An austenitic structure steel and a preparation process thereof

Abstract

The application discloses an austenitic structure steel and a preparation process thereof, and belongs to the technical field of metal materials. The Nb and N element content in the austenitic steel should satisfy: Lg([N][Nb])<-1.5 and -1.5 The precipitate in the austenitic structure steel is mainly nitride, the size of the nitride is smaller than 200 nm, and there is no large-size primary MX phase precipitate. Through the steel composition and process control, the austenitic structure steel is more reasonable in the solid solution stage dispersion nitride precipitate size and distribution, the precipitation strengthening effect is improved, and the low-temperature strength of the austenitic structure steel is improved, meanwhile, the generation of the primary MX phase in the solidification stage is reduced, and the toughness and plasticity of the austenitic structure steel in the super-low temperature environment are ensured.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, specifically to an austenitic structural steel and its preparation process. Background Technology

[0002] China's nuclear fusion program began in the 1950s. Currently, the mainstream equipment for controlled nuclear fusion experimental research is the tokamak toroidal device. The tokamak contains numerous superconducting coil structures. This structure uses ultra-high current to create an ultra-high magnetic field, controlling the movement of plasma and thus achieving a nuclear fusion reaction to generate energy. The ultra-high magnetic field will cause the superconducting wires to generate enormous electromagnetic forces, therefore requiring low-temperature, high-strength armor materials to ensure the safe operation of the superconducting wires. Furthermore, the entire superconducting magnet structure is still under a high-intensity magnetic field, and its structural support components—the superconducting coil box—must also possess excellent ultra-low temperature high-strength plasticity. Therefore, the development of novel ultra-low temperature austenitic structural steel is urgently needed. Summary of the Invention

[0003] The purpose of this invention is to provide an austenitic structural steel and its preparation process, which is particularly suitable for use at ultra-low temperatures (-269°C) and can still guarantee certain strength and toughness under ultra-low temperature conditions.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] An austenitic structural steel, the chemical composition of which is as follows (by weight percentage):

[0006] C < 0.03%; Cr 18–25%; Ni 10–15%; Mn 1–10%; Mo 1–3%; Si < 1.0%; N 0.25–0.40%; Nb ≤ 0.08%; V 0.05%–0.3%; S < 0.05%; P < 0.05%; Fe: balance.

[0007] The chemical composition of this austenitic structural steel is preferably as follows: Mn content is 3-6%, Ni content is preferably 12.5-15%, Cr content is preferably 19.5-23%, and N content is preferably 0.26-0.35%.

[0008] The contents of Nb, N and V in this austenitic steel should meet the requirements of formulas (1)-(2):

[0009] Lg([N][Nb])<-1.5 (1);

[0010] -1.5<Lg([N][V])<-0.8(2);

[0011] In formulas (1)-(2), [N], [Nb], and [V] represent the mass percentages of N, Nb, and V elements, respectively.

[0012] The precipitates in the austenitic structural steel are nitrides with a size of <200 nm; and there are no large-sized primary MX phase precipitates.

[0013] The austenitic structural steel exhibits the following properties when tested at -269℃: yield strength of 1300–1700 MPa, tensile strength of 1650–1900 MPa, elongation >25%, and fracture toughness >130 MPa·m. 1 / 2 Impact toughness >100J at -196℃.

[0014] The preparation process of the austenitic structural steel is as follows: first, the austenitic structural steel is prepared into ingots according to the chemical composition of the austenitic structural steel, and then forged or extruded. Finally, the austenitic structural steel is obtained by solution heat treatment.

[0015] The solution heat treatment temperature is 1020–1140℃.

[0016] The formula for calculating the holding time of the solution heat treatment is H×(2-4min / mm), where H is the thickness of the steel ingot in mm.

[0017] The solution heat treatment is followed by water cooling.

[0018] The advantages and beneficial effects of this invention are as follows:

[0019] (1) By controlling the ratio of Nb, V and N elements and reasonably controlling the content of other elements, the present invention makes the precipitation size and distribution of dispersed nitrides in the solid solution stage of the austenitic structural steel more reasonable, improves its precipitation strengthening effect, and thus improves the low temperature strength of the austenitic structural steel. At the same time, it reduces the generation of primary MX phase in the solidification stage and ensures its toughness and plasticity when used in ultra-low temperature environment.

[0020] (2) The present invention further controls the content of Mn and Cr elements. This is mainly because Mn and Cr elements are the main substitution solid solution elements. Their solid solution in the matrix will significantly affect the octahedral interstitial configuration of the face-centered cubic structure, and thus affect the solid solubility of nitrogen element. Studies have found that when the content of Mn element is 3-6% and Cr is 19.5-23%, nitrogen element can be fully dissolved, thereby ensuring the solid solution strengthening effect of nitrogen element at low temperature, thus ensuring low temperature strength.

[0021] (3) This invention further controls the content of Ni and N elements. This is mainly because N and Ni are the main austenite stabilizing elements in this invention. Studies have shown that only by controlling the content of N element to 0.26-0.35% and the content of Ni element to 12.5-15% can the formation of high-temperature ferrite be avoided, thus meeting the service conditions of ultra-high magnetic field strength. At the same time, it can also ensure the stability of its austenite structure, so that even if it is used for a long time at an ultra-low temperature of -269℃, it will not undergo temperature-induced martensitic phase transformation. Attached Figure Description

[0022] Figure 1 This is a scanning electron microscope image of Embodiment 1 of the present invention.

[0023] Figure 2 This is a scanning electron microscope image of Embodiment 2 of the present invention. Detailed Implementation

[0024] Unless otherwise specified, the experimental methods described in the following embodiments of the present invention are generally performed under conventional conditions or as recommended by the manufacturer. All commonly used chemical reagents used in the embodiments are commercially available products.

[0025] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0026] The terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product, or device that includes a series of steps is not limited to the steps or components listed, but may optionally include steps not listed, or may optionally include other steps or components inherent to such process, method, product, or device.

[0027] In this invention, "multiple" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0028] This invention provides an austenitic structural steel in which the precipitates are mainly nitrides such as NbN and VN, wherein the size of the nitrides is <200 nm; and there are no large-sized primary MX phase precipitates. "Large-sized" refers to primary MX phase precipitates with a size exceeding 10 μm.

[0029] The properties of the aforementioned austenitic structural steel are as follows: Under tensile testing at -269℃, its yield strength is 1300–1700 MPa; its tensile strength is 1650–1900 MPa, and its elongation is >25%; its fracture toughness at -269℃ is >130 MPa·m. 1 / 2 Impact toughness >100J at -196℃.

[0030] The improvement in the preparation process of the above-mentioned austenitic structural steel lies in the optimization of the Nb, V, and N element ratio to obtain uniformly dispersed precipitation strengthening phase characteristics, thereby achieving excellent low-temperature strength and toughness.

[0031] Preferably, the solution heat treatment temperature is 1050℃ to 1100℃, the heating rate is 4~6℃ / min, and the holding time is calculated using the formula H×(2-4min / mm), where H is the thickness of the steel ingot in mm. If the thickness of the steel ingot is 240mm, the holding time needs to be greater than 480min. The purpose of controlling the holding time is to ensure sufficient precipitation of the strengthening phase.

[0032] Furthermore, the Nb, N, and V composition of the above-mentioned austenitic structural steel must satisfy the following formulas (1)-(2):

[0033] Lg([N][Nb])<-1.5 (1);

[0034] -1.5<Lg([N][V])<-0.8 (2);

[0035] Where [N], [Nb], and [V] represent the mass percentage of each corresponding element.

[0036] Specifically, the composition of the austenitic structural steel is as follows: C < 0.03 wt%; Cr: 18-25 wt%; Ni: 10-15 wt%; Mn: 1-10 wt%; Mo: 1-3 wt%; Si: < 1.0 wt%; N: 0.25-0.40 wt%; Nb ≤ 0.08 wt%; V: 0.05%-0.3 wt%; S < 0.05 wt%; P < 0.05 wt%; Fe: balance.

[0037] Example 1:

[0038] The preparation process of austenitic structural steel in this embodiment includes the following steps:

[0039] (1) Smelting: Electric furnace smelting is adopted and VOD is used for refining. The final elemental composition of the ingot after electroslag remelting is shown in Table 1 below (wt.%).

[0040] Table 1. Chemical composition (wt.%) of austenitic structural steel in Example 1

[0041] C N Cr Ni Mn Mo Nb V Si Fe 0.012 0.29 22.3 14.5 5 1.9 0.08 0.29 0.6 margin

[0042] In this embodiment, Lg([N][Nb]) = -1.635; Lg([N][V]) = -1.075.

[0043] (2) Forging: The ingot is forged, and the temperature range of the forging process is 950℃~1200℃;

[0044] (3) Solution heat treatment: The forged steel ingot is subjected to solution heat treatment at a temperature of 1065℃.

[0045] The specific process of solution heat treatment is as follows: load the furnace at room temperature, set the heating rate to 5℃ / min, and start holding the temperature after heating to the set temperature. The holding time is H×3min / mm, where H is the thickness of the steel ingot in mm.

[0046] (4) After solution heat treatment, cooling treatment is carried out by water cooling.

[0047] Figure 1 This is a scanning electron microscope image from this embodiment. Figure 1 As can be seen from the data, the precipitates of this austenitic structural steel are mainly nitrides such as NbN and VN, and the size of the nitrides is <200nm; and there are no primary MX phase precipitates with a size exceeding 10μm.

[0048] Example 2:

[0049] The preparation method of the austenitic structural steel in this embodiment is the same as that in Example 1, and the measured composition is shown in Table 2 below:

[0050] Table 2 Chemical composition (wt.%) of austenitic structural steel in Example 2

[0051] C N Cr Ni Mn Mo Nb V Si Fe 0.008 0.30 20.5 13.2 4.6 2.2 0.05 0.2 0.55 margin

[0052] In this embodiment, Lg([N][Nb]) = -1.824; Lg([N][V]) = -1.22.

[0053] Figure 2 This is a scanning electron microscope image of the austenitic structural steel in this embodiment. Similarly, Figure 2 It can be seen that the precipitates of this austenitic structural steel are mainly dispersed fine nitrides.

[0054] Examples 3-4:

[0055] Examples 3-4 are prepared using the same process as Example 1, and the specific component ratios are shown in Table 3 below.

[0056] Table 3 Chemical composition (wt.%) of austenitic structural steels in Examples 3-4

[0057]

[0058] Comparative Examples 1-4:

[0059] To better illustrate the technical effects of the present invention, several comparative examples were designed, with the same preparation process as in Example 1. The specific component ratios are shown in Table 4 below:

[0060] Table 4. Chemical composition (wt.%) of austenitic structural steels in Comparative Examples 1-4

[0061]

[0062] The performance of the products in the above embodiments and comparative examples is shown in Table 5 below:

[0063] Table 5

[0064]

[0065] To meet the requirements for cryogenic applications, the yield strength is typically required to be 1350–1550 MPa, the tensile strength 1750–1900 MPa, and the elongation >25% under test conditions at -269℃, with a fracture toughness >130 MPa·m. 1 / 2 Impact toughness at -196℃ >100J; Comparative Examples 1 and 2 have high yield strength but poor toughness; Comparative Examples 3 and 4 have poor toughness.

[0066] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. An austenitic structural steel, characterized in that: The chemical composition of this austenitic structural steel, by weight percentage, is as follows: C<0.03%; Cr 18~25%; Ni 10~15%; Mn 1~10%; Mo 1~3%; Si<1.0%; N 0.25~0.40%; Nb≤0.08%; V 0.05%~0.3%; S < 0.05%; P < 0.05%; Fe: balance; The precipitates in the austenitic structural steel are dispersed nitrides with a size of <200nm; and there are no primary MX phase precipitates with a size exceeding 10μm. The Nb and N content in this austenitic structural steel should meet the requirements of formulas (1)-(2): Lg([N][Nb])<-1.5 (1; -1.5<Lg([N][V])<-0.8 (2); In formulas (1)-(2), [N], [Nb], and [V] represent the mass percentages of N, Nb, and V elements, respectively.

2. The austenitic structural steel according to claim 1, characterized in that: The chemical composition of this austenitic structural steel contains 3-6% Mn.

3. The austenitic structural steel according to claim 1, characterized in that: The chemical composition of this austenitic structural steel is as follows: Ni content is 12.5-15%, and Cr content is 19.5-23%.

4. The austenitic structural steel according to claim 1, characterized in that: The chemical composition of this austenitic structural steel contains 0.26-0.35% nitrogen.

5. The austenitic structural steel according to claim 1, characterized in that: The austenitic structural steel exhibits the following properties when tested at -269℃: yield strength of 1300~1700 MPa, tensile strength of 1650~1900 MPa, elongation >25%, and fracture toughness >130 MPa·m. 1 / 2 Impact toughness >100J at -196℃.

6. The preparation process of austenitic structural steel according to claim 1, characterized in that: The process first involves distributing the austenitic structural steel according to its chemical composition, preparing it into an ingot, then forging or extruding it, and finally obtaining the austenitic structural steel through solution heat treatment.

7. The preparation process of austenitic structural steel according to claim 6, characterized in that: The solution heat treatment temperature is 1020~1140℃.

8. The preparation process of austenitic structural steel according to claim 6, characterized in that: The formula for calculating the holding time of the solution heat treatment is H×(2-4min / mm), where H is the thickness of the steel ingot in mm.

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