Special-corrosion-resistant martensitic heat-resistant steel and preparation method thereof

Abstract

The invention relates to special-corrosion-resistant martensitic heat-resistant steel and a preparation method thereof, belongs to the technical field of nuclear heat-resistant steel, and solves the problems of poor oxidation corrosion resistance, low strength and poor impact toughness and plasticity of ferrite / martensitic steel in high-temperature liquid lead bismuth in the prior art. The heat-resistant steel comprises the following chemical components in percentage by mass: 0.10-0.13% of C; 0.9 to 1.4 percent of Si; mn: 0.5 to 0.7%; 8.5 to 9.0 percent of Cr; 0.4 to 0.6 percent of W; 0.7 to 0.8 percent of Mo; 0.25 to 0.40 percent of Ni; 0.15% to 0.20% of V; 0.05 to 0.07 percent of Nb; the content of N is 0.045 to 0.065 percent; compared with the prior art, the heat-resistant steel disclosed by the invention has the advantages that a compact Si-rich oxide layer is ensured to exist in the heat-resistant steel by increasing the content of Si, so that the high-temperature liquid lead / lead bismuth corrosion resistance of the material is improved; by adjusting the content of Mn, Cr, Ni, W, Mo and other elements, it is ensured that the material has a single martensite structure, the heat-resistant steel has good room-temperature and high-temperature mechanical properties at the same time, and the heat-resistant steel is suitable for lead-cooled fast reactor structural materials.

Description

Technical Field

[0001] This invention relates to the field of nuclear heat-resistant steel technology, and in particular to a special corrosion-resistant martensitic heat-resistant steel and its preparation method. Background Technology

[0002] As one of the fourth-generation reactors, the lead-cooled fast reactor (LFR) benefits from the low melting point, high boiling point, and good neutron performance of lead / lead-bismuth alloys, giving it unique advantages in inherent safety, nuclear waste management, and miniaturization design, and is considered one of the most promising reactor types. However, lead / lead-bismuth alloys, when used as a coolant, can cause severe corrosion damage to the reactor structural materials in contact with them. Therefore, the structural materials of the lead-cooled fast reactor must possess excellent resistance to corrosion from liquid lead / lead-bismuth.

[0003] 9-12% Cr ferritic / martensitic steel (F / M steel) and austenitic steel are important candidate structural materials for lead-cooled fast reactors. Due to the very high solubility of Ni in liquid lead-bismuth, the application of 316 series austenitic steels with high Ni content at temperatures of 500°C and above is severely limited. In contrast, F / M steel exhibits excellent radiation resistance and a low susceptibility to corrosion from liquid lead-bismuth, making it potentially suitable for long-term service in liquid lead-bismuth environments up to 550°C.

[0004] However, existing F / M steels, such as T91 steel, suffer severe oxidation corrosion in high-temperature liquid lead-bismuth environments. Although existing EP823 exhibits good corrosion resistance in high-temperature liquid lead-bismuth environments, the addition of high Cr and high Si leads to the formation of a dual-phase structure of ferrite and martensite, with the ferrite content reaching 10%. This severely deteriorates properties such as impact toughness, creep, and fatigue, failing to meet the mechanical property requirements of lead-bismuth structural materials.

[0005] Therefore, there is an urgent need to develop candidate structural materials for lead-bismuth stacks that combine resistance to high-temperature liquid lead-bismuth corrosion with comprehensive mechanical properties. Based on the shortcomings of existing candidate structural materials, this invention obtains an F / M steel with good resistance to lead-bismuth corrosion and comprehensive mechanical properties through composition optimization and process adjustment. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a special corrosion-resistant martensitic heat-resistant steel and its preparation method, in order to solve at least one of the problems of poor oxidation corrosion resistance, low strength, poor impact toughness, and poor plasticity of ferritic / martensitic steel in high-temperature liquid lead-bismuth in the prior art.

[0007] On one hand, embodiments of the present invention provide a special corrosion-resistant martensitic heat-resistant steel. The chemical composition of the heat-resistant steel, by mass percentage, includes: C: 0.10–0.13%; Si: 0.9–1.4%; Mn: 0.5–0.7%; Cr: 8.5–9.0%; W: 0.4–0.6%; Mo: 0.7–0.8%; Ni: 0.25–0.40%; V: 0.15–0.20%; Nb: 0.05–0.07%; N: 0.045–0.065%; B: 0.001–0.003%, with the balance being Fe and unavoidable impurity elements.

[0008] Furthermore, by mass percentage, the impurity elements include: S ≤ 0.003%; P ≤ 0.005%; Cu ≤ 0.01%; Al ≤ 0.03%; O ≤ 0.003%; H ≤ 0.0002%.

[0009] Furthermore, the composition of the heat-resistant steel is as follows: Cr eq =Cr + 2Si + 5V + 1.75Nb + 1.5Mo + 0.75W and Ni eq The chromium equivalent Cr calculated for Ni + 30C + 25N + 0.5Mn respectively eq and Ni equivalent Ni eq Among them, 12.5 < Cr eq <14.6, 4.6<Ni eq <6.3.

[0010] Furthermore, the W and Mo content in the heat-resistant steel satisfies 0.9≤1 / 2W+Mo≤1.1, and 1.1≤Mo / W≤2.

[0011] Furthermore, the microstructure of the heat-resistant steel comprises tempered martensite plus second-phase particles, wherein the second-phase particles comprise M23C6 type carbides and MX dispersion strengthening phases.

[0012] On the other hand, embodiments of the present invention also provide a method for preparing the heat-resistant steel, the method comprising:

[0013] (1) Determine the raw material ratio according to the composition of the heat-resistant steel and smelt it to obtain an ingot;

[0014] (2) The ingot is subjected to homogenization, forging, normalizing and tempering heat treatment in sequence to obtain martensitic heat-resistant steel.

[0015] Furthermore, in step (2), the homogenization heat treatment is carried out at a temperature of 1150-1220°C for 20-100 hours.

[0016] Furthermore, in step (2), the forging specifically involves forging the ingot after homogenization heat treatment, wherein the initial forging temperature is 1140-1180℃ and the final forging temperature is ≥900℃.

[0017] Furthermore, in step (2), the normalizing temperature is 1020-1080℃ and the time is 0.5-3h.

[0018] Furthermore, in step (2), the tempering temperature is 730-790°C and the time is 1-3 hours.

[0019] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0020] 1. Compared with existing ferritic / martensitic steel (F / M steel), this invention increases the Si content to ensure that the heat-resistant steel contains a dense Si-rich oxide layer, which can effectively prevent the diffusion of elements inside and outside the oxide layer when it is used in a high-temperature liquid lead-bismuth environment, thereby significantly improving the material's oxidation and corrosion resistance. By adjusting elements such as C, N, Ni, Mn, and Cr, the high-temperature ferrite introduced into the martensitic structure due to the increased Si content is balanced, thereby ensuring that the heat-resistant steel has a single and uniform martensitic structure.

[0021] 2. This invention promotes solid solution strengthening and precipitation strengthening of heat-resistant steel by adding W and Mo, and enhances the high-temperature strength and microstructure stability of the material through W and Mo composite strengthening.

[0022] 3. This invention strengthens grain boundaries by adding boron, suppresses the coarsening of M23C6 type carbides, and enhances the stability of martensitic lath microstructure during long-term service in high-temperature lead / lead-bismuth environment.

[0023] 4. The special corrosion-resistant ferritic / martensitic heat-resistant steel provided by this invention, through adjustments to the normalizing and tempering processes, can achieve the following properties without introducing high-temperature ferrite: room temperature tensile strength ≥730MPa, yield strength ≥540MPa, elongation after fracture ≥20%, reduction of area ≥65%; 550℃ tensile strength ≥415MPa, yield strength ≥350MPa, elongation after fracture ≥30%, reduction of area ≥85%; room temperature impact absorption energy ≥140J; and corrosion layer thickness ≤10μm after immersion in liquid lead-bismuth at 550℃ for 500h.

[0024] 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 may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0025] 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.

[0026] Figure 1 Here is a scanning electron microscope image of the martensitic heat-resistant steel in Example 1;

[0027] Figure 2 This is a photograph of the corrosion cross-section morphology of the martensitic heat-resistant steel in Example 1 after corrosion in liquid lead-bismuth at 550°C for 500 hours.

[0028] Figure 3 The image shows the corrosion cross-sectional morphology of T91 steel after corrosion in liquid lead-bismuth at 550℃ for 500 hours as a comparative example. Detailed Implementation

[0029] 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.

[0030] As an important candidate structural material for lead-cooled fast reactors, although 9-12% Cr ferritic / martensitic steel (F / M steel) has excellent radiation resistance and low susceptibility to liquid lead-bismuth dissolution corrosion, and has the potential to serve for a long time in a liquid lead-bismuth environment with a high temperature of 550℃, it cannot meet the requirements of lead-cooled fast reactor structural materials for good resistance to high-temperature liquid lead-bismuth corrosion and comprehensive mechanical properties in practical applications. For example, the existing T91 steel will undergo severe oxidation corrosion in high-temperature liquid lead-bismuth. Although the existing EP823 has good corrosion resistance in high-temperature liquid lead-bismuth environment, the addition of high Cr and high Si will form a dual-phase structure of ferrite and martensite, with the ferrite content reaching 10%, which seriously deteriorates the impact toughness, creep and fatigue properties.

[0031] Therefore, embodiments of the present invention provide a special corrosion-resistant martensitic heat-resistant steel, the chemical composition of which, by mass percentage, includes: C: 0.10-0.13%; Si: 0.9-1.4%; Mn: 0.5-0.7%; Cr: 8.5-9.0%; W: 0.4-0.6%; Mo: 0.7-0.8%; Ni: 0.25-0.40%; V: 0.15-0.20%; Nb: 0.05-0.07%; N: 0.045-0.065%; B: 0.001-0.003%, with the balance being Fe and unavoidable impurity elements.

[0032] The roles and content ranges of alloying elements in this invention are explained below:

[0033] Carbon: C is an austenite-forming element that can expand the austenite phase region and inhibit the formation of high-temperature ferrite. It also participates in the tempering precipitation of the strengthening phases M23C6 and MX phases. Excessive carbon content can lead to coarsening of the M23C6 phase and deterioration of weldability. Therefore, the carbon content in this invention is controlled at 0.10–0.13%.

[0034] Chromium (Cr): Cr is a major corrosion-resistant element in heat-resistant steel. In a high-temperature liquid lead-bismuth environment, it participates in the formation of an Fe-Cr spinel oxide layer on the steel surface. As its content increases, the Fe-Cr spinel oxide layer becomes thinner and denser, significantly improving oxidation resistance. However, Cr is also a ferrite-forming element; excessive content can promote the formation of high-temperature ferrite. Therefore, the Cr content in this invention is controlled at 8.5%–9.0%.

[0035] Silicon (Si): Si is a corrosion-resistant element. In a high-temperature liquid lead-bismuth environment, it can form SiO2 in the inner oxide layer and inner oxide region, effectively hindering the diffusion of elements inside and outside the oxide layer, thereby significantly improving the oxidation resistance of the material. In addition, Si can also improve the strength of heat-resistant steel to some extent. However, Si is a ferrite-forming element; excessive addition will significantly promote the formation of high-temperature ferrite, deteriorating the impact toughness and creep resistance of the material. Therefore, the Si content in this invention is controlled at 0.9–1.4%.

[0036] Tungsten and Mo: Both W and Mo are ferrite-forming elements, dissolving in the matrix to form substitutional solid solutions, enhancing solid solution strengthening. They also participate in the formation of the Laves phase during high-temperature service, promoting precipitation strengthening. However, excessively high W and Mo contents can promote the precipitation of δ-ferrite, worsening impact toughness and other properties. W has a larger atomic radius than Mo, therefore, W has a higher solid solution strengthening effect. W can also inhibit the coarsening of M23C6 and martensitic laths, improving the stability of the microstructure. However, at the same Mo equivalent, W has a stronger promoting effect on the precipitation of the Laves phase, which tends to coarsen during service, negatively impacting the fracture toughness and creep rupture properties of the steel. Therefore, in this invention, the W content is controlled at 0.4–0.6%, and the Mo content is controlled at 0.7–0.8%.

[0037] Manganese: Mn is an austenite forming element that can increase the stability of austenite and inhibit the precipitation of δ-ferrite. Therefore, the content of manganese in this invention is controlled at 0.5-0.7%.

[0038] Vanadium and niobium (V and Nb) in steel primarily combine with C and N to form MX-sized nanoscale dispersed strengthening phases, pinning dislocations and promoting precipitation strengthening. Excessive Nb will form large-sized NbC, causing a decrease in toughness. Conversely, excessively high V content will also form coarse V(C,N), reducing creep strength. Therefore, in this invention, the mass percentage of Nb ranges from 0.05% to 0.07%, and the mass percentage of V ranges from 0.15% to 0.20%.

[0039] Boron (B) is an interstitial solid solution element. During normalizing, it tends to segregate at grain boundaries, combining with grain boundary vacancies to strengthen the grain boundaries. During tempering, B atoms enter the grain boundaries of heat-resistant steel and the M23C6 carbides precipitated near the grain boundaries, inhibiting the coarsening rate of M23C6 during long-term service. However, excessively high B content will severely deteriorate the hot working and welding properties of the material. Therefore, the mass percentage of B in this invention ranges from 0.001% to 0.003%.

[0040] Nitrogen (N): N is an interstitial solid solution element that promotes solid solution strengthening. It also readily combines with V and Nb to form fine, dispersed MX phases, effectively hindering dislocation movement. However, the N content should not be too high; otherwise, it easily combines with B to form coarse BN inclusions, which not only consumes the beneficial B content but also deteriorates creep strength. To prevent BN formation, the N content must be controlled. Therefore, in the embodiments of this invention, the mass percentage of N is controlled within the range of 0.045% to 0.065%.

[0041] Specifically, by mass percentage, the impurity elements include: S≤0.003%; P≤0.005%; Cu≤0.01%; Al≤0.03%; O≤0.003%; H≤0.0002%. Among them, Al is a ferrite-forming element, and the addition of even a trace amount will lead to the formation of ferrite, which will reduce the various properties of the material, especially its resistance to lead-bismuth corrosion and creep resistance. Therefore, Al needs to be strictly limited.

[0042] It should be noted that, in order to ensure the comprehensive improvement of the structural stability, mechanical properties and corrosion resistance of heat-resistant steel, this invention needs to limit the magnitude of the chromium equivalent and nickel equivalent in the composition of heat-resistant steel.

[0043] Specifically, the composition of the heat-resistant steel is as follows: Cr eq =Cr + 2Si + 5V + 1.75Nb + 1.5Mo + 0.75W and Ni eq Calculate the chromium equivalent Cr of Ni + 30C + 25N + 0.5Mn respectively. eq and Ni equivalent Ni eq Among them, 12.5 < Cr eq <14.6, 4.6<Ni eq <6.3. If the chromium equivalent Cr eq Too much Ni will lead to the precipitation of ferrite at high temperatures, worsening the impact toughness of heat-resistant steel; too little Ni will result in insufficient high-temperature structural stability and corrosion resistance of the heat-resistant steel; if Ni... eq If the carbonitrides are too large, firstly, it will result in coarse carbonitrides in the heat-resistant steel, reducing the stability of the high-temperature structure; secondly, it will increase the risk of dissolution and corrosion of the heat-resistant steel in the liquid lead-bismuth environment. If the carbonitrides are too small, it will weaken the ability to balance the high-temperature ferrite, which is not conducive to the overall improvement of mechanical and corrosion properties.

[0044] It should be noted that, in order to ensure the high-temperature strength and microstructure stability of heat-resistant steel during service while guaranteeing solid solution strengthening and precipitation strengthening, this invention not only needs to limit the W and Mo content in the heat-resistant steel, but also needs to consider the quantitative relationship between the two.

[0045] Specifically, the W and Mo content in the heat-resistant steel satisfies 0.9≤1 / 2W+Mo≤1.1, and 1.1≤Mo / W≤2.

[0046] It should be noted that, in order to improve the room temperature and high temperature strength and high temperature microstructure stability of heat-resistant steel, this invention needs to define the microstructure of the heat-resistant steel.

[0047] Specifically, the microstructure of the heat-resistant steel comprises tempered martensite plus second-phase particles. The second-phase particles include M23C6 type carbides and MX dispersed strengthening phases. The M23C6 type carbides have a content of 1.0–2.5 wt.% and a size ≤500 nm; the MX dispersed strengthening phases have a content of 0.08–0.3 wt.% and a size ≤50 nm. Excessively large precipitate sizes weaken the pinning ability of dislocations, leading to decreased microstructure stability. The M23C6 type carbides include (Cr,Fe)23C6 type carbides, (Fe,Cr,W,Mo)23C6, etc.; the MX dispersed strengthening phases are NbC and V(C,N) nanoscale dispersed strengthening phases.

[0048] The special corrosion-resistant martensitic heat-resistant steel provided by this invention has a single, uniform martensitic structure. At room temperature, its tensile strength is ≥730 MPa, yield strength is ≥540 MPa, elongation after fracture is ≥20%, and reduction of area is ≥65%. At 550℃, its tensile strength is ≥415 MPa, yield strength is ≥350 MPa, elongation after fracture is ≥30%, and reduction of area is ≥85%. Its room temperature impact absorption energy is ≥140 J, and the corrosion layer thickness after immersion in liquid lead-bismuth at 550℃ for 500 hours is ≤10 μm. Compared with existing F / M steels, it has higher resistance to high-temperature liquid lead-bismuth corrosion, such as… Figure 2 , 3 As shown.

[0049] On the other hand, embodiments of the present invention also provide a method for preparing the heat-resistant steel, the method comprising:

[0050] (1) Determine the raw material ratio according to the composition of the heat-resistant steel and smelt it to obtain an ingot;

[0051] (2) The ingot is subjected to homogenization, forging, normalizing and tempering heat treatment in sequence to obtain martensitic heat-resistant steel.

[0052] Specifically, in step (1), the smelting process employs vacuum induction or vacuum induction + electroslag remelting or electric furnace + ladle refining + electroslag remelting. The electroslag remelting process can significantly improve the purity of heat-resistant steel, improve the uniformity of composition and structure, and further enhance the strength, toughness, and high-temperature performance of heat-resistant steel.

[0053] It should be noted that, in order to eliminate dendritic segregation inside the smelted ingot, promote uniform diffusion of elements, dissolve non-equilibrium phases, refine the microstructure, improve thermoplasticity, and reduce the tendency for hot cracking, the present invention requires homogenization heat treatment of the smelted ingot.

[0054] Specifically, in step (2), the homogenization heat treatment temperature is 1150-1220℃, which can be 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃, 1210℃, or 1220℃. If the homogenization temperature is too high, it will lead to grain boundary oxidation and overheating, abnormal grain growth, and promote the formation of δ-ferrite. If the temperature is too low, it will lead to dendritic segregation residue, uneven structure after subsequent performance heat treatment, and even precipitation of harmful phases.

[0055] Specifically, in step (2), the homogenization heat treatment time is 20-100 hours, which can be 20 hours, 25 hours, 30 hours, 35 hours, 40 hours, 45 hours, 50 hours, 55 hours, 60 hours, 65 hours, 70 hours, 75 hours, 80 hours, 85 hours, 90 hours, 95 hours, or 100 hours. If the homogenization time is too long, it will lead to excessively coarse grains; if the time is too short, it will lead to reduced uniformity of the microstructure, precipitation of harmful phases, and poor uniformity of mechanical properties.

[0056] Specifically, in step (2), the forging is to forge the ingot after homogenization heat treatment, which helps to control the dimensional accuracy of the ingot, prevent grain growth, and improve production efficiency.

[0057] More specifically, the forging process is as follows: forging deformation temperature 900-1180℃, initial forging temperature range 1140-1180℃, final forging temperature ≥900℃, total forging ratio greater than 3, and single-fire deformation 40-70%.

[0058] It should be noted that, in order to ensure the grain refinement and dissolution of coarse precipitates during non-uniform deformation in the forging process, the present invention requires normalizing treatment of the forging.

[0059] Specifically, in step (2), the normalizing temperature is 1020 to 1080°C, which can be 1020°C, 1030°C, 1040°C, 1050°C, 1060°C, 1070°C, or 1080°C. If the normalizing temperature is too high, it will lead to the precipitation of δ-ferrite and grain coarsening. If the temperature is too low, it will lead to insufficient re-dissolution of carbides and residual mixed crystal structure.

[0060] Specifically, in step (2), the normalizing time is 0.5-3h, 0.5h, 1h, 1.5h, 2h, 2.5h, or 3h. If the normalizing time is too long, it will lead to grain coarsening; if the time is too short, it will lead to insufficient austenitization and insufficient re-dissolution of carbides.

[0061] It should be noted that, in order to adjust the strength and toughness matching of the heat-resistant steel and obtain a tempered martensite structure with dispersed carbide precipitation, the present invention requires tempering treatment of the forging.

[0062] Specifically, in step (2), the tempering temperature is 730 to 790°C, which can be 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, or 790°C. If the tempering temperature is too high, it will cause coarsening of M23C6 carbides and excessive recovery of martensite laths. If the temperature is too low, it will cause insufficient precipitation of MX phase and low plasticity and toughness.

[0063] Specifically, in step (2), the tempering time is 1-3 hours, specifically 1 hour, 1.5 hours, 2 hours, 2.5 hours, and 3 hours. If the tempering time is too long, the strength will be significantly reduced; if the time is too short, the carbides will not be fully released and the impact toughness of the heat-resistant steel will be insufficient.

[0064] The special corrosion-resistant martensitic heat-resistant steel prepared by the method of this invention has a single, uniform martensitic structure. At room temperature, its tensile strength is ≥730 MPa, yield strength is ≥540 MPa, elongation after fracture is ≥20%, and reduction of area is ≥65%. At 550℃, its tensile strength is ≥415 MPa, yield strength is ≥350 MPa, elongation after fracture is ≥30%, and reduction of area is ≥85%. Its room temperature impact absorption energy is ≥140 J, and the corrosion layer thickness after immersion in liquid lead-bismuth at 550℃ for 500 h is ≤10 μm. Compared with existing F / M steels, it has higher resistance to high-temperature liquid lead-bismuth corrosion, such as… Figure 2 , 3 As shown.

[0065] The technical solution of the present invention will be further explained and illustrated below through specific embodiments and comparative examples.

[0066] Example 1

[0067] A method for preparing a special corrosion-resistant martensitic heat-resistant steel, wherein the chemical composition of the heat-resistant steel, by weight percentage, is C: 0.11%; Si: 0.92%; Mn: 0.63%; Cr: 8.61%; W: 0.57%; Mo: 0.77%; Ni: 0.38%; V: 0.19%; Nb: 0.057%; B: 0.0024%; N: 0.049%, with the remainder being Fe and impurities, wherein the impurities are: S: 0.0028%; P: 0.0034%; O: 0.0024%; H: 0.00008%; Cu: 0.008%; Al: 0.025%, specifically including the following steps:

[0068] (1) Smelting: The raw material ratio is determined according to the composition of heat-resistant steel, and heat-resistant steel ingots are obtained by vacuum induction + electroslag remelting. The smelting temperature is 1530-1600℃ and the time is 70-105min.

[0069] (2) The ingot is subjected to homogenization, forging, normalizing and tempering heat treatment in sequence. The homogenization treatment specifically involves heating the smelted ingot to 1200°C and holding it at that temperature for 24 hours, then cooling it to room temperature in the furnace.

[0070] The forging process specifically involves forging the homogenized ingot at an initial forging temperature of 1150℃ and a final forging temperature of 980℃. The deformation per forging pass is 60%, and the total forging ratio is 6, resulting in an ingot with a diameter of [missing information]. steel bars;

[0071] The normalizing heat treatment is as follows: hold at 1050℃ for 1 hour and then air cool to room temperature.

[0072] The tempering heat treatment is specifically as follows: hold at 750℃ for 2 hours and then air cool to room temperature.

[0073] Example 2

[0074] The same preparation method as in Example 1 was used, except that the chemical composition of the heat-resistant steel in Example 2 was: C: 0.11%; Si: 1.0%; Mn: 0.64%; Cr: 8.85%; W: 0.55%; Mo: 0.79%; Ni: 0.31%; V: 0.18%; Nb: 0.06%; B: 0.0029%; N: 0.049%, with the remainder being Fe and impurities, namely: S: 0.0021%; P: 0.0032%; O: 0.0022%; H: 0.00011%; Cu: 0.005%; Al: 0.025%.

[0075] Example 3

[0076] The same preparation method as in Example 1 was used, except that the chemical composition of the heat-resistant steel in Example 3 was: C: 0.11%; Si: 1.19%; Mn: 0.66%; Cr: 8.52%; W: 0.55%; Mo: 0.79%; Ni: 0.31%; V: 0.19%; Nb: 0.055%; B: 0.0025%; N: 0.050%, with the remainder being Fe and impurities, namely: S: 0.0023%; P: 0.0033%; O: 0.0024%; H: 0.00015%; Cu: 0.007%; Al: 0.022%.

[0077] Example 4

[0078] The same preparation method as in Example 1 was used, except that the chemical composition of the heat-resistant steel in Example 4 was: C: 0.12%; Si: 1.32%; Mn: 0.70%; Cr: 8.56%; W: 0.57%; Mo: 0.75%; Ni: 0.31%; V: 0.18%; Nb: 0.06%; B: 0.0024%; N: 0.048%, with the remainder being Fe and impurities, namely: S: 0.0023%; P: 0.0035%; O: 0.0021%; H: 0.00008%; Cu: 0.006%; Al: 0.027%.

[0079] Example 5

[0080] The same preparation method as in Example 1 was used, except that the normalizing temperature in Example 5 was 1030°C, and the temperature was maintained for 1 hour before being air-cooled to room temperature; the tempering temperature was 730°C, and the temperature was maintained for 2 hours before being air-cooled to room temperature.

[0081] Example 6

[0082] The same preparation method as in Example 1 was used, except that the normalizing temperature in Example 6 was 1060°C, and the temperature was maintained for 1 hour before being air-cooled to room temperature; the tempering temperature was 760°C, and the temperature was maintained for 2 hours before being air-cooled to room temperature.

[0083] Example 7

[0084] The same preparation method as in Example 1 was used, except that the normalizing temperature in Example 7 was 1070°C, and the temperature was maintained for 1 hour before being air-cooled to room temperature; the tempering temperature was 770°C, and the temperature was maintained for 2 hours before being air-cooled to room temperature.

[0085] Comparative Example 1

[0086] T91 steel was prepared using the same preparation method as in Example 1, except that the chemical composition of Comparative Example 1 was: C: 0.10%; Si: 0.38%; Mn: 0.51%; Cr: 9.48%; Mo: 1.05%; Ni: 0.31%; V: 0.22%; Nb: 0.066%; N: 0.049%, with the remainder being Fe and impurities, namely: S: 0.0031%; P: 0.0041%; O: 0.0022%; H: 0.00017%; Cu: 0.008%; Al: 0.025%.

[0087] Comparative Example 2

[0088] The same preparation method as in Example 1 was used, except that the chemical composition of the heat-resistant steel in Comparative Example 2 was: C: 0.12%; Si: 1.32%; Mn: 0.70%; Cr: 8.56%; W: 0.48%; Mo: 0.42%; Ni: 0.31%; V: 0.18%; Nb: 0.06%; B: 0.0024%; N: 0.048%, with the remainder being Fe and impurities, namely: S: 0.0023%; P: 0.0035%; O: 0.0021%; H: 0.00008%; Cu: 0.006%; Al: 0.027%.

[0089] Comparative Example 3

[0090] The same preparation method as in Example 1 was used, except that the chemical composition of the heat-resistant steel in Comparative Example 3 was: C: 0.10%; Si: 1.4%; Mn: 0.5%; Cr: 10%; W: 0.6%; Mo: 0.77%; Ni: 0.38%; V: 0.19%; Nb: 0.057%; B: 0.0024%; N: 0.049%, with the remainder being Fe and impurities, namely: S: 0.0028%; P: 0.0034%; O: 0.0024%; H: 0.00008%; Cu: 0.008%; Al: 0.025%.

[0091] Comparative Example 4

[0092] The same preparation method as in Example 1 was used, except that the chemical composition of the heat-resistant steel in Comparative Example 4 was: C: 0.08%; Si: 0.10%; Mn: 0.35%; Cr: 8.85%; W: 0.45%; Mo: 0.77%; Ni: 0.21%; V: 0.19%; Nb: 0.057%; B: 0.0024%; N: 0.049%, with the remainder being Fe and impurities, namely: S: 0.0028%; P: 0.0034%; O: 0.0024%; H: 0.00008%; Cu: 0.008%; Al: 0.025%.

[0093] Comparative Example 5

[0094] The same preparation method as in Example 1 was used, except that the normalizing temperature in Comparative Example 5 was 1100°C, and the temperature was maintained for 1 hour before being air-cooled to room temperature; the tempering temperature was 750°C, and the temperature was maintained for 2 hours before being air-cooled to room temperature.

[0095] Comparative Example 6

[0096] The same preparation method as in Example 1 was used, except that the normalizing temperature in Comparative Example 6 was 1050°C, and the temperature was maintained for 1 hour before being air-cooled to room temperature; the tempering temperature was 800°C, and the temperature was maintained for 2 hours before being air-cooled to room temperature.

[0097] The properties of the heat-resistant steels prepared in Examples 1-7 and Comparative Examples 1-6 at room temperature and 500°C are shown in Tables 1, 2, and 3.

[0098] Table 1. Properties of heat-resistant steels prepared in Examples 1-7 and Comparative Examples 1-3 at room temperature.

[0099]

[0100] Table 2 shows the properties of the heat-resistant steels prepared in Examples 1-7 and Comparative Examples 1-6 at 550°C.

[0101]

[0102] Table 3 shows the corrosion layer thickness after immersion in liquid lead-bismuth at 550°C for 500 hours in Examples 1-4 and Comparative Example 1.

[0103] category Corrosion layer thickness / μm Example 1 10 Example 2 9.3 Example 3 8.7 Example 4 6.6 Comparative Example 1 20

[0104] As can be seen from Tables 1, 2 and 3, the heat-resistant steels prepared in Examples 1 to 7 have significantly better tensile strength and yield strength at room temperature and 550°C than those in Comparative Examples 1 to 6, and also possess excellent plasticity and impact toughness.

[0105] As can be seen from Examples 1 to 4, gradually increasing the Si content within the range specified in this invention not only significantly improves the corrosion resistance of liquid lead-bismuth, but also has a beneficial effect on improving the tensile strength and yield strength of the material at room temperature and 550°C.

[0106] As can be seen from Examples 5 to 7, within the range of heat treatment processes specified in this invention, the material exhibits excellent room temperature and 550°C tensile strength, yield strength, and room temperature ductility and toughness.

[0107] Comparative Examples 1-4 show that materials containing elements such as Si, Cr, W, and Mo that are not within the scope specified in this invention exhibit significantly lower room temperature and high temperature tensile strengths than those in Examples 1-7. Comparative Examples 5 and 6 show that materials obtained when the heat treatment process exceeds the scope specified in this invention exhibit significantly lower room temperature and high temperature strengths than those in Examples 1-7.

[0108] The heat-resistant steels prepared in Example 1 and Comparative Example 1 were placed in liquid lead-bismuth at 550°C and kept there for 500 hours. The corrosion was then observed. The cross-sectional morphology of the corroded heat-resistant steel is shown in the following photograph. Figure 2 , 3 As shown, it can be seen that a 10μm thick, protective double-layer oxide film was formed on the steel surface of Example 1. The outer layer is an Fe-rich Fe3O4 magnetite layer, and the inner layer is a Cr-rich FeCr2O4 spinel layer. Due to the high Si content, the thickness of the oxide film formed is much smaller than the 20μm thick oxide film of the steel in Comparative Example 1. The increased Si content in the special corrosion-resistant ferritic / martensitic heat-resistant steel of the present invention can effectively hinder the inward diffusion of O and the outward diffusion of Fe by the Si oxide formed in the spinel layer, thereby effectively reducing the oxidation rate of the heat-resistant steel and significantly improving the lead-bismuth corrosion resistance.

[0109] 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 martensitic heat-resistant steel resistant to special corrosion, characterized in that, The chemical composition of the heat-resistant steel, by mass percentage, includes: C: 0.10–0.13%; Si: 0.9–1.4%; Mn: 0.5–0.7%; Cr: 8.5–9.0%; W: 0.4–0.6%; Mo: 0.7–0.8%; Ni: 0.25–0.40%; V: 0.15–0.20%; Nb: 0.05–0.07%; N: 0.045–0.065%; B: 0.001–0.003%, with the balance being Fe and unavoidable impurity elements.

2. The heat-resistant steel according to claim 1, characterized in that, The impurity elements, by mass percentage, include: S ≤ 0.003%; P ≤ 0.005%; Cu ≤ 0.01%; Al ≤ 0.03%; O ≤ 0.003%; H ≤ 0.0002%.

3. The heat-resistant steel according to claim 1, characterized in that, The composition of the heat-resistant steel is as follows: Cr eq =Cr + 2Si + 5V + 1.75Nb + 1.5Mo + 0.75W and Ni eq The chromium equivalent Cr calculated for Ni + 30C + 25N + 0.5Mn respectively eq and Ni equivalent Ni eq Among them, 12.5 < Cr eq <14.6, 4.6<Ni eq <6.

3.

4. The heat-resistant steel according to claim 1, characterized in that, The W and Mo content in the heat-resistant steel satisfies 0.9≤1 / 2W+Mo≤1.1, and 1.1≤Mo / W≤2.

5. The heat-resistant steel according to claim 1, characterized in that, The microstructure of the heat-resistant steel comprises tempered martensite plus second-phase particles, wherein the second-phase particles comprise M23C6 type carbides and MX dispersion strengthening phases.

6. A method for preparing heat-resistant steel according to any one of claims 1-5, characterized in that, The method includes: (1) The raw material ratio is determined according to the composition of the heat-resistant steel according to any one of claims 1-5 and then smelted to obtain an ingot; (2) The ingot is subjected to homogenization, forging, normalizing and tempering heat treatment in sequence to obtain martensitic heat-resistant steel.

7. The preparation method according to claim 6, characterized in that, In step (2), the homogenization heat treatment is performed at a temperature of 1150-1220℃ for 20-100 hours.

8. The preparation method according to claim 6, characterized in that, In step (2), the forging specifically involves forging the ingot after homogenization heat treatment, wherein the initial forging temperature is 1140-1180℃ and the final forging temperature is ≥900℃.

9. The preparation method according to claim 6, characterized in that, In step (2), the normalizing temperature is 1020-1080℃ and the time is 0.5-3h.

10. The preparation method according to claim 6, characterized in that, In step (2), the tempering temperature is 730-790℃ and the time is 1-3h.

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