CN114250349A - Method for improving hardness of heat-treated steel rail tread - Google Patents
Method for improving hardness of heat-treated steel rail tread Download PDFInfo
- Publication number
- CN114250349A CN114250349A CN202111499600.3A CN202111499600A CN114250349A CN 114250349 A CN114250349 A CN 114250349A CN 202111499600 A CN202111499600 A CN 202111499600A CN 114250349 A CN114250349 A CN 114250349A
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- cooling
- steel rail
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Abstract
The invention relates to a method for improving the hardness of a heat-treated steel rail tread, belonging to the technical field of steel rail rolling production. The technical scheme is as follows: the cooling inlet temperature is controlled at 750-; substantial cooling is divided into two stages: the first stage, cooling speed is 2.5-4.0 ℃/s from the end of pre-cooling to 530 ℃ and 580 ℃; the second stage is from the end of the first stage to 480-530 ℃, the cooling speed is 1.5-2.5 ℃/s; and then air-cooling to room temperature. According to the invention, on the premise of not increasing additional equipment investment, after the water-cooling heat treatment is carried out on the steel rail with large surface iron scale, the tread hardness is improved by more than 10HBW on the basis of the required lower limit.
Description
Technical Field
The invention relates to a method for improving the hardness of a heat-treated steel rail tread, belonging to the technical field of steel rail rolling production in the metallurgical industry.
Background
The on-line waste heat quenching is the most economical and effective heat treatment method for improving the hardness performance of the steel rail, the common cooling medium is water, compressed air and air mist which are mixed, and the main production flow is heating-rolling-heat treatment. Through heat treatment, the interlayer spacing of the pearlite plates is refined, so that the hardness performance of the steel rail is improved. Thinning the pearlite block spacing requires increasing the rail cooling rate, and increasing the cooling intensity is most directly and effectively achieved by increasing the cooling medium pressure. In the online heat treatment production process with water as a medium, when the iron scale on the surface of the steel rail is large (the maximum size of the iron scale exceeds 5mm, and the surface area ratio exceeds 5%), increasing water pressure easily causes the steel rail to generate unqualified structures such as martensite at the iron scale. In actual production, when the maximum size of the scale on the tread of the steel rail exceeds 5mm and the surface area ratio exceeds 5%, in order to avoid generating local abnormal structures, small water pressure is often set, so that the overall cooling strength of the steel rail is insufficient, and the hardness performance of the tread is lower than the lower limit.
In the prior art and the published patent, in the heat treatment production using water as a cooling medium, when the scale on the tread of a steel rail is larger and more, the research on how to improve the hardness is less involved.
For example, chinese patent application No. 201910020036.9 discloses a method for eliminating R350LHT rail heat treatment black speck, in the method, in the heat treatment process, the cooling water pressure of the first three groups of cooling sections M1-M3 is gradually increased, and the water pressure of the last six groups of cooling sections M4-M9 is kept the same as the water pressure of M3. The speed of the roller way is controlled to be 0.55-0.85 m/s according to the water temperature. Although the method can effectively eliminate the abnormal structure, the method is directed at the normal and good state of the surface state of the steel rail and is not suitable for the state of large and large scale on the surface of the steel rail.
For example, a Chinese patent with the application number of 201720753938.X discloses a surface descaling device for steel rail heat treatment, which discloses a device for removing iron scale on the surface of a heat-treated steel rail, but does not describe the relationship between the hardness improvement and the structure control of the heat-treated steel rail and the iron scale, and the invention needs special equipment for removing the iron scale, thereby increasing the additional cost.
Further, as disclosed in chinese patent No. 202011074845.7, "a heat treatment method for full-length residual heat quenching of heavy rail steel", and chinese patent No. 201610914511.3, "an on-line heat treatment method and an on-line heat treatment system for steel rails", and the like, which use water or water mist as a cooling medium, do not involve the influence of iron scale on the surface of the steel rail on cooling, and may cause generation of local defective structures when increasing the cooling strength.
Disclosure of Invention
The invention aims to provide a method for improving the hardness of a tread of a heat-treated steel rail, which can improve the hardness of the tread by more than 10HBW on the basis of the required lower limit after the water-cooling heat treatment of the steel rail with larger surface scale without adding extra equipment investment, and solves the problems in the background art.
The technical scheme of the invention is as follows:
a method for improving the hardness of a heat-treated steel rail tread comprises a rolling process and a cooling process, wherein in the cooling process, the temperature of a cooling inlet is controlled to be 750-; substantial cooling is divided into two stages: the first stage, cooling speed is 2.5-4.0 ℃/s from the end of pre-cooling to 530 ℃ and 580 ℃; the second stage is from the end of the first stage to 480-530 ℃, the cooling speed is 1.5-2.5 ℃/s; and then air-cooling to room temperature.
The water spraying pressure of the pre-cooling water nozzle is set to be 5.0-7.0 bar; the water spray pressure of the water nozzles in the first stage of the substantial cooling is set to be 3.0-5.0bar, and the water spray pressure of the water nozzles in the second stage of the substantial cooling is set to be 1.5-3.0 bar.
In the pre-cooling process, the distance between the nozzles for applying water spray cooling is controlled to be 1.0-2.0m according to the advancing direction of the steel rail, the distance between the adjacent nozzles is 0.5m, water spray cooling is carried out by 1-3 nozzles at intervals, and the 1-3 nozzles are closed, so that the distance adjustment of the nozzles for applying water spray cooling is realized.
The running speed of the steel rail in the cooling process is 0.6-0.8 m/s.
In the rolling process, the water spraying pressure of a descaling water nozzle of a finished product rolling mill is set to be 12.0-15.0 MPa.
The invention has the beneficial effects that: on the premise of not increasing the investment of additional equipment, after the water-cooling heat treatment of the steel rail with large surface scale, the hardness of the tread is improved by more than 10HBW on the basis of the required lower limit.
Detailed Description
The invention is further illustrated by the following examples.
A method for improving the hardness of a heat-treated steel rail tread comprises a rolling process and a cooling process, wherein in the cooling process, the temperature of a cooling inlet is controlled to be 750-; substantial cooling is divided into two stages: the first stage, cooling speed is 2.5-4.0 ℃/s from the end of pre-cooling to 530 ℃ and 580 ℃; the second stage is from the end of the first stage to 480-530 ℃, the cooling speed is 1.5-2.5 ℃/s; and then air-cooling to room temperature.
The water spraying pressure of the pre-cooling water nozzle is set to be 5.0-7.0 bar; the water spray pressure of the water nozzles in the first stage of the substantial cooling is set to be 3.0-5.0bar, and the water spray pressure of the water nozzles in the second stage of the substantial cooling is set to be 1.5-3.0 bar.
In the pre-cooling process, the distance between the nozzles for applying water spray cooling is controlled to be 1.0-2.0m according to the advancing direction of the steel rail, the distance between the adjacent nozzles is 0.5m, water spray cooling is carried out by 1-3 nozzles at intervals, and the 1-3 nozzles are closed, so that the distance adjustment of the nozzles for applying water spray cooling is realized.
The running speed of the steel rail in the cooling process is 0.6-0.8 m/s.
In the rolling process, the water spraying pressure of a descaling water nozzle of a finished product rolling mill is set to be 12.0-15.0 MPa.
The principle of the invention is as follows: after the steel rail is subjected to UF rolling, a certain gap exists between the surface iron scale and the steel rail matrix, and the gap has a certain storage and retention effect on water, so that the local cooling strength of the water on the steel rail is increased. The larger the scale, the stronger the effect on water retention and the higher the local cooling strength, and abnormal structures such as martensite are formed. Practice shows that when the maximum size of the iron scale on the surface of the steel rail is not more than 5mm and the surface area accounts for less than 3%, the iron scale on the surface of the steel rail has weak water storage effect, and in addition, the heat return is generated in the steel rail, so that the cooling strength at the position is prevented from being obviously increased, and the generation of local unqualified structures of the steel rail is inhibited.
The field observation statistics shows that if the descaling water pressure of the finished product rolling mill is less than 12Mpa, the surface scale is easy to be bonded with the matrix after the rolling is finished. Therefore, in order to loosen the scale and reduce the binding force, the descaling hydraulic pressure of the finished rolling mill is required to be not less than 12 MPa. Meanwhile, the descaling hydraulic pressure of the finished rolling mill is set to be 12-15MPa in consideration of the capacity of descaling equipment and the fact that the steel rail is overcooled due to overhigh hydraulic pressure.
The pre-cooling function is to clean the iron scale on the surface of the steel rail by applying larger water pressure, so that the maximum size of the iron scale is controlled to be less than 5mm, and the surface area is controlled to be less than 3%. In order to ensure effective impact force on the scale, water pressure of not less than 5.0bar needs to be applied. Because the cooling water pressure is large, the distance between the nozzles for applying water spray cooling needs to be properly adjusted, so that the steel rail is properly reheated in a cooling gap, and abnormal tissue generation caused by continuous strong cooling of the steel rail is avoided. Practice shows that the water pressure is controlled to be 5.0-7.0bar, the distance between the nozzles for applying water spray cooling is adjusted to be 1.0-2.0m according to the water pressure, the effect of impact cleaning of the iron oxide scale can be ensured, and abnormal tissues in the pre-cooling process can be avoided. Because in the precooling process, the nozzles are closed at intervals, the steel rail has temperature return, so that the transformation from austenite to pearlite occurs, and the pearlite lamellar spacing is enlarged. The pre-cooling process should be completed before the austenite begins to transform, preventing the formation of pearlite with a large interplate distance, and therefore the pre-cooling termination temperature should be above 680 ℃. Meanwhile, in order to ensure the acting time of pre-cooling, the pre-cooling termination temperature is below 720 ℃. Comprehensively, the pre-cooling termination temperature is controlled in the range of 680-720 ℃.
The substantial cooling is to complete the austenite transformation of the rail and to generate a fine lamellar pearlite structure. The first stage is from austenite to finish transformation, so the temperature in the stage is from 680-720 ℃ to 530-580 ℃, the process needs to keep larger cooling speed, increase supercooling degree, form pearlite with fine-piece spacing and improve hardness. Practice shows that the cooling speed of 2.5-4.0 ℃/s is selected at the stage, so that the pearlite inter-sheet distance can be fully refined, and abnormal tissues generated due to overlarge cooling strength are prevented. In the second stage of substantial cooling, a certain cooling speed is continuously applied in order to maintain the cooling strength of the internal structure of the steel rail, but in order to avoid the generation of abnormal structures on the surface, the cooling speed in the second stage cannot be too high, and the cooling speed is kept at 1.5-2.5 ℃/s.
Preferred embodiments of the invention:
table 1: process parameters of the examples
Table 2: surface condition of the rails of each example
Tread hardness performance was measured for the rails of examples 1 to 6, and the specific measurement results are shown in Table 3.
Table 3: structural Properties of the Heat treated rails of the examples
As can be seen from Table 3, the hardness of the tread of the heat-treated steel rail is improved by 11-22HBW on the basis of the required lower limit; the structure is pearlite, and no abnormal structures such as martensite and bainite are found.
Claims (5)
1. A method for improving the hardness of a heat-treated steel rail tread comprises a rolling process and a cooling process, and is characterized in that: in the cooling process, the cooling inlet temperature is controlled to be 750-820 ℃, the cooling process comprises pre-cooling and substantial cooling, the steel rail is pre-cooled from the inlet temperature to be 680-720 ℃, and after the pre-cooling is finished, the maximum size of the scale on the surface of the steel rail is controlled to be within 5mm, and the surface area of the scale is less than 3%; substantial cooling is divided into two stages: the first stage, cooling speed is 2.5-4.0 ℃/s from the end of pre-cooling to 530 ℃ and 580 ℃; the second stage is from the end of the first stage to 480-530 ℃, the cooling speed is 1.5-2.5 ℃/s; and then air-cooling to room temperature.
2. The method for improving the hardness of the tread of the heat-treated steel rail according to claim 1, wherein the method comprises the following steps: the water spraying pressure of the pre-cooling water nozzle is set to be 5.0-7.0 bar; the water spray pressure of the water nozzles in the first stage of the substantial cooling is set to be 3.0-5.0bar, and the water spray pressure of the water nozzles in the second stage of the substantial cooling is set to be 1.5-3.0 bar.
3. The method for improving the hardness of the tread of the heat-treated steel rail according to claim 1, wherein the method comprises the following steps: in the pre-cooling process, the distance between the nozzles for applying water spray cooling is controlled to be 1.0-2.0m according to the advancing direction of the steel rail.
4. The method for improving the hardness of the tread of the heat-treated steel rail according to claim 1, wherein the method comprises the following steps: the running speed of the steel rail in the cooling process is 0.6-0.8 m/s.
5. The method for improving the hardness of the tread of the heat-treated steel rail according to claim 1, wherein the method comprises the following steps: in the rolling process, the water spraying pressure of a descaling water nozzle of a finished product rolling mill is set to be 12.0-15.0 MPa.
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| Application Number | Priority Date | Filing Date | Title |
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| CN202111499600.3A CN114250349A (en) | 2021-12-09 | 2021-12-09 | Method for improving hardness of heat-treated steel rail tread |
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| CN202111499600.3A CN114250349A (en) | 2021-12-09 | 2021-12-09 | Method for improving hardness of heat-treated steel rail tread |
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| CN114250349A true CN114250349A (en) | 2022-03-29 |
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Citations (12)
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|---|---|---|---|---|
| JPS63297521A (en) * | 1987-05-28 | 1988-12-05 | Nkk Corp | Heat treatment method for rail |
| JPH08332501A (en) * | 1995-06-07 | 1996-12-17 | Nippon Steel Corp | Manufacture of high depth/high hardness rail |
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| JP2015209590A (en) * | 2014-04-30 | 2015-11-24 | Ｊｆｅスチール株式会社 | Rail production method |
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2021
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| CN113403465A (en) * | 2021-05-18 | 2021-09-17 | 邯郸钢铁集团有限责任公司 | Method for controlling uniformity of structure performance of two sides of heat-treated steel rail head |
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