Annealing method for improving intergranular corrosion resistance of super ferrite stainless steel

Abstract

The present invention relates to the technical field of ferritic stainless steel, specifically an annealing method for improving the intergranular corrosion resistance of super ferritic stainless steel. To address the problem of how to improve the recrystallization annealing process to enhance the intergranular corrosion resistance of stainless steel plates while maintaining fine grains, a novel annealing method for improving the intergranular corrosion resistance of super ferritic stainless steel is provided. The method comprises three continuous annealing stages: a pre-precipitation annealing to pre-precipitate a nanoscale Laves phase; a recrystallization annealing to pin grain boundaries and refine grains; and finally, a resolubility annealing to dissolve the Laves phase at high temperature, dissolving Nb, Mo, and Si elements near the grain boundaries and precipitating Nb (C, N) particles. The drag effect of the solute Nb, Mo, and Si suppresses grain coarsening, thereby enhancing the intergranular corrosion resistance of the super ferritic stainless steel while maintaining fine grains and ensuring its mechanical properties.

Description

Technical Field

[0001] The present invention relates to the technical field of ferritic stainless steel, in particular to an annealing method for improving the intergranular corrosion resistance of super ferritic stainless steel. Background Art

[0002] Super ferritic stainless steel is a high-chromium, high-molybdenum, aluminum-containing ferritic stainless steel. Its main elements Cr and Mo are used to improve the corrosion resistance of ferritic stainless steel. Nb and Ti elements are added to stabilize the C and N content in the steel to improve the intergranular corrosion resistance of the steel plate and ensure the formation of Laves phase. Compared with traditional super ferritic stainless steel, the addition of Al element fundamentally inhibits the precipitation of σ phase, promotes the precipitation of nano-scale Laves phase, and plays a role in pinning grain boundaries and refining grains. For example, the super ferritic stainless steel described in the patent with announcement number CN115652224B and patent name is a super super ferritic stainless steel and its preparation method. During the preparation process of the super super ferritic stainless steel, Nb2C particles are transformed into nano-scale Laves phase (Fe2Nb) and precipitate at grain boundaries and within grains, releasing some C elements into the matrix, which easily increases the intergranular corrosion sensitivity of the material, thereby reducing the intergranular corrosion resistance of the stainless steel.

[0003] Existing technologies, such as Chinese patent CN114150115B, disclose an annealing method for high-chromium super ferritic stainless steel sheets resistant to intergranular corrosion. By performing a homogenization annealing at a temperature of 750-830°C after recrystallization annealing, chromium atoms diffuse into the chromium-depleted regions at the grain boundaries, thereby improving intergranular corrosion resistance. However, since the temperature used during the recrystallization annealing is 1040-1080°C, grain coarsening still occurs. Therefore, how to improve the recrystallization annealing process to improve the intergranular corrosion resistance of the stainless steel sheet while ensuring fine grains is a current research challenge. Summary of the Invention

[0004] In order to solve the problem of how to improve the recrystallization annealing process to improve the intergranular corrosion resistance of stainless steel plates while ensuring fine grains, the present invention provides a new annealing method for improving the intergranular corrosion resistance of super ferritic stainless steel.

[0005] The present invention is achieved by adopting the following technical solutions:

[0006] An annealing method for improving the intergranular corrosion resistance of super ferrite stainless steel comprises the following steps:

[0007] 1) Pre-precipitation annealing: Heat the super-super ferritic stainless steel cold-rolled sheet to 950-1000°C and hold for 5-30 minutes to obtain a pre-precipitation annealed sheet. (Since the Al element in the super-super ferritic stainless steel does not form a brittle phase when held at 950-1000°C, sufficient nano-scale Laves phase can be pre-precipitated, forming nano-scale Nb (C, N) particles to reduce the C and N content in the matrix. At the same time, chromium atoms are fully diffused into the chromium-depleted areas at the grain boundaries, reducing the degree of chromium depletion and improving intergranular corrosion resistance. If the first annealing temperature is less than 950°C, sufficient nano-scale Laves phase cannot be precipitated, making it difficult to achieve the subsequent grain refinement effect.)

[0008] 2) Recrystallization annealing: The pre-precipitated annealed plate obtained in step 1) is further heated to 1000-1050°C for 1-60 minutes to obtain a recrystallized annealed plate. (This step completes recrystallization, utilizing the nanoscale Laves phase pre-precipitated in step 1) to pin grain boundaries, refine grains, and improve the strength and toughness of the stainless steel plate. Recrystallization annealing temperatures below 1000°C will result in incomplete recrystallization, while recrystallization annealing temperatures above 1050°C will coarsen grains and reduce mechanical properties.)

[0009] 3) Solution Annealing: The recrystallized annealed sheet obtained in step 2) is further heated to 1050-1100°C for a holding time of 1-15 minutes, with a controlled cooling rate of 50-150°C / s. (This high temperature, short holding time, dissolves the Laves phase at the grain boundaries and nearby, dissolving Nb, Mo, and Si near the grain boundaries. Simultaneously, Nb(C, N) particles are precipitated, further reducing the C and N content in the matrix and improving the intergranular corrosion resistance of the stainless steel sheet. The drag effect of the Nb, Mo, and Si solute elements further suppresses grain coarsening, ensuring good mechanical properties. A third-stage annealing temperature below 1050°C makes it difficult to dissolve the Laves phase at the grain boundaries, preventing the formation of sufficient Nb, Mo, and Si solute elements and the precipitation of Nb(C, N) particles, insufficiently improving the material's intergranular corrosion resistance. A third-stage annealing temperature above 1100°C results in rapid grain growth and reduced mechanical properties.

[0010] Furthermore, the weight percentages of the elements in the super ferritic stainless steel are as follows: 25% ≤ Cr ≤ 30.0%, 1.5% ≤ Mo ≤ 5.0%, 1.0% ≤ Ni ≤ 4.0%, C ≤ 0.015%, N ≤ 0.015%, 0.5% ≤ Al ≤ 6.0%, 0.10% ≤ Nb ≤ 0.60%, 0.10% ≤ Ti ≤ 0.30%, Mn ≤ 0.4%, Si ≤ 0.6%, S ≤ 0.005%, P ≤ 0.005%, O ≤ 0.01%, and satisfying Cr + 3.3 × Mo ≥ 35%, with the remainder being Fe and unavoidable impurities. The super ferritic stainless steel composed of these weight percentages is particularly suitable for this annealing method, and exhibits particularly good intergranular corrosion resistance and mechanical properties.

[0011] Furthermore, the thickness of the super super ferrite stainless steel cold-rolled sheet in step 1) is ≤3.0 mm.

[0012] The beneficial effects of the present invention are as follows: The present invention provides an annealing method for improving the intergranular corrosion resistance of super ferritic stainless steel, which uses three-stage continuous annealing to pre-precipitate a nanoscale Laves phase, which plays a role in pinning grain boundaries and refining grains during the recrystallization process. The Laves phase is then dissolved at high temperature to dissolve Nb, Mo, and Si elements near the grain boundaries, precipitating Nb (C, N) particles. The drag effect of the solute elements Nb, Mo, and Si is used to suppress grain coarsening, thereby improving the intergranular corrosion resistance of the super ferritic stainless steel while ensuring the fine grains of the super ferritic stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0015] Figure 1 : is a microstructure diagram of the super ferrite stainless steel plate after being treated by the annealing method of Example 1;

[0016] Figure 2 This is a surface morphology of the super ferrite stainless steel plate treated by the annealing method of Example 1 after boiling for 120 hours using the copper-copper sulfate-50% sulfuric acid corrosion test method. DETAILED DESCRIPTION

[0017] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features therein can be combined with each other.

[0018] In the description, it should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance. It should be noted that, unless otherwise expressly specified or limited, the terms "installed," "connected," and "connected" should be understood broadly. For example, they can refer to fixed connections, removable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms based on specific circumstances.

[0019] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all the embodiments.

[0020] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. Example 1:

[0021] Taking a 3 mm thick super ferritic stainless steel cold-rolled sheet as an example, the weight percentages of the elements of the super ferritic stainless steel are as follows: 30.0% Cr, 1.7% Mo, 1.0% Ni, 0.015% C, 0.015% N, 6.0% Al, 0.10% Nb, 0.30% Ti, 0.4% Mn, 0.6% Si, 0.005% S, 0.005% P, 0.01% O, and the rest are Fe and unavoidable impurities. The annealing steps include the following:

[0022] 1) Pre-precipitation annealing: Heat the super ferritic stainless steel cold-rolled sheet to 1000°C and hold for 30 minutes to obtain a pre-precipitation annealed sheet;

[0023] 2) Recrystallization annealing: The pre-precipitation annealed sheet obtained in step 1) is further heated to 1050°C and kept at this temperature for 60 minutes to obtain a recrystallization annealed sheet;

[0024] 3) Solution annealing: The recrystallized annealed plate obtained in step 2) was further heated to 1100°C for 15 min, with a cooling rate of 50°C / s. Example 2:

[0025] Taking a 2 mm thick super ferritic stainless steel cold-rolled sheet as an example, the weight percentages of the elements of the super ferritic stainless steel are as follows: 25.0% Cr, 5.0% Mo, 4.0% Ni, 0.015% C, 0.015% N, 0.5% Al, 0.60% Nb, 0.10% Ti, 0.4% Mn, 0.6% Si, 0.005% S, 0.005% P, 0.01% O, and the remainder being Fe and unavoidable impurities. The annealing steps include the following:

[0026] 1) Pre-precipitation annealing: Heat the super ferritic stainless steel cold-rolled sheet to 950°C and hold for 5 minutes to obtain a pre-precipitation annealed sheet;

[0027] 2) Recrystallization annealing: The pre-precipitation annealed sheet obtained in step 1) is further heated to 1050°C and kept at this temperature for 1 minute to obtain a recrystallization annealed sheet;

[0028] 3) Solution annealing: The recrystallized annealed plate obtained in step 2) was further heated to 1100°C, kept at this temperature for 1 min, and the cooling rate was controlled at 150°C / s. Example 3:

[0029] Taking a 2.5 mm thick super ferritic stainless steel cold-rolled sheet as an example, the weight percentages of the elements of the super ferritic stainless steel are as follows: 27.5% Cr, 3.5% Mo, 1.8% Ni, 0.015% C, 0.015% N, 1.5% Al, 0.28% Nb, 0.16% Ti, 0.4% Mn, 0.6% Si, 0.005% S, 0.005% P, 0.001% O, and the rest are Fe and unavoidable impurities. The annealing steps include the following:

[0030] 1) Pre-precipitation annealing: Heat the super ferritic stainless steel cold-rolled sheet to 980°C and hold for 15 minutes to obtain a pre-precipitation annealed sheet;

[0031] 2) Recrystallization annealing: The pre-precipitation annealed sheet obtained in step 1) is further heated to 1030°C and kept at this temperature for 10 minutes to obtain a recrystallization annealed sheet;

[0032] 3) Solution annealing: The recrystallized annealed plate obtained in step 2) was further heated to 1080°C for 10 min, and the cooling rate was controlled at 80°C / s.

[0033] Examples 1-3 use the annealing method of the present invention to improve the intergranular corrosion resistance of the stainless steel plate while ensuring fine grains. The microstructure is observed through an optical microscope. Figure 1, and the average grain size is 40.36μm; according to Method Y in GB / T 32571-2016 "Corrosion of Metals and Alloys - Test Method for Intergranular Corrosion of High-Chromium Ferritic Stainless Steel", the copper-copper sulfate-50% sulfuric acid corrosion test method was used, and the test was boiled for 120h. No signs of intergranular corrosion (i.e., grain shedding) were observed under a metallographic microscope. Figure 2 Since the microstructure diagrams and corrosion resistance test diagrams of the three embodiments are basically the same, this application only includes the diagram of embodiment 1)

[0034] The above description is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Although detailed descriptions have been made with reference to the aforementioned embodiments, those skilled in the art should understand that they may still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents; and such modifications or replacements do not deviate from the essence of the corresponding technical solutions within the scope of the technical solutions of the embodiments, and they should all be included in the scope of protection of the claims.

Claims

1. An annealing method for improving the intergranular corrosion resistance of super ferrite stainless steel, characterized in that The weight percentage of each element in super ferritic stainless steel is: 25%≤Cr≤30.0%, 1.5%≤Mo≤5.0%, 1.0%≤Ni≤4.0%, C≤0.015%, N≤0.015%, 0.5%≤Al≤6.0%, 0.10%≤Nb≤0.60%, 0.10%≤Ti≤0.30%, Mn≤0.4%, Si≤0.6%, S≤0.005%, P≤0.005%, O≤0.01%, and satisfy Cr+3.3×Mo≥35%, the rest is Fe and unavoidable impurities The annealing method comprises the following steps: 1) pre-precipitation annealing: heating the ultra-super ferritic stainless steel cold-rolled sheet to 950-1000° C. and holding the sheet at this temperature for 5-30 minutes to obtain a pre-precipitation annealed sheet; 2) recrystallization annealing: continuing to heat the pre-precipitation annealed sheet obtained in step 1) to 1000-1050° C. and holding the sheet at this temperature for 1-60 minutes to obtain a recrystallization annealed sheet; and 3) dissolution annealing: continuing to heat the recrystallization annealed sheet obtained in step 2) to 1050-1100° C. and holding the sheet at this temperature for 1-15 minutes, with the cooling rate controlled at 50-150° C. / s.

2. The annealing method for improving the intergranular corrosion resistance of super ferrite stainless steel according to claim 1, characterized in that: The thickness of the super ferritic stainless steel cold-rolled plate in step 1) is ≤3.0 mm.

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