A method for preparing stainless steel strips based on Ce-modified solution-free gradient aging strengthening

Abstract

This invention relates to the field of stainless steel thin strip continuous casting technology, and provides a method for preparing stainless steel thin strip based on Ce-modified solution-free gradient aging strengthening. The method includes steel composition design, smelting, thin strip continuous casting, residual heat hot rolling, and gradient aging treatment. By adding Ce element to synergize with gradient aging treatment and omitting the solution annealing step, the preparation process is simplified, energy consumption is reduced, and a short-process preparation of high-strength, high-corrosion-resistant 304 stainless steel thin strip is achieved. The strengthening element Ce forms L12-type nanoprecipitates with Ni in the 304 matrix, and gradient aging optimizes their morphology, size, and distribution, while also improving corrosion resistance and oxidation resistance. Twin-roll thin strip continuous casting combined with residual heat single-pass hot rolling completes solidification and deformation in one step. Rapid cooling during continuous casting refines the primary microstructure, and residual heat hot rolling utilizes residual heat energy to reduce energy consumption. Finally, gradient aging treatment is used to preferentially precipitate fine precipitates, then control their growth and adjust their number density. Through gradient heat treatment, both initial precipitation strengthening and later toughness recovery are considered, achieving a balance between strength and ductility.

Description

Technical Field

[0001] This invention relates to the field of stainless steel strip continuous casting technology, specifically to a method for preparing stainless steel strip based on Ce-modified solution-free gradient aging strengthening. Background Technology

[0002] 304 austenitic stainless steel is widely used in construction, food, medical, and machinery industries due to its good corrosion resistance, formability, and cost-effectiveness. However, its relatively low strength (typically tensile strength of 500–700 MPa) presents a significant performance bottleneck in applications requiring both lightweight and high strength and toughness.

[0003] Thin strip continuous casting integrates continuous casting, rolling, and heat treatment processes, allowing the produced thin strip to be formed into an industrial finished product in one step after only a short subsequent rolling process. This simplifies the production process, shortens the production cycle, and makes the production process more compact, continuous, efficient, and environmentally friendly. At the same time, production costs are significantly reduced, and the quality and performance of the produced thin strip products are no less than or even better than those produced by traditional processes.

[0004] Precipitation strengthening offers a range of advantages, including extremely high strengthening efficiency, excellent strength-toughness / plasticity balance, good high-temperature stability, and designability and controllability of properties. The conventional processing flow for precipitation-strengthened steels includes: smelting, homogeneous annealing, hot rolling / cold rolling, solution annealing (1050–1150℃), and aging treatment (500–650℃). Multiple high-temperature treatments are energy-intensive and increase costs.

[0005] The existing 304 stainless steel production process has the following drawbacks: the process flow is long, with a production cycle of up to 5 to 7 steps; cold working / reheating is difficult to carry out continuously, which is not conducive to process continuity.

[0006] In summary, there is an urgent need for a method to prepare stainless steel strips based on Ce-modified, solution-free gradient aging strengthening to solve the problems existing in the prior art. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing 304 stainless steel strip with Ce-modified, solution-free gradient aging strengthening. By combining twin-roll continuous casting technology with a simplified heat treatment process, a high-strength and high-toughness structure can be efficiently constructed. Simultaneously, the traditional solution annealing step is omitted; instead, gradient aging treatment is directly performed by retaining dislocations and utilizing residual heat from continuous casting, improving precipitation strengthening efficiency, simplifying the preparation process, and reducing energy consumption, thus demonstrating promising industrial application prospects. The specific technical solution is as follows:

[0008] A method for preparing stainless steel strips based on Ce-modified, solution-free gradient aging strengthening includes the following steps:

[0009] The steel liquid composition design and smelting are as follows: 304 austenitic stainless steel is selected, including C≤0.03%, Cr17–19%, and Ni8–11% by mass percentage; the strengthening element Ce is added, with Ce accounting for 0.05–0.5% by mass percentage; after smelting, an alloy liquid is obtained.

[0010] Thin strip casting involves pouring molten alloy into the space between two rollers under an inert atmosphere, and then using a twin-roll thin strip casting process to directly solidify and form a thin strip.

[0011] Residual heat hot rolling utilizes the residual heat after the continuous casting process for hot rolling.

[0012] The gradient aging treatment specifically includes: low-temperature aging treatment, in which the hot-rolled strip is fed into an aging furnace and held at T1–T2℃; medium-temperature aging treatment, in which the low-temperature aging steel sample is fed into an aging furnace and held at T3–T4℃; and high-temperature short-time conditioning treatment, in which the steel sample that has undergone medium-temperature aging treatment is briefly heated to T5–T6℃ and held at that temperature for a short time, and then air-cooled to room temperature to obtain high-strength 304 stainless steel strip.

[0013] Preferably, in thin strip continuous casting: a twin-roll thin strip continuous casting process is used to directly solidify and form a thin strip with a thickness of 1.0–1.9 mm; the casting temperature is 1490–1540℃, the roll speed is 70–100 m / min, and the cooling rate is 1000–2000℃ / s.

[0014] Preferably, in the residual heat hot rolling: the residual heat of the material at a temperature of 800–1100℃ after the continuous casting process is used to perform 1–2 passes of hot rolling, with a single pass reduction rate not exceeding 50%.

[0015] Preferably, the low-temperature aging treatment specifically involves feeding the hot-rolled strip directly into an aging furnace without cooling, and holding it at 450–500℃ for 1–3 hours.

[0016] The intermediate temperature aging treatment is specifically as follows: the steel sample that was aged at low temperature is sent into the aging furnace and held at 550–600℃ for 15–30 minutes;

[0017] The high-temperature short-time conditioning treatment is as follows: the steel sample that has undergone medium-temperature aging treatment is heated to 620–650℃ for 1–10 seconds and held at that temperature for 2–5 minutes. After completion, it is air-cooled to room temperature.

[0018] The present invention describes a method for preparing 304 stainless steel strip based on Ce-modified solution-free gradient aging strengthening. This method includes steel composition design, smelting, strip continuous casting, residual heat hot rolling, and gradient aging treatment. It omits the traditional solution annealing step, simplifying the preparation process and reducing energy consumption. The basic elemental composition and content of 304 stainless steel remain unchanged. The addition of the strengthening element Ce forms L12-type nanoprecipitates with Ni in the matrix, and gradient aging optimizes their morphology, size, and distribution. Furthermore, Ce purifies the molten steel, improving corrosion resistance. Twin-roll strip continuous casting combined with residual heat single-pass hot rolling completes solidification and deformation in one step, avoiding traditional multi-pass rolling and multiple solution treatments. Rapid cooling during continuous casting refines the primary microstructure, while hot rolling utilizes residual heat to reduce energy consumption. Finally, gradient aging treatment prioritizes the precipitation of fine precipitates, then controls their growth and adjusts their number density. This gradient heat treatment balances initial precipitation strengthening and later toughness recovery, achieving a balance between strength and ductility.

[0019] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0020] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0021] Figure 1 This is a schematic diagram of the method for preparing 304 stainless steel strip based on Ce-modified solution-free gradient aging strengthening in this invention. Detailed Implementation

[0022] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered by the claims.

[0023] A method for preparing 304 stainless steel strip based on Ce-modified solution-free gradient aging strengthening, see details. Figure 1 This includes the following steps:

[0024] The first step is the design and smelting of the molten steel. The design of the molten steel composition is as follows: 304 austenitic stainless steel is selected, and by mass percentage, it includes: C≤0.03%, Cr is 17–19%, and Ni is 8–11%; the strengthening element Ce is added, and by mass percentage, Ce is 0.05–0.5%; after smelting, an alloy liquid is obtained.

[0025] The second step is thin strip continuous casting, in which the alloy liquid is poured between the two rollers under the protection of an inert atmosphere, and the thin strip is directly solidified to form a thin strip using the twin-roll thin strip continuous casting process.

[0026] The third step is hot rolling with residual heat, which involves hot rolling using the residual heat after the continuous casting process.

[0027] The fourth step is gradient aging treatment, which includes: low-temperature aging treatment, in which the hot-rolled strip is sent into an aging furnace and held at T1–T2℃; medium-temperature aging treatment, in which the low-temperature aging steel sample is sent into an aging furnace and held at T3–T4℃; and high-temperature short-time conditioning treatment, in which the steel sample that has undergone medium-temperature aging treatment is briefly heated to T5–T6℃ to obtain high-strength 304 stainless steel strip.

[0028] In this preferred embodiment, the thin strip continuous casting process employs a twin-roll thin strip continuous casting process to directly solidify and form a 1.0–1.9 mm thick strip; the casting temperature is 1490–1540℃, the roll speed is 70–100 m / min, and the cooling rate is 1000–2000℃ / s. The high cooling rate refines the δ-ferrite and austenite microstructure, reduces macroscopic segregation, and the high dislocation density provides nucleation sites for subsequent precipitates.

[0029] In this preferred embodiment, during residual heat hot rolling: the residual heat at the material temperature of 800–1100℃ after the continuous casting process is used for 1–2 passes of hot rolling, with a single pass reduction rate not exceeding 50%. This improves the as-cast microstructure, breaks up dendrites, induces recrystallization and dislocation energy storage, and further increases the precipitation nucleation rate.

[0030] In this embodiment, the preferred low-temperature aging treatment is as follows: the hot-rolled strip is directly fed into the aging furnace without cooling and held at 450–500℃ for 1–3 hours; during this stage, a large number of dispersed nano-precipitated phase nuclei are formed, which, together with dislocations and grain boundaries, form a uniformly dispersed distribution, strongly hindering dislocation movement, thereby improving the yield strength.

[0031] The intermediate temperature aging treatment is as follows: the steel sample aged at low temperature is sent into the aging furnace and held at 550–600℃ for 15–30 minutes; in the higher temperature range, some precipitated phases grow moderately, reducing dislocation density and residual stress, reducing the risk of embrittlement, controlling the growth rate, so that the precipitates are still within the critical size range that can be cut / bypassed, thus maintaining the strengthening effect.

[0032] The high-temperature short-time conditioning treatment specifically involves raising the steel sample, which has undergone medium-temperature aging treatment, to 620–650℃ for 1–10 seconds and holding it at that temperature for 2–5 minutes, followed by air cooling to room temperature. This short-term exposure to the high-temperature zone promotes stress release and improves fracture toughness.

[0033] Examples 1-3:

[0034] The specific parameters of the technical solution adopted in this invention are detailed in Table 3.

[0035] In Examples 1-3, Ce not only acts as a precipitate strengthening element, but also modifies molten steel. The mechanism is as follows:

[0036] (1) Deep purification of molten steel: control of sulfur / oxygen inclusion morphology

[0037] Transformation of harmful inclusions: The high affinity of Ce for S (sulfur) and O (oxygen) in molten steel (Ce₂O₃ formation free energy ΔG) f = -1792 kJ / mol, Ce2S3ΔG f =-1070kJ / mol). Plastic MnS inclusions (easily extended into strips) are transformed into spherical Ce2O2S or Ce2S3.

[0038] The experimental results are detailed in Table 1:

[0039] Table 1 Experimental Results of Ce Deep Purification of Molten Steel

[0040] Inclusion type Average size (μm) Aspect Ratio Pitting Induction Probability MnS 5-20 >5:1 85％ <![CDATA[Ce2O2S]]> 0.5-2 ≈1:1 <10％

[0041] (2) Strengthening the passivation film: Improving the stability of the Cr2O3 film

[0042] Ce's "pinning effect": Ce 3+ Ions accumulate at the passivation film / substrate interface, reducing internal stress in the Cr2O3 film and inhibiting film rupture. This enhances the self-healing ability of the passivation film (Ce). 3+ (Preferential oxidation to fill defects).

[0043] Electrochemical verification:

[0044] Ce-modified steel: The passivation range is widened to +350mV, while that of conventional steel is only +220mV.

[0045] The passivation current density decreased by 50%, specifically from 2.1 × 10⁻⁶. -7 A / cm 2 Reduced to 1.0×10 -7 A / cm 2 .

[0046] (3) Grain boundary engineering: suppressing Cr carbide precipitation

[0047] Ce segregation: Ce atoms preferentially occupy grain boundary vacancies, thereby inhibiting Cr segregation. 23 C6 carbide precipitation avoids Cr-depleted areas at grain boundaries (resistance to intergranular corrosion).

[0048] The gradient aging process (500-600℃) further suppresses the carbide volume fraction. In Examples 1-3, the carbide volume fraction is <0.3%, while in the conventional process, the carbide volume fraction is >1.5%.

[0049] (4) Synergistically enhance high-temperature antioxidant properties

[0050] Oxide film structure optimization: Ce promotes the formation of a dense Cr2O3 / Al2O3 composite oxide film (CeO2 acts as an oxygen diffusion barrier). The oxide film adhesion is improved and the anti-peeling ability is enhanced, as shown in Table 2.

[0051] Table 2 Results of oxidation experiment at 650℃ / 100h

[0052] sample <![CDATA[Oxidation weight gain (mg / cm 2 )]]> Oxide film structure Traditional 304 stainless steel 2.15 <![CDATA[Porous Fe2O3 outer layer + inner layer cracks]]> Ce modified steel 0.82 <![CDATA[Dense Cr2O3 + CeO2 dispersion layer]]>

[0053] In the schemes of Examples 1-3, rapid solidification + residual heat rolling is adopted, skipping solution annealing. The specific principle is: during the thin strip continuous casting and residual heat rolling process, the solute elements are dissolved into the matrix. There is no cooling process, so the solute elements do not precipitate. Since there are no coarse precipitates in the matrix, it is not necessary to perform solution annealing to dissolve the solute elements and form an element-saturated matrix.

[0054] The advantages of skipping solution annealing are: First, significant energy saving and cost reduction: Utilizing the residual temperature field (≈600–700℃) after rolling, the aging process can be directly initiated, eliminating the need for secondary heating → reducing energy consumption by 35–40% (energy saving per ton of steel >200kWh); Second, preserving deformation defects and promoting precipitation: Dislocation density is preserved, and residual heat rolling introduces a high dislocation density (10... 14 -10 15 m -2 Skipping solid solution avoids defect recovery, allowing dislocations to become nucleation sites for the precipitation phase. Precipitation kinetics are accelerated; the combination of supersaturated vacancies and high dislocation density reduces the activation energy for κ-phase precipitation by 25% (from 180 kJ / mol to 135 kJ / mol) and shortens the aging time by 50%.

[0055] The schemes in Examples 1-3 utilize twin-roll thin strip continuous casting combined with residual heat single-pass hot rolling to achieve short-process microstructure refinement and dislocation strengthening. Gradient aging treatment finely controls the morphology and distribution of precipitates, achieving synergistic optimization of high strength and corrosion resistance of 304 stainless steel, while also taking into account ductility.

[0056] Comparative Examples 1-3:

[0057] The differences between Comparative Examples 1-3 and Example 1 are detailed in Table 3.

[0058] Table 3 Comparison of technical solutions and key parameters between Examples 1-3 and Comparative Examples 1-3

[0059]

[0060]

[0061] By combining Examples 1-3 and Comparative Examples 1-3, it can be seen that:

[0062] I. Technical solutions and effects of Examples 1-3

[0063] The commonalities of Examples 1-3 are as follows:

[0064] ① Composition design: Introduce Ce (0.05–0.5 wt.%) into the 304 stainless steel system, while maintaining the traditional C, Cr, and Ni ratio.

[0065] ② Process route: Twin-roll thin strip continuous casting (rapid solidification, thickness 1.0–1.9 mm, cooling rate 1000–2000℃ / s); residual heat hot rolling (800–1100℃, 1–2 passes, ≤50% reduction); gradient aging treatment (450–500℃ low temperature → 550–600℃ medium temperature → 620–650℃ short-time high temperature); skipping traditional solution annealing.

[0066] ③ Strengthening Mechanism: Ce and Ni form L12-type nanoprecipitates, which are fine, dispersed, and controllably distributed; Ce purifies the molten steel and controls the morphology of inclusions (MnS→Ce2O2S / Ce2S3), reducing pitting corrosion sources; Ce agglomerates at grain boundaries, inhibiting Cr... 23 C6 precipitation prevents intergranular corrosion; Ce promotes the formation of a dense and stable Cr2O3 / CeO2 composite oxide film, improving corrosion resistance and oxidation resistance; residual heat rolling retains a high dislocation density, accelerates precipitation kinetics, and shortens aging time.

[0067] ④ Overall effect: Both achieve higher strength than traditional 304 stainless steel (yield strength increased by about 2 times), while maintaining good ductility (≥50% elongation), and taking into account corrosion resistance and oxidation resistance.

[0068] II. Technical solutions and effects of comparative examples 1–3

[0069] The detailed analysis of Comparative Examples 1-3 is as follows:

[0070] Comparative Example 1 (without Ce addition): The process was the same as in Example 1, except that Ce was removed; performance: yield strength 280 MPa, tensile strength 550 MPa, elongation 60%. This demonstrates that Ce is a key factor in improving strength and corrosion resistance.

[0071] Comparative Example 2 (Traditional process: billet casting + homogeneous annealing + hot rolling + solution annealing + aging): The process is complex, lengthy, and energy-intensive. Performance: Yield strength 400 MPa, tensile strength 650 MPa, elongation 45%. Compared to Examples 1-3, both strength and plasticity are significantly inferior, and energy consumption is even higher.

[0072] Comparative Example 3 (strip directly cooled after continuous casting, without residual heat rolling and aging): The process was simplified, but the precipitation strengthening mechanism was not activated. Performance: Yield strength 300 MPa, tensile strength 600 MPa, elongation 50%. This demonstrates that residual heat rolling combined with gradient aging is a necessary condition for performance improvement.

[0073] III. As can be seen from Examples 1-3 and Comparative Examples 1-3, the technical solution of the present invention has the following characteristics:

[0074] 1. Innovative ingredient design:

[0075] For the first time, the Ce system has been introduced into the short process of continuous casting-residual heat rolling-gradient aging of 304 stainless steel strip. Ce simultaneously plays a role in precipitation strengthening, purifying molten steel, stabilizing the passivation film, regulating grain boundaries, and resisting oxidation, forming a "multifunctional" mechanism.

[0076] 2. Technological route innovation:

[0077] A refined microstructure and high dislocation matrix are obtained directly through thin strip continuous casting followed by residual heat hot rolling. Solution annealing is skipped, and direct aging using residual heat is achieved, resulting in energy savings of 35–40% and a shorter process cycle.

[0078] 3. Innovative heat treatment methods:

[0079] The design employs a gradient aging process (low-temperature precipitation → medium-temperature stabilization → high-temperature short-term conditioning) to enhance strength, restore toughness, and maintain ductility. This multi-stage aging process controls the morphology, size, and number density of the precipitated phases, achieving an optimal balance between strengthening effect and plasticity.

[0080] 4. Performance Breakthrough: Compared with traditional processes, the yield strength is increased by approximately 40–100%, and the elongation is increased by 10–15 percentage points. At the same time, it significantly improves resistance to pitting corrosion, intergranular corrosion, and high-temperature oxidation.

[0081] 5. Industrial application value: The process is continuous and the flow is shortened, resulting in a significant reduction in energy consumption and a remarkable energy-saving and emission-reduction effect; it has important promotion value in fields that combine high strength, corrosion resistance and formability (construction, medical, food machinery, lightweight transportation).

[0082] In summary, the technical solution of this invention is not merely a simple superposition of "Ce element addition + short process", but rather achieves a multi-dimensional breakthrough in strength, ductility, corrosion resistance and oxidation resistance through the systematic coupling of composition, process and heat treatment, possessing outstanding creativity and significant practical value.

[0083] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing stainless steel thin strips based on Ce-modified, solution-free gradient aging strengthening, characterized in that, Includes the following steps: The steel liquid composition design and smelting are as follows: 304 austenitic stainless steel is selected, including C≤0.03%, Cr17–19%, and Ni8–11% by mass percentage; Ce is added as a strengthening element, with Ce accounting for 0.05–0.5% by mass percentage; after smelting, an alloy liquid is obtained. Thin strip casting involves pouring molten alloy into the space between two rollers under an inert atmosphere, and then using a twin-roll thin strip casting process to directly solidify and form a thin strip. Residual heat hot rolling utilizes the residual heat after the continuous casting process for hot rolling. The gradient aging treatment specifically includes: low-temperature aging treatment, in which the hot-rolled strip is fed into an aging furnace and held at 450–500℃ for 1–3 hours; medium-temperature aging treatment, in which the low-temperature aging steel sample is fed into an aging furnace and held at 550–600℃ for 15–30 minutes; and high-temperature short-time conditioning treatment, in which the medium-temperature aging steel sample is heated to 620–650℃ in 1–10 seconds and held for 2–5 minutes, and then air-cooled to room temperature to obtain high-strength 304 stainless steel strip.

2. The method for preparing stainless steel thin strip based on Ce-modified solution-free gradient aging strengthening according to claim 1, characterized in that, In thin strip continuous casting: a twin-roll thin strip continuous casting process is used to directly solidify and form a thin strip with a thickness of 1.0–1.9 mm; the casting temperature is 1490–1540℃, the roll speed is 70–100 m / min, and the cooling rate is 1000–2000℃ / s.

3. The method for preparing stainless steel thin strip based on Ce-modified solution-free gradient aging strengthening according to claim 1, characterized in that, In residual heat hot rolling: the residual heat at the material temperature of 800–1100℃ after the continuous casting process is used to carry out 1–2 passes of hot rolling, with a single pass reduction rate not exceeding 50%.

4. The method for preparing stainless steel thin strip based on Ce-modified solution-free gradient aging strengthening according to claim 1, characterized in that, During the low-temperature aging process, the hot-rolled strip is directly fed into the aging furnace without cooling.

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