JP5354156B2 - Method for producing galvannealed steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a hot-dip galvannealed steel sheet capable of preventing a plating surface after the galvannealing from being non-uniformed attributable to the presence of scales on a surface of a steel sheet before the plating, and having the excellent uniformity of the surface even after the coating. <P>SOLUTION: In the manufacturing method for producing the hot-dip galvannealed steel sheet in which the hot dip galvanizing is executed after executing the reduction-annealing of a base steel sheet, and the galvannealing thereof is further executed, the surface oxygen amount Y of the base steel sheet is regulated to be &le;150 mg/m<SP>2</SP>, the base steel sheet is heated before executing the reduction-annealing, and the base steel sheet is subjected to oxidation, and successively, reduction so that the oxygen adhesion amount X on the surface of the base steel sheet is in a range of 25-500 mg/m<SP>2</SP>, and X and Y satisfy inequality (1): Y&times;2.5&le;X. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet that is excellent in plating surface uniformity.

  In the fields of automobiles, home appliances, building materials, etc., surface-treated steel sheets imparted with rust prevention properties to raw steel sheets, especially alloyed hot-dip galvanized steel sheets that can be manufactured at low cost and have excellent rust prevention properties are used.

  In general, alloyed hot-dip galvanized steel sheets use thin steel sheets that are hot-rolled, cold-rolled or further heat-treated as slabs as base material steel sheets. After degreasing and / or pickling and washing, or by omitting the pretreatment process and burning off the oil on the surface of the base steel plate in the preheating furnace, it is 600 to 900 in a non-oxidizing atmosphere or a reducing atmosphere. Recrystallization annealing is performed by heating to a temperature range of about 0 ° C., and then the steel sheet is cooled to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere without touching the atmosphere. It is manufactured by dipping in a hot dip galvanizing bath to which about 1% by mass of Al is added and hot dip galvanizing, and then alloying the plating layer in an alloying furnace.

  In recent years, alloyed hot-dip galvanized steel sheets have been widely used for automobile outer panels. In this application, the alloyed hot-dip galvanized steel sheet is used after being coated, and a uniform plated surface that does not cause uneven appearance after coating is required. However, in the galvannealed steel sheet in which the plating layer is alloyed after hot dip galvanization, even if the appearance of the base steel sheet is uniform, there is a slight difference in the chemical composition of the steel sheet surface. Since the plating adhesion and the growth of the alloy phase during the alloying process are different, the plated surface after the alloying process is not uniform, and the plating appearance is not only inferior, but also the appearance is uneven after coating.

  One of the factors that make the plating surface uneven is the remaining scale (oxide mainly composed of iron) on the surface of the steel plate before plating. If scale remains on the surface of the steel plate before plating, the plating adhesion will be different between where the scale is present and where there is no scale, and the reactivity with the base steel will be different during the alloying treatment. The composition and the amount of coating are different and the plating surface becomes non-uniform.

As a technique for improving the plating surface appearance, for example, Patent Document 1 describes that a surface layer of a steel sheet is removed by 0.1 μm or more prior to plating. Such a technique can also be expected to improve defects caused by remaining scale, which is a problem here. However, it is not easy to uniformly remove the steel sheet surface layer. For removal, for example, if a grindstone is used, a scratch made of the grindstone causes a negative effect that the uniformity of the appearance is impaired.
JP-A-8-269663

  The present invention has been made in view of such circumstances, and prevents the plating surface after alloying from becoming non-uniform due to the remaining scale on the surface of the steel sheet before plating, and has a good plating appearance. It is another object of the present invention to provide a method for producing an alloyed hot-dip galvanized steel sheet that does not cause uneven appearance after coating.

The means of the present invention for solving the above-mentioned problems is a method for producing an alloyed hot-dip galvanized steel sheet in which a base steel sheet is subjected to reduction annealing, followed by hot-dip galvanizing and further alloying treatment.
The surface oxygen amount Y of the base steel plate is specified to be 150 mg / m ² or less, the base material steel plate is heated and oxidized, and the oxygen amount X on the base steel plate surface after oxidation is in the range of 25 to 500 mg / m ² , and An oxidation galvanized steel sheet manufacturing method characterized in that an oxidation treatment is performed so that X and Y satisfy formula (1), followed by a reduction annealing treatment.
Y × 2.5 ≦ X (1)

  ADVANTAGE OF THE INVENTION According to this invention, it can prevent that the plating surface becomes non-uniform | heterogenous after alloying processing resulting from the scale remaining on the steel plate surface before plating, and can manufacture the galvannealed steel plate which has a favorable plating external appearance. The alloyed hot-dip galvanized steel sheet produced by the method of the present invention does not cause uneven appearance even after coating.

  The present inventors examined improvement and elimination of non-uniformity of the plating surface of the galvannealed steel sheet caused by the scale (oxide mainly composed of iron) existing on the surface of the steel sheet before plating. As a result, prior to the reduction annealing before plating, the surface of the steel sheet is oxidized, and then the oxide is reduced, thereby improving the non-uniformity of the plating surface after the alloying treatment and alloying with a good plating appearance. It has been found that hot-dip galvanized steel sheets can be manufactured. The present invention is based on this finding.

  The present invention will be described in detail below. In the present invention, the amount of scale on the surface of the steel sheet (the amount of oxide mainly composed of iron) is defined by the amount of oxygen on the surface of the steel sheet.

  In this invention, before carrying out reduction annealing of a base material steel plate, the oxidation process which heats and oxidizes a base material steel plate with a heating furnace is performed. After performing the oxidation treatment, the base steel plate is subjected to reduction annealing, then hot dip galvanized, and further subjected to alloying treatment to produce an alloyed hot dip galvanized steel plate.

The base steel sheet has a surface oxygen content Y of 150 mg / m ² or less. If the amount of oxide on the surface of the base steel plate exceeds 150 mg / m ² when the oxygen content is evaluated, the surface of the base steel plate surface will not be damaged due to the scale of the original base steel plate surface even if the steel plate surface is oxidized and then reduced. This is because the uniformity is not eliminated, and a region where a large amount of scale remains is left even after reduction, and unevenness of the plating surface due to the scale occurs after the alloying treatment.

Setting the amount of oxygen Y on the surface of the base steel sheet to 150 mg / m ² or less is to suppress the biting of the scale due to hot rolling, and by strengthening descaling during hot rolling, This is achieved by optimizing the pickling conditions such as extending the pickling time in the washing step and raising the pickling temperature.

A base steel plate having a surface oxygen amount within the above range is oxidized in a heating furnace. At that time, the amount of oxygen X on the surface of the base steel plate after oxidation is in the range of 25 to 500 mg / m ² , and the amount of oxygen X on the surface after oxidation and the amount of oxygen Y on the surface before oxidation are expressed by the formula (1). Try to be satisfied.
Y × 2.5 ≦ X (1)

The oxygen amount X on the steel sheet surface after oxidation in the heating furnace is regulated to 25 mg / m ² or more and 500 mg / m ² or less for the following reason. If the amount is less than 25 mg / m ² , the amount of oxygen is small, so the scale that has existed unevenly on the surface of the base steel sheet cannot be made uniform, and there remains a large amount of remaining scale. Since much remains, plating adhesion is different and a uniform appearance cannot be obtained after alloying treatment. If the amount exceeds 500 mg / m ² , the scale amount is too large, so that the scale cannot be sufficiently reduced in the subsequent reduction annealing process, and the portion where the scale is reduced and the portion where the scale remains coexist. A uniform plating appearance cannot be obtained after the crystallization treatment.

  The oxygen amount X on the base steel plate surface after oxidation is defined to satisfy Y × 2.5 ≦ X with respect to the oxygen amount Y on the base steel plate surface before oxidation for the following reason. If the difference between X and Y is small, the scale that was unevenly present on the surface of the base steel sheet cannot be made uniform. In order to eliminate the non-uniformity of the scale on the surface of the base steel plate and make the scale uniformly exist after the oxidation treatment, it is necessary to make the oxide more than 2.5 times the initial oxide amount after the oxidation treatment.

  There are two main types of furnaces that oxidize steel sheets: a direct fire type that directly heats the steel sheet with a flame, and a radiant type that heats the atmosphere in the furnace equipped with a radiant tube at a high temperature. is there. If the amount of oxygen on the surface of the steel sheet can be oxidized so as to satisfy the above relationship in the heating furnace, the type of the heating furnace will not matter, but considering that it can be oxidized efficiently in a short time, the direct fire type is better preferable.

  In the case of a direct-fired type heating furnace, the steel sheet can be oxidized so that the amount of oxygen on the steel sheet surface satisfies the above by appropriately controlling the air ratio that is the ratio of air to fuel supplied to the heating furnace burner. . For example, the amount of oxidation can be increased by increasing the air ratio and approaching 1, and the amount of oxidation can be further increased by increasing the air ratio to 1 or more.

  In the case of a radiant type heating furnace, the atmosphere in the heating furnace is controlled to oxidize the steel sheet so that the oxygen content on the steel sheet surface satisfies the above. For example, the amount of oxidation can be increased by controlling the oxygen concentration in the heating furnace and increasing the concentration.

  The steel sheet heated and oxidized in the heating furnace as described above is subjected to reduction annealing. Reduction annealing is performed in a reducing atmosphere to reduce oxides on the surface of the steel sheet. The reducing atmosphere may be a normally used reducing atmosphere. In reduction annealing, the steel sheet is heated to a temperature range of 600 to 900 ° C. and recrystallized.

  After reductive annealing, cooling, hot dip galvanizing, and alloying treatment are performed in order to prevent the plating surface from becoming non-uniform, and the appearance of plating is good. A hot-dip galvanized steel sheet can be obtained.

  Hereinafter, the present invention will be specifically described based on examples.

  In order to confirm the effect of the present invention, the following production experiment was conducted in a continuous hot dip galvanizing facility equipped with a direct-fire type heating furnace.

Cold rolled steel sheets having a plate thickness of 0.75 mm and a plate width of 1500 mm and having different surface scale amounts were prepared as base steel sheets. The base steel sheet was heated in a direct-fired heating furnace to oxidize, and then the steel sheet was heated to 600 ° C. in 60 seconds in a reduction annealing (H ₂ : 4 vol% -N ₂ atmosphere). Then, the temperature range of 600 to 850 ° C. is raised in 100 seconds and held at 850 ° C. for 20 seconds.) After cooling, the Al concentration in the plating bath is 0.14% by mass and the bath temperature is 460 ° C. Dipping, adjusting the plating adhesion to 45 to 55 g / m ² per side, further alloying treatment at 520 ° C., so that the Fe% in the plating film is in the range of 9 to 11% by mass. A galvannealed steel sheet was prepared.

  By changing the air ratio of the heating furnace burner, the amount of oxide on the steel sheet surface after oxidation in the heating furnace was adjusted. For example, in Comparative Example 23, the air ratio of the direct furnace was set to 0.93. The oxide amount (oxygen amount) on the surface of the steel sheet after oxidation in the heating furnace was measured with an on-line infrared interference type oxide meter immediately after the heating furnace.

The plating appearance of the produced galvannealed steel sheet was evaluated. For the evaluation of the plating appearance, a sample having a plate width × length of 1000 mm was collected and visually evaluated according to the following criteria. Minor defects are those in which the boundary with the surrounding is unclear and the size of the defect is 5 mm or less in the longitudinal direction of the coil, and those that are longer than this or that have a clear and conspicuous boundary with the surrounding are severe defects. It was.
A: Appearance is good (no defects are observed)
○: Appearance is good (less than 3 minor defects are observed per side)
Δ: poor appearance (more than 3 minor defects are observed per side)
X: Appearance defect (even one severe defect is observed)

  Table 1 summarizes the conditions of the manufacturing experiment and the evaluation results of the experimental materials.

  As apparent from Table 1, in the invention examples in which the oxygen amount Y on the surface of the steel sheet before oxidation in the heating furnace and the oxygen amount X on the surface of the steel sheet after oxidation treatment in the heating furnace are within the scope of the present invention, the appearance evaluation is good. An alloyed hot-dip galvanized steel sheet having a good plating appearance has been obtained. Those with a good plating appearance were good with no unevenness in the appearance after coating.

  The present invention can be used as a method for producing an alloyed hot-dip galvanized steel sheet having a good plating appearance. Since the alloyed hot-dip galvanized steel sheet produced by the method of the present invention has no uneven appearance even after painting, it can be used as an alloyed hot-dip galvanized steel sheet used for automobile outer plate applications.

Claims (1)

In the manufacturing method of the alloyed hot-dip galvanized steel sheet, after subjecting the base steel sheet to reduction annealing, galvanizing and further alloying,
The surface oxygen amount Y of the base steel plate is specified to be 150 mg / m ² or less, the base material steel plate is heated and oxidized, and the oxygen amount X on the base steel plate surface after oxidation is in the range of 25 to 500 mg / m ² , and The manufacturing method of the galvannealed steel plate characterized by performing the oxidation process which makes X and Y satisfy Formula (1), and performing a reduction annealing process after that.
Y × 2.5 ≦ X (1)