KR102364255B1 - steel pipe and plate - Google Patents

Abstract

This steel pipe has a base metal part made of a normal steel plate having a predetermined chemical composition, and a welding part provided in a butt part of the steel plate and extending in the longitudinal direction of the steel plate, from the surface of the base metal part to 1.0 mm in the depth direction. The surface layer metal structure as the metal structure in the range includes polygonal ferrite and granular bainite, the area ratio of the polygonal ferrite in the surface layer metal structure is 0 to 70%, the polygonal ferrite and the granular bay The area ratio of the total of the knights is 50% or more, the maximum hardness in the metal structure of the surface layer part is 270 Hv or less, and the metal structure in the range from more than 1.0 mm in the depth direction from the surface of the base material part to the center of the plate thickness. The metal structure contains 40% or less polygonal ferrite in terms of area ratio, and the maximum hardness in the internal metal structure is 248 Hv or less, and the average hardness is 150 to 220 Hv.

Description

steel pipe and plate

The present invention relates to a steel pipe and a steel plate.

In recent years, the demand for oil, natural gas, etc. has increased, and the diversification of energy sources is progressing. Therefore, in the severe corrosive environment which conventionally abandoned development, for example, the corrosive environment containing hydrogen sulfide, carbon dioxide gas, chlorine ion, etc., the extraction of crude oil and natural gas is made to be performed actively. In connection with this, the improvement of SSC resistance and HIC resistance is calculated|required with respect to the steel pipe (steel pipe for line pipe) used for the pipeline which transports crude oil, natural gas, etc.

In addition, the steel pipe for line pipe is required to increase in strength in order to save material by reducing the thickness and to reduce the weight of the product. However, if the addition amount of an alloying element is increased for the purpose of increasing strength, or if the amount of heat input is increased for high efficiency welding, the low-temperature toughness of the heat-affected zone HAZ decreases.

As disclosed in Patent Documents 1 and 2, for example, a steel pipe having HIC resistance is a steel pipe with high purity, reduction of inclusions, shape control of sulfide inclusions by addition of Ca, light pressure at the time of casting, and center by accelerated cooling. It has been manufactured by making full use of techniques such as suppression of segregation.

However, in the steel pipes disclosed in Patent Documents 1 and 2, SSC resistance was not considered at all. Therefore, in the steel pipes of Patent Documents 1 and 2, although the HIC resistance is excellent, it is estimated that the sulfide stress cracking resistance (SSC) resistance is not sufficient.

Japanese Patent Publication No. 63-001369 Japanese Patent Laid-Open No. 62-112722

An object of the present invention is to provide a steel pipe suitable for line pipe use having a strength of X52 to 70 class in API standards, and excellent in SSC resistance and HIC resistance, and a steel sheet used in the base material part of the steel pipe. .

MEANS TO SOLVE THE PROBLEM This invention was made|formed in view of said subject, The summary is as follows.

(1) A steel pipe according to an aspect of the present invention has a base material part made of a normal steel plate, and a weld part provided in a butt part of the steel plate and extending in the longitudinal direction of the steel plate, wherein the steel plate has a chemical composition, In mass%, C: 0.030 to 0.070%, Si: 0.005 to 0.50%, Mn: 1.05 to 1.65%, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.045%, Ca: 0.0010 to 0.0050%, N: 0.0015 to 0.0070%, Ni: 0 to 0.50%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, V: 0 to 0.100%, Mg: 0 to 0.0100 %, REM: 0 to 0.0100%, P: 0.015% or less, S: 0.0015% or less, O: 0.0040% or less, the balance: Fe and impurities, and in the chemical composition, the following formula Ceq determined in (i) is 0.300 to 0.400, and the surface layer metal structure, which is a metal structure ranging from the surface of the base material to 1.0 mm in the depth direction, contains polygonal ferrite and granular bainite, and in the surface layer metal structure The area ratio of the polygonal ferrite is 0 to 70%, the total area ratio of the polygonal ferrite and the granular bainite is 50% or more, and the maximum hardness in the surface layer metal structure is 270 Hv or less, , an internal metal structure that is a metal structure ranging from more than 1.0 mm in the depth direction to the center of the plate thickness from the surface of the base metal part contains polygonal ferrite of 40% or less in area ratio, in the internal metal structure, The maximum hardness is 248 Hv or less, and the average hardness is 150 to 220 Hv.

Here, [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] in the formula are in the mass% of C, Mn, Ni, Cu, Cr, Mo, and V content by

(2) In the steel pipe described in (1) above, the chemical composition is, in mass%, Ni: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Cu: 0.05 to 0.50%, V : 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100% You may contain 1 type, or 2 or more types.

(3) In the steel pipe according to (1) or (2), the balance of the surface layer metal structure consists of one or two types of bainite and pseudo pearlite, and the balance of the internal metal structure is the granular bay It may consist of 1 type, or 2 or more types of nite, bainite, and pseudo pearlite.

(4) The steel sheet according to another aspect of the present invention is used for the base material portion of the steel pipe according to any one of (1) to (3).

According to the above aspect of the present invention, a steel sheet suitable for line pipe having a strength of X52 to 70 class in API standards and excellent in SSC resistance and HIC resistance, and SSC resistance and HIC resistance using the steel sheet as a base material A steel pipe having excellent properties can be provided. Specifically, it is possible to provide a steel pipe having excellent SSC resistance (resistance to sulfide stress corrosion cracking) and HIC resistance (resistance to hydrogen induced cracking) and a steel sheet used for a base material of the steel pipe. A steel pipe having excellent sour resistance (SSC resistance and HIC resistance) is suitable as a line pipe for transporting petroleum, natural gas, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram of the steel pipe which concerns on this embodiment.
It is a figure which shows an example of the cooling curve of a steel plate.
3A is a view showing the result of measuring the hardness (load 100 g) of the surface layer metal structure of 0.1 to 1.0 mm from the surface at the position corresponding to 3 o'clock when the welded portion of the steel pipe is set to the 0 o'clock position.
3B is a diagram showing the results of measuring the hardness (load 100 g) of the surface layer metal structure of 0.1 to 1.0 mm from the surface at the position corresponding to 6 o'clock when the welded portion of the steel pipe is set to the 0 o'clock position.
Fig. 3C is a diagram showing the results of measuring the hardness (load 100 g) of the surface layer metal structure of 0.1 to 1.0 mm from the surface at the position corresponding to 9:00 when the welded portion of the steel pipe is set to the 0 o'clock position.
It is a figure which shows an example of the SEM photograph of the surface layer part metal structure.
It is a figure which shows an example of the SEM photograph of an internal metal structure.

A steel pipe according to an embodiment of the present invention (hereinafter referred to as a steel pipe according to the present embodiment),

It has a base material part made of a normal steel plate, and a welding part provided in a butt part of the steel plate and extending in the longitudinal direction of the steel plate,

The steel sheet has a chemical composition, in mass%, C: 0.030 to 0.070%, Si: 0.005 to 0.50%, Mn: 1.05 to 1.65%, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.045%, Ca: 0.0010 to 0.0050%, N: 0.0015 to 0.0070%, P: 0.015% or less, S: 0.0015% or less, O: 0.0040% or less, if necessary, Ni: 0.05 to One or two of 0.50%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Cu: 0.05 to 0.50%, V: 0.010 to 0.100%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100% Including the above, the balance consists of Fe and impurities, preferably Ceq=[C]+[Mn]/6+([Ni]+[Cu])/15+([Cr]+[Mo]+[V] ]) Ceq represented by /5 is 0.300 to 0.400,

The surface layer metal structure, which is a metal structure ranging from the surface of the base material to 1.0 mm in the depth direction, contains polygonal ferrite and granular bainite, and the area ratio of the polygonal ferrite in the surface layer metal structure is 0 to 70%, the total area ratio of the polygonal ferrite and the granular bainite is 50% or more, and the balance contains bainite (including tempered bainite), pseudo pearlite, or a mixture thereof,

The maximum hardness in the surface layer metal structure is 270 Hv or less, preferably 250 Hv or less,

The internal metal structure, which is a metal structure ranging from more than 1.0 mm in the depth direction from the surface of the base metal part to the center of the plate thickness, contains polygonal ferrite of 40% or less in area ratio, and the balance is granular bainite and bainite , pseudo-perlite or mixtures thereof,

Maximum hardness in the said internal metal structure is 248 Hv or less, and average hardness is 150-220 Hv.

In addition, a steel plate (hereinafter, referred to as a steel plate according to the present embodiment) according to an embodiment of the present invention is a steel plate used for the base material portion of the steel pipe.

Hereinafter, the steel pipe which concerns on this embodiment, the steel plate which concerns on this embodiment, and a preferable manufacturing method thereof are demonstrated.

First, the base material part of the steel pipe concerning this embodiment (that is, the steel plate which concerns on this embodiment) is demonstrated.

(I) chemical composition

The reason for limiting the chemical composition of the base material part (steel plate concerning this embodiment) of the steel pipe which concerns on this embodiment is demonstrated. Hereinafter, % regarding a chemical composition means mass %.

C: 0.030 to 0.070%

C is an element necessary for improving the strength of steel. When the C content is less than 0.030%, the effect of improving the strength cannot be sufficiently obtained. Therefore, the C content is made 0.030% or more. Preferably it is 0.040 % or more.

On the other hand, when the C content exceeds 0.070%, the strength of the steel increases too much, and the hardness of the surface layer metal structure and internal metal structure, particularly, the central segregation portion exceeds 248 Hv, and the SSC resistance and HIC resistance decrease. do. Therefore, the C content is made 0.070% or less. From the viewpoint of suppressing deterioration in weldability and toughness, the C content is preferably 0.050% or less.

Si: 0.005 to 0.50%

Si is an element which functions as a deoxidizer at the time of steelmaking. In addition, it is an element that is unavoidably mixed in the steelmaking step. When Si content is less than 0.005 %, the said effect cannot fully be acquired. Therefore, the Si content is made 0.005% or more. It is preferable to set it as 0.050 % or more at the point which fully acquires a deoxidation effect.

On the other hand, when Si content exceeds 0.50 %, the toughness of a weld heat affected zone (HAZ) will fall. Therefore, the Si content is made 0.50% or less. Preferably it is 0.35 % or less.

Mn: 1.05 to 1.65%

Mn is an element contributing to the improvement of the strength and toughness of steel. When the Mn content is less than 1.05%, the effect of improving strength and toughness cannot be sufficiently obtained. Therefore, the Mn content is made 1.05% or more. Preferably it is 1.15 % or more.

On the other hand, when Mn content exceeds 1.65 %, while MnS which deteriorates HIC resistance is produced|generated abundantly, the hardness of an internal metal structure, especially a central segregation part exceeds 248 Hv, and HIC resistance falls. Therefore, the Mn content is set to 1.65% or less. Preferably it is 1.50 % or less.

Al: 0.010 to 0.070%

Al is an element added for deoxidation. When Al content is less than 0.010 %, the said effect cannot fully be acquired. Therefore, the Al content is made 0.010% or more. Preferably it is 0.020 % or more.

On the other hand, when the Al content exceeds 0.070%, Al oxides are accumulated to form clusters, and the HIC resistance is lowered. Therefore, the Al content is made 0.070% or less. Preferably it is 0.045 % or less.

Ti: 0.005 to 0.020%

Ti is an element that combines with N to form a nitride. This nitride contributes to refinement|miniaturization of a crystal grain. When Ti content is less than 0.005 %, the said effect cannot fully be acquired. Therefore, the Ti content is made 0.005% or more. Preferably it is 0.008 % or more.

On the other hand, when the Ti content exceeds 0.020%, coarse nitride is formed and the HIC resistance is lowered. Therefore, the Ti content is made 0.020% or less. Preferably it is 0.015 % or less.

Nb: 0.005 to 0.045%

Nb is an element that expands the non-recrystallization temperature range and makes crystal grains fine, forms carbides and nitrides, and contributes to the improvement of the strength of steel. When Nb content is less than 0.005 %, the said effect is not fully acquired. Therefore, the Nb content is made 0.005% or more. Preferably it is 0.010 % or more.

On the other hand, when the Nb content exceeds 0.045%, coarse carbides and nitrides are formed, and the HIC resistance decreases. Therefore, the Nb content is made 0.045% or less. Preferably it is 0.035 % or less.

Ca: 0.0010 to 0.0050%

Ca is an element contributing to the improvement of HIC resistance by bonding with S to generate CaS and suppressing the generation of MnS elongated in the rolling direction. When Ca content is less than 0.0010 %, the said effect is not fully acquired. Therefore, the Ca content is made 0.0010% or more. Preferably it is 0.0020% or more.

On the other hand, when Ca content exceeds 0.0050 %, Ca oxide accumulates and HIC resistance falls. Therefore, the Ca content is made 0.0050% or less. Preferably it is 0.0040 % or less.

N: 0.0015 to 0.0070%

N is an element that forms nitride and contributes to suppression of coarsening of austenite grains during heating. When the N content is less than 0.0015%, the above effect cannot be sufficiently obtained. Therefore, the N content is made 0.0015% or more. Preferably it is 0.0020% or more.

On the other hand, when the N content exceeds 0.0070%, coarse carbonitrides are formed and the HIC resistance decreases. Therefore, the N content is made 0.0070% or less. Preferably it is 0.0050 % or less.

The chemical composition of the base material portion (steel sheet according to the present embodiment) of the steel pipe according to the present embodiment is within a range not to decrease the characteristics of the steel sheet according to the present embodiment in order to improve strength, toughness, and other characteristics other than the above elements. In , instead of a part of Fe, one or two or more selected from Ni, Mo, Cr, Cu, V, Mg, and REM may be included in the range described later. These elements are arbitrary elements, and it is not necessary to contain them. That is, the lower limit of the content of these elements is 0%.

Ni: 0 to 0.50%

Ni is an element contributing to the improvement of the toughness and strength of steel, and the improvement of corrosion resistance. When obtaining these effects, it is preferable to make Ni content into 0.05 % or more. More preferably, it is 0.10 % or more.

On the other hand, when Ni content exceeds 0.50 %, while intensity|strength rises too much and toughness falls, SSC resistance falls by grain boundary selective corrosion of the surface. Therefore, even when it is made to contain, Ni content shall be 0.50 % or less. Preferably it is 0.35 % or less.

Mo: 0 to 0.50%

Mo is an element contributing to the improvement of the quenching properties of steel. When this effect is acquired, it is preferable to make Mo content into 0.05 % or more. More preferably, it is 0.10 % or more. On the other hand, when Mo content exceeds 0.50 %, intensity|strength will rise too much and toughness will fall. Therefore, even when it is made to contain, Mo content shall be 0.50 % or less. Preferably it is 0.35 % or less.

Cr: 0 to 0.50%

Cr is an element contributing to the improvement of the strength of steel. When this effect is acquired, it is preferable to make Cr content into 0.05 % or more. More preferably, it is 0.10 % or more. On the other hand, when the Cr content exceeds 0.50%, the strength increases too much and the toughness decreases. Therefore, even when it is made to contain, Cr content shall be 0.50 % or less. Preferably it is 0.35 % or less.

Cu: 0 to 0.50%

Cu is an element contributing to the improvement of the strength of steel and the improvement of corrosion resistance. When obtaining these effects, it is preferable to make Cu content into 0.05 % or more. More preferably, it is 0.10 % or more. On the other hand, when Cu content exceeds 0.50 %, intensity|strength will rise too much and toughness will fall. Therefore, even when it is made to contain, Cu content shall be 0.50 % or less. Preferably it is 0.35 % or less.

V: 0 to 0.100%

V is an element contributing to the improvement of the strength of steel by forming carbides and/or nitrides. When this effect is acquired, it is preferable to make V content into 0.010 % or more. More preferably, it is 0.030 % or more. On the other hand, when V content exceeds 0.100 %, toughness will fall. Therefore, even when it is made to contain, V content shall be 0.100 % or less. Preferably it is 0.080 % or less.

Mg: 0 to 0.0100%

Mg is an element contributing to the improvement of toughness of steel by forming fine oxides and suppressing coarsening of crystal grains. When this effect is acquired, it is preferable to make Mg content into 0.0001 % or more. More preferably, it is 0.0010 % or more.

On the other hand, when the Mg content exceeds 0.0100%, the oxide aggregates and coarsens, and the HIC resistance and toughness decrease. Therefore, even when it is made to contain, Mg content shall be 0.0100 % or less. Preferably it is 0.0050 % or less.

REM: 0 to 0.0100%

REM is an element contributing to improvement of SSC resistance, HIC resistance, and toughness by controlling the shape of sulfide inclusions. In order to obtain these effects, the REM content is preferably 0.0001% or more. More preferably, it is 0.0010 % or more.

On the other hand, when the REM content exceeds 0.0100%, oxides are generated to lower the cleanliness of the steel, and as a result, the HIC resistance and toughness are lowered. Therefore, even when it is made to contain, REM content is made into 0.0100 % or less. Preferably it is 0.0060 % or less.

In the present embodiment, REM means a rare earth element, and is a generic term for 17 elements of Sc, Y, and lanthanoids, and the REM content represents the total content of these 17 elements.

As described above, the base material portion of the steel pipe according to the present embodiment (the steel sheet according to the present embodiment) contains the above-described essential elements and the balance is based on a chemical composition containing Fe and impurities, but the above-described essential elements may contain the above-described arbitrary elements as needed, and the remainder may be a chemical composition composed of Fe and impurities.

Impurities are components that are mixed from raw materials such as ore or scrap or from various environments in the manufacturing process when steel is industrially manufactured, and are allowed in a range that does not adversely affect steel.

It is preferable to control in the range mentioned later especially about P, S, O, Sb, Sn, Co, As, Pb, Bi, and H among impurities.

P: 0.015% or less

P is an impurity element, and it is a preferable element, so that there is little content. When the P content exceeds 0.015%, the HIC resistance remarkably decreases. Therefore, the P content is made 0.015% or less. Preferably it is 0.010 % or less.

Since it is so preferable that there is little P content, 0 % is included in a minimum. However, if the P content is reduced to less than 0.003%, the manufacturing cost will increase significantly, so that 0.003% is a practical lower limit of the P content on a practical steel sheet.

S: 0.0015% or less

S is an element which forms MnS extended in a rolling direction at the time of hot rolling. This stretched MnS reduces HIC resistance. When the S content exceeds 0.0015%, the HIC resistance remarkably decreases. Therefore, the S content is made 0.0015% or less. Preferably it is 0.0010 % or less.

Since it is so preferable that there is little S content, 0 % is included in a minimum. However, when the S content is reduced to less than 0.0001%, the manufacturing cost increases significantly, so that 0.0001% is a practical upper limit of the S content on a practical steel sheet.

O: 0.0040% or less

O is an element which remains unavoidably after deoxidation, and it is so preferable that there is little content. When the O content exceeds 0.0040%, a large amount of oxides are generated and the HIC resistance is remarkably reduced. Therefore, the O content is made 0.0040% or less. Preferably it is 0.0030 % or less.

Since it is so preferable that there is little O content, 0 % is included in a minimum. However, if the O content is reduced to less than 0.0010%, the manufacturing cost will rise significantly, so that 0.0010% is a practical lower limit of the O content on a practical steel sheet.

Further, in consideration of the influence on the steel sheet properties and the steel pipe properties, for example, it is preferable that Sb, Sn, Co and As are each 0.10% or less, Pb and Bi are each 0.005% or less, and H is 0.0005% or less.

Ceq: 0.300 to 0.400

In the steel pipe according to the present embodiment, in order to further improve strength, SSC resistance, and HIC resistance, the chemical composition of the steel sheet used for the base material portion of the steel pipe is defined by the following formula (1) as well as the content of each element Ceq (carbon equivalent) to be 0.400 or less.

[C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] in the formula are the contents of C, Mn, Ni, Cu, Cr, Mo, and V (mass%) am.

When Ceq exceeds 0.400, quenching property becomes too high, the maximum hardness of the surface layer part metal structure of the base material part (steel plate) mentioned later exceeds 270 Hv, As a result, SSC resistance falls. Moreover, the maximum hardness of the internal metal structure exceeds 248 Hv, and HIC resistance falls. Therefore, Ceq is made 0.400 or less. Preferably it is 0.350 or less. The lower limit of Ceq is set to 0.300 or more in order to secure a predetermined strength.

(II) metal structure

Next, the metal structure (structure and hardness contained) of the base material part of the steel pipe which concerns on this embodiment is demonstrated.

When the steel sheet is controlled and cooled, the surface layer portion of the steel sheet is rapidly cooled compared to the inside of the steel sheet. This means that a difference occurs between the metal structure of the surface layer portion of the steel sheet and the metal structure inside the steel sheet, and thus a difference occurs in mechanical properties. In particular, with regard to hardness, the surface layer portion of the steel sheet becomes higher in hardness than the inside. The present inventors have found that, in the steel sheet and the steel pipe having such a structure, the SSC resistance is inferior in the range (surface layer part) from the surface to 1.0 mm in the depth direction (thickness direction).

On the other hand, the present inventors have found that, in controlled cooling of a steel sheet, by using recuperative heat, it is possible to control the metal structure of the surface layer part of the steel sheet and the metal structure inside the steel sheet, respectively, and as a result, it is possible to suppress the hardening of the surface layer part of the steel sheet found out

In the steel pipe according to the present embodiment, in order to ensure excellent SSC resistance and HIC resistance, the metal structure of the steel sheet in the base material portion is (i) a structure in a range from the surface of the steel sheet to 1.0 mm in the depth direction (thickness direction). (Surface layer metal structure) and (ii) a structure (internal metal structure) ranging from more than 1.0 mm in the depth direction from the surface of the steel sheet to the center of the sheet thickness, in each metal structure, the type of structure included and fraction (area ratio), and hardness.

In the steel pipe according to the present embodiment, a surface layer portion (hereinafter, simply referred to as "steel plate surface layer portion") is defined as a surface layer portion up to 1.0 mm in the depth direction from the surface of the steel sheet of the base material portion. According to accelerated cooling, especially, since hardness in the range from the surface to a depth of 1.0 mm becomes high, let the surface layer part metal structure be a structure|tissue in the range from the steel plate surface to 1.0 mm in depth direction.

Surface layer metal structure from the surface of the steel sheet of the base material to a depth of 1.0 mm: contains polygonal ferrite and granular bainite, the area ratio of polygonal ferrite is 0 to 70%, the sum of polygonal ferrite and granular bainite The area ratio is 50% or more, and the maximum hardness is 270 Hv or less.

In the surface layer portion, when the area ratio of polygonal ferrite exceeds 70%, a high concentration of C is accumulated in the remainder to form a hardened region, and as a result, the SSC resistance deteriorates. Therefore, the area ratio of polygonal ferrite is set to 70% or less. Preferably it is 50 % or less. In addition, in order to ensure SSC resistance, the total area ratio of polygonal ferrite and granular bainite is set to 50% or more.

It is preferable that the remainder of the surface layer part metal structure consists of 1 type or 2 types of bainite and pseudo pearlite. However, the remainder does not have to be included. That is, the total area ratio of polygonal ferrite and granular bainite may be 100%.

When the maximum hardness of the surface layer part metal structure exceeds 270 Hv, SSC resistance will fall. Therefore, the maximum hardness of the surface layer metal structure is 270 Hv or less. Preferably it is 250 Hv. Although it is not necessary to set the lower limit from a viewpoint of SSC resistance, it is 160 Hv or more substantially.

The measurement of the area ratio of each structure is obtained by observing a metal structure at a magnification of, for example, 1000 times using a scanning electron microscope (SEM). The surface layer metal structure is obtained by observing the positions of 0.1 mm, 0.2 mm, and 0.5 mm from the surface of the steel sheet, and averaging the area ratio at each position.

In the present embodiment, polygonal ferrite is a structure observed as a lumped structure that does not contain coarse precipitates such as cementite and MA in the grains.

Bainite has a clear old austenite grain boundary, a fine lath structure is developed in the grain, and is a structure in which fine carbides and austenite-martensite hybrids are interspersed in laths and between laths. Here, bainite also includes tempered bainite.

In granular bainite, the prior austenite grain boundary is unclear, and needle-shaped ferrite (the austenite-martensite hybrid is not present in the grain) is a structure in which acicular ferrite is generated in random crystal orientations, It is produced at an intermediate transformation temperature of nite, where old austenite grain boundaries are partially visible, a coarse lath structure exists in the grains, and fine carbides and austenite-martensite hybrids are interspersed in laths and between laths; It is a structure in which former austenite grain boundaries are unclear and acicular or irregular ferrite parts are mixed.

The pseudo-perlite is a pearlite in which cementite is arranged in lacerations.

An example of the metal structure (imaging with a scanning electron microscope: 1000 times magnification) is shown in FIG. 4 from the surface of a steel plate. In FIG. 4, a portion with a smooth interior surrounded by a smooth curve is polygonal ferrite, and a portion with white dots inside is granular bainite.

The measurement of the maximum hardness of the surface layer metal structure is performed as follows.

First, the positions of 1/4, 1/2, and 3/4 of the width of the steel sheet in the width direction of the steel sheet from the end of the steel sheet in the width direction (corresponding to the butt portion in the case of a steel pipe) In one case, a rectangular steel sheet having a side of 300 mm was cut by gas cutting from the positions of 3, 6, and 9 o’clock, respectively), and a block test piece having a length of 20 mm and a width of 20 mm was machined from the center of the cut steel sheet. It collects by cutting, and grind|polishes by machine grinding|polishing. For each block test piece, with a Vickers hardness tester (load: 100 g), starting at 0.1 mm from the surface, 10 points at intervals of 0.1 mm in the plate thickness direction, 10 points at intervals of 1.0 mm in the width direction with respect to the same depth, a total of 100 measure points. As a result of the above measurement, in any of the test pieces, if two or more measurement points exceeding 270 Hv are not continuously revealed in the sheet thickness direction, the maximum hardness of the surface layer metal structure is judged to be 270 Hv or less.

When two or more measurement points exceeding 270 Hv exist continuously in the plate thickness direction, the hardness is not an ideal value, a structure with high hardness is formed, and SSC resistance is lowered, so it is not allowed. However, in this embodiment, even if there is one measurement point exceeding 270 Hv, if two or more points are not continuously revealed in the plate thickness direction, the point is not adopted as an outlier, and the next highest value is the maximum hardness . When two or more measurement points exceeding 270 Hv exist continuously in the plate thickness direction, their highest value is employed as the maximum hardness.

3A to 3C show the results of measuring the hardness of the surface layer metal structure at three locations corresponding to 3 o'clock, 6 o'clock, and 9 o'clock when the welded portion of the steel pipe is set at the 0 o'clock position. The hardness of the metal structure of the surface layer was measured using a Vickers hardness tester in the area from the surface layer to a depth of 0.1 mm to a depth of 1.0 mm with a load of 100 g at intervals of 0.1 mm and 10 points at the same depth. In any location, the maximum hardness is 270 Hv or less, and it turns out that it has the outstanding SSC resistance.

Metal structure (internal metal structure) ranging from more than 1.0 mm in the depth direction from the surface of the steel sheet of the base material part to the center of the sheet thickness: the area ratio of polygonal ferrite is 40% or less, the maximum hardness is 248 Hv or less, and the average hardness is 150 to 220Hv

In the internal metal structure, when the area ratio of polygonal ferrite exceeds 40%, it becomes difficult to secure the required strength and HIC resistance. Therefore, the area ratio of polygonal ferrite is set to 40% or less. Preferably it is 30 % or less, More preferably, it is 25 % or less.

The remainder of the internal metal structure consists of one or two or more types of granular bainite, bainite, and pseudo-perlite.

When the maximum hardness in an internal metal structure exceeds 248 Hv, HIC resistance will fall. Therefore, the maximum hardness is 248 Hv or less. In addition, if the average hardness is less than 150 Hv, the necessary mechanical properties cannot be ensured. Therefore, average hardness is made into 150 Hv or more. Preferably it is 160 Hv or more. On the other hand, when the average hardness exceeds 220 Hv, HIC resistance and toughness are lowered. Therefore, average hardness shall be 220 Hv or less. Preferably it is 210 Hv or less.

The tissue fraction (area ratio) of the internal metal structure is observed using a scanning electron microscope (SEM) at a position of 1/4 (t/4) of the plate thickness from the surface of the steel plate, for example, at a magnification of 1000 times. obtained by doing The observation position is set as the position of t/4 because the position structure at t/4 represents a typical structure of the internal metal structure.

In FIG. 5, an example of the metal structure (imaging with a scanning electron microscope: 1000 times magnification) at the position of t/4 is shown. In Fig. 5, a portion surrounded by smooth curves and having a smooth interior is polygonal ferrite. In addition, the portion where white dots or lines are visible inside is granular bainite or pseudo pearlite, surrounded by jagged white lines, and thinly visible inside is bainite.

The maximum hardness and average hardness in an internal metal structure can be measured with the following method.

1/4, 1/2, and 3/4 positions in the width direction of the steel sheet from the edge of the steel sheet in the width direction (corresponding to the butt portion in the case of a steel pipe) 3 o'clock, 6 o'clock, and 9 o'clock), a rectangular steel plate with a side of 300 mm was cut by gas cutting, and a block test piece with a length of 20 mm and a width of 20 mm was taken by machine cutting from the center of the cut steel plate. , polished by machine polishing. For each block test piece, with a Vickers hardness tester (load: 1 kg), starting at a depth of 1.2 mm from the surface, 10 points at intervals of 0.2 mm in the plate thickness direction and at intervals of 1.0 mm in the width direction with respect to the same depth Measure hardness. As a result of the above measurement, if two or more measurement points exceeding 248Hv are not continuously revealed in the plate thickness direction, it is determined that the maximum hardness of the surface layer metal structure is 248Hv or less.

In the base material of the steel pipe according to the present embodiment, a value of high hardness (ideal value) may be revealed locally. However, even if such an outlier is revealed, HIC resistance can be ensured. On the other hand, when two or more measurement points exceeding 248 Hv exist continuously in the plate thickness direction, HIC resistance is lowered, so it is not allowed. Therefore, in this embodiment, even if there is one measurement point exceeding 248 Hv, if two or more points are not continuously revealed in the plate thickness direction, the point is not adopted as an outlier, and the next highest value is called the maximum hardness . On the other hand, when two or more measurement points exceeding 248 Hv exist continuously in the plate|board thickness direction, those highest value is employ|adopted as maximum hardness.

In addition, average hardness is computed by averaging the hardness of all measurement points.

Next, the welding part of the steel pipe which concerns on this embodiment is demonstrated.

The steel pipe according to the present embodiment is obtained by processing the steel sheet according to the present embodiment normally, butting both ends of the steel sheet (ends in the width direction of the steel sheet), and welding. Therefore, as shown in FIG. 1 , the steel pipe 1 according to the present embodiment has a welded portion 3 that is provided in the butt portion of the steel plate 2 and extends in the longitudinal direction of the steel plate. The welding part 3 is normally provided continuously from the edge part of the longitudinal direction of the steel plate 2 to the other edge part.

In general, in steel pipe welding, the welded portion is constructed to have a greater thickness than the base material portion. Moreover, a weld metal is a high alloy than a base material, and corrosion resistance is also high. Therefore, there is hardly any case where the welded portion becomes the starting point of destruction. Therefore, the welded portion of the steel pipe according to the present embodiment is not particularly limited as long as it is obtained under normal conditions by SAW welding or the like.

The steel pipe according to the present embodiment preferably has a strength satisfying X52 to X70 specified in API5L in consideration of application to a line pipe.

Next, a preferred method for manufacturing a steel pipe according to the present embodiment will be described.

The steel pipe according to the present embodiment is not limited to the manufacturing method, and if it has the above-described configuration, the effect can be obtained.

That is, the steel sheet according to the present embodiment,

(i) a hot rolling process in which a steel piece having a predetermined chemical composition is heated to 1050 to 1250° C. and subjected to hot rolling, and the hot rolling is terminated at 830 to 1000° C.;

(ii) the steel sheet after the completion of hot rolling, from a temperature range of 750 to 950 ° C. to a temperature range of 400 to 650 ° C., at an average cooling rate of 15 to 100 ° C./sec, and the temperature rise due to recuperation in the middle is 5 An accelerated cooling process of accelerated cooling to perform two or more recuperations to 65 ° C.

It is obtained by a manufacturing method comprising

Further, the steel pipe according to the present embodiment, in addition to (i) and (ii),

(iii) a forming step of normally forming the steel sheet according to the present embodiment obtained as described above;

(iv) a welding process of butt welding both ends of a normal steel plate

It is obtained by a manufacturing method comprising further.

Hereinafter, preferable conditions are demonstrated about each process.

<Hot rolling process>

Steel piece heating temperature: 1050 to 1250 °C

A steel piece produced by casting molten steel having the same chemical composition as the base material of the steel pipe according to the present embodiment is heated to 1050 to 1250° C. and subjected to hot rolling. Casting of molten steel and manufacturing of a steel piece prior to hot rolling may be performed in accordance with a conventional method.

When the steel slab heating temperature is less than 1050°C, coarse carbonitrides of undissolved Nb and Ti are produced, and the HIC resistance is lowered. Therefore, it is preferable that the steel piece heating temperature shall be 1050 degreeC or more. More preferably, it is 1100 degreeC or more. On the other hand, when the steel slab heating temperature exceeds 1250°C, the grain size becomes large and the low-temperature toughness decreases. Moreover, as a result of the austenite particle size coarsening and hardening property becoming high excessively, hardening phase is formed in the surface layer part metal structure and internal metal structure, and SSC resistance and HIC resistance fall. Therefore, it is preferable that the steel piece heating temperature shall be 1250 degrees C or less. More preferably, it is 1200 degrees C or less.

In hot rolling, the steel piece heated to the said temperature is hot-rolled by the normal rolling-reduction|draft ratio, and it is set as a steel plate. The plate thickness is not particularly limited because it may be set according to the required thickness of the line pipe.

Rolling end temperature: 830 to 1000°C

In order to obtain a predetermined surface layer part metal structure and internal metal structure by accelerated cooling after finish rolling, the rolling completion|finish temperature (finishing temperature) shall be 830-1000 degreeC. When the rolling end temperature is less than 830°C, it is difficult to obtain the surface layer part metal structure and the internal metal structure, so that the finish rolling temperature is preferably 830°C or more. More preferably, it is 850 degreeC or more.

On the other hand, when the rolling completion temperature exceeds 1000°C, the crystal grains become coarse and the low-temperature toughness decreases. Therefore, it is preferable that rolling completion temperature shall be 1000 degrees C or less. More preferably, it is 900 degrees C or less.

<Accelerated cooling process>

Cooling start temperature Ts: 750 to 950 °C

Cooling stop temperature Tf: 400 to 650°C

Average cooling rate Vc: 15 to 100°C/sec

Number of repetitions: 2 or more

Temperature rise due to recuperation: 5 to 65°C (excluding recuperation after the last stop of water cooling)

In the accelerated cooling step, the surface temperature of the steel sheet after the completion of hot rolling is from a temperature range of 750 to 950° C. to a temperature range of 400 to 650° C., an average cooling rate of 15 to 100° C./sec, and from the start of cooling to the stop of cooling. Accelerated cooling is carried out to include two or more recuperations in which the temperature rise is 5 to 65°C.

Accelerated cooling by sandwiching a double row in the middle is a cooling facility in which a cooling zone is divided into a plurality in the longitudinal direction (transport direction) of the steel plate, and the amount of cooling water sprayed to the steel plate can be adjusted for each cooling zone. .

An example of the cooling curve of a steel plate is shown in FIG. The four cooling curves are, from the top, a cooling curve at the center of the plate thickness (1/2 part of the plate thickness), a cooling curve at a position of 1/4 of the plate thickness (t/4 part) from the surface, and a depth of 1.0 mm from the surface. These are the cooling curve of the area and the cooling curve of the surface of the steel plate. The whole steel plate is accelerated cooling from 830 degreeC of cooling start temperature (Ts) to about 620 degreeC in about 10 seconds, so that it may include three times of recuperation on the way.

In this cooling, the cooling start temperature Ts and the cooling stop temperature Tf are the points shown, and the average cooling rate Vc is the temperature change ΔT (cooling start temperature Ts−cooling stop temperature Tf) and the cooling time Δt (water cooling time) ) by dividing by

According to FIG. 2, as a result of adjusting the injection amount of cooling water in each cooling zone, as for the steel plate surface, it turns out that the surface temperature temporarily increased during cooling by the recuperation by sensible heat inside a steel plate. On the other hand, although there is an effect of recuperative heat on the cooling curve of the steel plate surface and the portion at a depth of 1.0 mm from the surface, the effect of recuperation on the cooling curve of the central plate thickness (1/2 part thickness) and the cooling curve of the plate thickness 1/4 part is not seen, and it turns out that the inside of a steel plate is cooled at a substantially constant cooling rate.

When the cooling start temperature Ts is less than 750°C, coarse ferrite is formed after rolling in the surface layer metal structure, and a structure having high hardness such as martensite is formed as the remainder. As a result, the SSC resistance deteriorates. Moreover, when cooling start temperature Ts is less than 750 degreeC, a ferrite fraction becomes excess in an internal metal structure, and the hardness of a hardened phase also becomes high. Therefore, it is preferable that cooling start temperature Ts shall be 750 degreeC or more. More preferably, it is 780 degreeC or more.

On the other hand, when cooling start temperature Ts exceeds 950 degreeC, even if it reheats twice or more, the maximum hardness of surface layer part metal structure will exceed 270 Hv, and SSC resistance will fall. Therefore, it is preferable that the cooling start temperature Ts shall be 950 degrees C or less. More preferably, it is 880 degrees C or less.

When cooling stop temperature Tf is less than 400 degreeC, the average hardness of an internal metal structure exceeds 220 Hv, and HIC resistance falls. Therefore, the cooling stop temperature Tf is preferably 400°C or higher. More preferably, it is 480 degreeC or more. On the other hand, when cooling stop temperature Tf exceeds 650 degreeC, the average hardness of an internal metal structure will become less than 150 Hv, and it may become impossible to satisfy|fill predetermined intensity|strength. Moreover, on the other hand, a structure with high hardness is locally formed, and SSC resistance and HIC resistance may fall. Therefore, it is preferable that cooling stop temperature Tf shall be 650 degrees C or less. More preferably, it is 580 degrees C or less.

When the average cooling rate Vc is less than 15°C/sec, polygonal ferrite exceeding 70% in area ratio is generated in the surface layer metal structure. Further, in the internal metal structure, polygonal ferrite exceeding 40% in area ratio is generated. In that case, since the strength as a line pipe cannot be ensured, it is preferable that the average cooling rate Vc is set to 15 degreeC/sec or more. More preferably, it is 25 degreeC/sec or more.

On the other hand, when average cooling rate Vc exceeds 100 degreeC/sec, martensitic transformation will generate|occur|produce, the hardness of surface layer part metal structure will exceed 270 Hv, and SSC resistance will fall. Moreover, the maximum hardness of an internal metal structure exceeds 248 Hv, and HIC resistance falls. Therefore, it is preferable that average cooling rate Vc shall be 100 degrees C/sec or less. More preferably, it is 80 degrees C/sec or less.

When the number of times of recuperation in which the recuperation temperature during accelerated cooling is within the predetermined range is one or less, the hardness of the surface layer metal structure exceeds 270 Hv, and the SSC resistance decreases. Therefore, the number of times of recuperation is made two or more.

Although the cooling curve of 3 times of recuperation times was shown in FIG. 2, what is necessary is just to determine the number of recuperation times suitably according to the steel type and sheet-feeding speed between cooling start temperature and cooling stop temperature.

In the steel sheet according to the present embodiment, in order to generate a predetermined structure, cooling is performed in a film boiling state. In order to set it as cooling in a film boiling state, at the time of recuperation in the middle of water cooling, cooling is performed so that the temperature rise by recuperation may be 65 degrees C or less without complete recuperation. If the temperature rise due to recuperation exceeds 65° C., coarse ferrite is generated and a predetermined structure cannot be obtained. On the other hand, if the temperature rise due to recuperation is less than 5°C, the effect of recuperation cannot be obtained. Therefore, it is preferable that the temperature rise width by recuperation shall be 5-65 degreeC. More preferably, it is 10-65 degreeC. However, about the last recuperation after stopping water cooling, it is not necessary to make the temperature rise width|variety into 5-65 degreeC.

When the temperature of the steel sheet during cooling is increased by induction heating or the like instead of recuperation, the temperature rises to the inside. Therefore, even if heating by induction heating or the like is performed instead of recuperation, a predetermined structure cannot be obtained.

When recuperating with a temperature rise of 5-65 degreeC is performed twice or more, it is preferable to perform 1st recuperation so that the surface temperature of the steel plate after recuperation may become 500 degreeC or more. Even if the surface of the steel sheet after the first recuperation is less than 500°C, a surface layer metal structure having excellent SSC resistance and an internal metal structure having excellent HIC resistance can be secured, but a surface layer metal structure having excellent SSC resistance and , in order to stably secure an internal metal structure having excellent HIC resistance, it is preferable to perform the first recuperation so that the surface temperature of the steel sheet after recuperation becomes 500° C. or higher.

After the water cooling stops, the temperature difference between the surface temperature and the center temperature disappears after a while. For example, in FIG. 2, the temperature difference between the steel plate surface layer part (surface temperature) and the steel plate inside (center temperature) at about 620 degreeC disappears, and the steel plate temperature is stabilized. After this, it is preferable to cool to 300 degrees C or less at an average cooling rate of 0.5 degreeC/sec or more and 5.0 degreeC/sec or less. If the average cooling rate is 0.5°C/sec or more and 5.0°C/sec or less, it may be left to cool. When the average cooling rate is less than 0.5°C/sec, the predetermined strength is not obtained. On the other hand, when the average cooling rate exceeds 5.0°C/sec, the toughness of the central portion deteriorates.

<Forming process>

The forming of the steel pipe according to the present embodiment is not limited to specific forming. Although warm working can also be used, cold working is preferable from the point of dimensional accuracy.

<Welding process>

Next, the both ends of the normally formed steel plate are butt|matched and welded. Although welding is also not limited to a specific welding, Submerged arc welding (SAW) is preferable. What is necessary is just to perform welding conditions under well-known conditions according to plate|board thickness etc.

In the method for manufacturing a steel pipe according to the present embodiment, heat treatment (deep heat treatment) may be performed on the welded portion so that a structure (ferrite and pearlite exceeding 10% in area ratio) that reduces the toughness of the welded portion is not generated. Although a normal temperature range may be sufficient as heat processing temperature, the range of 300-Ac1 point is preferable.

Since the base metal part of the steel pipe according to the present embodiment is not subjected to heat treatment, the metal structure of the base metal part is the same as that of the steel sheet according to the present embodiment. The base metal part of the steel pipe according to the present embodiment inherits the metal structure of the steel sheet according to the present embodiment, and has mechanical properties for line pipes and excellent local weldability. Moreover, since the welded part of the steel pipe which concerns on this embodiment is excellent in the weldability of the steel plate which concerns on this embodiment, it is a welded part which is excellent in mechanical characteristics. Therefore, the steel pipe according to the present embodiment is a steel pipe suitable as a steel pipe for a line pipe.

Example

Next, although the Example of this invention is described, the conditions in an Example are one condition example employ|adopted in order to confirm the practicability and effect of this invention, and this invention is not limited to this one condition example. . Various conditions can be employ|adopted for this invention, as long as the objective of this invention is achieved without deviating from the summary of this invention.

(Example 1)

A steel piece having the chemical composition and Ceq shown in Table 1 was hot rolled and cooled under the conditions shown in Table 2 to manufacture a steel sheet. In Table 2, the number of recuperations is the number of recuperations with a temperature rise of 5°C or higher. In addition, the maximum recuperative temperature width is the temperature rise width in recuperating which had the largest temperature rise width.

A test piece was taken from the manufactured steel sheet, and the surface layer metal structure (positions of 0.1 mm, 0.2 mm, and 0.5 mm) and the internal metal structure (position of t/4) were observed using SEM at a magnification of 1000 times, and polygonal ferrite , granular bainite, and the remainder fraction (area ratio) were calculated. The remainder of the surface layer metal structure was one or two types of bainite and pseudo-pearlite, and the remainder of the internal metal structure was all one type or two or more types of granular bainite, bainite, and pseudo-pearlite.

Moreover, the JIS5 tensile test piece was produced, the tensile test prescribed|regulated to JISZ2241 was done, and the yield strength and tensile strength were measured.

In addition, hardness was measured using a Vickers hardness meter. In the surface layer part metal structure, 0.1-1.0 mm in depth was measured from the surface layer by the load of 100 g every 10 points|pieces of the same depth at intervals of 0.1 mm. On the other hand, in the internal metal structure, 1.2 mm in depth - the center of plate|board thickness from the surface layer were measured at intervals of 0.2 mm, and 10 points|pieces of the same depth were measured with 1 kg of load. From this result, the maximum hardness was calculated|required about the surface layer part metal structure, and the maximum hardness and average hardness were calculated|required about the internal metal structure.

Moreover, the test piece was extract|collected from the manufactured steel plate, the following test was done, and HIC resistance and SSC resistance were evaluated.

Evaluation of HIC resistance

Conduct a test based on TM0284 of NACE (National Association of Corrosion and Engineer), observe the presence or absence of HIC (hydrogen induced cracking), and if the HIC area ratio is 5% or less, the HIC resistance is excellent (OK), When it was more than 5%, it was evaluated that the HIC resistance is inferior (NG).

The NACE test is a test in which hydrogen sulfide gas is saturated in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, the steel sheet is immersed in the solution, and whether cracks occur after 96 hours is observed.

Evaluation of SSC resistance

A full-thickness test piece with a width of 15 mm and a length of 115 mm was taken from the steel sheet in the width direction, and the SSC resistance was evaluated by a four-point bending test based on NACE's TM0284m and ASTM (American Society for Testing and Materials) G39. did.

In the 4-point bending test, a test piece to which a stress equivalent to 90% of the 0.2% yield strength derived from the tensile test was applied was subjected to 5% sodium chloride + 0.5% acetic acid at room temperature (24°C) saturated with 1 atm hydrogen sulfide gas, pH2 It was immersed in the aqueous solution of .7 for 720 hours, and in order to judge the presence or absence of generation|occurrence|production of SSC, the surface of the test piece was observed at 10 times magnification.

A case in which SSC did not occur was referred to as pass (OK), and a case in which SSC occurred was referred to as fail (NG). A result is shown in Table 3.

(Example 2)

The steel sheet shown in Table 3 was formed into a tubular shape by C press, U press and O press, the end faces were tack welded, and main welding was performed from the inner and outer faces. After the pipe expansion, the steel pipe for line pipe was made. Submerged arc welding was applied to the main welding. The production No. of the steel sheet and the production No. of the steel pipe are related, for example, the steel pipe of Production No. T1 is manufactured using the steel plate of Production No. S1, and the steel pipe of Production No. T2 is manufactured by No. T2. It has shown that it was manufactured using the steel plate of S2.

A test piece was taken from the manufactured steel sheet, and the surface layer metal structure (positions of 0.1 mm, 0.2 mm, and 0.5 mm) and the internal metal structure (position of t/4) were observed using a scanning electron microscope at a magnification of 1000 times, Gonal ferrite, granular bainite, and the remainder fraction (area ratio) were calculated.

Moreover, the JIS5 tensile test piece was produced, the tensile test prescribed|regulated to JISZ2241 was done, and the yield strength and tensile strength were measured.

Moreover, hardness was measured with a Vickers hardness meter. In the surface layer part metal structure, 0.1-1.0 mm in depth from the surface layer was measured at 0.1 mm intervals, 10 points|pieces at a time with respect to the same depth with 100 g of load. On the other hand, in the internal metal structure, 10 points|pieces were measured with respect to the same depth at intervals of 0.2 mm from 1.2 mm in depth - the center of plate|board thickness from the surface layer, and a load of 1 kg.

Moreover, the test piece was extract|collected from the manufactured steel plate, the following test was done, and HIC resistance and SSC resistance were evaluated.

Evaluation of HIC resistance

A test based on TM0284 of NACE (National Association of Corrosion and Engineer) was conducted, the presence or absence of HIC (hydrogen induced cracking) was observed, the HIC area ratio was 5% or less, and the HIC resistance was excellent (OK). It was evaluated that the HIC resistance was inferior (NG) to more than 5%.

The NACE test is a test in which hydrogen sulfide gas is saturated in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, the steel sheet is immersed in the solution, and whether cracks occur after 96 hours is observed.

Evaluation of SSC resistance

From a steel plate, a full-thickness test piece with a width of 15 mm and a length of 115 mm was taken from the width direction (direction perpendicular to the rolling direction), and subjected to a four-point bending test based on NACE's TM0284m and ASTM (American Society for Testing and Materials) G39. Thus, the SSC resistance was evaluated.

In the 4-point bending test, a test piece to which a stress equivalent to 90% of the 0.2% yield strength derived from the tensile test was applied was subjected to 5% sodium chloride + 0.5% acetic acid at room temperature (24°C) saturated with 1 atm hydrogen sulfide gas, pH2 It was immersed in the aqueous solution of .7 for 720 hours, and in order to judge the presence or absence of generation|occurrence|production of SSC, the surface of the test piece was observed at 10 times magnification. A case in which SSC did not occur was referred to as pass (OK), and a case in which SSC occurred was referred to as fail (NG). A result is shown in Table 4.

According to the present invention, a steel sheet for a line pipe having a strength of X52 to 70 class in API standards and excellent SSC resistance and HIC resistance, and a line pipe having excellent SSC resistance and HIC resistance using the steel sheet as a base material Steel pipes can be provided. Accordingly, the present invention has high applicability in the steel sheet manufacturing industry and the energy industry.

1: steel pipe
2: Steel plate (base material part)
3: Weld

Claims (6)

A base material part made of a normal steel plate,
A welding portion provided in the butt portion of the steel sheet and extending in the longitudinal direction of the steel sheet
Have,
The steel sheet, as a chemical composition, in mass%,
C: 0.030 to 0.070%;
Si: 0.005 to 0.50%;
Mn: 1.05 to 1.65%;
Al: 0.010 to 0.070%;
Ti: 0.005 to 0.020%;
Nb: 0.005 to 0.045%;
Ca: 0.0010 to 0.0050%,
N: 0.0015 to 0.0070%,
Ni: 0 to 0.50%;
Mo: 0 to 0.50%,
Cr: 0 to 0.50%;
Cu: 0 to 0.50%;
V: 0 to 0.100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100%
including,
P: 0.015% or less,
S: 0.0015% or less,
O: 0.0040% or less
limited to,
Balance: consisting of Fe and impurities,
In the chemical composition, Ceq determined by the following formula (1) is 0.300 to 0.400,
The surface layer metal structure, which is a metal structure ranging from the surface of the base material to 1.0 mm in the depth direction, contains polygonal ferrite and granular bainite, and the area ratio of the polygonal ferrite in the surface layer metal structure is 0 to 70%, the total area ratio of the polygonal ferrite and the granular bainite is 50% or more,
The maximum hardness in the surface layer metal structure is 270 Hv or less,
The internal metal structure, which is a metal structure in the range from more than 1.0 mm in the depth direction from the surface of the base material part to the center of the plate thickness, contains polygonal ferrite of 40% or less in area ratio,
The maximum hardness in the internal metal structure is 248 Hv or less, and the average hardness is 150 to 220 Hv.
Steel pipe, characterized in that.

Here, [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] in the formula are in the mass% of C, Mn, Ni, Cu, Cr, Mo, and V content by According to claim 1, wherein the chemical composition, in mass%,
Ni: 0.05 to 0.50%;
Mo: 0.05 to 0.50%,
Cr: 0.05 to 0.50%;
Cu: 0.05 to 0.50%;
V: 0.010 to 0.100%,
Mg: 0.0001 to 0.0100%;
REM: 0.0001 to 0.0100%
containing one or two or more of
Steel pipe, characterized in that. The balance of the metal structure of the surface layer part consists of 1 type or 2 types of bainite and pseudo pearlite according to claim 1 or 2,
The remainder of the internal metal structure is composed of one or two or more of granular bainite, bainite, and pseudo-perlite.
Steel pipe, characterized in that. A steel sheet used for the base material portion of the steel pipe according to claim 1 . A steel sheet for use in the base material portion of the steel pipe according to claim 2 . A steel sheet used for the base material portion of the steel pipe according to claim 3 .

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