JPH08109440A - High toughness rail with pearlitic metallic structure - Google Patents

Abstract

PURPOSE: To produce a highly wear resistant rail excellent in ductility and toughness by refining the grain size of the pearlite block in respectively specified ranges from railhead surface and from railfoot, respectively, in the cross section of a rail having high carbon content and pearlitic structure. CONSTITUTION: A bloom of a steel, containing, by weight, 0.60-0.85% C, 0.10-1.20% Si, and 0.40% Mn or further containing one or >=2 kinds among 0.05-2.00% Cr, 0.01-0.30% Mo, 0.02-0.10% V, 0.002-0.01% Nb, and 0.1-2.0% Co, is roughed and then hot-rolled at a relatively small rolling reduction of 5-30% in reduction of area, and hot rolling is finished at about 1000 deg.C. The average grain size of the pearlite block in the cross section of a rail is refined so that it is 20-50μm in the range between the railhead surface and a position at a depth of at least 20mm from the railhead surface and in the range between the railfoot and a position at a depth of 15mm from the railfoot and also 35-100μm in the part other than the above. Further, in the former parts, elongation value and U-notch Charpy value are regulated to >=12% and 25J/cm<2> , respectively. By this method, the pearlitic rail, excellent in strength, hardness, ductility, toughness, and wear resistance, 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 toughness rail which is used for railways and other industrial machines and which has toughness imparted to steel having a high carbon pearlite structure which is excellent in strength and wear resistance.

[0002]

2. Description of the Related Art Steel with a high carbon and pearlite metallographic structure is used as a structural material because it has high strength and good wear resistance. Corresponding to the trend, rails are often used.

As a method for producing such a steel material, for example, Japanese Patent Application Laid-Open No. 55-2768 describes that "steel of a specific component that easily exhibits a pearlite structure is cooled from a heating temperature of Ac ₃ or higher to 450 to 600 ° C. The method of manufacturing a hard rail that produces a fine pearlite structure by isothermal transformation at the temperature of
Further, JP-A-58-212229 discloses "C: 0.6
A rail containing 5 to 0.85% and Mn: 0.5 to 2.5% and having high temperature heat was rapidly cooled to improve wear resistance by making the structure of the rail or rail head fine pearlite. "Heat treatment method for rails" is disclosed, and further, Japanese Patent Laid-Open No.
9-133322 gazette "A rolling rail of a specific component capable of stably obtaining a pearlite structure is immersed in a molten salt bath at a temperature of Ar ₃ or higher to a specific temperature to reach about 10 mm below the surface of the top of the rail top. Many techniques for obtaining a high-performance rail are known, as is disclosed in "Method of heat treatment of rail having fine pearlite structure having hardness of Hv>350".

However, although the strength and wear resistance of pearlite steel can be easily obtained by heat treatment and addition of alloying elements to the required standard, the toughness is significantly lower than that of steel mainly composed of ferrite structure, For example, for pearlite rail steel, the room temperature test value in the JIS No. 3 U notch Charpy test is about 10 to ²⁰ J / cm ² . Thus ductility
When steel with low toughness is used as a structural member in a field subject to repeated loading and vibration, there is a concern that low stress brittle fracture may occur from minute initial defects and fatigue cracks.

[0005]

SUMMARY OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and overcomes the problems of controlled rolling that depended on low temperature or large reduction in rail forming.
It is an object of the present invention to provide a high toughness rail having not only wear resistance but also ductility and toughness by taking measures such as steel composition suitable for eutectoid steel and pearlite grain control.

[0006]

It is generally said that the means for improving the toughness of steel is achieved by grain refinement of the metal structure, that is, grain refinement of the austenite structure and intragranular transformation. The fine graining of the austenite structure is performed by, for example, low temperature heating during rolling, or combination of controlled rolling and heat treatment as disclosed in JP-A-63-277721, and low temperature heat treatment after rolling. ing.
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. It is being improved.
However, this method also has problems such as high manufacturing cost and low productivity as the development of labor-saving and productivity improving technology for each steel product is advanced in recent years, and early solution of these problems is desired. ing.

The inventors of the present invention have conducted many experiments from the steel components and the manufacturing method thereof in order to manufacture a steel having a fine grained pearlite structure and improved toughness, and as a result, have found the following. . That is, the rail head is mainly required to have wear resistance, and the bottom portion is required to have bending fatigue resistance and ductility. By using this part as the eutectoid C component and controlling the fine-grained pearlite block size, wear resistance, ductility, and ductility are improved. The inventors have found that a rail having toughness can be obtained, and completed the present invention.

It has been found that a high carbon steel close to the eutectoid carbon steel is recrystallized immediately after rolling at a relatively low temperature and with a small reduction amount during the working in the austenite state, and by continuous rolling under a small reduction. It was found that fine sized austenite grains were obtained, and as a result, fine pearlite structure was obtained.

As shown in FIG. 1, the pearlite block is a set of pearlites 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 forming pearlite are laminated. Then, the pearlite block becomes a destruction unit at the time of pearlite grain destruction.

The present invention is constructed on the basis of such findings, and the gist of the present invention is% by weight.
C: 0.60 to 0.85%, Si: 0.10 to 1.
20%, Mn: 0.40 to 1.50%, and if necessary, Cr: 0.05 to 2.00%, Mo:
0.01-0.30%, V: 0.02-0.10%,
Co: 0.1-2.0%, Nb: 0.002-0.0
Steel containing 1% or more of 1% or more, the balance being Fe and unavoidable impurities and having a pearlite structure, and the average particle size of the pearlite block in the rail cross section from the rail crown surface to the rail crown surface. At least 2 as a starting point
0 to 50 mm in the range of 0 mm and the bottom of the rail from the bottom of the rail in the range of at least 15 mm, and 35 to 100 μm in other parts, and the elongation value at the part where the pearlite block average particle diameter of the rail is 20 to 50 μm. 12% or more, U-notch Charpy value is 25 J /
A high toughness rail having a pearlite metallographic structure characterized by having a size of not less than cm ² .

The present invention will be described in detail below. First, the reason why the steel composition is limited as described above in the present invention will be described. C: C is required to be contained in an amount of 0.60% or more as an effective component for generating a pearlite structure and ensuring wear resistance. However, at a high content exceeding 0.85%, a large amount of cementite structure is precipitated and the hardness is increased, but the ductility is slightly decreased, and the high toughness, which is the object of the present invention, cannot be obtained sufficiently stably. Therefore, the C content is set to 0.60 to 0.85%.

Si: Si is contained in an amount of 0.1% or more as an effective component for strengthening the ferrite in the pearlite structure. However, if the content exceeds 1.20%, there is a problem that a martensite structure is generated and the steel is embrittled. Therefore,
The amount of Si was 0.10 to 1.20%. Mn: Mn is an element effective in strengthening the pearlite structure,
If the content is less than 0.40%, the effect is small. On the other hand, if it exceeds 1.50%, a martensitic structure is generated and the steel becomes brittle. Therefore, the Mn content is 0.40 to 1.
It was set to 50%.

Cr: Cr is an element that raises the equilibrium transformation point of pearlite, resulting in a finer pearlite structure. If it is less than 0.05%, its effect is small, and if it is added in excess of 2.0%, martensite is added. It produces a structure and embrittles the steel. Therefore, the Cr content is 0.05 to 2.00%
And Mo, Nb: Mo and Nb are effective elements for strengthening pearlite, and if the content is less than 0.01% and 0.002%, the effect is small. On the other hand, 0.30%,
Addition of more than 0.01% suppresses recrystallization of austenite grains during rolling, which is effective for grain refinement of the metal structure, as described later, produces elongated coarse austenite, and embrittles pearlite. Therefore, the amount of Mo is 0.01 to 0.
30% and the amount of Nb were 0.002-0.01%.

V, Co: 0.02 to 0.1% of V, Co
0.10 to 2.0% is an effective content for strengthening the pearlite structure of each component. If the amount is less than the lower limit, the strengthening effect is small, and if the amount is more than the upper limit, the strengthening effect is saturated. Reach Since the present invention is based on the knowledge of the recrystallization behavior of austenite peculiar to high carbon steel, which will be described later, there is no problem in the range where the metal structure exhibits pearlite even if the above-mentioned various components are added if necessary.

The average particle size of the pearlite block of the rail is 2
A range of 0 to 50 μm, a range of at least 20 mm from the rail crown surface starting from the rail crown surface, and at least 15 mm from the rail bottom surface starting from the rail bottom surface.
The range of mm is less than 20 mm from the top surface of the rail in consideration of the wear of the rail, because the rail head is damaged due to the contact with the wheels due to the passage of the train.
Also, the range in which the tensile stress generated at the bottom affects the damage is less than 15 mm from the bottom of the rail.

The average particle size of the pearlite block near the top and bottom surfaces of the rail is set to be in the range of 20 to 50 μm. When the particle size is reduced to less than 20 μm, the hardness required for ensuring wear resistance, which is a basic characteristic of the rail, is obtained. Not obtained, 50 μm
If it exceeds, the ductility and toughness will deteriorate.

The average particle size of the pearlite block except the vicinity of the top and bottom surfaces of the rail top is set to 35 to 100 μm. If the particle size is reduced to less than 35 μm, the strength required for the rail base material cannot be obtained and 100 μm is obtained. This is because if it exceeds, the ductility and toughness of the rail base material deteriorates.

Perlite block average particle size 20 to 50
The rail elongation in the μm range is 12% or more, and the U-notch Charpy value is 25 J / cm ² or more because the elongation is 12 or more.
If it is less than%, it may not be possible to stably handle elongation strain when passing a severe train, and there is a concern that cracking may occur due to long-term use, and if the U-notch Charpy value is less than 25 J / cm ^2, it will also be stable to shock when passing a severe train. It is because there is a concern that it cannot be dealt with and may be destroyed by long-term use.

Next, it will be explained that the rail of the present invention can be obtained by the following manufacturing method. In rail steel rolling, in the finish rolling stage after rough rolling of the slab,
From the viewpoint of securing the formability of the rail, the reduction amount per one pass is a relatively small range of 5 to 30% in terms of cross-section reduction rate, and the finishing temperature is about 1000 ° C. On the other hand, recently, rolling at a lower temperature and controlled rolling for the purpose of improving ductility and toughness have also been performed.

In general, in the case of controlled rolling of steel mainly composed of ferrite, the rolling temperature is lowered to a non-recrystallized region of austenite and the strain is introduced into the worked austenite to increase the ferrite nuclei to obtain fine grained ferrite. The controlled rolling method is used. However, in the case of pearlite steel, the transformation growth rate of pearlite is high due to the eutectoid transformation, and the transformation nuclei in the degree of work strain in the austenite grains do not act effectively, and it is practically difficult to obtain fine grain pearlite. It was Therefore, it was found that it is necessary to obtain fine-grained austenitic steel for pearlite grain refinement.

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

After the rolling utilizing the recrystallization behavior of the austenite is completed, the metal structure of the rail composed of the steel in the above-mentioned composition range is obtained by allowing the material to cool or accelerate it to further improve the strength. The diameter of the pearlite block inside becomes smaller, and the diameter of the pearlite block is 20 to 50μ in the head that contacts the wheel and the bottom that receives bending stress.
m and other parts exhibit a fine pearlite structure of 35 to 100 μm, and have particularly high ductility and toughness in addition to the strength and hardness that the rail should originally have.

By the way, since pearlite grows from an austenite grain boundary as a starting point, it is necessary to refine the austenite grains in order to miniaturize the pearlite block size. By hot working the steel in the austenite temperature range, Austenite grains are refined. Since the austenite grains are recrystallized every hot working, the austenite grains are refined by repeating the hot working and increasing the rolling reduction. On the other hand, since the austenite crystal grains start grain growth in a short time after rolling, the crystal grains become coarse when the rolling interval is long. 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.

[0024]

EXAMPLES Table 1 shows the chemical composition of rails. Table 2 shows heating conditions and finish rolling conditions. Table 3 shows the cooling conditions after rolling. Table 4 shows the mechanical properties of the invention rail and the comparative rail when rails were manufactured by combining the steel components, rolling conditions and cooling conditions shown in Tables 1 to 3.
The average particle size of the pearlite block was determined from the number of crystal grains cut by a line segment in the microscope visual field by the crystal grain size test method according to JIS G 0552. It can be seen that the strength of the rail of the present invention changes depending on the steel composition and cooling conditions, but the ductility value and toughness value are significantly higher than those of the comparative method.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

[0029]

As described above, according to the present invention, it is possible to provide a pearlite rail having not only strength, hardness and ductility but also extremely excellent toughness.

[Brief description of drawings]

FIG. 1 shows a schematic view of pearlite crystal grains.

Claims (2)

[Claims] 1. By weight%, C: 0.60 to 0.85%, Si: 0.10 to 1.20%, Mn: 0.40 to 1.50% are contained, and the balance is Fe and unavoidable. With a pearlite structure in steel consisting of
The average particle size of the pearlite block in the rail cross section is 20 to 50 μm in the range of at least 20 mm from the rail top surface starting from the rail top surface and at least 15 mm from the rail bottom surface in the rail bottom surface. The pearlite block of the rail has an average grain size of 20 to 50 μm, an elongation value of 12% or more, and a U-notch Charpy value of 25.
High toughness rail with a pearlite metallographic structure characterized by J / cm ² or more. 2. By weight%, C: 0.60 to 0.85%, Si: 0.10 to 1.20%, Mn: 0.40 to 1.50% are contained, and Cr: 0. 05-2.00%, Mo: 0.01-0.30%, V: 0.02-0.10%, Co: 0.1-2.0%, Nb: 0.002-0.01% 1 or 2 or more of, and the balance being Fe and unavoidable impurities in the balance and having a pearlite structure, the average particle size of the pearlite block in the rail cross section is from the rail top surface to the rail top surface as a starting point. 20 to 50 μm in a range of at least 20 mm, and in a range of at least 15 mm from the bottom of the rail starting from the bottom of the rail, and 35 to 100 μm in other regions.
The average particle size of the pearlite block of the rail is 2
A high toughness rail having a pearlite metallographic structure characterized by an elongation value of 12% or more and a U-notch Charpy value of 25 J / cm ² or more in a portion of 0 to 50 μm.