WO2014157215A1

WO2014157215A1 - Steel plate with excellent hydrogen-induced cracking resistance and toughness of the weld heat affected zone, and steel tube for use as line pipe

Info

Publication number: WO2014157215A1
Application number: PCT/JP2014/058311
Authority: WO
Inventors: 喜一郎田代; 加藤　拓; 進佑佐藤; 孝司三宅; 晴弥川野
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: KR20150119958A; CN105074036A; EP2980235A4; EP2980235A1; JP6165088B2; CN105074036B; EP2980235B1; KR101709033B1; JP2014208891A

Abstract

This steel plate has excellent hydrogen-induced cracking resistance and toughness of the weld heat affected zone (HAZ toughness), and this steel tube is obtained using said steel plate. This steel plate 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 Ca content, S content and O content satisfy (Ca-1.25S)/O ≦ 1.8, and further, in the region from the surface to 5mm deep in the thickness direction, there are 2.0 per mm² or fewer Ca-based inclusions that have a 50μm or longer major axis or long side, and 5×10² per μm² or more TiN's that have a 300nm or shorter major axis or long side.

Description

Steel sheets and line pipe steel pipes with excellent resistance to hydrogen-induced cracking and toughness of weld heat affected zone

The present invention is suitable for use in natural gas / crude oil transportation line pipes, pressure vessels, storage tanks, etc., and is obtained by using a steel plate excellent in hydrogen-induced crack resistance and toughness of weld heat affected zone, and the steel plate. The present invention relates to a steel pipe for a line pipe having excellent resistance to hydrogen-induced cracking and toughness of a heat affected zone.

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. Hydrogen-induced cracking (hereinafter sometimes referred to as “HIC”) is the intrusion of hydrogen into the steel material due to the corrosion reaction caused by hydrogen sulfide or the like, and this invaded hydrogen is converted into MnS or Nb (C , N) and other non-metallic inclusions are known to be cracks caused 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, in Patent Document 1, S / Ca <0.5, a large amount of Ca is contained with respect to S, and the degree of segregation of Mn at the center of the plate thickness is reduced and MnS is suppressed to suppress hydrogen-induced cracking. A steel material with improved is disclosed. 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 a Ca-based oxysulfide by a parameter formula including Ca, O, and S contents.

Although these conventional technologies can suppress HIC, the weld heat-affected zone on the surface layer may undergo brittle fracture and the toughness may deteriorate significantly. Hydrogen-induced crack resistance and the toughness of the weld heat-affected zone It has been difficult to achieve both (HAZ toughness).

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

The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to realize a steel plate and a steel pipe excellent in hydrogen-induced crack resistance and weld heat-affected zone toughness (HAZ toughness). is there.

The steel sheet excellent in the hydrogen-induced crack resistance of the present invention and the toughness of the weld heat affected zone, which has solved the above problems,
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%,
Ti: 0.003 to 0.030%,
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 Ca, S and O satisfy (Ca-1.25S) /O≦1.8,
Further, in the region from the surface to the depth of 5 mm in the plate thickness direction, the number of Ca-based inclusions having a long diameter or long side of 50 μm or more is 2.0 pieces / mm ² or less, and TiN having a long diameter or long side of 300 nm or less. Is 5 × 10 ² pieces / μm ² or more.

The steel sheet, 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 1.0% or less,
Ni: more than 0% and 1.5% or less,
Cr: more than 0% and 1.0% or less,
One or more elements selected from the group consisting of Mo: more than 0% and 1.0% or less, and Nb: more than 0% and 0.06%,
(B) Mg: more than 0% and 0.01% or less,
One or more elements selected from the group consisting of REM: more than 0% and 0.02% or less and Zr: more than 0% and 0.010% or less may be included.

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 inclusions present in the surface layer portion of the steel sheet are appropriately controlled, it is possible to provide a steel sheet or steel pipe excellent in hydrogen-induced crack resistance and toughness of the weld heat affected zone.

The inventors of the present invention have made extensive studies to solve the above-mentioned problems. First, the present inventors performed HIC tests specified in NACE (National Association of Corrosion and Engineer) TM0284 for various steel plates, and evaluated HIC resistance. In the NACE test, HIC is generated after a test piece, that is, a steel sheet, is immersed in a mixed solution of pH 2.7 of 5% NaCl solution and 0.5% acetic acid saturated with 1 atm hydrogen sulfide gas for 96 hours. It is a test to evaluate. On the other hand, in order to confirm the toughness of the weld heat affected zone (HAZ), a welding reproduction test simulating welding with a heat input of 40 kJ / cm was performed, and then a Charpy impact test was performed as shown in the examples described later. It was.

As a result of these tests, the HAZ toughness of the steel sheet surface layer part was significantly reduced compared to other parts (for example, the center part of the plate thickness) even though excellent HIC resistance was ensured in the NACE test. There was an example. As a result of investigating this cause in detail, it was first found that coarse Ca-based inclusions having a long diameter or a long side of 50 μm or more existed in the surface layer of the steel sheet, and this was the starting point for brittle fracture. In addition, the above-mentioned “major axis or long side” means the major axis when the shape of the inclusion is a circle or an ellipse, etc. Means the long side.

From these facts, conventionally, a large amount of Ca is added for the purpose of suppressing MnS, and therefore, Ca-based inclusions having a large contact angle with the molten steel, particularly Ca-based oxides, are formed, which is in the course of solidification in the manufacturing process. Forms agglomerates, coarsens, floats, and easily accumulates on the surface layer of the steel sheet. As a result, in the heat affected zone of the surface layer, coarse Ca-based inclusions become the starting point of brittle fracture, and HAZ toughness deteriorates it is conceivable that.

Therefore, the inventors investigated the relationship between the HAZ toughness and coarse Ca-based inclusions having a major axis or long side of 50 μm or more existing in a region from the surface to a depth of 5 mm in the thickness direction, that is, in the surface layer portion of the steel plate. As a result, excellent HAZ toughness, ΔvTrs ([1/2 t vTrs] − [surface vTrs]) is 0 ° C. or more and the fracture surface transition temperature of the surface layer is room temperature (25 ° C.). In order to achieve the following, it has been found that the number density of coarse Ca-based inclusions having a major axis or long side of 50 μm or more must be suppressed to 2.0 / mm ² or less. In the present invention, the “Ca inclusions” are, as described in Examples below, the amount of Ca (% by mass) when all elements except S, O, and N are 100% by mass. This refers to inclusions of 60% by mass or more. Examples of the Ca-based inclusion include Ca oxide, Ca sulfide, Ca oxysulfide, and composite inclusions of these and other inclusions. The number density of the Ca inclusions of 50 μm or more is preferably 1.8 pieces / mm ² or less, more preferably 1.5 pieces / mm ² or less, and most preferably 0 piece / mm ² .

In the present invention, in order to further secure HAZ toughness, a large number of TiN having a major axis or a long side of 300 nm or less is dispersed. TiN suppresses the coarsening of austenite grains during welding heating, and acts as a transformation nucleus of intragranular ferrite in the cooling process after welding heating, contributing to refinement of the structure of the weld heat affected zone. In order to obtain this effect, the number density of TiN having a long diameter or a long side of 300 nm or less is set to 5 × 10 ² pieces / μm ² or more. The number density of TiN is preferably 8 × 10 ² pieces / μm ² or more, more preferably 10 × 10 ² pieces / μm ² or more, and further preferably 20 × 10 ² pieces / μm ² or more. The more TiN is, the more preferable, and the upper limit of the number density is not particularly set, but the upper limit is about 150 × 10 ² pieces / μm ² from the component composition range of the present invention.

The lower limit of the target TiN size is approximately 50 nm or more, which can be recognized, for example, at an observation magnification of 100,000 using a transmission electron microscope (Transmission Electron Microscope, TEM), as shown in the examples described later. . The number density of the Ca-based inclusions and the number density of TiN are obtained by the method described in the examples described later.

In order to ensure excellent HIC resistance and HAZ toughness, it is necessary to control the component composition of the steel material such as a steel plate and a steel pipe obtained by using the steel plate, in addition to the control of the steel plate surface layer. Furthermore, in order to ensure properties other than the above-mentioned HIC resistance, such as high strength and excellent weldability, which are required as steel plates for line pipes and steel plates for pressure vessels, for example, 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. On the other hand, if the amount of Si is excessive, island-shaped martensite is generated, and HIC is generated and propagated, and the HAZ toughness deteriorates. 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. The amount of Mn is preferably 1.8% or less, more preferably 1.5% or less, and still more preferably 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.

[Ti: 0.003-0.030%]
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, the Ti amount is set to 0.003% or more. The amount of Ti is preferably 0.005% or more, 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 the content is made 0.030% or less. The amount of Ti is preferably 0.025% or less, more preferably 0.022% or less, still more preferably 0.020% or less, and still more preferably 0.018% 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 0.0040% or less, more preferably 0.0035% or less, and still more preferably 0.0030% 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. The N amount is preferably 0.008% or less, and more preferably 0.0060% or less.

[O (oxygen): 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]
As described above, S forms MnS as a sulfide inclusion, and as a result of extending by rolling, the HIC resistance is most deteriorated. For this reason, Ca is added to control the form of the sulfide inclusions in the steel as CaS, thereby detoxifying S against HIC resistance. In order to fully exhibit this effect, Ca / S needs to be 2.0 or more. Ca / S is 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.

[(Ca-1.25S) /O≦1.8]
In order to suppress CaO that easily forms agglomerates among Ca-based inclusions, the Ca amount (Ca-1.25S) obtained by subtracting the Ca component present as sulfide (CaS) from the total Ca amount in the steel, It must be ensured that the amount of O is not excessive. When the amount of Ca (Ca-1.25S) is excessive with respect to the amount of O, CaO is likely to be formed as oxide inclusions, and the aggregated coalescence (coarse Ca inclusions) of CaO is formed in the surface layer portion. It becomes easy to be formed in large quantities. In order to suppress this, the present inventors examined the relationship between (Ca-1.25S) / O and HAZ toughness. To obtain excellent HAZ toughness, (Ca-1.25S) / O was used. I found that it was necessary to make it 1.8 or less. (Ca-1.25S) / O is preferably 1.40 or less, more preferably 1.30 or less, still more preferably 1.20 or less, and particularly preferably 1.00 or less. Note that the lower limit of (Ca-1.25S) / O is about 0.1 from the viewpoint of suppressing Al ₂ O ₃ which is likely to form an aggregated coal like CaO.

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) Strength and toughness can be further increased by containing one or more elements selected from the group consisting of B, V, Cu, Ni, Cr, Mo, and Nb in the following amounts, b) By containing one or more elements selected from the group consisting of Mg, REM, and Zr in the following amounts, HAZ toughness can be further enhanced, desulfurization can be promoted, and HIC resistance can be further improved. . 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. The amount of B is more preferably 0.0005% or more, and further preferably 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. The amount of B is more preferably 0.004% or less, and still more preferably 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 1.0% 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 1.0%, the toughness deteriorates, so it is preferable to make it 1.0% or less. The amount of Cu is more preferably 0.5% or less, still more preferably 0.35% 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 1.0% 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 1.0%, the HAZ toughness deteriorates. Therefore, the Cr content is preferably 1.0% or less. The amount of Cr is more preferably 0.5% or less, and still more preferably 0.35% or less.

[Mo: more than 0% and 1.0% 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 1.0%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount is preferably 1.0% or less, more preferably 0.5% or less, and still more preferably 0.35% 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.

[Mg: more than 0% and 0.01% or less]
Mg is an element effective for improving the HAZ toughness through refinement of crystal grains, and is an element that exhibits a desulfurization action and is effective for improving the HIC resistance. In order to obtain these effects, the Mg content is preferably 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 0.0050% or less.

[REM: more than 0% and 0.02% or less]
REM (rare earth element) is an element that effectively suppresses the generation of MnS by the desulfurization action and improves the resistance to hydrogen-induced cracking, and forms an oxide to effectively improve the HAZ toughness. 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 exhibits a desulfurization action and contributes to improvement of HIC resistance, 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. As a method for easily obtaining a steel sheet having a steel sheet surface layer part as defined above, a method for producing so as to satisfy all of the following (1) to (4) can be mentioned.

〔Production method〕
(1) Ca addition rate In order to easily obtain a steel sheet having the above specified steel sheet surface layer portion, for example, in the Ca addition process after performing LF and RH treatment, Ca (when using a compound, the amount of Ca alone) Conversion rate) is recommended to be 0.002 kg / min · t to 0.020 kg / min · t.

When the addition rate of Ca is less than 0.002 kg / min · t, the chemical reaction due to the addition of Ca is gentle, so that stirring of the molten steel becomes insufficient and the oxide composition cannot be homogenized. As a result, inclusions with high Ca concentration and other inclusions are separated, and coarse Ca inclusions are likely to be present in the surface layer portion. Therefore, the Ca addition rate is preferably 0.002 kg / min · t or more, and more preferably 0.004 kg / min · t or more. On the other hand, when the addition rate of Ca exceeds 0.020 kg / min · t, the chemical reaction due to the addition of Ca becomes excessively intense, and the molten metal surface is disturbed. Therefore, since the molten steel is in direct contact with the atmosphere, the amount of oxygen involved is increased and the absolute amount of oxide is increased. As a result, inclusions with a high Ca concentration also relatively increase and aggregate and coalesce, and coarse Ca-based inclusions are easily formed on the surface layer. For these reasons, the Ca addition rate is preferably 0.020 kg / min · t or less, and more preferably 0.018 kg / min · t or less.

(2) Time from the addition of Ca to the start of supply of molten steel to the tundish (TD) In order to homogenize the oxide composition after the addition of Ca, the time from the addition of Ca to the start of supply of molten steel to the tundish is set to 10 It is preferable to secure at least minutes. The time is more preferably 15 minutes or longer. From the viewpoint of productivity and the like, the upper limit of the time is about 120 minutes.

(3) Time from supply of molten steel to tundish (TD) to start of casting After starting supply of molten steel from ladle to tundish, 3 minutes or more by start of casting, more preferably 5 minutes or more, upper limit is It is preferable to start casting after holding for approximately 40 minutes or less. Thereby, aggregation and coalescence of inclusions in the tundish can be promoted, and Ca-based inclusions remaining in part can be floated and separated.

(4) Average cooling rate from 1500 ° C. to 1000 ° C. in the cooling stage during casting From the viewpoint of securing the necessary TiN number density, the average cooling rate from 1500 ° C. to 1000 ° C. in the cooling stage during casting is 10 It is preferable to set it as ° C / min or more. Thereby, coarsening of TiN can be suppressed, and a specified amount of fine TiN can be secured in the present invention. The average cooling rate is more preferably 15 ° C./min or more. On the other hand, when the average cooling rate exceeds 35 ° C./min, TiN does not precipitate and remains in the steel material as solute Ti and solute N, and in this case as well, a specified amount of fine TiN can be secured in the present invention. difficult. Therefore, the average cooling rate is preferably 35 ° C./min or less. The average cooling rate is more preferably 30 ° C./min or less. In addition, the said average cooling rate is an average cooling rate during the temperature of the slab surface being cooled from 1500 degreeC to 1000 degreeC.

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 HAZ toughness.

This application claims the benefit of priority based on Japanese Patent Application No. 2013-074705 filed on March 29, 2013. The entire contents of Japanese Patent Application No. 2013-074705 filed on March 29, 2013 are 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 the following Ca addition method and casting method. The conditions from Ca addition to continuous casting in the production process are as shown in Table 2. That is, in the Ca addition step after LF and RH treatment, the case where the Ca addition rate is 0.002 kg / min · t to 0.020 kg / min · t, “(1) Ca addition in Table 2” In the column of “Speed”, “◯” is indicated, and otherwise “X” is indicated. In addition, when the time from the addition of Ca to the start of molten steel supply to the tundish (TD) is set to 10 minutes or more, in the column of “(2) Time from the addition of Ca to the start of TD supply” in Table 2, “ ◯ ”, otherwise“ × ”. When the time from the start of the molten steel supply to the tundish (TD) to the start of casting is 3 minutes or more, “○” in the column of “(3) Time from the start of TD supply to the start of casting” in Table 2 When it was not, it was set as "x". Further, when the average cooling rate from 1500 ° C. to 1000 ° C. in the cooling stage during casting is 10 to 35 ° C./min, “(4) Average cooling rate of 1500 to 1000 ° C. in the cooling stage during casting is shown in Table 2. "" In the column of "", and "x" otherwise.

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: 20 to 51 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, as shown below, the number density of Ca inclusions and TiN was measured. Moreover, the HIC test was conducted to evaluate the HIC resistance and the HAZ toughness.

[Measurement of number density of Ca inclusions]
The Ca inclusions were measured using a scanning electron microscope (SEM) as follows. First, the observation magnification was 400 times, and the cross section perpendicular to the rolling direction (surface in the plate width direction × plate thickness direction) was observed at 10 or more at equal intervals from the steel plate surface to a depth of 5 mm in the plate thickness direction. One visual field size of the observation is about 50 mm ² .

And about the inclusion detected during observation, when the shape of the inclusion was circular or elliptical, the major axis was measured, and when the inclusion was rectangular, the long side was measured to determine the size of the inclusion. In the above measurement, one having an interval between inclusions of 10 μm or less was treated as one inclusion. Next, quantitative analysis was carried out by EDX (Energy Dispersive X-ray spectroscopy) for inclusions having a major axis or a long side of 50 μm or more. And the Ca amount (mass%) when all elements except S, O, and N are detected as 100% by mass from the detected elements is obtained, and the inclusion whose Ca amount is 60% by mass or more is determined to be a Ca-based inclusion. It was. In each of the 10 or more cross sections, the number of Ca inclusions was measured and converted to the number per unit area (mm ² ). Then, the maximum value among the number densities obtained in a plurality of cross sections was the number density of Ca-based inclusions having a major axis or a long side of 50 μm or more.

[Measurement of TiN Number Density]
TiN was measured using a transmission electron microscope (TEM) as follows. First, arbitrary five places were observed in the position of depth 5mm from the steel plate surface to the plate thickness direction. The observation magnification was 60,000 times or more, and the visual field size was 1.5 μm × 1.5 μm or more. Thus, by increasing the observation magnification, the number of inclusions can be measured more accurately. In addition, by widening the observation field and increasing the number of observations and adopting the average value, variation in the number of TiNs depending on the observation location can be reduced.

And the major axis or long side was measured about the inclusion detected during observation, and it was set as the size of the inclusion. Further, the inclusion having the major axis or the long side of 300 nm or less was quantitatively analyzed by EDX, and the inclusion containing 10% by mass or more of Ti and N was confirmed as TiN. Then, the number of TiN was measured, and the number per unit area (μm ² ) was calculated and obtained. The average value of the five locations was defined as the number density of TiN having a major axis or a long side of 300 nm or less.

[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. The test piece was immersed in an 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 FIGURE 3. , 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).

[Evaluation of HAZ toughness]
In order to evaluate the toughness of the weld heat affected zone (HAZ), the following welding reproduction test was performed by simulating welding with a heat input of 40 kJ / cm for each steel plate. That is, samples (each size is 12 mm × 33 mm × 55 mm) cut out from the surface layer (6 mm below the surface of the steel plate) and the central part (1/2 t) of the plate thickness are placed at the notch position with the Charpy test piece after the thermal cycle test. After heating to a temperature of 1350 ° C., it was held for 5 seconds and cooled. The average cooling rate at this time was adjusted so that the cooling time to 800 to 500 ° C. was 27 seconds.

From the sample simulating this welding, one side with a V notch in the plate thickness direction of the steel plate is 10 mm at the surface layer (6 mm below the steel plate surface) and the plate thickness center (1/2 t) as defined in ASTM A370. Three Charpy test pieces were collected. And the Charpy impact test was implemented by the method prescribed | regulated to ASTM A370, and the fracture surface transition temperature was measured. In this example, the difference between the fracture surface transition temperature of the surface layer (vTrs of the surface layer) and the fracture surface transition temperature of the central part of the plate thickness (vTrs of 1/2 t): ΔvTrs ([1/2 t vTrs] − [vTrs of the surface layer] ]) Was 0 ° C. or higher and the fracture surface transition temperature of the surface layer was room temperature (25 ° C.) or lower, it was evaluated as having excellent HAZ toughness.

Table 1 and Table 2 show the following. No. 1-14 and no. It can be seen that Nos. 23 to 26 are excellent in HIC resistance and HAZ toughness because coarse Ca-based inclusions are suppressed in the surface layer portion of the steel sheet and the number density of TiN is not less than a certain value.

On the other hand, No. Nos. 15 and 27 were inferior in HAZ toughness due to insufficient TiN number density. No. 16-20 and no. Nos. 28 to 30 were inferior in HAZ toughness due to the presence of many Ca-based inclusions in the surface layer portion of the steel sheet. No. In No. 20, since there were significantly more Ca-based inclusions in the surface layer of the steel sheet, the HAZ toughness was considerably inferior. No. Nos. 21 and 31 had a low Ca / S value and a large amount of MnS, resulting in poor HAZ toughness. No. Nos. 22 and 32 had a large value of (Ca-1.25S) / O and a large amount of Ca-based inclusions, particularly CaO, resulting in poor HAZ toughness.

Since the steel sheet according to the present invention is excellent in hydrogen-induced cracking resistance and HAZ 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%,
Ti: 0.003 to 0.030%,
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 Ca, S and O satisfy (Ca-1.25S) /O≦1.8,
Further, in the region from the surface to the depth of 5 mm in the plate thickness direction, the number of Ca-based inclusions having a long diameter or long side of 50 μm or more is 2.0 pieces / mm 2 or less, and TiN having a long diameter or long side of 300 nm or less. Is a steel sheet having excellent resistance to hydrogen-induced cracking and toughness of the heat-affected zone of welding, characterized by being 5 × 10 2 pieces / μm 2 or more.
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 1.0% or less,
Ni: more than 0% and 1.5% or less,
Cr: more than 0% and 1.0% or less,
Mo: more than 0% 1.0% or less, and Nb: more than 0% 0.06% or less (b) Mg: more than 0% 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.