JPH06279929A - High strength rail excellent in toughness and ductility and its production - Google Patents

Abstract

(57) [Summary] [Purpose] A high-strength rail excellent in toughness and ductility and a manufacturing method thereof. [Structure] (Ti), (Ti, Mn), (Ti, S
i), (Ti, Mn, Si) is subjected to a deoxidizing treatment with one set of deoxidizing elements, hot rolling is performed, and accelerated cooling is performed to remove Ti carbonitrides precipitated on MnS in austenite grains. A high-strength rail excellent in toughness and ductility, which is characterized by producing pearlite as a core, and a method for producing the same.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength rail in which the pearlite structure of rail steel is refined to improve toughness and ductility, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, railway transportation has been aimed at higher loads and higher speeds, and the characteristics required for rails have become increasingly severe. For heavy-duty railways, measures against wear on sharp curves and internal fatigue damage to rail heads are required. On high-speed railways, surface damage mainly on straight sections is cited as an issue. In addition to these, it is recognized that rail rupture tends to occur intensively in winter in cold regions, and improving the toughness of rail materials in cold region railways is a characteristic that is essential for safe rail transportation. ing.

Further, in order to improve the efficiency of rail transportation, speeding up and heavy loading of cargo have been promoted, but along with this, wear and fatigue damage of rail heads are rapidly increasing. In order to cope with such harsh environment of use of rail materials, especially increase in wear, technological development for strengthening rail steel has been accelerated, and rail materials in curved sections are mostly used in Japan and abroad. All of the high-strength rails now dominate.

On the other hand, on the other hand, as the wear resistance of the rail steel is improved, a fatigue damage layer that should be scraped off due to wear remains on the rail head surface, especially on the gauge corner (GC) surface where the wheel flange root is pressed, A tendency to produce surface damage has become apparent. Further, the improvement of the wear resistance of the rail steel means that the stress concentration inside the rail GC due to the wheel load is fixed at one point, and the fatigue damage from the inside of the rail head rapidly increases. In order to improve such rail head surface damage and resistance to internal fatigue damage, it is important to improve the toughness and ductility of the rail material.

The following methods are conceivable as methods for improving the toughness and ductility of high-strength rails. (1) A method in which a rail head that has been once cooled to room temperature after normal rolling is reheated at a low temperature and then accelerated cooling is performed. (2) A method of accelerating and cooling the rail head after refining austenite grains by controlled rolling. (3) A method in which after controlled rolling, it is reheated to a low temperature before pearlite transformation and then accelerated cooling is performed. As a toughness evaluation method for rail steel, there is impact absorption energy at + 20 ° C in the 2mm U-notch Charpy test defined by the Russian GOST standard. According to this standard, the impact absorption energy at + 20 ° C of the high-strength heat-treated rail is 0.25 MJ / m ² or more is required. Also,
The ductility of the rail affects the generation of fatigue damage to the rail head, and the ductility requirement of the high-strength rail in China is that the diameter of the parallel part is 6 mm from the depth of 10 mm inside the rail head GC.
It is said that an elongation value of 12% or more is required in a tensile test with a parallel length of 30 mm.

[0006]

In the above method (1), in order to improve the toughness and ductility to a great extent, JP-A-55-12 is used.
Reheating to a low temperature of 850 ° C. or lower, which is lower than the normal heating temperature as described in Japanese Patent No. 5231, attempts to improve toughness and ductility by making the austenite grain size fine. When heating and deepening the heating to the inside of the rail head, it is necessary to lower the amount of heat input and heat for a long time. Therefore, there is a problem that heat treatment productivity is significantly impaired and manufacturing cost is increased. Also,
As described in JP-A-52-138427 and JP-A-52-138428, the method (2) is toughness due to austenite grain refinement during rolling.
In order to improve the ductility, a large reduction at high temperature is required, which causes a problem from the viewpoint of rail rolling mill capacity or rail shape control. Furthermore, the method (3) is described in Japanese Patent Publication No.
After rolling 5% or more at 0 ° C. or less, 750 to 750 again
This is a method in which the austenite grains are made fine by heating at 900 ° C. Since a heating furnace for low-temperature reheating after rolling is required, there are many problems from the viewpoint of workability, productivity, and manufacturing cost.

The present invention is intended to solve such a problem, and by producing pearlite transformation nuclei from within austenite grains, a high strength rail having a fine structure and excellent in toughness and ductility, and a method for producing the same. The purpose is to provide.

[0008]

The present invention is fundamentally different from the above-mentioned method, in that (Ti), (Ti and Mn), (Ti
And Si), (Ti, Mn, and Si), one set of four sets of deoxidizing elements was added, and the mixture was subjected to deoxidation treatment to be melted, and C: 0.55 to 0.85% by weight. , Si:
0.20 to 1.20%, Mn: 0.50 to 1.50%,
S: 0.002-0.035%, Cr: 0.1
1.0%, Ti: 0.0006 to 0.075%,
N: A steel slab containing 0.0005 to 0.0300% and the balance of iron and unavoidable impurities produced by the ingot-agglomeration method or the continuous casting method is hot-rolled to 0. A high-strength rail having excellent toughness and ductility, characterized in that the number of MnS having a size of 1 to 15 μm is 30 to 12000 per 1 mm ² in the rail steel, and 700 to when cooling from an austenitic temperature range. 5
This is a manufacturing method of accelerating cooling between 0 ° C and 1 to 5 ° C / sec.

In the present invention, the pearlite transformation, which was conventionally said to be generated only from the austenite grain boundaries,
It is characterized by arranging Ti carbonitride, which is a nucleus of pearlite transformation, in MnS in the austenite grains to generate pearlite transformation from inside the austenite grains. In addition to pearlite transformed from grain boundaries, By controlling / promoting the MnS production nuclei, the structure is remarkably refined by the pearlite produced from within the grains, and along with this, the toughness and ductility can be greatly improved.

[0010]

The present invention will be described in detail below. First,
The necessity of deoxidation and the reason why the deoxidation elements are limited to four groups of (Ti), (Ti and Mn), (Ti and Si), (Ti, Mn and Si) will be described.

There are two purposes of deoxidation in the present invention, one of which is to control the oxide serving as the core of MnS, and Ti is the number of oxides produced by forming fine oxides. Is added for the purpose of suppressing the decrease of Further, Mn and Si are added as a constituent element of manganese silicate (MnO—SiO ₂ ) which is effective as a nucleation site for MnS, for the purpose of controlling the number and distribution of MnS.

Another purpose of deoxidation is (Ti), (T
i and Mn), (Ti and Si), and (Ti, Mn and Si), one set of four deoxidizing elements is added to reduce the amount of oxygen in the molten steel as much as possible. The amount of oxygen is reduced in advance so that Ti added as a product-forming element does not precipitate as an oxide. That is, (Ti), (Ti and M
n), (Ti and Si), (Ti, Mn and Si)
By strengthening the deoxidation by adding one of the four deoxidizing elements, the oxygen content in the molten steel should be 15ppm or less before adding Ti, and the added Ti should be efficient without forming an oxide. The purpose is to generate carbonitrides. Al, which is generally used as a deoxidizer,
Al does not add Al because it produces alumina clusters that are harmful to the occurrence of fatigue damage from inside the rail.

The reason why the number of MnS of 0.1 to 15 μm after deoxidation is limited to 30 to 12,000 per mm ² will be described. Oxygen is reduced by sufficient deoxidation, and as a result, a fine oxide is generated, MnS is finely dispersed in austenite with this oxide as a core, and Ti carbonitride: Ti (C, N) is generated. The pearlite transformation is generated from the Ti carbonitride whose core is MnS in the austenite grains.
With MnS having a size of less than μm, it is difficult to form nuclei of Ti carbonitride, and when MnS having a size of more than 15 μm is generated,
The absolute number of MnS is reduced, and as a result, the number of MnS, which is a nucleus of pearlite, is reduced. Therefore, the size of MnS is limited to 0.1 to 15 μm. The reason for limiting the number of MnS to 30 to 12,000 per 1 mm ² is that less than 30 MnS cannot secure a sufficient nucleation site for improving toughness and ductility, and more than 12,000. When MnS is generated, the rail steel itself is contaminated and the toughness and ductility are rather reduced.
^The number of MnS per ² was limited to 30 to 12000.

Next, the reason why the chemical composition of the deoxidized molten steel is limited as described above will be described. C is an essential element for strengthening and generating a pearlite structure,
Further, it is an element that uniquely exerts an effect on wear resistance as well, but if it is less than 0.55%, a large amount of proeutectoid ferrite which is unfavorable for wear resistance and damage resistance is generated in the austenite grain boundary, If it exceeds 85%, not only harmful harmful pro-eutectoid cementite that embrittles the austenite grain boundaries is generated, but also martensite is generated in the heat-separated layer of the rail head and the minute segregation part of the welded portion, which significantly impairs toughness and ductility. ．
It was limited to 55 to 0.85%.

Si not only contributes to the strengthening of the ferrite phase in the pearlite structure by solid solution hardening, but also contributes to a slight improvement in the toughness and ductility of the rail steel. Further, Si is an important element that constitutes a manganese silicate-based oxide that becomes a core of MnS together with Mn, and if less than 0.2%, its effect cannot be expected, and Si is added as a deoxidizing element in an amount of 0.2% or more. Is required, and if it exceeds 1.20%, embrittlement is caused and weld bondability is also reduced, so 0.20 to
Limited to 1.20%.

Like C, Mn is an element that lowers the pearlite transformation temperature and enhances the hardenability to contribute to higher strength. Further, like Mn, it acts as a constituent element of manganese silicate as the core of MnS and as a deoxidizer. It is indispensable as an acid element. However, if it is less than 0.5%, its effect is small, and if it exceeds 1.50%, the content is limited to 0.50 to 1.50% so that a martensite structure is easily generated in the segregated portion.

Although S is generally known as a harmful element, in the present invention, a precipitate based on MnS whose core is an oxide such as manganese silicate in austenite grains: Ti (C, N) is formed. It is an essential element because it produces a pearlite structure with this as a transformation nucleus. However, if it is less than 0.002%, the amount of MnS as pearlite transformation nuclei is reduced, and pearlite intragranular transformation cannot be secured. On the other hand, if it exceeds 0.035%, a large amount of MnS is formed and the toughness and ductility are remarkably reduced, so 0.002 to 0.002%.
It was limited to 0.035%.

[0018] Cr contributes to higher strength by lowering the pearlite transformation, and at the same time contributes to improving wear resistance by strengthening the cementite phase in the pearlite structure. On the other hand, it lowers the impact toughness of cementite. It also has the effect of causing it. However, the cementite strengthening effect of Cr cannot be ignored, and addition of a small amount of Cr is also desirable from the viewpoint of preventing softening of the welded joint. Therefore, it is limited to 0.1 to 1.0% within a range in which a certain contribution is expected to secure the strength and the toughness and ductility are not impaired.

Although Ti is an important constituent of the present invention,
By finding the formation of pearlite transformation centered on Ti carbonitrides precipitated on MnS during cooling, pearlite transformation nuclei that were conventionally limited to austenite grain boundaries can be expected from within the austenite grains. It became possible to obtain rail steel composed of fine pearlite grains, and it was possible to achieve a significant improvement in toughness. However, if less than 0.0006%, this effect is weak, and if more than 0.075% is added, Ti precipitates become coarse, and this becomes the starting point of fatigue crack initiation from inside the rail head.
The amount of i added was limited to the range of 0.0006 to 0.075%.

N acts as a transformation nucleus of pearlite M
It is a constituent element of TiN on nS, and 0.0005% or more is necessary for effectively precipitating TiN.
If it exceeds 0300%, coarse TiN is generated, which becomes the starting point of fatigue cracks inside the rail, so the N addition amount is 0.0005-
It was limited to 0.0300%. It is desirable to reduce P, which is an unavoidable impurity element, as much as possible in order to improve the toughness of the rail steel.

The rail steel having the above-described composition is subjected to smelting including the above-mentioned deoxidation in a commonly used smelting furnace such as a converter or an electric furnace, and this molten steel is agglomerated and agglomerated. Method or continuous casting method, and then hot rolling. The rail that has undergone hot rolling also undergoes pearlite transformation from the Ti carbonitride that precipitates MnS in the austenite grains during cooling, and forms fine pearlite grains together with the pearlite produced from the austenite grain boundaries. As a result, a high-strength rail having excellent toughness can be manufactured as rolled.

When high strength and high toughness are required, 1 to 5 ° C./700 to 500 ° C. from the austenite region temperature reheated for the purpose of heat treatment after completion of rolling or once cooled to room temperature. Rail steel that has been accelerated and cooled in sec can achieve even higher toughness. That is, as a characteristic of the pearlite structure steel, pearlite transformation is caused at a low temperature by accelerated cooling, whereby the generation rate of pearlite transformation nuclei is improved, and as a result, pearlite grains can be made fine. Therefore, the austenite intragranular transformation of the pearlite structure from the Ti carbonitride precipitated on MnS and the pearlite transformation from the austenite grain boundaries by accelerated cooling can be superimposed to further improve the toughness of the rail steel. At this time, the cooling medium is
It is desirable to use a gas-liquid mixture such as air or mist and to set the strength of the rail head or bottom to 1100 MPa or more.

[0023]

EXAMPLES Next, examples of production of high strength rails having high toughness produced according to the present invention will be described. Table 1 shows (T
i), (Ti and Mn), (Ti and Si), (T
The chemical composition of the sample steel when a deoxidizing treatment is performed by adding one set of deoxidizing elements out of four sets of i, Mn and Si) is shown. For comparison, if deoxidation control was not performed,
Furthermore, Table 1 also shows the chemical composition of the sample steel in which Ti was not added with deoxidation control. Also, the measurement results of the number of MnS of 0.1 to 15 μm existing in the microstructure after cooling of these test steels and whether or not the microstructure after cooling contains the pearlite intragranular transformation with MnS as the nucleus are included. The results of observation are shown in Table 2. In the steel of the present invention and the comparative steel that were appropriately deoxidized, the formation of a predetermined amount of fine MnS was confirmed, and in the steel of the present invention in which Ti was added, the Ti carbon with MnS from the austenite grains as the nucleus was obviously used. It was confirmed that a pearlite structure starting from nitride was generated.

In Table 3, as-rolled and 700-700 from the austenite range temperature for each chemical component in order to keep the strength constant.
Tensile strength, elongation, and 2 of the rail steel after accelerated cooling in which the cooling rate was changed in the range of 1 to 5 ° C / sec.
The measurement result of the impact absorption energy in +20 degreeC in mmU notch Charpy test is shown. Tensile test for rail head G
A test piece having a parallel part diameter of 6 mm and a parallel part length of 30 mm was sampled from a depth position of 10 mm inside C. As a result, the steel of the present invention is
A sufficient improvement in ductility as an effect of the pearlite microstructure was observed as compared with the comparative steel. Impact test piece is rail head 1
It was collected from the bottom of mm. This test condition is based on the Russian GOST standard that regulates the toughness of heat treated rails. According to this standard, the impact absorption energy at + 20 ° C of high strength heat treated rails must be 0.25 MJ / m ² or more. Therefore, the fine pearlite structure steels in which the pearlite transformation is generated even in the austenite grains by performing the appropriate deoxidation according to the present invention all sufficiently satisfy the Charpy absorbed energy specified in the GOST standard.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

INDUSTRIAL APPLICABILITY The rail steel of the present invention is controlled by M
By controlling the number of nS to utilize Ti carbonitrides precipitated in MnS in austenite grains as pearlite transformation nuclei, the pearlite grains become finer,
Impact absorption energy of 0.25 MJ / m ² or more can be obtained. Furthermore, by the generation of fine pearlite structure,
The ductility of rail steel as well as toughness can be significantly improved.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masamitsu Wakao 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Co., Ltd. Technology Development Division (72) Inventor Fusao Ishikawa 20-1 Shintomi, Futtsu-shi, Chiba Shinnihon Steel Engineering Co., Ltd.

Claims (2)

[Claims] 1. A rail produced by deoxidizing molten steel into a steel slab, which is manufactured by a process including hot working, wherein C: 0.55 to 0.85% Si: 0.20 by weight%. 1.20% Mn: 0.50 to 1.50% S: 0.002 to 0.035% Cr: 0.1 to 1.0% Ti: 0.0006 to 0.075% N: 0.0005 0.00.0% and the balance iron and inevitable impurities, M
nS has a size of 0.1 to 15 μm, and the number is 1 mm ²
A high-strength rail with excellent toughness and ductility, characterized by the presence of 30 to 12,000 pieces. 2. A molten steel containing (Ti), (Ti and Mn),
(Ti and Si), (Ti, Mn, and Si), one set of four sets of deoxidizing elements was added, and the mixture was subjected to deoxidizing treatment to be melted. C: 0.55 to 0. 85% Si: 0.20 to 1.20% Mn: 0.50 to 1.50% S: 0.002 to 0.035% Cr: 0.1 to 1.0% Ti: 0.0006 to 0. 075% N: A steel piece containing 0.0005 to 0.0300% of molten steel, the balance of which is iron and unavoidable impurities, produced through the ingot-casting method or the continuous casting method. Or, after heating to a high temperature for the purpose of heat treatment, when cooling the head or the bottom of the rail from the austenite region temperature, accelerated cooling at 700 to 500 ° C. at 1 to 5 ° C./sec, M
A method for producing a high-strength rail excellent in toughness and ductility, which comprises producing pearlite having Ti carbonitride as a nucleus deposited on nS.