A method of heat treating a f65 steel forging

Abstract

This invention relates to the field of steel production technology, specifically to a heat treatment method for F65 steel forgings. The method involves a normalizing temperature of 920–940°C, followed by holding at that temperature for a period and then air cooling; a quenching temperature of 900–920°C, followed by holding at that temperature for a period and then water cooling; and a tempering temperature of 620–640°C, followed by holding at that temperature for a period and then water cooling. The normalizing process can be repeated 1–2 times, and 1–2 pre-tempering treatments can be added between quenching and tempering, with air cooling after each pre-tempering treatment. Furthermore, the F65 steel forgings are required to undergo furnace cooling at the end of the forging process. By furnace cooling after forging, lowering the normalizing temperature, performing multiple normalizing processes, adding pre-tempering, increasing the tempering temperature, and water cooling during tempering, the grain size is refined, the matrix structure is strengthened, temper brittleness is avoided, and the mechanical properties of the F65 steel forgings are improved.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, specifically to a heat treatment method for F65 steel forgings. Background Technology

[0002] ASTM A694, "Forged Carbon and Alloy Steel Pipe Flanges, Forged Fittings, Valves, and Parts for High Pressure Applications," specifies F65 as a low-carbon, low-alloy steel with extremely high strength and good low-temperature toughness. The specifications for its performance are as follows: after heat treatment, the longitudinal and transverse tensile strength at T / 2 and T / 4 positions is ≥530 MPa; the yield strength is ≥450 MPa; the elongation is ≥20%; the reduction of area is ≥35%; and the impact toughness (Akv) at -46℃ has a single impact value ≥60 J and an average value ≥73 J. Commonly used heat treatment processes in the current technology are: 1) Normalizing: heating to 960℃, holding for 5–6 hours, and air cooling; 2) Quenching: heating to 920℃, holding for 5–6 hours, and water cooling; 3) Tempering: heating to 350℃, holding for 4 hours, then further heating to 550℃, holding for 6–7 hours, and air cooling.

[0003] However, the heat treatment process in the above-mentioned existing technology can only enable F65 steel forgings with an effective wall thickness of less than 300mm to meet the performance specifications in a low-temperature environment. For F65 steel forgings with an effective wall thickness of 400 to 700mm, due to the coarse grains in their original cast structure and the limitations of the chemical composition and hardenability of the material, it is difficult to fully refine the grains and homogenize the composition in the core through large-scale deformation. After forging, there are still coarse grains and cast structure residues. After conventional heat treatment, the strength index and low-temperature impact index are difficult to meet the technical requirements. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defect that the heat treatment process of F65 steel in the prior art cannot make F65 steel forgings with an effective wall thickness of 400 to 700 mm meet the mechanical property specifications, thereby providing a heat treatment method for F65 steel forgings.

[0005] Therefore, the present invention provides the following technical solution:

[0006] This invention provides a heat treatment method for F65 steel forgings, comprising the following steps:

[0007] S1: Normalizing, heat the F65 steel forging to the normalizing temperature of 920-940℃, hold for t1, remove from the furnace, and air cool to <50℃;

[0008] S2: Quenching, heating to the quenching temperature, holding at t2, removing from the furnace, and water cooling to <50℃.

[0009] S3: Tempering. First, heat to 310-330℃ and hold for 2-4 hours. Then, continue heating to the tempering temperature of 620-640℃ and hold for t3. Remove from the furnace and water cool to <50℃.

[0010] The F65 steel forgings described in this invention are obtained by sawing and forging F65 steel continuously cast billets. Forging is a conventional process in the field. Typically, but not exclusively, the specific operation is as follows:

[0011] (1) Cutting: Cut the continuous casting billet into the required specifications, with a length / diameter ratio of 0.3 to 1.0;

[0012] (2) Forging heating: The billet is heated to 1200-1240℃ in a forging heating furnace, held for 4-5 hours, and then forged.

[0013] (3) First drawing: Lengthen the billet, with a drawing ratio of 2.0 to 2.5;

[0014] (4) First upsetting: Upsetting ratio is 2.0 to 2.5;

[0015] (5) Second elongation: The elongation ratio is 2.0 to 2.5;

[0016] (6) Second upsetting: Upsetting ratio is 2.0 to 2.5;

[0017] (7) Third stretching: Stretch to the final size;

[0018] (8) Cooling.

[0019] In this invention, F65 steel continuous casting billets can be obtained through commercial channels or made in-house. The preparation process is conventional in the field and generally includes electric furnace smelting or converter smelting, ladle refining, vacuum degassing and other processes.

[0020] Preferably, during forging cooling, the furnace is first cooled to 350-400°C, then removed from the furnace and air-cooled to <150°C. Furnace cooling after forging can obtain a balanced microstructure composed of pearlite and ferrite, avoiding the formation of non-equilibrium microstructures such as bainite due to excessively rapid cooling, thus providing a good microstructure basis for subsequent normalizing and tempering.

[0021] Preferably, the final forging temperature of the F65 steel forging is ≥850℃.

[0022] Optionally, the final forging temperature is 850–900℃.

[0023] Preferably, step S1 can be performed 1 to 2 times.

[0024] Preferably, one or two pre-tempering processes can be added between steps S2 and S3.

[0025] Preferably, the added pre-tempering treatment involves first heating to 310–330°C, holding at that temperature for 2–4 hours, then further heating to the tempering temperature of 620–640°C, holding at that temperature for the specified time t3, and then removing the product from the furnace and air-cooling it to <50°C.

[0026] Preferably, in step S1, the heat preservation time t1 is 6 to 8 hours.

[0027] Preferably, in step S2, the quenching temperature is 900–920°C.

[0028] Preferably, in step S2, the heat preservation time t2 is 5 to 7 hours.

[0029] Preferably, in step S2, the water cooling uses circulating water.

[0030] Preferably, the heat preservation time t3 is 8 to 14 hours.

[0031] Preferably, in step S1, the F65 steel forging is first heated to 670-690°C and held for 4-6 hours, and then heated to a normalizing temperature of 920-940°C.

[0032] Preferably, in step S2, the F65 steel forging treated in step S1 is first heated to 670-690°C and held for 4-6 hours, and then heated to the quenching temperature of 900-920°C.

[0033] Preferably, the F65 steel forging is obtained by sawing and forging an F65 steel continuous casting billet.

[0034] Preferably, the chemical composition of the F65 steel forging, by weight percentage, includes: C: ≤0.30%, Si: 0.15%~0.35%, Mn: ≤1.60%, P: ≤0.025%, S: ≤0.025%, Cr: ≤0.35%, Ni: ≤1.00%, Mo: ≤0.35%, Alt: ≤0.03%, V: ≤0.06%, Nb: ≤0.025%, N: ≤0.015%, and carbon equivalent (CEV) ≤0.46%.

[0035] Preferably, the chemical composition of the F65 steel forging, by weight percentage, includes: C: 0.07%–0.15%, Si: 0.25%–0.35%, Mn: 0.80%–1.50%, P: ≤0.015%, S: ≤0.010%, Cr: 0.10%–0.35%, Ni: 0.50%–1.00%, Mo: 0.10%–0.35%, Alt: 0.01%–0.03%, V: 0.015%–0.06%, Nb: 0.005%–0.025%, N: 0.006%–0.011%, and carbon equivalent (CEV): 0.40%–0.46%.

[0036] The heat treatment method for F65 steel forgings provided by this invention involves first normalizing the forgings by heating them to a normalizing temperature of 920–940°C, holding them at that temperature for a period of time, removing them from the furnace, air-cooling them to below 50°C, then quenching them, and finally tempering them by first heating them to 310–330°C, holding them for 2–4 hours, then further heating them to a tempering temperature of 620–640°C, holding them at that temperature for a period of time, removing them from the furnace, and water-cooling them to below 50°C. This heat treatment process can effectively improve the mechanical properties of F65 steel forgings. The normalizing treatment further refines the grains, promotes the diffusion of elements in the alloy, and homogenizes the overall chemical composition of the alloy. It also reduces the influence of as-cast microstructure and compositional segregation on the final microstructure and properties. Furthermore, the relatively low normalizing temperature of 920–940°C prevents excessive grain growth during subsequent holding, which could lead to coarse grains. The tempering temperature is relatively high, ranging from 620 to 640°C, which allows the quenched martensite and bainite to fully decompose into dispersed carbides, strengthening the matrix structure and further improving the low-temperature impact performance of the forgings. After tempering, the forgings are rapidly cooled to <200°C with water and then air-cooled to room temperature to avoid temper brittleness.

[0037] In the heat treatment method for F65 steel forgings provided by this invention, adding multiple normalizing operations and pre-tempering operations can further improve the mechanical properties of the forgings.

[0038] In the heat treatment method for F65 steel forgings provided by this invention, the chemical composition of the F65 steel forgings is further optimized. The carbon content is 0.07%–0.15%, because increasing the carbon content can improve the strength index of F65 steel after quenching and tempering. However, carbon itself is an element that reduces toughness; controlling its appropriate amount can enhance the strength of F65 steel. The silicon content is 0.25%–0.35%, because silicon is a major deoxidizing element and is an essential element. However, if the silicon content is too high, it will reduce the toughness and plasticity of the steel, and at the same time increase the ductile-brittle transition temperature of the steel, resulting in poor low-temperature impact toughness. The manganese content is 0.80%–1.50%, because manganese mainly functions to deoxidize and inhibit the harmful effects of sulfur, and the combined use of Mn and Si has a good deoxidizing effect. The Cr content is 0.10%–0.35%, as it is a chromium carbide-forming element and can improve the strength of F65 steel; the Mo content is 0.10%–0.35%, as molybdenum is also a carbide-forming element and can improve the strength of F65 steel; the Ni content is 0.50%–1.00%, as nickel is the most beneficial element for improving low-temperature toughness; the Alt content is 0.01%–0.03%, as aluminum has a strong bonding force with oxygen and is a commonly used element for final deoxidation. Aluminum and nitrogen can also form AlN dispersed precipitation, which can inhibit the growth of austenite grains and play a role in refining grains. If the Alt content is too low, less AlN will be formed, and the grain refining effect will not be obvious. If the Alt content is too high, AlN will aggregate and grow, and AlN itself is a brittle phase, which will reduce the toughness of the steel. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the heat treatment process curve of Embodiment 1 of the present invention.

[0040] Figure 2 This is a schematic diagram of the heat treatment process curve for Embodiment 2 of the present invention.

[0041] Figure 3 This is a schematic diagram of the heat treatment process curve for Embodiment 3 of the present invention.

[0042] Figure 4 This is a schematic diagram of the heat treatment process curve for Embodiment 4 of the present invention.

[0043] Figure 5 This is a schematic diagram of the heat treatment process curve for Embodiment 5 of the present invention.

[0044] Figure 6 This is a schematic diagram of the heat treatment process curve for Comparative Example 1 of the present invention. Detailed Implementation

[0045] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0046] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0047] The chemical composition of the F65 steel forgings used in the embodiments and comparative examples of this invention is the same, as follows: C: 0.11%, Si: 0.32%, Mn: 1.04%, P: 0.004%, S: 0.003%, Cr: 0.13%, Ni: 0.79%, Mo: 0.33%, Alt: 0.024%, V: 0.043%, Nb: 0.018%, N: 0.0095%, with the balance being Fe and unavoidable impurities, CEV = 0.44%.

[0048] The forging method and parameters of the F65 steel forgings used in the embodiments and comparative examples of this invention are consistent, as follows:

[0049] The continuously cast billet was sawn into Φ600mm×380mm billets, weighing 850kg; the billets were placed in a forging furnace and heated to 1240℃, held for 4 hours, and then removed from the furnace; a press was used to draw the billets to Φ390mm×880mm; the drawn billets were then upset with a forging hammer to Φ600mm×380mm; they were drawn again to Φ390mm×880mm; upset again to Φ600mm×380mm; and finally drawn to 415mm×415mm×600mm; the final forging temperature was controlled at 850℃; the billets were furnace cooled to 500℃ and then air cooled to obtain the F65 steel forgings in the embodiments and comparative examples of this invention.

[0050] Example 1

[0051] This embodiment provides a heat treatment method for F65 steel forgings, and the heat treatment process curve is as follows: Figure 1 As shown, the specific steps and parameters are as follows:

[0052] (1) Normalizing: The F65 steel forging is put into the furnace at 250°C, the heating rate is 80°C / h, the temperature is raised to 680°C, and held for 4h; the heating rate is 80°C / h, the temperature is raised to 930°C, held for 6h, and then air-cooled to 40°C.

[0053] (2) Quenching: Heating rate 80℃ / h, heating to 680℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 910℃, holding for 6h, water cooling to 45℃.

[0054] (3) Tempering: Heating rate 80℃ / h, heating to 320℃, holding for 3h; heating rate 80℃ / h, continuing to heat to 630℃, holding for 8h, water cooling to 30℃.

[0055] Example 2

[0056] This embodiment provides a heat treatment method for F65 steel forgings, and the heat treatment process curve is as follows: Figure 2 As shown, the specific steps and parameters are as follows:

[0057] (1) Normalizing: The F65 steel forging is put into the furnace at 250°C, the heating rate is 80°C / h, the temperature is raised to 680°C, and held for 4h; the heating rate is 80°C / h, the temperature is raised to 930°C, held for 6h, and then air-cooled to 40°C.

[0058] (2) Normalizing: Heating rate 80℃ / h, heating to 680℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 930℃, holding for 6h, then air cooling to 40℃;

[0059] (3) Quenching: Heating rate 80℃ / h, heating to 680℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 910℃, holding for 6h, water cooling to 45℃;

[0060] (4) Tempering: Heating rate 80℃ / h, heating to 320℃, holding for 3h; heating rate 80℃ / h, continuing to heat to 630℃, holding for 8h, water cooling to 30℃.

[0061] Example 3

[0062] This embodiment provides a heat treatment method for F65 steel forgings, and the heat treatment process curve is as follows: Figure 3 As shown, the specific steps and parameters are as follows:

[0063] (1) Normalizing: The F65 steel forging is put into the furnace at 250°C, the heating rate is 80°C / h, the temperature is raised to 680°C, and held for 4h; the heating rate is 80°C / h, the temperature is raised to 930°C, held for 6h, and then air-cooled to 40°C.

[0064] (2) Quenching: Heating rate 80℃ / h, heating to 680℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 910℃, holding for 6h, water cooling to 45℃.

[0065] (3) Preheating: Heating rate 80℃ / h, heating to 320℃, holding for 3h; heating rate 80℃ / h, continuing to heat to 630℃, holding for 8h, air cooling to 30℃;

[0066] (4) Tempering: Heating rate 80℃ / h, heating to 320℃, holding for 3h; heating rate 80℃ / h, continuing to heat to 630℃, holding for 8h, water cooling to 30℃.

[0067] Example 4

[0068] This embodiment provides a heat treatment method for F65 steel forgings, and the heat treatment process curve is as follows: Figure 4 As shown, the specific steps and parameters are as follows:

[0069] (1) Normalizing: Put the F65 steel forging into the furnace at 100℃, with a heating rate of 80℃ / h, heat up to 680℃, hold for 4h; continue heating at 80℃ / h to 920℃, hold for 6h, and air cool to 40℃.

[0070] (2) Quenching: Heating rate 80℃ / h, heating to 680℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 900℃, holding for 5h, water cooling to 45℃;

[0071] (3) Tempering: Heating rate 80℃ / h, heating to 320℃, holding for 2h; heating rate 80℃ / h, continuing to heat to 620℃, holding for 8h, water cooling to 30℃.

[0072] Example 5

[0073] This embodiment provides a heat treatment method for F65 steel forgings, and the heat treatment process curve is as follows: Figure 5 As shown, the specific steps and parameters are as follows:

[0074] (1) Normalizing: Put the F65 steel forging into the furnace at 100℃, with a heating rate of 80℃ / h, heat up to 680℃, hold for 4h; continue heating at 80℃ / h to 940℃, hold for 8h, and air cool to 40℃.

[0075] (2) Quenching: Heating rate 80℃ / h, heating to 680℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 920℃, holding for 7h, water cooling to 45℃;

[0076] (3) Tempering: Heating rate 80℃ / h, heating to 320℃, holding for 2h; heating rate 80℃ / h, continuing to heat to 640℃, holding for 14h, water cooling to 30℃.

[0077] Comparative Example 1

[0078] This comparative example provides a heat treatment method for F65 steel forgings, and the heat treatment process curve is as follows: Figure 6 As shown, the specific steps and parameters are as follows:

[0079] (1) Normalizing: Put the F65 steel forging into the furnace at 250°C, with a heating rate of 80°C / h, heat up to 680°C, hold for 4h; continue heating at 80°C / h to 960°C, hold for 5h, and air cool to 40°C.

[0080] (2) Quenching: Heating rate 80℃ / h, heating to 680℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 920℃, holding for 5h, water cooling to 45℃;

[0081] (3) Tempering: Heating rate 80℃ / h, heating to 350℃, holding for 4h; heating rate 80℃ / h, continuing to heat to 550℃, holding for 7h, then air cooling to 30℃.

[0082] The heat-treated F65 steel forgings obtained in the embodiments and comparative examples of the present invention were tested for tensile strength, yield strength, elongation, shrinkage rate and low temperature impact toughness in accordance with the "Standard Test Methods and Definitions for Mechanical Properties of Steel Products" (ASTM A370). The results are shown in Table 1.

[0083] Table 1 Mechanical properties of F65 steel forgings after heat treatment in the embodiments and comparative examples of the present invention.

[0084]

[0085]

[0086] As can be seen from the mechanical property data in the table, the mechanical properties of the F65 steel forgings treated by the heat treatment method of the present invention in Examples 1 to 5, especially the impact toughness at -46℃, are significantly better than those of the F65 steel forgings treated by the prior art in Comparative Example 1. Furthermore, the F65 steel forgings in Example 2, which underwent an additional normalizing treatment, and the F65 steel forgings in Example 3, which underwent an additional pre-tempering treatment, also showed improved mechanical properties compared to the F65 steel forgings in Example 1, which only underwent one normalizing treatment and one tempering treatment. Therefore, the heat treatment method of the present invention effectively improves the mechanical properties of F65 steel forgings.

[0087] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A heat treatment method for F65 steel forgings, characterized in that, Includes the following steps: S1: Normalizing, heat the F65 steel forging to the normalizing temperature of 930~940℃, hold for t1, remove from the furnace, and air cool to <50℃; S2: Quenching, heating to the quenching temperature, holding at t2, removing from the furnace, and water cooling to <50℃. S3: Tempering, first heat to 310~330℃, hold for 2~4 hours, then continue to heat to the tempering temperature of 630~640℃, hold for t3, remove from the furnace, and water cool to <50℃. The effective wall thickness of the F65 steel forging is 400~700mm; The chemical composition of the F65 steel forging, by weight percentage, includes: C: ≤0.30%, Si: 0.15%~0.35%, Mn: ≤1.60%, P: ≤0.025%, S: ≤0.025%, Cr: ≤0.35%, Ni: ≤1.00%, Mo: ≤0.35%, Alt: ≤0.03%, V: ≤0.06%, Nb: ≤0.025%, N: ≤0.015%, CEV: ≤0.46%.

2. The heat treatment method for F65 steel forgings according to claim 1, characterized in that, Step S1 is performed twice; And / or, add 1 to 2 pre-tempering processes between steps S2 and S3.

3. The heat treatment method for F65 steel forgings according to claim 2, characterized in that, The added pre-tempering treatment involves first heating to 310~330℃, holding at that temperature for 2~4 hours, then further heating to the tempering temperature of 630~640℃, holding at that temperature for the specified time t3, then removing from the furnace and air-cooling to <50℃.

4. The heat treatment method for F65 steel forgings according to any one of claims 1 to 3, characterized in that, In step S1, the heat preservation time t1 is 6~8h.

5. The heat treatment method for F65 steel forgings according to any one of claims 1 to 3, characterized in that, In step S2, the quenching temperature is 900~920℃; And / or, in step S2, the heat preservation time t2 is 5~7h; And / or, in step S2, the water cooling uses circulating water.

6. The heat treatment method for F65 steel forgings according to any one of claims 1 to 3, characterized in that, The heat preservation time t3 is 8~14h.

7. The heat treatment method for F65 steel forgings according to any one of claims 1 to 3, characterized in that, In step S1, the F65 steel forging is first heated to 670~690℃ and held for 4~6 hours, and then heated to the normalizing temperature of 930~940℃. And / or, in step S2, the F65 steel forgings treated in step S1 are first heated to 670~690℃, held for 4~6 hours, and then heated to the quenching temperature of 900~920℃.

8. The heat treatment method for F65 steel forgings according to any one of claims 1 to 3, characterized in that, The F65 steel forging is obtained by sawing and forging an F65 steel continuous casting billet.

9. The heat treatment method for F65 steel forgings according to claim 8, characterized in that, The chemical composition of the F65 steel forging, by weight percentage, includes: C: 0.07%~0.15%, Si: 0.25%~0.35%, Mn: 0.80%~1.50%, P: ≤0.015%, S: ≤0.010%, Cr: 0.10%~0.35%, Ni: 0.50%~1.00%, Mo: 0.10%~0.35%, Alt: 0.01%~0.03%, V: 0.015%~0.06%, Nb: 0.005%~0.025%, N: 0.006%~0.011%, and carbon equivalent (CEV): 0.40%~0.46%.

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