Method for controlling primary NbC re-dissolution in niobium-containing ferritic / martensitic heat-resistant steel

Abstract

The application discloses a method for controlling the resolubilization of primary NbC in niobium-containing ferrite / martensite heat-resistant steel, and belongs to the technical field of hot working of nuclear heat-resistant steel. The method comprises the following steps: hot deformation pretreatment (total deformation amount is 50%-75%); high-temperature solid solution treatment at 1300 DEG C for 2h-8h; normalizing treatment at 1100 DEG C-1140 DEG C for 1h-4h; and tempering treatment at 720 DEG C-780 DEG C for 1h-3h. The unique process combination of 'high-temperature solid solution at 1300 DEG C + normalizing' reduces the number density of primary NbC by more than 85%, and simultaneously reduces the excessive delta-ferrite generated by high-temperature solid solution to less than 15% through normalizing, thereby solving the contradiction between NbC resolubilization and the increase of delta-ferrite in the traditional process. The room-temperature impact energy of the treated steel can reach 33J, which is increased by about 43% compared with the conventional quenched and tempered state, and the tensile strength is not reduced, so that the comprehensive mechanical properties of the material are significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of hot working technology of nuclear ferritic / martensitic heat-resistant steel, specifically involving a method for controlling the re-dissolution of NbC in niobium-containing ferritic / martensitic heat-resistant steel, which is particularly suitable for the preparation of key components such as the cladding tubes of fourth-generation lead-cooled fast reactors. Background Technology

[0002] Ferritic / martensitic (F / M) heat-resistant steel with 9–12 wt.% Cr is a candidate material for key components such as cladding tubes in fourth-generation nuclear reactors due to its excellent high-temperature mechanical properties, corrosion resistance, and radiation resistance. To improve strength and toughness, elements such as Nb, Ti, C, and N are usually added to F / M steel to form nanoscale MX phases (M = Nb, Ti, etc.; X = C, N) in the matrix, which plays a role in precipitation strengthening. However, these high-melting-point MX phases are prone to precipitation, aggregation, and growth into primary carbides (mainly NbC) in the liquid phase during the solidification process of the ingot due to the segregation of alloying elements. The size can reach several micrometers or even more than ten micrometers. The hardness of primary NbC is as high as 1500–2000 HV, which is much higher than that of the matrix. This is inconsistent with the deformation of the matrix and easily causes stress concentration during processing and service, becoming a source of crack initiation and seriously deteriorating the toughness and long-term creep performance of the material.

[0003] Therefore, how to fully dissolve primary NbC through subsequent hot working and heat treatment is key to improving the comprehensive mechanical properties of F / M steel. Currently, the main method for dissolving primary carbides is high-temperature solution treatment (also known as high-temperature diffusion annealing). The principle is to utilize the increased solubility and diffusion rate of Nb and C elements in the matrix at high temperatures, allowing NbC to gradually dissolve. However, F / M steel contains a large number of ferrite-forming elements (such as Si, Cr, W, V, Nb, etc.), which undergo a phase transformation from austenite to δ-ferrite during high-temperature solution treatment—the higher the solution temperature and the longer the holding time, the greater the δ-ferrite content. Existing research shows that when the solution temperature exceeds 1250℃, the δ-ferrite content increases sharply (for example, after holding at 1300℃ for 8 hours, the δ-ferrite content can reach over 36%), and the excessive presence of δ-ferrite significantly reduces the toughness and strength of the material. This creates a dilemma: If the solution temperature is too low (e.g., 1150-1200℃), the NbC re-dissolution effect is limited, the amount of primary carbides remaining is large, and the improvement on toughness is not obvious. If the solution temperature is too high (e.g., 1250–1300℃), although NbC can be dissolved in large quantities, the δ-ferrite content increases sharply, which also impairs the toughness of the material.

[0004] Therefore, how to achieve sufficient NbC re-dissolution in one step while controlling the δ-ferrite content at a low level is a technical problem that urgently needs to be solved in the field of F / M steel hot working. Summary of the Invention

[0005] This invention aims to overcome the aforementioned difficulties in the prior art and provide a method for controlling the re-dissolution of primary NbC in niobium-containing F / M heat-resistant steel. This method, through the coordinated design of a four-stage process—"hot deformation pretreatment → high-temperature solution treatment → normalizing → tempering"—achieves a significant reduction in primary NbC number density (≥80%) while controlling the δ-ferrite content below 15%. This results in a significant improvement in the material's impact toughness while maintaining essentially the same strength or even slightly increasing it.

[0006] Technical solution

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for controlling the remelting of NbC in niobium-containing F / M heat-resistant steel, characterized by comprising the following steps: Step S1: Heat Deformation Pretreatment Niobium-containing F / M heat-resistant steel billets are subjected to hot deformation processing, with the total deformation controlled at 50% to 75%, to obtain deformed billets.

[0008] The hot deformation process is forging, rolling, or a combination thereof. The deformation temperature range is 950℃~1150℃.

[0009] Step S2: High-temperature solution treatment

[0010] The deformed billet obtained in step S1 is placed in a heat treatment furnace and heated to 1300°C under a protective atmosphere. It is held for 2 to 8 hours and then rapidly cooled to room temperature to obtain a solution-treated billet.

[0011] Step S3: Normalizing treatment

[0012] The solution-treated billet obtained in step S2 is heated to 1100℃~1140℃ and held for 1h~4h. After the holding period, it is air-cooled to room temperature to obtain the normalized billet.

[0013] Step S4: Tempering

[0014] The normalized billet obtained in step S3 is heated to 720℃~780℃ and held for 1h~3h. After the holding period, it is air-cooled to room temperature to obtain the finished product.

[0015] Further, in step S1, the total deformation is preferably 60% to 65%. More preferably, the total deformation is 62.5%.

[0016] Furthermore, in step S2, the high-temperature solution treatment is carried out in a sealed quartz tube or protective atmosphere furnace under argon protection, with a preferred holding time of 8 hours, and rapid cooling is achieved by water cooling.

[0017] Furthermore, in step S3, the normalizing temperature is preferably 1120°C, and the holding time is preferably 4 hours.

[0018] Furthermore, in step S4, the tempering temperature is preferably 750°C, and the holding time is preferably 2 hours.

[0019] The chemical composition of the niobium-containing F / M heat-resistant steel, by mass percentage, includes: C: 0.08%–0.12%, Cr: 8.0%–11.0%, Mn: 0.4%–0.7%, V: 0.1%–0.3%, Si: 0.8%–1.2%, W: 0.4%–1.0%, N: 0.02%–0.07%, Ti: 0.01%–0.03%, Nb: 0.2%–0.3%, with the balance being Fe and unavoidable impurities. Preferably, the steel also contains Ce: 0.005%–0.05%.

[0020] Technical Principle Explanation

[0021] The core innovation of this invention lies in identifying the contradictory relationship between NbC dissolution and the increase of δ-ferrite, and resolving this issue through the coordinated operation of four processes: "hot deformation pretreatment → 1300℃ high-temperature solution treatment → normalizing → tempering". The technical principle is as follows: (a) Mechanism for controlling deformation amount between 50% and 75% Deformation processing has a mechanical crushing effect on coarse primary carbides, reducing their size and aspect ratio, and shortening the diffusion path of alloying elements during subsequent solid solution treatment. Simultaneously, the numerous dislocations introduced by deformation can serve as rapid diffusion channels for Nb and C atoms ("short-circuit diffusion"), promoting the dissolution of primary carbides. Experiments show that as the deformation amount increases from 45% to 92.5%, the number density, average size, and aspect ratio of primary NbC all decrease, which is beneficial for subsequent solid solution treatment. However, excessively high deformation amounts (e.g., ≥77.5%) lead to a significant increase in the density of geometrically necessary dislocations (GNDs) in the matrix (e.g., the GND density reaches 9.08 × 10¹ at 92.5% deformation). 4 Even after solution recovery, the yield strength and impact toughness of the material will still suffer significant losses. A deformation of 50%–75% can achieve a good balance between carbide fragmentation and dislocation density control, while around 62.5% has been proven to be the optimal choice that balances both.

[0022] (ii) High-temperature solid solution causes significant re-dissolution of NbC, but is accompanied by a dramatic increase in δ-ferrite.

[0023] A systematic comparison of the number density of NbC after solution treatment at 1150℃, 1200℃, 1250℃, and 1300℃ revealed that NbC showed almost no re-dissolution at 1150℃ (the number density remained essentially the same as before solution treatment); at 1200℃ and 1250℃, although there was some coarsening or fluctuation, the net re-dissolution of NbC was still very limited; only when the solution temperature reached 1300℃ did NbC show significant re-dissolution. After holding at 1300℃ for 2h, 4h, and 8h, the number density of NbC decreased to 0.63×10¹¹ cells / m³, 0.52×10¹¹ cells / m³, and 0.34×10¹¹ cells / m³, respectively, representing decreases of approximately 73%, 78%, and 85.7% compared to before solution treatment (approximately 2.37×10¹¹ cells / m³). The fundamental reason is that, on the one hand, the solid solubility product of NbC in austenite increases sharply with increasing temperature; on the other hand, the diffusion rate of Nb and C elements is much higher at 1300℃ than at lower temperatures, and the diffusion rate in δ-ferrite is higher than that in austenite, which promotes atomic migration.

[0024] However, 1300℃ falls within the δ+γ dual-phase region of this steel, and during the heat treatment process, austenite continuously transforms into δ-ferrite. After solution treatment at 1300℃ for 8 hours, the δ-ferrite area accounts for as high as approximately 36%. Such a high δ-ferrite content will severely deteriorate subsequent mechanical properties; therefore, the solution-treated state cannot be directly used as the operational state.

[0025] (iii) Normalizing treatment to eliminate excess δ-ferrite

[0026] The solution-treated billet is normalized at 1100℃~1140℃ (preferably 1120℃), a temperature within the austenitic single-phase region of the steel. During the holding process, the excess δ-ferrite formed in the solution treatment stage undergoes a reverse transformation to austenite, significantly reducing the δ-ferrite content. Experiments have shown that after solution treatment at 1300℃ for 8 hours, the δ-ferrite content of the sample can be reduced from approximately 36% to approximately 12% after normalizing at 1120℃ for 4 hours. Simultaneously, austenite exhibits high carbon solubility during normalizing, allowing some residual primary NbC to continue dissolving, resulting in a slight decrease in the average NbC size. Air cooling after normalizing yields a martensitic structure, laying the microstructure foundation for subsequent tempering.

[0027] (iv) Tempering treatment to restore toughness

[0028] Tempering (750℃×2h) precipitates fine and dispersed M at the martensite lath boundaries and the original austenite grain boundaries. 23 The C6 and MX phases eliminate quenching stress, reduce dislocation density, and significantly restore the material's plasticity and toughness.

[0029] Beneficial effects: Compared with the prior art, the present invention has the following beneficial effects: (1) The effect of primary NbC re-dissolution is significant: the number density of primary NbC decreased from about 2.37 × 10¹¹ particles / m³ before treatment to about 0.34 × 10¹¹ particles / m³ after treatment (a reduction of about 85.7%), and the average size decreased from about 2.76 μm to below about 2.5 μm. Needle-shaped and strip-shaped large particle carbides basically disappeared, and the morphology tended to be spherical. This effectively reduced stress concentration points and potential crack initiation points during service.

[0030] (2) The content of δ-ferrite is effectively controlled: through normalizing treatment after solution treatment, the area ratio of δ-ferrite is reduced from about 36% in the solution state to about 12%, avoiding the deterioration of toughness and strength caused by excessive δ-ferrite.

[0031] (3) Significantly improved impact toughness without reduced strength: Compared with bars with the same amount of deformation treated only by quenching and tempering, the V-shaped impact energy at room temperature increased from about 23J to about 33J (an increase of about 43.5%), and the room temperature tensile strength increased from about 801MPa to about 807MPa, achieving "toughening without reducing strength". The elongation after fracture also increased from about 18% to about 22%, and the plasticity was significantly improved.

[0032] (4) The process is highly operable: the heating temperature and holding time of each step are within the range of conventional heat treatment equipment, no special tooling is required, and it is easy to promote and apply under industrial conditions. Attached Figure Description

[0033] Figure 1 The diagram shows a comparison of the NbC number density changes under different solution temperatures and times according to the method of the present invention. (a) Statistical results under different conditions, and (b) SEM results at typical solution temperatures and times.

[0034] Figure 2 SEM images of δ-ferrite before and after solution treatment at 1300℃ for 8 hours (a) and after normalizing treatment (b), along with their statistical results (c).

[0035] Figure 3 The graph shows a comparison of tensile strength (a, b) and impact energy (c) between Example 1 and Comparative Example 1. Detailed Implementation

[0036] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0037] Example 1 (Optimal Example)

[0038] This embodiment provides a method for controlling the primary NbC re-dissolution in niobium-containing F / M heat-resistant steel. The chemical composition of the F / M heat-resistant steel used, by mass percentage, is shown in Table 1: Table 1. Chemical composition (wt.%) of F / M heat-resistant steel in Example 1

[0039] The ingots were prepared using a vacuum induction melting + electroslag remelting process. The electroslag remelting slag system was "30% Al2O3 + 70% CaF2, mass fraction".

[0040] The specific process steps are as follows: Step S1: Heat Deformation Pretreatment The electroslag remelted ingot was forged into a 200mm×200mm square billet after being held at 1200℃ for 3 hours. After flaw detection and removal of defective areas, the billet was sawn into 200mm×200mm×200mm forging blocks. The forging blocks were held at 1200℃ for 8 hours, and then forged in the range of 950℃~1150℃, with the total deformation controlled at about 62.5%, to obtain a rod-shaped deformed billet with a diameter of about 75mm.

[0041] Tests showed that the δ-ferrite content under this deformation was approximately 5.5% to 6.5%, the average grain size of the original austenite was approximately 40.5 μm, the primary NbC number density was approximately 2.37 × 10¹¹ grains / m³, the average size was approximately 2.76 μm, and the aspect ratio was approximately 1.85.

[0042] Step S2: High-temperature solution treatment

[0043] Cut a 20mm×20mm×20mm sample from the deformed blank obtained in step S1, seal it in a quartz tube protected by argon gas, and keep it at 1300℃ for 8 hours. After the heat preservation is completed, cool it to room temperature with water to obtain a solid solution sample.

[0044] Testing revealed that the NbC number density in the solid solution sample decreased to approximately 0.34 × 10¹¹ cells / m³ (a reduction of approximately 85.7%), and the average NbC size was approximately 3.25 μm (smaller NbC preferentially dissolved, while the remaining larger NbC showed slight coarsening). Needle-shaped and elongated NbC largely disappeared, and the morphology tended towards spherical. However, the δ-ferrite content increased sharply to approximately 36.4%.

[0045] Step S3: Normalizing treatment

[0046] The solid solution sample obtained in step S2 is placed in a heating furnace and kept at 1100-1120℃ for 2-4 hours. After the heat preservation is completed, it is air-cooled to room temperature to obtain a normalized sample.

[0047] Testing revealed that when the normalizing process was 1100 ℃ + 2 h, the δ-ferrite content in the sample decreased to approximately 12%, a significant drop compared to the solution-treated state. The NbC number density showed little change compared to the solution-treated state (normalizing has a limited contribution to NbC re-dissolution), but the average NbC size decreased slightly (because the matrix's ability to dissolve carbon is enhanced after δ-ferrite transforms into austenite, and some NbC continues to re-dissolve).

[0048] Step S4: Tempering

[0049] The normalized sample obtained in step S3 was placed in a heating furnace and kept at 750°C for 2 hours. After the holding period, it was air-cooled to room temperature to obtain the finished product.

[0050] Mechanical property testing: The finished products obtained in steps S1 to S4 (referred to as Example 1) and the control sample obtained in steps S1+S3+S4 (without solution treatment in S2, referred to as Comparative Example 1) were subjected to room temperature tensile tests and Charpy impact tests, respectively. The results are shown in Table 2: Table 2 Mechanical properties of the finished product from Example 1 and Comparative Example 1

[0051] As can be seen from Table 2, after processing using the method of the present invention: The impact energy increased from 23.0J to 33.0J, an increase of approximately 43.5%. The tensile strength increased slightly from 800.9 MPa to 806.9 MPa. The elongation after fracture increased from 18% to 22%, and the plasticity was significantly improved; The yield strength decreased from 680.2 MPa to 629.4 MPa, mainly due to the recovery of dislocation density (GND) caused by solution treatment. However, the enhanced strain hardening capacity compensated for this decrease, allowing the tensile strength to be maintained.

[0052] Fracture analysis: The fibrous region accounts for approximately 20.0% of the tensile fracture surface of the deformed billet. Numerous shallow dimples are present within this region, with fractured NbC particles at their centers, indicating that primary carbides induced extensive microporous nucleation. In the finished product, the fibrous region accounts for approximately 46.5% of the tensile fracture surface, with a significant reduction in shallow dimples and the appearance of deep dimples. This indicates a substantial decrease in microporous nucleation sources after primary carbide re-dissolution, resulting in a significant enhancement in the material's resistance to crack initiation.

[0053] Comparative Example 1 (conditioning only, without high-temperature solution treatment)

[0054] Using the same O75 modified billet as in step S1 of Example 1, without the high-temperature solution treatment in step S2 and the normalizing in step S3, the material was directly subjected to quenching and tempering: 1120℃×4h air cooling + 750℃×2h air cooling. Its mechanical properties are listed in Table 2. It can be seen that without high-temperature solution treatment, almost no NbC re-dissolution occurs in a single step, and the impact energy is only 23.0J, far lower than the 33.0J in Example 1.

[0055] Comparative Example 2 (using a lower solution temperature)

[0056] To compare the effects of different solution temperatures on the re-dissolution of NbC, the inventors conducted a series of solution experiments. Specifically, samples cut from O75 billets were held at 1150℃, 1200℃, and 1250℃ for 8 hours (water-cooled), and then observed using metallography and scanning electron microscopy. The results showed that after solution treatment at 1150℃ for 8 hours, the NbC number density was approximately 2.34 × 10¹¹ cells / m³ (essentially the same as before solution treatment); after solution treatment at 1200℃ for 8 hours, the number density was approximately 2.32 × 10¹¹ cells / m³; and after solution treatment at 1250℃ for 8 hours, although the number density fluctuated, it only reached a minimum of 1.45 × 10¹¹ cells / m³. That is, within the range of 1150–1250℃, the re-dissolution of NbC was very limited. Only when the solution temperature reached 1300℃ did the NbC number density experience a sharp drop, demonstrating a significant re-dissolution effect. This fully demonstrates the necessity of solid solution at 1300℃ for achieving a large amount of NbC re-dissolution in a single step.

[0057] Based on the results of the above embodiments and comparative examples, it can be seen that the process route of "50% to 75% deformation pretreatment → 1300℃ high-temperature solution treatment → 1100℃ to 1140℃ normalizing → tempering" proposed in this invention achieves the unity of "significant re-dissolution of carbides in one step" and "controllable δ-ferrite content", providing a practical and feasible process solution for the manufacturing of large forgings of nuclear F / M heat-resistant steel.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for controlling the remelting of NbC in niobium-containing ferrite / martensitic heat-resistant steel, characterized in that, Includes the following steps: Step S1, Hot deformation pretreatment: The niobium-containing F / M heat-resistant steel billet is hot-deformed, and the total deformation is controlled at 50% to 75% to obtain the deformed billet. Step S2, high-temperature solution treatment: The deformed billet obtained in step S1 is heated to 1300℃ under a protective atmosphere and held for 2h to 8h. After the holding period, it is rapidly cooled to room temperature to obtain a solution-treated billet. Step S3, normalizing treatment: Heat the solution-treated billet obtained in step S2 to 1100℃~1140℃, hold for 1h~4h, and air cool to room temperature after holding to obtain normalized billet; Step S4, tempering treatment: Heat the normalized billet obtained in step S3 to 720℃~780℃, hold for 1h~3h, and air cool to room temperature after holding to obtain the finished product.

2. The method according to claim 1, characterized in that, In step S1, the total deformation is 60% to 65%.

3. The method according to claim 2, characterized in that, The total deformation was 62.5%.

4. The method according to claim 1, characterized in that, In step S2, the heat preservation time is 8 hours, and the rapid cooling is water cooling.

5. The method according to claim 1, characterized in that, In step S3, the normalizing temperature is 1100-1120℃, and the holding time is 4h.

6. The method according to claim 1, characterized in that, In step S4, the tempering temperature is 750°C and the holding time is 2 hours.

7. The method according to claim 1, characterized in that, The chemical composition of the niobium-containing F / M heat-resistant steel, by mass percentage, includes: C: 0.08%–0.12%, Cr: 9.0%–11.0%, Mn: 0.4%–0.7%, V: 0.1%–0.3%, Si: 1.0%–1.2%, W: 0.4%–0.6%, N: 0.03%–0.05%, Ti: 0.01%–0.02%, Nb: 0.2%–0.3%, with the balance being Fe and unavoidable impurities.

8. The method according to claim 7, characterized in that, The niobium-containing F / M heat-resistant steel also contains Ce: 0.005% to 0.01%.

9. The method according to claim 1, characterized in that, After treatment by the method described above, the number density of primary NbC is reduced by ≥80% compared to before treatment, and the area ratio of δ-ferrite is ≤15%.

10. The method according to claim 1, characterized in that, The protective atmosphere described in step S2 is argon.

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