JP3374659B2 - Ultra-high tensile ERW steel pipe and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automobile member such as a door impact beam, an ultrahigh-strength electric resistance welded steel pipe used as a member for mechanical structure, a member for civil engineering and a method for manufacturing the same.

[0002]

2. Description of the Related Art A reinforcing material called a door impact beam is provided inside a vehicle door of an automobile or the like from the viewpoint of safety. Press molding of high-strength cold-rolled steel sheet was often used for the conventional door impact beam, but in recent years, in order to reduce the weight, the tensile strength is 980 N / mm.
High-strength ERW steel pipes with extremely high strength of ² or more have been adopted.

Up to now, regarding ultra-high-strength steel pipes, JP-A 1-205032, JP-A 4-131327, JP-A 4-187319,
JP-A-6-57375, JP-A-6-88129, JP-A6-179913
A method for obtaining a high-strength electric resistance welded steel pipe is proposed, in which steel having a predetermined chemical composition is made into a high-strength steel sheet having a tensile strength of 980 N / mm ² or more and then electric resistance welding is performed, which is disclosed in each of the publications. .

Further, JP-A-3-122219 and JP-A-4-63227.
The steel pipe having a predetermined chemical composition disclosed in each of the publications has a tensile strength of 1180 N / mm ²
A method for obtaining the above high-strength electric resistance welded steel pipe has been proposed.

[0005]

[Problems to be Solved] JP-A 1-205032, JP-A 4-131327, JP-A 4-187319, JP-A 6-57375,
In the methods disclosed in JP-A-6-88129 and JP-A-6-179913, there is residual strain associated with pipe making. Therefore, it is necessary to consider hydrogen delayed cracking in practical use.

However, in the methods presented so far, hydrogen delayed cracking is not taken into consideration, or even if it is not taken into consideration, the expansion of demand for ultra high strength steel pipes is limited.

On the other hand, JP-A-3-122219 and JP-A-4-63227
Although the method disclosed in each of the publications has no residual tensile strain, there is a problem that the strength of the tubular body is deteriorated if corrosion progresses during its use.

The present invention has been made in view of the above circumstances, and is an ultra-high-strength electric resistance welded steel pipe having a high tensile strength and an excellent hydrogen-delayed cracking resistance, or in addition to this, an excellent corrosion resistance. It is intended to provide a manufacturing method.

[0009]

Means for Solving the Problems As a result of many experimental studies to achieve the above-mentioned object, the inventors have adjusted the steel composition and optimized the heat treatment conditions and pipe forming conditions of the steel sheet. It was found that it is possible to obtain an ultra-high-strength electric resistance welded steel pipe which is excellent in hydrogen delayed cracking properties or, in addition to this, excellent in corrosion resistance by adjusting the structure.

The present invention has been made based on these findings,

% By weight, C: 0.10 to 0.19%, S
i: 0.01 to 0.5%, Mn: 0.8 to 2.2%, A
1: 0.01 to 0.06%, Cr: 0.05 to 0.6
%, P: 0.02% or less, S: 0.003% or less,
N: 0.005% or less, the balance Fe and unavoidable impurities
When the temperature of the Ar ₃ transformation point of the steel is TAr ₃ , the finishing temperature Tf is (TAr ₃ +3
0) to (TAr ₃ +100) ° C. The finishing temperature Tf is controlled so as to be in the temperature range, and hot rolling is performed. At the time of the hot rolling, 30% in the temperature range of Tf to (Tf + 30) ° C.
Immediately after hot rolling with the above reduction ratio, 60 to 200 ° C
After cooling to a temperature Tc in the temperature range of 150 to 250 ° C. at a cooling rate of / sec, it is retained in a temperature range of 150 ° C. or more and Tc or less for 2 seconds or more, and wound at a temperature of less than 150 ° C. to obtain a hot rolled steel sheet, Provided is a method for producing an ultra-high-strength electric resistance welded steel pipe, which is characterized in that the hot-rolled steel plate is produced at a width reduction ratio Q satisfying the following formula (1).

1000 ≦ Q / (t / D) ² ≦ 3000 (1) where t (mm) is the thickness of the steel plate, D (mm) is the outer diameter of the electric resistance welded steel pipe, and Q (%) is the width The aperture ratio is defined by the following equation (2). Q = [{width of steel sheet−π (D−t)} / π (D−t)] × 100 (2)

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The ultrahigh-strength electric resistance welded steel pipe of the present invention can be achieved only by controlling the composition and structure of the steel. First Embodiment and Second Embodiment of the Present Invention
Therefore, the embodiment defines heat treatment conditions and pipe forming conditions of a steel sheet having a specific composition, and the third embodiment defines the composition of steel and the structure itself.

Hereinafter, each embodiment will be described in detail. (1) First Embodiment (Chemical composition) C: 0.1 in order to obtain a tensile strength of 980 N / mm ² or more and excellent hydrogen delayed cracking resistance.
0 to 0.19%, Si: 0.01 to 0.5%, Mn:
0.8-2.2%, Al: 0.01-0.06%, N
b: 0.005-0.03%, B: 0.0005-0.
Including 0030%, P: 0.02% or less, S:
0.003% or less, N: 0.005% or less, Ti: 0.
The composition is limited to 015% or less. Also, Cu:
0.05-0.50% is added as a selective ingredient. In that case, Ni may be added, but Ni: 0.1
It is 0% or less.

The reasons for limiting each element will be described below. C: C is an essential element for generating desired martensite and ensuring a target strength. However, if the content is less than 0.10%, the target is 980 N / m.
A strength of m ² or more cannot be obtained, while the content is 0.19%
If it exceeds, the tensile strength becomes too high, or the size of carbides precipitated during tempering becomes large, and the hydrogen delayed cracking resistance deteriorates in any case. Therefore, the content of C is set to 0.10 to 0.19%.

Si: Si is added to secure the soundness of the electric resistance welded portion, and its effect is that the content is 0.01.
~ 0.5%, so the Si content is 0.01
~ 0.5%.

Mn: Mn is an essential element for improving the hardenability of austenite to generate desired martensite and to secure a target strength. But,
Target content of less than 0.8% 980 N / m
A strength of m ² or more cannot be obtained, and on the other hand, if the content exceeds 2.2%, the hydrogen delayed cracking resistance deteriorates. Therefore,
The Mn content is 0.8 to 2.2%.

Al: Al is added as a deoxidizing element,
Further, N existing as an impurity in the steel is fixed as AlN to improve hydrogen delayed cracking resistance. However, the effect of addition is small if it is less than 0.01%, while if it exceeds 0.06%, inclusions increase and the hydrogen delayed cracking resistance deteriorates. Therefore, the content of Al is set to 0.01 to 0.06%.

Nb: Nb is an element that suppresses austenite grain growth during heating in a continuous annealing furnace, refines the martensite structure, and improves hydrogen delayed cracking resistance. The effect of addition is recognized at 0.005% or more, while the effect of addition is saturated even if added over 0.02%. Therefore, the content of Nb is 0.005 to 0.02.
%.

B: B is an element necessary for forming a desired martensite and securing a target strength. However, if the addition amount is less than 0.0005%, the target strength of 980 N / mm ² or more cannot be obtained, while
Even if the added amount exceeds 0.0030%, the effect of addition is saturated. Therefore, the content of B is 0.0005 to 0.00
30%.

P: P deteriorates the delayed fracture resistance, so it is necessary to regulate P to 0.02% or less. S: S is present as inclusions and deteriorates the hydrogen delayed cracking resistance, so it is necessary to regulate S to 0.003% or less.

N: If N is contained in excess of 0.005%, hydrogen delayed cracking resistance is deteriorated, so it is necessary to regulate the content to 0.005% or less. Ti: When Ti is deposited as a coarse nitride, it deteriorates hydrogen delayed cracking characteristics, so it is preferable not to add Ti. However, when solid solution N is fixed as TiN and it is unavoidably added to secure the hardenability of B, the addition amount must be 0.015% or less.

Cu: Cu is an element that suppresses the progress of corrosion of the steel pipe, suppresses the intrusion of hydrogen into the steel pipe, and improves the hydrogen delayed cracking property. The effect of addition is 0.0
It is observed at 5% or more, while the addition effect is saturated even if it is added over 0.50%. Therefore, when Cu is added, its content is set to 0.05 to 0.50%.

FIG. 1 shows the relationship between the amount of Cu added and the amount of change in the crack addition limit additional strain (Δε). From this figure, it is understood that the addition of Cu increases the crack generation limit additional strain (Δε) and suppresses hydrogen delayed cracking.

Ni: Ni is preferably not added because it promotes local corrosion due to casting segregation and deteriorates hydrogen delayed cracking resistance. However, when it is unavoidably added in order to avoid Cu defects during hot rolling, the content is set to 0.10% or less at which the hydrogen-delayed cracking resistance does not significantly decrease.

FIG. 2 shows the relationship between the amount of Ni added and the amount of change in the crack addition limit additional strain (Δε). From this figure, it is understood that the crack addition limit additional strain (Δε) is reduced by the addition of Ni and hydrogen delayed cracking is promoted.

(Production conditions) 1 steel slab having the above chemical composition
After soaking at 150 to 1300 ° C., the slab is hot-rolled at a finishing temperature of Ar ₃ points or more, and then 500
The hot-rolled steel strip is wound at ~ 650 ° C, and the hot-rolled steel sheet is pickled, cold-pressed, uniformly heated to 800-900 ° C in a continuous annealing furnace, then rapidly cooled, and further tempered at 150-250 ° C. ,
The obtained steel sheet is formed into a pipe with a width reduction ratio Q satisfying the following formula (1) to obtain 80 to 100% tempered martensite + the balance ferrite structure.

A. Hot rolling conditions a. Slab heating temperature The slab heating temperature needs to be 1150 ° C or higher in order to form a solid solution with Nb. If the slab heating temperature is less than 1150 ° C, Nb will be sufficient when heating in a continuous annealing furnace.
Since the lute drug effect is not exerted, the martensite structure does not become fine, and the effect of improving the hydrogen delayed cracking resistance property due to the addition of Nb cannot be obtained. On the other hand, from the viewpoint of workability, the upper limit of the slab heating temperature is 1300 ° C.

B. Finishing rolling temperature The finishing rolling temperature must be at least Ar ₃ point. If the finish rolling temperature is below the Ar ₃ point, the ferrite transformation part
Due to the strain-induced precipitation of Nb carbonitride, Nb does not exert a sufficient solute drug effect during heating in a continuous annealing furnace, so the martensite structure does not become fine, and the effect of improving hydrogen-delayed cracking resistance by adding Nb is I can't get it.

C. Winding temperature The winding temperature is 500 to 650 ° C. Winding temperature is 650
When the temperature exceeds ℃, Nb carbides become coarse, do not re-dissolve during heating in a continuous annealing furnace, and do not exert a sufficient solute drug effect, so the martensite structure does not become fine and N
The effect of improving the delayed hydrogen cracking resistance by adding b cannot be obtained. On the other hand, if the winding temperature is lower than 500 ° C, the hot-rolled steel strip becomes hard and becomes a problem in operation.

B. Heat treatment conditions in continuous annealing furnace a. Heating temperature The heating temperature in the continuous annealing furnace is 800 to 900 ° C. If it is less than 800 ° C, a sufficient amount of martensite cannot be obtained after quenching, and the target strength cannot be obtained. on the other hand,
If it exceeds 900 ° C., a fine martensite structure cannot be obtained due to coarsening of austenite grains during heating, and hydrogen delayed cracking resistance deteriorates.

B. Tempering heat treatment conditions Steel strips having 80 to 100% martensite + balance ferrite structure obtained by heating-quenching are 150 to 25
Tempering is performed in the temperature range of 0 ° C. Tempering temperature 150
If it is less than ℃, martensitic transformation strain remains, and the hydrogen cracking resistance after pipe forming decreases. On the other hand, if the tempering temperature exceeds 250 ° C., the cementite phase that precipitates with tempering becomes coarse and the delayed fracture resistance deteriorates.

C. Pipe-making conditions Width reduction in the electric-resistance welding-sizing pipe-making process is an important requirement for improving the hydrogen delayed cracking resistance of steel pipes. For this purpose, the width reduction ratio Q can be calculated by the formula (1). Pipe forming is carried out with control within the range shown.

1000 ≦ Q / (t / D) ² ≦ 3000 (1) where t (mm) is the thickness of the steel plate, D (mm) is the outer diameter of the electric resistance welded steel pipe, and Q (%) is the width The aperture ratio is defined by the following equation (2). Q = [{width of steel sheet −π (D−t)} / π (D−t)) × 100 (2) In FIG. 3, Q / (t / D) ² and hydrogen delayed cracking limit additional strain Δε The relationship of _c is shown. As a result of many experimental studies conducted by the present inventors on pipe forming conditions and hydrogen delayed cracking resistance,
As shown in FIG. 3, the hydrogen delay cracking limit additional strain of the steel pipe has a width drawing ratio Q of 1000 (t / D) ^{2 to} 3000 (t).
It has been found that a steel pipe having a peak between / D) ² and controlling the width reduction ratio in this range can provide a steel pipe having excellent hydrogen delayed cracking resistance. This appropriate width reduction ratio depends on the product (plate thickness / outer diameter) ratio, and in order to obtain a steel pipe with excellent hydrogen delayed cracking resistance, it is necessary to take a different width reduction ratio for each (plate thickness / outer diameter) ratio. There is.

The resistance to hydrogen delayed cracking of the steel pipe depends on the width drawing ratio Q.
The reason why it has a peak between 1000 (t / D) ^{2 and} 3000 (t / D) ² is considered as follows. That is,
If the width reduction ratio is less than 1000 (t / D) ² ,
If the maximum residual strain of the steel pipe increases and the hydrogen delayed cracking resistance of the steel pipe deteriorates, and conversely, if the width reduction ratio exceeds 3000 (t / D) ² , the pipe-forming rolling texture will become As a result, the steel pipe becomes more susceptible to hydrogen delayed cracking and the hydrogen pipe delayed cracking resistance deteriorates.

The critical additional strain Δε _{c for} hydrogen delayed cracking
Is calculated by cutting out a C-ring test piece having a width of 20 mm from the electric resistance welded steel tube, bolting it to the outer diameter before cutting out, applying a strain equivalent to the residual strain of the steel pipe, and further calculating by the following formula (3). 20% in 0.1N hydrochloric acid by adding additional strain (Δε)
It refers to the additional strain at the limit of cracking when it is dipped for 0 hour and checked for cracking. This value is used as an index of hydrogen delayed cracking resistance. That is, the higher this value is, the more preferable it is for the delayed hydrogen cracking resistance.

Δε = (4 · 10 ⁶ · t · δ) / (π · D · (D−t)) (3) where t is the plate thickness, D is the outer diameter of the steel pipe before cutting, δ is D
-(Outer diameter after addition of additional strain).

80% to 100% by the above method
By forming tempered martensite + balance ferrite structure, the tensile strength is 98 which is excellent in hydrogen delayed cracking resistance.
ERW steel pipe of 0 N / mm ² or more is manufactured.

(2) Second Embodiment (Chemical composition) In order to obtain excellent hydrogen delayed cracking characteristics with a tensile strength of 980 N / mm ² or more, in a weight percentage,
C: 0.10 to 0.19%, Si: 0.01 to 0.5
%, Mn: 0.8 to 2.2%, Al: 0.01 to 0.0
6%, Cr: 0.05-0.6%, P: 0.0
The composition is limited to 2% or less, S: 0.003% or less, and N: 0.005% or less. Also, Nb: 0.005
-0.03%, V: 0.005-0.03%, at least one kind, B: 0.0005-0.0030%, C
u: 0.05 to 0.50% is added as a selective component. Further, Ni may be added when Cu is added, but Ni: 0.30% or less.

The reasons for limiting each element will be described below. The reason for limiting C, Si, Mn, and Al is the same as in the first embodiment. Cr: An element for enhancing the hardenability of steel by the interaction with Mn and ensuring the target strength. If its content is less than 0.05%, its effect is poor, while 0.
If it exceeds 6%, the hydrogen delayed cracking resistance is deteriorated. Therefore, the content of Cr is set to 0.05 to 0.6%.

P, S and N are limited to the above range for the same reason as in the first embodiment. Nb and V: Nb and V are both preferable elements for improving hydrogen-delayed cracking resistance because they can refine austenite grains before transformation and refine martensite packets after transformation. However, 0.005% each
If it is less than 0.03%, the effect is small. On the other hand, if it exceeds 0.03%, the hydrogen-delayed cracking resistance is rather deteriorated. Therefore, the contents of Nb and V are each 0.005
It is set to 0.03%.

B: B is added as necessary in order to form a desired martensite and to secure a target strength. However, if the added amount is less than 0.0005%, the target strength of 980 N / mm ² or more cannot be obtained, while if the added amount exceeds 0.0030%, the effect of addition is saturated. Therefore, when adding the content of B, 0.0
It is 005 to 0.0030%.

When Cu is added for the same reason as in the first embodiment, the range is 0.05 to 0.50%. Increasing the amount of Cu may cause surface defects called Cu defects in some cases, which can be prevented by adding Ni. However, since Ni is an element harmful to hydrogen delayed cracking characteristics, Add an amount of 0.
It is preferably limited to 3% or less.

(Manufacturing conditions) For the steel slab having the above composition,
When the temperature of the Ar ₃ transformation point of the steel is TAr ₃ , the finishing temperature Tf is (TAr ₃ +30) to (TAr ₃ +10).
The finishing temperature Tf is controlled so that it falls within the temperature range of 0) ° C., hot rolling is performed, and during the hot rolling, Tf to (Tf
In the temperature range of +30) ° C., a reduction rate of 30% or more is applied, and immediately after hot rolling, at a cooling rate of 60 to 200 ° C./sec, 1
After cooling to a temperature Tc in the temperature range of 50 to 250 ° C.,
Let it stay in the temperature range of 150 ° C or higher and Tc or lower for 2 seconds or longer,
The hot-rolled steel sheet is wound at a temperature of less than 150 ° C., and the hot-rolled steel sheet is pipe-formed at a width reduction ratio Q satisfying the above formula (1).

A. Hot rolling conditions a. Finishing temperature Finishing temperature Tf is (TAr ₃ +30) to (TAr ₃ +1)
The temperature range is 00) ° C. The finishing temperature is (TAr ₃ +3
If it is less than 0) ° C., the volume ratio of martensite for obtaining the strength of 980 N / mm ² or more cannot be obtained. on the other hand,
If it exceeds (TAr ₃ +100) ° C., the martensite packet becomes coarse and the hydrogen delayed cracking resistance deteriorates.

B. Rolling condition In order to make martensite fine and to improve hydrogen delayed cracking resistance, strong rolling just before the end of hot rolling is necessary. Therefore, hot rolling is performed in a temperature range of Tf to (Tf + 30) ° C. with a reduction rate of 30% or more.

B. Cooling condition after hot rolling Immediately after hot rolling, the material is rapidly cooled to Tc in a temperature range of 150 to 250 ° C at a cooling rate of 60 to 200 ° C / sec. As a result, the martensite volume ratio for obtaining the strength of 980 N / mm ² or more can be secured. Cooling rate is 6
If it is less than 0 ° C./sec, martensite having a desired volume ratio cannot be obtained. The cooling rate is 200 ℃ / s
If it exceeds ec, operational troubles occur. If the cooling stop temperature is higher than 250 ° C, martensite having a desired volume ratio cannot be obtained.

After quenching in this way, the temperature of 150 ° C. or higher T
Let it stay in the temperature range of c or lower for 2 seconds or more. This allows
Hard tempered martensite is produced. FIG. 4 shows the relationship between the holding time when the quenched steel plate is held in the temperature range of 150 to 250 ° C. and the hydrogen delayed cracking limit additional strain Δε. From this figure, it is understood that a high hydrogen delayed cracking limit additional strain Δε _c close to 2000 μm can be stably obtained by holding for 2 seconds or more. If it is less than 2 seconds, quenching strain remains, so high Δε _{c of} 1900 μm or more
Can't get stable.

C. Winding temperature Winding is performed at a temperature lower than 150 ° C. This temperature is 150 ℃
With the above, a hard tempered martensite phase is not formed, and 9
A strength of 80 N / mm ² or more cannot be obtained.

D. Pipe-making conditions A hot-rolled steel sheet manufactured under the above conditions is used to make an ultra-high-strength electric resistance welded steel pipe. At that time, the above formula (1) must be satisfied, as in the first embodiment. There is.

(3) Third Embodiment (Chemical composition and structure) In order to obtain excellent hydrogen delayed cracking resistance and corrosion resistance with a tensile strength of 980 N / mm ² or more, C: 0.13 to 0.19. %, Mn: 1.0-
80% to 100% obtained by quenching heat treatment, having a composition containing 2.0% and Cu: 0.05 to 0.50%
Of martensite or tempered martensite. When Ni and Mo are added, Ni: 0.
It is limited to 1% or less and Mo: 0.3% or less.

The reasons for limiting each element will be described below. C: C is an essential element for generating desired martensite and ensuring a target strength. However, if the content is less than 0.13%, the target is 1180 N /
A strength of mm ² or more cannot be obtained, while the content is 0.19
If it exceeds%, the strength of the pipe body is deteriorated due to hydrogen delayed cracking or corrosion, and the durability is deteriorated. Therefore C
Content of 0.13 to 0.19%.

Mn: Mn is an essential element for forming a desired martensite and securing a target strength. However, if the content is less than 1.0%, the target strength of 1180 N / mm ² or more cannot be obtained, while on the other hand, hydrogen resistance delayed cracking with the content exceeding 2.0% or the corrosion characteristics deteriorate. . Therefore, the Mn content is 1.0 to
2.0%.

Cu: Cu is an element that lowers the hydrogen delayed cracking susceptibility of the steel pipe, further suppresses the progress of deterioration of the strength of the pipe body due to corrosion, and improves the durability of the ultra-high-strength electric resistance welded steel pipe. The effect of addition is recognized at 0.05% or more, while the effect of addition is saturated even if added over 0.50%. Therefore, when Cu is added, its content should be 0.0
5 to 0.50%.

FIG. 5 shows the relationship between the added amount of Cu and the residual strength rate after the corrosion test. From this figure, it is understood that the addition of Cu increases the residual strength rate and increases the durability of the steel pipe. The residual strength rate can be expressed by the following formula.

Residual strength rate (%) = {TS after immersion test
(N / mm ₂ ) / TS before immersion test (N / mm ₂ )} ×
100 TS before immersion test (N / mm ₂ ) = Tensile breaking load before immersion test (N) / Pipe cross-sectional area before immersion test (mm ² ) TS after immersion test (N / mm ₂ ) = It is the tensile breaking load (N) after the immersion test / tube cross-sectional area (mm ² ) before the immersion test.

Ni: Ni is preferably not added because it promotes local corrosion due to cast segregation and deteriorates hydrogen delayed cracking resistance. However, when it is unavoidably added to avoid Cu defects during hot rolling, the content is set to 0.10% or less at which the reduction of the residual strength rate is not remarkable.

Mo: Mo is desirable not to be added because it promotes local corrosion due to casting segregation and deteriorates hydrogen delayed cracking resistance. However, when it is unavoidably added in order to secure the hardenability, the content is set to 0.30% or less at which the reduction of the residual strength rate is not remarkable.

FIG. 6 shows the relationship between the added amount of Ni and the residual strength after the corrosion test, and FIG. 7 shows the relationship between the added amount of Mo and the residual strength rate after the corrosion test. From these figures, it is understood that the addition of 0.1% or less of Ni and 0.3% or less of Mo reduces the residual strength ratio and reduces the durability of the steel pipe.

Elements other than these do not particularly affect the durability of the steel pipe, that is, the hydrogen delayed cracking resistance and the corrosion resistance, and therefore Si, P, Al, Nb,
It is permissible to appropriately add alloying addition elements such as B, Ti and Cr in usual amounts according to other purposes.

The steel having the above composition is quenched and heat treated to obtain a martensite or tempered martensite structure of 80 to 100%. With the composition and structure as described above, an ultrahigh-strength electric resistance welded steel pipe having a tensile strength of 980 N / mm ² or more and excellent durability, that is, hydrogen delayed cracking resistance and corrosion resistance can be obtained.

(Manufacturing Conditions) When manufacturing the electric resistance welded steel pipe according to the third embodiment, a quenching heat treatment is applied to obtain 80
As long as a martensite or tempered martensite structure of ˜100% is obtained, the manufacturing method thereof is not limited, and the manufacturing conditions of the first and second embodiments can be used.

[0063]

EXAMPLES Examples of the present invention will be described below. (Example 1) Six kinds of steels A to F shown in Table 1 were melted, and as shown in Table 2, hot rolling conditions, heat treatment conditions in a continuous annealing furnace, and pipe forming conditions specified in the present invention were used. 0.8 mmφ x
A 1.6 mmt electric resistance welded steel pipe was produced.

The tensile strength and the maximum load of three-point bending of these steel pipes were measured, and a hydrogen resistance delayed cracking test was carried out. The three-point bending test was conducted with a pressing metal fitting radius of 152 mm and a supporting span of 600 mm. In the hydrogen-resistant delayed cracking test, a C-ring test piece with a width of 20 mm is cut out from a steel pipe, bolted to the outer diameter before cutting, a strain equivalent to the residual strain of the steel pipe is added, and then calculated by the above formula (3). The additional strain (Δε) to be added was added and the sample was dipped in 0.1N hydrochloric acid for 200 hours to check for cracks. The critical strain for crack initiation was used as an index for hydrogen delayed cracking resistance. The results are shown in Table 3.

[0065]

[Table 1]

[0066]

[Table 2]

[0067]

[Table 3]

As can be understood from Table 3, Steels A to E satisfying the composition specified in the present invention have higher critical strain for crack initiation and higher hydrogen delayed cracking resistance than Comparative Steel F. confirmed.

(Example 2) Using the above-mentioned steels A to E, various hot rolling conditions, heat treatment conditions in a continuous annealing furnace, pipe forming conditions and (sheet thickness / outer diameter) ratio as shown in Table 4 were changed. Made into ERW steel pipe. Table 5 shows the mechanical properties and the results of hydrogen delayed cracking test.

[0070]

[Table 4]

[0071]

[Table 5]

As can be seen from Table 5, the electric resistance welded steel pipes of the examples in which the hot rolling conditions, the heat treatment conditions in the continuous annealing furnace, and the pipe forming conditions satisfy the conditions specified in the present invention have a tensile strength of 980 N / It was confirmed that the crack resistance was higher than that of mm ² and the crack initiation critical strain was high, and that it had excellent hydrogen delayed cracking resistance.

Example 3 Six kinds of steels G to L shown in Table 6 were melted, and as shown in Table 7, 34.8 mmφ × 2 under the hot rolling conditions and pipe forming conditions specified in the present invention. A 3 mmt electric resistance welded steel pipe was produced. Then, the hydrogen delayed crack initiation limit additional strain Δε _c , which is an index of the tensile strength and hydrogen cracking resistance of these steel pipes, was measured. The results are shown in Table 8.

[0074]

[Table 6]

[0075]

[Table 7]

[0076]

[Table 8]

As shown in Table 8, all of the steels G to J satisfying the composition specified in the present invention have a strength of 980 N / mm ² or more and a high hydrogen delayed cracking limit additional strain Δε of 1900 μm or more. _c was stably obtained. Also,
Structurally, it was 100% tempered martensite as shown in Table 7. On the other hand, steel L whose C content is out of the range defined by the present invention has no problem in strength, but it is confirmed that the hydrogen delayed cracking limit critical additional strain Δε _c is extremely low and the hydrogen delayed cracking resistance is inferior. .

(Example 4) Steels G to L in Table 6 are used in Table 9
As shown in Fig. 4, electric resistance welded steel sheets were produced by variously changing hot rolling conditions and pipe forming conditions, and hydrogen delayed cracking limit additional strain Δ which is an index of tensile strength and hydrogen cracking resistance of these steel pipes was obtained.
ε _c was measured. The results are shown in Table 10.

[0079]

[Table 9]

[0080]

[Table 10]

As shown in Table 10, the electric resistance welded steel pipe whose hot rolling condition and pipe forming condition are within the scope of the present invention has a tensile strength of 980.
A high hydrogen cracking limit strain Δε _{c of} N / mm ₂ and 1900 μm or more can be stably obtained. Further, as shown in Table 9, the structure was a composite structure composed of 80% or more of tempered martensite and ferrite. On the other hand, in the case where the heat treatment condition and the pipe forming condition were out of the range of the present invention, the tensile strength was insufficient, the hydrogen delay cracking limit critical additional strain Δε _c was as low as 950 μm, and a stable Δε _c value could not be obtained. It was

(Embodiment 5) Seven kinds of steels M to S shown in Table 11 were melted and manufactured by the method shown in Table 12 to obtain 31.8 mmφ × 1.
A 6 mmt electric resistance welded steel pipe was produced. These steel pipes 0.1
It was immersed in N hydrochloric acid for 200 hours, a tensile test was performed before and after the immersion, and the residual strength ratio was determined and used as an index of durability. The residual strength rate (%) was determined by the method described above. The results are shown in Table 1.
3 shows.

[0083]

[Table 11]

[0084]

[Table 12]

[0085]

[Table 13]

As is clear from Table 13, the electric resistance welded steel pipes of the invention examples satisfying the conditions defined in the present invention in the steel composition and structure have a tensile strength of 1180 N / mm ₂ or more and a high residual strength ratio. It was confirmed that it has excellent durability.

[0087]

As described above, according to the present invention,
Tensile strength 980 N / m used for automobile parts such as door impact beam, mechanical structural members, civil engineering and building materials
A structural ultrahigh-strength electric resistance welded steel pipe excellent in hydrogen-delayed cracking resistance of m ² or more can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Cu addition amount and a crack generation limit additional strain change amount.

FIG. 2 is a diagram showing the relationship between the amount of Ni added and the amount of change in additional strain that causes cracking.

FIG. 3 is a diagram showing the relationship between Q / (t / D) ² and hydrogen delay cracking limit additional strain.

FIG. 4 is a diagram showing a relationship between a holding time in a temperature range of 150 to 250 ° C. and a hydrogen delayed cracking limit additional strain Δε _c .

FIG. 5 is a diagram showing a relationship between a Cu addition amount and a residual strength rate after a corrosion test.

FIG. 6 is a diagram showing the relationship between the amount of Ni added and the residual strength rate after a corrosion test.

FIG. 7 is a diagram showing the relationship between the amount of Mo added and the residual strength rate after a corrosion test.

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Claims (5)

(57) [Claims] 1. C: 0.10 to 0.19% by weight,
Si: 0.01 to 0.5%, Mn: 0.8 to 2.2%,
Al: 0.01 to 0.06%, Cr: 0.05 to 0.6
%, P: 0.02% or less, S: 0.003% or less, N:
0.005% or less, consisting of balance Fe and unavoidable impurities
To steel slab, the temperature of the Ar ₃ transformation point of the steel TA that
When r ₃ is set, the finishing temperature Tf is (TAr ₃ +30)
To (TAr ₃ +100) ° C., the finishing temperature Tf is controlled to carry out hot rolling, and during the hot rolling, reduction of 30% or more in the temperature range of Tf to (Tf + 30) ° C. Rate, 60 ~ 200 ℃ / s immediately after hot rolling
temperature T in the temperature range of 150 to 250 ° C. at a cooling rate of ec
After cooling to c, it is retained in a temperature range of 150 ° C. or higher and Tc or lower for 2 seconds or longer, and wound at a temperature of lower than 150 ° C. to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet satisfies the formula (1) below. A method for manufacturing an ultra-high-strength electric resistance welded steel pipe, which is characterized in that the pipe is produced at a rate Q. 1000 ≦ Q / (t / D) ² ≦ 3000 (1) where t (mm) is the thickness of the steel plate, D (mm) is the outer diameter of the electric resistance welded steel pipe, and Q (%) is the width reduction ratio. , Is defined by the following equation (2). Q = [{width of steel sheet−π (D−t)} / π (D−t)] × 100 (2) 2. Nb: 0.005 by weight%
At least 1 sort (s) is contained among 0.03% and V: 0.005-0.03%, The manufacturing method of the ultrahigh-strength electric resistance welded steel pipe of Claim 1 characterized by the above-mentioned. 3. Further, B: 0.0005 to 5% by weight.
The composition according to claim 1, which contains 0.0030%.
Alternatively, the method for manufacturing an ultra-high-strength electric resistance welded steel pipe according to claim 2 . 4. Further, in weight%, Cu: 0.05-.
0.50% is contained, It is characterized by the above-mentioned.
The method for manufacturing an ultra-high tensile electric resistance welded steel pipe according to claim 3 . 5. The method for producing an ultra-high-strength electric resistance welded steel pipe according to claim 4 , further comprising Ni: 0.3% by weight or less.