A method for regulating performance of niobium-containing precipitates of super austenitic stainless steel by boron and niobium

Abstract

The application discloses a method for improving the performance of a super austenitic stainless steel containing niobium precipitated phase by boron and niobium, and belongs to the technical field of stainless steel preparation. The method introduces trace elements B and Nb, utilizes B and Mo dissolved in the Nb(C, N) precipitated phase to form the (Nb, Mo)(B, C, N) precipitated phase, utilizes the grain boundary segregation effect of B to inhibit the formation of sigma, reduces the grain boundary precipitated phase, and makes the grain boundary precipitated phase smaller; the (Nb, Mo)(B, C, N) fine and dispersed distribution is further regulated by slow cooling and holding aging treatment; meanwhile, the Mo-containing precipitated phase at the grain boundary is delayed, and the Mo precipitated phase grain is reduced. The combination of the (Nb, Mo)(B, C, N) and Mo-rich precipitated phase which are fine and dispersed in the grain and at the grain boundary improves the strength and corrosion resistance of the super austenitic stainless steel.

Description

Technical Field

[0001] This invention relates to the field of stainless steel preparation technology, and in particular to a method for improving the performance of super austenitic stainless steel by regulating the niobium-containing precipitates with boron and niobium. Background Technology

[0002] Super austenitic stainless steel is an austenitic stainless steel with high Cr and Mo content. It possesses excellent corrosion resistance and mechanical properties and has been widely used in high-end equipment manufacturing fields such as energy conservation and environmental protection, marine engineering, and petrochemicals. With the development of these fields, higher requirements are being placed on the strength, toughness, and corrosion resistance of the materials.

[0003] Super austenitic stainless steel is one of the most technically challenging types of stainless steel to produce. During solidification, Mo segregation is severe, and the precipitation of precipitates is sensitive during hot rolling. The hot working window is particularly narrow, making its manufacturing extremely difficult. This is closely related to the sensitivity of precipitates to precipitation. How to suppress the precipitation of precipitates and simultaneously improve the strength and corrosion resistance of super austenitic stainless steel has become a serious problem restricting its development, and relevant research is urgently needed.

[0004] In stainless steel, alloying elements such as Ni, Mn, Ce, and N can stabilize the austenitic structure of super austenitic stainless steel and inhibit the precipitation of precipitates. Microalloying element B can affect the microstructure, corrosion resistance, and mechanical properties of super austenitic stainless steel; even extremely low amounts of microalloying elements can significantly improve material properties. Precipitation strengthening is generally the most effective method to improve material strength, but since precipitates affect corrosion resistance, super austenitic stainless steel primarily relies on solid solution strengthening to improve strength, and precipitation strengthening is rarely used. However, solid solution strengthening alone offers limited strength gains and is insufficient to meet the higher strength requirements of super austenitic stainless steel. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the performance of super austenitic stainless steel by controlling the niobium-containing precipitates using boron and niobium composites, thereby solving the problems existing in the prior art. This invention combines the beneficial effects of boron and niobium in super austenitic stainless steel, utilizing the dissolution of boron and molybdenum in the niobium (C,N) precipitates to form (Nb,Mo)(B,C,N) precipitates; and utilizing the grain boundary segregation effect of boron to suppress σ formation, thus reducing the number and finer size of the grain boundary precipitates. Through slow cooling and heat preservation aging treatment, the fine and dispersed distribution of (Nb,Mo)(B,C,N) precipitates is further controlled; simultaneously, the precipitation of molybdenum-containing precipitates at grain boundaries is delayed, resulting in a smaller molybdenum grain size. Through the combination of finely dispersed (Nb,Mo)(B,C,N) and moly-rich precipitates within and at grain boundaries, the strength and corrosion resistance of super austenitic stainless steel are improved, meeting the higher requirements of "high strength, high toughness, and high corrosion resistance" for engineering-grade super austenitic stainless steel.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] One of the technical solutions of the present invention is a method for improving the performance of super austenitic stainless steel by regulating the niobium-containing precipitate phase with boron and niobium, comprising the following steps: homogenizing a super austenitic stainless steel ingot containing elements B and Nb and rolling it into a steel plate, then performing solution treatment and slow cooling and heat preservation aging treatment in sequence, and finally water cooling to room temperature to obtain super austenitic stainless steel.

[0008] Furthermore, the content of element B in the super austenitic stainless steel is 0.003–0.011 wt.%, and the content of element Nb in the super austenitic stainless steel is 0.04–0.15 wt.%.

[0009] Furthermore, the super austenitic stainless steel has the following chemical composition by mass percentage: C: 0.01-0.03%, Si: 0.5-1.0%, P: 0.01-0.03%, S: 0.005-0.01%, Mn: 1.5-2.0%, Cr: 19-24%, Ni: 17.0-22.0%, Mo: 5.5-6.5%, Cu: 0.5-1.5%, N: 0.15-0.3%, B: 0.003-0.011%, Nb: 0.04-0.15%, with the balance being Fe and other unavoidable impurity elements.

[0010] Furthermore, the homogenization treatment is carried out at a temperature of 1200–1260°C for 16–24 hours.

[0011] Furthermore, the rolling temperature is 1000–1220°C.

[0012] Furthermore, the solution treatment is performed at a temperature of 1160–1240°C for a duration of 3–8 hours.

[0013] Furthermore, the slow cooling and heat preservation aging treatment specifically includes: after solution treatment, cooling to 1020-1100℃ at a cooling rate of 250-300℃ / h, and holding for 10-60 minutes.

[0014] The main role of C is to stabilize and strengthen the austenite phase. Reducing the C content can reduce the precipitation of Cr carbides, thereby reducing the Cr-depleted zone formed by Cr carbides and improving the corrosion resistance of the material. It also combines with Nb and N to form a fine, dispersed Nb(C,N) phase, which plays a role in precipitation strengthening.

[0015] Si has a strong deoxidizing ability, and adding a certain amount of Si to steel helps to reduce the oxygen content in the steel. Si helps to form a dense SiO2 oxide film between the stainless steel matrix and the oxide layer, improving the oxidation resistance of austenitic stainless steel. When austenitic stainless steel containing Si is in a chloride corrosive medium, Si can shift the pitting potential of the stainless steel to the positive potential, thus improving the corrosion resistance of the stainless steel.

[0016] Mn is an element that stabilizes austenite. Mn works synergistically with Ni to effectively increase the solubility of N in austenitic stainless steel.

[0017] Cr can significantly improve the corrosion resistance of austenitic stainless steel. The reaction of Cr with O can form a complete and dense Cr2O3 passivation film. The dense oxide film can prevent the stainless steel matrix from being penetrated by corrosive media, thereby improving the corrosion resistance of austenitic stainless steel.

[0018] Ni is a typical austenitizing element that promotes the formation of austenite in stainless steel. In super austenitic stainless steel, it can reduce the precipitation of the σ phase, thereby improving the strength and hot working properties of super austenitic stainless steel. Ni can also improve the structure and properties of the passivation film on the surface of stainless steel, and enhance its thermodynamic stability and high-temperature oxidation resistance.

[0019] Mo can significantly improve the resistance of super austenitic stainless steel to sulfur and chloride ion corrosion, and significantly improve its resistance to pitting corrosion and intergranular corrosion.

[0020] Cu can improve the corrosion resistance of stainless steel in reducing corrosive media. Cu tends to segregate in the passivation film, preventing oxygen from diffusing into the stainless steel matrix and thus protecting the matrix.

[0021] Nitrogen (N) is a commonly used austenite-stabilizing element that can expand the austenite region in steel. In stainless steel, it exhibits significant solid solution strengthening, improving strength, creep resistance, fatigue resistance, and work hardening ability. Ni also simultaneously enhances the stability of the passivation film in super austenitic stainless steel, improving the steel's corrosion resistance.

[0022] As a microalloying element, B is generally difficult to dissolve in the Fe matrix and is easily distributed at grain boundaries, thereby improving grain boundary strength and high-temperature hot working performance. This invention utilizes the fact that B is easily distributed at grain boundaries, which affects the segregation of elements such as Cr, Mo, and Nb at grain boundaries and controls the precipitation of precipitated phases.

[0023] Nb, as a precipitation strengthening element, is almost never added in super austenitic stainless steel. The formation of Nb(C,N) reduces corrosion resistance. This invention utilizes the (Nb,Mo)(B,C,N) precipitate phase formed by the composite regulation of B and Nb to compensate for the shortcomings of Nb(C,N) in terms of weak corrosion resistance.

[0024] This invention, based on the composition of existing super austenitic stainless steel, adds microalloying elements B and Nb, and appropriately increases the C and N content. B and Mo dissolve in the Nb(C,N) precipitate, forming a (Nb,Mo)(B,C,N) precipitate. The grain boundary segregation effect of B is utilized to suppress the precipitation of σ, reducing the number and fineness of the grain boundary precipitates. Slow cooling and holding aging treatment further regulates the fine and dispersed precipitation of (Nb,Mo)(B,C,N), delaying the precipitation of Mo-containing grain boundary precipitates, resulting in fewer and finer grain boundary precipitates. The combination of finely dispersed (Nb,Mo)(B,C,N) and Mo-rich precipitates within and at grain boundaries improves the strength and corrosion resistance of the super austenitic stainless steel.

[0025] The second technical solution of the present invention: a super austenitic stainless steel prepared by the above method.

[0026] The third technical solution of the present invention: an application of the above-mentioned super austenitic stainless steel in the fields of energy conservation and environmental protection, marine engineering or petrochemicals.

[0027] The present invention discloses the following technical effects:

[0028] The method of this invention can improve the strength and corrosion resistance of super austenitic stainless steel, and can meet the higher requirements of "high strength and toughness, high corrosion resistance" for super austenitic stainless steel used in engineering. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 The grain boundary precipitation phase diagrams of the super austenitic stainless steel prepared in Example 1 and Comparative Examples 1-2 of the present invention are shown. (a) is the super austenitic stainless steel sample prepared in Comparative Example 1, (b) is the super austenitic stainless steel sample prepared in Comparative Example 2, and (c) is the super austenitic stainless steel sample prepared in Example 1.

[0031] Figure 2 The intragranular Nb precipitation phase diagram of the super austenitic stainless steel prepared in Example 1 of this invention;

[0032] Figure 3 The image shows the energy spectrum of the intracrystalline Nb precipitate phase of the super austenitic stainless steel prepared in Example 1 of this invention.

[0033] Figure 4The polarization curve of the super austenitic stainless steel prepared in Example 1 of this invention;

[0034] Figure 5 The stress-strain curve of the super austenitic stainless steel prepared in Example 1 of this invention;

[0035] Figure 6 The following are grain boundary precipitation phase diagrams of the super austenitic stainless steel samples prepared in Examples 2-3 and Comparative Example 3 of the present invention: (a) is the super austenitic stainless steel sample prepared in Example 2, (b) is the super austenitic stainless steel sample prepared in Example 3, and (c) is the super austenitic stainless steel sample prepared in Comparative Example 3. Detailed Implementation

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0038] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0039] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0040] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0041] Example 1

[0042] A method for improving the properties of super austenitic stainless steel by regulating niobium-containing precipitates using boron and niobium:

[0043] Super austenitic stainless steel, by mass percentage, consists of the following components: C: 0.03%, Si: 0.8%, P: 0.03%, S: 0.01%, Mn: 2.0%, Cr: 20%, Ni: 18%, Mo: 6%, Cu: 1%, N: 0.25%, B: 0.005%, Nb: 0.05%, with the balance being Fe and other unavoidable impurity elements.

[0044] (1) Mix the raw materials according to the above mass percentages and smelt them in a vacuum induction furnace (heat up by powering on and then vacuuming, and control the temperature at 1500-1600℃). Then cast them into ingots under vacuum and demold them when cooled to room temperature to obtain super austenitic stainless steel ingots.

[0045] (2) The super austenitic stainless steel ingot was homogenized at 1250℃ for 20h and then rolled into steel plate at 1200℃.

[0046] (3) Cut a 10×10mm steel plate sample for solution treatment (the treatment temperature is 1200℃ and the time is 4h), and then cool it down to 1050℃ with the furnace at a cooling rate of 250℃ / h. Hold it for 30min (slow cooling and heat preservation aging treatment) and then water cool it to room temperature to obtain a super austenitic stainless steel sample.

[0047] Comparative Example 1

[0048] Same as Example 1, except that the super austenitic stainless steel does not contain B and Nb in its composition.

[0049] Comparative Example 2

[0050] Same as Example 1, except that the super austenitic stainless steel does not contain Nb in its composition.

[0051] The super austenitic stainless steel samples prepared in Example 1 and Comparative Examples 1-2 were mechanically polished and metallographically etched after being ground with sandpaper of different grades. The microstructure of the samples after aging at 1050℃ for 30 min was analyzed using a scanning electron microscope. The results are shown in […]. Figure 1 (Grain boundary precipitation phase diagram). Figure 1 (a) is the super austenitic stainless steel sample (0B sample) prepared in Comparative Example 1, (b) is the super austenitic stainless steel sample (B sample) prepared in Comparative Example 2, and (c) is the super austenitic stainless steel sample (B+Nb sample) prepared in Example 1.

[0052] from Figure 1As can be seen, after aging at 1050℃ for 30 min, coarse precipitates appeared at the grain boundaries of the OB sample. After adding B, compared with the OB sample, the B sample had fewer grain boundary precipitates and the size of the precipitates was smaller. After adding Nb to B, compared with the OB sample and the B sample, the number of grain boundary precipitates in the B+Nb sample was further reduced, and the size of the precipitates changed from micrometers to nanometers.

[0053] In the B+Nb sample, B and Nb work synergistically. B and Mo dissolve in the Nb(C,N) precipitate, consuming Mo and reducing element enrichment at grain boundaries. Simultaneously, the grain boundary segregation effect of B suppresses σ precipitation, resulting in fewer and smaller grain boundary precipitates. Slow cooling and holding aging treatment further regulates the fine and dispersed distribution of (Nb,Mo)(B,C,N); simultaneously, it delays the precipitation of Mo-rich precipitates at grain boundaries. Through the combination of finely dispersed (Nb,Mo)(B,C,N) precipitates within and at grain boundaries, the strength and corrosion resistance of the super austenitic stainless steel are improved.

[0054] The intragranular Nb precipitation phase diagram of the super austenitic stainless steel prepared in Example 1 is shown below. Figure 2 .

[0055] from Figure 2 As can be seen, after B+Nb aging, there are finely dispersed (Nb,Mo)(B,C,N) precipitates in the crystal. The size of the precipitates is on the nanoscale, which plays a good role in dispersion precipitation strengthening and pinning dislocations, thereby improving mechanical properties.

[0056] The energy spectrum of the intragranular Nb precipitate phase in the super austenitic stainless steel prepared in Example 1 is shown in Figure 1. Figure 3 .

[0057] from Figure 3 As can be seen, some of the intragranular ((Nb,Mo)(B,C,N) precipitates have a size of around 20 nm. Analysis of their composition revealed that these precipitates contain elements such as Nb, Mo, B, C, and N. This indicates that during the formation of the Nb(C,N) precipitate, B and Mo dissolve into it, forming the (Nb,Mo)(B,C,N) precipitate. This finely dispersed distribution within the grains provides excellent precipitation strengthening and also influences the distribution of elements at grain boundaries, reducing the precipitation of Mo-containing precipitates at grain boundaries and refining the size of the grain boundary precipitates.

[0058] Electrochemical experiments were conducted on samples B and B+Nb in a three-electrode corrosion cell equipped with a platinum foil counter electrode and a saturated calomel (SCE) reference electrode. The results are shown in [Figure number missing]. Figure 4 .

[0059] from Figure 4As can be seen, the B+Nb sample has a lower current density, indicating that the passivation film dissolves more slowly, has a denser structure, and has better corrosion resistance. The B sample has a relatively low self-corrosion potential, indicating that the passivation film forms slowly and has poor stability. Therefore, the passivation film on the surface of the B+Nb sample is more stable, and the material has better pitting corrosion resistance.

[0060] The stress-strain curves of specimen B and specimen B+Nb were measured using a small tensile testing machine. The results are shown in [Figure number missing]. Figure 5 .

[0061] from Figure 5 As can be seen, compared with sample B, the yield strength and tensile strength of sample B+Nb are improved. Although the plasticity is slightly reduced, the overall plasticity is good.

[0062] Example 2

[0063] Same as Example 1, except that step (3) specifically involves cutting a 10×10mm steel plate sample for solution treatment (treatment temperature 1200℃, time 4h), then cooling it to 1050℃ in the furnace at a rate of 300℃ / h, holding it at that temperature for 30min (slow cooling and holding aging treatment), and then water cooling it to room temperature to obtain a super austenitic stainless steel sample. The grain boundary precipitation phase diagram is shown in [reference needed]. Figure 6 .

[0064] Example 3

[0065] Same as Example 1, except that the mass percentage of B is 0.01% and the mass percentage of Nb is 0.15%. See the grain boundary precipitation phase diagram below. Figure 6 .

[0066] Comparative Example 3

[0067] Same as Example 1, except that step (3) is as follows: a 10×10mm steel plate sample is cut and subjected to solution treatment (temperature 1200℃, time 4h). Without slow cooling and heat preservation, the solution-treated sample is directly heated at 1050℃ for 30min and then water-cooled to room temperature to obtain a super austenitic stainless steel sample. The sample is then ground, polished, and metallographically etched for observation of the microstructure (see Example 1). Figure 6 ).

[0068] Figure 6 In the figures, (a) is a grain boundary precipitate (B+Nb) diagram of the super austenitic stainless steel sample prepared in Example 2, (b) is a grain boundary precipitate (B+Nb) diagram of the super austenitic stainless steel sample prepared in Example 3, and (c) is a grain boundary precipitate (B+Nb) diagram of the super austenitic stainless steel sample prepared in Comparative Example 3.

[0069] from Figure 6As can be seen from the data, the samples prepared in Examples 2 and 3 are the same as those prepared in Example 1. However, Comparative Example 3 was not subjected to slow cooling and heat preservation treatment, and no ((Nb,Mo)(B,C,N) precipitates appeared in the crystal. Therefore, the Mo element segregated to the grain boundary. Compared with Examples 1 to 3, the number of grain boundary precipitates increased and some precipitates were larger in size.

[0070] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for improving the performance of super austenitic stainless steel by regulating niobium-containing precipitates with boron and niobium, characterized in that, Includes the following steps: Super austenitic stainless steel ingots containing elements B and Nb are homogenized and rolled into steel plates. Then, they are subjected to solution treatment and slow cooling and holding aging treatment in sequence, and finally water-cooled to room temperature to obtain super austenitic stainless steel. The content of element B in super austenitic stainless steel is 0.003~0.011 wt.%, and the content of element Nb in super austenitic stainless steel is 0.04~0.15 wt.%. The super austenitic stainless steel has the following chemical composition by mass percentage: C: 0.01~0.03%, Si: 0.5~1.0%, P: 0.01~0.03%, S: 0.005~0.01%, Mn: 1.5~2.0%, Cr: 19~24%, Ni: 17.0~22.0%, Mo: 5.5~6.5%, Cu: 0.5~1.5%, N: 0.15~0.3%, B: 0.003~0.011%, Nb: 0.04~0.15%, with the balance being Fe and other unavoidable impurity elements; The homogenization process is carried out at a temperature of 1200~1260℃ for 16~24h. The rolling temperature is 1000~1220℃; The solution treatment is performed at a temperature of 1160~1240℃ for 3~8 hours. The slow cooling and heat preservation aging treatment specifically includes: after solution treatment, cooling to 1020~1100℃ at a cooling rate of 250~300℃ / h, and holding for 10~60min.

2. A super austenitic stainless steel prepared by the method of claim 1.

3. The application of the super austenitic stainless steel according to claim 2 in the fields of energy conservation and environmental protection, marine engineering or petrochemicals.

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