Patent Application
20160194729
This application claims priority to Chinese Application No. 201510006016.8, filed on Jan. 7, 2015, entitled “High-Strength and Highly Fatigue-Resistant Steel Rail and Production Method Thereof”, which is specifically and entirely incorporated by reference.
The present invention relates to a high-strength, highly wear-resistant, and highly contact-fatigue-resistant steel rail and a production method thereof, and belongs to the field of black steel manufacturing technology.
With the rapid development of China's heavy haul railways, the axle load on the freight railways in China has been increased continuously. For example, the axle load on most mixed passenger and freight transport railways is 21 tons or 23 tons, the axle load on special major freight railways, such as Daqin Railway and Shuohuang Railway, is 25 tons, and the axle load on the South Central Channel, which has just be built up and put into trial run, is 30 tons. It is foreseeable that increasing the axle load of freight trains will be the most optimal and the most economical method among the methods for further improving the efficiency of railway freightage in China. In addition, as the axle load on freight railways is increased continuously, the need for high wear resistance performance and high contact fatigue resistance performance of freight railways becomes more urgent. Therefore, it is an urgent task to develop new rail steel to adapt to the new railway transport pattern in China. Thus, in the present invention, a heavy-duty steel rail with high wear resistance and high contact fatigue resistance performance is developed, against the complex and heavy-haul railway transportation conditions in China.
In recent years, considering the present situation of steel rail production and railway operation in China, relevant domestic and foreign steel rail manufacturers have applied for patents related with steel rail techniques in China, such as hyper-eutectoid steel rail production technique, low alloy steel rail production technique, etc.
Relevant patent applications include as indicated below.
The technical problem to be solved in the present invention is to provide a high-strength, highly wear-resistant, and highly contact-fatigue-resistant steel rail and a production method thereof.
The technical solution of the present invention is as indicated below.
A high-strength and highly fatigue-resistant steel rail, comprising the following chemical components by weight percentage: C: 0.76%˜0.86%; Si: 0.60%˜1.00%; Mn: 0.70%˜1.50%, Cr: 0.10%˜0.50%, and 0.80%≦Mn %+Cr %≦1.60%; V: 0.05%˜0.30%, Ni: 0.10%˜0.35%, and 0.15%≦V %+Ni %≦0.40%; Mo: ≦0.03%; P: ≦0.020%; S: ≦0.015%; Fe and inevitable impurities: the remaining content, wherein, the metallurgical structure of the steel rail is fine pearlite+A, where, A is proeutectoid ferrite or proeutectoid cementite, and A≦2%.
The steel rail further comprises the following components by weight percentage: Ti: 0.05%˜0.30%, and Nb: 0.005%˜0.10%.
Preferably, A is proeutectoid ferrite or proeutectoid cementite, and A≦1%.
The tensile strength of the steel rail is 1,260 MPa˜1,420 MPa, the hardness of the rail head tread is 390HB˜432HB, the hardness of the part at 10 mm below the surface of rail head is 380HB˜420HB, and the hardness of the part at 24 mm below the surface of rail head is 370HB˜401HB.
The present invention further provides a method for producing the high-strength and highly fatigue-resistant steel rail, comprising the procedures in turn: converter smelting-LF refining-RH vacuum treatment-continuous casting-rolling-cooling-straightening, wherein, the finish rolling temperature in the rolling procedure is controlled to be 930° C.˜1,000° C.; the initial cooling temperature in the cooling procedure is controlled to be 780° C.˜880° C., the final cooling temperature is controlled to be 300° C.˜400° C., and the cooling rate is controlled to be 4.0° C.˜10.0° C./s.
Preferably, the cooling method is at least one selected from the group consisting of air blast cooling, water mist cooling and water cooling.
The present invention has the following beneficial effects. Compared with the inventions reported in the prior art, in the present invention, the second-phase structure (e.g., proeutectoid ferrite and proeutectoid cementite) is less, and the full rail profile is pearlite structure; especially, there is no martensite or bainite structure in tempered state in the rail web; thus, the risk of occurrence of horizontal cracks in the rail web in the service life of the steel rail is avoided. In addition, the present invention has the following advantages.
The present invention provides a high-strength and highly fatigue-resistant steel rail, comprising the following chemical components by weight percentage: C: 0.76%˜0.86%; Si: 0.60%˜1.00%; Mn: 0.70%˜1.50%, Cr: 0.10%˜0.50%, and 0.80%≦Mn %+Cr %≦1.60%; V: 0.05%˜0.30%, Ni: 0.10%˜0.35%, and 0.15%≦V %+Ni %≦0.40%; Mo: ≦0.03%; P: ≦0.020%; S: ≦0.015%; Fe and inevitable impurities: the remaining content, wherein, the metallurgical structure of the steel rail is fine pearlite+A, where, A is proeutectoid ferrite or proeutectoid cementite (also referred to as secondary cementite), and A≦2%.
Proeutectoid ferrite: it means ferrite precipitating from austenite lower than the eutectoid composition as the austenite is cooled down from a high temperature before eutectoid transformation (eutectoid transition) takes place.
Proeutectoid cementite: it is also referred to as secondary cementite, which refers to Fe3C precipitating from austenite A. The secondary cementite precipitates from austenite along the grain boundaries as the carbon content changes during temperature drop. It appears in hypereutectoid steel in most cases and is usually in mesh form. In view that the secondary cementite meshes have adverse effects to the performance of the steel, they can be broken by normalization to improve performance.
Preferably, 0.05%˜0.30% Ti and 0.005%˜0.10% Nb may be added, depending on the requirement for strengthening. Hereinafter, the design and control ranges of the elements will be described.
C is a positive element for improving wear resistance of steel rail. However, if the carbon content is too high, a large amount of proeutectoid cementite (also referred to as secondary cementite) may appear; if the carbon content is too low, a large amount of proeutectoid ferrite may precipitate; both cases have adverse effects to the contact fatigue resistance of the steel rail. Hence, in the present invention, the carbon content is controlled to be within a range of 0.76%˜0.86%, so that proeutectoid ferrite and proeutectoid cementite can be controlled to be 2.0% or lower.
Si is a major element for solution strengthening, and can improve the strength and wear resistance of steel rail. In addition, in eutectoid steel rails, Si is an element that promotes the precipitation of ferrite, and has a function of inhibiting the precipitation of cementite. However, if the Si content is too high, the plasticity and toughness of the steel rail will be decreased; consequently, the contact fatigue resistance of the steel rail will be decreased. Hence, in the present invention, the Si content is controlled to be within a range of 0.60%˜1.00%.
Mn and Cr are strong-hardenability alloying elements, an optimal effect can be attained if both elements are added at the same time. In addition, Mn and Cr are maj or elements for improving wear resistance of steel rail. However, if the Mn content and Cr content are too high, harmful bainite and martensite structures may appear in the steel rail. Therefore, the total addition amount of Mn and Cr must be controlled strictly. Hence, in the present invention, to ensure the structure of the steel rail is pearlite structure, the Mn content is controlled to be within a range of 0.70%˜1.50%, the Cr content is controlled to be within a range of 0.10%˜0.50%, and 0.80%≦Mn %+Cr %≦1.60%.
Mo is an element that can strongly increase hardenability; especially, when Mn and Cr are used in combination, even a small amount of Mo can result in mixed bainite and martensite structure in the rail web, which is quite harmful to the performance of the rail web. For example, in the Mn, Cr and Mo alloy system described in the patent document CN86106894A, typical bainite structure appears in the rail web. In the present invention, microalloy elements Mn and Cr are mainly utilized to ensure hardenability performance. Hence, in the present invention, the Mo content is controlled to be lower than 0.03%.
V and Ni are elements that can improve the strength-toughness of steel rail without compromising the plasticity of steel rail. However, excessive V and Ni have little contribution to further improvement of toughness but have adverse effects, and result in significantly increased production cost at the same time. Hence, in the present invention, the V content is controlled to be within a range of 0.05%˜0.30%, the Ni content is controlled to be within a range of 0.10%˜0.35%, and 0.15%≦V %+Ni %≦0.40%.
Both P and S are elements that can not be eliminated completely. P segregates at the grain boundaries in the steel rail structure, which is very harmful to the toughness of the steel rail; S mainly produces MnS inclusion in the steel, which is harmful to the contact fatigue resistance of the steel rail. Hence, in the present invention, according to the actual production control ability of the manufacturer, the P content is controlled to be lower than 0.020%, and the S content is controlled to be lower than 0.015%.
In addition, to improve the strength and wear resistance of the rail steel, prevent the welding heat affected zone from softening, and improve the strength and hardness of welded joints, Ti and Nb elements can be added for grain refining. However, if the Ti content and Nb content are too high, a second phase such as TiC or NbC will precipitate in the steel rail at a high temperature. In the precipitation process of the above second phase, the content of cementite lamellas in the pearlite structure of the steel rail will be reduced, because the content of carbon dissolved in the steel rail is decreased actually. Consequently, the strength and hardness of the steel will be decreased instead. Hence, in the present invention, only a very small amount of Ti and Nb can be added. The Ti content is controlled to be within a range of 0.05%˜0.30%, and the Nb content is controlled to be within a range of 0.005%˜0.10%.
The tensile strength of the obtained steel rail is 1,260 MPa˜1,420 MPa, and the testing positions are shown in
The present invention further provides a method for producing the above high-strength and highly fatigue-resistant steel rail, comprising the procedures in trun: converter smelting-LF refining-RH vacuum treatment-continuous casting-rolling-cooling-straightening-test-surface inspection, wherein, the finish rolling temperature in the rolling procedure is controlled to be 930° C.˜1,000° C.; the initial cooling temperature in the cooling procedure is controlled to be 780° C.˜880° C., the final cooling temperature is controlled to be 300° C.˜400° C., and the cooling rate is controlled to be 4.0° C.˜10.0° C./s.
The steel rail is treated by in-line heat treatment. Usually, the finish rolling temperature of the steel rail is 930° C.˜1,000° C. Forced cooling (including a combination of one or more selected from the group consisting of air blast cooling, water mist cooling, and water cooling) is required in the high-temperature stage in order to inhibit the precipitation of proeutectoid ferrite or proeutectoid cementite in the steel rail, and the initial cooling temperature of the steel rail is controlled to be between 780° C.˜880° C. Moreover, in view of the high initial cooling temperature, high cooling efficiency is required, so that the core part of rail head can be quenched fully. Usually, the cooling is controlled at 4.0° C./s˜10.0° C./s cooling rate, till the temperature at the center of the rail head surface drops to 300° C.˜400° C. Finally, the hardness of the part at 24 mm below the rail head surface reaches 370HB or more, while the hardness of the rail head surface increased to 390HB.
The production procedures include: rail steel smelting in a converter-refining in a LF furnace and molten steel heating-RH composition control and homogenization-bloom continuous casting under six-strand protection-bloom heating-7-stand universal rolling-feeding the rolled steel with ends into heat treatment equipment for forced cooling-discharging from the heat-treatment equipment at 300° C.˜400° C. final cooling temperature-cooling on walking beam cooler-combined vertical and horizontal straightening-non-destructive test, cross sectional dimension and straightness inspection-combined cutting and drilling, and cutting to specified length-surface inspection-warehousing.
The temperature of the rolled part is 930° C.˜1,000° C. when the rolled part is discharged from the UF finish rolling section. To ensure the initial cooling temperature is 780° C.˜880° C., the running speed of the roller way must be increased, and relevant pause procedures must be cancelled, so that the rolled part enters into the heat treatment equipments at a desired temperature. The range of adjustment is 4.0° C./s˜10.0° C./s, according to the composition of the steel rail in smelting and the cooling rate of the steel rail controlled by temperature.
Hereinafter, some examples will be described with reference to the above embodiment. The chemical composition of the rail steel in smelting in Examples 1-10 is shown in Table 1, wherein the remaining is Fe and inevitable impurities; the process conditions controlled in Examples 1-10 as shown in Table 2; the properties and metallurgical structure of the finished product in Examples 1-10 are shown in Table 3. In the examples and comparative examples of the present invention, the properties of the steel rail, including tensile strength, specific elongation, and hardness of tread, etc., are respectively tested as per GB/T228.1 “Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature”, and GB/T 231.1 “Metallic Materials-Brinell Hardness Test-Part 1: Test Method”.
The chemical compositions of the steel rails in smelting in the comparative examples 1-4 are shown in Table 4, wherein the remaining is Fe and inevitable impurities. The production procedures involved in comparative examples are the same as those in the examples, and the process conditions involved in comparative examples 1-4 are the same as those in Example 1.
The process conditions involved in comparative examples 5-7 are shown in Table 5. The chemical compositions of the steel rails in smelting in the comparative examples 5-7 are respectively the same as those involved in Example 5-7.
In the Table 3, P represents pearlite, a represents proeutectoid ferrite, and FeC represents proeutectoid cementite (also referred to as secondary cementite).
In the Table 6, P represents pearlite, at represents proeutectoid ferrite, and FeC represents proeutectoid cementite (also referred to as secondary cementite), M represents martensite.
It can be seen from Table 1 to 6 that the second-phase structure (e.g., proeutectoid ferrite and proeutectoid cementite) is less and the steel rail in the present invention has high-strength, highly wear-resistant, and highly contact-fatigue-resistant performance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201510006016.8 | Jan 2015 | CN | national |
