KR20150121193A - Steel plate with excellent hydrogen-induced cracking resistance and toughness, and steel pipe for line pipe - Google Patents

Abstract

Particularly, in the surface layer portion of the steel sheet, which is high in hydrogen concentration and in a strict environment, fine HIC of about several micrometers is sufficiently suppressed, thereby realizing a steel sheet excellent in hydrogen-induced organic cracking and toughness. The steel sheet having excellent hydrogen-induced organic cracking resistance and toughness according to the present invention has a prescribed element and the balance of iron and inevitable impurities. The ratio of Ca amount to S amount (Ca / S) is 2.0 or more, (Cmax / Cave) of the maximum Ca concentration (Cmax) in the region from the surface to the depth of 5 mm and the average Ca concentration (Cave) in the above region is 1.20 or less.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel pipe and a steel pipe for a line pipe having excellent hydrogen cracking resistance and toughness,

The present invention relates to a steel sheet excellent in resistance to hydrogen-organic cracking and toughness, which is suitable for use in a line pipe for transporting natural gas or crude oil, a pressure vessel, a storage tank, etc., and a line pipe Steel pipe.

BACKGROUND ART [0002] Along with the development of resources such as crude oil and gas containing hydrogen sulfide, line pipes, pressure vessels, and storage tanks used for transportation, refining, and storage thereof have various problems such as hydrogen hydrogen organic cracking resistance, So-called sourness is required. Hydrogen-Induced Cracking (hereinafter also referred to as " HIC ") is a phenomenon in which hydrogen invades into a steel material due to a corrosion reaction with hydrogen sulfide, N), and the like, are known to be cracks caused by gasification.

Particularly under a sour environment, it is known that the hydrogen concentration in the region from the surface to the depth of 5 mm (hereinafter, this region may be referred to as "surface layer portion of the steel sheet") in the sheet thickness direction becomes higher than the central portion of the steel sheet, It is known that cracks tend to be generated starting from a system oxide or an Al oxide.

BACKGROUND ART [0002] There have heretofore been proposed several techniques for increasing hydrogen-organic cracking resistance (hereinafter referred to as " HIC resistance "). For example, Patent Document 1 discloses a steel material having improved hydrogen-organic cracking resistance by suppressing the degree of segregation of Mn, Nb and Ti at the center of the plate thickness. In this method, although it is possible to improve the HIC characteristics of the center segregation portion, it is considered that it is difficult to suppress the cracks at the portions other than the center segregation portion because the inclusions at the portions other than the center segregation portion are not sufficiently controlled. Patent Document 2 discloses a method of suppressing HIC with MnS or Ca-based oxysulfide as a starting point based on a parameter equation consisting of Ca, O and S contents. By such a method, it is possible to ensure the HIC property, but it is considered that it is difficult to ensure the high penetrability of the surface layer portion in the surface layer portion of the steel sheet where the hydrogen concentration is particularly high, because minute HIC is easily generated as described later.

Japanese Patent Application Laid-Open No. 2010-209461 Japanese Patent Application Laid-Open No. 06-136440

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and has an object to provide a hydrogen-containing hydrogen-organic material which is sufficiently suppressed even in a fine HIC of several micrometers in a surface layer portion of a steel sheet, And to realize a steel sheet or steel pipe excellent in cracking and toughness.

In the steel sheet excellent in hydrogen-organic cracking resistance and toughness according to the present invention,

C: 0.02 to 0.15% (% means mass%, the same applies hereinafter)

Si: 0.02 to 0.50%

Mn: 0.6 to 2.0%

P: more than 0% and not more than 0.030%

S: more than 0% and not more than 0.003%

Al: 0.010 to 0.08%

Ca: 0.0003 to 0.0060%,

N: 0.001 to 0.01%, and

O (oxygen): more than 0% and not more than 0.0045%, the balance being iron and inevitable impurities,

The ratio of Ca to S (Ca / S) is 2.0 or more,

(Cmax / Cave) of the maximum Ca concentration (Cmax) in the region from the surface to the depth of 5 mm in the plate thickness direction and the average Ca concentration (Cave) in the region is 1.20 or less.

The steel sheet may further contain at least one element selected from the group consisting of the following (a) and (b) as further different elements.

(a) B: more than 0% and not more than 0.005%

V: more than 0% and not more than 0.1%

Cu: more than 0% to 1.5%

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

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

Mo: more than 0% and not more than 1.5%, and

Nb: more than 0% and not more than 0.06%

(b) Ti: more than 0% and not more than 0.03%

Mg: more than 0% and not more than 0.01%

REM: more than 0% and not more than 0.02%, and

Zr: more than 0% and not more than 0.010%

The steel sheet is suitable for a line pipe or a pressure vessel. The present invention also includes a steel pipe for a line pipe manufactured using the steel sheet.

According to the present invention, since the distribution of the Ca concentration in the plate thickness direction of the steel sheet is homogenized, even the fine HIC of the order of several micrometers can be sufficiently suppressed in the surface layer portion of the steel sheet where the hydrogen concentration becomes particularly high, It is possible to provide a steel sheet or steel pipe excellent in property and productivity.

FIG. 1 is a graph showing the incidence of HICs according to Ca concentration in inclusions which are the starting points of HIC.
2 is a diagram showing the relationship between Cmax / Cave and upper shelf energy.

The present inventors have conducted intensive studies in order to solve the above problems. First, the inventors of the present invention have found that the occurrence of HIC in the surface layer of the steel sheet, which is the most severe situation in the sour environment, is determined by using NACE (National Association of Corrosion and Engineer) TM0284 HIC test (NACE test) was conducted. This NACE test is a test for evaluating the occurrence of HIC after immersing a test piece, that is, a steel sheet for 96 hours, in a mixed aqueous solution of 5% NaCl solution saturated with 1 atm of hydrogen sulfide gas and pH 2.7 of 0.5% acetic acid.

Next, the present inventors carried out a Charpy test on the surface portion of the steel sheet after the HIC test according to ASTM A370. As a result, even when cracks were not observed in the "microscopic observation at a magnification of 100 times" prescribed in the NACE test, the Charpy test result after the HIC test was bad, that is, the toughness was poor.

In order to investigate the cause, when the microscope observation was carried out at a high magnification, it was found that a large number of microscopic cracks were generated from the inclusions. That is, a number of microscopic HICs below the observation limit which can not be observed by microscopic observation at the magnification of 100 times defined in the above NACE test are generated from the inclusions, and they were firstly identified as factors that deteriorate the toughness after the HIC test .

The inclusion composition which is the origin of the HIC including the fine HIC was investigated. In detail, the steel sheet subjected to the HIC test (NACE test) described in the following Examples was observed for its texture. The Ca concentration of the observed inclusions was also determined. The Ca concentration in the inclusions is the proportion of Ca (mass%, hereinafter simply expressed as%) in the composition of the constituents excluding O and N constituting the inclusions. (%) Of the inclusions which became the origin of the HIC out of the inclusions having the Ca concentration of 50% or more among the inclusions and the ratio (%) of the inclusions which became the origin of HIC out of the inclusions having the Ca concentration of 20% Respectively. The results are shown in Fig. In Fig. 1, the ratio of the inclusions that have been the origin of the HIC is denoted by the " HIC generation rate (%) " As shown in Fig. 1, in particular, inclusions whose Ca concentration is 50% or higher (hereinafter, inclusions whose Ca concentration is 50% or more are referred to as " Ca inclusions "), .

The Ca-based inclusions tend to aggregate and become locally aggregated during casting. Due to the presence of many Ca-based inclusions in the surface layer region of the steel sheet, the conventional Ca-based inclusions have been confirmed It is believed that a large number of fine HICs, which are difficult to obtain, are locally generated, which causes deterioration of toughness.

In the present invention, in the case where Ca-based inclusions are present in the surface layer portion of the steel sheet in controlling the Ca-based inclusions in the surface layer portion from the surface to the depth of 5 mm in the sheet thickness direction, I thought there was a point. Therefore, as shown in Examples to be described later, the maximum Ca concentration (Cmax) and the average Ca concentration (Cmax) of the plurality of sites measured when a plurality of Ca concentrations are measured at equal intervals from the surface to the depth of 5 mm in the plate thickness direction (Cmax / Cave) of the average Ca concentration in the region from the surface to the depth of 5 mm in the plate thickness direction, Cave) is used as a control factor of the Ca-based intervening amount in the surface layer of the steel sheet.

Next, the relationship between the Cmax / Cave and the toughness of the steel plate surface portion after the HIC test, specifically, the Charpy absorbed energy, particularly the upper shelf energy, was examined. As a result, a clear correlation was recognized in both of them, as shown in the following examples. That is, the inventors of the present invention first discovered that the toughness of the surface layer of the steel sheet after the HIC test can be improved by controlling the above (Cmax / Cave). Further, as evaluated in Examples to be described later, it has been found that Cmax / Cave can be set to 1.20 or less in order to achieve an upper shelf energy of 125 J or more with excellent toughness. The Cmax / Cave is preferably 1.19 or less, more preferably 1.18 or less, and further preferably 1.15 or less. From the viewpoint of improvement in toughness, the Cmax / Cave is preferably as small as possible, but the lower limit is about 1.00 where the amount of Ca in the steel sheet surface portion and the steel becomes equal.

In order to ensure excellent HIC resistance, it is necessary to control the composition of the steel material such as a steel pipe or a steel pipe obtained by using the steel sheet together with the control of the surface layer portion of the steel sheet. Further, in order to secure characteristics other than the above-mentioned HIC properties such as excellent HAZ toughness and weldability required for a steel sheet for a line pipe or a steel sheet for a pressure vessel, for example, it is necessary to form the steel sheet as follows. Hereinafter, the reason for defining each component will be described.

[Composition of components]

[C: 0.02-0.15%]

C is an indispensable element for securing the strength of the base material and the welded portion, and it is necessary to contain C in an amount of 0.02% or more. The amount of C is preferably 0.03% or more, and more preferably 0.05% 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 HIC or destruction propagation route is easily generated. Therefore, the C content should be 0.15% or less. The C content is preferably 0.12% or less, and more preferably 0.10% or less.

[Si: 0.02-0.50%]

Si has an effect of deoxidizing and is an element effective for improving the strength of the base material and the welded portion. In order to obtain these effects, the amount of Si is made 0.02% or more. The amount of Si is preferably 0.05% or more, and more preferably 0.15% or more. However, if the amount of Si is excessively large, the weldability and toughness are deteriorated. If the amount of Si is excessive, on the other hand, martensite is formed on the surface and HIC is generated and developed. Therefore, it is necessary to suppress the amount of Si to 0.50% or less. The amount of Si is preferably 0.45% or less, and more preferably 0.35% or less.

[Mn: 0.6 to 2.0%]

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% or more in the present invention. The amount of Mn is preferably 0.8% or more, and more preferably 1.0% 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 2.0% or less. , Preferably 1.8% or less, more preferably 1.5% or less, further preferably 1.2% or less.

[P: more than 0% to less than 0.030%]

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

[S: more than 0% and not more than 0.003%]

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

[Al: 0.010-0.08%]

Al is a strong acid element. If the amount of Al is small, the Ca concentration in the oxide increases, that is, the Ca-based inclusion tends to be easily formed in the surface layer portion of the steel sheet, and fine HIC is generated. Therefore, in the present invention, Al must be 0.010% or more. The amount of Al is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, if the Al content is excessively large, the Al oxide is generated in a cluster shape and becomes a starting point of hydrogen organic cracking. Therefore, the amount of Al needs to be 0.08% or less. The amount of Al is preferably 0.06% or less, and more preferably 0.05% or less.

[Ca: 0.0003 to 0.0060%]

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, the amount of Ca needs to be 0.0003% or more. The amount of Ca is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the amount of Ca exceeds 0.0060%, a large amount of HIC occurs from Ca based inclusions as a starting point. Therefore, in the present invention, the upper limit of the amount of Ca is set to 0.0060%. The amount of Ca is preferably 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.

[N: 0.001 to 0.01%]

N is an element that precipitates as TiN in the steel structure to inhibit coarsening of the austenite grains in the HAZ portion and also promotes ferrite transformation to improve the toughness of the HAZ portion. In order to obtain this effect, N should be contained in an amount of 0.001% or more. The amount of N is preferably 0.003% or more, and more preferably 0.0040% or more. However, if the amount of N is too large, the toughness of HAZ tends to deteriorate due to the presence of solid solution N. Therefore, the amount of N needs to be 0.01% or less. It is preferably not more than 0.008%, more preferably not more than 0.0060%.

[O: more than 0% and not more than 0.0045%]

O (oxygen) is preferably low in view of improvement in cleanliness, and when O is included in a large amount, not only the toughness is deteriorated but HIC is generated from the oxide as the starting point, and the hydrogen-organic cracking resistance is deteriorated. From this viewpoint, the amount of O needs to be 0.0045% or less, preferably 0.0030% or less, and more preferably 0.0020% or less.

[Ca / S (mass ratio): 2.0 or more]

When S is excessive with respect to Ca, MnS is generated around the central portion of the plate thickness, and HIC is generated starting from MnS. In order to suppress this, Ca / S must be 2.0 or more, preferably 2.5 or more, and more preferably 3.0 or more. On the other hand, the upper limit of Ca / S is about 15 from the Ca amount and S amount defined in the present invention.

The components of the steel material (steel plate, steel pipe) of the present invention are as described above, and the balance consists of iron and unavoidable impurities. Further, in addition to the above elements,

(a) one or more kinds of elements selected from the group consisting of B, V, Cu, Ni, Cr, Mo and Nb in the following quantities are contained to further increase strength and toughness,

(b) one or more elements selected from the group consisting of Ti, Mg, REM, and Zr in the following quantities can be contained to further enhance the HAZ toughness and accelerate the desulfurization to further improve the HIC property . These elements will be described in detail below.

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

B improves the hardenability, increases the strength of the base material and the welded portion, and 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 austenite lips Therefore, HAZ toughness is improved. In order to obtain this effect, it is preferable that the B content is 0.0002% or more. , More preferably 0.0005% or more, and even more preferably 0.0010% 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% or less. More preferably, it is 0.004% or less, and more preferably 0.0030% or less.

[V: more than 0% to 0.1% or less]

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

[Cu: more than 0% to 1.5% or less]

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

[Ni: more than 0% to 1.5% or less]

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, the amount of Ni is preferably 0.01% or more. The amount of Ni is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Ni is contained in a large amount, it becomes extremely expensive as a structural steel, and therefore, from an economical viewpoint, it is preferable that the amount of Ni is 1.5% or less. The amount of Ni is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Cr: more than 0% to 1.5% or less]

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% or more. The amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the amount of Cr exceeds 1.5%, the HAZ toughness is deteriorated. Therefore, the amount of Cr is preferably 1.5% or less. The amount of Cr is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Mo: more than 0% to 1.5% or less]

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

[Nb: more than 0% and not more than 0.06%]

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

[Ti: more than 0% to 0.03% or less]

Ti precipitates as TiN in the steel to prevent coarsening of the austenite grains in the HAZ portion at the time of welding and to accelerate the ferrite transformation, and thus is an element necessary for improving the toughness of the HAZ portion. Moreover, Ti is an element effective for improving the HIC resistance because it exhibits a desulfurizing effect. In order to obtain these effects, Ti is preferably contained in an amount of 0.003% or more. The amount of Ti is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, if the Ti content is excessive, the toughness of the base material and the HAZ portion deteriorates due to the solidification of Ti and the precipitation of TiC, and therefore, the Ti content is preferably 0.03% or less. The amount of Ti is more preferably 0.02% or less.

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

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

[REM: more than 0% and not more than 0.02%]

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% or more. The amount of REM is more preferably 0.0005% or more, and still more preferably 0.0010% 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%. The amount of REM is preferably 0.015% or less, more preferably 0.010% or less, still more preferably 0.0050% or less, from the viewpoint of suppressing the occlusion of the immersion nozzle during casting and improving productivity. Meanwhile, in the present invention, the REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y.

[Zr: more than 0% and not more than 0.010%]

Zr contributes to the improvement of the HIC toughness by the desulfurizing action, and also contributes to the improvement of the HAZ toughness by forming oxides and being finely dispersed. In order to exhibit these effects, the amount of Zr is preferably 0.0003% or more. The amount of Zr is more preferably 0.0005% or more, more preferably 0.0010% or more, still more preferably 0.0015% 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% or less. The amount of Zr is more preferably 0.0070% or less, more preferably 0.0050% or less, still more preferably 0.0030% or less.

The steel sheet specified in the present invention has been described above. The method for producing the steel sheet of the present invention is not particularly limited as long as it is a method for obtaining the above-described steel sheet surface layer portion. As a method for easily obtaining the above-described steel sheet having the surface layer portion of the steel sheet, the following method can be mentioned.

[Manufacturing method]

The molten steel is injected into the mold through the ladle and the tundish after the solvent has been obtained so as to have the composition of the above components. In order to obtain the steel sheet having the steel plate surface layer portion defined in the present invention, molten steel is injected into the tundish, In the step of casting, it is recommended that all of the following (1) to (3) are satisfied.

(1) In the tundish, the cross-sectional area of the flow path at the molten steel injection position into the mold is made larger than the cross-sectional area of the flow path at the molten steel injection position from the bore. Specifically, a tundish having such a sectional area of each flow passage is used.

(2) Ar is cast at a flow rate of 0.04 to 9.7 L (liter) / t (ton) from a position of 50 mm or more from the discharge hole of the injection nozzle.

(3) The solidification speed at the position of 1 to 3m from the meniscus position of the molten steel in the drawing toward the drawing direction is set to 0.26 mm / s or less.

The conditions (1) to (3) will be described below in order.

(1) Cross-sectional area of the flow path

The Ca-based inclusions have a high melting point and a large contact angle with the molten steel, so that a coagulated aggregate can easily be formed, which is likely to be a coarse inclusion. Therefore, it is necessary to sufficiently float the Ca-based inclusion inside the tundish. If the floating separation is insufficient, for example, the coarse Ca-based inclusions rise at the curved portion in continuous casting, and are easily accumulated on the surface layer. In order to sufficiently float and separate the inclusions in the tundish, it is preferable to reduce the average flow velocity of the molten steel in the tundish. By reducing the average molten steel flow rate, the lifting time can be prolonged and the floating separation can be promoted by the turbulence at the time of pouring. In order to reduce the average molten steel flow rate in the tundish, a tundish is used in which the cross-sectional area of the channel at the molten steel injection position in the tundish is larger than the cross-sectional area of the molten steel at the molten steel injection position. (The cross-sectional area of the flow passage at the molten steel injection position into the mold) / (the cross-sectional area of the flow passage at the molten steel injection position from the bore) exceeds 1.00, but the ratio is preferably 1.50 or more. On the other hand, the upper limit of the ratio is about 5.0.

(2) Ar injection

The casting is performed while blowing Ar at a position of 50 mm or more from the upper part of the discharge hole where the molten steel in the nozzle becomes non-filled, so that the Ca-based inclusion and the Ar bubble are incorporated in the nozzle and the mold to promote flotation separation. In order to obtain this effect, it is preferable to set the Ar flow rate to 0.04 L / t or more. The Ar flow rate is more preferably 0.10 L / t or more, and still more preferably 0.20 L / t or more. On the other hand, when the Ar flow rate exceeds 9.7 L / t, Ar bubbles remain in the surface layer of the slab, and the Ar bubble easily remains as defects on the steel sheet. Therefore, the Ar flow rate is preferably 9.7 L / t or less, more preferably 9.0 L / t or less, and further preferably 8.0 L / t or less.

(3) Solidification rate

Generally, when the solidification rate is high, inclusions existing in the vicinity of the solidification interface tend to be introduced into the interface, and when the solidification rate is small, a part of the inclusions is pushed out from the solidification interface to the center of the non-solidification. In the present invention, the solidification rate is made small so that inclusions are not accumulated in the surface layer portion of the steel sheet. Concretely, the solidification rate at a position of 1 to 3 m from the meniscus position of the molten steel in the mold to which the " area from the surface to the depth of 5 mm " Or less. The solidification rate is preferably 0.22 mm / s or less, more preferably 0.18 mm / s or less. On the other hand, the lower limit of the solidification rate is about 0.05 mm / s from the viewpoint of productivity and the like. The solidification rate can be adjusted by controlling the water density of the cooling water and the casting speed.

In the present invention, the steel sheet can be produced by performing hot rolling in accordance with a conventional method, or by further subjecting the steel sheet to reheating after the hot rolling, and then heat-treating the steel sheet after the casting as described above . In addition, a steel pipe for a line pipe can be produced by a general method using the steel sheet. The steel pipe for a line pipe obtained by using the steel sheet of the present invention is also excellent in HIC resistance and toughness.

This application claims the benefit of priority based on Japanese Patent Application No. 2013-073310 filed on March 29, 2013. The entire contents of the specification of Japanese Patent Application No. 2013-073310 filed on March 29, 2013 are incorporated herein by reference.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is of course not limited by the following Examples, and it is of course possible to carry out the present invention by appropriately modifying it within the range suitable for the purposes of the preceding and latter parts And they are all included in the technical scope of the present invention.

A steel having the composition shown in Table 1 was dissolved and continuous casting was conducted to obtain a slab (slab) having a thickness of 280 mm. The conditions of the continuous casting in the production process are as shown in Table 2. In the column of "(1) flow path cross-sectional area" in Table 2, when a tundish having a cross-sectional area of the flow path at the molten steel injection position to the mold is larger than the cross-sectional area of the flow path at the molten steel injection position from the take- Quot ;, and " x " if not. On the other hand, in the case of "O" in the present embodiment, a tundish having a ratio of (cross-sectional area of the flow path at the molten steel injection position from the thread) / (cross-sectional area of the flow path at the molten steel injection position to the mold) of 1.05 or more was used . In the column of " (2) Ar blowing " in Table 2, when casting was performed while blowing Ar at a flow rate of 0.04 to 9.7 L / t from a position of 50 mm or more from the discharge hole of the injection nozzle, &Quot; x ". In the column of "(3) solidification rate" in Table 2, the solidification rate at a position of 1 to 3 m from the meniscus position of molten steel in the mold toward the drawing direction was 0.26 mm / s or less, Quot ;, and the case of not performing the solidification at the above solidification rate was " x ".

Thereafter, the steel strip produced by the continuous casting was heated so as to have a temperature of 1050 to 1250 占 폚, and then "TMCP" (Thermo Mechanical Control Process) or "QT" (Quenching (sheet thickness: 12 to 90 mm) having various compositional compositions were obtained by two hot rolling and cooling methods as shown in Fig. In the above-mentioned "TMCP", hot rolling is performed so that the cumulative reduction rate of 900 ° C. or higher is 30% or more at the surface temperature of the steel sheet, and the hot rolling is performed so that the cumulative rolling reduction of 700 ° C. or more and 900 ° C. or less is 20% So that the rolling finish temperature was 700 ° C or more and less than 900 ° C. Thereafter, water cooling was started from a temperature of 650 ° C or higher, and water cooling was stopped at a temperature of 350-600 ° C, and then air was further cooled to room temperature. In the above-mentioned "QT", hot rolling was performed, followed by air cooling to room temperature, reheating at a temperature of 850 ° C. or more and 950 ° C. or less to quench and tempering at 600 to 700 ° C.

Cmax / Cave was measured using each steel sheet as shown below. Further, the HIC test was performed to evaluate the HIC property, and the Charpy impact test was carried out to evaluate the toughness.

[Measurement of Cmax / Cave]

The distribution of Ca concentration in a region from the surface to a depth of 5 mm in the plate thickness direction of the steel sheet was measured by fluorescence spectroscopy. Specifically, in order to peel off the scale layer of the steel sheet at first, the surface of the steel sheet was ground to 0.5 mm, and the Ca concentration of the grinding surface corresponding to the surface of the steel sheet was measured. Subsequently, after 0.5 mm of grinding in the plate thickness direction, the Ca concentration of the grinding surface was measured. This was repeatedly performed at a pitch of 0.5 mm in the plate thickness direction to measure the Ca concentration at a 10-section cross-section from the surface to a depth of 5 mm in the plate thickness direction. Cmax / Cave was obtained by setting the maximum value of Ca concentration in 10 sections as Cmax and the average value of Ca concentration in 10 sections as Cave.

[HIC test (NACE test)]

The HIC test was conducted and evaluated in accordance with NACE standard TM0284-2003. Specifically, three test specimens (size: sheet thickness x (width) 100 mm x (rolling direction) 20 mm) from the 1/4 W position and the 1/2 W position in the width direction of each steel sheet, . The test piece was immersed in a mixed aqueous solution containing 0.5% NaCl and 0.5% acetic acid at 25 ° C saturated with 1 atm of hydrogen sulfide for 96 hours and subjected to a cross section evaluation according to NACE standard TM0284-2003 FIGURE 3 to calculate a Crack Length Ratio , The ratio (%) of the crack length to the width of the test piece, and the crack length ratio). When the CLR was 3% or less, the HIC performance was evaluated to be excellent (O), and when the CLR was 3% or more, the HIC performance was poor (X).

[Charpy impact test]

After the NACE test, three Charpy test pieces having a thickness of 5 mm in the sheet thickness direction and 10 mm in the rolling direction were taken in the direction perpendicular to the rolling direction from the surface of the test piece according to ASTM A370, and notched in the thickness direction of the steel sheet. The Charpy impact test was carried out in accordance with ASTM A370, and the test temperature was varied from 0 ° C to 80 ° C in various ways to obtain the Charpy absorbed energy at 0% brittle fracture surface, that is, the upper shelf energy. When the upper shelf energy was 125 J or more, it was evaluated as excellent in toughness.

The results are shown in Table 2.

The following can be seen from Table 1 and Table 2. No. 1 to 13 and No. 22 to 26 show that the composition satisfies the composition of the present invention and the Cmax / Cave of the surface layer portion of the steel sheet satisfies the range defined by the present invention, so that the HIC resistance is excellent and the toughness is also excellent have.

On the contrary, 14 and 27, the Cmax / Cave of the surface layer portion of the steel sheet satisfied the range specified in the present invention, but the component composition (Ca / S) deviates from the present invention, resulting in poor HIC resistance. In addition, 15 to 21 and No. 28 to 31, toughness deteriorated because Cmax / Cave of the surface layer portion of the steel sheet did not satisfy the range specified in the present invention. Especially No. 15 to 19, 21, 28, 29 and 31, although the HIC property was secured, the toughness deteriorated.

Fig. 2 is a graph showing the relationship between Cmax / Cave and upper shelf energy obtained using the results of Table 2 above. Fig. It can be seen from Fig. 2 that the Cmax / Cave should be 1.20 or less in order to obtain excellent toughness with an upper shelf energy of 125 J or more.

Since the steel sheet according to the present invention has excellent resistance to organic hydrogen cracking and toughness, they are suitably used for line pipes, pressure vessels, storage tanks, etc. for transporting natural gas and crude oil.

Claims (5)

C: 0.02 to 0.15% (% means mass%, the same applies hereinafter)
Si: 0.02 to 0.50%
Mn: 0.6 to 2.0%
P: more than 0% and not more than 0.030%
S: more than 0% and not more than 0.003%
Al: 0.010 to 0.08%
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and
O (oxygen): more than 0% and not more than 0.0045%, the balance being iron and inevitable impurities,
The ratio of Ca to S (Ca / S) is 2.0 or more,
(Cmax / Cave) of the maximum Ca concentration (Cmax) in the region from the surface to the depth of 5 mm in the plate thickness direction and the average Ca concentration (Cave) in the region is 1.20 or less. Steel plate. The method according to claim 1,
The steel sheet further contains at least one element selected from the group consisting of the following (a) and (b) as another element.
(a) B: more than 0% and not more than 0.005%
V: more than 0% and not more than 0.1%
Cu: more than 0% to 1.5%
Ni: more than 0% and not more than 1.5%
Cr: more than 0% and not more than 1.5%
Mo: more than 0% and not more than 1.5%, and
Nb: more than 0% and not more than 0.06%
(b) Ti: more than 0% and not more than 0.03%
Mg: more than 0% and not more than 0.01%
REM: more than 0% and not more than 0.02%, and
Zr: more than 0% and not more than 0.010% 3. The method according to claim 1 or 2,
Steel plate for line pipe. 3. The method according to claim 1 or 2,
Steel plates for pressure vessels. A steel pipe for a line pipe produced by using the steel sheet according to claim 1 or 2.

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