JP3620319B2 - Martensitic stainless steel with excellent corrosion resistance and weldability - Google Patents

Description

【００２９】
【表２】

【００３０】
【発明の効果】
以上説明したように、本発明によれば、湿潤炭酸ガス、湿潤硫化水素を含む環境下で優れた耐食性、且つ、良好な溶接性を有するマルテンサイト系ステンレス鋼が得られ、石油、天然ガス用ラインパイプに使用でき、工業上著しい効果を有する。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a martensitic stainless steel excellent in weldability suitable for a line pipe used in an environment containing wet carbon dioxide and wet hydrogen sulfide.
[0002]
[Prior art]
Steel materials used in oil and natural gas transportation pipelines have corrosion resistance and on-site weldability according to the environment in which they are used (preheating necessary to prevent weld cracking in the joint welding at the site where the pipeline is installed, It is required to be excellent in heat treatment), and X50 and X65 grade carbon steel pipes are often used.
[0003]
In recent years, the environment containing wet carbon dioxide and wet hydrogen sulfide has increased, and from the viewpoint of corrosion resistance, the use of stainless steel has been studied. However, existing stainless steel does not always have sufficient performance for line pipes. New development has been desired.
[0004]
That is, the 0.2C-13Cr system having good corrosion resistance with respect to the environment containing wet carbon dioxide and wet hydrogen sulfide is for oil well pipes that do not require welding, and has high preheating and postheating temperatures to prevent weld cracking. And is not suitable for pipelines where on-site weldability is important. Duplex stainless steels such as 22Cr and 25Cr do not require preheating and post heat treatment, but are expensive and difficult to use in pipelines that require large amounts of steel.
[0005]
Thus, JP-A-6-1000094, JP-A-4-268018, JP-A-8-100265, JP-A-8-100366, etc. propose a 13Cr system with a reduced amount of C. It is difficult to say that the corrosion resistance and field weldability in an environment containing both wet carbon dioxide and wet hydrogen sulfide are satisfied with sufficient performance at the same time.
[0006]
[Problems to be solved by the invention]
The present invention provides a martensitic stainless steel that can be used in an environment containing both wet carbon dioxide and wet hydrogen sulfide, and is excellent in weldability and manufacturability.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have studied various components of martensitic stainless steel and obtained the following knowledge. Cr is effective for the corrosion resistance against acids in wet carbon dioxide gas, and the sulfide stress corrosion cracking resistance, which is a problem in an environment containing wet hydrogen sulfide, also reduces the amount of hydrogen entering the steel. Therefore, it is important to improve the corrosion resistance against wet hydrogen sulfide, adding a certain amount or more of Mo together with Cr, reducing the desulfurization element Mn and deoxidation element Si is effective, and weldability As for the manufacturability, it has been found that the control of the amount of C and N is effective. That is, the present invention is martensitic stainless steel having good corrosion resistance, weldability and manufacturability to both wet carbon dioxide and wet hydrogen sulfide, and has the following configuration. (Good manufacturability 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: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: 5.0% to 8% or less, Mo: 1.5 to 3%, balance Fe and inevitable impurities, C + N: 0.020% to 0.04% or less and excellent corrosion resistance and weldability Martensitic stainless steel.
[0008]
2. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: More than 5.0% and 8% or less, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, or 1 or 2 types, the balance A martensitic stainless steel consisting of Fe and inevitable impurities and excellent in corrosion resistance and weldability satisfying C + N: more than 0.020% and 0.04% or less.
[0009]
3. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: more than 5.0% and 8% or less, Mo: 1.5 to 3%, further containing 0.01 to 0.1% of one or two of Ti and Nb, from the remaining Fe and inevitable impurities C + N: martensitic stainless steel excellent in corrosion resistance and weldability that satisfies 0.020% and 0.04% or less.
[0010]
4). In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: 5.0% to 8%, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, 1 type or 2 types, Ti, Nb Martensite with excellent corrosion resistance and weldability, containing 0.01 to 0.1% of one or two, consisting of the balance Fe and inevitable impurities, satisfying C + N: more than 0.020% and 0.04% or less Stainless steel.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In the following, the components of the invention steel and its limited range will be described.
[0012]
C: 0.02% or less, an element that forms carbide with Cr in steel and increases strength, but when added in excess, the Cr content effective for corrosion resistance is reduced. Further, the hardness of the weld heat affected zone is increased, and post-weld heat treatment is required, so the upper limit is made 0.02%.
[0013]
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 made 0.02% in order to increase the hardness of the weld heat affected zone.
[0014]
Si: 0.1 to 0.3%, added as a deoxidizer, but less than 0.1% has no deoxidation effect. Since Si is excessively added by the delta ferrite decreases corrosion resistance and crystallization, to maintain the phase balance, since the increase of the Ni content is required, the upper limit is 0.3%.
[0015]
Mn: 0.1% or more and less than 0.2% , added as a desulfurizing agent on steelmaking, but if it exceeds 0.1%, there is no effect and hot workability is reduced, and if added excessively, carbon dioxide, hydrogen sulfide Since the corrosion resistance under the environment is lowered, the content is made less than 0.2% .
[0016]
Cr: 10 to 13%, an element effective for improving corrosion resistance in an environment containing wet carbon dioxide gas, but the effect cannot be obtained at 10% or less. As the content increases, the corrosion resistance improves, but it is a strong ferrite-forming element, and since it requires a martensite structure, it is necessary to increase the amount of Ni, which is an expensive austenite-generating element, so the upper limit is made 13%.
[0017]
Ni: More than 5.0% and 8% or less , an element necessary for obtaining a martensite structure. However, if it is 5.0% or less , the ferrite phase increases, and the toughness and corrosion resistance are impaired. If it exceeds 8%, it is expensive. Since it is an element, the economic efficiency is lowered, so the content range exceeds 5.0% and is 8% or less .
[0018]
Mo: 1.5 to 3%, which is an element effective for corrosion resistance, but the effect is not sufficient if it is less than 1.5%. Since it is a ferrite-forming element, if it is added in excess of 3%, it is necessary to add expensive Ni in order to secure the phase balance, so the content range is 1.5 to 3%.
[0019]
C + N: More than 0.020% and 0.04% or less , and the individual elements of C and N are added within the above-mentioned limited range, but the present invention further defines C + N. To obtain a target strength of 600 to 700 MPa, C + N exceeds 0.020% , and to suppress the hardness of the weld heat affected zone to 350 Hv or less of the target hardness, C + N of 0.04% or less.
[0020]
W, Cu: 0.1 to 3%, all of which are effective elements for strength and corrosion resistance. When added, the effect is not sufficient if it is less than 0.1%, and hot workability deteriorates if it exceeds 3%. Therefore, the content is set to 0.1 to 3%.
[0021]
Ti, Nb: 0.01 to 0.1%, both of which are elements that improve strength and toughness due to the effect of forming carbides with C in the steel and 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, so 0.01 to 0.1% is made.
[0022]
The steel of the present invention may be melted by any method such as a converter, an electric furnace, or a combination hot water thereof as long as it can be adjusted to a predetermined component range. After melting, billets and slabs are made with a continuous casting machine or mold, then processed into a predetermined shape such as a steel pipe and steel plate by hot rolling, and the target strength is obtained by heat treatment. The heat treatment may be performed after cooling or normalizing to a transformed martensite structure and then adjusting the strength by tempering.
[0023]
【Example】
Steels having chemical components shown in Table 1 were melted by using a vacuum melting furnace and made into a steel plate having a thickness of 12 mm by hot rolling, and then tempered by tempering with a proof stress of 600 to 700 MPa. Assuming actual operation, the slab heating temperature was 900 ° C. ± 10 ° C., followed by water cooling, followed by tempering at 640 ° C. ± 5 ° C.
[0024]
After the heat treatment, the corrosion resistance and weldability were investigated. The corrosion resistance test against wet carbon dioxide gas was conducted with a 5% NaCl-30 atm CO ₂ solution at 180 ° C. for 96 hours, and the corrosion amount was 0.3 mm / y or less. The sulfide stress corrosion cracking test (SSC test) as a test for evaluating the corrosion resistance against wet hydrogen sulfide was in accordance with the TM0177 test method defined by NACE.
[0025]
As test conditions, 60% of the proof stress was loaded in 5% NaCl + 0.5% acetic acid aqueous solution saturated with 1 atm of H ₂ S, and the case where no fracture occurred in 720 hours was regarded as acceptable. The weldability test was conducted for the purpose of determining the necessity of preheating and postheating in on-site welding, and a reproduced HAZ part was created, and the hardness was determined to be 350 Hv or less.
[0026]
Table 2 shows the test results. The steels A to J of the present invention show good results in strength, corrosion resistance, sulfide stress corrosion cracking resistance (SSC resistance: corrosion resistance against wet hydrogen sulfide), and hardness. On the other hand, the comparative steel K has a small amount of Cr and does not exhibit sufficient corrosion resistance. Since the comparative steel L has a large amount of Si as a deoxidizing agent and the comparative steel M has a large amount of Mn as a desulfurizing agent, the resistance to sulfide stress corrosion cracking (SSC resistance) is poor.
[0027]
Further, since the comparative steel N has a low Mo content, the resistance to sulfide stress corrosion cracking (SSC resistance) is poor. Since the comparative steel O has a low Ni content, delta ferrite is precipitated and the corrosion resistance is lowered. The comparative steel P has a low C + N amount and cannot provide sufficient strength. Since comparative steel Q had high C and N contents, the strength was high and the hardness in the weldability test was rejected.
[0028]
[Table 1]

[0029]
[Table 2]

[0030]
【The invention's effect】
As described above, according to the present invention, martensitic stainless steel having excellent corrosion resistance and good weldability in an environment containing wet carbon dioxide and wet hydrogen sulfide can be obtained. It can be used for line pipes and has a significant industrial effect.

Claims (4)

In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: 5.0% to 8% or less, Mo: 1.5 to 3%, balance Fe and inevitable impurities, C + N: 0.020% to 0.04% or less and excellent corrosion resistance and weldability Martensitic stainless steel. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: More than 5.0% and 8% or less, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, or 1 or 2 types, the balance A martensitic stainless steel consisting of Fe and inevitable impurities and excellent in corrosion resistance and weldability satisfying C + N: more than 0.020% and 0.04% or less. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: more than 5.0% and 8% or less, Mo: 1.5 to 3%, further containing 0.01 to 0.1% of one or two of Ti and Nb, from the remaining Fe and inevitable impurities C + N: martensitic stainless steel excellent in corrosion resistance and weldability that satisfies 0.020% and 0.04% or less. In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.1-0.3%, Mn: 0.1% or more and less than 0.2% , Cr: 10-13% Ni: 5.0% to 8%, Mo: 1.5 to 3%, W: 0.1 to 3%, Cu: 0.1 to 3%, 1 type or 2 types, Ti, Nb Martensite with excellent corrosion resistance and weldability, containing 0.01 to 0.1% of one or two, consisting of the balance Fe and inevitable impurities, satisfying C + N: more than 0.020% and 0.04% or less Stainless steel.