A method for timing control of TiN in titanium microalloyed steel based on continuous casting secondary cooling zone

Abstract

This invention discloses a method for time-sequential control of TiN in titanium microalloyed steel based on the secondary cooling zone of continuous casting, belonging to the field of iron and steel metallurgical technology. The method includes a zoned cooling process in the secondary cooling zone of continuous casting, employing a weak cooling phase in the initial stage and a strong cooling phase in the later stage. Through this staged and differentiated time-sequential cooling process, this invention precisely eliminates the potential formation of large-sized harmful TiN inclusions, completely solving problems such as uneven TiN inclusion size and excessive coarse inclusions in traditional cooling processes. Simultaneously, it significantly improves the initial nitrogen fixation efficiency in the molten steel, effectively optimizing the internal quality of the cast billet.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, specifically relating to a method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting. Background Technology

[0002] Titanium microalloying technology, with its excellent precipitation strengthening effect and cost advantages, is widely used in the production of low-alloy high-strength steel and is one of the important technical means to improve the comprehensive mechanical properties of steel. Titanium is chemically reactive and readily combines with nitrogen in steel during molten steel smelting and continuous casting solidification to form titanium nitride (TiN) inclusions. The precipitation morphology, size, and distribution of TiN play a decisive role in the properties of titanium microalloyed steel, exhibiting a distinct duality: on the one hand, nanoscale fine TiN particles can effectively pin austenite grain boundaries, inhibit grain growth, and simultaneously induce the formation of intragranular acicular ferrite, improving the toughness of the steel; on the other hand, micron-sized large TiN inclusions that precipitate prematurely during continuous casting solidification can form stress concentration sources, severely impairing the steel's low-temperature impact toughness and other properties, reducing the quality and reliability of the finished steel product.

[0003] Therefore, precisely controlling the precipitation behavior, size, and distribution of TiN during continuous casting is crucial for the stable production of high-performance titanium microalloyed steel. During the solidification of molten steel, solute elements such as Ti and N accumulate at the solidification front, with the local concentration product exceeding the solubility product, thus inducing TiN precipitation. Existing research confirms that the secondary cooling intensity during continuous casting is an important process parameter for controlling TiN size. Appropriately increasing the secondary cooling rate allows the molten steel to pass through the solidification two-phase region quickly, shortening the residence and growth time of TiN in the high-temperature zone, thereby effectively suppressing the formation of large-sized TiN inclusions.

[0004] Currently, relevant technologies have been used to optimize the refinement of TiN inclusions. Chinese Patent (Patent No.: ZL202111409012.6) discloses a method for reducing the size of titanium nitride inclusions in high-titanium alloy steel. This method effectively reduces the size of TiN inclusions in high-titanium alloy steel by controlling low superheat, constant casting speed, and increasing secondary cooling rate. Chinese Patent (Patent No.: ZL 201910814081.1) discloses a continuous casting method for controlling the liquid precipitation of TiN in titanium-containing microalloyed steel. This method effectively optimizes the TiN inclusion state inside the billet by adjusting the cooling intensity of a specific region at the end of solidification and combining it with a light reduction process.

[0005] However, existing technologies mostly employ a control strategy of uniform or localized strong cooling throughout the process. The core idea is to passively suppress the precipitation and growth of TiN. Although this can reduce the size of TiN to some extent, it has obvious technical limitations and cannot simultaneously ensure steel quality and production stability. The specific drawbacks are as follows: First, the nitrogen fixation efficiency is limited. While the strong cooling process inhibits TiN growth, it also reduces the number of effective TiN nuclei. This makes it difficult to fully fix the harmful free nitrogen atoms in the steel through the Ti and N precipitation reaction, thus making it difficult to completely eliminate the adverse effects of free nitrogen on the steel's properties. The nitrogen fixation advantages of titanium microalloying cannot be fully realized.

[0006] Second, the control mode is singular. Existing technologies do not take into account the thermodynamic and kinetic differences at different stages of continuous casting solidification, but instead adopt a uniform cooling mode. This makes it impossible to provide differentiated guidance and active control for the entire process of TiN precipitation, nucleation, and growth, and it is difficult to adapt to the TiN optimization control requirements at different solidification stages.

[0007] Third, it can easily lead to defects in the quality of the cast billet. An unreasonable forced cooling system will increase the internal temperature gradient of the cast billet, which can easily induce internal defects such as central cracks and triangular cracks at the end of solidification, thereby increasing the risk of billet scrap and reducing the production qualification rate and process stability.

[0008] In summary, there is currently a lack of a continuous casting secondary cooling control scheme that can simultaneously achieve efficient nitrogen fixation, TiN size refinement, active controllability, and ensure billet quality. Summary of the Invention

[0009] To address the aforementioned technical deficiencies, this invention provides a method for controlling the TiN content in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting. This method is of great significance for the stable production of high-performance titanium microalloyed steel and for improving steel quality and production stability.

[0010] The technical solution of the present invention is as follows: A method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting includes: adopting a partitioned cooling process of weak cooling in the front section and strong cooling in the back section in the secondary cooling zone of continuous casting.

[0011] This invention breaks away from the traditional mindset of uniform cooling. It adopts a zoned differentiated cooling process in the secondary cooling zone of continuous casting, with weak cooling in the front and strong cooling in the back. It establishes a phased time-series cooling control system of "inducing precipitation in the early stage and inhibiting growth in the later stage", actively constructs conditions suitable for TiN fractional precipitation, and precisely controls the precipitation behavior and size distribution of TiN inclusions in steel.

[0012] The initial weak cooling process creates relatively gentle cooling conditions, promoting the preferential nucleation and growth of some Ti and N atoms in the molten steel at temperatures above 1300℃. This induces the precipitation of a batch of TiN inclusions with a concentrated size distribution of 3~8μm and an average target size of approximately 5μm. These pre-precipitated TiN inclusions can effectively consume 60%~70% of the dissolved nitrogen in the molten steel, achieving a pre-nitrogen fixation effect. This process consumes free nitrogen elements at the source, avoiding the problem of subsequent large-scale precipitation and abnormal growth of TiN.

[0013] The subsequent strong cooling process, through enhanced cooling regime, drives the residual molten steel in the core of the billet to rapidly pass through the critical temperature zone of 1300℃ to the liquidus line where TiN concentrates and precipitates. On the one hand, it can effectively inhibit the growth of new TiN crystal nuclei caused by solute enrichment during solidification; on the other hand, it can prevent the TiN inclusions precipitated in the early stage from further coarsening. Through precise cooling control in this stage, the size of the TiN inclusions precipitated in the middle and later stages can be strictly controlled within 5μm, and most of them are uniformly distributed in the form of fine dispersed particles of 3~6μm.

[0014] This invention, through the aforementioned phased and differentiated sequential cooling process, precisely eliminates the potential for the formation of large-sized harmful TiN inclusions, completely solves the problems of uneven TiN inclusion size and excessive coarse inclusions in traditional cooling processes, and at the same time significantly improves the early fixation efficiency of nitrogen in molten steel, effectively optimizing the internal quality of the billet.

[0015] Preferably, the secondary cooling process of continuous casting with weak cooling in the front section and strong cooling in the back section includes: in the area from the solidification point of the billet to 1 / 4 to 1 / 3 of the thickness of the solidified shell, the secondary cooling water volume is controlled at 0.6 to 0.9 L / kg, and the average cooling rate of the billet surface is 5 to 15 °C / min; in the area from 1 / 4 to 1 / 3 of the billet thickness to the complete solidification of the billet, the secondary cooling water volume is controlled at 1.2 to 1.8 L / kg, and the average cooling rate of the billet surface is 25 to 45 °C / min.

[0016] More preferably, in the region from the solidification start point of the billet to 1 / 4 to 1 / 3 of the billet thickness, the secondary cooling water volume is controlled to be 0.7 to 0.8 L / kg, and the average cooling rate of the billet surface is 8 to 12 °C / min.

[0017] More preferably, in the region from 1 / 4 to 1 / 3 of the billet thickness to the complete solidification of the billet, the secondary cooling water volume is controlled to be 1.5 to 1.8 L / kg, and the average cooling rate of the billet surface is 35 to 45 °C / min.

[0018] Preferably, the composition of the titanium microalloyed steel, by mass percentage, includes: 0.01%~0.12% titanium (Ti) and 0.002%~0.007% nitrogen (N).

[0019] More preferably, Ti is 0.07%~0.12%.

[0020] More preferably, N is 0.003% to 0.005%.

[0021] Preferably, the supporting process parameters for the partitioned cooling process of the secondary cooling zone of continuous casting, which uses weak cooling in the front section and strong cooling in the back section, include: the superheat of the tundish is 15~25℃.

[0022] Preferably, the supporting process parameters for the partitioned cooling process of the secondary cooling zone of continuous casting, which uses weak cooling in the front section and strong cooling in the back section, include: keeping the casting speed constant and the fluctuation range not exceeding ±0.05m / min.

[0023] More preferably, the pulling speed is 0.9~1.2m / min.

[0024] The beneficial effects of this invention are as follows: (1) Active hierarchical control of TiN size was achieved: By adopting a "weak first, strong later" cooling strategy, the TiN precipitation process was decoupled in space and time. In the early stage, a batch of micron-sized TiN with controllable size was formed for nitrogen fixation; in the later stage, a large amount of submicron-sized TiN with low hazard was generated. Overall, it showed a bimodal distribution and effectively eliminated the extremely harmful coarse (>8μm) TiN.

[0025] (2) Improved nitrogen fixation efficiency and stability in the early stage: In the early stage, free nitrogen in the molten steel is actively consumed through induced precipitation, thereby reducing the nitrogen concentration in the liquid phase region during subsequent solidification, and thus reducing the driving force for TiN precipitation in the later stage from a thermodynamic perspective, creating favorable conditions for the formation of finer TiN.

[0026] (3) Comprehensive improvement of billet quality: The method provided by this invention can effectively control the TiN size and accelerate the overall solidification rate of the billet by relying on the strong cooling process in the later stage, which helps to reduce the center segregation; at the same time, by reasonably designing the weak cooling process, the risk of cracks in the surface and subcutaneous area of ​​the billet due to excessive cooling can be avoided, thereby achieving a balance between internal and surface quality.

[0027] (4) Strong process adaptability: The core of the method provided by the present invention lies in the optimization of the secondary cooling zone process of continuous casting. It does not require major modifications to the smelting and refining processes. It can be easily implemented on existing continuous casting machines by upgrading the secondary cooling water distribution model and control system, thus having good prospects for industrial promotion.

[0028] Implementation methods The specific embodiments listed in this invention are merely examples, and the invention is not limited to the specific embodiments described below. For those skilled in the art, any equivalent modifications and substitutions to the embodiments described below are also within the scope of this invention. Therefore, all equivalent transformations and modifications made without departing from the spirit and scope of this invention should be covered within its scope. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. All reagents or instruments whose manufacturers are not specified are commercially available conventional products. To better illustrate this invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this invention can be practiced even without certain specific details. In other embodiments, methods, means, equipment, and steps well known to those skilled in the art are not described in detail in order to highlight the main points of this invention.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Unless otherwise specified, all units used in this specification are International Standard Units (SI), and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors unavoidable in industrial production.

[0030] The key to implementing this invention lies in adopting a zoned cooling system with a weaker initial cooling followed by a stronger cooling in the secondary cooling zone of continuous casting. All embodiments and comparative examples use the production of titanium microalloyed Q355B steel continuous casting slabs with a cross-sectional thickness of 230 mm as an example. The target composition (mass percentage) of the molten steel is: C 0.16%, Si 0.30%, Mn 1.40%, Ti 0.06%, N 0.007% (70 ppm). The casting speed is kept constant at 1.0 m / min during continuous casting, the superheat of the tundish is controlled at 15~25℃, and full-process protective casting is employed.

[0031] The features and performance of the present invention will be further described in detail below with reference to embodiments and comparative examples.

[0032] I. Examples and Comparative Examples Example 1 In this embodiment, the tundish superheat is 25°C, and a zoned cooling process with weak cooling in the front and strong cooling in the back is adopted for the secondary cooling zone of continuous casting. The specific parameters are as follows: Front-end weak cooling process: In the area from the solidification start point of the billet (i.e., the outlet of the crystallizer) to 1 / 4 of the billet thickness (this position is measured from the outer surface of the billet inward), the secondary cooling water volume is controlled at 0.8L / kg, and the average cooling rate of the billet surface is 10℃ / min. Post-stage forced cooling process: In the area from 1 / 4 of the billet thickness to the complete solidification of the billet, the secondary cooling water volume is controlled at 1.5L / kg, and the average cooling rate of the billet surface is 35℃ / min.

[0033] Example 2 In this embodiment, the superheat of the tundish is 15°C, and the secondary cooling zone of the continuous casting adopts a zoned cooling process of weak cooling in the front section and strong cooling in the back section. The specific parameters are as follows: Pre-stage weak cooling process: In the area from the solidification start point of the billet to 1 / 4 of the billet thickness, the secondary cooling water volume is controlled at 0.7L / kg, and the average cooling rate of the billet surface is 8℃ / min. Post-stage forced cooling process: In the area from 1 / 4 of the billet thickness to the complete solidification of the billet, the secondary cooling water volume is controlled at 1.7L / kg, and the average cooling rate of the billet surface is 40℃ / min.

[0034] Example 3 In this embodiment, the superheat of the tundish is 22°C, and the secondary cooling zone of the continuous casting adopts a zoned cooling process of weak cooling in the front section and strong cooling in the back section. The specific parameters are as follows: Pre-stage weak cooling process: In the area from the solidification start point of the billet to 1 / 3 of the billet thickness, the secondary cooling water volume is controlled at 0.6L / kg, and the average cooling rate of the billet surface is 5℃ / min; Post-stage forced cooling process: In the area from 1 / 3 of the billet thickness to the complete solidification of the billet, the secondary cooling water volume is controlled at 1.2L / kg, and the average cooling rate of the billet surface is 25℃ / min.

[0035] Example 4 In this embodiment, the superheat of the tundish is 20°C, and the secondary cooling zone of the continuous casting adopts a zoned cooling process of weak cooling in the front section and strong cooling in the back section. The specific parameters are as follows: Pre-stage weak cooling process: In the area from the solidification start point of the billet to 1 / 3 of the billet thickness, the secondary cooling water volume is controlled at 0.9L / kg, and the average cooling rate of the billet surface is 15℃ / min; Post-stage forced cooling process: In the area from 1 / 3 of the billet thickness to the complete solidification of the billet, the secondary cooling water volume is controlled at 1.8L / kg, and the average cooling rate of the billet surface is 45℃ / min.

[0036] Comparative Example 1 In this comparative example, the tundish superheat is 20°C, and the traditional uniform strong cooling process is used in the secondary cooling zone of the continuous casting. The specific parameters are as follows: The secondary cooling water volume is 1.4 L / kg, and the average cooling rate of the billet surface is 30 °C / min.

[0037] Comparative Example 2 In this comparative example, the tundish superheat is 20°C, and a weak cooling process is used throughout the secondary cooling zone of the continuous casting process. The specific parameters are as follows: The secondary cooling water volume is 0.8 L / kg, and the average cooling rate of the billet surface is 10 °C / min.

[0038] II. Performance Testing and Characterization 1. TiN size distribution and low-magnification rating analysis of cast billets Sampling was performed on the fully solidified billets of Examples 1-2 and Comparative Examples 1-2. Metallographic observation was performed at 1 / 4 of the billet thickness (near the end of the initial weak cooling zone) and the core (corresponding to the end of the later strong cooling zone). The size distribution of TiN was statistically analyzed using image analysis. The TiN size distribution and the low-magnification rating of the billet are shown in Table 1.

[0039] Table 1. TiN Size Distribution and Low-Magnification Rating of Cast Billets

[0040] As can be seen from Table 1, (1) The present invention (Example 1 and Example 2) successfully achieved graded precipitation control of TiN particles, which can accurately obtain two TiN groups with expected size ranges, while completely suppressing the generation of harmful large particles with a particle size greater than 8 μm, thus eliminating the potential threat of coarse TiN to material performance from the source.

[0041] (2) Compared with the traditional uniform strong cooling process (Comparative Example 1), the present invention (Example 1 and Example 2) forms finer late-stage precipitates while significantly reducing the internal thermal stress of the billet due to the use of a weak cooling process in the front stage, thus performing better in terms of center segregation and crack control, and effectively improving the overall billet quality.

[0042] (3) Compared with the insufficient cooling process (Comparative Example 2), although the present invention adopts a strong cooling process in the later stage, it can still reduce the proportion of large-size TiN (particle size greater than 8μm) from 4.0% to a level close to zero, fundamentally eliminating the key factors that cause the deterioration of the low-temperature toughness of steel and significantly improving the service performance of the material.

[0043] (4) Compared with Example 1, Example 2 further obtained finer TiN particles and achieved better billet center quality by finely adjusting the cooling process. This shows that the front-weak and back-strong partitioned cooling process established in this invention has good process adaptability and can flexibly adjust the cooling rate of each stage according to different steel grades and quality requirements to achieve customized optimization of performance and quality.

[0044] 2. Comparison of nitrogen fixation release modes and analysis of efficiency advantages in the early stage of solidification The nitrogen fixation release patterns of Example 1 and Comparative Example 1 are significantly different. Specifically, the nitrogen fixation rate of Example 1 is 63% in the early stage of solidification and 37% in the later stage; while the nitrogen fixation rate of Comparative Example 1 is 35% in the early stage and 65% in the later stage.

[0045] It can be seen that the nitrogen fixation efficiency in the early stage of solidification of Example 1 (63%) is significantly better than that of Comparative Example 1 (35%), indicating that it can convert free nitrogen into stable nitrides more quickly in the early stage of treatment, thereby more effectively suppressing the adverse effects of free nitrogen on material properties in the later stage, and is expected to obtain better comprehensive mechanical properties.

[0046] In summary, this invention provides a novel method for actively and precisely controlling the size distribution of TiN inclusions in titanium microalloyed steel by controlling the timing of the secondary cooling zone in continuous casting. This method enhances the performance potential of the steel while ensuring the quality of the cast billet, and has significant industrial application value.

[0047] The embodiments described above merely illustrate specific implementation methods of this application, and while the descriptions are detailed, they should not be construed as limiting the scope of protection of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the technical solution of this application, and these modifications and improvements all fall within the scope of protection of this application.

Claims

1. A method for controlling the TiN content in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting, characterized in that, include: A zoned cooling process with weak cooling in the front section and strong cooling in the back section is adopted for the secondary cooling zone of continuous casting.

2. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting, as described in claim 1, is characterized in that... The partitioned cooling process for the secondary cooling zone of continuous casting, which employs weak cooling in the front section and strong cooling in the back section, includes: controlling the secondary cooling water volume at 0.6-0.9 L / kg and the average cooling rate of the billet surface at 5-15℃ / min in the region from the solidification point of the billet to 1 / 4 to 1 / 3 of the billet thickness; and controlling the secondary cooling water volume at 1.2-1.8 L / kg and the average cooling rate of the billet surface at 25-45℃ / min in the region from 1 / 4 to 1 / 3 of the billet thickness to the complete solidification of the billet.

3. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting, as described in claim 2, is characterized in that... Within the region from the solidification starting point of the billet to 1 / 4 to 1 / 3 of the billet thickness, the secondary cooling water volume is controlled to be 0.7 to 0.8 L / kg, and the average cooling rate of the billet surface is 8 to 12 °C / min.

4. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting according to claim 2, characterized in that, Within the region from 1 / 4 to 1 / 3 of the billet thickness to complete solidification, the secondary cooling water volume is controlled at 1.5 to 1.8 L / kg, and the average cooling rate of the billet surface is 35 to 45 °C / min.

5. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting according to claim 1, characterized in that, The composition of the titanium microalloy steel, by mass percentage, includes: Ti 0.01%~0.12%; N 0.002%~0.007%.

6. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting, as described in claim 5, is characterized in that... The Ti content is 0.07% to 0.12%.

7. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting, as described in claim 5, is characterized in that... The value of N is 0.003% to 0.005%.

8. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting according to claim 1, characterized in that, The supporting process parameters for the partitioned cooling process of the secondary cooling zone of continuous casting, which adopts weak cooling in the front section and strong cooling in the back section, include: the superheat of the tundish is 15~25℃.

9. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting according to claim 1, characterized in that, The supporting process parameters for the partitioned cooling process of the secondary cooling zone of continuous casting, which adopts weak cooling in the front section and strong cooling in the back section, include: the casting speed is kept constant, and the fluctuation range does not exceed ±0.05m / min.

10. The method for controlling TiN in titanium microalloyed steel based on the timing of the secondary cooling zone in continuous casting according to claim 9, characterized in that, The pulling speed is 0.9~1.2m / min.

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