A high-strength, high-toughness, ultra-fine bainitic railway steel and its production method

Abstract

This invention discloses a high-strength, high-toughness, ultrafine-structure bainitic railway steel and its production method. The method involves smelting and casting a steel billet with a predetermined cross-sectional area according to the specified composition, and then cooling it to room temperature at a first cooling rate. The billet is then subjected to secondary heating for descaling, universally rolled to a specific shape, and cooled to room temperature at a second cooling rate. After tempering, it is cooled to room temperature at a third cooling rate. By adjusting the heating temperature, holding time, and cooling method during the heat treatment process, the bainitic composition, morphology, and properties obtained after heat treatment meet the requirements. This results in a high-strength, ultrafine-structure bainitic railway steel.

Description

Technical Field

[0001] This invention relates to the field of railway steel production technology, and in particular to a high-strength, high-toughness, ultra-fine bainitic railway steel and its production method. Background Technology

[0002] With the increase in railway operating speed, axle load and traffic volume, the weakness of pearlitic railway steel in terms of low rolling contact fatigue resistance, i.e., low resistance to spalling, has become increasingly apparent.

[0003] To obtain railway steel with higher strength and toughness, the idea of ​​developing bainitic railway steel has been proposed. Research on bainitic railway steel has a history of 30-40 years internationally. Comparative analysis with pearlitic railway steel shows that, in addition to achieving higher strength and rigidity, it also exhibits superior contact fatigue properties.

[0004] Compared to pearlitic railway steel, bainitic railway steel has a more complex microstructure, primarily composed of bainite, martensite, and retained austenite, containing both soft and hard phases. This presents a significant challenge in microstructure control for improving the contact fatigue resistance of bainitic railway steel. Enhancing the strength and toughness of bainitic railway steel is the most effective way to improve its contact fatigue resistance.

[0005] By altering the chemical composition or microstructure, the strength and toughness of steel materials can be significantly improved. Among existing metal strengthening mechanisms, grain refinement is one that can simultaneously enhance both the strength and toughness of the metal matrix. This is because as the grains become finer, the "grain size effect" becomes significant and continuously changes, manifested as an increase in the volume fraction of grain boundaries per unit area and a decrease in the ability of small grains to accommodate defects such as dislocations and vacancies, thus macroscopically altering the mechanical properties of the metal material. Ultrafine-structured bainitic railway steel, due to its excellent mechanical properties, has gradually attracted considerable attention from experts and scholars in the field of advanced steel material development and research both domestically and internationally.

[0006] There are two main approaches to preparing ultrafine-structure bainitic railway steel in the laboratory: large plastic deformation and thermomechanical control. Currently, there is a general consensus that both methods can be used to prepare ultrafine-structure bainitic railway steel on a large scale. Both methods can control grain size and material properties through a combination of phase transformation and plastic deformation, which has significant fundamental research value in enriching grain refinement and improving strength. This approach is increasingly gaining attention from researchers as one of the effective processes for preparing ultrafine-structure bainitic railway steel.

[0007] Grain size standards are divided into 8 levels, with level 1 being the largest and level 8 the smallest. Under a 100x microscope, the size of austenite grains is compared with the standard grain size. Steels with grain sizes of 1-5 are classified as inherently coarse-grained steel, and those of 5-8 are classified as inherently fine-grained steel. The size of the austenite grains in steel is formed under specific heating conditions and is called the actual austenite grain size. Its size has a significant impact on the properties of the steel after cooling transformation. The finer the austenite grains, the finer the grains after heat treatment, and the better the mechanical properties, especially the impact toughness. Therefore, to obtain fine and uniform austenite grains when heating steel, firstly, inherently fine-grained steel must be selected; secondly, the heating temperature must be controlled and not too high.

[0008] When alloying elements such as chromium, molybdenum, titanium, and vanadium are added to steel, the tendency for grain growth is greatly reduced, thus lowering the steel's overheating sensitivity.

[0009] The prior art discloses an ultra-high strength, ductile, ultra-fine bainitic precision-rolled threaded steel bar and its preparation method, belonging to the field of metallurgical technology. The method includes: austenitizing the threaded steel bar to be treated; then cooling it at a first cooling rate to a first preset temperature, where the first cooling rate is 1℃ / s to 20℃ / s and the first preset temperature is 450℃ to 500℃; cooling it at a second cooling rate to a second preset temperature, where the second cooling rate is 0.1℃ / s to 5℃ / s and the second preset temperature is Ms+100℃ to Ms℃; cooling it at a third cooling rate to a third preset temperature, where the third cooling rate is 0.01℃ / s to 0.3℃ / s and the third preset temperature is Ms+5℃ to Ms-80℃, where Ms is the starting temperature of martensitic transformation; air cooling to room temperature; and tempering the threaded steel bar cooled to room temperature to obtain ultra-high strength threaded steel of PSB1200 grade and above.

[0010] The prior art discloses an ultrafine bainitic wear-resistant steel that rapidly undergoes phase transformation and its preparation method, comprising the following weight percentage components: C: 0.20–0.30%, Si: 1.15–1.35%, Mn: 0.85–1.05%, Cr: 0.95–1.24%, Ni: 1.05–1.18%, Al: 0.80–1.20%, with the remainder being Fe and unavoidable impurities. The preparation method includes S1 melting and casting; S2 hot rolling; S3 secondary warm rolling; S4 low-temperature phase transformation; and S5 high-temperature phase transformation. This invention solves the technical problem of excessively long isothermal phase transformation time in existing ultrafine bainitic steel preparation processes, which is detrimental to industrial production.

[0011] This invention discloses a method for preparing medium-carbon ultrafine bainitic steel with good thermal stability in the field of steel materials. The bainitic steel composition is: C 0.15~0.25%; Si 0.8~1.3%; Mn 1.0~1.5%; Cr 0.5~1.5%; Al 1.0~1.5%; Mo 0.5~0.8%; Ni 0.2~0.5%; Nb 0.020~0.030%, with the remainder being iron and unavoidable impurities. The billet is held at 80~120℃ above the full austenitizing temperature for 0.3~0.5h; then rapidly cooled at a rate >20℃ / s to 10~30℃ below the ferrite transformation temperature, held for 3~10min; then rapidly cooled at a rate >20℃ / s to Ms~Ms+10℃ above the martensitic transformation temperature, held for 0.5~1.5h; finally, air-cooled to room temperature.

[0012] The prior art discloses an ultrafine-grained bainitic gear steel and its production method, the composition of which includes the following chemical composition by weight percentage: C: 0.18-0.22%, Si: 1.30-1.60%, Mn: 1.00-1.25%, P: ≤0.010%, S: ≤0.010%, Cr: 0.25-0.55%, Mo: 0.30-0.50%, Nb: 0.015-0.25%, Al: 0.030-0.050%, V: 0.10-0. 0.20%, B: 0.0030~0.0050%, [N]: 90~160ppm, the remainder being Fe and unavoidable impurity elements. The steel is produced by electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-rolling (finishing). The end hardenability of the resulting steel meets the following requirements: J9: 40~47HRC, J15: 40~46HRC, J25: 35~44HRC. The steel is produced by carburizing followed by gas quenching and low-temperature tempering. The carburized layer grain size is ≥11.0 grade, and the surface hardness is ≥650HV.

[0013] The prior art discloses a method for producing ultrafine-grained bainitic forklift steel. The forklift steel contains 0.03–0.15% V and 100–200 ppm N by mass. The method includes the following steps: S1, heating the steel billet obtained after smelting and casting at 1050–1150°C; S2, rolling or forging the heated steel billet, wherein the compression ratio of rolling or forging is greater than or equal to 7:1, and the reduction in the first three rolling passes or the initial forging stage is 30–50%; S3, cooling the forklift obtained after rolling or forging; S4, tempering the cooled forklift. This invention can produce ultrafine-grained forklift steel, improving the overall mechanical properties of the forklift.

[0014] However, the steel products obtained by the aforementioned existing technologies still have room for improvement in terms of tensile strength and contact fatigue resistance.

[0015] Therefore, existing technologies still need improvement. Summary of the Invention

[0016] To address the aforementioned technical problems, this invention proposes a high-strength, high-toughness, ultra-fine bainitic railway steel and its production method, thereby resolving the technical issues existing in the tensile strength and contact fatigue resistance of steel products in the prior art.

[0017] To solve the above-mentioned technical problems, on the one hand, some embodiments of the present invention disclose a method for producing high-strength and tough ultrafine bainitic railway steel, wherein a steel billet with a predetermined cross-sectional area is smelted and cast according to the content composition and cooled to room temperature at a first cooling rate;

[0018] The steel billet is subjected to secondary heating to remove phosphorus, and then universally rolled to a specific shape before being cooled to room temperature at a second cooling rate.

[0019] After tempering, it is cooled to room temperature at the third cooling rate.

[0020] In some embodiments, the composition of the steel billet, by weight percentage, includes:

[0021] 0.20-0.35% carbon content, 1.30-1.80% silicon content, 1.50-2.50% manganese content, 0.005-0.020% phosphorus content, 0.001-0.008% sulfur content, 0.50-1.50% chromium content, 0.3-0.70% molybdenum content, 0.05-0.12% vanadium content, 0.02-0.70% nickel, with the remainder being Fe.

[0022] In some embodiments, the first cooling rate is 0.5-2 °C / min.

[0023] In some embodiments, the predetermined cross-sectional area is 1.05 × 10⁻⁶. 5 mm 2 -1.35×10 5 mm 2 .

[0024] In some embodiments, the second cooling rate is 3-7°C / min.

[0025] In some embodiments, the tempering temperature of the tempering treatment is 200-400°C.

[0026] In some embodiments, the tempering time for the tempering process is 5-12 hours.

[0027] In some embodiments, the composition of the molten steel, by weight percentage, is 30ppm≤N≤70ppm; O content≤10ppm; H content≤1.5ppm.

[0028] Some embodiments of the present invention also disclose a high-strength, high-toughness, ultra-fine-structure bainitic railway steel, which is prepared by the aforementioned production method of high-strength, high-toughness, ultra-fine-structure bainitic railway steel.

[0029] In some embodiments, the grain size is ≥9; the grain size is 0.010-0.016.

[0030] In some embodiments, the yield strength is greater than 1150 MPa, the tensile strength is ≥1280 MPa, the elongation is 13%-18%, and the reduction of face is 51%-56%.

[0031] In some embodiments, the contact fatigue cycles are 400,000 to 450,000 when the contact stress is 1350 MPa, the slip is 5%, and the rotation speed is 1000 rpm.

[0032] By adopting the above technical solution, the present invention has at least the following beneficial effects:

[0033] This invention provides a high-strength, high-toughness, ultrafine-structure bainitic railway steel and its production method. By adjusting the heating temperature, holding time, and cooling method during the heat treatment process, the composition, morphology, and properties of the bainite obtained after heat treatment meet the requirements. This results in a high-strength, ultrafine-structure bainitic railway steel. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a flowchart illustrating a method for producing high-strength, high-toughness, ultrafine-structure bainitic railway steel, as disclosed in some embodiments of the present invention. Detailed Implementation

[0036] The embodiments of this disclosure will be further described in detail below with reference to the accompanying drawings and examples. The detailed description of the embodiments and the accompanying drawings are used to illustrate the principles of this disclosure by way of example, but should not be used to limit the scope of this disclosure. This disclosure can be implemented in many different forms and is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

[0037] These embodiments are provided to make the disclosure thorough and complete, and to fully express the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​set forth in these embodiments should be interpreted as exemplary only and not as limiting.

[0038] It should be noted that, in the description of this disclosure, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," and "outer," etc., indicating orientation or positional relationship, are only for the convenience of describing this disclosure and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this disclosure. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0039] Furthermore, the terms "first," "second," and similar terms used in this disclosure do not indicate any order, quantity, or importance, but are merely used to distinguish different parts. "Vertical" is not strictly vertical, but within the permissible margin of error. "Parallel" is not strictly parallel, but within the permissible margin of error. Terms such as "including" or "contains" mean that the element preceding the word encompasses the element listed after the word, and do not exclude the possibility of encompassing other elements as well.

[0040] It should also be noted that, in the description of this disclosure, unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this disclosure depending on the specific circumstances. When a particular device is described as being located between a first device and a second device, an intermediary device may or may not be present between the particular device and the first or second device.

[0041] All terms used in this disclosure have the same meaning as understood by one of ordinary skill in the art to which this disclosure pertains, unless otherwise specifically defined. It should also be understood that terms defined in general dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant art, and not as idealized or highly formalized, unless expressly defined herein.

[0042] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, they should be considered part of the specification.

[0043] With the development of railway technologies, the requirements for the performance of railway steel are becoming increasingly stringent. Due to differences in heating temperature, holding time, and cooling methods during heat treatment, the morphology and properties of the bainitic microstructure obtained after heat treatment vary significantly. Generally, lower bainite exhibits superior overall mechanical properties compared to upper bainite. This is because the bainite ferrite (Bf) substructure of lower bainite appears as a high-density dislocation structure under a lens, and it also has a lower phase transformation temperature and higher carbon supersaturation. Lower bainite possesses excellent strength, toughness, hardness, and wear resistance, with lower notch sensitivity and brittle transition temperature, making it an ideal quenching microstructure with extremely high application value. Therefore, obtaining this microstructure through different heat treatment processes can improve the service life of the steel.

[0044] This invention discloses a method for producing high-strength, high-toughness, ultrafine-structure bainitic railway steel, such as... Figure 1 As shown, the specific process includes: smelting and casting a steel billet with a predetermined cross-sectional area according to the content composition and cooling it to room temperature at a first cooling rate; subjecting the steel billet to secondary heating for descaling, and then universally rolling it to a specific shape, followed by cooling it to room temperature at a second cooling rate; performing a tempering treatment, and then cooling it to room temperature at a third cooling rate. The first cooling rate is 0.5-2℃ / min. The second cooling rate is 3-7℃ / min. The tempering temperature for the tempering treatment is 200-400℃. The tempering time for the tempering treatment is 5-12 hours.

[0045] This embodiment controls the grain size and material properties by cooling the steel billet at a predetermined cooling rate after smelting and casting, combined with secondary heating for descaling and rolling, and by specifically controlling the cooling rate and tempering process parameters. This is achieved through a combination of phase transformation and plastic deformation. Ultimately, the resulting railway steel has a smaller grain size, with a grain size of not less than grade 9. At the same time, various mechanical properties such as yield strength and tensile strength are also effectively improved, and the number of contact fatigue cycles is also significantly increased.

[0046] This invention also discloses a high-strength, high-toughness, ultra-fine bainitic railway steel, wherein the composition of the billet, by weight percentage, includes:

[0047] 0.20-0.35% carbon content, 1.30-1.80% silicon content, 1.50-2.50% manganese content, 0.005-0.020% phosphorus content, 0.001-0.008% sulfur content, 0.50-1.50% chromium content, 0.3-0.70% molybdenum content, 0.05-0.12% vanadium content, 0.02-0.70% nickel, with the remainder being Fe.

[0048] To achieve excellent mechanical properties and resistance to contact fatigue, the molten steel composition is as follows: N ≤ 70 ppm; O ≤ 10 ppm; H ≤ 1.5 ppm. This high-strength, high-toughness, ultrafine-grained bainitic railway steel has a grain size ≥ 9; a grain size of 0.010-0.016; a yield strength greater than 1150 MPa; a tensile strength ≥ 1280 MPa; an elongation of 13%-18%; a reduction of face of 51%-56%; and a contact fatigue life of 400,000-450,000 cycles at a contact stress of 1350 MPa, a slip of 5%, and a rotational speed of 1000 rpm.

[0049] Some embodiments of the present invention disclose a method for producing high-strength, high-toughness, ultrafine-structure bainitic railway steel, wherein the composition of the steel billet, by weight percentage, includes:

[0050] The steel composition comprises 0.20-0.35% carbon, 1.30-1.80% silicon, 1.50-2.50% manganese, 0.005-0.020% phosphorus, 0.001-0.008% sulfur, 0.50-1.50% chromium, 0.3-0.70% molybdenum, 0.05-0.12% vanadium, 0.02-0.70% nickel, with the remainder being Fe. Furthermore, to achieve excellent mechanical properties and resistance to contact fatigue, the molten steel composition includes 30ppm ≤ N ≤ 70ppm; O ≤ 10ppm; and H ≤ 1.5ppm.

[0051] The specific production method is as follows: Based on the Si-Mn-Cr bainitic rail composition system, through alloy composition design, the steel is smelted and cast into a steel billet with a certain cross-sectional size, which is generally 1.05×10⁻⁶. 5 mm 2 -1.35×10 5 mm 2 The steel billet is then cooled to room temperature at a specific cooling rate, typically 0.5-2℃ / min. After austenitization and rolling to a specific size, it is cooled to room temperature a second time at a specific cooling rate, typically 3-7℃ / min. Finally, it undergoes a tempering process at 200-400℃ for 5-12 hours, followed by a third cooling to room temperature at a specific cooling rate, typically 1-2℃ / min. This embodiment, through multiple phase transformations and different cooling rates, refines the microstructure, improves strength and toughness, and enhances fatigue performance. Finally, the steel rail described in this patent is processed into finished rails using a straightening machine and a sawing line before shipment.

[0052] The obtained high-strength and high-toughness ultrafine bainitic railway steel has a grain size ≥9; a grain size of 0.010-0.016; a yield strength greater than 1150 MPa; a tensile strength ≥1280 MPa; an elongation of 13%-18%; a reduction of face of 51%-56%; and a contact fatigue life of 400,000-450,000 cycles at a contact stress of 1350 MPa, a slip of 5%, and a rotational speed of 1000 rpm.

[0053] This invention also demonstrates the superior properties of the production method and the obtained high-strength, high-toughness, ultrafine-structure bainitic railway steel through specific embodiments and comparative examples. Table 1 below shows the specific chemical composition and gas content of Examples 1-5 and Comparative Examples 1 and 2. Examples 1-5 are carried out using the methods listed in the above embodiments within the range of process control parameters. The parameters of Comparative Examples 1 and 2 are partially or entirely outside the range of process control parameters of the above embodiments. Specifically, Table 2 gives the specific control parameters of Examples 1-5 and Comparative Examples 1 and 2 in each process of casting, rolling, cooling, and tempering.

[0054] Table 1 Chemical composition and gas content (%) of railway steel used in the examples and comparative examples

[0055]

[0056] Table 2. Smelting, First Heating, and Rolling Processes of Examples and Comparative Examples

[0057]

[0058] The austenite grain size of the obtained high-strength and high-toughness ultrafine bainitic railway steel was tested, and the test results are shown in Table 3.

[0059] Table 3. Inspection results of non-metallic inclusions in railway steel used in the examples and comparative cases.

[0060]

[0061] Then, in accordance with the requirements of TB / T 2344 "Technical Conditions for Ordering 43kg / m-75kg / m Rails", tensile test specimens and metallographic specimens were processed and inspected at the fillet of the rail head for railway use. The statistics of tensile and metallographic decompression are shown in Table 4.

[0062] Table 4 Tensile properties of railway steel in examples and comparative cases

[0063]

[0064] Contact fatigue specimens were taken from the railheads of railway steel rails obtained in each embodiment and comparative example, and the test results are shown in Table 5.

[0065] Table 5 Contact Fatigue of Railway Steel in Embodiments and Comparative Examples of the Invention

[0066]

[0067] In summary, the ultrafine-structure bainitic railway steel and its production method disclosed in this invention employ a reasonable component ratio and undergo multiple phase transformations with varying cooling rates to refine the microstructure, thereby improving the strength, toughness, and fatigue resistance of the steel product. This enhances the overall performance of railway steel.

[0068] The embodiments of this disclosure have now been described in detail. To avoid obscuring the concept of this disclosure, some details known in the art have not been described. Those skilled in the art can fully understand how to implement the technical solutions disclosed herein based on the above description.

[0069] While specific embodiments of this disclosure have been described in detail by way of examples, those skilled in the art should understand that the examples are for illustrative purposes only and not intended to limit the scope of this disclosure. Those skilled in the art should understand that modifications can be made to the above embodiments or equivalent substitutions can be made to some technical features without departing from the scope and spirit of this disclosure. In particular, as long as there is no structural conflict, the technical features mentioned in the various embodiments can be combined in any manner.

Claims

1. A method for producing high-strength, high-toughness, ultrafine-structure bainitic railway steel, characterized in that, Steel billets with a predetermined cross-sectional area are smelted and cast according to their content and composition and cooled to room temperature at a first cooling rate. The steel billet is subjected to secondary heating to remove phosphorus, and then universally rolled to a specific shape before being cooled to room temperature at a second cooling rate. After tempering, cool to room temperature at the third cooling rate; The composition of steel billets, by weight percentage, includes: The composition comprises 0.20-0.35% carbon, 1.30-1.80% silicon, 1.50-2.50% manganese, 0.005-0.020% phosphorus, 0.001-0.008% sulfur, 0.50-1.50% chromium, 0.3-0.70% molybdenum, 0.05-0.12% vanadium, 0.02-0.70% nickel, with the remainder being Fe; the first cooling rate is 0.5-2℃ / min; the second cooling rate is 3-7℃ / min; and the tempering time for the tempering treatment is 5-12 hours.

2. The method for producing high-strength, high-toughness, ultrafine-structure bainitic railway steel according to claim 1, characterized in that, The predetermined cross-sectional area is 1.05 × 10⁻⁶. 5 mm 2 -1.35×10 5 mm 2 .

3. The method for producing high-strength, high-toughness, ultrafine-structure bainitic railway steel according to claim 1, characterized in that, The tempering temperature for the tempering treatment is 200-400℃.

4. The method for producing high-strength, high-toughness, ultrafine-structure bainitic railway steel according to claim 1, characterized in that, By weight percentage, the composition of molten steel is as follows: 30ppm≤N≤70ppm; O content≤10ppm; H content≤1.5ppm.

5. A high-strength, high-toughness, ultrafine-structure bainitic railway steel, characterized in that, It is prepared by the production method of high-strength and tough ultrafine bainitic railway steel according to any one of claims 1-4.

6. The high-strength, high-toughness, ultrafine-structure bainitic railway steel according to claim 5, characterized in that, Grain size ≥ 9; grain size 0.010-0.016; Alternatively, the yield strength is greater than 1150 MPa, the tensile strength is ≥1280 MPa, the elongation is 13%-18%, and the reduction of face is 51%-56%. Alternatively, with a contact stress of 1350 MPa, a slip of 5%, and a rotation speed of 1000 rpm, the contact fatigue cycle is 400,000 to 450,000 cycles.

Images