JPH06340951A - High strength rail excellent in toughness and its production - Google Patents

Abstract

PURPOSE:To refine the pearlitic structure of a rail material and to produce a rail having excellent toughness and safe even in a cold district by subjecting a rail material made of steel subjected to hot rolling and forming to heat treatment under specified conditions. CONSTITUTION:A slab having a compsn. contg., by weight, 0.55 to 0.85% C, 0.20 to 1.20% Si, 0.50 to 1.50% Mn, 0.006 to 0.035% S, 0.1 to 1.0% Cr and 0.01 to 1.0% V, and the balance Fe is subjected to hot rolling, and in a high temp. state immediately after being worked into a rail, or it is once cooled to a room temp. and is thereafter reheated for the purpose of heat treatment, and the head and bottom of the rail are subjected to accelerated cooling from the temp. of an austenitic region to the temp. range of 700 to 500 deg.C at 1 to 5 deg.C/sec cooling rate. By this heat treatment, pearlitic transformation occurs even from V carbides precipitated into MnS in austenite grains in the structure to form its structure into a one of fine pearlite grains together with pearlite formed from austenite grain boundaries, by which the high strength rail excellent in toughness as rolled can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength rail in which the pearlite structure of rail steel is refined to improve toughness and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, railway transportation has been aimed at higher loads and higher speeds, and the characteristics required for rails have become increasingly severe. For heavy-duty railways, measures against wear on sharp curves and internal fatigue damage to rail heads are required. On high-speed railways, surface damage mainly on straight sections is cited as an issue. In addition to these, it is recognized that rail rupture tends to occur intensively in winter in cold regions, and improving the toughness of rail materials in cold region railways is a characteristic that is essential for safe rail transportation. ing. The following methods can be considered as measures to improve the toughness of high strength rails. (1) A method in which a rail head that has been once cooled to room temperature after normal rolling is reheated at a low temperature and then accelerated cooling is performed. (2) A method of accelerating and cooling the rail head after refining austenite grains by controlled rolling. (3) A method in which after controlled rolling, it is reheated to a low temperature before pearlite transformation and then accelerated cooling is performed.

[0003]

In the above method (1), in order to significantly improve the toughness, JP-A-55-12523 is used.
It is intended to improve the toughness by reheating to a temperature of 850 ° C. or lower, which is lower than the normal heating temperature as described in JP-A No. 1, to improve the toughness, and heating at a low temperature. Moreover, if it is attempted to deepen the heating to the inside of the rail head, it is necessary to lower the amount of heat input and heat for a long time. Therefore, there is a problem that heat treatment productivity is significantly impaired and manufacturing cost is increased. The method (2) is disclosed in JP-A-52-138427 and JP-A-52-1384.
As described in Japanese Patent Publication No. 28, in order to improve the toughness by refining the austenite grains during rolling, large reduction at high temperature is required, and the ability of the rail rolling mill or the shape control of the rail is controlled. It also contains problems. Furthermore, the method of (3), as described in Japanese Patent Publication No. 4-4371, carries out rolling of 5% or more at 800 ° C. or less and then heats it again to 750 to 900 ° C. to form austenite grains. This is a method for making it fine, and since it requires a heating furnace for low-temperature reheating after rolling, there are many problems from the viewpoint of workability, productivity, and manufacturing cost.

[0004]

The present invention is fundamentally different from the above-mentioned invention, in which C: 0.55 to 0.85% by weight, Si: 0.20 to 1.
20%, Mn: 0.50 to 1.50%, S: 0.0
06-0.035%, Cr: 0.1-1.0%,
V: 0.01-1.0%, the balance being a rail steel consisting of iron and unavoidable impurities, and the presence of pearlite having MnS in the austenite grains as the core, excellent in toughness and high. It concerns strength rails. In the present invention, the pearlite transformation, which has been conventionally said to be generated only from the austenite grain boundaries, is arranged with MnS in the austenite grains with V carbide serving as the nucleus of the pearlite transformation to generate the pearlite transformation from the austenite grains. In addition to the pearlite transformed from the grain boundary, the pearlite structure is remarkably refined by the pearlite generated from within the grain, and accordingly, the toughness can be significantly improved.

In addition to this, the rail steel having the above composition is usually rolled or reheated to a normal temperature including the rail head or bottom, and then 700 to 500 is used for cooling from the austenite region temperature in the cooling process. 1 to between ℃
By adding a method of accelerated cooling at 5 ° C / sec,
It is possible to lower the transformation temperature of pearlite having MnS as a nucleus in the austenite grains, and to further improve the toughness through the refinement of the structure by pearlite having MnS as a nucleus. That is, as shown in FIG. 1, the degree of improvement in toughness by accelerated cooling when V is added and intragranular pearlite is significantly greater than the degree of improvement in toughness by accelerated cooling when V is not added. large. In addition, high strength rails having excellent wear resistance can be manufactured by increasing the strength by accelerated cooling.

[0006]

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 slightly 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 the strengthening by lowering the pearlite transformation temperature and enhancing hardenability. Further, like Mn, it is a constituent element of manganese silicate as the 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 0.006%, MnS as pearlite transformation nuclei
The amount decreases, and it becomes impossible to secure the pearlite intragranular transformation. If it is more than 0.035%, a large amount of MnS is formed and the toughness is remarkably lowered, so 0.006 to 0.035%.
Limited to.

[0010] Cr contributes to the increase in strength by lowering the pearlite transformation, and at the same time contributes to the improvement of wear resistance by strengthening the cementite phase in the pearlite structure. On the other hand, it 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 has been conventionally limited to austenite grain boundaries by the formation of pearlite transformation centered on V carbide precipitated on MnS during cooling. 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 toughness can be significantly 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%. 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 of components is melted by the above-mentioned deoxidizing process in a commonly used melting furnace such as a converter or an electric furnace, and this molten steel is ingoted or 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, a high-strength rail having excellent toughness can be manufactured as rolled.

When high strength and high toughness are required, a temperature of 700 to 500 ° C. is set after completion of rolling or when cooling from the austenite region temperature reheated once for the purpose of heat treatment after cooling to room temperature. Rail steels that have been accelerated cooled at 1 to 5 ° C / sec can achieve even higher toughness.
That is, as a characteristic of the pearlite structure steel, pearlite transformation is caused at a low temperature by accelerated cooling, whereby the generation rate of pearlite transformation nuclei is improved, and as a result, pearlite grains can be made fine.
Therefore, the austenite intragranular transformation of the pearlite structure from the V carbides precipitated on MnS and the pearlite transformation from the austenite grain boundaries due to accelerated cooling can be superimposed to further improve the toughness of the rail steel. At this time, it is desirable that the cooling medium is a gas-liquid mixture such as air or mist, and the strength of the rail head or bottom is 1100 MPa or more.

As a method for evaluating the toughness of rail steel, there is the impact absorbed energy at + 20 ° C. in the 2 mm U-notch Charpy test defined by the Russian GOST standard. According to this standard, the impact of the high-strength heat-treated rail at + 20 ° C. Absorbed energy is required to be 0.25 MJ / m ² or more. By utilizing the above-mentioned V carbides precipitated in MnS in the austenite grains as pearlite transformation nuclei, the pearlite grains are refined in the rail steel of the present invention, and impact absorption energy of 0.25 MJ / m ² or more can be obtained. it can.

[0015]

EXAMPLES Next, examples of production of high strength rails having high toughness produced according to the present invention will be described. Table 1 shows the results of observing whether the chemical composition of the sample steel and whether or not the V-added steel and the V-free steel each contain pearlite intragranular transformation having MnS as a nucleus in the microstructure after cooling. In addition, Table 2 shows the tensile test of the rail steel as it is rolled and after accelerated cooling in which the cooling rate is changed between 700 and 500 ° C for each chemical component in the range of the cooling rate of 1 to 5 ° C / sec in order to keep the strength constant. The results of strength and impact absorbed energy measurement at + 20 ° C. in a 2 mm U-notch Charpy test are shown.

The impact test piece was taken from 1 mm below the rail head. This test condition is based on the Russian GOST standard that regulates the toughness of heat treated rails. According to this standard, the impact absorption energy at + 20 ° C of high strength heat treated rails must be 0.25 MJ / m ² or more. Therefore, the fine pearlite structure steels in which the pearlite transformation is generated from the austenite grains of the present invention all sufficiently satisfy the Charpy absorbed energy defined in the GOST standard.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

As described above, according to the present invention, pearlite is finely generated not only from the austenite grain boundaries but also from inside the grain, and it has excellent toughness even in the as-rolled state and is safe in cold regions. Can provide strength rails.

[Brief description of drawings]

FIG. 1 is a diagram showing a Charpy impact value defined by a GOST standard of a rail with or without addition of V.

 ─────────────────────────────────────────────────── ─── 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: 0.006 to 0.035% Cr: 0.1 to 1.0% V: 0.01 to 1.0%, the balance consisting of iron and unavoidable impurities, and the presence of pearlite having MnS as a nucleus in austenite grains. High strength rail with excellent toughness. 2. By weight%, C: 0.55 to 0.85% Si: 0.20 to 1.20% Mn: 0.50 to 1.50% S: 0.006 to 0.035% Cr: 0.1 to 1.0% V: 0.01 to 1.0% is contained, and the balance is steel and iron and inevitable impurities. After hot rolling is completed, or after heating to a high temperature for the purpose of heat treatment, When cooling the head or the bottom of the rail from the austenitic temperature, the temperature must be between 700 and 500 ° C for 1
Accelerated cooling at ~ 5 ° C / sec, MnS in austenite grains
A method for producing a high-strength rail with excellent toughness, which is characterized by generating pearlite with a core of.