CN117210771B - Thick high-performance nitrogen-containing austenitic stainless steel for nuclear power and manufacturing method thereof - Google Patents

Abstract

The invention provides a thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power and a manufacturing method thereof. The components of the steel plate are as follows by weight percentage: C: 0.01%-0.03%; Si: 0.50%-0.70%; Mn: 4.00%-5.00%; P≤0.030%; S≤0.015%; Ni: 9.10%-9.90%; Cr: 18.00%-20.00%; Mo: 3.50%-4.50%; Al: ≤0.010%; Nb: 0.26%-0.35%; Ti: 0.15%-0.20%, N: 0.12%-0.16%; the remaining content is Fe and inevitable impurities; the manufacturing method comprises smelting, continuous casting, rolling and solid solution treatment; after the steel grade of the invention is solid solution treated, the elongation A ₅₀ of the steel plate at room temperature and 350°C high temperature state is ≥39.5%. The section shrinkage rate is ≥59.5%, which fully meets the index requirements and has a large margin.

Description

Thick gauge high performance nitrogen-containing austenitic stainless steel for nuclear power and its manufacturing method

Technical Field

The invention belongs to the field of metal materials, and in particular relates to a thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power and a manufacturing method thereof.

Background technique

Nuclear energy has gradually become an important energy source used by countries around the world due to its many advantages such as cleanliness and economy, and is an important part of the primary energy consumption structure of developed countries in the world. Since nuclear power plants consume very little nuclear fuel during operation, they can generate a large amount of electricity, and their power generation efficiency is much higher than that of thermal power generation. Because of this, vigorously developing nuclear power has become an inevitable choice for my country to optimize the structure of its power industry, achieve diversification of energy security, and enhance its comprehensive economic strength, industrial technology level and international status.

Among the key components of nuclear power plants, it is particularly important to have good mechanical properties, corrosion resistance, organizational stability and radiation resistance under extreme service conditions such as neutron irradiation and cooling medium corrosion. Austenitic stainless steel, as a steel type that meets the above conditions, is widely used in large pressure vessels such as nuclear reactor pressure vessels, steam generators and pressurizers in nuclear power plants.

The relevant patents of nitrogen-containing austenitic stainless steel currently produced are as follows:

Zhejiang Dalong Alloy Steel Co., Ltd. applied for a patent named "Production Method of 316LN Austenitic Stainless Steel for Nuclear Power", with patent application number 201310234968.6 and publication number CN 103320718 B, C≤0.020%, Mn≤2.00%, Si≤0.75%, P≤0.025%, S≤0.005%, Cr: 16.00～18.00%, Ni: 11.00～14.00%, Mo: 2.00～3.00%, Cu≤0.10%, Co≤0.050%, N: 0.10～0.16%, and the balance is Fe. The manufacturing method of this invention adopts electric furnace + vacuum refining + forging + annealing + quenching + tempering heat treatment. The steel plate produced by this method has good room temperature mechanical properties, with yield strength Rp0.2 reaching 233Mpa and tensile strength Rm reaching 558Mpa. However, the claims of the comparative document use a forging method to produce 316LN steel, which is somewhat inferior to rolled steel plates in terms of material uniformity and manufacturing cost. In addition, its yield strength at room temperature is 233MPa and its tensile strength is only 558MPa, and its overall performance is poor.

The patent application of Shanxi Taigang Stainless Steel Co., Ltd. is entitled "A high-nitrogen, high-strength, low-magnetic austenitic stainless steel medium plate and its manufacturing method", with patent application number 202110363149.6 and publication number CN 113088822A, C≤0.03%, Si≤1.00%, Mn: 3.00-6.00%, P≤0.030%, S≤0.015%, Cr: 18.00-23.00%, Ni: 12.00-18.00%, Mo: 2.00-4.00%, N: 0.20-0.30%, Nb: 0.15-0.25%, and the balance is Fe and unavoidable impurities. The yield strength of the steel plate with a thickness of 8mm in this invention is ≥480MPa, the tensile strength is ≥790MPa, and the impact at -40℃ is ≥119J, but the chemical composition of this invention is different from that of the present invention. The high-nitrogen design of this invention will increase the difficulty and cost of smelting control. Furthermore, the invention only provides the mechanical properties of 8 mm thick steel plates, and the invention does not take high temperature mechanical properties into consideration, and thus cannot meet the demand for steel used in reactor nuclear stations.

Taiyuan University of Technology applied for a patent for "super austenitic stainless steel with low precipitation phase precipitation and its preparation and heat treatment method", application number 202111210754.6, publication number CN 113943903 A, C≤0.02%, Si≤1.00%, Mn≤1.00%, P≤0.03%, S≤0.005%, Cr: 19.0～24.0%, Ni: 17.0～25.0%, Mo: 5.5～6.5%, Cu: 0.5～1.5%, N: 0.15～0.30%, B: 0.003～0.005%, Ce: 0.003～0.007%, the balance is Fe and other unavoidable impurity elements. It can significantly inhibit the precipitation of Cr-rich and Mo-rich hard and brittle σ phases, and obtain an austenitic stainless steel with good corrosion resistance and good thermoplasticity. However, the invention has high alloy components such as Cr and Ni, and rare earth elements are added, which leads to high production costs, and no mechanical properties and corrosion resistance parameters are provided.

Summary of the invention

The object of the present invention is to overcome the above problems and deficiencies and to provide a thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power with good comprehensive mechanical properties and internal quality and a method for manufacturing the same.

The object of the invention is achieved in this way:

A high-performance nitrogen-containing austenitic stainless steel with thick specifications for nuclear power, the components of the steel plate are as follows by weight percentage: C: 0.01%-0.03%; Si: 0.50%-0.70%; Mn: 4.00%-5.00%; P≤0.030%; S≤0.015%; Ni: 9.10%-9.90%; Cr: 18.00%-20.00%; Mo: 3.50%-4.50%; Al: ≤0.010%; Nb: 0.26%-0.35%; Ti: 0.15%-0.20%, N: 0.12%-0.16%; the rest is Fe and unavoidable impurities.

Furthermore, in the stainless steel, Cr:Si=28.0~37.0.

Furthermore, in the stainless steel, Ti:N=1.10-1.50.

Furthermore, the microstructure of the steel plate is fine austenite with a grain size of 7 to 9; the steel plate also includes TiN second phase particles of 5 to 20 nm.

Furthermore, the thickness of the stainless steel plate is 20-60 mm.

Furthermore, the thickness of the stainless steel plate is 20 to 60 mm; the yield strength Rp _0.2 ≥302 MPa, the tensile strength R _m ≥576 MPa; the tensile yield strength Rp _0.2 ≥176 MPa, the tensile strength R _m ≥469 MPa at a high temperature of 350°C; and the low-temperature impact KV ₈ ≥278 J at -196°C.

The reasons for the composition design of the present invention are as follows:

(1) C: The content of carbon in stainless steel and its distribution form largely affect the performance and structure of stainless steel: carbon can stabilize austenite elements and has a large effect, about 30 times that of nickel, which can significantly improve the mechanical properties of steel. However, as the carbon content in steel increases, more chromium forms carbides with carbon, which will reduce the corrosion resistance of steel. The carbon content in steel should not be too high, so the C content in the present invention is controlled within the range of 0.01 to 0.03%.

(2) Si: Si atoms can form solid solutions in the austenite lattice in the form of substitution, produce lattice distortion, play a role in solid solution strengthening, improve the strength of steel, and Si can improve the oxidation resistance of steel. At the same time, the Si element has the function of stabilizing irradiation defects, which is beneficial to improving the service life of nuclear power equipment. However, when the Si addition exceeds 3%, the plasticity and toughness of the steel decrease significantly. Therefore, it is required that the Si content in the steel be controlled at 0.50-0.70%, and the Cr:Si alloy element content ratio be controlled at 28.0-37.0, so as to ensure that the steel plate has both good plasticity and toughness, as well as excellent corrosion resistance.

(3) Mn: In stainless steel, manganese remains in the steel as a deoxidizing element. One of the important roles of manganese is reflected in nickel-saving stainless steel and high-nitrogen stainless steel. Manganese replaces nickel to save nickel resources and increase the solubility and improvement of nitrogen. Another important role of manganese is to form MnS, inhibit the harmful effects of sulfur, and improve the high-temperature thermoplasticity of high-chromium-nickel austenitic stainless steel. However, the increase in Mn content will reduce the corrosion resistance and plastic toughness of stainless steel. It is required that the Mn content in the steel be controlled within an appropriate range, that is, controlled within 4.00-5.00%.

(4) Ni: Within the nickel content range where martensitic transformation may occur, as the nickel content increases, the strength of the steel decreases and the plasticity increases. When a stable austenite structure is present, the addition of nickel can further improve its plasticity and toughness, and austenitic stainless steel has better stainlessness and corrosion resistance; however, an increase in nickel content will increase the intergranular corrosion sensitivity of austenitic stainless steel. Therefore, the present invention requires that the Ni content in the steel be controlled to be 9.10-9.90%.

(5) Cr: Chromium is one of the most important elements in stainless steel. In austenitic stainless steel, the interaction between chromium and nickel forms a stable austenitic structure. In a single austenitic stainless steel, the chromium content will not have a significant effect on the mechanical properties. The Cr element can cause the electrode potential of the steel to change suddenly, from a negative potential to a positive electrode potential. Therefore, the corrosion resistance of the steel can be significantly improved. Therefore, the present invention requires that the Cr content in the steel be controlled to be 18.00-20.00%.

(6) Mo: Mo is an important alloying element widely used in stainless steel. In austenitic stainless steel, Mo has a significant solid solution strengthening effect. In addition, the interaction between Mo and Cr can significantly improve the corrosion resistance of stainless steel. Therefore, the present invention requires that the Mo content in the steel be controlled to be 3.50-4.50%.

(7) Al: Al can significantly improve the oxidation resistance of steel and reduce smelting costs. Studies have shown that adding an appropriate amount of Al to austenitic steel can form nitrides to improve the mechanical properties of the steel. However, an increase in Al content will reduce the plasticity and toughness of the steel. Therefore, the present invention requires that the Al content in the steel be controlled to ≤0.010%.

(8) Nb: Nb can increase the yield strength of steel, reduce the brittle transition temperature, and is beneficial to the welding performance of steel. At the same time, Nb is less sensitive to radiation. Therefore, the present invention requires that the Nb content of steel be controlled to 0.26-0.35%.

(9) Ti: Ti can form carbides and nitrides, which can reduce the precipitation of Cr ₂₃ C6 in steel. The finely dispersed distribution of TiC and TiN serves as the core for the nucleation and growth of austenite, and has the effect of refining the grains. At the same time, TiC and TiN hinder the movement of dislocations, which can significantly improve the strength of austenite. Therefore, the present invention requires that the N content in the steel be controlled at 0.12-0.16%, and the alloy element content ratio of Ti:N be controlled at 1.10-1.50. Adding Ti and N elements according to this ratio is beneficial to the formation of TiN and TiC compounds on the one hand. On the other hand, it can ensure that the porosity is at a low level, ensuring that the steel plate has good surface quality.

(10) P: P is a harmful element that increases the brittleness of steel, significantly increases the yield point and yield strength ratio, deteriorates plasticity and toughness, and has an adverse effect on welding. At the same time, irradiation tests show that P is also very sensitive to irradiation embrittlement. Therefore, the lower the P content in the steel, the better. The present invention requires it to be less than 0.030%.

(11) S: S forms FeS and MnS sulfides at the austenite grain boundaries in steel, which will reduce the impact toughness and welding performance of the steel. At the same time, S also tends to accelerate radiation embrittlement. Therefore, the S content in steel should be limited to less than 0.015%.

(12) N: N is a solid solution forming element that can stabilize austenite. Its ability is about 40 times that of Ni. However, excessive N will form defects such as pores during the smelting process, affecting the surface quality of the steel plate. Therefore, the present invention requires that the N content in the steel be controlled at 0.12-0.16%.

The second technical solution of the present invention is to provide a method for manufacturing thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power, including smelting, continuous casting, rolling and solution treatment;

(1) Smelting and continuous casting:

In order to ensure the quality of continuous casting billet and control the segregation and looseness of the billet, high-quality scrap carbon steel is used for EAF electric furnace smelting, AOD converter controls the N element content, LF furnace controls the steel tapping temperature at 1505-1515℃, the tundish superheat is controlled at 15-25℃, the continuous casting speed is controlled at 1.0-1.2m/min, and the billet section thickness is 195-210mm.

(2) Rolling:

The two-stage rolling technology is adopted. The heating temperature of the steel billet is 1180℃～1250℃, and the holding time is 3～4min/mm, so that the steel billet is evenly heated and sufficient hot working temperature is provided for the rolling process. The rolling temperature of the first stage is controlled above the recrystallization temperature to break the dendrites of the ingot, reduce segregation, and uniform the structure. At the same time, it provides the driving force for recrystallization, so that the recrystallized grains in the structure are nucleated with nano-scale second phase particles such as TiN and TiC, which promotes the growth of grains to achieve the effect of grain refinement. Therefore, the first-stage rolling temperature is controlled at 1150℃～1195℃, the final rolling temperature is 1050℃～1100℃, and the single-pass reduction rate is 15～22%. The second-stage rolling is controlled at a temperature below the recrystallization temperature. Its purpose is to produce sufficient deformation inside the steel plate. On the other hand, a large amount of deformation energy is introduced to facilitate the formation of grains in the subsequent solution treatment process. Therefore, the second-stage rolling temperature is controlled at 970°C to 1000°C, the final rolling temperature is controlled at 920°C to 960°C, and the single-pass reduction rate is controlled at 5 to 10%; the steel plate is naturally cooled after rolling.

(3) Solution treatment:

The solution temperature is 1060±10℃, the net solution holding time is 1.5～2.0min/mm, and then it is cooled rapidly at a cooling rate of 7～10℃/s; the solution treatment is adopted, and a higher temperature and a longer temperature holding are selected, the purpose of which is to make Si, Ni and other elements fully dissolved into the Fe matrix to form a supersaturated solid solution to improve the performance of the steel. On the other hand, the heat preservation at a higher temperature is to make the deformation energy introduced during the rolling process interact with each other, promote the formation and growth of grains, and obtain uniform and fine austenite grains. After the solution treatment is completed, the steel plate is rapidly cooled in water at a cooling rate of 7℃/s～10℃/s to reduce the formation of precipitation phases.

The beneficial effects of the present invention are:

(1) The products produced by the process of the present invention have excellent strength indicators through alloying treatment and solution treatment. After solution treatment, the yield strength (Rp _0.2 ) ≥ 302MPa and the tensile strength (R _m ) ≥ 576MPa; the 350℃ high temperature tensile yield strength (Rp _0.2 ) ≥ 176MPa and the tensile strength (R _m ) ≥ 469MPa. The tensile properties of steel plates of different thicknesses are much higher than the index requirements, which are greatly improved compared with other steel grades, and can meet the mechanical property requirements under different service conditions.

(2) After the solution treatment of the steel of the present invention, the impact absorption energy at -196°C is ≥278J, which is also maintained at a relatively high level, fully meeting the requirements of the index and having a large margin.

(3) After solution treatment, the steel of the present invention has a post-fracture elongation A ₅₀ of ≥39.5% at room temperature and 350°C high temperature, and a cross-sectional shrinkage of ≥59.5%, which fully meets the requirements of the index and has a large margin.

(4) The steel of the present invention can effectively control the segregation of components inside the organization through rational smelting, rolling and heat treatment process control, and at the same time form fine and dispersed second phase particles. It can obtain an internal organization with fine austenite grain size and small inclusion size, and has excellent comprehensive mechanical properties.

Detailed ways

The present invention will be further described below by way of examples.

The embodiment of the present invention performs smelting, continuous casting, rolling and solution treatment according to the component ratio of the technical solution.

(1) Rolling:

The heating temperature of the steel billet is 1180℃～1250℃, and the holding time is 3～4min/mm; the first-stage rolling temperature is 1150℃～1195℃, the final rolling temperature is 1050℃～1100℃, and the single-pass reduction rate is 15%～22%; the second-stage rolling temperature is 970℃～1000℃, the final rolling temperature is 920℃～960℃, and the single-pass reduction rate is 5～10%; the steel plate is naturally cooled after rolling;

(2) Solution treatment:

The solution temperature is 1060±10℃, the net solution holding time is 1.5～2.0min/mm, followed by rapid cooling with a cooling rate of 7～10℃/s.

Furthermore, during the smelting process, the LF furnace controls the tapping temperature at 1505-1515°C, and the tundish superheat is controlled at 15-25°C;

During the continuous casting process, the continuous casting speed is controlled at 1.0-1.2 m/min, and the cross-sectional thickness of the ingot is 195-210 mm.

The composition of the steel of the present invention is shown in Table 1. The main process parameters of smelting and continuous casting of the steel of the present invention are shown in Table 2. The main process parameters of rolling of the steel of the present invention are shown in Table 3. The room temperature and low temperature mechanical properties of the steel of the present invention are shown in Table 4. The high temperature mechanical properties of the steel of the present invention at 350°C are shown in Table 5.

Table 1 Composition of steel according to the present invention (wt%)

Table 2 Main process parameters of steel smelting and continuous casting in the embodiment of the present invention

Example Steel plate thickness mm LF furnace tapping temperature ℃ Continuous casting speed m/min Continuous casting slab section thickness mm 1 20 1235 1165 196 2 30 1230 1160 199 3 40 1200 1150 202 4 50 1220 1150 205 5 60 1250 1155 209 6 20 1198 1152 195 7 30 1215 1170 200 8 40 1240 1190 203 9 50 1245 1195 207 10 60 1250 1190 210

Table 3 Main process parameters of steel rolling in the embodiment of the present invention

Table 4 Properties of steel in the examples of the present invention

Table 5 High temperature properties of steel according to the present invention at 350°C

Example R _p0.2 ,MPa Rm,MPa A,% Z,% 1 201 497 42.0 65.0 2 194 492 44.0 63.0 3 186 479 43.0 64.0 4 183 483 50.0 70.5 5 176 469 42.0 62.0 6 195 492 37.0 59.5 7 192 487 36.0 58.0 8 190 481 36.0 57.5 9 180 475 35.0 56.5 10 178 470 35.0 55.5

The performance indicators of the steel plate produced by the present invention fully meet the requirements, and the steel plate has good high-temperature performance and good toughness and strength matching. The room temperature yield strength Rp _0.2 of the stainless steel plate is ≥302MPa, and the tensile strength R _m is ≥576MPa; the tensile yield strength Rp _0.2 of the steel plate at a high temperature of 350°C is ≥176MPa, and the tensile strength R _m is ≥469MPa; the low-temperature impact absorption energy KV ₈ at -196°C is ≥278J, which fully meets the performance requirements of steel for reactor nuclear power.

In order to describe the present invention, the present invention is appropriately and fully illustrated by the embodiments above. The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims (7)

1. A high-performance nitrogen-containing austenitic stainless steel with thick specifications for nuclear power, characterized in that the components of the stainless steel are as follows by weight percentage: C: 0.01% to 0.03%; Si: 0.50% to 0.70%; Mn: 4.00% to 5.00%; P≤0.030%; S≤0.015%; Ni: 9.10% to 9.90%; Cr: 18.00% to 20.00%; Mo: 3.50% to 4.50%; Al: ≤0.010%; Nb: 0.26% to 0.35%; Ti: 0.15% to 0.20%, N: 0.12% to 0.16%; the remaining content is Fe and unavoidable impurities; the thickness of the stainless steel plate is 20 to 60 mm; the yield strength Rp _0.2 ≥302 MPa, the tensile strength R _m ≥576 MPa; the tensile yield strength Rp of the steel plate at a high temperature of 350°C _0.2 ≥176MPa, tensile strength R _m ≥469MPa; -196℃ low temperature impact KV ₈ ≥278J. 2. A thick-gauge, high-performance nitrogen-containing austenitic stainless steel for nuclear power according to claim 1, characterized in that Cr:Si in the stainless steel is 28.0 to 37.0. 3. A thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power according to claim 1, characterized in that Ti:N in the stainless steel is 1.10-1.50. 4. The thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power according to claim 1, characterized in that the stainless steel microstructure is fine austenite with a grain size of 7 to 9. 5. The thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power use according to claim 4, characterized in that the stainless steel further comprises TiN second phase particles of 5 to 20 nm. 6. A method for manufacturing a thick-gauge, high-performance nitrogen-containing austenitic stainless steel for nuclear power use as claimed in any one of claims 1 to 5, comprising smelting, continuous casting, rolling and solution treatment; characterized in that: (1) Rolling: The heating temperature of the steel billet is 1180℃～1250℃, and the holding time is 3～4min/mm; two-stage rolling is adopted, the first-stage starting rolling temperature is 1150℃～1195℃, the final rolling temperature is 1050℃～1100℃, and the single-pass reduction rate is 15%～22%; the second-stage starting rolling temperature is 970℃～1000℃, the final rolling temperature is 920℃～960℃, and the single-pass reduction rate is 5～10%; the steel plate is naturally cooled after rolling; (2) Solution treatment: The solution temperature is 1060±10℃, the net solution holding time is 1.5～2.0min/mm, followed by rapid cooling with a cooling rate of 7～10℃/s. 7. The method for manufacturing thick-gauge high-performance nitrogen-containing austenitic stainless steel for nuclear power according to claim 6, characterized in that: During the smelting process, the LF furnace controls the tapping temperature at 1505-1515°C, and the tundish superheat is controlled at 15-25°C; During the continuous casting process, the continuous casting speed is controlled at 1.0-1.2 m/min, and the cross-sectional thickness of the ingot is 195-210 mm.