JP6137043B2 - Rail manufacturing method - Google Patents

Description

  The present invention relates to a method for producing rails, in particular rails with high ductility and toughness.

Rails are used for various purposes such as railways, tracks for construction sites, cranes and elevators. The performance required by the application varies, but high ductility and toughness are required as basic performance.
For example, in mining railways (high-axle heavy railways) that mainly transport ores, the load applied to the axles of freight cars is high and the use environment of the rails is harsh. Is used steel having a pearlite structure with a high carbon content. This type of steel is excellent in wear resistance, but has a drawback in that it has low ductility and toughness.

  In order to improve the ductility and toughness, it is necessary to refine the pearlite structure, for example, refine the austenite grains before the pearlite transformation or refine the structure during the pearlite transformation. For that purpose, it is an effective means to reduce the rolling temperature during hot rolling, but even if the austenite grains can be refined immediately after hot rolling, the time until the start of heat treatment after hot rolling can be achieved. As a result of the grain growth, the rail had poor ductility.

  On the other hand, Patent Document 1 proposes to refine the pearlite structure by performing accelerated cooling after low-temperature reheating after rolling into a rail shape. Furthermore, Patent Document 2 describes that the growth of austenite grains is suppressed by precipitates.

JP 63-128123 A JP 2010-1500

However, any of the above-described methods has a problem of increasing the manufacturing cost. That is, in the technique described in Patent Document 1, since low temperature reheating and accelerated cooling are added, an increase in cost required for these heat treatments is inevitable.
Moreover, since the technique described in Patent Document 2 is to finely precipitate Ti precipitates and suppress austenite grain growth after rolling, there is a concern that the rolling contact fatigue life of the rails may decrease due to Ti precipitates. The

  Therefore, the present invention aims to propose a method for producing a rail having high ductility and toughness without causing an increase in production cost and without adding Ti as an alloy element. .

The inventors diligently studied the means for solving the above-mentioned problems, and it was found that regulating the composition of the components and the various hot rolling conditions is effective for realizing a refined pearlite structure at the product stage. As a result, the present invention has been completed.
That is, the gist configuration of the present invention is as follows.

(1) In mass%,
C: 0.75 to 0.85%
Si: 0.2-1.2%
Mn: 0.4-1.5%
P: 0.035% or less,
S: 0.035% or less and
Nb: 0.0001-0.05%
In order to produce a pearlite rail by cooling after performing hot rolling on the steel material having a component composition of Fe and inevitable impurities,
Heating the steel material to 1200 ° C. or higher and 1330 ° C. or lower prior to the hot rolling; and
In the hot rolling, the cumulative area reduction in a temperature range of 950 ° C. or lower is set to 40% or higher, the finishing temperature is set to 800 ° C. or higher at the head of the rail, and 600 ° C. or higher at the foot. The rail manufacturing method is characterized in that the temperature difference with the section is within 200 ° C.

(2) The method for manufacturing a rail according to (1), wherein the cooling is performed from a temperature range equal to or higher than a pearlite transformation start temperature to a temperature range equal to or lower than 500 ° C. at a cooling rate of 2 ° C./s or lower.

(3) The steel material is further mass%,
V: 0.1% or less,
Cr: 1.5% or less
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less and
Ca: One or two or more kinds selected from 0.015% or less are contained, The method for producing a rail as described in (1) or (2).

  According to the present invention, it is possible to manufacture a pearlite steel rail having extremely high ductility and toughness as compared with the conventional one at a low cost, and in particular, it is possible to provide a rail suitable for a high-axle railway.

It is sectional drawing which shows the collection position of the tensile test piece from a rail head.

The present invention is a steel material having a predetermined composition, formed into a rail shape by hot rolling under a predetermined condition, and then cooled to form a rail. First, the composition of the steel material Will be described in order.
The steel material contains C: 0.75 to 0.85%, Si: 0.2 to 1.2%, Mn: 0.4 to 1.5%, P: 0.035% or less, S: 0.035% or less, and Nb: 0.001 to 0.05%, the balance being Fe and Based on the component composition of inevitable impurities. Below, the reason for limitation of each component is shown. In addition, unless otherwise indicated, the "%" display regarding a component means "mass%".

C: 0.75-0.85%
In order to refine the austenite grains before pearlite transformation, increase the hot deformation in the temperature range below the recrystallization temperature, specifically, increase the recrystallization temperature to increase the recrystallization temperature in hot rolling. It is effective to increase the following processing. Therefore, C is limited to 0.85% or less in order to increase the recrystallization temperature. Furthermore, if limited to 0.85% or less, it is advantageous in that the total amount of pro-eutectoid cementite is suppressed and ductility and toughness can be improved. On the other hand, if C is less than 0.75%, the formation of cementite in the pearlite structure becomes insufficient, and it becomes difficult to ensure the wear resistance necessary for the rail due to a decrease in hardness. .

Si: 0.2-1.2%
Si is required to be 0.2% or more as a deoxidizing material and to strengthen the pearlite structure. However, if it exceeds 1.2%, it promotes the formation of rail surface defects, so the upper limit of its content is 1.2%. To do. Therefore, the Si content is 0.2 to 1.2%.

Mn: 0.4-1.5%
Mn is an effective element for maintaining high hardness even inside the rail because it has the effect of lowering the transformation temperature to pearlite and making the pearlite lamellar spacing dense. For that purpose, 0.4% or more is necessary, but when it exceeds 1.5%, the pearlite equilibrium transformation temperature is lowered and bainite is easily formed. Therefore, the amount of Mn is 0.4 to 1.5%.

P: 0.035% or less If the P content exceeds 0.035%, ductility and toughness deteriorate, so the P content is 0.035% by mass or less. Preferably it is 0.020% or less. In addition, in order to make it less than 0.001%, since the increase in steelmaking cost will be forced, content of 0.001% or more is permitted.

S: 0.035% or less If the S content exceeds 0.035%, MnS that progresses in the rolling direction is formed and the ductility and toughness of the rail deteriorate, so the P content is 0.035% by mass or less. Preferably it is 0.020% or less. In addition, in order to make it less than 0.0005%, since the increase in steelmaking cost will be forced, containing 0.0005% or more is accept | permitted.

Nb: 0.0001-0.05%
Nb is an important component in the basic composition and has the effect of suppressing the formation of ferrite and raising the recrystallization temperature. And by raising the non-recrystallization temperature range by adding Nb, rolling in the non-recrystallization temperature range (below the recrystallization temperature), which will be described later, is facilitated and contributes to the improvement of ductility. Furthermore, there is an effect of promoting the precipitation of carbides, and it becomes possible to compensate for the decrease in strength due to the limitation of the amount of C described above. For that purpose, it is necessary to make Nb amount 0.0001% or more. On the other hand, if the Nb content exceeds 0.05%, coarse Nb carbonitrides are generated during the solidification process and the cleanliness is deteriorated, so the upper limit of the Nb content is 0.05%. Preferably, it is 0.001 to 0.03%.

  In addition to the basic components described above, V: 0.1% or less, Cr: 1.5% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, and Ca: 0.015% or less as necessary 1 type or 2 types or more selected from can be contained. The preferred addition amount of each component is as follows.

V: 0.1% or less V is an element that forms VC or VN and precipitates finely in ferrite, and is effective for increasing the strength through precipitation strengthening of ferrite. It also functions as a hydrogen trap site and can be expected to suppress delayed fracture. However, addition exceeding 0.1% causes an increase in alloy cost. Therefore, V can be added with an upper limit of 0.1%. It should be noted that V is preferably contained in an amount of 0.001% or more in order to exhibit the effect of increasing the strength by adding V and suppressing delayed fracture resistance.

Cr: 1.5% or less
Cr raises the equilibrium transformation temperature of pearlite and contributes to refinement of the pearlite lamellar spacing, thereby increasing hardness and strength. However, when Cr is added in excess of 1.5%, the occurrence of weld defects is increased, the hardenability is increased, and the formation of martensite is promoted. Therefore, Cr can be added with an upper limit of 1.5%.

Cu: 1.0% or less
Cu is an element that can be further strengthened by solid solution strengthening. It is also effective in suppressing decarburization. However, if Cu is added in excess of 1.0%, surface cracks are likely to occur during continuous casting or rolling, so Cu can be added up to 1.0%.

Ni: 1.0% or less
Ni is an effective element that improves toughness and ductility. Moreover, since it is an effective element which suppresses Cu crack by adding together with Cu, when adding Cu, it is preferable to add Ni. However, if Ni is added in an amount exceeding 1.0%, the hardenability is increased and the formation of martensite is promoted, so Ni can be added up to 1.0%.

Mo: 1.0% or less
Mo is an element effective for increasing the strength, so it can be added as necessary.However, if Mo is added in excess of 1.0%, the hardenability is improved and martensite is formed as a result. Life and ductility are extremely reduced. Therefore, when adding Mo, the upper limit of the content is made 1.0%.

Ca: 0.015% or less
Ca can be added as it combines with O and S in the steel during solidification to form granular oxysulfide, and has the effect of improving ductility, toughness, and delayed fracture resistance. If added to, the cleanliness of the steel deteriorates, so the upper limit when adding Ca is 0.015%.
In the component composition of the steel material, the balance other than the elements described above is Fe and inevitable impurities.

  The steel material having the above composition is preferably used as a steel slab obtained by continuous casting, with the molten steel adjusted in the blast furnace, hot metal pretreatment, converter, RH degassing, etc. . The steel material thus made into a steel slab is hot-rolled and formed into a rail shape, and then cooled to become a rail product. At that time, prior to hot rolling, the steel material is heated to 1200 ° C or higher and 1330 ° C or lower, and hot rolling has a cumulative area reduction in a temperature range of 950 ° C or lower and a finish temperature It is important that the temperature of the head of the rail is 800 ° C. or higher and the foot is 600 ° C. or higher, and the temperature difference between the head and the foot is within 200 ° C.

[Steel material heating temperature: 1200 ℃ to 1330 ℃]
In hot rolling, in order to generate austenite grains and to stably recrystallize the austenite grains, it is advantageous to perform solid solution of the added Nb with certainty. For this purpose, it is necessary to heat the steel material to 1200 ° C. or higher and 1330 ° C. or lower and then subject it to hot rolling. That is, if the heating temperature is less than 1200 ° C., the solid solution of Nb is insufficient, and solid solution Nb or a fine precipitate of Nb that raises the upper limit temperature of the non-recrystallization temperature range cannot be obtained. Rolling in the crystallization temperature range becomes difficult, austenite grains cannot be refined, and as a result, sufficient ductility cannot be imparted to the rail. On the other hand, when the heating temperature exceeds 1330 ° C., the raw material is melted and hot rolling described later may be difficult. Therefore, the heating temperature of the steel material is set to 1200 ° C or higher and 1330 ° C or lower.

Hot rolling conditions [cumulative reduction in area in the temperature range below 950 ℃ is 40% or more]
In order to stably realize the refinement of the pearlite structure by performing hot rolling in the non-recrystallization temperature range, the cumulative area reduction rate at 950 ° C. or lower is set to 40% or higher. This is because if the cumulative area reduction is less than 40%, the austenite grains before the pearlite transformation are not sufficiently refined, and as a result, the rail ductility cannot be improved.

  Here, the non-recrystallization temperature range may be 950 ° C. or less if the component composition is described above, and therefore, hot rolling with a cumulative area reduction rate of 40% or more is performed in a temperature range of 950 ° C. or less. And

In addition, the cumulative area reduction in the temperature range of 950 ° C. or lower is the hot area of the cross-sectional area of the portion corresponding to the rail head of the material to be rolled immediately before the start of rolling at 950 ° C. or lower. When the cross-sectional area of the rail head at the end of rolling is B,
(AB) / A × 100
Can be calculated according to The cross-sectional areas A and B can be adjusted by setting a roll hole shape and a reduction gap pattern in hot rolling.

[Finishing temperature: The rail head is 800 ° C or higher, the foot is 600 ° C or higher, and the temperature difference between the head and foot is within 200 ° C]
As mentioned above, when performing hot rolling in the temperature range below the recrystallization temperature, the legs are likely to be colder than the rail head due to the unique shape of the rail, and cracks and scratches during rolling May occur. Therefore, the finishing temperature in hot rolling is set to 800 ° C. or higher at the head of the rail and 600 ° C. or higher at the foot. In other words, the temperature of 800 ° C or higher at the head of the rail is to prevent the generation of pro-eutectoid ferrite and pro-eutectoid cementite and to prevent the ductility from decreasing. This is to prevent the foot from being damaged.
Furthermore, the temperature difference between the head and the foot must be within 200 ° C. This is because if the temperature difference between the head and the foot exceeds 200 ° C., the amount of warping of the rail accompanying the temperature difference becomes large and rolling becomes difficult.

  Here, in order to control the finishing temperature as described above, for example, when rolling into a rail shape, rolling up to rough rolling is performed at over 950 ° C., and then the material to be rolled is waited in front of a finish rolling mill, Or it is suitable to make it reduce to 950 degrees C or less by forced cooling, and to perform the rolling in which a cumulative area reduction rate becomes 40% or more with a finishing mill after that. If the rail head has a large cross-sectional area and the temperature of the material to be rolled is reduced to 950 ° C after rough rolling, cooling with air cooling requires a long time. It is preferable to perform mist cooling for the purpose of conversion.

The product is formed into a rail shape through the above hot rolling and then cooled to become a product. In order to reliably lead the refinement effect of the austenite grains realized up to this hot rolling process to refinement of the pearlite structure in the rail product, it is preferable to control the cooling conditions. That is, it is preferable to carry out from a temperature range above the pearlite transformation start temperature to a temperature range below 500 ° C. at a cooling rate of 2 ° C./s or less.
First, the reason why the cooling start temperature is preferably equal to or higher than the pearlite transformation start temperature is to obtain a pearlite structure with certainty.
Next, the reason why the cooling rate is 2 ° C./s or less is to suppress the generation of bainite and martensite which are crack starting points during use of the rail.
Furthermore, the cooling stop temperature is set to a temperature range of 500 ° C. or lower in order to obtain a pearlite structure over the entire head.

  The cooling rate from the temperature range above the pearlite transformation start temperature to the temperature range below 500 ° C. is preferably 0.3 ° C./s or more. This is because when the cooling rate is less than 0.3 ° C./s, spheroidization of cementite occurs after pearlite transformation, and the hardness of the head of the rail decreases.

  A steel material having the composition shown in Table 1 was hot-rolled and cooled under the conditions shown in Table 2 to produce a pearlite steel rail. In addition, cooling was performed only on the head of the rail, and after cooling was stopped, it was allowed to cool. The pearlite steel rail was examined for structure and particle size, and mechanical properties were evaluated. The results are shown in Table 3.

  In addition, the finishing temperature in Table 2 shows the value which measured the temperature of the side surface layer of the rail head of the entrance of a final rolling mill and the side surface of a foot | leg with the radiation thermometer as rolling finishing temperature. The cooling stop temperature indicates a value obtained by measuring the temperature of the rail head side surface layer on the exit side of the cooling facility with a radiation thermometer as the cooling stop temperature. The cooling rate was defined as a value obtained by dividing the change in temperature from the start of cooling to the stop of cooling by the change in time. The structures in Table 3 were examined with an optical microscope.

As mechanical properties, hardness, yield point (0.2% yield strength) (YP), tensile strength (TS) and total elongation (El) were evaluated as follows.
(Hardness)
A portion including the parietal surface of the rail head portion was cut out and polished to a depth of 0.3 mm to remove a decarburized layer on the parietal surface, and the Brinell hardness was measured on the polished surface.

(Yield point, tensile strength and total elongation)
As shown in Fig. 1, a tensile test piece 2 with a parallel part diameter of 12.7mmφ and a gauge length of 50.8mm is taken from the rail head 1 and subjected to a tensile test in accordance with ASTM A370, and the undulation point (0.2% proof stress) YP, tensile strength TS and total elongation El were measured.

  Furthermore, about the rail after rolling, the presence or absence of the wrinkle on the surface was confirmed visually. The wrinkles were evaluated by the presence or absence of scale wrinkles and the presence or absence of chipped wrinkles present on the surface of the foot 3 (see FIG. 1). In all cases, the case where no wrinkle was observed was shown in Table 3, and the case where no wrinkle was observed was shown in Table 3.

  From the results shown in Table 3, it can be seen that in the comparative example in which the component composition or manufacturing conditions are out of the range, any of YP, TS, El, and surface flaws is inferior to the inventive example.

1 Rail head 2 Tensile specimen 3 Foot

Claims (2)

% By mass
C: 0.75 to 0.85%
Si: 0.2-1.2%
Mn: 0.4-1.5%
P: 0.035% or less,
S: 0.035% or less and
Nb: 0.001 to 0.05%
In order to produce a pearlite rail by cooling after performing hot rolling on the steel material having a component composition of Fe and inevitable impurities,
A child heating the steel material to 1200 ° C. or higher 1330 ° C. or less prior to the hot rolling,
In the hot rolling, the cumulative area reduction in a temperature range of 950 ° C. or lower is set to 40% or higher, the finishing temperature is set to 800 ° C. or higher at the head of the rail, and 600 ° C. or higher at the foot. and that the temperature difference between the parts as within 200 ° C.,
The method of manufacturing a rail, wherein the cooling is performed from a temperature range higher than a pearlite transformation start temperature to a temperature range of 500C or less at a cooling rate of 2C / s or less . The steel material is further mass%,
V: 0.1% or less,
Cr: 1.5% or less
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less and
Ca: 0.015% or less
The method for producing a rail according to claim 1, comprising one or more selected from among the above .

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