A steel for high-level radioactive waste disposal containers and its preparation method

Abstract

This invention relates to a steel for high-level radioactive waste disposal containers and its preparation method, belonging to the field of engineering steel technology. It solves the problem that existing low-carbon steel systems lack materials that simultaneously meet the comprehensive requirements of corrosion resistance, mechanical properties, weldability, radiation resistance, lifespan predictability, and material cost. The high-level radioactive waste disposal container steel of this invention, by mass percentage, comprises: C, 0.06-0.08%, Si, 0.15-0.35%, Mn, 0.30-0.60%, Ni, 2.0-3.0%, Cr, 0.60-1.00%, P≤0.010%, S≤0.005%, rare earth elements, 0.002-0.004%, with the balance being Fe and unavoidable impurities. This invention yields a material with excellent comprehensive performance.

Description

Technical Field

[0001] This invention relates to the field of engineering steel technology, and in particular to a steel for high-radioactive waste disposal containers and its preparation method. Background Technology

[0002] With the rapid development of the nuclear industry, the safe disposal of nuclear waste generated during the use of nuclear energy has also become a pressing issue. Among these, high-level radioactive waste is highly radioactive, has a large calorific value, is highly toxic, and has a long half-life, causing serious and prolonged harm and making it difficult to handle.

[0003] The performance requirements for disposal container materials mainly include: mechanical stability within a multi-barrier system of the disposal facility; corrosion resistance and radiation resistance in a multi-environmental disposal environment involving buffer / backfill materials, groundwater, and radioactive waste; and weldability for container fabrication. Extensive research has been conducted worldwide on disposal container materials. Germany, France, and Japan, among others, primarily focus on low-carbon steel and low-alloy steels such as TSTE355 carbon steel and JIS SF340A carbon steel. Domestic research also mainly concentrates on low-carbon steel due to its high strength, good machinability, mature processing technology, low cost, low sensitivity to radiation embrittlement, and good weldability. Low-carbon steel exhibits a certain corrosion rate in natural aquatic environments, making it easy to predict the service life of disposal tanks in the operational environment. If low-carbon steel corrodes at a low rate using a uniform corrosion pattern, sufficient thickness can meet the corrosion life requirements of disposal projects. This allows for single-material, single-structure designs, avoiding the problems caused by corrosion galvanic couples formed between dissimilar metals in composite metal tanks. However, the steels currently being researched in my country for disposal containers, such as Q235 low-carbon steel, have low strength and toughness, poor mechanical stability, and poor corrosion resistance in simulated geological disposal environments, making it difficult to meet the service life requirements of high-level radioactive waste disposal containers. Summary of the Invention

[0004] In view of the above analysis, the present invention aims to provide a steel for high-radioactive waste disposal containers and a method for its preparation, in order to solve at least one of the problems in the existing low-carbon steel system that is unable to simultaneously meet the comprehensive requirements of corrosion resistance, mechanical properties, weldability, radiation resistance, lifespan predictability and low material cost.

[0005] The objective of this invention is mainly achieved through the following technical solutions:

[0006] On the one hand, the present invention provides a steel for high radioactive waste disposal containers, the chemical composition of which, by mass percentage, includes: C, 0.06-0.08%, Si, 0.15-0.35%, Mn, 0.30-0.60%, Ni, 2.0-3.0%, Cr, 0.60-1.00%, P≤0.010%, S≤0.005%, rare earth elements, 0.002-0.004%, with the balance being Fe and unavoidable impurities.

[0007] Optionally, the rare earth element is Ce.

[0008] Optionally, the microstructure of the steel is tempered martensite.

[0009] Optionally, the steel has a yield strength ≥490MPa, elongation after fracture ≥27.0%, and impact energy at -20℃ ≥80J.

[0010] On the other hand, the present invention also provides a method for preparing steel for high-level radioactive waste disposal containers, which includes the following steps:

[0011] Step 1: Smelt steel billets according to the designed alloy composition;

[0012] Step 2: Hot-roll the steel billet to obtain a hot-rolled plate;

[0013] Step 3: The hot-rolled plate is quenched and tempered in sequence to obtain steel for high radioactive waste disposal containers.

[0014] Optionally, in step 2, hot rolling includes roughing and finishing rolling.

[0015] Optionally, the initial rolling temperature of the roughing roll is 1050-1100℃, and the final rolling temperature is 950-1000℃.

[0016] Optionally, the finishing rolling temperature is 850-900℃.

[0017] Optionally, in step 3, the quenching cooling rate is 5°C / min.

[0018] Optionally, quenching includes heating the steel plate to 900-1000℃, holding it at that temperature for 1-2 hours, and then water quenching it to room temperature.

[0019] Optionally, tempering includes heating the quenched steel plate to 550-650℃, holding it at that temperature for 0.5-1h, and then air-cooling it to room temperature.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] (1) The present invention adopts the concept of combining composition design and microstructure control (using quenching and tempering heat treatment process) to generate a martensitic structure with high corrosion resistance (the existing microstructure of steel used for high radioactive waste disposal containers is ferrite + pearlite), thereby comprehensively improving the corrosion resistance, mechanical properties, weldability, radiation resistance and other properties of the material.

[0022] (2) This invention uses Ni and Cr, which have lower radiation sensitivity, instead of Cu, which is more sensitive to uniform corrosion, to increase the corrosion potential of the disposal container material. This generates protective corrosion products such as NiFe2O4 and Cr2O3 during the oxygen-containing stage of the disposal tank. In addition, the hardenable elements Ni and Cr improve the material strength through solid solution treatment. Combined with the quenching and tempering heat treatment process, a tempered martensite structure with high strength and toughness is obtained (the microstructure of steel used for high-radioactive waste disposal containers in the prior art is ferrite + pearlite). This ensures the mechanical stability of the disposal container within the multi-barrier system of the disposal tank. The single tempered martensite structure does not form a corrosion galvanotype, further improving the material's resistance to uniform corrosion.

[0023] Furthermore, since the present invention incorporates alloying elements such as Ni and Cr, it provides a certain margin for material properties, thereby improving the weldability of the material while reducing the C content.

[0024] (3) Reduce the content of inclusions by controlling the content of impurity elements S and P. By adding rare earth elements to the alloy composition, the inclusions are further modified to reduce the tendency of inclusions to induce local corrosion in the container material, thereby ensuring that the container material does not experience local corrosion and has a low uniform corrosion rate in the disposal environment.

[0025] (4) The steel for the high-radioactivity waste disposal container of the present invention has high mechanical properties, for example, yield strength R eL ≥440MPa, elongation after fracture A≥27%, impact energy KV2≥80J at -20℃ (TStE355R adopted in France) eL ≥355MPa, A≥21%, room temperature KV2≥34J; Q235R studied domestically eL With a strength of ≥235MPa, A≥26%, and KV2≥27J at -20℃, it exhibits good corrosion resistance (corrosion rate ≤50μm / a) and good weldability (carbon equivalent ≤0.58, compared to 0.55 for the existing TSTE355 and 0.45 for Q235), meeting the comprehensive performance requirements for high-level radioactive waste disposal containers.

[0026] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages will become apparent from the description or may be learned by practicing the invention. Attached Figure Description

[0027] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0028] Figure 1 This is a metallographic image of the microstructure of Example 1.

[0029] Figure 2 The image shows the macroscopic morphology of the rusted sample from Example 2.

[0030] Figure 3 Macroscopic morphology of the acid-washed sample in Example 3

[0031] Figure 4 The image shows the microstructure of the corrosion rust layer in Example 4.

[0032] Figure 5 The XPS results for the rust layer in Example 5 are shown. Detailed Implementation

[0033] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0034] In a first aspect, the present invention provides a steel for a high-radioactive waste disposal container, the chemical composition by weight percentage being: C, 0.06-0.08%, Si, 0.15-0.35%, Mn, 0.30-0.60%, Ni, 2.0-3.0%, Cr, 0.60-1.00%, P≤0.010%, S≤0.005%, rare earth elements, 0.002-0.004%, with the remainder being Fe and unavoidable impurities.

[0035] Specifically, the rare earth element is Ce. This is because Ce has a stronger ability to modify inclusions compared to other rare earth elements.

[0036] The effects / synergistic effects and proportions of each element in this invention are based on the following:

[0037] C: A fundamental element in steel, it has a significant solid solution strengthening effect and can improve the hardenability of steel. However, an increase in C content is very detrimental to the low-temperature toughness and weldability of steel. Therefore, this invention controls the C content at 0.06-0.08%.

[0038] Si: A basic element in steel, it has a certain solid solution strengthening effect and mainly plays a deoxidation role in the steelmaking process. When the content is high, it is detrimental to the low-temperature toughness and weldability of steel. Therefore, the Si content in this invention is controlled at 0.15-0.35 wt.%.

[0039] Mn: a solid solution strengthening element and an austenite stabilizing element, and also has the effect of improving hardenability. Excessive Mn content is prone to segregation and is also detrimental to weldability. Therefore, the Mn content in this invention is controlled at 0.30-0.60 wt.%.

[0040] Ni and Cr: Ni and Cr can increase the corrosion potential of materials and generate protective corrosion products such as NiFe2O4 and Cr2O3 in the aerobic stage of the disposal reservoir. In this invention, the Ni content is controlled at 2.0-3.0 wt.% and the Cr content is controlled at 0.60-1.00 wt.%. In addition, Ni and Cr are also key hardenability elements in this invention. On the one hand, they can improve the hardenability of steel; on the other hand, Ni and Cr improve the strength of materials through solid solution. Combined with the quenching and tempering heat treatment process, a tempered martensitic structure with high strength and toughness is obtained (the microstructure of steel used for high-radioactive waste disposal containers in the prior art is ferrite + pearlite), which ensures the mechanical stability of the disposal container in the multi-barrier system of the disposal reservoir. The single tempered martensitic structure will not form a corrosion galvanocoupler, further improving the material's resistance to uniform corrosion.

[0041] P: P is generally a harmful element in steel, which can form non-metallic inclusions and impair the plasticity and toughness of steel. Therefore, it should be minimized. Considering the cost of P removal, this invention controls the P content to ≤0.010%.

[0042] Sulfur (S): Sulfur is generally considered a harmful element in steel. During high-temperature pressure processing, it forms sulfide inclusions, which easily lead to hot brittleness. The higher the sulfur content, the more severe the hot brittleness. Furthermore, sulfide inclusions are also a major source of pitting corrosion initiation; large sulfide inclusions in the treatment chamber environment easily induce pitting corrosion. Therefore, the sulfur content must be controlled. This invention controls the sulfur content to ≤0.005%.

[0043] Rare earth elements: These elements modify inclusions in steel, reducing the tendency for inclusions to induce localized corrosion in container materials, thereby ensuring that the container materials do not experience localized corrosion and possess a low uniform corrosion rate under the treatment environment. In this invention, the content of rare earth elements is 0.002-0.004%. Specifically, the rare earth element is Ce. This is because Ce has a stronger inclusion modification ability compared to other rare earth elements.

[0044] Secondly, the present invention also provides a method for preparing steel for high-radioactive waste disposal containers, comprising the following steps:

[0045] Step 1: Smelt according to the above chemical composition and cast into billets;

[0046] Step 2: Hot-roll the billet obtained in Step 1;

[0047] Step 3: The hot-rolled plates are quenched and tempered in sequence to obtain steel for high radioactive waste disposal containers.

[0048] Specifically, in step 2, hot rolling includes roughing and finishing. The initial rolling temperature for roughing is 1050-1100℃, and the final rolling temperature is 950-1000℃. The roughing reduction is 65-75%. The finishing rolling temperature is 850-900℃, and the finishing reduction is 50-60%.

[0049] Specifically, in step 3, quenching includes: heating the steel plate to 900-1000℃, holding it at that temperature for 1-2 hours, and then water quenching it to room temperature.

[0050] In one embodiment, the quenching temperature can be, for example, 900℃, 920℃, 940℃, 950℃, 960℃, 980℃, or 1000℃. The holding time can be, for example, 1h, 1.5h, or 2h.

[0051] It should be noted that 900-1000℃ is the austenitizing temperature. The purpose of heating the steel plate to 900-1000℃ in this invention is twofold: first, to generate an austenitic structure; and second, to allow alloying elements to dissolve into the matrix.

[0052] The cooling rate for water quenching is 5℃ / min. After water quenching, the austenitic structure transforms into martensite, thus ensuring the strength of the material.

[0053] Tempering involves heating the quenched steel plate to 550-650℃, holding it at that temperature for 0.5-1 hour, and then air-cooling it to room temperature. The purpose of tempering is to eliminate internal stress and generate tempered martensite that combines strength and toughness.

[0054] In one embodiment, the tempering temperature may be, for example, 550°C, 570°C, 590°C, 600°C, 620°C, 630°C, or 650°C. The holding time may be, for example, 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, or 1h.

[0055] Examples and comparative examples:

[0056] Preparation of steel for high-level radioactive waste disposal containers:

[0057] Step 1: In a 100kg vacuum induction furnace, smelt steel billets according to the alloy composition in Table 1 and add desulfurizing and dephosphorizing agents to obtain steel billets;

[0058] Step 2: Hot-roll the steel billet according to the process conditions in Table 2 to obtain hot-rolled plate;

[0059] Step 3: Quench and temper the hot-rolled plate sequentially according to the process conditions in Table 2 to obtain steel for high radioactive waste disposal containers.

[0060] It should be noted that Comparative Example 1 and Comparative Example 2 are TSTE 355 and Q235, respectively, which are typical candidate materials for disposal containers.

[0061] The mechanical properties of the steel used for high-level radioactive waste disposal containers were tested, and the test results are shown in Table 3.

[0062] Table 1. Chemical composition of the examples and comparative examples (unit: wt%)

[0063]

[0064] Table 2 Specific process conditions

[0065]

[0066]

[0067] Note: Temperatures in the table are in °C, and holding times are in hours. Comparative Examples 1 and 2 are hot-rolled steel plates, without post-rolling heat treatment process flow.

[0068] Table 3 Mechanical property test results

[0069]

[0070] Table 3 shows that the steel used for high-level radioactive waste disposal containers of the present invention possesses excellent mechanical properties, with a yield strength ≥490 MPa, elongation after fracture ≥27.0%, and impact energy at -20℃ ≥80 J. The yield strengths of Comparative Example 1 (TStE 355) and Comparative Example 2 (Q235) are 384 MPa and 255 MPa, respectively, significantly lower than those of the present invention. The elongation after fracture of Comparative Example 1 (TStE 355) and Comparative Example 2 (Q235) are 22.0% and 25.0%, respectively, significantly lower than those of the present invention. The impact energy at -20℃ of Comparative Example 1 (TStE 355) and Comparative Example 2 (Q235) are 50 J and 41 J, respectively, significantly lower than those of the present invention.

[0071] As can be seen, compared with Comparative Example 1 (TStE 355) and Comparative Example 2 (Q235), the steel for high-radioactive waste disposal containers obtained by the present invention, which combines composition design and microstructure control (using a tempering heat treatment process of quenching + tempering), has significantly improved strength and high toughness, and has a high resistance to geological disturbance in deep geological disposal environments.

[0072] In addition, immersion tests were conducted on Examples 1-5 and Comparative Examples 1 and 2 in simulated treatment environments. The corrosive solution was an underground aqueous solution simulating the treatment environment in Beishan, Gansu: CaCl2 0.5792 g / L, NaHCO3 0.1384 g / L, MgSO4·7H2O 0.5716 g / L, NaSO4 1.5777 g / L, NaNO3 0.0372 g / L, KCl 0.0382 g / L, NaCl 1.4876 g / L, pH = 7.6. The immersion periods were 20 days, 40 days, and 60 days. The corrosion rate (μm / a) results are shown in Table 4.

[0073] Table 4 Results of simulated environmental immersion tests

[0074]

[0075] Table 4 shows that the corrosion rates of the high-radioactive waste disposal container steel described in this invention were all ≤50 μm / a in the 20-day, 40-day, and 60-day corrosion tests in a simulated deep geological disposal environment. These rates were significantly lower than those of Comparative Examples 1 and 2. The gradually decreasing corrosion rate is attributed to the formation of a protective passivation film on the material surface as corrosion progresses. The corrosion rate is mainly related to the Ni content. Ni increases the corrosion potential of the rust layer and generates protective corrosion products such as NiFe2O4, thus improving the resistance to uniform corrosion under simulated disposal conditions. No pitting was found on the surface of the corroded samples after acid washing, demonstrating that the control of P and S elements and the addition of rare earth elements in Examples 1-5 reduced the tendency for localized corrosion.

[0076] In addition, the present invention also confirmed the microstructure of the steel in Example 1, and the results are shown in […]. Figure 1 .Depend on Figure 1 It can be seen that the microstructure of Example 1 is tempered martensite.

[0077] Furthermore, this invention also observed the macroscopic and microscopic morphology of the steel during the corrosion test, see... Figures 2-4 .Depend on Figure 2 and Figure 3 It can be seen that the corroded surface is relatively smooth. The corrosion behavior of Examples 2 and 3 is mainly uniform corrosion, and no obvious pitting corrosion occurs. Figure 4 This indicates that the rust layer in Example 4 is relatively dense and can effectively inhibit further corrosion of the material by the corrosive medium.

[0078] Figure 5 The image shown is the XPS result of the rust layer in Example 5. Figure 5 It can be seen that the rust layer products contain Ni oxides and hydroxides such as NiFe2O4, which effectively improves the corrosion resistance of the rust layer in the embodiment.

[0079] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A steel for a high-level waste disposal container, characterized by, Its chemical composition, by mass percentage, includes: C 0.06-0.08%, Si 0.15-0.35%, Mn 0.30-0.60%, Ni 2.0-3.0%, Cr 0.60-1.00%, P 0.005-0.008%, S≤0.005%, Ce 0.002-0.004%, with the balance being Fe and unavoidable impurities; The method for preparing the steel for high-level radioactive waste disposal containers includes the following steps: Step 1: Smelt steel billets according to the designed alloy composition; Step 2: Hot-roll the steel billet to obtain a hot-rolled plate; Step 3: The hot-rolled plate is quenched and tempered sequentially to obtain steel for high-radioactive waste disposal containers; The quenching process includes: heating the steel plate to 900-1000℃, holding it at that temperature for 1-2 hours, and then water quenching it to room temperature; The tempering includes heating the quenched steel sheet to 550 650°C, holding for 0.5 1 h, and then air cooling to room temperature. The microstructure of the steel used for the high-radioactive waste disposal container is a single tempered martensite structure.

2. The steel for a high-level waste disposal container according to claim 1, characterized by, The steel has a yield strength ≥490MPa and an elongation after fracture ≥27.0%.

3. Steel for high-level waste disposal containers according to claim 2, characterized in that, The steel has an impact energy of ≥80J at -20℃.

4. A method of producing a steel for a high-level waste disposal container, characterized by, The method for preparing the steel for high-radioactive waste disposal containers according to any one of claims 1-3 comprises the following steps: Step 1: Smelt steel billets according to the designed alloy composition; Step 2: Hot-roll the steel billet to obtain a hot-rolled plate; Step 3: The hot-rolled plate is quenched and tempered in sequence to obtain steel for high radioactive waste disposal containers.

5. The preparation method according to claim 4, characterized in that, In step 2, hot rolling includes roughing and finishing rolling.

6. The preparation method according to claim 5, characterized in that, The initial rolling temperature for rough rolling is 1050-1100℃, and the final rolling temperature is 950-1000℃.

7. The preparation method according to claim 5, characterized in that, The finishing rolling temperature is 850-900℃.

8. The preparation method according to claim 4, characterized in that, In step 3, the quenching cooling rate is 5℃ / min.

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