Classifications

• • 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
  • 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
  • C21C7/10—Handling in a vacuum
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
  • 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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • 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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • 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

Definitions

• the present invention relates to a steel plate with excellent hydrogen-induced cracking resistance and toughness and a steel pipe for a line pipe, and relates specifically to a steel plate with excellent hydrogen-induced cracking resistance and toughness and a steel pipe for a line pipe with excellent hydrogen-induced cracking resistance and toughness obtained using the steel plate which are suitable to a line pipe for natural gas/crude oil transportation, a pressure vessel, a storage tank and the like.
• the hydrogen-induced cracking is known to be cracking caused by that the hydrogen having intruded to the inside of steel material accompanying a corrosion reaction by hydrogen sulfide and the like described above gathers in non-metal inclusions and the like such as MnS and Nb (C, N) and is gasified.
• HIC Hydrophilic Chemical Vapor Deposition
• HIC resistance Hydrophilic Chemical Vapor Resistance
• Patent Literature 1 a steel is disclosed in which the hydrogen-induced cracking resistance is improved by suppressing the segregation degree of Mn, Nb, and Ti of the plate thickness center part.
• Patent Literature 2 a method is disclosed in which the HIC originated from MnS and Ca-based oxy-sulfide is suppressed by a parameter expression formed of the content of Ca, O, and S.
• 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 for a line pipe with excellent hydrogen-induced cracking resistance and toughness.
• the steel plate with excellent hydrogen-induced cracking resistance and toughness of the present invention which could solve the problems described above satisfies, in terms of mass%, C: 0.02-0.15%, 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%, Ca: 0.0003-0.0060%, N: 0.001-0.01%, and O: over 0% and 0.0045% or less, with the remainder consisting of iron and inevitable impurities, in which Ca/S that is the ratio of the Ca and the S is 2.0 or more, the Ca, the S, and the O satisfy (Ca-1.25S)/O ⁇ 1.80, and Ar gas content in steel is 0.50 microliter/cm 3 or less. Below, microliter is expressed as " ⁇ L".
• the steel plate may further contain (a) one element or more 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 1.5% or less, Ni: over 0% and 1.5% or less, Cr: over 0% and 1.5% or less, Mo: over 0% and 1.5% or less, and Nb: over 0% and 0.06% or less, and (b) one element or more selected from a group consisting of Ti: over 0% and 0.03% or less, 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.
• B over 0% and 0.005% or less
• V over 0% and 0.1% or less
• Cu over 0% and 1.5% or less
• Ni over 0% and 1.5% or less
• Cr over 0% and 1.5% or less
• Mo over 0% and 1.5% or less
• Nb over 0% and 0.06% or less
• Ti over
• 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. Below, there is a case the steel plate and the steel pipe are collectively referred to as a steel material.
• the steel plate is made to satisfy the specified componential composition and to suppress the Ar gas content in steel to within a specified range, the steel plate and the steel pipe for a line pipe with excellent hydrogen-induced cracking resistance and toughness 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 in which hydrogen sulfide gas is made to saturate in the aqueous solution of pH 2.7 of 5% NaCl solution+0.5% acetic acid and generation of the HIC is evaluated after 96 hours.
• the present inventors measured the Charpy impact resistance after the HIC test according to ASTM A370 with respect to the steel plate surface layer part which is known in that the hydrogen concentration in particular increased during the HIC test as shown in CAMP-ISIJ Vol. 24 (2011)-p. 671 for example. As a result, it was found that the values of the Charpy impact absorption energy dispersed.
• the Ar gas content in steel described above only had to be 0.50 ⁇ L/cm 3 or less.
• the Ar gas content described above is preferably 0.30 ⁇ L/cm 3 or less, and more preferably 0.25 ⁇ L/cm 3 or less.
• the componential composition of the steel In order to secure excellent HIC resistance and toughness, it is necessary to control the componential composition of the steel in addition to control the Ar gas content in steel described above. Further, in order to also secure other properties such as the high strength and excellent weldability required as a steel plate for a line pipe and a pressure vessel for example, the componential composition of the steel plate should be made as described below. Below, the reasons for specifying each component will be described.
• 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%.
• 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 material.
• P amount exceeds 0.030%, the toughness of the base plate and the HAZ part significantly deteriorate, 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 large 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 small as possible.
• Al is a strong deoxidizing element.
• Al amount is preferably 0.020% or more, and more preferably 0.030% or more.
• Al amount is preferably 0.08% or less.
• Al amount is preferably 0.06% or less, and more preferably 0.05% 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 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% 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 namely oxygen is preferable to be smaller from the viewpoint of improving the cleanliness.
• 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 17 from Ca amount and S amount specified in the present invention.
• (Ca-1.255)/O and the toughness studied the relationship of (Ca-1.255)/O and the toughness, and found out that (Ca-1.25S)/O should be 1.80 or less in order to secure the excellent toughness.
• (Ca-1.25S)/O described above 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.
• the lower limit value of (Ca-1.25S)/O becomes approximately 0.1.
• composition of the steel plate and the 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,
• 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 more 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. In contrast, 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 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 1.5%, 1.5% or less is preferable. Cu amount is more preferably 1.0% or less, and still more preferably 0.50% 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 large amount of Ni is contained, the cost increases extremely as a structural steel material, 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 1.5%, the HAZ toughness deteriorates. Therefore it is preferable to make Cr amount 1.5% or less. Cr amount is more preferably 1.0% or less, and still more preferably 0.50% 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 1.5%, the HAZ toughness and weldability deteriorate. Therefore Mo amount is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% 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.
• Ti is an element effective in improving the toughness of the HAZ part because coarsening of the austenitic grains is prevented and ferritic transformation is promoted in the HAZ part at the time of welding by precipitating as TiN in steel. Further, Ti is an element also effective in improving the HIC resistance because Ti exhibits the desulfurization action. In order to secure these effects, it is preferable to contain Ti by 0.003% or more. Ti amount is more preferably 0.005% or more, and still more preferably 0.010% or more. In contrast, when Ti amount becomes excessive, solid solution Ti and TiC precipitate, the toughness of the base plate and the HAZ part deteriorate, and therefore 0.03% or less is preferable. Ti amount is more preferably 0.02% or less.
• Mg is an element effective in improving the toughness through miniaturization of the grain, and is effective also in improving the HIC resistance because Mg exhibits the desulfurizing action. In order to secure these effects, it is preferable to contain Mg by 0.0003% or more. Mg amount is more preferably 0.001% or more. In contrast, 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 0.005% or less.
• REM Radar Earth Metal
• 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 namely 15 elements from La to Lu, Sc, and Y.
• Zr is an element improving the HIC resistance by the desulfurizing action, and contributing to improvement of the HAZ toughness by forming and finely dispersing oxide.
• 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.
• 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 that satisfies the stipulation of the Ar gas content described above.
• a method described below is recommended.
• the inclusions with the melting point of 1,550°C or above and the length of 3 ⁇ m or more are present preferably by 3 pieces/cm 2 or more of the number density inside the mold in the continuous casting step.
• the inclusions described above with the melting point of 1,550°C or above Al 2 O 3 , CaO, and complex inclusions thereof can be cited for example.
• the melting point can be obtained by executing quantitative analysis of the inclusions by energy dispersive X-ray spectroscopic analysis method and the like, preparing artificial inclusions simulating the composition thereof, and measuring the temperature at which the artificial inclusions start to melt by a laser microscope and the like.
• the inclusions with the aspect ratio of 1.3 or more may be handled as the inclusions with the melting point of 1,550°C or above utilizing that the liquid inclusions inside the mold are observed to be spherical after being solidified.
• the number density of the inclusions described above is more preferably 5 pieces/cm 2 or more and still more preferably 10 pieces/cm 2 or more, even when the number density of the inclusions described above becomes excessive, the toughness of the base plate and the HAZ part deteriorates, and therefore the upper limit of the number density of the inclusions described above is approximately 100 pieces/cm 2 .
• the methods of making the recirculation time in the RH in the smelting step 45 minutes or less, and, after 15 minutes or more elapses after adding Ca in the RH,
• 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. 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 toughness.
• the present application is to claim the benefit of the right of priority based on Japanese Patent Application No. 2013-138178 applied on July 1, 2013 . Entire contents of the description of Japanese Patent Application No. 2013-138178 applied on July 1, 2013 are incorporated by reference into the present application.
• the number of pieces of the inclusions inside the mold described above was observed by a SEM (Scanning Electron Microscope) with the molten steel sample being taken from the inside of the mold. In the observation, the magnification was made 400 times, and the number of the field of view was made 30. Also, the inclusions with the length of 3 ⁇ m or more and the aspect ratio of 1.3 or more were deemed the inclusions having the melting point of 1,550°C or above, and the number density thereof was obtained.
• Table 2 the case the inclusions described above were made to present inside the mold by the method described above by the number density of 3 pieces/cm 2 or more is marked with "OK", and the case other than that is marked with "NG”.
• hot rolling was executed so that the cumulative compression reduction at 900°C or above of the surface temperature of the steel plate and at 1,000°C or above of the steel plate average temperature obtained by calculation became 40% or more and the pass whose compression reduction per one pass was 10% or more became 2 passes or more, hot rolling was further executed thereafter so that the cumulative compression reduction of 700°C or above and below 900°C became 20% or more with the rolling finish temperature being made 700°C or above and below 900°C, water cooling was thereafter started from a temperature of 650°C or above and was stopped at a temperature of 350-600°C, and air cooling was thereafter executed to the room temperature.
• the steel plates having the size of 9-50 mm plate thickness ⁇ 2,000-3,500 mm widthx12,000-35,000mm length and various componential compositions were obtained.
• the Ar gas content in steel was measured as shown below. Further, the HIC test was executed to evaluate the HIC resistance, and the Charpy impact test was executed to evaluate the toughness.
• a specimen of the product plate thicknessx15 mmx15 mm cut out from the steel plate surface was charged into a vacuum chamber, the degree of vacuum was made 2 ⁇ 10 -5 Torr or less, drilling was executed using a G-Straight Drill made by Mitsubishi Materials Corporation from the steel plate surface to 5 mm below the surface, the gas component in steel was thereby extracted, and the gas component was quantitatively analyzed thereafter using an M-101QA-TDM type made by Anelva Corporation with the mass number measurement rage: 1-100 amu as a quadrupole mass analyzer.
• the product number GSDD 3000 was used which had the diameter D1: 3.0 mm, the groove length L3: 32 mm, the total length: 71 mm, and the blade diameter: 3.0 mm. Also, the ratio of the Ar amount with respect to the volume of the steel material that was drilled by drilling described above ( ⁇ L/cm 3 ) was obtained. This measurement was executed at 10 optional positions of the steel plate, and the maximum value among the 10 positions was made "Ar gas content in steel".
• the HIC test was executed for evaluation according to NACE standard TM0284-2003. More specifically, from 1/4 position and 1/2 position in the width W of each steel plate, 20 pieces each of the specimens were taken respectively. The size of the specimen was made the plate thicknessxwidth: 100 mmxrolling direction: 20 mm. 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 the crack length ratio that was the rate (%) of the total of the crack length with respect to the specimen width was measured.
• CLR Cell Length Ratio
• the Charpy test specimens with 10 mm in the plate thickness directionx10 mm in the rolling direction were taken by 10 pieces in the direction perpendicular to the rolling direction, and a notch was worked in the plate thickness direction of the steel plate.
• the Charpy impact test was executed according to ASTM A370, the test temperature was made 20°C, and the Charpy absorption energy and the percent brittle fracture were evaluated.
• 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 of natural gas and crude oil, a pressure vessel, a storage tank and the like.

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