JP2006322028A - Hot dip galvanizing method for p-added steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method where, when a P-added steel sheet is subjected to hot dip galvanizing, its satisfactory performances such as plating appearance and plating adhesion are improved as the alloying speed thereof is improved. <P>SOLUTION: In the hot dip galvanizing method for a P-added steel sheet, a P-added steel sheet is annealed and is subjected to pickling treatment several times, thereafter, its surface is activated, and the same is plated with a hot dip galvanizing bath containing 0.05 to 0.2 mass% Al and is then subjected to heating alloying treatment. In the method for performing pickling treatment several times, the first pickling treatment is performed in a hydrochloric acid aqueous solution, and the second pickling treatment is performed in hydrochloric acid aqueous solution. Further, in the method of activating the surface, heating is performed in a nonoxidizing atmosphere, and, moreover, in the method of activating the surface, Ni preplating is applied, and heating is performed at 430 to 500°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an alloying hot dip galvanizing method for P-added steel sheets.

  The alloyed hot-dip galvanized steel sheet is excellent in corrosion resistance, paint adhesion and the like, and is widely used particularly as a steel sheet for automobiles. Recently, demands for safety, durability, and weight reduction of automobiles are increasing, and by adding P (phosphorus) as a material that satisfies these requirements, the steel sheet is made high tension and safety and durability are increased. An alloyed hot-dip galvanized steel sheet using a P-added steel sheet that can improve the thickness and reduce the thickness of the steel sheet because of its high tension and can realize a reduction in weight has come to be used.

  However, it is known that P in steel inhibits and delays the alloying reaction of zinc, and usually requires a longer alloying treatment time than a steel sheet, which is a factor inhibiting productivity. Also, when manufacturing both steel sheets with high alloying speed (for example, ultra-low carbon steel sheets with addition of Ti and Nb) and P-added steel sheets on the same line, the Al concentration in the hot-dip Zn plating bath and the alloying treatment conditions It is necessary to manage each etc. optimally, and the operation becomes complicated.

From the background as described above, it is strongly desired to improve the alloying speed of the P-added steel sheet, and various efforts have been made for this purpose.
For example, Patent Document 1 discloses a method in which a P-added steel sheet is annealed, the surface is cleaned after pickling treatment, galvanized, and then heated and alloyed after plating.
Further, Patent Document 2 discloses that Ni high-strength steel plate containing P is subjected to Ni pre-plating, heated under predetermined conditions, then hot-dip Zn-plated, and heat-alloyed under predetermined conditions. .
Japanese Patent Publication No. 7-9055 Japanese Patent No. 2526320

  Although the method of Patent Document 1 can be expected to some extent, the plating appearance tends to be non-uniform, and it is difficult to apply it to automobiles, particularly for the use of outer plates. This method is effective when the Al concentration in the molten Zn plating bath is extremely low, but in the case of a normal alloyed hot dip galvanizing bath containing 0.05 to 0.2% Al. The effect was small.

  Although the method of Patent Document 2 can be expected to have a certain effect, the heat alloying treatment time is still long, and it has been difficult to obtain a good appearance that can be applied to automobiles, particularly for use in outer panels. Accordingly, an object of the present invention is to provide a method of improving performance such as good plating appearance and plating adhesion while improving the alloying speed when P-added steel sheet is subjected to alloying hot dip galvanizing.

  With reference to the description of Patent Document 1, the present inventors have studied various conditions for improving the alloying speed and obtaining a good plating appearance even when the Al concentration in the hot-dip Zn plating bath is high. As a result, it has been found that it is effective to pass a plurality of pickling treatments after annealing the P-added steel sheet, thereby completing the present invention.

That is, the gist of the present invention is as follows.
(1) Annealing the P-added steel sheet, passing through multiple pickling treatments, activating the surface, plating in a hot dip galvanizing bath containing 0.05 to 0.2% by mass of Al, and then heating alloying An alloying hot dip galvanizing method for a P-added steel sheet, characterized in that the treatment is performed.
(2) The alloy of the P-added steel sheet according to (1), wherein the plurality of pickling treatment methods include a first pickling treatment with a hydrochloric acid aqueous solution and a second pickling treatment with a sulfuric acid aqueous solution. Hot dip galvanizing method.
(3) The method of galvanizing alloying of P-added steel sheet according to (1) or (2) above, wherein the method for activating the surface is performed in a non-oxidizing atmosphere.
(4) Alloying of P-added steel sheet according to any one of (1) to (3) above, wherein the method of activating the surface is performed by performing Ni pre-plating and heating to 430 to 500 ° C. Hot dip galvanizing method.
(5) The heating alloying treatment method is characterized in that rapid heating is performed at a temperature increase rate of 20 ° C./sec or more at 470 to 600 ° C., and cooling is performed after soaking for less than 15 seconds. The alloying hot-dip galvanizing method for the P-added steel sheet according to any one of (1) to (4).
(6) The alloyed hot-dip galvanizing method for a P-added steel sheet according to any one of (1) to (5), wherein the P-added steel sheet contains 0.02% by mass or more of P.

  According to the present invention, alloyed hot dip galvanizing can be applied to the P-added steel sheet with high productivity, and good plating appearance and plating adhesion can be obtained.

The P-added steel sheet of the present invention is not particularly limited, and is manufactured by a process such as hot rolling, cold rolling, annealing, and can be applied to any of a low carbon steel sheet, an extremely low carbon steel sheet, and the like. Further, a steel plate containing a so-called trump element such as Cr, Cu, Ni, or Sn can be applied.
The present invention aims to obtain a good plating appearance with a high alloying speed, and is particularly effective for cold-rolled ultra-low carbon steel sheets that require a good plating appearance. In addition, it is known that when the P content is 0.02% or more, alloying is significantly delayed and the productivity is remarkably lowered, so that it is particularly effective for 0.02% or more P-added steel sheet. It is.

  In the present invention, after the P-added steel sheet is annealed, a plurality of pickling treatments are performed. Here, the effects of the first pickling treatment after annealing are as shown in Patent Document 1, and after crystal grain boundaries are formed by annealing, in particular, P present in the crystal grain boundaries is pickled. Reduction by removal contributes to an improvement in alloying speed. However, according to the study by the present inventors, when P is removed in this step, since only the crystal grain boundary is deeply eroded and the surface becomes rough, the subsequent plating appearance tends to be non-uniform. Further, since the effect of removing P existing in the crystal grain surface layer is not sufficient, the effect of improving the alloying speed is small.

  Therefore, in the present invention, the pickling treatment is further performed after the pickling treatment. Here, it is the concept of the number of pickling treatments in the present invention. For example, when a steel plate does not dry between tanks as it passes through a plurality of pickling treatment tanks, even if there are a plurality of tanks, acid The washing process is considered as one time. This is because the grain boundaries are eroded by the pickling treatment and then dried (usually after washing with water), so that the surface is thinly oxidized by oxygen in the atmosphere, and the oxidized steel sheet surface is re-oxidized. This is because P can be uniformly and effectively removed by the washing treatment, and as a result, a high alloying speed and a uniform plating appearance can be obtained. That is, it is meaningful that the steel sheet is dried between the pickling treatment.

  Although it does not specifically limit as a method of a pickling process, The method as shown in patent document 1, ie, the method of processing for 1 to 10 seconds at the temperature of 60-90 degreeC with 1-5% hydrochloric acid aqueous solution is suitable. Used for. In addition, about the 2nd pickling process (when performing the pickling process more than twice, the last pickling process), when performing the 1st pickling process (when performing the pickling process more than 2 times) The treatment with sulfuric acid is preferable to the treatment with hydrochloric acid because it also has the meaning of adjusting the rough surface state formed by the pickling treatment one time before). In this case, a method of treating with a 5 to 15% sulfuric acid aqueous solution at a temperature of normal temperature to 70 ° C. for 1 to 10 seconds is preferable.

  After the pickling treatment as described above, before the hot dip galvanization, a surface activation treatment for ensuring the wettability of the plating is required. This is to reduce the thin oxide film formed by exposure to the air and to ensure the wettability of the plating. As the surface activation treatment, a method of heating a steel sheet in a non-oxidizing atmosphere is used. In this case, it is necessary to perform the treatment at a temperature lower than the recrystallization temperature and at which sufficient plating wettability can be secured. Specifically, the treatment is required at about 600 to 700 ° C. for 10 seconds to several minutes. is there.

  When the heat treatment as described above is performed, the crystal grains may be coarsened and the material may be changed depending on the case. In order to avoid this, in order to ensure the plating wettability by heating at a lower temperature, than Fe A method of pre-plating a metal that is not easily oxidized is also preferably used. Specifically, a method of performing Ni pre-plating and heating to 430 to 500 ° C. is used.

  After performing the above surface activation treatment, plating is performed in a hot dip galvanizing bath containing 0.05 to 0.2% of Al. The reason why Al is set to 0.05 to 0.2% is that if it is less than 0.05%, an extremely high alloying speed can be obtained, but the plating adhesion deteriorates, and if it exceeds 0.2% This is because even with the method of the present invention, a sufficient alloying rate cannot be obtained.

  As a preferable form of the alloying treatment conditions after plating, rapid heating is performed at a temperature increase rate of 20 ° C./sec or more at 470 to 600 ° C., and cooling is performed without taking a soaking time, or soaking for less than 15 seconds. A method of cooling after holding can be used. According to this treatment, the plating appearance and plating adhesion are good and the productivity is not hindered.

Hereinafter, the present invention will be described in detail by way of examples.
In the following examples, cold-rolled and annealed steel sheets having the components shown in Table 1 were used.
(Examples 1-5 and Comparative Examples 1-3)
Table 2 shows the combinations of the original plate used and the processing conditions. After the first pickling treatment, it was washed with water and dried. Further, except for Comparative Example 1 and Comparative Example 2, the second pickling treatment was performed, followed by washing with water and drying. The conditions for the pickling treatment are shown in Table 3.

Then, it heated at 650 degreeC in 5% hydrogen + 95% nitrogen atmosphere, and hold | maintained for 1 minute.
Thereafter, the mixture was allowed to cool to 460 ° C., immersed in a molten Zn plating bath containing Al kept at 455 ° C., and adjusted to 60 basis weight by wiping. Table 2 also shows the Al concentration in the hot-dip Zn plating bath. Heat up to the specified temperature shown in Table 2 at a heating rate of 50 ° C / sec immediately above the wiping, soak for 10 seconds at a rate of 10 ° C / sec after soaking for a predetermined time, and then at room temperature at 20 ° C / sec. Until cooled.

Evaluation was performed as follows.
[Plating appearance] Appearance was evaluated as “◯” when the appearance was uniform with no unevenness on the appearance, and “×” was indicated when the appearance was uneven or patterned and was impractical (especially for automotive exterior panels). .
[Degree of alloying] The plating layer of the sample was dissolved in hydrochloric acid, the component amount was determined by chemical analysis, and Fe% in the plating layer was calculated. The case where 9% or more of Fe was obtained was designated as “◯”, and the case where less than 9% was designated as “X”.
[Plating adhesion] After 60 ° V bending, plating peeling at the bent portion was evaluated by a tape peeling method. The peeling width of less than 2 mm was evaluated as “◯”, and the peeling width was evaluated as “×”.
As is apparent from Tables 1 to 3, according to the present invention, an excellent alloying degree, plating appearance, and plating adhesion were obtained in a short-time alloying treatment.

(Examples 6 to 8 and Comparative Example 4)
Table 4 shows the combinations of the original plate and the processing conditions used. After the first pickling treatment, water washing and drying were performed, and in addition to Comparative Example 4, the second pickling treatment was performed and water washing was performed, and then the Ni pre-plating amount was 0.3 g by electroplating under the conditions shown in Table 5. / m ² applied. The conditions for the pickling treatment are shown in Table 3.

Thereafter, the mixture was heated to 460 ° C. at 40 ° C./sec in a 3% hydrogen + 95% nitrogen atmosphere, immediately immersed in a molten Zn plating bath containing Al kept at 455 ° C., and adjusted to 60 basis weight by wiping. Table 4 also shows the Al concentration of the molten Zn plating bath. Heat to the specified temperature shown in Table 4 at a rate of 50 ° C / sec immediately above the wiping, soak for 10 seconds at a rate of 10 ° C / sec after soaking for a predetermined time, and then at room temperature at 20 ° C / sec. Until cooled.
Evaluation was performed in the same manner as in the above example.
As is apparent from Tables 4 and 5, according to the present invention, an excellent alloying degree, plating appearance, and plating adhesion were obtained in a short-time alloying treatment.

  According to the present invention, alloyed hot dip galvanizing can be performed on the P-added steel sheet with high productivity, and a good plating appearance and plating adhesion can be obtained. Therefore, the industrial utility value is great.

Claims (6)

  After annealing the P-added steel sheet and performing a plurality of pickling treatments, the surface is activated, plated with a hot dip galvanizing bath containing 0.05 to 0.2% by mass of Al, and then subjected to heat alloying treatment. An alloying hot dip galvanizing method for a P-added steel sheet.   2. The alloyed molten zinc of P-added steel sheet according to claim 1, wherein the plurality of pickling treatment methods include a first pickling treatment with a hydrochloric acid aqueous solution and a second pickling treatment with a sulfuric acid aqueous solution. Plating method.   The method for galvanizing an alloyed P-added steel sheet according to claim 1 or 2, wherein the method for activating the surface is performed in a non-oxidizing atmosphere.   The method for activating hot galvanizing of a P-added steel sheet according to any one of claims 1 to 3, wherein the method for activating the surface includes applying Ni pre-plating and heating to 430 to 500 ° C.   The heating alloying treatment method is characterized in that rapid heating is performed at a temperature increase rate of 20 ° C./sec or more to 470 to 600 ° C., and cooling is performed after taking a soaking time of less than 15 seconds. Item 5. An alloying hot-dip galvanizing method for a P-added steel sheet according to any one of Items 1 to 4. The P-added steel sheet according to any one of claims 1 to 5, wherein the P-added steel sheet contains 0.02 mass% or more of P.