A high-temperature grain coarsening resistant low-strength austenitic stainless steel and a method for manufacturing the same

Abstract

This invention discloses a low-strength austenitic stainless steel resistant to high-temperature grain coarsening and its preparation method. The chemical composition of the stainless steel is: C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr: 15~17%, Ni: 8~10%, Cu: 2.5~3.0%, N≤0.015%, Nb: 0.5~1.0%. The preparation method includes smelting, continuous casting, hot rolling, cold rolling, and solution annealing processes, wherein the solution temperature is ≥1120℃ and the holding time is 2~4 min / mm. This invention effectively reduces the yield strength of austenitic stainless steel, improves formability, reduces material processing difficulty, and significantly reduces the grain coarsening sensitivity of austenitic stainless steel, improves high-temperature grain coarsening resistance, and reduces the performance deterioration caused by high-temperature grain coarsening during high-temperature brazing.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel production technology, specifically relating to a low-strength austenitic stainless steel resistant to high-temperature grain coarsening and its preparation method. Background Technology

[0002] Copper is the main material used in air conditioning and refrigeration equipment piping. However, as the price of copper continues to rise, and because austenitic stainless steel has much higher corrosion resistance than copper and is the best type of stainless steel in terms of formability, relevant companies have begun to use austenitic stainless steel such as SUS304L as a substitute for the materials used in air conditioning and refrigeration equipment piping.

[0003] Traditional SUS304L stainless steel, due to its high strength, requires 2-3 softening annealing processes after pipe bending. Furthermore, for assembly with various modules in air conditioning and refrigeration equipment, high-temperature brazing is still necessary. The brazing process involves holding at approximately 1100 degrees Celsius for 30 minutes, followed by slow cooling in the furnace. This process requires SUS304L stainless steel to undergo approximately four heat treatments in total. Ultimately, this results in severe grain coarsening in the SUS304L pipe, especially in areas of intense bending and deformation. Because the stress cannot be completely eliminated, this grain coarsening at these locations leads to the most severe brittleness, causing a significant decrease in corrosion resistance. During the actual operation of air conditioning and refrigeration equipment, this can easily lead to vibration fractures and economic losses.

[0004] Therefore, developing an austenitic stainless steel with low strength to improve formability and low grain coarsening sensitivity to enhance resistance to high-temperature grain coarsening, for use as piping material for air conditioning and refrigeration equipment, reducing the difficulty of pipe processing and the deterioration of pipe performance caused by high-temperature grain coarsening, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] To address the aforementioned technical problems in the prior art, this invention provides a low-strength austenitic stainless steel resistant to high-temperature grain coarsening and its preparation method.

[0006] The chemical composition of the high-temperature resistant, grain-coarsening, low-strength austenitic stainless steel provided by this invention is controlled by mass percentage as follows: C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr: 15~17%, Ni: 8~10%, Cu: 2.5~3.0%, N≤0.015%, Nb: 0.5~1.0%, with the balance being Fe and unavoidable impurity elements.

[0007] Furthermore, the aforementioned high-temperature resistant, low-strength austenitic stainless steel with coarsened grains has a yield strength ≤200MPa, elongation ≥60%, and hardness ≤120HV.

[0008] The method for preparing low-strength austenitic stainless steel with high-temperature resistant grain coarsening provided by this invention includes smelting, continuous casting, hot rolling, cold rolling, and solution annealing processes, wherein: (1) In the smelting process, the chemical composition of the molten steel obtained by smelting is controlled by mass percentage as follows: C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr:15~17%, Ni:8~10%, Cu:2.5~3.0%, N≤0.015%, Nb:0.5~1.0%, with the balance being Fe and unavoidable impurity elements; (2) In the continuous casting process, the molten steel is continuously cast into slabs; (3) In the hot rolling process, the slab is heated in a heating furnace and then hot rolled; (4) In the cold rolling process, the hot-rolled steel strip is pickled and then cold-rolled into stainless steel cold-rolled steel strip of the target specifications; (5) In the solution annealing process, the cold-rolled stainless steel strip obtained by cold rolling is subjected to solution annealing treatment. The solution temperature is controlled at ≥1120℃ and the holding time is controlled at 2~4min / mm according to the thickness of the steel strip.

[0009] Furthermore, in the above-mentioned method for preparing low-strength austenitic stainless steel with high-temperature grain coarsening, molten steel is obtained through electric furnace smelting, AOD refining, VOD refining, and LF refining in the smelting process, and Nb is alloyed in the LF refining stage.

[0010] Furthermore, in the above-mentioned method for preparing low-strength austenitic stainless steel with high-temperature grain coarsening, in the hot rolling process, the slab heating temperature is controlled at ≥1240℃, the holding time is controlled at ≥200min, the roughing rolling exit temperature is controlled at 1100℃~1200℃, the finishing rolling temperature is controlled at ≥950℃, and the coiling temperature is controlled at ≤680℃.

[0011] Furthermore, in the above-mentioned method for preparing low-strength austenitic stainless steel with high-temperature grain coarsening, in the hot rolling process, the slab heating temperature is controlled at 1260±20℃, the holding time is controlled at 210±10min, the roughing rolling exit temperature is controlled at 1150℃±20℃, the finishing hot rolling final rolling temperature is controlled at 1050℃±20℃, and the coiling temperature is controlled at ≤680℃.

[0012] Furthermore, in the above-mentioned method for preparing low-strength austenitic stainless steel with high-temperature resistant grain coarsening, in the solution annealing process, the solution temperature is controlled at 1130±10℃, the holding time is controlled at 2.5min / mm according to the steel strip thickness, and the grain size of the finished material is controlled at grade 5~8.

[0013] The high-temperature resistant, low-strength austenitic stainless steel with coarsening properties and its preparation method of the present invention have the following advantages and beneficial effects: This invention develops a low-strength austenitic stainless steel resistant to high-temperature grain coarsening and its preparation method through chemical composition control and solid solution treatment design. It effectively reduces the yield strength of austenitic stainless steel, thereby improving formability, reducing material processing difficulty, and reducing the number of softening annealing cycles. At the same time, it significantly reduces the grain coarsening sensitivity of austenitic stainless steel, thereby improving its resistance to high-temperature grain coarsening and reducing the performance degradation caused by high-temperature grain coarsening during high-temperature brazing. This promotes the rapid and widespread application of austenitic stainless steel in the field of air conditioning and refrigeration equipment manufacturing. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art 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. In the drawings: Figure 1 This is a photograph of the metallographic roughening structure of the austenitic stainless steel of Example 2 of the present invention after high-temperature treatment.

[0015] Figure 2 The image shows the metallographic coarsening structure of conventional SUS304L stainless steel after high-temperature treatment, as shown in Comparative Example 3. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0017] This invention develops a low-strength, high-temperature-resistant austenitic stainless steel by coarsening chemical composition and solid solution treatment in the preparation process. This reduces the yield strength and grain coarsening rate of austenitic stainless steel, thereby reducing the difficulty of material processing and the performance degradation caused by high-temperature grain coarsening, and promoting the rapid and widespread application of austenitic stainless steel in the field of air conditioning and refrigeration equipment manufacturing.

[0018] The chemical composition of the high-temperature resistant, grain-coarsening, low-strength austenitic stainless steel provided by this invention is controlled by mass percentage as follows: C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr: 15~17%, Ni: 8~10%, Cu: 2.5~3.0%, N≤0.015%, Nb: 0.5~1.0%, with the balance being Fe and unavoidable impurity elements.

[0019] The chemical composition design of the high-temperature resistant, grain-coarsening, low-strength austenitic stainless steel of this invention aims to produce an austenitic structure at room temperature. The functions of the required elements and their content control ranges are described below: (1) Carbon and nitrogen elements are easy to combine with niobium to form Nb(C, N) compounds. In order to avoid the formation of coarse Nb(C, N) compounds during the production of austenitic stainless steel when the Nb content is high, which may lead to rolling cracks and serious deterioration of elongation, it is necessary to reduce the C and N content as much as possible. Therefore, in this invention, the C and N contents are controlled to C≤0.015% and N≤0.015%, respectively.

[0020] (2) Chromium mainly plays a solid solution strengthening role in materials. Excessive chromium content will lead to an increase in the yield strength of the material. Chromium is also the most critical element to ensure the corrosion resistance of the material. Therefore, in this invention, the Cr content is controlled at 15~17%.

[0021] (3) Nickel and copper are key elements for the formation of stable austenitic phase in stable materials. At the same time, they can provide high stacking fault energy and promote dislocation cross-slip during the plastic deformation of materials. They are key elements to further ensure that the yield strength of materials remains at a low level. Therefore, in this invention, the contents of Ni and Cu are controlled as Ni: 8~10% and Cu: 2.5~3.0%, respectively.

[0022] (4) On the one hand, the influence of niobium on the mechanical properties of austenitic stainless steel: Niobium will increase the recrystallization temperature of austenitic stainless steel and prevent grain coarsening. At the same time, niobium will also greatly increase the solid solution temperature and time required in the raw material production process. Therefore, the niobium content should be limited to no more than 1.0%. Meanwhile, niobium is easy to form Nb(C, N) compounds with carbon and nitrogen. When the niobium content is less than 0.5%, due to the small size of the Nb(C, N) compounds, the yield strength of the material will increase due to the grain refinement of stainless steel and the strong effect of Nb on the solid solution strengthening of the lattice. Therefore, the niobium content should be controlled to no less than 0.5% to avoid the formation of too small Nb(C, N) compounds. On the other hand, the influence of niobium on the intergranular corrosion resistance of austenitic stainless steel: During the brazing process or during use at 450~850℃, carbon will form chromium carbonate Cr with Cr. 23C6 promotes the formation of chromium-depleted zones, reducing their corrosion resistance. During brazing, Nb preferentially forms compounds with Cr (C, N), preventing the formation of chromium-depleted zones and ensuring the material's corrosion resistance. Therefore, considering the above factors, the Nb content is controlled at 0.5~1.0% in this invention.

[0023] (5) Manganese and silicon have solid solution strengthening effect on materials. In order to achieve low material strength, they should be kept at a low level. Therefore, in this invention, the content of Mn and Si is controlled to Mn≤1.3% and Si≤0.5%, respectively.

[0024] (6) Phosphorus and sulfur are harmful elements in materials and should be kept as low as possible. Therefore, in this invention, the content of P and S is controlled to P≤0.040% and S≤0.010%, respectively.

[0025] The method for preparing low-strength austenitic stainless steel with high-temperature resistant grain coarsening provided by this invention includes smelting, continuous casting, hot rolling, cold rolling, and solution annealing processes, wherein: (1) In the smelting process, the chemical composition of the molten steel obtained by smelting is controlled by mass percentage as follows: C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr:15~17%, Ni:8~10%, Cu:2.5~3.0%, N≤0.015%, Nb:0.5~1.0%, with the balance being Fe and unavoidable impurity elements; (2) In the continuous casting process, the above-mentioned molten steel is continuously cast into slabs; (3) In the hot rolling process, the above-mentioned slab is heated in a heating furnace and then hot rolled; (4) In the cold rolling process, the hot-rolled steel strip is pickled and then cold-rolled into stainless steel cold-rolled steel strip of the target specifications; (5) In the solution annealing process, the cold-rolled stainless steel strip obtained by cold rolling is subjected to solution annealing treatment. The solution temperature is controlled at ≥1120℃ and the holding time is controlled at 2~4min / mm according to the thickness of the steel strip.

[0026] Preferably, in the smelting process, molten steel is obtained by electric furnace smelting, AOD refining, VOD refining and LF refining, and Nb alloying is carried out in the LF refining stage.

[0027] Preferably, in the hot rolling process, the slab heating temperature is controlled at ≥1240℃, the holding time is controlled at ≥200min, the roughing mill exit temperature is controlled at 1100℃~1200℃, the finishing mill final rolling temperature is controlled at ≥950℃, and the coiling temperature is controlled at ≤680℃.

[0028] Preferably, in the hot rolling process, the slab heating temperature is controlled at 1260±20℃, the holding time is controlled at 210±10min, the roughing mill exit temperature is controlled at 1150℃±20℃, the finishing mill final rolling temperature is controlled at 1050℃±20℃, and the coiling temperature is controlled at ≤680℃.

[0029] Preferably, in the cold rolling process, the cold rolling reduction is controlled at 80%.

[0030] Preferably, in the solution annealing process, the solution temperature is controlled at 1130±10℃, the holding time is controlled at 2.5min / mm according to the steel strip thickness, the grain size of the finished material is controlled at grade 5~8, and after pickling, a low-strength austenitic stainless steel product with high temperature resistance and coarsened grains is obtained.

[0031] The following describes in detail the high-temperature resistant, grain-coarsening, low-strength austenitic stainless steel and its preparation method, with reference to embodiments and comparative examples of the present invention.

[0032] Example 1 The high-temperature resistant, grain-coarsening, low-strength austenitic stainless steel of Example 1 has the following chemical composition (mass percentage, %): C: 0.012%, Si: 0.40%, Mn: 1.10%, P: 0.025%, S: 0.002%, Cr: 15.0%, Ni: 8.5%, Cu: 2.8%, N: 0.013%, Nb: 0.6%, with the balance being Fe and unavoidable impurity elements.

[0033] The specific implementation steps of the method for preparing low-strength austenitic stainless steel with high-temperature resistant grain coarsening in Example 1 include: (1) Smelting Molten steel is produced by smelting in an electric furnace, AOD refining, and LF refining. The chemical composition of the molten steel by mass percentage is as follows: C: 0.012%, Si: 0.40%, Mn: 1.10%, P: 0.025%, S: 0.002%, Cr: 15.0%, Ni: 8.5%, Cu: 2.8%, N: 0.013%, Nb: 0.6%, with the balance being Fe and unavoidable impurity elements.

[0034] (2) Continuous casting The molten steel was continuously cast into a slab with a thickness of 200 mm.

[0035] (3) Hot rolling The continuously cast slab is heated in a heating furnace to 1260±20℃, held for 210±10 min, and then hot rolled. The roughing exit temperature is 1150℃±20℃, the finishing rolling temperature is 1050℃±20℃, and the coiling temperature is ≤680℃.

[0036] (4) Cold rolling The hot-rolled steel strip is pickled and then cold-rolled with a cold rolling reduction of 80% to obtain a stainless steel cold-rolled strip with a target thickness of 0.7 mm. (5) Solution annealing The cold-rolled stainless steel strip was subjected to solution annealing at a temperature of 1130±10℃ and a holding time of 2.5min. The grain size of the finished material was controlled to be grade 5~8. After pickling, a low-strength austenitic stainless steel product with high temperature resistance and coarsened grain was obtained.

[0037] Other embodiments and comparative examples The chemical compositions of the austenitic stainless steels of Examples 2-3 and Comparative Examples 1-3 are shown in Table 1 below, wherein the austenitic stainless steel of Comparative Example 3 is conventional SUS304L stainless steel.

[0038] C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr: 15~17%, Ni: 8~10%, Cu: 2.5~3.0%, N≤0.015%, Nb: 0.5~1.0%. Table 1 shows the composition (%) of austenitic stainless steel in the examples and comparative examples. ; The preparation methods of austenitic stainless steel in Examples 2-3 are similar to those in Example 1. The preparation methods of Comparative Examples 1-3 are consistent with the existing general austenitic stainless steel production methods, and will not be described in detail here.

[0039] Mechanical properties of the austenitic stainless steels of Examples 1-3 and Comparative Examples 1-3 of the present invention were tested, and the results are shown in Table 2.

[0040] Table 2 Performance test results of austenitic stainless steel in the examples and comparative examples ; As can be seen from Table 2, the present invention, through coordinated control of chemical composition (especially control of Nb element content) and solid solution system design in the preparation process, achieves a finished product yield strength ≤200MPa, elongation ≥60%, and hardness ≤120HV, exhibiting good formability. In contrast, the existing austenitic stainless steel, due to its chemical composition and content exceeding the scope of the present invention, has significantly higher yield strength and hardness, resulting in poorer formability.

[0041] The austenitic stainless steels of Examples 1-3 and the conventional SUS304L of Comparative Example 3 were subjected to two high-temperature treatments at 1130℃ for 30 min each, and the changes in grain size were detected. The results are shown in Table 3. For the metallographic coarsening microstructure of the austenitic stainless steel of Example 2 after high-temperature treatment, please refer to [the table / document name]. Figure 1For Comparative Example 3, see the metallographic coarsening structure of conventional SUS304L stainless steel after high-temperature treatment. Figure 2 .

[0042] From Table 3 and Figure 1-2 It can be seen that by coordinating and controlling the chemical composition and designing the solid solution regime in the preparation process, this invention can effectively reduce the grain coarsening sensitivity of austenitic stainless steel, and the grain coarsening rate can be controlled below 70%, preventing the material from deteriorating in performance due to high-temperature grain coarsening, such as embrittlement and reduced corrosion resistance.

[0043] Table 3. Comparison of grain size after two high-temperature treatments at 1130℃+30min in the examples and comparative examples.

[0044] In summary, this invention, through chemical composition control and solid solution treatment design, develops a low-strength austenitic stainless steel resistant to high-temperature grain coarsening and its preparation method. This effectively reduces the yield strength of the austenitic stainless steel, thereby improving its formability, reducing material processing difficulty, and decreasing the number of softening annealing cycles. Simultaneously, it significantly reduces the grain coarsening sensitivity of the austenitic stainless steel, thereby improving its resistance to high-temperature grain coarsening and reducing performance degradation caused by high-temperature grain coarsening during high-temperature brazing. This promotes the rapid and widespread application of austenitic stainless steel in the manufacturing of air conditioning and refrigeration equipment.

[0045] In the description of this specification, references to terms such as "embodiment," "example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, those skilled in the art can combine or combine the different embodiments or examples described in this specification and the features therein without causing contradiction.

[0046] It should be noted that, unless otherwise specified, the terms used herein have the meanings commonly understood by those skilled in the art. Furthermore, when a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum. Further, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Moreover, when multiple ranges are provided to describe features, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0047] It should also be noted that, in this document, the term "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A low-strength austenitic stainless steel resistant to high-temperature grain coarsening, characterized in that, The chemical composition of the high-temperature resistant, grain-coarsening, low-strength austenitic stainless steel is controlled by mass percentage as follows: C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr: 15~17%, Ni: 8~10%, Cu: 2.5~3.0%, N≤0.015%, Nb: 0.5~1.0%, with the balance being Fe and unavoidable impurity elements.

2. The low-strength austenitic stainless steel resistant to high-temperature grain coarsening according to claim 1, characterized in that, The high-temperature resistant, grain-coarsened, low-strength austenitic stainless steel has a yield strength ≤200MPa, elongation ≥60%, and hardness ≤120HV.

3. A method for preparing low-strength austenitic stainless steel resistant to high-temperature grain coarsening, comprising smelting, continuous casting, hot rolling, cold rolling, and solution annealing processes, characterized in that: (1) In the smelting process, the chemical composition of the molten steel obtained by smelting is controlled by mass percentage as follows: C≤0.015%, Si≤0.5%, Mn≤1.3%, P≤0.040%, S≤0.010%, Cr:15~17%, Ni:8~10%, Cu:2.5~3.0%, N≤0.015%, Nb:0.5~1.0%, with the balance being Fe and unavoidable impurity elements; (2) In the continuous casting process, the molten steel is continuously cast into slabs; (3) In the hot rolling process, the slab is heated in a heating furnace and then hot rolled; (4) In the cold rolling process, the hot-rolled steel strip is pickled and then cold-rolled into stainless steel cold-rolled steel strip of the target specifications; (5) In the solution annealing process, the cold-rolled stainless steel strip obtained by cold rolling is subjected to solution annealing treatment. The solution temperature is controlled at ≥1120℃ and the holding time is controlled at 2~4min / mm according to the thickness of the steel strip.

4. The method for preparing low-strength austenitic stainless steel with high-temperature grain coarsening resistance according to claim 3, characterized in that, In the smelting process, molten steel is produced through electric furnace smelting, AOD refining, VOD refining, and LF refining, and Nb is alloyed during the LF refining stage.

5. The method for preparing low-strength austenitic stainless steel with high-temperature grain coarsening resistance according to claim 3, characterized in that, In the hot rolling process, the slab heating temperature is controlled at ≥1240℃, the holding time is controlled at ≥200min, the roughing mill exit temperature is controlled at 1100℃~1200℃, the finishing mill final rolling temperature is controlled at ≥950℃, and the coiling temperature is controlled at ≤680℃.

6. The method for preparing low-strength austenitic stainless steel with high-temperature resistant grain coarsening according to claim 3, characterized in that, In the hot rolling process, the slab heating temperature is controlled at 1260±20℃, the holding time is controlled at 210±10min, the roughing mill exit temperature is controlled at 1150℃±20℃, the finishing mill final rolling temperature is controlled at 1050℃±20℃, and the coiling temperature is controlled at ≤680℃.

7. The method for preparing low-strength austenitic stainless steel with high-temperature grain coarsening resistance according to claim 3, characterized in that, In the solution annealing process, the solution temperature is controlled at 1130±10℃, the holding time is controlled at 2.5min / mm according to the steel strip thickness, and the grain size of the finished material is controlled at grade 5~8.

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