US20190105694A1

US20190105694A1 - High-carbon and high-strength and toughness pearlitic rail and manufacturing method thereof

Info

Publication number: US20190105694A1
Application number: US16/155,205
Authority: US
Inventors: Zhenyu Han; Ming Zou; Hua Guo; Jun Yuan
Original assignee: Pangang Group Research Institute Co Ltd
Current assignee: Pangang Group Research Institute Co Ltd
Priority date: 2017-10-10
Filing date: 2018-10-09
Publication date: 2019-04-11
Also published as: CN107675084A; AU2018247222B2; AU2018247222A1; CN107675084B

Abstract

In view of the problem of uneven performance of railhead sections of pearlitic rail manufactured with existing technique and the poor performance of the pearlitic rail obtained, the invention provides a manufacturing method for high-carbon and high-strength and toughness pearlitic rail, including the following steps to: a. hot roll the steel billet into rail, with a final rolling temperature of 900-1000° C.; b. blow a cooling medium to the top surface of railhead, wherein, the two sides of railhead and the lower jaws on two sides of railhead when the center of top surface of rail is air-cooled to 800-850° C.; then air-cool the rail to room temperature after the center of top surface of rail is cooled to 480-530° C. By controlling the composition of steel and adopting a two-stage accelerated cooling, a high-carbon rail is produced with better strength and excellent toughness which is suitable for heavy-haul railway.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from CN 201710934583.9 filed Oct. 10, 2017, the contents of which are incorporated by reference in their entireties.

FIELD OF INVENTION

The invention relates to a rail, particularly to a high-carbon and high-strength and toughness pearlitic rail and the manufacturing method thereof.

BACKGROUND OF THE INVENTION

The rapid development of railway has higher requirements for the service performance of rail. With the continuous improvement of China's high-speed railway network, heavy-haul transformation will be conducted gradually for the existing main railway lines with passenger and freight mixed traffic. And the heavy-haul railway will develop towards large volume, high axle load and high density. As a key component of railway, the quality and performance of rail are closely related to the transport efficiency of railway and the safety of traffic. With the improvement of the transportation capacity of railway, the service environment of rail has become increasingly harsh and complex and all kinds of defects and failures have occurred. Some rails at small radius curves have defects and failures such as rapid abrasive wear and peeling-off simultaneously, making their service life inconsistent with that of the main line rail, thus limiting the further development of railway transportation.
Currently, the method of on-line or off-line heat treatment for pearlitic rail is mainly adopted to improve performance of the rail at curves. By blowing compressed air or water-air spray mixture to the railhead of austenitic rail, the railhead is rapidly cooled, and it is able to produce refined and lamellar perlite structure from the surface of the railhead to a certain depth. The strength and toughness of rail can be improved synchronously through grain refinement, so that the wear resistance and contact fatigue resistance can be improved simultaneously. In terms of accelerated cooling process, few research reports on the influence of cooling nozzle layout to the performance of rail are available at home and abroad.
Patent CN101646795B, Internal High-Hardness Type Pearlitic Rail with Excellent Wear Resistance and Fatigue Damage Resistance and Manufacturing Method Thereof, specifies a manufacturing method for an internal high-hardness pearlitic rail, characterized in that, the steel is hot rolled into a rail-like shape with a final rolling temperature of 850-950° C., and then the surface of railhead is rapidly cooled from the temperature above the pearlitic transformation temperature to 400-650° C. at a rate of 1.2-5° C./s. The patent only specifies the temperature to start and end cooling as well as corresponding range of cooling rate at different stages of heat treatment for rail, but does not specify any cooling methods.
Patent CN105483347A discloses A Heat Treatment Technique for Hardening Pearlitic Rail, characterized in that a rail is heated to 880-920° C. and insulated for 10-15 min, and then cooled to specific temperature range at specific range of cooling rate according to different steel types and insulated for 30 s, and then air-cooled, with specific conditions as follows: the process for hardening U75V pearlitic rail is: to insulate the rail at 880-920° C. for 10-15 min, and cool the rail to 570-600° C. at a cooling rate of 8-15° C./s, and then air cool the rail to 20-25° C. at a cooling rate of 0.2-0.5° C./s; the process for hardening U76CrRE pearlitic rail is: to insulate the rail at 850-900° C. for 10-15 min, and cool the rail to 590-610° C. at a cooling rate of 6-10° C./s, and then air cool the rail to 20-25° C. at a cooling rate of 0.2-0.5° C./s. The heat treatment technique for the two grades of materials, i.e. U75V and U76CrRE, disclosed by the patent also does not specify any cooling methods.
Patent CN103898303A discloses A Heat Treatment Method for Turnout Rail and Turnout Rail, characterized in that, accelerated cooling is carried out for a turnout rail to be treated with temperature of the top surface of the railhead of 650-900° C. to get the turnout rail with fully pearlitic structures, wherein, the accelerated cooling rate of the working side of the railhead of the turnout rail is higher than that of the non-working side of the railhead of the turnout rail, with a difference of 0.1-1.0° C./s. The patent specifies the benefits of different cooling rates on two sides of the railhead for the rail, especially for improving performance and controlling flatness of the rail with asymmetric section, but it does not clarify the influence of nozzle layout and cooling rate at different stages to the performance of rail after heat treatment.
In prior art, the heat treatment for rail is mainly focused on controlling different cooling rates in different temperature ranges to control heat treatment processes, it does not specify the refined control such as various nozzle layout and blowing method, therefore, no high-carbon pearlitic rail with excellent strength and toughness can be obtained.

SUMMARY OF THE INVENTION

The technical problem to be solved by the invention is that: in prior art, the methods adopting different cooling rates in different temperature ranges are used for the heat treatment of rail, therefore the pearlitic rail obtained has poor performance.
The technical scheme of the invention to solve the technical problem is to provide a manufacturing method for a high-carbon and high-strength and toughness pearlitic rail, comprising the following steps:
a. Rolling of rail
to hot roll steel billet into rail, with a final rolling temperature range of 900-1000° C.;
b. Cooling of rail
to blow a cooling medium to the top surface of railhead, wherein, the two sides of railhead and the lower jaws on two sides of railhead when the center of top surface of rail is air-cooled to 800-850° C.; then to air-cool the rail to the room temperature after the center of top surface of rail is cooled to 480-530° C.
Wherein, in the manufacturing method for high-carbon and high-strength and toughness pearlitic rail, the composition (in weight percentage) of the rail in step a is: C: 0.86%-1.05%, Si: 0.20%-0.50%, Mn: 0.50%-0.95%, Cr: 0.20%-0.40%, at least one of V, Nb and Ti, wherein V of 0.02%-0.10% if any, Ti of 0.001%-0.030% if any and Nb of 0.005%-0.08% if any, and the rest of Fe and inevitable impurities.
Wherein, in the manufacturing method for high-carbon and high-strength and toughness pearlitic rail, the cooling medium in steps b and c is at least one of compressed air and water-air spray mixture.
Wherein, in the manufacturing method for high-carbon and high-strength and toughness pearlitic rail, the cooling rate in step b is 2.0-5.0° C./s.
The invention also provides a high-carbon high-strength and toughness pearlitic rail, with the composition (in weight percentage) of: C: 0.86%-1.05%, Si: 0.20%-0.50%, Mn: 0.50%-0.95%, Cr: 0.20%-0.40%, at least one of V, Nb and Ti, wherein V of 0.02%-0.10% if any, Ti of 0.001%-0.030% if any and Nb of 0.005%-0.08% if any, and the rest of Fe and inevitable impurities.
Compared with the prior art, the beneficial effects of the invention lie in that: the invention uses a rail with specific composition and adopts a method of two-stage accelerated cooling, therefore, compared with the existing single method for heat treatment, the pearlitic rail manufactured in this method has more excellent strength, hardness, toughness and plasticity, especially a much better strength and toughness. The method of the invention can be easily conducted and has low requirement for equipment, and the high-carbon and high-strength and toughness pearlitic rail manufactured can enhance the overall strength and toughness of railhead and prolong the service life of rail under the same conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a manufacturing method for a high-carbon and high-strength and toughness pearlitic rail, comprising the following steps:
a. Rolling of rail
to hot roll steel billet into rail, with a final rolling temperature range of 900-1000° C.;
b. Cooling of rail
to blow a cooling medium to the top surface of railhead, wherein, the two sides of railhead and the lower jaws on two sides of railhead when the center of top surface of rail is air-cooled to 800-850° C.; then to air-cool the rail to the room temperature after the center of top surface of rail is cooled to 480-530° C.
The high-carbon and high-strength and toughness pearlitic rail of the invention has the composition (in weight percentage) of: C: 0.86%-1.05%, Si: 0.20%-0.50%, Mn: 0.50%-0.95%, Cr: 0.20%-0.40%, at least one of V, Nb and Ti, wherein V of 0.02%-0.10% if any, Ti of 0.001%-0.030% if any and Nb of 0.005%-0.08% if any, and the rest of Fe and inevitable impurities.
C is the most important and cheapest element to improve strength and hardness of pearlitic rail and to promote pearlitic transformation. Under the conditions of the present invention, when the content of C<0.86%, the strength and hardness obtained by using the manufacturing technique of the invention are too low and the wear resistance required for the heavy-haul railway cannot be guaranteed; when the content of C>1.05%, massive secondary cementite is precipitated at grain boundaries even though accelerated cooling is adopted after final rolling, thus deteriorating the toughness and plasticity of the rail. Therefore, the content of C is limited within the range of 0.86%-1.05%.
As the solid-solution strengthening element of steel, Si is present in ferrite and austenite to improve strength of structure, meanwhile, it can suppress precipitation of proeutectoid cementite, thus improving the toughness and plasticity of the rail. Under the conditions of the present invention, when the content of Si<0.20%, the solid solubility is relatively low, leading to low strengthening effects; when the content of Si>0.50%, the toughness and plasticity of the rail degrades and the transverse performance of the rail deteriorates. Therefore, the content of Si is limited within the range of 0.20%-0.50%.
Mn can form solid solution with Fe, strengthening ferrite and austenite. Meanwhile, Mn is also a carbide former and can partially replace Fe atom after entering cementite, improving hardness of carbide and finally improving hardness of the rail. Under the conditions of the present invention, when the content of Mn<0.50%, the strengthening effect is not obvious and the performance of steel can only be slightly improved through solid-solution strengthening; when the content of Mn>0.95%, the hardness of the carbide in steel is too high and the toughness and plasticity significantly degrade; meanwhile, Mn has obvious diffusion effect to carbon when in steel, and the segregation zone of Mn can still produce B, M and other abnormal structures even under the air-cooling condition. Therefore, the content of Mn is limited within the range of 0.50%-0.95%.
As a medium carbide former, Cr can form multiple carbides with carbon in steel; meanwhile, Cr can produce even distribution of carbides in steel, reduce the size of carbides and improve wear resistance of the rail. Under the conditions of the present invention, when the content of Cr<0.20%, the carbide formed has low hardness and low proportion, and aggregates in the form of sheet, in this way, the wear resistance of the rail cannot be improved effectively; when the content of Cr>0.40%, coarse carbide is prone to form, thus deteriorating the toughness and plasticity of the rail. Therefore, the content of Cr is limited within the range of 0.20%-0.40%.
V has low solubility in steel when under room temperature, and if V is present in austenite grain boundaries and other zones during hot rolling, it is precipitated through fine-grained V carbonitride [V (C, N)] or together with Ti in steel, suppressing the growth of austenite grains and thus refining grain and improving performance. Under the conditions of the present invention, when the content of V<0.02%, the precipitation of V carbonitride is limited and the rail cannot be strengthened effectively; when the content of V>0.10%, coarse carbonitride is prone to form, thus deteriorating the toughness and plasticity of the rail. Therefore, the content of V is limited within the range of 0.02%-0.10%.
The main function of Ti in steel is to refine austenite grains during heating, rolling and cooling, and finally improve extensibility and rigidity of the rail. When the content of Ti<0.001%, the amount of carbides formed in the rail is extremely limited. Under the condition of the present invention, when the content of Ti>0.030%, on one hand, excessive TiC forms since Ti is a strong carbonitride former, making the hardness of the rail too high, and on the other hand, excessive TiN and TiC may lead to segregation enrichment and form coarse carbonitride, degrading the toughness and plasticity and making the contact surface of the rail prone to crack under impact load and leading to fracture. Therefore, the content of Ti is limited within the range of 0.001%-0.030%.
The main function of Nb in steel is similar to that of V, i.e., to refine austenite grains with the Nb carbonitride precipitated and to make precipitation strengthening occur with the carbonitride produced during the cooling process after rolling. Nb can improve hardness of the rail, enhance toughness and plasticity of the rail and help prevent softening of welded joints. Under the condition of the present invention, when the content of Nb<0.005%, the precipitation of Nb carbonitride is limited and the rail cannot be strengthened effectively; when the content of Nb>0.08%, coarse carbonitride is prone to form, thus deteriorating the toughness and plasticity of the rail. Therefore, the content of Nb is limited within the range of 0.005%-0.08%.
The common smelting method in the art is adopted to smelt steel for the above rail: to conduct continuous casting for the molten steel in compliance with the above composition requirements to produce steel billet with the section of 250 mm×250 mm-450 mm×450 mm, cool the steel billet, put it into a heating furnace to heat to 1200-1300° C., insulate the steel billet for a certain period of time and take it out of the furnace, remove phosphorus with high-pressure water, and then roll the billet into 50-75 kg/m rail with the required section by universal rolling or groove rolling.
Currently, the main method to conduct heat treatment for rail is to carry out accelerated cooling to the railhead of the austenitic rail, while the cooling nozzles are mainly arranged on the top surface and two sides of the railhead. This is determined by the characteristics of rail: the top surface and one side of the rail bear multiple complex stress of the wheel, and the rail has a symmetrical section along the vertical direction. Both sides may be subjected to the stress of the wheel since their installation location varies. Therefore, the performance of the in-service top surface and two sides of the railhead should be higher than that of other parts of the rail.
In the process of accelerated cooling of the top surface and both sides of the railhead, with the sudden drop of surface temperature, the core of railhead transfers heat with the surface, the performance of the surface of railhead may not degrade but improve instead with the release of latent heat during phase change of pearlite. This means the supercooling of the core of railhead drops during phase change. Eventually, under room temperature, not only the hardness of the core of railhead is obviously lower than that of the surface, but also the toughness is relatively low. The invention adopts the method of adding nozzles at lower jaws on two sides of railhead to blow cooling medium. During the heat treatment, since the difference of cooling rate decreases between the core of railhead and the surface of railhead, the phase change of the surface of railhead can start at a much lower temperature, and the strength and toughness of the rail can be further improved. Although the improvement is quite limited, it can still improve the comprehensive strength and toughness of steel rail such as pearlite heat-treated rail, with the strength and toughness already reaching the limit.
In the invention, when the rail is air-cooled to 800-850° C., the “top surface of railhead, the two sides of railhead and the lower jaws on two sides of railhead” are cooled to 480-530° C. at a cooling rate of 2.0-5.0° C./s.
During the cooling process, the surface temperature drops rapidly under the action of the cooling medium, and the heat from the core of railhead and the rail web is continuously circulated and supplemented to the surface of railhead and a certain depth, leading to a drop of the supercooling of the core of railhead, which shows a decrease of strength and toughness of the rail under room temperature; if cooling for lower jaws of railhead is adopted simultaneously, new channels for heat losses are provided for the railhead, and the heat supplement for the core of railhead is significantly reduced, thus raising the supercooling of the section of railhead, especially the core of railhead. Since the grains of the whole railhead are further refined, the rail will have more excellent comprehensive strength and toughness under room temperature. When the temperature of the top surface of the railhead decreases to 480-530° C., air cooling is carried out until the railhead reaches room temperature, and straightening, flaw detection and processing, etc. are carried out in later stages to obtain finished rail.
The preferred embodiments of the invention will be further illustrated as follows, but it does not indicate that the protection scope of the invention is limited as described in the embodiments.

Examples 1-6 Manufacturing Pearlitic Rail with the Method of the Invention

The chemical composition of the steel billet for the pearlitic rail in Examples 1-6 is shown in Table 1:

TABLE 1

List of Chemical Composition of the Steel Billet for the Pearlitic Rail (%)

	C	Si	Mn	P	S	Cr	V/Ti/Nb

Example 1	0.96	0.37	0.59	0.011	0.005	0.37	0.008Ti
Example 2	0.91	0.43	0.90	0.008	0.004	0.20	0.09V
Example 3	1.05	0.34	0.50	0.012	0.006	0.40	0.04Nb
Example 4	0.94	0.50	0.72	0.009	0.004	0.32	0.07Nb
Example 5	0.86	0.29	0.76	0.010	0.003	0.28	0.026Ti
Example 6	0.98	0.20	0.84	0.010	0.005	0.25	0.03V

The steel billets shown in the above table are all rolled into 60 kg/m rails and cooled by the following method:
a. Rolling of rail
to hot roll steel billet into rail, with a final rolling temperature range of 900-1000° C.;
b. Cooling of rail
to blow a cooling medium to the top surface of railhead, wherein, the two sides of railhead and the lower jaws on two sides of railhead when the center of top surface of rail is air-cooled to 800-850° C.; then to air-cool the rail to the room temperature after the center of top surface of rail is cooled to 480-530° C.
The cooling rate in Examples 1-6 is shown in table 2.

TABLE 2

Cooling Rate under Different Methods

	Temperature to start	Average	Temperature to end
	accelerated	accelerated cooling	accelerated
	cooling/° C.	rate ° C./s	cooling/° C.

Example 1	832	2.9	517
Example 2	841	2.0	530
Example 3	800	3.0	526
Example 4	846	4.2	504
Example 5	850	4.7	496
Example 6	811	5.0	480

Comparative Examples 1-6 Manufacturing Pearlitic Rail with Existing Methods

The composition of the steel billet used in Comparative Examples 1-6 is the same as that of Examples 1-6, wherein the steel billet of Comparative Example 1 is the same as that of Example 1, and so forth.
Comparative Examples 1-6 adopt existing cooling method: a cooling medium is blown only to the top surface of railhead and two sides of railhead, and the rail is air-cooled to room temperature after the surface of railhead is cooled to 480-530° C.
The cooling rate in Comparative Examples 1-6 is shown in table 3:

TABLE 3

Cooling Rate under Different Methods

	Temperature	Average	Temperature
	to start	accelerated	to end
	accelerated	cooling	accelerated
Joint	cooling/° C.	rate ° C./s	cooling/° C.

Comparative Example 1	829	3.0	527
Comparative Example 2	840	2.2	536
Comparative Example 3	800	3.1	530
Comparative Example 4	848	4.2	509
Comparative Example 5	850	4.6	499
Comparative Example 6	814	4.9	535

Air cool the rail treated according to the Examples and the Comparative Examples to room temperature, take double-shoulder circular tensile specimen with d₀=10 mm, l₀=5d₀at 10 mm and 30 mm below the surface of railhead of the rail respectively, and detect R_p0.2, R_m, A and Z respectively according to GB/T 228.1; and take U-type Charpy impact specimen of 10 mm×10 mm×55 mm at the same position, and detect impact energy according to GB/T 229. Besides, take transverse hardness specimen from the railhead of rail respectively, and test Rockwell hardness respectively at the upper corner and the center of the top surface at 10 mm and 30 mm from the surface of railhead according to GB/T 230.1. The test positions and methods are the same for the Examples and the Comparative Examples. The detailed results are shown in tables 4 and 5.

TABLE 4

Mechanical Properties of Rails Prepared
with Different Methods (10 mm below the surface of railhead)

Room

Hardness/HRC

Tensile properties

temperature

Top

	Rp/MPa	Rm/MPa	A/%	Z/%	Aku/J	Corner	surface

Example 1	847	1398	12.0	25	18	40.6	40.4
Example 2	845	1390	12.5	24	20	40.1	40.3
Example 3	862	1426	11.5	22	18	41.9	41.6
Example 4	821	1368	11.5	22	19	40.0	40.1
Example 5	826	1359	12.0	25	21	390	39.1
Example 6	866	1420	11.0	22	17	41.2	41.4
Comparative	831	1376	11.0	21	16	40.2	40.1
Example 1
Comparative	823	1359	12.0	22	18	39.8	39.6
Example 2
Comparative	858	1410	10.5	17	16	41.8	41.7
Example 3
Comparative	852	1366	11.0	18	15	39.2	39.1
Example 4
Comparative	812	1339	11.0	20	18	38.5	38.7
Example 5
Comparative	852	1402	10.0	19	15	40.2	40.4
Example 6

TABLE 5

Mechanical Properties of Rails Prepared with
Different Methods (30 mm below the surface of railhead)

Room

Hardness/HRC

Tensile properties

temperature

Top

	Rp/MPa	Rm/MPa	A/%	Z/%	Aku/J	Corner	surface

Example 1	832	1327	11.5	21	20	39.7	39.6
Example 2	798	1308	11.0	24	22	38.1	38.0
Example 3	828	1367	11.5	22	20	38.9	38.8
Example 4	792	1298	11.5	21	22	37.2	37.5
Example 5	767	1268	11.5	23	24	36.3	36.7
Example 6	832	1376	10.5	21	22	39.2	39.4
Comparative	762	1274	10.5	19	19	36.8	36.7
Example 1
Comparative	754	1258	11.0	22	20	36.2	36.4
Example 2
Comparative	769	1298	10.5	19	17	37.2	37.1
Example 3
Comparative	753	1269	10.0	15	16	36.2	36.0
Example 4
Comparative	739	1241	10.0	19	18	35.2	35.4
Example 5
Comparative	800	1331	10.0	15	17	39.0	38.8
Example 6

It can be concluded from the above Examples and Comparative Examples that the Examples adopting the heat treatment technique of the invention are compared with the Comparative Examples adopting the existing heat treatment technique for the material with the same chemical composition. In the Examples, the cooling medium is blown to the top surface of railhead and the two sides of railhead and the lower jaws on two sides of railhead after the heat treated rail is air-cooled to 800-850° C., and the rail is air-cooled again to room temperature after the center of top surface of rail is cooled to 480-530° C. at a cooling rate of 2.0-5.0° C./s. On the contrary, the existing technique adopts the single method for heat treatment for the top surface of railhead and the two sides of railhead at a cooling rate of 2.0-5.0° C./s. The comparison results in tables 4 and 5 indicate that the strength, hardness, toughness and plasticity of the part within 10 mm below the surface of the railhead under the technique of the invention are slightly higher than those of the Comparative Examples; more importantly, the strength and toughness of the part at 30 mm below the surface of the railhead is obviously higher than those with the existing heat treatment technique. Thus, it can be concluded that, adding accelerated cooling for the lower jaws of railhead can enhance the overall strength and toughness of railhead and prolong the service life of rail under the same conditions.
The invention provides a high-carbon and high-strength and toughness rail and its manufacturing method. With the same composition and the same manufacturing technique, the method can improve the strength and toughness of rail. The product is suitable for heavy-haul railway with high requirements for contact fatigue resistance and wear resistance.

Claims

What is claimed is:

1. A manufacturing method for high-carbon and high-strength and toughness pearlitic rail, said manufacturing method comprising the following sequential steps:

(a) hot rolling a steel billet to form the rail with a final rolling temperature range of 900-1000° C.;

(b) air cooling the rail until a center of a top surface of a railhead of the rail is cooled to 800-850° C.;

(c) blowing a cooling medium to the top surface of the railhead of the rail, two sides of the railhead and lower jaws on the two sides of the railhead until the center of the top surface of the railhead of the rail is cooled to 480-530° C.; and

(d) further air cooling the rail to room temperature.

2. The manufacturing method according to claim 1, wherein the rail comprises:

(i) 0.86 wt. % to 1.05 wt. % C;

(ii) 0.20 wt. % to 0.50 wt. % Si;

(iii) 0.50 wt. % to 0.95 wt. % Mn;

(iv) 0.20 wt. % to 0.40 wt. % Cr;

(v) at least one of 0.02 wt. % to 0.10 wt. % V, 0.001 wt. % to 0.030 wt. % Ti and 0.005 wt. % to 0.08 wt. % Nb; and

(vi) Fe.

3. The manufacturing method according to claim 1, wherein the cooling medium is at least one of compressed air and a water-air spray mixture.

4. The manufacturing method according to claim 1, wherein a cooling rate in step (c) is 2.0-5.0° C./s.

5. A high-carbon and high-strength and toughness pearlitic rail manufactured by the manufacturing method according to claim 1, wherein the rail comprises:

(i) 0.86 wt. % to 1.05 wt. % C;

(ii) 0.20 wt. % to 0.50 wt. % Si;

(iii) 0.50 wt. % to 0.95 wt. % Mn;

(iv) 0.20 wt. % to 0.40 wt. % Cr;

(vi) Fe.