Method for improving the strength of 316 series 2000 mm wide hot-rolled stainless steel coils

Abstract

The application discloses a method for improving the strength of 316 system 2000mm wide hot-rolled stainless steel coil, which comprises four steps of annealing, stretching, straightening and plastic extension control: the 316 system wide hot-rolled black skin coil is subjected to precise solid solution annealing with different thicknesses, and the grain size is controlled to be 5.5-6.5 levels; the annealed steel strip is subjected to large tension stretching with a yield strength of 60%-80%; a multi-roller straightening machine is used to precisely control the roll pressure depth of the stretching and straightening; and the total plastic extension rate of the steel strip is stably controlled to be 0.5%-7% through stretching and straightening linkage control. The application does not need to change the stainless steel alloy composition, can be realized by using the existing wide hot coil annealing and pickling line, and finally obtains the product with a yield strength Rp0.2 of greater than or equal to 300 MPa (more than 20% higher than that of a traditional process), and the product has flat shape and excellent corrosion resistance, is suitable for 316 system (especially 316L) stainless steel coil with a thickness of 2-14 mm and a width of 1800-2100 mm, can provide high cost-effective materials for customers, reduces the production material cost, and has outstanding market competitiveness.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and in particular relates to a method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series. Background Technology

[0002] 316 series wide hot-rolled stainless steel coils (width 1800-2100mm, typical width 2000mm) are a distinctive high-end product in the metallurgical industry. Due to their excellent surface quality, good weldability, and large width, they can be flexibly cut to meet customer needs and are widely used in large storage tanks and pipelines in the chemical and petroleum industries, food processing equipment containers, ship bulkheads and decks, and large building curtain walls. The core advantage of wide-width products lies in reducing weld seams and improving the overall integrity and sealing of equipment, thus leading to continuously growing demand in industrial applications.

[0003] As competition in the stainless steel market intensifies, downstream customers are demanding higher cost-effectiveness from their products. While ensuring product safety, they seek materials with higher strength for the same thickness or thinner thickness for the same strength, aiming to reduce material consumption and production costs per unit. In engineering design, allowable stress is a key design parameter, calculated as "Allowable Stress = Ultimate Strength / Safety Factor." For ductile materials like 316 series stainless steel, the ultimate strength is typically chosen as the yield strength. Therefore, increasing the yield strength is the core approach to enhancing allowable stress and meeting customer cost-effectiveness requirements.

[0004] The existing standard for yield strength of 316 series wide hot-rolled stainless steel coils is Rp0.2≥230MPa, which cannot meet the high-strength requirements of some high-end applications. Traditional methods to improve the strength of stainless steel mainly include adjusting the alloy composition (such as increasing the content of Cr, Ni, and Mo) or using offline strengthening processes. However, adjusting the alloy composition will significantly increase production costs, while offline strengthening processes will reduce production efficiency and increase process complexity, neither of which meets the economic requirements of industrial production.

[0005] The main equipment of existing wide-width hot-rolled coil annealing and pickling lines includes uncoilers, welding machines, annealing furnaces, high-strength tension levelers (phosphorus breaking machines), shot blasting machines, pickling equipment, and coilers. Their core function is to eliminate work hardening during hot rolling, improve surface quality, and enhance corrosion resistance. However, because 316 series stainless steel is an austenitic material with a significant tendency for work hardening, and wide-width products (especially those over 2000mm in width) are prone to uneven deformation and shape defects (such as wavy edges and warping) during stretching and straightening, existing processes cannot achieve precise improvement in yield strength, let alone consistently reach the target of Rp0.2≥300MPa.

[0006] Therefore, how to achieve efficient and stable improvement of yield strength while maintaining the original alloy composition of 316 series stainless steel and utilizing the equipment of existing wide-width hot-rolled annealing and pickling lines through process innovation, while ensuring the quality of product shape, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To at least partially solve the technical problems existing in the prior art, the present invention provides a method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series.

[0008] The method of the present invention for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series includes: Annealing: Solution annealing is performed on 316 series wide hot-rolled black strip raw material. The furnace temperature is controlled at 1130±20℃, the plate temperature is controlled at 1130℃, and the annealing speed is kept uniform according to the thickness of the steel strip to control the grain size of the steel strip at 5.5~6.5 after annealing, and then cooling is performed. Tensioning: The annealed steel strip is subjected to high-tension mechanical tension, with the tension value being 60%~80% of the yield strength of 316 series stainless steel, so that the steel strip initially produces plastic elongation. Straightening: A multi-roll straightener is used to bend and straighten the steel strip. The pressing depth of the straightening rolls is controlled. The initial roll gap is set to 90%~95% of the steel strip thickness, the difference between adjacent roll gaps is controlled to 3%~8% of the steel strip thickness, and the single-pass pressing amount is 0.3~0.5mm. Plastic elongation control: Through the synergistic effect of stretching and straightening, the total plastic elongation of the steel strip is controlled at 0.5%~7%, and finally a 316 series wide hot-rolled stainless steel coil with a yield strength Rp0.2≥300MPa is obtained. The width range of the 316 series wide hot-rolled stainless steel coil is 1800~2100mm.

[0009] Furthermore, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series, the 316 series stainless steel is 316L stainless steel, whose microstructure is a typical austenitic γ phase structure, containing a small amount of δ ferrite, with a mass fraction of <5%.

[0010] Furthermore, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series, in the annealing step, when the nominal thickness of the 316 series wide hot-rolled stainless steel coil is ≥10mm, the annealing furnace temperature is 1110±20℃ and the plate temperature is 1110℃.

[0011] Furthermore, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of the 316 series, in the annealing step, the cooling is carried out using a water cooling + air cooling composite cooling method to cool to below 80°C.

[0012] Furthermore, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series, in the straightening step, the multi-roll straightener used includes at least two sets of straightening rolls, and the reduction amount of the last set of straightening rolls is 50% ± 10% of the reduction amount of the preceding straightening rolls.

[0013] Preferably, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series, the straightening rolls are made of a high-hardness, wear-resistant material with a Rockwell hardness of HRC≥60.

[0014] Furthermore, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series, in the plastic elongation control step, the high-tension mechanical stretching and tension straightening bending are linked and controlled simultaneously, adjusting the tension, roll gap and steel strip running speed to make the steel strip deform uniformly.

[0015] Furthermore, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series, in the plastic elongation control step, when the total plastic elongation of the steel strip exceeds 5%, the excessive transformation of martensite is suppressed by adjusting the water cooling rate, and the volume fraction of martensite transformation is controlled at 5%~10%.

[0016] Furthermore, in the above method for improving the strength of 2000mm wide hot-rolled stainless steel coils of the 316 series, in the annealing step, the thickness range of the 316 series wide hot-rolled stainless steel coil is 2-14mm, and the corresponding annealing speed is adjusted according to the thickness as follows: When the thickness 2≤h<6mm, the speed is 30±10m / min; When the thickness is 6≤h≤8mm, the speed is 190±20m / min; When the thickness is 8 < h ≤ 10.5 mm, the speed is 20 ± 6 m / min; When the thickness is 10.5 < h ≤ 12 mm, the speed is 14 ± 6 m / min; When the thickness is 12 < h ≤ 14 mm, the speed is 12 ± 5 m / min; In case of unit malfunction, in order to ensure plate temperature, the process speed of steel strip of all thicknesses can be reduced to a minimum of 5m / min.

[0017] The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of the 316 series of the present invention has the following advantages and beneficial effects: This invention effectively increases the yield strength of 316 series wide-width hot-rolled stainless steel coils to ≥300MPa, which is more than 20% higher than that of traditional processes. The allowable stress is also increased simultaneously, meeting the demand for high-strength materials in high-end industrial scenarios. The original alloy composition of stainless steel is maintained, without the need to add additional alloying elements or offline strengthening processes. It can be achieved using existing wide-width hot-rolled coil annealing and pickling lines, resulting in high production efficiency and controllable costs. At the same time, it ensures that the corrosion resistance of the product is no less than that of traditional processes. Detailed Implementation

[0018] 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.

[0019] The method of the present invention for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series includes: Annealing: The 316 series stainless steel is 316L stainless steel, whose microstructure is a typical austenitic γ phase structure, containing a small amount of δ ferrite, with a mass fraction of <5%. M23C6 type carbides are easily precipitated at the grain boundaries. The austenitic grains are elongated along the rolling direction and there are deformed twins. It is necessary to restore the microstructure through solution annealing. This invention precisely designs the annealing parameters. Solution annealing is performed on 316 series wide hot-rolled black stainless steel coil raw materials. When the nominal thickness of 316 series wide hot-rolled stainless steel coil is <10mm, the furnace temperature is controlled at 1130±20℃ and the plate temperature is controlled at 1130℃. When the nominal thickness of 316 series wide hot-rolled stainless steel coil is ≥10mm, the annealing furnace temperature is 1110±20℃ and the plate temperature is 1110℃. The annealing rate is kept constant during annealing, specifically: When the thickness 2≤h<6mm, the speed is 30±10m / min; When the thickness is 6≤h≤8mm, the speed is 190±20m / min; When the thickness is 8 < h ≤ 10.5 mm, the speed is 20 ± 6 m / min; When the thickness is 10.5 < h ≤ 12 mm, the speed is 14 ± 6 m / min; When the thickness is 12 < h ≤ 14 mm, the speed is 12 ± 5 m / min; In case of unit malfunction, in order to ensure plate temperature, the process speed of steel strip of all thicknesses can be reduced to a minimum of 5m / min; After annealing, the temperature is cooled to below 80°C using a combination of water cooling and air cooling to inhibit the re-precipitation of carbides. This ensures that the grain size of the annealed steel strip is controlled at 5.5~6.5, providing a uniform and stable base material structure for subsequent tensile and straightening processes. Tensioning: Based on the work hardening characteristics of 316 series stainless steel, precise tension is applied through a tensioning device to induce initial plastic elongation of the steel strip, eliminate internal residual stress, and thus correct plate defects such as wavy edges and warping. The tension value is 60% to 80% of the yield strength of 316 series stainless steel, which ensures effective plastic deformation while avoiding excessive tension that could lead to steel strip breakage or plate shape deterioration. The control method is to link with the subsequent tensioning and straightening process, and dynamically adjust the tension based on the real-time plate shape and elongation feedback of the steel strip. Straightening: A multi-roll straightener is used. The straightening rolls are made of high-hardness, wear-resistant material with a Rockwell hardness of HRC≥60. The steel strip is repeatedly bent and straightened. Uniform deformation is achieved by precisely adjusting the roll gap by controlling the pressing depth of the straightening rolls. The initial roll gap is set to 90%~95% of the steel strip thickness to ensure full contact between the steel strip and the straightening rolls. The difference between adjacent roll gaps is controlled at 3%~8% of the steel strip thickness to avoid abrupt deformation. This is designed for 316 series stainless steel with low stacking fault energy (approximately 18mJ / m). 2 The characteristics of the multi-roller straightener are as follows: the single-pass reduction is 0.3~0.5mm to prevent excessive deformation twinning; the multi-roller straightener includes at least two sets of straightening rolls, and the reduction of the last set of straightening rolls is 50%±10% of the reduction of the previous straightening rolls, further refining the deformation uniformity. Plastic elongation control: With total plastic elongation as the core control target, through the synergistic effect of stretching and straightening, the total plastic elongation of the steel strip is controlled at 0.5%~7%, ultimately obtaining 316 series wide hot-rolled stainless steel coils with a yield strength Rp0.2≥300MPa. The width range of the 316 series wide hot-rolled stainless steel coils is 1800~2100mm. Specifically, the high-tension mechanical stretching and tension bending straightening adopts linkage control, synchronously adjusting the tension, roll gap and steel strip running speed to make the steel strip deform uniformly. The strength is improved by utilizing work hardening (increased dislocation density) and moderate martensitic transformation (when elongation is >5%). Each 1% cold deformation can increase the yield strength by 60-90MPa, and the martensitic transformation can contribute an additional 10-20MPa / 1% strength improvement. When the elongation exceeds 5%, the martensite transformation amount is controlled at 5%-10% (volume fraction) by adjusting the cooling rate to avoid excessive martensite leading to a decrease in plasticity.

[0020] Example 1 The material used is 316L wide hot-rolled black steel coil, with a specification of 7.5mm×1950mm, an original yield strength of 230MPa, and an original grain size of 4.0. Annealing: Solution annealing was performed on 316L wide hot-rolled black strip at a furnace temperature of 1130±15℃ and a plate temperature of 1130℃. The annealing speed was maintained at a uniform annealing rate of 180m / min, so that the steel strip was cooled to 70℃ after annealing and the grain size was 5.8. Tensile: The annealed steel strip is subjected to high-tension mechanical tension, with the tension set at 138 MPa, which is 60% of the current yield strength of 230 MPa; Straightening: A multi-roll straightener is used to bend and straighten the steel strip. The pressing depth of the straightening rolls is controlled. The initial roll gap is set to 7.1mm, which is 94.7% of the steel strip thickness of 7.5mm. The difference between adjacent roll gaps is 0.45mm, which is 6% of the steel strip thickness of 7.5mm. The single-pass pressing amount is 0.35mm, and the pressing amount of the last set of rolls is 0.18mm. Plastic elongation control: Through the synergistic effect of stretching and straightening, the total plastic elongation of the steel strip is controlled at 3.8%; Tests showed that the yield strength Rp0.2 = 305 MPa, tensile strength = 638 MPa, elongation = 38%, the plate was flat, and the corrosion resistance met the standards.

[0021] Comparative Example 1 The 316L wide coil with a diameter of 13.8mm × 2080mm was produced using a traditional annealing and pickling process. The annealing temperature was 1100℃ and the speed was 20m / min. There was no precise tension control. The initial roll gap of the straightening roll was 85% of the steel strip thickness. The total elongation was 1.2%. The yield strength was tested to be 228MPa, which did not reach the target of 300MPa. The strip shape had slight wavy edges.

[0022] By comparing Example 1 and Comparative Example 1, it can be seen that the present invention, through a combination of processes including precise control of grain size during annealing, high-tension stretching, coordinated pressing depth of straightening rolls, and regulation of plastic elongation, can stably increase the yield strength of 316 series wide-width hot-rolled stainless steel coils to over 300 MPa without changing the alloy composition. Moreover, the product has excellent plate shape and corrosion resistance, fully meeting the needs of industrial applications.

[0023] It should be noted that, unless otherwise expressly specified and limited, the term "connection" or its synonyms should be interpreted broadly in this document. For example, "connection" can be a fixed connection or a detachable connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be the internal communication of two elements or the interaction between two elements. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances. Furthermore, expressions such as "first" and "second" are merely used to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. At the same time, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0024] Finally, it should 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 spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series, characterized in that, The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of the 316 series includes: Annealing: Solution annealing is performed on 316 series wide hot-rolled black strip raw material. The furnace temperature is controlled at 1130±20℃, the plate temperature is controlled at 1130℃, and the annealing speed is kept uniform according to the thickness of the steel strip to control the grain size of the steel strip at 5.5~6.5 after annealing, and then cooling is performed. Tensioning: The annealed steel strip is subjected to high-tension mechanical tension, with the tension value being 60%~80% of the yield strength of 316 series stainless steel, so that the steel strip initially produces plastic elongation. Straightening: A multi-roll straightener is used to bend and straighten the steel strip. The pressing depth of the straightening rolls is controlled. The initial roll gap is set to 90%~95% of the steel strip thickness, the difference between adjacent roll gaps is controlled to 3%~8% of the steel strip thickness, and the single-pass pressing amount is 0.3~0.5mm. Plastic elongation control: Through the synergistic effect of stretching and straightening, the total plastic elongation of the steel strip is controlled at 0.5%~7%, and finally a 316 series wide hot-rolled stainless steel coil with a yield strength Rp0.2≥300MPa is obtained. The width range of the 316 series wide hot-rolled stainless steel coil is 1800~2100mm.

2. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 1, characterized in that, The 316 series stainless steel is 316L stainless steel, whose microstructure is a typical austenitic γ phase structure, containing a small amount of δ ferrite, with a mass fraction of <5%.

3. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 1, characterized in that, In the annealing process, when the nominal thickness of the 316 series wide hot-rolled stainless steel coil is ≥10mm, the annealing furnace temperature is 1110±20℃ and the plate temperature is 1110℃.

4. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 1, characterized in that, During the annealing process, cooling is achieved using a combination of water cooling and air cooling to bring the temperature down to below 80°C.

5. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 1, characterized in that, In the straightening step, the multi-roll straightener used includes at least two sets of straightening rolls, and the pressing amount of the last set of straightening rolls is 50% ± 10% of the pressing amount of the preceding straightening rolls.

6. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 5, characterized in that, The straightening rollers are made of high-hardness, wear-resistant material with a Rockwell hardness of HRC≥60.

7. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 1, characterized in that, In the plastic elongation control step, high-tension mechanical stretching and tension bending straightening are coordinated and linked, and the tension, roll gap and steel strip running speed are adjusted synchronously to make the steel strip deform uniformly.

8. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 1, characterized in that, In the plastic elongation control step, when the total plastic elongation of the steel strip exceeds 5%, the excessive transformation of martensite is suppressed by adjusting the water cooling rate, and the volume fraction of martensite transformation is controlled at 5%~10%.

9. The method for improving the strength of 2000mm wide hot-rolled stainless steel coils of 316 series according to claim 1, characterized in that, In the annealing step, the thickness range of the 316 series wide hot-rolled stainless steel coil is 2-14mm, and the corresponding annealing speed is adjusted according to the thickness as follows: When the thickness 2≤h<6mm, the speed is 30±10m / min; When the thickness is 6≤h≤8mm, the speed is 190±20m / min; When the thickness is 8 < h ≤ 10.5 mm, the speed is 20 ± 6 m / min; When the thickness is 10.5 < h ≤ 12 mm, the speed is 14 ± 6 m / min; When the thickness is 12 < h ≤ 14 mm, the speed is 12 ± 5 m / min; In case of unit malfunction, in order to ensure plate temperature, the process speed of steel strip of all thicknesses can be reduced to a minimum of 5m / min.