Method for producing a steel for frog with excellent toughness

Abstract

The application discloses a production method of a frog steel with excellent toughness, and the frog steel comprises the following components: 0.24-0.30% of C, 1.40-1.70% of Si, 1.50-1.9% of Mn, 0.002-0.015% of P, 0.002-0.010% of S, 0.7-1.40% of Cr, 0.30-0.60% of Mo, 0.30-0.70% of Ni, 0.03-0.15% of V, and the rest is Fe and inevitable impurity elements, and the method comprises the following steps: S1, heating the steel billet obtained after smelting and casting to 1250-1300 DEG C; S2, rolling or forging the heated steel billet; S3, performing heat treatment on the frog steel obtained after rolling or forging, and the heat treatment comprises the following steps: after the frog steel obtained after rolling or forging is heated and kept warm again, the frog steel is cooled to the final cooling temperature of 100-300 DEG C at a cooling rate of 1.5-3 DEG C / s by using air cooling; and S4, tempering the frog steel after cooling. The method can improve the toughness of the frog steel by controlling the components of the bainite frog and reducing the cooling rate.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a method for producing frog steel with excellent strength and toughness. Background Art

[0002] As a critical component that guides vehicle steering and carries train loads, frogs are subject to the harshest conditions on railway lines, are subject to the most concentrated stresses, are most susceptible to damage, and have the shortest service life. Consequently, a frog steel with high strength, high toughness, high wear resistance, and excellent contact fatigue resistance is urgently needed. Bainitic frogs typically weigh up to 400 million tons and have an average lifespan 3-5 times longer than high-Mn steel frogs. Due to their excellent performance, bainitic frogs have become the preferred material for frogs both domestically and internationally. However, existing bainitic frogs often suffer from damage such as cracks and chipping during service, resulting in poor process quality and performance stability, severely hindering their widespread adoption.

[0003] Existing bainitic rails are designed with a low carbon content (0.15-0.25%), supplemented with appropriate amounts of Si, Mn, Cr, Ni, Mo, and other elements. Carbide-free bainite or a B / M duplex structure is achieved through air cooling. Meanwhile, through medium-temperature tempering, the rails achieve a good balance of strength and toughness.

[0004] The development of the railway industry places increasingly higher demands on frog performance, and it is necessary to further improve the mechanical properties of bainite frogs. Summary of the Invention

[0005] The main purpose of the present invention is to provide a method for producing frog steel with excellent strength and toughness, and to improve the toughness of the bainite frog by controlling the composition of the bainite frog and reducing the cooling rate.

[0006] In order to solve at least one of the above technical problems, the present invention adopts the following technical solutions:

[0007] According to the present invention, a method for producing frog steel with excellent strength and toughness is provided, wherein the frog steel comprises the following components: 0.24-0.30% C, 1.40-1.70% Si, 1.50-1.9% Mn, 0.002-0.015% P, 0.002-0.010% S, 0.7-1.40% Cr, 0.30-0.60% Mo, 0.30-0.70% Ni, 0.03-0.15% V, and the remainder being Fe and unavoidable The method comprises the following steps: S1, heating the steel billet obtained after smelting and casting to 1250-1300°C; S2, rolling or forging the heated steel billet; S3, heat treating the frog steel obtained after rolling or forging, wherein the heat treatment comprises: heating the frog steel obtained after rolling or forging again and keeping the temperature thereof, and then cooling the frog steel to a final cooling temperature of 100-300°C at a cooling rate of 1.5-3°C / s by air cooling; and S4, tempering the cooled frog steel.

[0008] According to one embodiment of the present invention, in step S1, the heating time is controlled to be 200 to 500 minutes.

[0009] According to one embodiment of the present invention, in step S2, the final cooling temperature of rolling is 950-1050°C, and the compression ratio is ≥4.

[0010] According to one embodiment of the present invention, in step S3, the frog steel obtained after rolling or forging is heated again to 920°C, kept at this temperature for 2 hours, cooled to the cooling temperature of 740-820°C, and then cooled by air cooling.

[0011] According to one embodiment of the present invention, in step S4, the cooled frog steel is tempered at a temperature of 100 to 450° C. for 5 to 120 hours.

[0012] According to one embodiment of the present invention, the impurity elements include, by mass percentage, 0.001-0.02% Sn, 0.05-0.15% Cu, and 0.01-0.02% As.

[0013] According to one embodiment of the present invention, the hydrogen content of the frog steel is ≤1.2 ppm, the oxygen content is ≤10 ppm, and the nitrogen content is 40-60 ppm.

[0014] According to one embodiment of the present invention, protective casting is performed throughout the entire casting process.

[0015] According to one embodiment of the present invention, protective casting is performed by controlling the sulfur content to 0.002-0.010%.

[0016] In a method for producing high-strength and high-toughness frog steel according to an embodiment of the present invention, the toughness of bainitic frogs is enhanced by rationally designing the composition, heating, rolling, and heat treatment parameters. Bainitic frogs produced using this method have a tensile strength of ≥1350 MPa and a reduction of area of ​​≥45%, making them particularly suitable for heavy-haul railway frogs. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 A flow chart showing a method for producing frog steel having excellent strength and toughness according to an embodiment of the present invention. DETAILED DESCRIPTION

[0019] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0020] Among the various structures of steel, ferrite and paratenite are both soft structures, and the higher the content, the higher the corresponding elongation, cross-sectional shrinkage and impact toughness. Ferrite is mainly obtained by low cooling temperature, and paratenite is mainly obtained by increasing the cooling rate during heat treatment. Bainite structure is a structure with moderate strength and toughness, which is generally obtained by a relatively slow cooling rate or natural cooling. The toughness index is relatively low compared to ferrite and paratenite, and the tensile strength is lower than that of martensite. Martensite is the main contributing structure to tensile strength, which is mainly obtained by a larger cooling rate. In order to obtain moderate strength and excellent toughness, the present invention obtains a composite structure of ferrite, bainite, paratenite and martensite at a certain cooling rate to achieve the performance requirements of excellent strength and toughness.

[0021] Figure 1A flow chart of a production method of the rail frog steel with excellent strength and toughness according to an embodiment of the present application is shown, wherein the rail frog steel has the following composition: 0.24-0.30% of C, 1.40-1.70% of Si, 1.50-1.9% of Mn, 0.002-0.015% of P, 0.002-0.010% of S, 0.7-1.40% of Cr, 0.30-0.60% of Mo, 0.30-0.70% of Ni, 0.03-0.15% of V, and the rest of Fe and inevitable impurity elements. In some embodiments of the present application, the impurity elements include: 0.001-0.02% of Sn, 0.05-0.15% of Cu and 0.01-0.02% of As, and preferably the hydrogen content is controlled to be ≤1.2 ppm, the oxygen content is controlled to be ≤10 ppm, and the nitrogen content is controlled to be 40-60 ppm.

[0022] In the production method of the rail frog steel with excellent strength and toughness according to an embodiment of the present application, the composition design of 0.24-0.30% of C is adopted to facilitate the expansion of the γ phase region, and with the increase of the carbon concentration in the austenite, the critical cooling speed is significantly reduced, and the hardenability of the steel is increased. Si is a reducing agent and deoxidizer in the steelmaking process, and as an alloying element, it mainly narrows the γ phase region and forms γ phase; no carbide is formed, and in the bainite, it mainly inhibits the formation of carbides, and is the main alloying element of the non-carbide bainite steel. Mn is not only a good deoxidizer and desulfurizer in the steelmaking process, but also can improve the hardenability and improve the hot working performance. The high-strength and high-toughness bainite rail frog steel of the present application reduces the cold brittleness of the steel by limiting the P content to be not greater than 0.015%, and at the same time, reduces the hot brittleness of the steel by limiting the sulfur content to be less than 0.010%, which is beneficial to maintain good plasticity, ductility and toughness. Cr is a γ phase region narrowing element, can form γ phase, can infinitely dissolve in α iron, is a medium carbide forming element, with the increase of Cr content, the hardenability of the steel can be increased, and the wear resistance of the steel can be improved. Ni can expand the γ phase region and form an infinite solid solution, which can not only improve the strength of the steel and maintain good plasticity and toughness, but also refine the ferrite grains, and under the same strength conditions, improve the plasticity and toughness of the steel, especially the low temperature toughness.

[0023] 0.30-0.60% of Mo can refine the grains of the steel, improve the hardenability and thermal strength performance, and also can inhibit the temper brittleness of the bainite alloy steel. 0.03-0.15% of vanadium is an excellent deoxidizer, which can improve the strength and toughness of the bainite steel; forms carbides with carbon and narrows the γ phase region, forms γ phase; improves the hardenability of the steel; has the effect of refining the grains, and is beneficial to the low temperature impact toughness.

[0024] N combined with V can improve the strength and low temperature toughness of the steel, and increase the aging sensitivity. The present application adds Mn, Cr, Mo and other strong hardenability elements to improve the hardenability of the product matrix, and improves the hardening layer depth of the product.

[0025] The production method of frog steel with excellent toughness according to an embodiment of the present invention generally comprises the following steps:

[0026] S1, heating the steel billet obtained after smelting and casting to 1250-1300°C;

[0027] S2, rolling or forging the heated steel billet;

[0028] S3, heat treating the frog steel obtained after rolling or forging, wherein the heat treatment comprises: reheating the frog steel obtained after rolling or forging and keeping it warm, and then cooling it to a final cooling temperature of 100 to 300° C. by air cooling at a cooling rate of 1.5 to 3° C. / s;

[0029] S4, tempering the cooled frog steel.

[0030] In some embodiments, steps S1 and S2 may employ the following parameters: After smelting and casting, the steel slab is heated to 1250-1300°C for 200-500 minutes. Increasing the heating temperature and time can increase the original grain size, improve hardenability, and increase the depth of the hardened layer. After dephosphorization, the steel slab is rolled or forged, with the final cooling temperature of the rolling process being 950-1050°C and the compression ratio being ≥4.

[0031] In some embodiments, in step S3, the frog steel obtained after rolling or forging is heated again to 920°C, kept warm for 2 hours, and then cooled to the cooling temperature of 740-820°C, and then air-cooled at a cooling rate of 1.5-3°C / s to a final cooling temperature of 100-300°C. Low-speed cooling corresponds to low strength and hardness indicators, and the structure is mainly bainite, but the toughness and plasticity indicators will increase. Generally, the greater the cooling, the higher the strength and the higher the tendency of martensite to appear. In some embodiments, by cooling at a cooling rate of 1.5-3°C / s, while appropriately reducing the strength, the toughness and plasticity are greatly improved, and a bainite structure with excellent strength and toughness matching is obtained.

[0032] In some embodiments, in step S4, after the cooled frog steel is straightened, it is tempered at a temperature of 100 to 450°C for 5 to 120 hours to eliminate residual stress. The tempering time is mainly related to the heating medium, furnace temperature, chemical composition of the steel, structure, sample size and shape, furnace loading method and furnace loading amount, and the sample size and shape and structure have the most significant impact on the tempering time. The cross-sectional thickness of the frog steel can usually reach 116 mm. According to the commonly used empirical formula (1) for tempering:

[0033] T=a×K×D (1)

[0034] Where, T is the heating time in min; a is the heating coefficient in min / mm, which is related to the workpiece size, heating medium, and chemical composition of the steel; K is the furnace charging correction factor (take 1.5-2.0); D is the effective thickness of the part in mm.

[0035] The minimum calculated tempering time is 300 minutes, or 5 hours.

[0036] Furthermore, in the embodiments of the present invention, the frog steel exhibits a composite microstructure of bainite, martensite, and paratenite. The martensite and paratenite content is relatively low, and the tempering process is affected by the compression of the bainite structure, resulting in an extremely low transformation rate and a long tempering time. Experimental studies have shown that the longest tempering time is 120 hours, while the longest tempering time for international bainite materials is over 30 days.

[0037] In some embodiments, protective casting is performed throughout the casting process, preferably using 0.002-0.010% low sulfur control to control the nitrogen, hydrogen, and oxygen contents of the frog steel.

[0038] The following describes the specific embodiments.

[0039] The frog steels of the embodiments and comparative examples of the present invention are cast through full-process protection, preferably with a low sulfur content of 0.002-0.010%. The main chemical compositions of Examples 1 to 5, calculated in percentage by mass, are shown in Table 1, and the compositions of unavoidable impurity elements are shown in Table 2. Comparative Examples 1 to 4 have the same compositions as Examples 1 to 4.

[0040] Table 1 Chemical composition of frog steel in the embodiment and comparative example (%)

[0041]

[0042] Table 2 Impurity element composition of frog steel in Examples and Comparative Examples (%)

[0043]

[0044]

[0045] The embodiments and comparative examples used different heating processes, rolling processes and heat treatment processes, as shown in Table 3.

[0046] Table 3 Heating, rolling and heat treatment processes of the embodiments and comparative examples

[0047]

[0048] After cooling and straightening, the rails of the embodiment and the comparative example are tempered at the same temperature of 100-450° C. for 5-120 min.

[0049] In accordance with the requirements of TB / T 3467, "Alloy Steel Composite Frog," tensile testing was performed on the examples and comparative examples. Metallographic examination was also performed. The tensile and metallographic data are summarized in Table 4.

[0050] Table 4 Tensile properties and metallographic structure statistics of the rails of the embodiment and comparative example

[0051]

[0052]

[0053] Combining Tables 1 to 3, the parameters related to the composition design, heating process, and rolling process of the comparative example are outside the predetermined range. As shown in Table 4, the bainitic frog produced using this method has a tensile strength of ≥1350 MPa and a reduction of area of ​​≥45%. The ferrite and martensite contents in the metallographic structure are both higher than those of the comparative example, resulting in superior strength and toughness, making it particularly suitable for heavy-duty railway frogs.

[0054] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples. Within the spirit of the embodiments of the present invention, the technical features of the above embodiments or different embodiments may be combined, and there are many other variations of the different aspects of the embodiments of the present invention described above, which are not provided in detail for the sake of clarity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included within the scope of protection of the embodiments of the present invention.

Claims

1. A method for producing frog steel with excellent strength and toughness, characterized in that: The frog steel has the following composition: 0.24-0.30% C, 1.40-1.70% Si, 1.50-1.9% Mn, 0.002-0.015% P, 0.002-0.010% S, 0.7-1.40% Cr, 0.30-0.60% Mo, 0.30-0.70% Ni, 0.03-0.15% V, and the remainder is Fe and unavoidable impurity elements. The method comprises the following steps: S1, heating the steel billet obtained after smelting and casting to 1250~1300℃; S2, rolling or forging the heated steel billet; S3, heat treating the frog steel obtained after rolling or forging, wherein the heat treatment comprises: heating the frog steel obtained after rolling or forging again to 920° C. and holding the temperature for 2 hours, cooling the temperature to a starting cooling temperature of 740° C. to 820° C., and then cooling the temperature to a final cooling temperature of 100° C. to 300° C. by air cooling at a cooling rate of 1.5° C. to 3° C. per second; S4, tempering the cooled frog steel.

2. The method according to claim 1, characterized in that In step S1, the heating time is controlled within 200-500 min.

3. The method according to claim 1, characterized in that In step S2, the final cooling temperature of rolling is 950-1050°C, and the compression ratio is ≥4.

4. The method according to claim 1, wherein In step S4, the cooled frog steel is tempered at a temperature of 100-450° C. for 5-120 hours.

5. The method according to claim 1, wherein Calculated by mass percentage, the impurity elements include: 0.001-0.02% Sn, 0.05-0.15% Cu and 0.01-0.02% As.

6. The method according to claim 1, characterized in that The hydrogen content of the frog steel is ≤1.2ppm, the oxygen content is ≤10ppm, and the nitrogen content is 40-60ppm.

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

8. The method according to claim 7, characterized in that Protective casting is carried out using a 0.002-0.010% low sulfur control method.

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