A method for controlling the microstructure of a low-alloy high-strength steel continuous casting billet

Abstract

The present application relates to a kind of low-alloy high-strength steel continuous casting billet organization regulation method, belong to continuous casting billet hot charging technical field.The method includes: the continuous casting billet just out of furnace is sent into sealed heat sending channel by foot roller, entrance and exit are installed the elastic sealing curtain of high-temperature resistant rubber material, side wall uses heat preservation cotton heat preservation, and nitrogen is used as protective atmosphere;Control channel temperature is 900~920 ℃, and heat preservation 1.2~1.8 h;Continuous casting billet temperature is reduced to 630~670 ℃, and heat preservation 0.8~1.2 h;Natural cooling, rolling or offline stack cooling.The present application is by optimizing heat sending temperature control system, while reducing the quantity of iron oxide skin, using continuous casting billet waste heat to eliminate as-cast defect, improve continuous casting billet quality, to improve steel organization uniformity and processing formability, suitable for batch production of heat sending hot charging technology.

Description

Technical Field

[0001] This invention relates to a method for controlling the microstructure of low-alloy high-strength steel continuous casting billets, belonging to the technical field of hot charging and hot delivery of continuous casting billets. Background Technology

[0002] Low-alloy high-strength steel is a type of steel strengthened through microalloying and controlled rolling and cooling processes. It can replace ordinary carbon steel in thinner dimensions and can be used in construction, engineering machinery, energy equipment, and other fields. It is one of the most widely used structural steels in the industrial sector. Hot-feeding and hot-charging of continuously cast billets is a production process shortening technology that efficiently connects continuous casting and rolling processes. The core is that after continuous casting, the billet is directly fed into a heating furnace or rolling mill for rolling via hot-feeding rollers and a sealed insulation cover.

[0003] Currently, three core problems constrain the hot-feeding and hot-charging technology for low-alloy high-strength steel. First, in existing hot-feeding processes, the high-temperature continuously cast billet comes into direct contact with air, generating a large amount of iron oxide scale on its surface. To avoid the unevenly deposited and loosely structured iron oxide scale affecting product quality, subsequent cleaning treatments (such as grinding and pickling) are performed. This leads to the loss of effective steel matrix, increasing raw material consumption and thus raising costs. Second, uncontrolled cooling rates frequently occur during hot-feeding, resulting in the inability to eliminate Widmanstätten and banded structures in the as-cast microstructure, thereby deteriorating subsequent processing and forming performance. Third, existing hot-feeding processes only focus on rapidly loading the high-temperature continuously cast billet into the furnace, failing to utilize the residual heat during the conveying process for microstructure control, resulting in wasted thermal energy resources and lost opportunities for microstructure optimization. Summary of the Invention

[0004] To address the technical problems of excessive iron oxide scale formation on the surface of continuously cast billets, poor machinability, and insufficient utilization of residual heat during the hot charging process of low-alloy high-strength steel continuously cast billets, this invention provides a method for controlling the microstructure of low-alloy high-strength steel continuously cast billets. This invention optimizes the hot charging temperature control system, reducing the amount of iron oxide scale while utilizing the residual heat of the continuously cast billet to eliminate as-cast defects, thereby improving the quality of the continuously cast billet and ultimately enhancing the uniformity of the steel microstructure and its machinability. This method is suitable for mass production using hot charging technology.

[0005] A method for controlling the microstructure of low-alloy high-strength steel continuous casting billets includes the following steps:

[0006] S1: The freshly cast billet is fed into a sealed hot delivery channel by foot rollers. The channel inlet and outlet are equipped with elastic sealing curtains made of high-temperature resistant rubber. The side walls of the channel are insulated with heat-insulating cotton, and nitrogen is used as a protective atmosphere. S2: Control the temperature of the front section of the hot delivery channel, which is 15~20 m long, to 900~920℃ and keep it warm for 1.2~1.8 h; S3: The temperature of the continuous casting billet is reduced to 630~670℃ by the gradient air cooling device in the channel and held at that temperature for 0.8~1.2 h; S4: The continuously cast billet is cooled naturally in the channel, the outlet sealing curtain is opened, and it is directly hot-charged into the heating furnace for rolling, or it is stacked off the line to cool.

[0007] In the above technical solution, in S1, the continuously cast billet can be obtained by existing technology, specifically including converter smelting, LF refining, RH refining and continuous casting processes.

[0008] Furthermore, based on mass fraction, the chemical composition of the continuously cast billet includes: C: 0.12%~0.20%, Si: 0.20%~0.60%, Mn: 1.20%~1.80%, P≤0.025%, S≤0.015%, and at least one of the microalloying elements: Nb, V, and Ti, and the total content of the microalloying elements is 0.01%~0.06%, with the remainder being Fe and unavoidable impurities.

[0009] In the above technical solution, in step S1, the continuously cast billet with a surface temperature of 1000~1200℃ that has just come out of the furnace is fed into a sealed hot delivery channel through foot rollers. Both the inlet and outlet of the channel are equipped with elastic sealing curtains made of high-temperature resistant rubber material. The side walls of the channel are insulated with heat-insulating cotton with a thickness of 100~150 mm to ensure that the atmosphere leakage rate in the channel is 3%~5% / h.

[0010] In the above technical solution, in step S1, nitrogen gas is introduced through nozzles at the top of the channel, wherein the nozzle spacing is 450~550 mm; and the gas flow rate is 0.8~1.2 m³ / s based on the surface area of ​​the continuously cast billet. 3 / (h·m 2 The positive pressure inside the channel is 10~20 Pa.

[0011] Furthermore, the purity of the nitrogen gas is ≥99.5%.

[0012] In the above technical solution, in step S2, the temperature is monitored in real time by an infrared temperature measuring device on the inner wall of the channel to ensure that the temperature fluctuation is ≤±10℃; when the temperature is below 900℃, an auxiliary electric heating device with a power of 20~50 kW in the channel is activated to supplement the heat; when the temperature is above 920℃, the auxiliary electric heating device is turned off and the air volume gradient air cooling slow cooling heat dissipation is activated.

[0013] In the above technical solution, the cooling rate in step S3 is 6~10℃ / min.

[0014] In the above technical solution, in step S4, the continuously cast billet is naturally cooled to 480~520℃ through the channel.

[0015] During continuous casting, the billet is kept in a high-temperature environment of around 1200℃ at the upper part of the secondary cooling chamber. At this temperature, microalloyed carbonitrides remain dissolved in the matrix and fail to precipitate, thus failing to pin grain boundaries and inhibit grain growth, resulting in coarse, primitive austenite grains. When the billet enters the lower section of the secondary cooling chamber, it is subjected to forced cooling by secondary cooling water spray, rapidly reducing the surface temperature to the 800℃ ferrite transformation range. Under the combined conditions of coarse austenite grains and a relatively high cooling rate, acicular ferrite precipitates and forms Widmanstätten structure. After the billet leaves the secondary cooling spray chamber, the residual heat from the billet core is conducted to the surface, causing the surface temperature to rise to around 1100℃. Although this temperature is within the austenite single-phase region, without prolonged constant-temperature holding, the atomic diffusion rate is insufficient, making it difficult for the formed Widmanstätten structure to completely dissolve and transform into austenite, ultimately remaining permanently in the billet matrix.

[0016] During continuous casting solidification, grains grow in a dendritic manner. Alloying and impurity elements such as Mn, Si, S, P, Nb, V, and Ti segregate between the dendrites, forming a structure with alternating alloy-rich and alloy-poor regions. In the subsequent solid-state phase transformation, the segregated regions gradually evolve into a layered structure with alternating ferrite and pearlite, which, after permanent solidification, forms the primary banded structure of the cast billet.

[0017] In step S2 of the method described in this invention, the residual heat of the continuously cast billet is used to homogenize the composition and microstructure at high temperatures, promoting the full dissolution of the acicular Widmanstätten structure into the matrix. Simultaneously, atomic diffusion at high temperatures weakens the center segregation of the continuously cast billet, reducing the segregation index to below 1.05. The complete austenite transformation temperature of low-alloy high-strength steel Ac3 is approximately 860℃. 900~920℃ is 50~70℃ higher than the Ac3 temperature, placing it in a stable austenitic single-phase region. Under the conditions of holding at 900~920℃ for 1.2~1.8 h, the acicular ferrite corresponding to the Widmanstätten structure can be fully redissolved into the austenitic matrix. The austenite then evolves into fine, uniform equiaxed austenite grains through static recrystallization. Simultaneously, the high temperature above 900℃ accelerates the atomic diffusion of alloying elements such as C and Mn in the steel, effectively homogenizing the composition and weakening the original center segregation of the continuously cast billet.

[0018] In step S3 of the method described in this invention, holding at 630~670℃ for 0.8~1.2 h promotes the uniform and regular growth of ferrite grains, optimizes the morphology of pearlite, and achieves overall microstructure homogenization. A protective atmosphere is continuously introduced during the holding process, and a slight positive pressure is maintained within the channel. At 630~670℃, the ferrite + pearlite dual-phase region of low-alloy high-strength steel is reached, and holding for 0.8~1.2 h can make the ferrite grain size tend to be uniform. Pearlite is a lamellar structure with alternating distributions of ferrite and cementite. Under constant temperature conditions of 630~670℃, carbon atoms diffuse sufficiently, which can release internal stress, stabilize the pearlite structure, and make the ferrite and pearlite arrangement more uniform and regular.

[0019] The beneficial effects of this invention are as follows: This invention employs a sealed hot delivery channel with an elastic sealing curtain and a slightly positive pressure gas supply, controlling the atmosphere leakage rate to within 5% / h, effectively suppressing oxidation on the surface of the continuously cast billet. Furthermore, the two-stage residual heat utilization process—using 900~920℃ to eliminate Widmanstätten structure and 630~670℃ to homogenize the structure—eliminates casting defects without additional heating, avoiding subsequent offline heat treatment. Moreover, by fully utilizing the residual heat of the high-temperature continuously cast billet during transportation, microstructure control can be achieved solely through temperature control of the hot delivery channel, eliminating the need for an additional heating furnace, thus achieving a synergistic effect of energy saving, cost reduction, and quality improvement. Detailed Implementation

[0020] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0021] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0022] In the following embodiments, the oxide scale thickness of the continuously cast billet was measured according to GB / T 6462-2005 "Measuring the Thickness of Metallic and Oxide Coatings by Microscopic Method". Specifically, samples were taken at 1 / 4 of the width and 1 / 2 of the length of the continuously cast billet, and the samples were prepared by mounting, grinding and polishing. The arbitration test was conducted using metallographic microscopy, and the average value of 5 fields of view was taken by the 200-500x lower intercept point method.

[0023] In the following examples, the Widmanstätten structure elimination rate of the continuously cast billet was tested according to GB / T 13299-2022 "Evaluation Method of Free Cementite, Pearlite and Widmanstätten Structure in Steel". Specifically, longitudinal samples were taken at 1 / 4 of the thickness of the continuously cast billet and etched with 4% nitric acid alcohol. The structure elimination rate was calculated by comparing the standard rating chart with the metallographic microscope at 100x magnification.

[0024] In the following embodiments, the ferrite grain size of the continuously cast billet was measured according to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals". Specifically, samples were taken from the core, 1 / 4 and 1 / 2 thickness of the billet, respectively, and etched with 4% nitric acid alcohol. The average grain size was quantitatively calculated using metallographic microscopy, the 100x rating image comparison method, and the 400x intercept point method combined with image analysis software.

[0025] In the following examples, the banded structure grade of the continuously cast billet was tested according to GB / T 34474-2025 "Evaluation Method for Banded Structure in Steel". Longitudinal samples were taken along the rolling direction and etched with 2% nitric acid alcohol; the grade was evaluated by metallographic microscopy at 100x magnification compared with the standard rating chart.

[0026] Example 1 A method for controlling the microstructure of low-alloy high-strength steel continuous casting billets includes the following steps: (1) Preparation of continuous casting billet: The raw materials were weighed according to the chemical composition, and the process was carried out by converter smelting → LF refining → RH refining → continuous casting to obtain low alloy high strength steel continuous casting billet. According to the mass fraction, its chemical composition is as follows: C: 0.15%; Si: 0.20%; Mn: 1.50%; P: 0.020%; S: 0.015%; Nb: 0.012%; V: 0.010%; Ti: 0.008%; the remainder is Fe and unavoidable impurities.

[0027] (2) Sealed hot delivery channel layout The continuously cast billet, with a surface temperature of 1100℃, is fed into a sealed hot delivery channel via foot rollers. Both the inlet and outlet of the channel are equipped with elastic sealing curtains made of high-temperature resistant rubber. The channel sidewalls are insulated with 100 mm thick insulation cotton to ensure an atmosphere leakage rate of 3%~5% / h. Nitrogen (N2) is used as the protective atmosphere, introduced through nozzles at the top of the channel with a nozzle spacing of 500 mm and a gas flow rate of 1 m³ / h. 3 / (h·m 2 Maintain a slight positive pressure of 0.015 MPa within the channel to prevent air from entering.

[0028] (3) Waste heat homogenization The first stage involves eliminating Widmanstätten structure. Specifically, the temperature of the 20-m long front-end hot-feeding channel is controlled at 910℃ and held for 1.5 hours. The temperature is monitored in real-time using an infrared thermometer on the inner wall of the channel. When the temperature drops below 900℃, an auxiliary electric heating device with a power of 50 kW is activated to ensure the holding temperature fluctuation is within 910±8℃. This stage utilizes the residual heat of the continuously cast billet for high-temperature homogenization, promoting the decomposition of Widmanstätten structure into ferrite and pearlite, while simultaneously eliminating center segregation and ensuring the segregation index drops below 1.05.

[0029] The second stage, microstructure homogenization, involves the following steps: After the initial heat treatment, a gradient air-cooling device within the channel is used to control the cooling rate at 8℃ / min, reducing the continuous casting billet temperature to 650℃. The billet is then held at 650℃ for 1 hour to promote uniform ferrite grain growth and refine the pearlite lamellar spacing, thus achieving microstructure homogenization. A protective atmosphere is continuously introduced during the heat treatment process to maintain a slight positive pressure within the channel.

[0030] (4) Follow-up processing After the heat preservation is completed, the continuously cast billet is naturally cooled to 480°C through the channel. The outlet sealing curtain is then opened, and the billet is removed from the line and stacked for further cooling.

[0031] The thickness of the iron oxide scale on the continuous casting billet without the above-mentioned control method is 0.35 mm; the ferrite grain size is 78 μm, and the banded structure grade is 3.2. The thickness of the iron oxide scale on the continuous casting billet after treatment by the method described in this invention is 0.12 mm; the Widmanstätten structure elimination rate is 100%, the ferrite grain size is 45 μm, and the banded structure grade is 0.8.

[0032] Example 2 A method for controlling the microstructure of low-alloy high-strength steel continuous casting billets includes the following steps: (1) The preparation method of the continuous casting billet is the same as in Example 1. According to the mass fraction, the chemical composition of the obtained low alloy high strength steel continuous casting billet includes: C: 0.17%; Si: 0.23%; Mn: 1.4%; P: 0.023%; S: 0.012%; Nb: 0.007%; V: 0.008%; Ti: 0.005%; the remainder is Fe and unavoidable impurities.

[0033] (2) Sealed hot delivery channel layout The continuously cast billet, with a surface temperature of 1100℃, is fed into a sealed hot delivery channel via foot rollers. Both the inlet and outlet of the channel are equipped with elastic sealing curtains made of high-temperature resistant rubber. The channel sidewalls are insulated with 100 mm thick insulation cotton to ensure an atmosphere leakage rate of 3%~5% / h. Nitrogen (N2) is used as the protective atmosphere and is introduced through nozzles at the top of the channel with a nozzle spacing of 500 mm and a gas flow rate of 1.1 m³ / h. 3 / (h·m 2 Maintain a slight positive pressure of 0.018 MPa within the channel to prevent air from entering.

[0034] (3) Waste heat homogenization The first stage involves eliminating Widmanstätten structure. Specifically, the temperature of the 20-m long front-end hot-feeding channel is controlled at 905℃ and held for 1.5 hours. The temperature is monitored in real-time using an infrared thermometer on the inner wall of the channel. When the temperature drops below 900℃, a 50 kW auxiliary electric heating device is activated to ensure the holding temperature fluctuation is within 905±9℃. This stage utilizes the residual heat of the continuously cast billet for high-temperature homogenization, promoting the decomposition of Widmanstätten structure into ferrite and pearlite, while simultaneously eliminating center segregation and ensuring the segregation index drops below 1.05.

[0035] The second stage, microstructure homogenization, involves the following steps: After the initial heat treatment, a gradient air-cooling device within the channel is used to control the cooling rate at 8℃ / min, reducing the continuous casting billet temperature to 650℃. The billet is then held at 650℃ for 1 hour to promote uniform ferrite grain growth and refine the pearlite lamellar spacing, thus achieving microstructure homogenization. A protective atmosphere is continuously introduced during the heat treatment process to maintain a slight positive pressure within the channel.

[0036] (4) Follow-up processing After the heat preservation is completed, the continuously cast billet is naturally cooled to 500°C through the channel. The outlet sealing curtain is then opened, and the billet is removed from the production line and stacked for further cooling.

[0037] The continuous casting billet treated by the method described in this invention has an iron oxide scale thickness of 0.14 mm; Widmanstätten structure elimination rate of 100%; ferrite grain size of 48 μm; and banded structure grade of 0.9.

[0038] Example 3 A method for controlling the microstructure of low-alloy high-strength steel continuous casting billets includes the following steps: (1) The preparation method of the continuous casting billet is the same as in Example 1. According to the mass fraction, the chemical composition of the obtained low alloy high strength steel continuous casting billet includes: C: 0.17%; Si: 0.35%; Mn: 1.47%; P: 0.019%; S: 0.010%; Nb: 0.020%; V: 0.018%; Ti: 0.012%; the remainder is Fe and unavoidable impurities.

[0039] (2) Sealed hot delivery channel layout The continuously cast billet, with a surface temperature of 1100℃, is fed into a sealed hot delivery channel via foot rollers. Both the inlet and outlet of the channel are equipped with elastic sealing curtains made of high-temperature resistant rubber. The channel sidewalls are insulated with 100 mm thick insulation cotton to ensure an atmosphere leakage rate of 3%~5% / h. Nitrogen (N2) is used as the protective atmosphere and is introduced through nozzles at the top of the channel with a nozzle spacing of 500 mm and a gas flow rate of 1.2 m³ / h. 3 / (h·m 2 Maintain a slight positive pressure of 0.017 MPa within the channel to prevent air from entering.

[0040] (3) Waste heat homogenization The first stage involves eliminating Widmanstätten structure. Specifically, the temperature of the 20-m long front-end hot-feeding channel is controlled at 915℃ and held for 1.5 hours. The temperature is monitored in real-time using an infrared thermometer on the inner wall of the channel. If the temperature drops below 915℃, a 50 kW auxiliary electric heating device is activated to ensure the holding temperature fluctuation is within 915±7℃. This stage utilizes the residual heat of the continuously cast billet for high-temperature homogenization, promoting the decomposition of Widmanstätten structure into ferrite and pearlite, while simultaneously eliminating center segregation and ensuring the segregation index drops below 1.05.

[0041] The second stage, microstructure homogenization, involves the following steps: After the initial heat treatment, a gradient air-cooling device within the channel is used to control the cooling rate at 8℃ / min, reducing the continuous casting billet temperature to 650℃. The billet is then held at 650℃ for 1 hour to promote uniform ferrite grain growth and refine the pearlite lamellar spacing, thus achieving microstructure homogenization. A protective atmosphere is continuously introduced during the heat treatment process to maintain a slight positive pressure within the channel.

[0042] (4) Follow-up processing After the heat preservation is completed, the continuously cast billet is allowed to cool naturally to 490°C through the channel. Then, the outlet sealing curtain is opened, and the billet is removed from the line and stacked for cooling.

[0043] The continuous casting billet treated by the method described in this invention has an iron oxide scale thickness of 0.13 mm; Widmanstätten structure elimination rate of 100%; ferrite grain size of 47 μm; and banded structure grade of 0.9.

Claims

1. A method for controlling the microstructure of low-alloy high-strength steel continuously cast billets, characterized in that: Includes the following steps: S1: The freshly cast billet is fed into a sealed hot delivery channel by foot rollers. The channel inlet and outlet are equipped with elastic sealing curtains made of high-temperature resistant rubber. The side walls of the channel are insulated with heat-insulating cotton, and nitrogen is used as a protective atmosphere. S2: Control the temperature of the front section of the hot delivery channel, which is 15-20 m long, to 900-920℃ and keep it at that temperature for 1.2-1.8 h; S3: The temperature of the continuous casting billet is reduced to 630~670℃ by the gradient air cooling device in the channel and held at that temperature for 0.8~1.2 h; S4: The continuously cast billet is cooled naturally in the channel, the outlet sealing curtain is opened, and it is directly hot-charged into the heating furnace for rolling, or it is stacked off the line to cool.

2. The method for controlling the microstructure of low-alloy high-strength steel continuous casting billets according to claim 1, characterized in that: In S1, the continuously cast billet is obtained through converter smelting, LF refining, RH refining and continuous casting processes.

3. The method for controlling the microstructure of low-alloy high-strength steel continuous casting billets according to claim 1 or 2, characterized in that: The chemical composition of the continuously cast billet, based on mass fraction, includes: C: 0.12%~0.20%, Si: 0.20%~0.60%, Mn: 1.20%~1.80%, P≤0.025%, S≤0.015%, and at least one of the microalloying elements: Nb, V, and Ti, with the total content of the microalloying elements being 0.01%~0.06%, the remainder being Fe and unavoidable impurities.

4. The method for controlling the microstructure of low-alloy high-strength steel continuous casting billets according to claim 1, characterized in that: In step S1, the continuously cast billet with a surface temperature of 1000~1200℃ is fed into a sealed hot delivery channel by foot rollers. Both the inlet and outlet of the channel are equipped with elastic sealing curtains made of high-temperature resistant rubber. The side walls of the channel are insulated with 100~150 mm thick insulation cotton to ensure that the atmosphere leakage rate in the channel is 3%~5% / h.

5. The method for controlling the microstructure of low-alloy high-strength steel continuous casting billets according to claim 1, characterized in that: In step S1, nitrogen gas is introduced through nozzles at the top of the channel, with a nozzle spacing of 450-550 mm; the gas flow rate is 0.8-1.2 m³ / s based on the surface area of ​​the continuously cast billet. 3 / (h·m 2 The positive pressure inside the channel is 10~20 Pa.

6. The method for controlling the microstructure of low-alloy high-strength steel continuous casting billets according to claim 1, characterized in that: In step S2, the temperature is monitored in real time by an infrared temperature measuring device on the inner wall of the channel to ensure that the temperature fluctuation is ≤±10℃. When the temperature is below 900℃, an auxiliary electric heating device with a power of 20~50 kW is activated in the channel to supplement the heat. When the temperature is above 920℃, the auxiliary electric heating device is turned off and the air volume gradient air cooling is activated for slow cooling.

7. The method for controlling the microstructure of low-alloy high-strength steel continuous casting billets according to claim 1, characterized in that: In S3, the cooling rate is 6~10℃ / min.

8. The method for controlling the microstructure of low-alloy high-strength steel continuous casting billets according to claim 1, characterized in that: In step S4, the continuously cast billet is naturally cooled to 480~520℃ through the channel.