JPH075972B2 - Method for manufacturing low carbon martensitic stainless steel line pipe - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a low carbon martensitic stainless steel line pipe, and more specifically, as a line pipe used for transportation of oil / natural gas, for example, as a wet pipe. The present invention relates to a method for manufacturing a steel pipe having excellent corrosion resistance in an environment containing carbon dioxide gas and wet hydrogen sulfide and having excellent weldability.

(Prior Art) In recent years, oil and natural gas produced in recent years are often containing a large amount of wet carbon dioxide gas. It is well known that carbon steel and low alloy steel are significantly corroded in such an environment. Therefore, addition of a corrosion inhibitor has been conventionally performed as an anticorrosion measure for line pipes used for transporting oil and natural gas. However, in many cases, the corrosion inhibitor loses its effect at high temperatures, and the cost required for adding and recovering the corrosion inhibitor in a submarine pipeline is enormous, and thus it is often not applicable. Therefore, the need for a corrosion resistant material that does not require the addition of a corrosion inhibitor has been increasing recently.

As a corrosion resistant material for oil and natural gas that contains a large amount of carbon dioxide, the application of stainless steel with good corrosion resistance was first examined. For example, LJ Klein, Corrosion (Corrosio
n) '84, Paper No. 211, martensite containing about 0.2% C and about 12 to 13% Cr, as typified by AISI420 steel, which has high strength and is relatively inexpensive. Series stainless steels are beginning to be widely used. However, these steels are added with a relatively large amount of carbon in order to obtain high strength.

AISI420 steel contains 0.16% or more and 0.22% or less of carbon. In line pipes, pipes are usually connected by welding during laying, but if such a martensitic stainless steel containing a relatively large amount of carbon is welded by a normal welding method, the weld heat affected zone The hardness significantly increases and the impact toughness deteriorates. Further, the increase in hardness of the weld heat affected zone makes the risk of sulfide stress cracking and pipeline destruction extremely high when hydrogen sulfide is contained in the transport fluid. It is possible to reduce the hardness of the heat affected zone by performing post-weld heat treatment such as holding at 600 ° C or higher after welding.However, performing such post-weld heat treatment at the line pipe laying site requires temperature control and The quality assurance is extremely difficult and enormous cost is required. For this reason, there is a demand for steel in which the hardness of the heat-affected zone of the weld is less increased even if the welding is carried out by the usual welding method.

On the other hand, if the carbon content in the martensitic stainless steel is reduced, the hardness increase in the weld heat affected zone can be suppressed and a steel having practically sufficient characteristics can be obtained.
However, in this case, it is difficult to manufacture by the seamless steel pipe rolling method, which is a process in which the conventional martensitic stainless steel pipe has been manufactured. According to the study by the present inventors,
It has been found that the cause is that a large amount of ferrite structure is produced during heating of the seamless steel pipe before rolling because the carbon content is reduced.

(Problems to be Solved by the Invention) In view of these circumstances, an object of the present invention is to provide a method for producing a martensitic stainless steel having reduced C as a steel pipe in order to improve weldability and corrosion resistance.

(Means for Solving the Problem) In order to achieve the above object, the present inventors first studied the components of martensitic stainless steel, and as a result, reduced the carbon content in order to improve the weldability and corrosion resistance. It was found that the above is extremely effective, and that the effect is particularly remarkable when the carbon content is 0.08% or less. Further, the present inventors have further studied the manufacturing method and set the carbon content to 0.08.
As a result of various studies on the manufacturing process using martensitic stainless steel as a steel pipe with a content of 10% or less, it is difficult to manufacture by the ordinary seamless steel pipe rolling method. After being formed into a strip by electric resistance welding, it is manufactured as a steel pipe, and if the subsequent heat treatment conditions are properly selected, the necessary characteristics as a line pipe can be satisfied and the welding heat affected zone of the circumferential welded part of the line pipe can be It was confirmed that it is possible to manufacture martensitic stainless steel line pipes whose characteristics far exceed those of conventional martensitic stainless steel pipes.

Here, the gist of the present invention is a method for producing a low-carbon martensitic stainless steel line pipe in which martensitic stainless steel pieces having a carbon content of 0.08% by weight or less are sequentially steel pipes in the following steps. It is in.

After heating the billet to a temperature of 1050-1300 ℃, the plate thickness 4.
A step of hot rolling to 0 mm or more and 25.4 mm or less and further winding as a hot coil at a temperature of 600 ° C. or more, after cutting the above hot coil into a predetermined width, continuously shaping the steel strip at both ends while forming it into a cylindrical shape. Process of welding by electric resistance welding to make a steel pipe, the above electric resistance welded steel pipe in the temperature range of 950 ℃ to 1100 ℃
After heating above min, then re-heated to at least 300 ° C. or less steps by cooling with air or the cooling rate is more than 50 volume% and accounting are tissue Matensaito, to a temperature of 550 ° C. or higher A _C1 less transformation point For 1 min or more, and then cooling at least 300 ° C or less at a cooling rate of air cooling or more.

Alternatively, the gist of the present invention is also a method for producing a low carbon martensitic stainless steel line pipe, which is a steel pipe in the next step.

After heating the billet to a temperature of 1050-1300 ℃, the plate thickness 4.
A step of hot rolling to 0 mm or more and 25.4 mm or less and further winding as a hot coil at a temperature of 600 ° C. or more, after cutting the above hot coil into a predetermined width, continuously shaping the steel strip at both ends while forming it into a cylindrical shape. ERW welded to form a steel pipe, and immediately thereafter, at least 2 mm on both sides of the ERW part
Reheating the part including the part within 500 ℃ or more to a temperature of A _C1 transformation point or less, after heating the electrosewn steel pipe to a temperature range of 950 ℃ or more and 1100 ℃ or less for 1 min or more, until at least 300 ℃ or less a step of cooling to 50 volume% in air over the cooling rate is to occupy is tissue martensite, after holding above 1min reheated to a temperature of 550 ° C. or higher a _C1 less transformation point to at least 300 ° C. or less The step of cooling at a cooling rate higher than air cooling.

(Operation) The martensitic stainless steel which is the subject of the present invention has a carbon content of 0.08% or less and has a substantially fine structure of 50%.
% Is all martensitic stainless steels having a martensitic structure. Hereinafter, the standard components of martensitic stainless steel which are desirable as the subject of the present invention will be exemplified, and the reason for selecting the content will be described. The content of each element is% by weight.

C: 0.08% or less If C is added in excess of 0.08%, the hardness of the heat-affected zone of the line pipe at the time of local circumferential welding becomes too high, which deteriorates the properties and corrosion resistance. 0.
It should be below 08%.

Si: 1% or less Si is effective as a deoxidizer and a strengthening element, but if it exceeds 1%, the toughness decreases, so the upper limit content is 1%.

Mn: 2% or less or more than 2% and 5% or less Mn is effective as a deoxidizer and a strengthening element, but even if added in an amount of more than 5%, the effect is not only saturated, but also contains hydrogen sulfide. Since the stress corrosion cracking resistance in the environment is reduced, the upper limit content is 5%.

Cr: 7.5 to 14% Cr is the most basic element that constitutes martensitic stainless steel, and it is necessary to add 7.5% or more to ensure corrosion resistance in a carbon dioxide environment, but 14%
If added in excess of 10%, it will be difficult to secure the strength required as a line pipe no matter how the other components are adjusted, so the addition range should be 7.5-14%.

Al: 0.1% or less Al is very effective as a deoxidizing element, but the content is 0.1
%, The alumina inclusions increase and the toughness decreases, so the upper limit content is made 0.1%.

N: 0.02% or less If N is present in a large amount like C, the hardness of the weld heat affected zone during field circumferential welding of the line pipe becomes too high and the characteristics deteriorate, so the content should be 0.02% or less. Should be.

In addition to the above-mentioned components, the balance of Fe and inevitable impurities is the most basic steel targeted by the present invention. In addition to this, it is also possible to manufacture a line pipe by using steel to which the following elements have been added or reduced if necessary.

Ni: Ni is effective in further improving the corrosion resistance in a humid carbon dioxide environment, but even if added in excess of 4%, the effect saturates and, conversely, stress corrosion cracking in an environment containing hydrogen sulfide. The upper limit of the content is 4% because it lowers the resistance.

Cu: Cu is also effective in reducing the corrosion rate of martensitic stainless steel in a wet carbon dioxide environment and adding it to steels with adjusted C and N contents to improve the toughness of the weld heat affected zone. However, even if added in excess of 4.5%, not only the effect will be saturated, but also the hot workability will be deteriorated, so the upper limit content is made 4.5%.

Co: Co is also effective in further improving the corrosion resistance in a wet carbon dioxide environment, but if it is added in excess of 4%, the effect will not only be saturated, but it will increase the cost of mischief. The content is 4%.

Mo: Mo is effective in improving the corrosion resistance in a wet carbon dioxide environment, but if it is added in excess of 2%, the effect will not only be saturated, but it will also reduce other properties such as toughness. Therefore, the upper limit content is 2%.

W: W is also effective in improving the corrosion resistance in a wet carbon dioxide environment, but if it is added in excess of 4%, the effect will not only be saturated, but other properties such as toughness will be deteriorated. Therefore, the upper limit content is 4%.

P: P is an element that deteriorates hot workability, so it is preferable that the amount is small. However, reducing it to a level that is too small unnecessarily increases the cost and saturates the effect of improving the characteristics. In the case of the present invention, the hot workability is further improved by reducing the content to 0.02% or less, which is sufficiently small to secure the hot workability required for manufacturing the target line pipe.

Similar to P, S: S is an element that deteriorates hot workability, so it is preferable that the amount is small, but reducing it to an excessively small level unnecessarily increases the cost and saturates the effect of improving the characteristics. In the case of the present invention, the stress corrosion cracking resistance is further improved when the content is reduced to 0.003% or less as a necessary and sufficient amount for ensuring the hot workability required for producing the target line pipe. .

V, Ti, Nb, Zr, Ta, Hf, B: V, Ti, Ta, Zr, Nb, Hf, B are effective elements to further improve the corrosion resistance, but Ti, Zr, Ta, Hf
When added in excess of 0.2% in V, Nb, 0.5% in V, Nb, and 0.01% in B, coarse precipitates / inclusions are formed to reduce stress corrosion cracking resistance, so the upper limit content is T
i, Zr, Ta, and Hf were 0.2%, V and Nb were 0.5%, and B was 0.01%.

Ca, rare earth element: Ca and rare earth element (REM) are elements that are effective in improving hot workability and corrosion resistance.
When added in excess of 0.01% and in rare earth elements in excess of 0.02%, coarse non-metallic inclusions are formed, respectively, which adversely deteriorates hot workability and corrosion resistance. Therefore, the upper limit content of Ca is 0.01%. , And 0.02% for rare earth elements.

In the present invention, the rare earth element has an atomic number of 57 to 71.
No. 89-104 and Y.

Next, the process of the present invention will be described.

Billet heating temperature: It is necessary to uniformly heat the billet to the center thereof to ensure hot workability in hot rolling. If the heating temperature exceeds 1300 ° C., the material loss due to the formation of oxide scale becomes significant and the yield decreases, which is not preferable. On the other hand, if the heating temperature is less than 1050 ° C, the deformation resistance in hot rolling becomes too large, which is not preferable. Therefore, the billet heating temperature is 1050.
~ 1300 ℃

Hot rolling: Hot rolling can be ordinary strip rolling. For practical use as a line pipe, the plate thickness should be 4.0 mm or more and 25.4 mm or less. From the viewpoint of productivity in the subsequent electric resistance welding, the plate shape is a hot coil.

Winding: When the hot coil is wound after hot rolling, if the winding temperature is less than 600 ° C, the winding force becomes too strong to make winding difficult, and the strength increases after winding and the subsequent electric sewing Since it interferes with welding, it is necessary to wind it at a temperature of 600 ° C or higher.

Forming and ERW welding: Normal ERW welding steel pipe manufacturing process can be applied to forming and ERW welding. After cutting into a predetermined width according to the outer diameter required as a line pipe, forming and ERW welding It may be manufactured as a steel pipe.

Heat treatment: In the first stage heat treatment (quenching), the heating temperature is
This is because austenitization is not sufficient at a temperature lower than 950 ° C, and thus it is difficult to obtain the required strength,
If the heating temperature exceeds 1100 ° C, the crystal grains will be remarkably coarsened and the stress corrosion cracking resistance will decrease, so it is necessary to set the heating temperature to 950 to 1100 ° C.

The cooling rate in the cooling after austenitization is that the cooling rate of at least 300 ° C. or lower is the cooling rate of air cooling or higher,
This is because martensite is not sufficiently generated at a cooling rate slower than that of air cooling, and it becomes difficult to secure a predetermined strength.If this cooling rate is not secured until the temperature of the steel pipe reaches 300 ° C or lower, This is because it becomes difficult to secure the strength of.

It is difficult to obtain the necessary strength as a line pipe unless at least 50% by volume is occupied by martensite after the first stage heat treatment.

In the second stage heat treatment (tempering), the heating temperature was set to 550
℃ or more and A _c1 temperature or less means that if the heating temperature is less than 550 ° C., sufficient tempering is not performed, and if the heating temperature exceeds the A _c1 temperature, part of the material becomes austenite, and fresh martensite is generated during subsequent cooling. This is to increase the susceptibility to stress corrosion cracking because martensite that is formed and is not sufficiently tempered remains.

The cooling rate in the cooling after the tempering is set to be the cooling rate equal to or higher than the air cooling because the toughness decreases at the cooling rate lower than the air cooling.

In addition to the above steps, if necessary, after cutting the hot coil into a predetermined width immediately after forming into a cylindrical shape by electric resistance welding of both ends of the steel strip to form a pipe, immediately at least both sides of the electric resistance portion A including the part within 2mm is 500 ℃ or more A
A step of reheating to a temperature not _higher than the _c1 temperature may be added, but the purpose of this step is the hardening structure produced by electric resistance welding, especially martensite, which is used until the final heat treatment (quenching and tempering). This is to prevent cracks from occurring in the sewn welded portion. From this purpose, when the reheating temperature is less than 500 ° C, the effect of softening the hardened structure is not remarkable, and when the reheating temperature exceeds the _Ac1 temperature, a part of the material becomes austenite, and fresh martensite is generated during the subsequent cooling. However, since the martensite that has not been sufficiently tempered remains, the risk of cracking increases on the contrary until the final heat treatment. Therefore, set the reheating temperature to 500
The temperature shall be above ℃ and below A _c1 temperature. In addition, since such a hardened structure is most prominently generated within 2 mm on both sides of the electric resistance welded portion, it is necessary to reheat the portion including at least this portion, but only this portion including the portion must be reheated. It may be reheated or the entire steel pipe may be reheated. In any case, the effect of preventing cracks in the electric resistance welded portion is greater as the reheating treatment is performed as soon as possible after electric resistance welding.

Next, examples of the present invention will be described.

(Examples) Stainless steel Nos. 1 to 10 having the components shown in Table 1 were melted and formed into a hot coil having a thickness of 12.7 mm by hot rolling, and then formed and electric resistance welded to form a steel pipe. , The yield strength was 45.7kg after heat treatment under the conditions shown in Table 1.
Steel pipe for line pipe with f / mm ² or more. During hot rolling, the heating temperature was 1200 ° C and the winding temperature was 650 ° C. Further, the No. 2 and No. 5 steel pipes were reheated to 650 ° C on both sides of the electric resistance welded portion immediately after being made into steel pipes by high frequency electric resistance welding. Cooling during quenching was water cooling to room temperature, and cooling after tempering was air cooling to room temperature. On the other hand, Comparative Example No. 11 is AISI 420 steel, No. 12 is 9Cr-1Mo.
Steel is a conventional steel that has been conventionally used in a wet carbon dioxide environment. Comparative Examples Nos. 11 and 12 were made into steel pipes by the conventional seamless rolling method.
The heat treatment was performed under the conditions shown in the table together.

Next, these steel pipes were welded by hand welding to prepare a joint as welding corresponding to local circumferential welding when laying a line pipe. The welding heat input was 17 kJ / cm. A JIS No. 4 impact test piece (full size) was sampled from the base material and the heat-affected zone of the weld zone to carry out an impact test. The maximum hardness of the heat affected zone was measured as Vickers hardness with a load of 5 kg. On the other hand, a test piece was taken from the base material and a corrosion test was performed in a wet carbon dioxide gas environment. For a corrosion test in a wet carbon dioxide environment, a test piece with a thickness of 3 mm, a width of 15 mm and a length of 50 mm was used. The test piece was placed in an autoclave at a test temperature of 120 ° C under a carbon dioxide partial pressure of 40 atm in a 3% NaCl aqueous solution. After being immersed for a day, the corrosion rate was calculated from the weight change before and after the test.
Although the unit of corrosion rate is expressed in mm / y, generally, when the corrosion rate of a material in an environment is less than 0.1 mm / y, the material is considered to be sufficiently corrosion resistant and usable.

The test results are also shown in Table 1. In Table 1, in the impact test results, ◯ indicates that the fracture surface transition temperature is −30 ° C. or lower, × indicates that the fracture surface transition temperature exceeds −30 ° C. and is 0 ° C. or less, and XX indicates that the fracture surface transition temperature exceeds 0 ° C. In the welding heat affected zone maximum hardness, ○ indicates that the maximum hardness is less than 300,
X has a maximum hardness of 300 or more and less than 450, and xx has a maximum hardness of 450
In the corrosion test results, ◎ indicates a corrosion rate of less than 0.05 mm / y, ○ indicates a corrosion rate of 0.05 mm / y or more and less than 0.10 mm / y, and × indicates a corrosion rate of 0.1 mm / y. y
Above 0.5 mm / y, and XX indicate that the corrosion rate was 0.5 mm / y or more.

As is apparent from Table 1, Nos. 1 to 10 which are examples of the present invention
Is remarkably excellent in impact toughness of the base material and the weld heat affected zone,
The maximum hardness of the heat-affected zone is sufficiently low, and it has practically sufficient corrosion resistance even in a very high temperature of 120 ° C in a wet carbon dioxide environment, and has excellent weldability and corrosion resistance. You know that you have. On the other hand, Comparative Examples Nos. 11 and 12 are significantly inferior in the impact toughness of the base metal and the weld heat affected zone, the maximum weld heat affected hardness, and the corrosion resistance.

(Effects of the Invention) As described above, the present invention makes it possible to provide a method for producing a low carbon martensitic stainless steel pipe which is extremely useful as a line pipe, and contributes greatly to industrial development. Is large.

Claims (5)

[Claims] 1. A method for producing a low-carbon martensitic stainless steel line pipe in which martensitic stainless steel pieces having a carbon content of 0.08% by weight or less are sequentially formed into steel pipes in the following steps. After heating the billet to a temperature of 1050-1300 ℃, the plate thickness 4.
A step of hot rolling to 0 mm or more and 25.4 mm or less and further winding as a hot coil at a temperature of 600 ° C. or more, after cutting the above hot coil into a predetermined width, continuously shaping the steel strip at both ends while forming it into a cylindrical shape. Process of welding by electric resistance welding to make a steel pipe, the above electric resistance welded steel pipe in the temperature range of 950 ℃ to 1100 ℃
After heating for more than min, cool at least 300 ° C or less at a cooling rate of air cooling or more to make a structure in which 50% by volume or more of martensite is occupied, and reheat to a temperature of 550 ° C or more and A _c1 transformation point or less. For 1 min or more, and then cooling at least 300 ° C or less at a cooling rate of air cooling or more. 2. A method for producing a low-carbon martensitic stainless steel line pipe in which martensitic stainless steel pieces having a carbon content of 0.08% by weight or less are sequentially formed into steel pipes in the following steps. After heating the billet to a temperature of 1050-1300 ℃, the plate thickness 4.
A step of hot rolling to 0 mm or more and 25.4 mm or less and further winding as a hot coil at a temperature of 600 ° C. or more, after cutting the above hot coil into a predetermined width, continuously shaping the steel strip at both ends while forming it into a cylindrical shape. ERW welded to form a steel pipe, and immediately thereafter, at least 2 mm on both sides of the ERW part
Reheating the part including the part within the temperature range of 500 ℃ or more and A _c1 transformation point or less, the electric resistance welded steel pipe in the temperature range of 950 ℃ or more and 1100 ℃ or less 1
After heating for more than min, cool at least 300 ° C or less at a cooling rate of air cooling or more to make a structure in which 50% by volume or more of martensite is occupied, and reheat to a temperature of 550 ° C or more and A _c1 transformation point or less. For 1 min or more, and then cooling at least 300 ° C or less at a cooling rate of air cooling or more. 3. A low carbon martensitic stainless steel comprising the following first group, second group, third group, fourth group, fifth group, sixth group,
The method for producing a low-carbon martensitic stainless steel line pipe according to claim 1 or 2, which comprises either the seventh group or the eighth group of components. Group 1 weight%, C0.08% or less, Si1% or less, Mn2% or less, Cr7.5
Low carbon martensitic stainless steel containing ~ 14%, Al 0.1% or less, N 0.02% or less, and the balance Fe and unavoidable impurities. 2nd group, wt%, C0.08% or less, Si1% or less, Mn2% or less, Cr7.5
-14%, Al 0.1% or less, N 0.02% or less, Ni 4% or less, Cu 4.5% or less, Co 4% or less 1 type or 2 or more types, and the balance Fe and unavoidable impurities Low carbon martensitic stainless steel. 3rd group, wt%, C0.08% or less, Si1% or less, Mn2% or less, Cr7.5
-14%, Al 0.1% or less, N 0.02% or less, and one or two of Mo2% or less and W4% or less, and low carbon martensitic stainless steel with the balance Fe and unavoidable impurities. steel. 4th group, wt%, C0.08% or less, Si1% or less, Mn2% or less, Cr7.5
~ 14%, Al 0.1% or less, N 0.02% or less, in addition to Ni4% or less, Cu4.5% or less, Co4% or less one or more kinds are contained, and further, Mo2% or less, Low carbon martensitic stainless steel containing one or two of W4% or less and the balance Fe and inevitable impurities. 5% by weight, C0.08% or less, Si1% or less, Mn2% or more 5
%, Cr 7.5 to 14%, Al 0.1% or less, N 0.02% or less, and a low carbon martensitic stainless steel containing the balance Fe and unavoidable impurities. 6th group, C0.08% or less, Si1% or less, Mn2% or more 5% by weight
% Or less, Cr 7.5 to 14%, Al 0.1% or less, N 0.02% or less, and also contains one or more of Ni 4% or less, Cu 4.5% or less, Co 4% or less, Low carbon martensitic stainless steel with balance Fe and unavoidable impurities. 7th group, by weight%, C0.08% or less, Si1% or less, Mn2% more than 5
% Or less, Cr 7.5 to 14%, Al 0.1% or less, N 0.02% or less, and one or two of Mo 2% or less and W 4% or less, and the balance Fe and inevitable impurities. Low carbon martensitic stainless steel. 8th group: C0.08% or less, Si1% or less, Mn2% or more 5% by weight
% Or less, Cr 7.5 to 14%, Al 0.1% or less, N 0.02% or less, and also contains one or more of Ni 4% or less, Cu 4.5% or less, Co 4% or less, Furthermore, a low carbon martensitic stainless steel containing one or two of Mo2% or less and W4% or less and the balance Fe and inevitable impurities. 4. The low carbon martensitic stainless steel according to claim 3, wherein the low carbon martensitic stainless steel is one in which one or both of P and S among impurity elements are reduced to the following ranges. Steel line pipe manufacturing method. P: 0.02 wt% or less, S: 0.003 wt% or less 5. A low carbon martensitic stainless steel,
The method for producing a low carbon martensitic stainless steel line pipe according to claim 3 or 4, which contains one or more elements of one or both of the following ninth and tenth groups. Group 9 wt%, V0.5% or less, Ti0.2% or less, Nb0.5% or less, Zr
0.2% or less, Ta 0.2% or less, Hf 0.2% or less, B 0.01% or less 10th group weight%, Ca 0.01% or less, rare earth element 0.02% or less