JPH07216500A - High strength steel material excellent in corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To produce a steel material combining superior HIC resistance, SSC resistance, and carbon dioxide corrosion resistance and having high strength satisfying the strength level of API specification X80 class. CONSTITUTION:This steel material has a composition, containing 0.03-0.07% C, 0.01-0.5% Si, 0.7-1.1% Mn, <=0.015% P, <=0.002% S, 0.3-0.7% Cr, 0.005-0.030% Ti, 0.005-0.05% Al, 0.0005-0.0050% Ca, and 0.005-0.010% N or further containing one or more kinds among 0.05-0.5% Cu, 0.05-0.5% Ni, 0.03-0.3% Mo, 0.01-0.1% Nb, and 0.01-0.1% V, and also has a dual phase structure of ferrite and bainite. Moreover, at the time of manufacture, a steel of this composition is heated to 1050-1250 deg.C, subjected to rolling where cumulative draft at <=950 deg.C is regulated to >=50%, finished at a temp. not lower than the Ar3 point, water-cooled from >=[Ar, point -30 deg.C] down to 400-550 deg.C at (10 to 25) deg.C/s cooling rate, and air-cooled or coiled.

Description

Detailed Description of the Invention

[0001]

This invention relates to hydrogen induced cracking resistance (hereinafter referred to as "HIC resistance"), sulfide stress corrosion cracking resistance (hereinafter referred to as "SSC resistance") and carbon dioxide corrosion resistance. Excellent in performance and API (American Petroleum Institute) standard X
It has high strength of 80 class (yield stress of 80 ksi or more) and is suitable for line pipes used for transportation of crude oil, natural gas, etc., tank tanks for tankers or pressure vessels for oil refining. The present invention relates to a steel material and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, a line pipe or a tanker for transporting crude oil or natural gas containing hydrogen sulfide.
Steels used for steel tanks, and for steels used for refining towers and handling tanks for crude oil or natural gas containing hydrogen sulfide include hydrogen-induced cracking (hereinafter referred to as "HIC") or sulfide stress corrosion cracking (hereinafter "SSC"). Is called as a problem), and many studies have been made on the HIC and SSC.

Incidentally, HIC is a crack that occurs in a steel material in the absence of external stress, and SSC is a crack that occurs under static stress.
It has been clarified that each of SCs is a phenomenon appearing as a result of "hydrogen embrittlement caused by hydrogen invading into steel when hydrogen is corroded in a wet hydrogen sulfide environment".

Then, as a method of preventing the above-mentioned HIC and SSC, for example, the following measures have been proposed. a) By adding Cu as a steel component, it is possible to suppress the entry of hydrogen into the steel in a wet hydrogen sulfide environment and to withstand HIC.
Morphology control of sulfides (A type inclusions) by addition of Ca from the viewpoint of improving the heat resistance and SSC resistance and eliminating the starting point of HIC, and further reducing nonmetallic inclusions (improving the cleanliness of steel) Method (see Japanese Patent Publication No. 60-35982). b) Since a hardened structure is formed in the central segregated portion of the steel material where the Mn and P concentrations are high and the HIC and SSC sensitivities are increased, segregation is reduced by soaking diffusion or the hardened structure of the hardened structure is controlled by controlled cooling after rolling. Method to prevent generation (Japanese Patent Publication Sho 63-
1369, etc.].

However, with the depletion of high-quality petroleum resources, the development of oil wells and gas wells in a harsh environment where the concentration of hydrogen sulfide is high has increased, and steel materials such as line pipes have a higher than conventional usage environment. Further, since the environment where the pH is low and the hydrogen sulfide pressure is high is imposed, and the accident prevention measures are strengthened from the viewpoint of global environmental problems, these specifications and standards are becoming more and more severe. As a result, the "NACE bath" called "0.5% acetic acid + 5% saline solution at 25 ° C saturated with hydrogen sulfide at 1 atm" is resistant to H
It has come to be used for evaluation of IC property and SSC resistance.

The current general specification standard is HIC.
Regarding "the crack length in the width direction (CLR) when immersed in a NACE bath for 96 hours is 5 to 15% or less. {Crack area ratio (CAR)
Is equivalent to less than 2%} ”,
Regarding C, "Specified minimum yield stress (SMYS) in NACE bath
Has a crack generation limit stress (σth) of 80% or more.

However, in recent years, from the viewpoint of economic efficiency, the strength of steel has been increased and the operating pressure has been increased, and the X65 class (yield stress: 65 ksi or higher level) that has been conventionally used has been used.
Demand for steel materials such as X70 (yield stress: 70 ksi or higher level) or X80 grade instead of the above steel materials has been increasing. However, it has been extremely difficult to manufacture a high-strength steel sheet for line pipes satisfying the above-mentioned specifications at a low cost only with the conventional technique as described above.
This is because if the amount of Mn is increased to increase the strength, the HIC sensitivity of the hardened structure that tends to occur in the central segregation portion increases, and it was difficult to prevent the occurrence of HIC only by using controlled cooling. Further, if the strength becomes high, the hardness of the welded part inevitably increases and the SSC sensitivity also increases.

On the other hand, recently, for steel materials for line pipes, in addition to solving the problem of cracking such as HIC and SSC, it is desired that the steel material has a higher resistance to carbon dioxide corrosion than conventional steels. I'm getting started.

Therefore, the strength of line pipe materials and the resistance to H
Cr known as an element effective in ensuring IC property and SSC resistance (see, for example, Japanese Patent Publication No. 2-50967 and Japanese Patent Publication No. 3-68101, which disclose X70-class sour-resistant environmental line pipes) Based on the knowledge that "the higher the content, the more the carbon dioxide corrosion resistance of steel improves," a line was designed to improve the carbon dioxide corrosion resistance by adding a specific amount of Cr. A proposal for a steel material for pipes has also been made (see JP-A-4-341540). However, the present situation is that no technology has yet been found for simultaneously satisfying HIC resistance, SSC resistance, and carbon dioxide corrosion resistance, which are sufficiently satisfactory for high-strength X80 grade steel for line pipes. .

Therefore, the object of the present invention is to provide excellent HIC resistance, SSC resistance, and carbon dioxide gas corrosion resistance as well as X80 grade (yield strength 80 ksi (1 ksi = It is to provide a high-corrosion-resistant high-strength steel material that sufficiently satisfies the strength level of 6.89 MPa) or more}.

[0011]

Therefore, as a result of intensive studies to achieve the above object, the present inventor was able to obtain the following findings. a) First, as already known, HIC occurs from the inclusions in the steel material and the hardening structure of the central segregation part. Therefore, to improve the HIC resistance, morphology control and reduction of inclusions by Ca treatment are required. It is an indispensable technology.

B) Here, since the center segregation of the steel material which causes the formation of the hardened structure is promoted by C, Mn and P,
From this point of view, it can be said that it is desirable to suppress the content of any of the above elements to a low content, but from the viewpoint of securing strength, it is necessary to refrain from extremely reducing C and Mn. That is, it is absolutely necessary to add a certain amount of C and Mn in order to secure X80 grade strength in the steel material. However, if the C content is up to 0.07% (hereinafter,% representing the component ratio is weight%) and the Mn content is up to 1.3%, the HIC resistance of the steel material is not particularly impaired, and If C and Mn are added even at a moderate level, it is possible to sufficiently secure the strength of X80 class by using it together with other means.

C) In other words, fortunately, Cr that exerts the effect of improving the carbon dioxide corrosion resistance according to its content is
On the other hand, it also has the function of improving the hardenability and increasing the strength of the steel material by the precipitation of chromium carbide. Therefore, Cr, which has the effect of improving carbon dioxide corrosion resistance, can be used in place of Mn as a strengthening element, and the basic component system of the steel material can be a low C-low Mn-
By using Cr steel, it becomes possible to secure high strength, excellent HIC resistance, and carbon dioxide corrosion resistance.

D) On the other hand, Cr which affects the SSC resistance of the base metal part
As a result of investigating the effects of the Mn content and the Mn content, “the strength of X80 grade” and “the current standard specifications for general SSC resistance”, “the critical crack initiation stress in NACE bath (σt
h) is 80% or more of the specified minimum yield stress (ie σth ≥ 80
%) ”, The addition range of Cr and Mn must be limited to a narrower range, and it is a general sub-marque welding (S
It is more difficult to secure stable "σth ≥ 80%" in the parts subjected to (AW), and it is difficult to secure stable high X80 grade strength and desired SSC resistance for low C-low Mn-Cr steel. There was a concern that it might not exist.

E) However, as a component of the above steel material, 0.00
When 5% or more of N is added together with Ti, its SSC resistance is improved, and stable addition of Cr to the steel material and its welded portion with sufficient strength and HIC resistance gives “σth ≧ 80.
It was found that the added N forms Ti and TiN to prevent coarsening of the austenite grains, and the SSC resistance through the hydrogen trapping effect. In particular, the effect of improving the SSC resistance due to the prevention of coarsening of the austenite grains in the heat-affected zone of the welding during welding is remarkable, and it is stable even in the portion subjected to sub-marque welding. It comes to satisfy “σth ≧ 80%”.

F) Further, the strength of the above composition, the HIC resistance,
In order to stably produce a steel material having excellent SSC resistance and carbon dioxide corrosion resistance, it is necessary to heat and roll the material steel under specific conditions, and then cool it under controlled specific conditions. Became clear.

The present invention has been completed after a comprehensive study of the above findings and the like. "The steel material is C: 0.03 to 0.0
7%, Si: 0.01 to 0.5%, Mn: 0.7 to 1.1%,
P: 0.015% or less, S: 0.002% or less, Cr:
0.3 to 0.7%, Ti: 0.005 to 0.030%, Al (= sol.A
l): 0.005 to 0.05%, Ca: 0.0005 to 0.0050%, N:
Contains 0.005 to 0.010%, or Cu: 0.05 to 0.5
%, Ni: 0.05 to 0.5%, Mo: 0.03 to 0.3%, N
b: 0.01 to 0.1%, V: 0.01 to 0.1% and at least one of Fe and unavoidable impurities in the balance, and a two-phase structure of ferrite and bainite. Excellent corrosion resistance (HIC resistance, SSC resistance, carbon dioxide corrosion resistance) and AP
It has a major feature in that "a steel material that has high strength and satisfies the I standard X80 class can be realized." Furthermore, "C: 0.03 to 0.07%, Si: 0.01 to 0.5%, Mn:
0.7 to 1.1%, P: 0.015% or less, S: 0.002% or less, Cr: 0.3 to 0.7%, Ti: 0.005 to 0.030%,
Al (= sol.Al): 0.005 to 0.05%, Ca: 0.0005 to 0.0050
%, N: 0.005 to 0.010% or Cu:
0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.03 to
Steel containing at least one of 0.3%, Nb: 0.01 to 0.1%, V: 0.01 to 0.1% and the balance being Fe and inevitable impurities is heated to 1050 to 1250 ° C and then 950 ° C or less. Rolling with a cumulative rolling reduction of 50% or more was completed, and after this rolling was completed at a temperature of Ar ₃ or higher,
400 to 5 from the temperature range above the temperature of [Ar ₃ points -30 ° C]
Excellent corrosion resistance (HIC resistance, SSC resistance, carbon dioxide corrosion resistance) by water cooling to a temperature range of 50 ° C at a cooling rate of 10 to 25 ° C / s, and then cooling or winding.
And that it enables stable manufacturing of steel materials with high strength that satisfies the API standard X80 class. "

Next, the reasons why the composition of the steel material and the manufacturing conditions are limited as described above in the present invention will be described in detail together with their actions.

[Action]

A) Constituent component of steel [C] C has a function of stably securing the strength of steel, but if its content is less than 0.03%, the desired strength (API
It becomes difficult to secure the standard X80 class), while 0.
If C is added in an amount of more than 07%, the HIC resistance deteriorates, the hardness of the weld heat affected zone (HAZ) increases, and the HA
A pearlite band is generated in the vicinity of the Z outermost line and S resistance is increased.
The SC property also decreases. Therefore, although the C content is set to 0.03 to 0.07%, it is desirable to adjust it to 0.04 to 0.06%.

[Si] Si is necessary as a deoxidizing agent during steel making, but if its content is less than 0.01%, a sufficient deoxidizing effect cannot be secured. On the other hand, when Si is contained in excess of 0.5%, the toughness of the steel material is significantly reduced. Therefore, the Si content was set to 0.01 to 0.5%, but 0.1 to 0.3% is desirable.
It is good to adjust to.

[Mn] Mn also has a function of stably securing the strength of the steel material, but if the content thereof is less than 0.7%, it becomes difficult to secure a desired strength (X80 class of API standard), On the other hand, when Mn is contained in excess of 1.1%, the SSC resistance of the base metal part deteriorates, and an abnormal structure (hardened structure) due to the concentration segregation of Mn and P occurs in the central segregation part.
C property is also impaired. Therefore, although the Mn content is set to 0.7 to 1.1%, it is desirable to adjust it to 0.8 to 1.0%.

[P] If the P content exceeds 0.015%, an abnormal structure due to the concentration segregation of Mn and P is generated in the central segregation portion of the steel material, and the HIC resistance is impaired.
Although it has been determined that the P content is not more than%, it is desirable that the P content is as low as possible.

When the [S] S content exceeds 0.002%, MnS is generated in the central segregation portion and HIC resistance is impaired even when the sulfide morphology is controlled by Ca.
Although it was set to 0.002% or less, it is desirable that the S content is as low as possible.

[Cr] Cr has an action of improving the strength and the carbon dioxide corrosion resistance of the steel material, but if the content thereof is less than 0.3%, the desired strength (X80 class of API standard) is secured. In addition to the difficulty, the effect of improving the carbon dioxide gas corrosion resistance is hardly recognized. On the other hand, if the content of Cr exceeds 0.7%, the SSC resistance of the base material part is deteriorated. Therefore, the Cr content was set to 0.3-0.7%, but 0.4-0.6% is desirable.
It is good to adjust to. Although it depends on the test method, 0.
By including 3 to 0.7% of Cr, the corrosion rate in the carbon dioxide environment is reduced to about half that of the conventional steel containing no Cr (line pipe steel).

[Ti] Ti has the effect of improving the SSC resistance of the steel by forming N and TiN to prevent coarsening of austenite grains, but its content is less than 0.005%. Therefore, it becomes impossible to secure sufficient SSC resistance in the base metal part, while if Ti is contained in excess of 0.030%, the toughness of the base metal part and the welded part deteriorates. Therefore, although the Ti content is set to 0.005 to 0.030%, the desirable content is 0.01 to 0.02%.

[Al] Al is also necessary as a deoxidizing agent during steel making, but if its content is less than 0.005%, a sufficient deoxidizing effect cannot be secured. On the other hand, if Al is contained in excess of 0.05%, the cleanliness and toughness of the steel will be impaired. Therefore, although the Al content was set to 0.005 to 0.05%,
Desirably, the content is adjusted to 0.01 to 0.03%.

[Ca] Ca has the effect of controlling the morphology of sulfide-based inclusions to improve the HIC resistance, but if its content is less than 0.0005%, MnS is formed and the desired resistance is obtained. HIC
The effect of improving the sex cannot be secured. On the other hand, 0.0050%
If Ca is contained in an amount exceeding the above range, excess Ca forms a cluster of oxides, and conversely the HIC resistance is impaired. Therefore, the Ca content is set to 0.0005 to 0.0050%, but it is desirable to adjust it to 0.001 to 0.003%.

[N] N is a very important constituent element for the steel material of the present invention in which Cr also plays a role of ensuring strength in place of Mn whose addition amount is limited. That is, as described above, Cr
When the content is large, the SSC resistance tends to deteriorate, but the addition of N together with Ti has the effect of improving the SSC resistance, and the Cr content that contributes to the improvement of strength and carbon dioxide corrosion resistance. This is because the addition range of can be expanded. However, if the N content is less than 0.005%, it becomes difficult to secure the desired SSC resistance in the base metal part, while if the N content exceeds 0.010%, the toughness of the base metal part and welded part deteriorates. Will be invited. Therefore, the N content is
Although it has been set as 0.005 to 0.010%, it is preferably 0.005 to 0.00
It is good to adjust to 8%.

[Cu, Ni, Mo, Nb and V] These elements not only do not impair the excellent characteristics of the steel material of the present invention, but further improve them to exert the action of expanding the range of applications of the steel material of the present invention. Therefore, if necessary, one kind or more than one kind of addition is performed. Cu: Cu has the effect of improving the corrosion resistance and, in an environment of high pH, suppressing hydrogen intrusion and improving the HIC resistance. However, if the content is less than 0.05%, the desired effect due to the above-mentioned action cannot be obtained, while if it exceeds 0.5%, Cu checking occurs during hot rolling, which makes manufacturing difficult.
Therefore, the Cu content is set to 0.05 to 0.5%, but the desirable range is 0.2 to 0.35%. Ni: Ni has the effect of improving the corrosion resistance and, in an environment having a low pH such as the NACE bath, suppressing hydrogen intrusion to some extent and improving the HIC resistance. Furthermore, it also has the effect of improving strength and toughness and the effect of preventing Cu checking. However, if the Ni content is less than 0.05%, the desired effect due to the above action cannot be obtained, while if it exceeds 0.5%, the effect is saturated, which is not desirable from the economical point of view. Therefore, the Ni content is set to 0.05 to 0.5%, but the desirable range is 0.1 to 0.2%. Mo: Mo not only improves strength and toughness, but also has the effect of suppressing hydrogen intrusion to some extent and improving HIC resistance by synergistic action with Ni in an environment of low pH such as NACE bath. However, if the Mo content is less than 0.03%, the desired effect due to the above-mentioned action cannot be obtained, while if it exceeds 0.3%, the toughness decreases and the SSC resistance decreases with the increase in hardness of the welded portion. Therefore, the Mo content is set to 0.03 to 0.3%, but the desirable range is 0.05 to 0.15%. Nb: Nb has the effect of improving strength and toughness by fine graining and precipitation of carbides, and improving SSC resistance by fine graining. However, if the content is less than 0.01%, the desired effect due to the above-mentioned action cannot be obtained, while if it exceeds 0.1%, the toughness is deteriorated. Therefore, the Nb content is 0.
Although it has been set as 01 to 0.1%, it is preferably 0.02 to 0.05%. V: V has the effect of improving the strength and toughness by grain refinement and carbide precipitation, as well as Nb, and improving the SSC resistance by grain refinement. However, if the content is less than 0.01%, the desired effect due to the above-mentioned action cannot be obtained.
If it exceeds, the toughness will be deteriorated. Therefore, the V content is set to 0.01 to 0.1%, preferably 0.02 to 0.07.
%.

B) Structure of Steel Material The structure of the steel material is defined as "two-phase structure of ferrite and bainite" in order to impart excellent HIC resistance and SSC resistance. It is desirable to have a "structure in which C is enriched with retained austenite", "hard martensite structure in the central segregation part", "block bainite structure" or "structure containing pearlite". HIC resistance and SSC resistance are not provided. The two-phase structure of the ferrite and bainite is
It can be stably achieved by appropriately adjusting the composition of the steel material and the cooling conditions after the rolling is completed.

C) Steel Manufacturing Conditions [Heating Temperature] The heating temperature of the material steel during rolling is 105
If the temperature is lower than 0 ° C., the carbides are not sufficiently solid-dissolved, the desired high strength cannot be obtained after rolling, and it becomes difficult to secure a predetermined rolling finishing temperature. On the other hand, the heating temperature is 12
If the temperature exceeds 50 ° C., the particles are coarsened and the product toughness is lowered, and the SSC resistance is also lowered. Therefore, although the heating temperature is set to 1050 to 1250 ° C, the desirable range is 1100 to 1200 ° C.

[Amount of reduction at 950 ° C. or lower] 9 during rolling
If the cumulative amount of reduction at 50 ° C. or lower is less than 50%, fine graining by recrystallization of austenite grains and stretching by rolling are insufficient, and fine ferrite grains cannot be obtained, resulting in toughness of the product and The SSC resistance becomes insufficient. Therefore, 95
It was decided to perform rolling so that the cumulative reduction rate at 0 ° C or lower was 50% or higher. However, the cumulative reduction rate should be 60 if possible.
It is desirable to secure at least%.

[Rolling finishing temperature] The rolling finishing temperature is Ar ₃
When the temperature falls below the point, accelerated cooling starts from the two-phase region where a large amount of pro-eutectoid ferrite is precipitated, and the resulting product has a structure in which residual austenite enriched with C and a hard "martensite structure" in the central segregation part are formed. "Or a" blocky bainite structure "is formed, and HIC resistance and SSC resistance are lowered. Therefore, the rolling is determined to be completed at a temperature of Ar ₃ points or higher, but it is desirable to finish the rolling at a temperature of [Ar ₃ points + 30 ° C.] or higher.

[Accelerated Cooling Start Temperature] Accelerated cooling after the completion of rolling is a process necessary for obtaining a high strength of API standard X80 class, but the accelerated cooling start temperature is [Ar ₃ points -30
[° C], accelerated cooling will start from "a two-phase region in which a large amount of pro-eutectoid ferrite is precipitated", and the resulting product has a structure "a structure in which residual austenite enriched with C is mixed". Or "hard martensite structure in the center segregation part" or "block-shaped bainite structure" and becomes HI resistant
C property and SSC resistance decrease. Therefore, the accelerated cooling start temperature after the completion of rolling is determined to be the temperature of [Ar ₃ point −30 ° C.] or higher, but it is desirable that Ar ₃ in which proeutectoid ferrite is not yet present.
It is better to start from a temperature above the point.

[Cooling rate during accelerated cooling] When the cooling rate (average cooling rate) during accelerated cooling is less than 10 ° C / s, two-phase separation of ferrite / pearlite proceeds and HIC resistance is improved at the center segregation portion. A poorly hardened structure is formed. This pearlite impairs the SSC resistance of the base material. On the other hand, if the cooling rate exceeds 25 ° C./s, hardened block bainite is likely to be formed, the HIC resistance and the SSC resistance are lowered, and it is industrially controlled to a predetermined accelerated cooling stop temperature. Becomes difficult. Therefore, the cooling rate at the time of accelerated cooling is set to 10 to 25 ° C / s, but the desirable range is 15 to 20 ° C / s.

[Accelerated Cooling Stop Temperature] The accelerated cooling performed after rolling is required to be stopped in the temperature range of 400 to 550 ° C. Because the accelerated cooling stop temperature is 400 ℃
If it is below the range, hardened block bainite is likely to be formed, and the HIC resistance and SSC resistance of the base material are deteriorated. When accelerated cooling is stopped in a temperature range higher than 550 ° C., untransformed austenite remains when the accelerated cooling is stopped, so that pearlite is generated after that, C is concentrated in the segregated portion, and HIC resistance is high. descend. Moreover, the generated part
Light impairs the SSC resistance of the base material. Therefore, although it has been determined that the accelerated cooling is stopped in the temperature range of 400 to 550 ° C as described above, the desired accelerated cooling stop temperature is 450 to 5
It can be said that the temperature is 00 ° C. After the accelerated cooling is stopped, for example, it is allowed to cool as it is for steel plates for large-diameter line pipes rolled in a thick plate mill, and it is wound in hot coils for ERW steel pipes rolled in a hot rolling mill. I'm fine.

Next, the present invention will be described with reference to examples.

EXAMPLE First, a steel piece having the composition shown in Tables 1 and 2 was obtained by a continuous casting apparatus and a vacuum melting casting apparatus.

[0037]

[Table 1]

[0038]

[Table 2]

Next, heating, rolling and accelerated cooling under the conditions shown in Tables 3 to 5 were applied to these steel slabs.
A 25.4 mm thick steel plate was produced. In each case, cooling was performed after accelerated cooling.

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

Then, test pieces were sampled from each of the steel plates thus obtained, and the strength and HIC resistance of the base material were measured.
, SSC resistance and carbon dioxide corrosion resistance, and SSC resistance of welds were investigated. The results of these investigations are also shown in Tables 3 to 5.

Here, the HIC resistance is "NACE TM-0.
It was evaluated by the method described in “2-84”. That is, NAC
The crack area ratio (CAR) when immersed in E bath (0.5% acetic acid + 5% saline solution, 25 ° C., 1 atm H ₂ S saturated) for 96 hours was measured,
Those with “CAR ≦ 2%” were considered to have excellent HIC resistance.
That is, FIG. 1 shows a HIC test piece (base material thickness × 20 mm width × 100
mm length) is shown, but 3 test pieces are collected for each condition in this way, and these are put in a NACE bath for 96 times.
After immersion in hr, the H of the cross section shown in Fig. 2 by ultrasonic flaw detection
After measuring the IC area, HIC sensitivity was evaluated with CAR using the formula

The HIC resistance evaluation results shown in Tables 3 to 5 are averages of three test pieces, and CLR (crack length in the width direction) corresponds to a value obtained by multiplying CAR by about three times. It is empirically known that "CAR ≤ 2%" is "CL
"C≤2%" corresponds to "R≤5%" and is an index of excellent HIC resistance. Therefore, in Tables 3 to 5, "CAR≤2%" is marked as excellent in HIC resistance and marked with "CAR>". Two
% "Was marked as inferior to HIC resistance and indicated by x.

Further, the SSC resistance is "NACE TM-01-
77 METHOD A ”. That is, after collecting a round bar test piece having a parallel portion of 6.35 mmφ × 25.4 mm from the center of the plate thickness of the steel plate, 80% of it was placed in a NACE bath.
It is examined whether or not breakage occurs by applying tensile stress of SMYS (here, SMYS is set to 80 ksi), and whether or not breakage occurs until 720 hr has passed, and those that did not break are marked with excellent SSC resistance, marked with ○. Those having fractures were shown to have poor SSC resistance and marked with X in Tables 3 to 5, respectively.

Further, among the above steel plates, the SS resistance of the base material is
Select the one with good C property, perform submarine welding on both sides in parallel with the rolling direction with a heat input of 20 kJ / cm, and then center the weld metal part from the plate thickness center as shown in Fig. 3. The SSC test pieces were sampled so as to be located at, and the same test as above was performed on these to evaluate the SSC resistance of the welded portion. The SSC resistance evaluation results in this case are also shown in Tables 3 to 5 in the same manner as that of the base metal portion.

On the other hand, the carbon dioxide corrosion resistance was calculated from the corrosion weight loss when the test pieces cut out from each steel sheet were immersed in "50 ° C. artificial seawater saturated with carbon dioxide at 1 atm" for 96 hours. The corrosion rate was evaluated. Table 3
In addition, in the column of the characteristic evaluation result in Table 5, those marked with "-" indicate that the evaluation was not performed.

From the results shown in Tables 3 to 5, the following can be understood. That is, the results of Test Nos. 1, 2, 3 and 8 show that when the Mn content of the steel sheet is higher than the range specified in the present invention, the SSC resistance of the base material is deteriorated, and particularly the Mn content exceeds 1.3. Test number 3, which is too high, shows that the HIC resistance of the base material also deteriorates. The results of test numbers 4 and 9 show that when the Mn and Cr contents of the steel sheet are lower than the range specified in the present invention, the API standard X80 is used.
It means that the strength of is not satisfied. From the results of test numbers 13, 14 and 15, there is sufficient SSC resistance for the base metal part.
It is clear that it is necessary to adjust the Cr content to 0.7% or less in order to impart the properties.

From the results of Test Nos. 16, 17 and 18, it is understood that it is necessary to secure the N content of 0.005% or more in order to sufficiently improve the SSC resistance of the welded portion. From the results of test numbers 19, 20 and 21, it is clear that the Ti content must be 0.005% or more in order to sufficiently improve the SSC resistance of the welded portion. In addition, test numbers 22, 23 and
From the results of No. 24, it can be seen that if the C content of the steel sheet is higher than the range specified in the present invention, the SSC resistance of the welded portion deteriorates.

On the other hand, test numbers 25, 27, 28, 31, 32, 33,
36, 37 and 39 are examples in which the steel plate manufacturing conditions deviate from the conditions specified in the present invention. In this case, sufficient strength or corrosion resistance (HIC resistance, SSC resistance, carbon dioxide corrosion resistance) is required. You can see that it cannot be secured in a stable manner.

On the other hand, when the specified conditions of the present invention are satisfied, the resulting steel sheet has a high strength of API standard X80 grade and excellent HIC resistance of CAR ≦ 2% under NACE conditions.
And excellent SSC resistance of σth ≧ 80% or more under NACE conditions
It is also possible to confirm from the results of Tables 3 to 5 that it has both excellent corrosion resistance and excellent carbon dioxide corrosion resistance with a corrosion rate of about half that of conventional steel. It has been confirmed that the steel sheets obtained according to the specified conditions of the present invention each have a two-phase structure of ferrite and bainite.

[0053]

[Summary of Effects] As described above, according to the present invention, a steel material having high strength of API standard X80 class and having excellent HIC resistance, SSC resistance, and carbon dioxide corrosion resistance is provided. Therefore, it is possible to obtain extremely useful effects in industry, such as high reliability even when used for materials such as line pipes used under severe conditions.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing a method of collecting HIC test pieces.

FIG. 2 is an explanatory diagram relating to a method for evaluating HIC resistance.

FIG. 3 is a schematic explanatory view showing a method of collecting an SSC test piece from a welded portion.

Claims (4)

[Claims] 1. A weight ratio of C: 0.03 to 0.07%, Si:
0.01 to 0.5%, Mn: 0.7 to 1.1%, P: 0.015% or less, S: 0.002% or less, Cr: 0.3 to 0.7%, Ti:
0.005 to 0.030%, Al: 0.005 to 0.05%, Ca: 0.0005
.About.0.0050%, N: 0.005 to 0.010%, and the balance is a component composition of Fe and unavoidable impurities, and has a two-phase structure of ferrite and bainite. Strength steel material. 2. A weight ratio of C: 0.03 to 0.07%, Si:
0.01 to 0.5%, Mn: 0.7 to 1.1%, P: 0.015% or less, S: 0.002% or less, Cr: 0.3 to 0.7%, Ti:
0.005 to 0.030%, Al: 0.005 to 0.05%, Ca: 0.0005
~ 0.0050%, N: 0.005 to 0.010% inclusive, and Cu:
0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.03 to 0.3
%, Nb: 0.01 to 0.1%, V: 0.01 to 0.1% of 1
A high-strength steel material excellent in corrosion resistance, characterized in that it also contains at least one kind and has a composition with the balance being Fe and unavoidable impurities and having a two-phase structure of ferrite and bainite. 3. A weight ratio of C: 0.03 to 0.07%, Si:
0.01 to 0.5%, Mn: 0.7 to 1.1%, P: 0.015% or less, S: 0.002% or less, Cr: 0.3 to 0.7%, Ti:
0.005 to 0.030%, Al: 0.005 to 0.05%, Ca: 0.0005
To 0.0050%, N: 0.005 to 0.010% and the balance Fe and unavoidable impurities
After heating to 250 ° C., rolling at which the cumulative rolling reduction at 950 ° C. or less is 50% or more is performed, and after this rolling is finished at a temperature of Ar ₃ points or more, a temperature of [Ar ₃ points −30 ° C.] or more 10 to 25 ℃ / s from the temperature range of 400 to 550 ℃
A method for producing a high-strength steel material having excellent corrosion resistance, which comprises cooling with water at a cooling rate of 1, and then cooling or winding. 4. A weight ratio of C: 0.03 to 0.07%, Si:
0.01 to 0.5%, Mn: 0.7 to 1.1%, P: 0.015% or less, S: 0.002% or less, Cr: 0.3 to 0.7%, Ti:
0.005 to 0.030%, Al: 0.005 to 0.05%, Ca: 0.0005
~ 0.0050%, N: 0.005 to 0.010% inclusive, and Cu:
0.05 to 0.5%, Ni: 0.05 to 0.5%, Mo: 0.03 to 0.3
%, Nb: 0.01 to 0.1%, V: 0.01 to 0.1% of 1
A steel containing not less than one kind and the balance consisting of Fe and unavoidable impurities is heated to 1050-1250 ° C., and then 95
After rolling at a cumulative rolling reduction of 50% or more at 0 ° C. or lower, and after finishing the rolling at a temperature of Ar ₃ or higher, [Ar ₃
400 to 550 ° C from the temperature range above the point -30 ° C]
The method for producing a high-strength steel material having excellent corrosion resistance, which comprises water-cooling to a temperature range of 10 to 25 ° C./s at a cooling rate of 10 to 25 ° C./s, and then cooling or winding.