JP5472071B2 - Steel for line pipe - Google Patents

Description

    The present invention relates to a steel for line pipes corresponding to API 5L X60-70 grade (tensile strength is 520 MPa to 760 MPa and yield strength is 415 MPa to 635 MPa), and particularly a line having excellent deformation performance even after strain aging. It relates to steel for pipes.

  Large diameter line pipes are used when transporting natural gas, crude oil, etc. over long distances. Steels for line pipes are required to have high strength and toughness, and are designed based on strain (Strain-Based Design) to prevent damage to the pipeline due to ground movement during earthquakes, thawing / freezing of frozen soil, etc. In other words, it is required to have a high deformation performance.

  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 hardening of steel pipes due to strain caused by pipe making and heating during coating. In general, it is 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 structure have been disclosed. For example, Patent Documents 1 and 2 disclose steels having a three-phase structure of ferrite, bainite, and island martensite.

JP 2008-248328 A JP 2008-248330 A

  In general, it is known that a structure including a soft phase and a hard phase is effective for obtaining a steel having a low yield ratio and a high uniform elongation. In Patent Documents 1 and 2, it is said that a low yield ratio is obtained by including ferrite and 3% or more of island-like martensite, but in that example, the yield ratio is 80% or less as a standard. The yield ratio in the invention examples of Patent Documents 1 and 2 is 75% or more.

  Here, in order to realize high deformation performance after being subjected to strain aging, it is necessary to lower YR before strain 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, it is important that the YR is less than 0.75 in the state of the steel plate before strain aging, that is, before the line pipe production. By setting YR before strain aging to less than 0.75, high deformation performance after strain aging can be stably secured. On the other hand, when the YR before strain aging is 0.75 or more, high deformation performance after strain aging cannot be achieved depending on the components, production conditions, and microstructure.

  Furthermore, in the techniques disclosed in Patent Documents 1 and 2, it is said that a maximum of 87% ferrite can be included. However, the increase in ferrite is disadvantageous for securing the strength, and the strength corresponding to the API 5L X60-70 grade is not always satisfied.

  Therefore, the present invention is excellent in deformation performance (specifically, steel materials satisfying the X60 to 70 grade as the material of the line pipe, that is, steel materials having a yield strength of 415 to 635 MPa and a tensile strength of 520 to 760 MPa (specifically, It is an object to provide a steel material for a line pipe having a yield ratio of less than 0.75).

  In addition, according to API 5L / ISO 3183 which prescribed | regulated the intensity | strength of a line pipe material, X60 is a yield strength: 415-565MPa, tensile strength: 520-760MPa, X70 is a yield strength: 485-635MPa, tensile strength : 570 to 760 MPa.

  As a result of various studies to solve the above problems, the present inventors have obtained knowledge shown in the following (a) to (e).

(A) obtaining high deformation performance after strain aging, ie low YR and high U.D. In order to make El compatible, it is necessary to make it the composite structure which has the microstructure advantageous for each characteristic. The composite structure is substantially composed of ferrite, bainite, and island martensite (hereinafter referred to as “MA”).

(B) The greater the difference in strength between the soft phase and the hard phase, the lower the YR and the higher the U.V. This is effective in achieving both El and it is necessary to increase the amount of MA.

(C) The increase in the ferrite occupation area ratio is advantageous for improving the deformation performance, but it is disadvantageous for improving the strength. Therefore, it is necessary to set the ferrite occupation area ratio from the balance of these performances. Further, the finer the crystal grain size of ferrite, the more advantageous it is to ensure the strength.

  Accordingly, in order to achieve both high strength and deformation performance, it is essential to adjust the area ratio of the ferrite structure and the average crystal grain size.

(D) An increase in the area ratio occupied by bainite is advantageous for securing the strength, but causes a decrease in deformation performance. Therefore, the ferrite occupation area ratio also needs to be set from the balance between strength and deformation performance.

(E) An increase in the MA occupation area ratio is advantageous in improving the deformation performance and the strength, but an excessive increase in the MA occupation area ratio saturates the deformation characteristics while causing a decrease in the base material toughness. Furthermore, the target strength may be exceeded. Therefore, it is necessary to set the MA occupation area ratio from the balance between deformation performance, strength and toughness.

  Based on the above findings, the present inventors have made extensive studies and completed the present invention. The gist of the present invention is a steel material for a line pipe shown in the following (1) to (3).

  (1) A steel product for a line pipe having a tensile strength of 520 to 760 MPa, a yield strength of 415 to 635 MPa, and a yield ratio of less than 0.75, and in mass%, C: 0.04 to 0.10% , Si: 0.05 to 0.60%, Mn: 1.3 to 1.9%, Cr: 0.01 to 0.60%, V: 0.01 to 0.09%, Nb: 0.001 -0.09%, Ti: 0.005-0.040% and sol. Al: 0.005 to 0.060% is contained, the balance is made of Fe and impurities, and P, S, N and O as impurities are respectively P: 0.02% or less, S: 0.005% or less, Ferrite having a chemical composition of N: 0.010% or less and O: 0.005% or less and having an area ratio and an average crystal grain size of 10 μm or less: 40 to 80%, bainite: 20 to 60%, Island-like martensite: Steel for line pipes having a microstructure composed of 1.0 to 5.0%.

  (2) In place of a part of Fe, further selected by mass%, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 0.5% or less, and B: 0.01% or less (1) The steel material for line pipes of the above (1) containing 1 or more types.

  (3) In place of a part of Fe, further containing at least one selected from mass%, Ca: 0.01% or less, REM: 0.02% or less, and Mg: 0.008% or less (1) or (2) line pipe steel.

  ADVANTAGE OF THE INVENTION According to this invention, although it is steel materials which satisfy X60-70 grade, the steel material for line pipes which has the outstanding deformation | transformation performance can be provided.

  Embodiments of the present invention will be described below. In the following description, “%” for the content of each element means “mass%”, and “%” for the microstructure means “area ratio (area%)”.

(A) Chemical composition C: 0.04 to 0.10%
C is an element necessary for increasing the strength of the steel material. In order to stably obtain a tensile strength of 520 to 760 MPa, C needs to be a content of 0.04% 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 content of C is set to 0.04 to 0.10%. C is preferably contained in an amount exceeding 0.05%, and more preferably contained in an amount exceeding 0.06%. Moreover, the upper limit with preferable C content is 0.08%.

Si: 0.05-0.60%
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 obtain these effects, 0.05% or more of Si is contained. 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 0.05 to 0.60%. Si is preferably contained in an amount exceeding 0.20%. Moreover, the upper limit with preferable Si content is 0.50%.

Mn: 1.3 to 1.9%
Mn has the effect | action which raises the intensity | strength of steel materials. In order to sufficiently obtain this effect, 1.3% or more of Mn is contained. On the other hand, if the content is excessive, weld cracks are likely to occur. Also, if the Mn content is excessive, good deformation characteristics, ie, low YR and high U.D. It becomes difficult to obtain El. Therefore, the Mn content is set to 1.3 to 1.9%. The minimum with preferable Mn content is 1.4%, and a more preferable minimum is 1.5%. Moreover, the upper limit with preferable Mn content is 1.8%, and a more preferable upper limit is 1.7%.

Cr: 0.01-0.60%
Cr is contained because it is an effective element for improving the strength of the steel material. In order to obtain this effect, it is necessary to contain 0.01% or more of Cr. On the other hand, if the Cr content is excessive, weld cracks are likely to occur. Therefore, the Cr content is set to 0.01 to 0.60%. The minimum with preferable Cr content is 0.04%, and a more preferable minimum is 0.08%. On the other hand, the preferable upper limit of the Cr content is 0.50%.

V: 0.01 to 0.09%
V is an element effective for improving the strength of the steel material. In order to acquire this effect, it is necessary to contain V 0.01% or more. On the other hand, when the V content is excessive, ductility and toughness deteriorate. Therefore, the V content is set to 0.01 to 0.09%. The minimum with preferable content of V is 0.02%, and a more preferable minimum is 0.03%. On the other hand, the upper limit with preferable V content is 0.08%.

Nb: 0.001 to 0.09%
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 0.001% or more. However, when there is too much the content, the toughness of a base material and HAZ will deteriorate. Therefore, the Nb content is set to 0.001 to 0.09%. A preferred lower limit is 0.01%, and a more preferred lower limit is. Moreover, a preferable upper limit is 0.08% and a more preferable upper limit is 0.07%.

Ti: 0.005-0.040%
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 0.040%, the toughness of the base material and the HAZ deteriorates. Therefore, the Ti content is set to 0.005 to 0.040%. A preferred lower limit is 0.010% and a preferred upper limit is 0.030%. Furthermore, the ratio of Ti and N content (Ti / N) is preferably 4.0 or more.

sol. Al: 0.005-0.060%
Al is an element having a deoxidizing action. Since it is effective in improving El, sol. It is contained 0.005% or more as Al (acid-soluble Al). 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 0.060%. Note that sol. The lower limit of the Al content is preferably 0.010%, and the upper limit is preferably 0.050%.

  The chemical composition of the steel product for line pipes according to the present invention contains each of the above elements, and the balance consists of Fe and impurities. 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, S, N and O as impurities can be allowed to the following ranges.

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. In addition, it is better that the N content is small, 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. In addition, it is better that the O content is small, and the preferable upper limit is 0.0020%, and the more preferable upper limit is 0.0015%.

  The steel material for line pipes according to the present invention may contain the following elements instead of a part of Fe.

Cu: 1.0% or less Cu has an effect of improving the strength of the steel material, so it may be contained as necessary. However, if the content is large, the surface properties and toughness of the steel material are significantly deteriorated. . Therefore, the content when Cu is contained is set to 1.0% or less. The above effect becomes remarkable when 0.05% or more is contained. The minimum with preferable Cu content is 0.1%, and a more preferable minimum is 0.2%. Moreover, a preferable upper limit is 0.6% and a more preferable upper limit is 0.5%.

Ni: 1.0% or less Ni has an effect of improving the strength of the steel material, and also has an effect of improving toughness. Therefore, Ni may be contained if necessary, but the Ni content is 1.0. If it exceeds%, an effect commensurate with the cost increase cannot be obtained. Therefore, the content when Ni is contained is set to 1.5% or less. The above effect becomes remarkable when 0.05% or more is contained. The minimum with preferable Ni content is 0.1%, and a more preferable minimum is 0.2%. Moreover, a preferable upper limit is 0.8% and a more preferable upper limit is 0.6%.

Mo: 0.5% or less Mo has an effect of improving the strength of the steel material, and further has an effect of accelerating the formation of MA. Therefore, Mo may be contained as necessary, but its content is excessive. As a result, the 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 content when Mo is contained is set to 0.50% or less. This effect becomes remarkable when the content is 0.04% or more. The minimum with preferable Mo content is 0.05%, and a more preferable minimum is 0.07%. Moreover, a preferable upper limit is 0.4% and a more preferable upper limit is 0.3%.

B: 0.01% or less B is an element effective for improving the strength of a steel material, and may be contained as necessary. However, when the content is excessive, ductility and toughness deteriorate. There is a fear. Therefore, when B is contained, the content is set to 0.01% or less. The above effect becomes remarkable when the content is 0.0004% or more. A preferable lower limit of the B content is 0.0008%. A preferred upper limit is 0.002%.

Ca: 0.01% or less,
REM: 0.02% or less Ca and REM are effective elements for controlling the form of sulfide (especially MnS) and improving low-temperature toughness. Therefore, Ca and REM may be contained as necessary. Is over 0.01%, or when REM is over 0.02%, inclusions containing Ca and REM may be coarsened and clustered, impairing the cleanliness of the steel material, and weldability May also be adversely affected. For this reason, content in the case of containing Ca and REM shall be 0.01% or less and 0.02% or less, respectively. . The above-mentioned effects become remarkable when Ca is contained in an amount of 0.0005% or more and REM is contained in an amount of 0.001% or more. In particular, the upper limit of the Ca content is preferably 0.006% from the viewpoint of weldability. Note that REM is a generic name 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 oxide finely dispersed and suppresses the coarsening of the particle size of HAZ and exhibits the effect of improving low-temperature toughness. However, if the content is excessive, a coarse oxide may be generated and the toughness may be deteriorated. For this reason, when it contains Mg, the content shall be 0.008% or less. The above effect becomes remarkable when Mg is contained in an amount of 0.0005% or more.

(B) About microstructure The steel for line pipes according to the present invention has a microstructure composed of ferrite, bainite and MA (island martensite), and by optimizing the composition ratio of the structure, High strength and high deformation performance, that is, low YR and high U.D. It has El. When a steel pipe is manufactured using the steel material for steel pipe (steel plate) of the present invention, the pre-formed steel plate has a high deformation performance. And U.S. El hardly deteriorates.

Ferrite having an average grain size of 10 μm or less: 40 to 80%
Ferrites are low YR and high U.V. Although effective for realizing El, an increase in the ferrite structure decreases the strength of the steel material. Therefore, the ferrite occupation area ratio in the composite structure is set to 40 to 80%. A preferable upper limit of the ferrite occupation area ratio is 70%. Moreover, the minimum with a preferable ferrite occupation area rate is 50%.

  As described above, when the ferrite structure is increased, the strength of the steel material is reduced, but when the ferrite structure is refined, the strength reduction can be suppressed. Therefore, the average crystal grain size of the ferrite structure is set to 10 μm or less. The lower the ferrite average crystal grain size, the more effective the strength improvement, so no lower limit is particularly defined. However, according to the manufacturing method described later, the average crystal grain size of ferrite is 5.0 μm even if it is small.

Bainite: 20-60%
As described above, when the ferrite structure is increased, the strength of the steel material is reduced, and it becomes difficult to obtain a steel material of API 5L X60-70 grade. In order to obtain this effect, the bainite occupation area ratio needs to be 20% or more. However, when the bainite occupation area ratio exceeds 60%, the strength may increase excessively as the X70 grade is exceeded. Therefore, the bainite occupation area ratio in the composite structure is set to 20 to 60%. The bainite occupation area ratio is preferably less than 50%.

MA: 1.0-5.0%
The increase in the area occupied by the MA increases the tensile strength, as well as the low YR and high U.D. It is effective for obtaining El. This effect is exhibited when the MA occupation area ratio is 1.0% or more. On the other hand, when the MA occupation area ratio exceeds 5.0%, the effect of improving the deformation performance is slightly saturated, and the base material toughness is deteriorated. In addition, the tensile strength may increase, and the strength may increase excessively as it exceeds the X70 grade. Therefore, the MA occupation area ratio is set to 1.0 to 5.0%. A preferable lower limit of the MA occupation area ratio is 1.5%. The MA occupation area ratio is preferably less than 3.0%.

  The steel material for line pipes according to the present invention has a microstructure composed of the above ferrite, bainite and MA, but some of them may have different microstructures such as pearlite and / or cementite. Good. However, when a structure other than ferrite, bainite, and MA is present, it is difficult to achieve both strength and deformation performance. Therefore, the occupied area ratio of the structure other than ferrite, bainite, and MA is preferably as small as possible. The upper limit of the rate is preferably 3%.

(C) 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 1100 ° C. or higher. This is because Nb is contained in the chemical composition of the slab, so that the effect of Nb can be obtained with certainty if it is present as solid solution Nb in the matrix by heating the slab. Moreover, hot rolling of steel materials becomes easy by heating a slab to such temperature. Note that if the heating temperature of the slab is too high, the austenite crystal grains may become coarse and the low temperature toughness may deteriorate, so the heating temperature is desirably 1200 ° 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 desirably 800 to 650 ° 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 750-650 degreeC. When the rolling finishing temperature is less than 650 ° C., the strength of the steel sheet is insufficient and processed ferrite is generated, and the toughness is remarkably deteriorated.

  In accelerated cooling after rolling (also simply referred to as “cooling”), the cooling start temperature is preferably set to 700 to 630 ° C. The accelerated cooling after rolling is performed in order to obtain a predetermined tensile strength. If the cooling start temperature is less than 630 ° C., this effect may be reduced. The lower limit of the cooling start temperature is more preferably 650 ° C. On the other hand, the upper limit of the cooling start temperature is more preferably 680 ° 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.

  The accelerated cooling after rolling requires a cooling stop temperature of 500 to 300 ° C. in order to ensure good deformation performance. By setting the cooling stop temperature to 500 ° C. or lower, it becomes easy to obtain a low YR. If the cooling stop temperature is 300 ° C. or less, it is difficult to ensure the strength properly. In order to suppress the occurrence of hydrogen cracking, it is effective to stop the accelerated cooling halfway, and the cooling stop temperature is preferably set to 300 ° C. or higher. In addition, it is preferable to cool or slowly cool after stopping the cooling.

  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.

  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 26 mm. In rolling, the thickness of the steel sheet on the exit side of each pass is set to the thickness of the first pass: 130 mm, the second pass: 110 mm, the third pass: 92 mm, the fourth pass: 76 mm, and the fifth pass: 64 mm. The sixth pass: 54 mm, the seventh pass: 45 mm, the eighth pass: 38 mm, the ninth pass: 32 mm, the tenth pass: 28 mm, and the eleventh pass: 26 mm. In addition, each temperature shown in Table 2 is the surface temperature of the to-be-rolled material measured using the radiation thermometer.

  About each obtained steel plate, while observing a structure | tissue, investigation of the tensile characteristic was performed.

  In the structure observation, the occupied area ratio of ferrite (α) and bainite (B) shows the microstructure of the plate thickness cross section parallel to the rolling direction in nital, and observes the center of the plate thickness at 500 times using an optical microscope Then, image analysis was performed. For the MA occupation area ratio, MA was revealed by repeller corrosion on a plate thickness section parallel to the rolling direction, and the center of the plate thickness was observed 1000 times using an optical microscope, and image analysis was performed. The results are also shown in Table 2.

  The tensile test was performed at room temperature by collecting a round bar tensile test piece having a parallel part diameter of 8.5 mm and a gauge distance of 42.5 mm from the center of the thickness of the steel sheet before and after the aging treatment. In this tensile test, 0.5% yield strength and tensile strength were determined for tensile test specimens taken from the direction perpendicular to the rolling direction (C direction). In tensile test specimens taken from the direction parallel to the rolling direction (L direction), 0.5% proof stress, tensile strength, uniform elongation and total elongation after aging, YR (0.5% proof stress / tensile) Strength). The results are shown in Table 3.

  As for aging conditions, 0.3% L-direction tensile prestrain (nominal strain) was applied, and then heat treatment was performed at 250 ° C. for 5 minutes in a salt bath. This condition is a condition that is severer than the strain age hardening phenomenon caused by normal pipe making and coating (that is, the degree of strain aging is large and the deformation performance is easily impaired).

  The deformation performance of each obtained steel sheet before and after strain aging is YR and U.S. Evaluation was based on El. In addition, since the strength in the C direction before pipe making (before strain aging) is a criterion for determining API grades, the evaluation of whether it corresponds to X60 to X70 grades is based on the tensile strength and yield strength of the test piece as it is. did.

  As shown in Table 3, No. 1 satisfying the conditions defined in the present invention. Nos. 3 to 10 had excellent deformation performance in which the YR in the L direction was less than 0.75 before simulated aging treatment (hereinafter simply referred to as “aging”) and 0.80 or less even after aging. Moreover, these have the tensile strength and yield strength of X60-70 grade. In addition, U.I. El is also a large value of 11.0% or more before and after the aging treatment, and has high deformation performance.

  No. which does not satisfy the chemical composition defined in the present invention. 1 has a tensile strength and a yield strength of X60 to 70 grade, but the amount of island-like martensite produced is small, the L direction YR before aging is large, and as a result, the YR after aging is 0.82. It became bigger. In addition, U.S. Since El is also a relatively small value, high deformation performance is not obtained.

  No. which does not satisfy the chemical composition defined in the present invention. In No. 2, since not only the component but also the cooling stop temperature is low, the amount of island martensite produced is large, and the tensile strength before aging in the C direction exceeds the specified grade strength.

  No. No. 11 satisfies the chemical composition defined in the present invention, but the cooling start temperature is low, the occupied area ratio of ferrite and bainite is outside the range defined in the present invention, and the ferrite grain growth progresses and the ferrite The average crystal grain size exceeded 10 μm. For this reason, the yield strength before aging in the C direction was lower than the prescribed grade strength.

  No. 12 also satisfies the chemical composition defined in the present invention, but the finishing temperature and the cooling start temperature are high, and the occupied area ratio of ferrite and bainite is out of the range defined in the present invention. The tensile strength exceeded the specified grade strength. Since the amount of MA produced is within the range defined by the present invention, YR is satisfactory, but the U.S. El is low and does not have deformation performance with excellent elongation balance.

  No. 13 also satisfied the chemical composition defined in the present invention, but because the cooling stop temperature was high, the amount of island martensite produced was small. For this reason, a low YR could not be obtained.

  According to the present invention, it is possible to provide a steel material for a line pipe having excellent strain aging resistance, a tensile strength of 520 to 760 MPa, a yield strength of 415 to 635 MPa, and a yield ratio of less than 0.75. 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 (3)

A steel for line pipes having a tensile strength of 520 to 760 MPa, a yield strength of 415 to 635 MPa, and a yield ratio of less than 0.75,
In mass%, C: 0.04 to 0.10%, Si: 0.05 to 0.60%, Mn: 1.3 to 1.9%, Cr: 0.01 to 0.60%, V: 0.01-0.09%, Nb: 0.001-0.09%, Ti: 0.005-0.040% and sol. Al: 0.005 to 0.060% is contained, the balance is made of Fe and impurities, and P, S, N and O as impurities are respectively P: 0.02% or less, S: 0.005% or less, N: 0.010% or less and O: 0.005% or less of the chemical composition, and the area ratio of ferrite having an average crystal grain size of 10 μm or less: 40-80%, bainite: 20-60%, Island-like martensite: A steel material for line pipes having a microstructure composed of 1.0 to 5.0%.   In place of a part of Fe, in addition to 1% by mass, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 0.5% or less, and B: 0.01% or less The steel material for line pipes according to claim 1 containing the above.   Instead of a part of Fe, it is further characterized by containing at least one selected from Ca: 0.01% or less, REM: 0.02% or less, and Mg: 0.008% or less in mass%. The steel material for line pipes according to claim 1 or 2.