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
  • C22C38/02—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys 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
  • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C22C38/04—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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

Definitions

• This disclosure relates to a steel plate that is suitable for structural steel used in an extremely low-temperature environment such as a storage tank of liquefied gas, in particular, a steel plate excellent in corrosion resistance in a salinity corrosive environment, and a method for manufacturing the same.
• the hot-rolled steel plate In using a hot-rolled steel plate in a structure for a storage tank of liquefied gas, the operating environment is at extremely low temperatures. Therefore, the hot-rolled steel plate needs to have not only high strength but also toughness at extremely low temperatures. For example, for a hot-rolled steel plate used for a storage tank of liquefied natural gas, excellent toughness needs to be guaranteed at the boiling point of liquefied natural gas, that is, -164 °C or lower. When a steel material has poor low-temperature toughness, the safety as a structure for an extremely low-temperature storage tank may not be maintained. Thus, there is a growing demand for steel materials with improved low-temperature toughness that are applied to such a structure.
• austenitic stainless steel which has an austenite microstructure exhibiting no brittleness at extremely low temperatures
• 9 % Ni steel, or five thousand series aluminum alloys have been conventionally used.
• the alloy cost and manufacturing cost of those metal materials are high, and thus there is a demand for steel plates which are inexpensive and excellent in extremely low-temperature toughness.
• high-Mn steel added with a large amount of Mn which is a relatively inexpensive austenite-stabilizing element to form an austenite microstructure is considered to be used as a structural steel plate used in an extremely low-temperature environment.
• JP 2015-508452 A (PTL 1) describes a steel material having improved machinability by cutting and Charpy impact properties at -196 °C of a weld heat-affected zone (HAZ) through addition of Mn in an amount of 15 % to 35 %, Cu in an amount of 5 % or less, and C and Cr in a suitable amount.
• HZ weld heat-affected zone
• JP 2016-84529 A (PTL 2) describes a high-Mn steel material having improved low temperature toughness through addition of C: 0.25 % to 0.75 %; Si: 0.05 % to 1.0 %; Mn: more than 20 % to 35 % or less; Ni: 0.1 % or more and less than 7.0 %; and Cr: 0.1 % or more and less than 8.0 %.
• JP 2016-196703 A (PTL 3) describes a high-Mn steel material having a base metal and a welded portion with improved extremely low-temperature toughness through addition of C in an amount of 0.001 % to 0.80 % and Mn in an amount of 15 % to 35 %, and addition of elements such as Cr, Ti, Si, Al, Mg, Ca, and REM.
• the addition amount of Cr and the amount of dissolved Cr is properly controlled to thereby make it possible to delay an initial corrosion reaction on a steel plate surface in a salinity corrosive environment. In this way, the hydrogen amount entering into steel can be reduced to suppress the stress corrosion cracking of austenite steel.
• a measure of improving crystal grain boundary strength is valid for effectively suppressing rupture of austenite originating from crystal grain boundaries.
• P is an element which is easily segregated, as with Mn, during a solidification process of a slab and lowers the crystal grain boundary strength of a portion crossing such a segregation portion. Therefore, impurity elements such as P need to be reduced.
• excellent in corrosion resistance means a fracture stress of 400 MPa or more when a test in accordance with the Slow Strain Rate Test Method based on NACE Standard TM0111-2011 is performed by immersing in artificial seawater (chloride ion concentration of 18000 ppm) at 23 °C and performing a constant-rate tensile test at a strain rate of 4 ⁇ 10 -7 inch/s.
• a steel plate excellent in corrosion resistance, in particular, corrosion resistance in a salinity corrosive environment it is possible to provide a steel plate excellent in corrosion resistance, in particular, corrosion resistance in a salinity corrosive environment. Therefore, when our steel plate is used for a steel structure used in an extremely low-temperature environment such as a tank for a storage tank of liquefied gas, the safety and the service life of the steel structure is significantly improved, thus producing significantly advantageous effects in industrial terms. Further, our steel plate is inexpensive compared with conventional materials, and thus excellent in economic efficiency.
• the C content is set to be 0.20 % or more and 0.70 % or less, and preferably 0.25 % or more and 0.60 % or less.
• Si 0.05 % or more and 1.00 % or less
• Si acts as a deoxidizer, is necessary for steelmaking, and is effective at increasing the strength of a steel plate by solid solution strengthening when dissolved in steel. To obtain such an effect, the Si content needs to be 0.05 % or more. On the other hand, a Si content beyond 1.00 % may deteriorate weldability and surface characteristics, lowering stress corrosion cracking resistance. Therefore, the Si content is set to 0.05 % or more and 1.00 % or less, and preferably 0.07 % or more and 0.50 % or less.
• Mn 15.0 % or more and 35.0 % or less
• Mn is a relatively inexpensive austenite-stabilizing element.
• Mn is an important element for achieving both high strength and extremely low-temperature toughness.
• the Mn content needs to be 15.0 % or more.
• the Mn content beyond 35.0 % causes saturation of the effect of improving the extremely low-temperature toughness, increasing alloy costs. Further, such a high Mn content deteriorates weldability and cuttability, and further promotes segregation as well as the occurrence of stress corrosion cracking. Therefore, the Mn content is set to 15.0 % or more and 35.0 % or less, and preferably 18.0 % or more and 28.0 % or less.
• the upper limit of the P content is 0.030 %, and desirably, the P content is kept as small as possible. Properties are improved as the P content is lower, and thus, the P content is preferably set to 0.024 % or less, and more preferably 0.020 % or less. On the other hand, reducing the P content to less than 0.001 % involves high steelmaking costs and impairs economic efficiency. Thus, the content of 0.001 % or more is allowable.
• the upper limit of the S content is 0.0200 %, and desirably, the S content is kept as small as possible. Therefore, the S content is set to 0.0200 % or less, and preferably 0.0180 % or less.
• reducing the S content to less than 0.0001 % involves high steelmaking costs and impairs economic efficiency. Thus, the content of 0.0001 % or more is allowable.
• Al 0.010 % or more and 0.100 % or less
• Al acts as a deoxidizer and is used most commonly in molten steel deoxidizing processes to obtain a steel plate.
• Al also has an effect of fixing solute N in steel to form AlN, thus suppressing coarsening of crystal grains.
• Al has an effect of suppressing deterioration of toughness caused by decrease in solute N.
• the Al content needs to be 0.010 % or more.
• a Al content beyond 0.100 % may form coarse nitrides which would become an origin of corrosion and fracture, thus lowering stress corrosion cracking resistance.
• Al diffuses to a weld metal portion during welding to deteriorate toughness of the weld metal.
• the Al content is set to 0.100 % or less, and preferably 0.020 % or more and 0.070 % or less.
• Cr is an important element because it is contained in a suitable amount to thereby produce an effect of delaying an initial corrosion reaction on a steel plate surface in a salinity corrosive environment to decrease the amount of hydrogen entering a steel plate and improve stress corrosion cracking resistance.
• the Cr content is increased, corrosion resistance can be improved.
• Cr inevitably precipitates in the form of, for example, a nitride, a carbide, or a carbonitride during rolling and those precipitates may become an origin of corrosion and fracture to deteriorate stress corrosion cracking resistance. Therefore, the Cr content is set to 0.5 % or more and 8.0 % or less.
• the amount of solute Cr is preferably 1.0 % or more and 6.0 % or less, and more preferably 1.2 % or more and 5.5 % or less.
• the solid solution state refers to a state in which solute atoms exist as atoms without forming precipitates.
• Nb 0.003 % or more and 0.030 % or less
• Nb precipitates as carbonitrides and the formed carbonitrides serve as a diffusible hydrogen trapping site.
• Nb is an element which has an effect of suppressing stress corrosion cracking.
• Nb is preferably contained in an amount of 0.003 % or more.
• the Nb content is more than 0.030 %, coarse carbonitrides may precipitate to become an origin of fracture. Further, the precipitates may be coarsened to deteriorate base metal toughness.
• V 0.01 % or more and 0.10 % or less
• V precipitates as carbonitrides and the formed carbonitrides serve as a diffusible hydrogen trapping site.
• V is an element which has an effect of suppressing stress corrosion cracking.
• V is preferably contained in an amount of 0.01 % or more.
• the V content is more than 0.10 %, coarse carbonitrides may precipitate to become an origin of fracture. Further, the precipitates may be coarsened to deteriorate base metal toughness. Therefore, when V is contained, the V content is preferably set to 0.01 % or more and 0.10 % or less, more preferably 0.02 % or more and 0.09 % or less, and further preferably 0.03 % or more and 0.08 % or less.
• Ti precipitates as nitrides or carbonitrides and the formed nitrides or carbonitrides serve as a diffusible hydrogen trapping site.
• Ti is an element which has an effect of suppressing stress corrosion cracking.
• Ti is preferably contained in an amount of 0.003 % or more.
• the precipitates may be coarsened to deteriorate base metal toughness. Further, coarse carbonitrides may precipitate to become an origin of fracture.
• the Ti content is preferably set to 0.003 % or more and 0.040 % or less, more preferably 0.005 % or more and 0.035 % or less, and further preferably 0.007 % or more and 0.032 % or less.
• the chemical composition may optionally contain at least one selected from the group of Cu: 0.01 % or more and 0.50 % or less, Ni: 0.01 % or more and 0.50 % or less, Sn: 0.01 % or more and 0.30 % or less, Sb: 0.01 % or more and 0.30 % or less, Mo: 0.01 % or more and 2.0 % or less, and W: 0.01 % or more and 2.0 % or less.
• Cu, Ni, Sn, Sb, Mo, and W are elements which are added with Cr to thereby improve corrosion resistance of high-Mn steel in a salinity corrosive environment.
• Cu, Sn, and Sb have an effect of increasing hydrogen overvoltage in a steel material to thereby suppress a hydrogen evolution reaction corresponding to the cathodic reaction.
• Ni forms a precipitation layer on a steel material surface and physically suppresses permeation of corrosive anions such as Cl - into a steel substrate.
• Cu, Ni, Sn, Sb, Mo, and W are liberated as a metal ion from a steel material surface during corrosion and densify corrosion products, thereby suppressing permeation of corrosive anions into a steel interface (interface between a rust layer and a steel substrate).
• Mo and W are liberated as MO 4 2- and WO 4 2- , respectively, and adsorbed in the corrosive products or to the steel plate surface, thereby imparting cation selective permeability and electrically suppressing permeation of corrosive anions into the steel substrate.
• More preferred contents are: a Cu content of 0.02 % or more and 0.40 % or less; a Ni content of 0.02 % or more and 0.40 % or less; a Sn content of 0.02 % or more and 0.25 % or less; a Sb content of 0.02 % or more and 0.25 % or less; a Mo content of 0.02 % or more and 1.9 % or less; and a W content of 0.02 % or more and 1.9 % or less.
• the chemical composition may optionally contain at least one selected from the group of Ca: 0.0005 % or more and 0.0050 % or less, Mg: 0.0005 % or more and 0.0100 % or less, and REM: 0.0010 % or more and 0.0200 % or less.
• Ca, Mg, and REM are elements useful for morphological control of inclusions and can be contained as necessary.
• the morphological control of inclusions means granulating elongated sulfide-based inclusions.
• the morphological control of inclusions improves ductility, toughness, and sulfide stress corrosion cracking resistance.
• Ca and Mg are preferably contained in an amount of 0.0005 % or more and REM is preferably contained in an amount of 0.0010 % or more.
• the Ca content is preferably set to 0.0005 % or more and 0.0050 % or less
• the Mg content is preferably set to 0.0005 % or more and 0.0100 % or less
• the REM content is preferably set to 0.0010 % or more and 0.0200 % or less.
• a Ca content of 0.0010 % or more and 0.0040 % or less, a Mg content of 0.0010 % or more and 0.0040 % or less, and a REM content is 0.0020 % or more and 0.0150 % or less are more preferable.
• temperatures refer to the temperature of the mid-thickness part of a steel plate.
• Heating a steel raw material to 1000 °C or higher is for dissolving carbonitrides in the microstructure to make the crystal grain size and the like uniform. Specifically, when the heating temperature is lower than 1000 °C, carbonitrides are not sufficiently dissolved and thus desired properties cannot be obtained. Further, heating at a temperature higher than 1300 °C deteriorates material properties due to coarsening of crystal grain size and needs excessive energy, lowering productivity.
• the upper limit of the heating temperature is set to 1300 °C, preferably 1050 °C or higher and 1250 °C or lower, and more preferably 1070 °C or higher and 1250 °C or lower.
• the rolling reduction ratio in hot rolling is limited to 3 or more.
• the upper limit of the rolling reduction ratio needs to be 30 for the reasons given below.
• the rolling reduction ratio is defined by a plate thickness of a material to be rolled / a plate thickness of a steel plate after hot rolling.
• the finish rolling temperature is lower than 750 °C, the amount of carbide precipitates during rolling is significantly increased, and even when the time for which a material to be rolled resides within a temperature range of 600 °C to 950 °C is 30 minutes or less, a sufficient amount of solute Cr may not be obtained, lowering corrosion resistance. Further, when rolling is performed at a temperature of lower than 750 °C, deformation resistance is increased to apply an excessive load to a manufacturing apparatus.
• the finish rolling temperature is set to 750 °C or higher. From the viewpoint of suppressing significant coarsening of crystal grain size, the upper limit of the finish rolling temperature is preferably 1050 °C or lower.
• the time for which a material to be rolled resides within a temperature range of 600 °C to 950 °C (residence time) is more than 30 minutes, a large amount of carbonitrides and carbides precipitate during rolling and the necessary amount of solute Cr cannot be obtained, lowering corrosion resistance and extremely-low temperature toughness. Therefore, the time for which a material to be rolled resides within a temperature range of 600 °C to 950 °C is limited to 30 minutes or less.
• the time for which a material to be rolled resides within a temperature range of 600 °C to 950 °C is preferable as short as possible, and thus, no lower limit is placed thereon.
• the length of the material to be rolled is made to be 5000 mm or less and the rolling reduction ratio of the material to be rolled is limited to 30 or less as described above. This is because when the length of the material to be rolled is more than 5000 mm or the rolling reduction ratio is more than 30, the rolling time becomes long and as a result, the time for which a material to be rolled resides within a temperature range of 600 °C to 950 °C exceeds 30 minutes.
• the average cooling rate within a temperature range of 600 °C to 700 °C is less than 3 °C/s, a large amount of precipitates such as Cr carbides are formed, the average cooling rate is limited to 3 °C/s or more.
• the average cooling rate is preferable as fast as possible, and thus, no upper limit is placed thereon.
• Steels of Nos. 1 to 28 listed in Table 1 were produced by steelmaking to obtain slabs, subsequently the slabs were formed into steel plates of sample Nos. 1 to 34 having a plate thickness of 6 mm to 50 mm under the manufacturing conditions listed in Table 2, and the steel plates were subjected to the following test.
• the corrosion resistance test was performed in accordance with the Slow Strain Rate Test Method based on NACE Standard TM0111-2011 (hereinafter, referred to as "SSRT test"). Test pieces having a shape of notched Type A round bar were used. The test pieces were immersed in artificial seawater (having chloride ion concentration of 18000 ppm) at 23 °C and subjected to a constant-rate tensile test at a strain rate of 4 ⁇ 10 -7 inch/s. In this disclosure, a test piece having a fracture stress of 400 MPa or more was considered as having excellent stress corrosion cracking resistance. The results thus obtained are listed in Table 2. Table 2 Sample No. Material No.

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• 2018
  • 2018-09-13 SG SG11202002379QA patent/SG11202002379QA/en unknown
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