A method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate

Abstract

This invention discloses a method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plates, comprising: using continuously cast long billets as raw materials, with the thickness of the continuously cast long billets controlled at 200-230 mm; rough rolling the continuously cast long billets into intermediate billets, with the thickness of the intermediate billets controlled at 35-45 mm; finish rolling the intermediate billets into ultrapure ferritic stainless steel hot-rolled coils with a thickness of 6.0-13.0 mm, with the finish rolling exit temperature controlled at 750-850℃ and the coiling temperature controlled at 500-600℃; straightening and leveling the hot-rolled coils, cutting them into multiple individual medium plates, and then sequentially heat-treating and pickling the individual medium plates, with the heat treatment temperature controlled at 850-1000℃ and the heat treatment time controlled at 1.0-2.5 min / mm, and pickling using a mixed acid of nitric acid and hydrofluoric acid, with the pickling time controlled at 10-25 min. This invention significantly improves the production efficiency and yield of hot-rolled medium plates of ultra-pure ferritic stainless steel, with a yield rate of over 90%. The average grain size of the hot-rolled medium plates of ultra-pure ferritic stainless steel is 100-250 μm, and the elongation after fracture in the tensile test reaches over 35%, meeting the requirements for manufacturing thick-gauge flanges and other components.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel product manufacturing technology, and more specifically relates to a method for manufacturing ultra-pure ferritic stainless steel hot-rolled medium plate. Background Technology

[0002] Ferritic stainless steel offers advantages such as low cost and price stability due to its low alloying element content and the absence of expensive alloying elements like Ni. Ultra-pure ferritic stainless steel, in particular, achieves excellent corrosion resistance, especially against intergranular corrosion, thanks to its extremely low C and N content (typically [C+N] ≤ 250 ppm) and the addition of stabilizing elements like Ti and Nb. Furthermore, ultra-pure ferritic stainless steel exhibits a relatively low coefficient of thermal expansion and high thermal conductivity, making it widely used in automotive exhaust systems, construction, home appliances, and heat exchangers.

[0003] Thick-gauge flange components are commonly used in automotive exhaust systems, heat exchangers, and other fields, with thicknesses reaching 6.0 mm or more, even exceeding 10 mm, but generally not exceeding 13 mm. The material used for manufacturing thick-gauge flange components is typically ultra-pure ferritic stainless steel medium plates. Currently, the common manufacturing method for ultra-pure ferritic stainless steel medium plates with thicknesses ranging from 6.0 to 13.0 mm is single-sheet rolling of short billets. This involves continuously casting molten stainless steel meeting composition requirements to obtain short billets, which are then rolled using a reversible rolling mill to obtain single-sheet black medium plates of the target thickness. These black-sheet medium plates are then heat-treated and pickled to obtain single-sheet medium plates with a clean surface that meets processing performance requirements. However, the existing short-bill single-sheet rolling method suffers from disadvantages such as low production efficiency, low yield, and high cost. The yield is only around 80%, and the toughness and mechanical properties of the ultra-pure ferritic stainless steel medium plates produced by this method are insufficient to meet the requirements for manufacturing flanges and other components.

[0004] Therefore, developing a method for manufacturing ultrapure ferritic stainless steel medium plates with higher production efficiency and yield, better cost and performance has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] To address the aforementioned technical problems in the existing technology, this invention provides a method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plates, comprising the following steps:

[0006] (1) Material preparation: The billet is a continuous casting billet, and the thickness of the continuous casting billet is controlled at 200-230mm.

[0007] (2) Hot continuous rolling roughing, the roughing is carried out in multiple passes to roll the continuous casting billet into an intermediate billet, and the thickness of the intermediate billet is controlled at 35-45mm.

[0008] (3) Hot continuous rolling finishing: a seven-stand hot continuous rolling mill is used for finishing rolling to roll the intermediate billet into a hot-rolled coil of ultra-pure ferritic stainless steel with a thickness of 6.0 to 13.0 mm. The exit temperature of the hot continuous rolling finishing is controlled at 750 to 850°C, and the coiling temperature is controlled at 500 to 600°C.

[0009] (4) Optimization of the opening, heat treatment and pickling process: First, the ultra-pure ferritic stainless steel hot-rolled coil is straightened and leveled, and cut into multiple single medium plates. Then, the single medium plates are heat treated and pickled in sequence. In the heat treatment process, the heat treatment temperature is controlled at 850-1000℃ and the heat treatment time is controlled at 1.0-2.5min / mm. After the medium plates are taken out of the furnace, they are water-cooled to room temperature. In the pickling process, shot blasting is performed first, and then pickling is performed using mixed acid. The mixed acid is nitric acid and hydrofluoric acid. The concentration of nitric acid is controlled at 90-180g / L, the concentration of hydrofluoric acid is controlled at 3-10g / L, and the pickling time is controlled at 10-25min.

[0010] Furthermore, in the above-mentioned method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plates, the continuously cast billet is heated to 1150-1250°C before the hot continuous rolling roughing step.

[0011] Furthermore, in the above-mentioned method for manufacturing ultrapure ferritic stainless steel hot-rolled intermediate plates, in the hot continuous rolling roughing step, the continuously cast billet is rolled into an intermediate billet in three passes.

[0012] Furthermore, in the above-mentioned method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plates, the width of the ultrapure ferritic stainless steel hot-rolled coil is 1200-2000 mm in the hot continuous rolling finishing step.

[0013] Furthermore, in the above-mentioned manufacturing method of ultrapure ferritic stainless steel hot-rolled medium plate, in the heat treatment process, a continuous solution furnace is used for heat treatment; in the pickling process, shot blasting is carried out by vertical shot blasting and pickling is carried out by vertical spray pickling.

[0014] Furthermore, in the above-mentioned manufacturing method of hot-rolled medium plates of ultrapure ferritic stainless steel, the length of a single medium plate is 3000-9000 mm in the plate-cutting process.

[0015] Furthermore, in the above-mentioned method for manufacturing ultra-pure ferritic stainless steel hot-rolled medium plate, the ultra-pure ferritic stainless steel hot-rolled medium plate is a 409L series stainless steel medium plate, a 436L series stainless steel medium plate, a 439 series stainless steel medium plate, a 441 series stainless steel medium plate, or a 444 series stainless steel medium plate.

[0016] The method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plates of the present invention has the following advantages and beneficial effects:

[0017] (1) The billet used in this invention for producing hot-rolled medium plates of ultrapure ferritic stainless steel is a continuously cast billet, which is then hot-rolled to obtain a hot-rolled coil of the target thickness. The hot-rolled coil is then straightened, leveled, and cut into individual medium plates. Finally, the individual medium plates are subjected to heat treatment and pickling in sequence to prepare the hot-rolled medium plates of ultrapure ferritic stainless steel. Compared with the existing technology of rolling short billets into individual sheets, the implementation of this invention can significantly improve the production efficiency and product yield of hot-rolled medium plates of ultrapure ferritic stainless steel, with a yield of over 90%.

[0018] (2) In the implementation of this invention, the performance of ultrapure ferritic stainless steel hot-rolled medium plates is improved by controlling the rolling reduction rate, hot continuous rolling finishing temperature, and optimizing the heat treatment and pickling processes of the open-plate heat treatment and pickling processes. This control is achieved through measures such as the thickness of the continuously cast billet, the thickness of the hot continuous rolling intermediate billet, the finishing exit temperature, the coiling temperature, and the heat treatment and pickling process parameters. The ultrapure ferritic stainless steel hot-rolled medium plate produced using this invention exhibits good toughness and mechanical properties, with an average grain size of 100–250 μm and an elongation after fracture exceeding 35% in tensile tests. All performance indicators of the product meet the requirements for manufacturing thick-gauge flanges and other components. Attached Figure Description

[0019] 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, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a metallographic diagram of a 439 stainless steel hot-rolled medium plate manufactured using the manufacturing method of the ultra-pure ferritic stainless steel hot-rolled medium plate of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0022] The purpose of this invention is to seek a method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plates with higher production efficiency and yield, better cost, and better product toughness and mechanical properties without significantly changing the existing tooling and equipment. The basic technical concept is as follows: the billet is a continuously cast billet, which is hot-rolled into a coil of the target thickness through hot continuous rolling including rough rolling and finish rolling. Then the coil is sheared to obtain a black-skinned medium plate. Finally, heat treatment and surface pickling are performed to obtain ultrapure ferritic stainless steel hot-rolled medium plates that meet the requirements.

[0023] Due to the relatively thick thickness of the finished medium plate, the hot-rolling deformation reduction rate is relatively low, which is not conducive to grain refinement. At the same time, ferritic stainless steel exhibits a significant size effect, with large grain sizes in the medium plate, resulting in deteriorated toughness compared to thinner plates. As thickness increases, the ductile-brittle transition temperature also rises. For ultra-pure ferritic stainless steel medium plates with a thickness greater than 6.0 mm, the ductile-brittle transition temperature exceeds 20℃. Below this temperature, the material's toughness decreases significantly. To ensure that hot-rolled coils can be leveled without fracture, a certain level of toughness must be maintained. Improving the toughness of hot-rolled medium plates requires corresponding modifications to the hot-rolling process.

[0024] The manufacturing method of the ultrapure ferritic stainless steel hot-rolled medium plate of the present invention is mainly used for producing ultrapure ferritic stainless steel hot-rolled medium plates with a thickness of 6.0 to 13.0 mm. It involves the control of rolling reduction rate, the control of hot continuous rolling finishing temperature, the optimization of heat treatment of the plate and pickling process, and specifically includes the following steps:

[0025] (1) Material preparation

[0026] Because the medium plate is relatively thick, only a thicker billet can achieve a larger hot rolling reduction rate, which is beneficial for grain refinement. However, when the billet thickness is too large, defects such as shrinkage cavities are easily generated inside the billet. Therefore, the billet thickness should be as large as possible while ensuring that defects such as shrinkage cavities do not occur.

[0027] Therefore, in the manufacturing method of the ultrapure ferritic stainless steel hot-rolled medium plate of the present invention, the billet is a continuously cast billet, and the thickness of the continuously cast billet is controlled at 200-230 mm.

[0028] (2) Hot continuous rolling roughing

[0029] Before rough rolling, the continuously cast billet is heated to 1150–1250℃, ensuring sufficient holding time to allow the billet to be thoroughly heated. The rough rolling process is performed in multiple passes to roll the continuously cast billet into an intermediate billet. To ensure the rolling reduction rate of the subsequent hot continuous rolling finish rolling, the thickness of the intermediate billet is controlled at 35–45 mm.

[0030] Preferably, in the hot continuous rolling roughing process, the continuously cast billet is rolled into an intermediate billet with a thickness of 35-45 mm in three passes.

[0031] (3) Hot continuous rolling finishing

[0032] A seven-stand hot continuous rolling mill is used for finishing rolling, and the intermediate billet is rolled into hot-rolled coils of ultra-pure ferritic stainless steel with a thickness of 6.0 to 13.0 mm.

[0033] The control of hot strip rolling temperature has a significant impact on the toughness of ultra-pure ferritic stainless steel. In particular, a lower finishing rolling temperature can reduce the ductile-brittle transition temperature and improve the toughness of hot-rolled coils of ultra-pure ferritic stainless steel. At the same time, the rolling temperature should not be too low, otherwise it will cause an increase in the strength of the rolled material and increase the rolling load on the rolling mill. Therefore, in the manufacturing method of ultra-pure ferritic stainless steel hot-rolled medium plate of the present invention, the finishing exit temperature of the hot strip rolling is controlled at 750-850°C, and the coiling temperature is controlled at 500-600°C.

[0034] Preferably, in hot continuous rolling finishing, the width of the ultra-pure ferritic stainless steel hot-rolled coil is 1200-2000 mm.

[0035] (4) Optimization of plate opening, heat treatment and pickling processes

[0036] First, the hot-rolled coils of ultra-pure ferritic stainless steel are straightened, leveled, and cut into multiple individual medium plates. These medium plates are then subjected to heat treatment and pickling sequentially to obtain finished products with good processing performance and surface cleanliness. In the heat treatment process, the temperature is controlled at 850–1000℃, and the treatment time is controlled at 1.0–2.5 min / mm. After exiting the furnace, the medium plates are water-cooled to room temperature to reduce the impact of precipitation on material properties. In the pickling process, shot blasting is performed first. Shot blasting not only removes the oxide scale on the surface of the steel plate but also breaks down the denser oxide layer inside, improving the efficiency of subsequent pickling. After shot blasting, a mixed acid is used for pickling. The mixed acid consists of nitric acid and hydrofluoric acid, with the nitric acid concentration controlled at 90–180 g / L and the hydrofluoric acid concentration controlled at 3–10 g / L. The pickling time is controlled at 10–25 min.

[0037] Preferably, in the plate-making process, the length of a single plate is 3000-9000 mm.

[0038] Preferably, in the heat treatment process, a continuous solution furnace is used for heat treatment; in the pickling process, shot blasting is carried out by vertical shot blasting and pickling is carried out by vertical spray pickling.

[0039] The manufacturing method of the ultrapure ferritic stainless steel hot-rolled medium plate of the present invention is described below with reference to Examples 1-5. The billets used in Examples 1-5 are all continuously cast billets, used to produce ultrapure ferritic stainless steel medium plates of series 409L, 436L, 439, 441, and 444, with a finished product thickness ranging from 6.0 to 13.0 mm. The specific steel grades and continuously cast billet thicknesses in Examples 1-5 are as follows:

[0040] Example steel grades Continuous casting billet thickness (mm) 1 409L 200 2 SUS436L 200 3 439 200 4 441 200 5 444 200

[0041] The specific implementation process of Examples 1 to 5 includes:

[0042] (1) A continuously cast billet of a specified thickness is prepared by continuously casting molten steel with a chemical composition that meets the requirements;

[0043] (2) After heating the continuous casting billet, hot continuous rolling roughing is performed, and the continuous casting billet is rolled into an intermediate billet through three passes of roughing;

[0044] (3) The intermediate billet is hot-rolled and finished using a seven-stand hot rolling mill, and the finishing exit temperature and coiling temperature are controlled to produce ultra-pure ferritic stainless steel hot-rolled coils. The width of the ultra-pure ferritic stainless steel hot-rolled coils is 1200-2000 mm, and the maximum coil weight can reach 28 tons.

[0045] (4) The hot-rolled coil is straightened and leveled, and cut into multiple single medium plates with a length of 3000-9000 mm. Then, the single medium plates are heat-treated and pickled in sequence. The heat treatment is carried out in a continuous solution furnace. After exiting the furnace, the plates are cooled to room temperature by water. After heat treatment, the plates are shot-blasted in a vertical manner, and then pickled in a vertical spray pickling manner using a mixture of nitric acid and hydrofluoric acid.

[0046] In the implementation of Examples 1-5, rolling reduction rate control and hot continuous rolling finishing temperature control were carried out. Specifically, the intermediate billet thickness, finishing exit temperature, coiling temperature, and hot-rolled coil thickness in Examples 1-5 were controlled as follows:

[0047] Example Intermediate billet thickness (mm) Finishing mill exit temperature (°C) Winding temperature (°C) Thickness of hot-rolled coil (mm) 1 42 780 542 12.10 2 38 832 572 8.95 3 39 805 560 10.15 4 38 820 562 9.93 5 37 838 587 8.12

[0048] Furthermore, the heat treatment and pickling processes were optimized during the implementation of Examples 1-5. Specifically, the main process parameters for heat treatment and pickling in Examples 1-5 were controlled as follows:

[0049]

[0050] Actual production measurements show that the yield of hot-rolled medium plates of ultra-pure ferritic stainless steel (409L, 436L, 439, 441, and 444 series) produced using Examples 1-5 of this invention all exceeds 90%. Furthermore, testing revealed that the average grain size of the ultra-pure ferritic stainless steel hot-rolled medium plates produced using Examples 1-5 of this invention is 100-250 μm, for example, see [reference needed]. Figure 1The microstructure of the 439 ultra-pure ferritic stainless steel hot-rolled medium plate produced using Example 3 is shown. Furthermore, tensile tests verified that the elongation after fracture of the ultra-pure ferritic stainless steel hot-rolled medium plates produced using Examples 1-5 of this invention were as follows: Example 1: 40%, Example 2: 37%, Example 3: 38%, Example 4: 36%, and Example 5: 35%, respectively. Clearly, the elongation after fracture of the ultra-pure ferritic stainless steel hot-rolled medium plates produced using this invention is all above 35%, exhibiting good toughness and mechanical properties, fully meeting the requirements for manufacturing thick-gauge flanges and other components.

[0051] In summary, compared with the prior art, the manufacturing method of the ultrapure ferritic stainless steel hot-rolled medium plate of the present invention has the following advantages and beneficial effects:

[0052] This invention produces ultra-pure ferritic stainless steel hot-rolled medium plates using continuously cast billets as raw material. The billets are then hot-rolled to the target thickness to obtain hot-rolled coils. These coils are then straightened, leveled, and cut into individual medium plates. Finally, each medium plate undergoes heat treatment and pickling to obtain the ultra-pure ferritic stainless steel hot-rolled medium plate. Compared to existing technologies that involve rolling short billets into single sheets, this invention significantly improves the production efficiency and yield of ultra-pure ferritic stainless steel hot-rolled medium plates, achieving a yield of over 90%.

[0053] In the implementation of this invention, the performance of hot-rolled medium plates of ultra-pure ferritic stainless steel is improved by controlling the rolling reduction rate, hot continuous rolling finishing temperature, and optimizing the heat treatment and pickling processes of the slab. This is achieved through control of the continuous casting billet thickness, hot continuous rolling intermediate billet thickness, finishing exit temperature, coiling temperature, heat treatment, and pickling process parameters. The ultra-pure ferritic stainless steel hot-rolled medium plates produced using this invention exhibit good toughness and mechanical properties, with an average grain size of 100–250 μm and an elongation after fracture exceeding 35% in tensile tests. All performance indicators of the product meet the requirements for manufacturing thick-gauge flanges and other components.

[0054] It should be noted that, in this document, the term "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device.

[0055] It should also be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A method for manufacturing a hot-rolled medium plate of ultra-pure ferritic stainless steel, characterized in that, Includes the following steps: (1) Material preparation: The billet is a continuous casting billet, and the thickness of the continuous casting billet is controlled at 200-230mm. (2) Hot continuous rolling roughing, the roughing is carried out in multiple passes to roll the continuous casting billet into an intermediate billet, and the thickness of the intermediate billet is controlled at 35-45mm. (3) Hot continuous rolling finishing: a seven-stand hot continuous rolling mill is used for finishing rolling to roll the intermediate billet into a hot-rolled coil of ultra-pure ferritic stainless steel with a thickness of 6.0 to 13.0 mm. The exit temperature of the hot continuous rolling finishing is controlled at 750 to 850°C, and the coiling temperature is controlled at 500 to 600°C. (4) Optimization of the opening, heat treatment and pickling process: First, the ultra-pure ferritic stainless steel hot-rolled coil is straightened and leveled, and cut into multiple single medium plates. Then, the single medium plates are heat treated and pickled in sequence. In the heat treatment process, the heat treatment temperature is controlled at 850-1000℃ and the heat treatment time is controlled at 1.0-2.5min / mm. After the medium plates are taken out of the furnace, they are water-cooled to room temperature. In the pickling process, shot blasting is performed first, and then pickling is performed using mixed acid. The mixed acid is nitric acid and hydrofluoric acid. The concentration of nitric acid is controlled at 90-180g / L, the concentration of hydrofluoric acid is controlled at 3-10g / L, and the pickling time is controlled at 10-25min.

2. The method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate as described in claim 1, characterized in that, The continuously cast billet is heated to 1150-1250℃ before the roughing step of hot continuous rolling.

3. The method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate as described in claim 1, characterized in that, In the hot continuous rolling roughing step, the continuously cast billet is rolled into an intermediate billet through three passes.

4. The method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate as described in claim 1, characterized in that, In the hot continuous rolling finishing process, the width of the ultra-pure ferritic stainless steel hot-rolled coil is 1200-2000 mm.

5. The method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate as described in claim 1, characterized in that, In the heat treatment process, a continuous solution furnace is used for heat treatment; in the pickling process, shot blasting is carried out using a vertical shot blasting method, and pickling is carried out using a vertical spray pickling method.

6. The method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate as described in claim 1, characterized in that, In the plate-making process, the length of a single plate is 3000-9000 mm.

7. The method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate according to any one of claims 1 to 6, characterized in that, The ultra-pure ferritic stainless steel hot-rolled medium plate is a 409L series stainless steel medium plate, a 436L series stainless steel medium plate, a 439 stainless steel medium plate, a 441 series stainless steel medium plate, or a 444 series stainless steel medium plate.

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