CN115505713B - Heat treatment process for reducing residual stress of hundred-meter online heat-treated bainitic steel rail - Google Patents
Heat treatment process for reducing residual stress of hundred-meter online heat-treated bainitic steel rail Download PDFInfo
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Abstract
The invention discloses a heat treatment process for reducing the residual stress of a hundred-meter online heat-treated bainite steel rail, which comprises the following steps: molten iron pretreatment, smelting in a combined blowing converter, LF refining, VD vacuum degassing, bloom continuous casting, billet slow cooling, billet heating, hundred-meter rail rolling, hundred-meter online heat treatment, hundred-meter cooling bed cooling, composite straightening, flaw detection, processing, inspection and hundred-meter tempering. The invention controls the temperature of the heat treatment line at different positions of the hundred-meter steel rail, the cooling speed at different temperature stages and the temperature returning temperature, so that the hundred-meter steel rail has better flatness on the premise that the strength, the hardness and the toughness of the online heat treatment bainite steel rail meet the requirements, and the residual stresses of the rail head and the rail bottom at different positions of the hundred-meter steel rail are less than or equal to 250MPa by combining a straightening process and a tempering process, and the fluctuation range of the total length residual stress of the hundred-meter steel rail is less than or equal to 20MPa.
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 a hundred-meter online heat-treated bainite steel rail.
Background
Along with the progress of railway heavy load transportation technology, the heavy load transportation capability of the railway in China is continuously improved, the axle weight of heavy load trucks is gradually improved, and the heavy load railway has the characteristics of large axle weight, large transportation capacity, high departure density and the like, for example, the large Qin line traffic of the Taiyuan in 2021 reaches 5.5 hundred million tons/year. The rapid development of the steel rail for the heavy haul railway brings more severe requirements on the toughness, the wear resistance and the fatigue resistance of the steel rail material of a steel mill. The strength and toughness of the pearlitic steel rail commonly used at present are basically limited due to the characteristics of components and tissue structures, and the pearlitic steel rail is low in impact toughness and fracture toughness, so that the service requirements of heavy haul railways cannot be completely met. Under the background, the method promotes the development of the steel rail suitable for heavy-duty transportation, 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 the laying of heavy-duty railway turnouts and positive lines.
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 bainitic steel rail is produced by adopting an online controlled cooling process, the proportion of the steel rail structure is changed by online controlled cooling, the proportion of martensite and the transformation of lower bainite are improved, and the spacing between structural pieces is further refined, so that various mechanical performance indexes of the steel rail are obviously improved.
After the online heat treatment, the bainitic steel rail can generate higher structural stress in the structural transformation, and meanwhile, as the yield strength of the online heat treatment steel rail is higher, 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 macroscopic and micro-area residual stress. In addition, in the hundred-meter steel rail production process, uneven and large fluctuation conditions exist in the total length residual stress, so that high residual stress can occur in the part of the upper line steel rail, and the residual stress is too high. When a train passes through, after the contact stress, the dynamic bending stress, the acting force of the wheel rail and the residual stress are superposed, the inside of the steel rail is subjected to higher stress, and when very small inclusions exist in the inside, stress concentration is generated at the position, fatigue cracks are initiated, nuclear damage in the rail head is formed, and the use and service of the steel rail are seriously affected.
At present, the online heat treatment process of the bainitic steel rail is not researched and invented for controlling flatness, straightening additional stress and residual stress in the production process of the bainitic steel rail, and the residual stress and the service state of the steel rail head are important in the actual service process of the steel rail without any description of the residual stress of the steel rail head. Therefore, the reasonable online cooling control process and tempering process further reduce the residual stress of the steel rail, particularly 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 publication number of CN106755900B provides a high-strength high-toughness bainitic steel rail and an online cooling control process thereof, the online cooling control of the patent is divided into three stages to cool the steel rail to 180-210 ℃, the patent provides specific data for tensile strength, tread hardness, room temperature impact and elongation, does not give clear description for the residual stress index of the steel rail, does not show whether the steel rail is subjected to a hundred-meter heat treatment process, does not carry out tempering treatment on the steel rail, and martensite generated by phase transformation is brittle, which is unfavorable for improving the service performance of the steel rail. From the practical application result of the steel rail, the tempering treatment of the bainite steel rail is an indispensable step, and the residual stress of the steel rail directly influences the service state of the steel rail, in particular the stability and the fluctuation of the residual stress of the rail head and the residual stress of the hundred meters steel rail.
Patent publication No. CN 102534403A provides a bainite heat-treated steel rail and a heat treatment method thereof, and the patent accelerates cooling to 200 ℃ at a cooling rate of 0.4-8 ℃/s after rolling and forming the steel rail on line, and then air-cooling to room temperature. The hardness of the obtained steel rail is 340-350HB, the room temperature impact is 81-87J, the hardness and toughness of the steel rail are low, the requirement of heavy haul railway application is not met, tempering treatment is not carried out on the steel rail, the structure, the residual stress and the service stability of the steel rail are not facilitated, and specific fluctuation indexes and stability are not provided for the throughout 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, wherein the patent is a heat treatment process route of normalizing, adjusting treatment, quenching and tempering, and the steel rail is heated to 900-940 ℃ and the heat preservation time is more than 7 hours; directly quenching to 320-350 ℃ by water cooling, carrying out isothermal phase change treatment, and keeping the temperature for more than 5 hours. Because the steel rail is required to be subjected to re-austenitizing and normalizing treatment, online waste heat treatment of the steel rail is not adopted, so that the excessively high energy consumption is increased, and the energy conservation and emission reduction are not facilitated. The isothermal transformation treatment at 320-350 ℃ is carried out on the steel rail for more than 5 hours, which is not beneficial to the actual production of the steel rail, and rail production factories at home and abroad do not have equipment for carrying out heat preservation and isothermal treatment on the steel rail after heat treatment of hundred meters.
Disclosure of Invention
The invention aims to provide a heat treatment process for reducing the residual stress of a hundred-meter online heat-treated bainitic steel rail, which is characterized in that the hundred-meter steel rail has better flatness on the premise that the strength, the hardness and the toughness of the online heat-treated bainitic steel rail meet the requirements by controlling the temperature of a heat treatment line at different positions of the hundred-meter steel rail, the cooling speed at different temperature stages and the temperature return temperature, and the residual stress of rail heads and rail bottoms at different positions of the hundred-meter steel rail is less than or equal to 250MPa by combining a straightening process and a tempering process, and the fluctuation range of the total length residual stress of the hundred-meter steel rail is less than or equal to 20MPa.
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 hundred-meter online heat-treated bainitic steel rail, which comprises the following steps of: molten iron pretreatment, smelting by a combined blown converter, LF refining, VD vacuum degassing, bloom continuous casting, billet slow cooling, billet heating, hundred-meter rail rolling, hundred-meter online heat treatment, hundred-meter cooling bed cooling, composite straightening, flaw detection, processing, inspection and hundred-meter tempering; wherein:
the large square billet rolling process route comprises the following steps: heating a bloom casting blank with a section of 280mm multiplied by 380mm, descaling by high-pressure water, cogging by a BD1 roughing mill, rolling by a BD2 roughing mill, descaling by high-pressure water and rolling by a CCS finishing mill;
after the steel rail is subjected to CCS high-precision rolling, the final rolling temperature is 930-980 ℃, and the rail head of the steel rail is air-cooled to 710-780 ℃ at a cooling rate of 1.0-2.0 ℃/s;
the temperature of the full length A-E rail of the hundred-meter steel rail is gradually reduced before the hundred-meter steel rail enters the on-line heat treatment unit, the temperature of the front end (A rail) of the hundred-meter steel rail is 780-800 ℃, the temperature of the rear end (E rail) of the hundred-meter steel rail is 690-710 ℃, and the temperature difference of the front end and the rear end of the hundred-meter steel rail is about 90-100 ℃;
in order to further stabilize the full length performance of the hundred-meter online heat-treated steel rail, have better flatness and more uniform full length residual stress, online induction heating compensation equipment is used for carrying out heat compensation on the B-E rail of the hundred-meter steel rail, and the full length temperature of the hundred-meter steel rail is ensured to be 780-800 ℃;
after thermal compensation is carried out on the hundred-meter steel rail B-E, the full length temperature is 780-800 ℃, and at the moment, the hundred-meter steel rail enters an online heat treatment production line, and the online heat treatment roller speed of the steel rail is controlled to be 0.5-1.0 m/s;
performing online rail control cooling by using more than 19 groups of fan groups, performing air-jet treatment on the tread of the rail head, the side surface of the rail head and the rail bottom, controlling the air pressure of the online control cooling at 15-28 KPa, controlling the air cooling section at 200-300S, controlling the outlet temperature of the rail head of the A-E section at 250-300 ℃, and controlling the outlet temperature of the rail bottom of the A-E section at 340-390 ℃;
immediately carrying out heat preservation and slow cooling treatment on the hundred-meter steel rail after the hundred-meter steel rail is discharged from an online heat treatment production line, wherein the highest temperature return of the sections A-E of the heat preservation section steel rail is between 350 and 400 ℃;
hundred-meter steel rail is slowly cooled to room temperature on a cooling bed at a cooling speed of 0.01-0.1 ℃/s;
the hundred-meter steel rail is straightened by adopting a flat-vertical composite straightener, the straightening process adopts a straightening thought of minimum rolling reduction under the premise of meeting the requirement of the steel rail flatness, the rolling reduction of the 2# roller is less than or equal to 18mm, the rolling reduction of the 4# roller 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 performed once; 12 Tempering the straightened and flaw-detected hundred-meter steel rail in time, wherein the heating temperature of the tempering process is between 250 and 350 ℃, the heat preservation is carried out for 30 to 50 hours, and the hundred-meter steel rail is slowly cooled to below 200 ℃ along with a furnace after the heat preservation is finished, and is discharged from the furnace and cooled to the room temperature.
Further, the steel rail comprises the following chemical components in percentage by weight: 0.16 to 0.25 percent, si:0.70 to 1.20 percent, mn:1.60 to 2.45 percent, cr:0.6 to 1.20 percent, less than or equal to 0.70 percent of Ni, 0.15 to 0.60 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 and Al: less than or equal to 0.010 percent, and the balance of Fe and unavoidable impurities.
Further, the cross section of the bloom was 280mm by 380mm.
Compared with the prior art, the invention has the beneficial technical effects that:
the hundred-meter online heat treatment bainite steel rail obtained by the invention has the advantages that the rail head residual stress is reduced to 229-249 MPa, the rail bottom residual stress is reduced to 231-246 MPa, and the full length residual stress fluctuation range of the hundred-meter 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%, the normal temperature impact is more than or equal to 110J, the tread hardness is 423-430 HBW, the fluctuation of the full length mechanical property of the hundred-meter 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 described with reference to the following description of the drawings.
FIG. 1 is a corresponding organization of different heat treatment processes.
Description of the preferred embodiments
A heat treatment process for reducing the residual stress of a hundred-meter online heat-treated bainite steel rail comprises the following steps: molten iron pretreatment, smelting by a combined blown converter, LF refining, VD vacuum degassing, bloom continuous casting, billet slow cooling, billet heating, hundred-meter rail rolling, hundred-meter online heat treatment, hundred-meter cooling bed cooling, composite straightening, flaw detection, processing, inspection and hundred-meter tempering; the method specifically comprises the following steps:
1) The steel rail takes C, mn, si, cr, ni, mo as a main alloy element, and micro-alloying elements such as Nb, V, ce and the like are added, wherein the weight percentage of the chemical components of the steel rail is C:0.16 to 0.25 percent, si:0.70 to 1.20 percent, mn:1.60 to 2.45 percent, cr:0.6 to 1.20 percent, less than or equal to 0.70 percent of Ni, 0.15 to 0.60 percent of Mo, less than or equal to 0.06 percent of Nb, less than or equal to 0.09 percent of V, 5 to 10ppm of Ce, less than or equal to 0.022 percent of P, less than or equal to 0.015 percent of S and Al: less than or equal to 0.010 percent, and the balance of Fe. Smelting, continuous casting and slow cooling the steel billet to obtain a bloom with the section of 280mm multiplied by 380mm.
2) Heating a bloom casting blank with a section of 280mm multiplied by 380mm, descaling by high-pressure water, cogging by a BD1 roughing mill, rolling by a BD2 roughing mill, descaling by high-pressure water and rolling by a CCS finishing mill; the billet rolling compression ratio is not less than 9:1, ensuring the grain size of original austenite of a steel rail; in the rolling process of the steel rail, multiple stages of high-pressure injection descaling is adopted to effectively remove the oxide skin, and surface defects caused by the pressing of the oxide skin and abnormal reflected waves of rail web and rail bottom flaw detection are prevented.
3) After the steel rail is rolled with high precision by CCS, the final rolling temperature of the hundred-meter steel rail is between 930 and 980 ℃, and the rail head of the steel rail is cooled to 710 to 780 ℃ by air at a cooling rate of 1.0 to 2.0 ℃/s.
4) Before the hundred-meter steel rail enters the online heat treatment unit, online induction heating compensation equipment is used for carrying out heat compensation on the B-E rail of the hundred-meter steel rail. After the steel rail is subjected to thermal compensation, the full length temperature of the section A-E of the hundred-meter steel rail entering the online heat treatment unit is 780-800 ℃.
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) And (3) performing online rail control cooling by using more than 19 groups of fan groups, performing air-jet treatment on the tread of the rail head, the side surface of the rail head and the rail bottom, controlling the cooling air pressure to be 15-28 KPa, controlling the air cooling section to be 200-300S, controlling the outlet temperature of the rail head of the A-E section to be 250-300 ℃, and controlling the outlet temperature of the rail bottom of the A-E section to be 340-390 ℃.
7) And immediately carrying out heat preservation and slow cooling treatment on the hundred-meter steel rail after the hundred-meter steel rail is discharged from an online heat treatment production line, wherein the highest return temperature of the section A-E of the heat preservation section steel rail is between 350 and 400 ℃, and the hundred-meter steel rail is slowly cooled to room temperature on a cooling bed at a cooling speed of 0.01 to 0.1 ℃/s.
8) The hundred-meter steel rail is straightened by adopting a flat-vertical composite straightener, the straightening rolling reduction of a No. 2 roller is less than or equal to 18mm, the straightening rolling reduction of a No. 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) Tempering the straightened and flaw-detected hundred-meter steel rail in time, wherein the heating temperature of the tempering process is between 250 and 350 ℃, the heat preservation is carried out for 30 to 50 hours, and the hundred-meter steel rail is slowly cooled to below 200 ℃ along with a furnace after the heat preservation is finished, and is discharged from the furnace and cooled to the room temperature.
Table 1 comparison of different heat treatment processes for the examples
As can be seen from Table 1, the rail compositions corresponding to examples 1 to 4 are the same, and the rail compositions are C:0.16 to 0.25 percent, si:0.70 to 1.20 percent, mn:1.60 to 2.45 percent, cr:0.6 to 1.20 percent, less than or equal to 0.70 percent of Ni, 0.15 to 0.60 percent of Mo, less than or equal to 0.06 percent of Nb, less than or equal to 0.09 percent of V, 5 to 10ppm of Ce, less than or equal to 0.022 percent of P, less than or equal to 0.015 percent of S and Al: less than or equal to 0.010 percent, and the balance of Fe. Different heat treatment processes, performance and residual stress comparisons were performed at the same composition.
Compared with the embodiment 1 and the embodiment 2, the temperature of the heat treatment unit entering the A-E sections of the hundred-meter steel rails in the embodiment 3 and the embodiment 4 is controlled between 780 ℃ and 800 ℃, the uniformity of the full-length temperature field of the hundred-meter steel rails is effectively improved, the temperature difference of the heat treatment unit exiting the A-E sections of the embodiment 3 and the embodiment 4 is less than or equal to 8 ℃, the full-length temperature difference of the hundred-meter steel rails is smaller, and the uniformity and the stability of the full-length residual stress and the mechanical property of the hundred-meter steel rails are favorably controlled.
Compared with the embodiment 3, the embodiment 4 further improves the temperature of the steel rail heat-treatment unit, combines the heat conduction and the phase change latent heat of the rail head center part and the rail waist part in the steel rail heat treatment process, ensures that the temperature of the steel rail is between 350 and 410 ℃, combines the heat preservation slow cooling treatment (the cooling speed is between 0.01 and 0.1 ℃/s), and ensures that the steel rail is subjected to long-time and sufficient phase change in the phase change region of bainite, thereby achieving the purposes of controlling the structure proportion and reducing the structure stress. By increasing the proportion of bainite, the flatness of the steel rail can be effectively ensured, the steel rail has good flatness, and the straightening reduction and the straightening additional stress in the straightening process can be reduced.
Table 2 mechanical properties of the rails according to the examples
Table 3 examples correspond to the residual stresses at different positions of the hundred meter rail
As can be seen from tables 2 and 3, the wide-length tensile strength fluctuation and the wide-length tread hardness fluctuation of the hundred-meter rails of example 3 and example 4 are relatively small, and example 4 has better flatness and lowest performance fluctuation, compared with examples 1 and 2. The straightening process of the straightening machine No. 2 roller and the straightening process of the straightening machine No. 4 roller and the hundred-meter steel rail tempering treatment are combined, the residual stresses of the rail head and the rail bottom of the hundred-meter steel rail obtained in the embodiment 4 are all less than or equal to 250MPa, and the fluctuation range of the total length residual stress of the hundred-meter steel rail is less than or equal to 20MPa.
The hundred-meter online heat treatment bainite steel rail obtained by the invention has the advantages that the rail head residual stress is reduced to 229-249 MPa, the rail bottom residual stress is reduced to 231-246 MPa, and the full length residual stress fluctuation range of the hundred-meter 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%, the normal temperature impact is more than or equal to 110J, the tread hardness is 423-430 HBW, the fluctuation of the full length mechanical property of the hundred-meter 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 structures of the embodiments 1-4, the steel rail structure is not deteriorated, and the steel rail structure is a complex phase structure of bainite and martensite, and has higher structure stability.
The above embodiments are only illustrative of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention, and various modifications and improvements made by those skilled in the art to the technical solutions of the present invention should fall within the protection scope defined by the claims of the present invention without departing from the design spirit of the present invention.
Claims (1)
1. A heat treatment process for reducing the residual stress of a hundred-meter online heat-treated bainite steel rail is characterized by comprising the following steps of: the bainitic steel rail material production process comprises the following steps: molten iron pretreatment, smelting by a combined blown converter, LF refining, VD vacuum degassing, bloom continuous casting, billet slow cooling, billet heating, hundred-meter rail rolling, hundred-meter online heat treatment, hundred-meter cooling bed cooling, composite straightening, flaw detection, processing, inspection and hundred-meter tempering; wherein:
the large square billet rolling process route comprises the following steps: heating a bloom casting blank with a section of 280mm multiplied by 380mm, descaling by high-pressure water, cogging by a BD1 roughing mill, rolling by a BD2 roughing mill, descaling by high-pressure water and rolling by a CCS finishing mill;
after the steel rail is subjected to CCS high-precision rolling, the final rolling temperature is 930-980 ℃, and the rail head of the steel rail is air-cooled to 710-780 ℃ at a cooling rate of 1.0-2.0 ℃/s;
the temperature of the full length A-E rail of the hundred-meter steel rail is gradually reduced before the hundred-meter steel rail enters an online heat treatment unit, the temperature of the front end of the hundred-meter steel rail is 780-800 ℃, the temperature of the rear end of the hundred-meter steel rail is 690-710 ℃, and the temperature difference of the front end and the rear end of the hundred-meter steel rail is 90-100 ℃;
in order to further stabilize the full length performance of the hundred-meter online heat-treated steel rail, and have excellent flatness and uniform full length residual stress, online induction heating compensation equipment is used for carrying out heat compensation on the B-E rail of the hundred-meter steel rail, so that the full length temperature of the hundred-meter steel rail is ensured to be 780-800 ℃;
after thermal compensation is carried out on the hundred-meter steel rail B-E, the full length temperature is 780-800 ℃, and at the moment, the hundred-meter steel rail enters an online heat treatment production line, and the online heat treatment roller speed of the steel rail is controlled to be 0.5-1.0 m/s;
performing online rail control cooling by using more than 19 groups of fan groups, performing air-jet treatment on the tread of the rail head, the side surface of the rail head and the rail bottom, controlling the air pressure of the online cooling to be 15-28 kPa, controlling the air cooling period to be 200-300 s, controlling the outlet temperature of the rail head of the section A-E to be 250-300 ℃, and controlling the outlet temperature of the rail bottom of the section A-E to be 340-390 ℃;
immediately carrying out heat preservation and slow cooling treatment on the hundred-meter steel rail after the hundred-meter steel rail is discharged from an online heat treatment production line, wherein the highest temperature return of the sections A-E of the heat preservation section steel rail is between 350 and 400 ℃;
slowly cooling the hundred-meter steel rail to room temperature on a cooling bed at a cooling speed of 0.01-0.1 ℃/s;
the hundred-meter steel rail is straightened by adopting a flat-vertical composite straightener, the straightening process adopts a straightening thought of minimum rolling reduction under the premise of meeting the requirement of the steel rail flatness, the rolling reduction of the 2# roller is less than or equal to 18mm, the rolling reduction of the 4# roller 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 performed once;
tempering the straightened and flaw-detected hundred-meter steel rail in time, wherein the heating temperature of the tempering process is between 250 and 350 ℃, the heat preservation is carried out for 30 to 50 hours, and the hundred-meter steel rail is slowly cooled to below 200 ℃ along with a furnace after the heat preservation is finished, and is discharged from the furnace and cooled to the room temperature;
the steel rail comprises the following chemical components in percentage by weight: 0.16 to 0.25 percent, si:0.70 to 1.20 percent, mn:1.60 to 2.45 percent, cr:0.6 to 1.20 percent, less than or equal to 0.70 percent of Ni, 0.15 to 0.60 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 and Al: less than or equal to 0.010 percent, and the balance of Fe and unavoidable impurities.
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