A method for preparing a strengthened 18-8 austenitic stainless steel

Abstract

The application discloses a preparation method of reinforced 18-8 austenitic stainless steel. In the basic 18-8 austenitic stainless steel component, more than 0.05 wt.% and less than 0.1 wt.% of metal Ti is added, and smelting is carried out according to the following component element adding sequence: pure iron and metal Ni are added first, then metal Cr with a target content of 40% to 60% is added, then metal Ti is added, and finally the remaining target content of metal Cr and other elements in the component are added; before the metal Ti is added, the free oxygen content in the molten steel is controlled to be 30 to 60 ppm, and the N element content is not more than 100 ppm; thereby forming the reinforced 18-8 austenitic stainless steel. The application further discloses a reinforced 18-8 austenitic stainless steel. Compared with the prior art, the application does not need additional thermal mechanical processing or heat treatment, and the preparation of the reinforced 18-8 austenitic stainless steel can be realized through a basic smelting process, and the size of the prepared steel material is not limited.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical process technology, specifically relating to a method for preparing strengthened 18-8 series austenitic stainless steel. Background Technology

[0002] Austenitic stainless steel is one of the most widely used steel grades in industrial production, possessing excellent corrosion resistance and oxidation resistance. In the early 20th century, Maurer and Strauss jointly developed the 18-8 series of austenitic stainless steels. Commonly used 18-8 stainless steel grades suitable for high-temperature service environments include 304H, 347H, 316H, and 321H. However, with the deterioration of service environments, it was found that the strength of 18-8 series austenitic stainless steel at high temperatures was insufficient. Subsequently, engineers mainly used two methods to further strengthen 18-8 series austenitic stainless steel: adding microalloying elements and performing special thermomechanical processing and heat treatment. Based on this, more high-performance 18-8 series austenitic stainless steels were developed. Sumitomo Metal Industries and Mitsubishi Heavy Industries of Japan developed Super304H austenitic stainless steel by adding 3% Cu and 0.5% Nb to 304H austenitic stainless steel. This requires aging treatment to precipitate NbCrN, Nb(C,N), M23C6, and nanoscale precipitates to improve its high-temperature mechanical properties. 347HFG, on the other hand, utilizes special thermomechanical processing and heat treatment processes to achieve a grain size level of 8 or higher, effectively improving the oxidation resistance and mechanical properties of 347H. Both of these methods for strengthening 18-8 series austenitic stainless steels require additional thermomechanical processing or heat treatment, and involve a significant increase in alloying elements, increasing manufacturing costs and production cycles.

[0003] Oxide dispersion strengthened (ODS) primarily improves the properties of steel by precipitating small, dispersed spherical, high-temperature resistant oxide particles within the steel. It has already been applied in nickel-based superalloys, 9Cr steel, and ferritic stainless steel. A common method for preparing ODS steel is to thoroughly break down and fuse high-purity metal powder with fine oxide particles using mechanical alloying (MA) to form a supersaturated solid solution. However, due to limitations in the manufacturing method, the MA method cannot produce large profiles.

[0004] The inventors of this application first proposed a stainless steel smelting method in Chinese invention patent application CN113737081A. Through precise smelting process control, it is possible to spontaneously generate in-situ dispersed micro-Ti2O3 particles in 430 stainless steel or T4003 stainless steel. These Ti2O3 particles have significantly higher stability than TiN and carbides, effectively refining the stainless steel microstructure and thus enhancing its properties. Furthermore, this method can be inherited during welding to effectively inhibit the growth of the heat-affected zone microstructure, thereby optimizing the weldability of stainless steel. However, through extensive experiments, the inventors found that this technical solution is not applicable to 18-8 series austenitic stainless steel, as it cannot precipitate fine and dispersed spherical oxides in situ, thus failing to achieve dispersion strengthening. Therefore, further research is necessary on the oxide dispersion strengthening process for 18-8 series austenitic stainless steel. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for preparing strengthened 18-8 series austenitic stainless steel. This method does not require additional thermomechanical processing or heat treatment. The preparation of strengthened 18-8 series austenitic stainless steel can be achieved through basic smelting processes, and there is no limitation on the size of the steel produced.

[0006] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0007] A method for preparing strengthened 18-8 series austenitic stainless steel involves adding more than 0.05 wt.% and less than 0.1 wt.% metallic Ti to a standard 18-8 series austenitic stainless steel composition, and smelting in the following order of element addition: first, pure iron and metallic Ni are added, then metallic Cr with a target content of 40% to 60% is added, followed by metallic Ti, and finally the remaining target content of metallic Cr and other elements in the composition are added; before adding metallic Ti, the free oxygen content in the molten steel is controlled to be 30 to 60 ppm, while the nitrogen content is not greater than 100 ppm; thereby forming strengthened 18-8 series austenitic stainless steel.

[0008] Preferably, if the free oxygen content in the molten steel is greater than 60 ppm before the addition of metallic Ti, the free oxygen content in the molten steel is adjusted to 30-60 ppm by adding silicon-calcium powder for deoxidation.

[0009] Preferably, after adding pure iron and metallic Ni, and before adding metallic Cr, a steelmaking protective slag (with the following composition: CaO ~ 60%, SiO2 ~ 30%, MgO ~ 5%, CaF2 ~ 5%) is added to the molten steel to isolate the molten steel from the air, preventing Cr from absorbing nitrogen from the atmosphere during the melting process and increasing the N content in the molten steel, so that Ti combines with N to form titanium nitrides.

[0010] A reinforced 18-8 series austenitic stainless steel is prepared using the method described in any of the above technical solutions.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] This invention achieves dispersion strengthening by precisely controlling the smelting process, enabling the spontaneous in-situ precipitation of fine, dispersed, and sufficiently dense titanium oxide particles in 18-8 series austenitic stainless steel during smelting and solidification. No further processing of the precipitated titanium oxides is required. Furthermore, titanium oxides exhibit excellent thermal stability at high temperatures and are not easily dissolved, thus improving the high-temperature strength of 18-8 series austenitic stainless steel. Compared to other strengthening methods, this invention offers a simpler, lower-cost process, and the ability to produce large components, making it suitable for large-scale steel production, such as vacuum oxygen decarburization (VOD), argon-oxygen decarburization refining (AOD), and ladle refining (LF). Attached Figure Description

[0013] Figure 1 The morphology of the precipitated phases in the 347H reference sample is shown in (a)-(c), where (a)-(c) represent the morphology of the precipitated phases at different internal locations.

[0014] Figure 2 The images show the morphology of Ti-containing 347H precipitates after being smelted with metallic Cr in two batches, where (a)-(c) show the morphology of the precipitates at different internal locations.

[0015] Figure 3 The images show the morphology of Ti-containing 347H precipitates obtained by smelting with metallic Cr in a single step, where (a)-(c) show the morphology of the precipitates at different internal locations, respectively.

[0016] Figure 4 The images show the morphology of Ti-containing 316H precipitates after being smelted with metallic Cr in two batches, where (a)-(c) show the morphology of the precipitates at different internal locations.

[0017] Figure 5 The images show the morphology of Ti-containing 316H precipitates obtained by smelting with metallic Cr in a single step, where (a)-(c) show the morphology of the precipitates at different internal locations. Detailed Implementation

[0018] Currently, all methods for strengthening 18-8 series austenitic stainless steel require additional thermomechanical processing or heat treatment, which increases the preparation cycle and production cost. Furthermore, the MA method for preparing ODS 18-8 series austenitic stainless steel can only produce small-volume steel. To address this issue, the inventors conducted extensive experimental research and discovered that by selecting suitable strengthening agents and strictly controlling the smelting process, fine and dispersed spherical oxides can be directly precipitated in situ within 18-8 series austenitic stainless steel, achieving dispersion strengthening. Moreover, the high-temperature stability of the metal oxides can significantly improve the strength of 18-8 series austenitic stainless steel at high temperatures. This method eliminates the need for special thermomechanical processing or heat treatment of the precipitated oxide particles and overcomes the limitation of the MA method in producing large-scale steel, enabling large-scale production and improving the strength of 18-8 series austenitic stainless steel at both room temperature and high temperatures.

[0019] The core of this invention lies in the strict control of the smelting process, mainly in the following two aspects: First, conventional austenitic stainless steel smelting processes do not specify the order of alloy addition. However, this invention requires strict control over the order of addition of each alloying element based on the following considerations: To avoid Cr absorbing atmospheric N during smelting, resulting in excessively high N content in the molten steel, Cr is added after steelmaking, Ni melting, and slag formation. To avoid deoxidizing elements such as Cr, Si, and Mn consuming the free oxygen content in the molten steel, approximately half of the Cr is added first, and Ti is added before Si, Mn, and other deoxidizing elements, with the added amount of Ti being greater than 0.05 wt.% and less than 0.1 wt.%. In general, the order of addition of each alloying element is as follows: first, add pure iron and Ni; then, add approximately half of the target content (specifically, 40%–60% of the target content) of Cr; next, add Ti; and finally, add the remaining target content of Cr, as well as other alloying elements such as Si and Mn. 2. Before adding metallic Ti, control the free oxygen content in the molten steel to be between 30-60 ppm and the nitrogen content to be less than 100 ppm.

[0020] The method for preparing strengthened 18-8 series austenitic stainless steel proposed in this invention is as follows:

[0021] Add more than 0.05 wt.% and less than 0.1 wt.% of metallic Ti to the standard 18-8 series austenitic stainless steel composition, and smelt according to the following order of element addition: first add pure iron and metallic Ni, then add metallic Cr at a target content of 40% to 60%, then add metallic Ti, and finally add the remaining target content of metallic Cr and other elements in the composition; before adding metallic Ti, control the free oxygen content in the molten steel to be 30 to 60 ppm, and the N element content to be no more than 100 ppm; thereby forming a strengthened 18-8 series austenitic stainless steel.

[0022] To better control the nitrogen content and absorb large inclusions in the molten steel, preferably, a steelmaking protective slag is added to the molten steel after adding pure iron and metallic Ni, and before adding metallic Cr.

[0023] Using the above-described smelting method, spherical titanium oxide particles can spontaneously precipitate in situ during molten steel and solidification. These particles, with a size less than 1 μm, can play a role in dispersion strengthening. Further experiments revealed that the titanium oxide particles can maintain their properties during subsequent forging, hot rolling, and heat treatment, and exhibit excellent thermal stability during high-temperature service. They are difficult to dissolve in the matrix and can improve the high-temperature strength of 18-8 series austenitic stainless steel.

[0024] The smelting process can employ commonly used smelting processes in the preparation of 18-8 series austenitic stainless steel, such as vacuum oxygen decarburization (VOD), argon oxygen decarburization refining (AOD), and ladle refining (LF).

[0025] The method of this invention is applicable to the strengthening of various 18-8 series austenitic stainless steels.

[0026] To facilitate public understanding, the technical solution of the present invention will be described in detail below through specific embodiments and in conjunction with the accompanying drawings:

[0027] Example 1

[0028] In this embodiment, 347H from the 18-8 series austenitic stainless steel was selected for performance enhancement. Two smelting processes were used to smelt Ti-containing 347H stainless steel, and the composition was compared with that of a 347H stainless steel benchmark sample.

[0029] When smelting 347H stainless steel with the standard composition, the following chemical composition and mass fractions are used for batching: Cr: 17.5-18.5 wt.%, C: 0.04-0.07 wt.%, Si: 0.20-0.40 wt.%, Mn≤2.00 wt.%, Ni: 9.1-9.6 wt.%, Nb: 0.55-0.70 wt.%, with the remainder being Fe and unavoidable impurities.

[0030] Smelting Process 1: Metallic Cr is added in two batches. The smelting process is as follows: first, pure iron, metallic Ni, and steelmaking protective slag are added, then 9% metallic Cr is added, then 0.09wt.% metallic Ti is added, and finally 9% metallic Cr, as well as metallic Si, Mn, and Nb are added.

[0031] Smelting Process Two: All metallic Cr is added at once. In actual smelting, pure iron, metallic Ni, and steelmaking protective slag are added first, followed by 18% metallic Cr, then 0.09 wt.% metallic Ti, and finally metallic Si, Mn, and Nb. Oxygen determination is performed before adding metallic Ti to ensure the free oxygen content in the molten steel is between 30-60 ppm. If the free oxygen content exceeds 60 ppm, calcium silicate powder is added for deoxidation until the free oxygen content is around 60 ppm. Table 1 shows the composition of the steel sample in the example.

[0032] Figure 1 The images show the morphology of precipitates in Ti-free 347H austenitic stainless steel after metallographic processing. It is evident that the number density of precipitates is significantly low, with most being spherical and short rod-shaped. SEM spot scanning results show that the precipitates in 347H stainless steel are predominantly spherical chromium-manganese oxides and niobium carbide (as shown in Table 2). The chromium-manganese oxides have low content and large size (approximately 1.11 μm), thus their strengthening effect is limited. Figure 2 The image shows the morphology of precipitates in 347H austenitic stainless steel with a Ti content of 0.09 wt.% obtained by smelting process one after metallographic software processing. The precipitates are diffusely distributed, small in size, mostly spherical and short rod-shaped, and their number density is significantly higher than that of the 347H reference composition sample. Table 3 lists the point scan energy dispersive spectroscopy (EDS) results for each type of precipitate. It was found that, in addition to niobium carbide, a considerable amount of titanium oxide precipitates were present, as well as a small amount of composite precipitates of titanium oxide, niobium carbide, and manganese sulfide. Statistical analysis revealed that titanium oxide accounted for approximately 20% of all precipitates, with an average size of 0.93 μm, and was spherical, thus playing a role in dispersion strengthening of 347H austenitic stainless steel. Figure 3 The images show the morphology of precipitated phases in 347H austenitic stainless steel with a Ti content of 0.08 wt.% obtained from smelting process two after metallographic processing. The precipitates are mostly spherical, short rod-shaped, and square. Table 4 shows the spot scan energy dispersive spectroscopy (EDS) results, indicating that the precipitates are a composite of niobium carbide, titanium nitride, and titanium oxide. It is evident that when all the metallic Cr is added at once, the free oxygen in the molten steel is consumed by Cr, resulting in insufficient O atoms to combine with Ti atoms when metallic Ti is subsequently added. The excess Ti then combines with N to form titanium nitride, which then precipitates in combination with titanium oxide. When titanium nitride precipitates, the size of the Ti-containing precipitate increases to 1.29 μm, and the shape changes from spherical to square, which will impair the strength of 347H stainless steel. Furthermore, the quantity density of Ti-containing 347H stainless steel obtained from smelting process two is significantly lower than that from smelting process one.

[0033] Table 5 shows the strengths of the three example steel ingots after forging, hot rolling, and solution treatment at room temperature and 800℃. The Ti-containing 347H stainless steel prepared by adding metallic Cr in two batches can precipitate in situ, dispersed, small-sized titanium oxides that remain stable at high temperatures, thus improving the strength of 347H at both room temperature and high temperatures. Adding all metallic Cr at once causes the titanium oxides to transform into a composite precipitation of titanium nitrides and titanium oxides, which impairs the strength of 347H stainless steel; therefore, the smelting process needs to be controlled.

[0034] Table 1. Composition of 347H austenitic stainless steel sample in Example 1 (wt.%)

[0035]

[0036] Table 2. Energy dispersive spectroscopy (EDS) results of precipitates in 347H austenitic stainless steel, wt.%.

[0037]

[0038] Table 3. Energy dispersive spectroscopy (EDS) results of precipitated phases in Ti-containing 347H austenitic stainless steel obtained by smelting process 1, at.%

[0039]

[0040] Table 4. Energy dispersive spectroscopy (EDS) results of precipitated phases in Ti-containing 347H austenitic stainless steel obtained by smelting process II, at.%.

[0041]

[0042] Table 5. Strength of 347H austenitic stainless steel samples in Example 1, MPa

[0043]

[0044]

[0045] Example 2

[0046] This embodiment uses 316H from the 18-8 series austenitic stainless steel for performance optimization. For comparison, two different smelting processes were used to prepare Ti-containing 316H stainless steel. The molten steel for smelting the basic composition of 316H austenitic stainless steel was batched according to the following chemical composition and mass fraction: Cr: 16.0-18.0 wt.%, C: 0.04-0.10 wt.%, Si: ≤0.75 wt.%, Mn ≤2.00 wt.%, Ni: 10.0-14.0 wt.%, Mo: 2.00-3.00 wt.%, with the remainder being Fe and unavoidable impurities. Smelting Process 1: Metallic Cr was added in two batches. The smelting process involved first adding pure iron, metallic Ni, and steelmaking protective slag, then adding 9% metallic Cr, then 0.07 wt.% metallic Ti, and finally adding 9% metallic Cr, metallic Si, and Mn. Smelting Process Two: All metallic Cr is added at once. In actual smelting, pure iron, metallic Ni, and steelmaking protective slag are added first, followed by 18% metallic Cr, then 0.07 wt.% metallic Ti, and finally metallic Si and Mn. Oxygen determination is performed before adding metallic Ti to ensure the free oxygen content in the molten steel is between 30-60 ppm. If the free oxygen content exceeds 60 ppm, calcium silicate powder is added for deoxidation until the free oxygen content is around 60 ppm. Table 6 shows the composition of the steel sample from the implementation example.

[0047] Figure 4 , Figure 5 The images show the morphology of precipitated phases in Ti-containing 316H austenitic stainless steel obtained through two different smelting processes after metallographic software processing. The comparison reveals that when metallic Cr is added in two batches during the smelting process, the number density of precipitated phases inside the 316H austenitic stainless steel is significantly higher, and these precipitates are mostly small-sized (e.g., ...). Figure 4 (As shown in Table 7). Its strengthening effect is shown in Table 7, with significant improvements in both room temperature strength and high-temperature strength at 800℃.

[0048] Table 6. Composition of 316H austenitic stainless steel samples in Example 2 (wt.%)

[0049]

[0050] Table 7. Strength of 316H austenitic stainless steel samples in Example 2, MPa

[0051]

Claims

1. A method for preparing strengthened 18-8 series austenitic stainless steel, characterized in that, Add more than 0.05 wt.% and less than 0.1 wt.% of metallic Ti to the standard 18-8 series austenitic stainless steel composition, and smelt according to the following order of element addition: first add pure iron and metallic Ni, then add metallic Cr at a target content of 40% to 60%, then add metallic Ti, and finally add the remaining target content of metallic Cr and other elements in the composition; before adding metallic Ti, control the free oxygen content in the molten steel to be 30 to 60 ppm, and the N element content to be no more than 100 ppm; thereby forming a strengthened 18-8 series austenitic stainless steel.

2. The method for preparing strengthened 18-8 series austenitic stainless steel as described in claim 1, characterized in that, If the free oxygen content in the molten steel is greater than 60 ppm before the addition of metallic Ti, the free oxygen content in the molten steel can be adjusted to 30-60 ppm by adding silicon-calcium powder for deoxidation.

3. The method for preparing strengthened 18-8 series austenitic stainless steel as described in claim 1, characterized in that, After adding pure iron and metallic Ni, and before adding 40% to 60% metallic Cr, a steelmaking protective slag is added to the molten steel.

4. A strengthened 18-8 series austenitic stainless steel, characterized in that, It is prepared using the method described in any one of claims 1 to 3.

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