KR20130105941A - High-strength steel sheet and high-strength steel pipe having excellent hydrogen-induced cracking resistance for use in line pipe - Google Patents

Abstract

Steel pipe for line pipes and line pipe steel pipes with excellent HIC resistance, which is most suitable for steel pipes used for transporting line pipes such as petroleum and natural gas, and includes C: 0.02 to 0.08%, Si: 0.01 to 0.5%, and Mn: 1.0. -1.6%, Nb: 0.001-0.10%, Ca: 0.0001-0.0050%, N: 0.0010-0.0050%, O: 0.0001-0.0030%, P: 0.01% or less, S: 0.0020% or less, Al: 0.0005 It consists of steel which restrict | limits to -0.030%, Ti: 0.030% or less, has a steel composition which satisfy | fills S / Ca <0.5, and limits the length of the uncompressed part of a center segregation part to 0.1 mm or less.

Description

High strength steel pipe for high strength line pipe and steel pipe for high strength line pipe with excellent organic crack resistance

TECHNICAL FIELD The present invention relates to a steel sheet for line pipes and a steel pipe for line pipes having excellent hydrogen organic crack resistance (called HIC resistance), which is most suitable for applications such as line pipes for transportation such as petroleum and natural gas.

In transport line pipes such as petroleum and natural gas, which contain a large amount of hydrogen sulfide (H ₂ S) containing water, generation of hydrogen organic cracks (called HIC) is concerned. The reason for this is that in an H ₂ S environment containing water (called a sour environment), hydrogen easily enters the surface from the steel.

In particular, HIC accumulates around defects in steel, such as stretched MnS present in the central segregation of steel, carbon nitride of Ti or Nb, or oxide inclusions in an oxide integrated zone. It is due to one hydrogen.

That is, in a sour environment, when hydrogen penetrated into steel accumulates around a defect and becomes a gas, a crack occurs when the pressure exceeds the fracture toughness value K _IC of the steel. If the central segregation portion of the steel, the periphery of the inclusions, and the like are hardened, the cracks tend to propagate.

Therefore, in the line pipe used in the sour environment, conventionally, suppression of the production | stretching of stretched MnS, suppression of accumulation of carbonitrides and oxides, such as Ti and Nb, or formation of a hardened phase by center segregation is carried out. Measures such as suppression of the

For example, Mn is an element which is easy to segregate in the center of a steel plate, and the method of suppressing segregation of Mn is proposed (for example, patent documents 1-3). In patent document 1, the steel plate which suppressed ratio of Mn content of a segregation part with respect to average Mn content in steel is proposed. In addition, Patent Documents 2 and 3 propose high-strength line pipes that limit the P concentration of the segregation portion and further utilize Ca in addition to the size of the Mn segregation spot.

In addition to the segregation of Mn, a hot rolled steel sheet excellent in HIC resistance has also been proposed, focusing on segregation of Nb (for example, Patent Document 4). Moreover, the method of suppressing inclusions, such as carbide and nitride of Ti and Nb, is proposed (for example, patent document 5, 6).

(Patent Document 1) Japanese Unexamined Patent Application Publication No. 6-220577

(Patent Document 2) Japanese Unexamined Patent Application Publication No. 6-256894

(Patent Document 3) Japanese Unexamined Patent Application Publication No. Hei 6-271974

(Patent Document 4) Japanese Unexamined Patent Application Publication No. 2002-363689

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

(Patent Document 6) Japanese Unexamined Patent Application Publication No. 2008-7841

Although development of suppression of segregation of Mn and control of MnS morphology using Ca has been frequently performed, the maximum Mn content of segregation part / average Mn content in steel and the size of Mn segregation spots are controlled. Just by doing so, we can't say that we can completely prevent HIC, and we know that we need to control them more closely.

Moreover, when the segregation of Mn was eliminated, segregation of Nb became a problem next. Also regarding this segregation of Nb, it was found that the control of (maximum Nb content of segregation part) / (average Nb content in steel) was insufficient and it was necessary to control more precisely. In addition, even if the length of the Nb-Ti-C-N-based inclusions and the surface density and length of the (Ti, Nb) (C, N) -based inclusions were controlled, the generation of HIC could not be prevented.

This invention is made | formed in view of such a situation, and makes it a subject to provide the steel plate for line pipes and the line pipe steel pipe which are excellent in HIC resistance, which is optimal for the steel pipe used for the line pipe for transportation, such as petroleum and natural gas. will be.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly research about the conditions which are necessary in order to obtain the steel plate for high strength line pipes, and the steel pipes for high strength line pipes which are excellent in the hydrogen-inorganic crack resistance of 500 Mpa or more of tensile strength, and the new ultrahigh strength line pipe steel plates and high strength lines Invented a steel pipe for pipes. The gist of the present invention is as follows.

(1) in mass%

C: 0.02-0.08%,

Si: 0.01% to 0.5%

Mn: 1.0 to 1.6%,

Nb: 0.001-0.10%,

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,

Limited to S, Ca content,

S / Ca <0.5

Satisfies the above, and the remainder is made of Fe and an unavoidable impurity element,

In addition, the length of the uncompressed part of the center segregation part is 0.1 mm or less.

Steel sheet for high-strength line pipe excellent in hydrogen-resistant organic cracking resistance, characterized in that limited to.

(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%

The steel sheet for high strength line pipe which is excellent in the hydrogen-induced organic cracking property as described in said (1) characterized by containing 1 type, or 2 or more types, and remainder consists of iron and an unavoidable impurity.

(3) at 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% to 0.005%

The steel sheet for high strength line pipe excellent in the hydrogen-induced organic cracking property as described in said (1) or (2) characterized by further containing 1 type (s) or 2 or more types.

(4) also,

Max Mn Segregation: 2.0 or less,

Nb segregation: 4.0 or less,

Ti segregation: 4.0 or less,

The steel sheet for high strength line pipe excellent in the hydrogen-induced organic cracking property as described in said (1) or (2) characterized by the above-mentioned.

(5) The steel plate for high strength line pipe excellent in the hydrogen-induced organic cracking property as described in said (1) or (2) characterized by the highest hardness of a center segregation part of 300 Hv or less.

(6) The base material is in mass%,

C: 0.02-0.08%,

Si: 0.01% to 0.5%

Mn: 1.0 to 1.6%,

Nb: 0.001-0.10%,

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

Limited to S, Ca content,

S / Ca <0.5

Satisfies the above, and the remainder is made of Fe and an unavoidable impurity element,

In addition, the steel pipe for high-strength line pipe excellent in hydrogen-induced organic crack resistance, characterized in that the length of the uncompressed portion of the center segregation portion of the base material is limited to 0.1 mm or less.

(7) The base material is 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%

The steel pipe for high-strength line pipes excellent in the hydrogen-induced organic crack resistance as described in said (6) characterized by containing 1 type, or 2 or more types, and remainder consists of iron and an unavoidable impurity.

(8) The base material is 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% to 0.005%

The steel pipe for high-strength line pipe excellent in the hydrogen-induced organic cracking property as described in said (6) or (7) characterized by further containing 1 type, or 2 or more types.

(9) In addition, the base metal

Max Mn Segregation: 2.0 or less,

Nb segregation: 4.0 or less,

Ti segregation: 4.0 or less,

The steel pipe for high strength line pipe excellent in the hydrogen-induced organic cracking property as described in said (6) or (7) characterized by the above-mentioned.

(10) The steel pipe for high strength line pipe excellent in the hydrogen-induced organic cracking property as described in said (6) or (7) characterized by the maximum hardness of the center segregation part of a base material being 300 Hv or less.

According to the present invention, the segregation degree of Mn, Nb, Ti is reduced, the rise of the length and the maximum hardness of the uncompressed part of the center segregation part is suppressed, and the steel sheet for line pipes and the steel pipes for line pipes excellent in hydrogen organic cracking resistance Industrial contributions, such as the possibility of manufacturing of the metal, are very remarkable.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship of ratio S / Ca of content of S and Ca, and CAR in a HIC test.

The present inventors performed NACE (National Association of Corrosion and Engineer) test using the various steel sheets for line pipes, and evaluated the presence or absence of HIC generation. The NACE test is a test method in which a hydrogen sulfide gas is saturated in a 5% NaCl solution + 0.5% acetic acid and a pH 2.7 solution to investigate whether cracks are formed after 96 hours.

After the test, the test piece was extract | collected from the steel plate in which the crack generate | occur | produced, and the generation place of HIC was observed in detail. As a result, it has been found that the uncompressed portion of the central segregation portion is the starting point of a particularly important HIC.

The non-compression part of this center segregation part needs to suppress the length to 0.1 mm or less. This is a rule based on the fact that the wavefront of the crack of the test piece in which the HIC was generated after the NACE test was observed with a scanning electron microscope (SEM), and the minimum value of the length of the uncompressed part was more than 0.1 mm.

The uncompressed part means that the voids generated in the steel piece at the time of solidification remain in the steel sheet without being pressed by hot rolling, and when the size is large, the length can be measured by nondestructive inspection such as ultrasonic waves.

The cause of the uncompressed part remaining in the central segregation part is hydrogen contained in the steel piece before hot rolling. Since the steel solidifies, cools and shrinks when continuously casting after the steel is melted by converter and secondary refining, in particular, voids are generated in the center portion of the steel piece. When this space | gap is negative pressure, when the amount of hydrogen of a steel piece is large, hydrogen gas will enter in a space | gap. When the solvent is melted by secondary refining, the amount of hydrogen contained in the steel remains mostly in the voids of the steel pieces after continuous casting as it is.

At the time of heating of hot rolling, the structure of a steel piece is austenite, and hydrogen is hard to be dissipated to the outside of a steel piece. The reason is that austenite is a face-centered cubic crystal, and therefore there is a large amount of hydrogen that can be dissolved. When the steel pieces are heated and pressed, the voids of the steel pieces decrease, but the pressure of the hydrogen gas contained in the voids increases in inverse proportion to the size of the voids. Therefore, a space | gap cannot be crimped also by hot rolling, and an uncompressed part remains in a steel plate.

As a result of investigating the relationship between the amount of hydrogen in the steel and the length of the uncompressed part in detail, it was evident that when the amount of hydrogen was suppressed to 2.5 ppm or less, the length of the uncompressed part remaining in the central segregation portion of the steel sheet became 0.1 mm or less. Therefore, in order to suppress the length of the uncompressed part of a center segregation part to 0.1 mm or less, it is necessary to restrict the amount of hydrogen in steel to 2.5 ppm or less. In addition, analysis of hydrogen is the measurement of the steel collected after secondary refining by the combustion method.

In addition, after hot rolling, the steel sheet is cooled, and when the metal structure is transformed from austenite to ferrite, bainite, martensite, pearlite, and the like, hydrogen is dissipated to the outside. Therefore, the amount of hydrogen remaining in the steel sheet is lower than the amount of hydrogen after secondary refining.

The present inventors have further examined the origin of HIC, and in addition, (a) stretched MnS, (b) precipitated Nb, Ti precipitates, and (c) oxides integrated are also the starting points of HIC. When it suppresses these, it discovered that the HIC of the steel plate for line pipes and the steel pipe for line pipes can be prevented remarkably.

In order to suppress the stretched coarse MnS, it is necessary to make S amount less than 0.002%, and to make ratio S / Ca of content of S and 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 ratio of S / Ca becomes 0.5 or more, HIC starts to be generated, and it can be seen that S / Ca needs to be less than 0.5.

In addition, in order to suppress accumulation of an oxide, it is necessary to make O amount 0.0030% or less and Al amount 0.030% or less. When the amount of O is large, it becomes clear that coarse oxides are easy to accumulate, and when Al is added more than 0.030%, clusters of oxides of Al become easy to accumulate.

In addition, in order to suppress the stretched coarse MnS, it is preferable that the maximum Mn segregation degree of a steel plate and a steel pipe shall be 2.0 or less, and in addition to these, the suppression of the carbonitride of Ti and Nb which were integrated is assuredly. It was found that the HIC of the steel sheet for line pipe and the steel pipe for line pipe can be prevented remarkably.

In order to suppress accumulation of carbon nitride of Ti and Nb, it is preferable to satisfy the following conditions. N amount is made 0.0050% or less, C amount is made 0.06% or less, and segregation degree of Nb and Ti is made 4 or less, respectively.

Here, the maximum Mn segregation degree is the ratio of the maximum Mn amount of the center segregation portion to the average Mn amount excluding the central segregation portion in the Mn amounts of the steel sheet and the steel pipe, that is, the maximum Mn of the center segregation portion. Amount) / (average Mn amount excluding center segregation).

Similarly, Nb segregation degree and Ti segregation degree are ratio of the average amount of Nb (Ti amount) of the center segregation part with respect to the average Nb amount (Ti amount) except the center segregation part of a steel plate and steel pipe.

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

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

The concentration distribution of Mn in the measurement area | region of 20 mm width (HIC test piece width) x 20 mm thickness (HIC test piece thickness) is measured by EPMA with a beam diameter of 50 micrometers. Next, in the place (central segregation part) where Mn amount was concentrated most, Mn density | concentration of the area | region of 1 mm (width) x 1 mm (thickness) is measured again with a beam diameter of 2 micrometers. And the maximum Mn segregation degree is calculated | required from this Mn concentration distribution. At that time, 500 points | pieces of 500 points | pieces data are accumulated. The ratio to the average Mn concentration except for the central segregation portion of the maximum Mn concentration in the 250,000 points was defined as the maximum Mn segregation value to obtain the value.

In addition, also about Nb segregation degree and Ti segregation degree, it can obtain | require by EPMA or CMA by measuring Nb concentration distribution and Ti concentration distribution, respectively. Similarly with regard to the Nb segregation degree and the maximum Ti segregation degree, it was found that segregation can be appropriately evaluated by setting the beam diameter to 2 µm.

In fact, also regarding the degree of Nb and Ti segregation, the concentrations of Nb and Ti 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 μm by EPMA. After measuring distribution and finding average Nb concentration and average Ti concentration, in the place (central segregation part) where Nb and Ti amount were concentrated most, 1mm (width) x 1mm (thickness) with a beam diameter of 2 micrometers again The concentration of Nb and Ti in the region of) is measured. At that time, the average of 500 points measured in the plate width direction is taken to derive the concentrations of Nb and Ti of the mean of the central segregation portion. The ratio of the average Nb concentration (Ti concentration) to the average Nb concentration (Ti concentration) of the central segregation portion is defined as the Nb segregation degree and the Ti segregation degree, respectively, to obtain the value.

If inclusions such as MnS are present, the segregation degree of each element increases in appearance, and therefore, the inclusions are evaluated without the value.

Moreover, it is preferable that the highest hardness of the center segregation part of the steel plate and steel pipe which the segregation of Mn, Nb, Ti was suppressed shall be 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 generation of HIC. Mn and Nb are elements that increase the hardenability, and Ti contributes to precipitation strengthening, and therefore, hardening of the central segregation portion can be suppressed by suppressing segregation of these elements.

In addition, the center segregation part is a site | part in which the density | concentration of Mn measured by EPMA and CMA becomes the maximum, and the highest hardness of a center segregation part is corroded with 3% nitric acid + 97% nital solution, and to JIS Z 2244. Based on this, the Vickers hardness test may be performed under a load of 25 g, and the measurement may be performed.

EMBODIMENT OF THE INVENTION This invention made based on the above examination result is demonstrated in detail below.

First, the reason for limitation of the base material component in the steel plate and steel pipe of this invention is demonstrated. Below,% of content of an element shall mean the mass%.

C: C is an element which improves the strength of steel, and addition of 0.02% or more is necessary as the effective lower limit. On the other hand, when the amount of C exceeds 0.08%, the formation of carbides is accelerated and the HIC resistance is impaired, so the upper limit is made 0.08%. Moreover, in order to suppress the fall of HIC property, weldability, and toughness, it is preferable to make C amount into 0.06% or less.

Si: Si is a deoxidation element and 0.01% or more of addition is required. On the other hand, when the amount of Si exceeds 0.5%, the toughness of the weld heat affected zone HAZ is lowered, so the upper limit is made 0.5%.

Mn: Mn is an element which improves strength and toughness, and addition of 1.0% or more is required. On the other hand, when Mn amount exceeds 1.6%, since HAZ toughness will fall, an upper limit shall be 1.6%. In addition, in order to suppress HIC, it is preferable to make Mn amount less than 1.5%.

Nb: Nb is an element which forms carbides and nitrides and contributes to the improvement of strength. In order to acquire the effect, it is necessary to add 0.001% or more. However, excessive addition of Nb increases the Nb segregation degree, leads to the accumulation of carbonitrides of Nb, and lowers HIC resistance. Therefore, in this invention, the upper limit of Nb amount is made into 0.10%. In addition, when considering HIC property, it is preferable to make Nb amount into 0.05% or less.

N: N is an element which forms nitrides, such as TiN and NbN, and in order to make austenite particle diameter at the time of heating using a nitride fine, it is necessary to make the lower limit of N amount into 0.0010%. However, when the content of N exceeds 0.0050%, carbonitrides of Ti and Nb tend to accumulate and impair HIC resistance. Therefore, the upper limit of N amount is made into 0.0050%. In addition, when toughness etc. are requested | required, in order to suppress the coarsening of TiN, it is preferable to make N amount into 0.0035% or less.

P: P is an impurity, and when content exceeds 0.01%, HIC resistance will be impaired and the toughness of HAZ will fall. Therefore, content of P is restrict | limited to 0.01% or less.

S: S is an element which produces | generates MnS extending | stretching to a rolling direction at the time of hot rolling, and reduces HIC resistance. Therefore, in this invention, it is necessary to reduce S amount, and content is restrict | limited to 0.0020% or less. In addition, in order to improve toughness, it is preferable to make S amount into 0.0010% or less. Although the amount of S is so preferable that it is small, it is difficult to make it less than 0.0001%, and it is preferable to set it as 0.0001% or more from a viewpoint of a manufacturing cost.

Ti: Ti is an element normally used for atomization of crystal grains as a deoxidizer or a nitride forming element, but in the present invention, it is an element that reduces HIC resistance and toughness by formation of carbonitride. Therefore, content of Ti is restrict | limited to 0.030% or less.

Al: Al is a deoxidation element. However, in the present invention, when the addition amount exceeds 0.030%, an integrated cluster of Al oxide is confirmed, and therefore, it is limited to 0.030% or less. When toughness is required, it is preferable to make the upper limit of Al amount into 0.017% or less. Although the lower limit of Al amount is not specifically limited, In order to reduce the amount of oxygen in molten steel, it is preferable to add Al 0.0005% or more.

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

Ca: Ca is an element which produces sulfide CaS, suppresses the production of MnS which extends in the rolling direction, and contributes remarkably to the improvement of HIC resistance. If the addition amount of Ca is less than 0.0001%, no effect is obtained, so the lower limit is made 0.0001%. The content of 0.0005% or more is preferable. On the other hand, when the addition amount of Ca exceeds 0.0050%, oxides accumulate and impair HIC resistance, so the upper limit is made 0.0050%.

In this invention, in order to fix S by adding Ca and forming CaS, the ratio of S / Ca in content of S and Ca is an important index. If the ratio of S / Ca is 0.5 or more, MnS is produced, and MnS stretched at the time of rolling is formed. As a result, the HIC resistance deteriorates. Therefore, the value of the ratio of S / Ca was made into less than 0.5.

In addition, in this invention, 1 type, or 2 or more types of element can be added among Ni, Cu, Cr, Mo, W, V, Zr, Ta, and B as an element which improves strength and toughness.

Ni: Ni is an element effective for improving the toughness and strength, and in order to obtain the effect, Ni: Ni is required to be added at 0.01% or more. However, since the HIC property and weldability are lowered at an addition exceeding 2.0%, the upper limit thereof is 2.0%. It is preferable.

Cu: Cu is an element effective for increasing the strength without lowering the toughness. However, Cu: Cu is ineffective at less than 0.01%. If Cu: Cu is more than 1.0%, it is likely to cause cracks during steel sheet heating or welding. Therefore, it is preferable to make the content into 0.01 to 1.0%.

Cr: Cr is effective to add 0.01% or more in order to improve the strength of the steel due to precipitation strengthening. However, when Cr: Cr is added in a large amount, the hardenability is increased, bainite structure is generated, and the toughness is reduced. Therefore, it is preferable to make the upper limit into 1.0%.

Mo: Mo is an element which improves hardenability and forms carbonitrides to improve strength, and in order to obtain the effect, 0.01% or more of addition is preferable. On the other hand, when Mo is added in a large amount exceeding 1.0%, since a cost will rise, it is preferable to make an upper limit into 1.0%. In addition, when the strength of the steel is increased, the HIC property and the toughness may decrease, so the upper limit is more preferably 0.20%.

W: W is an element effective for improving the strength, and an addition of 0.01% or more is preferable. On the other hand, when W exceeding 1.0% is added, the toughness may be lowered, so the upper limit is preferably 1.0%.

V: V is an element which forms carbides and nitrides and contributes to the improvement of strength, and in order to obtain the effect, 0.01% or more of addition is preferable. On the other hand, when V exceeding 0.10% is added, since toughness may be reduced, it is preferable to make an upper limit into 0.10%.

Zr, Ta: Zr and Ta are elements which form carbides and nitrides and contribute to the improvement of strength, similar to V, and in order to obtain the effect, it is preferable to add 0.0001% or more. On the other hand, when Zr and Ta are added in excess of 0.050%, since the toughness may be reduced, it is preferable to make the upper limit into 0.050%.

B: B is an element which segregates at grain boundaries of steel and contributes remarkably to improvement of hardenability. In order to acquire this effect, addition of 0.0001% or more of B is preferable. In addition, since B is an element which produces BN, lowers the solid solution N, and contributes to the improvement of the toughness of the weld heat affected zone, the addition of 0.0005% or more is more preferable. On the other hand, when B is excessively added, segregation to the grain boundaries becomes excessive, which may cause a decrease in toughness. Therefore, the upper limit is preferably made 0.0020%.

Moreover, in order to control inclusions, such as an oxide and a sulfide, you may contain 1 type, or 2 or more types of REM, Mg, Zr, Ta, Y, Hf, and Re.

REM: REM is an element added as a deoxidizer and a desulfurization agent, and 0.0001% or more of addition is preferable. On the other hand, when it adds exceeding 0.010%, a coarse oxide may generate | occur | produce, and HIC property and the toughness of a base material and HAZ may fall, and a preferable addition amount is 0.010% or less.

Mg: Mg is an element added as a deoxidizer and a desulfurization agent, and in particular, generates a fine oxide and contributes to the improvement of HAZ toughness. In order to acquire this effect, it is preferable to add Mg 0.0001% or more. On the other hand, when Mg is added more than 0.010%, oxides tend to aggregate and coarsen, resulting in deterioration of HIC properties and deterioration of toughness of the base material and HAZ. Therefore, it is preferable to make the addition amount of Mg into 0.010% or less.

Y, Hf, Re: Y, Hf, and Re are elements which, like Ca, form sulfides, suppress the production of MnS elongated in the rolling direction, and contribute to the improvement of HIC resistance. In order to acquire such an effect, it is preferable to add Y, Hf, and Re 0.0001% or more. On the other hand, when the amount of Y, Hf, and Re exceeds 0.0050%, the oxide increases, and when the aggregation and coarsening increase the HIC resistance, it is preferable to make the addition amount 0.0050% or less.

Next, the method to make the length of the uncompressed part of the center segregation part of a steel plate into 0.01 mm or less is demonstrated.

As described above, the cause of the uncompressed portion remaining in the central segregation portion is hydrogen contained in the steel sheet before hot rolling. Therefore, by reducing the amount of hydrogen in secondary refining, the pressure of the hydrogen gas contained in the space | gap of a steel piece can be reduced. When the amount of H in the secondary refining is 0.00025% or less, the uncompressed part is almost eliminated after hot rolling, and even if present, it can be 0.1 mm or less in length.

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

In addition, in this invention, it is preferable to make maximum Mn segregation degree, Nb segregation degree, and Ti segregation degree in the base material of a steel plate and steel pipes respectively 2.0 or less, 4.0 or less and 4.0 or less.

By setting the maximum Mn segregation ratio to 2.0 or less, generation of coarse MnS is suppressed, and generation of HIC starting from MnS stretched in the rolling direction can be prevented. In addition, when the Nb segregation degree is 4.0 or less, generation of integrated Nb (C, N) is suppressed. When the Ti segregation degree is 4.0 or less, generation of integrated TiN is suppressed, and deterioration of HIC property can be prevented.

The maximum Mn segregation rate is the ratio of the maximum Mn amount of the center segregation portion to the average Mn amount excluding the center segregation portion of the steel sheet and the steel pipe, and the Mn of the steel sheet and the steel pipe by EPMA or CMA having a beam diameter of 2 μm. The concentration distribution can be measured and found. The same applies to the Nb segregation degree and Ti segregation degree, and the Nb concentration distribution and the Ti concentration distribution are respectively measured by EPMA or CMA having a beam diameter of 2 µm, and the average Nb except the central segregation portion of the steel sheet and the steel pipe is measured. The ratio of the average amount of Nb to the amount of the center segregation portion (Nb segregation degree) and the average amount of Ti amount of the center segregation portion to the average Ti amount excluding the center segregation portion of the steel sheet and steel pipe (Ti segregation degree) We shall ask.

The method for suppressing the maximum Mn segregation degree, Nb segregation degree and Ti segregation degree will be described below.

In order to suppress segregation of Mn, Nb and Ti, the low pressure at the time of final solidification in continuous casting is optimal. It is performed in order to eliminate the mixing of the solidification part and the non-solidification part resulting from the nonuniformity of the cooling of casting casting at the time of final coagulation | solidification, and it can carry out final solidification uniformly in the width direction by this.

In continuous casting, the steel piece is usually water-cooled, but the end portion in the width direction is cooled quickly, and the cooling in the center portion in the width direction is enhanced. Therefore, even if it solidifies at the edge part and center part of the width direction of a steel piece, solidification is delayed at the 1/4 part of the width direction, and an unsolidified part remains inside a steel piece. Therefore, in the width direction of a steel piece, a solidified part and an unsolidified part do not become uniform, for example, the shape of the interface of a solidified part and an unsolidified part may become W-shaped in the width direction. If nonuniform coagulation occurs in such a width direction, segregation is encouraged and the HIC resistance is deteriorated.

On the other hand, in continuous casting, if the pressure is reduced at the time of final solidification, the unsolidified portion can be extruded to uniformly solidify in the width direction. In addition, if the pressure is reduced after uneven solidification occurs in the width direction, the deformation resistance of the solidified portion is large, and thus the unsolidified portion cannot be extruded effectively.

Therefore, in order to prevent such W-shaped solidification from occurring, it is preferable to reduce the pressure while controlling the amount of reduction in accordance with the distribution in the width direction of the central solid phase at the final solidification position of the cast piece. By doing in this way, center segregation is suppressed also in the width direction, and maximum Mn segregation degree, Nb segregation degree, and Ti segregation degree can be made smaller.

The steel containing the said component is melted in a steelmaking process, and it becomes a steel piece by continuous casting continuously, reheats the steel piece, and carries out thick plate rolling, and becomes a steel plate.

In this manufacturing process, when the reheating temperature of the steel piece is set to 950 ° C or higher, the rolling reduction is performed with the reduction ratio in the recrystallization temperature range to 2 or more, and the reduction ratio in the unrecrystallized region to 3 or more, and average guustenite. A particle diameter can be 20 micrometers or less. Moreover, although water cooling is performed after completion | finish of rolling, it is preferable to start water cooling from the temperature of 750 degreeC or more, and to stop water cooling in the temperature range of 400-500 degreeC.

The recrystallization temperature range is a temperature range in which recrystallization occurs after rolling, and is about 900 ° C. or more in the steel component of the present invention. On the other hand, the unrecrystallized temperature range is a temperature range in which recrystallization and ferrite transformation do not occur after rolling, and is about 750 to 900 ° C in the steel component used in the present invention. Rolling in the recrystallized temperature range is called recrystallized rolling or rough rolling, and rolling in the unrecrystallized temperature range is called unrecrystallized rolling or finish rolling.

After unrecrystallization rolling, by starting water cooling from the temperature of 750 degreeC or more and making water cooling stop temperature 400 degreeC or more, as described below, the maximum hardness of center segregation can be 300 Hv or less. First, when the water cooling start temperature is less than 750 ° C., a large amount of ferrite is generated before cooling start, and C (carbon) is discharged from the ferrite to austenite. After cooling, the austenite phase in which C is concentrated is transformed into hard martensite containing a large amount of C.

Therefore, when water-cooling start temperature is set to 750 degreeC or more, and generation | occurrence | production of hard martensite is suppressed, hardness can be suppressed to 300 Hv or less. In addition, when the water cooling stop temperature is 400 ° C or higher, the hard martensite after transformation is partially decomposed, and the hardness can be suppressed to 300 Hv or less. Moreover, since water intensity stops that a water cooling stop temperature is too high, 500 degrees C or less is preferable.

<Examples>

Next, an Example demonstrates this invention further in detail.

The steel which has the chemical component shown in Table 1 was melted, and it was set as the steel piece whose thickness is 240 mm by continuous casting. In addition, Table 1 also shows the analysis value of the amount of hydrogen in the molten steel. In continuous casting, the pressure was reduced under final solidification. The obtained steel piece was heated to 1000-1250 degreeC, hot rolling was carried out in the recrystallization temperature area | region exceeding 900 degreeC, and then hot rolling was performed in the unrecrystallization temperature range of 750-900 degreeC. After hot rolling, water cooling was started at 750 degreeC or more, water cooling was stopped at the temperature of 400-500 degreeC, and the steel plate of the various plate | board thickness shown in Table 2 was produced.

Further, the steel sheet was formed into a tubular shape by C press, U press, and O press, and the end face was temporarily welded, and main welding was performed from the inner and outer surfaces, and after expansion, the steel sheet was formed. In addition, submerged-arc welding was used for this welding.

Tensile test pieces, HIC test pieces, and macro test pieces were taken from the obtained steel sheets and steel pipes, and used for each test. The HIC test was performed based on NACETM0284. In addition, the segregation degree of Mn, Nb, Ti was measured by EPMA using the macro test piece. In the measurement of segregation degree by EPMA, the concentration distribution of Mn, Nb, Ti was measured in a measuring area of the total thickness x 20 mm width with a 50 µm beam system, and then each element in the test piece thickness direction was In the concentrated place (center segregation part), the density | concentration of each element was measured in the area | region of 1 mm x 1 mm with a 2 micrometers beam system.

In addition, the Vickers hardness of center segregation was measured based on JISZ2244. Vickers hardness was measured at a site having the highest Mn concentration in the distribution of the Mn concentration in the thickness direction measured by EPMA with 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 uncompressed part, the maximum hardness of the central segregation part, the tensile strength of the steel sheets obtained by the steels 1 to 34 of Table 1, respectively. The area ratio (CAR) of the crack obtained by the HIC test is shown.

Table 3 also shows the thickness of the steel pipes obtained from the steels 1 to 34 in Table 1, the heat input of the main welding, and the area ratio of the cracks determined by the HIC test. In addition, the maximum Mn segregation degree, Nb segregation degree, Ti segregation degree, length of uncompressed part, and maximum hardness of central segregation part of steel pipe are equivalent to steel plate, and tensile strength of steel pipe is 1 to 5% larger than steel plate.

Steels 1 to 23 are examples of the present invention, and the steel sheets obtained from these steels have a maximum Mn segregation rate of 1.6 or less, an Nb segregation degree of 4.0 or less, Ti segregation degree of 4.0 or less, and maximum hardness of the central segregation portion. It is 300 Hv or less, and the crack by HIC test does not generate | occur | produce. The same applies to steel pipes made of these steel sheets.

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

Claims (10)

In mass%,
C: 0.02-0.08%,
Si: 0.01% to 0.5%
Mn: 1.0 to 1.6%,
Nb: 0.001-0.10%,
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
Limited to S, Ca content,
S / Ca <0.5
Satisfies the above, and the remainder is made of Fe and an unavoidable impurity element,
The steel sheet for high-strength line pipe excellent in hydrogen-induced organic cracking resistance, wherein the length of the uncompressed portion of the central segregation portion is limited to 0.1 mm or less. The method of claim 1,
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%
The steel sheet for high-strength line pipe which is excellent in hydrogen-induced organic cracking property, containing 1 type, or 2 or more types, and remainder consists of iron and an unavoidable impurity. 3. The method according to claim 1 or 2,
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% to 0.005%
The steel sheet for high-strength line pipe excellent in the hydrogen-induced organic cracking property characterized by further containing 1 type (s) or 2 or more types. 3. The method according to claim 1 or 2,
Also,
Max Mn Segregation: 2.0 or less,
Nb segregation: 4.0 or less,
Ti segregation: 4.0 or less,
The steel sheet for high-strength line pipe excellent in the hydrogen-resistant organic cracking property characterized by the above-mentioned. 3. The method according to claim 1 or 2,
The steel sheet for high-strength line pipe excellent in the hydrogen-induced organic cracking property whose maximum hardness of a center segregation part is 300 Hv or less. The base material is in mass%,
C: 0.02-0.08%,
Si: 0.01% to 0.5%
Mn: 1.0 to 1.6%,
Nb: 0.001-0.10%,
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
Limited to S, Ca content,
S / Ca <0.5
Satisfies the above, and the remainder is made of Fe and an unavoidable impurity element,
In addition, the length of the uncompressed part of the center segregation part of a base material was limited to 0.1 mm or less, The steel pipe for high strength line pipes excellent in the hydrogen-induced organic cracking property. The method according to claim 6,
The base material is 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%
A steel pipe for high-strength line pipes having excellent organic hydrogen cracking resistance, characterized in that it contains one kind or two or more kinds thereof, and the remainder is made of iron and unavoidable impurities. 8. The method according to claim 6 or 7,
The base material is 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% to 0.005%
The steel pipe for high-strength line pipe excellent in the hydrogen-induced organic cracking property further containing 1 type, or 2 or more types of these. 8. The method according to claim 6 or 7,
Also, the base material
Max Mn Segregation: 2.0 or less,
Nb segregation: 4.0 or less,
Ti segregation: 4.0 or less,
Steel pipe for high-strength line pipe excellent in hydrogen-induced organic cracking property, characterized in that limited to. 8. The method according to claim 6 or 7,
A steel pipe for high-strength line pipes having excellent hydrogen cracking resistance, characterized in that the maximum hardness of the center segregation portion of the base material is 300 Hv or less.

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