Process for producing a steel for carbide-free bainite rail head

Abstract

The application discloses a production method of a carbide-free bainite frog steel, and the frog steel contains 0.20-0.30% of C, 1.50-2.10% of Si and 0.001-0.004% of Al in percentage by mass, and the method comprises the following steps: S1, heating or forging the steel billet obtained after smelting and casting; S2, rolling the heated steel billet; S3, heating the frog steel obtained after rolling or forging to 900-980 DEG C at 10-15 DEG C / min, and keeping the temperature for 60-120 min; S4, heat treating the frog steel after keeping the temperature, the open cooling temperature of the heat treatment is 700-820 DEG C, the cooling rate is 5-15 DEG C / s, and the final cooling temperature is 200-300 DEG C; and S5, tempering the frog steel after cooling. The method can obtain the carbide-free bainite frog steel by controlling the components and the heat treatment process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgy, in particular to a production method of carbide-free bainite frog steel. BACKGROUND

[0002] As a key component for guiding train turning and bearing train load, frog serves the most severe, the most concentrated stress, the most prominent damage and the shortest service life in railway line. The material of frog mainly includes high manganese steel, bainite steel and maraging steel. As a new emerging steel for railway line, bainite frog is gradually affirmed by line users due to its excellent strength and toughness performance and excellent contact fatigue performance.

[0003] Bainite frog is successfully applied to many fields due to its good comprehensive mechanical properties, such as bainite frog and bainite rail in railway line, high-strength bainite steel plate in engineering machinery, bainite wear-resistant steel plate for mine machinery, bainite seamless steel pipe for oil transportation and building steel in construction industry. However, the performance of bainite frog still has room for further improvement. SUMMARY

[0004] The main purpose of the present application is to provide a production method of carbide-free bainite frog steel, which obtains carbide-free bainite frog steel by controlling the composition and heat treatment process.

[0005] In order to solve at least one of the above technical problems, the present application adopts the following technical scheme:

[0006] According to the present application, a production method of carbide-free bainite frog steel is provided, the frog steel contains 0.20-0.30% of C, 1.50-2.10% of Si and 0.001-0.004% of Al by mass percentage, and the method comprises the following steps: S1, heating the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet; S3, heating the frog steel obtained after rolling or forging to 900-980℃ at 10-15℃ / min, and keeping for 60-120min; S4, heat treating the frog steel after keeping, the open cooling temperature of the heat treatment is 700-820℃, the cooling rate is 5-15℃ / s, and the final cooling temperature is 200-300℃; and S5, tempering the frog steel after cooling.

[0007] According to one embodiment of the present application, the frog steel has the following composition in percentage by mass: 0.20-0.30% of C, 1.50-2.10% of Si, 1.10-2.00% of Mn, 0.002-0.020% of P, 0.002-0.020% of S, 0.30-1.70% of Cr, 0.10-0.60% of Mo, 0.002-0.70% of Ni, 0.01-0.15% of V, 0.001-0.004% of Al, and the rest of Fe and inevitable impurities.

[0008] According to one embodiment of the present application, in step S1, the soaking temperature of the steel billet is 1200-1250℃, and the holding time is 40-200 min.

[0009] According to one embodiment of the present application, in step S2, the final cooling temperature of rolling or forging is 950-1000℃, and the rolling compression ratio or forging ratio is ≥4:1.

[0010] According to one embodiment of the present application, in step S4, the frog steel after heat treatment to the final cooling temperature is naturally cooled.

[0011] According to one embodiment of the present application, in step S5, the tempering heating rate is 5-15℃ / min, the tempering holding temperature is 200-400℃, and the tempering holding time is 5-120 h.

[0012] According to one embodiment of the present application, in step S5, the frog steel after tempering is furnace-cooled at a cooling rate of 1-3℃ / min until the furnace temperature is room temperature.

[0013] According to one embodiment of the present application, the frog steel has aluminum oxide inclusions ≤1.0 level.

[0014] According to one embodiment of the present application, the whole casting process is carried out by protective casting.

[0015] According to one embodiment of the present application, the protective casting is carried out by 0.002-0.010% low-sulfur control.

[0016] In the production method of the carbide-free bainite frog steel according to the embodiment of the present application, through the component design of alloying elements such as Si, supplemented by heat treatment and tempering process, the carbide-free bainite frog steel with excellent strength and toughness and excellent contact fatigue resistance is obtained, the excellent carbide-free bainite structure is obtained, and the performance advantages of the bainite structure are fully utilized. The bainite frog produced by the method has a tensile strength of ≥1350 MPa, an elongation of ≥12%, an improved wear resistance of more than 30%, a bainite lath thickness of 200-300 nm, and a residual austenite film content of 10-20%. The total amount of line passing of the carbide-free bainite frog steel is 3-5 billion tons, and the total amount of line passing of the ordinary high manganese steel frog steel is 1.5-2 billion tons. Therefore, the service cycle of the carbide-free bainite frog steel produced by the method is 2-3 times that of the ordinary high manganese steel frog. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort.

[0018] Figure 1 A flowchart of the production method of the carbide-free bainite frog steel according to the embodiment of the present application is shown;

[0019] Figure 2 The metallographic structure of the carbide-free bainite frog steel according to the embodiment 1 of the present application is shown;

[0020] Figure 3 It is a schematic diagram of sampling the contact fatigue sample of the bainite frog steel. DETAILED DESCRIPTION

[0021] In order to make the objects, technical solutions and advantages of the present application clearer, the embodiments of the present application are further described in detail below, with reference to the drawings.

[0022] Compared with the ordinary bainite frog, the carbide-free bainite frog has no carbide precipitation in the structure, and the bainite ferrite and residual austenite can be solid-solved with more carbon atoms, so that the performance has higher strength, hardness and toughness. In order to obtain the carbide-free bainite frog, the alloying elements + heat treatment process are usually used.

[0023] Figure 1A flow chart showing a production method of the carbide-free bainite frog steel according to the embodiments of the present application is shown, the frog steel containing 0.20-0.30% of C, 1.50-2.10% of Si, 0.001-0.004% of Al by mass percentage, the method comprising the following steps:

[0024] S1, heating the steel billet obtained after smelting and casting;

[0025] S2, rolling or forging the heated steel billet;

[0026] S3, heating the frog steel obtained after rolling or forging to 900-980℃ at 10-15℃ / min, and holding for 60-120min;

[0027] S4, heat treating the frog steel after holding, the open cooling temperature of the heat treatment being 700-820℃, the cooling rate being 5-15℃ / s, and the final cooling temperature being 200-300℃;

[0028] S5, tempering the frog steel after cooling.

[0029] In the production method of the carbide-free bainite frog steel according to the embodiments of the present application, the components of the frog steel are preferably 0.24-0.35% of C, 1.20-1.8% of Si, 1.50-2.50% of Mn, 0.002-0.020% of P, 0.002-0.020% of S, 0.30-1.70% of Cr, 0.10-0.60% of Mo, 0.002-0.70% of Ni, 0.01-0.15% of V, 0.001-0.004% of Al by mass percentage, and the rest is Fe and inevitable impurity elements.

[0030] In the above components, by using 1.20-1.8% of Si, the precipitation of carbide is effectively inhibited in the bainite phase transition, while not excessively delaying the bainite phase transition and other adverse effects. By using 0.001-0.004% of Al, the precipitation of carbide in the phase transition process is inhibited, the bainite phase transition is also accelerated, while avoiding the increase of excessive Al hard inclusions B-type inclusions (aluminum oxide type), and ensuring the frog or rail contact fatigue performance.

[0031] In some embodiments, steps S1 to S2 can use the following parameters: after smelting and casting, the steel billet is heated to 1200-1250℃, and the holding time is 40-200min. After dephosphorization, the steel billet is rolled or forged, wherein the final cooling temperature of rolling or forging is 950-1000℃, and the rolling compression ratio or forging ratio is XX.

[0032] After rolling or forging forming, in step S3, heating to 900-980℃ again at 10-15℃ / min, and holding for 60-120min. The holding treatment can homogenize the austenite structure and improve the stability of the structure after phase transition.

[0033] In some embodiments, steps S4 to S5 can adopt the following parameters: heat treatment at an open cooling temperature range of 740-820℃, a cooling rate of 5-15℃ / s, and a final cooling temperature of 200-300℃. After cooling, the frog uses steel is tempered at a temperature of 200-400℃, and the tempering time is 5-120h to eliminate residual stress. Among them, the tempering time is mainly related to the heating medium, furnace temperature, chemical composition of steel, structure, sample size and shape, charging method and charging amount, etc., and the sample size and shape and the structure have the most obvious effect on the tempering time. The cross-sectional thickness of the frog steel can usually reach 116mm, and the tempering time can be calculated according to the commonly used empirical formula (1):

[0034] T=a x K x D (1)

[0035] In the formula, T is the heating time min; a is the heating coefficient min / mm, which is related to the workpiece size, heating medium, and chemical composition of steel; K is the charging correction coefficient (1.5-2.0); and D is the effective thickness of the part mm.

[0036] The calculated tempering time is at least 300min, i.e. 5h.

[0037] Meanwhile, in the embodiments of the present application, the frog steel has a structure of bainite + martensite + residual austenite, and the content of martensite and residual austenite in the structure is small. During the tempering process, the transformation rate is extremely low due to the extrusion of bainite structure, and a long tempering time is required. Experimental research shows that the longest tempering time required is 120h, and the longest time for international bainite material reaches more than 30 days.

[0038] Further, in step S5, the tempered frog steel is cooled at a cooling rate of 1-3℃ / min until the temperature in the furnace is room temperature.

[0039] In some embodiments, the whole casting process is protected casting to control the nitrogen, hydrogen and oxygen content of the frog steel.

[0040] The following will be described according to specific embodiments.

[0041] The frog steel of the embodiments and the comparative examples is casted by whole process protection, and low sulfur of 0.002-0.010% is preferentially used, wherein the main chemical components of examples 1-5 are shown in table 1, and the components of comparative examples 1-4 are the same as those of examples 1-4.

[0042] Table 1 Chemical composition (%) of the steel used for turnouts in the examples and comparative examples

[0043]

[0044]

[0045] The examples and comparative examples used the same heating temperature, with a soaking temperature of 1200–1250°C and a heating time of 40–200 min. They also used the same rolling or forging parameters, with an initial rolling or forging temperature of 1100–1150°C and a final rolling or forging temperature of 950–1000°C, and a compression ratio of ≥4:1. Different heat treatment processes were employed, as shown in Table 3.

[0046] Table 3. Heating, rolling, or heat treatment processes for the examples and comparative cases.

[0047]

[0048] In both the examples and comparative examples, after the forks were heat-treated to the final cooling temperature, accelerated cooling was stopped until the forks cooled to room temperature in the furnace. The tempering heating rate was 5–15 °C / min, the tempering holding temperature was 200–400 °C, and the tempering holding time was 5–120 h. After tempering, the bainitic forks were cooled in the furnace at a cooling rate of 1–3 °C / min until the furnace temperature reached room temperature.

[0049] In the examples and comparative examples, tensile specimens were taken and tested according to the requirements of TB / T 2344 "Technical Conditions for Ordering 43kg / m~75kg / m Rails". Simultaneously, metallographic microstructure was examined according to the specified testing locations for metallographic specimens. The statistical data for tensile and metallographic analysis are shown in Table 4.

[0050] Table 4. Tensile properties and microstructure of the turnouts in the examples and comparative examples.

[0051]

[0052]

[0053] Figure 2 The metallographic structure of a carbide-free bainitic turnout steel according to Embodiment 1 of the present invention is shown.

[0054] according to Figure 3 The samples from the examples and comparative examples were sampled and subjected to contact fatigue tests. The contact fatigue resistance of the examples and comparative examples is shown in Table 5.

[0055] Table 5. Contact fatigue data of turnouts in the examples and comparative cases.

[0056]

[0057] In combination with Table 1 to Table 3, the component design, heating process, and rolling process related parameters of the comparative example are not within the predetermined range. As shown in Table 4-5, the tensile strength of the bainite frog produced by the method is ≥1350MPa, the elongation is ≥12%, the wear resistance is increased by more than 30%, the bainite lath thickness is 200-300nm, and the residual austenite film content is 10-20%, which improves the strength and toughness of the bainite rail, and improves the contact fatigue resistance and rail running safety.

[0058] Those skilled in the art should understand that the above discussion of any of the embodiments is only exemplary and is not intended to imply that the scope (including claims) of the embodiments disclosed herein is limited to these examples; the technical features of the above embodiments or different embodiments can also be combined, and there are many other changes of different aspects of the embodiments as described above. In order to be brief, they are not provided in detail. Therefore, any omission, modification, equivalent replacement, improvement, etc. made within the spirit and principle of the embodiments should be included in the protection scope of the embodiments.

Claims

1. A method for producing carbide-free bainitic turnout steel, characterized in that, The steel used for the forklift, by mass percentage, comprises: 0.20-0.30% C, 1.50-2.10% Si, 1.10-2.00% Mn, 0.002-0.020% P, 0.002-0.020% S, 0.30-1.70% Cr, 0.10-0.60% Mo, 0.002-0.70% Ni, 0.01-0.15% V, 0.001-0.004% Al, with the remainder being Fe and unavoidable impurity elements. The method includes the following steps: S1, heating the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet; S3, heat the steel for the turnout obtained after rolling or forging to 900-980℃ at 10-15℃ / min and hold for 60-120min; S4. Heat treatment is performed on the heat-insulated steel for the turnout. The initial cooling temperature of the heat treatment is 700~820℃, the cooling rate is 5~15℃ / s, and the final cooling temperature is 200~300℃. S5 involves tempering the cooled turnout steel to obtain carbide-free bainitic turnout steel with a bainitic lath thickness of 200-300 nm and a retained austenitic film content of 10-20%.

2. The method according to claim 1, characterized in that, In step S1, the heat treatment temperature of the steel billet is 1200~1250℃, and the heat treatment time is 40~200min.

3. The method according to claim 1, characterized in that, In step S2, the final cooling temperature of rolling or forging is 950~1000℃, and the rolling compression ratio or forging ratio is ≥4:

1.

4. The method according to claim 1, characterized in that, In step S4, the turnout steel that has been heat-treated to the final cooling temperature is allowed to cool naturally.

5. The method according to claim 1, characterized in that, In step S5, the tempering heating rate is 5~15℃ / min, the tempering holding temperature is 200~400℃, and the tempering holding time is 5~120h.

6. The method according to claim 5, characterized in that, In step S5, the tempered fork steel is cooled in the furnace at a rate of 1~3℃ / min until the furnace temperature reaches room temperature.

7. The method according to claim 1, characterized in that, The alumina inclusions in the steel used for the turnouts are ≤1.0 grade.

8. The method according to claim 1, characterized in that, Protective casting is carried out throughout the entire casting process.

9. The method according to claim 8, characterized in that, The casting process employs a low-sulfur control method of 0.002-0.010%.

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