JP2007023347A - Galvannealed steel sheet manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a galvannealed steel sheet having a uniform oxide layer on its surface and excellent in press forming property. <P>SOLUTION: The galvannealed steel sheet excellent in press forming property is obtained by successively allowing a steel sheet to be subjected to a galvanizing step, an alloying step, a temper rolling step, and an oxide layer forming step in which an acid solution is brought into contact with a surface of the steel sheet, a liquid film of the acid solution is deposited thereon, the steel sheet is squeezed by two or more pairs of continuous squeezing rolls, the acid solution is reacted with the galvannealed layer to deposit an oxide layer on the surface of the galvannealed layer. Thus, the oxide layer of the predetermined thickness or more can be formed with less dispersion in the sheet width direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet, and more particularly to a stable method for producing an alloyed hot-dip galvanized steel sheet having excellent press formability.

  In general, alloyed hot-dip galvanized steel sheets are superior in weldability and paintability compared to hot-dip galvanized steel sheets, and are therefore used in a wide range of fields, particularly for automobile bodies. Normally, steel plates for automobile bodies are used after being subjected to press forming. However, the alloyed hot-dip galvanized steel sheet has a disadvantage that its press formability is inferior to that of a cold-rolled steel sheet. This is due to the fact that the sliding resistance in the press mold is larger in the galvannealed steel sheet than in the cold-rolled steel sheet.

  An alloyed hot-dip galvanized steel sheet is subjected to a hot-dip galvanizing process on the steel sheet, and then heated to perform an alloying process in which Zn in the hot-dip galvanized layer reacts with Fe of the steel sheet to form an Fe-Zn alloy phase. The alloyed hot-dip galvanized layer. Since this Fe-Zn alloy phase increases in hardness and melting point as the Fe concentration increases, it is preferable to have a high Fe concentration from the viewpoint of slidability. The plated steel sheet is subjected to an alloying treatment set to a condition in which the Fe concentration of the galvannealed layer is increased.

However, the alloyed hot-dip galvanized layer with a high Fe concentration has a problem that a hard and brittle Γ phase is easily formed at the interface between the plated layer and the ground iron, and so-called powdering is likely to occur when the plated layer is peeled off during processing.
For example, Patent Document 1 discloses an excellent press-formability in which an oxide layer having a thickness of 10 nm or more is formed on the surface layer of the flat portion of the plating layer after the alloying process. A method for producing an alloyed hot-dip galvanized steel sheet has been proposed. According to the technique described in Patent Document 1, an alloyed hot-dip galvanized steel sheet having stable and excellent press formability is easily obtained. As the oxidation treatment, Patent Document 1 exemplifies a method such as immersion in an aqueous solution containing an oxidizing agent, spraying an aqueous solution containing an oxidizing agent, or electrolytic treatment in an aqueous solution containing an oxidizing agent.

  Further, in Patent Document 2, after the alloying treatment, the substrate is further immersed in an aqueous solution containing an oxidizing agent, adjusted for immersion time, subjected to oxidation treatment, washed with water, and the flat portion surface has a thickness of 10 nm or more. An alloyed hot-dip galvanized steel sheet in which an oxide layer having a Zn / Al ratio of 2.0 to 8.0 is formed on the surface layer has been proposed. According to the technique described in Patent Document 2, an alloyed hot-dip galvanized steel sheet having excellent press formability and excellent chemical conversion property is obtained.

Patent Document 3 proposes a method for producing an alloyed hot-dip galvanized steel sheet excellent in press formability in which an acidic solution having a pH buffering effect is brought into contact with the steel sheet to form an oxide layer on the surface of the plated layer. ing.
Japanese Patent Laid-Open No. 2001-131772 JP 2002-302753 A Japanese Patent Laid-Open No. 2003-306781

If there is a large amount of acidic solution in contact with the steel sheet, the pH will not rise even if Zn dissolves, it will only dissolve Zn one after another, and it will take a long time to form an oxide layer and melt. It is conceivable that the galvanized layer is severely damaged and the rust prevention property is lowered. For this reason, in the technique described in Patent Document 3, it is desirable that the acidic solution be present in a thin liquid film state on the surface of the plated steel sheet in order to form an oxide layer. The liquid film amount is preferably adjusted to m ² or less.

  However, if the roll pressing load is increased by, for example, the two choc pressing type squeezing rolls 7 in order to adjust the liquid film amount of the acidic solution in the plated steel sheet to an appropriate amount, the squeezing rolls 7 become large in the barrel direction as shown in FIG. It will bend. As a result, there is a variation in the amount of liquid film in the plate width direction, which obstructs the formation of a uniform oxide layer in the plate width direction, resulting in different press formability in the plate width direction.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to propose a method for producing an galvannealed steel sheet having a uniform oxide layer on the surface of the galvannealed layer and having excellent press formability. And The “steel plate” in the present invention includes a steel plate and a steel strip.

  In order to achieve the above-mentioned problems, the present inventors diligently studied about uniformizing the liquid film amount of the acidic solution attached to the surface of the hot dip galvanized layer. As a result, it is important that the pressing load applied to the squeeze roll is within a proper range and a small load, and for that purpose, it has been conceived that two or more pairs of squeeze rolls may be provided in the transport direction of the steel sheet. . As a result, it was found that variations in the liquid film amount of the acidic solution can be reduced in the plate width direction, and therefore, an oxide layer having a uniform thickness can be formed at each position in the plate width direction.

The present invention has been completed based on such findings. That is, the gist of the present invention is as follows.
(1) A hot dip galvanizing process for forming a hot dip galvanized layer on the steel sheet surface, an alloying process for heating and alloying the hot dip galvanized layer to form an alloyed hot dip galvanized layer, and the alloying and melting A temper rolling step for flattening the galvanized layer, and further contacting an acidic solution on the surface of the flattened alloyed hot-dip galvanized layer to attach a liquid film of the acidic solution, and the acidic solution and the alloy When an alloyed hot-dip galvanized steel sheet is produced by sequentially applying the oxide layer forming step of reacting with the hot-dip galvanized layer to form an oxide layer on the surface of the alloyed hot-dip galvanized layer. A method for producing an alloyed hot-dip galvanized steel sheet, characterized in that the liquid film of the acidic solution is squeezed with two or more pairs of squeezing rolls arranged in a row and reacted with the acidic solution and the alloyed hot-dip galvanized layer.

  (2) The method for producing an galvannealed steel sheet according to (1), wherein the pressing pressure of the squeezing roll is 0.2 MPa or less.

  ADVANTAGE OF THE INVENTION According to this invention, the oxide layer with few thickness dispersion | variation can be formed in the surface, the alloyed hot-dip galvanized steel plate which has uniform press formability can be manufactured stably, and there exists a remarkable effect on industry.

In the present invention, the steel sheet is sequentially subjected to a hot dip galvanizing process, an alloying process, a temper rolling process, and an oxide layer forming process to obtain an alloyed hot dip galvanized steel sheet.
An example of the production line of the galvannealed steel plate suitable for implementation of this invention is shown in FIG.
The steel sheet S is preferably heated to 500 to 1000 ° C. in the annealing furnace 1 and continuously led to the hot dip galvanizing bath 2 without being exposed to the atmosphere, subjected to a hot dip galvanizing process, and has a hot dip galvanized layer on the surface. Is formed. In addition, it is preferable from the viewpoint of ensuring appearance and corrosion resistance that the plating adhesion amount per side is 15 to 150 g / m ² .

  Next, the steel sheet S is pulled up from the hot dip galvanizing bath 2 and adjusted to a predetermined plating adhesion amount by gas wiping or the like (not shown), and then led to the alloying furnace 3 to be subjected to an alloying treatment step. The In the alloying process, the steel sheet S is heated to a predetermined temperature. Thereby, in the hot dip galvanized layer, Fe diffuses and an Fe—Zn alloying reaction occurs to form an Fe—Zn alloy phase, which becomes an alloyed hot dip galvanized layer. The predetermined temperature in the alloying treatment is preferably 440 to 500 ° C. from the viewpoint of ensuring the powdering resistance.

The steel sheet S heated in the alloying furnace and alloyed with the plating layer is then cooled to near room temperature by the cooling device 4.
The steel sheet S cooled to near room temperature is then guided to the temper rolling mill 5 and subjected to a temper rolling process. By the temper rolling process, the convex portion on the surface of the plating layer is flattened, and a flat portion is formed on the plating surface. The presence of a flat portion on the plating surface is effective for improving the slidability during press molding. On the other hand, the recess has a function of holding lubricating oil, and effectively acts to prevent oil shortage during press molding.

  The steel sheet S that has been subjected to the temper rolling step is then guided to the acidic solution contact device 6 and brought into contact with the acidic solution, and a liquid film of the acidic solution is allowed to adhere to the steel sheet surface. As a preferable acidic solution to be used, hydrochloric acid, sulfuric acid, nitric acid or the like adjusted to pH 1.0 to 4.0 so that Zn in the plating layer can be dissolved, or compounds such as chloride, sulfate, and nitrate were added. A solution etc. can be illustrated. The acid solution contact device 6 is not particularly limited as long as the acid solution contact device 6 can contact the steel plate and the acid solution liquid film can be attached to the surface of the steel plate. An apparatus capable of applying an acidic solution or an apparatus capable of applying an acidic solution can be exemplified.

In the present invention, the steel sheet S to which the acidic solution liquid film is adhered by the acidic solution contact device 6 immediately squeezes the acidic solution with the squeezing roll 7 so that the amount of the acidic solution liquid film becomes a predetermined amount. In the present invention, the acidic solution is squeezed by connecting two or more squeezing rolls in series. FIG. 1 shows a case where two pairs of squeezing rolls 71 and 72 are provided.
By squeezing the acidic solution using two or more pairs of squeezing rolls 7, the amount of the acidic solution adhering to the steel sheet can be gradually reduced, and the pressing pressure applied to the pair of squeezing rolls Can be set to an appropriate and small pressure. As a result, the deflection of each squeezing roll is minimized, and the liquid film amount can be made uniform in the plate width direction. In addition, by attaching two or more pairs of squeezing rolls to squeeze the liquid film of the acidic solution, the liquid that adheres even if the pressing pressure of the squeezing rolls is the same as compared with the case of using only one pair of squeezing rolls. The film amount can be adjusted to be small. If the amount of the liquid film is large, the pH of the solution does not increase even if zinc dissolution occurs, and only zinc dissolution occurs one after another, and it takes a long time to form the oxide layer. If the liquid film thickness can be reduced, the resulting oxide layer can be made thick and uniform. The number of squeezing roll pairs is preferably determined by the thickness of the oxide layer formed on the plating layer surface. In order to form an oxide layer having a thickness described later, it is preferable to squeeze the liquid film by connecting two pairs of squeezing rolls.

  In the present invention, since the oxide layer is formed on the surface of the plating layer so that the variation of the thickness in the plate width direction is reduced, the pressing pressure of each squeeze roll is preferably 0.2 MPa or less. When two pairs of squeezing rolls are used, the pressing pressures at the front and rear stages may be the same or different, and each is preferably 0.2 MPa or less. The pressing pressure can be measured by arranging a pressure gauge at the end of the roll to which the pressing load shown in FIG. 2 is applied.

  The steel sheet S in which the liquid film amount of the attached acidic solution is adjusted is guided to the reaction zone 8 and exposed to the atmosphere to dry and concentrate the oxidizing solution, and to react the plating layer with the acidic solution to form an oxide layer. To promote. The reaction zone 8 is preferably set to 1 to 120 s as a standing time during which the steel sheet S is exposed to the atmosphere. If the standing time is short, the acidic solution is not sufficiently dried and concentrated, and an oxide layer having a desired thickness is not formed. On the other hand, if the standing time is too long, there is a problem that the line length becomes too long. An oxide layer having a thickness of 10 nm or more can be formed on the surface of the plating layer by ensuring the above-described standing time after setting the pressing pressure of each squeezing roll.

  The steel sheet S that has passed through the reaction zone 8 is then guided to the cleaning device 9 and is preferably subjected to a cleaning process to wash away the acidic solution components remaining on the surface. If the acidic solution component remains on the surface of the steel sheet, corrosion may be promoted when an actual product is obtained. In addition, as the washing | cleaning apparatus 9, the apparatus etc. which combined the water-washing processing apparatus which wash | cleans a steel plate with water, or the neutralization processing apparatus (not shown) and the water-washing processing apparatus which neutralize an acidic solution can be illustrated.

The steel plate S that has been subjected to the cleaning treatment is preferably guided to the drying device 10 and then subjected to a drying treatment that completely removes moisture adhering to the steel plate surface.
Next, the present invention will be described in more detail with reference to examples.

A hot dip galvanizing treatment process using a production line of alloyed hot dip galvanized steel sheet shown in FIG. An alloying hot-dip galvanized steel sheet was manufactured by sequentially performing an alloying treatment process, a temper rolling process, and an oxide layer forming process. In addition, the adhesion amount of the plating layer was 48 g / m ² per side, the alloying treatment was 500 ° C., and the elongation of temper rolling was 0.8%.

  In the oxide layer forming step, the alloyed hot-dip galvanized steel sheet subjected to temper rolling is guided to the acidic solution contact device 6 and immersed in a sulfuric acid acidic solution (pH: 2.0, liquid temperature: 50 ° C.). The pressing pressure is changed in the range of 0.40 to 0.15 MPa, and the attached acidic solution is squeezed using one pair (comparative example) or two pairs (example of the present invention) and left in the reaction zone 8 for 10 s. And promoted the reaction.

The steel plate that passed through the reaction zone 8 was washed with water by the cleaning device 9 and dried by the drying device 10 to obtain a product plate.
About the obtained product plate (alloyed hot-dip galvanized steel plate), the thickness of the oxide layer formed on the surface is the center in the width direction and both ends in the width direction (plates) at any five points in the length direction of the steel plate. Measurements were made at a total of 15 points (positions 50 mm inside from the width end).
The thickness of the oxide layer was measured as follows.

About the test piece extract | collected from each above-mentioned place, the oxygen count was measured by the fluorescent X ray analysis, and the oxide thickness was calculated | required from the analytical curve previously calculated | required. The calibration curve was prepared by measuring the oxygen count of a standard sample with a known oxide layer thickness by fluorescent X-ray analysis.
The obtained results are shown in Table 1. In addition, the oxide layer thickness of the width direction center part shows the average value of five places of a length direction. In addition, the oxide layer thickness at the end in the width direction was obtained by calculating the average value of 10 measured points and expressing it as a ratio to the oxide layer thickness (average value) at the center in the width direction.

  In the present invention example, the thickness of the oxide layer can be formed without variation in the width direction. On the other hand, in all of the comparative examples that are outside the scope of the present invention, the oxide layer thickness decreases, or the oxide layer thickness is thick but the variation in the width direction is large.

It is explanatory drawing which shows typically an example of the manufacturing line of the galvannealed steel plate suitable for implementation of this invention. It is explanatory drawing which shows typically the bending condition of the squeeze roll at the time of squeezing of an acidic solution.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Annealing furnace 2 Hot dip galvanizing bath 3 Alloying furnace 4 Cooling device 5 Temper rolling mill 6 Acidic solution contact device 7, 71, 72 Drawing roll 8 Reaction zone 9 Cleaning device
10 Drying equipment

Claims (2)

  A hot dip galvanizing process for forming a hot dip galvanized layer on the surface of the steel sheet; an alloying process for heating and alloying the hot dip galvanized layer to form an alloyed hot dip galvanized layer; and the alloyed hot dip galvanized layer A temper rolling step for flattening, and further, an acidic solution is brought into contact with the surface of the flattened alloyed hot-dip galvanized layer to adhere a liquid film of the acidic solution. In the production of an alloyed hot-dip galvanized steel sheet by sequentially subjecting the steel sheet to an oxide layer forming step of forming an oxide layer on the surface of the alloyed hot-dip galvanized layer by reacting with the plating layer, the attached acidic solution The liquid film is squeezed with two or more pairs of squeezing rolls arranged in a row, and the acidic solution and the galvannealed alloy layer are reacted with each other.   The method for producing an galvannealed steel sheet according to claim 1, wherein the pressing pressure of the squeezing roll is 0.2 MPa or less.

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