JPH0354006B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to cold-rolled steel sheets that are subjected to forming processes such as press working, and in particular, the present invention relates to cold-rolled steel sheets that are subjected to forming processes such as press working. This relates to cold rolled steel sheets with excellent properties.

<Prior art and its problems> In general, cold-rolled steel sheets for forming processing are
It is manufactured by degreasing, cleaning, annealing, and temper rolling.

Here, one of the purposes of temper rolling is to give the steel plate an appropriate surface roughness by performing light rolling using work rolls with a dull surface finish, and to prevent premature seizing during press forming. Improvements can be made.

To dull-finish the surface of a work roll used in such skin pass rolling, methods using shot blasting and electrical discharge machining have been put into practical use.

In the case of dull finishing of these work rolls for temper rolling, an irregular and non-directional roughness profile is formed on the roll surface, so the surface of the steel plate after temper rolling with such work rolls is It exhibits a so-called rough surface consisting of irregular, non-directional peaks and valleys.

If a steel plate with such a rough surface is pressed, lubricating oil will remain in the valleys, reducing the friction between the steel plate and the press die, making the press work easier, and at the same time making the press work easier. Seizing can be prevented by trapping the metal powder that has come off due to the frictional force in the valley.

However, on the other hand, it is also known that the smaller the roughness, the better in terms of image clarity after painting.
It is considered difficult to improve both press formability and image clarity at the same time.

In recent years, the quality of the finish after press processing and painting of automotive steel sheets has become an extremely important quality control item, as it allows customers to directly visually demonstrate the high overall quality of the vehicle. It is becoming.

Additionally, in order to simplify the press forming process, the shapes of press formed products are becoming more complex.
A steel plate that can withstand such press forming is required.

Improvements in press formability have traditionally focused on the steel sheet material itself. That is, the main countermeasures are considered to be to improve the Rankford value (γ value) and elongation (El).

However, it has recently been revealed that press forming is affected not only by the deformability of the steel plate itself but also by the magnitude of the frictional resistance between the steel plate and the mold as important factors. Frictional resistance varies slightly depending on surface roughness and properties, but conventionally only rough adjustment has been carried out by rolling using shot blast rolls.

That is, at present, the average roughness is managed simply by shot blasting numbers (for example, #50, #80, #120, etc.).

The frictional resistance between the steel plate and the mold is closely related to the occurrence of galling during press forming and the flow of the metal. Generally speaking, the smaller the frictional resistance, the better the press formability, and the larger the roughness. It is said that frictional resistance is reduced and mold galling resistance is improved especially when subjected to severe press processing, that is, when oil runs out. In the prior art, roughness is imparted to the surface of a steel plate by applying a shot dull to a roll and performing skin bath rolling using this roll, but this method does not allow fine control of roughness.

<Objective of the Invention> The present invention solves these problems in the conventional technology, and by highly controlling the surface roughness of the steel sheet, it achieves the same steel sheet performance (γ value, El) while maintaining the same steel sheet performance (γ value, El).
The object of the present invention is to provide a cold-rolled steel sheet with excellent press formability, which also achieves both high image clarity, which was previously thought to be difficult, and which has excellent formability.

<Structure of the Invention> That is, according to the present invention, in the surface roughness pattern of a cold rolled steel sheet, the regularity parameter S representing the regularity of the steel sheet surface roughness expressed by the following formula is 0.25 or less in two orthogonal directions. , and the values ₁ and ₂ in each direction satisfy the following formula,
Furthermore, the three-dimensional average roughness SRa is 0.4 to 1.2 μm,
Average roughness Ra for the direction where at least is small
The product of the mean valley diameter Lmv at the surface roughness center plane is 50
A cold-rolled steel sheet with excellent formability characterized by a thickness in the range of ~250 μm is provided.

=1/n _{o 〓} ⁱ⁼¹ Xk _i S=1/n _o 〓 ⁱ⁼¹ _| X _/ --Xk _i /X _{2 |} is the distance between k peaks of convex portions on the surface of the steel sheet, k is the minimum integer such that the value of S is 0.25 or less, and ₁ and ₂ are the values obtained in two orthogonal directions.

The present invention will be explained in more detail below.

As a result of repeated research, the present inventors have discovered that a cold-rolled steel sheet with various properties regulated as described below has excellent press formability.

First, the most important thing is the steel sheet surface roughness pattern.

The regularity parameter S representing the regularity of the steel sheet surface roughness in the present invention can be expressed as follows, where the distance between the peaks k of the convex portions on the steel sheet surface is defined as Xk _i . In addition, in the formula, k is the value of S.
The smallest positive integer less than or equal to 0.25.

=1/n _o 〓 ⁱ⁼¹ Xk _i S=1/n _o 〓 ⁱ⁼¹ |X/--Lk _i |/X Regularity parameter S representing the regularity of surface roughness
must be 0.25 or less in two orthogonal directions.

In other words, the difference in the distance between the k convex peaks should be 25% or less with respect to the average distance between the k convex peaks, and the distribution of the convex roughness where the steel plate and mold come into contact should be uniformly controlled. By controlling the contact surface pressure to a constant level, it is possible to suppress fluctuations in the frictional resistance between the steel plate and the mold, and to improve mold galling resistance.

In addition, in order to manage the directionality of roughness, if the values of the average peak _{-to-peak distance in each direction in the two orthogonal directions are 1 and 2 ,}
It is essential that the following formula is satisfied.

2 |X/- ₁ -X/- ₂ |/X _{1 +} Directional roughness can be achieved by setting the difference in values to 25% or more.

As a result, the frictional resistance between the steel plate and the mold can take different values depending on the relationship between the direction and the directionality of roughness.

The reason for changing the roughness depending on the direction is to increase the roughness in the direction where die galling is more likely to occur in combination with the press forming conditions, thereby preventing the occurrence of die galling, or to create an anisotropic property of the steel sheet. This is because, if there is any friction, it is possible to perform press molding that minimizes the influence of the frictional resistance with the mold by controlling the frictional resistance with the mold. Furthermore, from the viewpoint of the sharpness of the steel sheet, the smaller the roughness of the steel sheet, the better, and roughness characteristics that are contradictory to improving press formability (die galling resistance) are required, but the minimum required for each direction is By giving the steel plate a roughness with a limited roughness, it is possible to avoid increasing the average roughness of the steel plate, and as a result, it is possible to obtain superior mold galling resistance in a specific direction than when giving a non-directional roughness. In addition, the sharpness of the steel plate can also be improved.

It is possible to change the frictional resistance depending on the direction in this way by changing the average peak-to-peak distance by 25% or more in two orthogonal directions. This is because a difference of less than 25% does not result in a large difference in the frictional resistance in both directions.

Furthermore, the three-dimensional average roughness SRa of the steel plate is ultimately required to be in the range of 0.4 to 1.2 μm.

When SRa is less than 0.4 μm, deterioration of press formability is unavoidable, and when it exceeds 1.2 μm, it becomes difficult to ensure image clarity.

Here, SRa is a parameter for accurately evaluating the average roughness of regular laser dulls such as the steel plate surface of the present invention, and its definition is a parameter that extends the normal average roughness Ra to three dimensions. It is. That is, from the roughness curved surface (f(x, y)), a portion with area SM is extracted on its center plane, and orthogonal coordinate axes (X axis, Y
axis) and the axis perpendicular to the center plane is expressed as the Z axis.
The value given by the following equation is expressed in μm.

Note that the central plane here refers to a virtual plane in which the volumes of convex portions and concave portions are equal in terms of surface roughness.

SRa=1/SW∫ ₀ ^L ∫ ₀ ^L | F (x, y) | dx, dy (However, L x L = SM) Furthermore, at least in the direction where press formability is required, that is, in the direction where the above is small Average roughness Ra and average valley diameter Lmv at the surface roughness center plane
It is necessary that the product of roughness is in the range of 50 to 250 μm, and both roughness parameters are correlated with mold galling resistance.

When Ra×Lmv is less than 50, sliding resistance is large and mold galling is likely to occur. Moreover, even if it exceeds 250, the effect will not be very large and the sharpness of the image may be impaired.

In order to obtain such a directional and regular steel sheet surface roughness pattern, the surface roughness pattern of the work roll must also necessarily be directional and regular. As a method for processing the work roll for this purpose, a processing method using a high-density laser energy source is suitable.

Additionally, the steel sheet of the present invention is basically a steel sheet with almost no roughness (bright steel sheet) that is rolled using the work rolls to impart a directional and regular surface roughness pattern. However, especially when workability is required in all directions and the processing conditions in a particular direction are severe, the work roll is used to roughen the steel plate uniformly in all directions. It is also possible to provide a regular roughness.

<Example> The present invention will be specifically described below based on an example.

Example 1 Various cold-rolled steel sheet samples for press forming shown in Table 1 were temper-rolled using work rolls that had been subjected to laser dulling and shot dulling. Note that since shot blasting provides random surface roughness, it is not possible to obtain each of the two orthogonal directions and S.

Using these samples, a test was conducted according to the CRM type mold galling test method shown in FIG. In this test method, the first sheet is molded into the shape shown in the figure with oil applied, and the second and subsequent sheets are molded continuously without oil. It was determined at what number of sheets mold galling occurred at the arrow () in Figure 2, and mold galling resistance was evaluated based on the number of sheets until mold galling occurred.
That is, the results of the mold galling property test investigated in the rolling direction (direction A) and the direction perpendicular to A (direction B) indicate that regular roughness with directionality is provided within the scope of the present invention, and , the average distance between the convex peaks in the B direction is relative to the average value in the two directions.
It can be seen that when the difference is 25% or more (a to d and j in Table 1), there is a difference in mold galling resistance in both directions, and the effect of changing the roughness depending on the direction can be obtained. Table 1 also shows the results of basic press forming using a 200 ton press as a press formability test. In the case of a basic mold that requires material to flow from direction B to direction A, pattern a, which emphasizes mold galling resistance in direction A, is equivalent to pattern e, which places equal emphasis on both directions A and B. It was a good result. Further, the sharpness of the image was determined by a visual judgment method (ranking method), and the sharpness of the steel plate a was combined with the effect of the regular pattern, and better results than those of the shot were obtained. However, when the roughness is large in both directions A and B, such as e-steel, the sharpness deteriorates, and it is important to apply roughness only in the required directions at the minimum necessary level. It shows.

■■■ Turtle Shell [0018] ■■■ <Effects of the Invention> The cold-rolled steel sheet of the present invention has the same steel sheet performance but excellent press forming by imparting regularity and anisotropy to the steel sheet surface. It is possible to ensure image quality and sharpness.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a CRM type mold galling test method. FIG. 2 is a perspective view of the shape of the test piece after the mold galling test. Explanation of symbols: 1...Wrinkle presser (bottom), 2...Wrinkle presser (top), 3...Test piece, 4...Punch.

Claims (1)

[Claims] 1. In the surface roughness pattern of a cold-rolled steel sheet, the regularity parameter S representing the regularity of the steel sheet surface roughness expressed by the following formula is expressed in two orthogonal directions.
0.25 or less, and the values ₁ and ₂ in each of the above directions satisfy the following formula, and the three-dimensional average roughness SRa is 0.4 to 1.2 μm, and the average roughness Ra and surface roughness in the direction where at least A cooling steel sheet with excellent formability, characterized in that the product of the average valley diameter Lmv at the center plane is in the range of 50 to 250 μm. =1/n _{o 〓} ⁱ⁼¹ Xk _i S=1/n _o 〓 ⁱ⁼¹ ｜X/ _-- XK _i ｜/X _{2 |} Here, Xk _i is the distance between k peaks of the steel sheet surface, k is the minimum positive integer such that the value of S is 0.25 or less, and ₁ and ₂ are the values obtained in two orthogonal directions.