JPH0354003B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a cold rolled steel sheet with excellent coating clarity and press workability.

(Conventional technology) User needs for automobiles are becoming more and more sophisticated year by year. In particular, regarding the appearance of automobiles, the brightness of the gloss, so-called paint clarity, has rapidly come into the spotlight in recent years as an important factor that determines the product image.

Furthermore, the good clarity of painted surfaces further increases the product value of external panels of home appliances, building exterior materials, and the like. Furthermore, in order to withstand complex press forming, the press formability required of steel sheets is becoming increasingly sophisticated.

BACKGROUND ART The semi-macroscopic geometrical shape of the surface of a rolled product such as a steel plate, that is, the surface roughness, has been known as a factor that strongly influences paint clarity and press workability.

Methods for imparting roughness to the roll surface include the shot blasting method shown in JP-A-51-101774, etc., the electrical discharge machining method shown in JP-A-49-17331, USP3619881, and the like, and JP-A-58-1989. 25557
Various methods are known, such as the method using laser light irradiation as shown in Japanese Patent Publication No. 62-11922.
Both methods have the following problems.

(Problems to be Solved by the Invention) The current roughness imparting method commonly used in production sites is a method using shot blasting.
In this shot blasting method, the roll is first ground and polished to have a smooth surface, and then hard particles such as iron called grit are sprayed onto the roll surface to make it rough. This shot blasting method is simple and has the advantage of hardening the surface and increasing the wear resistance of the roll, but on the other hand, the surface becomes irregular and unevenly rough, making it difficult to control the roughness. There is a drawback.

The method of irradiating laser light has excellent controllability,
It is possible to provide the roll surface with an extremely regular uneven pattern (hereinafter referred to as a matte surface). However, compared to the shot blasting method, the processing equipment is expensive, the streaks are noticeable due to the high regularity and the appearance after painting is spoiled, and the uneven pattern on the roll surface is caused by the relatively short rolling time. The disadvantage is that it wears out.

In the electrical discharge machining method, an electrode is placed opposite the roll, and while the roll is rotated and the electrode is moved in the direction of the roll axis, a spark discharge is generated between the roll and the electrode, and this discharge energy creates a group of fine craters on the roll surface. It is something that generates.

In this method, it is possible to give the roll surface any desired roughness mainly by changing the electric discharge machining current peak value, electric discharge pulse width, electric discharge pause width, etc. The uneven pattern on the roll surface formed by this method is slightly less regular than that produced by the laser method, but much more regular than that produced by the shot blasting method, and the roll processing method is also relatively simple. Moreover, it has the advantage that the uneven pattern has high wear resistance. The sharpness of steel sheets rolled using rolls processed by the electrical discharge machining method has been evaluated on average to be intermediate between those obtained by the shot blasting method and the laser method. Some of them have been found to have better values than others, but they have the drawback of not necessarily having a small amount of variation.

(Means for Solving the Problems) The present invention provides stable and high image clarity and press workability by combining the conditions for imparting a satin finished surface to the roll by electrical discharge machining and the manufacturing conditions. This method was developed as a result of various studies on methods for obtaining steel sheets with a roughness of 2 to 4 μm. After annealing, skin pass rolling is performed using a roll that has been polished to a surface roughness of Ra = 1 to 3 μm by electrical discharge machining. Excellent method for producing cold-rolled steel sheets, (2) After skin pass rolling and satin finishing, 5~
A method for producing a cold-rolled steel sheet with excellent paint clarity and press workability according to claim 1, which is carried out using a roll plated with a hard metal of 20 μm.

Figure 1A schematically shows a steel plate S rolled with a satin-finished roll using the method of the present invention, with a coating film P formed on the surface. The surface of the membrane is smooth.

FIG. 1B shows a comparative example, in which large irregularities (hereinafter referred to as waviness) are present in addition to small irregularities on the surface of the steel plate, and these remain as waviness on the surface of the coating film. Therefore, it is explained that the light incident on the painted surface is reflected from more directions than the painted surface, reducing the sharpness of the image.

Figure 2A schematically shows the lubricating oil L applied to the surface of the steel plate S of the present invention, and the lubricating oil L is uniformly held between the regular and relatively deep unevenness of the steel plate surface. There is.

On the other hand, FIG. 2B shows the state of the lubricating oil L applied to the surface of the steel plate S as a comparative example, and the oil film is broken in some places, such as at the top of the undulations.
Therefore, friction increases significantly at the portion where the oil film is broken, leading to galling of the mold and an increase in processing force.

In this way, the size of the waviness of the steel plate affects the sharpness of the image,
Affects press workability. Furthermore, if the surface roughness represented by small irregularities in the figure is too small, it becomes difficult for an oil film to adhere to the surface of the steel plate, and this also causes galling of the mold.

Hereinafter, waviness is abbreviated as Wca and surface roughness as Ra.
The values measured using the methods prescribed by JIS are used for each, but in order to satisfy the coating clarity and press workability, it is necessary to reduce the Wca value and increase the Ra value. The first one that meets these conditions
The conditions are as follows: The final stand work roll surface in cold rolling has a centerline average roughness obtained by electrical discharge machining.
It is necessary to perform satin finish so that Ra (roll) is 2 to 4 μm. When the roll Ra is less than 2 μm,
The unevenness transferred to the steel plate, that is, the Ra value of the steel plate, becomes smaller, which increases the tendency for shape defects during rolling and seizure after annealing, resulting in a significant decrease in yield.

If the roll Ra exceeds 4 μm, no matter how you change the rolling conditions of the skin pass rolling performed after annealing, it will not be possible to reduce the waviness Wca and increase Ra of the final steel sheet.

Therefore, the roughness Ra of the roll surface of the final stand of cold rolling is limited to 2 to 4 μm by electric discharge machining, but even if Ra is adjusted to this range by shot blasting, it will not be possible to adjust the roughness Ra to 2 to 4 μm later in the example. As shown in etc.
It is not possible to keep Wca to a small value. In addition,
The roll Ra in the first stage of cold rolling is the final roll of the steel plate.
Since there are few places that affect Wca and Ra, there is no need to limit it in particular, and there is no need to limit the reduction distribution etc. as long as normal cold rolling conditions are adopted.

Steel plates are annealed after cold rolling, but there are no particular conditions for this either, and normal box annealing or continuous annealing can be performed.

The second condition is that the surface of the roll used in skin pass rolling is polished by electric discharge machining so that the roll Ra is in the range of 1 to 3 μm. Role Ra
When it becomes less than 1 μm, the Wca of the steel plate becomes small, but
As Ra becomes smaller, the ability to retain lubricating oil during press working of the steel sheet decreases, so press workability tends to deteriorate.

Moreover, when Ra exceeds 3 μm, Wca increases rapidly.

Further, even if the roughness of the skin pass roll is adjusted to the same range Ra = 1 to 3 μm using the shot blasting method, the Wca of the steel sheet becomes larger than that obtained using the electric discharge machining method. When skin pass rolling is performed multiple times using multiple stands, Ra of at least the final rolling rolls may be adjusted within a predetermined range, and the rolls for the first rolling may be brightly finished.

In addition, skin pass rolling is performed at a normal rolling reduction rate.
Lubrication can be done by dry or wet methods, but Wca is smaller and more stable if the dry method is used, although it is slightly better. The satin finishing state of the final skin pass rolling roll is particularly closely related to the Wca and Ra of the steel sheet, and it is important to minimize the wear of the roll that occurs over the course of rolling.

An effective means for this purpose is to plate the roll electrical discharge machined surface with a hard metal such as Cr.
The plating thickness is suitably in the range of 5 to 20 μm; if it is less than 5 μm, the abrasion resistance effect will be small, and if it exceeds 20 μm, the plating adhesion will be poor and it will be easy to peel off.

It is desirable to strictly control the electrical discharge machining conditions in the cold rolling roll and the skin pass rolling roll in order to stabilize the Wca and Ra values of the steel sheet within a suitable range.

In addition to precisely controlling the peak value and pulse width of the applied pulse current and the voltage, it is especially important to keep the distance between the rolls and the discharge electrodes as constant as possible. The gap between poles can be precisely controlled by comparing the average machining voltage or average machining current with the reference surface during electrical discharge machining, outputting the difference to the servo valve solenoid and moving the hydraulic cylinder. .

(Function) The surface of the steel plate rolled by the method of the present invention, in which the final stand roll of cold rolling and the skin pass rolling roll are subjected to precisely controlled electric discharge machining to a predetermined roughness, is as shown in Fig. 3. As shown in A, a dense and uniform satin pattern was transferred. On the other hand, it can be seen that the surface of the steel sheet (FIG. 3B) is inferior in density when at least one of the cold rolling rolls or the skin pass rolling rolls is subjected to a satin finish using the shot blasting method.

Figure 4 shows the invention material and comparative material (products made by shot blasting or electrical discharge machining, but outside the range of roll Ra of the present invention).
The distribution of Wca and Ra is shown.

The material of the present invention has a small and stable Wca, a high and stable Ra, and is within the range of excellent image clarity and press workability.

On the other hand, the Wca of shot blasted material is
Although the roll Ra is within a large range and is an electrical discharge machined material, the roll Ra is outside the range of the present invention.
It can be seen that one of the values of Ra deviates in an undesirable direction.

(Example) Table 1 shows the main manufacturing conditions and the characteristic values of the product sheets in comparison with the inventive material and the comparative material. The steel sheets were cold-rolled to 0.7 mm under normal rolling conditions, box-shaped annealed at 680°C, and then subjected to 1.0% skin pass rolling, and the press workability in an actual machine was investigated.

Paint sharpness was determined based on the NSIC value determined by a mapping sharpness meter for steel plates coated under the conditions shown in Table 2.

In this method, a rectangular light beam is incident on the surface of a steel plate, the reflected light is captured at a predetermined position, and the distortion of the rectangular shape of the light beam is used as an index.
100 (maximum image sharpness), and the lower the NSIC value, the lower the image sharpness.

Note that, as the characteristic values of the steel sheet, the average value of several points in total of the rolled material immediately after the matte finishing on the roll and the values for every five coils was used. It can be seen that the Wca of the invention materials of the present invention when both cold rolled and skin pass rolls were subjected to a predetermined satin finish by electric discharge machining was 0.6 μm or less, indicating that waviness was small.

In addition, the evaluation value NSIC value for image sharpness is high, indicating good image sharpness. Furthermore, the Ra value is stable at 0.8 μm or more, and the relatively deep recesses have a high ability to retain lubricating oil, so press workability in actual machines is also good.

For comparison materials 8, 9, and 10, the matte finishing of the rolls was all done by electric discharge, but in 8, because the skin pass roll Ra was low, the Ra of the steel sheet was also low.
Poor workability. Comparative material 9 is skin pass roll.
Ra is high, and the image sharpness evaluated by the steel plate's Wca and NSIC value is poor. Comparative material 10 is a cold-rolled material.
Seizure occurred after annealing due to low Ra. Comparative materials 11 and 12 are cold-rolled rolls that have been subjected to a satin finish by shot blasting, and both have large waviness Wca, low NSIC values, and poor image clarity. Comparison material 1
Nos. 3 and 14 are skin-pass rolls subjected to shot blast processing, and in this case, regardless of the processing method of the cold-rolled rolls, both have high Wca.

■■■ Tortoise shell [0008] ■■■ ■■■ Tortoise shell [0009] ■■■ (Effect of the invention) The present application uses a relatively simple device and a highly controllable electric discharge machining method to produce cold As a result of rolling using rolls and skin pass rolls that have been subjected to highly accurate satin finishing to a predetermined roughness, we are able to consistently produce steel sheets with excellent paint clarity and press workability. This is an invention with great industrial benefits.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the sharpness of the coating film surface, Fig. 2 is a schematic diagram showing the state of lubricating oil on the surface of the steel plate, Fig. 3 is the property of the surface of the steel plate, and Fig. 4 is the steel plate of the present invention. It is a graph showing the measurement results of surface waviness and surface roughness for comparative materials.

Claims (1)

[Claims] 1. The final rolling of cold rolling is performed using a roll that has been polished to a surface roughness Ra of 2 to 4 μm by electrical discharge machining, and after annealing, the surface roughness is improved by skin pass rolling by electrical discharge machining. A method for producing a cold-rolled steel sheet with excellent paint clarity and press workability, characterized in that the process is carried out using a roll that has been satin finished to a degree Ra of 1 to 3 μm. 2 After skin pass rolling and satin finishing, 5~
A method for producing a cold-rolled steel sheet with excellent coating clarity and press workability according to claim 1, which is carried out using a roll plated with a hard metal of 20 μm.