JP2002105602A - Martensitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel having excellent corrosion resistance and weldability and also having high production stability. SOLUTION: This steel has a composition containing, by mass, <=0.02% C, 0.1 to 0.3% Si, 0.1 to 0.3% Mn, 11 to 15% Cr, 1 to 5% Ni, <=0.060% Al and <=0.020% N, also satisfying [Al]×[N]: <=6.0× 10-4, in which one or more kinds selected from Ti, V, Ca, Zr and Mg are added according to desired characteristics, and the balance substantially Fe with inevitable impurities; wherein, [Al]×[N] denotes the product of the concentration (mass%) of Al and N.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel excellent in corrosion resistance and weldability suitable for line pipes and the like used in an environment containing wet carbon dioxide gas, and in particular, to hot-rolled steel whose production conditions are liable to change. Stability of mechanical properties even in the production of steel strips: related to those with excellent material stability.

[0002]

2. Description of the Related Art Steel materials used for line pipes for transporting oil and natural gas are required to be excellent in corrosion resistance and on-site weldability according to the use environment. In recent years, corrosion resistance in an environment containing wet carbon dioxide gas has been required, and the use of stainless steel has been considered.However, existing stainless steel is not necessarily sufficient for line pipes and new Has been desired.

[0003] For example, 0.2C-13Cr type is for oil country tubular goods which does not require welding, and is suitable for line pipes which require high preheating and post-heating temperatures to prevent welding cracks and where on-site weldability is important. Not. Further, duplex stainless steel such as 22Cr does not require heat treatment after preheating, but is expensive as a raw material and is difficult to use in pipelines requiring a large amount of steel. Therefore, Japanese Patent Application Laid-Open Nos. 6-100943, 4-268018, 8-100266 are disclosed.
Japanese Patent Application Laid-Open Publication No. H11-15064 and the like have proposed a 13Cr-based material in which the amount of C is reduced.

[0004]

However, it is difficult to say that these prior art 13Cr-based alloys provide high levels of corrosion resistance, on-site weldability, and strength stability against variations in manufacturing conditions. In particular, since the mechanical properties of the material greatly fluctuate depending on the heat treatment conditions, when a hot-rolled steel strip is used, the material fluctuates depending on the portion of the steel strip, and it has been difficult to stably obtain an appropriate strength.

An object of the present invention is to provide a martensitic stainless steel excellent in corrosion resistance and on-site weldability and excellent in material stability even in a hot-rolled steel strip in which manufacturing conditions are liable to change. To provide steel.

[0006]

Means for Solving the Problems The present inventors have proposed 13Cr.
For martensitic stainless steels, we investigated the effect of the composition of the materials, especially on the strength,
It has been found that the control of nitride morphology by adjusting the amounts of N and Al affects the stability of strength.

In addition, it has been found that Cr is effective for corrosion resistance to acid in wet carbon dioxide gas, and that deoxidation and reduction of desulfurization elements are effective.

The present invention has been made based on the above findings and further studied. That is, the present invention provides: In mass%, C: 0.02% or less, Si: 0.1 to
0.3%, Mn: 0.1 to 0.3%, Cr: 11 to 15
%, Ni: 1 to 5%, Al: 0.060% or less, N:
5. 0.020% or less, and [Al] × [N]: 6.
A martensitic stainless steel characterized by being substantially equal to or less than 0 × 10 ⁻⁴ and the balance being Fe and unavoidable impurities.

Here, [Al] × [N] indicates the product of the concentration (% by mass) of Al and N.

[0010] 2. In mass%, Ti: 0.01 to 0.1
%, V: Martensitic stainless steel according to 1, containing one or two of 0.01 to 0.1%.

3. 3. The martensitic stainless steel according to 1 or 2, which contains, by mass%, one or more of Ca: 0.005% or less, Zr: 0.005% or less, and Mg: 0.005% or less.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the components of the present invention will be described.

C. C forms a carbide with a carbide-forming element such as Cr in steel and is added to improve the strength. On the other hand, if added excessively, the amount of Cr effective for corrosion resistance is reduced, the hardness of the heat affected zone is increased, and heat treatment after welding is required.
02% or less.

Si Si is added as a deoxidizing agent. If it is less than 0.1%, the effect is not obtained. On the other hand, if it exceeds 0.3%, the effect is saturated, and delta ferrite crystallizes, and it is necessary to increase the amount of Ni to maintain phase balance. 0.1-0.3
% (0.1% or more, 0.3% or less).

Mn Mn is added as a desulfurizing agent on steelmaking. If it is less than 0.1%, there is no effect and the hot workability also decreases. On the other hand, 0.3%
If added in excess of, the corrosion resistance under carbon dioxide deteriorates, so the content is made 0.1 to 0.3%.

Cr Cr is added because it is effective for improving corrosion resistance in an environment containing wet carbon dioxide gas. Less than 11% has no effect, while 1
If it exceeds 5%, a martensite structure cannot be obtained even when other component elements are adjusted, so that the content is set to 11 to 15%.

Ni Ni is added in an appropriate amount according to the amount of Cr in order to form an austenite phase and obtain a martensite structure. If it is less than 1%, phase stability is insufficient, and the material of the welded part is impaired.
On the other hand, if the content exceeds 5%, the effect is saturated and the material cost also increases.

Al Al is added for deoxidizing element and for making the structure finer.
If it is added in excess of 0.060%, alumina-based inclusions and nitrides increase, and the productivity and toughness decrease.

NN forms a compound with Cr in steel, and is effective for corrosion resistance.
In addition, the upper limit is set to 0.020% in order to reduce the amount of nitride, precipitate the nitride, deteriorate the stability of the material with respect to the fluctuation of the manufacturing conditions, and further increase the hardness of the heat affected zone.

[Al] × [N] [Al] × [N] is an important control factor in the present invention.
It controls the form of precipitation of the l-based nitride. In the present invention, the amounts of Al and N are set within the above ranges, and [Al] ×
[N] is set to 6.0 × 10 ⁻⁴ or less. If [Al] × [N] exceeds 6.0 × 10 ⁻⁴ , the precipitation amount of AlN increases,
The stability of the material with respect to the fluctuation of the manufacturing condition is deteriorated.

The basic component composition of the present invention is as described above.
Although the object can be sufficiently achieved, one or more of the following elements are selectively added depending on desired properties.

Ti, V Each of Ti and V forms carbides with C in the steel, refines crystal grains, and improves the strength and toughness of the steel. When these elements are added, the effect is not sufficient if it is less than 0.01%, while the effect is saturated even if it exceeds 0.1%, so that each is set to 0.01 to 0.1%. .

Ca, Zr, Mg Ca, Zr, Mg effectively suppresses the formation of AlN precipitates in steel. However, if these elements exceed 0.005%, they form oxide-based inclusions and deteriorate toughness. Therefore, the amount of each of these elements is set to 0.005% or less.

[0024] The steel of the present invention is melted in a converter, an electric furnace or a combination thereof, and the method is not particularly limited.
After smelting, it is made into a slab by a continuous casting machine or a mold, processed into a predetermined shape by hot rolling, and brought to a target strength by heat treatment.
It is desirable that the strength be adjusted by cooling after working or by normalizing to a martensitic structure and then tempering.

[0025]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted using a vacuum melting furnace, and hot-rolled to form a steel sheet having a thickness of 8 mm.
Normalization was performed with a proof stress of 600 to 650 MPa as a target, and tempering was performed. Assuming actual operation, hot-rolled steel sheet is 780 ℃ ±
After normalizing at 10 ° C., tempering was performed at 650 ° C. ± 10 ° C. After the heat treatment, the stability of the material with respect to the fluctuation of the manufacturing conditions (hereinafter referred to as manufacturability), corrosion resistance and weldability were investigated.

The manufacturability was determined by melting ten steel sheets for each steel type and subjecting each steel sheet to a tensile test (JISZ
2203) to determine mechanical properties (YS, TS).
The target strength was YS: 600 to 650 MPa.

The corrosion resistance is 5% NaCl-30 atm CO
_It was immersed in the solution of No. ₂ at 180 ° C. for 96 hours, and the corrosion resistance to wet carbon dioxide gas was determined. Corrosion 0.3mm /
The target was set to y or less. The weldability test aimed at obtaining preheating and postheating conditions in on-site welding, investigated the hardness of the reproduced HAZ, and set the target at Hv 350 or less.

Table 2 shows the test results. In the table, the circles indicate that the test results were good.
If more than a percentage satisfied the target strength, it was determined to be good. When the corrosion resistance and the weldability respectively satisfied the target values, it was determined to be good.

With the steels A to J of the present invention, good results were obtained in all of the manufacturability, corrosion resistance and weldability. On the other hand, the comparative steel K
The amount is small and the corrosion resistance is inferior. The comparative steel L has a large amount of the deoxidizing material Si and the comparative steel M has a large amount of the desulfurizing material Mn outside the range of the present invention. The comparative steel N has a small amount of Ni, and delta ferrite precipitates to lower the corrosion resistance. Comparative steel O has a large C content, and has high strength and hardness of the reproduced HAZ portion, and is inferior in weldability. Comparative steel P
ＳS has a high Al or N concentration or any one of the values of [Al] × [N], and the manufacturability is poor.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

According to the present invention, a martensitic stainless steel having excellent corrosion resistance and weldability, and having a high material stability even when the heat treatment conditions fluctuate, can be used for oil and natural gas line pipes. It is extremely useful in industry.

Continuing on the front page (72) Inventor Yusuke Minami 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Yukio Maho 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Stock In company

Claims (3)

[Claims] Claims: 1. A mass% of C: 0.02% or less, Si:
0.1-0.3%, Mn: 0.1-0.3%, Cr: 1
1-15%, Ni: 1-5%, Al: 0.060% or less, N: 0.020% or less, and [Al] ×
[N]: martensitic stainless steel characterized by being 6.0 × 10 ⁻⁴ or less and substantially consisting of Fe and inevitable impurities. However, [Al] × [N] indicates the product of the concentration (% by mass) of Al and N. 2. Ti: 0.01 to 0.1% by mass%,
The martensitic stainless steel according to claim 1, which contains one or two kinds of V: 0.01 to 0.1%. 3. Ca: 0.005% or less by mass%, Z:
The martensitic stainless steel according to claim 1 or 2, which contains one or more of r: 0.005% or less and Mg: 0.005% or less.