Bainite steel rail steel with excellent rolling contact fatigue resistance and preparation method of bainite steel rail steel

Abstract

The invention discloses alloy steel which comprises the following element components in percentage by mass: 0.08%-0.13% of C, 0.60%-1.10% of Mn, 0.40%-0.80% of Si, 0.15%-0.45% of Ni, 0.25%-0.55% of Mo, 0.05%-0.12% of V, 1.2%-1.6% of Cr, 0.02%-0.08% of Nb, less than or equal to 0.025% of P, less than or equal to 0.025% of S and the balance of Fe and inevitable impurities. The invention further provides a preparation method of the bainite steel. The preparation method comprises the steps that rolling and online heat treatment are conducted on steel billets; wherein the online heat treatment comprises continuous first-stage cooling and second-stage cooling; in the first-stage cooling, the temperature for starting cooling is 850-950 DEG C, the temperature for stopping cooling is 280-520 DEG C, and the cooling rate is 2-10 DEG C / s; the second-stage cooling is followed by the first-stage cooling, and the temperature is reduced to the room temperature at the cooling speed of 0.1-1 DEG C / s. The bainite steel is high in strength and good in toughness, more importantly, the bainite steel has remarkable rolling contact fatigue resistance, and the bainite steel is particularly suitable for being used as steel rail steel of railways, especially high-speed railways.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a bainite rail steel with excellent rolling contact fatigue resistance and a preparation method thereof. Background Art

[0002] Pearlitic rail steel is currently the most widely used type of rail steel in my country. Common pearlitic rail steels include U71Mn, U75V, U77MnCr, and U78CrV. U71Mn and U75V are commonly used for high-speed railway rails. Pearlitic steels offer advantages such as simple processing, low cost, stable lamellar structure, and excellent weldability. However, the lamellar cementite structure of pearlitic steel is susceptible to interlaminar cracking and coarsening, resulting in poor strength-toughness synergy, poor rolling contact fatigue resistance, and limited serviceability in extreme environments.

[0003] Unlike the lamellar structure of pearlite steel, bainite steel has a non-lamellar structure with a ferrite matrix and a small amount of carbides and retained austenite, resulting in significantly improved strength and toughness. Low-carbon bainite steel, a type of bainitic steel, generally has a carbon content of 0.02% to 0.10%. Due to its low carbon content, carbides are less likely to precipitate or form large M / A islands during the bainite phase transformation. Therefore, compared with medium- and high-carbon bainite steels, low-carbon bainite steel often has higher toughness. The main strengthening methods of low-carbon bainite steel are grain refinement and dislocation strengthening. By increasing the density of high-angle grain boundaries and introducing a certain amount of retained austenite, low-carbon bainite steel can achieve outstanding low-temperature toughness (-40°C) while maintaining high strength. Currently, low-carbon bainite steel is mainly used in high-end engineering fields such as bridges, pipeline transportation, ships, and offshore platforms. There are no public reports of its application in the field of rail steel.

[0004] For rail steel, the microstructure formation is primarily influenced by a variety of factors, including chemical composition and controlled rolling and cooling processes. The formation of bainitic steel relies on a controlled, medium-temperature phase transformation process, the core mechanism of which is a mixed diffusion-shear transformation. This transformation promotes the formation of bainitic ferrite from supercooled austenite through semi-coherent shear. Simultaneously, carbon is enriched in the retained austenite through controlled diffusion or precipitated as carbides, ultimately forming a non-lamellar bainite structure, such as the feather-like structure of upper bainite or the needle-like structure of lower bainite. This process requires precise control of temperature-time parameters and the addition of alloying elements to suppress competing phase transformations (pearlite / martensite), thereby achieving directional control of the bainite structure.

[0005] The current heat treatment process for bainitic steel mainly uses offline heat treatment (tempering). The tempering process releases residual stress, making the austenite more stable and effectively reducing the martensite content in the bainitic steel. For example, Chinese invention patent application publication number CN118639091A, "A method for manufacturing a low-carbon Nb, V and RE micro-alloyed bainitic rail," discloses a bainitic rail with a chemical composition including: C: 0.10-0.20%; Si: 0.90-1.50%; Mn: 1.80-2.40%; P ≤ 0.010%; S ≤ 0.005%; Cr: 0.80-1.20%; Mo : 0.20-0.40%; Ni: 0.10-0.50%; Nb: 0.03-0.10%; V: 0.06-0.15%; RE: 0.002-0.010%, with the remainder being Fe and unavoidable impurities. This bainitic rail is manufactured using an offline heat treatment process: after controlled cooling, the rail is tempered at a temperature of 280-400°C for 20 hours or longer. This tempering process places high demands on the heat treatment process and equipment, increasing manufacturing complexity and costs. Furthermore, the tempering time typically exceeds 20 hours, resulting in long production cycles and low efficiency.

[0006] In addition, the performance of bainite rails in the prior art in terms of rolling contact fatigue resistance needs to be further improved. Summary of the Invention

[0007] In order to solve the above technical problems, the present invention provides a new bainite steel and a preparation method thereof. The bainite steel provided by the present invention has excellent rolling contact fatigue resistance and is particularly suitable for manufacturing rails, especially high-speed railway rails.

[0008] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0009] An alloy steel comprises the following elemental components in mass percentage: C: 0.08% to 0.13%, Mn: 0.60% to 1.10%, Si: 0.40% to 0.80%, Ni: 0.15% to 0.45%, Mo: 0.25% to 0.55%, V: 0.05% to 0.12%, Cr: 1.2% to 1.6%, Nb: 0.02% to 0.08%, P≤0.025%, S≤0.025%, and the balance being Fe and unavoidable impurities.

[0010] As a preferred embodiment, the present invention provides an alloy steel comprising the following elemental components in mass percentage: C: 0.09% to 0.11%, Mn: 0.70% to 0.09%, Si: 0.50% to 0.70%, Ni: 0.25% to 0.40%, Mo: 0.30% to 0.40%, V: 0.07% to 0.10%, Cr: 1.30% to 1.45%, Nb: 0.04% to 0.07%, P≤0.015%, S≤0.015%, and the remainder being Fe and unavoidable impurities.

[0011] A second object of the present invention is to provide a method for preparing bainitic steel, wherein the bainitic steel comprises the following elemental compositions by mass percentage: C: 0.08-0.13%, Mn: 0.60-1.10%, Si: 0.40-0.80%, Ni: 0.15-0.45%, Mo: 0.25-0.55%, V: 0.05-0.12%, Cr: 1.2-1.6%, Nb: 0.02-0.08%, P≤0.025%, S≤0.025% , the balance is Fe and inevitable impurities; the preparation method comprises rolling and online heat treatment of a steel billet, the chemical composition of the steel billet being the same as that of the bainitic steel; wherein the online heat treatment comprises continuous first-stage cooling and second-stage cooling; the first-stage cooling comprises a starting cooling temperature of 850-950°C, an ending cooling temperature of 280-520°C, and a cooling rate of 2-10°C / s; the second-stage cooling follows the first-stage cooling and is cooled to room temperature at a cooling rate of 0.1-1°C / s.

[0012] Preferably, the bainitic steel comprises the following elemental components in mass percentage: C: 0.09% to 0.11%, Mn: 0.70% to 0.09%, Si: 0.50% to 0.70%, Ni: 0.25% to 0.40%, Mo: 0.30% to 0.40%, V: 0.07% to 0.10%, Cr: 1.30% to 1.45%, Nb: 0.04% to 0.07%, P≤0.015%, S≤0.015%, and the balance is Fe and unavoidable impurities; the chemical composition of the steel billet is the same as that of the bainitic steel.

[0013] Preferably, the first stage cooling starts at a temperature of 880-900°C, ends at a temperature of 320-480°C, and has a cooling rate of 8-10°C / s; the second stage cooling is performed at a cooling rate of 0.5-1°C / s to room temperature.

[0014] Preferably, in the rolling step, before starting rolling, the steel billet is reheated to 1200° C. and kept warm for 3 hours.

[0015] Preferably, in the rolling step, the starting rolling temperature is 1140-1180°C and the finishing rolling temperature is 950-980°C.

[0016] Preferably, the starting rolling temperature is 1150°C and the finishing rolling temperature is 950°C.

[0017] Preferably, in the rolling step, the compression ratio is 7:1 to 10:1.

[0018] Also preferably, the rolling step is divided into 5 passes.

[0019] Before rolling, the preparation method of the steel billet is already known to those skilled in the art, including, for example, smelting, casting, forging, etc.

[0020] Therefore, the present invention also provides bainite steel obtained by the preparation method.

[0021] The bainitic steel of the present invention has a microstructure composed of lath bainite, granular bainite, and retained austenite, wherein the lath bainite is the main structure and the retained austenite is distributed in the form of a thin film between and within the bainite laths. Figure 1 and Figure 2 .

[0022] The present invention also provides the use of the above-mentioned bainite steel in the preparation of steel rails, preferably in the preparation of high-speed railway rails.

[0023] Therefore, the present invention also provides a steel rail, which is prepared from the bainite steel of the present invention.

[0024] The preparation technology and process of rails are well known to those skilled in the art.

[0025] The bainitic steel provided by the present invention has excellent mechanical properties, including a tensile strength greater than 1000 MPa, a yield strength greater than 700 MPa, an elongation greater than 16%, a hardness greater than 300 HB, and an impact energy greater than 130 J at 20°C. In particular, the bainitic steel of the present invention exhibits excellent resistance to rolling contact fatigue. In tests conducted in accordance with "YB / T 5345-2014 Test Method for Rolling Contact Fatigue of Metallic Materials," rail samples made from the bainitic steel of the present invention experienced a weight loss of only 0.002 to 0.004 g after 40,000 cycles. Furthermore, no peeling or shedding of the rail sample surface was observed after 400,000 cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below with reference to the accompanying drawings.

[0027] Figure 1This is a transmission electron microscope micrograph of the low-carbon bainite steel of Example 1, magnified 8600 times. The photograph shows that the microstructure of the bainite steel is mainly composed of lath bainite.

[0028] Figure 2 This is a transmission electron microscope micrograph of the low-carbon bainite steel of Example 1, magnified 130,000 times. The photograph shows the presence of film-like retained austenite inside the bainite laths of the bainite steel.

[0029] Figure 3 Schematic diagram of the rolling contact fatigue test of the alloy steels of Example 3 and Comparative Example 1.

[0030] Figure 4 The photograph shows the surface condition of the low carbon bainite steel of Example 3 after the rolling contact fatigue test. Figure 4 It shows that there is no obvious fatigue crack on the surface of the bainite steel after 400,000 revolutions.

[0031] Figure 5 The photograph shows the surface condition of the pearlite steel of Comparative Example 1 after the rolling contact fatigue test. Figure 5 It is shown that after 40,000 revolutions, large areas of peeling blocks appear on the surface of the pearlite steel.

[0032] Figure 6 The photograph shows the surface wear of the bainite steel of Comparative Example 3 after the rolling contact fatigue test. Figure 6 It shows that after 40,000 revolutions, large areas of peeling blocks appeared on the surface of the bainite steel.

[0033] Figure 7 The photograph shows the surface wear of the bainite steel of Comparative Example 4 after the rolling contact fatigue test. Figure 7 It is shown that after 40,000 revolutions, pieces of bainite steel are peeled off from the surface. DETAILED DESCRIPTION

[0034] The present invention is described below with reference to specific examples. Those skilled in the art will appreciate that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention in any way.

[0035] The present invention provides a new bainitic steel comprising the following elemental compositions in mass percentages:

[0036] C: 0.08~0.13%, Mn: 0.60%~1.10%, Si: 0.40~0.80%, Ni: 0.15%~0.45%, Mo: 0.25%~0.55%, V: 0.05%~0.12%, Cr: 1.2%~1.6%, Nb: 0.02~0.08%, P: ≤0.025%, S: ≤0.025%, the balance is Fe and unavoidable impurities.

[0037] As a preferred embodiment, the bainitic steel contains the following elemental components in mass percentage: C: 0.09-0.11%, Mn: 0.70%-0.09%, Si: 0.50-0.70%, Ni: 0.25%-0.40%, Mo: 0.30%-0.40%, V: 0.07%-0.10%, Cr: 1.30%-1.45%, Nb: 0.04-0.07%, P: ≤0.015%, S: ≤0.015%, and the balance is Fe and unavoidable impurities.

[0038] Generally, the C content of bainite steel is above 0.2%. The bainite steel of the present invention is a low-carbon bainite steel with a significantly reduced C content, which is beneficial to improving the weldability of the bainite steel.

[0039] Mn and Cr elements play a role in solid solution strengthening, improving the strength of bainitic steel. At the same time, an appropriate amount of Mn can also accelerate the bainite transformation.

[0040] The Si element can significantly inhibit the precipitation of carbides, thereby improving the plasticity and toughness of bainitic steel.

[0041] The Ni element accelerates the transformation of bainite and inhibits the transformation of martensite, thus avoiding the formation of martensite under large undercooling conditions.

[0042] The Mo element plays a role in solid solution strengthening and improving the hardenability of bainite steel, and its composite effect with the Ni element can have a certain corrosion resistance effect.

[0043] V and Nb can form precipitates with elements such as C and N in alloy steel, hindering the movement of recrystallized grain boundaries after rolling and increasing dislocation density. Precipitates can also serve as nucleation sites, significantly increasing the nucleation rate during bainite transformation and refining grain size.

[0044] P and S elements are generally harmful elements in alloy steel. They will greatly increase the crack sensitivity of alloy steel, while increasing the low-temperature brittle transition temperature of alloy steel and reducing the low-temperature impact toughness of alloy steel. Therefore, the lower the content of P and S elements, the better.

[0045] The preparation process of the novel bainite steel provided by the present invention comprises:

[0046] Smelting → casting → forging → ingot rolling → online heat treatment.

[0047] The process flow of smelting, casting and forging is already known to those skilled in the art.

[0048] Ingot rollingThe forged ingot is reheated to 1200°C and held for 3 hours to ensure full austenitization. Rolling is started at 1140-1180°C, preferably 1150°C, with a reduction ratio of 7:1-10:1, in 5 passes, and the final rolling temperature is 950-980°C, preferably 950°C.

[0049] Online heat treatment The online heat treatment process includes a continuous first-stage cooling and a second-stage cooling process without insulation in between. The second-stage cooling process starts directly after the first-stage cooling process ends. The first-stage cooling process starts at a temperature of 850-950°C, preferably 880-900°C, and ends at a temperature of 280-520°C, preferably 320-480°C, at a cooling rate of 2-10°C / s, preferably 8-10°C / s; the second-stage cooling process starts at a cooling rate of 0.1-1°C / s, preferably 0.5-1°C / s, and cools to room temperature.

[0050] The strengthening mechanism of the novel bainite steel of the present invention is grain boundary strengthening and dislocation strengthening. The improvement of room temperature impact toughness is mainly achieved by forming thin film retained austenite between and inside bainite laths, refining bainite laths, and increasing the density of large-angle grain boundaries.

[0051] The experimental methods in the following examples are conventional methods unless otherwise specified. The raw materials, reagents, etc. used in the following examples are commercially available products unless otherwise specified.

[0052] Example 1 A new type of low-carbon bainitic steel

[0053] The forged ingot is reheated to 1200°C and held at this temperature for 3 hours. Rolling begins at 1150°C with a reduction ratio of 7:1. Rolling is performed in five passes, with a final rolling temperature of 950°C. The resulting plate has a thickness of 12mm. Online heat treatment is then performed. The specific process for online heat treatment is as follows: cooling from the initial cooling temperature of 900°C to the first stage cooling end temperature of 450°C at a cooling rate of 8°C / s, followed by cooling from 450°C to 20°C in the second stage at a cooling rate of 1°C / s, to obtain low-carbon bainitic steel.

[0054] The steel ingot and bainitic steel are composed of the following element compositions in mass percentage: C: 0.10%, Mn: 0.80%, Si: 0.60%, Ni: 0.35%, Mo: 0.35%, V: 0.08%, Cr: 1.35%, Nb: 0.06%, P≤0.01%, S≤0.01%, and the balance is Fe and unavoidable impurities.

[0055] Example 2 A new type of low-carbon bainitic steel

[0056] The chemical composition of the bainite steel of Example 2 is the same as that of Example 1, and the production process and steps are also basically the same, except that:

[0057] The final cooling temperature of the first stage of online heat treatment is 340℃.

[0058] Example 3 A new type of low-carbon bainitic steel

[0059] The chemical composition of the bainite steel of Example 3 is the same as that of Example 1, and the production process and steps are also basically the same, except that:

[0060] The cooling rate of the first stage of online heat treatment is 9.5℃ / s.

[0061] Example 4 A new type of low-carbon bainitic steel

[0062] The chemical composition of the bainite steel of Example 4 is the same as that of Example 1, and the production process and steps are also basically the same, except that:

[0063] The final rolling temperature is 960℃.

[0064] Comparative Example 1 A pearlite steel

[0065] In the prior art, the 880 MPa grade pearlite steel U71Mn is composed of the following elemental components in mass percentage: C: 0.71%, Mn: 0.82%, Si: 0.45%, Cr: 0.10%, Nb: 0.01%, Mo: 0.01%, Ni: 0.05%, Cu: 0.09%, P≤0.025%, S≤0.025%, and the balance is Fe and unavoidable impurities.

[0066] Comparative Example 2 A bainitic steel

[0067] In the prior art, heavy-duty line rail U20Mn2SiCrNiMo (bainite rail, manufactured by a certain steel company) is composed of the following elemental composition by mass percentage: C: 0.21%, Mn: 2.04%, Si: 1.43%, Cr: 0.53%, Nb: 0.03%, Mo: 0.34%, P≤0.022%, S≤0.015%, and the balance is Fe and unavoidable impurities.

[0068] Comparative Example 3 A bainitic steel

[0069] Comparative Example 3 adopts the same preparation process as Example 1, but the steel billet and bainitic steel are composed of the following element compositions in mass percentage: C: 0.15%, Mn: 1.12%, Si: 0.83%, V: 0.10%, Cr: 1.25%, P≤0.01%, S≤0.01%, and the balance is Fe and unavoidable impurities.

[0070] Comparative Example 4 A bainitic steel

[0071] The chemical composition of the bainitic steel in Comparative Example 4 is the same as that in Example 1, but the production process and steps are different. Specifically, the forged ingot is reheated to 1200°C and held at this temperature for 3 hours. Rolling is then initiated at 1150°C with a reduction ratio of 7:1 in five passes, with a final rolling temperature of 950°C. The resulting plate has a thickness of 12 mm. Online heat treatment is then performed; the specific process for this online heat treatment is as follows: cooling from the initial cooling temperature of 900°C to the first stage cooling end temperature of 280°C at a cooling rate of 3°C / s, followed by cooling from 280°C to 20°C at a cooling rate of 1°C / s, to obtain low-carbon bainitic steel.

[0072] Test Case Performance measurement of the alloy steels of various embodiments and comparative examples

[0073] 1. Microstructure of the Low Carbon Bainite Steel of Example 1

[0074] Micrographs of the low carbon bainite steel of Example 1 at different magnifications were obtained using a FEI Talos F200X transmission electron microscope (Thermo Fisher Scientific, USA). Figure 1 and Figure 2 .in, Figure 1 The photo is magnified 8600 times. Figure 2 The photo was magnified 130,000 times.

[0075] Figure 1 and Figure 2 The microstructure of the low carbon bainite steel of the present invention is a mixed microstructure of lath bainite, granular bainite and retained austenite, wherein the lath bainite is the main microstructure (see Figure 1 ), the retained austenite is distributed in the form of a film between the bainite laths and inside the laths (see Figure 2 ).

[0076] 2. Mechanical Properties of the Alloy Steels of the Examples and Comparative Examples

[0077] The tensile strength, yield strength, elongation after fracture, hardness and 20°C impact energy of the alloy steel samples of each embodiment and comparative example were tested respectively. The results are shown in Table 1.

[0078] Table 1 Mechanical properties test results of alloy steels of Examples and Comparative Examples

[0079] Sample name tensile strength Yield strength Elongation at break hardness Impact energy at 20℃ Example 1 1031MPa 773MPa 17.5％ 305HB 158J Example 2 1072MPa 761MPa 17.2％ 310HB 172J Example 3 1081MPa 782MPa 19.8％ 317HB 196J Example 4 1153MPa 807MPa 16.2％ 335HB 132J Comparative Example 1 933MPa 653MPa 14.0％ 274HB 42J Comparative Example 2 1338MPa 1224MPa 13.5％ 417HB 56J Comparative Example 3 1231MPa 831MPa 15.6％ 340HB 52J Comparative Example 4 955MPa 652MPa 17.1％ 231HB 85J

[0080] 2. Rolling contact fatigue resistance of the new bainite steel of the present invention

[0081] Using a GPM-40 wheel-rail wear tester, rolling contact fatigue resistance tests were conducted on the bainitic steel of Example 1, the bainitic steel of Example 3 (which exhibits the best overall mechanical properties), the pearlitic steel of Comparative Example 1, and the bainitic steels of Comparative Examples 3 and 4. Overall performance was assessed by the number of cycles of surface delamination and surface damage. The rolling contact fatigue tests involved pre-wear testing of the rail steels of the Example and Comparative Examples, followed by rolling contact fatigue testing under oil lubrication. The pre-wear test conditions were: load: 3000N; rotational speed: 500r / min; creep rate: 5%; and number of cycles: 20,000 revolutions. The sample used for the wear test against the rail steel was pearlitic wheel steel ER8. The rolling contact fatigue test conditions are as follows: load: 3000N; rotation speed: 500r / min; creep rate: 0.1%; cycle number: 400,000 revolutions (stop and observe every 40,000 revolutions, and stop when large areas of the rail steel sample surface peel off); lubrication condition: oil lubrication; test is carried out in accordance with "YB / T 5345-2014 Rolling Contact Fatigue Test Method for Metallic Materials"; the rolling contact fatigue test schematic diagram of the rail steel of the embodiment and comparative example is shown in FIG. Figure 3 shown.

[0082] The measurement results are shown in Table 2.

[0083] Table 2 Rolling contact fatigue performance test results

[0084]

[0085] The damaged surface of the bainite steel after the rolling contact fatigue test of Example 3 is as follows: Figure 4 As shown, the surface condition of the bainite steel of Example 1 is similar to that of Example 3 (photographs not shown). The damaged surfaces of the steels of Comparative Examples 1, 3 and 4 after rolling contact fatigue tests are shown in FIG. Figure 5 、 Figure 6 and Figure 7 As shown. Figure 4 and Figure 5-7 As can be seen, after the same 40,000 revolutions, the low-carbon bainitic steel of Example 3 exhibited no obvious fatigue cracks on its worn surface, whereas the pearlitic steel of Comparative Example 1 (with the same strength grade as the bainitic steel of the present invention) and the bainitic steels of Comparative Examples 3 and 4 exhibited extensive surface spalling. Therefore, compared to Comparative Examples 1, 3, and 4, the low-carbon bainitic steel of the present invention exhibited significant advantages in rolling contact fatigue resistance.

[0086] In summary, the present invention provides a novel alloy steel formulation, a method for preparing bainitic steel, and the resulting bainitic steel. In addition to high strength and excellent toughness, the bainitic steel of the present invention also exhibits significant resistance to rolling contact fatigue, making it particularly suitable for use as rail steel in railways, particularly high-speed railways.

Claims

1. An alloy steel comprising the following elemental compositions in mass percentage: C: 0.08% to 0.13%, Mn: 0.60% to 1.10%, Si: 0.40% to 0.80%, Ni: 0.15% to 0.45%, Mo: 0.25% to 0.55%, V: 0.05% to 0.12%, Cr: 1.2% to 1.6%, Nb: 0.02% to 0.08%, P≤0.025%, S≤0.025%, the remainder being Fe and unavoidable impurities.

2. The alloy steel according to claim 1, characterized in that The alloy steel includes the following elemental components in mass percentage: C: 0.09% to 0.11%, Mn: 0.70% to 0.09%, Si: 0.50% to 0.70%, Ni: 0.25% to 0.40%, Mo: 0.30% to 0.40%, V: 0.07% to 0.10%, Cr: 1.30% to 1.45%, Nb: 0.04% to 0.07%, P≤0.015%, S≤0.015%, and the balance is Fe and unavoidable impurities.

3. A method for preparing bainitic steel, the bainitic steel comprising the following elemental compositions by mass percentage: C: 0.08-0.13%, Mn: 0.60-1.10%, Si: 0.40-0.80%, Ni: 0.15-0.45%, Mo: 0.25-0.55%, V: 0.05-0.12%, Cr: 1.2-1.6%, Nb: 0.02-0.08%, P≤0.025%, S≤0.025%, the balance being Fe and unavoidable impurities; the method comprising rolling and online heat treating a steel billet, the chemical composition of the steel billet being the same as that of the bainitic steel; wherein: The online heat treatment includes continuous first-stage cooling and second-stage cooling; in the first-stage cooling, the starting cooling temperature is 850-950°C, the ending cooling temperature is 280-520°C, and the cooling rate is 2-10°C / s; the second-stage cooling is immediately followed by the first-stage cooling, and is cooled to room temperature at a cooling rate of 0.1-1°C / s.

4. The preparation method according to claim 3, characterized in that The bainite steel comprises the following elemental components in mass percentage: C: 0.09% to 0.11%, Mn: 0.70% to 0.09%, Si: 0.50% to 0.70%, Ni: 0.25% to 0.40%, Mo: 0.30% to 0.40%, V: 0.07% to 0.10%, Cr: 1.30% to 1.45%, Nb: 0.04% to 0.07%, P≤0.015%, S≤0.015%, and the balance is Fe and unavoidable impurities; The chemical composition of the steel billet is the same as that of the bainitic steel.

5. The preparation method according to claim 3, characterized in that The first stage cooling starts at a temperature of 880-900°C, ends at a temperature of 320-480°C, and has a cooling rate of 8-10°C / s; the second stage cooling is cooled to room temperature at a cooling rate of 0.5-1°C / s.

6. The preparation method according to claim 3, characterized in that In the rolling step, before rolling, the steel billet is reheated to 1200° C. and kept at this temperature for 3 hours; Preferably, in the rolling step, the starting rolling temperature is 1140-1180°C, and the finishing rolling temperature is 950-980°C; More preferably, the starting rolling temperature is 1150°C and the finishing rolling temperature is 950°C.

7. The preparation method according to claim 3 or 6, characterized in that In the rolling step, the compression ratio is 7:1 to 10:1; Preferably, the rolling step is divided into 5 passes.

8. Bainite steel obtained by the preparation method according to any one of claims 3 to 7.

9. Use of the bainite steel according to claim 8 in the manufacture of steel rails, preferably in the manufacture of high-speed railway rails.

10. A steel rail made from the bainite steel according to claim 8.

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