JP2002220634A - High tension steel superior in resistance to stress aging, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high tension steel superior in resistance to stress aging, with a tensile strength of 600 MPa or more. SOLUTION: The steel comprises including 0.01-0.10% C, 0.30% or less Si, 1.00-2.50% Mn, 0.005% or less S, 0.010% or less P, 0.005-0.06% Nb, 0.004-0.025% Ti, 0.05% or less sol.Al, 0.0050% or less N, 0.003% or less O, 0-1.5% Cu, 0-2.5% Ni, 0-0.80% Mo, 0-1.0% Cr, 0-0.1% V 0-0.03% Zr, 0-0.0030% Ca, and 0-0.002% B, and having a value A defined in an expression 'A=50N+C+0.3Si+10(P+O)' of 0.44 or less, and a value B defined in an expression 'B=(50N+C+100)/(4Nb+10Ti+2sol.Al)' of 1.6 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a steel sheet having a tensile strength of 600.
High-strength steel material excellent in strain aging resistance of not less than MPa, preferably not less than 760 MPa, and more particularly suitable for use in manufacturing welded structures such as line pipes and various pressure vessels for transporting natural gas and crude oil. The present invention relates to various steel materials (steel plates, steel pipes, steel bars, steel bars, etc.) and methods for producing the same.

[0002]

2. Description of the Related Art In pipelines for transporting natural gas and crude oil over long distances, reduction of transportation costs is a universal need, and there is a need to improve transportation efficiency by increasing operating pressure. In order to increase the operating pressure, a method of increasing the wall thickness of a conventional strength grade line pipe can be considered. However, this method has a problem that the welding efficiency at the site is reduced and the efficiency of the welding is reduced due to an increase in the mass of the structure.

On the other hand, there is an increasing need to increase the strength of the linepipe material itself to limit the increase in wall thickness. At present, the API (American Petroleum Institute) uses X80 grade (tensile strength of 620 MPa or more) steel. It has been standardized and put to practical use.

Further, high-strength line pipe materials are disclosed in JP-A-8-199292 and JP-A-8-269.
No. 546, X100 grade (tensile strength of 760 MPa or more) with a high Mn content
There is super-grade high-strength steel.

[0005] However, UOE, which is the mainstay of line pipe,
For large-diameter welded steel pipes represented by steel pipes and medium- and small-diameter welded steel pipes represented by ERW steel pipes, the forming of a steel sheet into a tube at room temperature is performed at room temperature, that is, by cold working, which causes so-called processing distortion. Even if a high-performance material (steel) is used as the material steel plate, the properties of the material steel plate may not be maintained in the welded steel pipe of the product.

Further, even in a steel material that cannot be subjected to cold working, distortion such as thermal distortion generated during welding or heat treatment and distortion generated during long-term use as a structural member may be partially accumulated. is there.

[0007] Such distortion is a small amount of distortion of 5% or less, but when the distorted steel material is used for a longer period of time, it is necessary to pay attention to deterioration of the steel material due to strain aging.

The strain aging occurs when the steel material is used at room temperature for a long period of time, but is accelerated by heating the steel material to 100 to 250 ° C. for a coating treatment or the like.

[0009] It is known that the deterioration of steel due to strain aging occurs to some extent even in steels having a tensile strength of 392 MPa or more and 490 MPa or more, which are conventionally used frequently. As a result, it has been confirmed that there is almost no practical problem.

On the other hand, in the high-strength steels having a tensile strength of 620 MPa or more and the above-mentioned X80 grade steel grade or more, at present, no systematic investigation has been made on the deterioration of steel materials due to strain aging. In particular, it is necessary to take measures against work hardening characteristics, such as a decrease in uniform elongation during a tensile test, as well as a decrease in toughness such as Charpy characteristics.

However, it is generally considered that the higher the strength of the metal material, the lower the elongation and ductility, particularly the uniform elongation characteristics, and therefore, it is necessary to deteriorate the work hardening characteristics due to the strain aging treatment.

As a result of studying various welded structural steel materials having a tensile strength of 600 MPa or more, the present inventor has found that many welded structural steel materials have been subjected to cold working with a strain amount of 3% and held at 250 ° C. for one hour. It has been found that the toughness and the work hardening characteristics deteriorate after the aging treatment by air cooling. In particular, with respect to uniform elongation, a remarkable decrease may occur, and the safety as a structural member may be impaired. However, in response to these problems of high-strength steel, technology that improves strain aging resistance and enhances safety as a structural material has
It was almost nothing.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a strain aging resistance and a weld toughness having a tensile strength of 600 MPa or more, preferably 760 MPa or more. High tensile strength steel material, more specifically, has good toughness and weldability, and also has excellent toughness in the welded portion. For example, the uniform elongation after 3% strain aging treatment is one It is used as a material for welded structures such as welded steel pipes having excellent work hardening characteristics and low temperature toughness even after strain aging treatment such as 0.45 times or more, more preferably 0.6 times or more of the same elongation. It is an object of the present invention to provide a high-strength steel material represented by a suitable steel plate and a method for producing the same.

[0014]

In order to achieve the above-mentioned object, the present inventor has developed a method for increasing the toughness and weldability of a high-strength steel for a welded structure having a tensile strength of 600 MPa or more, preferably 760 MPa or more. As a result of various investigations on the improvement of the strain and the improvement of the strain aging resistance, the following was found. (A) N and C contents which directly cause strain aging, and Si, P and O which promote strain aging of high strength steel
The content of (oxygen) is limited, specifically, the formula “A = 50N +
C + 0.3Si + 10 (P + O) (where each element symbol means the content (% by mass) of each element contained in the steel) "so that the A value is 0.44 or less. Is adjusted within the range described below, the low-temperature toughness and uniform elongation after strain aging are improved. (B) Nb, Ti and Al have a tensile strength of 600 MPa
It has the effect of reducing the strain aging characteristics of the high-strength steel described above, and at least three elements of Nb, Ti, and Al are added as essential components, and the formula "B = (50N + C + 10
O) / (4Nb + 10Ti + 2sol.Al) (where each element symbol means the content (% by mass) of each element contained in steel) so that the B value becomes 1.6 or less. If the content of each element is adjusted within the range described later, the aging characteristic sensitivity is reduced, and the deterioration due to strain aging is suppressed. (C) When both of the above formulas are satisfied within the composition range described later, the weldability and toughness and the toughness of the weld are excellent, and 3% regardless of the manufacturing conditions.
As a material for welded structures such as welded steel pipes with excellent work hardening characteristics and good low temperature toughness, the uniform elongation after strain aging treatment is 0.45 times or more the uniform elongation before strain aging treatment A high-strength steel material typified by a suitable steel plate can be obtained. (D) In addition, the above-mentioned high-strength steel material represented by a steel plate is obtained by heating a material steel to 950 to 1200 ° C., and then finishing at a temperature of 850.
Hot rolling is completed at ~ 650 ° C and accelerated cooling at a cooling rate of 4 ° C / sec or more from a temperature range not lower than 500 ° C to 300 ° C or less, so that uniformity after the above 3% strain aging treatment is achieved. It is possible to stably produce a product having an excellent work hardening property in which the elongation is 0.6 times or more the uniform elongation before the strain aging treatment.

The gist of the present invention completed on the basis of the above findings is as follows: (1) and (2) a high-strength steel material excellent in strain aging resistance; A method for producing a high-strength welded steel pipe excellent in strain aging resistance (4) and a method for producing a high-strength welded steel pipe (5) below. (1) Chemical composition in mass%, C: 0.01 to 0.10
%, Si: 0.30% or less, Mn: 1.00 to 2.50
%, P: 0.010% or less, S: 0.005% or less, N
b: 0.005 to 0.06%, Ti: 0.004 to 0.
025%, sol. Al: 0.05% or less, N: 0.0
050% or less, O (oxygen): 0.003% or less, Cu: 0
-1.5%, Ni: 0-2.5%, Mo: 0-0.80
%, Cr: 0 to 1.0%, V: 0 to 0.1%, Zr: 0
~ 0.03%, Ca: 0 ~ 0.0030%, B: 0 ~
0.002%, with the balance being substantially Fe,
It is made of steel having an A value defined by the formula (1) of 0.44 or less and a B value defined by the formula (2) of 1.6 or less, and having a tensile strength of 60 or less.
High-strength steel with excellent strain aging resistance of 0 MPa or more.

A = 50N + C + 0.3Si + 10 (P + O) (1) B = (50N + C + 10O) / (4Nb + 10Ti + 2sol.Al) (2) Here, equation (1) and ( 2) The symbol of the element in the formula means the content (% by mass) of each element contained in the steel. (2) The high-strength steel material according to (1), wherein the steel material is a steel plate. (3) In mass%, C: 0.01 to 0.10%, Si:
0.30% or less, Mn: 1.00 to 2.50%, P:
0.010% or less, S: 0.005% or less, Nb: 0.
005 to 0.06%, Ti: 0.004 to 0.025
%, Sol. Al: 0.05% or less, N: 0.0050
%, O (oxygen): 0.003% or less, Cu: 0-1.
5%, Ni: 0 to 2.5%, Mo: 0 to 0.80%, C
r: 0 to 1.0%, V: 0 to 0.1%, Zr: 0 to 0.
03%, Ca: 0 to 0.0030%, B: 0 to 0.00
2%, the balance being substantially composed of Fe, and the following (1)
The A value defined by the formula is 0.44 or less, and the B value defined by the formula (2) is 1.6 or less. After the steel is heated to 950 to 1200 ° C., hot rolling is performed, and the finishing temperature is 850 to 850 ° C. Rolling is completed at 650 ° C.
A method for producing a high-strength steel material excellent in strain aging resistance having a tensile strength of 600 MPa or more, which is accelerated to a temperature of 4 ° C. or lower at a cooling rate of 4 ° C./sec or more.

A = 50N + C + 0.3Si + 10 (P + O) (1) B = (50N + C + 10O) / (4Nb + 10Ti + 2sol.Al) (2) where (1) and (2) 2) The symbol of the element in the formula means the content (% by mass) of each element contained in the steel. (4) A high-strength welded steel pipe in which a base material portion is the high-strength steel sheet according to (2) above, which has excellent strain aging resistance. (5) A method for producing a high-strength welded steel pipe excellent in strain aging resistance, in which the high-strength steel sheet according to (2) is formed into a tubular shape in a temperature range of 600 ° C or lower and the butted portions are welded and joined.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the reasons for defining the present invention as described above will be described in detail. In the following description, “%” means “% by mass” unless otherwise specified.

First, the chemical composition of steel will be described.

C: 0.01 to 0.10% C is contained for the purpose of ensuring the strength of the base material.
If it is less than 01%, the hardenability is insufficient and it is difficult to secure a tensile strength of 600 MPa or more, and the toughness of the base material is not sufficient. On the other hand, when the content exceeds 0.10%, not only the toughness and weldability of the base material, but also the toughness of the heat affected zone of the base metal are reduced, and the strain aging resistance is deteriorated. Therefore,
The C content was 0.01 to 0.10%. A preferred range is 0.01 to 0.06%.

Si: 0.30% or less Si is usually added as a deoxidizing agent, but if its content exceeds 0.30%, not only will the toughness of the base metal and its weld be reduced, but also The strain aging resistance is also reduced. Therefore, the Si content is set to 0.30% or less. A preferred upper limit is 0.15%, and a more preferred upper limit is 0.10%.
It is.

Mn: 1.00 to 2.50% Mn is added to improve the hardenability of the steel and to increase the strength. If the content is less than 1.00%, the desired strength is reduced. It is difficult to secure. On the other hand, 2.50
%, The toughness of both the base metal and the welded portion is reduced. Therefore, the Mn content is 1.00-2.
50%. In addition, from the viewpoint of improving the toughness of the welded portion, it is desirable that Mn is small, and the preferable range of the Mn content is 1.00 to 1.90%, and the more preferable range is 1.
It is 00 to 1.70%.

P: not more than 0.010% P is an impurity element, which not only impairs the low-temperature toughness of the base material and its heat affected zone, but also reduces the weldability.
Further, the strain aging resistance is also reduced. Therefore, the lower the P content, the better. However, an excessive reduction leads to an increase in cost, and there is no particular problem up to 0.010%. Therefore, the upper limit is set to 0.010%. A preferred upper limit is 0.005%.

S: not more than 0.005% S is an impurity element similar to that of P described above, and not only impairs the low-temperature toughness of the base metal and its heat affected zone but also lowers the weldability. Therefore, the S content is above the above P
Similarly to the above, the lower the better, the better. However, an excessive reduction leads to an increase in cost, and there is no particular problem up to 0.005%, so the upper limit was made 0.005%. A preferred upper limit is 0.002%.

Nb: 0.005 to 0.06% Nb not only refines the microstructure of the base material to greatly improve the toughness of the high-strength steel, but also C, an element harmful to strain aging resistance, It has the effect of stabilizing these elements by bonding with N and improving the strain aging resistance. However, if the content is less than 0.005%, the above effects cannot be obtained. Conversely, if the content exceeds 0.06%, not only the weldability of the base material is impaired, but also the toughness and strain aging resistance are reduced. Instead, lower it. Therefore, the Nb content is 0.005 to 0.06.
%. A preferred range is 0.005 to 0.03%, and a more preferred range is 0.005 to 0.02%.

Ti: 0.004% to 0.025% Ti combines with elements C and N which are harmful to strain aging resistance, stabilizes these elements, and greatly improves strain aging resistance. Rather, the microstructure of the base metal and its heat affected zone is refined to improve the low temperature toughness of the high strength steel base metal and its heat affected zone. However, if the content is less than 0.004%, the above effects cannot be obtained. Conversely, if the content exceeds 0.025%, not only the strain aging resistance is impaired, but also the weldability and toughness are lowered. Let it. Therefore, the Ti content is 0.004 to 0.025%
And A preferred range is 0.004 to 0.015%.

Sol. Al: 0.05% or less Al is added as a deoxidizing agent, but has an effect of stabilizing by combining with N which is an element harmful to strain aging resistance, and significantly improving strain aging resistance. The effect of improving the strain aging resistance by Al is very small. Al content is obtained, but its effect is 0.002% or more of sol. Al
It becomes remarkable in the content. However, the content of sol. A
If the l content exceeds 0.05%, not only the toughness of the welded portion is deteriorated, but also the strain aging resistance and the weldability are rather reduced. Therefore, sol. The Al content was 0.05% or less. A preferred upper limit is 0.03%.

N: 0.0050% or less N is an impurity element which is extremely harmful to strain aging resistance. If its content exceeds 0.0050%, the toughness of the base metal and its welded portion is significantly reduced. In addition, even if other measures are taken to improve strain aging resistance, good strain aging resistance cannot be obtained. Therefore, the N content is 0.0050
% Or less. Note that the lower the N content, the more desirable, and a preferable upper limit is 0.0030%.

O (oxygen): 0.003% or less O, like N, is an extremely harmful impurity element for strain aging resistance, and when its content exceeds 0.003%, Further, not only does the toughness of the welded portion significantly decrease, but even if other measures are taken to improve the strain aging resistance, good strain aging resistance cannot be obtained. Therefore, the O content is set to 0.003% or less. The lower the O content is, the more desirable it is. A preferable upper limit is 0.0020%, and a more preferable upper limit is 0.0015%.

The following elements need not be added, but if they are added and contained within the following ranges, the strength, toughness, corrosion resistance, etc. of the base metal and the weld heat-affected zone can be improved without impairing the strain aging resistance. Can be improved. For this reason, when manufacturing a thicker steel plate or a high-strength high-strength steel material, one or more of these elements may be added as necessary.

Cu: Cu has an effect of improving hardenability and toughening a base material without significantly impairing weldability.
The effect becomes remarkable at a content of 0.1% or more. However, when the content exceeds 1.5%, not only the toughness of the base material and the welded portion thereof is impaired, but also the hot ductility may be significantly reduced. Therefore, when added, the Cu content is preferably set to 0.1 to 1.5%.

Ni: Ni has an effect of improving low-temperature toughness, brittle crack propagation stopping performance and weldability of high-strength steel,
The effect becomes remarkable at a content of 0.20% or more. However, when the content exceeds 2.5%, excessive quenching-tempering generates excessive retained austenite, and the yield strength may decrease. Therefore, when N is added,
The i content is preferably set to 0.20 to 2.5%.

Cr: Cr has an effect of improving hardenability and improving strength and toughness by precipitation strengthening during tempering, and the effect becomes remarkable at a content of 0.10% or more. However, if the content exceeds 1.0%, the strength is excessively increased, and the toughness of the base material and its welded part is impaired. Therefore, when Cr is added, the content of Cr is 0.1.
The content is preferably set to 10 to 1.0%.

Mo: Mo not only improves hardenability but also improves strength and toughness by solid solution strengthening.
At the time of addition with Nb, it has the effect of promoting the refinement of the structure and dispersing an appropriate amount of retained austenite in the steel to improve the resistance to strain embrittlement.
% Or more becomes significant. However, when the content exceeds 0.80%, the strength is excessively increased, and the toughness of the base material and the welded portion thereof is impaired. Therefore, when Mo is added,
The content is preferably 0.10 to 0.80%.

V: V has the effect of improving hardenability, improving strength and toughness, stabilizing elements harmful to strain aging resistance, and also improving strain aging resistance. Becomes remarkable at a content of 0.005% or more. However, when the content exceeds 0.1%, the strength is excessively increased, and the toughness of the base material and the welded portion thereof is impaired. For this reason,
When added, the V content is preferably 0.005 to 0.1%.

Ca: Ca controls the form of inclusions in the steel, improves the toughness and corrosion resistance of the base metal and its welds, and stabilizes the elements harmful to the strain aging resistance to thereby prevent the strain aging. Has the effect of improving the
It becomes significant at a content of 05%. However, 0.0030%
When the content exceeds the above range, not only the cleanliness of the steel decreases, but also the toughness of the base metal and the welded portion thereof decreases, as well as the strain aging resistance. For this reason, the Ca content when added is preferably 0.0005 to 0.0030%.

Zr: Zr controls the morphology of inclusions in the steel, improves the toughness and corrosion resistance of the base metal and its welds, and also stabilizes elements harmful to the strain aging resistance and strain aging resistance. It also has the effect of improving
It becomes significant at a content of 5%. However, if the content exceeds 0.03%, the cleanliness of the steel decreases, and not only the toughness of the base metal and the welded portion thereof, but also the strain aging resistance decreases. For this reason, when added, the Zr content is 0.1.
005-0.03% is good.

B: B has an effect of improving hardenability and improving strength and toughness, and its effect is 0.0003.
% Or more becomes significant. However, when the content exceeds 0.003%, the strength is excessively increased, and the toughness of the base material and the welded portion thereof is impaired. Therefore, when B is added,
The content is preferably set to 0.0003 to 0.003%.

Relationship between C, Si, P, Nb, Ti, Al, N and O: The content of these elements is within the above range, and the A value defined by the following equation (1) is 0.44. Less than,
(2) The content must be such that the B value defined by the equation is 1.6 or less. That is, when the A value exceeds 0.44 or the B value exceeds 1.6, the strain aging sensitivity increases, and the desired strain aging resistance cannot be secured. This is clear from the results of the examples described later. The preferred upper limit of the A value is 0.35, and the preferred upper limit of the B value is 0.80.

A = 50N + C + 0.3Si + 10 (P + O) (1) B = (50N + C + 10O) / (4Nb + 10Ti + 2sol.Al) (2) Here, the equation (1) and ( 2) The symbol of the element in the formula means the content (% by mass) of each element contained in the steel.

Next, the manufacturing method will be described.

Heating temperature of raw steel: 1200 ° C.
If it exceeds, after the subsequent hot rolling, elements harmful to strain aging cannot be stabilized, and it becomes difficult to secure better strain aging resistance, and at the same time, to secure good low-temperature toughness as a structural steel material It becomes difficult. If the temperature is lower than 950 ° C., it is difficult to stably secure the target tensile strength of 600 MPa or more. For this reason, the heating temperature of the raw steel is desirably 950 to 1200 ° C.

Finishing temperature of hot rolling: Rolling finishing temperature is 85
If the temperature exceeds 0 ° C., it becomes difficult to stably secure a desired low-temperature toughness even if water cooling (direct quenching) is performed immediately after the finish rolling. If the temperature is lower than 650 ° C., it is difficult to stably secure the target tensile strength of 600 MPa or more. For this reason, the finishing temperature of hot rolling is 650 to
It is desirable that the temperature be 850 ° C. The preferred range is 700 to
800 ° C.

Water cooling (accelerated cooling) start temperature: If the water cooling start temperature during direct quenching is lower than 500 ° C., good strength and low-temperature toughness can be stably maintained even if the rolling finish temperature is in the range of 850 to 650 ° C. Not only is it difficult to ensure, but it is also difficult to ensure better strain aging resistance. For this reason, it is desirable that the water cooling start temperature be 500 ° C. or higher.

Accelerated cooling rate: Accelerated cooling after hot rolling is as follows:
It is necessary to prevent the decomposition of retained austenite in steel and to further improve the strain aging resistance, but at a cooling rate of less than 4 ° C./sec, the decomposition of residual austenite cannot be sufficiently prevented, and It becomes difficult to further improve the aging characteristics. For this reason, the accelerated cooling rate is desirably set to 4 ° C./sec or more. More preferably, it is more preferably at least 10 ° C./sec. The faster the accelerated cooling rate, the better. The upper limit does not need to be particularly defined. However, if the rate is too high, the cost tends to increase.
It is good to be less than seconds.

Stop temperature of accelerated cooling: If the cooling stop temperature during accelerated cooling exceeds 300 ° C., the target 600 MPa
Not only is it difficult to ensure the above tensile strength and desired toughness, but also an appropriate amount of retained austenite effective for suppressing strain aging embrittlement is decomposed, making it difficult to ensure better strain aging resistance. Become. For this reason, it is desirable that the stop temperature of the accelerated cooling be 300 ° C. or less. Note that there is no restriction on cooling until the room temperature is reached after the accelerated cooling is stopped, and air cooling or slow cooling may be used.

Further, a method for manufacturing a welded steel pipe will be described.

The welded steel pipe of the present invention is manufactured by forming a steel plate manufactured under the above conditions into a tube and welding and joining the butt portions. At this time, the steel plate is formed into a tube. It is necessary to perform in a temperature range of 600 ° C. or less. This is because if the molding temperature exceeds 600 ° C., the function of the hard phase may be reduced.

When the steel plate is formed into a tubular shape, the (t / D) value is 0.1 or less, where D (mm) is the outer diameter of the formed steel pipe and t (mm) is the wall thickness (steel plate thickness). It is desirable to mold so that This is because, when the (t / D) value exceeds 0.1, toughness and strain aging embrittlement resistance deteriorate, and it becomes difficult to secure desired toughness and strain aging embrittlement resistance.

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

[0051]

EXAMPLES Slabs composed of 14 types of steels having the chemical compositions shown in Table 1 were prepared.

[0052]

[Table 1] Each of the prepared slabs had a thickness of 20 under various conditions shown in Table 2.
mm. At that time, each steel plate whose accelerated cooling stop temperature was other than room temperature was allowed to cool to the air (air cooling) until it reached room temperature after accelerated cooling.

[0053]

[Table 2] Each of the obtained steel plates is formed by joining an X groove on one side of the steel plate, butted, and subjected to submerged arc welding at a heat input of 7 kJ / mm.
No. test specimen and a V-notch test specimen specified in JIS Z 2202, while the welded heat-affected zone (H
AZ) JIS Z whose center in the width direction is located at the bottom of the notch
A 2202-specified V-notch specimen was taken.

Each specimen was subjected to a tensile test and a Charpy impact test, and the yield strength (MPa), tensile strength (MPa), uniform elongation (%), and fracture surface transition temperature (vTs) of the base metal were obtained. : H) at −30 ° C.
Was examined for Charpy absorbed energy (vE- _{30 ° C} : J).

Also, a part of each steel plate has a strain amount of 1.5.
% And 3.0% tensile strain, and then subjected to a strain aging treatment of heating and holding at 250 ° C. for 1 hour, and from each steel plate after this strain aging treatment, a No. 4 test piece specified in JIS Z 2201 and JIS A V-notch test piece defined by Z 2202 was sampled and subjected to a tensile test and a Charpy impact test to determine a uniform elongation (%) and a fracture surface transition temperature (vTs: ° C). Also shown.

As can be seen from the results shown in Table 2, the steel sheets Nos. 1 to 11 in which the chemical composition and the manufacturing conditions of the steel are within the ranges specified in the present invention have a high tensile strength of 632 MPa or more. Uniform elongation is 3.9% or more, vTs is -78
Not only is the HAZ vE- _{30 °} C or lower being not more than 148 J or better, the uniform elongation after strain aging treatment of 3.0% is maintained at a value of 60% or more before strain aging treatment.

Although the chemical composition of the steel is within the range specified in the present invention, the steel sheets of Nos. 12 to 15 whose production conditions are out of the range desired in the present invention have a tensile strength of 692 MPa or more. And the uniform elongation is 9.1% or more,
vTs is −52 ° C. or lower, vE of HAZ _{−30 ° C.} is 185
Not only is it good as J or more, but the uniform elongation after strain aging treatment of 3.0% maintains a value of 45% or more before strain aging treatment.

On the other hand, when the B value defined by the above-mentioned formula (2) exceeds the upper limit of 1.6 defined in the present invention, the sample No. 1
Sample No. 6 or a sample No. 16 in which the chemical composition of the steel is out of the range specified by the present invention, such as a sample No. 17 steel plate whose A value defined by the equation (1) exceeds the upper limit of 0.44 specified by the present invention. ~ 2
The steel sheet of No. 0 has a uniform elongation after strain aging treatment of 3.0% and a uniform elongation rate of 9 to 37% before strain aging treatment, even though the manufacturing conditions are within a range desirable in the present invention. bad.

[0059]

The high-strength steel material of the present invention, for example, a steel plate,
It has excellent strain aging resistance and is suitable for use in obtaining a welded structure such as a high-strength welded steel pipe having good strain aging resistance, and is extremely useful in industry.

Claims (5)

[Claims] (1) a chemical composition having a mass ratio of C: 0.01 to
0.10%, Si: 0.30% or less, Mn: 1.00
2.50%, P: 0.010% or less, S: 0.005%
Hereinafter, Nb: 0.005 to 0.06%, Ti: 0.00
4 to 0.025%, sol. Al: 0.05% or less,
N: 0.0050% or less, O (oxygen): 0.003% or less, Cu: 0 to 1.5%, Ni: 0 to 2.5%, Mo:
0 to 0.80%, Cr: 0 to 1.0%, V: 0 to 0.1
%, Zr: 0 to 0.03%, Ca: 0 to 0.0030
%, B: 0 to 0.002%, the balance being substantially Fe
A value defined by the following equation (1) is 0.44 or less,
(2) A high-strength steel material comprising a steel having a B value defined by the formula of 1.6 or less and having a tensile strength of 600 MPa or more and excellent in strain aging resistance. A = 50N + C + 0.3Si + 10 (P + O) (1) B = (50N + C + 10O) / (4Nb + 10Ti + 2sol.Al) (2) where (1) and (2) The symbol of the element in the formula means the content (% by mass) of each element contained in the steel. 2. The high-strength steel according to claim 1, wherein the steel is a steel plate. 3. C: 0.01 to 0.10% by mass%, S:
i: 0.30% or less, Mn: 1.00 to 2.50%,
P: 0.010% or less, S: 0.005% or less, Nb:
0.005 to 0.06%, Ti: 0.004 to 0.02
5%, sol. Al: 0.05% or less, N: 0.005
0% or less, O (oxygen): 0.003% or less, Cu: 0 to 0
1.5%, Ni: 0 to 2.5%, Mo: 0 to 0.80
%, Cr: 0 to 1.0%, V: 0 to 0.1%, Zr: 0
~ 0.03%, Ca: 0 ~ 0.0030%, B: 0 ~
0.002%, with the balance being substantially Fe,
A steel having an A value defined by the formula (1) of 0.44 or less and a B value defined by the formula (2) of 1.6 or less is heated to 950 to 1200 ° C., and then hot-rolled to finish. Temperature 850-650
Rolling is completed at a temperature of not more than 500 ° C.
A method for producing a high-strength steel material excellent in strain aging resistance with a tensile strength of 600 MPa or more, which is accelerated to a temperature of 00 ° C. or less at a cooling rate of 4 ° C./sec or more. A = 50N + C + 0.3Si + 10 (P + O) (1) B = (50N + C + 10O) / (4Nb + 10Ti + 2sol.Al) (2) where (1) and (2) The symbol of the element in the formula means the content (% by mass) of each element contained in the steel. 4. A high-strength welded steel pipe whose base material is the high-strength steel sheet according to claim 2, which is excellent in strain aging resistance. 5. The high-strength steel sheet according to claim 2,
A method for producing a high-strength welded steel pipe having excellent strain aging resistance, which is formed into a tubular shape in the following temperature range and the butt portion is welded and joined.