Patent Application
20160194730
This application claims priority to Chinese Application No. 201510006025.7, filed on Jan. 7, 2015, entitled “High-Impact-Toughness Steel Rail and Production Method Thereof”, which is specifically and entirely incorporated by reference.
The invention relates to a high-impact-toughness steel rail and a production method of the high-impact-toughness steel rail, and belongs to the field of steel rail material production.
As the railway transportation industry is developed rapidly, a high-capacity, high-axle load, and high-intensity railway transportation pattern has been established preliminarily. Under increasingly harsh railway line conditions, the problem of damage of railway tracks and steel rails becomes prominent increasingly. Steel rails not only are an important facility for railroad connections and crossings, but also are a critical link that has influences on railway line operation efficiency and safety. Steel rails bear dynamic loads transferred from the train wheels during the operation of railway lines, and have an increased trend of fracture and damage under the action of long-time alternate stress. Especially, at low temperatures, steel rails may have brittle ruptures and damages more easily as the brittleness of the steel rail material increases. In recent years, the railway construction cause has been developed on a large scale in cold regions in China; for example, in the Qinghai-Tibet Railway Project, the steel rails are subject to a minimum environmental temperature as low as −45° C. Therefore, steel rails must have enough toughness and ductility, besides increased strength, especially in high-altitude and extremely cold regions. Hence, there is a higher requirement for the toughness of steel rails. However, the requirement for production of such steel rails can't be met with the existing methods. There is an urgent need for high-impact-toughness steel rails.
A technical problem solved in the present invention is to provide a high-impact-toughness steel rail.
The high-impact-toughness steel rail provided in the present invention is a pearlite steel rail, in which the inter-lamellar spacing is 0.05˜0.09 μm, and ballistic work at normal temperature is 30˜35 J; the chemical components of the steel rail in weight percentage are: C: 0.71-0.82 wt %, Si: 0.25-0.45 wt %, Mn: 0.75-1.05 wt %, V: 0.03-0.15 wt %, P: ≦0.030 wt %, S: ≦0.035 wt %, Al: ≦0.040 wt %, and Fe and inevitable impurities of the remaining content.
Preferably, the mechanical properties of the steel rail are: Rp0.2: 800˜860 MPa, Rm: 1,300˜1,350 MPa, A: 13˜15%, and Z: 31˜35%.
As a preferred embodiment, the chemical components of the steel rail in weight percentage are: C: 0.7˜10.82 wt %, Si: 0.25˜0.45 wt %, Mn: 0.75˜1.05 wt %, V: 0.03˜0.15 wt %, P: ≦0.030 wt %, S: ≦0.035 wt %, Al: ≦0.020 wt %, and Fe and inevitable impurities of the remaining content.
As a preferred embodiment, the chemical components of the steel rail in weight percentage are: C: 0.72˜0.76 wt %, Si: 0.35˜0.37 wt %, Mn: 0.95˜0.99 wt %, V: 0.05˜0.09 wt %, P: ≦0.012 wt %, S: ≦0.011 wt %, Al: ≦0.04 wt %, and Fe and inevitable impurities of the remaining content.
Another aspect of the present invention is to provide a method for producing the high-impact-toughness steel rail disclosed in the present invention. The method comprises steelmaking, casting, rolling, and post-rolling heat treatment, wherein, the post-rolling heat treatment comprises the following steps:
a. accelerated cooling: applying a cooling medium to the rail head tread, two sides of rail head, and central part of rail base of the rolled steel rail for accelerated cooling at 1.0˜5.0° C./s cooling rate, wherein, the temperature at the central part of rail head tread, two sides of rail head, and central part of rail base of the rolled steel rail is 650˜900° C.;
b. air cooling: stopping the accelerated cooling when the temperature at the rail head tread drops to 400˜550° C., and cooling the steel rail by air cooling to room temperature, to obtain a pearlite steel rail with 0.05˜0.09 μm of inter-lamellar spacing.
Preferably, in the steelmaking procedure, low-sulfur melted iron is charged into a steelmaking furnace while adding a high-alkalinity refining slag, and blind coal and a low-nitrogen alloy are used as a carburant for steelmaking.
Preferably, the steelmaking procedure comprises smelting in a convertor or electric furnace, refining in a LF furnace, and RH or VD vacuum treatment, wherein, a foaming agent is used in a heating process during the refining in the LF furnace.
Preferably, the casting procedure is an overall-protection casting, and the steel billet is subjected to a slow cooling after the casting.
Preferably, after slow cooling, the steel billet is heated up for austenization before rolling, and the tapping temperature after the heating process is 1,000° C.
Preferably, the cooling medium is compressed air or a water mist-air mixture.
The U-type impact toughness of rail head of the steel rail manufactured with the method disclosed in the present invention can be 30 J or more, and the tensile strength is about 1,300 MPa or higher. The steel rail has good strength and toughness matching, and high rolling contact fatigue property and wear resistance property in the process of use, and is suitable for use as steel rails for railways in high-altitude and extremely cold regions.
The high-impact-toughness steel rail provided in the present invention is a pearlite steel rail, with 0.05˜0.09 μm of inter-lamellar spacing and 30˜35 J of ballistic work at normal temperature; the chemical components of the steel rail in weight percentage are: C: 0.71-0.82 wt %, Si: 0.25-0.45 wt %, Mn: 0.75-1.05 wt %, V: 0.03-0.15 wt %, P: ≦0.030 wt %, S: ≦0.035 wt %, Al: ≦0.020 wt %, and Fe and inevitable impurities of the remaining content.
Preferably, the mechanical properties of the steel rail are: Rp0.2: 800˜860 MPa, Rm: 1,300˜1,350 MPa, A: 13˜15%, and Z: 31˜35%.
As a preferred embodiment, the chemical components of the steel rail in weight percentage are: C: 0.71˜0.82 wt %, Si: 0.25˜0.45 wt %, Mn: 0.75˜1.05 wt %, V: 0.03˜0.15 wt %, P: ≦0.030 wt %, S: ≦0.035 wt %, Al: ≦0.040 wt %, and Fe and inevitable impurities of the remaining content.
As a preferred embodiment, the chemical components of the steel rail in weight percentage are: C: 0.72˜0.76 wt %, Si: 0.35˜0.37 wt %, Mn: 0.95˜0.99 wt %, V: 0.05˜0.09 wt %, P: ≦0.012 wt %, S: ≦0.011 wt %, Al: ≦0.04 wt %, and Fe and inevitable impurities of the remaining content.
The method for producing the high-impact-toughness steel rail disclosed in the present invention comprises steelmaking, casting, rolling, and post-rolling heat treatment, wherein, the post-rolling heat treatment comprises the following steps:
a. accelerated cooling: applying a cooling medium to the rail head tread, two sides of rail head, and central part of rail base of the rolled steel rail for accelerated cooling at 1.0˜5.0° C./s cooling rate, wherein, the temperature at the central part of rail head tread, two sides of rail head, and central part of rail base of the rolled steel rail is 650˜900° C.;
b. air cooling: stopping the accelerated cooling when the temperature at the rail head tread drops to 400˜550° C., and cooling the steel rail by air cooling to room temperature, to obtain a pearlite steel rail with 0.05˜0.09 μm of inter-lamellar spacing.
The steel rail shall be cooled by natural cooling to 650˜900° C. before accelerated cooling, if the temperature at the central part of rail head tread, two sides of rail head, and central part of rail base of the steel rail is higher than 900° C. after finish rolling. Hereunder the reason for setting the initial accelerated cooling temperature to 650-900° C. will be explained: If the temperature is higher than 900° C., under the shock chilling action of the cooling medium, the temperature of the surface layer of the steel rail will drop very quickly. If the temperature is lower than 650° C., since the temperature is close to the phase transformation temperature, the risk of occurrence of abnormal structures, such as bainite and martensite, etc., in the surface layer of the steel rail and within a certain range of depth under the surface layer will significantly increases; consequently, the steel rail has to be discarded owing to the occurrence of such abnormal structures, resulting in severe loss. Therefore, the initial accelerated cooling temperature is limited within the range of 650˜900° C.
In the accelerated cooling process, the cooling rate at the rail head tread, two sides of the rail head, and central part of the rail base is set to 1.0˜5.0° C./s, because: If the cooling rate is ≦1.0° C./s, in the initial stage of cooling, the temperature in the surface layer of the steel rail will drop significantly; after a while, the temperature in the surface layer will not drop further, but may even increase, owing to the heat supply from the core part; consequently, the purpose of accelerated cooling can't be attained; if the cooling rate is >5.0° C./s, the cooling in the surface layer of the rail head and within a certain range of depth under the surface layer will be too quick; therefore, abnormal structures, such as bainite and martensite, etc., may occur, resulting in discard of the steel rail.
The accelerated cooling shall be stopped with the temperature in the rail head tread fall drops to 400˜550° C., and the steel rail shall be cooled by air cooling to room temperature, because: It is desirable that the core part of steel rail head should be cooled to a degree of super-cooling as higher as possible to accomplish phase transformation, in order to ensure more outstanding performance in the core part. Usually, in the actual production process, it is difficult to monitor the temperature in the core part of rail head with physical means; instead, the temperature in the core part of rail head has to be obtained by monitoring the surface temperature and then calculating according to the surface temperature. If the final accelerated cooling temperature is >550° C., the temperature in the core part of rail head will be higher than 600° C., at which the steel rail has had phase transformation in entirety or in part, i.e., the phase transformation has not completed yet. If the accelerated cooling is stopped at such a temperature, the heat from the rail web part will diffuse quickly to the core part, resulting in increased temperature and lowered cooling rate for phase transformation; consequently, the steel rail obtained finally has lower overall properties, and the purpose of heat treatment is not attained. If the final accelerated cooling temperature is <400° C., the phase transformation in the entire cross section of rail head and in the central part of rail base has already completed at that temperature, and applying forced cooling further will have no significance. Therefore, the final accelerated cooling temperature is set to 400-550° C. After the accelerated cooling is completed, the steel rail is held in air and cooled naturally to room temperature; then, follow-up procedures, such as straightening, flaw detection, and processing, etc., are carried out, and finally a finished product of heat-treated steel rail is obtained.
Preferably, in the steelmaking procedure, low-sulfur melted iron is charged into a steelmaking furnace while adding a high-alkalinity refining slag to reduce the sulfur content in the molten steel, and blind coal and a low-nitrogen alloy are preferably used as a carburant for steelmaking. Preferably, the steelmaking procedure comprises steel smelting in a convertor or electric furnace, refining in a LF furnace, and RH or VD vacuum treatment, wherein, a foaming agent is used in a heating process during the refining in the LF furnace.
Preferably, the high-alkalinity refining slag is composed of the following components in weight percentage: CaO: 65˜85 wt %, SiO2: 0.5˜5 wt %, CaF2: 7˜15 wt %, Al2O3: <0.50 wt %, P: <0.005 wt %, S: <0.05 wt %, and inevitable impurities of the remaining content. More preferably, high-alkalinity refining slag composed with the following chemical components in weight percentage is used: CaO: 81.85 wt %, SiO2: 0.73 wt %, CaF2: 9.25 wt %, S: 0.019 wt %, Al2O3: <0.50 wt %, P: <0.005 wt %, and inevitable impurities of the remaining content.
Preferably, the casting procedure is an overall-protection casting, to prevent absorption of excessive N incurred by contact with the air; after casting, the steel billet is subjected to a slow cooling. Preferably, after slow cooling, the steel billet is heated up for austenization before rolling, and the tapping temperature after the heating process is 1,000° C.
Preferably, the cooling medium is compressed air or a water mist-air mixture.
The method disclosed in the present invention can employs the following process: Low-sulfur melted iron is charged into a convertor or electric furnace, and is smelted into molten steel for producing a pearlite steel rail, high-alkalinity refining slag is used for an overall-protection casting, blind coal and a low-nitrogen alloy are used as a carburant for steelmaking, a foaming agent is used in the refining procedure in a LF furnace, RH or VD vacuum treatment is carried out, and then the molten steel is continuously cast into a steel billet with appropriate cross section dimensions; then, the steel billet is fed into a heating furnace for heating. Usually, the tapping temperature in the heating furnace is 1,000° C.; the steel billet is treated by dephosphorization at multiple points with high pressure water, and rolled in a universal mill; after the rolling is completed, the accelerated cooling medium is blown to the central part of rail head tread, two sides of rail head, and central part of rail base of the steel rail, utilizing the residual heat in the steel rail. Here, the cooling medium can be compressed air or a water mist-air mixture.
Hereunder the present invention will be further detailed in some embodiments, but the present invention is not limited to these embodiments.
In the following examples:
The inter-laminal spacing is measured with the method described in GB/T 16594-2008 “General Rules for Measurement of Length in Micron Scale by SEM”;
The ballistic work at normal temperature is measured with the method described in GB/T 229-2007 “Metallic Materials—Charpy Pendulum Impact Test Method”;
Rp0.2 refers to specific plastic elongation strength, which is measured with the method described in GB/T 228.1-2010 “Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature”;
Rm refers to tensile strength, and A refers to specific elongation, which are measured with the method described in GB/T 228.1-2010 “Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature”;
Z % refers to reduction of cross sectional area, which is measured with the method described in GB/T 228.1-2010 “Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature”;
The microstructure is measured with a MeF3 optical microscope with the method described in GB/T 13298-1991 “Metal—Inspection Method of Microstructure”.
The chemical components of the steel rail in weight percentage are: C: 0.72 wt %, Si: 0.35 wt %, Mn: 0.98 wt %, V: 0.05 wt %, Al: 0.04 wt %, P: 0.011 wt %, S: 0.006 wt %, and Fe and inevitable impurities of the remaining content.
Low-sulfur melted iron is charged into a convertor to smelt into molten steel for pearlite steel rail, high-alkalinity refining slag (composition is: alkalinity R=5, Al2O3: 23 wt %, BaO: 10 wt %, CaF2: 5 wt %) is added for an overall-protection casting, blind coal and a low-nitrogen alloy (at 90:100 weight ratio) are used as a carburant (dosage: 1.2-1.3 kg/tmolten steel), a foaming agent (dosage: 1.27 kg/tmolten steel, LFP-III slag foaming agent) is used in the refining process in the LF furnace, RH vacuum treatment is carried out, and the molten steel is continuously cast into a steel billet with 280 mm×320 mm cross section dimensions, and then the steel billet is fed into a heating furnace for heating. The tapping temperature in the heating furnace is 1,000° C.; the steel billet is treated by dephosphorization at multiple points with high pressure water, and is rolled in a universal mill, and the rolling conditions include: initial rolling temperature in BD1: not lower than 950° C., finish rolling temperature: not lower than 700° C.
After the rolling is completed, an accelerated cooling medium (compressed air) is blown to the central part of rail head tread, two sides of rail head, and central part of rail base of the steel rail, utilizing the residual heat of the steel rail. Accelerated cooling is carried out from 812° C. initial cooling temperature, at 4.0° C./s cooling rate, and is stopped at 480° C.; then, the product is cooled by air cooling to room temperature; thus, a steel rail with good impact properties is obtained.
Microstructure specimens are taken from round corners of the steel rail head, to test the tensile properties and microstructure of the steel rail. The test result is shown in Table 3.
Specimens are taken from four positions of the steel rail head as indicated in
The chemical composition of steel rail and the parameters of heat treatment process in example 1 are modified, to obtain examples 2˜5. Table 1 lists the chemical compositions of the steel rails in the examples 1˜5, Table 2 lists the parameters of the heat treatment process in the examples 1˜5 (including initial accelerated cooling temperature, cooling rate, and finial accelerated cooling temperature), Table 3 lists the tensile properties and metallographic structures of steel rails in the examples 1˜5, and Table 4 lists the impact properties at normal temperature of steel rails in the examples 1˜5.
The heat treatment process in the embodiments is changed, so that the steel rail is directly cooled by air cooling to room temperature; thus, comparative examples 1˜5 are implemented, wherein, Table 1 lists the chemical compositions of the steel rails in the comparative examples 1˜5, Table 3 lists the tensile properties and metallographic structures of the steel rails in the comparative examples 1˜5, and Table 4 lists the impact properties at normal temperature of the steel rails in the comparative examples 1˜5.
The four points (1, 2, 3 and 4) in Table 4 are the test points of the impact specimen of steel rail head respectively. The notching direction of the U-type impact specimen is oriented to the rail head side.
Five groups of steel rails with different chemical compositions are selected in the present invention for comparison. All of the five processing methods used in the examples are the method disclosed in the present invention. The results of comparison in Tables 1˜4 indicate: under the condition of the same chemical composition and smelting process, since the conventional steel rails are pearlite steel rails, the impact toughness of the steel rails cooled by natural cooling after rolling doesn't meet the requirement for steel rails used for railways in high-altitude and extremely cold regions. The heat treatment of steel rail after rolling has significant influences on the final properties of the steel rail, specifically: with the method disclosed in the present invention, the properties of the steel rail, including tensile property and impact toughness, etc., are effectively improved, on the premise that the microstructure is pure pearlite; in addition, the toughness and ductility of the steel are kept at the present level; thus, the impact resistance, wear resistance, and fatigue properties of the steel rail can be effectively improved.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201510006025.7 | Jan 2015 | CN | national |
