Method for improving fracture toughness of steel rail by refining grains

Abstract

The invention discloses a method for improving fracture toughness of a steel rail by refining grains. The method is characterized by comprising the following steps: 1) increasing the section size of a steel rail casting blank from 280mm * 380mm to 320mm * 415mm; 2) large deformation rolling; and 3) on-line heat treatment: the steel rail at 730-780 DEG C enters an on-line heat treatment line for rapid air spraying cooling, is discharged from the on-line heat treatment production line after being cooled to 520 DEG C, and then enters a cooling bed for natural air cooling to room temperature. The method is suitable for achieving grain refinement of the steel rail by controlling austenite dynamic recrystallization and phase transformation behaviors, and the low-temperature fracture toughness and the fatigue crack propagation resistance of the steel rail are remarkably improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material processing, and particularly relates to a method for improving the fracture toughness of a rail by refining grains. BACKGROUND

[0002] Pearlitic rail (such as U71Mn, U75V, etc.) is the most widely used rail material in global railway transportation due to its excellent strength, hardness, wear resistance and good cost performance. Its service safety and long service life are crucial to guarantee the efficiency and safety of railway transportation. Under the action of long-term complex alternating load (wheel-rail impact, vibration, bending stress) and environmental factors (corrosion, temperature change), micro-cracks (such as rolling contact fatigue cracks, wear cracks, internal defect induced cracks, etc.) will inevitably occur on the surface of the rail. Fatigue cracks are prone to occur in the rail under alternating stress and low temperature environment, and if the fracture toughness is insufficient, it will lead to disastrous rail breaking accidents.

[0003] Research shows that the fracture toughness of the rail is closely related to the grain size. According to the Hall-Petch relationship, for every 1 grade (ASTM standard) of grain size refinement, the ductile-brittle transition temperature can be reduced by 10-15℃, and the crack propagation resistance can be increased by more than 20%. The existing technology mainly relies on alloying (such as adding V, Nb micro-alloy elements) or heat treatment (such as induction quenching) to improve the toughness, but there are problems such as high cost, high energy consumption and complex process. The austenite grain size of the rail produced by the conventional rolling process is mostly in the range of 30-50μm, which restricts the space for improving the toughness.

[0004] Therefore, the existing technology fails to provide an effective method for significantly and stably improving the fracture toughness of the pearlitic rail material, which is simple to operate, controllable in cost, and can reduce the risk of rail breaking due to low toughness and prolong the safe service life of the rail. SUMMARY

[0005] The purpose of the application is to provide a method for improving the fracture toughness of a rail by refining grains, which is particularly suitable for realizing grain refinement of the rail by controlling the austenite dynamic recrystallization and phase change behavior, and significantly improving the low-temperature fracture toughness and fatigue crack propagation resistance of the rail.

[0006] To solve the above technical problems, the application adopts the following technical scheme:

[0007] The application provides a method for improving the fracture toughness of a rail by refining grains, which comprises the following steps:

[0008] 1) Large-section rail casting billet smelting: increasing the rail casting billet section from 280mm x 380mm to 320mm x 415mm section size;

[0009] 2) large deformation rolling: the 320mm*415mm section pearlitic billet provided in step 1) is heated to 1280±10 DEG C and kept for 2.5-3.5 hours, and is rolled at 1200±10 DEG C, the reduction of each pass is increased by more than 20% compared with the traditional 280mm*380mm section cast billet, and the final section size is the iron standard 60kg / m rail, and the finish rolling temperature is 920-950 DEG C;

[0010] 3) online heat treatment: the rail is cooled to 520 DEG C by entering the online heat treatment line at 730 DEG C-780 DEG C and then is naturally air-cooled to room temperature after entering the cooling bed, and the rail meets the requirements of-20 DEG C fracture toughness KIC≥34MPa*m 1 / 2 , and the full section grain size is≥8.5 level.

[0011] Further, the 320mm*415mm section pearlitic billet provided in step 1) is heated to 1280 DEG C and kept for 3 hours.

[0012] Further, the rolling is carried out at 1200 DEG C.

[0013] Further, the method is at least applicable to U71MnHG 60kg / m rail for high-speed railway and U75VH 60kg / m heat-treated rail for general speed freight railway.

[0014] Further, the 320mm*415mm section U71MnHG billet provided in step 1) is heated to 1280 DEG C and kept for 3 hours, and the rolling is carried out at 1200 DEG C, the reduction of each pass is increased by 22% compared with the traditional 280mm*380mm section cast billet, and the final section size is the iron standard 60kg / m rail, and the finish rolling temperature is 936 DEG C.

[0015] Further, the rail is cooled to 520 DEG C by entering the online heat treatment line at 768 DEG C and then is naturally air-cooled to room temperature after entering the cooling bed.

[0016] Further, the rail is detected to meet the requirement of-20 DEG C fracture toughness KIC=39MPa*m 1 / 2 ; and the full section grain size is 9 level.

[0017] Compared with the prior art, the beneficial technical effects of the present application are:

[0018] The present application increases the section size of the billet, excites the dynamic recrystallization of austenite through multi-stage large deformation rolling, and realizes the full section grain size≥8.5 level by combining the online heat treatment process. DETAILED DESCRIPTION

[0019] The application will be further described in detail below with reference to specific examples, so as to make the application more clearly understood.

[0020] Example 1: U71MnHG 60kg / m rail for high-speed railway.

[0021] (1) Large-section rail billet smelting: combined with the existing U71MnG rail smelting process system, the rail billet section is increased from 280mmx380mm to 320mmx415mm section size;

[0022] (2) Large deformation rolling: the 320mmx415mm section U71MnHG steel billet provided in step (1) is heated to 1280℃ for 3 hours, and rolled at 1200℃, with 13 rolling passes, and the reduction per pass is increased by 22% compared with the traditional 280mmx380mm section billet, and the final section size is the iron standard 60kg / m rail, and the finish rolling temperature is 936℃.

[0023] (3) Online heat treatment: the rail is cooled to 520℃ by entering the online heat treatment line at 768℃, and then enters the cooling bed for natural air cooling to room temperature, and the rail-20℃ fracture toughness KIC is 39MPa·m 1 / 2 . The full-section grain size is 9 levels.

[0024] (4) The traditional 280mmx380mm section billet produces U71MnHG 60kg / m rail, and the rail-20℃ fracture toughness KIC is 33MPa·m 1 / 2 , and the full-section grain size is 8 levels. Therefore, the rail-20℃ fracture toughness KIC of U71MnHG 60kg / m rail produced by large-section billet is increased by 18% compared with the traditional process.

[0025] Example 2: U75VH 60kg / m heat-treated rail for general speed freight railway.

[0026] (1) Large-section rail billet smelting: combined with the existing U75VH rail smelting process system, the rail billet section is increased from 280mmx380mm to 320mmx415mm section size;

[0027] (2) Large deformation rolling: the 320mmx415mm section U75VH steel billet provided in step (1) is heated to 1280℃ for 3 hours, and rolled at 1200℃, with 13 rolling passes, and the reduction per pass is increased by 22% compared with the traditional 280mmx380mm section billet, and the final section size is the iron standard 60kg / m rail, and the finish rolling temperature is 948℃.

[0028] (3) Online heat treatment: the rail enters the online heat treatment line at 772℃, is rapidly cooled by air blowing, is discharged from the online heat treatment production line at 520℃, and then enters a cooling bed and is naturally air-cooled to room temperature, and sampling detection of the rail-20℃ fracture toughness KIC is 37 MPa·m 1 / 2 Full cross-section grain size 9th grade

[0029] (4) The traditional 280mm*380mm cross-section cast slab is used to produce U75VH 60kg / m rail, and the rail-20℃ fracture toughness KIC is 32 MPa·m 1 / 2 Full cross-section grain size 8th grade. The U71MnHG 60kg / m rail-20℃ fracture toughness KIC produced by using the large cross-section cast slab is increased by 15% compared with the traditional process.

[0030] As can be seen from Example 1 and Example 2, the rail realizes full cross-section grain size ≥8.5th grade by increasing the cross-section size of the blank, exciting austenite dynamic recrystallization through multi-stage large deformation rolling, and combining with the online heat treatment process. Compared with the conventional process, the rail head-20℃ fracture toughness KIC is increased by 15%-18%.

[0031] The above-described embodiments are only used to describe the preferred modes of the present application, and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method of improving the fracture toughness of a steel rail by refining the grain size, characterised in that: Comprising: 1) Large section rail bloom smelting: increasing the rail bloom section from 280mm x 380mm to 320mm x 415mm section size; 2) Large deformation rolling: heating the 320mm x 415mm section pearlite steel bloom provided by step 1) to 1280±10℃ for 2.5-3.5 hours, rolling at 1200±10℃, rolling passes 13 times, each pass compared to the traditional 280mm x 380mm section bloom, the reduction is increased by more than 20%, the final section size is 60kg / m rail, the finish rolling temperature is 920-950℃; 3) On-line heat treatment: the rail enters the on-line heat treatment line at 730℃-780℃ for air blast rapid cooling, and is cooled to 520℃ to exit the on-line heat treatment production line, and then enters the cooling bed for natural air cooling to room temperature.

2. The method of increasing the fracture toughness of a steel rail by refining the grain size according to claim 1, characterized in that: The 320mm x 415mm section pearlite steel bloom provided by step 1) is heated to 1280℃ for 3 hours.

3. The method of increasing the fracture toughness of a steel rail by refining the grain size according to claim 1, characterized in that: Rolling at 1200℃.

4. The method of increasing the fracture toughness of a steel rail by refining the grain size according to claim 1, characterized in that: At least applicable to U71 MnHG 60kg / m rail for high-speed railway, U75VH 60kg / m heat-treated rail for general speed freight railway.

5. The method of increasing the fracture toughness of a steel rail by refining the grain size according to claim 4, characterized in that: The 320mm x 415mm section U71 MnHG steel bloom provided by step 1) is heated to 1280℃ for 3 hours, rolled at 1200℃, rolling passes 13 times, each pass compared to the traditional 280mm x 380mm section bloom, the reduction is increased by 22%, the final section size is 60kg / m rail, the finish rolling temperature is 936℃.

6. The method of increasing the fracture toughness of a steel rail by refining the grain size according to claim 5, characterized in that: The rail enters the on-line heat treatment line at 768℃ for air blast rapid cooling, and is cooled to 520℃ to exit the on-line heat treatment production line, and then enters the cooling bed for natural air cooling to room temperature.

7. The method of increasing the fracture toughness of a steel rail by refining the grain size according to claim 6, characterized in that: The steel rail -20℃ fracture toughness KIC = 39 MPa · m 1 / 2 ; Full section grain size 9 levels.