JPH09253703A - Production of high strength rail - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable utilization of contained elements restraining the micro- segregation and improving the strength by applying a working having >=5% plastic deforming ratio in the austenitic range to a continuously cast bloom completing the solidification, heating to a prescribed temp. add rolling to a target shape. SOLUTION: Since the steel cast bloom after the continuous casting is cooled to the austenitic developing temp. zone and held to apply the plastic working havxng >=5% plastic deforming ratio, the working strain is effectively added. Thereafter, since this blank is heated to the hot-rolling temp. of 1100-1300 deg.C and rolled to a rail-shape, the micro-segregation of the elements, can effectively be restrained by promoting the diffusion the elements according to the hot- rolling, and the elements of strengthening elements of the rail, such as C, Si, Cr, Mo, can efficiently be utilized. In the production of the high strength rail, the cooling is executed at 2-6 deg.C/sec after hot-rolling to the shape of the rail, and thus, the rail is formed as fine pearlite structure or bainite structure and the high strength and high hardness can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-strength rail used for industrial machines such as cranes or railroad tracks.

[0002]

2. Description of the Related Art Conventional rails include carbon steel, high carbon steel,
Or, a material obtained by heat-treating manganese steel or the like is used. Especially, the gauge corner part of the outer rail that is laid in the curved section of the railroad receives the lateral pressure of the wheel due to the centrifugal force, so it is subjected to severe wear conditions. High-strength rails that can withstand the demand are required. As represented by the Shinkansen and the like in recent years, the speed of trains in railway transportation and the heavy loading of cargo have been promoted, and the wear life of rails has become extremely short. Also,
In many cases, the rail itself used in cranes has an excessive load, and even in this case, extending the life is taken up as an important issue. As an effective means for improving the wear resistance of such rails, the head of the rail is heated and cooled to obtain a structure having a specific property and to increase the strength effective for improving the wear resistance. The method to achieve is becoming mainstream.

For example, Japanese Examined Patent Publication (Kokoku) No. 54-25490 discloses a heat treatment rail for super-heavy load in which the head of a rail having a specific component composition exhibits a sorbite structure or a fine pearlite structure. Kosho 63-
Japanese Patent No. 23244 discloses that after the hot rolling is finished, the austenite region temperature to 850 to 500 ° C. is cooled at a cooling rate of 1 to 4 ° C./sec to reduce wear and fatigue damage, and to improve weldability. A method of obtaining a good high strength rail is shown. Further, in Japanese Examined Patent Publication No. 59-19173, the head surface layer of a rail made of steel of a specific component has a temperature of 8
After heating to 50 ° C or higher to austenite, it is cooled by gas or gas liquid to make the microstructure of the rail head surface layer fine pearlite, and its tensile strength is 120 kgf /
mm ² or more, and Brinell hardness of the surface of the rail head and (H _B) to 330 or more, a method of improving the reduction in the hardness of the welded joint portion between the rails as well as wear resistance are described, Japanese Patent Application No. Japanese Unexamined Patent Publication No. 6-229822 describes a rail using a bainite structure for the purpose of improving rolling contact fatigue characteristics of the rail surface in addition to wear resistance.

[0004]

However, Japanese Patent Publication No. 54-25490 and Japanese Patent Publication No. 63-232
In the methods described in Japanese Patent Publication No. 44, Japanese Patent Publication No. 59-19173, and Japanese Patent Application No. 6-229822, the state of the metal phase in each part of the rail is regulated, but Mn and Cr are added to the steel slab after continuous casting. Alternatively, when an element such as hydrogen is unevenly distributed, it is difficult to diffuse each element into the steel sufficiently uniformly, because the element is dispersed only through thermal diffusion accompanying the heat treatment. Therefore, even if the heat treatment is repeated thereafter, such uneven distribution of elements is not easily eliminated, and finally these elements are micro-segregated in the structure of the steel, and the hardenability which is the purpose of alloy addition is reduced. There is a limit to the control of the structure by heat treatment, as it becomes insufficient and the improvement of strength and hardness is reduced, or micro martensite may occur during the cooling process to the low temperature range, and it is not possible to achieve sufficient high strength. There was That is, due to the solidification segregation generated in the casting stage of the steel ingot or the slab, the temperature of 1200 ° C. for rail rolling
Even if it is heated to the above temperature range, the uniform diffusion of strengthening elements such as C, Si, Cr, and Mo, which are effective for improving the strength of rail steel, is insufficient and each element remains microsegregated. There was a problem that the strength of the rail steel could not be sufficiently increased by transformation into pearlite or bainite. The present invention has been made in view of such circumstances, and it is possible to effectively eliminate microsegregation due to elements such as Mn and Cr existing in steel, and to provide high strength with excellent wear resistance and internal fatigue damage resistance. It is intended to provide a rail manufacturing method.

[0005]

According to the present invention, there is provided a semiconductor device comprising:
The manufacturing method of the high-strength rail described is that after the solidification of the steel slab in the continuous casting is completed, the steel slab is subjected to plastic working such that the plastic working ratio becomes 5% or more in the austenite forming region, and then 1
It is heated to a hot rolling temperature of 100 to 1300 ° C. and hot rolled into a rail shape. The method for manufacturing a high-strength rail according to claim 2 is the method for manufacturing a high-strength rail according to claim 1, wherein after the steel slab is hot-rolled into a rail shape,
Cool at a cooling rate of 6 ° C / sec. The method for manufacturing a high-strength rail according to claim 3 is the method for manufacturing a high-strength rail according to claim 1, wherein after the steel slab is hot-rolled into a rail shape,
After cooling to room temperature and reheating the rail head or the entire rail to the austenite production zone temperature, 2 to 6 ° C.
Cool at a cooling rate of / sec.

[0006] The steel slab refers to general steel containing Mn and Cr components such as carbon steel, high carbon steel, stainless steel and alloy steel. After the completion of solidification of the steel slab in continuous casting, the molten steel is cooled in the mold, and then directly cooled with cooling water, air, etc. Say. The austenite formation region is a temperature range in which, in the process of solidifying and cooling molten steel, γ iron having a face-centered cubic structure and austenite, which is a microstructure of a phase in which other elements are solid-dissolved in γ iron, are formed. , At least the A ₃ transformation point, which is the transformation point between γ iron (face-centered cubic structure) and α iron (body-centered cubic structure). Further, as will be described later, the work strain introduced into the steel slab is easily recovered in a high temperature state. Therefore, it is desirable that the upper limit of the austenite formation region temperature is 1250 ° C. so that the work strain can be maintained. The plastic working rate refers to a working rate when plastically working a steel slab, and a reduction rate or the like during rolling is equivalent to this. For example, when a steel slab having a thickness of 100 mm is rolled into a steel slab having a thickness of 95 mm, the rolling reduction is (100-95) /100=0.05 (5%).
If the plastic working ratio at the austenite formation region temperature is less than 5%, the amount of working strain added to the steel slab due to such plastic working becomes insufficient, and this working strain remains in the subsequent heat treatment process. As a result, the diffusion promoting effect of each element in the steel cannot be exhibited.

The hot rolling temperature after rolling is 110
When the temperature is lower than 0 ° C, the degree of diffusion of each element in the steel is small and the microsegregation cannot be effectively eliminated. On the other hand, when the hot rolling temperature after rolling exceeds 1300 ° C., the grain boundary composed of γ iron or its solid solution begins to dissolve,
The upper limit is preferably 1300 ° C. The cooling rate of 2 to 6 ° C./sec for cooling the rail after hot rolling, which is the austenite formation region temperature, or after reheating, means that when the steel slab is cooled from the austenite state, the austenite is ferrite and cementite. The cooling rate is such that eutectoid decomposition occurs in and. If the cooling rate is lower than 2 ° C./sec, the crystal structure becomes coarse, the strength and hardness are lowered, and the cooling work efficiency is lowered, which is not preferable. On the contrary, when the cooling rate exceeds 6 ° C / sec, localized formation of martensite structure starts, and it is difficult to homogenize the steel slab from austenite structure to pearlite structure. Becomes

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Next, referring to the attached drawings, an embodiment in which the present invention is embodied will be described to provide an understanding of the present invention. Here, FIG. 1A shows the first embodiment of the present invention.
It is a schematic explanatory drawing of the rail manufacturing equipment 10 which applies the manufacturing method of the high strength rail which concerns on embodiment of this. The rail manufacturing equipment 10 uses the molten steel 11 through the immersion nozzle 13 to cast a steel slab 15
Tundish 12 and continuous casting mold 14 for casting
A pinch roll 16 for pulling out the steel slab 15 from the continuous casting mold 14, a strain addition rolling machine 17 for rolling the steel slab 15 to a predetermined reduction degree using a rolling roll 18, and a rolled steel cast. It has a rail processing rolling mill 19 that reheats the piece 15 and hot-rolls it into a rail shape, and a cooling device 21 that cools the hot-rolled rail 20 at a predetermined cooling rate. The continuous casting mold 14 has a long side length of 38 as an example.
It is a copper mold having a rectangular cross-sectional shape of 0 mm and a short side length of 320 mm, and is cooled by supplying cooling water to a flow path provided in the mold. The strain-applying rolling mill 17 is a rolling mill that imparts a predetermined working strain by applying an external force to the steel slab 15 obtained by continuous casting through the rolling rolls 18 to plastically deform it. Alternatively, a plurality of them may be arranged and continuously rolled. The rolling mill 19 for rail processing is used for steel slab 1 such as bloom.
5 is a hot rolling mill including a combination of a caliber mill for performing hot rolling from 5 to a rail shape, a universal mill, or the like. The cooling device 21 is a device that blows cooling air, steam, cooling water, oil, or the like onto the heated rail 20 to cool it.

Next, a method of manufacturing the high-strength rail according to the first embodiment of the present invention using the rail manufacturing facility 10 will be described in detail with reference to FIG. Here, FIG. 1B shows the molten steel 1 in the rail manufacturing facility 10.
It is a schematic diagram which shows the temperature transition of the molten steel 11, the steel slab 15, and the rail 20 in each process from 1 to the rail 20 is manufactured, corresponding to each process of the rail manufacturing equipment 10 shown in FIG. Is displayed. Table 1 is a composition table of the three types of steel (A, B, C) used here. Steel A is a high carbon steel containing 0.78 wt% of carbon (C), and Steel B is 0.76 wt% of carbon. % High carbon content, Steel C is carbon, chromium (C
r) and manganese (Mn) are each 0.30 wt%,
This indicates that the steel belongs to the high chromium manganese steel containing 1.16 wt% and 1.14 wt%.

[0010]

[Table 1]

First, molten iron is prepared by a secondary refining furnace, a converter or the like, and molten steel 1 having each component ratio shown in Table 1 is prepared.
No. 1 is prepared, this is injected into the tundish 12, and injection into the continuous casting mold 14 is started. The temperature of the molten steel 11 when it is poured into the continuous casting mold 14 is about 14
20 to 1520 ° C. The molten steel 11 is cooled through the continuous continuous casting mold 14 to form a solidified shell. This solidified shell is pulled out through a pinch roll 16 and further directly cooled by air or cooling water to completely solidify to the inside, and a steel slab 15 having a temperature of 1250 ° C. to 900 ° C.
Becomes At this time, the steel slab 15 forms an austenite structure, but uneven distribution of additive elements other than iron, which causes microsegregation thereafter, is recognized. Further, here, since the processing strain due to the mechanical processing is at a stage where it has not yet been introduced into the steel slab 15, the strain due to the mechanical processing is applied to the steel slab 15 by the next rolling operation by the strain-applying rolling mill 17. It is necessary to add it intentionally to enhance the potential diffusion capacity of elements in steel in the subsequent heat treatment process. That is, as shown in FIG. 1 (b), in the strain-applying rolling mill 17, the long side and the short side have lengths of 380 mm and 320 mm, respectively, in a temperature range not lower than the A ₃ line which is the austenite generation range temperature. The steel slab 15 having the cross-sectional shape is rolled in the direction of the short side, and the length of the short side after rolling is 2
It was rolled so as to have a length of 88 mm. The reduction rate (or processing rate) in this case is (320 mm-288 mm) / 320
mm = 0.1 (10%).

In addition to the rolling process described above, in addition to the rolling process described above, all or a part of the metal material is used as a mold for the plastic processing method in which the metal material is plastically deformed by applying an external force to obtain a desired shape and size. Or, compress it with a tool to adjust the height, diameter, width,
There are forging methods that reduce the thickness to form materials and products, extrusion methods that extrude blooms from die holes for processing, and plate material processing methods such as press processing or spinning processing. It is also possible to use

Next, the steel slab 1 thus rolled.
5 is cut into a predetermined shape as necessary, and stored in a heat-retaining furnace or the like to hold at 1250 ° C. without lowering the temperature, or
After rolling, it is reheated to a temperature of about 1250 ° C., and hot rolling is performed by using the rail rolling mill 19 so that the rail 20 has a desired shape. At this reheat treatment temperature, the strain due to the processing inherent in the steel slab 15 added in the rolling step remains, which promotes the diffusion of elements such as Mn and Cr segregated in the slab. Conjectured to be. Therefore, microsegregation due to Mn, Cr, etc. in the steel of the rail 20 is reduced, and a uniform and high-strength structure can be obtained. Then, after the steel slab 15 is roll-formed into a rail shape, cooling water or cooling air is sprayed,
2-6 ° C, which is the cooling rate that does not cause martensitic transformation
The rail 20 having a desired strength, hardness and wear resistance can be obtained by cooling at a cooling rate of / sec. Here, Examples 1 to 6 shown in Table 2 show respective specifications and the resulting hardness of the rail 20.

[0014]

[Table 2]

For example, in Example 1, a steel cast piece 15 having the composition of steel A in Table 1 was used to perform rolling at a rolling temperature of 1100 ° C. and a reduction of 5%, and then 1250. Reheated to ℃ and hot-rolled into a rail shape to obtain rail 20, and then reheat up to 4 ℃
An example is shown in which the rail 20 is cooled to room temperature at a cooling rate of / sec, which indicates that the surface of the rail 20 has a Vickers hardness (H _V ) of 419. Further, as shown in Table 2, in the conventional examples 1 to 3 in which the rolling process was not performed at the austenite formation region temperature, the values of the Vickers hardness of the rail surface after such treatment were 38, respectively.
7, 391, 380, which is clearly lower than the minimum Vickers hardness value 418 shown in Examples 1 to 6.

Further, FIG. 2 shows Mn in the rail 20,
The distribution state of each Cr element was analyzed by an X-ray microanalyzer (E
2 is a graph of the results of the investigation using PMA), and FIG.
(A) and (b) correspond to Conventional Example 1 and Example 1 shown in Table 2, respectively. In the figure, the horizontal axis represents the scan length of the electron beam in the EPMA microscope field, and the vertical axis represents M in the irradiation spot of the electron beam.
The intensity of the characteristic X-ray corresponding to n and Cr is represented by the numerical value converted into its component amount. As is clear from the figure,
In the conventional example 1, the variations in the amounts of Mn and Cr components are about 1.0 wt% and 0.1 wt%, respectively, whereas in the example 1, the variations in Mn and Cr are 0.3 wt% and 0 wt%, respectively. It can be seen that it has decreased to 0.03 wt%. Accordingly, since the distribution state of the elements such as Mn and Cr that form the alloy is made uniform, the strength and hardness are improved, and the resistance of the rail 20 required at high speed operation or high load operation is improved. It becomes possible to dramatically increase the wear resistance.

Next, a method of manufacturing the high strength rail according to the second embodiment of the present invention will be described in detail with reference to FIG. The rail manufacturing facility 22 shown in FIG. 3A is a heat treatment furnace 2 for heating the rail 20 to a predetermined temperature and cooling the rail 20 in the rail manufacturing facility 10 of FIG. 1A.
3 is added, the description of each device from the tundish 12 to the cooling device 21 is omitted, and the same reference numerals as those in FIG. 1 are used in the following description. Here, FIG. 3B is a schematic diagram showing the temperature transitions of the molten steel 11, the steel slab 15, and the rail 20 in each process from the molten steel 11 to the production of the rail 20 in the rail manufacturing facility 22. It is displayed corresponding to each process of the rail manufacturing facility 22 shown in FIG.

First, molten iron is prepared by a secondary refining furnace, a converter or the like, and molten steel 11 having the composition ratio shown in Steel A in Table 1 is prepared.
Is prepared, and this is injected into the tundish 12, and injection into the continuous casting mold 14 is started. This continuous casting mold 1
The temperature of the molten steel 11 when it is poured into No. 4 is about 1420-1
It is 520 ° C. In addition, molten steel 11 tundish 12
Since the steps from the injection to the rail 20 to the cooling of the rail 20 to room temperature are the same as the steps shown in the first embodiment, this description is omitted and the procedure of heat-treating the rail 20 cooled to room temperature is omitted. The following will be described. As shown at point P in FIG. 3B, the rail 20 is loaded into the heat treatment furnace 23,
Heat to austenite formation zone temperature.

After the rail 20 is held in the heat treatment furnace 23 for a predetermined time, it is cooled to room temperature at a cooling rate that does not cause martensitic transformation, in this case, a maximum cooling rate of 4 ° C./sec. Since the depth of quenching from the surface of the rail 20 varies depending on the holding time in the heat treatment furnace 23, the heating rate, etc., it is necessary to adjust the depth to a predetermined depth while considering the heat treatment cost and the like. By such heat treatment, non-equilibrium states such as work strain and segregation caused by casting and working are eliminated, the structure is homogenized, and the material properties are improved by utilizing the phase change of the metal material caused by temperature change. be able to. In this way, the distribution state of the additional elements such as Mn and Cr is made uniform, so that the strength and hardness are improved, and the wear resistance of the rail required at high speed operation or high load operation is further increased. It becomes possible.

FIG. 4 is a graph comparing the Vickers hardness in the cross section of the rail 20 between Example 1 of Table 2 and Conventional Example 1 shown in Table 2, wherein Example 1 and Conventional Example 1 are indicated by ● and ○, respectively. It is indicated by a mark, and the horizontal axis shows each rail 2
The distance (L) from the head surface of 0 is shown. As is clear from the figure, it can be seen that the hardness decreases and the state of the structure changes from the surface of the rail 20 toward the inside. Thus, the rail 20 that is required to have abrasion resistance
The hardness of the head surface of the rail 20 can be increased, and the toughness of the base portion of the rail 20 can be enhanced. In the first embodiment, the hardness of the head portion of the rail 20 can be significantly improved as compared with the conventional example 1. Is. 1 (b) and 2
In (b), the bloom after casting is 1250 to 900
Similar effects were obtained even when the product was once cooled to room temperature and then reheated as shown by the dotted line in FIG.

The embodiment of the present invention has been described above.
The present invention is not limited to these embodiments, and all changes in conditions without departing from the gist are within the scope of the present invention. For example, in the first embodiment, the case where carbon steel or the like in which the carbon component in the steel is 0.3 to 0.78 wt% is used has been described, but the present invention is not limited to such a component range. The invention can be applied. Also, the case where a rolling process is used to add work strain to the steel slab 15 at the austenite formation region temperature is shown, but other work, such as forging, adds necessary work strain to the steel slab 15. You can also do such things. Further, by using the heat treatment furnace 23, the rail 2
Although the heat treatment of 0 was performed over the entire rail 20, it is also possible to perform the heat treatment by heating only the head of the rail 20 partially using a high frequency heating device, a burner or the like.

[0022]

According to the method for producing a high-strength rail according to claims 1 to 3, the steel slab after continuous casting is cooled and held at the austenite formation region temperature, and the plastic working ratio is 5% or more. Since the work is performed, work strain is effectively added to the steel slab. And after that, 1100 to 130
Since it is heated to a hot rolling temperature of 0 ° C. and hot-rolled into a rail shape, it is possible to effectively suppress elemental microsegregation by promoting diffusion of the elements accompanying this, and C The elements that strengthen the steel of the rail, such as Si, Cr, and Mo, can be utilized more effectively.

Particularly, in the method for manufacturing a high-strength rail according to claim 2, after the steel slab is hot-rolled into the shape of the rail, 2
Since cooling is performed at a cooling rate of ~ 6 ° C / sec, the rail can be formed as a fine pearlite structure or bainite structure, and higher strength and higher hardness can be achieved more effectively. Further, in the method for manufacturing a high-strength rail according to claim 3, after hot rolling the steel slab into a rail shape, the steel slab is cooled to room temperature, and the head of the rail or the entire rail is brought to the austenite formation zone temperature. After reheating, cooling is performed at a cooling rate of 2 to 6 ° C./second, so that it is possible to efficiently improve the wear resistance and hardness of the rail head where abrasion resistance is required, and at the same time, add It is possible to more effectively utilize the quenching effect of steel depending on the element contained in the steel to transform austenite into pearlite or bainite, thereby further strengthening the structure. Thus, the segregation of the steel structure is also greatly improved, so that the rail can be made to have higher strength (hardness), and a high-strength rail excellent in wear resistance and internal fatigue damage resistance can be manufactured.

[Brief description of drawings]

1A and 1B are explanatory views of rail manufacturing equipment to which a method for manufacturing a high-strength rail according to a first embodiment of the present invention is applied, and molten steel, steel slab, and rail temperature. It is a schematic diagram which shows a transition.

2A and 2B are graphs showing distribution states of Mn and Cr elements in a rail in a conventional example and an example, respectively.

3 (a) and 3 (b) are explanatory views of rail manufacturing equipment to which the method for manufacturing a high-strength rail according to the second embodiment of the present invention is applied, and molten steel, steel slab, and rail temperature. It is a schematic diagram which shows a transition.

FIG. 4 is a graph showing Vickers hardness in a rail cross section.

[Explanation of symbols]

10 Rail Manufacturing Equipment 11 Molten Steel 12 Tundish 13 Immersion Nozzle 14 Continuous Casting Mold 15 Steel Slab 16 Pinch Roll 17 Strain Adding Roller 18 Rolling Roll 19 Rail Processing Rolling Machine 20 Rail 21 Cooling Device 22 Rail Manufacturing Equipment 23 Heat Treatment Furnace

Claims (3)

[Claims] 1. After completion of solidification of a steel slab in continuous casting,
The steel slab is subjected to plastic working in the austenite forming region so that the plastic working ratio is 5% or more, and then 1100 to 1300.
A method for producing a high-strength rail, which comprises heating to a hot rolling temperature of ℃ and hot rolling into a rail shape. 2. The method for manufacturing a high-strength rail according to claim 1, wherein the steel slab is hot-rolled into a rail shape and then cooled at a cooling rate of 2 to 6 ° C./sec. 3. The steel slab is hot-rolled into the shape of a rail, then cooled to room temperature, and the head of the rail or the entire rail is reheated to the austenite formation zone temperature, and then 2 to 6 ° C.
The method for manufacturing a high-strength rail according to claim 1, wherein the cooling is performed at a cooling rate of / sec.