JP3113137B2 - Manufacturing method of high toughness rail with pearlite metal structure - 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 producing a high toughness rail obtained by imparting toughness to steel having a high carbon pearlite structure excellent in strength and wear resistance used for railways and other industrial machines. Things.

[0002]

2. Description of the Related Art High carbon steel having a pearlitic metal structure is used as a structural material because of its high strength and good wear resistance. Many rails are specified in response to the change.

[0003] As a method of manufacturing such a steel material, for example,
Japanese Patent Application Laid-Open No. 55-2768 discloses that a steel having a specific component which easily exhibits a pearlite structure is cooled from a heating temperature of _three or more Ac and is isothermally transformed at a temperature of 450 to 600 ° C. to form a fine pearlite structure. Production method of hard rail "and JP-A-58-221229 disclose" C: 0.65
0.85%, Mn: 0.5-2.5%, Mn high rail which has high temperature heat and quench rapidly to improve wear resistance by making the structure of rail or rail head fine pearlite. Heat treatment method for rails "
No. 33322 discloses that a rolling rail of a specific component capable of stably obtaining a pearlite structure is immersed in a molten salt bath at a specific temperature from a temperature of ₃ or more points of Ar to a height of about 10 mm below the top surface of the rail. Many techniques are known, as disclosed in "Method of heat treatment of rail exhibiting fine pearlite structure having hardness of 350".

[0004] However, the strength and wear resistance of pearlite steel can be easily obtained as required standard rails by the addition of alloying elements, but the toughness is significantly lower than that of steel mainly composed of ferrite. For example, in the case of pearlite rail steel, the room temperature test value in the JIS No. 3 U notch Charpy test is about 1 to 2 kgfm. When such low toughness steel is used as a structural member in a field where repeated loads and vibrations are applied, there is a problem that low stress brittle fracture is caused by minute initial defects and fatigue cracks.

In general, it is said that the means for improving the toughness of steel is achieved by reducing the grain size of the metal structure, that is, by reducing the grain size of the austenite structure and intragranular transformation. Therefore, the grain refinement of the austenite structure is achieved by, for example, low-temperature heating during rolling, a combination of controlled rolling and heat treatment as disclosed in JP-A-63-277721, and low-temperature heat treatment after rolling. Have been. However, in the rail manufacturing method, low-temperature heating during rolling, low-temperature rolling in controlled rolling, and large reduction rolling are difficult to apply from the viewpoint of ensuring formability. Is planned. However, this method also has problems such as high manufacturing cost and low productivity in the development of labor saving and productivity improvement technologies for recent steel products, and it is desired to solve these problems early. ing.

[0006]

SUMMARY OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and overcomes the problem of controlled rolling that has been dependent on low temperatures or large pressures in forming rails.
An object of the present invention is to provide a method of performing controlled rolling specific to eutectoid steel to improve the toughness of a rail of eutectoid carbon steel.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted a number of experiments on steel components and their production methods in order to obtain a fine-grained pearlite structure and produce steel with improved toughness.
High-carbon steel, which is close to eutectoid carbon steel, is found to recrystallize immediately after rolling even at a relatively low temperature and a small rolling reduction in its austenite processing. It has been found that sized fine austenite grains are obtained by rolling, and as a result, a fine pearlite structure can be obtained.

The present invention has been made based on such findings, and the gist of the present invention is that C:
After roughly rolling a slab of carbon steel or low alloy steel containing 0.60 to 1.00% into a rail shape, when the surface temperature of the rail is between 850 to 1000 ° C., the cross-sectional reduction reduction per pass is reduced. Continuous finish rolling is performed in which 5% to 30% of rolling is performed in 3 passes or more and a rolling pass is 10 seconds or less, followed by being allowed to cool or from 2 to 15 ° C. from a temperature of 700 ° C. or more to 700 to 500 ° C. This is a method for producing a high toughness rail exhibiting a pearlite metal structure cooled in seconds.

Hereinafter, the present invention will be described in detail. First, the reason why the steel components are limited as described above in the present invention will be described. Carbon steel slabs produced by continuous casting or ingot slabging of molten steel produced in a normal melting furnace, or a small amount of strength toughness-improving elements such as Cr, Mo, V, and Ni. In low alloy billet, C
Is an effective component for producing a pearlite metal structure in the manufactured rail and ensuring abrasion resistance, and it is necessary to contain 0.60% or more. However, including an excessive amount exceeding 1.00% has a problem that a large amount of cementite metal structure precipitates to increase hardness and decrease ductility, and the present invention significantly lowers the target toughness. Therefore, in the present invention, the content of C contained in the carbon steel slab or the low alloy steel slab is limited to 0.60 to 1.00%.

[0010] The slab thus produced is heated to a usual high temperature exceeding 1050 ° C, rough-rolled into a rail shape, and then subjected to continuous finish rolling. The rough rolling end temperature is not particularly limited, but is preferably 1000 ° C. or more in consideration of formability in the finish rolling step. The continuous finish rolling is to form and finish into a rail shape of the final size. The continuous finish rolling is started from a high temperature at which the rough rolling is completed, and when the surface temperature of the rail is between 850 to 1000 ° C., 5 to 30 per pass. The finish rolling is performed continuously at a cross-sectional reduction rate of%.

The conditions for the continuous finish rolling are such that the grain size is necessary for obtaining a fine-grained pearlite metal structure and a fine-grained austenitic metal structure can be obtained. That is, the present invention
Contains a relatively large amount of C, so that the fine-grained austenite metal 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 recrystallization behavior is easy to complete quickly, recrystallization is repeated each time rolling is performed continuously even with a small rolling reduction, and grain growth of austenite metal structure is suppressed until rolling in the next pass. A metallographic structure is obtained. As a continuous finish rolling method in which such a phenomenon is obtained, 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 metal structure is in an unrecrystallized state, and the formation of a fine pearlite metal structure is impaired. Also, 1
In finish rolling at a temperature higher than 000 ° C., the austenite metal structure grows in grains, and subsequently, when the pearlite structure is transformed, a coarse austenite metal structure is formed, and a uniform and fine pearlite metal structure cannot be obtained.

In the meantime, the cross-sectional reduction rate per pass is 5 to 5.
A rolling reduction of 30% is an effective degree of working to produce a fine-grained austenitic metallographic structure, while a light rolling reduction of less than 5% is not sufficient for the austenite metallographic structure to recrystallize, and vice versa. If the rolling reduction is excessively more than 30%, it is difficult to form the rail. In continuous finish rolling, in order to easily generate a fine-grained austenitic metal structure with a cross-sectional reduction rate of 30% or less, rolling of three or more passes is performed so as to suppress recrystallization and grain growth of the austenitic metal structure. There is a need to do.

Furthermore, since the rail between passes to be rolled retains heat at a high temperature, an austenitic metal structure grows to generate coarse grains, thereby deteriorating properties 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, immediately perform the next rolling, and perform continuous finish rolling to refine the austenite metal structure and further generate a fine pearlite metal structure. . The time between passes in normal reverse rolling is about 20 to 25 seconds. Therefore, the austenite metallographic grains rolled during this time become so large that recovery of strain, recrystallization, and further grain growth cannot be performed, and the effect of refining the austenite grains by rolling recrystallization is reduced. Therefore, it is necessary to reduce the time between rolling passes as much as possible. A rail formed into a predetermined size under such rolling conditions and having high-temperature heat is immediately cooled to a low temperature and provided to a product.

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

According to the method of the present invention as described above, a rail having a fine pearlite structure and improved toughness can be manufactured.

[0016]

EXAMPLES Table 1 shows the chemical composition of a test steel having a pearlite metal structure. Table 2 shows heating conditions and finish rolling conditions when working steel having the components shown in Table 1 into rails, together with the present invention and comparative methods. Table 3 shows the cooling conditions after rolling. Table 4 shows the mechanical properties of the rail steel according to the method of the present invention and the comparative method when the rail is 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)
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]

As described above, the rail obtained by the method of the present invention has a fine pearlite structure when manufactured by finish rolling and cooling under specified conditions, and is a rail having extremely excellent toughness. be able to.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00, 9/04 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. After roughly rolling a slab of carbon steel or low alloy steel containing C: 0.60 to 1.00% into a rail shape, the surface temperature of the rail is between 850 to 1000 ° C.
A pearlitic metal structure characterized by being subjected to continuous finish rolling in which 3 or more passes of rolling with a cross-sectional reduction reduction rate of 5 to 30% per pass and 10 seconds or less between rolling passes are performed, followed by cooling. Method of manufacturing high toughness rails. 2. After roughly rolling a slab of carbon steel or low alloy steel containing C: 0.60 to 1.00% into a rail shape, the surface temperature of the rail is between 850 and 1000 ° C.
Continuous finish rolling is performed in which three or more passes are performed with a rolling reduction of 5 to 30% in cross-section per one pass and the interval between the rolling passes is 10 seconds or less.
A method for producing a high toughness rail exhibiting a pearlite metal structure, characterized by cooling at a temperature of 0 ° C at a rate of 2 to 15 ° C / sec.