AU2020364505B2

AU2020364505B2 - Rail and method for manufacturing same

Info

Publication number: AU2020364505B2
Application number: AU2020364505A
Authority: AU
Inventors: Keisuke Ando; Kazuya Tokunaga
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-10-11
Filing date: 2020-07-27
Publication date: 2023-08-03
Anticipated expiration: 2040-07-27
Also published as: JPWO2021070452A1; JP7063400B2; EP4023777A4; CA3157401A1; AU2020364505A1; EP4023777A1; US20230250505A1; CN114502761B; CA3157401C; WO2021070452A1; CN114502761A

Abstract

A rail 1 comprises a foot section 2, a web section 3, and a head section 4. The component composition of the web section 3 includes C: 0.70-1.20 wt%, Si: 0.20-1.20 wt%, Mn: 0.20-1.50 wt%, P: 0.035 wt% or less, and Cr: 0.20-2.50 wt%, with the remainder comprising Fe and unavoidable impurities. The area ratio of pearlite in the web section 3 is 95% or more, and the average size of the pearlite blocks is 60 µm or less.

Description

Title of Invention: RAIL AND METHOD FOR PRODUCING THE SAME

Technical Field

[0001]

The present invention relates to a railroad rail having

a foot, a web, and a head and to a method for producing the

rail.

Background Art

[0002]

In heavy haul railroads mainly used to transport ore

and other materials, the load on the axles of freight cars

is much higher than that on the axles of passenger cars, and

rails are used in harsh environments. The efficiency of

transportation in railroads has been improved by further

increasing the carrying capacity of freight cars. There is

thus a need for improvements in wear resistance, fatigue

damage resistance, and delayed fracture resistance.

[0003]

There have been various proposes for improving the wear

resistance and other properties of rails, such as

controlling the materials of rails, or using a special heat

treatment in the production method (e.g., see Patent

Literature 1 to Patent Literature 8). Patent Literature 1

and Patent Literature 2 each disclose a rail with its wear

resistance improved by increasing the C content to more than

0.85 mass% and 1.20 mass% or less. Patent Literature 3 and

Patent Literature 4 each disclose a rail with its wear

resistance improved by setting the C content to more than

0.85 mass% and 1.20 mass% or less and increasing the

cementite fraction by heating the head of the rail.

[0004]

Patent Literature 5 proposes a rail with its fatigue

damage resistance improved by suppressing formation of pro

eutectoid cementite by addition of Al and Si. Patent

Literature 6 discloses a rail with its service life improved

by setting, to Hv 370 or higher, the Vickers hardness in a

region from the surface of the corners and top of the head

of the rail to a depth of at least 20 mm.

[0005]

Patent Literature 7 discloses a method for forming a

tempered martensite microstructure having high toughness in

a web. The method includes rapidly cooling the web at a

cooling rate of 15 °C/sec or higher, then stopping cooling

at a temperature of 250 to 4500C, and when the bainite

transformation reaches 30% or more, cooling the web to the

Ms temperature or lower, forming a martensite

microstructure. Patent Literature 8 discloses that the

crack growth resistance of the web is provided by imparting

compressive residual stress by cooling a rail from the top

of the head to the upper neck or to the web with high pressure gas or water-containing gas.

Citation List

Patent Literature

[0006]

PTL 1: Japanese Unexamined Patent Application

Publication No. 8-109439

PTL 2: Japanese Unexamined Patent Application

Publication No. 8-144016

PTL 3: Japanese Unexamined Patent Application

Publication No. 8-246100

PTL 4: Japanese Unexamined Patent Application

Publication No. 8-246101

PTL 5: Japanese Unexamined Patent Application

Publication No. 2002-69585

PTL 6: Japanese Unexamined Patent Application

Publication No. 10-195601

PTL 7: Japanese Unexamined Patent Application

Publication No. 62-99438

PTL 8: Japanese Unexamined Patent Application

Publication No. 59-47326

[0007]

According to the rails in Patent Literature 1 to Patent

Literature 6, the head of each rail which mainly come into contact with wheel flanges has high wear resistance.

However, the materials of the web of the rail are not

sufficiently controlled, and the web may undergo crack

growth depending on production method.

[0008]

The technique of Patent Literature 7 requires

maintaining the temperature until bainite transformation

starts, which reduces production efficiency. The technique

disclosed in Patent Literature 8 puts the most importance on

the wear resistance/fatigue damage resistance of the head

and may not provide the web with desired crack growth

resistance, and the martensite microstructure highly

susceptible to cracking may be formed depending on

production conditions.

[0009]

The present invention has been made in light of the

above circumstances. The present invention therefore seeks

to provide a rail and a method for producing the rail in

which rail breakage is prevented by suppressing web crack

growth while the production efficiency is improved.

Summary of the Invention

[0010]

The gist of the present invention is as described

below.

[1] A rail includes a foot, a web, and a head, wherein the web has a chemical composition containing:

C: 0.70 to 1.20 mass%,

Si: 0.20 to 1.20 mass%,

Mn: 0.20 to 1.50 mass%,

P: 0.035 mass% or less,

S: 0.0005 to 0.012 mass%, and

Cr: 0.20 to 2.50 mass%, with the balance being Fe and

incidental impurities,

an area fraction of pearlite in the web is 95% or more,

and

an average size of pearlite blocks is 30 pm or less.

Also described is 60 pm or less.

[2] In the rail according to [1], the chemical composition

further contains one or two or more selected from Cu: 1.0

mass% or less, Ni: 1.0 mass% or less, Nb: 0.05 mass% or

less, Mo: 1.0 mass% or less, V: 0.005 to 0.10 mass%, W: 1.0

mass% or less and B: 0.005 mass% or less.

[3] In the rail according to [1] or [2], a crack growth rate

da/dN (m/cycle) in the web at a stress intensity factor AK =

1 2 MPa-m is 8.0 x 10-8 or less.

[4] A rail production method for producing the rail

according to any one of [1] to [3] includes:

performing finish rolling at a finishing temperature of

1000°C or lower in such a manner that a reduction in area of

a web is 10% or more; and after finish rolling, cooling the web at a cooling rate of 1 to 5 °C/s from a temperature higher than or equal to a pearlite transformation start temperature to a temperature range of 400 0 C to 600 0 C.

[5] In the rail production method according to [4], wherein

the finishing temperature in finish-rolling the web is in a

range of 800 0 C to 900 0 C.

[0011]

According to the present invention, it is possible to

reduce the crack growth rate in the web and thus to suppress

the crack growth in the web and the breakage of the rail.

Brief Description of Drawings

[0012]

[Fig. 1] Fig. 1 is a perspective view of a rail

according to a preferred embodiment of the present

invention.

[Fig. 2] Fig. 2 is a plan view of the rail according to

the preferred embodiment of the present invention.

[Fig. 3] Fig. 3 is a schematic view of an example rail

production system used in a rail production method according

to the present invention.

[Fig. 4] Fig. 4 is a schematic view of an example test

specimen used in a web crack growth test.

Description of Embodiments

[0013]

Embodiments of the present invention will be described

below. Fig. 1 is a schematic view of a rail according to a

preferred embodiment of the present invention. A rail 1 in

Fig. 1 supports the load for passenger railroads or freight

railroads and guides railroad cars in the running direction

(direction of arrow Y). The rail 1 has a foot (bottom) 2, a

web 3, and a head 4.

[0014]

The foot 2 will be placed on railroad ties and has a

cross section widening in the width direction (direction of

arrow X). The web 3 has a shape extending vertically

(direction of arrow Z) from the foot 2 and has a function of

ensuring bending stiffness as a beam of the rail 1 itself.

The head 4, which is located on the web 3, comes into

contact with the wheels of trains and directly supports the

load of the trains. As a train runs on the rail 1, the load

from the wheels of the train is transmitted from the head 4

to the web 3 and from the web 3 to the foot 2.

[0015]

Since the web 3 does not directly come into contact

with the wheels unlike the head 4, the web 3 does not need

to have wear resistance equivalent to that of the head 4.

The web 3 transmits the wheel weight on the head 4 to the foot 2. When the wheel weight is applied eccentrically from the center of the head 4 in the width direction, the web 3 may undergo bending stress to cause horizontal cracks. For this, the web 3 needs to have high crack growth resistance.

The web 3 of the rail 1 thus has the following chemical

composition and steel microstructure.

[0016]

The rail 1 contains C: 0.70 to 1.20 mass%, Si: 0.20 to

1.20 mass%, Mn: 0.20 to 1.50 mass%, P: 0.035 mass% or less,

S: 0.0005 to 0.012 mass%, and Cr: 0.20 to 2.50 mass%. The

composition components will be described below separately.

[0017]

C: 0.70 to 1.20 mass%

Carbon C is an essential element for ensuring the

strength, or fatigue damage resistance, of a pearlite

microstructure. The fatigue damage resistance increases as

the C content increases. With a C content of less than 0.70

mass%, it is difficult to provide higher fatigue damage

resistance than that of a conventional heat-treated type

pearlite steel rail. With a C content of more than 1.20

mass%, considerable amount of pro-eutectoid cementite is

formed at austenite grain boundaries during pearlite

transformation after hot rolling, and the fatigue damage

resistance remarkably decreases. Pro-eutectoid cementite is

also found when the C content is 1.20 mass% or less, but it is formed in trace amounts. Pro-eutectoid cementite thus has a minor effect on the fatigue damage resistance.

Therefore, the C content is 0.70 to 1.20 mass%. The C

content is preferably 0.75 to 1.00 mass%. The C content is

more preferably 0.75 to 0.85 mass%.

[0018]

Si: 0.20 to 1.20 mass%

Silicon Si needs to be contained in an amount of 0.20

mass% or more to serve as a deoxidizer and an element that

strengthens the pearlite microstructure. The presence of

more than 1.20 mass% Si promotes generation of surface

defects on rails. Therefore, the Si content is 0.20 to 1.20

mass%. The Si content is preferably 0.50 to 1.00 mass%.

[0019]

Mn: 0.20 to 1.50 mass%

Manganese Mn is an element effective for maintaining

high hardness inside the rail 1 since it has an effect of

lowering the pearlite transformation temperature to make the

interlamellar spacing fine. With a Mn content of less than

0.20 mass%, the above effect is not enough. With a Mn

content of more than 1.50 mass%, a martensite microstructure

tends to be formed, and hardening and brittleness tend to

occur during heat treatment and welding to degrade material

properties. Further, due to Mn increasing hardenability,

more bainite microstructure is formed on the surface layer of an internal high hardness-type rail to degrade the wear resistance. Furthermore, addition of excess Mn lowers the pearlite equilibrium transformation temperature and decreases the degree of supercooling to make the interlamellar spacing coarse. Therefore, the Mn content is

0.20 to 1.50 mass%. The Mn content is preferably 0.40 to

1.20 mass%.

[0020]

P: 0.035 mass% or less

The presence of more than 0.035 mass% P results in low

ductility. Therefore, the P content is 0.035 mass% or less.

The P content is preferably 0.020 mass% or less. If the P

content is less than 0.001%, the steelmaking costs

unavoidably increase. A P content of 0.001% or more is

therefore acceptable.

[0021]

S: 0.0005 to 0.012 mass%

Sulfur S is found in a steel material mainly in the

form of A type inclusions. With a S content of more than

0.012 mass%, the amount of the inclusions significantly

increases, and coarse inclusions are formed at the same

time, which lowers the cleanliness of the steel material.

To reduce the S content to less than 0.0005 mass%, the

steelmaking costs increase. Therefore, the S content is

0.0005 to 0.012 mass%. The S content is preferably 0.0005 to 0.010 mass%. The S content is more preferably 0.0005 to

0.008 mass%.

[0022]

Cr: 0.20 to 2.50 mass%

Chromium Cr is an element that raises the pearlite

equilibrium transformation temperature to make the

interlamellar spacing fine and also increases the strength

through solid solution strengthening. However, enough

internal hardness is not obtained with a Cr content of less

than 0.20 mass%. Addition of more than 2.50 mass% Cr

increases hardenability and tends to form a martensite

microstructure. Moreover, under production conditions where

no martensite microstructure is formed, pro-eutectoid

cementite is formed at prior austenite grain boundaries.

This results in low wear resistance and low fatigue damage

resistance. Therefore, the Cr content is 0.20 to 2.50

mass%. The Cr content is preferably 0.60 to 1.30 mass%.

[0023]

In addition to these composition components, the

chemical composition of the rail according to the present

invention may further contains one or two or more selected

from Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.05

mass% or less, Mo: 1.0 mass% or less, V: 0.005 to 0.10

mass%, W: 1.0 mass% or less and B: 0.005 mass% or less. The

composition components will be described below separately.

[0024]

Cu: 1.0 mass% or less

Copper Cu is an element that can further strengthen the

steel through solid solution strengthening like Cr. The

presence of more than 1.0 mass% Cu tends to cause Cu

cracking. Therefore, when the chemical composition contains

Cu, the Cu content is preferably 1.0 mass% or less. The Cu

content is more preferably 0.005 to 0.5 mass%.

[0025]

Ni: 1.0 mass% or less

Nickel Ni is an element that can strengthen the steel

without degrading the ductility. When the rail 1 contains

Cu, Ni is preferably added in combination with Cu because

addition of Ni can prevent or reduce Cu cracking. If the Ni

content exceeds 1.0 mass%, the steel hardenability further

increases, and martensite tends to be formed, which results

in low fatigue damage resistance. Therefore, when Ni is

contained, the Ni content is preferably 1.0 mass% or less.

The Ni content is more preferably 0.005 to 0.5 mass%.

[0026]

Nb: 0.05 mass% or less

Niobium Nb combines with C in the steel and

precipitates as a carbide during and after hot rolling for

forming a rail, which effectively reduces the prior

austenite grain size. As a result, the resistance, the fatigue damage resistance, and the ductility are greatly improved to extend the service life of the rail. With a Nb content of more than 0.05 mass%, the effect of improving the wear resistance and the fatigue damage resistance is saturated, and the effect corresponding to an increase in content is not obtained. Therefore, the upper limit of the

Nb content may be 0.05 mass%. With a Nb content of less

than 0.001 mass%, it is difficult to obtain a sufficient

effect of extending the service life of the rail. The

presence of 0.001 mass% or more Nb provides an effect of

extending the service life. Therefore, when Nb is

contained, the Nb content is preferably 0.001 mass% or more.

The Nb content is more preferably 0.001 mass% to 0.03 mass%.

[0027]

Mo: 1.0 mass% or less

Molybdenum Mo is an element that can improve

hardenability and can further strengthen the steel through

solid solution strengthening. If the Mo content exceeds 1.0

mass%, martensite tends to be formed in the steel, which

results in low wear resistance and low fatigue damage

resistance. Therefore, when the chemical composition of the

rail contains Mo, the Mo content is preferably 1.0 mass% or

less. The Mo content is more preferably 0.005 to 0.5 mass%.

[0028]

V: 0.005 to 0.10 mass%

Vanadium V is an element that forms a carbonitride and

is dispersedly precipitated in the matrix to improve the

fatigue damage resistance and the delayed fracture

resistance. With a V content of less than 0.005 mass%, the

above effect is not enough. The presence of more than 0.10

mass% V results in low workability and high alloy costs and

thus increases the costs for producing the rail material.

Therefore, the V content is 0.005 mass% to 0.10 mass% or

less. The V content is preferably 0.01 to 0.08 mass%.

[0029]

W: 1.0 mass% or less

Tungsten W is an element that precipitates as a carbide

during and after hot rolling for forming a rail shape and

improves the strength and ductility of the rail through

precipitation strengthening. If the W content exceeds 1.0

mass%, martensite is formed in the steel, resulting in low

ductility. Therefore, when W is added, the W content is

preferably 1.0 mass% or less. The lower limit of the W

content is not limited, but preferably 0.001 mass% or more

in order to provide the effect of improving the strength and

the ductility. The W content is more preferably 0.005 to

0.5 mass%.

[0030]

B: 0.005 mass% or less

Boron B is an element that segregates at prior

austenite grain boundaries and improves hardenability to

improve the strength of the rail. If the B content exceeds

0.005 mass%, a martensite microstructure is formed,

resulting in low wear resistance and low fatigue damage

resistance. Therefore, when B is contained, the B content

is preferably 0.005 mass% or less. The lower limit of the B

content is not limited, but preferably 0.001 mass% or more

in order to provide the effect of improving the strength and

the ductility. The B content is more preferably 0.001 to

0.003 mass%.

[0031]

The balance other than the above composition components

in the rail 1 includes Fe and incidental impurities.

Incidental impurities refer to impurities that are found in

raw materials or incidentally incorporated during the

production process and are basically unnecessary but allowed

to be contained because they are found in trace amounts and

do not affect the properties. Examples of incidental

impurities include N and 0. An N content of up to 0.0080

mass% is allowable, and an 0 content of up to 0.004 mass% is

allowable. Titanium Ti forms an oxide and degrades the

fatigue damage resistance, which is a fundamental feature of

the rail, and the Ti content is thus preferably controlled

at 0.0010 mass% or less.

[0032]

The web 3 of the rail 1 includes 95% or more area

fraction of pearlite microstructure. The web 3 of the rail

1 may include trace amounts, or 5% or less in total, of

bainite microstructure, martensite microstructure, pro

eutectoid cementite microstructure, and pro-eutectoid

ferrite microstructure. The pearlite microstructure

(pearlite blocks) is a lamellar microstructure composed of

alternating layers of ferrite and cementite, and the

pearlite blocks are composed of pearlite grains having the

same orientation. There is a relationship between the

pearlite microstructure and the crack growth, and the

pearlite grain boundaries serve as a barrier to crack

growth. If the web 3 includes less than 95% area fraction

of pearlite microstructure, there is a shortage of pearlite

grain boundaries for blocking crack growth. The web 3 of

the rail 1 thus contains 95% or more area fraction of

pearlite microstructure.

[0033]

According to the present disclosure, the pearlite

blocks have an average size of 60 Lm or less. There is also

a relationship between the size of the pearlite blocks and

the fatigue crack growth. As described above, the pearlite

grain boundaries serve as a barrier to crack growth and

- 16A

block crack growth. When the pearlite blocks have a fine size, there is a high possibility that crack growth may pass through the grain boundaries having a crack growth retardation effect. This suppresses crack growth as a result. If the pearlite blocks have an average size of more than 60 pm, the crack growth retardation effect is not enough. For this, the average size of the pearlite blocks is 60 Lm or less, preferably 40 pm or less.

[0034]

Fig. 3 is a schematic view of an example rail

production system. A rail production system 10 in Fig. 3

includes a BD (breakdown) rolling mill 11, rough rolling

mills 12 and 13, a finish rolling mill 14, and a cooling

facility 15. The BD rolling mill 11, the rough rolling

mills 12 and 13, and the finish rolling mill 14 hot-roll a

slab, and the cooling facility 15 cools the hot-rolled slab.

The finish rolling mill 14 rolls the slab through, for

example, a caliber rolling process. The finish rolling mill

14 has two upper and lower rolls with grooves according to a

desired cross section and directly presses the web 3, the

head 4, and the foot 2. The amounts of rolling reduction of

the web 3, the head 4, and the foot 2 are controlled by

adjusting the shapes of the grooves in the upper and lower

rolls.

[0035]

Next, the method for producing a rail will be described

below with reference to Fig. 3. First, a slab SS (material

slab) reheated in a heating furnace is rolled in the BD

(breakdown) rolling mill 11 into an approximate shape of the

rail 1. The slab SS rolled in the BD rolling mill 11 is

hot-rolled in the rough rolling mills 12 and 13.

Accordingly, the austenite grains coarsened by heating are

repeatedly subjected to rolling and recrystallization in a

recrystallization temperature range in the BD rolling mill

11 and the rough rolling mills 12 and 13 to form fine

grains.

[00361

Next, the slab SS is hot-rolled in finish rolling in

the finish rolling mill 14 in such a manner that the

finishing temperature of the web 3 is 10000C or lower and

the reduction in area of the web 3 is 10% or more. The

finishing temperature refers to the surface temperature of

the web 3 during finish rolling, but the surface temperature

of the head 4 may be regarded as the finishing temperature

of the web 3.

[0037]

When the slab SS is rolled in a non-recrystallization

temperature range (low temperature range), such as 10000C or

lower, in which recrystallization is unlikely to occur, the

austenite grains are elongated without being recrystallized, and deformation bands are formed in the grains. During transformation from austenite to pearlite, the deformation bands in the grains serve as nucleation sites for pearlite transformation together with austenite grain boundaries.

The pearlite grains become finer accordingly. At a

finishing temperature above the recrystallization

temperature range, recovery by recrystallization occurs, and

the average size of the pearlite blocks cannot be reduced to

pm or less. To make the crystal grains finer by rolling

in a non-recrystallization temperature range (low

temperature range), the finishing temperature during finish

rolling is set to 10000C or lower, which is a non

recrystallization temperature range (low temperature range).

If the finishing temperature is below 8000C, a significantly

large load is applied to the rolls during rolling. In

addition, rolling in an austenite low temperature range

introduces remarkable working strain into the austenite

grains, so that a desired crack growth retardation effect

cannot be obtained as a result. Therefore, the finishing

temperature is preferably 8000C to 9000C during finish

rolling.

[00381

To make the pearlite blocks finer, it is necessary to

press the web 3 to induce strain. The slab SS is thus

finish-rolled in the finish rolling mill 14 in such a manner that the reduction in area of the web 3 is 10% or more. The reduction in area is expressed by reduction in area (%) =

((Al - A2)/Al) x 100, where Al represents the cross

sectional area before finish rolling, and A2 represents the

cross-sectional area after finish rolling. If the reduction

in area is less than 10%, the average size of the pearlite

blocks cannot be reduced to 60 pm or less, and the crack

growth retardation effect cannot be obtained. The reduction

in area is more preferably 30% or more.

[00391

After finish rolling in the finish rolling mill 14, the

web 3 of the rail is subjected to accelerated cooling in the

cooling facility 15 at a cooling rate of 1 to 5 °C/s from a

temperature higher than or equal to a pearlite

transformation start temperature to a temperature range of

4000C to 6000C. The cooling stop temperature refers to, for

example, the surface temperature at a central portion of the

rail web 4 measured with a radiation thermometer when

cooling is stopped. The cooling rate (°C/sec) refers to a

temperature change per unit time (sec) from cooling start to

cooling stop.

[0040]

At a cooling rate higher than 5 °C/s, the area fraction

of pearlite microstructure decreases and the area fraction

of martensite microstructure and other microstructures increases, so that the pearlite microstructure cannot occupy

% or more area fraction. Accelerated cooling at a cooling

rate of 1 to 5 °C/s can form the web 3 composed of pearlite

and containing 95% or more area fraction of pearlite

microstructure. In addition, the production efficiency can

be improved because there is no need to maintain the

temperature until bainite transformation starts unlike a

conventional manner.

EXAMPLE 1

[0041]

The structure and operational effects of the present

invention will be more specifically described below by way

of Example. The present invention is not limited by the

following Example and can be appropriately modified within

the range of the spirit of the present invention. All of

the modifications are included in the technical scope of the

present invention.

[0042]

First, steels Al to A15 and B1 to B6, which have

different chemical compositions, are prepared. Table 1

below shows the components of the steels Al to A15 and B1 to

B6. The blanks in Table 1 mean that the component is absent

or negligible because of the content being in the range of

incidental impurities.

[0043]

[Table 1] Chemical Composition (mass%) Note SteelNo.C Si Mn P S Cr Cu Ni Nb Mo V W B Al 0.710.810.63 0.014 0.004 1.35 Ref. Example A2 0.82 0.65 1.02 0.0110.006 1.15 Ref. Example A3 0.94 0.23 0.92 0.0110.012 0.95 Ref. Example A4 0.99 0.58 0.24 0.010 0.005 0.35 Invention Example A5 1.081.05 0.78 0.010 0.010 0.57 Ref. Example A6 1.12 1.16 0.42 0.010 0.009 1.03 Ref. Example A7 1.18 0.77 1.46 0.010 0.005 0.21 Invention Example S0.65 0.42 1.150.014 0.008 0.40 Comparative Example 2 1.220.840.860.0140.005 1.51 Comparative Example 3 1.010.180.240.0120.006n0.65 Comparative Example B4 0.83 0.63 1.53 0.0110.0110.60 Comparative Example B6 0.710.77 1.420.014 0.0100.19 Comparative Example A8 0.810.75 0.68 0.015 0.005 1.23 0.89 Ref. Example A9 1.03 0.661.010.0120.0061.06 0.85 Ref. Example A1 1.10 0.2t 0.94 0.013 0.003 0.93 0.04 InventionExample All 1.18 0.53 0.210.0090.0040.36 082 076 InventionExample A12 0.750.93 0.44 0.014 0.005 0.45 008 Ref. Example A13 0.920.310.63 0.012 0.0110.640.77 091 Ref. Example A14 -- 0.850 -.78 0.84 0.0110.009 0.87 -- 0.03 0.004 Invention Example. Al - 0.840.25 0210.0100.005 2.49 ------......Ref. Example-- -- B6 0.910.56 0.36 0.011 0.0082.52__ _ _ _ Comrtive Exa mle

Next, rails (No. 1 to No. 25 in Table 2 below) were

produced in the rail production system 10 in Fig. 3 under

predetermined production conditions using the steels Al to

A15 and B1 to B6 containing the composition components in

Table 1. Subsequently, steel materials were extracted from

the webs 3 of the produced rails 1 to prepare test

specimens. Fig. 4 is a schematic view of an example test specimen. In Fig. 4, the test specimen has, for example, a plate shape with width W = 20 mm, height H = 100 mm, and thickness B = 5 mm. The test specimen has a notch at a half

H/2 of the height H. The notch has length L = 2 mm and

width C = 0.2 mm. The end of the notch has curvature radius

R = 0.1 mm.

[0045]

Table 2 shows the rail production conditions and the

test results.

[0046]

= .) N.) N.) .. . N. . . . . -. . -. . - . .

. 0.. . .H

. -) 0

. Cp:~ ~~ CtC):

C)n (71 9) T) (CDC

CD

C D, 0 nIc )0 )N o c;(D;c ;c mCm I~~nC 0o 0... .. 0~ CL

* 0

0) CD 0) .. . .2

CD C

0.. Ca)

Cl) CD c

Cl)

SD C C

) CDI CL~~~... ;: C.... CD0.

c.

+. .C) .. .

. . . . . C D - _ .0 .- ._ . . . .

.(71 . . . .> . . . . . . .

.

2 :- W : 4 -:Cn ) .Cn Cn lN ; l ; 4 ; lM W :Cn:C CD -Is n:(= - 4: N):1 - 00 00: Cn 00 = n CDI = P -P= = DI 00 CD -4 N : -4 0 : 0:C.

2 CCD C. .Cl .l.Cl).. . . .. . . . . . . . . . .

Cl). .l). .C. C .C. . .l).l. C.

[0047]

A fatigue crack growth test was conducted at a stress

ratio R = 0.1 using the test specimen in Fig. 4, and the

fatigue crack growth rate da/dN (m/cycle) was measured at a

1 2 stress intensity factor AK = 20 MPa-m to evaluate the

surface damage resistance of the web. When the fatigue

crack growth rate was 8.0 x 10-8 or less, the web was

determined to have crack growth resistance.

[0048]

In the production conditions in Table 2, the finishing

temperature is obtained by measuring the surface temperature

of the web 3 at the inlet of the finish rolling mill 14

using a radiation thermometer, and the cooling stop

temperature is obtained by measuring the surface temperature

of the web 3 using a radiation thermometer when cooling is

stopped.

[0049]

The size of the pearlite blocks in Table 2 was obtained

as follows: a test specimen for microscopic observation of L

cross sections was sampled at a middle point of the rail

web, embedded followed by mirror polishing, and then

subjected to orientation analysis using an EBSD (electron

backscatter diffraction pattern), and the sizes of pearlite

grains of respective orientations were measured as equivalent circular diameters and averaged. The grain boundaries at which a difference in orientation between adjacent crystal orientations was 5° or more were determined to define different pearlite blocks. The measurement region was 300 pm square, the step size was 0.3 pm, and measurement points at which the Confidence index indicating the reliability of measured orientation was 0.1 or less were removed from analysis targets. Crystal grains on the edges of the measurement region were also removed from analysis targets.

[00501 "P" In Table 2, means that the web includes 95% or more

area fraction of pearlite microstructure, and trace amounts,

or 5% or less in total, of bainite microstructure,

martensite microstructure, pro-eutectoid cementite

microstructure, and pro-eutectoid ferrite microstructure.

The area fraction of pearlite microstructure can be measured

by using a known technique. For example, the sampled test

specimen is polished and then etched in nital, the type of

microstructure is identified through cross-sectional

observation using an optical microscope at a magnification

of 400 times, and the area fraction of pearlite

microstructure is calculated by image analysis.

[0051]

Rails No. 1 to No. 7 and No. 24 were produced by using compatible steels that satisfy the mass percentages of the component composition of the present invention in accordance with production methods that satisfy the finish rolling conditions and the cooling conditions. The area fraction of pearlite in each of the webs 3 was 95% or more, and the average size of pearlite blocks was 60 pm or less. As a result, the crack growth rate da/dN (m/cycle) in each of the

1 2 webs 3 at a stress intensity factor AK = 20 MPa-m was 8.0

x 10-8 or less. In addition, the crack growth rate da/dN

(m/cycle) in the web 3 was 8.0 x 10-8 or less even when a

predetermined mass% of at least one of Cu, Ni, Nb, Mo, V, W,

and B was contained as in No. 17 to No. 23 (Steel No. A8 to

No. A14).

[0052]

When the reduction in area in finish rolling is less

than 10% as in No. 8 and No. 9, the average size of pearlite

blocks is larger than 60 pm. As a result, the crack growth

rate da/dN (m/cycle) in the web 3 does not satisfy 8.0 x 10-8

or less.

[0053]

When the cooling rate after finish rolling is higher

than 5 °C/sec as in No. 10, the area fraction of martensite

microstructure is high, and the area fraction of pearlite

microstructure in the web 3 is less than 95%. As a result,

the crack growth rate da/dN (m/cycle) in the web 3 does not satisfy 8.0 x 10-8 or less.

[0054]

When, as in No. 11 to No. 16 and No. 25, steels are out

of the range of the mass percentages of the chemical

composition according to the present invention, even when

the temperature conditions and the reduction in area in

finish rolling and the cooling rate satisfy the conditions

defined in the present invention, the area fraction of

pearlite microstructure in the web 3 is less than 95%, or

the average size of pearlite blocks is larger than 60 pm.

As a result, the crack growth rate da/dN (m/cycle) in the

web does not satisfy 8.0 x 10-8 or less.

[0055]

According to the present invention, the microstructure

of the web 3 of the rail 1 is controlled by controlling the

components of the steel, the finish rolling conditions, and

the cooling conditions to lower the web crack growth rate in

the web 3 of the rail 1 and thus to suppress the crack

growth in the web 3 and the breakage of the rail.

[0056]

The embodiment of the present invention is not limited

to the above embodiment, and various modifications can be

made to the embodiment of the present invention. For

example, the conditions for producing the web 3 are

illustrated in the embodiment, in which the foot 2 and the head 4 are hot-rolled at the same time when the web 3 is hot-rolled. There, for example, a rail 1 may be produced by preparing a slab having chemical compositions that satisfy the performance requirements for both the web 3 and the head

4, and hot rolling and cooling the web 3 and the head 4

under different conditions such that both the crack growth

resistance of the web 3 and the wear resistance of the head

4 and other properties are satisfied.

Reference Signs List

[0057]

1 Rail

2 Foot (bottom)

3 Web

4 Head

10 Rail production system

11 BD rolling mill

12, 13 Rough rolling mill

14 Finish rolling mill

15 Cooling facility

SS Slab

[0058]

The reference in this specification to any prior

publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0059]

Throughout this specification and the claims which follow,

unless the context requires otherwise, the word "comprise",

and variations such as "comprises" and "comprising", will be

understood to imply the inclusion of a stated integer or step

or group of integers or steps but not the exclusion of any

other integer or step or group of integers or steps.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

[Claim 1]

A rail comprising a foot, a web, and a head,

wherein the web has a chemical composition comprising:

C: 0.70 to 1.20 mass%,

Si: 0.20 to 1.20 mass%,

Mn: 0.20 to 1.50 mass%,

P: 0.035 mass% or less,

S: 0.0005 to 0.012 mass%, and

Cr: 0.20 to 2.50 mass%, with the balance being Fe and

incidental impurities,

an area fraction of pearlite in the web is 95% or more,

and

an average size of pearlite blocks is 30 pm or less.
[Claim 2]

The rail according to Claim 1, wherein the chemical

composition further comprises one or two or more selected

from Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.05

mass% or less, Mo: 1.0 mass% or less, V: 0.005 to 0.10

mass%, W: 1.0 mass% or less and B: 0.005 mass% or less.
[Claim 3]

The rail according to Claim 1 or 2, wherein a crack growth rate da/dN (m/cycle) in the web at a stress intensity 2 factor AK = 20 MPa-m/ is 8.0 x 10-8 or less.
[Claim 4]

A rail production method for producing the rail

according to any one of claims 1 to 3 , the method

comprising:

performing finish rolling at a finishing temperature of

1000°C or lower in such a manner that a reduction in area of

a web is 10% or more; and

after finish rolling, cooling the web at a cooling rate

of 1 to 5 °C/s from a temperature higher than or equal to a

pearlite transformation start temperature to a temperature

range of 400°C to 600°C.
[Claim 5]

The rail production method according to Claim 4,

wherein the finishing temperature in finish-rolling the web

is in a range of 800 to 900°C.