Manufacturing method of ultra-pure ferritic stainless steel hot-rolled medium plate

Abstract

The invention discloses a manufacturing method of an ultra-pure ferritic stainless steel hot-rolled medium plate, which comprises the following steps: a continuous casting long blank is adopted as a blank, and the thickness of the continuous casting long blank is controlled to be 200-230 mm; the continuous casting long blank is roughly rolled into an intermediate blank, and the thickness of the intermediate blank is controlled to be 35-45 mm; the intermediate billet is subjected to finish rolling to form an ultra-pure ferrite stainless steel hot-rolled coiled plate with the thickness of 6.0-13.0 mm, the finish rolling outlet temperature is controlled to be 750-850 DEG C, and the coiling temperature is controlled to be 500-600 DEG C; a hot-rolled coil plate is straightened, flattened and cut into a plurality of single medium plates, then the single medium plates are sequentially subjected to heat treatment and acid pickling, the heat treatment temperature is controlled to be 850-1000 DEG C, the heat treatment time is controlled to be 1.0-2.5 min / mm, mixed acid of nitric acid and hydrofluoric acid is adopted for acid pickling, and the acid pickling time is controlled to be 10-25 min. The production efficiency and the product yield of the ultra-pure ferritic stainless steel hot-rolled medium plate are greatly improved, the yield reaches 90% or above, the average grain size of the ultra-pure ferritic stainless steel hot-rolled medium plate product is 100-250 microns, the tensile test percentage elongation after fracture reaches 35% or above, and the use requirements for manufacturing thick flanges and other parts are met.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel product production, and more specifically relates to a method for manufacturing an ultra-pure ferrite stainless steel hot-rolled middle plate. Background Art

[0002] Ferritic stainless steel has the advantages of low cost and stable price due to its low content of alloy elements and the fact that it does not require the addition of expensive alloy elements such as Ni. Ultrapure ferritic stainless steel, in particular, has extremely low C and N content, generally [C+N]≤250ppm, and by adding a certain amount of stabilizing elements such as Ti and Nb, it can obtain excellent corrosion resistance, especially resistance to intergranular corrosion. At the same time, ultrapure ferritic stainless steel has the characteristics of relatively low thermal expansion coefficient and high thermal conductivity, and is widely used in automotive exhaust systems, construction, home appliances, heat exchangers and other fields.

[0003] Thick flange parts are often used in the fields of automobile exhaust systems, heat exchangers, etc., and their thickness can reach more than 6.0mm, even more than 10mm, but generally not more than 13mm. Ultra-pure ferritic stainless steel middle plates are generally used as materials for making thick flange parts. At present, the commonly used manufacturing method for ultra-pure ferritic stainless steel middle plates with a thickness of 6.0 to 13.0mm is to use short billet single sheet rolling, and the stainless steel liquid that meets the composition requirements is continuously cast to obtain a short billet, and the short billet is rolled by a reversible rolling mill to obtain a black skin single sheet middle plate of the target thickness, and then the black skin single sheet middle plate is heat treated and pickled to obtain a single sheet middle plate with a clean surface that meets the processing performance requirements. The above-mentioned short billet single sheet rolling method in the prior art has the disadvantages of low production efficiency, low yield rate, and high cost. The yield rate is only about 80%, and the toughness and mechanical properties of the ultra-pure ferritic stainless steel middle plate produced by this method are difficult to meet the requirements for making flanges and other parts.

[0004] Therefore, developing a method for manufacturing ultra-pure ferritic stainless steel middle plates with higher production efficiency and yield rate, better cost and excellent performance has become a technical problem that needs to be urgently solved by technical personnel in this field. Summary of the invention

[0005] In order to solve the above technical problems existing in the prior art, the present invention provides a method for manufacturing an ultra-pure ferritic stainless steel hot-rolled medium plate, comprising the following steps:

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

[0007] (2) Hot rolling rough rolling, which is carried out in multiple passes to roll the continuous casting long billet into an intermediate billet with a thickness of 35 to 45 mm;

[0008] (3) hot rolling finishing rolling, using a seven-stand hot rolling unit to carry out finishing rolling, rolling the intermediate billet into an ultra-pure ferritic stainless steel hot-rolled coil with a thickness of 6.0 to 13.0 mm, wherein the hot rolling finishing outlet temperature is controlled at 750 to 850°C, and the coiling temperature is controlled at 500 to 600°C;

[0009] (4) Optimization of flattening, heat treatment and pickling processes: first, the ultra-pure ferritic stainless steel hot-rolled coil is straightened and flattened, cut into multiple single medium plates, and 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°C, the heat treatment time is controlled at 1.0-2.5 min / mm, and the medium plate is water-cooled to room temperature after being taken out of the furnace; in the pickling process, shot blasting is first performed, and then mixed acid is used for pickling after shot blasting. The mixed acid uses nitric acid and hydrofluoric acid, the nitric acid concentration is controlled at 90-180 g / L, the hydrofluoric acid concentration is controlled at 3-10 g / L, and the pickling time is controlled at 10-25 min.

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

[0011] Furthermore, in the method for manufacturing the ultrapure ferritic stainless steel hot-rolled medium plate, in the hot continuous rolling rough rolling step, the continuously cast long slab is rolled into an intermediate slab in three passes.

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

[0013] Furthermore, in the above-mentioned method for manufacturing 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 method for manufacturing the ultrapure ferritic stainless steel hot-rolled middle plate, in the flat plate slab process, the length of a single middle plate is 3000 to 9000 mm.

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

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

[0017] (1) The present invention uses a continuous casting long billet as the billet for producing ultra-pure ferritic stainless steel hot-rolled medium plate, obtains a hot-rolled coil of target thickness through hot continuous rolling, and then straightens and flattens the hot-rolled coil, cuts it into a single medium plate, and finally sequentially heat treats and pickles the single medium plate to prepare an ultra-pure ferritic stainless steel hot-rolled medium plate. Compared with the single-sheet rolling of short billets in the prior art, the implementation of the present invention can greatly improve the production efficiency and product yield of ultra-pure ferritic stainless steel hot-rolled medium plate, and the yield rate reaches more than 90%;

[0018] (2) In the implementation process of the present invention, the thickness of the continuous casting slab, the thickness of the hot rolling intermediate slab, the finishing rolling outlet temperature, the coiling temperature, the heat treatment and pickling process parameters are controlled by rolling reduction rate control, hot rolling finishing rolling temperature control, plate opening heat treatment and pickling process optimization, thereby improving the performance of the ultra-pure ferritic stainless steel hot-rolled middle plate. The ultra-pure ferritic stainless steel hot-rolled middle plate product produced by the present invention has good toughness and mechanical properties, an average grain size of 100 to 250 μm, and an elongation of more than 35% after tensile test. The various performance indicators of the product meet the use requirements for manufacturing thick flanges and other parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 The present invention discloses a metallographic structure diagram of a 439 stainless steel hot-rolled middle plate produced by the method for producing the ultra-pure ferrite stainless steel hot-rolled middle plate of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0022] The purpose of the present invention is to seek a method for manufacturing ultra-pure ferritic stainless steel hot-rolled medium plates with higher production efficiency and yield rate, better cost, and good product toughness and mechanical properties on the basis of not significantly changing the existing tooling equipment. The basic technical concept is: the billet adopts a continuous casting long billet, and obtains a hot-rolled coil of target thickness through hot continuous rolling including rough rolling and finish rolling. The coil is then flattened and sheared to obtain a black medium plate, and finally heat treated and surface pickled to obtain an ultra-pure ferritic stainless steel hot-rolled medium plate that meets the requirements.

[0023] Since the finished product of the medium plate is thicker, the hot rolling deformation reduction rate is relatively low, which is not conducive to grain refinement. At the same time, the size effect of ferritic stainless steel is obvious, the grain size of the medium plate is large, and the toughness is worse than that of thin-gauge plates. With the increase of thickness, the toughness-brittle transition temperature will also increase. For ultra-pure ferritic stainless steel medium plates with a thickness greater than 6.0mm, the toughness-brittle transition temperature will exceed 20°C. Below the toughness-brittle transition temperature, the toughness of the material will decrease significantly. To ensure that the hot-rolled coil can be flattened without breaking, a certain toughness must be guaranteed. To improve the toughness of the hot-rolled medium plate, the hot rolling process needs to be improved accordingly.

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

[0025] (1) Preparation

[0026] Since the thickness of the middle plate is relatively thick, only a thicker billet can achieve a larger hot rolling reduction rate, which is conducive to the refinement of the microstructure grains; however, when the billet thickness is too large, shrinkage holes and other defects are very likely to appear inside the billet. Therefore, the billet thickness should be as thick as possible while ensuring that there are no shrinkage holes and other defects.

[0027] Therefore, in the method for manufacturing the ultrapure ferritic stainless steel hot-rolled medium plate of the present invention, the billet is a continuous casting long billet, and the thickness of the continuous casting long billet is controlled to be 200-230 mm.

[0028] (2) Hot rolling and rough rolling

[0029] Before rough rolling, the continuous casting long billet is heated to 1150-1250℃, and sufficient holding time is ensured to make the billet burn through. The rough rolling adopts multiple passes to roll the continuous casting long billet into an intermediate billet. In order to ensure the rolling reduction rate of the subsequent hot rolling and finishing, the thickness of the intermediate billet is controlled at 35-45mm.

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

[0031] (3) Hot rolling and finishing

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

[0033] The control of hot rolling temperature has a significant effect on the toughness of ultrapure ferritic stainless steel, especially the lower finishing rolling temperature, which can reduce the toughness-brittle transition temperature and improve the toughness of ultrapure ferritic stainless steel hot rolled coil; at the same time, the rolling temperature should not be too low, otherwise it will increase the strength of the rolled material and increase the rolling load of the rolling mill. Therefore, in the method for manufacturing ultrapure ferritic stainless steel hot rolled medium plate of the present invention, the hot rolling finishing outlet temperature is controlled at 750-850°C, and the coiling temperature is controlled at 500-600°C.

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

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

[0036] First, the ultra-pure ferritic stainless steel hot-rolled coil is straightened and flattened, cut into multiple single middle plates, and then the single middle plates are heat treated and pickled in sequence to obtain single middle plate products with good processing performance and surface cleanliness. Among them, in the heat treatment process, the heat treatment temperature is controlled at 850-1000℃, the heat treatment time is controlled at 1.0-2.5min / mm, and the middle plate is water-cooled to room temperature after it is taken out of the furnace to reduce the influence of precipitation on the material properties; in the pickling process, shot blasting is first performed, which can not only remove the oxide scale on the surface of the steel plate, but also break the denser oxide layer in the inner layer to improve the efficiency of subsequent pickling. After shot blasting, mixed acid is used for pickling. The mixed acid uses nitric acid and hydrofluoric acid, the nitric acid concentration is controlled at 90-180g / L, the hydrofluoric acid concentration is controlled at 3-10g / L, and the pickling time is controlled at 10-25min.

[0037] Preferably, in the flat plate forming process, the length of a single middle plate is 3000 to 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 performed by vertical shot blasting, and pickling is performed by vertical spray pickling.

[0039] The following is a method for manufacturing ultrapure ferritic stainless steel hot-rolled medium plate of the present invention in conjunction with Examples 1 to 5. The billets of Examples 1 to 5 are all continuous casting long billets, which are used to produce ultrapure ferritic stainless steel medium plates of 409L series, 436L series, 439 series, 441 series, 444 series, etc. The finished product thickness ranges from 6.0 to 13.0 mm. The specific steel grades and continuous casting long billet thicknesses of Examples 1 to 5 are:

[0040] Example Steel Type Thickness of continuous casting long billet (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) preparing a continuous casting long billet of a specified thickness by continuous casting of molten steel having a chemical composition that meets the requirements;

[0043] (2) heating the continuous casting long billet and performing hot continuous rough rolling, and rolling the continuous casting long billet into an intermediate billet through three rough rolling passes;

[0044] (3) hot rolling and finishing rolling are performed on the intermediate billet by a seven-stand hot rolling mill, and the finishing rolling outlet temperature and the coiling temperature are controlled to produce an ultra-pure ferritic stainless steel hot-rolled coil, wherein the width of the ultra-pure ferritic stainless steel hot-rolled coil is 1200 to 2000 mm, and the maximum coil weight can reach 28 tons;

[0045] (4) The hot-rolled coil is straightened and flattened, and cut into a plurality of single medium plates with a length of 3000 to 9000 mm. The single medium plates are then heat treated and pickled in sequence. The heat treatment is carried out in a continuous solution furnace, and the plates are water-cooled to room temperature after being taken out of the furnace. After the heat treatment, the plates are first shot blasted by a vertical shot blasting method, and then pickled by a mixed acid of nitric acid and hydrofluoric acid in a vertical spray pickling method.

[0046] Among them, during the implementation of Examples 1 to 5, the rolling reduction rate control and the hot rolling finishing temperature control are performed. Specifically, the intermediate billet thickness, finishing rolling outlet temperature, coiling temperature, and hot rolled coil thickness of Examples 1 to 5 are respectively controlled as follows:

[0047] Example Intermediate billet thickness (mm) Finishing rolling outlet temperature (℃) Coiling temperature(℃) Hot rolled coil thickness (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] In addition, the heat treatment and pickling process optimization were performed during the implementation of Examples 1 to 5. Specifically, the main process parameters of the heat treatment and pickling of Examples 1 to 5 were controlled as follows:

[0049]

[0050] According to actual production measurement, the yield rate of the 409L series, 436L series, 439 series, 441 series, and 444 series ultra-pure ferritic stainless steel hot-rolled plates produced by Examples 1 to 5 of the present invention is more than 90%. In addition, it is found through testing that the ultra-pure ferritic stainless steel hot-rolled plates produced by Examples 1 to 5 of the present invention have an average grain size of 100 to 250 μm, for example, see Figure 1 , which shows the metallographic structure of the 439 ultra-pure ferritic stainless steel hot-rolled middle plate produced by Example 3. In addition, through tensile tests, the ultra-pure ferritic stainless steel hot-rolled middle plates produced by Examples 1 to 5 of the present invention have tensile test elongations after fracture, respectively: Example 1: 40%, Example 2: 37%, Example 3: 38%, Example 4: 36%, Example 5: 35%. Obviously, the ultra-pure ferritic stainless steel hot-rolled middle plates produced by the present invention have tensile test elongations after fracture of more than 35%, have good toughness and mechanical properties, and fully meet the requirements for manufacturing thick flanges and other parts.

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

[0052] The present invention uses a continuous casting long billet as the billet for producing ultra-pure ferritic stainless steel hot-rolled medium plate, obtains a hot-rolled coil of target thickness through hot continuous rolling, then straightens and flattens the hot-rolled coil, cuts it into a single medium plate, and finally sequentially heat treats and pickles the single medium plate to prepare the ultra-pure ferritic stainless steel hot-rolled medium plate. Compared with the single sheet rolling of short billets in the prior art, the implementation of the present invention can greatly improve the production efficiency and product yield of ultra-pure ferritic stainless steel hot-rolled medium plate, and the yield rate reaches more than 90%.

[0053] In the implementation process of the present invention, the thickness of the continuous casting billet, the thickness of the hot rolling intermediate billet, the finishing rolling outlet temperature, the coiling temperature, the heat treatment and pickling process parameters are controlled by rolling reduction rate control, hot rolling finishing rolling temperature control, plate opening heat treatment and pickling process optimization, thereby improving the performance of the ultra-pure ferritic stainless steel hot-rolled middle plate. The ultra-pure ferritic stainless steel hot-rolled middle plate product produced by the present invention has good toughness and mechanical properties, an average grain size of 100 to 250 μm, and an elongation after fracture of a tensile test of more than 35%. The various performance indicators of the product meet the use requirements for manufacturing thick-specification flanges and other parts.

[0054] It should be noted that, in this article, the term "comprise" or any other variation thereof is intended to cover non-exclusive inclusion, so that an article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such 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, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements 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 ultrapure ferritic stainless steel hot-rolled medium plate, characterized in that: The steps include: (1) Material preparation: the billet is a continuous casting long billet, and the thickness of the continuous casting long billet is controlled at 200-230 mm; (2) Hot rolling rough rolling, which is carried out in multiple passes to roll the continuous casting long billet into an intermediate billet with a thickness of 35 to 45 mm; (3) hot rolling finishing rolling, using a seven-stand hot rolling unit to carry out finishing rolling, rolling the intermediate billet into an ultra-pure ferritic stainless steel hot-rolled coil with a thickness of 6.0 to 13.0 mm, wherein the hot rolling finishing outlet temperature is controlled at 750 to 850°C, and the coiling temperature is controlled at 500 to 600°C; (4) Optimization of flattening, heat treatment and pickling processes: first, the ultra-pure ferritic stainless steel hot-rolled coil is straightened and flattened, cut into multiple single medium plates, and 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°C, the heat treatment time is controlled at 1.0-2.5 min / mm, and the medium plate is water-cooled to room temperature after being taken out of the furnace; in the pickling process, shot blasting is first performed, and then mixed acid is used for pickling after shot blasting. The mixed acid uses nitric acid and hydrofluoric acid, the nitric acid concentration is controlled at 90-180 g / L, the hydrofluoric acid concentration is controlled at 3-10 g / L, and the pickling time is controlled at 10-25 min.

2. The method for manufacturing ultrapure ferrite stainless steel hot-rolled middle plate according to claim 1, characterized in that: The continuous casting long billet is heated to 1150-1250°C before the hot rolling rough rolling step.

3. The method for manufacturing the ultrapure ferrite stainless steel hot-rolled medium plate according to claim 1, characterized in that: In the hot rolling rough rolling step, the continuously cast long billet is rolled into an intermediate billet in three passes.

4. The method for manufacturing the ultrapure ferrite stainless steel hot-rolled middle plate according to claim 1, characterized in that: In the hot rolling finishing step, the width of the ultra-pure ferritic stainless steel hot rolled coil is 1200 to 2000 mm.

5. The method for manufacturing ultrapure ferrite stainless steel hot-rolled medium plate according to 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 by vertical shot blasting, and pickling is carried out by vertical spray pickling.

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

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

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