JP2910566B2 - Manufacturing method of high strength, high toughness, large diameter welded steel pipe material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a large-diameter welded steel pipe material having a high base material strength of at least X80 of the API standard and capable of ensuring excellent toughness in the entire area of the HAZ. About.

[0002]

2. Description of the Related Art UOE steel pipes used for long-distance transportation of petroleum and natural gas are required to have an X65-X70 class of -30.degree. Have been. In the future, even higher grade products of the X80 to X100 class will be required to have a specification of -30 ° C or less, but at present, the limit is about -20 ° C.

[0003] For example, NKK Technical Report, No. 138 (1992), p.
The X100 UOE steel pipes shown in ~ 31 are X100 of -20 ℃ specification.
Presumed to be of class. Under the manufacturing conditions of the steel sheet reported in this report, the heating temperature for hot rolling is 1150 ° C, which is almost the same as the solid solution temperature of Nb (CN), based on the contents of C and Nb in the base material, and the rolling end temperature is DWTT (Drop Weight Tearing). test, 85% SATT [Shear Area (ductile fracture rate) temperature at which 85%] 700 ° C. immediately above in consideration of the energy absorbed in the Charpy test and Ar ₃ point large Charpy test), the target cooling speed 30 ° C./sec.

However, in the above prior art, the content of alloying elements such as C, Si, Mn, Nb, and Mo must be increased in order to achieve high strength. This results in deterioration of the toughness of the welded joint, which is the main reason that the specification of only about -20 ° C can be satisfied.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an X80.
It is an object of the present invention to provide a method of manufacturing a high-strength, high-toughness, large-diameter welded steel pipe material capable of satisfying a specification of −30 ° C. or less in consideration of a cold district specification of up to X100 class.

[0006]

The gist of the present invention is as follows.
(1)-(3) The method for producing a high-strength, high-toughness, large-diameter welded steel pipe material.

(1) By weight%, C: 0.04 to 0.09%, Si: 0 to
0.5%, Mn: 1.5-2.5%, Nb: 0.005-0.060%, Ti:
0.005 to 0.025%, Mo: 0.05 to 0.50%, sol.Al: 0.05%
The following and N: 0.0050-0.0100% are contained, and Cu: 0.
05-2.5%, Cr: 0.05-1.0%, Ni: 0.05-1.0% and V: 0.01-0.1%, the balance being Fe
And unavoidable impurities, and P in the impurities is 0.010
%, And S is 0.005% or less. The material steel (hereinafter, referred to as the first material steel) is heated to 900 to 1200 ° C and hot-rolled. 50 ℃ /
A method for producing a high-strength, high-toughness, large-diameter welded steel pipe material characterized by cooling to 350 ° C. or less at a cooling rate of seconds and then performing tempering at 650 ° C. or less (hereinafter referred to as a first method).

(2) By weight%, C: 0.04 to 0.09%, Si: 0 to
0.5%, Mn: 1.5-2.5%, Nb: 0.005-0.060%, Ti:
0.005 to 0.025%, Mo: 0.05 to 0.50%, sol.Al: 0.02%
The following and O: 0.0010 to 0.0070% are contained, and Cu: 0.
05-2.5%, Cr: 0.05-1.0%, Ni: 0.05-1.0% and V: 0.01-0.1%, the balance being Fe
And unavoidable impurities, and P in the impurities is 0.010
% Or less, S is 0.005% or less, and the relationship between the Al mole% and the Mn mole% in the oxide with respect to all metal elements constituting the oxide satisfies the following formula and Steel) is heated to 900-1200 ° C and hot-rolled. After rolling at 650-800 ° C,
A method for producing a high-strength, high-toughness, large-diameter welded steel pipe material, characterized in that the material is cooled to 350 ° C. or less at a cooling rate of second and then tempered at 650 ° C. or less (hereinafter referred to as a second method).

(Al mol% + Mn mol%) ≧ 40 (mol%) (Al mol% / Mn mol%) = 1.0 to 5.0 (3) 900 to 120
After hot-rolling by heating to 0 ° C and completing rolling at 650 to 800 ° C, cooling is started at a cooling rate of 5 to 50 ° C / sec, and the cooling is stopped at 350 to 600 ° C. A method for producing a high-strength, high-toughness, large-diameter welded steel pipe material characterized by being left to cool (hereinafter, referred to as a third method).

In any of the above-mentioned material steels, Si may not be added. When Si is positively added, the lower limit is desirably 0.05%.

The specific strength and toughness target values of the large-diameter welded steel pipe material according to the method of the present invention are as follows.
And 85% SATT in DWTT is -30 ° C or less, and vE- _{30 of the} welded joint HAZ is 47J or more.

When a high-strength material of X80 grade or higher is used, it is preferable to add a combination of Nb and Mo as a raw material component, and although this is actually performed as described above, Mo causes deterioration of HAZ toughness. The present inventor studied a method for manufacturing a large-diameter welded steel pipe material in order to increase the toughness in a HAZ of a composite additive of Nb and Mo, and obtained the following new knowledge.

After reducing the Si content, which is said to increase the formation of island martensite, or promoting the precipitation of ferrite by lowering the Al content, the N content is controlled to be slightly higher to obtain finely dispersed TiN. Γ grains in the HAZ can be prevented from becoming coarse.

[0014] The addition conditions are adjusted so as to secure the amount of dissolved oxygen after the addition of Al, a deoxidation product containing Al is formed in the steel, and the final deoxidation is carried out to obtain an Al-Mn-based oxide. By finely dispersing the substance, it can be used as a nucleation site of intragranular acicular ferrite to refine the HAZ structure.

[0015]

[Action]

(I) Chemical Composition of Material Steel First, the reason why the chemical composition of the first material steel to be subjected to the method of the present invention is limited as described above will be described. % Means% by weight.

C: 0.04 to 0.09% C is an element necessary for securing strength. C content
If it is less than 0.04%, the required strength cannot be obtained, so the lower limit was made 0.04%. On the other hand, if it exceeds 0.09%, the toughness of the material and the welded joint deteriorates, so the upper limit was made 0.09%.

Si: 0 to 0.5% Although Si may not be added, it acts as a deoxidizing agent when smelting steel and is effective in improving strength. When adding positively to obtain these effects, the lower limit is desirably set to 0.05%. However, if it exceeds 0.5%, the formation of island-like martensite is promoted and the HAZ toughness is degraded, so the upper limit was made 0.5%. 0.1% is desirable
It is as follows.

Mn: 1.5 to 2.5% Mn is effective as a deoxidizing agent or for improving the strength and toughness of a material. To secure material strength of X80 grade or more, Mn content of 1.5% or more is required. On the other hand, 2.5
%, The toughness of the joint is significantly deteriorated.
2.5%.

Nb: 0.005 to 0.060% Nb forms fine carbonitrides and has the effect of increasing the strength. To obtain this effect, its content must be 0.005% or more. On the other hand, if it exceeds 0.060%, the harmful effect of embrittlement becomes greater, so the upper limit was made 0.060%.

Mo: 0.05 to 0.50% Mo increases the controlled rolling effect through the effect of improving hardenability and suppressing austenite recrystallization.
It is effective for increasing strength. This effect is particularly enhanced by the combined addition with Nb. To get these effects,
0.05% or more is required. On the other hand, if it exceeds 0.50%, the toughness deteriorates, so the upper limit was made 0.50%.

Ti: 0.005 to 0.025% Ti is effective for forming fine nitrides to prevent coarsening of γ grains and improving toughness. To achieve this effect, it must be at least 0.005%.

On the other hand, if the content exceeds 0.025%, the toughness decreases due to the precipitation of carbides, so the upper limit was made 0.025%.

Sol. Al: 0.05% or less Al is necessary as a deoxidizing element. On the other hand, if the sol.Al content exceeds 0.05%, the cleanliness of steel is impaired, so the upper limit is
0.05%. Since lower Al contributes to higher toughness of HAZ through promotion of ferrite precipitation, it is desirable to use sol.
The content is 0.02% or less.

N: 0.0050 to 0.0100% N forms TiN and contributes to suppression of coarsening of γ grains in HAZ. To obtain this effect, the content needs to be 0.0050% or more. In the first material steel of the method of the present invention, since the finely dispersed oxide of Al-Mn described later cannot be utilized for improving the HAZ toughness, the lower limit of the N content must be determined and TiN must be precipitated. is there. On the other hand, if the N content exceeds 0.0100%, the HAZ toughness deteriorates. Therefore, the range of the N content is set to 0.0050 to 0.0100%.

The first material steel further contains at least one selected from the following Cu, Cr, Ni and Mo.

Cu: 0.05-2.5% Cu is an element effective for increasing the strength. To get this effect
A content of 0.05% or more is required. On the other hand, if it exceeds 2.5%, the weldability deteriorates.

Cr: 0.05-1.0% Cr is an element effective for increasing the strength. To get this effect
A content of 0.05% or more is required. On the other hand, if it exceeds 1.0%, the weldability is degraded.

Ni: 0.05 to 1.0% Ni is an element effective for improving strength and toughness. To obtain this effect, a content of 0.05% or more is required. On the other hand, 1.0
Exceeding% impairs economic efficiency.

V: 0.01 to 0.1% V forms carbonitrides like Nb and increases the strength. However, since it is not as effective as Nb, the content needs to be at least 0.01%. On the other hand, if it exceeds 0.1%, toughness is impaired.

P: 0.010% or less P is an unavoidable impurity harmful to steel, such as promoting segregation. Particularly, in the case of a high-strength material, if the P content exceeds 0.010%, the steel becomes extremely embrittled, so the upper limit is made 0.010%.

S: 0.005% or less S forms MnS and is an unavoidable impurity harmful to steel quality.
In particular, when the S content exceeds 0.005%, MnS precipitates compositely on the Al-Mn finely dispersed oxide described later, which adversely affects the securing of HAZ toughness. Therefore, the upper limit is set to 0.005%.

Next, the reasons for limiting the chemical composition of the second material steel in the method of the present invention will be described.

This second material steel is to improve the HAZ toughness by using a finely dispersed oxide of Al-Mn. That is, the contents of Si, Mn, Nb, Ti, Mo, Cu, Cr, Ni, V, P and S are the same as above, sol.Al is 0.02% or less, O is 0.0010 to 0.0070%, and The relationship between the Al mole% and the Mn mole% in the oxide with respect to all the metal elements constituting the oxide satisfies the following formulas.

(Al mol% + Mn mol%) ≧ 40 (mol%) (Al mol% / Mn mol%) = 1.0 to 5.0 The reason is described below. Except in formulas and,% means% by weight.

Sol.Al: 0.02% or less Al is essential as a constituent element of the dispersed oxide, so it must be contained in a small amount. The preferred calculated lower limit for the sol.Al content is 0.0001%. This value exceeds the analysis limit and is usually in the range called tr. (Trace, trace amount).

That is, the lower limit of sol.Al may be tr.

The reason that the lower limit of sol.Al may be very small as described above is that even if the number of dispersed oxides is considerably small, the formed Al-Mn finely dispersed oxide exerts a sufficient effect. This is because there is no problem in improving the HAZ toughness.

HAZ using finely dispersed oxide of Al-Mn
When improving the toughness, if the sol.Al content exceeds 0.02%, the formation of an oxide effective for ferrite nucleation is inhibited, so that the object cannot be achieved. Therefore, in this case, it must be kept at 0.02% or less.

O: 0.0010 to 0.0070% When HAZ toughness is improved by an Al-Mn-based finely dispersed oxide, O is also an essential element as a constituent element of the dispersed oxide. To obtain the desired dispersed oxide, the O content is 0.0010
% Or more. On the other hand, if it exceeds 0.0070%, the cleanliness of the steel is impaired, so the upper limit was made 0.0070%.

When the relationship between Al mole% and Mn mole% in the oxide with respect to all the metal elements constituting the Al-Mn-based oxide does not satisfy the above-mentioned formulas, the nucleus for forming an acicular ferrite of the oxide is obtained. Function is reduced.

At this time, the desirable size of the Al-Mn-based finely dispersed oxide is 0.2 to 20 μm, and the desirable average density per 1 mm ² is 4 to 1000 pieces. In the case of such a dispersed state, this functions most effectively as a nucleus for producing acicular ferrite, and makes the HAZ finer. Under the above conditions, the Si content may be set to the upper limit of 0.5%.

(II) Manufacturing Method Next, the reasons for limiting the steps and conditions of the method of the present invention will be described.

According to the first and second methods of the present invention, the first and second steel materials are treated according to the following steps and conditions.

The material is heated to 900 to 1200 ° C. and hot rolled.

The rolling is completed at a finishing temperature of 650 to 800 ° C.

Subsequently, at a cooling rate of 5 to 50 ° C./sec.
Cool to below.

Next, tempering treatment is performed at 650 ° C. or less.

Heating temperature: Heat to 900 ° C. or more to make 100% γ (austenite). The temperature is preferably 1050 ° C. or more from the viewpoint of promoting the solid solution of Nb. On the other hand, 1200
If the temperature exceeds ℃, γ grains become coarse and the toughness is deteriorated.

Hot rolling finish temperature: If less than 650 ° C., work hardening of α (ferrite) is remarkable, and toughness is deteriorated. On the other hand, if the temperature is higher than 800 ° C., the microstructure is not sufficiently refined.

Cooling rate: Immediately after the completion of the above-mentioned hot rolling, accelerated forced cooling is performed immediately or with air unavoidable in equipment. If the cooling rate is less than 5 ° C./sec, the effect of accelerated cooling cannot be obtained. On the other hand, if the temperature exceeds 50 ° C./sec, an excessively quenched structure results, and the toughness is deteriorated.

Cooling stop temperature: The purpose of accelerated forced cooling to 350 ° C. or less is to form a low-temperature transformation product which significantly increases strength.

Tempering temperature: The low-temperature transformation product formed by the cooling described above is subjected to a tempering treatment because it causes deterioration of toughness. However, when the tempering temperature exceeds 650 ° C, the strength is significantly reduced. A desirable temperature range in consideration of the tempering effect is 450 to 650 ° C.

In the third method of the present invention, the above-mentioned first or second material steel is treated according to the following steps and conditions, and the tempering treatment is omitted.

The material is heated to 900 to 1200 ° C. and hot-rolled.

The rolling is completed at a finishing temperature of 650 to 800 ° C.

Subsequently, cooling is started at a cooling rate of 5 to 50 ° C./sec, and stopped at 350 to 600 ° C.

Thereafter, it is left to cool.

The hot rolling conditions in the above steps and the reasons for limiting them are the same as in the above-described first and second methods of the present invention. After the completion of the hot rolling, in order to obtain the same effect as in the above-described first and second methods, it is continued at 5 to 50 ° C.
Start accelerated cooling at a cooling rate of / sec.

The range of the cooling stop temperature after the cooling is 350 to
The reason for setting the temperature at 600 ° C. is to omit the final tempering treatment. When the cooling stop temperature is 350 ° C. or higher, the amount of low-temperature transformation products contributing to an increase in strength decreases, but the formation of martensite that causes embrittlement is also suppressed, so that the risk of deterioration in toughness is reduced. On the other hand, if the cooling stop temperature exceeds 600 ° C., the effect of accelerated cooling cannot be obtained.

After the accelerated forced cooling is stopped in the temperature range of 350 to 600 ° C., it is allowed to cool to room temperature (natural cooling) in order to perform self-tempering.

[0061]

EXAMPLE A steel slab having the chemical composition shown in Table 1 was produced by continuous casting, and was used as a raw steel sheet for a large-diameter welded steel pipe under the conditions shown in Table 2. Using these steel sheets, the properties and dispersion density of Al-Mn-based oxides, 85% SATT and vE- ₃₀
Was investigated and measured. Table 3 shows the results.

[0062]

[Table 1]

[0063]

[Table 2]

[0064]

[Table 3]

As can be seen from Table 3, high strength meeting the X80 to X100 grade and high toughness satisfying the specification of −30 ° C. or less in the entire steel plate and its HAZ can be obtained by manufacturing under the conditions specified in the present invention. Has been obtained.

[0066]

According to the method of the present invention, a material for a large-diameter welded steel pipe having a high strength satisfying the X80 to X100 grades and a high toughness satisfying the specification of -30 ° C or less over the entire area of the HAZ is obtained. Obtainable.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-278736 (JP, A) JP-A-8-20838 (JP, A) JP-A-4-362126 (JP, A) JP-A-2- 125812 (JP, A) JP-A-64-25917 (JP, A) JP-A-5-271864 (JP, A) JP-A-5-255801 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 8 / 02,8 / 10

Claims (3)

(57) [Claims] C .: 0.04 to 0.09% by weight, Si: 0 to 0.5% by weight.
%, Mn: 1.5 to 2.5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.025%, Mo: 0.05-0.50%, sol. Al: 0.05% or less and N: 0.0050-0.0100%, Cu: 0.05-
2.5%, Cr: 0.05-1.0%, Ni: 0.05-1.0% and V: 0.01-0.1%, the balance being Fe and unavoidable impurities, and P in the impurities is 0.010%.
In the following, S is steel material of 0.005% or less, 900-1200 ℃
After hot rolling at 650-800 ° C, it is cooled to 350 ° C or less at a cooling rate of 5-50 ° C / sec, and then tempered at 650 ° C or less. Method for producing high-strength, high-toughness, large-diameter welded steel pipe material. 2. C .: 0.04 to 0.09% by weight, Si: 0 to 0.5% by weight.
%, Mn: 1.5 to 2.5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.025%, Mo: 0.05-0.50%, sol. Al: 0.02% or less and O: 0.0010-0.0070%, further Cu: 0.05-
2.5%, Cr: 0.05-1.0%, Ni: 0.05-1.0% and V: 0.01-0.1%, the balance being Fe and unavoidable impurities, and P in the impurities is 0.010%.
In the following, S is 0.005% or less, and a material steel in which the relationship between the Al mole% and the Mn mole% in the oxide with respect to all the metal elements constituting the oxide satisfies the following formula and 900-1200 ° C.
After hot rolling at 650-800 ° C, it is cooled to 350 ° C or less at a cooling rate of 5-50 ° C / sec, and then tempered at 650 ° C or less. Method for producing high-strength, high-toughness, large-diameter welded steel pipe material. (Al mol% + Mn mol%) ≧ 40 (mol%) (Al mol% / Mn mol%) = 1.0 to 5.0 3. The steel material according to claim 1, which is heated to 900 to 1200 ° C. and hot-rolled.
After rolling at ~ 800 ° C, cooling is started at a cooling rate of 5-50 ° C / sec, then stopped at 350-600 ° C, and then allowed to cool. Manufacturing method for large diameter welded steel pipe material.