Steel for magnesium reduction tank, production method of steel, high-temperature-resistant and high-strength magnesium reduction tank and manufacturing method of high-temperature-resistant and high-strength magnesium reduction tank

Abstract

The invention provides steel for a magnesium reduction tank, a production method of the steel, the high-temperature-resistant and high-strength magnesium reduction tank and a manufacturing method of the high-temperature-resistant and high-strength magnesium reduction tank. The steel comprises 0.40%-0.50% of C, 0.05%-0.20% of Si, 1.70%-2.00% of Mn, 8.50%-9.50% of Cr, 0.30%-0.50% of Mo, 0.010%-0.025% of Al, 0.030%-0.060% of Nb, smaller than or equal to 0.020% of P, smaller than or equal to 0.010% of S, 0.040%-0.070% of N, smaller than or equal to 0.0040% of O and the balance Fe and other inevitable impurities. Compared with the prior art, the high-temperature-resistant and high-strength magnesium reduction tank is obtained through designed components and a production method, and the service life of the magnesium reduction tank exceeds 2000 hours.

Description

Technical Field

[0001] The present invention belongs to the field of alloy steel, and specifically relates to a steel for magnesium reduction tanks, its production method, a high-temperature and high-strength magnesium reduction tank, and its manufacturing method. Background Art

[0002] Magnesium is one of the light metal elements with the most abundant resource reserves in the earth's crust. Due to the high strength and light weight of magnesium alloys, the demand for magnesium is gradually increasing, and it is expected that the annual demand will reach 700,000 tons in the future.

[0003] Producing magnesium requires high-temperature reduction, and the reduction tank is an important component in magnesium smelting. Magnesium reduction tanks are usually made of cast superalloys, and the commonly used materials are ZG40Cr28Ni16 heat-resistant steel, ZG2Cr25Ni20Si2 heat-resistant steel, and ZG3Cr24Ni7N heat-resistant steel. There are the following problems with cast magnesium reduction tanks: First, cast magnesium reduction tanks contain high levels of Ni (7-16%) and high levels of Cr (24-28%), resulting in high costs for magnesium reduction tanks, generally up to 40,000 yuan / ton. Second, the strength of the heat-resistant steel in cast magnesium reduction tanks is low, resulting in less primary charge and low efficiency for magnesium reduction. Third, the density of cast magnesium reduction tanks is not high, the tank body has not undergone pressure processing, and the casting structure is coarse, resulting in easy corrosion, deformation of the tank body, and short service life. Fourth, the service life of cast magnesium reduction tanks is relatively short, and they generally need to be replaced after 50-60 days.

[0004] With the need for energy conservation, emission reduction, and green development, the vertical reduction tank for producing magnesium has been vigorously developed. This change has increased the pressure on the reduction tank, and the magnesium reduction tanks made of traditional materials cannot meet the requirements. Therefore, new magnesium reduction tank materials need to be developed to meet the needs of vertical magnesium reduction.

[0005] The patent with the publication number CN 115261730 A, published on November 1, 2022, discloses a heat-resistant stainless steel for magnesium reduction tanks and its preparation method. Its technical solution is "prepared by elements such as C, Si, Mn, P, S, Ni, Cr, N, Ce, Fe through batching, electric arc furnace melting, AOD double slag refining, LF furnace secondary refining, ingot casting, hot piercing, etc.". This patent mainly focuses on the production process of stainless steel. The cost of raw materials containing 20-25% Cr and 9%-13% Ni in the material is high, and the raw material for production is an ingot, without mentioning key indicators such as the strength, creep, and life of the magnesium reduction tank. Summary of the Invention

[0006] The purpose of the present invention is to provide a steel for magnesium reduction tanks and its production method. Through the design of the composition of the new magnesium reduction tank material, a steel for high-temperature and high-strength magnesium reduction tanks is obtained, without the need to add Ni, and the cost is low.

[0007] Another object of the present invention is to provide a magnesium reduction tank with high temperature resistance and high strength and a manufacturing method thereof. The magnesium reduction tank is made of steel, and through a designed heat treatment method, the product has good high temperature strength at 800°C - 1300°C for magnesium reduction, and the high temperature yield strength at 1280°C is ≥30 MPa. At 1280°C and 3 MPa stress, the creep rate ≤ 3.0×10 -5 h -1 . The service life of the magnesium reduction tank made of this steel exceeds 2000 hours.

[0008] The specific technical solution of the present invention is as follows:

[0009] A steel for magnesium reduction tank, comprising the following components by mass percentage: C 0.40% - 0.50%, Si 0.05% - 0.20%, Mn 1.70% - 2.00%, Cr 8.50% - 9.50%, Mo 0.30% - 0.50%, Al

[0010] 0.010% - 0.025%, Nb 0.030% - 0.060%, P ≤ 0.020%, S ≤ 0.010%, N 0.040 - 0.070%, O ≤ 0.0040%, and the balance is Fe and other inevitable impurities.

[0011] The composition of the steel for magnesium reduction tank satisfies: 96.25% ≤ 12×%C + 5×%Mn - 4×%Si + 9×%Cr + 8×%Mo + 3×%Nb + 85×%N ≤ 105.45%;

[0012] The composition of the steel for magnesium reduction tank also satisfies: 3.90% ≤ (%C + 6×%N) / 9 - (8×%Si + 3

[0013] ×%Mn) / 7 + (2×%Cr + 3×%Mo + 13×%Nb) / 4.

[0014] A production method of a steel for magnesium reduction tank provided by the present invention includes the following technological processes:

[0015] Arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → Round billet continuous casting.

[0016] For the round billet continuous casting, round billet continuous casting with a diameter of Φ380mm - Φ1200mm is adopted;

[0017] For the round billet continuous casting, the superheat is 30°C - 70°C. The diameter R of the continuous casting billet and the casting speed v should comply with R 2×v = 107800, where the continuous casting billet diameter R is in mm and the casting speed v is in m / min; the final electromagnetic stirring frequency f and the continuous casting billet diameter R should comply with R / f = 200, where the final electromagnetic stirring frequency f is in Hz and the continuous casting billet diameter R is in mm. When calculating using the above formulas, substitute the numerical values before the units for calculation.

[0018] The present invention provides a manufacturing method of a high-temperature resistant and high-strength magnesium reduction tank, which is manufactured using the steel for the magnesium reduction tank as described above and includes heat treatment, and the heat treatment includes normalizing and tempering.

[0019] For the normalizing, normalizing: the heating temperature T is 1100 - 1200 °C, the holding time t1 is determined by the steel pipe wall thickness S, t1 = 3×S, and air cooling. Among them, the unit of the holding time t1 is min and the unit of the steel pipe wall thickness S is mm.

[0020] For the tempering, tempering: the tempering temperature is 600 - 700 °C, the holding time t2 is determined by the steel pipe wall thickness S, t2 = 4×S, and air cooling. Among them, the unit of the holding time t2 is min and the unit of the steel pipe wall thickness S is mm.

[0021] A high-temperature resistant and high-strength magnesium reduction tank provided by the present invention is obtained by using the above manufacturing method. For the high-temperature resistant and high-strength magnesium reduction tank, the grain size of the 1 / 2 radius of the steel pipe is 85 - 95 μm; the high-temperature yield strength at 1280 °C of the 1 / 2 wall thickness of the steel pipe is ≥ 30 MPa, and the high-temperature tensile strength at 1280 °C is ≥ 35 MPa. At 1280 °C and under a stress of 3 MPa, the creep rate ≤ 3.0×10 -5 h -1 . The service life of the magnesium reduction tank exceeds 2000 hours.

[0022] The design concept of the present invention is as follows:

[0023] C: C is the cheapest strengthening element in steel. At room temperature, for every 0.1% increase in solid solution C, the strength can be increased by about 450 MPa. At 1280 °C, for every 0.1% increase in carbon, the strength can be increased by 1.2 MPa. In addition, C forms precipitation phases with alloying elements in the steel, playing a role in precipitation strengthening. However, if the carbon content is too high, it is easy to cause the tendency of graphitization, so C is controlled at 0.40% - 0.50%.

[0024] Si: Si is an effective solid solution strengthening element in steel, which improves the strength and hardness of the steel. Si can play a role in deoxidation during steelmaking and is a commonly used deoxidizer. However, Si is prone to instability at high temperatures, so the Si content in high-temperature steel should not be too high. Therefore, the Si content is controlled at 0.05% - 0.20%.

[0025] Mn: Mn can play a role in solid solution strengthening. For every 0.1% increase in Mn, the high-temperature strength can be increased by 2.5 MPa. The combination of Mn and S can prevent the hot brittleness caused by S. However, excessive Mn will reduce the plasticity of the steel. Therefore, the Mn content is controlled at 1.70% - 2.00%.

[0026] Cr: Cr is a carbide-forming element. Cr can improve both the hardenability and strength of the steel. Cr can improve the oxidation resistance of the steel and increase the corrosion resistance. Cr is the main element to improve the high-temperature strength of the steel, and the content is usually 5% - 25%. However, Cr has a high cost. Excessive addition cannot significantly improve the strength but increase the cost. On the other hand, too high a Cr content will lead to an increase in the viscosity of the steel during production, which is not conducive to casting. The Cr content should be controlled at 8.50% - 9.50%.

[0027] Mo: Mo mainly improves the heat resistance of the steel. Mo dissolved in the matrix can keep the structure of the steel at a relatively high stability during the tempering process, and can effectively reduce the segregation of impurity elements such as P, S, and As at the grain boundaries, thereby improving the toughness of the steel and reducing the temper brittleness. Mo can significantly improve the high-temperature performance of the steel. Usually, adding 0.1% of Mo can increase the high-temperature strength by 0.6 MPa. However, Mo reduces the stability of M7C3. When the Mo content is relatively high, needle-like Mo2C will be formed, which will lead to a decrease in the Mo content in the matrix. Therefore, Mo is controlled at 0.30% - 0.50%.

[0028] Nb: Nb is a strong C, N compound-forming element. Nb(C, N) is fine and dispersed, and maintains a coherent relationship with the matrix, which can play a role in strengthening and refining the structure. The strengthening of the matrix can increase the resistance to the initiation and propagation of fatigue cracks, thereby improving the fatigue strength. The Nb content is controlled at 0.030% - 0.060%.

[0029] Al: Al is the main deoxidizer in steelmaking. Al combines with N to form fine and dispersed AlN, and maintains a coherent relationship with the matrix, which can play a role in strengthening and refining the structure, and can increase the resistance to the initiation and propagation of fatigue cracks, thereby improving the creep strength of the steel. The Al content is controlled at 0.010% - 0.025%.

[0030] N: N is an inexpensive strengthening element. The solid solution strengthening effect of N is the same as that of C. At room temperature, for every 0.1% increase in N, the strength is increased by 450 MPa. At high temperatures, for every 0.1% increase in N, the strength can be increased by 15 MPa. However, too high an N content in the steel grade is likely to cause bubbles. And increasing nitrogen in the steel requires high pressure and is not easy to operate. Therefore, the nitrogen content is controlled at 0.04% - 0.07%.

[0031] The steel used for magnesium reduction vessels requires high strength. Generally, materials with high room temperature strength also have high high temperature strength. However, through research, it is found that this rule is not completely followed at a temperature of 1280°C. At high temperatures, grain-refining elements cannot improve the high temperature strength of the steel. Solution strengthening and dispersion strengthening are the main means to improve high strength. Through research, it is determined that at high temperature environments, C can effectively improve high temperature strength, and its contribution coefficient to high strength is 12; N also has a solution strengthening effect, but the high temperature solution effect of N is stronger, and its contribution coefficient to high temperature strength is 85; Mo improves high temperature strength through improving tempering stability and solution strengthening, and its contribution coefficient to high temperature strength is 8; Mn improves strength by increasing the expansion of the high temperature phase region during the phase transformation process, and its contribution coefficient to high temperature strength is 5; Cr has an atomic size similar to that of Fe, can achieve substitutional solid solution, and Cr can improve the high temperature oxidation effect, making a large contribution to high strength, with a contribution coefficient of 9; Si is a non-metallic element and is also the main solution strengthening element in steel, but in high temperature environments, Si is not beneficial to high strength, so the contribution coefficient is -4; Nb can provide the high temperature strength of the steel through both solution and dispersion methods, but Nb has a pinning effect on grain boundaries when combined with N, which is not conducive to high temperature strength, so the contribution coefficient to high temperature strength is 3. To ensure the high temperature strength and toughness of the steel, it is not enough to only ensure the high temperature strength of the steel. Therefore, let the high temperature strengthening factor in the steel be represented by A, then 96.25% ≤ A value ≤ 105.45%,

[0032] A value = 12 × %C + 5 × %Mn - 4 × %Si + 9 × %Cr + 8 × %Mo + 3 × %Nb + 85 × %N.

[0033] Magnesium reduction vessels require excellent high temperature creep resistance during service. The high temperature creep resistance is related to the strength of the steel on the one hand, and also related to the grain boundary stability, precipitation and solution stability of the steel. Therefore, it is necessary to limit the ratios of C, Si, Mn, Cr, Mo, N, and Nb. Since the atomic scales of C and N are small, they can only exist in the form of interstitials during the strengthening process and can generate slip under high temperature stress, so their influence on high temperature creep is relatively small. Si and Mn contribute greatly to the room temperature strength of the steel, but because Si and Mn are not beneficial to the homogenization of the steel and are prone to instability deviation at high temperatures, they are not beneficial to the high temperature creep performance. Cr, Mo, and Nb can form second phases with C and N in the steel, and the second phases can effectively pin dislocations and stacking faults in the steel, thereby slowing down the creep rate. Let the high temperature creep factor in the steel be represented by Y, then 3.90% ≤ Y value, Y value = (%C + 6 × %N) / 9 - (8 × %Si + 3 × %Mn) / 7 + (2 × %Cr + 3 × %Mo + 13 × %Nb) / 4. The above coefficients are obtained through inductive analysis based on the different effects of alloying elements.

[0034] In view of the stress characteristics of vertical magnesium reduction tanks, the present invention has developed a steel for magnesium reduction tanks that meets the requirements of high efficiency, long life, high temperature resistance, and long life. Compared with the prior art, through the designed composition and production method, the present invention obtains a magnesium reduction tank with high temperature resistance and high strength. The product has good high temperature strength at 800°C - 1300°C for magnesium reduction, and the high temperature yield strength at 1280°C is ≥30 MPa. At 1280°C and a stress of 3 MPa, the creep rate is ≤3.0×10 -5 h -1 . The service life of the magnesium reduction tank made of this steel exceeds 2000 hours. Detailed implementation manners

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0036] Examples 1 - 4

[0037] A steel for magnesium reduction tanks includes the mass percentage components as shown in Table 1: The balance not shown in Table 1 is Fe and unavoidable impurities.

[0038] Table 1 Chemical compositions of the examples and comparative examples of the present invention (wt%)

[0039] Steel grade C Si Mn Cr Mo Nb N P S O Al Value A Value Y Example 1 0.42 0.08 1.75 8.7 0.32 0.035 0.042 0.015 0.004 0.0035 0.012 98.01 3.94 Example 2 0.45 0.12 1.84 8.9 0.42 0.045 0.052 0.013 0.007 0.0032 0.015 102.14 4.07 Example 3 0.47 0.10 1.82 8.8 0.39 0.052 0.061 0.014 0.009 0.0028 0.018 102.00 4.06 Example 4 0.41 0.19 1.98 9.2 0.46 0.032 0.048 0.016 0.008 0.0025 0.023 104.72 4.06 Comparative example 1 0.48 0.15 1.76 9.5 0.48 0.054 0.064 0.012 0.002 0.0031 0.019 108.90 4.46 Comparative example 2 0.43 0.14 1.95 9.2 0.44 0.048 0.056 0.018 0.004 0.0024 0.022 105.57 4.18 Comparative example 3 0.40 0.17 1.72 8.6 0.32 0.034 0.042 0.016 0.005 0.0033 0.015 96.35 3.79 Comparative example 4 0.46 0.09 1.81 9.1 0.43 0.058 0.067 0.015 0.006 0.0032 0.013 105.42 4.28

[0040] Comparative examples 1 - 4

[0041] A steel for magnesium reduction tanks includes the mass percentage components as shown in Table 1: The balance not shown in Table 1 is Fe and unavoidable impurities.

[0042] The production methods of the steels for magnesium reduction tanks in the above-mentioned examples and comparative examples include the following technological processes:

[0043] Converter smelting → LF furnace refining → RH vacuum degassing → Round billet continuous casting.

[0044] Specifically, it is mainly:

[0045] Converter smelting: Determine the oxygen content before tapping, and adopt the operation of retaining steel during tapping to avoid slagging;

[0046] LF furnace refining: Adjust elements such as C, Si, Mn, Cr, Mo, and Nb to the target values;

[0047] RH vacuum degassing: The pure degassing time is ≥15 minutes to ensure that the [H] content after vacuum treatment is ≤1.5 ppm, avoid the appearance of white spots in the steel and cause hydrogen embrittlement. Nitrogen is used as the lifting gas;

[0048] Round bloom continuous casting: The target temperature of the tundish molten steel is controlled at 30 - 70 °C above the liquidus temperature, and round blooms with a diameter of φ380 mm - φ1200 mm are continuously cast. Specifically, for the continuous casting billet with a diameter of 800 mm, the superheat is 45 °C, the casting speed is 0.168 m / min, and the final electromagnetic stirring is 4 Hz.

[0049] Using the steel for magnesium reduction tanks in the above-mentioned examples and comparative examples to manufacture magnesium reduction tanks, the manufacturing process is as follows: Round bloom continuous casting with a diameter of Φ380 mm - Φ1200 mm → Drilling → Heating of round bloom → Pipe threading → Sizing → Heat treatment → Machining → Nondestructive testing → Packaging and warehousing. The key heat treatment process is as follows:

[0050] Normalizing: The heating temperature (T, °C) is 1100 - 1200 °C, and the holding time (t1, min) depends on the wall thickness (S, mm) of the steel pipe, t1 = 3×S, air cooling.

[0051] Tempering: The tempering temperature is 600 - 700 °C, and the holding time (t2, min) depends on the wall thickness (S, mm) of the steel pipe, t2 = 4×S, air cooling.

[0052] The heat treatment processes of each example and comparative example are shown in Table 2.

[0053] Table 2 List of heat treatment process conditions of the examples and comparative examples of the present invention

[0054]

[0055]

[0056] The performance detection method is as follows:

[0057] Microstructure: Samples are taken at the 1 / 2 radius of the steel pipe for metallographic and grain size analysis.

[0058] Performance: Tensile, impact, and high-temperature creep specimens are taken at the 1 / 2 radius of the steel pipe, and mechanical property tests are carried out with reference to GB / T228, GB / T229, and GB / T38822. The mechanical properties are shown in Table 3.

[0059] Table 3 List of performance detection situations of the examples and comparative examples of the present invention

[0060]

[0061] The chemical compositions and production methods of the steels in Examples 1-4 are appropriately controlled. Their chemical compositions meet the requirements, and the strength and high-temperature creep properties of the steels are both good. The chemical compositions of Comparative Examples 1 and 2 meet the requirements of the present invention, but the A value does not meet the requirements of the present invention. The A value is relatively high. Even though heat-treated according to the process of the present invention, the high-temperature properties of Product at 1280 °C are relatively low, and the creep rate is relatively high under a stress of 3 MPa at 1280 °C. The chemical composition of Comparative Example 3 meets the requirements of the present invention, but the Y value does not meet the requirements of the present invention. The Y value is relatively low, and it is not produced according to the heat treatment process of the present invention, resulting in poor properties at 1280 °C and a high creep rate under a stress of 3 MPa. The chemical composition of Comparative Example 4 is properly controlled, but the heat treatment process is improper, resulting in unsatisfactory overall properties.

[0062] The data underlined above do not meet the requirements of the present invention.

[0063] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A steel for magnesium reduction tank, characterized in that, The steel for magnesium reduction tank comprises the following components by mass percentage: C 0.40% - 0.50%, Si 0.05% - 0.20%, Mn 1.70% - 2.00%, Cr 8.50% - 9.50%, Mo 0.30% - 0.50%, Al 0.010% - 0.025%, Nb 0.030% - 0.060%, P ≤ 0.020%, S ≤ 0.010%, N 0.040 - 0.070%, O ≤ 0.0040%, and the balance is Fe and other inevitable impurities.

2. The steel for magnesium reduction tank according to claim 1, characterized in that, The components of the steel for magnesium reduction tank satisfy: 96.25% ≤ 12×%C + 5×%Mn - 4×%Si + 9×%Cr + 8×%Mo + 3×%Nb + 85×%N ≤ 105.45%.

3. The steel for a magnesium reduction tank according to claim 1 or 2, characterized in that, The components of the steel for magnesium reduction tank also satisfy: 3.90% ≤ (%C + 6×%N) / 9 - (8×%Si + 3×%Mn) / 7 + (2×%Cr + 3×%Mo + 13×%Nb) / 4.

4. A production method of the steel for magnesium reduction tank according to any one of claims 1-3, characterized in that, The production method comprises the following technological processes: Smelting in an electric arc furnace or a converter → Refining in an LF furnace → Vacuum degassing in an RH or VD → Continuous casting of round billets.

5. The production method of the steel for magnesium reduction tank according to claim 4, characterized in that, For the continuous casting of round billets, the superheat is 30°C - 70°C.

6. The production method of the steel for magnesium reduction tank according to claim 4, characterized in that, For the round billet continuous casting, the diameter R of the continuous casting billet and the casting speed v should meet the requirement that R 2 × v = 107800, where the unit of the diameter R of the continuous casting billet is mm and the unit of the casting speed v is m / min; the frequency f of the final electromagnetic stirring and the diameter R of the continuous casting billet should meet the requirement that R / f = 200, where the unit of the frequency f of the final electromagnetic stirring is Hz and the unit of the diameter R of the continuous casting billet is mm.

7. A manufacturing method of a magnesium reduction tank with high temperature resistance and high strength, characterized in that, The manufacturing method uses the steel for magnesium reduction tank according to any one of claims 1 - 3, and includes heat treatment, and the heat treatment includes normalizing and tempering.

8. The manufacturing method according to claim 7, characterized in that, For the normalizing, normalizing: the heating temperature T is 1100 - 1200°C, and the holding time t1 is determined by the wall thickness S of the steel pipe, t1 = 3×S, air cooling; wherein, the unit of the holding time t1 is min, and the unit of the wall thickness S of the steel pipe is mm.

9. The manufacturing method according to claim 7 or 8, characterized in that, For the tempering, tempering: the tempering temperature is 600 - 700°C, and the holding time t2 is determined by the wall thickness S of the steel pipe, t2 = 4×S, air cooling; wherein, the unit of the holding time t2 is min, and the unit of the wall thickness S of the steel pipe is mm.

10. A high-temperature resistant and high-strength magnesium reduction tank manufactured by the manufacturing method according to any one of claims 7 to 9, characterized in that: For the high-temperature and high-strength magnesium reduction tank, the grain size at 1 / 2 radius of the steel pipe is 85 - 95 μm; the high-temperature yield strength at 1280°C of the 1 / 2 wall thickness of the steel pipe is ≥ 30 MPa, and the high-temperature tensile strength at 1280°C is ≥ 35 MPa; The creep rate is ≤ 3.0×10 -5 h -1 at 1280 °C under a stress of 3 MPa; the service life of the magnesium reduction tank exceeds 2000 hours.

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