A corrosion-resistant high-strength duplex stainless steel and its preparation method

Abstract

The present invention provides a corrosion-resistant high-strength duplex stainless steel, comprising, by weight percentage: C 0.09% to 0.14%, Si ≤ 0.80%, Mn ≤ 0.80%, P ≤ 0.035%, S ≤ 0.030%, Cr 20.00% to 22.00%, Ni 4.80% to 5.80%, 0 < Mo ≤ 0.20%, Ti 0.35% to 0.65%, 0 < Al ≤ 0.13%, 0 < N ≤ 0.035%, with the remainder being iron and unavoidable impurities. The present invention also provides a method for preparing the corrosion-resistant high-strength duplex stainless steel, comprising: (1) smelting to obtain a steel ingot; (2) performing upsetting, drawing and forging on the steel ingot to obtain an intermediate billet; (3) performing a staged heat treatment on the intermediate billet, and then cooling. The corrosion-resistant high-strength duplex stainless steel provided by the present invention has significantly improved hot working properties and excellent intergranular corrosion resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of stainless steel production and manufacturing, and in particular relates to a corrosion-resistant high-strength duplex stainless steel and a preparation method thereof. Background Art

[0002] Ferrite-austenite corrosion-resistant high-strength duplex stainless steel has high strength and good corrosion resistance in oxidizing and organic acids. Therefore, it can be used to manufacture components of equipment that are resistant to oxidizing acid corrosion and have high strength. It is widely used in industry, food industry, medical industry, etc.

[0003] However, due to its high Ti content, the presence of TiC hard and brittle phase in the steel makes the current ferrite-austenite corrosion-resistant high-strength duplex stainless steel difficult to manufacture and has poor intergranular corrosion performance, which seriously restricts its production and application. Summary of the Invention

[0004] In order to solve all or part of the above problems, the purpose of the present invention is to provide a corrosion-resistant high-strength duplex stainless steel and a preparation method thereof, so as to solve the problems of the current ferrite-austenite corrosion-resistant high-strength duplex stainless steel, such as the difficulty in manufacturing and the poor intergranular corrosion performance.

[0005] Specifically, the present invention is achieved through the following technical solutions:

[0006] In one aspect, the present invention provides a corrosion-resistant, high-strength duplex stainless steel comprising, by weight percentage:

[0007] C 0.09%~0.14%, Si≤0.80%, Mn≤0.80%, P≤0.035%, S≤0.030%, Cr20.00%~22.00%, Ni 4.80%~5.80%, 0<Mo≤0.20%, Ti 0.35%~0.65%, 0<Al≤0.13%, 0<N≤0.035%, the balance is iron and unavoidable impurities.

[0008] Optionally, the contents of Ti and C satisfy Ti / (C-0.02)≥5.

[0009] Optionally, the contents of C, N, and Al satisfy 0.85≤(C+N) / Al≤2.12.

[0010] Optionally, the composition comprises, by weight percentage:

[0011] C 0.10%~0.11%, Si≤0.35%, Mn≤0.45%, P≤0.020%, S≤0.001%, Cr21.40%~21.60%, Ni 5.40%~5.45%, 0<Mo≤0.02%, Ti 0.45%~0.50%, 0<Al≤0.08%, 0<N≤0.012%, the balance is iron and unavoidable impurities.

[0012] On the other hand, the present invention provides a method for preparing corrosion-resistant high-strength duplex stainless steel, comprising the following steps:

[0013] (1) Smelting to obtain steel ingots;

[0014] (2) The steel ingot is subjected to upsetting, drawing and forging to obtain an intermediate billet;

[0015] (3) The intermediate billet is subjected to a stepwise heat treatment and then cooled.

[0016] Optionally, in step (2), the heating temperature of the upsetting and drawing forging is 1220°C to 1260°C, and the final forging temperature is not lower than 950°C.

[0017] Optionally, in step (2), the upsetting deformation in each upsetting and drawing forging pass is not less than 35%.

[0018] Optionally, in step (2), the number of passes of upsetting and drawing forging of the steel ingot is determined according to the diameter of the corrosion-resistant high-strength duplex stainless steel; wherein:

[0019] The diameter of the corrosion-resistant high-strength duplex stainless steel is less than 300 mm, and the steel ingot is subjected to one upsetting and drawing forging process;

[0020] Alternatively, the diameter of the corrosion-resistant high-strength duplex stainless steel is 300 mm to 500 mm, and the steel ingot is subjected to two passes of upsetting and drawing forging;

[0021] Alternatively, the diameter of the corrosion-resistant high-strength duplex stainless steel is greater than 500 mm, and the steel ingot is subjected to upsetting, drawing and forging for more than three times.

[0022] Optionally, in step (3), the staged heat treatment includes performing a solution treatment on the intermediate billet at a temperature T1 for a time t1 and a stabilization heat treatment at a temperature T2 for a time t2;

[0023] Wherein, T1=exp[a+b×Ti+c×(Ti)^2], t1=(1-4)×d / 2 minutes, where Ti is the weight percentage of elemental titanium, a, b, and c are dimensionless, with values ​​of a=7.1937, b=-0.4018, and c=0.1793, respectively, and d is the diameter of the corrosion-resistant high-strength duplex stainless steel in millimeters;

[0024] Among them, T2 is 880℃~930℃, t2=t1 / 2.

[0025] It can be seen from the above technical solutions that the corrosion-resistant high-strength duplex stainless steel and the preparation method thereof of the present invention have at least the following beneficial effects:

[0026] The present invention optimizes and improves the elemental composition and production process of corrosion-resistant high-strength duplex stainless steel, thereby significantly improving its hot working performance and intergranular corrosion resistance, and perfectly meeting downstream production and application requirements. DETAILED DESCRIPTION

[0027] In order to fully understand the purpose, features and effects of the present invention, the present invention is described in detail through the following specific embodiments. Except for the following contents, the process of the present invention adopts conventional methods or devices in the art. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0028] In response to the common problems of ferrite-austenite corrosion-resistant high-strength duplex stainless steel, such as the difficulty in manufacturing and poor intergranular corrosion performance, the inventors of the present invention conducted in-depth research on the elemental composition and preparation methods of stainless steel, and thus creatively proposed a corrosion-resistant high-strength duplex stainless steel and its preparation method.

[0029] In a first aspect, the present invention provides a corrosion-resistant high-strength duplex stainless steel comprising, by weight percentage:

[0030] C 0.09%~0.14%, Si≤0.80%, Mn≤0.80%, P≤0.035%, S≤0.030%, Cr20.00%~22.00%, Ni 4.80%~5.80%, 0<Mo≤0.20%, Ti 0.35%~0.65%, 0<Al≤0.13%, 0<N≤0.035%, the balance is iron and unavoidable impurities.

[0031] The present invention optimizes the element composition and ratio of stainless steel to achieve synergistic effects between the elements, as follows:

[0032] C primarily plays a role in solid solution strengthening. Since the present invention requires higher strength, the minimum C content is controlled to be above 0.09%. However, excessive C will lead to the precipitation of a large amount of carbides, which has a harmful effect, especially on intergranular corrosion resistance. Therefore, the maximum C content is limited to below 0.14%.

[0033] Si and Mn are harmful elements in the stainless steel of the present invention and will reduce the thermoplasticity of the alloy. Therefore, the content of Si and Mn is controlled to be below 0.80%.

[0034] Cr is a crucial alloying element in stainless steel. Its corrosion resistance increases with increasing chromium content. When the chromium content is high, more nickel is needed to form a duplex (ferrite-austenite) structure. Therefore, the Cr content is controlled between 20.00 and 22.00%.

[0035] Nickel is the main element of steel. The nickel content of duplex stainless steel is generally 1.5-7%. Nickel is an element that stabilizes austenite. Adding nickel delays the formation of harmful intermetallic phases in stainless steel. However, when the Ni content exceeds 20%, the manufacturing cost increases and the microstructure control of duplex stainless steel is not conducive. Therefore, the Ni content is controlled at 4.80% to 5.80%.

[0036] Titanium (Ti) has a strong affinity for nitrogen, oxygen, and carbon, and its affinity for sulfur is even stronger than that of iron. It is an excellent deoxidizer and degasser, and an effective element for fixing nitrogen and carbon. In stainless steel, it is often used to fix carbon, eliminating chromium depletion at grain boundaries, thereby eliminating or reducing intergranular corrosion. However, the steel of this invention contains a large amount of nitrogen. If the Ti content is too high, excessive titanium nitride will form, severely reducing the material's plasticity. Therefore, the Ti content is controlled to 0.35-0.65%.

[0037] Nitrogen is a strong austenite-forming element. It can partially replace nickel in duplex stainless steel and be adjusted with the nickel content to achieve an appropriate phase balance. However, excessive nitrogen can reduce the hot plasticity of the steel and, in the steel of this invention, form hard titanium nitride with titanium. Therefore, the nitrogen content should be controlled to ≤ 0.035%.

[0038] P and S are harmful elements in steel, and their content needs to be controlled at P≤0.035% and S≤0.030%.

[0039] Mo is a ferrite-forming element. While a moderate amount of Mo can improve stainless steel's resistance to pitting and crevice corrosion, it also increases the tendency of stainless steel to form intermetallic phases. Excessive Mo can also form a large amount of σ phase, severely reducing the material's plasticity and toughness. Therefore, the Mo content should be controlled below 0.20%.

[0040] Al is a ferrite-forming element. Its addition refines the grain size of duplex steel, thereby increasing its strength and raising the brittle temperature range of stainless steel. However, excessive Al can form inclusions, reducing the material's metallurgical quality. Therefore, the Al content should be controlled to ≤ 0.13%, preferably ≤ 0.08%.

[0041] Furthermore, elements such as C, Ni, Mn, and N are austenite-forming elements, while Cr, Mo, Ti, and Al are ferrite-forming elements. The present invention is a duplex stainless steel requiring the simultaneous presence of austenite and ferrite in its structure, with the single-phase content being within a range of 40% to 60%. Furthermore, to improve steel quality, the present invention also strictly controls elements such as Mo and Al within a certain range.

[0042] In a preferred embodiment, the contents of Ti and C satisfy Ti / (C-0.02) ≥ 5, for example, Ti / (C-0.02) is 5.5, 5.75, or 6. The inventors have discovered that by further optimizing the contents of Ti and C to meet the aforementioned requirements, Ti can better play its role in fixing carbon in stainless steel, thereby eliminating the adverse effects of C.

[0043] In another preferred embodiment, the contents of C, N, and Al satisfy 0.85≤(C+N) / Al≤2.12, for example, (C+N) / Al is 1.5, 1.6, 1.8, or 1.9. The duplex stainless steel of the present invention effectively controls the C and N contents by adding an appropriate amount of Al. By ensuring that the C, N, and Al contents meet the aforementioned requirements, the C and N contents can be effectively controlled, intergranular corrosion resistance can be ensured, and the strength of the duplex steel can be guaranteed.

[0044] In a more preferred embodiment, the corrosion-resistant high-strength duplex stainless steel of the present invention comprises, by weight percentage:

[0045] C 0.10%~0.11%, Si≤0.35%, Mn≤0.45%, P≤0.020%, S≤0.001%, Cr21.40%~21.60%, Ni 5.40%~5.45%, 0<Mo≤0.02%, Ti 0.45%~0.50%, 0<Al≤0.08%, 0<N≤0.012%, the balance is iron and unavoidable impurities.

[0046] In a second aspect, the present invention provides a method for preparing corrosion-resistant high-strength duplex stainless steel, comprising:

[0047] (1) Smelting

[0048] According to the requirements of the above-mentioned elemental composition, conventional processes are adopted to smelt steel ingots. For example, the "electric furnace smelting + VOD refining + electroslag remelting" process is adopted to smelt steel ingots. For details, please refer to the relevant technical solutions in the prior art, which will not be described here.

[0049] However, it should be noted that the C and Ti contents must be strictly controlled throughout the smelting process. Specifically, the C content is 0.09% to 0.14%, and should be kept within the lower limit as excessive C can cause a decrease in the material's intergranular corrosion resistance. The Ti content is 0.35% to 0.65%, and the C, N, and Al contents must meet a certain ratio of 0.85 ≤ (C + N) / Al ≤ 2.12. Otherwise, Ti will not effectively solidify C and improve performance.

[0050] (2) Forging

[0051] In the present invention, a multi-pass upsetting and drawing process is used to forge the steel ingot. The heating temperature during the forging process is 1220°C to 1260°C, and the final forging temperature is not less than 950°C.

[0052] Based on the inventor's research, the present invention relates to stainless steel materials with a narrow hot working range, and the forging heating temperature and final forging temperature must be strictly controlled. If the heating temperature is too high, the material will be overburned and then cracked during forging. If the final forging temperature is too low, a large amount of σ phase will be produced, reducing the plastic toughness of the material.

[0053] During the upsetting and drawing forging process, the upsetting deformation per pass must be no less than 35%. The upper limit of the upsetting deformation can be adjusted based on the deformation process designed for the finished product specifications and performance requirements, and is set based on the principle of preventing cracking after upsetting deformation. The product of this invention requires high performance, requiring sufficient internal deformation of the material and achieving a uniform and fine grain size. Therefore, the upsetting deformation limit must not be less than 35%, otherwise the deformation will be insufficient.

[0054] The number of upsetting passes during the forging process (i.e., the number of upsetting and drawing passes) corresponds to the product specifications.

[0055] In one embodiment, if the diameter of the stainless steel product is less than 300 mm, the steel ingot is subjected to one upsetting and drawing forging process, that is, upsetting once.

[0056] In another embodiment, the diameter of the stainless steel product is 300-500 mm, and the steel ingot is subjected to two upsetting and drawing forging passes, that is, upsetting twice.

[0057] In another embodiment, if the diameter of the stainless steel product is greater than 500 mm, the steel ingot is subjected to upsetting and drawing forging for more than three times, that is, the number of upsetting times is ≥3 times.

[0058] Based on the inventor's research, forging deformation includes upsetting deformation and drawing deformation, and the total deformation is the combined effect of the two. If the product diameter is large, that is, the deformation of drawing forging is relatively small, then the upsetting deformation needs to be increased. Conversely, if the product diameter is small, that is, the deformation of drawing forging is relatively large, then an appropriate upsetting deformation can meet the requirements of the total forging deformation.

[0059] (3) Heat treatment

[0060] The present invention adopts a staged heat treatment process. The intermediate billet obtained in the forging step is first subjected to a solution treatment at a temperature T1 for a time t1, and then subjected to a stabilization heat treatment at a temperature T2 for a time t2. After the heat treatment, it is cooled, for example, by water cooling.

[0061] In a preferred embodiment, the temperature T1 of the solution treatment is T1 = exp[a + b × Ti + c × (Ti)^2], wherein Ti is the weight percentage of elemental titanium. For example, the content of elemental titanium in the stainless steel of the present invention is 0.50%, then the value of Ti in the above formula is 0.50; a, b, and c are dimensionless, and the values ​​are a = 7.1937, b = -0.4018, and c = 0.1793, respectively; the time t1 of the solution treatment is t1 = (1 to 4) × d / 2 minutes, for example, t1 = 1.5 × d / 2 minutes, t1 = 2 × d / 2 minutes, or t1 = 2.5 × d / 2 minutes, wherein d is the diameter of the corrosion-resistant high-strength duplex stainless steel (i.e., the final product) in millimeters.

[0062] It should be noted that exp is an exponential function. For example, EXP{F(X)} is e raised to the power of F(X). "^2" represents a quadratic function. For example, a^2 is a 2 .

[0063] In another preferred embodiment, the temperature T2 of the stabilization heat treatment is 880° C. to 930° C., and the holding time is 1 / 2 of the time of the solution treatment, that is, t2=t1 / 2.

[0064] In the present invention, the purpose of solution treatment is to improve the strength and hardness of the material, refine the grains, and enhance the plasticity and toughness of the material. The purpose of stabilization treatment is to form fully stable TiC from Ti and C. Stabilization treatment can greatly improve the intergranular corrosion resistance of the steel. By adopting the solution treatment and stabilization treatment system of the present invention, it is possible to effectively refine the grains and improve the intergranular corrosion resistance.

[0065] Example

[0066] The present invention is further illustrated by way of examples below, but the present invention is not limited to the scope of the examples. In the following examples, the experimental methods without specific conditions are based on conventional methods and conditions.

[0067] Table 1 Unit: weight %

[0068] C Si Mn P S Cr Ni Mo Ti Al N Fe Example 1 0.1 0.34 0.43 0.018 0.001 21.47 5.41 0.01 0.48 0.07 0.011 margin Example 2 0.1 0.33 0.41 0.016 0.001 21.53 5.42 0.02 0.46 0.06 0.012 margin Example 3 0.11 0.34 0.42 0.017 0.001 21.51 5.43 0.01 0.50 0.08 0.011 margin

[0069] Example 1:

[0070] The actual composition of the corrosion-resistant, high-strength duplex stainless steel in this example is shown in Table 1 above. The finished forging dimensions are φ290 × 6500 mm. The ingot was heated to 1240°C and finished at 960°C. It underwent a single upset forging process, with an upset deformation of 40%. The heat treatment process consisted of first cooling at 1150°C (5 hours) followed by water cooling, and then cooling at 900°C (2.5 hours) followed by water cooling. Intergranular corrosion performance tested according to GB / T 4334E was satisfactory.

[0071] Example 2:

[0072] The actual composition of the corrosion-resistant, high-strength duplex stainless steel in this example is shown in Table 1 above. The finished forging dimensions are φ350 × 4500 mm. The ingot was heated to 1250°C and finished at 950°C. The steel was subjected to two upset forging passes, with upset deformations of 40% and 35%, respectively. The heat treatment process consisted of a 6-hour water cooling at 1150°C followed by a 3-hour water cooling at 890°C. Intergranular corrosion performance was tested according to GB / T4334E and met the requirements.

[0073] Example 3:

[0074] The actual composition of the corrosion-resistant, high-strength duplex stainless steel of this example is shown in Table 1 above. The ingot was heated to 1260°C and finished at 970°C. The finished forging had dimensions of φ550 × 1800 mm. It underwent three upsetting forging passes, with upsetting deformations of 40%, 35%, and 35%, respectively. The heat treatment process consisted of first cooling at 1150°C (8 hours) with water, and then cooling at 910°C (4 hours) with water. Intergranular corrosion performance was tested according to GB / T4334E and met the requirements.

[0075] The above embodiments are preferred implementations of the present invention, but the implementation of the present invention is not limited to the above embodiments. Any other substitutions, modifications, combinations, changes, simplifications, etc. that do not deviate from the spirit and principles of the present invention should be considered equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A corrosion-resistant high-strength duplex stainless steel, characterized in that: In percentage by weight: C 0.10%~0.11%, Si≤0.35%, Mn≤0.45%, P≤0.020%, S≤0.001%, Cr 21.40%~21.60%, Ni5.40%~5.45%, 0<Mo≤0.02%, Ti 0.45%~0.50%, 0<Al≤0.08%, 0<N≤0.012%, the balance is iron and unavoidable impurities; Among them, the contents of C, N and Al satisfy 0.85≤(C+N) / Al≤2.12; The method for preparing the corrosion-resistant high-strength duplex stainless steel comprises the following steps: (1) smelting to obtain a steel ingot; (2) performing upsetting, drawing and forging on the steel ingot to obtain an intermediate billet; (3) performing a segmented heat treatment on the intermediate billet and then cooling the intermediate billet; the segmented heat treatment comprises: T 1. Solution treatment at temperature t 1 time and T 2. Temperature insulation t 2. Stabilization heat treatment is performed for a period of time; in, T 1=exp[a+b×Ti+c×(Ti)^2], t 1 = (1-4) × d / 2 minutes, where Ti is the weight percentage of elemental titanium; a, b, and c are dimensionless, with values ​​of a = 7.1937, b = -0.4018, and c = 0.1793, respectively; d is the diameter of the corrosion-resistant high-strength duplex stainless steel, in millimeters; in, T 2 is 880℃~930℃, t 2= t 1 / 2.

2. The corrosion-resistant high-strength duplex stainless steel according to claim 1, characterized in that: The contents of Ti and C satisfy Ti / (C-0.02)≥5.

3. A method for preparing the corrosion-resistant high-strength duplex stainless steel according to any one of claims 1 to 2, characterized in that: The steps include: (1) Smelting to obtain steel ingots; (2) The steel ingot is subjected to upsetting, drawing and forging to obtain an intermediate billet; (3) The intermediate billet is subjected to segmented heat treatment and then cooled.

4. The preparation method according to claim 3, characterized in that In step (2), the heating temperature of the upsetting and drawing forging is 1220°C to 1260°C, and the final forging temperature is not less than 950°C.

5. The preparation method according to claim 3, characterized in that In step (2), the upsetting deformation of each upsetting and drawing forging pass is not less than 35%.

6. The preparation method according to claim 5, characterized in that In step (2), the number of passes of upsetting and drawing forging of the steel ingot is determined according to the diameter of the corrosion-resistant high-strength duplex stainless steel; wherein: The diameter of the corrosion-resistant high-strength duplex stainless steel is less than 300 mm, and the steel ingot is subjected to one upsetting and drawing forging process; Alternatively, the diameter of the corrosion-resistant high-strength duplex stainless steel is 300 mm to 500 mm, and the steel ingot is subjected to two upsetting and drawing forging passes; Alternatively, the diameter of the corrosion-resistant high-strength duplex stainless steel is greater than 500 mm, and the steel ingot is subjected to upsetting, drawing and forging for more than three times.