JP2000328190A - High-strength pearlite rail excellent in toughness and ductility and method for producing the same - Google Patents

Abstract

(57) [Summary] [Problem] To produce fine Mg sulfide by adding Mg to molten steel, and to use this Mg sulfide and MnS with sulfide as a nucleus to achieve high toughness in cold regions. And a high-strength pearlitic rail with high ductility. SOLUTION: In weight%, C: 0.55-1.20%, Mg: 0.0004-
Mg sulfide having a diameter of 0.01 to 10 μm and a diameter of 0.01 to 10 μm in an arbitrary cross section in the pearlite structure containing at least 0.02% and at least a rail head substantially in a pearlite structure.
^2. High-strength pearlitic rail with excellent toughness and ductility, characterized by the presence of 500 to 100,000 pieces in ² . Also,
After forming a steel slab composed of a predetermined amount of components on a rail by hot rolling, the temperature of the rail is maintained at austenite temperature by hot rolling or heating after hot rolling. Manufacturing method of accelerated cooling at / sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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 of manufacturing the same.

[0002]

2. Description of the Related Art At present, railway transportation is being increased in load for improving transportation efficiency and speeding up for speeding transportation, and the requirements for rail characteristics are becoming stricter. Heavy load railways require more abrasion resistance and head internal fatigue damage resistance in sharply curved sections, and high-speed railways have a problem of increasing the rate of rail replacement mainly due to head surface damage in straight sections. . In addition to these, rail replacement due to the occurrence of rail cracks is concentrated in winter in railways in cold regions, and improving the toughness of rail materials has become an important issue for extending rail life.

[0003] Heavy loading for improving transport efficiency promotes wear of the rail head, and the life of the rail has been shortened due to increased fatigue damage. In order to improve the shortening of rail life in heavy-duty railways, technical development of high-strength rail steel with excellent wear resistance has been actively conducted. As a result, almost high-strength rails have been used in curved sections.

On the other hand, along with the improvement of the wear resistance of the rail steel,
A fatigue damage layer, which should be scraped off by abrasion, remains on the surface of the rail head or the surface of the gauge corner (GC) against which the root of the wheel flange is pressed, and surface damage has been recognized. Further, the improvement of the wear resistance of the rail steel causes the stress concentration due to the wheel load to be fixed at one point inside the rail GC, and the fatigue damage from inside the rail GC is increased. In order to improve the head surface damage resistance and the internal fatigue damage resistance of such a rail, it is important to improve the toughness and ductility of the rail material.

The following methods can be considered as measures for improving the toughness and ductility of a high-strength rail. (1) A method in which, after ordinary rolling, a rail once cooled to room temperature is reheated at a low temperature, and then acceleratedly cooled. (2) A method in which austenite grains are refined by controlled rolling and then the rail head is accelerated and cooled. (3) A method of reheating to a low temperature before controlled pearlite transformation after controlled rolling, and then accelerated cooling.

[0006]

In the above method (1), for example, as described in Japanese Patent Application Laid-Open No. 55-125321, reheating is performed to a low temperature of 850 ° C. or lower, which is lower than a normal heating temperature. However, it is intended to significantly improve toughness and ductility by reducing the size of austenite grains. However, when heating at a low temperature and deepening the heating to the inside of the rail head, it is necessary to reduce the amount of heat input and perform heating for a long time. This heat treatment impairs productivity and raises manufacturing costs. .

The method (2) is disclosed in, for example,
As described in JP-B-138427 and JP-A-52-138428, it is intended to improve toughness and ductility by making austenite grains finer by controlled rolling. However, there are also problems from the viewpoint of the rolling mill's ability to require a large rolling force or the like, or the shape controllability in that the cross-sectional shape of the rail and the dimensional accuracy in the longitudinal direction cannot be easily obtained.

The method (3) is described in, for example,
No. 4371, after rolling at 5% or more at 800 ° C. or less, again at 750 to 900 ° C.
In this case, the austenite grains are refined by heating to improve toughness and ductility. But,
Since this method requires a heating furnace for reheating at a low temperature after rolling, it has problems such as workability, productivity, and production cost.

Methods for improving the toughness of rail steel include, for example, JP-A-8-104946 and JP-A-8-104946.
As described in JP-A-104947 and JP-A-8-109438, Mg is added as a deoxidizing element, and the number of MnS of 0.1 to 10 μm is 6 per 1 mm ^2.
There are 00 to 12,000 high-strength pearlite-based rails having excellent toughness and ductility, and this method has enabled the production of rails having excellent toughness and ductility. However, further heavy load and higher speed are being studied in heavy load railways, and further improvements in toughness and ductility characteristics have been demanded.

[0010]

In order to achieve the above object, the present invention has the following constitution. (1) C: 0.55 to 1.20% by weight, Mg: 0.0004 to 0.02% by weight, at least the rail head has a substantial pearlite structure, and any of the pearlite structures 1 mm of Mg sulfide with a diameter of 0.01 to 10 μm in cross section
^2. A high-strength pearlite rail excellent in toughness and ductility, characterized in that 500 to 100,000 of them are present in ² . (2) In weight%, C: 0.55 to 1.20%, Si: 0.10 to 1.20%, Mn: 0.10 to 1.50%, S: 0.002 to 0.050% , Mg: 0.0004 to 0.02%, O: 0.0002 to 0.0100%, the balance being Fe and unavoidable impurities,
At least the rail head has a substantially pearlite structure, and a diameter of 0. 0 in any cross section in the pearlite structure.
Mg sulfide size of 01~10μm is in 1 mm ² 50
A high-strength pearlite rail excellent in toughness and ductility, characterized by having 0 to 100,000 pieces.

(3) The steel having the composition described in (2) above is further added by weight%: 0.0005 to 0.05%, Nb: 0.001 to 0.05%, Ni: 0.1 to 0.1%. A high-strength pearlite rail excellent in toughness and ductility, characterized by containing one or more of 4.0% and Cu: 0.1 to 4.0%. (4) The steel having the composition described in the above (2) or (3) is further added by weight: Cr: 0.10 to 1.0%, Mo: 0.01 to 0.50%, V: 0.01 A high-strength pearlitic rail excellent in toughness and ductility, characterized in that it contains one or more of B: 0.0001 to 0.00050%.

(5) After forming a steel slab comprising the components described in any one of the above (2) to (4) on a rail by hot rolling, the steel slab is left as hot rolled or is heated after hot rolling. A method for producing a high-strength pearlitic rail excellent in toughness and ductility, characterized in that at least the head of the rail is accelerated and cooled at a temperature of 700 to 500 ° C at 1 to 5 ° C / sec at an austenite temperature.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors first studied in detail the formation of Mg inclusions. As a result, when Mg is added, fine Mg sulfide is generated, and when Mg is added more than the stoichiometric composition with the free oxygen concentration in the molten steel, a large amount of fine Mg sulfide is stably generated, Mg sulfide at the time of pearlite transformation and MnS generated by using this Mg sulfide as a nucleus suppress austenite grain growth more effectively than finely dispersed Mg oxide and increase transformation sites at austenite grain boundaries. To make the pearlite structure fine.

Further, Mg sulfide finely dispersed in austenite grains is more likely to become an intragranular transformation nucleus than Mg oxide itself, and furthermore, MnS, which is more likely to undergo intragranular transformation than Mg-based inclusions, also contains Mg. Since it is easier to precipitate than oxides, it has been found that a large number of transformations from within the grains in addition to the transformation from the grain boundaries cause the pearlite structure after the transformation to be fine. FIGS. 1A and 1B show CMA mapping results of the MnS distribution in the case where Mg was added and in the case where Mg was not added. As is clear from this mapping diagram, FIG. MnS is finely dispersed in the case.

By adding Mg as described above, austenite grain refinement by finely dispersing Mg sulfide, and Mg sulfide finely dispersed in austenite grains and grains from MnS formed on the nuclei thereof. The present inventors have found that a rail steel having excellent toughness and ductility can be obtained by refining the pearlite structure by two effects of internal transformation, and completed the present invention.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components of the rail steel will be described. The content of the components is% by weight. C: C is an essential element for increasing the strength of the rail steel and forming the pearlite structure. If it is less than 0.55%, it is difficult to obtain a required high-strength pearlite structure, and if it exceeds 1.20%, not only harmful pro-eutectoid cementite which embrittles austenite grain boundaries is formed, but also a rail head heat treatment layer. Or martensite is formed in the micro segregation part of the welded part and the toughness and ductility are remarkably reduced.
To 1.20%.

Si: Si not only contributes to high strength by solid solution strengthening in the ferrite phase in the pearlite structure, but also
There is a slight toughness and ductility improvement effect. If it is less than 0.10%, the effect is small, and if it exceeds 1.20%, embrittlement is caused and weldability is reduced.
%.

Mn: Like Mn, Mn is an element that lowers the pearlite transformation temperature and enhances hardenability to contribute to higher strength. However, if the content is less than 0.10%, the effect is small, and if it exceeds 1.50%, a martensite structure is easily generated in the segregated portion.
Limited to 50%.

S: S is generally known as a harmful element. However, in the present invention, MnS is formed by using sulfides in austenite as nuclei to suppress coarsening of austenite grains and MnS becomes a transformation nucleus of pearlite. This is one of the important elements for the refinement of the pearlite structure.
However, if it is less than 0.002%, a sufficient amount of MnS cannot be obtained, and if it exceeds 0.050%, coarse MnS
Began to form and markedly reduced toughness and ductility, so the content was limited to 0.002 to 0.050%.

Mg: Mg is an important constituent element of the present invention. The Mg-based sulfide and MnS precipitated using the sulfide as nuclei have the effect of suppressing the growth of austenite grains by the pinning effect, and refine the pearlite structure after transformation. In addition to this effect, pearlite is generated from Mg sulfide itself and MnS precipitated using the sulfide as a nucleus, and has a function of further refining the pearlite structure. As a result, it was possible to significantly improve the toughness and ductility of the rail steel. However, if the content is less than 0.0004%, there is almost no effect of suppressing austenite grain growth and the pearlite structure is not refined due to intragranular transformation, and if it exceeds 0.02%, coarse Mg sulfide is formed and the toughness is remarkably reduced. It was limited to the range of 0.0004 to 0.02%. Further, in order to positively precipitate Mg sulfide, it is desirable that the amount of Mg is higher than the stoichiometric composition as MgO.

O: O is generally dissolved as an impurity in the molten steel, and it is better to reduce the oxygen in the molten steel in order to increase the amount of Mg sulfide, but if the concentration is to be reduced to less than 0.0002%. Productivity during smelting decreases. When the oxygen concentration exceeds 0.0100%, coarse oxides are formed and the toughness is remarkably reduced.
Limited to 0100%.

In the present invention, in addition to the above components, one or more of Al, Nb, Ni,
By adding Cu, Cr, Mo, V, and B, high toughness can be obtained by improving the toughness of ferrite ground and reducing the size of austenite grains during heating for rail rolling or austenite grains during controlled rolling. In addition, high strength and high toughness can be obtained at the same time by accelerated cooling in the cooling process. In addition, in the addition of the above components, Al, Nb, Ni, and Cu improve toughness, and C,
The main objects of r, Mo, V, and B are to improve high strength and hardness at the same time as toughening.

The reasons for limiting these chemical components will be described below. Al: Al contains a deoxidizing agent remaining during steelmaking. Al forms a composite oxide with Mg, suppresses austenite grain growth, and this Mg-Al composite oxide forms M
It becomes a nucleus of nS precipitation, and pearlite transformation is performed from these precipitates to make the structure fine. The effect of this composite precipitation is 0.0
005% or more is effective.
Since the oxide and the Mg-Al oxide are coarsened and lower the toughness, the content of Al is set to 0.0005 to 0.0005.
Limited to 0.05%.

Nb: By heating Nb at a low temperature during hot rolling, the carbonitride of Nb suppresses austenite grain growth and contributes to grain refinement. Further, austenite grains after hot rolling are refined by high-temperature heating / low-temperature finish rolling, and the pearlite structure obtained after accelerated cooling is refined. In order to obtain this effect, 0.001% or more is necessary. If it exceeds 0.05%, coarse Nb carbides, Nb nitrides, and Nb carbonitrides are formed, thereby lowering toughness. Therefore, it was limited to the range of 0.001 to 0.05%.

Ni: Ni is a solid solution in ferrite and is an element effective for improving the toughness of ferrite.
When the content is less than 1%, the effect is extremely small.
Even if the content exceeds 0%, the effect is saturated. Therefore, from the viewpoint of improving the toughness, the range is limited to 0.1 to 4.0%.

Cu: Cu, like Ni, forms a solid solution in ferrite and is an effective element for improving the toughness of ferrite. When the content is less than 0.1%, the effect is extremely small. Even if the content exceeds 0%, the effect is saturated. Therefore, from the viewpoint of improvement in toughness, 0.1 to 4.0%
Limited to the range.

Cr: Cr contributes to high strength by lowering the pearlite transformation temperature and has the effect of strengthening the cementite phase in the pearlite structure. Therefore, from the viewpoint of preventing the weld joint from softening, it is 0.1%. The above content is effective. On the other hand, when the content exceeds 1.0%, bainite and martensite are formed not only at the element segregation portion at the time of forced cooling but also at the shoulder portion of the rail where the supercooling tendency is strong, resulting in a decrease in toughness. Therefore, the range is limited to 0.1 to 1.0% as long as a certain contribution is expected to secure the strength and the toughness and ductility are not impaired.

Mo: Mo is an element effective for suppressing the transformation speed of pearlite and refining the pearlite structure, thereby improving toughness. Further, Mo prevents the induction of transformation at a high temperature in conjunction with the heat generated by the pearlite transformation of the surface layer inside the rail during accelerated cooling, and contributes to increasing the strength inside the rail to increase the strength. However, if the content is less than 0.01%, the above effect is small, and if the content exceeds 0.50%, the pearlite transformation rate is reduced, and bainite and martensite are generated in the pearlite structure, resulting in a decrease in toughness. Therefore, it was limited to the range of 0.01 to 0.50%.

V: V is an element which precipitates in ferrite and is effective for improving the strength. If it is less than 0.01%, the strength cannot be increased, and if it exceeds 1%, coarse V carbides are formed. The toughness decreases. Therefore, 0.01 to 1.0
%.

B: B is an element that segregates at austenite grain boundaries even when added in a small amount and significantly improves hardenability by delaying transformation. To get this effect,
0.0001% or more is required, and if it exceeds 0.0050%, carbonitride B is generated, and the toughness is significantly reduced. Therefore, the range is limited to 0.0001 to 0.0050%. P, which is an unavoidable element, is desirably reduced as much as possible to lower the toughness of the rail steel, and is desirably 0.03% or less.

The rail steel having the above-mentioned composition is melted in a commonly used melting furnace such as a converter or an electric furnace, and the molten steel is solidified by ingot making, lumping or continuous casting. Then, it is manufactured through a hot rolling method. In the rail after hot rolling, pearlite is generated from MnS in addition to Mg sulfide in austenite grains during cooling, and a fine pearlite structure is formed together with pearlite generated from austenite grain boundaries. As a result, a high-strength rail with excellent toughness can be manufactured as-rolled.

Further, when high strength of 1200 MPa or more is required together with high toughness, the steel sheet is reheated to the austenite region temperature after the end of rolling, or once cooled to room temperature and then subjected to heat treatment. 1-5 ° C
It is desirable to perform accelerated cooling at / sec. In addition, since accelerated cooling causes pearlite transformation at a low temperature, the generation rate of pearlite transformation nuclei in rail steel is improved, and pearlite grains become fine. When the cooling rate during this accelerated cooling is less than 1 ° C./sec, the required strength cannot be obtained, and when it exceeds 5 ° C./sec, martensite is generated. Therefore, the cooling rate was limited to 1 to 5 ° C / sec.

As described above, accelerated cooling not only increases the strength, but also increases the austenite grain boundaries, Mg sulfides and M
g In pearlite transformation from MnS with sulfides as nuclei, transformation nuclei are increased, and as a result, pearlite is refined, so that the toughness of the rail steel can be further improved. At this time, it is desirable to use a gas-liquid mixture such as air or mist as a cooling medium.

Next, the number of Mg sulfides with a diameter of 0.01 to 10 μm in the pearlite structure of the rail steel
The reason why the number is limited to 500 to 100,000 in mm ² will be described. Mg sulfide in the rail steel suppresses the grain growth of fine γ grains after processing by pinning, and makes the pearlite structure after transformation fine. Further, the pearlite transformation from Mg sulfide in the γ grains and MnS generated using the Mg sulfide as a nucleus makes the pearlite structure finer and improves the toughness value. At this time, M
g The Mg sulfide whose equivalent circle diameter (diameter) in the cross-section calculated from the cross-sectional area of the sulfide is less than 0.01 μm does not become a transformation nucleus in pearlite grains, and when it exceeds 10 μm, it becomes a starting point of fracture and becomes tough. M value
g The size of the sulfide was limited to 0.01 to 10 μm.

If the number is less than 500 per 1 mm ² , the effect of pinning and intra-granular transformation is weak, and the effect of improving toughness cannot be obtained. Further, when the number exceeds 100,000, the amount of S that becomes Mg sulfide increases and the number of MnS having high intragranular transformation ability decreases due to a decrease in S linked as MnS, resulting in a decrease in toughness. , M in 1mm ²
g The number of sulfides was limited to 500 to 100,000. Such a pearlite structure needs to be formed at least at the head of the rail.

[0036]

Next, an example of manufacturing a high-strength rail having high toughness manufactured according to the present invention will be described. Rails were manufactured from molten steel having the components shown in Table 1, and the number of Mg sulfides, the state of the grain structure, and mechanical properties were investigated. In Table 2,
Measurement results of the number of Mg sulfides having a diameter of 0.01 to 10 μm in an arbitrary cross section present in the pearlite structure after cooling of the rail steel, the γ particle number of the sample quenched after processing, and the The observation result of presence or absence is shown.

The particle size of Mg sulfide can be determined by using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) at an arbitrary cross section at a magnification of 1,000 to 50,000 times. The area was determined and the diameter of a circle equal to this area was used. In addition, Mg sulfide in MnS is also S
Since the contrast is different in the images of EM and TEM, the portion where the contrast is different is regarded as Mg sulfide, and the particle size is calculated in the same manner as in the case of Mg sulfide. The number of Mg sulfides is determined by measuring the number of Mg sulfides having a diameter of 0.01 to 10 μm in 20 visual fields, averaging the 20 visual fields according to a normal distribution, and calculating the average from the number in the area of 1 visual field to 1 mm ^2. The value converted to was used.

As is clear from Table 2, in the steel of the present invention to which Mg is added, a predetermined amount of Mg sulfide is finely dispersed, and the Mg sulfide in the grains and the Mg sulfide are used as nuclei. It was confirmed that the pearlite undergoes intragranular transformation from the generated MnS and the pearlite structure became fine.

Table 3 shows the tensile test strength of the rail steel in which the cooling rate between 700 ° C. and 500 ° C. was changed in the range of 1 ° C./s to 5 ° C./s for each steel type in order to keep the strength as-rolled and the strength constant. 2 shows the results of measuring the impact absorption energy at + 20 ° C. in the elongation and 2 mm U notch Charpy tests. The tensile test was performed on a test piece having a parallel part diameter of 6 mm and a parallel part length of 30 mm taken from a depth of 10 mm inside the rail head gauge corner.

As a result, the steel of the present invention was found to have sufficiently improved ductility due to the refinement of the pearlite structure as compared with the conventional steel. The impact test piece was taken from 1 mm below the rail head. The test conditions are based on the Russian Γ oct standard, which requires that the high-strength heat-treated rail has an impact absorption energy at + 20 ° C. of 25 J / cm ² or more. The fine pearlite structure is formed by the suppression of grain growth of the austenite fine grains after processing and the intra-pearlite transformation from inside the austenite grains, all of which sufficiently satisfy the Charpy absorbed energy specified in the Γ oct standard.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

As described above, by controlling the size and the number of Mg sulfides by adding Mg or the like, the austenite grains after processing become fine and the pearlite structure transformed from the grain boundaries becomes fine. In addition, the pearlite structure after transformation is fine because the pearlite is transformed from the inside of the austenite grains from the Mg sulfide existing in the austenite grains and MnS generated from the Mg sulfide as a nucleus. Further, the pearlite grains are refined by accelerated cooling, and a shock absorption energy of 25 J / cm ² or more can be stably obtained. That is, the present invention can provide a high-strength pearlite rail excellent in toughness and ductility or a method for producing the same.

[Brief description of the drawings]

FIG. 1 shows the C of MnS at the rail head after rail rolling.
It is a MA mapping diagram, (a) is a thing of a Mg additive material,
(B) shows a comparative material without Mg added.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kenichi Kamine 1-1 Futaba-cho, Tobata-ku, Kitakyushu Nippon Steel Corporation Yawata Works F-term (reference) 4K042 AA04 BA02 BA05 CA02 CA05 CA08 CA09 CA10 CA13 DA01

Claims (5)

[Claims] 1. A steel composition comprising, by weight%, C: 0.55 to 1.20% and Mg: 0.0004 to 0.02%, wherein at least the rail head has a substantial pearlite structure. 1 mm of Mg sulfide having a diameter of 0.01 to 10 μm in an arbitrary cross section of
^2. A high-strength pearlite-based rail having excellent toughness and ductility, characterized in that there are 500 to 100,000 of these in ² . 2. C: 0.55 to 1.20%, Si: 0.10 to 1.20%, Mn: 0.10 to 1.50%, S: 0.002 to 0. 050%, Mg: 0.0004-0.02%, O: 0.0002-0.0100%, the balance being Fe and unavoidable impurities,
At least the rail head has a substantially pearlite structure, and a diameter of 0. 0 in any cross section in the pearlite structure.
Mg sulfide size of 01~10μm is in 1 mm ² 50
A high-strength pearlitic rail excellent in toughness and ductility, characterized by having 0 to 100,000 pieces. 3. The steel according to claim 2, further comprising, by weight%, 0.0005 to 0.05% of Al, 0.001 to 0.05% of Nb, and 0.1 to 4% of Ni. A high-strength pearlite rail excellent in toughness and ductility, characterized by containing one or more of 0% and Cu: 0.1 to 4.0%. 4. The steel according to claim 2 or 3, further comprising, by weight%, Cr: 0.1-1.0%, Mo: 0.01-0.50%, V: 0.01-%. A high-strength pearlitic rail excellent in toughness and ductility, characterized by containing one or more of 1.00% and B: 0.0001 to 0.00050%. 5. A steel slab comprising the component according to claim 2 formed on a rail by hot rolling, and then the temperature is set to an austenite zone temperature by hot rolling or heating after hot rolling. , At least the head of the rail
A method for producing a high-strength pearlite-based rail having excellent toughness and ductility, characterized by accelerated cooling at a temperature of 0 to 500 ° C at 1 to 5 ° C / sec.