JPH0931595A - Corrosion resisting steel for electric resistance welded tube, excellent in toughness at low temperature, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion resisting steel for electric resistance welded tube, capable of securing HIC resistance and removing toughness deterioration in a resistance welded zone by specifying respective contents of C, Si, Mn, P, S, Al, and Ca in the steel and also specifying the ratio between the contents of Ca and O. SOLUTION: The steel for electric resistance welded tube has a composition consisting of 0.01-0.20% C, 0.01-0.50% Si, 0.50-1.80% Mn, <=0.020% P, <=0.001% S, 0.005-0.100% Al, 0.0005-0.0050% Ca, and the balance Fe with inevitable impurities. In this composition, the ratio between the contents of Ca and O, (Ca)/(O), is regulated to 1.5-2.0, and further, one or >=2 kinds among 0.20-0.80% Cu, 0.05-0.60% Ni, <=1.00% Cr, <=1.00% Mo, 0.01-0.10% Ti, 0.01-0.10% Nb, and 0.10% V are incorporated at need. Moreover, it is desirable to regulate the amount of O in the molten steel before the addition of Ca and Si to <=0.0020%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in low-temperature toughness of the electric resistance welded portion mainly used for line pipes having strength level of X-42 to X-80 class of American Petroleum Institute (API) standard and high resistance. The present invention relates to a corrosion resistant steel for electric resistance welded pipe excellent in hydrogen induced cracking property (hereinafter referred to as HIC resistance) and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, oil wells and natural gas gas wells have been excavated in deep oil fields that have not been neglected in the past, due to the depletion of petroleum resources expected in the near future,
Development is being actively conducted on a global scale in areas such as the sour gas field where development was once abandoned.

Such oil wells and gas wells are generally extremely deep, and their atmospheres are CO ₂ , H ₂ S, Cl ^−.
In an extremely harsh corrosive environment containing, for example, H ₂ S is very often contained in the produced petroleum and natural gas. Further, oil and natural gas containing a large amount of H ₂ S not only corrode the steel surface when coexisting with seawater and fresh water, but also
Hydrogen generated by corrosion may enter the steel and cause destruction, which is a problem. This fracture is different from sulfide stress corrosion cracking, which has long been observed in high-strength steel.
Occurrence is recognized even if there is no additional stress from the outside.

This failure is caused by MnS and Al existing in the steel.
₂ O ₃ inclusions, and CaO inclusions / CaS inclusions with high melting points become cluster-like inclusions that are stretched or broken in the rolling direction when rolling steel, and hydrogen that has penetrated from the environment intervenes. It accumulates at the boundary between the object and the ground iron, gasifies, and a minute crack is generated by the gas pressure, which is generated by connecting and propagating. The damage that occurs when hydrogen penetrates into steel is called hydrogen-induced cracking, hydrogen blistering cracking, etc. (hereinafter referred to as HIC).

In the case of a line pipe for transporting oil or natural gas, if the HIC occurs and penetrates in the plate thickness direction, the line pipe may be broken such as oil leak or gas leak. Further, in recent years, an electric resistance welded steel pipe is often used as a line pipe, and it is important to prevent the occurrence of HIC.

Conventionally, as a method of preventing HIC, a method of adding Cu depending on the environment in which the steel pipe is used to form a corrosion resistant coating to prevent hydrogen invasion (Japanese Patent Laid-Open No.
97515), and Mn which is the starting point of the generation of HIC.
A method in which inclusions in steel such as S are treated with Ca, Zr, a noble earth element (REM), etc. and spheroidized to control the morphology (for example, JP-A-54-38214 and JP-A-54-31020). ) Is often used. In addition, A-based sulfide inclusions such as MnS and M at the center segregation portion, which is common in continuous cast materials, and M
In order to prevent the occurrence of cracks due to the formation of bainite, which is a low temperature transformation structure due to segregation of n and P, and martensite, a low S
It is also a commonly practiced technique to reduce the P and lower the P (for example, JP-A-52-111815 and JP-A-54-131522). Further, it has been pointed out that Nb carbonitride also becomes a starting point of HIC generation, and an example in which the amounts of Nb, C and N are limited (Japanese Patent Laid-Open No. 56-119759).
There is also. To date, steels that can withstand considerably severe environments have been developed and manufactured by the electric resistance welded pipe method from continuous cast materials by these technologies.

On the other hand, the areas where oil and natural gas are produced in recent years have spread to extremely cold regions such as Alaska, Russia, and the Arctic Ocean, and the pH used in Alaska, Canada, etc .:
It extends to the so-called BP environment of 4.8 to 5.4 and the so-called NACE environment of pH: 4.5 or less, which is severer than this. Line pipes, oil well pipes, and gas well pipes used in such an environment are naturally required to have excellent HIC resistance and low-temperature toughness both in the base material and the electric resistance welded portion.

However, since the toughness of the welded portion of the electric resistance welded pipe is lower than that of the base metal portion, various studies have heretofore been made on the electric resistance welded steel pipe having excellent low temperature toughness including the electric resistance welded portion. Various methods and ERW steel pipes have been proposed. For example, improving the toughness of the material by limiting the finishing temperature and winding temperature in the hot rolling process, post-heat-treating the electric resistance welded portion after pipe making to the austenitizing temperature, and immediately thereafter, about 10 to 20 mm in the circumferential direction from the welding line. A method for locally forcibly cooling only the periphery of a distant weld (Japanese Patent Laid-Open No. 54-1365).
No. 12), refinement of crystal grains by addition of Nb and V,
This is, for example, heat treatment of the pipe body after pipe making.

However, the above-mentioned electric resistance welded steel pipe is used in a normal environment, is not subjected to low P, low S, Ca addition, etc., and has a specification in a sour-resistant environment taken into consideration. is not. However, the demands for HIC resistance and low temperature toughness of electric resistance welded steel pipes are increasing more and more in recent years due to the severer usage environment, and the Ca addition method is effective for satisfying the requirements of these composite properties. Known to be.

As a conventional technique of the Ca addition method, a CaO-containing flux is blown into a molten steel which has been previously deoxidized in a ladle by a carrier gas for deoxidation and desulfurization, and then a Ca alloy is blown into the molten steel to melt the molten steel. A method for controlling the sulfide form in the solution (Japanese Patent Publication No. 59-22765), in which molten steel pre-deoxidized in a ladle is CaO1.
5 to 4.5 kg / molten steel Ton (hereinafter referred to as TS) is blown, and then Ca-Si 0.3 to 0.7 kg / at atmospheric pressure
The concentration of [Ca], [Al], and [O] in TS blown and molten steel is [O] ≦ 20 ppm, [% Ca] / [% Al]
² ≦ 1.45 × 10 ⁻⁷ and [% Ca] ³ · [% Al] ² ≧
Method of controlling to 1.06 × 10 ^-11 (Japanese Patent Publication No. 6-967)
33).

However, it has been found that the electric resistance welded steel pipe to which Ca is added may have the toughness of the electric resistance welded portion remarkably lower than that of the base metal portion. This decrease in toughness of the electric resistance welded part
As a result of various investigations, the cause was that inclusions existing in the electric resistance joint and its vicinity were heated to near the melting point of the steel due to the heat effect during electric resistance welding, and were pressed from both sides by the welded upset. It became clear that this was due to the plate-shaped deformation. Also, the inclusions that deform into a plate shape are
As a result of the component analysis, mCaO.nAl ₂ O ₃ (m,
It was found that the compound inclusion has a molecular composition ratio of n is an integer.

Various methods have been proposed so far for solving the problem of deterioration in toughness of the electric resistance welded portion. For example, C: 0.01 to 0.35%, Si: 0.
02 to 0.50%, Mn: 0.10 to 1.80%, A
1: 0.005% to 0.050%, Ca: 0.000
In addition to 5 to 0.008%, Zr: 0.001 to 0.01
5% and P: 0.015% or less, S: 0.0
Al ₂ O contained in inclusions in the heat-affected zone of welding, using as a raw material steel for electric resistance welded pipes, which has a Zr / Al value of less than 2 and has a Zr / Al value of less than 2 and which consists of the balance Fe and inevitable impurities ERW steel pipe having a welded portion in which the concentration of ₃ is 50% or less (Japanese Patent Laid-Open No. 63-137144), C: 0.0
1 to 0.35%, Si: 0.02 to 0.50%, Mn:
0.10 to 1.80%, Al: 0.001 to 0.100
%, Ca <0.001%, P ≦ 0.020%,
S ≦ 0.0012% and P (%) + 25 × S (%)
≦ 0.04%, Cu: 0.2-0.6
%, Ni: 0.1 to 1.0%, Cr: 0.2 to 3.0%
One or more of and Mo: 0.1-1.0%,
Nb: 0.01 to 0.15%, V: 0.01 to 0.15
%, Ti: 0.01 to 0.10% of one or two or more, and an electric resistance welded steel pipe made of steel composed of the balance Fe and unavoidable impurities, the electric resistance welded portion of which is BP. An electric resistance welded steel pipe (Japanese Patent Publication No. 53857/1993) having excellent sour resistance under the environment and excellent toughness at the minimum operating temperature of -46 ° C, C: 0.05-0.
35%, Si: 0.02-0.5%, Mn: 0.5-2
%, Ca 0.0005 to 0.008% and Al 0.005 to 0.1%, the balance Fe and unavoidable impurities, and the content of S, O and Ca is 1.0. ≤
(% Ca) {1-72 (% O)} / 1.25 (% S) ≦
After satisfying 2.5, the deoxidation product is (CaO) m
A composite inclusion of (Al ₂ O ₃ ) n is used, and its molecular composition ratio is m
By setting the range of / n <1, Al ₂ O ₃ in the inclusions
A method of increasing the fraction to generate composite inclusions having a melting point of 1600 ° C. or higher, ensuring HIC resistance, and avoiding stretching in the vicinity of the electric resistance welded portion (Japanese Patent Publication No. 5-87582).
Has been proposed.

As a method for controlling the CaO-Al ₂ O ₃ -based inclusion composition, a CaO-containing flux is blown into a molten steel which has been previously deoxidized in a ladle with a carrier gas for deoxidation and desulfurization, and then the Method for controlling sulfide morphology in molten steel by injecting Ca alloy into molten steel (JP-A-59-227)
No. 65), the content of S, O, Ca is 1.0 ≦ [%
Ca] {1-72 [% O]} / 1.25 [% S] ≤2.
5 is satisfied, the deoxidation product is (CaO) m (Al
₂ O ₃ ) n as a composite inclusion, and its molecular structure ratio is m / n <
A steel plate having a range of 1 (Japanese Patent Laid-Open No. 2-290947) and a molten steel pre-deoxidized in a ladle in an amount of 0.1 to 1 to
rr: CaO 1.5 to 4.5 kg / molten steel Ton (hereinafter T
S.) and then Ca-Si0.
3 to 0.7 kg / TS blowing, [Ca] in molten steel,
The concentration of [Al] and [O] is [O] ≦ 20 ppm, [%
Ca] ・ [% Al] ² ≦ 1.45 × 10 ⁻⁷ and [% C
a] A method of controlling ³ · [% Al] ² ≧ 1.06 × 10 ⁻¹¹ (Japanese Patent Publication No. 6-96733) has been proposed.

[0014]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method disclosed in Japanese Patent No. 7144 is to improve the melting point by reforming the inclusions into a composite inclusion of ZrO ₂ · Al ₂ O ₃ by adding Zr to avoid stretching during electric resistance welding.
Zr addition is not industrially common because Zr addition is expensive in steel making, its oxidizability is large, the yield of alloy addition is unstable, and the immersion nozzle is likely to be clogged during continuous casting. The method disclosed in Japanese Examined Patent Publication No. 5-53857 discloses that Ca <0.001% or C
When a is not added, it is necessary to reduce the amount of S to the limit in order to eliminate MnS that is the starting point of HIC generation.
Not only the desulfurization treatment cost for conversion increases, but M
The Ca addition method of controlling the form of nS to render it harmless is more advantageous in the production process on an industrial scale. Moreover, when the S content cannot be reduced to the limit of the harmless region, there is a problem that the HIC resistance cannot be ensured in the recent severe environment. Further, according to the method disclosed in Japanese Examined Patent Publication No. 5-87582, according to the CaO—Al ₂ O ₃ equilibrium diagram shown in FIG. ₂ , 3CaO · 5Al ₂ is a complex inclusion having a molecular constitution ratio of m / n <1. It is a composite inclusion with O ₃ , CaO · 6Al ₂ O ₃ and Al ₂ O ₃ . Among these complex inclusions, since they are liquid in the molten steel at the molten steel temperature (about 1600 ° C) when Ca is added, the HIC is caused by the interfacial tension.
3CaO ・ 5A may have a harmless spherical shape
It is well known that only l ₂ O ₃ is contained, and molten steel Ca
When controlled only by the contents of Al and Al, the formation of CaO-Al ₂ O ₃ -based inclusions that are not spheroidized is considered to be sufficient.
This causes the generation of C, and the composition control of inclusions is insufficient. Further, the Al content disclosed in JP-B-5-87582 is the same as that of Insol. Al
In addition to the solid solution Sol. It is considered that Al is also included, and it cannot be said that the composition of the CaO—Al ₂ O ₃ type inclusions is exhibited.

Further, the above-mentioned JP-A-59-22765, JP-A-2-290947 and JP-B-6-
The method disclosed in Japanese Patent No. 96733 is premised on desulfurization by CaO flux blowing, and further control of molten steel components after CaSi blowing requires a long time for analysis under secondary refining where this treatment is performed, If you make adjustments while checking the analysis values, there is a problem of molten steel temperature drop, and it is very difficult to implement in actual operation.

The object of the present invention is to solve the above-mentioned drawbacks of the prior art, to add no special element such as Zr, and to use CaSi.
By controlling the composition of the inclusions in the steel without controlling the concentrations of [Ca], [Al], and [O] in the molten steel after addition, it is possible to secure HIC resistance and It is an object of the present invention to provide a corrosion resistant steel for electric resistance welded pipe having excellent low temperature toughness capable of eliminating deterioration of toughness and a method for producing the same.

[0017]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made a detailed investigation on the toughness value of the electric resistance welded portion and the shape and composition of inclusions existing in the welded portion of the steel added with Ca to achieve the above object. Was carried out. As a result, the plate-like inclusions in the electric resistance welded portion that cause a decrease in the toughness of the electric resistance welded portion are CaO.
Is a composite inclusion of Al ₂ O ₃ and m (CaO) · n (A
It was found that the inclusions had a molecular ratio of 1 ₂ O ₃ ) of 1 ≦ m / n ≦ 3. This inclusion having a molecular ratio of 1 ≦ m / n ≦ 3 is 1 in the CaO—Al ₂ O ₃ system equilibrium diagram shown in FIG.
It is a low melting point inclusion of 400 ° C. or less, and it is sufficiently conceivable that it extends near the electric resistance welded portion. On the other hand, in some steels, the HIC resistance and low temperature toughness of the electric resistance welded portion did not deteriorate,
When the inclusions were investigated, m (CaO) n (Al
It was found that the compound composition of ₂ O ₃ ) had a molecular composition ratio of m / n ≧ 3.

Further, it was also confirmed that the CaO inclusions dispersed and spheroidized in the base metal portion did not deform into a plate shape in the electric resistance welded portion because of the high melting point. From the above results, it is the inclusions, which are low melting point deoxidation products that plate-form during electric resistance welding, that deteriorate the low temperature toughness of the electric resistance welds, and suppress the plate formation of inclusions. Therefore, it has been found that it is necessary to use a high melting point inclusion. To increase the melting point of inclusions, C
From the aO-Al ₂ O ₃ equilibrium diagram (CaO) and (Al
The weight composition ratio of ₂ O ₃ ) is set to a high (CaO) composition, or it is set to a high (A) as disclosed in Japanese Patent Publication No. 5-87582.
1 ₂ O ₃ ) composition.

However, in the case of a high (Al ₂ O ₃ ) composition, as described above, the complex inclusion (CaO) having a high (Al ₂ O ₃ ) composition ratio is used.
・ 6Al ₂ O ₃ ) does not cause harmless spheroidization in HIC,
Further, since Ca has a stronger affinity with O than S, Ca added to the molten steel first reacts with (Al ₂ O ₃ ) to form a composite inclusion, and then reacts with S to form (CaS). Since most of the added Ca is consumed in the reaction with (Al ₂ O ₃ ), the amount of Ca required for the reaction with S is insufficient and MnS, which has a serious adverse effect on HIC resistance, is added. It is highly likely to be generated and is not preferable. In addition, high (Ca
Even in the (O) composition, excessive Ca addition is not limited to (CaO) inclusions dispersed and spheroidized, but (CaO), (Ca
It is well known that a large amount of (S) cluster-like inclusions are formed, which is harmful to HIC resistance (JP-A-59-76).
818 and Japanese Patent Laid-Open No. 2-290947).

Next, the present inventors first carried out a detailed investigation on inclusions causing HIC. As a result, inclusions in Ca-treated steel are CaO, CaS, C
aO-Al ₂ O ₃ composite inclusions and, further CaO-Al ₂ O ₃ -ZrO ₂ composite inclusions produced by the reaction of the ladle refractory is the CaO-Al ₂ O ₃ -MgO ₂ composite inclusions I clarified that.

Further, the inventors of the present invention investigated the relationship between inclusions and HIC generation in the steel material in which HIC was generated.
l ₂ O ₃ of high-containing composition CaO-Al ₂ O ₃ composite inclusions and Mn
Two types of inclusion morphologies were observed: when S remains, and when CaO and CaS, which are considered to be caused by excessive addition of Ca, are very large and the inclusions themselves are clustered.

Therefore, the inventors of the present invention conducted various tests to control the composition of the deoxidized product, and as a result, the composition of inclusions (Ca
O), (CaS), (Al ₂ O ₃ ), (ZrO 2) and (M
Focusing on gO), a method of controlling the content of Ca and O in steel was considered. Most of the [Ca] content in the product steel is in the form of inclusions, and CaO and CaO-Al ₂
O _{3 in the} O ₃ composite inclusions constitutes CaO and CaS, while [O] in the product steel is an oxidation of CaO, Al ₂ O ₃ and ZrO ₂ , MgO, etc. in CaO and Ca-based composite inclusions. It is what constitutes a thing.

From the above results, the [Ca] content producing Sol from the [Ca] content in the product steel and Sol. [C
a) and [O] content to reduce the [O] content that forms oxides such as ZrO ₂ , MgO, etc., to obtain CaO and Al from the [Ca] and [O] content in the product steel.
It is possible to derive the composition ratio of Ca-based inclusions composed of ₂ O ₃ . However, [Ca], [S],
Analysis of [O] is easy, but Sol. [Ca], In
sol. Trace analysis of [Al], [Zr], and [Mg] is difficult.

Therefore, the present inventors have adopted [S] as a simple method.
If ≦ 0.001%, the amount of CaS produced and ZrO ₂ ,
Assuming that the amount of oxides such as MgO does not change so much, the ratio [Ca] of the [Ca] and [O] contents in the product steel
/ [O] and CaO and Al ₂ O ₃ obtained by quantitative analysis
The relationship between the composition ratios of CaO and Al ₂ O ₃ depending on the amount was investigated.
The results are shown in FIG. 1, and CaO—Al ₂ O ₃ shown in FIG.
Compared with the theoretical line of [Ca] / [O] in the product steel calculated from the composition of the phase diagram, it is almost the same in the high melting point region, but in the low melting point region, it is on the high [Ca] / [O] side It turned out to be a big shift. It is considered that this is because the amount of CaS produced and the amount of oxides such as ZrO ₂ and MgO did not change so much, but it was ignored. However, depending on the [Ca] / [O] value in the product steel, CaO-Al
It is sufficient to estimate the composition of ₂ O ₃ -based composite inclusions. 1 and 2, C in 3C · A, 3C · 5A, etc. is CaO and A is Al ₂ O ₃ .

From the above results, [Ca] / in product steel
The relationship between the [O] value and the composition of the CaO-Al ₂ O ₃ -based composite inclusion is appropriate to consider as follows. [Ca] / [O]> about 1.5 ... Coexistence of high melting point inclusions and low melting point inclusions about 1.5 ≧ [Ca] / [O]> about 0.8 ... low melting point inclusions 0.8 ≧ [Ca] / [O] ...... High melting point inclusion Here, the range of about 1.5 ≧ [Ca] / [O]> about 0.8 is also shown in the prior art. Is a low-melting spherical inclusion that deforms into a plate shape during electric resistance welding and reduces the toughness of the electric resistance welded portion. The range of [Ca] / [O]> about 1.5 indicates the toughness of the electric resistance welded portion. This is a region where high-melting-point inclusions and low-melting-point inclusions that do not deteriorate coexist.

Furthermore, the present inventors have found that [C
When the relationship between a] / [O] and the occurrence of HIC was investigated, as shown in FIG. 3, [Ca] / [O] = 0.8-
It was confirmed that HIC did not occur in the region of 2.0. From the results of this investigation, not only the low melting point inclusions that have been conventionally known, but also the high melting point inclusions / low melting point inclusions coexisting region, unless the inclusions cluster during rolling, do not adversely affect the HIC resistance. I clarified that.
However, when [Ca] / [O]> 2, CaO and C
It was clarified that a large amount of aS was generated and became inclusions clustered during rolling, and when [Ca] / [O] <0.8, MnS remained and became a starting point for generating HIC.

Therefore, the inventors of the present invention [Ca] / [O] = 1.5-in the product steel in which the toughness of the electric resistance welded portion is not deteriorated.
A study was made on a method of obtaining 2.0. Ca in the ladle
When Ca is added, when the amount of Ca added is insufficient, MnS, which has a serious adverse effect on HIC resistance, is generated. On the contrary, excessive addition of Ca causes cluster formation of (CaO) and (CaS). As described above, a large amount of substances are produced, which is harmful to HIC resistance.

The inventors of the present invention focused on the Al ₂ O ₃ content and the O content in the molten steel before the addition of CaSi from the reaction mode of Ca in the molten steel, and by changing the CaSi addition amount, the Ca ₂ The relationship between O content in molten steel and [Ca] / [O] in product steel was investigated. As a result, as shown in FIG. 4, [O] in molten steel before addition of CaSi
If the content exceeds 0.0020%, the [Ca] / [O] in the product steel after addition of CaSi varies greatly, and a correlation with the addition amount of CaSi that can be used in actual operation cannot be obtained. The content of [O] in the previous molten steel is 0.00
When the content is 20% or less, in the product steel after addition of CaSi [C
It has been clarified that the variation of a] / [O] is very small, a good correlation with the amount of CaSi added is obtained, and the value of [Ca] / [O] in the product steel can be accurately controlled only by the amount of CaSi added.

Therefore, the inventors of the present invention have [Ca] / [O] = in the product steel after addition of CaSi for molten steel in which the [O] content in the molten steel before addition of CaSi is 0.0020% or less.
CaSi addition amount and CaSi to obtain 1.5 to 2.0
As a result of investigating the relationship between [Ca] / [O] in the product steel after addition, as shown in FIG.
If it is between 2 and 1.8 kg / TS, a low melting point CaO-Al that deforms plate-like during electric resistance welding and deteriorates the toughness of the welded part
The inventors have found that it is possible to prevent the formation of ₂ O ₃ -based complex inclusions as well as the formation of (CaO), (CaS) -based clustered inclusions due to excess Ca, and arrived at the present invention.

That is, the invention of claim 1 of the present application is C:
0.01 to 0.20%, Si: 0.01 to 0.50%,
Mn: 0.50 to 1.80%, P: 0.020% or less,
S: 0.001% or less, Al: 0.005 to 0.100
%, Ca: 0.0005 to 0.0050%, the balance Fe and unavoidable impurities, and the ratio [Ca] / [O] of Ca and O is 1.5. ~
It is a corrosion-resistant electric resistance welded steel pipe steel excellent in low-temperature toughness characterized by being set in a range of 2.0.

The invention according to claim 2 of the present application is C: 0.
01 to 0.20%, Si: 0.01 to 0.50%, M
n: 0.50 to 1.80%, P: 0.020% or less,
S: 0.001% or less, Al: 0.005 to 0.100
%, Ca: 0.0005 to 0.0050%, Cu: 0.20 to 0.80%, Ni: 0.
05-0.60%, Cr: 1.00% or less, Mo: 1.
00% or less, Ti: 0.01 to 0.10%, Nb: 0.
01 to 0.10%, V: 0.10% or less of 0.1% or less of the steel containing the balance Fe and unavoidable impurities, the ratio of Ca, O content [Ca ] /
It is a corrosion resistant steel for electric resistance welded steel pipe excellent in low temperature toughness, characterized in that [O] is in the range of 1.5 to 2.0.

Further, the invention of claim 3 of the present application is C:
0.01 to 0.20%, Si: 0.01 to 0.50%,
Mn: 0.50 to 1.80%, P: 0.020% or less,
S: 0.001% or less, Al: 0.005 to 0.100
%, Ca: 0.0005 to 0.0050%, and the balance Fe and unavoidable impurities.
[Ca] / [O] content ratio of [Ca] / [O] is 1.5 to 2.0.
The amount of [O] in the molten steel before addition is 0.0020% or less,
By adding 1.2 to 1.8 kg / Ton-Steel of CaSi, the ratio [Ca] / [O] of the contents of [Ca] and [O] in the product steel is 1.5 to 2.0. It is a method for producing a corrosion resistant steel for electric resistance welded steel pipe having excellent low temperature toughness, which is characterized by setting the range.

Furthermore, the invention of claim 4 of the present application is as follows.
C: 0.01 to 0.20%, Si: 0.01 to 0.50
%, Mn: 0.50 to 1.80%, P: 0.020% or less, S: 0.001% or less, Al: 0.005 to 0.1
In addition to 00%, Ca: 0.0005 to 0.0050% is contained, and further Cu: 0.20 to 0.80%, Ni:
0.05-0.60%, Cr: 1.00% or less, Mo:
1.00% or less, Ti: 0.01 to 0.10%, Nb:
0.01 to 0.10%, V: 0.10% or less 1
Or two or more, and the balance Fe and unavoidable impurities, and the ratio of Ca and O contents [Ca] / [O]
When producing a steel having 1.5 to 2.0, the amount of [O] in the molten steel that has been deoxidized and desulfurized before addition of CaSi is 0.0020% or less, and CaSi is 1.2 to 1.8k.
It is characterized in that the ratio [Ca] / [O] of the contents of [Ca] and [O] in the product steel is made to be in the range of 1.5 to 2.0 by adding g / Ton-Steel. This is a method for producing a corrosion resistant steel for electric resistance welded steel pipe having excellent low temperature toughness.

[0034]

BEST MODE FOR CARRYING OUT THE INVENTION The steel to which the present invention is applied is a steel for electric resistance welded pipe for high strength corrosion resistant line pipe.
The chemical composition is limited as follows. C is a basic element that improves the strength of steel, and 0.01% is required to secure the strength.
The above is required, but if it exceeds 0.20%, it is not desirable for toughness and weldability, etc., and it has an effect on use, and it also prevents the occurrence of bainite and martensite, which are abnormal structures of the central segregation zone in continuous cast materials. 0.01 ~
0.20%.

Si is used as a deoxidizing agent during refining, and 0.01% or more is necessary for improving strength.
If it exceeds 0.50%, brittleness increases, so 0.01 to 0.
50%. Mn is an element necessary for imparting the strength and toughness of steel, and is required to be 0.50% or more to secure the strength, but if it exceeds 1.80%, the weldability and toughness deteriorate, so 0 It was set to 0.50 to 1.80%.

Since P is an element that promotes the generation of bainite and martensite, which are abnormal structures of the central segregation zone in the continuous cast material, it is set to 0.020% or less. If the content of S is large, sulfide-based inclusions are formed, which has a great adverse effect on the HIC resistance, so the content was made 0.001% or less. Al is an element necessary for deoxidation at the time of refining, and if it is less than 0.005%, it has no effect, and if it exceeds 0.10%, it causes deterioration of the steel quality due to the increase of inclusions, etc. .10
%.

Ca is used for controlling the morphology of inclusions,
At least 0.0005% or more is necessary in order to improve the HIC resistance by spheroidizing MnS, and 0.005%
If not less than 1.0, not only is there saturation and there is no further effect, but Ca-based inclusion clusters increase and HIC resistance decreases, so the content was made 0.0005-0.005%.
In addition, the important thing is that the composition ratio with O is 1.5 ≦ [Ca] /
That is, [O] ≦ 2.0 is satisfied.

Cu is an element having an effect on the strength improvement and HIC resistance of steel, but if it is less than 0.20%, it has no effect, and if it exceeds 0.80%, the weldability deteriorates and hot workability increases. Since it has an adverse effect, it was set to 0.20 to 0.80%. Ni is an element that has the effect of improving the toughness of steel, but it is preferable that it is harmful and small in preventing penetration of hydrogen into the steel. However, when Cu is added in an amount of 0.30% or more, Ni
If not added in an amount of 0.05% or more, Cu brittleness occurs, which adversely affects the surface quality and the like. Cu of this invention steel ≧ 0.80%
Then, Ni ≦ 0.
6% is sufficient. Also, if Cu is not added depending on the use environment, Ni may not be added for the above reason.

Ti and Nb are elements having the effect of improving the strength of steel, but if less than 0.01%, the effect of improving the strength cannot be expected, and if over 0.10%, the toughness is impaired.
It was set to 0.01 to 0.10%. In the NACE environment, it is not necessary to add Cu because there is no corrosion resistance effect of Cu, but in order to secure the strength of the steel when Cu is not added, T
It is necessary to add 0.01% or more of i and Nb.
Cr, Mo and V are elements that improve the strength of steel,
Even if added in excess of 1.0%, the strength increasing effect is small and it is economically disadvantageous, so the content was made 1.0% or less. Further, these optional additional elements are used in combination depending on the usage environment of the target steel and the required strength level.

In the present invention, the amount of [O] in the molten steel before adding CaSi is 0.0020% or less is that CaSi
If the amount of [O] in the molten steel before addition exceeds 0.0020%, the [Ca] / [O] in the product steel after addition of CaSi varies greatly, and the correlation with the amount of CaSi added so that it can be used in actual operation However, when the amount of [O] in the molten steel before adding CaSi is 0.0020% or less, the variation of [Ca] / [O] in the product steel after adding CaSi is very small, and the amount of CaSi added is This is because a good correlation with is obtained and the value of [Ca] / [O] in the product steel can be accurately controlled only by the amount of CaSi added.

CaS added to molten steel in the present invention
i was 1.2 to 1.8 kg / TS was 1.2 kg
If less than / TS, [Ca] / [O] in the product steel is less than 1.5, and the CaO-Al ₂ O ₃ -based composite inclusions have a low melting point.
It cannot be used as a coexisting region of high melting point inclusions, and low melting point inclusions that deform plate-like at the welded part and deteriorate toughness are formed during electric resistance welding, and when it exceeds 1.8 kg / TS, it is due to excess Ca (CaO). , (CaS) -based cluster inclusions are formed, and it is not possible to obtain a steel for electric resistance welded pipe having excellent weld toughness and HIC resistance.

[C] in the product steel according to the present invention
a], [O] content ratio [Ca] / [O] = 1.5-
The range of 2.0 is that when [Ca] / [O] exceeds 2, a large amount of CaO and CaS are produced, which become clustered inclusions during rolling, and HIC occurs due to the clustered inclusions. also, [Ca] / [O] is less than 1.5, the low-melting-a CaO-Al ₂ O ₃ composite inclusions
This is because it cannot be a coexistence region of high-melting point inclusions, and low-melting point inclusions that deform in a plate shape at the welded portion during electric resistance welding and deteriorate toughness are generated.

The feature of the present invention is that the desulfurization method before adding CaSi is not regulated, and the desulfurization method by blowing CaO flux in the atmosphere or under vacuum or the slag on the molten steel in the ladle is desulfurized. This is a method in which only synthetic slag is substituted and desulfurization is performed only by stirring molten steel such as Ar bubbling.

A feature of the present invention is that the amount of [O] in the molten steel after desulfurization, that is, before addition of CaSi is 0.0020% or less. As a method of reducing the amount of [O] in the molten steel before adding CaSi to 0.0020% or less, since [O] in the molten steel is in the form of an oxide, the molten steel is refluxed in a vacuum refining furnace or The amount of [O] in molten steel can be easily reduced to 0.00 by promoting floating of oxide inclusions and adsorbing to slag by blowing Ar gas into molten steel in a ladle.
It can be 20% or less. The amount of [S] in the molten steel at this time does not need to be 0.001% or less because a desulfurization reaction occurs due to the subsequent addition of CaSi.
From the viewpoint of suppressing the generation of sulfides that adversely affect the HIC resistance, it is desirable to keep [S] ≦ 0.001% as much as possible. If this requirement is met, CaSi will be 1.2
.About.1.8 kg / TS, the ratio [Ca] / [O] of the product steel [Ca] / [O] = 1.
It can be controlled in the range of 5 to 2.0, and CaO-Al ₂ O ₃ composite inclusions can be used as a coexisting region of low-melting and high-melting inclusions, and plate-like deformation occurs at the welded portion during electric resistance welding, degrading toughness. It is possible to prevent the formation of low melting point inclusions that cause the excess Ca (C
It is possible to prevent the formation of aO) and (CaS) -based cluster inclusions, and obtain a steel for electric resistance welded pipe having excellent weld toughness and excellent HIC resistance.

[0045]

EXAMPLES Steel Nos. Shown in Tables 1 and 2 AK steels of the present invention and steel Nos. E-welded steel pipes of various sizes shown in Table 2 were manufactured by performing electric resistance welding while using as-rolled conventional steels L to Z in the as-roll state. From each of the obtained electric resistance welded steel pipes, a thickness of 5 to 13 mm including an electric resistance welded portion, a width of 20 mm, and a length of 10
A 0 mm test piece and a test piece having a thickness of 5 to 13 mm, a width of 20 mm, and a length of 100 mm were collected from the base material, and 0.5% CH ₃ COO was added to a 5% NaCl aqueous solution saturated with H ₂ S.
H-added solution (temperature 25 ° C., pH = 2.7-4.
The test piece was dipped in 0 (so-called NACE environment) for 96 hours, the crack area ratio (CAR (%)) was measured, and the HIC resistance was compared. For weld toughness, JIS Z2202 specified JIS No. 4 test piece or subsize No. 4 test piece was taken from the C direction of each electric resistance welded steel pipe, and a notch was made at the abutting part. Fracture surface transition temperature v at the weld measured according to the specified Charpy impact test
The comparison was made with Trs. The results are shown in Table 2 and FIG.

[0046]

[Table 1]

[0047]

[Table 2]

As shown in Table 2 and FIG. A
The electric resistance welded steel pipes using the steels of the present invention of ~ K do not cause HIC, and the fracture surface transition temperature vT even at the electric resistance welded portion.
rs-46 ° C. or less is obtained, and the low temperature toughness is remarkably excellent. On the other hand, steel No. The electric resistance welded steel pipes using the conventional steels of L to P do not generate HIC, but the low temperature toughness is low at the fracture surface transition temperature vTrs-19 to -28 ° C in the electric resistance welded portion. In addition, steel No. The electric resistance welded steel pipes using the conventional steels of Q to Z have a fracture surface transition temperature vTr at the electric resistance welded portion.
s-46 ° C. or less is obtained, and the low temperature toughness is remarkably excellent, but HIC occurs and the HIC resistance is poor.

[0049]

As described above, according to the present invention,
Good H resistance in severe sour environment (NACE environment)
We can provide steels for electric resistance welded pipes that have excellent low temperature toughness including IC properties and welded parts. In particular, even in sour environments in extremely cold regions, it is possible to lay line pipes without causing HIC and accidents due to low temperature toughness fracture. Becomes

[Brief description of drawings]

FIG. 1 Ratio [C] of [Ca] and [O] content in product steel [C
a] / [O] and CaO and Al obtained by quantitative analysis
It is a graph showing the relationship between the composition ratio of CaO and Al ₂ O ₃ by ₂ O ₃ amount.

FIG. 2 is a CaO—Al ₂ O ₃ system phase diagram.

FIG. 3 is a graph showing the relationship between [O] and [Ca] contents in product steel, the presence or absence of HIC occurrence, and [Ca] / [O].

FIG. 4 is a graph showing the relationship between the addition amount of CaSi and [Ca] / [O] in product steel when [O] ≦ 20 ppm in molten steel and [O]> 20 ppm in steel before Ca treatment.

FIG. 5 is a graph showing the relationship between [Ca] / [O] in product steel, fracture surface transition temperature vTrs, and occurrence of HIC in the example.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/50 C22C 38/50

Claims (4)

[Claims] 1. C: 0.01 to 0.20%, Si: 0.
01 to 0.50%, Mn: 0.50 to 1.80%, P:
0.020% or less, S: 0.001% or less, Al: 0.
In addition to 005 to 0.100%, Ca: 0.0005 to
A steel containing 0.0050% and the balance Fe and unavoidable impurities, wherein the ratio of the Ca and O contents [Ca]
/ [O] is in the range of 1.5 to 2.0, which is a corrosion resistant steel for electric resistance welded steel pipe excellent in low temperature toughness. 2. C: 0.01 to 0.20%, Si: 0.
01 to 0.50%, Mn: 0.50 to 1.80%, P:
0.020% or less, S: 0.001% or less, Al: 0.
In addition to 005 to 0.100%, Ca: 0.0005 to
0.0050%, further Cu: 0.20
0.80%, Ni: 0.05-0.60%, Cr: 1.
00% or less, Mo: 1.00% or less, Ti: 0.01 to
0.10%, Nb: 0.01 to 0.10%, V: 0.1
1% or more of 0% or less, and the balance F
steel consisting of e and unavoidable impurities, Ca, O
The ratio [Ca] / [O] of the content of the above is in the range of 1.5 to 2.0, which is a corrosion resistant steel for electric resistance welded steel pipe excellent in low temperature toughness. 3. C: 0.01 to 0.20%, Si: 0.
01 to 0.50%, Mn: 0.50 to 1.80%, P:
0.020% or less, S: 0.001% or less, Al: 0.
In addition to 005 to 0.100%, Ca: 0.0005 to
When producing a steel containing 0.0050%, balance Fe and unavoidable impurities, and having a Ca / O content ratio [Ca] / [O] of 1.5 to 2.0, deoxidation is performed beforehand. And the amount of [O] in the molten steel before addition of desulfurized CaSi to 0.
0020% or less, CaSi 1.2 ~ 1.8kg /
By adding molten steel Ton, [C
a], the ratio [Ca] / [O] of [O] contents is 1.5 to
A method for producing a corrosion resistant steel for electric resistance welded pipe excellent in low temperature toughness, characterized in that the range is 2.0. 4. C: 0.01 to 0.20%, Si: 0.
01 to 0.50%, Mn: 0.50 to 1.80%, P:
0.020% or less, S: 0.001% or less, Al: 0.
In addition to 005 to 0.100%, Ca: 0.0005 to
0.0050%, further Cu: 0.20
0.80%, Ni: 0.05-0.60%, Cr: 1.
00% or less, Mo: 1.00% or less, Ti: 0.01 to
0.10%, Nb: 0.01 to 0.10%, V: 0.1
1% or more of 0% or less, and the balance F
Molten steel before addition of CaSi, which is deoxidized and desulfurized in advance when producing a steel composed of e and unavoidable impurities and having a Ca / O content ratio [Ca] / [O] of 1.5 to 2.0. The ratio [Ca] of the [Ca] and [O] contents in the product steel is adjusted by adjusting the content of [O] in the product steel to 0.0020% or less and adding CaSi in the range of 1.2 to 1.8 kg / molten steel Ton.
/ [O] is in the range of 1.5 to 2.0. A method for producing a corrosion resistant ERW steel, which is excellent in low temperature toughness.