AU2021218136B2

AU2021218136B2 - Pearlite steel rail with rail head hardened layer having uniform hardness gradient and preparation method thereof

Info

Publication number: AU2021218136B2
Application number: AU2021218136A
Authority: AU
Inventors: Yong Deng; Ruoxi Li; Dawei Yang; Jun Yuan
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2020-10-19
Filing date: 2021-08-19
Publication date: 2023-02-02
Anticipated expiration: 2041-08-19
Also published as: CN112410659A; AU2021218136A1

Abstract

The present disclosure relates to the technical field of steel rail production, and discloses a pearlite steel rail with a rail head hardened layer having an uniform hardness gradient and a preparation method thereof. The method comprises the successively performed steps of smelting with a revolving furnace or an electric furnace, refining with a LF furnace, RH or VD vacuum treatment, continuous casting to obtain steel billets, subjecting the steel billets to rolling, heat treatment and processing the steel rail, wherein the heat treatment is a multi-stage cooling process, and the chemical components of the steel rail are controlled, the steel rail comprises: at least one of 0.65-0.85% of C, 0.1-1% of Si, 0.1-1.5% of Mn, less than or equal to 0.03% of P, less than or equal to 0.03% of S, 0.01-0.2% of Cr, 0.005 0.15% of Ni, 0.001-0.3% of Mo and 0.002-0.2% of V, and the balance of Fe and inevitable impurities. The rail head part of the steel rail manufactured with the method has a deep hardened layer with a depth more than 25mm, and has an uniform hardness gradient. To be published with FIG. 1. 1/1 Accompanying Drawings FIG. 1

Description

1/1

Accompanying Drawings

FIG. 1

PEARLITE STEEL RAIL WITH RAIL HEAD HARDENED LAYER HAVING UNIFORM HARDNESS GRADIENT AND PREPARATION METHOD THEREOF FIELD

[0001] The present disclosure relates to the technical field of steel rail

production, in particular to a pearlite steel rail with a rail head hardened

layer having an uniform hardness gradient and a preparation method

thereof.

BACKGROUND

[0002] The railway industry of China is operating in a high-speed

development stage, the existing passenger-cargo mixed transportation lines

gradually reduce the operation of passenger trains along with an increase

of the dedicated passenger transport lines, its operation has transformed

into taking freight transportation as the dominant service, in the meanwhile,

the dedicated freight transportation lines have also developed towards the

trend of heavy load, the overall trend of the freight transportation lines is a

development towards the directions of high freight volume, heavy axle and

high traffic density. The trend has put higher requirements on the service

performance and service life of the steel rails on the line, the high efficiency

and safety of the railway freight transportation can be ensured only by improving the quality and performance of the steel rails.

[00031 At present, in order to improve the service performance and service

life of the steel rail, the main solution of the passenger-cargo mixed

transportation line and the dedicated freight transportation line is to adopt

a heat-treated pearlite steel rail with high performance, the higher wear

resistance and contact fatigue resistance is obtained by improving the

strength and hardness of the steel rail thereof, so as to reduce the abrasion

speed of the steel rail and the occurrence of damages such as surface cracks,

peeling and chipping.

[00041 In recent years, the means for enhancing hardness of the pearlite

steel rail by the steel rail manufactures at home and abroad mainly

comprises the steps of carrying out an accelerated cooling on a rail head of

the steel rail by an off-line or on-line heat treatment mode, so as to refine

the pearlite structure of a part of the rail head of the steel rail and obtain

higher strength and hardness by means of refining the crystal grains, the

related patent technologies are specified as follows:

[00051CN10468632 has a tile of invention "Rail for straight-curve

transition section and a production method of rail", it discloses that the rail

is provided with three hardness zones from a rail head to a rail head comer

area, wherein the three hardness zones have different hardness, the first

hardness zone has a lowest hardness, the second hardness zone has high

hardness, andthe third hardness zone has the highest hardness, the steel rail having different hardness in each area is obtained by using different accelerated cooling strengths during the on-line heat treatment, so as to improve the service life and safety factor of the steel rail in the straight curve transition section. However, the characteristic of different hardness in each area of the rail head in the steel rail obtained in the patent is only distributed from the surface to a shallow hardened layer, and the abrasion condition of the steel rail after a long-term use is still not optimistic.

[00061 CN104060075 has a title of invention "Heat treatment method for

improving depth of hardened layer of steel rail", it discloses a steel rail heat

treatment method for increasing depth of hardened layer of steel rail by

using rolling waste heat in an online heat treatment mode to perform

multistage accelerated cooling on the steel rail, the heat treatment method

comprises the steps of naturally cooling the steel rail after finish rolling

until the center temperature of a rail head tread is 660-730°C, then carrying

out two accelerated cooling stages with different cooling speeds, subjecting

the steel rail to an air cooling to room temperature, such that the rail head

part can obtain a deep hardened layer with a thickness exceeding 25mm,

the position which is 25mm below the surface layer of a rail head has the

hardness value equal to that of the surface layer of the rail head, a wear test

is performed to prove that the wear resistance of the steel rail in a

continuous wear process under long-term service can be improved.

However, the patent does not consider that there is a large hardness difference between the high hardness area and the unhardened area during the actual service period of the steel rail, the rail part with a large hardness difference is prone to generate defects such as cracks under the impact of train wheels during its service life in the line, the defects will impose an adverse influence on the safe service of the steel rail.

[0007] CN102220545 has a title of invention "High-carbon and high

strength heat treated steel rail with high wear resistance and plasticity and

manufacturing method thereof', it discloses a high-carbon and high

strength heat treated steel rail meeting the use requirements of heavy load

railways, the steel rail comprises the following chemical components in

percentage by weight: C: 0.80%~-1.20%, Si: 0.20%~-1.20%, Mn:

0.20 %~-1.60 %, Cr: 0.15 %~-1.20 %, V: 0.01%~0.20 %, Ti: 0.002 %~

0.050%, P0.030%, SO0.030%, AlO0.010%, NGO0.0100%andthe

balance of iron and inevitable impurities; carrying out online heat treatment

after rolling, carrying out an accelerated cooling on the rail head and the

rail bottom to 400-500°C, and then carrying out air cooling to room

temperature. The tensile strength of the rail head of the steel rail produced

by the patent is more than 1,330MPa, the hardness of the rail head is more

than 380HB, and the depth of a hardened layer is more than 25 mm.

However, the steel rail produced by the patent has high content of alloy

elements such as Cr and Ti, high production cost and poor welding

performance of the steel rail, the accelerated cooling process and equipment used herein are complex, thus it is difficult to promote the process.

[0008] In the relevant patents for improving hardness of the pearlite steel

rail at present, a part of patents can improve the hardness of a surface layer

or an internal hardened layer of the steel rail, but there is a certain gap

between the obtained steel and the actual application requirement of the

rail line; although some of the steel rails disclosed in some patents have a

deep hardened layer in the rail head, both the chemical components and the

production process of the steel rails are complex, which is not conducive

to popularization and application of said patents.

SUMMARY

[0009] The present disclosure aims to solve the problems in the prior art

concerning a large hardness difference between the high hardness area and

the unhardened area during the actual service period of the steel rail, the

rail part with a large hardness difference is prone to generate cracks under

the impact of train wheels during its service life in the line, high production

cost, the complex production process of the steel rails, and poor welding

performance of the steel rail, and provides a pearlite steel rail with a rail

head hardened layer having an uniform hardness gradient and a preparation

method thereof.

[0010] In order to achieve the above objects, a first aspect of the present disclosure provides method for preparing a pearlite steel rail with a rail head hardened layer having an uniform hardness gradient, the method comprises the successively performed steps of smelting with a revolving furnace or an electric furnace, refining with a LF furnace, RH or VD vacuum treatment, continuous casting to obtain steel billets, subjecting the steel billets to rolling, heat treatment and machining the steel rail;

[0011] wherein the heat treatment is a multi-stage cooling process, and

specifically comprises the following steps:

[0012] (1) first stage cooling: when the temperature of the running surface

of the steel rail after the finish final rolling is within a range of 760-900°C,

subjecting the running surface of the steel rail, gauge corner and field

corner of the rail head, gauge face and field face of the rail head and two

rail head comers of the rail head to an accelerated cooling treatment for 50

150 seconds at a cooling rate of 2-6°C/s;

[0013] (2) second stage cooling: subjecting the running surface of the steel

rail, gauge corner and field corner of the rail head, gauge face and field

face of the rail head and two rail head comers of the rail head of the cooled

steel rail obtained after step (1) to an accelerated cooling treatment for 20

seconds at a cooling rate of 1-5°C/s;

[0014] (3) third stage cooling: subjecting the cooled steel rail obtained

after step (2) to an air cooling to room temperature;

[0015] controlling the chemical components of the steel rail, the steel rail comprises the following chemical components in percentage by weight: at least one of 0.65-0.85% of C, 0.1-1% of Si, 0.1-1.5% of Mn, less than or equal to 0.03% of P, less than or equal to 0.03% of S, 0.01-0.2% of Cr,

0.005-0.15% of Ni, 0.001-0.3% of Mo and 0.002-0.2% of V, and the

balance of Fe and inevitable impurities.

[0016] Preferably, the steel rail comprises the following chemical

components in percentage by weight: at least one of 0.68-0.82% of C, 0.2

0.7% of Si, 0.6-1.2% of Mn, less than or equal to 0.025% of P, less than or

equal to 0.02% of S, 0.05-0.15% of Cr, 0.005-0.1% of Ni, 0.001-0.2% of

Mo and 0.02-0.1% of V, and the balance of Fe and inevitable impurities.

[0017] Preferably, a cooling medium used in the heat treatment is water

mist and/or compressed air.

[0018] Preferably, the continuous casting is a bloom billet protection

continuous casting.

[0019] Preferably, the steel billet is descaled by using high pressure water

before subjecting the steel billet to rolling.

[0020] Preferably, the rolling is carried out in an universal rolling mill.

[0021] Preferably, the air cooling is performed on a walking beam cooling

bed.

[0022] Preferably, the processing course comprises: straightening the steel

rails by using a horizontal and vertical composite straightening machine.

[0023] A second aspect of the present disclosure provides a pearlite steel rail with a rail head hardened layer having an uniform hardness gradient prepared with the aforementioned method, wherein the running surface of the steel rail has a Brinell hardness within a range of 360-400 HB; the rail head part of the steel rail has a hardened layer with a depth more than mm, and when an inner depth of the rail head increases for every 5mm, the Brinell hardness of the hardened layer decreases by 3 ~ 6 HB.

[0024] Preferably, the steel rail has a tensile strength larger than 1,200MPa

and an elongation rate greater than or equal to 10%.

[0025] The present disclosure adopts the methods of controlling the

chemical components of the steel rail and the heat treatment process, the

manufactured and produced pearlite steel rail has an uniform hardness

gradient without the need of adding a plurality of micro-alloy elements, the

running surface of the steel rail has a Brinell hardness within a range of

360-400 HB; the rail head part of the steel rail has a deep hardened layer

with a depth more than 25mm, and deep hardened layer has an uniform

hardness gradient, when an inner depth of the rail head increases for every

mm, the Brinell hardness of the hardened layer decreases by 3-6 HB on

average, the deep hardened layer can be matched with the abrasion process

and the work hardening of the steel rail during the long-term service,

thereby reduce the abrasion speed of the steel rail and the occurrence of

surface defects such as cracks, peeling and chipping of the steel rail,

improve the service performance and the service life of the steel rail, enhance the safety of train operation, and the preparation method is simple and easy to operate.

BRIEF DESCRITION OF THE DRAWING

[0026] FIG. 1 is a schematic diagram of the section hardness measurement

position of the rail head in the test example.

DETAILED DESCRPITION

[0027] The following content describes in detail the specific embodiments

of the present disclosure with reference to the drawing. It should be

understood that the specific embodiments detailed herein only serve to

explain and illustrate the present disclosure, instead of imposing a

limitation thereto.

[0028] The terminals and any value of the ranges disclosed herein are not

limited to the precise ranges or values, such ranges or values shall be

comprehended as comprising the values adjacent to the ranges or values.

As for numerical ranges, the endpoint values of the various ranges, the

endpoint values and the individual point values of the various ranges, and

the individual point values may be combined with one another to produce

one or more new numerical ranges, which should be deemed have been

specifically disclosed herein.

[0029] A first aspect of the present disclosure provides a method for preparing a pearlite steel rail with a rail head hardened layer having an uniform hardness gradient, the method comprises the successively performed steps of smelting with a revolving furnace or an electric furnace, refining with a LF furnace, RH or VD vacuum treatment, continuous casting to obtain steel billets, subjecting the steel billets to rolling, heat treatment and processing;

[0030] wherein the heat treatment is a multi-stage cooling process, and

specifically comprises the following steps:

[0031] (1) first stage cooling: when the temperature of the running surface

subjecting the running surface of the steel rail, gauge corner and field

corner of the rail head, gauge face and field corner of the rail head and two

rail head comers of the rail head to an accelerated cooling treatment for 50

150 seconds at a cooling rate of 2-6°C/s;

[0032] (2) second stage cooling: subjecting the running surface of the steel

rail, gauge corner and field corner of the rail head, gauge face and field

face of the rail head and two rail head comers of the rail head of the cooled

steel rail obtained after step (1) to an accelerated cooling treatment for 20

seconds at a cooling rate of 1-5°C/s;

[0033] (3) third stage cooling: subjecting the cooled steel rail obtained

after step (2) to an air cooling to room temperature;

[0034] controlling the chemical components of the steel rail, the steel rail comprises the following chemical components in percentage by weight: at least one of 0.65-0.85% of C, 0.1-1% of Si, 0.1-1.5% of Mn, less than or equal to 0.03% of P, less than or equal to 0.03% of S, 0.01-0.2% of Cr,

0.005-0.15% of Ni, 0.001-0.3% of Mo and 0.002-0.2% of V, and the

balance of Fe and inevitable impurities.

[0035] The reasons for limiting the content of the main chemical elements

of the steel rail according to the present disclosure are described in detail

below:

[0036] Carbon (C) is the most important and cheapest element for

obtaining desirable comprehensive mechanical property of the pearlite

steel rail and promoting pearlite transformation of the steel rail. When the

content of C is less than 0.65%, the proper strength, hardness and the wear

resistance of the steel rail cannot be ensured under the production process

disclosed by the present disclosure; when the content of C is more than

0.85%, the steel rail has excessive strength parameter, too low toughness

and plasticity and excessively high proportion of carbide under the

production process of the present application, thereby affecting the fatigue

performance of the steel rail and having an adverse effect on the safe use

of the steel rail; thus the carbon content in the present disclosure is limited

within a range of 0.65-0.85%.

[0037] The main functions of Silicon (Si) in steel reside in suppressing the

formation of cementite and being used as a solid solution strengthening element, increasing hardness of the ferrite matrix, and improving the strength and hardness of steel. When the content of Si is less than 0.10%, the solid solution amount is low, so that the strengthening effect is not obvious; when the Si content is more than 1.00%, the steel is prone to generate a local segregation, the toughness and plasticity of steel will be reduced, the safe use of the steel rail is adversely affected. Therefore, the

Si content in the present disclosure is defined within a range of 0 .10-1.00%.

[0038] Manganese (Mn) is indispensable for improving the strength of

ferrite and austenite in steel. When the Mn content is less than 0.10%, it

can hardly achieve the effect of increasing the hardness of carbide thereby

increasing the strength and hardness of steel; when the content of Mn is

more than 1.50%, it will coarsen the crystal grain size, and obviously lower

the toughness and plasticity of steel; in addition, Mn has a significant

influence on the diffusion of C in steel, an abnormal structure such as

bainite or martensite may be generated in the Mn segregation region, and

the welding performance of the steel rail is affected. Therefore, the Mn

content in the present disclosure is defined within a range of 0.10-1.50%.

[0039] Chromium (Cr) is used as a carbide forming element, can form

various carbides with carbon in steel; moreover, Cr can be used for

uniformly distributing carbide in steel, reducing the size of carbide and

improving the wear resistance of the steel rail. When the Cr content is less

than 0.01%, the hardness and the proportion of the formed carbide are lower; when the Cr content is more than 0.20%, the hardenability of the steel rail is too high, the steel rail is prone to generate harmful bainite and martensite structures, the steel rail cannot be ensured to be a pearlite structure, and the safe use of the steel rail is adversely affected. Therefore, the Cr content in the present disclosure is limited to a range of 0.01- 0 .2 0 %.

[0040] Phosphorus (P) and sulfur (S) are impurity elements which cannot

be completely removed from the steel rail. P will perform segregation at

the grain boundary of the steel rail structure, so that the toughness of the

steel rail is seriously reduced; S is easy to form MnS inclusions in steel,

which is harmful to the wear resistance and contact fatigue resistance of

the steel rail. Therefore, the content of P in the present disclosure shall be

controlled below 0.030%; the content of S shall be controlled below

0.030%.

[0041] The main role of Nickel (Ni) in steel is a solid solution

strengthening element, increasing the ferrite matrix hardness to improve

the strength and hardness of pearlite steel rails. When the Ni content is less

than 0.005%, its effect is small, and cannot produce the solid solution

strengthening effect; when the Ni content is more than 0.15%, the

toughness of the ferrite phase in the steel will be decreased, resulting in a

reduction in the fatigue resistance of the steel rail. Therefore, the N content

in the present disclosure is limited to a range of 0 .0 0 5 -0 . 15 %.

[0042] The primary functions of Molybdenum (Mo) in steel reside in improving the equilibrium transformation temperature of pearlite steel, increasing the supercooling degree and enhancing the effect of refining the pearlite lamella subjected to an accelerated cooling, so as to improve hardness of the steel rail. When the Mo content is less than 0.001%, the effect of improving the supercooling degree is small, it cannot make contribution to improving hardness of the steel rail; when the Mo content is more than 0.30%, it results in a decrease in the phase transition rate of the pearlite structure, and it is prone to produce a martensite structure which adversely affects the toughness and plasticity of the steel rail. As a result, the Mo content in the present disclosure is defined within a range of

0.001-0.30%.

[0043] Vanadium (V) is a precipitation strengthening element in pearlite

steel, forms carbonitride in the cooling process of the steel rail, and

improves the strength and hardness of the steel rail, thereby enhancing

wear resistance of the steel rail. When the content of V is less than 0.002%,

the precipitation strengthening degree is too small to play a desired role;

when the content of V is more than 0.20%, its capability of improving the

strength and the hardness of the steel rail is reduced, and excessive

precipitation strengthening effect may cause excessive precipitated phases

in steel and have negative effect on the toughness of the steel rail. Therefore,

the V content in the present disclosure is limited within a range of 0.002

0.20%.

[0044] In a preferred embodiment, the steel rail comprises the following

chemical components in percentage by weight: at least one of 0.68-0.82%

of C, 0.2-0.7% of Si, 0.6-1.2% of Mn, less than or equal to 0.025% of P,

less than or equal to 0.02% of S, 0.05-0.15% of Cr, 0.005-0.1% of Ni,

0.001-0.2% ofMoand0.02-0.1% ofV, and the balance of Fe and inevitable

impurities.

[0045] The inventors of the present disclosure have found through a large

amount of researches that:

[0046] (1) In regard to the first stage cooling: when the temperature of the

running surface of the steel rail is within a range of 760-900°C, the steel

rail shall be subjected to an accelerated cooling at a high temperature stage

so as to inhibit precipitation of proeutectoid ferrite or proeutectoid

cementite in the steel rail and obtain the steel rail with uniform hardness

gradient; in addition, because the starting temperature of the cooling

process is high, a larger cooling rate is required to ensure the stable

performance of the interior of the rail head, and the cooling rate needs to

be controlled within a range of 2.0-6.0°C/s;

[0047] (2) In regard to the second stage cooling: after the steel rail is

subjected to the first-stage accelerated cooling for 50-150 seconds, the

internal temperature of the rail running surface of the steel rail is still within

a range of 500-600°C, the phase change of the steel rail is still performed

in the temperature range, if the accelerated cooling is stopped, the heat of the part of steel rail, which is not subjected to the accelerated cooling, can be quickly diffused to the rail head, the phase change cooling rate is reduced, such that the hardness of the final product steel rail is reduced, the hardness gradient is adversely affected, and the effect of a heat treatment process is insufficient; in the meanwhile, it is required to carry out the accelerated cooling of the second stage, and considering that the cooling rate of the central part of the rail head is lower than that of the surface at this time, if the cooling rate is adopted continuously at the same level as that of the first stage, the risk of abnormal structures on the surface is high, and it is difficult to obtain the uniform hardness gradient, so that the cooling intensity and the cooling time shall be reduced to obtain the pearlite steel rail with an uniform hardness gradient.

[0048] (3) In regard to the third stage cooling: after the first two cooling

stages are finished, the internal temperature of the rail head of the steel rail

shall be within a range of 400-450°C, the phase change of the steel rail has

been completed at the moment, there is not obvious significance to

continue the accelerated cooling process, the steel rail can be subjected to

an air cooling to room temperature to facilitate the subsequent process

treatment.

[0049] In a specific embodiment, a first stage accelerated cooling in step

(1) may be performed when the temperature of the running surface of the

steel rail after the finish final rolling is 760°C, 780°C, 800°C, 820°C,

840°C,860°C,880°C,or900°C.

[0050] In a specific embodiment, the cooling rate of the accelerated

cooling in step (1) may be 2°C/s, 3C/s, 4°C/s, 5°C or 6°C/s.

[0051] In a specific embodiment, the time for the accelerated cooling in

step (1) may be 50s, 60s, 70s, 80s, 90s, 100s, 10s, 120s, 130s, 140s or 150

s.

[0052] In a specific embodiment, the cooling rate of the accelerated

cooling in step (2) may be 1C, 2°C, 3C, 4C, or 5°C.

[0053] In a specific embodiment, the time for accelerated cooling in step

(2) may be 20s, 30s, 40s, 50s or 60s.

[0054] In a preferred embodiment, the cooling medium used in the heat

treatment is water mist and/or compressed air.

[0055] In a preferred embodiment, the continuous casting is a bloom billet

protection continuous casting.

[0056] In a preferred embodiment, the steel billet is descaled by using high

pressure water before subjecting the steel billet to rolling.

[0057] In a preferred embodiment, the rolling is carried out in an universal

rolling mill.

[0058] In a preferred embodiment, the air cooling is performed on a

walking beam cooling bed.

[0059] In a preferred embodiment, the processing course comprises:

straightening the steel rails by using a horizontal and vertical composite straightening machine.

[0060] In a specific embodiment, the method for preparing a pearlite steel

rail with a rail head hardened layer having an uniform hardness gradient

comprises the successively performed steps on low-sulfur molten steel of

smelting with a revolving furnace or an electric furnace, refining with a LF

furnace, RH or VD vacuum treatment, bloom billet protection continuous

casting, heating the steel billet with a heating furnace, descaling the steel

billet by using high pressure water before subjecting the steel billet to

rolling, carrying out rolling with an universal rolling mill, subjecting the

steel rail to an online heat treatment, air cooling with a walking beam

cooling bed at room temperature, straightening the steel rails by using a

horizontal and vertical composite straightening machine, inspecting

specification of the steel rails, treating the steel rails on a processing line,

surface inspection, and transporting the steel rails to a warehouse.

[0061] A second aspect of the present disclosure provides a pearlite steel

rail with a rail head hardened layer having an uniform hardness gradient

prepared with the aforementioned method, wherein the running surface of

the steel rail has a Brinell hardness within a range of 360-400 HB; the rail

head part of the steel rail has a hardened layer with a depth more than

mm, and the deep hardened layer has an uniform hardness gradient;

when an inner depth of the rail head increases for every 5mm, the Brinell

hardness of the hardened layer decreases by 3 ~ 6 HB; namely the Brinell hardness of the hardened layer at an inner depth of 5mm in the rail head is within a range of 355-397HB, the Brinell hardness of the hardened layer at an inner depth of 10mm in the rail head is within a range of 350-393HB, the Brinell hardness of the hardened layer at an inner depth of 15mm in the rail head is within a range of 345-390HB, the Brinell hardness of the hardened layer at an inner depth of 20mm in the rail head is within a range of 340-387HB, the Brinell hardness of the hardened layer at an inner depth of 25mm in the rail head is within a range of 335-383HB.

[0062] Preferably, the steel rail has a tensile strength larger than 1,200MPa

and an elongation rate greater than or equal to 10%.

[0063] The present disclosure will be described in detail below with

reference to examples, but the scope of the present disclosure is not limited

thereto.

[0064] Example 1

[0065] The specific process for preparing the steel rail comprised the

following steps:

[0066] smelting with an electric furnace, refining with a LF furnace, RH

vacuum treatment, performing a bloom billet protection continuous casting

to obtain steel billets, descaling the steel billets by using high pressure

water before subjecting the steel billets to rolling, rolling the steel billets in

an universal rolling mill, heat treatment and processing of the steel rail

were sequentially carried out;

[0067] wherein the cooling medium for heat treatment was compressed air,

and the heat treatment specifically comprised the following steps:

[00681 (1) First stage cooling: when the temperature of the running surface

of the steel rail after the finish final rolling was 855°C, the running surface

of the steel rail, gauge corner and field corner of the rail head, gauge face

and field face of the rail head and two rail head comers of the rail head

were subjected to an accelerated cooling treatment for 150 seconds at a

cooling rate of 2.1°C/s;

[0069] (2) Second stage cooling: the running surface of the steel rail, gauge

corner and field comer of the rail head, gauge face and field face of the rail

head and two rail head comers of the rail head of the cooled steel rail

obtained after step (1) were subjected to an accelerated cooling treatment

for 50 seconds at a cooling rate of 1.6°C/s;

[0070] (3) Third stage cooling: the cooled steel rail obtained after step (2)

was placed on a walking beam cooling bed and subjected to an air cooling

to room temperature;

[0071] The chemical components of the steel rail were as shown in Table

1.

[0072] Example 2

[0073] The specific process for preparing the steel rail comprised the

following steps:

[0074] smelting with an electric furnace, refining with a LF furnace, RH vacuum treatment, performing a bloom billet protection continuous casting to obtain steel billets, descaling the steel billets by using high pressure water before subjecting the steel billets to rolling, rolling the steel billets in an universal rolling mill, heat treatment and processing of the steel rail were sequentially carried out;

[0075] wherein the cooling medium for heat treatment was compressed air,

and the heat treatment specifically comprised the following steps:

[0076] (1) First stage cooling: when the temperature of the running surface

of the steel rail after the finish final rolling was 792°C, the running surface

of the steel rail, gauge corner and field corner of the rail head, gauge face

and field face of the rail head and two rail head comers of the rail head

were subjected to an accelerated cooling treatment for 85 seconds at a

cooling rate of 3.9°C/s;

[0077] (2) Second stage cooling: the running surface of the steel rail, gauge

corner and field comer of the rail head, gauge face and field face of the rail

head and two rail head comers of the rail head of the cooled steel rail

obtained after step (1) were subjected to an accelerated cooling treatment

for 35 seconds at a cooling rate of 2.7°C/s;

[0078] (3) Third stage cooling: the cooled steel rail obtained after step (2)

was placed on a walking beam cooling bed and subjected to an air cooling

to room temperature;

[0079] The chemical components of the steel rail were as shown in Table

1.

[0080] Example 3

[0081] The specific process for preparing the steel rail comprised the

following steps:

[0082] smelting with an electric furnace, refining with a LF furnace, RH

vacuum treatment, performing a bloom billet protection continuous casting

to obtain steel billets, descaling the steel billets by using high pressure

an universal rolling mill, heat treatment and processing of the steel rail

were sequentially carried out;

[0083] wherein the cooling medium for heat treatment was compressed air

and water mist, and the heat treatment specifically comprised the following

steps:

[0084] (1) First stage cooling: when the temperature of the running surface

of the steel rail after the finish final rolling was 846°C, the running surface

of the steel rail, gauge corner and field corner of the rail head, gauge face

and field face of the rail head and two rail head comers of the rail head

were subjected to an accelerated cooling treatment for 150 seconds at a

cooling rate of 5.6°C/s;

[0085] (2) Second stage cooling: the running surface of the steel rail, gauge

corner and field comer of the rail head, gauge face and field face of the rail

head and two rail head comers of the rail head of the cooled steel rail obtained after step (1) were subjected to an accelerated cooling treatment for 25 seconds at a cooling rate of 4°C/s;

[0086] (3) Third stage cooling: the cooled steel rail obtained after step (2)

was placed on a walking beam cooling bed and subjected to an air cooling

to room temperature;

[0087] The chemical components of the steel rail were as shown in Table

1.

[0088] Example 4

[0089] The steel rail was prepared according to the same method in

Example 1, except that the chemical components of the steel rail were

different, as shown in Table 1.

[0090] Example 5

[0091] The steel rail was prepared according to the same method in

Example 2, except that the starting temperature of the accelerated cooling

in step (1) is 760°C.

[0092] Comparative Example 1

[0093] The steel rail was prepared according to the same method in

Example 1, except that the heat treatment specifically comprised the

following steps:

[0094] (1) First stage cooling: when the temperature of the running surface

of the steel rail after the finish final rolling was 860°C, the running surface

of the steel rail, gauge corner and field corner of the rail head, gauge face and field face of the rail head and two rail head comers of the rail head were subjected to an accelerated cooling treatment for 210 seconds at a cooling rate of 1.8°C/s;

[0095] (2) Third stage cooling: the cooled steel rail obtained after step (1)

was placed on a walking beam cooling bed and subjected to an air cooling

to room temperature.

[0096] Comparative Example 2

[0097] The steel rail was prepared according to the same method in

Example 2, except that the heat treatment specifically comprised the

following steps:

[0098] (1) First stage cooling: when the temperature of the running surface

of the steel rail after the finish final rolling was 775°C, the running surface

of the steel rail, gauge corner and field corner of the rail head, gauge face

and field face of the rail head and two rail head comers of the rail head

were subjected to an accelerated cooling treatment for 190 seconds at a

cooling rate of 2°C/s;

[0099] (2) Third stage cooling: the cooled steel rail obtained after step (1)

was placed on a walking beam cooling bed and subjected to an air cooling

to room temperature.

[00100] Comparative Example 3

[00101] The steel rail was prepared according to the same method in

Example 3, except that the heat treatment specifically comprised the following steps:

[001021 (1) First stage cooling: when the temperature of the running

surface of the steel rail after the finish final rolling was 838°C, the running

surface of the steel rail, gauge corner and field corner of the rail head,

gauge face and field face of the rail head and two rail head corners of the

rail head were subjected to an accelerated cooling treatment for 170

seconds at a cooling rate of 2.4°C/s;

[001031 (2) Third stage cooling: the cooled steel rail obtained after step

(1) was placed on a walking beam cooling bed and subjected to an air

cooling to room temperature.

[001041 Table 1

Example 1 Example 2 Example 3 Example 4

C(%) 0.78 0.76 0.8 0.81

Si(%) 0.47 0.5 0.46 0.55

Mn(%) 1.11 1.13 1.08 0.74

Cr(%) 0.12 0.1 0.14 0.08

P(%) 0.012 0.02 0.01 0.012

S(%) 0.014 0.008 0.013 0.004

Ni(%) - - 0.02 0.01

Mo(%) - 0.01 -

V(%) 0.01 - -

Fe + Balance Balance Balance Balance inevitable impurities (%)

[001051 Test Example

[00106] The tensile properties and the section hardness of the rail head

of the Examples and Comparative Examples were tested according to the

HBW 2.5/187.5 test force scheme of the Standard CB/T231.1-2018

"Brinell hardness Test of Metal Material, Part I: Test Method", wherein the

measurement position of section hardness of the rail head were shown in

FIG. 1, the Brinell hardness measurement was carried out on three lines A,

B and C, the distance between measurement points of hardness was 5mm,

the first point was 5mm away from the rail surface, and the measurement

depth was up to 25 mm. The results were shown in Table 2.

[001071 Table 2 Fracture section hardness /HB Hardness Tensile Elongat of rail Numbers strength ionrate running Sites 5mm10mm15mm20mm25mm /MPa /% surface /HB A 360 357 351 348 344 Example B 362 357 355 351 347 1237 10.5 366 C 360 363 357 353 350 A 379 374 369 363 359 Example B 378 367 365 356 351 1290 10.0 379 2 C 380 373 369 364 357 A 382 379 373 369 362 Example B 381 375 376 367 364 1287 10.0 383 C 386 383 378 375 367 Comparati A 358 365 364 351 341 1228 10.5 362 ve B 351 359 350 336 328

Example C 362 364 359 347 339 1 Comparati A 373 374 375 363 361 ve B 368 376 368 357 350 1281 10.5 375 Example 2 C 369 377 369 364 359 Comparati A 373 380 369 372 367 ve B 374 374 370 367 356 1274 10.5 381 Example 3 C 373 386 380 375 366

[00108] The detection results in Table 2 demonstrates that the Brinell

hardness of the running surface of the steel rail prepared with the method

of the present disclosure is within a range of 360-400HB, the rail head part

of the steel rail is provided with a deep hardened layer with a depth more

than 25mm, the deep hardened layer of the steel rail has a more uniform

hardness gradient than the steel rail prepared in the Comparative Examples.

[00109] The above content describes in detail the preferred embodiment

of the present disclosure, but the present disclosure is not limited thereto.

A variety of simple modifications can be made in regard to the technical

solutions of the present disclosure within the scope of the technical concept

of the present disclosure, including a combination of individual technical

features in any other suitable manner, such simple modifications and

combinations thereof shall also be regarded as the content disclosed by the

present disclosure, each of them falls into the protection scope of the

present disclosure.

Claims

Claims 1. A method for preparing a pearlite steel rail with a rail head hardened

layer having an uniform hardness gradient, the method comprises the

successively performed steps of smelting with a revolving furnace or an

electric furnace, refining with a LF furnace, RH or VD vacuum treatment,

continuous casting to obtain steel billets, subjecting the steel billets to

rolling, heat treatment and processing the steel rail;

wherein the heat treatment is a multi-stage cooling process, and

specifically comprises the following steps:

(1) first stage cooling: when the temperature of the running surface of the

steel rail after the finish final rolling is within a range of 760-900°C,

subjecting the running surface of the steel rail, gauge corner and field

corner of the rail head, gauge face and field face of the rail head and two

rail head comers of the rail head to an accelerated cooling treatment for 50

150 seconds at a cooling rate of 2-6°C/s;

(2) second stage cooling: subjecting the running surface of the steel rail,

gauge corner and field corner of the rail head, gauge face and field face of

the rail head and two rail head corners of the rail head of the cooled steel

rail obtained after step (1) to an accelerated cooling treatment for 20-60

seconds at a cooling rate of 1-5°C/s;

(3) third stage cooling: subjecting the cooled steel rail obtained after step

(2) to an air cooling to room temperature;

controlling the chemical components of the steel rail, the steel rail

comprises the following chemical components in percentage by weight: at

least one of 0.65-0.85% of C, 0.1-1% of Si, 0.1-1.5% of Mn, less than or

equal to 0.03% of P, less than or equal to 0.03% of S, 0.01-0.2% of Cr,

0.005-0.15% of Ni, 0.001-0.3% of Mo and 0.002-0.2% of V, and the

balance of Fe and inevitable impurities.
2. The method of claim 1, wherein the steel rail comprises the following

chemical components in percentage by weight: at least one of 0.68-0.82%

of C, 0.2-0.7% of Si, 0.6-1.2% of Mn, less than or equal to 0.025% of P,

less than or equal to 0.02% of S, 0.05-0.15% of Cr, 0.005-0.1% of Ni,

0.001-0.2% ofMoand0.02-0.1% ofV, and the balance of Fe and inevitable

impurities.
3. The method of claim 1 or 2, wherein the cooling medium used in the

heat treatment is water mist and/or compressed air.
4. The method of claim 1 or 2, wherein the continuous casting is a bloom

billet protection continuous casting.
5. The method of claim 1 or 2, wherein the steel billet is descaled by using

high pressure water before subjecting the steel billet to rolling.
6. The method of claim 1 or 2, wherein the rolling is carried out in an

universal rolling mill.
7. The method of claim 1 or 2, wherein the air cooling is performed on a walking beam cooling bed.
8. The method of claim orclaim 2, wherein the processing course

comprises: straightening the steel rails by using a horizontal and vertical

composite straightening machine.
9. A pearlite steel rail with a rail head hardened layer having an uniform

hardness gradient prepared with the method of any one of claims 1 to 8,

wherein the running surface of the steel rail has a Brinell hardness within

a range of 360-400 HB; the rail head part of the steel rail has a hardened

layer with a depth more than 25mm, and when an inner depth of the rail

head increases for every 5mm, the Brinell hardness of the hardened layer

decreases by 3 - 6 HB.
10. The pearlite steel rail of claim 9, wherein the steel rail has a tensile

strength larger than 1,200MPa and an elongation rate greater than or equal

to 10%.