Method for refining functional boron stainless steel sheet structure through large-deformation machining

Abstract

The invention relates to the technical field of metallurgical materials, and particularly discloses a method for refining a functional boron stainless steel sheet structure through large deformation machining. Comprising the following steps that S1, smelting is conducted, specifically, a steel ingot is prepared through a vacuum induction smelting and electroslag remelting duplex technology; s2, sheath forging: carrying out surface treatment on the steel ingot to obtain a blank, filling the blank into a steel sheath for packaging to form a blank with a sheath, and carrying out hot forging on the blank with the sheath to obtain a forged plate with the sheath; s3, sheath rolling: performing multi-pass hot rolling on the forged plate to prepare a hot-rolled thin plate with a sheath; s4, solution treatment: carrying out solution treatment on the hot-rolled sheet to obtain a solution-treated sheet; and S5, surface treatment is conducted, specifically, after a steel ladle sleeve of the thin plate subjected to solution treatment is removed, surface treatment is conducted, and the functional boron stainless steel thin plate with the needed specification and size is obtained. By means of the method, the boron stainless steel sheet which is excellent in mechanical property, uniform in thermal neutron absorption and suitable for being used as a structure / shielding integrated material can be prepared.

Description

Technical Field

[0001] This invention relates to the field of metallurgical materials technology, specifically to a method for refining the microstructure of functional boron stainless steel sheets using large deformation processing. Background Technology

[0002] Boron-containing austenitic stainless steel (boron content of 0.25~2.25%) has good thermal neutron absorption and gamma-ray shielding properties. It can maintain the stability of its structure and mechanical properties even at high temperatures and has excellent corrosion resistance. It is currently widely used in spent fuel pool storage racks, transportation and dry storage containers, reactor shielding systems and spent fuel reprocessing.

[0003] In the American ASTM A887-89 standard, boron-containing austenitic stainless steel (19Cr-14Ni-0.2~2.25B) is divided into eight grades based on its boron content, with each grade further divided into A and B levels. The composition ranges of these two levels are identical, but the essential difference lies in the boron particle size and uniformity of the boron compound in B-grade boron stainless steel sheets compared to A-grade sheets. This results in B-grade sheets of the same thickness exhibiting inferior elongation after fracture, impact resistance, and uniformity of thermal neutron absorption compared to A-grade sheets. Therefore, in spent fuel reprocessing systems, A-grade high-boron stainless steel is more likely to be used as an integrated structural / shielding material, while B-grade stainless steel can only be used as a functional material for thermal neutron shielding, requiring structural materials as an auxiliary component. Thus, how to process A-grade high-boron stainless steel sheets more quickly and effectively has become one of the main research focuses for its application in spent fuel reprocessing procedures.

[0004] There are already many reports on the preparation methods of boron stainless steel plates, with powder metallurgy and smelting casting being the most mainstream methods.

[0005] Powder metallurgy is a highly efficient powder forming process. Through homogeneous powder mixing, pressure sintering for densification, and subsequent deformation control, it achieves a uniform and dispersed distribution of boron and borides in a stainless steel matrix, while suppressing boride coarsening and grain boundary segregation. Compared to melt casting, it offers greater control over composition and microstructure, balancing mechanical properties and neutron shielding performance. It can produce high-boron-content plates and avoids hot-working cracking problems. For example, Chinese patent documents CN105463293A and CN106435401A disclose methods for preparing thin plates (boron content of 0.2-3.0%) of boron-containing stainless steel powder metallurgy. Among them, document CN105463293A mainly uses "atomized powder preparation - pressure billet preparation - atmosphere sintering - forging - rolling - heat treatment" to prepare Grade A thin plates, while document CN106435401A mainly uses "atomized powder preparation - powder rolling - sintering" to prepare boron stainless steel thin plates. The elongation of the finished plates prepared by these inventions is only >3.0%. However, the powder metallurgy method for preparing high boron stainless steel has a long preparation process cycle, high equipment costs, or the final product may not meet the mechanical property requirements of the plates.

[0006] The core of melt casting is to melt metal or alloy raw materials, inject them into a mold, and then cool and solidify them to form ingots, castings, or billets. Because the steel ingots obtained by melt casting contain a large amount of hard, brittle, and coarse eutectic borides, the hot working process of boron-containing stainless steel sheets prepared by melt casting is extremely prone to cracking, making forming very difficult. Existing publicly available preparation methods are all unable to complete the large deformation processing of boron stainless steel. For example, Chinese patent documents CN106702287A, CN110527929A, and CN111826583A all disclose the preparation process of functional boron-containing stainless steel sheets based on melt casting. Among them, patent CN106702287A discloses a method to improve the hot working properties of high-boron stainless steel and facilitate room temperature plasticity. This invention mainly uses vacuum welding on the upper and lower surfaces of a stainless steel billet to assemble the billet, finally producing a three-layer sandwich structure composite sheet. However, this method results in a lower effective boron content in the final cross-sectional area of ​​the sheet, and edge cracking still occurs during the forming process. Patent CN110527929A and the literature (Fabrication of high borated austenitic stainless steel thick plates with enhanced ductility and toughness using ahot-roll-bonding method. Materials Science & Engineering A, 2021, 799.) disclose a boron-containing stainless steel thick plate with excellent ductility and toughness and its preparation method. This method mainly uses a "vacuum welding + stack rolling" process to prepare thick plates with a boron content of less than 1.5%. According to the microstructure evaluation results, although the interfacial bonding force is high after bending deformation, a small number of micropores can still be observed in the bonding interface area at the microscale. These pores will deteriorate the mechanical stability of the material during long-term service. Patent CN111826583A discloses a process route that uses centrifugal casting to prepare the billet, which not only requires high equipment capabilities and conditions, but also easily leads to gravity segregation during the casting process, deteriorating the uniformity of boron in the finished plate. Clearly, none of the technical solutions disclosed in the aforementioned documents can achieve large deformation processing of boron stainless steel ingots, thereby solving the problem of coarse microstructure in finished thin plates.

[0007] In summary, existing technologies are insufficient to produce boron stainless steel sheets with excellent mechanical properties and uniform thermal neutron absorption using the melt casting method. Summary of the Invention

[0008] In view of the above-mentioned shortcomings in the prior art, the core objective of the present invention is to provide a method for refining the microstructure of functional boron stainless steel sheets by large deformation processing, which can obtain boron stainless steel sheets with excellent mechanical properties and uniform thermal neutron absorption based on melt casting.

[0009] This invention is achieved through the following technical solution:

[0010] This invention provides a method for refining the microstructure of functional boron stainless steel sheets using large deformation processing, comprising the following steps: S1 Smelting: Steel ingots are prepared through a dual process of "vacuum induction melting + electroslag remelting"; S2 Forging with a sheath: After cleaning the surface of the steel ingot obtained in step S1, a billet is obtained. The billet is then placed in a steel sheath to form a sheathed billet. The sheathed billet is then hot-forged to obtain a sheathed forging plate. S3 Encasing Rolling: The forged plate obtained in step S2 is hot-rolled in multiple passes to produce a hot-rolled sheet with an encasing. S4 Solution treatment: The hot-rolled sheet obtained in step S3 is subjected to solution treatment to obtain a solution-treated sheet. S5 Surface Treatment: After removing the cladding from the solution-treated sheet obtained in step S4, surface treatment is performed to obtain a functional boron stainless steel sheet of the required specifications and dimensions.

[0011] This invention prepares steel ingots using a dual process of "vacuum induction melting + electroslag remelting," fully utilizing the rapid solidification effect of the water-cooled crucible in electroslag remelting. Combined with optimization of the electroslag remelting filling ratio, it initially achieves refined and homogenized microstructure control of boron stainless steel electroslag ingots, improving their hot working properties. Subsequently, a "clad forging + clad rolling" process is employed to ensure that each region of the forged bar and hot-rolled slab is essentially under triaxial compressive stress during deformation, preventing cracking of the boron stainless steel billet during hot working and enabling large-deformation forging and rolling of boron stainless steel, further refining the boron content in the boron stainless steel. The size of the boride particles was determined; subsequently, a solution treatment was performed to dissolve the boride precipitated at the grain boundaries after rolling into the austenitic matrix to a suitable extent, while simultaneously eliminating work hardening and internal stress generated during rolling, further refining the grains and boride particles; finally, a boron stainless steel sheet with refined grains, uniform boride dispersion, and a maximum particle size dMax of the brittle boride phase < 40 μm was prepared, and the sheet thickness met the requirements of 2.0~6.0 mm and an aspect ratio ≥ 250; it has excellent mechanical properties, room temperature elongation ≥ 12%, and good neutron absorption uniformity, meeting the requirements as an integrated structural / shielding material.

[0012] As a preferred embodiment, step S1 of the above preparation method is optimized as follows: The process begins with vacuum induction melting to obtain a vacuum induction ingot, followed by electroslag remelting.

[0013] Specifically, during vacuum induction melting, alloying elements are proportioned according to the designed composition. The furnace charge is melted and the molten steel is refined through vacuum induction melting, and boron stainless steel vacuum induction ingots are obtained through casting.

[0014] The preferred design composition of this invention satisfies the following: the chemical composition of the prepared functional boron stainless steel sheet is: 18.00%≤Cr≤20.00%, 12.00%≤Ni≤15.00%, 0.02%≤B≤2.25%, C≤0.08%, Mn≤2.0%, Si≤0.75%, P<0.045%, S<0.030%, Co<0.2%, with the balance being unavoidable impurities and Fe. Among these, the Mn / Si content ratio is ≥2.5, and the total amount of C, P, and S is <0.1%.

[0015] Since Mn and Si elements in boron stainless steel are not only the main pre-deoxidizing elements in the smelting process, their content ratio also significantly affects the strengthening effect of the finished sheet. Therefore, this invention specifies their ratio, limiting the Mn / Si content ratio to ≥2.5. Impurities such as C, P, and S significantly deteriorate the mechanical and corrosion properties of functional boron stainless steel sheets. Therefore, this invention specifies their total content, limiting the total amount of C, P, and S to <0.1%. From a composition design perspective, this further ensures the machinability of the ingot and the mechanical properties of the finished sheet.

[0016] After obtaining a vacuum induction ingot through vacuum induction melting, the solidification defects in the head and tail areas of the vacuum induction ingot are removed and the surface is machined to ensure that there is no obvious oxide scale on the surface of the machined vacuum induction ingot, so as to ensure the stability of the electric arc in the subsequent electroslag remelting process and avoid oxygenation of the slag.

[0017] After machining, the vacuum induction ingot is mounted on the cathode of the electroslag remelting furnace. A circular plate with the same composition as the vacuum induction ingot is used as the arc-starting plate. The slag is CaF2-Al2O3-CaO with a mass ratio of 50:25:25. Remelting is carried out under high-purity argon protection. The filling ratio of the vacuum induction ingot is 0.40~0.65. Further design of the slag system ensures desulfurization and deoxidation effects. Control of the filling ratio ensures the stability of the electroslag remelting process and avoids component segregation caused by molten pool fluctuations.

[0018] As a preferred embodiment, step S2 in the above preparation method is optimized as follows: First, in step S2, the thickness of the steel cladding is designed to be t; where, when the steel ingot is a round steel ingot, t ≥ 0.1D, where D is the diameter of the boron stainless steel round ingot; when the steel ingot is a square steel ingot, t ≥ 0.1H, where H is the thickness of the boron stainless steel square ingot. This ensures that the steel cladding effectively protects the billet. Furthermore, the steel cladding in this invention is made of 304 stainless steel or 316 stainless steel. After the billet is placed in the cladding, it is sealed on all six sides, and the gap between the cladding and the billet is less than 1mm.

[0019] Secondly, before encapsulating the steel cladding, the boron stainless steel electroslag remelted steel ingot obtained in step S1 is first cleaned by removing the oxide scale from the surface of the electroslag remelted steel ingot to obtain the billet. Then, both the steel cladding and the billet are pre-oxidized to ensure that the steel cladding does not stick to the boron stainless steel billet during the forging process. Subsequently, the pre-oxidized billet is loaded into the pre-oxidized steel cladding and the steel cladding is sealed and welded.

[0020] Preferably, the pre-oxidation treatment of the billet and steel cladding is carried out at 900~1000℃ in an air atmosphere for 6h~7h.

[0021] Most importantly, in step S2 of this invention, the encased billet is heated to a homogenization temperature T and held at that temperature before undergoing multi-pass forging. The homogenization temperature T = (1150-1200×b%+1000×c%) ±10℃, preferably T = (1150-1200×b%+1000×c%) ±5℃, and more preferably T = (1150-1200×b%+1000×c%) ±3℃, where b and c are the mass percentages of boron and carbon in the boron stainless steel ingot, respectively. Since the boron and carbon content in boron stainless steel significantly affects the melting point and processing temperature range of the material, this invention specifies the homogenization temperature range for forging boron stainless steel with different compositions based on variations in composition. This not only ensures the yield of seamless boron stainless steel billets but also avoids deterioration of the microstructure and properties. This invention precisely controls the degree of austenitization based on the boron and carbon content, ensuring that the borides are properly dissolved to improve thermoplasticity, while avoiding excessively high temperatures that could lead to coarse grains.

[0022] The present invention further limits the holding time of the encased billet after heating to the homogenization temperature T to ≥3h, so as to achieve uniform temperature inside and outside the ingot and reduce uneven forging deformation; further controls the deformation amount per forging cycle to <30% and the final forging temperature to ≥900℃, so as to avoid excessive deformation and failure of the protective effect of the encasing on the core billet; at the same time, the forging ratio is controlled to ≥3.0 to ensure that the borides in the solidification structure are fully broken.

[0023] After forging, boron stainless steel forged plates with a width-to-thickness ratio ≥10 are obtained. This provides billets with uniform thickness and dense structure for subsequent rolling processes, reducing the difficulty of rolling.

[0024] As a preferred embodiment, step S3 in the above preparation method is optimized as follows: In step S3, the forged plate obtained in step S2 is heated to a homogenization temperature T and held at that temperature, followed by multiple hot rolling passes. Preferably, T = (1150-1200×b%+1000×c%) ± 5℃, more preferably T = (1150-1200×b%+1000×c%) ± 3℃, where b is the mass percentage of boron in the boron stainless steel ingot, and c is the mass percentage of carbon in the boron stainless steel ingot. This invention preferably uses the same homogenization temperature for forging and rolling, and the rolling process follows the homogenization temperature used for forging to ensure a consistent microstructure and uniform dissolution and distribution of borides after heating.

[0025] Furthermore, after heating to the homogenization temperature, the holding time is ≥2h to eliminate the internal stress after forging and ensure uniform rolling deformation. In addition, during the rolling process in step S3, the deformation amount of a single pass is controlled to be ≤h, where h=25-4.0×b, and b is the mass percentage of boron in the boron stainless steel ingot. Since the higher the boron content, the worse the thermoplasticity, this invention determines the deformation amount of a single pass based on the boron content. The higher the boron content, the smaller the deformation amount of a single pass, thus avoiding cracking. This invention controls the large deformation of rolling deformation amount to ≥90%, which is the core of achieving microstructure refinement. Through strong plastic deformation, the grains undergo dynamic recrystallization, breaking down the remaining coarse grains and boride particles to form a fine and uniform equiaxed grain structure. The final rolling temperature is controlled to ≥900℃ to ensure that the rolling is carried out in the austenite stable region, avoiding cracking due to decreased plasticity caused by excessively low temperature, and at the same time laying a good microstructure foundation for subsequent solution treatment.

[0026] As a preferred embodiment, step S4 of the above preparation method is optimized as follows: In step S4, the solution treatment temperature is 900~1100℃, the holding time is 30~90min, and then water cooling is performed to obtain boron stainless steel solution treated sheet.

[0027] Preferably, in step S5 of the above preparation method, after removing the steel cladding of the boron stainless steel solution-treated sheet obtained in step S4, the sheet undergoes surface treatment such as pickling and surface grinding to obtain the functional boron stainless steel finished sheet.

[0028] The functional boron stainless steel sheet finally prepared by this invention has a thickness of 2.0~6.0 mm and an aspect ratio of ≥250; and the maximum particle size dMax of the brittle boride phase in the functional stainless steel sheet is <40 μm, with a room temperature elongation of ≥12%.

[0029] The present invention has at least the following advantages and beneficial effects: (1) Through complete process design, the present invention adopts a dual process of "vacuum induction + electroslag remelting" to initially achieve the refinement and homogenization control of the microstructure of boron stainless steel electroslag ingot, and improve the hot working performance of boron stainless steel electroslag ingot; then adopts "clad forging + clad rolling" to avoid cracking of boron stainless steel billet during hot working and to achieve large deformation forging and rolling of boron stainless steel, further refining the size of boride particles in boron stainless steel; finally, the grains and boride particles are further refined through solution treatment; thus, the preparation of grade A high boron stainless steel thin plate is realized based on melt casting method.

[0030] (2) By strictly controlling the homogenization temperature during the forging and rolling process, and adjusting the homogenization temperature according to the boron and carbon content in boron stainless steel, this invention not only ensures the yield of seamless boron stainless steel billet preparation and processing, but also avoids the deterioration of microstructure and properties.

[0031] (3) The entire preparation method of the present invention has a compact process flow, which is easy to realize industrial production. Moreover, through targeted process parameter control, the yield of the product is significantly improved, thereby reducing production energy consumption and cost. Attached Figure Description

[0032] Figure 1 The image shows the metallographic structure of the functional boron stainless steel sheet prepared in Example 1 of this invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the present invention will be further described below in conjunction with specific embodiments.

[0034] The following embodiments are intended to enable those skilled in the art to gain a more comprehensive understanding of the present invention, but do not limit the invention in any way.

[0035] Example 1 This embodiment provides a method for refining the microstructure of functional boron stainless steel sheets using large deformation processing, including the following steps: S1 Smelting The alloying elements are prepared according to the design composition. The prepared raw materials Fe, Cr, Ni, and Si are loaded into a crucible and heated in the furnace under a vacuum of less than 20 Pa. After a molten pool is formed and there is no other unmelted metal in the pool, the vacuum is turned off. Under the protection of high-purity argon, ferroboron and manganese metal / alloy, which have been placed in the vacuum induction furnace feeder beforehand, are added to the crucible in sequence for alloying. The temperature of the molten steel is adjusted to the casting temperature (1535~1540℃) by power regulation. The Φ350 mm boron stainless steel vacuum induction ingot is produced by die casting. The chemical composition of the boron stainless steel vacuum induction ingot, by mass percentage, is B 2.20%, Cr 19.8%, Ni 12.2%, Si 0.28%, Mn 1.50%, C 0.03%, P 0.01%, S 0.01%, Co 0.04%, with the remainder being Fe and unavoidable impurity elements.

[0036] After removing the solidification defects at the head and tail of the boron stainless steel vacuum induction ingot and removing the surface oxide scale by machining, it is placed on the cathode of an electroslag remelting furnace. A disc with the same composition as the boron stainless steel vacuum induction ingot is used as an arc ignition plate, and CaF2-Al2O3-CaO is used as the slag (the mass ratio of each component is 50:25:25). Remelting is carried out under the protection of high-purity argon to obtain a Φ540mm boron stainless steel electroslag remelting ingot (filling ratio is 0.42).

[0037] S2 forging After removing the oxide scale from the surface of the boron stainless steel electroslag remelting ingot obtained in step S1, a billet is obtained. The billet and its steel cladding are pre-oxidized at 1000℃ for 6 hours. Then, the billet is placed into a 55mm thick 304 stainless steel cladding and sealed on all six sides, with a gap of less than 1mm between the cladding and the billet. The clad billet is heated in a furnace to 1120℃ and held for 5 hours, followed by three-pass forging. The deformation per pass is <30%, the forging ratio is 3.2, and the final forging temperature is controlled at 920℃ to obtain a boron stainless steel forged plate with a width-to-thickness ratio of 11. This boron stainless steel forged plate is a crack-free forged plate.

[0038] S3 Encasing Rolling The boron stainless steel forged plate obtained in step S2 is heated in a furnace to 1110℃ and held for 3 hours, followed by 8 passes of hot rolling. The deformation per pass is h≤16mm, the rolling deformation is 95%, and the final rolling temperature is 905℃, resulting in a clad boron stainless steel hot-rolled sheet. This boron stainless steel sheet is a crack-free sheet.

[0039] S4 solution treatment The hot-rolled boron-clad stainless steel sheet obtained in step S3 is subjected to solution treatment at an annealing temperature of 900℃ and a holding time of 50 min, followed by water cooling to obtain the solution-treated boron-clad stainless steel sheet.

[0040] S5 Surface Treatment Remove the cladding from the solution-treated boron stainless steel sheet obtained in step S4, and remove the iron oxide scale by pickling and machining to obtain a functional boron stainless steel sheet with a width of 600 mm and a thickness of 2.0 mm.

[0041] The microstructure of the functional boron stainless steel sheet prepared in this embodiment was observed sequentially using an Olympus CX71 metallographic microscope and a FEI Nova / Nano 400 field emission scanning electron microscope. The metallographic characterization results are shown in the attached figure. Figure 1 As shown: In the microstructure of the finished boron stainless steel sheet, borides are uniformly dispersed in the matrix in the form of granules and short rods, without a continuous grain boundary boride network; and the size of the borides is mostly concentrated in 1-5 μm, which is relatively small and dispersed, without obvious coarsening and agglomeration. In the metallographic observation structure of this embodiment, the maximum particle size dMax of the boride particles is 32 μm; at the same time, the room temperature elongation of the finished boron stainless steel sheet was measured to be 13.0%.

[0042] Example 2 This embodiment provides a method for refining the microstructure of functional boron stainless steel sheets using large deformation processing, which differs from Embodiment 1 in that: (1) After vacuum induction melting, boron stainless steel vacuum induction ingots with a specification of Φ450 mm are produced by die casting. The chemical composition of the boron stainless steel vacuum induction ingots by mass percentage is B 0.21%, Cr 18.2%, Si 0.10%, Mn 0.8%, C 0.06%, P 0.02%, S 0.01%, Co 0.02%, with the remainder being Fe and unavoidable impurity elements.

[0043] (2) The boron stainless steel electroslag remelting ingot obtained has a specification of Φ620mm (filling ratio of 0.53).

[0044] (3) During the forging process, the wall thickness of the sleeve is 63mm. The sleeved billet is heated to 1145℃ in the furnace and held for 3.5 hours. Then it is forged in 4 fires with a single fire deformation amount of <30%, a forging ratio of 3.8, and the final forging temperature is controlled at 910℃ to obtain a boron stainless steel forging plate with a width-to-thickness ratio of 13.

[0045] (3) During the cladding rolling process, the boron stainless steel forging plate obtained in step S2 is heated to 1145°C in the furnace and held for 2.5 hours. Then, it is hot rolled in 7 passes with a single pass deformation amount h≤24mm, a rolling deformation amount of 92%, and a final rolling temperature of 920°C to obtain a cladding boron stainless steel hot-rolled sheet.

[0046] (4) During the solution treatment process, the annealing temperature is 1100℃, the holding time is 90min, and the solution is cooled with water to obtain a thin sheet of boron-clad stainless steel solution treated with a cladding.

[0047] (5) During the surface treatment process, a functional boron stainless steel sheet with a width of 1550 mm and a thickness of 6.0 mm is finally obtained.

[0048] The microstructure of the functional boron stainless steel sheet prepared in this embodiment was observed sequentially using an Olympus CX71 metallographic microscope and a FEI Nova / Nano 400 field emission scanning electron microscope. Metallographic characterization was performed, and its metallographic image and attached... Figure 1 Consistent with the results, the boron stainless steel finished sheet has a fine and dispersed boron compound particle size in its microstructure, with no obvious coarsening or agglomeration. The maximum particle size dMax of the boron compound is 28 μm. At the same time, the room temperature elongation of the boron stainless steel finished sheet was measured to be 42.5%.

[0049] Example 3 This embodiment provides a method for refining the microstructure of functional boron stainless steel sheets using large deformation processing, which differs from Embodiment 1 in that: (1) After vacuum induction melting, boron stainless steel vacuum induction ingots with a specification of Φ270 mm are produced by die casting. The chemical composition of the boron stainless steel vacuum induction ingots by mass percentage is B 1.27%, Cr 18.4%, Si 0.30%, Mn 1.6%, C 0.04%, P 0.03%, S 0.01%, Co 0.01%, with the remainder being Fe and unavoidable impurity elements.

[0050] (2) The boron stainless steel electroslag remelting ingot obtained has a specification of Φ350mm (filling ratio of 0.595).

[0051] (3) During the forging process, the wall thickness of the cladding is 36mm. The cladding billet is heated to 1130℃ in the furnace and held for 4 hours to obtain a boron stainless steel forging plate with a width-to-thickness ratio of 12.

[0052] (3) During the cladding rolling process, the boron stainless steel forging plate obtained in step S2 is heated to 1135°C in the furnace and held for 2.5 hours. Then, it is hot rolled in 7 passes with a single pass deformation amount h≤24mm, a rolling deformation amount of 92%, and a final rolling temperature of 920°C to obtain a cladding boron stainless steel hot-rolled sheet.

[0053] (4) During the solution treatment process, the annealing temperature is 1000℃, the holding time is 60min, and the solution is cooled with water to obtain a thin sheet of boron-clad stainless steel solution treated with a cladding.

[0054] (5) During the surface treatment process, a functional boron stainless steel sheet with a width of 800 mm and a thickness of 3.0 mm is finally obtained.

[0055] The microstructure of the functional boron stainless steel sheet prepared in this embodiment was observed sequentially using an Olympus CX71 metallographic microscope and a FEI Nova / Nano 400 field emission scanning electron microscope. Metallographic characterization was performed, and its metallographic image and attached... Figure 1 Consistent with the results, the boron stainless steel finished sheet has a fine and dispersed boron compound particle size in its microstructure, with no obvious coarsening or agglomeration. The maximum particle size dMax of the boron compound is 35 μm. At the same time, the room temperature elongation of the boron stainless steel finished sheet was measured to be 17.5%.

[0056] Comparative Example 1 This comparative example provides a method for preparing a functional boron stainless steel sheet. The difference from Example 1 is that this comparative example does not involve sheathing forging; instead, conventional forging is used. The specific operation of conventional forging is as follows: the billet is heated in a furnace to 1120°C and held for 5 hours, followed by three forging passes, with a single-pass deformation of <30%, a forging ratio of 3.2, and a final forging temperature controlled at 920°C. Because this boron stainless steel forged sheet lacks sheathing protection during forging, it suffers severe cracking and does not meet the requirements for further rolling.

[0057] Comparative Example 2 This comparative example provides a method for preparing functional boron stainless steel sheets, differing from Example 1 in that neither the steel cladding nor the billet undergoes pre-oxidation treatment. The billet is directly placed into a 55mm thick 304 stainless steel cladding and sealed on all six sides, with a gap of less than 1mm between the cladding and the billet. The clad billet is heated in a furnace to 1120℃ and held for 5 hours, followed by three-pass forging, with a single-pass deformation of <30%, a forging ratio of 3.2, and a final forging temperature of 920℃. Because neither the steel cladding nor the billet undergoes pre-oxidation treatment, after high-temperature forging deformation, the billet and cladding severely adhere, forming an irregular interface. On the one hand, it is difficult to remove the cladding smoothly; on the other hand, although some areas are cleaned by milling, it is difficult to determine whether the removal is complete in certain areas. Some areas of the finished sheet are mixed with a small amount of boron-free stainless steel components from the cladding, causing its chemical composition to deviate from the ingot composition.

[0058] Comparative Example 3 This comparative example provides a method for preparing a functional boron stainless steel sheet, which differs from Example 1 in that the homogenization temperature during the forging stage in this comparative example is 1150°C. Since the temperature exceeds the theoretical solidus temperature (1145°C) of the boron-containing stainless steel billet, partial liquefaction occurs during the billet heating process, resulting in severe cracking during forging and making it impossible to obtain a forged sheet suitable for subsequent forming.

[0059] Comparative Example 4 This comparative example provides a method for preparing functional boron stainless steel sheets, differing from Example 1 in that the deformation per pass during the cladding rolling process is 16mm ≤ h ≤ 18mm. Since boride particles are hard and brittle, they hardly undergo plastic deformation even at high temperatures. Therefore, when the deformation per pass is too large, the numerous pores formed by the breakage of the borides cannot be fully filled and healed by the austenitic matrix. Under excessive tensile stress, these pores expand and connect, eventually evolving into cracks. After two passes of large deformation rolling, the sheet material severely cracks, making it impossible to obtain finished sheets of the expected dimensions.

[0060] Comparative Example 5 This comparative example provides a method for preparing a functional stainless steel sheet, which differs from Example 1 in that the solution treatment step S4 is omitted. Because the sheet is a deformed form, its matrix contains numerous crystal defects such as dislocations and vacancies, and the borides at the grain boundaries do not adequately dissolve back into the matrix. Although the boride particle size is similar to that of Example 1, its room temperature plasticity is significantly deteriorated, with a room temperature tensile elongation of only 9%.

[0061] This invention is not limited to the above-described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

Claims

1. A method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing, characterized in that, Includes the following steps: S1 Smelting: Steel ingots are prepared through a dual process of "vacuum induction melting + electroslag remelting"; S2 Forging with a sheath: After cleaning the surface of the steel ingot obtained in step S1, a billet is obtained. The billet is then encapsulated in a steel sheath to form a sheathed billet. The sheathed billet is then hot-forged to obtain a sheathed forging plate. S3 Encasing Rolling: The forged plate obtained in step S2 is hot-rolled into a casing sheet through multiple passes of hot rolling; S4 Solution treatment: The hot-rolled sheet obtained in step S3 is subjected to solution treatment to obtain a solution-treated sheet. S5 Surface Treatment: After removing the steel cladding from the solution-treated sheet obtained in step S4, surface treatment is performed to obtain a functional boron stainless steel sheet of the required specifications and dimensions.

2. The method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing according to claim 1, characterized in that, In step S2, the thickness of the steel cladding is t; where, when the steel ingot is a round steel ingot, t≥0.1D, where D is the diameter of the boron stainless steel round ingot; when the steel ingot is a square steel ingot, t≥0.1H, where H is the thickness of the boron stainless steel square ingot.

3. The method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing according to claim 1, characterized in that, In step S2, both the steel sleeve and the billet are pre-oxidized before the billet is encapsulated in the steel sleeve.

4. The method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing according to claim 1, characterized in that, In step S2, the clad billet is heated to a uniform heating temperature T and held at that temperature before being forged in multiple passes. The uniform heating temperature T = (1150-1200×b%+1000×c%) ±10℃, where b and c are the mass percentages of boron and carbon in the boron stainless steel ingot, respectively.

5. The method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing according to claim 4, characterized in that, In step S2, the deformation amount per forging pass is <30%, the final forging temperature is ≥900℃, and the forging ratio is ≥3.

0.

6. The method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing according to claim 1, characterized in that, In step S3, the forged plate obtained in step S2 is heated to a uniform heating temperature T and held at that temperature, followed by multiple hot rolling passes; the uniform heating temperature T = (1150-1200×b%+1000×c%) ±10℃, where b is the mass percentage of boron in the boron stainless steel ingot and c is the mass percentage of carbon in the boron stainless steel ingot.

7. The method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing according to claim 6, characterized in that, In step S3, the deformation amount of a single rolling pass is ≤h, the rolling deformation amount is ≥90%, and the final rolling temperature is ≥900℃; where h=25-4.0×b, and b is the mass percentage of boron in the boron stainless steel ingot.

8. The method for refining the microstructure of functional boron stainless steel thin plates using large deformation processing according to any one of claims 1 to 7, characterized in that, In step S4, the solution treatment temperature is 900~1100℃, and the holding time is 30~90min.

9. The method for refining the microstructure of functional boron stainless steel sheet by utilizing large deformation processing according to any one of claims 1 to 7, characterized in that, In step S2, the steel sheath is made of 304 stainless steel or 316 stainless steel.

10. The method for refining the microstructure of functional boron stainless steel sheet using large deformation processing according to claim 1, wherein the chemical composition of the functional boron stainless steel sheet prepared therefrom is: 18.00%≤Cr≤20.00%, 12.00%≤Ni≤15.00%, 0.02%≤B≤2.25%, C≤0.08%, Mn≤2.0%, Si≤0.75%, P<0.045%, S<0.030%, Co<0.2%, with the balance being unavoidable impurities and Fe, wherein, The Mn / Si content ratio is ≥2.5, and the total amount of C, P and S is <0.1%.

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