JPH06100943A - Manufacturing method of stainless steel line pipe - Google Patents

Abstract

(57) [Summary] [Purpose] Providing a method for manufacturing martensitic stainless steel line pipes with excellent weldability. [Structure] C: 0.02 to 0.04% by weight, S
i: 0.1 to 0.3%, Mn: 0.3 to 1.0%,
S: 0.005% or less, P: 0.020% or less,
Cr: 12.0 to 14.0%, Ni: 3.5 to 4.5
%, Mo: 0.75 to 1.25%, Cu: 0.1
0.5%, N: 0.03 to 0.07%, balance Fe and unavoidable impurities, and the following formula (I) 4.0 ≦ Ni + Cu + 30 (C + N) −1.5 × Mo ≦ 5.5 (I ), A martensitic stainless steel is used as a steel pipe, austenitized at 900 ° C to 1000 ° C, cooled at a cooling rate of air cooling or higher, and then cooled to 600 ° C or higher.
_A method for producing a stainless steel line pipe, which comprises performing tempering treatment at a temperature lower than the _Cl point temperature and then performing air cooling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a martensitic stainless steel line pipe having excellent weldability, which has high corrosion resistance when transporting petroleum or natural gas containing wet carbon dioxide. , A method for producing a steel pipe having excellent weldability.

[0002]

2. Description of the Related Art Oil and natural gas produced in recent years often contain wet carbon dioxide gas due to depletion of wells in a mild environment. It is known that carbon steel and low alloy steel show a large corrosion weight loss in a humid carbon dioxide environment.
Conventionally, corrosion inhibitors have been added. However, the corrosion inhibitor may not be able to exert its effect when the temperature is high, and in a submarine pipeline, treatment of the corrosion inhibitor is expensive, which is not always practical. In the environment, steel containing about 13% Cr has excellent corrosion resistance, and AI containing about 0.2% C.
SI420 series martensitic stainless steel pipe API
It is standardized by (American Petroleum Institute) and is used in large quantities in oil well steel pipes without welded structures.

However, the line pipe is required to have corrosion resistance, strength sufficient to withstand the internal pressure of the transport fluid, and high impact value of the welded portion and the base metal. AISI42
Since the 0-system martensitic stainless steel pipe has a high C content, the welding preheating temperature is around 200 ° C, and the hardness of the weld heat-affected zone is high and the impact value is low, so it is applied to line pipes. Is difficult. From these points of view, duplex stainless steels have come to be widely applied to the line pipes of the environment (Iron and Steel Institute, February 1, 1990).
Held on the 4th, the 133rd Nishiyama Memorial Lecture, page 52), duplex stainless steel contains a large amount of Cr, Ni, Mo,
It was not economical because it could be over-performance in some environments.

On the other hand, although the line pipe using 12Cr type AISI410 steel was standardized by API in June 1988, the content of Ni was 0.5% or less. NKK Technical Report (1989) Issued annually, No. 129, 15
As shown in (Page 22), it has a drawback that the impact property of the weld is poor. Recently Duplex Stainless Steel and 13%
A 13% C martensitic stainless steel containing Ni and Mo with low C, which is intermediate in corrosion resistance and cost of Cr martensitic stainless steel, has been developed (for example, Iron and Steel Institute, CAMP-ISIJ Vol. 2 (1989). −
898, Vol. 4 (1991) -558), but these are all for oil well steel pipes and do not have the weldability required for line pipes.

[0005]

Therefore, the present invention has sufficient corrosion resistance even in a carbon dioxide gas environment, the fracture transition temperature of the base metal and the weld heat affected zone is -30 ° C or less, and the welding preheating temperature is 125 ° C. It is an object of the present invention to provide the following method for producing a martensitic stainless steel line pipe.

[0006]

DISCLOSURE OF THE INVENTION As a result of studying the influence of the components in order to achieve the above object, the present inventors reduced the C content and added Ni, Mo, and Cu as in the case of the oil well steel pipe. However, it has been found that if the components are controlled using certain parameters, the weldability required for the line pipe can be maintained and the corrosion resistance in a wet carbon dioxide environment can be improved.

That is, according to the present invention, in% by weight, C:
0.02-0.04%, Si: 0.1-0.3%, M
n: 0.3 to 1.0%, S: 0.005% or less,
P: 0.020% or less, Cr: 12.0 to 14.
0%, Ni: 3.5 to 4.5%, Mo: 0.75
1.25%, Cu: 0.1 to 0.5%, N: 0.0
3 to 0.07%, balance Fe and inevitable impurities, and martensite stainless steel satisfying the following formula (I) 4.0 ≦ Ni + Cu + 30 (C + N) −1.5 × Mo ≦ 5.5 (I) Steel pipe, then austenitized at 900 ° C to 1000 ° C, cooled at a cooling rate of air cooling or higher, and then 600 ° C or higher
_{The present} invention provides a method for producing a stainless steel line pipe, which comprises performing a tempering treatment at a temperature lower than the _Cl point temperature and then performing air cooling.

[0008]

The present invention will be described in more detail below. First,
The components of the martensitic stainless steel used in the present invention and the reasons for limitation thereof will be described.

C: 0.02 to 0.04%, the lower the content of C, the more preferable the corrosion resistance is, but if it is less than 0.02%, vacuum degassing treatment is required in the steelmaking stage, resulting in high cost. If it exceeds 0.04%, not only the corrosion resistance is adversely affected but also the weldability deteriorates, so the content was made 0.02% to 0.04%.

Si: 0. 1 to 0.3% , which is an element necessary for deoxidation, but an unnecessarily high content because it is also a ferrite-forming element, contains C, Ni, C from the viewpoint of structural balance.
Since it is necessary to increase the content of the austenite-forming element such as u or N above the original purpose, the content was made 0.1 to 0.3% which is sufficient for deoxidation.

Mn: 0.3 to 1.0% Mn is an element necessary not only for deoxidation but also for securing strength, but in this component system, addition of more than 1.0% is effective. Is saturated, the upper limit was made 1.0%.

S: 0.005% or less, P: 0.020%
In the following , any of them affects the corrosion resistance and impact properties and is preferably as small as possible, but the upper limit is set to a value that does not cause an industrially nuisance increase in cost and has no adverse effect.

Cr: 12.0 to 14.0% , Cr is a basic element necessary to secure a martensitic structure and to impart corrosion resistance. To obtain stable corrosion resistance 1
2.0% is necessary, and when it is contained in excess of 14.0%, the same amount of Ni is further required to secure the martensite structure, which leads to an increase in cost. As described above, the hardness of the heat-affected zone of welding is increased, and a long tempering process is required.

Ni: 3.5 to 4.5% , Ni supplements the amount of austenite-forming element due to C reduction, and is an element essential for improving general corrosion resistance and impact characteristics in the environment as well as C reduction. is there. If it is less than 3.5%, it is difficult to supplement the amount of austenite-forming element, and if it exceeds 4.5%, not only the effect is saturated but also the A _Cl point is significantly lowered, which requires a long tempering treatment. Or, the hardness of the weld heat affected zone is significantly increased.

Mo: 0.75 to 1.25% , oil and gas fields generally contain chloride ions in many cases, but in a wet carbon dioxide environment containing chloride ions, in addition to general corrosion, local corrosion Is a problem. Mo is an element that remarkably improves the local corrosion resistance, and if it is less than 0.75%, its effect cannot be sufficiently exerted, and if it is added in an amount of more than 1.25%, it only causes an increase in cost. Is saturated.

Cu: 0.1 to 0.5% , Cu is Ni, N
At the same time, it is added as an element that supplements the amount of austenite forming element due to C reduction, but it is also effective in improving the impact characteristics of the weld heat affected zone. However, if less than 0.1%, the effect is not recognized at all, and if more than 0.5% is added, some C
u does not form a solid solution but precipitates, and the hardness of the weld heat affected zone increases.

N: 0.03 to 0.07% , N is Ni, C
Although it is added together with u as an element that supplements the amount of austenite forming element due to C reduction, if N is reduced to 0.02% to 0.04% and N is usually 0.20 to 0.25%, a martensite single phase It may not become a structure, and causes deterioration of corrosion resistance and impact properties. Further, in order to have a single-phase structure, if Ni, Cu, and C are added up to the upper limit value of the predetermined range, the quench hardening ability increases, and the preheating temperature cannot be 125 ° C. or lower. Therefore, the lower limit is 0.0
It was 3%. The upper limit is 0.07% because if added in excess of 0.07%, the solubility may be exceeded during solidification and blowholes may occur.

4.0 ≦ Ni + Cu + 30 (C + N) −
1.5 × Mo ≦ 5.5 , this parameter is for making the steel constituted in the above composition range into a material having a low welding preheating temperature and excellent impact characteristics of the welding heat affected zone without impairing corrosion resistance. It is a thing. That is, if it exceeds 5.5, the quench-hardening ability is high, and the preheating temperature cannot be 125 ° C. or lower. If it is less than 4.0, the weld heat affected zone may not have a martensite single-phase structure due to the welding heat cycle, and the corrosion resistance and impact properties may deteriorate.

A line pipe is manufactured using the martensitic stainless steel having the above components. First, the above steel is used as a steel pipe, and then austenitized at a temperature of 900 to 1000 ° C. If the temperature is lower than 900 ° C., complete austenitization cannot be expected, sufficient strength cannot be secured after tempering, and strength is uneven. Further, if the temperature is higher than 1000 ° C., the grain size becomes coarse and the impact properties of the base material deteriorate.
After austenitizing, cool at a cooling rate higher than air cooling. This is to prevent deterioration of impact properties due to precipitation of Cr carbide.

Then, tempering treatment is performed at a temperature of 600 ° C. or higher and _lower than the A _Cl point. The tempering that exceeds the _ACl point partially transforms to austenite, so that the untempered martensite phase is re-precipitated by the subsequent cooling and the impact properties are significantly deteriorated. Further, if it is less than 600 ° C., sufficient tempering is not performed. After this, air cool.

[0021]

EXAMPLES The present invention will be specifically described below based on examples. (Example) Stainless steel having the components shown in Table 1 was manufactured by a hot rolling method into a seamless steel pipe having an outer diameter of 168.3 mm and a wall thickness of 8.94 mm, and then heat-treated as shown in Table 2 to obtain a product. A joint was produced by circumferential welding using a covered arc welding method. The strength of each steel pipe is 56kg with a 0.2% offset.
/ mm ² or more. The heat input for circumferential welding was 1.4 kJ / mm only for the root pass, and was 1.7 kJ / mm for all subsequent passes. A JIS No. 4 impact test piece (full size) was sampled from the base metal and the welding heat affected zone and subjected to an impact test. The hardness of the heat affected zone was measured by Vickers under a load of 500 g. Furthermore, two types of corrosion tests are conducted, and the thickness is 3m.
m-width, 20 mm, 45 mm long strip-shaped test piece and U-be obtained by bending a 3 mm-thick, 20 mm wide, 75 mm-long strip-shaped test piece into a U-shape with a bending radius of 8 mm.
It is a stress corrosion cracking test using a nd test piece. All the environments were immersed in 20% NaCl for 10 days in an autoclave at 150 ° C. under the condition of a partial pressure of carbon dioxide gas of 30 atm.

In the evaluation of each test, in the impact test, those having a fracture surface transition temperature of -30 ° C or lower are indicated by ◯, and those having a fracture surface transition temperature higher than -30 ° C are indicated by x. The maximum hardness of 300 or less is represented by O, and the hardness exceeding 300 is represented by X. For preheating temperature, use two steel pipes with a length of 1.0 m that were separately implemented as root face 2.
0 mm, root gap 1.6 mm, groove angle 35 on one side
After forming a groove shape with a degree of 100 degrees, a bead having a length of 100 mm was temporarily placed for one pass, and then a test was conducted to confirm the presence or absence of cracking after 4 hours. When the temperature was 125 ° C. or less, it was represented by ◯, and when it exceeded 125 ° C., it was represented by x. In the general corrosion test, those of 5 mils / year (0.125 mm / y) or less, which are generally regarded as corrosion resistant materials, are ○, those of 0.05 mm / y or less are ◎, 0.125 mm
Those exceeding / y are represented by x. In the stress corrosion cracking test, the cross section of the U-shaped portion was observed with an optical microscope to confirm the presence or absence of cracks. The case where cracks were confirmed was represented by x, and the case where cracks were not confirmed was represented by o. The results are summarized in Table 2. Obviously, the steel pipe manufactured according to the present invention has corrosion resistance,
It can be seen that it has excellent weldability and can be applied to line pipes.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

The present invention relates to the production of a seamless steel pipe, but it is obvious that the present invention can also be applied to a UOE steel pipe using a thick plate obtained by rolling a thick plate. As described above, the present invention provides a method for producing a high-strength martensitic stainless steel line pipe having both excellent corrosion resistance and excellent weldability in a wet carbon dioxide environment, and contributes to the development of industry. Is large.

Claims (1)

[Claims] 1. By weight%, C: 0.02 to 0.04%, Si: 0.1 to 0.3.
%, Mn: 0.3 to 1.0%, S: 0.005%
Below, P: 0.020% or less, Cr: 12.0 to
14.0%, Ni: 3.5-4.5%, Mo: 0.
75-1.25%, Cu: 0.1-0.5%, N:
A martensite system which comprises 0.03 to 0.07%, balance Fe and unavoidable impurities, and satisfies the following formula (I): 4.0 ≦ Ni + Cu + 30 (C + N) −1.5 × Mo ≦ 5.5 (I) After using stainless steel as a steel pipe, it is austenitized at 900 ° C to 1000 ° C, cooled at a cooling rate of air cooling or higher, and then 600 ° C or higher.
_A method for producing a stainless steel line pipe, which comprises performing tempering treatment at a temperature lower than the _Cl point temperature and then performing air cooling.