JP3081116B2 - High wear resistant rail with pearlite metal structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high wear-resistant rail which is used for railways and other industrial machines and which has high strength and wear resistance and has a high carbon pearlite structure and is provided with ductility and toughness. It is.

[0002]

2. Description of the Related Art High carbon steel having a pearlitic metal structure is used as a structural material because of its high strength and good wear resistance. It is used particularly often as a rail corresponding to the development.

[0003] As a method for producing such a steel material, for example, Japanese Patent Application Laid-Open No. 55-2768 discloses that a steel having a specific component which easily exhibits a pearlite structure is cooled from a heating temperature of _three or more Ac to 450 to 600 ° C. A method of manufacturing a hard rail that transforms at a constant temperature to generate a fine pearlite structure,
Japanese Patent Application Laid-Open No. 58-221229 discloses "C: 0.6.
The rail containing 5 to 0.85% and Mn: 0.5 to 2.5% and having high temperature heat was quenched to improve the wear resistance by making the structure of the rail or the rail head fine pearlite. Rail heat treatment method "
Japanese Patent Application Laid-Open No. 9-133322 discloses that a rolling rail of a specific component capable of stably obtaining a pearlite structure is immersed in a molten salt bath at a specific temperature from a temperature of ₃ or more Ar to a height of about 10 mm below the surface of the rail top. Many techniques for obtaining high-performance rails are known, as disclosed in "Heat treatment method for rail exhibiting fine pearlite structure having hardness of Hv>350".

[0004] However, it can be said that the strength and wear resistance of pearlite steel can be easily obtained by heat treatment or addition of alloying elements. However, in steels containing carbon above the eutectoid point, the austenitic grain boundaries can be obtained. Cementite is formed, significantly reducing ductility and toughness. For example, the elongation value in a JIS No. 4 tensile test is less than 10%, or JIS No. 3 U
It is less than 10 J / cm ^{2 as} a normal temperature test value in a notch Charpy test. When such low ductility and toughness steel is used as a structural member in a field where repeated loads and vibrations are applied, there is a concern that low stress brittle fracture may be caused by minute initial defects and fatigue cracks.

[0005]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and makes the carbon content of steel effective for improving wear resistance equal to or higher than the eutectoid point. It is an object of the present invention to provide a highly wear-resistant rail having ductility and toughness by taking measures such as controlling steel components and pearlite crystal grains in order to prevent a decrease in ductility and toughness of the resulting steel.

[0006]

Means for Solving the Problems The present inventors have found that when the wear resistance is improved by the pearlite structure, the use of hypereutectoid carbon having a temperature equal to or higher than the eutectoid point significantly improves the wear resistance. Fig. 1 compares the difference in wear resistance between the steel above the eutectoid point and the steel below the eutectoid point, the hardness on the horizontal axis, and the amount of wear after a certain time under rolling contact on the vertical axis. . It can be seen that a hypereutectoid steel having a temperature equal to or higher than the eutectoid point shows high wear resistance even at a low hardness. However, as described above, the addition of a carbon content above the eutectoid point impairs the ductility and toughness of the steel.

In general, it is said that a means for improving the toughness of steel is achieved by reducing the grain size of the metal structure, that is, by reducing the grain size of the austenite structure and intragranular transformation. The grain refinement of the austenite structure is achieved by, for example, low-temperature heating during rolling, a combination of controlled rolling and reheating as disclosed in JP-A-63-277721, or low-temperature heating after rolling. Have been. However, in the rail manufacturing method, it is difficult to apply low-temperature heating during rolling, low-temperature rolling in controlled rolling, and large reduction rolling from the viewpoint of ensuring formability. Is planned. However, this method also has problems such as high manufacturing cost and low productivity in the development of labor saving and productivity improvement technologies for recent steel products, and it is desired to solve these problems early. ing.

The inventors of the present invention have conducted a number of experiments on steel components and methods for producing the same in order to obtain a fine-grained pearlite structure and to improve the toughness of the steel, and have found the following. . In other words, the head of the rail is mainly required to have wear resistance and the bottom is required to have bending fatigue properties and ductility. This part is used as a hypereutectoid C component and the fine pearlite block size is controlled to provide wear resistance and ductility. The present inventors have found that a rail having excellent toughness can be obtained, and have reached the present invention.

It has been found that high-carbon steel is recrystallized immediately after rolling at a relatively low temperature and with a small rolling reduction in the processing in the austenitic state. Was obtained, and as a result, it was found that a fine-grained pearlite structure excellent in ductility and toughness was obtained.

As shown in FIG. 2, a pearlite block is a set of pearlite having the same crystal orientation and a set of pearlite colonies having the same crystal orientation and lamella direction. The lamella is a striped structure in which ferrite and cementite constituting pearlite are laminated. And this pearlite block becomes a destruction unit at the time of pearlite grain destruction.

The present invention has been made based on such knowledge, and the gist of the present invention is that the mass%
The lower% indicates mass) , and contains C: more than 0.85 to 1.20%, Si: 0.10 to 1.20%, Mn: 0.40 to 1.50%, and further required Accordingly, Cr: 0.05 to 2.00%, Mo: 0.01 to 0.30%, V: 0.02 to 0.10%, Nb: 0.002 to 0.01%, Co: 0 1 to 2.0% of one or more steels, the balance being steel consisting of Fe and unavoidable impurities, having a pearlite structure, and having a pearlite block average particle size in the rail cross section that is smaller than the rail top surface. A distance of at least 20 mm from the top surface of the rail and at least 15 mm from the bottom surface of the rail from the bottom surface of the rail.
５０50 μm, 35 to 100 μm in other parts, the average particle size of the pearlite block of the rail is 20 to
A highly wear-resistant rail having a pearlite metal structure characterized by an elongation value of 10% or more and a U-notch Charpy value of 15 J / cm ² or more at a portion of 50 μm.

Hereinafter, the present invention will be described in detail. First, the reason why the steel components are limited as described above in the present invention will be described. C: C is an effective component for generating a pearlite structure and ensuring wear resistance, and is usually 0.60 as a rail steel.
0.85 percent that have been found using. At this time, proeutectoid ferrite may be formed at the γ grain boundary in the pearlite structure,
From the viewpoint of improving the wear resistance and suppressing the starting point of the rail internal fatigue damage, the C content needs to be more than 0.85%. On the other hand, the amount of proeutectoid cementite in the austenite grain boundaries also increases with the increase in carbon, and if the C content exceeds 1.2%, the deterioration in ductility and toughness will occur even if the pearlite structure is refined as described below. I can't overlook it. Therefore, the amount of C is set to 0.85
% To 1.20%.

Si: Si contains 0.1% or more as an effective component for strengthening ferrite in the pearlite structure. However, when the content exceeds 1.20%, there is a problem that a martensite structure is generated and the steel is embrittled. Therefore,
The Si amount was 0.10 to 1.20%. Mn: Mn is an element that, in addition to strengthening the pearlite structure, lowers the pearlite transformation temperature and suppresses the formation of proeutectoid cementite. If it exceeds, a martensite structure is generated, and the steel is embrittled. Therefore, the amount of Mn is 0.4
0 to 1.50%.

Cr: Cr is an effective element that raises the equilibrium transformation point of pearlite, makes the pearlite structure finer, and suppresses the formation of proeutectoid cementite, and is selectively added. If it is less than 0.05%, the effect is small,
Excessive addition over 2.0% creates a martensitic structure and embrittles the steel. Therefore, the Cr content is 0.05 to
2.00%.

Mo, Nb: Mo and Nb are effective elements for strengthening pearlite, and are selectively added. The effect is small when it is less than 0.01% and 0.002%, respectively.
On the other hand, additions exceeding 0.30% and 0.01%, respectively, suppress the recrystallization of austenite grains during rolling, which is effective in refining the metal structure, as described later, and generate elongated coarse austenite grains. , Embrittles pearlite steel.
Therefore, the Mo amount is 0.01 to 0.30% and the Nb amount is 0.1%.
002 to 0.01%.

V, Co: 0.02-0.1% of V, Co
Of 0.10 to 2.0% of each component is an effective content for strengthening the pearlite structure, and is selectively added. If the amount is less than the lower limit, the strengthening effect is small, and if the amount exceeds the upper limit, the strengthening effect reaches a saturation range. Since the present invention contains carbon above eutectoid carbon as described above, and is based on the knowledge of the recrystallization behavior of austenite specific to high carbon steel, the metallographic structure can be added even if the above various components are added as necessary. There is no hindrance in the range in which pearlite is exhibited.

The pearlite block has an average particle size of 20 to 50.
The range of μm is 0 to 20 mm or more starting from the rail top surface from the rail top surface, and 0 to 15 mm or more starting from the rail bottom surface from the rail bottom surface. The area that causes a damage problem due to contact with the wheel is less than 20 mm from the top of the rail considering the wear of the rail, and the area where the tensile stress generated at the bottom affects the damage is 15 mm from the bottom of the rail.
Because it is less than.

The average particle size of the pearlite block in the vicinity of the top and bottom surfaces of the rail is in the range of 20 to 50 μm. When the particle size is reduced to less than 20 μm, the hardness required for securing the wear resistance, which is the basic characteristic of the rail, is reduced. Not obtained, 50 μm
This is because ductility and toughness are deteriorated when the ratio exceeds.

The reason why the average particle size of the pearlite block is 35 to 100 μm in the portion other than the vicinity of the top surface and the bottom surface of the rail is that when the particle size is reduced to less than 35 μm, the strength required for the rail base material cannot be obtained. If it exceeds, the ductility and toughness of the rail base material deteriorate.

The average particle size of the pearlite block is from 20 to 50.
The elongation of the rail in the range of μm was 10% or more, and the U-notch Charpy value was 15 J / cm ² or more.
Is less than% there is concern of cracks caused by long-term use can not correspond to elongation strain at a train pass, also U-notch Charpy value is destroyed by long-term use can not also correspond to impact during a train pass is less than 15 J / cm ² There is concern.

Next, the fact that the rail of the present invention can be obtained by the following manufacturing method will be described. In the rolling of rail steel, in the finish rolling stage after the rough rolling of the slab,
From the viewpoint of ensuring the formability of the rail, the rolling reduction per pass is within a relatively small range of usually 5 to 30% in terms of cross-sectional reduction rate, and the finishing temperature is about 1000 ° C. On the other hand, recently, controlled rolling for the purpose of improving ductility and toughness has been carried out at a lower temperature.

In general, in the case of controlled rolling of steel mainly containing ferrite, the rolling temperature is lowered to a non-recrystallized region of austenite, strain is introduced into the processed austenite to increase ferrite nuclei, and fine-grained ferrite is formed. The controlled rolling method to obtain However, in the case of pearlite steel, the transformation growth rate of pearlite is high due to eutectoid transformation, the transformation nucleus of the degree of processing strain in austenite grains does not work effectively, and it is found that it is difficult to obtain substantially fine pearlite. Was. Therefore, it was found that obtaining fine-grained austenitic steel was necessary for reducing pearlite.

From such a viewpoint, as a result of studying the recrystallization behavior of austenite of a high carbon steel in detail, it was found that recrystallization to a lower temperature and a lower workability than that of a low carbon steel;
After processing, the time required for complete recrystallization is very small, that is, recrystallization is completed immediately after rolling, and when processing is continuously performed even under a small pressure, it is found that recrystallization is repeated each time, and as a result, It was found that fine austenite grains were obtained by static recrystallization during this rolling. At this time, in the finishing rolling mill, a larger strain is applied to the upper part of the rail head and the lower part of the rail bottom as compared with other parts, and the fine grain size becomes relatively large.

After completion of the rolling utilizing the recrystallization behavior of austenite, if the steel is allowed to cool or to further improve the strength, accelerated cooling is performed to obtain the metallographic structure of the rail composed of steel in the above-described composition range. The diameter of the pearlite block inside becomes smaller, and the pearlite block diameter of the head in contact with the wheel and the bottom part subjected to bending stress is 20-50 μ
m and other parts have a fine pearlite structure of 35 to 100 μm, and have high ductility and toughness in addition to the inherent strength and wear resistance of the rail.

By the way, since pearlite grows from the austenite grain boundaries, it is necessary to make austenite grains fine in order to make the pearlite block size fine. Austenite grains are refined. Since the austenite grains are recrystallized every time hot working is performed, they are refined by repeating hot working and by increasing the rolling reduction. On the other hand, since the austenite crystal grains begin to grow in a short time after rolling, if the rolling interval is long, the crystal grains become coarse. Therefore, the perlite block size is
It can be controlled by the reduction ratio of hot rolling in the austenite temperature range, the number of rolling passes, the time between rolling passes, and the like.

[0026]

EXAMPLES Table 1 shows the chemical components of steel. Table 2 shows heating conditions and finish rolling conditions. Table 3 shows the cooling conditions after rolling. In Table 4, when rails were manufactured by combining steel components, rolling conditions and cooling conditions shown in Tables 1 to 3,
3 shows the mechanical properties of the inventive rail and the comparative rail. Here, the wear amount was measured by using a Nishihara type abrasion tester, using a ring-shaped test piece having a diameter of 30 mm and a wheel material as a mating material, and a slip rate of 9%.
Was measured after 500,000 times of rolling contact under the above conditions. The average particle size of the pearlite block is JIS G
It was determined from the number of crystal grains cut by a line segment in the visual field of the microscope by a crystal grain size test method according to 0552. It can be seen that the rail of the present invention has significantly improved wear resistance and almost no reduction in ductility and toughness.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

As described above, the present invention can provide a pearlitic rail having excellent wear resistance in addition to strength, hardness, ductility and toughness.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between hardness and wear (measured by a Nishihara-type abrasion tester) of steel having a C eutectoid point or higher and steel having the C or lower elongation.

FIG. 2 is a schematic view of pearlite crystal grains.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-47-7606 (JP, A) JP-A-52-138427 (JP, A) JP-A-2-305937 (JP, A) JP-A-Heisei 6 17135 (JP, A) JP-A-8-109440 (JP, A) JP-A-63-277721 (JP, A) JP-A-55-2768 (JP, A) US Patent 4,486,248 (US, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. A mass % of C: more than 0.85 to 1.20%, Si: 0.10 to 1.20%, Mn: 0.40 to 1.50%, the balance being Fe and Having a pearlite structure in steel consisting of unavoidable impurities, the pearlite block average particle size in the rail cross section is at least 20 mm from the rail top surface to the rail top surface, and from the rail bottom surface to the rail bottom surface. 20 to 50 μm in a range of at least 15 mm, and 35 to 100 in other portions
μm, and the elongation value at a portion where the pearlite block average particle size of the rail is 20 to 50 μm is 10% or more,
A highly wear-resistant rail having a pearlite metal structure characterized by a U-notch Charpy value of 15 J / cm ² or more. 2. The composition contains, by mass %, C: more than 0.85 to 1.20%, Si: 0.10 to 1.20%, and Mn: 0.40 to 1.50%, and further contains Cr: 0%. 0.05 to 2.00%, Mo: 0.01 to 0.30%, V: 0.02 to 0.10%, Nb: 0.002 to 0.01%, Co: 0.1 to 2.0 % Of steel and the balance consisting of Fe and unavoidable impurities and having a pearlite structure, wherein the average particle size of the pearlite block in the rail cross section is determined from the rail top surface to the rail top surface. At least 20mm
And the elongation value in a region where the average pearlite block particle size of the rail is 20 to 50 μm in a range of 20 to 50 μm in a range of at least 15 mm from the bottom of the rail, and 35 to 100 μm in other regions. Is 10% or more, U-notch Charpy value is 15 J / cm
Highly wear-resistant rail exhibiting a pearlite metal structure characterized by being ² or more.