Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations

Definitions

• 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.
• HIC hydrogen-induced cracking
• 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.
• 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.
• 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.
• 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.
• 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.
• 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).
• the steel plate may further include, as other elements,
• 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.
• the steel plate and the steel pipe with excellent hydrogen-induced cracking resistance and toughness of a weld heat affected zone can be provided.
• the present inventors made a lot of intensive studies in order to solve the problems described above.
• 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.
• 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.
• the long diameter or long side 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.
• the present inventors investigated the relationship between the coarse Ca-based inclusions with 50 ⁇ m 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.
• the HAZ toughness which corresponded to that ⁇ vTrs ([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 ⁇ m or more of the long diameter or long side should be suppressed to 2.0 or less as shown in the example described below.
• Ca-based inclusions 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.
• 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 ⁇ m or more described above is preferably 1.8 or less, more preferably 1.5 or less, and most preferably 0 piece/mm 2 .
• TiN 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.
• the number density of TiN with 300 nm or less of the long diameter or long side is made 5 ⁇ 10 2 pieces/ ⁇ m 2 or more.
• the number density of TiN described above is preferably 8 ⁇ 10 2 pieces/ ⁇ m 2 or more, more preferably 10 ⁇ 10 2 pieces/ ⁇ m 2 or more, and still more preferably 20 ⁇ 10 2 pieces/ ⁇ m 2 or more. Also, TiN is preferable to be as much as possible. The upper limit becomes 150 ⁇ 10 2 pieces/ ⁇ m 2 from the componential composition range and the like of the present invention.
• 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.
• TEM transmission electron microscope
• 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.
• 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.
• 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.
• C amount should be 0.15% or less.
• C amount is preferably 0.12% or less, and more preferably 0.10% or less.
• Si is an element having a deoxidizing action and effective in improving the strength of the base plate and the weld part.
• Si amount is made 0.02% or more.
• Si amount is preferably 0.05% or more, and more preferably 0.15% or more.
• 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 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.
• 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 is an element inevitably included in steel.
• 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 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.
• S amount is preferable to be as little as possible.
• A1 is a strong deoxidizing element.
• 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.
• Al amount should be 0.08% or less.
• Al amount is preferably 0.06% or less, and more preferably 0.05% or less.
• 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 has an action of controlling the form of sulfide, and has an effect of suppressing formation of MnS by forming CaS.
• Ca amount should be made 0.0003% or more.
• Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more.
• 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 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.
• N should be contained by 0.001% or more.
• N amount is preferably 0.003% or more, and more preferably 0.0040% or more.
• N amount is preferably 0.008% or less, and more preferably 0.0060% or less.
• O oxygen
• O amount should be 0.0045% or less, is preferably 0.0030% or less, and more preferably 0.0020% or less.
• Ca/S should be 2.0 or more.
• Ca/S is preferably 2.5 or more, and more preferably 3.0 or more.
• the upper limit of Ca/S is approximately 15 from Ca amount and S amount specified in the present invention.
• 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.
• 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.
• (Ca-1.25S)/O 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 Al 2 O 3 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.
• the strength and toughness can be improved further
• the HAZ toughness can be enhanced further, desulfurization is promoted, and the HIC resistance can be improved further.
• 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.
• B amount 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.
• 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 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.
• 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 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 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 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 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 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 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 rare earth element
• REM amount is more preferably 0.0005% or more, and still more preferably 0.0010% or more.
• 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.
• the REM means the lanthanoid elements (15 elements from La to Lu), Sc (Scandium), and Y.
• 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.
• a method for manufacturing the steel plate so as to satisfy all of (1)-(4) below can be cited.
• 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.
• the adding rate of Ca is preferably 0.020 kg/min ⁇ t or less, and more preferably 0.018 kg/min ⁇ t or less.
• the time after adding Ca until the start of molten steel supply to the tundish is more preferably 15 min or more. Also, from the viewpoint of the productivity, the upper limit of the time is approximately 120 min.
• 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.
• the average cooling rate described above is more preferably 15°C/min or more.
• the average cooling rate described above is more preferably 30°C/min or less.
• 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.
• 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.
• 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.
• TMCP Thermo Mechanical Control Process
• QT Quenching and Tempering
• 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.
• 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 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 mm 2 .
• 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
• the long diameter or the long side was made the size of the inclusion.
• the inclusions with 10 ⁇ m or less of the interval of the inclusion and inclusion were treated as one inclusion.
• the quantitative analysis was executed by EDX (Energy Dispersive X-ray spectrometry).
• the number of pieces of the Ca-based inclusion was measured and was converted to the number of pieces per unit area (mm 2 ). 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 ⁇ m or more of the long diameter or long side.
• 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 ⁇ m ⁇ 1.5 ⁇ m 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.
• TEM transmission electron microscope
• 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 ( ⁇ m 2 ) 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.
• 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).
• CLR Cell Length Ratio: the ratio (%) of the total of the crack length relative to the
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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