JPH09227943A - Production of high strength rail excellent in ductility and toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain the gamma transgranular transformed grain refinement of a rail steel by the use of oxide metallurgy and to improve toughness. SOLUTION: A bloom of a steel, which is prepared by adding one or >=2 deoxidizing elements among Zr, Mn, and Si to a molten steel to perform deoxidizing treatment and further successively adding, supplementarily, Ti-containing Zr, and has a composition consisting of, by mass, 0.55-0.85% C, 0.20-1.20% Si, 0.50-1.50% Mn, 0.002-0.035% S, 0.1-1.0% Cr, 0.001-0.020% Ti resultant from supplementarily added Zr, and the balance iron with inevitable impurities, is hot-rolled into rail. By this method, a high strength rail excellent in toughness and ductility, in which the number of MnS of a size of 0.1-10μm is regulated to (30 to 10000) pieces per mm<2> , is produced. This rail can clear the toughness specification specified by Russian GOST standard.

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 for a railway, which has a fine pearlite structure of a rail steel to improve toughness and ductility.

[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 the harsh environment in which rail materials are used, especially the increase in wear, technological development for increasing the strength of rail steel has been accelerated, and almost all rail materials in curved sections, both in Japan and abroad, are being used. 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, particularly on the gauge corner (GC) surface against which 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 considered as measures for improving the toughness and ductility of the high-strength rail. (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.

[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., and 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.

Further, as a conventional deoxidizing method for molten steel for rails, deoxidizing by adding Al in addition to Mn and Si has been carried out, but alumina (Al ₂ O ₃ ) is first formed by adding Al. As a result, the production of manganese silicate (MnO.SiO ₂ ) which is effective as a nucleus for producing MnS is hindered, and M
There was a problem that the fine pearlite structure could not be achieved because sufficient pearlite nucleation from nS did not occur. Further, since excessive addition of Al produces alumina clusters, which are one kind of coarse inclusions, and serves as the starting point of fatigue fracture from inside the rail head, deoxidation by Al tends to be avoided as much as possible.

[0008]

The present invention is fundamentally different from the above-mentioned invention, and one or more deoxidizing elements of Zr, Mn, and Si are added to molten steel to perform deoxidizing treatment. Subsequently, Zr containing Ti was additionally added, C: 0.55
~ 0.85%, Si: 0.20 to 1.20%, Mn:
0.50 to 1.50%, S: 0.002 to 0.035
%, Cr: 0.1-1.0% and Ti provided by additional Zr: 0.001-0.020%,
A steel slab having the balance of iron and unavoidable impurities is hot-rolled, and the number of MnS having a size of 0.1 to 10 μm is 30 to 10,000 per 1 mm ² in the rail steel. It is a high strength rail with excellent ductility. Further, in the present invention, after the hot rolling or after heating to the austenite region, the temperature between 700 and 500 ° C.
A method of accelerated cooling at 5 ° C / sec can also be adopted.

In the present invention, the pearlite transformation, which was conventionally said to be generated only from the austenite grain boundaries,
The feature is that MnS in the austenite grains also plays a role of nuclei for pearlite transformation, and pearlite transformation is generated also in the austenite grains.
After one or more kinds of Si deoxidizing elements are added for sufficient deoxidation, MnS, which is a pearlite structure forming nucleus, is finely dispersed in the austenite grains by the subsequent addition of Zr. The contained Ti precipitates on MnS, and in addition to the pearlite transformed from the grain boundary, it becomes possible to further promote the nucleation of MnS in the austenite grains. According to the present invention, the structure is remarkably refined by the pearlite efficiently generated from the inside of the grain, and accordingly, the toughness and the ductility can be greatly improved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The purpose of deoxidation in the present invention is to make Zr, Mn, Si
The oxygen content in the molten steel is reduced as much as possible by adding one or more kinds of deoxidizing element of No. 2 to perform deoxidizing treatment. Ti in Zr added during deoxidation floats with Zr for the purpose of effectively finely dispersing, but Ti in Zr added additionally remains in molten steel as it is, forming carbonitrides. , Eventually MnS
It precipitates on the surface and acts as a transformation nucleus of pearlite. At this time, Zr additionally added forms an oxide and finely disperses in the molten steel, and acts as nuclei of MnS. That is,
Add one or more deoxidizing elements of Zr, Mn and Si as deoxidizing agents to reduce the amount of oxygen in molten steel to 50 ppm or less,
By additionally adding Zr containing a small amount of Ti together with other alloy elements, it is intended to efficiently generate a carbonitride without forming an oxide by a small amount of Ti contained in Zr.

Here, in order to efficiently generate a carbonitride of Ti, Z containing 0.03 to 0.3% of Ti is used.
It is desirable to add 0.1 to 0.5% of r to the molten steel. If the Ti content in Zr is less than 0.03% or the amount of Zr added to the molten steel is less than 0.1%, the amount of Ti precipitated in MnS cannot be secured, and nuclei for pearlite transformation are not sufficiently generated. . Further, when the Ti content in Zr exceeds 0.3%, or the amount of Zr added to the molten steel is 0.5.
%, The generated TiN tends to be coarse and tends to be the starting point of rail fatigue damage. At this time, after directly deoxidizing, T
The reason why i is not added is that precipitates such as TiN generated by adding a large amount of Ti generate harmful precipitates as the starting point of fatigue cracks generated from inside the rail head when the rail is laid. is there. On the other hand, Ti in Zr added additionally has a small amount of Mn efficiently.
It has been confirmed that it has a feature of depositing on S and does not coarsen to a detrimental degree.

Next, the reason why the number of MnS of 0.1 to 10 μm after deoxidation is limited to 30 to 10,000 per 1 mm ² will be described. By adding one or more deoxidizing elements of Zr, Mn, and Si, the amount of oxygen in molten steel can be reduced as much as possible, and by sufficiently reducing oxygen, coarsening of oxides in molten steel can be prevented and additional addition can be made. A fine oxide is generated by Zr, and MnS is finely dispersed in austenite with this oxide as a nucleus. Further, carbonitride Ti (CN) is converted from Ti contained in Zr additionally added with this MnS as a nucleus. To generate. The pearlite transformation is generated from the Ti precipitates having MnS in the austenite grains as nuclei. At this time, in MnS having a size of 0.1 μm or less, Ti
It is difficult to form carbonitride nuclei and has Mn of 10 μm or more.
When S is generated, the absolute number of MnS decreases,
As a result, the number of MnS, which is the core of pearlite, decreases, so the number of MnS was limited to 0.1 to 10 μm. Further, the number of MnS is 30 to 1000 per 1 mm ² .
The reason for limiting the number to 0 is that 30 or less MnS cannot secure sufficient nucleation sites for improving toughness and ductility, and if 10000 or more MnS is generated, the rail steel itself is contaminated. On the contrary, since the toughness and ductility decrease, the number of MnS per 1 mm ² is 30 to
Limited to 10,000.

Next, the reason why the chemical composition of the steel slab obtained by deoxidizing and additionally adding Zr is limited as described above will be described. C is an essential element for strengthening and forming a pearlite structure, and is an element that also has a unique effect on wear resistance, but if it is less than 0.55%, wear resistance and resistance to austenite grain boundaries are obtained. A large amount of proeutectoid ferrite, which is unfavorable to damage, is generated, and when it exceeds 0.85%, not only harmful proeutectoid cementite that embrittles the austenite grain boundaries is generated, but also minute amounts of rail head heat treatment layers and welds are produced. Martensite is generated in the segregated portion, and the toughness and ductility are significantly impaired, so the content is limited to 0.55 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. In addition, Si is an important element that constitutes a manganese silicate-based oxide that becomes the core of MnS together with Mn, and an effective fine oxide can be generated by performing an appropriate deoxidizing method as in the present invention. If Si is 0.2% or less, the effect cannot be expected, and Si is 0.2 as a deoxidizing element.
% Or more is necessary, and if it exceeds 1.2%, embrittlement is caused and weld bondability is reduced, so 0.20 to 1.20.
%. Like C, Mn is an element that contributes to strengthening by lowering the pearlite transformation temperature and increasing hardenability, and, like Si, as a constituent element of manganese silicate as the core of MnS and as a deoxidizing element. This is also an essential element, and by adopting the deoxidizing method of the present invention, further miniaturization of the oxide is achieved, and it is effective in inducing the fine precipitation of MnS and the subsequent nucleation of pearlite. 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, MnS-based precipitate Ti (CN) having an oxide such as manganese silicate in the austenite grains as a nucleus is formed, which is formed. It is an essential element because it produces a pearlite structure as a transformation nucleus. However, if it is less than 0.002%, M as pearlite transformation nucleus
The amount of nS decreases, and it becomes impossible to secure the pearlite intragranular transformation. On the other hand, if it exceeds 0.035%, a large amount of MnS is formed and the toughness and ductility are remarkably lowered, so 0.002 to 0.002
It was limited to 0.035%.

[0016] 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 reduces 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. Ti is
As described above, Zr containing Ti is added by being added to the molten steel. But 0.001
If it is less than%, carbonitride cannot be sufficiently generated,
Further, when the content exceeds 0.020%, the Ti carbonitride coarsens and becomes harmful as a starting point of a fatigue crack generated from inside the rail head, so the range is set to 0.001.
Was set to 0.020%. Inevitable impurity element P
Is preferably reduced 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 melting including the above-mentioned deoxidation in a commonly used melting furnace such as a converter or an electric furnace, and this molten steel is ingoted or agglomerated. Method or continuous casting method, and then hot rolling. The rail that has undergone hot rolling also undergoes pearlite transformation from Ti carbonitrides precipitated in MnS in austenite grains during cooling, and constitutes fine pearlite grains together with pearlite produced from 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 ° C to 500 ° C after completion of rolling or reheating for the purpose of heat treatment after cooling to room temperature once. 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.

As a method for evaluating the toughness of rail steel, there is the impact absorbed energy at + 20 ° C. in the 2 mm U-notch Charpy test defined by the Russian GOST standard. According to this standard, the impact of the high strength heat-treated rail at + 20 ° C. Absorbed energy is required to be 0.25 MJ / m ² or more. By utilizing the Ti carbonitrides precipitated in MnS in the austenite grains as pearlite transformation nuclei, the pearlite grains are refined in the rail steel of the present invention, and impact absorption energy of 0.25 MJ / m ² or more is obtained. be able to.

The ductility of the rail has an influence on 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 portion is 6 mm and the length of the parallel portion is 10 mm inside the rail head GC. It is said that an elongation value of 12% or more is required in a 30 mm tensile test. In response to such material requirements, by producing a pearlite transformation from MnS produced in the austenite grains of the present invention, a fine pearlite structure is produced, and ductility of rail steel as well as toughness can be significantly improved. It was

[0021]

EXAMPLES Next, examples of manufacturing high-strength high-strength rails according to the present invention will be described. Table 1 shows the chemical composition of the test steel and the steel with deoxidation of one or more of Zr, Mn, and Si and without deoxidation control. And the composition of the additive-free steel are shown in the structure after cooling.
The MnS number of m was measured, and M was measured in the structure after cooling.
Table 2 shows the results obtained by observing whether or not the pearlite intragranular transformation having nS as the nucleus is included. 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. The tensile test is a parallel part diameter of 6 mm, which is sampled from a depth of 10 mm inside the rail head GC,
The test was performed on a test piece having a parallel length of 30 mm. As a result, it was confirmed that the steel of the present invention has a sufficiently improved ductility as an effect of the pearlite microstructure, as compared with the comparative steel. The impact test piece was taken from 1 mm below the rail head. This test condition is based on the Russian GOST standard that regulates the toughness of heat-treated rails. According to this standard, high-strength heat-treated rails require impact absorption energy at + 20 ° C of 0.25 MJ / m ² or more. The steels of the present invention all sufficiently satisfy the Charpy absorbed energy specified in the GOST standard.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, a rail having a fine pearlite structure in which pearlite transformation is generated even in the austenite grains can be manufactured by appropriately performing molten steel deoxidation. Has a ductility / toughness value that sufficiently satisfies the overseas standard values.

Claims (2)

[Claims] 1. A deoxidizing element of Zr, Mn and Si is added to molten steel in an amount of 1
1 or 2 or more kinds thereof are added to perform deoxidation treatment, and subsequently, Zr containing Ti is additionally added in a mass% of 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%, and Ti: 0.001 to be provided by additional Zr.
A steel slab containing 0.020% of which the balance is iron and unavoidable impurities is hot-rolled into a rail, and the number of MnS having a size of 0.1 to 10 μm is 30 to 1000 per 1 mm ² .
A method for producing a high-strength rail having excellent toughness and ductility, which is characterized by the presence of zero pieces. 2. The steel slab of the preceding paragraph is hot-rolled, or further heated to an austenite range, and then 700 to 500 ° C.
The method for producing a high-strength rail excellent in ductility and toughness according to claim 1, wherein the space is acceleratedly cooled at 1 to 5 ° C / sec.