CN115505713A - Heat treatment process for reducing residual stress of Baimi online heat treatment bainite steel rail - Google Patents
Heat treatment process for reducing residual stress of Baimi online heat treatment bainite steel rail Download PDFInfo
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Abstract
The invention discloses a heat treatment process for reducing the residual stress of Baimi online heat treatment bainite steel rails, which comprises the following production processes: molten iron pretreatment → combined blown converter smelting → LF refining → VD vacuum degassing → large square billet continuous casting → steel billet slow cooling → steel billet heating → hectometer steel rail rolling → hectometer online heat treatment → hectometer cooling → composite straightening → flaw detection → processing, inspection → hectometer tempering. According to the invention, the temperature of the Baimi steel rail placed in different positions in a heat treatment line, the cooling speed and the temperature return temperature in different temperature stages are controlled, so that the Baimi steel rail has better flatness on the premise that the strength, hardness and toughness of the Baimi steel rail subjected to online heat treatment meet the requirements, and the head residual stress and the bottom residual stress of the Baimi steel rail at different positions are less than or equal to 250MPa and the full-length residual stress fluctuation range of the Baimi steel rail is less than or equal to 20MPa by combining a straightening process and a tempering process.
Description
Technical Field
The invention relates to the field of steel rail heat treatment, in particular to a heat treatment process for reducing the residual stress of Baimi online heat-treated bainite steel rails.
Background
With the progress of railway heavy-load transportation technology, the railway heavy-load transportation capacity of China is continuously improved, the axle weight of heavy-load trucks is gradually improved, and the heavy-load railway has the characteristics of great axle weight, large transportation capacity, high departure density and the like, for example, the traffic volume of the Taiyuan Daqin line reaches 5.5 hundred million tons per year in 2021. The rapid development of the steel rail for the heavy-duty railway puts more rigorous requirements on the obdurability, wear resistance and fatigue resistance of steel rail materials in a steel mill. The pearlite steel rail commonly used at present basically exerts the toughness to the utmost limit due to the characteristics of components and tissue structures, has lower impact toughness and fracture toughness, and can not completely meet the service requirements of heavy haul railways. Under the background, the development of steel rails suitable for heavy haul railway is promoted, the bainite steel rail has good strength, plasticity, impact toughness, wear resistance and rolling contact fatigue resistance, and the bainite steel rail has better weldability and is very suitable for turnout and main track laying of heavy haul railway.
In order to meet the requirements of high strength and toughness, high wear resistance and excellent fatigue resistance of the steel rail for the heavy haul railway, the bainite steel rail is produced by adopting an online controlled cooling process, the steel rail structure proportion is changed by online controlled cooling, the martensite proportion and the lower bainite transformation are improved, and the structure piece spacing is further refined, so that various mechanical property indexes of the steel rail are obviously improved.
After the bainite steel rail is subjected to online heat treatment, higher structural stress can be generated in the structural transformation, and meanwhile, because the yield strength of the steel rail is higher in an online heat treatment state, the straightening pressure of the steel rail is correspondingly improved in the straightening process, so that the additional straightening stress of the steel rail is higher, and the structural stress and the straightening stress are superposed, so that the steel rail generates higher residual stress in macro and micro areas. In the production process of the hectometer steel rail, the situation that the through residual stress is uneven and fluctuates greatly exists, so that the high residual stress can occur on the part of the on-line steel rail, and the residual stress is too high. When a train passes through, after the contact stress of the wheel rail, the dynamic bending stress, the acting force of the wheel rail and the residual stress are superposed, the inside of the steel rail is forced to bear higher stress action, when small inclusions exist in the inside of the steel rail, stress concentration is generated at the position, fatigue cracks are generated, nuclear damage in a rail head is formed, and the use and the service of the steel rail are seriously influenced.
At present, the on-line heat treatment process of the bainite steel rail does not carry out research and invention aiming at the control of straightness, the control of straightening additional stress and the control of residual stress in the production process of the bainite steel rail, and all the research and invention do not explain the residual stress of a steel rail head, and the residual stress and the service state of the steel rail head are the key points in the actual service process of the steel rail. Therefore, the reasonable on-line controlled cooling process and tempering process further reduce the residual stress of the steel rail, especially the residual stress of the rail head on the premise of improving the comprehensive mechanical property of the bainite steel rail, thereby effectively improving the service performance of the steel rail.
The patent with the publication number of CN106755900B provides a high-strength high-toughness bainite steel rail and an online controlled cooling process thereof, wherein the steel rail is cooled to 180-210 ℃ in three stages by online controlled cooling, specific data are provided for tensile strength, tread hardness, room temperature impact and elongation, clear description is not given to the index of residual stress of the steel rail, whether the steel rail is a heat treatment process of a hectometer steel rail or not is not reflected, tempering treatment is not carried out on the steel rail, martensite generated by phase transformation is brittle, and the improvement of the service performance of the steel rail is not facilitated. From the practical application result of the steel rail, the bainite steel rail is an essential step for tempering treatment, and the residual stress of the steel rail directly influences the service state of the steel rail, particularly the residual stress of a rail head and the stability and the volatility of the residual stress of the hectometer steel rail.
The patent with publication number CN 102534403A provides a bainite heat-treated steel rail and a heat treatment method thereof, and the steel rail is subjected to on-line heat treatment, is subjected to accelerated cooling to 200 ℃ at a cooling speed of 0.4-8 ℃/s after being rolled and formed, and is then cooled to room temperature in air. The hardness of the obtained steel rail is 340-350HB, the room-temperature impact is 81-87J, the hardness and the toughness of the steel rail are low, the steel rail does not meet the application requirements of heavy haul railways, the steel rail is not tempered, the steel rail is not beneficial to the organization, the residual stress and the service stability of the steel rail, and specific volatility indexes and stability are not provided for the through-length performance and the residual stress of the steel rail after production.
The patent with publication number CN105385938B provides an alloy system, a heat treatment method of a bainite steel rail thereof and the bainite steel rail, the patent is a heat treatment process route of normalizing, adjusting, quenching and tempering, the steel rail is heated to 900-940 ℃, and the heat preservation time is more than 7h; water cooling and quenching directly to 320-350 ℃, isothermal phase change processing, and the heat preservation time is more than 5h. Because the steel rail needs to be subjected to re-austenitizing and normalizing treatment, the online waste heat treatment of the steel rail is not adopted, the overhigh energy consumption is increased, and the energy conservation and emission reduction are not facilitated. The steel rail is subjected to 320-350 ℃ isothermal phase change treatment for more than 5 hours, the actual production of the steel rail is not facilitated, rail production plants at home and abroad do not have equipment for carrying out heat preservation and isothermal treatment on the hundred-meter steel rail after heat treatment, the heat treatment process cannot be used for producing and applying the hundred-meter steel rail, can only be applied to producing short steel rails, and the application of bainite steel rails on the positive line is severely restricted.
Disclosure of Invention
The invention aims to provide a heat treatment process for reducing the residual stress of a hectometer on-line heat treatment bainite steel rail, which is characterized in that the hectometer steel rail has better flatness on the premise that the strength, hardness and toughness of the on-line heat treatment bainite steel rail meet the requirements by controlling the temperature of a heat treatment line at different positions of the hectometer steel rail, the cooling speed and the temperature return temperature in different temperature stages, and the residual stress of a rail head and a rail bottom at different positions of the hectometer steel rail is less than or equal to 250MPa and the fluctuation range of the full length residual stress of the hectometer steel rail is less than or equal to 20MPa by combining a straightening process and a tempering process.
In order to solve the technical problems, the invention adopts the following technical scheme:
the invention relates to a heat treatment process for reducing the residual stress of a Baimi online heat treatment bainite steel rail, which comprises the following steps: molten iron pretreatment → combined blown converter smelting → LF refining → VD vacuum degassing → bloom continuous casting → steel billet slow cooling → steel billet heating → hectometer steel rail rolling → hectometer online heat treatment → hectometer cooling → composite straightening → flaw detection → processing, inspection → hectometer tempering; wherein:
the bloom rolling process route comprises the following steps: heating a casting blank with a 280 mm-380 mm section of a large square billet → performing high-pressure water descaling → cogging a BD1 roughing mill → rolling a BD2 roughing mill → performing high-pressure water descaling → rolling by a CCS finishing mill;
after the steel rail is rolled by CCS high precision, the final rolling temperature is 930-980 ℃, and the steel rail head is air-cooled to 710-780 ℃ at the cooling speed of 1.0-2.0 ℃/s;
before the hectometer steel rail enters the on-line heat treatment unit, the temperature of the A-E rail of the full length of the hectometer steel rail is gradually reduced, the temperature of the front end (A rail) of the hectometer steel rail is between 780 and 800 ℃, the temperature of the rear end (E rail) is between 690 and 710 ℃, and the temperature difference of the front end and the rear end of the hectometer steel rail is between 90 and 100 ℃;
in order to further stabilize the full length performance of the hectometer steel rail in an online heat treatment state, have better flatness and more uniform full length residual stress, online induction heating compensation equipment is used for carrying out thermal compensation on a B-E rail of the hectometer steel rail, and the full length temperature of the hectometer steel rail is ensured to be between 780 and 800 ℃;
after the B-E rail of the hectometer steel rail is subjected to thermal compensation, the full length temperature is 780-800 ℃, the hectometer steel rail enters an online heat treatment production line at the moment, and the online heat treatment roller speed of the steel rail is controlled at 0.5-1.0 m/s;
using more than 19 groups of fan units to perform online steel rail control cooling, performing air spraying treatment on a rail head tread, a rail head side face and a rail bottom, controlling the online cooling air pressure to be 15-28 KPa, controlling the air cooling section to be 200-300S, controlling the rail head outlet temperature of an A-E section to be 250-300 ℃, and controlling the rail bottom outlet temperature of the A-E section to be 340-390 ℃;
immediately carrying out heat preservation and slow cooling treatment on the hectometer steel rail after the hectometer steel rail is taken out of the online heat treatment production line, wherein the maximum return temperature of A-E sections of the steel rail in a heat preservation section is between 350 and 400 ℃;
the hectometer steel rail is slowly cooled to room temperature on a cooling bed at a cooling speed of 0.01-0.1 ℃/s;
straightening a hundred-meter steel rail by using a vertical and horizontal composite straightening machine, wherein the straightening process follows the straightening idea of minimum rolling reduction on the premise of meeting the requirement of the straightness of the steel rail, the 2# roller straightening rolling reduction is less than or equal to 18mm, the 4# roller straightening rolling reduction is less than or equal to 15mm, the straightening speed is more than or equal to 1.5m/s, the straightening temperature is less than or equal to 60 ℃, and the straightening is only allowed to be carried out once; 12 The straightened and detected hectometer steel rail is immediately tempered, the heating temperature of the tempering process is between 250 and 350 ℃, the temperature is kept for 30 to 50 hours, the hectometer steel rail is slowly cooled to below 200 ℃ along with the furnace after the temperature is kept, and the hectometer steel rail is taken out of the furnace and cooled to the room temperature by air.
Further, the steel rail comprises the following chemical components in percentage by weight: 0.16 to 0.25%, si:0.70 to 1.20%, mn: 1.60-2.45%, cr:0.6 to 1.20 percent of Ni, less than or equal to 0.70 percent of Mo, less than or equal to 0.06 percent of Nb, less than or equal to 0.09 percent of V, less than or equal to 10ppm of Ce, less than or equal to 0.022 percent of P, less than or equal to 0.015 percent of S, al: less than or equal to 0.010 percent, and the balance of Fe and inevitable impurities.
Further, the cross section of the bloom is 280mm × 380mm.
Compared with the prior art, the invention has the beneficial technical effects that:
the hectometer online heat treatment bainite steel rail obtained by the method has the advantages that the residual stress of the rail head is reduced to 229-249 MPa, the residual stress of the rail bottom is reduced to 231-246 MPa, and the fluctuation range of the full-length residual stress of the hectometer steel rail is less than or equal to 20MPa. The tensile strength of the steel rail is 1340-1370 MPa, the yield strength is 1100-1210 MPa, the elongation is more than or equal to 16 percent, the normal temperature impact is more than or equal to 110J, the tread hardness is 423-430 HBW, the fluctuation of the mechanical property of the full length of the hectometer steel rail is very small, the fluctuation range of the full length tensile strength is less than or equal to 15MPa, and the fluctuation range of the full length tread hardness is less than or equal to 4HBW.
Drawings
The invention is further illustrated in the following description with reference to the drawings.
FIG. 1 shows different heat treatment processes corresponding to different textures.
Detailed description of the preferred embodiments
A heat treatment process for reducing the residual stress of Baimi online heat treatment bainite steel rails comprises the following production processes: molten iron pretreatment → combined blown converter smelting → LF refining → VD vacuum degassing → bloom continuous casting → steel billet slow cooling → steel billet heating → hectometer steel rail rolling → hectometer online heat treatment → hectometer cooling → composite straightening → flaw detection → processing, inspection → hectometer tempering; the method specifically comprises the following steps:
1) The steel rail of the invention takes C, mn, si, cr, ni and Mo as main alloy elements, micro-alloying elements such as Nb, V, ce and the like are added, and the steel rail comprises the following chemical components in percentage by weight: 0.16 to 0.25%, si:0.70 to 1.20%, mn: 1.60-2.45%, cr:0.6 to 1.20 percent of Ni, less than or equal to 0.70 percent of Mo, less than or equal to 0.15 to 0.60 percent of Nb, less than or equal to 0.06 percent of V, less than or equal to 0.09 percent of Ce, less than or equal to 0.022 percent of P, less than or equal to 0.015 percent of S, al: less than or equal to 0.010 percent, and the balance being Fe. The components are smelted, continuously cast and slowly cooled to obtain a bloom with the cross section of 280mm multiplied by 380mm.
2) Heating a casting blank with a section of a large square billet of 280mm multiplied by 380mm → high-pressure water descaling → BD1 roughing mill cogging → BD2 roughing mill rolling → high-pressure water descaling → CCS finishing mill rolling; the rolling reduction ratio of the steel billet is not less than 9:1, ensuring the grain size of original austenite of the steel rail; the steel rail is descaled by adopting multi-stage high-pressure jet in the rolling process so as to effectively remove oxide scale and prevent surface defects caused by pressing-in of the oxide scale and abnormal reflected waves of rail web and rail bottom flaw detection.
3) After the steel rail is rolled by CCS high precision, the finishing rolling temperature of the hectometer steel rail is between 930 and 980 ℃, and the head of the steel rail is cooled to 710 to 780 ℃ by air at a cooling speed of 1.0 to 2.0 ℃/s.
4) Before the hectometer steel rail enters the online heat treatment unit, the online induction heating compensation equipment is used for carrying out thermal compensation on the hectometer steel rail B-E. After the steel rail is subjected to thermal compensation, the full-length temperature of the A-E section of the hectometer steel rail entering the online heat treatment unit is between 780 and 800 ℃.
5) The roller speed of the hundred-meter steel rail on-line heat treatment production is controlled to be 0.5-1.0 m/s.
6) The on-line steel rail controlled cooling is carried out by using more than 19 groups of fan units, the air spraying treatment is carried out on the tread of the rail head, the side surface of the rail head and the rail bottom, the on-line cooling air pressure is controlled to be 15-28 KPa, the air cooling section control time is 200-300S, the outlet temperature of the rail head at the section A-E is controlled to be 250-300 ℃, and the outlet temperature of the rail bottom at the section A-E is controlled to be 340-390 ℃.
7) After the hectometer steel rail is taken out of the on-line heat treatment production line, the hectometer steel rail is immediately subjected to heat preservation and slow cooling treatment, the maximum return temperature of the A-E sections of the steel rails in the heat preservation section is between 350 and 400 ℃, and the hectometer steel rail of the hectometer steel rail is slowly cooled to room temperature on a cooling bed at the cooling speed of 0.01 to 0.1 ℃/s.
8) Straightening the hectometer steel rail by using a horizontal and vertical composite straightening machine, wherein the straightening reduction of a 2# roller is less than or equal to 18mm, the straightening reduction of a 4# roller is less than or equal to 15mm, the straightening speed is more than or equal to 1.5m/s, and the straightening temperature is less than or equal to 60 ℃.
9) And (3) tempering the straightened and flaw-detected hectometer steel rail in time, keeping the temperature of the tempering process at 250-350 ℃, keeping the temperature for 30-50 h, slowly cooling the hectometer steel rail to below 200 ℃ along with the furnace after the heat preservation is finished, discharging the steel rail from the furnace, and cooling the steel rail to room temperature in air.
TABLE 1 comparison of different heat treatment processes for the examples
As can be seen from table 1, the compositions of the rails according to examples 1 to 4 are the same, and the compositions of the rails are all C:0.16 to 0.25%, si:0.70 to 1.20%, mn: 1.60-2.45%, cr:0.6 to 1.20 percent of Ni, less than or equal to 0.70 percent of Mo, less than or equal to 0.15 to 0.60 percent of Nb, less than or equal to 0.06 percent of V, less than or equal to 0.09 percent of Ce, less than or equal to 0.022 percent of P, less than or equal to 0.015 percent of S, al: less than or equal to 0.010 percent and the balance of Fe. Different heat treatment processes, performances and residual stress comparisons were performed under the same composition.
Compared with the embodiment 1 and the embodiment 2, the temperature of the A-E section of the hectometer steel rail in the embodiment 3 and the embodiment 4 in the heat treatment unit is controlled between 780 ℃ and 800 ℃, the uniformity of the full-length temperature field of the hectometer steel rail is effectively improved, the temperature difference of the A-E section of the embodiment 3 and the embodiment 4 in the heat treatment unit is less than or equal to 8 ℃, and the full-length temperature difference of the hectometer steel rail is smaller, so that the uniformity and the stability of the full-length residual stress and the mechanical property of the hectometer steel rail are favorably controlled.
Compared with the embodiment 3, the embodiment 4 further increases the temperature of the steel rail out of the heat treatment unit, enables the temperature of the steel rail to be in the range of 350-410 ℃ by combining the heat conduction and the phase change latent heat of the rail head center part and the rail web part in the steel rail heat treatment process, and enables the steel rail to be subjected to long-time and sufficient phase change in the phase change interval of bainite by combining the heat preservation slow cooling treatment (the cooling speed is 0.01-0.1 ℃/s), thereby achieving the purposes of controlling the tissue proportion and reducing the tissue stress. By increasing the proportion of the bainite, the straightness of the steel rail can be effectively ensured, the good straightness of the steel rail can be ensured, and the straightening reduction and the straightening additional stress in the straightening process can be reduced.
TABLE 2 mechanical Properties of the corresponding rails of the examples
TABLE 3 residual stress at different positions of hectometer rail in each example
As can be seen from tables 2 and 3, the through-length tensile strength fluctuation and the through-length tread hardness fluctuation of the steel rails of examples 3 and 4 are relatively small, and example 4 has better flatness and the lowest performance fluctuation, compared to examples 1 and 2. By combining the straightening process of the 2# roller and the 4# roller of the straightening machine and the tempering treatment of the hectometer steel rail, the residual stress of the head and the bottom of the hectometer steel rail obtained in the embodiment 4 is less than or equal to 250MPa, and the fluctuation range of the full length residual stress of the hectometer steel rail is less than or equal to 20MPa.
The Bainite online heat treatment bainite steel rail obtained by the invention has the rail head residual stress reduced to 229-249 MPa, the rail bottom residual stress reduced to 231-246 MPa, and the full length residual stress fluctuation range of the Baimetre steel rail is less than or equal to 20MPa. The tensile strength of the steel rail is 1340-1370 MPa, the yield strength is 1100-1210 MPa, the elongation is more than or equal to 16 percent, the normal temperature impact is more than or equal to 110J, the tread hardness is 423-430 HBW, the fluctuation of the mechanical property of the full length of the hectometer steel rail is very small, the fluctuation range of the full length tensile strength is less than or equal to 15MPa, and the fluctuation range of the full length tread hardness is less than or equal to 4HBW.
In the process of the embodiment of the invention, compared with the steel rail structure of the embodiment 1-embodiment 4, the steel rail structure is not deteriorated, is a complex phase structure of bainite and martensite, and has higher structure stability.
The above-described embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements of the technical solutions of the present invention can be made by those skilled in the art without departing from the spirit of the present invention, and the technical solutions of the present invention are within the scope of the present invention defined by the claims.
Claims (3)
1. A heat treatment process for reducing the residual stress of Baimi online heat treatment bainite steel rails is characterized in that: the production process of the bainite steel rail material comprises the following steps: molten iron pretreatment → combined blown converter smelting → LF refining → VD vacuum degassing → bloom continuous casting → steel billet slow cooling → steel billet heating → hectometer steel rail rolling → hectometer online heat treatment → hectometer cooling → composite straightening → flaw detection → processing, inspection → hectometer tempering; wherein:
the bloom rolling process route comprises the following steps: heating a casting blank with a 280 mm-380 mm section of a large square billet → performing high-pressure water descaling → cogging a BD1 roughing mill → rolling a BD2 roughing mill → performing high-pressure water descaling → rolling by a CCS finishing mill;
after the steel rail is rolled by CCS high precision, the final rolling temperature is 930-980 ℃, and the steel rail head is air-cooled to 710-780 ℃ at the cooling speed of 1.0-2.0 ℃/s;
before the hectometer steel rail enters the online heat treatment unit, the temperature of the A-E rail of the full length of the hectometer steel rail is gradually reduced, the temperature of the front end of the hectometer steel rail is 780-800 ℃, the temperature of the rear end of the hectometer steel rail is 690-710 ℃, and the temperature difference of the front end and the rear end of the hectometer steel rail is about 90-100 ℃;
in order to further stabilize the full length performance of the hectometer steel rail in an online heat treatment state, have better flatness and more uniform full length residual stress, online induction heating compensation equipment is used for carrying out thermal compensation on a B-E rail of the hectometer steel rail, and the full length temperature of the hectometer steel rail is ensured to be between 780 and 800 ℃;
after the B-E rail of the hectometer steel rail is subjected to thermal compensation, the full length temperature is 780-800 ℃, the hectometer steel rail enters an online heat treatment production line at the moment, and the online heat treatment roller speed of the steel rail is controlled at 0.5-1.0 m/s;
using more than 19 groups of fan units to perform online steel rail control cooling, performing air spraying treatment on a rail head tread, a rail head side face and a rail bottom, controlling the online cooling air pressure to be 15-28 KPa, controlling the air cooling section to be 200-300S, controlling the rail head outlet temperature of an A-E section to be 250-300 ℃, and controlling the rail bottom outlet temperature of the A-E section to be 340-390 ℃;
immediately carrying out heat preservation and slow cooling treatment on the hectometer steel rail after the hectometer steel rail is taken out of the online heat treatment production line, wherein the maximum return temperature of A-E sections of the steel rail in a heat preservation section is between 350 and 400 ℃;
the hectometer steel rail is slowly cooled to room temperature on a cooling bed at a cooling speed of 0.01-0.1 ℃/s;
straightening a hundred-meter steel rail by using a vertical and horizontal composite straightening machine, wherein the straightening process follows the straightening idea of minimum rolling reduction on the premise of meeting the requirement of the straightness of the steel rail, the 2# roller straightening rolling reduction is less than or equal to 18mm, the 4# roller straightening rolling reduction is less than or equal to 15mm, the straightening speed is more than or equal to 1.5m/s, the straightening temperature is less than or equal to 60 ℃, and the straightening is only allowed to be carried out once; 12 Timely tempering the straightened and detected hectometer steel rail, keeping the heating temperature of the tempering process between 250 and 350 ℃, keeping the temperature for 30 to 50 hours, slowly cooling the hectometer steel rail to below 200 ℃ along with a furnace after the heat preservation is finished, discharging the hectometer steel rail from the furnace, and cooling the hectometer steel rail to room temperature.
2. The heat treatment process for reducing the residual stress of the hectometer online heat-treated bainite steel rail according to claim 1, wherein: the steel rail comprises the following chemical components in percentage by weight: 0.16 to 0.25%, si:0.70 to 1.20%, mn: 1.60-2.45%, cr:0.6 to 1.20 percent of Ni, less than or equal to 0.70 percent of Mo, less than or equal to 0.06 percent of Nb, less than or equal to 0.09 percent of V, less than or equal to 10ppm of Ce, less than or equal to 0.022 percent of P, less than or equal to 0.015 percent of S, al: less than or equal to 0.010 percent, and the balance of Fe and inevitable impurities.
3. The heat treatment process for reducing the residual stress of the hectometer online heat-treated bainite steel rail according to claim 1, wherein: the section of the bloom is 280mm multiplied by 380mm.
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