Ultrahigh-strength bainite track steel and preparation method thereof

Abstract

The invention provides ultrahigh-strength bainite track steel and a preparation method thereof. The ultrahigh-strength bainite track steel is subjected to Nb, B and V composite microalloying; the steel comprises the following chemical components in percentage by mass: 0.35-0.40% of C, 2.0-2.3% of Si and Al, more than 0.5% and less than 0.8% of Ni and Mo, 1.0-2.0% of Cr and Mn, 0.05-0.2% of Nb, B and V, and the balance of Fe and inevitable impurities. In the ultrahigh-strength bainite rail steel, the mass percent of chemical components of Si, Mn, Cr, Ni, V, Mo, Nb, Al and B is as follows: N is more than or equal to 0.13 and less than or equal to 2.56, and n is more than or equal to 0.40 and less than or equal to 0.92; wherein N = [1.1 * (Si + Al) * (Ni + Mo)-1] / [0.4 * (Cr + Mn)]; n = 0.8 * (Ni + Mo) / [3-(Si + Al)]. According to the ultrahigh-strength bainite rail steel and the preparation method thereof, the preparation efficiency is high, and the rail steel is excellent in comprehensive mechanical property and suitable for popularization.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical metallurgy deformation or changing the physical properties of rail steel, and specifically relates to an ultra-high strength bainite rail steel and a preparation method thereof. Background Art

[0002] Rails are important track components that support the normal operation of trains and are widely used in railway transportation and other fields. Since rails are subject to strong mechanical impact and fatigue damage throughout their service life, rails are required to have not only good impact resistance but also high strength. Therefore, it is necessary to study the impact toughness, strength and other properties of rails.

[0003] At present, pearlite rails are widely used in passenger and general freight railways in the world. Pearlite rails have the advantage of good wear resistance, but the disadvantage of low strength. Since the 21st century, with the continuous development of my country's railway technology, heavy-duty and high-density passenger lines have put forward service requirements for rails with good toughness and high strength. Therefore, bainite rail steel with higher strength and toughness has been developed and produced.

[0004] In order to further develop bainitic rail steel with excellent comprehensive mechanical properties, many research and development units have improved and developed many bainitic rail steels. The following is an introduction: The patent document with publication number CN103409694A proposes a low-carbon micro-alloyed bainitic rail steel and a manufacturing method. The components are calculated by weight percentage: C: 0.08-0.15, Si: 1.00-1.80, Mn: 2.00-2.60, Al: 0.05-1.00, Nb: 0.01-0.07, V: 0.10-0.22, Cr: 1.00-1.60, Mo: 0.20-0.50, RE: 0.01-0.03, and the rest are Fe and unavoidable impurities. The invention obtains the low-carbon microalloyed bainite rail steel through hot rolling + air cooling process, and the hot-rolled mechanical properties are as follows: tensile strength ≥ 1250MPa, yield strength ≥ 900MPa, elongation ≥ 14%, and room temperature impact energy ≥ 60J. The advantage is that the weldability is improved without reducing the hardness and strength.

[0005] The invention patent with the authorization announcement number CN116555669B provides "a niobium microalloyed high-strength bainite rail and its production method". The components and contents of the niobium microalloyed high-strength bainite rail are as follows: C: 0.19-0.22, Si: 0.92-0.95, Mn: 2.10-2.30, Nb: Cr: 0.87-0.92, Ni: 0.04-0.06, Mo: 0.36-0.39, the balance is Fe and inevitable impurities. In the production method disclosed in the present invention, a small amount of Nb alloy is added to replace a large amount of Ni alloy, and the comprehensive mechanics of the rail can be further optimized through the process of continuous casting + multi-pass hot rolling + tempering, and the tensile strength is ≥1366MPa, the yield strength is ≥1137MPa, the elongation is ≥14.5%, and the room temperature impact energy is ≥95J.

[0006] The patent document with authorization announcement number CN110468347A proposes a high-strength and toughness bainite rail and a manufacturing method thereof. The components of the rail are as follows by mass percentage: C: 0.20-0.30, Si: 1.00-1.80, Mn: 1.80-2.80, Cr: 0.50-1.00, Mo: 0.40-0.70, Nb: 0.02-0.08, V: 0.05-0.10, Ti: 0.003-0.020, N: 0.003-0.0060. The bainite rail obtained by the present invention through smelting-continuous casting-reheating-rolling-tempering process has a tensile strength of ≥1350MPa, a yield strength of ≥1150MPa, an elongation of ≥14%, and an impact absorption energy of ≥70J at room temperature.

[0007] The patent document with the authorization announcement number CN106435367A proposes a bainite rail and a preparation method thereof, wherein the composition (mass percentage) includes C: 0.20-0.30, Si: 1.40-1.60, Mn: 1.85-2.05, Cr: 1.00-1.25, Mo: 0.30-0.50, V: 0.07-0.09, 3.1≤Mn+Cr≤3.3, and Fe remainder. The content of bainite ferrite lath in the microstructure of the rounded corner of the rail head of the present invention is ≥90%, and the width of the bainite ferrite lath is 0.3-0.8 μm, the content of film-like retained austenite is ≤5%, and the width of the film-like retained austenite is less than 0.1 μm, and the content of martensite is ≤5%, and the rail head has excellent strength and toughness and low residual stress at the center.

[0008] The above-mentioned bainitic rails all have good strength and toughness, but the online heat treatment method adopted is relatively rough, and the detailed temperature control and cooling process in the process is not specified. Moreover, most patents for bainitic rails do not provide detailed descriptions of the type and size of the final cooled structure. In addition, in order to meet the urgent needs of the national major Sichuan-Tibet Railway construction, the service performance of rail steel in high-altitude and cold areas should be taken into account. Based on the above problems, the existing technology still needs to be improved.

[0009] Among them, low-temperature bainite has a finer size and higher strength and toughness, and is one of the research directions for improving the performance of rail steel. Therefore, by combining the application of new steel grades with the use of new online heat treatment, the ultra-high strength bainitic rail steel is developed to further improve the strength and toughness of bainitic rails and extend the service life of rails, which has an important role in promoting the heavy-load and high-speed development of my country's railways. In this regard, the following improvement technical solutions are proposed. Summary of the invention

[0010] The technical problem solved by the present invention is: to provide an ultra-high strength bainitic rail steel and a preparation method thereof, to solve the technical problem of how to further improve the strength and toughness of the ultra-high strength bainitic rail steel and improve its comprehensive mechanical properties.

[0011] The technical solution adopted by the present invention is: an ultra-high strength bainite rail steel: Nb, B, V composite microalloying;

[0012] The chemical composition of the ultra-high strength bainite rail steel is C: 0.35-0.40, Si+Al: 2.0-2.3, 0.5<Ni+Mo<0.8, Cr+Mn: 1.0-2.0, Nb+B+V: 0.05-0.2, and the rest is Fe and unavoidable impurities;

[0013] In the ultra-high strength bainite rail steel, the chemical composition mass percentage ratio of Si, Mn, Cr, Ni, V, Mo, Nb, Al and B meets the following requirements: 0.13≤N≤2.56, 0.40≤n≤0.92; wherein,

[0014] N=[1.1×(Si+Al)×(Ni+Mo)-1] / [0.4×(Cr+Mn)];

[0015] n=0.8×(Ni+Mo) / [3-(Si+Al)].

[0016] In the above technical solution: the content of film-like retained austenite in the ultra-high strength bainitic rail steel structure is 6-10%, and the total content of retained austenite in the ultra-high strength bainitic rail steel structure is less than 15%.

[0017] In the above technical solution: the yield strength of the ultra-high strength bainite rail steel is 1150-1350MPa, the tensile strength is 1450-1550MPa, the elongation is 14-20%, and the room temperature impact toughness is 80-120J / cm 2 , the impact toughness at -40℃ is 35~60J / cm 2 .

[0018] The present invention also seeks to protect a method for preparing ultra-high strength bainitic rail steel, the method being any method for preparing the ultra-high strength bainitic rail steel, the method comprising the following steps:

[0019] S1. Smelting: feeding materials according to the design requirements of the composition of ultra-high strength bainitic rail steel, smelting in a vacuum high-frequency induction furnace and casting into steel ingots after electroslag remelting; the composition of the ultra-high strength bainitic rail steel is as follows by mass percentage: C: 0.35-0.40, Si+Al: 2.0-2.3, 0.5<Ni+Mo<0.8, Cr+Mn: 1.0-2.0, Nb+B+V: 0.05-0.2, and the rest is Fe and unavoidable impurities.

[0020] S2, hot rolling: hot rolling the steel ingot obtained in step S1, and air cooling it to room temperature after hot rolling to obtain hot-rolled rail steel.

[0021] S3, austenitizing: heating the hot-rolled rail steel obtained in step S2 to 890-960° C. to obtain the rail steel to be treated after austenitizing treatment.

[0022] S4, first stage cooling rate control: cool the untreated rail steel after austenitization obtained in step S3 to a first preset temperature at a first stage cooling rate, and then perform a second stage cooling; prevent the rail steel after austenitization from forming ferrite and pearlite structures, and convert the high-temperature austenite structure into an undercooled austenite structure; the first cooling rate is 20°C / s~25°C / s, and the first preset temperature is 500°C~600°C.

[0023] S5. Second stage cooling rate control: cool the rail steel after the first stage cooling in step S4 to a second preset temperature at a second cooling rate, and then cool it to a third preset temperature at a third cooling rate; the second cooling rate is 1°C / s to 5°C / s, and the second preset temperature is 400°C to 500°C; the third cooling rate is 10°C / s to 20°C / s, and the third preset temperature is 370°C to 390°C.

[0024] S6, third stage cooling rate control: cool the rail steel after the third stage cooling in step S5 to a fourth preset temperature at a fourth cooling rate, and then cool to room temperature at a fifth cooling rate; the fourth cooling rate is 0.01°C / s to 0.1°C / s; the fifth cooling rate is 0.001°C / s to 0.01°C / s; the fourth preset temperature is 300°C to 370°C.

[0025] In the above technical solution: the microstructure of the ultra-high strength bainitic rail steel after multi-stage graded cooling control treatment consists of low-temperature bainite with a bimodal size distribution; the size of the bainite laths during the fifth cooling rate process is 60 to 150 nm.

[0026] In the above technical solution: the amount of bainite transformation pre-generated in the fourth cooling rate control process of the third cooling rate control stage in step S6 is 10-20%.

[0027] In the above technical solution: the amount of bainite transformation of the refined size in the fifth cooling rate control process of the third cooling rate control stage in step S6 is 70% to 80%.

[0028] In the above technical solution: the content of film-like retained austenite in the final structure of step S6 is 6-10%; the content of total retained austenite in the final structure of step S6 is less than 15%.

[0029] In the above technical solution: the yield strength of the ultra-high strength bainite rail steel is 1150-1350MPa, the tensile strength is 1450-1550MPa, the elongation is 14-20%, and the room temperature impact toughness is 80-120J / cm 2 , the impact toughness at -40℃ is 35~60J / cm 2 .

[0030] The advantages of the present invention compared with the prior art are:

[0031] 1. The present invention rationally designs the chemical components of the ultra-high strength bainitic rail steel and regulates the relative addition amounts of some key elements, thereby bringing into play the key regulating role of the relevant elements on the strength and toughness of the steel of the present invention; combined with a multi-stage and multi-level cooling control process, the ultra-high strength of the ultra-high strength bainitic rail steel is achieved while achieving coordinated deformation to obtain excellent comprehensive mechanical properties.

[0032] 2. The microstructure of the ultra-high strength bainitic rail steel after multi-stage graded cooling control treatment of the present invention is composed of low-temperature bainite with a bimodal size distribution. The size of the bainite laths in the fifth cooling rate control process of the third cooling stage is 60-150nm. It has excellent mechanical properties and good processing performance, effectively improves production efficiency and reduces costs, and is suitable for promotion.

[0033] 3. The amount of bainite transformation pre-generated during the fourth cooling rate control process of the third cooling stage of the ultra-high strength bainitic rail steel of the present invention is 10-20%, the amount of bainite transformation with refined size during the fifth cooling rate control process of the third cooling stage is 70%-80%, the content of film-like retained austenite in the final structure is 6-10%, and the total residual austenite content in the final structure is less than 15%.

[0034] 4. The yield strength of the ultra-high strength bainite rail steel of the present invention is 1150-1350MPa, the tensile strength is 1450-1550MPa, the elongation is 14-20%, and the room temperature impact toughness is 80-120J / cm2 , the impact toughness at -40℃ is 35~60J / cm 2 .

[0035] 5. The ultra-high strength bainitic rail steel alloy system of the present invention is reasonably controlled, the preparation process is simple and easy, which is conducive to industrial production and has high preparation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a microstructure scan of the ultra-high strength bainite rail steel prepared in Example 1 of the present invention;

[0037] Figure 2 This is a microstructure scan of the ultra-high strength bainite rail steel prepared in Example 2 of the present invention;

[0038] Figure 3 This is a microstructure scanning photograph of the ultra-high strength bainite rail steel prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0039] The following will be combined with the attached embodiment of the present invention Figure 1-3 , the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] An ultra-high strength bainitic rail steel: Nb, B, V composite microalloying;

[0041] The chemical composition of the ultra-high strength bainite rail steel is C: 0.35-0.40, Si+Al: 2.0-2.3, 0.5<Ni+Mo<0.8, Cr+Mn: 1.0-2.0, Nb+B+V: 0.05-0.2, and the rest is Fe and unavoidable impurities.

[0042] In the ultra-high strength bainite rail steel, the chemical composition mass percentage ratio of Si, Mn, Cr, Ni, V, Mo, Nb, Al and B meets the following requirements: 0.13≤N≤2.56, 0.40≤n≤0.92; wherein,

[0043] N=[1.1×(Si+Al)×(Ni+Mo)-1] / [0.4×(Cr+Mn)];

[0044] n=0.8×(Ni+Mo) / [3-(Si+Al)].

[0045] The present invention can effectively refine the grains of steel by precisely controlling the content of elements such as Si and Al. Grain refinement can not only improve the strength and toughness of steel, but also improve its wear resistance and fatigue resistance. The addition of elements such as Ni and Mo helps to promote the formation of bainite structure; under a specific ratio, these elements can optimize the morphology and distribution of bainite structure, thereby improving the overall performance of steel. The content of elements such as Mn and Cr has a significant effect on the strength of steel; under a specific ratio, these elements can significantly improve the yield strength and tensile strength of steel, so that it meets the requirements of ultra-high strength rail steel. By optimizing the content of elements such as Ni and Mo, the toughness of steel can be improved; in particular, Ni plays an important role in improving the low-temperature toughness and brittle fracture resistance of steel. Appropriate Si and Al content can ensure that the steel can undergo sufficient plastic deformation when subjected to force and is not easy to break, thereby improving its ductility. The addition of elements such as Si and Mn can significantly improve the wear resistance of steel; the hard phase and refined grains formed by these elements in the steel help to improve the wear resistance of steel. By optimizing the content of elements such as Cr, Ni, and Mo, the fatigue resistance of steel can be improved; these elements can reduce fatigue damage to steel under cyclic loads and extend its service life. By precisely controlling the content of elements such as Si and Al, it can be ensured that the steel is not prone to cracks and inclusions during welding, thereby improving its welding quality. The high strength and wear resistance of the ultra-high strength bainitic rail steel of the present invention make it an ideal choice for heavy-duty railways. It can withstand greater loads and impact forces, ensuring the safety and stability of the railway system. The high toughness and fatigue resistance of the steel also make it suitable for high-speed railway systems. Under extreme climatic and geological conditions, such as cold areas and desert areas, the ultra-high strength bainitic rail steel can maintain good performance and meet the needs of railway construction in special environments.

[0046] In the above embodiment: the content of film-like retained austenite in the ultra-high strength bainitic rail steel structure is 6-10%, and the total content of retained austenite in the ultra-high strength bainitic rail steel structure is less than 15%.

[0047] It should be understood that the film-like retained austenite is a high-temperature phase that is retained at room temperature. It is not easy to corrode and has high impact toughness. It can absorb deformation energy and buffer the quenching stress of the workpiece, thereby reducing the tendency of deformation cracking. To a certain extent, the film-like retained austenite can balance (compensate) the volume change caused by the tempering decomposition of martensite, which helps to improve the dimensional stability of the material. When the total retained austenite content is controlled below 15%, it can be ensured that the strength of the material will not be significantly reduced due to excessive retained austenite. At the same time, an appropriate amount of retained austenite can maintain the toughness of the material, so that the material has better fracture resistance when subjected to impact loads. An appropriate amount of retained austenite can increase the wear-resistant area of ​​the material and improve the wear resistance of the material. Under the action of alternating compressive stress, the film-like retained austenite can absorb energy and reduce the fatigue damage of the material. By precisely controlling the content and morphology of retained austenite, the fatigue resistance of the material can be optimized and the service life of the material can be extended. Controlling the content and morphology of retained austenite helps to stabilize the metallographic structure of the material and improve the dimensional stability of the material. An appropriate amount of retained austenite can improve the plasticity and toughness of the material, thereby improving the processing performance of the material. During the cutting process, the film-like retained austenite can absorb the cutting heat and cutting stress, reducing the wear and deformation of the cutting edge.

[0048] In the above embodiment: the yield strength of the ultra-high strength bainite rail steel is 1150-1350MPa, the tensile strength is 1450-1550MPa, the elongation is 14-20%, and the room temperature impact toughness is 80-120J / cm 2 , the impact toughness at -40℃ is 35-60J / cm 2 .

[0049] It should be understood that the yield strength and tensile strength of ultra-high strength bainitic rail steel are much higher than those of traditional rail steel, and it can withstand greater loads and impact forces, ensuring the safety and stability of the railway system. The steel exhibits excellent impact toughness at both room and low temperatures, and can maintain good fracture resistance in extreme environments, extending the service life of the track. The elongation of 14 to 20% means that the steel can undergo large plastic deformation when subjected to external forces and is not easy to break, which improves the durability and reliability of the track. Good ductility enables ultra-high strength bainitic rail steel to adapt to various complex shapes and sizes during processing and installation, reducing processing difficulty and cost. Due to the special properties of bainite structure, the steel has excellent wear resistance, can resist the wear of train wheels on the track, and reduce the frequency of track maintenance and replacement; the impact toughness at -40°C is 35 to 60 J / cm 2 , indicating that the steel can still maintain high toughness in low temperature environment and is not prone to brittle fracture, making it suitable for railway systems in cold regions.

[0050] The present invention also seeks to protect a method for preparing ultra-high strength bainitic rail steel, the method being any method for preparing the ultra-high strength bainitic rail steel, the method comprising the following steps:

[0051] Step S1, smelting: feeding materials according to the design requirements of the composition of ultra-high strength bainitic rail steel, smelting in a vacuum high-frequency induction furnace and casting into steel ingots after electroslag remelting; the composition of the ultra-high strength bainitic rail steel is as follows in mass percentage: C: 0.35-0.40, Si+Al: 2.0-2.3, 0.5<Ni+Mo<0.8, Cr+Mn: 1.0-2.0, Nb+B+V: 0.05-0.2, and the rest is Fe and unavoidable impurities.

[0052] In this step, the vacuum high-frequency induction furnace is used for smelting, which can effectively remove the gas and inclusions in the steel and improve the purity and quality of the steel. The electroslag remelting technology can further purify the molten steel, reduce the content of harmful elements, and improve the crystal structure and performance of the steel. Due to the precise control of chemical composition and advanced smelting process, ultra-high strength bainitic rail steel has extremely high yield strength and tensile strength, which can meet the needs of heavy load and high-speed rail transportation. While maintaining high strength, the steel also has excellent toughness and can maintain good fracture resistance in extreme environments.

[0053] Step S2, hot rolling: hot rolling the steel ingot obtained in step S1, and air cooling it to room temperature after hot rolling to obtain hot-rolled rail steel.

[0054] In this step, during the hot rolling process, the steel ingot is subjected to strong plastic deformation at high temperature, which helps to refine the grains, thereby improving the strength and toughness of the steel. Hot rolling can make the structure in the steel ingot more uniform, reduce the formation of segregation and inclusions, and improve the overall performance of the steel. The rail steel after hot rolling has a higher yield strength and tensile strength, which can meet the requirements of heavy-load and high-speed railway transportation for the strength of rail materials. The hot rolling process helps to maintain the toughness of the steel, enabling it to maintain good fracture resistance in extreme environments.

[0055] Step S3, austenitizing: heating the hot-rolled rail steel obtained in step S2 to 890-960° C. to obtain the rail steel to be treated after austenitizing treatment.

[0056] In this step, the austenitizing process transforms the structure in the steel into austenite, which is a lamellar microstructure whose property is a solid solution of a small amount of carbon dissolved in γ-Fe. Through the austenitizing treatment, the uneven structure and residual stress that may be generated during the hot rolling process can be eliminated, making the microstructure of the steel more uniform and stable. Due to its internal composition, austenitic stainless steel has extremely strong stability, strong steel plasticity and good toughness. Heating the hot-rolled rail steel to the austenitizing temperature range can make it obtain higher strength and toughness, which is crucial for the requirements of rail materials for heavy-duty and high-speed railway transportation. Austenitic stainless steel has excellent corrosion resistance and high temperature resistance. The austenitizing process can usually be completed in a shorter time and can be batch processed by automated equipment. This helps to improve production efficiency and reduce production costs.

[0057] Step S4, first stage cooling rate control: cool the untreated rail steel after austenitization obtained in step S3 to a first preset temperature at a first stage cooling rate, and then perform a second stage cooling; prevent the rail steel after austenitization from forming ferrite and pearlite structures, and convert the high-temperature austenite structure into an undercooled austenite structure; the first cooling rate is 20°C / s~25°C / s, and the first preset temperature is 500°C~600°C.

[0058] In this step, by controlling the cooling rate in the first stage, the formation of ferrite and pearlite structures in the rail steel after austenitization can be effectively avoided. These structures may cause performance degradation during the service of the rail steel, such as reduced strength, toughness and wear resistance. The first stage cooling rate control can ensure that the high-temperature austenite structure is transformed into an undercooled austenite structure during the cooling process, laying a good foundation for the subsequent heat treatment process. The undercooled austenite structure has high stability and excellent mechanical properties. By controlling the cooling rate in the first stage, the microstructure of the rail steel can be made more uniform and fine, thereby improving its strength and toughness. In addition, the undercooled austenite structure also helps to improve the wear resistance and fatigue resistance of the rail steel and extend its service life.

[0059] Step S5, second stage cooling rate control: cool the rail steel after the first stage cooling in step S4 to a second preset temperature at a second cooling rate, and then cool it to a third preset temperature at a third cooling rate; the second cooling rate is 1°C / s to 5°C / s, and the second preset temperature is 400°C to 500°C; the third cooling rate is 10°C / s to 20°C / s, and the third preset temperature is 370°C to 390°C.

[0060] In this step, the phase transformation process in the rail steel can be precisely controlled by controlling the cooling rate in sections. In the second cooling process, the second cooling rate is relatively slow, which is conducive to the transformation of austenite to bainite, while avoiding the formation of martensite caused by too fast cooling. Subsequently, the acceleration of the third cooling rate helps to further refine the bainite structure and improve the strength and toughness of the steel. Segmented cooling can make the microstructure of the rail steel more uniform and fine. In the second cooling process, the slower cooling rate helps to form a uniformly distributed bainite structure, while the subsequent rapid cooling helps to refine these structures, thereby improving the overall performance of the steel. The mechanical properties of the rail steel controlled by the second cooling rate are significantly improved, including higher strength, toughness, wear resistance and fatigue resistance. Segmented cooling also helps to enhance the stability of the steel. Although segmented cooling increases the complexity of heat treatment, by accurately controlling the cooling rate and temperature, the overall heat treatment time can be shortened and the production efficiency can be improved. At the same time, due to the optimization of microstructure and the improvement of mechanical properties, the product quality is also significantly improved.

[0061] Step S6, third-stage cooling rate control: cool the rail steel after the third-stage cooling in step S5 to a fourth preset temperature at a fourth cooling rate, and then cool to room temperature at a fifth cooling rate; the fourth cooling rate is 0.01°C / s to 0.1°C / s; the fifth cooling rate is 0.001°C / s to 0.01°C / s; the fourth preset temperature is 300°C to 370°C.

[0062] In this step, the microstructure of the rail steel can be further refined by controlling the cooling rate in the third stage, especially cooling at an extremely low fourth cooling rate. This refinement helps to improve the strength and toughness of the steel, while improving its wear resistance and fatigue resistance. During the third cooling process, as the temperature decreases, precipitates in the steel will gradually form and distribute. By accurately controlling the cooling rate and temperature, the distribution and morphology of the precipitates can be optimized, thereby improving the overall performance of the steel. For example, the miniaturization and uniform distribution of the precipitates help to improve the strength and toughness of the steel. The third stage cooling rate control also helps to reduce the residual stress in the rail steel. During the cooling process, a certain residual stress will be generated inside the steel. By accurately controlling the cooling rate and temperature, the residual stress can be released or reduced, thereby improving the stability and service life of the steel. The third stage cooling rate control can make the microstructure and performance of the rail steel more uniform; during the cooling process, each part of the steel will be affected by the same or similar cooling conditions, thereby avoiding performance differences caused by uneven cooling. By adjusting the cooling rate and preset temperature in the third stage, the mechanical properties and microstructure of rail steel can be flexibly adjusted to suit special application scenarios and service conditions. Similarly, although the third stage cooling rate control increases the complexity of heat treatment, by accurately controlling the cooling rate and temperature, the overall heat treatment time can be shortened and production efficiency can be improved.

[0063] It can be seen that the present invention reasonably designs the chemical components of the ultra-high strength bainitic rail steel and regulates the relative addition amounts of some key elements, thereby exerting the key regulating role of the relevant elements on the strength and toughness of the steel of the present invention; combined with the multi-stage and multi-level cooling control process, the ultra-high strength of the ultra-high strength bainitic rail steel is achieved while achieving coordinated deformation to obtain excellent comprehensive mechanical properties.

[0064] In the above embodiment: the microstructure of the ultra-high strength bainitic rail steel after the multi-stage graded cooling control treatment consists of low-temperature bainite with a bimodal size distribution; the size of the bainite laths during the fifth cooling rate process is 60-150nm.

[0065] It should be understood that the low-temperature bainite structure itself has high strength, and by precisely controlling the size of the bainite lath, the mechanical properties of the steel can be further optimized to reach ultra-high strength levels. The low-temperature bainite structure with a bimodal size distribution helps to improve the toughness of the steel, so that it can show better fracture resistance when subjected to impact loads. The fine bainite laths help to reduce the generation and expansion of fatigue cracks, thereby improving the fatigue resistance of the steel. Although ultra-high strength bainite rail steel has high strength, due to the particularity of its organizational structure, the steel shows good formability during processing. By precisely controlling the cooling rate and the size of the bainite lath, the welding performance of the steel can be optimized and the cracks and deformation that may occur during welding can be reduced. Multi-stage graded cooling control processing can more accurately control the organizational transformation of the steel, thereby shortening the production cycle and improving production efficiency. By optimizing the cooling process, energy consumption can be reduced and production costs can be reduced. Due to the optimization of steel properties, waste during processing and use can be reduced and material utilization can be improved.

[0066] In the above embodiment: the amount of bainite transformation pre-generated in the fourth cooling rate control process of the third cooling rate control stage in step S6 is 10-20%.

[0067] It should be understood that the pre-generated 10-20% bainite can act as a "core" or "seed" in the subsequent cooling process to promote the formation of more fine bainite, thereby refining the overall microstructure grain. This refinement effect helps to improve the strength and toughness of the steel. By controlling the pre-generated amount of bainite, the coarse or harmful microstructure (such as Widmanstätten structure, etc.) that may be formed during the subsequent cooling process can be reduced, thereby improving the comprehensive performance of the steel. The pre-generated bainite has a higher strength, and the increase in its transformation amount helps to improve the strength of the overall steel. However, too high a transformation amount may lead to a decrease in toughness, so a transformation amount of 10-20% is a relatively balanced choice. The right amount of bainite transformation can improve the toughness of the steel by refining the grains and reducing harmful microstructures while ensuring strength. This improvement in toughness helps the steel to show better fracture resistance when subjected to impact loads. By pre-generating a certain amount of bainite, the time in the subsequent heat treatment process can be reduced, thereby improving production efficiency. Optimizing the cooling rate and the amount of bainite transformation can reduce energy consumption during heat treatment, thereby reducing production costs.

[0068] In the above embodiment: the amount of bainite transformation to a refined size in the fifth cooling rate control process of the third cooling rate control stage in step S6 is 70% to 80%.

[0069] It should be understood that when the bainite transformation reaches 70% to 80%, a large amount of bainite is formed in the steel, and these fine bainite laths can significantly refine the grains of the steel. Grain refinement is an effective means to improve the strength and toughness of steel, and can significantly improve the comprehensive mechanical properties of steel. A high proportion of bainite transformation helps to form a more uniform organizational structure, reduce component segregation and microstructural inhomogeneity in steel, thereby improving the reliability and service life of steel. Bainite itself has high strength, and when the transformation reaches 70% to 80%, the strength of the steel will be significantly improved. This high-strength characteristic enables the steel to withstand greater loads and is suitable for applications with high strength requirements. Although high-strength steels tend to have lower toughness, the refined size of bainite transformation helps to improve toughness by optimizing the organizational structure while maintaining high strength. This combination of high strength and high toughness makes the steel have better fracture resistance when subjected to impact loads. Fine bainite laths can reduce the generation and expansion of fatigue cracks, thereby improving the fatigue resistance of steel. This is especially important for components that need to withstand cyclic loads for a long time. By precisely controlling the amount of bainite transformation, the heat treatment process can be optimized, the heat treatment time and energy consumption can be reduced, and production efficiency can be improved. The optimized heat treatment process not only improves production efficiency, but also helps reduce production costs and improve the competitiveness of enterprises.

[0070] In the above embodiment: the content of film-like retained austenite in the final structure of step S6 is 6-10%; the total content of retained austenite in the final structure of step S6 is less than 15%. In the above embodiment: the yield strength of the ultra-high strength bainitic rail steel is 1150-1350MPa, the tensile strength is 1450-1550MPa, the elongation is 14-20%, and the room temperature impact toughness is 80-120J / cm 2 , the impact toughness at -40℃ is 35~60J / cm 2 The technical advantages of the above parameters are described above and will not be repeated here.

[0071] The present invention is described in detail below in conjunction with specific embodiments:

[0072] Example 1: See attached Figure 1 In this embodiment, a method for preparing ultra-high strength bainite rail steel comprises the following steps:

[0073] Step S1, smelting: feeding materials according to the design requirements of the composition of ultra-high strength bainitic rail steel, smelting in a vacuum high-frequency induction furnace and casting into steel ingots after electroslag remelting; the composition of the ultra-high strength bainitic rail steel is C: 0.36, Si+Al: 2.10, Ni+Mo: 0.65, Cr+Mn: 1.20, Nb+B+V: 0.08 in mass percentage, and the rest is Fe and unavoidable impurities.

[0074] Step S2, hot rolling: hot rolling the steel ingot obtained in step S1, and air cooling it to room temperature after hot rolling to obtain hot-rolled rail steel.

[0075] Step S3, austenitizing: the hot-rolled rail steel obtained in step S2 is heated to 910° C. and kept at this temperature for 90 minutes to obtain the rail steel to be treated after austenitizing treatment.

[0076] Step S4, first stage cooling rate control: cool the untreated rail steel after austenitization obtained in step S3 to a first preset temperature at a first stage cooling rate, and then perform a second stage cooling; prevent the austenitized rail steel from forming ferrite and pearlite structures, and convert the high-temperature austenite structure into an undercooled austenite structure; the first cooling rate is 22°C / s, and the first preset temperature is 580°C.

[0077] Step S5, second stage cooling rate control: cool the rail steel after the first stage cooling in step S4 to a second preset temperature at a second cooling rate, and then cool it to a third preset temperature at a third cooling rate; the second cooling rate is 3°C / s, and the second preset temperature is 450°C; the third cooling rate is 12°C / s, and the third preset temperature is 380°C.

[0078] Step S6, third stage cooling rate control: cool the rail steel after the third stage cooling in step S5 to a fourth preset temperature at a fourth cooling rate, and then cool to room temperature at a fifth cooling rate; the fourth cooling rate is 0.05°C / s; the fifth cooling rate is 0.008°C / s; the fourth preset temperature is 365°C.

[0079] The ultra-high strength bainite rail steel prepared in the specific embodiment 1 of the present invention was subjected to scanning electron microscope (SEM) analysis, impact and tensile tests. The results are shown in Figure 1 .from Figure 1 It can be seen that the structure is a low-temperature bainite structure with a bimodal size, and its content is 84%. The amount of bainite transformation pre-generated during the fourth cooling rate control process in the third cooling stage is 13%, and the content of refined bainite during the fifth cooling rate control process in the third cooling stage is 71%. The total residual austenite content in the structure is 16%, of which the film-like residual austenite content is 9%. In this embodiment 1, ultra-high strength bainitic rail steel is prepared, in which N=1.04, n=0.58, the thickness of the second stage bainitic ferrite lath is 142nm, the yield strength is 1220MPa, the tensile strength is 1450MPa, the elongation is 18%, and the room temperature impact toughness is 115J / cm 2 , -40℃ impact toughness is 58J / cm 2 .

[0080] Example 2: See attached Figure 2 In this embodiment, a method for preparing ultra-high strength bainite rail steel comprises the following steps:

[0081] Step S1, smelting: feeding materials according to the design requirements of the composition of ultra-high strength bainitic rail steel, smelting in a vacuum high-frequency induction furnace and casting into steel ingots after electroslag remelting; the composition of the ultra-high strength bainitic rail steel is C: 0.38, Si+Al: 2.3, Ni+Mo: 0.8, Cr+Mn: 1.6, Nb+B+V: 0.18 in mass percentage, and the rest is Fe and unavoidable impurities.

[0082] Step S2, hot rolling: hot rolling the steel ingot obtained in step S1, and air cooling it to room temperature after hot rolling to obtain hot-rolled rail steel.

[0083] Step S3, austenitizing: the hot-rolled rail steel obtained in step S2 is heated to 940° C. and kept at this temperature for 90 minutes to obtain the rail steel to be treated after austenitizing treatment.

[0084] Step S4, first stage cooling rate control: cool the untreated rail steel after austenitization obtained in step S3 to a first preset temperature at a first stage cooling rate, and then perform a second stage cooling; prevent the rail steel after austenitization from forming ferrite and pearlite structures, and convert the high-temperature austenite structure into an undercooled austenite structure; the first cooling rate is 25°C / s, and the first preset temperature is 545°C.

[0085] Step S5, second stage cooling rate control: cool the rail steel after the first stage cooling in step S4 to a second preset temperature at a second cooling rate, and then cool it to a third preset temperature at a third cooling rate; the second cooling rate is 4°C / s, and the second preset temperature is 425°C; the third cooling rate is 18°C / s, and the third preset temperature is 372°C.

[0086] Step S6, third-stage cooling rate control: cool the rail steel after the third-stage cooling in step S5 to a fourth preset temperature at a fourth cooling rate, and then cool to room temperature at a fifth cooling rate; the fourth cooling rate is 0.08°C / s; the fifth cooling rate is 0.001°C / s; the fourth preset temperature is 320°C.

[0087] The ultra-high strength bainite rail steel prepared in the specific embodiment 2 of the present invention was subjected to scanning electron microscope (SEM) analysis, impact and tensile tests. The results are shown in Figure 2 .from Figure 2It can be seen that the organization is a low-temperature bainite organization with a bimodal size, and its content is 89%. The amount of bainite transformation pre-generated during the fourth cooling rate control process in the third cooling stage is 11%, and the content of refined bainite during the fifth cooling rate control process in the third cooling stage is 78%. The total residual austenite content in the organization is 11%, of which the content of film-like residual austenite is 8%. In this embodiment 2, ultra-high strength bainitic rail steel is prepared, in which N=1.6, n=0.91, the thickness of the bainitic ferrite lath in the second stage is 62nm, the yield strength is 1350MPa, the tensile strength is 1550MPa, the elongation is 16%, and the room temperature impact toughness is 108J / cm 2 , -40℃ impact toughness is 48J / cm 2 .

[0088] Example 3: See attached Figure 3 In this embodiment, a method for preparing ultra-high strength bainite rail steel comprises the following steps:

[0089] Step S1, smelting: feeding materials according to the design requirements of the composition of ultra-high strength bainitic rail steel, smelting in a vacuum high-frequency induction furnace and casting into steel ingots after electroslag remelting; the composition of the ultra-high strength bainitic rail steel is C: 0.37, Si+Al: 2.2, Ni+Mo: 0.72, Cr+Mn: 1.9, Nb+B+V: 0.12 in mass percentage, and the rest is Fe and unavoidable impurities.

[0090] Step S2, hot rolling: hot rolling the steel ingot obtained in step S1, and air cooling it to room temperature after hot rolling to obtain hot-rolled rail steel.

[0091] Step S3, austenitizing: the hot-rolled rail steel obtained in step S2 is heated to 930° C. and kept at this temperature for 90 minutes to obtain the rail steel to be treated after austenitizing treatment.

[0092] Step S4, first stage cooling rate control: cool the untreated rail steel after austenitization obtained in step S3 to a first preset temperature at a first stage cooling rate, and then perform a second stage cooling; prevent the austenitized rail steel from forming ferrite and pearlite structures, and convert the high-temperature austenite structure into an undercooled austenite structure; the first cooling rate is 23°C / s, and the first preset temperature is 520°C.

[0093] Step S5, second stage cooling rate control: cool the rail steel after the first stage cooling in step S4 to a second preset temperature at a second cooling rate, and then cool it to a third preset temperature at a third cooling rate; the second cooling rate is 2°C / s, and the second preset temperature is 480°C; the third cooling rate is 15°C / s, and the third preset temperature is 385°C.

[0094] Step S6, third stage cooling rate control: cool the rail steel after the third stage cooling in step S5 to a fourth preset temperature at a fourth cooling rate, and then cool to room temperature at a fifth cooling rate; the fourth cooling rate is 0.02°C / s; the fifth cooling rate is 0.005°C / s; the fourth preset temperature is 345°C.

[0095] The ultra-high strength bainite rail steel prepared in the specific embodiment 3 of the present invention was subjected to scanning electron microscope (SEM) analysis, impact and tensile tests. The results are shown in Figure 3 .from Figure 3 It can be seen that the organization is a low-temperature bainite organization with a bimodal size, and its content is 90%. The amount of bainite transformation pre-generated during the fourth cooling rate control process in the third cooling stage is 17%, and the content of refined bainite during the fifth cooling rate control process in the third cooling stage is 73%. The total residual austenite content in the organization is 10%, of which the content of film-like residual austenite is 7%. In this embodiment 3, ultra-high strength bainitic rail steel is prepared, in which N=0.98, n=0.72, the thickness of the second stage bainitic ferrite lath is 121nm, the yield strength is 1280MPa, the tensile strength is 1490MPa, the elongation is 15%, and the room temperature impact toughness is 86J / cm 2 , -40℃ impact toughness is 39J / cm 2 .

[0096] Regarding the present invention, it should be noted that: the present invention is based on clarifying the role and content control principle of the multi-alloying elements C, Si, Mn, Cr, Ni, V, Mo, Nb, Al and B in ultra-high strength bainitic rail steel, and rationally designs the chemical components (all in weight percentage) of the ultra-high strength bainitic rail steel.

[0097] By setting the following innovative technical requirements, the relative addition amounts of some key elements are regulated, so as to give full play to the key regulating role of the relevant elements on the strength and toughness of the steel of the present invention: By regulating the relationship: 0.13≤N≤2.56, 0.40≤n≤0.92,

[0098] The alloy elements Si, Mn, Cr, Ni, V, Mo, Nb, Al and B are required to be

[0099] N=[1.1×(Si+Al)×(Ni+Mo)-1] / [0.4×(Cr+Mn)];

[0100] n=0.8×(Ni+Mo) / [3-(Si+Al)] to adjust the element content ratio;

[0101] The N value is controlled between 0.13 and 2.56, and the n value is controlled between 0.40 and 0.92.

[0102] Among them, too high N and n values ​​will lead to extended process time and increased cost of bainitic rail steel, while too low N and n values ​​will make it difficult to prepare ultrafine low-temperature bainite structure and even deteriorate the comprehensive mechanical properties.

[0103] In the preparation method of the present invention, the multi-stage and multi-level cooling control process can control the tissue type in different stages.

[0104] Among them, the rapid cooling rate in the first stage transforms the high-temperature austenite structure into a supercooled austenite structure, thereby avoiding the pearlite and ferrite phase regions.

[0105] The two-stage cooling rate in the second stage shortens the time from supercooled austenite to bainite phase, and avoids the high temperature zone of bainite phase, preventing the formation of granular bainite and upper bainite.

[0106] In the third cooling stage, during the fourth cooling rate control process, some low-temperature bainite is pre-generated to divide the supercooled austenite area, and plays a role in heterogeneous nucleation, accelerating the bainite phase transformation. Then, during the fifth cooling rate control process, it is reduced to a lower bainite phase transformation area, increasing the driving force of the bainite phase transformation, generating refined bainite laths, and obtaining a double-peak size ultrafine bainite structure. While achieving ultra-high strength, coordinated deformation is achieved to obtain excellent comprehensive mechanical properties.

[0107] In summary, the present invention provides an ultra-high strength bainitic rail steel and a preparation method thereof through component design and multi-stage and multi-level cooling control process, thereby improving the comprehensive mechanical properties of bainitic rail steel. The alloy system of the ultra-high strength bainitic rail steel of the present invention is reasonably controlled, the preparation process is simple and easy, the preparation efficiency is high, and it is conducive to industrial production. Therefore, the ultra-high strength bainitic rail steel obtained by the scheme of the present invention has high preparation efficiency and excellent comprehensive mechanical properties, and is suitable for promotion.

[0108] Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0109] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification and equivalent replacement made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. An ultra-high strength bainite rail steel, characterized in that: Nb, B, V composite microalloying; The chemical composition of the ultra-high strength bainite rail steel is C: 0.35-0.40, Si+Al: 2.0-2.3, 0.5<Ni+Mo<0.8, Cr+Mn: 1.0-2.0, Nb+B+V: 0.05-0.2, and the rest is Fe and unavoidable impurities; In the ultra-high strength bainite rail steel, the chemical composition mass percentage ratio of Si, Mn, Cr, Ni, V, Mo, Nb, Al and B meets the following requirements: 0.13≤N≤2.56, 0.40≤n≤0.92; wherein, N=[1.1×(Si+Al)×(Ni+Mo)-1] / [0.4×(Cr+Mn)]; n=0.8×(Ni+Mo) / [3-(Si+Al)].

2. The ultra-high strength bainite rail steel according to claim 1, characterized in that: The content of film-like retained austenite in the ultra-high strength bainite rail steel structure is 6-10%, and the total content of retained austenite in the ultra-high strength bainite rail steel structure is less than 15%.

3. The ultra-high strength bainite rail steel according to claim 1, characterized in that: The ultra-high strength bainite rail steel has a yield strength of 1150-1350 MPa, a tensile strength of 1450-1550 MPa, an elongation of 14-20%, and a room temperature impact toughness of 80-120 J / cm 2 , the impact toughness at -40℃ is 35-60J / cm 2 .

4. A method for preparing ultra-high strength bainitic rail steel, the method being the method for preparing ultra-high strength bainitic rail steel according to any one of claims 1 to 3, the method comprising the following steps: S1. Smelting: feeding materials according to the design requirements of the composition of ultra-high strength bainitic rail steel, smelting in a vacuum high-frequency induction furnace and casting into steel ingots after electroslag remelting; the composition of the ultra-high strength bainitic rail steel is as follows by mass percentage: C: 0.35-0.40, Si+Al: 2.0-2.3, 0.5<Ni+Mo<0.8, Cr+Mn: 1.0-2.0, Nb+B+V: 0.05-0.2, and the rest is Fe and unavoidable impurities; S2, hot rolling: hot rolling the steel ingot obtained in step S1, and air cooling it to room temperature after hot rolling to obtain hot-rolled rail steel; S3, austenitizing: heating the hot-rolled rail steel obtained in step S2 to 890-960° C. to obtain the rail steel to be treated after austenitizing treatment; S4, first stage cooling rate control: cooling the untreated rail steel after austenitization obtained in step S3 to a first preset temperature at a first stage cooling rate, and then performing a second stage cooling; The rail steel after austenitization is prevented from forming ferrite and pearlite structures, and the high-temperature austenite structure is converted into an undercooled austenite structure; the first cooling rate is 20°C / s to 25°C / s, and the first preset temperature is 500°C to 600°C; S5, second stage cooling rate control: cool the rail steel after the first stage cooling in step S4 to a second preset temperature at a second cooling rate, and then cool it to a third preset temperature at a third cooling rate; the second cooling rate is 1°C / s to 5°C / s, and the second preset temperature is 400°C to 500°C; the third cooling rate is 10°C / s to 20°C / s, and the third preset temperature is 370°C to 390°C; S6, third stage cooling rate control: cool the rail steel after the third stage cooling in step S5 to a fourth preset temperature at a fourth cooling rate, and then cool to room temperature at a fifth cooling rate; the fourth cooling rate is 0.01°C / s to 0.1°C / s; the fifth cooling rate is 0.001°C / s to 0.01°C / s; the fourth preset temperature is 300°C to 370°C.

5. The preparation method according to claim 4, characterized in that: The microstructure of the ultra-high strength bainitic rail steel after multi-stage graded cooling control treatment consists of low-temperature bainite with a bimodal size distribution; the size of the bainite lath during the fifth cooling rate is 60-150nm.

6. The preparation method according to claim 4, characterized in that: The amount of bainite transformation pre-generated in the fourth cooling rate control process of the third cooling rate control stage in step S6 is 10-20%.

7. The preparation method according to claim 4, characterized in that: In the fifth cooling rate control process of the third cooling rate control stage in step S6, the amount of bainite transformation to a refined size is 70% to 80%.

8. The preparation method according to claim 4, characterized in that: The content of film-like retained austenite in the final structure of step S6 is 6-10%; the content of total retained austenite in the final structure of step S6 is less than 15%.

9. The preparation method according to claim 4, characterized in that: The yield strength of ultra-high strength bainitic rail steel is 1150-1350MPa, the tensile strength is 1450-1550MPa, the elongation is 14-20%, and the room temperature impact toughness is 80-120J / cm 2 , the impact toughness at -40℃ is 35~60J / cm 2 .

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