JPH09164417A - Mechanical descaling method of hot rolled steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent appearance of adverse effect due to a lubricant at the time of rolling in a pickling stage at the time of descaling the hot rolled steel strip by high-draft rolling. SOLUTION: After the hot rolled steel strip 1 to the surface of which hot rolled scales are stuck is rolled at high draft, at the time of mechanically removing the hot rolled scales by successively executing brushing 5 and, if necessary, spray process 6, the water which has large coefficient of friction or a water solution containing water-soluble rolling oil is supplied between the work rolls of a cold rolling mill 4 and the steel strip. As the water-soluble rolling oil, the oil which essentially consists of fats and oils, synthetic ester, mineral oil or the mixed oil of them and the portion stuck to the steel strip is easily cleaned by brushing and spray process after high-draf trolling is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a descaling method for reducing a pickling load by rolling a hot rolled steel strip under high pressure.

[0002]

2. Description of the Related Art A hot-rolled steel strip is covered with a scale mainly composed of an oxide. This hot rolled steel strip
If it is sent to a lower step such as cold rolling as it is, it causes defects such as surface flaws and cracks due to the hot rolling scale.
Therefore, the hot-rolled steel strip is usually removed by pickling, and the hot-rolled steel strip is transported to a lower step. This method has problems in pickling equipment, waste acid treatment, adjustment of descaling ability, and the like, and there is also a possibility that the properties of the steel material may be deteriorated due to intrusion of hydrogen generated during pickling.

[0003] In order to solve various problems caused by pickling, various methods for removing the scale of the hot-rolled steel strip sent to the pickling step have been studied. For example, cold rolling of a hot-rolled steel strip to which scale has been attached at a high pressure reduction (hereinafter referred to as black scale rolling) is disclosed in Japanese Patent Publication Nos. 54-133460, 57-41821 and 57-41821. This is introduced in, for example, Japanese Patent Publication No. -10917. Cold rolling at high pressure
Since the scale is cracked and the adhesion to the steel strip is reduced, it is easily separated from the steel strip after cold rolling by shot blasting, high pressure water injection, brushing, abrasive grinding, or the like. As a result, the scale attached to the hot-rolled steel strip carried into the pickling tank is reduced, and the load of the pickling step is reduced.

[0004]

When the hot-rolled steel strip is subjected to black scale rolling at a high pressure reduction rate, the load of the pickling process is certainly reduced, but the scale fragments peeled off from the steel strip surface are reduced. Easily adheres and remains on the surface. The scale in this case has a strong adhesion to the surface of the steel strip, unlike a scale generated when the hot-rolled steel strip is passed through a tension leveler. Therefore, many scales are sent to the pickling tank, and the pickling load cannot be reduced as expected. Moreover, scale fragments separated from the hot-rolled steel strip by high-pressure rolling are pressed or pressed into the surface of the steel strip, making it difficult to remove in the pickling process. It is likely to cause defects. For this reason, scale fragments are removed by, for example, abrasive grinding or the like, but some still remain on the steel strip surface.

The present inventors have studied various measures against residual scale which causes surface flaws in products, aiming to utilize the advantages of black leather rolling, which is effective in reducing the pickling load. As a result, almost all of the residual scale is removed when brushing and spraying the steel strip surface after black leather rolling, and the scale pieces transferred from the hot-rolled steel strip to the work rolls of the rolling mill are polished with a high pressure. We found that the scale itself that remained on the surface of the steel strip decreased significantly when it was removed with a water spray, scraper, etc., and we filed a separate application. In this way, the pickling load is reduced when the descaling is efficiently performed in the rolling before pickling and the scale brought into the pickling tank is reduced. The present invention is a further improvement of the descaling method by high-pressure rolling before pickling, by using water or aqueous solution easily removed by the subsequent spray treatment for lubrication during rolling,
It is an object of the present invention to prevent foreign matter from entering the pickling tank, and to carry in a steel strip having excellent surface properties into the pickling tank to enable pickling with a reduced load.

[0006]

In order to achieve the object of the mechanical descaling method of the present invention, a hot-rolled steel strip having a hot-rolled scale on its surface is rolled under high pressure and then brushed. When mechanically removing the hot-rolled scale by means of the above, water having a large friction coefficient or an aqueous solution containing a water-soluble rolling oil is supplied between the work roll of the cold rolling mill and the steel strip. As the water-soluble rolling oil, oils, synthetic esters, mineral oils or oils containing a mixed oil thereof as a main component are used, which are easily washed away from the steel strip when the steel strip rolled under high pressure is washed. The water-soluble rolling oil is
0.05- (0.15 + α × R + β × γ) [However,
α: 1/7500 (constant), β: −1/2500 (constant), γ: rolling reduction (%), R: rolling roll diameter (mm)]
It is preferred to have a coefficient of friction μ in the range.

The line according to the invention is constructed as shown in FIG. The hot-rolled steel strip 1 with the hot-rolled scale adhered is unwound from the payoff reel 2, passed through bridle rolls 3, and rolled under high pressure by a cold rolling mill 4. The hot-rolled scale is cracked and crushed by rolling under high pressure, and the steel strip 1
Peeled off from After removing the scale pulverized material remaining on the surface of the steel strip with the brush roll 5, the spray device 6 is further used.
And is sprayed with high pressure water from the spray nozzle 7 to clean the surface of the steel strip. At this time, the oil content adhering to the steel plate when the water-soluble rolling oil is used is also washed at the same time. Therefore, it is possible to prevent oil from being brought into the pickling tank.
There is no problem of oil contamination of the treatment equipment at the time of waste acid treatment. The steel strip treated in this way is then sent to a pickling tank 8, where scales slightly remaining on the surface are pickled and removed.

The cold rolling mill is preferably equipped with a work roll having a polisher, a spray nozzle or a scraper facing the circumferential surface. In this case, the crushed material of the hot-rolled scale transferred from the hot-rolled steel strip to the peripheral surface of the work roll is removed from the roll surface by a polisher, a spray nozzle, a scraper, or the like disposed opposite to the surface of the rolling roll, and is removed from the system. Is discharged. It is preferable that the polisher, the spray nozzle, the scraper, and the like are arranged facing the peripheral surface of the work roll at a position downstream of the rolling point in the rotation direction. Further, a similar polisher, spray nozzle, scraper and the like may be provided for a backup roll which rotates in contact with a work roll.

The cold rolling mill 4 preferably has a rolling reduction of 10
% Or more (preferably 40% or more). The present inventors considered the reason why such a high pressure reduction is effective for descaling of the hot-rolled steel strip 1 as follows. The scale formed on the surface of the hot rolled steel strip 1 is
It is mainly composed of Fe ₃ O ₄ , but conceptually, the FeO layer, F in which the oxygen concentration gradually increases from the inside toward the surface layer, F
It is considered to have a structure in which the e ₃ O ₄ layer and the Fe ₂ O ₃ layer are laminated on the surface of the base steel. In fact, the quenched steel strip tends to have a thicker FeO layer. The scale layer is relatively thick at 9 to 10 μm in weak deoxidized steel, and 6 in Ti deoxidized steel.
It is relatively thin, about 7 μm.

The Fe ₃ O ₄ layer and the Fe ₂ O ₃ layer, which occupy most of the scale layer, are hard and brittle, and are easily cracked even with a relatively low rolling force. For example, even if a rolling force (about 2%) of a conventional tension leveler, which is incorporated as a pre-step of pickling, or mechanical repeated bending work, cracks are generated and peeling occurs. The Fe ₂ O ₃ layer and the Fe ₃ O ₄ layer can be cracked even by a device that gives mechanical repeated bending as seen in sulfuric acid pickling. On the other hand, the FeO layer at the interface with the base steel is malleable and deforms according to the elongation of the base steel at a low rolling reduction. Therefore, it is brought into the pickling tank without being peeled off from the base steel by the rolling force of about the tension leveler. However, when the rolling reduction is set to be high, the difference in deformation amount between the base steel and the FeO layer becomes large, and cracks occur in the FeO layer that cannot follow the elongation of the base steel. In fact, when examining the pulverized material of the scale peeled from the surface of the hot-rolled steel strip by cold rolling, when the rolling reduction is low, the peeled scale is powdery and granular, but it is pulverized with the increase of the rolling reduction. It is observed that the size of the object becomes large and scale-like. The state change of the exfoliation scale depending on the rolling reduction is caused by an increase in the amount of scale exfoliation due to the crack starting from the inside of the scale layer, in other words, the crack entering the FeO layer, as the high pressure rolling is performed. It is speculated that there is. As a result, the scale remaining on the surface of the steel strip after rolling is significantly reduced.

However, the scale-like peeled scale pieces are
Even if it is peeled from the steel strip and has strong adhesion to the surface of the steel material, it may be transferred to the surface of the rolling roll and then pressed or pressed again on the surface of the steel strip. Therefore,
The present inventors have tried a method of brushing the surface of the steel strip after cold rolling to remove the residual scale from the surface of the steel strip and to remove the scale pieces adhering to the surface of the rolling roll. As a result, it was found that the scale was removed more than expected and the pickling conditions in the pickling tank 8 were significantly eased. It is considered that this is because cracks are deeply formed in the scale layer by high-pressure rolling, and the peeling force by brushing spreads to the deep portion of the scale layer from the crack as a starting point. Further, when the spray treatment is performed using high-pressure hot water during or after the brushing, the scale pieces remaining on the surface of the steel strip can be more reliably removed.

In the cold rolling mill 4, the hot rolled steel strip 1 is cold rolled at a high pressure reduction rate. Therefore, it is necessary to lubricate the work roll and the hot-rolled steel strip 1. However, if an ordinary oil-based lubricant is used, the oil content remaining on the surface of the steel strip after rolling is brought into the pickling tank 8 and the treatment of waste acid is hindered. Therefore, in the present invention, water or an aqueous solution containing a water-soluble rolling oil that is sufficiently washed away by the high pressure water sprayed from the spray nozzle 7 of the spray device 6 is used. Water or an aqueous solution is also effective in promoting scale separation from the steel strip during rolling. The metal flow during rolling is non-uniformly deformed as shown in FIG.
In addition, as shown in FIG.
There is a difference in the degree of deformation as shown in (b). Due to this rolling pattern and the difference in ductility between the scale layer and the base steel, peeling occurs in the scale layer.

To this extent, when the friction coefficient μ acting between the work roll and the steel plate surface (scale surface) is large, the shearing force τ (= μP) acting on the surface is also large. Therefore, the restraint force on the surface of the steel sheet becomes large, the uneven deformation in the sheet thickness direction is large, and as a result, the scale peels off. In normal cold rolling, rolling oil with fairly good lubricity is used to set the friction coefficient between the roll and the steel strip to about 0.03, the rolling load and the mill motor power are reduced, and a high pressure reduction rate is achieved. . As the rolling oil, an aqueous solution of about 1 to 5% is usually used, and it is intended to prevent seizure as well as to cool the roll bite portion with water. On the other hand, in the present invention, from the viewpoint of descaling by black skin rolling, it is important to cause a large deformation to the inside shown in FIG. Therefore, it is desirable to use a material having a high friction coefficient, and rolling is performed in a state in which the lubricating property of the rolling oil is slightly reduced. That is, the lubricity of the rolled material is appropriately adjusted with water or water-soluble rolling oil.

Among them, 0.05 to (0.15 + α × R
+ Β × γ) [where α: 1/7500 (constant), β:
-1/2500 (constant), γ: rolling reduction (%), R: rolling roll diameter (mm)] When using a water-soluble rolling oil having a friction coefficient μ, a large plastic deformation is obtained, The peeling of the scale layer is promoted. The friction coefficient μ is effectively 0.05 or more in order to improve the descaling property. However, if the friction coefficient μ is too high, the mill motor power and load during rolling increase. The rolling cost, which is occupied by the mill construction cost considering the power cost, load, and torque, decreases according to the rolling lubricity as shown in Fig. 4, but it is occupied by the pickling solution amount, the pickling section construction cost, etc. On the contrary, the pickling cost rises. In the present invention, water or water-soluble rolling oil is used in order to balance the rolling cost and the pickling cost. If the friction coefficient μ is too large, the rolling load will increase,
The surface pressure of the roll bite also rises. Therefore, the scale may be pressed against the base steel. Since this phenomenon is more remarkable as the roll diameter is smaller and the rolling reduction is larger, it is necessary to define the upper limit of the friction coefficient μ in relation to the roll diameter and the rolling reduction as seen in the conditional expression described above. .

When a hot-rolled steel strip is rolled under high pressure, cracks, delamination, etc. occur in the scale that cannot follow the expansion of the base steel, and the adhesion to the base steel decreases. When brushing such a steel strip, brush bristles enter the gaps generated in the scale layer, and the scale is removed from the surface of the steel strip. As the brush roll 5, a silica-based or alumina-based nylon brush containing abrasive grains is used. Further, when a brush roll containing abrasive grains is used, removal of scale is further promoted. Brushing acts on the entire surface of the steel strip with a large descaling force. The scale remaining after the brushing is carried into the spray device 6,
From the spray nozzle 7, for example 10 to 50 kgf / cm
High-pressure water of about ² is sprayed. As a result, the water or aqueous solution used for lubrication during rolling is washed away together with the residual scale without damaging the base steel. Further, even if there is residual rolling oil, since it is aqueous, it does not adversely affect the acid solution in the pickling tank 8 and the waste acid treatment.

Since most of the scale is removed from the surface of the steel strip by brushing and spraying, very little scale should be removed by the pickling treatment. Therefore, the pickling load is greatly reduced. When hot water held at 80 to 95 ° C is used as the high-pressure water used in the spray after brushing, the pickling tank is used without lowering the temperature of the steel strip heated by the working heat caused by cold rolling under high pressure. You can carry it in. Further, the water or the aqueous solution used for lubrication during rolling is also efficiently removed. Therefore, the temperature drop of the pickling bath and the inclusion of foreign matter are suppressed, the treatment under constant pickling conditions becomes possible, and the energy required for temperature compensation of the pickling bath is also saved.

[0017]

EXAMPLE A hot-rolled steel strip having a thickness of 2.7 mm was cold-rolled at a reduction rate of 50% prior to pickling in a descaling line shown in FIG. As the hot-rolled steel strip, a hot-rolled steel strip having the components and compositions shown in Table 1 and having a hot-rolled scale having an average thickness of 7 to 15 μm attached to the surface was used. Further, during rolling, an aqueous solution containing water and a water-soluble rolling oil was flown between the work roll having a diameter of 450 mm and the hot-rolled steel strip 1 at a flow rate of 4.5 Nm ³ /
Supplied in minutes. The coefficient of friction of water-soluble rolling oil is 0.05 to
0.19 (= 0.15 + 450 / 7500-50 / 25
It was adjusted within the range of (00).

[0018]

When a lubricant containing water is not used, that is, when dry rolling is performed, the temperature rises in the roll bite due to insufficient cooling capacity, which causes seizure. Also, when an oil-based lubricant is used, if the oil is not sufficiently washed in front of the acid tank, the oil will be mixed into the pickling tank and the waste acid treatment equipment will be contaminated with oil. The maintenance load of the nozzle filter increases due to the spray roasting that is performed. On the other hand, by using water or water-soluble rolling oil that easily cleans the adhered steel strips by brushing and spraying after high-pressure rolling, insufficient cooling at the roll bite due to insufficient cooling capacity does not occur, and the acid tank Since the oil content is easily removed by the brushing and spraying process for the previous black leather strip, there is no mixing of oil in the pickling tank. It is possible to easily achieve an appropriate balance between ensuring the friction coefficient of the above and reducing the power and load of the mill motor by adjusting the concentration, for example.

As described above, by rolling using water or water-soluble rolling oil, it is effective to achieve both insufficient cooling, contamination by oil in the pickling tank, descaling, and reduction of mill motor power and load. Was achieved. In addition, when the friction coefficient is back-calculated by the Hill's approximate formula which is usually used for load calculation of cold rolling, in the present embodiment, as shown in Table 2,
The coefficient of friction was about 0.2, which was considerably higher than that of about 0.03 for ordinary cold rolling. But,
This size was in a proper range for improving the descaling property. Further, when the surface of the steel strip after passing through the spray device 6 was observed, no foreign matter remained on the surface of the steel strip.

The spray-treated steel strip was passed through a pickling tank 8 containing a hydrochloric acid-based acid solution held at 90 ° C., and immersed for 6 seconds for pickling. The steel strip after pickling had a good surface with no residual scale in any case. Also,
When the effect of the friction coefficient μ on the scale peeling condition was investigated, as shown in Table 2, the shearing force τ (= μP) was small in the region where the friction coefficient μ was low, and the scale peeling property was deteriorated. However, if the friction coefficient μ is too large, the scale releasability rather deteriorates. This is a result of the scale being pressure-bonded to the base steel because the rolling load increases and the surface pressure of the roll bite increases as the friction coefficient μ increases.

[0022]

[0023]

As described above, in the present invention, when descaling a hot rolled steel strip by high pressure rolling,
Rolling is performed while supplying water and water-soluble rolling oil between the work roll and the steel strip. The supplied water and water-soluble rolling oil are
Since it is washed off from the steel strip surface in the subsequent spraying process, it is not brought into the pickling tank. Further, even if there is residual water-soluble rolling oil, since it is aqueous, it does not adversely affect the acid solution and its waste acid treatment. In this way, according to the present invention, the life of the pickling bath can be extended and the deterioration of the pickling bath can be suppressed. Further, the pickling load is reduced, and the pickling time can be shortened, the pickling equipment can be downsized, and a low-concentration acid solution can be used.

[Brief description of the drawings]

FIG. 1 shows a descaling line of a hot-rolled steel strip according to the present invention.

Fig. 2 Layer composition of scale formed on hot rolled steel strip surface

FIG. 3 shows a metal flow (a) and a deformation region (b) during high pressure rolling.

[Fig. 4] Effect of lubricity during rolling on rolling cost and pickling cost

[Explanation of symbols]

1: Hot-rolled steel strip 2: Pay-off reel 3: Bridle roll 4: Cold rolling machine 5: Brush 6: Spray device 7: Spray nozzle 8: Pickling tank
9: Take-up reel

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C10N 40:20 (72) Inventor Junya Hayakawa 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Sakai Nisshin Steel Co., Ltd. Inside the factory

Claims (3)

[Claims] 1. A work roll of a cold rolling mill when mechanically removing a hot-rolled scale by brushing after hot-rolling a hot-rolled steel strip having a hot-rolled scale adhered to the surface thereof, by brushing. A mechanical descaling method for a hot-rolled steel strip, characterized in that water having a large friction coefficient or an aqueous solution containing a water-soluble rolling oil is supplied between the steel strip and the steel strip. 2. One or two or more water-soluble rollings containing oils and fats, synthetic esters, mineral oils or mixed oils as main components, and from which oils adhered to steel strip can be easily washed off after rolling under high pressure. The mechanical descaling method according to claim 1, wherein oil is used. 3. 0.05 to (0.15 + α × R + β ×
γ) [However, α: 1/7500 (constant), β: -1 /
3. The mechanical descaling method according to claim 1, wherein a water-soluble rolling oil having a friction coefficient μ within the range of 2500 (constant), γ: rolling reduction (%), R: rolling roll diameter (mm)] is used.