Process for the production of large size s31254 super austenitic stainless steel bars

Abstract

The present application relates to the technical field of metal forging, and particularly relates to a production process of large-size S31254 super austenitic stainless steel rod, which comprises the following steps: smelting S31254 super austenitic stainless steel by adopting an electric furnace+AOD+LF smelting process to obtain an electrode blank with oxygen content of less than or equal to 20 ppm; electric-shock remelting the electrode blank to obtain an electric-shock ingot with oxygen content of less than or equal to 20 ppm; heating the electric-shock ingot to 1200-1250 DEG C and keeping the temperature for 50-60 hours to perform homogenization diffusion, then cooling to 1140-1180 DEG C and keeping the temperature for 6-12 hours, and then taking out the electrode blank to perform forging to obtain S31254 super austenitic stainless steel rod. The production process simultaneously controls the oxygen content to be less than or equal to 20 ppm by electric-shock remelting and homogenization diffusion, and large-size S31254 super austenitic stainless steel rod without precipitated phase is obtained, the rod surface is free of cracks, the yield is high, and the rod has excellent seawater corrosion resistance, and has high practical application value.

Description

Technical Field

[0001] This invention relates to the field of metal forging technology, and in particular to a production process for large-size S31254 super austenitic stainless steel bars. Background Technology

[0002] S31254 super austenitic stainless steel possesses excellent resistance to seawater corrosion and is widely used in harsh environments such as oil and petrochemical plants and offshore platforms. Its usage and demand are increasing year by year, and the specifications for bars made from S31254 super austenitic stainless steel are also expanding from the initial... Growth to However, S31254 super austenitic stainless steel has a high Mo content, which easily leads to segregation during production. When the molten steel solidifies, Mo- and Cr-rich intermetallic phases (i.e., precipitates) will form between the dendrites. These precipitates, rich in Mo and Cr, result in Mo and Cr depletion near the precipitates, thus reducing the material's resistance to seawater corrosion and its overall mechanical properties. Therefore, eliminating precipitates generated during hot working is crucial for obtaining large-size S31254 material with high seawater corrosion resistance and excellent overall mechanical properties.

[0003] Currently, when producing small-sized S31254 super austenitic stainless steel forgings, the segregation problem can be solved by using refining processes, casting small ingots, and increasing diffusion. However, this method is not suitable for producing larger-sized S31254 super austenitic stainless steel forgings. Summary of the Invention

[0004] To address the above technical problems, this invention provides a production process for large-size S31254 super austenitic stainless steel bars. By employing an electroslag remelting and homogenization diffusion process while controlling oxygen content, large-size S31254 super austenitic stainless steel bars with a diameter ≥350mm and a weight ≥3t without precipitates are obtained.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] The first aspect of this invention provides a production process for large-size S31254 super austenitic stainless steel bars, specifically including the following steps:

[0007] S1. S31254 super austenitic stainless steel is cast using the electric furnace + AOD + LF smelting process to obtain electrode billets with an oxygen content ≤20ppm.

[0008] S2. Electroslag remelting is performed on the electrode blank obtained in S1 to obtain an electroslag ingot with an oxygen content ≤20ppm.

[0009] S3. The electroslag ingot obtained from S2 is heated to 1200-1250℃ and held for 50-60h for homogenization and diffusion, then cooled to 1140-1180℃ and held for 6-12h, and then forged to obtain S31254 super austenitic stainless steel bar.

[0010] Through extensive research and experimentation on the precipitate problem in large-size S31254 super austenitic stainless steel bars, the inventors creatively proposed a process combining electroslag remelting and homogenization diffusion while simultaneously controlling the oxygen content of the electrode billet and electroslag ingot to 20 ppm or below. This process yields large-size S31254 super austenitic stainless steel without precipitates. Specifically, in the electroslag remelting step, the solidification rate of the electroslag molten steel is much higher than that of the cast ingot due to the circulating cooling water at the edge of the electroslag, effectively reducing the degree of segregation within the material. Combined with the homogenization diffusion treatment step, this achieves complete elimination of precipitates. Simultaneously, controlling the oxygen content in the electrode billet and electroslag ingot to 20 ppm or below ensures that the resulting large-size S31254 super austenitic stainless steel possesses good plasticity. Furthermore, cooling before forging after homogenization diffusion ensures that no cracks appear on the material surface during forging, thereby improving the yield.

[0011] In conjunction with the first aspect, the specifications of the S31254 super austenitic stainless steel bar described in S3 are as follows: Weight ≥ 3t.

[0012] Preferably, the specifications of the electrode blank in S1 are as follows: The weight of the electrode blank is ≥4t, and this weight can guarantee that the weight of the final rod is ≥3t.

[0013] Preferably, the specifications of the electroslag ingot in S2 are as follows: The specifications of this electroslag ingot are much higher than those of the precipitate-free S31254 super austenitic stainless steel forgings that can be obtained by existing production processes (the ingot side length is about 520mm), resulting in large-size precipitate-free S31254 super austenitic stainless steel bars.

[0014] Preferably, the specifications of the S31254 super austenitic stainless steel bar described in S3 are as follows:

[0015] In conjunction with the first aspect, the heating rate described in S3 is 80–120 °C / h, preferably 100 °C / h.

[0016] In conjunction with the first aspect, the final forging temperature of the forging step described in S3 is ≥900℃. Forging temperatures within this range can ensure that the resulting bar stock has good plasticity and excellent comprehensive properties.

[0017] Optionally, S1 uses at least one of aluminum powder and aluminum wire for deoxidation. Aluminum powder can be dispersed on the surface of molten steel for diffusion deoxidation, or aluminum wire can be inserted deep into the molten steel for deoxidation, or both aluminum powder and aluminum wire can be used for deoxidation.

[0018] Optionally, S2 uses aluminum powder for deoxidation.

[0019] In conjunction with the first aspect, the composition of the S31254 super austenitic stainless steel includes: C≤0.02%, Si≤0.80%, Mn≤1.00%, P≤0.03%, S≤0.01%, Cr: 19.5%~20.5%, Ni: 17.5%~18.5%, Cu: 0.50%~1.00%, Mo: 6.00%~6.50%, N: 0.18%~0.22%, with the balance being Fe and unavoidable impurities.

[0020] The second aspect of the present invention provides a large-size S31254 super austenitic stainless steel bar prepared according to the above-described production process. The bar has a diameter ≥350mm, a weight ≥3t, no surface cracks, and good resistance to seawater corrosion.

[0021] The beneficial effects of this invention are as follows: By combining electroslag remelting and homogenization diffusion with a production process that simultaneously controls the oxygen content to 20 ppm or below, this invention yields large-diameter S31254 super austenitic stainless steel bars with a diameter ≥350 mm and a weight ≥3 t. These large-diameter bars fill the gap in the production of large-diameter S31254 super austenitic stainless steel bars without precipitates, and exhibit excellent resistance to seawater corrosion, with no surface cracks, thus possessing high practical application value. Attached Figure Description

[0022] Figure 1 A surface photograph of the large-size S31254 super austenitic stainless steel bar in Example 4;

[0023] Figure 2 A surface photograph of the large-size S31254 super austenitic stainless steel bar in Example 4;

[0024] Figure 3 Metallographic image of the large-size S31254 super austenitic stainless steel bar in Example 4;

[0025] Figure 4 Metallographic image of the large-size S31254 super austenitic stainless steel bar in Example 5;

[0026] Figure 5 Metallographic image of the large-size S31254 super austenitic stainless steel bar in Comparative Example 1.

[0027] Figure 6 Metallographic image of the large-size S31254 super austenitic stainless steel bar in Comparative Example 2.

[0028] Figure 7 A surface photograph of the large-size S31254 super austenitic stainless steel bar in Comparative Example 3;

[0029] Figure 8 A photograph of the "pits" on the surface of the bar with severe surface cracks in Comparative Example 3 after turning and grinding.

[0030] Figure 9 The image shows the residual cracks on the surface of the bar obtained after turning and grinding the bar with severe surface cracks in Comparative Example 3.

[0031] Figure 10 This is a photograph of the cracks on the surface of the pipe after the bar with severe surface cracks in Comparative Example 3 was heat-processed into a pipe. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0033] The larger the size of the S31254 super austenitic stainless steel ingot, the more severe the segregation, and the more difficult it is to eliminate the precipitated phases. During experiments, the inventors discovered that the largest ingot size capable of producing precipitate-free S31254 super austenitic stainless steel forgings using existing technology is only an ingot with a side length of approximately 520 mm. Furthermore, a 70-hour diffusion treatment is required to produce precipitate-free bars. Moreover, as the ingot size increases further, due to the intensified segregation, no matter how long the diffusion time is extended (precipitates still exist even after 100 hours), it is impossible to produce precipitate-free bars. Further increasing the diffusion time would not only affect production efficiency but also cause severe overheating of the product, resulting in serious surface cracks during forging. Through extensive experimental research on the precipitate problem of large-size S31254 super austenitic stainless steel bars, the inventors creatively proposed a production process that combines electroslag remelting and homogenization diffusion while controlling the oxygen content to 20 ppm or below. This production process can solve the current problem of not being able to obtain large-size S31254 bars without precipitates.

[0034] The technical solution of the present invention will be described below through specific embodiments.

[0035] The composition of the S31254 super austenitic stainless steel used in the following embodiments, by weight percentage, specifically includes: C: 0.015%, Si: 0.42%, Mn: 0.45%, P: 0.029%, S: 0.001%, Cr: 20.0%, Ni: 17.8%, Cu: 0.70%, Mo: 6.25%, N: 0.19%, with the balance being Fe and unavoidable impurities.

[0036] Example 1

[0037] This embodiment provides a production process for large-size S31254 super austenitic stainless steel bars, specifically including the following steps:

[0038] S1, S31254 super austenitic stainless steel is smelted using an electric furnace + AOD + LF process, and cast to obtain an oxygen content of 15ppm, with specifications of... An electrode blank weighing 5.5t;

[0039] S2. Electroslag remelting is performed on the electrode blank obtained in S1 to obtain an oxygen content of 19 ppm and a specification of [specification missing]. Electroslag ingots;

[0040] S3. The electroslag ingot obtained in S2 is heated to 1225℃ at a rate of 100℃ / h and held for 55h for homogenization and diffusion. Then it is cooled to 1150℃ and held for 10h. It is then taken out of the furnace and forged at a final forging temperature of ≥900℃ to obtain S31254 super austenitic stainless steel bar.

[0041] Among them, S1 and S2 use aluminum powder to deoxidize molten steel.

[0042] Example 2

[0043] This embodiment provides a production process for large-size S31254 super austenitic stainless steel bars, specifically including the following steps:

[0044] S1. S31254 super austenitic stainless steel is smelted using an electric furnace + AOD + LF process, and cast to obtain an oxygen content of 10ppm and specifications. An electrode blank weighing 4.6t;

[0045] S2. Electroslag remelting is performed on the electrode blank obtained in S1 to obtain an oxygen content of 19 ppm and a specification of [specification missing]. Electroslag ingots;

[0046] S3. The electroslag ingot obtained in S2 is heated to 1200℃ at a rate of 80℃ / h and held for 50h for homogenization and diffusion. Then it is cooled to 1140℃ and held for 12h. It is then taken out of the furnace and forged at a final forging temperature of ≥900℃ to obtain S31254 super austenitic stainless steel bar.

[0047] S1 uses aluminum powder and aluminum wire for deoxidation, while S2 uses aluminum powder to deoxidize molten steel.

[0048] Example 3

[0049] This embodiment provides a production process for large-size S31254 super austenitic stainless steel bars, specifically including the following steps:

[0050] S1. S31254 super austenitic stainless steel is cast using an electric furnace + AOD + LF smelting process, yielding an oxygen content of 18ppm and a specification of... An electrode blank weighing 5.8t;

[0051] S2. Electroslag remelting is performed on the electrode blank obtained in S1 to obtain an oxygen content of 15 ppm and a specification of [specification missing]. Electroslag ingots;

[0052] S3. The electroslag ingot obtained in S2 is heated to 1250℃ at a rate of 120℃ / h and held for 60h for homogenization diffusion. Then it is cooled to 1180℃ and held for 6h. It is then taken out of the furnace and forged at 910~1060℃ to obtain S31254 super austenitic stainless steel bar.

[0053] Among them, S1 and S2 use aluminum powder to deoxidize molten steel.

[0054] Example 4

[0055] This embodiment provides a large-size S31254 super austenitic stainless steel bar, prepared according to the production process provided in Example 1. The specifications of this bar are as follows: It weighs 4.5 tons, and its surface photo is as follows. Figures 1-2 As shown, the metallographic diagram is as follows Figure 3 As shown.

[0056] Example 5

[0057] This embodiment provides a large-size S31254 super austenitic stainless steel bar, prepared according to the production process provided in Example 2. The specifications of this bar are as follows: It weighs 3.9t, and its metallographic diagram is as follows. Figure 4 As shown.

[0058] Example 6

[0059] This embodiment provides a large-size S31254 super austenitic stainless steel bar, prepared according to the production process provided in Example 3. The specifications of this bar are as follows: It weighs 3.4 tons.

[0060] Comparative Example 1

[0061] This comparative example provides a production process for large-size S31254 super austenitic stainless steel bars. This process involves smelting and directly casting 24-inch steel ingots (riseer size 600mm) according to step S1 in Example 1, without electroslag remelting. All other steps are the same as in Example 1, yielding large-size S31254 super austenitic stainless steel bars, the metallographic diagram of which is shown below. Figure 5 As shown.

[0062] Comparative Example 2

[0063] This comparative example provides a production process for large-size S31254 super austenitic stainless steel bars, specifically including the following steps: Following step S1 in Example 1, the steel is smelted and directly cast into 24-inch ingots (riseer size 600mm), without electroslag remelting, but the homogenization diffusion time is extended to 100 hours. Then, following step S3 in Example 1, the steel is cooled, held at room temperature, unloaded, and forged to obtain large-size S31254 super austenitic stainless steel bars, the metallographic diagram of which is shown below. Figure 6 As shown.

[0064] Comparative Example 3

[0065] This comparative example provides a production process for large-size S31254 super austenitic stainless steel bars. This process controls the oxygen content in the electrode billet and electroslag ingot to 35 ppm, and the remaining steps are the same as in Example 1, yielding large-size S31254 super austenitic stainless steel bars. A photograph of the bar surface is shown below. Figure 7 As shown.

[0066] It can be seen that when the oxygen content in the electrode blank and electroslag ingot is >20ppm, the surface of the obtained rod has obvious coarse cracks when the same production process as in Example 1 is followed. These coarse cracks extend into the interior of the rod, which seriously affects the subsequent use of the rod.

[0067] In practical applications, if these bars with large surface cracks are to be utilized, they need to be machined to a certain extent. However, even then, some cracks will still remain, so further grinding is necessary to remove them. But this will leave a lot of pits on the surface of the resulting bar (such as...). Figure 8 As shown), this results in uneven thickness of the rods, limiting their use; in addition, the polished rods may retain some microscopic cracks invisible to the naked eye (such as...). Figure 9 As shown, the presence of cracks can be confirmed by surface penetration testing, severely affecting the subsequent use of the bar stock. Even if the bar stock with surface cracks is processed into tubes through hot working, the resulting tubes will still have cracks on their surface (e.g., ...). Figure 10 As shown), its use is therefore restricted.

[0068] Test Example

[0069] The large-size S31254 super austenitic stainless steel bars prepared in Examples 4-6 and Comparative Examples 1-3, as well as conventional steel grades 304L and 316L, were subjected to corrosion resistance tests. The corrosion resistance was tested on the solid solution state of the materials, and the ASTM G48 Method A was used for 5 cycles, with each cycle lasting 48 hours. The specific test results are shown in Table 1.

[0070] Table 1. Relevant performance test results

[0071]

[0072] As shown in Table 1, the bars produced according to the manufacturing process of large-size S31254 super austenitic stainless steel bars provided by this invention exhibit excellent corrosion resistance. Meanwhile, data from Comparative Examples 1 and 2 show that without electroslag remelting, even extending the homogenization diffusion time to 100 hours cannot effectively eliminate the precipitated phases. Data from Comparative Example 3 shows that if the oxygen content in the electrode billet and electroslag ingot is not controlled to 20 ppm or below, the resulting bars suffer from severe surface cracking, rendering them unusable.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A production process for large-size S31254 super austenitic stainless steel bars, characterized in that, Specifically, the following steps are included: S1. S31254 super austenitic stainless steel is cast using the electric furnace + AOD + LF smelting process to obtain electrode billets with an oxygen content of ≤20ppm. S2. Electroslag remelting is performed on the electrode blank obtained in S1 to obtain an electroslag ingot with an oxygen content ≤20ppm. S3. The electroslag ingot obtained from S2 is heated to 1200~1250℃ and held for 50~60h for homogenization and diffusion, then cooled to 1140~1180℃ and held for 6~12h, and then forged to obtain S31254 super austenitic stainless steel bar. The electrode blank has a specification of φ=570±20mm and a weight of ≥4t, the electroslag ingot has a specification of φ=750±20mm, and the S31254 super austenitic stainless steel bar has a specification of φ≥350mm and a weight of ≥3t.

2. The production process of large-size S31254 super austenitic stainless steel bars as described in claim 1, characterized in that, The specifications of the S31254 super austenitic stainless steel bar mentioned in S3 are φ=350~550mm.

3. The production process for large-size S31254 super austenitic stainless steel bars as described in claim 1, characterized in that, The heating rate described in S3 is 80~120℃ / h.

4. The production process of large-size S31254 super austenitic stainless steel bars as described in claim 1, characterized in that, The final forging temperature of the forging step described in S3 is ≥900℃.

5. The production process for large-size S31254 super austenitic stainless steel bars as described in claim 1, characterized in that, S1 uses at least one of aluminum powder and aluminum wire for deoxidation; and / or S2 uses aluminum powder for deoxidation.

6. The production process for large-size S31254 super austenitic stainless steel bars as described in claim 1, characterized in that, The composition of the S31254 super austenitic stainless steel includes: C≤0.02%, Si≤0.80%, Mn≤1.00%, P≤0.03%, S≤0.01%, Cr: 19.5%~20.5%, Ni: 17.5%~18.5%, Cu: 0.50%~1.00%, Mo: 6.00%~6.50%, N: 0.18%~0.22%, with the balance being Fe and unavoidable impurities.

7. A large-size S31254 super austenitic stainless steel bar, characterized in that, It is prepared according to the production process described in any one of claims 1 to 6.

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