JP2012017522A - Steel material for line pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for a line pipe having a tensile strength of 760 MPa or more, excellent in deformability after strain-ageing.SOLUTION: A steel material having chemical-composition containing respective prescribed contents of C, Si, Mn, Cu, Ni, Mo. Nb, Ti and Al and the balance Fe with impurities of P, S, N and O in the respective prescribed contents or lower, is used to have a composite structure consisting a ferrite of 5-30%, an island martensite of 3-20% and bainite in microstructure.

Description

  The present invention relates to a steel material for a line pipe, and particularly relates to a high-tensile steel material having excellent deformability after strain aging and a tensile strength of 760 MPa or more.

  Large diameter line pipes are used when transporting natural gas, crude oil, etc. over long distances. Steel materials for line pipes are required to have high strength and toughness, and SBD (Strain-Based Design) with high deformation performance to prevent damage to the pipeline due to ground movement during earthquakes, thawing / freezing of frozen soil, etc. ) Corresponding steel is desired. In particular, the base material is required to be improved in deformation performance and strain aging resistance in order to prevent local buckling. Deformation performance is improved when the yield ratio (hereinafter referred to as “YR”) is low and has a high uniform elongation (hereinafter referred to as “U.El”). Deformation performance deteriorates due to strain aging caused by heating during pipe making and coating. By the way, it is generally said that the higher the strength, the more difficult it is to secure high deformation performance after strain aging.

  In response to such demands, techniques for improving the strain aging resistance of steel materials by controlling the chemical composition and production conditions have been disclosed. For example, Patent Document 1 discloses “a high-strength steel material excellent in strain aging characteristics and a manufacturing method thereof”. Patent Document 2 discloses “a steel pipe for high-strength line pipe excellent in strain aging resistance, a steel sheet for high-strength line pipe, and a manufacturing method thereof”.

JP 2002-220634 A JP 2007-314828 A

  The steel material disclosed in Patent Document 1 is said to be excellent in strain aging resistance. It only suppresses the decrease in El. The improvement in deformation performance is due to low YR and high U.D. It is necessary to have El. Therefore, the steel material obtained by this method may not have good deformability after strain aging.

  The steel pipe disclosed in Patent Document 2 is intended to reduce the change in yield strength before and after aging treatment, and is not intended to improve deformability such as a decrease in YR. For this reason, the steel plate obtained by the manufacturing method disclosed in Patent Document 2 may not have good deformability after strain aging. Moreover, the microstructure of the steel pipe disclosed in Patent Document 2 is a structure composed of bainite or bainite and martensite, and does not allow the formation of a ferrite structure.

  Accordingly, an object of the present invention is to provide a steel material for a line pipe which is suitable as a material for a line pipe, and particularly has a tensile strength of 760 MPa or more after strain aging and is excellent in deformability.

  In order to solve the above problems, the present inventors have made various studies. As a result, the knowledge shown in the following (a) to (h) was obtained.

  (A) High deformation performance after strain aging, that is, low YR and high U.V. In order to make El compatible, a composite structure having a microstructure advantageous for each characteristic may be used.

  (B) The microstructure is a composite structure substantially composed of ferrite, island martensite (hereinafter referred to as “MA”), and bainite.

  (C) Ferrite and MA are low YR and high U.V. Effective for both Elization. The larger the occupied volume ratio, the lower the YR and the higher the U.V. El.

  (D) As shown in FIG. 1, the increase in the ferrite occupation volume ratio is advantageous in improving the deformation performance. On the other hand, an increase in the ferrite volume ratio is disadvantageous for improving the strength, so it is necessary to keep the volume ratio below a certain level. Therefore, in order to achieve both high strength and deformation performance, it is essential to adjust the ferrite occupation volume ratio.

  (E) An increase in the volume ratio occupied by the MA is also advantageous for improving the deformation performance, and further advantageous for improving the strength. However, even if the volume ratio occupied by the MA is excessively increased, the improvement in these characteristics is saturated. Causes reduction in base metal toughness. Therefore, the MA occupation volume ratio needs to be a certain volume ratio or less from the viewpoint of toughness.

  (F) Strength deficiency due to increase in ferrite occupied volume fraction is reinforced by adjusting the composition and making part of the structure bainite.

  (G) In order to lose solute N which is harmful to the strain aging resistance, it is effective to add Ti.

  (H) In order to realize high deformability after strain aging, YR before strain aging needs to be a certain value or less.

  The present invention has been completed based on the above findings and has been completed. The gist of the present invention is as follows (1) to (3).

(1) In mass%,
C: 0.06 to 0.14%,
Si: 0.2-0.9%
Mn: 1.0 to 3.0%
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.5%,
Mo: 0.04 to 0.50%,
Nb: 0.005 to 0.08%,
Ti: 0.005 to 0.04% and sol. Al: 0.005 to 0.100%
The balance consists of Fe and impurities, and P, S, N and O as impurities are each P: 0.02% or less,
S: 0.005% or less,
N: a steel material having a chemical composition of 0.010% or less and O: 0.005% or less,
The microstructure is 5-30% by volume ferrite and 3-20% island martensite and the balance mainly consists of bainite,
A steel material for a line pipe, wherein the YR before aging is 0.72 or less and the tensile strength is 760 MPa or more.

  (2) Further, by mass%, containing at least one selected from Cr: 1.0% or less, V: 0.5% or less and B: 0.01% or less (1) ) Steel for line pipes as described in).

  (3) The above (1), further comprising at least one selected from the group consisting of Ca: 0.01% or less, REM: 0.02% or less, and Mg: 0.008% or less in terms of mass%. ) Or (2) line pipe steel.

  (4) The steel product for a line pipe according to any one of (1) to (3) above, further containing, by mass%, Sn: 0.5% or less.

  According to the steel material for a line pipe of the present invention, it is possible to produce a line pipe steel material that is excellent in deformation performance after strain aging and has a high strength of a tensile strength of 760 MPa or more.

It is a figure which shows the relationship between a yield ratio (0.5% yield strength / tensile strength) and a ferrite volume fraction.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Microstructure The steel for line pipes according to the present invention is a high-strength and high-deformation after strain aging by making the microstructure a composite structure of ferrite, MA and bainite and optimizing the composition ratio of the structure. Performance, ie low YR and high U.D. El steel sheets can be manufactured.

Ferrite structure: 5 to 30% by volume
The increase in ferrite occupied volume fraction is due to the low YR and high U.V. Effective for Elization. However, since the increase in ferrite occupied volume ratio causes a decrease in strength, the ferrite occupied volume ratio is set to 30% or less. On the other hand, if the ferrite volume fraction is less than 5%, it is insufficient to achieve the above characteristics. Therefore, the ferrite volume ratio was set to 5 to 30%. A preferred lower limit is 10%.

MA structure: 3 to 20% by volume
An increase in the volume fraction occupied by MA increases the tensile strength, resulting in a low YR and a high U.D. Effective for Elization. If the occupied volume ratio of MA is less than 3%, it is insufficient to achieve the above characteristics. On the other hand, if it exceeds 20%, the improvement in deformation performance is saturated and the base material toughness is further deteriorated. Therefore, the MA occupation volume ratio is set to 3 to 20%. A preferred lower limit is 5% and a preferred upper limit is 15%.

Bainite structure: main balance Bainite compensates for a decrease in strength due to an increase in ferrite occupied volume fraction. Therefore, the remaining structure is mainly bainite. The occupied volume ratio is preferably 50% or more.

  In the steel material for line pipes according to the present invention, different microstructures such as pearlite and / or cementite may exist in addition to ferrite, MA and bainite. When a structure other than ferrite, MA, and bainite is present, it is difficult to achieve both strength and deformability. For this reason, the occupied volume ratio of structures other than ferrite, MA, and bainite is preferably as small as possible, and the upper limit of the occupied volume ratio is preferably 3%.

(B) About chemical composition "%" of content of each element in a chemical composition means "mass%".

C: 0.06% to 0.14%
C is an element necessary for increasing the strength of the steel material. In order to stably obtain a tensile strength of 760 MPa or more, C needs to have a content of 0.06% or more. C also has an effect of promoting MA generation by interaction with Si. On the other hand, if the content of C becomes too large, not only the toughness and weldability of the base metal, but also the toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) is deteriorated, as well as the resistance to strain aging. Degradation occurs. Therefore, the C content is set to 0.06% or more and 0.14% or less. C is preferably contained in an amount exceeding 0.07%. Moreover, the upper limit with preferable C content is 0.12%.

Si: 0.2-0.9%
Si has the effect of suppressing the precipitation of cementite and promoting the formation of MA, and has good deformation performance before and after strain aging, that is, low YR and high U.V. Effective for obtaining El. In order to reliably obtain these effects, Si is contained in an amount of 0.2% or more. However, when the Si content is too large, the deterioration of the toughness of the base material and the HAZ becomes significant. Therefore, the Si content is set to 0.2 to 0.9%. Si is preferably contained in an amount exceeding 0.3%, and more preferably contained in an amount exceeding 0.5%. Moreover, the upper limit with preferable Si content is 0.8%, and a more preferable upper limit is 0.75%.

Mn: 1.0-3.0%
Mn has the effect | action which raises the intensity | strength of steel materials. In order to obtain this effect sufficiently, Mn is contained at 1.0% or more. On the other hand, if the content is excessive, weld cracks are likely to occur. In addition, when the Mn content is large, good deformation characteristics aimed by the present invention, that is, low YR and high U.D. It becomes difficult to obtain El. Therefore, the Mn content is set to 1.0 to 3.0%. The minimum with preferable Mn content is 1.2%, and a more preferable minimum is 1.5%. Moreover, the upper limit with preferable Mn content is 2.5%, and a more preferable upper limit is 2.0%.

Cu: 0.05 to 1.0%
Cu has the effect of improving the strength of the steel material, so 0.05% or more is contained. However, if the content is large, the surface properties and toughness of the steel material are significantly deteriorated. Therefore, the Cu content is set to 0.05 to 1.0%. A preferable lower limit of the Cu content is 0.1%. Moreover, a preferable upper limit is 0.6%. Furthermore, the lower limit of the Cu content is preferably 0.2%, and the upper limit is more preferably 0.5%.

Ni: 0.05 to 1.5%
Ni has the effect | action which improves the intensity | strength of steel materials, and also has the effect | action which improves toughness. In order to exhibit these effects, it is necessary to contain Ni 0.05% or more. However, if the Ni content exceeds 1.5%, an effect commensurate with the cost increase cannot be obtained. Therefore, the Ni content is set to 0.05 to 1.5%. A preferable lower limit of the Ni content is 0.1%. Moreover, a preferable upper limit is 1.0%. Further, the lower limit of the Ni content is preferably 0.2%, and the upper limit is more preferably 0.6%.

Mo: 0.04 to 0.50%
Mo has the effect of improving the strength of the steel material and further promotes the formation of MA, so it is necessary to contain 0.04% or more. However, if the content is excessive, an increase in yield strength due to strain aging increases, and the deformation characteristics are impaired. Moreover, it becomes easy to generate | occur | produce the toughness deterioration and weld crack of HAZ. Therefore, the Mo content is set to 0.04 to 0.50%. A preferable lower limit of the Mo content is 0.05%. Moreover, a preferable upper limit is 0.20%. Furthermore, the lower limit of the Mo content is preferably 0.07%, more preferably 0.13%.

Nb: 0.005 to 0.08%
Nb has the effect of improving the strength of the steel material, and also has the effect of increasing the base material toughness by combining with appropriate rolling conditions. For this reason, Nb needs to be contained by 0.005% or more. However, if the content is too large, the toughness of the base material and the HAZ deteriorates. Therefore, the Nb content is set to 0.005 to 0.08%. A preferred lower limit is 0.01%. Moreover, a preferable upper limit is 0.06%. Furthermore, the Nb content is preferably 0.02% at the lower limit, and more preferably 0.05% at the upper limit.

Ti: 0.005-0.04%
Ti forms precipitates (TiN) together with N, an element harmful to the strain aging resistance, stabilizes N atoms, greatly improves the strain aging resistance, and improves the base metal and HAZ structure. There is an effect of improving the low-temperature toughness of the high-strength steel base material and HAZ by refining. However, if the content is less than 0.005%, the above effect cannot be obtained. Conversely, if the content exceeds 0.04%, the toughness of the base material and the HAZ deteriorates. Therefore, the Ti content is set to 0.005 to 0.04%. A preferred lower limit is 0.01% and a preferred upper limit is 0.03%. Furthermore, the ratio of Ti and N content (Ti / N) is preferably 4.0 or more.

sol. Al: 0.005 to 0.100%
Al is an element having a deoxidizing action. Since it is effective in improving El, sol. As Al (“acid-soluble Al”), 0.005% or more is contained. However, sol. If the Al content becomes too large, the toughness of the HAZ will deteriorate. Therefore, sol. The Al content was 0.005 to 0.100%. Note that sol. It is preferable that the lower limit of the Al content is 0.010% and the upper limit is 0.060%.

  Moreover, although the main component of the remainder of the steel material which concerns on this invention is Fe, when the other component is contained by the various factors of a manufacturing process, the characteristic of steel materials may deteriorate. Therefore, in order to ensure the targeted good performance, the content of the following components contained in the impurities is particularly controlled. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

P: 0.02% or less P is an element that causes deterioration of toughness, and its content increases. In particular, when it exceeds 0.02%, the deterioration of toughness tends to be remarkable. Therefore, the content of P is set to 0.02% or less. In addition, it is better that the content of P is small, and it is preferably 0.01% or less.

S: 0.005% or less S increases the content of inclusions that are harmful to ductility or toughness. In particular, when it exceeds 0.005%, inclusions increase and the ductility and the toughness deteriorate significantly. Therefore, the content of S is set to 0.005% or less. In addition, it is better that the content of S is small, and it is preferable that the content is 0.003% or less.

N: 0.010% or less N is an impurity element that is extremely harmful to the strain aging resistance characteristics. If its content exceeds 0.010%, not only the toughness of the base metal and its welded part is significantly reduced, but also N Even if other measures for improving anti-strain aging characteristics are taken, good anti-strain aging characteristics cannot be obtained. Therefore, the N content is set to 0.010% or less. The N content is preferably as low as possible, and the preferable upper limit is 0.005%.

O: 0.005% or less O may be effective for the formation of oxides that form ferrite nuclei if the content is very small, but is extremely detrimental to the strain aging resistance characteristics as in the case of N described above. If it is an impurity element and its content increases, not only will the toughness of the base metal and its weld be significantly reduced, but it will not be possible to obtain good strain aging characteristics even if other measures for improving strain aging characteristics are taken. Therefore, the content of O is set to 0.005% or less. The lower the O content, the better. The preferable upper limit is 0.0020%, and the more preferable upper limit is 0.0015%.

  You may make the steel material which concerns on this invention contain 1 or more types selected from the following elements as needed.

Cr: 1.0% or less Since Cr is an element effective for improving the strength of a steel material, it may be contained. However, if the Cr content is excessive, weld cracks are likely to occur. Therefore, when it contains Cr, the content shall be 1.0% or less. The Cr content is preferably 0.5% or less. The above effect becomes remarkable when Cr is contained by 0.04% or more. A preferable lower limit of the Cr content is 0.08%.

V: 0.5% or less V is an element effective for improving the strength of a steel material, so may be contained. However, when the content is excessive, ductility and toughness may be deteriorated. Therefore, when V is contained, the content is set to 0.5% or less. The above effect becomes remarkable when the content is 0.004% or more. The minimum with preferable V content is 0.008%.

B: 0.01% or less B is an element effective for improving the strength of a steel material, and therefore may be contained. However, when the content is excessive, ductility and toughness may be deteriorated. Therefore, when B is contained, the content is set to 0.01% or less. The B content is preferably 0.002% or less. The above effect becomes remarkable when B is contained in an amount of 0.0004% or more. A preferable lower limit of the B content is 0.0008%.

Ca: 0.01% or less,
REM: 0.02% or less Ca and REM may be contained because they are effective elements for controlling the form of sulfide (particularly MnS) and improving low-temperature toughness. However, if the content is excessive, inclusions containing Ca and REM may be coarsened and clustered, which may impair the cleanliness of the steel material and adversely affect weldability. For this reason, when it contains 1 or more types of Ca and REM, the content shall be 0.01% or less and 0.02% or less, respectively. In particular, the upper limit of the Ca content is preferably 0.006% from the viewpoint of weldability. In order to obtain the above effects, it is preferable to contain Ca by 0.0005% or more and REM by 0.001% or more. Note that REM is a general term for a total of 17 elements of Sc, Y, and lanthanoid, and can contain one or more selected from these elements. The content of REM means the total amount of the above elements.

Mg: 0.008% or less Mg forms an finely dispersed oxide, and suppresses the coarsening of the particle size of the HAZ and exhibits the effect of improving the low temperature toughness. In order to obtain this effect, Mg may be contained. However, when Mg is contained in excess of 0.008%, a coarse oxide may be generated and toughness may be deteriorated. For this reason, when it contains Mg, the content shall be 0.008% or less. In order to acquire said effect, it is preferable to contain 0.0005% or more of Mg.

Sn: 0.50% or less Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, Sn is promptly reduced to Fe ^3+, by having an effect of reducing Fe ³⁺ concentration as oxidizing agent, since inhibit corrosion promoting effect of Fe ^3+, thereby improving the weather resistance in high airborne salt environments . Furthermore, Sn has the effect of suppressing the anodic dissolution reaction of steel and improving the corrosion resistance. In order to obtain this effect, Sn may be included. However, the above effect is saturated even if Sn is contained in excess of 0.50%. For this reason, when it contains Sn, the content shall be 0.50% or less. In order to acquire said effect, it is preferable to contain 0.03% or more of Sn. The minimum with preferable content of Sn is 0.05%, and a preferable upper limit is 0.30%.

In addition, when Sn and Cu are simultaneously contained in steel, when a steel plate is manufactured, rolling cracks are likely to occur. In order to prevent this, when Sn is contained, the Cu content is preferably less than 0.2% and the Cu content ratio (Cu / Sn ratio) is preferably 1.0 or less.

(C) YR In order to realize high deformability after strain aging, it is necessary to lower YR before aging. The increase in YR due to strain aging largely depends on the microstructure, and the reduction in YR is particularly effective in improving the buckling resistance. Since the YR after strain aging becomes lower as the YR before strain aging becomes lower, it is necessary to realize a low YR before strain aging. Specifically, the YR before aging is set to 0.72 or less. By setting the YR before aging to 0.72 or less, high deformability after aging can be stably secured. On the other hand, when YR before aging exceeds 0.72, high deformability after aging cannot be achieved depending on components, production conditions, and microstructure.

(D) Manufacturing conditions Although there is no restriction | limiting in the manufacturing method of the steel material for line pipes which concerns on this invention, For example, the following method is employable.

  The heating temperature before rolling is preferably 850 ° C. or higher. Heating the slab to such a temperature facilitates hot rolling of the steel material. The heating temperature before rolling is more preferably 950 ° C. or higher. However, if the heating temperature of the slab is too high, the austenite crystal grains may become coarse and the low temperature toughness may deteriorate. Therefore, the heating temperature is desirably 1200 ° C. or lower. The heating temperature is more preferably 1100 ° C. or lower.

  Rolling is preferably performed under the condition that the total rolling reduction in a temperature range of 900 ° C. or lower is 50% or more. The rolling finishing temperature is preferably 850 to 700 ° C.

  By setting the total rolling reduction in the temperature range of 900 ° C. or less to 50% or more, residual strain can be reliably imparted to austenite, and it becomes easy to ensure good toughness. The total rolling reduction in the temperature range of 900 ° C. or lower is more preferably 75% or higher. Here, the “total rolling reduction in a temperature range of 900 ° C. or lower” is {(thickness when reaching 900 ° C.) − (Final thickness)} / (thickness when reaching 900 ° C.) × 100 (%) is meant.

  Furthermore, favorable intensity | strength and toughness are obtained more reliably by making rolling finishing temperature into 850-700 degreeC. When the rolling finishing temperature is less than 700 ° C., the strength of the steel sheet may be insufficient. On the other hand, when it exceeds 850 ° C., it may be difficult to ensure good toughness. The rolling finishing temperature is more preferably 730 ° C. or higher. The rolling finishing temperature is more preferably 800 ° C. or lower.

  In accelerated cooling after rolling (also simply referred to as “cooling”), the cooling start temperature is preferably 850 to 700 ° C.

  The accelerated cooling after rolling is performed in order to obtain a predetermined tensile strength. If the cooling start temperature is less than 700 ° C., this effect may be reduced. On the other hand, if the cooling start temperature exceeds 850 ° C., good toughness may not be obtained. The upper limit of the cooling start temperature is more preferably 800 ° C. The lower limit is more preferably 750 ° C.

  In accelerated cooling after rolling, the cooling rate is preferably 10 ° C./s or more. If the cooling rate is less than 10 ° C./s, it may be difficult to ensure a predetermined tensile strength. In order to obtain a predetermined tensile strength more reliably, the cooling rate is preferably 20 ° C./s or more. In order to ensure good ductility of the steel sheet, the cooling rate is preferably 70 ° C./s or less.

  Each temperature mentioned above refers to the average temperature in the surface portion of the material to be rolled, and the “cooling rate” is a value obtained by dividing the temperature difference of the surface portion of the material at the start and stop of cooling by the cooling time. Point to. Here, the temperature when cooling is stopped means the maximum temperature reached after recuperation. However, when the cooling stop temperature is lower than 200 ° C., the temperature at the position in the thickness direction 1/4 (width direction 1/2 and length direction 1/2) of the material is used to start cooling. The cooling rate up to 200 ° C. is calculated.

  The accelerated cooling after rolling requires a cooling stop temperature of 400 ° C. or lower in order to ensure good deformability. Moreover, it becomes easy to obtain low YR by setting the cooling stop temperature to 400 ° C. or lower. In addition, in order to suppress generation | occurrence | production of a hydrogen crack, it is preferable that the cooling stop temperature shall be 200 degreeC or more. In addition, it is preferable to cool or slowly cool after stopping the cooling.

  A line pipe can be produced by forming the steel plate produced in the present invention into a tubular shape, joining the butt portions, and applying a coating for expanding the tube and preventing corrosion as necessary.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Using a steel piece having a chemical composition shown in Table 1 having a thickness of 140 mm, heating, rolling and accelerated cooling (water cooling) were performed under the production conditions shown in Table 2 to obtain a steel plate having a thickness of 19 mm. Each temperature shown in Table 2 is the surface temperature of the material to be rolled on the entry side of the fifth pass, the eleventh pass, and the thirteenth pass, measured using a radiation thermometer. In addition, the thickness of the steel plate on the exit side of each pass is the first pass: 130 mm, the second pass: 110 mm, the third pass: 92 mm, the fourth pass: 76 mm, the fifth pass: 64 mm, and the sixth pass: 54 mm. 7th pass: 45 mm, 8th pass: 38 mm, 9th pass: 32 mm, 10th pass: 27 mm, 11th pass: 23 mm, 12th pass: 21 mm, 13th pass: 19 mm.

  About each obtained steel plate, the tensile characteristic and the impact characteristic were investigated.

  Tensile properties are as follows: A round bar tensile test piece with a parallel part diameter of 8.5 mm and a gauge distance of 42.5 mm was taken in parallel to the rolling direction from the center of the plate thickness and subjected to a tensile test at room temperature. investigated. Specifically, 0.5% yield strength, tensile strength, uniform elongation, total elongation and drawing were obtained, and YR (0.5% yield strength / tensile strength) was calculated from these results.

  After applying 0.5% tensile prestrain (nominal strain) to the tensile test piece, heat treatment was performed at 250 ° C. for 5 minutes in a salt bath, and the tensile properties after aging were similarly investigated. This condition is a condition that is stricter than the strain aging condition by normal pipe making and coating (that is, the degree of strain aging is large and the deformation performance is easily impaired).

  Impact characteristics were measured by taking a V-notch specimen described in JIS Z 2242 (2005) perpendicularly to the rolling direction from the center of the plate thickness, performing a Charpy impact test, and at a fracture surface transition temperature and at −80 ° C. Absorbed energy was determined.

  The ferrite occupied volume ratio was obtained by revealing the microstructure of the sheet thickness cross section parallel to the rolling direction with nital, and observing the center of the sheet thickness at 500 times using an optical microscope, and performing image analysis. The MA occupied volume ratio was obtained by revealing MA by repeller corrosion on a plate thickness section parallel to the rolling direction, and observing the plate thickness central portion at 1000 times using an optical microscope, and performing image analysis.

  The test results are shown in Table 3. The deformability before and after strain aging of each steel plate obtained was YR and U.S. Evaluation was based on El.

  As shown in Table 3, No. 1 satisfying the provisions of the present invention. 1 to 10 and 15 to 22 have a ferrite occupied volume ratio of 5 to 30% and an MA occupied volume ratio of 3 to 20%. El is 9% or more, and YR after aging is 0.88 or less; El has a high deformability of 7% or more, and a tensile strength of 760 MPa or more before and after strain aging. No. Since 13 and 14 do not satisfy the component conditions defined in the present invention, high deformability is not obtained. No. No. 11 has a cooling stop temperature of 400 ° C. or higher, and does not satisfy the occupied volume ratio of ferrite and MA defined in the present invention in the microstructure, so that high strength and deformability are not obtained. No. No. 12 does not use accelerated cooling and the cooling rate is very slow, so the ferrite occupied volume ratio exceeds the upper limit and a hard bainite structure is not obtained. Therefore, although the yield ratio is low, a high tensile strength of 760 MPa or more has not been obtained.

  As described above, it is possible to produce a steel product for a line pipe having excellent strain aging resistance and a tensile strength of 760 MPa or more by the method of the present invention. This steel material is suitable as a material for a large-diameter, high-strength, high-toughness line pipe used for pipelines that transport natural gas and crude oil in large quantities.

Claims (4)

% By mass
C: 0.06% to 0.14%,
Si: 0.2-0.9%
Mn: 1.0 to 3.0%
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.5%,
Mo: 0.04 to 0.50%,
Nb: 0.005 to 0.08%,
Ti: 0.005 to 0.04% and sol. Al: 0.005 to 0.100%
The balance consists of Fe and impurities, and P, S, N and O as impurities are each P: 0.02% or less,
S: 0.005% or less,
N: a steel material having a chemical composition of 0.010% or less and O: 0.005% or less,
The microstructure is 5-30% by volume ferrite and 3-20% island martensite and the balance mainly consists of bainite,
A steel product for a line pipe, wherein the yield ratio before aging is 0.72 or less and the tensile strength is 760 MPa or more.   The composition according to claim 1, further comprising at least one selected from Cr: 1.0% or less, V: 0.5% or less, and B: 0.01% or less in mass%. Steel for line pipes.   Furthermore, it contains 1 or more types selected from Ca: 0.01% or less, REM: 0.02% or less, and Mg: 0.008% or less in the mass%, The Claim 1 or Claim characterized by the above-mentioned. 2. Steel for line pipes according to 2. The steel material for a line pipe according to any one of claims 1 to 3, further comprising, by mass%, Sn: 0.5% or less.

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