JP3267772B2 - Manufacturing method of high strength, high ductility, high toughness rail - Google Patents

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 in which a pearlite structure is refined to improve ductility and toughness.

[0002]

2. Description of the Related Art In recent years, railway transportation has been pursued to the utmost in the speed of passenger railways, and in overseas heavy-duty railways, the loading of freight cars has become more and more advanced, and the characteristics required of rails are becoming increasingly severe. Has become. In high-speed railways, in order to ensure safety as the speed increases, rails having improved ductility and toughness and excellent characteristics of fracture resistance are required. In heavy-load railways, high wear resistance is exhibited in sharply curved sections, and high strength is applied to the resistance to fatigue damage generated from the inside of the rail head. Rails with improved destruction resistance are required.

[0003] To date, high-strength rails having excellent ductility and toughness have been manufactured in many cases by mixing steel composition and further rolling conditions or tempering heat treatment conditions. The manufacturing method has the following problems. For example, 1) Japanese Patent Application Laid-Open No. 55-125231 discloses that "the surface layer of the head of a rolled rail is heated to a temperature of 850 ° C. or more to austenite, and then cooled to 800 to 550 ° C. by gas cooling. In a manufacturing method of performing a heat treatment of rolling the rail and then heating the rail head again and accelerated cooling, as described in "Method of manufacturing a heat-treated hard-head rail cooled in 50 to 400 seconds", the ductility is reduced by refining austenite grains. Although an attempt is made to improve toughness, there is a problem that the service life is extremely short because the degree of improvement is small. If the material-improved portion is to be deepened by heating to the inside of the rail head, there is a problem that productivity of heat treatment is significantly impaired and manufacturing cost is increased due to the necessity of long-time heating.

[0004] 2) A steel having a component composition containing elements such as Nb and, if necessary, Cr, V, etc., disclosed in Japanese Patent Publication No. Hei 5-30883, has a cross-section of 25% or more at a temperature of 800 ° C. or less. A method for producing a high-performance rail excellent in ductility and toughness by rolling at a low temperature at a finishing temperature of 700 ° C. or more, and optionally cooling the rail head immediately after the low-temperature rolling until the completion of pearlite transformation ”, and JP-A-52-138427. "Cr and Nb containing V and C if necessary
After heating steel containing o and the like to 1220 ° C. or higher, 1
Rolling equivalent to rough rolling including at least one pass at 100 ° C. or higher with a reduction of 30% or more in cross section and rolling at 1000 ° C. or lower with a cross-section reduction of 30% or less at the rail head, and then pearlite transformation on the rail head A method of manufacturing a rail that is cooled at a rate that causes cracking ”, and“ A steel containing Cr and Nb is heated to 1100 ° C. or less and a cross-sectional reduction rate is 1000 ° C. or less disclosed in JP-A-52-138428. Is performed at the time of rail rolling and forming, by performing rail forming rolling including at least one pass including a strong reduction of 30% or more, and then rapidly cooling to a transformation start temperature at a speed at which pearlite transformation occurs on the rail head. In the manufacturing method in which the austenite grains are refined by controlled rolling and then the rail head is accelerated and cooled, the rolling mill is used to reduce the austenitic grains by large rolling pressure and improve the toughness. There is a problem in the shape control of or rail.

3) Rolling of carbon steel or carbon steel containing a hardening element such as Cr, Mo or Ni at a temperature of 800 ° C. or less and a cross-sectional area of 5% or more is disclosed in Japanese Patent Publication No. 4-4371. After the application, before reaching the transformation point from austenite to pearlite, it is again heated to a low temperature of 750 to 900 ° C., and then cooled at a rate of 1 to 15 ° C./sec. As in the `` Production method, '' the production method of performing low-temperature controlled rolling during rail rolling forming, reheating before pearlite transformation, and then cooling, uses a low-temperature heating furnace after rolling to repeat workability and productivity. There was a problem in manufacturing cost.

[0006]

SUMMARY OF THE INVENTION In view of the current state of the art, the present invention provides high strength and high ductility required in recent years as the speed of passenger railways and the heavy loading of freight cars continue to advance. An object of the present invention is to provide a manufacturing method in which high-toughness rails can be mass-produced in a simplified manufacturing process.

[0007]

That is, the present invention generally relates to hot rolling of a high carbon steel rail formed into a rail shape, generally at an austenite grain coarsening temperature as disclosed in Japanese Patent Publication No. 4-4371. By heating to an avoiding temperature of 900 to 1050 ° C., sized and fine austenite grains are generated, and at that temperature, the cross-sectional reduction rate is 1
By performing finish forming and rolling of 0 to 20%, a rail having a fine pearlite structure and having satisfactory characteristics is manufactured, and the gist thereof is that the rail is roughly formed in a hot rolling step. C: 0.55 to 0.85%, S
i: 0.15 to 0.85%, Mn: 0.50 to 1.50
%, V: 0.03-0.50%, N: 0.010-0.
025%, and if necessary, Cr: 0.1
5 to 0.80%, Nb: 0.005 to 0.030%, T
i: A rail containing 0.003 to 0.030% of one or more kinds, the balance being Fe and inevitable impurities, having a cross-sectional reduction ratio of 10 to 10 at a temperature of 900 to 1050 ° C.
High strength, high ductility, high cooling in which 20% of finish forming and rolling are performed and then left to cool, or are further cooled from a temperature of 700 ° C or more to a temperature of 700 to 500 ° C at a rate of 1 to 5 ° C / sec. This is a method of manufacturing a tough rail.

Hereinafter, the present invention will be described in detail. First, the reason for defining the content of each steel component constituting the rail in the present invention as described above will be described.

The C component is an element necessary for improving the characteristics required for the rail, such as forming a pearlite structure to improve the strength and abrasion resistance. However, 0.
When the C content is less than 55%, a large amount of pro-eutectoid ferrite structure is formed at austenite grain boundaries, and the wear resistance and damage resistance are deteriorated. An excessive C content of more than 0.85% generates pro-eutectoid cementite at the austenite grain boundary to cause embrittlement, and also causes the surface layer of the rail head which has been subjected to the heat treatment of the present invention to be described later or There is a problem that a martensite structure is generated in a micro segregated portion of the welded heat affected zone, which significantly deteriorates toughness. Therefore, in the present invention, the C component is a content that provides high strength and wear resistance without affecting the brittleness of the rail and the deterioration of toughness after welding.
It was limited to 0.55 to 0.85%.

The Si component increases the strength by solid solution hardening into a ferrite phase in the pearlite structure, and slightly improves the toughness of the rail. These effects are 0.15%
If the content is less than 0.85%, and if the content exceeds 0.85%, the rail is embrittled.
Limited to 5%.

Like the C component, the Mn component is included as an essential element for forming a pearlite structure and lowering the pearlite structure formation temperature to improve the strength. However, if the Mn content is less than 0.50%, the effect is small, and if it exceeds 1.50%, a martensitic structure is generated in a fine component segregation part and the toughness is significantly impaired.
Its content was limited to 0.50 to 1.50%.

The V component suppresses the growth of austenite grains when V carbon / nitride produced at a high temperature such as hot rolling is cooled after rail forming and rolling and when heat treatment for heating to a high temperature is performed again. It is an effective component for reducing the austenite grains by exhibiting the effect of reducing the pearlite structure after cooling and improving the ductility and toughness required for the rail. However, if the V component is less than 0.03%, the effect cannot be obtained. Conversely, if the content exceeds 0.50%, no further effect can be expected.
Limited to 50%.

At least 0.010% of the N component is combined with the V component, and is contained in order to effectively increase the strength required for the rail by the precipitation hardening effect of the generated fine VN. . Particularly, the precipitation of fine VN has an effect of compensating for the strength that is reduced by the pearlite transformation point that rises in the hot rolling of the rail that refines austenite grains as in the present invention. However, an excessive content exceeding 0.025% generates coarse VN precipitates and deteriorates toughness, and bubbles generated in the ingot forming process during the production of steel slabs remain inside the rail and are required for the rail. There is a problem that the properties to be performed are significantly deteriorated. Thus, in the present invention, the N component is 0.010 to 0.0
It is necessary to contain 25%. A rail containing such components and having excellent properties such as high strength and high ductility manufactured by the method of the present invention can be manufactured.

In addition, P and S, which are unavoidable elements, need to be minimized as harmful components which hinder the manufacture of rails having the desired characteristics of the present invention.

Further, in the present invention, a small amount of Cr, Nb, or Ti is selectively contained in order to further improve various properties aimed at by the present invention. Like the V component, the Cr component is an effective element that compensates for the strength that decreases due to the pearlite transformation point that increases when the austenite grains are refined, and is an effective element that prevents the softening of the rail weld joint. . However, the effect cannot be obtained with a small content of less than 0.15%, and an excessive content exceeding 0.80% results in excessively high strength and deteriorates toughness. Therefore, the content of the Cr component was limited to 0.15 to 0.80%.

The Nb component is an effective element for forming Nb charcoal / nitride and refining austenite grains similarly to the V component and the like. (Approx. ° C) to improve the ductility and toughness of the rail. The effect is 0.005%
Cannot be expected with a small content of less than 0.030%
If the content exceeds the limit, no further effect can be expected.
Therefore, the Nb component needs to be contained in the range of 0.005 to 0.030%.

The Ti component has an effect of suppressing austenite grain growth even at the rolling heating temperature of the rail since the precipitate does not dissolve to a higher temperature than precipitates such as V and Nb. And improve the ductility and toughness required for the rail. However, if the content is less than 0.003%, such effects cannot be obtained. On the other hand, if the content exceeds 0.030%, the precipitates of Ti are coarsened to deteriorate ductility and toughness. In addition, when a rail is used, a fatigue crack originates from the inside of the rail head. Therefore, the Ti component must be contained in the range of 0.003 to 0.030%.

The rail steel having the above-mentioned composition is obtained by melting a steel melt produced through a normal steel-making process such as a melting furnace such as a converter or an electric furnace and a deoxidizing treatment, into an ingot and a lump. A slab is manufactured by a method or a continuous casting method, and is further formed into a rail through hot rolling. The heating temperature of the slab during hot rolling on the rail The rough forming and rolling conditions of the rail are not particularly limited and there is no problem under normal manufacturing conditions,
In the final step of hot rolling the rail, the temperature is 900 to 1
It is necessary to perform a process of performing finish forming and rolling at a cross-sectional reduction rate of 10 to 20% at a temperature of 050 ° C. and then allowing to cool.

The finish forming rolling is a production condition under which fine austenite grains are formed by sizing and a rail having a fine pearlite structure after rolling is obtained. In a high-carbon steel such as a rail steel, the temperature range of 900 to 1050 ° C. is a temperature at which austenite grains recrystallize, unlike the control rolling of ordinary low-carbon steel. At a low temperature of less than 900 ° C., a non-recrystallized region is formed, and a roll having excellent ductility and toughness cannot be obtained by rolling at this temperature. 1050 ° C
The austenite grains grow coarse even at temperatures exceeding
A rail with excellent ductility and toughness cannot be obtained. That is, in the high carbon steel, 900 to 1050 ° C. is the recrystallization temperature range of the austenite grains, and the area is reduced by 10% at the section reduction rate.
By performing rolling of up to 20%, fine austenite grains are produced in a uniform size without mixed grains or elongated grains, and as a result,
A rail exhibiting a fine pearlite structure excellent in ductility and toughness after cooling can be obtained. The rolling reduction is a necessary condition for obtaining fine austenite grains. If the cross-sectional reduction rate is less than 10%, recrystallization is incomplete. Also reaches the supersaturated range.

The rail of the present invention having the above-mentioned composition, which is manufactured through such manufacturing conditions, has a higher strength than an as-rolled rail used under relatively small wheel loads such as a domestic passenger railway. It has high ductility and high toughness and can be used for a long time.

Furthermore, the present invention relates to the above-mentioned rail when a high-strength rail particularly excellent in ductility and toughness used for a railway carrying heavy loads such as an overseas heavy-load railway is required. It is cooled at a rate of 1 to 5 ° C./sec from a temperature of 700 ° C. or more after the finish forming and rolling to a temperature of 700 to 500 ° C. In other words, the cooling start temperature after finish forming and rolling is 7
At a low temperature of 00 ° C. or less, the pearlite transformation starts near the pearlite transformation start temperature or before the start of cooling, and a coarse pearlite structure is mixed, and the high hardness and high strength characteristics required especially at the rail head cannot be obtained. . Therefore, in order to obtain a fine pearlite structure having excellent characteristics, cooling is started from a temperature of 700 ° C. or more, and cooling is performed at a rate of 1 to 5 ° C./sec so as to avoid a bainite structure generation range of 700 to 500 ° C. I do. At this time, high strength (1200 MPa or more) cannot be obtained at a slow cooling speed of 1 ° C / sec or less, and bainite or martensite, which is harmful to wear or damage, at the rail head corners at a high speed exceeding 5 ° C / sec. There is a problem that abnormal tissues are generated. Therefore, when manufacturing a high-strength rail with particularly excellent ductility and toughness, it is necessary to cool at a temperature of 700 to 500 ° C. at a rate of 1 to 5 ° C./sec so that a harmful bainite structure is not generated. There is.

[0022]

EXAMPLES Next, examples of manufacturing rail steel having high ductility and toughness manufactured according to the present invention will be described. Thirteen types of rail steels A to M shown in Table 1 were melted, and rails were manufactured according to the present invention and the comparative method. The specific method of finish forming and rolling of the rail is as follows. [Method 1 of the present invention] A rail steel having the chemical components of A to M,
After subjecting to finish forming and rolling at 900 to 1050 ° C. so as to have a reduction rate of 10 to 20%, natural cooling is performed. [Method 2 of the present invention] After the finish rolling of the present invention, the material is accelerated and cooled from 750 ° C to 700 to 500 ° C at 1 to 5 ° C / sec.

[Comparative Method 1] A rail steel having the chemical components of A to M is subjected to finish forming and rolling at 1100 ° C. so as to have a reduction rate of 5 to 15%, and then naturally cooled. [Comparative Method 2] After the above finish rolling, the temperature was changed from 750 ° C. to 700 to
It accelerates and cools between 500 ° C at 1 to 5 ° C / sec. [Comparative Method 3] A rail steel having chemical compositions of A to M
After finish forming and rolling at a low temperature of 0 ° C. with a reduction in area of 15% and 25%, natural cooling is performed. [Comparative method 4] After the low-temperature rolling, the temperature is reduced from 750 ° C to 700 to
It accelerates and cools between 500 ° C at 1 to 5 ° C / sec.

As a standard for evaluating ductility of rail steel, a standard in China, that is, a tensile test piece taken from a depth of 10 mm inside a rail head gauge corner, a parallel part diameter of 6 mm, and a parallel part length of 30 mm was used for a 12% test. Based on obtaining the above elongation value. As a method for evaluating the toughness of rail steel, 2 mm U specified by the Russian GOST standard is used.
The standard used was 0.25 MJ / m ² or more as the absorbed energy at + 20 ° C. in the notch Charpy test.

A tensile test piece having a parallel part diameter of 6 mm and a 2 mm U notch impact test piece were taken from below the rail head 2 mm manufactured under the above rolling and cooling conditions, and the results of the respective tests were carried out. -3. By the low-temperature rolling specific to the high carbon pearlite steel of the present invention, a finer pearlite structure than that of the conventional steel was obtained, and a significant improvement in ductility and toughness was achieved.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

According to the present invention, by subjecting rail steel within the chemical composition range of the present invention to finish forming and rolling according to the present invention, the elongation value of 12% or more specified in the Chinese regulations and the GOST standard of Russia are specified. Impact value 2 UE + 20 ° C. ≧ 0.
It is clear that 25 MJ / m ² can be secured, and not only can the ductility and toughness of the rail steel be significantly improved, but also it is possible to increase the strength at the same time, and in particular, wear resistance is required. It was possible to further improve the safety of the rails on the sharp curve section.

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60 C21D 9/04

Claims (4)

(57) [Claims] C .: 0.55 to 0.85%, Si: 0.15 to 0.85%, Mn: 0.50 to 1.50% by weight% roughly formed in the hot rolling step. A rail containing V: 0.03 to 0.50%, N: 0.010 to 0.025% and the balance being Fe and unavoidable impurities has a cross-sectional reduction rate of 10 at a temperature of 900 to 1050 ° C.
A method for producing a high-strength, high-ductility, and high-toughness rail, which is characterized in that it is subjected to finish forming rolling of up to 20% and then left to cool. 2. C: 0.55 to 0.85%, Si: 0.15 to 0.85%, Mn: 0.50 to 1.50% by weight% roughly formed in the hot rolling step. V: 0.03 to 0.50%, N: 0.010 to 0.025%, Cr: 0.15 to 0.80%, Nb: 0.005 to 0.030%, Ti: A rail containing 0.003 to 0.030% of one or more kinds, the balance being Fe and unavoidable impurities, having a cross-sectional reduction rate of 10% at a temperature of 900 to 1050 ° C.
A method for producing a high-strength, high-ductility, and high-toughness rail, which is characterized in that it is subjected to finish forming rolling of up to 20% and then left to cool. C .: 0.55 to 0.85%, Si: 0.15 to 0.85%, Mn: 0.50 to 1.50% by weight% roughly formed in the hot rolling step. A rail containing V: 0.03 to 0.50%, N: 0.010 to 0.025% and the balance being Fe and unavoidable impurities has a cross-sectional reduction rate of 10 at a temperature of 900 to 1050 ° C.
After performing finish forming and rolling of up to 20%, a high strength, a high ductility, and a high strength, characterized by cooling at a speed of 1 to 5 ° C./sec from a temperature of 700 ° C. or more to a temperature of 700 to 500 ° C. Manufacturing method for tough rails. C .: 0.55 to 0.85%, Si: 0.15 to 0.85%, Mn: 0.50 to 1.50% by weight% roughly formed in the hot rolling step. V: 0.03 to 0.50%, N: 0.010 to 0.025%, Cr: 0.15 to 0.80%, Nb: 0.005 to 0.030%, Ti: A rail containing 0.003 to 0.030% of one or more kinds, the balance being Fe and unavoidable impurities, having a cross-sectional reduction rate of 10% at a temperature of 900 to 1050 ° C.
After performing finish forming and rolling of up to 20%, a high strength, a high ductility, and a high strength, characterized by cooling at a speed of 1 to 5 ° C./sec from a temperature of 700 ° C. or more to a temperature of 700 to 500 ° C. Manufacturing method for tough rails.