A method for producing a high-strength quenched and tempered steel for a boiler boom

Abstract

The application discloses a production method of high-strength quenched and tempered steel for boiler suspender, adopts a process route of converter+LF furnace+RH furnace+continuous casting+hot rolling+induction heating+water quenching+roller bottom type continuous annealing furnace tempering+cutting off of head and tail of fixed-length round steel+packaging, produces 35CrMoA alloy steel with a diameter of 60-80mm. The material produced by the method has the performance of tensile strength of 880-1010Mpa, yield strength of 750-900Mpa, elongation of 16%-30%, reduction of area of 50-70%, normal temperature AKu2 of 80-160J, hardness of 260-290HBW, and is a kind of quenched and tempered steel which is free of normalizing, straightening and stress relieving and stable in performance, and can be directly used for feeding with full fixed length of 9-12 meters, and is applied to boiler suspender after thread processing of the fixed-length head and tail.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a high-strength quenched and tempered steel for boiler hangers and its production process. Background Technology

[0002] Boiler hangers are critical load-bearing components in the boiler industry, used to suspend the boiler body from the top beam. They are essential load-bearing parts. During operation, the hangers bear enormous static loads from the boiler body, as well as dynamic loads caused by temperature changes, pressure fluctuations, and system vibrations. Therefore, high requirements are placed on the strength, toughness, and fatigue performance of the materials used in the hangers. The quality of the round steel used in the hangers directly affects the safety and reliability of boiler operation.

[0003] Currently, 35CrMoA alloy structural steel is commonly used for boiler hanger rods. 35CrMoA steel is a medium-carbon low-alloy structural steel with good hardenability, and after quenching and tempering, it obtains excellent comprehensive mechanical properties.

[0004] The production process of quenched and tempered steel for boiler hangers typically includes electric arc furnace smelting, continuous casting, continuous rolling, and quenching and tempering. Traditional quenching and tempering processes have the following prominent problems and shortcomings: First, the production process is lengthy, has a long cycle, and consumes a lot of energy. The traditional production process for boiler hanger steel involves hot-rolled round steel normalizing treatment + straightening + quenching and tempering treatment (quenching + tempering) + straightening + stress-relieving annealing, etc.

[0005] Secondly, the uniformity of the microstructure is difficult to guarantee, leading to insufficient performance stability. 35CrMoA hot-rolled steel contains a large amount of ferrite and bainite in its microstructure after quenching and tempering. For large-diameter round bars of 60mm to 80mm, due to the large cross-sectional size, the cooling rate of the core and surface differs significantly during quenching and cooling, easily leading to asynchronous microstructure transformation. This results in coarse core microstructure and uneven performance, causing some products to have strength indicators lower than delivery requirements after quenching and tempering.

[0006] Third, the bending stability is poor, with defects such as "hook bend" and "S" bend. Traditionally produced quenched and tempered round steel is prone to bending deformation in the length direction, especially at both ends of the fixed length, where "hook bend" is likely to occur. This defect makes subsequent thread processing difficult and requires an additional pressure straightening process.

[0007] In summary, existing production technologies for quenched and tempered steel used in boiler hangers suffer from problems such as numerous processes, high energy consumption, uneven microstructure, unstable performance, and poor bending stability, making it difficult to meet the demands of modern boiler manufacturing for efficient, energy-saving, and high-performance materials. Therefore, developing a production method for high-strength quenched and tempered steel that eliminates the need for normalizing, straightening, and stress-relief annealing, and allows for direct feeding of ultra-long, fixed-length materials, has significant industrial application value and market prospects. Summary of the Invention

[0008] The purpose of this invention is to provide a production method for high-strength quenched and tempered steel for boiler hangers that requires no normalizing, straightening, or stress-relief annealing, enabling the production of 35CrMoA quenched and tempered steel with a diameter of 60-80mm and a fixed length of 9-12 meters, thus meeting the requirements for boiler hangers.

[0009] This invention is achieved through the following technical solution: A method for producing high-strength quenched and tempered steel for boiler hangers, comprising the following process route: converter + LF furnace + RH furnace + continuous casting + hot rolling + induction heating + water quenching + rolling hearth continuous annealing furnace tempering + head and tail trimming of standard-length round steel + packaging. The method is characterized by: steel diameter of 60-80mm, standard length of 9-12 meters, and material of 35CrMoA alloy steel; the process steps include: (1) Smelting: LF+RH double refining is adopted, RH vacuum degassing treatment is carried out, the vacuum holding time is not less than 10 min at a vacuum degree of 0.5 torr, and the H of the molten steel leaving the station is ≤1.0 ppm.

[0010] (2) Continuous casting: Full-process protective casting is adopted, the tundish superheat is ≤25℃ for continuous casting furnace and ≤30℃ for open casting furnace, the continuous casting speed is 0.75~0.90m / min, and the billet size is 280×280mm.

[0011] (3) Heating: The billet heating time is 180-360 min, the heating section temperature is 1150-1200℃, and the soaking section temperature is 1160-1200℃.

[0012] (4) Rolling and cooling: The final rolling temperature is 780~830℃, and the upper cooling bed temperature is 750~800℃; the cooling bed insulation cover is closed, one tooth per steel, and the cooling rate of the round steel is ≤1.5℃ / s; the round steel maintains regular rotation when stepping on the cooling bed to achieve uniform phase transformation and straightening, and the curvature q≤2.0mm / m.

[0013] (5) Induction heating and water quenching: Induction heating and water quenching are continuous production lines, using rotating bottom support inclined rollers, the round steel rotates forward, the forward speed of the round steel is 0.008m / s; the water outlet red temperature is 30~50℃, the red temperature difference along the length of the whole fixed length round steel is ≤20℃, and the red temperature difference at the same cross section is ≤5℃.

[0014] (6) Tempering in a roller hearth continuous annealing furnace: After quenching, the round steel is evenly distributed on the roller conveyor and advances at a constant speed. It is heated by natural gas and held at 610-630℃ for 4 hours. It is then air-cooled after being taken out of the furnace.

[0015] Further, in step (5) induction heating and water quenching: 12 sets of induction coils are used for heating, the water temperature is 890~940℃, and there are no black spots on the ends of the round steel. After heating, water quenching is carried out at intervals of 150mm. Four water rings are arranged to cool the round steel. The interval between each two water rings is 0.1m. 20 nozzles are evenly arranged in the cooling water rings. The water pressure of a single water ring is 0.08~0.12Mpa, the cooling water temperature is ≤40℃, and the round steel is cooled evenly.

[0016] Further, in step (6) the round steel is tempered in a roller hearth continuous annealing furnace: the bending degree of the round steel q ≤ 2.5 mm / m, there is no hook bend, and the bending degree within 1 meter at both ends of the fixed length q ≤ 1.5 mm / m; the remanence of the round steel ≤ 20 Gs.

[0017] Furthermore, the steel produced has a tensile strength of 880–1010 MPa, a yield strength of 750–900 MPa, an elongation of 16–30%, a reduction of area of ​​50%–70%, an AKu2 of 80–160 J at room temperature, and a hardness of 260–290 HBW.

[0018] Invention Point and Innovative Principle: This invention achieves the preparation of ultra-long, fixed-length, high-strength quenched and tempered steel without normalizing, straightening, or stress-relief annealing through the following technical means: (1) High-purity smelting: This invention adopts a converter + LF + RH dual refining process to achieve ultra-low gas and inclusion control. Low superheat casting is conducive to obtaining fine as-cast structure, reducing center segregation and shrinkage defects, and providing a microstructure basis for rolling and heat treatment.

[0019] (2) Microstructure control mechanism of controlled rolling, controlled cooling and online straightening: 1) Controlled rolling process: The final rolling temperature is controlled at 780-830℃, and the final rolling is completed in the austenite recrystallization zone. The reasonable final rolling temperature ensures that the austenite grain size is small and uniform, providing small original austenite grains for subsequent phase transformation. The small austenite grains are transformed into finer ferrite, pearlite or bainite structures in the subsequent cooling process, which is beneficial to improving the strength and toughness of steel; 2) Controlled cooling and online straightening: The temperature of the round steel on the cooling bed is 750-800℃, the cooling bed insulation cover is closed, and the cooling rate is ≤1.5℃ / s. This slow and uniform cooling process allows the round steel to complete the microstructure transformation from austenite to ferrite to pearlite on the cooling bed, avoiding the microstructure stress and deformation caused by rapid cooling. The round steel maintains regular rotation while stepping on the cooling bed, and achieves natural straightening by utilizing the synergistic effect of phase transformation expansion and gravity. The curvature can be controlled within q≤2.0mm / m, completely eliminating the offline straightening process in traditional processes.

[0020] (3) Online induction heating quenching: 1) Uniform heating and cooling by rotation: A rotating bottom support inclined roller is used to keep the round steel rotating and moving forward during induction heating and quenching. The rotation ensures the uniformity of the circumferential temperature field of the round steel and avoids the temperature difference between the top and bottom and the local overheating problem that is common when heating statically. Twelve sets of induction coils heat in sequence, so that the temperature of the round steel rises uniformly to 890-940℃, and there are no black spots at the ends, ensuring the consistency of temperature along the length of the entire round steel; 2) Uniform quenching with multiple water rings: After heating, the steel enters the water-cooled quenching zone at 150mm intervals. Four water rings are evenly arranged, with a 0.1-meter interval between every two water rings. Each water ring has 20 nozzles evenly distributed at an angle, and the water pressure of a single water ring is 0.08-0.12MPa. This quenching system design with multiple water rings, multiple nozzles, and uniform water pressure ensures the uniformity of cooling of the round steel in both the length and cross-sectional directions. The quenching temperature is controlled between 30 and 50℃, with a quenching temperature difference of ≤20℃ along the length of the entire round steel bar and a quenching temperature difference of ≤5℃ along the same cross-section, ensuring the uniformity of the quenched structure. The uniform stress distribution of the quenched structure also avoids quenching cracks and deformation problems.

[0021] (4) Microstructure and property control in roller hearth continuous tempering: The quenched round steel bars are evenly distributed on the roller conveyor of the roller hearth continuous tempering furnace, one layer at a time, moving forward at a constant speed, and held at 610-630℃ for 4 hours. The roller hearth continuous tempering furnace has the advantages of good temperature uniformity and precise control of holding time, ensuring the uniformity of the tempered microstructure of the entire round steel bar in the length direction. After tempering, the round steel bars are air-cooled after exiting the furnace. The curvature q ≤ 2.5 mm / m, there is no hook bend, and the curvature within 1 meter at both ends of the fixed length is q ≤ 1.5 mm / m, ensuring the straightness of the product.

[0022] (5) Integrated mechanism for eliminating normalizing, straightening, and stress-relief annealing: Through precise control of the entire process, the three processes of normalizing, straightening, and stress-relief annealing are eliminated: 1) Eliminating normalizing: By controlling the slow and uniform cooling (≤1.5℃ / s) on the rolling and cooling beds, the round steel directly completes the microstructure transformation from austenite to ferrite to pearlite on the cooling bed, obtaining a uniform and refined microstructure comparable to normalizing treatment, thus eliminating the need for an additional normalizing process; 2) Straightening: Through the regular rotation of the round steel as it moves on the cooling bed, natural straightening is achieved by utilizing the synergistic effect of phase transformation expansion and gravity, and the curvature can be controlled within q≤2.0mm / m. After tempering, the curvature can still be maintained within q≤2.5mm / m, meeting the usage requirements, and offline straightening is not required; 3) Eliminating stress-relief annealing: Through uniform induction heating quenching and continuous roll bottom tempering, the quenching stress and the microstructure stress during the tempering process are fully released. The tempered round steel has a low internal stress level and good bending stability, eliminating the need for additional straightening and corresponding stress-relieving annealing processes.

[0023] Significant improvements of this invention: (1) Excellent mechanical properties and stable overall performance. The tensile strength of the product reaches 880-1010MPa, yield strength 750-900MPa, elongation 16-30%, reduction of area 50-70%, room temperature AKu2 80-160J, and hardness 260-290HBW. All mechanical performance indicators meet the requirements for use of boiler hangers, and the performance stability is good with small batch-to-batch differences. (2) Stable product curvature with no "hook bend" or "S" bend defects. Through online natural straightening and uniform heat treatment, the product curvature q≤2.5mm / m, and the curvature within 1 meter at both ends of the fixed length q≤1.5mm / m. There is no "hook bend" defect, which meets the requirements for direct feeding of ultra-long fixed lengths of 9-12 meters. (3) Through the innovation of the whole process, this invention eliminates the three traditional processes of normalizing, straightening and stress-relief annealing, significantly shortening the production cycle and reducing energy consumption and cost. The process is simplified, the cycle is shortened, the cost is reduced, and it is energy-saving and environmentally friendly, which is in line with the national industrial policies of green manufacturing and sustainable development. Attached Figure Description

[0024] Figure 1 This is a microstructure diagram of 1 / 2R steel after quenching and tempering in Example 1.

[0025] Figure 2 This is a microstructure diagram of 1 / 2R steel after quenching and tempering in Example 2.

[0026] Figure 3 This is a 1 / 2R microstructure diagram of the steel after quenching and tempering in Example 3. Detailed Implementation

[0027] The present invention will be further illustrated by the following examples, but should not be construed as limiting the scope of the invention.

[0028] Example 1: A method for producing high-strength quenched and tempered steel for boiler hanger rods. Product specifications: diameter 60mm, fixed length 9m. Process steps: (1) Smelting: Steel is tapped from the converter and refined in a double LF+RH process. The RH vacuum treatment has a vacuum degree of 0.5 torr and a vacuum holding time of 12 min. The molten steel leaving the station has a H of 0.8 ppm.

[0029] (2) Continuous casting: full-process protective casting, tundish superheating furnace 22℃, opening furnace 28℃, continuous casting speed 0.80m / min, billet size 280×280mm.

[0030] (3) Heating: The billet heating time is 240 min, the heating section temperature is 1180℃, and the soaking section temperature is 1170℃.

[0031] (4) Rolling and cooling: The final rolling temperature is 800℃, and the upper cooling bed temperature is 770℃. The cooling bed insulation cover is closed, and one steel is rolled per tooth. The cooling rate of the round steel is 1.2℃ / s. The round steel maintains regular rotation when it moves on the cooling bed, and the curvature q=1.5mm / m.

[0032] (5) Induction heating and water quenching: The rotating base inclined rollers allow the round steel to rotate and advance at a speed of 0.008 m / s. 12 sets of induction coils are used for heating, with an initial water temperature of 920℃. After heating, water quenching is performed at 150 mm intervals using 4 water rings, with a 0.1 m interval between each two water rings. 20 nozzles are evenly distributed, with a water pressure of 0.10 MPa per water ring and a cooling water temperature of 35℃. The water temperature at the outlet is 45℃, with a length-wise reddening temperature difference of 15℃ and a cross-sectional reddening temperature difference of 3℃.

[0033] (6) Tempering in a roller hearth continuous annealing furnace: After quenching, the round steel bars are evenly distributed on the roller conveyor, one layer at a time, and moved forward at a constant speed. The temperature is maintained at 620℃ for 4 hours, and then the bars are air-cooled after exiting the furnace. The curvature of the round steel bars is q=2.0mm / m, with no hook bends. The curvature within 1 meter of both ends of the fixed length is q=1.2mm / m. The remanence of the round steel bars is 12Gs.

[0034] The product performance test results (samples were taken and tested according to GB / T 2975, at 1 / 2 diameter) are shown in Table 1.

[0035] Example 2: A method for producing high-strength quenched and tempered steel for boiler hanger rods. Product specifications: diameter 70mm, fixed length 10m. Process steps: (1) Smelting: Steel is tapped from the converter and refined in a double LF+RH process. The RH vacuum treatment has a vacuum degree of 0.5 torr and a vacuum holding time of 15 min. The H of the molten steel leaving the station is 0.7 ppm.

[0036] (2) Continuous casting: full-process protective casting, tundish superheating furnace 22℃, start furnace 26℃, continuous casting speed 0.80m / min, billet size 280×280mm.

[0037] (3) Heating: The billet is heated for 300 minutes, the temperature of the heating section is 1160℃, and the temperature of the soaking section is 1180℃.

[0038] (4) Rolling and cooling: The final rolling temperature is 790℃, and the upper cooling bed temperature is 760℃. The cooling bed insulation cover is closed, and one steel is rolled per tooth. The cooling rate of the round steel is 1.2℃ / s. The round steel maintains regular rotation while stepping on the cooling bed, and the curvature q=1.8mm / m.

[0039] (5) Induction heating and water quenching: The rotating base inclined rollers allow the round steel to rotate and advance at a speed of 0.008 m / s. 12 sets of induction coils are used for heating, with an initial water temperature of 910℃. After heating, water quenching is performed at 150 mm intervals using 4 water rings, with a 0.1 m interval between each two water rings. 20 nozzles are evenly distributed, with a water pressure of 0.09 MPa per water ring and a cooling water temperature of 32℃. The outflow temperature is 42℃, the length-direction temperature difference is 18℃, and the cross-sectional temperature difference is 4℃.

[0040] (6) Tempering in a roller hearth continuous annealing furnace: After quenching, the round steel bars are evenly distributed on the roller conveyor, one layer at a time, and moved forward at a constant speed. The temperature is maintained at 615℃ for 4 hours, and then the bars are air-cooled after exiting the furnace. The curvature of the round steel bars is q=2.2mm / m, with no hook bends. The curvature within 1 meter of both ends of the fixed length is q=1.3mm / m. The remanence of the round steel bars is 10Gs.

[0041] The product performance test results (samples were taken and tested according to GB / T 2975, at 1 / 2 diameter) are shown in Table 1.

[0042] Example 3: A method for producing high-strength quenched and tempered steel for boiler hanger rods. Product specifications: diameter 80mm, standard length 12m. Process steps: (1) Smelting: Steel is tapped from the converter and refined in a double LF+RH process. The RH vacuum treatment has a vacuum degree of 0.5 torr and a vacuum holding time of 14 min. The H of the molten steel exiting the station is 0.9 ppm.

[0043] (2) Continuous casting: full-process protective casting, tundish superheating furnace 24℃, start furnace 29℃, continuous casting speed 0.80m / min, billet size 280×280mm.

[0044] (3) Heating: The billet heating time is 340 min, the heating section temperature is 1190℃, and the soaking section temperature is 1185℃.

[0045] (4) Rolling and cooling: The final rolling temperature is 820℃, and the upper cooling bed temperature is 790℃. The cooling bed insulation cover is closed, and one steel is rolled per tooth. The cooling rate of the round steel is 1.4℃ / s. The round steel maintains regular rotation while stepping on the cooling bed, and the curvature q=1.9mm / m.

[0046] (5) Induction heating and water quenching: The rotating base inclined rollers allow the round steel to rotate and advance at a speed of 0.008 m / s. 12 sets of induction coils are used for heating, with an initial water temperature of 930℃. After heating, water quenching is performed at 150 mm intervals using 4 water rings, with a 0.1 m interval between each two water rings. 20 nozzles are evenly distributed, with a water pressure of 0.11 MPa per water ring and a cooling water temperature of 38℃. The water temperature at the outlet is 48℃, the temperature difference along the length is 19℃, and the temperature difference along the same cross section is 4℃.

[0047] (6) Tempering in a roller hearth continuous annealing furnace: After quenching, the round steel bars are evenly distributed on the roller conveyor, one layer at a time, and moved forward at a constant speed. The temperature is maintained at 625℃ for 4 hours, and then the bars are air-cooled after exiting the furnace. The curvature of the round steel bars is q=2.4mm / m, with no hook bends, and the curvature within 1 meter of both ends of the fixed length is q=1.4mm / m. The remanence of the round steel bars is 15Gs.

[0048] The product performance test results (samples were taken and tested according to GB / T 2975, at 1 / 2 diameter) are shown in Table 1.

[0049] Table 1. Test results of mechanical properties of the steel produced in the examples. .

[0050] As can be seen from the test results of Examples 1-3, the quenched and tempered steel for boiler hangers with diameters of 60-80mm and fixed lengths of 9-12 meters produced using the method of this invention meets all design requirements for mechanical properties and exhibits good performance stability. The product has excellent straightness, with no "hook bend" defects, and achieves integrated production without normalizing, straightening, or stress-relief annealing. This effectively meets the requirements of boiler hangers for high strength, high toughness, and direct feeding of ultra-long fixed lengths.

Claims

1. A method for producing high-strength quenched and tempered steel for boiler hangers, comprising the following process route: converter + LF furnace + RH furnace + continuous casting + hot rolling + induction heating + water quenching + tempering in a roller hearth continuous annealing furnace + trimming of the ends of the round steel to length + packaging, characterized in that: The steel has a diameter of 60–80 mm, a standard length of 9–12 meters, and is made of 35CrMoA alloy steel; the process steps include: (1) Smelting: LF+RH double refining is adopted, RH vacuum degassing treatment is carried out, the vacuum holding time is not less than 10 min at a vacuum degree of 0.5 torr, and the H of the molten steel leaving the station is ≤1.0 ppm; (2) Continuous casting: Full-process protective casting is adopted, the tundish superheat is ≤25℃ for continuous casting furnace and ≤30℃ for open casting furnace, the continuous casting speed is 0.75~0.90m / min, and the billet size is 280×280mm; (3) Heating: The billet heating time is 180-360 min, the heating section temperature is 1150-1200℃, and the soaking section temperature is 1160-1200℃; (4) Rolling and cooling: The final rolling temperature is 780~830℃, and the upper cooling bed temperature is 750~800℃; the cooling bed insulation cover is closed, one tooth per steel, and the cooling rate of the round steel is ≤1.5℃ / s; the round steel maintains regular rotation when stepping on the cooling bed to achieve uniform phase transformation and straightening, and the curvature q≤2.0mm / m; (5) Induction heating and water quenching: Induction heating and water quenching are continuous production lines, using rotating bottom support inclined rollers, the round steel rotates and moves forward, the forward speed of the round steel is 0.008m / s; the water outlet red temperature is 30~50℃, the red temperature difference along the length of the whole fixed length round steel is ≤20℃, and the red temperature difference at the same cross section is ≤5℃; (6) Tempering in a roller hearth continuous annealing furnace: After quenching, the round steel is evenly distributed on the roller conveyor and advances at a constant speed. It is heated by natural gas and held at 610-630℃ for 4 hours. It is then air-cooled after being taken out of the furnace.

2. The method for producing high-strength quenched and tempered steel for boiler hangers according to claim 1, characterized in that... Step (5) Induction heating and water quenching: 12 sets of induction coils are used for heating, and the water temperature is 890~940℃. There are no black spots on the ends of the round steel. After heating, water quenching is carried out at intervals of 150mm. Four water rings are arranged to cool the round steel. The interval between each two water rings is 0.1m. 20 nozzles are evenly arranged in the cooling water rings. The water pressure of a single water ring is 0.08~0.12Mpa. The cooling water temperature is ≤40℃. The round steel is cooled evenly.

3. The method for producing high-strength quenched and tempered steel for boiler hangers according to claim 1, characterized in that... Step (6) Tempering in a roller hearth continuous annealing furnace: the curvature of the round steel q≤2.5mm / m, there is no hook bend, the curvature within 1 meter of both ends of the fixed length q≤1.5mm / m; the residual magnetism of the round steel ≤20Gs.

4. The method for producing high-strength quenched and tempered steel for boiler hangers according to claim 1, characterized in that: The steel produced has a tensile strength of 880–1010 MPa, a yield strength of 750–900 MPa, an elongation of 16–30%, a reduction of area of ​​50%–70%, an AKu2 of 80–160 J at room temperature, and a hardness of 260–290 HBW.

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