WO2014157165A1

WO2014157165A1 - Steel plate with excellent hydrogen-induced cracking resistance and toughness, and line pipe steel tube

Info

Publication number: WO2014157165A1
Application number: PCT/JP2014/058222
Authority: WO
Inventors: 喜一郎田代; 加藤　拓; 進佑佐藤; 晴弥川野; 孝司三宅
Original assignee: 株式会社神戸製鋼所
Priority date: 2013-03-29
Filing date: 2014-03-25
Publication date: 2014-10-02
Also published as: KR101709034B1; JP6169025B2; JP2014208892A; EP2980238A1; EP2980238A4; KR20150121193A; EP2980238B1; CN105102652A; CN105102652B

Abstract

Provided is a steel plate which has excellent resistance against hydrogen-induced cracking and excellent toughness and which sufficiently suppresses minute HIC on the order of several μm's especially in the steel plate surface layer portion, which will be in a harsh environment with a high hydrogen concentration. This steel plate with excellent hydrogen-induced cracking resistance and toughness is characterized by comprising prescribed elements with the remainder being iron and unavoidable impurities, wherein the ratio (Ca/S) of Ca content to S content is 2.0 or greater, and the ratio (Cmax/Cave) of the greatest Ca concentration (Cmax) in the region from the surface to 5mm deep in the thickness direction to the average Ca concentration (Cave) of said region is 1.20 or less.

Description

Steel plates and line pipe steel pipes with excellent hydrogen-induced crack resistance and toughness

The present invention is suitable for natural gas / crude oil transportation line pipes, pressure vessels, storage tanks, and the like, a steel plate excellent in hydrogen-induced crack resistance and toughness, and hydrogen-induced crack resistance obtained using the steel plate The present invention relates to a steel pipe for line pipe having excellent toughness.

With the development of inferior resources such as crude oil and gas containing hydrogen sulfide, the line pipes, pressure vessels, and storage tanks used for transportation, refining, and storage of these materials are resistant to hydrogen-induced cracking resistance and stress corrosion cracking resistance. So-called sour resistance is required. In hydrogen-induced cracking (hereinafter sometimes referred to as “HIC”), hydrogen penetrates into the steel material due to the corrosion reaction caused by hydrogen sulfide or the like, and the invaded hydrogen is converted into MnS or Nb (C, It is known that the cracks are accumulated in non-metallic inclusions such as N) and are generated by gasification.

Particularly in the sour environment, it is known that the hydrogen concentration in the region from the surface to the depth of 5 mm in the thickness direction (hereinafter, this region may be referred to as “steel plate surface layer portion”) is higher than that in the central portion of the steel plate. It is known that cracks are likely to occur in the surface layer portion of the steel sheet starting from Ca-based oxide or Al-based oxide.

Conventionally, several techniques for improving hydrogen-induced crack resistance (hereinafter sometimes referred to as “HIC resistance”) have been proposed. For example, Patent Document 1 discloses a steel material that has improved resistance to hydrogen-induced cracking by suppressing the segregation degree of Mn, Nb, and Ti at the center of the plate thickness. Although this method can improve the HIC characteristics of the center segregation part, it is difficult to suppress cracks in parts other than the center segregation part because inclusions in parts other than the center segregation part are not sufficiently controlled. It seems to be. Patent Document 2 discloses a method of suppressing HIC starting from MnS or Ca-based oxysulfide by a parameter formula including Ca, O, and S contents. Although HIC resistance can be ensured by such a method, in the steel sheet surface layer portion where the hydrogen concentration is particularly high, as described later, it is difficult to ensure high toughness of the surface layer portion, because fine HIC is likely to occur. Seem.

JP 2010-209461 A Japanese Patent Laid-Open No. 06-136440

The present invention has been made by paying attention to the above-described circumstances, and its purpose is to provide a fine HIC of about several μm in a steel sheet surface layer portion that is particularly severe in a sour environment with a high hydrogen concentration. It is to realize a sufficiently suppressed steel sheet and steel pipe excellent in hydrogen-induced crack resistance and toughness.

The steel sheet excellent in hydrogen-induced crack resistance and toughness of the present invention that has solved the above problems is
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 0.030% or less,
S: more than 0% and 0.003% or less,
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 0.0045% or less, with the balance consisting of iron and inevitable impurities,
The ratio of Ca to S (Ca / S) is 2.0 or more, and 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 And the ratio (Cmax / Cave) is 1.20 or less.

The steel sheet may further contain one or more elements selected from at least one of the following groups (a) and (b) as other elements.
(A) B: more than 0% and 0.005% or less,
V: more than 0% and 0.1% or less,
Cu: more than 0% and 1.5% or less,
Ni: more than 0% and 1.5% or less,
Cr: more than 0% and 1.5% or less,
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 0.01% or less,
REM: a group consisting of more than 0% and 0.02% or less, and Zr: more than 0% and 0.010% or less

The above steel plate is suitable for line pipes and pressure vessels. Moreover, the steel pipe for line pipes manufactured using the said steel plate is also contained in this invention.

According to the present invention, since the Ca concentration distribution in the sheet thickness direction of the steel sheet is homogenized, even a fine HIC of about several μm is sufficiently suppressed in the steel sheet surface layer where the hydrogen concentration is particularly high, and as a result. It is possible to provide a steel plate or a steel pipe excellent in hydrogen-induced crack resistance and toughness.

FIG. 1 is a diagram showing the HIC generation rate for each Ca concentration of inclusions that are the origin of HIC. FIG. 2 is a graph showing the relationship between Cmax / Cave and upper shelf energy.

The inventors of the present invention have made extensive studies to solve the above-mentioned problems. First, the present inventors use various steel plates to re-specify the cause of the HIC occurrence in the steel plate surface layer, which is in the most severe condition in the sour environment. An HIC test (NACE test) was performed. In this NACE test, the generation of HIC after a test piece, that is, a steel sheet, was immersed for 96 hours in a mixed aqueous solution of 5% NaCl solution saturated with 1 atm hydrogen sulfide gas and 0.5% acetic acid at pH 2.7. It is a test to evaluate.

Next, the present inventors performed a Charpy test on the steel plate surface portion after the HIC test in accordance with ASTM A370. As a result, even when cracks were not observed in the prescribed “microscopic observation at a magnification of 100 times” in the NACE test, the Charpy test result after the HIC test was sometimes poor, that is, the toughness was inferior.

In order to investigate the cause, the above-mentioned microscopic observation was performed with the magnification increased, and it was found that many fine cracks were generated starting from the inclusions. That is, a large number of fine HIC below the observation limit, which is not observed in the NACE test at a magnification of 100 times as specified, is generated starting from inclusions, and these are factors that deteriorate the toughness after the HIC test. I first figured out that.

Furthermore, the inclusion composition which is the starting point of HIC generation including the fine HIC was investigated. In detail, the structure observation was performed about the steel plate which performed the HIC test (NACE test) as described in the Example mentioned later. Further, the Ca concentration of the observed inclusions was determined. The Ca concentration in the inclusion is a ratio (% by mass, hereinafter simply referred to as%) of Ca in the component composition excluding O and N constituting the inclusion. Of the inclusions with a Ca concentration in the inclusion of 50% or more, the ratio (%) of the inclusion that was the origin of HIC generation, and among the inclusions with a Ca concentration in the inclusion of 20% or less, HIC generation Each of the ratio (%) of the inclusion which became the starting point was calculated | required. The result is shown in FIG. In FIG. 1, the ratio of inclusions that are the starting point of HIC generation is indicated by “HIC generation rate (%)” on the vertical axis. As shown in FIG. 1, inclusions having a particularly high Ca concentration of 50% or more (hereinafter, inclusions having a Ca concentration of 50% or more are referred to as “Ca inclusions”) include HIC including the fine HIC. We found that it is likely to be the starting point of occurrence.

The Ca-based inclusions tend to agglomerate and coalesce locally during casting, and the presence of a large amount of Ca-based inclusions in the surface layer region of the steel sheet makes this Ca-based inclusion a starting point. It is considered that a large number of fine HICs that are difficult to confirm by the method are locally generated, which causes a decrease in toughness.

And in this invention, when controlling Ca area | region from the surface to a depth of 5 mm in the plate | board thickness direction, ie, the Ca type | system | group inclusion of a steel plate surface layer part, when there are many Ca type | system | group inclusions in a steel plate surface layer part, this steel plate It was considered that a portion with a high Ca concentration was present in the surface layer portion. Therefore, as shown in the examples described later, the maximum Ca concentration (Cmax) when the Ca concentration is measured at a plurality of locations at equal intervals from the surface to the depth of 5 mm in the plate thickness direction, and the average Ca concentration at the plurality of locations ( The ratio (Cmax / Cave) with the average Ca concentration (Cave) in the region from the surface to the depth of 5 mm in the plate thickness direction was used as a control factor for the amount of Ca-based inclusions in the surface layer portion of the steel plate.

Next, the relationship between the Cmax / Cave and the toughness of the steel sheet surface layer after the HIC test, specifically Charpy absorbed energy, particularly the upper shelf energy, was investigated. As a result, a clear correlation was recognized between the two as shown in Examples described later. That is, the present inventors first found that the toughness of the steel sheet surface layer portion after the HIC test can be improved by controlling the above (Cmax / Cave). Furthermore, as evaluated in the examples described later, it was found that Cmax / Cave should be 1.20 or less to achieve an upper shelf energy of 125 J or more as excellent toughness. The Cmax / Cave is preferably 1.19 or less, more preferably 1.18 or less, and still more preferably 1.15 or less. From the viewpoint of improving 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 plate surface layer and the steel is the same.

In order to ensure excellent HIC resistance, it is necessary to control the component composition of a steel material such as a steel plate and a steel pipe obtained by using the steel plate as well as the control of the surface layer portion of the steel plate. Furthermore, in order to ensure properties other than the above-mentioned HIC resistance such as excellent HAZ toughness and weldability, which are required, for example, as steel plates for line pipes and steel plates for pressure vessels, the component composition of the steel plates needs to be as follows: is there. Hereinafter, the reasons for defining each component will be described.

(Component composition)
[C: 0.02 to 0.15%]
C is an indispensable element for securing the strength of the base material and the welded portion, and needs to be contained by 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 too large, the HAZ toughness and weldability deteriorate. On the other hand, if the amount of C is excessive, NbC and island-shaped martensite that become the starting point of HIC and the fracture propagation path are likely to be generated. Therefore, the C amount needs to be 0.15% or less. The amount of C is preferably 0.12% or less, more preferably 0.10% or less.

[Si: 0.02 to 0.50%]
Si is an element that has a deoxidizing action and is effective in improving the strength of the base material and the welded portion. In order to obtain these effects, the Si content is set to 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 too large, weldability and toughness deteriorate. If the amount of Si is excessive, island martensite is generated and HIC is generated and progresses. Therefore, the amount of Si needs to be suppressed to 0.50% or less. The amount of Si is preferably 0.45% or less, 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, if the amount of Mn is too large, not only MnS is produced and the hydrogen-induced cracking resistance deteriorates, but also the HAZ toughness and weldability deteriorate. Therefore, the upper limit of the amount of Mn is 2.0% or less. Preferably it is 1.8% or less, More preferably, it is 1.5% or less, More preferably, it is 1.2% or less.

[P: more than 0% and 0.030% or less]
P is an element inevitably contained in the steel material. If the amount of P exceeds 0.030%, the toughness of the base material and the HAZ part is significantly deteriorated, and the resistance to hydrogen-induced cracking is also deteriorated. Therefore, in the present invention, the amount of P is suppressed to 0.030% or less. The amount of P is preferably 0.020% or less, more preferably 0.010% or less.

[S: more than 0% and 0.003% or less]
If S is too much, it is an element that produces a large amount of MnS and significantly deteriorates the resistance to hydrogen-induced cracking. Therefore, in the present invention, the upper limit of the amount of S 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. Thus, the smaller one is desirable from the viewpoint of improving hydrogen-induced crack resistance.

[Al: 0.010 to 0.08%]
Al is a strong deoxidizing element. When the amount of Al is small, the Ca concentration in the oxide increases, that is, Ca inclusions are easily formed in the surface layer of the steel sheet, and fine HIC is generated. Therefore, in the present invention, Al needs to be 0.010% or more. The amount of Al is preferably 0.020% or more, more preferably 0.030% or more. On the other hand, when there is too much Al content, the oxide of Al will produce | generate in cluster shape and will become the starting point of a hydrogen induced crack. Therefore, the Al amount 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 the effect | action which controls the form of sulfide, and there exists an effect which suppresses formation of MnS by forming CaS. In order to obtain this effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the Ca content exceeds 0.0060%, a large amount of HIC is generated starting from Ca-based inclusions. Therefore, in the present invention, the upper limit of the Ca amount is set to 0.0060%. The Ca content 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, suppresses coarsening of the austenite grains in the HAZ part, further promotes ferrite transformation, and improves the toughness of the HAZ part. In order to acquire this effect, it is necessary to contain N 0.001% or more. The N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, if the amount of N is too large, the HAZ toughness deteriorates due to the presence of solute N, so the amount of N needs to be 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.0060% or less.

[O: more than 0% and 0.0045% or less]
O (oxygen) is preferably low from the viewpoint of improving cleanliness. When a large amount of O is contained, in addition to deterioration of toughness, HIC is generated starting from oxide, and resistance to hydrogen-induced cracking is deteriorated. . From this viewpoint, the amount of O needs to be 0.0045% or less, preferably 0.0030% or less, 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 center of the plate thickness, and HIC is generated starting from MnS. In order to suppress this, Ca / S needs to be 2.0 or more, preferably 2.5 or more, more preferably 3.0 or more. The upper limit of Ca / S is about 15 from the Ca amount and S amount specified 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 inevitable impurities. In addition to the above elements,
(A) Inclusion of one or more elements selected from the group consisting of the following amounts of B, V, Cu, Ni, Cr, Mo, and Nb to further increase strength and toughness,
(B) To contain one or more elements selected from the group consisting of Ti, Mg, REM, and Zr in the following amounts to increase HAZ toughness and promote desulfurization to further improve HIC resistance. Can do. Hereinafter, these elements will be described in detail.

[B: more than 0% and 0.005% or less]
B enhances hardenability, increases the strength of the base metal and the welded portion, and bonds with N during the process of cooling the heated HAZ portion during precipitation, thereby precipitating BN and causing ferrite transformation from within the austenite grains. In order to promote, HAZ toughness is improved. In order to acquire this effect, it is preferable to contain B amount 0.0002% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.0010% or more. However, if the B content is excessive, the toughness between the base material and the HAZ part deteriorates or weldability deteriorates, so the B content is preferably 0.005% or less. More preferably, it is 0.004% or less, More preferably, it is 0.0030% or less.

[V: more than 0% and 0.1% or less]
V is an element effective for improving the strength. To obtain this effect, V is preferably 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%, weldability and base metal toughness deteriorate. Therefore, the V amount is preferably 0.1% or less, and more preferably 0.08% or less.

[Cu: more than 0% and 1.5% or less]
Cu is an element effective for improving the hardenability and increasing the strength. In order to acquire this effect, it is preferable to contain 0.01% or more of Cu. The amount of Cu is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Cu content exceeds 1.5%, the toughness deteriorates, so it is preferable to set it to 1.5% or less. The amount of Cu is more preferably 1.0% or less, still more preferably 0.50% or less.

[Ni: more than 0% and 1.5% or less]
Ni is an element effective for improving the strength and toughness of the base material and the welded portion. In order to obtain this effect, the Ni content 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, if Ni is contained in a large amount, it becomes extremely expensive as a structural steel material. Therefore, the Ni content is preferably 1.5% or less from an economical viewpoint. The amount of Ni is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Cr: more than 0% and 1.5% or less]
Cr is an element effective for improving the strength, and in order to obtain this effect, it is preferable to contain 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, if the Cr content exceeds 1.5%, the HAZ toughness deteriorates. Therefore, the Cr content 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% and 1.5% or less]
Mo is an element effective for improving the strength and toughness of the base material. In order to obtain this effect, the Mo amount 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, if the Mo amount exceeds 1.5%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount 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 0.06% or less]
Nb is an element effective for increasing strength and base metal toughness without degrading weldability. In order to obtain this effect, the Nb content is preferably 0.002% or more. The Nb amount is more preferably 0.010% or more, and still more preferably 0.020% or more. However, if the Nb content exceeds 0.06%, the toughness of the base material and the HAZ deteriorates. Therefore, in the present invention, the upper limit of the Nb amount is preferably 0.06%. The Nb amount is more preferably 0.050% or less, still more preferably 0.040% or less, and still more preferably 0.030% or less.

[Ti: more than 0% and 0.03% or less]
Ti is an element necessary for improving the toughness of the HAZ part in order to prevent coarsening of the austenite grains in the HAZ part during welding and promote ferrite transformation by precipitating as TiN in the steel. . Further, Ti is an element effective for improving the HIC resistance since it exhibits a desulfurization action. In order to obtain these effects, it is preferable to contain 0.003% or more of Ti. 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 part deteriorates due to solid solution of Ti and precipitation of TiC, so 0.03% or less is preferable. The amount of Ti is more preferably 0.02% or less.

[Mg: more than 0% and 0.01% or less]
Mg is an element effective for improving toughness through refinement of crystal grains, and is an element effective for improving HIC resistance since it exhibits a desulfurization action. In order to acquire this effect, it is preferable to contain Mg 0.0003% or more. The amount of Mg is more preferably 0.001% or more. On the other hand, since the effect is saturated even if Mg is excessively contained, the upper limit of the Mg content is preferably 0.01%. The amount of Mg is more preferably 0.005% or less.

[REM: more than 0% and 0.02% or less]
REM (rare earth element) is an element effective for suppressing the formation of MnS by the desulfurization action and enhancing the resistance to hydrogen-induced cracking. In order to exhibit such an effect, it is preferable to contain REM 0.0002% or more. The amount of REM is more preferably 0.0005% or more, and further preferably 0.0010% or more. On the other hand, the effect is saturated even if a large amount of REM is contained. Therefore, the upper limit of the REM amount is preferably 0.02%. From the viewpoint of increasing productivity by suppressing the clogging of the immersion nozzle during casting, the REM content is more preferably 0.015% or less, still more preferably 0.010% or less, and still more preferably 0.0050%. It is as follows. 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 0.010% or less]
Zr is an element that contributes to improvement of HIC resistance by desulfurization and also contributes to improvement of HAZ toughness by forming an oxide and finely dispersing it. In order to exert these effects, the Zr content is preferably 0.0003% or more. The amount of Zr is more preferably 0.0005% or more, still more preferably 0.0010% or more, and still more preferably 0.0015% or more. On the other hand, when Zr is added excessively, coarse inclusions are formed and the hydrogen-induced crack resistance and the base metal toughness are deteriorated. Therefore, the Zr content is preferably 0.010% or less. The amount of Zr is more preferably 0.0070% or less, still more preferably 0.0050% or less, and still more preferably 0.0030% or less.

In the above, the steel plate prescribed | regulated by this invention was demonstrated. The method for producing the steel plate of the present invention is not particularly limited as long as it is a method capable of obtaining the steel plate surface layer defined above. The following method is mentioned as a method for easily obtaining a steel plate having a steel plate surface layer part as defined above.

〔Production method〕
After melting to have the above component composition, the molten steel is poured into a mold through a ladle and tundish, but in order to obtain a steel sheet having a specified steel sheet surface layer portion in the present invention, the molten steel is poured into the tundish. It is recommended to satisfy all of the following (1) to (3) in the process of injecting and continuously casting.
(1) In the tundish, the flow path cross-sectional area at the molten steel injection position into the mold is made larger than the flow path cross-sectional area at the molten steel injection position from the ladle. Specifically, a tundish in which each channel cross-sectional area is designed in this way is used.
(2) Casting while blowing Ar at a flow rate of 0.04 to 9.7 L (liter) / t (ton) from a position of 50 mm or more from the upper part of the discharge hole of the injection nozzle.
(3) The solidification rate at a position of 1 to 3 m from the meniscus position of the molten steel in the mold in the drawing direction is set to 0.26 mm / s or less.

The conditions (1) to (3) will be described in order below.
(1) Channel cross-sectional area Ca-based inclusions have a high melting point and have a large contact angle with molten steel, so that agglomerates are easily formed and coarse inclusions are easily formed. Therefore, it is necessary to sufficiently float and separate this Ca-based inclusion inside the tundish. If this floating separation is insufficient, for example, the coarse Ca-based inclusion floats at the curved portion during continuous casting, and tends to accumulate on the surface layer. In order to sufficiently float and separate the inclusions in the tundish, it is preferable to reduce the average molten steel flow velocity in the tundish. By reducing the average molten steel flow velocity, the ascent time can be extended, and levitation separation can be promoted by turbulent flow during ladle injection. To reduce the average molten steel flow velocity in the tundish, use a tundish where the cross-sectional area at the pouring position of the molten steel into the mold in the tundish is larger than the cross-sectional area at the pouring position of the molten steel from the ladle. . The ratio represented by (flow path cross-sectional area at the molten steel injection position into the mold) / (flow path cross-sectional area at the molten steel injection position from the ladle) may be more than 1.00, but the ratio is preferably 1.50 or more. The upper limit of the ratio is about 5.0.

(2) Ar blowing By casting 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 is not filled, the Ca inclusions and Ar bubbles are combined in the nozzle and the mold. Floating separation can be promoted. In order to obtain this effect, the Ar flow rate is preferably 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 steel slab surface layer and easily remain as defects in 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 still more preferably 8.0 L / t or less.

(3) Solidification rate In general, when the solidification rate is high, inclusions existing in the vicinity of the solidification interface are easily taken into the interface, and when the solidification rate is low, some of the inclusions are unsolidified from the solidification interface to the central part. Extruded. In the present invention, by reducing the solidification rate, inclusions are prevented from accumulating in the steel sheet surface layer. Specifically, the solidification rate is 0.26 mm at a position 1 to 3 m from the meniscus position of the molten steel in the mold where the “region from the surface to a depth of 5 mm”, which is the object of the present invention, solidifies. / S or less. The solidification rate is preferably 0.22 mm / s or less, more preferably 0.18 mm / s or less. The lower limit of the solidification rate is approximately 0.05 mm / s from the viewpoint of productivity and the like. The solidification speed can be adjusted by controlling the water density of the cooling water and the casting speed.

In the present invention, the process after casting as described above is not particularly limited. Hot rolling is performed according to a conventional method, or after the hot rolling, the steel sheet is further reheated and heat treated. Can be manufactured. Moreover, the steel pipe for line pipes can be manufactured by the method generally performed using this steel plate. The steel pipe for line pipes obtained 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 content of the specification of Japanese Patent Application No. 2013-073310 filed on March 29, 2013 is incorporated herein by reference.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Steel with the composition shown in Table 1 was melted and a steel piece (slab) having a thickness of 280 mm was obtained by continuous casting. The conditions for continuous casting in the production process are as shown in Table 2. In the column of “(1) Channel cross-sectional area” in Table 2, a tundish in which the channel cross-sectional area at the molten steel injection position into the mold is larger than the channel cross-sectional area at the molten steel injection position from the ladle was used. In some cases, “◯” was indicated, and in other cases, “X” was indicated. In this example, in the case of “◯”, the ratio of (flow channel cross-sectional area at the molten steel injection position from the ladle) / (flow path cross-sectional area at the molten steel injection position into the mold) is 1.05 or more. A tundish was used. In addition, in the column of “(2) Ar blowing” in Table 2, casting is 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 upper part of the discharge hole of the injection nozzle. “○” was given, and “x” was given otherwise. Furthermore, in the column of “(3) Solidification rate” in Table 2, “○” indicates that the solidification rate at a position of 1 to 3 m from the meniscus position of the molten steel in the mold toward the drawing direction is 0.26 mm / s or less. The case where the solidification rate was not performed was defined as “x”.

Thereafter, the steel slab produced by continuous casting is heated to 1050 to 1250 ° C., and then “TMCP” (Thermo Mechanical Control Process) or “QT” in the “Hot rolling / cooling method” column of Table 2. As indicated by (Quenching and Tempering), steel plates (plate thickness: 12 to 90 mm) with various component compositions were obtained by two patterns of hot rolling and cooling methods. In the “TMCP”, hot rolling is performed so that the cumulative reduction rate of 900 ° C. or more at the surface temperature of the steel sheet is 30% or more, and further, the cumulative reduction rate of 700 ° C. or more and less than 900 ° C. is 20% or more. Rolling was performed so that the rolling end temperature was 700 ° C. or higher and lower than 900 ° C. Thereafter, water cooling was started from a temperature of 650 ° C. or higher, water cooling was stopped at a temperature of 350 to 600 ° C., and then air cooled to room temperature. In the “QT”, after hot rolling, air-cooled to room temperature, reheated to a temperature of 850 ° C. or higher and 950 ° C. or lower, quenched, and then tempered at 600 to 700 ° C.

And using each steel plate, Cmax / Cave was measured as shown below. Moreover, the HIC test was conducted to evaluate the HIC resistance, and the Charpy impact test was conducted to evaluate the toughness.

[Measurement of Cmax / Cave]
The distribution of Ca concentration in the region from the surface to a depth of 5 mm in the plate thickness direction of the steel plate was measured by fluorescence spectroscopic analysis. Specifically, in order to first peel off the scale layer of the steel sheet, 0.5 mm from the steel sheet surface was ground, and the Ca concentration of the ground surface corresponding to the steel sheet surface was measured. Next, after grinding 0.5 mm in the plate thickness direction, the Ca concentration of the ground surface was measured. This was repeated at a pitch of 0.5 mm in the plate thickness direction, and the Ca concentration of 10 cross sections from the surface to the depth of 5 mm was measured. Cmax / Cave was determined by setting Cmax as the maximum value of Ca concentration in 10 cross sections and Cave as the average value of Ca concentration in 10 cross sections.

[HIC test (NACE test)]
The HIC test was performed and evaluated according to NACE standard TM0284-2003. Specifically, three test pieces (size: plate thickness x (width) 100 mm x (rolling direction) 20 mm) were collected from each 1/4 W position and 1/2 W position in the width direction of each steel sheet. did. Then, 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 the cross section was evaluated according to NACE standard TM0284-2003 FIGURE3. , CLR (Crack Length Ratio, ratio (%) of crack length to crack width, crack length ratio) was measured. And when the said CLR was 3% or less, it evaluated that it was excellent in HIC resistance ((circle)), and the case where CLR was over 3% was evaluated as inferior to HIC resistance (x).

[Charpy impact test]
After the NACE test, three Charpy test pieces having a thickness of 5 mm and a rolling direction of 10 mm were sampled in a direction perpendicular to the rolling direction from just below the surface of the test piece in accordance with ASTM A370, and notched in the thickness direction of the steel sheet. The Charpy impact test was carried out according to ASTM A370, and the test temperature was changed in various ways from 0 ° C. to 80 ° C., and the Charpy absorbed energy, that is, the upper shelf energy when the brittle fracture surface ratio was 0% was determined. And it evaluated that the case where this upper shelf energy was 125J or more was excellent in toughness.

These results are shown in Table 2.

Table 1 and Table 2 show the following. No. 1 to 13, and No. 1 Nos. 22 to 26 satisfy the component composition specified in the present invention, and Cmax / Cave of the surface layer portion of the steel sheet satisfies the specified range in the present invention, so that they have excellent HIC resistance and toughness. I understand.

On the other hand, No. 14 and 27, Cmax / Cave of the steel sheet surface layer portion satisfies the range specified in the present invention, but the component composition (Ca / S) deviates from the range specified in the present invention, resulting in poor HIC resistance. became. No. 15-21 and no. In Nos. 28 to 31, Cmax / Cave of the surface layer portion of the steel sheet did not satisfy the range specified in the present invention, so that the toughness deteriorated. In particular, no. Nos. 15 to 19, 21, 28, 29 and 31 had poor toughness although HIC resistance could be secured.

FIG. 2 is a graph showing the relationship between Cmax / Cave and upper shelf energy obtained using the results shown in Table 2 above. From FIG. 2, it can be seen that 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 plates according to the present invention are excellent in hydrogen-induced crack resistance and toughness, they are suitably used for natural gas / crude oil transportation line pipes, pressure vessels, storage tanks, and the like.

Claims

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 0.030% or less,
S: more than 0% and 0.003% or less,
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 0.0045% or less, with the balance consisting of iron and inevitable impurities,
The ratio of Ca to S (Ca / S) is 2.0 or more, and 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 A steel sheet excellent in hydrogen-induced crack resistance and toughness, wherein the ratio (Cmax / Cave) is 1.20 or less.
Furthermore, the steel plate of Claim 1 which contains 1 or more types of elements selected from the group of at least any one of the following (a) and (b) as another element.
(A) B: more than 0% and 0.005% or less,
V: more than 0% and 0.1% or less,
Cu: more than 0% and 1.5% or less,
Ni: more than 0% and 1.5% or less,
Cr: more than 0% and 1.5% or less,
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 0.01% or less,
REM: a group consisting of more than 0% and 0.02% or less, and Zr: more than 0% and 0.010% or less
The steel plate according to claim 1 or 2, which is for line pipes.
The steel plate according to claim 1 or 2, which is for a pressure vessel.
A steel pipe for a line pipe manufactured using the steel sheet according to claim 1 or 2.