JP3406750B2 - Descaling method of hot rolled steel strip - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel strip in which scale is mechanically removed by high-pressure rolling before pickling and the necessary properties are produced. The present invention relates to a method for descaling a hot-rolled steel strip to be manufactured. [0002] The surface of 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. In order to solve various problems caused by the 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 is attached at a high pressure ratio (hereinafter referred to as black scale rolling) is disclosed in Japanese Patent Publication No. 54-133460.
JP, JP-A-57-41821, JP-A-57-41821,
No. 10917, and the like. Cold rolling at a high pressure ratio causes cracks in the scale and decreases the adhesive force to the steel strip, so it can be easily removed from the steel strip after cold rolling by shot blasting, high pressure water injection, brushing, abrasive grinding, etc. Is separated into 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. [0003] 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 scale fragments separated from the steel strip surface are generated. It easily adheres and remains on the steel strip 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. [0004] The present inventors have studied various measures against residual scale that causes surface flaws in products, with the aim of taking advantage of black scale 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 the black scale rolling, and the scale pieces transferred from the hot-rolled steel strip to the work roll of the rolling mill are polished with a high pressure When removing with a water spray, a scraper, etc., it was found that the scale itself remaining on the steel strip surface was significantly reduced, and a separate application was filed. In this way, when the scale before rolling is pickled efficiently by rolling before pickling and the scale brought into the pickling tank is reduced, the pickling load is reduced. The present invention further improves the descaling method by high-pressure rolling before pickling, and specifies the arrangement of bridle rolls that impart the tension required for rolling, thereby generating surface flaws due to residual scale. And to obtain a steel strip having excellent surface properties. In order to achieve the object, a method for descaling a hot-rolled steel strip according to the present invention is to reduce the hot-rolled steel strip having a hot-rolled scale attached to its surface by 10 to 50%. Cold rolling at a rolling reduction, then removing the hot-rolled scale from the base steel of the hot-rolled steel strip by high-pressure spraying after brushing, and then further transported to the pickling process to downstream of the brushing device when descaling The hot-rolled steel strip is cold-rolled while tension is applied to the hot-rolled steel strip with a bridle roll provided on the downstream side of the high-pressure spraying apparatus in the above, and hot water maintained at 80 to 95 ° C. is used as high-pressure water to be sprayed. And A line for implementing the method of the present invention is constructed as shown in FIG. The hot-rolled steel strip 1 with the hot-rolled scale attached is unwound from the payoff reel 2,
10-50% by cold rolling mill 4 after bridle roll 3
It is cold rolled at a rolling reduction. The hot-rolled scale is cracked and pulverized by high-pressure rolling, and is separated from the steel strip 1. After the scale pulverized matter remaining on the steel strip surface is removed by the brush roll 5, the scale pulverized product is further introduced into the spray device 6, and the surface of the steel strip is cleaned by spraying high-pressure water from the spray nozzle 7. 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 preferably has a work roll having a polisher, a spray nozzle or a scraper opposed to the peripheral 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 has a draft of 10 to 50%.
(Preferably 40 to 50%). The present inventors considered the reason why such a high-pressure reduction is effective for the descaling of the hot-rolled steel strip 1 as follows. As shown in FIG. 2, the scale formed on the surface of the hot-rolled steel strip 1 includes an FeO layer and an FeO layer in which the oxygen concentration increases gradually.
_It has a structure in which a ₃ O ₄ layer and a Fe ₂ O ₃ layer are laminated on the surface of a base steel. The Fe ₃ O ₄ layer and the Fe ₂ O ₃ layer are hard and brittle, and cracks occur even at a relatively low rolling force. easy. For example, cracks occur even in a mechanically repeated bending process and a rolling force (about 2%) of a conventional tension leveler incorporated as a pre-process of pickling, and peeling occurs at the scale layer interface. On the other hand, FeO at the interface with the base steel
The layers are malleable and deform at low rolling reductions according to the elongation of the base steel. For this reason, the steel sheet is not peeled off from the base steel by the rolling force of the tension leveler and is taken into the pickling tank. However, when the rolling reduction is set to 10% or more, the difference in the amount of deformation between the base steel and the FeO layer increases, and the FeO layer which cannot follow the elongation of the base steel delaminates from the base steel. generate. In fact, when examining the crushed material of the scale peeled from the surface of the hot-rolled steel strip by cold rolling, the peeled scale is powdery when the rolling reduction is low,
It is observed that the size of the pulverized material becomes large and scale-like as the rolling reduction increases. The state change of the peel scale according to the rolling reduction is that Fe ₃ O ₄
It means that the scale is peeled off from the layer, and that the scale is peeled off from the FeO layer in high-pressure rolling. When the scale is separated from the interface between the FeO layer and the base steel, the scale remaining on the surface of the steel strip after rolling is significantly reduced. However, even though FeO has strong adhesion to the steel material surface and is separated from the steel strip,
After being transferred to the surface of the rolling roll, it may be pressed or pressed again to the surface of the steel strip. Therefore, the present inventors brushed the steel strip surface after cold rolling, and then remove high-pressure water spray to remove the remaining scale from the steel strip surface, and remove the scale pieces adhering to the rolling roll surface. I tried to remove it. As a result, it was found that the scale was removed more than expected, and the pickling conditions in the pickling tank 8 were greatly reduced. When a hot-rolled steel strip is rolled under high pressure, cracks, delamination, and the like occur on a scale that cannot follow the spreading of the base steel, and the adhesion to the base steel decreases. When brushing such a steel strip, brush bristles enter gaps generated in the scale layer, and scale is removed from the steel strip surface. As the brush roll 5, a nylon brush containing abrasive grains of silica or alumina, a notch wire brush, or the like is used. When a brush roll containing abrasive grains is used, scale removal is further promoted. Brushing acts on the entire surface of the steel strip with a large descaling force. [0010] The scale remaining after the brushing is carried into the spray device 6, and high-pressure water of, for example, about 10 to 500 kgf / cm ² is sprayed from the spray nozzle 7. Thereby, the residual scale is removed without damaging the base steel. Brushing and spraying remove most of the scale from the steel strip surface, so that only a small amount of scale must be removed by pickling. Therefore, the pickling load is greatly reduced. In addition, when hot water maintained at 80 to 95 ° C. is used as high-pressure water to be used for spraying after brushing, the pickling tank is used without lowering the temperature of the steel strip heated by the processing heat caused by cold rolling under high pressure. Can be delivered to Therefore, a decrease in the temperature of the pickling bath is suppressed, the treatment can be performed under constant pickling conditions, and the energy required for temperature compensation of the pickling bath can be reduced. In this descaling line, it is necessary to apply tension to the hot-rolled steel strip 1 in order to cold-roll the hot-rolled steel strip 1 at a predetermined rolling reduction. From the viewpoint of achieving stability, load reduction, shape stability, and the like of the rolling mill 4, it is preferable to apply a large tension from the front and the rear. A bridle roll is usually used for applying the tension. From the upstream side of the rolling mill 4, necessary tension can be applied to the hot-rolled steel strip 1 by the bridle roll 3 without adversely affecting descaling. On the other hand, if it is attempted to apply tension by bridle rolls arranged downstream of the rolling mill 4, the steel strip 1 in which a part of the scale is peeled off or floats will pass between the bridle rolls. For this reason, the scale pieces adhere to the bridle roll, contaminate the subsequent coil, and cause dent defects in the bridle roll itself. Therefore, in the present invention, as shown in FIG. 3, the brush roll 5 and the spray processing 6 are arranged on the upstream side of the downstream bridle roll 10. That is, after stripping the easily stripped steel strip 1 by brushing and spraying the steel strip 1 in which the scale layer has been cracked or peeled by high-pressure rolling, the steel strip 1 is removed from the bridle roll 10.
Send to As a result, there is no scale piece transferred from the steel strip 1 to the bridle roll 10, and contamination of the subsequent coil and damage to the bridle roll itself due to the transfer scale are prevented. As a result, the steel strip 1 is carried into the pickling tank 8 while maintaining good surface properties. 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 with an average thickness of 10 to 15 μm adhered to the surface was used. During rolling, the scale pieces transferred to the work roll surface were removed by a polisher facing the roll peripheral surface. [0014] In order to investigate the influence of the processing conditions after rolling, the steel strip 1 was transported to a pickling tank in the following cases 1 to 3. Case 1: (FIG. 3) Nylon brush (three 1.6 mm thick wire rods, outer diameter 360 mm) at silica or alumina abrasive grain position 20
The steel strip brushed under the condition of rotating at 0 rpm was carried into a pickling tank via bridle rolls. Case 2: (FIG. 4) Temperature 8 on front of steel strip brushed as in case 1.
After spraying high-pressure water at 0 ° C., the mixture was carried into a pickling tank via a bridle roll. Case 3: (Comparative Example) Contrary to Case 1, the steel strip exiting the bridle roll was brushed under the same conditions, and then carried into an acid pickling tank. In the pickling tank, a hydrochloric acid solution (bath temperature: 90)
℃), each steel strip was pickled by dipping for 2 to 20 seconds. Then, the steel strip surface after pickling was observed, and cases 1 to 3 were observed.
Were compared. In case 3, the steel strip is bent along the bridle roll, so that debris of scale debris partially occurs. Further, since pressure is applied between the bridle roll and the steel strip, the detached scale pieces are pressed on the steel strip or the bridle roll. The press-bonded scale pieces are repeatedly pressed and peeled by the rotation of the bridle roll, but leave indentations on the steel strip or the bridle roll, and remain as unacceptable flaws on the steel strip in terms of quality. On the other hand, in Cases 1 and 2, the scale is almost completely removed with a brush or spray before the bridle roll, so that no new scale peels between the bridle rolls. As is clear from this comparison, by disposing the bridle roll 10 on the downstream side of the brush roll 5 and the spray device 6, the surface properties of the steel strip 1 fed into the pickling tank 8 are maintained well, and It can be seen that damage by 10 scale pieces is also prevented. As a result, the advantages of black scale rolling were utilized, and the pickling load could be reduced. As described above, according to the present invention, when descaling a hot-rolled steel strip by high-pressure rolling before pickling, a bridle roll for imparting tension necessary for rolling is brushed. It is located downstream of the roll and spray equipment. As a result, the scale pieces that are easily peeled have already been removed by brushing or spraying. Is suppressed. As a result, the steel strip fed into the pickling tank has a good surface condition, the advantages of black scale rolling can be utilized, and the pickling load can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a descaling line of a hot-rolled steel strip according to the present invention. FIG. 2 is a layer configuration of a scale formed on the surface of the hot-rolled steel strip. FIG. Bridle roll arranged on the side [Fig. 4] Bridle roll arranged downstream of the spray device [Description of symbols] 1: Hot rolled steel strip 2: Payoff reel 3: Bridle roll 4: Cold rolling mill 5: Brush roll 6 : Spray device 7: Spray nozzle 8: Pickling tank 9: Take-up reel 10: Bridle roll on the downstream side

Continuation of the front page (72) Inventor Junya Hayakawa 5th Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Corporation Sakai Works (56) References JP-A-60-145216 (JP, A) JP-A-50-127835 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B21B 27/06-28/04 B21B 45/04-45/08 B08B 1/02 C23G 5/00

Claims (1)

(57) [Claims 1] A hot-rolled steel strip having a hot-rolled scale attached to its surface is cold-rolled at a rolling reduction of 10 to 50%, and then hot-sprayed by brushing through high-pressure spraying. hot-rolled scale is removed from the rolled strip of base steel, Thereafter, when the descaling is transported to the pickling process, under the brushing device
The hot-rolled steel strip is cold-rolled with tension applied to the hot-rolled steel strip by bridle rolls provided downstream of the high-pressure spray device in the stream, and hot water maintained at 80 to 95 ° C is used as high-pressure water to be sprayed. Characteristic descaling method of hot rolled steel strip.