EP2383360B1 - Steel plate excellent in resistance of ductile crack initiation from welded heat-affected zone and base material and manufacturing method therefor - Google Patents
Steel plate excellent in resistance of ductile crack initiation from welded heat-affected zone and base material and manufacturing method therefor Download PDFInfo
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- EP2383360B1 EP2383360B1 EP09835126.5A EP09835126A EP2383360B1 EP 2383360 B1 EP2383360 B1 EP 2383360B1 EP 09835126 A EP09835126 A EP 09835126A EP 2383360 B1 EP2383360 B1 EP 2383360B1
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- ferrite
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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
- C22C38/02—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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- 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
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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
- C22C38/08—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 steel materials suitable for use in welded structures, such as pipelines, bridges, and architectural structures, requiring structural safety and a method for manufacturing the same and particularly relates to one excellent in resistance of ductile crack initiation from welded heat affected zone and a base material.
- the invention is targeted to steel materials for structures having excellent resistance of ductile crack initiation from welded heat affected zone and a base material and having strength of Tensile strength: 490 MPa or more in TS and high toughness of Ductile-brittle fracture transition temperature of Charpy impact test (according to the regulation of JIS Z 2242): vTrs of 0°C or lower.
- ductile crack initiates in a stress concentration zone, such as a weld toe, and the generated ductile crack serves as a trigger to cause brittle fracture, resulting in break and fracture of the structures in some cases.
- Patent Document 1 discloses a high tensile-strength steel material excellent in resistance of ductile crack initiation in which, in the microstructure a steel material surface zone, the ferrite area fraction is 10 to 40%, the bainite area fraction is 50% or more, and the average grain size is 5 ⁇ m or lower.
- Patent Document 2 discloses a steel plate excellent in arrestrability and resistance of ductile fracture in which the microstructure is substantially constituted by a ferrite structure, a pearlite structure, and a bainite structure and, when divided into three layers of both surface zones and the central zone in the plate thickness direction of the steel plate, each zone has a specific microstructure.
- Both the surface zones of the steel plate are constituted by a layer which has 50% or more of a ferrite structure containing ferrite grains in which the equivalent (circle) diameter is 7 ⁇ m or lower and the aspect ratio is 2 to 4 over 5% or more of the plate thickness of each of the structure zones and in which the bainite area fraction of the portion is 5 to 25% or lower.
- the central zone in the plate thickness direction of the steel plate is constituted by a layer which contains ferrite grains in which the equivalent (circle) diameter is 4 to 10 ⁇ m and the aspect ratio is 2 or lower over 50% or more of the plate thickness and in which the bainite area fraction of the zones is 10% or lower.
- Patent Document 2 is directed to a steel plate in which three layers having a ferrite/pearlite structure containing ferrite grains different in the aspect ratio are present in the plate thickness direction from the plate surface of the steel plate and further in which a bainite structure which is a hard phase is appropriately dispersed in a soft phase which is the ferrite/pearlite structure.
- the technique increases the arrestrability by positively forming processed ferrite grains having a high aspect ratio and also appropriately dispersing a bainite structure on each of both the surface zones of the three zones of the steel plate and, in contrast, increases extension characteristics, which are important to ductile fracture at room temperature, by controlling the central zone of the steel plate to have a uniform equiaxed ferrite grain structure and also suppressing a bainite structure, and thus satisfies both opposite characteristics of "arrestrability" and "ductile fracture characteristics” by controlling both the surface zones and the central zone of the steel plate to the three-layer structure.
- Patent Document 3 is directed to a technique of obtaining deformed ferrite grains on the steel plate surface zone of ferrite/pearlite steel and also controlling the microstructure of the central zone to a uniform equiaxed ferrite grain structure similarly as the technique of Patent Document 2.
- Patent Document 3 discloses a method for manufacturing a thick steel plate excellent in arrestrability and ductile fracture characteristics, in which the rolling conditions are strictly controlled so that the steel plate surface zone has a specific microstructure.
- an equivalent plastic strain ⁇ of ⁇ ⁇ 0.5 in a non-recrystallization temperature zone of Ar 3 transformation point or more and 900°C or lower is given to a surface layer zone of 0.05 t or more and 0.15 t or lower from both the surfaces in the plate thickness direction.
- the surface layer zone is cooled to a temperature range of 450 to 650°C at a cooling rate of 2 to 15°C /s while maintaining the temperature of the central zone defined as t/4 to 3t/4 of the plate thickness at the Ar 3 transformation point or more within a period of time when the residual and cumulative equivalent plastic strain ⁇ r of the surface layer zone satisfies ⁇ r ⁇ 0.5, and subsequently rolling is restarted.
- the residual and cumulative equivalent plastic strain ⁇ r of 0.35 ⁇ ⁇ r ⁇ 0.55 is given to the central zone to complete the rolling at the Ar 3 transformation point or more and also the surface layer is recuperated to the Ar 3 transformation point or lower by processing heat and internal sensible heat, and thereafter cooling is performed in such a manner that the average cooling rate is 1 to 10°C/s.
- Patent Documents 1 to 3 all relate to techniques of forming fine subgrains in austenite to miniaturize the structure after transformation by performing rolling in a non recrystallization zone (fine grain temperature zone) of austenite or performing rolling at a rolling finish temperature Ar 3 or more.
- a further method for manufacturing a high tensile steel is disclosed in Patent Document 4.
- the present inventors have conducted extensive researches on a microstructure of base material excellent in resistance of ductile crack initiation of a welded heat affected zone and have found that, when a microstructure of base material has ferrite and a hard phase in which the average aspect ratio of the ferrite and the area fraction of the hard phase are specified at the 1/4 position of the plate thickness exhibiting an average structure in the plate thickness direction of a steel plate, the resistance of ductile crack initiation is excellent also in a welded heat affected zone and such a steel material is excellent also in the resistance of ductile crack initiation of the base material, and further manufacturing conditions of a steel plate having the microstructure.
- the present invention has been accomplished based on the findings and further researches and is more specifically directed to a steel plate and a manufacturing method according to the claims.
- the steel plate according to the invention is also referred to as "the steel material”.
- a steel material capable of suppressing ductile crack initiation from welded heat affected zone and a base material that can suppress ductile crack initiation from a stress concentration zone, such as a weld toe, and prevent collapse or break of steel structures even when the steel structures greatly deform due to an earthquake or the like, for example, can be easily and stably mass-produced and industrially remarkable effects are demonstrated.
- the chemical composition and the microstructure are specified.
- % by mass is simply represented by % unless otherwise specified.
- the C is an element having an action of increasing the strength of steel and, particularly in the invention, contributes to the generation of a hard phase.
- the C content 0.02% or more is required.
- the C content exceeds 0.2%, the ductility or the bending workability are reduced and also the weldability decreases. Therefore, the C content is limited in the range of 0.02 to 0.2%. More preferably, the C content is 0.02 to 0.18%.
- Si acts as a deoxidizing agent and has an action of forming a solid solution to increase the strength of steel.
- the Si content 0.01% or more is required.
- the toughness is reduced and also the weldability is reduced.
- Si is limited in the range of 0.01 to 0.5%. More preferably, the Si content is 0.01 to 0.4%.
- Mn has an action of increasing the strength of steel and also increasing the toughness through an increase in hardenability. In order to obtain such an effect, the Mn content of 0.1% or more is required. In contrast, when the Mn content exceeds 2.5%, the weldability is reduced. Therefore, Mn is limited in the range of 0.1 to 2.5%. More preferably, the content is 0.5 to 2.0%.
- the P content is preferably reduced as much as possible, but the content up to 0.05% is permissible. Therefore, the P content is limited to 0.05% or lower. More preferably, the content is 0.04% or lower.
- the S content is preferably reduced as much as possible. However, the content up to 0.05% is permissible. Therefore, the S content is limited to 0.05% or lower. More preferably, the content is 0.04% or lower.
- Al is an element that acts as a deoxidizing agent and also contributes to pulverization of crystal grains.
- an excessive content of Al in a proportion exceeding 0.1% causes a reduction in toughness. Therefore, the Al content is limited to 0.1% or lower. More preferably, the content is 0.05% or lower.
- N is an element that increases the strength of steel by solid solution strengthening similarly as C.
- an excessive content of N causes a reduction in toughness. Therefore, the N content is limited to 0.01% or lower. More preferably, the content is 0.005% or lower.
- the chemical compositions described above are basic chemical compositions but, in the invention, one or two or more elements selected from Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1% or lower may be further contained according to the desired properties.
- Cu is an element that has an action of increasing the strength of steel through an increase in hardenability or solid solution.
- the content 0.01% or more is required.
- the content exceeds 2%, the weldability decreases and also cracks are likely to generate during manufacturing of steel materials. Therefore, when Cu is added, the content is in the range of 0.01 to 2%. More preferably, the content is 0.01 to 1%.
- Ni is added as required, because Ni contributes to an increase in low temperature toughness, an increase in hardenability, and prevention of hot ductility of Cu when Cu is contained. Such an effect is recognized when Ni is contained in the proportion of 0.01% or more. However, the addition of 5% or more causes a reduction in steel material cost and also a reduction in weldability. Therefore, when Ni is added, the content is in the range of 0.01 to 5%. More preferably, the content is 0.01 to 4.5%.
- Cr is added as required in order to increase the strength of steel materials through an improvement of hardenability or an increase in tempering softening resistance. Such an effect is recognized when Cr is contained in the proportion of 0.01% or more. In contrast, the addition exceeding 3% reduces weldability and toughness. Therefore, when Cr is added, the content is in the range of 0.01 to 3%. More preferably, the content is in the range of 0.01 to 2.5%.
- Mo is added as required in order to increase the strength of steel materials through an improvement of hardenability or an increase in tempering softening resistance. Such an effect is recognized when Mo is contained in the proportion of 0.01% or more. In contrast, the addition exceeding 2% reduces weldability or toughness. Therefore, when Mo is added, the content is in the range of 0.01 to 2%. More preferably, the content is in the range of 0.01 to 1%.
- Nb is an element that precipitates as a carbide or a carbonitride in tempering and increases the strength of steel through precipitation strengthening. Moreover, Nb also has an effect of pulverizing austenite grains during rolling to increase toughness. In order to obtain the effects, the content of 0.001% or more is preferable. However, the content exceeding 0.1% reduces toughness. Therefore, when Nb is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- V is an element that precipitates as a carbide or a carbonitride in tempering and increases the strength of steel through precipitation strengthening. Moreover, V also has an effect of pulverizing austenite grains during rolling to increase toughness. In order to obtain the effects, the content of 0.001% or more is preferable. However, the content exceeding 0.1% reduces toughness. Therefore, when Nb is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- Ti is added as required because Ti has an effect of pulverizing austenite in a welded heat affected zone to increase toughness. In order to obtain the effect, the content of 0.001% or more is preferable. However, the addition exceeding 0.1% reduces toughness and also causes a sudden rise of steel material cost. Therefore, when Ti is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- B is added as required because B has an effect of increasing hardenability and increasing the strength of steel with a small content thereof.
- the content is preferably 0.0001% or more.
- the addition exceeding 0.01% reduces weldability. Therefore, when B is added, the content is 0.01% or lower. More preferably, the content is 0.005% or lower.
- Ca is added as required because Ca increases the base material toughness by controlling the shape of a CaS inclusion and further increase the toughness of a welded heat affected zone.
- the content of 0.0001% or more is preferable.
- the addition exceeding 0.01% reduces toughness due to an increase in the amount of the CaS inclusion. Therefore, when Ca is added, the content is 0.01% or lower. More preferably, the content is 0.009% or lower.
- REM is an element that increases the toughness of a welded heat affected zone and is added as required.
- the content is preferably 0.0001% or more.
- the addition exceeding 0.1% causes a reduction in toughness. Therefore, when REM is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- REM is a general term of Y, Ce and the like that are rare earth elements and the addition amount as used herein refers to the total amount of these rare earth elements.
- the steel material according to the invention has a microstructure in which the structure at the 1/4 position of the plate thickness contains 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 or more.
- the area fraction of the hard phase is lower than 50% and exceeds 90% or the aspect ratio of the ferrite grain size is lower than 1.5, there is a possibility that ductile crack initiation occurs.
- the upper limit of the average aspect ratio of the ferrite grain size does not need to particularly specify and is 5 or lower in view of the capability and the like of a rolling mill.
- the area fraction of the hard phase is more preferably 52 to 90% and the average aspect ratio of the ferrite grain size is more preferably 1.6 or more.
- the average aspect ratio is more preferably 1.7 or more.
- the yield ratio(or Y/T ratio) of a base material decreases, and the strain concentration in a stress concentration zone is eased even in the base material as it is or even after a simulated heat cycle of simulating the welded heat affected zone. Such an effect is not obtained in the case of a single phase of ferrite or a single phase of a hard phase.
- the structure of the surface of a steel plate (1 mm position from the plate surface) contains ferrite and a hard phase, in which the area fraction of the ferrite exceeds 40% and is more preferably 50% or more.
- the average aspect ratio of the ferrite grain size exceeds 2.
- the hard phase is bainite, martensite, or a bainite/martensite mixed structure and contains 5% or lower, in terms of area fraction, of an island martensite (M-A constituent) (MA).
- Fig. 2 illustrates the results of examining the resistance of ductile crack initiation using a simulated heat cycle specimen of a welded zone (highest heating temperature of 1400°C). As illustrated in Fig. 2 , when the area fraction of the hard phase of the base material is 50 to 90% and the average aspect ratio of the ferrite thereof is 1.5 or more, ductile crack initiation is not observed also after the simulated heat cycle.
- a simulated heat cycle time for reaching the highest heating temperature: 6 s, cooling rate from the highest heating temperature to room temperature: 40°C/s
- Fig. 1 illustrates the specimen shape and the test conditions.
- Fig. 4 illustrates the results of examining the influence of the microstructure of the base material exerted on the resistance of ductile crack initiation. As illustrated in Fig. 4 , when the area fraction of the hard phase of the base material is 50 to 90% and the average aspect ratio of the ferrite is 1.5 or more, ductile crack initiation is not accepted.
- Fig. 3 illustrates the specimen shape and the test conditions.
- the sample material (specimen 1) in which a single through-thickness edge notch is introduced into the center was fixed with clamps 5, then a tensile load (arrow 6) was applied to 0.8 mm in terms of displacement of a clip gage 3 between knife-edges 4 that are screwed, the load was removed, and then the specimen was ground to the central zone and mirror polished. Then, the presence of crack initiation at the notch tip was evaluated. The case where the ductile crack from the notch bottom was 50 ⁇ m or more was defined as crack initiation.
- the aspect ratio refers to the ferrite grain size in the rolling direction (major axis)/the ferrite grain size in the plate thickness direction (minor axis) in a cross section parallel to the rolling direction.
- the steel material according to the invention is obtained by successively subjecting the steel material of the above-described chemical compositions to a hot rolling process, a water cooling process, or further a tempering process.
- the hot rolling includes reheating to 1000°C or more and performing rolling in such a manner that the rolling reduction rate in a temperature range of 900°C or more is 50% or more and the rolling finish temperature becomes Ar 3 -10°C to Ar 3 -50°C.
- a more preferable rolling finish temperature is lower than Ar 3 -10°C to Ar 3 -40°C.
- the cumulative rolling reduction rate at 900°C or more is lower than 50%, desired strength and toughness cannot be secured.
- the rolling finish temperature exceeds Ar 3 , the aspect ratio of ferrite does not reach 1.5 or more.
- the rolling finish temperature is lower than Ar 3 -50°C, the area fraction of the hard phase obtained by the subsequent water cooling does not reach 50% or more.
- the water cooling is started at Ar 3 -10°C to Ar 3 -70°C immediately after hot rolling, and then the water cooling is terminated at 500°C or lower.
- the water cooling start temperature exceeds Ar 3 -10°C, ferrite of lower than 10% in terms of area fraction (hard phase exceeding 90% in terms of area fraction) precipitates.
- the water cooling start temperature is lower than Ar 3 -70°C or water cooling is not started immediately after (within 300 seconds) hot rolling, ferrite exceeding 50% in terms of area fraction (hard phase not reaching 50% in terms of area fraction) or pearlite, which is not intended to precipitate in the invention, precipitates.
- desired characteristics cannot be satisfied.
- tempering treatment can be further performed at a temperature of lower than the Ac 1 point.
- tempering treatment By performing tempering treatment, toughness and ductility increase, and desired strength and toughness can be achieved.
- the tempering temperature exceeds the Ac 1 point, a large amount of island martensite generates to reduce the toughness.
- the Ar 3 point and the Ac 1 point can be calculated by the following equation based on the content (% by mass) of each chemical composition.
- the obtained steel plates were subjected to microstructure observation, a tensile test, a toughness test, a ductile crack initiation test after a simulated heat cycle, and a ductile crack initiation test of base materials.
- the test methods were performed as described in the following items (1) to (5).
- specimens were extracted in the cross section parallel to the rolling direction. Then, the specimens were mirror polished, and then etched with nital. Thereafter, the microstructure at the 1/4 position of the plate thickness and the microstructure 1 mm below the surface were observed. The observation of each of the positions was performed with Field number: 20 fields of view. The area fraction was determined by binarizing the ferrite and the hard phase and observing at a magnification of 200 ⁇ . The average aspect ratio of the ferrite was determined by determining the length in the rolling direction and the length in the plate thickness direction of each ferrite present in the field of view at a magnification of 400 ⁇ , determining the length in the rolling direction/the length in the plate thickness direction, and then determining the average value thereof.
- V notch specimens were extracted so that the longitudinal direction was in parallel to the rolling direction according to the regulation of JIS Z 2242 (2005), and then the ductile-brittle fracture transition temperature was determined to evaluate the toughness.
- the specimens were extracted in such a manner that the 1/4 position of the plate thickness when the plate thickness was 20 mm or more or the 1/2 position of the plate thickness when the plate thickness was lower than 20 mm was the center.
- the specimens were subjected to a simulated heat cycle of a welded heat affected zone in which the highest heating temperature was 760°C, 900°C, 1200°C, and 1400°C (time for reaching the highest heating temperature: 6s, Cooling rate from the highest heating temperature to room temperature: 40°C/s) using a Gleeble tester.
- a single through-thickness edge notch was introduced with the length of 3 mm in the plate thickness direction into the center of the simulated heat cycle zone.
- the notch processing was carried out by electrical discharge machining, and the notch tip radius was 0.1 mm.
- a tensile load was applied while gripping the specimens with both right and left ends thereof with a constraint length of 50 mm.
- the displacement between the knife-edges screwed near the notch was measured with the clip gage.
- a tensile load was applied to 0.6 mm in terms of clip gage displacement, and then the load was removed. Thereafter, the specimen was ground to the width center and mirror polished. Then, the crack initiation state at the notch bottom was analyzed under a microscope with a magnification of 50 ⁇ . It was defined that the ductile crack initiation occurred when a ductile crack extended in the length of 50 ⁇ m or more from the notch bottom.
- a single through-thickness edge notch was introduced with the length of 3 mm in the plate thickness direction into the center of the specimens as illustrated in Fig. 3 .
- the notch processing was carried out by electrical discharge machining, and the notch tip radius was 0.1 mm.
- the steel plate (Steel type K*) of No. 11 in which the C content does not satisfy the lower limit of the range of the invention has low tensile strength.
- the steel plate (Steel type L*) of No. 12 in which the content of each of C, P, and S exceeds the upper limit of the range of the invention has low toughness and has poor ductile crack initiation characteristics of a welded heat affected zone.
- the steel plate of No. 13 in which the reheating temperature of slab is lower than the invention and the cumulative rolling reduction rate at 900°C or more is outside the range of the invention has low toughness.
- the steel plate of No. 14 in which the rolling finish temperature and the water cooling start temperature exceed the range of the invention ferrite is not generated, the microstructure specified in the invention is not obtained, and the resistance of ductile crack initiation of a welded heat affected zone is poor.
- the hard phase area fraction and the average aspect ratio of ferrite do not satisfy the values specified in the invention and both the steel plates have low tensile strength and have poor resistance of ductile crack initiation of welded heat affected zones.
- the tempering temperature exceeds the range of the invention, since a large amount of island martensite is generated, the toughness is low and the resistance of ductile crack initiation of a welded heat affected zone is poor.
- the steel plate (Steel type W*) of No. 28 in which the C content does not satisfy the lower limit of the range of the invention has low tensile strength.
- the steel plate (Steel type X*) of No. 29 in which the content of each of C, P, and S exceeds the upper limit of the range of the invention has low toughness.
- the steel plate of No. 30 in which the reheating temperature of slab is lower than the range of the invention and the cumulative rolling reduction rate at 900°C or more does not satisfy the range of the invention has low toughness.
- the hard phase area fraction and the average aspect ratio of ferrite do not satisfy the values specified in the invention and both the steel plates have low tensile strength and have poor resistance of ductile crack initiation.
- the tempering temperature exceeds the value of the invention, a large amount of island martensite (M-A constituent) is generated, and thus the toughness is low and the resistance of ductile crack initiation is poor.
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Description
- The present invention relates to steel materials suitable for use in welded structures, such as pipelines, bridges, and architectural structures, requiring structural safety and a method for manufacturing the same and particularly relates to one excellent in resistance of ductile crack initiation from welded heat affected zone and a base material. Specifically, the invention is targeted to steel materials for structures having excellent resistance of ductile crack initiation from welded heat affected zone and a base material and having strength of Tensile strength: 490 MPa or more in TS and high toughness of Ductile-brittle fracture transition temperature of Charpy impact test (according to the regulation of JIS Z 2242): vTrs of 0°C or lower.
- It is known that when the welded structures, such as pipelines, bridges, and buildings, are exposed to large external force of an earthquake or the like, ductile crack initiates in a stress concentration zone, such as a weld toe, and the generated ductile crack serves as a trigger to cause brittle fracture, resulting in break and fracture of the structures in some cases.
- In order to avoid such break and fracture of the welded structures, it is important that steel materials constituting the same are excellent in resistance of ductile crack initiation.
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Patent Document 1 discloses a high tensile-strength steel material excellent in resistance of ductile crack initiation in which, in the microstructure a steel material surface zone, the ferrite area fraction is 10 to 40%, the bainite area fraction is 50% or more, and the average grain size is 5 µm or lower. -
Patent Document 2 discloses a steel plate excellent in arrestrability and resistance of ductile fracture in which the microstructure is substantially constituted by a ferrite structure, a pearlite structure, and a bainite structure and, when divided into three layers of both surface zones and the central zone in the plate thickness direction of the steel plate, each zone has a specific microstructure. - Both the surface zones of the steel plate are constituted by a layer which has 50% or more of a ferrite structure containing ferrite grains in which the equivalent (circle) diameter is 7 µm or lower and the aspect ratio is 2 to 4 over 5% or more of the plate thickness of each of the structure zones and in which the bainite area fraction of the portion is 5 to 25% or lower. The central zone in the plate thickness direction of the steel plate is constituted by a layer which contains ferrite grains in which the equivalent (circle) diameter is 4 to 10 µm and the aspect ratio is 2 or lower over 50% or more of the plate thickness and in which the bainite area fraction of the zones is 10% or lower.
- More specifically, the technique of
Patent Document 2 is directed to a steel plate in which three layers having a ferrite/pearlite structure containing ferrite grains different in the aspect ratio are present in the plate thickness direction from the plate surface of the steel plate and further in which a bainite structure which is a hard phase is appropriately dispersed in a soft phase which is the ferrite/pearlite structure. The technique increases the arrestrability by positively forming processed ferrite grains having a high aspect ratio and also appropriately dispersing a bainite structure on each of both the surface zones of the three zones of the steel plate and, in contrast, increases extension characteristics, which are important to ductile fracture at room temperature, by controlling the central zone of the steel plate to have a uniform equiaxed ferrite grain structure and also suppressing a bainite structure, and thus satisfies both opposite characteristics of "arrestrability" and "ductile fracture characteristics" by controlling both the surface zones and the central zone of the steel plate to the three-layer structure. - Also the technique of
Patent Document 3 is directed to a technique of obtaining deformed ferrite grains on the steel plate surface zone of ferrite/pearlite steel and also controlling the microstructure of the central zone to a uniform equiaxed ferrite grain structure similarly as the technique ofPatent Document 2. - More specifically,
Patent Document 3 discloses a method for manufacturing a thick steel plate excellent in arrestrability and ductile fracture characteristics, in which the rolling conditions are strictly controlled so that the steel plate surface zone has a specific microstructure. - Specifically, when the thickness during plate rolling is defined as t, an equivalent plastic strain ε of ε ≥ 0.5 in a non-recrystallization temperature zone of Ar3 transformation point or more and 900°C or lower is given to a surface layer zone of 0.05 t or more and 0.15 t or lower from both the surfaces in the plate thickness direction.
- Thereafter, the surface layer zone is cooled to a temperature range of 450 to 650°C at a cooling rate of 2 to 15°C /s while maintaining the temperature of the central zone defined as t/4 to 3t/4 of the plate thickness at the Ar3 transformation point or more within a period of time when the residual and cumulative equivalent plastic strain εr of the surface layer zone satisfies εr ≥ 0.5, and subsequently rolling is restarted.
- In the restarted rolling, the residual and cumulative equivalent plastic strain εr of 0.35 ≤ εr < 0.55 is given to the central zone to complete the rolling at the Ar3 transformation point or more and also the surface layer is recuperated to the Ar3 transformation point or lower by processing heat and internal sensible heat, and thereafter cooling is performed in such a manner that the average cooling rate is 1 to 10°C/s.
- The techniques of
Patent Documents 1 to 3 all relate to techniques of forming fine subgrains in austenite to miniaturize the structure after transformation by performing rolling in a non recrystallization zone (fine grain temperature zone) of austenite or performing rolling at a rolling finish temperature Ar3 or more. A further method for manufacturing a high tensile steel is disclosed inPatent Document 4. -
- [Patent Document 1] Japanese Unexamined Patent Application Publication No.
2008-202119 - [Patent Document 2] Japanese Unexamined Patent Application Publication No.
2000-328177 - [Patent Document 3] Japanese Unexamined Patent Application Publication No.
2003-221619 - [Patent Document 4]
US2002 043305 - However, according to the techniques of
Patent Documents 1 to 3, when the surface layer structure changes to the welded heat affected zone by welding or the like, there is a concern that the effect of resistance of ductile crack initiation is lost. - Moreover, in all of a scale breaker for use in treatment of the surface of a slab extracted from a heating furnace described in Examples of
Patent Document 1, two-stage rolling of rolling in a pulverization temperature range and rolling in a set temperature zone described in Examples ofPatent Document 2, and various kinds of rolling or temperature control for separately creating the structure of a surface layer and the structure inside a steel plate described inPatent Document 3, the manufacturing process is complicated. - Then, in view of the problems of such former techniques, it is an object of the present invention to provide steel materials excellent in resistance of ductile crack initiation from the welded heat affected zone and a base material by a simple method and a method for manufacturing the same.
- In order to achieve the object, the present inventors have conducted extensive researches on a microstructure of base material excellent in resistance of ductile crack initiation of a welded heat affected zone and have found that, when a microstructure of base material has ferrite and a hard phase in which the average aspect ratio of the ferrite and the area fraction of the hard phase are specified at the 1/4 position of the plate thickness exhibiting an average structure in the plate thickness direction of a steel plate, the resistance of ductile crack initiation is excellent also in a welded heat affected zone and such a steel material is excellent also in the resistance of ductile crack initiation of the base material, and further manufacturing conditions of a steel plate having the microstructure.
- The present invention has been accomplished based on the findings and further researches and is more specifically directed to a steel plate and a manufacturing method according to the claims.
- In this disclosure, the steel plate according to the invention is also referred to as "the steel material".
- According to the invention, a steel material capable of suppressing ductile crack initiation from welded heat affected zone and a base material that can suppress ductile crack initiation from a stress concentration zone, such as a weld toe, and prevent collapse or break of steel structures even when the steel structures greatly deform due to an earthquake or the like, for example, can be easily and stably mass-produced and industrially remarkable effects are demonstrated.
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Fig. 1 is a view illustrating a ductile crack initiation test method of a welded heat affected zone. -
Fig. 2 is a view illustrating influence of the area fraction of a hard phase and the average aspect ratio of ferrite on ductile crack initiation of a 1400°C simulated heat cycle material. -
Fig. 3 is a view illustrating a ductile crack initiation test method of a base material. -
Fig. 4 is a view illustrating influence of the area fraction of a hard phase and the average aspect ratio of ferrite on ductile crack initiation of a base material. Best Modes for Carrying Out the Invention - In the invention, the chemical composition and the microstructure are specified. In the description of the chemical composition, % by mass is simply represented by % unless otherwise specified.
- C is an element having an action of increasing the strength of steel and, particularly in the invention, contributes to the generation of a hard phase. In order to obtain such an effect, the C content of 0.02% or more is required. In contrast, when the C content exceeds 0.2%, the ductility or the bending workability are reduced and also the weldability decreases. Therefore, the C content is limited in the range of 0.02 to 0.2%. More preferably, the C content is 0.02 to 0.18%.
- Si acts as a deoxidizing agent and has an action of forming a solid solution to increase the strength of steel. In order to obtain such an effect, the Si content of 0.01% or more is required. In contrast, when the Si content exceeds 0.5%, the toughness is reduced and also the weldability is reduced. Therefor, Si is limited in the range of 0.01 to 0.5%. More preferably, the Si content is 0.01 to 0.4%.
- Mn has an action of increasing the strength of steel and also increasing the toughness through an increase in hardenability. In order to obtain such an effect, the Mn content of 0.1% or more is required. In contrast, when the Mn content exceeds 2.5%, the weldability is reduced. Therefore, Mn is limited in the range of 0.1 to 2.5%. More preferably, the content is 0.5 to 2.0%.
- Since P causes degradation of toughness, the P content is preferably reduced as much as possible, but the content up to 0.05% is permissible. Therefore, the P content is limited to 0.05% or lower. More preferably, the content is 0.04% or lower.
- Since S is present as an inclusion in steel and degrades the ductility and the toughness, the S content is preferably reduced as much as possible. However, the content up to 0.05% is permissible. Therefore, the S content is limited to 0.05% or lower. More preferably, the content is 0.04% or lower.
- Al is an element that acts as a deoxidizing agent and also contributes to pulverization of crystal grains. However, an excessive content of Al in a proportion exceeding 0.1% causes a reduction in toughness. Therefore, the Al content is limited to 0.1% or lower. More preferably, the content is 0.05% or lower.
- N is an element that increases the strength of steel by solid solution strengthening similarly as C. However, an excessive content of N causes a reduction in toughness. Therefore, the N content is limited to 0.01% or lower. More preferably, the content is 0.005% or lower.
- The chemical compositions described above are basic chemical compositions but, in the invention, one or two or more elements selected from Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1% or lower may be further contained according to the desired properties.
- Cu is an element that has an action of increasing the strength of steel through an increase in hardenability or solid solution. In order to secure such an effect, the content of 0.01% or more is required. In contrast, when the content exceeds 2%, the weldability decreases and also cracks are likely to generate during manufacturing of steel materials. Therefore, when Cu is added, the content is in the range of 0.01 to 2%. More preferably, the content is 0.01 to 1%.
- Ni is added as required, because Ni contributes to an increase in low temperature toughness, an increase in hardenability, and prevention of hot ductility of Cu when Cu is contained. Such an effect is recognized when Ni is contained in the proportion of 0.01% or more. However, the addition of 5% or more causes a reduction in steel material cost and also a reduction in weldability. Therefore, when Ni is added, the content is in the range of 0.01 to 5%. More preferably, the content is 0.01 to 4.5%.
- Cr is added as required in order to increase the strength of steel materials through an improvement of hardenability or an increase in tempering softening resistance. Such an effect is recognized when Cr is contained in the proportion of 0.01% or more. In contrast, the addition exceeding 3% reduces weldability and toughness. Therefore, when Cr is added, the content is in the range of 0.01 to 3%. More preferably, the content is in the range of 0.01 to 2.5%.
- Mo is added as required in order to increase the strength of steel materials through an improvement of hardenability or an increase in tempering softening resistance. Such an effect is recognized when Mo is contained in the proportion of 0.01% or more. In contrast, the addition exceeding 2% reduces weldability or toughness. Therefore, when Mo is added, the content is in the range of 0.01 to 2%. More preferably, the content is in the range of 0.01 to 1%.
- Nb is an element that precipitates as a carbide or a carbonitride in tempering and increases the strength of steel through precipitation strengthening. Moreover, Nb also has an effect of pulverizing austenite grains during rolling to increase toughness. In order to obtain the effects, the content of 0.001% or more is preferable. However, the content exceeding 0.1% reduces toughness. Therefore, when Nb is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- V is an element that precipitates as a carbide or a carbonitride in tempering and increases the strength of steel through precipitation strengthening. Moreover, V also has an effect of pulverizing austenite grains during rolling to increase toughness. In order to obtain the effects, the content of 0.001% or more is preferable. However, the content exceeding 0.1% reduces toughness. Therefore, when Nb is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- Ti is added as required because Ti has an effect of pulverizing austenite in a welded heat affected zone to increase toughness. In order to obtain the effect, the content of 0.001% or more is preferable. However, the addition exceeding 0.1% reduces toughness and also causes a sudden rise of steel material cost. Therefore, when Ti is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- B is added as required because B has an effect of increasing hardenability and increasing the strength of steel with a small content thereof. In order to obtain the effect, the content is preferably 0.0001% or more. However, the addition exceeding 0.01% reduces weldability. Therefore, when B is added, the content is 0.01% or lower. More preferably, the content is 0.005% or lower.
- Ca is added as required because Ca increases the base material toughness by controlling the shape of a CaS inclusion and further increase the toughness of a welded heat affected zone. In order to obtain the effects, the content of 0.0001% or more is preferable. However, the addition exceeding 0.01% reduces toughness due to an increase in the amount of the CaS inclusion. Therefore, when Ca is added, the content is 0.01% or lower. More preferably, the content is 0.009% or lower.
- REM is an element that increases the toughness of a welded heat affected zone and is added as required. In order to obtain the effect, the content is preferably 0.0001% or more. However, the addition exceeding 0.1% causes a reduction in toughness. Therefore, when REM is added, the content is 0.1% or lower. More preferably, the content is 0.05% or lower.
- REM is a general term of Y, Ce and the like that are rare earth elements and the addition amount as used herein refers to the total amount of these rare earth elements.
- The steel material according to the invention has a microstructure in which the structure at the 1/4 position of the plate thickness contains 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 or more. When the area fraction of the hard phase is lower than 50% and exceeds 90% or the aspect ratio of the ferrite grain size is lower than 1.5, there is a possibility that ductile crack initiation occurs.
- The upper limit of the average aspect ratio of the ferrite grain size does not need to particularly specify and is 5 or lower in view of the capability and the like of a rolling mill. The area fraction of the hard phase is more preferably 52 to 90% and the average aspect ratio of the ferrite grain size is more preferably 1.6 or more. The average aspect ratio is more preferably 1.7 or more.
- In a two phase mixed structure containing ferrite and a hard phase, the yield ratio(or Y/T ratio) of a base material decreases, and the strain concentration in a stress concentration zone is eased even in the base material as it is or even after a simulated heat cycle of simulating the welded heat affected zone. Such an effect is not obtained in the case of a single phase of ferrite or a single phase of a hard phase.
- In the steel material according to the invention, the structure of the surface of a steel plate (1 mm position from the plate surface) contains ferrite and a hard phase, in which the area fraction of the ferrite exceeds 40% and is more preferably 50% or more. The average aspect ratio of the ferrite grain size exceeds 2. When the area fraction of the ferrite is lower than 40% or the average aspect ratio of the ferrite grain size is 2 or lower, the resistance of ductile crack initiation in a welded heat affected zone is poor.
- In the invention, the hard phase is bainite, martensite, or a bainite/martensite mixed structure and contains 5% or lower, in terms of area fraction, of an island martensite (M-A constituent) (MA).
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Fig. 2 illustrates the results of examining the resistance of ductile crack initiation using a simulated heat cycle specimen of a welded zone (highest heating temperature of 1400°C). As illustrated inFig. 2 , when the area fraction of the hard phase of the base material is 50 to 90% and the average aspect ratio of the ferrite thereof is 1.5 or more, ductile crack initiation is not observed also after the simulated heat cycle. - The results illustrated in
Fig. 2 were obtained by specimens of 12 mm thickness (= plate thickness direction) × 12 mm width × 200 length from the 1/4 center of the plate thickness (1/2 center of the plate thickness in the case of a plate thickness of 25 mm or lower) from the steel materials obtained by producing steel having a composition in the range of the invention by various manufacturing methods and changing the microstructure, and then giving a simulated heat cycle (time for reaching the highest heating temperature: 6 s, cooling rate from the highest heating temperature to room temperature: 40°C/s) of a welded zone thereto by a Gleeble tester to obtain sample materials. -
Fig. 1 illustrates the specimen shape and the test conditions. The sample material (specimen 1), to which the simulated heat cycle was given, in which a single through-thickness edge notch is introduced with the length of 3 mm in the plate thickness direction into the center of a simulatedheat cycle zone 2 of the sample material (specimen 1) was fixed withclamps 5, then a tensile load (arrow 6) was applied to 0.6 mm in terms of displacement of aclip gage 3 between knife-edges 4 that are screwed, the load was removed, and then the specimen was ground to the central zone and mirror polished. Then, the presence of crack initiation at the notch tip was evaluated. The case where the ductile crack from the notch bottom was 50 µm or more was defined as crack initiation. - It is considered that the results illustrated in
Fig. 2 are obtained due to the fact that the yield ratio (or Y/T ratio) (0.2% proof stress/tensile strength) decreased also in the structure after the simulated heat cycle and the degree of distortion concentration at the notch tip zone decreased by the use of the base material having a complex structure of ferrite and a hard phase. - Such outstanding characteristics were observed in common also in a base material to which the simulated heat cycle was not given.
- More specifically,
Fig. 4 illustrates the results of examining the influence of the microstructure of the base material exerted on the resistance of ductile crack initiation. As illustrated inFig. 4 , when the area fraction of the hard phase of the base material is 50 to 90% and the average aspect ratio of the ferrite is 1.5 or more, ductile crack initiation is not accepted. - The results of the base material illustrated in
Fig. 4 were obtained by specimens of 12 mm thickness (= plate thickness direction) × 12 mm width × 200 length from the 1/4 center of the plate thickness (1/2 center of the plate thickness in the case of a plate thickness of 25 mm or lower) from steel materials obtained by producing steel having a composition in the range of the invention by various manufacturing methods and changing the microstructure. -
Fig. 3 illustrates the specimen shape and the test conditions. The sample material (specimen 1) in which a single through-thickness edge notch is introduced into the center was fixed withclamps 5, then a tensile load (arrow 6) was applied to 0.8 mm in terms of displacement of aclip gage 3 between knife-edges 4 that are screwed, the load was removed, and then the specimen was ground to the central zone and mirror polished. Then, the presence of crack initiation at the notch tip was evaluated. The case where the ductile crack from the notch bottom was 50 µm or more was defined as crack initiation. - It is considered that the results illustrated in
Fig. 4 are obtained due to the fact that the yield ratio(or Y/T ratio) (0.2% proof stress/tensile strength) decreased and the degree of distortion concentration at the notch tip zone decreased by the use of a base material having a complex structure of ferrite and a hard phase. - Moreover, it is also considered to be one of the factors that the slip plane greatly leaned to the crack initiation direction in the base material as it is and also after the simulated heat cycle by increasing the average aspect ratio of the ferrite, i.e., the development of the specific aggregate structure. The aspect ratio refers to the ferrite grain size in the rolling direction (major axis)/the ferrite grain size in the plate thickness direction (minor axis) in a cross section parallel to the rolling direction.
- The same results as those of
Fig. 2 were obtained also when the highest heating temperature of the simulated heat cycle was 760°C, 900°C, and 1200°C. - The steel material according to the invention is obtained by successively subjecting the steel material of the above-described chemical compositions to a hot rolling process, a water cooling process, or further a tempering process.
- The hot rolling includes reheating to 1000°C or more and performing rolling in such a manner that the rolling reduction rate in a temperature range of 900°C or more is 50% or more and the rolling finish temperature becomes Ar3-10°C to Ar3-50°C. A more preferable rolling finish temperature is lower than Ar3-10°C to Ar3-40°C. By setting the rolling finish temperature in the invention range, processing strain(or residual strain) can be added to ferrite generated during rolling to thereby increase the aspect ratio of the ferrite. When the reheating temperature is lower than 1000°C, hot rolling that gives a desired cumulative rolling reduction rate cannot be performed to the steel material.
- When the cumulative rolling reduction rate at 900°C or more is lower than 50%, desired strength and toughness cannot be secured. When the rolling finish temperature exceeds Ar3, the aspect ratio of ferrite does not reach 1.5 or more. When the rolling finish temperature is lower than Ar3-50°C, the area fraction of the hard phase obtained by the subsequent water cooling does not reach 50% or more.
- In the water cooling process, the water cooling is started at Ar3-10°C to Ar3-70°C immediately after hot rolling, and then the water cooling is terminated at 500°C or lower. When the water cooling start temperature exceeds Ar3-10°C, ferrite of lower than 10% in terms of area fraction (hard phase exceeding 90% in terms of area fraction) precipitates. When the water cooling start temperature is lower than Ar3-70°C or water cooling is not started immediately after (within 300 seconds) hot rolling, ferrite exceeding 50% in terms of area fraction (hard phase not reaching 50% in terms of area fraction) or pearlite, which is not intended to precipitate in the invention, precipitates. Thus, desired characteristics cannot be satisfied.
- After performing the cooling, tempering treatment can be further performed at a temperature of lower than the Ac1 point. By performing tempering treatment, toughness and ductility increase, and desired strength and toughness can be achieved. When the tempering temperature exceeds the Ac1 point, a large amount of island martensite generates to reduce the toughness.
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- Hereinafter, the invention will be described in more detail based on Examples.
- Steel materials containing the chemical compositions shown in Table 1 were subjected to hot rolling at the conditions shown in Table 2 to thereby obtain steel plates having a plate thickness of 12 to 100 mm.
- The obtained steel plates were subjected to microstructure observation, a tensile test, a toughness test, a ductile crack initiation test after a simulated heat cycle, and a ductile crack initiation test of base materials. The test methods were performed as described in the following items (1) to (5).
- From the obtained steel plates, specimens were extracted in the cross section parallel to the rolling direction. Then, the specimens were mirror polished, and then etched with nital. Thereafter, the microstructure at the 1/4 position of the plate thickness and the
microstructure 1 mm below the surface were observed. The observation of each of the positions was performed with Field number: 20 fields of view. The area fraction was determined by binarizing the ferrite and the hard phase and observing at a magnification of 200×. The average aspect ratio of the ferrite was determined by determining the length in the rolling direction and the length in the plate thickness direction of each ferrite present in the field of view at a magnification of 400×, determining the length in the rolling direction/the length in the plate thickness direction, and then determining the average value thereof. - From the obtained steel plates, full thickness JIS No. 5 specimens were extracted so that the tensile direction was perpendicular to the rolling direction of the steel plate according to the regulation of JIS Z 2201 (1998). The tensile test was performed according to JIS Z 2241 (1998), and then the 0.2% proof (σ0.2) and the tensile strength (TS) were determined to evaluate the static tensile properties.
- From the obtained steel plates, V notch specimens were extracted so that the longitudinal direction was in parallel to the rolling direction according to the regulation of JIS Z 2242 (2005), and then the ductile-brittle fracture transition temperature was determined to evaluate the toughness. The specimens were extracted in such a manner that the 1/4 position of the plate thickness when the plate thickness was 20 mm or more or the 1/2 position of the plate thickness when the plate thickness was lower than 20 mm was the center.
- From the obtained steel plates, specimens of 12 mm thickness (= plate thickness direction = t) × 12 mm width and 200 mm in full length were extracted at the 1/4 center of the plate thickness (1/2 center of the plate thickness when the plate thickness was 25 mm or lower). The specimens were subjected to a simulated heat cycle of a welded heat affected zone in which the highest heating temperature was 760°C, 900°C, 1200°C, and 1400°C (time for reaching the highest heating temperature: 6s, Cooling rate from the highest heating temperature to room temperature: 40°C/s) using a Gleeble tester.
- Thereafter, as illustrated in
Fig. 1 , a single through-thickness edge notch was introduced with the length of 3 mm in the plate thickness direction into the center of the simulated heat cycle zone. The notch processing was carried out by electrical discharge machining, and the notch tip radius was 0.1 mm. - In the test, a tensile load was applied while gripping the specimens with both right and left ends thereof with a constraint length of 50 mm. During the test, the displacement between the knife-edges screwed near the notch was measured with the clip gage. A tensile load was applied to 0.6 mm in terms of clip gage displacement, and then the load was removed. Thereafter, the specimen was ground to the width center and mirror polished. Then, the crack initiation state at the notch bottom was analyzed under a microscope with a magnification of 50×. It was defined that the ductile crack initiation occurred when a ductile crack extended in the length of 50 µm or more from the notch bottom.
- From the obtained steel plates, specimens of 12 mm thickness (= plate thickness direction = t) × 12 mm width and 200 mm in full length were extracted at the 1/4 center of the plate thickness (1/2 center of the plate thickness when the plate thickness was 25 mm or lower).
- To the obtained specimens, a single through-thickness edge notch was introduced with the length of 3 mm in the plate thickness direction into the center of the specimens as illustrated in
Fig. 3 . The notch processing was carried out by electrical discharge machining, and the notch tip radius was 0.1 mm. - In the test, a tensile load was applied while gripping the specimens with both right and left ends thereof with a constraint length of 50 mm. During the test, the displacement between the knife-edges screwed near the notch was measured with the clip gage. A tensile load was applied to 0.8 mm in terms of clip gage displacement, and then the load was removed. Thereafter, the test was ground to the width center and mirror polished. Then, the crack initiation state at the notch bottom was analyzed under a microscope with a magnification of 50×. It was defined that the ductile crack initiation occurred when a ductile crack extended in the length of 50 µm or more from the notch bottom.
- With respect to the specimens that were subjected to the simulated heat cycle, the obtained experimental results are shown in Table 3. All of the steel plates of Nos. 1 to 10 produced using the chemical compositions and the manufacturing method specified in the invention have the structure specified in the invention. It is found that the steel plates have excellent strength and toughness and have excellent resistance of ductile crack initiation of a welded heat affected zone.
- In contrast, the steel plate (Steel type K*) of No. 11 in which the C content does not satisfy the lower limit of the range of the invention has low tensile strength. The steel plate (Steel type L*) of No. 12 in which the content of each of C, P, and S exceeds the upper limit of the range of the invention has low toughness and has poor ductile crack initiation characteristics of a welded heat affected zone.
- The steel plate of No. 13 in which the reheating temperature of slab is lower than the invention and the cumulative rolling reduction rate at 900°C or more is outside the range of the invention has low toughness. In the steel plate of No. 14 in which the rolling finish temperature and the water cooling start temperature exceed the range of the invention, ferrite is not generated, the microstructure specified in the invention is not obtained, and the resistance of ductile crack initiation of a welded heat affected zone is poor.
- In the steel plate of No. 15 in which the cooling start temperature is lower than the range of the invention and the steel plate of No. 16 in which the water cooling stop temperature exceeds the range of the invention, the hard phase area fraction and the average aspect ratio of ferrite do not satisfy the values specified in the invention and both the steel plates have low tensile strength and have poor resistance of ductile crack initiation of welded heat affected zones. In the steel plate of No. 17 in which the tempering temperature exceeds the range of the invention, since a large amount of island martensite is generated, the toughness is low and the resistance of ductile crack initiation of a welded heat affected zone is poor.
- The obtained experimental results of the base material are shown in Table 4. All of the steel plates of Nos. 18 to 27 produced using the chemical compositions and the manufacturing method specified in the invention have the structure specified in the invention. It is recognized that the steel plates have excellent strength and toughness and have excellent resistance of ductile crack initiation of a welded heat affected zone.
- In contrast, the steel plate (Steel type W*) of No. 28 in which the C content does not satisfy the lower limit of the range of the invention has low tensile strength. The steel plate (Steel type X*) of No. 29 in which the content of each of C, P, and S exceeds the upper limit of the range of the invention has low toughness. The steel plate of No. 30 in which the reheating temperature of slab is lower than the range of the invention and the cumulative rolling reduction rate at 900°C or more does not satisfy the range of the invention has low toughness.
- In the steel plate of No. 31 in which the rolling finish temperature and the water cooling start temperature exceed the range of the invention, ferrite is not generated, the microstructure specified in the invention is not obtained, and the resistance of ductile crack initiation is poor.
- In the steel plate of No. 32 in which the cooling start temperature is lower than the range of the invention and the steel plate of No. 33 in which the water cooling stop temperature exceeds the range of the invention, the hard phase area fraction and the average aspect ratio of ferrite do not satisfy the values specified in the invention and both the steel plates have low tensile strength and have poor resistance of ductile crack initiation. In the steel plate of No. 34 in which the tempering temperature exceeds the value of the invention, a large amount of island martensite (M-A constituent) is generated, and thus the toughness is low and the resistance of ductile crack initiation is poor.
-
- 1.
- Specimen
- 2.
- Simulated heat cycle zone
- 3.
- Clip gage
- 4.
- Knife-edge
- 5.
- Clamp
- 6.
- Tensile load
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-
Table 2 No. Steel type Plate thickness (mm) Slab reheating temperature [°C] Cumulative rolling reduction rate at 900°C or more [%] Rolling finish temperature [°C] Water cooling start temperature [°C] Water cooling stop temperature [°C] Tempering temperature [°C] 1 A 14 1160 87 702 683 431 - 2 B 22 1190 75 679 665 378 - 3 C 12 1210 92 731 687 298 - 4 D 100 1150 56 782 779 421 - 5 E 75 1240 62 583 578 72 620 6 F 35 1190 73 672 634 388 - 7 G 24 1150 81 641 623 28 580 8 H 68 1240 55 701 687 426 - 9 I 34 1170 72 607 598 426 - 10 J 18 1160 82 718 704 388 - 11 K* 22 1120 78 748 726 315 - 12 L* 45 1180 68 725 706 248 - 13 C 73 970* 34* 741 732 42 650 14 A 14 1160 87 785* 777* 413 - 15 B 28 1230 72 695 600* 388 - 16 J 19 1240 78 712 699 638* - 17 D 75 1090 64 768 749 62 760* 18 M 15 1150 86 695 681 401 - 19 N 20 1180 76 686 664 308 - 20 O 12 1200 91 721 697 498 - 21 P 100 1130 55 792 777 418 - 22 Q 75 1250 61 602 589 25 600 23 R 35 1200 72 668 643 418 - 24 S 25 1160 73 647 625 72 500 25 T 72 1250 58 708 697 457 - 26 U 37 1170 71 618 605 412 - 27 V 15 1150 83 723 703 378 - 28 W* 25 1100 84 758 748 258 - 29 X* 48 1200 69 721 710 243 - 30 O 75 930* 33* 737 717 23 600 31 M 15 1150 88 755* 747* 428 - 32 N 25 1220 71 679 605* 352 - 33 V 18 1250 77 715 703 658* - 34 P 77 1080 62 776 748 245 750* Note: The steel types marked by * are outside the range of the invention. -
Table 3 No. Microstructure of 1/4 Plate thickness/4 Microstructure 1 mm below the surface σ 0.2 [MPa] TS [MPa] vTrs [°C] Ductile crack initiation characteristics Note (2) Classification Hard phase structure Note (1) Hard phase fraction [%] Ferrite average aspect ratio Hard phase structure Note (1) Ferrite phase fraction [%] Ferrite average aspect ratio 760°C 900°C 1200°C 1400°C 1 B 59 1.9 B 55 2.4 428 548 -57 ○ ○ ○ ○ Present invention example 2 B 75 2.2 B 68 3.1 563 728 -105 ○ ○ ○ ○ Present invention example 3 B,M 54 2.3 B,M 77 4.8 521 689 -33 ○ ○ ○ ○ Present invention example 4 B 64 1.6 B 48 2.2 408 521 -29 ○ ○ ○ ○ Present invention example 5 TB 90 1.7 TM 41 2.6 555 667 -98 ○ ○ ○ ○ Present invention example 6 B 62 1.8 B 59 2.3 473 621 -47 ○ ○ ○ ○ Present invention example 7 TM 55 2.1 TM 72 2.4 481 582 -92 ○ ○ ○ ○ Present invention example 8 B 83 1.8 B 42 2.1 529 683 -64 ○ ○ ○ ○ Present invention example 9 B 87 1.7 B 40 2.2 433 538 -41 ○ ○ ○ ○ Present invention example 10 B 75 2.5 M 58 3.3 428 548 -72 ○ ○ ○ ○ Present invention example 11 B 73 2.2 B 53 3.0 325 421* -18 ○ ○ ○ ○ Comparative example 12 B,M 72 1.7 B,M 43 2.3 677 991 15* ○ × × × Comparative example 13 TM 72 2.3 TM 40 2.5 521 609 8* ○ ○ ○ ○ Comparative example 14 B 100* - B 0* -* 548 678 -21 × × × × Comparative example 15 P 14* 1.1* P 87 1.4* 344 472* -11 × × × × Comparative example 16 P 21* 1.3* P 81 1.4* 388 488* -18 × × × × Comparative example 17 B,MA 63 1.8 M,MA 48 2.8 521 622 6* × × × × Comparative example Note: The cells marked by * are outside the range of the invention.
Note (1): B: Bainite., M:Martensite, P: Pearlite, TB: Tempered bainite, TM: Tempered martensite, MA: Island martensite
Note (2): ○: No ductile crack initiation × : Ductile crack initiation -
Table 4 No. Microstructure of 1/4 Plate thickness Microstructure 1 mm below the surface σ0.2 [MPa] TS [MPa] vTrs [°C] Ductile crack initiation characteristics Note (2) Classification Hard phase structure Note (1) Hard phase fraction [%] Ferrite average aspect ratio Hard phase structure Note (1) Ferrite phase fraction [%] Ferrite average aspect ratio 18 B 55 1.8 B 57 3.1 436 528 -48 ○ Present invention example 19 B 72 2.1 B 42 4.9 573 726 -121 ○ Present invention example 20 B,M 52 2.2 B,M 66 3.9 511 698 -21 ○ Present invention example 21 B 62 1.6 B 48 2.2 359 515 -28 ○ Present invention example 22 TB 89 1.8 TM 41 4.1 552 628 -111 ○ Present invention example 23 B 68 1.9 B 49 2.8 487 615 -35 ○ Present invention example 24 TM 59 2.0 TM 57 3.1 472 577 -98 ○ Present invention example 25 B 84 1.6 B 42 2.7 507 641 -63 ○ Present invention example 26 B 88 1.7 B 43 2.9 402 513 -34 ○ Present invention example 27 B 77 2.4 B 53 3.3 425 538 -66 ○ Present invention example 28 B 74 2.1 M 58 3.7 368 411* -38 ○ Comparative example 29 B,M 78 1.8 M 55 2.2 687 983 10* ○ Comparative example 30 TM 69 2.4 TM 42 2.4 513 618 7* ○ Comparative example 31 B 100* - B 0* -:* 558 688 -18 × Comparative example 32 P 12* 1.2* P 89 1.4* 358 451* -13 × Comparative example 33 P 18* 1.4* P 83 1.4* 398 473* -21 × Comparative example 34 B,MA 68 1.7 M,MA 44 2.8 535 637 5* × Comparative example Note: The cells marked by * are outside the range of the invention.
Note (1): B: Bainite., M: Martensite, P: Pearlite, TB: Tempered bainite, TM: Tempered martensite, MA: Island martensite
Note (2): O: No ductile crack initiation × : Ductile crack initiation
Claims (3)
- A steel plate excellent in resistance of ductile crack initiation from welded heat affected zone and a base material,
the steel plate consisting of a composition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al: 0.1% or lower, and N: 0.01% or lower, optionally one or more of Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1 % or lower, in terms of % by mass, and the balance being Fe with inevitable impurities,
the microstructure at the 1/4 position of the plate thickness containing ferrite and a hard phase,
the area fraction of the hard phase at the 1/4 position of the plate thickness being 50 to 90%, and
the average aspect ratio of the ferrite grain size at the 1/4 position of the plate thickness being 1.5 or more,
wherein the microstructure on the surface of the steel plate contains ferrite and a hard phase, the area fraction of the ferrite on the surface of the steel plate exceeds 40%, and the average aspect ratio of the ferrite grain size on the surface of the steel plate exceeds 2. - A method for manufacturing a steel plate excellent in resistance of ductile crack initiation from welded heat affected zone and a base material comprising: reheating a steel base material having the chemical compositions of Claim 1 to 1000°C or more, rolling the same in such a manner that a rolling reduction rate in a temperature range of 900°C or more is 50% or more and a rolling finish temperature is Ar3-10°C to Ar3-50°C, starting water cooling at Ar3-10°C to Ar3-70°C, and terminating the water cooling at 500°C or lower.
- The method for manufacturing a steel plate excellent in resistance of ductile crack initiation from welded heat affected zone and a base material according to Claim 2 further comprising, after the water cooling, performing tempering treatment at a temperature of lower than the AC1 point.
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