Method for preparing bainite steel rail and bainite steel rail

Abstract

The present invention belongs to the field of rail manufacturing, specifically to a method for preparing a bainitic rail and the bainitic rail obtained using this method. The method comprises: heating and soaking a continuously cast billet; rolling the soaked billet in multiple passes while controlling the rolling temperature and compression ratio; cooling the resulting rail in a controlled manner; and pre-bending the rail and cooling it to room temperature on a walking-beam cooling bed. The bainitic rail obtained using this method exhibits excellent mechanical properties and a long service life, meeting the high standards for rails required for modern railway transportation.

Description

Technical Field

[0001] The present invention belongs to the technical field of rail manufacturing, and in particular relates to a method for preparing a bainite rail and a bainite rail obtained by using the method. Background Art

[0002] The rapid development of the railway industry, particularly the expansion of heavy-haul and high-speed rail, has led to increasingly stringent performance requirements for rails. Traditional rail materials often fail to meet the high standards of modern railways due to issues such as insufficient strength, poor toughness, insufficient wear resistance, and weak impact resistance. To improve the service performance and lifespan of rails, the industry is continuously seeking to develop new rail materials with higher strength, improved toughness, enhanced wear resistance, and greater impact resistance.

[0003] Bainitic steel, due to its excellent combination of strength and toughness, has become a hot topic in rail material research. The strength and toughness of bainitic rails are closely related to their microstructure. In actual production, the preparation of bainitic rails typically involves heating the continuous casting billet, soaking treatment, multi-pass rolling, and natural cooling. However, traditional preparation methods have many limitations, such as uncontrollable cooling rates, microstructural heterogeneity, and unsatisfactory mechanical properties. These issues have restricted further improvement of bainitic rail performance.

[0004] In the prior art, natural cooling is typically used after rolling of bainitic rails. This process involves a wide temperature range, gradually decreasing from high temperature to room temperature. The natural cooling rate varies throughout the cooling process, decreasing from fast to slow as the rail temperature decreases. This cooling method may result in insufficient bainite transformation, resulting in the formation of non-bainitic structures such as proeutectoid ferrite, which affects the ultimate performance of the rail. In addition, due to the variation in cooling rate, the bainite may exhibit a variety of morphologies, such as acicular, granular, or massive. These different morphologies have different effects on the performance of the rail.

[0005] In actual production, it is found that there are huge challenges in producing bainitic rails with excellent comprehensive mechanical properties such as high strength, good toughness, wear resistance and impact resistance. Summary of the Invention

[0006] In view of this, and in order to solve the problems existing in the prior art, the present invention provides a method for preparing a bainitic rail and a bainitic rail obtained using the method, thereby providing technical support for obtaining a bainitic rail with excellent macroscopic properties (such as strength, toughness, wear resistance, corrosion resistance and impact resistance).

[0007] The technical solution adopted in the present invention is as follows:

[0008] A first aspect of the present invention provides a method for preparing a bainite rail, comprising the following steps:

[0009] Step S1: heating the continuous casting billet and performing soaking treatment;

[0010] Step S2: feeding the continuous casting slab into a rolling line for universal rolling, wherein the universal rolling includes cogging, rough rolling, intermediate rolling and finishing rolling, wherein the rough rolling passes are more than 3 passes, the intermediate rolling passes are more than 1 pass, the finishing rolling pass is 1 pass, the rolling temperature of the last intermediate rolling pass is 930-960°C, the finishing rolling temperature is controlled at 850-900°C, the rolling reduction ratio is ≥13:1, and the interval between the last two rolling passes is controlled within 30s;

[0011] Step S3: performing controlled cooling on the rail, wherein the controlled cooling comprises cooling the rail to a first temperature at a first cooling rate, and then cooling the rail to a bainite transformation start temperature range at a second cooling rate, wherein the first cooling rate is greater than the second cooling rate;

[0012] Step S4: The pre-bent rails are placed on a walking cooling bed to cool to room temperature.

[0013] According to one embodiment of the present invention, the continuous casting billet contains the following chemical components in percentage by mass: C: 0.21%~0.25%; Si: 2.0%~2.5%; Mn: 0.60%~1.00%; Cr: 0.60%~0.80%; Mo: 0.40%~0.45%; N: 0.005%~0.008%; Ti: 0.08%~0.13%; the balance is Fe and unavoidable impurities.

[0014] According to one embodiment of the present invention, Ti, C, and N in the continuous casting billet satisfy the following chemical equivalent ratio: Ti / C is 3.95-4.00, and Ti / N≤3.42.

[0015] According to one embodiment of the present invention, the original austenite grain size of the continuous casting billet is 50-90 μm.

[0016] According to one embodiment of the present invention, in step S1, the heating rate is ≤400°C / h, the heating time is ≥180min, and after heating to 1150-1250°C, a soaking treatment is performed, the soaking temperature is 1100-1200°C, and the soaking time is ≥40min.

[0017] According to one embodiment of the present invention, in step S2, the starting rolling temperature of the universal rolling is 990-1000°C, and the rolling reduction ratio is ≥10:1.

[0018] According to one embodiment of the present invention, in step S3, the bainite transformation starting temperature range is 390-480°C.

[0019] According to one embodiment of the present invention, in step S3, the first temperature is 480°C, the first cooling rate is 3-5°C / s, and the second cooling rate is 0.3-0.5°C / s.

[0020] According to one embodiment of the present invention, the cooling rate of the rail in step S4 is 0.1-0.3° C. / s.

[0021] According to another aspect of the present invention, a bainite rail is provided. The bainite rail is manufactured by the method described in any one of the above embodiments.

[0022] According to one embodiment of the present invention, the yield strength of the bainite rail is ≥1400 MPa, the tensile strength is ≥1450 MPa, the elongation at break is ≥15%, and the room temperature impact energy is ≥100 J.

[0023] The advantageous effects of the method for preparing bainite rails of the present invention compared with the prior art include at least one of the following:

[0024] The present invention controls the hot rolling process so that the strain and deformation defects accumulated during rolling are retained, and deformed austenite grains are generated during rolling. The deformed austenite is ellipsoidal. During the bainite transformation, atoms diffuse to defects such as grain boundaries or dislocations in a relatively short time, which serve as nucleation points to accelerate the bainite transformation. The previously formed bainite subunits can promote the formation of new bainite subunits at the austenite / bainite interface, accelerating the autocatalytic mechanism. Since the density of defects accumulated during rolling is high, they provide more bainite nucleation points, which is conducive to the formation of fine bainite.

[0025] During the rolling process, the present invention introduces more nucleation sites for strain-induced precipitation phases, causing nanoscale precipitation phases to precipitate from the matrix. These precipitates serve as nucleation sites for bainite transformation, promoting bainite transformation. The strain-induced precipitation phases enhance the autocatalytic effect by promoting the formation of bainite subunits. Simultaneously, by consuming carbon atoms, they reduce the carbon concentration in the residual austenite, thereby lowering the thermal stability of the austenite and accelerating bainite formation. Furthermore, during the bainite transformation, carbide precipitation competes with bainite transformation. Smaller precipitates precipitate after the bainite transformation, pinning dislocations in the bainite and surrounding residual austenite, further enhancing the rail's strength.

[0026] The present invention can obtain finer lath bainite structure by controlling the cooling path and accurately controlling the bainite transformation temperature range. This fine bainite structure not only improves the strength of the rail but also maintains good toughness and wear resistance.

[0027] The method of the present invention achieves fine-tuning of the bainite microstructure through precise control of the heating, rolling, and cooling processes, as well as optimized chemical composition, resulting in a bainite rail with excellent macroscopic properties. This method not only improves the mechanical properties of the rail but also extends its service life, meeting the high standards for rails in modern railway transportation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other embodiments can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 A flow chart of a method for preparing a bainite rail provided in some embodiments of the present invention;

[0030] Figure 2 The microstructure of the bainite rail prepared by the method of Example 2 is shown in FIG. DETAILED DESCRIPTION

[0031] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0032] like Figure 1 As shown, the first aspect of the present invention provides a method for preparing a bainite rail, which generally comprises the following steps:

[0033] Step S1: heating the continuous casting billet and performing soaking treatment;

[0034] Step S2: feeding the continuous casting slab into the rolling line for universal rolling, wherein the universal rolling includes slab opening, rough rolling, intermediate rolling and finishing rolling, wherein the rough rolling passes are more than 3 passes, the intermediate rolling passes are more than 1 pass, the finishing rolling pass is 1 pass, the rolling temperature of the last intermediate rolling pass is 930-960°C, the finishing rolling temperature is controlled at 850-900°C, the rolling reduction ratio is ≥13:1, and the interval between the last two rolling passes is controlled within 30 seconds;

[0035] Step S3: performing controlled cooling on the rail, wherein the controlled cooling includes cooling the rail to a first temperature at a first cooling rate, and then cooling the rail to a bainite transformation start temperature range at a second cooling rate, wherein the first cooling rate is greater than the second cooling rate;

[0036] Step S4: The pre-bent rails are placed on a walking cooling bed to cool to room temperature.

[0037] The preparation method of the present invention controls the hot rolling process so that the strain and deformation defects accumulated during rolling are retained, and deformed austenite grains are generated by rolling. The deformed austenite is ellipsoidal. During the bainite transformation, atoms diffuse to defects such as grain boundaries or dislocations in a relatively short time, which serve as nucleation points to accelerate the bainite transformation. The previously formed bainite subunits can promote the formation of new bainite subunits at the austenite / bainite interface, accelerating the autocatalytic mechanism. Since the defect density accumulated during rolling is large, it provides more bainite nucleation points, which is conducive to the formation of fine bainite. On the other hand, during the rolling process, more nucleation sites of strain-induced precipitation phases are introduced by rolling, so that nano-scale precipitation phases are precipitated in the matrix. The precipitation phases can serve as nucleation particles for bainite transformation to promote bainite transformation. The strain-induced precipitation phases enhance the autocatalytic effect by promoting the formation of bainite subunits, and at the same time reduce the carbon concentration of the residual austenite by consuming carbon atoms, thereby reducing the thermal stability of the austenite and accelerating the formation of bainite. Furthermore, during the bainite transformation process, carbide precipitation competes with bainite transformation. Smaller precipitates form after the bainite transformation, pinning dislocations in the bainite and surrounding retained austenite, further enhancing the rail's strength. The bainitic rails produced using the method according to the present invention possess excellent mechanical properties and a long service life, meeting the high standards for rails required by modern railway transportation.

[0038] In some embodiments, the continuous casting slab in step S1 contains the following chemical components in percentage by mass: C: 0.21%~0.25%; Si: 2.0%~2.5%; Mn: 0.60%~1.00%; Cr: 0.60%~0.80%; Mo: 0.40%~0.45%; N: 0.005%~0.008%; Ti: 0.08%~0.13%; the remainder is Fe and unavoidable impurities.

[0039] Titanium (Ti), as an important microalloying element, plays a key role in bainitic rails. Ti forms titanium carbide (TiC) and titanium nitride (TiN) in the steel. These fine, dispersed precipitates form at grain boundaries and dislocations, hindering grain boundary migration through a pinning effect, significantly improving the rail's strength and toughness. During the rolling stage, strain-induced precipitation significantly affects the recrystallization behavior of austenite. The addition of Ti can increase the recrystallization temperature of austenite, inhibit austenite recrystallization at lower rolling temperatures, maintain deformation accumulation during rolling, and thus promote the formation of a higher number of dislocations after rolling. This accumulated deformation provides more nucleation sites for the subsequent bainite transformation and reduces the energy barrier for bainite transformation, accelerating the bainite transformation process.

[0040] Ti is typically added to steel in trace amounts, generally not exceeding 0.08wt%, to facilitate the formation of numerous fine, dispersed precipitates during cooling. Ti precipitates in steel occur as TiN and TiC. Ti exists in steel as carbides and nitrides. These fine, dispersed nanoscale precipitates can form at grain boundaries and dislocations, hindering grain boundary migration through a pinning effect and thus affecting the recrystallization behavior of austenite. During hot rolling, Ti can influence the recrystallization behavior of austenite. The precipitation of TiN or Ti(C,N) inhibits austenite grain growth at high temperatures, resulting in finer grains during the phase transformation. Furthermore, Ti can form heterogeneous precipitations at the austenite-ferrite interface. These precipitations hinder interface migration, affecting grain size and distribution. Furthermore, Ti addition can increase the recrystallization temperature of cold-rolled steel, requiring higher temperatures to initiate recrystallization. This helps maintain a fine grain structure and defects such as dislocations within the crystal at lower temperatures. The ratios of Ti to C, and Ti to N, are crucial for the formation of nanoscale, fine TiC and TiN precipitates, which significantly impact the size of the prior austenite grains. To this end, in some embodiments, the Ti, C, and N ratios in the continuous casting ingot are controlled to meet the following chemical equivalence ratios: Ti / C is 3.95-4.00, and Ti / N is ≤ 3.42.

[0041] Furthermore, the original austenite grain size of the continuous casting ingot is 50~90μm. The original austenite grain size of the continuous casting ingot is small, and grain refinement means more grain boundary area, providing more potential nucleation sites. The smaller original austenite grain size may promote the formation of finer bainite subunits. The already formed bainite subunits can promote the formation of new bainite subunits at the austenite / bainite interface. The smaller austenite grain size can increase the chance of autocatalytic nucleation, that is, the difference between the activation energy of grain boundary nucleation and the activation energy of autocatalytic nucleation is greater, which is conducive to the formation of fine bainite. The refinement of the bainite microstructure can improve its strength, toughness, impact resistance, etc.

[0042] In some embodiments, in step S1, the heating rate is ≤400°C / h, the heating time is ≥180min, and after heating to 1150~1250°C, a soaking treatment is performed, the soaking temperature is 1100~1200°C, and the soaking time is ≥40min. Controlling the heating rate and time can avoid stress concentration and micro defects inside the material, help to obtain a uniform microstructure, thereby improving the uniformity and overall performance of the material. During the homogenization process, a portion of the titanium carbon and nitrides are redissolved into the matrix, and the Ti dissolved in the austenite can increase the recrystallization temperature, which is conducive to rolling in the non-recrystallized austenite area. High-pressure water descaling can be performed after the soaking treatment.

[0043] In some embodiments, in step S2, rolling is performed using a universal rolling method, which includes blooming, roughing, intermediate, and finishing rolling. For example, the rolling mill layout adopts a seven-stand 1-1-2-2-1 configuration, with the main mill area consisting of two reversing blooming mills (BD1 and BD2), a universal roughing mill unit (U1-E1), a universal and intermediate mill unit (U2-E2), and a universal finishing mill (UF).

[0044] The continuously cast rectangular billet is rolled through BD1 and BD2 and then in the pilot hole into an H-shaped rail blank for rolling in the universal mill. In the universal mill train, the heavy rail is reversibly rolled in the tandem mills U1-E1 in multiple passes, for example, three passes in the universal roughing mill U1 and two passes in the edger E1. The heavy rail is then rolled in the tandem mills U2-E2 in at least one pass, for example, one pass each in the universal roughing mill U2 and the edger E2, before finally being rolled in the universal finishing mill UF in one pass.

[0045] In some embodiments, the starting rolling temperature is 990-1000°C, and the rolling reduction ratio is ≥10:1. The number of rough rolling passes is 3 or more, the number of intermediate rolling passes is 1 or more, and the number of finishing rolling passes is 1. The rolling temperature of the last intermediate rolling pass is 930-960°C, and the finishing rolling temperature is controlled at 850-900°C. The interval between the last two rolling passes is controlled within 30 seconds, and the overall rolling reduction ratio is ≥13:1.

[0046] Starting rolling below 1000°C and finishing at 850-900°C introduces more nucleation sites for strain-induced precipitation, ultimately leading to a higher nucleation rate and shorter incubation time, accelerating the formation of precipitates. During hot rolling, Ti nanoprecipitates consume approximately 30%-40% of the total Ti content. These Ti nanoprecipitates range in size from 30 to 80 nm. Because Ti dissolved in austenite increases the recrystallization temperature, when the finishing temperature is controlled between 850 and 900°C, rolling occurs within the unrecrystallized austenite region. Without recrystallization, accumulated strain and deformation defects are retained, resulting in the formation of deformed austenite grains. Deformation is performed on the basis of the original austenite grain size of 50-90 μm in the continuous casting. The deformed austenite is ellipsoidal with a minor axis to major axis ratio of 1:3 to 1:5. Nanoscale precipitates preferentially nucleate on dislocations and subgrains within the deformed austenite.

[0047] The deformation temperature of the last two rolling passes is controlled to ensure that rolling is carried out in the unrecrystallized austenite region, suppressing dynamic recrystallization during rolling. This allows the accumulated strain and deformation defects to be retained, and the rolled austenite grains to be formed, which facilitates the refinement of the bainite structure. The interval between the last two rolling passes is controlled within 30 seconds to suppress static recrystallization, which allows the accumulated strain and deformation defects to be retained, which facilitates the refinement of the bainite structure.

[0048] In step S3, after the hot rolling is completed, the rail is subjected to controlled cooling, wherein the controlled cooling includes cooling the rail to a first temperature at a first cooling rate, and then cooling the rail to a bainite transformation start temperature range at a second cooling rate, wherein the first cooling rate is greater than the second cooling rate.

[0049] In some embodiments, the rail is cooled to 500°C at a first cooling rate of 3-5°C / s, accelerating cooling to the bainite transition temperature. Ti improves the stability of ultra-cooled austenite by delaying carbide formation during eutectoid decomposition, thereby inhibiting pearlite transformation. Compressed air is also used to accelerate cooling, preventing pearlite transformation before bainite transformation. Below 480°C, the cooling rate is controlled at 0.3-0.5°C / s to ensure entry into the bainite transition zone at the appropriate temperature. The cooling path is controlled to achieve fine, uniform lath-like bainite and a small amount of film-like retained austenite.

[0050] In some embodiments, the bainite transformation starting temperature range is 390-480° C. When the temperature is lowered to this temperature, bainite structure begins to form.

[0051] In step S4, the pre-bent rail is placed on a walking cooling bed to cool to room temperature. For example, the rail can be moved to the cooling bed for natural cooling at a cooling rate of 0.1-0.3°C / s. During the slow cooling process, the bainite transformation is completed.

[0052] After completing the above processing, the rails can be further sent to a horizontal and vertical composite straightening machine for straightening.

[0053] By refining the grain size of the continuous casting ingot and controlling the rolling process without recrystallization, the austenite grains do not grow and deformed austenite grains are generated, thus refining the austenite grain size before the bainite transformation. Grain refinement means more grain boundary area before the bainite transformation occurs, providing more potential nucleation sites, thereby refining the ferrite platelets. Furthermore, a smaller prior austenite grain size promotes the formation of finer bainite subunits. Existing bainite subunits can promote the formation of new bainite subunits at the austenite / bainite interface, accelerating the autocatalytic mechanism. Smaller austenite grain size increases the chance of autocatalytic nucleation, meaning the difference between the activation energy of grain boundary nucleation and that of autocatalytic nucleation is greater. Since there is no recrystallization during hot rolling, the accumulated deformation leads to a higher dislocation density. Dislocations can serve as effective sites for bainite nucleation, reducing the energy barrier required for bainite nucleation and thus accelerating bainite formation.

[0054] In addition, the strain-induced precipitates generated during hot rolling can act as nucleation sites for bainite transformation, and the dislocation- and strain-induced precipitates promote the formation of bainite subunits to enhance the autocatalytic effect. At the same time, they reduce the carbon concentration of the remaining austenite by consuming carbon atoms, thereby reducing the thermal stability of austenite and accelerating the formation of bainite.

[0055] Bainite transformed from deformed austenite effectively fixes defects in the deformed austenite. Atoms diffuse quickly to nucleation sites such as grain boundaries or dislocations, accelerating bainite transformation. Carbide precipitation competes with bainite transformation, with carbide precipitation occurring after bainite transformation, allowing smaller precipitates to pin onto dislocations in the bainite and surrounding retained austenite, further enhancing rail strength.

[0056] Another aspect of the present invention provides a bainite rail, which is prepared by the method described in the above embodiment.

[0057] In some embodiments, the bainitic rail has a yield strength of ≥1400 MPa, a tensile strength of ≥1450 MPa, an elongation at break of ≥15%, and an impact energy at room temperature of ≥100 J.

[0058] In order to further illustrate the technical solutions and beneficial effects of the present invention, the present invention also provides the following specific embodiments.

[0059] Example 1

[0060] In this embodiment, the bainite rail contains the following chemical composition by mass percentage: C: 0.21%; Si: 2.0%; Mn: 0.60%; Cr: 0.60%; Mo: 0.40%; N: 0.005%; Ti: 0.08%; the balance being Fe and unavoidable impurities. The original austenite grain size of the continuous casting ingot is 50 μm.

[0061] The continuous cast slab, meeting the aforementioned composition, was heated to 1150°C at a heating rate of 400°C / h, soaked for 60 minutes, and then descaled with high-pressure water. The square continuous cast slab was then fed into a rolling mill using a seven-stand universal rolling process. The rolling temperature was set at 990°C, and the rolling reduction ratio was 10:1. The H-shaped rail slab was then rolled through three reversible passes in a universal roughing mill, two passes in a universal intermediate mill, and finally one pass in a universal finishing mill. The final intermediate rolling temperature was controlled at 930°C, and the finishing temperature was 850°C. The interval between the last two rolling passes was kept within 30 seconds, and the rolling reduction ratio was 14:1. The rail was then cooled by compressed air injection at a cooling rate of 3°C / s to 480°C, and then at a cooling rate of 0.3°C / s to 390°C. The pre-bent rails are then placed on a step-type cooling bed and cooled to room temperature of 20-30°C at a cooling rate of 0.1°C / s, and then placed in a horizontal and vertical composite straightening machine for straightening.

[0062] After testing, the yield strength of the bainite rail is about 1420MPa, the tensile strength is about 1460MPa, the elongation at break is about 15%, and the impact energy at room temperature is about 105J.

[0063] Example 2

[0064] In this embodiment, the bainite rail contains the following chemical composition by mass percentage: C: 0.23%; Si: 2.3%; Mn: 0.80%; Cr: 0.70%; Mo: 0.42%; N: 0.006%; Ti: 0.11%; the balance being Fe and unavoidable impurities. The original austenite grain size of the continuously cast ingot is 70 μm.

[0065] The continuous cast slab, meeting the aforementioned composition, was heated to 1200°C at a heating rate of 390°C / h, soaked for 50 minutes, and then descaled with high-pressure water. The square continuous cast slab was then fed into a rolling mill using a seven-stand universal rolling process. The rolling temperature was set at 995°C, and the rolling reduction ratio was 11:1. The H-shaped rail slab was then rolled through three reversible passes in a universal roughing mill, one pass in a universal intermediate mill, and finally one pass in a universal finishing mill. The intermediate rolling temperature was controlled at 940°C, and the finishing temperature was controlled at 880°C. The interval between the last two rolling passes was kept within 30 seconds, and the rolling reduction ratio was 13:1. The rail was then cooled by compressed air spray at a cooling rate of 4°C / s to 480°C, and then at a cooling rate of 0.4°C / s to 430°C. The pre-bent rails are then placed on a step-type cooling bed, cooled to room temperature of 20-30°C at a cooling rate of 0.2°C / s, and then placed in a horizontal and vertical composite straightening machine for straightening.

[0066] After testing, the bainite rail has a yield strength of 1450MPa, a tensile strength of 1470MPa, an elongation at break of 18%, and an impact energy at room temperature of 130J. Figure 2 The microstructure of the bainite rail produced by this method is shown. In the figure, the green is a film-like retained austenite, and the rest is thin lath-like bainite. The bainite structure is fine and uniform.

[0067] Example 3

[0068] In this embodiment, the bainite rail contains the following chemical composition by mass percentage: C: 0.25%; Si: 2.5%; Mn: 1.00%; Cr: 0.80%; Mo: 0.45%; N: 0.008%; Ti: 0.13%; the balance being Fe and unavoidable impurities. The original austenite grain size of the continuous casting ingot is 90 μm.

[0069] The continuous cast slabs meeting the aforementioned composition were heated to 1250°C at a heating rate of 380°C / h, soaked for 40 minutes, and then descaled with high-pressure water. The square continuous cast slabs were then fed into a rolling mill using a seven-stand universal rolling process. The rolling temperature was set at 1000°C, and the rolling reduction ratio was 12:1. The H-shaped rail slabs were then rolled through three reversible passes in a universal roughing mill, two passes in a universal intermediate mill, and finally one pass in a universal finishing mill. The final intermediate rolling temperature was controlled at 960°C, and the finishing temperature was controlled at 900°C. The interval between the last two rolling passes was kept within 30 seconds, and the rolling reduction ratio was 13:1. The rails were then cooled by compressed air spray at a cooling rate of 5°C / s to 480°C, and then again at a cooling rate of 0.5°C / s to 480°C. The pre-bent rails are then placed on a step-type cooling bed, cooled to room temperature of 20-30°C at a cooling rate of 0.3°C / s, and then placed in a horizontal and vertical composite straightening machine for straightening.

[0070] After testing, the bainitic rail has a yield strength of 1460MPa, a tensile strength of 1480MPa, an elongation at break of 16%, and an impact energy at room temperature of 125J.

[0071] Comparative Example 1

[0072] In this comparative example, the bainite rail had the following chemical composition, by mass percentage: C: 0.23%; Si: 2.3%; Mn: 0.80%; Cr: 0.70%; Mo: 0.42%; N: 0.006%; Ti: 0.11%; the balance being Fe and unavoidable impurities. The original austenite grain size of the continuously cast ingot was 70 μm.

[0073] The continuous cast slabs meeting the aforementioned composition were heated to 1200°C at a heating rate of 390°C / h, soaked for 50 minutes, and then descaled with high-pressure water. The square continuous cast slabs were then fed into a rolling mill using a seven-stand universal rolling process. The rolling temperature was set at 1050°C, and the rolling reduction ratio was 11:1. The H-shaped rail slabs were then rolled through three reversible passes in a universal roughing mill, one pass in a universal intermediate mill, and finally one pass in a universal finishing mill. The final rolling temperature was controlled at 980°C, and the rolling reduction ratio was 13:1. The rails were then cooled by compressed air spray at a cooling rate of 4°C / s to 480°C, and then at a cooling rate of 0.4°C / s to 430°C. The pre-bent rails are then placed on a step-type cooling bed, cooled to room temperature of 20-30°C at a cooling rate of 0.2°C / s, and then placed in a horizontal and vertical composite straightening machine for straightening.

[0074] Testing revealed that the bainitic rail had a yield strength of 1150 MPa, a tensile strength of 1070 MPa, an elongation at break of 12%, and a room-temperature impact energy of 85 J. In this comparative example, due to the high final rolling temperature, dynamic recrystallization occurred during rolling. Rolling defects disappeared during the recrystallization process, failing to provide additional nucleation sites for subsequent bainite transformation. Consequently, the accumulation of rolling defects could not be used to refine the bainite structure.

[0075] Comparative Example 2

[0076] In this comparative example, the bainite rail had the following chemical composition, by mass percentage: C: 0.23%; Si: 2.3%; Mn: 0.80%; Cr: 0.70%; Mo: 0.42%; N: 0.006%; the balance being Fe and unavoidable impurities. The original austenite grain size of the continuously cast ingot was 70 μm.

[0077] The continuous cast slab, meeting the aforementioned composition, was heated to 1200°C at a heating rate of 390°C / h, soaked for 50 minutes, and then descaled with high-pressure water. The square continuous cast slab was then fed into a rolling mill using a seven-stand universal rolling process. The rolling temperature was set at 995°C, and the rolling reduction ratio was 11:1. The H-shaped rail slab was then rolled through three reversible passes in a universal roughing mill, one pass in a universal intermediate mill, and finally one pass in a universal finishing mill. The intermediate rolling temperature was controlled at 940°C, and the finishing temperature was controlled at 880°C. The interval between the last two rolling passes was kept within 30 seconds, and the rolling reduction ratio was 13:1. The rail was then cooled by compressed air spray at a cooling rate of 4°C / s to 480°C, and then at a cooling rate of 0.4°C / s to 430°C. The pre-bent rails are then placed on a step-type cooling bed, cooled to room temperature of 20-30°C at a cooling rate of 0.2°C / s, and then placed in a horizontal and vertical composite straightening machine for straightening.

[0078] Testing revealed that the bainitic rail exhibited a yield strength of 1320 MPa, a tensile strength of 1350 MPa, an elongation at break of 16%, and a room-temperature impact energy of 120 J. In this comparative example, since the chemical composition of the continuous casting ingot does not contain Ti, the recrystallization temperature during hot rolling is lower than that of Ti-containing rails. Consequently, rolling defects in the final few passes diminish as recrystallization recovers, failing to provide additional nucleation sites for the subsequent bainite transformation. Consequently, the accumulation of rolling defects cannot be used to refine the bainite structure. Furthermore, the lack of Ti also results in a lack of titanium carbonitrides in the matrix, reducing the number of precipitated phases and degrading the rail's mechanical properties.

[0079] Comparative Example 3

[0080] In this comparative example, the bainite rail contains the following chemical components in mass percentage: C: 0.23%, Si: 2.3%, Mn: 0.80%, Cr: 0.70%, Mo: 0.42%, N: 0.006%, Ti: 0.11%, and the balance is Fe and inevitable impurities.

[0081] The continuous cast slabs meeting the aforementioned composition were heated to 1200°C at a heating rate of 390°C / h, soaked for 50 minutes, and then descaled with high-pressure water. The square continuous cast slabs were then fed into a rolling mill using a seven-stand universal rolling process. The rolling temperature was set at 995°C, and the rolling reduction ratio was 11:1. The H-shaped rail slabs were then rolled through three reversible passes in a universal roughing mill, one pass in a universal intermediate mill, and finally one pass in a universal finishing mill. The intermediate rolling temperature was controlled at 940°C, and the finishing temperature was controlled at 880°C. The interval between the last two rolling passes was kept within 30 seconds, and the rolling reduction ratio was 13:1. The rails were then cooled to 430°C using compressed air injection at a cooling rate of 0.8°C / s. The pre-bent rails are then placed on a step-type cooling bed, cooled to room temperature of 20-30°C at a cooling rate of 0.2°C / s, and then placed in a horizontal and vertical composite straightening machine for straightening.

[0082] Testing revealed that the bainitic rail had a yield strength of 880 MPa, a tensile strength of 892 MPa, an elongation at break of 12%, and a room-temperature impact energy of 45 J. In this comparative example, due to the low cooling rate after final rolling, the rail formed ferrite and pearlite before the bainite transformation. Furthermore, the temperature was too high during the bainite transformation, resulting in the formation of granular bainite. These undesirable microstructures reduced the mechanical properties of the rail.

[0083] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0084] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art could make various modifications and improvements without departing from the spirit of the present application, all of which fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. A method for preparing a bainite rail, characterized in that: The steps include: Step S1: heating the continuous casting billet and performing soaking treatment; Step S2: feeding the continuous casting slab into a rolling line for universal rolling, wherein the universal rolling includes cogging, rough rolling, intermediate rolling and finishing rolling, wherein the rough rolling passes are more than 3 passes, the intermediate rolling passes are more than 1 pass, the finishing rolling pass is 1 pass, the rolling temperature of the last intermediate rolling pass is 930-960°C, the finishing rolling temperature is controlled at 850-900°C, the rolling reduction ratio is ≥13:1, and the interval between the last two rolling passes is controlled within 30s; Step S3: performing controlled cooling on the rail, wherein the controlled cooling comprises cooling the rail to a first temperature at a first cooling rate, and then cooling the rail to a bainite transformation start temperature range at a second cooling rate, wherein the first cooling rate is greater than the second cooling rate; Step S4: placing the pre-bent rails on a walking cooling bed to cool to room temperature; The continuous casting slab comprises the following chemical components by mass percentage: C: 0.21%-0.25%; Si: 2.0%-2.5%; Mn: 0.60%-1.00%; Cr: 0.60%-0.80%; Mo: 0.40%-0.45%; N: 0.005%-0.008%; Ti: 0.08%-0.13%; the balance being Fe and unavoidable impurities; The first temperature is 480° C., and the first cooling rate is 3-5° C. / s.

2. The method for preparing a bainite rail according to claim 1, characterized in that: The Ti, C, and N in the continuous casting billet satisfy the following chemical equivalent ratio: Ti / C is 3.95-4.00, and Ti / N is ≤3.

42.

3. The method for preparing a bainite rail according to claim 1, wherein: The original austenite grain size of the continuous casting billet is 50-90 μm.

4. The method for preparing a bainite rail according to claim 1, wherein: In step S1, the heating rate is ≤400°C / h, the heating time is ≥180 min, and after heating to 1150-1250°C, a soaking treatment is performed, the soaking temperature is 1100-1200°C, and the soaking time is ≥40 min.

5. The method for preparing a bainite rail according to claim 1, wherein: In step S2, the starting rolling temperature of the universal rolling is 990-1000°C, and the rolling reduction ratio is ≥10:

1.

6. The method for preparing a bainite rail according to claim 1, characterized in that: In step S3, the bainite transformation starting temperature range is 390-480°C.

7. The method for preparing a bainite rail according to claim 1, wherein: In step S3, the second cooling rate is 0.3-0.5°C / s.

8. The method for preparing a bainite rail according to claim 1, wherein: The cooling rate of the rail in step S4 is 0.1-0.3°C / s.

9. A bainite rail, characterized in that: The bainite rail is prepared by the method according to any one of claims 1 to 7.

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