CN116695000A - A kind of ultra-fine pearlitic steel rail for heavy-duty railway and its production method - Google Patents

Abstract

The invention relates to an ultrafine pearlitic steel rail for heavy-duty railways and a production method thereof. The chemical components in the steel are calculated by weight percentage: C 0.84%-0.94%, Si 0.50%-0.70%, Mn 0.75%-0.90%, Cr 0.3≤Cr/C≤0.5, V 0.07%～0.10%, Nb 0.01%～0.03%, P≤0.015%, S≤0.010%, the balance is Fe and unavoidable impurities. The invention is mainly applied to small-radius lines of heavy-duty railways (curved lines with a radius less than 300m), and prepares an ultra-fine pearlite with a pearlite lamellar spacing of 57-75nm through alloy composition design, controlled rolling and controlled cooling technology On-line heat treatment rail, rail tread hardness ≥ 435HBW, tensile strength ≥ 1430MPa, elongation ≥ 12%, the rail of the present invention maintains good toughness while improving the strength and hardness of the rail, and improves the contact fatigue resistance of the rail.

Description

A kind of ultra-fine pearlitic steel rail for heavy-duty railway and its production method

technical field

The invention belongs to the technical field of rails, and relates to a rail material, in particular to an ultrafine pearlitic rail for heavy-duty railways and a production method thereof.

Background technique

With the development of the economy, the axle load, speed and running density of freight trains continue to increase, and the side wear of the rail and the contact fatigue damage of the tread surface are becoming more and more serious. The side wear exceeds the limit and the tread contact fatigue damages are removed in advance, which greatly increases the maintenance workload of the railway. The wear and contact fatigue resistance of the rail are related to the strength and hardness of the rail. Increasing the hardness of the rail tread is conducive to improving its contact fatigue resistance. Studies have shown that increasing the tread hardness by 60HBW can increase its wear resistance by more than one time. Ultra-fine pearlitic steel rail can obtain better plasticity while obtaining higher strength and hardness with smaller pearlite group diameter and lamellar spacing. Steel rail trends for steel rails.

At present, the ultra-fine pearlitic steel rail mainly adopts the preparation method of alloying composition design and online heat treatment process. By using the waste heat of hot-rolled steel rails, the rail head is sprayed with compressed air or water mist mixture to make the rail head Bit rapid cooling to obtain a finer microstructure, improve the hardness of the rail head tread, so as to achieve the simultaneous improvement of wear resistance and contact fatigue resistance. The notification number CN104032222A provides a method for preparing a nano-pearlite steel rail. This method prepares a full pearlite steel rail with a lamellar spacing of 60nm and a tensile strength greater than 1300MPa. However, the alloying scheme provided by this patent document is complicated. In addition to conventional Si and Mn elements, 6 alloying elements are added to increase the difficulty of molten steel refining control and composition regulation, and the cost of alloying is high. At the same time, the online heat treatment method provided by this patent document requires long-term heat preservation, which will reduce production efficiency. . The notification number CN104087836B provides a preparation method of vanadium-chromium micro-alloyed ultra-fine pearlitic steel rail. The tensile strength of vanadium-chromium micro-alloyed ultra-fine pearlitic steel rail is prepared by adopting Cr and V alloying composition design and two-stage online heat treatment process. Strength ≥ 1400MPa, elongation ≥ 10%, hardness of the rail head tread ≥ 420HB, hardness ≥ 375HB at 20mm below the rail head tread, but the lamellar spacing of the ultra-fine pearlitic rail prepared by this method is 70-100nm, which cannot give full play to the superfine pearlite rail. Fine lamellar pearlite structure improves the properties of rail materials. The notification number CN105051220B provides a preparation method for corrosion-resistant nano-pearlite steel rail containing rare earth Ce and Nb elements. Rare earth Ce and Nb elements are added to the steel, and the pearlite is refined to 62nm to 83nm by using deformation heat treatment process and controlled cooling. Body lamellar structure, the tensile strength of the rail is not less than 1230MPa, the impact energy at room temperature is not less than 68J, and the tread hardness is not less than 438HB. The method provided by this patent improves the material by utilizing the precipitation of Ce and Nb in the rolling process Recrystallization temperature, the final rolling is rolled in the non-recrystallized area to refine the austenite grains, but the material work hardening is obvious during the final rolling of this method, which increases the difficulty of rolling and reduces the service life of the rolls and rolling mills.

In the above rail preparation technologies related to ultrafine pearlite, the combination of alloying and heat treatment is used to prepare nano-scale lamellar pearlite, but there are problems such as complex alloying and difficult rolling. The controlled rolling process is added on the basis of the controlled cooling process, and an easy-to-roll production method for ultra-fine pearlitic steel rails is provided.

Contents of the invention

The purpose of the present invention is to provide a kind of ultra-fine pearlitic steel rail and production method thereof for heavy-duty railways, mainly used in small-radius lines of heavy-duty railways (curved lines with a radius less than 300m), along with the increase of heavy-duty train axle load, The rails in small-radius curved road sections face greater contact stress, and the rail head tread and rail head side are prone to damage caused by contact fatigue and reduce the service life of the rail. The invention adopts alloy composition design, controlled rolling and controlled cooling technology to prepare An ultra-fine pearlite on-line heat-treated steel rail with a pearlite lamellar spacing of 57-75nm, the rail tread hardness ≥ 435HBW, the tensile strength ≥ 1430MPa, and the elongation ≥ 12%. The steel rail of the present invention improves the strength and hardness of the steel rail Good toughness is maintained, and the average fracture toughness of the steel rail at -20°C in the present invention is not less than 35MPa·m ^1/2 , which improves the contact fatigue resistance of the steel rail.

In order to achieve the above object, the present invention adopts the following technical solutions to realize:

An ultra-fine pearlitic steel rail for heavy-duty railways, the chemical composition of the steel is calculated by weight percentage: C 0.84%-0.94%, Si 0.50%-0.70%, Mn 0.75%-0.90%, Cr 0.3≤Cr/C≤ 0.5, V 0.07%-0.10%, Nb 0.01%-0.03%, P≤0.015%, S≤0.010%, and the balance is Fe and unavoidable impurities.

Above-mentioned each element adding amount (percentage by weight) selection and its effect description are as follows:

C element is the cheapest element used to improve the strength and hardness of the matrix. For the ultra-fine pearlite steel rail of the present invention, the C content greater than 0.84% is conducive to increasing the cementite content in the pearlite, improving the strength and hardness of the steel rail. Hardness, but too high C content at the rail waist will easily form a cementite network at the prior austenite grain boundary during the controlled cooling process, reducing the strength of the rail waist, so the present invention chooses the C content to be 0.84% to 0.94%.

Si is a solid-solution strengthening element mainly dissolved in ferrite. The main function of Si in the present invention is that solid-solution strengthening elements exist in ferrite to increase the strength of the structure, but excessive Si content will reduce the strength of the material. The reduction of area will deteriorate the plasticity of the material. Therefore, the present invention chooses the Si content to be 0.50%-0.70%.

Mn can be dissolved into cementite to form (Fe, Mn)3C. Mn can improve the stability of cold austenite and reduce the temperature range of pearlite transformation, so that the steel can obtain finer sheets and higher strength when cooling after hot rolling. Higher pearlescent. Therefore, the present invention chooses the Mn content to be 0.75%-0.90%.

Both Nb and V are microalloying elements, both of which can form C compounds. The present invention makes full use of the precipitation of Nb and V under different conditions in the process of controlled rolling and controlled cooling to achieve the effect of inhibiting the growth of austenite grains and precipitation strengthening. The Nb content of 0.01% to 0.04% is mainly used to control the induced precipitation during the rolling process to hinder the growth of austenite recrystallization grains, and the V content of 0.07% to 0.1% is mainly used to control the precipitation during the cooling process Strengthening effect, and at the same time inhibit the proeutectoid cementite to form a network at the grain boundary at the rail waist.

The Cr element can improve the hardenability of the material and improve the wear resistance of the rail. Cr is beneficial to increase the equilibrium phase transition temperature of pearlite, and obtain greater undercooling to refine the pearlite lamellae. The Cr content of the present invention must be proportioned to the C content, requiring 0.3≤Cr/C≤0.5, so that the overcooling Cold austenite decomposition can only produce Cr-rich alloy cementite.

P is a harmful element of the rail, and it is very easy to segregate at the grain boundary. On the premise of ensuring steelmaking conditions, steelmaking costs, etc., the present invention requires that the P content be ≤0.015%.

S is a harmful element of the rail of the present invention. Excessively high S content is likely to form large-size A-type non-metallic inclusions, resulting in quality defects in the welding heat-affected zone. Therefore, the lower the better, the lower the better without increasing unnecessary costs. The invention requires S≤0.010%.

The tensile strength of the rail of the invention is R _m ≥ 1430MPa, the elongation A ≥ 12%, the hardness of the rail head tread ≥ 435HBW, and the average fracture toughness at -20°C is not less than 35MPa·m ^1/2 .

The structure within 25 mm below the rail head tread of the present invention is a pearlite structure with a lamellar spacing of 57-75 nm, and the pearlite content accounts for 100%; the tissue at the rail waist is a pearlite structure with a lamellar spacing of 100-130 nm and long granular The cementite structure is analyzed, the pearlite content accounts for 95% to 98%, the proeutectoid cementite accounts for 2% to 5%, and there is no abnormal structure of martensite and bainite.

The invention adopts alloy composition design, controlled rolling and controlled cooling technology to improve the strength and hardness of the steel rail to ensure the contact fatigue resistance performance of the steel rail. The production method of ultra-fine pearlitic steel rails for heavy-duty railways includes continuous casting, continuous casting slab heating, rail rolling and on-line heat treatment systems, and the specific methods include:

1) The continuous casting slab adopts 280～320mm×380～410mm large-scale continuous casting slab to ensure that the rolling compression ratio of the rail is greater than 11, and after dephosphorization by high-pressure water, it is rolled by universal method or pass method to 50～75kg/m Required cross-section rails; continuous casting slabs need to be heated in a walking heating furnace before rolling. Carrying out heat preservation is conducive to the homogenization of C elements, and uniform C content is conducive to inhibiting the network precipitation of secondary cementite at the grain boundary of the rail waist after rolling. Rough rolling BD1 billet rolling temperature is 1180～1240℃, BD1 rolling deformation is greater than 70%, average deformation rate is 1.5～3.5s ^-1 , rough rolling BD2 rolling temperature is 980～1040℃, BD2 rolling deformation is greater than 100% , the average deformation rate is 1.2～3.2s ^-1 , after BD2 rolling, it is slowly cooled in the range of 1000℃～950℃, and the time is controlled at 200～240s, so that the material can be completely recrystallized and a small amount of NbC can be precipitated to eliminate the internal work hardening of the material. The final rolling temperature of the finishing universal rolling mill is 900-930°C, multi-pass rolling is adopted in the partial recrystallization zone, the deformation of each pass is greater than 30%, the average deformation rate is 1-2s ^-1 , and NbC in the steel is completely precipitated after rolling , pinning grain boundaries inhibits the growth of recrystallized grains and refines austenite grains;

2) After the final rolling of the rail, the residual heat temperature of the rail is between 900 and 850°C. Compressed air or water mist mixture with a cooling rate of 5.0 to 9.0°C/s is applied to the rail head tread and both sides of the rail for forced cooling. When the rail head surface When the temperature drops to 700-650°C, stop the strong cooling, and use air cooling for 20-50s to cool to above the pearlite transformation point at a high cooling rate. Short-term air cooling is beneficial to the dispersion and precipitation of VC in supercooled austenite. The role of dispersion strengthening is also conducive to reducing the temperature gradient from the center to the surface of the rail. Due to the refinement of austenite grains, the incubation period of supercooled austenite to pearlite transformation is reduced. The air cooling time should not be too long. Continue to use forced cooling with a cooling rate of 2-4°C/s. When the surface temperature of the rail head drops to 510-480°C, stop cooling, and the surface of the rail will start to produce pearlite phase transformation when it drops to the range of 500-600°C. The phase transformation occurs in a temperature range, and the moderate cooling rate is beneficial to the fact that most of the pearlite phase transformation within 25mm below the rail tread occurs in a lower temperature range, forming pearlite with a lamellar spacing of 57-75nm. After stopping the strong cooling Carry out air cooling, and the surface layer of the rail returns to the temperature range of 530-500°C. When the rail is cooled to 510-480°C, due to the high cooling rate of forced cooling, there may be a small amount of supercooled austenite that has not completed the phase transformation. Using the internal temperature difference of the rail to make The temperature of the surface layer of the rail is returned to 530-500°C to ensure that the structure is completely transformed into a pearlite structure. The rail continues to be air-cooled to room temperature, and then the finished rail is obtained through straightening, flaw detection, processing and other processes.

The steel rail of the present invention adopts three-stage cooling. In the first stage, the surface temperature of the rail is lowered to above the phase transition temperature of the cooling speed in the first stage to inhibit the growth of austenite grains, and in the second stage, the internal temperature of the material is reduced by air cooling. The gradient promotes the dispersion and precipitation of VC at the same time. The third stage adopts a moderate cooling rate to complete the pearlite phase transformation of the material at a lower temperature, and obtains a smaller pearlite group size and a pearlite sheet of 59-75nm. The purpose and characteristics of the structure and performance of the invented rail, the characteristics of the composition design and the selection of controlled rolling and controlled cooling are manifested in the following two aspects:

One is to make full use of the role of Nb and V microalloying elements in the process of controlled rolling and controlled cooling. During the process of controlled rolling, NbC pinning grain boundaries are formed to hinder the growth of austenite grains, and the austenite grains after recrystallization are refined. solid crystal grains, VC interphase precipitation strengthening is formed during the controlled cooling process, and an ultra-fine pearlite structure with a lamellar spacing of 57-75nm is obtained through composition design and segmental controlled cooling process, through the alloy composition provided by the invention, rolling , cooling process, within the scope of the present invention, tensile strength (Rm) ≥ 1430MPa, elongation (A) ≥ 12%, rail head tread hardness greater than or equal to 435HBW.

The second is to control the structure of the rail waist position into fine pearlite structure and intermittent long granular secondary cementite without martensite and bainite through reasonable composition design of the rail and continuous casting slab heating system The emergence of abnormal tissue, etc., to ensure the stability of the full-section structure and performance.

Compared with prior art, the beneficial effect of the present invention is:

(1) The tensile strength (Rm) ≥ 1430MPa of the online heat-treated steel rail produced under the alloy composition, rolling process and cooling process system provided by the present invention, the elongation (A) ≥ 12%, and the rail head surface hardness (HBW) is greater than or equal to 435HBW, the average -20°C fracture toughness of the rail of the present invention is not less than 35MPa·m ^1/2 .

(2) The structure within 25 mm of the rail head tread of the present invention is a pearlite structure with a lamellar spacing of 57 to 75 nm, the pearlite content accounts for 100%, and the tissue at the rail waist is a pearlite structure with a lamellar spacing of 100 to 130 nm and Long granular proeutectoid cementite structure, pearlite content accounts for 95% to 98%, proeutectoid cementite accounts for 2% to 5%, no martensite, bainite and other abnormal structures, ensuring full section Organizational and performance stability.

(3) The present invention eliminates the work hardening of the rail in the rolling process of the universal rolling mill by controlling the rolling, reduces the rolling difficulty of the higher alloy composition design, is beneficial to prolong the service life of the rolling mill and the roll, and improves the specification accuracy of the rail and the accuracy of the rail. At the same time, NbC particles hinder the growth of grains after recrystallization after final rolling, refine the austenite grain size, and cooperate with the subsequent controlled cooling process to improve the mechanical properties of the rail.

In summary, the ultra-fine pearlite heavy-duty railway small-radius curve online heat-treated steel rail of the present invention has good ductility and plasticity, and has relatively high tread hardness and is suitable for laying heavy-duty railways.

Detailed ways

In order to make the purpose of the present invention, technical solutions and advantages clearer, the specific implementation of the present invention will be further described below in conjunction with the examples. The following examples are used to specifically illustrate the contents of the present invention, and these examples are only general examples of the contents of the present invention. description, but not limitation of the content of the present invention.

In the embodiment of the present invention, the chemical composition of the rail, the rolling process parameters, the on-line heat treatment process parameters and the corresponding mechanical properties of the rail are shown in Table 1 to Table 5, within the given composition range of the present invention and according to the given rolling of the present invention The mechanical properties of the rail prepared by the cooling process are tensile strength (Rm) ≥ 1430MPa, elongation (A) ≥ 12%, and rail head surface hardness (HBW) greater than or equal to 435HBW.

Table 1 Chemical composition of rail

Table 2 Main process parameters of rail rolling

Table 3 embodiment online heat treatment process parameter

Table 4 comparative example on-line heat treatment process parameters

Opening and cooling temperature/℃ Average cooling rate/℃/s Final cooling temperature/℃ Inversion temperature/℃ Comparative example 1 780 2.7 510 542 Comparative example 2 774 3.3 505 545 Comparative example 3 781 3.1 513 547

Table 5 Rail Mechanical Properties

The steel rail prepared in the embodiment and the comparative example is carried out tensile test according to the TB/T2344-2020 standard, the tread hardness sample is sampled, the fracture toughness sample is sampled, and the specified pearlite lamellar inspection is carried out; the steel rail wear test of the present invention adopts thickness 10mm, A hollow circular sample with a diameter of 36 mm is tested under a dry friction environment with a test load of 150 kg and a slip of 10%.

Experiment is by using the slab of the composition of embodiment 1-3 as comparative example experiment raw material composition, and respectively corresponding comparative example 1-3, embodiment adopts the control rolling and control cooling process that the present invention provides to produce, comparative example adopts Existing rolling and on-line heat treatment technology for production. As can be seen from Table 4, adopting the method in the present invention, the strength and hardness of the standard position of the rail are improved while having good toughness; the spacing of the pearlite sheets at 25 mm below the rail head tread increases, and the strength and hardness decrease slightly. It can be seen from experiments The steel rail produced by the method provided by the invention has smaller interlaminar spacing, higher wear resistance and good toughness.

In summary, the present invention mentions a kind of ultra-fine pearlitic steel rail for heavy-duty railway and its production method. Under the same composition, the rolling process and waste heat online heat treatment process provided by the present invention can effectively improve the The strength and toughness index of the rail improves the anti-wear performance of the rail, and is suitable for small-radius curves with a radius of less than 300m for heavy-duty railways.

Claims (5)

1. The ultra-fine pearlite steel rail for heavy-duty railway is characterized in that the steel comprises the following chemical components in percentage by weight: 0.84 to 0.94 percent of C, 0.50 to 0.70 percent of Si, 0.75 to 0.90 percent of Mn, 0.3 to 0.5 percent of Cr/C, 0.07 to 0.10 percent of V, 0.01 to 0.03 percent of Nb, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, and the balance of Fe and unavoidable impurities.

2. The ultra-fine pearlite rail for heavy haul railway as set forth in claim 1, wherein rail tensile strength R _m More than or equal to 1430MPa, elongation A more than or equal to 12%, tread hardness of the rail head more than or equal to 435HBW, and fracture toughness average value at minus 20 ℃ not lower than 35 MPa.m ^1/2 。

3. The ultra-fine pearlite rail for heavy haul railway according to claim 1, characterized in that the internal structure 25mm below the rail head tread is a pearlite structure with a lamellar spacing of 57-75 nm, the pearlite content accounting for 100%; the rail web tissue is a pearlite tissue with a lamellar spacing of 100-130 nm and a long granular proeutectoid cementite tissue, the pearlite content is 95-98%, and the proeutectoid cementite content is 2-5%.

4. A method of producing a superfine pearlite rail for heavy haul railways as set forth in any one of claims 1-3, comprising:

1) The rolling compression ratio of the steel rail is larger than 11, the soaking temperature of the continuous casting billet is 1250-1310 ℃ before rolling, the heat preservation time is 2.5-3.5 hours, the cogging rolling temperature of the rough rolling BD1 is 1180-1240 ℃, the rolling deformation of the BD1 is larger than 70%, and the average deformation rate is 1.5-3.5 s ^-1 The method comprises the steps of carrying out a first treatment on the surface of the The rolling temperature of rough rolling BD2 is 980-1040 ℃, the actual rolling deformation of BD2 is more than 100%, and the average deformation rate is 1.2-3.2 s ^-1 After BD2 is rolled, slowly cooling is carried out at the temperature of 1000-950 ℃ for 200-240 s; the finish rolling temperature is 900-930 ℃, the rolling pass deformation is more than 30%, and the average deformation rate is 1-2 s ^-1 ；

2) The temperature of the steel rail after finishing rolling is 900-850 ℃, forced cooling with the cooling speed of 5.0-9.0 ℃/s is applied to the tread surface and two sides of the steel rail, when the surface temperature of the rail head is reduced to 700-650 ℃, forced cooling is stopped, air cooling is adopted for 20-50 s, cooling is carried out to above the pearlite transformation point, then forced cooling with the cooling speed of 2-4 ℃/s is continuously adopted, and when the surface temperature of the rail head is reduced to 510-480 ℃, cooling is stopped, and air cooling is carried out.

5. The method for producing ultra-fine pearlite rail for heavy haul railway according to claim 4, wherein the continuous casting slab is a large-size section continuous casting slab of (280-320 mm) × (380-410 mm), and the rail with the required section is rolled into the rail with the required section of 50-75 kg/m by a universal method or a grooved method after dephosphorization.