Method for regulating dynamic recrystallization structure of austenitic stainless steel thick plate

Abstract

The application discloses a method for regulating dynamic recrystallization structure of austenitic stainless steel thick plate, which comprises the following steps: (1) diffusing annealing austenitic stainless steel thick slab at 1230-1260 DEG C to obtain homogenized slab; (2) after hot rolling of the homogenized slab, immediately performing large deformation hot rolling with single pass reduction of 25-50% to obtain rough rolling plate with surface temperature not lower than 1090 DEG C; if the thickness of the rough rolling plate reaches the required thickness, then water cooling to room temperature to obtain controlled hot rolling plate; otherwise, the next step is performed; (3) after rapidly cooling the rough rolling plate to below 920 DEG C, performing small deformation hot rolling with single pass reduction of 5-10%, total reduction is greater than or equal to 25%, and then water cooling to room temperature to obtain controlled hot rolling plate. The application adopts special 'high-temperature large reduction assisted low-temperature small reduction' process to regulate dynamic recrystallization behavior of the material to obtain uniformly distributed recrystallization grain structure, aiming at the problem that it is difficult to control the hot rolling structure of the austenitic stainless steel thick slab. The process has strong operability and can realize industrial application.

Description

Technical Field

[0001] This invention relates to the field of steel alloy materials technology, specifically to a method for controlling the dynamic recrystallization structure of thick austenitic stainless steel plates. Background Technology

[0002] Austenitic stainless steel slabs are crucial structural and functional materials in nuclear power, military, and petrochemical industries. Their service environments are generally harsh, requiring them to withstand high-temperature or cyclic temperature stresses and corrosive atmospheres. This necessitates stringent requirements for high-temperature / room-temperature strength, fatigue, creep, and corrosion resistance. Consequently, strict control over the material's microstructure, particularly the size and uniformity of austenitic grains, is essential during processing. In on-site production, these thick-gauge materials are typically formed through hot rolling or hot forging processes. The control of grain size and distribution characteristics primarily relies on recrystallization behavior, with dynamic recrystallization being the most critical aspect. Dynamic recrystallization during hot deformation not only determines the final rolled microstructure and even the microstructure of the finished slab (for hot-rolled slabs), but also directly impacts the static recrystallization and grain growth during subsequent solution treatment. Therefore, controlling the dynamic recrystallization behavior during hot deformation is of paramount importance.

[0003] However, the differences in temperature and strain distribution along the thickness of hot-rolled thick slabs often lead to highly complex dynamic recrystallization behavior, resulting in chaotic hot-deformed structures and even significant mixed crystals, severely impacting product quality and overall performance. The main reason for this is that different slab thicknesses induce different stacking fault energies and recrystallization modes due to variations in deformation conditions, such as continuous / discontinuous dynamic recrystallization, or only recovery or deformation (dislocation multiplication). These different recrystallization modes directly lead to chaotic hot-deformed structures. Related studies have shown that when austenitic stainless steel (such as 304 and 316) is deformed in the range of 950 to 1100℃, discontinuous dynamic recrystallization is the main nucleation mechanism, and a morphology of mixed coarse and fine grains has been observed in the microstructure (Jafari M, et al. Materials Science and Engineering A, 2009, 501(1):16-25. etc.); at higher temperatures (around 1150℃) and under larger strain conditions, obvious continuous dynamic recrystallization will occur, forming uniform equiaxed fine grains (Babu KA, et al. Materials Science and Engineering A, 2018, 734:269-280. etc.). Patent ZL201910354680.X and literature (Wang Z G, et al. Materials Letters, 2022, 311:131590.) also show that using a small reduction in hot rolling can prevent dynamic recrystallization of austenitic stainless steel and regulate the grain boundary structure, thereby improving the material's resistance to intergranular corrosion. In other words, optimizing the hot deformation process can effectively control the dynamic recrystallization behavior of the material, thus obtaining an ideal deformed microstructure. Therefore, developing a hot deformation process suitable for thick austenitic stainless steel slabs is both necessary and feasible. This method not only needs to achieve control over dynamic recrystallization behavior but also needs to be easy to implement, especially for industrial applications.

[0004] Based on the above research background, this invention provides a method for controlling the dynamic recrystallization structure of austenitic stainless steel thick plates. Taking the deformation characteristics and dynamic recrystallization mode of austenitic stainless steel thick slabs as the starting point, a special "high temperature and high pressure assisted by low temperature and low pressure" process is used to make the material undergo continuous dynamic recrystallization or full recrystallization as much as possible under high temperature and high pressure conditions. Then, the non-continuous dynamic recrystallization temperature range is avoided, and low pressure hot rolling is used at low temperature to prevent dynamic recrystallization of the material and make the deformation uniform, thereby obtaining a uniformly distributed hot deformation structure. Summary of the Invention

[0005] The purpose of this invention is to address the above-mentioned problems by providing a method for controlling the dynamic recrystallization structure of austenitic stainless steel thick plates.

[0006] The objective of this invention is achieved as follows: A method for controlling the dynamic recrystallization microstructure of austenitic stainless steel thick plates, comprising the following steps: Step 1: Preparing austenitic stainless steel thick slab billets: the microstructure consists of austenitic grains, the maximum δ-ferrite content is <2%, and the slab thickness is 150-300 mm; Step 2: Diffusion annealing the thick slab billets at 1230-1260℃ for a holding time t = (1.5-2.5) h, where h is the slab thickness in mm and t is in min; obtaining homogenized slab billets; Step 3: After the homogenized slab billets are removed from the furnace, they undergo 1-2 passes... The first hot rolling of the billet involves a reduction of 6%-8%, followed immediately by a large reduction hot rolling, with a single-pass reduction of 25%-50%, to obtain a rough-rolled plate. The surface temperature of the rough-rolled plate should not be lower than 1090℃. If the thickness of the rough-rolled plate reaches the required thickness, the billet is directly water-cooled to room temperature to obtain a controlled hot-rolled plate; otherwise, proceed to step four. Step four: After cooling the rough-rolled plate to below 920℃ at a cooling rate of 30-100℃ / min, a small reduction hot rolling is performed, with a single-pass reduction of 5%-10% and a total reduction of 20%-40%. After rolling, the plate is water-cooled to room temperature to obtain a controlled hot-rolled plate.

[0007] In step one above, the thick slab is a continuously cast slab, an electroslag slab, or an intermediate slab that is half-rolled from a continuously cast slab or an electroslag slab, and the average austenite grain size is ≤110μm.

[0008] In steps two, three, and four above, the required temperature refers to the actual temperature reached on the surface of the steel plate, and the temperature difference between the edge and the middle of the slab surface shall not exceed 20°C.

[0009] In steps three and four above, the thickness of the controlled hot-rolled plate is between 20 and 100 mm; the microstructure of the controlled hot-rolled plate obtained in step three is 60 ≤ F. R <95%, the controlled microstructure FR of the hot-rolled plate obtained in step four is <25%, where FR is the dynamic recrystallization fraction; the grain size variance coefficient Cv of both types of controlled hot-rolled plates is less than 0.5, among which S represents the standard deviation of different grain sizes. This represents the average grain size.

[0010] The beneficial effects of the present invention are: (1) The present invention addresses the problem of controlling the hot-rolled structure of austenitic stainless steel thick slabs. Taking into account the characteristics of hot deformation of austenitic stainless steel thick slabs and the dynamic recrystallization mode, a special "high temperature and large pressure reduction assisted by low temperature and small pressure reduction" process is adopted to regulate the dynamic recrystallization behavior of the material in order to obtain a uniformly distributed recrystallized grain structure.

[0011] (2) The process design of this invention organically combines the theoretical knowledge of dynamic recrystallization with the conditions of industrial production. The process is highly operable and universal, and can be applied in industrial applications. Attached Figure Description

[0012] The present invention will now be further described with reference to the accompanying drawings.

[0013] Figure 1 The microstructure of the hot-rolled plate in Embodiment 1 of the present invention is shown in the EBSD image.

[0014] Figure 2 The microstructure of the hot-rolled plate in Embodiment 2 of the present invention is shown in the EBSD image.

[0015] Figure 3 The microstructure of the controlled hot-rolled plate is shown in the EBSD diagram for the comparative example of this invention. Detailed Implementation

[0016] The method of this invention includes the following steps: 1. Preparing austenitic stainless steel slabs; the microstructure of which consists of austenitic grains, with a maximum δ-ferrite content of <2%, and a slab thickness ≥150mm; 2. Diffusion annealing the slabs at 1230~1260℃ for a holding time t=(1.5~2.5)h, where h is the slab thickness in mm and t is in min; obtaining homogenized slabs; 3. After the homogenized slabs are removed from the furnace, they are first subjected to 1~2 passes of hot rolling, with a reduction of 6%~8%. Then immediately perform hot rolling with a large reduction, with a single-pass reduction of 25% to 50%, to obtain a rough-rolled plate; the surface temperature of the rough-rolled plate is not lower than 1090℃; if the thickness of the rough-rolled plate reaches the required thickness, the slab is directly water-cooled to room temperature to obtain a controlled hot-rolled plate; otherwise, proceed to step 4; 4. After cooling the rough-rolled plate to below 920℃ at a cooling rate of not less than 30℃ / min, perform hot rolling with a small reduction, with a single-pass reduction of 5% to 10%, and a total reduction of ≥25%, and water-cool to room temperature after rolling to obtain a controlled hot-rolled plate.

[0017] In step 1 above, the thick slab can be a continuously cast slab, an electroslag slab, or an intermediate slab, with an average austenite grain size ≤110μm.

[0018] In steps 2, 3, and 4 above, the required temperature refers to the actual temperature reached on the surface of the steel plate, and the temperature difference between the edge and the middle of the slab surface shall not exceed 20°C.

[0019] In steps 3 and 4 above, the thickness of the controlled hot-rolled plate is ≥20mm; the microstructure F of the controlled hot-rolled plate obtained in step 3 is... R >60%, the controlled microstructure F of the hot-rolled plate obtained in step 4 R <25%, F R The dynamic recrystallization fraction; the two control coefficients for grain size variance C of hot-rolled plates v All are less than 0.5, among which S represents the standard deviation of different grain sizes. This represents the average grain size.

[0020] The steel grades and compositions processed in this embodiment of the invention adopt the standard GB / T4238-2015.

[0021] In this embodiment of the invention, the dynamic recrystallization fraction was detected by the EBSD module in a ZEISS ULTRA™ 55 field emission scanning electron microscope. The data was processed on the built-in CHANNEL-5HKL software. The detection magnification and grain size statistics were performed in accordance with the standard GB / T 6394—2002.

[0022] In this embodiment of the invention, the standard used for detecting the δ-ferrite content in austenitic stainless steel thick slabs is GB / T13305-2008.

[0023] Example 1

[0024] The austenitic stainless steel slab (electroslag intermediate billet) used was grade 06Cr17Ni12Mo2, ASTM grade 316, with a thickness of 170 mm. The microstructure consisted of austenitic grains with a maximum δ-ferrite content of 0.9% and an average grain size of 35 μm. The slab was diffusion annealed at 1240℃ for 360 min (t = 1.8 h), where h is the slab thickness in mm, to obtain a homogenized slab. After the homogenized slab was removed from the furnace, it underwent a single-pass hot rolling with a reduction of 7%, followed immediately by a high-reduction hot rolling. The single-pass reductions were 25%, 33%, 40%, and 42%, respectively. The final rolling temperature on the steel plate surface was 1108℃, with a 5℃ temperature difference between the edge and center. After rolling, the slab was water-cooled to room temperature to produce a controlled hot-rolled plate.

[0025] EBSD testing and data analysis of hot-rolled steel plate samples revealed that the recrystallization fraction F of the steel plate... R The percentage is 72%, and the grain size variance coefficient C v The value is 0.35, and the microstructure is as follows Figure 1 As shown, it meets the requirements of this invention.

[0026] Example 2

[0027] The method is the same as in Example 1, except that: (1) the grade of austenitic stainless steel slab is 06Cr19Ni10, ASTM grade 304, the slab thickness is 200mm, the maximum δ-ferrite content in the microstructure is 0.8%, and the average grain size is 42μm; (2) the slab is diffusion annealed at 1250℃ and the holding time is 400min; a homogenized slab is obtained; (3) after the homogenized plate is taken out of the furnace, it is hot rolled in one pass with a reduction of 8%, and then hot rolled with a large reduction immediately. The single-pass reductions are 25%, 34%, and 35%, respectively, to obtain a rough-rolled plate with a steel plate surface temperature of 1118℃; (4) after the rough-rolled plate is water-cooled to 890℃, it is hot rolled with a small deformation of 6% in one pass, and the total reduction is 30%; after rolling, it is water-cooled to room temperature to obtain a controlled hot-rolled plate. (4) The recrystallization fraction F of the controlled hot-rolled plate is... R The percentage is 15%, and the grain size variance coefficient C v The value is 0.4, and the microstructure is as follows Figure 2 As shown, it meets the requirements of this invention.

[0028] Comparative Example

[0029] The experimental materials were the same as in Example 1, except that: (1) after the homogenized plate was hot rolled, it was hot rolled according to the traditional process with a reduction of 12%, 18%, 20%, 22%, 22%, 25%, 20%, and 16%. The final rolling temperature of the steel plate surface was 980°C, and the temperature difference between the edge and the middle was 22°C. After rolling, it was water-cooled to room temperature to obtain a hot-rolled plate.

[0030] (2) Through EBSD testing and data analysis of hot-rolled plate samples, it was found that the recrystallization fraction F of the steel plate was high. R The percentage is 35%, and the grain size variance coefficient C v The value is 0.68, and the microstructure is as follows: Figure 3 As shown, this process results in a deformed microstructure with a clear mixture of large and small grains, which is detrimental to the design of this invention.

[0031] The features and effects of the present invention have been described in detail above through embodiments and comparative examples, but they are not limited to these embodiments. There are many other equivalent embodiments without departing from the concept of the present invention.

[0032] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by those skilled in the art within the scope of the present invention are covered by the patent scope of the present invention.

Claims

1. A method for controlling the dynamic recrystallization microstructure of thick austenitic stainless steel plates, characterized in that: Includes the following steps: Step 1: Prepare austenitic stainless steel slabs: their microstructure consists of austenitic grains, with δ-ferrite content <2%, and slab thickness of 150~300mm. Step 2: Diffusion anneal the thick slab at 1230-1260℃ for a holding time t = (1.5-2.5)h, where h is the thickness of the slab in mm and t is in min; to obtain a homogenized slab. Step 3: After the homogenized slab is taken out of the furnace, it is first subjected to 1-2 passes of hot rolling with a reduction of 6%-8%, and then immediately subjected to large reduction hot rolling with a single pass reduction of 25%-50% to obtain a rough rolled plate; the surface temperature of the rough rolled plate is not lower than 1090℃. Step 4: After cooling the rough-rolled plate to below 920℃ at a cooling rate of 30~100℃ / min, perform hot rolling with a small reduction. The single-pass reduction is 5%-10%, and the total reduction is 20%~40%. After rolling, water cool to room temperature to obtain a controlled hot-rolled plate. In step one above, the thick slab is a continuously cast slab, an electroslag slab, or an intermediate slab that is hot-rolled half of a continuously cast slab or an electroslag slab, with an average austenite grain size ≤110μm. In steps three and four above, the thickness of the controlled hot-rolled plate is between 20 and 100 mm; the microstructure of the controlled hot-rolled plate obtained in step three is 60 ≤ F. R <95%, the controlled microstructure F of the hot-rolled plate obtained in step four R <25%, F R The dynamic recrystallization fraction; the grain size variance coefficient Cv for both controlled hot-rolled plates is less than 0.5, where Cv = S / S represents the standard deviation of different grains. This represents the average grain size.

2. The method for controlling the dynamic recrystallization microstructure of austenitic stainless steel thick plates according to claim 1, characterized in that: In steps two, three, and four above, the required temperature refers to the actual temperature reached on the surface of the steel plate, and the temperature difference between the edge and the middle of the slab surface shall not exceed 20°C.

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