JPH06279918A - High strength rail excellent in rolling fatigue damage resistance and its production - Google Patents

Abstract

PURPOSE:To grow a fine structure and to improve rolling fatigue damage resistance by specifying additive components in a rail steel and also specifying heat treatment conditions after hot rolling. CONSTITUTION:A high strength rail has a composition consisting of, by weight, 0.55-0.85% C, 0.20-1.20% Si, 0.50-1.50% Mn, 0.002-0.010% S, 0.1-1.0% Cr, 0.01-1.0% V, <=0.010% Al, and the balance Fe with inevitable impurities. After hot rolling or high temp. heating of this steel, accelerated cooling is applied to a railhead or further to a rail bottom part, from an austenite region temp. at a rate of (1 to 5) deg.C/sec through the temp. region between 700 and 500 deg.C, to form pearlite where MnS in an austenite grain is used as a nucleus. Further, in the railhead, the whole length of alumina cluster of >=100mum is regulated to <1000mum per centimeter. Because the additive quantity of Al is limited, the formation of Al oxide being the origin of fatigue crack can be effectively inhibited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention improves rail surface damage resistance and rail internal fatigue damage resistance by refining the pearlite structure of rail steel and further reducing non-metallic inclusions to improve ductility. The present invention relates to a high-strength rail having excellent rolling fatigue damage resistance and a method for manufacturing the same.

[0002]

2. Description of the Related Art In order to improve the efficiency of rail transportation, speeding up and heavy loading of cargo are being promoted, but along with this, wear and fatigue damage of rail heads are rapidly increasing.
In order to cope with such harsh environment of use of rail materials, especially increase in wear, technological development for strengthening rail steel has been accelerated, and rail materials in curved sections are mostly used in Japan and abroad. All of the high-strength rails now dominate.

On the other hand, on the other hand, as the wear resistance of the rail steel is improved, a fatigue damage layer that should originally be scraped off due to wear remains on the rail head surface, particularly on the gauge corner (GC) surface against which the wheel flange root is pressed, A tendency to produce surface damage has become apparent. Further, the improvement of the wear resistance of the rail steel means that the stress concentration inside the rail GC due to the wheel load is fixed at one point, and the fatigue damage from the inside of the rail head rapidly increases.

As a measure against such rail surface and internal fatigue damage, the maximum hardness of the rail top portion or GC portion or both, as disclosed in Japanese Patent Laid-Open No. 62-233301, is 2 from the rail surface. A high-strength rail with surface damage resistance to the surface of the head characterized by a depth of ~ 8 mm, the rail head disclosed in Japanese Patent Application Laid-Open No. 3-232925 has a fine pearlite structure and is internal from the GC. Vickers hardness Hv35 up to a depth of 20 mm
A rail excellent in GC internal fatigue damage resistance, characterized by having 0 to 420, and JP-A-2-2824.
As disclosed in Japanese Patent Laid-Open No. 48, the rail top has a Vickers hardness of Hv200 to 350, and the head corner has a Vickers hardness of Hv250 to 410, which is excellent in rolling fatigue damage resistance. rail,
and so on.

[0005]

However, both the rail surface and the rail against internal fatigue damage described above are intended to control the hardness distribution of the rail head by applying a special heat treatment method. That is, the hardness distribution on the rail top surface and the GC surface or on the head surface and inside is controlled by changing the cooling rate of the rail top surface or the GC part or both, or by changing the cooling rate during cooling. However, there is a problem that it is complicated and the cooling device is complicated.

The present invention provides a high-strength rail excellent in rolling fatigue damage resistance and a method for producing the same by solving the above problems, specifying additive components or further processing conditions, and improving the structure. The purpose is to do.

[0007]

In the present invention, MnS, which is one of the causes of rolling damage on the rail head surface, is finely dispersed, and a fine pearlite structure is generated from the finely dispersed MnS to form a fine grain structure. It is an Al-based material that forms a structure and improves ductility, which is effective for improving surface damage resistance, and further inhibits the formation of manganese silicate, an oxide that becomes the core of MnS by limiting the amount of Al added. While suppressing the formation of oxides,
It is also effective in suppressing the formation of Al-based oxides that are the starting points of fatigue cracks generated from inside the rail. In addition to this, after the rail steel having the above composition is normally rolled, or after the rail head or bottom is reheated to a normal temperature, 700 to 50 is used when cooling from the austenite region temperature in the cooling process.
By accelerating cooling at 0 ° C at 1-5 ° C / sec,
By lowering the pearlite transformation temperature and generating the pearlite transformation at a low temperature including the pearlite transformation with MnS in the austenite grains as the nucleus, it is possible to further refine the pearlite structure compared to the as-rolled rail steel and achieve a remarkable ductility. Not only can the improvement be achieved, but high strength rails having excellent resistance to fatigue cracks generated from inside the rails can be manufactured by increasing the strength by accelerated cooling.

That is, according to the present invention, (1) C by weight%:
0.55 to 0.85%, Si: 0.20 to 1.20%,
Mn: 0.50 to 1.50%, S: 0.002 to 0.0
10%, Cr: 0.1 to 1.0%, V: 0.01 to 1.
0%, Al: ≦ 0.010%, the balance consisting of iron and unavoidable impurities, and pearlite with MnS in the austenite grains as nuclei is present. Total length is 1
A high-strength rail with excellent rolling fatigue damage resistance, which is characterized in that it is less than 1,000 μm per cm ² .
(2) C: 0.55 to 0.85% by weight, Si: 0.
20 to 1.20%, Mn: 0.50 to 1.50%, S:
0.002-0.010%, Cr: 0.1-1.0%,
V: 0.01 to 1.0%, Al: ≤ 0.010%, and the balance of steel consisting of iron and unavoidable impurities after the hot rolling is finished, or after being heated to a high temperature for the purpose of heat treatment, When cooling the head or the bottom of the rail from the austenite temperature, the temperature should be between 1 and 5 at 700-500 ° C.
Accelerated cooling at ℃ / sec to generate pearlite with MnS in austenite grains as the nucleus, and the total length of alumina clusters of 100 μm or more at the rail head is 1 cm.
It is a method for producing a high-strength rail excellent in rolling fatigue damage resistance, which is characterized in that it is less than 1,000 μm per ² pieces.

[0009]

The present invention will be described in detail below. First,
The reason for setting the chemical composition of the rail as described above will be explained. C is an essential element for strengthening and forming a pearlite structure, and is an element that also has a unique effect on wear resistance, but if it is less than 0.55%, wear resistance and resistance to austenite grain boundaries are obtained. A large amount of proeutectoid ferrite, which is unfavorable to damage, is generated, and when it exceeds 0.85%, not only harmful proeutectoid cementite that embrittles the austenite grain boundaries is generated, but also minute amounts of rail head heat treatment layers and welds Martensite is generated in the segregated portion and the toughness is significantly impaired, so the content is limited to 0.55 to 0.85%.

Si not only contributes to the strengthening of the ferrite phase in the pearlite structure by solid solution hardening, but also contributes to the improvement of the toughness of the rail steel. Further, Si is an important element that constitutes a manganese silicate-based oxide that becomes a core of MnS together with Mn, and if 0.2% or less, its effect cannot be expected, and Si is a deoxidizing element of 0.1.
It is necessary to add 2% or more, and if it exceeds 1.2%, embrittlement is caused and weld bondability is reduced.
Limited to 0%.

Like C, Mn is an element that contributes to strengthening by lowering the pearlite transformation temperature and increasing hardenability. Further, like Mn, it is a constituent element of manganese silicate as a core of MnS, and deoxidation. It is indispensable as an acid element. However, if it is less than 0.5%, its effect is small, and if it exceeds 1.50%, the content is limited to 0.50 to 1.50% so that a martensite structure is easily generated in the segregated portion.

Although S is generally known as a harmful element, in the present invention, a precipitate (VC) based on MnS having an oxide such as manganese silicate in austenite grains as a nucleus is formed and transformed. It is an essential element because it produces a pearlite structure as the nucleus. But,
If less than 0.002%, MnS as pearlite transformation nuclei
The amount is extremely reduced, and it becomes impossible to secure pearlite intragranular transformation. On the other hand, if it is 0.010% or more, not only MnS becomes coarse and ductility is significantly lowered, but also it becomes a starting point of a rail surface damage crack, so the content is limited to 0.002 to 0.010%.

[0013] Cr contributes to higher strength by lowering the pearlite transformation and at the same time contributes to improving wear resistance by strengthening the cementite phase in the pearlite structure, but on the other hand reduces the impact toughness of cementite. It also has the effect of causing it. However, the cementite strengthening effect of Cr cannot be ignored, and addition of a small amount of Cr is also desirable from the viewpoint of preventing softening of the welded joint. Therefore, it is limited to 0.1 to 1.0% within a range in which a certain contribution is expected to ensure strength and the toughness is not impaired.

Although V is an important constituent of the present invention, it was conventionally limited to austenite grain boundaries by the fact that the formation of pearlite transformation centered on V carbide precipitated on MnS during cooling was found. The pearlite transformation nuclei can be expected also from within the austenite grains, and as a result, a rail steel composed of fine pearlite grains can be obtained, and the ductility can be greatly improved. However, 0.
If it is less than 01%, this effect is weak, and if it is added in an amount of 1.0% or more, V carbides are coarsened and become a starting point of fatigue crack initiation from inside the rail head.
It was limited to the range of 1.0%.

Al is effective as a deoxidizing agent, but the oxide of Al inhibits the formation of manganese silicate, which becomes a transformation nucleus of the pearlite structure, and not only produces alumina oxide first, but also this alumina system. Oxide is the starting point of fatigue crack generation from the inside of the rail, so it is limited to 0.010% or less. It is desirable to reduce P, which is an unavoidable impurity element, as much as possible in order to improve the toughness of the rail steel.

The rail steel having the above-mentioned composition is subjected to melting including the above-mentioned deoxidation in a commonly used melting furnace such as a converter or an electric furnace, and this molten steel is ingot-agglomerated. Method or continuous casting method, and then hot rolling. The rails that have undergone hot rolling also undergo pearlite transformation from V carbides precipitated in MnS in austenite grains during cooling, and form fine pearlite grains together with pearlite produced from austenite grain boundaries. as a result,
As-rolled, it is possible to manufacture a high-strength rail having excellent ductility.

In order to achieve high strength and high ductility, a temperature between 700 and 500 ° C. should be 1 to 700 after completion of rolling or when cooling from the reheated austenite region temperature once for the purpose of heat treatment after cooling to room temperature. Rail steel that has been accelerated cooled at 5 ° C./sec provides even higher ductility. That is, as a characteristic of pearlite structure steel, pearlite transformation is caused at a low temperature by accelerated cooling, which improves the production rate of pearlite and consequently makes pearlite grains fine. Therefore Mn
The austenite intragranular transformation of the pearlite structure from the V carbide precipitated on S and the pearlite transformation from the austenite grain boundary due to accelerated cooling can be superimposed to further improve the ductility of the rail steel. At this time, the cooling medium is
It is desirable to use a gas-liquid mixture such as air or mist and to set the strength of the rail head or bottom to 1,100 MPa or more.

The demand for ductility of rail steel has become strong from some users, and a machine using a tensile test piece with a parallel portion 6 mm diameter and a parallel portion length 30 mm taken from a rail head gauge / corner interior 10 mm depth. In the test, the rail having an elongation value of 12% or more and excellent cleanliness of inclusions is said to be a rail having excellent surface damage resistance and internal fatigue damage resistance. It is possible to secure a sufficient elongation value in the as-rolled high-strength rail steel having the above-described chemical composition and a fine pearlite structure in which the pearlite transformation with MnS in the austenite grains as the nucleus is introduced. It was possible to significantly improve the life of the surface damage occurrence.

Although the occurrence of fatigue damage from the inside of the rail head is observed inside the rail GC where the oil is sufficiently applied to the rail outside the sharp curve section of a private railway in Japan and wear is extremely suppressed. Most of the damage on overseas heavy-duty railways is fatigue damage originating from the inside of the GC. However, a test method for causing fatigue damage from inside the rail GC in a laboratory has not been established yet. However, it is known that alumina clusters, which are Al-based inclusions, are harmful to the occurrence of such rail internal fatigue damage, and in the study of the inventors, alumina clusters of 100 μm or more were collected from the rail head. 1 cm of test piece
^It has been clarified that when the total length per ² is 1,000 μm or more, internal fatigue damage occurs after laying. Therefore, 100 μm present in a 10 mm × 20 mm test piece sampled from a depth of 13 mm inside the GC of the rail under test.
The total length of the above alumina clusters was measured. As a result, it was found that all of the steels of the present invention had a few alumina clusters of 100 μm or more, and the occurrence of internal fatigue damage after rail laying could be sufficiently prevented.

When the rail steel having the above-described chemical composition and having a fine pearlite structure in which the pearlite transformation centered on MnS in the austenite grains is introduced, the temperature is 700 to 500 ° C. when cooled from the austenite region temperature after rolling. In a rail with high strength by accelerated cooling at a cooling rate of 1 to 5 ° C / sec, the strain concentration of the base metal around the alumina cluster can be reduced and the occurrence of fatigue cracks can be suppressed, so compared to the as-rolled rail. As a result, it was found that the life of the surface damage occurrence in the laboratory is increased by about 1.5 times, and the occurrence of fatigue damage inside the rail can be sufficiently prevented.

[0021]

EXAMPLES Next, production examples of high-strength rails produced according to the present invention will be described. Table 1 shows the results of observing the chemical composition of the sample steel and whether or not the rail structure after cooling contains a pearlite structure having MnS as a nucleus. Further, Table 2 shows the strength and elongation of a tensile test piece having a parallel part diameter of 6 mm and a length of 30 mm, which was sampled from the rail head gauge / corner 10 mm depth after cooling, and water using a Nishihara abrasion tester. The measurement result of surface fatigue damage occurrence life under lubrication condition is shown. The width of the test piece taken from just below the rail head surface is 8 m.
A cylindrical test piece with a width of 8 mm and a diameter of 30 mm having a chemical component equivalent to a wheel was used as a convex test piece having m, a diameter of 30 mm, and a radius of curvature of 15 mm. The test conditions are a load of 50 kg and a slip rate of 20.
The number of repetitions until surface damage occurred was measured in%.
Table 3 shows the measurement results of the number and total length of the alumina clusters of 100 μm or more existing in 1 cm ^{2 of} the test rail.

From these results, M in austenite
For rails confirmed to undergo pearlite transformation from nS, a sufficient improvement in surface fatigue damage life and prevention of internal fatigue damage have been achieved by ensuring a sufficient elongation value. Improvements have been achieved.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

Industrial Applicability According to the present invention, the surface fatigue damage life of the rail is greatly improved and the internal fatigue damage is prevented from occurring, which greatly contributes to the industrial development.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masamitsu Waka 20-1 Shintomi, Futtsu-shi, Chiba Shin Nippon Steel Co., Ltd.Technology Development Headquarters (72) Inventor Shuichi Funaki 20-1 Shintomi, Futtsu-shi, Chiba New Japan Iron & Steel Co., Ltd.

Claims (2)

[Claims] 1. By weight%, C: 0.55 to 0.85% Si: 0.20 to 1.20% Mn: 0.50 to 1.50% S: 0.002 to 0.010% Cr: 0.1-1.0% V: 0.01-1.0% Al: ≤0.010%, the balance consisting of iron and unavoidable impurities, and MnS in austenite grains serving as nuclei A high-strength rail with excellent rolling fatigue damage resistance characterized by the presence of pearlite and a total length of alumina clusters of 100 μm or more at the rail head of less than 1,000 μm per 1 cm ² . 2. C .: 0.55 to 0.85% Si: 0.20 to 1.20% Mn: 0.50 to 1.50% S: 0.002 to 0.010% Cr: 0.1-1.0% V: 0.01-1.0% Al: ≤0.010% Steel containing iron and unavoidable impurities in the balance, after the hot rolling is finished, or for the purpose of heat treatment After heating to a high temperature at 1, the head or even the bottom of the rail is cooled from austenite temperature to 700-500 ° C for 1
Accelerated cooling at ~ 5 ° C / sec, MnS in austenite grains
Of high-strength rails with excellent rolling fatigue damage resistance, characterized in that pearlite with nuclei as cores is produced, and the total length of alumina clusters of 100 μm or more at the rail head is less than 1,000 μm per 1 cm ² . Manufacturing method.