JPH0732056A - High tensile strength thin steel sheet having excellent press-formability - Google Patents

Abstract

PURPOSE:To enable the production of a high tensile strength thin steel sheet having excellent press-formability by forming a specific roughness on the steel sheet surface and sticking a specific quantity of oil having a specific viscosity on the surface or forming a specific thickness of solid lubricating film of resin or wax. CONSTITUTION:The oil having >=50cst (40 deg.C) viscosity at >=1g/m<2> or the solid lubricating film of resin or wax at >=1mum thickness is applied on the surface of the steel sheet having Ra <=0.4mum center line average roughness or Rz <=4mum ten point average roughness at least in the one side surface of the steel sheet surface. When the roughness of the steel sheet surface does not satisfy the condition, the oil film is broken and the lubricity is deteriorated. When the lubricating film does not satisfy the condition, the oil film is broken and the lubricity is deteriorated. Therefore, the press-formability of the steel sheet is drastically improve the controlling the condition of the lubrication and the surface roughness of the high tensile strength thin steel sheet.

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. The present invention is particularly advantageously applied to high strength steels having a tensile strength of 35 to 150 kgf / mm ² strength level.

[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 efficiency and running safety of automobiles. This automotive steel sheet 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. But,
In general, it is unavoidable that the mechanical properties of steel sheets deteriorate as the strength of steel sheets increases, and steel sheets with high mechanical strength (EL, r value) at the level of conventional mild steel sheets for automobiles are inevitable. Development 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 improving the material of the steel sheet itself, but the present invention changes the idea, and the press forming is performed with the deformability of the steel sheet itself and the steel sheet and the metal. The solution of the problem was examined focusing on the fact that the magnitude of the frictional resistance with the mold has an important influence. The frictional resistance slightly changes depending on the surface roughness and properties and the type of lubricating oil, but up to now, as a technique in which the press formability has been improved by controlling the surface structure of mild steel sheet, Japanese Patent Publication No. 3-54006. Japanese patent publication is disclosed.

However, the method and possibility of improving the press formability by controlling the surface roughness of the high-strength steel sheet have not been clarified, and the conventional surface roughness control of the high-strength steel sheet is the same as that of mild steel. The rough adjustment was limited to rolling by using a shot blast roll.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a high-strength thin steel sheet which has excellent press formability by highly controlling the surface roughness of the steel sheet and using a lubricating oil having a relatively high viscosity. The purpose is to do.

[0006]

That is, according to the present invention, the center line average roughness Ra of at least one surface of the steel sheet is Ra ≦
A viscosity of 50 cst is applied to a steel plate surface having a 0.4-μm or JIS B0601 ten-point average roughness Rz of Rz ≦ 4 μm.
(Centistokes, 40 ° C.) or more oil is 1 g / m ² or more, or resin-based or wax-based solid lubricating film is 0.
A high-strength thin steel sheet excellent in press formability, which is characterized by having a thickness of 1 μm or more. In this case, in the surface roughness pattern of the steel plate, a regularity parameter S representing the regularity of the steel plate surface roughness represented by the following formula.
Preferably satisfies S ≦ 0.25 in at least one direction.

[0007]

[Equation 3]

[0008]

[Equation 4]

Here, Xi is the distance between the peaks of the recesses on the surface of the steel sheet.

[0010]

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 a fine surface roughness structure must be limited in order to achieve the press-formability higher than the conventional one.

In particular, for the purpose of improving press formability, it was newly found that the optimum range of the surface roughness of the steel sheet is lowered by increasing the viscosity of the lubricating oil. Furthermore, we were able to clarify the effect of regular arrangement of surface roughness. Therefore, as a result of repeated studies based on this finding, the present inventors have found that it is possible to manufacture a high-strength thin steel sheet excellent in press formability by regulating the manufacturing conditions as follows.

First, the center line average roughness Ra of the steel sheet surface is R
a ≦ 0.4 μm, or the ten-point average roughness Rz is Rz ≦
4 μm. At that time, it is assumed that the lubricating oil having a viscosity of 50 cst or more has a thickness of 1 g / m ² or more, or the resin-based or wax-based solid lubricating film has a thickness of 0.1 μm or more. When Ra is larger than 0.4 μm or Rz is larger than 4 μm, the surface pressure of the contact sliding portion becomes high, the oil film is broken, and the lubricity deteriorates. Further, the steel plate surface roughness Ra is 0.4 μm.
m or less, or Rz is 4 μm or less, when the viscosity of the lubricating oil is less than 50 cst or the amount of lubricating oil applied is less than 1 g / m ² , or when the solid lubricating film has a thickness of less than 0.1 μm Then, the oil film is destroyed and the lubricity deteriorates.

More preferably, the regularity parameter S indicating the regularity of the surface roughness of the steel sheet is S ≦ 0.25 in at least one direction. This effectively improves the stability of the sliding state between the steel plate and the die. In order to obtain such a regular steel plate surface roughness pattern, the surface roughness pattern of the skin pass roll must also be regular. A laser dull processing method is suitable as a skin pass roll processing method therefor, but in the present invention, the same effect can be obtained if the steel plate surface roughness Ra is 0.4 μm or less or Rz is 4 μm or less. Is not limited.

[0014]

EXAMPLES The present invention will be specifically described below based on examples. Example 1 The surface roughness of a steel sheet was controlled by temper rolling various high-tensile steel sheets with a plate thickness of 0.7 mm produced by a CAL (continuous annealing line) process using various skin pass rolls. Further, under a lubricating condition with a high viscosity oil (400 cst / 40 ° C.) or a solid lubricant, a cylindrical drawing test with a punch diameter of 33 mmφ was carried out with various blank diameters, and the limiting drawing ratio (LDR) was measured. At this time, the amount of oil applied is 1.5 g /
m ² or the film thickness of the solid lubricant was 0.3 μm. The results are shown in Table 1. As can be seen from the results shown in Table 1, the steel sheets of the examples show significantly better press formability than the comparative examples, despite the same material. The influence of LDR on the surface roughness Ra and Rz of the same steel is shown in FIGS. 1 and 2. All steel sheets show excellent LDR when Ra is 0.4 μm or less or Rz is 4 μm or less.

Example 2 Various high-strength steel sheets having a plate thickness of 0.7 mm produced by the CAL process were temper-rolled to control the surface roughness of the steel sheet. Further, the LDR was measured in the same manner as in Example 1 under various lubricating conditions. However, the amount of oil applied is 1 g /
m ² or the film thickness of the solid lubricant was 0.2 μm. The results are shown in Table 2. As can be seen from the results shown in Table 2, the steel sheets of the examples show dramatically better press formability than the comparative examples, despite the same material. Further, FIG. 3 shows the effect of LDR on the lubricating oil viscosity in the same steel. LDR for any steel with a viscosity of 50 cst (40 ° C) or higher
Is high.

Example 3 A high-tensile steel plate having a plate thickness of 0.7 mm was transferred at the same reduction rate using skin pass rolls having various laser dull and shot dull patterns. Table 3 shows the results of LDR measurement using a lubricant having a viscosity of 80 cst in the same manner as in Example 1. Even if the surface roughness is at the same level, the LDR is high and the press molding is improved when S ≦ 0.25, which is a regular roughness pattern.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

[0020]

According to the present invention, by controlling the viscosity of the lubricating oil and the surface roughness of the high-strength thin steel sheet, the press formability of the steel sheet of the same material is remarkably improved and the range of use thereof is expanded. At the same time, it becomes possible to mold with difficulty.
It is possible to manufacture a high-strength thin steel sheet having excellent press formability.

The effects of the present invention can also be applied to high-strength thin plates including Al plates.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between Ra and LDR.

FIG. 2 is a graph showing the relationship between Rz and LDR.

FIG. 3 is a graph showing the relationship between viscosity and LDR.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takaaki Hira 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Co., Ltd. Technical Research Headquarters (72) Makoto Imanaka 1 Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Co., Ltd. Technical Research Headquarters (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Co., Ltd. Technical Research Headquarters

Claims (2)

[Claims] 1. The center line average roughness Ra of at least one surface of the steel sheet is Ra ≦ 0.4 μm or the ten-point average roughness Rz is R.
On the surface of a steel plate with z ≦ 4 μm, a viscosity of 50 cst (40
A high tensile strength thin steel sheet having excellent press formability, characterized in that oil of 1 ° C. or higher) is 1 g / m ² or more, or a solid lubricating film of resin or wax has a thickness of 0.1 μm or more. 2. A surface roughness pattern of a steel plate, wherein a regularity parameter S representing the regularity of the steel plate surface roughness represented by the following formula satisfies S ≦ 0.25 in at least one direction. A high-strength thin steel sheet excellent in press formability according to 1. [Equation 1] [Equation 2] Here, Xi: distance between peaks of steel plate surface recesses