JP3764710B2 - Method for producing pearlitic rail with excellent toughness and ductility - Google Patents

Description

【００４６】
【表２】

【００４７】
【表３】

【００４８】
【表４】

【００４９】
【表５】

【００５０】
【表６】

【００５１】
【発明の効果】
本発明により、圧延後に微細なオーステナイト組織が得られる。これから変態するパーライト組織も微細となり、靭性および延性に優れたパーライト系レールを提供できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a pearlite rail in which the pearlite structure of rail steel is refined to improve toughness and ductility.
[0002]
[Prior art]
In rail transport, heavy loads for improving transport efficiency and high speed for speeding up transport are being promoted, and demands on the characteristics of rails are becoming strict. Heavy loading promotes wear of the rail head in the sharp curve section and increases fatigue damage from the stress concentration portion inside the rail GC (gauge corner), thereby shortening the rail life.
In order to improve the shortening of the rail life in heavy-duty railways, technological development of high-strength rail steel having excellent wear resistance and internal fatigue damage resistance has been actively conducted. As a result, high-strength rails with a narrow lamellar spacing by heat treatment are widely used in sharply curved sections.
[0003]
On the other hand, in cold districts, rail replacement due to occurrence of rail cracks is concentrated in the winter, and improving the toughness of the rail material is a necessary issue for extending the rail life. It is also important to improve the toughness and ductility of the rail material in order to improve the internal fatigue damage resistance of the head. In the pearlite structure, the toughness and ductility can be improved by making the structure fine.
[0004]
Up to now, refinement of the structure, toughness and ductility have been improved by the following methods.
(1) A method in which after the hot rolling, the rail once cooled to room temperature is reheated to a temperature range as low as possible in the austenite temperature range and then accelerated cooling.
(2) A method in which the rail head is accelerated and cooled after the austenite grains are refined by controlled rolling.
(3) A method of obtaining a fine pearlite structure by promoting transformation from within the austenite crystal grain in addition to the austenite crystal grain boundary during pearlite transformation.
[0005]
[Problems to be solved by the invention]
In the above method (1), for example, as described in JP-A-63-128123, the rail is reheated to a low temperature of 950 ° C. or lower, which is lower in the austenite temperature range, and the austenite grains are refined. By granulating, the structure after pearlite transformation is refined, and the toughness and ductility are greatly improved.
However, if it is attempted to deepen the heating to the inside of the rail head at a low temperature, it is necessary to lower the input heat amount and to heat for a long time. This heat treatment impedes productivity and deteriorates the manufacturing cost.
[0006]
Further, the method (2) described above is carried out, for example, as described in JP-A-52-138427 and JP-A-52-138428, with a rolling material containing Nb at 1220 ° C. or higher or 1100 ° C. or lower. It is intended to improve toughness and ductility by reducing the austenite grain size by heating and temperature-controlled rolling under suitable conditions. Nb has the effect of preventing the fine recrystallized grains after processing from becoming coarse due to grain growth, making it easy to obtain a fine austenite structure, but high carbon steel tends to produce coarse Nb carbonitrides, Its use is accompanied by difficulties.
[0007]
Further, in the method (3), as described in, for example, Japanese Patent Application Laid-Open No. 6-279928, nucleation of pearlite transformation is performed using V carbonitride and Ti carbonitride deposited on MnS as transformation nuclei. This is a method of refining the structure by promoting the above. By this method, certain toughness and ductility improvements were obtained. However, when these carbonitrides are used, the transformation is completed at a high temperature as a result of active generation of transformation nuclei.
[0008]
[Means for Solving the Problems]
The pearlite structure consists of a layered structure of cementite phase and ferrite phase called lamellar structure. A region where the crystal orientations of ferrite are aligned is called a pearlite block and can be said to be a crystal grain unit in a pearlite structure. The finer the pearlite block, the better the impact value and ductility value of pearlite steel. When pearlite steel breaks, cracks are bent and progressed in units of pearlite blocks. Therefore, the finer the pearlite block, the larger the total area of the new interface, and the greater the energy required to form the interface, the impact value is considered to improve. Further, since the progress of cracks until reaching the final fracture is slower as the fine-grained structure, the ductility is also improved by increasing the amount of deformation until the fracture.
[0009]
Normally, rail steel contains a high amount of C. When this rail steel is cooled from the high temperature austenite temperature range, pearlite transformation starts to occur from about 700 ° C. Most of pearlite transformation nuclei are formed at austenite grain boundaries. For this reason, the finer the austenite structure that is the structure before transformation, the more pearlite transformation nuclei. As a result, the finer the austenite crystal grains, the finer the pearlite block size after completion of transformation.
[0010]
In order to obtain a rail having good ductility and toughness required in cold regions, the present invention aims to refine the pearlite structure by making austenite grains fine and generating a large number of transformation nuclei from the grain boundaries. The summary is as follows.
(1) In mass%,
C: 0.59 to 1.2%,
Si: 0.1-1.2%,
Mn: 0.4 to 1.4%
N: 0.005 (excluding 0.005) to 0.030%,
Al: 0.005 to 0.050%
In the reheating step for performing hot rolling using a rail rolling material comprising the balance Fe and inevitable impurities, the residence time in the heating furnace at 900 ° C. or higher is 40 minutes or longer and heating is performed. Ri range der of maximum temperature 1000 to 1200 ° C. in a furnace, and forming a rail by hot rolling within time 7 min to complete the finish rolling from material extraction from the heating furnace, A method for producing a high-strength pearlitic rail excellent in toughness and ductility.
(2) In addition to the component described in (1) above,
Mo: 0.015-0.070%, Cr: 0.1-1.0%,
V: 0.005-0.070%, Ni: 0.01-1.50%,
Nb: 0.004 to 0.050%, Cu: 0.01 to 1.50%,
Ti: 0.0001 to 0.0100%, S: 0.002 to 0.050%,
The manufacturing method of the high intensity | strength pearlite type rail excellent in toughness and ductility characterized by including 1 type, or 2 or more types of these.
(3) In the reheating step for performing hot rolling using the rolling material having the component described in (1) or (2) above, the residence time in the heating furnace at 900 ° C. or higher is 40 minutes or longer. In addition, the maximum heating temperature is in the range of 1000 to 1200 ° C., and the time from the material extraction from the heating furnace to finish rolling is completed within 7 minutes on the rail by hot rolling, and then austenite as hot rolled A high-strength pearlitic rail excellent in toughness and ductility, characterized by accelerated cooling at a temperature range of 700 to 550 ° C. at a temperature of 1 to 10 ° C./sec. Method.
(4) A rolling material having the component described in (1) or (2) above is reheated according to the reheating conditions and rolling conditions described in (1) above, and is formed into a rail by hot rolling. Is cooled to 500 ° C. or lower, and then reheated to an austenite region temperature of 800 ° C. or higher and lower than 1200 ° C., then the temperature range in which at least the surface layer of the rail is 700 to 550 ° C. is 1 to 10 ° C./sec. A method for producing a high-strength pearlitic rail excellent in toughness and ductility, characterized by accelerated cooling at a high temperature.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In order to manufacture rail steel, first, components are adjusted in a converter, an electric furnace, etc., and further solidified through secondary refining such as degassing treatment as necessary. Thereafter, it is heated to a high temperature in a reheating furnace and gradually formed into a rail shape by a plurality of rolling mills, and finally finished into a rail shape at 800 to 1100 ° C. The metal structure in the rolling stage is an austenite structure.
When a steel material is hot-rolled, the strain energy introduced into the steel becomes a driving force, and the austenite structure is recrystallized finely. Since the fine grain structure immediately after recrystallization has a high grain boundary energy, a grain growth phenomenon occurs in which the crystal grains bite each other and the grain size increases.
[0012]
The recrystallization phenomenon occurs in a short time during or immediately after the processing, but the grain growth phenomenon takes time to reach a stable value. For this reason, austenite crystal grains are gradually refined by repeating the processing in a short time. Further, if fine foreign particles that hinder the movement of grain boundaries are present in the structure, the movement of grain boundaries is hindered, so that the crystal grains are hardly crushed and the grain growth is suppressed. A phenomenon in which the movement of grain boundaries is suppressed by foreign particles in steel is called a pinning effect and a pinning effect.
Since the present inventors have the property that AlN precipitates finely in the steel, the inventors have studied the method of using it as pinning particles to suppress the grain growth after rolling of rail steel, and obtained the following knowledge It was.
[0013]
A steel material containing Al and N has a property that AlN having a fine size of several nm to several tens of nm is densely precipitated in a temperature range near 1000 ° C. to 1200 ° C. According to the experiments by the present inventors, the precipitation amount of AlN is small in the process of naturally cooling the steel material from a high temperature state. In order to deposit AlN finely and in a large amount, it is necessary to reheat to a temperature range of 1000 ° C. or more and less than 1200 ° C. If the reheating temperature is less than 1000 ° C., it takes a long time for precipitation, so it is not practical. If the reheating temperature exceeds 1200 ° C., the AlN once generated is re-dissolved, so that the pinning effect of AlN is obtained. Disappear.
[0014]
In order for AlN to precipitate, Al and N need to diffuse, move to adjacent positions, and combine with each other. Therefore, it is necessary to stay in the heating furnace for a certain time or more at a high temperature. According to the experiments by the present inventors, in order to obtain sufficient precipitation of AlN, in addition to the maximum heating temperature being 1000 to 1200 ° C., the residence time in the heating furnace in the temperature range of 900 ° C. or higher is 40 minutes. This is necessary.
[0015]
The present inventors have confirmed that when a steel material containing Al and N is reheated under such conditions, AlN precipitates finely. When a steel material in which AlN is finely precipitated is rolled, the fine recrystallized grains immediately after rolling are prevented from growing and a fine-grained austenite structure is obtained. When the steel material temperature becomes 700 ° C. or lower after the completion of rolling, pearlite transformation starts. The pearlite transformation starts with the formation of pearlite transformation nuclei at the grain boundaries of the austenite structure. At this time, a fine pearlite structure can be obtained from the austenite structure that has become fine due to the pinning effect of AlN.
[0016]
On the other hand, in a sharp curve part of a high-speed track and a heavy load track, high strength of 1000 MPa or more is required together with ductility. In order to obtain such high strength, it is desirable to perform accelerated cooling between 700 to 550 ° C., which is the pearlite transformation temperature range, from the austenite state after the end of rolling. When accelerated cooling is performed, pearlite transformation occurs from the state in which austenite is maintained at a lower temperature than the transformation temperature at the time of cooling. A state where transformation starts at a lower temperature is called a state where the degree of supercooling is high.
[0017]
When the degree of supercooling is increased, the lamellar interval is narrowed during pearlite transformation, the strength is increased, and the rate of transformation nuclei is increased, so the effect of making the pearlite block size fine is also obtained, further improving ductility and toughness Can be achieved. However, when the cooling rate during accelerated cooling is less than 1 ° C./sec, the strength required as a high-strength rail cannot be obtained, and when it exceeds 10 ° C./sec, bainite and martensite are easily generated, which is not preferable. .
[0018]
Further, when a cooling method using water spray or liquid immersion is adopted in the accelerated cooling, a film boiling phenomenon occurs in the steel material surface layer, which easily causes uneven cooling. Therefore, the cooling medium is preferably air or air-water mist. Moreover, the initial temperature of accelerated cooling needs to be in the austenite temperature range where the pearlite transformation does not start, and needs to be 720 ° C. or higher. According to the inventors, when the maximum heating temperature is limited to 1000 to 1200 ° C., in order to set the accelerated cooling start temperature to 720 ° C. or higher, the material is extracted from the heating furnace within at least 7 minutes. It is necessary to complete the rolling.
[0019]
As a method for obtaining high strength, the rolled rail is naturally cooled to near room temperature once and transformed to pearlite, and then reheated to the austenite temperature, and accelerated cooling is performed in the subsequent cooling process. There is also a method in which pearlite transformation is caused from a cold state and the structure is refined as the hardness increases. In that case, in the austenite temperature range, the finer austenite structure can be obtained as it is reheated to a lower temperature, and the pearlite structure after transformation becomes finer.
[0020]
In order to utilize the pinning effect of AlN, it is necessary to reheat to less than 1200 ° C. so that AlN deposited in the heating furnace prior to rolling does not re-dissolve. On the other hand, if the heating temperature is less than 800 ° C., uneven temperature in the cross section tends to occur, it is difficult to obtain a uniform austenite structure, and a stable hardness cannot be obtained. The cooling rate is desirably 1 ° C./s or more and 10 ° C./s or less for the same reason as in the case of direct acceleration after rolling.
[0021]
By the above manufacturing method, the pearlite block size after transformation becomes fine, and rail steel having excellent toughness and ductility can be obtained. Such a fine pearlite structure needs to be formed at least on the head portion of the rail where impact loads from the wheels are easily applied.
[0022]
The reason why the rail steel components are limited will be described below. The content of the component is mass%. C: C is an essential element for increasing the strength and generating a pearlite structure in rail steel. High strength of pearlite structure difficult to obtain in need is less than 0.59%, also for producing harmful pro-eutectoid cementite which embrittle austenite grain boundaries exceeds 1.20% from 0.59 to 1. Limited to 2%.
[0023]
Si: Si not only contributes to high strength by solid solution strengthening to the ferrite phase in the pearlite structure, but also has a slight toughness and ductility improving effect. If it is less than 0.1%, these effects are small, and if it exceeds 1.2%, embrittlement is caused and weldability is also lowered, so it is limited to 0.1 to 1.2%.
[0024]
Mn: Mn contributes to high strength by lowering the pearlite transformation temperature and improving hardenability. However, if it is less than 0.4%, the effect is small, and if it exceeds 1.4%, it is not preferable because a martensite structure is easily generated in the segregated portion.
[0025]
N: N is necessary for precipitating AlN that suppresses austenite grain growth by pinning, and 0.005 % or more ( excluding 0.005%) is necessary to obtain the effect. On the other hand, if N exceeds 0.030%, an embrittlement phenomenon occurs at a high temperature range, and an internal crack in the steel material in casting occurs, which is not preferable.
[0026]
Al: Al is necessary for precipitating AlN that suppresses austenite grain growth by pinning, and 0.005% or more is necessary to obtain the effect. On the other hand, if Al exceeds 0.050%, the Al oxide becomes coarse, resulting in a decrease in toughness, and there is a risk of becoming an internal fatigue starting point when used in heavy-duty railways.
[0027]
Furthermore, in the present invention, in addition to the above components, one or more Mo, Cr, V, Ni, Nb, Cu, Ti. By adding S, Ca and Mg , high toughness can be obtained by improving the toughness of ferrite ground, austenite grains during heating of the rail rolling material, or by refining austenite grains during rolling, and acceleration in the cooling process By cooling, higher strength and higher toughness can be obtained.
The reason for limiting these chemical components will be described below.
[0028]
Mo: Mo has an effect of suppressing the pearlite transformation speed and miniaturizing the pearlite block size. However, when the amount is less than 0.015%, this effect is small. On the other hand, when the amount of Mo exceeds 0.07%, the pearlite transformation rate is excessively decreased in the segregation part, and bainite and martensite are generated in the pearlite structure. .
[0029]
Cr: Cr contributes to increasing the strength by lowering the pearlite transformation temperature, and it is effective to contain 0.1% or more from the viewpoint of preventing weld joint softening. On the other hand, if the content exceeds 1.0%, bainite and martensite are generated not only at the element segregation portion but also at the shoulder portion of the rail having a strong supercooling tendency at the time of forced cooling, which is not preferable.
[0030]
V: V is an element that precipitates V carbonitrides serving as pearlite transformation nuclei and refines the pearlite structure by promoting the formation of transformation nuclei from the austenite grain boundaries and within the grains. But V is weak this effect is less than 0.005% is not preferable because the 0.070% ultra the V carbonitride is fracture origin become coarse.
[0031]
Ni: Ni is an element effective for improving the toughness of ferrite by dissolving in ferrite. However, when Ni is less than 0.01%, the effect is small, and even if it exceeds 1.5%, the effect is saturated.
[0032]
Nb: Nb is heated at a low temperature during hot rolling, so that the Nb carbonitride suppresses austenite grain growth and contributes to fine graining. Also, the austenite grains after hot rolling are refined by high temperature heating / low temperature finish rolling, and the pearlite block obtained after accelerated cooling is made fine. In order to obtain this effect, 0.004% or more of Nb is required. However, if it exceeds 0.050%, the toughness is reduced due to the formation of coarse Nb carbonitride, which is not preferable.
[0033]
Cu: Cu is an element effective for improving the toughness of ferrite by dissolving in ferrite similarly to Ni. However, when Cu is less than 0.01%, the effect is small, and even if it exceeds 1.5%, the effect is saturated.
[0034]
Ti: Ti, like V, is an element that precipitates Ti nitride serving as a pearlite transformation nucleus and refines the pearlite structure by promoting the formation of austenite grain boundaries and transformation nuclei from within the grains. When Ti is less than 0.0001%, the effect is small, and when Ti exceeds 0.010%, coarse Ti nitride is generated and toughness is lowered, which is not preferable.
[0035]
S: S is generally known as a harmful element, but forms MnS and MgS, and these sulfides become precipitation sites for V carbonitride and Ti nitride. When these inclusions are present in the austenite grains, in addition to the grain boundaries, the transformation from the grains is promoted, and the pearlite block size after the pearlite transformation is further refined. However, if S is less than 0.002%, a sufficient number of MgS and MnS cannot be obtained, and if it exceeds 0.05%, coarse MnS is generated and the toughness and ductility are remarkably lowered.
[0037]
Mg precipitates Mg oxide, Mg—Al oxide, and Mg sulfide, and further serves as a precipitation nucleus of MnS, V carbonitride using these as nuclei. These inclusions refine the pearlite block after pearlite transformation by the effect of promoting intragranular transformation. However, if it is less than 0.0004%, the pearlite block size is hardly miniaturized, and if it exceeds 0.02%, coarse inclusions are generated and the toughness is remarkably lowered.
[0038]
P is an element inevitably contained in the steel. P is preferably low, and is preferably 0.015% or less, because it causes embrittlement of the ferrite layer and lowers impact characteristics. Similarly, O is unavoidably present in the molten steel, but if O is 0.02% or more, coarse inclusions are generated and the toughness is lowered, which is not preferable.
[0039]
【Example】
Next, examples of manufacturing a high-strength rail having high toughness manufactured according to the present invention will be described.
A slab having a rectangular cross section containing various components shown in Table 1 was produced by continuous casting. When the slab is reheated in the heating furnace for hot rolling, the residence time in the heating furnace at which the slab temperature becomes 900 ° C. or higher is set to 60 minutes, and the temperature of the slab is set in the heating furnace. The temperature at which the maximum is 1120 ° C. After removing the slab from the heating furnace, it was rolled into a rail shape in 6 minutes.
A part of the rolled rail was left to cool in the atmosphere. Also, some of the rail head surface temperature is accelerated by air blowing from the time when the temperature of the rail head surface reaches 750 ° C., and the surface temperature is accelerated within a range of 700 to 550 ° C. at a cooling rate of 1 to 10 ° C./s. The lower cross-sectional hardness was set to Hv 350 or more.
[0040]
Table 2 shows the tensile test results of the rail steel manufactured from the components shown in Table 1, reheat conditions, and the results of a 2 mmU notch Charpy impact test at a test temperature of 20 ° C.
The Charpy test piece was taken in the longitudinal direction from 3 mm below the rail top surface, and the notch position was 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.
The tensile test piece was a round bar tensile test piece with a parallel part diameter of 6 mm and a parallel part length of 30 mm taken from the rail head cross section. The sampling position was such that the center of the tensile test piece was 10 mm below the surface of the rail head gauge corner.
As shown in Table 2, the steel of the present invention has higher impact values and ductility due to refinement of the pearlite structure than the comparative steel. Further, by performing accelerated cooling heat treatment directly from a high temperature region after rolling, hardness and strength are increased, and ductility and toughness are improved.
[0041]
Table 3 shows the results of rolling and manufacturing the rails under various heating conditions and rolling times for the steel types B of the examples shown in Table 1. The rolled rail was accelerated and cooled at a cooling rate of 1 to 10 ° C./s in a surface temperature range of 700 to 550 ° C. by air cooling so that the cross-sectional hardness 2 mm below the top surface was Hv 350 or more. The temperature of the head surface was 750 ° C. or more except for Comparative Example b4, but the start temperature of accelerated cooling decreased for Comparative Example b4 because the rolling time was long.
[0042]
Table 4 shows the tensile test results of the rail steel produced from the components shown in Table 3 and the reheating conditions, and the 2 mm U notch Charpy impact test results at a test temperature of 20 ° C. The shape and sampling position of the test piece were the same as those used in Table 2.
As shown by this result, the rail steel produced in the condition range of the present invention has a finer pearlite structure than the comparative steel rail, and has a high impact value and ductility.
On the other hand, in Comparative Example b4, an impact value and elongation equivalent to those of Invention Example B were obtained, but the start temperature of accelerated cooling was lowered, and sufficient hardness and strength were not obtained.
[0043]
Table 5 is the same heating and rolling process as in Example C shown in Table 1, after the rail was naturally cooled to room temperature and then reheated to various temperatures, the head surface temperature was 700-550 ° C. The results of a test in which accelerated cooling is performed at a cooling rate of 1 to 10 ° C./s while the cross-sectional hardness is 2 mm below the top of the head to be Hv 350 or higher while the temperature is in the range of.
[0044]
Table 6 shows the components shown in Table 4, the tensile test of rail steel cooled from reheating conditions, and the results of a 2 mmU notch Charpy impact test at a test temperature of 20 ° C. The shape and sampling position of the test piece were the same as those used in Table 2.
As shown in this result, the rail steel manufactured by reheating the rail to the temperature range of the present invention has a higher impact value and ductility due to the refinement of the pearlite structure compared to the rail heated to the other temperature range. It was.
[0045]
[Table 1]

[0046]
[Table 2]

[0047]
[Table 3]

[0048]
[Table 4]

[0049]
[Table 5]

[0050]
[Table 6]

[0051]
【The invention's effect】
According to the present invention, a fine austenite structure can be obtained after rolling. The pearlite structure transformed from now on becomes fine, and a pearlite rail excellent in toughness and ductility can be provided.

Claims (4)

% By mass
C: 0.59 to 1.2%,
Si: 0.1-1.2%,
Mn: 0.4 to 1.4%
N: 0.005 (excluding 0.005) to 0.030%,
Al: 0.005 to 0.050%
In the reheating step for performing hot rolling using a rail rolling material comprising the balance Fe and inevitable impurities, the residence time in the heating furnace at 900 ° C. or higher is 40 minutes or longer and heating is performed. Ri range der of maximum temperature 1000 to 1200 ° C. in a furnace, and forming a rail by hot rolling within time 7 min to complete the finish rolling from material extraction from the heating furnace, A method for producing a high-strength pearlitic rail excellent in toughness and ductility. In addition to the ingredients of claim 1, in mass%,
Mo: 0.015-0.070%,
Cr: 0.1 to 1.0%,
V: 0.005-0.070%,
Ni: 0.01 to 1.50%,
Nb: 0.004 to 0.050%,
Cu: 0.01 to 1.50%,
Ti: 0.0001 to 0.0100%,
S: 0.002 to 0.050%,
Mg: 0.0004 to 0.020%,
The method for producing a high-strength pearlite rail excellent in toughness and ductility according to claim 1, wherein one or more of the above are contained.   In the reheating process for performing hot rolling using the rolling material having the component according to claim 1 or 2, the residence time in the heating furnace of 900 ° C or higher is 40 minutes or more, and the maximum heating temperature is The rail is in the range of 1000 to 1200 ° C., and after the material is extracted from the heating furnace until the finish rolling is completed within 7 minutes, the rail is formed from the austenite region as it is hot-rolled by hot rolling. A method for producing a high-strength pearlite rail excellent in toughness and ductility, characterized by accelerated cooling at a temperature range of 700 to 550 ° C. at 1 to 10 ° C./sec in at least the surface layer of the head. A rolled material having the component according to claim 1 or 2 is reheated according to the reheating conditions and rolling conditions according to claim 1 and formed into a rail by hot rolling, and the rail is once cooled to 500 ° C or lower. Then, after reheating to 800 degreeC or more and less than 1200 degreeC austenite area temperature, the temperature range whose surface layer of the said rail at least head part is 700-550 degreeC is accelerated-cooled at 1-10 degreeC / sec. A method for producing a high-strength pearlite rail excellent in toughness and ductility.