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 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.
• HIC Hydrogen-induced cracking
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ m or
• 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.
• 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
• 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.
• 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.
• the present inventors performed HIC tests specified in NACE (National Association of Corrosion and Engineer) TM0284 for various steel plates, and evaluated HIC resistance.
• NACE National Association of Corrosion and Engineer
• 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.
• 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.
• coarse Ca-based inclusions having a long diameter or a long side of 50 ⁇ m or more existed in the surface layer of the steel sheet, and this was the starting point for brittle fracture.
• 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.
• the inventors investigated the relationship between the HAZ toughness and coarse Ca-based inclusions having a major axis or long side of 50 ⁇ m 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.
• excellent HAZ toughness, ⁇ vTrs [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.).
• the number density of coarse Ca-based inclusions having a major axis or long side of 50 ⁇ m or more must be suppressed to 2.0 / mm 2 or less.
• 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 ⁇ m 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 .
• TiN 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.
• 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 / ⁇ m 2 or more.
• the number density of TiN is preferably 8 ⁇ 10 2 pieces / ⁇ m 2 or more, more preferably 10 ⁇ 10 2 pieces / ⁇ m 2 or more, and further preferably 20 ⁇ 10 2 pieces / ⁇ m 2 or more.
• 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.
• 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.
• the component composition of the steel plates needs to be as follows: is there.
• 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.
• the amount of C is too large, the HAZ toughness and weldability deteriorate.
• 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 is an element that has a deoxidizing action and is effective in improving the strength of the base material and the welded portion.
• 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.
• the amount of Si is too large, weldability and toughness deteriorate.
• 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 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.
• 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.
• 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 is a strong deoxidizing element.
• 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.
• the oxide of Al will produce
• the amount of Al is preferably 0.06% or less, and more preferably 0.05% or less.
• Ti is an element necessary for improving the toughness of the HAZ part in order to prevent coarsening of the austenite grains in the HAZ part during welding and promote ferrite transformation by precipitating as TiN in the steel. . Further, Ti is an element effective for improving the HIC resistance since it exhibits a desulfurization action. In order to obtain these effects, 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 has the 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.
• 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 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.
• the N amount is preferably 0.003% or more, and more preferably 0.0040% or more.
• the N amount is preferably 0.008% or less, and more preferably 0.0060% or less.
• O (oxygen) is preferably low from the viewpoint of improving cleanliness.
• O (oxygen) is preferably low from the viewpoint of improving cleanliness.
• 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]
• S forms MnS as a sulfide inclusion, and as a result of extending by rolling, the HIC resistance is most deteriorated.
• Ca is added to control the form of the sulfide inclusions in the steel as CaS, thereby detoxifying S against HIC resistance.
• 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 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.
• 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.
• (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. .
• these elements will be described in detail.
• 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.
• HAZ toughness is improved.
• the amount of B is more preferably 0.0005% or more, and further preferably 0.0010% or more.
• 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 is an element effective for improving the strength.
• V is preferably contained in an amount of 0.003% or more. More preferably, it is 0.010% or more.
• the V amount is preferably 0.1% or less, and more preferably 0.08% 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 is an element effective for improving the strength and toughness of the base material and the welded portion.
• 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.
• 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.
• 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.
• 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.
• the 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 for increasing strength and base metal toughness without degrading weldability.
• 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.
• 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 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.
• the Mg content is preferably 0.0003% or more.
• the amount of Mg is more preferably 0.001% or more.
• 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
• the amount of REM is more preferably 0.0005% or more, and further preferably 0.0010% or more.
• 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%.
• 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.
• the REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the Ca addition rate is preferably 0.002 kg / min ⁇ t or more, and more preferably 0.004 kg / min ⁇ t or more.
• 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.
• the Ca addition rate is preferably 0.020 kg / min ⁇ t or less, and more preferably 0.018 kg / min ⁇ t or less.
• 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.
• 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.
• 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.
• 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.
• the average cooling rate is preferably 35 ° C./min or less.
• the average cooling rate is more preferably 30 ° C./min or less.
• 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.
• 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.
• TMCP Thermo Mechanical Control Process
• QT the “Hot rolling / cooling method” column of Table 2.
• steel plates plate thickness: 20 to 51 mm
• 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.
• 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 .
• 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 ⁇ m or more.
• 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 ⁇ m ⁇ 1.5 ⁇ m 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.
• TEM transmission electron microscope
• 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 ( ⁇ m 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).
• CLR Cell Length Ratio, ratio (%) of crack length to crack width, crack
• 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
• 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): ⁇ vTrs ([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.
• 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. 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.
• 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.

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