A preparation process of high niobium stainless steel strip, high niobium stainless steel strip and application thereof

Abstract

The present invention discloses a preparation process of a high-niobium stainless steel strip, the high-niobium stainless steel strip and an application, comprising the following steps: 1) Simulated annealing; 2) First thermal annealing: Pass the black skin coil of the high-niobium stainless steel strip air-cooled to room temperature through an annealing furnace at a speed of 5 mpm, with the temperature of the annealing furnace being 1200 °C. After the high-temperature and low-speed operation, a black skin coil with a grain size of 8.0 is obtained; 3) First mixed acid pickling: Subject the black skin coil treated in 2) to mixed acid pickling with 104 g / L of HNO3 and 45 g / L of HF to obtain a white skin coil with a grain size of 8.0; 4) Second thermal annealing; 5) Second mixed acid pickling to obtain a white skin coil with a grain size of 6.5. The beneficial effect of the present invention is that by adjusting the production process of the thermal annealing operation, the grain size of the 347H stainless steel strip reaches 6-7 grades, and it has high temperature resistance and good intergranular corrosion resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel production, and in particular relates to a preparation process of a high-niobium stainless steel strip, a high-niobium stainless steel strip and an application thereof. Background Art

[0002] 347H is a high-carbon, niobium-chromium-nickel austenitic heat-resistant stainless steel. Due to the stabilizing element niobium (Nb), it has good resistance to intergranular corrosion and polysulfate intergranular stress corrosion. It is mainly used in high-temperature pipes or containers for high-temperature medium transmission. 347H stainless steel has high requirements for high-temperature strength. Since Nb has a strong effect on grain refinement, the grain size is not easy to control during the manufacturing process. When the grains grow, the high-temperature performance will be reduced. If the grain size is too small, it cannot meet the requirements of the American standard and seriously affects the high-temperature performance of this steel at 600℃. On the other hand, if the grains are coarse, there will be fewer grain boundaries. During intergranular corrosion, alloy elements such as Cr and Ni will precipitate less at the grain boundaries, so the amount of grain boundaries is small, and the intergranular corrosion resistance is better. If the grains are too coarse, the toughness of the material will be reduced. Therefore, how to control the grain size grade during the preparation process is a technical problem that technicians in this field need to solve urgently. Summary of the invention

[0003] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a preparation process of a high niobium stainless steel strip, a high niobium stainless steel strip and its application. By adjusting the production process of the thermal annealing operation, the grain size of the 347H stainless steel strip reaches 6 to 7 levels, and has high temperature resistance and good intergranular corrosion resistance.

[0004] To achieve the above object, the present invention adopts the following technical solution:

[0005] A process for preparing a high niobium stainless steel strip is characterized by comprising the following steps:

[0006] 1) Simulated annealing: take a high niobium stainless steel strip black coil sample that has been air-cooled to room temperature, use a muffle furnace to control the temperature for simulated annealing, and determine the annealing furnace temperature during the first thermal annealing;

[0007] 2) First thermal annealing: The high niobium stainless steel strip black coil cooled to room temperature is passed through the annealing furnace at a speed of 5 mpm, and the annealing furnace temperature is 1200°C. After high temperature and low speed operation, the black coil has a grain size of 8.0;

[0008] 3) The first mixed acid pickling, the black coil treated in 2) is pickled with a mixed acid of 104g / L HNO3 and 45g / L HF to remove the oxide scale on the surface, and is leveled by a leveling unit to obtain a white coil with a grain size of 8.0;

[0009] 4) Second thermal annealing, the white paper coil obtained by treatment in 3) is passed through an annealing furnace at a speed of 5 mpm, the annealing furnace temperature is 1200°C, and after high temperature and low speed operation, the grain size is 6.5;

[0010] 5) Second mixed acid pickling: the white coil treated in 4) is pickled with a mixed acid of 104 g / L HNO3 and 45 g / L HF to remove the oxide scale on the surface, and is leveled by a leveling unit to obtain a white coil with a grain size of 6.5.

[0011] Furthermore, the high niobium stainless steel strip comprises, by weight percentage, the following components: C: 0.05-0.08; Si: 0.4-0.7; Mn: 1.0-1.4; P≤0.03; S≤0.001; Cr: 17.5-18.0; Ni: 9.0-9.4; N≤0.03; Mo≤0.3; Nb: 0.5-0.8; and the remainder is Fe and other inevitable impurity elements.

[0012] Furthermore, the preparation process of the high niobium stainless steel strip also includes rough rolling, selecting a 180 mm thick continuous casting slab with few cracks, removing the surface oxide scale through a dephosphorization device, and then sending it to a heating furnace for heating. The heating furnace temperature is 1240° C., the continuous heating time is within 240 minutes, and rough rolling is performed for 7 passes to obtain a 26 mm thick plate.

[0013] Furthermore, the preparation process of the high niobium stainless steel strip also includes finish rolling, the finish rolling temperature is controlled at 1000-1020°C, and after 7 passes of finish rolling, a 3.0 mm high niobium stainless steel strip black skin coil is finally obtained, and hot rolling is performed at a coiling temperature of 850°C. The coiled high niobium stainless steel strip black skin coil is placed on the site and air-cooled to room temperature.

[0014] A high niobium stainless steel strip, characterized in that, in terms of weight percentage, the components include: C: 0.05-0.08; Si: 0.4-0.7; Mn: 1.0-1.4; P≤0.03; S≤0.001; Cr: 17.5-18.0; Ni: 9.0-9.4; N≤0.03; Mo≤0.3; Nb: 0.5-0.8; the remainder is Fe and other inevitable impurity elements; the grain size is 6.5; the high niobium stainless steel strip is prepared according to the above-mentioned preparation process of the high niobium stainless steel strip.

[0015] Another object of the present invention is to provide a high niobium stainless steel strip prepared by the above-mentioned high niobium stainless steel strip preparation process for use in manufacturing large boiler superheaters, reheaters, steam pipelines, and heat exchanger pipes for petrochemical industry.

[0016] The functions of various elements used in the present invention and the preferred composition analysis are as follows:

[0017] C: Carbon is an element that strongly forms and stabilizes austenite and expands the austenite zone in austenitic stainless steel. Carbon forms austenite 30 times more than nickel. Carbon is an interstitial element that can significantly improve the strength of austenitic stainless steel through solid solution strengthening. Therefore, as the C content in the steel increases, the strength of austenitic stainless steel increases. Preferably, the C content in 347H is 0.05-0.08, and more preferably 0.7.

[0018] N: In stainless steel, like Ni, N is also one of the elements that contributes most to the stabilization of the austenite phase, and the increase in N content can also improve corrosion resistance and strength. However, if the N content is too high, the processability of the stainless steel will be reduced. If the N content is too low, Cr needs to be reduced accordingly to ensure phase balance, and it also has an adverse effect on stabilizing phase balance and ensuring strength. It is preferably ≤0.03, and more preferably 0.01.

[0019] Cr: is an element necessary to ensure the corrosion resistance of stainless steel. Increasing the Cr content can improve the corrosion resistance of stainless steel. However, too much Cr content will lead to the formation of too much ferrite in the stainless steel, and it is impossible to ensure that the stainless steel has sufficient tensile elongation. Therefore, the Cr content is preferably 17.5-18.0, and more preferably 17.6.

[0020] Ni: Like Mn and N, Ni is an austenite stabilizing element. Insufficient Ni content will reduce the corrosion resistance and workability of stainless steel. Ni is preferably 9.0 to 9.4, more preferably 9.05.

[0021] Si: It is a ferrite element and has a certain solid solution strengthening effect. It can improve the hardenability and strength of steel, and also has a certain optimization effect on the material's antioxidant properties. At the same time, its pitting corrosion resistance is enhanced.

[0022] Mo: The strengthening effect on austenitic stainless steel is not significant, and the addition of Mo has little effect on its room temperature mechanical properties; the addition of Mo content improves the high temperature strength of stainless steel, such as durability and creep properties. Mo is preferably ≤0.3.

[0023] Nb: Niobium strongly stabilizes carbon, thus having good resistance to intergranular corrosion. When the Nb / C ratio is greater than 8-10, intergranular corrosion does not occur. Niobium has a grain refinement effect, NbC and Nb(CN) precipitate during the heat treatment process, the "pinning" principle of the precipitated material points organizes the movement of grain boundaries, and the solute niobium atoms drag the grain boundaries, organize recrystallization, and refine the grains. The preferred Nb content is 0.5-0.8%.

[0024] Compared with the prior art, the beneficial effects of the present invention are: by designing the composition and adjusting the production process of the thermal annealing operation, the grain size of the high niobium stainless steel strip reaches 6 to 7 levels, and it has high temperature resistance and good intergranular corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described below in conjunction with the accompanying drawings.

[0026] Figure 1 This is a metallographic structure photograph of a black coil sample subjected to simulated annealing in a muffle furnace of the present invention and corroded by aqua regia.

[0027] Figure 2 This is a metallographic structure photograph of the present invention after the first annealing and nitric acid corrosion.

[0028] Figure 3 This is a metallographic structure photograph of the present invention after the second annealing and nitric acid corrosion. DETAILED DESCRIPTION

[0029] The present invention will be further described below in conjunction with specific embodiments.

[0030] A preparation process of a high-niobium stainless steel strip, wherein the high-niobium stainless steel strip comprises, by weight percentage, the following components: C: 0.05-0.08; Si: 0.4-0.7; Mn: 1.0-1.4; P≤0.03; S≤0.001; Cr: 17.5-18.0; Ni: 9.0-9.4; N≤0.03; Mo≤0.3; Nb: 0.5-0.8; the remainder is Fe and other unavoidable impurity elements. The preparation process comprises the following steps:

[0031] 1) Rough rolling: select 180mm thick continuous casting slab with few cracks, remove the surface oxide scale through dephosphorization equipment, and then send it to the heating furnace for heating. The heating furnace temperature is 1240℃, and the continuous heating time is within 240min. Rough rolling is performed for 7 passes to obtain 26mm thick plate.

[0032] 2) Finish rolling, wherein the finishing rolling temperature is controlled at 1000-1020° C. After 7 passes of finishing rolling, a 3.0 mm high niobium stainless steel strip black skin coil is finally obtained, and hot rolling is performed at a coiling temperature of 850° C. The coiled high niobium stainless steel strip black skin coil is placed on the site and air-cooled to room temperature.

[0033] 3) Simulated annealing: Take a high niobium stainless steel strip black coil sample that is air-cooled to room temperature, use a muffle furnace to control the temperature for simulated annealing, and determine the first thermal annealing temperature; the relationship between the simulated annealing temperature and time and the grain size is shown in Table 1.

[0034] Table 1 Relationship between simulated annealing temperature and time and grain size

[0035]

[0036] The temperature of the muffle furnace is 1180℃, the time in the furnace is 15 minutes, and the metallographic structure of the grain size is 7.0 is shown in the figure below. Figure 1As shown; referring to the performance parameters and grain size grade of the muffle furnace, the annealing furnace temperature is determined to be 1200℃.

[0037] 4) First thermal annealing: The high niobium stainless steel strip black coil cooled to room temperature is passed through the annealing furnace at a speed of 5mpm. The annealing furnace temperature is 1200℃. After high temperature and low speed operation, the black coil with a grain size of 8.0 has a metallographic structure as shown in the following figure: Figure 2 shown.

[0038] 5) The first mixed acid pickling, the black coil treated in 2) is pickled with a mixed acid of 104g / L HNO3 and 45g / L HF to remove the oxide scale on the surface, and is leveled by a leveling unit to obtain a white coil with a grain size of 8.0.

[0039] 6) Second thermal annealing: the white coil obtained by treatment in 3) is passed through the annealing furnace at a speed of 5mpm, and the annealing furnace temperature is 1200℃. After high temperature and low speed operation, the grain size is 6.5. The metallographic structure photo is as follows: Figure 3 shown.

[0040] 7) Second mixed acid pickling: the white coil treated in 4) is pickled with a mixed acid of 104 g / L HNO3 and 45 g / L HF to remove the oxide scale on the surface, and is leveled by a leveling unit to obtain a white coil with a grain size of 6.5.

[0041] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A process for preparing a high niobium stainless steel strip, characterized in that: The following steps are involved: 1) Simulated annealing: take a high niobium stainless steel strip black coil sample that has been air-cooled to room temperature, use a muffle furnace to control the temperature for simulated annealing, and determine the annealing furnace temperature during the first thermal annealing; 2) First thermal annealing: The high niobium stainless steel strip black coil cooled to room temperature is passed through an annealing furnace at a speed of 5 mpm, and the annealing furnace temperature is 1200°C. After high temperature and low speed operation, a black coil with a grain size of 8.0 is obtained; 3) The first mixed acid pickling, the black coil treated in 2) is pickled with a mixed acid of 104g / L HNO3 and 45g / L HF to remove the oxide scale on the surface, and is leveled by a leveling unit to obtain a white coil with a grain size of 8.0; 4) Second thermal annealing, the white paper coil obtained by treatment in 3) is passed through an annealing furnace at a speed of 5 mpm, the annealing furnace temperature is 1200°C, and after high temperature and low speed operation, the grain size is 6.5; 5) Second mixed acid pickling: the white coil treated in 4) is pickled with a mixed acid of 104 g / L HNO3 and 45 g / L HF to remove the oxide scale on the surface, and is leveled by a leveling unit to obtain a white coil with a grain size of 6.

5.

2. The process for preparing a high niobium stainless steel strip according to claim 1, characterized in that: The high niobium stainless steel strip comprises, by weight percentage, the following components: C: 0.05-0.08; Si: 0.4-0.7; Mn: 1.0-1.4; P≤0.03; S≤0.001; Cr: 17.5-18.0; Ni: 9.0-9.4; N≤0.03; Mo≤0.3; Nb: 0.5-0.8; the remainder is Fe and other inevitable impurity elements.

3. A process for preparing a high niobium stainless steel strip according to claim 1 or 2, characterized in that: The preparation process of the high niobium stainless steel strip also includes rough rolling, selecting a 180 mm thick continuous casting slab with few cracks, removing the surface oxide scale through a dephosphorization device, and then sending it to a heating furnace for heating. The heating furnace temperature is 1240° C., the continuous heating time is within 240 minutes, and rough rolling is performed 7 times to obtain a 26 mm thick plate.

4. The process for preparing a high niobium stainless steel strip according to claim 3, characterized in that: The preparation process of the high niobium stainless steel strip also includes finish rolling, wherein the finish rolling temperature is controlled at 1000-1020°C, and after 7 passes of finish rolling, a 3.0 mm high niobium stainless steel strip black skin coil is finally obtained, and hot rolling is performed at a coiling temperature of 850°C. The coiled high niobium stainless steel strip black skin coil is placed on the site for air cooling to room temperature.

5. A high niobium stainless steel strip, characterized in that: The composition includes, by weight percentage, C: 0.05-0.08; Si: 0.4-0.7; Mn: 1.0-1.4; P≤0.03; S≤0.001; Cr: 17.5-18.0; Ni: 9.0-9.4; N≤0.03; Mo≤0.3; Nb: 0.5-0.8; the balance is Fe and other inevitable impurity elements; the grain size is 6.5; the high niobium stainless steel strip is prepared according to the preparation process of the high niobium stainless steel strip according to any one of claims 1 to 4.

6. Use of a high niobium stainless steel strip prepared by the process for preparing a high niobium stainless steel strip according to any one of claims 1 to 4 in manufacturing large boiler superheaters, reheaters, steam pipes, and heat exchanger pipes for petrochemical industry.

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