AU2017204403B2

AU2017204403B2 - A turnout rail and preparation method thereof

Info

Publication number: AU2017204403B2
Application number: AU2017204403A
Authority: AU
Inventors: Yong Deng; Hua Guo; Zhenyu Han; Yuan Wang; Jun Yuan; Ming Zou
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2016-07-12
Filing date: 2017-06-28
Publication date: 2018-07-19
Anticipated expiration: 2037-06-28
Also published as: CN106048175A; CN106048175B; AU2017204403A1; DE102017115472A1

Abstract

Abstract The Invention relates to the field of railway, particularly to a turnout rail and preparation method thereof, wherein, the billet employed contains C: 0.2-0.3wt%, Si: 1.2-1.8wt%, Mn: 1.8-2.5wt%, Cr: 1.3-1.6wt% and Fe, of which the total 5 content of Mn and Cr is 3.2-3.7wt%. The turnout rail with excellent strength and toughness may be obtained by using the method claimed in the Invention in combination with the specific accelerated cooling process. In the preferred embodiments, the turnout rail claimed in the Invention contains a small amount of (0.01wt% below) or no such precious alloy elements as Mo, Ni, V and Nb, so 10 the preparation cost is low.

Description

A Turnout Rail and Preparation Method Thereof

Technical Field

The Invention relates to the field of railway, particularly to a turnout rail and preparation method thereof.

Background Art

The leapfrog development of railway puts forward higher requirements for the service performance of rails. The traditional pearlitic rail has been perfected after years’ development. Its service performance indicates that the pearlitic rail has excellent wear resistance but relatively poor contact fatigue resistance. Particularly, with the year-by-year increase of China’s railway traffic volume and traffic density, fatigue damage of the rail has become the principal factor affecting the service life of rail. Some studies show that applying bainite structure to the field of rail can effectively reduce the fatigue damage of the rail. Since the 1990s, China has conducted intensive studies on bainite rail and obtained fruitful research achievements. Nevertheless, compared to the pearlitic rail, the bainite rail has a significant difference in economic cost. For the common pearlitic rail, generally, high-carbon content of 0.65-0.95 wt% with appropriate Si and Mn, and little Cr, V and Nb etc. is added in the steel depending on demands. Therefore, the alloying cost is low and large-scale application may be realized. The bainite rail has a higher content of precious alloy elements particularly Mo, Ni, V and Nb etc. to obtain fined bainite structure under air cooling after rolling. The high cost limits the popularization and application of such rail with excellent performance.

Summary of the Invention

The Invention is aimed at providing a turnout rail and preparation method thereof to overcome the defects of high cost for preparing bainite turnout rail and poor performance in the prior art.

The inventor of the Invention finds in his studies that properly cooling control and strong fine-grain strengthening effect cannot only significantly increase the excellent comprehensive hardness and toughness properties, but also considerably decrease alloy elements in low-carbon steel under the condition of equivalent performance, particularly the content of such precious alloy elements as Mo, Ni, V and Nb, i.e. the action of precious alloy elements can be realized even surpassed by using waste heat after rolling of turnout rail and exact cooling control process.

Therefore, the Invention provides a method for preparing turnout rail, characterized in that, comprising the following steps: (1) hot-rolling a billet containing C: 0.2-0.3wt%, Si: 1.2-1.8wt%, Mn: 1.8-2.5wt%, Cr: 1.3-1.6wt% and Fe, and cooling it naturally, wherein, the total content of Mn and Cr is 3.2-3.7wt%; (2) applying a cooling medium respectively to the railhead tread center and both sides of the railhead when the railhead surface temperature of the said turnout rail decreases to 800 °C to make the said turnout rail undergo the first stage of accelerated cooling at a cooling speed of 10-20°C/s; (3) applying a cooling medium respectively to the railhead and rail base when the railhead tread center temperature decreases to 400-420 °C to make the said turnout rail undergo the second stage of accelerated cooling at a cooling speed of 5-8°C/s; and during the second stage of accelerated cooling, the cooling speed of working side of railhead of the said turnout rail is higher than that of non-working side of railhead by l-2°C/s; (4) stopping the second stage of accelerated cooling when the railhead tread center temperature decreases to 200-240 °C, and air-cooling the turnout rail to room temperature.

The Invention also provides a turnout rail made with the said preparation method.

The Invention is to prepare turnout rail with a high comprehensive performance by controlling the content of the alloy elements and the cooling process. The main alloy elements of the said turnout rail are C, Si, Mn and Cr. According to a preferred embodiment of the Invention, the rail does not contain any precious alloy elements such as Mo, Ni, V and Nb, and the preparation cost is low. In addition, the turnout rail prepared with the method claimed in the Invention is mainly composed of fine bainite and contains a small amount of martensite and retained austenite multi-phase structure and is suitable for being severed as heavy load railway turnout after cutting and milling by virtue of the uniform hardness distribution of full section of railhead. The said “a small amount” is relative to the amount of fine bainite in the turnout rail.

Other features and advantages of the Invention will be described in detail in the following embodiments.

Detailed Description of the Preferred Embodiments

The following is the detailed description of the embodiments of the Invention. It should be understood that the embodiments described herein are examples for explaining the Invention only, but cannot be interpreted as a limit to the Invention.

The endpoints and any values disclosed herein are not limited to the precise range or values; the range or values should be understood to include values close to these values or range. For the range of values, it can be combined with each other between the endpoint values in respective scope, the point values between the endpoint values in various ranges and single point value, and the individual point values to obtain a new range of values, the range of these values should be considered as specifically disclosed herein.

The Invention provides a method for preparing turnout rail, characterized in that, comprising the following steps: (1) hot-rolling a billet containing C: 0.2-0.3wt%, Si: 1.2-1.8wt%, Mn: 1.8-2.5wt%, Cr: 1.3-1.6wt% and Fe, and cooling it naturally, wherein, the total content of Mn and Cr is 3.2-3.7wt%; (2) applying a cooling medium respectively to the railhead tread center and both sides of the railhead when the railhead surface temperature of the said turnout rail decreases to 800 °C to make the said turnout rail undergo the first stage of accelerated cooling at a cooling speed of 10-20°C/s; (3) applying a cooling medium respectively to the railhead and rail base when the railhead tread center temperature decreases to 400-420 °C to make the said turnout rail undergo the second stage of accelerated cooling at a cooling speed of 5-8°C/s; and during the second stage of accelerated cooling, the cooling speed of working side of railhead of the said turnout rail is higher than that of non-working side of railhead by l-2°C/s; (4) stopping the second stage of accelerated cooling when the railhead tread center temperature decreases to 200-240 °C, and air-cooling the turnout rail to room temperature.

The reason why the content of the main alloy elements in the billet is limited to the above range is as follows:

Carbon (C) is the most important element for obtaining the desired hardness and overall mechanical properties. When the carbon content is less than 0.2wt%, the strengthening effect cannot be fully exerted, resulting in too low strength and hardness of the turnout rail and failure of ensuring wear resistance of the turnout rail. When the carbon content is higher than 0.3wt%, after the accelerated cooling process, the turnout rail has too high strength index but too low toughness and plasticity, not conducive to the service safety. Therefore, the carbon content is limited to 0.2-0.3wt %.

Silicon (Si), as the main addition element in steel, is usually in the form of solid solution in the ferrite and can improve the structure strength. When the content of silicon is less than 1.2wt%, it cannot effectively inhibit the precipitation of coarse and non-continuously distributed carbides in the steel, and cannot get the carbide-free bainitic steel; when the silicon content is higher than 1.8wt%, there is no significant improvement in the tissue and performance of the turnout rail but the increased probability of defects on the surface of the turnout rail. Therefore, the content of silicon is limited to 1.2-1.8wt%.

Manganese (Mn) can significantly reduce the start transition temperature of bainite structure and increase the hardness of the carbide. In particular, Mn is added as a main alloy element to the steel in the Invention without adding Mo. When the content of manganese is less than 1.8wt%, the positive effect on the bainite steel is difficult to be achieved. When the content of manganese is higher than 2.5wt%, the fatigue performance of the turnout rail is significantly reduced accompanied with the increased segregation degree of the steel. Under the cooling conditions, it is easy to form martensite structure, not conducive to the service safety of the turnout rail. Therefore, the content of manganese is limited to 1.8-2.5 wt%.

Chromium (Cr) can promote rightward movement of C curve and increase hardenability of the turnout rail, and is an important adding element to the bainite turnout rail. Under the condition that no Mo is added to the billet, chromium may reduce the bainite transition temperature and improve wear resistance of the turnout rail. When the content of chromium is lower than 1.3wt%, its function in the turnout is difficult to be realized. When the content of chromium is higher than 1.6wt%, it is beneficial for carbon in the steel to form complex carbides. Although it is good to increase the wear performance, it will decrease toughness and plasticity of the turnout rail. Therefore, the content of chromium is limited to 1.3-1.6wt%.

In order to ensure excellent serviceability for the said turnout rail claimed in Invention, the total content of Mn and Cr shall be 3.2-3.7wt%. The reason is that even accelerated cooling process is employed after rolling, it should be still ensured that the content of Mn and Cr in the turnout rail keeps in a certain range in order to ensure fine bainite structure as much as possible during continuous cooling process. When the total content of Mn and Cr is less than 3.2wt%, it is impossible to completely inhibit the precipitation of eutectoid ferrite in steel even if rapid cooling is used, and the objective of the Invention cannot be achieved. When the total content of Mn and Cr is higher than 3.7Wt%, a large amount of carbides will be precipitated on the ferrite matrix during the cooling process, and more severe segregation will occur, and the objective of the Invention cannot be achieved as well. Therefore, the total content of Mn and Cr shall be 3.2-3.7wt%.

According to the Invention, the inventor has found that when the component content of the billet is controlled within the above-mentioned composition range and the turnout rail with excellent toughness at a low cost can be obtained in combination with the cooling process in the said method claimed in the Invention, wherein, preferably (subject to the total weight of the said billet), the said billet may contain C: 0.28-0.3wt%, Si: 1.5-1.6wt%, Mn: 2.15-2.4wt%, Cr: 1.4-1.5wt%, Mn and Cr 3.6-3.7wt% totally.

According to the Invention, the billet also may contain P and S, of which the content of P may be 0.011-0.014wt%, S may be 0.002-0.004wt%. The main component in the billet is Fe, and the content of the said Fe may be 94-95wt%.

According to the Invention, the total content of the precious alloy elements in the billet is 0.01 wt% below, and the precious alloy elements include Mo, Ni, V and Nb. Preferably, the billet contains no any precious alloy element.

According to the Invention, the billet composed of the above elements can be obtained with the conventional method in the prior art, for example, by using a converter or an electric furnace to smelt the molten steel containing the above-mentioned components, through secondary refining, vacuum degassing, the steel can be continuously cast into a bloom, and the bloom can be sent to the heating furnace for heating and insulation, finally the billet claimed in the Invention can be obtained. As for the process, it is unnecessary to go into detail.

According to the Invention, there is no special limit to the hot rolling method, which may be a conventional hot rolling method in the prior art. For example, the said billet may be hot-rolled by a hole-pattern method or a universal method to obtain the turnout rail, and then the following cooling process may be conducted. There is no special limit to the conditions of hot rolling, which may be conventional ones in the prior art.

According to the Invention, after hot rolling, the head surface temperature of the turnout rail is reduced to 800 °C by means of natural cooling by using waste heat of hot rolling, and then the first stage of accelerated cooling is conducted.

In order to obtain excellent bainite turnout rail, it is necessary to obtain not only bainite structure but also fine bainite structure as much as possible (e.g., refined bainite ferrite slats) so as to ensure the excellent overall performance of the turnout rail. It has certain requirements not only for the composition of the billet, but also for strictly control of the accelerated cooling process.

According to the Invention, the cooling speed of the first stage of accelerated cooling is 10-20°C/s. By controlling the cooling speed, it is possible to rapidly pass through the ferrite-pearlite transition region during the continuous cooling process and to inhibit the precipitation of eutectoid ferrite. Preferably, the cooling speed of the first stage accelerated cooling is 12-15°C/s, further preferably 14-15°C/s.

According to the Invention, when the temperature of railhead tread center decreases to 400-420 °C, the second stage of accelerated cooling is conducted simultaneously for the railhead and the rail base of the turn rail. Preferably, in

Step (3), when the temperature of railhead tread center decreases to 415-419°C, the second stage accelerated cooling is conducted simultaneously for the railhead and the rail base of the said turnout rail. The purpose of applying the cooling medium to the railhead and the rail base of the said turnout rail is to avoid bending the rail for excessive cooling of railhead and failing to meet the flatness requirements.

According to a preferred embodiment of the Invention, the cooling speed of the second stage of accelerated cooling is 7-7.5 °C/s to obtain a rail having better overall performance.

According to the Invention, there is no special limit to the selection of the cooling medium for the first stage of the accelerated cooling and the second stage of accelerated cooling, as long as the desired effect of the Invention can be obtained. Preferably the cooling medium for the first stage of accelerated cooling is water-mist mixture, and the cooling medium for the second stage of accelerated cooling is compressed air.

According to the Invention, when the temperature of the railhead tread center decreases to 200-240 °C, preferably 220-230°C, the second stage of accelerated cooling should be stopped and the rail should be air-cooled to room temperature.

The Invention also provides a turnout rail prepared with the method described above. The said turnout rail has excellent tensile property, impact property and railhead tread hardness at normal temperature and small wear loss weight; meanwhile, it has a small amount of (0.01wt% or below) or no such precious alloy elements as Mo, Ni, V and Nb, so the preparation cost is low.

The Invention is further described in combination with embodiments as follows. In the following embodiments, "room temperature" means "25 °C".

The chemical components contained in the billet used in the following embodiments are shown in Table 1, and the ones contained in the billet used in the reference embodiments are shown in Table 2. In addition to the elements in Table 1 and 2, the allowances are Fe and unavoidable impurities:

Table 1

Table 2

Embodiments 1-6

Billets 1# to 6# in Table 1 are hot-rolled into 60ATI turnout rail at 1200°C. The 60AT1 turnout rail from billets 1# to 6# through hot rolling in Table 1 is respectively treated with accelerated cooling process numbered correspondingly in Table 3, and then air-cooled to room temperature to obtain turnout rails A1-A6.

Table 3

Reference embodiment 1-6

Billets 1# to 6# in Table 2 are hot-rolled into 60ATI turnout rail at 1200°C. The 60AT1 turnout rail is cooled directly to room temperature to obtain turnout rails D1-D6.

Test embodiment 1

The performance tests are carried out for the turnout rails A1-A6 and D1-D6 prepared in Embodiment 1-6 and Reference embodiment 1-6 according to the following method, particularly:

The tensile performance of the turnout rail is determined in accordance with, GB/T228.1-2010 "Test Methods for Tension Testing of Metallic Materials", and the results of measured Rm (tensile strength), A% (elongation) are shown in Table 4;

The impact properties and hardness of the turnout rail are determined with the method in the prior art, and the results are shown in Table 4;

The wear test is carried out on a MM200 wear tester to determine the average wear weight loss. The sample is taken from the railhead of the turnout rails A1-A6 and D1-D6. In all wear tests, the abrasive materials cut are the same and the test parameters are as follows:

Sample size: thickness 10mm, diameter 36mm, round; test load: 150kg; slip: 10%; for the sample material grinded: U75V hot rolled rail having a hardness of 280-310HB, equivalent to hardness of train wheel; rotation rate: 200r/min; total wear times: 100,000 times;

The wear weight loss is calculated as follows: Wear weight loss = weight of the sample before wear - weight of the sample after wear. The results are shown in Table 4.

Table 4

From the results of the above-described embodiments and reference embodiments, it was found that, the excellent comprehensive hardness and toughness indexes of the turnout rail have reached even exceeded that of bainite turnout rail added with such precious metal elements as Ni and Mo despite the turnout rail prepared with the method claimed in the Invention only contains appropriate conventional cheap elements like C, Si, Mn and Cr etc. by using the waste heat of hot rolling and stepped accelerated cooling after cooling. Therefore, under the premise of obtaining similar performance, the Invention can significantly reduce the content of precious alloy elements, thereby reducing the preparation cost. By virtue of the excellent comprehensive hardness and toughness, the product can be applied to heavy railway and line sections under complex conditions.

The above are the preferred embodiments rather than the limitations of specific details of the embodiments of the Invention. Any simple variations made to the Invention based on its technical spirits and principles shall be included in the protection scope of the Invention.

It is to be noted that the technical features related to the embodiments of the Invention described above can combine with each other when they do not conflict with each other. In order to avoid unnecessary repetition, any possible combination of the Invention is no longer described.

In addition, any combination of the various embodiments of the Invention may be carried out as long as it does not contravene the idea of the Invention and is likewise to be regarded as the disclosure of the Invention.

Claims

Claims

1. A method for preparing a turnout rail, characterized in that, comprising the following steps: (1) hot-rolling a billet containing C: 0.2-0.3wt%, Si: 1.2-1.8wt%, Mn: 1.8-2.5wt%, Cr: 1.3-1.6wt% and Fe, and cooling it naturally, wherein, the total content of Mn and Cr is 3.2-3.7wt%; (2) applying a cooling medium respectively to the railhead tread center and both sides of the railhead when the railhead surface temperature of the said turnout rail decreases to 800 °C to make the said turnout rail undergo the first stage of accelerated cooling at a cooling speed of 10-20°C/s; (3) applying a cooling medium respectively to the railhead and rail base when the railhead tread center temperature decreases to 400-420 °C to make the said turnout rail undergo the second stage of accelerated cooling at a cooling speed of 5-8°C/s; and during the second stage of accelerated cooling, the cooling speed of working side of railhead of the said turnout rail is higher than that of non-working side of railhead by l-2°C/s; (4) stopping the second stage of accelerated cooling when the railhead tread center temperature decreases to 200-240 °C, and air-cooling the turnout rail to room temperature.
2. A method according to Claim 1, characterized in that, the said billet contains C: 0.28-0.3wt%, Si: 1.5-1.6wt%, Mn: 2.15-2.4wt%, Cr: 1.4-1.5wt%, herein the total content of Mn and Cr is 3.6-3.7wt%.
3. A method according to Claim 1 or 2, characterized in that, the total content of precious alloy elements in the billet is 0.01wt% below, and the said precious alloy elements include Mo, Ni, V and Nb.
4. A method according to Claim 3, characterized in that, the said billet has no any precious alloy element.
5. A method according to Claim 1 or 2, characterized in that, the cooling medium in the said first stage of accelerated cooling is water-mist mixture, and that in the said second stage of accelerated cooling is compressed air.
6. A turnout rail prepared with the method according to any of Claims 1-5.