CA2152877C - Rail excellent in toughness and wear resistance and method of manufacturing same - Google Patents

Abstract

There is provided a rail of high toughness and high wear resistance, consisting essentially of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 3.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities. The rail has a metal structure of a bainite structure, a hardness of 400 Hv or more at each of a head top portion and a head corner portion thereof, a tensile strength of 1200 MPa or more, and a 2 mm, U-notch Charpy absorbed energy of 30 J or more at +20°C.

Description

-The present invention relates to a high-toughness, high wear-resistance rail, an more particularly to a rail which great wear-resistant and damage-resistant enough to serve in high-rate transportation railroads S and heavy-duty mine railroads and which has strength, toughness and wear-resistance great enough to serve in cold districts. The invention also relates to a method of manufacturing the rail.
Hitherto, rails for use in railroads have been developed to be stronger, in terms of wear-resistance, rolling-fatigue resistance and the like, which are considered important properties required of the rails.
Rails for use in railroads need to be even stronger since the railroad transportation has recently become more speedy and more heavy-duty, applying a high axle-load on the rails. Particularly in cold districts such as Russia and Canada, rails having excellent toughness are demanded. The ordinary rails at present have but insufficient toughness; their 2 mm, U-notch Charpy absorbed energy is as small as 10 J or less at 20C. It is difficult to increase their Charpy absorbed energy by 5 J.
One of methods of improving the toughness is heat treatment. Rails may be heat-treated to acquire an increased toughness in one of the following two alternative methods.
In the first method generally known as ~off-line - 21~2877 heat treatment." In this method, rails made by rolling are cooled. Thereafter, they are heated to AC3 point or a higher temperature and subjected to accelerated cooling. (See Jpn. Pat. Appln. KOKAI Publication No. 63-128123.) In the second method known as "on-line heat treatment." In this method, rails made by rolling and remaining at Ar3 point or a higher temperature are subjected to accelerated cooling. (See Jpn. Pat. Appln.
KOKAI Publication No. 63-23244.) The off-line method may increase the strength and toughness of the rail, since the microstructure of the steel is transformed at low temperatures by accelerated cooling and microstructure of the steel is made finer by repeatedly transforming the microstructure of the steel.
The method, however, is undesirable in view of heat efficiency because the rails must be heated before they are subjected to accelerated cooling.
The on-line method excels in heating efficiency because the rails are subjected to accelerated cooling immediately after having been made by rolling. However, the method can hardly improve the toughness of rails greatly, though strengthening the rails due to the accelerated cooling strengthens, if the rails are of the conventional steel composition.
Conventional rails is made of steel having fine pearlite structure to have high strength and high wear-resistance. Having pearlite structure, they cannot have their toughness increased greatly. It is proposed that rails be made of bainitic steel to have great toughness. However, bainite structure is said to less resistant to wear than fine pearlite structure, though it is superior to fine pearlite structure in terms of toughness.
Jpn. Pat. Appln. KOKAI Publication No. 2-282448 discloses a rail which is made of steel having C content slightly lower than the conventional steel, the top and corner portions of which differ in hardness, and which has an improved resistance to rolling-fatigue damage.
Jpn. Pat. Appln. KOKAI Publication No. 5-271871 dis-closes a rail which is made of bainitic steel having C
content slightly lower than the conventional steel so that its top portion may be sufficiently resistant to rolling-fatigue damage. Neither rail has an improved toughness, however. Further, the bainitic steel rail disclosed in Publication No. 5-271871 is disadvantageous in that its service life will be shorten if it is used in mine railroad and inevitably subjected to a high axle-load. This is because it is intended to increase wear amount of the rail to improve resistance to rolling-fatigue damage of the rail.
As mentioned above, conventional rails of pearlite structure can hardly have their toughness improved greatly, whereas conventional rails of bainite structure is intended to increase wear amount of the rail to improve the resistance to rolling-fatigue damage is increased. No technique is available which can provide rails which excels in both toughness and wear-resistance.
The present invention has been developed in view of the above-described circumstances, and has as its object to provide a rail excellent in toughness, strength and wear resistance, and method of manufacturing the same.
According to a first aspect of the present invention, there is provided a rail of high toughness and high wear resistance, consisting essentially of 0.2 to O.S wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities, the rail having a metal structure of a bainite structure, a hardness of 400 Hv or more at each of a head top portion and a head corner portion thereof, a tensile strength of 1200 MPa or more, and a 2 mm, U-notch Charpy absorbed energy of 30 J or more at +20C.
According to a second aspect of the present invention, there is provided a rail of high toughness and high wear resistance, consisting essentially of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities, the rail having a metal structure of 2152~77 a bainite structure, and a hardness of 400 Hv or more at each of a head top portion and a head corner portion thereof.
According to a third aspect of the invention, there is provided a method for manufacturing a rail of high toughness and high wear resistance, comprising the steps of:
(a) preparing a steel consisting essentially of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities;
(b) hot rolling the steel to have a rolling finishing temperature of 800 - 1000C, thereby forming a rail stock; and (c) cooling the rail stock at a cooling rate of 5C/sec. or less between a bainite transformation-starting temperature or more and 400C or less.
This invention can be more fully understood from the following detailed description when taken in con-junction with the accompanying drawings, in which:
FIG. 1 is a graph, showing the relationship between the hardnesses and the wear amounts of rails; and FIG. 2 is a graph, showing the relationship between the hardnesses and the wear loss ratios of rails.
As a result of having paid attention to a bainite structure of excellent toughness, the present invention provides a rail of high toughness, high strength and high wear resistance by forming its microstructure of bainite and adjusting its composition and manufacturing conditions so as to increase its hardness.
As is aforementioned, it is known that the conven-tional rail with a bainite structure has high toughness but low wear resistance. FIG. 1 shows the relationship between the amount of wear and the hardnesses of rails having a pearlite structure, a bainite structure and a tempering martensite structure, respectively. As is evident from FIG. 1, the amount of wear in the bainite structure is larger than that in the pearlite structure.
This means that in the conventional bainitic rail of high strength, bainite is employed to enhance the resis-tance against a rolling fatigue damage at the sacrifice of the resistance against wear. On the other hand, in the present invention, the bainite structure is made to have high hardness, thereby enhancing both its toughness and wear resistance.
According to a first embodiment of the present invention, there is provided a rail, which essentially consists of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt%
or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities; and has a metal struc-ture of a bainite structure, a hardness of 400 Hv or more at each of a head top portion and a head corner 21~2~77 portion thereof, a tensile strength of 1200 MPa or more, and a 2 mm, U-notch Charpy absorbed energy of 30 J or more at +20C.
The rail constructed as above can be used in high rate transportation railroads or heavy-duty railroads, and in cold districts.
Preferably, the rail further contains at least one selected from the group consisting of 0.1 to 1.0 wt%
of Ni and 0.1 to 1.0 wt% of Mo, in light of more increasing its strength. Preferably, the rail further contains at least one selected from the group consisting of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of V, in - light of more increasing its wear resistance. To increase both the strength and the wear resistance, the rail may contain both the above-described ingredients.
According to a second aspect of the invention, there is provided a rail, which essentially consists of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities; and has a metal structure of a bainite structure and a hardness of 400 Hv or more at each of a head top portion and a head corner portion thereof.
By virtue of this structure, the rail can have high toughness and wear resistance, and be used in high-rate transportation railroads or heavy-duty railroads.

Also in this case, the rail further preferably containing of at least one selected from the group consisting of 0.1 to 1.0 wt% of Ni and 0.1 to 1.0 wt%
of Mo, in light of more increasing its strength.
Preferably, the rail further contains of at least one selected from the group consisting of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of V, in light of more increasing its wear resistance. To increase both the strength and the wear resistance, the rail may contain both the above-described ingredients.
The reasons why the chemical composition, the microstructure, the hardness, the toughness and manufacturing conditions of the rail are set to the above-described values will be explained.
[Re: Chemical Composition]
0.2 to 0.5 wt% of C
C is an essential element for securing sufficient strength and wear resistance to a rail steel. If the C
content is less than 0.2 wt%, a steel of a hardness suitable to a rail cannot be prepared at low cost.
Further, if the C content exceeds 0.5 wt%, the rail head portion cannot have a uniform bainite structure, thereby deteriorating the toughness. In light of the above, the C content is set to 0.2 to 0.5 wt%.
0.1 to 2.0 wt% of Si Si is an element which is effective not only as a deoxidizer but also an element for increasing the - 21~2877 strength and hence the wear resistance by solving ferrite in the bainite structure. If the Si content is less than 0.1 wt%, no effect can be found. If the Si content is more than 2.0 wt%, the steel embrittles.
Therefore, the Si content is set to 0.1 to 2.0 wt%.
1.0 to 4.0 wt% of Mn Mn is an element for increasing the strength of the rail steel by lowering the transformation temperature of bainite to enhance the hardenability. If the Mn content is less than 1.0 wt%, little effect will be found. If the Mn content is more than 4.0 wt%, a martensite structure will easily be formed because of micro segregation of the steel. The martensite structure hardens or embrittles during heating and welding, thereby degrading the steel. Therefore, the Mn content is set to 1.0 to 4.0 wt%.
0.035 wt% or less of P
Since P reduces the toughness, its content is limited to less than 0.035 wt%.
0.035 wt% or less of S
S is contained in the steel, mainly in the form of inclusion. If the S content exceeds 0.035 wt%, the amount of inclusion significantly increases and hence the steel embrittles and degrades. To avoid this, the S
content is limited less than 0.035 wt%.
0.3 to 4.0 wt% of Cr Cr is an element for improving the hardenability of bainite. Cr is a very important element in the steel of the present invention for strengthening the bainite structure as the metal structure. If the Cr content is less than 0.3 wt%, the quenching properties of bainite are degraded, and the microstructure is not formed by a uniform bainite structure. If the Cr content exceeds 4.0 wt%, martensite may easily be formed. Therefore, the Cr content is set to 0.3 to 4.0 wt%.
0.1 to l.0 wt% of Ni, 0.1 to l.0 wt% of Mo Each of Ni and Mo is an element to be solved in bainite for improving the hardenability of bainite and strengthening the same. If the Ni or Mo content is less than 0.1 wt%, no effect can be found. Further, even if the Ni or Mo content exceeds 1.0 wt%, no more effect can be expected. In light of these, it is preferable that each of the Ni and Mo content is set to o.1 to l.0 wt%, and at least one of them is added.
0.01 to 0.1 wt% of Nb and 0.01 to 0.1 wt% of V
Since each of Nb and V is bonded with C contained in bainite and deposited after rolling, they are effective elements for increasing the hardness and wear resistance of the head portion of the rail until deep portion by precipitation hardening, thereby elongating the service life of the rail. If the Nb or V content is less than 0.01 wt%, a sufficient effect cannot be obtained. Further, even if the Nb or V content exceeds 0.1 wt%, no more effect can be expected. In light of the above, it is preferable that each of the Nb and V
content is set to 0.01 to 0.1 wt%, and at least one of them is added.
[Re: Metal Structure (Microstructure)]
In the present invention, the rail has a bainite structure. As compared with the conventional pearlitic rail, the bainite structure has a high dislocation density, and accordingly has high strength and high toughness. Therefore, the amount of C can be set smaller than that in the pearlitic rail.
[Re: Hardness]
Any portion of the head top portion and head corner portion of the rail has a hardness of 400 Hv or more.
In the case of the bainitic rail of the invention, if the hardness of the rail is 400 Hv or more, the amount of wear is less than that of an ordinary rail.
The reason why the hardness is set to the above-described values is based on the relationship between the hardness and the amount of wear.
Although it is most desirable to estimate the amount of wear of a rail actually used, it is also effective to estimate the same from comparison tests in which contact conditions of actually-used rails are simulated using the Nishihara type wear-testing system.
The above method can estimate the wear resistance (the relationship between the hardness and the wear loss ratio) in a short time. Hereinbelow, estimation results obtained using the method will be described.
FIG. 2 shows the influence of the hardness on the wear loss ratio. Steel samples of various compositions as shown in table 1 and tables 2, 4, 6, 8 and 10 (which will be later referred to) were prepared by varying the contents of C, Si, Mn, Cr, Ni, Mo, Nb and V. These samples were heated at 1250C, rolled at 920C, and acceleratedly cooled at 3C/sec. so that the rolling finishing temperature and the cooling rate of the invention could be satisfied, thereby forming steel plates of a thickness of 12 mm.
Nishihara type wear test pieces having a diameter of 30 mm and a width of 8 mm, were cut from the steel plates, and were subjected to a wear test carried out under the conditions of a contact load of 50 kg, a slippage ratio of -10%, and no lubricant. The wear loss of each test piece after 500,000 rotations was measured. Estimation was performed on the basis of the ratio of the wear loss of each test piece to the wear loss of an ordinary rail.
The hardness of an ordinary rail having a pearlite structure is about 250 Hv. As can be understood from FIG. 2, the wear loss ratio decreases in accordance with an increase in hardness, and at the same hardness, the bainite structure had a greater wear loss ratio than the pearlite structure. As regards the bainite structure, at the same hardness, bainite containing 1.0 wt% or more of Mn has a lower wear loss ratio than that containing less than 1.0 wt% of Mn. Specifically, the wear loss ratio of each steel in table 1 which contains less than 1.0 wt% of Mn is higher than that of each steel in tables 2, 4, 6, 8 and 10, which contains 1.0 wt% or more of Mn.
This is probably because an increase in Mn content reduces the bainite transformation temperature, thereby narrowing the width of the lath of bainite and hence enhancing the wear resistance. Further, in the case of containing 1.0 wt% or more of Mn, the wear loss ratio of the bainite structure is equal to or less than that of the usual rail when its hardness is 400 Hv or more.
Thus, if the content of Mn is 1.0 wt% or more and the hardness is 400 Hv or more, the bainite structure of the invention can have a wear resistance equal to or more than that of the ordinary rail, and can be put to practice.
Although in the invention, it suffices if the hardness is 400 Hv or more, the hardness is preferably 500 Hv or less to effectively prevent delayed fracture.

Table 1 Chemical Composition (wt%) Sample Hardness Wear Loss C Si Mn P S Cr (Hv) Ratio A-l 0.31 0.33 0.41 0.011 0.0083.03 347 2.91 A-2 0.30 0.32 0.60 0.011 0.0082.53 369 2.07 A-3 0.41 0.32 0.39 0.011 0.0081.52 390 1.55 A-4 0.29 0.32 0.82 0.010 0.007 2.50 414 1.21 [Re: Strength and Toughness]
In the case of using the rail of the present invention in cold districts, it is necessary to set the tensile strength to 1200 MPa or more, the 2 mm, U-notch Charpy absorbed energy to 30 J or more at +20C. These conditions are satisfied in the first embodiment of the invention. However, it is not always necessary to satisfy the conditions in districts other than cold ones.
[Re: Manufacturing Conditions]
In order to form the above-described bainite structure of a steel having a composition as above and obtain the above-described rail properties, the steel of the composition is hot-rolled to have a finishing temperature of 800 - 1000C, and is cooled at a cooling rate of 5C/sec. or less between a bainite transformation-starting temperature or more and 400C or less.
If the rolling finishing temperature is less than 800C, bainite transformation will start disadvan-tageously during rolling, thereby significantly reducing the strength. Moreover, if the rolling finishing temperature is more than 1000C, austenite grains are enlarged, making it difficult to secure a predetermined toughness after hot rolling. In light of these facts, the rolling finishing temperature is set to 800 -1000C.

21S2~77 As regards the cooling rate, the bainite structure having a desired strength and a desired toughness can be obtained even by air cooling. If, however, the cooling rate exceeds 5C/sec., martensite will appear and reduce the toughness. Therefore, the cooling rate is set to 5C or less.
It is preferable to perform on-line cooling in which the bainite structure is cooled on a rolling line immediately after it is rolled on the same line. The on-line cooling is advantageous in thermal efficiency, as compared with a treatment in which the bainite structure is cooled to a room temperature after hot rolling and then reheated.
In the rail manufactured by the above-described method, each of the head top portion and the head corner portion has a hardness of 400 Hv or more, and a tensile strength of 1200 MPa or more, and a 2 mm, U-notch Charpy absorbed energy of 30 J or more at +20C. The rail constructed as above can be used in high-speed transpor-tation railroads or heavy-duty railroads, or in cold districts.
Examples Examples of the invention will be explained.
In the description, the drawings and the tables, uE20 represents a 2 mm, U-notch Charpy absorbed energy at +20C.

Example 1 Steel samples having compositions shown in FIG. 2 were heated to 1250C, rolled at 920C, and acceler-atedly cooled at 3C/sec., thereby forming steel plates of a thickness of 12 mm. The steel plates were sub-jected to a tensile test, a Charpy impact test and a wear test. In the wear test, test pieces having a diameter of 30 mm and a width of 8 mm were cut from the steel plates, and were tested under the conditions of a contact load of 50 kg, a slippage ration of -10%, and no lubricant. The wear loss of each test piece after 500,000 rotations was measured. The ratio of the wear loss of each test piece to the wear loss of an ordinary rail was calculated. Table 3 shows the mechanical properties and the wear loss ratio of each steel sample.
As is shown in FIG. 3, a sample B-l, which has a C
content lower than the present invention, has a hardness of 333 Hv lower than the lower limit value of the present invention and a wear loss ratio of 2.64 higher than the invention. Accordingly, the sample B-1 cannot be put to practice.
Samples B-6 and B-7, which have C contents higher than the invention and a pearlite structure, have hardnesses and wear loss ratios falling within target ranges of the present invention. However, they have low toughnesses of uE20 = 20.5 J and 16.4 J.
On the other hand, samples s-2, B-3 B-4 and B-5, 21~2877 which satisfy the component ranges of the invention, have strengths, toughnesses and wear loss ratios falling within target ranges of the invention.
Table 2 (wt%) Sample C Si Mn P S Cr B-l 0.13 0.332.02 0.009 0.0072.01 B-2 0.21 0.332.03 0.011 0.0082.03 B-3 0.30 0.322.03 0.011 0.0082.03 B-4 0.41 0.322.02 0.011 0.0082.02 B-5 0.49 0.322.04 0.010 0.0072.00 s-6 0.60 0.321.03 0.010 0.0072.01 B-7 0.80 0.540.85 0.016 0.0092.00 Table 3 Sample TS uE20 Hard- Wear Micro-(MPa) (J) ness Loss Structure (Hv) Ratio B-l 1009 93.9 333 2.64 Bainite B-2 1224 60.9 406 0.99 Bainite B-3 1269 51.8 417 0.62 Bainite B-4 1331 39.4 445 0.50 Bainite B-5 1416 33.4 464 0.41 Bainite B-6 1088 20.5 302 1.30 Pearlite B-7 1228 16.4 348 0.46 Pearlite Example 2 Steel samples having compositions shown in table 4 were processed in the same manner as in Example 1, and resultant steel plates were subjected to the tensile test, the Charpy impact test and the wear test. All the samples had the bainite structure. Table 5 shows the mechanical properties and the wear loss ratio of each steel sample. A sample C-l, which has a Mn content lower than the present invention, has a hardness lower than the present invention and a wear loss ratio of 3.15 higher than the present invention.
On the other hand, samples C-2, C-3, C-4, C-5, C-6, - 2152~77 C-7 and C-8, which have Mn contents falling within the range of the present invention, have hardnesses of 400 Hv or more, and wear loss ratios of less than 1.
Furthermore, they show excellent tensile strengths of 1200 MPa or more and excellent toughnesses of uE20 = 30 J or more. However, in the case of a sample C-9 having a Mn content higher than the range of the invention, it is found that the Mn effect of increasing the wear resistance is saturated.
Table 4 (wt%) Sample C Si Mn P S Cr C-l 0.31 0.340.31 0.008 0.0072.51 C-2 0.31 0.341.02 0.010 0.0072.51 C-3 0.30 0.311.53 0.010 0.0072.53 C-4 0.30 0.311.99 0.010 0.0072.53 C-5 0.31 0.312.48 0.010 0.0072.52 C-6 0.32 0.303.04 0.009 0.0082.53 C-7 0.31 0.313.50 0.009 0.0072.52 C-8 0.30 0.313.99 0.009 0.0072.52 C-9 0.31 0.344.52 0.009 0.0082.53 2152~77 Table 5 Sample TS uE20 Hard- Wear (MPa) (J) ness Loss (HV) Ratio C-l 1122 32.4 340 3.15 C-2 1281 63.7 403 0.99 C-3 1356 61.6 411 0.88 C-4 1428 66.8 425 0.79 C-5 1509 62.2 447 0.65 C-6 1571 60.3 468 0.53 C-7 1613 58.8 479 0.46 C-8 1652 55.4 485 0.44 C-9 1707 48.1 506 0.48 Example 3 Steel samples having compositions shown in table 6 were processed in the same manner as in Example 1, and resultant steel plates were subjected to the tensile test, the Charpy impact test and the wear test. All the samples had the bainite structure. Table 7 shows the mechanical properties and the wear loss ratio of each steel sample. A sample D-l, which has a Cr content lower than the present invention, has a hardness lower 21~2877 than the present invention and a high wear loss ratio of 2.52.
On the other hand, samples D-2, D-3, D-4, D-S, D-6, D-7, D-8, D-9 and D-10, which have Cr contents falling within the range of the present invention, have hardnesses of 400 Hv or more, and wear loss ratios of less than 1. Furthermore, they show excellent tensile strengths of 1200 MPa or more and excellent toughnesses of uE20 = 30 J or more. However, in the case of a sample D-ll having a Cr content higher than the range of the invention, it is found that the Cr effect of increasing the wear resistance is saturated.

Table 6 ( wt%
Sample C Si Mn P S Cr D- 1 0.40 0.31 2.04 0.009 0.008 0.10 D- 2 0.40 0.32 2.05 0.009 0.007 0.35 D- 3 0.41 0.32 2.02 0.011 0.007 0.57 D- 4 0.42 0.31 2.01 0.010 0.008 1.00 D- 5 0.40 0.33 2.01 0.011 0.007 1.51 D- 6 0.41 0.32 2.02 0.011 0.008 2.02 D- 7 0.40 0.32 2.04 0.008 0.008 2.52 D- 8 0.40 0.32 2.04 0.009 0.008 3.04 D- 9 0.42 0.32 2.03 0.010 0.007 3.49 D-10 0.41 0.32 2.03 0.010 0.007 3.98 D-ll 0.41 0.32 2.01 0.010 0.006 4.50 - 21~2877 Table 7 Sample TS uE20 Hard- Wear (MPa) tJ) ness Loss (Hv) Ratio D- 1 1081 31.2 332 2.52 D- 2 1212 43.8 400 0.97 D- 3 1154 49.1 405 0.90 D- 4 1296 48.4 411 0.78 D- 5 1339 49.8 429 0.61 D- 6 1388 45.5 433 0.57 D- 7 1474 49.2 448 0.52 D- 8 1564 43.7 469 0.44 D- 9 1620 47.3 486 0.39 D-10 1677 46.6 502 0.36 D-ll 1701 40.2 524 0.37 Example 4 Steel samples having compositions shown in table 8 were processed in the same manner as in Example 1, and resultant steel plates were subjected to the tensile test, the Charpy impact test and the wear test. All the samples had the bainite structure. Table 9 shows the mechanical properties and the wear loss ratio of each steel sample. A sample E-l, which has a composition of the present invention and in which Ni and Mo are not contained, has a hardness of 400 Hv or more. Further, it shows a strength, a toughness and a wear loss ratio which fall within the target ranges of the invention.
Steel samples E-2 and E6, which contain less than 0.1 wt% of Ni and Mo, respectively, show substantially the same strength, toughness and wear loss ratio as the sample D-l containing no Ni and Mo. This means that addition of less than 0.1 wt% of Ni and Mo shows almost no effect. Samples E-3, E-4, E-7, E-8 and E-10, which contain 0.1 to 1.0 wt% of Ni and/or 0.1 to 1.0 wt%
of Mo, have hardnesses of 400 Hv or more, and show excellent strengths, toughnesses and wear loss ratios.
In particular, they show strengths higher than the sample E-l. Steel samples E-5 and E-9, which respec-tively contain more than 1.0 wt% of Ni and Mo, respectively, show substantially the same strength, toughness and wear loss ratio as the samples E-4 and E-8. This means that if the Ni or Mo content exceeds 1.0 wt%, its addition effect is saturated.

Table 8 ( wt% ) Sample C Si Mn P S Cr Ni Mo E- 1 0.41 0.32 1.02 0.011 0.007 2.02 - -E- 2 0.40 0.31 1.04 0.011 0.007 2.02 0.05 E- 3 0.40 0.31 1.04 0.011 0.007 2.02 0.21 E- 4 0.39 0.32 1.01 0.012 0.008 2.02 0.73 E- 5 0.39 0.32 1.01 0.012 0.008 2.02 1.50 E- 6 0.40 0.31 1.02 0.012 0.007 2.01 - O .06 ~ r~

E- 7 0.40 0.31 1.02 0.012 0.007 2.01 - O .22 00 E- 8 0.41 0.31 1.02 0.010 0.007 2.01 - 0.70 E- 9 0.41 0.31 1.02 0.010 0.007 2.01 - 1.49 E-10 0.40 0.31 1.04 0.011 0.007 2.02 0.21 0.23 21~2877 Table 9 Sample TS uE20 Hard- Wear (MPa) (J) ness Loss (HV) Ratio E- 1 1378 41.8 422 0.54 E- 2 1386 40.6 424 0.54 E- 3 1454 41.1 435 0.47 E- 4 1569 37.8 461 0.39 E- 5 1601 33.4 470 0.39 E- 6 1370 42.2 419 0.56 E- 7 1438 38.3 431 0.50 E- 8 1526 35.5 456 0.41 E- 9 1555 34.9 558 0.41 E-10 1481 38.6 442 0.45 Steel samples having compositions shown in table 10 were processed in the same manner as in Example 1, and resultant steel plates were subjected to the tensile test, the Charpy impact test and the wear test. All the samples had the bainite structure. Table 11 shows the mechanical properties and the wear loss ratio of each steel sample. A steel sample E-l, which has a composi-tion of the present invention and in which Nb and V are not contained, has a hardness of 400 Hv or more.
Further, it shows a strength, a toughness and a wear loss ratio which fall within the target ranges of the present invention.
Steel samples F-2, F-3, F-5, F-6 and F-8, which contain 0.01 to 0.1 wt% of Nb and/or Mo, show excellent strength, toughness and wear loss ratio. In particular, their strength and hardness are higher than those of the sample F-l. Steel samples F-4 and F-7, which respec-tively contain more than o.l wt% of Nb and v, show substantially the same strength, toughness and wear loss ratio as the samples F-3 and F-6. This means that if the Nb or V content exceeds 0.1 wt%, its addition effect is saturated.
Moreover, a steel sample F-9, which have Ni, Mo, Nb and V contents falling within the ranges of the present invention, shows excellent strength, toughness and wear loss ratio as compared with the sample E-10 containing Ni and Mo and the sample F-8 containing Nb and V.

Table 10 ( wt% ) Sample C Si Mn P S Cr Ni Mo Nb P

F-l 0.30 0.31 1.53 0.010 0.007 2.53 - - - -F-2 0.32 0.32 1.51 0.012 0.008 2.52 - - 0.03 F-3 0.32 0.32 1.51 0.012 0.008 2.52 - - 0.08 F-4 0.32 0.32 1.51 0.012 0.008 2.52 - - 0.15 F-5 0.31 0.31 1.51 0.010 0.008 2.53 - - - 0.03 F-6 0.31 0.31 1.50 0.010 0.008 2.53 - - - 0.10 cn oo F-7 0.31 0.31 1.50 0.010 0.008 2.53 - - - O .20 ~~

F-8 0.32 0.32 1.51 0.011 0.008 2.51 - - 0.09 0.09 F-9 0.32 0.31 1.51 0.011 0.008 2.510.20 0.19 0.09 0.09 Table 11 Sample TS uE20 Hard- Wear (MPa) (J) ness Loss (HV) Ratio F-l 1356 61.6 411 0.88 F-2 1400 60.1 419 0.84 F-3 1471 63.9 441 0.53 F-4 1503 59.4 446 0.51 F-5 1406 58.2 421 0.74 F-6 1512 61.3 448 0.50 F-7 1571 55.6 464 0.47 F-8 1558 55.6 460 0.42 F-9 1634 44.8 483 0.40 Example 6 Table 12 shows steel samples G-l and G-2. A rail stock was prepared by hot rolling the samples to the actual shape of a rail, with the rolling finishing temperature varied from 760 to 1030C. Thereafter, the rail stock was cooled with the cooling conditions varied from air cooling to accelerated cooling of 6. 5C/sec., thereby forming a rail. Table 13 shows the manufac-turing conditions.

Table 13 also shows the tensile properties, the 2 mm, U-notch Charpy absorbed energy at +20C, the hardness and the wear loss ratio of each rail sample manufactured as above. The wear loss ratio was estimated by sub;ecting the wear test samples in Example 1 extracted from the rolling material head portion, to the same test as in Example 1.
Under conditions 1 which satisfied the cooling rate but did not satisfy the rolling finishing temperature, the sample showed a low tensile strength of 1068 MPa and a high wear loss ratio of 3.11 (a hardness of 320 Hv).
Under conditions 2 - 6, 8 - 10 and 12 which satisfied both the cooling rate and the rolling finishing temperature, the samples showed excellent values, i.e. a hardness of 400 Hv or more, a wear loss ratio of 1 or less, a tensile strength of 1200 MPa or more and a toughness of uE20 = 30 J or more.
Under conditions 7, 11 and 13 which satisfied the rolling finishing temperature but did not satisfy the cooling rate, the samples showed low toughnesses of UE20 = 23.0 J, 28.5 J and 21.1 J, respectively.
Under conditions 14 and 15 which satisfied the cooling rate but did not satisfy the rolling finishing temperature, the samples showed low toughnesses of uE20 = 28.9 J and 22.0 J, respectively.

Table 12 (wt%) Steel C Si Mn P S Cr G-l 0.29 0.34 1.52 0.011 0.007 2.31 G-2 0.40 0.33 1.03 0.011 0.007 2.04 Table 13 Conditions Steel Rolling Cooling TS uE20 Hardness Wear Loss No. Temperature Rate (C/s) (MPa) (J) (Hv) Ratio ( C) 1 G-l 760Air Cooling 1068 32.8 320 3.11 2 G-l 820Air Cooling 1207 84.4 401 0.99 3 G-2 820 2.9 1309 47.3 413 0.66 4 G-2 870Air Cooling 1246 46.2 406 0.68 G-l 870 2.0 1310 62.2 410 0.87 6 G-l 870 3.2 1341 56.6 419 0.83 7 G-2 870 6.3 1421 23.0 430 0.53 8 G-2 920Air Cooling 1285 52.9 408 0.63 9 G-2 920 3.1 1378 41.8 422 0.54 G-l 920 4.9 1426 55.3 436 0.65 11 G-l 920 6.5 1476 28.5 444 0.55 12 G-2 970 3.1 1391 36.4 428 0.57 13 G-l 970 6.2 1494 21.1 446 0.63 14 G-l 1030Air Cooling 1372 28.9 421 0.73 G-2 1030 2.9 1436 22.0 428 0.53

Claims (12)

1. A rail of high toughness and high wear resistance, consisting essentially of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt%
or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities, the rail having a metal structure of a bainite structure, a hardness of 400 Hv or more at each of a head top portion and a head corner portion thereof, a tensile strength of 1200 MPa or more, and a 2 mm, U-notch Charpy absorbed energy of 30 J or more at +20°C.

2. The rail according to claim 1, further contain-ing at least one selected from the groups consisting of 0.1 to 1.0 wt% of Ni and 0.1 to 1.0 wt% of Mo.

3. The rail according to claim 1, further contain-ing at least one selected from the group consisting of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of V.

4. The rail according to claim 2, further contain-ing of at least one selected from the group consisting of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of V.

5. A rail of high toughness and high wear resistance, consisting essentially of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt%
or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities, the rail having a metal structure of a bainite structure, and a hardness of 400 Hv or more at each of a head top portion and a head corner portion thereof.

6. The rail according to claim 5, further contain-ing at least one selected from the groups consisting of 0.1 to 1.0 wt% of Ni and 0.1 to 1.0 wt% of Mo.

7. The rail according to claim 5, further contain-ing at least one selected from the group consisting of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of V.

8. The rail according to claim 6, further contain-ing at least one selected from the group consisting of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of v.

9. A method for manufacturing a rail of high toughness and high wear resistance, comprising the steps of:
(a) preparing a steel consisting essentially of 0.2 to 0.5 wt% of C, 0.1 to 2.0 wt% of Si, 1.0 to 4.0 wt% of Mn, 0.035 wt% or less of P, 0.035 wt% or less of S, 0.3 to 4.0 wt% of Cr, and the balance of iron and inevitable impurities;
(b) hot rolling the steel to have a rolling finishing temperature of 800 - 1000°C, thereby forming a rail stock; and (c) cooling the rail stock at a cooling rate of 5°C/sec. or less between a bainite transformation-starting temperature or more and 400°C or less.

10. The method according to claim 9, wherein the steel further contains at least one selected from the groups consisting of 0.1 to 1.0 wt% of Ni and 0.1 to 1.0 wt% of Mo.

11. The method according to claim 9, wherein the steel further contains at least one selected from the group consisting of 0.01 to 0.1 wt% of Nb and 0.01 to 1.0 wt% of v.

12. The method according to claim 10, wherein the steel further contains at least one selected from the groups consisting of 0.1 to 1.0 wt% of Ni and 0.1 to 1.0 wt% of Mo.