Annealing method for improving forming performance of ferritic stainless steel

Abstract

The invention relates to an annealing method for improving the forming performance of ferritic stainless steel, and belongs to the technical field of smelting. The annealing method comprises the following steps: (1) smelting molten steel according to set components, and casting a cast ingot and forging the cast ingot into a flat blank; performing air cooling after forging; (2) the casting blank is heated and then subjected to hot rolling, a hot rolled plate with the thickness being 2-3 mm is obtained, and air cooling is conducted; (3) annealing the air-cooled hot rolled plate; (4) performing air cooling on the hot-rolled annealed plate, performing acid pickling by using a hydrochloric acid solution, and performing five-pass cold rolling on the hot-rolled annealed plate subjected to acid pickling to 1mm; and (5) annealing the cold-rolled sheet according to the following process: carrying out 1050 DEG C * 3 minutes, and carrying out air cooling. According to the annealing method for improving the forming performance of the ferritic stainless steel, after the annealing process, the alpha fiber structure is gradually weakened, namely {111} lt; 112gt, 112gt; and {111} lt; 110gt, 110gt; the overall strength of the gamma fiber recrystallization texture of the component is remarkably enhanced, and the r value can be improved.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and more specifically to an annealing method for improving the formability of ferritic stainless steel. Background Technology

[0002] As a key device for the conduction and treatment of engine exhaust gases, the automotive exhaust system is exposed to harsh operating environments characterized by high temperatures, corrosive exhaust gases, vibration, and alternating hot and cold cycles. The material properties of its components directly determine the structural reliability, operational stability, and overall service life of the exhaust system, playing a crucial supporting role in the safety performance and environmental emission standards of the entire vehicle. With the rapid development of the global automotive industry, on the one hand, engines are iterating and upgrading towards higher power, higher compression ratios, and smaller sizes, causing exhaust temperatures to continue to rise. The exhaust manifold outlet temperature of some high-end models or high-performance engines has exceeded 900°C. Traditional materials such as carbon steel and ordinary heat-resistant steel are no longer able to meet the high-temperature service requirements of core components such as exhaust manifolds and three-way catalytic converter housings due to their insufficient high-temperature strength, poor oxidation resistance, and short thermal fatigue life. On the other hand, the automotive market has increasingly stringent requirements for lightweight vehicles and energy conservation and emission reduction, which also puts forward comprehensive performance demands for exhaust system materials that are "high-strength, lightweight, and low-cost." Against this industry background, the development of high-performance ferritic stainless steel with excellent high-temperature resistance, mechanical properties, and economy has gradually become the core technology research and development focus in the field of automotive exhaust system materials.

[0003] With the further improvement of the power and environmental performance of automobile engines, the structural design and manufacturing process of exhaust systems are becoming increasingly complex, placing more stringent and diversified requirements on the comprehensive performance of each component. From a manufacturing perspective, the exhaust system comprises multiple components, including the three-way catalytic converter housing, front pipe, exhaust manifold, and muffler housing. Not only is the overall structure complex, but the connecting pipes must be designed with varying diameters and bends to meet assembly space requirements. Manufacturing processes necessitate basic steps such as pipe expansion, diameter reduction, cold bending, and welding. More importantly, the three-way catalytic converter housing requires hydraulic expansion to achieve complex curved surface forming to accommodate the catalyst carrier installation. The exhaust manifold, designed to fit the engine block's exhaust port layout, often involves multi-directional bending and deep drawing composite forming. The front pipe requires precise spatial control through welded pipe bending. These complex forming processes place extremely high demands on the material's formability. The industry typically considers an average r-value ≥ 1.3 as a key formability indicator for assessing a material's suitability for these complex processes. Insufficient material formability can easily lead to defects such as cracking, wrinkling, and uneven wall thickness during processing, resulting in increased component scrap rates and significantly higher manufacturing costs.

[0004] To address these performance challenges, domestic and international companies and research institutions have conducted a series of research and development efforts on ferritic stainless steel materials, achieving certain technological results. For example, companies such as Nippon Steel and Kawasaki Steel have developed several targeted materials, forming certain advantages in specific performance dimensions. However, overall, the existing publicly available ferritic stainless steel materials still have significant performance shortcomings: some materials, while meeting high-temperature resistance requirements, have poor formability. When performing complex processes such as hydraulic forming and deep drawing, it is necessary to improve the forming pass rate by reducing the processing speed and adding intermediate annealing processes, leading to reduced production efficiency; some materials have good processing and forming properties, but are insufficient in high-temperature oxidation resistance or thermal fatigue performance, and are prone to failure problems such as surface oxide layer peeling and weld cracking after long-term service; and some high-performance materials rely on the large-scale addition of rare alloying elements (such as Nb and Mo), which can improve performance but significantly increases the production cost of the materials, making it difficult to meet the economic requirements of large-scale mass production in the automotive industry.

[0005] In summary, current ferritic stainless steel materials used in automotive exhaust systems generally suffer from technical bottlenecks, namely, the difficulty in simultaneously achieving high-temperature resistance and formability, and the challenge of balancing high performance and low cost. Existing materials still have significant room for improvement in their overall performance adaptability to complex forming processes and harsh high-temperature service environments. Therefore, developing a ferritic stainless steel material for automotive exhaust systems that combines excellent high-temperature oxidation resistance, high formability, and reasonable cost advantages to overcome existing technological deficiencies and meet the upgrading needs of the automotive industry for exhaust system materials has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] To address the technical problem of poor formability of existing ferritic stainless steel materials used in automotive exhaust systems at high temperatures, this invention provides an annealing method to improve the formability of ferritic stainless steel, thereby solving the aforementioned problem.

[0007] The technical solution of this invention is as follows: An annealing method for improving the formability of ferritic stainless steel includes the following steps: (1) Smelt steel according to the set composition, cast ingots and forge them into flat billets; air cool after forging; (2) The billet is heated and then hot-rolled to obtain a hot-rolled plate with a thickness of 2~3mm, and then air-cooled; (3) Anneal the air-cooled hot-rolled plate; (4) After the hot-rolled annealed plate is air-cooled, it is pickled with hydrochloric acid solution, and the pickled hot-rolled annealed plate is cold-rolled to 1mm in five passes. (5) The cold-rolled sheet is annealed according to the following process: 1050℃×3min, air cooling.

[0008] Furthermore, the composition of the ferritic stainless steel is as follows: C: 0~0.009%, Si: 0.45%~0.55%, Mn: 0.2%~0.3%, P: 0~0.008%, S: 0~0.008%, Cr: 18%~19%, Nb: 0.45%~0.55%, Ti: 0.15%~0.25%, N: 0~0.01%, Mo: 1.98%~2.22%, with the balance being Fe.

[0009] Furthermore, the composition of the ferritic stainless steel is as follows: C: 0.009%, Si: 0.52%, Mn: 0.32%, P: 0.008%, S: 0.008%, Cr: 19.5%, Nb: 0.45%, Ti: 0.155%, N: 0.072%, Mo: 1.97%, with the balance being Fe.

[0010] Furthermore, the smelting equipment is a vacuum induction furnace, and the smelting temperature is 1600℃~1650℃.

[0011] Furthermore, the casting temperature is 1520℃~1550℃.

[0012] Furthermore, the pre-forging temperature of the flat billet is 1180~1220℃, the initial forging temperature is 1050~1100℃, the final forging temperature is 800~850℃, and it is air-cooled after forging.

[0013] Furthermore, before forging the flat billet, it is peeled and the ends are cut off to ensure that there are no defects such as shrinkage cavities or inclusions on the surface.

[0014] Furthermore, the hot rolling parameters for the billet are as follows: the billet is heated to 1180℃~1220℃ and hot rolled in 5~7 passes, with an initial rolling temperature of 1130~1170℃ and a final rolling temperature of 900~950℃.

[0015] Furthermore, the annealing of the air-cooled hot-rolled plate is as follows: annealing is carried out in a box-type resistance furnace with annealing parameters of 950℃×8min, followed by air cooling.

[0016] The beneficial effects of this invention are as follows: The annealing method for improving the formability of ferritic stainless steel provided by this invention, after the annealing process, gradually weakens the α-fiber structure, {111} <112> and {111} <110> The overall strength of the recrystallized texture of the γ-fibers in the component is significantly enhanced, which is beneficial for improving the r-value. In the annealed plate, {111} <112> The maximum strength value of the texture is 11.31, corresponding to... The value is 1.69, indicating {111} <112> Texture can achieve good formability. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 These are metallographic images of cold-rolled steel sheets at different annealing temperatures. In the figures, (a) represents 950℃, (b) represents 1000℃, (c) represents 1050℃, and (d) represents 1100℃.

[0019] Figure 2 This is an orientation diagram of a specific crystal orientation in a cold-rolled sheet at different annealing temperatures. In the diagram, (a) is 950℃, (b) is 1000℃, (c) is 1050℃, and (d) is 1100℃.

[0020] Figure 3 These are annealed plates at different cold rolling annealing temperatures. value.

[0021] Figure 4 It represents the Δr value of the annealed plate at different cold rolling annealing temperatures. Detailed Implementation

[0022] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0023] Example 1 An annealing method for improving the formability of ferritic stainless steel, comprising the following specific steps: (1) Smelt steel according to the set composition. The smelting equipment is a 150kg vacuum induction furnace. The smelting temperature is 1600~1650℃. The casting temperature is 1520~1550℃. The ingot is forged into a flat billet of 600 mm×240 mm×50 mm. Before forging, peel off the skin and cut off the head and tail to make the surface free of shrinkage cavities, inclusions and other defects. The pre-forging heating temperature is 1100℃, the initial forging temperature is 1050℃, the final forging temperature is 850℃, and the steel is air-cooled after forging. (2) Heat the billet to 1180~1220℃ and perform 6 passes of hot rolling. The initial rolling temperature is 1150℃ and the final rolling temperature is 900℃ to obtain a hot-rolled plate with a thickness of 3.0mm, and then air-cool it. (3) After air cooling, the hot-rolled plate is placed in a box-type resistance furnace for annealing. The annealing process is 950℃×8min, followed by air cooling. (4) After the hot-rolled annealed plate is air-cooled, it is pickled with hydrochloric acid solution, and the pickled hot-rolled annealed plate is cold-rolled to 1mm in five passes. (5) The cold-rolled sheet is annealed according to the following process: 1050℃×3min, air cooling.

[0024] Depend on Figure 1 (c) and Figure 2 As can be seen in (c), the cold-rolled sheet completely recrystallizes after annealing at 1050℃, and the grains consist of a large number of uniform equiaxed {111} grains. <110> and {111} <112> The recrystallized grain composition of the oriented γ-fiber texture is beneficial to increasing the r value, and {111} <110> and {111} <112> These percentages, accounting for 33.4% and 50.7% respectively, ensured the experimental steel possessed good formability. (From...) Figure 3 and Figure 4 It can be seen that after annealing at 1050℃ The values ​​of Δr and Δr are 1.69 and 0.22, respectively, indicating good forming performance.

[0025] Comparative Example 1 An annealing method, the specific steps of which are as follows: (1) Smelt steel according to the set composition. The smelting equipment is a 150kg vacuum induction furnace. The smelting temperature is 1600~1650℃. The casting temperature is 1520~1550℃. The ingot is forged into a flat billet of 600 mm×240 mm×50 mm. Before forging, peel off the skin and cut off the head and tail to make the surface free of shrinkage cavities, inclusions and other defects. The pre-forging heating temperature is 1100℃, the initial forging temperature is 1050℃, the final forging temperature is 850℃, and the steel is air-cooled after forging. (2) Heat the billet to 1180~1220℃ and perform 5~7 passes of hot rolling. The initial rolling temperature is 1150℃ and the final rolling temperature is 900℃ to obtain a hot-rolled plate with a thickness of 3.0mm, and then air-cool it. (3) After air cooling, the hot-rolled plate is placed in a box-type resistance furnace for annealing. The annealing process is 950℃×8min, followed by air cooling. (4) After the hot-rolled annealed plate is air-cooled, it is pickled with hydrochloric acid solution, and the pickled hot-rolled annealed plate is cold-rolled to 1mm in five passes. (5) The cold-rolled sheet is annealed according to the following process: 950℃×3min, air cooling.

[0026] Depend on Figure 1 (a) and Figure 2As can be seen in (a), the cold-rolled sheet did not undergo complete recrystallization after annealing at 950℃, and some fine {111} particles were present in the cold-rolled annealed sheet. <110> and {111} <112> Recrystallized grains, and a relatively small amount of {100} <001> and {100} <011> Grains. From Figure 3 and Figure 4 It can be seen that after annealing at 950℃ The values ​​and Δr are 1.35 and 0.35 respectively, which are not conducive to obtaining good forming performance.

[0027] Comparative Example 2 An annealing method, the specific steps of which are as follows: (1) Smelt steel according to the set composition. The smelting equipment is a 150kg vacuum induction furnace. The smelting temperature is 1600~1650℃. The casting temperature is 1520~1550℃. The ingot is forged into a flat billet of 600 mm×240 mm×50 mm. Before forging, peel off the skin and cut off the head and tail to make the surface free of shrinkage cavities, inclusions and other defects. The pre-forging heating temperature is 1100℃, the initial forging temperature is 1050℃, the final forging temperature is 850℃, and the steel is air-cooled after forging. (2) Heat the billet to 1180~1220℃ and perform 5~7 passes of hot rolling. The initial rolling temperature is 1150℃ and the final rolling temperature is 900℃ to obtain a hot-rolled plate with a thickness of 3.0mm, and then air-cool it. (3) After air cooling, the hot-rolled plate is placed in a box-type resistance furnace for annealing. The annealing process is 950℃×8min, followed by air cooling. (4) After the hot-rolled annealed plate is air-cooled, it is pickled with hydrochloric acid solution, and the pickled hot-rolled annealed plate is cold-rolled to 1mm in five passes. (5) The cold-rolled sheet is annealed according to the following process: 1000℃×3min, air cooling.

[0028] Depend on Figure 1 (b) and Figure 2 As can be seen in (b), only some fine {111} particles exist in the cold-rolled annealed sheet after annealing at 1000℃. <110> and {111} <112> Recrystallized grains, and a relatively small amount of {100} <001> and {100} <011> Grains. From Figure 3 and Figure 4 It can be seen that after annealing at 1000℃ The values ​​and Δr are 1.51 and 0.27 respectively, which are not conducive to obtaining good forming performance.

[0029] Comparative Example 3 An annealing method, the specific steps of which are as follows: (1) Smelt steel according to the set composition. The smelting equipment is a 150kg vacuum induction furnace. The smelting temperature is 1600~1650℃. The casting temperature is 1520~1550℃. The ingot is forged into a flat billet of 600 mm×240 mm×50 mm. Before forging, peel off the skin and cut off the head and tail to make the surface free of shrinkage cavities, inclusions and other defects. The pre-forging heating temperature is 1100℃, the initial forging temperature is 1050℃, the final forging temperature is 850℃, and the steel is air-cooled after forging. (2) Heat the billet to 1180~1220℃ and perform 5~7 passes of hot rolling. The initial rolling temperature is 1150℃ and the final rolling temperature is 900℃ to obtain a hot-rolled plate with a thickness of 3.0mm, and then air-cool it. (3) After air cooling, the hot-rolled plate is placed in a box-type resistance furnace for annealing. The annealing process is 950℃×8min, followed by air cooling. (4) After the hot-rolled annealed plate is air-cooled, it is pickled with hydrochloric acid solution, and the pickled hot-rolled annealed plate is cold-rolled to 1mm in five passes. (5) The cold-rolled sheet is annealed according to the following process: 1100℃×3min, air cooling.

[0030] Depend on Figure 1 (d) and Figure 2 As can be seen in (d), the cold-rolled sheet completely recrystallized after annealing at 1100℃, and the grains grew significantly, reducing {111} <110> and {111} <112> Oriented γ-fiber texture. (From) Figure 3 and Figure 4 It can be seen that after annealing at 1100℃ The values ​​and Δr are 1.42 and 0.35 respectively, which are not conducive to obtaining good forming performance.

[0031] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. An annealing method for improving the formability of ferritic stainless steel, characterized by, It comprises the following steps: (1) smelting molten steel according to a set composition, casting an ingot and forging into a flat blank; air cooling after forging; (2) heating the cast blank and then hot rolling to obtain a hot-rolled plate with a thickness of 2-3 mm, air cooling; (3) annealing the air-cooled hot-rolled plate; (4) air cooling the annealed hot-rolled plate, pickling with a hydrochloric acid solution, and cold rolling the pickled hot-rolled annealed plate through five passes to 1 mm; (5) annealing the cold-rolled plate according to the following process: 1050℃×3min, air cooling.

2. The annealing method for improving the formability of ferritic stainless steel according to claim 1, wherein The ferritic stainless steel has the following composition: C: 0-0.009%, Si: 0.45%-0.55%, Mn: 0.2%-0.3%, P: 0-0.008%, S: 0-0.008%, Cr: 18%-19%, Nb: 0.45%-0.55%, Ti: 0.15%-0.25%, N: 0-0.01%, Mo: 1.98%-2.22%, and the balance of Fe.

3. The annealing method for improving formability of ferritic stainless steel according to claim 1, wherein The ferritic stainless steel has the following composition: C: 0.009%, Si: 0.52%, Mn: 0.32%, P: 0.008%, S: 0.008%, Cr: 19.5%, Nb: 0.45%, Ti: 0.155%, N: 0.072%, Mo: 1.97%, and the balance of Fe.

4. The annealing method for improving formability of ferritic stainless steel according to claim 1, wherein The smelting equipment is a vacuum induction furnace, and the smelting temperature is 1600-1650℃.

5. The annealing method for improving formability of ferritic stainless steel according to claim 1, wherein The casting temperature is 1520-1550℃.

6. The annealing method for improving formability of ferritic stainless steel according to claim 1, wherein The heating temperature before flat blank forging is 1180-1220℃, the initial forging temperature is 1050-1100℃, the final forging temperature is 800-850℃, and air cooling is performed after forging.

7. The annealing method for improving formability of ferritic stainless steel according to claim 1, wherein The flat blank is first peeled and cut at the head and tail before forging to make the surface free of shrinkage holes, inclusions and other defects.

8. The annealing method for improving formability of ferritic stainless steel according to claim 1, wherein The hot rolling parameters of the cast blank are as follows: heating the cast blank to 1180-1220℃, hot rolling through 5-7 passes, the initial rolling temperature is 1130-1170℃, and the final rolling temperature is 900-950℃.

9. The annealing method for improving formability of ferritic stainless steel according to claim 1, wherein The annealing of the air-cooled hot-rolled plate is performed in a box-type resistance furnace, and the annealing parameters are 950℃×8min, air cooling.

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