Classifications

• • 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
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22C38/06—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
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—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
  • C22C38/48—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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the disclosure relates to high-Mn steel that is suitable for structural steel used in an extremely low-temperature environment such as a storage tank of liquefied gas, in particular, high-Mn steel excellent in toughness at low temperature, and a production method therefor.
• the steel plate needs to have high strength and excellent toughness at extremely low temperature because the structure is used at extremely low temperature.
• excellent toughness needs to be guaranteed at the boiling point of the liquefied natural gas, that is, -164 °C or lower.
• a steel material has poor low-temperature toughness, the safety as a structure for an extremely low temperature storage tank may not be maintained.
• steel materials with improved low-temperature toughness that are applied to such a structure.
• austenitic stainless steel which has austenite as a structure of a steel plate, the austenite showing no brittleness at extremely low temperature, 9 % Ni steel, or five thousand series aluminum alloys have been conventionally used.
• the alloy cost and production cost are high, and thus there is a demand for steel materials which are inexpensive and excellent in extremely low-temperature toughness.
• JP 2016-196703 A proposes using, as structural steel used in an extremely low temperature environment, high-Mn steel added with a large amount of Mn which is relatively inexpensive and an austenite-stabilizing element.
• PTL 1 proposes a technique of properly controlling the austenite grain size to prevent carbides generated in crystal grain boundaries from becoming an origin of fracture and a propagation path of cracks.
• the technique may provide high-Mn steel which exhibits excellent low-temperature toughness in a base metal and a heat-affected zone after welding.
• the high-Mn steel material of PTL 1 has a thickness of about 15 mm to 50 mm, but for example, in an application such as a longitudinal member, the thickness is required to be less than 15 mm, in particular 10 mm or less.
• the "exhibiting excellent low-temperature toughness" means that the absorbed energy vE -196 °C in a Charpy impact test at -196 °C is 100 J or more.
• high-Mn steel excellent in low-temperature toughness and ductility.
• both a base metal and a heat-affected zone have excellent low-temperature toughness. Therefore, our high-Mn steel largely contributes to the improvement of the safety and the service life of a steel structure used in an extremely low temperature environment such as a tank for a storage tank of liquefied gas, and has industrially significant effects. Further, our production method does not decrease productivity or increase the production cost, and thus is excellent in economic efficiency.
• the C content is an inexpensive austenite-stabilizing element, and an important element to obtain austenite. To obtain this effect, the C content needs to be 0.30 % or more. On the other hand, a C content beyond 0.90 % generates excessive Cr carbides, deteriorating low-temperature toughness. Therefore, the C content is set to 0.30 % or more and 0.90 % or less.
• the lower limit of the C content is preferably 0.36 %, and more preferably 0.40 %.
• the upper limit of the C content is preferably 0.80 %, and more preferably 0.66 %.
• the upper limits and the lower limits can be arbitrarily combined.
• the C content is preferably set to 0.36 % or more and 0.80 % or less, and more preferably 0.40 % or more and 0.80 % or less.
• Si 0.05 % or more and 1.0 % or less
• the Si acts as a deoxidizer, is necessary for steelmaking, and is effective at increasing hardness 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 1.0 % deteriorates weldability. Therefore, the Si content is set to 0.05 % or more and 1.0 % or less. In particular, from the perspective of obtaining a steel plate with increased hardness, the lower limit of the Si content is preferably 0.07 %, more preferably 0.23 %, further preferably 0.26 %, and still further preferably 0.51 %.
• the upper limit of the Si content is set to 0.8 %, more preferably 0.7 %, further preferably 0.6 %, and still further preferably 0.5 %.
• the upper limit and the lower limit can be combined.
• the Si content is preferably set to 0.07 % or more and 0.8 % or less, 0.23 % or more and 0.7 % or less, and more preferably 0.26 % or more and 0.6 % or less.
• the Si content is preferably 0.07 % or more and 0.5 % or less.
• Mn 15.0 % or more and 30.0 % or less
• Mn is a relatively inexpensive austenite-stabilizing element.
• Mn is an important element for achieving both strength and extremely low-temperature toughness.
• the Mn content needs to be 15.0 % or more.
• a Mn content beyond 30.0 % does not increase the effect of improving extremely low-temperature toughness, but increases alloy cost. 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 30.0 % or less.
• the lower limit of the Mn content is preferably 16.0 %, more preferably 18.0 %, and further preferably 19.0 %.
• the upper limit of the Mn content is preferably 29.0 %, and more preferably 28.0 %.
• the Mn content is preferably set to 16.0 % or more and 29.0 % or less, and more 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. Therefore, the P content is set to 0.030 % or less. Further, from the perspective of decreasing the origin of stress corrosion cracking, the upper limit of the P content is preferably 0.028 % or less, and more preferably 0.024 % or less. Excessively reducing P, however, involves high refining cost and is economically disadvantageous. Therefore, the lower limit of the P content is preferably set to 0.002 %, and more preferably 0.005 %.
• the upper limit of the S content is 0.0070 %, and desirably, the S content is kept as small as possible. Therefore, the S content is set to 0.0070 % or less. From the perspective of preventing deterioration in base metal low-temperature toughness and ductility, the upper limit of the S content is preferably 0.0060 % or less. Excessively reducing S, however, involves high refining cost and is economically disadvantageous. Therefore, the lower limit of the S content is preferably set to 0.001 % or more. The S content is preferably set to 0.0020 % or more and 0.0060 % or less.
• Al 0.01 % or more and 0.07 % or less
• the Al acts as a deoxidizer and is used most commonly in molten steel deoxidizing processes to obtain a steel plate. To obtain such an effect, the Al content needs to be 0.01 % or more. On the other hand, when the Al content is beyond 0.07 %, Al is mixed into a weld metal portion during welding, deteriorating toughness of the weld metal. Therefore, the Al content is set to 0.07 % or less. Therefore, the Al content is set to 0.01 % or more and 0.07 % or less. In particular, from the perspective of obtaining an effect as a deoxidizer, the lower limit of the Al content is preferably 0.02 %, more preferably 0.046 %, and further preferably 0.052 %.
• the upper limit of the Al content is preferably set to 0.065 %, and more preferably 0.06 %.
• the upper limit and the lower limit can be combined.
• the Al content is preferably set to 0.02 % or more and 0.06 % or less.
• the Cr content is an element which stabilizes austenite with an appropriate amount of addition and is effective at improving extremely low-temperature toughness and base metal strength. To obtain such effects, the Cr content needs to be 0.5 % or more. On the other hand, a Cr content beyond 7.0 % generates Cr carbides, deteriorating low-temperature toughness and stress corrosion cracking resistance. Therefore, the Cr content is set to 0.5 % or more and 7.0 % or less. In particular, from the perspective of improving extremely low-temperature toughness and base metal strength, the lower limit of the Cr content is preferably 1 % or more, more preferably 1.2 %, and further preferably 2.0 %.
• the upper limit of the Cr content is preferably set to 6.7 % or less, more preferably 6.5 % or less, and further preferably 6.0 %.
• the upper limits and the lower limits can be arbitrarily combined.
• the Cr content is preferably set to 1.0 % or more and 6.7 % or less, and more preferably 1.2 % or more and 6.5 % or less.
• the Cr content is further preferably 2.0 % or more and 6.0 % or less.
• N 0.0050 % or more and 0.0500 % or less
• N is an austenite-stabilizing element and an element which is effective at improving extremely low-temperature toughness. To obtain such an effect, the N content needs to be 0.0050 % or more. On the other hand, the N content beyond 0.0500 % coarsens nitrides or carbonitrides, deteriorating toughness. Therefore, the N content is set to 0.0050 % or more and 0.0500 % or less. In particular, from the perspective of improving extremely low-temperature toughness, the lower limit of the N content is preferably 0.0060 % or more, more preferably 0.0355 %, and further preferably 0.0810 %.
• the upper limit of the N content is preferably set to 0.0450 % or less, and more preferably 0.0400 % or less.
• the upper limits and the lower limits of the N content can be arbitrarily combined.
• the N content is preferably set to 0.0060 % or more and 0.0400 % or less.
• the O content is set to 0.0050 % or less.
• the upper limit of the O content is preferably 0.0045 % or less.
• the lower limit of the O content is preferably 0.0023 % or more.
• the upper limits and the lower limits of the O content can be arbitrarily combined.
• the O content is preferably set to 0.0023 % or more and 0.0050 % or less.
• Ti and Nb form carbonitrides with a high melting point in steel to prevent coarsening of crystal grains, then becoming an origin of fracture and a propagation path of cracks.
• Ti and Nb hinder structure control for enhancing low-temperature toughness and improving ductility, and thus, need to be intentionally limited.
• Ti and Nb are components which is inevitably mixed from raw materials into steel, and Ti of 0.005 % or more and 0.010 % or less and Nb of 0.005 % or more and 0.010 % or less are typically mixed.
• each Ti and Nb By limiting the content of each Ti and Nb to less than 0.005 % to eliminate the adverse effect of carbonitrides, excellent low-temperature toughness and ductility can be guaranteed. Therefore, from the perspective of excellent low-temperature toughness and ductility, the content of each Ti and Nb is preferably set to 0.004 % or less, and more preferably 0.003 % or less.
• the balance other than the aforementioned components includes Fe and inevitable impurities.
• the inevitable impurities include H, and a total content of 0.01 % may be allowed.
• the steel material When a steel material has a body centered cubic (bcc) crystal structure, the steel material may cause brittle fracture in a low temperature environment, and thus, is not suitable for use in a low temperature environment.
• the steel material When the steel material is assumed to be used in a low temperature environment, the steel material is required to have, as its matrix phase, an austenite structure which has a face centered cubic (fcc) crystal structure.
• austenite as its matrix phase means that area ratio of austenite phase is 90 % or more. When area ratio of austenite phase is 90 % or more, low-temperature toughness can be guaranteed.
• the balance other than austenite phase is ferrite or martensite phase.
• volume fraction of ⁇ martensite is preferably less than 1.0 %, more preferably less than 0.5 %, and further preferably less than 0.1 %.
• a type means sulfide inclusions
• B type means cluster-type inclusions
• C type means granular oxide inclusions.
• coarsening of crystal grains is effective at guaranteeing low-temperature toughness in austenite steel.
• the biggest crystal grain size of the microstructure needs to be 50 ⁇ m or more, which is achieved by using high-Mn steel satisfying the aforementioned requirements.
• the following elements can be contained as necessary. At least one of Cu: 0.01 % or more and 1.00 % or less, Ni: 0.01 % or more and 1.00 % or less, Mo: 2.0 % or less, V: 2.0 % or less, W: 2.0 % or less, Ca: 0.0005 % or more and 0.0050 % or less, Mg: 0.0005 % or more and 0.0050 % or less, and REM: 0.0010 % or more and 0.0200 % or less.
• Cu, Ni, Mo, V, and W stabilize austenite and improve base metal strength.
• Cu and Ni are each preferably contained in an amount of 0.01% or more
• Mo, V, and W are each preferably contained in an amount of 0.001 % or more.
• the content of each Cu and Ni is beyond 1.00 %, or the content of each Mo, V, and W is beyond 2.0 %, coarse carbonitrides are generated, which may become an origin of fracture, and additionally increase production cost. Therefore, when these alloying elements are contained, the content of each Cu and Ni is preferably 1.00 % or less, and the content of each Mo, V, and W is preferably 2.0 % or less.
• the content of each Cu and Ni is more preferably 0.05 % or more and 0.70 % or less.
• the content of each Mn, V, and W is more preferably 0.003 % or more and 1.7 % 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.
• these elements are contained in a large amount, not only the amount of nonmetallic inclusions may be increased, ending up deteriorating ductility, toughness, and sulfide stress corrosion cracking resistance, but also an economic disadvantage may be entailed.
• the content of each element is set to 0.0005 % or more and 0.0050 % or less.
• the content is set to 0.0010 % or more and 0.0200 % or less.
• the Ca content is set to 0.0010 % or more and 0.0040 % or less
• the Mg content is set to 0.0010 % or more and 0.0040 % or less
• the REM content is set to 0.0020 % or more and 0.0150 % or less.
• Our high-Mn steel can be obtained from molten steel having the aforementioned chemical composition which is prepared by steelmaking using a publicly-known method such as using a converter and an electric heating furnace.
• the high-Mn steel may also be subjected to secondary refinement in a vacuum degassing furnace.
• secondary refinement to limit the contents of Ti and Nb which hinder suitable structure control within the aforementioned range, it is necessary to prevent Ti and Nb from being inevitably mixed from raw materials or the like into steel and decrease the contents of Ti and Nb. For example, by decreasing the basicity of slag in the refining stage, these alloy elements are concentrated in the slag to be discharged, thus decreasing Ti and Nb concentrations in a final slab product.
• a method may be used in which oxygen is injected for oxidation, floating and separating alloy of Ti and Nb during circulation. Subsequently, it is preferable to make the steel into a steel material such as a slab having a determined size by a publicly-known steel making method such as continuous casting or ingot casting and blooming.
• production conditions are defined to make the aforementioned steel material into a steel material exhibiting excellent low-temperature toughness.
• Heating temperature of a steel material 1100 °C or more and 1300 °C or less
• the heating temperature before hot rolling is set to 1100 °C or more. Further, when the lower limit of the heating temperature of the steel material is less than 1100 °C, the amount of nonmetallic inclusions in steel is increased, thus deteriorating extremely low-temperature toughness and ductility. However, the heating temperature beyond 1300 °C may trigger local melting. Thus, the upper limit of the heating temperature is set to 1300 °C. The temperature is controlled based on surface temperature of the steel material.
• Rolling finish temperature 800 °C or more and less than 950 °C
• a steel ingot or a billet is heated and subsequently subjected to hot rolling.
• To make coarse crystal grains it is preferable to increase the cumulative rolling reduction at high temperature.
• the crystal grains become excessively coarse in a temperature range of 950 °C or more, and thus desired yield stress cannot be obtained. Therefore, final finish rolling of at least one pass at less than 950 °C is necessary.
• hot rolling at low temperature refines the microstructure and excessively introduces working strain, thus deteriorating low-temperature toughness. Therefore, the lower limit of the rolling finishing temperature is set to 800 °C.
• the upper limit of cooling start temperature is set to 900 °C.
• the average rate of cooling a surface of the steel plate from a temperature at or above (rolling finish temperature - 100 °C) to a temperature range of 300 °C or more and 650 °C or less is set to 1.0 °C/s or more.
• the cooling rate is 1 °C/s or more.
• tempering treatment may be performed to adjust strength of the steel plate.
• Steel slabs having the chemical compositions listed in Table 1 were made by a process for refining with converter and ladle and continuous casting. Next, the obtained steel slabs were charged into a heating furnace and heated to 1150 °C, and subsequently, hot rolled into steel plates having a thickness of 10 mm to 30 mm. Tensile properties and toughness of the steel plates were evaluated as described below.
• JIS NO. 5 tensile test pieces were collected from each steel plate. Then, the test pieces were subjected to a tensile test in conformity with JIS Z 2241 (1998) to investigate tensile test properties.
• a test piece had a yield stress of 400 MPa or more and a tensile strength of 800 MPa or more, the corresponding steel plate was determined to have excellent tensile properties. Further, when a test piece had a total elongation of 30 % or more upon breakage, the corresponding steel plate was determined to have excellent ductility.
• Charpy V-notch test pieces were collected from each steel plate having a plate thickness of more than 20 mm at a 1/4 position of the plate thickness or from each steel plate having a plate thickness of 20 mm or less at a 1/2 position of the plate thickness, in a direction orthogonal to the rolling direction in conformity with JIS Z 2202 (1998). Then, the test pieces were subjected to Charpy impact test in conformity with JIS Z 2242 (1998), where three test pieces were used for each steel plate, to determine absorbed energy at -196 °C and evaluate base metal toughness. When the three test pieces had an average absorbed energy (vE- 196 ) of 100 J or more, the corresponding steel plate was determined to have good base steel toughness.
• High-Mn steel of the disclosure was confirmed to satisfy the aforementioned desired performance (a base metal yield stress of 400 MPa or more, a total elongation upon breakage of 30 % or more, an average of absorbed energy (vE- 196 ) of 100 J or more for low-temperature toughness).
• a base metal yield stress of 400 MPa or more a total elongation upon breakage of 30 % or more, an average of absorbed energy (vE- 196 ) of 100 J or more for low-temperature toughness.
• comparative examples out of the scope of the disclosure did not satisfy the aforementioned desired performance in terms of any one or more of total elongation, yield stress, and low-temperature toughness.
• the steel materials were subjected to heat cycle treatment under conditions of peak temperature of 1400 °C and a cooling rate of 10 °C/s.
• the results are listed in Table 2.
• the steel materials according to the disclosure were excellent in welded portion toughness as well as in base metal toughness at low temperature. Specifically, in welding which gave input heat of 0.5 kJ/cm to 5 kJ/cm, the maximum crystal grain size was 50 ⁇ m or more, and the absorbed energy in Charpy impact test at -196 °C was 100 J or more.

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  • 2018-04-25 CN CN201880026635.5A patent/CN110573642A/zh active Pending
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  • 2018-04-25 JP JP2018544293A patent/JP6460292B1/ja active Active
  • 2018-04-25 SG SG11201907930QA patent/SG11201907930QA/en unknown
  • 2018-04-25 BR BR112019022088A patent/BR112019022088A2/pt not_active Application Discontinuation
  • 2018-04-25 WO PCT/JP2018/016764 patent/WO2018199145A1/fr unknown
  • 2018-04-25 EP EP18790123.6A patent/EP3617337A4/fr active Pending
  • 2018-04-25 KR KR1020197031996A patent/KR102331032B1/ko active IP Right Grant
• 2019
  • 2019-08-30 PH PH12019501995A patent/PH12019501995B1/en unknown

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