EP-4739811-A1 - A DUPLEX STAINLESS STEEL AND USE THEREOF

Abstract

The present disclosure relates to a corrosion resistant duplex stainless steel (ferritic austenitic alloy) which is suitable for use in a plant for the production of urea; and uses thereof. The disclosure also relates to objects made of said duplex stainless steel. Furthermore, the present disclosure also relates to a method for the production of urea and to a plant for the production of urea comprising one or more parts made from said duplex stainless steel, and to a method of modifying an existing plant for the production of urea.

Inventors

• GULLBERG, Daniel
• ÖHLIN, Oscar

Assignees

• Alleima Tube AB

Dates

Publication Date

20260513

Application Date

20240706

Claims (1)

1. Claims 1. A duplex stainless steel comprising in weight% (wt%): C < 0.030; Si < 0.5; Mn <1.5; Cr more than 28.0 to 29.0; Ni 7.1 to 7.8; Mo 2.0 to less than 3.0; N 0.30 to 0.36; Cu < 1.0; S < 0.02; P < 0.03; balance Fe and unavoidable occurring impurities. 2. The duplex stainless steel according to claim 1, wherein said duplex stainless steel fulfills the following requirement: 28.4-[Cr]-29*[N]+0.44*[Ni]-1.66*[Si]+0.027*[Cr] 2 *[N] is greater than 0 (1) wherein the values of the elements are given in weight%. 3. The duplex stainless steel according to claim 1 or claim 2, wherein said duplex stainless steel fulfills the following requirement: [Cr] * [N] is less than 10 (2), wherein the values of the elements are given in weight%. 4. The duplex stainless steel according to any one of claims 1 to 3, wherein the content of C is < 0.020 wt%. 5. The duplex stainless steel according to any one of claims 1 to 4 , wherein the content of Si is in the range of 0.05 to 0.5 wt%. 6. The duplex stainless steel according to any one of claims 1 to 5, wherein the content of Si is in the range of 0.1 to 0.4 wt%. 7. The duplex stainless steel according to any one of claims 1 to 6, wherein the content of Mn is in the range of 0.5 to 1.5 wt%. 8. The duplex stainless steel according to any one of claims 1 to 7, wherein the content of Cr is more than 28.00. 9. The duplex stainless steel according to claims 1 to 8, wherein the content of Mo is in the range of 2.1 to 2.7 wt%. 10. The duplex stainless steel according to claims 1 to 9, wherein the content of Cu is in the range of Cu is < 0.8 wt%, such as < 0.4 wt%. 11. An object comprising the duplex stainless steel according to any one of claims 1 to 10. 12. The duplex stainless steel according to claims 1 to 10 or an object according to claim 11, wherein the duplex stainless steel or the object has an average impact toughness at 0°C of at least 50 J, ISO 148-1. 13. The object according to claim 11 or claim 12, wherein said object is selected from a valve, such as a valve body or an ejector; a tube and a pipe, such as a seamless tube or large pipe; a pressurized component such as a vessel, a tube or a bearing component; or a plate. 14. Use of an object according to any one of claims 11 to 13. . . in a carbamate environment.

Description

A DUPLEX STAINLESS STEEL AND USE THEREOF Technical field The present disclosure relates to a corrosion resistant duplex stainless steel (ferritic austenitic alloy) suitable for use for example in a plant for the production of urea. The disclosure also relates to an object of said duplex stainless steel, more specifically to a bar or a formed object made of said bar or to a forged object. Duplex stainless steel refers to ferritic-austenitic alloy. Such alloys have a microstructure comprising ferritic and austenitic phases. Background references in this respect include for example WO 95/00674 and US 7,347,903 and WO 2017/013180. The duplex stainless steels described therein are highly corrosion resistant and can therefore be used, e.g., in the highly corrosive environment of a urea manufacturing plant. However, even though the duplex stainless steels disclosed above work for most applications, they have, as all duplex stainless steels, limitations in which applications they can be used due to their transition temperature. The transition temperature is the temperature where the material will change from ductile to brittle. This means that for certain applications these steels are not suitable as they do not fulfill the criteria of ductility. Hence, there is still a need for a duplex stainless steel having higher impact toughness, i.e., higher strength and ductility, in order to avoid brittle fracture as per definition by the requirement of impact strength at the minimum design temperature. Additionally, the microstructural stability of the duplex stainless steel is dependent on the composition, and it is important to have a material with a stable microstructure in order to assure proper corrosion resistance as well as sufficient mechanical properties. Thus, there is thus still a need for duplex stainless steels having a stable microstructure. Further, there still exists a need for a further improvement of the duplex stainless steel materials used in for example the plants for the production of urea, for example for parts which are exposed to high pressure at low temperatures, such as high pressure vessels. Summary of the disclosure An aspect of the present disclosure is therefore to provide a corrosion resistant duplex stainless steel having an improved impact toughness, especially when used in an environment of high pressures at low temperatures or exposed to said conditions. Another aspect of the present disclosure is to provide a duplex stainless steel with improved structure stability. Another aspect of the present disclosure is to provide a duplex stainless steel which will have a transitional temperature which will allow the steel to be ductile at low temperatures. In order to address one or more of the foregoing aspects, the present disclosure provides a duplex stainless steel comprising C < 0.030; Si < 0.5; Mn <1.5; Cr more than 28.0 to 29.0; Ni 7.1 to 7.8; Mo 2.0 to less than 3.0; N 0.30 to 0.36; Cu < 1.0; S < 0.02; P < 0.03; balance Fe and unavoidable occurring impurities. The present inventors have by thorough research found a specific duplex stainless steel composition which will provide for a high impact toughness at low design temperatures, i.e., temperatures less than about 20 °C. The present duplex stainless steel will additionally have improved ductility and improved structure stability. Furthermore, the present duplex stainless steel will provide for that an object of said stainless steel will have proper corrosion resistance as well as sufficient mechanical properties. These before-mentioned properties are very useful when an object of said duplex stainless steel is to be used in applications where it is for example exposed to high pressures and low temperatures. Additionally, the present disclosure relates to an object of the hereinabove or hereinafter defined duplex stainless steel and to the use thereof. The present disclosure also relates to a bar of the hereinabove or hereinafter defined duplex stainless steel. Examples of an object is a formed object, such as a machined component, a pressure bearing component, a large pipe or a tube or a plate. Additional examples but not limited of an object are a part, such as a formed or a machined component; a valve, such as a valve body or an ejector; a tube or a pipe, such as a seamless tube or a large pipe; a pressurized component such as a vessel, a tube or a bearing component; or a plate. The object may be manufactured from a bar of the present duplex stainless steel or may be forged. Additionally, the present disclosure relates to the use of the duplex stainless steel as defined hereinabove or hereinafter in carbamate environment. The present disclosure also relates to a method for producing urea wherein at least one part of the equipment is made from an object of a duplex stainless steel as defined hereinabove or hereinafter and to a plant for the production of urea comprising one or more parts comprising a duplex stainless steel as defi