Isothermal annealing method for 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet

Abstract

The application belongs to the technical field of metal material heat treatment, and particularly relates to an isothermal annealing treatment method for 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billets, which comprises the following steps: step one, furnace loading: placing the 10Cr9MoW2VNbBN continuous casting round billet in a heat treatment furnace; step two, high-temperature homogenization treatment: controlling the heat treatment furnace to be heated at a heating speed of less than or equal to 80 DEG C / h, and heated to 800-850 DEG C and kept for 18-36 hours; step three, rapid cooling to isothermal temperature: cooling the continuous casting round billet to 720-750 DEG C at a cooling speed of more than or equal to 30 DEG C / h; step four, isothermal keeping: keeping for 36-48 hours at the isothermal temperature of 720-750 DEG C; and step five, slow cooling and furnace unloading: after the isothermal keeping, slowly cooling to below 200 DEG C at a cooling speed of not more than 20 DEG C / h, and then air cooling to room temperature to obtain the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet. The application can improve the cutting performance of the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet, eliminate the casting residual stress, and provide a uniform structure basis for subsequent quenching and tempering treatment.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, specifically relating to an isothermal annealing method for continuously cast round billets of 10Cr9MoW2VNbBN heat-resistant steel. Background Technology

[0002] 10Cr9MoW2VNbBN steel is a martensitic heat-resistant steel, conforming to standard NB / T47008-2017. It is equivalent to grade F92 in the SA182 and SA336 sections of the American ASME standard. Its main chemical composition is: C: 0.07–0.13%, Si: ≤0.50%, Mn: 0.30–0.60%, P: ≤0.020%, S: ≤0.010%, Cr: 8.50–9.50%, Mo: 0.30-0.60%, B: 0.001-0.006%, V: 0.15-0.25%, Nb: 0.04-0.09%, Ni: ≤0.4%, W: 1.50-2.00%, N: 0.030-0.070%. It is mainly used in power plant equipment and other workpieces that operate at high temperatures and require certain corrosion resistance and oxidation resistance, such as high-pressure steam pipes and reheat steam pipes on boilers, heating furnace pipes in the petrochemical industry, flanges, valve discs, valve bodies, etc.; 10Cr9MoW2VNbBN steel has excellent high-temperature strength and creep resistance, which can effectively reduce the weight of boilers and pipes and improve thermal fatigue resistance. Compared with other austenitic stainless steels, it has good thermal conductivity and low expansion rate. Therefore, the use of 10Cr9MoW2VNbBN steel is increasing.

[0003] However, due to the harsh operating environment of 10Cr9MoW2VNbBN steel, there are high requirements for the quality uniformity of the finished product, ultrasonic flaw detection, and processing quality. The control requirements for segregation and low magnification quality are becoming increasingly stringent, making the production process quite difficult. The main production processes (including the main inspection processes) of hot-rolled seamless steel pipes typically include: billet preparation and inspection → billet heating → piercing → rolling → reheating of rough pipe → sizing (reduction) → heat treatment → straightening of finished pipe → finishing → inspection (non-destructive, physical and chemical, bench inspection) → warehousing.

[0004] Due to factors such as cost and efficiency, the production of 10Cr9MoW2VNbBN billets has gradually shifted from ingot casting to continuous casting. In actual production, 10Cr9MoW2VNbBN steel is typically produced using continuous casting to prepare large-sized round billets (diameter ≥ Φ250mm). However, due to its low carbon content, it is a hypoperiterite steel with a complex solidification process. Furthermore, its high alloy content makes it a crack-sensitive steel. During the solidification and subsequent cooling of large-sized continuously cast billets, compositional segregation, coarse dendrites, residual δ-ferrite, and high casting residual stress are easily caused. If the original microstructure and residual stress of the billet are not improved and eliminated, direct machining or subsequent heat treatment can easily lead to problems such as severe tool wear, workpiece deformation, and even cracking.

[0005] Currently, the "continuous annealing process" is commonly used in industry as the annealing route for continuously cast billets (such as holding at 750-800℃ for a long time). Although this process can achieve a softened state that meets processing requirements, the annealing cycle is long and cannot effectively improve the as-cast microstructure, making it difficult to improve the microstructure and compositional uniformity of the continuously cast billet. Under the influence of microstructure inheritance, it will affect the quality of the final pipe. Some companies have tried to use full annealing (heating to above Ac3 and then slow cooling), but because G115 steel contains a large amount of strong carbide-forming elements such as W, Mo, V, and Nb, coarse Laves phase (Fe2W type) and MX type carbonitrides are easily precipitated during slow cooling, which seriously impairs the high-temperature creep strength and toughness of the material. Currently, the relevant domestic technology, namely, the heat treatment method for large-size continuous casting billets of low-carbon high-chromium heat-resistant steel, as described in TISCO CN117070718B, adopts a slow cooling + tempering heat treatment process for the billets. The slow cooling process involves slowly cooling the large-size continuous casting billets of low-carbon high-chromium heat-resistant steel to 100℃~150℃, either in the furnace or in a pit. The tempering process involves placing the large-size continuous casting billets of low-carbon high-chromium heat-resistant steel, which have been slowly cooled to 100℃~150℃, into the furnace and heating them to 765±10℃ at a heating rate of less than 100℃ / h. The temperature is held for 14~16 hours, and then cooled in the furnace to ≤500℃. After being removed from the furnace, the billets are air-cooled to room temperature. Baosteel's patent CN122007305A, "A Forging Method for G115 Large-Size Continuously Cast Round Billets," focuses on the forging process of G11 approximation continuously cast round billets. It refines grains, breaks dendrites, and reduces central porosity by controlling the heating temperature (≤1180℃), deformation amount, and final forging temperature. However, it does not involve isothermal annealing. This demonstrates that existing technologies largely focus on forging, welding, or standard quenching and tempering processes, lacking systematic research on microstructure control during the pretreatment stage. No existing technology specifically targeting isothermal annealing of 10Cr9MoW2VNbBN continuously cast round billets has been found.

[0006] Therefore, how to achieve softening and stress relief through isothermal annealing without inducing the precipitation of harmful phases in 10Cr9MoW2VNbBN steel, and effectively improve the uniformity of billet quality, has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] To achieve the above objectives, this invention discloses an isothermal annealing method for continuously cast round billets of 10Cr9MoW2VNbBN heat-resistant steel, comprising the following steps: Preparation process of 1.10Cr9MoW2VNbBN continuously cast round billet (≥Φ450mm): The tundish is properly baked and kept clean, with a baking temperature of 1000~1200℃; argon pre-blowing is performed during the initial pouring, and the liquid level in the tundish is not lower than 750mm; the cooling water flow rate of the crystallizer is 3600±100L / min, and the secondary cooling water ratio is 0.11±0.01L / kg steel; the liquid level fluctuation in the crystallizer is ≤±3mm, the thickness of the protective slag layer is 7~12mm, and the slag consumption is 0.40±0.05kg / t steel; the superheat of the tundish in normal heats is controlled at a target of 20~35℃, and the target casting speed is 0.20-0.35m / min; during the electromagnetic stirring process in the crystallizer, the current is 200~400A / m, and the frequency is 2~6Hz; the current of the electromagnetic stirring at the end of solidification is 400~600A / m, and the frequency is 4~6Hz; the billet straightening temperature is ≥920℃.

[0008] 2. Heat treatment (isothermal annealing) process for continuously cast billets: (1) Loading the furnace: The continuous casting billets are heat-treated using an annealing furnace or an annealing slow cooling pit. The bottom of the furnace is lined with hot billets of other steel grades from the previous casting. The billets are preheated before being loaded into the furnace, with a preheating temperature of not less than 600℃. After the billets are cut to length, they are transferred to the annealing furnace within 30 minutes. The loading temperature is ≥600℃. The billets are arranged in a grid pattern inside the furnace to ensure isolation of the billets. After loading into the furnace, the top is covered with hot billets of other steel grades. (2) High temperature homogenization treatment: Place the 10Cr9MoW2VNbBN continuous casting round billet in a heat treatment furnace, start heating, heating rate ≤80℃ / h, heat to 800–850℃, hold for 18–36 hours to allow alloy elements to diffuse fully and reduce dendrite segregation. (3) Rapid cooling to isothermal temperature: Cool the billet to 720–750°C at a cooling rate of not less than 30°C / h; the cooling rate is preferably 35–40°C / h; (4) Isothermal holding: Hold at 720–750℃ for 36–48 hours to promote carbide spheroidization, decomposition of residual austenite, and release casting residual stress; the isothermal temperature is preferably 735±5℃, and the holding time of Φ650mm continuous casting billet is preferably 48 hours. Slow cooling after unloading: After isothermal treatment, slowly cool to below 200°C at a cooling rate not exceeding 20°C / h, and then air cool to room temperature.

[0009] 3. Inspection of cast billets After the billet exits the heat treatment furnace, it should be stored away from the vents to ensure uniform cooling.

[0010] Each 10Cr9MoW2VNbBN heat-resistant steel continuously cast round billet was subjected to Brinell hardness testing (hardness required ≤250HB). The 10Cr9MoW2VNbBN heat-resistant steel continuously cast round billets were subjected to low magnification inspection (center crack required <2.0 grade), metallographic inspection and residual stress inspection according to the heat batch.

[0011] The hardness of the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet is ≤250HB, and the hardness deviation of each billet measurement point is <10HB. The center crack of the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet is <2.0 grade.

[0012] Furthermore, the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet treated by the method of the present invention has a Brinell hardness of 210–235 HB, a residual stress relief rate of more than 85%, and a microstructure consisting of uniformly distributed spherical carbides dispersed in a ferrite / tempered sorbite matrix without continuous network δ-ferrite.

[0013] By adopting the above technical solution, the beneficial effects of the present invention are as follows: (1) Significantly softens materials: after annealing, the hardness is stable at 210–235HB, which meets the requirements of rough machining such as turning and boring, and the tool life is increased by more than 30%; (2) Highly efficient stress relief: X-ray diffraction measurements show that the residual stress relief rate is >85%, which greatly reduces the risk of deformation in subsequent processing; (3) Suppress harmful phase precipitation: By avoiding the Laves phase coarsening sensitive zone of 10Cr9MoW2VNbBN steel in the range of 550–650℃, the aggregation of Fe2W type phase is effectively prevented, ensuring high-temperature performance; (4) Compatible with subsequent heat treatment: After subsequent processing, the billet produced by this annealing process has a room temperature tensile strength ≥620 MPa and a 650℃ / 10-ton tensile strength ≥620 MPa. 5 h-permanent strength ≥100 MPa; (5) The process can be industrialized: only conventional bogie-type or bell-type annealing furnaces are required, no special equipment is needed, and it is easy to integrate into existing production lines. Attached Figure Description

[0014] Figure 1 Metallographic structure diagram after annealing in Example 1; Figure 2 This is a metallographic diagram of the structure after annealing in Example 2; Figure 3 This is a metallographic diagram of the structure after annealing in Example 2; Figure 4This is a photograph of the surface cracks after sawing in Comparative Example 1; Figure 5 Metallographic structure diagram for Comparative Example 1; Figure 6 This is a photograph of the surface after sawing in Example 1. Detailed Implementation

[0015] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0016] The detection methods for each performance index in the embodiments and comparative examples of this invention are as follows: Brinell hardness: According to GB / T 231.1 "Metallic materials - Brinell hardness test - Part 1: Test method"; Residual stress (surface): According to GB / T 31310-2014 "Determination of Residual Stress in Metallic Materials - Drilling Strain Method"; Metallographic structure: According to GB / T 13298 "Metallic Microstructure Examination Method"; Rm (tensile strength), Rp0.2 (0.2% yield strength), and A (elongation): according to GB / T 228.1 Metallic materials, tensile testing—Part 1: Test method at room temperature; 650℃ / 10 5 h Creep strength: GB / T 2039-2024 "Metallic materials uniaxial tensile creep test method".

[0017] Example 1 A Φ650mm continuously cast round billet of 10Cr9MoW2VNbBN has the following chemical composition (wt.%): C 0.10, Si 0.35, Mn 0.45, P 0.014, S 0.005, Cr 9.0, Mo 0.45, W 1.80, V 0.22, Nb 0.08, N 0.05, B 0.008, balance Fe. It is processed according to the method of this invention: Step (1) Preparation of continuous casting round billets: The tundish is well baked and kept clean, with a baking temperature of 1120℃; argon is pre-blown during the initial pouring, and the liquid level in the tundish is 780mm; the cooling water flow rate of the crystallizer is 3700L / min, and the secondary cooling water ratio is 0.11L / kg steel; the liquid level fluctuation in the crystallizer is ±3mm, the slag layer thickness of the protective slag is 9mm, and the slag consumption is 0.42kg / t steel; the superheat of the tundish in a normal heat is 26℃, and the casting speed is 0.22m / min; during the electromagnetic stirring process in the crystallizer, the current is 200A / m and the frequency is 5Hz; the current of the electromagnetic stirring at the end of solidification is 600A / m and the frequency is 4Hz; the billet straightening temperature is 926℃.

[0018] Step (2), Furnace loading: The bottom of the furnace is laid with the previous 40Cr hot billet. The billet is preheated before loading into the furnace at a temperature of 640℃. After the billet is cut to length, it is transferred to the annealing furnace within 26 minutes at a loading temperature of 660℃. The furnace is arranged in a grid pattern to ensure the isolation of the billet inside the furnace. After loading into the furnace, the top is covered with 40Cr hot billet.

[0019] Step (3), high temperature homogenization treatment: the heat treatment furnace is heated at a rate of 65℃ / h, heated to 830℃, and held for 32 hours; Step (4) Rapid cooling to isothermal temperature: Cool the billet to 735℃ at a cooling rate of 35℃ / h; Step (5), isothermal insulation: Insulate at 735℃ for 48 hours; Step (6) Slow cooling and unloading: Slowly cool to 190°C at 15°C / h, then air cool to room temperature.

[0020] Test results: No surface cracks after sawing of the cast billet (e.g.) Figure 6 (as shown) Brinell hardness: 225 HB; Residual stress (surface): reduced from the original 320 MPa to 42 MPa (relief rate 86.9%). Metallographic structure: The metallographic structure of the 10Cr9MoW2VNbBN heat-resistant steel continuously cast round billet in this embodiment is shown in the figure below. Figure 1 As shown, from Figure 1 It can be seen that uniform spherical carbides (size <1μm) are distributed in the ferrite matrix, with no δ-ferrite network; After subsequent heat treatment of the pipe: Rm (tensile strength) = 635 MPa, Rp0.2 (0.2% yield strength) = 520 MPa, A (elongation) = 22%, 650℃ / 10 5 h_endurance strength = 108 MPa.

[0021] Example 2: A Φ500mm continuously cast round billet of 10Cr9MoW2VNbBN has the following chemical composition (wt.%): C 0.10, Si 0.37, Mn 0.44, Cr 9.06, Mo 0.44, W 1.81, V 0.21, Nb 0.08, N 0.0488, B 0.008, balance Fe. It is processed according to the method of this invention: Step (1) Preparation of continuous casting round billets: The tundish is well baked and kept clean, with a baking temperature of 1118℃; argon is pre-blown during the initial pouring, and the liquid level in the tundish is 780mm; the cooling water flow rate of the crystallizer is 3700L / min, and the secondary cooling water ratio is 0.11L / kg steel; the liquid level fluctuation in the crystallizer is ±3mm, the slag layer thickness of the protective slag is 9.5mm, and the slag consumption is 0.43kg / t steel; the superheat of the tundish in a normal heat is 22℃, and the casting speed is 0.29m / min; during the electromagnetic stirring process in the crystallizer, the current is 300A / m and the frequency is 6Hz; the current of the electromagnetic stirring at the end of solidification is 600A / m and the frequency is 5Hz; the billet straightening temperature is 925℃.

[0022] Step (2), Furnace loading: The bottom of the furnace is laid with the previous cast 45 steel hot billet. The billet is preheated before being loaded into the furnace at a temperature of 655℃. After the billet is cut to length, it is transferred to the annealing furnace 24 minutes later at a loading temperature of 658℃. The furnace is arranged in a grid pattern to ensure the isolation of the billet inside the furnace. After the furnace is loaded, the top is covered with 45 steel hot billet.

[0023] Step (3), high temperature homogenization treatment: the heat treatment furnace is heated at a rate of 65℃ / h, heated to 835℃, and held for 28 hours; Step (4) Rapid cooling to isothermal temperature: Cool the billet to 735℃ at a cooling rate of 35℃ / h; Step (5), isothermal insulation: Insulate at 735℃ for 45 hours; Step (6) Slow cooling and unloading: Slowly cool to 190℃ at 15℃ / h, then air cool.

[0024] Test results: Brinell hardness: 223HB; Residual stress (surface): reduced from the original 320 MPa to 40 MPa (relief rate 87.5%). Metallographic structure: The metallographic structure of the 10Cr9MoW2VNbBN heat-resistant steel continuously cast round billet in this embodiment is shown in the figure below. Figure 2 , Figure 3 As shown, from Figure 2 , Figure 3 It can be seen that uniform spherical carbides (size <1μm) are distributed in the ferrite matrix, with no δ-ferrite network; After subsequent heat treatment of the pipe: Rm=642 MPa, Rp0.2=523MPa, A=22%, 650℃ creep strength=110MPa·h.

[0025] Comparative Example 1 A Φ500mm billet was prepared using the continuous casting round billet preparation method of step (1) in Example 2; The heat treatment is carried out by continuous annealing process: heat treatment at 800℃ for 48 hours, followed by furnace cooling (≤20℃ / h), and then unloading at 350℃.

[0026] Test results: Brinell hardness 337 HB; Surface cracks appear during the sawing process of the cast billet (such as...) Figure 4 As shown in the figure, segregation leads to insufficient local toughness; Metallographic structure: The metallographic structure of the 10Cr9MoW2VNbBN continuously cast round billet in this comparative example is shown in the figure below. Figure 5 As shown, from Figure 5 It can be seen that the structure is martensitic.

[0027] As can be seen from the examples and comparative examples, the present invention effectively maintains high-temperature performance while ensuring processability.

Claims

1. A method for isothermal annealing of continuously cast round billets of 10Cr9MoW2VNbBN heat-resistant steel, characterized in that, Includes the following steps: Step 1: Loading the furnace: Place the 10Cr9MoW2VNbBN continuously cast round billet into the heat treatment furnace; Step 2, High-temperature homogenization treatment: Control the heat treatment furnace to raise the temperature at a rate of ≤80℃ / h, raise the temperature to 800–850℃, and hold for 18–36 hours; Step 3: Rapid cooling to isothermal temperature: Cool the continuously cast round billet to 720–750℃ at a cooling rate of ≥30℃ / h; Step 4: Isothermal Insulation: Insulate at an isothermal temperature of 720–750℃ for 36–48 hours; Step 5, slow cooling and unloading: After the isothermal process is completed, the furnace is slowly cooled to below 200°C at a cooling rate not exceeding 20°C / h, and then air-cooled to room temperature.

2. The isothermal annealing method according to claim 1, characterized in that, Step one is as follows: The continuous casting round billets are heat-treated using an annealing furnace or an annealing slow cooling pit. The bottom of the furnace is made of hot billets of other steel grades cast previously. The billets are preheated before being loaded into the furnace, and the preheating temperature is not lower than 600℃. After the billets are cut to length, they are transferred to the annealing furnace within 30 minutes. The loading temperature is ≥600℃. The billets are arranged in a grid pattern in the furnace to ensure that the billets are isolated inside the furnace. After loading into the furnace, the top is covered with hot billets of other steel grades.

3. The isothermal annealing method according to claim 1, characterized in that, In step three, the cooling rate is 35–40 °C / h.

4. The isothermal annealing method according to claim 1, characterized in that, In step four, the isothermal temperature is 735±5℃.

5. The isothermal annealing method according to claim 1, characterized in that, When the diameter of the continuously cast round billet is 650mm, the holding time of the continuously cast round billet in step four is 48h.

6. The isothermal annealing method according to claim 1, characterized in that, The 10Cr9MoW2VNbBN continuously cast round billet is prepared using the following process: The tundish is properly baked and kept clean, with a baking temperature of 1000~1200℃; argon pre-blowing is performed during the initial pouring, and the liquid level in the tundish is not lower than 750mm; the cooling water flow rate of the crystallizer is 3600±100L / min, and the secondary cooling water ratio is 0.11±0.01L / kg steel; the liquid level fluctuation in the crystallizer is ≤±3mm, the thickness of the protective slag layer is 7~12mm, and the slag consumption is 0.40±0.05kg / t steel; the superheat of the tundish in normal heats is controlled at a target of 20~35℃, and the target casting speed is 0.20-0.35m / min; during the electromagnetic stirring process in the crystallizer, the current is 200~400A / m, and the frequency is 2~6Hz; the current of the electromagnetic stirring at the end of solidification is 400~600A / m, and the frequency is 4~6Hz; the billet straightening temperature is ≥920℃.

7. The isothermal annealing method according to claim 1, characterized in that, The Brinell hardness of the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet treated by isothermal annealing is ≤250HB, and the center crack is <2.0 grade.

8. The isothermal annealing method according to claim 1, characterized in that, The Brinell hardness of the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet after isothermal annealing is 210-235HB, and the residual stress relief rate is greater than 85%.

9. The isothermal annealing method according to claim 1, characterized in that, The microstructure of the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet after isothermal annealing is uniformly distributed spherical carbides dispersed in the ferrite or tempered sorbite matrix, without continuous network δ-ferrite.

10. The isothermal annealing method according to claim 1, characterized in that, The Brinell hardness of the 10Cr9MoW2VNbBN heat-resistant steel continuous casting round billet after isothermal annealing is 223~225HB, and the residual stress relief rate is 86.9~87.5%.

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