JPH0760332A - Descaling method for hot rolled steel strip - Google Patents

Abstract

(57) [Summary] [Constitution] Work rolls (2) and (3) with dull that have hot rolled steel strip (1) with scale attached to the surface and chrome plated on the surface (surface roughness: 1.0 μmRa or more Rolling axis of each roll axis has an inclination angle θ (0.1 to 2.0 degrees) with respect to the width direction of the steel strip (1), and the rolling ratio is reduced by the rolling mills arranged so as to intersect each other. After pressing down at 0.5% or more and less than 10%, pickling treatment is performed. [Effect] Since the scale braking effect can be enhanced, the pickling time can be greatly shortened, which is extremely effective in improving the line speed during pickling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing scale formed on the surface of a hot rolled steel strip.

[0002]

2. Description of the Related Art Since scale is attached to the surface of a hot rolled steel strip that has been hot rolled, it is necessary to remove this scale before performing cold rolling. The descaling method is roughly classified into a pickling method and a mechanical descaling method. The pickling method requires a long pickling tank and large-scale waste acid treatment equipment, and these equipments occupy a large space,
Equipment costs are high. Further, since a large amount of acid is used, it is not economical and the working environment is bad. On the other hand, the mechanical descaling method is a method of mechanically removing scales by shot blasting, grinding, etc., but has a drawback that uniform descaling cannot be performed unlike the pickling method.

Therefore, in the descaling process, the pickling method is the mainstream although the above problems occur, and the mechanical descaling method is used complementarily to shorten the time required for pickling. is the current situation.

Regarding mechanical descaling, for example, in JP-A-55-30363, a hot-rolled steel sheet is prepared by using a dull work roll having a maximum surface roughness Hmax of 5 to 35 μm.
A method is disclosed in which a scale is applied at a reduction rate of 0.5 to 2%, scale braking is performed, and then pickling is performed. However, the scale breaking effect is small, and the roll wear is fast.

Japanese Unexamined Patent Publication (Kokai) No. 57-118816 discloses a descaling device for removing scale by passing a steel sheet between a pair of rolls arranged such that their roll axes intersect at an intersection angle θ. Is proposed. This device is to perform positive rubbing between the steel plate and the roll to cause relative slip in the width direction of the steel plate to shear and separate the scale, but the descaling effect is not so large. Further, nothing is mentioned about the surface condition of the roll.

[0006]

SUMMARY OF THE INVENTION The present invention further improves the conventional mechanical descaling method to enhance the scale breaking effect, shorten the pickling time, improve the line speed during pickling, etc. The object of the present invention is to provide a method for descaling a hot-rolled steel strip capable of achieving the above.

[0007]

The gist of the present invention resides in the following descaling method for hot-rolled steel strip.

The center line average roughness (Ra) of the surface is 1.0 μm.
(Hereinafter referred to as 1.0 μmRa) and less than 10 μmRa,
It is equipped with a pair of dull work rolls with chrome plating on its surface, and the work rolls cross each other at an inclination angle θ of 0.1 to 2.0 degrees with respect to the width direction of the steel strip of the roll axis. Depending on the rolling mill installed, the rolling reduction of the hot-rolled steel strip with scale attached on the surface is 0.5% or more 10
A method for descaling a hot-rolled steel strip, which comprises performing a pickling treatment after applying a rolling reduction of less than%.

As described above with reference to FIG. 1, the above-mentioned "arranged so as to cross each other" means that the pair of work rolls are in opposite directions with respect to the width direction of the steel strip, that is, the work. If one of the rolls has an inclination angle of θ with respect to the above-mentioned reference, the other of the work rolls is arranged so as to have an inclination angle of −θ with respect to the reference. means.

[0010]

The method of the present invention will be described in detail below.

According to the method of the present invention, a hot-rolled steel strip having a scale attached to the surface thereof is provided with a work roll with a dull whose surface is plated with chrome before the pickling treatment, and the roll axes of the work rolls intersect each other. It is characterized in that rolling mills arranged in this way perform reduction to improve the scale braking effect.

FIG. 1 is a view showing the arrangement of work rolls (FIGS. 1 and 2 are drawn with an inclination angle larger than the actual one for convenience of explanation). The upper work roll 2 and the lower work roll 3 are shown. (Hereinafter, both are also referred to as work rolls 2 and 3), and the roll axes are arranged in a state of intersecting with the width direction of the steel strip 1 at an inclination angle of θ. Since the work rolls 2 and 3 have inclination angles of θ in the opposite directions with respect to the width direction of the steel strip 1, the angle formed by the upper work roll 2 and the lower work roll 3 is 2θ.
When the work rolls 2 and 3 have different inclination angles instead of θ, the meandering of the steel strip during rolling increases.
As described above, it is preferable to incline symmetrically with respect to the width direction of the steel strip 1.

FIG. 2 is a diagram schematically showing the shearing force acting on the steel strip when the steel strip is rolled by the work rolls arranged in the state shown in FIG. In this figure, R ₁ −R ₁ ′
And R ₂ -R ₂ ′ are paired work rolls 2, 3 respectively.
, V _R1 and V _R2 are the roll peripheral speeds of the work rolls 2 and 3, respectively, and the speed of the steel strip 1 is V,
The shear forces τ _R1 and τ _R2 act in a direction perpendicular to the traveling direction of the steel strip, that is, in the width direction of the steel strip. Therefore, the steel strip 1 is formed by the work rolls 2 and 3 arranged in the state shown in FIG.
When rolling, the shearing forces τ _R1 and τ _R2 act on the surface of the steel strip, and at the same time, a compressive force (not shown) acts in the longitudinal direction of the steel strip as in the case of rolling with a normal work roll. The scale breaking effect is greater than in the case of normal rolling.

In order to make full use of the above-mentioned shear forces τ _R1 and τ _R2 for scale braking, it is better to use a dull roll having a rough surface as a work roll than a bright roll having a smooth surface. Is even more effective.

The roll axis of a pair of work rolls (hereinafter,
The inclination angle θ of the work roll axis, or simply roll axis) is set to 0.1 to 2.0 degrees with reference to the width direction of the steel strip, because the scale braking effect is too large when the inclination angle θ is smaller than 0.1 degrees. This is because the effect of reducing the time required for pickling is small because it is not present, while the effect is saturated and does not change even if the tilt angle θ is set to be larger than 2.0 degrees.

FIG. 3 is a diagram showing the influence of the inclination angle θ of the roll shaft on the pickling time. A roll having a diameter of 220 mm (material: JIS
SKD61) was used to roll down a hot rolled steel strip (low carbon steel) having a thickness of 2.0 mm and a width of 300 mm at a rolling reduction of 3%, and then 5% hydrochloric acid (liquid temperature: 80
This is a comparison of the time required for descaling (pickling time) when pickling at (° C.). The surface roughness of the work roll immediately before the start of rolling was 5 μmRa and was plated with chromium. The vertical axis represents the ratio of the time required for pickling (pickling time ratio) based on the case where the inclination angle θ of the roll axis is 0.

As shown in this figure, when the roll shaft inclination angle θ becomes 0.1 degree, the pickling time ratio is remarkably reduced. However, when it exceeds 2.0 degrees, the effect saturates,
No further reduction in pickling time ratio was observed.

In the method of the present invention, the surface roughness of the dull work roll is set to 1.0 μmRa or more and less than 10 μmRa because the above-mentioned scale breaking effect is small when the surface roughness is less than 1.0 μmRa, and when it is 10 μmRa or more. This is because scale indentation may occur.

FIG. 4 is a diagram showing the influence of the presence or absence of inclination of the roll shaft and the surface roughness of the roll on the pickling time ratio. As in the case of FIG. 3, a roll having a diameter of 220 mm (material:
According to JIS SKD61), a hot rolled steel strip (low carbon steel) with a thickness of 2.0 mm and a width of 300 mm was rolled down at a rolling reduction of 3%, and then 5% hydrochloric acid (liquid temperature:
80 ° C) when pickled. The surface roughness of the work roll immediately before the start of rolling was 5 μmRa and was plated with chromium.

From this figure, it is possible to reduce the pickling time ratio by inclining the roll axis and increasing the surface roughness of the roll. For example, when the surface roughness of the roll is 1.0 μmRa, To obtain the same effect as when the roll shaft is tilted (tilt angle θ is 1.0 degree) without tilting the roll shaft, the surface roughness of the roll is 3.5 μm.
You can see that it has to be mRa.

The surface of the work roll is plated with chromium in order to prevent the surface roughness from being lowered due to the wear of the roll. For the chrome plating, a conventionally used method may be applied. If the plating thickness exceeds 10 μm, peeling is likely to occur, so the thickness is preferably 10 μm or less.

FIG. 5 is a diagram showing the effect of chromium plating on the reduction of the surface roughness of the work roll. As in the case of FIGS. 3 and 4, a roll having a diameter of 220 mm (material: JIS SKD61)
The hot rolled steel strip (low carbon steel) with a thickness of 2.0 mm and a width of 300 mm is rolled at a rolling reduction of 3%, and then pickled with 5% hydrochloric acid (liquid temperature: 80 ° C). The results are compared between cases where chrome plating is performed and cases where chrome plating is not performed. The surface roughness of the work roll just before the start of rolling is 5 μmRa
Is. In addition, when the work roll inclination angle θ is 0
The case of 1.5 was compared. The horizontal axis is the total rolling length, and the vertical axis is the pickling time ratio expressed based on the case where the work roll inclination angle θ is 1.5. From the results in this figure, regardless of the value of the inclination angle θ, the surface roughness is reduced and the effect of scale braking is reduced in the roll not plated with chrome, so that the pickling time ratio is increased. I understand.

The reduction ratio of 0.5% or more and less than 10% is
If the rolling reduction is less than 0.5%, the scale braking effect is small, while if it is more than 10%, there is a high possibility of causing scale indentation flaws in the steel strip.

The hot-rolled steel sheet is subjected to reduction under the above conditions and then subjected to pickling treatment. The pickling may be performed by a conventionally used method.

FIG. 6 is a view showing the constitution of an example of an apparatus for carrying out the method of the present invention. 1 is a steel strip, 2 is an upper work roll, 3 is a lower work roll, 4 is these work rolls 2 and 3. Is a rolling mill arranged at an inclination angle θ, 5 is a rewinding machine, 6 is a winding machine, 7 is a temperature raising tank, 8 is a pickling tank, and 9 is a tension leveler. The backup rolls 2'and 3'are
You may incline like work rolls 2 and 3,
Only the work rolls 2 and 3 may be tilted, and the backup rolls 2'and 3'may be arranged parallel to the plate width direction as usual.

The tension leveler 9 is installed on the exit side of the rolling mill to prevent the operation trouble due to the meandering of the steel strip. Further, the tension leveler 9 can correct the shape of the steel strip to improve the scale peeling effect. To do. A roller leveler may be used instead of the tension leveler 9.

By descaling the hot-rolled steel strip using this apparatus, the scale breaking effect is improved, so that the pickling time can be shortened and the line speed during pickling can be improved.

[0028]

EXAMPLE A hot rolled steel strip (low carbon steel, scale thickness: 7 to 8 μm) having a plate thickness of 2.0 mm and a plate width of 300 mm was passed through an apparatus (pickling line) having the configuration shown in FIG. The pickling treatment was performed, and the effects of the work roll inclination angle θ and the roll surface roughness on the time required for pickling were investigated.

The inclination angle θ of the work rolls 2 and 3 is 0.
The work rolls were dull-free or dull-like (surface roughness: 5.0 μmRa). The surface of the work roll was plated with chrome. The rolling reduction by the rolling mill 4 is 3%, the temperature of the temperature raising tank 7 and the pickling tank 8 is 80 ° C., and the hydrochloric acid concentration in the pickling tank is 5%.

The results of the investigation are shown in Table 1. The results in Table 1 are expressed as a pickling time ratio, where a work roll with a dull is used and the tilt angle is 0 degree, that is, the pickling time required when rolling with a rolling mill arranged without a tilt is 1. . As is clear from this result, when using the work roll with dull,
Alternatively, when the roll is inclined, the scale breaking effect is enhanced and the pickling time is shortened. In particular, when the work roll with dull is arranged so as to be inclined, a remarkable effect is recognized. When the surface roughness of the roll was investigated after rolling for 400 km, there was almost no change compared with that before rolling.

[0031]

[Table 1]

[0032]

When the method of the present invention is applied to the descaling of hot rolled steel strip, the scale breaking effect can be enhanced, and the pickling time can be greatly shortened. This method is extremely effective in improving the line speed during pickling and reducing the size of the pickling tank.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an arrangement state of work rolls in the present invention.

FIG. 2 is a diagram schematically showing a shearing force acting on a steel strip when the steel strip is rolled by the work rolls arranged in the state shown in FIG.

FIG. 3 is a view showing a relationship between a work roll shaft inclination angle θ and a pickling time ratio.

FIG. 4 is a diagram showing the influence of the presence or absence of inclination of the work roll axis and the surface roughness of the work roll on the pickling time ratio.

FIG. 5 is a diagram showing the effect of chromium plating on the reduction of the surface roughness of the work roll.

FIG. 6 is a diagram showing a configuration of an example of an apparatus for carrying out the method of the present invention.

[Explanation of symbols]

1: steel strip, 2: upper work roll, 2'and 3 ': backup roll, 3: lower work roll, 4: rolling mill,
5: rewinder, 6: winder, 7: temperature raising tank, 8: pickling tank,
9: Tension leveler.

Claims (1)

[Claims] 1. A pair of dull work rolls having a center line average roughness (Ra) of 1.0 μm or more and less than 10 μm and having chromium plating on the surface thereof, and the work roll is The rolling mills are installed so that the roll shafts have an inclination angle θ of 0.1 to 2.0 degrees with respect to the width direction of the steel strip, and the rolling reduction is 0.5% or more and 10% or more on the hot-rolled steel strip with scale attached to the surface. A method for descaling a hot-rolled steel strip, which comprises performing a pickling treatment after applying a rolling reduction of less than 1.