JP2003129180A - Pearlitic rail superior in toughness and ductility, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pearlitic rail with high toughness and high ductility. SOLUTION: The pearlitic rail superior in toughness and ductility has a steel composition including, by mass%, 0.6-1.2% C, 0.1-1.2% Si, 0.1-1.5% Mn, 0.005-0.070% V, 0.005-0.025% N, 0.0005-0.0100% Ti, and 0.015% or less P, and one or more of 0.1-1.0% Cr, 0.01-0.50% Mo, 0.1-2.0% Ni, 0.004-0.05% Nb, 0.1-2.0% Cu, 0.0001-0.0015% B, 0.0004-0.0100% Mg, 0.002-0.020% S, and 0.002-0.050% Al, as needed, has substantially a pearlitic structure in at least a top part of the rail, and has 2-100 per 1 mm<2> of (Ti,V)N particles with diameters of 0.1-5.0 μm, in an arbitrary cross section of the top part of the rail. The method for manufacturing the pearlitic rail superior in toughness and ductility is characterized by making a staying time of the steel material in a furnace to be 60 minutes or shorter, when re-heating and rolling the above steel slab, in the furnace which can heat the steel material to 1,200 deg.C or higher.

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 pearlite block size of rail steel is miniaturized to improve toughness and ductility, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Rail transportation is under heavy load to improve transportation efficiency and speeding up to speed up transportation, and requirements for rail characteristics are becoming strict. The heavy load promotes wear of the rail head in a sharp curve section and increases fatigue damage from a stress concentration part inside the GC (gauge corner) of the rail, so that the rail life is becoming shorter. In order to improve the shortening of rail life in heavy-duty railway, technological development of high-strength rail steel with excellent wear resistance and internal fatigue damage resistance has been actively conducted. As a result, this high-strength rail is becoming widespread in curved sections.

On the other hand, rail replacement in the cold regions is concentrated due to the occurrence of rail cracks in winter, and improvement of the toughness of the rail material has become an issue necessary for extending the rail life. Further, to improve the internal fatigue damage of the head, it is important to improve the toughness and ductility of the rail material. As an impact value standard for rail steel, there is an example in which a ² mm U notch test value at a test temperature of 20 ° C. is 25 J / cm ² or more in the Russian Γoct standard, and the toughness and ductility are improved by the following methods. (1) A method in which after normal rolling, the rail once cooled to room temperature 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.

[0004]

In the above method (1), as described in, for example, JP-A-55-125321, reheating to a low temperature of 850 ° C. or lower, which is lower than the normal heating temperature, is carried out. However, the fineness of the austenite grains is intended to significantly improve the toughness and ductility. However, if heating at a low temperature and trying 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, and this heat treatment hinders productivity and raises the manufacturing cost. .

The method (2) is described in, for example, Japanese Patent Laid-Open No.
JP-A-2-138427 and JP-A-52-13842.
As described in Japanese Patent Publication No. 8, the fine graining of austenite grains by controlled rolling is intended to improve toughness and ductility. However, there are problems from the viewpoint of the ability of the rolling mill that requires a large rolling force, or the shape controllability that the rail cross-sectional shape and dimensional accuracy in the longitudinal direction cannot be easily obtained.

Further, the method (3) described above is, for example, as described in Japanese Patent Publication No. 4-4371, 800 ° C.
After rolling 5% or more below, again 750-90
By heating to 0 ° C., the austenite grains are made fine and the toughness and ductility are improved. However, since this method requires a heating furnace for low-temperature reheating after rolling, there are problems such as workability, productivity, and manufacturing cost.

As a method for improving the toughness of rail steel, Mg is added as a deoxidizing element to 0.1 as described in, for example, JP-A-8-104946 and JP-A-8-109438. A pearlite rail having excellent toughness and ductility, in which the number of MnS having a particle size of 10 μm is 600 to 12000 per 1 mm ^2, and JP-A-6-27
There is a pearlite-based rail whose structure is refined by causing transformation from V carbonitride and Ti carbonitride, as seen in Japanese Patent Publication No. 9850, Japanese Unexamined Patent Publication No. 7-150235, and Japanese Unexamined Patent Publication No. 8-104947. These methods have enabled rails with excellent toughness and ductility to be manufactured. However, in heavy-duty railways, further heavy loading and higher speed are being studied, and further improvement in toughness and ductility characteristics has been demanded.

[0008]

When the pearlite steel breaks, the grain boundaries serve as resistance to crack propagation, so that the crack bends and progresses on a grain-by-grain basis. Therefore, the finer the crystal grains,
The energy required for destruction increases and the impact value increases. When carbon steel undergoes austenite to pearlite transformation, the transformation mainly starts from austenite grain boundaries. At this time, if there is a precipitate in the austenite having a high crystal lattice matching with the ferrite, it becomes a transformation nucleus and the transformation easily occurs. As a result, many pearlite modules grow and the pearlite structure after transformation is fine.

When the rail steel material contains Ti, V, N, (Ti, V) N is precipitated in the steel. (Ti, V) N
The ratio of Ti and V in the inside changes from 0 to 1, and (Tix, V
1-x) N, x = 0 to 1 The crystal structures of TiN with x = 1 and VN with x = 0 are similar, and both have high matching of crystal lattice with ferrite iron. As a result, a large number of pearlite modules grow with (Ti, V) N as the nucleus, and fine pearlite is formed.

On the other hand, when precipitates are present in steel, depending on the size and material of the steel, the precipitates may become fracture starting points when a load is applied. T for finer pearlite structure
Addition of i, V, and N is effective, but precipitation (Ti,
V) N is hard and difficult to deform. Therefore, (Ti, V) N
If the size of the steel becomes large, stress concentration will occur in the adjacent steel materials, and the impact properties and ductility values will rather deteriorate.

The present invention provides conditions for obtaining good impact characteristics and ductility required in cold regions, the gist of which is as follows. (1) Mass%, C: 0.6 to 1.2%, Si: 0.1 to 1.2%, Mn: 0.1 to 1.5%, V: 0.005 to 0.070% , N: 0.005-0.025%, Ti: 0.0005-0.010%, P: 0.015% or less, at least the rail head has a substantially pearlite structure, and The diameter is 0.
1 to 5.0 μm (Ti, V) N is ² to 10 in 1 mm 2.
Perlite rail with excellent toughness and ductility, characterized by the presence of zero rails. (2) Further, if necessary, in mass%, Cr: 0.1 to 1.0%, Mo: 0.01 to 0.50%, Ni: 0.1 to 2.0%, Nb: 0. 004 to 0.05%, Cu: 0.1 to 2.0%, B: 0.0001 to 0.0015%, Mg: 0.0004 to 0.0100%, S: 0.002 to 0.020%. , Al: 0.002 to 0.050% of one kind or two kinds or more, and a pearlite rail excellent in toughness and ductility. (3) When the steel slab according to (1) or (2) above is reheated and rolled, the in-furnace time is set to 60 minutes or less in a heating furnace in which the steel material is 1200 ° C. or higher. A method for producing a pearlite rail having excellent toughness and ductility. (4) Further, after the above steel slab is formed into a rail by hot rolling, it is brought to an austenite temperature by as-hot-rolling or by heating after hot-rolling, and at least 700 to 500 ° C. is applied to the head during cooling of the rail. A method for producing a pearlite rail having excellent toughness and ductility, characterized by accelerating cooling at an interval of 1 to 5 ° C / sec.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. Rail steel is generally manufactured by the steps of refining-casting-cooling-reheating-hot rolling-cooling. First, the converter
Molten steel, refined and adjusted in composition in an electric furnace, is solidified by a method such as continuous casting. The cast slab and steel ingot are reheated to 1200 ° C. or higher during hot rolling. The steel material heated to a high temperature passes through a plurality of rolling mills, is gradually formed into a rail shape, and is finished into a rail shape at 900 to 1100 ° C. After the rolling is completed, when the temperature falls below the eutectoid point, pearlite transformation occurs. The pearlite transformation generally initiates and grows at austenite grain boundaries.

When the rail steel material contains Ti, V, N, (Ti, V) N precipitates as the temperature decreases after casting. When this (Ti, V) N is reheated to 1200 ° C. or higher before rolling, part of it is dissolved in the base material, but it precipitates again as a nitride as the temperature decreases during or after rolling. The ratio of Ti and V in (Ti, V) N varies from 0 to 1, and is expressed in the form of (Tix, V1-x) N, x = 0 to 1. The crystal structures of TiN with x = 1 and VN with x = 0 are very similar, and both have good crystal lattice matching with ferritic iron. Therefore, ferrite transformation easily occurs in steel alone or as (Ti, V) N nuclei precipitated on precipitates such as MnS.

When the ferrite transformation occurs with (Ti, V) N as the nucleus, cementite is immediately precipitated in the adjacent austenite, and the transition to the pearlite transformation occurs. As a result, a pearlite module having a large number of different crystal orientations centered on (Ti, V) N grows, fine pearlite is formed, and a rail steel having excellent toughness and ductility can be obtained. Such a fine pearlite structure is
It must be formed at least on the head of the rail that is prone to shock loads from the wheels.

The reason for defining the number frequency of (Ti, V) N in the cross section will be described below. Effective as a transformation nucleus (Ti,
V) The N size is not clear at this stage, but the size that can be identified by a commercial analyzer such as a modern scanning electron microscope SEM is up to about 0.1 μm.
V) The lower limit of the size for defining the number of N is 0.1 μm. On the other hand, if it exceeds 5 μm, it tends to become a fracture starting point when a load is applied, which is not preferable. Therefore, the upper limit of the size for defining the number of (Ti, V) N is set to 5 μm.

If the frequency of (Ti, V) N in the range of 0.1 to ⁵ μm is less than 2 / mm ² in an arbitrary cross section of the rail head, finer structure cannot be obtained, and if it exceeds 100 / mm ^2. 5
The number of coarse (Ti, V) N exceeding μm also increases, and ductility is rather deteriorated, which is not preferable. Coarse (Ti, V)
In order to prevent the generation of N, it is desirable to carry out solution treatment by dissolving it in steel during reheating prior to hot rolling. But (T
Once i, V) N precipitates, it has a low solubility and it is difficult to form a complete solution.

When it is difficult to form a complete solution such as (Ti, V) N, grains are rather grown due to Ostwald's growth during heating at a high temperature, and the starting point of fracture tends to occur when a load is applied. In order to suppress the growth of (Ti, V) N during heating, it is necessary to limit the in-furnace time within 60 minutes in a heating furnace at 1200 ° C. or higher. The material temperature is controlled by measuring the material temperature with a stationary material thermometer installed in the heating furnace or with a radiation thermometer from the furnace monitoring window.

On the other hand, in the steeply curved portion of heavy-duty railways and high-speed railways, rail steel is exposed to severe loads and friction, and therefore high strength and high hardness are required. For the rail steel material to be used in such a use environment, the pearlite transformation temperature range is 700 to 500, which is the pearlite transformation temperature range, either directly from the austenite range temperature after completion of rolling or after reheating to the austenite range temperature.
Accelerated cooling between ℃ is desirable. Accelerated cooling subcools the austenite to a lower temperature, lowering the pearlite transformation temperature. When the degree of subcooling increases in this way,
Since the ferrite-cementite layer spacing in pearlite is reduced to increase the strength and the rate at which transformation nuclei are generated is increased, the effect of refining the pearlite structure can be obtained. As a result, the toughness can be improved in addition to the increase in strength. However, the cooling rate during accelerated cooling is 1 ℃ / s
When it is less than ec, the required strength cannot be obtained and it is 5 ℃ / sec.
If it exceeds, martensite is generated, which is not preferable.

Next, the reasons for limiting the components of the rail steel will be described. Hereinafter,% means mass%. C: C reduces ductility and toughness, but is a basic element that determines strength and wear resistance, which are extremely important for rail safety. If C is less than 0.6%, it is difficult to obtain the required high-strength pearlite structure. Further, if it exceeds 1.2%, proeutectoid cementite is formed, and the toughness and ductility are remarkably lowered, which is not preferable.

Si: Si is necessary as a deoxidizing material for molten steel,
Indispensable for refining rail steel. Also, Si
Has the effect of improving the strength by solid solution strengthening in the ferrite phase in the pearlite structure and improving the toughness and ductility to some extent. However, if less than 0.1%, the effect is small.
If it exceeds 2%, embrittlement is caused, and weldability is also deteriorated, which is not preferable.

Mn: Mn is an element that contributes to higher strength by lowering the transformation temperature and enhancing hardenability. It is also essential to render Sn harmless by precipitating MnS in combination with S in steel. Precipitated Mn
S becomes a precipitation site of (Ti, V) N, which also becomes a pearlite transformation nucleus. If Mn is less than 0.1%, these effects cannot be obtained, and if it exceeds 1.5%, martensite structure is likely to be generated in the segregated portion, which is not preferable.

V: V becomes a pearlite transformation nucleus (Ti,
V) An element that is indispensable for forming N. When V is less than 0.005%, the precipitation amount of (Ti, V) N is small, and the effect of improving ductility is small. On the other hand, V is 0.0
If it exceeds 7%, V that precipitates after completion of pearlite transformation
C is increased and ductility is rather deteriorated, which is not preferable. Moreover, it is desirable that the ratio of the mass contents of V and N and the V / N value are low for the following reasons. In order to use the V precipitate as a pearlite transformation nucleus, it is necessary to use the V nitride that precipitates at a temperature higher than the transformation point. If N is not sufficient, the precipitation amount of V carbides increases below the transformation point, causing an increase in strength and a decrease in ductility.

N: N is an element necessary for precipitating (Ti, V) N, and for this purpose, 0.005% or more is necessary. If N exceeds 0.025%, fine cracks are likely to occur inside the slab during continuous casting, so 0.0
It is necessary to limit it to 25% or less.

Ti: Ti serves as a pearlite transformation nucleus (T
i, V) It is an essential element for the precipitation of N. Ti is 0.0
If it is less than 005%, the effect cannot be obtained, and if it exceeds 0.01%, coarse (Ti, V) N is generated and the ductility is deteriorated, which is not preferable.

P: P is an unavoidable element in steel and embrittles the ferrite phase in the pearlite structure. Therefore, P is 0.015% in rails for cold regions where ductility is important.
The following is desirable.

Further, in the present invention, in addition to the above components, one or more of Cr, Mo, Ni, and
By adding Nb, Cu, B, Mg, S, and Al, high toughness can be obtained by improving the toughness of the ferrite base material, austenite grains at the time of heating the rail rolling material, or by making the austenite grains fine during rolling. . Also,
Accelerated cooling in the cooling process makes it possible to obtain higher strength and higher toughness at the same time. The reason for limiting these chemical components will be described below.

Cr: Cr contributes to strengthening by lowering the pearlite transformation temperature, and it is effective to contain 0.1% or more from the viewpoint of preventing softening of the welded joint. 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 shoulder portion of the rail having a strong tendency to be supercooled during forced cooling, which is not preferable.

Mo: Mo suppresses the transformation rate of pearlite, lowers the transformation temperature, contributes to high strength, and
It is an element effective in improving toughness because it refines the pearlite structure. However, if it is less than 0.01%, the above effect is small, and if it exceeds 0.50%, the pearlite transformation rate is too low, and bainite or martensite is generated in the pearlite structure, resulting in a decrease in toughness. Absent.

Ni: Ni is a solid solution in ferrite and is an effective element for improving the toughness of ferrite. However, when the Ni content is less than 0.1%, the effect is weak, and when the Ni content exceeds 2.0%, the effect is saturated.

Nb: Nb suppresses austenite grain growth by Nb carbonitride during hot rolling and contributes to grain refinement. To obtain this effect, Nb needs to be 0.004% or more, but if it exceeds 0.05%, the toughness decreases due to the formation of coarse Nb carbonitride, which is not preferable.

Cu: Cu, like Ni, is a solid solution in ferrite and is an effective element for improving the toughness of ferrite. However, when Cu is less than 0.1%, its effect is weak, and even when it is contained over 2.0%, its effect is saturated.

B: B is an element which segregates at the austenite grain boundaries even when added in a small amount and delays the transformation to remarkably improve the hardenability. To get this effect,
B is required to be 0.0001% or more, and if it exceeds 0.0015%, an iron carbon boride is generated, and the toughness is significantly reduced, which is not preferable.

Mg: Mg precipitates Mg oxide, Mg-Al oxide, and Mg sulfide, and further uses these as nuclei for Mn.
It becomes a precipitation nucleus of S and (Ti, V) N. These inclusions make the pearlite block after pearlite transformation finer by promoting the generation of pearlite transformation nuclei. However, when Mg is 0.
If it is less than 0004%, the pearlite block size is hardly refined, and if it exceeds 0.01%, coarse inclusions are formed, and the toughness is remarkably lowered, which is not preferable.

S: S is an unavoidable impurity in steel, but M
It forms gS and MnS and becomes a (Ti, V) N precipitation site. These inclusions suppress the grain growth of austenite grains after rolling and promote the intragranular transformation to reduce the pearlite block size after the pearlite transformation. However, if S is less than 0.002%, a sufficient number of MgS, Mn
S cannot be obtained, and if it exceeds 0.02%, coarse MnS starts to be generated, and toughness and ductility are deteriorated, which is not preferable.

Al: Al ₂ O ₃ and Al-Mg composite oxide serve as MnS precipitation nuclei. Further, AlN has an effect of suppressing the growth of austenite grains and contributes to the refinement of the pearlite structure. However, when Al is less than 0.002%, this effect is weak, and when it exceeds 0.05%, the oxide becomes coarse and there is a risk of becoming an internal fatigue starting point when used in heavy-duty railway.

[0036]

EXAMPLES The present invention will be described in more detail with reference to examples of the present invention in which (Ti, V) N is used to refine the pearlite structure and comparative examples in which the amounts of V, N and Ti are changed. Table 1 shows its chemical composition in mass%. Code A1,
The compositions of the comparative examples shown in B1, C1, and D1 are the same as those of Example A,
It is similar to B, C, and D, respectively. A rail was manufactured by hot rolling from the steel material having the above components. In the table, the in-furnace time of 1200 ° C. or higher when reheating the material is shown. The material temperature was measured by using a stationary thermometer installed in the heating furnace and a temperature measurement using a radiation thermometer from a monitoring window provided on the furnace wall.

The steel material composition of Comparative Example A2 is the same as that of Example A, but the in-furnace time in the temperature range of 1200 ° C. or higher during reheating exceeds 60 minutes. Further, in the table, the measurement results of the number of (Ti, V) N in the rail longitudinal direction and the longitudinal section at 5 mm below the crown surface of the rail head after the rail molding are shown. The number of samples was measured by mirror-polishing the sample and using a scanning electron microscope equipped with an analyzer at a magnification of 5000 times in the range of 1 × 1 mm.

Table 2 shows the tensile strength TS = for each rail.
Aiming at about 1300 MPa, the cooling rate between 700 ° C and 500 ° C from the austenite temperature range after rolling is 1 to 5 ° C / s.
2 mm U-notch Charpy impact value and round bar tensile test value at a test temperature of 20 ° C. when cooled in the range of 1, and after cooling in the air after rolling. Charpy test piece
The position 3 mm below the top surface of the rail was taken as the top surface of the test piece, 3 rows were taken in the rail width direction and 4 rows were taken in the longitudinal direction, and the notch position was made on the top surface side. Since the notch depth is 2 mm, the position of the notch bottom corresponds to 5 mm below the rail top surface. Table 2 shows the average of 12 impact test values. In the tensile test, the center of the rail head gauge corner is set to 10 mm inward from the center of the test piece.
The test was performed with IS4 subsize test pieces.

As shown in Table 2, the impact value and the elongation tend to decrease as the amount of C increases, but the steels of the present invention in which the amounts of V, N and Ti are appropriate are (Ti, V) N. As a result of pearlite transformation progressing as a nucleus, a fine pearlite structure is obtained,
Good impact and elongation values were obtained. Further, the present invention example A,
B, C, and D are heat treated, and are Γoct standard 25J /
Good impact values over cm ² were obtained.

On the other hand, in Comparative Examples A1 to D1 and A2, the impact value and the elongation value were remarkably lowered as compared with Examples A to D of the present invention.
The reason is as follows. In Comparative Example A1, since V was not added, (Ti, V) N was not generated. In Comparative Example B1, since the amount of N was small, the amount of (Ti, V) N produced was small. Comparative Example C1 has a finer structure,
Since Ti was too high, (Ti, V) N was excessively generated, and a coarse one of them was a fracture starting point, and the fact that P was high affected ductility. Comparative Example D1 was an example in which V was too high, and although the structure was refined, a large amount of VC was precipitated after the completion of transformation, and the ductility was deteriorated due to the adverse effect of precipitation strengthening. In Comparative Example A2, since the in-furnace time at high temperature was long, (Ti, V) N grew and coarsened, and when a load was applied, it became a fracture starting point and deteriorated in ductility.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

According to the present invention, by precipitating an appropriate size and number of (Ti, V) N, the pearlite structure after transformation becomes fine and coarse (Ti, V) N is generated to cause destruction. It is possible to prevent the starting point and obtain a pearlitic rail with high ductility.

Continued front page    F-term (reference) 4K042 AA04 BA02 CA02 CA05 CA06                       CA08 CA09 CA10 CA12 CA13                       DA06 DE01

Claims (5)

[Claims] 1. By mass%, C: 0.6 to 1.2%, Si: 0.1 to 1.2%, Mn: 0.1 to 1.5%, V: 0.005 to 0. 070%, N: 0.005-0.025%, Ti: 0.0005-0.010%, P: 0.015% or less, at least the rail head has a substantially pearlite structure, and the rail head (Ti, V) with a diameter of 0.1-5.0 μm in any cross section
A pearlite rail with excellent toughness and ductility, characterized by the presence of 2 to 100 N in 1 mm ² . 2. As a steel component, in mass%, Cr: 0.1 to 1.0%, Mo: 0.01 to 0.50%, Ni: 0.1 to 2.0%, Nb: 0. 0.004 to 0.05%, Cu: 0.1 to 2.0%, B: 0.0001 to 0.0015%, Mg: 0.0004 to 0.0100%, S: 0.002 to 0.020. %, Al: 0.002 to 0.050% of one kind or two or more kinds thereof are contained, and the pearlite rail having excellent toughness and ductility according to claim 1. 3. When the steel slab according to claim 1 or 2 is reheated and rolled, the in-furnace time is set to 60 minutes or less in a heating furnace where the steel material reaches 1200 ° C. or higher. A method for manufacturing a pearlite rail having excellent toughness and ductility. 4. A method for producing a high-strength pearlite rail, which comprises reheating the steel slab according to claim 1 or 2 to form a rail by hot rolling, and then hot rolling or hot rolling. After heating to an austenite temperature range, at least 700 to 5 parts of the head are cooled when the rail is cooled.
A method for producing a pearlite rail having excellent toughness and ductility, which comprises accelerating cooling at a temperature of 00 ° C at 1 to 5 ° C / sec. 5. A method for producing a high-strength pearlite rail, wherein the steel slab according to claim 3 is formed into a rail by hot rolling, and then the austenite temperature is maintained by hot rolling or by heating after hot rolling. In cooling the rail, at least the head is 700 to 500 ° C and the temperature is 1 to 5 ° C / s.
A method for manufacturing a pearlite rail having excellent toughness and ductility, which is characterized by accelerated cooling with ec.