A kind of heat-resistant stainless steel for high-end electric heating pipe and its manufacturing method

Abstract

The present invention discloses a method for manufacturing heat-resistant stainless steel for high-end electric heating pipes, comprising the following steps: S1: smelting and continuous casting: smelting by the "EAF+AOD+VOD" method, and then continuous casting to form a cast billet; S2: heating and heat preservation of the cast billet: heating and heat preservation of the cast billet by a heating furnace; S3: hot rolling rough rolling: rough rolling mill is used to perform 5 to 13 passes of rough rolling and rolling of the cast billet, wherein the rough rolling outlet temperature is not less than 1020°C; the deformation of the first and second rough rolling passes is controlled at 15 to 22%; S4: hot rolling finishing rolling: 5 to 11 passes of finishing rolling and rolling of the cast billet, wherein the finishing rolling outlet temperature is not less than 960°C; S5: coiling: the cast billet after finishing rolling is cooled to below 650°C at a rate greater than 20°C / S for coiling to obtain a coil; S6: annealing and pickling to obtain heat-resistant stainless steel. The present invention has the advantages of simple process, high yield rate, low cost and excellent performance.

Description

Technical Field

[0001] The invention relates to the technical field of heat-resistant stainless steel production, and in particular to heat-resistant stainless steel for electric heating pipes and a preparation method thereof. Background Art

[0002] Stainless steel is an indispensable and important material in modern industry. Stainless steel is widely used in aerospace, automobile manufacturing, mechanical processing and other fields. With the development of industrial technology, the requirements for materials are getting higher and higher, especially heat-resistant stainless steel. Electric heating tubes are commonly used parts for industrial-grade heat treatment furnaces, fusible metal forging heating furnaces, gas heat treatment furnaces, and dryer equipment. The operating conditions of high-end electric heating tubes are generally 600-1000℃, and heat-resistant stainless steel or alloy materials are required. These materials not only need to have mechanical properties such as high strength and high hardness, but also need to have good high temperature resistance, oxidation resistance, corrosion resistance and other properties.

[0003] In the existing technology, due to the presence of easily oxidized elements such as Al and Ti in heat-resistant steel, the preparation of heat-resistant alloy materials mainly adopts vacuum induction melting, electroslag remelting, vacuum electron beam melting and other methods. These methods can effectively improve the purity and performance of the materials, but the equipment cost is high, the production process is complicated, the energy consumption is large, and the production efficiency is low; secondly, the existing materials still have certain limitations in high temperature resistance, oxidation resistance, corrosion resistance, etc., and it is difficult to meet the needs of certain special application scenarios; in addition, after using Ni, Cr and other alloys to improve oxidation resistance, due to the high high temperature strength, deformation rolling is difficult, cracks and cracking are more, and the rolling yield is low.

[0004] Therefore, how to develop a heat-resistant stainless steel material with low cost, simple process, high yield rate and excellent performance is the main problem facing current technology. Summary of the invention

[0005] The first object of the present invention is to provide a high-end heat-resistant stainless steel for electric heating tubes which has low cost, simple process, high yield rate, excellent surface quality, and excellent yield strength and tensile strength.

[0006] To achieve the above object, the present invention adopts the following technical solutions: 1. A heat-resistant stainless steel for high-end electric heating pipes, characterized in that: its chemical composition comprises, by weight percentage: C: ≤0.06%, Mn: 0.3-0.8%, Si: 0.35-0.75%, P: ≤0.04%, S: ≤0.002%, Cr: 19-22%, Ni: 18-22%, Al: 0.15-0.6%, Ti: 0.15-0.6%, N: 0.01-0.1%, and the rest are iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is ≤1500;

[0007] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0008] The heat-resistant stainless steel has a yield strength of not less than 130 MPa at 600°C and a tensile strength of not less than 300 MPa;

[0009] The yield strength of the heat-resistant stainless steel at 800° C. is not less than 80 MPa, and the tensile strength is not less than 160 MPa.

[0010] The second object of the present invention is to provide a method for manufacturing high-end heat-resistant stainless steel for electric heating tubes with low cost, simple process, high yield rate, excellent surface quality, and excellent yield strength and tensile strength.

[0011] To achieve the above object, the present invention adopts the following technical solution: a method for manufacturing heat-resistant stainless steel for high-end electric heating tubes, comprising the following steps:

[0012] S1: Smelting and continuous casting: The “EAF+AOD+VOD” method is used for smelting, and then continuous casting is formed into a cast billet; the chemical composition of the cast billet composition comprises, by weight percentage: C: ≤0.06%, Mn: 0.3-0.8%, Si: 0.35-0.75%, P: ≤0.04%, S: ≤0.002%, Cr: 19-22%, Ni: 18-22%, Al: 0.15-0.6%, Ti: 0.15-0.6%, N: 0.01-0.1%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is ≤1500;

[0013] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0014] S2: Heating and insulation of the ingot: The ingot is heated and insulated by a heating furnace. The temperature of the heating furnace's soaking zone is 1220-1280°C, and the ingot is kept in the furnace for no less than 180 minutes.

[0015] S3: hot rolling rough rolling: the rough rolling mill is used to perform 5 to 13 passes of rough rolling on the ingot, wherein the rough rolling outlet temperature is not less than 1020°C; and the deformation of the first and second rough rolling passes are controlled at 15 to 22%;

[0016] S4: hot rolling and finishing rolling: the ingot is subjected to 5 to 11 passes of finishing rolling, wherein the finishing rolling outlet temperature is not less than 960°C;

[0017] S5: Coiling: Cool the finished billet to below 650°C at a rate of more than 20°C / S to obtain a coil;

[0018] S6: Annealing and pickling: annealing the coil at a temperature of 1050-1200°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide scale to obtain heat-resistant stainless steel;

[0019] The heat-resistant stainless steel has a yield strength of not less than 130 MPa at 600°C and a tensile strength of not less than 300 MPa;

[0020] The yield strength of the heat-resistant stainless steel at 800° C. is not less than 80 MPa, and the tensile strength is not less than 160 MPa.

[0021] Furthermore, in the aforementioned method for manufacturing heat-resistant stainless steel for high-end electric heating tubes, after heating and heat-insulating the ingot and before hot rolling and rough rolling, the surface oxide scale of the ingot needs to be removed, and the surface oxide scale of the ingot is removed by a high-pressure dephosphorization machine, and the dephosphorization pressure is 25MPa.

[0022] Furthermore, the aforementioned method for manufacturing heat-resistant stainless steel for high-end electric heating tubes, wherein: during the hot rolling rough rolling process, the rough rolling mill first performs 1 to 3 passes of rough rolling on the ingot to obtain an intermediate ingot, and then the intermediate ingot is reheated in a heating furnace according to the S2 process and then sent to the rough rolling mill for the remaining passes of rough rolling, followed by hot rolling, finishing rolling and coiling.

[0023] Furthermore, in the aforementioned method for manufacturing heat-resistant stainless steel for high-end electric heating tubes, during the hot rolling and rough rolling process, before the rough rolling intermediate billet is heated again, the surface of the intermediate billet needs to be ground to remove surface cracks.

[0024] Furthermore, in the aforementioned method for manufacturing heat-resistant stainless steel for high-end electric heating tubes, in which: in S6, the concentration of nitric acid in the mixed acid solution is 70~120 mg / L, and the concentration of hydrofluoric acid is 20~40 mg / L.

[0025] Through the implementation of the above technical scheme, the beneficial effects of the present invention are: (1) The heat-resistant stainless steel of the present invention has excellent surface quality and excellent performance, and the yield strength at 600°C is not less than 130MPa, and the tensile strength is not less than 300MPa; the yield strength at 800°C is not less than 80MPa, and the tensile strength is not less than 160MPa; (2) The manufacturing method of the present invention has simple process, high yield rate and low cost. The heat-resistant stainless steel manufactured not only has excellent surface quality and no marble pattern, but also has excellent performance. The yield strength at 600°C is not less than 130MPa, and the tensile strength is not less than 300MPa; the yield strength at 800°C is not less than 80MPa, and the tensile strength is not less than 160MPa. The heat-resistant stainless steel manufactured is particularly suitable for making high-end electric heating pipes. DETAILED DESCRIPTION

[0026] A heat-resistant stainless steel for high-end electric heating tubes, wherein the chemical composition comprises, by weight percentage: C: ≤0.06%, Mn: 0.3-0.8%, Si: 0.35-0.75%, P: ≤0.04%, S: ≤0.002%, Cr: 19-22%, Ni: 18-22%, Al: 0.15-0.6%, Ti: 0.15-0.6%, N: 0.01-0.1%, and the rest is iron and other inevitable impurities; and the chemical composition related value TN expressed by formula (1) is ≤1500;

[0027] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0028] The heat-resistant stainless steel has a yield strength of not less than 130 MPa at 600°C and a tensile strength of not less than 300 MPa;

[0029] The yield strength of the heat-resistant stainless steel at 800° C. is not less than 80 MPa, and the tensile strength is not less than 160 MPa.

[0030] The present invention also relates to a method for manufacturing heat-resistant stainless steel for high-end electric heating pipes, comprising the following steps:

[0031] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a 180-220 mm thick ingot; the temperature of the molten steel during casting is between 1450 and 1500 °C; the chemical composition of the ingot composition comprises, by weight percentage: C: ≤0.06%, Mn: 0.3-0.8%, Si: 0.35-0.75%, P: ≤0.04%, S: ≤0.002%, Cr: 19-22%, Ni: 18-22%, Al: 0.15-0.6%, Ti: 0.15-0.6%, N: 0.01-0.1%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is ≤1500;

[0032] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0033] S2: Heating and insulation of ingot: The ingot is heated and insulated by a heating furnace. The temperature of the heating furnace's equalizing zone is 1220-1280°C, and the ingot is kept in the furnace for no less than 180 minutes. The heating of the heating furnace can make the material uniform on the one hand, and ensure sufficient slab temperature to reduce the resistance of hot rolling on the other hand. However, if the temperature of the heating furnace is higher than 1280°C, oxidation will be serious, so the temperature of the heating furnace is between 1220 and 1280°C.

[0034] Preferably, after heating and heat preservation of the ingot and before hot rolling and rough rolling, the surface oxide scale of the ingot needs to be removed, and the surface oxide scale of the ingot is removed by a high-pressure dephosphorization machine, and the dephosphorization pressure is 25MPa;

[0035] S3: hot rolling rough rolling: the rough rolling mill is used to perform 5 to 13 rough rollings on the ingot, wherein the rough rolling outlet temperature is not less than 1020°C; and the deformation of the first and second rough rollings are controlled at 15 to 22%; the product of the present invention has no phase change in the high-temperature heating stage, has a high alloy content, a high temperature during the initial rolling, and a relatively small rolling resistance, so a reduction rate of not less than 15% is set to reduce the subsequent rolling pressure, but due to the absence of lubrication of the ferrite second phase, there may still be a trace amount of low-melting point inclusions or precipitates at the grain boundary, and the strength of the grain boundary at high temperature is less than the strength within the grain. In the case of an excessively high reduction rate, the surface layer of the slab will crack under the action of the rolling tangential stress, so the upper limit of the rolling reduction rate is controlled to be 22%;

[0036] Preferably, during the hot rolling rough rolling process, the rough rolling mill first performs 1 to 3 passes of rough rolling on the cast billet to obtain an intermediate billet with a thickness of 140 to 160 mm, and then the intermediate billet is heated again in a heating furnace according to the S2 process before being sent to the rough rolling mill for the remaining passes of rough rolling. The purpose of the secondary heating is, on the one hand, to increase the rolling temperature of the slab to compensate for the temperature drop after rolling and reduce the rolling resistance, and on the other hand, to check the surface quality of the slab after rough rolling to avoid large cracks and the risk of strip breakage during rolling;

[0037] Preferably, during the hot rolling rough rolling process, before the rough rolling intermediate billet is heated again, the surface of the intermediate billet needs to be ground to remove surface cracks;

[0038] S4: hot rolling and finishing rolling: the cast slab is subjected to 5 to 11 passes of finishing rolling, wherein the finishing rolling outlet temperature is not less than 960°C to ensure the rollability of the slab;

[0039] Preferably, the hot rolling and finishing rolling is carried out on a Steckel mill. Compared with a continuous rolling mill, the interval between two rolling passes of the Steckel mill is longer. During this time, after the dynamic recrystallization of the grains, the rolling resistance will decrease, which is more conducive to rolling forming.

[0040] S5: Coiling: Cool the finished ingot to below 650℃ at a rate of more than 20℃ / S to obtain a coil. If the supercooling rate is too slow, high chromium brittle phase will easily precipitate in the temperature range of 650~900℃. If the coiling temperature is too low, the coiling resistance will be too large and it will be difficult to coil.

[0041] S6: Annealing and pickling: annealing the coil at a temperature of 1050-1200°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove oxide scale after annealing, wherein the nitric acid concentration in the mixed acid solution is 70-120 mg / L, and the hydrofluoric acid concentration is 20-40 mg / L; heat-resistant stainless steel is obtained;

[0042] The heat-resistant stainless steel has a yield strength of not less than 130 MPa at 600°C and a tensile strength of not less than 300 MPa;

[0043] The yield strength of the heat-resistant stainless steel at 800° C. is not less than 80 MPa, and the tensile strength is not less than 160 MPa.

[0044] The following specifically explains the reasons for limiting the composition of heat-resistant stainless steel for high-end electric heating tubes:

[0045] The content of C (carbon) is ≤0.06%; carbon mainly forms carbides, which are one of the important strengthening phases in steel. The type, composition, quantity, size, morphology and distribution of carbides have a decisive influence on the performance of steel. Since the Ti content of the alloy of the present invention is relatively high, TiC is first formed in the initial crystallizer solidified shell. TiC is a brittle phase, which is densely distributed at the grain boundary, causing the slab cracks to expand inward along the grain boundary, resulting in deeper longitudinal cracks. In addition, the electric heating tube is used at high temperature for a long time, and the intergranular corrosion resistance is improved, so C is required to be as low as possible; therefore, in order to ensure good corrosion resistance, weldability and processability, the present invention determines the content of carbon element to be ≤0.06%.

[0046] The content of chromium (Cr) is 19-22%. Chromium is the main element that determines the corrosion resistance of steel. A small amount of chromium can only improve the corrosion resistance of steel, but cannot make it rust-proof. Chromium increases the electrode potential of the solid solution and forms a dense oxide film on the surface. Chromium can effectively increase the pitting potential value of steel and reduce the sensitivity of steel to pitting. Chromium is an element that strongly forms and stabilizes ferrite and reduces the austenite zone. However, as the chromium content increases, the tendency of some intermetallic compounds to precipitate increases. The presence of these intermetallic compounds will significantly reduce the plasticity and toughness of steel. Therefore, the present invention determines the content of chromium to be 19-22%.

[0047] The content of nickel (Ni) is 18-22%. Nickel is an austenite-forming element, which expands the austenite stable zone. As the nickel content in the steel increases, the austenite phase zone moves toward the high chromium direction, that is, the chromium in the steel can be increased without forming a single ferrite structure. At the same time, nickel is also an important element for improving heat resistance and high temperature strength. However, excessive addition will lead to a sharp increase in raw material costs, which does not reduce production costs. Therefore, the present invention determines the content of nickel to be 18-22%.

[0048] The content of manganese (Mn) is 0.3-0.8%. Mn is an element that expands the austenite zone. The ability of Mn to stabilize the austenite structure in steel is second only to Ni, and it is an element that strongly improves the hardenability of steel. Therefore, Mn can partially replace Ni. However, a large amount of manganese addition will damage the oxidation resistance, and will slightly reduce the chromium equivalent, affecting the corrosion resistance. Therefore, the present invention determines the content of manganese to be 0.3-0.8%.

[0049] The content of silicon (Si) is 0.35-0.75%. Silicon is a strong ferrite strengthening element. Silicon has a significant effect on improving the corrosion resistance of iron-based and nickel-based corrosion-resistant alloys in strong oxidizing media. At high temperatures or in strong oxidizing media, adding a certain amount of silicon to steel can form a silicon-rich surface layer SiO2 on the surface, thereby significantly improving the oxidation resistance or corrosion resistance of the steel. Adding silicon also has a certain effect on sulfuric acid corrosion resistance. Adding silicon can also inhibit the tendency of pitting corrosion in chloride ion media. Silicon element helps deoxidation and has the effect of adjusting the oxygen concentration to below 50ppm. However, at the same time, too high a silicon content will also cause smelting segregation and decreased ductility. Therefore, the present invention determines the content of silicon element to be 0.35-0.75%.

[0050] The content of titanium (Ti) is 0.15-0.6%. Titanium is a necessary element because it is a property required for high-end electric heating tubes. In other words, it is an effective element in the formation of a dense and highly emissive blackened film. However, when added in excess, TiN inclusions will form and cause surface defects. TiN inclusions are inclusions attached to the inner wall of the immersion nozzle and are harmful. When inclusions are attached to the immersion nozzle, they will also promote the formation of heavier attached deposits of the base metal, which will fall off and be transported into the mold together with the molten alloy, captured by the solidified shell, and become the cause of surface defects. Therefore, the present invention determines the titanium content to be 0.15-0.6%.

[0051] The content of aluminum (Al) is 0.15-0.6%. Aluminum is a required property for high-end electric heating tubes, so it is a necessary element. It is an effective element for forming a dense and highly emissive blackened film. Moreover, aluminum is usually added to steel as a deoxidizer. It is an important element for deoxidation. It has the function of adjusting the oxygen concentration to below 50ppm, and also has the function of controlling oxide inclusions. Al will also form precipitation phases with Ni and Ti to improve the strength of stainless steel, but aluminum will also strongly reduce the austenite phase region in steel. Therefore, the present invention determines the content of aluminum to be 0.15-0.6%.

[0052] The content of phosphorus (P) is ≤0.04%. Phosphorus is a harmful element that segregates at grain boundaries and causes cracks during hot working, so it should be reduced as much as possible. Therefore, the present invention determines the content of phosphorus to be ≤0.04%.

[0053] The sulfur (S) content is ≤0.002%. Sulfur is a harmful element that segregates at grain boundaries to form low-melting point compounds and causes thermal cracking during manufacturing. Therefore, it should be reduced as much as possible. Therefore, the present invention determines the sulfur content to be ≤0.002%.

[0054] The content of N (nitrogen) is 0.01-0.1%. Nitrogen is also an element that stabilizes austenite and can improve the corrosion resistance of steel. It can also improve the strength of the material, so the lower limit of N is set at 0.01%. However, too high a nitrogen content, on the one hand, is easy to form TiN inclusions with Ti to block the water outlet, and on the other hand, it is easy to cause the material to have poor toughness during hot rolling, making it difficult to roll and not conducive to hot processing. Therefore, the upper limit of nitrogen is set at 0.1%.

[0055] The rest of the steel is Fe (iron) and some inevitable impurities.

[0056] The content and function of the individual elements in the above composition are not the key points of the design of the present invention, but are played by the overall coordinated combination of the contents of multiple elements. At the same time, by controlling the melting temperature of titanium nitride during casting, the influence of titanium nitride precipitation on the casting process and the surface of the product during casting is evaluated, and the formula for the melting temperature of titanium nitride inclusions expressed by formula (1) is obtained. The chemical composition related value TN expressed by formula (1) is ≤1500;

[0057] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0058] When TN>1500, the melting point of TiN in the molten steel is too high, and large TiN inclusions will exist in the molten steel during casting. TiN will easily block the water inlet during casting, and will also remain on the surface of the ingot to form scars or marble patterns. The precipitation of large-sized TiN will also reduce the toughness of the material. Therefore, in the present invention, TN is controlled at 1500 or below.

[0059] The present invention is further described in detail below through specific examples and comparative examples. In the following examples and comparative examples, the TN value is rounded to the nearest unit; and when the TN value is specifically calculated, according to industry rules, the values ​​of the components in the TN value formula are directly used within the percentage content, and the values ​​are not converted without the percentage sign. For example, when the component C is 0.015%, the value 0.015 is directly used in the formula. Example 1

[0060] A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes comprises the following steps:

[0061] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a cast billet; the temperature of the molten steel during casting is 1480°C; the chemical composition of the cast billet composition comprises, by weight percentage: C: 0.015%, Mn: 0.5%, Si: 0.35%, P: 0.03%, S: 0.002%, Cr: 19%, Ni: 20%, Al: 0.35%, Ti: 0.48%, N: 0.055%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is 1500;

[0062] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0063] S2: Heating and insulation of the ingot: The ingot is heated and insulated in a heating furnace. The temperature of the heating furnace's soaking zone is 1280°C, and the ingot is in the furnace for 240 minutes.

[0064] After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot is removed. The ingot surface oxide scale removal is carried out by a high-pressure dephosphorization machine with a dephosphorization pressure of 30 MPa.

[0065] S3: Hot rolling and rough rolling: The rough rolling mill is used to perform three passes of rough rolling on the ingot to obtain an intermediate billet, and the deformation of the first rough rolling is controlled at 17%, and the deformation of the second rough rolling is controlled at 18%; then the surface of the intermediate billet is ground to remove surface cracks, and then the intermediate billet is heated again in a heating furnace according to the S2 process and then sent to the rough rolling mill for two passes of rough rolling; the rough rolling outlet temperature is 1065℃;

[0066] S4: hot rolling and finishing rolling: the cast slab is subjected to 8 passes of finishing rolling by a Steckel mill, wherein the finishing rolling outlet temperature is 1005°C to ensure the rollability of the slab;

[0067] S5: Coiling: Cool the finished billet to 630°C at a rate of 30°C / S and coil it to obtain a coil;

[0068] S6: Annealing and pickling: annealing the coil at 1050°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide scale, wherein the nitric acid concentration in the mixed acid solution is 80 mg / L and the hydrofluoric acid concentration is 30 mg / L; heat-resistant stainless steel with good surface quality is obtained;

[0069] The heat-resistant stainless steel has a yield strength of 150 MPa at 600°C and a tensile strength of 325 MPa;

[0070] The heat-resistant stainless steel has a yield strength of 102 MPa at 800° C. and a tensile strength of 180 MPa. Example 2

[0071] A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes comprises the following steps:

[0072] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a cast billet; the temperature of the molten steel during casting is 1450°C; the chemical composition of the cast billet composition comprises, by weight percentage: C: 0.03%, Mn: 0.8%, Si: 0.6%, P: 0.03%, S: 0.001%, Cr: 21.1%, Ni: 22%, Al: 0.15%, Ti: 0.6%, N: 0.06%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition correlation value TN expressed by formula (1) is 1498;

[0073] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0074] S2: Heating and insulation of the ingot: The ingot is heated and insulated in a heating furnace. The temperature of the heating furnace's soaking zone is 1270°C, and the ingot is kept in the furnace for 250 minutes.

[0075] After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot is removed. The ingot surface oxide scale removal is carried out by a high-pressure dephosphorization machine with a dephosphorization pressure of 30 MPa.

[0076] S3: Hot rolling and rough rolling: The rough rolling mill is used to perform 5 passes of rough rolling on the ingot to obtain the intermediate billet. The deformation of the first rough rolling is controlled at 15%, and the deformation of the second rough rolling is controlled at 19%. Then the surface of the intermediate billet is ground to remove surface cracks, and then the intermediate billet is heated again in the heating furnace according to the S2 process and then sent to the rough rolling mill for 8 passes of rough rolling. The rough rolling outlet temperature is 1035℃.

[0077] S4: hot rolling and finishing rolling: the cast slab is subjected to 5 passes of finishing rolling by a Steckel mill, wherein the finishing rolling outlet temperature is 1015°C to ensure the rollability of the slab;

[0078] S5: Coiling: Cool the finished billet to 600°C at a rate of 40°C / S to obtain a coil;

[0079] S6: Annealing and pickling: annealing the coil at 1100°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide scale, wherein the nitric acid concentration in the mixed acid solution is 70 mg / L and the hydrofluoric acid concentration is 40 mg / L; heat-resistant stainless steel with good surface quality is obtained;

[0080] The heat-resistant stainless steel has a yield strength of 167 MPa at 600°C and a tensile strength of 350 MPa;

[0081] The heat-resistant stainless steel has a yield strength of 118 MPa at 800° C. and a tensile strength of 215 MPa. Example 3

[0082] A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes comprises the following steps:

[0083] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a cast billet; the temperature of the molten steel during casting is 1500°C; the chemical composition of the cast billet composition comprises, by weight percentage: C: 0.019%, Mn: 0.5%, Si: 0.51%, P: 0.02%, S: 0.001%, Cr: 19.5%, Ni: 19%, Al: 0.6%, Ti: 0.15%, N: 0.01%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is 1375;

[0084] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0085] S2: Heating and insulation of the ingot: The ingot is heated and insulated in a heating furnace. The temperature of the heating furnace's soaking zone is 1280°C, and the ingot is in the furnace for 240 minutes.

[0086] After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot is removed. The ingot surface oxide scale removal is carried out by a high-pressure dephosphorization machine with a dephosphorization pressure of 30 MPa.

[0087] S3: Hot rolling rough rolling: The rough rolling mill is used to perform 4 passes of rough rolling on the ingot to obtain the intermediate billet, and the deformation of the first pass of rough rolling is controlled at 18%, and the deformation of the second pass of rough rolling is controlled at 22%; then the surface of the intermediate billet is ground to remove surface cracks, and then the intermediate billet is heated again in the heating furnace according to the S2 process and then sent to the rough rolling mill for 7 passes of rough rolling; the rough rolling outlet temperature is 1070℃;

[0088] S4: hot rolling and finishing rolling: the cast slab is subjected to 9 passes of finishing rolling by a Steckel mill, wherein the finishing rolling outlet temperature is 1002°C to ensure the rollability of the slab;

[0089] S5: Coiling: Cool the finished ingot to 590°C at a rate of 25°C / S and coil it to obtain a coil;

[0090] S6: Annealing and pickling: annealing the coil at 1200°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide scale, wherein the nitric acid concentration in the mixed acid solution is 120 mg / L and the hydrofluoric acid concentration is 20 mg / L; heat-resistant stainless steel with good surface quality is obtained;

[0091] The heat-resistant stainless steel has a yield strength of 130 MPa at 600°C and a tensile strength of 306 MPa;

[0092] The heat-resistant stainless steel has a yield strength of 84 MPa at 800° C. and a tensile strength of 160 MPa. Example 4

[0093] A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes comprises the following steps:

[0094] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a cast billet; the temperature of the molten steel during casting is 1460°C; the chemical composition of the cast billet composition comprises, by weight percentage: C: 0.06%, Mn: 0.3%, Si: 0.48%, P: 0.04%, S: 0.001%, Cr: 22%, Ni: 21.8%, Al: 0.42%, Ti: 0.55%, N: 0.03%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is 1407;

[0095] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0096] S2: Heating and insulation of the ingot: The ingot is heated and insulated in a heating furnace. The temperature of the heating furnace's soaking zone is 1260°C, and the ingot is in the furnace for 220 minutes.

[0097] After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot is removed by using a high-pressure dephosphorization machine with a dephosphorization pressure of 25 MPa;

[0098] S3: Hot rolling rough rolling: The rough rolling mill is used to perform 5 passes of rough rolling on the ingot to obtain the intermediate billet, and the deformation of the first rough rolling is controlled at 19%, and the deformation of the second rough rolling is controlled at 15%; then the surface of the intermediate billet is ground to remove surface cracks, and then the intermediate billet is heated again in the heating furnace according to the S2 process and then sent to the rough rolling mill for 6 passes of rough rolling; the rough rolling outlet temperature is 1052℃;

[0099] S4: hot rolling and finishing rolling: the cast slab is subjected to 11 passes of finishing rolling by a Steckel mill, wherein the finishing rolling outlet temperature is 985°C to ensure the rollability of the slab;

[0100] S5: Coiling: Cool the finished billet to 620°C at a rate of 25°C / S and coil it to obtain a coil;

[0101] S6: Annealing and pickling: annealing the coil at 1080°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide scale, wherein the nitric acid concentration in the mixed acid solution is 100 mg / L and the hydrofluoric acid concentration is 30 mg / L; heat-resistant stainless steel with good surface quality is obtained;

[0102] The heat-resistant stainless steel has a yield strength of 156 MPa at 600°C and a tensile strength of 338 MPa;

[0103] The heat-resistant stainless steel has a yield strength of 103 MPa at 800° C. and a tensile strength of 198 MPa. Example 5

[0104] A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes comprises the following steps:

[0105] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a cast billet; the temperature of the molten steel during casting is 1480°C; the chemical composition of the cast billet composition comprises, by weight percentage: C: 0.052%, Mn: 0.6%, Si: 0.75%, P: 0.01%, S: 0.002%, Cr: 19.6%, Ni: 18%, Al: 0.28%, Ti: 0.48%, N: 0.044%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition correlation value TN expressed by formula (1) is 1463;

[0106] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0107] S2: Heating and insulation of the ingot: The ingot is heated and insulated in a heating furnace. The temperature of the heating furnace's soaking zone is 1220°C, and the ingot is in the furnace for 180 minutes.

[0108] After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot is removed by using a high-pressure dephosphorization machine with a dephosphorization pressure of 25 MPa;

[0109] S3: hot rolling rough rolling: the rough rolling mill is used to perform 7 passes of rough rolling on the ingot, wherein the rough rolling outlet temperature is 1020°C; and the deformation amount of the first pass of rough rolling is controlled at 22%, and the deformation amount of the second pass of rough rolling is controlled at 20%;

[0110] S4: hot rolling and finishing rolling: the cast slab is subjected to 10 passes of finishing rolling by a Steckel mill, wherein the finishing rolling outlet temperature is 960°C to ensure the rollability of the slab;

[0111] S5: Coiling: Cool the finished ingot to below 550°C at a rate of 40°C / S to obtain a coil;

[0112] S6: Annealing and pickling: annealing the coil at 1070°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide scale, wherein the nitric acid concentration in the mixed acid solution is 110 mg / L and the hydrofluoric acid concentration is 23 mg / L; heat-resistant stainless steel with good surface quality is obtained;

[0113] The heat-resistant stainless steel has a yield strength of 153 MPa at 600°C and a tensile strength of 330 MPa;

[0114] The heat-resistant stainless steel has a yield strength of 100 MPa at 800° C. and a tensile strength of 185 MPa.

[0115] Comparative Example 1

[0116] A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes comprises the following steps:

[0117] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a cast billet; the temperature of the molten steel during casting is 1440°C; the chemical composition of the cast billet composition comprises, by weight percentage: C: 0.01%, Mn: 0.5%, Si: 0.6%, P: 0.02%, S: 0.002%, Cr: 19.2%, Ni: 20%, Al: 0.35%, Ti: 0.18%, N: 0.06%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is 1518;

[0118] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0119] S2: Heating and insulation of the ingot: The ingot is heated and insulated in a heating furnace. The temperature of the heating furnace's soaking zone is 1270°C, and the ingot is in the furnace for 240 minutes.

[0120] After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot is removed by using a high-pressure dephosphorization machine with a dephosphorization pressure of 25 MPa;

[0121] S3: Hot rolling and rough rolling: The rough rolling mill is used to perform 4 passes of rough rolling on the ingot to obtain the intermediate billet. The deformation of the first rough rolling is controlled at 18%, and the deformation of the second rough rolling is controlled at 19%. Then the surface of the intermediate billet is ground to remove surface cracks, and then the intermediate billet is heated again in the heating furnace according to the S2 process and then sent to the rough rolling mill for 6 passes of rough rolling. The rough rolling outlet temperature is 1045℃.

[0122] S4: hot rolling and finishing rolling: the cast billet is subjected to 10 passes of finishing rolling by a Steckel mill, wherein the finishing rolling outlet temperature is 985°C;

[0123] S5: Coiling: Cool the finished billet to 650°C at a rate of 20°C / S to obtain a coil;

[0124] S6: Annealing and pickling: annealing the coil at 1060°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove oxide scale, wherein the nitric acid concentration in the mixed acid solution is 90 mg / L and the hydrofluoric acid concentration is 23 mg / L; obtaining heat-resistant stainless steel; the surface of the obtained heat-resistant stainless steel is marbled;

[0125] The heat-resistant stainless steel has a yield strength of 159 MPa at 600°C and a tensile strength of 334 MPa;

[0126] The heat-resistant stainless steel has a yield strength of 112 MPa at 800° C. and a tensile strength of 188 MPa.

[0127] Comparative Example 2

[0128] A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes comprises the following steps:

[0129] S1: Smelting and continuous casting: The smelting is carried out by the method of "EAF (electric arc furnace melting) + AOD (argon oxygen furnace refining) + VOD (vacuum degassing furnace degassing)", and then continuous casting is formed into a cast billet; the temperature of the molten steel during casting is 1460°C; the chemical composition of the cast billet composition comprises, by weight percentage: C: 0.019%, Mn: 0.75%, Si: 0.5%, P: 0.02%, S: 0.001%, Cr: 18.2%, Ni: 10%, Al: 0.003%, Ti: 0.12%, N: 0.048%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is 1490;

[0130] Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273;

[0131] S2: Heating and insulation of the ingot: The ingot is heated and insulated in a heating furnace. The temperature of the heating furnace's soaking zone is 1260°C, and the ingot is in the furnace for 230 minutes.

[0132] After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot is removed. The ingot surface oxide scale removal is carried out by a high-pressure dephosphorization machine with a dephosphorization pressure of 30 MPa.

[0133] S3: hot rolling rough rolling: the rough rolling mill is used to perform 6 passes of rough rolling on the ingot to obtain the intermediate ingot. In this embodiment, during the first rough rolling, the deformation of the first rough rolling is controlled at 23%, which causes the machine to stop due to overload. Therefore, the deformation of the first rough rolling is reset to 18% before continuing the rough rolling.

[0134] The deformation of the second rough rolling is controlled at 18%; then the surface of the intermediate billet is ground to remove surface cracks, and then the intermediate billet is heated again in the heating furnace according to the S2 process and then sent to the rough rolling mill for 7 passes of rough rolling; the rough rolling outlet temperature is 1048℃;

[0135] S4: hot rolling and finishing rolling: the cast billet is subjected to 7 passes of finishing rolling by a Steckel mill, wherein the finishing rolling outlet temperature is 966°C;

[0136] S5: Coiling: Cool the finished billet to 650°C at a rate of 32°C / S to obtain a coil;

[0137] S6: Annealing and pickling: annealing the coil at 1090°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove oxide scale, wherein the nitric acid concentration in the mixed acid solution is 80 mg / L and the hydrofluoric acid concentration is 35 mg / L; heat-resistant stainless steel is obtained;

[0138] The heat-resistant stainless steel has a yield strength of 117 MPa at 600°C and a tensile strength of 280 MPa;

[0139] The heat-resistant stainless steel has a yield strength of 66 MPa at 800° C. and a tensile strength of 138 MPa.

[0140] In order to more intuitively understand the data changes of each embodiment and comparative example in the present invention, the data of each embodiment and comparative example are explained below by forming a table, wherein the ingot composition and TN value of each product in each embodiment and comparative example are listed in Table 1; the heating temperature, furnace time, dephosphorization pressure, one-pass deformation amount and two-pass deformation amount during hot rough rolling, whether secondary heating is performed during hot rough rolling, and whether surface grinding is performed after secondary heating, hot rough rolling outlet temperature, and hot finishing outlet temperature in each embodiment and comparative example are listed in Table 2; and the surface quality of the product in each embodiment and comparative example, the yield strength and tensile strength at 600°C, and the yield strength and tensile strength at 800°C are listed in Table 3;

[0141]

[0142]

[0143]

[0144] The advantages of the present invention are: (1) the heat-resistant stainless steel of the present invention has excellent surface quality and excellent performance, and the yield strength at 600°C is not less than 130MPa, and the tensile strength is not less than 300MPa; the yield strength at 800°C is not less than 80MPa, and the tensile strength is not less than 160MPa; (2) the manufacturing method of the present invention has simple process, high yield rate and low cost. The heat-resistant stainless steel manufactured not only has excellent surface quality and no marble pattern, but also has excellent performance. The yield strength at 600°C is not less than 130MPa, and the tensile strength is not less than 300MPa; the yield strength at 800°C is not less than 80MPa, and the tensile strength is not less than 160MPa. The heat-resistant stainless steel manufactured is particularly suitable for making high-end electric heating pipes.

[0145] The above description is only a preferred embodiment of the invention and does not constitute any other form of limitation on the invention. Any modification or equivalent change made based on the technical essence of the invention still falls within the scope of protection of the invention.

Claims

1. A heat-resistant stainless steel for high-end electric heating tubes, characterized by: Its chemical composition includes by weight percentage: C: ≤0.06%, Mn: 0.3-0.8%, Si: 0.35-0.75%, P: ≤0.04%, S: ≤0.002%, Cr: 19-22%, Ni: 18-22%, Al: 0.15-0.6%, Ti: 0.15-0.6%, N: 0.01-0.1%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is ≤1500; Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273; The heat-resistant stainless steel has a yield strength of not less than 130 MPa at 600°C and a tensile strength of not less than 300 MPa; The yield strength of the heat-resistant stainless steel at 800° C. is not less than 80 MPa, and the tensile strength is not less than 160 MPa.

2. A method for manufacturing heat-resistant stainless steel for high-end electric heating tubes, characterized in that: The following steps are involved: S1: Smelting and continuous casting: The "EAF+AOD+VOD" method is used for smelting, and then continuous casting is formed into a cast billet; the chemical composition of the cast billet composition includes, by weight percentage: C: ≤0.06%, Mn: 0.3-0.8%, Si: 0.35-0.75%, P: ≤0.04%, S: ≤0.002%, Cr: 19-22%, Ni: 18-22%, Al: 0.15-0.6%, Ti: 0.15-0.6%, N: 0.01-0.1%, and the rest is iron and other inevitable impurities; at the same time, the chemical composition related value TN expressed by formula (1) is ≤1500; Formula (1): TN = 9240 / (5.26 + 0.04 × Ti + 0.022 × Cr - 8.83 × N) - 273; S2: Heating and insulation of the ingot: The ingot is heated and insulated by a heating furnace. The temperature of the heating furnace's soaking zone is 1220-1280°C, and the ingot is kept in the furnace for no less than 180 minutes. S3: hot rolling rough rolling: the rough rolling mill is used to perform 5 to 13 passes of rough rolling on the ingot, wherein the rough rolling outlet temperature is not less than 1020°C; and the deformation of the first and second rough rolling passes are controlled at 15 to 22%; S4: hot rolling and finishing rolling: the ingot is subjected to 5 to 11 passes of finishing rolling, wherein the finishing rolling outlet temperature is not less than 960°C; S5: Coiling: Cool the finished billet to below 650°C at a rate of more than 20°C / S to obtain a coil; S6: Annealing and pickling: annealing the coil at a temperature of 1050-1200°C, and pickling the coil with a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide scale to obtain heat-resistant stainless steel; The heat-resistant stainless steel has a yield strength of not less than 130 MPa at 600°C and a tensile strength of not less than 300 MPa; The yield strength of the heat-resistant stainless steel at 800° C. is not less than 80 MPa, and the tensile strength is not less than 160 MPa.

3. The method for manufacturing heat-resistant stainless steel for high-end electric heating tubes according to claim 2 is characterized in that: After the ingot is heated and kept warm and before the hot rolling and rough rolling, the surface oxide scale of the ingot needs to be removed. The surface oxide scale of the ingot is removed by a high-pressure dephosphorization machine with a dephosphorization pressure of 25 MPa.

4. The method for manufacturing heat-resistant stainless steel for high-end electric heating tubes according to claim 2 is characterized in that: During the hot rolling rough rolling process, the rough rolling mill first performs 1 to 3 passes of rough rolling on the ingot to obtain an intermediate billet, and then the intermediate billet is reheated in the heating furnace according to the S2 process and then sent to the rough rolling mill for the remaining passes of rough rolling, followed by hot rolling, finishing rolling and coiling.

5. The method for manufacturing heat-resistant stainless steel for high-end electric heating tubes according to claim 4 is characterized in that: During the hot rolling rough rolling process, before the rough rolling intermediate billet is heated again, the surface of the intermediate billet needs to be ground to remove surface cracks.

6. The method for manufacturing heat-resistant stainless steel for high-end electric heating tubes according to claim 2 is characterized in that: In S6, the concentration of nitric acid in the mixed acid solution is 70-120 mg / L, and the concentration of hydrofluoric acid is 20-40 mg / L.

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