Nb-containing austenitic stainless steel having corrosion resistance superior to 304l under copper brazing process and method of manufacturing the same

Abstract

The Nb-containing austenitic stainless steel with better corrosion resistance than 304L under copper brazing process and its manufacturing method, the composition in mass percentage is: C≤0.03%, Si≤1.0%, Mn 4.0-7.0%, Ni 3.5-5.5%, Cr 17.5-19.5%, Mo≤0.6%, Cu 0.5-3.0%, N 0.2-0.3%, P<0.05%, S<0.01%, Nb 0.02-0.20%, the rest contains Fe and inevitable impurities. The stainless steel base is austenitic structure, no carbide precipitates in the austenite after copper brazing process, the base structure is still single austenite phase, and the pitting potential of the above copper brazed austenitic stainless steel is >300mV, the pitting corrosion rate is <10g / (m 2 *h), the crystal corrosion rate under mixed acid, i.e. 10% HNO3+3% HF, is <400g / (m 2 *h); the room temperature yield strength Rp 0.2 >400MPa, the elongation A 50 >40%, which is suitable for application in assembled building stainless steel core plate requiring high strength, high corrosion resistance and excellent welding performance.

Description

Technical Field

[0001] This invention relates to Nb-containing austenitic stainless steel and its manufacturing method, and particularly to an Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process and its manufacturing method. Background Technology

[0002] In recent years, with the development of modern industrial technology, prefabricated buildings have been rapidly promoted in China due to their advantages of fast construction speed and low production cost. By simply transporting prefabricated house components to the construction site for assembly, houses can be manufactured in batches, much like machine production. Prefabricated building wall panels are made of stainless steel plates and core tubes welded together with copper brazing. Copper has a melting point of 1083℃; to achieve good weld quality, copper rings are fixed at the connection points of the stainless steel plates and core tubes (e.g.,...). Figure 1 As shown), the entire assembly is heated to above 1083℃ to fuse copper into the gap between the stainless steel plate and the core tube. The copper brazing process is as follows: room temperature - 100 min - 1000℃ - 30 min - 1100℃ - 15 min - 1000℃ - 25 min - 900℃ - 15 min - 800℃ - 15 min - 700℃ - 13 min - 600℃ - 10 min - 500℃ - air cooling. During the copper brazing process, the stainless steel is exposed to temperatures above 1000℃ for an extended period, resulting in coarse grains and reduced material strength. This is detrimental to the load-bearing capacity of buildings. Therefore, high strength performance requirements are imposed on the stainless steel material used for the core plate. Furthermore, during the cooling process of copper brazing, the material undergoes a prolonged sensitization temperature range of 600–800℃, forming interstitial carbides (Cr). 23 C6 content results in chromium depletion at the grain boundaries of stainless steel, which is detrimental to its corrosion resistance. Therefore, stainless steel used in core panels requires low C content and high corrosion resistance. In summary, stainless steel used in prefabricated building core panels needs to meet the requirements of low C content, high strength, and high corrosion resistance.

[0003] Traditional low-carbon austenitic stainless steel 304L has relatively low strength and contains a high amount of the precious metal nickel, resulting in high alloy costs and making it unsuitable for the requirements of prefabricated building core panels. In recent years, nickel-based alloying techniques have been used to obtain nickel-saving or even nickel-free 200-series austenitic stainless steels, significantly reducing costs and improving strength to some extent. However, currently available nickel-saving 200-series stainless steels have lower chromium content, resulting in corrosion resistance far below that of 304 stainless steel, thus failing to meet the performance requirements of prefabricated building core panels.

[0004] Chinese patent publication No. CN112501493B discloses a kind of nickel-saving high-nitrogen austenitic stainless steel and manufacturing method with excellent pitting corrosion and sulfuric acid corrosion resistance.The composition and mass percentage of the alloy material are as follows:C:0.03-0.12%, Si:0.25-1.0%, Mn:6.0-7.0%, Cr:18.0-20.5%, Ni:2.0-3.0%, Cu:1.0-2.0%, P≤0.045%, S≤0.0020%, N:0.2-0.3%, the balance is Fe and inevitable impurities.The steel is a typical nickel-saving austenitic stainless steel with Mn and N replacing Ni.Although the base material has high strength, the yield strength Rp 0.2 >400MPa, and the corrosion resistance reaches the level of 304 stainless steel.However, the material has high C content, and a large amount of Cr 23 C6 is precipitated after copper brazing, which seriously reduces the pitting corrosion and intergranular corrosion resistance of the material, and the grain structure is coarsened during copper brazing, which significantly reduces the strength, and cannot meet the performance requirements of the steel for assembled building core board.

[0005] Chinese patent publication No. CN114250421B discloses a kind of high-nitrogen austenitic stainless steel and manufacturing method with excellent post-weld intergranular corrosion and pitting corrosion resistance, which is superior to 316L.The chemical composition and mass percentage are as follows:C:0.03-0.06%, Si:0.25-0.80%, Mn:2.6-5.0%, Cr:20.0-22.0%, Ni:6.0-7.0%, Cu:0.5-2.0%, Mo≤1.0%, V≤0.07%, P≤0.045%, S≤0.0020%, N:0.2-0.3%, the balance is Fe and other inevitable impurities.The steel is a kind of austenitic stainless steel with 316L as the reference.Although the post-weld intergranular corrosion resistance is excellent, the Ni content is still high, the cost is high, and the C content is high, which significantly reduces the pitting corrosion and intergranular corrosion resistance after copper brazing, and cannot avoid the phenomenon of grain structure coarsening and strength reduction after copper brazing.

[0006] Chinese patent publication No. CN102199738A discloses a low-carbon and low-nickel austenitic stainless steel with high strength and high toughness and corrosion resistance, which has the following chemical composition: C≤0.03, Si≤0.8, Mn: 15-22, Ni≤2.0, Cr: 15-22, N: 0.4-0.8, Mo: 1.0-4.0, Cu≤0.5, Nb≤0.5, V≤0.5, S≤0.025, P≤0.03, and the balance of Fe and other inevitable impurities. The material also has high strength and corrosion resistance by replacing Ni with Mn and N, and has low C content, but the N content is too high (more than 0.4%), which cannot be realized by continuous casting in large-scale industrial production, and nitrogen pores are prone to occur in the continuous casting slab, so the material can only be obtained by vacuum induction melting and then electroslag remelting in the laboratory, which has low production efficiency and high production cost.

[0007] In summary, the existing conventional austenitic and nickel-reduced austenitic stainless steel cannot meet the performance requirements of high strength, pitting corrosion resistance and intergranular corrosion resistance after copper brazing, and cannot be realized by continuous casting in industrial production. SUMMARY

[0008] The purpose of the present application is to provide a Nb-containing austenitic stainless steel with corrosion resistance better than 304L under copper brazing process and a manufacturing method thereof, which has low cost, a single austenitic phase matrix, a room temperature yield strength Rp 0.2 > 400 MPa, and an elongation A 50 > 40%; no carbide is precipitated in the austenite after copper brazing, the matrix is still a single austenitic phase, the room temperature yield strength is still maintained at Rp 0.2 > 400 MPa, the pitting corrosion resistance and intergranular corrosion resistance after welding are better than those of 304L stainless steel, and the hot-rolled coil has no edge crack and good surface quality, which is suitable for application in prefabricated building stainless steel core plates requiring high strength, high corrosion resistance and excellent welding performance.

[0009] To achieve the above purpose, the technical scheme of the present application is as follows:

[0010] The Nb-containing austenitic stainless steel with corrosion resistance better than 304L under copper brazing process has the following composition:

[0011] C: ≤0.03%,

[0012] Si: ≤1.0%,

[0013] Mn: 4.0-7.0%,

[0014] Ni: 3.5-5.5%,

[0015] Cr: 17.5-19.5%,

[0016] Mo: ≤0.6%,

[0017] Cu: 0.5-3.0%,

[0018] N: 0.2-0.3%,

[0019] P: <0.05%,

[0020] S: <0.01%,

[0021] Nb: 0.02-0.20%,

[0022] the balance comprising Fe and other inevitable impurities, and the following relationships are simultaneously satisfied:

[0023] PREN≥18.5,

[0024] PREN=Cr+3.3Mo+20N-0.3Mn-30C;

[0025] intergranular corrosion resistance equivalent≥21,

[0026] intergranular corrosion resistance equivalent=1.2Cr+4.5Mo+0.5Ni+0.3Cu+20N-2.5Mn+55Nb-100C;

[0027] NbC precipitation temperature≥800℃,

[0028] NbC precipitation temperature=800C+5Cr+15Ni+20*Cu-200N+1000Nb+605;

[0029] NbN precipitation temperature≤1050℃,

[0030] NbN precipitation temperature=100C+500N+1000Nb+732.

[0031] Further, the balance is Fe and other inevitable impurities.

[0032] The stainless steel substrate according to the present application is of austenitic structure, and no carbide precipitates in the austenite after copper brazing, so that the substrate structure is still single austenite phase, the point corrosion potential of the substrate after copper brazing is >300mV, the point corrosion rate is <10g / (m 2 *h), the intergranular corrosion rate in mixed acid, i.e. 10% HNO3+3% HF, is <400g / (m 2 *h); the room temperature yield strength Rp 0.2 >400MPa, and the elongation A 50 >40%.

[0033] In the component design of the high-nitrogen austenitic stainless steel according to the application:

[0034] C is an austenite forming element, and the increase of C helps to obtain an austenite structure and reduce the ferrite content in the solidification process. Meanwhile, C as an interstitial atom can play a solid solution strengthening role. However, too high C content is easy to form Cr 23 C6, which reduces the pitting corrosion resistance and intergranular corrosion resistance. Especially in the copper brazing process, the stainless steel is in the sensitization temperature range of 600-800 DEG C for a long time, and Cr 23 C6 is more likely to be formed, which greatly reduces the corrosion resistance of the material. Although C as an austenite forming element is beneficial to reduce the ferrite content, it is obviously more harmful than beneficial in the copper brazing process. Therefore, the carbon content is controlled to be C≤0.03%.

[0035] Si is an element commonly contained in steel smelting and is used as a deoxidizer, so Si is contained in general stainless steel. However, the Si content should not be too high, because it is easy to form a low-melting-point oxide scale adhered to the surface of the base body, which is difficult to remove, affects the surface quality and reduces the pitting corrosion resistance of the material, so the Si content is controlled to be Si≤1.0%.

[0036] Mn can improve the solubility of N in the molten steel, so that more N is dissolved in the molten steel. The commonly used Mn-N combination replaces the noble metal Ni to improve the stability of the austenite phase and reduce the alloy cost. However, the Mn content should not be too high, because too high Mn is easy to form MnS inclusions with impurity elements S in the steel, which reduces the pitting corrosion resistance and intergranular corrosion resistance, so the Mn content is controlled to be 4.0-7.0%.

[0037] Cr is the main added element in the stainless steel according to the application, and the content in the prior art is generally above 10.5%, which can generate Cr2O3 passivation film, which is the most fundamental reason for the corrosion resistance of the stainless steel. The stability of the protective film is relatively improved with the increase of the Cr content, and the corrosion resistance is correspondingly improved, including the pitting corrosion resistance and the intergranular corrosion resistance. In order to ensure that the material has better corrosion resistance than 304L, the chromium content in the steel according to the application is controlled to be above 17.5%. Chromium is also a ferrite forming element, and too high Cr content greatly increases the ferrite content. In order to obtain a full austenite structure, a large amount of Ni element must be added, which further increases the alloy cost. Therefore, the Cr content in the application is controlled to be 17.5-19.5%.

[0038] Ni is an austenite forming element, which expands the austenite phase region and improves the stability of the austenite. Ni in the solid solution state in the austenite hinders the migration of C elements to the grain boundary, suppresses the precipitation of Cr 23 C6, thereby improving the intergranular corrosion resistance. Too high Ni content will reduce the strength of the austenitic stainless steel and greatly increase the alloy cost. Considering the cost and performance, the Ni content is controlled to be 3.5-5.5%.

[0039] N, nitrogen is a very strong austenite stabilizing element, which can improve the strength of the steel, and can also improve the stability of Cr2O3 passivation film, significantly improve the resistance to pitting and intergranular corrosion. But too high N content will increase the difficulty of melting and hot working, leading to difficult to industrialized production. In addition, the solubility of N in molten steel is limited, too high will make the slab porosity, after rolling into the surface peeling defects, affect the material appearance, the defect position will also become the corrosion source, reduce the corrosion resistance of the material. Therefore, the content of N in the steel is controlled in 0.2-0.3%.

[0040] P, in general, P is a harmful element in steel, increases the cold brittleness of the steel, makes the welding and cold bending performance worse, so the content of P is controlled in <0.05%.

[0041] S, sulfur is usually a harmful element, which can cause hot brittleness of the steel, reduce the ductility and toughness of the steel, and form cracks during rolling. In addition, S is easy to form MnS inclusions with Mn, which becomes a preferential corrosion source, which is not conducive to the pitting and intergranular corrosion resistance, so the content of S is controlled within 0.01%.

[0042] Mo, as an element for significantly improving corrosion resistance, especially for improving the pitting and intergranular corrosion resistance in chloride ion environment, its corrosion resistance is about 3.3 times that of Cr. But its price is very expensive, and too high Mo is easy to form brittle SIGMA phase, which greatly damages the hot working performance. Unless the corrosion resistance of stainless steel in coastal environment is very high, such as 316L stainless steel containing Mo, generally 304 stainless steel and below corrosion resistance grade stainless steel do not contain Mo. Therefore, the content of Mo is controlled to be ≤0.6%.

[0043] Cu, in stainless steel, it can improve the toughness and cold working performance of the material, especially in high nitrogen austenitic stainless steel. Too high Cu content, continuous casting slab heating process is easy to appear Cu enrichment or even Cu liquefaction, which sharply reduces the hot workability of the steel, causes edge cracking and surface peeling defects. Therefore, the content of Cu in the present application is controlled in 0.5-3.0%.

[0044] Nb, a small amount of Nb in the steel can combine with C, N and other elements, precipitate and disperse the second phase particles NbC, NbN, which can hinder the growth of grains and structure, refine the grains, and improve the strength. At the same time, Nb preferentially combines with C during copper brazing, inhibits the combination of Cr and C to form Cr7C3, and improves the corrosion resistance of the steel. Therefore, the content of Nb in the present application is controlled in 0.01-0.1%. r23C6, to improve the intergranular corrosion resistance of the steel; however, the content of Nb should not be too high, Nb is a strong ferrite forming element, which greatly promotes the formation of ferrite, and is not conducive to the formation of full austenite structure, in addition, during the rolling process of high nitrogen steel, the content of NbN precipitated phase formed by the combination of Nb and N is too high, which is easy to cause hot rolling cracking phenomenon, therefore, the content of Nb is controlled to be 0.02-0.2%.

[0045] In the component design of the Nb-containing austenitic stainless steel according to the application:

[0046] 1. In order to ensure that the pitting corrosion resistance of the steel after copper brazing is better than that of 304L, the pitting resistance equivalent PREN value should be higher than that of 304L.

[0047] The traditional PREN value calculation formula is PREN=Cr+3.3Mo+30N-Mn, which considers the pitting corrosion resistance of the solid solution state stainless steel, and during the copper brazing process, the Cr 23 The C6 phase will become the pitting source preferentially, reducing the pitting corrosion resistance of the material.

[0048] The application considers the negative impact of C element on the pitting corrosion resistance of the material after copper brazing, and modifies the traditional pitting resistance equivalent PREN in the solid solution state, and the modified pitting resistance equivalent PREN after copper brazing is Cr+3.3Mo+20N-0.3Mn-30C. Figure 2 The traditional PREN pitting resistance equivalent calculation formula and the pitting corrosion rate diagram of each steel after actual copper brazing have a confidence R 2 of 0.80%; Figure 3 The modified PREN pitting resistance equivalent calculation formula according to the application and the pitting corrosion rate diagram of each steel after actual copper brazing have a confidence R 2 of 0.87%. It can be seen that the modified PREN calculation formula corresponds better to the actual situation. The actual composition of 304L is C: 0.02%, Si: 0.45%, Mn: 1.2%, Cr: 18.1%, Ni: 8.01%, Cu: 0.15%, Mo: 0.03%, N: 0.07%, and the modified PREN is 18.5. In order to ensure that the pitting corrosion resistance of the steel after copper brazing is better than that of 304L, the components of the steel according to the application also need to satisfy:

[0049] The pitting resistance equivalent PREN=Cr+3.3Mo+20N-0.3Mn-30C is greater than or equal to 18.5.

[0050] 2、In order to obtain the intergranular corrosion resistance of the austenitic stainless steel after copper brazing is better than that of 304L, in addition to the component design of low C, high Cr and high N for the material, Nb element is added to the stainless steel, in addition to the effect of refining grains and improving strength, it can ensure that Nb is combined with C preferentially in the copper brazing sensitization temperature range of 600-800 DEG C, and Cr 23 C6 is formed to inhibit the combination of Cr

[0051] In order to ensure that the intergranular corrosion resistance of the steel composition of the present application is better than that of 304L, through a large number of experimental researches, it is found that the intergranular corrosion resistance and the content of each element satisfy the following relationship:

[0052] The intergranular corrosion resistance equivalent = 1.2Cr + 4.5Mo + 0.5Ni + 0.3Cu + 20N - 2.5Mn + 55Nb - 100C. Among them, Cr, Mo and N are important elements for improving the stability of Cr2O3 passivation film in stainless steel, the higher the content, the more stable the passivation film, and the better the intergranular corrosion resistance; the combination of Nb and C is higher than that of Cr and C, which reduces the Cr 23 C6 precipitation, reduces the degree of intergranular chromium depletion, and improves the intergranular corrosion resistance; Ni and Cu elements in solid solution state in austenite hinder the migration of C, reduce the precipitation of Cr 23 C6, and improve the intergranular corrosion resistance; the higher the content of C, the more C that is difficult to be dissolved, which greatly promotes the precipitation of Cr 23 C6, and reduces the intergranular corrosion resistance. The actual composition of 304L is substituted into the formula to obtain the intergranular corrosion resistance equivalent of 21. Therefore, the steel composition of the present application still needs to satisfy the following intergranular corrosion resistance equivalent constraint relationship:

[0053] The intergranular corrosion resistance equivalent is greater than or equal to 21,

[0054] The intergranular corrosion resistance equivalent = 1.2Cr + 4.5Mo + 0.5Ni + 0.3Cu + 20N - 2.5Mn + 55Nb - 100C.

[0055] 3、Although Nb element is added to the steel, in order to ensure that C in the steel is combined with Nb preferentially rather than with Cr to form Cr 23 C6 in the sensitization temperature range of copper brazing, it is necessary to ensure that the precipitation temperature of NbC is higher than that of Cr 23 C6 in the steel with low C, the precipitation temperature of Cr 23 C6 is not higher than 800 DEG C, and above 800 DEG C, due to the high temperature, the precipitated Cr 23C6 will also be re-dissolved into austenite in the form of Cr atoms and C atoms. Therefore, it is necessary to control the precipitation temperature of NbC to be higher than 800℃. The precipitation temperature of NbC can be expressed by 800C+5Cr+15Ni+20Cu-200N+1000Nb+605(℃), wherein C and Nb promote the precipitation of NbC, and the influence coefficient is large, because the higher the content of C and Nb, the more C and Nb that cannot be dissolved, and the excess C and Nb can only be precipitated in the form of NbC precipitate phase, thereby increasing the precipitation temperature of NbC. Cr, Ni and Cu also promote the precipitation of NbC, because Cr, Ni and Cu exist in the form of solid solution atoms in austenite, thereby reducing the solubility of C in austenite and promoting the precipitation of NbC. Only N element can inhibit the precipitation of NbC and reduce the precipitation temperature of NbC, because the binding force between N and Nb is stronger than that between C and Nb.

[0056] 4. The Nb element added in the steel is easy to combine with N element to form NbN during high-temperature rolling. Due to the plastic difference between the NbN precipitate phase and the austenite matrix, micro-cracks are easily generated at the interface of the NbN precipitate phase, and the cracks are expanded in the subsequent rolling passes, thereby causing edge cracks or surface peeling defects of the hot-rolled steel coil, affecting the appearance of the product and reducing the corrosion resistance. Therefore, it is necessary to control the NbN precipitate phase not to precipitate in the rolling temperature range. The finish rolling temperature of hot rolling is generally above 1050℃, and therefore it is necessary to ensure that the precipitation temperature of NbN is lower than 1050℃. The precipitation temperature of NbN can be expressed by 100C+500N+1000Nb+732(℃), wherein N and Nb promote the precipitation of NbN, and the influence coefficient is large, because the higher the content of N and Nb, the more N and Nb that cannot be dissolved, and the excess N and Nb can only be precipitated in the form of NbN precipitate phase, thereby increasing the precipitation temperature of NbN. C also promotes the precipitation of NbN, because C as an interstitial solid solution atom reduces the space for N which is also an interstitial atom, thereby reducing the solubility of N and promoting the precipitation of NbN.

[0057] The manufacturing method of the Nb-containing austenitic stainless steel with corrosion resistance better than that of 304L according to the copper brazing process comprises the following steps:

[0058] 1) Smelting and casting

[0059] The slab is obtained by smelting, AOD, LF refining and continuous casting according to the above-mentioned composition; wherein the pouring temperature is controlled to be 1450℃-1500℃, and is preferably controlled to be 1460-1480℃; the cooling rate of the secondary cooling section is controlled to be 60-90℃ / min, and is preferably controlled to be 70-80℃ / min;

[0060] 2) Hot rolling

[0061] The slab heating is carried out in a heating furnace, the heating temperature is controlled to be 1200-1300℃, preferably controlled to be 1200-1250℃; the heating time is controlled to be 180-280min, preferably controlled to be 200-240min, and then rolled to the required thickness through rough rolling and finish rolling, and then coiled, and the coiling temperature is controlled to be 600-900℃, preferably controlled to be 600-700℃;

[0062] 3) annealing, pickling

[0063] The annealing is carried out in an annealing furnace, the annealing temperature is 1050-1150℃, preferably 1100-1150℃; the annealing time is 1-10min, preferably 3-10min; after the annealing, mechanical dephosphorization is carried out, and finally the required steel coil is obtained through pickling.

[0064] Preferably, the finish rolling outlet temperature of the hot rolling in step 2) is ≥1000℃.

[0065] Preferably, the pickling in step 3) adopts a sulfuric acid section + a mixed acid section; the sulfuric acid section has a sulfuric acid concentration of 250-400g / L, preferably 300-350g / L; the mixed acid section is nitric acid + hydrofluoric acid, the nitric acid concentration is 140-240g / L, preferably 180-220g / L; and the hydrofluoric acid concentration is 10-25g / L, preferably 15-20g / L.

[0066] In the manufacturing method of the austenitic stainless steel described in the application:

[0067] In the continuous casting process, the pouring temperature is controlled to be 1450-1500℃, preferably 1460-1480℃, which is lower than the conventional 304L steel pouring temperature of 1475-1490℃. The high-nitrogen steel has the risk of nitrogen gas escaping in the continuous casting process, and once the primary solidification shell captures the nitrogen gas, there will be the risk of needle-shaped pores on the surface of the slab, which deteriorates the surface quality of the slab. Reducing the pouring temperature is beneficial to improving the N solubility of the molten steel and reducing the risk of nitrogen gas escaping. In addition, the cooling rate of the secondary cooling section is controlled to be 60-90℃ / min, preferably 70-80℃ / min, which is much higher than the cooling rate of 40-50℃ / min of the conventional 304L steel. Nb and N elements combine to form NbN in the high-temperature solidification structure, although the subsequent heating process will re-dissolve the NbN back into the matrix, but if the NbN content in the cast slab structure is too high, it may not be completely dissolved in the heating furnace, and the undissolved NbN will generate micro-cracks in the subsequent rolling process, which is easy to produce edge cracks and surface peeling defects. Therefore, it is necessary to as high as possible to improve the cooling rate of the secondary cooling section to reduce the precipitation of NbN.

[0068] In the hot rolling process, the heating temperature is controlled to be 1200-1300 DEG C, preferably 1200-1250 DEG C. The high-nitrogen steel has high strength and large deformation resistance, compared with the conventional 304L steel, the heating temperature should be higher, the softening slab is beneficial to the smooth rolling. However, the heating temperature should not be too high, otherwise the Cu in the steel will be enriched to form liquid phase copper in the oxidation process, causing "copper brittleness" to cause the steel coil surface to peel. The heating time is controlled to be 180-280 min, preferably 200-240 min, in order to fully dissolve the NbN precipitated phase in the solidification process of the slab, while avoiding the austenite organization being too large, affecting the hot working performance. In particular, during the subsequent rolling process, it must be ensured that the finish rolling temperature at the end of rolling is higher than 1000 DEG C, NbN will re-precipitate from the austenite organization below 1000 DEG C, if the rolling has not ended, NbN will produce cracks in the subsequent rolling process due to the mismatch of plasticity and matrix. The coiling temperature is controlled to be 600-900 DEG C, preferably 600-700 DEG C, in the coiling process, the lower coiling temperature can reduce the Cr 23 C6 precipitation, which is beneficial to subsequent annealing solid solution.

[0069] In the annealing pickling process, the annealing temperature is 1050-1150 DEG C, preferably 1100-1150 DEG C; the annealing time is 1-10 min, preferably 3-10 min; in the annealing recrystallization process of high-nitrogen Nb-containing steel, N and Nb can improve the recrystallization nucleation energy and hinder the grain growth, in order to ensure that the grains grow fully and obtain high-strength steel with good plasticity, it is necessary to as high as possible to improve the annealing temperature and prolong the annealing time.

[0070] The pickling process preferably adopts sulfuric acid section + mixed acid section (nitric acid + hydrofluoric acid) process, the sulfuric acid concentration of the sulfuric acid section is 250-400 g / L, preferably 300-350 g / L; the nitric acid concentration of the mixed acid section is 140-240 g / L, preferably 180-220 g / L; the hydrofluoric acid concentration is 10-25 g / L, preferably 15-20 g / L. The main role of the sulfuric acid section is to remove the loose outer layer of iron oxide scale, and the main role of the mixed acid section is to remove the dense inner layer of chromium oxide scale and re-passivate to form a stable passivation film.

[0071] The beneficial effects of the present application are:

[0072] The present application obtains full austenitic structure with excellent comprehensive performance in solid solution state through the component design of Mn-N instead of Ni, and on this basis, through the design idea of low C and micro Nb alloying, no carbide is precipitated in the austenite after the copper brazing process, and the matrix organization is still single austenite phase. Combined with the control of the pitting resistance equivalent, the intergranular corrosion resistance equivalent and the precipitation temperature of NbC, the pitting and intergranular corrosion resistance of the Nb-containing austenitic stainless steel after copper brazing is better than that of 304L, and at the same time, higher strength (yield strength Rp0.2 >400MPa).

[0073] The present application avoids the precipitation of NbN in the rolling process by controlling the precipitation temperature of NbN and the finishing temperature of hot rolling, so that the material has good hot working performance, so that the present application can realize the industrialization process of continuous casting + slab + hot rolling, and obtain a steel coil without edge cracking and good surface quality.

[0074] The present application reduces the amount of NbN precipitation in the slab by optimizing the pouring temperature and the cooling speed of the secondary cooling section in the continuous casting stage; in the hot rolling stage, the heating temperature and time are optimized to promote the decomposition of NbN during heating, and to create good conditions for avoiding edge cracking during rolling. BRIEF DESCRIPTION OF DRAWINGS

[0075] Figure 1 Copper brazing physical diagram.

[0076] Figure 2 The PREN value calculation formula before correction and the fitting relationship diagram of the pitting rate of each steel grade.

[0077] Figure 3 The PREN value calculation formula after correction and the fitting relationship diagram of the pitting rate of each steel grade.

[0078] Figure 4 The Cr 23 C6 precipitation SEM diagram.

[0079] Figure 5 The physical diagram of the edge cracking stainless steel prepared in Comparative Example 3.

[0080] Figure 6 The NbN precipitation SEM diagram of the edge cracking stainless steel prepared in Comparative Example 3.

[0081] Figure 7 The physical diagram of the surface peeling stainless steel prepared in Comparative Example 4.

[0082] Figure 8 The Cu enrichment SEM diagram of the surface peeling stainless steel prepared in Comparative Example 4.

[0083] Figure 9 The solid solution state metallographic OM diagram of Example 1 of the present application.

[0084] Figure 10 The metallographic OM diagram after copper brazing of Example 1 of the present application. DETAILED DESCRIPTION

[0085] The present application will be further described below in conjunction with examples and drawings.

[0086] The components of the examples and the comparative examples of the present application are shown in Table 1, and the balance comprises Fe and other inevitable impurities; the manufacturing process and performance of the examples and the comparative examples are shown in Table 2 and Table 3, respectively.

[0087] The tensile property test of the material is performed according to GB / T 228.1 Metallic materials - Tensile test - Part 1: Method of test at room temperature.

[0088] The intergranular corrosion rate-D method test is performed according to GB / T 4334-2008 Stainless steels - Method for intergranular corrosion testing, and the test solution is 10% nitric acid + 3% hydrofluoric acid, and the test temperature is 70±1℃.

[0089] The pitting potential test is performed according to GB / T 17899-1999 Stainless steels - Method for pitting potential measurement, and the test solution is 3.5% NaCl solution, and the test temperature is 30±1℃.

[0090] The pitting corrosion rate test is performed according to GB / T 17897-2016 Stainless steels - Ferric chloride pitting test method - Method B, and the test solution is 6% FeCl3+0.16% HCl solution, and the test temperature is 35±1℃.

[0091] In the comparative example 1, the Mn content exceeds the upper limit of the present application, and the Cr content exceeds the lower limit, resulting in that the pitting resistance equivalent PREN is only 18.4, which does not reach the level of 304L stainless steel pitting resistance equivalent PREN value 18.5. After copper brazing, the pitting potential is 286mV, and the pitting corrosion rate is 10.63g / (m 2 *h), and the pitting resistance performance after copper brazing is not as good as that of 304L.

[0092] In the comparative example 2, the C content exceeds the upper limit of the present application, and the Nb content exceeds the lower limit, resulting in that the intergranular corrosion resistance equivalent is only 18.5, which does not reach the level of 304L stainless steel intergranular corrosion resistance equivalent 21.0. Moreover, the NbC precipitation temperature is lower than the Cr 23 C6 precipitation temperature 800℃, and it is inevitable to produce precipitated phase after copper brazing (see Figure 4 ), through electron probe micro-area composition analysis, the C and Cr element contents in precipitated phase 1, precipitated phase 2 and precipitated phase 3 are 6%-8% and 30%-35% respectively, which are much higher than the contents 0.035% and 19.3% in the matrix organization, and the precipitated phase is judged as Cr 23 C6. The presence of the precipitated phase Cr 23 C6 further reduces the intergranular corrosion resistance of the material, and the intergranular corrosion rate after copper brazing is 500g / (m 2 *h), and the intergranular corrosion resistance performance after copper brazing is not as good as that of 304L.

[0093] The N content in Comparative Example 3 exceeds the upper limit of the present application, the precipitation temperature of NbN is 1080℃, and the final rolling temperature is controlled to be low, i.e. 950℃, and a large amount of precipitates are generated during hot rolling (see Figure 5 ). According to the micro-area composition analysis by electron probe, the N and Nb elements in precipitate 1, precipitate 2 and precipitate 3 are 5% to 8% and 7% to 9% respectively, which are much higher than the contents of 0.3% and 0.19% in the matrix structure. The precipitates are judged to be NbN. A large amount of NbN precipitates generated during hot rolling cause cracking at the edge of the rolled sheet (see Figure 6 ).

[0094] In Comparative Example 4, the Cu content exceeds the upper limit of the present application, and the heating temperature is 1280℃, which is not in the preferred heating temperature range of 1200 to 1250℃. Serious element enrichment occurs due to oxidation during the heating of the slab (see Figure 7 ). According to the micro-area composition analysis by electron probe, the Cu element in precipitate 1, precipitate 2 and precipitate 3 is as high as 20% or more, which is much higher than the content of about 1% in the matrix structure. The Cu enrichment is serious, and the copper precipitates liquefy during high-temperature heating, weakening the intergranular bonding force and causing the peeling defect on the surface of the steel coil (see Figure 8 ).

[0095] As can be seen from Table 3, the yield strength of 304L austenitic stainless steel after copper brazing is much lower than that of the present application, and cannot meet the strength requirements of the steel for the pressure-bearing core plate of the assembled building.

[0096] Table 3 also lists the PREN (pitting resistance equivalent number) and the intergranular corrosion resistance equivalent of the comparative patents of the present application. The intergranular corrosion resistance equivalents of Chinese patents CN112501493 B and CN102199738 A are 5.59 and 1.68 respectively, and it can be predicted that the intergranular corrosion resistance after copper brazing is far from the level of 304L. The PREN and the intergranular corrosion resistance equivalent of Chinese patent CN114250421 B both meet the requirements, and it can be predicted that the pitting resistance and the intergranular corrosion resistance after copper brazing are both superior to those of 304L, but the alloy Ni content is high, the alloy cost is high, and there is no price advantage compared with the present application.

[0097] Examples 1 to 10 are all prepared according to the composition design, composition constraints and processing technology of the present application, and the stainless steel products prepared thereby are qualified in terms of quality and performance. The solid solution state metallographic structure of the stainless steel prepared in Example 1 is shown in Figure 9 , which is composed of a single austenitic structure and contains a large amount of annealing twins. The metallographic structure after copper brazing of Example 1 is shown in Figure 10The grain size is coarsened to a certain extent, but still consists of single austenite, and no carbide or other precipitated phase is precipitated at the grain boundary. The intergranular corrosion rate, pitting corrosion rate and pitting potential of the copper brazing measured in examples 1-10 are all superior to those of 304L stainless steel, and the yield strength is much higher than that of 304L, which can meet the requirements of the stainless steel for the pressure core plate of the fabricated building.

[0098] In summary, by the component design of replacing Ni with Mn-N, the full austenitic structure with excellent comprehensive performance in solid solution state is obtained, and on this basis, by the design idea of low C and micro-Nb alloying, no carbide is precipitated in the austenite after the copper brazing process, and the matrix structure is still single austenite phase, combined with the control of the pitting resistance equivalent, intergranular corrosion resistance equivalent and NbC precipitation temperature, the Nb-containing economic high-nitrogen austenitic stainless steel with the pitting corrosion and intergranular corrosion resistance superior to 304L after copper brazing is obtained, and at the same time, higher strength (yield strength Rp 0.2 > 400 MPa) is obtained. By controlling the NbN precipitation temperature and the hot rolling final rolling temperature, the stainless steel coil with excellent edge and surface quality is obtained, and the consistent industrial production process of smelting + continuous casting + hot rolling + annealing is realized. The Nb-containing economic high-nitrogen austenitic stainless steel of the application is more suitable for the copper brazing of the stainless steel core plate of the fabricated building than 304L.

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Claims

1. The corrosion resistance of Nb-containing austenitic stainless steel under copper brazing process is superior to that of 304L. Its composition by mass percentage is as follows: C：≤0.03%， Si: ≤1.0%, Mn: 4.0~7.0%, Ni: 3.5~5.5%, Cr：17.5~19.5%， Mo: ≤0.6%, Cu: 0.5~3.0%, N：0.2~0.3%， P：＜0.05%， S：＜0.01%， Nb: 0.02~0.20%, with the balance being Fe and other unavoidable impurities, and the following relationship must be satisfied simultaneously: Pitting resistance equivalent PREN ≥ 18.5 PREN=Cr+3.3Mo+20N-0.3Mn-30C; Intergranular corrosion resistance equivalent ≥21, Intergranular corrosion resistance equivalent = 1.2Cr + 4.5Mo + 0.5Ni + 0.3Cu + 20N - 2.5Mn + 55Nb - 100C; NbC precipitation temperature ≥ 800℃ NbC precipitation temperature = 800°C + 5Cr + 15Ni + 20Cu - 200N + 1000Nb + 605°C; NbN precipitation temperature ≤1050℃ NbN precipitation temperature = 100°C + 500N + 1000Nb + 732°C.

2. The corrosion resistance of Nb-containing austenitic stainless steel under the copper brazing process as described in claim 1 is superior to that of 304L, characterized in that... The austenitic stainless steel has an austenitic phase matrix in the solution-treated state, and after copper brazing, no carbides precipitate in the austenite, so the matrix remains a single austenitic phase.

3. The Nb-containing austenitic stainless steel with better corrosion resistance than 304L under the copper brazing process as described in claim 1 or 2, characterized in that, After copper brazing, the pitting potential of the austenitic stainless steel is >300mV, and the pitting corrosion rate is <10g / (m). 2 h), under mixed acid conditions (10% HNO3 + 3% HF), the intergranular corrosion rate is <400 g / (m). 2 h); Room temperature yield strength R p0.2 >400MPa, elongation A 50 >40%.

4. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 1, 2, or 3, characterized in that, Includes the following steps: 1) Smelting and casting The smelting, AOD, LF refining, and continuous casting into slabs are carried out according to the composition described in claim 1; wherein the casting temperature is controlled at 1450℃~1500℃; and the cooling rate of the secondary cooling section is controlled at 60~90℃ / min. 2) Hot-rolled The slab is heated in a heating furnace at a temperature of 1200-1300℃ and a heating time of 180-280 minutes. After being rolled to the required thickness by rough rolling and finish rolling, it is coiled at a temperature of 600-900℃. 3) Annealing and pickling Annealing is carried out in an annealing furnace at a temperature of 1050~1150℃ for 1~10 minutes. After annealing, the steel coils are mechanically descaled and then pickled to obtain the required steel coils.

5. The method for manufacturing Nb-containing austenitic stainless steel with superior corrosion resistance to 304L under copper brazing process as described in claim 4, characterized in that, In step 1), the casting temperature is controlled at 1460~1480℃.

6. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under the copper brazing process as described in claim 4 or 5, characterized in that, In step 1), the cooling rate of the second cooling section is controlled at 70~80℃ / min.

7. The method for manufacturing Nb-containing austenitic stainless steel with superior corrosion resistance compared to 304L under copper brazing process as described in claim 4, characterized in that, In step 2), the heating temperature is controlled at 1200~1250℃.

8. A method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 4 or 7, characterized in that, In step 2), the heating time is controlled to be 200~240 min.

9. A method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 4 or 7, characterized in that, In step 2), the winding temperature is controlled at 600~700℃.

10. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 8, characterized in that, In step 2), the winding temperature is controlled at 600~700℃.

11. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under the copper brazing process as described in claim 4 or 7, characterized in that, In step 2), the hot rolling finishing temperature, i.e. the final rolling temperature, is ≥1000℃.

12. The method for manufacturing Nb-containing austenitic stainless steel with superior corrosion resistance to 304L under copper brazing process as described in claim 8, characterized in that, In step 2), the hot rolling finishing temperature, i.e. the final rolling temperature, is ≥1000℃.

13. The method for manufacturing Nb-containing austenitic stainless steel with superior corrosion resistance to 304L under copper brazing process as described in claim 9, characterized in that, In step 2), the hot rolling finishing temperature, i.e. the final rolling temperature, is ≥1000℃.

14. The method for manufacturing Nb-containing austenitic stainless steel with superior corrosion resistance to 304L under copper brazing process as described in claim 4, characterized in that, In step 3), the annealing temperature is 1100~1150℃.

15. A method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 4 or 14, characterized in that, In step 3), the annealing time is 3~10 minutes.

16. A method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 4 or 14, characterized in that, Step 3) Pickling uses a sulfuric acid section + a mixed acid section; wherein, the sulfuric acid concentration in the sulfuric acid section is 250~400g / L; the mixed acid section is nitric acid + hydrofluoric acid, with a nitric acid concentration of 140~240g / L and a hydrofluoric acid concentration of 10~25g / L.

17. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 16, characterized in that, In step 3), the sulfuric acid concentration in the sulfuric acid section is 300~350g / L.

18. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 16 or 17, characterized in that, In step 3), the concentration of nitric acid is 180~220g / L.

19. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 16 or 17, characterized in that, In step 3), the concentration of hydrofluoric acid is 15~20g / L.

20. The method for manufacturing Nb-containing austenitic stainless steel with corrosion resistance superior to 304L under copper brazing process as described in claim 18, characterized in that, In step 3), the concentration of hydrofluoric acid is 15~20g / L.

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