A production method for effectively controlling edge defects of hot-rolled high-strength steel

Abstract

This invention belongs to the field of steel production technology and discloses an effective production method for controlling edge defects in hot-rolled high-strength steel. The process includes converter smelting → LF refining → continuous casting → soaking furnace heating → hot continuous rolling → cooling and coiling. Specifically: the LF refining process controls the nitrogen content in the molten steel to be ≤40ppm at the end point; the continuous casting uses a funnel-shaped crystallizer with a taper ratio of 1.1%~1.3%, and the narrow face taper is adjusted during casting to maintain the heat flux density of the narrow face at 55%~85% of the heat flux density of the wide face; the secondary cooling intensity is controlled to ensure that the corner temperature of the billet is >900℃ in the straightening zone; the soaking furnace heating temperature is ≥1150℃, and the time in the furnace is ≥30min; in the vertical roll fixed-width side pressing pass of hot continuous rolling, the rolling force of the vertical roll mill is controlled to be >300kN. This invention significantly reduces the incidence of edge quality defects such as edge cracks, burrs, and broken edges in high-manganese and high-titanium series high-strength steel by optimizing the steel composition, continuous casting, and hot rolling processes.

Description

Technical Field

[0001] This invention belongs to the field of steel production technology, specifically relating to a production method for effectively controlling edge defects in hot-rolled high-strength steel. Background Technology

[0002] In the production of hot-rolled high-strength steel (yield strength ≥ 450 MPa), the high manganese and high titanium content of the steel makes controlling edge quality defects (such as edge cracks, burrs, and broken edges) during solidification and rolling extremely difficult. These defects lead to a high product rejection rate, resulting in significant economic losses. For example, a certain factory's annual production line of high-strength steel, with an annual output of approximately 800,000 tons, had an edge quality rejection rate as high as 1.14%, resulting in annual losses of several million yuan.

[0003] Currently, there is a lack of systematic research on control measures for edge defects in high-strength steel. Often, the processes used for low-alloy steel are applied, resulting in poor control. The root causes of these defects involve multiple factors, including the solidification characteristics of molten steel, stress and strain during continuous casting, and subsequent rolling deformation. Adjusting a single factor is insufficient to fundamentally solve the problem. With increasing market demand for high-value-added products such as high-strength steel, developing a production method that can systematically control edge defects in hot-rolled high-strength steel is of great significance for improving product yield, reducing production costs, and enhancing market competitiveness. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an effective production method for controlling edge defects of hot-rolled high-strength steel, which addresses the shortcomings of the existing technology. By optimizing the steel composition, continuous casting and hot rolling processes, the occurrence rate of edge quality defects such as edge cracks, burrs and broken edges in high-manganese and high-titanium series high-strength steel is significantly reduced, thereby reducing the product re-judgment rate and improving economic benefits.

[0005] To solve the technical problem proposed in this invention, this invention provides a production method for effectively controlling edge defects in hot-rolled high-strength steel, the production process of which includes: converter smelting → LF refining → continuous casting → soaking furnace heating → hot continuous rolling → cooling and coiling.

[0006] In the above scheme, the LF refining adopts a large amount of slag, and the slag layer thickness is 80~120mm.

[0007] In the above scheme, the LF refining process involves a small amount of aluminum heating for slag formation in the early stage (initial stage of electric heating), with an aluminum particle addition of 0.3~0.8 kg / ton of steel; and an aluminum addition process for slag adjustment in the middle stage (after slag formation), with an aluminum particle addition of 0.5~1.5 kg / ton of steel.

[0008] In the above scheme, the LF refining process is carried out with weak argon stirring, and the argon flow rate is 20~50 NL / min.

[0009] In the above scheme, the N content of the molten steel at the LF refining endpoint is ≤40ppm.

[0010] In the above scheme, the continuous casting adopts full-process protective casting, specifically including: the ladle to the tundish uses a long nozzle and is sealed with argon gas, the tundish to the crystallizer uses an immersion nozzle, and a covering agent is added to the tundish.

[0011] Furthermore, the amount of the covering agent added is sufficient to ensure that the molten steel surface is completely covered without any exposure, preferably 10~20 kg / ton of steel.

[0012] In the above scheme, the number of continuous casting heats in the tundish is ≤8.

[0013] In the above scheme, the continuous casting adopts a funnel-shaped crystallizer with a taper ratio of 1.1% to 1.3%, and the taper of the narrow face is adjusted online during the casting process to maintain the heat flux density of the narrow face at 55% to 85% of the heat flux density of the wide face.

[0014] In the above scheme, the continuous casting crystallizer uses a copper plate with deep-drilled cooling water channels.

[0015] In the above scheme, the continuous casting uses a mold protective slag with a basicity of 0.87~1.07 and a viscosity of 0.16~0.50 Pa·s at 1300℃.

[0016] In the above scheme, the continuous casting dynamically adjusts the secondary cooling intensity according to the casting speed. Within the casting speed range of 3.6~4.2 m / min, the secondary cooling water ratio is controlled at 1.60~1.70 L / kg, and the arrangement angle and flow distribution of the nozzles in the secondary cooling zone are controlled to ensure that the cooling intensity in the central area is greater than that in the corner areas, so that the corner temperature of the billet is >900℃ when the billet is in the straightening zone. The straightening zone refers to the interval between the end of the second sector section and the beginning of the third sector section of the continuous casting machine, that is, the physical section where the billet undergoes straightening deformation after complete solidification.

[0017] In the above scheme, the continuous casting sector adopts a liquid core pressing mode, and the pressing amount of the second sector is controlled to be greater than that of the first sector.

[0018] Furthermore, when the target thickness of the billet is 58~62mm, the exit thickness of the first sector is set to the target thickness plus 6~8mm, and the exit thickness of the second sector is set to the target thickness.

[0019] In the above scheme, the continuous casting uses electromagnetic braking to control the fluctuation range of the molten steel level in the crystallizer to within ±3mm.

[0020] In the above scheme, the heating temperature of the homogenizing furnace is ≥1150℃, and the furnace time is ≥30min.

[0021] Preferably, the heating temperature in the homogenizing furnace is 1150~1250℃, and the time spent in the furnace is 30~50min.

[0022] In the above scheme, during the hot continuous rolling vertical roll fixed width side pressing pass, the rolling force of the vertical roll mill is controlled to be >300kN, preferably 300~400 kN, so that the edge of the slab produces dog bone deformation.

[0023] The method of the present invention is applicable to high-strength steel with a yield strength ≥450MPa produced by thin slab continuous casting and rolling process, wherein the high-strength steel has a C content of 0.04~0.08%, a Mn content of 1.0~2.2%, and a Ti content of 0.05~0.20%.

[0024] The hot-rolled high-strength steel produced according to the method of the present invention has an edge defect rate of <0.6%.

[0025] The main technical concept of this invention is as follows: The core of this invention lies in the synergistic optimization of the entire high-strength steel production process, with a focus on controlling the purity of molten steel, the solidification process of continuously cast billets, and the deformation conditions during hot rolling. Key improvements include: (1) Control of steel composition and purity: Given the high titanium content of high-strength steel, strict control of nitrogen content is crucial. Excessive nitrogen promotes the precipitation of numerous fine TiN particles at austenite grain boundaries, severely deteriorating the steel's high-temperature plasticity and edge quality. Therefore, this invention considers nitrogen control as a core aspect of purity control. The LF refining slag-forming process is optimized (using a large slag volume, initial low-aluminum heating for slag formation, mid-stage aluminum addition for slag adjustment, and weak argon stirring throughout) to stably control the nitrogen content of the finished product below 0.004%. Simultaneously, full-process protective casting is implemented to reduce secondary oxidation and nitrogen accumulation in the molten steel, and the number of consecutive castings in the tundish is controlled to minimize the formation and accumulation of titanium oxides.

[0026] (2) Optimization of continuous casting process: a. Crystallizer and Primary Cooling: A funnel-shaped crystallizer with a taper ratio of 1.1%~1.3% is used. During casting, the taper of the narrow face is adjusted online to maintain its heat flux density within 55%~85% of the heat flux of the wide face. The core function of this measure is to ensure uniform and synchronous shrinkage of the initial billet shell within the crystallizer, reducing billet shell thickness fluctuations and thermal stress concentrations caused by uneven cooling, thereby suppressing the initiation of longitudinal cracks at the edges (especially corners) from the source. A deep-drilled cooling channel crystallizer copper plate is used instead of a water-trough copper plate. The deep-drilled design greatly increases the distribution density and uniformity of the cooling channels, eliminating the "cooling blind zone" between water troughs, and achieving more uniform and efficient primary cooling of the billet perimeter. Uniform primary cooling is the foundation for obtaining a billet shell with uniform thickness and good initial quality, effectively reducing weak points in the billet shell caused by localized overcooling or insufficient cooling. These weak points are the origin of defects such as edge burrs and cracks in subsequent rolling.

[0027] b. Protective Slag: TiO2 inclusions generated during the solidification process of high-titanium steel are absorbed by the protective slag, which easily leads to slag modification, increased basicity, and deterioration of lubrication performance. This invention selects a protective slag with low basicity (R=0.87~1.07) and high viscosity (0.16~0.50 Pa•s at 1300℃). After absorbing titanium compounds, its properties change slowly, maintaining a stable molten layer and lubricating film for a long time. This ensures the uniformity of heat transfer between the crystallizer and the billet shell and the continuity of lubrication, avoiding a sharp increase in friction and uneven heat transfer caused by abrupt changes in the properties of the protective slag. This is crucial for controlling edge longitudinal cracks and surface quality.

[0028] c. Secondary Cooling: The secondary cooling system was redesigned for high-strength steel, with the cooling intensity dynamically adjusted according to the casting speed. For example, within a typical casting speed range of 3.6~4.2 m / min, the secondary cooling water volume was controlled at 1.60~1.70 L / kg, a reduction of approximately 8% compared to the general process. The nozzle arrangement angle and flow distribution in the secondary cooling zone were optimized to enhance cooling of the central area of ​​the billet while relatively weakening cooling of the edges and corners. The purpose of these optimizations was to increase the corner temperature of the billet when it enters the straightening zone, ensuring it remains consistently above 900℃. This temperature avoids the low plasticity temperature range (700~900℃) where carbonitrides of microalloying elements such as Nb, V, and Ti precipitate in large quantities. When the corner temperature is >900℃, the material exhibits higher high-temperature plasticity, enabling it to withstand the tensile strain generated during the straightening process without easily developing cracks, thereby significantly reducing transverse cracks and corner cracks originating from the billet.

[0029] d. Liquid Core Reduction: Optimize the liquid core reduction mode for the fan-shaped segments, reducing the reduction amount in the first fan-shaped segment and increasing the reduction amount in the second fan-shaped segment. This makes the edge shape of the billet more uniform and reduces the risk of internal cracks caused by improper reduction. For example, when the target billet thickness is in the range of 58~62mm, the input thickness of the first fan-shaped segment is set to "target thickness + 6~8mm", while the second fan-shaped segment is directly reduced to the target thickness. In the early stage of solidification (first fan-shaped segment), the billet shell is thinner and has lower strength. Excessive reduction will lead to severe deformation and stress concentration of the billet shell, which can easily cause internal cracks at the edge. Shifting the main reduction amount to the second fan-shaped segment, which has a thicker and stronger billet shell, can make the compression deformation of the billet edge shape more uniform and gradual, thereby effectively reducing the risk of internal cracks caused by improper liquid core reduction at the billet edge. These internal cracks will be exposed and expand into edge surface defects during subsequent rolling.

[0030] e. Electromagnetic braking: Optimizing electromagnetic braking parameters such as current and frequency stabilizes the molten steel flow field within the crystallizer, controlling the fluctuation range of the molten steel surface within ±3mm. This reduces the impact on the meniscus, minimizes surface fluctuations, promotes uniform melting and lubrication of the protective slag, and ultimately allows for more uniform and symmetrical growth of the initial billet shell. A uniform billet shell is fundamental to avoiding localized weaknesses and cracks; therefore, optimizing electromagnetic braking parameters is an important auxiliary means to control edge longitudinal cracks and improve corner quality.

[0031] (3) Optimization of heating and rolling processes: a. Soaking furnace: Use a high soaking temperature (not lower than 1150℃) and appropriately extend the time in the furnace. Control the time in the furnace according to the thickness of the slab to ensure that the slab is thoroughly heated, so as to promote the solid solution of microalloying elements in the slab and improve the edge thermoplasticity.

[0032] b. Hot continuous rolling: In the initial stage of the hot continuous rolling process, in the vertical roll rolling pass where the slab is subjected to width side pressure (or fixed width), the rolling force of the vertical roll mill is increased (set to >300kN). By applying sufficient vertical roll rolling force, the edge of the slab is compressed in the width direction to produce a 'dog bone' bulge, thereby compressing and welding the potential microcracks at the edge and reducing the risk of their propagation in subsequent finishing rolling.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention addresses the unique mechanisms of edge defects in the production of high-manganese and high-titanium high-strength steel using thin-slab continuous casting and rolling processes (such as sensitivity to TiN precipitation, easy entry into the brittle zone in the straightening zone, and stress concentration under liquid core pressure). It employs a synergistic matching and innovative optimization approach, from metallurgical principles to the entire process parameters. These interconnected and synergistic measures solve the technical challenges in the production of high-manganese and high-titanium high-strength steel, significantly reducing the edge defect correction rate from over 1.14% in traditional processes to below 0.6%. This achieves remarkable and unexpected technical results, effectively reducing quality losses due to defect corrections and resulting in significant economic benefits. Stable and controllable edge quality improves the yield and pass rate of high-strength steel products, reduces production costs, and enhances competitiveness in the high-end steel market. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the morphology of edge defects (burrs, edge cracks, broken edges) in high-strength steel.

[0035] Figure 2 This is a schematic diagram showing the morphology and location of the crack in the middle of a typical sample (longitudinal section) from the liquid core pressing process.

[0036] Figure 3 This is a schematic diagram illustrating the formation process of edge cracks during rolling. Detailed Implementation

[0037] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0038] The following examples and comparative examples use typical compositions of high-strength steel with a yield strength ≥450MPa. Taking BST850X steel as an example, its target composition is as follows: C: 0.055%, Si: 0.16%, Mn: 1.90%, P≤0.015%, S≤0.004%, Alt: 0.025%, Ti: 0.175%, N≤0.004%. The production process is as follows: hot metal pretreatment → converter smelting → LF refining → continuous casting → soaking furnace heating → hot continuous rolling → laminar flow cooling → coiling. 1) LF Refining A large slag volume is adopted, with a slag layer thickness of 80~120mm; in the early stage (initial stage of electric heating), a small amount of aluminum is used for slag formation, with an aluminum particle addition of 0.3~0.8 kg / ton of steel; in the middle stage (after slag formation), aluminum is added to adjust the slag, with an aluminum particle addition of 0.5~1.5 kg / ton of steel; weak argon stirring is carried out throughout the process, with an argon flow rate of 20~50 NL / min; after LF treatment, the nitrogen content of the molten steel is ≤40ppm; 2) Continuous casting The casting process is protected throughout. The ladle to tundish uses a long nozzle and is sealed with argon gas. The tundish to crystallizer uses an immersion nozzle. A covering agent is added to the tundish at a rate of 10-20 kg / ton of steel. The number of consecutive heats cast from the tundish is ≤8. A funnel-shaped crystallizer with a taper ratio of 1.1% to 1.3% is used, and the taper of the narrow face is adjusted online during the casting process to maintain the heat flux density of the narrow face at 55% to 85% of that of the wide face. The crystallizer uses a copper plate with deeply drilled cooling water channels and a crystallizer protective slag with a basicity of 0.87 to 1.07 and a viscosity of 0.16 to 0.50 Pa·s at 1300℃. The fluctuation range of the molten steel level in the crystallizer is controlled within ±3 mm by electromagnetic braking. The secondary cooling intensity is dynamically adjusted by adjusting the casting speed. Within the casting speed range of 3.6~4.2 m / min, the secondary cooling water ratio is controlled at 1.60~1.70 L / kg. The arrangement angle and flow distribution of the nozzles in the secondary cooling zone are also controlled to ensure that the cooling intensity in the central area is greater than that in the corner area, so that the corner temperature of the billet is >900℃ when the billet is in the straightening zone. The sector section adopts the liquid core pressing mode, and the pressing amount of the second sector section is controlled to be greater than that of the first sector section. When the target thickness of the billet is 58~62mm, the exit thickness of the first sector section is set to the target thickness plus 6~8mm, and the exit thickness of the second sector section is set to the target thickness. 3) Heating in a soaking furnace The heating temperature in the soaking furnace is ≥1150℃, and the furnace time is ≥30min; 4) Hot continuous rolling In the vertical roll fixed width side pressing pass, the rolling force of the vertical roll mill is controlled to be >300kN, so that the edge of the slab will produce dog bone deformation; the finishing rolling start temperature is controlled to be 1000~1050℃; the finishing rolling finish temperature is controlled to be 860℃~920℃. 5) Laminar flow cooling and winding The system employs either centralized or sparse cooling at the front end, with the winding temperature controlled between 550 and 650°C.

[0039] The differences in process parameter control between the examples and the comparative examples are shown in the table below: Table 1 Key parameters for LF refining

[0040] Table 2 Key parameters for continuous casting

[0041] Table 3 Key Parameters for Continuous Casting (Continued)

[0042] Table 4 Key parameters and production results of soaking furnace heating and hot continuous rolling

[0043] After implementing the above control methods, the edge defect incidence rate was reduced to below 0.6%. In actual production, statistics on the edge quality of high-strength steel products were compiled. Taking 2025 production data as an example, the total output of high-strength steel was approximately 419,000 tons, and the amount of steel with edge defects requiring reassessment was approximately 1,939 tons, reducing the edge quality defect incidence rate to 0.46%. Simultaneously, the rates of accidents such as steel leakage and adhesion during production decreased, and the stability of continuous casting and rolling was improved.

[0044] In contrast, when using traditional low-alloy steel process control methods to produce high-strength steel with the same composition, without strict nitrogen control, without using special protective slag and secondary cooling water meters, and without optimizing the liquid core pressing and vertical roller parameters, the statistical results show that the edge defect correction rate is above 1.14%, which is much higher than the method of this invention.

[0045] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A production method for effectively controlling edge defects in hot-rolled high-strength steel, the process comprising converter smelting → LF refining → continuous casting → soaking furnace heating → hot continuous rolling → cooling and coiling, characterized in that, Including the following control methods: 1) LF refining: The final molten steel N content ≤ 40ppm; 2) Continuous casting: A funnel-shaped crystallizer with a taper ratio of 1.1%~1.3% is used. During the casting process, the taper of the narrow face is adjusted to maintain the heat flux density of the narrow face at 55%~85% of that of the wide face. The intensity of secondary cooling is controlled so that the corner temperature of the billet is >900℃ when the billet is in the straightening zone. 3) Heating in a soaking furnace: The temperature of the soaking furnace is ≥1150℃, and the time spent in the furnace is ≥30min; 4) Hot continuous rolling: In the fixed width side pressing pass of the vertical roll mill, the rolling force of the vertical roll mill is controlled to be >300kN.

2. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, The LF refining process uses a large amount of slag with a slag layer thickness of 80~120mm. During the heating and slag-forming stage, the amount of aluminum particles added is controlled at 0.3~0.8 kg / ton of steel, and during the slag conditioning stage, the amount of aluminum particles added is controlled at 0.5~1.5 kg / ton of steel.

3. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, The LF refining process involves weak argon stirring throughout, with an argon flow rate of 20~50 NL / min.

4. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, The continuous casting adopts full-process protective casting. The tundish to tundish uses a long nozzle and is sealed with argon gas. The tundish to crystallizer uses an immersion nozzle, and a covering agent is added to the tundish. The continuous casting controls the number of consecutive heats cast from the tundish to be ≤8.

5. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, The continuous casting process uses electromagnetic braking to control the fluctuation of the molten steel level in the crystallizer to within ±3mm. The crystallizer is made of copper plate with deep-drilled cooling channels and uses a crystallizer protective slag with an alkalinity of 0.87~1.07 and a viscosity of 0.16~0.50 Pa·s at 1300℃.

6. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, During continuous casting, the secondary cooling water ratio is controlled at 1.60-1.70 L / kg within the casting speed range of 3.6-4.2 m / min. The arrangement angle and flow distribution of the nozzles in the secondary cooling zone are also controlled to ensure that the cooling intensity in the central area is greater than that in the corner areas.

7. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, The continuous casting sector adopts a liquid core pressing mode, and the pressing amount of the second sector is controlled to be greater than that of the first sector.

8. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 7, characterized in that, When the target thickness of the billet is 58~62mm, the exit thickness of the first sector is set to the target thickness plus 6~8mm, and the exit thickness of the second sector is set to the target thickness.

9. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, The heating temperature in the soaking furnace is 1150~1250℃, and the time spent in the furnace is 30~50min; in the hot continuous rolling vertical roll fixed width side pressing pass, the rolling force of the vertical roll mill is controlled to be 300~400 kN.

10. The production method for effectively controlling edge defects in hot-rolled high-strength steel according to claim 1, characterized in that, The production method is applicable to high-strength steel with a yield strength ≥450MPa, wherein the high-strength steel has a C content of 0.04~0.08%, a Mn content of 1.0~2.2%, and a Ti content of 0.05~0.20%; the high-strength steel produced by the method has an edge defect rate of <0.6%.

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