A smelting method for controlling the precipitation of large-size titanium nitride in the center of a casting blank

Abstract

The application discloses a smelting method for controlling large-size titanium nitride precipitation in the center of a casting blank, and belongs to the technical field of iron and steel metallurgy. The method is characterized by the following steps: controlling titanium during converter tapping, LF refining segmented alloying, VD / RH vacuum deep denitrogenation, precise calcium treatment and soft blowing, continuous casting low superheat and weak cooling and electromagnetic stirring, and non-uniform light pressing at the solidification end, so as to reduce the supersaturation of titanium and nitrogen in the center equiaxed crystal zone at the solidification end, and inhibit the precipitation of coarse angular titanium nitride in the center segregation band of the casting blank. After the application is implemented, the average size of the titanium nitride in the center of the casting blank is reduced from 8-12 mu m to 3-5 mu m, the rating is less than or equal to 1.0, and the subsequent hot working cracking and fatigue performance of die steel, bearing steel, high-grade automobile steel and titanium-containing microalloy steel are significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, and relates to a smelting method for controlling the precipitation of large-sized titanium nitride in the core of cast billets. Background Technology

[0002] In titanium-containing microalloyed steel, mold steel, and clean steel, titanium nitride, due to its high melting point and hardness, tends to precipitate as coarse angular particles in the central segregation zone at the end of the billet solidification period, severely impacting the subsequent processing and performance of the steel. Existing technologies suffer from the following prominent problems: 1) Inappropriate timing of titanium alloying. Most companies add titanium only once after tapping or supplement it in the later stages of LF refining. This results in significant titanium loss due to the reaction with oxidizing substances in the slag, leading to unstable yields, generally between 40% and 60%. This forces frequent fine-tuning of titanium content before continuous casting, causing localized supersaturation of titanium and nitrogen at the solidification end, which in turn exacerbates the precipitation of coarse TiN in the core. 2) Inadequate vacuum denitrification control. LLF refining nitrogen addition is not strictly controlled, with nitrogen content often fluctuating between 40 and 60 ppm. The thermodynamically sensitive range for TiN precipitation is Ti×N ≥ 400 ppm², and existing processes struggle to stabilize the titanium-nitrogen activity product below the precipitation sensitivity line. 3) Unreasonable soft-blowing process. Insufficient soft blowing time or excessive flow rate leads to exposed molten steel absorbing nitrogen, or violent churning of the slag surface causes... 4) Severe segregation during continuous casting solidification. High superheat and strong secondary cooling lead to well-developed columnar crystals and severe central segregation. Titanium and nitrogen are mutually enriched in the "concentrated soup zone" of the final solidification, providing kinetic conditions for TiN coarsening. 5) "One-size-fits-all" light reduction parameters. Existing light reduction parameters mostly apply equal reduction within a fixed solid fraction range, without optimization for TiN-sensitive steel grades. The reduction amount and casting speed are poorly matched, failing to effectively "squeeze out" the titanium- and nitrogen-rich concentrated liquid.

[0003] In summary, existing processes have shortcomings in controlling both the thermodynamic titanium-nitrogen activity product and kinetic segregation enrichment, resulting in large and unevenly distributed TiN inclusions in the core of titanium-containing steel billets, high defects in rolled products, mold cracking, and insufficient bearing fatigue life. Therefore, there is an urgent need for a smelting method that suppresses TiN precipitation in the core through synergistic control of the entire process. Summary of the Invention

[0004] The present invention aims to provide a smelting method for controlling the precipitation of large-sized titanium nitride in the core of the billet, which can overcome the defects of the prior art, suppress the precipitation of large-sized titanium nitride inclusions in the core of the billet, and make TiN precipitate as dispersed fine particles during solidification rather than coarsening and agglomerating in the core, thereby improving the hot working performance and final service performance of titanium-containing steel.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet includes the following process steps: Step 1, converter tapping stage: slag blocking during tapping, controlling the acid-soluble aluminum (Als) in the tapped steel to be ≥0.030wt%, adding ferrotitanium when the steel is 1 / 3 full, controlling the titanium yield to be 65%~75%, preventing secondary nitrogen absorption during tapping, and ensuring the slag layer thickness is ≤50mm; Step 2, LF refining stage: Low-nitrogen submerged arc slag operation is adopted. After slag formation by electric current and before vacuum treatment, the first titanium alloying is completed. The temperature of molten steel is controlled at ≥1580℃, so that the titanium content reaches 60% to 70% of the target value. Step 3, Vacuum Treatment Stage: Transfer the molten steel to VD or RH for vacuum degassing, with a vacuum degree ≤67Pa and a holding time ≥15min. Before vacuum breaking, the nitrogen content [N] in the molten steel should be ≤35ppm, and the titanium content loss should be ≤0.002wt%. After vacuum breaking, return to LF for a second titanium replenishment, precisely adjusting the titanium content to 0.010~0.020wt%. Step 4, calcium treatment and soft blowing stage: feed the silicon-calcium wire and control the Ca / Al ratio to 0.08-0.12, then perform soft blowing with argon for ≥15 minutes. During the soft blowing process, the molten steel is not exposed and the slag surface moves slightly. Step 5, continuous casting stage: the superheat of the tundish is controlled at 15-25℃, the crystallizer is stirred by electromagnetic stirring, the secondary cooling section adopts a weak cooling water supply system, the specific water volume is ≤0.25L / kg, and the casting speed fluctuation is ≡±0.05m / min; Step 6, light reduction at the end of solidification: Apply a non-uniform reduction of 3-6 mm in the range of solid fraction fs = 0.3-0.7, with the reduction rate synchronized with the pulling speed.

[0006] Furthermore, in step 1, the converter tapping stage: before tapping, slag-blocking operation is carried out using slag-blocking balls or slag-blocking plates, and the tapping time is controlled at 3 to 5 minutes. During the tapping process, long furnace cap tapping or nitrogen sealing tapping is used to prevent air entrainment and nitrogen increase.

[0007] Further, in step 2, the LF refining stage: the LF refining process involves adding titanium in two stages. The first stage involves adding 60% to 70% of the total titanium 5 minutes after energizing, and the second stage involves adding the remaining 30% to 40% before soft blowing, in order to prevent the titanium added to the molten steel from reacting prematurely with the oxides in the slag.

[0008] Further, in step 3, the vacuum treatment stage: RH vacuum treatment is used, with a cycle factor ≥3.5, increasing the gas flow rate to 800~1200NL / min, and a vacuum denitrification efficiency ≥40%; or VD vacuum treatment is used, with a holding time ≥15min, and a vacuum chamber pressure ≤67Pa.

[0009] Furthermore, in step 3, the vacuum treatment stage: from the moment the vacuum is broken until casting, a nitrogen-absorbing covering agent is used throughout the process, and the nitrogen increase in the molten steel is ≤3ppm.

[0010] Further, in step 4, the calcium treatment and soft blowing stage: the calcium silicate wire is fed in while the powder spraying process is carried out. The length of the calcium silicate wire fed in is 30-50m, and the powder spraying time is 2-4min.

[0011] Further, in step 5, the continuous casting stage: the electromagnetic stirring parameters of the crystallizer are: current 300-400A, frequency 4-6Hz, and the stirring direction is switched every 30 minutes.

[0012] Further, in step 6, light reduction at the end of solidification: the starting point of light reduction is calibrated by a nail gun test, and the reduction amount is non-uniformly distributed in the direction of the width of the billet cross section, with the reduction amount being larger in the middle and smaller at the edges. The reduction amount in the middle is 60% to 80% of the total reduction amount.

[0013] Furthermore, the total nitrogen content in the molten steel throughout the entire process is ≤10ppm, and the proportion of titanium nitride inclusions with a unit area size of ≤5μm in the core of the billet is ≥90%.

[0014] The control principle of this invention is based on the following thermodynamic and kinetic analysis: (1) Thermodynamically sensitive range of titanium nitride precipitation. The sensitive temperature for TiN precipitation is 1450~1500℃, corresponding to a solid fraction of fs≈0.4~0.8 in the billet. This range is the "thick soup zone" of the final solidification. By strictly controlling the nitrogen content to ≤35ppm and accurately maintaining the titanium content at 0.010~0.020wt%, the titanium-nitrogen activity product Ti×N≤350ppm² can be achieved, which is lower than the critical threshold for the nucleation and coarsening of coarse TiN, forcing TiN to precipitate as fine and dispersed particles.

[0015] (2) Kinetic advantages of deep vacuum denitrification. VD / RH vacuum treatment can efficiently remove dissolved nitrogen from steel, with a denitrification efficiency of ≥40%, reducing the nitrogen content from 45-55 ppm at the LF endpoint to ≤35 ppm, creating conditions for subsequent precise titanium control. The key innovation lies in the dual-point titanium control mechanism of "pre-vacuum titanium addition and post-vacuum titanium refinement"—adding titanium before vacuum can make full use of the low oxygen potential of the vacuum environment to reduce titanium oxidation loss, while adding titanium after vacuum can accurately compensate for the titanium loss during the vacuum process and finely adjust to the target range.

[0016] (3) Macroscopic control of weak segregation solidification. In the continuous casting stage, a synergistic system of low superheat (15-25℃) plus weak cooling (specific water volume ≤0.25L / kg) plus electromagnetic stirring is adopted to expand the proportion of equiaxed crystal zone, dilute the central segregation, and reduce the volume of mutual enrichment of the final solidified liquid from a macroscopic perspective.

[0017] (4) Micro-control under non-uniform light pressure. Applying non-uniform light pressure in the range of fs=0.3 to 0.7, i.e. the solidification "thick soup zone", the pressure distribution of large in the middle and small at the edges can "squeeze out" the residual liquid pool rich in titanium and nitrogen, interrupt the continuous growth window of TiN, and make the existing fine TiN particles remain in the matrix instead of migrating and aggregating to the center.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1) The average size of titanium nitride in the core of the billet is significantly reduced from 8-12 μm to 3-5 μm, and there are no 10 μm angular TiN precipitates; 2) The activity product of titanium and nitrogen in the core of the billet is Ti×N≤350ppm², which is far below the threshold for coarse precipitation, and TiN exists in the form of dispersed particles; 3) The center segregation level is ≤C class 1.0 grade (according to GB / T10561), and the total rating of non-metallic inclusions is significantly reduced; 4) The pass rate of flaw detection of rolled products is increased from 82% to over 98%, and the tendency of hot cracking in titanium-containing low alloy steel is significantly reduced; 5) The total nitrogen increase in the whole process is ≤10ppm, and the process stability and repeatability are excellent. Attached Figure Description

[0019] Figure 1 A microstructure of titanium nitride in the core of a steel plate produced by existing technology and process for casting and rolling.

[0020] Figure 2 This is a microstructure of titanium nitride in the core of a steel plate produced by the process of this invention. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to specific embodiments. Example 1

[0022] A method for smelting high-strength titanium-containing steel for engineering machinery, with a cross-sectional dimension of 300mm × 2270mm and a casting temperature of 1540℃, includes the following process steps: (1) Converter tapping: The tapping volume was 143t, with a double slag-blocking system using slag-blocking balls and plates, resulting in a slag layer thickness of 40mm after tapping. Before tapping, the carbon content of the molten pool was controlled at 0.08-0.12%, and the oxygen content was ≤600ppm. 180kg of ferroaluminum was added first for deoxidation, followed by 800kg of ferromanganese to stabilize the slag composition. When the tapping volume was 1 / 3, 240kg of ferrotitanium was added, with a titanium recovery rate of 68%. Nitrogen sealing was used at the furnace mouth during the tapping process to prevent nitrogen absorption, and the tapping time was 3.5min. At the end of tapping, Als was measured to be 0.032%.

[0023] (2) LF refining: Add low-nitrogen submerged arc slag (60% lime, 20% alumina, 15% calcium fluoride, and 5% calcium silicate deoxidizer), and slag by electrolysis. After slag formation and before vacuum treatment, add 140 kg of titanium supplement—titanium iron—to bring the titanium content to the target value of 65%. The LF process is carried out with argon blowing and stirring throughout, and the temperature is controlled at 1595–1605℃. The final nitrogen content of the LF process is 42 ppm, and the titanium content is 0.013%.

[0024] (3) Vacuum treatment with VD: Molten steel is transferred into a VD tank, and a vacuum is drawn until the pressure is ≤50Pa. The vacuum is maintained for 18 minutes, followed by circulation degassing. During the vacuum process, the titanium content decreases from 0.013% to 0.011% (loss of 0.002%), and the nitrogen content decreases from 42ppm to 33ppm (denitrification efficiency 39%). Before vacuum treatment, [N] is 33ppm and [Ti] is 0.011%.

[0025] (4) Titanium replenishment after venting: After venting, the nitrogen content in the molten steel is increased by 2 ppm (total nitrogen increase ≤ 4 ppm). 100 kg of ferrotitanium is added for a second precise titanium replenishment, adjusting the titanium content to 0.016%, and the temperature is raised back to 1605℃. Subsequently, 45 m of SiCa wire is fed in, controlling Ca / Al = 0.10, and powder spraying is performed for 3 min. Soft blowing of argon is performed for 16 min at an argon flow rate of 0.8 m³ / min, with slight movement of the slag surface and no exposed molten steel. Final inspection: [N] = 34 ppm, [Ti] = 0.016%.

[0026] (5) Continuous casting: The superheat of the tundish is 21℃, the electromagnetic stirring parameters of the crystallizer are 350A / 5Hz, and the stirring direction is switched every 30min. The secondary cooling section adopts a weak cooling regime, with a specific water volume of 0.22L / kg, a casting speed of 0.78m / min, and a casting speed fluctuation of ±0.03m / min. The crystallizer and the secondary cooling section adopt a full-arc electromagnetic stirring combined with gas and slag protection.

[0027] (6) Light pressing at the end of solidification: Calibrated by nail shooting test, light pressing begins at solid fraction fs=0.45, with a total pressing amount of 5mm, distributed in a "larger in the middle and smaller at the edge" pattern (3.5mm in the middle and 1.5mm at the edge). The pressing rate is synchronized with the pulling speed, and the pulling speed is maintained at 0.78m / min during the pressing period.

[0028] Results of Example 1: After sampling and dissection of the continuously cast billet, the maximum size of the titanium nitride in the core was 4.8 μm, with an average size of 3.6 μm. No angular TiN particles larger than 10 μm were found. The center segregation grade was Class C, Grade 1.0 (GB / T 10561). The rolled product underwent flaw detection, and the pass rate reached 98.5%. Example 2

[0029] The production of high-strength ship plates using titanium-containing steel, with a cross-section of 300mm × 2270mm, compares the differences between the process of this invention and the traditional process: Original process parameters: one-time addition of ferrotitanium at tapping (titanium yield 45%), LF refining without nitrogen control (endpoint [N] = 50ppm), no vacuum treatment, superheat 32℃, strong secondary cooling (specific water volume 0.35L / kg), no light pressure reduction.

[0030] The process parameters of this invention are: titanium control in multiple tapping stages (70% yield), LF staged titanium addition and VD deep denitrification ([N]=32ppm), low superheat 18℃, weak cooling (specific water volume 0.20L / kg), and non-uniform light pressure (fs=0.35~0.65, total pressure reduction 4mm).

[0031] Comparison results: The maximum size of TiN in the core of the billet produced by the original process is 11.2 μm, with obvious angular aggregation, central segregation of Class C 2.0, and a flaw detection pass rate of 82% for rolled products; The maximum size of TiN in the core of the billet produced by the process of this invention is 5.2 μm, with an average of 3.8 μm, central segregation of Class C 1.0, and a flaw detection pass rate of 98.2%.

[0032] Comparative Example 1: Comparative Example 1 uses the existing common process of "one-time titanium addition after vacuum": LF refining endpoint [N] = 45ppm, titanium content 0.008%, followed by direct VD vacuum treatment (15min), after which only 80kg of ferrotitanium is added to make the titanium content reach 0.014%, without segmented titanium control, without precise calcium treatment soft blowing optimization, and continuous casting adopts the traditional strong cooling system.

[0033] Comparative results: Although the nitrogen content decreased to 36 ppm, a significant amount of titanium was lost during the vacuum process (loss of 0.005%). After titanium replenishment, the titanium distribution was uneven, with local Ti×N still reaching 420 ppm² at the end of solidification. TiN angular particles with a maximum size of 9.5 μm appeared in the core of the cast billet, receiving a rating of C-class 1.5. This demonstrates that the dual-point titanium control mechanism of "pre-vacuum titanium addition and post-vacuum titanium refinement" is the core feature of this invention's inventiveness. Simply replacing the existing "post-vacuum titanium addition" approach with "pre-vacuum titanium addition" without supporting other processes cannot achieve the same effect as this invention.

[0034] Table 1 compares the process parameters of the embodiments and comparative examples of the present invention.

[0035] Table 1. Comparison of process parameters between the embodiments of the present invention and the comparative examples. .

Claims

1. A smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet, characterized in that... The process includes the following steps: Step 1, converter tapping stage: slag blocking during tapping, controlling the acid-soluble aluminum (Als) in the tapped steel to be ≥0.030wt%, adding ferrotitanium when the steel is 1 / 3 full, controlling the titanium yield to be 65%~75%, preventing secondary nitrogen absorption during tapping, and ensuring the slag layer thickness is ≤50mm; Step 2, LF refining stage: Low-nitrogen submerged arc slag operation is adopted. After slag formation by electric current and before vacuum treatment, the first titanium alloying is completed. The temperature of molten steel is controlled at ≥1580℃, so that the titanium content reaches 60% to 70% of the target value. Step 3, Vacuum Treatment Stage: Transfer the molten steel to VD or RH for vacuum degassing, with a vacuum degree ≤67Pa and a holding time ≥15min. Before vacuum breaking, the nitrogen content [N] in the molten steel should be ≤35ppm, and the titanium content loss should be ≤0.002wt%. After vacuum breaking, return to LF for a second titanium replenishment, precisely adjusting the titanium content to 0.010~0.020wt%. Step 4, calcium treatment and soft blowing stage: feed the silicon-calcium wire and control the Ca / Al ratio to 0.08-0.12, then perform soft blowing with argon for ≥15 minutes. During the soft blowing process, the molten steel is not exposed and the slag surface moves slightly. Step 5, continuous casting stage: the superheat of the tundish is controlled at 15-25℃, the crystallizer is stirred by electromagnetic stirring, the secondary cooling section adopts a weak cooling water supply system, the specific water volume is ≤0.25L / kg, and the casting speed fluctuation is ≡±0.05m / min; Step 6, light reduction at the end of solidification: Apply a non-uniform reduction of 3-6 mm in the range of solid fraction fs = 0.3-0.7, with the reduction rate synchronized with the pulling speed.

2. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that: Step 1: Converter tapping stage: Before tapping, slag-blocking operation is carried out using slag-blocking balls or slag-blocking plates. The tapping time is controlled at 3-5 minutes. During the tapping process, long furnace cap tapping or nitrogen sealing tapping is used to prevent air entrainment and nitrogen increase.

3. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that: Step 2, LF refining stage: The LF refining process involves adding titanium in two stages. The first stage involves adding 60% to 70% of the total titanium 5 minutes after powering on, and the second stage involves adding the remaining 30% to 40% before soft blowing.

4. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that... Step 3, Vacuum treatment stage: Use RH vacuum treatment, with a cycle factor ≥3.5, increase the gas flow rate to 800~1200NL / min, and the vacuum denitrification efficiency ≥40%; or use VD vacuum treatment, with a holding time ≥15min and a vacuum chamber pressure ≤67Pa.

5. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that... Step 3, Vacuum treatment stage: From the moment the vacuum is broken until casting, a nitrogen-absorbing covering agent is used throughout the process, and the nitrogen content in the molten steel is ≤3ppm.

6. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that... Step 4, calcium treatment and soft blowing stage: Simultaneously with feeding the calcium silicate wire, powder spraying is carried out. The feeding length of the calcium silicate wire is 30-50m, and the powder spraying time is 2-4min.

7. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that... Step 5, continuous casting stage: The electromagnetic stirring parameters of the crystallizer are: current 300-400A, frequency 4-6Hz, and the stirring direction is switched every 30 minutes.

8. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that... Step 6, light reduction at the end of solidification: The starting point of light reduction is determined by the nail shooting test. The reduction amount is non-uniformly distributed in the direction of the width of the billet cross section, with the larger amount in the middle and the smaller amount at the edges. The reduction amount in the middle is 60% to 80% of the total reduction amount.

9. The smelting method for controlling the precipitation of large-sized titanium nitride in the core of a cast billet according to claim 1, characterized in that: The total nitrogen content in molten steel throughout the entire process is ≤10ppm, and the proportion of titanium nitride inclusions with a unit area size of ≤5μm in the core of the billet is ≥90%.

Images