JPH0852501A - Manufacture of cold rolled steel sheet excellent in press formability - Google Patents

Abstract

PURPOSE:To provide a method for manufacturing a high tensile steel sheet, mild steel sheet and surface treated steel sheet by which press formability is improved by controlling the rugged structure on the surface of steel sheet. CONSTITUTION:Rolling at the final stand of cold rolling is executed with bright rolls whose average height Ra is <=0.3mum or ten-point mean roughness Rz is <=2.5mum and further skin pass rolling after annealing is executed with rolls whose Ra is 0.5-l.5mum, Rz is 5.0-20.0mum, the average interval between projecting parts is 10-80mum and in the range of 0.5-1.5% elongation percentage in skin pass rolling. Or, by executing rolling at the final stand of the cold rolling with the rolls whose average height Ra is 0.5-l.5mum, ten-point mean roughness Rz is 5.0-20.0mum and average interval between the projecting parts is 1080mum and further executing skin pass rolling after annealing with the bright rolls whose Ra is <=0.3mum or Rz is <=2.5mum and in the range of 0.5-1.5% elongation percentage in skin pass rolling, the cold rolled steel sheet is manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a method for producing a high-strength steel sheet, a mild steel sheet, and a surface-treated steel sheet for the purpose of improving press formability by regulating the uneven surface structure of the steel sheet. These are also applied to aluminum plates, aluminum alloy plates and the like.

[0002]

2. Description of the Related Art In the past, improvement of press formability has been promoted as a problem of improvement of steel sheet material itself, whereas the present invention changes the idea. The improvement of press formability was examined by focusing on the fact that the size of the friction resistance between the steel plate and the die influences as an important factor together with the deformability of. It is known that this frictional resistance depends not only on the properties of the lubricant but also on the surface irregularity structure. A technology that has improved press formability by controlling the geometrical surface irregularity structure so far. As the above, the inventions disclosed in Japanese Patent Application Laid-Open No. 2-280902 are disclosed.

However, the invention for controlling these surface irregularities to improve the press formability is managed only by controlling the roll surface irregularities used for temper rolling. Of course, the surface uneven structure after the temper rolling mainly depends on the surface uneven structure of the temper rolling roll, but also depends on the surface uneven structure after the cold rolling to some extent. Therefore, in order to highly control the steel sheet surface unevenness structure after temper rolling, it is necessary to manage the surface unevenness structure of both the cold rolling roll used for cold rolling and the temper rolling roll used for temper rolling. It is believed that there is.

[0004]

DISCLOSURE OF THE INVENTION The present invention controls the steel plate surface unevenness structure by controlling the surface unevenness structure of both the cold rolling roll and the temper rolling roll. An object of the present invention is to provide a method for producing a cold-rolled steel sheet having a highly controlled structure and good press formability.

[0005]

That is, according to the present invention, the final stand of cold rolling is performed by a bright roll having an average roughness Ra of 0.3 μm or less or a ten-point average roughness Rz of 2.5 μm or less. Ra is 0.5 when temper-rolled after annealing.
~ 1.5 μm, Rz is 5.0 to 20.0 μm, and the average interval of the convex portions is 10 to 80 μm. The temper rolling elongation is 0.5 to 1.5%, or cold rolling. The final stand of rolling is performed with a roll having an average roughness Ra of 0.5 to 1.5 μm, an average roughness Rz of 5.0 to 20.0 μm, and an average interval of convex portions of 10 to 80 μm, and further annealing. The subsequent temper rolling was performed with a bright roll having Ra of 0.3 μm or less or Rz of 2.5 μm or less at a temper rolling elongation of 0.5 to 1.5%, which was excellent in press formability. A method of manufacturing a cold rolled steel sheet is provided.

Hereinafter, the present invention will be described in more detail. As for the surface uneven structure of the cold-rolled steel sheet obtained by cold rolling, the average roughness is generally controlled by the grain size number of the shot conventionally used for shot blasting. However, it was 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 plate surface and the mold during press-forming. . Further, it was found that the uneven surface structure of the steel sheet having excellent sliding characteristics was such that recesses independent of other recesses were densely distributed in a flat part having an average roughness of 0.3 μm or less. This is because when the steel plate surface concave portion is sealed in the mold during press molding, the lubricating oil therein flows out and the frictional resistance decreases. However, if the recesses are connected or the flat portion has a large roughness, it becomes difficult to seal the recesses with a mold. That is, sealing the recesses with a mold during press molding leads to a reduction in frictional resistance, but for this purpose, it is desirable that each recess be independent. Moreover, the effect is increased when the independent recesses are densely distributed.

As a result of repeated studies based on this finding, the inventors of the present invention regulated the manufacturing conditions as shown below.
It has been clarified that it is possible to manufacture a thin steel sheet with good press formability.

Manufacturing Method (1) First, the final stand of cold rolling has an average roughness Ra of 0.
3, preferably 0.2 μm or less or ten-point average roughness R
Bright rolls having z of 2.5, preferably 1.5 μm or less are used. Thereby, Ra of the steel sheet surface after cold rolling is set to 0.3 μm or less. This is because the average roughness of the flat portion on the surface of the steel sheet after temper rolling is 0.3 μm or less, and the concave portions are independently distributed in the flat portion. At this time, if the average roughness of the flat portion exceeds 0.3 μm, it becomes difficult to seal the concave portion with the mold.

Further, the temper rolling after annealing is carried out when Ra is 0.5 to
1.5, preferably 0.8 to 1.0 μm and Rz 5.0 to 20.0, preferably 10 to 15 μm and the average interval of the convex portions is 10 to 80, preferably 30 to 70 μm.
Rolling is performed at a temper rolling elongation of 0.5 to 1.5%. If Ra of the temper rolling roll is less than 0.5 μm or Rz is less than 5.0 μm, the surface roughness of the steel sheet also becomes small, and the amount of lubricating oil for press working stored in the recesses on the surface of the steel sheet becomes insufficient, causing baking. Pressing problems such as scraping and mold scoring occur. Further, when Ra of the temper rolling roll exceeds 1.5 μm or Rz exceeds 20.0 μm, the surface roughness of the steel sheet becomes excessive and it is difficult for the lubricating oil for press working to flow to the flat portion during press forming. become. Further, the temper rolling elongation percentage is 0.5% or more in order to prevent stretcher strain during press forming. However, the temper rolling elongation percentage is set to 1.5% or less in order to suppress deterioration of the mechanical properties of the steel sheet.

Further, the average interval between the convex portions of the temper rolling roll is set to 10 to 80 μm. When it exceeds 80 μm, the frictional resistance increases due to excessive dispersion of the lubricating oil supply source. If the thickness is less than 10 μm, independent recesses may be connected to each other at the initial stage of deformation during press molding. The method for processing the temper rolling roll is not particularly limited, but it is preferable to perform dull processing by shot blasting, which enables formation of dense protrusions.

Manufacturing method (2) First, the final stand of cold rolling has an average roughness Ra of 0.5.
To 1.5, preferably 0.8 to 1.0 μm, and a ten-point average roughness Rz of 5.0 to 20.0, preferably 10 to 15.
μm and the average interval of the convex portions is 10 to 80, preferably 3
It is performed with a roll of 0 to 70 μm. As a result, the average distance between the recesses on the surface of the steel sheet after cold rolling is set to 10 to 80 μm.
This is because the intervals between the recesses on the surface of the steel sheet after temper rolling are densely distributed. If Ra of the cold rolling roll is less than 0.5 μm or Rz is less than 5.0 μm, the steel plate surface roughness after temper rolling with the bright roll becomes small, so that the lubricating oil for press working in the steel plate surface recess is reduced. Insufficient storage volume causes press problems such as baking and galling.
Further, when Ra of the cold rolling roll exceeds 1.5 μm or Rz exceeds 20.0 μm, the surface roughness of the steel sheet after temper rolling by the bright roll becomes excessive, and the lubricating oil for press working at the time of press forming. Is difficult to flow to the flat part.

The temper rolling after annealing is performed with a bright roll having an average roughness Ra of 0.3, preferably 0.2 μm or less, or a ten-point average roughness Rz of 2.5, preferably 1.5 μm or less. The temper rolling elongation is 0.5 to 1.5%. The content of 0.5% or more is for flattening the convex portion and for preventing yield elongation. The reason why the content is 1.5% or less is to suppress deterioration of mechanical properties of the steel sheet.
At this time, the method for processing the cold rolling roll is not particularly limited, but it is preferable to perform the dull processing by shot blasting capable of forming dense protrusions.

Here, Ra means the center line average roughness representing the surface roughness of industrial products. The centerline average roughness is obtained by extracting a portion of the measurement length L from the roughness curve in the direction of the centerline, the centerline of the extracted portion is the X axis, and the vertical magnification direction is the Y axis. When expressed by f (x), the value obtained by the following formula is expressed in micrometers (μm).

[Equation 1]

The roughness curve is a contour represented by a cut when the surface to be measured is cut along a plane perpendicular to the surface to be measured. The center line is a straight line in which when a straight line parallel to the average line of the roughness curve is drawn, the areas surrounded by this straight line and the roughness curve are equal on both sides of this straight line. The average line is a straight line that has the geometric shape of the surface to be measured in the extracted portion of the roughness curve, and is set so that the sum of squares of the deviation from that line to the section curve or roughness curve is minimized. Say.

Rz means ten-point average roughness. This is because the portion of the roughness curve a shown in FIG. 1 extracted by the reference length L is parallel to the average line and measured from a straight line that does not cross the sectional curve in the direction of longitudinal magnification to the 5th highest peak. The value of the difference between the average value of the altitude and the average value of the altitudes of the valley bottoms from the deepest to the fifth is expressed in micrometers (μm).

[0016] That _{is, R 1, R 3, R} 5, R 7, R 9 shown in Figure 1: elevation summit from best sampled portion corresponding to the reference length L to the fifth R _2, R _4, R ₆ , R ₈ , R ₁₀ : As the elevation of the valley bottom from the deepest to the fifth of the extracted portion corresponding to the reference length L

[Equation 2] Is.

The interval between the convex portions is obtained by observing and recording the surface of the object to be measured with an optical microscope and performing image processing analysis to obtain the convex portions (or concave portions).
And the flat portion are binarized, and the interval between the convex portions is measured in units of micrometers (μm). The average interval of the convex portions is the arithmetic mean value of the measured values.

[0018]

EXAMPLES The present invention will be specifically described below based on examples, but the present invention is not limited thereto.
Cold rolling a hot rolled steel sheet with a thickness of 4.0 mm to 0.7 mm,
After annealing, temper rolling was performed to produce cold rolled steel sheets having various surface irregularity structures. Moreover, in order to evaluate the press formability, a normal rust preventive oil (viscosity 16.3 cst-40 ° C.) was added to 1.
Under the lubrication condition of 5 g / m ² applied, the limiting drawing ratio (LD
R) was investigated. This LDR is the ratio of the maximum blank diameter and punch diameter that can be deep-drawn using a die with a punch diameter of 33 mm. Tables 1 and 2 show the surface roughness structure and the LDR after cold rolling and temper rolling, and temper rolling. The example in Table 1 corresponds to the production method (1) of the present invention, and the example in Table 2 corresponds to the production method (2) of the present invention. The steel sheet produced by the method of the present invention exhibits better press formability than the comparative example.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

According to the method of the present invention, by controlling the surface uneven structure of the rolling rolls used in cold rolling and temper rolling, the slidability is improved even in the steel sheet of the same material,
A steel sheet having significantly improved press formability can be obtained. For this reason, it becomes possible to manufacture a cold-rolled steel sheet having a good press formability, such as widening the range of use thereof and enabling difficult forming.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating a method of obtaining a ten-point average roughness Rz.

[Explanation of symbols]

a Roughness curve L Reference length R ₁ , R ₃ , R ₅ , R ₇ , R ₉ Elevation of peaks from the highest to the 5th peak of the extracted portion that can correspond to the reference length L R ₂ , R ₄ , R ₆ , R ₈ , R ₁₀ Elevation of the valley bottom from the deepest to the 5th of the extracted part that can correspond to the reference length L

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takaaki Hira 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Steel Research & Development Division Steel Research Laboratory (72) Inventor Toshiyuki Kato Chiba 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Japan Kawasaki Steel Co., Ltd.

Claims (2)

[Claims] 1. The cold rolling final stand has an average roughness Ra of 0.3 μm or less or a ten-point average roughness Rz of 2.5 μm.
Performed by the following bright rolls, and further temper-rolled after annealing with Ra of 0.5 to 1.5 μm and Rz of 5.0 to 2.
A method for producing a cold-rolled steel sheet, which comprises performing rolling in a temper rolling elongation ratio of 0.5 to 1.5% with a roll having a diameter of 0.0 μm and an average interval of convex portions of 10 to 80 μm. 2. The final stand for cold rolling has an average roughness Ra of 0.5 to 1.5 μm and a ten-point average roughness Rz of 5.0 to.
20.0 μm and the average interval of the convex portions is 10 to 80 μm, and temper rolling after annealing is performed with Ra of 0.
A method for producing a cold-rolled steel sheet, which is performed with a bright roll of 3 μm or less or Rz of 2.5 μm or less in a temper rolling elongation ratio of 0.5 to 1.5%.