An online heat treatment process for precisely controlling the hardness distribution of the rail head cross section

Abstract

This invention belongs to the field of high-strength rail production technology and discloses an online heat treatment process for precisely controlling the hardness distribution of the rail head cross section. First, the rail head cross section is divided into four regions from the surface inwards: Region 1 (≤6mm from the surface), Region 2 (6mm < ≤ 11mm), Region 3 (11mm < ≤ 21mm), and Region 4 (> 21mm). Then, the hot-rolled rail, which is in the austenitic state, undergoes online accelerated cooling. Different cooling intensities are applied to the rail head surface during the pearlitic transformation in Regions 1, 2, 3, and 4, respectively. After accelerated cooling, the rail is allowed to cool naturally to room temperature. This invention divides the rail head cross section into four regions from the surface inwards and applies different cooling intensities during the phase transformation of the metal in different regions, thereby controlling the Brinell hardness distribution of the rail head cross section to meet the comprehensive requirements of the European rail standard BS EN 13674.1.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel rail production technology, specifically relating to an online heat treatment process for precisely controlling the hardness distribution of the rail head cross section. Background Technology

[0002] With the significant increase in train axle load, operating speed, and operating density, wear and fatigue damage on rails during service have increased significantly, severely impacting their service life. Therefore, higher requirements are placed on rail quality. Domestic and international rail manufacturers generally employ online heat treatment to accelerate the cooling of hot-rolled rails, improving their strength and hardness to enhance their wear resistance and fatigue resistance. To ensure the effectiveness and service life of heat-treated rails, the European rail standard BS EN 13674.1 specifies the hardness requirements for heat-treated rails in detail, particularly outlining clear requirements for the hardness distribution on the rail head cross section.

[0003] The European rail standard BS EN13674.1 specifies the distribution of Brinell hardness test points on the cross section of the rail head of heat-treated rails. Figure 1 This standard sets strict criteria for the distribution of Brinell hardness on the rail head cross-section. First, it specifies hardness requirements for positions 1, 2, 3, and 4, with the lower limit of hardness at position 2 being 10 HB lower than that at position 1, and the lower limit of hardness at position 3 being 10 HB lower than that at position 2. Second, the hardness at positions 1, 2, and 3 must satisfy HBW2 > HBW3 + 0.3 × (HBW1 - HBW3), where HBW1, HBW2, and HBW3 are the average hardness values ​​at positions 1, 2, and 3, respectively. Furthermore, the difference between any two of HBW1, HBW2, and HBW3 must be ≤ 30 HBW. This requires the Brinell hardness on the rail head cross-section to gradually decrease from the surface inwards, but current manufacturing processes often result in the hardness distribution on the rail head cross-section failing to meet this standard.

[0004] In summary, there is an urgent need for an online heat treatment process that can precisely control the hardness distribution of the rail head cross section, so that the Brinell hardness distribution of the rail head cross section meets the requirements of the European rail standard BS EN 13674.1. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an online heat treatment process for precisely controlling the hardness distribution of the rail head cross section, which addresses the shortcomings of the existing technology. The rail head cross section is divided into four regions from the surface to the inside. Different cooling intensities are applied when the metal in different regions undergoes phase transformation, thereby regulating the Brinell hardness distribution of the rail head cross section to meet the comprehensive requirements of the European rail standard BS EN13674.1.

[0006] To address the technical problem proposed in this invention, this invention provides an online heat treatment process for precisely controlling the hardness distribution of the rail head cross section, comprising the following steps:

[0007] 1) Divide the cross section of the rail head into four regions from the surface to the inside: Region 1 is ≤6mm from the surface, Region 2 is 6mm < ≤11mm from the surface, Region 3 is 11mm < ≤21mm from the surface, and Region 4 is >21mm from the surface.

[0008] 2) The hot-rolled rails in the austenitic state are subjected to online accelerated cooling. The initial accelerated cooling temperature is 680-820℃. When pearlitic phase transformation occurs in regions one, two, three and four in sequence, different cooling intensities are applied to the rail head surface so that the cooling rate changes in the following order: phase transformation in region one > phase transformation in region three > phase transformation in region two > phase transformation in region four. The total accelerated cooling time is controlled to be 55-115s, and the rail tread temperature is accelerated to 420-520℃, and then naturally cooled to room temperature.

[0009] In the above scheme, the cooling rate for accelerated cooling is in the range of 0.6 to 6.5℃ / s.

[0010] In the above scheme, the cooling rate during phase change in each region is controlled in the following manner:

[0011] A. When the metal in region A begins to undergo a phase transition, the cooling rate is controlled at 2.0–6.5 °C / s;

[0012] B. When the metal in region II begins to undergo a phase transition, the cooling rate becomes 55-75% of that in region I.

[0013] C. When the phase transition begins in the metal of region III, the cooling rate becomes 76-90% of that in region I;

[0014] D. When the metal in region four begins to undergo a phase transition, the cooling rate becomes 30-50% of that in region one.

[0015] In the above scheme, the total time for accelerated cooling is 55 to 115 seconds.

[0016] Furthermore, the total cooling time is adjusted according to the rail cross-sectional specifications:

[0017] A. When 45kg / m ≤ rail cross-section specification ≤ 50kg / m, the total time for accelerated cooling is 55~95s;

[0018] B. When 50kg / m < rail cross-section specification ≤ 50kg / m, the total time for accelerated cooling is 60-105s;

[0019] C. When 55kg / m < rail cross-section specification ≤ 60kg / m, the total time for accelerated cooling is 65~115s.

[0020] Furthermore, the cooling time during phase transitions in Region 1, Region 2, Region 3, and Region 4 accounts for 10–20%, 25–40%, 25–40%, and 20–30% of the total cooling time, respectively.

[0021] In the above scheme, the test points on the cross section of the rail head are arranged according to the European rail standard BS EN13674.1. The hardness of position 1 is within 25 HB higher than the lower limit of the standard, the hardness of position 2 is within 30 HB higher than the lower limit of the standard, and the hardness of position 3 is within 20 HB higher than the lower limit of the standard. In addition, HBW2 > HBW3 + 0.3 × (HBW1 - HBW3) is satisfied, and the difference between any two of HBW1, HBW2, and HBW3 is ≤ 30 HBW. HBW1, HBW2, and HBW3 are the average hardness of positions 1, 2, and 3, respectively.

[0022] This method applies to all heat-treated steel grades in the European rail standard BS EN13674.1.

[0023] This invention is mainly based on the following design concept:

[0024] This invention utilizes the residual heat of the rolled rail for accelerated cooling, increasing the supercooling degree of the austenite-to-pearlite transformation, resulting in pearlite with finer interlamellar spacing, thereby improving the rail's hardness. During rail heat treatment, the pearlite phase transformation in the rail head occurs from the surface inwards. As accelerated cooling continues, the remaining heat in the rail head gradually decreases. If the applied cooling intensity is unreasonable, the cooling rate increases further inwards, causing the internal hardness to gradually increase, thus failing to meet the requirements of the European rail standard BS EN 13674.1 for the hardness distribution of the rail cross section.

[0025] Therefore, this invention divides the cross-section of the rail head into four regions from the surface to the interior. Region 1 corresponds to the rail tread and position 4, Region 2 corresponds to position 1, Region 3 corresponds to position 2, and Region 4 corresponds to position 3. Different cooling rates are applied when the metal in different regions undergoes phase transformation. Based on the cooling rate of Region 1, the hardness of position 1 is controlled to be within 25 HB higher than the standard lower limit by a combination of strong-weak-strong-weak cooling intensity, the hardness of position 2 is within 30 HB higher than the standard lower limit, and the hardness of position 3 is within 20 HB higher than the standard lower limit. This achieves a gradual decrease in Brinell hardness from the surface to the interior, better meeting the requirement that HBW2 > HBW3 + 0.3 × (HBW1 - HBW3) and the difference between any two of HBW1, HBW2, and HBW3 is ≤ 30 HBW.

[0026] The present invention controls the total time of accelerated cooling according to the size of the rail cross-section because the smaller the rail cross-section, the smaller the size of the rail head, and the shorter the required cooling time. Controlling the appropriate accelerated cooling time can save energy and improve production efficiency, and also avoid the formation of undesirable structures such as martensite.

[0027] The present invention sets the cooling time of Region 1, Region 2, Region 3 and Region 4 to 10-20%, 25-40%, 25-40% and 20-30% of the total cooling time, respectively, because the pearlitic phase transformation process of the rail head metal proceeds gradually from the surface to the interior. The settings are based on the amount of metal covered by the four regions and their distance from the rail head surface.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] This invention provides an online heat treatment process for precisely controlling the hardness and distribution of the rail head cross-section. This method precisely controls the hardness at positions 1, 2, 3, and 4 of the rail head cross-section by rationally setting the cooling rate during the pearlite phase transformation at different depths of the rail head cross-section. This regulates the Brinell hardness distribution of the rail head cross-section to meet the comprehensive requirements of the European rail standard BS EN 13674.1. The process is simple, highly operable, and easy to promote and apply. Attached Figure Description

[0030] Figure 1 This is a distribution diagram of the Brinell hardness test points on the cross section of the rail head of heat-treated steel rails, as specified in the European rail standard BS EN 13674.1. Detailed Implementation

[0031] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0032] In the following embodiments, the conventional rail production process is first carried out, including converter smelting, refining, vacuum treatment, continuous casting, billet heating, and rolling, to obtain hot-rolled rails. Then, the method of the present invention is used for online heat treatment. The online heat treatment process includes the following steps:

[0033] 1) Divide the cross section of the rail head into four regions from the surface to the inside: Region 1 is ≤6mm from the surface, Region 2 is 6mm < ≤11mm from the surface, Region 3 is 11mm < ≤21mm from the surface, and Region 4 is >21mm from the surface.

[0034] 2) The hot-rolled rails in the austenitic state are subjected to online accelerated cooling. The initial accelerated cooling temperature is 680-820℃. When pearlitic phase transformation occurs in regions one, two, three and four in sequence, different cooling intensities are applied to the rail head surface so that the cooling rate changes in the following pattern: phase transformation in region one > phase transformation in region three > phase transformation in region two > phase transformation in region four, and the cooling rate is in the range of 0.6-6.5℃ / s.

[0035] Specifically, the cooling rate during phase change in each region is controlled in the following manner:

[0036] A. When the metal in region A begins to undergo a phase transition, the cooling rate is controlled at 2.0–6.5 °C / s.

[0037] B. When the metal in region II begins to undergo a phase transition, the cooling rate becomes 55-75% of that in region I.

[0038] C. When the phase transition begins in the metal of region III, the cooling rate becomes 76-90% of that in region I;

[0039] D. When the metal in region four begins to undergo a phase transition, the cooling rate becomes 30-50% of that in region one.

[0040] The total accelerated cooling time is 55–115 seconds, and the total accelerated cooling time is adjusted according to the rail cross-section specifications.

[0041] A. When 45kg / m ≤ rail cross-section specification ≤ 50kg / m, the total time for accelerated cooling is 55~95s;

[0042] B. When 50kg / m < rail cross-section specification ≤ 50kg / m, the total time for accelerated cooling is 60-105s;

[0043] C. When 55kg / m < rail cross-section specification ≤ 60kg / m, the total time for accelerated cooling is 65-115s;

[0044] Furthermore, the cooling time during phase transitions in Region 1, Region 2, Region 3, and Region 4 accounts for 10–20%, 25–40%, 25–40%, and 20–30% of the total cooling time, respectively.

[0045] After the temperature of the rail tread surface is accelerated to 420-520℃, it is allowed to cool naturally to room temperature.

[0046] The specific process parameters for each embodiment are detailed in Tables 1 to 3.

[0047] Table 1 shows the steel grades, rail cross-sectional specifications, and accelerated cooling process parameters used in each embodiment.

[0048]

[0049] Table 2 shows the accelerated cooling parameters for Region 1 and Region 2 in each embodiment.

[0050]

[0051] Table 3 shows the accelerated cooling parameters for Region 3 and Region 4 in each embodiment.

[0052]

[0053] Test points were set up on the cross section of the rail head obtained in each embodiment according to the European rail standard BS EN13674.1, and the Brinell hardness of each point was tested. The results are shown in Tables 4 to 6.

[0054] Table 4. Brinell hardness distribution of railhead cross section of R350HT steel rail.

[0055]

[0056] Table 5. Brinell hardness distribution of railhead cross section of R370CrHT steel rail.

[0057]

[0058] Table 6. Brinell hardness distribution of the railhead cross section of R400HT steel rail.

[0059]

[0060] As can be seen from Tables 4 to 6, the Brinell hardness distribution of the rail head cross section obtained by the present invention meets the comprehensive requirements of the European rail standard BS EN 13674.1.

[0061] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An online heat treatment process for precisely controlling the hardness distribution of the rail head cross section, characterized in that, Includes the following steps: 1) Divide the cross section of the rail head into four regions from the surface to the inside: Region 1 is ≤6mm from the surface, Region 2 is 6mm < ≤11mm from the surface, Region 3 is 11mm < ≤21mm from the surface, and Region 4 is >21mm from the surface. 2) Online accelerated cooling is applied to the austenitic rails after hot rolling. The initial accelerated cooling temperature is 680~820℃. Different cooling intensities are applied to the rail head surface as pearlitic phase transformations occur sequentially in regions one, two, three, and four, resulting in a cooling rate of: A. When the pearlite phase transition first occurs in the region, the cooling rate is controlled at 2.0~6.5℃ / s; B. When the pearlite phase transition begins in region two, the cooling rate becomes 55-75% of that in region one; C. When the pearlite phase transition begins in region three, the cooling rate becomes 76-90% of that in region one; D. When the pearlite phase transition begins in region four, the cooling rate becomes 30-50% of that in region one; The total time for accelerated cooling is controlled to be 55~115s. The cooling time during phase transformation in Zone 1, Zone 2, Zone 3 and Zone 4 accounts for 10~20%, 25~40%, 25~40% and 20~30% of the total cooling time, respectively. The rail tread temperature is accelerated to 420~520℃ and then naturally cooled to room temperature.

2. The online heat treatment process for precisely controlling the hardness distribution of the rail head cross section according to claim 1, characterized in that, The accelerated cooling rate is in the range of 0.6~6.5℃ / s.

3. The online heat treatment process for precisely controlling the hardness distribution of the rail head cross section according to claim 1, characterized in that, The total cooling time is adjusted according to the rail cross-section specifications: A. When 45kg / m≤rail section specification≤50kg / m, the total time for accelerated cooling is 55~95s; B. When 50kg / m < rail cross-section specification ≤ 50kg / m, the total time for accelerated cooling is 60~105s; C. When 55kg / m < rail cross-section specification ≤ 60kg / m, the total time for accelerated cooling is 65~115s.

4. The online heat treatment process for precisely controlling the hardness distribution of the rail head cross section according to claim 1, characterized in that, The cross-section of the obtained rail head is marked with test points according to the European rail standard BS EN 13674.

1. The hardness of position 1 is within 25 HB higher than the lower limit of the standard, the hardness of position 2 is within 30 HB higher than the lower limit of the standard, and the hardness of position 3 is within 20 HB higher than the lower limit of the standard. It also satisfies that HBW2 > HBW3 + 0.3 × (HBW1 - HBW3), and the difference between any two of HBW1, HBW2, and HBW3 is ≤ 30 HBW. HBW1, HBW2, and HBW3 are the average hardness of positions 1, 2, and 3, respectively.

5. The online heat treatment process for precisely controlling the hardness distribution of the rail head cross section according to claim 1, characterized in that, This method applies to all heat-treated steel grades in the European rail standard BS EN 13674.1.

Images