EP2980235B1

EP2980235B1 - 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: EP2980235B1
Application number: EP14772647.5A
Authority: EP
Inventors: Kiichirou Tashiro; Taku Kato; Shinsuke Sato; Takashi Miyake; Haruya KAWANO
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2013-03-29
Filing date: 2014-03-25
Publication date: 2018-05-30
Anticipated expiration: 2034-03-25
Also published as: WO2014157215A1; KR20150119958A; EP2980235A1; CN105074036A; JP2014208891A; EP2980235A4; JP6165088B2; CN105074036B; KR101709033B1

Description

[Technical Field]

The present invention relates to a steel plate with excellent hydrogen-induced cracking resistance and toughness of a weld heat affected zone and a steel pipe for a line pipe with excellent hydrogen-induced cracking resistance and toughness of a weld heat affected zone obtained using the steel plate suitable to a line pipe for transportation, a pressure vessel, a storage tank and the like of natural gas and crude oil.

[Background Art]

Accompanying development of bony resources such as crude oil and gas containing hydrogen sulfide, with respect to a line pipe, pressure vessel, and storage tank used for transportation, refining, and storage of them, so-called sour resistance such as hydrogen-induced cracking resistance and stress corrosion cracking resistance is required. The hydrogen-induced cracking (may be hereinafter referred to as "HIC") is known to be cracking caused by that hydrogen intrudes to the inside of steel accompanying a corrosion reaction by hydrogen sulfide and the like described above, and the hydrogen intruded gathers in non-metal inclusions and the like to begin with MnS and Nb (C, N) and is gasified.
Under a sour environment in particular, it is known that the hydrogen concentration of the region from the surface to 5 mm depth in the plate thickness direction (this region may be hereinafter referred to as "steel plate surface layer part") becomes higher compared to the steel plate center part, and it is known that cracking is liable to be caused from the origin of Ca-based oxide, Al-based oxide and the like of the steel plate surface layer part.
Conventionally, several proposals have been made with respect to the technology for improving the hydrogen-induced cracking resistance (may be hereinafter referred to as "HIC resistance"). For example, in Patent Literature 1, a steel is disclosed in which the hydrogen-induced cracking resistance is improved by achieving S/Ca<0.5 to contain much amount of Ca relative to S, lowering the segregation degree of Mn of the plate thickness center part, and suppressing MnS. According to this method, although the HIC property of the center segregation part can be improved, the inclusion of the portion other than the center segregation part is not controlled sufficiently, and therefore it is considered to be hard to suppress cracking of the portion other than the center segregation part. Also, in Patent Literature 2, a method is disclosed in which HIC originated from the MnS and Ca-based oxy-sulfide is suppressed by a parameter expression formed of the content of Ca, O, and S.
According to these prior arts, although suppression of HIC was possible, there was a case that the brittle fracture occurred and the toughness extremely deteriorated in the weld heat affected zone of the surface layer, and it was hard to achieve both of the hydrogen-induced cracking resistance and the toughness of the weld heat affected zone (HAZ toughness). EP 1 719 821 A1 discloses a steel product for line pipe excellent in resistance to hydrogen-induced cracking (HIC) and a line pipe produced by using the steel product.

[Citation List]

[Patent Literature]

[Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2010-209461
[Patent Literature 2] Japanese Unexamined Patent Application Publication No. H06-136440

[Summary of Invention]

[Technical Problem]

The present invention has been developed in view of such circumstances as described above, and its object is to achieve a steel plate and a steel pipe with excellent hydrogen-induced cracking resistance and toughness of a weld heat affected zone (HAZ toughness).

[Solution to Problem]

The steel plate with excellent hydrogen-induced cracking resistance and toughness of a weld heat affected zone of the present invention which could solve the problems described above is characterized to satisfy:

C: 0.02-0.15% (% means mass%, hereinafter the same);
Si: 0.02-0.50%;
Mn: 0.6-2.0%;
P: over 0% and 0.030% or less;
S: over 0% and 0.003% or less;
Al: 0.010-0.08%;
Ti: 0.003-0.030%;
Ca: 0.0003-0.0060%;
N: 0.001-0.01%; and
O (oxygen): over 0% and 0.0045% or less,

²

The steel plate may further include, as other elements,

(a) at least one element selected from a group consisting of:
- B: over 0% and 0.005% or less;
- V: over 0% and 0.1% or less;
- Cu: over 0% and 1.0% or less;
- Ni: over 0% and 1.5% or less;
- Cr: over 0% and 1.0% or less;
- Mo: over 0% and 1.0% or less; and
- Nb: over 0% and 0.06% or less, and
(b) at least one element selected from a group consisting of:
- Mg: over 0% and 0.01% or less;
- REM: over 0% and 0.02% or less; and
- Zr: over 0% and 0.010% or less.

The steel plate described above is suitable to the use of a line pipe and the use of a pressure vessel. Further, a steel pipe for a line pipe manufactured using the steel plate described above is also included in the present invention.

[Advantageous Effects of Invention]

According to the present invention, because the inclusions present in the steel plate surface layer are properly controlled, the steel plate and the steel pipe with excellent hydrogen-induced cracking resistance and toughness of a weld heat affected zone can be provided.

[Description of Embodiments]

The present inventors made a lot of intensive studies in order to solve the problems described above. First, with respect to various steel plates, the present inventors executed the HIC test specified in NACE (National Association of Corrosion Engineers) TM0284, and evaluated the HIC resistance. The NACE test is a test for evaluating occurrence of HIC after a specimen that is a steel plate is immersed for 96 hours in a mixed aqueous solution of pH 2.7 of 5% NaCl solution and 0.5% acetic acid saturated with hydrogen sulfide gas of 1 atm. On the other hand, in order to confirm the toughness of the weld heat affected zone (HAZ), the weld reproducibility test simulating welding with the weld heat input of 40 kJ/cm was executed and the Charpy impact test was subsequently performed as shown in the example described below.
As a result of executing these tests, there was a case in which the HAZ toughness of the steel plate surface layer part extremely deteriorated compared to other portions (the plate thickness center part for example) even in the steel plate that could secure excellent HIC resistance in the NACE test. As a result of detailed investigation on this reason, it was found out first that coarse Ca-based inclusions with 50 µm or more of the long diameter or long side were present in the steel plate surface layer part, and they became the origins of the brittle fracture. Also, "the long diameter or long side" described above means the long diameter when the shape of the inclusions is a circular shape, an elliptic shape and the like, and means the long side when the shape of the inclusions is a rectangular shape as described in the measurement of the size of the inclusions executed in the example described below.
From these facts, it is considered that, in the prior arts, because much amount of Ca was added with the aim of suppressing MnS, the Ca-based inclusions, particularly the Ca-based oxide, with large contact angle with respect to the molten steel were formed, they formed agglomerates in the middle of solidification in the manufacturing step, were coarsened and were made to float, were liable to gather in the steel plate surface layer part, as a result, the coarse Ca-based inclusions became the origins of the brittle fracture in the weld heat affected zone of the surface layer, and the HAZ toughness deteriorated.
Therefore, the present inventors investigated the relationship between the coarse Ca-based inclusions with 50 µm or more of the long diameter or long side present in the region from the surface to 5 mm depth in the plate thickness direction which was the steel plate surface layer part and the HAZ toughness. As a result, it was found out that, in order to achieve the excellent HAZ toughness which corresponded to that ΔvTrs ([vTrs of 1/2t]-[vTrs of surface layer]) was 0°C or more and the fracture appearance transition temperature of the surface layer was room temperature (25°C) or below, the number density of the coarse Ca-based inclusions with 50 µm or more of the long diameter or long side should be suppressed to 2.0 or less as shown in the example described below. Also, in the present invention, "Ca-based inclusions" described above means inclusions whose Ca amount (mass%) is 60 mass% or more when all elements excluding S, O and N are assumed to be 100 mass% as described in the example shown below. As the Ca-based inclusions, in addition to Ca oxide, Ca sulfide, and Ca oxysulfide, the complex inclusions of them and other inclusions and the like can be cited as an example. The number density of the Ca-based inclusions of 50 µm or more described above is preferably 1.8 or less, more preferably 1.5 or less, and most preferably 0 piece/mm².
In the present invention, in order to further secure the HAZ toughness, much of TiN with 300 nm or less of the long diameter or long side is dispersed. TiN suppresses coarsening of the austenitic grain in heating of welding, acts as the transformation nuclei of ferrite within the grain in the cooling step after the heating of welding, and contributes to miniaturization of the microstructure of the weld heat affected zone. In order to secure this effect, the number density of TiN with 300 nm or less of the long diameter or long side is made 5×10² pieces/µm² or more. The number density of TiN described above is preferably 8×10² pieces/µm² or more, more preferably 10×10² pieces/µm² or more, and still more preferably 20×10² pieces/µm² or more. Also, TiN is preferable to be as much as possible. The upper limit becomes 150×10² pieces/µm² from the componential composition range and the like of the present invention.
Further, the lower limit value of the size of TiN that becomes the object described above is 50 nm or more which can be recognized using a transmission electron microscope (TEM) with 100,000 magnifications for example as shown in the example below. The number density of the Ca-based inclusions and the number density of TiN described above are obtained by a method described in the example shown below.
In order to secure excellent HIC resistance and HAZ toughness, it is necessary to control the componential composition of the steel such as a steel plate and a steel pipe obtained using the steel plate in addition to control the steel plate surface layer part described above. Further, in order to also secure the properties other than the HIC resistance described above such as the high strength and excellent weldability required as a steel plate for a line pipe and a steel plate for a pressure vessel for example, the componential composition of the steel plate should be made as described below. Below, the reasons for determining each component will be described.

[Componential composition]

[C: 0.02-0.15%]

C is an indispensable element for securing the strength of the base plate and the weld part, and should be contained by 0.02% or more. C amount is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, when C amount is too much, the HAZ toughness and the weldability deteriorate. Also, when C amount is excessive, NbC and island martensite which become an origin of HIC and a fracture development route are liable to be formed. Therefore, C amount should be 0.15% or less. C amount is preferably 0.12% or less, and more preferably 0.10% or less.

[Si: 0.02-0.50%]

Si is an element having a deoxidizing action and effective in improving the strength of the base plate and the weld part. In order to secure such effects, Si amount is made 0.02% or more. Si amount is preferably 0.05% or more, and more preferably 0.15% or more. However, when Si amount is too much, the weldability and the toughness deteriorate. Also, when Si amount is excessive, island martensite is formed, HIC is generated and develops and the HAZ toughness deteriorates. Therefore, Si amount should be suppressed to 0.50% or less. Si amount is preferably 0.45% or less, and more preferably 0.35% or less.

[Mn: 0.6-2.0%]

Mn is an element effective in improving the strength of the base plate and the weld part, and is contained by 0.6% or more in the present invention. Mn amount is preferably 0.8% or more, and more preferably 1.0% or more. However, when Mn amount is too much, MnS is formed, not only the hydrogen-induced cracking resistance deteriorates, but also the HAZ toughness and the weldability deteriorate. Therefore, the upper limit of Mn amount is made 2.0% or less. Mn amount is preferably 1.8% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.

[P: over 0% and 0.030% or less]

P is an element inevitably included in steel. When P amount exceeds 0.030%, deterioration of the toughness of the base plate and the HAZ part is extreme, and the hydrogen-induced cracking resistance also deteriorates. Therefore, in the present invention, P amount is suppressed to 0.030% or less. P amount is preferably 0.020% or less, and more preferably 0.010% or less.

[S: over 0% and 0.003% or less]

S is an element that forms much amount of MnS and extremely deteriorates the hydrogen-induced cracking resistance when it is contained excessively, and therefore the upper limit of S amount is made 0.003% in the present invention. S amount is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. Thus, from the viewpoint of improving the hydrogen-induced cracking resistance, S amount is preferable to be as little as possible.

[Al: 0.010-0.08%]

A1 is a strong deoxidizing element. When Al amount is less, the Ca concentration in oxide is liable to increase or the Ca-based inclusions are liable to be formed in the steel plate surface layer part, and fine HIC is generated. Therefore, in the present invention, Al should be made 0.010% or more. Al amount is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, when Al amount is too much, the oxide of Al is formed in a cluster shape and becomes the origin of the hydrogen-induced cracking. Therefore, Al amount should be 0.08% or less. Al amount is preferably 0.06% or less, and more preferably 0.05% or less.

[Ti: 0.003-0.030%]

Ti is an element required for improving the toughness of the HAZ part because Ti prevents coarsening of the austenitic grain and promotes the ferritic transformation in the HAZ part in welding by precipitating as TiN in steel. Also, because Ti exhibits the desulfurizing action, Ti is an element effective also in improving the HIC resistance. In order to secure these effects, Ti amount is made 0.003% or more. Ti amount is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, when Ti amount becomes excessive, the toughness of the base plate and the HAZ part deteriorates because of solid solution of Ti and precipitation of TiC, and therefore Ti amount is made 0.030% or less. Ti amount is preferably 0.025% or less, more preferably 0.022% or less, further more preferably 0.020% or less, and still more preferably 0.018% or less.

[Ca: 0.0003-0.0060%]

Ca has an action of controlling the form of sulfide, and has an effect of suppressing formation of MnS by forming CaS. In order to secure this effect, Ca amount should be made 0.0003% or more. Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when Ca amount exceeds 0.0060%, much amount of HIC is generated from the origins of the Ca-based inclusions. Therefore, in the present invention, the upper limit of Ca amount is made 0.0060%. Ca amount is preferably 0.0040% or less, more preferably 0.0035% or less, and still more preferably 0.0030% or less.

[N: 0.001-0.01%]

N is an element precipitating as TiN in the steel microstructure, suppressing coarsening of the austenitic grain of the HAZ part, promoting the ferritic transformation, and improving the toughness of the HAZ part. In order to secure these effects, N should be contained by 0.001% or more. N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, when N amount is too much, the HAZ toughness deteriorates adversely because of presence of solid-solutionized N, and therefore N amount should be 0.01% or less. N amount is preferably 0.008% or less, and more preferably 0.0060% or less.

[O (oxygen): over 0% and 0.0045% or less]

O (oxygen) is preferable to be less from the viewpoint of improving the cleanliness. When much amount of O is contained, in addition to that the toughness deteriorates, HIC is generated from the origin of the oxide, and the hydrogen-induced cracking resistance deteriorates. From this viewpoint, O amount should be 0.0045% or less, is preferably 0.0030% or less, and more preferably 0.0020% or less.

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

As described above, S forms MnS as the sulfide-based inclusions and develops by rolling. As a result, S deteriorates the HIC resistance most. Therefore, Ca is added, the form of the sulfide-based inclusions in steel is controlled as CaS, and S is made harmless with respect to the HIC resistance. In order to exert this action and effect sufficiently, Ca/S should be 2.0 or more. Ca/S is preferably 2.5 or more, and more preferably 3.0 or more. Also, the upper limit of Ca/S is approximately 15 from Ca amount and S amount specified in the present invention.

[(Ca-1.25S)/O≤1.8]

In order to suppress CaO that is particularly liable to form agglomerates among the Ca-based inclusions, it should be made that the Ca amount obtained by deducting the Ca portion present as sulfide (CaS) from the whole Ca amount in steel (Ca-1.25S) does not become excessive with respect to O amount. When Ca amount (Ca-1.25S) is excessive with respect to O amount, CaO is liable to be formed as the oxide-based inclusions, and much amount of the agglomerates of the CaO (coarse Ca-based inclusions) are liable to be formed in the surface layer part. In order to suppress this event, the present inventors studied the relationship of (Ca-1.25S)/O and the HAZ toughness, and found out that (Ca-1.25S)/O should be 1.8 or less in order to secure excellent HAZ toughness. (Ca-1.255)/O is preferably 1.40 or less, more preferably 1.30 or less, further more preferably 1.20 or less, and especially preferably 1.00 or less. Also, from the viewpoint of suppressing Al₂O₃ that is liable to form agglomerates similarly to CaO, the lower limit value of (Ca-1.255)/O becomes 0.1.
The composition of steel (steel plate, steel pipe) of the present invention is as described above, and the remainder is iron and inevitable impurities. Also, in addition to the elements described above, (a) by further containing at least one element selected from a group consisting of B, V, Cu, Ni, Cr, Mo, and Nb of the amount described below, the strength and toughness can be improved further, and (b) by further containing at least one element selected from a group consisting of Mg, REM, and Zr of the amount described below, the HAZ toughness can be enhanced further, desulfurization is promoted, and the HIC resistance can be improved further. Below, these elements will be described in detail.

[B: over 0% and 0.005% or less]

B enhances the quenchability, increases the strength of the base plate and the weld part, is bonded with N in the process the HAZ part having been heated in welding is cooled to precipitate BN, promotes ferritic transformation from inside the austenitic grain, and therefore improves the HAZ toughness. In order to secure these effects, it is preferable to contain B amount by 0.0002% or more. B amount is more preferably 0.0005% or more, and still more preferably 0.0010% or more. However, when B amount becomes excessive, the toughness of the base plate and the HAZ part deteriorate, deterioration of the weldability is caused, and therefore B amount is preferably 0.005% or less. B amount is more preferably 0.004% or less, and still more preferably 0.0030% or less.

[V: over 0% and 0.1% or less]

V is an element effective in improving the strength, and, in order to secure this effect, it is preferable to contain V by 0.003% or more, and more preferably 0.010% or more. On the other hand, when V content exceeds 0.1%, the weldability and the base plate toughness deteriorate. Therefore, V amount is preferably 0.1% or less, and more preferably 0.08% or less.

[Cu: over 0% and 1.0% or less]

Cu is an element effective in improving the quenchability and increasing the strength. In order to secure these effects, it is preferable to contain Cu by 0.01% or more. Cu amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, because the toughness deteriorates when Cu content exceeds 1.0%, 1.0% or less is preferable. Cu amount is more preferably 0.5% or less, and still more preferably 0.35% or less.

[Ni: over 0% and 1.5% or less]

Ni is an element effective in improving the strength and toughness of the base plate and the weld part. In order to secure the effect, it is preferable to make Ni amount 0.01% or more. Ni amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when much amount of Ni is contained, the cost increases extremely as a structural steel, and therefore it is preferable to make Ni amount 1.5% or less from the economical viewpoint. Ni amount is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Cr: over 0% and 1.0% or less]

Cr is an element effective in improving the strength, and, in order to secure this effect, it is preferable to contain Cr by 0.01% or more. Cr amount is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when Cr amount exceeds 1.0%, the HAZ toughness deteriorates. Therefore it is preferable to make Cr amount 1.0% or less. Cr amount is more preferably 0.5% or less, and still more preferably 0.35% or less.

[Mo: over 0% and 1.0% or less]

Mo is an element effective in improving the strength and toughness of the base plate. In order to secure the effects, it is preferable to make Mo amount 0.01% or more. Mo amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when Mo amount exceeds 1.0%, the HAZ toughness and weldability deteriorate. Therefore Mo amount is preferably 1.0% or less, more preferably 0.5% or less, and still more preferably 0.35% or less.

[Nb: over 0% and 0.06% or less]

Nb is an element effective in enhancing the strength and base plate toughness without deteriorating the weldability. In order to secure the effects, it is preferable to make Nb amount 0.002% or more. Nb amount is more preferably 0.010% or more, and still more preferably 0.020% or more. However, when Nb amount exceeds 0.06%, the toughness of the base plate and HAZ deteriorates. Therefore, in the present invention, it is preferable that the upper limit of Nb amount is made 0.06%. Nb amount is more preferably 0.050% or less, further more preferably 0.040% or less, and still more preferably 0.030% or less.

[Mg: over 0% and 0.01% or less]

Mg is an element effective in improving the HAZ toughness through miniaturization of the grain, and is an element exhibiting the desulfurizing action and effective also in improving the HIC resistance. In order to secure these effects, it is preferable to make Mg amount 0.0003% or more. Ng amount is more preferably 0.001% or more. On the other hand, even when Mg is contained excessively, the effects saturate, and therefore it is preferable that the upper limit of Mg amount is made 0.01%. Mg amount is more preferably 0.0050% or less.

[REM: over 0% and 0.02% or less]

REM (rare earth element) is an element suppressing formation of MnS by the desulfurizing action to enhance the hydrogen-induced cracking resistance, and forming oxide to effectively act in improving the HAZ resistance. In order to exert such effects, it is preferable to contain REM by 0.0002% or more. REM amount is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, even when much amount of REM is contained, the effects saturate. Therefore, it is preferable that the upper limit of REM amount is made 0.02%. From the viewpoint of suppressing blockage of the immersion nozzle in casting and improving the productivity, REM amount is more preferably 0.015% or less, further more preferably 0.010% or less, and still more preferably 0.0050% or less. Also, in the present invention, the REM means the lanthanoid elements (15 elements from La to Lu), Sc (Scandium), and Y.

[Zr: over 0% and 0.010% or less]

Zr is an element exhibiting the desulfurizing action and contributing to improvement of the HIC resistance, and contributing also to improvement of the HAZ toughness by forming and finely dispersing oxide. In order to exert these effects, it is preferable to make Zr amount 0.0003% or more. Zr amount is more preferably 0.0005% or more, further 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 cracking resistance and the base plate toughness are deteriorated. Therefore, it is preferable to make Zr amount 0.010% or less. Zr amount is more preferably 0.0070% or less, further more preferably 0.0050% or less, and still more preferably 0.0030% or less.
The steel plate specified in the present invention has been described above. The method for manufacturing the steel plate of the present invention is not particularly limited as far as it is a method of obtaining the steel plate surface layer part specified above. As the method for easily obtaining the steel plate having the steel plate surface layer part specified above, a method for manufacturing the steel plate so as to satisfy all of (1)-(4) below can be cited.

[Manufacturing method]

(1) Ca adding rate

In order to easily obtain the steel plate having the steel plate surface layer part specified above, it is recommendable in the Ca adding process after performing the LF and RH treatment for example to make the adding rate of Ca (when a compound is used, the amount of the compound is converted to the amount of Ca only) 0.002 kg/min ·t-0.020 kg/min ·t.
When the adding rate of Ca is less than 0.002 kg/min ·t, because the chemical reaction by adding Ca is mild, stirring of the molten steel becomes insufficient, and the oxide composition cannot be homogenized. As a result, inclusions with high Ca concentration and inclusions other than that are separated, and coarse Ca-based inclusions are liable to be present in the surface layer part. Therefore, the adding rate of Ca 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 adding rate of Ca exceeds 0.020 kg/min ·t, the chemical reaction by adding Ca becomes excessively intense, and the molten steel surface becomes turbulent. Accordingly, because the molten steel directly contacts the atmospheric air, the entangled amount of oxygen increases, and the absolute amount of oxide increases. As a result, the inclusions with high Ca concentration relatively increase, coagulate, and are integrated, and coarse Ca-based inclusions are liable to be formed in the surface layer. From these facts, the adding rate of Ca is preferably 0.020 kg/min ·t or less, and more preferably 0.018 kg/min ·t or less.

(2) Time after adding Ca until start of molten steel supply to tundish (TD)

In order to homogenize the oxide composition after adding Ca, it is preferable to secure the time after adding Ca until the start of molten steel supply to the tundish by 10 min or more. The time is more preferably 15 min or more. Also, from the viewpoint of the productivity, the upper limit of the time is approximately 120 min.

(3) Time from start of molten steel supply to tundish (TD) until start of casting

It is preferable to start casting after holding the steel for 3 min or more, more preferably 5 min or more, and approximately 40 min or less as the upper limit after the start of molten steel supply from the ladle to the tundish until the start of casting. Thus, coagulation and integration of inclusions inside the tundish can be promoted, and the Ca-based inclusions remaining in a part can be made to float and separate.

(4) Average cooling rate from 1,500°C to 1,000°C in cooling stage in the middle of casting

From the viewpoint of securing required number density of TiN, it is preferable to make the average cooling rate from 1,500°C to 1,000°C in the cooling stage in the middle of casting 10°C/min or more. Thus, coarsening of TiN can be suppressed, and fine TiN of the amount specified in the present invention can be secured. The average cooling rate described above 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 steel as solid-solutionized Ti and solid-solutionized N, and it is hard to secure fine TiN of the amount specified in the present invention also in this case. Therefore, it is preferable to make the average cooling rate described above 35°C/min or less. The average cooling rate described above is more preferably 30°C/min or less. Also, the average cooling rate described above is the average cooling rate while the temperature of the surface of the slab is cooled from 1,500°C to 1,000°C.
In the present invention, the step after casting as described above is not particularly the object, and the steel plate can be manufactured by performing hot rolling according to an ordinary method, or by reheating and performing heat treatment after the hot rolling. Also, using the steel plate, a steel pipe for a line pipe can be manufactured by a method generally employed. The steel pipe for a line pipe obtained using the steel plate of the present invention is also excellent in the HIC resistance and the HAZ toughness.

[Examples]

Although the present invention will be described below more specifically referring to examples, the present invention is not to be limited by the examples below, it is a matter of course that the present invention can be also implemented with modifications being appropriately added within the range adaptable to the purposes described above and below, and any of them is to be included within the technical range of the present invention.
Steel with the componential composition shown in Table 1 was molten, and a slab with 280 mm thickness was obtained by the Ca adding method and the casting method described below. The conditions of Ca adding until continuous casting in the manufacturing step are as shown in Table 2. More specifically, the case the adding rate of Ca was made 0.002 kg/min ·t-0.020 kg/min ·t in the Ca adding step after performing the LF and RH treatment was marked with "○" and the case other than that was marked with "×" in the column "(1) Ca adding rate" of Table 2. Also, the case the time after adding Ca until the start of molten steel supply to the tundish (TD) was made 10 min or more was marked with "o" and the case other than that was marked with "x" in the column "(2) time after adding Ca until start of supply to TD" of Table 2. The case the time after the start of molten steel supply to the tundish (TD) until the start of casting was made 3 min or more was marked with "o" and the case other than that was marked with "×" in the column "(3) time from start of supply to TD until start of casting" of Table 2. Further, the case the average cooling rate from 1,500°C to 1,000°C in the cooling stage in the middle of casting was made 10-35°C/min was marked with "○" and the case other than that was marked with "×" in the column "(4) average cooling rate of 1,500°C-1,000°C in cooling stage in casting" of Table 2.
Thereafter, after heating the slab manufactured by continuous casting so as to become 1,050-1,250°C, the steel plate (plate thickness: 20-51 mm) with various componential compositions was obtained by the hot rolling/cooling method of 2 patterns as shown "TMCP" (Thermo Mechanical Control Process) or "QT" (Quenching and Tempering) in the column "hot rolling/cooling method" of Table 2. In "TMCP" described above, hot rolling was performed so that the cumulative draft of 900°C or above in terms of the surface temperature of the steel plate became 30% or more, and hot rolling was further performed so that the cumulative draft of 700°C or above and below 900°C became 20% or more with the rolling finish temperature of 700°C or above and below 900°C. Thereafter, water cooling was started from a temperature of 650°C or above, the water cooling was stopped at a temperature of 350-600°C, and air cooling was thereafter performed to the room temperature. Also, in "QT" described above, air cooling was performed to the room temperature after the hot rolling, the steel plate was reheated to a temperature of 850°C or above and 950°C or below and was quenched, and was thereafter subjected to tempering treatment at 600-700°C.
Also, using each steel plate, the number density of the Ca-based inclusions and TiN was measured as shown below. Further, the HIC test was performed, and evaluation of the HIC resistance and evaluation of the HAZ toughness were executed.

[Measurement of number density of Ca-based inclusions]

Measurement of the Ca-based inclusions was performed as follows using a scanning electron microscope (SEM). First, with the observation magnification of 400 times, 10 or more cross sections were observed with respect to the cross section perpendicular to the rolling direction (the plane of the plate width directionxplate thickness direction) at equal intervals from the steel plate surface to 5 mm depth in the steel thickness direction. The size of one field of view of the observation is approximately 50 mm².
Also, with respect to the inclusions detected during the observation, the long diameter was measured when the shape of the inclusion was a circular shape, an elliptic shape and the like, the long side was measured when the shape of the inclusion was a rectangular shape, and the long diameter or the long side was made the size of the inclusion. In the measurement, the inclusions with 10 µm or less of the interval of the inclusion and inclusion were treated as one inclusion. Next, with respect to the inclusion with 50 µm or more of the long diameter or long side, the quantitative analysis was executed by EDX (Energy Dispersive X-ray spectrometry). Also, the Ca amount (mass%) when the mass of all elements excluding S, O, and N from the detected elements was 100 mass% was obtained, and the inclusion in which this Ca amount was 60 mass% or more was made the Ca-based inclusion. In each cross section of 10 or more cross sections described above, the number of pieces of the Ca-based inclusion was measured and was converted to the number of pieces per unit area (mm²). Also, the maximum value among the number density obtained in the plural cross sections was made the number density of the Ca-based inclusions with 50 µm or more of the long diameter or long side.

[Measurement of number density of TiN]

Measurement of TiN was performed as follows using a transmission electron microscope (TEM). First, optional 5 locations were observed at the position of 5 mm depth from the steel plate surface in the plate thickness direction. The observation magnification was made 60,000 times or more, and the size of one field of view was made 1.5 µm×1.5 µm or more. By thus making the observation magnification large, the number of pieces of the inclusions can be measured more accurately. Also, by widening the observation field of view and increasing the number of observations and adopting the average value thereof, dispersion of the number of pieces of TiN according to the observation location can be reduced.
Further, with respect to the inclusion detected during the observation, the long diameter or the long side was measured which was made the size of the inclusion. Furthermore, with respect to the inclusion with 300 nm or less of the long diameter or long side, the quantitative analysis was executed by EDX, and the inclusion containing Ti and N by 10 mass% or more respectively was confirmed to be TiN. Also, the number of pieces of the TiN was measured, the number of pieces per unit area (µm²) was calculated and obtained, and the average value of 5 locations described above was made the number density of TiN with 300 nm or less of the long diameter or long side.

[HIC test (NACE test)]

The HIC test was performed for evaluation according to NACE standard TM0284-2003. More specifically, from 1/4W position and 1/2W position in the width direction of each steel plate, 3 pieces each or 6 pieces in total of the specimens (size: plate thickness×100 mm (width)×20 mm (rolling direction)) were taken. Also, the specimen was immersed for 96 hours in an aqueous solution containing 0.5% NaCl and 0.5% acetic acid of 25°C saturated with hydrogen sulfide of 1 atm, evaluation of the cross section was executed according to NACE standard TM0284-2003 FIGURE 3, and CLR (Crack Length Ratio: the ratio (%) of the total of the crack length relative to the specimen width) was measured. Further, the case the CLR was 3% or less was evaluated to be excellent in the HIC resistance (o), and the case the CLR exceeded 3% was evaluated to be poor in the HIC resistance (x).

[Evaluation of HAZ toughness]

In order to evaluate the toughness of the weld heat affected zone (HAZ), with respect to each steel plate, the welding reproducibility test described below was executed simulating welding with the weld heat input of 40 kJ/cm. More specifically, the sample cut out from each of the surface layer (6 mm below the steel plate surface) and the plate thickness center part (1/2t) (the size was 12 mm×33 mm×55mm for the both) was heated so that the portion that would become the notch position in the Charpy test specimen after the thermal cycle test became 1,350°C, was thereafter held for 5 s, and was cooled. The average cooling rate then was adjusted so that the cooling time to 800-500°C became 27 s.

From this sample simulating the welding, in each of the surface layer (6 mm below the steel plate surface) and the plate thickness center part (1/2t), the Charpy test specimens whose one side was 10 mm in which a V-notch was worked in the plate thickness direction of the steel plate were taken by 3 pieces each as specified in ASTM A370. Also, the Charpy impact test was executed by the method specified in ASTM A370, and the fracture appearance transition temperature was measured. In the present example, the case the difference between the fracture appearance transition temperature of the surface layer (vTrs of surface layer) and the fracture appearance transition temperature of the plate thickness center part (vTrs of 1/2t): ΔvTrs ([vTrs of 1/2t]-[vTrs of surface layer]) was 0°C or more and the fracture appearance transition temperature of the surface layer was the room temperature (25°C) or below was evaluated to be excellent in the HAZ toughness.

[Table 2]

No.	(1) Ca adding rate	(2) Time after adding Ca until start of supply to TD	(3) Time from start of supply to TD until start of casting	(4) Average cooling rate of 1,500°C - 1,000°C in cooling stage in casting	Hot rolling/cooling method	Number density of Ca-based inclusions (pieces/mm²)	Number density of TiN (×10² pieces/µm²)	HIC resistance CLR	HAZ toughness
No.	(1) Ca adding rate	(2) Time after adding Ca until start of supply to TD	(3) Time from start of supply to TD until start of casting		Hot rolling/cooling method	Number density of Ca-based inclusions (pieces/mm²)	Number density of TiN (×10² pieces/µm²)	HIC resistance CLR	vTrs of surface layer (°C)	vTrs of 1/2t (°C)	ΔvTrs (°C)
1	○	○	○	○	TMCP	0	15.3	○	-3	-26	5
2	○	○	○	○	TMCP	0	7.9	○	5	10	5
3	○	○	○	○	TMCP	0	19.9	○	-36	-29	7
4	○	○	○	○	TMCP	0.2	54.2	○	-6	4	10
5	○	○	○	○	TMCP	0	27.9	○	-27	-17	10
8	○	○	○	○	TMCP	0.1	60.8	○	-24	-18	8
7	○	○	○	○	TMCP	0	29,5	○	-2	7	9
8	○	○	○	○	TMCP	0	70.8	○	6	19	13
9	○	○	○	○	TMCP	0	42.3	○	2	8	8
10	○	○	○	○	TMCP	0	31.6	○	15	20	5
11	○	○	○	○	TMCP	0	26.7	○	8	14	8
12	○	○	○	○	TMCP	0	55.6	○	3	18	18
13	○	○	○	○	TMCP	0	21.3	○	-1 0	4	14
14	○	○	○	○	TMCP	0.8	48.8	○	3	6	3
15	○	○	○	×	TMCP	0	2.1	○	31	33	2
16	×	○	○	○	TMCP	3.2	48.4	○	23	16	-7
17	×	○	○	○	TMCP	2.5	38.1	○	8		-1
18	×	×	○	○	TMCP	5.1	13.3	○	31	13	-18
19	○	×	○	○	TMCP	8.5	63.8	○	33	21	-17
20	○	×	×	○	TMCP	22.3	55.1	○	52	17	-35
21	○	○	○	○	TMCP	0,3	31.6	×	20	15	-5
22	○	○	○	○	TMCP	18.9	22.7	×	37	8	-29
23	○	○	○	○	QT	0	50	○	-2	9	11
24	○	○	○	○	QT	0.1	18	○	8	14	6
25	○		○	○	QT	0	35	○	11	19	8
26	○	○	○	○	QT	0	45	○	-3	3	6
27	○	○	○	×	QT	0	3.2	○	29	35	6
28	×	○	○	○	QT	2.9	58	○	24	16	-8
29	○	×	○	○	QT	9.2	43	○	37	18	-18
30	○	○	×	○	QT	3.5	47	○	28	14	-12
31	○	○	○	○	QT	0.4	39	×	13	9	-4
32	○	○	○	○	QT	15.8	33	×	36	11	-27

From Table 1 and Table 2, followings are found out. It is found out that Nos. 1-14 and Nos. 23-26 are excellent in the HIC resistance and is excellent also in the HAZ toughness because coarse Ca-based inclusions are suppressed and the number density of TiN is a constant or more in the steel plate surface layer part.
On the other hand, Nos. 15 and 27 resulted to be poor in the HAZ toughness because the number density of TiN was insufficient. Nos. 16-20 and Nos. 28-30 resulted to be poor in the HAZ toughness because much amount of the Ca-based inclusions was present in the steel plate surface layer part. Also, in No. 20, because extremely much amount of the Ca-based inclusions was present in the steel plate surface layer part, the HAZ toughness also resulted to be quite poor. Nos. 21 and 31 resulted to be poor in the HAZ toughness because the value of Ca/S was small and much amount of MnS was formed. Also, Nos. 22 and 32 resulted to be poor in the HAZ toughness because the value of (Ca-1.25S)/O was large and much amount of the Ca-based inclusions, particularly CaO, was formed.

[Industrial Applicability]

Because the steel plates related to the present invention are excellent in the hydrogen-induced cracking resistance and the HAZ toughness, they are used suitably to a line pipe for transportation, a pressure vessel, a storage tank and the like of natural gas and crude oil.

Claims

A steel plate with excellent hydrogen-induced cracking resistance and toughness of a weld heat affected zone, consisting of:
C: 0.02-0.15% (% means mass%, hereinafter the same);

Si: 0.02-0.50%;

Mn: 0.6-2.0%;

P: over 0% and 0.030% or less;

S: over 0% and 0.003% or less;

Al: 0.010-0.08%;

Ti: 0.003-0.030%;

Ca: 0.0003-0.0060%;

N: 0.001-0.01%; and

O (oxygen): over 0% and 0.0045% or less;
optionally, at least one element selected from at least either group of (a) and (b) below:
(a) a group consisting of:
B: over 0% and 0.005% or less;

V: over 0% and 0.1 % or less;

Cu: over 0% and 1.0% or less;

Ni: over 0% and 1.5% or less;

Cr: over 0% and 1.0% or less;

Mo: over 0% and 1.0% or less; and

Nb: over 0% and 0.06% or less;

(b) a group consisting of:
Mg: over 0% and 0.01% or less;

REM: over 0% and 0.02% or less; and

Zr: over 0% and 0.010% or less,

with the remainder consisting of iron and inevitable impurities; wherein
the ratio of the Ca and the S (Ca/S) is 2.0 or more,
the Ca, the S, and the O satisfy (Ca-1.25S)/O≤1.8 with a lower limit of 0.1, and in a region from the surface to 5 mm depth in the plate thickness direction, Ca-based inclusions having a Ca amount of 60 mass% or more when all elements excluding S, O and N are assumed to be 100 mass% with 50 µm or more of the long diameter or long side are 2.0 pieces/mm² or less, and TiN with 300 nm or less and a lower limit of 50 nm or more of the long diameter or long side is 5×10² pieces/µm² or more with an upper limit of 150×10² pieces/µm².
Use of the steel plate according to claim 1 for a line pipe.
Use of the steel plate according to claim 1 for a pressure vessel.
A steel pipe for a line pipe manufactured using the steel plate according to claim 1.