Patent Application
20160060736
This application claims priority to Chinese Application No. 201410443977.0, filed on Sep. 2, 2014, entitled “Pearlitic Steel Rail with High Strength and Toughness and Producing Method Thereof”, which is specifically and entirely incorporated by reference.
The present invention relates to a steel rail, in particular to a pearlitic steel rail with high strength and toughness and a producing method thereof.
The rapid development of railroads has raised higher requirements for the service performance of steel rails. Especially, for heavy-duty railroads with high traffic volume and high axle load, the early replacement of steel rails as a consequence of quick wearing has become a major factor that has impacts on the efficiency of railroad transportation. Usually, alloying, heat treatment or a combination of them is utilized in the production of steel rails, in order to improve the overall property such as wear resistance et al. However, improved strength and hardness of pearlitic steel rails will result in further degraded toughness and plasticity that are low originally. Especially, as the carbon content is increased, the specific elongation of steel rails still tends to decrease even though heat treatment is utilized to give full play of the strong refined crystalline strengthening effect. Consequently, the development of pearlitic steel rails towards higher carbon content and higher wear resistance is limited. The traditional research on pearlitic steel rails mainly focuses on chemical composition and heat treatment process, but neglects the influence of the rolling process of steel rails on the microscopic structure and mechanical properties of the steel rails. As a result, it is very difficult to further improve the overall properties of pearlitic steel rails.
The object of the present invention is to provide a pearlitic steel rail with high strength and toughness, which has outstanding strength and toughness.
The present inventor has found in the research: on the premise of meeting the rolling conditions, by means of refined control of the rolling process, in conjunction with appropriate chemical composition and post-rolling cooling process, the toughness and plasticity of the steel rails, especially the specific elongation, can be improved significantly, while the strength grade is maintained, and thereby the steel rails for railroad transportation can meet the requirements for long service life and service safety. Through further research, the present inventor has found: to refine austenite grains and finally obtain a steel rail product with outstanding performance, the chemical composition of steel billet, heating and holding temperature, initial rolling temperature, final rolling temperature, rolling reduction in the last two passes, and air cooling of steel rail with residual heat in a cooling bed must be controlled in a coordinated manner. Only in that way the production of a steel rail product with high-performance can be realized.
Based on the findings described above, the present invention provides a producing method of a pearlitic steel rail with high strength and toughness, comprising: controlling the following processing conditions in a rolling procedure for producing the pearlitic steel rail with high strength and toughness: initial rolling temperature of 1,120-1,180° C., final rolling temperature of 840-880° C., and rail profile reduction in last two rolling passes of 6%-12%; the steel rail is cooled to 600° C. or lower at a cooling rate ≦2.0° C./s after final rolling, and then air-cooled to room temperature;
wherein the chemical composition of the steel rail meets the following requirements: C: 0.75%-0.84%, Si: 0.30%-0.80%, Mn: 0.50%-1.50%, V: 0.04%-0.12%, Ti: 0.004%-0.02%, and 0.10%≦V+10Ti≦0.25%, [N]≦30 ppm, P≦0.020%, S≦0.008%, with the remaining content consisting of Fe and inevitable impurities.
The present invention further provides a pearlitic steel rail with high strength and toughness produced by the producing method described above.
The steel rail obtained by the method of the present invention has significantly refined pearlitic grains, tensile strength≧1000 MPa, and specific elongation≧14%, and has high overall properties of strength and toughness.
Other characteristics and advantages of the present invention will be further detailed in the embodiments hereunder.
Hereunder some embodiments of the present invention will be detailed, with reference to the accompanying drawings. It should be appreciated that the embodiments described here are only provided to describe and explain the present invention, but shall not be deemed as constituting any limitation to the present invention.
Unless otherwise stated, the “room temperature” described in the present invention refers to 5-40° C., and [N] represents nitrogen content.
In the present invention, the following processing conditions are controlled in a rolling procedure to produce the pearlitic steel rail with high strength and toughness: initial rolling temperature of 1,120-1,180° C., final rolling temperature of 840-880° C., and rail profile reduction in last two rolling passes of 6%-12%; the steel rail is cooled to 600° C. or lower at a cooling rate ≦2.0° C./s after final rolling, and then air-cooled to room temperature;
the chemical composition of the steel rail meets the following requirements (calculated in weight percentage): C: 0.75%-0.84%, Si: 0.30%-0.80%, Mn: 0.50%-1.50%, V: 0.04%-0.12%, Ti: 0.004%-0.02%, and 0.10%≦V+10Ti≦0.25%, [N]≦30 ppm, P≦0.020%, S≦0.008%, with the remaining content consisting of Fe and inevitable impurities.
Hereunder the reasons for confining the major chemical elements in the steel rail of the present invention within the above ranges will be explained:
Carbon (C) is the most important and cheapest element for improving the strength and hardness of pearlitic steel rails and promoting pearlitic transformation. When the content of C is <0.75%, the strength and hardness of the steel rail produced through the production process of the present invention are too low to ensure required wear resistance of the steel rail; when the content of C is >0.84%, a trace of secondary cementite will still precipitate at the grain boundaries even though the steel rail is cooled at a cooling rate ≦2.0° C./s after final rolling, resulting in degraded toughness and plasticity of the steel rail. Hence, the content of C should be confined within a range of 0.75%-0.84%.
Silicon (Si) exists as a solution strengthening element for steel in ferrite and austenite and can improve the strength of the structure. When the content of Si is <0.30%, the quantity of solid solution is low, and the strengthening effect is not obvious; when the content of Si is >0.80%, it will decrease the toughness and plasticity of the steel rail, resulting in degraded transverse mechanical properties of the steel rail, and adverse to application safety of the steel rail. Hence, the content of Si should be confined within a range of 0.30%-0.80%.
Manganese (Mn) can work with Fe to form solid solution and improve the strength of ferrite and austenite. In addition, Mn is a carbide forming element, and can partially substitute Fe atoms when it enters into cementite, and thereby improve the hardness of carbides and ultimately increase the hardness of steel. When the content of Mn is <0.50%, the strengthening effect is not obvious, and the performance of the steel can be improved slightly only by solution strengthening; when the content of Mn is >1.50%, the hardness of carbides in the steel will be too high, while the toughness and plasticity will be decreased severely; moreover, Mn has severe impacts on the diffusion of carbon in steel, and abnormal structures such as B and M still can occur in regions with Mn segregation even under an air cooling condition. Hence, the content of Mn should be confined within a range of 0.50%-1.50%.
Vanadium (V) has very low solubility in steel at room temperature. If V exists at austenite grain boundaries or in other regions during a hot rolling process, it will precipitate in a form of fine-grain vanadium carbonitrides or compound with Ti in the steel and then precipitate. Thus, it inhibits the growth of austenite grains and thereby attains a purpose of refining grains and improving performance. When the content of V is <0.04%, the precipitation of V-containing carbonitrides is limited, and it is unable to give full play to the strengthening effect; when the content of V is >0.12%, bulky carbonitrides may be formed, causing compromised toughness and plasticity of the steel rail. Hence, the content of V should be confined within a range of 0.04%-0.12%.
Titanium (Ti) has a main role of refining austenite grains in steel during heating, rolling, and cooling, and thereby increasing the specific elongation and rigidity of the steel rail. It is one of the most important additional elements in the present invention. When the content of Ti is <0.004%, the quantity of carbides formed in the steel rail is very limited; when the content of Ti is >0.02%, on one hand, an excessive amount of TiC will be produced since Ti is a strong carbonitride forming element, resulting in excessive high hardness of the steel rail; on the other hand, the excessive amount of TiC will segregate and concentrate to form bulky carbides, which will not only cause degraded toughness and plasticity but also increase the probability of cracking in the contact surface of the steel rail under impact loads and result in fractures. Hence, the content of Ti should be confined within a range of 0.004%-0.02%.
In the present invention, the total content of V and Ti is confined to meet 0.10%≦V+10Ti≦0.25%, because: though V and Ti have different affinities with C, N etc. in the steel and can form carbides that are quite different in quantity and shape, they have similar austenite grain refining effects provided by carbonitrides formed by them. Take V for example, in steel with low N content, the percentage of V dissolved in the ferritic matrix exceed 50%; whereas, in steel with high N content, the percentage of V dissolved in the steel is about 20%, while the remaining 70% has precipitated in the form of vanadium carbonitrides. In the present invention, the improvement of performance is not obvious when V or Ti is added separately. If 0.10% V is added without adding Ti, the strength of the steel rail still can reach 1,000MPa but the specific elongation is usually lower than 12%; if Ti is added separately for micro-alloying, the strength of the steel rail can't reach 1,000MPa. Wherein “V+10Ti” means V content+10×Ti content.
In the present invention, the content of nitrogen (N) is confined to be lower than 30 ppm, mainly for inhibiting the generation of bulky nitrides that contain V and Ti, and thereby attaining a purpose of inhibiting the growth of austenite grains by forming fine and dispersed carbides.
Phosphorus (P) can severely decrease the ductility and toughness of the steel and increase the brittleness of the same. When the content of P is ≦0.020%, the adverse effects are limited, and will not have severe impacts on the service performance and safety of the steel rail.
Sulfur (S) exists in steel mainly in the form of MnS inclusions, which increase the brittleness of the steel and cause difference between longitudinal and transverse mechanical properties. When the content of S is ≦0.008%, the purity of the steel is high, and possible defects formed at the inclusions and cavities formed by the inclusions under wheel load, which have adverse impacts on the service performance of the steel rail, can be effectively avoided.
The object of the present invention can be attained, as long as the conditions described above are met in the steel billet rolling process. There is no particular restriction on the approach of acquisition of the steel billet. For example, the steel billet can be obtained by a method comprising: molten steel with composition described above is smelted using a rotary furnace or electric furnace, Al-free deoxidation, reducing S content by LF refining, RH vacuum treatment or VD treatment; then, the composition is tuned into the target range, and the molten steel is continuously cast into a steel billet with 250 mm×250 mm-450 mm×450 mm cross section, the steel billet is cooled, and then fed into a heating furnace, the heating temperature is ≧1200° C., and kept at the temperature for a duration not longer than 3h, to ensure homogenous temperature across the cross section, and then the billet is discharged from the furnace, and treated with high pressure water to remove iron scale, and the steel billet is obtained.
In the present invention, the initial rolling temperature is controlled at 1,120-1,180° C., mainly to enable the bulky V and Ti precipitates formed in the continuous casting process to be re-dissolved fully or largely and ensure precipitation of fine V and Ti carbonitrides in rolling process of the steel rail, which pin at the austenite grain boundaries of the austenite phase region and refine the austenite grains, so that the effect of the present invention can be attained.
In the present invention, the final rolling temperature is controlled at 840-880° C., mainly to ensure stable and uniform precipitation of precipitate at appropriate granularity; when the temperature is higher than 880° C., the precipitates tend to become bulky, and consequently the precipitation-based effect will be weakened; when the temperature is lower than 840° C., the precipitates will be too fine to provide an effect of pinning austenite grain boundary; in addition, within the range of 840-880° C., the lower the final rolling temperature is, the higher the precipitation effect of V and Ti carbonitrides in the steel is.
In the present invention, the rail profile reduction (area reduction or amount of deformation) in last two rolling passes is controlled at 6%-12%, in order to ensure a steel rail with good performance can be obtained on one hand and control the amount of deformation within a range that is permitted by the mill load on the other hand.
In the present invention, the steel rail is cooled at a cooling rate ≦2.0° C./s (preferably at 1-2° C./s) to 600° C. or lower (preferably 500-600° C.) after final rolling, mainly to ensure the formation of bulky austenite structures prior to the phase transformation of pearlitic structures, so that the cooling process can work with the steps described above to attain the object of the present invention. Here, the cooling medium used for cooling can be any cooling medium commonly used in the art, such as compressed air. The cooling can be carried out in a conventional approach, such as holding in a cooling bed.
In the present invention, the steel rail is cooled to 600° C. or lower at a cooling rate ≦2.0° C./s after final rolling, and then air-cooled to room temperature, i.e., when the temperature of steel rail is lower than 600° C., the steel rail is cooled by natural cooling in the air to an appropriate temperature. In the process of air-cooling to room temperature, procedures such as straightening, flaw detection, and machining, etc., can be carried out as required, so as to obtain a finished steel rail product.
The cross section of the steel rail obtained with the method described in the present invention can be 50-75kg/m.
The cross section for the steel rail obtained by the present method may be 50-75kg/m.
The present invention further provides a pearlitic steel rail with high strength and toughness produced by the producing method described above.
Hereinafter the pearlitic steel rail with high strength and toughness and its producing method of the present invention will be described in combination of examples. Unless otherwise stated, all the experimental methods used in the following examples are conventional methods.
The chemical compositions of the steel rails involved in the examples and comparative examples are shown in Table 1.
Molten steel with a composition described above is smelted in a rotary furnace, Al-free deoxidation, reducing S content by LF refining, and RH vacuum treatment; then, the composition is tuned into the target range, and the molten steel is continuously cast into a steel billet with 280 mm×380 mm cross section; next, the steel billet is cooled, and then fed into a heating furnace, the heating temperature is 1,250° C., and discharged after holding for 2.5 h, and treated with high pressure water to remove iron scale, so as to obtain a steel billet to be rolled.
In the examples, a steel billet in a chemical composition described above is rolled into a 60 kg/m steel rail. The rolling process and the cooling process after final rolling are shown in Table 2; in the comparative example, the rolling and cooling are carried out in a conventional way, as shown in Table 3.
The steel rails treated as described above are cooled by air-cooling to room temperature (25° C.), and tested by rail head tensile impact test and microscopic structure test respectively. The results are shown in Table 4, where, Rm represents tensile strength, and A is percentage elongation after fracture, wherein the tensile impact test is carried out as per GB/T 228.1-2010.
In the present invention, six examples and corresponding comparative examples, in which steel rails with the same chemical composition are treated through different rolling and cooling processes after rolling, are selected for comparison. The structures of all the steel rails are pearlite. In the examples, the processes are the method of the present invention. With uniform and fine V and Ti carbonitride grains precipitating from the steel and pinning at the austenite grain boundaries, the toughness and plasticity of the steel rail, especially the specific elongation, are significantly improved. All of the six groups of steel rails have strength within the range of 1,020MPa-1,100MPa, while the specific elongation is as high as 14.5%-16.0%, which means the steel rails have superior strength-toughness matching. In contrast, the steel rails in the comparative examples, which are obtained through the existing matured process (i.e., the rolling and the air cooling after rolling of the steel rail are not controlled), have strength equivalent to that of the steel rails in the example, but have specific elongation apparently lower than that of the steel rails in the examples. It is observed that the steel rail of the present invention has higher plasticity, which not only can ensure operation safety of trains, but also is helpful for improving the contact fatigue property of steel rails.
While a pearlitic steel rail with high strength and toughness and a producing method thereof provided in the present invention are described above, those skilled in the art should appreciate that various modifications can be made without departing from the spirit and scope of the present invention.
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| Number | Date | Country | Kind |
|---|---|---|---|
| 201410443977.0 | Sep 2014 | CN | national |
