A third-generation nuclear power plant high-strength heat-resistant ferritic stainless steel plate and a manufacturing method thereof

Abstract

The application belongs to the field of ferrous materials, and particularly relates to a high-strength heat-resistant ferritic stainless steel plate for third-generation nuclear power plants and a manufacturing method thereof. The chemical composition of the steel plate is as follows in terms of percentage by weight: C: 0.01% to 0.12%, Si: 0.8% to 1.5%, Mn: 0.02% to 0.70%, S: 0.020% to 0.050%, Ni: 0.01% to 1.2%, Cr: 12.5% to 14.5%, Mo: 0.2% to 0.9%, Zr: 0.001% to 0.010%, Mg: 0.010% to 0.020%, Ce: 5 to 15 ppm, Ca: 0.00032% to 0.003%, and the balance of Fe and inevitable impurities. The product produced according to the chemical composition and the production process requirements of the steel plate has high strength and toughness, low hardness and high-temperature resistance.

Description

Technical Field

[0001] This invention belongs to the field of ferrous metal materials, and in particular relates to a high-strength heat-resistant ferritic stainless steel plate for third-generation nuclear power plants and its manufacturing method. Background Technology

[0002] As one of the most critical main equipment in a nuclear power unit, the steam generator plays a vital role in transferring the heat energy generated by the reactor pressure vessel from the primary loop to the secondary loop. It is a key piece of equipment connecting the primary and secondary loops in the nuclear island. The ferritic stainless steel support plate, as a major component of the steam generator, plays an important role in the barrier between the primary and secondary loops. Its safety level is nuclear safety level 1, and its quality level is QA1.

[0003] The main manufacturing challenge of ferritic stainless steel for steam generator support plates is that the steel plate is constantly in use in a high-temperature and corrosive environment. Therefore, the steel plate must ensure high comprehensive mechanical properties under long-term high-temperature conditions. Ferritic stainless steel is prone to precipitating carbides in a high-temperature environment for a long time. How to avoid the formation of carbides and damage to the performance and corrosion resistance of the steel plate is the main challenge in steel plate production.

[0004] This invention utilizes the special thickness of ferritic stainless steel supported by a steam generator, which places high demands on key processes such as steel plate smelting, billet heating, forming, and heat treatment, thereby ensuring that the performance of the steel plate meets the requirements.

[0005] Patent application number 201910821783.2 discloses a method for eliminating surface cracks in ultra-wide ferritic stainless steel medium-thick plates: the isometric crystal ratio of the continuously cast billet is not less than 75%; the homogenization temperature of the billet is 750-850℃, the furnace time is 1-2 hours, and it is water-cooled to room temperature; the surface roughness Ra of the billet after grinding is ≤70μm, and it is sprayed with high-temperature anti-oxidation coating; the billet is treated in a preheating section, a heating section and a soaking section in sequence before exiting the furnace; the preheating section temperature is 950-1180℃, the heating section temperature is 1180-1260℃, the soaking section temperature is 1220-1250℃, and the furnace time is 4-5.5 hours; the rough rolling start temperature is ≥1150℃; the finish rolling start temperature is ≥1150℃, the final rolling temperature is ≥950℃, the single-pass reduction rate is ≤20%, the annealing temperature is 750-880℃, the furnace time is 2-5 minutes / mm, and it is air-cooled. The product width is 2500–4000 mm, and the thickness is 8–30 mm. The scrap rate due to surface cracks has been reduced from over 6% to below 0.5%. The steel plate of this invention has a yield strength ≥230 MPa, tensile strength ≥450 MPa, and elongation ≥28%. However, the mechanical properties of this invention are relatively low, and the high-temperature mechanical properties of the steel plate are not considered, which cannot guarantee the requirements for steel used in nuclear power.

[0006] Patent application number 201910821783.2 discloses a method for eliminating surface cracks in ultra-wide ferritic stainless steel medium-thick plates: the isometric crystal ratio of the continuously cast billet is not less than 75%; the homogenization temperature of the billet is 750-850℃, the furnace time is 1-2 hours, and it is water-cooled to room temperature; the surface roughness Ra of the billet after grinding is ≤70μm, and it is sprayed with high-temperature anti-oxidation coating; the billet is treated in a preheating section, a heating section and a soaking section in sequence before exiting the furnace; the preheating section temperature is 950-1180℃, the heating section temperature is 1180-1260℃, the soaking section temperature is 1220-1250℃, and the furnace time is 4-5.5 hours; the rough rolling start temperature is ≥1150℃; the finish rolling start temperature is ≥1150℃, the final rolling temperature is ≥950℃, the single-pass reduction rate is ≤20%, the annealing temperature is 750-880℃, the furnace time is 2-5 minutes / mm, and it is air-cooled. The product width is 2500–4000 mm, and the thickness is 8–30 mm. The scrap rate due to surface cracks has been reduced from over 6% to below 0.5%. The steel plate of this invention has a yield strength ≥230 MPa, tensile strength ≥450 MPa, and elongation ≥28%. However, the mechanical properties of this invention are relatively low, and it does not consider the high-temperature mechanical properties of the steel plate, thus failing to meet the requirements for steel used in nuclear power.

[0007] The invention patent document "Method for controlling grain size of medium-thick plates of high-carbon austenitic stainless steel" (acceptance number CN111549276A) also discloses a three-stage heat treatment method, with temperatures of 600℃, 1010℃ and 1050℃ for each stage. The above heat treatment method not only ensures that the steel plate has uniform grain size throughout the thickness direction, but also has good resistance to intergranular corrosion. However, it does not discuss whether the above heat treatment process is conducive to the elimination of residual ferrite in the steel. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a high-strength, heat-resistant ferritic stainless steel plate for third-generation nuclear power plants and its manufacturing method. Products manufactured according to the chemical composition and production process requirements of this invention exhibit high strength and toughness, low hardness, and high-temperature resistance.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A high-strength, heat-resistant ferritic stainless steel plate for third-generation nuclear power plants, wherein the chemical composition of the steel by weight percentage is: C: 0.01%–0.12%, Si: 0.8%–1.5%, Mn: 0.02%–0.70%, S: 0.020%–0.050%, Ni: 0.01%–1.2%, Cr: 12.5%–14.5%, Mo: 0.2%–0.9%, Zr: 0.001%–0.010%, Mg: 0.010%–0.020%, Ce: 5–15 ppm, Ca: 0.00032%–0.003%; the balance being Fe and unavoidable impurities.

[0011] The reasons for using the above-mentioned components are as follows:

[0012] 1) Carbon: In steel, carbon (C) often forms carbides with other alloying elements, which plays a strengthening role. However, C easily combines with alloying elements in steel to form carbides, causing segregation within the steel and resulting in a significant decrease in the toughness and plasticity of the steel plate. This invention addresses this problem by employing a low carbon content design. Therefore, the steel of this invention requires a C content of 0.01% to 0.12% in its design composition.

[0013] 2) Silicon: In steelmaking, silicon acts as a reducing agent and deoxidizer. It is an inexpensive alloying element; adding an appropriate amount of silicon to steel, dissolved in austenite, can improve the steel's hardness and strength. However, when the silicon content is too high, because silicon has a stronger affinity for oxygen than iron, it easily forms low-melting-point silicates during welding. This increases the fluidity of the slag and molten metal, causing spattering and affecting weld quality. Therefore, the recommended silicon content is 0.8%–1.5%.

[0014] 3) Manganese (Mn): Mn is infinitely soluble in Fe and is a strong austenite stabilizer, playing a role in fixing austenite in steel and replacing some of the role of nickel. While increasing the strength of steel, Mn has a relatively small impact on plasticity and can lower the lower critical point of steel, increasing the undercooling during austenite cooling, thereby refining the microstructure and improving the mechanical properties of steel plates. In addition, it is relatively inexpensive. However, excessively high Mn content will increase the tendency of grain coarsening in steel; therefore, the recommended Mn content is 0.02%–0.70%.

[0015] 4) Sulfur: S easily forms sulfide inclusions in steel, which reduces the impact toughness of steel, impairs weldability, and aggravates defects such as center segregation and porosity, and increases irradiation embrittlement. However, an appropriate amount of sulfur is beneficial to the cutting action of steel plates. Therefore, this invention requires S: 0.020% to 0.050%.

[0016] 5) Nickel: Ni is an important element in austenitic stainless steel. While expanding the austenite region, Ni also inhibits the formation of ferrite in the steel. Furthermore, its combination with Cr ensures the steel plate has good resistance to oxidation and corrosion. Due to its high price, the Ni content should be controlled to 0.01%–1.2%.

[0017] 6) Chromium: Cr forms various carbides with C in steel and dissolves in austenite, playing a role in dispersion, grain refinement and solid solution strengthening. It has a positive effect on the strength and toughness of steel plates. However, excessive addition of Cr will reduce the toughness of steel plates and increase their brittleness. In this invention, the Cr content is controlled between 12.5% ​​and 15.5%.

[0018] 7) Molybdenum: Mo is a strong carbide-forming element. Ferritic stainless steel generally has poor corrosion resistance and high-temperature resistance. However, with the increase of molybdenum content, it can strengthen the grains and avoid grain boundary depletion corrosion through solid solution strengthening and synergistic strengthening with other alloys. Therefore, this invention requires Mo to be 0.2% to 0.9%.

[0019] 8) Zirconium: It has the effect of refining grains, which is beneficial to the low-temperature toughness of steel. Zirconium can form nano-sized ZrO2 with free oxygen in steel, which can not only play a deoxidation role, but also refine grains and increase corrosion resistance. The ZrO2 content added to this steel is controlled at 0.001% to 0.010%.

[0020] 9) Magnesium: Trace amounts of Mg can improve the crystal structure of steel, destroy the network structure of alloy carbides, thereby reducing the size of carbides and promoting their uniform dispersion. Excessive Mg can easily cause the aggregation and growth of composite inclusions. Therefore, the Mg content in this invention is controlled at 0.010% to 0.020%.

[0021] 10) Cerium: During the smelting process, this element undergoes inclusion modification, transforming it into soft, fine inclusions. These fine rare earth inclusions can act as nucleation sites for high-temperature ferrite, refining the dendritic structure, thereby reducing microsegregation, decreasing the size of primary carbides, and making the distribution of primary carbides more uniform. Therefore, the amount of Ce added to steel is 5ppm to 15ppm.

[0022] 11) Calcium: Ca plays a role in controlling the morphology of sulfides in steel, inhibiting the formation of MnS by forming CaS. To achieve this effect, the Ca content needs to be above 0.0003%. Furthermore, if the Ca content exceeds 0.006%, the CaS particles become too large, increasing brittleness and making them more prone to becoming fracture initiation points. Therefore, the amount of Ca added to steel should be 0.00032%–0.003%.

[0023] The steel plate of this invention has a room temperature tensile yield strength ≥416MPa, tensile strength ≥568MPa, and elongation ≥20%; a 350℃ high temperature tensile yield strength ≥332MPa and tensile strength ≥477MPa; a 400℃ high temperature tensile yield strength ≥322MPa and tensile strength ≥445MPa; an impact energy of -23℃ ≥55J; all 180° bending tests are qualified; and a Brinell hardness of 143~149HB.

[0024] The steel plate has a thickness of 15–150 mm and a width of 2000–4300 mm. The microstructure of the steel plate, by volume, is 50%–70% martensite and 30%–50% ferrite.

[0025] A method for manufacturing high-strength, heat-resistant ferritic stainless steel plates for third-generation nuclear power plants includes smelting, billet heating, high-pressure water descaling, rolling, and heat treatment, specifically comprising the following steps:

[0026] 1) The smelting process includes: controlling the tapping temperature at 1570–1620℃; refining the molten steel in an LF furnace to adjust the composition and performing deep desulfurization treatment; feeding silicon-calcium wire at a speed of 2.0–3.9 m / s and a feed rate of 3.9–6.6 m / t; employing VD vacuum treatment and further optimization, with a vacuum treatment time of 19–32 min; ensuring composition control through ladle refining, and ensuring gas content control and zirconium oxide formation through vacuum treatment, resulting in molten steel composition and temperature meeting the target requirements. Furthermore, prolonged argon blowing, stirring, and killing after refining effectively ensure the modification of inclusions by calcium and cerium elements in the steel, fundamentally guaranteeing steel purity, and also serving as an effective means of microalloying the steel.

[0027] 2) The billet is sent to the heating furnace for heating. The billet goes through a preheating section, a heating section, and a soaking section before being taken out of the furnace. The temperature range of the preheating section is 700-800℃, which can control the construction of a micro oxide layer and inhibit deep oxidation. The temperature range of the heating section is 800-1230℃, which reconstructs the low-melting-point oxide of Mns / SiO2-MnO and significantly improves the crack resistance. The temperature range of the soaking section is 1120-1200℃, which ensures the maximum austenitization. The total time of the billet in the heating furnace is controlled within 5-7 hours.

[0028] 3) High-pressure water descaling: Before rolling, the billet after exiting the furnace is descaled twice using high-pressure water. The first descaling lasts 15-30 seconds with a descaling machine pressure of 20-30 MPa, and the second descaling lasts 5-20 seconds with a descaling machine pressure of 5-19 MPa. High-pressure water descaling can control the surface quality of the steel plate and can produce an instantaneous self-tempering effect, reducing the surface hardness of the steel plate and ensuring the uniformity of strength between the surface and the core of the steel plate.

[0029] 4) Rolling: The first-stage rolling temperature is 1000–1180℃, with a single-pass reduction rate of 10%–20% for the first three passes, and a single-pass reduction rate controlled at 10%–15% for the remaining passes; the intermediate billet thickness is determined based on the finished product thickness, generally 1.5–2 times the thickness of the finished steel plate. During the entire rolling process, do not use mill cooling water for descaling to prevent recrystallization from stopping due to excessively rapid temperature drop on the billet surface; the second-stage rolling temperature is 900–1000℃, with a single-pass reduction rate of 5%–10%; the rolled steel plate is directly subjected to laminar flow cooling, with the final cooling temperature controlled at 400–500℃ on the steel plate surface to reduce the hardness of the steel plate.

[0030] 5) Heat treatment is as follows: quenching temperature 920~1150℃, heating rate 0.5~5min / mm, holding time 1~10min / mm, ensuring maximum austenite content during quenching and avoiding austenite recrystallization and refinement; tempering temperature 600~800℃, heating rate 0.5~5min / mm, holding time 1~10min / mm.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] 1) This invention achieves the basic requirement of meeting the mechanical properties of stainless steel through special chemical composition design. Special elements such as Zr, Mg, Ce, and Ca are introduced to ensure the special performance requirements of the steel plate, laying the foundation for the production of a high-strength heat-resistant ferritic stainless steel plate for third-generation nuclear power plants from the source.

[0033] 2) Through special billet heating, the billet is processed in sequence through the preheating section, heating section and soaking section before exiting the furnace, completely removing the thermal stress of the billet assembly and maximizing the austenite content.

[0034] 3) Two-stage descaling can reduce the surface hardness of steel plates. At the same time, two short-time descalings can achieve the effect of surface descaling and ensure that the surface roughness of the rolled steel plate is ≤7.0.

[0035] 4) This invention adopts a controlled rolling combined with a special quenching and tempering heat treatment process. The combination of the two can ensure the high stability of the ferrite structure in the final steel plate.

[0036] 5) This invention ultimately produces ferritic stainless steel with a thickness of 15mm to 150mm, a width of 2000mm to 4300mm, and an unlimited length, solving the problem of producing medium and thick plates of ferritic stainless steel for nuclear power plants. The average mechanical properties of the steel plate are: room temperature tensile yield strength ≥416MPa, tensile strength ≥568MPa, elongation ≥20%; 350℃ high temperature tensile yield strength ≥332MPa, tensile strength ≥477MPa; 400℃ high temperature tensile yield strength ≥322MPa, tensile strength ≥445MPa, -23℃ impact energy ≥55J, all 180° bending tests are qualified, and Brinell hardness is 143 to 149HB. Attached Figure Description

[0037] Figure 1 It is the microstructure of steel plate after quenching and tempering heat treatment. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0039] A ferritic stainless steel plate for supporting key equipment in a third-generation nuclear power plant and its manufacturing method are disclosed. The specific implementation methods are as follows: Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting process of the steel in the examples; Table 3 shows the heating and descaling methods for the steel billets in the examples; Table 4 shows the rolling and heat treatment methods for the steel in the examples; and Table 5 shows the properties of the stainless steel in the examples.

[0040] Table 1. Chemical composition (wt%) of steel in each embodiment.

[0041]

[0042] Note: Since phosphorus (P) is a harmful element in steel, the P content should be controlled below 0.010%, unless otherwise specified.

[0043] Table 2. Smelting process of steel in the examples

[0044]

[0045] Table 3. Heating and descaling methods for steel billets in the examples.

[0046]

[0047] Table 4. Rolling and heat treatment methods for the steel in the examples.

[0048]

[0049] Table 5 Stainless Steel Properties in Examples

[0050]

[0051] As can be seen from the examples, the average mechanical properties of the produced steel plates are as follows: room temperature tensile yield strength ≥ 416 MPa, tensile strength ≥ 568 MPa, elongation ≥ 20%; 350℃ high temperature tensile yield strength ≥ 332 MPa, tensile strength ≥ 477 MPa; 400℃ high temperature tensile yield strength ≥ 322 MPa, tensile strength ≥ 445 MPa, -23℃ impact energy ≥ 55 J, and all 180° bending tests are qualified.

[0052] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A high-strength, heat-resistant ferritic stainless steel plate for third-generation nuclear power plants, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.01%–0.12%, Si: 0.8%–1.5%, Mn: 0.02%–0.70%, S: 0.020%–0.050%, Ni: 0.01%–1.2%, Cr: 12.5%–14.5%, Mo: 0.2%–0.9%, Zr: 0.001%–0.010%, Mg: 0.010%–0.020%, Ce: 5–15 ppm, Ca: 0.00032%–0.003%; the balance is Fe and unavoidable impurities. Tensile yield strength at room temperature ≥416MPa, tensile strength ≥568MPa, elongation ≥20%; tensile yield strength at 350℃ ≥332MPa, tensile strength ≥477MPa; tensile yield strength at 400℃ ≥322MPa, tensile strength ≥445MPa; impact energy at -23℃ ≥55J; all 180° bending tests are qualified; Brinell hardness 143~149HB. The microstructure of the steel plate consists of 50%–70% martensite and 30%–50% ferrite by volume. The manufacturing method of high-strength heat-resistant ferritic stainless steel plates for third-generation nuclear power plants includes smelting, billet heating, high-pressure water descaling, rolling, and heat treatment, specifically including the following steps: 1) The temperature range of the billet heating and preheating section is 700-800℃, the temperature range of the heating section is 800-1230℃, the temperature range of the soaking section is 1120-1200℃, and the total time of the billet in the heating furnace is controlled at 5-7 hours. 2) High-pressure water descaling: Before rolling, the billet after exiting the furnace is descaled twice using high-pressure water. The first descaling lasts 15-30 seconds with a descaling machine pressure of 20-30 MPa, and the second descaling lasts 5-20 seconds with a descaling machine pressure of 5-19 MPa. 3) Rolling: The first stage rolling temperature is 1000-1180℃; no mill cooling water is used for descaling during the entire rolling process; the second stage rolling temperature is 900-1000℃; the rolled steel plate is directly subjected to laminar flow cooling, and the final cooling temperature is controlled at 400-500℃ on the surface of the steel plate. 4) Heat treatment is as follows: quenching temperature 920~1150℃, heating rate 0.5~5min / mm, holding time 1~10min / mm; tempering temperature 600~800℃, heating rate 0.5~5min / mm, holding time 1~10min / mm.

2. The high-strength heat-resistant ferritic stainless steel plate for third-generation nuclear power plants according to claim 1, characterized in that, The thickness of the steel plate is 15-150mm.

3. A method for manufacturing high-strength, heat-resistant ferritic stainless steel plates for third-generation nuclear power plants as described in claim 1 or 2, comprising smelting, billet heating, high-pressure water descaling, rolling, and heat treatment, characterized in that, Specifically, the methods and steps are as follows: 1) The temperature range of the billet heating and preheating section is 700-800℃, the temperature range of the heating section is 800-1230℃, the temperature range of the soaking section is 1120-1200℃, and the total time of the billet in the heating furnace is controlled at 5-7 hours. 2) High-pressure water descaling: Before rolling, the billet after exiting the furnace is descaled twice using high-pressure water. The first descaling lasts 15-30 seconds with a descaling machine pressure of 20-30 MPa, and the second descaling lasts 5-20 seconds with a descaling machine pressure of 5-19 MPa. 3) Rolling: The first stage rolling temperature is 1000-1180℃; no mill cooling water is used for descaling during the entire rolling process; the second stage rolling temperature is 900-1000℃; the rolled steel plate is directly subjected to laminar flow cooling, and the final cooling temperature is controlled at 400-500℃ on the surface of the steel plate. 4) Heat treatment is as follows: quenching temperature 920~1150℃, heating rate 0.5~5min / mm, holding time 1~10min / mm; tempering temperature 600~800℃, heating rate 0.5~5min / mm, holding time 1~10min / mm.

4. The method for manufacturing a high-strength, heat-resistant ferritic stainless steel plate for third-generation nuclear power plants according to claim 3, characterized in that, The smelting process includes: controlling the tapping temperature at 1570–1620℃; refining the molten steel in an LF furnace to adjust its composition and performing deep desulfurization treatment; feeding silicon-calcium wire at a speed of 2.0–3.9 m / s and a wire feeding rate of 3.9–6.6 m / t; and employing VD vacuum treatment for a duration of 19–32 min.

5. The method for manufacturing a high-strength, heat-resistant ferritic stainless steel plate for third-generation nuclear power plants according to claim 3, characterized in that, In step 3), the thickness of the intermediate billet in the first-stage rolling and the second-stage rolling is 1.5 to 2 times the thickness of the finished steel plate.

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