JP2003231939A - High-strength steel sheet superior in sr resistance, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet which has higher strength than the API X65 grade, without containing a large amount of alloying elements therein, and superior SR (stress relief annealing) resistance of keeping the strength and the toughness after SR treatment. <P>SOLUTION: The high-strength steel sheet superior in SR resistance comprises, by mass% 0.02-0.08% C, 0.01-0.50% Si, 0.5-1.8% Mn, 0.02% or less P, 0.005% or less S, 0.05-0.50% Mo, 0.005-0.04% Ti, 0.01-0.07% Al, 0.005-0.07% Nb, and/or 0.005-0.10% V, and the balance substantially Fe, wherein C/(Mo+Ti+Nb+V), which is a ratio by atom.% of an amount of C to the total amount of Mo, Ti, Nb, and V, is 0.6-2.0, a metal structure is substantially a single phase of ferrite, and precipitates containing Ti and Mo are dispersed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet having strength exceeding API standard X65 grade used for manufacturing steel pipes and the like, and particularly excellent strength and toughness even after stress relief annealing (SR) performed after welding. The present invention relates to a high-strength steel sheet having excellent SR resistance and a manufacturing method thereof.

[0002]

2. Description of the Related Art Riser steel pipes used for oil or gas excavation and the like are often used by welding forged products (for example, connectors) having a large amount of alloying elements by circumferential welding, and after welding, residuals due to welding A stress relief annealing (SR) process is performed for the purpose of stress relief. Further, even when a steel material or a steel plate used as a steel pipe for piping or other strength members of a power plant or the like is welded to Cr-Mo steel or the like, SR treatment is performed for the purpose of removing residual stress by welding. Since the steel pipe or the like which is the base material may be heat-treated by the SR treatment to lower the strength and toughness, the steel pipe and the steel material subjected to the SR treatment need to ensure the strength and toughness even after the SR treatment. It must have excellent SR resistance. In recent years, in order to improve operating efficiency by increasing internal pressure when using steel pipes and reduce material costs by using thinner steel materials, demand for high-strength steel pipes or steel materials exceeding API X65 grades such as API X80 grades has also increased. ing.
In response to such a request, Japanese Patent Application Laid-Open Nos. 11-50188 and 2001-158939 disclose steel sheets or steel pipes having API SR80 grade or more excellent in SR resistance.

[0003]

However, JP-A-11-
The steel plate of Japanese Patent No. 50188 suffers from strength reduction due to SR treatment by S
Since it is compensated by precipitation of Cr carbide during R treatment,
Since a large amount of Cr needs to be added, not only the manufacturing cost is high, but also the weldability and toughness are deteriorated. On the other hand, in the steel pipe disclosed in JP 2001-158939 A, it is necessary to limit the seam weld metal to a specific composition range, and the base metal has a sufficiently high strength even if the strength is lowered by SR treatment. This is a technology that copes with the decrease in strength. Therefore, the base material strength is lowered by the SR treatment.

Therefore, an object of the present invention is to solve the problems of the prior art and to provide a high-strength steel sheet exceeding API X65 grade, which has strength and toughness after SR treatment without addition of a large amount of alloying elements. An object of the present invention is to provide a high-strength steel sheet having excellent SR resistance characteristics that does not deteriorate.

[0005]

The features of the present invention for solving the above problems are as follows.

(1) C: 0.02 to 0.08% by mass%, Si:
0.01 to 0.50%, Mn: 0.5 to 1.8%, P: 0.02% or less, S: 0.0
05% or less, Mo: 0.05 to 0.50%, Ti: 0.005 to 0.04%, Al: 0.
01-0.07%, Nb: 0.005-0.07% and / or
V: 0.005 to 0.10% is contained, the balance is substantially Fe, and is the ratio of the amount of C in atomic% to the total amount of Mo, Ti, Nb, and V.
C / (Mo + Ti + Nb + V) is 0.6 ~ 2.0, the metallographic structure is substantially a ferrite single phase, characterized in that precipitates containing Ti and Mo are dispersed precipitation. High strength steel plate with excellent SR resistance.

(2) Further, in mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Ca: 0.0005 to 0.00
A high-strength steel sheet having excellent SR resistance properties according to (1), which contains one or more selected from 25%.

Steel having the composition as described in (3), (1) or (2) is hot-rolled at a heating temperature of 1000 to 1250 ° C. and a rolling end temperature of 750 ° C. or higher, and then at 2 ° C. A method for producing a high-strength steel sheet having excellent SR resistance, comprising cooling at a cooling rate of / s or more and then winding the steel strip at a temperature of 550 to 700 ° C.

Steel having the composition as described in (4), (1) or (2) is hot-rolled under the conditions of heating temperature: 1000 to 1250 ° C., rolling end temperature: 750 ° C. or higher, and then 2 ° C. A method for producing a high-strength steel sheet excellent in SR resistance, which comprises cooling at a cooling rate of / s or more and then holding at a temperature of 550 to 700 ° C. for 5 minutes or more.

Steel having the composition as described in (5), (1) or (2) is hot-rolled at a heating temperature of 1000 to 1250 ° C. and a rolling end temperature of 750 ° C. or higher, and then at 2 ° C. cooling at a cooling rate of / s or more, then from the temperature of 550 ~ 700 ℃ to 0.1 ℃ /
S-resistant, characterized by cooling at a cooling rate of s or less
A method for producing a high-strength steel sheet having excellent R characteristics.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have made detailed studies on changes in the microstructure of steel materials by SR treatment in order to improve both SR resistance and high strength. In general, steel sheets for welded steel pipes and steel sheets for welded structures are severely limited in chemical composition from the viewpoint of weldability, so high-strength steel sheets of API X65 grade or higher are manufactured by accelerated cooling after hot rolling. Therefore, the microstructure is mainly composed of bainite or martensite, but when SR treatment is performed on steel having such a structure, strength reduction due to tempering of bainite or martensite is unavoidable. Further, there is a method of precipitating Cr carbide or the like at the time of SR in order to compensate for the strength decrease due to tempering, but the toughness is deteriorated because the carbide easily coarsens. Therefore, there is a limit in securing strength and toughness even after SR treatment in the transformation strengthening mainly composed of bainite or martensite. As a result of earnest studies on the microstructure morphology with which excellent SR resistance properties are obtained, the inventors of the present invention have found that it is important to make the microstructure of steel a microstructure that does not cause a morphological change before and after SR treatment. Have found that the matrix can be made substantially a ferrite single phase and strengthened by dispersing and precipitating thermally stable fine precipitates. Then, as a result of studying various precipitates that precipitate in the steel, the composite carbide composed of Mo and Ti is an extremely fine precipitate of 10 nm or less under the proper balance of components, and is also thermally stable. I found out. Therefore, even if the matrix is essentially a ferrite single phase, the strength exceeding API X65 grade can be easily obtained by precipitation strengthening, and the morphology of the composite carbide containing Mo and Ti does not change due to the SR treatment. We also obtained the knowledge that there is almost no change. Further, the carbide composed of Mo and Ti is Nb and / or
It was also found that Nb or V can be used because it also forms a composite with V or exhibits similar morphology and thermal stability.

The steel sheet having a ferrite structure in which the precipitates containing Ti and Mo as a base are dispersed and precipitated can be easily manufactured in a thin steel sheet by using a general hot rolling process of winding in a specific temperature range. Can be manufactured. Further, even a thick steel plate can be manufactured by holding the temperature for a certain period of time or more or gradually cooling it using the manufacturing process of the thick steel plate. The steel sheet manufactured in this manner has a higher material strength than the steel sheet mainly composed of bainite or martensite obtained by conventional accelerated cooling, etc., even with the addition of a small amount of alloying elements. Weldability is also obtained at the same time.

The high strength steel sheet of the present invention will be described in detail below. First, the structure of the high strength steel sheet of the present invention will be described.

The metal structure of the steel sheet of the present invention is substantially a ferrite single phase. Generally, the ferrite phase obtained by hot rolling has a low dislocation density, so the microstructure does not change due to heating below the transformation point such as SR treatment, and since it is rich in ductility, the grain size should be appropriate. Provides high toughness. Bainite in the ferrite phase,
When different metallic structures such as martensite and pearlite are mixed, the strength of these phases is lowered by the SR treatment, and therefore the smaller the structural fraction other than the ferrite phase, the better. However, if the volume fraction of the structure other than ferrite is low, the effect can be ignored, so the total volume fraction is
It may contain 10% or less of another metal structure, that is, bainite, martensite, pearlite, cementite, etc., alone or in combination.

Next, the precipitates dispersed and precipitated in the steel sheet in the present invention will be described. The steel sheet according to the present invention is one in which precipitates containing Mo and Ti as the base are dispersed and precipitated in the ferrite phase. This precipitate is extremely fine and has high thermal stability, and its shape does not change even by SR treatment, so that high strength can be maintained even after SR treatment. Mo and Ti are elements that form carbides in steel, and strengthening the steel by precipitation of MoC and TiC has been conventionally performed, but in the present invention, Mo and Ti are added in combination, and Mo is added.
By finely precipitating a composite carbide containing steel and Ti as the base in the steel, a larger strength improving effect can be obtained as compared with the case of precipitation strengthening of MoC and / or TiC. This unprecedented large strength improving effect is due to the fact that since the composite carbide containing Mo and Ti as the base is stable and has a slow growth rate, an extremely fine precipitate having a particle size of less than 10 nm can be obtained. .

When the composite carbide containing Mo and Ti as a base is composed of only Mo, Ti and C, the total of Mo and Ti and C are combined at an atomic ratio of about 1: 1. Although it has a high thermal stability and is very effective in increasing the strength, there is a problem that the toughness of the welded portion deteriorates as the content of Ti increases. In the present invention, in a composite carbide composed of only Mo, Ti, C, to improve the weld toughness by substituting a part of Ti with another element, in addition to Mo and Ti, Nb The present invention has been completed by adding and / or V to precipitate a composite carbide containing Mo and Ti and Nb and / or V to obtain similar precipitation strengthening and excellent SR resistance.

In the present invention, the composite carbide containing Mo and Ti and Nb and / or V, which are the precipitates dispersed and precipitated in the steel sheet in the present invention, uses the steel material and the production method of the components of the present invention described below. It can be obtained by dispersing it in the ferrite phase by producing a steel sheet using the above method. When the high-strength steel sheet of the present invention contains a precipitate other than the composite carbide mainly composed of Mo and Ti, the effect of increasing the strength by the composite carbide of Mo and Ti is not impaired and the SR resistance is not deteriorated. And

Next, the chemical composition of the high strength steel sheet of the present invention will be described.

C: 0.02 to 0.08%. C is an element that contributes to precipitation strengthening as a carbide, but if it is less than 0.02%, sufficient strength cannot be secured, and if it exceeds 0.08%, toughness and SR resistance are deteriorated, so the C content is made 0.02 to 0.08%. Stipulate.

Si: 0.01 to 0.50% Si is added for deoxidation, but if it is less than 0.01%, the deoxidizing effect is not sufficient and 0.5
If it exceeds 0%, the toughness and weldability are deteriorated, so the Si content is specified to be 0.01 to 0.50%.

Mn: 0.5 to 1.8% Mn is added for strength and toughness, but if it is less than 0.5%, its effect is not sufficient,
If it exceeds 1.8%, the weldability will deteriorate, so the Mn content should be 0.5.
Specify to ~ 1.8%.

P: 0.02% or less. Since P is an unavoidable impurity element that deteriorates weldability and toughness after SR, the upper limit of P content is specified to be 0.02%.

S: 0.005% or less. Since S also deteriorates the toughness after SR, the smaller the content, the better. However, if it is 0.005% or less, there is no problem, so the upper limit of the S content is specified to 0.005%.

Mo: 0.05 to 0.50%. Mo is an important element in the present invention, and by containing 0.05% or more,
While suppressing the pearlite transformation during cooling after hot rolling, it forms fine composite precipitates with Ti and greatly contributes to the strength increase. However, if added in excess of 0.50%, the structural fraction other than ferrite such as bainite and martensite increases, so the SR treatment causes a decrease in strength. Therefore, Mo
The content is specified to be 0.05 to 0.50%.

Ti: 0.005 to 0.04%. Ti, like Mo, is an important element in the present invention. When added in excess of 0.04%, it forms a complex precipitate with Mo and greatly contributes to the strength increase. However, if it exceeds 0.04%, the toughness of the heat-affected zone will be significantly deteriorated, so the Ti content is 0.005 to 0.04%.
Prescribed in.

Al: 0.01 to 0.07%. Al is added as a deoxidizer, but if it is less than 0.01%, it has no effect, and if it exceeds 0.07%, the cleanliness of the steel decreases and the toughness deteriorates.
The content is specified as 0.01 to 0.07%.

It contains one or two of Nb and V.

Nb: 0.005 to 0.05% Nb improves toughness by making the structure finer, but forms a composite precipitate with Ti and Mo, and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.05%, the toughness of the weld heat affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.05%.

V: 0.005 to 0.10% V is Ti as well as Nb
Also forms a complex precipitate with Mo and contributes to strength increase. However, if it is less than 0.005%, it has no effect, and if it exceeds 0.1%, the toughness of the weld heat affected zone deteriorates, so the V content is 0.005%.
Specified to ~ 0.1%.

It is the ratio of the amount of C to the total amount of Mo, Ti, Nb and V,
C / (Mo + Ti + Nb + V): 0.6 to 2.0. In C / (Mo + Ti + Nb + V), each element symbol indicates the content (at%) in atomic% of the component. The strengthening of the steel sheet of the present invention is due to the composite precipitate (carbide) containing Ti, Mo, and Nb and / or V. In order to effectively utilize the precipitation strengthening by this composite precipitate, the C content and the carbide forming elements Mo, Ti, Nb, V
The amount relationship is important, and by adding these elements in a proper balance, a thermally stable and extremely fine composite precipitate can be obtained. At this time, the value of C / (Mo + Ti + Nb + V), which is the ratio of the content of C in atomic% and the total content of Mo, Ti, Nb, and V in atomic%, is 0.6 to Set to 2.0. When the value of C / (Mo + Ti + Nb + V) is less than 0.6 or exceeds 2.0, the amount of any element is excessive, and Ti and Mo of the present invention are present.
Since a hardened structure other than the complex precipitates including and is excessively formed, the SR resistance and the toughness are deteriorated.
The value of (o + Ti + Nb + V) is specified to be 0.6 to 2.0. In addition, when using the content of mass%, it calculates using the following formula | equation (1) and makes the value 0.6-2.0.

(C / 12.01) / (Mo / 95.9 + Nb / 92.91 + V / 50.94 + Ti / 47.9) (1) In the present invention, for the purpose of further improving the strength and toughness of the steel sheet,
You may contain 1 type, or 2 or more types of Cu, Ni, Cr, and Ca shown below.

Cu: 0.50% or less. Cu is an element effective in improving toughness and increasing strength, but if added in a large amount, the weldability deteriorates, so if it is added, the upper limit is 0.50%.

Ni: 0.50% or less. Ni is an element effective in improving toughness and increasing strength, but if added in a large amount, S
Since the R characteristic deteriorates, 0.50% is made the upper limit when added.

Cr: 0.50% or less. Cr is low C like Mn
However, it is an element effective for obtaining sufficient strength, but if added in large amounts, the weldability deteriorates.
The upper limit is%.

Ca: 0.0005 to 0.0025%. Ca is an element effective in improving the toughness by controlling the morphology of sulfide inclusions, but if it is less than 0.0005%, its effect is not sufficient, and if it is added in excess of 0.0025%, the effect saturates. Since the toughness deteriorates due to the decrease in cleanliness, the Ca content is specified to 0.0005 to 0.0025% when added.

The balance other than the above consists essentially of Fe.
The fact that the balance consists essentially of Fe means that those containing other trace elements including unavoidable impurities can be included in the scope of the present invention unless the effects of the present invention are lost.

Next, a method for manufacturing the high strength steel sheet of the present invention will be described.

The high-strength steel sheet of the present invention is a steel having the above-described composition, and is hot-rolled at a heating temperature of 1000 to 1250 ° C. and a rolling end temperature of 750 ° C. or higher, and then cooled at 2 ° C./s or higher. By cooling at a rate and then maintaining the temperature at 550 to 700 ° C. for a certain period of time, fine composite carbide containing Ti and Mo and Nb and / or V can be dispersed and precipitated to be produced. 55
As a method of holding at a temperature of 0 to 700 ° C for a certain time, 550 to 7
Winding steel strip at a temperature of 00 ℃ (first manufacturing method), 550
Hold isothermal for 5 minutes or more at a temperature of ~ 700 ° C (second manufacturing method), gradually cool at a cooling rate of 0.1 ° C / s or less from a temperature of 550 ~ 700 ° C (third manufacturing method), There are three manufacturing methods. Hereinafter, each manufacturing method will be described in detail.

Heating temperature: 1000 to 1250 ° C. If the heating temperature is less than 1000 ° C, the solid solution of carbide is insufficient to obtain the required strength, and if it exceeds 1250 ° C, the toughness deteriorates.
~ 1250 ℃

Rolling end temperature: 750 ° C. or higher. When the rolling end temperature is low, rolling strain remains in the ferrite phase and SR
Since the treatment causes recovery and lowers the strength after SR, the rolling end temperature is set to 750 ° C or higher. In addition, although the upper limit of the rolling end temperature is not particularly specified, excellent SR resistance and strength can be obtained, but it is preferable to end the rolling at a temperature of 950 ° C. or lower in order to prevent deterioration of toughness due to coarsening of the structure.

After the completion of rolling, cooling is performed at a cooling rate of 2 ° C./s or more. If cooling or slow cooling is performed after the rolling is completed, precipitates are deposited from the high temperature region, and the precipitates easily coarsen and the strength decreases. Therefore, quenching to the optimum temperature for precipitation strengthening,
Preventing precipitation from a high temperature range is an important manufacturing condition in the present invention. If the cooling rate is less than 2 ° C / s, the effect of preventing precipitation in the high temperature region is insufficient and the strength decreases, so the cooling rate after rolling is specified to be 2 ° C / s or more. As a cooling method at this time, any cooling equipment can be used depending on the manufacturing process.

After cooling at a cooling rate of 2 ° C./s or more, in order to obtain the ferrite structure and the fine precipitates of the present invention, it is necessary to hold at a high temperature for a certain period of time. The first manufacturing method is to manufacture a thin steel sheet, and after hot rolling, after cooling with water cooling on a run-out table, etc., in a hot rolling process of winding on a steel strip, winding is performed at a predetermined temperature. Thus, the steel strip is kept isothermally to deposit the precipitate of the present invention.

The cooling end temperature may be higher than the subsequent winding temperature, isothermal holding temperature, or slow cooling start temperature, but if the cooling end temperature is too high, coarsening of precipitates occurs. Since sufficient strength cannot be obtained, it is desirable to set it to 750 ° C or lower.

First manufacturing method: Winding temperature: 550 to 700 ° C.
And 2 if the steel strip is produced by the hot rolling process
After cooling at a cooling rate of ℃ / s or more, coiling temperature 550 to 700 ℃
Take up with. If the coiling temperature is lower than 550 ° C, SR resistance deteriorates due to the formation of bainite, and if it exceeds 700 ° C, the precipitates become coarse and sufficient strength cannot be obtained. Specified at ~ 700 ℃.

The second manufacturing method and the third manufacturing method are suitable for manufacturing thick steel plates without winding, and after cooling in a water cooling facility after finish rolling in a thick plate mill. The second manufacturing method is a method of precipitating the precipitate of the present invention by holding the material in a soaking furnace isothermally for a predetermined time or longer. In the third production method, after cooling with water, the steel sheet of the present invention is produced by precipitating the precipitate of the present invention while maintaining a high temperature by performing gradual cooling such as cover gradual cooling. These cases will be described below.

Second production method: After cooling at a cooling rate of 2 ° C./s or more, the temperature is kept at 550 to 700 ° C. for 5 minutes or more. The cooling end temperature is preferably equal to or higher than the isothermal holding temperature and equal to or lower than 750 ° C. If the steel strip is not wound, as in the hot rolling process, following cooling after rolling,
It is possible to obtain a ferrite single structure in which precipitates containing Mo and Ti are dispersed and precipitated by carrying out isothermal holding for a certain period of time or more. At this time, if the temperature is lower than 550 ° C, SR resistance is deteriorated due to the formation of bainite, and if the temperature exceeds 700 ° C, the precipitates are coarsened and sufficient strength cannot be obtained. Therefore, the holding temperature is specified to be 550 to 700 ° C. . Further, if the holding time is less than 5 minutes, the ferrite transformation is not completed, and after the cooling, bainite or pearlite is generated, so that the S resistance is low.
Since the R characteristic deteriorates, the holding time is specified to be 5 minutes or longer. Note that, if the ferrite transformation is completed by the isothermal holding, the cooling rate thereafter may be any rate.

Third manufacturing method: After cooling at a cooling rate of 2 ° C./s or more, the temperature is gradually cooled from a temperature of 550 to 700 ° C. at a cooling rate of 0.1 ° C./s or less. It is possible to manufacture the steel sheet of the present invention by performing slow cooling from a predetermined temperature subsequent to cooling after rolling without performing the above isothermal holding. If the cooling rate at this time exceeds 0.1 ° C./s, bainite is generated and the SR resistance deteriorates, so the upper limit of the cooling rate is specified to 0.1 ° C./s. The temperature at which the slow cooling is started is 550 to 700 ° C. This is because if the temperature is lower than 550 ° C, the SR resistance is deteriorated due to the formation of bainite, and if the temperature exceeds 700 ° C, the precipitates become coarse and sufficient strength cannot be obtained.

The steel sheet of the present invention produced by the above-mentioned first, second and third production methods which can use a conventional hot rolling mill or thick plate mill is subjected to press bend forming, roll forming, UOE forming and the like. It can be formed into a steel pipe and used as a steel pipe (electric resistance welded pipe, spiral steel pipe, UOE steel pipe) for transporting crude oil or natural gas.

[0049]

[Examples] Test steels with chemical compositions shown in Table 1 (steel types A to K)
Was used to produce steel plates with plate thicknesses of 12, 18, and 26 mm.

[0050]

[Table 1]

The hot rolled steel strip (No. 1 to 17) having a plate thickness of 12 mm is
It was manufactured by cooling after rolling and winding at a predetermined temperature. Table 2 shows the slab heating temperature of each steel sheet and the end of rolling (finishing)
The temperature, the cooling rate after rolling, and the winding temperature are shown. Board thickness 18 mm and
26 mm thick steel plates (No. 18 to 28) were manufactured by hot rolling (thick plate process) using steel types B, C, E, I and K under the conditions shown in Table 3. In Table 3, when the treatment method after cooling is described as "temperature retention", after the rolling was performed by the accelerated cooling device, the gas combustion furnace was subjected to isothermal retention (soaking treatment). Table 3 also shows the holding temperature and the holding time for those that were held isothermally. In addition, in the case where the treatment method after cooling is described as "slow cooling", after cooling by an accelerated cooling device after rolling, the steel sheets were stacked to perform slow cooling to room temperature. Table 3 also shows the slow cooling start temperature and the average cooling rate from the slow cooling start to 300 ° C. for the slow cooling. Further, No. 28 was manufactured by accelerating cooling after completion of rolling to 350 ° C. and then by air cooling.

The microstructure of the steel sheet manufactured as described above was observed with an optical microscope and a transmission electron microscope (TEM). The components of the precipitate were analyzed by energy dispersive X-ray spectroscopy (EDX). Further, in order to investigate the SR resistance property, each steel sheet was subjected to SR treatment using a gas atmosphere furnace. The heat treatment condition at this time was 650 ° C. for 2 hours, after which it was taken out of the furnace and cooled to room temperature by air cooling. Then, the tensile properties and the Charpy impact properties of the steel sheet before and after the SR treatment were measured. As for the tensile properties, a tensile test was performed using a full-thickness test piece in the rolling vertical direction as a tensile test piece, and the yield strength and tensile strength were measured. Then, in consideration of manufacturing variations, those having a yield strength of 520 MPa or more and a tensile strength of 600 MPa or more before and after SR treatment were evaluated as high strength steel sheets exceeding API X65 grade. Regarding toughness, in the Charpy impact test after SR treatment, -1
A steel sheet having an absorbed energy of 70 J or more at 0 ° C. was evaluated as a high toughness steel sheet. In addition, in order to evaluate the weld heat affected zone toughness (HAZ toughness), a Charpy impact test was performed after applying a heat history equivalent to a heat input of 15 kJ / cm using a welding heat cycle reproduction device using a steel sheet before SR treatment. It was Then, those having a fracture surface transition temperature of −10 ° C. or less were judged to have good HAZ toughness. The measurement results are also shown in Tables 2 and 3.

[0053]

[Table 2]

[0054]

[Table 3]

In Table 2, Nos. 1 to 8 which are examples of the present invention
All of the samples had a chemical composition and a manufacturing method within the scope of the present invention, and had high tensile strength of 600 MPa or more before and after SR treatment and excellent toughness. The structure of the steel sheet was substantially a ferrite single layer, and fine carbide precipitates containing Ti and Mo and Nb and / or V and having a particle size of less than 10 nm were dispersed and precipitated.

The chemical compositions of Nos. 9 to 12 are within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, the metal structure is not substantially a ferrite single phase, and Ti and Mo are Since the precipitates containing P were not dispersed and deposited, sufficient strength could not be obtained after SR treatment, or the toughness was low.

Nos. 13 to 17 had chemical components outside the range of the present invention, and thus sufficient strength could not be obtained after SR treatment, or toughness was low.

In Table 3, No. 18-
Each of No. 21 had a chemical composition and a manufacturing method within the scope of the present invention, and had high strength of 600 MPa or more and high toughness before and after SR treatment. The structure of the steel sheet was substantially a ferrite single layer, and fine carbide precipitates containing Ti and Mo and Nb and / or V and having a particle size of less than 10 nm were dispersed and precipitated.

The chemical compositions of Nos. 22 to 25 are within the scope of the present invention, but the production method is outside the scope of the present invention, the metal structure is not substantially a ferrite single phase, and Ti and Mo are Since the precipitates containing P were not dispersed and deposited, sufficient strength could not be obtained after SR treatment, or the toughness was low.

In Nos. 26 and 27, the chemical composition was out of the range of the present invention, and sufficient strength could not be obtained after SR treatment, or the toughness was low.

No. 28 had a chemical composition within the range of the present invention, but the cooling condition after hot rolling was different from the range of the present invention, so that sufficient strength could not be obtained after SR treatment.

[0062]

As described above, according to the present invention, the AP
A steel sheet having high strength of IX65 grade or higher and excellent in strength and toughness even after SR treatment can be obtained. Therefore, it is possible to manufacture steel pipes such as electric resistance welded steel pipes, spiral steel pipes and UOE steel pipes having excellent characteristics.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toyohisa Shingu             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Minoru Suwa             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F-term (reference) 4K032 AA01 AA04 AA08 AA11 AA14                       AA16 AA19 AA22 AA23 AA27                       AA29 AA31 AA35 AA36 BA01                       CC03 CC04 CD02 CD03 CE01                       CE02

Claims (5)

[Claims] 1. In mass%, C: 0.02 to 0.08%, Si: 0.01 to
0.50%, Mn: 0.5-1.8%, P: 0.02% or less, S: 0.005% or less, Mo: 0.05-0.50%, Ti: 0.005-0.04%, Al: 0.01-
0.07%, Nb: 0.005-0.07% and / or V: 0.
005-0.10% is contained, the balance is essentially Fe, and the ratio of the amount of C in atomic% to the total amount of Mo, Ti, Nb, and V is C / (Mo +
Ti + Nb + V) is 0.6 to 2.0, the metallographic structure is substantially a ferrite single phase, and the precipitates containing Ti and Mo are dispersed and precipitated, which is excellent in SR resistance characteristics. High strength steel plate. 2. Further, in mass%, Cu: 0.50% or less, N
i: 0.50% or less, Cr: 0.50% or less, Ca: 0.0005 to 0.0025%, and one or more kinds selected from the group is contained, which is excellent in SR resistance according to claim 1 and is high. Strength steel plate. 3. A steel having the chemical composition according to claim 1 or 2 is hot-rolled at a heating temperature of 1000 to 1250 ° C. and a rolling end temperature of 750 ° C. or higher, and then 2 ° C./s. A method for producing a high-strength steel sheet excellent in SR resistance, characterized by cooling at the above cooling rate and then winding the steel strip at a temperature of 550 to 700 ° C. 4. A steel having the composition of claim 1 or 2 is hot-rolled at a heating temperature of 1000 to 1250 ° C. and a rolling end temperature of 750 ° C. or higher, and then 2 ° C./s. A method for producing a high-strength steel sheet excellent in SR resistance, which comprises cooling at the above cooling rate, and then maintaining isothermal at a temperature of 550 to 700 ° C. for 5 minutes or more. 5. A steel having the composition according to claim 1 or 2 is hot-rolled at a heating temperature of 1000 to 1250 ° C. and a rolling end temperature of 750 ° C. or higher, and then 2 ° C./s. Cool at the above cooling rate, then from the temperature of 550 ~ 700 ℃ to 0.1 ℃ / s
SR resistance, characterized by cooling at the following cooling rates
A method for producing a high strength steel plate having excellent characteristics.