A short process preparation method of high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation

Abstract

The present application relates to the technical field of stainless steel thin strip continuous casting, and provides a short-process preparation method of high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation, which comprises the following steps: steel liquid component design and smelting; double-roller thin strip continuous casting; afterheat hot rolling; cyclic solid solution treatment; aging strengthening treatment, and 304 stainless steel thin strip is obtained. By introducing specific amounts of Al and Ti elements into the 304 stainless steel, controlling the continuous casting and rolling process parameters, relying on the sub-rapid solidification advantage of thin strip continuous casting, omitting homogenizing annealing and multi-pass rolling, and implementing cyclic solid solution treatment after rolling, the grains are refined, and a highly uniform supersaturated solid solution and a large number of dislocations are obtained. Finally, 500-600 DEG C aging strengthening is performed, so that Al and Ti are combined with elements such as Ni in the austenitic stainless steel matrix to form a dispersed precipitate phase. Through the synergistic effect of the three of precipitation strengthening, fine-grain strengthening and dislocation strengthening, the strength level is significantly improved while the toughness of the stainless steel is maintained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of stainless steel thin strip continuous casting, in particular to a short-process preparation method of high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation. BACKGROUND

[0002] 304 austenitic stainless steel is widely used due to its excellent corrosion resistance, formability and weldability, but its conventional yield strength is 250-280 MPa, and the ultimate tensile strength is only 550-650 MPa, which cannot meet the requirements of high-strength structures or flexible applications.

[0003] Twin-roll strip casting (TRC) can directly produce thin strips with a thickness of 2-4 mm from molten steel, and combined with hot rolling, it can realize short-process casting and rolling integration, which is beneficial to energy saving and material refinement.

[0004] Traditional 304 austenitic stainless steel has low yield strength and tensile strength, which cannot meet the requirements of high-strength structural parts or flexible thin strip applications. Traditional strengthening methods mainly rely on cold work hardening or fine-grain strengthening, but do not effectively combine precipitation strengthening mechanisms, resulting in limited improvement of strength and plasticity. The existing precipitation strengthening process is usually: casting blank, homogenization annealing, multi-pass hot rolling, cold rolling, solid solution treatment and aging. High-temperature and long-time refinement of macrosegregation in the casting blank is needed to diffuse solute elements and provide precursors for precipitation strengthening. In addition, the casting blank is thick and needs to be rolled multiple times at high pressure to reach the application level, which is costly. And single long-time solid solution will inevitably lead to coarse austenite grains, which is not conducive to the subsequent mechanical properties.

[0005] In summary, there is an urgent need for a short-process stainless steel thin strip preparation method with high performance to solve the problems existing in the prior art. SUMMARY

[0006] The present application aims to provide a short-process preparation method of high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation, which introduces 0.3-0.8wt.% Al and 0.1-0.4wt.% Ti elements into 304 stainless steel, integrates twin-roll thin strip continuous casting, waste heat hot rolling, solid solution treatment and aging strengthening, and realizes the unity of strength improvement and production efficiency. The specific technical scheme is as follows:

[0007] A short-process preparation method of high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation, comprising the following steps:

[0008] The molten steel composition design and smelting, the molten steel composition design is: selecting 304 stainless steel, including: C≤0.03%, Si is 0.3-0.6%, Mn is 1.0-1.5%, Cr is 17-18.5%, Ni is 8-9.5%, Ti is 0.1-0.4%, Al is 0.3-0.8%, N is 0.02-0.06%, the balance is iron; after smelting, alloy liquid is obtained;

[0009] Double-roller thin strip continuous casting, the alloy liquid is poured between the double rollers under the protection of inert atmosphere, and the initial thin strip is formed by adopting the double-roller thin strip continuous casting process; the temperature of continuous casting is controlled at 1490-1540 DEG C; the roller speed is controlled at 70-100 m / min; the cooling rate is 1000-2000 DEG C / s;

[0010] Waste heat hot rolling, the residual heat after the continuous casting process is utilized for hot rolling processing, and the hot rolled thin strip is obtained;

[0011] Cyclic solid solution treatment, specifically including: the thin strip is subjected to at least two times of solid solution treatment;

[0012] Ageing strengthening treatment, specifically including: the thin strip after cyclic solid solution treatment is heated to 500-600 DEG C and kept for 15 min-3 h; after cooling, the 304 stainless steel thin strip is obtained.

[0013] Preferably, in the double-roller thin strip continuous casting: the inert gas is nitrogen with purity≥99.99%; the material of the double roller is copper, the diameter of the copper double roller is 300-500 mm; the thickness of the obtained initial thin strip is 1.0-1.9 mm. Preferably, in the waste heat hot rolling: the temperature of the initial thin strip is 1000-1200 DEG C; 1-2 passes of rolling are performed by utilizing its own residual heat; the total reduction is≤50%; the thickness of the hot rolled thin strip is 0.5-1.3 mm; after rolling, the spray water cooling mode is adopted for cooling.

[0014] Preferably, the cyclic solid solution treatment specifically includes: the first solid solution treatment, specifically including: the hot rolled thin strip is heated to 600-1000 DEG C and kept for 10-30 min, and then water quenching is performed to cool to room temperature, and the stainless steel thin strip is obtained; the second solid solution treatment, specifically including: the stainless steel thin strip is heated to 600-1000 DEG C and kept for 10-30 min, and then water quenching is performed to cool to room temperature.

[0015] Preferably, the cyclic solid solution treatment further includes the third solid solution treatment, specifically including: the stainless steel thin strip after the second solid solution treatment is heated to 600-1000 DEG C and kept for 10-30 min, and then water quenching is performed to cool to room temperature.

[0016] The application discloses a short process preparation method of high-strength stainless steel thin strips based on cyclic solid solution and aging precipitation.

[0017] In addition to the objects, features and advantages described above, the present application has other objects, features and advantages. The present application will be further described in detail below with reference to the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0018] The accompanying drawings, which form a part of this application, are intended to provide further understanding of the application and are incorporated herein for a description of the application. In the drawings:

[0019] Figure 1 The application is based on the principle of a short process preparation method of high-strength stainless steel thin strips based on cyclic solid solution and aging precipitation. DETAILED DESCRIPTION

[0020] The embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0021] A short process preparation method of high-strength stainless steel thin strips based on cyclic solid solution and aging precipitation, as shown in Figure 1 , comprises the following steps:

[0022] The first step is steel liquid composition design and smelting. The steel liquid composition design is: selecting 304 stainless steel, including, in terms of mass percentage: C≤0.03%, Si is 0.3-0.6%, Mn is 1.0-1.5%, Cr is 17-18.5%, Ni is 8-9.5%, Ti is 0.1-0.4%, Al is 0.3-0.8%, N is 0.02-0.06%, and the balance is iron; and the alloy liquid is obtained after smelting. Controlling the Ti content to be 0.1-0.4% can promote the formation of precipitated strengthening phase, while avoiding the precipitation of coarse TiN. Preferably, an induction furnace, an electric arc furnace or other industrial smelting equipment is used, and the furnace temperature is controlled to be greater than or equal to 1600°C during the smelting process; Al or Ti can be used for deoxidization to ensure purity.

[0023] The second step is double-roller thin strip continuous casting. The alloy liquid is poured between the double rollers under the protection of an inert atmosphere, and an initial thin strip is formed by using a double-roller thin strip continuous casting process; the temperature of the continuous casting is controlled to be 1490-1540°C; the roller speed is controlled to be 70-100 m / min; and the cooling rate is 1000-2000°C / s.

[0024] Preferably, the inert gas is nitrogen with a purity of ≥99.99% to prevent surface oxidation; the material of the double rollers is copper, the diameter of the copper double rollers is 300-500 mm; and the thickness of the obtained initial thin strip is 1.0-1.9 mm.

[0025] The third step is waste heat hot rolling. The residual heat after the continuous casting process is used for hot rolling to obtain a hot-rolled thin strip. In the waste heat hot rolling, the temperature of the initial thin strip is 1000-1200°C; 1-2 passes of rolling are performed by using the residual heat thereof; the total reduction rate is ≤50%; the thickness of the hot-rolled thin strip is 0.5-1.3 mm; and the cooling is performed by using a spray water cooling method after rolling.

[0026] The fourth step is cyclic solid solution treatment, specifically including: subjecting the thin strip to at least two solid solution treatments. Preferably, the cyclic solid solution treatment specifically includes:

[0027] The first solid solution treatment specifically includes: heating the hot-rolled thin strip to 600-1000°C, maintaining for 10-30 min, and water quenching to room temperature to obtain a stainless steel thin strip. A box-type furnace, a roller hearth furnace or a belt-type continuous furnace can be used.

[0028] The second solid solution treatment specifically includes: heating the stainless steel thin strip to 600-1000°C, maintaining for 10-30 min, and water quenching to room temperature. A box-type furnace, a roller hearth furnace or a belt-type continuous furnace can be used.

[0029] In addition, the cyclic solid solution treatment further comprises a third solid solution treatment, specifically comprising: heating the stainless steel thin strip after the second solid solution treatment to 600-1000℃, maintaining for 10-30min, and water quenching to room temperature. A box furnace, roller hearth furnace or belt continuous furnace can be used.

[0030] The fifth step is aging strengthening treatment, specifically comprising: heating the thin strip after the cyclic solid solution treatment to 500-600℃, maintaining for 15min-3h; and obtaining the 304 stainless steel thin strip after cooling. A box furnace, roller hearth furnace or belt continuous furnace can be used. Since the final product is in the form of a steel coil, a curling process is required, and the heat dissipation is slow in this process, so the aging treatment is performed by utilizing the residual heat and curling. The target is to induce the formation of dispersed strengthening phases such as Ni3Ti and NiAl, effectively hinder the dislocation movement, significantly improve the yield and tensile strength, and release the residual stress. The cooling method is air cooling.

[0031] Embodiments 1-3:

[0032] The technical scheme of the present application is adopted, and the specific parameters are shown in Table 1.

[0033] The key point of embodiments 1-3 is to realize the breakthrough of the performance of 304 stainless steel by using ultra-fine grains and precipitation strengthening, and the principle is as follows:

[0034] (1) Composition design breakthrough:

[0035] In the traditional 304 stainless steel system, trace amounts of Al elements (0.3-0.8 wt.%) and Ti elements (0.1-0.4 wt.%) are introduced, and the Ni element contained in the 304 stainless steel is used to form a dispersed precipitation strengthening phase (Ni3Ti and NiAl) under appropriate heat treatment conditions, which fills the long-term lack of precipitation strengthening mechanism in the 304 system.

[0036] (2) Short process integration:

[0037] The Al and Ti precipitation strengthening mechanism is combined with the double-roller thin strip continuous casting, residual heat single-pass hot rolling, and cyclic solid solution treatment and aging strengthening, and a continuous manufacturing process path is constructed.

[0038] Process characteristics: short process, high efficiency; realizing the organic combination of precipitation strengthening, fine-grain strengthening and dislocation strengthening; cyclic solid solution and aging combination; cost and performance balance;

[0039] Double-roller thin strip continuous casting (TRC): directly preparing 1-5mm thin strip from melt, skipping the traditional ingot-hot rolling multi-pass process, saving energy by more than 50%. Ultra-high cooling rate (1000-2000℃ / s) makes the δ-ferrite to austenite phase transition complete quickly, refines the structure, and inhibits coarse dendrites and segregation.

[0040] Residual heat single-pass hot rolling: using TRC outlet ≥1000℃ residual temperature, single-pass rolling thinning 30-50%, avoiding secondary heating. Breaking part of the as-cast structure, inducing deformation energy storage and dislocation density increase, providing nucleation sites for subsequent aging precipitation.

[0041] Cyclic solid solution treatment: the core principle of cyclic solid solution annealing is to break through the limitations of conventional single solid solution treatment by using multiple "thermal cycles" to optimize the structure through repeated dynamic recrystallization and element diffusion. Its specific mechanism can be divided into the following aspects:

[0042] First, grain ultra-fining, which is the most core role of cyclic solid solution, details as follows:

[0043] Single solid solution defects: heating to solid solution temperature (such as 820℃) and holding, although it can dissolve compounds, but the grain inevitably grows. The higher the temperature, the longer the time, the coarser the grain, which is not conducive to the subsequent mechanical properties.

[0044] The specific effect of the present application using cyclic solid solution is: ①, the first heating: dissolve part of the compound, the dislocation dense area and subgrain boundary structure formed after continuous casting and rolling are partially recovered / recrystallized, the grain is preliminarily refined, but the grain may still be uneven; ②, the second and subsequent heating: after the first solid solution-quenching, the high dislocation area remaining in the matrix becomes a new recrystallization nucleation point, and the coarse grains that have not been refined are further divided and refined; ③, "crystal breaking" effect: each "heating-recrystallization-fast cooling" cycle is equivalent to "resetting" and further refining the existing grain structure, effectively preventing the trend of grain growth, and ultimately obtaining uniform and fine austenite grains, accompanied by a large number of dislocations.

[0045] Second, promote element diffusion and complete dissolution of compounds, details as follows:

[0046] Single solid solution defects: some stable intermetallic compounds (such as Ti and Mo-rich phases) may not be completely dissolved within the holding time of single solid solution. Undissolved compounds cannot play a role during aging and will become stress concentration points, damaging performance.

[0047] The application adopts the effect of cyclic solid solution: ①, short path diffusion: the fine grains produced by each cycle of recrystallization means a shorter atomic diffusion path, greatly promoting the uniform distribution of Al, Ti and other elements in the austenitic matrix; ②, repeated dissolution: some stable Ti or Al-rich micro phases may not be completely dissolved in a single solid solution. The cyclic treatment provides multiple opportunities for complete dissolution of these phases through multiple heating, ensuring that all alloying elements are fully solid-soluted into the austenitic matrix to form a highly supersaturated and compositionally uniform solid solution; ③, each heating cycle provides new opportunities and shorter diffusion paths for compound dissolution and element homogenization (because the structure is finer). The cumulative effect of multiple cycles is much better than single long time holding, which can more completely dissolve all unbalanced phases and achieve atomic-level high homogenization of elements.

[0048] Third, eliminate genetic organization, specifically: in some high-alloy steels, the original organization (such as dendritic segregation, banded structure) has "genetic" characteristics, and single solid solution treatment is difficult to completely eliminate this microscopic heterogeneity. The cyclic solid solution of the application can completely break down this genetic structure through repeated recrystallization to obtain a more isotropic homogeneous structure.

[0049] Fourth, cyclic solid solution creates the best conditions for aging precipitation, specifically: ①, prepare supersaturated solid solution: through the above effects, the cyclic solid solution ensures that Al and Ti atoms are uniformly and supersaturated in the form of single atoms in the matrix, which is a prerequisite for forming dispersed, nanoscale precipitates instead of coarse, harmful precipitates during subsequent aging; ②, provide a large number of nucleation sites: the fine grains produced by cyclic solid solution mean a large grain boundary area, and the recrystallization process will introduce a large number of crystal defects (such as dislocations), which will become the nucleation core for the preferential formation of precipitates during subsequent aging, making the precipitates more dispersed and fine, and the strengthening effect more significant.

[0050] In the aging strengthening stage: the precipitates are mainly nanoscale Ni3(Al, Ti) (L12 type) and a small amount of NiAl or Ni3Ti phase. The precipitation mechanism is as follows: after cyclic solid solution, Al and Ti are uniformly distributed in the austenite, and aging heating promotes the combination of Al, Ti and Ni to precipitate, the precipitates are coherent, the interface energy is low, and the strengthening effect is significant. The application also has a significant advantage in energy saving: element uniform diffusion is a prerequisite for obtaining good precipitation strengthening effect, and due to the characteristics of microsegregation and low thickness of thin strip continuous casting, homogenization annealing and multi-pass rolling of traditional cast blanks are not required.

[0051] Mechanism source: synergistic effect of precipitation strengthening (large amount of nanoscale precipitates), fine-grain strengthening (ultra-fine grains formed by cyclic solid solution) and dislocation strengthening (dislocations accompanying grain boundaries), realizing strong and tough stainless steel. The three synergistically improve the comprehensive performance of strength and plasticity, breaking through the traditional inverted relationship.

[0052] (3) Performance is improved significantly: by using the technical scheme of the application, the yield strength is improved to 550-600 MPa, the tensile strength reaches 750-850 MPa, and the performance is improved significantly on the basis of maintaining good elongation.

[0053] Comparative Examples 1-3:

[0054] The differences between Comparative Examples 1-3 and Example 1 are shown in Table 1.

[0055] Table 1 Comparison of technical schemes and key parameters of Example 1-3 and Comparative Examples 1-3

[0056]

[0057]

[0058] From Examples 1-3 and Comparative Examples 1-3, it can be seen that:

[0059] I. Technical scheme and effect of Examples 1-3

[0060] The common features of Examples 1-3 are as follows:

[0061] ① Creativity of component design

[0062] A small amount of Ti (0.1-0.4%) and Al (0.3-0.8%) are introduced into the 304 system to form Ni3Ti and NiAl nano precipitates in the matrix by Ni.

[0063] Previous studies mainly rely on cold working or Nb / Mo strengthening, while Ti-Al-Ni precipitation strengthening has not been applied in the short process of 304 steel, which is obviously novel.

[0064] ② Creativity of process route

[0065] Traditional precipitation strengthening requires "continuous casting billet → homogenization annealing → multi-pass rolling → solid solution → aging".

[0066] The application directly prepares thin strips by TRC, with small segregation scale, and realizes element redistribution by combining with cyclic solid solution, thereby fundamentally omitting homogenization annealing and multi-pass rolling, which belongs to process innovation.

[0067] ③ Uniqueness of strengthening mechanism

[0068] Cyclic solid solution-induced recrystallization breaks down existing grain boundaries, forming new fine grains. This process generates numerous grain boundaries while introducing dislocations, ensuring elemental homogeneity. During the aging stage, nanoscale Ni3(Al,Ti) precipitates, synergistically strengthening the grains with the high dislocation density, achieving a triple superposition of precipitation strengthening, grain refinement strengthening, and dislocation strengthening. This mechanism effectively breaks through the traditional strength-plasticity inversion relationship of 304 stainless steel, representing a novel mechanism.

[0069] ④ Significance of the overall effect

[0070] Compared to conventional 304 stainless steel (UTS≤650MPa, YS≤280MPa), the embodiments of this invention achieve UTS 830 MPa and YS 600MPa while maintaining an elongation rate of ≥50%. Simultaneously, it achieves process shortening and energy consumption reduction of 20–30%, demonstrating significant engineering advantages in industrial applications.

[0071] This invention, through a systematic coupling of composition, process, microstructure, and properties, achieves for the first time an organic combination of Ti-Al-Ni precipitation strengthening and short-process continuous casting-rolling-cyclic solution treatment in 304 stainless steel. This approach not only achieves nearly twice the strength of traditional 304 stainless steel strips in terms of mechanical properties, but also maintains excellent ductility and corrosion resistance, while significantly reducing process energy consumption, demonstrating outstanding inventiveness and significant practical value.

[0072] II. Comparison of Comparative Examples 1–3 with the technical solutions of the present invention

[0073] ① The strength of the stainless steel strip of the present invention is significantly improved, as follows:

[0074] Comparative Example 1 (without Ti / Al) and Comparative Example 3 (without heat treatment) both showed UTS≤600MPa, which is much lower than that of Examples 1-3.

[0075] Although Comparative Example 2 was processed using traditional methods, its UTS was only 650 MPa, indicating that even with precipitation, the strengthening effect was limited due to grain coarsening.

[0076] All embodiments of the present invention achieve a UTS of 770–830 MPa and a yield strength of 550–600 MPa, thus doubling the strength.

[0077] ② The elongation remains good, as follows:

[0078] The elongation rate of Comparative Example 2 was only 45%, indicating that traditional processes are difficult to achieve both high strength and high plasticity.

[0079] All embodiments of the present invention maintain an elongation of 50–60%, which is comparable to or even better than that of cold-rolled 304, indicating that the synergistic effect of precipitation strengthening, grain refinement strengthening, and dislocation strengthening effectively overcomes the problem of strength-plasticity inversion.

[0080] ③, organizational control advantage, as follows:

[0081] Example 3 is directly cooled, and the strength is insufficient. Although example 2 is subjected to multiple heat treatments, the energy consumption is high, and the grain is coarse.

[0082] Embodiments 1-3 of the present application obtain 5-10 μm ultra-fine grains and dispersed nano precipitates by TRC rapid solidification + cyclic solid solution + aging, and have strength, ductility and organizational stability.

[0083] ④, process simplification and energy saving, as follows:

[0084] Example 2: requires homogenization annealing (high temperature for a long time) + multiple hot rolling, complex process, high energy consumption.

[0085] In embodiments 1-3: omitting homogenization annealing and multiple hot rolling, using continuous casting waste heat + cyclic solid solution, the overall energy consumption is reduced by 20-30%, and the production cycle is shortened by about 50%.

[0086] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A short process method for producing a high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation, characterized by, The method comprises the following steps: The steel liquid component design and smelting, the steel liquid component design comprises: selecting 304 stainless steel, including: C≤0.03%, Si is 0.3-0.6%, Mn is 1.0-1.5%, Cr is 17-18.5%, Ni is 8-9.5%, Ti is 0.1-0.4%, Al is 0.3-0.8%, N is 0.02-0.06%, and the balance is iron, by mass fraction; after smelting, an alloy liquid is obtained; Double-roller thin strip continuous casting, the alloy liquid is poured between double rollers under the protection of inert gas atmosphere, and an initial thin strip is formed by adopting a double-roller thin strip continuous casting process; the temperature of continuous casting is controlled at 1490-1540 DEG C; the roller speed is controlled at 70-100 m / min; and the cooling rate is 1000-2000 DEG C / s; Waste heat hot rolling, residual heat after the continuous casting process is utilized for hot rolling processing, and a hot-rolled thin strip is obtained; Cyclic solid solution treatment, specifically including: the thin strip is subjected to at least two times of solid solution treatment; Aging strengthening treatment, specifically including: the thin strip after the cyclic solid solution treatment is heated to 500 DEG C-600 DEG C and kept for 15 min-3 h; and the 304 stainless steel thin strip is obtained after cooling; In the waste heat hot rolling: the temperature of the initial thin strip is 1000-1200 DEG C; 1-2 passes of rolling are performed by utilizing the self residual heat; the total reduction rate is less than or equal to 50%; the thickness of the hot-rolled thin strip is 0.5-1.3 mm; and the spray water cooling mode is adopted for cooling after rolling; The cyclic solid solution treatment specifically includes: The first solid solution treatment, specifically including: the hot-rolled thin strip is heated to 600-1000 DEG C and kept for 10-30 min, and then water-quenched to room temperature to obtain a stainless steel thin strip; The second solid solution treatment, specifically including: the stainless steel thin strip is heated to 600-1000 DEG C and kept for 10-30 min, and then water-quenched to room temperature.

2. The short process production method of high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation according to claim 1, characterized in that, In the double-roller thin strip continuous casting: the inert gas is nitrogen with a purity of greater than or equal to 99.99%; the material of the double roller is copper, the diameter of the copper double roller is 300-500 mm; and the thickness of the obtained initial thin strip is 1.0-1.9 mm.

3. The short process method for preparing a high-strength stainless steel thin strip based on cyclic solid solution and aging precipitation according to claim 1, characterized in that, The cyclic solid solution treatment further includes a third solid solution treatment, specifically including: the stainless steel thin strip after the second solid solution treatment is heated to 600-1000 DEG C and kept for 10-30 min, and then water-quenched to room temperature.

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