Production method of high-plasticity rare earth microalloyed R350HT steel rail

Abstract

The invention discloses a production method of a high-plasticity rare earth microalloyed R350HT steel rail, and belongs to the technical field of ferrous metallurgy. The method comprises the following steps: pretreating molten iron; smelting in a converter; lF refining, wherein an aluminum-free deoxidation process is adopted; vD vacuum treatment; continuous casting; rolling and heat treatment; the steel comprises the following chemical components in percentage by mass: 0.72%-0.80% of C, 0.15%-0.58% of Si, 0.70%-1.20% of Mn, less than or equal to 0.020% of P, less than or equal to 0.025% of S, less than or equal to 0.15% of Cr, less than or equal to 0.04% of V, 0.0010%-0.0020% of rare earth elements and the balance of Fe and impurities. According to the process optimized through precise rare earth addition and cooperation of other components, the high strength and high hardness of the steel rail are guaranteed, meanwhile, the ductility of the steel rail is remarkably improved, inclusions are effectively refined, and the comprehensive performance of the steel rail is comprehensively improved.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and in particular relates to a method for producing high-plasticity rare earth microalloyed R350HT steel rails. Background Technology

[0002] R350HT rails are a type of high-strength heat-treated rail according to the International Union of Railways (UIC) standard, requiring high strength, hardness, and good wear resistance. However, while pursuing high strength, the rail's ductility and toughness often become weak points; a lower elongation rate affects the rail's impact fatigue resistance and service safety. Furthermore, non-metallic inclusions in the steel (such as oxides and sulfides) are stress concentration points, easily becoming fatigue crack initiations and worsening the rail's fatigue life. While traditional manufacturing processes can control the number of inclusions, they struggle to effectively alter their morphology and distribution; brittle inclusions are prone to early damage under stress.

[0003] Rare earth elements possess strong deoxidizing and desulfurizing capabilities in steel, and can modify brittle inclusions such as high-melting-point Al2O3 into low-melting-point rare earth oxysulfides, making them easier to deform during hot working. This refines and spheroidizes inclusions, significantly improving the steel's transverse impact toughness and plasticity. However, rare earth elements are chemically reactive, and improper control of their addition timing, method, and content can easily lead to problems such as nozzle nodules and component segregation, restricting their industrial application in rail steel. Summary of the Invention

[0004] The purpose of this invention is to provide a production method for high-plasticity rare earth microalloyed R350HT rails. Through precise addition of rare earth elements and optimized processes with synergistic effects of other components, the method significantly improves the elongation of the rails while ensuring their high strength and hardness, effectively refines inclusions, and comprehensively enhances the overall performance of the rails.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention discloses a method for producing high-plasticity rare-earth microalloyed R350HT steel rails, comprising the following sequential steps:

[0007] Hot metal pretreatment: Hot metal pretreatment is the first critical step in the entire smelting process. A hot metal ladle desulfurization process is adopted, using a passivating magnesium-based desulfurizing agent to reduce the sulfur content in the hot metal from the initial 0.040%-0.070% to below 0.005%. Simultaneously, the phosphorus content is strictly controlled to ensure that the [P] in the hot metal entering the furnace is ≤0.120%. During pretreatment, the hot metal temperature is maintained at 1250-1280℃ to provide favorable thermodynamic conditions for subsequent converter smelting.

[0008] Converter smelting: Top-and-bottom blowing converter smelting is adopted, and double-slag operation is implemented. The first slag dumping is carried out 5-8 minutes after the blowing starts, at which time most of the phosphorus is removed. The final control is P≤0.010%, tapping carbon≥0.08%, tapping temperature≥1650℃, and the final slag basicity is strictly controlled at 3.0±0.2 to ensure the dephosphorization effect. A pneumatic slag-blocking device is used during the tapping process to strictly control the slag amount to ≤50mm.

[0009] LF refining: An aluminum-free deoxidation process is adopted, using silicon, calcium, barium, etc. for deoxidation, with the addition amount controlled at 1.2-1.8 kg / ton of steel; argon blowing and stirring are carried out throughout the refining process, with fine-tuning of composition and heating operations; the argon flow rate is adjusted according to the refining stage: 80-120 NL / min during slag formation, 60-80 NL / min during heating, and 20-40 NL / min during soft blowing; the refining time is controlled at 45-60 minutes to ensure uniform steel composition and stable temperature, with the final refining temperature controlled at 1580-1600℃;

[0010] VD vacuum treatment: Vacuum degassing is performed, and the vacuum level is rapidly reduced to below 0.10 kPa within 8 minutes. The deep vacuum is maintained for ≥20 minutes, and the soft argon blowing time is guaranteed to be ≥15 minutes to promote the flotation of inclusions. Rare earth lanthanum alloy cored wire is added through a special wire feeder, and the wire feeding speed is controlled at 180-220 m / min, with the rare earth recovery rate stable at 45.5%-55.0%.

[0011] Continuous casting: Low superheat pouring ΔT≤30℃, constant casting speed operation, casting speed controlled at 0.70-0.75m / min; electromagnetic stirring technology is used; the continuously cast billet is stacked and slowly cooled for more than 48 hours to prevent hydrogen-induced cracking;

[0012] Rolling and heat treatment: The billet is heated using a three-stage heating furnace: preheating section ≤800℃, heating section 1150-1250℃, and soaking section 1200-1280℃; the total heating time is ≥3 hours to ensure uniform billet temperature; the initial rolling temperature is 1100-1150℃, and the final rolling temperature is controlled at 880-920℃; after rolling, an online heat treatment process is adopted: quenching temperature 700-850℃, exit temperature 400-600℃; the heat treatment cooling rate is precisely controlled at 1.5-2.5℃ / s, and segmented cooling control is adopted. The first stage starts cooling at 730-800℃ with a cooling rate of 2.0-2.3℃ / s, and the first stage ends cooling at 570-590℃; the second stage starts cooling at 570-590℃ with a cooling rate controlled at 1.6-1.9℃ / s, and the second stage ends cooling at 400-580℃ to ensure the acquisition of a fine lamellar pearlite structure.

[0013] Its chemical composition, by mass percentage, is as follows:

[0014] C: 0.72%–0.80%, Si: 0.15%–0.58%, Mn: 0.70%–1.20%, P: ≤0.020%, S: ≤0.025%, Cr: ≤0.15%, V: ≤0.04%, rare earth elements: 0.0010%–0.0020%, the remainder being Fe and unavoidable impurities.

[0015] Furthermore, the electromagnetic stirring current of the crystallizer is 350A and the frequency is 3.5Hz.

[0016] Furthermore, the produced rails meet the following performance requirements:

[0017] Tensile strength R m ≥1235MPa, elongation A: ≥14.5%, tread center hardness: 358HBW-380HBW, metallographic structure is uniform and fine pearlite with a small amount of ferrite.

[0018] Furthermore, its chemical composition by mass percentage is: C 0.76, Si 0.35, Mn 0.95, P 0.012, S 0.005, RE 0.0015, with the balance being Fe and unavoidable impurities, of which the residual elements are Cr 0.08% and V 0.03%.

[0019] Furthermore, its chemical composition by mass percentage is: C 0.75, Si 0.32, Mn 0.92, P 0.011, S 0.004, RE 0.0010, Cr 0.07%, V 0.02%, with the balance being Fe and unavoidable impurities.

[0020] Furthermore, its chemical composition by mass percentage is: C 0.78, Si 0.38, Mn 0.98, P 0.013, S 0.006, RE 0.0020, Cr 0.09%, V 0.04%, with the balance being Fe and unavoidable impurities.

[0021] Furthermore, the K3 level of non-metallic inclusions is controlled at 2.0-3.0.

[0022] Furthermore, the continuous casting billet specifications are 280mm × 380mm.

[0023] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0024] This invention effectively solves the problems of insufficient plasticity and toughness and poor inclusion control in traditional R350HT rails by optimizing the composition design and precisely controlling the rare earth addition process, and by coordinating the optimized process settings, thus providing rail products with superior performance for railway transportation.

[0025] This invention, through a process setting optimized by rare earth microalloying, significantly increases the elongation of the rail to over 14.5% without sacrificing strength, effectively refines inclusions in the steel, improves their morphology and distribution, and comprehensively enhances the overall mechanical properties and service reliability of the R350HT rail. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0027] Example 1

[0028] According to the composition range designed in this invention, a batch of R350HT rare earth treated steel rails was smelted. The specific process steps are as follows:

[0029] Raw material preparation and molten iron pretreatment: Blast furnace molten iron was used, with the following initial composition: C 4.2%, Si 0.35%, Mn 0.45%, P 0.085%, S 0.028%. The molten iron was pretreated at 1350℃, and the composition of the molten iron after desulfurization was: [S] = 0.003%, [P] = 0.075%.

[0030] The converter smelting adopts a 150-ton top and bottom combined blowing converter and implements double slag operation: the first slag dumping is carried out 6 minutes after the blowing starts, and the endpoint control is: [C] = 0.12%, [P] = 0.008%, the tapping temperature is 1655℃, and strict slag blocking is carried out during the tapping process, and the slag discharge is controlled at 40mm.

[0031] LF refining adopts an aluminum-free deoxidation process, adding 1.5 kg / ton of silicon-calcium-barium composite deoxidizer, refining time is 55 minutes, final temperature is 1590℃, argon blowing and stirring are carried out throughout the process, and the argon flow rate is controlled in stages according to process requirements.

[0032] The total VD vacuum treatment time was 25 minutes. When the vacuum degree reached 0.08 kPa, rare earth lanthanum alloy cored wire was added at a wire feeding speed of 200 m / min. The rare earth yield was 47.2%, and the soft argon blowing time was 18 minutes.

[0033] The tundish superheating temperature in the continuous casting process is 25℃, the casting speed is 0.72m / min, electromagnetic stirring is used throughout the process, the billet size is 280mm×380mm, and the billet is stacked and slowly cooled for 48 hours.

[0034] Billet heating regime: preheating section 750℃, heating section 1220℃, soaking section 1260℃. Total heating time 3.5 hours. Initial rolling temperature 1120℃, final rolling temperature 910℃. In-line heat treatment: quenching temperature 780℃, exit temperature 550℃, first stage initial cooling temperature 780℃, cooling rate 2.0℃ / s, final cooling temperature 580℃; second stage initial cooling temperature 580℃, cooling rate 1.8℃ / s, final cooling temperature 485℃.

[0035] The chemical composition (mass percentage) of the obtained rail is: C 0.76, Si 0.35, Mn 0.95, P 0.012, S 0.005, RE 0.0015, with the balance being Fe and unavoidable impurities, of which the residual elements are Cr 0.08% and V 0.03%.

[0036] Rail performance test results

[0037]

[0038] Example 2

[0039] By changing the amount of rare earth added and the process parameters, while keeping other conditions the same as in Example 1:

[0040] Rare earth element addition: RE 0.0010%; VD vacuum treatment: rare earth yield 46.8%; continuous casting superheat: 28℃; final rolling temperature: 900℃. In-line heat treatment: quenching temperature 800℃, exit temperature 560℃; first stage cooling temperature 800℃, cooling rate 2.3℃ / s, final cooling temperature 590℃; second stage cooling temperature 590℃, cooling rate 1.6℃ / s, final cooling temperature 560℃. Chemical composition of the resulting rail: C 0.75, Si 0.32, Mn 0.92, P 0.011, S 0.004, RE 0.0010, Cr 0.07%, V 0.02%, balance Fe and unavoidable impurities.

[0041] Rail performance test results

[0042]

[0043] Example 3

[0044] Further optimization of process parameters: Rare earth addition: RE 0.0020%, VD vacuum treatment: rare earth yield 48.5%, continuous casting superheat: 22℃, final rolling temperature: 880℃, online heat treatment: quenching temperature 760℃, exit temperature 530℃, first stage starting cooling temperature 760℃, cooling rate 2.1℃ / s, final cooling temperature 590℃; second stage starting cooling temperature 590℃, cooling rate 1.9℃ / s, final cooling temperature 530℃. The resulting rail chemical composition: C 0.78, Si 0.38, Mn 0.98, P 0.013, S 0.006, RE 0.0020, Cr 0.09%, V 0.04%, balance Fe and unavoidable impurities.

[0045] Rail performance test results

[0046]

[0047] Comparative Example 1

[0048] The same matrix composition as in Example 1 was used, but no rare earth elements were added. The production process was the same except that no rare earth elements were added.

[0049] Rail performance test results

[0050]

[0051] A comparison of the three embodiments with the comparative example shows that:

[0052] Significant improvement in plasticity: The elongation of the examples with added rare earth elements was ≥14.5%, which is more than 20% higher than that of the comparative examples without added rare earth elements. Among them, Example 3 achieved the best effect of 15.8%.

[0053] Excellent inclusion control: The non-metallic inclusion K3 level of rare earth treated rails is controlled at 2.0-3.0, which is a significant improvement compared to the 5.0 level of the comparative example.

[0054] Good process stability: The rare earth yield in all three embodiments remained above 46.8%, indicating stable and reliable process control. After adding rare earth elements, the R350HT rail achieved a good balance between high strength and high plasticity while maintaining high strength.

[0055] The above embodiments fully demonstrate that the present invention effectively solves the problems of insufficient plasticity and toughness and poor inclusion control of traditional R350HT rails by optimizing the composition design and precisely controlling the rare earth addition process, thus providing rail products with better performance for railway transportation.

[0056] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for producing a high-plasticity rare-earth microalloyed R350HT steel rail, characterized in that, include: Hot metal pretreatment: Hot metal pretreatment is the first critical step in the entire smelting process; The hot iron ladle desulfurization process is adopted, which reduces the sulfur content in the hot iron from the initial 0.040%-0.070% to below 0.005% by injecting passivating magnesium-based desulfurizing agent; at the same time, the phosphorus content is strictly controlled to ensure that the [P] in the hot iron entering the furnace is ≤0.120%; during the pretreatment process, the temperature of the hot iron is maintained at 1250-1280℃ to provide good thermodynamic conditions for subsequent converter smelting. Converter smelting: Top-and-bottom blowing converter smelting is adopted, and double-slag operation is implemented. The first slag dumping is carried out 5-8 minutes after the blowing starts, at which time most of the phosphorus is removed. The final control is P≤0.010%, tapping carbon≥0.08%, tapping temperature≥1650℃, and the final slag basicity is strictly controlled at 3.0±0.2 to ensure the dephosphorization effect. A pneumatic slag-blocking device is used during the tapping process to strictly control the slag amount to ≤50mm. LF refining: An aluminum-free deoxidation process is adopted, using silicon, calcium, barium, etc. for deoxidation, with the addition amount controlled at 1.2-1.8 kg / ton of steel; argon blowing and stirring are carried out throughout the refining process, with fine-tuning of composition and heating operations; the argon flow rate is adjusted according to the refining stage: 80-120 NL / min during slag formation, 60-80 NL / min during heating, and 20-40 NL / min during soft blowing; the refining time is controlled at 45-60 minutes to ensure uniform steel composition and stable temperature, with the final refining temperature controlled at 1580-1600℃; VD vacuum treatment: Vacuum degassing is performed, and the vacuum level is rapidly reduced to below 0.10 kPa within 8 minutes. The deep vacuum is maintained for ≥20 minutes, and the soft argon blowing time is guaranteed to be ≥15 minutes to promote the flotation of inclusions. Rare earth lanthanum alloy cored wire is added through a special wire feeder, and the wire feeding speed is controlled at 180-220 m / min, with the rare earth recovery rate stable at 45.5%-55.0%. Continuous casting: Low superheat pouring ΔT≤30℃, constant casting speed operation, casting speed controlled at 0.70-0.75m / min; electromagnetic stirring technology is used; the continuously cast billet is stacked and slowly cooled for more than 48 hours to prevent hydrogen-induced cracking; Rolling and heat treatment: The billet is heated using a three-stage heating furnace: preheating stage ≤800℃, heating stage 1150-1250℃, and soaking stage 1200-1280℃; the total heating time is ≥3 hours to ensure uniform billet temperature; the initial rolling temperature is 1100-1150℃, and the final rolling temperature is controlled at 880-920℃; after rolling, an online heat treatment process is adopted: the entire cooling process is carried out in two stages; the heat treatment cooling rate is precisely controlled at 1.5-2.5℃ / s, and a segmented cooling control is adopted. The first stage starts at a cooling temperature of 730-800℃, with a cooling rate of 2.0-2.3℃ / s, and the first stage ends at a cooling temperature of 570-590℃; the second stage starts at a cooling temperature of 570-590℃, with a cooling rate controlled at 1.6-1.9℃ / s, and the second stage ends at a cooling temperature of 400-580℃ to ensure the acquisition of a fine lamellar pearlite structure. Its chemical composition, by mass percentage, is as follows: C: 0.72%–0.80%, Si: 0.15%–0.58%, Mn: 0.70%–1.20%, P: ≤0.020%, S: ≤0.025%, Cr: ≤0.15%, V: ≤0.04%, rare earth elements: 0.0010%–0.0020%, the remainder being Fe and unavoidable impurities.

2. The production method of high-plasticity rare-earth microalloyed R350HT steel rail according to claim 1, characterized in that, The electromagnetic stirring current of the crystallizer is 350A and the frequency is 3.5Hz.

3. The production method of high-plasticity rare-earth microalloyed R350HT steel rail according to claim 1, characterized in that, The produced rails meet the following performance requirements: Tensile strength R m ≥1235MPa, elongation A: ≥14.5%, tread center hardness: 358HBW-380HBW, metallographic structure is uniform and fine pearlite with a small amount of ferrite.

4. The production method of high-plasticity rare-earth microalloyed R350HT steel rail according to claim 1, characterized in that, Its chemical composition by mass percentage is: C 0.76, Si 0.35, Mn 0.95, P 0.012, S 0.005, RE 0.0015, with the balance being Fe and unavoidable impurities, of which the residual elements are Cr 0.08% and V 0.03%.

5. The method for producing high-plasticity rare-earth microalloyed R350HT steel rails according to claim 1, characterized in that, Its chemical composition by mass percentage is: C 0.75, Si 0.32, Mn 0.92, P 0.011, S 0.004, RE 0.0010, Cr 0.07%, V 0.02%, with the balance being Fe and unavoidable impurities.

6. The method for producing high-plasticity rare-earth microalloyed R350HT steel rails according to claim 1, characterized in that, Its chemical composition by mass percentage is: C 0.78, Si 0.38, Mn 0.98, P 0.013, S 0.006, RE 0.0020, Cr 0.09%, V 0.04%, with the balance being Fe and unavoidable impurities.

7. The method for producing high-plasticity rare-earth microalloyed R350HT steel rails according to any one of claims 4-6, characterized in that, The K3 level of non-metallic inclusions is controlled between 2.0 and 3.

0.

8. The method for producing high-plasticity rare-earth microalloyed R350HT steel rails according to claim 1, characterized in that, The dimensions of the continuously cast billet are 280mm × 380mm.

Images