JP5326343B2 - Manufacturing method of high internal hardness rail - Google Patents

Description

  The present invention relates to a rail used in heavy-duty railways, and more particularly to a pearlite rail with increased hardness up to the inside of the rail head.

  In overseas heavy-duty railways, as a means of improving the efficiency of railway transportation, train speeds are increased and train loading weight is increased. A high strength (high hardness) rail having a pearlite structure using carbon steel is used.

  However, due to the further increase in axle load and high speed transportation in recent years, the rail wear speed has also become faster, and in order not to increase the frequency of rail replacement, a rail that can be used up to a larger amount of wear. Therefore, it is necessary to develop a rail having a head that is more homogeneous and harder to the inside, for example, a rail having a hardness higher than HB370 from the surface to a depth of 20 mm or more.

  In general, perlite-type rails are heat-treated to increase the hardness by heat-treating the rail head, thereby improving wear resistance. During the heat treatment, if the rail is cooled at a constant cooling rate from the temperature of the Ac1 point or higher, the cooling rate inside the head is slower than the cooling rate on the head surface. The hardness decreases as time goes on, and it is impossible to increase the hardness uniformly to the inside.

  As a method for solving such a problem and increasing the hardness to the inside, for example, in Patent Document 1, a region within 5 mm below the surface starting from the surface of the rail head from the temperature of Ar1 point or higher is pearlite. After cooling at a head surface cooling rate of 1 to 10 ° C./s until transformation is started, a region of 20 mm or more below the surface from the surface starts at a cooling rate of 2 to 20 ° C./s until the end of pearlite transformation. A method of manufacturing a deep deep high strength rail for cooling is disclosed.

  Further, in Patent Document 2, the head of a high-temperature rail that holds heat at or above the austenite region temperature is increased while increasing the amount of cooling medium with the passage of cooling time, or nearing the nozzle distance, or both. A method of heat treating a rail that is controlled and cooled while operating is disclosed.

  In the manufacturing method of Patent Document 1, the formation of bainite and martensite metal structure on the rail top surface is suppressed, the entire surface of the rail head is made a pearlite metal structure, and the hardness difference within 20 mm from the rail top surface is HB30 or less. Manufactures rails. Also in the manufacturing method of Patent Document 2, a rail whose hardness difference within at least 20 mm from the head surface layer is HB30 or less is manufactured by changing the amount of the cooling medium.

  However, in these techniques, it is necessary to accelerate cooling to a temperature of 550 ° C. or lower in order to increase the hardness to HB370 or higher up to a high depth. In this case, the boundary region between the rail head and the base of the column is required. It has been found by the inventor's research that there is a problem that a fine martensitic metal structure (micromartensite) is generated inside.

  This micro martensite is inconsistent with the pearlite structure and is likely to become the starting point of cracking. Even if such a rail has high hardness to the inside, it should be used for actual use due to safety issues. I can't.

JP-A-9-241747 JP-A-64-87720

  Therefore, the present invention has a hardness of HB370 or higher from the rail head surface to a point at least 20 mm inside, and generates micro martensite in the boundary region between the rail head and the base of the pillar. It aims at providing the high internal hardness rail which suppressed and increased the hardness to the inside.

  In the present invention, when accelerated cooling is performed to 550 ° C. or lower in order to increase the hardness to HB370 or higher to a depth of 20 mm or higher using the conventional technology, the base portion of the rail head portion and the column portion becomes thin. It has been found that the cooling rate is likely to increase, so that when the end temperature of the accelerated cooling is lowered, excessive cooling occurs and micro martensite is generated in the boundary region between the rail head portion and the column portion corresponding to the root.

  And, in order to obtain a rail that suppresses the generation of micro martensite in the rail head portion and in the boundary region between the head portion and the column portion, it is important not to lower the head surface temperature at the end of cooling excessively. It has been found that by adjusting the element content to a specific range, even when the cooling end temperature is raised, sufficient hardness can be secured up to the deep part.

This invention is made | formed based on such knowledge, and achieves the said subject by setting it as the following rails.
(1) By mass%, C: 0.60 to 0.86%, Si: 0.10 to 1.20%, Mn: 0.40 to 1.50%, Cr: 0.05 to 2.00% containing the following formula (1) being defined Ceq is 1.00 or more, QP defined by the following formula (2) is satisfied, respectively 7.0 or less, the balance of Fe and unavoidable impurities was after the hot rolled rail by shape using a steel starts accelerated cooling from a temperature above Ar1 point, cooling the range of 600 to 650 ° C. at a cooling rate of 1 to 5 ° C. / sec, followed by By cooling to a range of 520 ° C. to less than 550 ° C. at a cooling rate of 2 to 20 ° C./sec, accelerating cooling is completed, and after that, the entire surface of the rail head exhibits a pearlite metal structure, and the rail head surface The hardness up to the point 20mm inside from the starting point is HB370 or higher, rail top table And a hardness difference between at least 20 mm from the surface and HB30 or less, and the boundary region between the rail head portion and the rail pillar portion has a pearlite metal structure. Hardness rail manufacturing method .
Ceq = C + Si / 10 + Mn / 4.75 + Cr / 5.0 ... Formula (1)
QP = (0.06 + 0.4 × C) × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 2.33 × Cr) (2)
Here, C, Si, Mn, and Cr are values in mass% of the content of each element.
(2) The steel is further in mass%, Mo: 0.01 to 0.50%, Nb: 0.002 to 0.050, B: 0.0001 to 0.0050%, Co: 0.003. -2.00%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, Ti: 0.0050-0.0500%, Mg: 0.0005-0.0200%, Ca: 0.0005 to 0.0150, Zr: 0.0001 to 0.2000%, N: 0.0060 to 0.0200%, The high inside according to (1) above Hardness rail manufacturing method .

  In the present invention, the rail head 1 means a portion located above the horizontal line passing through the point A that intersects each other when the lower surface of the head side portion 3 is extended as shown in FIGS. The boundary region 4 between 1 and the rail pillar 2 is a 15 mm front and back region including the A.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the high depth and high intensity | strength rail which suppressed the bainite metal structure and micro martensite in a rail head surface layer and a rail head inside.

In the present invention, when accelerated cooling is performed to a temperature of 550 ° C. or lower in order to increase the hardness to HB370 or higher to a depth of 20 mm or higher using conventional technology, the base portion of the rail head portion and the column portion is thin. Thus, it has been found that the cooling rate is likely to be increased, and therefore, when the cooling end temperature is lowered, excessive cooling occurs, and micromartensite is generated in the boundary region between the head and the pillar corresponding to the root.
And in order to suppress the production | generation of the micro martensite in the rail head part inside and the boundary area | region of a head part and a column part, it discovered that the head surface temperature at the time of completion | finish of cooling was important.

  FIG. 3 shows a steel composed of C: 0.85%, Si: 0.55%, Mn: 1.15%, Cr: 0.22% by mass (Ceq = 1.19, QP = 4.7). The rail formed using is cooled at a cooling rate of 2.9 ° C./s over a temperature range of 770 ° C. to 630 ° C. and then 5.5 ° C./s from that temperature to an end temperature of 550-500 ° C. In the case of cooling at a cooling rate and then allowing to cool, whether or not micromartensite is generated in the boundary region between the rail head and the column is examined. As shown in the figure, when the cooling end time was less than 520 ° C., micromartensite was generated.

Next, the conditions of the content of the component elements were examined so that the micromartensite was not generated and the hardness at the point 20 mm inside from the rail top surface was HB370 or more.
As a result, the hardness at the point entering the inside of 20 mm is the formula Ceq = C + Si / 10 + Mn / 4.75 + Cr / 5.0 (C, Si, Mn, Cr are values of mass% of the content of each element. It was found that it can be organized by the value of Ceq defined in).
In addition, the coefficient of this formula is based on the contribution ratio to the hardness of each element at a point within 20 mm obtained experimentally.

  FIG. 4 shows the relationship between Ceq and hardness at a point within 20 mm when cooling of steel rails having various Ceqs is completed at 530 ° C. As shown in the figure, HB370 or higher is obtained when Ceq is 1.00 or higher.

The present invention based on the above findings will be sequentially described below.
The chemical composition of steel used for the rail according to the present invention will be described.
In the present invention, by controlling the cooling rate, the hardness from the head surface to the point 20 mm inside becomes HB370 or more, so that C: 0.60-0.86% by mass%, Si: Steel containing 0.10 to 1.20%, Mn: 0.40 to 1.50%, Cr: 0.05 to 2.00%, the balance being Fe and inevitable impurities is used as the steel of the basic composition .
The reasons for limitation of each element are as follows.

C: Steel having a high carbon and pearlite metal structure is often used for rails because of its high strength and good wear resistance. In order to obtain this pearlite structure, C needs to be contained by 0.60% or more. However, when the content exceeds 0.86%, there is a problem that a large amount of cementite metal structure is precipitated and ductility and toughness are lowered. Therefore, the C content range is limited to 0.60 to 0.86%.
Moreover, in order to obtain intensity | strength required as rail steel, the containing range of S, Mn, and Cr was limited as follows.

  Si: Si contains 0.1% or more as an effective component for strengthening ferrite in the pearlite structure. However, if the content exceeds 1.20%, there is a problem that a martensite structure is generated and the steel is embrittled. Therefore, the Si content range is set to 0.10 to 1.20%.

  Mn: Mn is an element effective for strengthening the pearlite structure. However, when the content is less than 0.40%, the effect is small. Conversely, when the content exceeds 1.50%, a martensite structure is formed, and the steel becomes brittle. Make it. Therefore, the Mn content range is set to 0.40 to 1.50%.

  Cr: Cr is an element that raises the equilibrium transformation point of pearlite and, as a result, refines the pearlite structure. If its content is less than 0.05%, its effect is small. , Generates a martensitic structure and embrittles the steel. Therefore, the content range of Cr is set to 0.05 to 2.00%.

In the present invention, the steel composed of the above components is used as a basis, but in addition to the above elements, the following elements may be added in the following ranges for various purposes. The effect of each additive element can be obtained without impairing the effect of the present invention.
Mo: 0.01~0.50%, N b: 0.002~0.050, B: 0.0001~0.0050%, Co: 0.003~2.00%, Cu: 0.01~ 1.00%, Ni: 0.01~1.00%, Ti: 0.0050~0.0500%, Mg: 0.0005~0.0200%, Ca: 0.0005~0.0150, Z r : 0.0001 to 0.2000%, N: 0.0060 to 0.0200%.
The reason why the addition amount of each element is within the above range is as follows.

  Mo: Mo is an element that contributes to high hardness (high strength) by making the pearlite structure fine, and improves the hardness (strength) of the pearlite structure. However, if the amount of Mo is less than 0.01%, the effect is small, and if the amount of Mo exceeds 0.50%, a martensite structure harmful to ductility is generated, so the amount of added Mo is 0.01 to 0. .50% is desirable.

  Nb: Nb is an element effective for forming nitrides and carbonitrides, improving ductility, and at the same time improving hardness (strength). Further, it is an element that raises the temperature range of non-recrystallized austenite and stabilizes the non-recrystallized austenite structure. However, if the amount of Nb is less than 0.002%, the effect cannot be expected, and if the amount of Nb exceeds 0.050%, coarse precipitates that start fatigue damage are generated. 0.002 to 0.050% is desirable.

  B: B is an element that reduces the ductility of the rail and increases the life by minimizing the formation of a pro-eutectoid cementite structure and uniforming the hardness distribution of the head. However, if the amount of B is less than 0.0001%, the effect is not sufficient, and if the amount of B exceeds 0.0050%, coarse precipitates are generated, so the amount of B added is 0.0001 to 0.0050. % Is desirable.

  Co: Co is an element that improves the hardness (strength) of the pearlite structure. Further, the wear surface of the rail head further increases the fine lamella structure of the pearlite structure directly below the rolling surface formed by contact with the wheel. It is an element that refines and improves wear resistance. However, if the Co content is less than 0.003%, the effect cannot be expected. Further, if the Co content exceeds 2.00%, spalling damage occurs on the rolling surface, so the Co addition amount is preferably 0.003 to 2.00%.

  Cu: Cu is an element that improves the hardness (strength) of the pearlite structure. However, if the amount of Cu is less than 0.01%, the effect cannot be expected. Further, if the amount of Cu exceeds 1.00%, a martensite structure harmful to wear resistance is generated, so the amount of Cu added is preferably 0.01 to 1.00%.

  Ni: Ni is an element for increasing the hardness (high strength) of pearlite steel. However, if the amount of Ni is less than 0.01%, the effect is remarkably small. If the Ni content exceeds 1.00%, spalling damage occurs on the rolling surface. For this reason, the Ni addition amount is desirably 0.01 to 1.00%.

  Ti: Ti is an effective component for forming nitrides and carbonitrides to improve ductility and at the same time improve hardness (strength). Further, it is an element that raises the temperature range of non-recrystallized austenite and stabilizes the non-recrystallized austenite structure. However, the effect is small when the Ti content is less than 0.0050%. On the other hand, if the Ti content exceeds 0.0500%, coarse precipitates are generated and the ductility of the rail is greatly reduced. Therefore, the Ti addition amount is preferably 0.0050 to 0.0500%.

  Mg: Mg is an element effective in reducing the austenite grains and the pearlite structure and improving the ductility of the pearlite structure. However, if the amount of Mg is less than 0.0005%, the effect is weak. Further, if the Mg content exceeds 0.0200%, a coarse oxide of Mg is generated and the ductility of the rail is lowered. Therefore, the Mg addition amount is desirably 0.0005 to 0.0200%.

  Ca: Ca is an element that contributes to the generation of pearlite transformation and, as a result, is effective in improving the ductility of the pearlite structure. However, if the Ca content is less than 0.0005%, the effect is weak. Further, if the Ca content exceeds 0.0150%, a coarse oxide of Ca is generated and the ductility of the rail is lowered. Therefore, the Ca addition amount is preferably 0.0005 to 0.0150%.

  Zr: Zr is an element that suppresses the formation of a pro-eutectoid cementite structure generated in the segregation part. However, if the amount of Zr is 0.0001% or less, a pro-eutectoid cementite structure is formed and the ductility of the rail is lowered. On the other hand, if the amount of Zr exceeds 0.2000%, a large amount of coarse Zr-based inclusions are generated and the ductility of the rail is lowered. Therefore, the amount of Zr added is preferably 0.0001 to 0.2000%.

  N: N is an element effective for enhancing the ductility of the pearlite structure and at the same time improving the hardness (strength). However, if the N content is less than 0.0060%, the effect is weak. On the other hand, if the N content exceeds 0.0200%, it becomes difficult to make a solid solution in the steel, and bubbles are generated as a starting point of fatigue damage. Therefore, the N addition amount is 0.0060 to 0.0200%. desirable. In the rail steel, N is contained as a maximum of about 0.0050% as an impurity. Therefore, in order to make N amount into the above range, it is necessary to intentionally add N.

Next, the hardness and metal structure required for the rail head and conditions for obtaining them will be described.
The hardness required for the head surface of the high-strength rail is HB370 or higher, and particularly the hardness required for the rail used in heavy-duty railways is HB370 (AREMA: American Rail Standard). Therefore, the required hardness is set to HB370 or higher.

  In order to use the rail stably for a long period of time, it is necessary that the rail has a uniform hardness from the head surface to a depth that is a wear limit. If the decrease in hardness is greater from the head surface to the inside, wear progresses faster as the rail is used, which is not preferable. Considering the difference in wear progress, the maximum hardness difference between the top surface and at least 20 mm inside the head was HB30.

  The hardness conditions as described above are at least satisfied within a range of 20 mm in depth with respect to the surfaces of the top of the head and the corner of the head as indicated by hatching in FIG.

Therefore, in the present invention, as described above, the content of C, Si, Mn, and Cr is limited so that the value of Ceq defined by the following formula (1) is 1.00 or more, and the above hardness Meet the requirements of
Ceq = C + Si / 10 + Mn / 4.75 + Cr / 5.0 Formula (1)
Here, C, Si, Mn, and Cr are values in mass% of the content of each element.

In the present invention, the entire rail head portion exhibits a pearlite metal structure, and the metal structure in the boundary region between the rail head portion and the rail column portion is also pearlite.
Steel with a high carbon and pearlite metal structure has high strength and good wear resistance, so it can cope with higher axle load and higher speed transportation accompanying the increase in the weight of railway vehicles. On the other hand, when the cooling rate of the head surface becomes excessive, a bainite metal structure is formed, but the structure is lower in hardness and inferior in wear resistance than the pearlite structure. For this reason, when a bainite structure is generated, the wear rate is increased and the rail life is shortened.

  In the prior art, micro martensite may be generated in the boundary region between the rail head portion and the rail column portion. This micro martensite may become a starting point of rail breakage due to mismatch with the pearlite structure, and its generation must be suppressed for safety.

Therefore, in the present invention, from the viewpoint of hardenability, C, Si, Mn, so that the value of QP defined by the following formula (2) is 7.0 or less, preferably 5.0 or less. Limit the Cr content.
QP = (0.06 + 0.4 × C) × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 2.33 × Cr) (2)
Here, C, Si, Mn, and Cr are values in mass% of the content of each element.

  Thereby, generation | occurrence | production of the micro martensite in the boundary area | region of a rail head part and a column part can be suppressed, and the structure | tissue of the rail head part and the whole boundary region of a head part and a column part can be made into a pearlite metal structure.

A method for manufacturing a rail according to the present invention will be described.
Melting is performed in a commonly used melting furnace such as a converter or an electric furnace, and this molten steel is made into a steel strip for rolling having the above-mentioned chemical composition by ingot-making, slabbing or continuous casting. After this steel strip is hot-rolled into the shape of a rail, it is subsequently accelerated and cooled from a temperature of the Ar1 point or higher to make the entire surface of the rail head a pearlite metal structure. Or after hot rolling, it is once cooled, and after being heated again to Ac1 point or more, accelerated cooling is similarly performed.

In accelerated cooling, first, the surface layer is cooled at a slow cooling rate so that the difference in cooling rate between the surface layer and the inside does not become large, the surface layer is pearlite transformed, and then rapidly cooled to remove the heat inside. By performing in two stages, the above-mentioned hardness condition is satisfied.
In this case, the following conditions are preferable for the temperature range and cooling rate of accelerated cooling in each stage.

  In order to obtain high hardness in the surface layer and inside of the rail head, the start temperature of accelerated cooling needs to be higher than the Ac1 point that is the austenite temperature range temperature. The cooling rate of the first stage is preferably 1 ° C./s to 5 ° C./s in order to transform the surface layer to pearlite so that the difference in cooling rate between the surface layer part and the inside does not increase. If the cooling rate is less than 1 ° C / s, it takes a very long time to transform pearlite and the austenite grains become coarse, which is not preferable. On the other hand, if it exceeds 5 ° C / s, the surface layer has a different structure other than pearlite (bainite Etc.) is not preferable.

Further, the temperature range for cooling at this rate is preferably set to a temperature between Ac1 point and 600 to 650 ° C. Cooling at this cooling rate until the head surface temperature falls below this temperature range is not preferable because the range in which pearlite transformation starts from the surface layer becomes deep and internal heat removal becomes difficult.
In addition, although it can implement suitably about the cooling method after completion | finish of hot rolling until it starts accelerated cooling, natural cooling is enough.

  As described above, after the surface layer starts pearlite transformation, it is preferable to cool at a faster cooling rate of 2 to 20 ° C./s in order to heat the inside and make the entire head a pearlite structure. . If the cooling rate is less than 2 ° C / s, the internal heat removal cannot be increased, and if the cooling rate exceeds 20 ° C / s, the difference in cooling rate between the surface layer and the inside increases, This is not preferable because it exhibits a metal structure other than pearlite.

Further, the cooling end temperature at this cooling rate is preferably set to 520 ° C. or more at the head surface temperature so as not to generate micromartensite or bainite structure inside the rail head. However, if the cooling is terminated at a head surface temperature of 550 ° C. or higher, the cooling is stopped before the pearlite transformation is completed at a high depth, and sufficient high depth hardness cannot be obtained. Therefore, the cooling end temperature is preferably between 520 ° C. and less than 550 ° C., but a more preferable temperature is 520 ° C. to 530 ° C.
What is necessary is just to cool to room temperature as it is after completion | finish of the above accelerated cooling.

  Examples of the present invention will be described below. The conditions used in the examples are one example of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to these examples.

[Example 1]
After forming a rolling material having various chemical components into a rail by hot rolling, or after reheating the rail once cooled to a temperature below the Ar1 point after hot rolling to a temperature above the Ac1 point, the rail head Cooling is performed at a cooling rate of 1 to 5 ° C./s until the surface temperature of the part reaches from 600 ° C. to 650 ° C. from the temperature of the Ar1 point or higher, and then until the temperature reaches 520 ° C. to less than 550 ° C. It cooled at 2 degreeC-20 degreeC / s, and it stood to cool after that, and used for evaluation.
Table 1 shows the chemical composition of the test rail steel (the balance is Fe and inevitable impurities), and the microstructure of the rail head and the boundary region, rail head surface and surface in the rail manufactured under the above conditions. The hardness at the lower 20 mm position was shown.
As shown in Table 1, those satisfying the conditions of the present invention satisfied the hardness and structure conditions defined in the present invention.

[Example 2]
Rails manufactured using the A9 steel of Table 1 were cooled under various cooling conditions shown in Table 2.
Table 2 also shows the microstructure of the rail head and the boundary region in the rail after cooling, and the hardness at the rail head surface and 20 mm below the surface.
As shown in Table 2, by adopting appropriate cooling conditions, a rail satisfying the hardness and structure conditions defined in the present invention was obtained.

It is a figure which shows the cross section of the rail which concerns on the embodiment of this invention. It is a figure which shows the high hardness area | region which concerns on the embodiment of this invention. It is a figure which shows the relationship between the completion | finish temperature of accelerated cooling, and the presence or absence of the production | generation of the micro martensite in the boundary area | region of a rail head part and a pillar part. It is a figure which shows the relationship between Ceq of steel, and the hardness in the point of 20 mm inside.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Rail head part 2 Rail pillar part 3 Rail head side part 4 Boundary area of head part and pillar part

Claims (2)

In mass%, C: 0.60 to 0.86%, Si: 0.10 to 1.20%, Mn: 0.40 to 1.50%, Cr: 0.05 to 2.00% the following formula (1) being defined Ceq is 1.00 or more, QP defined by the following formula (2) is satisfied, respectively 7.0 or less, using a steel the balance of Fe and unavoidable impurities after the hot rolled rail by shape, starts accelerated cooling from above Ar1 point temperature, range or in cooling of 600 to 650 ° C. at a cooling rate of 1 to 5 ° C. / sec, followed by 2 20 ° C. / sec range or in cooling of 520 ° C. to 550 below ° C. at a cooling rate of the accelerated cooling and exit, by cooling thereafter, the rail head entire surface exhibits a metallic structure of pearlite, the rail head surface The hardness from the starting point to the point 20 mm inside is HB370 or more, the rail top surface and the surface A high internal hardness rail characterized in that the hardness difference between at least 20 mm inside from the surface is HB30 or less and the boundary region between the rail head and the rail pillar is a pearlite metal structure Manufacturing method .
Ceq = C + Si / 10 + Mn / 4.75 + Cr / 5.0 ... Formula (1)
QP = (0.06 + 0.4 × C) × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 2.33 × Cr) (2)
Here, C, Si, Mn, and Cr are values in mass% of the content of each element. Further, the steel is, in mass%, Mo: 0.01 to 0.50%, Nb: 0.002 to 0.050, B: 0.0001 to 0.0050%, Co: 0.003 to 2. 00%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Ti: 0.0050 to 0.0500%, Mg: 0.0005 to 0.0200%, Ca: 0 .0005~0.0150, Zr: 0.0001~0.2000%, N : 0.0060~0.0200% of the production of high internal hardness rail according to claim 1, characterized by containing either Way .

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