JPS6256523A - Manufacture of high strength rail providing weldability - Google Patents

Abstract

PURPOSE:To deepen hardened depth and to provide good weldability, by hot rolling steel contg. specified quantities of C, Si, Mn, Cr to rail, and controlling apparent pearlite transformation and cooling condition of a range thereafter. CONSTITUTION:Steel composed of, by weight 0.55-0.85% C, 0.20-1.20% Si, 0.5-1.50% Mn, 0.10-0.80% Cr and the balance Fe with inevitable impurity, if necessary >=added one kind among 0.01-0.05% Nb, 0.05-0.20% V, 0.01-0.05% Ti is used and hot rolled to rail. Then the surface or part less than 5mm from the surface of the top of the rail is cooled at the rate of 2-15 deg.C/sec from 800 deg.C to starting temperature of apparent pearlite transformation. It is continuously cooled in transformation domain succeeding thereto while maintaining apparent transformation temp. Successively it is forcedly cooled by 0.5-10.0 deg.C/sec at temp. between apparent end temp. of pearlite transformation and 450 deg.C further lower than preceding step cooling. In this way, high strength rail superior in weldability is obtd. by adding a small quantity of alloy.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention provides a bath welding joint that is required under heavy load conditions such as mine railways overseas, or under high speed transfer conditions. The present invention relates to a method for manufacturing a high-strength rail with bath weldability, which has a fine pearlite structure that is effective in improving wear resistance and damage resistance caused by numerical wear.

(Conventional technology) In recent years, rail materials with a fine pearlite structure have been made to have higher strength as railway transportation is becoming more and more heavy-duty and related.

It is a well-known fact that the RJ's wear resistance and damage resistance have been significantly improved.1゜Especially with regard to wear resistance, The use of high-strength rails is an essential requirement in sharply curved sections.
Furthermore, the American Railway Standards (AiA) are also applied to straight sections.
) is being studied to further increase the strength of the conventional seven-way carbon steel rail.

The following methods are used to increase the strength and intermediate strength of such rails.

(2) As-rolled alloy copper rail to which alloying elements such as Cr and 810 are added in large amounts (Japanese Patent Laid-Open No. 140316/1983).

(1) A heat-treated rail manufactured by accelerated cooling of the rail head or the entire rail without adding any alloy (Japanese Patent Publication No. 55-23885).

■ Addition of relatively low content of alloy for wear resistance.

A low-alloy steel heat-treated rail that is not a damage-resistant scale, but has improved the hardness loss of welded parts.

■ High-strength as-rolled rail designed to improve damage resistance in the oiled section by adding a relatively low amount of dowel.

(Problems to be Solved by the Invention) In order to achieve high strength due to the fine pearlite structure in the alloy steel rail (2) as rolled, a large amount of expensive alloy must be added. Moreover, there is a natural limit to the amount of alloy added in order to prevent foreign structures such as martensitic structure from being mixed into the rolled steel. This alloy steel rail is manufactured by Eri.

In terms of strength, it is 125 to 4i/ltm2 at most, and the strength achieved at the rail head surface is lower than that of the heat-treated rail described later. Additionally, fuller rails for longer rails are available.
When contacting the bath, the cooling rate during rolling (800'~
Mikage's? -Light transformation starting temperature IVl 0.4~0
．． 7℃/,,c) The cooling rate of the joint part & (same temperature range 0.7~b) A martensitic structure is generated in the welded joint part of the steel rail.In order to prevent this, it is necessary to Preheating or post-heat treatment is required, which significantly impedes welding efficiency.However, as a characteristic of the alloy steel rail base material, there is almost no decrease in hardness toward the inside of the rail head, so normal reheating and heat treatment described later is not possible. The hardness distribution of the rail shows a high hardness that is reversed from about 15m below the head surface.Also, alloy steel rails can be manufactured as rolled, so they have the advantage of high production efficiency.

On the other hand, heat-treated rails (1) and (2) are mainly manufactured by a method in which the head of the rail is cooled to room temperature after rolling, reheated to austenite, and then air quenched with compressed air or the like. This method not only requires energy for reheating, but also causes a decrease in hardness corresponding to the heating temperature gradient in the inner direction of the rail head. Therefore, as the rail is used after installation, wear progresses toward the inside where the hardness decreases, and the wear resistance tends to gradually decrease over time.

In addition, the medium strength rail of ■ is HB269 in terms of hardness.
→Ha is around 286, and even in heavily oiled sections, it is limited to gentle curves and sections. Therefore, the lower limit HB with higher strength
It is thought that medium strength rails having a rating of 300 will be widely used in sharply curved sections in combination with oil application.

(Means for Solving the Problems) The present invention improves the difficulty of welding the alloy steel rail or the difficulty in achieving the strength, and improves the stability of hardness in the depth direction, which is an advantage of the alloy steel rail. The present invention relates to a manufacturing method that directly heat-treats after rolling to produce high-strength rails with high production efficiency while saving reheating energy required by existing heat-treated rails.

That is, in the present invention, C: 0.55 to 0.85 yen, St
: 0.20~1.20%, Mn: 0.50~1°50. , 〈 is based on a component system in which 0.10 to 0.80% of Cr is added to this, and Nb, V, as necessary.
A steel rail containing one or more types of TI is heated from the austenitic region after hot rolling to the rail head surface or at a depth of less than 5++ m below the head surface between 800°C and the apparent pearlite transformation start temperature. 2-b cooling, followed by continuous cooling of the 2-lite transformation region without interrupting the pearlite transformation while maintaining the apparent transformation start temperature, and cooling the area between the apparent end temperature of pearlite transformation and 450°C to 0. It is characterized by forced cooling at -5 to 10-0'C75ec.

(Structure of the invention) The present invention will be explained in detail below.

In the present invention, steel for rails having the following composition range is used, which is melted in a converter or an electric furnace.

(1) C: o, ss~0.85%, Si: 0.
20~1.20%, Mn: 0.50~1.50
%, Cr: Steel containing 0.10 to 0.80%, with the balance consisting of iron and inevitable impurities.

(2) C: 0.55-0185chi, Si: 0.2
0-1620%, Mn 0.50-1.50%
, Cr: 0.10 to 0.80 Ti, one or more types of Nb*v*Ti, and the balance consists of ferrite and inevitable impurities.

Among these chemical components, C is an essential element for increasing strength and forming a pearlite structure, and is also an element that has a unique effect on wear resistance. A large amount of pro-eutectoid ferrite, which is unfavorable for wear resistance, is generated at the boundaries, and if it exceeds 0.85%, not only is pro-eutectoid cementite harmful to the austenite grain boundaries generated.

The thickness was limited to 0.55 to 0.85 inch because martensite is generated in minute segregation areas of heat-treated layers and welded parts and causes embrittlement. S
i is an element that increases strength and improves wear resistance by solidly dissolving in ferrite in pearlite structure, but it also needs to be added in an amount of 0.20% or more as a deoxidizing element, and 1, 20 If it exceeds 0.2%, embrittlement will occur and the welding bondability will be reduced, so it is limited to 0.20 to 1.20%. M
Like C, r is an element that contributes to high strength by lowering the R-lite transformation temperature and increasing hardenability. However, if it is less than 0.50%, its contribution is small and 1,5%
In order to facilitate the formation of martensite in the segregated portions when it exceeds 0.0, the range is limited to 0.50 to 1.50. It has been found that Cr contributes to high strength by lowering the temperature at which pearlite transformation starts, but it has also been found to contribute to wear resistance by strengthening the cementite in 9-lite. It is an indispensable element for preventing part softening and is an important component of the present invention.

By adding 0.10% or more of Cr, an increase in strength becomes apparent during accelerated cooling, and in the range of 0.10 to 0.80% Cr, even under the latter cooling conditions of the present invention, marten is formed in the micro-segregation area at the center of the rail head. There is no risk that the site will generate.

However, when Cr is added in an amount of 0.8 or more, bainite and martensite are generated not only in the segregated areas but also in the rail shoulders where there is a strong tendency to overcool. Therefore, the component range of Cr is 0.10
It was limited to ~0.80%.

In this case, by adding austenite grain refining elements such as Nb, V, and Ti, it is possible to ensure not only high strength but also ductility. By heating the teeth at a low temperature during hot rolling, Nb (C, N) precipitates suppress austenite grain growth and contribute to grain refinement. Also,
The austenite grains after hot rolling are refined by high-temperature heating and low-temperature finish rolling, and the pearlite block size obtained after forced cooling is made fine. An increase in the pearlite transformation point due to these grain refinements tends to reduce strength, but by adding reinforcing elements such as Or, rails with excellent strength and ductility and improved welded joint softening can be manufactured. becomes possible. At this time, the effective amount of Nb added is 0.01%, and if it exceeds 0.05%, NbC is generated, which instead causes embrittlement. Therefore, the Nb component range is set to 0.
It was limited to 0.01 to 0.05%. V shows almost the same tendency as Nb, but V(C,N) precipitated during heating is more similar to Nb (
C, N), since the melting temperature is low, the primary Kl austenite grains contribute to refinement only during low-temperature heating during rail rolling. Further, V (C, N) melted by normal heating is reprecipitated during cooling, resulting in an increase in strength due to precipitation hardening.

However, if the amount of V added is less than 0.0 s %, the number of precipitates will be small and the desired effect cannot be expected. 0,20 again
Addition of more than % of V will cause embrittlement due to coarsening of V(C,N). The component range of this V is 0.05
It was limited to ~0.20%. TI is precipitated Ti (C.

It is known that N) does not melt even at high temperatures, and is effective in reducing the initial grain size of austenite even at normal rail rolling heating temperatures. However, like Nb, T1 is also 0
．． If it is less than 0.01 inch, the effect is small, and if it exceeds 0.05%, it is likely that the TiN will become coarse and become the starting point of damage, so the range of T+ components was limited to 0.01 to 0.05 inch.

Next, in the present invention, the rail steel having the above chemical composition is heated from the austenitic region after hot rolling to the apparent pearlite transformation starting temperature of 800°C at the depth of the rail head surface or five grooves below the head surface. 2-15'c/. . The subsequent pearlite transformation region is cooled continuously without interrupting the pearlite transformation while maintaining the apparent transformation start temperature to the apparent end temperature of pearlite transformation ~ 45
Forced cooling is performed between 0°C at a rate of 0.5 to 10.0°C/3ee and at a slow speed in the front cooling area. The details of the heat treatment conditions will be explained below.

In the present invention, the forced cooling temperature after hot rolling of the rail is set between 800°C and the temperature at which the light transformation starts, at the rail head surface or below the head surface at a depth of 5° to 2°.
The reason for setting it at 15'c/sec is that the cooling rate until the start of transformation has a decisive influence on suppressing the vanillite transformation temperature, which has a strong correlation with the strength of the rail. Therefore, the cooling rate from 800°C until the apparent /4'-light change signal starts is set at 2 to 15 degrees. .

Closed. The apparent start of /4'-lite transformation referred to here refers not to the inflection point of the tension curve at the time of CCTC drawing, but to the thermal analysis inflection point due to the heat of salite transformation on the cooling curve. In addition, the forced cooling referred to herein means cooling within the above temperature range at a cooling rate that exceeds normal cooling after rolling at the position of the above measurement method. This 2~15VS, l! The cooling rate range is 115kl for the head surface strength from large cross-section rails to small cross-section rails! /+m2 dust at the cooling rate required to ensure more than 2 m2, is a critical component of the invention for producing rails of a wide range of strength levels for the requirements of medium strength rails, which are different from high strength rails. . In other words, medium-strength rails are intended to replace expensive high-strength rails and be installed on curved sections where the wear environment is not very severe, or on steep curved sections coated with oil, and are a new field of rail that has emerged in recent years. and the lower limit of the cooling rate is 2℃
/sec is the lower limit value for obtaining an intermediate strength of 115 kpm2 or more. Please set the upper limit cooling rate to 15℃.
The reason why the cooling rate is limited to 15ec is that if the cooling rate is higher than this, martensite or bainite structure will be generated on the rail head surface and the rail material will be damaged. 800 for the above reasons
℃～Mikageno/? -The cooling rate to the light transformation start temperature is set to 2~b.In the subsequent coolite transformation region, while maintaining the apparent pearlite transformation start temperature, the rail cross section or chemical composition is further reduced to the apparent pearlite transformation start temperature. The heat generated by pearlite transformation, which varies depending on the cooling rate, is suppressed and the pearlite transformation is continuously cooled without interruption.

Then, on the rail head surface or at a depth less than 5 degrees below the head surface, forced cooling is performed between the apparent pearlite transformation end temperature and 450°C at a rate of 0.5 to 10.0'c/see and at a slower rate than the previous stage cooling. do. The temperature at which the pearlite transformation of Umikayu ends is defined as the temperature at which the pearlite transformation ends at the rail head surface or below the head surface.
The apparent value of 2 on the cooling curve obtained at a depth below
- the inflection point temperature, meaning the end point of light transformation. Here, forced cooling is performed for the purpose of increasing the hardening depth of the rail head, following the head surface layer which has undergone the superficial 2-light transformation.

The cooling rate range was set to 0.5 to 10.0'C/see because a cooling rate slower than 0.5'C/see would not provide sufficient hardening depth; In this case, harmful martensite is generated inside the head of the rail steel with Cr added and whose salite transformation rate has been reduced, or in the micro-segregation part that exists in the center of the rail head.

Also, the purpose of forced cooling at a slower rate than the cooling rate from 800°C to the apparent pearlite transformation start temperature in the previous stage is to ensure that the pearlite that transforms in the latter stage concentrates alloying elements including the segregated parts. Because the rate of metamorphosis decreases significantly as the degree increases.

The purpose is to complete the pearlite transformation at as high a temperature as possible and prevent the formation of bainite and martensite from the untransformed portion. Therefore, the temperature between the apparent No9-lite transformation end temperature and 450°C is 0.5 to 10.0°C, which is slower than the previous stage cooling.
Forced cooling was performed at a cooling rate of 8 ec.

Note that any of these forced cooling methods can be compressed air cooling, air/water cooling, water cooling, hot water cooling, fluidized bed cooling, or a combination thereof, and the cooling treatment is performed successively after the hot rolling is completed. At this time, forced cooling or constraint cooling of parts other than the rail head may be performed to control bending.

(Example) Examples of the present invention are shown below.

Table 1 shows the chemical compositions of the inventive steel and comparative conventional steel.

Table 2 shows the combinations of cooling rates of the steel of the present invention and comparative conventional steel. Table 3 shows the mechanical properties of rail steel obtained by these cooling methods. In addition, Fig. 1 shows the cross-sectional hardness distribution of the rail head of the inventive steel (A and B) and comparative steel (B and C), and Fig. 2 shows the cross-sectional hardness distribution of the inventive steel (AIB).
and shows the cross-sectional hardness distribution of the two-lash butt weld joint of comparative steels (C, D). 'From these figures, the steel of the present invention can be directly heat treated after rolling by adding a relatively small amount of alloy, resulting in a high-strength base material with a deeper hardening depth than the conventional reheating heat-treated rail, and also has a high strength base metal with a deeper hardening depth than the conventional reheating heat treated rail. With regard to the cross-sectional hardness of the welded joint, by combining the amount of alloy added and the cooling rate, it is possible to obtain a high-strength rail with weldability, which has a wide range of strength levels not found in conventional rails, and the hardness of the base material and the joint part are unified. Therefore, the steel of the present invention can be used not only for steep curves with the most severe installation conditions, but also for gentle curve sections that are oiled.
It is a high-strength rail with excellent wear resistance, damage resistance, and weldability that can be applied to a wide range of installation conditions.

[Brief explanation of drawings]

Figure 1 (B), (C), and @ are graphs showing the cross-sectional hardness distribution of the Renil head of the invention steels A and B and comparative examples C and D, respectively. and the flash of comparative examples C and D.
It is a graph showing the hardness distribution of a butt weld joint.

Claims (2)

[Claims] (1) Contains C: 0.55-0.85% Si: 0.20-1.20% Mn: 0.50-1.50% Cr: 0.10-0.80% in weight%, Steel, the remainder of which is iron and unavoidable impurities, is cooled from the austenitic region after rail hot rolling by cooling from 800°C to the apparent pearlite transformation start temperature at a rate of 2 to 15°C/sec, followed by subsequent pearlite transformation. While maintaining the apparent transformation start temperature of the region, the area is continuously cooled without interrupting the pearlite transformation, and the temperature between the apparent end temperature of the pearlite transformation and 450°C is 0.5 to 10°C.
．． A method for manufacturing a high-strength rail with weldability, characterized by forced cooling at 0° C./sec and at a slower rate than front and rear cooling. (2) Contains C: 0.55-0.85% Si: 0.20-1.20% Mn: 0.50-1.50% Cr: 0.10-0.80% in weight%, Nb: 0.01-0.05%, V: 0.05-0.20
%, Ti: 0.01 to 0.05% of one or more kinds, with the balance consisting of iron and inevitable impurities, from the austenite region after rail hot rolling, to 800 ° C.
~2 to 15℃/s between the apparent pearlite transformation start temperature
EC to cool the subsequent pearlite transformation region while maintaining the apparent transformation start temperature and continuously cooling the pearlite transformation without interrupting the pearlite transformation apparent end temperature to 450°C by 0.5 to 10°C. A method for manufacturing a high-strength rail with weldability, characterized by forced cooling at a rate of .0°C/sec and slower than the cooling rate of the previous stage.