Process for producing a deep hardening rail steel

Abstract

The application discloses a production method of deep-hardening frog steel, the frog steel contains 0.20-0.35% of C, 0.80-1.70% of Cr and 0.002-0.006% of B in percentage of mass, and the method comprises the following steps: S1, heating and keeping the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet, the final cooling temperature of rolling or forging is 700-950 DEG C, and the compression ratio is greater than or equal to 4; S3, heat treating the frog steel obtained after rolling or forging, the heat treatment comprises: cooling by using waste heat, the open cooling temperature is 740-820 DEG C, the cooling rate is 5-15 DEG C / s, and the final cooling temperature is 100-400 DEG C; and S4, tempering the frog steel after cooling. The method can obtain the deep-hardening frog steel by controlling the composition of bainite frog and reducing the final cooling temperature of rolling or forging.

Description

TECHNICAL FIELD

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

[0002] As a key component for guiding the turning of vehicles and bearing the load of trains, frogs serve in the most severe railway lines, bear the most concentrated stress, suffer the most prominent damage, and have the shortest service life. Therefore, a high-strength, high-toughness, high-wear-resistance, and excellent contact fatigue resistance rail steel is urgently needed. Among them, the total weight of the bainite frog can reach 400 million tons, and the average service life is 3-5 times longer than that of high manganese steel frog. The bainite frog has become the first choice for frog materials at home and abroad due to its excellent performance. At present, the annual demand for bainite frogs is about 2000-3000 tons, and the economic benefit is 25-55 million yuan per year.

[0003] However, the existing bainite steel frog has cracks, block shedding and other damages during service, and the process quality and performance stability are poor, which seriously restricts the large-scale use of bainite frogs. The development of the railway industry puts forward higher and higher requirements for the performance of frogs, and it is necessary to further improve the mechanical properties of bainite frogs. SUMMARY

[0004] The main purpose of the present application is to provide a production method of deep-hardening frog steel, which obtains deep-hardening frog steel by controlling the composition of bainite frog and reducing the final cooling temperature of rolling or forging.

[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 deep-hardening frog steel is provided, the frog steel contains 0.20-0.35% of C, 0.80-1.70% of Cr, and 0.002-0.006% of B by mass percentage, and the method comprises the following steps: S1, heating and holding the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet, the final cooling temperature of rolling or forging is 700-950℃, and the compression ratio is ≥4; S3, heat treating the frog steel obtained after rolling or forging, the heat treatment comprises: cooling by using waste heat, the open cooling temperature is 740-820℃, the cooling rate is 5-15℃ / s, and the final cooling temperature is 100-400℃; and S4, tempering the cooled frog steel.

[0007] According to one embodiment of the present application, the frog steel has the following components in percentage by mass: 0.20-0.35% of C, 1.40-1.80% of Si, 1.50-2.50% of Mn, 0.002-0.020% of P, 0.002-0.020% of S, 0.80-1.70% of Cr, 0.10-0.60% of Mo, 0.01-0.15% of V, 0.002-0.006% of B, 0.001-0.004% of Al, and the rest of Fe and inevitable impurity elements.

[0008] According to one embodiment of the present application, in step S1, the billet soaking section temperature is 1250-1300℃, and the holding time is 200-500min.

[0009] According to one embodiment of the present application, in step S4, the cooled frog steel is tempered at a temperature of 100-450℃ for 5-120h.

[0010] According to one embodiment 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 in percentage by mass.

[0011] According to one embodiment of the present application, the frog steel has a hydrogen content of ≤1.2ppm, an oxygen content of ≤10ppm, and a nitrogen content of ≤60ppm.

[0012] According to one embodiment of the present application, the whole casting process is conducted by protective casting.

[0013] According to one embodiment of the present application, the protective casting is conducted by low-sulfur control of 0.002-0.010%.

[0014] According to one embodiment of the present application, the frog steel has a tensile strength of ≥1350MPa, an elongation of ≥12%, and a hardness of 30mm below the surface of ≥40HRC.

[0015] According to one embodiment of the present application, the frog steel has the following microstructure in percentage by volume: 5-25% of ferrite, 65-75% of bainite, and a small amount of residual austenite and martensite.

[0016] In the production method of the deep-hardening frog steel according to the embodiment of the present application, by reasonably designing the components and the heating, rolling and heat treatment parameters, the ferrite / bainite composite structure is controlled to obtain the deep-hardening frog steel. The bainite frog steel produced by the method has a tensile strength ≥ 1350 MPa, an elongation ≥ 12%, a hardness of 30 mm below the surface layer ≥ 40 HRC, and contains 5-25% of ferrite, 65-75% of bainite and a small amount of residual austenite and martensite in the structure. It is particularly suitable for heavy-load railway frog materials. 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 following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0018] Figure 1 A flow chart of the production method of the deep-hardening frog steel according to the embodiment of the present application is shown. DETAILED DESCRIPTION

[0019] In order to make the objects, technical solutions and advantages of the present application more clear, the following will further describe the embodiments of the present application in combination with specific embodiments and with reference to the drawings.

[0020] Figure 1 A flow chart of the production method of the deep-hardening frog steel according to the embodiment of the present application is shown, wherein the frog steel contains 0.20-0.35% of C, 0.80-1.70% of Cr and 0.002-0.006% of B in terms of mass percentage, as shown in Figure 1 The method generally includes the following steps:

[0021] S1, heating and holding the steel billet obtained after smelting and casting;

[0022] S2, rolling or forging the heated steel billet, the final cooling temperature of rolling or forging is 700-950℃, and the compression ratio is ≥ 4;

[0023] S3, heat treating the frog steel obtained after rolling or forging, the heat treatment includes: cooling by using the residual heat, the open cooling temperature is 740-820℃, the cooling rate is 5-15℃ / s, and the final cooling temperature is 100-400℃;

[0024] S4, tempering the cooled frog steel.

[0025] In the production method of the deep-hardening frog steel according to the embodiments of the present application, the carbide-free bainite or B / M complex phase structure is obtained under specific cooling conditions by adopting 0.80-1.70% Cr, and the hardenability of the steel is improved by using 0.002-0.006% B to improve the uniformity of the structure, so that the high-strength, deep-hardening frog steel is obtained, which is suitable for use as frog material for heavy-load railway.

[0026] In some embodiments, the components of the frog steel can be, in terms of mass percentage: 0.20-0.35% C, 1.40-1.80% Si, 1.50-2.50% Mn, 0.002-0.020% P, 0.002-0.020% S, 0.80-1.70% Cr, 0.10-0.60% Mo, 0.01-0.15% V, 0.002-0.006% B, 0.001-0.004% Al, and the rest is Fe and inevitable impurity elements. In some embodiments of the present application, the impurity elements include: 0.001-0.02% Sn, 0.05-0.15% Cu, and 0.01-0.02% As, and the hydrogen content is preferably controlled to be ≤1.2 ppm, the oxygen content is ≤10 ppm, and the nitrogen content is ≤60 ppm.

[0027] In some embodiments, step S1 can adopt the following parameters: the billet soaking temperature is 1250-1300°C, and the holding time is 200-500 min. By increasing the heating temperature and prolonging the heating time, in combination with the lower final cooling temperature in step S2, the original grain size can be increased, and the product hardening layer depth can be increased. Increasing the heating temperature or prolonging the heating time can increase the austenite grain size of the steel. The increase of the austenite grain size increases the hardenability and the hardening layer depth. Reducing the final cooling temperature can also increase the martensite content, thereby increasing the core hardness.

[0028] In step S3, the frog steel is subjected to online heat treatment by preheating rolling or forging. On the one hand, the bainite steel rail produced by relying on the strong fine-grain strengthening effect of online heat treatment has a more uniform and refined structure and can effectively inhibit the formation of residual austenite; on the other hand, by accurately controlling the online heat treatment temperature, the proportion of martensite can be maximally reduced, and the strength and toughness and plasticity indexes can be greatly improved at the same time, so that the bainite steel rail can fully play the excellent fatigue performance while the wear resistance reaches the level of the existing pearlitic heat-treated steel rail.

[0029] In some embodiments, in step S4, after the straightening process, the cooled frog steel is tempered at a temperature of 100-450 DEG C for 5-120 hours to eliminate residual stress and ensure the toughness of the steel. The tempering time is mainly related to the heating medium, furnace temperature, chemical composition of the 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 frog steel can usually have a thickness of 116 mm. According to the commonly used empirical formula (1) for tempering:

[0030] T=a x K x D (1)

[0031] In the formula, T is the heating time in minutes; 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 charging correction coefficient (1.5-2.0); and D is the effective thickness of the part in mm.

[0032] The calculated tempering time is at least 300 minutes, i.e. 5 hours.

[0033] In the embodiments of the present application, the frog steel has a microstructure of bainite + martensite + residual austenite, and the content of martensite and residual austenite is small. During the tempering process, the transformation rate is extremely low due to the extrusion of the bainite structure, and a long tempering time is required. Experimental studies show that the longest tempering time is 120 hours, and the longest time for international bainite materials is more than 30 days.

[0034] In some embodiments, the entire casting process is protected casting, and low-sulfur control of 0.002-0.010% is preferably used to control the nitrogen, hydrogen and oxygen contents of the frog steel.

[0035] The following will be described according to specific embodiments.

[0036] The frog steel of the embodiments and the comparative examples is subjected to full-process protected casting, and low-sulfur control of 0.002-0.010% is preferably used, wherein the main chemical components of Examples 1-5 are shown in Table 1, and the inevitable impurity element components are shown in Table 2. The components of Comparative Examples 1-4 are the same as those of Examples 1-4.

[0037] Table 1 Chemical components of frog steel of the embodiments and the comparative examples / %

[0038]

[0039] Table 2 Impurity element components of frog steel of the embodiments and the comparative examples / %

[0040]

[0041] When the examples and comparative examples adopt different heating processes, rolling or process and heat treatment processes, as shown in Table 3.

[0042] Table 3 Heating, rolling and heat treatment processes of examples and comparative examples

[0043]

[0044] After the cooling and straightening of the steel rails of the examples and comparative examples, the same tempering at 100-450℃ is adopted, and the tempering time is 5-120h.

[0045] The tensile samples of the examples and comparative examples are processed and inspected according to the requirements of TB / T 2344 “Technical Conditions for Ordering 43kg / m-75kg / m Steel Rails”, and the tensile and metallographic data statistics are shown in Table 4.

[0046] Table 4 Tensile properties of steel rails of examples and comparative examples

[0047]

[0048] In combination with Tables 1-3, the component design, heating process and rolling process related parameters of the comparative examples are not within the predetermined range. As shown in Table 4, the tensile strength of the bainite frog produced by the method is ≥1350MPa, the elongation is ≥12%, the hardness of the 30mm below the surface is ≥40HRC, and the microstructure contains 5-25% ferrite, 65-75% bainite and a small amount of residual austenite and martensite. It is particularly suitable for heavy load railway frog materials.

[0049] It should be understood by those skilled in the art that the above discussion of any of the embodiments is merely exemplary and is not intended to suggest that the scope of the embodiments disclosed herein (including the claims) is limited to these examples; nor do the embodiments underlying the present embodiments suggest that the embodiments between the technical features of the above examples or different embodiments can not be combined, and there are many other changes of the 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 principles of the embodiments should be included in the scope of protection of the embodiments.

Claims

1. A method for producing deep hardened frog steel, characterized in that: The composition of the frog steel is as follows, calculated by mass percentage: 0.35% C, 1.80% Si, 2.50% Mn, 0.015% P, 0.004% S, 1.70% Cr, 0.60% Mo, 0.15% V, 0.006% B, 0.7% Ni, 0.003% Al, and the remainder being Fe and unavoidable impurity elements. The method comprises the following steps: S1, heating and keeping the steel billet obtained after smelting and casting, wherein the temperature of the steel billet soaking section is 1250~1300℃, and the holding time is 200~500min; S2, rolling or forging the heated steel billet, with the final cooling temperature of rolling or forging being 700~950℃ and the compression ratio being ≥4; S3, heat treating the frog steel obtained after rolling or forging, wherein the heat treatment comprises: cooling the steel using residual heat to obtain a carbide-free bainite or bainite / martensite multiphase structure, with an initial cooling temperature of 740-820° C., a cooling rate of 5-15° C. / s, and a final cooling temperature of 100-400° C.; S4, tempering the cooled frog steel.

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

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

4. The method according to claim 1, wherein The hydrogen content of the frog steel is ≤1.2ppm, the oxygen content is ≤10ppm, and the nitrogen content is ≤60ppm.

5. The method according to claim 1, wherein Protective casting is carried out throughout the casting process.

6. The method according to claim 5, characterized in that Protective casting is carried out using a low sulfur control method of 0.002~0.010%.

7. The method according to claim 1, characterized in that The tensile strength of the frog steel is ≥1350 MPa, the elongation is ≥12%, and the hardness 30 mm below the surface is ≥40 HRC.

8. The method according to claim 1, characterized in that Calculated by volume percentage, the structure of the frog steel contains 5-25% ferrite, 65-75% bainite and a small amount of retained austenite and martensite.

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