A method for controlling iron oxide scale on the surface of hot-rolled strip steel

Abstract

This invention discloses a method for controlling iron oxide scale on the surface of hot-rolled strip steel, belonging to the field of hot-rolled strip steel production technology. The method includes: furnace heating, post-furnace descaling, pre-roughing mill descaling, roughing, pre-finishing mill descaling, finishing, ultra-fast cooling, and coiling. During furnace heating, the slab exit temperature is 1100–1200℃; the descaling pressure after furnace heating is 19–20 MPa; the descaling pressure before the roughing mill is 16–19 MPa; a purging device is installed on both sides of the bottom of the rolling channel in the roughing mill, and a backflushing water device is installed at the top of the exit of the E2 and E3 mill channels in the roughing mill; the descaling pressure before the finishing mill is 19–20 MPa; a backflushing water device is installed at the top of the exit of the F6, F7, and F8 mill channels in the finishing mill; the strip coiling temperature is controlled at 600–700℃. This invention solves the technical problems of easily generating regenerated iron oxide scale in vertically coiled strip steel and the unsatisfactory effect of traditional descaling methods.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled strip steel production technology, and particularly relates to a method for controlling iron oxide scale on the surface of hot-rolled strip steel. Background Technology

[0002] Hot-rolled strip steel refers to the strip steel product obtained by hot rolling steel billets, and it occupies an important position in steel production. The surface quality of hot-rolled strip steel directly affects its performance and subsequent processing effects, among which the control of iron oxide scale is a key factor affecting surface quality. Iron oxide scale mainly includes primary iron oxide scale, secondary iron oxide scale, and regenerated iron oxide scale, which are formed during heating, rolling, and cooling processes, respectively.

[0003] Currently, hot-rolled strip steel production lines typically use high-pressure water descaling to remove iron oxide scale. Specifically, a descaling device is installed before the rolling mill, using high-pressure water to impact the steel surface and remove the iron oxide scale. Simultaneously, cooling water is used during the rolling process to control the rolling temperature and clean the surface.

[0004] However, the following technical problems exist in the existing technology:

[0005] First, relying solely on descaling equipment is insufficient to completely remove iron oxide scale. Some residual iron oxide scale will be pressed into the steel surface during subsequent rolling processes, forming indentation defects.

[0006] Second, newly generated iron oxide scale during the rolling process is prone to accumulate in the rolling channel, causing scratches on the surface of the strip steel;

[0007] Third, improper coordination of temperature and pressure parameters in each process can lead to unsatisfactory control of iron oxide scale, or even cause new surface defects.

[0008] These problems seriously affect the surface quality and product performance of hot-rolled strip steel. Therefore, there is an urgent need for an effective method to control iron oxide scale during the hot-rolled strip steel production process. Summary of the Invention

[0009] The purpose of this invention is to provide a method for controlling iron oxide scale on the surface of hot-rolled strip steel, so as to solve the technical problem mentioned in the background art of how to effectively control iron oxide scale throughout the entire process and avoid the indentation of oxide impurities and scratches on the surface of strip steel.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] A method for controlling iron oxide scale on the surface of hot-rolled strip steel includes the following steps: heating in a heating furnace, descaling after the furnace, descaling before the roughing mill, roughing rolling, backflushing water purging during roughing rolling, descaling before the finishing mill, finishing rolling, backflushing water purging during finishing rolling, ultra-fast cooling treatment, and coiling; wherein...

[0012] When the furnace is heated, the slab exit temperature is controlled at 1100-1200℃ to reduce the formation of iron oxide scale.

[0013] The descaling pressure after the furnace is 19-20 MPa to remove the iron oxide scale generated on the surface of the steel during the furnace exit process.

[0014] The descaling pressure before the roughing mill is 16-19 MPa, which removes the iron oxide scale generated on the surface of the steel during transportation.

[0015] Blowing devices are installed on both sides of the bottom of the rolling channel of the roughing mill to prevent iron oxide scale from accumulating in the rolling channel and to avoid the accumulated iron oxide scale being pressed into the surface of the strip steel during the rolling process, thus forming defects. In addition, backflushing water devices are installed at the top of the exit of the E2 and E3 rolling mill channels of the roughing mill to blow and rinse the residual iron oxide scale on the surface of the strip steel, ensuring the cleanliness of the strip steel surface.

[0016] The descaling pressure before the finishing mill is 19-20 MPa; the descaling effect is enhanced by slightly increasing the high-pressure water pressure, while avoiding excessive impact force that could cause defects on the steel surface.

[0017] A backflushing water device is installed at the top of the exit of the F6, F7, and F8 mill passages in the finishing mill to blow away the iron oxide scale on the surface of the strip steel.

[0018] The strip winding temperature is controlled at 600-700℃ during winding to reduce the growth of the oxide layer and improve the surface quality of the strip.

[0019] Preferably, the descaling uses a descaling box, which includes a water spray pipe and a descaling nozzle, with the water outlet direction of the nozzle forming an angle of 15 degrees with the water spray pipe.

[0020] Preferably, the ultra-fast cooling process uses an ultra-fast cooling nozzle, and the strip coiling temperature is controlled by adjusting the flow rate and pressure of the ultra-fast cooling nozzle.

[0021] Preferably, the purging device includes a nozzle on each side of the bottom of each rolling mill stand, with the water outlet direction of the nozzles pointing towards the center of the bottom surface of the rolling channel.

[0022] Preferably, the backwashing device includes a row of nozzles arranged laterally along the width of the strip, with the water outlet direction of the nozzles facing the mill inlet side, and the water outlet direction of the nozzles inclined downward at 30 to 60 degrees to the horizontal plane.

[0023] Preferably, the nozzle of the backwash device is made of stainless steel, with a nozzle diameter of 8mm and a water spray pressure of 0.8 to 1MPa during operation; it can effectively rinse the surface of the strip steel.

[0024] Preferably, the angle between the water outlet direction of the nozzle and the horizontal plane is 30 degrees or 60 degrees.

[0025] Preferably, when the heating furnace is heating, the slab exit temperature is controlled at 1100℃ or 1200℃; and the strip coiling temperature is controlled at 600℃ or 700℃.

[0026] Preferably, the descaling pressure of the descaling after the furnace is 19 MPa or 20 MPa; the descaling pressure of the descaling before the roughing mill is 16 MPa or 19 MPa; and the descaling pressure of the descaling before the finishing mill is 19 MPa or 20 MPa.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] The method for controlling iron oxide scale on the surface of hot-rolled strip steel provided by the present invention effectively solves the technical problems of surface indentation of oxide impurities and scratches caused by insufficient control of iron oxide scale in the prior art by optimizing the process parameters and equipment configuration of the whole process, and significantly improves the surface quality of hot-rolled strip steel.

[0029] Specifically, this invention achieves the following technical effects through the following means: By controlling the slab exit temperature at 1100–1200℃, the amount of primary iron oxide scale generated is reduced at the source. By setting up multiple descaling processes with different pressure levels (19–20 MPa after furnace, 16–19 MPa before roughing, and 19–20 MPa before finishing), precise removal of iron oxide scale at different stages is achieved. Furthermore, by installing purging devices and backflushing water devices at key locations in the roughing and finishing mills, iron oxide scale accumulation in the rolling channel is effectively prevented, and residual scale is promptly removed, avoiding surface indentation and scratch defects. Finally, by controlling the coiling temperature at 600–700℃, the formation of regenerated iron oxide scale during strip cooling is reduced.

[0030] Through the synergistic effect of the above-mentioned multiple steps, the present invention not only improves the surface quality of strip steel and reduces the surface defect rate, but also improves production efficiency, reduces production costs, and enhances product market competitiveness by reducing rework and scrap rates. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0032] A method for controlling iron oxide scale on the surface of hot-rolled strip steel includes the following steps: heating in a heating furnace, descaling after the furnace, descaling before the roughing mill, roughing, descaling before the finishing mill, finishing, ultra-fast cooling, and coiling.

[0033] The specific steps are as follows:

[0034] (1) Slab heating: Control the heating temperature at the lower limit, and control the slab exit temperature at 1100-1200℃ (inclusive) to prevent forced steel burning. (Previously 1200-1250℃)

[0035] Specifically, controlling the heating temperature at the lower limit means selecting the lowest possible heating temperature while ensuring that the slab can be rolled normally, avoiding controlling the temperature near the upper limit of the allowable range; the purpose is to reduce the formation of iron oxide scale, because the higher the temperature, the more iron oxide scale is formed.

[0036] The slab exit temperature is controlled at 1100–1200℃, which is a significantly lower temperature range compared to the original process (1200–1250℃), thus reducing the amount of iron oxide scale formed. This is because temperature is closely related to the formation of iron oxide scale during the steel heating process. When steel materials are in a high-temperature environment, their surface will undergo an oxidation reaction with oxygen in the air, generating iron oxide scale mainly composed of FeO (ferrous oxide), while small amounts of Fe3O4 (magnetite) and Fe2O3 (hematite) will also be formed.

[0037] In the high-temperature range of 1200–1250℃, the oxidation reaction rate on the steel surface accelerates significantly due to the higher temperature. This is because, according to Arrhenius's law, the oxidation reaction rate has an exponential relationship with temperature; an increase in temperature leads to a sharp increase in the oxidation rate. The iron oxide scale formed in this temperature range is not only abundant but also, due to its rapid growth rate, easily forms a loosely structured multilayered oxide layer. This layer is more likely to detach or be pressed into the steel surface during subsequent rolling processes, affecting product quality.

[0038] Controlling the slab exit temperature at a lower range of 1100–1200℃, while oxidation still occurs, significantly reduces the reaction rate. This slower oxidation rate promotes the formation of a more uniform and denser oxide layer, reducing the risk of cracking and peeling. This not only directly reduces the amount of iron oxide scale generated but also makes subsequent descaling processes more effective.

[0039] After numerous experiments, it was found that when the slab exit temperature is below 1100℃, the slab temperature in subsequent roughing and finishing rolling processes will be lower than the austenitizing temperature (approximately 900℃). Some areas will detach from the austenite region, forming a mixed-grain structure, resulting in uneven product microstructure and severely affecting product performance. Simultaneously, low-temperature rolling is also prone to causing quality defects such as cracks on the product surface. To ensure rolling quality, it may be necessary to reduce rolling speed or increase the number of rolling passes. This not only disrupts the normal production rhythm but also reduces production efficiency and leads to a sharp increase in deformation resistance during rolling. At this point, the rolling force will increase significantly, not only accelerating roll wear and shortening its service life but also potentially exceeding the mill's design load limit.

[0040] Therefore, controlling the slab exit temperature at 1100℃~1200℃ can ensure that the entire rolling process takes place within the austenitic range, ensuring that the material has good plastic deformation ability, and can also inhibit the excessive formation of iron oxide scale to a certain extent.

[0041] (2) First descaling: High-pressure water descaling after the furnace, with the descaling pressure controlled at 19-20 MPa (including 19 MPa and 20 MPa).

[0042] (Previously 15-18 MPa)

[0043] (3) Second descaling: High-pressure water descaling before the roughing mill, with the descaling pressure controlled at 16-19 MPa (including 16 MPa and 19 MPa).

[0044] (Previously 14-15 MPa)

[0045] Specifically, high-pressure water descaling is a crucial process for removing iron oxide scale from the surface of steel on hot-rolled production lines. This process utilizes the mechanical impact of high-pressure water jets to wash away the iron oxide scale from the steel surface, while also achieving rapid cooling. Because water jets are used as the descaling medium, the higher the pressure, the greater the impact force of the water jet, and the better the effect of removing iron oxide scale.

[0046] Hot rolling production lines typically employ two descaling processes: post-furnace descaling and pre-roughing mill descaling. Post-furnace descaling uses a higher pressure of 19–20 MPa to remove the iron oxide scale formed on the steel in the heating furnace; while pre-roughing mill descaling uses a pressure of 16–19 MPa to remove newly formed iron oxide scale during steel transportation. This dual descaling setup ensures the smooth operation of subsequent rolling processes and improves product surface quality.

[0047] The pressure values ​​at both stations have been increased compared to before (from 15-18 MPa to 19-20 MPa for descaling after the furnace, and from 14-15 MPa to 16-19 MPa for descaling before the roughing mill). Increasing the water pressure enhances the descaling effect, thus better ensuring rolling quality. Furthermore, the slight increase will not lead to excessively high water pressure, which could cause a sharp drop in the steel surface temperature, affecting the uniformity of rolling temperature and potentially resulting in uneven rolling force, thereby impacting the dimensional accuracy of the product. Secondly, excessive impact force can easily create pits or other surface defects on the steel surface. In addition, excessively high water pressure will exacerbate nozzle wear, increase equipment maintenance costs, and significantly increase energy consumption.

[0048] (4) Rough rolling: A purging device is provided on both sides of the bottom of the rough rolling channel. The purging device includes a nozzle on each side of the bottom of each mill stand, with the water outlet direction of the nozzle pointing towards the center of the bottom surface of the rolling channel. The continuous purging action prevents iron oxide scale from accumulating in the rolling channel and avoids the accumulated iron oxide scale from being pressed into the surface of the strip steel during the rolling process, thus preventing defects.

[0049] A backflushing water device is installed at the top of the exit passage of the roughing mills E2 and E3. The backflushing water device includes a row of nozzles arranged transversely along the width of the strip. The water outlet direction of the nozzles faces the mill inlet side, and the water outlet direction of the nozzles is inclined downwards at an angle of 30 to 60 degrees (inclusive) to the horizontal plane. The nozzles of the backflushing water device are made of stainless steel, with a nozzle diameter of 8 mm, and the water spray pressure during operation is 0.8 to 1 MPa. It is used to blow away and rinse residual iron oxide scale on the surface of the strip, ensuring the cleanliness of the strip surface.

[0050] (5) Third descaling: High-pressure water descaling before the finishing mill, with the descaling pressure controlled at 19-20 MPa (including 19 MPa and 20 MPa).

[0051] (Previously 17-19 MPa)

[0052] Specifically, similar to the first and second descaling processes, the descaling effect is enhanced by slightly increasing the high-pressure water pressure, while avoiding excessive impact that could cause defects on the steel surface.

[0053] (6) Three sets of backwash water devices are installed between the finishing mills F6~F7, F7~F8, and F8~F9 to blow away the iron oxide scale on the surface of the strip.

[0054] (7) Coil the strip steel. By adjusting the flow rate and pressure of the ultra-fast cooling nozzle, the coiling temperature of the strip steel is reduced from 700-750℃ to 600-700℃ (including 600℃ and 700℃). Reducing the coiling temperature can reduce the growth of the oxide layer and improve the surface quality of the strip steel. Add a blower system to remove the surface iron oxide scale in time, prevent the iron scale from being rolled into the interlayer, and at the same time assist in the cooling of the strip steel.

[0055] During installation, adjust the height and assembly angle of the descaling nozzle in the descaling process. The water outlet direction of the nozzle should be at a 15-degree angle to the water spray pipe. When high-pressure water is ejected at a 15-degree angle, the impact force will be decomposed into two components, one perpendicular to the steel surface and the other parallel to it. The perpendicular component provides the main force for peeling off the iron oxide scale, while the parallel component helps to scrape off and transport the scale, ensuring the best descaling effect.

[0056] During routine maintenance, take advantage of downtime such as repairs to check the condition of the descaling nozzles for each process to prevent excessive wear, clogging, or detachment of the nozzles.

[0057] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling iron oxide scale on the surface of hot-rolled strip steel, characterized in that, Includes the following steps: The process includes furnace heating, post-furnace descaling, pre-roughing mill descaling, roughing, pre-finishing mill descaling, finishing mill descaling, ultra-fast cooling, and coiling; among these steps... When the heating furnace is used for heating, the temperature of the slab exiting the furnace is controlled at 1100-1200℃; The descaling pressure after the furnace is 19-20 MPa; The descaling pressure before the roughing mill is 16-19 MPa; A purging device is provided on both sides of the bottom of the rolling passage of the roughing mill, and a backflushing water device is provided at the top of the exit of the E2 and E3 mill passages of the roughing mill. The descaling pressure before the finishing mill is 19-20 MPa; A backflushing water device is installed at the top of the exit passages of the finishing mills F6, F7, and F8; The strip winding temperature is controlled at 600-700℃ during the winding process; The purging device includes a nozzle on each side of the bottom of each mill stand, with the water outlet direction of the nozzle pointing towards the center of the bottom surface of the rolling channel; The backwashing device includes a row of nozzles arranged laterally along the width of the strip, with the water outlet direction of the nozzles facing the mill inlet side, and the water outlet direction of the nozzles inclined downward at 30 to 60 degrees with the horizontal plane. The nozzle of the backwash device is made of stainless steel, with a nozzle diameter of 8mm and a water pressure of 0.8 to 1MPa during operation.

2. The method for controlling iron oxide scale on the surface of hot-rolled strip steel according to claim 1, characterized in that, The descaling process uses a descaling box, which includes a water spray pipe and a descaling nozzle. The angle between the water outlet direction of the nozzle and the water spray pipe is 15 degrees.

3. The method for controlling iron oxide scale on the surface of hot-rolled strip steel according to claim 1, characterized in that, The ultra-fast cooling process employs an ultra-fast cooling nozzle, and the strip coiling temperature is controlled by adjusting the flow rate and pressure of the ultra-fast cooling nozzle.

4. The method for controlling iron oxide scale on the surface of hot-rolled strip steel according to claim 1, characterized in that, The angle between the water outlet direction of the nozzle and the horizontal plane is 30 degrees or 60 degrees.

5. The method for controlling iron oxide scale on the surface of hot-rolled strip steel according to claim 1, characterized in that, When the heating furnace is used for heating, the temperature of the slab exiting the furnace is controlled at 1100℃ or 1200℃; when the strip is coiled, the coiling temperature is controlled at 600℃ or 700℃.

6. The method for controlling iron oxide scale on the surface of hot-rolled strip steel according to claim 1, characterized in that, The descaling pressure of the descaling after the furnace is 19 MPa or 20 MPa; the descaling pressure of the descaling before the roughing mill is 16 MPa or 19 MPa; and the descaling pressure of the descaling before the finishing mill is 19 MPa or 20 MPa.