and inevitable impurities is provided with an in-line annealing furnace. When manufacturing with continuous hot-dip plating equipment, the annealing is performed under the condition that the reducing gas atmosphere and dew point in the annealing furnace satisfy the following formulas (1) to (3). A method for producing a hot-dip Zn-Al-Si alloy-plated steel sheet that excels in design without sublimation. (1) + 10 ° C ≧ (dew point) ≧ -40 ° C (2) 2% ≦ (hydrogen concentration) ≦ 30%, balance nitrogen (3) (dew point) ≧ [(hydrogen concentration) × 2− (annealing temperature) / 10 + 10 ]
° C 2. The molten Zn produced by the method according to claim 1.
-A method for producing a chemically treated hot-dip Zn-Al-Si alloy plated steel sheet, comprising subjecting an Al-Si alloy plated steel sheet to a chromate treatment. 3. Molten Zn produced by the method according to claim 1.
-A thin-film resin coating having a thickness of 5 µm or less is applied to the Al-Si alloy-plated steel sheet or the chemical conversion-treated hot-dip Zn-Al-Si alloy-plated steel sheet produced by the method according to claim 2.
A method for producing an organic-coated hot-dip Zn-Al-Si alloy-plated steel sheet.