JP3068938B2 - Method for producing galvannealed steel sheet with excellent formability - Google Patents

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 an alloyed hot-dip galvanized steel sheet for use in forming such as press working, and more particularly to an alloy having excellent formability by controlling the surface roughness of the steel sheet. The present invention relates to a method for producing a galvanized steel sheet.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets have the largest production volume among surface-treated steel sheets, and their applications have been expanded from those for construction materials and civil engineering to home electric appliances and automobiles. Recently, alloyed hot-dip galvanized steel sheets, in which iron has been diffused from the steel sheet to the coating layer by heat treatment after hot-dip galvanizing, have the same material properties as cold-rolled steel sheets and have excellent surface quality such as paintability. , Are beginning to be used in car bodies. In particular,
In recent years, there has been an extremely strong demand for improvement in corrosion resistance of automotive body materials, and the demand for thicker galvannealed steel sheets having excellent corrosion resistance has been rapidly increasing.

Conventionally, the surface roughness of a cold-rolled steel sheet subjected to press working has been adjusted for the purpose of improving the formability, the surface properties of a product, the life of a press die, and the like. That is, from the viewpoint of the finish of the coating performed after the press working, the surface roughness of the steel sheet is preferably small,
Smooth steel sheets are likely to have insufficient lubrication between the mold and the steel sheet during press working, which increases the coefficient of friction and reduces formability. Temper rolling using a roughened dull roll is performed to make the steel plate surface uneven, thereby ensuring pressability. If the surface has moderate irregularities, lubricating oil is sealed in the concave portions during the press working to form micropools, and by increasing the degree of fluid lubrication, the occurrence of frictional resistance and mold galling are reduced. Recently, as disclosed in, for example, Japanese Patent Publication No. 03-54006, a regular uneven pattern is formed on a temper rolling roll with a laser beam, and the surface pattern of a steel sheet to be transferred is controlled to cold-roll. A method for effectively improving the formability of a steel sheet has been developed and applied to the manufacture of automotive steel sheets.

[0004]

The formability of an alloyed hot-dip galvanized steel sheet is basically determined by the formability of a cold-rolled steel sheet, which is a raw material. Inferior. It is considered that the reason for this is that the plating layer adheres to the press die and the frictional resistance increases. The surface roughness of alloyed hot-dip galvanized steel sheets is adjusted by temper rolling using dull rolls as in the case of cold-rolled steel sheets. However, the pressability of the alloyed hot-dip galvanized steel sheet has not been effectively improved. Therefore, despite the fact that the galvannealed steel sheet has better corrosion resistance than the cold-rolled steel sheet, its use is limited to relatively lightly pressed products and parts.

[0005] The present invention solves these problems, and by controlling and optimizing the surface roughness, can significantly improve the press formability and expand its use. It is intended to provide a method for manufacturing a steel sheet.

[0006]

According to the present invention, there is provided, in accordance with the present invention, a temper rolling in a manufacturing process of a galvannealed steel sheet, wherein the elongation of the steel sheet by rolling is 3.0% or less. And the three-dimensional center line average roughness SRa1 and the three-dimensional ten-point average roughness S
Rz1 and three-dimensional center line average roughness SRa2 after temper rolling
And a method for producing an alloyed hot-dip galvanized steel sheet excellent in formability, characterized in that the surface roughness is controlled so that the three-dimensional ten-point average roughness SRz2 has the following relationship. SRa2-SRa1 ≦ 0 (μm) 30 (μm) ≧ SRz2-SRz1 ≧ 2 (μm)

Hereinafter, the present invention will be described in more detail.

[0008] The alloyed hot-dip galvanized steel sheet according to the present invention is composed of various components whose components are adjusted for the purpose of optimizing the formability represented by the r value and the strength represented by the TS according to the application. The steel sheet is used as a raw material, and hot-dip galvanizing of 30 to 100 (g / m ² ) is applied to both surfaces or one surface, respectively, and the amount of Fe in the plating layer is reduced to 5 to 20 watts by heat treatment.
The steel sheet adjusted to t% has an SRa of about 0.5 to 3.0 μm and an SRz of about 8 to 20 μm.

The present inventors performed temper rolling of an alloyed hot-dip galvanized steel sheet using rolls having various surface roughnesses, and transferred a predetermined surface pattern formed in advance on the roll surface to the steel sheet. Using the steel sheet adjusted to the surface roughness, the influence of the surface roughness of the alloyed hot-dip galvanized steel sheet on the press forming limit and slidability was investigated. As a result, qualitatively, they found that the area of the flat part roll-transferred to the alloyed hot-dip galvanized surface was larger, and that the sliding characteristics when the concave part functioning as a micro pool was moderately excellent. Was. At this time, the relationship between the surface roughness and the formability can be evaluated by the three-dimensional average surface roughness and the three-dimensional ten-point average roughness. In other words, the three-dimensional average surface roughness indicates the size of the area of the flat portion on which the roll is transferred, and the three-dimensional ten-point average roughness indicates the average depth of the concave portion functioning as a micropool.

[0010] The three-dimensional average roughness SRa is a parameter for accurately evaluating the average roughness of the surface-treated steel sheet, and its definition is a parameter obtained by extending the normal average roughness Ra to three-dimensional. It is. That is, a portion of the area SM is extracted from the roughness phase (f (x, y)), and orthogonal coordinate axes (X axis, Y axis) are placed on the center plane of the extracted portion.
, And the axis orthogonal to the center plane is represented by the Z axis, and the value given by the following equation is represented in μm units, and SRa is obtained. The term “center plane” as used herein refers to an imaginary plane in which the areas of the concave portions and the convex portions are equal in surface roughness.

[0011]

(Equation 1)

Also, the three-dimensional ten-point average roughness SR
z is a parameter for accurately evaluating the concave portion formed on the surface of the surface-treated steel sheet, and its definition is a parameter obtained by expanding the normal ten-point roughness Rz three-dimensionally. In other words, at the portion where the area SM is extracted from the roughness surface (f (x, y)), the fifth from the highest measured in the direction of the vertical magnification from a plane parallel to the center plane and not crossing the roughness surface SRz is expressed by the following equation, which expresses the difference between the average value of the altitude of the peak up to and the average value of the valley bottom from the deepest to the fifth in μm.

[0013]

(Equation 2)

Here, R ₁ , R ₃ , R ₅ , R ₇ , and R ₉ are the altitudes (μm) of the top to fifth peaks of the portion corresponding to the reference area L × L, R ₂ , R ₄ , R ₆ , R ₈ , and R ₁₀ are the altitudes (μm) of the valley bottoms from the highest to the fifth in the portion corresponding to the reference area L × L.

The concave portions formed on the surface of the surface-treated steel sheet can be evaluated by the three-dimensional maximum roughness SRmax. However, if there is a minute flaw on the surface of the steel sheet, there is a high possibility of counting the altitude of the flaw. Therefore, practically, it is necessary to evaluate the three-dimensional ten-point average roughness SRz from which noise can be eliminated.

The method of adjusting the surface roughness of an alloyed hot-dip galvanized steel sheet is to form arbitrary irregularities on a skin pass roll and transfer the irregularities by rolling. Blasting a shot having a grain size of (2) on the roll surface, (2) making crater-like irregularities on the roll surface using a laser beam, (3)
There are electric discharge machining, (4) roll etching, and the like, but a method using a laser beam or etching is preferable in terms of roughness accuracy. Conventionally, the adjustment of the surface roughness of a steel sheet has been controlled only by the average roughness Ra. However, a feature of the present invention is that Ra and Rz are independently optimized.

Next, the reasons for limiting the constituent elements of the present invention will be described.

In temper rolling, the reason why the elongation percentage of the galvannealed steel sheet by rolling is set to 3.0% or less is as follows.
Even if it exceeds 3.0%, the transfer of the roll roughness is sufficient, so that good sliding characteristics can be obtained at the time of press forming. However, if the elongation is excessive, the yield strength of the steel sheet increases, the n value decreases, and press forming is performed. The elongation was limited to 3.0% or less, since the elongation properties (extended formability, deep drawability) deteriorated. On the other hand, the lower limit of the elongation is naturally limited by controlling the surface roughness of the galvannealed steel sheet limited by the present invention.

Next, the three-dimensional center line average roughness SRa1 of the alloyed hot-dip galvanized steel sheet before temper rolling and the three-dimensional center line average roughness SRa2 after temper rolling are SRa2−SRa1 ≦ 0.
The reason why the surface roughness is controlled so as to satisfy the relationship of (μm) is that if SRa2 is excessively large, powdering during press molding is remarkable and the slidability is deteriorated.
a2-SRa1 ≦ 0 (μm)

The three-dimensional ten-point average roughness S of the steel sheet before temper rolling
Rz1 and three-dimensional ten-point average roughness SRz2 after temper rolling are 3
The reason for controlling the surface roughness so that 0 (μm) ≧ SRz2−SRz1 ≧ 2 (μm) is that if SRz2 is excessively large, the sealing of the lubricating oil in the recesses of the steel plate during press forming becomes insufficient and the die If it is too small, micropools are not formed at the time of press molding and powdering is apt to occur remarkably, so that 30 (μm) ≧ SRz2-SRz
It was limited to 1 ≧ 2 (μm).

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to embodiments.

Example 1 Galvannealed steel sheet (sheet thickness 0.7 mm, basis weight 60 g / m) manufactured in the on-site process
² (both sides, base material: ultra-low carbon steel sheet) were subjected to temper rolling under the conditions shown in Table 1, and then surface roughness was measured and a cup forming test by deep drawing was performed to evaluate press formability. In the cup forming test, after applying blanks of various diameters to lubricating oil, deep drawing is performed with a punch having a diameter of 33 mm, and the limit drawing ratio (LDR) = R / r was calculated and press formability was evaluated. Further, the surface of the formed steel sheet was observed with a scanning electron microscope to observe the degree of powdering. Table 2 summarizes the results. For comparison, samples outside the limits of the present invention were added and evaluated similarly. According to the method of the present invention, the critical draw ratio exceeds 2.2 by controlling the surface roughness of the alloyed hot-dip galvanized steel sheet by temper rolling, and it can be seen that excellent press formability can be obtained.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

The present invention is configured as described above, and according to the temper rolling method of the present invention, the formability of a galvannealed steel sheet can be effectively improved,
The use of alloyed hot-dip galvanized steel sheet with excellent corrosion resistance can be dramatically expanded.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takaaki Hira 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corp. Technical Research Division (56) References JP-A-2-274858 (JP, A) Kaihei 2-59101 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B21B 1/22

Claims (1)

(57) [Claims] In a temper rolling in a manufacturing process of an alloyed hot-dip galvanized steel sheet, an elongation percentage of the steel sheet by rolling is set to 3.0% or less, and a three-dimensional center line average roughness of the steel sheet before the temper rolling is performed. SRa1 and three-dimensional ten-point average roughness SRz1
And an alloy having excellent formability, characterized in that the surface roughness is controlled so that the three-dimensional center line average roughness SRa2 and the three-dimensional ten-point average roughness SRz2 after temper rolling have the following relationship: Manufacturing method of galvannealed steel sheet. SRa2-SRa1 ≦ 0 (μm) 30 (μm) ≧ SRz2-SRz1 ≧ 2 (μm)