High-strength heat-resistant ferritic stainless steel plate for third-generation nuclear power station and manufacturing method thereof

Abstract

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

Description

Technical Field

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

[0002] As one of the most core main equipment in a nuclear power unit, the steam generator is responsible for converting the heat energy generated by the reactor pressure vessel from the primary circuit to the secondary circuit. It is a key equipment connecting the primary and secondary circuits of the nuclear island. The ferritic stainless steel support plate is the main component of the steam generator and plays an important role as a barrier between the primary and secondary circuits. Its safety level is Nuclear Safety Level 1 and its quality level is QA1.

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

[0004] The present invention uses the special thickness of the ferritic stainless steel for steam generator supports to place higher requirements 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] The patent document with application number 201910821783.2 discloses a method for eliminating surface cracks in ultra-wide ferritic stainless steel medium and thick plates: the proportion of equiaxed crystals in the continuous casting billet is not less than 75%; the billet homogenization temperature is 750-850℃, the furnace time is 1-2h, and it is water-cooled to room temperature; the surface roughness Ra of the billet after grinding is ≤70μm, and high-temperature anti-oxidation coating is sprayed; the billet is sequentially treated in the preheating section, heating section and soaking section before being taken out of 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 4-5.5h; the rough rolling start temperature is ≥1150℃; the finishing rolling start temperature is ≥1150℃, the final rolling temperature is ≥950℃, the rolling single pass reduction rate is ≤20%, the annealing temperature is 750-880℃, the furnace time is 2-5min / mm, and air cooling is performed. The product width is 2500-4000mm and the thickness is 8-30mm. 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 of ≥230MPa, a tensile strength of ≥450MPa, and an elongation of ≥28%. However, the mechanical properties of this invention are relatively low, and the invention does not consider the high-temperature mechanical properties of the steel plate, which cannot meet the demand for steel used in nuclear power plants.

[0006] The patent document with application number 201910821783.2 discloses a method for eliminating surface cracks in ultra-wide ferritic stainless steel medium and thick plates: the proportion of equiaxed crystals in the continuous casting billet is not less than 75%; the billet homogenization temperature is 750-850℃, the furnace time is 1-2h, and it is water-cooled to room temperature; the surface roughness Ra of the billet after grinding is ≤70μm, and high-temperature anti-oxidation coating is sprayed; the billet is sequentially treated in the preheating section, heating section and soaking section before being taken out of 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 4-5.5h; the rough rolling start temperature is ≥1150℃; the finishing rolling start temperature is ≥1150℃, the final rolling temperature is ≥950℃, the rolling single pass reduction rate is ≤20%, the annealing temperature is 750-880℃, the furnace time is 2-5min / mm, and air cooling is performed. The product width is 2500-4000mm and the thickness is 8-30mm. 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 of ≥230MPa, a tensile strength of ≥450MPa, and an elongation of ≥28%. However, the mechanical properties of this invention are relatively low, and the invention does not consider the high-temperature mechanical properties of the steel plate, which cannot meet the demand for steel used in nuclear power plants.

[0007] The invention patent document "Grain size control method for medium and thick plates of high-carbon austenitic stainless steel" (acceptance number CN111549276A) also discloses a three-stage heat treatment method, with the temperatures of each stage being 600°C, 1010°C and 1050°C respectively. The above heat treatment method not only ensures that the steel plate has uniform grains 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 present invention aims to overcome the shortcomings of the prior art and provides a high-strength, heat-resistant ferritic stainless steel plate for a third-generation nuclear power plant and a method for manufacturing the same. The product produced according to the chemical composition and production process requirements of the steel of the present invention has high strength and toughness, low hardness, and high-temperature resistance.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A high-strength, heat-resistant ferritic stainless steel plate for a third-generation nuclear power plant. The chemical composition of the steel is as follows by weight: 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.

[0011] The reasons for adopting the above components are as follows:

[0012] 1) Carbon: Carbon often forms carbides with other alloying elements in steel, contributing to its strengthening properties. Carbon easily combines with alloying elements in steel to form carbides, causing segregation within the steel and significantly reducing the toughness and ductility of the steel plate. To address this issue, the present invention adopts a low-carbon design. Therefore, the steel composition of the present invention requires a carbon content of 0.01% to 0.12%.

[0013] 2) Silicon: Serving as a reducing agent and deoxidizer during the steelmaking process, silicon is an inexpensive alloying element. Adding an appropriate amount of silicon to steel, dissolving in austenite, can increase the steel's hardness and strength. Excessive silicon content, due to its stronger oxygen-binding ability than iron, can easily form low-melting-point silicates during welding. This increases the fluidity of the slag and molten metal, causing spattering and affecting weld quality. Therefore, the silicon content should be within a range of 0.8% to 1.5%.

[0014] 3) Manganese: Mn can form an infinite solid solution with Fe and is a strong austenite stabilizing element. It stabilizes austenite in steel and can partially replace nickel. While increasing the strength of steel, Mn has a relatively small effect on plasticity. It can also lower the lower critical point of steel and increase the degree of undercooling during austenite cooling, thereby refining the microstructure and improving the mechanical properties of the steel plate. Furthermore, it is relatively inexpensive. However, excessive Mn content increases the tendency of grain coarsening in steel. Therefore, Mn should be within a range of 0.02% to 0.70%.

[0015] 4) Sulfur: Sulfur (S) easily forms sulfide inclusions in steel, which reduces the impact toughness of the steel and impairs welding performance. It also aggravates defects such as central segregation and porosity and increases radiation embrittlement. However, an appropriate amount of sulfur is beneficial to the cutting effect of the steel plate. Therefore, the present invention requires S: 0.020% to 0.050%.

[0016] 5) Nickel: Nickel is a key element in austenitic stainless steel. It expands the austenite region while inhibiting the formation of ferrite. Combined with Cr, it ensures excellent resistance to oxidation and corrosion. Due to its high price, the nickel content should be kept to 0.01% to 1.2%.

[0017] 6) Chromium: Cr forms various carbides with C in steel and dissolves in austenite, playing a role in strengthening through dispersion, grain refinement, and solid solution, and has a positive impact on the strength and toughness of the steel plate. However, excessive addition of Cr will reduce the toughness of the steel plate and increase its brittleness. In the present 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 and high-temperature resistance. However, increasing the molybdenum content can strengthen grains and prevent grain boundary depletion corrosion through solid solution strengthening and combined strengthening with other alloys. Therefore, the present invention requires Mo content of 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 deoxidize 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 complex inclusions to aggregate and grow. Therefore, the Mg content in the present invention is controlled to be 0.010% to 0.020%.

[0021] 10) Cerium: This element modifies inclusions during the smelting process, transforming them into soft, fine inclusions. These fine rare earth inclusions serve as nucleation sites for high-temperature ferrite, refining the dendritic structure, thereby alleviating microsegregation, reducing the size of primary carbides, and making their distribution more uniform. Therefore, the amount of Ce added to steel is 5ppm to 15ppm.

[0022] 11) Calcium: Ca controls the morphology of sulfides in steel, suppressing the formation of MnS by forming CaS. To achieve this effect, the Ca content must be at least 0.0003%. However, if the Ca content exceeds 0.006%, the resulting CaS becomes too large, increasing its brittleness and becoming a potential initiation point for fracture cracks. Therefore, the recommended Ca content in steel is 0.00032% to 0.003%.

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

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

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

[0026] 1) Smelting includes: controlling the tapping temperature at 1570-1620°C, refining the molten steel in an LF furnace to adjust its composition and undergo deep desulfurization. The silicon-calcium feed rate is 2.0-3.9 m / s, and the feed rate is 3.9-6.6 m / t. VD vacuum treatment is employed and further optimized, with a treatment time of 19-32 minutes. Composition control is ensured through refining outside the furnace, and vacuum treatment ensures gas content control and the formation of zirconia, ensuring that the molten steel composition and temperature meet control targets. Furthermore, after refining, a long period of argon blowing, stirring, and sedation effectively ensures that calcium and cerium in the steel modify inclusions, ultimately ensuring the purity of the steel. This also effectively implements microalloying.

[0027] 2) The slab is heated in a heating furnace, passing through preheating, heating, and soaking stages before exiting the furnace. The preheating stage, which operates at a temperature range of 700-800°C, allows for the controlled formation of a micro-oxidation layer to inhibit deep oxidation. The heating stage, which operates at a temperature range of 800-1230°C, reconstructs the Mns / SiO2-MnO low-melting-point oxide, significantly improving crack resistance. The soaking stage, which operates at a temperature range of 1120-1200°C, ensures maximum austenitization. The total time the slab remains in the heating furnace is limited to 5-7 hours.

[0028] 3) High-pressure water descaling: Before rolling, high-pressure water is used to descale the ingot twice after it leaves the furnace. The first descaling takes 15 to 30 seconds, and the descaling machine pressure is 20 to 30 MPa. The second descaling takes 5 to 20 seconds, and the descaling machine pressure is 5 to 19 MPa. High-pressure water descaling can control the surface quality of the steel plate, and can produce an instantaneous self-tempering effect, reduce the surface hardness of the steel plate, and ensure the uniformity of the strength of the surface and core of the steel plate.

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

[0030] 5) Heat treatment: quenching temperature 920 ~ 1150 ℃, heating rate 0.5 ~ 5 min / mm, holding time 1 ~ 10 min / mm, quenching stage to ensure maximum austenite content and avoid austenite recrystallization refinement; tempering temperature 600 ~ 800 ℃, heating rate 0.5 ~ 5 min / mm, holding time 1 ~ 10 min / mm.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1) The present invention achieves the basis for meeting the mechanical properties of stainless steel through the design of a special chemical composition, wherein 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.

[0033] 2) Through special billet heating, the billet is sequentially processed through the preheating section, heating section and soaking section before being taken out of the furnace, completely removing the thermal stress of the billet and maximizing the austenite content.

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

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

[0036] 5) The present invention ultimately produces ferritic stainless steel with a thickness of 15 mm to 150 mm, a width of 2000 mm to 4300 mm, and no length limit, solving the problem of producing medium and thick ferritic stainless steel plates for nuclear power plants. The average mechanical properties of the steel plates are as follows: room temperature tensile yield strength ≥416 MPa, tensile strength ≥568 MPa, and elongation ≥20%; high-temperature tensile yield strength ≥332 MPa and tensile strength ≥477 MPa at 350°C; high-temperature tensile yield strength ≥322 MPa and tensile strength ≥445 MPa at 400°C; impact energy ≥55 J at -23°C; all qualified in 180° bending; and Brinell hardness 143 to 149 HB. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is the structure of steel plate after quenching and tempering heat treatment. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the specific implementation methods of the present invention are further described below in conjunction with examples. The following examples are used to specifically illustrate the contents of the present invention. These examples are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0039] A ferritic stainless steel plate for supporting key equipment in a third-generation nuclear power plant and a manufacturing method thereof are described. Specific embodiments are as follows: Table 1 shows the chemical composition of the example steel; Table 2 shows the smelting process system of the example steel; Table 3 shows the heating and descaling methods of the example steel ingot; Table 4 shows the rolling and heat treatment methods of the example steel; and Table 5 shows the properties of the example stainless steel.

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

[0041]

[0042] Note: Since P is a harmful element in steel, the P content can be controlled below 0.010% without special instructions.

[0043] Table 2 Smelting process system of example steel

[0044]

[0045] Table 3 Heating and descaling method of the steel casting according to the embodiment

[0046]

[0047] Table 4 Rolling and heat treatment methods of example steel

[0048]

[0049] Table 5 Stainless steel properties of the examples

[0050]

[0051] It can be seen from the examples that the average mechanical properties of the steel plates 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, 180° bending all qualified.

[0052] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A high-strength, heat-resistant ferritic stainless steel plate for a third-generation nuclear power plant, characterized in that: 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: 5~15ppm, Ca: 0.00032%~0.003%; the balance is Fe and unavoidable impurities.

2. The high-strength heat-resistant ferritic stainless steel plate for a third-generation nuclear power plant according to claim 1, characterized in that: 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, 180° bending all qualified, Brinell hardness 143~149HB.

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

4. The high-strength heat-resistant ferritic stainless steel plate for a third-generation nuclear power plant according to claim 1, characterized in that: The volume ratio of the steel plate is 50% to 70% martensite and 30% to 50% ferrite.

5. A method for manufacturing high-strength, heat-resistant ferritic stainless steel plates for third-generation nuclear power plants according to any one of claims 1 to 4, comprising smelting, billet heating, high-pressure water dephosphorization, rolling, and heat treatment, characterized in that: The specific method steps include: 1) The temperature range of the preheating section of the billet is 700-800°C, the temperature range of the heating section is 800-1230°C; the temperature range of the soaking section is 1120-1200°C, and the total time the slab stays in the heating furnace is controlled at 5-7 hours; 2) High-pressure water dephosphorization: Before rolling, high-pressure water is used to descale the billet after it leaves the furnace twice. The first dephosphorization takes 15 to 30 seconds, and the descaling machine pressure is 20 to 30 MPa. The second dephosphorization takes 5 to 20 seconds, and the descaling machine pressure is 5 to 19 MPa. 3) Rolling: The first stage rolling temperature is 1000-1180℃; no mill cooling water is used for dephosphorization during the entire rolling process; the second stage rolling temperature is 900-1000℃; the steel plate is directly laminar cooled after rolling, and the final cooling temperature is controlled at 400-500℃, the surface temperature of the steel plate; 4) Heat treatment: 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.

6. The method for manufacturing a high-strength, heat-resistant ferritic stainless steel plate for a third-generation nuclear power plant according to claim 5, characterized in that: The smelting process includes: controlling the tapping temperature at 1570-1620° C., refining the molten steel in an LF furnace to adjust the composition and perform deep desulfurization treatment, feeding calcium silicon at a linear speed of 2.0-3.9 m / s and a feed rate of 3.9-6.6 m / t; and adopting VD vacuum treatment for a vacuum treatment time of 19-32 minutes.

7. The method for manufacturing a high-strength, heat-resistant ferritic stainless steel plate for a third-generation nuclear power plant according to claim 5, characterized in that: In the step 3), the thickness of the intermediate billet during 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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