KR20190065337A - Steel pipe for steel plate and line pipe, and manufacturing method thereof - Google Patents

Abstract

P: not less than 0% by mass but not more than 0.030% by mass, S: not less than 0% by mass and not more than 0.003% by mass, Al: 0.010 to less than 0.05% 0.001 to 0.01 mass% of Ca, 0.0003 to 0.0060 mass% of Ca, 0.001 to 0.01 mass% of N and 0.0045 mass% or more of O to 0 mass% and further satisfying the following expressions (1) and (2) , The balance being iron and inevitable impurities, and an area ratio of a portion where the defect echo height is 20% or more is 0.05% or less.
3.0? [Ca] / [S] (1)
([Ca] -1.25 x [S]) / [O]? 1.80 (2)
Here, [Ca], [S] and [O] are contents (mass%) of Ca, S and O, respectively.

Description

Steel pipe for steel plate and line pipe, and manufacturing method thereof

This disclosure relates to a steel pipe and a steel pipe for a line pipe, and a manufacturing method thereof.

BACKGROUND ART [0002] In line pipes and storage tanks mainly for transportation of oil and gas, so-called "sour resistance" such as hydrogen-organic cracking resistance and stress corrosion cracking resistance is developed with the development of inferior resources containing hydrogen sulfide It becomes necessary. Hydrogen Induced Cracking (hereinafter also referred to as " HIC ") is a method in which hydrogen penetrated into a steel material in response to a corrosion reaction with hydrogen sulfide or the like is decomposed into a nonmetallic inclusion including MnS and Nb (C, N) It is known that cracks are generated due to gasification. When HIC occurs, there is a problem that the toughness of the structure is lowered. Particularly, since hydrogen penetrates from the surface layer of the steel sheet, the surface layer portion of the plate thickness is easier than the central portion of the plate thickness, and it is required to improve the HIC characteristics of the surface layer portion.

Hitherto, a technique for improving the HIC resistance of the surface layer has been studied. For example, Patent Document 1 discloses that the amount of Ar gas blown into the molten steel is set to a predetermined value or less, whereby MnS, Ca-Al-based and Ca-based inclusion clusters and Ti-based and Nb- And the uncompacted bubbles of the Ar gas in the steel material causing the segregation zone are reduced to improve the HIC resistance.

Patent Document 2 proposes to improve the HIC resistance by controlling the Ca concentration in the slab to a predetermined range at the time of slab manufacturing and controlling the contents of Ca, S and O in the steel and the Ar gas content to a predetermined range. .

JP-A-07-136748 Japanese Patent Application Laid-Open No. 2016-125140

However, in Patent Document 1, studies have been made to reduce the number of bubbles in the slab, but the uncompacted bubbles in the steel product as the final product are not considered. Therefore, defects caused by the uncompacted bubbles remaining in the steel can not be controlled, and HIC due to the uncompacted bubble can not be suppressed.

Further, in the method of Patent Document 2, studies have been made to reduce the content of Ar gas bubbles in the steel material, but the size of the bubbles and the unfired bubbles of the steel material are not considered. Therefore, HIC can not be sufficiently suppressed when a small amount of coarse Ar bubbles are present.

The main object of the present invention is to provide a steel plate and a steel pipe for a line pipe excellent in hydrogen hydrogen organic cracking property.

The steel sheet according to the embodiment of the present invention comprises 0.02 to 0.15 mass% of C, 0.02 to 0.50 mass% of Si, 0.6 to 2.0 mass% of Mn, more than 0 mass% and less than 0.030 mass% of P, more than 0 mass% 0.003 mass% or less, Al: 0.010 to 0.080 mass%, Ca: 0.0003 to 0.0060 mass%, N: 0.001 to 0.01 mass%, and O: 0 to 0.0045 mass% And an area ratio of a portion where the remainder is iron and inevitable impurities and whose defect echo height is 20% or more is 0.05% or less.

3.0? [Ca] / [S] (1)

([Ca] -1.25 x [S]) / [O]? 1.80 (2)

Here, [Ca], [S] and [O] are contents (mass%) of Ca, S and O, respectively.

The steel sheet according to the embodiment of the present invention is characterized by comprising at least one of B, 0 to 0.005 mass%, V: more than 0 mass% to 0.1 mass%, Cu: more than 0 mass% to 1.5 mass% 0% by mass or more, Cr: 0% by mass or more and 1.5% by mass or less, Mo: 0 to 1.5% by mass, Nb: 0 to 0.06% More than 0 mass% and not more than 0.01 mass%, REM: not less than 0 mass% and not more than 0.02 mass%, and Zr: not less than 0 mass% and not more than 0.010 mass%.

The steel sheet according to the embodiment of the present invention may be used for a line pipe.

The steel pipe for a line pipe according to the embodiment of the present invention is formed of a steel plate according to the embodiment of the present invention.

The steel sheet according to the embodiment of the present invention may be used for a pressure vessel.

The method of manufacturing a steel sheet according to the embodiment of the present invention is a method of manufacturing a steel sheet according to an embodiment of the present invention that has a chemical composition of a steel sheet according to an embodiment of the present invention and has a density of bubbles having a circle- ) Is not more than 0.15 / cm < ^{2 &} gt ;.

According to an embodiment of the present invention, there is provided a steel plate and a steel pipe for a line pipe excellent in hydrogen hydrogen organic cracking property, and a manufacturing method thereof.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the CLR at the surface layer portion and the number density of bubbles having a circle-equivalent diameter of 0.2 mm or more in the slab aggregate.
2 is a graph showing the relationship between the CLR of the surface layer portion and the area ratio of a portion having a defect echo height of 20% or more.

In order to solve the above problems, the inventors of the present invention have found that the CLR (Crack Length Ratio, ratio of the sum of the crack lengths to the width of the test piece [%] and crack length ratio) of the surface layer measured by the HIC test, The correlation between the internal defects of the steel sheet measured by the method of the present invention. As a result, the chemical composition of the steel sheet was controlled within a predetermined range so that the content of Ca, S and O satisfied the predetermined relational expression, and the internal defects were controlled so that the area ratio of the portion having the defect echo height of 20% It was found that excellent HIC resistance was obtained.

Hereinafter, a steel sheet and a manufacturing method thereof according to an embodiment of the present invention will be described in detail.

<1. Steel plate>

(1-1. Chemical composition)

The steel sheet according to the embodiment of the present invention comprises 0.02 to 0.15 mass% of C, 0.02 to 0.50 mass% of Si, 0.6 to 2.0 mass% of Mn, more than 0 mass% and less than 0.030 mass% of P, more than 0 mass% 0.001 to 0.03 mass% Al, 0.010 to 0.080 mass% Al, 0.0003 to 0.0060 mass% Ca, 0.001 to 0.010 mass% of N, and 0.0045 mass% or more of O and 0 mass% And (2) below, and the balance of iron and inevitable impurities.

3.0? [Ca] / [S] (1)

([Ca] -1.25 x [S]) / [O]? 1.80 (2)

Here, [Ca], [S] and [O] are contents (mass%) of Ca, S and O, respectively.

By controlling the chemical composition as described above, it is possible to obtain a steel sheet having excellent hydrogen-organic cracking resistance.

[C: 0.02 to 0.15 mass%]

C is an indispensable element for ensuring the strength of the base material and the welded portion, and it is necessary to contain C in an amount of 0.02 mass% or more. The C content is preferably 0.03 mass% or more, and more preferably 0.04 mass% or more. On the other hand, if the amount of C is excessively large, the HAZ toughness and weldability deteriorate. In addition, if the amount of C is excessive, NbC or island-shaped martensite, which is a starting point of the HIC or destruction propagation path, is likely to be generated. Therefore, the C content should be 0.15 mass% or less. Preferably 0.12 mass% or less, and more preferably 0.10 mass% or less.

[Si: 0.02 to 0.50 mass%]

Si is an element effective for deoxidizing and improving the strength of a base material and a welded portion. In order to obtain these effects, the amount of Si is set to 0.02 mass% or more. The amount of Si is preferably 0.05 mass% or more, and more preferably 0.15 mass% or more. However, if the amount of Si is excessively large, the weldability and toughness are deteriorated. If the amount of Si is excessive, island-shaped martensite is generated and HIC is generated and evolved. Therefore, it is necessary to suppress the amount of Si to 0.50 mass% or less. The amount of Si is preferably 0.45 mass% or less, and more preferably 0.35 mass% or less.

[Mn: 0.6 to 2.0 mass%]

Mn is an element effective for improving the strength of the base material and the welded portion, and is contained in an amount of 0.6 mass% or more in the embodiment of the present invention. The amount of Mn is preferably 0.8% by mass or more, and more preferably 1.0% by mass or more. However, when the amount of Mn is excessively large, MnS is generated and not only the hydrogen-organic cracking property is deteriorated, but also the HAZ toughness and weldability are deteriorated. Therefore, the upper limit of the amount of Mn is set to 2.0% by mass. The amount of Mn is preferably 1.8 mass% or less, more preferably 1.5 mass% or less, further preferably 1.2 mass% or less.

[P: more than 0 mass% and not more than 0.030 mass%]

P is an element inevitably contained in the steel. If the P content exceeds 0.030 mass%, the toughness deterioration of the base material and the HAZ part is remarkable, and the hydrogen-organic cracking resistance also deteriorates. Therefore, in the embodiment of the present invention, the amount of P is suppressed to 0.030 mass% or less. The P content is preferably 0.020 mass% or less, and more preferably 0.010 mass% or less.

[S: more than 0% by mass and less than 0.003% by mass]

S is an element which forms a large amount of MnS and significantly deteriorates the hydrogen-organic cracking resistance when it is excessively large. Therefore, in the embodiment of the present invention, the upper limit of the amount of S is set to 0.003 mass%. The amount of S is preferably 0.002 mass% or less, more preferably 0.0015 mass% or less, further preferably 0.0010 mass% or less. From the viewpoint of improving the resistance to hydrogen-induced organic cracking, it is preferable that the amount is small.

[Al: 0.010 to 0.080 mass%]

Al is a strong acid element. When the amount of Al is small, the Ca concentration in the oxide increases, that is, the Ca-based inclusion tends to be formed in the surface layer portion of the steel sheet, and fine HIC is generated. Therefore, in the embodiment of the present invention, it is necessary to set Al to 0.010 mass% or more. The amount of Al is preferably 0.020 mass% or more, and more preferably 0.030 mass% or more. On the other hand, if the Al content is excessively large, the Al oxide is generated in a cluster phase and becomes a starting point of hydrogen organic cracking. Therefore, the amount of Al needs to be 0.080 mass% or less. The amount of Al is preferably 0.060 mass% or less, and more preferably 0.050 mass% or less.

[Ca: 0.0003 to 0.0060 mass%]

Ca has an effect of controlling the form of sulfides and has the effect of inhibiting the formation of MnS by forming CaS. In order to obtain this effect, it is necessary to set the amount of Ca to 0.0003 mass% or more. The amount of Ca is preferably 0.0005 mass% or more, and more preferably 0.0010 mass% or more. On the other hand, if the amount of Ca exceeds 0.0060 mass%, a large amount of HIC is generated starting from Ca-based inclusions. Therefore, in the embodiment of the present invention, the upper limit of the amount of Ca is set to 0.0060 mass%. The amount of Ca is preferably 0.0045 mass% or less, more preferably 0.0035 mass% or less, further preferably 0.0025 mass% or less.

[N: 0.001 to 0.01% by mass]

N is precipitated as TiN in the steel structure to suppress the coarsening of the austenite grains in the HAZ portion, further promoting the ferrite transformation and improving the toughness of the HAZ portion. In order to obtain this effect, it is necessary to contain N in an amount of 0.001 mass% or more. The N content is preferably 0.003 mass% or more, and more preferably 0.0040 mass% or more. However, if the amount of N is too large, the toughness of the HAZ tends to deteriorate due to the presence of the solid solution N. Therefore, the N content should be 0.01 mass% or less. Preferably 0.008 mass% or less, and more preferably 0.0060 mass% or less.

[O: 0 mass% or more and 0.0045 mass% or less]

O (oxygen) is preferably low in terms of improvement in cleanliness, and when O is included in a large amount, in addition to deterioration of toughness, HIC is generated from the oxide as a starting point, and hydrogen hydrogen organic cracking resistance deteriorates. From this viewpoint, the amount of O needs to be 0.0045 mass% or less, preferably 0.0035 mass% or less, and more preferably 0.0030 mass% or less.

[[Ca] / [S]: 3.0 or more]

The steel sheet according to the embodiment of the present invention satisfies the following expression (1).

3.0? [Ca] / [S] (1)

Here, [Ca] and [S] are contents (% by mass) of Ca and S, respectively.

The technical significance of the above equation (1) will be described below.

S forms MnS as a sulfide inclusion, and HIC occurs from MnS as a starting point. Therefore, by adding Ca to control the form of sulfide inclusions in the steel as CaS, the formation of MnS is suppressed and the HIC resistance is prevented from being lowered. The present inventors have found that it is necessary to set [Ca] / [S] to 3.0 or more in order to sufficiently exert this action effect. [Ca] / [S] is preferably 3.5 or more, and more preferably 4.0 or more. On the other hand, in consideration of the amounts of Ca and S specified in the embodiment of the present invention, the upper limit of [Ca] / [S] is about 15.

[(Ca) -1.25 x [S]) / [O]: 1.80 or less]

The steel sheet according to the embodiment of the present invention satisfies the following expression (2).

([Ca] -1.25 x [S]) / [O]? 1.80 (2)

Here, [Ca], [S] and [O] are contents (mass%) of Ca, S and O, respectively.

The technical significance of the above equation (2) will be described below.

In order to suppress the generation of HIC by Ca-based oxysulfide, it is effective to suppress the formation of CaO, which easily forms a coagulum, among Ca-based inclusions. For that purpose, the amount of Ca ([Ca] -1.25 x [S]) obtained by subtracting the Ca content present as sulfide (CaS) from the total amount of Ca in the steel should not be excessive with respect to the amount of O. If the amount of Ca ([Ca] -1.25 x [S]) is excessive with respect to the amount of O, CaO is easily formed as an oxide inclusion, and a coagulated aggregate of CaO (coarse Ca inclusion) . In order to suppress this, the present inventors have studied the relationship between ([Ca] -1.25 x [S]) / [O] and the HIC property, S]) / [O] needs to be 1.80 or less. ([Ca] -1.25 x [S]) / [O] is preferably 1.40 or less, more preferably 1.30 or less, further preferably 1.20 or less, particularly preferably 1.00 or less. On the other hand, the lower limit of (Ca) -1.25 x [S] / [O] is about 0.1 in view of suppressing Al ₂ O _3, which is likely to form a coagulum like CaO.

The basic components of the steel sheet according to the embodiment of the present invention are as described above, and the balance is iron and unavoidable impurities. However, it is a matter of course that inevitable impurities other than P and S are brought into the steel depending on the conditions of raw materials, materials or manufacturing facilities.

On the other hand, as described above, P and S are inevitably included elements (inevitable impurities), but their composition ranges are separately defined as described above. For this reason, in the present specification, the term "inevitable impurities" included as the remainder means an element that is inevitably included, excluding elements whose composition ranges are separately specified.

Further, the steel sheet according to the embodiment of the present invention may further contain the following elements in addition to the above-mentioned elements, and the characteristics of the steel sheet are further improved depending on the kinds of elements contained therein.

[B: more than 0% by mass and not more than 0.005% by mass]

B improves the hardenability and improves the strength of the base material and the welded portion. In addition, at the time of welding, BN is precipitated in the process of cooling the heated HAZ to precipitate BN to promote ferrite transformation from within the austenitic grains Therefore, HAZ toughness is improved. To obtain this effect, the amount of B is preferably 0.0002 mass% or more. The amount of B is more preferably 0.0005 mass% or more, and still more preferably 0.0010 mass% or more. However, if the B content is excessive, the toughness of the base material and the HAZ part deteriorates, or the weldability deteriorates. Therefore, the B content is preferably 0.005 mass% or less. The amount of B is more preferably 0.004 mass% or less, and still more preferably 0.0030 mass% or less.

[V: more than 0 mass% and not more than 0.1 mass%]

V is an element effective for improving the strength. In order to obtain this effect, it is preferable to contain V in an amount of 0.003 mass% or more. More preferably, it is 0.010 mass% or more. On the other hand, if the V content exceeds 0.1% by mass, the weldability and the toughness of the base material deteriorate. Therefore, the amount of V is preferably 0.1 mass% or less, and more preferably 0.08 mass% or less.

[Cu: more than 0% by mass and not more than 1.5% by mass]

Cu is an element effective for improving hardenability and enhancing strength. In order to obtain this effect, it is preferable to contain Cu in an amount of 0.01% by mass or more. The amount of Cu is more preferably 0.05% by mass or more, and still more preferably 0.10% by mass or more. However, if the Cu content exceeds 1.5% by mass, the toughness deteriorates, and therefore, the Cu content is preferably 1.5% by mass or less. The amount of Cu is more preferably 1.0 mass% or less, and still more preferably 0.50 mass% or less.

[Ni: more than 0% by mass and not more than 1.5% by mass]

Ni is an element effective for improving the strength and toughness of a base material and a welded portion. In order to obtain this effect, it is preferable that the amount of Ni is 0.01 mass% or more. The amount of Ni is more preferably 0.05% by mass or more, and still more preferably 0.10% by mass or more. However, when Ni is contained in a large amount, it becomes extremely expensive as a structural steel, and therefore, from an economic point of view, the amount of Ni is preferably 1.5 mass% or less. The amount of Ni is more preferably 1.0 mass% or less, and still more preferably 0.50 mass% or less.

[Cr: more than 0% by mass and not more than 1.5% by mass]

Cr is an element effective for improving the strength. In order to obtain this effect, Cr is preferably contained in an amount of 0.01 mass% or more. The amount of Cr is more preferably 0.05 mass% or more, and still more preferably 0.10 mass% or more. On the other hand, when the amount of Cr exceeds 1.5% by mass, the HAZ toughness is deteriorated. Therefore, the amount of Cr is preferably 1.5% by mass or less. The amount of Cr is more preferably 1.0 mass% or less, and still more preferably 0.50 mass% or less.

[Mo: more than 0% by mass and not more than 1.5% by mass]

Mo is an element effective for improving the strength and toughness of the base metal. In order to obtain this effect, it is preferable to set the amount of Mo to 0.01% by mass or more. The amount of Mo is more preferably 0.05 mass% or more, and still more preferably 0.10 mass% or more. However, when the amount of Mo exceeds 1.5% by mass, HAZ toughness and weldability deteriorate. Therefore, the amount of Mo is preferably 1.5 mass% or less, more preferably 1.0 mass% or less, and still more preferably 0.50 mass% or less.

[Nb: more than 0 mass% and 0.06 mass% or less]

Nb is an effective element for increasing strength and toughness of a base material without deteriorating weldability. To obtain this effect, the Nb content is preferably 0.002 mass% or more. The amount of Nb is more preferably 0.010 mass% or more, and still more preferably 0.020 mass% or more. However, if the amount of Nb exceeds 0.06 mass%, the toughness of the base material and the HAZ deteriorates. Therefore, in the embodiment of the present invention, the upper limit of the amount of Nb is preferably 0.06 mass%. The amount of Nb is more preferably 0.050 mass% or less, still more preferably 0.040 mass% or less, still more preferably 0.030 mass% or less.

[Ti: more than 0 mass% and 0.03 mass% or less]

Ti precipitates as TiN in the steel, thereby preventing coarsening of the austenite grains in the HAZ portion at the time of welding and promoting ferrite transformation, which is an element effective for improving the toughness of the HAZ portion. Moreover, Ti shows an effect of desulfurization and is an effective element for improving the HIC resistance. In order to obtain these effects, Ti is preferably contained in an amount of 0.003 mass% or more. The amount of Ti is more preferably 0.005 mass% or more, and still more preferably 0.010 mass% or more. On the other hand, if the Ti content is excessively high, solid Ti or TiC precipitates to deteriorate the toughness of the base material and the HAZ portion, so that the content is preferably 0.03 mass% or less. The amount of Ti is more preferably 0.02 mass% or less.

[Mg: more than 0 mass% and not more than 0.01 mass%]

Mg is an element effective for improving toughness through refinement of crystal grains and also exhibits a desulfurizing action and is therefore effective in improving the HIC resistance. In order to obtain these effects, Mg is preferably contained in an amount of 0.0003 mass% or more. The amount of Mg is more preferably 0.001 mass% or more. On the other hand, if Mg is added excessively, the effect is saturated, so the upper limit of the amount of Mg is preferably 0.01% by mass. The amount of Mg is more preferably 0.005 mass% or less.

[REM: more than 0 mass% and 0.02 mass% or less]

REM (rare earth element) is an element effective for suppressing the formation of MnS by the desulfurization action and enhancing the hydrogen-organic cracking property. In order to exhibit such an effect, it is preferable to contain REM in an amount of 0.0002 mass% or more. The amount of REM is more preferably 0.0005 mass% or more, and still more preferably 0.0010 mass% or more. On the other hand, even if a large amount of REM is contained, the effect is saturated. Therefore, the upper limit of the amount of REM is preferably 0.02% by mass. From the viewpoint of suppressing the occlusion of the immersion nozzle during casting to improve the productivity, the amount of REM is preferably 0.015 mass% or less, more preferably 0.010 mass% or less, still more preferably 0.0050 mass% or less. On the other hand, in the embodiment of the present invention, the REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y.

[Zr: more than 0 mass% and 0.010 mass% or less]

Zr is an element contributing to improvement of HAZ toughness by improving the HIC resistance by the desulfurizing action, and by forming oxide and being finely dispersed. In order to exhibit these effects, the amount of Zr is preferably 0.0003 mass% or more. The amount of Zr is more preferably 0.0005 mass% or more, still more preferably 0.0010 mass% or more, even more preferably 0.0015 mass% or more. On the other hand, when Zr is excessively added, coarse inclusions are formed to deteriorate hydrogen hydrogen organic cracking property and base material toughness. Therefore, the amount of Zr is preferably 0.010 mass% or less. The amount of Zr is more preferably 0.0070 mass% or less, further preferably 0.0050 mass% or less, still more preferably 0.0030 mass% or less.

(1-2 internal defects)

The steel sheet according to the embodiment of the present invention can suppress the HIC from the steel sheet accumulation even when the area ratio of the portion having the defect echo height of 20% or more is 0.05% or less and the bubbles remain on the steel sheet accumulation table .

This will be described in detail below.

In the casting process of the slab, it is necessary to blow the Ar gas into the molten steel, for example, in order to suppress clogging of the injection nozzle, reflux for degassing in RH, and stirring of the molten steel in the tundish have.

The slab collecting zone is a surface portion of the slab and is a portion that is solidified in the early stage because it is easier to cool than the center portion in the slab forming step. Therefore, the bubbles caused by the Ar gas blown at the time of slab casting float on the slab collecting bed, but are trapped in the solidified portion of the curved portion, and bubbles are likely to remain.

Since it is difficult to completely press the bubbles remaining in the slab collecting table in the rolling process, they are likely to remain as bubbles in the steel plate collecting table. Hydrogen is liable to be accumulated in the bubbles remaining in the steel plate accumulation zone, so that HIC may be generated starting from the remaining bubbles. Therefore, by reducing bubbles in the steel plate accumulation bed, it is possible to improve the HIC resistance.

Here, the &quot; slab integration zone &quot; means an area of about t / 8 to t / 4 from the surface of the slab among the slabs having a plate thickness t, and &quot; steel plate integration zone &quot; / T &quot; / 4 &quot; from the surface of the steel sheet among the steel sheets having a thickness t 'obtained by hot rolling the steel sheet.

When hot rolling a slab, typically the slab is rolled almost uniformly (i. E., The slab integration zone and other portions are rolled at approximately the same reduction rate). Therefore, a region of about t / 8 to t / 4 from the surface of the slab is a portion corresponding to a region of about t '/ 8 to t' / 4 from the surface of the steel sheet obtained by hot rolling. That is, the &quot; slab integration zone &quot; corresponds to &quot; steel plate integration zone &quot; of the steel sheet obtained by hot rolling.

The relationship between the CLR at the surface layer measured by the HIC test and the area ratio of a portion having a defect echo height of 20% or more as measured by an ultrasonic flaw test is shown in Fig.

Here, the defect echo height means the ratio [%] of the intensity of the defect echo reflected from the defect inside the test piece to the intensity of the bottom echo reflected from the bottom of the steel sheet (or a test piece obtained by taking a part of the steel sheet) .

The area ratio of the portion having the defect echo height of 20% or more means the ratio [%] of the area of the portion where the defect echo height is 20% or more with respect to the total area scanned by the probe.

From these results, the present inventors have found a correlation between the CLR of the surface layer and the area ratio. That is, even when the bubbles remain on the steel plate accumulation zone, if the area ratio of the portion having the defect echo height of 20% or more is 0.05% or less, the CLR of the surface layer portion of the steel sheet can be 10% or less, And found that HIC can be suppressed. From the viewpoint of obtaining a steel sheet excellent in HIC resistance, the defect echo height is preferably 30% or less, more preferably 25% or less, and the area ratio of the portion having a defect echo height of 20% or more is preferably 0.04% or less , And more preferably 0.03% or less.

On the other hand, since it is difficult to eliminate all the bubbles in the steel sheet, the defect echo height and the area ratio of the portion where the defect echo height is 20% or more are usually 0% or more.

The steel sheet according to the embodiment of the present invention and the steel pipe for a line pipe formed using the same may be suitably used in a line pipe for transporting natural gas and crude oil, a storage tank, and a pressure vessel for purification.

<2. Method of Manufacturing Steel Sheet>

The steel sheet manufacturing method according to the embodiment of the present invention uses a slab having the chemical composition described above and having density of bubbles having a circle equivalent diameter of 0.2 mm or more in the slab collecting table of 0.15 pieces / cm ² or less. By using the slab, a steel sheet having excellent HIC resistance can be produced.

This will be described in detail below.

(2-1 Slabs having the number density of bubbles having a diameter of 0.2 mm or more and equivalent to the inner circle of the slab aggregation zone of 0.15 pieces / cm ² or less)

As described above, since hydrogen easily accumulates in bubbles remaining in the steel plate accumulating bands, HIC may be generated starting from the remaining bubbles. Therefore, by reducing bubbles in the steel plate accumulation bed, it is possible to improve the HIC resistance.

Since the "slab accumulation zone" corresponds to the "steel plate accumulation zone" of the steel sheet obtained by hot rolling, as a concrete means for reducing the air bubbles in the steel plate accumulation zone, by reducing the air bubbles in the slab accumulation zone, It is effective to reduce bubbles in the steel plate accumulation zone of the obtained steel sheet, and the HIC resistance can be improved.

Fig. 2 shows the relationship between the CLR of the surface layer measured by the HIC test and the number density of bubbles having a circle-equivalent diameter of 0.2 mm or more in the slab accumulation zone. As a result, the present inventors have found that by producing a steel sheet using a slab in which the number density of bubbles having a circle-equivalent diameter of 0.2 mm or more in the slab integration zone is 0.15 pieces / cm ² or less, Can be reduced. The steel sheet produced using such a slab has an area ratio of not more than 0.05% at a portion where the defect echo height is not less than 20%, CLR at the surface layer portion of the steel sheet can be made not more than 10%, and HIC I can do it.

Circle-equivalent diameter of the bubbles in the slab for the integrated, the number density of the circle-equivalent diameter of 0.2mm or more bubbles in a desirable, and, more preferably not more than 0.15mm slab integrated for 0.17mm or less is preferably 0.10 pieces / cm ² or less Cm &lt; ² &gt; or less.

On the other hand, since it is difficult to eliminate all of the bubbles in the slab collecting platform, the circle equivalent diameter of bubbles in the slab collecting platform is usually 0 mm or more, and the number density of bubbles having a circle- 0 pieces / cm ² or more.

The method of measuring the circle-equivalent diameter of the bubbles and the number density of the bubbles is not particularly limited, and for example, the following methods can be mentioned.

The specimens taken from the slab collection are observed using an optical microscope, and the circle equivalent diameter of the bubbles is measured using a binocular micrometer to count the number of bubbles having a circle equivalent diameter of 0.2 mm or more in the observation field.

Next, the density of the bubbles is calculated from the area of the observation field and the number of bubbles having a circle equivalent diameter of 0.2 mm or more.

(2-2 a step of casting the slab)

In order to cast a slab having a density of the bubbles having a diameter of 0.2 mm or more corresponding to the circle of the slab collecting zone of 0.15 pieces / cm ² or less in the steelmaking process, the amount of Ar gas blown into the nozzle when molten steel is supplied from the tundish to the mold And the bubble diameter.

In the case of using the Ar gas, the inner tube diameter of 70mm more than 115mm or less and the average pore diameter of 30μm or less than 60μm porous from the kite (煉瓦), the back pressure to 3L (liter in an Ar gas is 1.4kgf / cm ² or more, 1.8kgf / cm ² or less ) / t (ton) to 10 L / t or less.

The inner diameter of the inner tube is preferably 75 mm or more, more preferably 80 mm or more, and preferably 110 mm or less, and more preferably 105 mm or less.

The average pore diameter is preferably 35 m or more, more preferably 40 m or more, and most preferably 55 m or less, and more preferably 50 m or less.

Back pressure is, 1.45kgf / cm ^2, it is preferable, and 1.5kgf / cm ² and more preferably at least, it is preferable 1.75kgf / cm ² or less is more preferable 1.7kgf / cm ² or less or more.

The blowing amount is preferably 5 L / t or more, more preferably 7 L / t or more, more preferably 12 L / t or less, and more preferably 10 L / t or less.

By blowing the Ar gas within this range, the nozzle clogging hardly occurs, and the Ar gas having a large diameter is blown into the molten steel, so that the bubbles of the Ar gas are liable to rise in the mold. As a result, bubbles of the Ar gas tend to be released from the accumulation area, so that the bubbles of the Ar gas captured by the accumulation zone can be reduced.

On the other hand, a means for reducing the amount of Ar gas blown together with the molten steel from the injection nozzle for injecting the molten steel into the mold is taken into consideration. However, since the molten steel is hardly stirred by the Ar gas near the molten steel surface of the mold, It is not recommended because there is worry about it.

The conditions other than the above are not particularly limited, and the steel having the above-described chemical composition may be cast in accordance with a conventional steelmaking method and the slab may be cast by a continuous casting process.

The method of manufacturing the steel sheet according to the embodiment of the present invention using the slab is not particularly limited as long as the area ratio of the portion where the defect echo height of the steel sheet is 20% or more is 0.05% or less, , And then cooling is performed to produce a steel sheet.

Hereinafter, "temperature" refers to the temperature of the material.

In order to achieve the above-mentioned steel sheet defective area ratio, for example, in the temperature range where the surface temperature is 900 ° C or higher, the steel sheet is rolled by 5 passes or more at a reduction ratio of 20% or less per one pass so that the cumulative rolling reduction becomes 50% It is recommended to perform hot rolling.

By hot rolling under the above conditions, since the plate thickness portion is preferentially deformed from the plate thickness inside portion, the bubbles trapped on the accumulation portion can be pressed more efficiently.

As the cooling conditions after hot rolling, it is recommended that cooling is performed at an average cooling rate of 10 ° C / s or more from the cooling start temperature of, for example, the Ar3 transformation point or higher.

By cooling under the above conditions, HIC generated near the center of the steel sheet can be effectively suppressed.

Further, a steel pipe for a line pipe can be manufactured by a general method using the steel sheet according to the embodiment of the present invention. The steel pipe for a line pipe obtained by using the steel sheet according to the embodiment of the present invention is also excellent in HIC resistance and toughness. Further, the steel sheet according to the embodiment of the present invention may be used in a pressure vessel by a method generally used.

As described above, the method of manufacturing the steel sheet according to the embodiment of the present invention has been described. However, those skilled in the art who understand the desired characteristics of the steel sheet according to the embodiment of the present invention can perform trial and error, As a method of producing a steel sheet, there is a possibility of finding a method other than the above-mentioned manufacturing method.

As described above, since bubbles are likely to remain in the slab collecting zone and HIC is liable to be generated starting from the bubbles remaining in the steel plate collecting bed, in particular, attention is paid to the slab collecting table and the steel plate collecting table, And a method of manufacturing the same. However, since the bubbles in the portion other than the accumulating portion are usually smaller than those in the accumulating portion, by controlling the bubbles in the accumulating portion as described above to improve the HIC resistance of the accumulating portion, . That is, it is to be noted that the effect of the present invention is not limited to the integrated structure but extends to the entire steel plate.

Example

Hereinafter, the embodiments of the present invention will be described in more detail with reference to Examples. However, the present invention is not limited by the following Examples in any way, and the present invention is not limited to the following Examples. Of course, all of which are included in the technical scope of the present invention.

The steel having the chemical composition of the steel types A to K shown in Table 1 was dissolved and a slab (cast piece) was obtained under the casting conditions shown in Table 2.

For the casting conditions in Table 2, &quot;?&Quot; indicates that the Ar gas was blown into the tundish at a rate of 5 to 9 L / t under a back pressure of 1.4 to 1.8 kgf / cm ² from a porous crucible having an inner diameter of 90 mm and an average pore diameter of 45 μm , And a slab having a thickness of 280 mm is obtained by continuous casting.

As to the casting conditions in Table 2, &quot; x &quot; indicates a condition in which an Ar gas was supplied to the tundish at a rate of 5 to 9 L / t under a back pressure of 1.4 to 1.8 kgf / cm ² from a porosity tube having an inside diameter of 120 to 150 mm and an average pore diameter of 45 [ And a slab having a thickness of 280 mm is obtained by continuous casting.

The obtained slab was reheated at 1050 to 1250 占 폚 and then subjected to a two-pattern process shown in Table 2 to obtain a test specimen. 1 to 12 steel sheets were produced.

"TMCP" is defined as follows: (1) At a temperature range of 900 ° C or higher, by rolling over 5 passes at a reduction rate of 20% or less per pass, the cumulative reduction rate is 50% (2) hot rolling is carried out so that the cumulative rolling reduction becomes 20% or more and the rolling finish temperature is 850 to 900 占 폚 at a temperature range of 850 占 폚 to 900 占 폚; (3) Cooling at an average cooling rate of 10 to 50 占 폚 / s from the cooling start temperature of 750 to 850 占 폚, stopping at a temperature range of 350 to 600 占 폚, and air cooling to room temperature.

Quot; QT &quot; means that (1) the cumulative rolling reduction is 50% or more by rolling at 5 or more passes at a reduction rate of 20% or less per pass at a temperature range of 900 DEG C or more, (3) heating and reheating at a temperature of 850 to 950 占 폚, and (4) tempering at 600 to 700 占 폚. to be.

For each of the above-mentioned slabs and steel plates, the number density of bubbles having a circle equivalent diameter of 0.2 mm or more and the area ratio of a portion having a defect echo height of 20% or more in the slab integrated zone and the HIC test I did.

[One. Density of the bubbles having a diameter of 0.2 mm or more corresponding to the circle of the slab integrated zone]

At a position of 45 to 60 mm in the thickness direction of the slab from the surface of the slab having a thickness of 280 mm, a position of 1/4 of the width of the slab toward the width direction of the slab (direction perpendicular to the casting direction) Test specimens having a plate thickness of 15 mm, a width of 70 mm and a length of 15 mm including an L section (surface perpendicular to the casting direction of the slab) were collected from two places (slab collecting zone). The end face of the L was polished by using the emery abrasive paper (# 320 to # 1500), and the mirror finish was performed by buff polishing. Next, the L section was observed using an optical microscope (magnification: 5 times), and the circle equivalent diameter of the bubbles was measured using a binocular micrometer (magnification: 5 times) mm &lt; / RTI &gt; The density of bubbles was calculated from the area of the observation field and the number of bubbles having a circle-equivalent diameter of 0.2 mm or more. The maximum density among the densities obtained from the two places was determined as the number density of bubbles having a circle-equivalent diameter of 0.2 mm or more in the slab collecting table.

[2. Area ratio of the portion where the defect echo height is 20% or more]

From the two positions (steel sheet accumulation zone) at a position of ¼ of the width of the steel sheet and one-half of the width of the steel sheet toward the width direction (direction perpendicular to the rolling direction) of the steel sheet, Test specimens were collected.

(A steel sheet having a thickness of 30 mm or less)

Three test specimens having a plate thickness x 20 mm width x 100 mm length (rolling direction) of the steel sheet were taken at the two positions, and a total of six specimens were prepared.

(Steel plate having a plate thickness exceeding 30 mm)

(I) a position perpendicular to the surface from the surface of the steel sheet, (ii) a position 1/2 of the thickness of the sheet, and (iii) a thickness Test specimens of 30 mm x 20 mm wide x 100 length were sampled to prepare a total of six specimens.

Each test piece was subjected to an ultrasonic test at a pitch of 0.4 mm × 0.4 mm using a GSONIC SCAN 8AX1500SR ultrasonic inspection device manufactured by JINESS Co., Ltd. and a water-immersion probe (frequency 10 MHz, diameter 0.5 inches, focal depth 4.5 inches) And the area ratio of the portion where the defect echo height of each test piece was 20% or more was measured, and the average value was made the area ratio of the portion where the defect echo height of the steel sheet was 20% or more.

[3. HIC test]

The HIC test was carried out using the test piece used in the ultrasonic flaw test according to the method specified in NACE standard TM0284-2003. Specifically, after immersing in an aqueous solution of 25 캜 (5.0% NaCl + 0.5% acetic acid) saturated with 1 atm of hydrogen sulfide for 96 hours, the cross-sectional evaluation of each test piece (NACE standard TM0284- 2003 FiGURE 2-8) was performed, and the CLR was measured. Here, the section is a surface defined by the thickness direction and the width direction of the test piece.

(A steel sheet having a thickness of 30 mm or less)

The end face is divided into three portions evenly in the plate thickness direction, and three sections of the front face side, the center portion and the back face side are defined. CLR was measured at the cross section on the surface side, and the average value was defined as &quot; CLR at the surface layer. &Quot; Further, the CLR was measured at the cross section of the center portion, and the average value was defined as &quot; CLR at the center portion &quot;.

(Steel plate having a plate thickness exceeding 30 mm)

The CLR of the test piece taken from the surface of the steel sheet in a direction perpendicular to the surface was measured and the average value was defined as &quot; CLR at the surface layer. &Quot; Further, the CLR of the test specimen collected at a position 1/2 of the plate thickness was measured, and the average value was defined as &quot; CLR at the center. &Quot;

A steel sheet having a CLR at the surface layer portion and a CLR at the center portion of 10% or less was found to be practically usable and the HIC resistance was judged to be excellent.

Table 2 shows the number density of bubbles having a diameter of 0.2 mm or more, the area ratio of the portion having the defect echo height of 20% or more, the CLR of the surface layer portion and the CLR of the center portion. With respect to the CLR at the surface layer portion and the CLR at the central portion, those having a value of 10% or less are indicated by "?".

On the other hand, among the Tables 1 and 2, the underlining means that it deviates from the specification of the embodiment of the present invention.

From the results in Table 2, the following can be considered. Test No. 1 to 5 and 12 are examples satisfying all of the requirements specified in the embodiment of the present invention and have excellent HIC resistance.

On the other hand, 6 to 11 are examples that do not satisfy any of the requirements specified in the embodiment of the present invention.

Test No. 6 and 7 are examples of a steel sheet produced by using a slab having a high number density of bubbles having a circle equivalent diameter of 0.2 mm or more and having a circle equivalent diameter in a slab integrated zone without proper casting conditions and the area of a portion having a defect echo height of 20% The CLR of the surface layer was deteriorated, and the desired HIC resistance was not achieved.

Test No. 8 and 9 are examples of a steel sheet produced using steel grades G and H each having a small [Ca] / [S], and a large amount of MnS was generated and the CLR at the central portion was deteriorated. On the other hand, 8, the number density of air bubbles is not evaluated because the CLR at the center portion is deteriorated.

Test No. 10 and 11 are examples of the steel sheets produced using the steel types I and J having large ([Ca] -1.25 x [S]) / [O], and the coarse Ca inclusions are generated in the steel sheet accumulation zone, The desired HIC performance was not achieved.

The disclosure of the present specification includes the following aspects.

Sun 1:

C: 0.02 to 0.15% by mass,

0.02 to 0.50% by mass of Si,

Mn: 0.6 to 2.0 mass%

P: more than 0% by mass, 0.030%

S: more than 0 mass% and less than 0.003 mass%

Al: 0.010 to 0.080% by mass,

Ca: 0.0003 to 0.0060% by mass,

N: 0.001 to 0.01% by mass, and

O: more than 0 mass% and less than 0.0045 mass%

And satisfies the following formulas (1) and (2)

The balance being iron and inevitable impurities,

And an area ratio of a portion having a defect echo height of 20% or more is 0.05% or less.

3.0? [Ca] / [S] (1)

([Ca] -1.25 x [S]) / [O]? 1.80 (2)

Here, [Ca], [S] and [O] are contents (mass%) of Ca, S and O, respectively.

Sun 2:

B: more than 0 mass% and less than 0.005 mass%

V: more than 0 mass% to less than 0.1 mass%

Cu: more than 0% by mass and not more than 1.5%

Ni: more than 0% by mass and not more than 1.5%

Cr: more than 0% by mass and not more than 1.5%

Mo: more than 0 mass% to 1.5 mass%

Nb: more than 0 mass%, 0.06 mass% or less,

Ti: more than 0 mass% and 0.03 mass% or less,

Mg: more than 0 mass% and 0.01 mass% or less,

REM: more than 0 mass% and 0.02 mass% or less, and

Zr: more than 0 mass% and less than 0.010 mass%

Wherein the steel sheet further contains at least one member selected from the group consisting of the following.

Sun 3:

The steel sheet described in the above 1 or 2 for use as a line pipe.

Sun 4:

A steel pipe for a line pipe formed from a steel sheet according to any one of Sun 1 to 3.

Sun 5:

A steel sheet for a pressure vessel as described in the above 1 or 2.

Sun 6:

A method of manufacturing a steel sheet according to aspect 1 or 2, having a composition the chemical components described in the aspect 1 or 2, and an integrated process for producing a slab for a circle-equivalent diameter of the steel sheet using the above cell density of 0.15 pcs / cm ² or less in the slab 0.2mm .

The present application is based on Japanese patent application filed on November 16, 2016, Japanese Patent Application No. 2016-223416 filed on November 16, 2016, and Japanese Patent Application No. 2017-176045 filed on September 13, It is accompanied. Sections 2016-223416 and 2017-176045 are hereby incorporated by reference.

Claims (7)

C: 0.02 to 0.15% by mass,
0.02 to 0.50% by mass of Si,
Mn: 0.6 to 2.0 mass%
P: more than 0% by mass, 0.030%
S: more than 0 mass% and less than 0.003 mass%
Al: 0.010 to 0.080% by mass,
Ca: 0.0003 to 0.0060% by mass,
N: 0.001 to 0.01% by mass, and
O: more than 0 mass% and less than 0.0045 mass%
And satisfies the following formulas (1) and (2)
The balance being iron and inevitable impurities,
And an area ratio of a portion having a defect echo height of 20% or more is 0.05% or less.
3.0? [Ca] / [S] (1)
([Ca] -1.25 x [S]) / [O]? 1.80 (2)
Here, [Ca], [S] and [O] are contents (mass%) of Ca, S and O, respectively. The method according to claim 1,
B: more than 0 mass% and less than 0.005 mass%
V: more than 0 mass% to less than 0.1 mass%
Cu: more than 0% by mass and not more than 1.5%
Ni: more than 0% by mass and not more than 1.5%
Cr: more than 0% by mass and not more than 1.5%
Mo: more than 0 mass% to 1.5 mass%
Nb: more than 0 mass%, 0.06 mass% or less,
Ti: more than 0 mass% and 0.03 mass% or less,
Mg: more than 0 mass% and 0.01 mass% or less,
REM: more than 0 mass% and 0.02 mass% or less, and
Zr: more than 0 mass% and less than 0.010 mass%
Wherein the steel sheet further contains at least one member selected from the group consisting of iron and iron. 3. The method according to claim 1 or 2,
Steel plate for line pipe. A steel pipe for a line pipe formed from the steel sheet according to claim 1 or 2. A steel pipe for a line pipe formed of the steel sheet according to claim 3. 3. The method according to claim 1 or 2,
Steel plates for pressure vessels. Claim 1 or a method for manufacturing a steel sheet according to claim 2, wherein one of the preceding claims having a composition the chemical components described in 2, wherein the slab also integrated for circle-equivalent diameter of more than 0.2mm cell density of 0.15 pcs / cm ² less than the slab Is used.

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