Highly corrosion resistant and easily copper brazing wettable austenitic stainless steel and method of making

Abstract

The application provides an austenitic stainless steel with high corrosion resistance and easy copper brazing infiltration and a manufacturing method thereof, the main chemical components of the austenitic stainless steel are as follows in percentage by weight (%): C: 0.002-0.03; Si: 0.1-0.8; Mn: 0.05-2.0; Cr: 15-23; Ni: 6-15; Mo: 0.005-0.50; Cu: 0.1-3.5; N: 0.002-0.20; Al: 0.005-0.05%; one or more of Nb, V and Ti; rare earth elements: 0.0005-0.20, the rare earth elements are selected from one or more of Ce and Y; Sn: 0.001-0.15; and the following condition is met: 5<= (Cu / (Sn+5* rare earth elements))<=15. The stainless steel of the application is suitable for manufacturing fluid conveying pipe parts in the air conditioning and heat exchange industries.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of stainless steel smelting, in particular to an austenitic stainless steel with high corrosion resistance and easy copper brazing infiltration and a manufacturing method thereof. BACKGROUND

[0002] Copper pipe for air conditioning refrigeration is an extremely important variety in copper pipe products, accounting for about 20% of the total amount of copper pipe, and belongs to high-end products, mainly used in household air conditioners, central air conditioners, industrial and commercial refrigerators. The development of copper pipe technology and its wide application has driven the development of the air conditioning industry, but at the same time, the high cost of copper pipe has also restricted the subsequent development of the air conditioning industry. In order to reduce product cost and improve competitiveness, international leading enterprises including foreign brand air conditioner enterprises have begun to seek low-cost material replacement solutions. Under the premise of ensuring that the heat exchange capacity is not reduced and the processability is met, developing new stainless steel pipes to replace copper pipes has become a consensus.

[0003] The material and performance characteristics of the heat exchange pipe of an air conditioner determine its manufacturing cost and service life. The main differences between copper pipe as heat exchange pipe and stainless steel heat exchange pipe are: 1) heat exchange performance, the thermal conductivity of copper pipe is 100 W / m℃, and the thermal conductivity of stainless steel pipe is 13 W / m℃, which is 13% of the thermal conductivity of copper pipe as heat exchange pipe, and if other conditions are the same, the heat exchange effect of copper pipe is better. 2) Physical and chemical properties, since the medium transported in the heat exchange pipe is not necessarily hot water, but more corrosive liquid, which requires the heat exchange pipe to have certain corrosion resistance; and since the heat exchange pipe is in contact with air during actual use, especially in the presence of local corrosive environment, which also requires the heat exchange pipe to have certain corrosion resistance, and from the perspective of corrosion resistance, stainless steel pipe is obviously superior to copper pipe, and the oxide layer formed by oxidation of copper pipe will cause more and more internal dirt, which will further lead to poorer heat transfer effect. Stainless steel heat exchange pipe has higher strength and hardness than copper pipe, making it resistant to impact and vibration. From the perspective of physical and chemical properties, stainless steel pipe can be used as heat exchange pipe. 3) Economic and environmental performance, copper is a pure metal, while stainless steel is an alloy material containing multiple elements, and has lower production cost, so the price of stainless steel is much lower than that of copper when calculated based on the same weight of material. Stainless steel pipe has the characteristics of safety and reliability, hygiene and environmental protection, economy and applicability, and has been proven by domestic and foreign engineering practice to be one of the new, energy-saving and environmentally friendly pipe materials with good comprehensive performance in heat exchange systems. Currently, there is no relevant research on stainless steel products specifically used to replace copper pipe as heat exchange pipe, and existing stainless steel products used to make air conditioner heat exchanger components have the problems of low corrosion resistance and poor copper brazing stability. Therefore, in combination with the development needs of the industry, a high-corrosion-resistant stainless steel with good forming processability is needed to be developed for making air conditioner heat exchanger components, which can replace existing copper pipes, thereby achieving cost reduction, carbon reduction and environmental friendliness. SUMMARY

[0004] One of the purposes of the present application is to provide an austenitic stainless steel with high corrosion resistance and easy copper brazing infiltration, which is mainly aimed at fluid conveying pipe components and their service environment in the air conditioning, heat exchange and other industries.

[0005] To achieve the above-mentioned purposes, the high corrosion-resistant and easy-to-infiltrate copper brazing austenitic stainless steel of the present application has the following main chemical components in percentage by weight (%): C: 0.002~0.03; Si: 0.1~0.8; Mn: 0.05~2.0; P<0.04; S<0.01; Cr: 15~23; Ni: 6~15; Mo: 0.005~0.50; Cu: 0.1~3.5; N: 0.002~0.20; Al: 0.005~0.05%; one or more of Nb, V, Ti, wherein Nb: 0.001~0.50, V: 0.001~0.2, Ti: 0.001~0.2; rare earth elements: 0.0005~0.20, the rare earth elements are selected from one or more of Ce and Y; Sn: 0.001~0.15; and the relationship among Cu, Sn and rare earth elements needs to satisfy: 5≤(Cu / (Sn+5*rare earth elements))≤15.

[0006] The stainless steel of the present application has good corrosion resistance, forming processability and copper brazing infiltration, and the theoretical basis for obtaining good corrosion resistance, forming processability and brazing infiltration performance is that:

[0007] The stainless steel contains 15-23% of Cr and 6-15% of Ni, which ensures good corrosion resistance and plastic processing performance of the stainless steel. Meanwhile, the contents of C, N and Al are comprehensively controlled to avoid too high C and N contents from causing chromium carbide to precipitate and reduce corrosion resistance, and in the manufacturing process, too high C content can cause atoms to easily segregate or combine with other atoms to form atomic clusters or precipitates, which can change the mechanical properties of the material. Al can play a deoxidizing role to reduce inclusions and improve the intrinsic quality of the material. By controlling the content of Al, the present application avoids too high Al content from forming aluminum oxide and affecting the purity and processability of the steel. The present application also adds Nb, V and Ti, which are carbon and nitride forming elements. The precipitation of carbon and nitride and the partial solid solution can improve the uniformity, formability and weldability of the steel. The addition of Sn in the present application can improve the passivation ability of the base material surface similar to Mo. The cost of Sn is much lower than that of Mo, which reduces the alloy cost and makes the passivation film denser and reduces the thickness of the passivation film, which is beneficial to improve the corrosion resistance of the steel and can ensure that the thickness of the surface oxide film of the stainless steel is controlled within 1-10 nm. The present application also adds Cu, which can form ε-Cu precipitation on the surface of the stainless steel during the manufacturing process. The synchronous addition of Sn and Cu and the strict control of the addition amount of the two (5≤(Cu / (Sn+5*Rare Earth Elements)≤15) are beneficial to improve the copper area ratio of the passivation film, so that the local copper content on the surface of the stainless steel is ≥5% and the area fraction is not less than 20%, which is beneficial to improve the wettability and capillary flow of the copper brazing filler metal and improve the brazing quality. At the same time, the addition of Sn and rare earth elements (Ce or Y) is also beneficial to improve the corrosion resistance, so as to ensure better corrosion resistance of the stainless steel (avoiding internal dirt of the stainless steel pipe to affect the heat conduction capacity of the material) and higher brazing wettability. Therefore, the present application controls and matches the content of each element to provide a high corrosion-resistant stainless steel with good forming and processing performance, which is suitable for manufacturing fluid conveying pipe parts in the air conditioning and heat exchange industries.

[0008] Next, the effects of each element in the austenitic stainless steel of the present application will be described in detail.

[0009] (1) C: C (carbon) solid solution in stainless steel can improve the strength of the steel, but too high will reduce the plasticity of the steel and reduce the corrosion resistance. C (carbon) is an alloying element required to ensure the strength of the austenitic stainless steel, and at least 0.002% or more carbon content is required. However, when the carbon content exceeds 0.03%, the chromium carbide content increases, which affects the corrosion performance of the subsequent brazing structure. Therefore, the upper limit is set to 0.03%. The preferred content of C is 0.005-0.02%.

[0010] (2) N: N (nitrogen) is an alloying element, and in the case of adding microalloying elements Nb and Ti, it has solid solution and precipitation strengthening effects due to the carbonitride of Nb and Ti. If the nitrogen content is 0.002% or less, the effect cannot be obtained, but on the other hand, if the nitrogen content exceeds 0.2%, massive nitrides are generated. Such nitrides not only reduce the quality of the steel material, but also inhibit the fine dispersion precipitation of the nitrides. Therefore, the nitrogen content is set to 0.002 to 0.20%, and the optimal range of the nitrogen content is 0.005 to 0.03%.

[0011] (3) Si: Si (silicon) can function as a deoxidizer, increase the strength of the steel to some extent, and reduce the workability of the material. Although silicon is added as a deoxidizer in the steelmaking process, it is also an effective element for improving the steam oxidation resistance of the steel. Proper deoxidization must be performed in the steelmaking process, and a Si content of at least 0.1% or more is required. However, if the silicon content is too high, the workability of the steel will be biased, so the upper limit of the silicon content is set to 0.8%. The optimal range of the silicon content is 0.1 to 0.75%.

[0012] (4) Mn: Mn (manganese) forms MnS in combination with impurities S contained in the steel, thereby improving hot workability. However, if the manganese content is less than 0.05%, this effect cannot be obtained. On the other hand, if the manganese content is reduced, the steel becomes hard and brittle, and the workability and weldability of the steel are reduced. Therefore, the upper limit of the manganese content is set to 2%, and the preferred manganese content is 0.2 to 1.7%.

[0013] (5) P, S: P and S (phosphorus and sulfur) are considered to be harmful elements in stainless steel, and should be controlled as low as possible. In this design, P < 0.04 and S < 0.01.

[0014] (6) Cr: Cr (chromium) is the most important alloying element in stainless steel, and easily forms a dense oxide film of Cr2O3 by contacting oxygen, which can ensure oxidation resistance, steam oxidation resistance, and corrosion resistance. Too low affects corrosion resistance, and too high Cr affects high temperature plasticity and makes it difficult to pickle. The Cr content required for austenitic stainless steel is at least 15%. The higher the chromium content, the more the corresponding corrosion resistance is improved. However, higher chromium reduces the structural stability of austenitic stainless steel. Therefore, in order to stabilize the austenitic structure, it is necessary to increase the expensive nickel content. Therefore, the Cr content is set to 15 to 23%, and the preferred range of the Cr content is 17 to 20%.

[0015] (7) Ni: Ni (nickel) is an alloying element that stabilizes the austenite structure in the austenitic stainless steel and is important to ensure corrosion resistance. The lower limit of the Ni content is 6%, which is balanced with the Cr content described above. On the other hand, too high a content of Ni not only increases the cost but also reduces the strength. Therefore, the upper limit is set to 15%, and the preferable range is 8.5-11.0%.

[0016] (8) Mo: Mo (molybdenum) is added to improve corrosion resistance, and too low a content does not improve corrosion resistance, and too high a content increases the cost and causes the generation of precipitates during hot rolling, which affects corrosion resistance and workability. The present invention selects a Mo content of 0.005-0.5%, preferably 0.01-0.3%. This Mo content ensures that the corrosion resistance and workability of the stainless steel meet the requirements and also reduces the manufacturing cost.

[0017] (9) Nb and Ti: Nb (niobium) and Ti (titanium) are both alloying elements that further improve the corrosion resistance of the austenitic stainless steel, and their combination with C reduces the tendency of C to combine with Cr to form Cr23C6-type precipitates, thereby improving corrosion resistance. To effectively improve corrosion resistance, a Nb content of at least 0.001% is required. If the Nb content is 0.5% or more, very coarse Fe2Nb-type precipitates are precipitated, which reduces the strength, and therefore, the Nb content is set to 0.001-0.5%. A small amount of Ti (titanium) produces a uniformly dispersed phase of Ti2O3 in the stainless steel matrix, and the Ti2O3 precipitates can serve as nucleation cores to refine the grains. When the Ti content is less than 0.001%, Ti2O3 is not substantially produced, and no effect is produced. On the other hand, when the Ti content exceeds 0.2%, coarse TiN is produced, which coarsens the structure and deteriorates the workability. Therefore, the Ti content should be between 0.001 and 0.20%, and the optimal range of Ti is 0.002-0.10%.

[0018] (10) V: V (vanadium) also has a similar effect to Nb, which suppresses the precipitation of Cr23C6-type precipitates and also improves the strength of the stainless steel. A small amount of vanadium also improves the morphology of inclusions, obtaining fine inclusions that are nearly spherical, thereby improving the deformation workability of the inclusions. The present invention selects a range of 0.001-0.2%.

[0019] Among them, Nb, V, and Ti are micro-alloying elements that are strong carbide-forming elements, can form carbides with carbon in preference to chromium, prevent intergranular corrosion, and improve corrosion resistance. Nb, V, and Ti can be added alone or in combination with two or more.

[0020] (11) Al: Al (aluminum) is added as a deoxidizer to improve the purity of the steel and improve the processing and forming properties of the stainless steel. However, if a large amount of aluminum is added, the additional effect of Ti will disappear, so the upper limit of the aluminum content is set to 0.05%. On the other hand, in order to obtain sufficient deoxidizing effect, an aluminum content of 0.005% or more is necessary. The preferred aluminum content is 0.005 to 0.05%.

[0021] (12) Cu: The addition of Cu (copper) can improve the strength and toughness and improve the cold working formability. At the same time, the addition of copper can also precipitate ε-copper during the manufacture of stainless steel, so that the surface of the stainless steel has a local copper content ≥5% by area fraction of not less than 20%, which is beneficial to improve the wettability of copper and stainless steel during copper brazing, to ensure that the brazing material uniformly covers the contact surface after melting, and to minimize defects such as shrinkage after solidification of the filler metal, and to improve the overall performance of the weld. When the copper content is less than 0.1%, the improvement effect is microscopic, and the lower limit of the design is 0.1%. Higher copper content is prone to copper brittleness during hot working, and when the copper content exceeds 3.5%, the hot working plasticity decreases and even leads to hot cracking defects, so the present invention selects 0.1 to 3.5%.

[0022] (12) Sn: Sn (tin) is enriched in the passive film on the surface of the stainless steel in the form of metallic tin or tin oxide, which can improve the re-passivation ability of the passive film and reduce the thickness of the passive film, so that the thickness of the surface oxide film of the stainless steel can be controlled in the range of 1 nm to 10 nm, and good stability and corrosion resistance can be achieved. Under the conditions of brazing process, it is beneficial to the reaction of the mixture of borax, boric acid and alkaline fluoride in the filler metal with Sn oxide on the surface of the stainless steel to improve the cleanliness of the base material, improve the wettability and capillary flow of the copper filler. Too high Sn content will also cause rolling liquid cracking defects, and the range of Sn content is controlled in the range of 0.001 to 0.15%.

[0023] (13) Rare earth elements: such as Ce (cerium), Y (yttrium) and the like are added to the stainless steel, which can be slightly dissolved in the matrix, purify the grain boundary, modify the inclusions, uniform the structure, reduce the precipitation and segregation of precipitates in the grain boundary, thereby improving the corrosion resistance and mechanical properties of the steel. Rare earth oxides also have a pinning effect on the base metal, which is beneficial to improve the processing yield of the parts. The addition amount of rare earth elements should not be too much, otherwise the quality of the steel will be deteriorated, therefore the total amount of rare earth elements Ce and Y added in the present patent is controlled in the range of 0.0005 to 0.20%.

[0024] (14) In the present invention, it is required that 5 ≤ (Cu / (Sn+5*Rare earth elements)) ≤ 15, wherein the upper limit value is controlled in order to improve the hot working performance. If the upper limit value exceeds 15, the rare earth elements do not have obvious effect on improving the hot plasticity, which is prone to cause hot rolling edge cracking; if the lower limit is less than 5, too much Sn and rare earth elements are added, which reduces the grain boundary strength and increases the inclusions, affecting the corrosion resistance and processability.

[0025] Further, the weight percentage (%) of C, Si, Mn, Cr, Ni, Mo, Cu, N, Al, Ti in the chemical composition is preferably: C: 0.005~0.02, Si: 0.1~0.75, Mn: 0.2~1.7, Cr: 17~20, Ni: 8.5~11, Mo: 0.005~0.50, Cu: 0.1~3.5, N: 0.005~0.03, Al: 0.005~0.02, Ti: 0.002~0.1.

[0026] Further, the addition of rare earth elements also needs to satisfy: 0.005≤5*rare earth elements≤(Si+0.5*Mn)≤1.8, which improves the corrosion resistance of the stainless steel and reduces the thickness of the surface oxide skin, thereby facilitating the pickling of the stainless steel.

[0027] Further, the chemical composition of the austenitic stainless steel also needs to satisfy: Crep=Cr+2.0Si+5.5V, Niep=Ni+0.5Mn+30C+25N+0.35Cu, wherein 18.5≤Crep≤24, 8≤Niep≤18, and Crep / Niep>1.5. The requirement for the flanging performance is that the flanging height satisfies the condition of ≥5~10mm without cracking at the edge. The flanging performance is mainly determined by the formability of the material, which is related to the nickel equivalent. If the nickel equivalent is too low, the formability is low, which is not conducive to forming processing; if the nickel equivalent is too high, the contribution of Ni and Cu will lead to high cost, and if the nickel equivalent is too high due to the contribution of C and N, it will affect the intergranular corrosion performance. Therefore, the nickel equivalent needs to be controlled. Similarly, the chromium equivalent is also considered for corrosion resistance and cost. If the chromium equivalent is too low, the corrosion resistance is insufficient, and if the chromium equivalent is too high, it will easily lead to the formation of precipitates, thereby reducing the formability and corrosion resistance. In order to ensure the forming performance of the stainless steel, such as the effective height of the flanging of the welded pipe, the alloy design needs to satisfy the following principles: Crep=Cr+2.0Si+5.5V, Niep=Ni+0.5Mn+30C+25N+0.35Cu, wherein 18.5≤Crep≤24, 8≤Niep≤18, and Crep / Niep>1.5.

[0028] The performance requirements of the stainless steel material of the present application are as follows: 125Mpa≤yield strength≤225Mpa; tensile strength: 425Mpa≤tensile strength≤550Mpa; elongation: ≥42%; grain size grade: 5~10 grade.

[0029] The second object of the present application is to provide a manufacturing method of the high corrosion-resistant and easy-to-infiltrate copper brazing austenitic stainless steel, which is based on the formula system of the chemical composition design of the high corrosion-resistant and easy-to-infiltrate copper brazing austenitic stainless steel according to the first object of the present application, and the manufacturing method comprises the following steps: induction furnace smelting, mold casting, skinning forging, hot rolling, solid solution annealing treatment and pickling, cold rolling, cold rolling annealing pickling (the product is used in a cold-rolled state), aging heat treatment, and finally the high corrosion-resistant and easy-to-infiltrate copper brazing austenitic stainless steel is obtained. After the aging heat treatment, the yield strength of the stainless steel material is not less than 125 MPa and not more than 225 MPa, the tensile strength is not less than 425 MPa and not more than 550 MPa, the elongation is not less than 42%, and the grain size is 5-10 levels. At the same time, ε-copper is precipitated in the passivation film on the surface of the stainless steel pipe after the aging heat treatment, and the local copper content on the surface of the stainless steel is not less than 20% in terms of area fraction. The stainless steel has good brazing performance, and can ensure that the stainless steel welded pipe after brazing can withstand a pressure of 18 MPa without leakage. The stainless steel of the present application has a higher pitting corrosion potential and a higher corrosion resistance.

[0030] The manufacturing method of the present application comprises the following steps:

[0031] (1) Induction furnace smelting and mold casting

[0032] The formula system of the present application is melted by an induction furnace, and then a ingot is obtained by mold casting;

[0033] (2) Skin forging

[0034] After the ingot obtained in the above step (1) is cooled and reheated in the furnace, it is heated to 1200±50℃ and held for 100-150 minutes, and then forged into a billet, and then air-cooled or water-cooled after forging;

[0035] (3) Hot rolling and solid solution annealing

[0036] The surface oxide skin of the forged billet is milled off, heated to 1210±30℃ and held for 60-100 minutes, and then rolled to 3-6 mm, and then air-cooled or water-cooled after rolling; and then cold-rolled and annealed after pickling, the annealing temperature is 950-1100℃, and the grain size of the stainless steel plate is controlled to be 5-10 levels;

[0037] (4) Cold rolling and cold rolling annealing pickling

[0038] The steel plate obtained after solid solution annealing is polished on the surface and then cold-rolled for 1-2 times to obtain a steel plate with a thickness of 0.3-2.5 mm, and the total reduction rate of the cold rolling pass is not less than 50%; the cold rolling annealing temperature is 950-1100℃, and the grain size of the stainless steel sheet is controlled to be 6-10 levels, which can ensure the deep drawing performance and improve the corrosion resistance.

[0039] (5) aging heat treatment

[0040] The aging heat treatment temperature is set to 680-820°C, and the aging time is 5-60 minutes, so that a stainless steel plate with a thickness of 0.20-2.5 mm is finally obtained.

[0041] The stainless steel plate prepared by the manufacturing method has good brazing performance, so that the brazed stainless steel pipe can withstand a pressure of 18Mpa without leakage. Meanwhile, the stainless steel has a pitting corrosion resistance potential of 350 or above, and has higher corrosion resistance.

[0042] Further, the cold rolling annealing is preferably performed in a reducing atmosphere, and the annealing time is ≤10 minutes.

[0043] In the forging in step (2), the initial forging temperature is not less than 1160°C, and the final forging temperature is greater than 900°C.

[0044] In the hot rolling in step (3), the initial rolling temperature is greater than 1180°C, and the final rolling temperature is controlled to be above 900°C. Preferably, the hot rolling temperature is 1220-1235°C, and the final rolling temperature is 980-995°C.

[0045] Preferably, the solid solution annealing temperature in step (3) is 1030-1100°C, and the cold rolling annealing temperature in step (4) is 1050-1080°C. Embodiment

[0046] The following describes in detail the embodiment of the high-corrosion-resistance and copper-brazing-infiltration-easy austenitic stainless steel and the manufacturing method thereof:

[0047] The high-corrosion-resistance and copper-brazing-infiltration-easy austenitic stainless steel has the following main chemical components in terms of weight percentage (%): C: 0.002-0.03; Si: 0.1-0.8; Mn: 0.05-2.0; P<0.04; S<0.01; Cr: 15-23; Ni: 6-15; Mo: 0.005-0.50; Cu: 0.1-3.5; N: 0.002-0.20; Al: 0.005-0.05%; one or more of Nb, V and Ti, wherein Nb: 0.001-0.50, V: 0.001-0.2, Ti: 0.001-0.2; rare earth element: 0.0005-0.20, the rare earth element being selected from one or more of Ce and Y; Sn: 0.001-0.15; and the relationship between Cu, Sn and the rare earth element needs to satisfy: 5≤(Cu / (Sn+5*rare earth element))≤15.

[0048] The manufacturing method of the austenitic stainless steel with high corrosion resistance and easy copper brazing infiltration comprises the following steps:

[0049] (1) induction furnace smelting and mold casting

[0050] The formula system of the present application is smelted by an induction furnace, and then a cast ingot is obtained through mold casting;

[0051] (2) skinning forging

[0052] The cast ingot obtained in the above step (1) is cooled and heated in the furnace, and then heated to 1200±50°C and kept for 100-150 minutes, and forged into a billet (about 50×30×300mm square billet); wherein the initial forging temperature is not less than 1160°C, the final forging temperature is greater than 900°C, and air cooling or water cooling is performed after forging;

[0053] (3) hot rolling and solid solution annealing

[0054] The surface oxide skin of the forged billet is milled, heated and kept at 1210±30°C for 60-100 minutes, and then rolled, the initial rolling temperature is greater than 1180°C, the final rolling temperature is controlled to be above 900°C, and rolled to 3-6mm, and then air cooled or water cooled; after cooling, solid solution annealing is performed, the annealing temperature is 950-1100°C, and the grain size of the stainless steel plate is controlled to be 5-10 levels; the annealing surface is subjected to pickling or other polishing methods to obtain a hot-rolled steel plate with a silver surface for cold rolling;

[0055] (4) cold rolling and cold rolling annealing pickling

[0056] The steel plate obtained after solid solution annealing is subjected to surface polishing (to obtain a silver surface) by pickling or other polishing methods, and then used for cold rolling, 1-2 times of cold rolling are performed, and finally a 0.3-2.5mm thick steel plate is obtained, the total reduction rate of the cold rolling pass is ≥50%; the cold rolling annealing temperature is 950-1100°C, and the grain size of the stainless steel sheet is controlled to be 6-10 levels;

[0057] (5) aging heat treatment

[0058] The aging heat treatment temperature is set to 680-820°C, and the aging time is 5-60 minutes, and finally a stainless steel plate with a thickness of 0.20-2.5mm is obtained.

[0059] Part of the test data is as follows:

[0060] The weight percentage (%) of the main chemical components of each group of tests (Example 1-Example 8 and Comparative Example 2, Comparative Example 3) is shown in Table 1, the performance index of the corresponding stainless steel product is shown in Table 2, and the technical parameters of the corresponding manufacturing method are shown in Table 3, and SUS304L stainless steel is used as a comparative test (i.e. Comparative Example 1), and the manufacturing method is the same as the present application.

[0061] Table 1

[0062]

[0063] The following Table 2 is the grain size grade, mechanical property, corrosion resistance and local ε-copper content ≥5% surface area fraction ratio of the ferritic stainless steel of the embodiments 1-5 of the present application.

[0064] Table 2

[0065]

[0066] Generally speaking, the copper tube of the heat exchange tube of the air conditioner usually needs to meet the following requirements: it can withstand 18Mpa pressure without leakage. Therefore, the brazing performance requirement in Table 2 means that the stainless steel welded tube after brazing can withstand 18Mpa pressure without leakage. If the stainless steel welded tube after brazing leaks when subjected to 18Mpa pressure, it does not meet the brazing performance requirement. The applicant also found in the test process that when (Cu / (Sn+5*Ce(Y)) is greater than 15, the effect of rare earth elements on improving thermal plasticity is not obvious, which is easy to cause hot rolling edge cracking, so it is not used.

[0067] The manufacturing process conditions of each experimental group are shown in Table 3.

[0068] Table 3

[0069]

[0070] Since the mechanical properties of Comparative Examples 1-3 cannot meet the requirements of the present application (in order to improve the processing performance of the material, the present application needs to obtain an austenitic stainless steel with lower mechanical strength), there is no need for aging treatment, so Comparative Examples 1-3 do not perform aging treatment after cold rolling, annealing and pickling.

[0071] The present application has the advantages that: by adding Sn, Ce and Cu elements in the stainless steel, the thickness of the surface passivation film of the metal matrix is reduced and the corrosion resistance is improved. By adding V, Nb, Ti and other elements, the corrosion resistance can be further improved. The yield strength of the stainless steel material obtained by the present application is not less than 125 MPa and not more than 225 MPa, the tensile strength is not less than 425 MPa and not more than 550 MPa, the elongation is greater than or equal to 42%, and the grain size is 5-10 levels. After annealing, the stainless steel coil is subjected to aging heat treatment with a temperature setting of 680-820 DEG C and an aging time of 5-60 minutes. The ε-copper is precipitated in the surface passivation film of the stainless steel, so that the surface with a copper content of ≥5% accounts for not less than 20% in area fraction, realizing good corrosion resistance, formability and brazing wettability. The brazed stainless steel pipe can withstand a pressure of 18 MPa without leakage. The part yield rate, safety and service life are improved. Compared with the copper pipe, the stainless steel of the present application not only has the advantages of lower production cost and price and better corrosion resistance, but also has good copper brazing wettability, can better adapt to the whole production line of the air conditioner (such as brazing welding process and copper heat exchange pipe connecting parts), and can replace the copper pipe as the air conditioner heat exchange pipe without making too many adjustments to the production line, saving cost.

[0072] Further, the weight percentage (%) of C, Si, Mn, Cr, Ni, Mo, Cu, N, Al and Ti in the chemical composition of the stainless steel of the present application is preferably: C: 0.005-0.02, Si: 0.1-0.75, Mn: 0.2-1.7, Cr: 17-20, Ni: 8.5-11, Mo: 0.005-0.50, Cu: 0.1-3.5, N: 0.005-0.03, Al: 0.005-0.02, and Ti: 0.002-0.1.

[0073] Further, the addition of Ce also needs to satisfy: 0.005≤5*Ce≤(Si+0.5*Mn)≤1.8, which improves the corrosion resistance of the stainless steel and reduces the surface oxide skin thickness, which is beneficial to the pickling of the stainless steel.

[0074] Further, the chemical composition of the austenitic stainless steel also needs to satisfy: Crep=Cr+2.0Si+5.5V, Niep=Ni+0.5Mn+30C+25N+0.35Cu, wherein 18.5≤Crep≤24, 8≤Niep≤18, and Crep / Niep>1.5. The requirement for the flanging performance is that the flanging height satisfies the condition of ≥5-10 mm without cracking at the edge. Compared with the conventional corrosion-resistant stainless steel, the austenitic stainless steel of the present application has further reduced strength on the basis of maintaining good corrosion resistance, improves the flanging performance and brazing performance of the welded pipe axial opening, and greatly improves the welding processing efficiency.

[0075] The present application can be implemented in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the application should be determined not with reference to the above description but instead with reference to the appended claims.

Claims

1. A highly corrosion-resistant and easily brazed austenitic stainless steel, characterized in that, The weight percentage (%) of its main chemical components is as follows: C: 0.002~0.03; Si: 0.1~0.8; Mn: 0.05~2.0; P<0.04; S<0.01; Cr: 15~23; Ni: 6~15; Mo: 0.005~0.50; Cu: 0.1~3.5; N: 0.002~0.20; Al: 0.005~0.05%; one or more of Nb, V, and Ti, wherein Nb: 0.001~0.50, V: 0.001~0.2, Ti: 0.001~0.2; rare earth elements: 0.0005~0.05, wherein the rare earth element is selected from one or more of Ce and Y; Sn: 0.001~0.15; and the relationship between Cu, Sn and rare earth element must satisfy: 5≤(Cu / (Sn+5*rare earth element))≤15.

2. The highly corrosion-resistant and easily brazed austenitic stainless steel according to claim 1, characterized in that, The weight percentages (%) of C, Si, Mn, Cr, Ni, Mo, Cu, N, Al, and Ti in the chemical composition are as follows: C: 0.005~0.02, Si: 0.1~0.75, Mn: 0.2~1.7, Cr: 17~20, Ni: 8.5~11, Mo: 0.005~0.50, Cu: 0.1~3.5, N: 0.005~0.03, Al: 0.005~0.02, Ti: 0.002~0.

1.

3. The highly corrosion-resistant and easily brazed austenitic stainless steel according to claim 1, characterized in that, The addition of rare earth elements must also meet the following condition: 0.005 ≤ 5 * rare earth elements ≤ (Si + 0.5 * Mn) ≤ 1.

8.

4. The highly corrosion-resistant and easily brazed austenitic stainless steel according to claim 1, characterized in that, The chemical composition of the austenitic stainless steel also needs to meet the following requirements: Crep=Cr+2.0Si+5.5V, Niep=Ni+0.5Mn+30C+25N+0.35Cu, where 18.5≤Crep≤24, 8≤Niep≤18, and Crep / Niep>1.

5.

5. The highly corrosion-resistant and easily brazed austenitic stainless steel according to claim 1, characterized in that, The properties of the austenitic stainless steel material are: 125 MPa ≤ yield strength ≤ 225 MPa; tensile strength: 425 MPa ≤ tensile strength ≤ 550 MPa; Elongation: ≥42%; Grain size grade: 5~10.

6. A method for manufacturing austenitic stainless steel with high corrosion resistance and easy copper brazing, characterized in that: The formulation system designed according to the chemical composition of the highly corrosion-resistant and easily brazed austenitic stainless steel according to any one of claims 1 to 5, the manufacturing method comprising the following steps: induction furnace smelting—ingot casting—peeling and forging—hot rolling—solution annealing and pickling—cold rolling—cold rolling annealing and pickling—aging heat treatment, finally obtaining the highly corrosion-resistant and easily brazed austenitic stainless steel. Weld-wetted austenitic stainless steel.

7. The method for manufacturing austenitic stainless steel with high corrosion resistance and easy copper brazing according to claim 6, characterized in that, The specific steps are as follows: (1) Induction furnace smelting and ingot casting The above-mentioned formula system is smelted in an induction furnace and then cast into ingots. (2) Peeling and forging After cooling the ingot obtained in step (1), heat it in the furnace to 1200±50°C and hold it for 100~150 minutes. Then forge it into a steel billet and air-cool or water-cool it after forging. (3) Hot rolling and solution annealing After milling off the surface oxide scale of the forged steel billet, it is heated at 1210±30°C and held for 60~100 minutes before rolling to 3~6mm. After rolling, it is air-cooled or water-cooled. After cooling, it is solution annealed at 950~1100°C, and the grain size of the stainless steel plate is controlled at 5-10 grades. (4) Cold rolling and cold rolling annealing After pickling, solution annealing and steel plate, the surface is polished and then cold rolled. The cold rolling is carried out 1 to 2 times to finally obtain a steel plate with a thickness of 0.3 to 2.5 mm. The total reduction rate of the cold rolling passes is ≥50%. The cold rolling annealing temperature is 950 to 1100°C, and the grain size of the stainless steel sheet is controlled at 6 to 10 levels. (5) Aging heat treatment The aging heat treatment temperature is set at 680~820°C, and the aging time is 5~60 minutes, finally obtaining a stainless steel plate with a thickness of 0.20~2.5mm.

8. The method for manufacturing austenitic stainless steel with high corrosion resistance and easy copper brazing according to claim 7, characterized in that, The cold rolling annealing is carried out in a reducing atmosphere, and the annealing time is ≤10 minutes.

9. The method for manufacturing austenitic stainless steel with high corrosion resistance and easy copper brazing according to claim 7, characterized in that: In step (2), the initial forging temperature is not lower than 1160°C and the final forging temperature is greater than 900°C.

10. The method for manufacturing austenitic stainless steel with high corrosion resistance and easy copper brazing according to claim 7, characterized in that: The hot rolling temperature of step (3) is 1220~1235°C, and the final rolling temperature is 980~995°C; the solution annealing temperature of step (3) is 1030~1100°C; and the cold rolling annealing temperature of step (4) is 1050~1080°C.

Images