Economical high-plasticity duplex stainless steel with excellent stress corrosion resistance and its manufacturing method

Abstract

An economical high-plasticity duplex stainless steel with excellent stress corrosion resistance and its manufacturing method. The mass percentages of its components are: C 0.03 - 0.09%, Si 0.3 - 1.0%, Mn 2.5 - 4.5%, Ni 0.5 - 1.5%, Cr 19.6 - 21.5%, Mo ≤ 0.3%, Cu ≤ 0.3%, N 0.06 - 0.13%, V ≤ 0.08%, P ≤ 0.04%, S ≤ 0.0015%, Al 0.01 - 0.06%, and the balance contains Fe and other inevitable impurities. The stainless steel of the present invention has low cost, high strength, and better stress corrosion resistance than 304 stainless steel. The room temperature yield strength Rp 0.2 ≥ 450 MPa, the tensile strength R m ≥ 730 MPa, the elongation A 50 ≥ 45%. The hot-rolled coil has no edge crack and good surface quality, and can replace 304 stainless steel for automotive high-pressure fuel tanks, greatly improving the stress corrosion resistance of welded joints.

Description

Technical Field

[0001] The present invention relates to duplex stainless steel and a manufacturing method thereof, and particularly to an economical high-plasticity duplex stainless steel with excellent stress corrosion resistance and a manufacturing method thereof. Background Art

[0002] 304 austenitic stainless steel is widely used in the field of stamping products due to its excellent corrosion resistance and cold forming performance. Currently, 304 stainless steel is used for stamping the upper and lower shells of automotive high-pressure fuel tanks in the market. After the upper and lower shells of the fuel tank are respectively stamped, they must be joined by roller resistance welding. Roller resistance welding is a method of welding by applying pressure through a roller electrode and using the resistance heat generated by the current passing through the contact surface and the adjacent area. This welding method is a gasless welding method. Therefore, the heat-affected area on the surface of the weld is easily oxidized, greatly reducing the corrosion resistance of the weld. Moreover, due to the existence of welding stress, stress corrosion cracking is extremely likely to occur in the weld area of 304 stainless steel fuel tanks, resulting in fuel leakage from the fuel tank.

[0003] Compared with ferritic stainless steel and austenite + ferritic duplex stainless steel, the stress corrosion sensitivity of austenitic stainless steel is a natural shortcoming. Although ferritic stainless steel has excellent stress corrosion resistance, its strength and elongation are insufficient, making it difficult to meet the performance requirements of high-pressure fuel tanks.

[0004] Chinese Patent CN102634740A developed an economical duplex stainless steel with high plasticity and a manufacturing method thereof. Its chemical composition by weight percentage is as follows: C: 0.01 - 0.06%, Si: 0.1 - 1.0%, Mn: 0.5 - 4.0%, Cr: 19.5 - 22.0%, Ni: 1.8 - 3.5%, N: 0.1 - 0.18%, Mo: 0.5 - 1.3%, Cu: 0.1 - 1.0%, and the rest is Fe and inevitable impurities. This patent uses Mn and N to replace Ni to reduce costs, and at the same time adjusts the contents of Cr and Mo to control Md 30 / 50 At 50 - 100 °C, the elongation reaches about 40%, and it can be used in environments with high requirements for corrosion resistance and plasticity, such as heat exchangers and water tanks. However, the contents of precious metals Ni and Mo in this patent are still relatively high. Compared with 304 stainless steel, the cost reduction is only about 20%. Moreover, although the elongation has been improved to some extent, it still cannot meet the stamping requirements of high-pressure fuel tanks (elongation ≥ 45%).

[0005] Chinese Patent CN115466902B developed a niobium-containing economical high-plasticity duplex stainless steel with excellent intergranular corrosion resistance and its manufacturing method. The mass percentages of its components are as follows: C: 0.01 - 0.06%, Si: 0.2 - 0.9%, Mn: 3.0 - 5.0%, Ni: 0.8 - 2.0%, Cr: 20.6 - 22.5%, Mo ≤ 1.0%, Cu ≤ 1.0%, N: 0.08 - 0.15%, V < 0.07%, P < 0.04%, S < 0.0015%, Nb: 0.02 - 0.30%, B: 0.001 - 0.003%, RE 0.01 - 0.03%, and the balance contains Fe and other inevitable impurities. This patent also uses Mn and N to replace Ni to reduce costs. By controlling the martensite content M(ε40) generated by a 40% tensile strain to be between 10% and 30%, the elongation reaches about 40%. Compared with 304 stainless steel, the cost reduction of this patent reaches more than 40%, but the elongation still cannot meet the stamping requirements of high-pressure fuel tanks.

[0006] The strength and stress corrosion resistance of traditional duplex stainless steels such as 2205 and 2507, as well as economical duplex stainless steels such as 2101 and 2304, can meet the requirements, but there are the following problems: 1. The elongation is insufficient, about 30%, making it difficult to meet the stamping performance requirements of fuel tanks (elongation ≥ 45%); 2. The alloy cost is too high, and there is no cost advantage in replacing 304 stainless steel. Therefore, there is an urgent need to develop a high-plasticity economical duplex stainless steel to replace 304 stainless steel for high-pressure fuel tanks. Summary of the Invention

[0007] The purpose of the present invention is to provide an economical high-plasticity duplex stainless steel with excellent stress corrosion resistance and its manufacturing method. The room-temperature yield strength Rp of the duplex stainless steel 0.2 ≥ 450 MPa (about 1.7 times that of 304 stainless steel), the tensile strength R m ≥ 730 MPa, the elongation A 50 ≥ 45%, the crack initiation time in 42% boiling magnesium chloride solution ≥ 24 h, the stress corrosion resistance is far better than that of 304 austenitic stainless steel, the hot-rolled coil has no edge cracks and good surface quality, is suitable for stamping into high-pressure fuel tanks, and solves the problem of stress corrosion cracking of 304 stainless steel high-pressure fuel tanks; and the cost of stainless steel is low, suitable for large-scale production.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] An economical high-plasticity duplex stainless steel with excellent stress corrosion resistance, the mass percentages of its components are as follows:

[0010] C: 0.03 - 0.09%,

[0011] Si: 0.3~1.0%,

[0012] Mn: 2.5~4.5%,

[0013] Ni: 0.5~1.5%,

[0014] Cr: 19.6~21.5%,

[0015] Mo ≤ 0.3%,

[0016] Cu ≤ 0.3%,

[0017] N: 0.06~0.13%,

[0018] V ≤ 0.08%,

[0019] P ≤ 0.04%,

[0020] S ≤ 0.0015%,

[0021] Al: 0.01~0.06%,

[0022] The balance contains Fe and other inevitable impurities, and the following relational expressions need to be satisfied simultaneously:

[0023] Chromium equivalent Cr eq : 21.4~22.6%, Cr eq = Cr + Mo + 1.5Si + 5V + 3Al;

[0024] Nickel equivalent Ni eq : 8.0~9.2%, Ni eq = Ni + 0.87Mn + 0.33Cu + 30(C + N) - 1.35;

[0025] The ratio of final rolling ferrite F ≥ 50%,

[0026] F = -286.25*C + 6.58*Si - 4*Mn + 10.87*Cr - 14.68*Ni + 8.75*Mo - 10.3*Cu - 164*N + 67.75*V + 69.5*Al - 110.35;

[0027] The ratio of annealing austenite A ≥ 55%,

[0028] A = 240.25*C - 15.8*Si + 2.63*Mn - 8.73*Cr + 6.8*Ni - 4.5*Mo + 11.1*Cu + 222.25*N - 17.25*V - 24*Al + 188.24.

[0029] Furthermore, the balance is Fe and other inevitable impurities.

[0030] The microstructure of the duplex stainless steel of the present invention is austenite+ferrite, and the volume proportion of austenite is ≥55%.

[0031] The crack initiation time of the duplex stainless steel in 42% boiling magnesium chloride solution is ≥24h; the room temperature yield strength Rp 0.2 ≥450MPa, tensile strength R m ≥730MPa, elongation A 50 ≥45%.

[0032] The design principles of the chemical elements of the economical high-plasticity duplex stainless steel with excellent stress corrosion resistance described in the present invention are as follows:

[0033] C is an austenite-forming element. It can partially replace Ni in duplex stainless steel to reduce alloy cost and balance the ratio of austenite and ferrite in stainless steel. At the same time, C, as an interstitial atom, can play a role in solid solution strengthening. However, if the C content is too high, it is easy to form M with chromium. 23 C 6 Reduce the corrosion resistance of the heat affected zone of the oil tank welding and increase the sensitivity of intergranular corrosion. Therefore, from the perspective of organizational control, strength improvement and corrosion resistance, the role of carbon in stainless steel is contradictory. In summary, the present invention controls the C content to 0.03-0.09%.

[0034] Si is an element required for deoxidation and ferrite phase formation, but the content should not be too high. The Si element will significantly reduce the melting point of the oxide scale and promote the oxide scale to pin the substrate and be difficult to remove by pickling. Therefore, the present invention controls the Si content to 0.3-1.0%.

[0035] Mn is an austenite forming and stabilizing element, and is also an element required for ensuring strength. Mn can also increase the solubility of N in molten steel, so that more N is dissolved in the molten steel. Mn-N is often used to replace the precious metal Ni, improve the stability of austenite phase, regulate the tendency of deformation martensite formation, and balance the two-phase structure in duplex stainless steel. However, too high Mn is not good for corrosion resistance, so the present invention controls the Mn content to 2.5~4.5%.

[0036] Cr is the most important additive element for ensuring high corrosion resistance of the duplex stainless steel of the present invention. Chromium is a ferrite-forming element. When the chromium content is low, the corrosion resistance will decrease significantly. Therefore, the minimum chromium content of the present invention is controlled to be 19.6%. However, when the chromium content is too high, in order to obtain a dual-phase equilibrium structure, the content of austenite-forming elements needs to be increased, thereby increasing the cost. Therefore, the present invention controls the Cr content to 19.6~21.5%.

[0037] Ni, an austenite-forming element, expands the austenite phase region and simultaneously improves the stability of austenite. In combination with the ferrite-forming element Cr, it ensures that the two-phase ratio at room temperature in the present invention is within a reasonable range. The influence of Ni on the stress corrosion resistance of stainless steel shows a "U" - shaped trend. Among them, the stress corrosion resistance is the worst in the range of 3 - 8%, at the bottom of the "U" - shape. In addition, if the Ni content is too high, the two-phase ratio is unbalanced and the alloy cost increases significantly. Considering both cost and performance (especially stress corrosion resistance), therefore, the Ni content in the present invention is controlled within 0.5 - 1.5%.

[0038] N can improve the strength of steel and can also significantly improve the resistance to pitting corrosion. Its role in enhancing the pitting corrosion resistance is 30 times that of Cr. Nitrogen is also a very strong austenite stabilizing element. If the nitrogen content is too high, it will increase the risk of nitrogen pores and significantly increase the 2 precipitation temperature of Cr 2 and significantly promote the formation of Cr

[0039] N brittle phases, thus affecting the hot working performance and being prone to edge cracks and surface defects during forging and rolling. Therefore, the N content in the present invention is controlled within 0.06 - 0.13%.

[0040] P is generally a harmful element in steel, increasing the cold brittleness of steel and making the welding and cold bending properties worse. The P content in the present invention is controlled to be ≤0.040%.

[0040] S is usually a harmful element, making the steel have hot brittleness, reducing the ductility and toughness of the steel, and forming cracks during rolling. Especially, the hot working performance of duplex stainless steel is extremely sensitive to the S element. To reduce the adverse effect of S on the hot rolling performance, the S content in the present invention is controlled to be ≤0.0015%.

[0041] V is a ferrite-forming element. More importantly, V is an element that strongly promotes the precipitation of Cr 2 N (a brittle phase with a close-packed hexagonal structure). Among the influences of various alloying elements on the precipitation temperature of Cr 2 N, it is the most significant, seriously damaging the hot working performance of duplex stainless steel. And Cr 2 N in the room temperature structure will significantly reduce the ductility and plasticity of the material. In the present invention, V is a harmful element and is controlled at a low level as much as possible. The V content in the present invention is controlled to be ≤0.08%.

[0042] Mo is an element that can significantly improve the corrosion resistance, especially can improve the pitting corrosion resistance in a chloride ion environment. Its corrosion resistance effect is about 3.3 times that of Cr. However, its price is very expensive, and too much Mo is prone to form brittle phases, greatly damaging the hot working performance. Therefore, the Mo content in the present invention is controlled to be ≤0.3%.

[0043] Cu. A small amount of Cu in stainless steel can improve the ductility and cold working performance of the material, but excessive Cu will seriously deteriorate the hot working performance of duplex stainless steel. Therefore, in the present invention, the Cu content is controlled to be ≤0.3%.

[0044] Al. A ferrite-forming element and a commonly used deoxidizer in steel. In the present invention, it mainly serves the purpose of refining the grain size and improving the hot working performance. However, excessive Al will promote the agglomeration and growth of inclusions, reducing the metallurgical quality and castability of the material. Therefore, in the present invention, the Al content is controlled to be 0.01 - 0.06%.

[0045] In addition, the composition design of the duplex stainless steel described in the present invention also needs to satisfy:

[0046] Chromium equivalent Cr eq : 21.4 - 22.6%, Cr eq = Cr + Mo + 1.5Si + 5V + 3Al;

[0047] Nickel equivalent Ni eq : 8.0 - 9.2%, Ni eq = Ni + 0.87Mn + 0.33Cu + 30(C + N) - 1.35;

[0048] To obtain a high-plasticity duplex stainless steel, it is necessary to maximize the TRIP (Transformation Induced Plasticity) effect of the transformation from austenite to martensite during the deformation process of the material, which requires minimizing the alloy element content of the material. However, it should not be too low, otherwise, during the solidification of the billet, the austenite transforms into martensite in a non-steady state, which is not only unfavorable for the plasticity of the material but even deteriorates the workability of the billet. Therefore, to obtain a high-plasticity duplex stainless steel, it is necessary to control the austenite structure in the critical region of the austenite → martensite structure transformation. According to the Delong diagram (see Figure 1 )), in the present invention, the chromium equivalent Cr eq is controlled to be 21.4 - 22.6%, and the nickel equivalent Ni eq is controlled to be 8 - 9.2%, and the austenite structure is controlled within the critical region range of the austenite → martensite structure transformation.

[0049] To further improve the plasticity of the material, it is necessary to increase the proportion of austenite structure in the material after annealing. In this way, more austenite structures participate in the deformation during the deformation process, enhancing the TRIP (Transformation Induced Plasticity) effect and improving the plasticity. Through research, it is found that when the proportion of austenite in the material after annealing is ≥55%, during the plastic deformation process, the retained austenite in the material is more likely to transform into martensite, improving the plasticity and strength of the material. Therefore, it is necessary to control the proportion of austenite A after annealing to be ≥55%.

[0050] A=228.25*C-15.8*Si+2.63*Mn-8.73*Cr+6.8*Ni-4.5*Mo+11.1*Cu+222.25*N-17.25*V-24*Al+162.24.

[0051] In the above formula, C, Mn, Ni, Cu, and N are austenite forming elements, and Si, Cr, Mo, V, and Al are ferrite forming elements.

[0052] As a strong austenite-forming element, C has a stabilizing effect on austenite that is about 30 times greater than Ni when used as an interstitial solid solution element. C can promote the formation of metastable austenite and improve its stability. This metastable austenite can be transformed into martensite through the TRIP effect (transformation induced plasticity) during the subsequent deformation process, thereby improving the plasticity of the material.

[0053] N is also a strong austenite-forming element, which can expand the austenite phase region, increase austenite stability, and help maintain the two-phase (austenite and ferrite) structure of duplex steel during annealing, thereby improving its mechanical properties.

[0054] As an austenite stabilizing element, Ni can reduce the temperature of transformation from austenite to ferrite and prolong the phase transformation time, thus affecting the final structure and properties of duplex steel.

[0055] Mn is a relatively weak austenite stabilizing element, and its effect is about half that of Ni. The role of Mn is not to form an austenite phase, but to reduce the critical quenching rate of steel, thereby inhibiting the decomposition of the austenite phase and improving stability during annealing cooling.

[0056] Cu is a weak austenite-forming element that can reduce the cold work hardening rate of austenite, thereby improving the plasticity of the material.

[0057] Cr and Mo, as ferrite-forming elements, can close the austenite phase region, which means that during the annealing process, it helps to form more ferrite, thus affecting the microstructure and properties of duplex steel.

[0058] Si, V, and Al are strong ferrite stabilizing elements. Their addition will cause a large amount of ferrite structure to appear in the steel, thereby reducing the proportion of the austenite phase.

[0059] In addition, duplex stainless steel is prone to cracking due to the incoordination of the two-phase deformation during hot rolling. In order to improve the hot working performance of the material, the high-temperature ferrite content can be increased as a soft phase to bear more deformation without causing cracking due to the incoordination of the two phases. Through research, it was found that when the final rolling ferrite ratio is less than 50%, the frequency of severe cracking at the edge of the steel coil during rolling is higher than that when the final rolling ferrite ratio is ≥50%. Therefore, the final rolling ferrite ratio F must be controlled to be ≥50%.

[0060] F = -286.25 * C + 6.58 * Si - 4 * Mn + 10.87 * Cr - 14.68 * Ni + 8.75 * Mo - 10.3 * Cu - 164 * N + 67.75 * V + 69.5 * Al - 110.35。

[0061] In the above formula, Si, Cr, Mo, V, and Al are ferrite - forming elements, and C, Mn, Ni, Cu, and N are austenite - forming elements. Among them,

[0062] As a ferrite - forming element, Cr can not only increase the content of the ferrite phase but also affect the phase transformation process from austenite to ferrite, enabling the dual - phase steel to obtain an ideal phase composition and distribution during hot rolling.

[0063] As an element that expands the ferrite phase region, Mo can strongly hinder the precipitation of pro - eutectoid ferrite, which helps to control the phase transformation process of dual - phase steel and makes the ratio of ferrite and austenite more balanced.

[0064] Si is a ferrite - forming element. During hot rolling, Si can inhibit grain growth, thereby obtaining a fine and uniform grain structure, which is very beneficial to improving the comprehensive mechanical properties of the steel.

[0065] V is a relatively strong ferrite - forming element. An excessive V content will promote the precipitation of Cr 2 an element that promotes the precipitation of N (a brittle phase with a close - packed hexagonal structure), seriously damaging the hot - working performance of duplex stainless steel.

[0066] Al is a strong ferrite - forming element. Its addition can refine the grains of the dual - phase steel structure, thereby increasing the strength of the dual - phase steel and simultaneously raising the brittle temperature range of the stainless steel. The addition of the above ferrite elements, as soft phases, can bear more deformation force without cracking caused by the incoordination of the two phases during hot rolling.

[0067] As a strong austenite - forming element, C can reduce the ferrite content during solidification, thereby reducing the nitrogen content discharged due to the precipitation of ferrite during solidification and increasing the nitrogen solubility of the material.

[0068] As a strong austenite - stabilizing element, N means that at a certain temperature, the presence of nitrogen helps to stabilize the austenite phase and greatly reduces the formation of the ferrite phase.

[0069] As an austenite - forming element, if an excessive amount of Ni is added, it will cause an imbalance in the ratio of ferrite and austenite phases, a decrease in the ferrite content, and the elements will be enriched in it, thus forming more intermediate phases, which will affect the toughness and corrosion resistance of the material.

[0070] Mn and Cu are weak austenite stability elements. Adding them will cause the appearance of austenite structure in the steel, thereby reducing the proportion of the ferrite phase region.

[0071] The manufacturing method of the economical high-nitrogen and easy-to-form austenitic stainless steel described in the present invention includes the following steps:

[0072] 1) Smelting and casting

[0073] Smelt according to the above composition, refine by AOD and LF, and continuously cast into billets; control the cooling rate in the secondary cooling section to 1 - 1.5 °C / s;

[0074] 2) Hot rolling

[0075] Control the heating temperature of the billet to 1200 - 1250 °C; control the heating time to 160 - 240 min, and then coil after rough rolling and finish rolling, where the finish rolling exit temperature, i.e., the final rolling temperature ≥ 1000 °C;

[0076] 3) First annealing and first pickling

[0077] The first annealing is carried out in an annealing furnace, the annealing temperature is 1050 - 1120 °C; the annealing time is 3 - 10 min; after annealing, mechanical descaling is carried out, and finally a hot-rolled steel coil is obtained after the first pickling;

[0078] 4) Cold rolling

[0079] Roll to the required thickness specification;

[0080] 5) Second annealing and second pickling

[0081] The second annealing is carried out in an annealing furnace, the annealing temperature is 1050 - 1120 °C, the annealing time is 1 - 5 min, and then the second pickling is carried out.

[0082] Preferably, in step 3), the first pickling adopts a sulfuric acid section + mixed acid section, where the sulfuric acid concentration in the sulfuric acid section is 200 - 300 g / L; the mixed acid section is nitric acid + hydrofluoric acid, the nitric acid concentration is 180 - 240 g / L, and the hydrofluoric acid concentration is 15 - 25 g / L.

[0083] Preferably, in step 4), the rolling adopts a conventional continuous rolling mill or a reversible single rolling mill.

[0084] Preferably, in step 5), the second pickling is an electrolysis + mixed acid process, the medium in the electrolysis section is Na 2 SO 4 , the electrolysis current is 3500 - 4000 A; the mixed acid section is nitric acid + hydrofluoric acid, the nitric acid concentration is 180 - 240 g / L, and the hydrofluoric acid concentration is 15 - 25 g / L.

[0085] In the manufacturing method of the duplex stainless steel described in the present invention:

[0086] During the continuous casting process, the cooling rate of the second cooling section is controlled at 1~1.5℃ / s, which is lower than the cooling rate of 2~3℃ / s of conventional 304 steel. Too fast cooling rate will increase the tendency of solidified austenite structure to transform into martensite, increasing the risk of brittle fracture of the ingot. However, the cooling rate should not be too slow, otherwise the ingot will not be completely solidified. Therefore, the cooling rate of the second cooling section is controlled at 1~1.5℃ / s.

[0087] During the hot rolling process, the heating temperature is controlled at 1200~1250℃ and the heating time is controlled at 160~240min to ensure that the Cr in the original billet 2 The N brittle phase is completely decomposed and the alloy elements are fully re-dissolved, and the organization will not be coarse due to too long heating time, which will affect the hot processing performance. Finally, it is rolled to the required thickness on the hot rolling unit and then coiled. The final rolling temperature is ≥1000℃, which can ensure that the ferrite content is ≥50% during the rolling process, and there will be no cracking caused by the uncoordinated deformation of the two phases.

[0088] During the primary annealing and secondary annealing process, the annealing temperature is 1050~1120℃, and the annealing time is 3~10min and 1~5min respectively, which is lower than the annealing temperature (1080~1130℃) of 304 stainless steel and the annealing time (1~3min) is longer. The higher the annealing temperature, the lower the proportion of austenite in the finished product after annealing, which is not conducive to the plasticity of the material. The longer the annealing time is, the more conducive it is to the full recrystallization growth of austenite + ferrite structure and improve plasticity.

[0089] Preferably, the primary pickling process adopts a sulfuric acid section + mixed acid section (nitric acid + hydrofluoric acid) process, with a sulfuric acid concentration of 200~300g / L. The main function is to remove rust, and the mixed acid (nitric acid 180~240g / L, hydrofluoric acid 15~25g / L) mainly plays a passivation role.

[0090] As a preferred method, the secondary pickling process adopts electrolysis + mixed acid process (nitric acid + hydrofluoric acid) process, and the electrolysis medium is Na 2 SO 4 The electrolysis current is 3500~4000A. Its main function is to remove rust. The mixed acid (nitric acid 180~240g / L, hydrofluoric acid 15~25g / L) mainly plays a passivation role.

[0091] Beneficial effects of the present invention:

[0092] The present invention adopts Mn-N alloying, high Cr and low Ni composition design, and further controls the chromium equivalent Cr eq : 21.4~22.6%, nickel equivalent Ni eq: 8 - 9.2%, making the austenite structure in the stainless steel fall into the critical region of austenite → martensite transformation, controlling the final rolling ferrite ratio F ≥ 50% to avoid cracking of the steel coil during rolling, and controlling the austenite ratio A ≥ 55% after annealing to increase the proportion of austenite in the structure and improve the strength and plasticity of the material.

[0093] Based on the composition design, the present invention controls the cooling rate in the secondary cooling section of continuous casting at 1 - 1.5 °C / s to obtain duplex stainless steel with excellent slab quality; through low-V and micro-Al alloying design, it creates good conditions for avoiding edge cracking during hot rolling, and combined with controlling the hot rolling heating temperature at 1200 - 1250 °C, heating time at 160 - 240 min, and final rolling temperature ≥ 1000 °C, it ensures that the ferrite content during rolling is ≥ 50%, obtaining a duplex stainless steel coil without edge cracking, enabling the present invention to realize the industrial process of continuous casting + hot rolling + cold rolling.

[0094] The elongation of the duplex stainless steel obtained in the present invention is ≥ 45%, the yield strength is ≥ 450 MPa, the tensile strength R m ≥ 730 MPa, and the stress crack initiation time is ≥ 24 h, obtaining duplex stainless steel with both high strength and plasticity. The strength is 1.7 times that of 304 stainless steel, with higher plasticity and lower cost than traditional duplex stainless steel. The elongation ≥ 45% can meet the stamping performance requirements of high-pressure fuel tanks and greatly improve the stress corrosion resistance of the fuel tank welding joints; the elongation of traditional duplex stainless steel is often less than 40%, and the content of precious metals Ni and Mo added is relatively high; the stress corrosion resistance of the duplex stainless steel obtained in the present invention is far better than that of 304 austenitic stainless steel, and the material cost is reduced by more than 50% compared with 304 austenitic stainless steel, being more economical. Description of the Drawings

[0095] Figure 1 It is a schematic diagram of the Delong diagram;

[0096] Figure 2 It is a schematic diagram of the position of the solidification structure of Comparative Example 2 in the Delong diagram;

[0097] Figure 3 It is a metallographic structure photo of the slab of Comparative Example 2;

[0098] Figure 4 It is a physical diagram of the surface crack of the slab prepared in Comparative Example 2;

[0099] Figure 5 It is a physical diagram of the edge cracking of the steel coil prepared in Comparative Example 3;

[0100] Figure 6 It is a physical diagram of the steel coil prepared in Example 1;

[0101] Figure 7This is a metallographic structure photograph of the finished product prepared in Example 1. DETAILED DESCRIPTION

[0102] The present invention will be further described below in conjunction with the embodiments and drawings.

[0103] The compositions of the embodiments and comparative examples of the present invention are shown in Tables 1 and 2, with the remainder containing Fe and other inevitable impurities; the manufacturing processes and properties of the embodiments and comparative examples are shown in Tables 3 and 4, respectively.

[0104] The tensile properties test of the material is carried out according to GB / T 228.1 "Tensile test of metallic materials Part 1: Room temperature test method";

[0105] Stress corrosion test is carried out in accordance with YB / T 5362-2006 "Stress corrosion test method for stainless steel in boiling magnesium chloride solution", and the test conditions are 42% boiling magnesium chloride solution;

[0106] The austenite content of the material is measured by the magnetic method according to GB / T 1954-2008 "Method for measuring the ferrite content of chromium-nickel austenitic stainless steel welds".

[0107] The Si and Mn contents in Comparative Example 1 exceed the upper limit of the present invention, resulting in Cr eq 、Ni eq It exceeds the upper limit of the constraint of the present invention and is far away from the constraint area of ​​the present invention. At the same time, the annealing temperature exceeds the upper limit of the constraint of the present invention and the annealing time exceeds the lower limit of the constraint of the present invention, resulting in a low austenite ratio in the finished material and an elongation of 38%, which does not meet the requirement of the present invention and cannot meet the stamping performance requirements of the high-pressure fuel tank.

[0108] The Cr and Ni contents in Comparative Example 2 exceed the lower limits of the present invention, resulting in Cr eq 、Ni eq Exceeding the lower limit of the present invention, Figure 2 It is a schematic diagram of the position of the solidification structure of comparative example 2 in the Delong diagram. It can be seen from the figure that in the Delong diagram of the solidification structure, it is in the three-phase region of ferrite + austenite + martensite; Figure 3 The metallographic structure photograph of the ingot of comparative example 2 shows that brittle martensite structure exists in the ingot. In addition, the cooling rate of the second cooling stage of continuous casting exceeds the upper limit of the constraint of the present invention, further promoting the transformation of austenite → martensite structure. Figure 4 This is a real picture of the surface cracks of the ingot prepared in Comparative Example 2. It can be seen from the figure that the ingot has serious cracks and is scrapped and returned to the furnace to terminate production.

[0109] In Comparative Example 3, the hot rolling heating temperature and the final rolling temperature exceeded the lower limit of the present invention, and the final rolling ferrite ratio F was only 45.71%. During the rolling process, the deformation of the austenite-ferrite two phases was not coordinated, and severe cracking occurred at the edge of the hot-rolled steel coil. Figure 5Physical drawing of the edge cracking of the steel coil prepared in Comparative Example 3.

[0110] As can be seen from Table 4, stress corrosion cracks appeared in Comparative Example 4 (304 austenitic stainless steel) in 42% boiling magnesium chloride solution in only 1 h, and its stress corrosion resistance was much lower than that of the present invention, failing to meet the performance requirements of high-pressure fuel tanks.

[0111] Cr in Comparative Examples 5-8 eq , Ni eq exceeded the upper limit of the constraints of the present invention, were far from the constraint region of the present invention, and the austenite content in the finished stainless steel structure was lower than the constraint value of the present invention. Therefore, the elongation rate was low and could not meet the stamping performance requirements of high-pressure fuel tanks.

[0112] Examples 1-10 were all controlled in accordance with the ingredient design, ingredient constraints and processing technology of the present invention. Figure 7 This is the metallographic structure photo of the stainless steel obtained in Example 1 of the present invention. It can be seen from the figure that the product has good economy and processability. The duplex stainless steel structure is austenite-ferrite duplex, and the volume ratio of the finished austenite is 55.81%.

[0113] Figure 6 This is the physical drawing of the steel coil prepared in Example 1. It can be seen from the figure that the obtained steel coil has no edge cracking, and the surface quality is excellent. On the basis of high strength and high elongation rate, it also has excellent stress corrosion resistance, realizing the application of economical high-plasticity duplex stainless steel in high-pressure fuel tanks.

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[0115]

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[0117]

Claims

1. Economical high-plasticity duplex stainless steel with excellent stress corrosion resistance, characterized by: The mass percentages of its components are as follows: C：0.03~0.09%， Si: 0.3~1.0%, Mn: 2.5~4.5%, Ni: 0.5~1.5%, Cr：19.6~21.5%， Mo≤0.3%, Cu≤0.3%, N：0.06~0.13%， V≤0.08%， P≤0.04%， S≤0.0015%， Al：0.01~0.06%， The balance includes Fe and other unavoidable impurities and must satisfy the following relationship at the same time: Chromium equivalent Cr eq :21.4~22.6%,Cr eq =Cr+Mo+1.5Si+5V+3Al; Nickel equivalent Ni eq :8.0~9.2%,Ni eq =Ni+0.87Mn+0.33Cu+30(C+N)-1.35; Final rolling ferrite ratio F ≥ 50%, F=-286.25*C+6.58*Si-4*Mn+10.87*Cr-14.68*Ni+8.75*Mo-10.3*Cu-164*N+67.75*V+69.5*Al-110.35; Annealed austenite ratio A≥55%, A= 240.25*C-15.8*Si+2.63*Mn-8.73*Cr+6.8*Ni-4.5*Mo+11.1*Cu+222.25*N-17.25*V-24*Al+188.

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2. The economical high-plasticity duplex stainless steel with excellent stress corrosion resistance according to claim 1, characterized in that: The microstructure of the duplex stainless steel is austenite+ferrite, and the volume proportion of austenite is ≥55%.

3. The economical high-plasticity duplex stainless steel with excellent stress corrosion resistance according to claim 1 or 2, characterized in that: The crack initiation time of the duplex stainless steel in 42% boiling magnesium chloride solution is ≥24h; the room temperature yield strength Rp 0.2 ≥450MPa, tensile strength R m ≥730MPa, elongation A 50 ≥45%.

4. The method for producing an economical high-plasticity duplex stainless steel having excellent stress corrosion resistance according to any one of claims 1 to 3, characterized in that: The steps include: 1) Smelting and casting According to claim 1, the components are smelted, AOD, LF refined, and continuously cast into ingots; the cooling rate of the secondary cooling section is controlled to be 1-1.5°C / s; 2) Hot rolling The heating temperature of the ingot is controlled at 1200-1250°C; the heating time is controlled at 160-240 minutes, and then the ingot is coiled after rough rolling and finish rolling, wherein the finish rolling outlet temperature, i.e., the final rolling temperature, is ≥1000°C; 3) One annealing and one pickling The primary annealing is carried out in an annealing furnace at a temperature of 1050-1120°C for 3-10 minutes. After annealing, the steel is descaled and pickled once to obtain a hot-rolled steel coil. 4) Cold rolling Rolling to the required thickness specification; 5) Secondary annealing and secondary pickling The secondary annealing is carried out in an annealing furnace at a temperature of 1050-1120°C for 1-5 minutes, followed by secondary pickling.

5. The manufacturing method according to claim 4, characterized in that: In step 3), the primary pickling adopts a sulfuric acid section + a mixed acid section, wherein the sulfuric acid concentration in the sulfuric acid section is 200-300 g / L; the mixed acid section is nitric acid + hydrofluoric acid, the nitric acid concentration is 180-240 g / L, and the hydrofluoric acid concentration is 15-25 g / L.

6. The manufacturing method according to claim 4, characterized in that: In step 4), the cold rolling adopts a conventional continuous rolling mill or a reversible single rolling mill.

7. The manufacturing method according to claim 4, characterized in that: In step 5), the secondary pickling adopts an electrolysis section + a mixed acid section, the electrolysis section medium is Na2SO4, the electrolysis section current is 3500~4000A; the mixed acid section is nitric acid + hydrofluoric acid, the nitric acid concentration is 180~240g / L, and the hydrofluoric acid concentration is 15~25g / L.

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