Austenitic stainless steel containing nickel and molybdenum

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

Austenitic stainless steel containing nickel and molybdenum

[0001] The present invention relates to an austenitic stainless steel containing nickel and molybdenum.

[0002] Existing 304STS and 316STS materials used in LNG vessels stabilize the austenite phase down to temperatures below room temperature, but the addition of expensive nickel (Ni) carries a cost burden. Therefore, the development of materials with superior price competitiveness compared to 304STS and 316STS is necessary. Austenitic materials should reduce the expensive nickel content and add inexpensive manganese (Mn), carbon (C), and nitrogen (N) elements to stabilize the austenite phase even at room temperature. Furthermore, according to International Maritime Organization (IMO) regulations, materials used in LNG vessels must have an impact absorption energy of at least 41J at -196°C and a pitting potential equivalent to or higher than that of 304STS and 316STS.

[0003] One object of the present invention is to provide an austenitic stainless steel having improved impact absorption energy and corrosion resistance by adding nickel and molybdenum.

[0004] In order to achieve the above object, the present invention provides an austenitic stainless steel comprising chromium (Cr) in a weight ratio of 16 to 20, manganese (Mn) in a weight ratio of 4.5 to 9, nickel (Ni) in a weight ratio of 3 to 6, carbon (C) in a weight ratio of 0.05 to 0.3, nitrogen (N) in a weight ratio of 0.1 to 0.35, molybdenum (Mo) in a weight ratio of more than 0 and less than or equal to 5.0, and the remainder iron (Fe) and other unavoidable impurities.

[0005] According to the present invention, the addition of inexpensive manganese (Mn), carbon (C), and nitrogen (N) elements provides the advantage of lower cost compared to conventional 304STS and 316STS. Furthermore, the austenite phase can be stabilized even at room temperature, and the impact absorption energy can be greater than 41 J at -196°C.

[0006] Figure 1 is a polarization curve graph of an embodiment according to the present invention.

[0007] Figure 2 is a graph showing the results of the formal potential evaluation of an embodiment according to the present invention.

[0008] Figure 3 is a graph showing the impact absorption energy results of an embodiment according to the present invention.

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. The present invention can be modified in various ways and can take various forms, and thus specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to a specific disclosed form, but should be understood to include all modifications, equivalents, and substitutes included in the spirit and technical scope of the present invention. In describing each drawing, similar reference numerals are used to indicate similar components. In the attached drawings, the dimensions of structures are shown larger than actual size to ensure clarity of the present invention.

[0010] While terms like "first" and "second" may be used to describe various components, these components should not be limited by these terms. These terms are used solely to distinguish one component from another. For example, without departing from the scope of the present invention, a first component could be referred to as a "second component," and similarly, a second component could also be referred to as a "first component."

[0011] The terminology used in this application is only used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, it should be understood that the terms "comprise" or "have" indicate the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0012] Meanwhile, unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning within the context of the relevant technology, and shall not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

[0013]

[0014] An austenitic stainless steel according to an embodiment of the present invention may include chromium (Cr) in a weight ratio of about 16 to 20, manganese (Mn) in a weight ratio of about 4.5 to 9, nickel (Ni) in a weight ratio of about 3 to 6, carbon (C) in a weight ratio of about 0.05 to 0.3, nitrogen (N) in a weight ratio of about 0.1 to 0.35, molybdenum (Mo) in a weight ratio of more than about 0 and less than or equal to 5.0, and the remainder iron (Fe) and other unavoidable impurities.

[0015] In one embodiment, the austenitic stainless steel may include the nickel and the molybdenum in a weight ratio of about 4.0 to 7.0.

[0016] In one embodiment, the austenitic stainless steel may include the nickel and the molybdenum in a weight ratio of about 4.6 or more and less than 5.0.

[0017] In one embodiment, the austenitic stainless steel may include the nickel and the molybdenum in a weight ratio of about 5.0 or more and 7.0 or less.

[0018] Below, the reason for the numerical limitation of the present invention is explained.

[0019] (1) Chromium (Cr): Approximately 16 to 20 weight ratio

[0020] Chromium is added because it can form a passive film on the surface of stainless steel, imparting corrosion resistance. It also stabilizes the austenite phase. Therefore, to ensure corrosion resistance and austenite phase stabilization in austenitic stainless steel, chromium is required in an amount of about 16 wt% or more. If chromium exceeds about 20 wt%, a sigma phase, which weakens corrosion resistance, may form. Therefore, the austenitic stainless steel may contain chromium in an amount of about 16 to 20 wt%.

[0021] (2) Manganese: about 4.5 to 9 weight ratio

[0022] Manganese can stabilize the austenite phase and increase the solubility of nitrogen. The above effects can be achieved by adding manganese in an amount of about 4.5 weight percent or more. However, if manganese exceeds about 9 weight percent, it can combine with impurities such as sulfur (S) or oxygen (O) to form manganese sulfide or manganese oxide, which can deteriorate the corrosion resistance and mechanical properties of the austenitic stainless steel. Therefore, the austenitic stainless steel may contain manganese in an amount of about 4.5 to 9 weight percent.

[0023] (3) Nickel: Approximately 3 to 6 weight ratio

[0024] Nickel can stabilize the austenite phase and increase toughness and formability. It also improves shock absorption at approximately -196°C by minimizing the increase in formal dislocations and the amount of interstitial elements. However, because nickel is an expensive element, adding excessive amounts increases manufacturing costs. Therefore, the austenitic stainless steel may contain nickel in a weight ratio of approximately 3 to 6.

[0025] (4) Carbon: about 0.05 to 0.3 weight ratio

[0026] Carbon can stabilize the austenite phase and inhibit martensitic transformation. However, carbon causes carbides to form, reducing corrosion resistance. Therefore, the austenitic stainless steel may contain carbon in a weight ratio of approximately 0.05 to 0.3.

[0027] (5) Nitrogen: about 0.1 to 0.35 weight ratio

[0028] Nitrogen can stabilize the austenite phase and partially replace nickel. However, nitrogen causes nitride formation, which reduces corrosion resistance and toughness. Therefore, the austenitic stainless steel may contain nitrogen in a weight ratio of about 0.1 to 0.35.

[0029] (6) Molybdenum: Approximately 0 to 5.0 weight ratio or less

[0030] Molybdenum forms a passive oxide on the surface of stainless steel, thereby improving corrosion resistance. Excessive addition of molybdenum forms a sigma phase, which reduces impact energy absorption at approximately -196°C. Therefore, the austenitic stainless steel may contain molybdenum in a weight ratio greater than about 0 and less than or equal to 5.0.

[0031] (7) Nickel and molybdenum: Weight ratio of approximately 4.0 to 7.0

[0032] Nickel and molybdenum can interact to improve the pitting potential and corrosion resistance of stainless steels. For example, when molybdenum alone was added to a stainless steel containing manganese and nitrogen, there was no change in pitting potential. However, when molybdenum was added together with nickel to a stainless steel containing manganese and nitrogen, pitting potential significantly improved. Therefore, it can be concluded that nickel and molybdenum interact to improve the properties of stainless steel.

[0033] When the weight ratio of nickel and molybdenum is less than 4.0, the shock absorption energy at -196 ℃ may be less than 41 J. When the weight ratio of nickel and molybdenum exceeds 7.0, the manufacturing cost may increase because nickel and molybdenum are expensive elements.

[0034]

[0035] In one embodiment, the formal potential of the austenitic stainless steel containing the nickel and the molybdenum in a weight ratio of 4.0 to 7.0 may follow the following equation 1:

[0036] [Formula 1]

[0037]

[0038] In the above equation 1, E pit is the formal potential, Ni is the weight ratio of nickel, and Mo is the weight ratio of molybdenum.

[0039] In one embodiment, the austenitic stainless steel may have an impact absorption energy of about 41 J or more at about -196°C. If the impact absorption energy is less than about 41 J, the steel may not be suitable for use in LPG vessels.

[0040] In one embodiment, the formal potential of the austenitic stainless steel is about 0.2 V. SCE You can have ideals.

[0041]

[0042] Hereinafter, to aid understanding of the present invention, examples will be given in detail. However, the following examples are intended only to illustrate the scope of the present invention and are not intended to limit its scope. These examples are provided to more fully explain the present invention to those of average skill in the art.

[0043]

[0044] <Example 1>

[0045] Nitrogen gas was injected into a vacuum induction melting furnace (VIM), and commercial electrolytic iron, chromium, manganese, nickel, molybdenum, and carbon powder (purity 95% or higher) were used as a master alloy, melted, and then manufactured into small ingots. The small ingots were heated to approximately 1200°C and hot rolled to manufacture them into sheets with an average thickness of approximately 11.5 mm. The nickel and molybdenum contents were set at a weight ratio of approximately 4.35.

[0046]

[0047] <Example 2>

[0048] Small steel ingots were manufactured in the same manner as in Example 1, except that the content of nickel and molybdenum was set to a weight ratio of about 4.606.

[0049]

[0050] <Example 3>

[0051] Small ingots were manufactured in the same manner as in Example 1, except that the content of nickel and molybdenum was set to a weight ratio of about 4.766.

[0052]

[0053] <Example 4>

[0054] Small ingots were manufactured in the same manner as in Example 1, except that the content of nickel and molybdenum was set to a weight ratio of about 5.01.

[0055]

[0056] <Example 5>

[0057] Small steel ingots were manufactured in the same manner as in Example 1, except that the content of nickel and molybdenum was set to a weight ratio of about 5.21.

[0058]

[0059] <Example 6>

[0060] Small ingots were manufactured in the same manner as in Example 1, except that the content of nickel and molybdenum was set to a weight ratio of about 5.353.

[0061]

[0062] <Example 7>

[0063] Small ingots were manufactured in the same manner as in Example 1, except that the content of nickel and molybdenum was set to a weight ratio of about 5.36.

[0064]

[0065] <Example 8>

[0066] Small steel ingots were manufactured in the same manner as in Example 1, except that the content of nickel and molybdenum was set to a weight ratio of about 6.692.

[0067]

[0068] <Comparative Example 1>

[0069] Commercial 304STS was used.

[0070]

[0071] <Comparative Example 2>

[0072] Commercial 316STS was used.

[0073]

[0074] Experimental Example 1

[0075] Figure 1 is a polarization curve graph of an example according to the present invention. The polarization potential was measured at about 25°C in a 3.5 wt% NaCl aqueous solution using the potentiodynamic polarization method (GAMRY, REFERENCE600) at a scan rate of about 0.2 mV / s. The pitting potential refers to the potential at which pitting begins to occur on the surface of an austenitic stainless steel, and the higher the pitting potential, the later the pitting occurs. It was confirmed that the polarization potential and passive behavior of both the examples and comparative examples were similar to each other.

[0076] Figure 2 is a graph showing the results of the formal potential evaluation of an embodiment according to the present invention. The formal potential of Comparative Example 1 is approximately 0.3 V. SCE and Comparative Example 2 is 0.35 to 0.41 V SCE It was confirmed that the nickel and molybdenum contents had a formal potential value higher than that of Comparative Example 1 from 4.766 or higher, and that the nickel and molybdenum contents had a formal potential value higher than that of Comparative Example 2 from 5.353 or higher. In addition, it was confirmed that when the total nickel and molybdenum contents were in a weight ratio of 4.27 to 6.69, it followed the following equation 1.

[0077] [Formula 1]

[0078]

[0079] In the above equation 1, E pit is the formal potential, Ni is the weight ratio of nickel, and Mo is the weight ratio of molybdenum.

[0080] Figure 3 is a graph showing the impact absorption energy results of examples according to the present invention. The impact absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) according to ASTM A370: Metal Charpy Impact Test. For LNP ship use, the impact absorption energy must be at least about 41 J at about -196°C. All examples 1 to 8 had impact absorption energies exceeding about 41 J.

[0081]

[0082] Although the present invention has been described above with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below.

Claims

An austenitic stainless steel comprising chromium (Cr) in a weight ratio of 1.16 to 20, manganese (Mn) in a weight ratio of 4.5 to 9, nickel (Ni) in a weight ratio of 3 to 6, carbon (C) in a weight ratio of 0.05 to 0.3, nitrogen (N) in a weight ratio of 0.1 to 0.35, molybdenum (Mo) in a weight ratio of more than 0 and less than or equal to 5.0, and the remainder iron (Fe) and other unavoidable impurities.

2. In paragraph 1, The above austenitic stainless steel is an austenitic stainless steel containing nickel and molybdenum in a weight ratio of 4.0 to 7.

0.

3. In paragraph 2, The above austenitic stainless steel is an austenitic stainless steel containing nickel and molybdenum in a weight ratio of 4.27 or more and less than 6.

69.

4. In paragraph 2, An austenitic stainless steel comprising the above nickel and the above molybdenum in a weight ratio of 4.0 to 7.0, wherein the formal potential of the austenitic stainless steel conforms to the following formula 1: [Formula 1] In the above equation 1, E pit is the formal potential, Ni is the weight ratio of nickel, and Mo is the weight ratio of molybdenum.

5. In paragraph 1, The above austenitic stainless steel is an austenitic stainless steel having an impact absorption energy of 41 J or more at -196 ℃.

6. In paragraph 1, The formal potential of the above austenitic stainless steel is 0.2 V. SCE Austenitic stainless steel having an ideal properties.