A high-impact-toughness steel for rail transit support seats and a method for manufacturing the same

Abstract

The application provides a high-impact-toughness rail transit support seat steel and a preparation method thereof. The chemical components of the steel are as follows in percentage by weight: C is less than or equal to 0.15%, Si is 0.20-0.28%, Mn is 0.65-0.70%, P is less than or equal to 0.012%, S is less than or equal to 0.003%, Cr is 0.10-0.30%, Al is 0.022-0.035%, gas O is less than or equal to 15ppm, N is less than or equal to 80ppm, H is less than or equal to 2ppm, and the balance is Fe and inevitable impurities. The preparation method comprises the following steps: after the molten steel is subjected to KR pretreatment, converter smelting, LF refining and RH vacuum treatment, the molten steel is subjected to continuous casting, the continuous casting pulling speed is 1.4-1.6 m / min, the specific water consumption of the secondary cooling section is 0.8-1 L / kg, the proportion of columnar crystal organization of the continuous casting billet is more than 85%, and then the continuous casting billet is subjected to forging and air cooling. Through the continuous casting process of low pulling speed and strong cooling, the proportion of columnar crystal organization is actively increased, the impact energy at-20 DEG C is more than 105 J without adding valuable alloy, and the continuous casting production efficiency is considered.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a high-impact toughness steel for rail transit support seats and its preparation method. Background Technology

[0002] Rail transit support seats are key load-bearing components in rail transit systems, responsible for bearing train loads and transmitting vibration and impact forces. As my country's rail transit system develops towards higher speeds and heavier loads, train speeds and loads are continuously increasing. The dynamic stresses, vibrations, impacts, and complex load environments experienced by the support seats are becoming increasingly severe, placing higher demands on the strength, toughness, and impact resistance of the materials. In particular, low-temperature impact toughness is directly related to the safe operation of rail transit in cold regions. Existing research indicates that one of the key performance indicators for steel used in support seats is its low-temperature impact performance, generally requiring an impact energy of no less than 27 J at -20℃.

[0003] Existing technologies generally believe that increasing the proportion of equiaxed crystals is beneficial to improving the impact toughness and comprehensive mechanical properties of steel. For example, Chinese patent CN116426832B discloses a hot-rolled steel plate for casing in medium-deep well mining. This technical solution clearly proposes to increase the equiaxed crystal ratio through electromagnetic stirring in the secondary cooling zone to improve the quality of the billet. However, in the continuous casting process, in order to achieve a higher proportion of equiaxed crystals, weak cooling measures are often required, which directly limits the output per unit time of the continuous casting machine, resulting in limited production efficiency.

[0004] Chinese patent CN117802412A discloses "A 500MPa grade quenched and tempered steel plate with excellent weldability and its production method". The plate has an equiaxed crystal ratio of ≤25% and an impact toughness of KV2≥145J at -20℃, which is excellent. However, this method requires the addition of multiple alloying elements such as Ni, Mo, V, Nb, and B, as well as metallurgical processes such as Mg treatment and Ti deoxidation, resulting in high production costs and complex smelting processes.

[0005] Given the above, how to improve the low-temperature impact toughness of steel used in rail transit support seats without adding complex alloy systems or sacrificing continuous casting production efficiency is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a high-impact toughness steel for rail transit support seats and its preparation method, solving the technical problem of "how to improve the low-temperature impact toughness of low alloy steel while ensuring continuous casting production efficiency".

[0007] To achieve the above objectives, the present invention is implemented using the following technical solution: This invention provides a high-impact toughness steel for railway transportation support seats, comprising the following components by weight percentage: C≤0.15%, Si 0.20~0.28%, Mn 0.65~0.70%, P≤0.012%, S≤0.003%, Cr 0.10~0.30%, Al 0.022~0.035%, with additional gaseous O content ≤15ppm, N content ≤80ppm, H content ≤2ppm, and the balance being Fe and unavoidable impurities.

[0008] The design principle of the chemical composition of the steel plate of this invention is as follows: Carbon (C): Carbon is the most economical and basic strengthening element in steel, and it has a significant effect on improving the strength of steel through solid solution strengthening. However, excessive carbon content will significantly reduce the plasticity and low-temperature impact toughness of steel. Therefore, this invention controls the carbon content to ≤0.15% to ensure that the steel matrix has a good toughness and plasticity foundation.

[0009] Silicon (Si): Silicon is the main deoxidizing element in steel and also has a certain solid solution strengthening effect. However, excessive silicon content will reduce the low-temperature impact toughness of steel. This invention controls the Si content to 0.20~0.28%, ensuring the deoxidation effect while avoiding adverse effects on toughness.

[0010] Manganese (Mn): Manganese is a good deoxidizer and solid solution strengthening element, which can improve the strength and hardenability of steel, while lowering the phase transformation temperature and refining ferrite grains. However, excessive manganese content will exacerbate center segregation in continuously cast billets, adversely affecting impact toughness. This invention controls the Mn content to 0.65~0.70%, ensuring both strength and toughness.

[0011] Phosphorus (P): Phosphorus tends to segregate at grain boundaries in steel, significantly reducing the low-temperature impact toughness of the steel. Its content should be minimized as much as possible. In this invention, P is controlled to be ≤0.012%.

[0012] Sulfur (S): Sulfur readily combines with manganese in steel to form MnS inclusions, significantly reducing the steel's impact toughness and density. Therefore, its content should be minimized as much as possible. This invention controls S ≤ 0.003%.

[0013] Chromium (Cr): Chromium improves the hardenability of steel and promotes the formation of a tough microstructure with excellent low-temperature toughness. A small amount of chromium dissolved in solid solution can also inhibit high-temperature austenite grain boundary migration and refine the original austenite grains, which is beneficial for improving low-temperature impact toughness. However, when the chromium content is too high, excessive solid solution strengthening of the matrix leads to a decrease in the plasticity of the steel and an increase in the ductile-brittle transition temperature, which in turn worsens the low-temperature impact performance. This invention controls the Cr content to be 0.10~0.30%.

[0014] Aluminum (Al): Aluminum acts as a deoxidizer in steel, and also combines with nitrogen (N) in the steel to form fine AlN particles, which can pin austenite grain boundaries and refine grains. If the aluminum content is too low, deoxidation is insufficient; if it is too high, coarse alumina inclusions are easily formed, reducing the cleanliness and impact toughness of the steel. This invention controls the Al content to be 0.022~0.035%.

[0015] Oxygen (O), nitrogen (N), and hydrogen (H) are harmful gaseous elements in steel. O easily forms oxide inclusions, N easily forms coarse nitride inclusions or causes free nitrogen solid solution embrittlement, and H easily causes hydrogen-induced cracks and white spots, all of which seriously impair impact toughness. This invention controls O ≤ 15 ppm, N ≤ 80 ppm, and H ≤ 2 ppm.

[0016] Another object of the present invention is to provide a method for preparing steel for high impact toughness rail transit support seats, comprising the following steps: S1. Smelting: After pretreatment, the molten iron is sent to the converter for smelting. The carbon content of the tapped steel is controlled at 0.05~0.10%. The tapping process adopts slag-blocking tapping and bottom blowing argon gas stirring throughout the process. S2, LF Refining: The molten steel after tapping from the converter enters the LF refining furnace for refining. Lime and fluorite are added for slag formation, and SiC is used for diffusion deoxidation. The FeO+MnO content in the slag is controlled to be less than 1%. White slag is refined for 20 minutes with argon stirring throughout the process, and the oxygen content in the steel is controlled to be ≤20ppm. S3, RH vacuum treatment: The molten steel after LF refining is sent into RH vacuum treatment, pure calcium wire is fed in, and finally soft argon blowing is performed for 15~25 minutes. S4. Continuous Casting: The molten steel is poured into round billets. The superheat and casting speed are strictly controlled. The casting speed is 1.4~1.6 m / min. The liquid level fluctuation in the crystallizer is controlled within ±3 mm. The crystallizer uses a special protective slag for continuous casting of low-carbon steel. The cooling water flow rate is 120~150 m³ / min. 3 / h, the water flow rate in the secondary cooling section is 0.8~1L / kg, electromagnetic stirring is carried out, and if the liquid level fluctuation exceeds ±5mm, waste is removed to obtain a continuous casting billet; the ratio of columnar crystals and equiaxed crystals in the continuous casting billet is controlled; S5. Forging: Heat the continuously cast billet to 1250℃ and hold for 30 minutes. The initial forging temperature is 1180~1230℃, and the final forging temperature is 1050~1100℃. After forging, air cool.

[0017] Preferably, in S1, the smelting process further includes controlling the amount of molten iron fed into the furnace: P ≤ 0.10%, Si ≤ 0.70%, and slag content ≤ 0.5%.

[0018] Preferably, in S2, the amount of lime added is 6~8 kg / t steel, and the amount of fluorite added is 1~2 kg / t steel.

[0019] Preferably, in S3, the vacuum degree of the RH vacuum treatment is ≤67Pa, and the vacuum is maintained for 12~20 minutes; the feeding speed of the pure calcium wire is 2~4m / s, and the feeding amount is 0.5~1.5kg / t steel.

[0020] Preferably, in S4, the basicity R of the special protective slag for continuous casting of low carbon steel is 1.0~1.2, the viscosity at 1300℃ is 0.8~1.2 Pa·s, and the melting point is 1100~1150℃; the superheat is 15~45℃; the electromagnetic stirring parameters of the crystallizer are 60A / 4Hz, and the electromagnetic stirring parameters of the end are 150A / 10Hz.

[0021] Preferably, in S4, the proportion of columnar crystals in the continuously cast billet is 85-93%, and the proportion of equiaxed crystal structure is 7-15%.

[0022] Compared with the prior art, the beneficial effects achieved by the present invention are: (1) This invention breaks away from the traditional technical bias that "increasing the proportion of equiaxed crystals is beneficial to improving toughness." By controlling the process of low casting speed combined with strong cooling in the continuous casting process, a large temperature gradient is obtained at the solidification front, providing sufficient driving force for the directional growth of columnar crystals and actively increasing the proportion of columnar crystal structure in the continuous casting billet to over 85%. The high proportion of directional columnar crystals is fully broken up and dynamically recrystallized during the subsequent forging process through high-temperature deformation, forming a uniform and refined grain structure. The results of the examples show that the impact energy at -20℃ using the method of this invention reaches over 105J.

[0023] (2) This invention achieves excellent low-temperature impact toughness simply by optimizing and controlling the continuous casting process parameters. It eliminates the need for expensive alloying elements such as Ni, Mo, V, Nb, and B, as well as additional hardware such as light reduction equipment. This significantly reduces alloy costs and equipment investment costs while ensuring continuous casting production efficiency. Compared to existing technologies that rely on complex alloy systems or additional hardware equipment, this invention offers significant economic advantages and is particularly suitable for large-scale industrial production of low-alloy steel.

[0024] (3) This invention, through the combination of low-carbon composition design and trace Cr microalloying, ensures a good toughness and plasticity foundation for the steel matrix while improving hardenability and refining austenite grains through Cr element, further improving low-temperature impact toughness. This invention controls the continuous casting speed at 1.4~1.6m / min, combined with strong cooling to allow columnar crystals to grow fully in a directional manner, ensuring the solidification quality of the billet while taking into account the production efficiency of the continuous casting machine. Too low a casting speed will reduce output, while above this range, the proportion of columnar crystals will decrease and the impact toughness will be insufficient. This critical range ensures the impact toughness target while avoiding the forced reduction of casting speed to pursue equiaxed crystals, which would limit production capacity. Attached Figure Description

[0025] Figure 1 The images show the solidification morphology of the continuously cast billets after low-magnification pickling in Examples 1 and 2 of this invention. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] A low-carbon steel for rail transit support seats has the following chemical composition by weight percentage: C≤0.15%, Si0.20~0.28%, Mn0.65~0.70%, P≤0.012%, S≤0.003%, Cr0.10~0.30%, Al0.022~0.035%, with additional gaseous contents of O≤15ppm, N≤80ppm, H≤2ppm, and the balance being Fe and unavoidable impurities.

[0028] The preparation method of the high impact toughness rail transit support steel includes: smelting, LF refining, RH vacuum treatment, continuous casting and forging; S1. The molten iron is pretreated by the KR method, and the P content, Si content and slag content of the molten iron entering the furnace are controlled to be ≤0.10%, ≤0.70%, and ≤0.5. The pretreated molten iron is sent to the converter for high-speed drawing and blowing smelting. The carbon content of the tapped steel is controlled to be 0.05~0.10%. The tapping process is stirred by bottom blowing argon gas throughout, and slag-blocking tapping is adopted to control the slag content to be ≤5cm. S2. After the steel is tapped from the converter, it enters the LF refining furnace for slag making, stirring, deoxidation and alloying. Lime and fluorite are added to the furnace. The amount of lime added is 6~8 kg / t steel and the amount of fluorite added is 1~2 kg / t steel. SiC is used for diffusion deoxidation. The FeO+MnO in the slag is controlled to be <1%. White slag is refined for 20 minutes. Argon gas is used for stirring throughout the process. The oxygen content in the steel is controlled to be ≤20ppm. After refining by S3 and LF, the molten steel is sent to RH vacuum treatment. The vacuum is evacuated to ≤67Pa and maintained for 12~20 minutes. Pure calcium wire is fed in at a feeding speed of 2~4m / s and the feeding amount is 0.5~1.5kg / t steel. Soft argon blowing is carried out for 15~25 minutes. S4. Pour the above molten steel into 180mm cross-section round billets. The continuous casting superheat is 15-45℃, the casting speed is 1.4-1.6m / min, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. The crystallizer uses a special protective slag for continuous casting of low-carbon steel, and the cooling water flow rate is 120-150m³. 3 / h, secondary cooling section specific water flow rate 0.8~1L / kg, crystallizer electromagnetic stirring parameters 60A / 4Hz, end electromagnetic stirring parameters 150A / 10Hz, process liquid level fluctuation exceeding ±5mm is discarded to obtain continuous casting billet; after low-magnification pickling of continuous casting billet, the proportion of columnar crystal structure is controlled to be 85~93%, and the proportion of equiaxed crystal structure is controlled to be 7~15%; S5. Heat the continuously cast billet to 1250℃ and hold for 30 minutes. The initial forging temperature is 1180~1230℃, and the final forging temperature is 1050~1100℃. After forging, air cool.

[0029] This invention, through low-carbon composition design and converter endpoint carbon content control, ensures that the carbon content of the tapped steel is 0.05~0.10%. Subsequent refining and continuous casting further guarantee that the carbon content in the finished steel is ≤0.15%. This low carbon content ensures a good toughness and plasticity foundation for the steel matrix, allowing the toughness gains obtained through subsequent columnar crystal structure regulation to be fully realized. KR pretreatment and slag-blocking tapping processes further guarantee low P, low S, and low O levels in the molten steel, providing a cleanliness guarantee for obtaining a high columnar crystal ratio in the continuously cast billet. This invention adds 0.10~0.30% trace amounts of Cr. Solid-solution Cr can improve the hardenability of the steel, induce the formation of a tough acicular bainite structure, and simultaneously inhibit high-temperature austenite grain boundary migration, achieving grain refinement and further improving the low-temperature impact performance of the steel. Limiting the Cr content within this range ensures the stable performance of its grain refinement and hardenability gains. Excessive Cr content will result in excessive solid solution strengthening, increasing the ductile-brittle transition temperature of the steel and deteriorating its low-temperature impact toughness. This invention achieves excellent low-temperature impact toughness in rail transit support steel by synergistically combining three processes: low-carbon matrix toughness and plasticity control, Cr microalloying for fine grain toughening, and high-proportion columnar grain optimization, without adding precious alloying elements such as Ni, Mo, V, and Nb.

[0030] In S4, the continuous casting process uses a low casting speed of 1.4~1.6m / min combined with strong cooling (primary cooling water flow rate of 120~150m³ / min). 3 The high-efficiency cooling (0.8-1 L / kg for secondary cooling) increases the temperature gradient at the solidification front, providing a driving force for the directional growth of columnar crystals. Low casting speed extends the residence time of the billet in the secondary cooling zone, allowing the strong cooling to fully act on the solidification front. The crystallizer electromagnetic stirring (60 A / 4 Hz) only acts on the meniscus region to improve surface quality, while the end electromagnetic stirring (150 A / 10 Hz) is positioned further back. Neither of these is sufficient to disrupt the continuous directional growth of columnar crystals. The combined effect of these three mechanisms results in a columnar crystal structure accounting for over 85% of the continuously cast billet. This high proportion of directional columnar crystals is fully broken up along the deformation direction and undergoes dynamic recrystallization during subsequent forging, forming a uniform and refined grain structure. This eliminates the directional structural characteristics of the columnar crystals, ultimately yielding forgings with excellent low-temperature impact toughness.

[0031] In this invention, the basicity R of the special protective slag for continuous casting of low carbon steel is 1.0~1.2, the viscosity (1300℃) is 0.8~1.2 Pa·s, and the melting point is 1100~1150℃.

[0032] Example 1; The chemical composition of the steel used for the rail transit support seat in this embodiment, by weight percentage, is as follows: C≤0.15%, Si0.20%, Mn0.65%, P≤0.012%, S≤0.003%, Cr0.10%, Mo0.011%, Al0.022%, with gaseous O content≤15ppm, N content≤80ppm, H content≤2ppm, and the balance being Fe and unavoidable impurities.

[0033] S1. Smelting: The molten iron is pretreated by the KR method, and the P content, Si content and slag content of the molten iron entering the furnace are controlled to be ≤0.10%, ≤0.70%, and ≤0.5. The pretreated molten iron is sent to the converter for high-speed drawing and blowing smelting. The carbon content of the tapped steel is controlled to be 0.05%. The tapping process is stirred by bottom blowing argon gas throughout, and slag-blocking tapping is adopted to control the slag content to be ≤5cm. S2, LF Refining: After the steel is tapped from the converter, it enters the LF refining furnace for slag making, stirring, deoxidation and alloying. Lime and fluorite are added to the furnace. The amount of lime added is 6 kg / t steel and the amount of fluorite added is 1 kg / t steel. SiC is used for diffusion deoxidation. The FeO+MnO in the slag is controlled to be <1%. White slag refining is carried out for 20 minutes. Argon gas is used for stirring throughout the process. The oxygen content in the steel is controlled to be ≤20ppm. S3, RH vacuum treatment: The molten steel after LF refining is sent to RH vacuum treatment, and the vacuum degree is ≤67Pa. The vacuum is maintained for 12 minutes. Pure calcium wire is fed in at a wire feeding speed of 2m / s. The wire feeding amount is 0.5kg / t steel. Soft argon blowing is performed for 15 minutes. S4. Continuous Casting: The molten steel is poured into 180mm cross-section round billets. The superheating temperature is 15℃, the casting speed is 1.4m / min, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. The crystallizer uses a special protective slag for continuous casting of low-carbon steel, and the cooling water flow rate is 120m³. 3 / h, secondary cooling section specific water flow rate 0.8L / kg, crystallizer electromagnetic stirring parameters 60A / 4Hz, end electromagnetic stirring parameters 150A / 10Hz, process liquid level fluctuation exceeding ±5mm is discarded to obtain continuous casting billet; S5. Forging: Heat the continuously cast billet to 1250℃ and hold for 30 minutes. The initial forging temperature is 1180℃ and the final forging temperature is 1050℃. After forging, air cool.

[0034] Example 2; The chemical composition of the steel used for the rail transit support seat in this embodiment, by weight percentage, is: C≤0.15%, Si0.25%, Mn0.68%, P≤0.012%, S≤0.003%, Cr0.2%, Al0.03%, with additional gaseous O content≤15ppm, N content≤80ppm, H content≤2ppm, and the balance being Fe and unavoidable impurities.

[0035] S1. Smelting: The molten iron is pretreated by the KR method, and the P content, Si content and slag content of the molten iron entering the furnace are controlled to be ≤0.10%, ≤0.70%, and ≤0.5. The pretreated molten iron is sent to the converter for high-speed drawing and blowing smelting. The carbon content of the tapped steel is controlled at 0.08%. The tapping process is stirred by bottom blowing argon gas throughout, and slag-blocking tapping is adopted to control the slag content to be ≤5cm. S2 and LF Refining: After the steel is tapped from the converter, it enters the LF refining furnace for slag making, stirring, deoxidation and alloying. Lime and fluorite are added to the furnace. The amount of lime added is 7 kg / t steel and the amount of fluorite added is 1.5 kg / t steel. SiC is used for diffusion deoxidation. The FeO+MnO in the slag is controlled to be <1%. White slag refining is carried out for 20 minutes. Argon gas is used for stirring throughout the process. The oxygen content in the steel is controlled to be ≤20ppm. S3, RH vacuum treatment: The molten steel after LF refining is sent to RH vacuum treatment, and the vacuum degree is ≤67Pa. The vacuum is maintained for 15 minutes. Pure calcium wire is fed in at a wire feeding speed of 3m / s, and the wire feeding amount is 1kg / t steel. Soft argon blowing is carried out for 20 minutes. S4. Continuous Casting: The molten steel is poured into 180mm cross-section round billets. The superheating temperature is 30℃, the casting speed is 1.5m / min, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. The crystallizer uses a special protective slag for continuous casting of low-carbon steel, and the cooling water flow rate is 135m³. 3 / h, secondary cooling section specific water flow rate 0.9L / kg, crystallizer electromagnetic stirring parameters 60A / 4Hz, end electromagnetic stirring parameters 150A / 10Hz, process liquid level fluctuation exceeding ±5mm is discarded to obtain continuous casting billet; S5. Forging: Heat the continuously cast billet to 1250℃ and hold for 30 minutes. The initial forging temperature is 1200℃ and the final forging temperature is 1080℃. After forging, air cool.

[0036] Example 3; The chemical composition of the steel used for the rail transit support seat in this embodiment, by weight percentage, is as follows: C:≤0.15%, Si0.28%, Mn0.70%, P≤0.012%, S≤0.003%, Cr0.30%, Al0.035%, with additional gaseous O content≤15ppm, N content≤80ppm, H content≤2ppm, and the balance being Fe and unavoidable impurities.

[0037] S1. Smelting: The molten iron is pretreated by the KR method, and the P content, Si content and slag content of the molten iron entering the furnace are controlled to be ≤0.10%, ≤0.70%, and ≤0.5. The pretreated molten iron is sent to the converter for high-speed drawing and blowing smelting. The carbon content of the tapped steel is controlled to be 0.10%. The tapping process is stirred by bottom blowing argon gas throughout, and slag-blocking tapping is adopted to control the slag content to be ≤5cm. S2, LF Refining: After the steel is tapped from the converter, it enters the LF refining furnace for slag making, stirring, deoxidation and alloying. Lime and fluorite are added to the furnace. The amount of lime added is 8 kg / t steel and the amount of fluorite added is 2 kg / t steel. SiC is used for diffusion deoxidation. The FeO+MnO in the slag is controlled to be <1%. White slag refining is carried out for 20 minutes. Argon gas is used for stirring throughout the process. The oxygen content in the steel is controlled to be ≤20ppm. S3, RH vacuum treatment: The molten steel after LF refining is sent to RH vacuum treatment, and the vacuum degree is ≤67Pa. The vacuum is maintained for 20 minutes. Pure calcium wire is fed in at a wire feeding speed of 4m / s. The wire feeding amount is 1.5kg / t steel. Soft argon blowing is performed for 25 minutes. S4. Continuous Casting: The molten steel is poured into 180mm cross-section round billets. The superheating temperature is 45℃, the casting speed is 1.6m / min, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. The crystallizer uses a special protective slag for continuous casting of low-carbon steel, and the cooling water flow rate is 150m³. 3 / h, secondary cooling section specific water flow rate 1L / kg, crystallizer electromagnetic stirring parameter 60A / 4Hz, end electromagnetic stirring parameter 150A / 10Hz, process liquid level fluctuation exceeding ±5mm is discarded to obtain continuous casting billet; S5. Forging: Heat the continuously cast billet to 1250℃ and hold for 30 minutes. The initial forging temperature is 1180~1230℃, and the final forging temperature is 1050~1100℃. After forging, air cool.

[0038] Comparative Example 1; The difference between Comparative Example 1 and Example 2 lies in the difference in S4. S4 is changed to: Continuous casting: The above molten steel is poured into a 180mm cross-section round billet. The continuous casting superheat is 30℃, the casting speed is 1.8m / min, the liquid level fluctuation in the crystallizer is controlled within ±3mm, the crystallizer uses special protective slag for continuous casting of low carbon steel, the primary cooling water flow is 135m³ / h, the secondary cooling section specific water flow is 0.9L / kg, the electromagnetic stirring parameters of the crystallizer are 60A / 4Hz, and the final electromagnetic stirring parameters are 150A / 10Hz. If the liquid level fluctuation exceeds ±5mm during the process, waste is discarded to obtain a continuously cast billet; the remaining steps are the same as in Example 2.

[0039] Comparative Example 2; The difference between Comparative Example 2 and Example 2 lies in S4. S4 is changed to: Continuous casting: The above molten steel is poured into 180mm cross-section round billets. The continuous casting superheat is 30℃, the casting speed is 1.5m / min, the liquid level fluctuation in the crystallizer is controlled within ±3mm, the crystallizer uses a special protective slag for continuous casting of low carbon steel, and the cooling water flow is 100m³. 3 / h, secondary cooling section specific water flow rate 0.5L / kg, crystallizer electromagnetic stirring parameters 60A / 4Hz, end electromagnetic stirring parameters 150A / 10Hz, process liquid level fluctuation exceeding ±5mm is discarded to obtain continuous casting billet; the remaining steps are the same as in Example 2.

[0040] Comparative Example 3; The difference between Comparative Example 3 and Example 2 is that the C content in the steel used for rail transit support seats is increased to 0.25%; the rest is the same as Example 2.

[0041] Comparative Example 4; The difference between Comparative Example 4 and Example 2 is that the Cr content in the steel used for rail transit support is increased to 0.50%; the rest is the same as Example 2.

[0042] Test and Results Analysis The continuous casting billets obtained in S4 of Examples 1-3 were subjected to low-magnification pickling according to GB / T226, with columnar crystal structure accounting for 85-93% and equiaxed crystal structure accounting for 7-15%; the continuous casting billets obtained in S4 of Comparative Examples 1-2 had columnar crystal structure accounting for 60-70% and equiaxed crystal structure accounting for 35-40% after low-magnification pickling. Figure 1 (a) and Figure 1 (b) are photographs of Examples 1 and 2 after low-magnification acid washing, respectively.

[0043] Low-temperature impact toughness: The forged finished products of each embodiment and comparative example were sampled from different parts of the forging according to the test method of GB / T229 and subjected to Charpy impact test at -20℃. The results are shown in Table 1.

[0044] Table 1

[0045] Note: The accuracy of the impact testing machine is ±1J, and the average value is the rounded result.

[0046] As shown in Table 1, the average impact energy at -20℃ for Examples 1 to 3 were 105J, 112J, and 108J, respectively, all of which are far higher than the basic requirements for steel used in rail transit support seats. Moreover, the data within each example showed small fluctuations and good repeatability, indicating that by using the low casting speed combined with strong cooling continuous casting process described in this invention, the proportion of columnar crystal structure can be actively increased to more than 85%, which can effectively reduce the segregation in the center of the billet and obtain a forging product with uniform structure after forging, significantly improving the low-temperature impact toughness of the steel.

[0047] The average impact energy at -20℃ in Comparative Example 1 was 69 J, significantly lower than the 112 J in Example 2. The difference between Comparative Example 1 and Example 2 was that the continuous casting speed was increased to 1.8 m / min, while all other process conditions remained the same. With the increased casting speed, the residence time of the billet in the secondary cooling zone was shortened, the temperature gradient at the solidification front decreased, and the driving force for the directional growth of columnar crystals was insufficient, resulting in a decrease in the proportion of columnar crystals to 62%. This indicates that a low casting speed is a prerequisite for ensuring sufficient directional growth of columnar crystals. Once the casting speed exceeds the range of this invention, the proportion of columnar crystals decreases, central segregation intensifies, and low-temperature impact toughness deteriorates accordingly.

[0048] Comparative Example 2: Cold water volume is only 100m³ 3 With a secondary cooling water flow rate reduced to 0.5 L / kg, under weak cooling conditions, the solidification rate of the billet was slow, the temperature gradient at the solidification front was insufficient, and the proportion of columnar crystals decreased to 68%. Due to the lack of clear directionality, it was difficult to form a directional columnar crystal structure that penetrated the cross-section, resulting in limited control over central segregation. Furthermore, under weak cooling conditions, the dendrite spacing was large, the solute distribution time was longer, and local segregation was more likely to occur. Ultimately, the average impact energy at -20℃ was only 54 J. These results indicate that simply reducing the casting speed without sufficient cooling intensity is insufficient to fully utilize the segregation-inhibiting effect of columnar crystals. Only a synergistic effect of low casting speed and strong cooling can achieve the ideal columnar crystal structure and impact toughness.

[0049] Comparative Example 3 used the same continuous casting process as Example 2, with the columnar crystal ratio also reaching over 85%. The only difference was that the carbon content increased from ≤0.15% to 0.25%. With the increased carbon content, the pearlite content in the steel increased, grain boundary embrittlement intensified, and central segregation worsened. Even though the high columnar crystal ratio improved central segregation, it was still difficult to offset the toughness loss caused by the increased carbon content. This indicates that the improvement in impact toughness from a high columnar crystal ratio has an upper limit and cannot infinitely compensate for the embrittlement effect caused by changes in chemical composition. This comparative result shows that a low carbon content (≤0.15%) is itself an important contributing factor to impact toughness. The combined effect of the high columnar crystal ratio microstructure and the low carbon content design enables this invention to achieve excellent low-temperature impact toughness without the addition of precious alloys.

[0050] The average impact energy at -20℃ for Comparative Example 4 was 86 J, which, although still higher than the minimum requirement of 27 J, was significantly lower than the 112 J of Example 2. The difference between Comparative Example 4 and Example 2 was that the chromium content was increased from 0.2% to 0.50%. With the increased chromium content, the precipitation of carbides in the steel increased and became unevenly distributed, which impaired the impact toughness to some extent. This indicates that within the range of this invention (0.10~0.30%), the chromium content is sufficient to exert its beneficial effect. Further increasing the chromium content not only fails to improve toughness but may also adversely affect impact toughness due to carbide precipitation.

[0051] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No markings in the claims should be construed as limiting the scope of the claims.

Claims

1. A high-impact toughness steel for rail transit support seats, characterized in that, It includes the following components by weight percentage: C≤0.15%, Si 0.20~0.28%, Mn 0.65~0.70%, P≤0.012%, S≤0.003%, Cr 0.10~0.30%, Al 0.022~0.035%, and gaseous contents of O≤15ppm, N≤80ppm, H≤2ppm, with the balance being Fe and unavoidable impurities; The method for preparing the high impact toughness rail transit support steel includes the following steps: S1. Smelting: After pretreatment, the molten iron is sent to the converter for smelting. The carbon content of the tapped steel is controlled at 0.05~0.10%. The tapping process adopts slag-blocking tapping and bottom blowing argon gas stirring throughout the process. S2, LF Refining: The molten steel after tapping from the converter enters the LF refining furnace for refining. Lime and fluorite are added for slag formation, and SiC is used for diffusion deoxidation. The FeO+MnO content in the slag is controlled to be less than 1%. White slag is refined for 20 minutes with argon stirring throughout the process, and the oxygen content in the steel is controlled to be ≤20ppm. S3, RH vacuum treatment: The molten steel after LF refining is sent into RH vacuum treatment, pure calcium wire is fed in, and finally soft argon blowing is performed for 15~25 minutes. S4. Continuous Casting: The molten steel is poured into round billets. The superheat and casting speed are strictly controlled. The casting speed is 1.4~1.6 m / min. The liquid level fluctuation in the crystallizer is controlled within ±3 mm. The crystallizer uses a special protective slag for continuous casting of low-carbon steel. The cooling water flow rate is 120~150 m³ / min. 3 / h, the water flow rate in the secondary cooling section is 0.8~1L / kg, electromagnetic stirring is carried out, and if the liquid level fluctuation exceeds ±5mm, waste is removed to obtain a continuous casting billet; the ratio of columnar crystals and equiaxed crystals in the continuous casting billet is controlled; S5. Forging: Heat the continuously cast billet to 1250℃ and hold for 30 minutes. The initial forging temperature is 1180~1230℃, and the final forging temperature is 1050~1100℃. After forging, air cool.

2. The steel for rail transit support seats according to claim 1, characterized in that, In S4, the columnar crystals account for 85-93% of the continuous casting billet, and the equiaxed crystal structure accounts for 7-15%.

3. The steel for rail transit support seats according to claim 1, characterized in that, In S3, the feeding speed of the pure calcium wire is 2~4m / s, and the feeding amount is 0.5~1.5kg / t steel.

4. The steel for rail transit support seats according to claim 1, characterized in that, In S1, the smelting process also includes controlling the amount of molten iron fed into the furnace: P ≤ 0.10%, Si ≤ 0.70%, and slag content ≤ 0.5%.

5. The steel for rail transit support seats according to claim 1, characterized in that, In S2, the amount of lime added is 6~8 kg / t steel, and the amount of fluorite added is 1~2 kg / t steel.

6. The steel for rail transit support according to claim 1, characterized in that, In S4, the basicity R of the special protective slag for continuous casting of low carbon steel is 1.0~1.2, the viscosity at 1300℃ is 0.8~1.2 Pa·s, and the melting point is 1100~1150℃.

7. The steel for rail transit support according to claim 1, characterized in that, In S4, the superheat is 15–45°C.

8. The steel for rail transit support according to claim 1, characterized in that, In S4, the electromagnetic stirring parameters of the crystallizer are 60A / 4Hz, and the electromagnetic stirring parameters of the end are 150A / 10Hz.

9. The steel for rail transit support according to claim 1, characterized in that, In S3, the vacuum degree of the RH vacuum treatment is ≤67Pa, and the vacuum is maintained for 12~20 minutes.

Images