Stainless steel materials that are easy to cast and shape

Abstract

This invention discloses a stainless steel material comprising 14.6-16.1 wt% chromium (Cr), 5.4-6.2 wt% nickel (Ni), 1.5-2.2 wt% silicon (Si), 0.001-0.05 wt% carbon (C), 0.3-0.8 wt% manganese (Mn), 2.5-3.2 wt% copper (Cu), 1.2-2.0 wt% cobalt (Co), and 0.2-0.3 wt% niobium (Nb), with the remainder containing iron (Fe) and unavoidable impurities. This stainless steel material provides better fluidity during casting, facilitating the formation of thin castings, and allows for adjustment of yield strength as needed, making shaping easier, while maintaining high tensile strength and impact resistance. Its fracture strength is significantly improved compared to traditional duplex stainless steel.

Description

Technical Field

[0001] This invention relates to stainless steel materials, and more particularly to a type of stainless steel material suitable for casting and shaping. By adjusting the specific proportions of each material, it can achieve better formability when used to manufacture thin castings such as propellers and golf club heads, and facilitates the adjustment of the casting shape and specifications. Prior Technology

[0002] The shipbuilding industry includes not only shipyards responsible for hull construction, but also many manufacturers of related parts, such as ship propeller manufacturers, ship hardware manufacturers, ship information equipment manufacturers, and ship fitting manufacturers. Propellers are used in everything from large container ships, cruise ships, and warships to small and medium-sized fishing boats, yachts, and jet skis. The propeller is driven by a motor to rotate and propel the boat forward. Materials used to manufacture marine propellers can be plastic or metal. Since propellers are immersed in water for a long time, they need to have good corrosion resistance and impact resistance, so high-strength and corrosion-resistant materials are usually selected. Chinese Patent Publication Nos. 103643160A and 116426725A respectively disclose 17-4 and 15-5 stainless steel materials. 17-4 and 15-5 stainless steel materials are precipitation hardening stainless steels, which are widely used in aerospace, golf equipment, ship parts or industrial parts and other fields. However, when used for casting, they have the disadvantages of poor fluidity and obvious chromium pitting, which can easily lead to defects in the forming surface and failure, thereby increasing the manufacturing cost.

[0003] To achieve better flowability during manufacturing, US Patent Publication No. 20040042926A1 discloses a high-silicon-content stainless steel material. While this material achieves high tensile strength, it sacrifices impact resistance. If used to manufacture ship propellers or golf club heads, it would increase the risk of propeller and golf club head breakage. Furthermore, during casting, the varying cooling rates in different areas create shrinkage stress, leading to deformation of the casting. Deformed castings deviate from specifications, requiring reshaping. However, castings with high yield strength are difficult to reshape; therefore, appropriately controlling the yield strength is beneficial for reshaping.

[0004] Therefore, the inventors sought to provide a stainless steel material with good casting fluidity and easy shaping to reduce the probability of surface defects during product forming and to adjust the size to meet specifications. This material also possesses superior impact toughness compared to traditional 15-5 stainless steel and higher yield strength and tensile strength than traditional duplex steel. Summary of the Invention

[0005] Given the numerous shortcomings of existing stainless steel materials, the main objective of this invention is to provide a stainless steel material that is easy to cast and shape. This is achieved by adding appropriate proportions of materials such as chromium (Cr), nickel (Ni), cobalt (Co), and silicon (Si) to create a stainless steel material with properties such as high flowability, easy shaping, corrosion resistance, impact resistance, and high tensile strength.

[0006] To achieve the aforementioned main objectives, the present invention provides a stainless steel material for manufacturing ship propellers. The stainless steel material comprises 14.6-16.1 wt% chromium (Cr), 5.4-6.2 wt% nickel (Ni), 1.5-2.2 wt% silicon (Si), 0.001-0.05 wt% carbon (C), 0.3-0.8 wt% manganese (Mn), 2.5-3.2 wt% copper (Cu), 1.2-2.0 wt% cobalt (Co), and 0.2-0.3 wt% niobium (Nb), with the remainder comprising iron (Fe) and unavoidable impurities.

[0007] By utilizing the aforementioned technical characteristics, stainless steel materials with an appropriate silicon (Si) content can significantly improve their fluidity. In the manufacturing process of ship propellers, this not only aids in propeller forming but also, for stainless steel materials with higher chromium (Cr) content, reduces chromium pitting on the surface of castings, thus increasing the product manufacturing success rate. Furthermore, excessively fluid stainless steel materials can lead to reduced impact toughness. Therefore, the content of materials such as chromium (Cr), nickel (Ni), cobalt (Co), and silicon (Si) in stainless steel materials is controlled within a specific range to achieve optimal mechanical properties.

[0008] Preferably, an alloy containing chromium (Cr), nickel (Ni), silicon (Si), carbon (C), manganese (Mn), copper (Cu), cobalt (Co), niobium (Nb), and iron (Fe) is heated to 1600℃-1700℃ to form a molten iron mixture. This molten iron mixture is then added to a ceramic mold at 1100℃-1150℃. After cooling, the mold is removed, and a solution treatment is performed at 1050℃-1100℃ for 1.5 hours. H1100 precipitation hardening can be selectively performed to obtain stainless steel. Therefore, compared with conventional 17-4 and 15-5 stainless steel and duplex stainless steel, the stainless steel material of this invention exhibits superior mechanical properties.

[0009] Preferably, the stainless steel material further contains 0.01-0.3 wt% molybdenum (Mo). Molybdenum can combine with carbon to precipitate carbides, thereby increasing the hardness and corrosion resistance of the stainless steel material. However, to maintain a certain level of impact resistance, the molybdenum content is controlled below 0.3 wt%.

[0010] Preferably, the stainless steel material further contains 0.01-0.05 wt% vanadium (V). Vanadium can then combine with carbon to precipitate carbides, increasing the hardness of the stainless steel material. However, to maintain a certain level of impact resistance, the vanadium content is controlled below 0.05 wt%.

[0011] Preferably, the stainless steel material further contains 0.01-0.06 wt% nitrogen (N). Nitrogen is used to improve the strength of the stainless steel material, but the nitrogen content is controlled below 0.06 wt% to maintain a certain level of impact resistance and avoid nitrogen porosity defects.

[0012] Preferably, the stainless steel material further contains 0.01-0.04 wt% phosphorus (P). Phosphorus is an impurity in stainless steel, and its content needs to be minimized. Controlling the phosphorus content to below 0.04 wt% is a more economically viable ratio, based on the cost of refining and purification.

[0013] Preferably, the stainless steel material further contains 0.001-0.03 wt% sulfur (S). Sulfur is an impurity in the stainless steel material, and its content needs to be minimized. Furthermore, to maintain better mechanical properties, the sulfur content is controlled below 0.03 wt%.

[0014] Preferably, the silicon (Si) content of the stainless steel material is 1.52-1.98 wt%. This allows for the production of stainless steel materials in a more optimal ratio, resulting in products with the best mechanical properties.

[0015] Detailed descriptions of the structure, characteristics, assembly, and use of the stainless steel material provided by this invention will be provided in the subsequent detailed descriptions of the embodiments. However, those skilled in the art will understand that such detailed descriptions and the specific embodiments listed for implementing this invention are merely illustrative and not intended to limit the scope of the patent application. Simple Explanation of the Diagram

[0016] Figure 1 is a Schaeffler diagram of the present invention, showing the metallographic structure of the embodiments of the present invention and other stainless steel materials. Implementation

[0017] The stainless steel material of this invention mainly comprises 14.6-16.1 wt% chromium (Cr), 5.4-6.2 wt% nickel (Ni), 1.5-2.2 wt% silicon (Si), 0.001-0.05 wt% carbon (C), 0.3-0.8 wt% manganese (Mn), 2.5-3.2 wt% copper (Cu), 1.2-2.0 wt% cobalt (Co), and 0.2-0.3 wt% niobium (Nb), with the remainder containing iron (Fe) and unavoidable impurities.

[0018] The stainless steel material of this invention is suitable for manufacturing ship propellers. In order to test the properties of this stainless steel material, the stainless steel material of this invention is also used to make standard test pieces for mechanical property testing. The standard test pieces for mechanical property testing are made in accordance with the G5121 standard of Japanese Industrial Standards (JIS) that specifies stainless steel castings.

[0019] First, molds for ship propellers and standard test pieces for mechanical property testing are made separately. After the wax pattern of the mold is made, it goes through five repeated processes of dipping, stuccoing, and drying. After removing the wax with steam dewaxing, a ceramic shell is obtained.

[0020] Materials containing chromium, nickel, silicon, carbon, manganese, copper, cobalt, niobium, and iron are heated to 1620°C and mixed into a molten iron mixture. The mold is then heated to 1150°C, and the molten iron mixture is poured into the mold. After cooling, the ingate is removed through a de-shell process, and sandblasting is performed to obtain a rough casting.

[0021] The rough blank is then subjected to heat treatment to improve its strength and corrosion resistance. The product is placed in a vacuum furnace and subjected to solution treatment at 1050°C for 1.5 hours. Some mechanical property tests are performed using standard test pieces with H1100 precipitation hardening. The resulting propeller is further shaped and processed to meet product specifications, and then surface treated to increase the smoothness, roughness, and gloss of the product surface.

[0022] In the embodiments, multiple standard test specimen blanks for testing the mechanical properties of stainless steel were prepared in different proportions, and then processed into tensile test bars conforming to the American Society for Testing and Materials (ASTM) standard E8 / E8M for tensile testing of metals and impact test specimens conforming to the Japanese Industrial Standards (JIS) standard Z2202 for impact testing of metallic materials, in order to test the mechanical properties of stainless steel. The stainless steel materials will be described and tested using the following three embodiments, X804-1, X804-2, and X804-3.

[0023] The composition of Examples X804-1, X804-2 and X804-3 is shown in Table 1.

[0024] Table 1: Example chromium nickel Silicon carbon manganese molybdenum vanadium copper cobalt niobium nitrogen X804-1 14.83 5.81 1.73 0.02 0.40 0.10 0.03 2.62 1.62 0.24 0.04 X804-2 15.36 5.97 1.82 0.02 0.39 0.09 0.03 2.84 1.61 0.23 0.04 X804-3 16.08 5.85 1.87 0.02 0.37 0.09 0.03 2.81 1.59 0.23 0.04

[0025] Chromium forms a protective chromium oxide film on the surface of stainless steel, primarily to isolate it from oxygen and iron, thus preventing rust. Adding a higher chromium content enhances the rust resistance of stainless steel. Nickel possesses excellent corrosion resistance; when used in combination with chromium to produce stainless steel, it promotes the formation of the chromium oxide film, maintaining the rust-resistant properties of the stainless steel.

[0026] Traditional stainless steel materials 15-5 or 17-4 typically contain less than 1 wt% silicon. However, Table 1 shows that the silicon content of this invention is increased to 1.5-2.2 wt%. A silicon content of 1.5 wt% or more can improve the fluidity of the molten iron mixture during casting, which is beneficial for the forming of propellers or thin castings and reduces the occurrence of chromium pitting on the surface of propellers and thin castings. In order to avoid the decrease in impact resistance caused by adding too much silicon, the silicon content is also limited to less than 2.2 wt%, but the optimal silicon content is 1.52-1.98 wt% to achieve the best balance between fluidity and impact resistance.

[0027] Carbon is mainly used to increase the hardness of stainless steel, but too much carbon will lead to the precipitation of too many carbides, which will reduce impact toughness and affect corrosion resistance. Therefore, the carbon content is limited to 0.001-0.05 wt%, and other elements are used to increase the hardness of stainless steel.

[0028] Cobalt and niobium can improve the stability of stainless steel at high temperatures and also give it good impact resistance.

[0029] Molybdenum and vanadium can combine with carbon to precipitate carbides, thereby increasing the hardness of stainless steel. Molybdenum can also repair the chromium oxide protective film and improve its stability to maintain the corrosion resistance of stainless steel. In order to maintain a certain impact resistance, the content of molybdenum is limited to 0.01-0.3 wt%, and the content of vanadium is limited to 0.01-0.05 wt% to achieve the optimal ratio. However, in this invention, it is also possible to choose not to add molybdenum and vanadium.

[0030] Table 1 does not list the composition of phosphorus and sulfur. Phosphorus and sulfur are considered impurities in stainless steel. Although sulfur makes stainless steel easier to process, it also means that mechanical properties will decrease. Therefore, the sulfur content is limited to 0.001-0.03 wt%. The lower the phosphorus content, the better. Except for the use of more expensive refining methods, the phosphorus content is usually limited to 0.01-0.04 wt% in the example of simply heating to produce a molten iron mixture.

[0031] Furthermore, while nitrogen can increase the strength of stainless steel, it also reduces its impact resistance. Excessive nitrogen content can also lead to nitrogen porosity defects. Therefore, the nitrogen content is limited to 0.01-0.06 wt%.

[0032] The nickel equivalent (Nieq) and chromium equivalent (Creq) values ​​for Examples X804-1, X804-2, and X804-3 are shown in Table 2, and Figure 1 can also be consulted, which is a Schaeffler diagram of conventional 15-5 and 17-4 stainless steel materials and the three examples.

[0033] Table 2: Material Nieq Creq 15-5 8.0 17.4 17-4 8.2 18.5 X804-1 10.02 19.01 X804-2 10.23 19.69 X804-3 10.07 20.51

[0034] The formula for calculating nickel equivalent is: Nieq = Ni + Co + 0.5 (manganese) + 0.3 (copper) + 25 (nitrogen) + 30 (carbon).

[0035] The formula for calculating chromium equivalent is: Creq = Cr + 2(silicon) + 1.5(molybdenum) + 5(vanadium) + 5.5(aluminum) + 1.75(niobium) + 1.5(titanium) + 0.75(tungsten).

[0036] As shown in Figure 1, the positions of X804-1, X804-2 and X804-3 in the Schaeffler diagram are basically stainless steel materials with a dual-phase structure of Austenite + Ferrite, but they are closer to the metallographic structure of Austenite.

[0037] In stainless steel, manganese and sulfur form manganese sulfide (MnS), which reduces the chance of ferrous sulfide (FeS) formation. Therefore, the example contains 0.3-0.8 wt% manganese, which can reduce hot tearing during casting. However, if more than 0.8 wt% manganese is added, the hardness of the stainless steel will decrease significantly.

[0038] Examples X804-1, X804-2, and X804-3 were subjected to tensile and impact tests in the cast, solid solution, and aging states, respectively. The test results for the cast state are shown in Table 3, the test results for the solid solution state are shown in Table 4, and the test results for the aging state are shown in Table 5.

[0039] Table 3 (As-cast condition): Material YS(Mpa) TS (MPa) EL(%) Impact toughness (J) X804-1 469 974 4 53.2 X804-2 391 832 9 71.3 X804-3 302 647 9 169

[0040] Table 4 (Solid Solution State): Material sample YS(Mpa) TS (MPa) EL(%) Impact toughness (J) X804-1 A 561 1035 15 69.6 B 561 1039 14 C 586 1022 14 average 569 1032 14 X804-2 A 461 1006 19 70.5 B 464 1009 14 C 457 1012 16 average 461 1009 16 X804-3 A 367 946 16 80.5 B 386 971 16 C 368 959 twenty one average 374 959 18

[0041] Table 5 (Precipitated State): Material sample YS(Mpa) TS (MPa) EL(%) Impact toughness (J) X804-1 A 561 1082 19 64.6 B 579 1062 17 C 574 1069 17 average 571 1071 18 X804-2 A 503 1083 15 46 B 495 1066 18 C 502 1067 15 average 500 1072 16 X804-3 A 412 1038 16 59.3 B 407 1030 15 C 402 1023 19 average 407 1030 17

[0042] In Tables 3 to 5, YS represents yield strength, TS represents tensile strength, EL represents elongation, and impact toughness represents impact resistance.

[0043] Based on the mechanical properties exhibited after testing in the three embodiments X804-1, X804-2, and X804-3, the yield strength decreased significantly with increasing chromium equivalent, regardless of whether it was in the cast, solution, or precipitated state. Therefore, the required yield strength can be achieved by adjusting the chromium equivalent. The tensile strength remained relatively stable between 1000-1100 MPa. The impact toughness remained above 45 J, while traditional 15-5 and 17-4 stainless steel materials exhibited impact toughness of only 10-30 J, or even below 10 J. This indicates that the impact toughness of the stainless steel material of this invention is significantly superior to that of traditional stainless steel materials.

[0044] Compared with the duplex steel for casting specified in ASTM A890, which has a yield strength of 415-485 MPa and a tensile strength of 620-690 MPa, the three embodiments of this invention, X804-1, X804-2 and X804-3, have a yield strength of 302-586 MPa and a tensile strength of 647-1083 MPa, which are significantly better than traditional duplex steel for casting in terms of strength.

[0045] In addition, to prevent the mechanical properties from deteriorating due to excessively high chromium equivalent, the chromium content should ideally be controlled at 14.6-15.2 wt%. Alternatively, depending on the ease of shaping, the chromium content can be controlled at 15.3-16.1 wt%. However, the nickel content should be controlled at 5.4-6.2 wt%, and the cobalt content at 1.2-2.0 wt%, in order to maintain a certain strength and resistance to deformation of the stainless steel material, while also ensuring good impact resistance.

[0046] In summary, the stainless steel material provided by this invention has at least the following advantages compared with the prior art:

[0047] 1. The stainless steel material of the present invention, by adjusting the silicon content to increase the fluidity during casting, can help form the shape of ship propellers or other thin products during casting, and also reduce the defects of chromium pitting on the surface, thereby increasing the success rate of manufacturing thin castings and effectively reducing manufacturing costs.

[0048] 2. The stainless steel material of the present invention is composed of appropriate amounts of chromium, nickel, cobalt and other materials to form a stainless steel material with properties such as corrosion resistance, impact resistance and high strength. After testing, the impact toughness is significantly better than that of traditional 15-5 and 17-4 stainless steel materials, and the strength is also better than that of traditional duplex steel materials.

[0049] Finally, it must be stated again that the constituent elements disclosed in the foregoing embodiments of the present invention are merely illustrative examples and are not intended to limit the scope of this application. Substitutions or variations of other equivalent elements should also be covered by the scope of the patent application in this application.

Claims

1. A stainless steel material for manufacturing ship propellers, the stainless steel material comprising 14.6-16.1 wt% chromium (Cr), 5.4-6.2 wt% nickel (Ni), 1.5-2.2 wt% silicon (Si), 0.001-0.05 wt% carbon (C), 0.3-0.8 wt% manganese (Mn), 2.5-3.2 wt% copper (Cu), 1.2-2.0 wt% cobalt (Co), and 0.2-0.3 wt% niobium (Nb), the remainder comprising iron (Fe) and unavoidable impurities; wherein, An alloy containing chromium (Cr), nickel (Ni), silicon (Si), carbon (C), manganese (Mn), copper (Cu), cobalt (Co), niobium (Nb), and iron (Fe) is heated to 1600℃-1700℃ and mixed into a molten iron mixture. This molten iron mixture is then added to a ceramic mold at 1100℃-1150℃. After cooling, the mold is removed, and a solution treatment is performed at 1050℃-1100℃ for 1.5 hours. H1100 precipitation hardening can be selectively performed to obtain the stainless steel material.

2. The stainless steel material as claimed in claim 1, further comprising 0.01-0.3 wt% molybdenum (Mo).

3. The stainless steel material as claimed in claim 1, further comprising 0.01-0.05 wt% vanadium (V).

4. The stainless steel material as claimed in claim 1, further comprising 0.01-0.06 wt% nitrogen (N).

5. The stainless steel material as claimed in claim 1, further comprising 0.01-0.04 wt% phosphorus (P).

6. The stainless steel material as claimed in claim 1, further comprising 0.001-0.03 wt% sulfur (S).

7. The stainless steel material as described in claim 1, wherein the silicon (Si) content of the stainless steel material is 1.52-1.98 wt.

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