A method for strengthening and toughening low-Cr ferritic stainless steel

Abstract

This invention discloses a method for strengthening and toughening low-Cr ferritic stainless steel, comprising: controlling the mass percentage content of C, N, and Ni in the low-Cr ferritic stainless steel to be C: 0.02%–0.03%, N: 0.02%–0.03%, and Ni: 0.5%–1.0%, respectively; controlling the hot rolling coiling temperature at 660℃±10℃ during hot rolling; employing a two-stage hot wire annealing process for heat treatment, with the first hot wire annealing temperature controlled at (AC3+30℃)±10℃ and the heat treatment time controlled at 2.5 min / mm, and the second hot wire annealing temperature controlled at [AC1+(AC3-AC1) / 3]±10℃ and the heat treatment time controlled at 2.0 min / mm; and after hot wire annealing, the hot-rolled strip is leveled in a finishing mill, with the leveling pressure controlled at 300–400t and the leveling roll crown controlled at 0.02–0.06. This invention can maintain the coexistence of ferrite and martensite phases in low-Cr ferritic stainless steel and refine the grains through multiple phase transformations, thus achieving strong and tough control of low-Cr ferritic stainless steel with a yield strength of 450-500 MPa and an elongation after fracture of 25%-30%.

Description

Technical Field

[0001] This invention belongs to the field of ferritic stainless steel production technology, and more specifically relates to a method for strengthening and toughening low-Cr ferritic stainless steel. Background Technology

[0002] Low-Cr ferritic stainless steel is a type of ferritic stainless steel with excellent corrosion resistance and oxidation resistance. It boasts high strength, good machinability, and weldability. Furthermore, due to its low alloy content, it offers significant economic advantages and is widely used in building panels, rail transportation, and home appliances and bathroom fixtures. With societal development and increasing demands for raw materials across various sectors, customers are placing ever higher requirements on the strength and toughness of low-Cr ferritic stainless steel, making the development of strengthening and toughening processes for low-Cr stainless steel essential.

[0003] Because low-Cr ferritic stainless steel has a low alloy content, maintaining stable performance has always been a challenge in actual production. Due to its low Cr content, and the addition of austenite-forming elements such as Ni to improve weldability (e.g., 022Cr12Ni), low-Cr ferritic stainless steel exhibits a two-phase structure at high temperatures, consisting of α-ferrite and γ-austenite. Therefore, the heat treatment process can control the proportion of the high-temperature austenite phase through temperature regulation, providing multiple options for subsequent heat treatment processes. If low-temperature α-ferrite annealing is used, the final microstructure is a single ferrite structure, which, while exhibiting good toughness, often fails to meet expected strength. Conversely, if high-temperature γ-austenite annealing is used, the final product has a single martensitic structure, which, while possessing high strength, has poor ductility and toughness. While annealing in the α-ferrite + γ-austenite dual-phase region can achieve a coexistence of ferrite and martensite binary structures, practical experience reveals several drawbacks. Due to the high heat treatment temperature and the fact that austenite nucleates later than ferrite, ferrite grain growth without austenite pinning is uncontrolled, easily leading to large grains. Furthermore, the long growth time required for austenite grains makes it difficult to achieve an ideal ferrite to austenite ratio in a short treatment period. Therefore, it is challenging to achieve the strengthening and toughening of low-Cr ferritic stainless steel through fixed-temperature heat treatment. Summary of the Invention

[0004] To address the technical problems existing in the prior art, the present invention provides a method for strengthening and toughening low-Cr ferritic stainless steel, comprising:

[0005] (1) Component optimization

[0006] The mass percentage content of C, N, and Ni in the low-Cr ferritic stainless steel is controlled as follows: C: 0.02%–0.03%, N: 0.02%–0.03%, Ni: 0.5%–1.0%.

[0007] (2) Hot rolling coiling temperature control

[0008] During the hot rolling process, laminar cooling water is introduced during coiling, and the hot rolling coiling temperature is controlled within the range of 660℃±10℃;

[0009] (3) Optimization of hot wire annealing process

[0010] A two-stage hot-wire annealing process was used for the strengthening and toughening heat treatment of low-Cr ferritic stainless steel. The temperature of the first hot-wire annealing was controlled at (AC3+30℃)±10℃, and the heat treatment time was controlled at 2.5 min / mm based on the thickness of the hot-rolled strip. After the hot-rolled strip was air-cooled to room temperature after the first hot-wire annealing, a second hot-wire annealing was performed. The temperature of the second hot-wire annealing was controlled at [AC1+(AC3-AC1) / 3]±10℃, and the heat treatment time was controlled at 2.0 min / mm based on the thickness of the hot-rolled strip.

[0011] (4) Adjustment of finishing process

[0012] After hot-rolled annealing, the hot-rolled strip is leveled in the finishing mill, where the leveling pressure is controlled at 300-400t and the crown of the leveling roll is controlled at 0.02-0.06.

[0013] As one specific implementation method, in the above-mentioned strengthening and toughening treatment method for low-Cr ferritic stainless steel, the low-Cr ferritic stainless steel is 022Cr12Ni, S41003, or X2CrNi12.

[0014] Furthermore, in the above-mentioned strengthening and toughening treatment method for low-Cr ferritic stainless steel, the yield strength of the low-Cr ferritic stainless steel is in the range of 450-500 MPa, the tensile strength is in the range of 500-650 MPa, and the elongation after fracture is in the range of 25%-30%.

[0015] The method for strengthening and toughening low-Cr ferritic stainless steel of the present invention has the following advantages and beneficial effects:

[0016] This invention, through technical measures such as composition optimization, hot rolling temperature control, hot-wire annealing process optimization, and finishing process adjustment, can maintain the coexistence of ferrite and martensite phases in low-Cr ferritic stainless steel and controllably adjust the phase ratio of ferrite and martensite. Furthermore, it can refine the grain size through multiple phase transformations. As a result, the strength and toughness of low-Cr ferritic stainless steel are significantly improved, with yield strength in the range of 450–500 MPa, tensile strength in the range of 500–650 MPa, and elongation after fracture in the range of 25%–30%, achieving controlled strength and toughness improvement in low-Cr ferritic stainless steel. Detailed Implementation

[0017] 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.

[0018] The toughening and strengthening treatment method for low-Cr ferritic stainless steel of the present invention effectively controls the ratio of ferrite to martensite in low-Cr ferritic stainless steel through technical measures such as composition optimization, hot rolling coiling temperature control, hot wire annealing process optimization, and finishing process adjustment. It also achieves grain refinement through multiple phase transformations and ultimately realizes the toughening and strengthening control of low-Cr ferritic stainless steel.

[0019] Specifically, the method for strengthening and toughening low-Cr ferritic stainless steel according to the present invention includes:

[0020] (1) Component optimization

[0021] Unlike existing technologies that refine grains and improve product strength and toughness by adding precipitation strengthening elements such as Nb and Ti, this invention increases the content of austenite-forming elements C, N, and Ni. The C content is controlled at 0.02% to 0.03% by mass, and the N content is controlled at 0.02% to 0.03% by mass to ensure the existence of a single austenite phase range during subsequent heat treatment. The Ni content is controlled at 0.5% to 1.0% by mass to ensure the stability of the high-temperature austenite structure and avoid the decrease in product strength caused by excessive decomposition of austenite during subsequent heat treatment.

[0022] (2) Hot rolling coiling temperature control

[0023] During hot rolling, laminar cooling water is introduced during coiling to reduce the hot rolling coiling temperature, which is controlled within the range of 660℃±10℃. By strictly controlling the hot rolling coiling temperature range, on the one hand, the performance stability of multiple batches of hot-rolled strip is ensured, avoiding fluctuations in the final product performance caused by uneven microstructure and properties of different batches of hot-rolled raw materials; on the other hand, reducing the hot rolling coiling temperature controls the proportion of residual martensite phase in the microstructure of the hot-rolled raw materials, providing a microstructure basis for phase transformation during subsequent heat treatment.

[0024] (3) Optimization of hot wire annealing process

[0025] In existing technologies, some methods improve the strength and toughness of low-Cr ferritic stainless steel by adjusting the bell-type furnace process, but these measures do not consider the efficiency issues during annealing. Other methods use hot-wire annealing, but this involves fixed-temperature annealing in the dual-phase region, which lacks the effect of multiple phase transformations to refine the grains. This invention innovatively develops a two-stage hot-wire annealing process for strengthening and toughening low-Cr ferritic stainless steel, first austenitizing and then partially ferrititizing. The first hot-wire annealing completely austenitizes the product through a relatively long high-temperature austenitic phase region heat treatment, forming a fully martensitic structure after cooling. The second hot-wire annealing is a shorter ferrite + austenite dual-phase region heat treatment. At this time, due to the lower temperature, the austenite grains do not grow excessively, and some of the martensite transforms into austenite before decomposing back into ferrite. After cooling, the high-temperature austenite portion transforms into martensite. Therefore, by using the secondary hot wire annealing process, on the one hand, the coexistence of ferrite and martensite in the product can be maintained and the phase ratio of ferrite and martensite in the product can be controlled. On the other hand, the grains can be refined through multiple phase transformations, thus achieving the strengthening and toughening control of low-Cr ferritic stainless steel.

[0026] Specifically, in the aforementioned two-stage hot-wire annealing process, the temperature of the first hot-wire annealing is controlled at (AC3 temperature + 30℃) ± 10℃, and the heat treatment time is controlled at 2.5 min / mm based on the thickness of the hot-rolled strip. After the first hot-wire annealing, the material microstructure is fully martensitic, with a yield strength in the range of 550–650 MPa, a tensile strength in the range of 600–750 MPa, and an elongation after fracture in the range of 12%–16%. After the hot-rolled strip has completed the first hot-wire annealing, it is air-cooled to room temperature before undergoing a second hot-wire annealing. The heat treatment temperature is controlled at [AC1 temperature + (AC3 temperature - AC1 temperature) / 3] ± 10℃, and the heat treatment time is controlled at 2.0 min / mm based on the thickness of the hot-rolled strip. During the second hot wire annealing, some martensitic structures undergo austenitic and ferrite transformations, which further refines the grains while ensuring the coexistence of the two phases. After the second hot wire annealing, the grain size is above grade 8.5, the yield strength is in the range of 400-450 MPa, the tensile strength is in the range of 500-650 MPa, and the elongation after fracture is in the range of 25%-30%.

[0027] (4) Adjustment of finishing process

[0028] After hot-line annealing, the hot-rolled strip undergoes leveling treatment in the finishing mill to further improve its strength. The leveling pressure is controlled at 300–400 t, and the crown of the leveling rolls is controlled at 0.02–0.06 to ensure the strip shape. Through the leveling treatment in the finishing mill, the yield strength of the low-Cr ferritic stainless steel is increased to 450–500 MPa.

[0029] The following detailed description of the strengthening and toughening treatment method for low-Cr ferritic stainless steel, in conjunction with specific embodiments of the present invention, provides a clear explanation.

[0030] Example 1

[0031] Example 1 was applied to the production process of 022Cr12Ni hot-rolled coil. The target thickness of the hot-rolled strip was 3mm. The chemical composition of 022Cr12Ni by weight percentage was C: 0.023%, Si: 0.035%, Mn: 1.20%, P: 0.017%, S: 0.001%, Cr: 12.22%, Ni: 0.90%, N: 0.024%, with the remainder being Fe and unavoidable impurities.

[0032] During the hot rolling process, laminar cooling water was introduced during coiling and the hot rolling coiling temperature was set to 660℃. In actual coiling operations, the measured hot rolling coiling temperature was 657℃.

[0033] Based on the iron-carbon phase diagram and / or theoretical calculations, the AC1 temperature for 022Cr12Ni is 700℃, and the AC3 temperature is 810℃. The first hot-wire annealing temperature was set at 840℃, with an actual average temperature of 845℃ and a heat treatment time of 7.5 min. After the hot-rolled strip completed the first hot-wire annealing, it was air-cooled to room temperature before undergoing a second hot-wire annealing. The second hot-wire annealing temperature was set at 730℃, with an actual average temperature of 732℃ and a heat treatment time of 6 min.

[0034] After hot-rolled annealing, the hot-rolled strip is leveled in the finishing mill. The crown of the leveling rolls in the leveling mill is 0.04 to 0.06, and the leveling pressure is 300t.

[0035] Using Example 1, a total of 7 hot-rolled coils of 022Cr12Ni stainless steel were produced. Actual testing showed that the average yield strength of the products was 467 MPa, the average tensile strength was 598 MPa, and the average elongation after fracture was 26.5%.

[0036] Example 2

[0037] Example 2 was applied to the production process of S41003 hot-rolled coil. The target thickness of the hot-rolled strip was 4 mm. The chemical composition of S41003 by weight percentage was C: 0.021%, Si: 0.038%, Mn: 1.18%, P: 0.019%, S: 0.001%, Cr: 12.31%, Ni: 0.87%, N: 0.022%, with the remainder being Fe and unavoidable impurities.

[0038] During the hot rolling process, laminar cooling water was introduced during coiling and the hot rolling coiling temperature was set to 660℃. In actual coiling operations, the measured hot rolling coiling temperature was 658℃.

[0039] Based on the iron-carbon phase diagram and / or theoretical calculations, the AC1 temperature of S41003 is 710℃, and the AC3 temperature is 820℃. The first hot-wire annealing temperature was set at 850℃, with an actual average temperature of 849℃ and a heat treatment time of 10 minutes. After the hot-rolled strip completed the first hot-wire annealing, it was air-cooled to room temperature before undergoing a second hot-wire annealing. The second hot-wire annealing temperature was set at 740℃, with an actual average temperature of 743℃ and a heat treatment time of 8 minutes.

[0040] After hot-rolled annealing, the hot-rolled strip is leveled in the finishing mill. The crown of the leveling rolls in the leveling mill is 0.04 to 0.06, and the leveling pressure is 300t.

[0041] Using Example 2, a total of 6 hot-rolled coils of S41003 stainless steel were produced. Actual testing showed that the average yield strength of the products was 457 MPa, the average tensile strength was 587 MPa, and the average elongation after fracture was 27.0%.

[0042] Example 3

[0043] Example 3 was applied to the production process of X2CrNi12 hot-rolled coil. The target thickness of the hot-rolled strip was 4 mm. The chemical composition of S41003 by weight percentage was C: 0.025%, Si: 0.034%, Mn: 1.31%, P: 0.015%, S: 0.001%, Cr: 12.19%, Ni: 0.95%, N: 0.027%, with the remainder being Fe and unavoidable impurities.

[0044] During the hot rolling process, laminar cooling water was introduced during coiling and the hot rolling coiling temperature was set to 660℃. In actual coiling operations, the measured hot rolling coiling temperature was 664℃.

[0045] Based on the iron-carbon phase diagram and / or theoretical calculations, the AC1 temperature of X2CrNi12 is 685℃, and the AC3 temperature is 795℃. The first hot-wire annealing temperature was set at 825℃, with an actual average temperature of 819℃ and a heat treatment time of 10 minutes. After the hot-rolled strip completed the first hot-wire annealing, it was air-cooled to room temperature before undergoing a second hot-wire annealing. The second hot-wire annealing temperature was set at 715℃, with an actual average temperature of 720℃ and a heat treatment time of 8 minutes.

[0046] After hot-rolled annealing, the hot-rolled strip is leveled in the finishing mill. The crown of the leveling rolls in the leveling mill is 0.04 to 0.06, and the leveling pressure is 300t.

[0047] Using Example 3, a total of 9 hot-rolled X2CrNi12 stainless steel coils were produced. Actual testing showed that the average yield strength of the products was 478 MPa, the average tensile strength was 601 MPa, and the average elongation after fracture was 26.5%.

[0048] In summary, the toughening and strengthening treatment method for low-Cr ferritic stainless steel of the present invention, through technical measures such as composition optimization, hot rolling temperature control, hot wire annealing process optimization, and finishing process adjustment, can maintain the coexistence of ferrite and martensite phases in low-Cr ferritic stainless steel and controllably adjust the phase ratio of ferrite and martensite. On the other hand, it can also refine the grains through multiple phase transformations. Through the present invention, the toughness of low-Cr ferritic stainless steel is significantly improved, with yield strength in the range of 450-500 MPa, tensile strength in the range of 500-650 MPa, and elongation after fracture in the range of 25%-30%, thus achieving the control of toughening and strengthening of low-Cr ferritic stainless steel.

[0049] 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.

[0050] 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 strengthening and toughening a low-Cr ferritic stainless steel, characterized by, Comprise: (1) Component optimization The mass percentage content of C, N and Ni in the low-Cr ferrite stainless steel is controlled as C: 0.02%~0.03%, N: 0.02%~0.03%, and Ni: 0.5%~1.0%, respectively; (2) Hot rolling coiling temperature control During the hot rolling process, laminar cooling water is injected when coiling, and the hot rolling coiling temperature is controlled in the range of 660℃±10℃; (3) Hot wire annealing process optimization The twice hot wire annealing process is adopted for the strengthening and toughening heat treatment of the low-Cr ferrite stainless steel, wherein the first time hot wire annealing temperature is controlled at (AC3+30℃)±10℃, and the heat treatment time is controlled at 2.5min / mm based on the thickness of the hot rolling strip; after the hot rolling strip is cooled to room temperature after the first time hot wire annealing, the second time hot wire annealing is carried out, the second time hot wire annealing temperature is controlled at [AC1+(AC3-AC1) / 3]±10℃, and the heat treatment time is controlled at 2.0min / mm based on the thickness of the hot rolling strip; (4) Finishing process adjustment After the hot wire annealing treatment, the hot rolling strip is subjected to flattening treatment in the finishing unit, wherein the flattening pressure is controlled at 300~400t, and the flattening roller crown is controlled at 0.02~0.06, Wherein, the low-Cr ferrite stainless steel is 022Cr12Ni, S41003, or X2CrNi12.

2. The method of the low Cr ferritic stainless steel according to claim 1, wherein The yield strength of the low-Cr ferrite stainless steel is in the range of 450~500MPa, the tensile strength is in the range of 500~650MPa, and the elongation after fracture is in the range of 25%~30%.