WO2015178320A1 - 厚鋼板 - Google Patents
厚鋼板 Download PDFInfo
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- WO2015178320A1 WO2015178320A1 PCT/JP2015/064099 JP2015064099W WO2015178320A1 WO 2015178320 A1 WO2015178320 A1 WO 2015178320A1 JP 2015064099 W JP2015064099 W JP 2015064099W WO 2015178320 A1 WO2015178320 A1 WO 2015178320A1
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- upper 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
- 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
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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
- 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
- C22C38/12—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
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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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
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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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
Definitions
- the present invention relates to a thick steel plate. Specifically, the present invention relates to a thick steel plate that is mainly used as a structural material for ships, buildings, bridges, construction machines, and the like, and has a tensile strength of 490 MPa or more and less than 650 MPa and excellent fatigue characteristics.
- Non-Patent Document 1 shows the effects of various influencing factors on fatigue strength, including solid solution strengthening, precipitation strengthening, grain refinement and Although the fatigue properties are improved by the second phase strengthening, the dislocation strengthening is accompanied by an increase in movable dislocations, so that it is difficult to improve the fatigue properties.
- the process of fatigue failure can be divided into (1) a process until a repeated load is applied and a crack is generated, and (2) a process until the generated crack progresses and breaks.
- it is effective to suppress the accumulation of dislocations in the process (1), and solid solution strengthening, precipitation strengthening, grain refinement, etc. are considered effective. It is done.
- the above process (2) it is effective to prevent the crack from progressing, so it is considered that crystal grain refinement and second phase strengthening are effective.
- Patent Document 1 a two-phase structure of fine ferrite and hard martensite is used, and the hardness difference is regulated to 150 or more in terms of Vickers hardness, thereby suppressing the crack growth rate and improving the fatigue life after the occurrence of cracks. It has been proposed.
- Patent Document 2 proposes a technique for reducing the crack growth rate by making the steel structure a mixed structure of fine ferrite and bainite. By using this technique, it can be expected to improve the fatigue life after the occurrence of cracks even in fatigue fracture, but no consideration is given to the fatigue characteristics until crack initiation.
- Patent Document 3 proposes improving fatigue strength by precipitating carbides in the ferrite structure. However, there is no description about the fatigue characteristics after the occurrence of cracks. Further, since Patent Document 3 is intended for thin steel plates, no consideration is given to satisfying other characteristics required for large structures such as toughness.
- the present invention has been made in view of the above circumstances, and a main object thereof is to provide a thick steel plate having excellent fatigue characteristics.
- the thick steel plate of the present invention that can solve the above problems is C: 0.02 to 0.10% by mass, Mn: 1.0 to 2.0% by mass, Nb: more than 0% by mass, 0.05% by mass.
- Ti more than 0% by mass, 0.05% by mass or less
- Al 0.01 to 0.06% by mass
- Si 0.1 to 0.6% by mass
- Cu 0.1%, respectively.
- the metallographic structure at the surface layer includes at least one of ferrite and upper bainite in a fraction of 80 area% or more,
- the effective crystal grain size of at least one crystal grain of the ferrite and upper bainite is 10.0 ⁇ m or less,
- the average equivalent circle diameter of the crystal grains of the remaining structure excluding ferrite and upper bainite is 3.0 ⁇ m or less,
- the value of the dislocation density ⁇ as measured by X-ray diffraction is 2.5 ⁇ 10 15 m ⁇ 1 or less. It is characterized by that.
- the “average equivalent circle diameter” means the diameter when the tissue is converted into a circle having the same area, that is, the average value of the “equivalent circle diameter”.
- the ratio of the island-like martensite in the remaining structure is preferably 5 area% or less.
- the chemical composition preferably satisfies at least one of the following requirements (a) to (c), and the properties of the thick steel plate are further improved according to the type of element contained: Is done.
- Ni more than 0% by mass and 0.6% by mass or less; [Ni] / [Cu] which is a ratio of Ni amount [Ni] and Cu amount [Cu] is 1.2 Is less than.
- B Further, from the group consisting of V: more than 0 mass%, 0.5 mass% or less, Cr: more than 0 mass%, 1.0 mass% or less, and Mo: more than 0 mass%, 0.5 mass% or less. Contains one or more selected.
- C Further, B: more than 0% by mass and 0.005% by mass or less are contained.
- the bainite transformation start temperature Bs calculated from the chemical composition based on the following formula (1) is 640 ° C. or higher.
- Bs (° C.) 830 ⁇ 270 ⁇ [C] ⁇ 90 ⁇ [Mn] ⁇ 37 ⁇ [Ni] ⁇ 70 ⁇ [Cr] ⁇ 83 ⁇ [Mo] (1)
- [C], [Mn], [Ni], [Cr] and [Mo] indicate the contents (mass%) of C, Mn, Ni, Cr and Mo, respectively.
- the other thick steel plate of the present invention that has solved the above problems is C: 0.02 to 0.10% by mass, Mn: 1.0 to 2.0% by mass, Nb: more than 0% by mass, 0.05% by mass or less, Ti: more than 0% by mass, 0.05% by mass
- B more than 0% by mass, 0.005% by mass or less
- the metallographic structure at the surface layer includes at least one of ferrite and upper bainite in a fraction of 80 area% or more,
- the effective crystal grain size of at least one crystal grain of the ferrite and upper bainite is 10.0 ⁇ m or less, Of the metal structure in the surface layer, the average equivalent circle diameter of the crystal grains of the remaining structure excluding ferrite and upper bain
- the proportion of island martensite in the remaining structure excluding ferrite and upper bainite in the metal structure in the surface layer is preferably 5 area% or less.
- the chemical component composition preferably satisfies at least one of the following requirements (a) and (b), and the properties of the thick steel plate are further improved according to the type of element contained. .
- Ni more than 0% by mass and 0.6% by mass or less; [Ni] / [Cu] which is a ratio of Ni amount [Ni] and Cu amount [Cu] is 1.2 Is less than.
- B Further, from the group consisting of V: more than 0 mass%, 0.5 mass% or less, Cr: more than 0 mass%, 1.0 mass% or less, and Mo: more than 0 mass%, 0.5 mass% or less. Contains one or more selected.
- the bainite transformation start temperature Bs calculated from the chemical composition based on the following formula (1) is 640 ° C. or higher.
- Bs (° C.) 830 ⁇ 270 ⁇ [C] ⁇ 90 ⁇ [Mn] ⁇ 37 ⁇ [Ni] ⁇ 70 ⁇ [Cr] ⁇ 83 ⁇ [Mo] (1)
- [C], [Mn], [Ni], [Cr] and [Mo] indicate the contents (mass%) of C, Mn, Ni, Cr and Mo, respectively.
- the EBSP A crystal grain having a GAM (Grain Average Misoration) of 1 ° or more in one crystal grain observed by the method is 20 to 80 area% by fraction. It is preferable to include.
- the metal structure at the t / 4 position contains crystal grains having a GAM of 1 ° or more in one crystal grain observed by the EBSP method in a fraction of 20 area% or more and 80 area% or less, the following formula ( It is preferable that the relationship of 2) and Formula (3) is satisfied.
- FIG. 1 is a schematic explanatory view showing a steel sheet according to the present invention.
- FIG. 2 is a schematic explanatory view showing a test piece used for measurement of fatigue characteristics.
- FIG. 3 is a schematic explanatory view showing a compact test piece used for measuring the crack growth rate.
- FIG. 4 is a conceptual diagram of grain boundaries, KAM, and GAM.
- the present inventors investigated the ratio of the life before the crack until the occurrence of cracking and the ratio of the life after the crack until the failure until the fatigue failure.
- the pre-stage life until crack initiation accounts for about 50% of the total life until fatigue failure, and the proportion of pre-stage life until crack occurrence occupies as the total life becomes longer by lowering the stress level. It turned out to increase.
- the ratio of the previous stage life until the occurrence of cracking tends to increase, so it is considered effective to increase the previous stage life.
- the present inventors examined the requirement for extending the previous stage life from various angles. As a result, while appropriately controlling the chemical composition, the fraction of the main structure, the effective crystal grain size of the crystal grains of the structure, the average equivalent circular diameter of the remaining structure excluding the main structure, X-ray diffraction In the thick steel plate that appropriately controlled the value of the dislocation density ⁇ when measured, etc., it was found that the pre-stage life can be extended, and as a result, the total life until fatigue failure can be extended. Completed the invention.
- Embodiment 1 of the present invention will be described.
- the inventors first investigated the relationship between additive elements and fatigue strength for various thick steel plates. As a result, it became clear that the fatigue strength was remarkably improved by the addition of Si and Cu. In general, fatigue cracks are generated due to irreversible motion of dislocations that move due to repetitive stress. At this time, it is known that dislocations form a cell structure, but the addition of Si and Cu so that the total amount is 0.3 mass% or more can suppress the formation of this cell structure. It became clear.
- these elements do not form precipitates, and are not significantly dissolved in carbides, etc. present in the steel sheet. It is done. That is, if these elements are sufficiently present in a solid solution state in the matrix, it is considered that the irreversible motion of dislocations is suppressed, so that the fatigue life before crack generation is improved.
- This dislocation celling suppression effect does not occur significantly with other additive elements such as Mn and Cr, but rather lowers the transformation point to produce lower bainite with a high dislocation density, and fatigue strength is higher than static strength improvement. It turned out not to improve.
- dislocation density ⁇ is preferably 2.0 ⁇ 10 15 m ⁇ 1 or less, more preferably 1.5 ⁇ 10 15 m ⁇ 1 or less.
- the lower limit of the dislocation density ⁇ is generally 5.0 ⁇ 10 13 m ⁇ 1 or more.
- FIG. 1 is a schematic explanatory view showing a steel sheet according to the present invention.
- 1A is a schematic perspective view of a steel plate according to the present invention
- FIG. 1B is a schematic side view of the steel plate according to the present invention.
- L indicates the rolling direction
- W indicates the width direction
- D indicates the plate thickness direction
- S1 indicates the surface of the steel plate
- S2 indicates a cross section in the plate thickness direction D parallel to the rolling direction L.
- the present inventors investigated the fatigue characteristics of various steel plates with different chemical composition and structure.
- the structure in the longitudinal section parallel to the rolling direction that is, the section S2 in FIG. 1 in the surface layer near the steel sheet surface S1, for example, the surface layer at a depth of about 1 to 3 mm in the thickness direction from the steel sheet surface S1.
- a thick steel plate having excellent fatigue characteristics can be obtained by controlling as follows.
- the surface layer was set at a depth of about 1 to 3 mm from the surface of the steel sheet, since the scale layer was present on the surface of the steel sheet immediately after the production, depending on the production conditions, about 0.1 to 2 mm. This is for evaluating the surface layer of the steel plate itself.
- Ferrite and upper bainite are structures in which movable dislocations are relatively less likely to be introduced at the time of formation of the structure than other structures. This suppresses a decrease in the fatigue limit ratio and improves the life until crack initiation. it can.
- the metallographic structure in the surface layer includes at least one of ferrite and upper bainite in a fraction of 80 area% or more.
- the fraction of at least one of ferrite and upper bainite is preferably 85 area% or more, more preferably 90 area% or more.
- the upper limit of the fraction of at least one of ferrite and upper bainite may be 100 area%, but is generally 98 area% or less.
- the average length in the plate thickness direction of the crystal grain (hereinafter, “ (Sometimes referred to as “effective crystal grain size”) was set to 10.0 ⁇ m or less.
- the effective crystal grain size of at least one crystal grain of ferrite and upper bainite is preferably 6 ⁇ m or less, more preferably 5 ⁇ m or less.
- the lower limit of the effective crystal grain size of at least one crystal grain of ferrite and upper bainite is not limited, but generally exceeds 2 ⁇ m.
- the upper bainite can be made finer in structure size than ferrite, but since it is accompanied by shear deformation at the time of transformation, movable dislocations are easily introduced. In particular, when a bainite transformation is performed at a low temperature, a lower bainite structure containing a large amount of mobile dislocations tends to be formed. In order to suppress the generation of the lower bainite structure, it is preferable to appropriately adjust the bainite transformation start temperature Bs. From such a viewpoint, the bainite transformation start temperature Bs calculated based on the formula (1) is preferably 640 ° C. or higher, and more preferably 660 ° C. or higher.
- the remaining structure excluding ferrite and upper bainite on the surface layer needs to have an average equivalent circle diameter of 3.0 ⁇ m or less.
- the reason why the remaining structure has an average equivalent circle diameter of 3.0 ⁇ m or less is that if the average size of the remaining structure exceeds 3.0 ⁇ m, other characteristics such as toughness may be greatly deteriorated.
- a preferable upper limit of the average equivalent circle diameter of the remaining tissue is 2.5 ⁇ m or less, and more preferably 2.0 ⁇ m or less.
- a preferable lower limit is approximately 0.5 ⁇ m or more.
- the remaining structure excluding ferrite and upper bainite on the surface layer is basically martensite, island-like martensite (MA), pearlite, and pseudo-pearlite.
- island-like martensite generated in the cooling process after rolling, etc. causes expansion transformation in the generation process, which introduces movable dislocations in the matrix and causes a decrease in the life until crack initiation. Become. Therefore, it is preferable that the island-like martensite in the remaining structure of the surface layer is 5% or less in terms of area ratio.
- the area ratio of island-like martensite is better as it is smaller, but is more preferably 3% or less, and even more preferably 1% or less. Most preferably, it is 0%.
- the chemical composition of the steel sheet that improves the fatigue characteristics will be described.
- a fine ferrite structure or an upper bainite structure is ensured by appropriately adding alloy elements such as C, Mn, and Nb, and at the same time, fatigue cracks are generated by appropriately adjusting the addition amount of Si and Cu. Therefore, it is possible to realize a thick steel plate having excellent fatigue characteristics by suppressing the formation of dislocation cells. From such a viewpoint, each component is adjusted as follows.
- C 0.02 to 0.10% by mass C is an important element for ensuring the strength of the steel sheet. Therefore, the C amount is determined to be 0.02% by mass or more.
- the amount of C is preferably 0.03% by mass or more, and more preferably 0.04% by mass or more.
- the C amount is determined to be 0.10% by mass or less.
- the amount of C is preferably 0.08% by mass or less, and more preferably 0.06% by mass or less.
- Mn 1.0 to 2.0% by mass Mn is an important element for ensuring hardenability in order to obtain a fine structure.
- the amount of Mn needs to be 1.0 mass% or more.
- the amount of Mn is preferably 1.2% by mass or more, more preferably 1.4% by mass or more.
- the amount of Mn needs to be 2.0 mass% or less.
- the amount of Mn is preferably 1.8% by mass or less, more preferably 1.6% by mass or less.
- Nb more than 0% by mass and 0.05% by mass or less
- Nb is an element effective for improving hardenability and refining the structure.
- the Nb content is preferably 0.01% by mass or more. More preferably, it is 0.02 mass% or more.
- the Nb amount needs to be 0.05% by mass or less. Preferably it is 0.04 mass% or less, More preferably, it is 0.03 mass% or less.
- Ti More than 0% by mass, 0.05% by mass or less Ti is useful for improving the hardenability and at the same time forming TiN to make the structure of the heat-affected zone fine during welding and to suppress the decrease in toughness. Element. For this reason, it is preferable to contain Ti 0.01 mass% or more. More preferably, it is 0.02 mass% or more. However, when the amount of Ti is excessive, coarse TiN is generated, which may deteriorate characteristics such as toughness. Therefore, the Ti amount needs to be 0.05% by mass or less. Preferably it is 0.04 mass% or less, More preferably, it is 0.03 mass% or less.
- Al 0.01 to 0.06% by mass
- Al is an element useful for deoxidation, and the deoxidation effect is not exhibited unless the content is less than 0.01% by mass.
- it is 0.02 mass% or more, More preferably, it is 0.03 mass% or more.
- the Al amount needs to be 0.06% by mass or less.
- it is 0.05 mass% or less, More preferably, it is 0.04 mass% or less.
- Si 0.1 to 0.6 mass% and Cu: 0.1 to 0.6 mass%
- Si is a large amount of solid solution strengthening and is necessary to ensure the strength of the base material
- the Si amount needs to be 0.1% by mass or more.
- the amount of Si is preferably 0.2% by mass or more, more preferably 0.3% by mass or more.
- the amount of Si needs to be 0.6% by mass or less.
- it is 0.55 mass% or less, More preferably, it is 0.5 mass% or less.
- Cu is an element effective in suppressing cell formation by suppressing cross slip of dislocations, and the Cu content needs to be 0.1% by mass or more in order to effectively exhibit this action.
- the amount of Cu is preferably 0.2% by mass or more, more preferably 0.3% by mass or more.
- the amount of Cu needs to be 0.6% by mass or less.
- it is 0.55 mass% or less, More preferably, it is 0.5 mass% or less.
- Si and Cu can exhibit a common action in terms of suppressing dislocation cell formation. From such a viewpoint, each of them may be contained alone or in combination. Further, the effect of suppressing dislocation cell formation by Si and Cu can be effectively exhibited when the total of [Si] + [Cu] becomes 0.3 mass% or more. Preferably, it is 0.4 mass% or more. In addition, the preferable upper limit of [Si] + [Cu] is the sum of the respective preferable upper limits.
- the basic components in the thick steel plate of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that unavoidable impurities brought into the steel, such as P, S, and N, depending on the situation of raw materials, materials, manufacturing equipment, and the like are contained in the steel. In the thick steel plate of the present invention, it is also effective to positively contain the following elements, and the properties of the thick steel plate are further improved according to the type of the contained element.
- Ni more than 0% by mass and 0.6% by mass or less Ni has the effect of improving hardenability and making the structure finer, and at the same time, suppressing the cracking during hot working that is likely to occur due to the addition of Cu. is there.
- Ni is preferably contained in an amount of 0.1% by mass or more. More preferably, it is 0.2 mass% or more. However, if Ni is excessively contained, the hardenability becomes excessive, and the dislocation density ⁇ becomes excessive, so that desired fatigue characteristics cannot be obtained. Therefore, the amount of Ni is preferably 0.6% by mass or less. More preferably, it is 0.5 mass% or less, More preferably, it is 0.4 mass% or less.
- the ratio of Ni amount [Ni] and Cu amount [Cu] is preferably less than 1.2, more preferably 1.1 or less.
- the lower limit of this ratio ([Ni] / [Cu]) is approximately 0.5 or more.
- V more than 0% by mass, 0.5% by mass or less
- Cr more than 0% by mass, 1.0% by mass or less
- Mo more than 0% by mass, 0.5% by mass or less
- V, Cr, and Mo are elements that have the effect of improving the hardenability of the steel sheet, and are effective in refining the structure.
- V 0.01% by mass or more
- Cr 0.1% by mass or more
- Mo 0.01% by mass or more alone, or two or more types should be contained. Is preferred. However, if these elements are contained excessively, the hardenability becomes excessive, the dislocation density ⁇ becomes excessive, and desired fatigue characteristics cannot be obtained.
- the respective amounts be V: 0.5% by mass or less, Cr: 1.0% by mass or less, and Mo: 0.5% by mass or less. More preferably, they are V: 0.4 mass% or less, Cr: 0.8 mass% or less, Mo: 0.4 mass% or less.
- B More than 0% by mass and 0.005% by mass or less B is an element that improves hardenability, and particularly an element that suppresses the formation of a coarse ferrite structure and easily generates a fine upper bainite structure. .
- the B content is preferably set to 0.0005 mass% or more. More preferably, it is 0.001 mass% or more. However, if the amount of B is excessive, the hardenability becomes excessive and the dislocation density ⁇ is excessively high, so that the desired fatigue characteristics cannot be obtained. More preferably, it is 0.004 mass% or less.
- the plate thickness of the thick steel plate according to the present invention is not particularly limited, but when the plate thickness is small, the contribution to the improvement of the crack growth life is reduced. From such a viewpoint, the plate thickness is preferably 6 mm or more, and more preferably 10 mm or more.
- the steel plate of the present invention satisfies the above-mentioned requirements, and its manufacturing method is not particularly limited.
- the steel plate series manufacturing process in which hot rolling is performed after melting and casting the steel, fatigue characteristics are obtained.
- the steel slab having the above-mentioned chemical composition for example, a slab is used, the heating temperature before hot rolling, the cumulative reduction ratio in the entire hot rolling process, and the finish rolling finish temperature. It is preferable to control the average cooling rate and cooling stop temperature from a lower temperature to 600 ° C. of either the finish rolling end temperature or 800 ° C. as follows.
- Heating temperature before hot rolling 1000-1200 ° C
- Cumulative rolling reduction in all hot rolling processes 70% or more
- Finishing rolling finish temperature Ar 3 transformation point to Ar 3 transformation point + 150 ° C
- the average cooling rate from the lower temperature to 600 ° C 15 ° C / second or less
- Cooling stop temperature 500 ° C or more
- the steel slab It is preferable to heat the steel slab to a temperature range of 1000 to 1200 ° C. before hot rolling. More preferably, it is 1050 degreeC or more. It is preferable to heat to a temperature range of 1000 ° C. or higher so that the cumulative reduction ratio during hot rolling can be 70% or more, which will be described later, while preventing coarsening of the crystal grains. However, if the heating temperature becomes too high and exceeds 1200 ° C., the tissue size cannot be reduced even if sufficient reduction is applied, so it is preferable that the heating temperature is 1200 ° C. or less. More preferably, it is 1150 degrees C or less.
- the cumulative reduction ratio in the all hot rolling process is 70% or more. More preferably, it is 75% or more. In order to reduce the structure size, particularly the effective crystal grain size, it is necessary to apply sufficient reduction in the non-recrystallization temperature range.
- the finish rolling end temperature is in the range of Ar 3 transformation point to Ar 3 transformation point + 150 ° C. It is preferable.
- the finish rolling end temperature is more preferably in the range of Ar 3 transformation point + 20 ° C. or higher and Ar 3 transformation point + 100 ° C. or lower.
- t 0 is the rolling start thickness of the steel slab when the temperature at a position 3 mm from the surface is in the rolling temperature range (unit: mm)
- t 1 is the temperature at a position 3 mm from the surface in the rolling temperature range.
- the finished thickness of the steel slab (unit: mm) and t 2 indicate the thickness of the steel slab before rolling, for example, the slab.
- Ar 3 transformation point employs a value obtained by equation (5).
- Ar 3 transformation point 910-230 ⁇ [C] + 25 ⁇ [Si] ⁇ 74 ⁇ [Mn] ⁇ 56 ⁇ [Cu] ⁇ 16 ⁇ [Ni] ⁇ 9 ⁇ [Cr] ⁇ 5 ⁇ [Mo] ⁇ 1620 ⁇ [Nb] (5)
- [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [Nb] are respectively C, Si, Mn, Cu, Ni, Cr, Mo and The content of Nb in mass% is shown.
- the hot rolling After the hot rolling is finished, it is preferable to cool from the finish rolling finish temperature or 800 ° C. to a lower temperature to at least 600 ° C. at an average cooling rate of 15 ° C./second or less. If the average cooling rate at this time exceeds 15 ° C./second, the structure transformation is substantially completed at a low temperature unless isothermal holding or the like is performed, so that dislocations are excessively introduced and desired fatigue characteristics are obtained. Absent.
- the average cooling rate is more preferably 10 ° C./second or less.
- the stop temperature at the above average cooling rate, that is, the cooling stop temperature to 500 ° C. or more, generation of a coarse ferrite structure can be suppressed, and a fine ferrite or upper bainite structure can be secured.
- the cooling stop temperature is lower than 500 ° C., the transformation is completed at a low temperature, so that dislocations are excessively introduced and the desired fatigue characteristics may not be obtained.
- the temperature range for cooling at the average cooling rate is from 800 ° C. to 600 ° C. when the finish rolling finish temperature is higher than 800 ° C., and from the finish rolling finish temperature to 600 ° C. when the finish rolling finish temperature is lower than 800 ° C. Up to °C.
- the lower limit of the average cooling rate is preferably 3.0 ° C./second or more from the viewpoint of controlling the structure inside the steel sheet described later.
- the present inventors in addition to the structure control in Embodiment 1, as shown in FIG.
- the position of the thickness t / 4 was selected in order to evaluate at an average position within the thickness.
- the longitudinal section parallel to the rolling direction at the position t / 4 of the sheet thickness is basically a line area, but when actually observing the structure, an area having a certain spread around that position is used. It will be observed (see Examples below).
- the metal structure at the t / 4 position of the steel sheet contains upper bainite in a fraction of 80% by area, the effective crystal grain size of the upper bainite crystal grains is 10.0 ⁇ m or less, and the remainder excluding the upper bainite
- the average equivalent circle diameter of the tissue is preferably 3.0 ⁇ m or less.
- the upper bainite structure is a structure in which fine crystal grain boundaries can be uniformly dispersed in the structure, thereby suppressing the progress of cracks.
- the metal structure at the t / 4 position of the steel sheet contains 80 bai% or more of upper bainite in a fraction.
- the upper bainite fraction inside the steel plate is more preferably 85 area% or more, and still more preferably 90 area% or more.
- the upper limit of the upper bainite fraction inside the steel sheet may be 100 area%, but is generally 98 area% or less.
- the crystal grain size of the upper bainite at the position t / 4 of the steel plate affects the fatigue crack growth characteristics, and when it becomes coarse, the crack growth cannot be sufficiently suppressed. Therefore, the average length in the plate thickness direction of the crystal grains, that is, the effective crystal grain size, is 10 when the region surrounded by the large-angle grain boundaries where the orientation difference between adjacent upper bainite crystals is 15 ° or more is used. It is preferably 0.0 ⁇ m or less.
- the effective crystal grain size is more preferably 8 ⁇ m or less, and even more preferably 7 ⁇ m or less.
- the remaining structure other than the upper bainite is preferably 3.0 ⁇ m or less in terms of the average equivalent circle diameter.
- the reason why the average equivalent circle diameter of the remaining structure is set to 3.0 ⁇ m or less is that when it exceeds 3.0 ⁇ m, other characteristics such as toughness may be greatly deteriorated.
- the remaining structure basically includes martensite and MA as in the case of the surface layer, and these hard remaining structures can reduce the crack growth rate.
- a preferable lower limit is approximately 0.5 ⁇ m or more.
- KAM Karl Average Misorientation
- GAM Garnier Average Misorientation
- FIG. 4 shows a conceptual diagram of grain boundaries, KAM, and GAM.
- the hexagons in FIG. 4 indicate EBSP measurement points, and the outer periphery of the region indicated by the thick line in FIG. 4A is a large-angle grain boundary of 15 ° or more, and the region surrounded by the outer periphery is a crystal grain.
- the average value of the orientation difference in this single crystal grain is KAM
- FIG. 4 (b) schematically shows how to obtain KAM.
- KAM can be calculated by the following formula.
- GAM is an average value of KAM in one crystal grain
- FIG. 4 (c) schematically shows how to obtain GAM.
- m 9
- GAM of KAM in the same grain 0.64.
- GAM can be calculated by the following formula.
- the crystal grains having a large misorientation in the crystal grains and the crystal grains having a small misorientation in the crystal grains are present in a moderately dispersed manner in the structure, crack propagation is suppressed.
- the upper limit of the area ratio may be 80% or less.
- upper bainite is composed of bainitic ferrite having a large misorientation and granular bay having a small misorientation.
- nittic ferrite it is not limited to this.
- the element in the formula (2) is an element having a weak carbide-forming ability.
- any transformation curves of ferrite, bainitic ferrite, and granular bainitic ferrite are increased to a long time side. shift. That is, if the cooling rate is the same and the value of equation (2) is large, bainitic ferrite and granular bainitic ferrite are likely to be generated.
- the element in the formula (3) is an element having a strong carbide-forming ability, and when the value of the formula (3) increases, only the transformation curves of ferrite and granular bainitic ferrite shift to the long time side.
- the transformation curve of bainitic ferrite hardly changes. That is, when the value of the equation (3) is increased, bainitic ferrite having a large orientation difference is relatively easily generated as compared with granular bainitic ferrite.
- the ratio can be controlled after a mixed structure of bainitic ferrite and granular bainitic ferrite.
- the values of the formulas (2) and (3) may be appropriately adjusted in consideration of the ratio of crystal grains exceeding 1 ° in the GAM value, and are not particularly limited. It is preferable that the calculated value is 40 or more and the value calculated by the expression (3) is 2 or less.
- the value of formula (2) is more preferably 45 or more, and still more preferably 50 or more.
- the value of formula (3) is more preferably 1.5 or less, and still more preferably 1.0 or less.
- the upper limit of the value of formula (2) or the lower limit of the value of formula (3) is inevitably determined by the content range of each element.
- the conditions for controlling the structure form in the surface layer are satisfied, and the cumulative reduction ratio during hot rolling and the reduction ratio in the non-recrystallization temperature range are as follows: It is preferable to control as described above. Cumulative rolling reduction ratio of all hot rolling processes: 80% or more Rolling ratio in non-recrystallization temperature range: less than 85% From either the finish rolling finish temperature or 800 ° C, from the lower temperature to at least 600 ° C Average cooling rate: 3.0 ° C / second or more
- the rolling reduction is preferably 80% or more. If this cumulative rolling reduction is insufficient, even if the surface layer structure becomes fine, the structure inside the steel sheet does not become sufficiently fine, and the crack growth rate does not sufficiently decrease. More preferably, it is 85% or more.
- the average cooling rate after hot rolling that is, the finish rolling end temperature or the temperature of either 800 ° C. is lower.
- the average cooling rate from the temperature of at least 600 ° C. is preferably 3.0 ° C./second or more. More preferably, it is 5 ° C./second or more.
- the cumulative rolling reduction in the non-recrystallization temperature range is less than 85%. More preferably, it is 80% or less.
- Example 1 After steels having chemical composition compositions of steel types A to W shown in Table 1 were melted and cast according to a normal melting method, rolling conditions No. 1 shown in Table 2 were obtained. Hot rolling was performed under various conditions a to l to obtain a steel plate having a thickness of 20 mm.
- Table 1 the column indicated by “ ⁇ ” indicates no addition, and [Si] + [Cu] indicates the total content of Si and Cu.
- the Ar 3 transformation point shown in Table 1 is a value obtained by the above formula (5).
- the “total hot rolling process cumulative reduction ratio” is the cumulative reduction ratio in the total hot rolling process.
- the structure of the steel sheet For each steel sheet, the structure of the steel sheet, the effective crystal grain size, the size of the remaining structure as the second phase, the tensile strength, the fatigue characteristics, and the dislocation density ⁇ were measured according to the following procedures. In any measurement, the test piece was sampled so that the position at a depth of 3 mm from the steel sheet surface was the evaluation position.
- the metal structure was fractionated by an image analysis process using image analysis software “Image Pro Plus ver. 4.0”. The values of the three visual fields were averaged to obtain the area ratio of each metal structure. Note that the observation area was photographed with a visual field of 166 ⁇ m in the plate thickness direction and 222.74 ⁇ m in the rolling direction centered at a position 3 mm deep from the surface of the steel plate.
- the measurement conditions at this time are: measurement area: 200 ⁇ m ⁇ 200 ⁇ m, measurement step: 0.5 ⁇ m interval, and measurement points whose confidence index (Confidence Index) indicating the reliability of the measurement direction is smaller than 0.1 are analyzed. Excluded. With respect to the crystal grain boundaries thus obtained, the cutting lengths at 100 locations in the plate thickness direction were measured, and the average value was taken as the effective crystal grain size. However, an effective crystal grain size of 2.0 ⁇ m or less was judged as measurement noise and excluded. The observation region was a region having a width of 3 mm from the surface of the steel plate and having a spread of 100 ⁇ m on both sides in the plate thickness direction.
- the size of the remaining structure other than ferrite and upper bainite was cut out from the sample so that a plane parallel to the rolling direction of the steel sheet at a depth of 3 mm from the steel sheet surface and perpendicular to the steel sheet surface was exposed, and this was # 150. Polishing was performed using up to # 1000 wet emery paper, followed by mirror polishing using a diamond abrasive as an abrasive. This mirror surface test piece was etched with a 2% nitric acid-ethanol solution, that is, a nital solution, and then observed with an observation area of 3.71 ⁇ 10 ⁇ 2 mm 2 and an observation magnification of 400 times.
- the observation region at this time was a region having a spread of 100 ⁇ m on both sides in the plate thickness direction centered at a position 3 mm deep from the steel plate surface.
- the image was subjected to image analysis processing using the image analysis software, the area per crystal grain of the remaining tissue was calculated, and the equivalent circle diameter of the crystal grains of the remaining tissue was obtained from the calculated area.
- the average value of the three fields of view was averaged to obtain the equivalent circle diameter.
- MA in the remaining tissue was prepared by adding A solution (3 g picric acid + 100 ml ethanol solution), B solution (1 g sodium disulfite + 100 ml distilled water solution), and ethanol to the mirror surface test piece subjected to the above mirror polishing (A Liquid: B liquid: Ethanol)
- a Liquid: B liquid: Ethanol After etching using a repeller corrosive liquid mixed at a ratio of (5: 6: 1), the observation area was 3.71 ⁇ 10 ⁇ 2 mm 2 and the observation magnification was 400 times.
- the white-corroded phase was subjected to image analysis processing using the above-described image analysis software as MA, and after the metal structure was separated, the values of these five fields of view were averaged to obtain the area ratio of MA.
- the fatigue characteristics were obtained by cutting a 4 mm thick steel plate from a position where the depth from the surface of each steel plate was 2 to 6 mm and producing a test piece as shown in FIG.
- the surface of the test piece was polished to # 1200 with emery paper to remove the influence of the surface condition.
- the fatigue test was performed on condition of the following using the electrohydraulic servo type fatigue tester by an Instron company.
- Fatigue properties are affected by the tensile strength TS.
- the fatigue limit ratio of 5 million times was determined, and the fatigue limit ratio of 5 million times exceeded 0.51.
- the 5 million times fatigue limit ratio is a value obtained by dividing the 5 million times fatigue limit by the tensile strength TS, and the 5 million times fatigue limit was determined as follows. In each test piece, a fatigue test was performed with a stress amplitude at which the value obtained by dividing the stress amplitude ⁇ a by the tensile strength TS ( ⁇ a / TS) was 0.51, and the presence or absence of fracture when reaching 5 million times was examined.
- the strain ⁇ is a value calculated by applying the Hall method and based on the equations (7) and (8).
- ⁇ cos ⁇ / ⁇ 0.9 / D + 2 ⁇ sin ⁇ / ⁇ (7)
- ⁇ 2 ⁇ m 2 ⁇ s 2
- ⁇ is the true half width (unit: rad)
- ⁇ is the Bragg angle (unit: °)
- ⁇ is the incident X-ray wavelength (unit: nm)
- D is the crystal size (unit: nm)
- ⁇ m is the measured half width
- ⁇ s is the half width (equipment constant) of the unstrained sample.
- ⁇ is calculated from ⁇ m and ⁇ s by the above equation (8), this value is substituted into the above equation (7), and ⁇ cos ⁇ / ⁇ sin ⁇ / ⁇ is plotted, and (110), (211) And three points of (220) were fitted by the method of least squares. Then, the strain ⁇ was calculated from the inclination (2 ⁇ ) of the fitting straight line, and was substituted into Equation (6) to calculate the dislocation density ⁇ .
- Table 3 shows the structure of the steel sheet, the effective crystal grain size, the size of the remaining structure, the tensile strength TS, the fatigue characteristics, and the dislocation density ⁇ .
- test no. Nos. 18 to 34 are examples in which any of the requirements defined in the present invention is not met, and all of them resulted in poor fatigue characteristics.
- test no. 18 is an example using a steel plate of steel type K with a small amount of C, and a predetermined tensile strength TS has not been achieved. Therefore, properties other than the organization are not evaluated.
- Test No. 19 is an example using a steel sheet of steel type L with an excessive amount of C, and the tensile strength TS is too high. Therefore, properties other than the organization are not evaluated.
- Test No. No. 20 is an example using a steel sheet of steel type M that deviates from the requirement that “the total content of Si and Cu is 0.3% or more”, the dislocation celling suppression was not exhibited, and the fatigue characteristics deteriorated.
- Test No. No. 21 is an example in which a steel sheet of steel type N having an excessive amount of Si was used. The size of the remaining structure was increased, and the fatigue characteristics were deteriorated.
- Test No. No. 22 is an example using a steel sheet of steel type O having an excessive amount of Mn.
- the tensile strength TS was increased, the dislocation density ⁇ was increased, and the fatigue characteristics were deteriorated.
- Test No. No. 23 is an example using a steel sheet P of steel type P with a low Mn content.
- the predetermined tensile strength TS was not achieved, the effective crystal grain size became too large, and the fatigue characteristics deteriorated.
- Test No. No. 24 is an example in which a steel sheet of steel type Q having an excessive amount of Cu was used, and the dislocation density ⁇ was excessive, and the fatigue characteristics deteriorated.
- Test No. 25 is an example using a steel sheet of steel type R with an excessive amount of Ni, and the requirement of [Ni] / [Cu] ⁇ 1.2 is also deviated, the dislocation density ⁇ becomes excessive, and the fatigue characteristics deteriorate. did.
- Test No. No. 26 is an example using a steel sheet of steel type S with an excessive amount of Cr.
- the dislocation density ⁇ was excessive, and the fatigue characteristics deteriorated.
- Test No. No. 27 is an example using a steel sheet of steel type T with an excessive amount of Mo.
- the dislocation density ⁇ was excessive, and the fatigue characteristics deteriorated.
- Test No. No. 28 is an example using a steel sheet of steel type U with an excessive amount of V.
- the dislocation density ⁇ was excessive, and the fatigue characteristics deteriorated.
- Test No. No. 29 is an example using a steel sheet of steel type V having a bainite transformation start temperature Bs lower than 640 ° C., and the dislocation density ⁇ became excessive, and the fatigue characteristics deteriorated.
- Test No. 30 is a rolling condition No. 30 with a high heating temperature of hot rolling. In this example, the effective crystal grain size became too large, and the fatigue characteristics deteriorated.
- Test No. No. 31 is a rolling condition No. 31 with a low cumulative rolling reduction during hot rolling. In this example, the effective crystal grain size becomes too large, and the fatigue characteristics are deteriorated.
- Test No. No. 32 is a rolling condition No. having a low finish rolling end temperature. In this example, the dislocation density ⁇ was excessive and the fatigue characteristics were deteriorated.
- Test No. No. 33 is a rolling condition no. In this example, the dislocation density ⁇ was excessive and the fatigue characteristics were deteriorated.
- Test No. 34 is a rolling condition No. 34 having a low cooling stop temperature. In this example, the dislocation density ⁇ was excessive and the fatigue characteristics were deteriorated.
- Example 2 Test No. shown in Table 3
- the inside of the steel plate that is, the fraction of the upper bainite at the position of t / 4 where the plate thickness is t, the effective crystal grain size, and the size of the remaining structure as the second phase were carried out. Evaluation was carried out in the same manner as in Example 1.
- the method of collecting the test piece is the same as described above except that the position is equivalent to t / 4 of the plate thickness t.
- the ratio of the crystal grain in which GAM becomes 1 degree or more and the crack growth rate were measured by the following method.
- the ratio of crystal grains with a GAM of 1 ° or more was measured by SEM-EBSP.
- an EBSP apparatus (trade name: “OIM”) manufactured by TEX SEM Laboratories is used in combination with SEM, and a region surrounded by a large-angle grain boundary in which the orientation difference between adjacent crystal grains is 15 ° or more is defined as a crystal grain. As a result, the crystal grain size was measured.
- Measurement conditions at this time are centered on the t / 4 position of the steel sheet, a region having a spread of 100 ⁇ m on both sides in the plate thickness direction is a measurement region of 200 ⁇ m ⁇ 200 ⁇ m, a measurement step: an interval of 0.5 ⁇ m, and a measurement orientation Measurement points having a confidence index CI (Confidence Index) indicating reliability of less than 0.1 were excluded from the analysis target.
- the average value GAM in the crystal grain of KAM which is the orientation difference between each measurement point and the adjacent point, was measured, and the crystal grain having a GAM of 1 ° or more was obtained.
- This crystal grain means a crystal grain into which high strain is introduced. This measurement was performed for each steel type with three fields of view, and the average value of the area fraction at which the GAM was 1 ° or more was calculated.
- test no. 1 to 6, 10, 11, 13, and 15 satisfy the preferable requirements in the steel sheet because the chemical composition and production conditions of the steel are appropriately controlled, and the crack growth rate is 4.0 ⁇ 10. -5 mm / cycle or less, and it was confirmed that it had further excellent fatigue crack growth characteristics.
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Abstract
Description
表層での金属組織がフェライトおよび上部ベイナイトの少なくとも一方を、分率で80面積%以上含み、
前記フェライトおよび上部ベイナイトの少なくとも一方の結晶粒の有効結晶粒径が10.0μm以下であり、
前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織の結晶粒の平均円相当径が3.0μm以下であり、
X線回折で測定したときの転位密度ρの値が2.5×1015m-1以下である、
ことを特徴とする。
(a)更に、Ni:0質量%超、0.6質量%以下を含有し、Ni量[Ni]と前記Cu量[Cu]との比である[Ni]/[Cu]が1.2未満である。
(b)更に、V:0質量%超、0.5質量%以下、Cr:0質量%超、1.0質量%以下およびMo:0質量%超、0.5質量%以下よりなる群から選択される1種以上を含有する。
(c)更に、B:0質量%超、0.005質量%以下を含有する。
Bs(℃)=830-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] …(1)
但し、[C]、[Mn]、[Ni]、[Cr]および[Mo]は、夫々C、Mn、Ni、CrおよびMoの含有量(質量%)を示す。
C:0.02~0.10質量%、Mn:1.0~2.0質量%、Nb:0質量%超、0.05質量%以下、Ti:0質量%超、0.05質量%以下、Al:0.01~0.06質量%を夫々含有すると共に、Si:0.1~0.6質量%およびCu:0.1~0.6質量%から選択される1種以上を含有し、SiとCuを合計で0.3質量%以上含み、更に、B:0質量%超、0.005質量%以下を含有し、残部が鉄および不可避的不純物の厚鋼板であって、
表層での金属組織がフェライトおよび上部ベイナイトの少なくとも一方を、分率で80面積%以上含み、
前記フェライトおよび上部ベイナイトの少なくとも一方の結晶粒の有効結晶粒径が10.0μm以下であり、
前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織の結晶粒の平均円相当径が3.0μm以下であり、
X線回折で測定したときの転位密度ρの値が2.5×1015m-1以下であり、
且つ、
板厚をtとしたとき、表面から板厚方向に沿って板厚tのt/4位置における、圧延方向に平行な縦断面での金属組織が、上部ベイナイトを分率で80面積%以上含み、
前記上部ベイナイトの結晶粒の有効結晶粒径が10.0μm以下であり、
上部ベイナイトを除いた残部組織の結晶粒の平均円相当径が3.0μm以下である、
ことを特徴とする。
(a)更に、Ni:0質量%超、0.6質量%以下を含有し、Ni量[Ni]と前記Cu量[Cu]との比である[Ni]/[Cu]が1.2未満である。
(b)更に、V:0質量%超、0.5質量%以下、Cr:0質量%超、1.0質量%以下およびMo:0質量%超、0.5質量%以下よりなる群から選択される1種以上を含有する。
Bs(℃)=830-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] …(1)
但し、[C]、[Mn]、[Ni]、[Cr]および[Mo]は、夫々C、Mn、Ni、CrおよびMoの含有量(質量%)を示す。
35×[Si]+18×[Mn]+17×[Ni]+16×[Cu]≧40…(2)
21×{[Ti]-3.4[N]}+19×[Cr]+11×{[Nb]-7.7[C]}+10×[Mo]≦2 …(3)
但し、[Si]、[Mn]、[Ni]、[Cu]、[Ti]、[N]、[Cr]、[Nb]、[C]および[Mo]は、夫々Si、Mn、Ni、Cu、Ti、N、Cr、Nb、CおよびMoの含有量(質量%)を示し、{[Ti]-3.4[N]}、{[Nb]-7.7[C]}が負となるときは、「0」として計算する。
本発明の実施の形態1について説明する。本発明者らは、まず種々の厚鋼板について、添加元素と疲労強度の関係を調査した。その結果、SiおよびCuの添加により疲労強度が顕著に向上することが明らかになった。一般的に疲労亀裂は繰り返し応力により運動する可動転位が交差すべり等により非可逆的運動となることに起因して発生する。この際、転位はセル構造を形成することが知られているが、SiおよびCuを合計で0.3質量%以上となるように添加することで、このセル構造の形成が抑制されることが明らかとなった。
Cは、鋼板の強度を確保するために重要な元素である。そのため、C量は0.02質量%以上と定めた。C量は、好ましくは0.03質量%以上であり、より好ましくは0.04質量%以上である。一方、C量が過剰になると、高強度となり過ぎて所望の引張強度が得られないだけでなく、加速冷却を用いる場合には焼入れ性が過剰となり、転位密度ρが大きくなるため疲労特性が低下する。そこでC量は0.10質量%以下と定めた。C量は、好ましくは0.08質量%以下であり、より好ましくは0.06質量%以下である。
Mnは、微細な組織を得るために焼入れ性を確保するうえで重要な元素である。こうした作用を有効に発揮させるためには、Mn量は1.0質量%以上とする必要がある。Mn量は好ましくは1.2質量%以上であり、より好ましくは1.4質量%以上である。しかしMn量が過剰になると、焼入れ性が過剰となり転位密度ρが増加し、十分な疲労特性が得られない。そのため、Mn量は2.0質量%以下とする必要がある。Mn量は好ましくは1.8質量%以下であり、より好ましくは1.6質量%以下である。
Nbは、焼入れ性を向上させ、組織を微細化させるために有効な元素である。こうした作用を有効に発揮させるためには、Nb量は0.01質量%以上とすることが好ましい。より好ましくは0.02質量%以上である。しかしながら、Nb量が過剰になると焼入れ性が過剰になり、所望の疲労特性が得られない。そのため、Nb量は0.05質量%以下とする必要がある。好ましくは0.04質量%以下、より好ましくは0.03質量%以下である。
Tiは、焼入れ性を向上させると同時にTiNを形成することで溶接時の熱影響部の組織を微細とし、靱性の低下を抑制するなどに有用な元素である。このため、Tiは0.01質量%以上含有させることが好ましい。より好ましくは0.02質量%以上である。しかしながら、Ti量が過剰になると、粗大なTiNが生じることで靱性などの特性を低下させる恐れがある。そのため、Ti量は0.05質量%以下とする必要がある。好ましくは0.04質量%以下、より好ましくは0.03質量%以下である。
Alは脱酸のために有用な元素であり、0.01質量%に満たないと脱酸効果が発揮されない。好ましくは0.02質量%以上であり、より好ましくは0.03質量%以上である。しかしながら、Al量が過剰になると焼入れ性が過剰となり、転位密度ρが増加することで所望する疲労特性が得られない。そのため、Al量は0.06質量%以下とする必要がある。好ましくは0.05質量%以下、より好ましくは0.04質量%以下である。
Siは、固溶強化量が大きく母材の強度を確保するために必要な元素であると同時に、転位の運動を抑制させることでセル化抑制に有効な元素である。この作用を有効に発揮させるためには、Si量は0.1質量%以上とする必要がある。Si量は好ましくは0.2質量%以上、より好ましくは0.3質量%以上である。しかし、Si量が過剰になると残部組織が粗大かつ過剰に生じるため、靱性等他の特性を低下させる恐れがある。そのため、Si量は0.6質量%以下とする必要がある。好ましくは0.55質量%以下、より好ましくは0.5質量%以下である。
Niは、焼入れ性を向上させ、組織を微細にする効果があると同時に、Cu添加により生じやすくなる熱間加工時の割れを抑制する効果がある。このような効果を発揮させるためには、Niは0.1質量%以上含有させることが好ましい。より好ましくは0.2質量%以上である。しかし、Niを過剰に含有させると焼入れ性が過剰となり、転位密度ρが過大となることで所望とする疲労特性が得られない。そのため、Ni量は0.6質量%以下とすることが好ましい。より好ましくは0.5質量%以下、更に好ましくは0.4質量%以下である。
V、CrおよびMoは、鋼板の焼入れ性を向上させる効果のある元素であり、組織を微細化させることに有効である。このような作用を発揮させるためには、V:0.01質量%以上、Cr:0.1質量%以上、Mo:0.01質量%以上のいずれか単独、または2種以上を含有させことが好ましい。しかしながら、これらの元素を過剰に含有させると焼入れ性が過剰となり、転位密度ρが過大となって所望の疲労特性が得られない。そこで、夫々の量をV:0.5質量%以下、Cr:1.0質量%以下、Mo:0.5質量%以下とすることが好ましい。より好ましくは、V:0.4質量%以下、Cr:0.8質量%以下、Mo:0.4質量%以下である。
Bは、焼入れ性を向上させる元素であり、特に粗大なフェライト組織の生成を抑制して、微細な上部ベイナイト組織を生じさせやすくする元素である。こうした効果を発揮させるためには、B量を0.0005質量%以上とすることが好ましい。より好ましくは0.001質量%以上である。しかし、B量が過剰になると焼入れ性が過剰となり、転位密度ρが過大となって所望の疲労特性が得られないため、0.005質量%以下とすることが好ましい。より好ましくは、0.004質量%以下である。
全熱間圧延工程での累積圧下率:70%以上
仕上げ圧延終了温度:Ar3変態点~Ar3変態点+150℃
仕上げ圧延終了温度または800℃のいずれかの温度のうち、より低温の温度から600℃での平均冷却速度:15℃/秒以下
冷却停止温度:500℃以上
累積圧下率=(t0-t1)/t2×100 …(4)
(4)式中、t0は表面から3mmの位置の温度が圧延温度範囲にあるときの鋼片の圧延開始厚み(単位:mm)、t1は表面から3mmの位置の温度が圧延温度範囲にあるときの鋼片の圧延終了厚み(単位:mm)、t2は圧延前の鋼片、例えばスラブの厚みを、夫々示す。
Ar3変態点=910-230×[C]+25×[Si]-74×[Mn]-56×[Cu]-16×[Ni]-9×[Cr]-5×[Mo]-1620×[Nb]…(5)
但し、[C],[Si],[Mn],[Cu],[Ni],[Cr],[Mo]および[Nb]は、夫々C,Si,Mn,Cu,Ni,Cr,MoおよびNbの質量%での含有量を示す。
次に、本発明の実施の形態2について説明する。大型構造物に用いられる鋼板においては、亀裂進展速度の低下、即ち亀裂進展特性の向上もまた求められている。それは、万が一疲労亀裂が発生した場合でも、亀裂進展速度が遅ければ、破壊に至るまでに損傷部位を発見し、補修することが可能であるからである。
21×{[Ti]-3.4[N]}+19×[Cr]+11×{[Nb]-7.7[C]}+10×[Mo]≦2 …(3)
但し、[Si]、[Mn]、[Ni]、[Cu]、[Ti]、[N]、[Cr]、[Nb]、[C]および[Mo]は、夫々Si、Mn、Ni、Cu、Ti、N、Cr、Nb、CおよびMoの含有量(質量%)を示し、{[Ti]-3.4[N]}、{[Nb]-7.7[C]}が負となるときは、「0」として計算する。
全熱間圧延工程の累積圧下率:80%以上
未再結晶温度域での圧下率:85%未満
仕上げ圧延終了温度または800℃のいずれかの温度のうち、より低温の温度から少なくとも600℃までの平均冷却速度:3.0℃/秒以上
表1に示す鋼種A~Wの化学成分組成の鋼を、通常の溶製法に従って溶製し鋳造した後、表2に示す圧延条件No.a~lの各種条件にて熱間圧延を行ない、厚さ20mmの鋼板を得た。尚、表1において、「-」で示した欄は無添加であることを示し、[Si]+[Cu]はSiとCuの合計含有量を示す。また表1に示したAr3変態点は、前記式(5)によって求められた値である。表2において、「全熱間圧延工程累積圧下率」とは、全熱間圧延工程での累積圧下率である。
鋼板表面から深さ3mm位置の鋼板の圧延方向に平行で且つ鋼板の表面に対して垂直な面が露出するようにサンプルを切り出し、これを#150~#1000までの湿式エメリー紙を用いて研磨し、その後に研磨剤としてダイヤモンド研磨剤を用いて鏡面研磨仕上げした。この鏡面試験片を、2%硝酸-エタノール溶液、即ちナイタール溶液でエッチングした後、観察面積:3.71×10-2mm2、観察倍率400倍で3視野観察し、その画像をMedia Cybernetics社製の画像解析ソフト“Image Pro Plus ver.4.0”を使用した画像解析処理により、金属組織の分別を行った。この3視野の値を平均してそれぞれの金属組織の面積率とした。尚、観察領域は、鋼板表面から深さ3mm位置を中心とし、板厚方向に166μm、圧延方向に222.74μmの視野で撮影した。
鋼板表面から深さ3mm位置の鋼板の圧延方向に平行な縦断面において、SEM(Scanning Electron Microscope:走査型電子顕微鏡)-EBSP(Electron Backscatter Pattern:電子後方散乱解析像法)によってフェライトおよび上部ベイナイトの有効結晶粒径を測定した。具体的には、TEX SEM Laboratries社のEBSP装置(商品名:「OIM」)をSEMと組み合わせて用い、隣り合う結晶粒の方位差が15°以上の大角粒界で囲まれた領域を結晶粒として結晶粒径を測定した。このときの測定条件は、測定領域:200μm×200μm、測定ステップ:0.5μm間隔とし、測定方位の信頼性を示すコンフィデンス・インデックス(Confidence Index)が0.1よりも小さい測定点は解析対象から除外した。このようにして求められる結晶粒界について、板厚方向に100箇所の切断長さを測定し、その平均値を有効結晶粒径とした。但し、有効結晶粒径が2.0μm以下は測定ノイズとして判断し、除外した。観察領域は、鋼板表面から深さ3mm位置を中心とし、板厚方向両側に100μmの広がりのある領域とした。
フェライトおよび上部ベイナイト以外の残部組織のサイズは、鋼板表面から深さ3mm位置の鋼板の圧延方向に平行で且つ鋼板の表面に対して垂直な面が露出するようにサンプルを切り出し、これを#150~#1000までの湿式エメリー紙を用いて研磨し、その後に研磨剤としてダイヤモンド研磨剤を用いて鏡面研磨仕上げした。この鏡面試験片を、2%硝酸-エタノール溶液、即ちナイタール溶液でエッチングした後、観察面積:3.71×10-2mm2、観察倍率400倍で観察した。このときの観察領域は、鋼板表面から深さ3mm位置を中心とし、板厚方向両側に100μmの広がりのある領域とした。その画像を、上記画像解析ソフトを用いて画像解析処理し、残部組織の結晶粒1個あたりの面積を算出し、算出した面積から、残部組織の結晶粒の円相当直径を求めた。尚、本実施例では、3視野の平均値を平均して、円相当直径とした。
各鋼板の表面からの深さが2~6mmとなる位置から、板厚4mm、標点距離35mmの引張試験片を採取し、JIS Z2241:2011にしたがって引張試験を行なうことによって、引張強度TS(Tensile Strength)を測定した。
疲労特性は、各鋼板の表面からの深さが2~6mmとなる位置から、4mm厚の鋼板を切り出し、図2に示すような試験片を作製して行なった。尚、試験片表面はエメリー紙にて#1200まで研磨を行なって、表面状態の影響を除去した。得られた試験片について、インストロン社製電気油圧サーボ式疲労試験機を用いて、以下の条件で疲労試験を行なった。
制御方法:荷重制御
制御波形:正弦波
応力比:R=-1
試験速度:20Hz
試験終了サイクル数:5000000回
転位密度ρはX回折測定を行ない、得られたα-Feの半価幅より算出した。以下に、測定条件および測定原理を説明する。分析装置はX線回折装置「RAD-RU300」(商品名:理学電機株式会社製)を用い、ターゲットにはCo乾球を用いた。得られたX線回折測定結果より、ピークフィッティングによりピーク半価幅を算出し、転位密度ρを計算した。転位密度ρは式(6)より求めた。
ρ(m-1)=-14.4ε2/b2 …(6)
但し、εは歪みを、bはバーガースべクトル(=0.25×10-9m)を夫々示す。
βcosθ/λ=0.9/D+2εsinθ/λ …(7)
β2=βm 2-βs 2 …(8)
尚、βは真の半価幅(単位:rad)、θはブラッグ角(単位:°)、λは入射X線波長(単位:nm)、Dは結晶の大きさ(単位:nm)、βmは実測した半価幅、βsは無歪試料における半価幅(装置定数)である。また、上記式(8)により、βmとβsからβを計算し、この値を上記式(7)に代入してβcosθ/λ-sinθ/λをプロットし、(110)、(211)および(220)の3点を最小自乗法でフィッティングした。そして、フィッティング直線の傾き(2ε)から歪みεを算出し、式(6)に代入して転位密度ρを計算した。
表3に示した試験No.1~17の各鋼板について、鋼板内部、即ち板厚をtとしたときのt/4の位置での上部ベイナイトの分率、有効結晶粒径、第2相となる残部組織のサイズについて、実施例1に示した方法と同様にして評価した。試験片の採取方法については、板厚tのt/4に相当する位置とする以外は、上記と同様である。また、これらの鋼板につき、下記の方法によって、GAMが1°以上となる結晶粒の割合および亀裂進展速度を測定した。
鋼板のt/4位置において、SEM-EBSPによって、GAMが1°以上となる結晶粒の割合の測定を行った。具体的には、TEX SEM Laboratries社のEBSP装置(商品名:「OIM」)をSEMと組み合わせて用い、隣り合う結晶粒の方位差が15°以上の大角粒界で囲まれた領域を結晶粒として結晶粒径を測定した。このときの測定条件は、鋼板のt/4位置を中心とし、板厚方向両側に100μmの広がりのある領域を、200μm×200μmの測定領域とし、測定ステップ:0.5μm間隔とし、測定方位の信頼性を示すコンフィデンス・インデックスCI(Confidence Index)が0.1よりも小さい測定点は解析対象から除外した。各測定点と隣接点との方位差であるKAMの結晶粒内の平均値GAMを測定し、GAMが1°以上となる結晶粒とした。この結晶粒は、高ひずみが導入された結晶粒を意味する。この測定を各鋼種3視野の測定を行い、GAMが1°以上となる面積分率の平均値を計算した。
ASTM(American Society for Testing Materials) E647に従って、コンパクト試験片を用い、電気油圧サーボ式疲労試験機にて下記の条件で疲労亀裂進展試験を行い、亀裂進展速度を測定した。尚、コンパクト試験片は、鋼板のt/4の位置から採取し、図3に示す形状のものを用いた。また、亀裂長さはコンプライアンス法を用いた。
試験環境:室温、大気中
制御方法:荷重制御
制御波形:正弦波
応力比:R=-1
試験速度:5~20Hz
但し、式(9)中、a:亀裂長さ(単位:mm)、n:繰り返し数(単位:cycle)、C,p:材料、荷重等の条件で決まる定数を夫々示す。
Claims (16)
- C:0.02~0.10質量%、Mn:1.0~2.0質量%、Nb:0質量%超、0.05質量%以下、Ti:0質量%超、0.05質量%以下、Al:0.01~0.06質量%を夫々含有すると共に、Si:0.1~0.6質量%およびCu:0.1~0.6質量%から選択される1種以上を含有し、SiとCuを合計で0.3質量%以上含み、残部が鉄および不可避的不純物の厚鋼板であって、
表層での金属組織がフェライトおよび上部ベイナイトの少なくとも一方を、分率で80面積%以上含み、
前記フェライトおよび上部ベイナイトの少なくとも一方の結晶粒の有効結晶粒径が10.0μm以下であり、
前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織の結晶粒の平均円相当径が3.0μm以下であり、
X線回折で測定したときの転位密度ρの値が2.5×1015m-1以下である、
ことを特徴とする厚鋼板。 - 前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織中の島状マルテンサイトの割合が、分率で5面積%以下である請求項1に記載の厚鋼板。
- 化学成分組成が、下記(a)~(c)の少なくともいずれかの要件を満足する請求項1に記載の厚鋼板。
(a)更に、Ni:0質量%超、0.6質量%以下を含有し、Ni量[Ni]と前記Cu量[Cu]との比である[Ni]/[Cu]が1.2未満である。
(b)更に、V:0質量%超、0.5質量%以下、Cr:0質量%超、1.0質量%以下およびMo:0質量%超、0.5質量%以下よりなる群から選択される1種以上を含有する。
(c)更に、B:0質量%超、0.005質量%以下を含有する。 - 前記残部組織中の島状マルテンサイトの割合が、分率で5面積%以下である請求項3に記載の厚鋼板。
- 化学成分組成から下記式(1)に基づいて計算されるベイナイト変態開始温度Bsが640℃以上である請求項1~4のいずれかに記載の厚鋼板。
Bs(℃)=830-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] …(1)
但し、[C]、[Mn]、[Ni]、[Cr]および[Mo]は、夫々C、Mn、Ni、CrおよびMoの含有量(質量%)を示す。 - C:0.02~0.10質量%、Mn:1.0~2.0質量%、Nb:0質量%超、0.05質量%以下、Ti:0質量%超、0.05質量%以下、Al:0.01~0.06質量%を夫々含有すると共に、Si:0.1~0.6質量%およびCu:0.1~0.6質量%から選択される1種以上を含有し、SiとCuを合計で0.3質量%以上含み、更に、B:0質量%超、0.005質量%以下を含有し、残部が鉄および不可避的不純物の厚鋼板であって、
表層での金属組織がフェライトおよび上部ベイナイトの少なくとも一方を、分率で80面積%以上含み、
前記フェライトおよび上部ベイナイトの少なくとも一方の結晶粒の有効結晶粒径が10.0μm以下であり、
前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織の結晶粒の平均円相当径が3.0μm以下であり、
X線回折で測定したときの転位密度ρの値が2.5×1015m-1以下であり、
且つ、
板厚をtとしたとき、表面から板厚方向に沿って板厚tのt/4位置における、圧延方向に平行な縦断面での金属組織が、上部ベイナイトを分率で80面積%以上含み、
前記上部ベイナイトの結晶粒の有効結晶粒径が10.0μm以下であり、
上部ベイナイトを除いた残部組織の結晶粒の平均円相当径が3.0μm以下である、
ことを特徴とする厚鋼板。 - 前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織中の島状マルテンサイトの割合が、分率で5面積%以下である請求項6に記載の厚鋼板。
- 化学成分組成が、下記(a)、(b)の少なくともいずれかの要件を満足する請求項6に記載の厚鋼板。
(a)更に、Ni:0質量%超、0.6質量%以下を含有し、Ni量[Ni]と前記Cu量[Cu]との比である[Ni]/[Cu]が1.2未満である。
(b)更に、V:0質量%超、0.5質量%以下、Cr:0質量%超、1.0質量%以下およびMo:0質量%超、0.5質量%以下よりなる群から選択される1種以上を含有する。 - 前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織中の島状マルテンサイトの割合が、分率で5面積%以下である請求項8に記載の厚鋼板。
- 前記t/4位置における金属組織において、EBSP法により観察した一つの結晶粒内におけるGAMが1°以上の結晶粒を、分率で20面積%以上80面積%以下含む請求項6に記載の厚鋼板。
- 更に、下記式(2)および式(3)の関係を満足する請求項10に記載の厚鋼板。
35×[Si]+18×[Mn]+17×[Ni]+16×[Cu]≧40…(2)
21×{[Ti]-3.4[N]}+19×[Cr]+11×{[Nb]-7.7[C]}+10×[Mo]≦2 …(3)
但し、[Si]、[Mn]、[Ni]、[Cu]、[Ti]、[N]、[Cr]、[Nb]、[C]および[Mo]は、夫々Si、Mn、Ni、Cu、Ti、N、Cr、Nb、CおよびMoの含有量(質量%)を示し、{[Ti]-3.4[N]}、{[Nb]-7.7[C]}が負となるときは、「0」として計算する。 - 化学成分組成が、下記(a)、(b)の少なくともいずれかの要件を満足する請求項10に記載の厚鋼板。
(a)更に、Ni:0質量%超、0.6質量%以下を含有し、Ni量[Ni]と前記Cu量[Cu]との比である[Ni]/[Cu]が1.2未満である。
(b)更に、V:0質量%超、0.5質量%以下、Cr:0質量%超、1.0質量%以下およびMo:0質量%超、0.5質量%以下よりなる群から選択される1種以上を含有する。 - 前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織中の島状マルテンサイトの割合が、分率で5面積%以下である請求項12に記載の厚鋼板。
- 化学成分組成が、下記(a)、(b)の少なくともいずれかの要件を満足する請求項11に記載の厚鋼板。
(a)更に、Ni:0質量%超、0.6質量%以下を含有し、Ni量[Ni]と前記Cu量[Cu]との比である[Ni]/[Cu]が1.2未満である。
(b)更に、V:0質量%超、0.5質量%以下、Cr:0質量%超、1.0質量%以下およびMo:0質量%超、0.5質量%以下よりなる群から選択される1種以上を含有する。 - 前記表層での金属組織のうちフェライトおよび上部ベイナイトを除いた残部組織中の島状マルテンサイトの割合が、分率で5面積%以下である請求項14に記載の厚鋼板。
- 化学成分組成から下記式(1)に基づいて計算されるベイナイト変態開始温度Bsが640℃以上である請求項6~15のいずれかに記載の厚鋼板。
Bs(℃)=830-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] …(1)
但し、[C]、[Mn]、[Ni]、[Cr]および[Mo]は、夫々C、Mn、Ni、CrおよびMoの含有量(質量%)を示す。
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WO2018034071A1 (ja) * | 2016-08-19 | 2018-02-22 | 株式会社神戸製鋼所 | 厚鋼板およびその製造方法 |
CN109563598A (zh) * | 2016-08-19 | 2019-04-02 | 株式会社神户制钢所 | 厚钢板及其制造方法 |
CN113166897A (zh) * | 2018-11-30 | 2021-07-23 | 株式会社Posco | 具有优异的可冷加工性和ssc抗力的超高强度钢及其制造方法 |
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CN105839003B (zh) * | 2016-05-31 | 2017-09-26 | 江阴兴澄特种钢铁有限公司 | 一种正火态交货的180~200mm厚EH36钢板及其制备方法 |
CN109023048B (zh) * | 2018-08-06 | 2020-08-25 | 首钢集团有限公司 | 一种460MPa级高强抗震耐火耐候钢热轧卷板及其生产方法 |
KR20220147153A (ko) | 2020-06-30 | 2022-11-02 | 가부시키가이샤 고베 세이코쇼 | 후강판 및 그의 제조 방법 |
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EP3656886A1 (en) | 2020-05-27 |
KR20180115352A (ko) | 2018-10-22 |
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