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 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
  • 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
  • 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
  • C22C38/04—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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
• • 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
  • C22C38/26—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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—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
  • 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 disclosure relates to steel material suitable for steel structures such as ships, marine structures, pressure vessels, line pipes, and offshore wind power generators. Specifically, the present disclosure relates to a thick, high tensile strength steel plate having a thickness exceeding 100 mm, that not only has excellent base metal strength and toughness but also has excellent joint CTOD properties in multilayer-fill-welded portions, and a method of producing same.
• a mid-thickness part tends to have a decreased cooling rate and coarser crystal grains because of the increased thickness. Therefore, crystal grain refinement of the mid-thickness part is important to produce a steel plate where the mid-thickness part has excellent strength and toughness.
• CTOD test a crack tip opening displacement test
• the CTOD test evaluates resistance to occurrence of brittle cracks by introducing a fatigue precrack into a test piece at the location to be evaluated for toughness, subjecting the test piece to three-point bending, and measuring the amount of crack opening (plastic deformation) immediately before fracture.
• the heat-affected zone (hereinafter also referred to as "HAZ”) of a multilayer fill weld is subjected to a plurality of different thermal cycles from each of the welding passes, forming a mixture of various microstructures.
• HAZ in the vicinity of the weld line
• CGHAZ coarse grain heat-affected zone
• MA martensite austenite constituent
• ICCGHAZ inter-critically reheated coarse grain heat-affected zone
• ICCGHAZ inter-critically reheated coarse grain heat-affected zone
• ICCGHAZ inter-critically reheated coarse grain heat-affected zone
• SCHAZ sub-critically reheated HAZ
• joint CTOD properties are required for the CGHAZ in the vicinity of the weld line and the SC/inter-critically reheated HAZ (SC/ICHAZ) boundary, which is the boundary of untransformed/transformed zones of the base metal during welding.
• SC/ICHAZ SC/inter-critically reheated HAZ
• CTOD testing of welded joints is basically performed at full thickness, and therefore when the CGHAZ is the subject of evaluation, the region where a fatigue precrack is introduced includes the ICCGHAZ microstructure. That is, joint CTOD properties obtained by a joint CTOD test depend on toughness of the most brittle microstructure in the evaluation region, and therefore joint CTOD properties of the CGHAZ reflect the toughness of ICCGHAZ microstructure as well as CGHAZ microstructure.
• the HAZ microstructures described above are CGHAZ, ICHAZ, and SCHAZ in order of proximity from the weld line in the microstructure formed in one welding pass during multilayer fill (multi-pass) welding.
• ICCGHAZ is microstructure formed in multilayer fill welding when the CGHAZ is heated to a two phase region of ferrite and austenite by the thermal hysteresis of subsequent passes, and the location and frequency at which ICCGHAZ microstructure is formed can vary depending on how welding passes are layered.
• PTL 2 describes a technique to suppress austenite grain growth and improve toughness of welded portions by adding REM in combination with Ti and dispersing the fine particles in steel.
• PTL 5 describes a technique to improve low-temperature toughness by controlling hardness of a central segregation area.
• CTOD specification temperature in standards that specify joint CTOD properties is typically -10 °C.
• PTL 1 proposes rolling condition control for average effective crystal grain size refinement of mid-thickness microstructure
• the technique has not been applied to steel plates having a thickness exceeding 100 mm.
• refinement of average effective crystal grain size is insufficient; maximum effective crystal grain size also needs to be refined.
• the technique proposed in PTL 2 to suppress coarsening of the austenite structure of the HAZ by adding REM in combination with Ti and dispersing fine particles in steel is targeted at steel material that has relatively low strength and low alloying element content. Therefore, the technique is not applicable to higher strength steel material with higher alloying element content, because the HAZ microstructure does not contain ferrite.
• the technique proposed in PTL 3 is effective when HAZ microstructure is mainly composed of ferrite and a cooling rate of the heat-affected zone is slow, as in large-heat input welding.
• the heat input is relatively small, and further, the alloy component in the base metal is relatively large for steel plates exceeding 100 mm thickness. Therefore, in multilayer fill welding of steel plates, the HAZ microstructure becomes mainly bainite, and the HAZ toughness improvement effect described above cannot be obtained.
• PTL 5 proposes a technique for satisfactory joint CTOD properties at normal specification temperature (-10 °C) in steel plates having a thickness of 100 mm or less.
• normal specification temperature -10 °C
• mechanical properties equivalent to those of steel plates having thicknesses of 100 mm or less have not yet been obtained, and joint CTOD properties at even lower specification temperatures, as mentioned above, have not been investigated.
• CTOD properties of the SC/ICHAZ boundary have also not been investigated.
• the present disclosure is made in view of the problems described above faced by conventional technologies, and it would be helpful to provide a steel plate having a thickness exceeding 100 mm, high strength, excellent base metal toughness at low temperature, and excellent multilayer-fill-welded joint CTOD properties, as well as a method of producing same.
• high strength is defined as a yield stress of 325 MPa or more in the mid-thickness part in a tensile test.
• Excellent base metal toughness at low temperature is defined as having an absorbed energy of 100 J or more in the Charpy test at -40 °C in the mid-thickness part.
• Excellent multilayer-fill-welded joint CTOD properties is defined as a crack opening displacement of 0.4 mm or more at notch location CGHAZ and at the SC/ICHAZ boundary, respectively, at a test temperature of -20 °C.
• the inventors have found that the desired strength and toughness can be secured by making the maximum effective crystal grain size to 150 ⁇ m or less while keeping the average effective crystal grain size to 20 ⁇ m or less, in the base metal microstructure in the mid-thickness part, as described below.
• the inventors discovered that such problems can be solved by eliminating coarse crystal grains that have a maximum effective crystal grain size exceeding 150 ⁇ m.
• the mid-thickness part is a region from the center in the thickness direction (1/2 position) to a thickness of 10 % of the steel plate toward both main surfaces of the steel plate.
• the inventors discovered that by using composite inclusions containing two types of inclusion, that is, sulfides containing Ca and Mn and oxides containing Al, the composite inclusions can exist stably even in a region in the vicinity of the weld line where the temperature is increased to a high temperature, and the effect of suppressing austenite grain coarsening can be fully exhibited. Further, the inventors discovered that a Mn-poor layer is formed around the composite inclusions, which has a nucleation effect (nucleation site effect) on bainite and the like.
• the inventors discovered that the size of such composite inclusions needs to be 0.1 ⁇ m or larger in circle equivalent diameter in order to fully exhibit the nucleation site effect due to the composite inclusions described above.
• At least one composite inclusion needs to be present in the austenite grains of the HAZ when weld temperature is increasing in order to fully utilize refinement of the HAZ microstructure due to the nucleation site effect.
• austenite grain size in the vicinity of the weld line reaches about 200 ⁇ m or more, and therefore the inventors discovered that the number density of composite inclusions need to be 25/mm 2 or more in order for the finally obtained HAZ microstructure to be sufficiently fine.
• toughness of the composite inclusions themselves is low, and therefore the presence of an excessive amount of composite inclusions can instead reduce the HAZ toughness.
• the number of composite inclusions needs to be appropriately controlled in the mid-thickness part, where element segregation exists and multilayer-fill-welded HAZ toughness is poor.
• the inventors discovered that good multilayer-fill-welded joint CTOD properties are obtainable by making the number density of the composite inclusions 250/mm 2 or less.
• the inventors discovered that rolling at a high mid-thickness temperature of T 1 °C or more, with an average rolling reduction per pass of 3 % or more and a cumulative rolling reduction ratio of 25 % or more, can increase the strain applied to the mid-thickness part, elongate and break up coarse inclusions, and distribute fine inclusions to a high density. Further, the inventors discovered that such inclusions can help secure the HAZ toughness improvement effect.
• a steel plate having high strength and excellent toughness of base metal and multilayer-fill-welded joint CTOD properties at low temperatures is provided, as well as a method of producing same.
• C is an element that increases hardenability and improves strength of steel.
• C content needs to be 0.03 % or more. However, the C content exceeding 0.13 % increases the hardness of C-enriched portions and degrades joint CTOD properties.
• the C content is therefore in the range from 0.03 % to 0.13 %.
• the C content is preferably 0.04 % or more.
• the C content is preferably 0.12 % or less.
• the C content is more preferably 0.06 % or more.
• the C content is more preferably 0.10 % or less.
• Si is also used as a deoxidizer, but is an element inevitably included as an impurity, and excessive Si content exceeding 0.60 % decreases joint CTOD properties.
• the Si content is therefore limited to an upper limit of 0.60 %.
• the upper limit is preferably 0.50 % or less.
• a lower limit is not particularly limited, but lowering Si excessively leads to increased refining time and higher costs, and therefore the lower limit of the Si content is preferably around 0.02 %.
• the Si content is more preferably 0.04 % or more.
• Mn is an element that has the effect of improving strength of the base metal and welded portion through improvement of hardenability of steel. To obtain these effects, addition of 0.9 % or more is required. However, addition exceeding 2.7 % not only decreases weldability, but also causes excessive hardenability, which reduces toughness of the base metal and welded portion, resulting in degradation of joint CTOD properties.
• the Mn content is therefore in the range from 0.9 % to 2.7 %.
• the Mn content is preferably 1.1 % or more.
• the Mn content is preferably 2.5 % or less.
• the Mn content is more preferably 1.2 % or more.
• the Mn content is more preferably 2.3 % or less.
• P is an element that has a large effect of embrittling grain boundaries, and when added in large amounts decreases HAZ toughness and degrades joint CTOD properties.
• P content is therefore limited to 0.050 % or less.
• the P content is preferably 0.030 % or less. Decreasing P content as much as possible is desirable, and therefore a lower limit of the P content is not particularly limited. However, excessively low P content leads to increased refining time and higher costs.
• the P content is therefore preferably 0.001 % or more.
• the P content is more preferably 0.005 % or more.
• S is an element that degrades joint CTOD properties, and therefore an upper limit of S content is limited to 0.0050 %.
• the upper limit is preferably 0.0030 % or less. Decreasing S content as much as possible is desirable, and therefore a lower limit of the S content is not limited. However, excessively low S content leads to increased refining time and higher costs.
• the S content is therefore preferably 0.0001 % or more.
• the S content is more preferably 0.0005 % or more.
• Al is an element required for formation of composite inclusions to improve toughness of multilayer-fill-welded HAZ and to improve joint CTOD properties, and needs to be added at 0.002 % or more. However excessive addition of more than 0.100 % causes the amount of composite inclusions to become excessive and joint CTOD properties in a low temperature range to degrade.
• the Al content is therefore in the range from 0.002 % to 0.100 %.
• the Al content is preferably 0.005 % or more.
• the Al content is preferably 0.090 % or less.
• the Al content is more preferably 0.020 % or more.
• the Al content is more preferably 0.075 % or less.
• Precipitated TiN has an effect of inhibiting coarsening of austenite grains in the base metal and HAZ, refining HAZ microstructure and improving joint CTOD properties.
• addition of 0.002 % or more is required.
• Ti content exceeds 0.055 %, Ti nitrides coarsen and toughness of the heat-affected zone instead degrades, resulting in degradation of joint CTOD properties.
• the Ti content is therefore in the range from 0.002 % to 0.055 %.
• the Ti content is preferably 0.005 % or more.
• the Ti content is preferably 0.050 % or less.
• the Ti content is more preferably 0.010 % or more.
• the Ti content is more preferably 0.045 % or less.
• Nb is an element that broadens the non-recrystallization temperature range of austenite phase and has an effect of improving strength and toughness of base metal via efficient non-recrystallization range rolling to obtain a fine grain microstructure.
• the non-recrystallization temperature T 2 becomes too low, and the rolling temperature for non-recrystallization range rolling for fine grain formation becomes too low.
• Rolling at low temperatures increases the deformation resistance of the rolled material and increases load on the rolling mill, resulting in an increase in the number of rolling passes, which reduces production efficiency and makes it difficult to increase pass rolling reduction.
• appropriate introduction of strain to 1/2 t in steel plates having a thickness exceeding 100 mm becomes impossible, making obtaining desired properties difficult.
• the Nb content needs to be 0.005 % or more. However, when the Nb content exceeds 0.070 %, joint CTOD properties degrade.
• the Nb content is therefore in the range from 0.005 % to 0.070 %.
• the Nb content is preferably 0.010 % or more.
• the Nb content is preferably 0.060 % or less.
• the Nb content is more preferably 0.015 % or more.
• the Nb content is more preferably 0.050 % or less.
• Ca is an element that improves toughness of multilayer-fill-welded HAZ and improves joint CTOD properties by forming acid sulfides that are highly stable at high temperatures.
• the Ca content needs to be 0.0005 % or more.
• the content exceeding 0.0200 % causes excessive precipitation of acid sulfides, which instead degrades joint CTOD properties.
• the Ca content is therefore in the range from 0.0005 % to 0.0200 %.
• the Ca content is preferably 0.0010 % or more.
• the Ca content is preferably 0.0170 % or less.
• the Ca content is more preferably 0.0015 % or more.
• the Ca content is more preferably 0.0150 % or less.
• N is an element that reduces HAZ toughness and degrades joint CTOD properties, and therefore an upper limit of N content is limited to 0.0120 %. Decreasing N content as much as possible is desirable, and therefore a lower limit of the N content is not limited. However, excessively low N content leads to increased refining time and higher costs.
• the N content is therefore preferably 0.0005 % or more.
• the N content is more preferably 0.0020 % or more.
• the N content is more preferably 0.0110 % or less.
• the N content is more preferably 0.0030 % or more.
• the N content is more preferably 0.0090 % or less.
• the chemical composition may further contain at least one optional element selected from the group consisting of Ni, Cu, Cr, Mo, V, W, B, REM, and Mg, in a quantity indicated below.
• Ni is an element which can increase strength of a steel plate without greatly degrading toughness of both base metal and joints. However, when Ni content exceeds 2.5 %, production costs and environmental impact increase. The Ni content is therefore limited to 2.5 % or less. The Ni content is more preferably 2.0 % or less. However, when added, the Ni content is preferably 0.1 % or more.
• Cu is an element that can increase strength of steel plates without significantly degrading toughness of the base metal and joints.
• the Cu content is therefore limited to 2.0 % or less.
• the Cu content is more preferably 1.8 % or less.
• the Cu content is preferably 0.05 % or more.
• the Cu content is more preferably 0.1 % or more.
• Cr is an element that has an effect of increasing strength of steel through improving hardenability. However, Cr content exceeding 1.5 % degrades joint CTOD properties, and therefore the Cr content is limited to 1.5 % or less.
• the Cr content is more preferably 1.3 % or less. However, when added, the Cr content is preferably 0.05 % or more.
• the Cr content is more preferably 0.1 % or more.
• Mo is an element that has an effect of increasing strength of steel through improving hardenability. However, Mo content exceeding 1.5 % degrades joint CTOD properties, and therefore the Mo content is limited to 1.5 % or less.
• the Mo content is more preferably 1.3 % or less. However, when added, the Mo content is preferably 0.05 % or more. The Mo content is more preferably 0.1 % or more.
• V 0.25 % or less
• W is an element that improves strength of the base metal, but when W content exceeds 0.45 %, HAZ toughness decreases and joint CTOD properties degrade, and therefore the W content is limited to 0.45 % or less.
• the W content is more preferably 0.40 % or less. However, when added, the W content is preferably 0.05 % or more.
• the W content is more preferably 0.15 % or more.
• Mg is an element that suppresses growth of austenite grains in the heat-affected zone by forming oxide inclusions and improves toughness of the heat-affected zone.
• the Mg content is therefore limited to 0.005 % or less.
• the Mg content is more preferably 0.004 % or less.
• the Mg content is preferably 0.0005 % or more.
• the Mg content is more preferably 0.001 % or more.
• the [Ti] / [N] range is preferably 1.80 or more.
• the [Ti] / [N] range is preferably 4.50 or less.
• the [Ti] / [N] range is more preferably 2.00 or more.
• the [Ti] / [N] range is more preferably 4.00 or less. 0 ⁇ Ca ⁇ 0.18 + 130 Ca ⁇ O / 1.25 / S ⁇ 1.50
• ⁇ [Ca] - (0.18 + 130 [Ca]) x [O] ⁇ / 1.25 / [S] is the atomic concentration ratio (ACR) of Ca, O, and S in steel.
• ACR atomic concentration ratio
• MnS main form of sulfide inclusions
• MnS has a low melting point and melts in the vicinity of the weld line during welding, and therefore the effect of suppressing austenite grain coarsening in the vicinity of the weld line and the effect of transformation during cooling after welding cannot be obtained, resulting in joint CTOD properties degrading.
• the ACR exceeds 1.50, the main form of sulfide inclusions is CaS.
• the square brackets [] in Expressions (1) to (4) indicate content in mass% of an element enclosed in the brackets and have a value of 0 when the element is not contained.
• Average effective crystal grain size at mid-thickness part meaning a range of 10 % of the thickness of the steel plate towards each surface, centered at the 1/2 thickness position of the steel plate: 20 ⁇ m or less
• average effective crystal grain size of microstructure in the mid-thickness part of the steel plate having a thickness exceeding 100 mm is 20 ⁇ m or less. Crystal grains in the mid-thickness part in which segregation is easily caused are refined as described above to improve base metal toughness, thereby increasing joint CTOD properties at SC/ICHAZ boundaries.
• the smaller the average effective crystal grain size, the more advantageous, and therefore a lower limit of the average effective crystal grain size is not particularly limited. Typically, the lower limit is about 1 ⁇ m.
• effective crystal grain size is defined as the circle equivalent diameter of a crystal grain surrounded by grain boundaries of crystal grains having an orientation difference of 15° or more, that is, large-angle grain boundaries. Further, the average effective crystal grain size in the mid-thickness part can be measured by a method described in the following EXAMPLES section.
• the maximum effective crystal grain size of the microstructure in the mid-thickness part is 150 ⁇ m or less.
• the average effective crystal grain size is 20 ⁇ m or less
• coarse crystal grains having an effective crystal grain size exceeding 150 ⁇ m are mixed into the mid-thickness part, the coarse crystal grains become fracture origins and lead to a decrease in base metal strength, base metal toughness, and SCHAZ toughness in the mid-thickness part.
• the maximum effective crystal grain size is therefore 150 ⁇ m or less.
• the maximum effective crystal grain size can be measured by the method described in the EXAMPLES section below.
• the present disclosure limits the number density at the 1/2 thickness position of composite inclusions containing sulfides containing Ca and Mn and oxides containing Al and having a circle equivalent diameter of 0.1 ⁇ m or more to a range from 25/mm 2 to 250/mm 2 .
• Mn-containing sulfides When Mn-containing sulfides are formed, Mn-poor regions forming around the composite inclusions are effective as nucleation sites. Further, the inclusion of Ca in such sulfides results in a high melting point and allows the inclusions to remain at the temperatures reached by the HAZ in the vicinity of the weld line. As a result, the austenite grain growth suppression and nucleation site effects are exhibited, and joint CTOD properties are improved. In order to fully exhibit the above effect, the number density of composite inclusions at the 1/2 thickness position needs to be 25/mm 2 or more. On the other hand, the presence of excessive amounts of composite inclusions can degrade joint CTOD properties.
• the number density of composite inclusions having a circle equivalent diameter of 0.1 ⁇ m or more at the 1/2 thickness position is 250/mm 2 or less.
• the number density is preferably 30/mm 2 or more.
• the number density is preferably 215/mm 2 or less.
• the number density is more preferably 50/mm 2 or more.
• the number density is more preferably 200/mm 2 or less.
• the number density can be measured by the method described in the EXAMPLES section below.
• the frequency of measurement of average effective crystal grain size, maximum effective crystal grain size, and composite inclusions may be such that one or two cross-sections at the mid-thickness part of any one steel plate are measured among steel plates produced under the same material smelting conditions and rolling conditions. As long as the material steelmaking method and rolling conditions are not changed, the crystal grain size and inclusion number density are highly reproducible, and therefore measurement results at the above measurement frequency are representative of the whole.
• temperature means temperature at the mid-thickness part, unless otherwise noted. Further, temperature at the mid-thickness part may be measured as described in the following EXAMPLES section. However, for example, on an actual production line, temperature at a surface of a steel plate may be measured using a radiation thermometer and temperature at the mid-thickness part may be determined by heat transfer calculation.
• a method of preparing the material is not particularly limited, and any known steelmaking method may be applied, such as a converter, an electric furnace, a vacuum melting furnace, and the like.
• the material may be produced, for example, by a continuous casting method. Further, molten steel from which the material is produced may be further subjected to secondary refining such as ladle refining.
• the material having the chemical composition and produced as described above is heated to a temperature of 990 °C or more and 1210 °C or less.
• the heating temperature is lower than 990 °C, the following conditions of hot rolling cannot be met, and a sufficient effect cannot be obtained.
• the heating temperature is higher than 1210 °C, austenite grains become coarse and the desired fine grain microstructure cannot be obtained after controlled rolling.
• the range of the heating temperature is 990 °C or more to 1210 °C or less.
• the temperature is preferably 1010 °C or more.
• the temperature is preferably 1190 °C or less.
• the temperature is more preferably 1030 °C or more.
• the temperature is more preferably 1170 °C or less.
• Rolling in the recrystallization temperature range defined as the mid-thickness temperature being the T 1 temperature or more is carried out with an average rolling reduction per pass of 3 % or more and cumulative rolling reduction ratio of 25 % or more.
• the purpose of rolling in the recrystallization temperature range is to refine the microstructure by recrystallization, homogenize microstructure, and to refine and disperse coarse inclusions, even in steel plates having a thickness exceeding 100 mm.
• the mid-thickness temperature is T 1 °C to T 2 °C
• a mixed-grain-size microstructure of recrystallized grains and coarse recovered grains is formed and the desired uniformly-sized grain microstructure cannot be obtained. Therefore, it is necessary to roll at the mid-thickness temperature of T 1 °C or more while avoiding rolling at the mid-thickness temperature of T 1 °C to T 2 °C.
• the average rolling reduction per pass is less than 3 %, it is not possible to introduce sufficient strain into the mid-thickness part in a steel plate having a thickness exceeding 100 mm thick, and the mid-thickness part microstructure cannot be sufficiently refined. Further, even when the average rolling reduction per pass is 3 % or more, when cumulative rolling reduction ratio is less than 25 %, recrystallization does not progress sufficiently and uniform microstructure cannot be obtained. For this reason, rolling in the recrystallization temperature range is performed with an average rolling reduction per pass of 3 % or more and a cumulative rolling reduction ratio of 25 % or more.
• the cumulative rolling reduction ratio is preferably 30 % or more.
• the cumulative rolling reduction ratio is more preferably 35 % or more.
• the conditions for rolling in the non-recrystallization temperature range are not particularly limited.
• a larger average rolling reduction per pass (average of rolling reduction in each pass) is preferable, and specifically, the average rolling reduction per pass is preferably 3 % or more.
• the obtained hot-rolled steel plate is cooled.
• the cooling can be done by any method as long as the conditions described below are met.
• the cooling can be done by water cooling.
• Average cooling rate 1.0 °C/s to 50.0 °C/s
• the average cooling rate at the mid-thickness position is therefore 1.0 °C/s to 50.0 °C/s.
• the average cooling rate at the mid-thickness position is preferably 1.2 °C/s or more.
• the average cooling rate is preferably 45 °C/s or less.
• the average cooling rate is more preferably 1.5°C/s or more.
• the average cooling rate is more preferably 40°C/s or less.
• the range when the cooling stop temperature is 500 °C or less, the range is 700 °C to 500 °C. When the cooling stop temperature is greater than 500 °C, the range is 700 °C to the cooling stop temperature.
• Cooling stop temperature 600 °C or less
• the hot-rolled steel plate is cooled to a cooling stop temperature where the mid-thickness temperature is 600 °C or less.
• the cooling stop temperature at the mid-thickness temperature is therefore 600 °C or less.
• the cooling stop temperature is preferably 580 °C or less.
• the cooling stop temperature is more preferably 560 °C or less.
• a lower limit of the cooling stop temperature is not particularly limited. The lower limit is preferably about 200 °C.
• Tempering Temperature 700 °C or less
• the steel plate may be subjected to tempering treatment.
• Tempering treatment can further improve base metal toughness.
• a tempering temperature higher than 700 °C generates a coarse ferrite phase, thus degrading base metal toughness and SCHAZ toughness.
• the tempering temperature at the mid-thickness temperature is 700 °C or less.
• the tempering temperature is more preferably 650 °C or less.
• a lower limit of the tempering temperature is not particularly limited as long as the effect of improving base metal toughness can be obtained. The lower limit may be around 300 °C.
• Material having a chemical composition listed in Table 1 was used to produce steel plates under the production conditions listed in Table 2. Rolling at T 1 °C or more was performed with an average rolling reduction per pass ⁇ 3 %. During hot rolling, a thermocouple was attached in a central position in the longitudinal direction, width direction, and thickness direction of each steel material to be hot rolled to measure the temperature of the mid-thickness part. At the same time, surface temperature of the steel material was measured with a radiation thermometer.
• Average effective crystal grain size, maximum effective crystal grain size, number density of composite inclusions containing sulfides containing Ca and Mn and oxides containing Al, yield stress, toughness, and CTOD properties were measured for each of the steel plates obtained by the following methods.
• a sample was collected from each obtained steel plate so that a measurement position was located at a central position in the longitudinal direction, width direction, and thickness direction of the steel plate, so as to include the mid-thickness part. Then, after mirror polishing a surface of the sample, electron backscatter pattern (EBSP) analysis was performed under the following conditions. From an obtained crystal orientation map, a circle equivalent diameter of a microstructure surrounded by a large-angle grain boundary having an orientation difference of 15° or more from adjacent crystal grains was determined, and an average of the circle equivalent diameters in the following analysis region was defined as an average effective crystal grain size. The maximum value of circle equivalent diameter obtained was defined as the maximum effective grain size.
• EBSP electron backscatter pattern
• Samples were taken from the center of the steel plate in the longitudinal, width, and thickness directions, and mirror polished, finishing with diamond buffing + alcohol. Then, using a field emission scanning electron microscope (FE-SEM), composite inclusions having a circle equivalent diameter of 0.1 ⁇ m or more in the 1 mm ⁇ 1 mm evaluation area, the center of which is the center of the area, were identified by EDX (energy dispersive X-ray spectroscopy) analysis, and the number density of the composite inclusions was also evaluated. Evaluation of inclusion type was performed such that an inclusion was judged to contain an element when the chemical composition of the inclusion, quantified by the ZAF method, contained 3 % or more of the element in terms of atomic fraction.
• FE-SEM field emission scanning electron microscope
• V-notch test pieces as specified in Japanese Industrial Standard JIS Z2242 were taken from the 1/2 thickness position of the steel plate so that the longitudinal direction of each test piece was perpendicular to the rolling direction of the plate.
• the absorbed energy vE- 40 °C at -40 °C was measured by the Charpy impact test.
• the base metal toughness was considered to be good when the average vE- 40 °C of three such test pieces was 100 J or more.
• each steel plate was used to produce a multilayer-fill-welded joint.
• Each obtained multilayer-fill-welded joint was subjected to a joint CTOD test to measure the amount of crack opening displacement in the CGHAZ and the amount of crack opening displacement in the SC/ICHAZ. The conditions for production of the multilayer-fill-welded joints and the conditions of the joint CTOD tests are described below.
• Welded joints used for the joint CTOD tests were produced by submerged arc welding (multilayer fill welding) with K groove geometry and heat input of 5.0 kJ/mm.
• the test method was based on BS EN10225-4 (2019), and the crack opening displacement (CTOD value ( ⁇ )) was evaluated at the test temperature of -20 °C using test pieces each having a square cross-section of t x t (where t is plate thickness).
• a higher CTOD value ( ⁇ ) indicates that a brittle crack is less likely to occur.

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• 2023
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