Steel for magnesium reduction tank, production method of steel, magnesium reduction large-diameter vertical reduction tank and heat treatment process of magnesium reduction large-diameter vertical reduction tank

Abstract

The invention provides steel for a magnesium reduction tank, a production method of the steel, a magnesium reduction large-diameter vertical reduction tank and a heat treatment process of the magnesium reduction large-diameter vertical reduction tank. The steel comprises the following components: 0.45%-0.65% of C, 0.05%-0.20% of Si, 1.10%-1.60% of Mn, 7.50%-8.50% of Cr, 0.40%-0.70% of Mo, 0.60%-0.80% of Ni, 0.010%-0.025% of Al, 0.030%-0.060% of Nb, less than or equal to 0.020% of P, less than or equal to 0.010% of S, 0.050%-0.090% of N, 0-0.0040% of O and the balance of Fe and inevitable impurities. Compared with the prior art, a component system with the low Cr content and the low Ni content is obtained through component design; and after heat treatment, the magnesium is reduced at 800-1300 DEG C to have good high-temperature strength, and the service life exceeds 2000 h.

Description

Technical Field

[0001] The present invention belongs to the field of high-alloy steels, and particularly relates to a steel for magnesium reduction tanks, a production method thereof, a large-diameter vertical magnesium reduction tank, and a heat treatment process thereof. Background Art

[0002] The magnesium smelting industry is an energy-intensive industry with high energy consumption. A large amount of three wastes will be generated during the production of magnesium, and the greenhouse gas emissions per unit of metal are 5 times that of the steel industry. In order to improve the smelting efficiency, the magnesium reduction tank in magnesium smelting is developing towards large diameter and vertical tanks. The diameter of the magnesium reduction tank has developed from the original 300 mm to 600 mm, and even more than 900 mm. And with the development of the emphasis on green environmental protection, it is necessary to implement the key transformation project of clean production of vertical reduction magnesium smelting in the magnesium smelting industry.

[0003] Compared with the horizontal tank, due to the gravity effect, the vertical tank has higher requirements for the strength of the material, and the requirements for the central quality and homogenization of the material for the large-diameter reduction tank are also significantly improved. The traditional horizontal tank is produced by the casting process. Although the casting process is simpler than the forging and seamless steel pipe processes, the material density of the casting process is not high enough, the as-cast structure has not undergone hot deformation, the structure is generally coarse, and the segregation at the structure interface is relatively large. Therefore, high-alloy steel is usually used for casting magnesium reduction tanks to make up for the deficiencies in the process. However, the use of high-alloy steel, especially high-Ni and high-Cr steel, on the one hand, increases the casting difficulty and reduces the production efficiency. On the other hand, the service life of the final product is short and it is not economical enough.

[0004] The patent with the publication number CN 107267778 A, which was published on October 20, 2017, discloses a new type of high-efficiency magnesium reduction tank and its manufacturing method, designing a high-Al content of 2%-3%, high-Cr and high-Ni duplex heat-resistant steel (Cr: 20% - 27%, Ni: 8% - 12). However, for high-alloy heat-resistant steel, how to achieve the addition of a high content of easily oxidized Al in light metals is a difficult problem and this element is not beneficial to high-temperature performance. Summary of the Invention

[0005] The purpose of the present invention is to provide a steel for magnesium reduction tanks and a production method thereof. Through composition design, a steel for magnesium reduction tanks is obtained, with both Cr and Ni contents being relatively low, low cost; and having high creep rupture performance and low creep rate, which can improve production efficiency and increase the service life of the reduction tank.

[0006] Another purpose of the present invention is to provide a large-diameter vertical magnesium reduction tank and a heat treatment process thereof. The large-diameter vertical magnesium reduction tank is produced by using the above-mentioned steel for magnesium reduction tanks, and the product satisfies the requirements of having good high-temperature strength at 800°C - 1300°C for magnesium reduction, the high-temperature yield strength at 1280°C ≥ 33 MPa, and the tensile strength ≥ 37 MPa. At 1280°C and under a stress of 3 MPa, the creep rate ≤ 2.5×10-5 h -1 The service life of the manufactured magnesium reduction tank exceeds 2,000 hours. The large diameter means a diameter ≥ 508 mm.

[0007] The specific technical solution of the present invention is as follows:

[0008] A steel for magnesium reduction tank, comprising the following components by mass percentage:

[0009] C 0.45% - 0.65%, Si 0.05% - 0.20%, Mn 1.10% - 1.60%, Cr 7.50% - 8.50%, Mo 0.40% - 0.70%, Ni 0.60 - 0.80%, Al 0.010% - 0.025%, Nb 0.030% - 0.060%, P ≤ 0.020%, S ≤ 0.010%, N 0.050 - 0.090%, O - 0.0040%, and the balance is Fe and unavoidable impurities.

[0010] The composition of the steel for magnesium reduction tank satisfies: 9.3% ≤ 1.8 × %Cr + 1 × %Si + 2.5 × %Mo + 3

[0011] × %Nb - 3 × %Ni - 0.6 × %Mn - 25 × %N - 3 × %C ≤ 10.5%.

[0012] A production method of a steel for magnesium reduction tank provided by the present invention includes the following technological processes:

[0013] Arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → Round billet continuous casting.

[0014] For the round billet continuous casting, round billet continuous casting with a diameter of Φ380 mm - Φ1200 mm is carried out; the superheat is controlled at 40℃

[0015] - 80℃. The diameter R of the continuous casting billet and the casting speed v should comply with R2 × v = 93100, 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 comply with R / f = 150, 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.

[0016] A heat treatment process of a large - diameter vertical magnesium reduction tank provided by the present invention uses the above - mentioned steel for magnesium reduction tank to manufacture a large - diameter vertical magnesium reduction tank, and the heat treatment process includes quenching and tempering.

[0017] For the quenching: the heating temperature T1 is 1100 - 1200℃, and the holding time t1 is determined by the wall thickness S of the steel pipe, 5 × S - T1 / 4 ≤ t1 ≤ 5 × S - T1 / 16, and 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.

[0018] The tempering: the tempering temperature T2 is 600 - 700 °C, and the holding time t2 is determined by the wall thickness S of the steel pipe, 5×S - T2 / 2 ≤ t2 ≤ 5×S - T2 / 10, with air cooling. Herein, the unit of the holding time t2 is min, and the unit of the wall thickness S of the steel pipe is mm.

[0019] A large-diameter vertical magnesium reduction retort provided by the present invention is manufactured by adopting the above heat treatment process. The grain size of the manufactured large-diameter vertical magnesium reduction retort is 85 - 90 μm; the high-temperature yield strength of the large-diameter vertical magnesium reduction retort at 1280 °C is ≥33 MPa, and the tensile strength is ≥37 MPa. At 1280 °C and under a stress of 3 MPa, the creep rate is ≤2.5×10 -5 h -1 . The service life of the manufactured magnesium reduction retort exceeds 2000 hours.

[0020] The design concept of the present invention is as follows:

[0021] 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 steel, playing a role in precipitation strengthening. However, too high a carbon content is likely to cause a graphitization tendency, so C is controlled at 0.45% - 0.65%.

[0022] Si: Si is an effective solid solution strengthening element in steel, which improves the strength and hardness of steel. Si can play a deoxidizing role during steelmaking and is a commonly used deoxidizer. However, Si is prone to instability at high temperatures, so the content of Si in high-temperature steel should not be too high. Therefore, the content of Si is controlled at 0.05% - 0.20%.

[0023] 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. Mn combines with S to prevent the hot brittleness caused by S. However, excessive Mn will reduce the plasticity of steel. Therefore, the content of Mn is controlled at 1.10% - 1.60%.

[0024] Cr: Cr is a carbide-forming element. Cr can improve both the hardenability and strength of steel. Cr can improve the oxidation resistance of steel and increase the corrosion resistance. Cr is the main element to improve the high-temperature strength of steel, and its content is usually 5% - 25%. However, Cr has a high cost, and 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 steel during the production process, which is not conducive to casting. The content of Cr should be controlled at 7.50% - 8.50%.

[0025] Mo: Mo mainly improves the heat resistance of steel. Mo dissolved in the matrix can keep the structure of steel highly stable during the tempering process, and can effectively reduce the segregation of impurity elements such as P, S, and As at grain boundaries, thereby improving the toughness of steel and reducing temper brittleness. Mo can significantly improve the high-temperature performance of 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 high, needle-like Mo2C will be formed, which will lead to a decrease in the Mo content of the matrix. Therefore, the Mo content is controlled at 0.40% - 0.70%.

[0026] Nb: Nb is a strong C and 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%.

[0027] Ni: Ni is the main element to improve toughness and reduce stacking fault energy. The addition of Ni is beneficial to reducing the creep rate, but the price of Ni is high. Adding too much will lead to an increase in cost. Therefore, the Ni content is controlled at 0.60% - 0.80%.

[0028] 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 steel. The Al content is controlled at 0.010% - 0.025%.

[0029] 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 steel is likely to cause bubbles. And increasing nitrogen in steel requires high pressure and is not easy to operate. Therefore, the nitrogen content is controlled at 0.05% - 0.09%.

[0030] The high-temperature strength and creep rate of the steel for magnesium reduction tanks are affected by the interaction of elements. At high temperatures, elements that refine grains cannot improve the high-temperature strength of the steel. Solid solution strengthening and dispersion strengthening are the main means to improve strength. The high-temperature creep rate is also significantly affected by the grain boundary strength, atomic binding force, etc. In addition, during the transformation from liquid to solid state in high-alloy steels, various complex phases will form, such as high-temperature ferrite, peritectic reaction, etc., which will reduce the creep properties of the steel. Therefore, considering the interaction between various elements comprehensively, determining the contribution coefficient of each element, and determining that the steel needs to meet the following requirements to obtain high-temperature strength and excellent creep rate at 1280°C. Let the high-performance factor be the A value. According to the different effects of gold elements, the coefficients obtained by inductive analysis are 9.3% ≤ A value ≤ 10.5%. The A value is 1.8×%Cr + 1×%Si + 2.5×%Mo + 3×%Nb - 3×%Ni - 0.6×%Mn - 25×%N - 3×%C.

[0031] Compared with the prior art, through composition design, the present invention obtains a composition system with low Cr and Ni contents and low cost; after being produced by the designed heat treatment process, the product has good high-temperature strength at 800°C - 1300°C for magnesium reduction, the high-temperature yield strength at 1280°C ≥ 33 MPa, and the tensile strength ≥ 37 MPa. At 1280°C and under a stress of 3 MPa, the creep rate ≤ 2.5×10 - 5 h -1 . The service life of the manufactured magnesium reduction tank exceeds 2000 hours. Detailed implementation manners

[0032] 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 part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0033] Examples 1 - 4

[0034] A steel for magnesium reduction tanks includes the following mass percentage components: The balance not shown in Table 1 is Fe and unavoidable impurities.

[0035] Comparative Examples 1 - 4

[0036] A steel for magnesium reduction tanks includes the mass percentage components as described in Table 1: The balance not shown in Table 1 is Fe and unavoidable impurities.

[0037] Table 1 Chemical compositions of the examples and comparative examples of the present invention (wt%)

[0038] Steel grade C Si Mn Cr Mo Ni Nb N P S O Al Value A Example 1 0.47 0.11 1.18 7.7 0.42 0.62 0.036 0.051 0.016 0.003 0.0036 0.013 9.88 Example 2 0.58 0.14 1.58 8.2 0.65 0.79 0.039 0.078 0.017 0.006 0.0036 0.018 9.63 Example 3 0.53 0.14 1.59 7.6 0.54 0.67 0.049 0.058 0.015 0.008 0.0038 0.022 9.31 Example 4 0.48 0.18 1.21 8.1 0.62 0.76 0.039 0.062 0.013 0.005 0.0035 0.011 10.43 Comparative example 1 0.46 0.18 1.41 8.1 0.53 0.78 0.057 0.087 0.015 0.004 0.0034 0.028 9.52 Comparative example 2 0.53 0.13 1.35 7.9 0.55 0.75 0.048 0.088 0.017 0.005 0.0035 0.008 9.02 Comparative example 3 0.64 0.16 1.52 8.2 0.49 0.68 0.053 0.069 0.014 0.005 0.0032 0.014 9.71 Comparative example 4 0.48 0.16 1.46 8.4 0.48 0.62 0.052 0.064 0.018 0.003 0.0036 0.027 10.86

[0039] The production methods of the steel for magnesium reduction tanks in the above examples and comparative examples have the following process flow:

[0040] Converter smelting → LF furnace refining → RH vacuum degassing → Continuous casting of Φ800mm round billets;

[0041] The main processes are as follows:

[0042] Converter smelting: Determine the oxygen content before tapping. During the tapping process, the operation of leaving steel is adopted to avoid slagging;

[0043] LF furnace refining: Adjust elements such as C, Si, Mn, Cr, Mo, Ni, and Nb to the target values;

[0044] 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 phenomenon, and use nitrogen as the lifting gas;

[0045] Continuous casting of Φ800mm round billets: Produce continuous casting billets with a diameter of 800mm, with a superheat of 65°C, a drawing speed of 0.145m / min, and an end electromagnetic stirring of 5.3Hz.

[0046] The manufacturing process of the large-diameter vertical reduction tank for magnesium reduction using the steel in the above examples and comparative examples is as follows: Continuous casting of Φ800mm round billets → Punching → Heating of round billets → Pipe threading → Sizing → Heat treatment → Machining → Flaw detection → Packaging and warehousing. The key heat treatment processes are quenching and tempering:

[0047] The quenching: The heating temperature T1 is 1100 - 1200°C, and the holding time t1 is determined by the wall thickness S of the steel pipe, 5×S - T1 / 4 ≤ t1 ≤ 5×S - T1 / 16, air cooling. Among them, the unit of the holding time t1 is min, and the unit of the wall thickness S of the steel pipe is mm.

[0048] The tempering: The tempering temperature T2 is 600 - 700°C, and the holding time t2 is determined by the wall thickness S of the steel pipe, 5×S - T2 / 2 ≤ t2 ≤ 5×S - T2 / 10, air cooling. Among them, the unit of the holding time t2 is min, and the unit of the wall thickness S of the steel pipe is mm.

[0049] The main heat treatment parameters of each example and comparative example are shown in Table 2. The diameter is 1056mm.

[0050] Table 2 List of heat treatment process conditions of the examples and comparative examples of the present invention

[0051]

[0052] The performance detection method is as follows:

[0053] Organization: Samples are taken at the 1 / 2 radius of the steel pipe for metallographic and grain size analysis.

[0054] 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.

[0055] Table 3 List of performance detection conditions for the examples and comparative examples of the present invention

[0056]

[0057]

[0058] For Examples 1-4, the chemical composition and production method of the steel are properly controlled. Its chemical composition meets the requirements, and the strength and high-temperature creep performance of the steel are both good. In Comparative Example 1, the composition and heat treatment do not meet the requirements, resulting in poor effects. In Comparative Example 2, the requirement of value A is not met. Even according to the heat treatment process of this application, the high-temperature performance of the product is reduced, and the high-temperature creep performance is also reduced. In Comparative Example 3, the composition meets the requirements of the present invention, but the heat treatment is not carried out according to the present invention, resulting in a reduction in grain size, high-temperature strength, and creepability. In Comparative Example 4, the value of A does not meet the requirements, resulting in poor effects.

[0059] The underlined above are the data that do not meet the requirements of the present invention.

[0060] The above description of the embodiments is for the convenience of 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 labor. Therefore, the present invention is not limited to the above embodiments, and the 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 magnesium reduction tank steel comprises the following components in mass percentage: C 0.45%~0.65%, Si 0.05%~0.20%, Mn 1.10%~1.60%, Cr 7.50%~8.50%, Mo0.40%~0.70%, Ni 0.60~0.80%, Al 0.010%~0.025%, Nb 0.030%~0.060%, P≤0.020%, S≤0.010%, N 0.050~0.090%, O~0.0040%, and the balance is Fe and unavoidable impurities.

2. The steel for magnesium reduction tank according to claim 1, characterized in that: The composition of the steel for the magnesium reduction tank satisfies: 9.3%≤1.8×%Cr+1×%Si+2.5×%Mo+3×%Nb-3×%Ni-0.6×%Mn-25×%N-3×%C≤10.5%.

3. A method for producing the steel for magnesium reduction tank according to claim 1 or 2, characterized in that: The production method comprises the following process flow: electric arc furnace or converter smelting→LF furnace refining→RH or VD vacuum degassing→round billet continuous casting.

4. The production method according to claim 3, characterized in that: The round billet continuous casting controls the superheat at 40°C to 80°C.

5. The production method according to claim 3, characterized in that: For the round billet continuous casting, the billet diameter R and the drawing speed v should meet the following equation: R2×v=93100, wherein the billet diameter R is in mm and the drawing speed v is in m / min.

6. The production method according to claim 3, characterized in that: For the round billet continuous casting, the terminal electromagnetic stirring frequency f and the continuous casting billet diameter R should meet the requirement of R / f=150, wherein the terminal electromagnetic stirring frequency f is in Hz and the continuous casting billet diameter R is in mm.

7. A heat treatment process for a large-diameter vertical reduction tank for magnesium reduction, characterized in that: The magnesium reduction tank steel according to claim 1 or 2 is used to manufacture a large-diameter vertical reduction tank for magnesium reduction, and the heat treatment process includes quenching and tempering.

8. The heat treatment process according to claim 7, characterized in that: The quenching: heating temperature T1 is 1100-1200°C, and the holding time t1 is determined by the steel pipe wall thickness S, 5×S-T1 / 4≤t1≤5× S-T1 / 16, air cooling; the unit of holding time t1 is min, and the unit of steel pipe wall thickness S is mm.

9. The heat treatment process according to claim 7 or 8, characterized in that: The tempering: the tempering temperature T2 is 600-700°C, the holding time t2 is determined by the steel pipe wall thickness S, 5×S-T2 / 2≤t2≤5×S-T2 / 10, and air cooling; wherein the unit of the holding time t2 is min, and the unit of the steel pipe wall thickness S is mm.

10. A large-diameter vertical reduction tank for magnesium reduction, characterized in that: The magnesium reduction large diameter vertical reduction tank is manufactured by the heat treatment process described in any one of claims 7 to 9, wherein the grain size is 85-90 μm; the high temperature yield strength at 1280°C is ≥33 MPa, and the tensile strength is ≥37 MPa. The creep rate at 1280°C and 3 MPa stress is ≤2.5×10 -5 h -1 ;Service life exceeds 2000 hours.

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