Heat treatment steel rail with good comprehensive mechanical property and preparation method thereof

Abstract

The invention discloses a heat-treated steel rail with good comprehensive mechanical properties. The steel rail comprises the following chemical components in percentage by weight: 0.70-0.80% of C; 0.50% to 0.70% of Si; 0.80% to 1.00% of Mn; 0.02 to 0.10 percent of V; 0.02 to 0.07 percent of Nb; ti: 0.005% to 0.020%; less than or equal to 0.020% of P; less than or equal to 0.015% of S; al is less than or equal to 0.01%; the balance is Fe and inevitable impurities; the invention further discloses a preparation method of the heat treatment steel rail with the good comprehensive mechanical property. The preparation method comprises the steps of molten iron pretreatment, converter smelting, LF ladle refining, VD vacuum degassing, continuous casting, steel billet heating and steel rail rolling. The invention aims to provide the heat-treated steel rail with good comprehensive mechanical properties, and the heat-treated steel rail with good strength and toughness ratio can be prepared, and the service life of the steel rail under freight heavy load conditions can be prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and metal materials, and in particular relates to a heat-treated steel rail with good comprehensive mechanical properties. Background Art

[0002] Rail transit plays an irreplaceable role in my country's economic construction. With the expansion of railway laying area year by year, although the complex service environment has increasingly obvious requirements for special performance of rails such as corrosion resistance and low temperature resistance, for freight lines, the excessive fatigue peeling cracks, core damage, surface damage and wear of rails caused by heavy loads are still the main problems, and the methods to improve these problems still mainly depend on improving the tensile strength, toughness, hardness and other indicators of rails. Therefore, from the perspective of practicality and economy, further improving the comprehensive mechanical properties of rails and controlling the increase in their production costs are of great significance to enterprises and society. Summary of the invention

[0003] The purpose of the present invention is to provide a heat-treated rail with good comprehensive mechanical properties, which can be prepared with a good strength and toughness ratio and extend the service life of the rail under heavy freight load conditions.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] The invention discloses a heat-treated steel rail with good comprehensive mechanical properties. The chemical components of the steel rail include, by weight percentage, C: 0.70-0.80%; Si: 0.50-070%; Mn: 0.80-1.00%; V: 0.02-0.10%; Nb: 0.02-0.07%; Ti: 0.005-0.020%; P≤0.020%; S≤0.015%; Al≤0.01%; and the balance is Fe and unavoidable impurities.

[0006] Furthermore, the chemical composition of the rail includes by weight: C: 0.77%; Si: 0.64%; Mn: 0.94%; V: 0.042%; Nb: 0.034%; Ti: 0.011%; P: 0.014%; S: 0.004%; Al: 0.003%; the remainder is Fe and unavoidable impurities.

[0007] Furthermore, the chemical composition of the rail includes by weight: C: 0.79%; Si: 0.66%; Mn: 0.96%; V: 0.043%; Nb: 0.032%; Ti: 0.009%; P: 0.012%; S: 0.003%; Al: 0.003%; the remainder is Fe and unavoidable impurities.

[0008] Furthermore, the chemical composition of the rail includes by weight: C: 0.78%; Si: 0.63%; Mn: 0.94%; V: 0.047%; Nb: 0.031%; Ti: 0.008%; P: 0.014%; S: 0.002%; Al: 0.002%; the remainder is Fe and unavoidable impurities.

[0009] A method for preparing a heat-treated rail with good comprehensive mechanical properties, comprising: molten iron pretreatment - converter smelting - LF ladle refining - VD vacuum degassing - continuous casting - billet heating - rail rolling; wherein:

[0010] 1) Converter production

[0011] In order to ensure product quality and accurately control the composition of finished products, the requirements for molten iron for converters are as follows: the phosphorus content is less than or equal to 0.15% by mass, and the sulfur content is less than or equal to 0.04%; slag-making auxiliary materials are added during converter smelting, and ferrosilicon, silicon manganese and aluminum silicon manganese are added during steelmaking for deoxidation and alloying according to the requirements of the finished product composition; aluminum-free deoxidation and alloying are used in converter smelting, and argon is blown in the whole process as normal refining;

[0012] 2) LF and VD furnace production

[0013] According to the requirements of producing low-oxygen steel, LF uses medium basicity and strong reducing refined slag to treat molten steel, deep vacuum time is 15-20min, deep vacuum degree is 7-9KPa, soft blowing time is 18-22min, and the temperature after VD is 1536-1546℃;

[0014] 3) Continuous casting production

[0015] In continuous casting production, the superheat is controlled between 15-30℃ and the casting speed is between 0.6-0.7m / min;

[0016] (2) The rail rolling process is: continuous casting billet - sawing - heating - BD1 rolling - BD2 rolling - CCS universal rolling mill continuous rolling - sawing - cooling - head and tail cutting - straightening - inspection - packaging - weighing - storage; the billet preheating section temperature is not more than 800°C; the heating temperature is 1200°C, and the heating time is not less than 3 hours; the furnace temperature is not less than 1100°C, the rolling temperature is 1080-1160°C, and the final rolling temperature is 890-930°C;

[0017] (3) The cooling medium for online waste heat quenching is pure air medium; the heat treatment temperature is determined based on the phase change temperature parameters in the steel and the actual capacity of the industrial equipment; the sample starts with waste heat heat treatment at a temperature of 760-790°C, and leaves the heat treatment production line after 100-160s of online heat treatment. The top surface, both sides, and the lower jaw and bottom of the rail head are cooled. The actual cooling rate in the first stage is 1.5-2.5°C / s, and the cooling time is 50-80s; the second stage is fast cooling, with a cooling rate of 2.5-3.5°C / s. After cooling, the outlet rail head temperature is 450-500°C, and then naturally air-cooled to room temperature.

[0018] Furthermore, the rail structure is pearlite, a small amount of ferrite is allowed, and the decarburization layer is ≤0.5mm.

[0019] The role and mechanism of each alloy element in the present invention:

[0020] The main role of C in pearlite rails is to improve strength and hardness, promote pearlite transformation and form carbides. With the increase of C content, the strength and hardness of steel increase, while plasticity and toughness decrease. If the C content is too low, the proportion of carbides in pearlite steel decreases and the proportion of ferrite increases, resulting in insufficient strength and hardness of the rail; if the C content is too high, the hardness of the rail is too high, which accelerates the expansion of fatigue cracks and reduces the welding performance of the rail. When the C content is greater than 0.70%, the strength and hardness of the rail are good, but the plasticity, toughness and low temperature resistance are reduced. Therefore, in order to improve the strength and hardness of the rail, the C content range is set to 0.70-0.80%.

[0021] The main function of Si in steel is to inhibit the formation of cementite and, as a solid solution strengthening element, to improve the strength and hardness of the ferrite matrix. When the Si content is less than 0.40%, its solid solution amount is low and the strengthening effect is small; when the Si content is greater than 0.90%, since Si is a crack-sensitive element in the rail, it will aggravate the deterioration of fatigue fracture performance and produce local segregation. The lower heat transfer will also reduce the weldability of the rail. Therefore, the Si content range is set to 0.50-0.70%.

[0022] As a solid solution strengthening element and carbide-forming element in steel, Mn can strengthen ferrite and cementite at the same time, and improve the strength and hardness of the rail. In addition, Mn can also replace Fe and S elements in steel to form MnS, avoiding the formation of FeS (FeS and Fe are easy to form low-melting point compounds) and causing hot brittleness of the rail. Mn can also reduce the pearlite transformation temperature, thereby reducing the pearlite interlamellar spacing. However, the increase in Mn content will greatly reduce the welding performance of steel, increase the grain size, and increase the sensitivity to the formation of white spots and ingot segregation. When the Mn content is less than 0.40%, its strengthening effect on carbides is limited, and the improvement of rail performance is small; when the Mn content is greater than 1.00%, excessive Mn elements lead to an increase in grain size, affecting the structural changes of steel during heat treatment, and significantly reducing the plastic toughness of the rail. Mn also has a significant effect on the diffusion of C in steel, and may also cause abnormal structures in the Mn segregation area and affect the weldability of the rail. Therefore, the Mn content is set in the range of 0.80-1.00%.

[0023] V exists in steel mainly in the form of carbides, nitrides, or oxides, and plays a role in precipitation strengthening. Since V will dissolve in austenite under high temperature conditions, especially when there is no precipitation phase under the condition of the final rolling temperature of the rail at 900°C, its effect on the grain refinement of rail steel is not obvious. Too high vanadium content will lead to the appearance of aggregated carbides, which will reduce the strength. The precipitation of carbides in the crystal will also reduce the room temperature toughness, so the vanadium content in most steels is not high. Vanadium carbides are the hardest and most wear-resistant of metal carbides. Dispersed carbides can increase the hardness and wear resistance of tool steels and improve the welding performance of steel to a certain extent. Therefore, the Mn content range is set to 0.02-0.10%.

[0024] The main function of Nb in steel is to precipitate fine niobium carbonitrides to hinder grain growth, thereby refining austenite grains and achieving the purpose of simultaneously improving the strength and toughness of steel. When the Nb content is less than 0.01%, less niobium carbonitrides precipitate, and the strengthening effect on steel is not obvious; when the Nb content is greater than 0.08%, coarse carbonitrides are easily formed, which in turn reduces the toughness of steel. Therefore, the Nb content range is set to 0.02-0.07%.

[0025] Ti exists in steel mainly in the form of precipitation phase. The fine TiN precipitated early after solidification can hinder the coarsening and growth of austenite grains under high temperature conditions, achieving the purpose of fine grain strengthening. On the other hand, the formation of TiN will also reduce the N content in the steel billet, thereby improving the performance of the rail. However, at lower temperatures, since Nb (C, N) and V (C, N) will be compound precipitated on the formed TiN particles, the strengthening effect is reduced, and the Ti element is active in nature and is easy to form a variety of compounds. It is easy to burn during the steelmaking process, and the impact on the performance of the steel is difficult to control, so the Ti content must be strictly controlled. Therefore, the Ti content range is set to 0.005-0.02%.

[0026] P and S are both impurity elements that cannot be completely removed from the rails, which will greatly increase the crack sensitivity of the steel, and at the same time increase the low-temperature brittle transition temperature of the steel and reduce the low-temperature impact performance of the steel. Therefore, under the premise of not affecting the performance of the rails, the P and S contents are required to be as low as possible. The P content in the present invention needs to be controlled below 0.020%; the S content needs to be controlled below 0.015%.

[0027] The reasons for selecting the above process parameters are as follows:

[0028] The rail head is mainly pearlite structure, and the spacing between pearlite lamellae is the main factor determining its mechanical properties. In order to improve the toughness and wear resistance of the rail, the present invention adopts an online heat treatment process to refine the spacing between pearlite lamellae. During the heat treatment process of the rail head, a segmented cooling approach is adopted. The cooling rate in the first stage is 1.5-2.5℃ / s, which quickly takes away the heat from the surface of the rail head to form refined pearlite lamellae and avoid the appearance of abnormal tissue; the cooling rate in the second stage is 2.5-3.5℃ / s. After the surface tissue of the rail is stable in the first stage, the internal heat needs to be transferred to the surface to dissipate, and the heat dissipation efficiency is reduced. Therefore, the cooling rate is appropriately increased, and the return temperature is reduced to make the cross-sectional hardness of the rail head more uniform.

[0029] Compared with the prior art, the beneficial technical effects of the present invention are:

[0030] By adopting microalloying and heat treatment processes, fine grain strengthening and precipitation strengthening are carried out, and the microstructure transformation is made uniform, while ensuring that the various properties of the rails meet the use requirements, the strength and toughness of the rails are further improved, the maintenance cost of the line rails is reduced, and the service safety performance and service life of the rails are improved. The rail production method is simple, highly operable, and easy to promote and apply. DETAILED DESCRIPTION

[0031] A rare earth treated rail with good corrosion resistance and low temperature toughness and a preparation method thereof, the smelting production process of the rail is: molten iron → converter smelting → LF refining → VD → continuous casting. The converter smelting adopts aluminum-free deoxidation alloying, and the whole process is argon blown as normal refining. LF uses medium alkalinity and strong reducing refining slag to treat molten steel, the deep vacuum time is 15-20min; the deep vacuum degree is 7-9KPa, the soft blowing time is 18-22min, and the temperature after VD is 1536-1546℃. In continuous casting production, the superheat is controlled between 15-30℃, and the pulling speed is between 0.6-0.7m / min. The smelting chemical composition control of Examples 1-3 of the present invention is shown in Table 1, and Comparative Examples 1 and 2 are two conventional composition heat-treated rails respectively.

[0032] Table 1 Comparison of different components during implementation (wt.%)

[0033]

[0034] The balance is Fe and inevitable impurities.

[0035] The rail rolling process is: continuous casting billet - sawing - heating - BD1 rolling - BD2 rolling - CCS universal rolling mill continuous rolling - sawing - cooling - cutting head and tail - straightening - inspection - packaging - weighing - storage; the billet heating preheating section temperature is not more than 800℃; the heating temperature is 1200℃, and the heating time is not less than 3 hours; the furnace temperature is not less than 1100℃, the rolling temperature is 1080-1160℃, and the final rolling temperature is 890-930℃;

[0036] Rail heat treatment process: The cooling medium for online waste heat quenching is wind medium, and the heat treatment temperature is determined based on the phase change temperature parameters in the steel and the actual capacity of the industrial equipment; the sample starts with waste heat heat treatment at 760-790℃, and exits the heat treatment production line after 100-160s of online heat treatment, with an end temperature of 450-500℃. In the first stage of online cooling, the top surface, both sides, and the jaw of the gauge angle and the bottom of the rail are cooled. The actual cooling section has a cooling rate of 1.5-2.5℃ / s and a cooling time of 50-80s; the second stage is fast cooling, with a cooling rate of 2.5-3.5℃ / s. After cooling, the outlet rail head temperature is 450-500℃, and then naturally air-cooled to room temperature.

[0037] The rail structure is pearlite, a small amount of ferrite is allowed, and the decarburization layer is ≤0.5mm.

[0038] The tensile strength, elongation after fracture, surface hardness and cross-sectional hardness properties of Examples 1-3 and Comparative Examples 1-2 are shown in Tables 2 and 3.

[0039] Table 2 Mechanical properties test results of various embodiments of the present invention and comparative examples

[0040] project Tensile strength / MPa Elongation after break / % Surface hardness / HBW Example 1 1297 13.5 368 Example 2 1289 12.5 363 Example 3 1304 13.5 371 Comparative Example 1 1195 10.5 344 Comparative Example 2 1256 10.0 357

[0041] Table 3 Cross-sectional hardness test results of various embodiments of the present invention and comparative examples

[0042]

[0043]

[0044] By comparing the examples and comparative examples, it can be seen that the heat-treated rail produced by the method of the present invention has better comprehensive mechanical properties, so that the rail has better service performance under heavy load conditions, extends the service life of the rail, and reduces operation and maintenance costs.

[0045] The raw materials listed in the present invention, as well as the upper and lower limits and interval values ​​of the raw materials of the present invention, and the upper and lower limits and interval values ​​of the process parameters (such as temperature, time, etc.) can all realize the present invention, and the embodiments are not listed one by one here.

[0046] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A heat-treated rail with good comprehensive mechanical properties, characterized by: The chemical composition of the rail includes by weight: C: 0.70-0.80%; Si: 0.50-070%; Mn: 0.80-1.00%; V: 0.02-0.10%; Nb: 0.02-0.07%; Ti: 0.005-0.020%; P≤0.020%; S≤0.015%; Al≤0.01%; the remainder is Fe and unavoidable impurities.

2. The heat-treated rail with good comprehensive mechanical properties according to claim 1, characterized in that: The chemical composition of the rail includes by weight: C: 0.77%; Si: 0.64%; Mn: 0.94%; V: 0.042%; Nb: 0.034%; Ti: 0.011%; P: 0.014%; S: 0.004%; Al: 0.003%; the remainder is Fe and unavoidable impurities.

3. The heat-treated rail with good comprehensive mechanical properties according to claim 1, characterized in that: The chemical composition of the rail includes by weight: C: 0.79%; Si: 0.66%; Mn: 0.96%; V：0.043％； Nb: 0.032%; Ti: 0.009%; P: 0.012%; S: 0.003%; Al: 0.003%; the balance is Fe and inevitable impurities.

4. The heat-treated rail with good comprehensive mechanical properties according to claim 1, characterized in that: The weight percentage of the chemical composition of the rail includes: C: 0.78%; Si: 0.63%; Mn: 0.94%; V ：0.047％； Nb: 0.031%; Ti: 0.008%; P: 0.014%; S: 0.002%; Al: 0.002%; the balance is Fe and inevitable impurities.

5. The method for preparing a heat-treated rail with good comprehensive mechanical properties according to any one of claims 1 to 4, characterized in that: include: Hot metal pretreatment - converter smelting - LF ladle refining - VD vacuum degassing - continuous casting - billet heating - rail rolling; including: 1) Converter production In order to ensure product quality and accurately control the composition of finished products, the requirements for molten iron for converters are as follows: the phosphorus content is less than or equal to 0.15% by mass, and the sulfur content is less than or equal to 0.04%; slag-making auxiliary materials are added during converter smelting, and ferrosilicon, silicon manganese and aluminum silicon manganese are added during steelmaking for deoxidation and alloying according to the requirements of the finished product composition; aluminum-free deoxidation and alloying are used in converter smelting, and argon is blown in the whole process as normal refining; 2) LF and VD furnace production According to the requirements of producing low-oxygen steel, LF uses medium basicity and strong reducing refined slag to treat molten steel, deep vacuum time is 15-20min, deep vacuum degree is 7-9KPa, soft blowing time is 18-22min, and the temperature after VD is 1536-1546℃; 3) Continuous casting production In continuous casting production, the superheat is controlled between 15-30℃ and the casting speed is between 0.6-0.7m / min; (2) The rail rolling process is: continuous casting billet - sawing - heating - BD1 rolling - BD2 rolling - CCS universal rolling mill continuous rolling - sawing - cooling - head and tail cutting - straightening - inspection - packaging - weighing - storage; the billet preheating section temperature is not more than 800°C; the heating temperature is 1200°C, and the heating time is not less than 3 hours; the furnace temperature is not less than 1100°C, the rolling temperature is 1080-1160°C, and the final rolling temperature is 890-930°C; (3) The cooling medium for online waste heat quenching is pure air medium; the heat treatment temperature is determined based on the phase change temperature parameters in the steel and the actual capacity of the industrial equipment; the sample starts with waste heat heat treatment at a temperature of 760-790°C, and leaves the heat treatment production line after 100-160s of online heat treatment. The top surface, both sides, and the lower jaw and bottom of the rail head are cooled. The actual cooling rate in the first stage is 1.5-2.5°C / s, and the cooling time is 50-80s; the second stage is fast cooling, with a cooling rate of 2.5-3.5°C / s. After cooling, the outlet rail head temperature is 450-500°C, and then naturally air-cooled to room temperature.

6. The method for preparing a heat-treated rail with good comprehensive mechanical properties according to claim 5, characterized in that: The rail structure is pearlite, a small amount of ferrite is allowed, and the decarburization layer is ≤0.5mm.

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