A method for controlling the casting of a low alloy series of steel grades

Abstract

The application discloses a control method for low-alloy series steel casting, which utilizes formulas to calculate the length and starting position of the transfer blank, and utilizes system information transmission to ensure the whole-process process adjustment, so that the performance of the final product can be effectively controlled. The method can effectively reduce the product loss in the whole-process production of the low-alloy steel, improve the product benefit, realize the performance control of the transfer blank without increasing the product cost and the control and quality risk, make the transfer blank meet the performance of the original grade product, be suitable for the conventional thickness specification product with large market demand, achieve the normal sales purpose, solve the problem of the transfer blank reduction, and improve the overall benefit of the low-alloy series product.

Description

Technical Field

[0001] This invention relates to a method for casting billets, and more particularly to a method for controlling the mixed casting of low-alloy steel grades. Background Technology

[0002] Lightweighting of automobiles can effectively improve the energy efficiency of automobiles and promote energy conservation and emission reduction. The application of high-strength steel achieves a balance between lightweighting and safety. Low-alloy high-strength steel, by adding Nb and Ti elements, utilizes fine grain strengthening and precipitation strengthening effects to give the steel advantages such as high yield strength ratio, good formability, weldability, and wear resistance. It is one of the most widely used high-strength steels in the current automotive industry and is widely used in body structural parts, reinforcement parts, seat rails, and other parts.

[0003] Currently, in response to the different application requirements of the vehicle body, low alloy high strength steel has achieved a coverage of 260-800MPa. The large number of steel grades and complex product specifications often lead to steel companies needing to cast different grades together in the same batch during steelmaking, which in turn produces mixed billets with different steel grades. Usually, such billets are downgraded to ordinary plates, resulting in a waste of alloy costs.

[0004] For steel companies, the problem of mixing different steel grades is common. Various solutions have been proposed to address this issue. For example, Chinese patent CN110961587A provides a "method for isolating mixed-grade molten steel to reduce the length of mixed-grade billets". This method is basically the same as the method for mixing steel grades with ferrules. It requires lowering the molten steel volume in the tundish and stopping pouring while adding isolation cold material (device) to the crystallizer to reduce the length of the mixed-grade billets. This method lowers the tundish liquid level, and subsequent pouring in the ladle will cause a sharp change in the flow field inside the tundish, making it difficult to control the cleanliness of the molten steel. The operation is also relatively complex. Improper control or excessive operation time will lead to thickening of the billet shell inside the crystallizer, affecting the control of the fan-shaped section and posing a risk of steel leakage or abnormal shutdown. Chinese patent CN106475544A discloses a "control method for continuous casting of dissimilar steel grades." This method aims to reduce the tonnage of the mixed casting transfer billet by adjusting the casting speed before and after mixing dissimilar steel grades. However, this speed adjustment affects the liquid level fluctuation in the crystallizer, increasing the risk of quality defects such as inclusions and bubbles in the cast billet, thus affecting the surface quality of the product. Furthermore, all of the above methods affect the continuous casting production rhythm and reduce production efficiency. Although they reduce the amount of transfer billets, they do not provide a reasonable application method for the transfer billets. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for controlling the mixed casting of low alloy steel grades, so as to effectively control the performance of the final product.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: Calculate the length L of the single-flow mixed casting junction blank according to the following formula (Ⅰ):

[0007] (I)

[0008] In the formula, W represents the mass of molten steel in the impact zone of the continuous casting ladle, in tons.

[0009] ρ—Slab density, t / m³ 3 ;

[0010] a— is the length of the narrow face of the cast billet, in meters;

[0011] b—is the length of the wide side of the billet, in meters;

[0012] n—the number of continuous casting machines corresponding to the tundish;

[0013] The mass W of molten steel in the impact zone of the continuous casting ladle in formula (Ⅰ) is calculated using the following formula (Ⅱ):

[0014] (II)

[0015] w0—Width of the bottom of the middle package, in meters;

[0016] θ—Angle between the sidewall of the liner and the vertical plane, °;

[0017] h—Liquid level in the intermediate ladle when the main ladle is poured, in meters;

[0018] l—Distance between the retaining walls on both sides of the drain outlet of the large package, in meters;

[0019] ρ0—density of molten steel, t / m³ 3 .

[0020] Furthermore, the starting position of the transfer blank is the time t after the ladle is opened for casting, and t is calculated using the following formula (Ⅲ):

[0021] (III)

[0022] W1—The mass of molten steel in the tundish at the start of pouring;

[0023] W—Millage of molten steel in the impact zone of the continuous casting ladle, in tons;

[0024] n—the number of continuous casting machines corresponding to the tundish;

[0025] ρ0 — Density of molten steel, t / m³ 3 ;

[0026] a— is the length of the narrow face of the cast billet, in meters;

[0027] b—is the length of the wide side of the billet, in meters;

[0028] v—continuous casting speed, m / min.

[0029] Furthermore, the steel grade is continuously cast using a full ladle mixing method, and the casting speed is not reduced during the mixing process.

[0030] Furthermore, the steel grade is divided into a level for every 40MPa increase in the lower limit of yield strength. The cross-cutting billets of adjacent levels are determined to be the steel grade of that furnace based on the furnace batch. Cross-cutting billets that are mixed and cast across two or more levels are downgraded and are not determined to be the original grade.

[0031] Furthermore, when hot rolling is carried out on the transfer billet of the original steel grade, the original steel grade process is used for production. The hot rolling of the first mixed billet adopts the hot coil box straight-through mode, and the hot rolling of the second mixed billet adopts the hot coil box coiling mode.

[0032] Furthermore, when the transfer billet is determined to be of the original steel grade and is being annealed as a transfer billet mixed with a lower grade, the annealing speed is increased by 8 to 13 m / min in the length range of 0.22 to 0.30 L from the beginning of the transfer billet to the corresponding billet, and the annealing speed is increased by 25 to 35 m / min for the remaining part.

[0033] When annealing a transfer billet that is determined to be of the original steel grade and is being mixed with a higher grade transfer billet, the annealing process for the non-transfer billet position shall follow the original steel grade. The annealing speed shall be reduced by 8 to 13 m / min in the 0.22 to 0.30 L length range from the start of the transfer billet to the corresponding billet, and the annealing speed shall be reduced by 25 to 35 m / min for the remaining part.

[0034] The beneficial effects of adopting the above technical solution are as follows: This invention uses formulas to calculate the length and starting position of the transfer billet, and utilizes system information transmission to ensure process adjustment throughout the entire process, effectively controlling the performance of the final product. This invention further controls full-ladle casting in continuous casting without adjusting the billet casting speed, which is beneficial for the stable control of the flow field and liquid level in the continuous casting process, avoiding billet quality risks. This invention further provides rules for judging mixed-cast billets, providing a basis for optimizing production scheduling. Combined with the use of a hot-rolled width-fixing mill, it can minimize the risk of mixed-cast billet rejection. Utilizing the hot-rolled coil box to unify the transition direction of the transfer billet composition facilitates process control in the annealing process, reducing the risk of process control anomalies.

[0035] In summary, the present invention can effectively reduce product losses throughout the entire production process of low alloy steel grades, thereby improving product efficiency. It can achieve performance control of transfer billets without increasing product costs and control or quality risks, enabling transfer billets to meet the performance requirements of the original grade products and be suitable for conventional thickness specifications with high market demand, thus achieving normal sales. This solves the problem of transfer billet downgrades and improves the overall efficiency of low alloy series products. Detailed Implementation

[0036] The present invention will now be described in further detail with reference to specific embodiments.

[0037] The control method for mixed casting of low-alloy steel grades applies to full-lap mixed casting of cold-rolled products. The following rules for mixed casting and billet determination are adopted: Based on the cold-rolled grade, each 40MPa increase in the lower limit of yield strength defines a new grade. The interleaving billets between adjacent grades are determined to be the original steel grade of that heat. Two interleaving billets are generated per flow during the mixed casting process. For interleaving billets mixed with lower grades, an "L" is added to the end of the grade determination; for interleaving billets mixed with higher grades, an "H" is added to the end of the grade determination. Interleaving billets spanning two or more grades are downgraded and not considered part of the original grade.

[0038] The control method for mixed casting of this low-alloy steel series includes continuous casting, hot rolling, cold rolling, and annealing processes. After continuous casting, the transfer billet determined to be of the original steel grade undergoes subsequent hot rolling, cold rolling, and annealing processes. The processes for each step are described below:

[0039] (1) Continuous casting: Full ladle mixing is adopted, and the continuous casting speed is not reduced during mixing; based on the calculated transfer billet length and the actual cutting situation of the continuous casting billet, the actual transfer billet length and the total length L of the transfer billet involved in the cut billet are noted in the system. The starting position of the transfer billet is the time t after the ladle starts casting.

[0040] The length L of the single-flow mixed casting joint blank is calculated using the following formula (Ⅰ):

[0041] (I)

[0042] In the formula, W represents the mass of molten steel in the impact zone of the continuous casting ladle, in tons.

[0043] ρ—Slab density, t / m³ 3 Using 7.8t / m 3 calculate;

[0044] a— is the length of the narrow face of the cast billet, in meters;

[0045] b—is the length of the wide side of the billet, in meters;

[0046] n—the number of continuous casting machines corresponding to the tundish;

[0047] The mass W of molten steel in the impact zone of the continuous casting ladle in formula (Ⅰ) is calculated using the following formula (Ⅱ):

[0048] (II)

[0049] w0—Width of the bottom of the middle package, in meters;

[0050] θ—Angle between the sidewall of the liner and the vertical plane, °;

[0051] h—Liquid level in the intermediate ladle when the main ladle is poured, in meters;

[0052] l—Distance between the retaining walls on both sides of the drain outlet of the large package, in meters;

[0053] ρ0—density of molten steel, t / m³ 3 Using 7.0t / m 3 calculate.

[0054] The starting position of the transfer blank is the time t after the large ladle is poured, and t is calculated using the following formula (Ⅲ):

[0055] (III)

[0056] W1—The mass of molten steel in the tundish at the start of pouring;

[0057] W—Millage of molten steel in the impact zone of the continuous casting ladle, in tons;

[0058] n—the number of continuous casting machines corresponding to the tundish;

[0059] ρ0 — Density of molten steel, t / m³ 3 Using 7.0t / m 3 calculate;

[0060] a— is the length of the narrow face of the cast billet, in meters;

[0061] b—is the length of the wide side of the billet, in meters;

[0062] v—continuous casting speed, m / min.

[0063] (2) Hot rolling: When the billet is determined to be the original steel grade, the original steel grade process is used for hot rolling. The hot rolling of the first mixed billet adopts the hot coil box straight-through mode, and the hot rolling of the second mixed billet adopts the hot coil box coiling mode. The purpose of the coil box is not to control the hot rolling process, but to realize the exchange of the head and tail of the billet through the coil box. This is conducive to the consistency of the cold rolling process adjustment and reduces the instability of human identification.

[0064] (3) Cold rolling: The thickness of the cold-rolled coils rolled to the finished thickness in the cold rolling process should be controlled at 1.0 to 1.6 mm after rolling, based on the above requirements for the original steel grade of the transfer billet.

[0065] (4) Annealing: When the transfer billet is determined to be of the original steel grade and is a transfer billet mixed with a lower grade, i.e., the grade is followed by L, the annealing speed is increased by 8 to 13 m / min for the length range of 0.22 to 0.30 L from the start of the transfer billet to the corresponding billet, and preferably by 10 m / min for the length range of 0.25 L. The annealing speed for the remaining part is increased by 25 to 35 m / min, and preferably by 30 m / min. The L in 0.25 L is the length of the single-flow mixed-cast transfer billet.

[0066] When annealing a transfer billet of the original steel grade that is being mixed with a higher grade billet (i.e., the grade ends with an "H"), the annealing process for the non-transfer billet position follows the original steel grade. The annealing speed is reduced by 8-13 m / min for the length range of 0.22-0.30L from the start of the transfer billet to the corresponding billet, preferably by 10 m / min for the length range of 0.25L. The annealing speed is reduced by 25-35 m / min for the remaining part, preferably by 30 m / min. The "L" in "0.25L" represents the length of the single-flow mixed-cast transfer billet.

[0067] Example 1: The specific control method for mixed casting of this low alloy series steel grades is as follows.

[0068] The 260MPa and 300MPa products were mixed and cast. The casting sequence and composition control of the two furnaces are shown in Table 1. The angle between the side wall of the tundish and the vertical plane was 12.1°, the distance between the retaining walls was 4.2m, the bottom width of the tundish was 1.0m, and the liquid level height of the tundish at the start of the ladle casting was 1.06m. The mass W of the molten steel in the impact zone was calculated using formula (II):

[0069] ;

[0070] The two-strand casting cross-section, i.e., the width of the billet, is 1.35m long, and the thickness, i.e., the narrow side of the billet, is 0.23m long. The length L of the mixed-cast junction billet is calculated according to formula (Ⅰ):

[0071] ;

[0072] When the ladle is opened for casting, the mass of molten steel in the tundish is 68.2t, the number of continuous casting machines corresponding to the tundish is 2, and the continuous casting speed is 1.4m / min. The starting position of the transfer billet is the time t after the ladle is opened for casting. According to formula (III), t is calculated as follows:

[0073]

[0074] The actual blank cutting situation, product thickness, annealing production process, and corresponding position performance control are shown in Table 2.

[0075] Example 2: The specific control method for mixed casting of this low alloy series steel grades is as follows.

[0076] The 380MPa and 340MPa products were mixed and cast. The casting sequence and composition control of the two furnaces are shown in Table 1. The angle between the side wall of the tundish and the vertical plane was 12.1°, the distance between the retaining walls was 4.2m, the bottom width of the tundish was 1.0m, and the liquid level height of the tundish at the start of the ladle casting was 1.12m. The mass of molten steel W in the impact zone was calculated using the formula:

[0077] ;

[0078] The cross-section (length of the wide side of the billet) of both castings is 1.45m, and the thickness (length of the narrow side of the billet) is 0.23m. Calculate the length L of the joint casting section using the formula:

[0079] ;

[0080] When the ladle is opened for casting, the mass of molten steel in the tundish is 66.5t, the number of continuous casting machines corresponding to the tundish is 2, and the continuous casting speed is 1.3m / min. The starting position of the transfer billet is the time t after the ladle is opened for casting. According to formula (III), t is calculated as follows:

[0081] ;

[0082] The actual blank cutting situation, product thickness, annealing production process, and corresponding position performance control are shown in Table 2.

[0083] Example 3: The specific control method for mixed casting of this low alloy series steel grades is as follows.

[0084] The 420MPa and 460MPa products were mixed and cast. The casting sequence and composition control of the two furnaces are shown in Table 1. The angle between the side wall of the tundish and the vertical plane was 12.1°, the distance between the retaining walls was 4.2m, the bottom width of the tundish was 1.0m, and the liquid level height in the tundish at the start of the ladle casting was 1.08m. The mass of molten steel W in the impact zone was calculated using the formula:

[0085] ;

[0086] The cross-section (length of the wide side of the billet) of both castings is 1.10m, and the thickness (length of the narrow side of the billet) is 0.23m. Calculate the length L of the joint casting section using the formula:

[0087] ;

[0088] When the ladle is opened for casting, the mass of molten steel in the tundish is 64.3t, the number of continuous casting machines corresponding to the tundish is 2, and the continuous casting speed is 1.45m / min. The starting position of the transfer billet is the time t after the ladle is opened for casting. According to formula (III), t is calculated as follows:

[0089] ;

[0090] The actual blank cutting situation, product thickness, annealing production process, and corresponding position performance control are shown in Table 2.

[0091] Table 1 Composition and casting sequence of each mixed-casting furnace in various embodiments

[0092]

[0093] Table 2. Process and performance control of each embodiment

[0094]

[0095] Note: The position of the billet corresponding to the finished product inspection is calculated from the end with normal composition.

Claims

1. A method for controlling the mixed casting of low-alloy steel grades, characterized in that, Calculate the length L of the single-flow mixed casting joint blank according to the following formula (Ⅰ): （Ⅰ） In the formula, W represents the mass of molten steel in the impact zone of the continuous casting ladle, in tons. ρ—Slab density, t / m³ 3 ; a— is the length of the narrow face of the cast billet, in meters; b—is the length of the wide side of the billet, in meters; n—the number of continuous casting machines corresponding to the tundish; The mass W of molten steel in the impact zone of the continuous casting ladle in formula (Ⅰ) is calculated using the following formula (Ⅱ): （Ⅱ） w0—Width of the bottom of the middle package, in meters; θ—Angle between the sidewall of the liner and the vertical plane, °; h—Liquid level in the intermediate ladle when the main ladle is poured, in meters; l—Distance between the retaining walls on both sides of the drain outlet of the large package, in meters; ρ0—density of molten steel, t / m³ 3 ; The steel grades are divided into a level for every 40MPa increase in the lower limit of yield strength. The transfer billets of adjacent mixed-casting grades are determined to be the steel grade of that furnace based on the furnace batch. Transfer billets that are mixed-casting across two or more levels are downgraded and are not determined to be the original grade. When annealing a transfer billet that is determined to be of the original steel grade and is a transfer billet mixed with a lower grade, the annealing speed in the range of 0.22 to 0.30L from the beginning of the transfer billet to the corresponding billet should be increased by 8 to 13 m / min, and the annealing speed in the remaining part should be increased by 25 to 35 m / min. When annealing a transfer billet that is determined to be of the original steel grade and is being mixed with a higher grade transfer billet, the annealing process for the non-transfer billet position shall follow the original steel grade. The annealing speed shall be reduced by 8 to 13 m / min in the 0.22 to 0.30 L length range from the start of the transfer billet to the corresponding billet, and the annealing speed shall be reduced by 25 to 35 m / min for the remaining part.

2. The method for controlling the mixed casting of low-alloy steel grades according to claim 1, characterized in that: The starting position of the transfer blank is the time t after the ladle is opened for casting, and t is calculated using the following formula (Ⅲ): （Ⅲ） W1—The mass of molten steel in the tundish at the start of pouring; W—Millage of molten steel in the impact zone of the continuous casting ladle, in tons; n—the number of continuous casting machines corresponding to the tundish; ρ0 — Density of molten steel, t / m³ 3 ; a— is the length of the narrow face of the cast billet, in meters; b—is the length of the wide side of the billet, in meters; v—continuous casting speed, m / min.

3. The method for controlling the mixed casting of low-alloy steel grades according to claim 1, characterized in that: The steel grade is continuously cast using a full ladle mixing method, and the casting speed is not reduced during the mixing process.

4. The method for controlling the mixed casting of low-alloy steel grades according to claim 1, characterized in that: When the transfer billet determined to be of the original steel grade is hot rolled, the original steel grade process is used for production. The hot rolling of the first mixed billet adopts the hot coil box straight-through mode, and the hot rolling of the second mixed billet adopts the hot coil box coiling mode.

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