WO2014157036A1 - 鋼矢板及びその製造方法 - Google Patents
鋼矢板及びその製造方法 Download PDFInfo
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- WO2014157036A1 WO2014157036A1 PCT/JP2014/057985 JP2014057985W WO2014157036A1 WO 2014157036 A1 WO2014157036 A1 WO 2014157036A1 JP 2014057985 W JP2014057985 W JP 2014057985W WO 2014157036 A1 WO2014157036 A1 WO 2014157036A1
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- Prior art keywords
- steel sheet
- sheet pile
- ferrite
- pearlite
- strength
- Prior art date
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 170
- 239000010959 steel Substances 0.000 title claims abstract description 170
- 238000004519 manufacturing process Methods 0.000 title claims description 23
- 238000000034 method Methods 0.000 title description 11
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 87
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 46
- 239000002184 metal Substances 0.000 claims abstract description 27
- 229910052751 metal Inorganic materials 0.000 claims abstract description 27
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 21
- 239000002244 precipitate Substances 0.000 claims abstract description 19
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 14
- 238000005096 rolling process Methods 0.000 claims description 43
- 238000005098 hot rolling Methods 0.000 claims description 27
- 230000009467 reduction Effects 0.000 claims description 16
- 239000013078 crystal Substances 0.000 claims description 14
- 230000001186 cumulative effect Effects 0.000 claims description 11
- 238000010438 heat treatment Methods 0.000 claims description 11
- 239000000126 substance Substances 0.000 claims description 11
- 238000001816 cooling Methods 0.000 claims description 8
- 239000012535 impurity Substances 0.000 claims description 7
- 229910052804 chromium Inorganic materials 0.000 claims description 6
- 239000000203 mixture Substances 0.000 claims description 4
- 239000002245 particle Substances 0.000 abstract description 6
- 239000000463 material Substances 0.000 description 31
- 239000010953 base metal Substances 0.000 description 15
- 229910045601 alloy Inorganic materials 0.000 description 13
- 239000000956 alloy Substances 0.000 description 13
- 230000000694 effects Effects 0.000 description 13
- 238000001556 precipitation Methods 0.000 description 9
- 230000007423 decrease Effects 0.000 description 8
- 229910052758 niobium Inorganic materials 0.000 description 7
- 238000005728 strengthening Methods 0.000 description 7
- 229910052720 vanadium Inorganic materials 0.000 description 7
- 229910052759 nickel Inorganic materials 0.000 description 6
- 229910052750 molybdenum Inorganic materials 0.000 description 5
- 229910001567 cementite Inorganic materials 0.000 description 4
- 238000010276 construction Methods 0.000 description 4
- 229910052802 copper Inorganic materials 0.000 description 4
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 4
- 229910052748 manganese Inorganic materials 0.000 description 4
- 229910052757 nitrogen Inorganic materials 0.000 description 4
- 230000003287 optical effect Effects 0.000 description 4
- 229910052710 silicon Inorganic materials 0.000 description 4
- 230000009466 transformation Effects 0.000 description 4
- 238000003466 welding Methods 0.000 description 4
- 229910052782 aluminium Inorganic materials 0.000 description 3
- 229910001566 austenite Inorganic materials 0.000 description 3
- 238000009863 impact test Methods 0.000 description 3
- 230000006872 improvement Effects 0.000 description 3
- 230000002411 adverse Effects 0.000 description 2
- 229910001563 bainite Inorganic materials 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 125000004429 atom Chemical group 0.000 description 1
- 230000033228 biological regulation Effects 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 125000004432 carbon atom Chemical group C* 0.000 description 1
- 238000005256 carbonitriding Methods 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 229910000175 cerite Inorganic materials 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000000605 extraction Methods 0.000 description 1
- 229910000734 martensite Inorganic materials 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000000691 measurement method Methods 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
- -1 ore or scrap Substances 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
Images
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
- B21B1/026—Rolling
-
- 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
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
- B21B45/004—Heating the product
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
- B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
- B21B45/0203—Cooling
-
- 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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
-
- 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
-
- 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
- C21D2211/009—Pearlite
Definitions
- the present invention relates to a steel sheet pile used for earth retaining and water retaining in the field of civil engineering and construction and a method for producing the same.
- the steel sheet pile has a hat shape, a U shape, a Z shape, a straight shape, or an H shape in cross section, and has connecting portions (joints) at both ends thereof.
- steel sheet piles are widely used as steel materials for earth retaining and water retaining in the field of civil engineering and construction.
- Steel sheet piles are subject to high stress when they are used for revetment in deep harbors and soft ground.
- Strengthening of steel sheet pile base metal and welded parts is required. Therefore, a steel sheet pile having a yield stress of 460 MPa or more is required.
- the steel sheet piles are welded together during use, the steel sheet piles are also required to have high toughness at their welds.
- As one means for increasing the toughness of the welded portion of the steel sheet pile it is conceivable to reduce the hardenability of the steel sheet pile. However, when the hardenability is lowered, the yield stress of the steel sheet pile is lowered.
- controlled rolling can be employed as a means for realizing high strength of the steel sheet pile base metal and welded parts.
- an upward warp and / or a downward warp may occur in the steel sheet pile during controlled rolling.
- a steel sheet pile controlled rolling method has been proposed in which the warp shape is controlled by performing controlled rolling under predetermined rolling conditions and cooling conditions (see, for example, Patent Document 4).
- Patent Document 5 a technique is disclosed in which a part of the steel structure is transformed into ferrite before hot rolling by increasing the Al content in the steel material (Al: 0.3 to 2.0 mass%).
- the yield strength (YP) of a steel sheet pile obtained by hot rolling is set to 390 N / mm 2 or more while suppressing an increase in rolling load (deformation resistance of steel material) in a temperature range of 1000 ° C. or lower.
- a method of manufacturing a steel sheet pile that can be used has been proposed.
- Japanese Unexamined Patent Publication No. 09-287052 Japanese Unexamined Patent Publication No. 10-001721 Japanese Unexamined Patent Publication No. 2003-253379 Japanese Unexamined Patent Publication No. 2006-249513 Japanese Unexamined Patent Publication No. 2007-332414
- the steel sheet pile needs to have high yield strength, high tensile strength, and high toughness, and also has low hardenability. Furthermore, in the manufacturing method of a steel sheet pile, it is necessary to suppress the content of the alloy element, simplify the rolling conditions, and reduce the load on the rolling roll. However, there is no prior art that satisfies all these conditions. In view of such problems, the present invention is directed to alloy saving that suppresses excessive addition of expensive alloy elements, easy manufacturability that can be produced without impairing productivity, and improvement of yield strength to 460 MPa or more. The task is to solve the conflicting problems.
- a steel sheet pile that requires weldability and toughness and further requires high strength is manufactured without impairing cost performance and productivity, and a steel sheet pile having a yield stress of 460 MPa or more and a method for manufacturing the steel sheet pile are provided. This is the issue.
- the inventors of the present invention have earnestly studied a method for controlling precipitates in a steel material by hot rolling a steel material that limits the carbon equivalent and contains an alloy element at a high temperature. did.
- the yield strength can be increased without significantly impairing toughness by containing a predetermined amount of V and Nb and by promoting the precipitation of carbonitride by increasing the rolling reduction at high temperature and performing hot rolling.
- the present invention has been made based on such knowledge, and the gist of the invention is as follows.
- the chemical composition is mass%, C: 0.05 to 0.18%, Si: 0.10 to 0.50%, Mn: 0.50 to 1.50%, Nb: 0.040 to 0.050%, V: 0.20 to 0.30%, Cu: 0 to 0.40%, Ni: 0 to 1.00%, Mo: 0 to 1 .00%, and Cr: 0 contains ⁇ 1.00%, Al: limited to less than 0.05%, balance being Fe and impurities, the carbon equivalent CE N obtained by the following formulas 1 and 2 0.260 to 0.500, and the metal structure includes ferrite, pearlite, Widmanstatten ferrite, and a precipitate, and the precipitate is one of Nb carbonitride and V carbonitride Or the total number density of the precipitates is 0.10 to 0.30 / ⁇ m 2 , The total area ratio of the cerite and the pearlite is 70% or more, the area ratio of the Widmanstatten ferrite is 1% or more
- CE N [C] + f (C) ⁇ ⁇ [Mn] / 6 + [Si] / 24 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5 ⁇
- f (C) 0.75 + 0.25 ⁇ tanh ⁇ 20 ⁇ ([C] ⁇ 0.12) ⁇ (Expression 2)
- [C], [Mn], [Si], [Ni], [Cr], [Mo], [Nb], and [V] indicate the content of each element, and the inclusion of elements that are not contained The amount is considered 0%.
- the chemical component is, by mass, Cu: 0.05 to 0.40%, Ni: 0.10 to 1.00%, Mo: 0.10.
- One or more of 1.00 to 1.00% and Cr: 0.10 to 1.00% may be contained.
- a method for producing a steel sheet pile according to another aspect of the present invention includes a step of heating a steel piece made of the chemical component according to the above [1] or [2] to 1100 to 1350 ° C., and 900 ° C. A step of hot rolling the steel slab to obtain a steel sheet pile under the condition that the cumulative rolling reduction is 90% or more and a finishing temperature is 850 ° C. or more; and a step of cooling the steel sheet pile. .
- the alloy is not added excessively, the productivity is improved and the roll roll is prevented from being broken by rolling at a high temperature, the yield stress is 460 MPa or more, and the tensile strength TS is 550 MPa or more.
- the productivity is improved and the roll roll is prevented from being broken by rolling at a high temperature
- the yield stress is 460 MPa or more
- the tensile strength TS is 550 MPa or more.
- the welding part of a steel sheet pile shows the weld metal and heat affected zone of the welded steel sheet pile.
- the base material of the steel sheet pile indicates a portion other than the welded portion in the welded steel sheet pile (substantially the same as “the steel sheet pile before welding”).
- the strength of the base material is reduced. In this case, it is usual to improve the strength of the base material by performing hot rolling at a low temperature in the production stage. On the other hand, it is desirable to perform hot rolling of steel sheet piles at a high temperature in consideration of improvement of productivity and reduction of load on the rolling roll. Therefore, it is difficult to satisfy all of the high strength and toughness of the steel sheet pile base material, the toughness of the welded portion of the steel sheet pile, and the productivity only by means of adjusting the hardenability. . Then, the present inventors examined securing of the strength of the steel sheet pile base metal and the welded portion by precipitation strengthening.
- the present inventors have further studied and attempted to optimize the temperature and reduction ratio of hot rolling, the content of Nb and V, and the hardenability, thereby precipitating Nb carbonitride and V carbonitride.
- the steel sheet pile of high strength whose yield stress is 460 MPa or more was successfully obtained without controlling the toughness of the base metal and the welded portion.
- CE N [C] + f (C) ⁇ ⁇ [Mn] / 6 + [Si] / 24 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5 ⁇
- f (C) 0.75 + 0.25 ⁇ tanh ⁇ 20 ⁇ ([C] ⁇ 0.12) ⁇ (Expression 2)
- [C], [Mn], [Si], [Ni], [Cr], [Mo], [Nb], and [V] indicate the content of each element in unit mass% and are contained. The content of no element is regarded as 0%.
- the unit “%” for the chemical component means “mass%”.
- C (C: 0.05-0.18%) C is an element effective for increasing the strength of steel.
- the lower limit value of the C amount is set to 0.05% in order to ensure strength.
- the upper limit value of the C amount is set to 0.18%.
- the lower limit of the C content is preferably 0.10%.
- Si is a deoxidizing element.
- the lower limit value of the Si amount is set to 0.10%.
- Si is also an element that improves the strength.
- the Si content is preferably 0.20% or more.
- the Si amount becomes excessive, the base material of the steel sheet pile and the toughness of the heat-affected zone decrease, so the upper limit value of the Si amount is set to 0.50%.
- Mn is an element that improves the hardenability of steel, and is useful for refining the metal structure and securing strength and toughness.
- the lower limit value of the amount of Mn is 0.50%.
- the upper limit of the amount of Mn is set to 1.50%. In order to further optimize the ratio between strength and toughness, the upper limit of the amount of Mn is preferably 1.30%.
- Nb is an extremely important element in the present embodiment.
- Nb combines with C and / or N to form Nb carbonitride.
- the Nb carbonitride improves the strength of the steel sheet pile base metal and the welded portion by precipitation strengthening. In order to obtain this effect, the Nb content is 0.040% or more.
- the upper limit value of the Nb amount is 0.050%.
- the Nb content is 0.040 to 0.050%.
- the Nb amount is preferably 0.040 to 0.045%.
- V 0.20-0.30%)
- V is an extremely important element in the present embodiment.
- V combines with C and / or N to form V carbonitride.
- This V carbonitride improves the strength of the steel sheet pile base metal and the welded portion by precipitation strengthening.
- the V amount is set to 0.20% or more.
- the upper limit value of the V amount is set to 0.30%. Therefore, the V amount is 0.20 to 0.30%.
- the amount of V is preferably 0.20 to 0.23%.
- Al is a deoxidizing element, but is not necessarily required when Si, which is another deoxidizing element, is contained. Therefore, the lower limit of the amount of Al is not particularly limited. On the other hand, when the amount of Al becomes excessive, coarse Al oxide is produced, and the toughness of the steel sheet pile is lowered. Therefore, the Al content is limited to less than 0.05%.
- the upper limit of the amount of Al is preferably 0.03%, and more preferably 0.02%.
- the balance of the chemical components of the steel sheet pile according to the present embodiment is Fe and impurities.
- Impurities are raw materials such as ore or scrap, or components mixed in due to various factors in the manufacturing process when industrially manufacturing steel materials, and adversely affect the characteristics of the steel sheet pile according to the present embodiment. It means what is allowed in the range not given.
- typical impurities include P and S.
- P is a harmful component that may embrittle the steel sheet pile and further reduce the strength of the steel sheet pile base metal and the weld. If the amount of P is 0.040% or less, its inclusion is allowed, but a smaller amount of P is preferred.
- S is a harmful component that lowers the strength and toughness of the steel sheet pile base metal and the welded portion. If the amount of S is 0.040% or less, its inclusion is allowed, but it is preferable that the amount of S is small.
- Cu is an element that improves the strength of the base material of the steel sheet pile and the welded portion by dissolving in the metal structure of the steel sheet pile.
- the Cu content is preferably 0.05% or more.
- the upper limit value of the amount of Cu is preferably set to 0.40%.
- Ni is an element that enhances the hardenability of the steel sheet pile and improves the strength and toughness of the steel sheet pile base metal and the welded portion by dissolving in the metal structure of the steel sheet pile.
- the Ni content is preferably set to 0.10% or more.
- the upper limit of the Ni amount is 1.00%. More preferably, the upper limit value of the Ni amount is 0.50%, and more preferably, the upper limit value of the Ni amount is 0.30%.
- Cu when Cu is contained, it is preferable to contain both Cu and Ni in order to prevent deterioration of surface properties.
- Mo is an element that improves the strength of the base material of the steel sheet pile and the welded portion even if its content is very small. In order to obtain this effect, the Mo amount is preferably set to 0.10% or more.
- Mo is an element that enhances the strength of steel at high temperature (ie, steel sheet pile before rolling), so when it is excessively contained, the deformation resistance of the steel sheet pile before rolling increases, and this is the case during rolling. May cause breakage of the rolling roll. Therefore, the upper limit of the Mo amount is preferably 1.00%. More preferably, the upper limit of the amount of Mo is 0.50%. Further, the lower limit of the Mo amount is preferably 0.30%.
- Cr 0 to 1.00% Cr is an element that enhances the hardenability of the steel sheet pile and is effective in increasing the strength.
- the Cu content is preferably set to 0.10% or more.
- the upper limit of the Cr amount is 1.00%. More preferably, the upper limit value of Cr amount is 0.50%, and still more preferably, the upper limit value of Cr amount is 0.30%.
- Carbon equivalent CE N 0.260 to 0.500
- the following known (Formula 1) and (Formula 2) are used to ensure the strength of the base metal of the base metal and the welded portion without reducing the toughness of the welded portion. and from 0.260 to 0.500 the carbon equivalent CE N to be.
- Carbon equivalent CE N is an indicator of the hardenability.
- the lower limit of carbon equivalent CE N shall the lower limit of carbon equivalent CE N to 0.260.
- CE N [C] + f (C) ⁇ ⁇ [Mn] / 6 + [Si] / 24 + [Ni] / 20 + ([Cr] + [Mo] + [Nb] + [V]) / 5 ⁇
- f (C) 0.75 + 0.25 ⁇ tanh ⁇ 20 ⁇ ([C] ⁇ 0.12) ⁇ (Expression 2)
- [C], [Mn], [Si], [Ni], [Cr], [Mo], [Nb], and [V] are the contents of each element.
- the metal structure of the steel sheet pile according to this embodiment will be described.
- regulates the metal structure of a steel sheet pile is not specifically limited, For example, the position of 1/6 of the web width in a steel sheet pile (position spaced apart from the edge part of the web by 1/6 of the web width)
- the metal structure is controlled as described below, it can be considered that the metal structure is appropriately controlled over substantially the entire steel sheet pile.
- the metal structure of the steel sheet pile according to the present embodiment preferably includes ferrite, pearlite, and Widmanstatten ferrite, and further includes precipitates.
- Precipitates are carbonitrides such as V (C, N) and Nb (C, N).
- Precipitation strengthening due to fine precipitates and refinement of the metal structure due to the pinning effect of fine precipitates ensure the toughness of the steel sheet pile base metal and welds, and the steel sheet pile base metal and welds Strength is improved.
- V (C, N) is referred to as V carbonitride
- Nb (C, N) is referred to as Nb carbonitride.
- the pearlite described here is a layered structure of cementite and ferrite, as is generally well known.
- the ferrite described here is a normal ferrite having a granular shape.
- Widmanstetten ferrite is a metal structure that grows at a diffusion-controlled rate of carbon atoms, and is a plate-like ferrite structure in which Fe atoms are transformed and grown by a shear transformation mechanism, similar to the martensitic transformation. It is distinguished from ordinary ferrite.
- Ferrite contained in pearlite and acicular ferrite also have a plate shape, but Widmanstatten ferrite is distinguished from ferrite and acicular ferrite contained in pearlite in the following points.
- Widmanstatten ferrite forms a laminated structure without cementite.
- Acicular ferrite grows radially around non-metallic inclusions.
- Widmanstatten ferrite grows in a plate shape from austenite grain boundaries or already transformed ferrite.
- plate-like ferrite having an aspect ratio L / W of length L and width W of 4 or more, not accompanied by layered cementite, and grown from austenite grain boundaries or already transformed ferrite is obtained. Defined as Domanstetten ferrite.
- This definition distinguishes Widmanstatten ferrite from normal ferrite, ferrite contained in pearlite, and acicular ferrite when the structure of a two-dimensional section of a steel sheet pile is observed with an optical microscope of 200 to 500 times. be able to.
- Toughness can be improved by containing ferrite.
- pearlite the strength can be improved.
- Widmanstatten ferrite the effect of preventing the ferrite and pearlite from coarsening can be obtained. Improvement of the toughness of the steel sheet pile is achieved by preventing the ferrite and pearlite from coarsening.
- Total area ratio of ferrite and pearlite 70% or more
- Widman Stetten ferrite area ratio 1% or more
- the total area ratio of ferrite and pearlite of the steel sheet pile according to the present embodiment is 70% or more.
- strength and toughness of the base material of a steel sheet pile and a welding part can fully be improved.
- Metal structures other than ferrite, pearlite, and Widmanstatten ferrite, such as bainite, may be generated as the remaining structure. The inclusion of such a remaining structure is allowed as long as the total area ratio of the above ferrite and pearlite is maintained. It is not necessary to define the content ratio of ferrite, pearlite, and Widmanstatten ferrite.
- Widmanstatten ferrite is 1% or more in terms of area ratio.
- the above-described effects produced by Widmann Stetten ferrite cannot be obtained when the area ratio is less than 1%.
- the total area ratio of ferrite and pearlite and the area ratio of Widmanstatten ferrite were measured in accordance with the method of International Institute of Welding. Specifically, a grid having 10 crosswise points in total, that is, a total of 100 crossing points, was placed on the optical microscopic structure photograph, and the structure of each crossing point was visually identified (point-counting). This was repeated over 5 fields of view and averaged to quantify the fraction of each constituent tissue.
- Total number density of precipitates 0.10 to 0.30 / ⁇ m 2 If the total number of precipitates per unit area of one or two of Nb carbonitride and V carbonitride is less than 0.10 / ⁇ m 2 , sufficient strength cannot be obtained. On the other hand, when the total number of Nb carbonitrides and V carbonitrides per unit area is more than 0.30 / ⁇ m 2 , the toughness of the steel sheet pile is lowered. Therefore, the total number density of Nb carbonitride and V carbonitride is 0.10 to 0.30 / ⁇ m 2 . The total number of preferable Nb carbonitrides and V carbonitrides per unit area is 0.11 to 0.25 / ⁇ m 2 .
- the total number of Nb carbonitrides and V carbonitrides per unit area can be measured by taking an extracted replica as a sample and analyzing the sample with a transmission electron microscope. It is not necessary to define the sizes of Nb carbonitride and V carbonitride. However, it is considered that Nb carbonitride and V carbonitride having a major axis smaller than 10 nm do not substantially affect the properties of the steel sheet pile and cannot be observed by the above-described extraction replica method. . Therefore, the substantial lower limit of the major axis of Nb carbonitride and V carbonitride is 10 nm.
- Nb carbonitride and V carbonitride are contained at the number density described above, Nb carbonitride and V carbonitride having a major axis exceeding 300 nm are not generated.
- Nb carbonitride and V carbonitride having a major axis larger than 300 nm were not confirmed. Therefore, the substantial upper limit of the major axis of Nb carbonitride and V carbonitride is 300 nm.
- the average crystal grain size of ferrite and pearlite in the steel sheet pile (hereinafter sometimes abbreviated as “average crystal grain size” or “crystal grain size”) exceeds 80 ⁇ m, the toughness of the steel sheet pile base material and welded portion and The strength may decrease.
- the average crystal grain size of the steel sheet pile is less than 10 ⁇ m, the decrease in the elongation of the steel sheet pile may increase.
- the toughness decreases. Therefore, the average grain size of the steel sheet pile is preferably 10 to 80 ⁇ m.
- the average crystal grain size of the metal structure of the steel sheet pile according to the present embodiment is determined by observing with an optical microscope in accordance with JIS G 0551.
- tissue is calculated
- the “average crystal grain size of ferrite and pearlite” means the average crystal grain size of both ferrite and pearlite.
- the average crystal grain size of each of ferrite and pearlite is less than 10 ⁇ m or more than 80 ⁇ m, if the average crystal grain size of both ferrite and pearlite is 10 ⁇ m to 80 ⁇ m, it is judged that the above regulations are satisfied.
- the Widmanstatten ferrite is ignored when measuring the average grain size of the metal structure described above.
- the major axis of the Widmanstatten ferrite is usually about 5 to 30 ⁇ m, and the change in the major axis within this range does not affect the characteristics of the steel sheet pile according to the present embodiment. Therefore, it is not necessary to define the size of the Widmanstatten ferrite in this embodiment.
- FIG. 1 shows the relationship between the average grain size ( ⁇ m), strength [YP (MPa)], and elongation (%) of ferrite and pearlite of a steel sheet pile based on the test results using some samples. It is a figure.
- the yield strength may be less than 460 MPa, and when the average grain size of ferrite and pearlite in steel sheet piles is less than 10 ⁇ m, elongation may decrease. .
- yield strength: 460 to 550 MPa Yield strength: 460 to 550 MPa
- tensile strength: 550 to 740 MPa The yield strength of the base material of the steel sheet pile of this embodiment is set to 460 MPa or more in order to obtain the effect of reducing the plate thickness by increasing the strength.
- the tensile strength of the base material of the steel sheet pile of the present embodiment is set to 550 MPa or more in order to obtain the effect of reducing the plate thickness by increasing the strength. If the yield strength and tensile strength of the steel sheet pile base material are set to the above values or more and the welding cost, transportation and construction costs are suppressed, it is advantageous in terms of economy.
- the upper limit value of the yield strength should be 550 MPa and the upper limit value of the tensile strength should be 740 MPa. Is desirable. Such yield strength and tensile strength are obtained when the steel sheet pile contains the above-mentioned predetermined amount of alloy elements and has a predetermined metal structure.
- the manufacturing method in the steel sheet pile of this embodiment is as follows: C: 0.05 to 0.18%, Si: 0.10 to 0.50%, Mn: 0.50 to 1.50%, Nb: 0.040 to 0.050%, V: 0.20 to 0.30%, Cu: 0 to 0.40%, Ni: 0 to 1.00%, Mo: 0 to 1.00%, and Cr: 0 to 1. containing 00% Al: limited to less than 0.05%, balance being Fe and impurities, the carbon equivalent CE N is a steel strip is 0.260 to 0.500, and heated to 1100 ⁇ 1350 ° C.
- the chemical composition of the steel slab is, by mass, Cu: 0.05 to 0.40%, Ni: 0.10 to 1.00%, Mo: 0.10 to One or more of 1.00% and Cr: 0.10 to 1.00% may be contained.
- the chemical composition of the molten steel is adjusted by a conventional method and then cast to obtain a steel piece.
- the casting is preferably continuous casting.
- the thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation and time required for heating.
- the steel sheet pile of the present embodiment is manufactured by hot rolling a steel piece. After hot rolling, air cooling may be performed, but accelerated cooling may be performed in order to increase the strength and toughness of the base material of the steel sheet pile and the welded portion.
- Step slab heating temperature before hot rolling 1100-1350 ° C
- the heating temperature of the steel slab before hot rolling is 1100 ° C. or higher. This is because if the heating temperature is too low, the temperature of the steel slab decreases during hot rolling, and the deformation resistance of the steel slab becomes too high. On the other hand, when the heating temperature of the steel slab before hot rolling exceeds 1350 ° C., the load on the heating device increases, and the amount of scale generated on the surface of the steel slab increases. Therefore, the upper limit of the heating temperature of the steel slab before hot rolling is set to 1350 ° C.
- Hot rolling After the steel slab is heated, hot rolling is performed. In hot rolling, the cumulative rolling reduction at 900 ° C. or higher is set to 90% or higher. This hot rolling condition can improve productivity, reduce the load on the roll, and prevent breakage. If the cumulative rolling reduction at 900 ° C. or higher is less than 90%, the total number density of Nb carbonitride and V carbonitride is less than 0.10 pieces / ⁇ m 2 , and the Widmanstatten ferrite The area ratio is less than 1%. This causes coarsening of ferrite and pearlite. When the reduction rate on the lower temperature side is increased in the range of 900 ° C.
- the “cumulative rolling reduction” is the cumulative rolling amount (sheet thickness before entering the first pass and the last thickness) with respect to the sheet thickness at the start of rolling (ie, immediately before being put into the first pass of the rolling mill). It is the percentage of the difference between the thickness after leaving the pass.
- the “cumulative rolling reduction at 900 ° C. or higher” in the present embodiment is obtained by the following equation.
- r 900 (t 0 ⁇ t) / t 0 ⁇ 100 (Formula 3)
- r 900 is the cumulative rolling reduction at 900 ° C. or higher
- t 0 is the plate thickness at the start of rolling
- t is the rolling pass for starting rolling in a state where the steel slab temperature is less than 900 ° C. It is the plate thickness just before being put in.
- “Increasing the reduction rate on the lower temperature side within the range of 900 ° C. or higher” means a pass at a relatively low temperature among passes that are rolled at 900 ° C. or higher (especially a pass that is rolled at 900 to 1000 ° C.). ) In the pass having a relatively high temperature (particularly, the pass in which rolling is performed at 1000 ° C. or higher).
- the finishing temperature of hot rolling is 850 ° C. or higher. This is because, when hot rolling is performed at a temperature lower than 850 ° C., ferrite transformation has started, and therefore, two-phase rolling is performed. When two-phase rolling is performed, processed ferrite is generated, the toughness of the base material is reduced, the deformation resistance is increased, and the load on the roll is increased.
- the steel sheet pile obtained by hot rolling is cooled.
- the cooling method is not particularly limited. If the steel sheet pile hot-rolled as described above is air-cooled, for example, as in a normal steel sheet pile manufacturing method, a total of 70 area% or more of ferrite and pearlite, and 1 area% or more of Widmannstatten ferrite and A steel sheet pile containing precipitates is obtained.
- the Charpy absorbed energy (impact value) obtained by conducting the Charpy impact test is equal to or greater than the target value, it is determined that the toughness is good.
- the yield strength YP was 460 MPa or more
- the tensile strength TS was 550 MPa or more
- the impact value was 32 J or more, which was a target value of mechanical properties.
- No. Examples 1 to 12 are examples, and all satisfy the materials.
- the metal structures of these examples were mainly composed of ferrite, pearlite, and Widmanstatten ferrite within the range of the metal structure observation described above, and the area ratio of Widmanstatten ferrite was 1% or more. .
- no. Reference numerals 13 to 27 are comparative examples in which the strength and / or Charpy absorbed energy does not reach the target value.
- No. 13 and 26, 15, 17, 19 and 21 have less C, Si, Mn, Nb and V, respectively, and the yield strength does not satisfy the target.
- No. In 14, 16, 18, 20, 22, and 23, C, Si, Mn, Nb, V, and Al are excessive and the toughness is lowered.
- No. 24 is reduced toughness CE N is too high.
- No. 25 since CE N is too low, the impact value is low.
- the alloy is not added excessively, the productivity is improved and the roll is prevented from being broken by rolling at a high temperature, the yield stress is 460 MPa or more, the tensile strength TS is 550 MPa or more, and the impact is reduced. It is possible to provide a high-strength steel sheet pile having a value of 32 J or more and a manufacturing method thereof, and the industrial contribution is extremely remarkable.
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Abstract
Description
本願は、2013年3月28日に、日本に出願された特願2013-070394号に基づき優先権を主張し、その内容をここに援用する。
本発明は、このような問題点に鑑み、高価な合金元素の過剰添加を抑制する省合金化、生産性を阻害することなく製造が可能な易製造性、および降伏強度の460MPa以上への向上という、相反する諸問題を解決することを課題とする。つまり、溶接性及び靱性が要求され、さらに高強度も必要とされる鋼矢板を、コストパフォーマンスおよび生産性を損なうことなく製造し、降伏応力が460MPa以上である鋼矢板及びその製造方法を提供することを課題とする。
CEN=[C]+f(C)×{[Mn]/6+[Si]/24+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5}……(式1)
f(C)=0.75+0.25×tanh{20×([C]-0.12)}……(式2)
ここで、[C]、[Mn]、[Si]、[Ni]、[Cr]、[Mo]、[Nb]、[V]は各元素の含有量を示し、含有されていない元素の含有量は0%とみなす。
CEN=[C]+f(C)×{[Mn]/6+[Si]/24+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5}……(式1)
f(C)=0.75+0.25×tanh{20×([C]-0.12)}……(式2)
ここで、[C]、[Mn]、[Si]、[Ni]、[Cr]、[Mo]、[Nb]、[V]は各元素の含有量を単位質量%で示し、含有されていない元素の含有量は0%とみなす。
Cは、鋼の強度を高めるのに有効な元素である。本実施形態に係る鋼矢板では、強度を確保するために、C量の下限値を0.05%とする。一方、Cを過剰に含有すると、鋼矢板の母材および熱影響部の靱性が低下するので、本実施形態では、C量の上限値を0.18%とする。強度と靱性とのバランスをさらに適正化させるためには、C量の下限値は0.10%が好ましい。
Siは、脱酸元素である。脱酸を十分に行うために、Si量の下限値を0.10%とする。また、Siは強度を向上させる元素でもあり、この効果を得るためにはSi量を0.20%以上とすることが好ましい。一方、Si量が過剰になると、鋼矢板の母材および熱影響部の靱性が低下するので、Si量の上限値を0.50%とする。
Mnは、鋼の焼入れ性を向上させる元素であり、金属組織を細粒化するとともに強度及び靱性を確保するために有用である。本実施形態では、鋼矢板の母材の強度を確保するために、Mn量の下限値を0.50%とする。一方、Mn量が過剰になると、焼入れ性が増大して鋼矢板の溶接部の靱性が低下する。したがって、本実施形態では、Mn量の上限値を1.50%とする。強度と靱性との比率をさらに適正化させるためには、Mn量の上限値を1.30%にすることが好ましい。
Nbは、本実施形態では極めて重要な元素である。NbはCおよび/またはNと結びついてNb炭窒化物を形成する。このNb炭窒化物は析出強化によって鋼矢板の母材及び溶接部の強度を向上させる。この効果を得るために、Nb量を0.040%以上とする。一方、0.050%超のNbが含有された場合、Nb炭窒化物によって鋼矢板の母材の靱性が低下し、且つ焼入れ性の上昇によって鋼矢板の溶接部の靱性を損なう。したがって、Nb量の上限値を0.050%とする。上述の理由により、Nb量は0.040~0.050%とする。Nb量は、好ましくは0.040~0.045%である。
Vは、本実施形態では極めて重要な元素である。VはCおよび/またはNと結びついてV炭窒化物を形成する。このV炭窒化物は、析出強化によって鋼矢板の母材及び溶接部の強度を向上させる。この効果を得るために、V量を0.20%以上とする。一方、0.30%超のVを添加すると、鋼矢板の母材の靱性が低下し、且つ焼入れ性の上昇によって鋼矢板の溶接部の靱性も損なう。したがって、V量の上限値を0.30%とする。したがって、V量は0.20~0.30%とする。V量は、好ましくは0.20~0.23%である。
Alは、脱酸元素であるが、他の脱酸元素であるSiが含有されている場合、必ずしも必要とされない。従って、Al量の下限値は特に限定しない。一方、Al量が過剰になると、粗大なAl酸化物が生じることにより鋼矢板の靱性が低下する。したがって、Al量を0.05%未満に制限する。Al量の上限値は、0.03%が好ましく、0.02%がさらに好ましい。
本実施形態に係る鋼矢板の化学成分の残部は、Feおよび不純物である。不純物とは、鋼材を工業的に製造する際に、鉱石若しくはスクラップ等のような原料、又は製造工程の種々の要因によって混入する成分であって、本実施形態に係る鋼矢板の特性に悪影響を与えない範囲で許容されるものを意味する。本実施形態において、代表的な不純物としてはPおよびSが挙げられる。Pは、鋼矢板を脆化し、さらに鋼矢板の母材及び溶接部の強度を低下させる場合がある有害な成分である。P量が0.040%以下であれば、その含有が許容されるが、P量は少ない方が好ましい。また、Sは、鋼矢板の母材および溶接部の強度および靭性を低下させる有害な成分である。S量が0.040%以下であれば、その含有が許容されるが、S量は少ない方が好ましい。
Cuは、鋼矢板の金属組織中に固溶することにより、鋼矢板の母材及び溶接部の強度を向上させる元素である。その効果を得るためには、Cu量を0.05%以上とすることが好ましい。一方、Cuを過剰に含有させると、CuSの析出および鋼矢板の表面性状の悪化を招くことがあるので、Cu量の上限値を0.40%とすることが好ましい。
Niは、鋼矢板の焼入れ性を高め、鋼矢板の金属組織中に固溶することにより、鋼矢板の母材および溶接部の強度及び靭性を向上させる元素である。その効果を得るためには、Ni量を0.10%以上とすることが好ましい。一方、Niは高価な元素であるので、Ni量の上限値を1.00%にすることが好ましい。より好ましくは、Ni量の上限値は0.50%であり、更に好ましくは、Ni量の上限値は0.30%である。なお、Cuが含有される場合、表面性状の劣化を防止するために、CuとNiとをともに含有させることが好ましい。
Moは、その含有量が微量であっても、鋼矢板の母材および溶接部の強度を向上させる元素である。この効果を得るためには、Mo量を0.10%以上とすることが好ましい。一方、Moは高温時の鋼(すなわち、圧延前の鋼矢板)の強度を高める元素であるので、過剰に含有される場合、圧延前の鋼矢板の変形抵抗が上昇し、このことが圧延時の圧延ロールの割損を引き起こす場合がある。したがって、Mo量の上限値は1.00%が好ましい。より好ましくは、Mo量の上限値を0.50%である。また、Mo量の下限値は0.30%が好ましい。
Crは、鋼矢板の焼入れ性を高め、強度上昇に有効な元素である。この効果を得るためには、Cu量を0.10%以上とすることが好ましい。一方、Crを過剰に含有させる場合、鋼矢板の溶接部及び母材の靱性が低下することがある。したがって、Cr量の上限値を1.00%にすることが好ましい。より好ましくは、Cr量の上限値を0.50%とし、更に好ましくは、Cr量の上限値を0.30%とする。
本実施形態に係る鋼矢板では、溶接部の靱性を低下させることなく、母材の母材および溶接部の強度を確保するために、下記の公知の(式1)及び(式2)で求められる炭素当量CENを0.260~0.500とする。炭素当量CENは焼入れ性の指標である。460MPa以上の降伏応力を確保するために、炭素当量CENの下限値を0.260にしなければならない。一方、鋼矢板の母材及び溶接部の靱性を確保するために、炭素当量CENの上限値を0.500にすることが必要である。
CEN=[C]+f(C)×{[Mn]/6+[Si]/24+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5}……(式1)
f(C)=0.75+0.25×tanh{20×([C]-0.12)}……(式2)
ここで、[C]、[Mn]、[Si]、[Ni]、[Cr]、[Mo]、[Nb]、[V]は各元素の含有量である。
(析出物:Nb炭窒化物およびV炭窒化物のうちの1種または2種)
本実施形態に係る鋼矢板の金属組織は、フェライトと、パーライトと、ウイッドマンステッテンフェライトとを含み、更に、析出物を含むことが好ましい。析出物とは、V(C,N)、およびNb(C,N)などの炭窒化物である。微細な析出物による析出強化と、微細な析出物のピン止め効果による金属組織の微細化により、鋼矢板の母材および溶接部の靱性を確保し、かつ、鋼矢板の母材および溶接部の強度を向上させている。V(C,N)をV炭窒化物と称し、Nb(C,N)をNb炭窒化物と称する。
ウイッドマンステッテンフェライトとは、炭素原子の拡散律速で成長する金属組織であり、且つ、マルテンサイト変態と同様にFe原子が剪断変態機構で変態成長した、板状のフェライト組織であり、上述した通常のフェライトとは区別される。
パーライトに含まれるフェライト、およびアシキュラーフェライトも板状の形状を有するが、ウイッドマンステッテンフェライトと、パーライトに含まれるフェライトおよびアシキュラーフェライトとは以下の点において区別される。パーライトに含まれるフェライトは、上述のようにセメンタイトとともに層状組織を構成している。これに対して、ウイッドマンステッテンフェライトは、セメンタイトを伴わずに積層構造を構成している。アシキュラーフェライトは、非金属介在物などを中心として放射状に成長する。これに対して、ウイッドマンステッテンフェライトは、オーステナイト粒界または既に変態したフェライトから板状に成長する。
本実施形態では、長さLと幅Wとのアスペクト比L/Wが4以上であり、層状のセメンタイトを伴わず、且つオーステナイト粒界または既に変態したフェライトから成長した板状のフェライトを、ウイッドマンステッテンフェライトと定義する。この定義により、鋼矢板の二次元断面を200~500倍の光学顕微鏡によって組織観察した場合に、ウイッドマンステッテンフェライトと、通常のフェライト、パーライトに含まれるフェライト、およびアシキュラーフェライトとを区別することができる。
(ウイッドマンステッテンフェライトの面積率:1%以上)
本実施形態に係る鋼矢板の、フェライトおよびパーライトの合計面積率は70%以上である。これにより、鋼矢板の母材および溶接部の強度及び靱性を十分に向上させることができる。フェライト、パーライト、およびウイッドマンステッテンフェライト以外の金属組織、例えばベイナイト等が残部組織として生成する場合がある。このような残部組織の含有は、上述のフェライトおよびパーライトの合計面積率が保たれている限り許容される。フェライトと、パーライトと、ウイッドマンステッテンフェライトとの含有比率を規定する必要はない。しかし、ウイッドマンステッテンフェライトの含有量は、面積率で1%以上である。上述した、ウイッドマンステッテンフェライトが奏する効果は、その面積率が1%未満である場合には得られない。
フェライトおよびパーライトの合計面積率、およびウイッドマンステッテンフェライトの面積率は、国際溶接学会(International Institute of Welding)の方式に従って計測された。すなわち、光学顕微鏡組織写真上に縦横各々10個すなわち計100個の交叉点を有するグリッドを載せ、各交叉点の組織を目視にて同定(point-counting)した。これを5視野にて繰り返し、平均することにより、各構成組織の分率を定量化した。
Nb炭窒化物およびV炭窒化物のうち1種または2種である析出物の単位面積当たりの個数の合計が0.10個/μm2よりも少ない場合、十分な強度が得られない。一方、Nb炭窒化物およびV炭窒化物の単位面積当たりの個数の合計が0.30個/μm2よりも多い場合、鋼矢板の靭性が低下する。したがって、Nb炭窒化物およびV炭窒化物の合計個数密度を0.10~0.30個/μm2とする。好ましいNb炭窒化物およびV炭窒化物の単位面積当たりの合計個数は、0.11~0.25個/μm2である。Nb炭窒化物およびV炭窒化物の単位面積当たりの合計個数は、抽出レプリカを試料とし、この試料を透過型電子顕微鏡によって分析することにより測定することができる。
なお、Nb炭窒化物およびV炭窒化物の大きさを規定する必要はない。しかし、長径が10nmよりも小さいNb炭窒化物およびV炭窒化物は、鋼矢板の特性に実質的な影響を及ぼすことがないと考えられ、また、上述の抽出レプリカ法で観察することができない。従って、Nb炭窒化物およびV炭窒化物の長径の実質的な下限値は10nmとなる。また、上述した個数密度でNb炭窒化物およびV炭窒化物を含有させた場合、300nmを超える長径を有するNb炭窒化物およびV炭窒化物が生成することはない。本発明者らが本実施形態に係る種々の鋼矢板の金属組織を観察した結果、300nmよりも大きい長径を有するNb炭窒化物およびV炭窒化物は確認されなかった。従って、Nb炭窒化物およびV炭窒化物の長径の実質的な上限値は300nmとなる。
鋼矢板のフェライトおよびパーライトの平均結晶粒径(以下、「平均結晶粒径」または「結晶粒径」と略する場合がある)が80μmを超えると、鋼矢板の母材および溶接部の靱性及び強度が低下することがある。一方、鋼矢板の平均結晶粒径が10μm未満になると、鋼矢板の伸びの低下が大きくなることがある。伸びが低下した場合、靱性が低下する。したがって、鋼矢板の平均結晶粒径は、10~80μmが好ましい。
本実施形態に係る鋼矢板の金属組織の平均結晶粒径は、JIS G 0551に準拠して、光学顕微鏡によって観察することにより求められる。なお、パーライト組織の粒径測定は、パーライトコロニー(JIS G 0551に記載の「パーライトの島」)に対して、上述のフェライト粒の粒径測定方法を適用することにより求められる。「フェライトおよびパーライトの平均結晶粒径」とは、フェライトおよびパーライト両者の平均の結晶粒径を意味する。もしフェライトまたはパーライト各々の平均結晶粒径が10μm未満または80μm超であっても、フェライトおよびパーライト両者の平均の結晶粒径が10μm~80μmであれば、上述の規定が満たされていると判断される。
ウイッドマンステッテンフェライトは、上述した金属組織の平均結晶粒径を計測する際には無視される。また、ウイッドマンステッテンフェライトの長径は通常5~30μm程度となり、この範囲内における長径の変化は、本実施形態に係る鋼矢板の特性に影響しない。従って、本実施形態においてウイッドマンステッテンフェライトの大きさを規定する必要はない。
(引張強さ:550~740MPa)
本実施形態の鋼矢板の母材の降伏強度は、高強度化による板厚の低減の効果を得るために、460MPa以上とする。また、本実施形態の鋼矢板の母材の引張強さは、高強度化による板厚の低減の効果を得るために、550MPa以上とする。鋼矢板の母材の降伏強度および引張強さを上述の値以上とし、溶接コスト、運送や施工のコストを抑制すれば、経済性の面で有利になる。一方、鋼矢板の母材および溶接部の靭性の確保や、合金量の低減による溶接性の向上を考慮すると、降伏強度の上限値を550MPaにし、且つ引張強さの上限値を740MPaにすることが望ましい。このような降伏強度および引張強さは、上述した所定の量の合金元素を鋼矢板が含有し、かつ所定の金属組織となった場合に得られる。
熱間圧延前の鋼片の加熱温度は、1100℃以上とする。加熱温度が低すぎる場合、熱間圧延中に鋼片の温度が低下し、鋼片の変形抵抗が高くなりすぎるからである。一方、熱間圧延前の鋼片の加熱温度が1350℃を超える場合、加熱装置の負荷が増大し、さらに、鋼片の表面に生成するスケールの量が増加する。したがって、熱間圧延前の鋼片の加熱温度の上限値を1350℃とする。
鋼片を加熱した後、熱間圧延を行う。熱間圧延では、900℃以上での累積圧下率を90%以上とする。この熱間圧延条件によって、生産性を向上させ、ロールへの負荷を低減して割損を防止することができる。もし、900℃以上での累積圧下率が90%未満であった場合、Nb炭窒化物およびV炭窒化物の合計個数密度が0.10個/μm2未満となり、且つウイッドマンステッテンフェライトの面積率が1%未満となる。このことは、フェライトおよびパーライトの粗大化を生じさせる。900℃以上の範囲で、より低温側での圧下率を増加させると、フェライトおよびパーライトの組織が微細化され、鋼矢板の母材及び溶接部の強度及び靭性をさらに高めることができる。
一般的に「累積圧下率」とは、圧延開始時(即ち、圧延装置の最初のパスに投入される直前)の板厚に対する累積圧下量(最初のパスに入る前の板厚と、最後のパスを出た後の板厚との差)の百分率である。一方、本実施形態における「900℃以上での累積圧下率」とは、以下の式によって得られる。
r900=(t0-t)/t0×100 (式3)
上記式3において、r900は900℃以上での累積圧下率であり、t0は圧延開始時の板厚であり、tは鋼片温度が900℃未満である状態で圧延を開始する圧延パスに投入される直前の板厚である。
「900℃以上の範囲内でより低温側での圧下率を増加させる」とは、900℃以上で圧延を行うパスのうち、比較的温度が低いパス(特に900~1000℃で圧延を行うパス)における圧下率を、比較的温度が高いパス(特に1000℃以上で圧延を行うパス)における圧下率よりも大きくすることを意味する。
熱間圧延の仕上温度は、850℃以上とする。これは、850℃未満で熱間圧延を行うと、フェライト変態が開始しているので、2相域圧延となるためである。2相域圧延を行った場合、加工フェライトが生じて母材の靱性が低下し、変形抵抗が大きくなり、ロールへの負荷が高くなる。
Claims (3)
- 化学成分が、質量%で、
C:0.05~0.18%、
Si:0.10~0.50%、
Mn:0.50~1.50%、
Nb:0.040~0.050%、
V:0.20~0.30%、
Cu:0~0.40%、
Ni:0~1.00%、
Mo:0~1.00%、および
Cr:0~1.00%
を含有し、
Al:0.05%未満
に制限し、残部がFe及び不純物であり、
下記式1及び式2によって求められる炭素当量CENが0.260~0.500であり、
金属組織がフェライトと、パーライトと、ウイッドマンステッテンフェライトと、析出物とを含み、
前記析出物がNb炭窒化物およびV炭窒化物のうちの1種または2種であり、
前記析出物の個数密度が0.10~0.30個/μm2であり、
前記フェライトおよび前記パーライトの合計面積率が70%以上であり、
前記ウイッドマンステッテンフェライトの面積率が1%以上であり、
前記フェライトおよび前記パーライトの平均結晶粒径が10~80μmであり、
降伏強さが460~550MPa且つ引張強さが550~740MPaである
ことを特徴とする鋼矢板。
CEN=[C]+f(C)×{[Mn]/6+[Si]/24+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5}……(式1)
f(C)=0.75+0.25×tanh{20×([C]-0.12)}……(式2)
ここで、[C]、[Mn]、[Si]、[Ni]、[Cr]、[Mo]、[Nb]、[V]は各元素の含有量を単位質量%で示し、含有されていない元素の含有量は0%とみなす。 - 前記化学成分が、質量%で、
Cu:0.05~0.40%、
Ni:0.10~1.00%、
Mo:0.10~1.00%、
Cr:0.10~1.00%、
のうち1種又は2種以上を含有することを特徴とする請求項1に記載の鋼矢板。 - 請求項1又は2に記載の前記化学成分からなる鋼片を、1100~1350℃に加熱する工程と、
900℃以上での累積圧下率が90%以上であり、且つ仕上温度が850℃以上である条件で前記鋼片を熱間圧延して鋼矢板を得る工程と、
前記鋼矢板を冷却する工程と、
を備える鋼矢板の製造方法。
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