JP2016199806A - Steel plate for high strength line pipe and steel pipe for high strength line pipe excellent in low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel plate for a high strength line pipe excellent in low temperature toughness, specifically excellent in both CTOD characteristics and DWTT characteristics.SOLUTION: A steel plate for a high strength line pipe satisfies prescribed ingredients, and includes 10 pieces/mmor more oxides whose circular equivalent diameter is 2 μm or more, at a position of t/2 when plate thickness is set as t, with 10 μm or less of the circle-equivalent average diameter of crystal grains surrounded by large-angle grain boundaries with orientation difference of neighboring two crystals at the position of t/2 being 15° or larger. The fraction of hard tissue at the position of t/2 satisfies 5 area% or less. The separation index SI measured with a fracture of a Charpy test piece at a designated temperature satisfies 0.15 mm/mmor less.SELECTED DRAWING: Figure 2

Description

  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 specifically, the present invention relates to a steel plate for high-strength line pipe and a steel pipe for high-strength line pipe excellent in both CTOD (Crack Tip Opening Displacement) characteristics and DWTT (Drop Weight Tear Test) characteristics. .

  Line pipes used for transportation of natural gas and crude oil tend to increase the operating pressure for the purpose of improving transportation efficiency, and there is a demand for higher strength in steel materials for line pipes. In addition, from the viewpoint of safety, it is required to have excellent CTOD characteristics and DWTT characteristics, which are one of evaluation indices of fracture toughness, as a brittle fracture prevention characteristic.

  From the viewpoint of increasing the strength, it is conceivable that the strengthening mechanism of the steel material includes solid solution strengthening, precipitation strengthening, transformation strengthening, and dislocation strengthening. Among these, the dislocation strengthening, which increases the strength of the material by increasing the dislocation density, is the cumulative rolling reduction in the so-called two-phase temperature range where ferrite has transformed and precipitated from the austenite single-phase structure in the rolling process during steel plate production. Since the effect can be obtained by increasing the strength, it is a strengthening mechanism that is easier to apply than other strengthening mechanisms.

  However, by increasing the cumulative rolling reduction in this two-phase region temperature range, the crystal orientation rotates with the increase of dislocation density, and the texture develops. Due to the development of this texture, the difference in toughness between the rolling surface direction and the sheet thickness direction becomes large, so that during the Charpy test or COTD test using a test specimen taken from the rolling surface direction, the fracture surface of the test specimen In addition, a minute opening in the thickness direction called separation is generated. This separation occurs due to a large difference in toughness between the rolling surface direction and the sheet thickness direction. Therefore, in addition to the influence of the texture, S present in the steel mainly causes the central segregation at the center of the sheet thickness. It also occurs when MnS stretched in the rolling surface direction is generated.

  When performing the CTOD test, if the above-mentioned separation occurs before the brittle crack occurs, it is determined that the opening is stable only up to the position where the separation occurs, and the critical CTOD value, which is an index of the CTOD characteristic, is originally evaluated. Lower than the value. For this reason, in a material in which separation occurs, for example, the limit CTOD value cannot be improved only by improving the base material toughness evaluated by the fracture surface transition temperature vTrs.

  For this reason, for example, in Patent Document 1, in order to obtain solid DWTT characteristics while achieving solid solution strengthening by adding an expensive element and suppressing separation, the rolling temperature in the austenite non-recrystallized region, particularly the rolling end temperature. A method for controlling the above is disclosed.

JP 2013-47393 A

  In the technology described in Patent Document 1, it is necessary to employ solid solution strengthening by adding expensive elements, a complicated manufacturing process that combines online water-cooling equipment and heating equipment, and special rolling conditions. This leads to a decrease in productivity.

  The present invention has been made in view of the circumstances as described above, and its purpose is to easily produce a steel sheet for high-strength line pipe excellent in both low-temperature toughness, particularly CTOD characteristics and DWTT characteristics, at low cost. It is to provide technology that can.

The steel sheet for a high-strength line pipe of the present invention that has solved the above-mentioned problems is in mass%, C: 0.02-0.2%, Si: 0.02-0.5%, Mn: 0.6 -2.5%, P: more than 0% and 0.03% or less, S: more than 0% and 0.01% or less, Al: 0.010 to 0.08%, Nb: 0.001 to 0.1%, Ti: 0.003-0.03%, Ca: 0.0003-0.006%, N: 0.001-0.01%, O: more than 0% and 0.0045% or less, REM: 0.0001- 0.005%, and Zr: 0.0001 to 0.005%, the balance is iron and inevitable impurities, and when the plate thickness is t, the equivalent circle diameter is 2 μm or more at the position of t / 2 the oxide contained 10 / mm ² or less, at a position of t / 2, the orientation difference between adjacent two crystals surrounded by a 15 ° or more high-angle grain boundaries In addition, the average equivalent circle diameter of the crystal grains is 10 μm or less, the hard structure fraction at the position of t / 2 is 5 area% or less, and the separation index SI measured from the Charpy test piece fracture surface at the specified temperature is 0. The main point is that it is 15 mm / mm ² or less.

  In a preferred embodiment of the present invention, the steel sheet is further mass%, Cu: more than 0% and 1.5% or less, Ni: more than 0% and 1.5% or less, Cr: more than 0% and 1.5% or less, It contains at least one selected from the group consisting of Mo: more than 0% and not more than 1.0%, 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 high-strength line pipe that is manufactured using the steel sheet for 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 μm or more at the position of t / 2, and the separation index SI measured from the Charpy test piece fracture surface at the specified temperature. The CTOD characteristics are improved by setting to an appropriate range, and the average equivalent circle diameter of the crystal grains surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2, And DWTT characteristics can be improved by appropriately controlling the hard tissue fraction. Therefore, according to the present invention, it is possible to realize a steel sheet for high-strength line pipe excellent in low-temperature toughness having a tensile strength of 520 MPa or more and excellent in both CTOD characteristics and DWTT characteristics without using an expensive alloy element.

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

  The present inventors have repeatedly studied in order to provide a steel plate for a high-strength line pipe excellent in both CTOD characteristics and DWTT characteristics. Specifically, regarding the improvement of CTOD characteristics, steel sheets for high-strength line pipes that do not completely suppress the occurrence of separation, but allow for the occurrence of separation to a certain extent and obtain an excellent limit CTOD value. In order to achieve this, the relationship between the occurrence of separation and the microstructure in the CTOD test was examined. As a result, the critical CTOD value obtained in the CTOD test has a correlation with the separation index SI in the Charpy test, and decreases the separation index SI measured from the Charpy test piece fracture surface at the specified temperature, and at the position of t / 2. It has been found that the number density of coarse oxides having a circle equivalent diameter of 2 μm or more should be suppressed. As for the improvement of DWTT characteristics, the average equivalent circle diameter of crystal grains surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2 is refined and hardened. The present inventors have found that the tissue fraction should be suppressed and completed the present invention.

  Hereinafter, each requirement prescribed | regulated with the steel plate for line pipes of this invention is demonstrated. In this specification, unless otherwise specified, t means a plate thickness.

(Average crystal grain size of crystal grains surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2: 10 μm or less)
In order to ensure good DWTT characteristics, it is necessary to ensure the toughness of the base material by refining crystal grains. Therefore, in the present invention, the upper limit of the circle-equivalent average diameter of the crystal grains surrounded by the large-angle grain boundaries where the orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2 is set to 10 μm or less. The average crystal grain size is preferably 8.0 μm or less, and more preferably 7.0 μm or less. The average crystal grain size is preferably as small as possible, but the lower limit is approximately 4 μm or more.

  In the present invention, the circle equivalent average diameter of crystal grains surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more, as well as an oxide having a hard structure fraction and an equivalent circle diameter of 2 μm or more described later. The reason for setting the number density measurement position to t / 2 instead of t / 4, which is a representative position for steel sheet characteristic evaluation, is to improve the toughness of the portion that becomes the starting point of fracture, and to achieve the desired DWTT characteristic. This is to ensure.

(Separation index SI measured from the fracture surface of Charpy specimen at the specified temperature: 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, the target limit CTOD value can be secured 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 ° C. The specified temperature can be obtained from the following equation (1). That is, the test temperature (designated temperature) at the time of performing the Charpy test varies depending on the plate thickness. In order to evaluate the target limit CTOD value when the test temperature is set to −10 ° C., the designated temperature (designated temperature) T ₁ ) must also be considered. However, T ₁ : Charpy test temperature (° C.), T ₂ : CTOD test temperature (° C.), and in this specification, −10 ° C. and t: plate thickness (mm), respectively.
T ₁ = T ₂ −6 × (t) ^1/2 +20 (1)

The separation index SI is obtained by dividing the total length of separation generated perpendicular to the thickness direction of the Charpy specimen fracture surface by the area (cross-sectional area) of the specimen fracture surface, as shown in the following formula (2). (See Fig. 1 below). However, L _n is the n-th separation length (mm), S _A represents the cross-sectional area of the fracture surface of (mm ²⁾ respectively.
SI = Σ (L _n ) / S _A (2)

In the steel sheet for high-strength line pipe of the present invention, the separation index SI obtained as described above needs to be 0.15 mm / mm ² or less. This separation index SI is preferably 0.12 mm / mm ² or less, more preferably 0.10 mm / mm ² or less. However, even if separation occurs, this separation index SI does not necessarily have to be 0 mm / mm ² from the viewpoint of showing a high limit CTOD value. From such a 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 is a plot of the relationship between the critical CTOD value measured as an index of the CTOD characteristic and the separation index SI based on the results of Examples described later. From this figure, it can be seen that when the separation index SI is 0.15 mm / mm ² or less, the critical CTOD value at −10 ° C. ≧ 0.25 mm is satisfied, which is an acceptance criterion for CTOD characteristics.

(At t / 2 position, 10 oxides / mm ² or less having an equivalent circle diameter of 2 μm or more)
Since coarse oxides adversely affect the improvement of CTOD characteristics, in the present invention, the number density of oxides of the above size is set to 10 pieces / mm ² or less. The number density of the oxide is preferably as low as possible, preferably 8 pieces / mm ² or less, more preferably 5 pieces / mm ² or less. The lower limit is not particularly limited from the above viewpoint, but in view of productivity in the slab manufacturing stage, the lower limit is preferably about 0.1 / mm ² or more.

  In addition, the oxide made into object by this invention means what is observed when it observes by the method as described in the Example mentioned later.

(The fraction of hard structure at the position of t / 2 is 5 area% or less)
Since the hard tissue adversely affects the improvement of the DWTT characteristics, in the present invention, the area fraction of the hard tissue occupying the entire tissue 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 3 area% or less, more preferably 1 area% or less. In addition, the minimum is not specifically limited from the said viewpoint, For example, 0 area% may be sufficient.

  In addition, as a hard structure | tissue made into object by this invention, an island-like martensite, a martensite, etc. are mentioned, for example.

  In the steel sheet for high-strength line pipe of the present invention, it is necessary to appropriately adjust the chemical composition. The reason for setting the range of the chemical composition is as follows. In addition, regarding a chemical component composition,% means the mass%.

(C: 0.02-0.2%)
C is an element indispensable for securing the strength of the steel plate and the welded portion as the base material, and for that purpose, C needs to be contained by 0.02% or more. The amount of C is preferably 0.03% or more, and more preferably 0.05% or more. However, when the amount of C is excessive, island-shaped martensite (MA) is easily generated, and the toughness of HAZ (heat affected zone) is lowered and weldability is also lowered. From such a viewpoint, the C amount needs to be 0.2% or less. The amount of C is preferably 0.15% or less, more preferably 0.12% or less.

(Si: 0.02-0.5%)
In addition to having a deoxidizing action, Si is effective for improving the strength of a steel plate and a welded portion that are base materials. In order to exert these effects, the Si content is 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 becomes excessive, weldability and toughness deteriorate. Therefore, the amount of Si needs to be suppressed to 0.5% or less. The amount of Si is preferably 0.45% or less, more preferably 0.35% or less.

(Mn: 0.6-2.5%)
Mn is an element effective for improving the strength of the base steel plate and the weld. In order to exert such an effect, it is necessary to contain 0.6% or more of Mn. The amount of Mn is preferably 1.0% or more, more preferably 1.2% or more. However, when the amount of Mn is excessive, not only MnS is generated and the generation of separation is promoted, but also HAZ toughness and weldability are deteriorated, so the upper limit of the amount of Mn is made 2.5% or less. The amount of Mn is preferably 2.0% or less, more preferably 1.9% or less, and still more preferably 1.8% or less.

(P: more than 0% and 0.03% or less)
P is an element inevitably contained in the steel material. When the amount of P exceeds 0.03%, the base material toughness and the HAZ toughness are significantly deteriorated. Therefore, in the present invention, the amount of P is suppressed to 0.03% or less. The amount of P 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 make it 0% industrially.

(S: more than 0% and 0.01% or less)
When the amount of S becomes excessive, MnS is generated and the generation of separation is promoted, so the upper limit is made 0.01% or less. The amount of S is preferably 0.008% or less, more preferably 0.006% or less, and still more preferably 0.005% or less. In this way, from the viewpoint of suppressing the occurrence of separation, a smaller amount of S is desirable, but it is difficult to make it 0% industrially. A preferable lower limit of the amount of S is approximately 0.0001% or more.

(Al: 0.010 to 0.08%)
Al is a strong deoxidizing element, and it is necessary to contain 0.010% or more in order to obtain a deoxidizing effect. The amount of Al is preferably 0.02% or more, 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, thereby reducing the HAZ toughness. Therefore, the Al amount 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 element effective for increasing strength and base metal toughness without degrading weldability. In order to exert such an effect, the Nb amount needs to be 0.001% or more. The Nb amount is preferably 0.005% or more, 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 Nb amount is set to 0.1% or less. The Nb amount is preferably 0.08% or less, more preferably 0.05% or less.

(Ti: 0.003-0.03%)
Ti precipitates in the steel as TiN, which is necessary for improving the base metal toughness by suppressing the coarsening of austenite grains during slab heating and for improving the HAZ toughness by coarsening of austenite grains in the HAZ during welding Element. In order to exert such effects, the Ti amount needs to be 0.003% or more. The amount of Ti is preferably 0.005% or more, more preferably 0.01% or more. On the other hand, if the amount of Ti is excessive, solute Ti or TiC is precipitated and the toughness of the base material and the HAZ deteriorates, so it is necessary to make it 0.03% or less. The amount of Ti is preferably 0.025% or less, more preferably 0.020% or less.

(Ca: 0.0003 to 0.006%)
Ca has the effect | action which controls the form of sulfide, and there exists an effect which suppresses formation of MnS by forming CaS. In order to exert such an effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.006% and becomes excessive, the toughness deteriorates, so the upper limit of the Ca content is 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 precipitates in the steel as TiN, and is necessary for improving the base metal toughness by suppressing the coarsening of austenite grains during slab heating and for improving the HAZ toughness by coarsening of austenite grains in the HAZ during welding Element. In order to exert these effects, N needs to be contained by 0.001% or more. The N amount is preferably 0.003% or more, and more preferably 0.004% or more. However, if the amount of N becomes excessive, the toughness in HAZ deteriorates due to the presence of solute N, so it is necessary to make it 0.01% or less. The N amount is preferably 0.008% or less, and more preferably 0.006% or less.

(O: more than 0% and 0.0045% or less)
A smaller amount of O is preferable from the viewpoint of suppressing coarse oxides. From such a viewpoint, in the present invention, the upper limit of the O amount is set to 0.0045% or less. Preferably it is 0.0040% or less, More preferably, it is 0.0035% or less. A smaller amount of O is better, but it is difficult to make it 0% industrially.

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

(Zr: 0.0001 to 0.005%)
Zr is an element that contributes to the improvement of CTOD characteristics by forming an oxide and finely dispersing it. In order to exert such an effect, the Zr amount needs to be 0.0001% or more. The amount of Zr is preferably 0.0003% or more, more preferably 0.0005% or more. On the other hand, if the amount of Zr is excessive, coarse inclusions are formed and the toughness of the base material is deteriorated, so 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, and still more preferably 0.001% or less.

  The chemical component composition in the steel sheet for high-strength line pipe of the present invention is as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Examples of the inevitable impurities include As, Sb, Sn, H, and the like.

  Moreover, it is also preferable that the steel plate for line pipes of the present invention further contains one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, V, and B in the following amounts as necessary. These elements are elements that improve the strength and toughness of the base material and HAZ, and may be contained alone or in combination of two or more. The reason for setting the range when these are contained is as follows.

(Cu: more than 0% and 1.5% or less)
Cu is an element effective for increasing the strength. In order to exhibit such an effect, it is preferable to contain 0.01% or more of Cu. The amount of Cu is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the amount of Cu becomes excessive, the toughness of the base material deteriorates, so it is preferable to set it to 1.5% or less. The amount of Cu is more preferably 1.0% or less, still more preferably 0.5% or less.

(Ni: more than 0% and 1.5% or less)
Ni is an element effective for improving the strength and toughness of the base material and the weld. In order to obtain such an effect, the Ni content 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 a large amount of Ni is contained, it becomes extremely expensive as a structural steel material. Therefore, the Ni content is preferably 1.5% or less from an economical viewpoint. The amount of Ni is more preferably 1.0% or less, still more preferably 0.5% or less.

(Cr: more than 0% and 1.5% or less)
Cr is an element effective for improving the strength, and in order to obtain such an effect, it is preferable to contain 0.01% or more. The amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.5%, the HAZ toughness deteriorates. Therefore, the Cr content is preferably 1.5% or less. The amount of Cr is more preferably 1.0% or less, and still more preferably 0.5% or less.

(Mo: more than 0% and 1.0% or less)
Mo is an element effective for improving the strength and toughness of the base material. In order to obtain such an effect, the Mo amount 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, if the Mo amount exceeds 1.0%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount is preferably 1.0% or less, and more preferably 0.5% or less.

(V: more than 0% and 0.2% or less)
V is an element effective for improving the strength. In order to obtain such an effect, V is preferably contained in an amount of 0.003% or more. The amount of V is more preferably 0.010% or more. On the other hand, if the V content exceeds 0.2%, the weldability and the base metal toughness deteriorate. Therefore, the V amount is preferably 0.2% or less, more preferably 0.1% or less, and still more preferably 0.08% or less.

(B: more than 0% and 0.0003% or less)
B has the effect of increasing the hardenability and increasing the strength of the base material and the welded portion. Further, B is combined with N in the process of cooling the HAZ portion heated during welding to precipitate BN and promote ferrite transformation from within the austenite grains, and thus has an effect of improving HAZ toughness. In order to obtain these effects, the B content is preferably 0.0001% or more. However, if the amount of B becomes excessive, the toughness of the base material and the HAZ part deteriorates or weldability deteriorates, so the upper limit of the amount of B is preferably made 0.0003% or less.

  In manufacturing the steel sheet of the present invention, it is necessary to appropriately control the manufacturing process. Hereinafter, it demonstrates in order of a process.

  First, deoxidation is performed using Mn, Si, and Al. Next, in order to float and separate the coarse oxide, an RH reflux time of 10 minutes or more is secured. As demonstrated in Examples described later, when the RH reflux time is short, the number density of coarse oxides of 2 μm or more increases and the CTOD characteristics deteriorate. The RH reflux time is preferably 15 minutes or more, and more preferably 20 minutes or more. Note that the upper limit of the RH reflux time is not particularly limited from the above viewpoint, but in consideration of productivity and the like, it is 60 minutes or less. Preferably there is.

  Next, elements are added in the order of Ti → (REM, Zr) → Ca. When each element is added in an order other than this addition order, the oxide does not have an appropriate composition, and the number density of coarse oxides of 2 μm or more increases as demonstrated in the examples described later, and the CTOD characteristics are improved. descend. In particular, since Ca has a very strong deoxidizing power, if Ca is added before REM and Zr are added, all oxygen bonded to REM and Zr disappears, and a desired oxide of REM and Zr is obtained. I can't. Note that (REM, Zr) indicates that the order of adding REM and Zr is not particularly limited. That is, as long as it is after Ti and before Ca, either the order of REM → Zr or the order of Zr → REM may be used. Alternatively, REM and Zr may be added simultaneously.

  In the present invention, it is necessary to secure 10 minutes or more from the addition of (REM, Zr) to the start of casting. When this time is less than 10 minutes, as demonstrated in the examples described later, the number density of coarse oxides of 2 μm or more increases and CTOD characteristics deteriorate. The time from the addition of (REM, Zr) to the start of casting is preferably 15 minutes or more, more preferably 20 minutes or more. In addition, although the preferable upper limit of the said time is not specifically limited from the said viewpoint, when productivity etc. are considered, it is preferable that it is 90 minutes or less.

For example, after producing a slab such as a slab as described above, the slab is reheated at a heating temperature of 1050 to 1200 ° C. in the normal temperature range, and after performing predetermined rough rolling, the Ar ₃ transformation point to In the temperature range of 950 ° C. (hereinafter referred to as “Ar ₃ point to 950 ° C.”), hot rolling is performed so that the cumulative rolling reduction is 50% or more. By setting the cumulative reduction ratio during this hot rolling to 50% or more, it corresponds to a circle of crystal grains surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2. The average diameter can be 10 μm or less, and the DWTT characteristics are improved. The cumulative rolling reduction at this time is preferably 55% or more, more preferably 60% or more. However, if the cumulative rolling reduction exceeds 80%, the texture develops, the separation index SI increases, and the CTOD characteristic decreases, so the upper limit is made 80% or less. The cumulative rolling reduction is preferably 70% or less.

The “cumulative rolling reduction” is a value calculated from the following equation (3). The temperature is defined as an average temperature obtained by calculation from the surface temperature of the slab or steel plate in consideration of the plate thickness and the like. In the following formula (3), t ₀ is the rolling start thickness (mm) of the steel sheet when the average temperature is in the rolling temperature range, and t ₁ is the rolling finish thickness (mm) of the steel sheet when the average temperature is in the rolling temperature range. , T ₂ indicate the thickness of the slab (for example, slab) before rolling.
Cumulative rolling reduction = (t ₀ −t ₁ ) / t ₂ × 100 (3)

Also, the Ar ₃ point adopts the value determined by the following equation (4). The values shown in Table 1 described later are also the same. In the following formula (4), [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are the contents (mass%) of C, Mn, Cu, Cr, Ni and Mo, respectively. ) And t indicates the plate thickness (mm) at the time of temperature measurement.
Ar ₃ (° C.) = 910−310 × [C] −80 × [Mn] −20 × [Cu] −15 × [Cr] −55 × [Ni] −80 × [Mo] −0.35 × (t -8) ... (4)

It cools after the said rolling. In the present invention, in order to ensure the desired low temperature toughness (particularly DWTT characteristics), it is recommended to perform rolling at a non-recrystallization temperature and perform cooling such as water cooling immediately after the end of rolling. Specifically, the average cooling rate in the range from the Ar ₃ point or higher (cooling start temperature) to 550 ° C. or lower (cooling stop temperature) is set to 10 ° C./second or higher. By controlling the cooling conditions in the above temperature range in this way, the hard tissue fraction at the t / 2 position can be controlled within 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. A preferable lower limit of the average cooling rate is 15 ° C./second or more. However, if the average cooling rate is too high, the strength is remarkably increased and the toughness is deteriorated, so the upper limit is made 50 ° C./second or less. A preferable upper limit of the average cooling rate is 45 ° C./second or less.

  In the above process, the cooling stop temperature is closely related to the hard tissue fraction at the t / 2 position, and the hard tissue fraction is suppressed to 5 area% or less by controlling the cooling stop temperature to 550 ° C. or lower. Can do.

  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 stop temperature based on a number of basic experiments conducted by the present inventors. From this figure, it can be seen that the hard tissue fraction can be suppressed to 5 area% or less by controlling the cooling stop temperature to 550 ° C. or less. FCT is preferably 530 ° C. or lower, and more preferably 500 ° C. or lower. In addition, since strength raises when FCT becomes low temperature and toughness deteriorates, the lower limit shall be 350 degreeC or more.

  The plate thickness of the steel sheet for 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 or more, more preferably 10 mm or more. Further, the upper limit of the plate thickness is preferably 30 mm or less, and more preferably 25 mm or less.

  The steel sheet for high-strength line pipes of the present invention is then used as a steel pipe for line pipes, and the obtained steel pipe reflects the characteristics of the raw steel sheet and has excellent low-temperature toughness.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with modifications within a range that can be adapted to the gist described below, and these are all included in the technical scope of the present invention.

Various steel materials A to O having chemical composition and Ar ₃ points shown in Table 1 below were produced by the following methods. The unit in Table 1 is mass%, and the balance is iron: unavoidable impurities. The REMs in Table 1 were added in the form of misch metal containing La and Ce.

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

The slab thus obtained was reheated at the heating temperature shown in Table 2, and then rough rolled so that the cumulative rolling reduction at 900 ° C. or higher was 40% or higher at the surface temperature of the steel sheet, and further Ar Hot rolling was performed at _{3 to} 950 ° C. at the cumulative reduction shown in Table 2 to obtain a steel sheet having a thickness t (20 mm).

  After the rolling, the range from SCT (cooling start temperature) to FCT (cooling stop temperature) shown in Table 2 was cooled at an average cooling rate of 30 ° C./second, and then allowed to cool to room temperature to produce various steel sheets. Obtained.

  With respect to the steel sheet thus obtained, the number density of oxides having an equivalent circle diameter of 2 μm or more, the average crystal grain size at the position of t / 2, the hard structure fraction at the position of t / 2, the tensile strength by the following method Characteristics (yield strength, tensile strength), Charpy characteristics (separation index SI), CTOD characteristics (limit CTOD value), and DWTT characteristics (85% ductile fracture surface transition temperature, 85% SATT) were measured.

(Measurement of number density of oxides with equivalent circle diameter of 2 μm or more)
The cross section (L cross section) perpendicular to the surface of the steel sheet and parallel to the rolling direction was observed at an observation magnification of 400 times and an observation field of view of about 50 mm ² using a Shimadzu EPMA-8705 at a measurement position of t / 2, and 2 μm. The component composition at the center of the inclusion was quantitatively analyzed by wavelength dispersion spectroscopy of characteristic X-rays for the above inclusion. The analysis target elements were Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM (La, Ce, Nd, Dy, Y), and Nb. The relationship between the X-ray intensity of each element and the element concentration is obtained in advance using a known substance as a calibration curve, and then the element concentration of the inclusion is determined from the X-ray intensity obtained from the inclusion and the calibration curve. did. Among the obtained quantitative results, inclusions having an oxygen content of 5% or more were used as oxides, and the number density was determined.

(Measurement of circle equivalent average diameter of crystal grains surrounded by a large-angle grain boundary whose orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2)
A test piece obtained by polishing a cross section (L cross section) perpendicular to the steel sheet surface and parallel to the rolling direction using colloidal silica was used. The crystal grain size was measured by the EBSD method with the t / 2 position as the measurement position. Specifically, using an EBSD (Electron Backscatter Diffraction) device from TexSEM Laboratories in combination with SEM, the area surrounded by a large-angle grain boundary where the orientation difference between adjacent crystal grains is 15 ° or more is used as a crystal grain. The diameter was determined. The measurement conditions at this time are as follows: measurement area: 200 μm × 200 μm (area centered at the t / 2 position of the steel sheet and 100 μm spread on both sides in the thickness direction), measurement step: 0.5 μm interval, and reliability of measurement orientation Measurement points with CI (Confidence Index) indicating sex were less than 0.1 were excluded from the analysis target.

(Measurement of hard tissue fraction at t / 2 position)
The test piece which grind | polished the cross section (L cross section) perpendicular | vertical to a steel plate surface and parallel to a rolling direction, and corroded with the repeller reagent was used. Using the position of t / 2 as a measurement position, a hard tissue was identified by image analysis from an optical microscope tissue photograph taken at a magnification of 400 times, and the fraction was determined.

(Measurement of tensile properties (yield strength, tensile strength))
Tensile properties were evaluated by measuring the yield strength YS and the tensile strength TS by a test method based on the API-5L standard using a full thickness tensile test piece based on API-5L.

(Measure Charpy characteristics (separation index SI))
Using a 2 mm V notch Charpy test piece compliant with the ASTM-A370 standard, evaluation was performed by a test method compliant with this standard. At that time, Charpy test specimens were sampled from the t / 2 position so as to be in the same direction as the CTOD test specimens, three tests were conducted at the separation index measurement temperatures shown in Table 3 below, and the separation index was measured. The value with the maximum value was adopted as the separation index SI. FIG. 1 is a diagram schematically showing a fracture surface of a Charpy specimen when measuring a separation index SI. In FIG. 1, 1 is a separation, 2 is a fractured surface, 3 is a 2 mmV notch, and 4 is a thickness direction. The separation index SI is measured by measuring the lengths L _{1 to} L ₃ of the separation generated on the fracture surface of the Charpy test piece and dividing the total length by the cross-sectional area of the fracture surface of the test piece according to the above equation (2). It is what.

(Measurement of CTOD characteristics (limit CTOD value))
Using a B × 2B-shaped three-point bending CTOD test piece conforming to BS7448, evaluation was performed by a test method conforming to the standard. The CTOD test was performed on each steel plate at −10 ° C., and the lower value of the two was adopted as the limit CTOD value. In this example, the case where the CTOD value at −10 ° C. was 0.25 mm or more was evaluated as having excellent CTOD characteristics (pass).

(Measurement of DWTT characteristics (85% ductile fracture surface transition temperature))
Using a DWTT test piece that was chevron notched according to the API5L3 standard, the DWTT characteristic was evaluated by a test method that conformed to this standard. Two tests were performed at various temperatures. In this example, the lowest temperature at which the ductile fracture surface ratio becomes 85% (85% ductile fracture surface transition temperature, 85% SATT) is obtained, and when this value is −10 ° C. or less, the DWTT characteristics are excellent (pass). evaluated.

  These results are shown in Table 3. In Table 3, the “circle equivalent average diameter of crystal grains surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2” is expressed as “at the position of t / 2”. It was abbreviated as “equivalent circle average diameter of large-angle grains”.

  From these results, it can be considered as follows.

  First, test No. in Table 3 1-8 are the examples of this invention manufactured on the conditions recommended using the steel materials AH of Table 1 which satisfy | fill the chemical component composition prescribed | regulated by this invention. These are the number density of coarse oxides having a circle equivalent diameter of 2 μm or more as defined in the present invention, the circle equivalent average diameter of crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more, Since both the hard tissue fraction and the separation index SI satisfy the requirements of the present invention, the critical CTOD value satisfies the target value of 0.25 mm or more in the CTOD test performed at a test temperature of −10 ° C. You can see that Furthermore, it can be seen that 85% SATT is -10 ° C. or lower and is excellent in DWTT characteristics.

  On the other hand, test no. Since Nos. 9 to 15 do not satisfy any of the requirements defined in the present invention, the limit CTOD value or 85% SATT does not reach the target value.

  For details, see Test No. in Table 3. 9 is an example using the steel material I of Table 1 in which REM and Zr are not added to the steel. Since the oxide composition is not appropriate, the number density of coarse oxides increases, and the critical CTOD value reaches the target value. Not reached.

Test No. in Table 3 No. 10 used the steel material J in Table 1 satisfying the chemical composition defined in the present invention. However, since the cumulative rolling reduction at the Ar ₃ point to 950 ° C. is low, the orientation difference between two adjacent crystals is 15 °. The circle-equivalent average diameter of the crystal grains surrounded by the above large-angle grain boundaries is increased, and the base material toughness is deteriorated. Both the critical CTOD value and 85% SATT do not reach the target values.

Test No. in Table 3 No. 11 used the steel material K in Table 1 that satisfies the chemical composition defined in the present invention. However, since the cumulative rolling reduction at Ar _{3 to} 950 ° C. is high, the separation index SI increases and the critical CTOD value is The goal has not been reached.

  Test No. in Table 3 12 used the steel material L in Table 1 satisfying the chemical composition defined in the present invention, but because the cooling stop temperature (FCT) after rolling is high, the hard structure area ratio at the t / 2 position is increased, 85% SATT has not reached the target value.

  Test No. in Table 3 No. 13 used the steel material M in Table 1 that satisfies the chemical composition defined in the present invention, but because the RH reflux time in the molten steel treatment process is short, the number density of coarse oxides increased, and the critical CTOD value was The goal has not been reached.

  Test No. in Table 3 No. 14 used the steel material N in Table 1 that satisfies the chemical composition defined in the present invention. However, since the addition order of elements in the molten steel treatment process was not appropriate, the number density of coarse oxides increased, and the limit CTOD The value has not reached the target.

  Test No. in Table 3 15 used the steel material O of Table 1 that satisfies the chemical composition defined in the present invention, but in the molten steel treatment process, the time from the addition of (REM, Zr) to the start of casting is short, so that the coarse oxide The number density has increased and the critical CTOD value has not reached the target.

Claims (3)

% 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 0.03% or less,
S: more than 0% and 0.01% or less,
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% and 0.0045% or less,
REM: 0.0001 to 0.005%, and Zr: 0.0001 to 0.005%
The balance is iron and inevitable impurities,
When the plate thickness is t,
at a position of t / 2, containing 10 oxides / mm ² or less of oxide having an equivalent circle diameter of 2 μm or more,
The fraction of hard structure at the position of t / 2 where the circle equivalent average diameter of the crystal grains surrounded by the large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more at the position of t / 2 is 10 μm or less Satisfies 5% by area or less,
A steel sheet for a high-strength line pipe excellent in low-temperature toughness, having a separation index SI measured from a fracture surface of a Charpy specimen at a specified temperature of 0.15 mm / mm ² or less. Furthermore, in mass%,
Cu: more than 0% and 1.5% or less,
Ni: more than 0% and 1.5% or less,
Cr: more than 0% and 1.5% or less,
Mo: more than 0% and 1.0% or less,
The steel plate for high-strength line pipes according to claim 1, comprising at least one selected from the group consisting of V: more than 0% and 0.2% or less, and B: more than 0% and 0.0003% or less.   A steel pipe for a high-strength line pipe excellent in low-temperature toughness manufactured using the steel sheet for a high-strength line pipe according to claim 1 or 2.

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