JP3368557B2 - High strength rail with excellent ductility and toughness and its manufacturing method - 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-strength rail in which the pearlite structure of rail steel is refined to improve toughness and ductility, 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 ruptures tend to occur intensively in winter in cold regions, and improving the toughness of rail materials in cold region railways is an essential property for safe rail transportation. There is.

Further, in order to improve the efficiency of rail transportation, speeding up and heavy loading of cargo are being promoted, and along with this, friction and fatigue damage of the rail head are rapidly increasing. In order to cope with the harsh environment in which rail materials are used, especially the increase in wear, technological development for increasing the strength of rail steel has been accelerated, and almost all rail materials in curved sections, both in Japan and abroad, are being used. 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 which should be originally scraped off by abrasion 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. In order to improve such rail head surface damage and resistance to internal fatigue damage, it is important to improve the toughness and ductility of the rail material.

The following methods are considered as measures for improving the toughness and ductility of the high-strength rail. (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.

[0006] As a toughness evaluation method for rail steel, there is an impact absorbed energy at + 20 ° C in a 2 mm U-notch Charpy test defined by the GOST standard of Russia. According to the standard, impact of a high strength heat treated rail at + 20 ° C is measured. Absorbed energy is required to be 0.25 MJ / m ² or more. In addition, the ductility of the rail affects the generation of fatigue damage to the rail head, and the ductility requirement of the high-strength rail in China is that the diameter of the parallel section is 6 mm and the length of the parallel section is 30 mm, which is sampled from a depth of 10 mm inside the rail head GC. In the tensile test, the elongation value of 12% or more is required.

[0007]

In the above method (1), in order to improve the toughness and ductility to a great extent, JP-A-55-12 is used.
Reheating to a low temperature of 850 ° C. or lower, which is lower than the normal heating temperature as described in Japanese Patent No. 5231, is intended to improve toughness and ductility by making austenite grains fine. When heating and deepening 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-52-238.
As described in Japanese Patent No. 427 and Japanese Patent Laid-Open No. 52-138428, when it is attempted to improve toughness and ductility by refining austenite grains during rolling, large reduction at high temperature is required, and rail rolling is required. There are problems from the viewpoint of machine capacity or rail shape control.

Furthermore, the method (3) is disclosed in Japanese Examined Patent Publication No. 4-437.
As described in Japanese Patent Publication No. 1-5, 5% at 800 ° C or lower
After performing the above rolling, it is a method of refining the austenite grains by heating again to 750 to 900 ° C., and since a heating furnace for low temperature reheating is required after rolling, workability and productivity are improved. However, there are many problems from the viewpoint of manufacturing cost.

The present invention is intended to solve such a problem, and by producing pearlite transformation nuclei from the inside of austenite grains, a high-strength rail having a fine structure of pearlite and excellent in toughness and ductility, and the same. It is intended to provide a manufacturing method.

[0011]

The present invention achieves the above object, and has the following features. That is, a rail manufactured by deoxidizing molten steel to form a billet, which is manufactured by a process including hot working, and has a weight ratio of C: 0.55.
~ 0.90%, Si: 0.10 to 1.20%, M
n: 0.50 to 1.50%, S: 0.002 to
0.035%, Mg: 0.0004 to 0.01%, A
1: 0.0005-0.05%, V: 0.001-
1.00%, N: 0.0005-0.030%, or, if necessary, Cr: 0.10-
1.0%, Ni: 0.10 to 4.0%, Mo:
0.10 to 0.50%, Nb: 0.01 to 0.05
%, One or two or more, and the balance being iron and unavoidable impurities such as P, pearlite with MnS in the austenite grains as the core, and V on MnS.
There is pearlite whose core is carbonitride, and
The number of MnS having a size of 10 μm is 600 to 1 mm ²
A strong rail with excellent toughness and ductility, characterized by the presence of 12,000 pieces. And, Mg was added to the molten steel as a deoxidizing element and subjected to deoxidizing treatment to be melted, and in weight%, C: 0.55 to 0.90%,
Si: 0.10 to 1.20%, Mn: 0.50
1.50%, S: 0.002-0.035%, M
g: 0.0004 to 0.01%, Al: 0.0005 to
0.05%, V: 0.001-1.00%, N:
0.0005 to 0.030%, or, if necessary, Cr: 0.10 to 1.0%, Ni:
0.10 to 4.0%, Mo: 0.10 to 0.50%,
Nb: 0.01 to 0.05% of one or more kinds of molten steel, the balance of which is inevitable impurities such as iron and P, is made into a steel piece through an ingot-casting method or a continuous casting method. When the steel slab is hot-rolled to form a rail shape, and after the rolling is finished, or after being heated to a high temperature for the purpose of heat treatment, when the head or the bottom of the rail is cooled from the austenite temperature, Accelerated cooling at 700 to 500 ° C at 1 to 5 ° C / sec to precipitate fine MnS in austenite grains, refine the austenite grains by MnS, generate pearlite with MnS as a nucleus, and further on MnS. The present invention relates to a method for producing a high-strength rail having excellent toughness and ductility, which is characterized in that pearlite having V carbonitrides deposited on the core thereof is generated.

[0012]

The present invention will be described in detail below. First,
As a result of detailed examination of the role of MnS in the formation of pearlite, MnS prevents coarsening of the austenite grain size and makes the austenite grains finer to increase the grain boundary transformation pearlite. It was found that the pearlite transformation occurs also from V carbonitrides precipitated on MnS. The pearlite transformation is first generated from austenite grain boundaries in the cooling process, and pearlite is also generated from MnS as a nucleus and V carbonitride precipitated on MnS as the temperature decreases. Therefore, in addition to a large number of pearlites from the austenite grain boundaries that have been refined, MnS and V
Fine pearlite is generated by superposition of pearlite generated from carbonitride. Such a pearlite structure is particularly excellent in ductility and toughness. As described above, it is necessary to finely disperse MnS that acts as a transformation nucleus of pearlite in the steel, and deoxidation control is an important factor for finely dispersing the oxide that becomes the nucleus of MnS.

In the present invention, when Mg is added as deoxidizing element to molten steel during deoxidation of C, Si, Mn or Al, the affinity with solid solution oxygen in the steel is stronger than that of other elements.
Although an oxide mainly composed of g is generated and a part of the oxide floats, the Mg oxide remaining in the molten steel does not aggregate but is finely dispersed and acts as a precipitation nucleus of MnS in the cooling process. As a result, M
The number and distribution of nS can be controlled, and it effectively acts as austenite grain coarsening prevention and transformation nucleus of pearlite.

Next, the reason why the chemical composition of the deoxidized rail steel is limited will be described. C is an essential element for strengthening and forming a pearlite structure. 0.55
If it is less than 0.1%, the necessary high-strength pearlite structure is hard to obtain, and if it exceeds 0.90%, not only harmful proeutectoid cementite that embrittles the austenite grain boundaries is generated, but also rail head heat treatment scraps and Martensite is generated in the minute segregation portion of the welded portion, which significantly impairs toughness and ductility.
It was limited to 55 to 0.90%.

Si not only contributes to the strengthening of the ferrite phase in the pearlite structure by solid solution hardening, but also contributes to a slight improvement in the toughness and ductility of the rail steel. 0.
If it is less than 10%, its effect is small, and if it exceeds 1.20%, embrittlement is caused and weld bondability is also reduced.
Limited to 1.20%.

Like C, Mn is an element that lowers the pearlite transformation temperature and enhances hardenability, thereby contributing to higher strength. 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% to facilitate the formation of a martensite structure in the segregated portion.

Although S is generally known as a harmful element, in the present invention, MnS is formed with an oxide in the austenite grain as a nucleus, which prevents coarsening of austenite and a pearlite structure with MnS as a transformation nucleus. It is an essential element for producing. But 0.00
If it is less than 2%, its effect is small, and if it exceeds 0.035%, a large amount of MnS is formed and the toughness and ductility are remarkably reduced, so the content is limited to 0.002 to 0.035%.

Mg is an important constituent of the present invention. M
The g-based oxide functions as a precipitation nucleus of MnS, and its dispersion controls the distribution of MnS, and the produced MnS contributes to austenite grain refinement and the formation of pearlite transformation with it as a nucleus. As a result, it became possible to obtain a rail steel composed of fine pearlite grains by superimposing grain boundary transformed pearlite and intragrain transformed pearlite, and it was possible to significantly improve the toughness. If the amount of Mg is less than 0.0004%, the effect as the nuclei for forming MnS is insufficient, and if it exceeds 0.01%, the Mg-based oxide is coarsened and the toughness is lowered. 0004-0.01
It was limited to the range of%.

Al is an effective deoxidizing element, and the addition of Mg contributes to further refinement of the oxide. 0.005%
In the following, the effect of deoxidation is insufficient, and if it exceeds 0.05%, the oxide becomes coarse and the toughness decreases, so the Al addition amount was limited to the range of 0.0005 to 0.05%.

V is precipitated as V carbonitride on MnS during cooling and becomes a pearlite transformation nucleus. V addition amount is 0.00
If it is less than 1%, this effect is weak, and if it exceeds 1.00%, the V carbonitride becomes coarse and the toughness is lowered, so the V addition amount is limited to the range of 0.001 to 1.00%.

N acts as a transformation nucleus of pearlite M
It is a constituent element of V carbonitride on nS, and 0.0005% or more is necessary for effectively precipitating V (C, N). If it exceeds 0.030%, coarse V (C, N, N) is generated and causes a decrease in toughness, so the N addition amount is 0.000.
It was limited to 5 to 0.030%.

Further, in the present invention, in addition to the above components, one or more kinds of Cr, Ni, Mo, and
By adding Nb or the like, it is possible to obtain high toughness by improving the toughness of the ferrite base material, austenite grains at the time of heating for rail rolling, or refining the austenite grains at the time of controlled rolling, and further, in the cooling process. Accelerated cooling can provide higher strength and higher toughness at the same time.

The reason for limiting the chemical composition is Cr
Has a function of strengthening the cementite phase in the pearlite structure at the same time as contributing to the strengthening by lowering the pearlite transformation temperature, and therefore, addition of about 0.1% is effective from the viewpoint of preventing softening of the weld joint. is there.
On the other hand, if the content exceeds 1.0%, bainite and martensite are generated not only in the element segregation portion but also in the rail shoulder portion having a strong tendency of supercooling during forced cooling, resulting in a decrease in toughness. Therefore, it is limited to 0.1 to 1.0% within a range in which a certain contribution is expected to secure strength and the toughness and ductility are not impaired.

Ni is an element effective as a solid solution in ferrite to improve the toughness of ferrite. If it is less than 0.1%, its effect is extremely small. Saturate. Therefore, from the viewpoint of improving the toughness, the range is limited to 0.1% to 4%.

Mo is an element effective for improving the toughness because it suppresses the transformation rate of pearlite and refines the pearlite structure. Further, Mo prevents the induction of transformation at high temperature in conjunction with the heat generation associated with the pearlite transformation of the surface layer inside the rail during accelerated cooling, contributes to the high strength inside the rail, and enhances the hardening strength. However, the addition of less than 0.1% of Mo has little effect on the above, and the addition of more than 0.50% lowers the pearlite transformation rate to form bainite or martensite in the pearlite structure, resulting in lower toughness. . Therefore, the amount of Mo added is 0.10 to 0.50%
Limited to the range.

By heating Nb at a low temperature during hot rolling, carbonitrides of Nb suppress austenite grain growth and contribute to grain refinement. Further, the austenite grains after hot rolling are refined by high temperature heating / low temperature finish rolling, and the pearlite block size obtained after accelerated cooling is refined. At this time, the amount of Nb added needs to be 0.01% or more,
If it exceeds 0.05%, coarse Nb carbide, Nb nitride,
The toughness decreases due to the formation of Nb carbonitride. Therefore, the amount of Nb added is limited to the range of 0.01 to 0.05%. 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-described composition is subjected to melting including the above-described deoxidation in a commonly used melting furnace such as a converter or an electric furnace, and this molten steel is agglomerated and agglomerated. Method or continuous casting method, and further hot rolling is used for production.
The rails that have undergone hot rolling also produce pearlite from MnS in the austenite grains during cooling, and form fine pearlite grains together with pearlite produced from the 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, 1 to 5 ° C./700 to 500 ° C. from the austenite region temperature reheated for the purpose of heat treatment after completion of rolling or once cooled to room temperature. Higher toughness can be obtained by accelerated cooling in sec. This is because pearlite transformation occurs at a low temperature by accelerated cooling, so that the rate of generation of pearlite transformation nuclei is improved and the pearlite grains become fine. When the cooling rate during this accelerated cooling is less than 1 ° C / sec, pearlite becomes coarse, and when it is 5 ° C / sec or more, martensite is produced, which causes a decrease in toughness. Therefore, the cooling rate was limited to 1 to 5 ° C / sec.

As described above, accelerated cooling causes an increase in transformation nuclei in the pearlite transformation from austenite grain boundaries and MnS, and contributes to grain refinement of pearlite. As a result, the toughness of rail steel can be further improved. it can. At this time, a gas-liquid mixture such as air or mist is used as the cooling medium, and the strength of the rail head or bottom is 110.
It is desirable to set it to 0 MPa or more.

Next, M of 0.1 to 10 μm in rail steel
The reason why the number of nS is limited to 600 to 12000 per 1 mm ² will be described. In the present invention, sufficient deoxidation reduces oxygen and produces fine oxides. MnS produced by using this oxide as a nucleus is finely dispersed in austenite to prevent the austenite from being finely divided and to act as a pearl generating nucleus. At this time, MnS having a size of less than 0.1 μm is unlikely to be a transformation nucleus of pearlite.
Since the absolute number of MnS decreases when MnS of more than 0 μm is generated, the size of MnS is limited to 0.1 to 10 μm. Further, the number of MnS is 600 to 120 per 1 mm ² .
The reason for limiting the number to 00 is that with MnS less than 600, the effects of austenite grain refinement and intragranular transformation nuclei are small, and the toughness / ductility improvement effect cannot be obtained. Again 120
If more than 00 MnS is generated, the rail steel itself is contaminated and the toughness and ductility are rather deteriorated. Therefore, the number of MnS per 1 mm ² is limited to 600 to 12000. Here, the number of MnS is obtained by sampling with an extraction replica, photographing the replica with a transmission electron microscope, and measuring the number.

[0031]

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 chemical composition of rail steel when Mg was added to molten steel to perform deoxidation treatment and when Mg was not added.

[0032]

[Table 1]

Table 2 shows the measurement results of the number of MnS of 0.1 to 10 μm (collected by an extraction replica, photographed by a transmission electron microscope, and measured) of MnS existing in the structure of the rail steel after cooling and after cooling. 2 shows the observation result of the presence or absence of pearlite intragranular transformation existing in the structure of. In the steel of the present invention which has been deoxidized by adding Mg, a predetermined amount of fine M
The formation of nS was confirmed, and the formation of a pearlite structure starting from the austenite grain boundaries as well as MnS and V carbonitrides on MnS from the austenite grains was confirmed. confirmed.

Table 3 shows from the austenite temperature range of 700 to 700 for each chemical component in order to keep the strength as it is rolled.
Tensile test strength, elongation and 2mm of rail steel after accelerated cooling with cooling rate changed in the range of 1 to 5 ° C / S between 500 ° C
The impact absorption energy measurement result in +20 degreeC in a U notch Charpy test is shown. The tensile test was carried out on a test piece having a parallel part diameter of 6 mm and a parallel part length of 30 mm taken from a position 10 mm deep inside the rail head GC.

[0035]

[Table 2]

[0036]

[Table 3]

As a result, it was confirmed that the steel of the present invention has a sufficiently improved ductility as an effect of the pearlite microstructure as compared with the conventional comparative steel. 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 toughness in heat treatment, and according to this standard, high-strength heat-treated rails require impact absorption energy at + 20 ° C of 0.25 MJ / m ² or more. , Mg of the present invention
All of the fine pearlite structure steels in which the pearlite transformation is generated from the austenite grains by performing the deoxidation by the addition sufficiently satisfy the Charpy absorbed energy defined in the GOST standard.

[0038]

As described above, deoxidation by addition of Mg controls the size and number of MnS to make fine austenite grains and fine pearlite generated from grain boundaries, and MnS itself. , And even M
By utilizing the V carbonitrides precipitated in nS as pearlite transformation nuclei, the pearlite grains become finer.
Furthermore, the pearlite grains are made finer by accelerated cooling,
A shock absorption energy of 0.25 MJ / m ² or more can be obtained. According to the present invention, a high-strength rail excellent in toughness and ductility can be manufactured.

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-8-104946 (JP, A) JP-A-7-238339 (JP, A) JP-A-7-188875 (JP, A) JP-A-6- 340951 (JP, A) JP-A-6-279928 (JP, A) JP-A-6-279927 (JP, A) JP-A-2-47240 (JP, A) JP-B-4-4371 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9 / 04,8 / 00

Claims (4)

(57) [Claims] 1. A rail produced by deoxidizing molten steel to obtain a steel slab, which is manufactured by a process including hot working, in which C: 0.55 to 0.90%, Si: 0. 10 to 1.20%, Mn: 0.50 to 1.50%, S: 0.002 to 0.035% Mg: 0.0004 to 0.01%, Al: 0.005 to 0.05%, V: 0.001 to 1.00%, N: 0.0005 to 0.030%, with the balance being inevitable impurities such as iron and P, and pearlite having MnS in the austenite grains as the core Furthermore, pearlite having V carbonitride on MnS as a nucleus is present, and Mn having a size of 0.1 to 10 μm.
A high-strength rail with excellent toughness and ductility, characterized in that there are 600 to 12,000 S per 1 mm ² . 2. A rail produced by deoxidizing molten steel to form a steel slab, which is produced by a process including hot working, in which C: 0.55 to 0.90%, Si: 0. 10 to 1.20%, Mn: 0.50 to 1.50%, S: 0.002 to 0.035% Mg: 0.0004 to 0.01%, Al: 0.005 to 0.05%, V: 0.001 to 1.00%, N: 0.0005 to 0.030%, and 1 or 2 or more weight% thereof, Cr: 0.10 to 1.0%, Ni: 0.10 to 4.0%, Mo: 0.10 to 0.50%, Nb: 0.01 to 0.05%, with the balance being iron and inevitable impurities such as P. , Pearlite having MnS in the austenite grains as a nucleus, and pearlite having V carbonitride on MnS as a nucleus are present, and M with a size of 0.1 to 10 μm
A high-strength rail with excellent toughness and ductility, characterized in that the number of nS is 600 to 12000 per 1 mm ² . 3. Mg is added to the molten steel as a deoxidizing element and is subjected to deoxidizing treatment to be melted, and by weight%, C: 0.55 to 0.90%, Si: 0.10 to 1.20%. , Mn: 0.50 to 1.50%, S: 0.002 to 0.035% Mg: 0.0004 to 0.01%, Al: 0.0005 to 0.05%, V: 0.001 to Molten steel containing 1.00%, N: 0.0005 to 0.030% and the balance consisting of unavoidable impurities such as iron and P is made into a billet through an ingot-agglomeration method or a continuous casting method. A steel slab is hot-rolled to form a rail shape, and after the rolling is finished or after being heated to a high temperature for the purpose of heat treatment, when the head or further the bottom of the rail is cooled from the austenite range temperature to 700 to Accelerated cooling at a temperature of 1 to 5 ° C / sec between 500 ° C and finely cooling the austenite grains. Toughness characterized by precipitating various MnS, refining austenite grains by MnS, producing pearlite with MnS as a nucleus, and further producing pearlite with V carbonitride as a nucleus on MnS. And a method of manufacturing high strength rails with excellent ductility. 4. A molten steel is prepared by adding Mg as a deoxidizing element and performing a deoxidizing treatment. % By weight, C: 0.55 to 0.90%, Si: 0.10 to 1.20%, Mn: 0.50 to 1.50%, S: 0.002 to 0.035% Mg: 0 0.0004 to 0.01%, Al: 0.0005 to 0.05%, V: 0.001 to 1.00%, N: 0.0005 to 0.030%, and 1 type or By weight% of two or more, Cr: 0.10 to 1.0%, Ni: 0.10 to 4.0%, Mo: 0.10 to 0.50%, Nb: 0.01 to 0.05. %, And the balance being iron and unavoidable impurities such as P as molten steel through ingot-agglomeration or continuous casting to form a slab, which is hot-rolled to form a rail shape, After completion of the rolling, or after heating to a high temperature for the purpose of heat treatment, the head or bottom of the rail is austenized. When cooling from the zone temperature, 700-500 ° C. is acceleratedly cooled at 1-5 ° C./sec to precipitate fine MnS in austenite grains, austenite grains are refined by MnS, and pearlite with MnS as nuclei. The method for producing a high-strength rail excellent in toughness and ductility, characterized in that pearlite having V carbonitrides deposited on MnS as nuclei is generated.