JP2004285482A - Martensitic stainless steel having excellent corrosion resistance and weldability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a martensitic stainless steel which exhibits high corrosion resistance in an environment including both of humid carbon dioxide and humid hydrogen sulfide (sulfide stress corrosion cracking resistance), and whose weldability can be improved. <P>SOLUTION: In the stainless steel, C and Mo are added, desulfurization elements and deoxidation elements are reduced, and further, productivity and weldability are improved by the control of the amounts of C and N to be added. Thus, the stainless steel has a composition comprising, by mass, ≤0.02% C, ≤0.02% N, 0.1 to 0.3% Si, 0.1 to 0.3% Mn, 10 to 13% Cr, 5 to 8% Ni and 1.5 to 3% Mo, and the balance Fe with inevitable impurities, and also satisfying C+N: 0.02 to 0.04%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a martensitic stainless steel excellent in weldability and suitable for a line pipe used in an environment containing wet carbon dioxide gas and wet hydrogen sulfide.

  Steel materials used for oil and natural gas transportation pipelines include corrosion resistance and on-site weldability according to the usage environment (the preheating necessary to prevent cracks in the welded joints at the site where the pipeline is constructed, X50, X65 grade carbon steel pipes were often used.

  In recent years, the environment containing wet carbon dioxide gas and wet hydrogen sulfide has increased, and the use of stainless steel has been considered from the viewpoint of corrosion resistance. However, existing stainless steel does not always have sufficient performance for line pipes. Instead, new development has been desired.

  That is, the 0.2C-13Cr system having good corrosion resistance to an environment containing wet carbon dioxide gas and wet hydrogen sulfide is used for oil country tubular goods that do not require welding, and has high preheating and post-heating temperatures for preventing welding cracks. It is not suitable for pipelines where local weldability is important. Duplex stainless steels such as 22Cr and 25Cr do not require preheating and post-heating, but are expensive, and are difficult to use in pipelines that require large amounts of steel.

Therefore, Patent Documents 1, 2, 3, and 4 propose 13Cr-based materials having a reduced C content. However, corrosion resistance in an environment containing both wet carbon dioxide gas and wet hydrogen sulfide has been proposed. It is difficult to say that the on-site weldability is satisfied with sufficient performance at the same time.
JP-A-6-100943 JP-A-4-268018 JP-A-8-100235 JP-A-8-100236

  The present invention provides a martensitic stainless steel that can be used in an environment containing both wet carbon dioxide gas and wet hydrogen sulfide, and is excellent in weldability and productivity.

The present inventors have studied various components of martensitic stainless steel in order to achieve the above object, and have obtained the following findings. Cr is effective for corrosion resistance to acid in wet carbon dioxide gas, and also reduces sulfide stress corrosion cracking resistance which is a problem in an environment containing wet hydrogen sulfide. Therefore, it is important to improve the corrosion resistance to wet hydrogen sulfide, and it is effective to add a certain amount of Mo together with Cr, and that desulfurization and reduction of deoxidizing elements are effective. It was found that the control of the C and N contents was effective. That is, the present invention is a martensitic stainless steel having good corrosion resistance to both wet carbon dioxide gas and wet hydrogen sulfide, and excellent weldability and productivity, and has the following configuration. (Incidentally, good productivity means that the mechanical properties are stable against fluctuations in manufacturing conditions such as heat treatment.)
1. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : Martensitic stainless steel comprising 5 to 8%, Mo: 1.5 to 3%, balance Fe and inevitable impurities, and excellent in corrosion resistance and weldability satisfying C + N: 0.02 to 0.04%.

  2. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : 5 to 8%, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, containing one or two kinds, the balance being Fe and unavoidable impurities , C + N: martensitic stainless steel excellent in corrosion resistance and weldability satisfying 0.02 to 0.04%.

  3. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : 5 to 8%, Mo: 1.5 to 3%, and 0.01 to 0.1% of one or two of Ti and Nb, the balance being Fe and unavoidable impurities. A martensitic stainless steel excellent in corrosion resistance and weldability satisfying 02 to 0.04%.

  4. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : 5 to 8%, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, 1 or 2 types, Ti, Nb 1 or 2 types Martensitic stainless steel containing 0.01 to 0.1%, the balance being Fe and unavoidable impurities, and satisfying C + N: 0.02 to 0.04% and having excellent corrosion resistance and weldability.

  According to the present invention, wet carbon dioxide gas, excellent corrosion resistance in an environment containing wet hydrogen sulfide, and martensitic stainless steel having good weldability is obtained, can be used for oil, natural gas line pipe, It has a significant industrial effect.

  Hereinafter, the components of the invention steel and the limited range thereof will be described.

  C: 0.02% or less, an element that forms a carbide with Cr in steel to increase the strength, but when added excessively, reduces the amount of Cr effective for corrosion resistance. Further, since the hardness of the heat affected zone is increased and heat treatment after welding is required, the upper limit is made 0.02%.

  N: 0.02% or less, forms a compound with Cr in steel, and reduces the amount of Cr effective for corrosion resistance. Further, the upper limit is set to 0.02% in order to increase the hardness of the heat affected zone.

  Si: 0.1 to 0.3%, which is added as a deoxidizing agent, but not more than 0.1% has no deoxidizing effect. If Si is excessively added, delta ferrite is crystallized and the corrosion resistance is reduced. Therefore, it is necessary to increase the amount of Ni in order to maintain the phase balance. Therefore, the upper limit is set to 0.3%.

  Mn: 0.1-0.3%, added as a desulfurizing agent on steelmaking, but less than 0.1% has no effect and deteriorates hot workability. , The upper limit is set to 0.3%.

  Cr: 10 to 13%, which is an element effective for improving corrosion resistance in an environment containing wet carbon dioxide gas. As the content increases, the corrosion resistance improves, but the amount of Ni, which is a strong ferrite-forming element and an expensive austenite-forming element in order to form a martensite structure, needs to be increased, so the upper limit is made 13%.

  Ni: 5 to 8%, an element necessary for obtaining a martensitic structure, but if less than 5%, ferrite phase increases, impairing toughness and corrosion resistance. Therefore, the content range is set to 5 to 8%.

  Mo: 1.5 to 3%, an element effective for corrosion resistance, but if less than 1.5%, the effect is not sufficient. Due to the ferrite-forming element, if it is added in excess of 3%, expensive Ni must be added to secure the phase balance. Therefore, the content range is set to 1.5 to 3%.

  C + N: 0.02 to 0.04%, individual elements of C and N are added within the above-mentioned limited range, but the present invention further defines C + N. C + N is set to 0.02% or more to obtain a yield strength of 600 to 700 MPa of the target strength, and C + N is set to 0.04% or less to suppress the hardness of the weld heat affected zone to 350Hv or less of the target hardness.

  W, Cu: 0.1 to 3%, both of which are effective elements for strength and corrosion resistance. When added, less than 0.1% does not have sufficient effect, and when more than 3%, hot workability deteriorates. Therefore, the content is set to 0.1 to 3%.

  Ti, Nb: 0.01 to 0.1%, both of which are elements that form carbides with C in steel and improve the strength and toughness due to the effect of refining the crystal grains. If it is less than 0.01%, the effect is not sufficient, and if it exceeds 0.1%, the effect is saturated.

  The steel of the present invention may be produced by any method, such as a converter, an electric furnace, or a combination thereof, as long as it can be adjusted to a predetermined component range. After the smelting, billets and slabs are formed by a continuous casting machine or a mold, and then processed into a predetermined shape such as a steel pipe or a steel plate by hot rolling, and a target strength is obtained by heat treatment. The heat treatment is preferably performed after cooling or normalizing to obtain a transformed martensitic structure, followed by tempering to adjust the strength.

  Steel having the chemical composition shown in Table 1 was melted using a vacuum melting furnace, and after hot rolling was performed to form a steel sheet having a thickness of 12 mm, quenching was performed with a target strength of 600 to 700 MPa, and tempering was performed. Assuming actual operation, the slab was heated from 900 ° C. ± 10 ° C. and water-cooled, and then tempered at 640 ° C. ± 5 ° C.

After heat treatment, corrosion resistance and weldability were investigated. The corrosion resistance test for wet carbon dioxide gas was performed using a 5% NaCl-30 atm CO ₂ solution at 180 ° C. for 96 hours, and a corrosion amount of 0.3 mm / y or less was accepted. The sulfide stress corrosion cracking test (SSC test) as a test for evaluating the corrosion resistance to wet hydrogen sulfide was based on the TM0177 test method specified by NACE.

The test conditions were such that a test piece was loaded with 60% of the proof stress in a 5% NaCl + 0.5% acetic acid aqueous solution saturated with 1 atm of H ₂ S and did not break in 720 hours. The weldability test was performed for the purpose of judging the necessity of preheating and post-heating in on-site welding, and a reproduced HAZ was created, and the hardness was 350 Hv or less.

  Table 2 shows the test results. The steels A to J of the present invention show good results in both strength, corrosion resistance, sulfide stress corrosion cracking resistance (SSC resistance: corrosion resistance to wet hydrogen sulfide), and hardness. On the other hand, the comparative steel K has a small Cr content and does not show sufficient corrosion resistance. Comparative steel L has a large amount of Si as a deoxidizing agent, and comparative steel M has a large amount of Mn as a desulfurizing agent, and thus has poor sulfide stress corrosion cracking resistance (SSC resistance).

Further, since the comparative steel N has a low Mo content, the sulfide stress corrosion cracking resistance (SSC resistance) is poor. Since the comparative steel O has a low Ni content, delta ferrite precipitates and the corrosion resistance decreases. The comparative steel P has a low C + N content and does not have sufficient strength. Comparative steel Q had high strength because both C and N contents were high, and the hardness in the weldability test was rejected.

Claims (4)

  In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : Martensitic stainless steel comprising 5 to 8%, Mo: 1.5 to 3%, balance Fe and unavoidable impurities, and excellent in corrosion resistance and weldability satisfying C + N: 0.02 to 0.04%.   In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : 5 to 8%, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, containing one or two kinds, the balance being Fe and unavoidable impurities , C + N: martensitic stainless steel excellent in corrosion resistance and weldability satisfying 0.02 to 0.04%.   In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : 5 to 8%, Mo: 1.5 to 3%, and 0.01 to 0.1% of one or two of Ti and Nb, the balance being Fe and unavoidable impurities. A martensitic stainless steel excellent in corrosion resistance and weldability satisfying 02 to 0.04%.   In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, Cr: 10 to 13%, Ni : 5 to 8%, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, 1 or 2 types, Ti, Nb 1 or 2 types Martensitic stainless steel containing 0.01 to 0.1%, the balance being Fe and unavoidable impurities, and satisfying C + N: 0.02 to 0.04% and having excellent corrosion resistance and weldability.