JPH0739952A - High tensile steel sheet having good press formability - Google Patents

Abstract

PURPOSE:To provide the high tensile steel sheet having good press formability. CONSTITUTION:This high tensile steel sheet has plural microrecessed parts on the steel sheet surfaces and has the good press formability. The average area of these recessed parts is 4X10<-3> to 30X10<-3> (mm<2>). The average value L (mm) of the spacings between the recessed parts and YS (kgf/mm<2>) in tensile characteristics satisfy the relation of equation I. Center line average height Ra(mum) and YS (kgf/mm<2>) satisfy a relation of equation II. ¦L-1.8/(YS-10)-0.015¦<=. -0.012XYS/3+0.065...I. 12/YS+0.3<=Ra<=12/YS+1.0...II.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength thin steel sheet whose press formability is remarkably improved by controlling the surface roughness of the steel sheet.

[0002]

2. Description of the Related Art In recent years, there has been a demand for higher tensile strength of thin steel sheets for automobile bodies for the purpose of improving fuel economy and running safety of automobiles. This high-strength steel sheet for automobiles is required to have excellent press formability as its characteristics. In order to improve press formability, high ductility (El) and high Rankford value (r value) are required as mechanical properties of the steel sheet. However, in general, the mechanical properties are inevitably deteriorated as the strength of the steel plate increases, and the mechanical properties (El, r values) of the conventional mild steel plate level for automobiles are high despite the high strength. The development of steel sheets is a very difficult task. Therefore, achieving both high strength and formability of the steel sheet has been an unachievable problem in the prior art.

Conventionally, the improvement of press formability has been promoted as a problem of improvement of the material of the steel sheet itself, but the present invention changes the idea, and the press formability is the deformability of the steel sheet itself and the steel sheet and the metal. The solution of the problem was examined paying attention to the fact that the magnitude of the frictional resistance with the mold affects as an important factor. The frictional resistance slightly changes depending on the surface roughness and properties, and thus far, Japanese Patent Publication No. 3-54006 has been disclosed as a technique in which the press formability is improved by controlling the surface structure of mild steel sheets. .

However, the method and possibility of improving the press formability by controlling the surface roughness and the surface pattern of the high-strength steel sheet have not been clarified, and the surface roughness control of the high-strength steel sheet up to now has been the mild steel. Similar to the above, the rough adjustment was limited to the rolling using the shot blast roll.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high-strength thin steel sheet which can obtain good press formability by highly controlling the surface roughness structure of the steel sheet.

[0006]

That is, the present invention is a high-strength thin steel sheet having a plurality of minute recesses on the surface of a steel sheet, the recesses having an average area of 4 × 10 ⁻³ to 30 × 10 ^−3. (M
m ² ), the average value L (mm) of the space between the recesses and YS (kgf / mm ² ) in the tensile properties satisfy the relationship of the equation, and the center line average roughness Ra (μm) and YS (kgf /
mm ² ) satisfies the relation of the formula, and provides a high-strength thin steel sheet with good press formability.
Here, the concave portion means a portion which is deeper by 1.5 μm or more than the flat portion whose average roughness Ra is 0.3 μm or less. | L-1.8 / (YS-10) -0.015 | ≤-0.012xYS / 3 + 0.065 ... 12 / YS + 0.3≤Ra≤12 / YS + 1.0 ......

[0007]

The present invention will be described in more detail below. As for the surface roughness of the high-strength thin steel sheet, the average roughness is generally controlled by the grain size number of the shot used for shot blasting. However, it has been found that fine control of the surface roughness structure is required to achieve press-formability better than before by improving the slidability between the steel sheet surface and the mold during press-forming. .

In particular, in order to reduce frictional resistance,
It was newly found that the optimum range of the surface roughness of the steel sheet and the average recess interval changes depending on the strength level of the material.
As a result of the inventors' detailed examination of this relationship, YS
The relationship between the value and the surface roughness (centerline average roughness Ra), the YS value, and the optimum range in the average recess interval L was found.

As a result of repeated studies based on this basic data, the inventors of the present invention regulated the conditions as follows:
It has been clarified that it is possible to manufacture a high-strength thin steel sheet with good press formability.

First, the relationship between YS and surface roughness is set to 12 / YS + 0.3≤Ra≤12 / YS + 1.0. The amount of crushing of the sliding portion when the surface of the steel sheet slides on the die decreases as the steel sheet YS rises. Therefore, the surface roughness for obtaining the sufficient containment effect for supplying the lubricating oil accumulated in the recess to the sliding surface decreases as the steel plate YS rises. If Ra is less than 12 / YS + 0.3, the amount of lubricating oil retained in the recess cannot be secured sufficiently, and there is a risk of seizure on the sliding surface during pressing.
Is larger than 12 / YS + 1.0, the pressure for containing the lubricating oil in the concave portion is not sufficient, so that the supply of the lubricating oil to the sliding surface fails and the lubricating state deteriorates.

Further, the relation between the average concave portion interval L and YS is | L-1.8 / (YS-10) -0.015 | ≤-0.012 * YS / 3 + 0.065. Although the detailed reason is not clear, when the recess interval is larger than the range of the formula, the sliding state deteriorates due to insufficient supply of the lubricating oil to the sliding surface. When the interval between the recesses is smaller than the range of the formula, the contact area ratio with the tool decreases, the surface pressure of the sliding part increases, and the boundary friction area in the flat part expands, so the sliding state deteriorates. It seems to be.

Here, the recessed portion has an average roughness Ra of 0.3 μm.
The depth is 1.5 μm or more deeper than the flat portion of m or less.

FIG. 1 shows a method for measuring the average concave portion spacing. The surface of the steel sheet is photographed, and the concave portion and the flat portion are distinguished based on the photograph. This is binarized by image analysis, and the degree of analysis at this time is 5 × 10 ⁻⁶ mm ² or less per pixel. Then, the flat portion distance (recess interval) is scanned in one direction,
The average value is defined as the average recessed portion spacing.

The average recess area is 4 × 10 ⁻³ to 30 ×.
It should be 10 ^-3 (mm ² ). If the average recess area is larger than 30 × 10 ⁻³ mm ² , it becomes difficult to contain the lubricating oil accumulated in the recess at the time of molding, and if the average recess area is less than 4 × 10 ⁻³ mm ² , it is accumulated in each recess. Since the amount of lubricating oil becomes insufficient, the sliding condition deteriorates.

Next, a brief description will be made on a method for producing a high-strength steel sheet having good press formability according to the present invention. In order to appropriately roughen the high-strength thin steel sheet as in the present application, various countermeasures can be considered in the manufacturing process, but the present invention is not particularly limited to that method. For example,
A method of using a roll subjected to laser dull processing is also effective as one example. This method is a technique for forming a concavo-convex to be given to the surface of a steel sheet by subjecting a bright roll to a laser dull process. This dulling roll is pressed against the surface of the steel sheet at a reduction rate that provides a desired roughness pattern. Thereby, if the roughness pattern can be controlled within the range of the present invention, a high-strength steel sheet having good press formability can be obtained. However, the same effect can be obtained by a method other than the laser dull processing method as long as the roughness can be controlled within the range of the present invention.

[0016]

EXAMPLES The present invention will be specifically described below based on examples. (Example) Steel sheets having the mechanical properties and the average recess spacing shown in Table 1 were prepared by temper rolling various high-tensile steel sheets having a plate thickness of 0.7 mm using various skin pass rolls. In addition, ordinary rust preventive oil (viscosity 16.3 cst-40
(° C) Punch diameter 33 with various blank diameters under lubrication conditions
The LDR was measured by performing a cylindrical aperture of mmφ. The results are shown in Table 2. As can be seen from the results shown in the table, the steel sheet having the characteristics within the claims of the present invention (invention example) shows dramatically better press formability than the comparative example, despite the same material. Further, the influence of LDR on Ra is shown in FIG. 2, and the influence of LDR on the average recess spacing is shown in FIG.
Shown in. It can be seen from FIGS. 2 and 3 that those within the scope of the present invention have excellent press formability.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

According to the present invention, by controlling the surface roughness and the interval between recesses of a high-strength thin steel sheet according to the strength of the steel sheet, the slidability of the steel sheet of the same material is remarkably improved.
It is possible to manufacture a high-strength thin steel sheet having good press formability, such as widening its use range and making it possible to form with a high degree of difficulty. Further, the effects of the present invention can be applied to high-tensile thin plates including high-tensile surface-treated steel plates.

[Brief description of drawings]

FIG. 1 is a diagram showing a method of measuring an average recess interval L.

FIG. 2 is a diagram showing a relationship between a surface roughness Ra and a limit aperture ratio LDR.

FIG. 3 is a diagram showing a relationship between an average recessed portion interval L and a limit aperture ratio LDR.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takaaki Hira 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division (72) Inventor Toshiyuki Kato Chuo-ku, Chiba-shi, Chiba Kawasaki Town No. 1 Kawasaki Steel Corporation Technical Research Division

Claims (1)

[Claims] 1. A high-strength thin steel sheet having a plurality of minute recesses on the surface of the steel sheet, wherein the recesses have an average area of 4 × 10 ^{−3 to} 3.
0 × 10 ⁻³ (mm ² ) and the average value L (m
m) and YS (kgf / mm ² ) in the tensile properties satisfy the relation of the formula, and the center line average roughness Ra (μm) and Y
A high-strength thin steel sheet with good press formability, characterized in that S (kgf / mm ² ) satisfies the relation of formula. | L-1.8 / (YS-10) -0.015 | ≤-0.012xYS / 3 + 0.065 ... 12 / YS + 0.3≤Ra≤12 / YS + 1.0 ......