Method for promoting complete bainite transformation of steel rail and bainite steel rail

Abstract

The invention belongs to the field of steel rail manufacturing, and discloses a method for promoting complete bainite transformation of a steel rail and the bainite steel rail. The method comprises the steps that a cast ingot is produced, the carbon equivalent CE in the cast ingot is lower than 0.43%, and the weight ratio of Si to Mn is larger than 0.5; carrying out multi-pass rolling on the cast ingot; and the rolled steel rail is cooled to the bainite phase change interval temperature at the first cooling speed of 20-30 DEG C / s, then cooled for preset time at the second cooling speed of 0.08-0.12 DEG C / s, and finally cooled to the room temperature at the third cooling speed lower than 0.5 DEG C / s. According to the method, the element ratio is accurately controlled, the bainite phase change driving force is optimized, the carbon content is controlled to be 0.19%-0.23%, and meanwhile, the carbon equivalent CE is limited to be smaller than or equal to 0.43%, so that under the condition that enough phase change driving force is provided, the steel rail weldability is ensured, the Si / Mn mass ratio is optimized, the austenite carbon activity coefficient is improved, and the bainite phase change dynamic characteristic is accelerated.

Description

Technical Field

[0001] The present invention relates to the technical field of rail manufacturing, and in particular to a method for promoting complete bainite transformation of a rail and a bainite rail. Background Art

[0002] The rapid development of the railway industry has led to increasingly stringent performance standards for rail and frog materials. Currently, the performance improvements of various pearlite rail steels widely used in my country primarily rely on alloying and heat treatment. However, within the current industrial production technology framework, these two methods alone are insufficient to achieve significant improvements in rail material strength and environmental performance. While high-manganese steel for rail applications offers excellent impact resistance, and the welding challenges between high-manganese steel frogs and rails have been overcome, integrally cast high-manganese steel frogs suffer from insufficient initial hardness. This, combined with defects introduced by the casting process, results in a low average service life and a high degree of dispersion in the lifespan distribution. Consequently, the industry urgently needs to explore and develop new steel grades to replace existing materials. Therefore, developing new steels suitable for rails and frogs is an inevitable choice to meet the development trends of the railway industry. Against this backdrop, bainitic steel, with its excellent overall performance, particularly in terms of contact fatigue and wear resistance, stands out as an ideal candidate for wheel-rail contact steel.

[0003] However, existing bainite rail manufacturing processes still have some drawbacks. Because the bainite phase transformation temperature range lies between the pearlite diffusion phase transformation and the martensite shear phase transformation, its phase transformation properties and microstructure are prone to diversification. The isothermal transformation time of bainite is too long, and incomplete transformation affects the overall mechanical properties of bainite. To shorten the bainite transformation time and increase the amount of bainite transformation, it is necessary to increase the driving force of the bainite phase transformation. Increasing the carbon content in the rail composition can enhance the driving force of the bainite phase transformation, but high-carbon designs significantly degrade the rail's weldability and easily form coarse carbides, resulting in reduced rail toughness. Low-carbon designs, on the other hand, rely on excess alloying elements (such as manganese and chromium) to supplement the driving force of the bainite phase transformation. However, excessive alloying elements result in an excessively wide bainite phase transformation temperature window. An excessively wide phase transition temperature window will make the bainite phase transition process complicated and difficult to precisely control. During the cooling process, different parts may be in different phase transition temperature ranges due to differences in cooling rates, resulting in poor tissue uniformity and tissue abnormalities in local areas, affecting the stability of the overall material performance. In addition, it is difficult to precisely control the cooling process parameters during production to obtain the ideal bainite structure, which is not conducive to ensuring product quality and production efficiency in large-scale industrial production.

[0004] In summary, there is an urgent need in this field for a technology that can promote complete bainite transformation of rails. Summary of the Invention

[0005] In view of the above problems, the present invention provides a method for promoting complete bainite transformation of a rail and a bainite rail.

[0006] According to one aspect of the present invention, a method for promoting complete bainite transformation of a rail is provided, the method comprising the following steps: An ingot comprising the following components by weight is produced: C: 0.19% to 0.23%, Si: 1.50% to 1.60%, Mn: 2.30% to 2.40%, Cr: 0.50% to 0.55%, Mo: 0.14% to 0.16%, V: 0.08% to 0.09%, N: 0.006% to 0.008%, Al ≤ 0.05%, the balance being Fe and unavoidable impurities, wherein The carbon equivalent CE is less than 0.43%, and the weight ratio of Si to Mn is greater than 0.5; The ingot is subjected to multiple-pass rolling; The rolled rail is first cooled to the bainite phase transformation range temperature at a first cooling rate of 20-30°C / s, then cooled for a predetermined time at a second cooling rate of 0.08-0.12°C / s, and finally cooled to room temperature at a third cooling rate of less than 0.5°C / s.

[0007] According to one embodiment of the present invention, the weight ratio of Si to Mn is between 0.62 and 0.68.

[0008] According to one embodiment of the present invention, the weight percentage of C in the ingot is 0.21%, the weight percentage of Si is 1.55%, and the weight percentage of Mn is 2.35%.

[0009] According to one embodiment of the present invention, the rolling temperature is controlled between 850 and 950°C.

[0010] According to one embodiment of the present invention, the cumulative rolling reduction rate is controlled to be above 60%.

[0011] According to one embodiment of the present invention, the bainite transformation temperature range is 350-480°C.

[0012] According to one embodiment of the present invention, the second cooling period lasts for 1000-1500 seconds.

[0013] According to one embodiment of the present invention, the ingot is prepared by vacuum induction melting, and is continuously cast into a billet after melting, and the cooling rate is controlled to be 2-3°C / s.

[0014] According to another aspect of the present invention, there is provided a bainite rail, which is manufactured by the method according to any one of the above embodiments.

[0015] According to one embodiment of the present invention, the rail microstructure comprises bainite with a volume fraction greater than 98% and a balance of film-like retained austenite, the bainite being an ultra-fine lath with an aspect ratio of ≥20:1, the lath thickness being ≤180nm, the retained austenite film thickness being less than 20nm, the rail tensile strength being ≥1450MPa, the elongation after fracture being ≥12%, the impact energy at -40°C being ≥120J, and the contact fatigue life being ≥1×10 7 Second-rate.

[0016] Due to the adoption of the above technical solution, the method for promoting complete bainite transformation of a rail and the bainite rail provided by the present invention have at least one of the following beneficial effects: (1) By precisely controlling the element ratio, the driving force of bainite phase transformation is optimized, the carbon content is controlled within 0.19% to 0.23%, and the carbon equivalent CE is limited to ≤ 0.43% to ensure the weldability of the rail while providing sufficient phase transformation driving force. The Si / Mn weight ratio is optimized, the austenite carbon activity coefficient is increased, and the carbon concentration gradient at the bainite lath interface is made ≥ 1.8wt%, thereby accelerating the bainite phase transformation dynamic characteristics. (2) The hardenability of the rail can be controlled by optimizing the content of each element. By controlling the hardenability factor (DI ≥ 2.5), the decomposition kinetics of austenite can be delayed, ensuring that bainite is formed within a wider cooling rate range, avoiding the formation of martensite due to insufficient cooling rate or over-quenching of the composition. By controlling the Mo content to 0.14%-0.16% and the V content to 0.08%-0.09%, the pearlite nucleation is delayed, and the pearlite transformation amount is close to zero, thereby improving the strength and fatigue life of the rail. (3) Utilizing the Mo-VN synergistic precipitation effect, combined with the aging process to refine the strips, improve the toughness of the rail, strengthen the matrix, inhibit the coarsening of the precipitate phase, and improve the uniformity of the precipitate phase distribution; (4) Through the coupling effect of cooling rate and composition, a controlled cooling process matching the composition is designed. First, the steel is cooled to the bainite phase transformation temperature at a first cooling rate of 20~30℃ / s to inhibit the decomposition of austenite and provide more power for bainite transformation. Then, the steel is slowly cooled at a cooling rate of 0.08~0.12℃ / s in the bainite phase transformation temperature range to promote bainite transformation and ensure that the bainite transformation amount at this stage is ≥98%. Finally, the steel is slowly cooled at a cooling rate of ≤0.5℃ / s to inhibit the formation of martensite and avoid the coarsening of bainite laths. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A flowchart of a method for promoting complete bainite transformation of a rail according to one embodiment of the present invention; Figure 2 FIG. 4 is a CCT continuous cooling curve diagram according to an embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0019] Bainitic rails are considered an important direction to replace traditional pearlitic rails due to their excellent strength-toughness balance characteristics. However, the alloy design and process control of bainitic rails in the existing technology still have significant defects, leading to problems such as incomplete phase transformation, coarsening of the structure and unstable performance. The present invention ensures that bainite is completely transformed during the continuous cooling process (transformation amount ≥ 98%) through the coordinated optimization of specific alloy composition design and continuous cooling process, while suppressing the formation of martensite, solving the problems of uneven structure, excessive retained austenite and performance fluctuations caused by incomplete phase transformation in traditional bainitic rails. The rails are suitable for application scenarios such as high-speed railways and heavy-load tracks that have strict requirements on strength, toughness and fatigue life.

[0020] like Figure 1 As shown, the method for promoting complete bainite transformation of rail provided by the present invention generally includes the following steps: Step S1: producing an ingot comprising the following components by weight percentage: C: 0.19%-0.23%, Si: 1.50%-1.60%, Mn: 2.30%-2.40%, Cr: 0.50%-0.55%, Mo: 0.14%-0.16%, V: 0.08%-0.09%, N: 0.006%-0.008%, Al≤0.05%, the balance being Fe and unavoidable impurities, wherein the carbon equivalent CE is less than 0.43%, and the weight ratio of Si to Mn is greater than 0.5; Step S2: rolling the ingot in multiple passes; Step S3: Cool the rolled rail to a bainite transformation temperature at a first cooling rate of 20-30°C / s, then cool it to a predetermined temperature at a second cooling rate of 0.08-0.12°C / s, and finally cool it to room temperature at a third cooling rate of less than 0.5°C / s.

[0021] In the present invention, the ingot includes the following components in weight percentage: C: 0.19%~0.23%, Si: 1.50%~1.60%, Mn: 2.30%~2.40%, Cr: 0.50%~0.55%, Mo: 0.14%~0.16%, V: 0.08%~0.09%, N: 0.006%~0.008%, Al≤0.05%, and the balance is Fe and unavoidable impurities.

[0022] Previous bainitic rail composition designs had limitations, including contradictions in carbon content and imbalances in element synergy. The carbon content contradiction refers to the fact that while a high-carbon design (C ≥ 0.25%) can improve strength, it significantly deteriorates weldability and easily forms coarse carbides, resulting in a decrease in toughness (-40°C impact energy ≤ 60J). While a low-carbon design (C ≤ 0.19%) can improve weldability, the driving force for bainite transformation is insufficient and requires compensation by excessive Mn and Cr (e.g., Mn ≥ 2.5%, Cr ≥ 0.8%). This, in turn, results in an excessively wide bainite transformation temperature window (> 150°C) and a retained austenite volume fraction ≥ 5%, triggering fatigue crack sensitivity (crack growth rate da / dN ≥ 1×10 -8 m / cycle). In addition, the Si / Mn ratio in the prior art is unreasonable, resulting in insufficient austenite carbon activity (a c ≤3.0), the carbon concentration gradient at the bainite lath interface is low (≤1.2wt%), resulting in slow phase transformation kinetics (bainite transformation amount ≤85%).

[0023] To this end, the present invention precisely controls the rail composition and element ratio to optimize the driving force of bainite phase transformation and the distribution of precipitation phases.

[0024] In some cases, the CE value needs to balance the driving force for the bainite transformation and weldability. Carbon content (0.19%-0.23%) and alloying elements (such as Mn, Cr, and Mo) jointly influence the driving force for the transformation. A CE of ≤ 0.43% ensures weldability (e.g., avoiding cold cracking) while optimizing carbon activity through a Si / Mn ratio (>0.5). The carbon equivalent CE is calculated using the following formula: CE = C + Mn / 20 + Cr / 20 + Mo / 15+V / 10+Si / 30.

[0025] In some embodiments, the weight ratio of Si to Mn is greater than 0.5, preferably between 0.62 and 0.68. By optimizing the Si / Mn ratio, the austenite carbon activity coefficient is increased to above 3.5, and the carbon concentration gradient at the bainite lath interface is maintained at ≥1.8 wt%, accelerating the phase transformation kinetics.

[0026] Furthermore, by coordinating the element ratios, the hardenability of the rail can be optimized, achieving a hardenability factor (DI) greater than 2.5. Controlling hardenability can prevent martensite formation. Bainitic rails require high strength and toughness for service performance. While martensite possesses high strength, it is also brittle and has high residual stresses, making it susceptible to cracking. Excessively high hardenability reduces the cooling rate requirement. During slow cooling, austenite may skip the bainite transformation window and enter the martensite transformation zone (Ms ≤ 200°C). Controlling the hardenability factor can slow the austenite decomposition kinetics, ensuring bainite formation over a wider cooling rate range and avoiding martensite formation caused by insufficient cooling rates or overquenching. Insufficient hardenability can easily lead to austenite decomposition into pearlite in the high-temperature range. Pearlite has low strength (tensile strength ≤ 800 MPa) and a coarse lamellar structure (interlamellar spacing ≥ 200 nm), which significantly reduces the fatigue life of the rail. By optimizing the content of microalloying elements such as Mo and V (Mo: 0.14%-0.16%, V: 0.08%-0.09%), pearlite nucleation is delayed, the bainite transformation window (350-480°C) is expanded, and the pearlite transformation amount approaches zero. The bainite transformation rate is controlled by the nucleation activation energy (QA = 240-250 kJ / mol) and the carbon diffusion capacity. A high Si content (1.50%-1.60%) improves austenite stability, inhibits cementite precipitation, maintains austenite carbon activity, and delays the formation of proeutectoid ferrite. Optimizing the Mn / Cr ratio (Mn: 2.30%-2.40%, Cr: 0.50%-0.55%) lowers the bainite nucleation barrier and promotes uniform nucleation of high-density laths (thickness ≤180 nm).

[0027] Furthermore, the Mo-VN synergistic precipitation effect, combined with aging treatment, optimizes the strengthening contribution. 0.14%-0.16% Mo, 0.08%-0.09% V, and 0.006%-0.008% N combine. In the temperature range of 350-480°C, V and Mo preferentially occupy the carbonitride lattice, forming a (Mo, V) (C, N) composite precipitate phase with an average size of 5-15nm and a spacing of ≤50nm. This refines the laths (thickness ≤180nm) and improves toughness.

[0028] Alternatively, in some embodiments, the ingot contains 0.21% by weight of C, 1.55% by weight of Si, and 2.35% by weight of Mn.

[0029] Optionally, in some embodiments, the ingot is prepared by vacuum induction melting and then continuously cast into billets. The cooling rate is controlled at 2-3°C / s to avoid macrosegregation. Segregation can form localized element-enriched regions, and the enrichment of alloying elements can significantly delay the bainite transformation.

[0030] Optionally, in some embodiments, the rolling temperature is controlled between 850°C and 950°C, which is within the austenite pre-recrystallization zone, and the cumulative rolling reduction is controlled to be above 60%. Using rolling parameters with large deformation amounts can increase the bainite nucleation rate, shorten the bainite incubation period, and achieve a more complete bainite transformation.

[0031] Optionally, in some embodiments, the rolled rail is first cooled at a first cooling rate of 20-30°C / s to a bainite transformation temperature range of 350-480°C, then cooled at a second cooling rate of 0.08-0.12°C / s for a predetermined time, and finally cooled to room temperature at a third cooling rate of less than 0.5°C / s. Conventional bainite rail cooling processes employ a single cooling rate (e.g., 1°C / s), which fails to meet the kinetic requirements of the bainite transformation. This results in excessively fast cooling rates in the early and middle stages of the transformation temperature range (forming martensite, Ms ≥ 250°C) and insufficient cooling rates in the later stages, causing the bainite laths to coarsen to ≥ 250nm and an aspect ratio ≤ 10:1. A high cooling rate during final cooling to room temperature destabilizes the retained austenite, leading to martensitic transformation (volume fraction ≥ 5%) and deteriorating impact toughness.

[0032] Figure 2 A CCT continuous cooling curve diagram according to one embodiment of the present invention is shown, visually demonstrating the phase transformation behavior of a rail under specific composition and cooling processes. The details are as follows: the horizontal axis represents time, and the vertical axis represents temperature (°C). Based on the CCT curve, a three-stage cooling design is implemented to completely avoid the pearlite (P) and martensite (M) regions. In the first stage, rapid cooling (20-30°C / s) suppresses the precipitation of the proeutectoid phase. In the second stage, slow cooling at a rate of 0.08-0.12°C / s in the bainite range (350-480°C) forms high-density nano-laths. In the third stage, cooling to room temperature at a rate of ≤0.5°C / s prevents the transformation of retained austenite into martensite.

[0033] This invention breaks down the cooling process into three stages, dynamically adapting to the different thermodynamic requirements of bainite nucleation, growth, and retained austenite stabilization, achieving coordinated optimization from macroscopic cooling parameters to microstructural evolution. In the first stage, rapid cooling at a rate of 20-30°C / s is used to avoid the precipitation of proeutectoid ferrite or pearlite while ensuring that the austenite single-phase region remains in the bainite transformation temperature range (350-480°C). This process effectively inhibits the premature precipitation of carbides by shortening the high-temperature residence time, resulting in a more uniform carbon concentration distribution in the austenite, laying the foundation for the refined growth of bainite laths.

[0034] After entering the second stage, a quasi-isothermal transformation environment is established in the bainite phase transformation temperature range at a slow cooling rate of 0.08-0.12°C / s. After the phase transformation lasts for 1000-1500 seconds, the bainite transformation volume reaches ≥98%. This not only provides sufficient diffusion time for carbon atoms to promote the lateral ordering of the bainite laths, but also continuously drives the phase transformation front forward through slow cooling. Compared to the conventional process, which results in lath coarsening (≥250nm) due to insufficient cooling rate during this stage, the present invention stabilizes the bainite lath width within 180nm and increases the aspect ratio to over 20:1, forming a reinforced structure with a high density of dislocations and interwoven nanoscale laths. This refined structure not only significantly enhances the material's tensile strength but also improves its impact toughness, achieving a low-temperature impact energy of -40°C exceeding 120J, doubling the impact energy of the conventional process.

[0035] In the third stage, final cooling at room temperature is completed at a slow cooling rate of less than 0.5°C / s. By reducing the cooling rate, the driving force of martensite transformation is suppressed, and the volume fraction of retained austenite is stabilized below 5%. At the same time, carbon enrichment in austenite is promoted, so that its carbon content reaches more than 1.5wt%, significantly improving the mechanical and chemical stability of retained austenite.

[0036] Another object of the present invention is to provide a bainitic rail, which is manufactured using the method described above. The rail microstructure includes bainite with a volume fraction greater than 98% and the remainder being film-like retained austenite. The bainite is an ultra-fine lath with an aspect ratio of ≥20:1. The orientation difference between adjacent laths determined by EBSD is ≥10°, and the thickness of the bainite lath observed by metallographic microscope is ≤180nm. The thickness of the retained austenite film is less than 20nm. The tensile strength of the rail tested by the tensile test (GB / T 228.1) is ≥1450MPa, the elongation after fracture is ≥12%, the impact energy tested by the -40℃ impact test (GB / T 229) is ≥120J at -40℃, and the contact fatigue life tested by the rolling contact fatigue test (EN 13674-1) is ≥1×10 7 Second-rate.

[0037] The present invention is further described below by way of examples, but the protection scope of the present invention is not limited thereto.

[0038] Example 1 Alloy Melting and Casting: Molten steel is prepared in a vacuum induction melting furnace (VIM) with the following precise proportions by weight: C: 0.23%, Si: 1.50%, Mn: 2.30%, Cr: 0.50%, Mo: 0.14%, V: 0.08%, N: 0.006%, Al ≤ 0.05%, with the balance being Fe and unavoidable impurities. The carbon equivalent is approximately 0.425, and the Si / Mn ratio is 0.65. After melting, the steel is continuously cast into billets with a cooling rate of 2°C / s to prevent macrosegregation.

[0039] Controlled rolling process: The square billet is heated to 1200℃ and kept at this temperature for 2 hours, and then multi-pass rolling is performed in the austenite non-recrystallization zone (950℃~850℃). The cumulative reduction rate is controlled at 62%, and the final rolling temperature is controlled at 850±10℃ to obtain a rolled billet with the original austenite grain size ≤20μm.

[0040] Controlled cooling process: Immediately after rolling, cool to 480℃ at a rate of 20℃ / s, then cool to 350℃ at a constant rate of 0.1℃ / s for a total time of 1300 seconds, and then slowly cool to room temperature at a rate of 0.5℃ / s to ensure that the volume fraction of retained austenite is ≤2% (thickness ≤20nm) and inhibit the formation of martensite (Ms≤200℃).

[0041] TEM images were used to analyze the content and size of bainite and retained austenite. The microstructure showed a bainite volume fraction of approximately 98.3%, an aspect ratio of bainite laths ≥20:1, an orientation difference of adjacent laths ≥10° as determined by EBSD, and a bainite lath thickness ≤180 nm under metallographic microscopy. The retained austenite film thickness was less than 20 nm. Tensile testing (GB / T 228.1) revealed a tensile strength of approximately 1480 MPa and an elongation of approximately 13%. An impact test (GB / T 229) at -40°C revealed an impact energy of approximately 123 J. A rolling contact fatigue test (EN 13674-1) revealed a contact fatigue life of ≥1×10 7 Second-rate.

[0042] Example 2 Alloy Melting and Casting: Molten steel was prepared in a vacuum induction melting furnace (VIM) with the following precise proportions by weight: C: 0.21%, Si: 1.55%, Mn: 2.35%, Cr: 0.53%, Mo: 0.15%, V: 0.085%, N: 0.007%, Al ≤ 0.05%, with the balance being Fe and unavoidable impurities. The carbon equivalent was approximately 0.421, and the Si / Mn ratio was 0.66. After melting, the steel was continuously cast into billets with a controlled cooling rate of 2.5°C / s to prevent macrosegregation.

[0043] Controlled rolling process: The square billet is heated to 1200℃ and kept at this temperature for 3 hours. Multi-pass rolling is performed in the austenite non-recrystallization zone (930℃~880℃). The cumulative reduction rate is controlled at 65%, and the final rolling temperature is controlled at 840±10℃ to obtain a rolled billet with the original austenite grain size ≤20μm.

[0044] Controlled cooling process: Immediately after rolling, cool to 470℃ at a rate of 25℃ / s, then cool to 350℃ at a constant rate of 0.08℃ / s for a total time of 1500 seconds, and then slowly cool to room temperature at a rate of ≤0.5℃ / s to ensure that the volume fraction of retained austenite is ≤2% (thickness ≤20nm) and to inhibit the formation of martensite (Ms≤200℃).

[0045] TEM images were used to analyze the content and size of bainite and retained austenite. The microstructure showed a bainite volume fraction of approximately 98.8%, an aspect ratio of bainite laths ≥ 20:1, EBSD-derived misorientation between adjacent laths ≥ 10°, and metallographic microscopy revealed a bainite lath thickness ≤ 180 nm. The retained austenite film thickness was less than 20 nm. Tensile testing (GB / T 228.1) revealed a tensile strength of approximately 1500 MPa and an elongation of approximately 12%. An impact test (GB / T 229) at -40°C revealed an impact energy of approximately 125 J. A rolling contact fatigue test (EN 13674-1) revealed a contact fatigue life of ≥ 1 × 10 7 Second-rate.

[0046] Example 3 Alloy Melting and Casting: Molten steel was prepared in a vacuum induction melting furnace (VIM) with the following precise proportions by weight: C: 0.19%, Si: 1.60%, Mn: 2.40%, Cr: 0.55%, Mo: 0.16%, V: 0.09%, N: 0.008%, Al ≤ 0.05%, with the balance being Fe and unavoidable impurities. The carbon equivalent was approximately 0.41, and the Si / Mn ratio was 0.67. After melting, the steel was continuously cast into billets with a controlled cooling rate of 3°C / s to prevent macrosegregation.

[0047] Controlled rolling process: The square billet is heated to 1200℃ and kept at this temperature for 2 hours. Multi-pass rolling is carried out in the austenite non-recrystallization zone (950℃~850℃). The cumulative reduction rate is about 68%. The final rolling temperature is controlled at 860±10℃ to obtain a rolled billet with the original austenite grain size ≤20μm.

[0048] Controlled cooling process: Immediately after rolling, cool to 480℃ at a rate of 30℃ / s, then cool to 350℃ at a constant rate of 0.12℃ / s for a total time of approximately 1083 seconds, and then slowly cool to room temperature at a rate of ≤0.5℃ / s to ensure that the volume fraction of retained austenite is ≤2% (thickness ≤20nm) and inhibit the formation of martensite (Ms≤200℃).

[0049] TEM images were used to analyze the content and size of bainite and retained austenite. The microstructure showed a bainite volume fraction of approximately 98.1%, an aspect ratio of bainite laths ≥20:1, an orientation difference of adjacent laths ≥10° as determined by EBSD, and a bainite lath thickness ≤180 nm under metallographic microscopy. The retained austenite film thickness was less than 20 nm. The tensile strength (GB / T 228.1) of the rails was ≥1450 MPa, and the elongation after fracture was ≥12%. The impact energy (GB / T 229) at -40°C was ≥120 J, and the contact fatigue life (EN 13674-1) was ≥1×10 7 Second-rate.

[0050] Comparative Example 1 In this comparative example, molten steel was prepared using a vacuum induction melting furnace (VIM) with the following precise proportions by weight: C: 0.30%, Si: 1.55%, Mn: 2.35%, Cr: 0.53%, Mo: 0.15%, V: 0.085%, N: 0.007%, Al ≤ 0.05%, with the balance being Fe and unavoidable impurities. The carbon equivalent was approximately 0.511, and the Si / Mn ratio was 0.66. All other operations and parameters were the same as those in Example 2.

[0051] After testing, coarse M3C carbides (size ≥ 500nm) appeared in the rails obtained from this comparative example, the impact energy dropped to 80~90J (-40℃), the elongation after fracture dropped to 8%, and the fatigue life was only 6×10 6 Second-rate.

[0052] Comparative Example 2 In this comparative example, molten steel was prepared using a vacuum induction melting furnace (VIM) with the following precise proportions by weight: C: 0.13%, Si: 1.55%, Mn: 2.35%, Cr: 0.53%, Mo: 0.15%, V: 0.085%, N: 0.007%, Al ≤ 0.05%, with the balance being Fe and unavoidable impurities. The carbon equivalent was approximately 0.34, and the Si / Mn ratio was 0.66. Other operations and parameters were the same as in Example 2.

[0053] After testing, it was found that the volume fraction of bainite in the rail obtained in this comparative example was about 93%, the volume fraction of retained austenite was higher than 5% (the film thickness exceeded 30nm), the tensile strength was only 1280MPa, the impact energy was ≤70J, and the fatigue crack sensitivity was significantly increased.

[0054] Comparative Example 3 In this comparative example, molten steel was prepared using a vacuum induction melting furnace (VIM) with the following precise proportions by weight: C: 0.21%, Si: 0.8%, Mn: 2.35%, Cr: 0.53%, Mo: 0.15%, V: 0.085%, N: 0.007%, Al ≤ 0.05%, with the balance being Fe and unavoidable impurities. The carbon equivalent was approximately 0.396, and the Si / Mn ratio was 0.34. All other operations and parameters were the same as in Example 2.

[0055] After testing, it was found that the volume fraction of bainite in the rail obtained in this comparative example was about 83%, the volume fraction of retained austenite was higher than 5% (the film thickness exceeded 25 nm), and the tensile strength was only 1230 MPa.

[0056] Comparative Example 4 In this comparative example, the alloy melting, casting and rolling operations and parameters are the same as those in Example 2, except that the cooling process is controlled to cool to room temperature at a constant rate of 1°C / s within the bainite phase transformation temperature range.

[0057] After testing, the rail microstructure of bainite laths coarsened to 250~300nm, the aspect ratio ≤8:1, the retained austenite stability was poor during final cooling, martensitic transformation occurred, the volume fraction was ≥6%, the impact energy dropped to 60~70J, the tensile strength was 1380MPa, and the elongation was 9%.

[0058] The above-described embodiments merely illustrate the implementation methods of the present invention. 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 would be able to make various modifications and improvements without departing from the spirit of the present invention, and all such modifications and improvements fall within the scope of protection of the present invention.

Claims

1. A method for promoting complete bainite transformation of rails, characterized in that: The following steps are involved: Producing an ingot comprising the following components by weight percentage: C: 0.19% to 0.23%, Si: 1.50% to 1.60%, Mn: 2.30% to 2.40%, Cr: 0.50% to 0.55%, Mo: 0.14% to 0.16%, V: 0.08% to 0.09%, N: 0.006% to 0.008%, Al ≤ 0.05%, the balance being Fe and unavoidable impurities, wherein the carbon equivalent CE is less than 0.43%, and the weight ratio of Si to Mn is greater than 0.5; The ingot is subjected to multiple-pass rolling; The rolled rail is first cooled to the bainite phase transformation range temperature at a first cooling rate of 20-30°C / s, then cooled for a predetermined time at a second cooling rate of 0.08-0.12°C / s, and finally cooled to room temperature at a third cooling rate of less than 0.5°C / s.

2. The method according to claim 1, characterized in that The weight ratio of Si to Mn is between 0.62 and 0.

68.

3. The method according to claim 1, characterized in that The ingot contains 0.21% by weight of C, 1.55% by weight of Si, and 2.35% by weight of Mn.

4. The method according to claim 1, wherein The rolling temperature is controlled between 850~950℃.

5. The method according to claim 1, wherein The cumulative rolling reduction rate is controlled above 60%.

6. The method according to claim 1, characterized in that The temperature range of bainite phase transformation is 350~480℃.

7. The method according to claim 1, characterized in that The second cooling rate lasts for 1000~1500s.

8. The method according to claim 1, characterized in that The ingot is prepared by vacuum induction melting, and is continuously cast into a billet after melting, with a cooling rate controlled at 2-3°C / s.

9. A bainite rail, characterized in that: The rail is manufactured by the method according to any one of claims 1-8.

10. The bainite rail according to claim 9, characterized in that: The rail microstructure comprises bainite with a volume fraction greater than 98% and the remainder being film-like retained austenite. The bainite is an ultra-fine lath with an aspect ratio of ≥20:1, a lath thickness of ≤180nm, and a retained austenite film thickness of less than 20nm. The rail has a tensile strength of ≥1450MPa, an elongation after fracture of ≥12%, an impact energy at -40°C of ≥120J, and a contact fatigue life of ≥1×10 7 Second-rate.

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