JPH04154913A - Production of high tensile strength bent pipe excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a high tensile strength bent pipe excellent in corrosion resistance by subjecting a pipe of a steel having a specific composition containing Cu, Ni, Cr, Mo, B, Nb, etc., to bending at specific temp., performing quenching to exert hardening treatment, and further applying specific tempering treatment to the above steel pipe. CONSTITUTION:A pipe of a steel having a composition which consists of, by weight, 0.01-0.06% C, <=0.50% Si, 1.00-2.00% Mn, 0.25-1.50% Cu, 0.10-1.50% Ni, <=0.50% Cr, <=0.50% Mo, <=0.01% N, 0.01-0.10% Al, <=0.002% B, and the balance Fe with inevitable impurities and further contains one or more kinds among 0.002-0.050% Nb, 0.01-0.10% V, and 0.005-0.03O% Ti and either or both of 0.001-0.005% Ca and 0.001-0.010% Te and in which the contents of P and S among the impurities are limited to <=0.020% and <=0.002%, respectively, is heated at 900-1050 deg.C, bent and is then quenched to undergo hardening treatment. Subsequently, this worked steel pipe is subjected to tempering treatment at 450-600 deg.C. By this method, the bent pipe excellent in hydrogen induced cracking resistance and sulfide stress corrosion cracking resistance and having high strength can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a high-tensile bent pipe that is resistant to hydrogen-induced cracking and sulfide stress corrosion cracking and has excellent corrosion resistance.

(Prior Art) Pipelines are used as a means of transporting natural gas and oil, but in recent years, in order to improve transport efficiency, there has been a trend toward thicker walls and higher tensile strength of pipeline transport pipes. For this reason, not only straight pipes but also bent pipes are required to have thick walls and high tension. In addition, in recent years, oil and natural gas produced often contain large amounts of hydrogen sulfide, and both straight and bent pipes are exposed to hydrogen-induced cracking (Tl) caused by hydrogen sulfide.
There is a need for materials that are resistant to SCC) and sulfide stress corrosion chipping (SSCC). These requirements are most severe for bent pipes used around platform A of submarine pipelines.

Conventionally, bent pipes have been manufactured by continuously pushing a bare straight steel pipe and bending it while heating it inside a high-frequency heating coil. Semi-materials have low strength, so they are not suitable for thick walls and high tensile strength. For this reason, increasing the thickness and tension is dealt with by quenching or quenching and tempering after bending.

However, although the quenched and tempered material has relatively high strength, it does not meet the high strength required for bent pipes used around the platform.On the other hand, the as-quenched material has poor hydrogen-induced cracking resistance. is insufficient.

(Problem to be solved by the invention) The problem to be solved by the invention is to solve the above-mentioned problem.
A method for manufacturing bent pipes with high strength and excellent resistance to hydrogen-induced cracking and sulfide stress corrosion cracking, which can fully meet the diverse requirements of bent pipes used around platforms. Our goal is to provide the following.

Specifically, the mechanical properties of the bent pipe targeted by the manufacturing method of the present invention are API standard 5L-X65 to X70 class, and the hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance are respectively NA. No cracks or breaks under CE conditions.

(Means for Solving the Problems) This invention is based on "C: 0.01 to 0.06%, S: 0.01 to 0.06%, S
i: 0.50% or less, Mn: 1.0 (1 to 2.00
%, Cu: 0.25-1.50%, Ni: 0.
10-1.50%, Cr: 0.50% or less, Mo:
0.50% or less, N: 0.01% or less, ^ffi:
Contains 0.01 to 0.10%, B: 0.002% or less, and further includes Nb: 0.002 to 0.050%, V
: 0.01-0.10% and Ti: 0.005
One or more of ~0.030% and Ca:
0.001-0.005% and Te: 0.001
~0.010%, the remainder consists of Fe and unavoidable impurities, and the content of P and S as impurities is P: 0.020% or less, S: 0
．． A steel pipe having a composition of 0.002% or less is
After heating and bending in a temperature range of ~1050°C, a quenching process is performed to rapidly cool it, and then a temperature range of 450 to 600°C is applied.
The subject matter is a method for producing hypertonic copper tubes with excellent corrosion resistance, which is characterized by subjecting them to tempering treatment in the temperature range of °C.

(Function) Hereinafter, in the method of the present invention, the reason why the component composition and heat treatment conditions of the steel pipe are numerically limited as described above will be explained.

A Composition of Steel Pipe (a) C C has the effect of increasing strength, and adding a large amount is advantageous in obtaining high strength. However, if more than necessary is added, hydrogen-induced cracking is likely to occur. From the viewpoint of hydrogen-induced cracking resistance, it is best to make the micro-Il weave of the bent pipe a mixed structure of ferrite and bainite, and the C content is 0.0.
If it exceeds 6%, striped martensite increases and becomes a starting point for hydrogen-induced cracking, so 0.06% is set as the upper limit. On the other hand, if it is less than 0.01%, the strength will be insufficient or it will be difficult to melt the steel, so 0.01% is set as the lower limit.

(B) Si Si has a deoxidizing effect, but on the other hand, like C, when added in large amounts, striped martinites are formed and hydrogen-induced cracking is more likely to occur, so the upper limit is 0.50%. And so.

(C) Moon Mn has the effect of increasing the strength of the rope, and in order to ensure the desired strength it is necessary to contain it at 1.00% or more, but if it is contained in excess of 2.00%, it may cause micro-segregation. Since this amount is unfavorable for hydrogen-induced cracking resistance, the content is set at 1.00 to 2.00%.

(d) Cu Cu has the effect of increasing strength, toughness, and hydrogen-induced cracking resistance. However, if the content is lower than 0.25%, the above-mentioned effects will be small, and if the content exceeds 1.50%, the cost will increase and the surface quality of the bent pipe will be impaired. It was set at 0.25 to 1.50%.

(e) Ni Ni has the effect of preventing Cu checking flaws.

In order to reliably prevent this flaw, at least 0.10
% content, the lower limit was set at 0.10%. Further, Ni has the effect of increasing strength and toughness.

In order to prevent Cu checking flaws and increase strength and toughness, it is advantageous to add a large amount of Ni, but this increases cost, so 1.50 was set as elm.

(f) Cr Cr has the effect of improving the strength of steel, but 0.5
If the content exceeds 0%, the toughness deteriorates, so the content was set to 0.50% or less.

(g) M.

Mo contributes to strength improvement, but if it is contained in an amount exceeding 0.50%, it impairs the toughness of the welded part and also impairs the corrosion resistance improvement effect of Cu, so its content is reduced to 0.5%.
It was set to 0% or less.

(h) N N has a precipitation-strengthening effect, but if added in large quantities, the toughness of the weld zone and weld heat-affected zone will be impaired, so its content was set to 0.01% or less.

(i) 8i AI is effective as a deoxidizing agent and must be present at least 0.0% to ensure cleanliness of steel. Since a content of % of old is required, the lower limit was set to 0.01%. On the other hand, if l exceeds 0.10% and Kaneko f! Since the number of 203 clusters increases and the steel becomes brittle, the upper limit was set at 0.10%.

(j) B B is an effective component that enhances hardenability and contributes to improving strength, but if it is contained in an amount exceeding 0.002%, it will impair corrosion resistance, so its content should be reduced to 0.002%. 0.002% or less.

In addition to the above-mentioned components, the steel pipe used as the raw material for the hend pipe also contains N.
One or more of b, ■ and Ti, and Ca
and one of Te. The reasons for limiting the content of these components are as follows.

(k) Nb Nb has the effect of refining the structure before heat treatment and increasing the toughness after heat treatment, but this effect is small when its content is less than 0.002%, and when it is contained more than 0.050%. , Nb remains in the steel as undissolved carbonitride and impairs corrosion resistance, so its content should be 0.002 to 0.
．． 050% is good.

(1)■ (2) has the effect of increasing the strength of steel through solid solution strengthening.

If the content is less than 0.01%, the above effect will be small, and if the content exceeds 0.10%, the surface quality of the hend tube will be impaired, so the content should be 0.01 to 0.10%.
% is good.

(m) Ti Ti has the effect of suppressing the coarsening of the weld heat affected zone and preventing the toughness of the weld heat affected zone from deteriorating due to the formation of fine TiN. However, if the content is less than 0.005%,
The above effect is small, and if the content exceeds 0.030, T
As iC is generated and the toughness deteriorates,
Its content is preferably 0.005 to 0.030%.

(n) Ca Ca has the effect of spheroidizing MnS to improve hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance.

However, if it is less than 0.001%, the above effect is small and 0.001% is less than 0.001%.
If the content exceeds 0.005%, Ca-0-3 inclusions will be formed and the corrosion resistance will deteriorate, so the content should be 0.00%.
1 to 0.005% is good.

(o) Te Te, like Ca, also has the effect of spheroidizing MnS to improve hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance.

However, if it is less than 0.001%, the above effect is small and 0.001% is less than 0.001%.
．． If the content exceeds 0.010%, corrosion resistance will deteriorate.
Its content is preferably 0.001 to 0.010%.

In addition to the above-mentioned components, the raw material steel pipe consists of Fe and unavoidable impurities. P and S in impurities are each P:
It is preferable to suppress it to 0.020% or less, and S: 0.002% or less.

P has a large effect on micro-segregation and has an adverse effect on hydrogen-induced cracking resistance. S has a strong affinity with Mn, and when it is present in large amounts, ductile MnS inclusions increase and the hydrogen-induced cracking resistance deteriorates.

Note that the hot rolling of the steel billet having the above-mentioned composition is preferably carried out by a so-called controlled rolling method. This is because it is preferable to perform controlled rolling in order to refine the structure and improve toughness after quenching and tempering the steel pipe while bending it in the subsequent process. In the controlled rolling, it is preferable to apply strong pressure at a temperature below the non-recrystallization temperature range so that the structure after induction hardening is refined even in the center of the wall thickness. Further, the upper limit of heating of the steel slab is preferably 1180°C to prevent coarsening of austenite grains, and the lower limit is preferably 1050°C to ensure solid solution of carbonitrides and ensure strength.

B. Heat treatment conditions (a) Hardening and bending The bending process is performed in a temperature range of 900 to 1050°C, and after completion of the process, it is immediately quenched, cooled, and hardened. Water cooling may be used for rapid cooling. Even if it is hardened at a temperature lower than 900°C, the desired strength cannot be secured. On the other hand, if the steel is quenched at a temperature exceeding 1050°C, the austenite grains will become coarser and the toughness will deteriorate.

(b) Tempering Tempering is performed at a temperature range of 450 to 600°C. 450℃
Even if tempering is performed at a lower temperature, microscopic hardened structures such as striped martensite during quenching are not sufficiently softened, and hydrogen-induced cracking resistance and the like are not improved. - On the other hand, tempering at a temperature higher than 600°C will result in the strength being lower than the desired value.

(Example) Welded steel pipes (outer diameter 24 to 36 inches) of various compositions shown in Table 1 were subjected to induction heating and bending to the quenching temperatures shown in Table 2, immediately water-cooled and quenched, and then A high tensile strength bent pipe was obtained by tempering. The bending radius is (3~
5) x outer diameter.

The obtained bent pipes were subjected to a tensile test, a hydrogen'-induced sidewall test, and a sulfide stress corrosion cracking test. These results are shown in Table 2. In the tensile test, tensile test pieces (71
57M (according to 4370)) and calculate the yield strength (Y
S) and tensile strength (TS) were determined. In the hydrogen-induced cracking test, a test piece was immersed in a solution for 96 hours, and the presence or absence of cracking was examined. The pH of the solution was 4.8 to 5.4 (BP conditions) and 3.0 to 4.0 (NACE conditions). Sulfide stress corrosion cracking tests test specimens in solution [80%XSMY
S.

The specimens were immersed for 720 hours under a stress of 100%, and the presence or absence of breakage was examined.

As is clear from Table 2, the bent pipes with Nα of 4 to 9 obtained by the manufacturing method of the present invention have high strength and are free from hydrogen-induced cracking and sulfide stress corrosion cracking. On the other hand, No. whose C content is outside the range specified by the present invention,
Although the bent pipes No. 1, No. 2, and the component compositions are within the range specified by the present invention, hydrogen-induced cracking and sulfide stress corrosion cracking occurred in the as-quenched bent pipe No. 13. , sulfide stress corrosion cracking occurs in a short period of time.

(Effects of the Invention) As shown in the examples, the bent pipe obtained by the manufacturing method of the present invention has high strength and excellent resistance to hydrogen-induced cracking and sulfide stress corrosion cracking. There is.

Claims (1)

[Claims] In weight%, C: 0.01-0.06%, Si: 0.50
% or less, Mn: 1.00-2.00%, Cu: 0.25
~1.50%, Ni:0.10~1.50%, Cr:0
．． 50% or less, Mo: 0.50% or less, N: 0.01%
Below, Al: 0.01-0.10%, B: 0.002%
Contains the following, further Nb: 0.002 to 0.050%,
V: 0.01-0.10% and Ti: 0.005-0
．． One or more of 030% and Ca: 0.00
1.0.005% and Te: 0.001-0.010
%, the remainder consists of Fe and unavoidable impurities, and the content of P and S as impurities is P: 0.020% or less and S: 0.002% or less, respectively. The steel pipe is heated in a temperature range of 900 to 1,050°C, subjected to bending processing, and then subjected to a quenching treatment in which it is rapidly cooled, and then subjected to a tempering treatment in a temperature range of 450 to 600°C. A method for manufacturing high-tensile bent pipes with excellent corrosion resistance.