A bainite railway steel and a production method for controlling the residual-austenite stability thereof

Abstract

This invention discloses a production method for bainitic railway steel and its residual stability control. The method involves heating the smelted and cast billet to a first temperature for a first time, followed by descaling and rolling, then cooling to room temperature at a first cooling rate. After bending deformation in all directions, the billet undergoes tempering treatment. This deformation and tempering process yields high-strength, high-toughness bainitic railway steel with high residual stability. While maintaining the strength and toughness of existing online heat-treated bainitic railway steel, it improves the residual stability of railway steel, enhancing operational safety and meeting the requirements for heavy-haul railway steel or turnout railway steel.

Description

Technical Field

[0001] This invention relates to the field of railway steel production technology, and in particular to a bainitic railway steel and a method for controlling its residual austenitic stability. Background Technology

[0002] The performance of railway steel directly affects railway transportation efficiency and traffic safety. Bainitic steel, as a newly emerging type of railway track steel, is increasingly being used in railway applications due to its superior performance and longer service life. To improve the strength and toughness of bainitic steel for railway use, medium-low carbon alloying is typically employed, using appropriate processes to obtain microstructures such as bainitic + disabled austenitic or bainitic + martensite + disabled austenitic. However, in the research and application of bainitic railway steel, the stability of the disabled austenitic structure directly impacts its mechanical and service performance.

[0003] Depending on the driving force (cooling or applied stress), the stability of retained austenite can be divided into thermal stability and mechanical stability. Factors affecting the stability of retained austenite mainly include: the chemical composition of the austenite, the grain size of the austenite, the morphology of the austenite, the dislocation density in the austenite, the strength of the matrix structure surrounding the austenite, the stress / strain distribution between different phases, the strain rate, the deformation temperature, and the grain orientation.

[0004] The prerequisite for austenite to undergo martensitic transformation is that the driving force of the transformation must be greater than the resistance to transformation. The driving force mainly includes chemical and mechanical forces. The chemical driving force is the free energy difference between austenite and martensite. Therefore, any element that increases the free energy difference between austenite and martensite can improve the stability of austenite. Common elements that can increase the stability of austenite include C, N, Mn, and Ni. The most direct reflection of the influence of chemical elements on the stability of austenite is their effect on the Ms point. Each 1 wt% increase in C and Mn can lower the Ms by approximately 300-400℃ and 30-40℃ respectively, having the greatest impact on the Ms point. Ni also has a certain effect on lowering the Ms; 1 wt% Ni can lower the Ms by approximately 16-17℃.

[0005] Generally, the smaller the austenite grain size during cooling and deformation, the less likely austenite is to undergo martensitic transformation. Therefore, grain refinement is often used during heat treatment to stabilize austenite at room temperature. Austenite grain refinement has almost no effect on the width of the martensite laths, and a single martensite lath represents a single variant. This may explain the relatively homogeneous nature of variants in small-sized austenite. To overcome the strain energy required for the phase transformation, a high driving force is needed, making it difficult for small-sized austenite to undergo martensitic transformation.

[0006] The smaller the grain size of retained austenite, the less stable the retained austenite becomes. This is because during deformation, a large number of mechanical twins are generated in large-sized austenite grains. These intragranular deformation structures stabilize the grains, leading to higher mechanical stability of the austenite. However, for small austenite grains, no mechanical twins form during deformation, and the austenite transforms into martensite under relatively small strain. The Ms point increases with decreasing austenite grain size, due to the segregation of Al at the original austenite grain boundaries.

[0007] Retained austenite in steel is generally obtained through direct and inverse phase transformation processes. Regardless of the process, it typically exhibits two morphologies: thin-film and blocky or spherical. The morphological dependence of austenite stability is frequently reported in different steels. Generally, during plastic deformation, blocky austenite is less stable than thin-film austenite. However, the influence of morphology on the stability of retained austenite usually involves contributions from other factors. Austenite grains with different morphologies generally possess different microstructural characteristics, including chemical composition, grain size, and adjacent phases. In multiphase steels, the influence of morphology on the stability of retained austenite is more complex. Generally, the higher the carbon content in austenite, the more stable the austenite.

[0008] The influence of matrix strength on austenite stability is mainly related to the distribution of stress / strain between austenite and the matrix during deformation; the higher the matrix strength, the smaller the stress distributed to the austenite. However, matrix strength can affect austenite stability not only by altering the stress / strain distribution but also by adjusting the phase transformation strain. This study investigated the effect of matrix strength on austenite stability by varying the strength of the martensitic matrix surrounding the austenite through different degrees of tempering. They found that the more severe the martensitic matrix recovery, the lower the mechanical stability of the austenite. This is because higher martensite strength results in greater resistance to the volume expansion accompanying the austenitic phase transformation, thus increasing the stability of the austenite.

[0009] Therefore, existing technologies still need improvement. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention proposes a production method for bainitic railway steel and its residual stability control, thereby resolving the technical issue of insufficient residual stability in existing bainitic railway steel.

[0011] To solve the above-mentioned technical problems, on the one hand, some embodiments of the present invention disclose a production method for controlling the residual stability of bainitic railway steel. After the smelting and casting of the billet is heated at a first temperature for a first time, it is descaled and rolled, cooled to room temperature at a first cooling rate, and then subjected to bending deformation in all directions before tempering.

[0012] In some embodiments, the first temperature is 1180-1230°C.

[0013] In some embodiments, the first time is 40-80 minutes; the rolling compression ratio is not less than 11:1.

[0014] In some embodiments, the final rolling temperature is 850-950°C.

[0015] In some embodiments, the first cooling rate is 5-10°C / min.

[0016] In some embodiments, the complete deformation in all directions is 15-25mm / 1600mm.

[0017] In some embodiments, the complete deformation of the up-down action is to bend and deform once each in the up, down, left, and right directions.

[0018] In some embodiments, the tempering treatment is performed at a temperature of 200-400°C for 12-48 hours.

[0019] In some embodiments, the bainitic railway steel comprises the following chemical composition by weight percentage: 0.20%-0.35% carbon, 1.30%-1.80% silicon, 1.50%-2.50% manganese, 0.005%-0.020% phosphorus, 0.001%-0.010% sulfur, 0.50%-1.50% chromium, 0.3%-0.70% molybdenum, 0.02%-0.12% vanadium, 0.02%-0.70% nickel, with the remainder being Fe and unavoidable impurities.

[0020] On the other hand, this invention also discloses a bainitic railway steel, which adopts the aforementioned production method for bainitic railway steel with paraaugment stability control, and has a tensile strength ≥1280MPa, an elongation ≥12%, and a paraaugment conversion rate ≤30%.

[0021] By adopting the above technical solution, the present invention has at least the following beneficial effects: This invention provides a production method for bainitic railway steel and its residual stability control. High-strength and high-toughness bainitic railway steel is obtained through deformation and tempering treatment. It has high residual stability, which ensures the strength and toughness of existing online heat-treated bainitic railway steel while improving the residual stability of railway steel, thereby improving the operational safety of railway steel and meeting the requirements of heavy-haul railway steel or turnout railway steel. Attached Figure Description

[0022] 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.

[0023] Figure 1 This is a diagram showing the locations of metallographic and tensile specimen inspections for a bainitic railway steel as disclosed in some embodiments of the present invention. Figure 2 Metallographic photographs of a bainitic railway steel disclosed in some embodiments of the present invention. Detailed Implementation

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] This invention discloses a production method for controlling the residual austenitic stability of bainitic railway steel, relating to a method for obtaining high-strength and high-toughness bainitic railway steel with controlled residual austenitic stability through deformation and low-temperature tempering. The steel composition, by mass percentage, includes: 0.20-0.35% carbon, 1.30-1.80% silicon, 1.50-2.50% manganese, 0.005-0.020% phosphorus, 0.001-0.010% sulfur, 0.50-1.50% chromium, 0.3-0.70% molybdenum, 0.02-0.12% vanadium, and 0.02-0.70% nickel. During production, following the above-mentioned composition design, after smelting and casting, the billet is heated to 1180-1230℃ for 40-80 minutes. After descaling, the billet undergoes universal rolling with a rolling compression ratio ≥11:1 and a final rolling temperature between 850-950℃. After rolling, it is cooled to room temperature at a cooling rate of 5-10℃ / min, and then bent at approximately 15-25mm / 1600mm. Finally, it is tempered at 200-400℃ for 12-48 hours.

[0032] The bainitic railway steel produced using the above method has a tensile strength ≥1280MPa, elongation ≥12%, and residual austenitic transformation rate ≤30%, exhibiting high residual austenitic stability. It is particularly suitable for heavy-haul railway steel or turnout railway steel. This embodiment achieves residual austenitic stability control of high-strength, high-toughness bainitic railway steel through appropriate composition design, heat treatment, high-compression-ratio rolling, bending deformation, and low-temperature tempering.

[0033] Examples 1-5 of this invention, based on the Si-Mn-Cr bainitic rail steel composition system, involve alloy composition design, smelting and casting steel billets, and then cooling to room temperature. The billets are then heated at 1180-1230℃ for 40-80 minutes, descaled, and then subjected to universal rolling with a compression ratio ≥11:1, with a final rolling temperature between 850-950℃. After rolling, the billets are cooled to room temperature at a cooling rate of 5-10℃ / min, and then bent at approximately 15-25mm / 1600mm. Finally, they are tempered at 200-400℃ for 12-48 hours. The bainitic railway steel produced using this method has a tensile strength ≥1280MPa, elongation ≥12%, and residual austenitic transformation rate ≤30%, exhibiting high residual austenitic stability. It is particularly suitable for railway steel used in heavy-haul railways or turnout railways. The composition systems of Comparative Examples 1 and 2 are outside the scope of Examples 1-5 above, and the control of heating process, rolling process, compression ratio, final cooling temperature, cooling rate, cold deformation amount and tempering process parameters also do not meet the requirements of Examples 1-5 above. Specifically, the chemical composition and gas content of Examples 1-5 and Comparative Examples 1 and 2 are shown in Table 1, and the control of their heating process, rolling process, compression ratio, final cooling temperature, cooling rate, cold deformation amount and tempering process parameters are shown in Table 2.

[0034] Table 1 Chemical composition and gas content (%) of railway steel used in the examples and comparative examples

[0035] Table 2 Heating, rolling, and heat treatment processes for the examples and comparative examples

[0036] Examples 1-5 and Comparative Examples 1 and 2 were prepared according to the requirements of TB / T 2344 "Technical Conditions for Ordering 43kg / m~75kg / m Rails" and attached... Figure 1 Tensile specimens are machined and inspected at the sampling locations. Simultaneously, according to the attached... Figure 1 As shown, metallographic microstructure was examined at the locations indicated on the metallographic specimens. Metallographic photographs are attached. Figure 2 As shown in Tables 3 and 4, the tensile and metallographic data are statistically analyzed.

[0037] Table 3 Tensile properties of railway steel in examples and comparative examples

[0038] The volume fraction of residual austenite before and after deformation and tempering in the examples and comparative examples was measured using a Cu target X-ray diffractometer (XRD). The measurement method and the calculation of austenite content were in accordance with the national standard GB / T 8362.

[0039] Table 4. Residual stability of railway steel in examples and comparative cases.

[0040] As shown in the table above, the para-auspicious transformation rate after deformation in the embodiments of this application is less than that in the comparative example. After tempering treatment, the final para-auspicious transformation rate of the bainitic railway steel and its para-auspicious stability control production method disclosed in Embodiments 1-5 of this application is between 22.8% and 29.7%, while the final para-auspicious transformation rate of the comparative example is between 31.9% and 38.0%. The para-auspicious stability control effect of the bainitic railway steel and its para-auspicious stability control production method of this application is better.

[0041] In summary, the production method for bainitic railway steel and its para-aortic stability control disclosed in the embodiments of the present invention, through specific composition design, involves reheating and rolling with a high compression ratio after smelting and casting, followed by bending deformation and low-temperature tempering treatment. This ensures that the strength and toughness of the bainitic railway steel are guaranteed while achieving stronger para-aortic stability.

[0042] 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.

[0043] 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 production method for controlling the residual austenitic stability of bainitic railway steel, characterized in that, After the smelted and cast billet is heated to the first temperature for the first time, it is descaled and rolled, then cooled to room temperature at the first cooling rate, and then bent and deformed in all directions before being tempered. The bainitic railway steel described herein comprises the following chemical composition by weight percentage: 0.20%-0.35% carbon, 1.30%-1.80% silicon, 1.50%-2.50% manganese, 0.005%-0.020% phosphorus, 0.001%-0.010% sulfur, 0.50%-1.50% chromium, 0.3%-0.70% molybdenum, 0.02%-0.12% vanadium, 0.02%-0.70% nickel, with the remainder being Fe and unavoidable impurities; The first temperature is 1180-1230℃, the first time is 40-80min; the rolling compression ratio is not less than 11:1, the final rolling temperature is 850-950℃, the first cooling rate is 5-10℃ / min, the bending deformation in all directions is 15-25mm / 1600mm, the tempering temperature is 200-400℃, and the tempering time is 12-48h.

2. The production method for residual austenitic stability control of bainitic railway steel according to claim 1, characterized in that, The bending deformation in all directions (up, down, left, right) is defined as bending once in each direction.

3. A bainitic railway steel, characterized in that, The production method for controlling the residual austenitic stability of bainitic railway steel according to claim 1 or 2 results in a tensile strength ≥1280MPa, an elongation ≥12%, and a residual austenitic conversion rate ≤30%.

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