Method for controlling oxide scale on surface of hot-rolled strip steel

Abstract

The invention discloses a method for controlling oxide scale on the surface of hot-rolled strip steel, which belongs to the technical field of hot-rolled strip steel production and comprises the steps of heating in a heating furnace, descaling behind the furnace, descaling in front of a roughing mill, rough rolling, descaling in front of a finishing mill, finish rolling, ultrafast cold treatment and coiling. When the heating furnace is used for heating, the tapping temperature of the plate blank is 1100-1200 DEG C; the descaling pressure for descaling after the furnace is 19 to 20 MPa; the descaling pressure of the front descaling of the roughing mill is 16-19 MPa; purging devices are arranged on the two sides of the bottom of a rough rolling channel, and back flushing devices are arranged on the tops of channel outlets of rough rolling E2 and E3 rolling mills; the descaling pressure for descaling in front of a finishing mill is 19-20 MPa; back flushing devices are arranged at the tops of outlets of channels of rolling mills F6, F7 and F8 for finish rolling; the coiling temperature of the strip steel is controlled to be 600-700 DEG C during coiling; the invention solves the technical problems that the vertical roll type strip steel is easy to generate regenerated scale and the traditional descaling mode is not ideal in effect.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled strip steel production, and in particular relates to a method for controlling iron oxide scale on the surface of hot-rolled strip steel. Background Art

[0002] Hot rolled strip refers to the strip steel product obtained after hot rolling of steel billets, which plays an important role in steel production. The surface quality of hot rolled strip directly affects its performance and subsequent processing effect, among which the control of iron oxide scale is the key factor affecting the 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 respectively.

[0003] At present, hot-rolled strip production lines usually use high-pressure water descaling to remove iron oxide. The specific method is to set up a descaling device before the rolling mill, and use high-pressure water to impact the surface of the steel to make the iron oxide fall off. At the same time, cooling water is also used during the rolling process to control the rolling temperature and clean the surface.

[0004] However, there are the following technical problems in the prior art:

[0005] First, it is difficult to completely remove the iron oxide scale by simply relying on the descaling device. Some of the remaining iron oxide scale will be pressed into the steel surface during the subsequent rolling process, forming a pressed-in defect;

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

[0007] Third, improper coordination of temperature and pressure parameters in each process will lead to unsatisfactory control of iron oxide scale and 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 for controlling iron oxide scale during the production process of hot-rolled strip steel. Summary of the invention

[0009] The purpose of the present invention is to provide a method for controlling iron oxide scale on the surface of hot-rolled strip to solve the technical problem of how to achieve effective control of iron oxide scale in the whole process and avoid the pressure of oxidized impurities and scratches on the surface of the strip proposed in the above background technology.

[0010] In order to achieve the above object, the present invention provides the following technical solutions:

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

[0012] When the heating furnace is heated, the slab out-of-furnace temperature is controlled at 1100-1200°C to reduce the generation of iron oxide scale;

[0013] The descaling pressure of the post-furnace descaling is 19-20MPa, which removes the iron oxide scale generated on the surface of the steel during the process of leaving the furnace;

[0014] The descaling pressure of the rough rolling mill is 16-19 MPa, which removes the iron oxide produced on the surface of the steel during transportation;

[0015] Blowing devices are arranged on both sides of the bottom of the rough rolling channel to prevent the accumulation of iron oxide scale in the rolling channel and to avoid the accumulated iron oxide scale being pressed into the surface of the strip to form defects during the rolling process; and a backwashing water device is arranged on the top of the rough rolling E2 and E3 mill channel outlets to blow and flush the iron oxide scale remaining on the surface of the strip to ensure the cleanliness of the strip surface.

[0016] The descaling pressure of the descaling 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 to cause defects on the steel surface.

[0017] A backwash device is arranged at the top of the outlet of the F6, F7 and F8 rolling mill channels for finishing rolling to blow away the iron oxide scale on the surface of the strip steel;

[0018] During the coiling, the coiling temperature of the strip steel is controlled at 600-700° C., thereby reducing the growth of the oxide layer and improving the surface quality of the strip steel.

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

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

[0021] Preferably, the purging device comprises a nozzle disposed on both sides of the bottom of each rolling mill frame, and the water outlet direction of the nozzle points to the center of the bottom surface of the rolling channel.

[0022] Preferably, the backwash water device comprises a row of nozzles arranged transversely along the width direction of the strip, the water outlet direction of the nozzles is toward the inlet side of the rolling mill, and the water outlet direction of the nozzles is inclined downward at 30 to 60 degrees to the horizontal plane.

[0023] Preferably, the nozzle of the backwash water device is made of stainless steel, the nozzle diameter is 8 mm, and the water spraying pressure during operation is 0.8-1 MPa; it can effectively flush 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 heated, the slab outlet temperature is controlled at 1100°C or 1200°C; and when the strip is coiled, the coiling temperature is controlled at 600°C or 700°C.

[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; the descaling pressure of the descaling before the finishing mill is 19 MPa or 20 MPa.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

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

[0029] Specifically, the present invention achieves the corresponding technical effects through the following technical means: by controlling the slab out-of-furnace temperature at 1100-1200°C, the amount of primary iron oxide scale generated is reduced from the source. By setting multiple descaling processes with different pressure levels (19-20MPa after the furnace, 16-19MPa before rough rolling, and 19-20MPa before finishing rolling), accurate removal of iron oxide scale at different stages is achieved. In addition, a purge device and a backwash water device are set at key positions of the roughing mill and the finishing mill, which effectively prevents the accumulation of iron oxide scale in the rolling channel and removes the residual iron scale in time, avoiding the generation of surface indentation and scratch defects. Finally, by controlling the coiling temperature at 600-700°C, the formation of regenerated iron oxide scale during the cooling process of the strip is reduced.

[0030] Through the synergistic effect of the above-mentioned multiple links, the present invention not only improves the surface quality of the strip steel and reduces the surface defect rate, but also improves production efficiency, reduces production costs and enhances product market competitiveness due to reduced rework and scrap rates. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. All other embodiments obtained by ordinary technicians in the field without making creative work based on the embodiments of the present invention shall fall 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 comprises the following steps: heating in a heating furnace, descaling after the furnace, descaling before a roughing mill, rough rolling, descaling before a finishing mill, finishing rolling, ultra-fast cooling, and coiling.

[0033] The specific operations are as follows:

[0034] (1) Slab heating: control the heating temperature according to the lower limit, and control the slab furnace temperature at 1100-1200℃ (including 1100℃ and 1200℃), and avoid forced steel burning. (Previously 1200-1250℃)

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

[0036] The slab furnace temperature is controlled at 1100-1200℃, which significantly reduces the temperature range compared to the original process (1200-1250℃), and can reduce the amount of iron oxide scale generated. Because in the process of heating steel, temperature is closely related to the generation of iron oxide scale. When the steel material is in a high temperature environment, its surface will react with oxygen in the air to generate iron oxide scale mainly composed of FeO (ferrous oxide), and a small amount 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 is significantly accelerated due to the high temperature. This is because according to the Arrhenius law, the oxidation reaction rate is exponentially related to the temperature, and the increase in temperature will cause the oxidation rate to increase sharply. The iron oxide scale formed in this temperature range is not only large in quantity, but also due to its fast growth rate, it is easy to form a loose multi-layer oxide structure, which is more likely to fall off or press into the steel surface during the subsequent rolling process, affecting the product quality.

[0038] When the slab temperature is controlled at a relatively low temperature range of 1100-1200℃, although oxidation reaction still occurs, the reaction rate is significantly reduced. Since the oxidation reaction rate is relatively slow, it is conducive to the formation of an oxide layer with a more uniform structure and higher density, reducing the risk of cracking and peeling of the oxide layer. This not only directly reduces the amount of iron oxide scale generated, but also makes the subsequent descaling process more effective.

[0039] After many experiments, it was found that when the slab temperature out of the furnace is lower than 1100℃, the temperature of the slab in the subsequent rough rolling and finishing rolling process will be lower than the austenitizing temperature (about 900℃), and some areas will be separated from the austenite zone to form a mixed crystal structure, resulting in uneven product structure and serious impact on product performance. At the same time, low-temperature rolling is also prone to produce quality defects such as cracks on the surface of the product. In order to ensure the quality of rolling, it may be necessary to reduce the rolling speed or increase the number of rolling passes, which will not only disrupt the normal production rhythm, but also reduce production efficiency; and cause the deformation resistance during the rolling process to increase sharply. At this time, the rolling force will increase significantly, which will not only aggravate the wear of the rollers and shorten the service life, but may also exceed the design load limit of the rolling mill.

[0040] Therefore, controlling the slab furnace temperature at 1100℃~1200℃ can not only ensure that the entire rolling process is carried out within the austenite range and ensure that the material has good plastic deformation ability, but also inhibit the excessive generation of iron oxide scale to a certain extent.

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

[0042] (Previously 15~18MPa)

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

[0044] (Previously 14~15MPa)

[0045] Specifically, high-pressure water descaling is an important process for removing iron oxide from the surface of steel on hot rolling production lines. This process uses the mechanical impact of high-pressure water jets to wash away the iron oxide on the surface of steel, while also achieving a rapid cooling effect. It is precisely because water jets are used as descaling media that the higher the pressure, the greater the impact of the water jet, and the better the effect of removing iron oxide.

[0046] There are usually two descaling processes in the hot rolling production line: descaling after the furnace and descaling before the roughing mill. Descaling after the furnace uses a higher pressure of 19-20MPa, which is mainly used to remove the iron oxide scale formed on the steel in the heating furnace; while descaling before the roughing mill uses a pressure of 16-19MPa, the purpose is to remove the newly generated iron oxide scale during the transportation of the steel. This double descaling setting can ensure the smooth progress of the subsequent rolling process and improve the surface quality of the product.

[0047] The pressure values ​​of these two workstations have been increased compared with before (the descaling after the furnace has been increased from 15-18MPa to 19-20MPa, and the descaling before the rough rolling mill has been increased from 14-15MPa to 16-19MPa). The descaling effect is enhanced by increasing the water pressure, thereby better ensuring the rolling quality; and a small increase will not lead to excessive water pressure. Excessive water pressure will cause the surface temperature of the steel to drop sharply, affecting the uniformity of the rolling temperature, and may cause uneven rolling force, thereby affecting the dimensional accuracy of the product; secondly, excessive impact force can easily form pits or other surface defects on the surface of the steel; in addition, excessive water pressure will aggravate nozzle wear, increase equipment maintenance costs, and also significantly increase energy consumption.

[0048] (4) Rough rolling: a purging device is provided on both sides of the bottom of the rolling channel of the rough rolling, and the purging device includes a nozzle provided on both sides of the bottom of each rolling mill frame, and the water outlet direction of the nozzle points to the center of the bottom surface of the rolling channel. The continuous purging action prevents the accumulation of iron oxide scale in the rolling channel, and prevents the accumulated iron oxide scale from being pressed into the surface of the strip during the rolling process to form defects.

[0049] A backwashing device is installed at the top of the rough rolling E2 and E3 mill channel outlets; the backwashing device includes a row of nozzles arranged transversely along the strip width direction, the water outlet direction of the nozzles is toward the mill inlet side, and the water outlet direction of the nozzles is inclined downward at 30 to 60 degrees (including 30 degrees and 60 degrees) to the horizontal plane. The nozzle of the backwashing device is made of stainless steel, the nozzle diameter is 8mm, and the water spraying pressure is 0.8 to 1MPa during operation; it is used to blow and flush the iron oxide scale remaining on the strip surface to ensure the cleanliness of the strip surface.

[0050] (5) The third descaling: high-pressure water descaling before the finishing mill, the descaling pressure is controlled at 19-20 MPa (including 19 MPa and 20 MPa).

[0051] (Previously 17~19MPa)

[0052] Specifically, similar to the first and second descaling steps, the descaling effect is enhanced by slightly increasing the high-pressure water pressure, while preventing excessive impact force from causing defects on the steel surface.

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

[0054] (7) When coiling the strip, the coiling temperature of the strip can be reduced from 700-750°C to 600-700°C (including 600°C and 700°C) by adjusting the flow rate and pressure of the ultra-fast cooling nozzle. Lowering the coiling temperature can reduce the growth of the oxide layer and improve the surface quality of the strip. A fan purge system can be added to promptly remove the surface oxide scale to prevent the scale from being drawn into the interlayer and assist in cooling the strip.

[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 an angle of 15 degrees to the water spray pipe. When high-pressure water is ejected at an angle of 15 degrees, the impact force will be decomposed into two components, one perpendicular to the surface of the steel and the other parallel to the surface of the steel. The vertical component provides the main force for stripping the oxidized iron scale, while the parallel component helps to scrape and transport the iron scale, ensuring the best descaling effect.

[0056] During routine maintenance, use downtime such as inspection to check the descaling nozzles in each process to prevent excessive wear, clogging, and falling off of the nozzles.

[0057] The above description is only 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 in the protection scope of the present invention.

Claims

1. A method for controlling iron oxide scale on the surface of hot-rolled strip, characterized in that: The following steps are involved: Heating in heating furnace, descaling after furnace, descaling before roughing mill, rough rolling, descaling before finishing mill, finishing rolling, ultra-fast cold treatment, coiling; among them, When the heating furnace is heated, the slab out-of-furnace temperature is controlled at 1100-1200°C; The descaling pressure of the post-furnace descaling is 19-20 MPa; The descaling pressure of the roughing mill before descaling is 16-19 MPa; A purge device is provided on both sides of the bottom of the rough rolling rolling channel, and a backwash device is provided on the top of the rough rolling E2 and E3 rolling mill channel outlets; The descaling pressure of the descaling before the finishing mill is 19-20MPa; A backwashing device is arranged at the top of the outlet of the F6, F7 and F8 rolling mill channels of the finishing rolling mill; During the coiling, the coiling temperature of the strip is controlled at 600-700°C.

2. The method for controlling iron oxide scale on the surface of hot-rolled strip according to claim 1, characterized in that: The descaling uses a descaling box, which includes a water spray pipe and a descaling nozzle, and 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 according to claim 1, characterized in that: The ultra-fast cooling treatment 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.

4. The method for controlling iron oxide scale on the surface of hot-rolled strip according to claim 1, characterized in that: The purging device comprises a nozzle arranged on both sides of the bottom of each rolling mill frame, and the water outlet direction of the nozzle points to the center of the bottom surface of the rolling channel.

5. The method for controlling iron oxide scale on the surface of hot-rolled strip according to claim 1, characterized in that: The backwash water device comprises a row of nozzles arranged transversely along the width direction of the strip, the water outlet direction of the nozzles is toward the inlet side of the rolling mill, and the water outlet direction of the nozzles is inclined downward at 30 to 60 degrees to the horizontal plane.

6. The method for controlling iron oxide scale on the surface of hot-rolled strip according to claim 5, characterized in that: The nozzle of the backwash water device is made of stainless steel, the nozzle diameter is 8mm, and the water spraying pressure during operation is 0.8-1MPa.

7. The method for controlling iron oxide scale on the surface of hot-rolled strip according to claim 5, characterized in that: The angle between the water outlet direction of the nozzle and the horizontal plane is 30 degrees or 60 degrees.

8. The method for controlling iron oxide scale on the surface of hot-rolled strip according to claim 1, characterized in that: When the heating furnace is heated, the slab out-of-furnace temperature is controlled at 1100° C. or 1200° C.; when the strip is coiled, the coiling temperature is controlled at 600° C. or 700° C.

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