A method for improving σ phase precipitation in super ferritic stainless steel

Abstract

The present invention relates to the technical field of ferritic stainless steel, specifically a method for improving the precipitation of σ phase in super ferritic stainless steel. Raw materials are prepared according to elemental ratios and smelted in an electric furnace. Blocks containing rare earth oxide particles are added to the molten steel at the end of the AOD furnace and the end of the LF furnace, respectively. The mass of the rare earth oxide particles in the blocks added in each stage accounts for 0.1% to 0.8% of the mass of the molten steel. Argon is blown bottom for stirring, and finally, billets are continuously cast. This method not only simplifies the overall production process but also reduces the size of the σ phase, improves its distribution, and refines the ferrite structure.

Description

Technical Field

[0001] The present invention relates to the technical field of ferritic stainless steel, in particular to a method for improving σ phase precipitation in super ferritic stainless steel. Background Art

[0002] Super ferritic stainless steel has excellent corrosion resistance in chloride media, comparable to super austenitic stainless steel and nickel-based alloys; it has high fatigue strength and hardness, and can withstand corrosion and erosion from seawater and mud; its seismic resistance is better than that of titanium tubes, and the tube ends are not easily blocked by microbial fouling; it has good thermal conductivity and a linear expansion coefficient close to that of carbon steel, while taking into account economy. It is the best choice for condenser cooling pipes in coastal power plants and a key variety of steel for major equipment in the field of marine engineering.

[0003] However, due to the high Cr and Mo content in super ferritic stainless steel, σ phase rich in Cr and Mo is easily precipitated at the grain boundaries during hot rolling, annealing and other processes (temperature range is about 750-960°C). On the one hand, it will reduce the mechanical properties of the material (especially impact toughness), and on the other hand, it will seriously affect the corrosion resistance.

[0004] At present, the precipitation of σ phase is mainly improved through high-temperature annealing and rapid cooling processes. The principle of high-temperature annealing is to avoid the temperature range of 750-960℃. The disadvantage of high-temperature annealing is the problems of grain coarsening and texture weakening. The principle of rapid cooling is to cross the sensitive temperature range before the σ phase is incubated and precipitated. The disadvantage of rapid cooling is that the requirements for process parameters such as cooling rate are extremely strict.

[0005] Chinese patent publication number CN112647026B discloses a "method for preparing high-chromium, high-molybdenum ferritic stainless steel." This approach addresses the aforementioned issues by strain-inducing the precipitation of the Laves phase at higher temperatures while avoiding the precipitation of the σ phase. This approach primarily avoids heating the material between 750°C and 960°C and holding it for extended periods. This method requires high temperature control during the preparation process and still carries the risk of σ phase precipitation. Chinese patent application publication number CN115652224A discloses a "super-ferritic stainless steel and its preparation method." This approach reduces the precipitation kinetics of the σ phase by introducing aluminum into the alloy system. However, the addition of aluminum increases the presence of alumina inclusions in the steel, requiring stricter control of inclusions. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, the present invention provides a method for improving the precipitation of σ phase in super ferritic stainless steel. This method not only simplifies the overall production process but also reduces the size of the σ phase, improves its distribution, and refines the ferrite structure.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for improving σ phase precipitation in super ferritic stainless steel is as follows:

[0009] Raw materials are configured according to element ratios and smelted in an electric furnace. Blocks containing rare earth oxide particles are added to the molten steel at the end of an AOD furnace and at the end of an LF furnace.

[0010] Control T[O] of AOD furnace to 15ppm before steel tapping, and control T[O] of LF furnace to 9ppm before steel tapping.

[0011] The mass of the rare earth oxide particles in the blocks added twice is 0.1% to 0.8% of the mass of the molten steel.

[0012] Argon is blown from the bottom for stirring, and finally the billet is continuously cast.

[0013] The method for preparing the block containing rare earth oxide particles specifically comprises the following steps:

[0014] 1. Weigh rare earth oxide particles, nickel powder, and iron powder and mix them.

[0015] The rare earth oxide particles are a mixture of at least two species of particles selected from cerium oxide (CeO 2 ), lanthanum oxide (La 2 O 3 ), yttrium oxide (Y 2 O 3 ), and lanthanum cerium oxide (LaCeO 2 ).

[0016] The particle size range of the rare earth oxide particles, nickel powder and iron powder is 1 to 10 μm.

[0017] The weight percentage of the rare earth oxide particles is 30-40%, the weight percentage of the nickel powder is 10-20%, and the balance is iron powder.

[0018] 2. Use hot isostatic pressing equipment to directly shape the mixed powder in a vacuum or inert gas environment at a molding temperature of 1000-1350°C to obtain a block.

[0019] Compared with the existing method, the present invention has the following beneficial effects:

[0020] 1. The blocks prepared by the present invention only need to be added to the molten steel through the silo at the end of the AOD and LF furnaces. The overall production process is simple and easy, and can meet the demand for additives in large-scale steelmaking production.

[0021] 2. After conventional rolling and heat treatment, the ingot produced by the present invention contains spherical rare earth oxide particles dispersed within the grains and at grain boundaries. These particles, with a size of 0.5 to 2.5 μm, can serve as nucleation sites for the σ phase, refining its size and achieving a more uniform distribution. Existing techniques typically add rare earth elements to molten steel, primarily to purify it, remove inclusions, and create microalloying. The present invention introduces rare earth oxide particles directly into the molten steel system, maximizing their distribution within the steel. These particles effectively serve as nucleation sites for the σ phase during subsequent rolling and heat treatment, controlling its size and distribution.

[0022] 3. After conventional hot rolling, the ingot produced by the present invention has no σ phase precipitation. After the hot-rolled plate is kept at 800°C for 1 hour, the average size of the σ precipitation phase is smaller, the distribution is more uniform, and the ferrite structure is finer.

[0023] In summary, the present invention introduces rare earth oxide particles into superferritic stainless steel, leveraging their role as nuclei for the σ phase, reducing the size of the σ phase and improving its distribution, while also refining the ferrite structure. Furthermore, the overall production process is simple and easy to implement, meeting the additive needs of large-scale steelmaking operations. DETAILED DESCRIPTION

[0024] The present invention discloses a method for improving the precipitation of σ phase in super ferritic stainless steel. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters to achieve the desired effect. It should be noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of the present invention to implement and apply the technology of the present invention.

[0025] Comparative Example:

[0026] The raw materials were configured according to the element ratio and stainless steel was smelted in an electric furnace. T[O] was measured to be 14ppm before AOD tapping and T[O] was measured to be 8ppm before LF tapping. Finally, the steel was continuously cast into billets.

[0027] Example 1:

[0028] 1. Rare earth oxide particles (particle size range 1 μm to 5 μm), nickel powder (particle size range 1 μm to 5 μm), and iron powder (particle size range 1 μm to 5 μm) are weighed and mixed according to mass percentages of 30%, 10%, and 60%, respectively. The rare earth oxide particles are composed of cerium oxide (CeO2) and yttrium oxide (Y2O3), and the ratio of the two is 1:1.

[0029] 2. Use hot isostatic pressing equipment to directly shape the mixed powder in a vacuum environment at a molding temperature of 1000°C to obtain a block.

[0030] 3. Raw materials were prepared according to the element ratio and stainless steel was smelted in an electric furnace. 8.5 kg / t of lump was added for the first time before tapping the AOD furnace (T[O]=14 ppm). 8.5 kg / t of lump was added for the second time before tapping the LF furnace (T[O]=6 ppm). Argon was blown from the bottom for stirring, and the steel was finally continuously cast into billets.

[0031] Example 2:

[0032] 1. Rare earth oxide particles (particle size range 5μm to 10μm), nickel powder (particle size range 5μm to 10μm), and iron powder (particle size range 5μm to 10μm) are weighed and mixed according to the mass percentages of 40%, 20%, and 40%, respectively. The rare earth oxide particles are composed of cerium oxide (CeO2), yttrium oxide (Y2O3), and lanthanum cerium oxide (LaCeO2), and the ratio of the three is 2:1:1.

[0033] 2. Use hot isostatic pressing equipment to directly shape the mixed powder under vacuum environment at a molding temperature of 1350°C to obtain a block.

[0034] 3. Raw materials were prepared according to the element ratio and stainless steel was smelted in an electric furnace. 12 kg / t of lump was added for the first time before tapping the AOD furnace (T[O]=10 ppm). 13 kg / t of lump was added for the second time before tapping the LF furnace (T[O]=7 ppm). Argon was blown from the bottom for stirring, and the steel was finally continuously cast into billets.

[0035] Example 3:

[0036] 1. Rare earth oxide particles (particle size range 3μm to 8μm), nickel powder (particle size range 3μm to 8μm), and iron powder (particle size range 3μm to 8μm) are weighed and mixed according to mass percentages of 35%, 15%, and 50%, respectively. The rare earth oxide particles are composed of cerium oxide (CeO2), lanthanum oxide (La2O3), yttrium oxide (Y2O3), and lanthanum cerium oxide (LaCeO2), and the ratio of the four is 2:1:1:1.

[0037] 2. Use hot isostatic pressing equipment to directly shape the mixed powder under vacuum environment at a molding temperature of 1150°C to obtain a block.

[0038] 3. Raw materials were prepared according to the element ratio and stainless steel was smelted in an electric furnace. 11 kg / t of lump was added for the first time before AOD tapping (T[O]=11 ppm), and 10 kg / t of lump was added for the second time before LF tapping (T[O]=8 ppm). Argon was blown from the bottom for stirring, and the steel was finally continuously cast into billets.

[0039] The steel grades of the comparative example and the example are shown in Table 1. The main chemical compositions of the four heats of steel are basically the same, and the S content of the comparative example steel is significantly reduced.

[0040] Table 1 Chemical composition of steel ingots of comparative examples and examples, wt%

[0041] Steel C P S Cr Ni Mo Nb Ti Al Comparative Example 0.011 0.007 0.0026 27.1 2.1 4.0 0.24 0.2 0.06 Example 1 0.013 0.006 0.0022 27.2 2.0 3.8 0.26 0.2 0.05 Example 2 0.012 0.006 0.0016 27.2 2.1 3.7 0.25 0.2 0.05 Example 3 0.013 0.006 0.0014 27.3 2.1 3.9 0.27 0.2 0.05

[0042] After hot-rolling four heats of steel into 5 mm plates, metallographic and scanning electron microscopic examinations of the core of the rolled plates were performed to observe the presence and distribution of rare earth oxide particles and σ phase. The results are shown in Table 2. As can be seen, compared with the comparative example, the hot-rolled plates of Examples 1-3 exhibit no σ phase precipitation, and the rare earth oxides are mostly dispersed in spherical or ellipsoidal shapes. This comparison reveals a significant improvement in the σ phase precipitation problem in super ferritic stainless steel.

[0043] Table 2 Hot rolled plate microstructure analysis results

[0044]

[0045] In addition to being easily present during hot rolling, the σ phase is even more likely to precipitate during annealing. To further observe the precipitation of σ, the comparative example and the example were heat treated at 1050°C for 10 minutes to dissolve the precipitated phase back into the matrix, followed by a 1-hour heat treatment at 800°C to induce a large amount of σ phase precipitation. This allows for more intuitive statistical analysis of the microstructure differences between the comparative example and the example. Metallographic and scanning electron microscopic observations were also performed, and the results are shown in Table 3.

[0046] A comparison revealed that after heat treatment, the σ phase in the comparative example significantly increased in size, with an average size of 20 μm, and the metallographic structure showed that the ferrite grains varied in size, with an average size of up to 160 μm. In Examples 1 to 3, the σ phase was significantly smaller, with an average size between 7 and 9 μm, and the metallographic structure showed that the ferrite grains were more uniform, with an average size between 75 and 85 μm. This shows that the present invention significantly improves the σ phase precipitation problem in heat-treated materials.

[0047] Table 3 Microstructure analysis results of hot-rolled plates after heat treatment

[0048]

[0049] The blocks prepared by the present invention only need to be added to the molten steel through the silo at the end of the AOD and LF furnaces. The overall production process is simple and easy, and can meet the demand for additives in large-scale steelmaking production.

[0050] After conventional rolling and heat treatment, the ingot produced by the present invention contains spherical rare earth oxide particles dispersed within the grains and at grain boundaries. These particles, with a size of 0.5 to 2.5 μm, can serve as nucleation sites for the σ phase, refining its size and achieving a more uniform distribution. Existing patents all incorporate rare earth elements into molten steel, primarily to purify the molten steel (removing oxygen and sulfur), modify inclusions (for example, transforming alumina inclusions into rare earth aluminum oxide, rare earth oxides, and rare earth oxysulfides), and to achieve microalloying. The present invention, however, introduces rare earth oxide particles directly into the molten steel system. This advancement is reflected in its ability to maximize the distribution of the rare earth oxide particles within the steel, effectively serving as nucleation sites for the σ phase during subsequent rolling and heat treatment, controlling its size and distribution.

[0051] After conventional hot rolling, the ingot produced by the present invention has no σ phase precipitation in the plate. After the hot-rolled plate is kept at 800° C. for 1 hour, the average size of the σ precipitation phase is smaller, the distribution is more uniform, and the ferrite structure is finer.

[0052] This invention introduces rare earth oxide particles into superferritic stainless steel, leveraging them to act as nuclei for the σ phase, reducing its size and improving its distribution while simultaneously refining the ferrite structure. The overall production process is simple and easy to implement, meeting the additive needs of large-scale steelmaking operations.

[0053] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for improving the precipitation of σ phase in super ferritic stainless steel, characterized in that: The raw materials are prepared according to the element ratio and smelted in an electric furnace, and the blocks containing rare earth oxide particles are added to the molten steel at the end of the AOD furnace and the end of the LF furnace respectively; The components and weight percentages of the block are as follows: rare earth oxide particles account for 30% to 40% by weight, nickel powder accounts for 10% to 20% by weight, and the balance is iron powder; Control T[O] before tapping of AOD furnace to 15ppm, and control T[O] before tapping of LF furnace to 9ppm; The mass of the rare earth oxide particles in the blocks added twice is 0.1% to 0.8% of the mass of the molten steel; Blow argon gas from the bottom for stirring, and finally continuously cast into billets; By introducing rare earth oxide particles into super ferritic stainless steel, the rare earth oxide particles can play the role of σ phase nucleation particles, reduce the size of σ phase, improve the distribution state of σ phase, and refine the ferrite structure.

2. The method for improving σ phase precipitation in super ferrite stainless steel according to claim 1, characterized in that: The preparation method of the block containing rare earth oxide particles specifically comprises the following steps: 1) Weighing rare earth oxide particles, nickel powder, and iron powder and mixing them; The rare earth oxide particles are at least two of cerium oxide CeO2, lanthanum oxide La2O3, yttrium oxide Y2O3, and lanthanum cerium oxide LaCeO2; the particle size of the rare earth oxide particles, nickel powder, and iron powder ranges from 1 to 10 μm; 2) Using molding equipment to directly mold the mixed powder in a vacuum or inert gas environment at a molding temperature of 1000-1350° C. to obtain a block.

3. The method for improving σ phase precipitation in super ferrite stainless steel according to claim 2, characterized in that: In step 2) of the method for preparing a block containing rare earth oxide particles, the molding equipment is a hot isostatic pressing equipment.

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