Ni-Cr-Ti type austenitic stainless steel and manufacturing method thereof

Abstract

The invention relates to the technical field of pipe preparation, and particularly discloses Ni-Cr-Ti type austenitic stainless steel and a manufacturing method thereof. The high-temperature performance of the material is remarkably improved by controlling the content of C in the electrode blank to be 0.06%-0.07% and the content of Mo to be 0.3%-0.4%. And meanwhile, a staged temperature control mode is adopted in the forging process, accurate regulation and control over grain crushing and recrystallization are achieved, the heat preservation time is shortened to 1-2 h in the last fire, abnormal grain growth is avoided, the problem of edge crystal mixing of a traditional process is thoroughly solved, and the material structure uniformity is guaranteed. Furthermore, low-temperature solid solution treatment is innovatively adopted, the excessive growth of austenite grains is reduced and a proper amount of fine TiC precipitated phase is reserved while sufficient solid solution of the Cr element is ensured to maintain corrosion resistance, and the precipitation phase coarsening is further inhibited in cooperation with water-cooling rapid cooling, so that the synergistic improvement of high-temperature strength and plasticity is finally realized.

Description

Technical Field

[0001] This invention relates to the field of pipe manufacturing technology, and in particular to a Ni-Cr-Ti type austenitic stainless steel and its manufacturing method. Background Technology

[0002] 06Cr18Ni11Ti stainless steel, as a typical austenitic stainless steel, is widely used in the manufacture of high-temperature components in aerospace, petrochemical, power plant boiler and other fields due to its good corrosion resistance and processing performance. These components include high-temperature pipes, heat exchanger cores, combustion chamber parts, etc. Their service environment often faces high-temperature conditions, which puts forward stringent requirements on the material's high-temperature strength, creep resistance and structural stability.

[0003] However, existing 06Cr18Ni11Ti stainless steel exhibits significant performance shortcomings during long-term high-temperature service: Firstly, the relatively low carbon and titanium content in its composition directly leads to poor high-temperature performance stability, making it difficult to meet the requirements of long-term high-temperature service; secondly, the Ti element within the material easily combines with the C element to form TiC precipitates, which can improve room temperature strength to some extent, but at high temperatures, the TiC phase is prone to coarsening, resulting in decreased grain boundary bonding, which in turn leads to high-temperature plasticity decay and increased creep deformation; thirdly, the traditional rolling + solution treatment manufacturing process makes it difficult to achieve uniform grain refinement, resulting in uneven grain size distribution and easy occurrence of mixed grains at the edges, directly affecting the high-temperature tensile properties of the material, making the components more susceptible to intergranular corrosion or fatigue crack propagation under high-temperature cyclic loading, significantly shortening their service life.

[0004] As high-end equipment develops towards higher parameters and longer service life, the high-temperature performance of existing 06Cr18Ni11Ti stainless steel can no longer meet the stringent service requirements. There is an urgent need to develop a low-cost and easily industrialized manufacturing method to fundamentally improve its high-temperature strength and structural stability, and ensure the safe and reliable operation of high-temperature components. Summary of the Invention

[0005] To address the issue that the high-temperature performance of existing 06Cr18Ni11Ti stainless steel needs further improvement, this invention provides a Ni-Cr-Ti type austenitic stainless steel and its manufacturing method.

[0006] To solve the above-mentioned technical problems, the technical solution provided by the embodiments of the present invention is as follows: A method for manufacturing Ni-Cr-Ti type austenitic stainless steel includes the following steps: S1, the raw materials are smelted in an electric arc furnace, refined by AOD, refined by LF, and cast to obtain an electrode billet; the electrode billet is then subjected to electroslag remelting to obtain an electroslag ingot; the electrode billet contains 0.06%~0.07% C and 0.3%~0.4% Mo; S2, the electroslag ingot is forged to obtain a forged steel billet with a total forging ratio ≥4; wherein, the heating temperature before the first fire drawing is 1160℃~1180℃, and after the first fire drawing is completed, the billet after the first fire drawing is subjected to several fire drawing processes. Before each drawing process, the billet is heated to 1020℃~1080℃; the holding time before the last drawing process is 1h~2h; except for the last drawing process, the holding time before the other drawing processes is 2h~3h. S3, the forged steel billet is heated to 940℃~980℃ for solution treatment and then water-cooled to obtain Ni-Cr-Ti type austenitic stainless steel.

[0007] Compared to existing technologies, the manufacturing method of Ni-Cr-Ti austenitic stainless steel provided by this invention, by controlling the C content in the electrode billet to 0.06%~0.07% and the Mo content to 0.3%~0.4%, not only avoids insufficient high-temperature strength caused by low C content, but also enhances grain boundary bonding through the solid solution strengthening effect of Mo, suppressing high-temperature coarsening of the TiC phase, thus significantly improving the high-temperature performance of the material. Simultaneously, the forging process employs a staged temperature control method (heating at 1160℃~1180℃ for the first heating stage and 1020℃~1080℃ for the drawing stage), achieving precise control of grain breakage and recrystallization. The final heating stage shortens the holding time to 1h~2h, avoiding abnormal grain growth and completely solving the problem of edge mixed grains in traditional processes, ensuring the uniformity of the material microstructure. Furthermore, this invention innovatively employs low-temperature (940℃~980℃) solution treatment, which ensures that the Cr element is fully dissolved to maintain corrosion resistance while reducing the excessive growth of austenite grains, retaining an appropriate amount of fine TiC precipitates, and further suppressing the coarsening of precipitates by water cooling, ultimately achieving a synergistic improvement in high-temperature strength and plasticity.

[0008] It should be noted that the Ni-Cr-Ti type austenitic stainless steel described in this invention is 06Cr18Ni11Ti stainless steel.

[0009] It should be noted that the electric arc furnace smelting, AOD refining, LF refining, casting and electroslag remelting processes described in this invention are all existing technologies and can be carried out using conventional processes of existing 06Cr18Ni11Ti stainless steel production. It is only necessary to control the C and Mo content in the above processes within the range defined by this invention.

[0010] Furthermore, in the electric arc furnace smelting process, the tapping temperature is 1630℃~1650℃, and the carbon content in the molten steel is 2.0%~2.5%.

[0011] The carbon content in the molten steel is set at 2.0%~2.5%. This carbon content level can be achieved by removing harmful gases such as N and H from the molten steel through the oxygen blowing decarburization process during the AOD refining stage, reducing the formation of subsequent inclusions, and helping to achieve the target carbon content of 0.06%~0.07% in the electrode billet, ensuring the compositional basis required for high-temperature performance.

[0012] Furthermore, in the AOD refining process, the C content in the molten steel is controlled to be 0.05%~0.06% and the Mo content to be 0.3%~0.4%.

[0013] Starting with an initial C content of 2.0%~2.5% in the electric arc furnace, the C content in the electrode billet is precisely controlled at 0.06%~0.07% through AOD refining and decarburization, LF refining and fine-tuning, and the purification effect of electroslag remelting. At the same time, 0.3%~0.4% of Mo element is introduced. This composition system not only avoids the insufficient high-temperature strength caused by low C, but also improves the grain boundary bonding force through the solid solution strengthening effect of Mo element, inhibits the high-temperature coarsening of TiC phase, and significantly improves the high-temperature stability of the material.

[0014] Furthermore, in the LF refining process, the ladle temperature is 1570℃~1580℃.

[0015] Further, the chemical composition of the electrode blank is: C 0.06%~0.07%, Si 0.35%~0.55%, Mn 1.00%~1.50%, P≤0.033%, S≤0.002%, Cr 17.30%~17.70%, Ni 9.10%~9.50%, Ti 0.55%~0.65%, Mo 0.30%~0.40%, with the balance being Fe and unavoidable impurities.

[0016] Furthermore, in S1, the slag system in the electroslag remelting process is CaF2:Al2O3:CaO=60%:20%:20%.

[0017] Furthermore, in S2, the total number of firing cycles for the lengthening process is 2.

[0018] Furthermore, in S2, the deformation amount of the final fire is 30%~60%.

[0019] This deformation ensures that both the edges and the core of the billet undergo sufficient deformation and recrystallization, effectively eliminating the mixed crystal phenomenon caused by insufficient edge deformation in traditional processes. This ensures that the grain size distribution of the entire billet cross section is consistent, avoids stress concentration caused by uneven microstructure, and improves the performance stability and reliability of the material during high-temperature service.

[0020] Furthermore, in S2, the final forging temperature is 800℃~850℃.

[0021] Furthermore, in S2, the forging process employs an octagonal forging method.

[0022] Furthermore, in S2, the final drawing process has 9 to 11 drawing passes, and the remaining drawing processes have 6 to 8 drawing passes.

[0023] Furthermore, the specific steps of S2 are as follows: S201, after heating the electroslag ingot to 1160℃~1180℃ and holding it for 4h~6h, it is then upsetting and drawing to obtain a fire-forged billet. S202, after heating the first-fired forging billet to 1020℃~1080℃ and holding it for 2h~3h, it is drawn out twice to obtain the second-fired forging billet; S203, the forged billet is heated to 1020℃~1080℃ and held for 1h~2h, and then drawn out three times to obtain a forged steel billet.

[0024] First, the electroslag ingot is heated to 1160℃~1180℃ and held for 4h~6h. On the one hand, the high temperature can eliminate the component segregation formed during the solidification process of the electroslag ingot, making the distribution of elements such as C, Ti, and Mo more uniform, and avoiding performance fluctuations caused by uneven composition in subsequent processing. On the other hand, the long holding time can promote the initial softening and recrystallization of coarse grains in the electroslag ingot, creating conditions for grain breakage in subsequent upsetting and drawing. After the first upsetting and drawing, the reheating temperature of the billet is reduced. This can avoid abnormal grain growth caused by high temperature (such as >1100℃), and the deformation resistance of the billet can be reduced by the medium temperature environment, so that the secondary drawing can achieve more uniform plastic deformation, further break up the grains, and make the grain size continue to decrease. The final heat treatment, through a combination of medium temperature (1020℃~1080℃) and short holding time (1h~2h), can ensure that the billet is fully softened and meets the requirements for elongation deformation, while suppressing grain growth to the greatest extent. This avoids the re-coarsening of previously refined grains due to excessive holding time, ultimately forming a uniform and fine grain structure and eliminating the problems of mixed grains at the edges and uneven structure in the core.

[0025] The forging process described above ensures that the microstructure and performance meet the standards while shortening the overall processing cycle, taking into account both product quality and industrial production efficiency, and is suitable for large-scale mass production.

[0026] Specifically, in S201, the billet is first roughened to 2 / 3 of the height of the electroslag ingot, and then drawn to an octagonal billet of Ф420mm.

[0027] Furthermore, in S4, the heat treatment time for the solution treatment is 15 min to 25 min.

[0028] Short-term heat preservation can inhibit the recombination of dissolved Ti and C elements to form coarse TiC precipitates, while retaining a small number of fine and dispersed TiC particles. These fine particles can not only improve the high-temperature strength of the material through dispersion strengthening, but also avoid the decrease in grain boundary bonding caused by TiC phase coarsening, thus significantly improving the performance stability of the material under high-temperature service environment.

[0029] The present invention also provides a Ni-Cr-Ti type austenitic stainless steel, characterized in that it is prepared by the manufacturing method of Ni-Cr-Ti type austenitic stainless steel described in any one of the above claims.

[0030] The Ni-Cr-Ti austenitic stainless steel provided by this invention lays the compositional foundation for the high-temperature stability and strength of the material by controlling the content of C and Mo elements in the electrode billet. At the same time, by controlling the heating temperature and holding time of the billet in the forging process in stages, and by controlling the total forging ratio ≥4 and the deformation amount of 30%~60% in the final heat, uniform grain refinement and elimination of structural defects are achieved. Furthermore, low-temperature solution treatment is used to ensure the full solidification of Cr elements while inhibiting TiC coarsening. Ultimately, the material has both excellent high-temperature strength and performance stability, which can meet the service requirements of components under high-temperature conditions in aerospace, petrochemical, power plant boiler and other fields, and has high practical value. Attached Figure Description

[0031] Figure 1 Metallographic image of the grain size of the 06Cr18Ni11Ti stainless steel bar prepared in Example 1; Figure 2 Metallographic image of the grain size of the 06Cr18Ni11Ti stainless steel bar prepared in Example 2; Figure 3 Metallographic image of the grain size of the 06Cr18Ni11Ti stainless steel bar prepared in Example 3; Figure 4 Metallographic image of the grain size of the 06Cr18Ni11Ti stainless steel bar prepared for Comparative Example 2. 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 embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] To better illustrate the present invention, further examples are provided below.

[0034] The chemical composition of the electrode blanks in the following embodiments is controlled according to the table below:

[0035] Example 1 This invention provides a method for manufacturing 06Cr18Ni11Ti stainless steel bars, the specific process of which is as follows: S1 involves smelting raw materials such as ferrochrome, nickel plate, ferromanganese, metallic titanium, and ferromolybdenum in an electric arc furnace, refining them using AOD and LF methods, with a ladle temperature of 1575℃, followed by casting to obtain an electrode billet. The composition of the electrode billet is controlled according to the table above, with a C content of 0.065% and a Mo content of 0.35%. S2, after polishing the surface of the electrode blank to remove defects, electroslag remelting is performed to obtain a Ф480mm electroslag ingot; wherein, the slag baking temperature is 700℃, the baking time is 6h, the slag system is CaF2:Al2O3:CaO=60%:20%:20%, and the amount of slag added to the Ф480mm ingot is 72kg / furnace. S3, the Ф480mm electroslag ingot is loaded into the furnace and heated to 1160℃. After holding for 4 hours, it is uprooted and drawn once to 2 / 3 of the height of the electroslag ingot. Then it is drawn into Ф420mm octagonal billet through 8 passes. S4. The Ф420mm octagonal billet is returned to the heating furnace, heated to 1080℃ and held for 2 hours. After being taken out of the furnace, it is drawn into a Ф290mm octagonal billet through 8 passes. S5, the 290mm octagonal billet is returned to the heating furnace, heated to 1040℃ and held for 1.5 hours, then drawn into a 290mm bar in 10 passes after exiting the furnace; the final forging temperature is 830℃, the total forging ratio is 9.32, and the final heat deformation is 47.56%; S6, after forging, is loaded into the furnace and subjected to a solution treatment process at 960℃, held for 20 minutes, then removed from the furnace and water-cooled until the material temperature is ≤200℃, to obtain 06Cr18Ni11Ti stainless steel bars.

[0036] Example 2 This invention provides a method for manufacturing 06Cr18Ni11Ti stainless steel bars, the specific process of which is as follows: S1 involves smelting raw materials such as ferrochrome, nickel plate, ferromanganese, metallic titanium, and ferromolybdenum in an electric arc furnace, refining them using AOD and LF methods, with a ladle temperature of 1570℃, followed by casting to obtain an electrode billet. The composition of the electrode billet is controlled according to the table above, with a C content of 0.068% and a Mo content of 0.39%. S2, after polishing the surface of the electrode blank to remove defects, electroslag remelting is performed to obtain a Ф480mm electroslag ingot; wherein, the slag baking temperature is 650℃, the baking time is 7h, the slag system is CaF2:Al2O3:CaO=60%:20%:20%, and the amount of slag added to the Ф480mm ingot is 72kg / furnace. S3, load the Ф480mm electroslag ingot into the furnace and heat it to 1170℃. After holding it at that temperature for 5 hours, perform a first upsetting and drawing process to upset it to 2 / 3 of the height of the electroslag ingot. Then, draw it through 6 passes to lengthen it into a Ф420mm octagonal billet. S4. The Ф420mm octagonal billet is returned to the heating furnace, heated to 1080℃ and held for 2 hours. After being taken out of the furnace, it is drawn into a Ф290mm octagonal billet in 6 passes. S5, the 290mm octagonal billet is returned to the heating furnace, heated to 1080℃ and held for 1 hour, then drawn into 290mm bars in 11 passes after exiting the furnace; the final forging temperature is 800℃, the total forging ratio is 9.32, and the final heat deformation is 47.56%; S6, after forging, is loaded into the furnace and subjected to a solution treatment process at 980℃, held for 15 minutes, then removed from the furnace and water-cooled until the material temperature is ≤200℃, to obtain 06Cr18Ni11Ti stainless steel bars.

[0037] Example 3 This invention provides a method for manufacturing 06Cr18Ni11Ti stainless steel bars, the specific process of which is as follows: S1 involves smelting raw materials such as ferrochrome, nickel plate, ferromanganese, metallic titanium, and ferromolybdenum in an electric arc furnace, refining them using AOD and LF methods, with a ladle temperature of 1580℃, followed by casting to obtain an electrode billet. The composition of the electrode billet is controlled according to the table above, with a C content of 0.062% and a Mo content of 0.30%. S2, after polishing the surface of the electrode blank to remove defects, electroslag remelting is performed to obtain a Ф480mm electroslag ingot; wherein, the slag baking temperature is 600℃, the baking time is 8h, the slag system is CaF2:Al2O3:CaO=60%:20%:20%, and the amount of slag added to the Ф480mm ingot is 72kg / furnace. S3, the Ф480mm electroslag ingot is loaded into the furnace and heated to 1180℃. After holding for 6 hours, it is uprooted and drawn once to 2 / 3 of the height of the electroslag ingot. Then it is drawn into a Ф420mm octagonal billet through 7 passes. S4. The Ф420mm octagonal billet is returned to the heating furnace, heated to 1020℃ and held for 3 hours. After being taken out of the furnace, it is drawn into a Ф290mm octagonal billet through 7 passes. S5, the 290mm octagonal billet is returned to the heating furnace, heated to 1020℃ and held for 1.5 hours, then drawn into 290mm bars in 9 passes after exiting the furnace; the final forging temperature is 850℃, the total forging ratio is 9.32, and the final heat deformation is 47.56%; S6, after forging, is loaded into the furnace and subjected to a solution treatment process at 940℃, held for 25 minutes, then removed from the furnace and water-cooled until the material temperature is ≤200℃, to obtain 06Cr18Ni11Ti stainless steel bars.

[0038] Comparative Example 1 This comparative example provides a method for manufacturing 06Cr18Ni11Ti stainless steel bars. The only difference from Example 1 is that the C content in the electrode blank is controlled at 0.05% and the Mo content at 0.2%, while the rest are exactly the same.

[0039] Comparative Example 2 This comparative example provides a method for manufacturing 06Cr18Ni11Ti stainless steel bars. The only difference from Example 1 is that the heating temperature of the octagonal billet returned to the heating furnace in steps S4 and S5 is 1160℃. All other aspects are exactly the same.

[0040] Comparative Example 3 This comparative example provides a method for manufacturing 06Cr18Ni11Ti stainless steel bars. The only difference from Example 1 is that the solution treatment temperature in step S6 is 920°C, and the rest is exactly the same.

[0041] Comparative Example 4 This comparative example provides a method for manufacturing 06Cr18Ni11Ti stainless steel bars. The only difference from Example 1 is that the solution treatment temperature in step S6 is 1150℃ and the holding time is 15min. All other aspects are exactly the same.

[0042] Performance testing The 06Cr18Ni11Ti stainless steel bars prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were tested according to the method of GB / T 228.2-2015. The tensile strength at 350℃, yield strength at 350℃, room temperature tensile strength, room temperature yield strength, room temperature elongation after fracture, and room temperature reduction after fracture were measured. The results are shown in Table 1.

[0043] Table 1

[0044] The grain size of the 06Cr18Ni11Ti stainless steel bars prepared in Examples 1-3 and Comparative Examples 1-4 was inspected at multiple points using a fan-shaped sampling method along the radius of the bars. The inspection results are shown in Table 2. Metallographic images of the grain size at the edge, half radius, and center of the 06Cr18Ni11Ti stainless steel bars prepared in Examples 1-3 and Comparative Example 2 are shown below. Figures 1-4 As shown in the figure, the results are shown in Table 2.

[0045] Table 2 Grain size test results (grade)

[0046] As shown in Table 2, the grain size of Examples 1 to 3 is around grade 5.5 with a range of 0.5, indicating uniform grain size. Comparative Example 1, with only composition adjustment, shows no significant difference in grain size compared to the Examples. Comparative Examples 2 and 4 have poorer grain size compared to other groups and exhibit edge mixing, mainly due to excessively high reheat and solution temperatures, respectively, leading to grain growth.

[0047] In summary, the process of this invention achieves grain refinement and microstructure homogenization by controlling the C and Mo content in the electrode blank and by controlling the forging and solution treatment processes. This significantly improves the high-temperature tensile strength and yield strength, meeting the manufacturing requirements of high-temperature components in aerospace, petrochemical and other fields. It combines technical reliability with industrial feasibility and has high application value.

[0048] 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 method of producing an austenitic stainless steel of the type Ni-Cr-Ti, characterized in that, It comprises the following steps: S1, the raw material is smelted by arc furnace, AOD refining, LF refining, and is poured to obtain an electrode blank; the electrode blank is subjected to electroslag remelting to obtain an electroslag ingot; the C content in the electrode blank is 0.06%~0.07%, and the Mo content is 0.3%~0.4%; S2, the electroslag ingot is subjected to forging to obtain a forged steel blank, and the total forging ratio is ≥4; wherein the heating temperature before one fire upsetting is 1160℃~1180℃, and after one fire upsetting, the blank after one fire upsetting is subjected to elongation treatment for several times; Before each elongation process, the blank is heated to 1020℃~1080℃; the holding time before the last elongation is 1h~2h; the holding time before the elongation of the rest of the times is 2h~3h; S3, the forged steel blank is heated to 940℃~980℃ for solid solution treatment, and is water cooled to obtain a Ni-Cr-Ti type austenitic stainless steel.

2. The method of producing the Ni-Cr-Ti type austenitic stainless steel according to claim 1, characterized by, The arc furnace smelting process is at a tapping temperature of 1630℃~1650℃, and the C content in the molten steel is 2.0%~2.5%.

3. The method of producing a Ni-Cr-Ti type austenitic stainless steel according to claim 1, characterized by, The AOD refining process controls the C content in the tapping molten steel to be 0.05%~0.06%, and the Mo content to be 0.3%~0.4%.

4. The method of producing a Ni-Cr-Ti type austenitic stainless steel according to claim 1, characterized by, The LF refining process is at a ladle temperature of 1570℃~1580℃.

5. The method of producing a Ni-Cr-Ti type austenitic stainless steel according to claim 1, characterized by, The chemical composition of the electrode blank is: C 0.06%~0.07%, Si 0.35%~0.55%, Mn 1.00%~1.50%, P≤0.033%, S≤0.002%, Cr 17.30%~17.70%, Ni 9.10%~9.50%, Ti 0.55%~0.65%, Mo 0.30%~0.40%, and the balance is Fe and unavoidable impurities.

6. The method of producing a Ni-Cr-Ti type austenitic stainless steel according to claim 1, characterized by, In S2, the total number of elongation times is 2 times; and / or In S2, the deformation amount of the last fire is 30%~60%; and / or In S2, the final forging temperature is 800℃~850℃.

7. The method of producing the Ni-Cr-Ti type austenitic stainless steel according to claim 1 or 6, characterized by, In S2, the forging process adopts an octagonal forging mode; and / or In S2, the elongation pass of the last elongation process is 9~11 passes, and the elongation pass of the elongation process of the rest of the times is 6~8 passes.

8. The method of producing the Ni-Cr-Ti type austenitic stainless steel according to claim 1 or 6, characterized by, The specific steps of S2 are: S201, the electroslag ingot is heated to 1160℃~1180℃ and held for 4h~6h, and then is subjected to one upsetting and elongation to obtain a one-fire forging blank; S202, the one-fire forging blank is heated to 1020℃~1080℃ and held for 2h~3h, and then is subjected to second elongation to obtain a two-fire forging blank; S203, the two-fire forging blank is heated to 1020℃~1080℃ and held for 1h~2h, and then is subjected to third elongation to obtain a forged steel blank.

9. The method of producing a Ni-Cr-Ti type austenitic stainless steel according to claim 1, characterized by, In S4, the holding time of the solid solution treatment is 15min~25min.

10. An austenitic stainless steel of the type Ni-Cr-Ti, characterized in that, Prepared by the manufacturing method of the Ni-Cr-Ti type austenitic stainless steel according to any one of claims 1~9. Prepared by the manufacturing method of the Ni-Cr-Ti type austenitic stainless steel according to any one of claims 1~9.

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