Production method of vanadium microalloyed bainite frog steel

Abstract

The present invention discloses a method for producing vanadium microalloyed bainitic frog steel, comprising 0.03-0.15% vanadium and 40-120 ppm nitrogen by mass. The method comprises the following steps: S1, heating a steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet to obtain a frog; S3, cooling the frog obtained after rolling or forging, wherein the initial cooling temperature is 740-820°C and the final cooling temperature is 200-400°C; and S4, tempering the cooled frog. The present invention can improve the performance of bainitic frogs through vanadium-nitrogen microalloying.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a method for producing vanadium microalloyed bainite frog steel. Background Art

[0002] A frog is a horizontal crossing device used in railway transportation to divert trains from one track to the other. Typically, a frog consists of a center rail, wing rails, and couplings. Based on their planar configuration, they are categorized as either straight frogs or curved frogs; and based on their structural type, they are classified as fixed frogs or movable frogs. Currently, frogs are commonly made of high-manganese steel cast or bainite forged steel. Bainite frog steel, due to its high strength, excellent toughness, and hardness, exhibits excellent contact fatigue and wear resistance, making it an ideal material for new railway frogs.

[0003] The development of the railway industry places increasingly higher demands on frog performance, and it is necessary to further improve the performance of bainite frogs. Summary of the Invention

[0004] The main purpose of the present invention is to provide a method for producing vanadium microalloyed bainite frog steel to improve the mechanical properties of the bainite frog.

[0005] According to one aspect of the present invention, a method for producing vanadium microalloyed bainitic frog steel is provided, wherein the frog steel contains 0.03-0.15% V and 40-120 ppm N by mass, the method comprising the following steps:

[0006] S1, heating the steel billet obtained after smelting and casting;

[0007] S2, rolling or forging the heated steel billet to obtain a frog;

[0008] S3, cooling the frog obtained after rolling or forging, wherein the initial cooling temperature is 740-820°C and the final cooling temperature is 200-400°C;

[0009] S4, tempering the cooled frog.

[0010] According to one embodiment of the present invention, the composition of the frog steel, calculated by mass percentage, is: 0.24-0.35% C, 1.20-2.1% Si, 1.50-2.50% 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, 0.03-0.15% V, 0.001-0.004% Al, and the remainder is Fe and unavoidable impurity elements.

[0011] According to one embodiment of the present invention, in step S1, heating is performed at 1250-1300° C. for 200-500 minutes.

[0012] According to one embodiment of the present invention, in step S3, the cooling rate is 5-15°C / s.

[0013] According to one embodiment of the present invention, in step S4, the tempering temperature is 200-450° C., and the tempering time is 5-12 hours.

[0014] According to one embodiment of the present invention, the tensile strength of the frog steel is greater than or equal to 1350 MPa, the elongation is greater than or equal to 12%, and the room temperature impact resistance is greater than or equal to 70J.

[0015] According to one embodiment of the present invention, protective casting is performed throughout the entire casting process.

[0016] In the method for producing vanadium microalloyed bainite frog steel according to an embodiment of the present invention, a composition design of 0.03-0.15% V and 40-120 ppm N is adopted, thereby improving bainite frog performance through vanadium-nitrogen microalloying. 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 vanadium microalloyed bainite frog steel 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] Vanadium has a precipitation strengthening effect in bainitic frog steel. In bainitic frog steel, vanadium significantly enhances strain rate sensitivity, and tensile strength increases with increasing strain rate. Furthermore, vanadium is a strong carbonitride-forming element, which can strongly inhibit austenite grain growth and refine ferrite grains. Vanadium can shrink the austenite phase region, increase the temperature of the phase transformation points Ac1 and Ac3, and increase the temperature at which supercooled austenite transforms to pearlite and ferrite, thereby expanding the bainite transformation region. Based on this understanding, the present application proposes the method described below to improve bainitic frog performance through vanadium-nitrogen microalloying.

[0021] Figure 1 A flow chart showing a method for producing a vanadium microalloyed bainite frog steel according to an embodiment of the present invention is provided, wherein the frog steel contains 0.03-0.15% V and 40-120 ppm N by mass. The method comprises the following steps:

[0022] S1, heating the steel billet obtained after smelting and casting;

[0023] S2, rolling or forging the heated steel billet to obtain a frog;

[0024] S3, cooling the frog obtained after rolling or forging, wherein the initial cooling temperature is 740-820°C and the final cooling temperature is 200-400°C; the heat treatment in step S3 can be performed using residual heat from rolling or forging, that is, using the residual heat to start cooling from the initial cooling temperature, without first cooling and then heating to the initial cooling temperature;

[0025] S4, tempering the cooled frog.

[0026] In the production method of microalloyed bainitic frog steel according to an embodiment of the present invention, a composition design with 0.03-0.15% vanadium and 40-120 ppm nitrogen is employed. This facilitates improving bainitic frog performance through vanadium-nitrogen microalloying, making the frog steel suitable for heavy-haul railway frog applications. The final cooling temperature is set between 200°C and 400°C to ensure bainite precipitation. When the carbon content in the steel is high, the bainite content in the metallographic structure of the frog steel can reach as high as 90%-95%. When the carbon content is low, the bainite content in the metallographic structure of the frog steel can be, for example, 20%-30%.

[0027] For medium-carbon steel, the initial cooling temperature during quenching is typically 30 to 50 degrees Celsius above Ac3 (727 to 912°C). In the embodiments of the present invention, the inventors have determined through process exploration that the optimal initial cooling temperature is 740 to 820°C. If the initial cooling temperature is too low, the austenite will slowly transform, the degree of undercooling will decrease, and the strength and toughness will be low. If the initial cooling temperature is too high, the microstructure will be coarse, and the mechanical properties after quenching will be reduced.

[0028] Heating the steel billet in step S1 is beneficial to improving the strength of the steel. In some embodiments, the steel billet may be heated at 1250-1300° C. for 200-500 min.

[0029] In some embodiments, the cooling rate in step S3 may be 5-15° C. / s.

[0030] Tempering in step S4 is beneficial to ensuring the toughness of the steel. In some embodiments, the tempering temperature is 200-450° C. and the tempering time is 5-12 hours.

[0031] In some embodiments, the entire casting process is performed by protective casting, and low sulfur control is adopted to control the sulfur content in the steel to 0.002-0.010 wt%.

[0032] In some embodiments, the frog steel has the following composition, by mass percentage: 0.24-0.35% C, 1.20-2.1% Si, 1.50-2.50% 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, 0.03-0.15% V, 0.001-0.004% Al, and the remainder is Fe and unavoidable impurity elements.

[0033] In an embodiment of the present invention, the frog steel produced according to the above method has a tensile strength greater than or equal to 1350 MPa, an elongation greater than or equal to 12%, and a room temperature impact resistance greater than or equal to 70J.

[0034] The following describes the specific embodiments.

[0035] The frogs of the embodiment of the present invention and the comparative example are cast through full-process protection, preferably using 0.002-0.010% low sulfur control, and their chemical compositions are shown in Table 1.

[0036] Table 1 Chemical composition of frogs in Examples and Comparative Examples (%)

[0037]

[0038]

[0039] Both the examples and comparative examples were heated at 1250-1300°C for 200-500 minutes. After rolling or forging, the residual heat was used for heat treatment, with the initial cooling temperature ranging from 740-820°C, a cooling rate of 5-15°C / s, and a final cooling temperature of 200-400°C. After cooling and straightening, the frogs were tempered at 200-450°C for 5-12 hours.

[0040] In accordance with the requirements of TB / T 2344, "Technical Conditions for Ordering 43kg / m to 75kg / m Rails," tensile specimens were processed and inspected according to the tensile specimen sampling locations in the Examples and Comparative Examples. Room-temperature impact tests were also conducted. The tensile and impact test data are summarized in Table 2.

[0041] Table 2 Tensile and impact properties of frogs in Examples and Comparative Examples

[0042]

[0043]

[0044] Combining Tables 1 and 2, the V content and / or N content of the comparative example is not within the preset range, resulting in the strength, elongation, end face shrinkage and room temperature impact of the produced frog being generally smaller than those of the embodiment.

[0045] In general, the production method of the present invention improves the strength and toughness of the bainite frog, while increasing the depth of the hardened layer and improving the safety of rail operation.

[0046] 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 vanadium microalloyed bainitic frog steel, characterized in that: The following steps are involved: S1, heating the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet to obtain a frog; S3, cooling the frog obtained after rolling or forging, wherein the initial cooling temperature is 740-820°C and the final cooling temperature is 200-400°C; S4, tempering the cooled frog; The frog steel has the following composition, by mass percentage: 0.28-0.35% C, 1.20-2.1% Si, 1.90-2.50% 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, 0.12-0.15% V, 0.001-0.004% Al, 100-120 ppm N, and the remainder being Fe and unavoidable impurity elements. In step S1, heating is performed at 1250-1300° C. for 200-500 min; In step S3, the cooling rate is 5-15°C / s.

2. The method according to claim 1, characterized in that In step S4, the tempering temperature is 200-450° C., and the tempering time is 5-12 hours.

3. The method according to claim 1, characterized in that The tensile strength of the frog steel is greater than or equal to 1350 MPa, the elongation is greater than or equal to 12%, and the room temperature impact is greater than or equal to 70J.

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

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