JPH07173530A - Production of high toughness rail having pearlite metallic structure - Google Patents

Abstract

PURPOSE:To produce a high toughness rail by imparting toughness to a high carbon steel, excellent in strength and wear resistance and having a pearlitic structure. CONSTITUTION:At the time of rolling a steel contg. 0.6 to 1.00% C to form into a rail, three passes of continuous rolling are performed in the finish rolling at temp. range of 850 to 1000 deg.C, at area reduction rate of 5 to 30% for each pass and with interpass time of <=8sec and then air cooling or accelerated cooling is executed. Based on the knowledge that, concerning a high carbon steel, austenitic grains are easy to recrystallize, and immediately after rolling, recrystallized grains close to dynamic recrystallized grains are produced, rolling is executed by the same method, by which fine rolling recrystallized grains can be obtd. to remarkably improve the roughness of the pearlitic steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a high toughness rail which is toughened to a steel having a high carbon pearlite structure excellent in strength and wear resistance which is used for railways and other industrial machines. It is a thing.

[0002]

2. Description of the Related Art Steel with a high carbon and pearlite metallographic structure is used as a structural material because it has high strength and good wear resistance. A lot of rails have been specified to meet the demands.

As a method for producing such a steel material, for example, Japanese Patent Application Laid-Open No. 55-276 discloses that "steel of a specific component that easily exhibits a pearlite structure is cooled from a heating temperature of Ac ₃ or higher to 450 to 600 ° C. Method for producing a hard rail in which a fine pearlite structure is generated by performing a constant temperature transformation at a temperature of "C: 0.65 to JP-A-58-212229.
The Mn high rail containing 0.85% and Mn: 0.5 to 2.5% and having high temperature heat was rapidly cooled to improve wear resistance by making the rail or rail head structure fine pearlite. Rail heat treatment method ", and Japanese Patent Laid-Open No. 59-13
3322 gazette "A rolling rail of a specific component that can stably obtain a pearlite structure is immersed in a molten salt bath at a specific temperature from the temperature of Ar ₃ or higher, and about 1 below the surface of the top of the rail top.
Many techniques are known, as is disclosed "a heat treatment method for a rail exhibiting a fine pearlite structure having a hardness of Hv> 350 by 0 mm".

However, although the strength and wear resistance of pearlite steel can be easily obtained by adding an alloying element to the rail of the required standard product, the toughness is significantly lower than that of steel mainly composed of ferrite structure, For example, for pearlite rail steel, the room temperature test value in the JIS No. 3 U notch Charpy test is about 1 to 2 kgf · m. When such low toughness steel is specified as a structural member in a field subject to repeated loads and vibrations, there is a problem of causing low stress brittle fracture from minute initial defects and fatigue cracks.

It is generally said that the means for improving the toughness of steel is achieved by refining the metal structure, that is, refining the austenite structure and intragranular transformation. Therefore, in order to refine the austenite structure, for example, low-temperature heating during rolling or a combination of controlled rolling and heat treatment as disclosed in JP-A-63-277721, or low-temperature heat treatment after rolling is used. Has been done. However, in the rail manufacturing method, from the viewpoint of ensuring formability, it is difficult to apply low temperature heating during rolling, low temperature rolling in controlled rolling, and large reduction rolling. Is being pursued. However, this method also has problems such as high manufacturing cost and low productivity as the development of labor-saving and productivity improving technology for each steel product is advanced in recent years, and early solution of these problems is desired. ing.

[0006]

SUMMARY OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and overcomes the problems of controlled rolling that depended on low temperature or large reduction in rail forming.
It is an object of the present invention to provide a method for improving the toughness of a rail of eutectoid carbon steel by performing controlled rolling peculiar to eutectoid steel.

[0007]

DISCLOSURE OF THE INVENTION The inventors of the present invention have conducted many experiments from steel components and their manufacturing methods in order to manufacture a steel having a fine grained pearlite structure and improved toughness.
It was found that high-carbon steel, which is close to eutectoid carbon steel, is recrystallized immediately after rolling at a relatively low temperature and with a small reduction amount during processing in the austenitic state. It was found that fine grained austenite grains were obtained by rolling, and as a result, fine grain pearlite structure was obtained.

The present invention is constructed on the basis of such knowledge, and the gist thereof is C in weight%:
After roughly rolling a steel slab of carbon steel or low alloy steel containing 0.60 to 1.00% into a rail shape, the reduction ratio of the cross-sectional area per pass when the surface temperature of the rail is 850 to 1000 ° C. Continuous finishing rolling with 5 to 30% rolling for 3 or more passes and for 10 seconds or less between rolling passes, followed by cooling or 2 to 15 ° C / from 700 ° C or higher to 700 to 500 ° C. It is a method for manufacturing high toughness rails that exhibit a pearlite metallographic structure that is cooled in seconds.

The present invention will be described in detail below. First, the reason why the steel composition is limited as described above in the present invention will be described. Carbon steel slab produced by continuous casting method or ingot-agglomeration method of molten steel melted in an ordinary melting furnace, or further containing a small amount of elements for improving strength and toughness such as Cr, Mo, V and Ni. In low alloy billet, C
Is an effective component that produces a pearlite metallographic structure in the manufactured rail and ensures wear resistance, and it is necessary to contain 0.60% or more. However, the inclusion of an excessive amount exceeding 1.00% has a problem that a large amount of cementite metallographic structure precipitates to increase hardness and lower ductility, and the present invention significantly lowers the toughness of interest. Therefore, in the present invention, the content of C contained in the carbon steel piece or the low alloy steel piece is limited to 0.60 to 1.00%.

The steel slab thus produced is heated to a normal high temperature of over 1050 ° C., then roughly rolled into a rail shape, and subsequently subjected to continuous finish rolling. The rough rolling end temperature is not particularly limited, but is preferably 1000 ° C. or higher in consideration of the formability in the finish rolling process. The continuous finish rolling is to finish and finish the rail shape of the final size, and the continuous finish rolling is started from the high temperature after finishing the rough rolling, and the surface temperature of the rail is 5 to 30 per pass between 850 and 1000 ° C. Finish rolling is continuously performed at a cross-sectional reduction rate of%.

The continuous finish rolling conditions are within a range in which the grain size is required to obtain a fine grained pearlite metallographic structure and a fine grained austenitic metallographic structure is obtained. That is, the present invention is
Since it contains a relatively large amount of C, the fine-grain austenite metallographic structure is easily recrystallized at a low temperature and a reduction rate.
After rolling, the time required for complete recrystallization is very short,
Fine-grained austenite is used because the recrystallization behavior is quick and easy to complete, and even when the rolling reduction is small, recrystallization is repeated every time rolling is continuously performed, and grain growth of the austenite metal structure is suppressed until the next pass rolling. A metallic structure is obtained. As a continuous finishing rolling method capable of obtaining such a phenomenon, the surface temperature of the rail is limited to a range of 850 to 1000 ° C. That is, at a low finishing temperature of less than 850 ° C., the austenite metallographic structure is in a non-recrystallized state and the generation of fine pearlite metallographic structure is impaired. Also, 1
In finish rolling exceeding 000 ° C., the austenite metallographic structure grows, and subsequently a coarse austenite metallographic structure is generated during transformation of the pearlite structure, so that a uniform and fine pearlite metallographic structure cannot be obtained.

The cross-sectional reduction rate per pass during this period is 5 to 5.
A rolling reduction of 30% is an effective workability for producing a fine-grained austenitic metallographic structure, and a light rolling reduction of less than 5% is not sufficient strain workability for recrystallization of the austenitic metallographic structure, and vice versa. If the rolling reduction is excessively higher than 30%, it becomes difficult to form the rail. Further, in continuous finish rolling, in order to easily generate a fine-grained austenite metallographic structure with a cross-sectional reduction rate of 30% or less, rolling of three passes or more is performed so as to suppress recrystallization of the austenite metallographic structure and grain growth. There is a need to do.

Moreover, since the rails between the passes to be rolled retain heat at a high temperature, the austenite metal structure grows and coarse grains are generated to deteriorate the characteristics required for the rail such as strength and toughness. Therefore, in the present invention, it is necessary to shorten the time between rolling passes to 10 seconds or less and immediately carry out the next rolling, thereby performing the refinement of the austenite metallographic structure and the continuous finish rolling for producing a fine pearlite metallographic structure. . The time between passes of normal reverse rolling is about 20 to 25 seconds. Therefore, the austenite metallographic grains rolled during this period are so large that strain recovery, recrystallization, and further grain growth cannot be performed, and the grain refining effect of austenite grains due to rolling recrystallization is reduced. The time between rolling passes must be shortened as much as possible since it is not possible to manufacture. The rail, which is molded into a predetermined size under such rolling conditions and retains high temperature heat, is immediately cooled to a low temperature and supplied to the product.

Further, when a rail having high strength is required, 700 ° C. which has a transformation strengthening function after continuous finish rolling.
From the above temperature, the cooling rate is within the temperature range related to the transformation of steel, that is, the temperature range of 700 to 500 ° C., and cooling is performed at a rate of 2 to 15 ° C./sec. If the speed at this time is less than 2 ° C / sec, the cooling becomes slow cooling and the transformation strengthening equivalent to that of standing cooling is obtained and the strengthening becomes insufficient. On the contrary, in the rapid cooling exceeding 15 ° C / sec bainite and martensite. An abnormal metal structure such as is generated, which significantly impairs the toughness and provides a brittle rail.

According to the method of the present invention as described above, it is possible to obtain a fine grain pearlite structure and manufacture a rail having improved toughness.

[0016]

[Examples] Table 1 shows the chemical composition of a test steel having a pearlite metal structure. Table 2 shows the heating conditions and finish rolling conditions when processing the steel having the components shown in Table 1 into rails, together with the present invention and the comparative method. Table 3 shows the cooling conditions after rolling. Table 4 shows the mechanical properties of rail steels according to the method of the present invention and the comparative method when rails are manufactured by combining the steel components, rolling conditions and cooling conditions shown in Tables 1 to 3.

In the method of the present invention, the strength of the rail changes depending on the steel composition and cooling conditions, but the ductility value (elongation) and the toughness value (2
It can be seen that UE + 20) shows a significantly higher value than that of the comparative method.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

[Table 4]

[0022]

INDUSTRIAL APPLICABILITY As described above, the rail obtained by the method of the present invention is made into a fine pearlite structure by being manufactured by finish rolling under the specified conditions and further cooling, and is made to have extremely excellent toughness. be able to.

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[Procedure amendment]

[Submission date] September 21, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

As a method of manufacturing such a steel material, for example,
Japanese Patent Application Laid-Open No. 55-2768 discloses that "a steel of a specific component which easily exhibits a pearlite structure is cooled from a heating temperature of Ac ₃ point or higher and is subjected to an isothermal transformation at a temperature of 450 to 600 ° C. to form a fine pearlite structure. A method for manufacturing a hard rail ", and" C: 0.65 "in JP-A-58-212229.
~ 0.85%, Mn: 0.5 ~ 2.5% containing Mn: 0.5-2.5% and rapidly heat the high temperature Mn high rail to improve wear resistance by making the structure of the rail or rail head fine pearlite. Heat treatment method for rails ", Japanese Patent Laid-Open No. 59-1
33322 gazette "A rolling rail of a specific component capable of stably obtaining a pearlite structure is immersed in a molten salt bath at a temperature from the Ar ₃ point or higher to a specific temperature, and Hv> about 10 mm below the surface of the rail crown. Many techniques are known as disclosed in "Method for heat treatment of rail having a fine pearlite structure having a hardness of 350".

Claims (2)

[Claims] 1. After roughly rolling a carbon steel or low alloy steel billet containing C: 0.60 to 1.00% into a rail shape, the surface temperature of the rail is between 850 and 1000 ° C.
A pearlite metallographic structure characterized by performing continuous finishing rolling in which the reduction rate of cross-section reduction per pass is 5 to 30% for 3 passes or more and the interval between rolling passes is 10 seconds or less, followed by cooling. Method for manufacturing high toughness rails. 2. A steel slab of carbon steel or low alloy steel containing C: 0.60 to 1.00% is roughly rolled into a rail shape, and then the surface temperature of the rail is between 850 and 1000 ° C.
Rolling with a cross-section reduction rolling reduction of 5 to 30% per pass is performed by continuous finishing rolling for 3 passes or more and between rolling passes for 10 seconds or less, and then 700 to 50 from a temperature of 700 ° C. or more.
A method of manufacturing a high toughness rail having a pearlite metallographic structure, characterized by cooling between 0 ° C and 2 to 15 ° C / sec.