KR101524397B1 - High-strength steel sheet and high-strength steel pipe having excellent hydrogen-induced cracking resistance for use in line pipe - Google Patents

Abstract

It is a line pipe steel pipe and line pipe steel pipe excellent in HIC resistance and optimal for steel pipes used for pipelines for transportation of petroleum and natural gas. It is composed of 0.02 to 0.08% of C, 0.01 to 0.5% of Si, 1.0 P: 0.01% or less, S: 0.0020% or less, Al: 0.0005% or less, and N: 0.001 to 0.10% To 0.030% of Ti, not more than 0.030% of Ti, a steel composition satisfying S / Ca < 0.5, and a length of an uncompacted portion of the center segregation portion is limited to 0.1 mm or less.

Description

BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention relates to a high strength steel pipe for a high-strength line pipe and a high-strength steel pipe for a high-strength line pipe,

The present invention relates to a steel sheet for a line pipe and a steel pipe for a line pipe, which are excellent in use for pipelines for transporting petroleum, natural gas, etc., and which have excellent hydrogen-organic cracking resistance (called HIC resistance).

In the case of pipelines for transportation such as petroleum and natural gas containing a large amount of hydrogen sulfide (H ₂ S) containing water, generation of hydrogen organic cracks (HIC) is a concern. The reason for this is that in the H ₂ S environment (referred to as "sour environment") containing water, hydrogen tends to invade from the surface of the steel.

The HIC is used particularly in the case of a steel material such as MnS grown in the central segregation portion of the steel, carbonitride of integrated Ti or Nb, or oxide inclusion in the oxide stack, It is due to one hydrogen.

That is, in a sour environment, cracks are generated when the hydrogen entering the steel accumulates around the defect to become a gas and the pressure exceeds the fracture toughness value (K _IC ) of the steel. Further, if the center segregation portion of the steel, the periphery of the inclusions, etc. are hardened, the cracks tend to propagate.

Accordingly, in the line pipe used in the sour environment, conventionally, it has been conventionally proposed to suppress the generation of MnS which is stretched, to suppress the accumulation of carbonitride or oxide of Ti and Nb, or to form a cured image And the like.

For example, Mn is an element easily segregated at the center of a steel sheet, and a method of suppressing segregation of Mn has been proposed (for example, Patent Documents 1 to 3). Patent Document 1 proposes a steel sheet in which the ratio of the Mn content in the segregation portion to the average Mn content in the steel is suppressed. Further, Patent Documents 2 and 3 propose a high-strength line pipe which limits the P concentration in the segregation portion in addition to the size of the Mn segregation spot and utilizes Ca.

In addition to Mn segregation, a hot-rolled steel sheet excellent in HIC resistance has also been proposed (see, for example, Patent Document 4). Further, a method of suppressing inclusions such as carbides and nitrides of Ti and Nb has been proposed (for example, Patent Documents 5 and 6).

(Patent Document 1) Japanese Patent Application Laid-Open No. 6-220577

(Patent Document 2) Japanese Patent Application Laid-Open No. 6-256894

(Patent Document 3) Japanese Patent Application Laid-Open No. 6-271974

(Patent Document 4) Japanese Patent Application Laid-Open No. 2002-363689

(Patent Document 5) Japanese Patent Application Laid-Open No. 2006-63351

(Patent Document 6) Japanese Patent Application Laid-Open No. 2008-7841

(Maximum Mn content in the segregated portion) / (average Mn content in the steel) and the size of the Mn segregation spot are controlled in a manner that controls the segregation of Mn and the shape control of MnS using Ca , It can not be said that the HIC can be completely prevented, and it is found that it is necessary to control them more strictly.

Further, when the segregation of Mn is eliminated, there is a problem of segregation of Nb next. This segregation of Nb is also insufficient for the control of (the maximum Nb content in the segregation portion) / (the average Nb content in the steel) so that it is necessary to control more strictly. In addition, even if the length of the Nb-Ti-C-N inclusions and the surface density and length of the (Ti, Nb) (C, N) inclusions were controlled, the occurrence of HIC could not be prevented.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet for a line pipe and a line pipe steel pipe excellent in HIC resistance, which is optimum for a steel pipe used for a pipeline for transporting oil, natural gas, will be.

The present inventors have made intensive studies on the conditions necessary for obtaining a steel plate for a high-strength line pipe and a steel pipe for a high-strength line pipe having a tensile strength of 500 MPa or more and excellent hydrogen-organic cracking resistance and to provide a steel plate for a new ultra- And has come to invent steel pipes for pipes. The gist of the present invention is as follows.

(1) in mass%

C: 0.02 to 0.08%

Si: 0.01 to 0.5%

Mn: 1.0 to 1.6%

Nb: 0.001 to 0.10%

Ca: 0.0001 to 0.0050%,

N: 0.0010 to 0.0050%,

O: 0.0001 to 0.0030%

&Lt; / RTI &gt;

P: 0.01% or less,

S: 0.0020% or less,

Al: 0.0005 to 0.030%

Ti: 0.030% or less,

, And the content of S and Ca is limited,

S / Ca < 0.5

, The balance being Fe and inevitable impurity elements,

Further, when the length of the unsealed portion of the center segregation portion is set to 0.1 mm or less

Wherein the steel sheet has excellent hydrogen crack resistance.

(2) in mass%

Ni: 0.01 to 2.0%

Cu: 0.01 to 1.0%

Cr: 0.01 to 1.0%

Mo: 0.01 to 1.0%

W: 0.01 to 1.0%

V: 0.01 to 0.10%,

Zr: 0.0001 to 0.050%

Ta: 0.0001 to 0.050%

B: 0.0001 to 0.0020%

(1), wherein the balance of the iron and inevitable impurities is at least one selected from the group consisting of iron and unavoidable impurities.

(3)% by mass

REM: 0.0001 to 0.01%

Mg: 0.0001 to 0.01%

Y: 0.0001 to 0.005%

Hf: 0.0001 to 0.005%,

Re: 0.0001-0.005%

(1) or (2), characterized by further comprising at least one of the following:

(4) Further,

Maximum Mn segregation degree: 2.0 or less,

Nb grain size: 4.0 or less,

Ti segregation degree: 4.0 or less,

(1) or (2), characterized in that the hydrogen-induced cracking resistance is limited to a range of from 0.1 to 10% by weight.

(5) The steel sheet for a high strength line pipe as described in (1) or (2) above, wherein the maximum hardness of the center segregation portion is 300 Hv or less.

(6) When the base material contains, by mass%

C: 0.02 to 0.08%

Si: 0.01 to 0.5%

Mn: 1.0 to 1.6%

Nb: 0.001 to 0.10%

Ca: 0.0001 to 0.0050%,

N: 0.0010 to 0.0050%,

O: 0.0001 to 0.0030%

&Lt; / RTI &gt;

P: 0.01% or less,

S: 0.0020% or less,

Al: 0.0005 to 0.030%

Ti: not more than 0.030%

, And the content of S and Ca is limited,

S / Ca < 0.5

, The balance being Fe and inevitable impurity elements,

A steel pipe for a high-strength line pipe excellent in resistance to hydrogen-induced organic cracking, characterized in that the length of an uncompacted portion of the center segregation portion of the base material is limited to 0.1 mm or less.

(7) When the base material contains, by mass%

Ni: 0.01 to 2.0%

Cu: 0.01 to 1.0%

Cr: 0.01 to 1.0%

Mo: 0.01 to 1.0%

W: 0.01 to 1.0%

V: 0.01 to 0.10%,

Zr: 0.0001 to 0.050%

Ta: 0.0001 to 0.050%

B: 0.0001 to 0.0020%

(6), characterized in that the steel pipe for high-strength line pipe excellent in resistance to hydrogen-induced hydrogen cracking is characterized in that the steel pipe comprises one or two or more kinds of iron and inevitable impurities.

(8) A method for producing a base material,

REM: 0.0001 to 0.01%

Mg: 0.0001 to 0.01%

Y: 0.0001 to 0.005%

Hf: 0.0001 to 0.005%,

Re: 0.0001-0.005%

(6) or (7), characterized in that the steel pipe further contains one or more of the following.

(9) Further,

Maximum Mn segregation degree: 2.0 or less,

Nb grain size: 4.0 or less,

Ti segregation degree: 4.0 or less,

(6) or (7), which is characterized in that the hydrogen permeation resistance is limited to the range of 0.1 to 5% by weight.

(10) The steel pipe for a high strength line pipe according to (6) or (7), wherein the maximum hardness of the central segregation portion of the base material is 300 Hv or less.

According to the present invention, the segregation degree of Mn, Nb, and Ti is reduced, the length and the maximum hardness of the unsealed portion of the center segregated portion are suppressed from increasing, and the steel plate for a line pipe and the steel pipe for a line pipe , And the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the ratio S / Ca of the contents of S and Ca and CAR in the HIC test. FIG.

The present inventors conducted a NACE (National Association of Corrosion and Engineer) test using various steel sheets for a line pipe, and evaluated the occurrence of HIC. The NACE test is a test method to determine whether cracks are formed after 96 hours by saturating a hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7.

After the test, specimens were taken from the cracked steel sheet and the location of the occurrence of HIC was observed in detail. As a result, it was proven that the uncompacted portion of the center segregation portion is a starting point of HIC which is particularly important.

It is necessary that the length of the uncompacted portion of the center seamed portion is suppressed to 0.1 mm or less. This is a rule based on that the minimum value of the length of the uncompacted portion was more than 0.1 mm by observing the fracture surface of the crack of the test piece on which HIC occurred after the above NACE test with a scanning electron microscope (SEM).

In the uncompacted portion, voids generated in the billet during solidification are not compressed by hot rolling and remain on the steel plate. When the size is large, the length can be measured by nondestructive inspection such as ultrasonic wave.

The reason why the uncompacted portion remains in the center segregation portion is the hydrogen contained in the billet before hot rolling. When the steel is melted by the converter and secondary refining and then continuously cast, the steel is solidified, cooled, and shrunk, so that voids are particularly generated in the center of the steel strip. When this gap is negative, hydrogen gas enters the voids when the amount of hydrogen in the billet is large. The amount of hydrogen contained in the steel when it is solvented by secondary refining remained mostly in the voids of the steel strip after continuous casting.

At the time of hot rolling, the structure of the steel strip is austenite, and hydrogen is hardly dissipated to the outside of the steel strip. This is because the amount of hydrogen that can be employed is large because the austenite is a face-centered cubic system. When the steel strip is heated and pressed down, the air gap of the steel strip becomes smaller, but the pressure of the hydrogen gas contained in the gap becomes higher in inverse proportion to the size of the gap. As a result, the gap can not be squeezed even by hot rolling, and the uncompacted portion remains on the steel sheet.

The relationship between the amount of hydrogen in the steel and the length of the uncompacted portion was examined in detail. As a result, it became apparent that the length of the uncompacted portion remained in the center segregation portion of the steel sheet became 0.1 mm or less when the amount of hydrogen was suppressed to 2.5 ppm or less. Therefore, in order to suppress the length of the uncompacted portion of the center segregation portion to 0.1 mm or less, it is necessary to limit the amount of hydrogen in the steel to 2.5 ppm or less. Also, the analysis of hydrogen is the measurement of the steel taken after the second refinement by the combustion method.

Further, after the hot rolling, the steel sheet is cooled, and when the metal structure is transformed from austenite into ferrite, bainite, martensite, pearlite, etc., hydrogen is dissipated to the outside. As a result, the amount of hydrogen remaining in the steel sheet is lowered compared with the amount of hydrogen after secondary refining.

As a result of further studies on the origin of HIC, the inventors of the present invention have found that (a) softened MnS, (b) precipitates of accumulated Nb and Ti, and (c) And it has been found that the occurrence of HIC in the steel sheet for a line pipe and the steel pipe for a line pipe can be remarkably prevented by suppressing these.

In order to suppress the coarsened coarse MnS, it is necessary to make the S content less than 0.002% and to make the S / Ca content ratio S / Ca less than 0.5.

FIG. 1 shows the relationship between CAR (crack area ratio) and S / Ca in the HIC test of 0.04% C - 1.25% Mn steel. As shown in FIG. 1, when the S / Ca ratio becomes 0.5 or more, HIC starts to be generated, and it is found that S / Ca needs to be less than 0.5.

In order to suppress the accumulation of the oxides, it is necessary to make the O content 0.0030% or less and the Al content to 0.030% or less. When the amount of O is large, coarse oxides tend to accumulate easily. When the amount of Al is more than 0.030%, it becomes clear that clusters of Al oxides tend to accumulate.

Further, in order to suppress the coarsened coarse MnS, it is preferable that the maximum Mn segregation degree of the steel sheet and the steel pipe is 2.0 or less. In addition, by suppressing the accumulated carbonitride of Ti and Nb, It is possible to remarkably prevent the occurrence of HIC in a steel pipe for a line pipe and a steel pipe for a line pipe.

In order to suppress the accumulation of carbonitrides of Ti and Nb, it is preferable to satisfy the following conditions. The amount of N is set to 0.0050% or less, the amount of C is set to 0.06% or less, and the degrees of segregation of Nb and Ti are set to 4 or less.

The maximum Mn segregation is a ratio of the maximum amount of Mn in the center segregation portion to the average amount of Mn excluding the center segregation in the amount of Mn in the steel sheet and the steel pipe, Amount) / (average amount of Mn excluding the center segregation part).

Similarly, the Nb content and the Ti content are the ratio of the average Nb amount (Ti amount) of the center segregation part to the average Nb amount (Ti amount) excluding the center segregation part of the steel sheet and the steel pipe.

The maximum Mn segregation can be obtained by measuring the Mn concentration distribution of the steel sheet and the steel pipe by using EPMA (Electron Probe Micro Analyzer) or CMA (Computer Aided Micro Analyzer) capable of image processing of measurement results by EPMA .

At this time, the numerical value of the maximum Mn segregation degree changes according to the probe diameter of the EPMA (or CMA). The present inventors have found that the segregation of Mn can be appropriately evaluated by setting the probe diameter (beam diameter) to 2 탆. Actually, measurement is performed as follows.

The concentration distribution of Mn in the measurement region of 20 mm width (HIC test piece width) x 20 mm thickness (HIC test piece thickness) with a beam diameter of 50 m is measured by EPMA. Next, at a place where the most amount of Mn is concentrated (central segregation part), the Mn concentration in a region of 1 mm (width) x 1 mm (thickness) is measured with a beam diameter of 2 m again. Then, the maximum Mn segregation degree is obtained from this Mn concentration distribution. At that time, 500 points x 500 points of data are accumulated. The ratio of the maximum Mn concentration among these 250000 points to the average Mn concentration excluding the center segregation portion was defined as the maximum Mn segregation degree, and the value was obtained.

Nb segregation and Ti segregation can also be obtained by measuring the Nb concentration distribution and the Ti concentration distribution by EPMA or CMA, respectively. It was found that the segregation can be appropriately evaluated by setting the beam diameter to 2 占 퐉 for the Nb segregation and the Ti segregation.

Actually, with regard to Nb and Ti segregation, the respective concentrations of Nb and Ti in the measurement region of 20 mm width (HIC test piece width) × 20 mm thickness (HIC test piece thickness) with a beam diameter of 50 μm by EPMA The distribution was measured to obtain an average Nb concentration and an average Ti concentration. Thereafter, at a site where the Nb and Ti concentrations were most concentrated (center segregation portion), a beam diameter of 2 占 퐉 was further changed to 1 mm ) &Lt; / RTI &gt; At that time, an average of 500 points measured in the plate width direction is taken to derive the average Nb and Ti concentrations of the center segregation portions. The ratio of the average Nb concentration (Ti concentration) of the central segregation portion to the average Nb concentration (Ti concentration) is defined as Nb and Ti segregation, respectively, and their values are obtained.

In the presence of inclusions such as MnS, the degree of segregation of each element is apparently increased. Therefore, when inclusions are in contact, their values are to be excluded.

The maximum hardness of the central segregation portion of the steel sheet and the steel pipe in which segregation of Mn, Nb and Ti is suppressed is preferably 300 Hv or less. By setting the upper limit of the maximum hardness of the center segregation portion to 300 Hv, it is possible to reliably prevent the occurrence of HIC. Mn and Nb are elements for increasing the quenching property, and Ti contributes to precipitation strengthening. Therefore, by suppressing segregation of these elements, curing of the center segregation portion can be suppressed.

The center segregation portion is a portion where the concentration of Mn measured by EPMA or CMA is the maximum and the maximum hardness of the center segregation portion is corroded with 3% nitric acid + 97% , A Vickers hardness test is carried out under a load of 25 g and measured.

The present invention based on the above-described examination results will be described in detail below.

First, the reason for restricting the base metal component in the steel sheet and the steel pipe of the present invention will be described. In the following, the% of the content of the element means% by mass.

C: C is an element that improves the strength of steel, and an effective lower limit of 0.02% or more is required. On the other hand, if the amount of C exceeds 0.08%, generation of carbide is promoted and the HIC property is impaired, so the upper limit is set to 0.08%. Further, in order to suppress the HIC property, the weldability and the deterioration of the toughness, the C content is preferably 0.06% or less.

Si: Si is a deoxidizing element, and it is necessary to add 0.01% or more. On the other hand, if the amount of Si exceeds 0.5%, the toughness of the weld heat affected zone (HAZ) is lowered, so the upper limit is set to 0.5%.

Mn: Mn is an element for improving the strength and toughness, and addition of 1.0% or more is required. On the other hand, when the amount of Mn exceeds 1.6%, the HAZ toughness is lowered, so the upper limit is set to 1.6%. Further, in order to suppress HIC, it is preferable that the Mn content is less than 1.5%.

Nb: Nb is an element which forms carbides and nitrides and contributes to improvement of strength. In order to obtain the effect, it is necessary to add 0.001% or more. However, if Nb is excessively added, the Nb segregation degree is increased, the Nb carbonitride is accumulated, and the HIC resistance is lowered. Therefore, in the present invention, the upper limit of the amount of Nb is set to 0.10%. Further, in consideration of the HIC property, the amount of Nb is preferably 0.05% or less.

N: N is an element which forms nitrides such as TiN and NbN. In order to make the austenite grain size at the time of heating by using a nitride, it is necessary to set the lower limit value of N amount to 0.0010%. However, if the content of N exceeds 0.0050%, the carbonitrides of Ti and Nb are likely to be accumulated, thereby impairing the HIC resistance. Therefore, the upper limit of the amount of N is set to 0.0050%. Further, when toughness or the like is required, it is preferable that the N content is 0.0035% or less in order to suppress the coarsening of TiN.

P: P is an impurity. When the content exceeds 0.01%, the HIC resistance is deteriorated and the toughness of the HAZ is lowered. Therefore, the content of P is limited to 0.01% or less.

S: S is an element that generates MnS stretched in the rolling direction during hot rolling, thereby lowering the HIC resistance. Therefore, in the present invention, it is necessary to reduce the amount of S, and the content is limited to 0.0020% or less. Further, in order to improve the toughness, it is preferable that the S content is 0.0010% or less. The S content is preferably as small as possible, but it is difficult to make the S content less than 0.0001%, and preferably 0.0001% or more from the viewpoint of production cost.

Ti: Ti is an element generally used for atomization of crystal grains as a deoxidizing agent or a nitride forming element, but in the present invention, it is an element which reduces the HIC property and toughness by the formation of carbonitride. Therefore, the content of Ti is limited to 0.030% or less.

Al: Al is a deoxidizing element. However, in the present invention, when the addition amount exceeds 0.030%, an integrated cluster of the Al oxide is confirmed, so that it is limited to 0.030% or less. When toughness is required, the upper limit of the amount of Al is preferably 0.017% or less. The lower limit of the amount of Al is not particularly limited, but it is preferable to add 0.0005% or more of Al to reduce the amount of oxygen in the molten steel.

O: O is an impurity, and its content is limited to 0.0030% or less in order to suppress the accumulation of the oxide and improve the HIC resistance. In order to suppress the formation of oxides and to improve the base material and the HAZ toughness, the O content is preferably 0.0020% or less.

Ca: Ca is an element that generates sulphide CaS, inhibits the formation of MnS extending in the rolling direction, and significantly contributes to the improvement of the HIC resistance. When the addition amount of Ca is less than 0.0001%, the effect can not be obtained, so the lower limit value is set to 0.0001%. The content is preferably 0.0005% or more. On the other hand, if the addition amount of Ca exceeds 0.0050%, the oxide is accumulated and damages the HIC property, so the upper limit is set to 0.0050%.

In the present invention, the ratio of S / Ca in the content of S and Ca is an important index for fixing S by adding Ca and forming CaS. When the ratio of S / Ca is 0.5 or more, MnS is generated, and MnS stretched at the time of rolling is formed. As a result, the internal HIC property deteriorates. Therefore, the ratio of S / Ca was made less than 0.5.

In the present invention, at least one element of Ni, Cu, Cr, Mo, W, V, Zr, Ta and B may be added as an element improving the strength and toughness.

Ni: Ni is an element effective for improvement of toughness and strength. In order to obtain the effect, it is necessary to add 0.01% or more. However, when the content exceeds 2.0%, HIC property and weldability are lowered. .

Cu: Cu is an element effective for increasing the strength without deteriorating toughness. However, when Cu is less than 0.01%, Cu is not effective. When Cu is more than 1.0%, cracks tend to occur during heating of the steel sheet or during welding. Therefore, the content thereof is preferably 0.01 to 1.0%.

Cr: Cr is added in an amount of 0.01% or more in order to improve the strength of steel by precipitation strengthening, but when added in a large amount, the quenching property is increased, the bainite structure is formed, and the toughness is lowered. Therefore, it is preferable to set the upper limit to 1.0%.

Mo: Mo is an element which improves the quenching property and improves the strength by forming a carbonitride. In order to obtain the effect, it is preferable to add Mo of at least 0.01%. On the other hand, when a large amount of Mo is added in an amount exceeding 1.0%, the cost increases, so that the upper limit is preferably 1.0%. Further, if the steel strength is increased, the HIC property and toughness may be lowered. Therefore, a more preferable upper limit is set to 0.20%.

W: W is an element effective for improving the strength, and it is preferable to add at least 0.01%. On the other hand, when W exceeding 1.0% is added, the toughness may be lowered. Therefore, the upper limit is preferably 1.0%.

V: V is an element which forms carbides and nitrides and contributes to improvement of strength. In order to obtain an effect, it is preferable to add V of at least 0.01%. On the other hand, when V exceeding 0.10% is added, the toughness may be deteriorated. Therefore, the upper limit is preferably 0.10%.

Zr, and Ta: Zr and Ta are elements that contribute to the improvement of strength by forming carbides and nitrides in the same manner as V. In order to obtain the effect, 0.0001% or more is preferably added. On the other hand, if Zr and Ta are added in an amount exceeding 0.050%, the toughness may be lowered. Therefore, the upper limit is preferably 0.050%.

B: B is an element which segregates in the grain boundary of the steel and significantly contributes to the improvement of the hardness. In order to obtain this effect, B is preferably added in an amount of 0.0001% or more. Further, B is an element that generates BN and lowers solvency N and contributes to improvement in toughness of the weld heat affected zone, and therefore, it is more preferable to add B by 0.0005% or more. On the other hand, if B is excessively added, the segregation in the grain boundaries may become excessive, resulting in a decrease in toughness. Therefore, the upper limit is preferably 0.0020%.

In addition, one or more of REM, Mg, Zr, Ta, Y, Hf, and Re may be contained in order to control inclusions such as oxides and sulfides.

REM: REM is an element to be added as a deoxidizing agent and a desulfurizing agent, and the addition of 0.0001% or more is preferable. On the other hand, if it is added in an amount exceeding 0.010%, coarse oxides may be generated to lower the HIC property, the toughness of the base material and the HAZ, and the preferable amount is less than 0.010%.

Mg: Mg is an element to be added as a deoxidizing agent and a desulfurizing agent, and particularly contributes to the improvement of HAZ toughness by generating a fine oxide. In order to obtain this effect, it is preferable to add 0.0001% or more of Mg. On the other hand, when Mg is added in an amount of more than 0.010%, the oxide tends to flocculate and coarsen, resulting in deterioration of the HIC property and deterioration of the toughness of the base material and HAZ. Therefore, the addition amount of Mg is preferably 0.010% or less.

Y, Hf, and Re: Y, Hf, and Re, as well as Ca, are elements that generate sulfides and inhibit the formation of MnS elongated in the rolling direction and contribute to improvement in HIC resistance. In order to obtain such an effect, it is preferable to add 0.0001% or more of Y, Hf and Re. On the other hand, if the amount of Y, Hf, or Re exceeds 0.0050%, the amount of the oxide increases and coagulation or coarsening deteriorates the HIC property, so that the addition amount is preferably 0.0050% or less.

Next, a method of setting the length of the uncompacted portion of the central segregation portion of the steel sheet to 0.01 mm or less will be described.

As described above, the cause of the uncombined portion remaining in the center segregation portion is the hydrogen contained in the billet before hot rolling. Therefore, by reducing the amount of hydrogen in secondary refining, the pressure of the hydrogen gas contained in the voids of the billet can be lowered. When the amount of H in the secondary refining is 0.00025% or less, the uncompacted portion hardly disappears after hot rolling, and even if it exists, it can be made 0.1 mm or less in length.

In order to reduce the amount of hydrogen in the secondary refining, it is preferable to lower the hydrogen partial pressure in the atmosphere at the time of secondary refining. For example, by blowing an inert gas or nitrogen into the atmosphere, the hydrogen partial pressure can be lowered.

Further, in the present invention, it is preferable that the maximum Mn segregation degree, Nb segregation degree and Ti segregation degree in the base material of the steel sheet and the steel pipe are 2.0 or less, 4.0 or less and 4.0 or less, respectively.

When the maximum Mn segregation degree is 2.0 or less, the formation of coarse MnS is suppressed, and the occurrence of HIC originating from MnS stretched in the rolling direction can be prevented. When the Nb grain size is 4.0 or less, the production of integrated Nb (C, N) is suppressed. When the Ti segregation degree is 4.0 or less, the formation of integrated TiN is suppressed and deterioration of HIC property can be prevented.

The maximum Mn segregation is a ratio of the maximum amount of Mn in the center segregation portion to the average amount of Mn except for the center segregation portion of the steel sheet and the steel pipe. The maximum Mn segregation degree is obtained by EPMA or CMA having a beam diameter of 2 탆, Can be obtained by measuring the concentration distribution. Nb grain size distribution and Ti concentration distribution were measured by EPMA or CMA having a beam diameter of 2 占 퐉 and the average Nb The ratio of the average amount of Nb in the center segregation part to the amount of Ti in the center segregation part (Ti segregation degree) with respect to the average amount of Ti except for the center segregation part of the steel plate and the steel pipe .

A method for suppressing the maximum amount of Mn segregation, Nb segregation and Ti segregation will be described below.

In order to suppress the segregation of Mn, Nb and Ti, the soft-hardening at the final solidification in continuous casting is optimum. Is carried out in order to eliminate the coexistence of the solidified portion and the non-solidified portion due to the uneven cooling of the casting during the final solidification, whereby the final solidification can be uniformly performed in the width direction.

In the continuous casting, generally, the steel strip is water-cooled, but the end portion in the width direction is fast in cooling and the cooling in the central portion in the width direction is strengthened. As a result, solidification is delayed at 1/4 of the width direction even though solidification occurs at the end portion and the center portion in the width direction of the billet, and the non-solidified portion remains in the billet. As a result, in the width direction of the steel strip, the solidified portion and the non-solidified portion are not uniform, for example, the shape of the interface between the solidified portion and the non-solidified portion becomes W-shaped in the width direction. If such non-uniform solidification occurs in the width direction, segregation is promoted and the HIC resistance is deteriorated.

On the other hand, in the continuous casting, if the softening at the final solidification is performed, the unfused portion can be extruded and uniformly solidified in the width direction. In addition, when light-hardening is performed after non-uniform solidification in the width direction, the un-solidified portion can not be effectively extruded due to the large deformation resistance of the solidified portion.

Therefore, in order to prevent such a W-type solidification from occurring, it is preferable to reduce the hardness while controlling the reduction amount in accordance with the distribution in the width direction of the center solidification rate at the final solidification position of the casting pieces. By doing so, center segregation is suppressed even in the width direction, and the maximum amount of Mn segregation, Nb segregation, and Ti segregation degree can be further reduced.

The steel containing the above-mentioned components is dissolved in the steelmaking process, is subsequently made into a steel billet by continuous casting, and the billet is reheated and subjected to heavy plate rolling to obtain a steel billet.

In this manufacturing process, when the reheating temperature of the billet is set to 950 DEG C or higher, the pressure ratio in the recrystallization temperature region is set to 2 or more, and the pressure ratio in the non-recrystallized region is set to 3 or more, The particle size can be made 20 mu m or less. In addition, although water cooling is performed after completion of rolling, it is preferable to start the water cooling from a temperature of 750 ° C or higher and stop the water cooling in a temperature range of 400 to 500 ° C.

The recrystallization temperature range is a temperature range in which recrystallization occurs after rolling, and is higher than about 900 占 폚 in the steel component of the present invention. On the other hand, the non-recrystallized temperature region is a temperature range in which recrystallization and ferrite transformation do not occur after rolling, and is about 750 to 900 占 폚 in the steel component used in the present invention. The rolling in the recrystallization temperature region is referred to as recrystallization rolling or rough rolling, and rolling in the non-recrystallization temperature region is referred to as non-recrystallization rolling or finish rolling.

After the non-recrystallized rolling, the maximum hardness of the center segregation can be made 300 Hv or less as described below by starting water-cooling from a temperature of 750 캜 or higher and setting the water-cooling stop temperature to 400 캜 or higher. First, when the water-cooling start temperature is lower than 750 占 폚, a large amount of ferrite is generated before the start of cooling, and C (carbon) is released to the austenite from the ferrite. Thereafter, when cooled, the austenite phase in which C is concentrated transforms into a hard martensite containing a large amount of C.

Therefore, when the water-cooling start temperature is set to 750 占 폚 or higher and the generation of hard martensite is suppressed, the hardness can be suppressed to 300 Hv or lower. When the water-cooling stop temperature is 400 占 폚 or higher, the hard martensite after the transformation is partially decomposed to suppress the hardness to 300 Hv or lower. The water-cooling stopping temperature is preferably 500 ° C or lower because the strength is lowered when the water-cooling stopping temperature is too high.

<Examples>

Next, the present invention will be described in more detail by way of examples.

A steel having the chemical composition shown in Table 1 was melted and made into a steel strip having a thickness of 240 mm by continuous casting. Table 1 also shows analytical values of the amount of hydrogen in molten steel. In the continuous casting, a slight reduction in the final solidification was carried out. The obtained billets were heated to 1000 to 1250 占 폚, hot-rolled in a recrystallization temperature range exceeding 900 占 폚, and then hot-rolled in the non-recrystallization temperature region of 750 to 900 占 폚. After hot rolling, water cooling was started at 750 DEG C or higher, and water cooling was stopped at a temperature of 400 to 500 DEG C to produce a steel sheet having various thicknesses as shown in Table 2.

Further, the steel sheet was formed into a tubular shape by a C press, a U press, and an O press, the end faces were welded together, welded from the inside and the outside, and then expanded to form a steel pipe. In addition, the present welding employs submerged-arc welding.

Tensile test specimens, HIC test specimens, and macro specimens were taken from the obtained steel sheet and steel pipe, and were provided for each test. The HIC test was conducted in accordance with NACETM0284. In addition, the degree of segregation of Mn, Nb and Ti was measured by EPMA using a macro test piece. In the measurement of the degree of segregation by EPMA, the concentration distribution of Mn, Nb and Ti was measured by a 50 占 퐉 beam system with a total thickness x 20 mm width measurement area, and then each element in the thickness direction of the test piece was measured The concentration of each element was measured in a concentrated area (center segregation part) in a region of 1 mm x 1 mm with a beam meter of 2 mu m.

The Vickers hardness of the center segregation was measured in accordance with JIS Z 2244. The Vickers hardness was measured at a region where the Mn concentration was the highest in the distribution of the concentration of Mn in the thickness direction measured by EPMA at a load of 25 g.

Table 2 shows the sheet thickness, the maximum Mn segregation degree, the Nb segregation degree, the Ti segregation degree, the length of the uncompacted portion, the maximum hardness of the center segregation portion, the tensile strength and the tensile strength of the steel sheet obtained respectively by the steels (1 to 34) (CAR) of the crack obtained by the HIC test.

Table 3 shows the thickness of the steel pipe obtained from the steel (1 to 34) in Table 1, the heat input amount of the main welding, and the area ratio of cracks obtained by the HIC test. In addition, the maximum hardness of the steel sheet is equal to that of the steel sheet, and the tensile strength of the steel sheet is 1 to 5% larger than that of the steel sheet.

The steels 1 to 23 are examples of the present invention and the steel sheet obtained from these steels had a maximum Mn segregation degree of 1.6 or less, a Nb segregation degree of 4.0 or less, a Ti segregation degree of 4.0 or less, 300 Hv or less, and cracks due to the HIC test do not occur. The same applies to steel pipes made of these steel plates.

On the other hand, the steels 24 to 34 show comparative examples outside the scope of the present invention. That is, since any one element of the basic components is out of the scope of the present invention, the CAR exceeds 3% in the HIC test.

Claims (10)

In terms of% by mass,
C: 0.02 to 0.08%
Si: 0.01 to 0.5%
Mn: 1.0 to 1.6%
Nb: 0.001 to 0.10%
Ca: 0.0001 to 0.0050%,
N: 0.0010 to 0.0050%,
O: 0.0001 to 0.0030%
&Lt; / RTI &gt;
P: 0.01% or less,
S: 0.0020% or less,
Al: 0.0005 to 0.030%
Ti: not more than 0.030%
H: 2.5 ppm or less
, And the content of S and Ca is limited,
S / Ca &lt; 0.5, the balance being Fe and inevitable impurity elements,
Further, the length of the uncompacted portion of the center segregation portion is limited to 0.1 mm or less,
Maximum Mn segregation degree: 2.0 or less,
Nb grain size: 4.0 or less,
Ti segregation degree: not more than 4.0,
Wherein the maximum hardness of the center segregation portion is 300 Hv or less. The method according to claim 1,
In terms of% by mass,
Ni: 0.01 to 2.0%
Cu: 0.01 to 1.0%
Cr: 0.01 to 1.0%
Mo: 0.01 to 1.0%
W: 0.01 to 1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%
Ta: 0.0001 to 0.050%
B: 0.0001 to 0.0020%
, And the balance being composed of iron and inevitable impurities. The steel sheet for a high-strength line pipe excellent in crack resistance in hydrogenation. 3. The method according to claim 1 or 2,
In terms of% by mass,
REM: 0.0001 to 0.01%
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%
Hf: 0.0001 to 0.005%,
Re: 0.0001-0.005%
Wherein the steel sheet further contains one or more of the following: delete delete The base material, in mass%
C: 0.02 to 0.08%
Si: 0.01 to 0.5%
Mn: 1.0 to 1.6%
Nb: 0.001 to 0.10%
Ca: 0.0001 to 0.0050%,
N: 0.0010 to 0.0050%,
O: 0.0001 to 0.0030%
&Lt; / RTI &gt;
P: 0.01% or less,
S: 0.0020% or less,
Al: 0.0005 to 0.030%
Ti: not more than 0.030%
H: 2.5 ppm or less
, And the content of S and Ca is limited,
S / Ca &lt; 0.5
, The balance being Fe and inevitable impurity elements,
Further, the length of the uncompacted portion of the central segregation portion of the base material is limited to 0.1 mm or less,
Of base metal
Maximum Mn segregation degree: 2.0 or less,
Nb grain size: 4.0 or less,
Ti segregation degree: 4.0 or less,
However,
A steel pipe for a high strength line pipe excellent in hydrogen hydrogen organic cracking property, characterized in that the maximum hardness of the center segregation part of the base material is 300 Hv or less. The method according to claim 6,
The base material, in mass%
Ni: 0.01 to 2.0%
Cu: 0.01 to 1.0%
Cr: 0.01 to 1.0%
Mo: 0.01 to 1.0%
W: 0.01 to 1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%
Ta: 0.0001 to 0.050%
B: 0.0001 to 0.0020%
, And the balance being iron and inevitable impurities. The steel pipe for a high strength line pipe is excellent in hydrogen hydrogen organic cracking property. 8. The method according to claim 6 or 7,
The base material, in mass%
REM: 0.0001 to 0.01%
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%
Hf: 0.0001 to 0.005%,
Re: 0.0001-0.005%
Wherein the steel pipe further contains one or more of the following. delete delete

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