WO2015025486A1 - 厚鋼板およびその製造方法 - Google Patents
厚鋼板およびその製造方法 Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—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
- 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
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- 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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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- 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
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- 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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- 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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- 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
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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
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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
Definitions
- the present invention relates to a steel plate that is used in various steel structures such as ships, marine structures, architecture, and steel pipe fields, and that is particularly suitable as a manufacturing method thereof to which large heat input welding is applied.
- steel structures used in the fields of ships, offshore structures, architecture, steel pipes, etc. are finished into structures having a desired shape by welding. Therefore, from the viewpoint of ensuring safety, these structures are based on the base material characteristics that are the basis of the steel materials used, that is, in addition to ensuring strength, toughness, and elongation, as well as characteristics of welds, mainly joint strength, There is a demand for excellent joint toughness.
- the strength and the plate thickness are not so excessive, the strength class is a tensile strength of about 550 to 750 N / mm grade 2 , and the plate thickness is 50 mm or less.
- the strength class is a tensile strength of about 550 to 750 N / mm grade 2
- the plate thickness is 50 mm or less.
- Application of steel sheets is expected to expand.
- thick steel plates with the above strength classes and thicknesses are manufactured by the so-called TMCP technology that combines controlled rolling and accelerated cooling for the purpose of improving the properties of steel plates, reducing alloy elements, and omitting heat treatment.
- TMCP technology can secure a high cooling rate, there is an advantage that the strength of the base material can be secured with a relatively low component.
- the surface of the steel plate is rapidly cooled compared to the inside of the steel plate, the hardness in the vicinity of the steel plate surface is higher than that inside the steel plate, and the hardness distribution in the thickness direction may vary.
- accelerated cooling may not be uniform over the entire surface of the steel sheet, and there is a concern about the influence on the material uniformity within the steel sheet, specifically the elongation characteristics of the steel sheet.
- Patent Document 1 discloses a relatively low cooling rate of 3 to 12 ° C./s for accelerated cooling.
- a method for suppressing an increase in surface hardness with respect to the center portion of the plate thickness by controlling the speed is disclosed.
- Patent Document 2 discloses that the structure of the steel sheet is made into a two-phase structure of ferrite and bainite by waiting in the temperature range where ferrite precipitates during the cooling process, and the difference in hardness between the surface layer and the center of the sheet thickness is indicated.
- a method for producing a reduced steel sheet with a small material difference in the thickness direction is disclosed.
- Patent Documents 3 and 4 reduce the uneven cooling due to the scale characteristics by performing descaling immediately before cooling.
- a method for improving the steel plate shape is disclosed. All of these techniques are interpreted as techniques capable of ensuring material uniformity and simultaneously achieving high toughness by refining crystal grains.
- Patent Document 6 there is disclosed a method for achieving both high strength of steel sheets and joint characteristics, particularly HAZ toughness, by optimizing the amount of Nb added in the steel.
- the effect of the high heat input welding countermeasure techniques represented by Patent Documents 5 and 6 on the homogeneity of the steel sheet material has not been verified.
- the present invention was developed in view of the above-described present situation, and has various characteristics required for a high heat input welded joint, and reduces variations in hardness in the plate thickness direction and the plate width direction of the steel plate. Therefore, it is to provide a thick steel plate having a thickness of 50 mm or less with improved material properties, particularly its total thickness elongation characteristic, together with its advantageous manufacturing method.
- the present invention in addition to optimizing the component design as a measure against large heat input welding, to examine the manufacturing conditions for improving the material homogeneity in the steel sheet, and to satisfy the predetermined homogeneity It was obtained after many experiments and examinations regarding the material threshold value to be included in. That is, the present invention 1.
- C 0.030 to 0.080%, Si: 0.01 to 0.10%, Mn: 1.20 to 2.40%, P: 0.008% or less, S: 0.0005 -0.0040%, Al: 0.005-0.080%, Nb: 0.003-0.040%, Ti: 0.003-0.040%, N: 0.0030-0.0100%, B: Contains 0.0003 to 0.0030%, with the balance being a component composition composed of Fe and inevitable impurities, and fluctuations in Vickers hardness (HV) in both the plate thickness direction and the plate width direction Width: A thick steel plate excellent in high heat input welding characteristics and material homogeneity, wherein ⁇ HV is 30 or less. 2.
- HV Vickers hardness
- a thick steel plate excellent in both material homogeneity and high heat input weld joint characteristics and a method for producing the same can be obtained, which is extremely useful industrially.
- C 0.030 to 0.080%
- C is an element that increases the strength of the steel material, and 0.030% or more of addition is necessary in order to ensure the strength necessary for structural steel.
- the upper limit is made 0.080%.
- it is in the range of 0.040 to 0.070%.
- Si 0.01 to 0.10% Si is an element added as a deoxidizer when melting steel, and it is necessary to add 0.01% or more. However, if it exceeds 0.10%, island martensite is generated in the high heat input welding HAZ, and the toughness is liable to be lowered. Therefore, Si is set in the range of 0.01 to 0.10%.
- Mn 1.20 to 2.40%
- Mn is an element that enhances the strength of the steel plate base material, and it needs to be added in an amount of 1.20% or more in order to ensure the strength necessary for structural steel. Moreover, since it is cheaper than other alloy components, positive addition is effective. However, if it exceeds 2.40%, the hardenability becomes excessive, the base material toughness is lowered, and the weldability is impaired. Therefore, the Mn content is 1.20 to 2.40%. Preferably, it is in the range of 1.50% to 2.20%.
- P 0.008% or less
- the content is made 0.008% or less. It is preferable to reduce as much as possible in consideration of economics at the time of melting the material.
- S 0.0005 to 0.0040%
- S is one of the elements contained in steel as an impurity. Unlike P, when it exists as sulfides such as MnS, CaS, and REM-S, it becomes a nucleus of ferrite formation and exhibits an effect of improving the high heat input HAZ toughness. This effect is effective when 0.0005% or more is added. On the other hand, excessive addition leads to a large amount of sulfide formation, and lowers the base material toughness. Therefore, the S content is in the range of 0.0005 to 0.0040%.
- Al 0.005 to 0.080%
- Al is an element added for deoxidation of steel, and it is necessary to contain 0.005% or more. On the other hand, if added over 0.080%, the amount of inclusions becomes excessive and the toughness of the base material is lowered. Therefore, Al is made 0.005 to 0.080% of range. Preferably, the content is 0.010 to 0.060%.
- Nb 0.003 to 0.040%
- Nb has an effect of expanding the non-recrystallization temperature region by addition, and is an element effective for ensuring the strength toughness of the steel plate base material. However, if the addition is less than 0.003%, the above effect is small. On the other hand, if the addition exceeds 0.040%, island martensite is generated in the high heat input welding HAZ, and the toughness is lowered. Therefore, Nb is set in the range of 0.003 to 0.040%. Preferably, it is 0.005 to 0.025% of range.
- Ti 0.003-0.040%
- Ti precipitates as TiN during solidification, and is an extremely useful element for increasing the toughness of high heat input weld HAZ, particularly suppressing the coarsening of austenite grains in the weld heat affected zone and becoming a transformation nucleus of ferrite. is there. In order to obtain this effect, 0.003% or more must be added. On the other hand, if added over 0.040%, the precipitated TiN becomes coarse, and the above effect is hardly obtained. Therefore, Ti is set to a range of 0.003 to 0.040%. Preferably, it is 0.005 to 0.025% of range.
- N 0.0030 to 0.0100%
- N is an element necessary for the formation of TiN described above and the formation of B nitride described later, and is one of the most important elements in the present invention.
- it is necessary to contain 0.0030% or more.
- N is set in the range of 0.0030 to 0.0100%.
- it is in the range of 0.0040 to 0.0070%.
- B 0.0003 to 0.0030%
- B When B exists in a solid solution state, it is unevenly distributed at grain boundaries to ensure hardenability and contribute to ensuring the strength of the base material, and when it exists as B nitride, it acts as a ferrite nucleus and has a large heat input. Contributes to both the effects of increasing welded HAZ toughness. For this reason, B is one of the most important elements in the present invention. If the content is less than 0.0003%, the former effect cannot be obtained. If the content exceeds 0.0030%, a large amount of solid solution B exceeding B nitride exists, and high heat input welding HAZ. Reduce toughness. Therefore, B is in the range of 0.0003 to 0.0030%.
- the basic component composition of the present invention is as described above, with the balance being Fe and inevitable impurities. Furthermore, when improving a desired characteristic, 1 type (s) or 2 or more types of Cu, Ni, Cr, Mo, and V can be added as a selection element.
- Cu 1.00% or less
- Cu is an element that can be added to increase the strength. However, if it is added in excess of 1.00%, the properties of the surface of the steel sheet base material are deteriorated due to hot brittleness. For this reason, when adding, it is preferable to make the quantity into the range of 1.00% or less.
- Ni 1.00% or less
- Ni is an element capable of improving the toughness while increasing the strength of the base material. If added over 1.00%, the effect is saturated and economically disadvantageous. For this reason, when adding, it is preferable to make the quantity into the range of 1.00% or less.
- Cr 1.00% or less Cr is an effective element for increasing the strength. However, if added over 1.00%, the base material toughness is degraded. For this reason, when adding, it is preferable to make the quantity into the range of 1.00% or less.
- Mo 0.50% or less Mo is an element effective for increasing the strength of the base material. However, if added over 0.50%, the toughness is remarkably deteriorated and the economy is impaired. For this reason, when adding, it is preferable to make the quantity into the range of 0.50% or less.
- V 0.10% or less V is an element effective for increasing the strength of the base material. However, if added over 0.10%, the toughness is remarkably deteriorated. For this reason, when adding, it is preferable to make the quantity into the range of 0.10% or less.
- one or more of Ca, Zr, and REM can be added as selective elements to the above component composition.
- Ca, Zr, and REM are effective in fixing S in steel and improving the toughness of the steel sheet.
- Ca, which is a strong sulfide-forming element is 0.0005% or more, and with respect to Zr and REM, effects can be obtained by adding 0.001% or more.
- the amount of each of Ca, Zr, and REM exceeds 0.0050%, 0.020%, and 0.020%, the amount of inclusions in the steel increases and the toughness is deteriorated. Accordingly, when these elements are added, it is preferable that Ca is 0.0005 to 0.0050%, Zr is 0.001 to 0.020%, and REM is 0.001 to 0.020%.
- HV Vickers hardness
- the fluctuation range (variation) of the hardness is set to a range of 30 or less, preferably 20 or less in terms of Vickers hardness.
- a steel plate is a steel plate excellent in material homogeneity in the steel plate. The hardness test method will be described in detail in Examples.
- Steels such as slabs for the production of steel sheets are produced by melting steels having the above-described composition using conventional methods such as converters or electric furnaces, and using conventional processes such as continuous casting or ingot casting. It is preferable to use a raw material.
- the steel material temperature in the present invention is the average temperature of the steel material surface and the central part (1/2 part of the plate thickness).
- Heating temperature 1000-1300 ° C
- a steel material such as a slab after casting is charged into a heating furnace after being cooled to room temperature or in a high temperature state, and the steel material temperature is set to 1000 ° C. or higher.
- the lower limit of the heating temperature of the steel material was set to 1000 ° C. from the viewpoint of mainly dissolving Nb carbonitride and ensuring sufficient solid solution Nb.
- the upper limit is set to 1300 ° C.
- a desirable steel material temperature is 1000 to 1250 ° C., more desirably 1050 to 1200 ° C.
- the steel slab that is rolled and heated with a cumulative reduction ratio of 40% or more in the non-recrystallization temperature range is subjected to controlled rolling in the non-recrystallization temperature range after hot rolling in the recrystallization temperature range.
- Rolling in the recrystallization temperature region is preferably performed in order to refine the austenite grains at the time of heating, and it is desirable to perform one pass or more, preferably 20% or more of the cumulative rolling reduction.
- the lower limit of the rolling rolling reduction ratio in the non-recrystallization temperature region is defined as 40%. A higher rolling reduction is preferable. However, the upper limit is about 80% industrially.
- the lower limit temperature of the recrystallization temperature range is generally in the range of 800 to 950 ° C. due to the influence of the crystal grain size, processing history, strain, etc. in addition to the steel composition. By conducting a preliminary test and investigating in advance, the lower limit temperature can be estimated.
- the rolling end temperature is preferably not less than the Ar3 transformation point from the viewpoint of the uniformity of the structure.
- Descaling is performed by causing a high collision pressure jet stream to collide with the steel plate surface immediately before accelerated cooling.
- a high collision pressure jet stream to collide with the steel plate surface immediately before accelerated cooling.
- it is necessary to reduce the variation in hardness within the steel plate, and in particular, while maintaining the strength inside the steel plate, the variation in hardness of the surface layer is reduced. It is important to suppress.
- unevenness in the thickness of the scale may occur in the width direction due to descaling or the like before and during rolling. Further, when the scale is thick, the scale may be partially peeled off. If the scale thickness varies during cooling after rolling, the cooling rate of the steel sheet surface changes according to the thickness, and the hardness of the steel sheet surface also changes according to the cooling rate. In order to increase the strength of the steel sheet, it is effective to increase the cooling rate during accelerated cooling. However, when cooling at a high cooling rate, the influence of the scale thickness on the surface hardness becomes prominent. Therefore, if the scale thickness is uneven, the variation in hardness increases and the material uniformity in the steel sheet deteriorates. .
- the descaling by the jet flow of high collision pressure is performed immediately before the accelerated cooling, and the thickness of the scale is uniformly reduced to 15 ⁇ m or less without causing a large difference in the cooling rate. That is, when the scale thickness of the steel sheet after accelerated cooling is 15 ⁇ m or less, the hardness variation in the plate thickness direction is ⁇ HV30 or less, and the hardness variation in the plate width direction is also ⁇ HV30 or less.
- the scale thickness before accelerated cooling can be estimated by the scale thickness after accelerated cooling, and the desired effect can be obtained by performing descaling immediately before cooling so that the scale thickness of the steel sheet after cooling is 15 ⁇ m or less. It was elucidated that it could be obtained.
- descaling by the jet flow of high impact pressure immediately before cooling both strength at high cooling speed and material uniformity in the steel sheet can be achieved.
- Descaling pressure impact pressure of jet flow on steel plate surface: 1 MPa or more
- the impact pressure of jet flow on the steel plate surface immediately before accelerated cooling is such that the scale thickness of the steel plate after cooling is 15 ⁇ m or less.
- Descaling is performed under conditions of 1 MPa or more. If the collision pressure of the jet flow on the surface of the steel sheet is less than 1 MPa, the descaling may be insufficient and unevenness in scale may occur, resulting in variations in surface hardness. Therefore, the collision pressure of the jet flow is set to 1 MPa or more. Descaling is performed using high pressure water. In addition, if the collision pressure of the jet flow on the steel plate surface is 1 MPa or more, there is no problem even if another jet flow is used. More preferably, it is 2 MPa or more.
- Average cooling rate of steel plate 10 ° C./s or more Accelerated cooling after descaling is carried out to ensure the strength of the steel plate. In accelerated cooling, it is necessary to select conditions that simultaneously ensure the material homogeneity of the steel sheet surface layer. When the average cooling rate of the steel sheet is less than 10 ° C./s, the cooling of the surface layer region becomes non-uniform even on the descaled surface layer, and the variation in hardness with the inside of the steel sheet becomes large. For this reason, an average cooling rate is prescribed
- the cooling start temperature is ideally not less than the Ar3 transformation point from the viewpoint of the uniformity of the obtained metal structure.
- the temperature may drop while being transported to the accelerated cooling facility through descaling after completion of rolling, and the cooling start temperature may be lower than the Ar3 transformation point.
- the accelerated cooling start temperature is preferably within the range of the rolling end temperature to (rolling end temperature-30 ° C.).
- Cooling stop temperature 200 to 600 ° C at the average temperature of the steel plate
- cooling is performed to 200 to 600 ° C., which is a temperature range of bainite transformation, and the inside of the steel sheet is transformed into a microstructure having a predetermined strength (bainite transformation in the present invention).
- the cooling stop temperature exceeds 600 ° C., the bainite transformation is incomplete and sufficient strength cannot be obtained.
- the cooling stop temperature is less than 200 ° C., part of martensite or island martensite (MA) is generated particularly in the surface layer portion, and material uniformity in the steel sheet cannot be obtained, resulting in a reduction in the elongation characteristics of the entire thickness. Therefore, the cooling stop temperature for accelerated cooling is set to 200 to 600 ° C.
- Ac1 751-26.6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B
- TS total thickness elongation
- the Vickers hardness was measured according to JIS Z 2244 for the cross section perpendicular to the rolling direction.
- the hardness distribution in the plate thickness direction and the hardness distribution in the plate width direction were determined.
- the hardness of the entire thickness was measured at a pitch of 1 mm
- the hardness of the full width was measured at a pitch of 20 mm.
- the hardness in the plate width direction was measured at a surface layer 1 mm position (a position 1 mm inside from the surface layer), a t / 4 position (plate thickness 1/4 position), and a t / 2 position (plate thickness center). Since all the steel sheets showed the largest variation in hardness at the surface layer of 1 mm, the variation in hardness in the plate width direction was evaluated at the surface layer of 1 mm. In addition, the test load at the time of hardness measurement was made constant at 10 kgf (98 N).
- a test piece having a width of 80 mm, a length of 80 mm and a thickness of 15 mm is taken from the thick steel plate, heated to 1450 ° C., and then cooled to 800 to 500 ° C. in 250 seconds.
- three 2 mm V notch Charpy test pieces were collected and subjected to the Charpy impact test in the same manner as described above.
- the impact test temperature was ⁇ 40 ° C.
- the toughness target was 50 J or more in terms of the average absorbed energy at ⁇ 40 ° C. (hereinafter referred to as vE-40 ° C.).
- Table 4 shows the steel plate base material characteristics and the high heat input welding HAZ toughness evaluation results.
- HV Vickers hardness
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Abstract
Description
1.質量%で、C:0.030~0.080%、Si:0.01~0.10%、Mn:1.20~2.40%、P:0.008%以下、S:0.0005~0.0040%、Al:0.005~0.080%、Nb:0.003~0.040%、Ti:0.003~0.040%、N:0.0030~0.0100%、B:0.0003~0.0030%を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、かつ、板厚方向と板幅方向のそれぞれでのビッカース硬さ(HV)の変動幅:ΔHVが30以下であることを特徴とする大入熱溶接特性と材質均質性に優れた厚鋼板。
2.更に、質量%で、Cu:1.00%以下、Ni:1.00%以下、Cr:1.00%以下、Mo:0.50%以下、V:0.10%以下の1種または2種以上を含有することを特徴とする1に記載の大入熱溶接特性と材質均質性に優れた厚鋼板。
3.更に、質量%で、Ca:0.0005~0.0050%、Zr:0.001~0.020%、REM:0.001~0.020%の1種または2種以上を含有することを特徴とする1または2に記載の大入熱溶接特性と材質均質性に優れた厚鋼板。
4.1乃至3のいずれか一つに記載の成分組成の鋼素材を1000~1300℃に加熱後、熱間圧延し、鋼板表面での噴射流の衝突圧が1MPa以上の条件で噴射流を鋼板表面に衝突させることによりデスケーリングを行い、その後、直ちに、鋼板の平均冷却速度:10℃/s以上、鋼板の平均冷却停止温度:200~600℃で加速冷却を行うことを特徴とする、板厚方向と板幅方向のそれぞれでのビッカース硬さ(HV)の変動幅:ΔHVが30以下である大入熱溶接特性と材質均質性に優れた厚鋼板の製造方法。
5.加速冷却停止後、Ac1変態点以下で焼き戻すことを特徴とする4記載の厚鋼板の製造方法。
C:0.030~0.080%
Cは、鋼材の強度を高める元素であり、構造用鋼として必要な強度を確保するためには、0.030%以上の添加が必要である。一方、0.080%を超えると、大入熱溶接HAZ中に島状マルテンサイトが生成し易くなるため、上限は0.080%とする。好ましくは、0.040~0.070%の範囲である。
Siは、鋼を溶製する際の脱酸剤として添加される元素であり、0.01%以上の添加が必要である。しかし、0.10%を超えると、大入熱溶接HAZ中に島状マルテンサイトが生成し、靱性の低下を招きやすくなる。よって、Siは0.01~0.10%の範囲とする。
MnはCと同様に、鋼板母材の強度を高める元素であり、構造用鋼として必要な強度を確保するために、1.20%以上の添加が必要である。また他の合金成分に比較して安価であることから、積極的な添加が有効である。しかしながら、2.40%を超えると焼入性が過剰となり、母材靱性が低下するとともに溶接性を損なう。従ってMn量は1.20~2.40%とする。好ましくは1.50%~2.20%の範囲である。
Pは不純物として鋼中に含有される元素の一つである。しかしながら、鋼板母材および、大入熱HAZ部の靱性を低下させるため、0.008%以下とする。素材溶製時の経済性を考慮した上で可能な範囲で低減することが好ましい。
SはPと同様に、不純物として鋼中に含有される元素の一つである。Pと異なり、MnSやCaS、REM-Sなどの硫化物として存在した場合にフェライトの生成核となり、大入熱HAZ部靱性を向上させる効果を現す。この効果は0.0005%以上の添加で有効である。一方で過剰の添加は多量の硫化物生成を招き、母材靱性を低下させるようになる。従って、S量は0.0005~0.0040%の範囲とする。
Alは、鋼の脱酸のために添加される元素であり、0.005%以上含有させる必要がある。一方で、0.080%を超えて添加すると、介在物量が過剰となり、母材の靱性を低下させる。従って、Alは0.005~0.080%の範囲とする。好ましくは0.010~0.060%とする。
Nbは、添加により未再結晶温度域を拡大させる効果を有し、鋼板母材の強度靱性を確保するのに有効な元素である。しかし、0.003%未満の添加では上記効果が小さく、一方で0.040%を超えて添加すると、大入熱溶接HAZに島状マルテンサイトを生成させ、靱性を低下させる。このため、Nbは0.003~0.040%の範囲とする。好ましくは、0.005~0.025%の範囲である。
Tiは、凝固時にTiNとして析出し、特に溶接熱影響部のオーステナイト粒の粗大化を抑制し、且つ、フェライトの変態核となるなど、大入熱溶接HAZの高靭化に極めて有用な元素である。この効果を得るためには、0.003%以上の添加が必要である。一方で、0.040%を超えて添加すると、析出したTiNが粗大化し、上記効果が得られにくくなる。よって、Tiは、0.003~0.040%の範囲とする。好ましくは、0.005~0.025%の範囲である。
Nは、上述したTiNの生成、また、後述するB窒化物の形成に必要な元素であり、本発明において最も重要な元素の一つである。これらの窒化物を大入熱溶接HAZ部において生成させ、靱性向上に有効に寄与させるためには、0.0030%以上含有させる必要がある。一方で、0.0100%を超えて添加すると、溶接入熱条件によってはTiNが溶解する領域における固溶N量が増加し、溶接HAZ部の靱性を低下させる場合がある。従って、Nは、0.0030~0.0100%の範囲とする。好ましくは、0.0040~0.0070%の範囲である。
Bは固溶状態で存在する場合は粒界に偏在して焼入性を確保し、母材強度の確保に寄与すると共に、B窒化物として存在する場合はフェライト核として作用し、大入熱溶接HAZ靱性を高める効果の両方に寄与する。このため、Bは本発明で最も重要な元素の一つである。含有量が0.0003%未満では前者の効果が得られず、また、0.0030%を超えて添加するとB窒化物を上回る固溶Bが多量に存在することになり、大入熱溶接HAZ靱性を低下させる。従ってBは0.0003~0.0030%の範囲とする。
Cuは強度を増加させるために添加することができる元素である。しかしながら、1.00%を超えて添加すると、熱間脆性により鋼板母材表面の性状を劣化させる。このため、添加する場合、その量は1.00%以下の範囲とすることが好ましい。
Niは母材の強度を増加させつつ靭性も向上させることが可能な元素である。1.00%を超えて添加した場合、効果が飽和するとともに経済的に不利となる。このため、添加する場合、その量は1.00%以下の範囲とすることが好ましい。
Crは強度を増加させるために有効な元素である。しかしながら、1.00%を超えて添加すると、母材靭性を劣化させる。このため、添加する場合、その量は1.00%以下の範囲とすることが好ましい。
Moは母材強度を増加するのに有効な元素である。しかしながら、0.50%を超えて添加すると、著しく靭性を劣化させるとともに経済性を損なう。このため、添加する場合、その量は0.50%以下の範囲とすることが好ましい。
Vは母材強度を増加するのに有効な元素である。しかしながら、0.10%を超えて添加すると、著しく靭性を劣化させる。このため、添加する場合、その量は0.10%以下の範囲とすることが好ましい。
Ca、Zr、REMは鋼中のSを固定して鋼板の靭性を向上させる効果がある。強い硫化物形成元素であるCaは0.0005%以上で、また、ZrおよびREMに関しては0.001%以上の添加でそれぞれ効果が得られる。しかしながら、Ca、Zr、REMのそれぞれの量が0.0050%、0.020%、0.020%を超えて添加すると鋼中の介在物量が増加し靭性をかえって劣化させる。従って、これらの元素を添加する場合、Caは0.0005~0.0050%、Zrは0.001~0.020%、REMは0.001~0.020%の範囲とすることが好ましい。
本規定は、本発明内でも最も重要な要件の一つで有り、材質の均質性、特に母鋼板全厚の伸び特性に多大な影響を及ぼす。板厚方向および板幅方向のそれぞれでのビッカース硬さ(HV)の変動幅(ΔHV)が30超えである鋼板は、母鋼板の引張試験時にその硬さが相対的に低位となる部位で優先的にくびれが生じるため、全厚の伸び特性が著しく低下する。このため、硬さの変動幅(ばらつき)はビッカース硬度で30以下の範囲、望ましくは20以下とする。このような鋼板を鋼板内の材質均質性に優れた鋼板とする。硬さ試験方法は実施例において詳述する。
鋳造後のスラブなどの鋼素材は、室温まで冷却した後、あるいは高温の状態のままで、加熱炉に装入し、鋼素材温度を1000℃以上とする。鋼素材の加熱温度は、主にNb炭窒化物を溶解せしめ、固溶Nbを十分に確保する観点から下限を1000℃とした。また、鋼素材温度が1300℃を超える場合、加熱時のオーステナイト粒の粗大化が起こり母材靱性に悪影響を及ぼすため上限は1300℃とした。なお、望ましい鋼素材温度は1000~1250℃、より望ましくは1050~1200℃である。
加熱された鋼スラブは、再結晶温度域での熱間圧延後、未再結晶温度域にて制御圧延を行う。再結晶温度域における圧延は、加熱時のオーステナイト粒を微細化するために実施することが好ましく、1パス以上、好ましくは累積圧下率20%以上行うのが望ましい。この再結晶温度域での熱間圧延後に未再結晶温度域において実施する制御圧延はその圧下率が小さい場合、所定の母材靱性を得ることが出来ない。このため、未再結晶温度域における圧延の累積圧下率の下限を40%と規定する。また、圧下率は高い方が好ましい。しかしながら、工業的には80%程度が上限となる。
加速冷却の直前に高衝突圧の噴射流を鋼板表面に衝突させることによるデスケーリングを行う。鋼板内の材質均一性に優れた厚鋼板とするためには、鋼板内の硬さのばらつきを低減することが必要であり、特に鋼板内部の強度を保ちながら、表層部の硬さのばらつきを抑制することが重要である。
本発明では、冷却後の鋼板のスケール厚みが15μm以下となるように加速冷却の直前に鋼板表面での噴射流の衝突圧が1MPa以上となる条件でデスケーリングを行う。鋼板表面での噴射流の衝突圧が1MPa未満では、デスケーリングが不十分でスケールむらが生じる場合があり、表層硬さのばらつきが生じるため、噴射流の衝突圧は1MPa以上とする。デスケーリングは高圧水を用いて行う。なお、鋼板表面での噴射流の衝突圧が1MPa以上であれば、他の噴射流を用いても問題はない。より好ましくは2MPa以上である。
デスケーリング後の加速冷却は、鋼板の強度を確保するために実施される。加速冷却においては、鋼板表層部の材質均質性を同時に担保する条件を選択する必要がある。鋼板の平均冷却速度が10℃/s未満の場合、デスケ-リングされた表層面であっても表層域の冷却が不均一となり、鋼板内部との硬さのばらつきが大きくなる。このため、平均冷却速度は10℃/s以上に規定する。また、より好ましい平均冷却速度は15℃/s以上である。なお、冷却開始温度は、得られる金属組織の均一性の観点から、理想的にはAr3変態点以上であることが好ましい。しかしながら、例えば板厚が薄い場合などにおいては、圧延完了からデスケーリングを経て、加速冷却設備に搬送される間に温度低下が起こり、冷却開始温度がAr3変態点を下回る場合がある。この影響が本発明の目的とするところの硬さの均質性を阻害しないためには、加速冷却開始温度は、圧延終了温度~(圧延終了温度-30℃)の範囲内であることが望ましい。
加速冷却は、ベイナイト変態の温度域である200~600℃まで冷却し、所定の強度が得られるミクロ組織に鋼板内部を変態(本発明ではベイナイト変態)させる。冷却停止温度が600℃を超えると、ベイナイト変態が不完全であり、十分な強度が得られない。また、冷却停止温度が200℃未満では、特に表層部において一部マルテンサイトや島状マルテンサイト(MA)が生成し、鋼板内の材質均一性が得られず全厚の伸び特性が低下する。このため、加速冷却の冷却停止温度は鋼板平均温度で200~600℃とする。所望の強度靭性が得られるように加速冷却停止後、Ac1変態点以下で焼き戻しても良い。Ac1変態点は下式によって求めることができる。但し、式において、各元素記号は各元素の含有量(質量%)を示す。
Ac1 =751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B
Claims (5)
- 質量%で、C:0.030~0.080%、Si:0.01~0.10%、Mn:1.20~2.40%、P:0.008%以下、S:0.0005~0.0040%、Al:0.005~0.080%、Nb:0.003~0.040%、Ti:0.003~0.040%、N:0.0030~0.0100%、B:0.0003~0.0030%を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、かつ、板厚方向と板幅方向のそれぞれでのビッカース硬さ(HV)の変動幅:ΔHVが30以下であることを特徴とする厚鋼板。
- 更に、質量%で、Cu:1.00%以下、Ni:1.00%以下、Cr:1.00%以下、Mo:0.50%以下、V:0.10%以下の1種または2種以上を含有することを特徴とする請求項1に記載の厚鋼板。
- 更に、質量%で、Ca:0.0005~0.0050%、Zr:0.001~0.020%、REM:0.001~0.020%の1種または2種以上を含有することを特徴とする請求項1または2に記載の厚鋼板。
- 請求項1乃至3のいずれか一つに記載の成分組成の鋼素材を1000~1300℃に加熱後、熱間圧延し、鋼板表面での噴射流の衝突圧が1MPa以上の条件で噴射流を鋼板表面に衝突させることによりデスケーリングを行い、その後、直ちに、鋼板の平均冷却速度:10℃/s以上、鋼板の平均冷却停止温度:200~600℃で加速冷却を行うことを特徴とする、板厚方向と板幅方向のそれぞれでのビッカース硬さ(HV)の変動幅:ΔHVが30以下である厚鋼板の製造方法。
- 加速冷却停止後、Ac1変態点以下で焼き戻すことを特徴とする請求項4記載の厚鋼板の製造方法。
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EP3279352A4 (en) * | 2015-03-31 | 2018-02-07 | JFE Steel Corporation | High strength/high toughness steel sheet and method for producing same |
US10640841B2 (en) | 2015-03-31 | 2020-05-05 | Jfe Steel Corporation | High-strength, high-toughness steel plate and method for producing the same |
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CN105473753B (zh) | 2017-09-26 |
BR112016001817A2 (pt) | 2017-08-01 |
JP2015040328A (ja) | 2015-03-02 |
KR20160028480A (ko) | 2016-03-11 |
BR112016001817B1 (pt) | 2021-04-06 |
TW201508069A (zh) | 2015-03-01 |
KR101795548B1 (ko) | 2017-11-08 |
TWI525199B (zh) | 2016-03-11 |
CN105473753A (zh) | 2016-04-06 |
JP5692305B2 (ja) | 2015-04-01 |
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