KR20170118899A - Steel plate for high-strength line pipe with excellent low temperature toughness and steel pipe for high strength line pipe - Google Patents

Abstract

A steel sheet for a high strength line pipe excellent in both low temperature toughness, particularly CTOD characteristics and DWTT characteristics, is provided. When a high strength line pipe steel sheet of the present invention, with the satisfaction of a predetermined component, t the plate thickness, the position of t / 2, and an oxide or more of circle equivalent diameter of 2μm 10 gae / mm ² or less, the average equivalent diameter of the crystal grains surrounded by the diagonal grain boundary having the azimuthal difference of the adjacent two crystals at the position of t / 2 is 15 占 or more is 10 占 퐉 or less, the fraction of the hard tissue at the position of t / 2 is 5% And the separation index SI measured from the Charpy test piece wave front at the specified temperature satisfies 0.15 mm / mm ² or less.

Description

Steel plate for high-strength line pipe with excellent low temperature toughness and steel pipe for high strength line pipe

The present invention relates to a steel plate for a high strength line pipe excellent in low temperature toughness and a steel pipe for a high strength line pipe manufactured from the steel plate for a high strength line pipe. More particularly, the present invention relates to a steel plate for a high-strength line pipe excellent in both CTOD (Crack Tip Opening Displacement) characteristic and DWTT (Drop Weight Tear Test) characteristic, and a steel pipe for a high strength line pipe.

The line pipe used for transporting natural gas or crude oil tends to increase the operating pressure to improve the transportation efficiency, and there is a demand for a higher strength in the steel for the line pipe. In addition, from the viewpoint of safety, it is required that the CTOD characteristic and the DWTT characteristic, which are one of the evaluation indexes of fracture toughness, are excellent as the brittle fracture preventing characteristics.

From the viewpoint of high strength, reinforcement by solid solution strengthening, precipitation strengthening, transformation strengthening and dislocation strengthening can be considered as reinforcing mechanisms of steel materials. Among them, the dislocation strengthening which increases the strength of the material by increasing the dislocation density is a phenomenon in which the cumulative reduction rate at the so-called two-phase temperature region where ferrite is transformed from austenite single phase structure in the rolling process It is an enhancement mechanism that is easy to apply as compared with other reinforcement mechanisms.

However, by increasing the cumulative reduction ratio at the two-phase temperature region, the crystal orientation is rotated with the increase of the dislocation density, and the texture develops. The development of this texture causes a difference in toughness between the rolled surface direction and the plate thickness direction. This is because, in the Charpy test or COTD test using the test piece taken from the rolled surface direction, A minute opening is generated in the direction. This separation is caused by an increase in the difference in toughness between the rolled surface direction and the plate thickness direction. Therefore, in addition to the influence of the aggregate structure, the separation is caused by the S existing in the steel, But also by the formation of MnS.

In the CTOD test, if the above separation occurs before brittle cracks occur, it is determined that the separation is stably opened only to the position where the separation has occurred, so that the limit CTOD value, which is an index of the CTOD characteristic, do. Therefore, in the material in which the separation occurs, the limit CTOD value can not be improved only by improving the toughness of the base material, which is evaluated, for example, by the wave-front transition temperature vTrs.

For this reason, for example, in Patent Document 1, in order to obtain solid DWTT characteristics with suppression of separation and enhancement of solubility by addition of expensive elements, the rolling temperature in the austenite non-recrystallized region, Temperature and the like are controlled.

Japanese Patent Application Laid-Open No. 2013-47393

In the technique described in Patent Document 1, it is necessary to adopt a hardening process by adding an expensive element, a complicated manufacturing process combining an on-line water-cooling facility and a heating facility, and special rolling conditions, .

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique capable of manufacturing a steel sheet for a high strength line pipe which is excellent in both low temperature toughness, particularly CTOD characteristics and DWTT characteristics, have.

A steel sheet for a high strength line pipe according to the present invention which can solve the above problems is a steel sheet for a high strength line pipe which contains 0.02 to 0.2% of C, 0.02 to 0.5% of Si, 0.6 to 2.5% of Mn, S: more than 0% to 0.01%, Al: 0.010 to 0.08%, Nb: 0.001 to 0.1%, Ti: 0.003 to 0.03%, Ca: 0.0003 to 0.006% %, REM: 0.0001 to 0.005%, and Zr: 0.0001 to 0.005%, with the balance being iron and unavoidable impurities, and assuming that the plate thickness is t, the at least oxide of 10 / mm ² or less contained, and, t / 2 in the position of the circle-equivalent crystal grain orientation difference surrounded to step 15 ° or more diagonal mouth of the adjacent two crystals mean diameter 10μm or less, t / 2 this fraction of the hard tissue at the location where the separation index SI determined from Charpy specimen wave front of the designated temperature, with 5% by area or less satisfied the base to 0.15mm / mm ² to less than or equal to Have.

In a preferred embodiment of the present invention, the steel sheet comprises, by mass%, Cu: not less than 0% but not more than 1.5%, Ni: not less than 0% but not more than 1.5%, Cr: not less than 0% %, V: more than 0% and not more than 0.2%, and B: more than 0% and not more than 0.0003%.

The present invention also includes a steel pipe for a high-strength line pipe, which is produced by using the steel plate for a high-strength line pipe and has excellent low-temperature toughness.

According to the present invention, when the plate thickness is t, the number density of coarse oxides having a circle equivalent diameter of 2 占 퐉 or more at a position of t / 2 and the separation index SI measured from the Charpy test piece wave front at a specified temperature, The CTOD characteristic is improved and the circle equivalent mean diameter of crystal grains surrounded by the diagonal grain boundary in which the azimuth difference of two adjacent crystals at the position of t / 2 is 15 degrees or more and the hard tissue fraction are appropriately It is possible to improve the DWTT characteristics. Therefore, according to the present invention, it is possible to realize a steel sheet for a high-strength line pipe excellent in both CTOD and DWTT characteristics and excellent in low-temperature toughness having a tensile strength of 520 MPa or more without using expensive alloying elements.

1 is a schematic diagram of a Charpy test piece wavefront for explaining a method of measuring the separation index SI.
2 is a graph showing the relationship between the separation index SI and the limit CTOD value, which is an index of the CTOD characteristic.
3 is a graph showing the relationship between the hard tissue fraction at the t / 2 position and the cooling stop temperature (FCT).

The inventors of the present invention have studied to provide a steel sheet for a high-strength line pipe excellent in both CTOD characteristics and DWTT characteristics. Specifically, with respect to the improvement of the CTOD characteristics, aiming at a steel sheet for a high-strength line pipe in which an excellent CTOD value can be obtained after some occurrence of separation is allowed, rather than completely suppressing the occurrence of separation, The occurrence of separation in the test and the relationship between the microstructure were examined. As a result, the limit CTOD value obtained by the CTOD test is correlated with the separation index SI in the Charpy test, and the separation index SI measured from the Charpy test piece wave front at the specified temperature is lowered, and the t / 2 position It is possible to suppress the number density of coarse oxides having a circle-equivalent diameter of 2 占 퐉 or more. Regarding the improvement of the DWTT characteristics, the average equivalent diameter of the crystal grains surrounded by the diagonal grain boundary in which the azimuth difference of two adjacent crystals at the position of t / 2 is 15 degrees or more is miniaturized, and the hard tissue fraction And the present invention has been completed.

Hereinafter, each of the requirements specified in the steel sheet for a line pipe of the present invention will be described. In this specification, unless otherwise specified, t means plate thickness.

(average crystal grain size surrounded by a diagonal grain boundary in which the azimuth difference of two adjacent crystals is 15 degrees or more at a position of t / 2: 10 mu m or less)

In order to secure good DWTT characteristics, it is necessary to secure the toughness of the base material by making crystal grains finer. Therefore, in the present invention, the upper limit of the circle equivalent mean diameter of the crystal grains surrounded by the diagonal grain boundary having the azimuth difference of two adjacent crystals at the position of t / 2 is 15 占 or less. The average crystal grain size is preferably 8.0 탆 or less, and more preferably 7.0 탆 or less. The smaller the average crystal grain size is, the better, but the lower limit is generally 4 탆 or more.

On the other hand, in the present invention, the measurement of the circle equivalent mean diameter of the crystal grains surrounded by the diagonal grain boundary having the azimuth difference of the adjacent two crystals of 15 degrees or more and the density number of the oxide grains having a circle- The reason for setting t / 2 instead of t / 4, which is a representative position of the characteristic evaluation, is to improve the toughness of the portion that becomes the origin of fracture and to secure the desired DWTT characteristics.

(Separation index SI measured from the Charpy specimen wave front at the specified temperature SI: 0.15 mm / mm ² or less)

By setting the separation index SI of the fracture surface of the Charpy test piece at the specified temperature to 0.15 mm / mm ² or less, it is possible to secure the target limit CTOD value even if separation occurs in the CTOD test. The target limit CTOD value is 0.25 mm or more when the test temperature is -10 占 폚. On the other hand, the designated temperature can be obtained from the following expression (1). That is, the test temperature (set temperature) are made different according to the plate thickness, in order to when the test temperature was -10 ℃ to evaluate the limit CTOD value aiming, (T ₁₎ is specified, the temperature at a Charpy test conducted . T ₁ : Charpy test temperature (캜), and T ₂ : CTOD test temperature (캜). In this specification, -10 캜 and t: plate thickness (mm), respectively.

T ₁ = T ₂ -6 × (t) ^1/2 + 20 (1)

The separation index SI can be obtained by dividing the total length of separation generated perpendicularly to the thickness direction of the Charpy test piece wave front face by the area (cross sectional area) of the fracture surface of the test piece as shown in the following formula (2) 1). Here, Ln denotes the n-th separation length (mm), and S _A denotes the cross-sectional area (mm ² ) of the wave front.

SI = Σ (Ln) / S A (2)

In the steel sheet for a high-strength line pipe of the present invention, it is necessary to set the separation index SI obtained as described above to 0.15 mm / mm ² or less. The separation index SI is preferably 0.12 mm / mm ² or less, and more preferably 0.10 mm / mm ² or less. However, the separation index SI does not necessarily have to be 0 mm / mm ² from the viewpoint of exhibiting a high limit CTOD value even if separation occurs. From this viewpoint, the separation index SI is preferably 0.05 mm / mm ² or more, and more preferably 0.10 mm / mm ² or more.

For reference, Fig. 2 shows the relationship between the separation index SI and the CTOD characteristic. This figure plots the relationship between the limiting CTOD value measured as an index of CTOD characteristics and the separation index SI based on the results of the later embodiments. From this figure, it can be seen that when the separation index SI is 0.15 mm / mm ² or less, the limit CTOD value at -10 캜, which is the acceptance criterion of the CTOD characteristic, satisfies 0.25 mm.

(10 pieces / mm ² or less of oxide having a circle-equivalent diameter of 2 탆 or more at a position of t / 2)

Since the coarse oxide adversely affects the improvement of the CTOD characteristics, the number density of the oxide of the above-mentioned size is set to 10 / mm ² or less in the present invention. The number density of the oxide is preferably as small as possible, preferably not more than 8 / mm ² , more preferably not more than 5 / mm ² . The lower limit is not particularly limited in view of the above, but in view of the productivity in the slab manufacturing step, it is preferable that the lower limit is about 0.1 pcs / mm ² or more.

On the other hand, the oxide to be used in the present invention means that the oxide is observed when observed by the method described in the later examples.

(the percentage of the hard tissue at the position of t / 2 is not more than 5% by area)

Since the hard tissue adversely affects the improvement of the DWTT characteristics, in the present invention, the area fraction of the hard tissue occupied in the entire structure observed at the position of t / 2 is set to 5% or less. The area fraction of the hard tissue is preferably as small as possible, preferably not more than 3% by area, more preferably not more than 1% by area. On the other hand, the lower limit is not particularly limited in view of the above, and may be, for example, 0 area%.

On the other hand, examples of the hard tissues to be subjected in the present invention include island-shaped martensite and martensite.

It is necessary to appropriately adjust the chemical composition of the steel sheet for a high strength line pipe of the present invention. The reasons for setting the range of the chemical composition are as follows. On the other hand, in terms of chemical composition,% means mass%.

(C: 0.02 to 0.2%)

C is an indispensable element for ensuring the strength of the steel sheet and the welded portion as the base material, and for this purpose, it is necessary to contain C in an amount of 0.02% or more. The C content is preferably 0.03% or more, and more preferably 0.05% or more. However, if the amount of C is excessive, the Martensite-Austenite contitant (MA) easily forms, and the toughness of the HAZ (Heat Affected Zone) is lowered and the weldability is lowered. From this point of view, the C content needs to be 0.2% or less. The C content is preferably 0.15% or less, and more preferably 0.12% or less.

(Si: 0.02 to 0.5%)

Si has a deoxidizing action and is effective for improving the strength of the steel sheet and the welded portion as the base material. In order to exhibit these effects, the amount of Si should be 0.02% or more. The amount of Si is preferably 0.05% or more, and more preferably 0.15% or more. However, when the amount of Si is excessive, the weldability and toughness are deteriorated. Therefore, it is necessary to suppress the amount of Si to 0.5% or less. The Si content is preferably 0.45% or less, and more preferably 0.35% or less.

(Mn: 0.6 to 2.5%)

Mn is an effective element for improving the strength of the steel sheet and the welded part which are the base materials. In order to exhibit such an effect, Mn should be contained in an amount of 0.6% or more. The amount of Mn is preferably 1.0% or more, and more preferably 1.2% or more. However, when the amount of Mn is excessive, MnS is generated to promote the occurrence of separation, and also the HAZ toughness and weldability are deteriorated. Therefore, the upper limit of the amount of Mn is set to 2.5% or less. The Mn content is preferably 2.0% or less, more preferably 1.9% or less, and further preferably 1.8% or less.

(P: more than 0% to 0.03% or less)

P is an element inevitably included in the steel. When the P content exceeds 0.03%, deterioration of the base material toughness and HAZ toughness is remarkable. Therefore, in the present invention, the amount of P is suppressed to 0.03% or less. The P content is preferably 0.020% or less, more preferably 0.015% or less, and still more preferably 0.010% or less. The amount of P is preferably as small as possible, but it is difficult to industrially make 0%.

(S: more than 0% and not more than 0.01%)

When the amount of S becomes excessive, MnS is generated and the occurrence of separation is promoted, so the upper limit is set to 0.01% or less. The S content is preferably 0.008% or less, more preferably 0.006% or less, and still more preferably 0.005% or less. From the viewpoint of suppressing generation of separation as described above, it is preferable that the amount of S be small, but it is difficult to industrially make it 0%. The preferable lower limit of the amount of S is about 0.0001% or more.

(Al: 0.010 to 0.08%)

Al is a strong acid element, and it is necessary to contain at least 0.010% in order to obtain a deoxidizing effect. The amount of Al is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, when the amount of Al becomes excessive, a large amount of AlN is generated, and the amount of TiN precipitation decreases, and HAZ toughness is lowered. Therefore, the amount of Al needs to be 0.08% or less. The amount of Al is preferably 0.06% or less, and more preferably 0.05% or less.

(Nb: 0.001 to 0.1%)

Nb is an effective element for increasing strength and toughness of a base material without deteriorating weldability. In order to exhibit such an effect, the amount of Nb must be 0.001% or more. The amount of Nb is preferably 0.005% or more, and more preferably 0.010% or more. However, if the amount of Nb becomes excessive and exceeds 0.1%, the toughness of the base material and the HAZ deteriorates. Therefore, the upper limit of the amount of Nb is set to 0.1% or less. The amount of Nb is preferably 0.08% or less, more preferably 0.05% or less.

(Ti: 0.003 to 0.03%)

Ti is precipitated as TiN in the steel to improve the toughness of the base material due to suppression of coarsening of the austenite during the heating of the slab and to improve the toughness of the HAZ by coarsening of the austenite grains in the HAZ at the time of welding to be. In order to exhibit such an effect, the Ti amount needs to be 0.003% or more. The amount of Ti is preferably 0.005% or more, and more preferably 0.01% or more. On the other hand, when the amount of Ti becomes excessive, Ti and TiC which are solid solution are precipitated, and the toughness of the base material and the HAZ deteriorates. Therefore, it is required to be 0.03% or less. The amount of Ti is preferably 0.025% or less, and more preferably 0.020% or less.

(Ca: 0.0003 to 0.006%)

Ca has an effect of controlling the form of sulfide and has the effect of inhibiting the formation of MnS by forming CaS. In order to exhibit such an effect, the amount of Ca needs to be 0.0003% or more. The amount of Ca is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the amount of Ca exceeds 0.006% and becomes excessive, the toughness deteriorates, so the upper limit of the amount of Ca is set to 0.006% or less. The amount of Ca is preferably 0.005% or less, and more preferably 0.004% or less.

(N: 0.001 to 0.01%)

N is precipitated as TiN in the steel to improve the toughness of the base material due to suppression of coarsening of the austenite during the heating of the slab and to improve the toughness of the HAZ by coarsening of the austenite grains in the HAZ at the time of welding to be. In order to exhibit these effects, N should be contained in an amount of 0.001% or more. The N content is preferably 0.003% or more, and more preferably 0.004% or more. However, if the amount of N becomes excessive, toughness in HAZ deteriorates due to the presence of solid solution N, and therefore, it is required to be 0.01% or less. The N content is preferably 0.008% or less, and more preferably 0.006% or less.

(O: more than 0% to less than 0.0045%)

O is preferable from the standpoint of suppressing coarse oxide. From this point of view, in the present invention, the upper limit of the amount of O is 0.0045% or less. , Preferably not more than 0.0040%, more preferably not more than 0.0035%. O amount is preferably small, but it is difficult to industrially make it 0%.

(REM: 0.0001-0.005%)

REM (rare earth element) is an element contributing to improvement of CTOD characteristics by forming oxides and finely dispersing them. In order to exhibit such an effect, it is necessary to contain REM in an amount of 0.0001% or more. The amount of REM is preferably 0.0003% or more, and more preferably 0.0005% or more. On the other hand, when a large amount of REM is contained, coarse inclusions are formed to deteriorate the toughness of the base material, so that the upper limit of the amount of REM is 0.005% or less. In the present invention, REM means 15 elements from La to Lu, which are lanthanoid elements, and scandium Sc and yttrium Y.

(Zr: 0.0001 to 0.005%)

Zr is an element contributing to the improvement of the CTOD characteristics by forming oxides and finely dispersing them. In order to exhibit such an effect, it is necessary to set the amount of Zr to 0.0001% or more. The amount of Zr is preferably 0.0003% or more, and more preferably 0.0005% or more. On the other hand, when the amount of Zr becomes excessive, coarse inclusions are formed to deteriorate the toughness of the base material. Therefore, the amount of Zr needs to be 0.005% or less. The amount of Zr is preferably 0.003% or less, more preferably 0.002% or less, still more preferably 0.001% or less.

In the steel plate for a high strength line pipe of the present invention, the chemical composition is as described above, and the balance is substantially iron. However, it is a matter of course that inevitable impurities included in raw materials, materials, manufacturing facilities, and the like are included in the steel. Examples of the inevitable impurities include As, Sb, Sn, H and the like.

The steel sheet for a line pipe of the present invention may further contain one or more kinds of elements selected from the group consisting of Cu, Ni, Cr, Mo, V and B in the following amounts, if necessary. These elements improve the strength and toughness of the base material and the HAZ, and may be used alone or in combination of two or more. The reason for setting the range for containing them is as follows.

(Cu: more than 0% to 1.5% or less)

Cu is an effective element for increasing the strength. In order to exhibit such an effect, it is preferable to contain Cu in an amount of 0.01% or more. The amount of Cu is more preferably 0.05% or more, and more preferably 0.10% or more. However, if the amount of Cu is excessive, the toughness of the base material deteriorates, and therefore, it is preferable to be 1.5% or less. The amount of Cu is more preferably 1.0% or less, and still more preferably 0.5% or less.

(Ni: more than 0% to 1.5% or less)

Ni is an effective element for improving the strength and toughness of a base material and a welded portion. In order to obtain such an effect, the amount of Ni is preferably 0.01% or more. The amount of Ni is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if Ni is contained in a large amount, it becomes extremely expensive as a structural steel, and therefore, from an economic viewpoint, it is preferable that the amount of Ni is 1.5% or less. The amount of Ni is more preferably 1.0% or less, and still more preferably 0.5% or less.

(Cr: more than 0% to 1.5% or less)

Cr is an element effective for improving the strength. In order to obtain such an effect, it is preferable that Cr is contained in an amount of 0.01% or more. The Cr content is more preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, when the amount of Cr exceeds 1.5%, the HAZ toughness is deteriorated. Therefore, the amount of Cr is preferably 1.5% or less. The Cr content is more preferably 1.0% or less, and still more preferably 0.5% or less.

(Mo: more than 0% to 1.0% or less)

Mo is an effective element for improving the strength and toughness of a base material. In order to obtain such an effect, the Mo content is preferably 0.01% or more. The amount of Mo is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the amount of Mo exceeds 1.0%, the HAZ toughness and weldability deteriorate. Therefore, the amount of Mo is preferably 1.0% or less, and more preferably 0.5% or less.

(V: more than 0% to 0.2% or less)

V is an element effective for improving the strength. In order to obtain such an effect, it is preferable that V is contained in an amount of not less than 0.003%. The V content is more preferably 0.010% or more. On the other hand, if the V content exceeds 0.2%, the weldability and the toughness of the base material deteriorate. Therefore, the amount of V is preferably 0.2% or less, more preferably 0.1% or less, and even more preferably 0.08% or less.

(B: more than 0% to less than 0.0003%)

B improves the hardenability and acts to increase the strength of the base material and the welded portion. Further, B also acts to improve HAZ toughness because it accelerates ferrite transformation from within the austenitic grains through precipitation of BN by binding with N in the course of cooling the heated HAZ during welding. In order to obtain these effects, it is preferable that the B content is 0.0001% or more. However, if the amount of B excesses, the toughness of the base material and the HAZ part deteriorates, or the weldability deteriorates. Therefore, the upper limit of the amount of B is preferably 0.0003% or less.

In the production of the steel sheet of the present invention, it is necessary to appropriately control the production process thereof. Hereinafter, explanation will be given in the order of steps.

First, deoxidation is performed using Mn, Si, and Al. Then, in order to float and separate the coarse oxide, the RH reflux time is secured for 10 minutes or more. As demonstrated in the later examples, when the RH reflux time is short, the number density of coarse oxides of 2 탆 or more increases and the CTOD characteristic is deteriorated. The RH reflux time is preferably 15 minutes or more, and more preferably 20 minutes or more. On the other hand, the preferable upper limit of the RH refluxing time is not particularly limited in view of the above, but it is preferably 60 minutes or less in consideration of productivity and the like.

Next, an element is added in the order of Ti → (REM, Zr) → Ca. When the elements are added in the order other than the order of addition, the oxides do not have an appropriate composition, and the number density of coarse oxides of 2 탆 or more increases as demonstrated in the later examples, and the CTOD characteristics are lowered. Particularly, since Ca has an extremely strong deoxidizing power, when Ca is added before REM and Zr are added, all oxygen bonded to REM and Zr disappears, and desired REM and Zr oxides can not be obtained. On the other hand, the description of (REM, Zr) means that the order of addition of REM and Zr is not particularly limited. That is, after Ti, when Ca is Ca, the order of REM? Zr and the order of Zr? REM may be used. Alternatively, REM and Zr may be added at the same time.

In the present invention, it is necessary to secure the time from addition of (REM, Zr) to the start of casting for 10 minutes or more. When this time is less than 10 minutes, the number density of coarse oxides of 2 탆 or more increases and, as demonstrated in the later examples, CTOD characteristics are lowered. (REM, Zr) to the start of casting is preferably 15 minutes or more, and more preferably 20 minutes or more. On the other hand, the preferable upper limit of the time is not particularly limited in view of the above, but it is preferably 90 minutes or less in consideration of productivity.

After, for example, as described above, making a cast of the slab or the like, re-heating the slab to a heating temperature in a range of 1050~1200 ℃ normal temperature, and then subjected to a predetermined rough rolling, Ar ₃ transformation temperature to 950 (Hereinafter, referred to as " Ar ₃ point ~ 950 ° C "), the hot rolling is performed so that the cumulative rolling reduction is 50% or more. By setting the cumulative reduction ratio at the time of hot rolling to 50% or more, the circle equivalent mean diameter of the crystal grains surrounded by the diagonal grain boundary having the azimuth difference of the adjacent two crystals at the position of t / 2 is 15 or more And the DWTT characteristics are improved. The cumulative reduction ratio at this time is preferably 55% or more, and more preferably 60% or more. However, if the cumulative rolling reduction exceeds 80%, the aggregate structure develops, the separation index SI becomes large, and the CTOD characteristic decreases. Therefore, the upper limit is set to 80% or less. The cumulative rolling reduction is preferably 70% or less.

The "cumulative reduction factor" is a value calculated from the following formula (3). The temperature is defined as an average temperature calculated from the surface temperature of the slab or the steel sheet in consideration of the plate thickness and the like. (Mm), t ₁ is the rolling finish thickness (mm) of the steel sheet when the average temperature is in the rolling temperature range, t ₀ is the rolling thickness (mm) of the steel sheet when the average temperature is in the rolling temperature range, , and t ₂ represents the thickness of the cast steel (for example, slab) before rolling.

Cumulative reduction ratio = (t ₀ -t ₁ ) / t ₂ × 100 ... (3)

Further, the value obtained by the following formula (4) is adopted for the Ar ₃ point. The values shown in Table 1 to be described later are also the same. (% By mass) of C, Mn, Cu, Cr, Ni and Mo, respectively, in [C], [Mn], [Cu], [Cr], [Ni] and [Mo] , And t represents a plate thickness (mm) at the time of temperature measurement.

Cr ₃ -05 × [Ni] -80 × [Mo] -0.35 × (t-8) )···(4)

After the rolling, it is cooled. In the present invention, it is recommended that cooling is carried out at a non-recrystallization temperature to ensure a desired low-temperature toughness (in particular, DWTT characteristics), and cooling with water immediately after completion of rolling. Concretely, the average cooling rate in the range from the Ar ₃ point or more (cooling start temperature) to 550 ° C or less (cooling stop temperature) is set to 10 ° C / second or more. By controlling the cooling conditions in this temperature range in this manner, the hard tissue fraction at the t / 2 position can be controlled to a predetermined range, and desired DWTT characteristics can be ensured. The upper limit of the cooling start temperature is preferably about Ar ₃ + 80 ° C or lower. The preferred lower limit of the average cooling rate is 15 deg. C / second or more. However, if the average cooling rate is excessively high, the strength remarkably increases to cause deterioration of toughness, so the upper limit is set at 50 ° C / sec or less. The preferred upper limit of the average cooling rate is 45 DEG C / second or less.

In this process, the cooling stop temperature is closely related to the hard tissue fraction at the t / 2 position, and by controlling the cooling stop temperature to 550 캜 or less, the hard tissue fraction can be suppressed to 5% or less by area have.

For reference, FIG. 3 shows the relationship between the hard tissue fraction at the t / 2 position and the cooling stop temperature (FCT). This figure plots the relationship between the hard tissue fraction and the cooling quiescence temperature based on a number of basic experiments of the present inventors. From this figure, it can be understood that the hard tissue fraction can be suppressed to 5% or less by controlling the cooling stop temperature to 550 캜 or less. The FCT is preferably 530 DEG C or less, and more preferably 500 DEG C or less. On the other hand, when the temperature of the FCT is low, the strength is increased and the toughness is deteriorated.

The plate thickness of the steel plate for a high strength line pipe according to the present invention is not particularly limited, but for application as a line pipe, the plate thickness is preferably at least 6 mm, more preferably at least 10 mm. The upper limit of the plate thickness is preferably 30 mm or less, and more preferably 25 mm or less.

The steel plate for a high-strength line pipe of the present invention is a steel pipe for a line pipe after that, but the obtained steel pipe reflects the characteristics of the steel sheet as a material and has excellent low-temperature toughness.

This application claims the benefit of priority based on Japanese Patent Application No. 2015-081206 filed on April 10, 2015. The entire contents of the specification of Japanese Patent Application No. 2015-081206 filed on April 10, 2015 are incorporated herein by reference.

Example

Hereinafter, the present invention will be described more specifically by way of examples. The present invention is not limited to the following embodiments, but may be practiced with modifications within the scope of the present invention, which are all within the technical scope of the present invention.

Various steel materials A to O having chemical composition and Ar ₃ points shown in the following Table 1 were produced by the following method. The units in Table 1 are mass% and the remainder are iron and unavoidable impurities. On the other hand, REM in Table 1 was added in the form of mis-metal including La and Ce.

First, in the molten steel treatment step, deoxidation was performed using Mn, Si, and Al. Next, RH reflux was carried out for the time shown in Table 2 to float and separate the coarse oxide. Subsequently, as shown in Table 2, except for a part of the steel N, Ti, (REM, Zr) and Ca were added in this order, and casting was started. Table 2 shows the time from the addition of (REM, Zr) to the start of casting.

In this way, the resulting slab was re-heated at a heating temperature shown in Table 2, and more than 900 ℃ cumulative rolling reduction in the surface temperature of the steel sheet is subjected to rough rolling is at least 40%, more in Ar ₃ point to ~950 ℃ Rolled at a cumulative reduction ratio shown in Table 2 to obtain a steel sheet having a thickness t (20 mm).

After the rolling, the range from the addition of the SCT (cooling start temperature) to the FCT (cooling stop temperature) shown in Table 2 was cooled at an average cooling rate of 30 占 폚 / sec and then cooled to room temperature to obtain various steel sheets.

With respect to the thus obtained steel sheet, the number density of oxides having a circle equivalent diameter of 2 탆 or more, the average crystal grain size at a position of t / 2, the hard tissue fraction at a position of t / 2, (Yield strength, tensile strength), Charpy characteristics (separation index SI), CTOD characteristics (limit CTOD values), and DWTT characteristics (85% ductile fracture transition temperature, 85% SATT).

(Measurement of number density of oxides having a circle equivalent diameter of 2 탆 or more)

(L section perpendicular to the steel sheet surface and parallel to the rolling direction) was observed at a viewing magnification of 400 times and an observation field of about 50 mm ² using Shimadzu EPMA-8705 with the position of t / 2 as the measurement position, The inclusion of 2 탆 or more was quantitatively analyzed for the composition of the inclusions at the central portion by wavelength dispersive spectroscopy of the characteristic X-rays. The elements to be analyzed were Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM (La, Ce, Nd, Dy, Y) and Nb. The relationship between the X-ray intensity and the element concentration of each element was previously obtained as a calibration curve using the base material, and then the element concentration of the inclusion was determined from the X-ray intensity obtained from the inclusion and the calibration curve. Among the obtained quantitative results, inclusions having an oxygen content of 5% or more were used as oxides, and the number density was obtained.

(Measurement of the circle equivalent mean diameter of crystal grains surrounded by the diagonal grain boundary in which the azimuthal difference between the adjacent two crystals at the position of t / 2 is 15 or more)

(L section) perpendicular to the steel sheet surface and parallel to the rolling direction was polished using colloidal silica. The crystal grain size was measured by the EBSD method using the position of t / 2 as the measurement position. Specifically, an EBSD (Electron Back Scatter Diffraction) apparatus manufactured by TexSEM Laboratries was used in combination with SEM, and a region surrounded by diagonal grain boundaries in which the azimuth difference of neighboring crystal grains was 15 degrees or more was defined as a crystal grain, and the average equivalent diameter was obtained. The measurement conditions at this time were as follows: measurement area: 200 占 퐉 x 200 占 퐉 (area with 100 占 퐉 on both sides in the sheet thickness direction centering on the t / 2 position of the steel sheet), measurement step: 0.5 占 퐉 intervals, (CI) of less than 0.1 was excluded from the analysis.

(Measurement of hard tissue fraction at the position of t / 2)

(L section) perpendicular to the steel sheet surface and parallel to the rolling direction was polished, and a specimen having corroded with Repera reagent was used. Using a position of t / 2 as a measurement position, a hard tissue was identified by image analysis from an optical microscope tissue photograph taken at 400 times, and the fraction was obtained.

(Measurement of Tensile Properties (Yield Strength, Tensile Strength)

The tensile properties were evaluated by measuring the yield strength YS and the tensile strength TS using a full-thickness tensile test specimen conforming to API-5L using a test method conforming to the API-5L standard.

(Measurement of Charpy Property (Separation Index SI)

A 2 mm V notch Charpy test specimen conforming to the ASTM-A370 standard was used and evaluated by a test method in accordance with this standard. At that time, the Charpy test piece was sampled from the position of t / 2 so as to be in the same direction as the CTOD test piece, and three tests were carried out at the separation index measuring temperature shown in Table 3 below. Is adopted as the separation index SI. Fig. 1 is a view schematically showing the Charpy test piece wavefront when measuring the separation index SI. Fig. 1, reference numeral 1 denotes a separation, 2 denotes a wavefront, 3 denotes a 2 mm V notch, and 4 denotes a plate thickness direction. The separation index SI is obtained by measuring the lengths L _{1 to} L ₃ of the separations occurring on the wavefront of the Charpy test piece and dividing the total length by the cross-sectional area of the wavefront of the test piece according to the above formula (2).

(Measurement of CTOD characteristics (limit CTOD value)

Three-point bending CTOD test specimens of B × 2B shape conforming to BS7448 were evaluated by a test method according to the standard. The CTOD test was carried out for each steel sheet at -10 캜 at a temperature of -10 캜, and the lower of the two values was adopted as the limit CTOD value. In the present embodiment, the CTOD value at -10 캜 was 0.25 mm or more, and the CTOD characteristic was evaluated to be excellent (acceptable).

(Measurement of DWTT characteristic (85% ductile wavefront transition temperature)

DWTT characteristics were evaluated by a test method conforming to this specification using a DWTT test piece that was not chevron-notched in accordance with the API5L3 standard. The test was carried out in duplicate at various temperatures. In this embodiment, the lowest temperature (85% ductile wavefront transition temperature, 85% SATT) at which the ductile wavefront ratio is 85% is obtained, and the DWTT characteristic is evaluated to be excellent .

These results are shown in Table 3. On the other hand, in Table 3, "the circle equivalent average diameter of the crystal grains surrounded by the diagonal grain boundary in which the azimuth difference of two adjacent crystals at the position of t / 2 is 15 degrees or more" is defined as " Average diameter of circle equivalent ".

From these results, it can be considered as follows.

First of all, 1 to 8 are examples of the present invention manufactured under recommended conditions using the steel materials A to H in Table 1 satisfying the chemical composition specified in the present invention. These are the number density of coarse oxides having a circle-equivalent diameter of 2 탆 or more defined in the present invention, the circle-equivalent average diameter of crystal grains surrounded by diagonal grain boundaries in which the azimuth difference of two adjacent crystals is 15 or more, the hard tissue fraction, and the separation index SI Satisfies the requirements of the present invention, it can be seen that the limit CTOD value satisfies the target value of 0.25 mm or more in the CTOD test performed at the test temperature -10 占 폚. Furthermore, it can be seen that the 85% SATT is all below -10 ° C, and the DWTT characteristic is excellent.

On the other hand, 9 to 15 do not satisfy any of the requirements specified in the present invention, the limit CTOD value or 85% SATT has not reached the target value.

Specifically, the test No. 3 of Table 3 was used. 9 is an example using steels I in Table 1 in which REM and Zr are not added in the steel. As the oxide composition is not appropriate, the number density of coarse oxides increases and the limit CTOD value does not reach the target value.

Table 3 Test No. 10 uses the steel material J in Table 1 satisfying the chemical composition specified in the present invention. However, since the cumulative reduction ratio at the Ar ₃ point to 950 ° C is low, the angle difference between the adjacent two crystals is 15 ° or more. The circle-equivalent average diameter of the surrounding crystal grains becomes large, and the toughness of the base material deteriorates, and both of the limit CTOD value and 85% SATT do not reach the target value.

Table 3 Test No. 11 used the steel material K in Table 1 satisfying the chemical composition specified in the present invention. However, since the cumulative reduction ratio at the Ar ₃ point to 950 ° C is high, the separation index SI increases and the limit CTOD value It is not.

Table 3 Test No. 12 used the steel material L in Table 1 satisfying the chemical composition specified in the present invention. However, since the cooling stop temperature (FCT) after rolling was high, the hard tissue area ratio at the t / 2 position increased, Is not reaching the target value.

Table 3 Test No. 13 used the steel material M in Table 1 satisfying the chemical composition specified in the present invention. However, since the RH reflux time in the molten steel treatment process is short, the number density of coarse oxides increases and the limit CTOD value It is not.

Table 3 Test No. 14 used the steel N in Table 1 satisfying the chemical composition specified in the present invention. However, since the order of addition of the elements in the molten steel treatment process is not appropriate, the number density of coarse oxides increases and the limit CTOD value This goal has not been reached.

Table 3 Test No. 15 used the steel material O of Table 1 satisfying the chemical composition specified in the present invention. However, since the time from the addition of (REM, Zr) to the start of casting in the molten steel treatment process is short, the number density And the limit CTOD value is not reaching the target.

Claims (3)

In terms of% by mass,
C: 0.02 to 0.2%
Si: 0.02 to 0.5%
Mn: 0.6 to 2.5%
P: more than 0% and not more than 0.03%
S: more than 0% and not more than 0.01%
Al: 0.010 to 0.08%
Nb: 0.001 to 0.1%
Ti: 0.003 to 0.03%
Ca: 0.0003 to 0.006%
N: 0.001 to 0.01%
O: more than 0% to less than 0.0045%
REM: 0.0001 to 0.005%, and
Zr: 0.0001 to 0.005%
, The balance being iron and inevitable impurities,
When the plate thickness is t,
at a position of t / 2, an oxide having a circle equivalent diameter of 2 탆 or more at 10 / mm ² or less,
the average equivalent diameter of the crystal grains surrounded by the diagonal grain boundary having the azimuthal difference of two adjacent crystals at the position of t / 2 of not less than 15 is not more than 10 mu m,
the percentage of the hard tissue at the position of t / 2 is not more than 5 area%
In addition,
And the separation index SI measured from the Charpy test piece wave front at the specified temperature is not more than 0.15 mm / mm ² . The method according to claim 1,
In terms of% by mass,
Cu: more than 0% to 1.5%
Ni: more than 0% and not more than 1.5%
Cr: more than 0% and not more than 1.5%
Mo: more than 0% and not more than 1.0%
V: more than 0% to not more than 0.2%, and
B: more than 0% and not more than 0.0003%
And at least one member selected from the group consisting of aluminum and aluminum. A steel pipe for a high strength line pipe excellent in low-temperature toughness, which is produced by using the steel plate for a high strength line pipe according to any one of claims 1 to 3.

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