JP4061141B2 - Perlite high-strength rail excellent in ductility and manufacturing method thereof - Google Patents

Description

【００３１】
表２はそれぞれの鋼種につき７００℃〜５５０℃間の冷却速度を１〜１０℃/secの範囲で変化させたレール鋼の引張試験強度、伸びおよび試験温度２０℃での２ｍｍＵノッチシャルピー衝撃試験結果、初析セメンタイト分率を示す。シャルピー試験片はレール頭頂面下３ｍｍが試験片上面になるように、レール長手方向に採取してノッチを頭頂面側に加工した。また初析セメンタイト分率は、レールの頭部垂直断面の頭頂面から３０ｍｍ下を倍率２００倍で３視野、顕微鏡撮影し、その写真から点算法により算出した。引張試験はレール頭部ゲージコーナー内部１０ｍｍ深さから採取しており、平行部直径６ｍｍ、平行部長さ３０ｍｍのＪＩＳ４号相似試験片とした。
【００３２】
本発明のＮ添加鋼は比較例に比べ、初析セメンタイトが減少して衝撃値、伸び値が改善した。
【００３３】
【表２】

【００３４】
【発明の効果】
本発明により均一なパーライト組織を有する高耐摩耗性レールが得られる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength rail in which the structure near the rail head surface is a uniform pearlite single-phase structure to improve ductility and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, rail transport has been increasingly loaded and speeded up to improve transport efficiency, and the demand for rail characteristics has become stricter. That is, the wear of the rail head increases in the sharp curve section, and the fatigue damage from the stress concentration portion inside the rail gauge corner (GC) increases. In order to improve the shortening of the life of such a rail, technical development of high strength rail steel having excellent wear resistance and internal fatigue damage resistance has been performed. As a result, hypereutectoid steel rails containing more than 0.85% C have recently been developed and put into practical use.
[0003]
On the other hand, in cold regions, rail replacement due to the occurrence of rail cracks is concentrated in winter, and improving the toughness and ductility of rail materials has become an important issue for extending the rail life. In steel materials, the hard and brittle pro-eutectoid cementite phase tends to coarsely precipitate on the prior austenite grain boundaries as the C content increases. When such coarse pro-eutectoid cementite is generated, it becomes a starting point of fracture, so that ductility is deteriorated.
[0004]
[Problems to be solved by the invention]
As a method of suppressing proeutectoid cementite, there is a method of adding a large amount of Al and Si as described in JP-A-2002-69585. However, such a large amount of addition has a problem that the production cost increases.
[0005]
[Means for Solving the Problems]
In the present invention, by adding N, the amount of C segregated at austenite grain boundaries is reduced, and the formation of cementite is suppressed, and the gist thereof is as follows.
(1) In mass%,
C: 0.72 to 1.4%, Si: more than 0.5 to 1.0%,
Mn: 0.4 to 1.4%, N: 0.012 to 0.030%
And the balance Fe and inevitable impurities, and the rail head cross section is a pearlite structure having a uniform metal structure of 2 mm or more from the steel surface and up to 30 mm from the steel surface. A pearlite high-strength rail excellent in ductility and having a pro-eutectoid cementite ratio of 0.
[0007]
( 2 ) Steel component is further in mass%,
Cr: 0.1 to 1.0%, Ni: 0.01 to 1.5%,
Cu: 0.01 to 1.5%, Al: 0.002 to 0.05%,
Mo: 0.015% to 0.040%
The pearlite high-strength rail excellent in ductility and having a pro-eutectoid cementite ratio of 0 described in (1) above , comprising one or more of the following, and the balance consisting of Fe and inevitable impurities.
[0008]
( 3 ) From the state in which the steel slab composed of the component described in (1) or (2) is a high temperature state after being formed into a rail by hot rolling or austenite region temperature by reheating after hot rolling, A high strength with excellent ductility with a pro-eutectoid cementite ratio of 0, characterized by accelerated cooling at a cooling rate of 1 to 10 ° C./sec during a period when the head of the rail is cooled from at least 700 ° C. to 550 ° C. A manufacturing method of pearlite rail.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The rolling material of rail steel is adjusted in components by a converter, an electric furnace, etc., and after degassing treatment as necessary, it is solidified and manufactured. Thereafter, the rolled material is reheated to 1200 to 1350 ° C., gradually hot formed by a plurality of hole rolling mills, and finished into a rail shape at 800 to 1100 ° C. The metal structure from the reheating in the heating furnace to the end of rolling is a face-centered cubic austenite structure.
[0010]
In order to ensure wear resistance and internal fatigue damage resistance, high strength of 1000 MPa or more is required in the curved portion of the high-speed track and heavy load track. The metal structure of rail steel has a fine layered structure of ferrite and cementite and is called a pearlite lamellar structure. The strength of pearlite steel increases as the layer spacing between cementite phase and ferrite phase, that is, lamellar spacing, is narrower. The lamellar spacing becomes narrower as the transformation temperature decreases. In order to obtain a high-strength rail steel material, it is necessary to reheat to the austenite region temperature after completion of rolling or once cooled to room temperature, and accelerate cooling to 700 to 550 ° C. which is a pearlite transformation temperature region. However, when the cooling rate during accelerated cooling is less than 1 ° C./sec, the required strength cannot be obtained, and when it exceeds 10 ° C./sec, a bainite structure and a martensite structure are generated, which is not preferable. If a spraying method using a liquid such as water or a cooling method by dipping is employed during accelerated cooling, a film boiling phenomenon occurs in the steel material surface layer, which easily causes uneven cooling. For this reason, the cooling medium is preferably air or air-water mist.
[0011]
Now, C and N having a small atomic radius are stabilized in terms of energy because they exist in the austenite grain boundary where the crystal lattice of Fe is disturbed. For this reason, C and N are easily segregated at the austenite grain boundaries. The grain boundary of the hypereutectoid composition steel containing a C amount equal to or greater than the eutectoid point is in a state where pro-eutectoid cementite is likely to precipitate due to C segregation. The precipitation of proeutectoid cementite also depends on the cooling rate, and the slower the cooling rate, the easier it is to precipitate. Cementite is hard and poor in ductility, so if it precipitates in a clustered form at the austenite grain boundary, it deteriorates the toughness of the steel material. Therefore, how to suppress this pro-eutectoid cementite is important in hypereutectoid steel.
[0012]
The present inventors have discovered that when the N content in the steel is increased, the transformation rate is delayed in a high temperature range around 700 ° C., and the precipitation of proeutectoid cementite is suppressed. The present inventors presume this cause as follows. Cementite has a crystal structure of Fe ₃ C, and a large amount of C is required for its precipitation. When the hypereutectoid steel is gradually cooled from the austenite stable temperature range, precipitation of cementite starts from around 700 ° C. At this time, segregation of N occurs in the austenite crystal grain boundary of the steel material having a high content of N, and the C content of the grain boundary is lower than that of the steel material having a low N content. For this reason, it is considered that the driving force for generating proeutectoid cementite at the grain boundaries is reduced in steel materials having a high N content.
[0013]
Such a uniform pearlite structure needs to be formed at least on the head portion of the rail where the shock load from the wheel is easily applied. However, in a very surface layer portion within 2 mm from the rail surface, C is released into the atmosphere, so that a decarburized layer is formed and pro-eutectoid ferrite is likely to be generated. For this reason, the area within 2 mm from the rail surface is not covered by this patent. In addition, the stress generated by contact with the wheel is reduced 30 mm or more from the surface of the steel material, so even if proeutectoid cementite occurs, there is no practical problem, so it was excluded.
[0014]
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. If it is less than 0.72%, the required high-strength pearlite structure is difficult to obtain, and if it exceeds 1.4%, the formation of proeutectoid cementite is inevitable, so the content was limited to 0.72 to 1.4%.
[0015]
Si: Si contributes to an increase in strength by solid solution strengthening in the ferrite phase in the pearlite structure, and has a slight toughness and ductility improvement effect. If the content is less than 0.5%, the effect is small. If the content exceeds 1.0%, embrittlement is caused and weldability is deteriorated, so the content is limited to more than 0.5 to 1.0%.
[0016]
Mn: Mn is an element that 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%, a martensite structure is easily generated in the segregated portion, which is not preferable.
[0017]
N: N segregates at the austenite grain boundary to reduce the amount of C grain boundary segregation, suppresses the formation of proeutectoid cementite at around 700 ° C., and promotes pearlite transformation at around 600 ° C. For that purpose, 0.012 % or more is required. However, if N exceeds 0.030%, a high temperature embrittlement phenomenon occurs, and an internal crack of the steel material in casting occurs, which is not preferable.
[0021]
Furthermore, in the present invention, in addition to the above components, if necessary, one or more Cr, Ni, Cu, Al, and Mo are added to improve the toughness of the ferrite base and to prevent the precipitation of proeutectoid cementite. Further, higher strength can be obtained by accelerated cooling in the cooling process.
[0022]
The reason for limiting these chemical components will be described below.
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 during forced cooling, resulting in a decrease in toughness.
[0023]
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.
[0024]
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.
[0025]
Al: Al has an effect of suppressing generation of pro-eutectoid cementite by reducing generation driving force of cementite. However, if the Al content is less than 0.002%, this effect is not obtained. If the Al content exceeds 0.05%, the Al oxide becomes coarse and the toughness is lowered.
[0026]
Mo: Mo suppresses the growth rate after pearlite nucleation and increases the chance of many nuclei growing, so that it has the effect of reducing the pearlite crystal grain size. However, if the content is less than 0.015%, the effect is small, and if the content exceeds 0.040%, the pearlite transformation rate is excessively lowered in the segregated portion of Mo, and bainite and martensite are generated in the pearlite structure, which is not preferable.
[0027]
In addition, P is inevitably contained in the steel, and it is preferably 0.015% or less in order to embrittle the ferrite layer and lower the impact characteristics. If O exceeds 0.01%, a coarse oxide is generated and the toughness is lowered, which is not preferable. S is inevitably contained in the steel, but if it is 0.02% or less, the effect on the material is small.
[0028]
Moreover, the transformation temperature is lowered by accelerated cooling from the surface to increase the hardness. The cooling rate is as fast as the surface layer, and the cooling rate inside the steel material decreases. For this reason, the hardness decreases toward the inside. On the other hand, since the rails are used over time and wear progresses, a hardness of about 30 mm from the surface layer is important. Further, the hardness of the very surface layer varies slightly due to the change of the decarburized layer state based on the production conditions. For this reason, it is convenient to use a position about 5 mm below the surface as a reference when setting the rail hardness. If the hardness at this 5 mm point is less than Hv330, the amount of wear is large when used in heavy-duty railways, and the rail replacement cycle is shortened, which is not preferable. On the other hand, hardness exceeding Hv460 is difficult to achieve with a pearlite structure, and there is a possibility that foreign structures such as fine martensite are mixed.
[0029]
【Example】
Next, examples of manufacturing a high-strength rail having high toughness manufactured according to the present invention will be described. Rails were produced from the rolled material comprising the components shown in Table 1.
[0030]
[Table 1]

[0031]
Table 2 shows the results of 2 mm U notch Charpy impact test at 20 ° C. for tensile strength, elongation, and test temperature of rail steel in which the cooling rate between 700 ° C. and 550 ° C. was changed in the range of 1 to 10 ° C./sec for each steel type. The pro-eutectoid cementite fraction is shown. The Charpy test piece was sampled in the rail longitudinal direction so that 3 mm below the rail top surface was the top surface of the test piece, and a notch was processed on the top surface side. Further, the pro-eutectoid cementite fraction was calculated from a point of view by taking a microscope image of three fields of view at a magnification of 200 times 30 mm below the top surface of the rail vertical section. The tensile test was taken from a depth of 10 mm inside the rail head gauge corner, and a JIS No. 4 similar test piece having a parallel part diameter of 6 mm and a parallel part length of 30 mm was used.
[0032]
In the N-added steel of the present invention, the pro-eutectoid cementite decreased and the impact value and elongation value improved compared to the comparative example.
[0033]
[Table 2]

[0034]
【The invention's effect】
According to the present invention, a highly wear-resistant rail having a uniform pearlite structure can be obtained.

Claims (3)

% By mass
C: 0.72-1.4%,
Si: more than 0.5 to 1.0%,
Mn: 0.4 to 1.4%
N: 0.012 to 0.030%
And the balance Fe and inevitable impurities, and the rail head cross section is a pearlite structure having a uniform metal structure of 2 mm or more from the steel surface and up to 30 mm from the steel surface. A pearlite high-strength rail excellent in ductility and having a pro-eutectoid cementite ratio of 0. Steel component is more mass%,
Cr: 0.1 to 1.0%,
Ni: 0.01 to 1.5%,
Cu: 0.01 to 1.5%,
Al: 0.002 to 0.05%,
Mo: 0.015% to 0.040%
The pearlite high-strength rail excellent in toughness having a pro-eutectoid cementite ratio of 0 according to claim 1 , wherein the pearlite high-strength rail has a pro-eutectoid cementite ratio of 0. From the state which made the steel slab which consists of a component of Claim 1 or 2 into the austenite area temperature by the high temperature state after forming in a rail by hot rolling, or the reheating after hot rolling, the head of the said rail is A method for producing a high-strength pearlite rail excellent in toughness with a pro-eutectoid cementite ratio of 0, characterized by accelerated cooling at a cooling rate of 1 to 10 ° C./sec during a period of cooling from at least 700 ° C. to 550 ° C. .