A method for improving the edge quality of high carbon steel in thin slab continuous casting and rolling

Abstract

The application discloses a method for improving the edge quality of high-carbon steel in thin slab continuous casting and rolling, and belongs to the field of steel production, which comprises the following steps: hot metal pretreatment, converter smelting, LF furnace refining, continuous casting and continuous rolling. In the continuous casting process, the meniscus flow velocity of the crystallizer is controlled to be 0.29-0.32 m / s; the water reduction of the secondary cooling zone and the edge temperature of the cast slab out of the fan-shaped section are controlled, so that the corner temperature of the cast slab is 1100-1020 DEG C. Compared with the prior art, the method can control the occurrence of defects such as edge opening and burr in the high-carbon production process of thin slab continuous casting and rolling, and improve the edge quality.

Description

Technical Field

[0001] This invention relates to a steel production method, and more particularly to a method for improving the edge quality of high-carbon steel produced by continuous casting and rolling of thin slabs. Background Technology

[0002] Carbon steel, also known as carbon steel, can be classified into low-carbon steel (≤0.25%), medium-carbon steel (0.25%-0.6%), and high-carbon steel (>0.6%) based on its carbon content. Low-carbon steel has low strength and low hardness, making it soft. Medium and high-carbon tool steels, on the other hand, possess good strength, hardness, and toughness, excellent wear resistance, superior overall performance, and a long service life. They are mainly used to manufacture knives, measuring tools, saw blades, precision hardware tools, pen nibs, needles, etc., and are widely used in engineering machinery, textiles, electronics, automobiles, and other fields.

[0003] With the development of technology, the production of high carbon steel by continuous casting and rolling of thin slabs has become the mainstream. For example, the method disclosed in "Method for producing thin-gauge high carbon steel based on ESP thin slab continuous casting and rolling process" (CN201611258861.5) includes hot metal pretreatment, converter smelting, LF furnace refining, ESP continuous casting and rolling, and cooling, wherein the thickness of the slab is 90mm to 110mm.

[0004] However, this process exhibits two types of edge quality defects: notches and burrs on the strip edges. These defects are related to the original defects in the cast slab during the process. Therefore, a method is urgently needed to improve the edge quality of high-carbon steel in thin slab continuous casting and rolling, thereby eliminating these defects and improving product quality. Summary of the Invention

[0005] The technical objective of this invention is to address the shortcomings of the prior art by providing a method for improving the edge quality of high-carbon steel produced by continuous casting and rolling of thin slabs, thereby controlling the occurrence of defects such as edge notches and burrs during the continuous casting and rolling process of high-carbon steel and improving edge quality.

[0006] The technical solution of this invention to solve its technical problem is: a method for improving the edge quality of high carbon steel in thin slab continuous casting and rolling, including molten iron pretreatment, converter smelting, LF furnace refining, continuous casting, and continuous rolling, characterized in that: in the continuous casting process, the flow velocity of the meniscus of the crystallizer is controlled at 0.29-0.32 m / s; the water reduction in the secondary cooling zone and the edge temperature of the slab in the fan-shaped section are controlled so that the corner temperature of the slab is 1100-1020℃.

[0007] Furthermore, in the aforementioned continuous casting process, a high-carbon protective slag is used. The composition of the high-carbon protective slag is as follows: BaS 0.88–0.91%; SiO2 25–27%; CaO 23.5–24.5%; MgO 4–5%; Al2O3 4–5%; Fe2O3 0.4–0.5%; MnO2 0.07–0.1%; Na2O 10–12%; K2O 0.16–0.25%; F 10–11%; Li2O 2.8–3.2%; C-tot 7.6–7.9%; and moisture 0.06–0.08%.

[0008] Furthermore, the density of the above-mentioned high-carbon protective slag is 0.7-0.75 g / ml; melting point is 840-860℃; and viscosity is 0.9-1.0 Poise.

[0009] Furthermore, in the aforementioned continuous casting process, the temperature of the billet edge is controlled by reducing the cooling at the edge of the billet and by using auxiliary heating at the edge of the billet. This reduces the water content in the secondary cooling zone and controls the temperature of the billet edge in the fan-shaped section.

[0010] Furthermore, the above-mentioned method of reducing the cooling of the billet edge is to use an eccentric nozzle at the edge of the billet in the curved section.

[0011] Furthermore, the aforementioned auxiliary heating of the billet edge is achieved by adding a flame heater at the casting machine outlet.

[0012] Furthermore, the temperature of the cast billet before entering the rough rolling mill is ≥990℃.

[0013] Compared with the prior art, the present invention has the following outstanding advantages:

[0014] 1. The method of the present invention improves the performance of the protective slag by increasing the meniscus flow rate, thereby achieving good lubrication between the billet and the crystallizer, reducing the friction between the narrow copper plate and the billet, and eliminating the edge notch defect of high carbon steel.

[0015] 2. The method of the present invention increases the temperature of the straightening section of the billet, fully avoiding the third brittle zone. After the temperature is increased by the heater, the plasticity of the billet is improved, and the burr defects on the edge of the strip are eliminated. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the eccentric nozzle outlet of the present invention.

[0017] Figure 2 This is a schematic diagram of the nozzle outlet of existing technology.

[0018] Figure 3 This is a schematic diagram of the curved section nozzle layout of the present invention.

[0019] Figure 4 This is a schematic diagram of the layout of the flame heater at the casting machine outlet of the present invention. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0021] For the purposes of the following detailed description, it should be understood that the invention may take various alternative variations and sequences of steps unless explicitly stated otherwise. Furthermore, except in any operational instance or where otherwise indicated, all figures representing quantities of ingredients as used, for example, in the specification and claims, should in all cases be understood to be modified by the term “about.” At least, and without attempting to limit the application of the principle of equivalents to the scope of the claims, each numerical parameter should be understood at least according to the number of significant figures reported and by applying common rounding techniques.

[0022] Although the numerical ranges and parameters illustrating the broad scope of the invention are approximate, the values ​​described in the specific examples are reported as precisely as possible. However, any numerical value inherently contains some error that is necessarily caused by the standard deviation found in its corresponding test measurement.

[0023] It should also be understood that any range of values ​​stated herein is intended to include all subranges included therein. For example, the range “1 to 10” is intended to include all subranges between (and including) the stated minimum value of 1 and the stated maximum value of 10, that is, a minimum value equal to or greater than 1 and a maximum value equal to or less than 10.

[0024] In this application, unless otherwise specified, the use of the singular includes the plural and the plural encompasses the singular. Additionally, in this application, unless otherwise expressly stated, the use of "or" means "and / or," even if "and / or" may be explicitly used in certain circumstances. Furthermore, in this application, unless otherwise specified, the use of "a" or "an" means "at least one / a." For example, "an" first material, "an" coating composition, etc., refer to one or more of any of these items.

[0025] This invention relates to a method for improving the edge quality of high-carbon steel in thin slab continuous casting and rolling. The process flow is as follows: hot metal pretreatment, converter smelting, LF furnace refining, continuous casting, continuous rolling, and slow cooling storage.

[0026] Based on the above process flow, the following controls were implemented in the continuous casting process:

[0027] I. Improve the performance of protective slag

[0028] The rigid connection of continuous casting and rolling of thin slabs means that the original defects of the slab or low temperature can be inherited by the strip. In the production of high carbon steel, the low casting speed and slow flow rate of the meniscus of the crystallizer lead to poor slag formation, which in turn affects the lubrication performance of the protective slag. The frictional resistance between the slab and the crystallizer increases, causing micro-cracks to form at the corners and narrow faces of the slab. After rolling, small gaps are formed on the edges of the strip.

[0029] High-carbon protective slag is used to improve its performance. The composition of the high-carbon protective slag is as follows: BaS 0.88–0.91%; SiO2 25–27%; CaO 23.5–24.5%; MgO 4–5%; Al2O3 4–5%; Fe2O3 0.4–0.5%; MnO2 0.07–0.1%; Na2O 10–12%; K2O 0.16–0.25%; F 10–11%; Li2O 2.8–3.2%; C-tot 7.6–7.9%; moisture 0.06–0.08%. Density 0.7–0.75 g / ml; melting point 840–860℃; viscosity 0.9–1.0 Poise. The above composition design ensures the melting of the protective slag and controls the slag film thickness within the range of 0.139-0.152mm, thus providing good lubrication between the billet and the crystallizer.

[0030] II. Dynamically Adjusting the EMBr Current

[0031] In addition to improving the performance of the protective slag, the lubrication can also be improved by dynamically adjusting the EMBr current in combination with the SEN insertion depth change, thereby increasing the meniscus velocity and controlling the crystallizer meniscus velocity at 0.29-0.32 m / s.

[0032] The specific method is as follows:

[0033] (1) Determine the electromagnetic braking (EMBr) current setting value

[0034] The EMBr current setting value is determined based on the pulling speed.

[0035] In this embodiment, the relationship between the pulling speed and the EMBr current setting is as follows:

[0036] Pulling speed (m / min) 3.5 4.0 4.5 5.0 5.5 EMBr current setting (A) 115.3 131.96 149.96 166.62 184.45

[0037] (2) Measure the flow velocity at the meniscus of the crystallizer under different currents of EMBr.

[0038] The measurement method was the nail plate method, and the EMBr current adjustment range was increased from -60A to +10A.

[0039] The formula for calculating the flow velocity at the meniscus of the crystallizer is:

[0040]

[0041] In the formula: φlump is the diameter of the steel block, in mm;

[0042] hlump is the height of the steel block, in mm;

[0043] v s denoted as molten steel flow velocity, in m / s.

[0044] In this embodiment, specifically: 80 sets of nail plates are prepared and the EMBr current is measured from -60A to +10A respectively. The flow velocity corresponding to the meniscus is calculated using the above formula.

[0045] (3) Determine the EMBr current adjustment amount

[0046] Based on the results of the flow velocity at the meniscus of the crystallizer under different EMBr currents measured by the nail plate method, the relationship between the change in the insertion depth of the crystallizer submersible nozzle (SEN) and the adjustment of the EMBr current was established.

[0047] In this embodiment, the relationship between the EMBr current adjustment and the SEN insertion depth change during the production process is shown in the table below:

[0048]

[0049]

[0050] (4) Dynamically adjust the current reduction

[0051] Based on the EMBr current setting, the current reduction is dynamically adjusted according to the change in SEN insertion depth.

[0052] By using EMBr current control measures, the flow velocity at the meniscus of the crystallizer was increased from 0.18-0.25 m / s to 0.29-0.32 m / s.

[0053] In the optimization scheme, the existing EMBr core edge pads are removed to reduce the electromagnetic braking effect at the narrow face position.

[0054] III. Temperature Control at the Edge of the Cast Billet

[0055] Lower drawing speeds prolong the cooling time of the billet within the fan-shaped section, resulting in lower edge temperatures and reduced plasticity of the billet. This leads to the formation of edge burrs on the strip after rolling.

[0056] In the continuous casting process, steel is poured from the ladle into the tundish, and then from the tundish into the crystallizer. The crystallizer forms a billet with a liquid core, which then passes through a fan-shaped secondary cooling zone to form a fully solidified billet. This invention controls the water reduction in the secondary cooling zone and the temperature of the billet edge exiting the fan-shaped section by reducing edge cooling and auxiliary heating of the billet edge, thus maintaining the corner temperature of the billet at 1100-1020℃.

[0057] (1) Reduce the cooling of the billet edge

[0058] Specifically, this can be achieved by using an eccentric nozzle 2 at the edge of the curved section of the billet, such as... Figure 1 As shown, the existing full-diameter symmetrical nozzle 3 is optimized into an eccentric nozzle 2, and the outlet 1 of the eccentric nozzle 2 becomes the full-diameter symmetrical nozzle 3 (as shown). Figure 2 As shown, half of the water outlet, i.e., the impact area, is reduced to half of its original size. Spraying water only on one side can reduce the cooling of the billet edge and also provide cooling protection for the fan-shaped roller section. The eccentric nozzles 2 are distributed as follows: Figure 3 As shown, the nozzle outlet faces away from the casting billet 4.

[0059] (2) Auxiliary heating of the billet edge

[0060] Specifically, it could be: such as Figure 4 As shown, a flame heater 7 is added at the outlet of the casting machine, specifically between the roughing mill 5 and the fan-shaped section 6, on both sides of the billet 4. Natural gas and oxygen are used for heating, and the maximum flame temperature can reach 2300℃, which can increase the temperature of the edge of the billet 4 by 20-30℃, so that the temperature of the billet before entering the roughing mill is ≥990℃, thereby improving the plasticity of the billet.

[0061] By weakening edge cooling and auxiliary heating of the billet edge, the billet temperature avoids the third brittle zone before the straightening section, and the billet corner temperature is increased from 910-930℃ in the existing technology to 1000-1020℃.

[0062] The present invention will be further described below with reference to specific embodiments.

[0063] Example 1 uses high-carbon protective slag to increase lubrication and improve meniscus flow rate, without dynamically adjusting the current based on changes in SEN insertion depth, and controls the billet edge temperature to 1000-1020℃.

[0064] Example 2 uses existing technology to protect the slag, and dynamically adjusts the EMBr current to increase the meniscus flow rate, while controlling the edge temperature of the billet to 1000-1020℃.

[0065] Example 3 is a comprehensive solution that uses high-carbon protective slag combined with changes in SEN insertion depth to dynamically adjust the EMBr current to increase the meniscus flow rate, and controls the edge temperature of the billet to 1000-1020℃.

[0066] Comparative experiments were conducted to better compare this application with existing technologies.

[0067] The control group used existing protective slag technology and the same EMBr current setting value as in each embodiment, without dynamic current adjustment based on changes in SEN insertion depth; no additional temperature control was performed on the corner of the billet, and the temperature was measured at 910-930℃.

[0068] The composition of the protective flux used in each group is shown in the table below:

[0069]

[0070] The properties of the protective flux used in each group are shown in the table below:

[0071] Grouping Density (g / ml) Melting point (°C) Viscosity Example 2, Control Group 0.70 878 1.05 Example 1, Example 3 0.75 852 0.96

[0072] The differences in process parameters among the groups are shown in the table below:

[0073] Grouping Meniscus velocity in the crystallizer (m / s) Slab edge temperature (°C) control group 0.19 920 Example 1 0.30 1019 Example 2 0.29 1006 Example 3 0.32 1010

[0074] The results show:

[0075] Examples 1 and 2, employing different control methods, both achieved a significantly increased meniscus flow rate in the crystallizer compared to the control group without a control strategy. Example 3, using two control methods, resulted in an even more pronounced increase. Examples 1-3, by increasing the meniscus flow rate, ensured good lubrication between the billet and the crystallizer, reducing friction between the narrow-face copper plate and the billet, and preventing the appearance of high-carbon steel edge notches. In contrast, the control group still exhibited high-carbon steel edge notches, with a defect rate of 13.45%.

[0076] In Examples 1-3, by increasing the temperature of the straightening section of the billet, the third brittle zone was effectively avoided, resulting in improved billet plasticity and the absence of burr defects on the strip edges. In contrast, the control group still exhibited burr defects on the strip edges, with a defect rate of 24.67%.

[0077] It should be noted that the specific embodiments of the present invention have been described in detail. For those skilled in the art, various obvious changes made to it without departing from the spirit and scope of the present invention are within the protection scope of the present invention.

Claims

1. A method for improving the edge quality of high-carbon steel in thin slab continuous casting and rolling, comprising hot metal pretreatment, converter smelting, LF furnace refining, continuous casting, and continuous rolling, characterized in that: In the continuous casting process, the flow velocity at the meniscus of the crystallizer is controlled at 0.29-0.32 m / s; the water reduction in the secondary cooling zone and the temperature of the billet edge in the fan-shaped section are controlled to ensure that the corner temperature of the billet is 1100-1020℃; in the continuous casting process, a high-carbon protective slag is used, the composition of which is: BaS 0.88-0.91%; SiO2 25-27%; CaO 23.5-24.5%; MgO 4-5%; Al2O3 4-5%; Fe2O3 0.4-0.5%; MnO2 0.07-0.1%; Na2O 10-12%; K2O 0.16-0.25%; F 10-11%; Li2O 2.8-3.2%; C-tot 7.6-7.9%; and moisture 0.06-0.08%.

2. The method for improving the edge quality of high-carbon steel in continuous casting and rolling of thin slabs according to claim 1, characterized in that: The high-carbon protective slag has a density of 0.7–0.75 g / ml, a melting point of 840–860 °C, and a viscosity of 0.9–1.0 Poise.

3. The method for improving the edge quality of high-carbon steel in continuous casting and rolling of thin slabs according to claim 1, characterized in that: In the continuous casting process, the temperature of the billet edge in the secondary cooling zone and the exit fan-shaped section is controlled by reducing the cooling of the billet edge and the auxiliary heating of the billet edge.

4. The method for improving the edge quality of high-carbon steel in continuous casting and rolling of thin slabs according to claim 1, characterized in that: To reduce cooling at the edge of the billet, eccentric nozzles are used at the edge of the billet in the curved section.

5. The method for improving the edge quality of high-carbon steel in continuous casting and rolling of thin slabs according to claim 1, characterized in that: The auxiliary heating of the billet edge is achieved by adding a flame heater at the outlet of the casting machine.

6. The method for improving the edge quality of high-carbon steel in continuous casting and rolling of thin slabs according to claim 5, characterized in that: The temperature of the billet before entering the rough rolling mill is ≥990℃.

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