A method for manufacturing an ultra-fine grained austenitic stainless steel strip

Abstract

The present application relates to the technical field of stainless steel strip preparation, and particularly relates to a preparation method of ultra-fine-grain austenitic stainless steel strip, which comprises smelting, double-roller thin strip continuous casting, on-line large-strain hot rolling, cooling, cold rolling, annealing and pickling, to obtain the ultra-fine-grain austenitic stainless steel strip. The specific mechanism is that: when the thin strip is subjected to large-strain hot rolling, dynamic recrystallization occurs, columnar crystals are broken to form equiaxed crystals, high-temperature ferrite is transformed into austenite, and after the hot rolling is completed, the rolling strip organization is austenite; the hot-rolled strip is subjected to deep cooling with liquid nitrogen, so that the rolling strip organization is kept as austenite, and the rapid cooling reduces element segregation; the deep-cooled rolling strip is subjected to cold rolling and the rolling strip is subjected to annealing treatment, the martensite produced by the deep-cooled cold rolling is more, and the reverse phase change mechanism of the martensite at a low temperature is diffusion mechanism, the austenite formed by the reverse phase change is nucleated on the martensite, so that the austenite after the reverse phase change is nanocrystalline / ultra-fine-grain, and the strength and plasticity of the austenitic stainless steel strip are improved.

Description

Technical Field

[0001] This invention relates to the field of stainless steel strip preparation technology, and specifically to a method for preparing ultrafine-grained austenitic stainless steel strip. Background Technology

[0002] Austenitic stainless steel refers to stainless steel that has an austenitic structure at room temperature. When the steel contains approximately 18% Cr, 8%-25% Ni, and 0.1% C, it has a stable austenitic structure. Austenitic stainless steel is non-magnetic and possesses high toughness and plasticity, but its strength is relatively low. It cannot be strengthened through phase transformation and can only be strengthened through cold working. Adding elements such as S, Ca, Se, and Te improves its machinability.

[0003] The prior art discloses a process for preparing austenitic stainless steel, as follows:

[0004] Invention application No. 202310649198.5 discloses a high-strength, high-plasticity austenitic stainless steel and its production method. Specifically, the method involves: smelting in a vacuum induction furnace and casting in a cast iron mold. High-purity nitrogen is used during the smelting process, with the nitrogen pressure maintained at 0.2 MPa, and the casting is protected throughout. The cast billet is held at 1100℃ for 45 minutes in a heating furnace and then forged to a thickness of 14 mm, followed by water cooling to room temperature. The forged billet is then held at 1200℃ for 3 hours in a box-type resistance furnace, followed by water cooling to room temperature to obtain a solution-treated steel sample. The solution-treated steel sample is held in liquid nitrogen for 15 minutes and then subjected to low-temperature rolling, undergoing at least 10 passes to form a steel plate with a thickness of 3.5 mm. During the rolling process, the sample is held in liquid nitrogen for 5 minutes every 4 passes. The low-temperature rolled steel plate is then annealed to obtain an annealed plate at 950℃ for 1 hour in a nitrogen atmosphere. The annealed board is further subjected to aging treatment to obtain an aged board. The aging treatment temperature is 550℃ and the time is 7h.

[0005] The invention application with application number 200610116419.9 discloses a thin strip continuously cast austenitic stainless steel strip and its manufacturing method, specifically: smelting, continuous casting to form a 1-5mm cast strip, and coiling; solution treatment of the cast strip at 1050-1100℃ for 5-8 minutes; pickling, cold rolling of the cast strip, annealing at 1050-1100℃ for 3-8 minutes; pickling again and cold rolling to achieve the product thickness.

[0006] Existing technologies have long process flows, unavoidable solution annealing, and high energy consumption; moreover, the production line is discontinuous, failing to fully utilize the short-process characteristics of twin-roll thin strip casting, while annealing at high temperatures does not fully save energy, and the process flow is further extended by multiple cold rolling processes.

[0007] In summary, there is an urgent need for a simplified process for preparing stainless steel strips with good strength and ductility to solve the problems existing in the current technology. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing ultrafine-grained austenitic stainless steel strip with simplified process steps and good strength and plasticity. The specific technical solution is as follows:

[0009] A method for preparing ultrafine-grained austenitic stainless steel strip includes the following steps:

[0010] Step 1, Smelting: Molten steel is obtained by smelting in an induction furnace;

[0011] Step 2, Twin-roll thin strip casting: Molten steel with a superheat of 10-30℃ is cast using a twin-roll thin strip casting machine to obtain a cast strip;

[0012] Step 3, Online High-Strain Hot Rolling: The cast strip is conveyed to the hot rolling mill via conveyor rollers to obtain the rolled strip. During the hot rolling process: the hot rolling temperature is 1100℃-1200℃, and the reduction rate is 60-80%.

[0013] Step 4: Cooling: Cool the strip.

[0014] Step 5, Cold Rolling: The cooled strip is cold rolled; the reduction rate during cold rolling is 30-50%.

[0015] Step 6: Annealing: Anneal the cold-rolled strip, wherein the annealing temperature is 400-700℃ and the annealing time is 60s-3600s.

[0016] Step 7, pickling: Pickling is performed on the annealed strip to remove oxide scale. After rinsing and drying, ultrafine-grained austenitic stainless steel strip is obtained.

[0017] Preferably, in step three: the hot rolling temperature is 1150℃-1190℃, and the reduction rate is 65-75%. More preferably, the reduction rate is 70%.

[0018] Preferably, step four specifically involves: cryogenic cooling using liquid nitrogen.

[0019] Preferably, the reduction rate during the cold rolling process in step five is 35-45%. More preferably, the reduction rate during the cold rolling process in step five is 40%.

[0020] Preferably, in step six: the annealing temperature is 400-700℃, and the annealing time is 60s-3600s. More preferably, in step six: the annealing temperature is 500-600℃, and the annealing time is 500s-2000s.

[0021] The technical solution of this invention includes smelting, twin-roll thin strip continuous casting, online high-strain hot rolling, cooling, cold rolling, annealing at 400℃-700℃, and pickling processes to obtain ultrafine-grained austenitic stainless steel strip. The specific mechanism is as follows: ① The initial microstructure of the thin strip continuously cast is columnar austenitic crystals on both sides and extremely fine equiaxed ferrite crystals in the core. During high-strain hot rolling, dynamic recrystallization occurs, the columnar crystals break down to form equiaxed crystals, and the high-temperature ferrite transforms into austenite. After hot rolling, the strip microstructure is austenitic, and the thickness of the cast strip has been significantly reduced; ② The hot-rolled strip is cryogenically cooled with liquid nitrogen to maintain the austenitic microstructure, while rapid cooling reduces element segregation and ensures that subsequent cold rolling can achieve a higher martensitic content even with a small reduction rate. ③ Cold rolling of the strip after cryogenic treatment: Because it has already undergone large strain hot rolling, the reduction rate required to achieve the target thickness through cold rolling is smaller, and the final deformation is smaller. However, due to the cryogenic treatment, the deformation after cold rolling is smaller, and there will still be more martensite. ④ Annealing of the strip: Because the deformation is smaller, the recrystallization temperature is higher. Therefore, annealing at 400℃-700℃ will only cause the residual austenite to recover, but the deformed martensite will undergo reverse transformation to form austenite. Because there is more martensite produced by cold rolling after cryogenic treatment, and the reverse transformation mechanism of martensite at lower annealing temperature is a diffusion mechanism, the austenite formed by reverse transformation nucleates on the martensite, so the austenite after reverse transformation is nanocrystalline / extremely fine grain. Therefore, the final product structure consists of recovered residual austenite and nano / extremely fine-grained austenite formed by reverse phase transformation. The final austenite structure provides good plasticity, the unrecrystallized residual austenite improves the strength of the finished product, and the extremely fine reverse-transformed austenite forms fine grains to strengthen and improve both strength and plasticity.

[0022] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0023] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0024] Figure 1 This is a schematic diagram of the process for preparing ultrafine-grained austenitic stainless steel strip in preferred embodiment 1 of the present invention;

[0025] Figure 2 This is a microscopic image of the austenitic stainless steel strip in the preferred embodiment 1 of the present invention under a micrometer scale;

[0026] Figure 3 This is a microscopic image of the austenitic stainless steel strip in the preferred embodiment 1 of the present invention under a nanoscale.

[0027] Figure 4 This is a microscopic image of the austenitic stainless steel strip in Comparative Example 1 on a micrometer scale. Detailed Implementation

[0028] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered by the claims.

[0029] Example 1:

[0030] A method for preparing ultrafine-grained austenitic stainless steel strip, see details. Figure 1 It includes the following steps:

[0031] Step 1, Smelting: Molten steel is obtained by smelting in an induction furnace;

[0032] Step 2, Twin-roll thin strip casting: Molten steel with a superheat of 20°C is cast using a twin-roll thin strip casting machine to obtain a cast strip;

[0033] Step 3, Online High-Strain Hot Rolling: The cast strip is conveyed to the hot rolling mill via conveyor rollers and subjected to multiple hot rolling passes to obtain the rolled strip. During the hot rolling process: the hot rolling temperature is 1150℃ and the reduction rate is 70%.

[0034] Step 4: Cooling: Cool the strip, preferably by deep cooling with liquid nitrogen to -20°C, in which the strip structure remains unchanged;

[0035] Step 5, Cold Rolling: The cooled strip is subjected to multiple cold rolling passes; the reduction rate during cold rolling is 40%; the microstructure of the rolled strip is 40% martensite and 60% austenite;

[0036] Step 6, Annealing: Anneal the cold-rolled strip, wherein: annealing temperature is 600℃ and annealing time is 900s; after annealing, martensite undergoes reverse phase transformation to form extremely fine austenite grains, and deformed austenite recovers.

[0037] Step 7, pickling: Pickling is performed on the annealed strip to remove oxide scale. After rinsing and drying, ultrafine-grained austenitic stainless steel strip is obtained.

[0038] Mechanical property tests were conducted on the ultrafine-grained austenitic stainless steel strip obtained in this embodiment, yielding a yield strength of 600 MPa, a tensile strength greater than 1 GPa, and an elongation greater than 40%. Further testing was performed, and... Figure 2 and Figure 3 The resulting product has a microstructure of nano / ultrafine austenite, which gives it a good balance of strength and plasticity.

[0039] Example 2:

[0040] The difference between this embodiment and embodiment 1 is as follows: in step two, molten steel with a superheat of 10°C is cast using a twin-roll thin strip continuous casting machine; in step three, the hot rolling temperature is 1100°C and the reduction rate is 60%; in step five, the reduction rate during cold rolling is 30%; in step six, the annealing temperature is 400°C and the annealing time is 120s.

[0041] Mechanical property tests were conducted on the ultrafine-grained austenitic stainless steel strip obtained in this embodiment. The results showed that the cold-rolled strip had a low martensite content, but the annealing temperature and time were low and short, resulting in small austenite size from the reverse transformation. The austenite deformed during cold rolling underwent a recovery process, and the final product microstructure consisted of nanoscale reverse-transformation austenite and deformed residual austenite, yet it possessed high strength. The yield strength reached 700 MPa, the tensile strength reached 1.3 GPa, and the elongation was 35-40%.

[0042] Example 3:

[0043] The difference between this embodiment and embodiment 1 is as follows: in step two, molten steel with a superheat of 30°C is cast using a twin-roll thin strip continuous casting machine; in step three, the hot rolling temperature is 1200°C and the reduction rate is 80%; in step five, the reduction rate during cold rolling is 50%; in step six, the annealing temperature is 700°C and the annealing time is 3600s.

[0044] Mechanical properties of the ultrafine-grained austenitic stainless steel strip obtained in this embodiment were tested, and the results showed that the sample had an extremely fine austenitic equiaxed grain structure, a yield strength of 450 MPa, a tensile strength of 1 GPa, and an elongation of 60%.

[0045] Comparative Example 1:

[0046] The preparation of stainless steel strip includes the following steps:

[0047] Induction furnace for melting 304 stainless steel;

[0048] Casting on a twin-roll thin strip continuous casting machine produces a dual-phase microstructure of austenitic and ferrite.

[0049] After water cooling, the cast strip is cold rolled to a thickness of 0.5 mm. The microstructure of the rolled strip consists of deformed austenite, ferrite, and martensite.

[0050] Stainless steel strip is obtained by annealing the rolled strip, wherein the annealing is performed at 800℃ and 950℃ for 2 minutes. During the annealing process, austenite undergoes reverse phase transformation to form austenite, deformed austenite recrystallizes, and ferrite transforms into austenite.

[0051] The mechanical properties of the annealed strip were tested and the microstructure was observed. The average yield strength was 400 MPa, the average tensile strength was 900 MPa, and the average elongation was 45%. See details. Figure 4The microstructure is austenitic equiaxed crystals. After annealing at 800℃, the grain size is 200µm, which is larger than that of Example 1. Incompletely transformed martensite is distributed in the austenitic matrix.

[0052] Comparative Example 2:

[0053] The preparation of stainless steel strip includes the following steps:

[0054] Induction furnace for melting 304 stainless steel;

[0055] Molten steel is poured into a continuous casting machine for continuous casting to obtain a strip;

[0056] The cast strip continues to undergo multi-pass online hot rolling at 1050-1100℃ with a reduction rate of 70%, and the microstructure is austenitic.

[0057] Hot-rolled plates are subjected to water cooling and pickling to remove surface oxide scale;

[0058] The hot-rolled strip is then cold-rolled with a reduction rate of 40%, resulting in deformed austenite and martensite, with a relatively low martensite content of 25%.

[0059] Stainless steel strip is obtained by annealing cold-rolled strip at 600℃ for 15 minutes.

[0060] The mechanical properties of the finished product were tested. The mechanical properties met the national standard but were lower than those of the example. The tensile strength was 700 MPa and the elongation was 40%. The microstructure was observed to show that the martensite reverse transformation was complete, the austenite grains of the reverse transformation grew, and the deformed austenite mainly underwent a recovery process. Compared with Example 1, the austenite grain size in the microstructure was larger, and the strength was reduced.

[0061] Comparative Example 3:

[0062] The preparation of stainless steel strip includes the following steps:

[0063] Induction furnace for melting 304 stainless steel;

[0064] Molten steel is poured into a continuous casting machine for continuous casting to obtain a strip;

[0065] The cast strip continues to undergo multi-pass online hot rolling, at 1050-1100℃, with a reduction rate of 40%, and the microstructure is austenitic.

[0066] Liquid nitrogen cryogenics;

[0067] The hot-rolled strip is then cold-rolled, wherein the cold rolling reduction rate is 60%, and the strip consists of deformed austenite and martensite, with a martensite content of 60%. As the martensite content increases with the reduction rate, the deformation resistance increases, and the number of cold rolling passes increases.

[0068] Stainless steel strip is obtained by annealing cold-rolled strip, specifically by annealing at 500℃ for 15 minutes. During annealing, the microstructure changes include reverse martensite transformation and austenite recovery. Due to the low annealing temperature, the reverse martensite transformation mechanism is a diffusion mechanism, resulting in a slow transformation rate and a high martensite content. Furthermore, the austenite recovery process occurs after the reverse martensite transformation. Therefore, the final microstructure consists of austenite undergoing reverse transformation, incompletely transformed martensite, and deformed austenite.

[0069] The final product performance was tested because the presence of a large amount of deformed austenite and martensite in the microstructure significantly reduced the product elongation and increased the yield strength. Specifically, the yield strength was 500 MPa, the tensile strength was 900 MPa, and the elongation was 30-40%.

[0070] Comparative Example 4:

[0071] The preparation of stainless steel strip includes the following steps:

[0072] Induction furnace for melting 304 stainless steel;

[0073] The molten steel is poured into a continuous casting machine for continuous casting.

[0074] The cast strip is then subjected to multiple online hot rolling passes with a reduction rate of 70%. The microstructure of the rolled strip is primarily austenitic.

[0075] The obtained hot-rolled strip can be cryogenically cooled to below MS using liquid nitrogen, resulting in the appearance of martensite in the strip microstructure.

[0076] The hot-rolled strip after cryogenic cooling is then cold-rolled with a reduction rate of 40%. The resulting strip microstructure consists of 70% martensite and 30% austenite.

[0077] The stainless steel strip, which is cold-rolled, is annealed at 600℃ for 15 minutes. After annealing, the martensite undergoes a reverse phase transformation, forming extremely fine austenite grains. The deformed austenite recovers. Due to the low annealing temperature, the reverse phase transformation mechanism of martensite is a diffusion mechanism, the transformation rate is slow, and the martensite phase content is high. At the same time, the recovery process of austenite occurs after the reverse phase transformation of martensite. Therefore, the final microstructure consists of austenite with reverse phase transformation, incompletely transformed martensite, and deformed austenite.

[0078] The mechanical properties of the finished product were tested, and the yield strength reached 500 MPa, the tensile strength was 1 GPa, and the elongation was less than 40%.

[0079] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing ultrafine-grained austenitic stainless steel strip, characterized in that, Includes the following steps: Step 1, Smelting: Molten steel is obtained by smelting in an induction furnace; Step 2, Twin-roll thin strip casting: Molten steel with a superheat of 10-30℃ is cast using a twin-roll thin strip casting machine to obtain a cast strip; Step 3, Online High-Strain Hot Rolling: The cast strip is conveyed to the hot rolling mill via conveyor rollers to obtain a rolled strip. During the hot rolling process: the hot rolling temperature is 1100℃-1200℃, and the reduction rate is 60-80%; after the hot rolling is completed, the microstructure of the rolled strip is austenitic. Step 4: Cooling: Cool the strip; use liquid nitrogen for deep cooling to maintain the austenitic microstructure of the strip. Step 5, Cold Rolling: The cooled strip is cold rolled; the reduction rate during cold rolling is 30-50%. Step 6: Annealing: Anneal the cold-rolled strip, wherein the annealing temperature is 400-700℃ and the annealing time is 60s-3600s. Step 7, pickling: Pickling is performed on the annealed strip to remove oxide scale. After rinsing and drying, ultrafine-grained austenitic stainless steel strip is obtained.

2. The method for preparing ultrafine-grained austenitic stainless steel strip according to claim 1, characterized in that, In step three: the hot rolling temperature is 1150℃-1190℃, and the reduction rate is 65-75%.

3. The method for preparing ultrafine-grained austenitic stainless steel strip according to claim 2, characterized in that, The reduction rate is 70%.

4. The method for preparing ultrafine-grained austenitic stainless steel strip according to claim 1, characterized in that, In step five, the reduction rate during cold rolling is 35-45%.

5. The method for preparing ultrafine-grained austenitic stainless steel strip according to claim 4, characterized in that, In step five, the reduction rate during the cold rolling process is 40%.

6. The method for preparing ultrafine-grained austenitic stainless steel strip according to claim 1, characterized in that, In step six: annealing temperature 400-700℃, annealing time 60s-3600s.

7. The method for preparing ultrafine-grained austenitic stainless steel strip according to claim 6, characterized in that, In step six: the annealing temperature is 500-600℃ and the annealing time is 500s-2000s.

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