WO2010074347A1 - 溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材およびその製造方法 - Google Patents
溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材およびその製造方法 Download PDFInfo
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- 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
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- 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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
- C21D2211/002—Bainite
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- 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
- C21D2211/005—Ferrite
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- 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
- C21D2211/008—Martensite
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- 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
- C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
Definitions
- the present invention relates to a steel material suitable for use in welded structures such as pipelines, bridges, and architectures where structural safety is required, and a method for manufacturing the same.
- the present invention relates to a heat affected zone (welded heat affected zone) and a material having excellent resistance to ductile crack initiation of a base metal portion. Specifically, it is excellent in ductile crack initiation characteristics of the weld heat affected zone and the base metal, and has a tensile strength of 490 MPa or more in TS and a Charpy impact test (as defined in JIS Z 2242). Compliant) ductile / brittle fracture transition temperature: For structural steels having high toughness with vTrs of 0 ° C. or less.
- Patent Document 1 discloses that the microstructure of a steel surface portion has a ferrite fraction (ferrite area fraction) of 10 to 40%, a bainite fraction of 50% or more, and an average grain size (average grain).
- a high-strength steel material having excellent ductile crack generation characteristics characterized in that the size) is 5 ⁇ m or less is described.
- Patent Document 2 discloses a steel sheet whose microstructure is substantially composed of a ferrite structure, a pearlite structure, and a bainite structure, and is composed of three layers of both surface portions of the steel plate and a central portion in the plate thickness direction. When divided, steel sheets each having a specific microstructure and excellent in arrestability and ductile crack fracture resistance are described.
- Both surface portions of the steel sheet have a ferrite structure having 50% or more of ferrite grains having an equivalent grain diameter of 7 ⁇ m or less and an aspect ratio of 2 to 4 over 5% or more of the plate thickness, and the portion
- the bainite fraction is composed of a layer having a ratio of 5 to 25% or less, and the central portion in the thickness direction of the steel sheet extends over 50% of the thickness of the sheet, and the equivalent circle average particle diameter is 4 to 10 ⁇ m, and the aspect ratio is 2 or less. It has a ferrite grain and is composed of a layer having a bainite fraction of 10% or less.
- Patent Document 2 is a steel sheet in which three layers having ferrite / pearlite structures composed of ferrite grains having different aspect ratios exist from the surface of the steel sheet in the thickness direction, and further the soft phase.
- a bainite structure as a hard phase is appropriately dispersed in the ferrite pearlite structure.
- both surface portions of the steel plate are actively formed with processed ferrite grains having a large aspect ratio, and the arrest characteristics are enhanced by appropriately dispersing the bainite structure, while the central portion of the steel plate is uniform, etc.
- Patent Document 3 is a technique for forming processed ferrite grains on the steel sheet surface layer of ferritic pearlite steel and making the microstructure inside the steel sheet uniform and equiaxed ferrite grains as in the technique of Patent Document 2. It is. That is, Patent Document 3 describes a method for producing a thick steel plate having excellent arrest characteristics and ductile fracture characteristics in which rolling conditions are strictly controlled and a steel sheet surface layer portion has a specific microstructure.
- the surface layer zone from 0.05 t to 0.15 t from both surfaces in the plate thickness direction is Ar 3 transformation point or higher and 900 ° C. or lower.
- Equivalent plastic strain ⁇ satisfying ⁇ ⁇ 0.5 is applied in the non-recrystallization temperature zone.
- the inner side on the core side from the thickness t / 4 position from both surfaces is within the time.
- the surface layer region is cooled to a temperature range of 450 to 650 ° C. at a cooling rate of 2 to 15 ° C./s, and then rolling is resumed.
- Patent Documents 1 to 3 are concerned that the effect of ductile crack generation is lost when the surface layer structure is changed to a weld heat affected zone by welding or the like.
- the present invention aims to provide a steel material having excellent ductile crack generation characteristics in a welding heat-affected zone and a base metal, and a method for producing the same, by a simple method. To do.
- the present inventors have conducted intensive research on a base material structure excellent in ductile crack generation characteristics of a weld heat-affected zone, and presenting an average structure in the thickness direction of a steel sheet.
- the base material structure is a ferrite and a hard phase that define the average aspect ratio of ferrite and the area fraction of the hard phase (hard phase)
- a ductile crack is also generated in the weld heat affected zone.
- the present inventors have found that the generation characteristics are excellent, and that such a steel material is also excellent in the ductile crack generation characteristics of the base metal portion, and further the manufacturing conditions of the steel sheet having the microstructure.
- the present invention has been completed on the basis of such findings, and has been completed.
- C 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, having a composition comprising the balance Fe and inevitable impurities, and the microstructure at the 1/4 position of the plate thickness is ferrite.
- It consists of a hard phase, the area fraction of the hard phase is 50 to 90%, and the average aspect ratio of the ferrite is 1.5 or more Steel with excellent crack initiation characteristics.
- a steel material with excellent ductile cracking characteristics in the weld heat-affected zone and the base metal is reheated to 1000 ° C. or higher, the rolling reduction in the temperature range of 900 ° C. or higher is 50% or higher, and the rolling finish temperature is Ar 3 points.
- the tempering treatment is further performed at a maximum heating temperature Ac of less than 1 point, so that the weld heat-affected zone and the base metal portion have ductile crack initiation characteristics as described in (4). Excellent steel manufacturing method.
- the present invention for example, even if a large deformation due to an earthquake or the like occurs in a steel structure, it is possible to suppress the occurrence of a ductile crack from a stress concentrated portion such as a weld toe, and prevent the steel structure from collapsing or breaking. Steel products that can suppress the occurrence of ductile cracks in the weld heat-affected zone and the base metal can be easily and stably mass-produced, and have remarkable industrial effects.
- the component composition and the microstructure are defined. Unless otherwise specified in the description of the component composition, the mass% is simply represented by%.
- [Ingredient composition] C 0.02 to 0.2% C is an element having an action of increasing the strength of steel, and in the present invention, contributes particularly to the formation of a hard phase. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, when the content exceeds 0.2%, ductility and bending workability are lowered, and weldability is lowered. For this reason, C is limited to the range of 0.02 to 0.2%. More preferably, it is 0.02 to 0.18%.
- Si acts as a deoxidizer and has the effect of improving the strength of the steel by solid solution. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, the content exceeding 0.5% reduces toughness and weldability. For this reason, Si was limited to the range of 0.01 to 0.5%. More preferably, it is 0.01 to 0.4%.
- Mn 0.1 to 2.5%
- Mn has the effect of increasing the strength of the steel and improving the toughness through improving the hardenability. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, the content exceeding 2.5% lowers the weldability. For this reason, Mn was limited to the range of 0.1 to 2.5%. More preferably, it is 0.5 to 2.0%.
- P 0.05% or less Since P leads to deterioration of toughness, it is preferable to reduce it as much as possible, but 0.05% is acceptable. For this reason, P was limited to 0.05% or less. More preferably, it is 0.04% or less.
- S 0.05% or less S is present as an inclusion in steel and deteriorates ductility and toughness. Therefore, it is desirable to reduce it as much as possible, but 0.05% is acceptable. For this reason, S was limited to 0.05% or less. More preferably, it is 0.04% or less.
- Al 0.1% or less Al is an element that acts as a deoxidizer and contributes to refinement of crystal grains, but excessive content exceeding 0.1% leads to a decrease in toughness. For this reason, Al was limited to 0.1% or less. More preferably, it is 0.05% or less.
- N 0.01% or less is an element that increases the strength of the steel by solid solution strengthening in the same manner as C. However, since excessive inclusion leads to a decrease in toughness, N is limited to 0.01% or less. More preferably, it is 0.005% or less.
- the above-mentioned components are basic components, but in the present invention, Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01-2%, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca: 0.01% or less, REM: 0 One or two or more selected from 1% or less may be contained.
- Cu 0.01-2% Cu is an element having an action of increasing the strength of steel through an increase in hardenability and solid solution. In order to ensure such an effect, a content of 0.01% or more is required. On the other hand, if the content exceeds 2%, weldability deteriorates and flaws are likely to occur during the production of the steel material. Therefore, when added, the content is made 0.01 to 2%. More preferably, it is 0.01 to 1%.
- Ni 0.01-5% Ni contributes to improvement of low temperature toughness, increase of hardenability, and prevention of hot brittleness of Cu when Cu is contained, so it is added as necessary. Although such an effect is recognized by addition of 0.01% or more, addition of 5% or more causes a reduction in steel material cost and weldability. Therefore, when added, the content is made 0.01 to 5%. More preferably, it is 0.01 to 4.5%.
- the content 0.01 to 3% Cr is added as necessary in order to increase the strength of the steel material by improving the hardenability and increasing the temper softening resistance. Such an effect is recognized when the content is 0.01% or more. On the other hand, addition exceeding 3% lowers weldability and toughness. Therefore, when added, the content is made 0.01 to 3%. More preferably, the content is in the range of 0.01 to 2.5%.
- Mo 0.01-2% Mo is added as necessary in order to increase the strength of the steel material by improving the hardenability and increasing the temper softening resistance. Such an effect is recognized when the content is 0.01% or more. On the other hand, addition exceeding 2% lowers weldability and toughness. Therefore, when added, the content is made 0.01 to 2%. More preferably, the content is in the range of 0.01 to 1%.
- Nb 0.1% or less
- Nb is an element that precipitates as carbide or carbonitride during tempering and increases the strength of the steel through precipitation strengthening. Nb also has the effect of improving the toughness by refining austenite grains during rolling. In order to obtain the effect, 0.001% or more is preferable. However, the content exceeding 0.1% lowers the toughness. For this reason, when adding, it is made into 0.1% or less. More preferably, it is 0.05% or less.
- V 0.1% or less
- V is an element that precipitates as carbide or carbonitride during tempering and increases the strength of steel through precipitation strengthening. Moreover, it has the effect of refining austenite grains during rolling and improving toughness. In order to obtain the effect, 0.001% or more is preferable. However, the content exceeding 0.1% lowers the toughness. For this reason, when adding, it is made into 0.1% or less. More preferably, it is 0.05% or less.
- Ti 0.1% or less Since Ti has the effect of refining austenite and improving toughness in the weld heat affected zone, it is added as necessary. In order to obtain the effect, 0.001% or more is preferable. However, addition exceeding 0.1% lowers toughness and leads to an increase in steel material cost. For this reason, when adding, it is made into 0.1% or less. More preferably, it is made 0.05% or less.
- B 0.01% or less B is added as necessary because it has the effect of improving the hardenability and increasing the strength of steel when contained in a small amount. In order to acquire the effect, 0.0001% or more is preferable. However, addition of 0.01% or less reduces weldability. For this reason, when adding, it is 0.01% or less. More preferably, it is 0.005% or less.
- Ca 0.01% or less Ca is added as necessary to improve the toughness of the base metal by controlling the form of the CaS inclusions and further to improve the toughness of the weld heat affected zone. In order to acquire the effect, 0.0001% or more is preferable. However, addition over 0.01% reduces toughness due to an increase in CaS inclusions. For this reason, when adding, it is 0.01% or less. More preferably, it is 0.009% or less.
- REM 0.1% or less REM is an element that improves the toughness of the weld heat affected zone, and is added as necessary. In order to acquire the effect, 0.0001% or more is preferable. However, addition exceeding 0.1% causes a decrease in toughness. For this reason, when adding, it is made into 0.1% or less. More preferably, it is made 0.05% or less.
- REM is a general term for rare earth elements such as Y and Ce, and the added amount here means the total amount of these rare earth elements.
- the structure at the 1/4 position of the plate thickness is composed of ferrite and a hard phase, the area fraction of the hard phase is 50 to 90%, and the average aspect ratio of the ferrite grain size is 1.5. It has the above microstructure. If the area fraction of the hard phase is less than 50% or more than 90%, or the average aspect ratio of the ferrite grain size is less than 1.5, ductile cracks may occur.
- the upper limit value of the average aspect ratio of the ferrite grain size is not particularly required, but is set to 5 or less from the capability of the rolling mill.
- the area fraction of the hard phase is more preferably 52 to 90%, and the average aspect ratio of the ferrite particle diameter is more preferably 1.6 or more. More preferably, it is 1.7 or more.
- the yield ratio of the base material decreases, and the stress concentration part remains in the base material or after a simulated heat cycle that simulates the weld heat affected zone.
- the strain concentration in is relaxed. Such an effect cannot be obtained when the ferrite single phase or the hard phase single phase is used.
- the structure of the steel plate surface (1 mm from the plate surface) is composed of ferrite and a hard phase, and the area ratio of ferrite is more than 40%, more preferably 50% or more. Further, the average aspect ratio of the ferrite grain size exceeds 2. When the ferrite area ratio is less than 40% or the average aspect ratio of the ferrite grain size is 2 or less, the ductile crack resistance in the weld heat affected zone is inferior.
- the hard phase is bainite, martensite, or a bainite / martensite mixed structure, and includes an island-shaped martensite (MA) (MA) having an area fraction of 5% or less.
- MA island-shaped martensite
- FIG. 2 shows the results of investigating ductile crack initiation characteristics using a simulated heat cycle specimen (maximum heating temperature 1400 ° C.) of the weld, and as shown in FIG.
- maximum heating temperature 1400 ° C. maximum heating temperature 1400 ° C.
- Fig. 1 shows the specimen shape and test conditions. Clamp clamped with a single through-thickness edge notch in the thickness direction of 3 mm in the center of the reproducible heat cycle part 2 of the test material (test piece 1) to which the reproducible heat cycle was applied ) Tensile load (arrow 6) up to 0.6 mm by displacement of clip gauge 3 between knife-edge 4 which is restrained and screwed by 5 and then unloaded. The presence or absence of cracks at the notch tip was evaluated by grinding and mirror polishing to the center of the specimen width. The case where the ductile crack from the notch bottom was 50 ⁇ m or more was defined as crack initiation.
- FIG. 4 shows the result of investigating the influence of the microstructure of the base material portion on the ductile crack initiation characteristics.
- the area fraction of the hard phase of the base material is 50 to 90. %, And when the average aspect ratio of the ferrite is 1.5 or more, ductile cracks are not observed.
- a test piece having a length of ⁇ 200 was taken from the center of 1/4 of the plate thickness (plate thickness of 25 mm or less is 1/2 center of the plate thickness) (FIG. 3).
- FIG. 3 shows the test piece shape and test conditions.
- a test piece (test piece 1) with a one-side through notch introduced in the thickness direction of 3mm in the center is restrained by a clamp 5, and the displacement of the clip gauge 3 between screwed knife edges 4 is 0.8mm.
- the unloading was performed, and the test piece was ground and mirror-polished to the center of the width of the specimen to evaluate the presence or absence of cracks at the notch tip.
- the case where the ductile crack from the notch bottom was 50 ⁇ m or more was defined as crack initiation.
- the results shown in FIG. 4 show that the yield ratio (0.2% proof stress / tensile strength) is reduced and the degree of strain concentration at the notch tip is reduced by making the base material a composite structure of ferrite and hard phase. This is probably due to this.
- the aspect ratio refers to the ferrite particle diameter in the rolling direction (long diameter) / plate thickness direction (short diameter) in the cross section parallel to the rolling direction.
- the steel material according to the present invention can be obtained by sequentially performing a hot rolling process, a water cooling process, or a tempering process on the steel material having the above components.
- the hot rolling re-heating is performed to 1000 ° C. or higher, and rolling is performed in a temperature range of 900 ° C. or higher so that the rolling reduction is 50% or higher and the rolling finishing temperature is Ar 3 points to Ar 3 ⁇ 50 ° C.
- a more preferable rolling finishing temperature is less than Ar 3 points to Ar 3 -40 ° C.
- the cumulative rolling reduction at 900 ° C. or higher is less than 50%, desired strength and toughness cannot be ensured.
- the rolling finishing temperature exceeds the Ar 3 point, the ferrite aspect ratio is not 1.5 or more. If finish rolling temperature falls below the Ar 3 -50 ° C., the fraction of hard phase obtained by the subsequent water cooling is not more than 50%.
- water cooling starts at Ar 3 ⁇ 10 ° C. to Ar 3 ⁇ 70 ° C., and ends at 500 ° C. or less.
- the water cooling start temperature exceeds Ar 3 ⁇ 10 ° C., it becomes a ferrite (hard phase exceeding 90% in area fraction) with an area fraction below 10%.
- the water cooling start temperature is lower than Ar 3 -70 ° C. or immediately after hot rolling (within 300 seconds)
- ferrite with an area fraction exceeding 50% (in area fraction) In the hard phase (less than 50%) and the present invention, pearlite that is not desired to precipitate is precipitated, and the desired characteristics cannot be satisfied.
- a tempering treatment can be further performed at less than Ac 1 point.
- the toughness and ductility can be improved and adjusted to the desired strength and toughness.
- the tempering temperature exceeds 1 point of Ac, a large amount of island martensite is generated, and the toughness is lowered.
- Ar 3 points and Ac 1 point can be calculated by the following formula based on the content (% by mass) of each component.
- the present invention will be described in more detail based on examples.
- the steel materials having the components shown in Table 1 were hot-rolled under the conditions shown in Table 2 to obtain steel plates having a thickness of 12 to 100 mm.
- the obtained steel sheet was subjected to a structure observation, a tensile test, a toughness test, a ductile crack generation test after a reproducible thermal cycle, and a ductile crack generation test of the base material.
- the test method was as follows (1) to (5).
- a one-side through notch was introduced in the thickness direction of 3 mm in the center of the reproduction heat cycle portion.
- the notch machining was performed by electric discharge machining and the notch tip radius was set to 0.1 mm.
- the left and right ends of the test piece were gripped with a restrained length of 50 mm, and a tensile load was applied.
- the displacement between the knife edges attached by screwing in the vicinity of the notch was measured with a clip gauge, and after the tension loading to 0.6 mm by the clip gauge displacement, the load was removed. Thereafter, the test was cut to the center of the width and mirror-polished, and the state of crack generation at the bottom of the notch was examined with a microscope of magnification x50.
- the definition of ductile crack initiation was defined as when the ductile crack extended from the notch bottom by 50 ⁇ m or more.
- the displacement between the knife edges attached by screwing in the vicinity of the notch was measured with a clip gauge, and after the tension loading to 0.8 mm by the clip gauge displacement, the load was removed. Thereafter, the test was cut to the center of the width and mirror-polished, and the state of crack generation at the bottom of the notch was examined with a microscope of magnification x50.
- the definition of ductile crack initiation was defined as when the ductile crack extended from the notch bottom by 50 ⁇ m or more.
- Table 3 shows the experimental results obtained for the test pieces to which the reproducible thermal cycle was applied.
- the components specified in the present invention No. produced by the production method. 1-No.
- Each of the 10 steel plates has the prescribed structure of the present invention. And while having the outstanding intensity
- No. C is less than the lower limit of the range of the present invention.
- No. 11 steel plate (steel type K *) has low tensile strength.
- steel type L * has low toughness and is inferior in ductile crack initiation characteristics in the weld heat affected zone.
- the reheating temperature of the slab is lower than that of the present invention, and the cumulative rolling reduction of 900 ° C. or more falls outside the scope of the present invention.
- No. 13 steel sheet has low toughness.
- the rolling finish temperature and the water cooling start temperature exceed the range of the present invention.
- the steel plate No. 14 does not form ferrite and does not have the structure defined by the present invention, and is inferior in the ductile crack initiation characteristics of the weld heat affected zone.
- the water cooling start temperature falls below the range of the present invention.
- Table 4 shows the experimental results obtained for the base metal part.
- the components specified in the present invention No. produced by the production method. 18 ⁇ No.
- the 27 steel plates all have the prescribed structure of the present invention. And while having the outstanding intensity
- No. C is less than the lower limit of the range of the present invention.
- 28 steel plates (steel type W *) have low tensile strength.
- 29 steel plate (steel type X *) has low toughness.
- the reheating temperature of the slab is below the range of the present invention, and the cumulative rolling reduction of 900 ° C.
- the 30 steel plate has low toughness.
- the rolling finish temperature and the water cooling start temperature exceed the range of the present invention.
- the steel plate No. 31 does not form ferrite and does not have the structure defined by the present invention, and is inferior in ductile crack resistance.
- the water cooling start temperature falls below the range of the present invention.
- No. 32 steel plate and water cooling stop temperature No. exceeding the range of the present invention.
- the steel plate No. 33 has a hard phase fraction and an average aspect ratio of ferrite that are not specified in the present invention, and both have low tensile strength and inferior ductility cracking characteristics. No. with tempering temperature exceeding the value of the present invention.
- the steel plate No. 34 has low toughness because a large amount of island-like martensite is generated, and is inferior in ductile crack resistance.
- Test piece 2 Reproduction thermal cycle part 3: Clip gauge 4: Knife edge 5: Clamp 6: Tensile load
Abstract
Description
すなわち、特許文献2の技術は、鋼板の板表面から板厚方向に向けて、アスペクト比の異なるフェライト粒からなるフェライト・パーライト組織を有する層が三層存在している鋼板であり、更に軟質相である当該フェライト・パーライト組織中に硬質相であるベイナイト組織を適切に分散させたものである。このうち鋼板の両表面部には各々、アスペクト比の大きな加工フェライト粒を積極的に形成させるとともに、ベイナイト組織を適切に分散させることによりアレスト特性を高め、一方、鋼板の中央部は均一な等軸フェライト粒組織に制御するとともに、ベイナイト組織を抑制することにより常温時の延性破壊に対して重要な伸び特性を向上させるものであり、この鋼板の両表面部及び中央部を上記三層構造に制御することにより、「アレスト特性」と「延性破壊特性」という相反する特性を両方満足させる技術である。
すなわち、特許文献3には圧延条件を厳格に制御して鋼板表層部を特定のミクロ組織としたアレスト特性および延性破壊特性に優れた厚鋼板の製造方法が記載されている。
また、特許文献1~3の技術は、いずれもオーステナイトの未再結晶域(細粒化温度域)で圧延あるいは、圧延仕上げ温度Ar3以上で圧延することで、オーステナイト中に微細なサブグレインを形成させ、変態後の組織を微細化する技術に関するものである。
(1) 質量%で、C:0.02~0.2%、Si:0.01~0.5%、Mn:0.5~2.5%、P:0.05%以下、S:0.05%以下、Al:0.1%以下、N:0.01%以下を含み、残部Feおよび不可避的不純物からなる組成を有し、板厚の1/4位置のミクロ組織がフェライトと硬質相からなり、前記硬質相の面積分率が50~90%で、かつ、前記フェライトの平均アスペクト比が1.5以上であることを特徴とする溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材。
(2)成分組成に、更に、質量%でCu:0.01~2%、Ni:0.01~5%、Cr:0.01~3%、Mo:0.01~2%、Nb:0.1%以下、V:0.1%以下、Ti:0.1%以下、B:0.01%以下、Ca:0.01%以下、REM:0.1%以下のうちから選ばれた1種または2種以上を含有することを特徴とする(1)に記載の溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材。
(3)上記(1)または(2)において、鋼板表面の組織が、フェライトと硬質相からなり、フェライトの面積率が、40%を超え、かつ、フェライト粒径の平均アスペクト比が、2を超えることを特徴とする溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材。
(4)(1)または(2)に記載の成分を有する鋼素材を1000℃以上に再加熱し、900℃以上の温度域での圧下率が50%以上で圧延仕上げ温度がAr3点~Ar3−50℃となる圧延を施した後、Ar3−10℃~Ar3−70℃で水冷を開始し、500℃以下で水冷を終了することを特徴とする溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材の製造方法。
(5)水冷を行った後、更に、最高加熱温度Ac1点未満で焼戻し処理を行うことを特徴とする(4)に記載の溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材の製造方法。
C:0.02~0.2%
Cは鋼の強度を増加させる作用を有する元素であり、本発明では特に硬質相の生成に寄与する。このような効果を得るためには0.02%以上の含有を必要とする。一方、0.2%を超えて含有すると、延性(ductility)や曲げ加工性(bending workability)を低下させるとともに、溶接性(weldability)が低下する。このため、Cは0.02~0.2%の範囲に限定した。より好ましくは、0.02~0.18%である。
Siは脱酸剤として作用するとともに、固溶して鋼の強度を向上させる作用を有する。このような効果を得るためには0.01%以上の含有を必要とする。一方、0.5%を超える含有は、靱性を低下させるとともに溶接性を低下させる。このため、Siは0.01~0.5%の範囲に限定した。より好ましくは0.01~0.4%である。
Mnは焼き入れ性の向上を通じて、鋼の強度を増加させるとともに、靱性を向上させる作用を有する。このような効果を得るためには、0.1%以上の含有を必要とする。一方、2.5%を超える含有は溶接性を低下させる。このため、Mnは0.1~2.5%の範囲に限定した。より好ましくは0.5~2.0%である。
Pは靱性の劣化に繋がるため、できるだけ低減することが好ましいが、0.05%までは許容できる。このため、Pは0.05%以下に限定した。より好ましくは、0.04%以下である。
Sは鋼中では介在物として存在し、延性、靱性を劣化させるため、できるだけ低減することが望ましいが、0.05%までは許容できる。このため、Sは0.05%以下に限定した。より好ましくは、0.04%以下である。
Alは脱酸剤として作用するとともに、結晶粒の微細化にも寄与する元素であるが、0.1%を超える過剰の含有は靱性の低下に繋がる。このため、Alは0.1%以下に限定した。より好ましくは0.05%以下である。
NはCと同様に固溶強化により鋼の強度を増加させる元素であるが、過剰の含有は靱性の低下に繋がるため、Nは0.01%以下に限定した。より好ましくは0.005%以下である。
Cuは焼き入れ性の増加や固溶を通じて鋼の強度を増加させる作用を有する元素である。このような効果を確保するためには0.01%以上の含有を必要とする。一方、2%を超える含有は、溶接性が低下するとともに、鋼材製造時に疵が生じやすくなる。このため、添加する場合には0.01~2%の範囲とする。より好ましくは、0.01~1%である。
Niは低温靱性の向上、焼入れ性の増加、Cu含有時にCuの熱間脆性の防止に寄与するため、必要に応じて添加する。このような効果は0.01%以上の添加で認められるが、5%以上の添加は鋼材コストの低下を招くとともに、溶接性が低下する。このため、添加する場合には0.01~5%の範囲とする。より好ましくは、0.01~4.5%である。
Crは焼き入れ性の向上や焼戻し軟化抵抗の増加を通じて、鋼材の強度を増加させるために必要に応じて添加する。このような効果は0.01%以上の含有で認められる。一方、3%を超える添加は溶接性と靱性を低下させる。このため、添加する場合には0.01~3%の範囲とする。より好ましくは0.01~2.5%の範囲とする。
Moは焼き入れ性の向上や焼戻し軟化抵抗の増加を通じて、鋼材の強度を増加させるために必要に応じて添加する。このような効果は0.01%以上の含有で認められる。一方、2%を超える添加は溶接性や靱性を低下させる。このため、添加する場合には0.01~2%の範囲とする。より好ましくは0.01~1%の範囲とする。
Nbは焼戻し時に炭化物(carbide)や炭窒化物(carbonitride)として析出し、析出強化(precipitation strengthening)を通じて鋼の強度を増加させる元素である。また、Nbは圧延時にオーステナイト粒を微細化させて靱性を向上させる効果も有する。その効果を得るためには、0.001%以上が好ましい。しかし、0.1%を超える含有は靱性を低下させる。このため、添加する場合には0.1%以下とする。より好ましくは0.05%以下である。
Vは焼戻し時に炭化物や炭窒化物として析出し、析出強化を通じて鋼の強度を増加させる元素である。また、圧延時にオーステナイト粒を微細化させて靱性を向上させる効果も有する。その効果を得るためには、0.001%以上が好ましい。しかし、0.1%を超える含有は靱性を低下させる。このため、添加する場合には0.1%以下とする。より好ましくは0.05%以下である。
Tiは溶接熱影響部においてオーステナイトを微細化させ靱性を向上させる効果を有するため、必要に応じて添加する。その効果を得るためには、0.001%以上が好ましい。しかし、0.1%を超える添加は靱性を低下させるとともに、鋼材コストの高騰に繋がる。このため、添加する場合には0.1%以下とする。より好ましくは0.05%以下とする。
Bは少量の含有で焼入れ性を向上させ、鋼の強度を増加させる効果を有するため必要に応じて添加する。その効果を得るためには、0.0001%以上が好ましい。しかし、0.01%以下の添加は溶接性を低下させる。このため、添加する場合には0.01%以下とする。より好ましくは0.005%以下とする。
CaはCaS介在物の形態制御により母材靱性を向上させ、さらには溶接熱影響部の靱性を向上させるため必要に応じて添加する。その効果を得るためには、0.0001%以上が好ましい。しかし、0.01%を超える添加はCaS介在物の増加により靱性を低下させる。このため、添加する場合には0.01%以下とする。より好ましくは、0.009%以下である。
REMは、溶接熱影響部の靱性を向上させる元素であり、必要に応じて添加する。その効果を得るためには、0.0001%以上が好ましい。しかし、0.1%を超える添加は靱性の低下を招く。このため、添加する場合には0.1%以下とする。より好ましくは0.05%以下とする。
本発明に係る鋼材は、板厚の1/4位置の組織がフェライトと硬質相からなり、硬質相の面積分率が50~90%で、かつ、フェライト粒径の平均アスペクト比が1.5以上のミクロ組織を備える。硬質相の面積分率が50%未満あるいは、90%超えあるいは、フェライト粒径の平均アスペクト比が1.5未満の範囲では、延性き裂の発生が生じる恐れがある。
なお、フェライト粒径の平均アスペクト比の上限値は、特に、規定する必要が無いが、圧延機の能力等から5以下とする。また、硬質相の面積分率は、より好ましくは52~90%、フェライト粒径の平均アスペクト比は、より好ましくは、1.6以上である。さらに好ましくは、1.7以上である。
すなわち、図4は、耐延性き裂発生特性に及ぼす母材部のミクロ組織の影響を調査した結果を示し、図4に示されるように、母材の硬質相の面積分率が50~90%であり、かつ、フェライトの平均アスペクト比が1.5以上の場合において、延性き裂の発生が認められない。
図4に示した母材部の結果は、本発明範囲内の組成の鋼を種々の製造方法で作製してミクロ組織を変化させた鋼材から、12mm厚さ(=板厚方向)×12mm幅×200長さの試験片を板厚の1/4中心(板厚25mm以下は板厚の1/2中心)から採取して行った(図3)。
図4に示す結果は、母材をフェライトと硬質相との複合組織とすることで、降伏比(0.2%耐力/引張強度)が低下し、ノッチ先端部におけるひずみ集中の度合いが減少したことによるものと考えられる。
Ar3(℃)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo
Ac1(℃)=723−14Mn+22Si−14.4Ni+23.3Cr
以下、実施例に基づいてさらに本発明を詳細に説明する。
得られた鋼板から、圧延方向に平行な断面において試験片を採取し、鏡面研磨、ナイタルエッチ後に、板厚の1/4位置および表面下1mmの組織観察を行った。観察は各々視野数:20視野で行った。面積分率はフェライトと硬質相を2値化し、倍率×200で求めた。フェライトの平均アスペクト比は、倍率×400でその視野にある個々のフェライトの圧延方向の長さと板厚方向の長さを求め、圧延方向の長さ/板厚方向の長さを求めて、それらの平均値として求めた。
得られた鋼板から、JIS Z 2201(1998)の規定に準拠して、引張方向が鋼板の圧延方向と直角方向になるように、全厚のJIS 5号試験片を採取した。引張試験は、JIS Z 2241(1998)に準拠して行い、0.2%耐力(σ0.2)、引張強度(TS)を求め、静的な引張特性を評価した。
得られた鋼板から、JIS Z 2242(2005)の規定に準拠して、長手方向が圧延方向と平行方向となるようにVノッチ試験片を採取し、延性/脆性破面遷移温度を求め、靱性を評価した。試験片は、板厚が20mm以上の場合は板厚の1/4位置、板厚が20mm未満の場合は板厚の1/2位置を中心となるように採取した。
得られた鋼板から、板厚の1/4中心(板厚25mm以下は板厚の1/2中心)で、12mm厚さ(=板厚方向=t)×12mm幅、全長200mmの試験片を採取した。この試験片に、グリーブル試験機を用い、最高加熱温度を760℃、900℃、1200℃、1400℃とする溶接熱影響部の再現熱サイクルを付与した(最高加熱温度までの到達時間:6s、最高加熱温度から室温までの冷却速度:40℃/s)。
その後、図1に示すように、再現熱サイクル部中央に長さ3mmの板厚方向へ片側貫通ノッチを導入した。ノッチ加工は放電加工により実施し、ノッチ先端半径は0.1mmとした。
試験は、試験片の左右両端部を拘束長さ50mmでグリップし、引張載荷を与えた。試験中は、ノッチ近傍にネジ止めにて取り付けたナイフエッジ間の変位をクリップゲージにて計測し、クリップゲージ変位で0.6mmまで引張載荷した後、除荷した。その後、試験を幅中央まで削り込んで鏡面研磨し、ノッチ底におけるき裂発生状況を倍率×50の顕微鏡で調べた。延性き裂発生の定義は、ノッチ底から延性き裂が50μm以上伸展している時とした。
得られた鋼板から、板厚の1/4中心(板厚25mm以下は板厚の1/2中心)で、12mm厚さ(=板厚方向=t)×12mm幅、全長200mmの試験片を採取した。
得られた試験片に、図3に示すように、試験片中央に長さ3mmの板厚方向へ片側貫通ノッチを導入した。ノッチ加工は放電加工により実施し、ノッチ先端半径は0.1mmとした。
試験は、試験片の左右両端部を拘束長さ50mmでグリップし、引張載荷を与えた。試験中は、ノッチ近傍にネジ止めにて取り付けたナイフエッジ間の変位をクリップゲージにて計測し、クリップゲージ変位で0.8mmまで引張載荷した後、除荷した。その後、試験を幅中央まで削り込んで鏡面研磨し、ノッチ底におけるき裂発生状況を倍率×50の顕微鏡で調べた。延性き裂発生の定義は、ノッチ底から延性き裂が50μm以上伸展している時とした。
一方、Cが本発明の範囲の下限に満たないNo.11の鋼板(鋼種K*)は低引張強度である。また、C、P、Sが本発明の範囲の上限を超えるNo.12の鋼板(鋼種L*)は靱性が低く、溶接熱影響部の延性き裂発生特性に劣る。
スラブの再加熱温度が本発明を下回り、かつ、900℃以上の累積圧下率が本発明の範囲から外れるNo.13の鋼板は靱性が低い。圧延仕上げ温度および水冷開始温度が本発明の範囲を上回るNo.14の鋼板はフェライトが生成せずに本発明が規定する組織とはならず、溶接熱影響部の耐延性き裂発生特性に劣る。
水冷開始温度が本発明の範囲を下回るNo.15の鋼板ならびに水冷停止温度が本発明の範囲を上回るNo.16の鋼板は硬質相分率、フェライトの平均アスペクト比が本発明に規定の値とならず、ともに低引張強度であるとともに、溶接熱影響部の耐延性き裂発生特性に劣る。焼戻し温度が本発明の範囲を超えるNo.17の鋼板は島状マルテンサイトが大量に生成したため低靱性であり、溶接熱影響部の耐延性き裂発生特性に劣る。
一方、Cが本発明の範囲の下限に満たないNo.28の鋼板(鋼種W*)は低引張強度である。また、C、P、Sが本発明の範囲の上限を超えるNo.29の鋼板(鋼種X*)は靱性が低い。スラブの再加熱温度が本発明の範囲を下回り、かつ、900℃以上の累積圧下率が本発明の範囲にみたないNo.30の鋼板は靱性が低い。
圧延仕上げ温度および水冷開始温度が本発明の範囲を上回るNo.31の鋼板はフェライトが生成せずに本発明が規定する組織とはならず、耐延性き裂発生特性に劣る。
水冷開始温度が本発明の範囲を下回るNo.32の鋼板ならびに水冷停止温度が本発明の範囲を上回るNo.33の鋼板は硬質相分率、フェライトの平均アスペクト比が本発明に規定の値とならず、ともに低引張強度であるとともに、耐延性き裂発生特性に劣る。焼戻し温度が本発明値を超えるNo.34の鋼板は島状マルテンサイトが大量に生成したため低靱性であり、耐延性き裂発生特性に劣る。
2:再現熱サイクル部
3:クリップゲージ
4:ナイフエッジ
5:クランプ
6:引張載荷
Claims (5)
- 質量%で、C:0.02~0.2%、Si:0.01~0.5%、Mn:0.5~2.5%、P:0.05%以下、S:0.05%以下、Al:0.1%以下、N:0.01%以下を含み、残部Feおよび不可避的不純物からなる組成を有し、板厚の1/4位置のミクロ組織がフェライトと硬質相からなり、前記硬質相の面積分率が50~90%で、かつ、前記フェライトの平均アスペクト比が1.5以上であることを特徴とする溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材。
- 成分組成に、更に、質量%でCu:0.01~2%、Ni:0.01~5%、Cr:0.01~3%、Mo:0.01~2%、Nb:0.1%以下、V:0.1%以下、Ti:0.1%以下、B:0.01%以下、Ca:0.01%以下、REM:0.1%以下のうちから選ばれた1種または2種以上を含有することを特徴とする請求項1に記載の溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材。
- 請求項1または2において、鋼板表面の組織が、フェライトと硬質相からなり、前記フェライトの面積率が、40%を超え、かつ、前記フェライト粒径の平均アスペクト比が、2を超える溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材。
- 請求項1または2に記載の成分を有する鋼素材を1000℃以上に再加熱し、900℃以上の温度域での圧下率が50%以上で圧延仕上げ温度がAr3点~Ar3−50℃となる圧延を施した後、Ar3−10℃~Ar3−70℃で水冷を開始し、500℃以下で水冷を終了することを特徴とする溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材の製造方法。
- 水冷を行った後、更に、最高加熱温度Ac1点未満で焼戻し処理を行うことを特徴とする請求項4に記載の溶接熱影響部および母材部の耐延性き裂発生特性に優れた鋼材の製造方法。
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US13/141,373 US20130000798A1 (en) | 2008-12-26 | 2009-12-25 | Steel material excellent in resistance of ductile crack initiation from welded heat affected zone and base material and method for manufacturing the same |
RU2011131056/02A RU2493287C2 (ru) | 2008-12-26 | 2009-12-25 | Стальной материал с высокой стойкостью к инициированию вязких трещин от зоны, подвергнутой действию сварочного тепла, и базовый материал, а также способ их производства |
EP09835126.5A EP2383360B1 (en) | 2008-12-26 | 2009-12-25 | Steel plate excellent in resistance of ductile crack initiation from welded heat-affected zone and base material and manufacturing method therefor |
KR1020117015996A KR101343747B1 (ko) | 2008-12-26 | 2009-12-25 | 용접열 영향부 및 모재부의 내연성 균열 발생 특성이 우수한 강재 및 그 제조 방법 |
CN2009801526488A CN102264934A (zh) | 2008-12-26 | 2009-12-25 | 焊接热影响部及母材部的耐延性破裂发生特性优良的钢材及其制造方法 |
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JP (2) | JP5712484B2 (ja) |
KR (1) | KR101343747B1 (ja) |
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