Austenitic stainless steel containing carbon and nitrogen

Abstract

NOT VISIBLE DUE TO STATUS OF PATENT

Description

Austenitic stainless steel containing carbon and nitrogen

[0001] The present invention relates to an austenitic stainless steel containing carbon and nitrogen.

[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 that are more cost-competitive than 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 the development of chemical compositions of austenitic stainless steel materials with corrosion-related weight loss equivalent to or greater than that of 304STS and 316STS is necessary.

[0003] One object of the present invention is to provide an austenitic stainless steel having improved weight loss due to corrosion by adding carbon and nitrogen.

[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 0.12, nitrogen (N) in a weight ratio of 0.15 to 0.30, the remainder being iron (Fe) and other unavoidable impurities.

[0005] According to the present invention, the addition of low-cost carbon (C) and nitrogen (N) elements provides advantages such as lower cost compared to existing 304STS and 316STS, and reduced weight loss according to immersion tests (ASTM G48-11). In addition, the impact absorption energy can be 41 J or more at approximately -196°C.

[0006] Figure 1 is an image showing the results of a corrosion test according to an embodiment of the present invention.

[0007] Figure 2 is a graph showing the results of a corrosion test according to an embodiment of 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] The austenitic stainless steel according to an embodiment of the present invention may include 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 0.12, nitrogen (N) in a weight ratio of 0.15 to 0.30, the remainder being iron (Fe) and other unavoidable impurities.

[0015] In one embodiment, the austenitic stainless steel may include the carbon and the nitrogen in a weight ratio of 0.25 to 0.32.

[0016] In one embodiment, the austenitic stainless steel may include the carbon and the nitrogen in a weight ratio of 0.25 to 0.28.

[0017] Below, the reason for the numerical limitation of the present invention is explained.

[0018] (1) Chromium (Cr): Approximately 16 to 20 weight ratio

[0019] 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%.

[0020] (2) Manganese: about 4.5 to 9 weight ratio

[0021] 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.

[0022] (3) Nickel: Approximately 3 to 6 weight ratio

[0023] 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.

[0024] (4) Carbon: about 0.05 to 0.12 weight ratio

[0025] 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 0.12.

[0026] (5) Nitrogen: about 0.15 to 0.30 weight ratio

[0027] 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 approximately 0.15 to 0.30.

[0028] (6) Carbon and nitrogen: weight ratio of approximately 0.25 to 0.32

[0029] Carbon and nitrogen interact to improve the weight loss in stainless steels according to the ASTM G48-11 corrosion test. As the carbon and nitrogen content increases, the weight loss in the corrosion test decreases linearly. This suggests that carbon and nitrogen interact to improve the properties of stainless steel.

[0030] If the weight ratio of carbon to nitrogen is less than 0.25, the weight loss resulting from corrosion testing may not improve. If the weight ratio of carbon to nitrogen exceeds 0.32, carbides and nitrides may form, degrading the properties of stainless steel.

[0031]

[0032] In one embodiment, the austenitic stainless steel may have a weight loss of 10 to 200 g / m2 according to a corrosion test (ASTM G48-11), and may be about 60 to 120 g / m2.

[0033] In one embodiment, the austenitic stainless steel may have an impact absorption energy of about 41 to 200 J at about -196°C, and may be about 105 to 140 J. If the impact absorption energy is less than about 41 J, the steel may not be suitable for use in LPG vessels.

[0034]

[0035] 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.

[0036]

[0037] <Example 1>

[0038] 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 about 1200 oAfter heating to C, it was hot rolled to produce a plate shape with an average thickness of approximately 11.5 mm. The nitrogen content was controlled by controlling the amount of carbon introduced into the VIM and the reaction time. The carbon and nitrogen content was set to a weight ratio of approximately 0.2469.

[0039]

[0040] <Example 2>

[0041] Small ingots were manufactured in the same manner as in Example 1, except that the content of carbon and nitrogen was set to a weight ratio of about 0.253.

[0042]

[0043] <Example 3>

[0044] Small ingots were manufactured in the same manner as in Example 1, except that the content of carbon and nitrogen was set to a weight ratio of about 0.2795.

[0045]

[0046] <Example 4>

[0047] Small ingots were manufactured in the same manner as in Example 1, except that the content of carbon and nitrogen was set to a weight ratio of about 0.3134.

[0048]

[0049] <Comparative Example 2>

[0050] Commercial 304STS was used.

[0051]

[0052] <Comparative Example 3>

[0053] Commercial 316STS was used.

[0054]

[0055] Experimental Example 1

[0056] Fig. 1 is an image showing the results of a corrosion test (ASTM G48-11) of an example according to the present invention, and Fig. 2 is a graph showing the results of a corrosion test (ASTM G48-11) of an example according to the present invention. Examples 1 to 4 and Comparative Examples 1 to 2 were subjected to a corrosion test in a solution of about 10% FeCl-ㆍ6H2O at about 21 to 23°C for about 72 hours according to ASTM G48-11, and the results are shown in Figs. 1 and 2. Referring to Figs. 1 and 2, it was confirmed that the example according to the present invention exhibited a weight loss due to corrosion at a level similar to that of existing 304 STS and 316 STS.

[0057] 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 ships, the impact absorption energy should be at least about 41 J at about -196°C. Examples 1 to 4 all recorded impact absorption energies of at least about 100 J.

[0058]

[0059] 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 containing 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 0.12, nitrogen (N) in a weight ratio of 0.15 to 0.30, the remainder being iron (Fe) and other unavoidable impurities.

2. In paragraph 1, The above austenitic stainless steel is an austenitic stainless steel containing carbon and nitrogen in a weight ratio of 0.25 to 0.

32.

3. In paragraph 2, The above austenitic stainless steel is an austenitic stainless steel containing carbon and nitrogen in a weight ratio of 0.25 to 0.

28.

4. In paragraph 1, The above austenitic stainless steel is an austenitic stainless steel having a weight loss of 10 to 200 g / ㎡ according to an immersion test (ASTM G48-11).

5. In paragraph 4, The above austenitic stainless steel is an austenitic stainless steel having a weight loss of 60 to 120 g / ㎡ according to an immersion test (ASTM G48-11).

6. In paragraph 1, The above austenitic stainless steel is an austenitic stainless steel having an impact absorption energy of 41 to 200 J at -196°C.

7. In paragraph 6, The above austenitic stainless steel is an austenitic stainless steel having an impact absorption energy of 105 to 140 J at -196°C.