JPS60213309A - Pickling method of hot rolled steel strip - Google Patents

Abstract

PURPOSE:To reduce remarkably a pickling time by previously providing a stretch larger than a specific rate to a hot rolled steel strip to be descaled by pickling. CONSTITUTION:A steel strip 10 is provided with a tension by brake rolls 11, 12 and tension providing rolls 14, 15 placed respectively at the inlet and outlet sides of a bending and stretching device 27. The device 27 is arranged within this tension-providing zone, and is constituted of many bending rolls 13 arranged zigzag. With respect to the stretch of steel strip 10 at the device 27, a specific rate of bending stretch is provided by detecting the number of revolutions of a brake motor 22 and a tension motor 24 and changing the speed of motor 24 by a control device 26 based on the difference between said number of revolutions. Further, when the device 27 is not provided, the steel strip can be stretched by increasing the tension larger than its yielding point. In any case, the stretch to be provided to the strip is regulated to >=4%. Next, the strip 18 is introduced to a pickling bath 19, and the pickling time is effectively reduced because the cracks produced by said stretch are sufficiently wetted by the acid liquid.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a pickling method for descaling scale generated on the surface of a hot rolled strip material (Vl!4 band) in a short time.

[Background of the invention]

The scale on the surface of the hot rolled strip material is descaled by pickling.

The pickling speed of the strip is usually about 300 m/-. However, the pickling time for one scale is about 24 seconds, and a long pickling tank with a capacity of 120 mK is used.

As described above, pickling equipment requires a large amount of space, and if this can be shortened, great advantages can be obtained not only in terms of space, but also in terms of maintenance and operation.

A method for shortening the pickling time is disclosed in Japanese Patent Application Laid-Open No. 57-47519.
A method of rolling and bending/stretching has been announced as shown in .

In this method, it is said that before pickling or mechanical descaling, it is recommended to stretch the film by 3 to 10% by one rolling process, and then bend and stretch it by 0.5 to 3 inches.

However, after trying out Kenpo, it was discovered that it had the following drawbacks.

First, when the systrilag is stretched by 3 to 10 inches using a rolling mill, one scale is also rolled as the strip base material is stretched, making it difficult to effectively crack the scale, which is effective for pickling. Further, when a scaled strip is continuously rolled by 3 to 10%, the surface of the rolling roll is damaged in a short period of time, making it difficult to continue the work for a long time.

Second, when the strip is bent and stretched by 0.5 to 3% after rolling as above, cracks do occur in the scale, but
As a result, no significant pickling effect was observed.

[Purpose of the invention]

The purpose of the present invention is to investigate an effective pretreatment method before pickling and to provide a pickling method that dramatically shortens the pickling time.

[Summary of the invention]

It was found that when the strip was bent and stretched by 0.5 to 3%, especially about 3%, cracks were observed in the scale, but the pickling effect was not as improved as expected.

We will discuss the results of our investigation into this phenomenon.

S) When IJ tubes are wound around multiple rolls arranged in a staggered manner, tension is applied and the strip is passed through the woman, causing the strip to stretch.

This expansion rate can be determined as follows.

In other words, the elongation (εt) of the steel strip when bending is applied in a tension field.
can be determined by the following formula (see Figure 4). In other words, the bending radius of the steel strip...
...Tension acting on the cross section of the strip steel...σI (~/
m”) Yield stress・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・σ.

Thickness of steel strip・・・・・・・・・・・・・・・・・・・・・
. . . h, the initial bending center X1 moves toward the bending center side to the position indicated by X2 in FIG. 1 due to the action of the tension σf. This moving distance a is given by aX (σ, -(-σ, )) = 2aσ, = σf・h,
becomes. In this case, the bending operation with the rollers at the - location is performed twice in total, taking into account rebending, so the elongation of the strip with one roll is 2a hσf RRσ with respect to the center of the plate. Become.

Also, since the elongation K includes the elastic elongation, the actual plastic elongation ε,t is as follows.In the case of n rollers, here, N: Number of rollers H: Plate thickness E: Longitudinal elastic modulus To establish.

The thickness of the test strip is typically 2.31111. If the unit tension applied to the steel strip is 8 kg/m" and the diameter of the bending roller is 100 m, the elongation of the strip per roll is given by the above formula. This elongation increases in proportion to the number of rollers by about 1%.

If the strip is stretched in this way, the breaking limit of the scale is about 0.5%, so if the strip is stretched by at least 3 inches, cracks equivalent to 25% will occur in the scale.

The pitch of cracks that occur on this scale is as small as 10μ, as large as 30μ, and on average is about 15μ, and it has been found that this pitch is not related to the elongation rate.

Therefore, as the elongation rate increases, the width of the crack increases, and this was also verified by observation results using a microscope.

That is, as shown in FIG. 1, when the strip 1 is stretched, cracks 4 occur in the scale 2 attached to the strip. The pitch p of this crack is 15μ on average, and as the elongation increases, the crack width δ increases.

When the strip width δ is the elongation rate ε, &b) of the strip, it can be determined as follows: δ=p(εpt O,s)/100. If εt”3% and p=15μ,
δ=0.375μ.

In this way, if cracks occur in the scale, the circumference S of the scale in contact with the acid solution as shown in Figure 1
If the thickness of the scale is f, then 5 = 2f + p In other words, the contact with the acid liquid when there are no cracks is So = p, so the contact length ratio r with the acid liquid when there are cracks is h γ = (2f + p n /p=1+-.

Since the scale thickness is normally f=10μ, if p=15, then γ=233 times.

Therefore, if cracks occur, the pickling time should be able to be reduced to 100% or less, but with bending and stretching of about 3%, the time reduction efficiency is only about 28% compared to the case where there are no cracks.

FIG. 3 shows the relationship between the strip elongation rate εf and the pickling time rate β. Note that the pickling time rate β=1.0 is the standard time rate when there are no cracks.

It can be seen that β increases with the stretching rate β. in this way,
Due to the stretching of the strip, cracks occur, and
Since the pitch of the cracks is constant, even though the number of cracks is the same, when the elongation (εf) is small, it is small, and when gt becomes large, β increases, and when the elongation rate tt becomes 4%, pickling It was confirmed that the time rate β becomes saturated, and in this case, the pickling time can be shortened by 60% compared to the case where there are many cracks. It has been confirmed that the above phenomenon occurs due to the following reasons.

That is, as shown in FIG. 2, when the crack width δ of the school 2 is small, a film 5 is formed in the crack 3 due to surface tension.
Since the acid solution cannot sufficiently penetrate into the cracks, the effect of shortening the pickling time is small.

This is because an air layer remains within the crack width δ in FIG. 2 and is not replaced by pickling.

However, when the elongation εf of the strip becomes about 4%, the pickling will sufficiently infiltrate the crack width δ shown in Figure 2, and the area where the acid solution contacts the scale 2 will become larger, so the pickling time will be reduced. The shortening effect will be greater.

However, even if the strip is stretched further than this, the effect of shortening the pickling time will not increase significantly.

This can also be understood from the fact that the acid liquid contact length does not increase.

The above effects have been shown for the case where the strip is bent and stretched before pickling, but according to the theoretical background of the present invention, the effect can also be obtained when the strip is simply stretched by tension without bending.
It has been demonstrated that if the elongation rate of the strip is the same, the pickling shortening effect is almost the same.

[Embodiments of the invention]

An embodiment of the invention is shown in FIG.

The strip 10 has rollers for tensioning devices, namely entry brake rollers 11, 12 and strip bending and stretching device 2.
Tension is applied by tension applying rollers 14 and 15 on the exit side of 7.

A strip bending and stretching device 27 is arranged within this tensioning section and is constituted by a number of strip bending rollers 13 arranged in a staggered manner.

The stretching of the strip 10 in this bending/stretching device 27 is provided by the driving power of the brake motor 22 and the tension motor 24. The rotation speed of the motor 22 and the motor 24 is measured by the tachometer 23.
．． 25, and based on the difference, a control device 26 speeds up or slows down the remoter 24 to give a predetermined bending elongation rate to the systrip.

Further, if the bending/stretching device 27 is not provided, the strip can be stretched by increasing the tension to a value higher than the yield point of the strip.

In any case, as mentioned above, the elongation for the strip is 4% or more.

The stretched strip 28 is guided by deflector rollers 16, 17 and led to the first pickling tank 19.

Conventionally, four pickling tanks of about 40 m were installed, but with the pickling tank of the present invention, the pickling time can be shortened to 40% of the conventional pickling tank, so two types of 24 m long pickling tanks can be arranged.

However, the strip passing speed in this case is 300 m/m.

By pickling 18 in the first pickling tank 19, the second pickling tank 2
At 9, each strip 28 is pickled in a pickling process 20.

The concentration of the acid solution 18 in the first tank is about 15%, and the concentration of the second acid solution is about 20%.

After being pickled in this way, the strip 29 passes through an adhesion pickling removal device disposed after the guide roller 21, and the descaling operation is completed.

In the above pickling method, the thickness of the copper strip is 1.6 to 4.5 cm, and the width of the copper strip is 7 cm.
00 to 1500 W is normally processed at a speed of 300 m/m.

When using the bending/stretching machine 27 shown in FIG. 5, normally about five bending rollers having a roller diameter of 50 to 100a+ are used, and the applied tension is 5 to 10 Kg/m''.

In FIG. 5, only the IJ tube bending and stretching device 27 is disposed, but of course a scale removing device such as a brush may be disposed immediately after this device.

However, even if a brush device for mechanically removing scale is arranged, the shape of the strip is not necessarily flat. Therefore, it is difficult to bring the brush into full contact with the strip surface, resulting in local areas where the brush does not make contact. Therefore, it is necessary to design a pickling device as shown in FIG. 5 in order to remove scale from such a portion, and the effects of the present invention can be exerted.

〔Effect of the invention〕

According to the present invention: (1) By stretching the strip by 4% or more before pickling, the pickling time could be shortened by 60% compared to the case without stretching.

(2) There is no need for rolling work before stretching the strip, improving maintainability.

0) Since the pickling time was shortened, the length of the pickling tank became less than half of the conventional length, improving acid maintenance and operability.

(4) If the length of the pickling tank is made different from the conventional one, the processing speed can be increased to more than twice that of the conventional one.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the occurrence of cracks on scale A when the strip is stretched, Fig. 2 is a diagram showing the pickling infiltration situation for scale cracks, and Fig. 3 is a diagram showing the elongation rate and pickling time rate of the strip. , FIG. 4 is a diagram of the bending and stretching situation of an eight-person trip, and FIG. 5 is a system diagram of the present invention and an embodiment. 26...Control device, 27...Bending/stretching device. Agent Patent Attorney Akio Takahashi 81st rod 2 items 5 63rd time God L14r-t ￥7412]

Claims (1)

[Claims] 1. A method for pickling a hot rolled steel strip, characterized in that before performing pickling descaling on the hot rolled steel strip, pickling is performed after giving the steel strip a plastic elongation rate of 4% or more. .