JPH0929304A - Highly formable cold rolled steel sheet excellent in resistance to galling - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly formable cold rolled steel sheet excellent in the resistance to galling irrespective of the magnitude of a surface pressure generated on the sheet at the time of press forming. SOLUTION: In this highly formable cold rolled steel sheet having surface- roughness structure in which recessed parts having depth of >=1μm from the flat part are independently distributed in the flat part of the average roughness Ra of the surface of the steel sheet which is 0.2<Ra<=0.4μm on both sides or one side, the depth of the recessed part is 10-30μm, the area of each recessed part is 0.0001-0.01mm<2> and the area rate of the recessed part is 5-30%.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention provides a high-strength steel sheet, a mild steel sheet, and a surface-treated steel sheet for the purpose of improving die galling resistance by regulating the geometric shape of the steel sheet surface. These are also applied to aluminum plates, aluminum alloy plates and the like.

[0002]

2. Description of the Related Art Conventionally, the improvement of mold gallability has been improved by increasing the hardness of a steel sheet, whereas in the present invention, the geometric shape of the surface of the steel sheet is an important factor for improving the mold gallability. Focusing on the influence, we examined the solution of the problem. It is known that the geometrical shape of the surface of the steel sheet affects the frictional resistance during press forming, and by controlling the geometrical surface shape so far, the frictional resistance can be reduced to improve the press formability. As a technique that has been improved, there is a technique disclosed in Japanese Patent Publication No. 3-54006. Further, in Japanese Patent Laid-Open No. 2-280902, the area of the flat portion and the closest spacing of the recesses are specified, but these are intended to reduce frictional resistance during molding and improve moldability. However, it does not disclose mold galling resistance at the time of molding, which has been a problem due to the trend of lowering the viscosity of lubricating oil in the press field in recent years. Further, a surface structure suitable for friction at a high surface pressure such as accompanied by mold galling is not disclosed. As a measure for avoiding the galling phenomenon of the steel sheet, it is conceivable to increase the steel sheet strength to increase the surface hardness and improve the die galling resistance, but the increase of the steel sheet strength causes deterioration of press formability. Therefore, there has been an increasing demand for a high formability steel sheet which is excellent in die galling resistance without increasing the strength of the steel sheet, that is, suppressing deterioration of press formability as much as possible.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a cold-rolled steel sheet in which the galling resistance obtained by controlling the geometrical shape of the steel sheet surface is good regardless of the surface pressure generated in the sheet during press forming. Is to provide.

[0004]

Means for Solving the Problems That is, according to the present invention, in a flat part having an average roughness Ra of the steel sheet surface of more than 0.2 μm and 0.4 μm or less, the depth from the flat part is 10 μm or more and 30 μm or less.
The following recesses are independently provided on both sides or one side of the steel plate surface, and the area of each recess is 0.0001 mm ² or more and 0.0
Provided is a highly formable cold-rolled steel sheet excellent in die galling resistance, which is 1 mm ² or less and a recess area ratio of 5% or more and 30% or less. Further, it is preferable that the above-described geometrical shape of the steel sheet surface of the present invention has no directivity with respect to the forming direction of the steel sheet or the shape of press forming.

The present invention will be described in more detail below. As for the geometrical shape of the surface of the cold-rolled steel sheet, the average roughness is generally controlled by the grain size number of the shot used in conventional shot blasting. However, the present inventors have found that more precise control of the surface geometrical shape is necessary in order to improve the die galling resistance of the steel sheet surface during press forming and avoid press forming defects.
That is, the geometric shape of the steel plate surface having excellent mold galling resistance is
A recess having a depth of 10 μm or more and 30 μm or less is provided in a flat part having Ra of more than 0.2 μm and 0.4 μm or less on both sides or one side of the steel sheet independently of other recesses, and the area of each recess is 0. 0001mm ² or more 0.01 mm ² or less, the recess area ratio is 30% or less than 5% was found. Further, it is preferable that the positions of the recesses are arranged substantially regularly (that is, uniform without directionality).

The die galling during press forming originates from the adhesion of metal on the surface of the steel sheet to the die, and the main factor that accelerates this adhesion phenomenon is the lack of oil that occurs on the surface of the steel sheet during forming. To be Therefore, avoiding oil shortage during molding can be considered as a means for improving mold galling resistance. The cause of this oil shortage may be that the recesses on the surface of the steel sheet are crushed and flattened during forming, or that oil travels through the groove portions on the surface of the steel sheet and escapes to the outside of the contact system. Therefore, even if the surface of the steel sheet is crushed to some extent during forming, a sufficient depth to leave recesses is provided, the flat area ratio is increased so that the surface of the steel sheet is less likely to be deformed, and oil is added to the steel sheet. It is considered that the mold galling resistance is improved by making the recesses independent so that they do not escape through the surface groove portion to the outside of the contact system. As a result of repeated studies based on this consideration, the inventors of the present invention can provide a thin steel sheet excellent in die galling resistance by regulating the geometric shape of the surface of the steel sheet as shown below. Clarified.

First, a friction test was conducted on materials having different recess depths. The test device pulls out the material 1 while pressing both surfaces of the material 1 with a tool 2 (material: SKD11) at a constant surface pressure P as shown in FIG. 2, and the friction coefficient μ is μ = D / from the pulling load D at that time. It was calculated by (2 · P). Ra is more than 0.2 μm to 0.4 μm, and the area of each independent recess is 0.
An experiment was conducted using a steel plate having a recess area ratio of 01 mm ² or less and a concave area ratio of 5 to 30% as a material 1, and a lubricating oil (viscosity: 1
5 cST / 40 ° C.) was applied to both sides of the material for the experiment.
FIG. 1 shows the results. The surface pressure has two levels: low surface pressure (P = 1 kgf / mm ² ) and high surface pressure (P = 20 kgf / mm ² ). From the observation of the material surface after drawing, it was found that when μ exceeds 0.2, linear flaws are likely to occur, leading to the occurrence of mold galling. In the case of a low surface pressure as shown in FIG. 1, the smaller the recess depth is, the smaller μ is. However, when it is 30 μm or more, μ is increased. This is because if the recess is deep, the surface pressure is small, so that hydrostatic pressure is not applied to the oil in the recess, and it is difficult to supply oil to the boundary. On the other hand, in the case of high surface pressure, conversely, the smaller the depth of the recess is, the larger μ is, but when it becomes 10 μm or more, μ decreases. This is because due to the high surface pressure, if the recess is too shallow, the recess will be flattened and the trapped oil will be lost during molding. Further, even if the concave portion is deep, due to the high surface pressure, the convex portion is crushed so that the hydrostatic pressure is sufficiently applied to the oil in the concave portion, and the oil is supplied to the boundary portion to reduce μ.

From FIG. 1, it was found that the condition that μ is small from low surface pressure to high surface pressure is that the depth of the recess is in the range of 10 μm to 30 μm. Based on these findings, it was found that if the geometric shape of the surface of the steel sheet is controlled as follows, the die galling resistance of the steel sheet is dramatically improved. The geometrical shape of the steel sheet surface is obtained from a known two-dimensional roughness profile or obtained by image processing analysis of the steel sheet surface using three-dimensional roughness data.

Flat part: Ra of the flat part is more than 0.2 μm and 0.
4 μm or less. If the thickness is less than 0.2 μm, slipping may occur during passing through, which may cause trouble. Also, 0.4
This is because when the thickness exceeds μm, the flat portion has a contact portion and a non-contact portion at the time of molding, and the recesses cannot be kept independent.

Depth of recess: 10 μm or more and 30 μm or less, preferably 15 to 25 μm. 10 μm or more means that when the thickness is less than 10 μm, the surface is crushed before the completion of molding to make the recess shallow, and the deformation of the steel plate causes the surface to be roughened, and the recess disappears due to the interaction of the shallow recess. This is because it runs out of oil. 30 μm or less is 30 μm
If it is larger than m, the hydrostatic pressure is not applied to the oil even if the recess becomes slightly shallower during molding, and the lubricating effect cannot be obtained.

Recess area: 0.0001 for each recess area
mm ² or more 0.01 mm ² or less, preferably 0.000
It is set to ¹ to 0.007 mm ² . The reason why it is set to 0.0001 mm ² or more is that it is considered that it is difficult to manufacture if it is less than 0.0001 mm ² . The reason why it is set to 0.01 mm ² or less is that if it is larger than this, it becomes difficult to obtain the effect of confining oil, and the contact surface pressure at the flat portion increases.

Recess area ratio: 5% to 30%, preferably 10 to 30%. The reason why the content is 5% or more is that if it is less than 5%, the holding power of oil before molding is poor. 30
The reason for setting it to be less than or equal to 5% is that if it is larger than this, the surface pressure in the flat portion during molding becomes large, and the amount of decrease in the depth of the recess becomes large, so that the oil outflow amount becomes excessively larger than the appropriate amount. .

[0013]

EXAMPLES The present invention will be specifically described below based on examples. Hot-rolled steel sheet having a thickness of 4.0 mm is cold-rolled to 0.8 mm, and after annealing, tempered rolling of 0.8 to 1.2% is formed on the surface by laser processing. Various skin pass rolls having different geometric shapes. The cold rolled steel sheet having the same geometrical shape on both sides was manufactured. Table 1 shows the mechanical properties of the obtained steel sheet.

[0014]

The steel plates having various surface geometric shapes shown in Table 2 were subjected to continuous frustoconical forming with a punch diameter of 100 mm, a blank diameter of 230 mm, and a forming height of 50 mm shown in FIG. The molded products of No. 1 were visually evaluated, and those having no external appearance problems were evaluated as ◯, and defective ones as × to evaluate the mold galling resistance. Lubricating oil is normal rust preventive oil (viscosity 16.
3 cst-40 ° C.) was applied on both sides at 1 g / m ² . The results are shown in Table 2. The steel sheet of the present invention exhibits better mold galling resistance than the comparative example. It should be noted that in the case where die galling occurs on only one surface due to a pressed part, it has been demonstrated that the cold rolled steel sheet having the geometrical shape of the present invention on only one surface also has good galling resistance.

[0016]

[0017]

According to the present invention, by controlling the geometrical shape of the surface of the cold rolled steel sheet, even in the cold rolled steel sheet of the same material, the die galling resistance is remarkably improved, so that the range of use thereof is expanded.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a recess depth and a friction coefficient.

FIG. 2 is a schematic diagram illustrating a test device for a friction test.

FIG. 3 is a perspective view showing a shape of a truncated cone for evaluating mold galling resistance (unit: mm).

[Explanation of symbols]

 1 Material 2 Tool P Surface pressure D Load

Claims (1)

[Claims] 1. The average roughness Ra of the steel sheet surface is more than 0.2 μm and not more than 0.4 μm.
The steel sheet surface is provided with recesses of μm or more and 30 μm or less independently, and the area of each recess is 0.0001.
mm ² or more 0.01 mm ² or less, the recess area ratio is 5
% Or more and 30% or less, a high formability cold rolled steel sheet excellent in die galling resistance.