WO2018030171A1 - 高強度厚鋼板およびその製造方法 - Google Patents
高強度厚鋼板およびその製造方法 Download PDFInfo
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- WO2018030171A1 WO2018030171A1 PCT/JP2017/027356 JP2017027356W WO2018030171A1 WO 2018030171 A1 WO2018030171 A1 WO 2018030171A1 JP 2017027356 W JP2017027356 W JP 2017027356W WO 2018030171 A1 WO2018030171 A1 WO 2018030171A1
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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
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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
-
- 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
Definitions
- the present invention relates to a high-strength thick steel plate and a method for producing the same.
- the present invention is a high-strength thick steel plate excellent in toughness and brittle crack propagation stopping properties of large heat input welds used for large structures such as ships, offshore structures, low-temperature storage tanks, and construction / civil engineering structures, and It relates to the manufacturing method.
- vessels such as container ships and bulk carriers use high-strength thick materials for the hull outer plates due to their structures, but recently, with the increase in size of the hull, the strength has increased further. It is out.
- the brittle crack propagation stop property of a steel sheet tends to deteriorate as the strength or thickness of the steel plate increases, the demand for the brittle crack propagation stop property is further advanced.
- Ni content As a means for improving the brittle crack propagation stopping characteristics of steel materials, a method of increasing the Ni content has been conventionally known. For example, in a storage tank of liquefied natural gas (LNG), 9% Ni steel is used. Used on a commercial scale. However, an increase in the amount of Ni added necessitates a significant increase in manufacturing cost, and therefore it is difficult to apply to applications other than the LNG storage tank.
- LNG liquefied natural gas
- Patent Document 1 focuses on the fact that shear lip (plastic deformation region) generated in a steel surface layer portion when brittle crack propagates is effective in improving brittle crack propagation stop characteristics, A method of absorbing propagation energy possessed by a brittle crack propagating by refining the layer is disclosed.
- the surface layer portion is cooled to the Ar 3 transformation point or less by controlled cooling after hot rolling, and then the control cooling is stopped to recover the surface layer portion to the transformation point or more.
- Patent Document 2 in order to improve brittle crack propagation stop characteristics in a steel material mainly composed of ferrite-pearlite, the surface portions of the front and back surfaces of the steel material have a circle-equivalent particle size of 5 ⁇ m or less and an aspect ratio: Consists of a layer having 50% or more of a ferrite structure having two or more ferrite grains, and suppressing the local recrystallization phenomenon by setting the maximum rolling reduction per pass during finish rolling to 12% or less. It is disclosed that it is important to suppress variation in diameter.
- Patent Document 3 describes a technique using the TMCP method in which not only the ferrite crystal grains are refined but also subgrains formed in the ferrite crystal grains are used to improve the brittle crack propagation stop characteristics. Specifically, for a steel sheet having a thickness of 30 to 40 mm, without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer, (a) rolling conditions for securing fine ferrite crystal grains, ( b) Rolling conditions for generating a fine ferrite structure in a portion of 5% or more of the steel plate thickness, (c) A texture is developed in the fine ferrite, and dislocations introduced by processing (rolling) are rearranged by thermal energy and sub- It is described that brittle crack propagation stopping characteristics are improved by rolling conditions for forming grains and (d) cooling conditions for suppressing coarsening of the formed fine ferrite crystal grains and fine subgrain grains.
- Patent Document 4 discloses that the (110) plane X-ray intensity ratio is 2 or more by controlled rolling, and the area ratio of coarse grains having a circle-equivalent diameter of 20 ⁇ m or more is 10% or less. An improved steel sheet is described.
- Patent Document 5 as a steel for welded structure having excellent brittle crack propagation stopping performance of a joint part, the stress load direction and the crack propagation direction are shifted by texture development.
- a steel sheet having an X-ray surface strength ratio of 1.5 or more is disclosed.
- Patent Document 6 discloses toughness and brittle crack propagation stop characteristics by setting the X-ray diffraction intensity ratio of (211) plane in a plane parallel to the plate surface to 1.5 or more in a region of 50% or more of the plate thickness. Describes a thin-walled thick steel plate.
- Patent Document 7 (100) X-ray plane intensity ratio parallel to the rolling surface, respectively, for the front and back layer portions up to 25% of the plate thickness from the front and back surfaces of the steel plate and the other plate thickness center portion, and
- a high-strength thick steel plate excellent in brittle crack propagation stopping performance having a texture defining a (111) and / or (211) X-ray plane strength ratio parallel to the rolling surface is described.
- Patent Document 8 (111) or / and (211) X-ray plane intensity ratio parallel to the rolling surface in the central region of 10% or more and less than 50% of the plate thickness with the center in the plate thickness direction of the steel plate as the center. Furthermore, in the front and back surface regions on the front and back sides from the central region, the brittle crack propagation stop having a texture that defines the (111) or / and (211) X-ray surface strength ratio parallel to the rolling surface A high strength thick steel plate with excellent performance is described.
- Patent Documents 9 to 12 describe structural high-strength thick steel plates having a texture that defines various X-ray plane strength ratios at the center portion of the plate thickness and 1 ⁇ 4 portion of the plate thickness.
- the bond part is exposed to a high temperature just below the melting point during high heat input welding, the austenite crystal grains are most likely to be coarsened, and then transformed into a fragile upper bainite structure by cooling. Martensite is formed and toughness is reduced.
- Patent Documents 15 and 16 Ti oxide and Mg oxide are used (Patent Documents 15 and 16), ferrite nuclei are generated by BN, and the form of sulfide is controlled by adding Ca or REM to improve toughness. It has been proposed to let In addition, a method for improving toughness by controlling the amounts of Ca, O, and S and finely dispersing Ca and Mn composite sulfides as ferrite nuclei has been proposed (Patent Document 17).
- Patent Document 18 discloses a steel for high heat input welding in which the brittle crack propagation stop property is improved by controlling the texture, and a manufacturing method thereof.
- Patent Documents 1 and 2 the steel materials excellent in brittle crack propagation stopping characteristics described in Patent Documents 1 and 2 are obtained by recooling only the steel surface layer part and then recovering the heat, and by applying processing during the recuperation, a specific structure is obtained. In actual production scale, it is a process that is difficult to control and has a heavy load on rolling and cooling equipment.
- Non-Patent Document 1 the brittle crack propagation stop property of a steel plate having a plate thickness of 65 mm is evaluated, and a result that a brittle crack does not stop in a large brittle crack propagation stop test of a base material is reported. Furthermore, Non-Patent Document 1 shows that the Kca value at the use temperature of ⁇ 10 ° C. in the ESSO test of the specimen is less than 3000 N / mm 3/2 , and the hull structure to which a steel plate having a thickness exceeding 50 mm is applied. In this case, it is suggested that ensuring safety is an issue.
- Patent Documents 9 to 12 require rolling in a temperature range lower than the Ar 3 point, that is, a ferrite-austenite two-phase region. For this reason, not only high-precision rolling technology is required, but also rolling in a lower temperature range than usual, production efficiency is reduced, and special consideration is also required for flattening the steel plate shape. Is done. For this reason, a technique for securing excellent brittle crack propagation stopping characteristics under manufacturing conditions that do not sacrifice productivity is desired.
- Patent Document 17 the toughness of the weld heat-affected zone exceeding 400 kJ / cm is ensured by using a composite sulfide of Ca and Mn, but no examination has been made on the brittle crack propagation stopping performance.
- Patent Document 18 discloses a steel sheet that ensures the toughness of the weld heat-affected zone and has excellent brittle crack propagation stopping performance. However, even when this technology is used, the toughness and brittle crack propagation characteristics of large heat input welds may not be sufficient when the plate thickness exceeds 70 mm.
- An object of the present invention is to provide a high-strength thick steel plate excellent in toughness and brittle crack propagation stopping characteristics of a high heat input weld and a manufacturing method thereof.
- the inventors of the present invention have made extensive studies to achieve the above-mentioned problems, and have high brittle crack propagation stopping characteristics even in thick steel sheets, and high strength thick steel sheets excellent in toughness of large heat input welds and the steel sheets. The following knowledge was obtained about the manufacturing method which obtains stably.
- the (211) plane X-ray intensity ratio in a plane parallel to the plate surface (rolled surface) of the steel plate surface is 1. .2 or more, the (211) plane X-ray intensity ratio is 1.5 or more and the (222) plane X-ray intensity ratio in a plane parallel to the plate surface (rolled surface) at the plate thickness center (plate thickness 1/2 position).
- the above texture can be obtained by following specific hot rolling conditions when rolling a steel having a specific chemical composition in the austenite region.
- the lower the temperature during rolling the higher the toughness value and the developed (211) plane texture.
- the plate thickness exceeds 50 mm, the temperature difference between the center of the plate thickness during rolling and the surface of the steel plate becomes large, so if the temperature is lowered too much, a ferrite structure is formed in the surface layer and deteriorates toughness. Cause problems.
- the toughness of the weld bond part of the steel sheet with the above-mentioned specific chemical composition is affected by the embrittlement structure, and the toughness of this embrittlement structure is greatly improved by refining the transformation nucleus that promotes the ferrite transformation during cooling after welding. To do.
- the amounts of Ca, S, and O are adjusted so as to satisfy the following formula (1).
- the (211) plane X-ray intensity ratio in the plane parallel to the plate surface of the steel plate surface is 1.2 or more and the plate surface at the plate thickness center (plate thickness 1/2 position) using the chemical composition and the manufacturing process described above.
- the (211) plane X-ray intensity ratio is 1.5 or more and the (222) plane X-ray intensity ratio is 2.5 or less in a plane parallel to the surface, extremely excellent brittle crack propagation stop characteristics are obtained.
- the bond portion has high toughness in high heat input welding.
- Component composition is mass%, C: 0.03 to 0.20%, Si: 0.01 to 0.30%, Mn: 1.5 to 3.0%, P: 0.02%
- the Ceq defined by the above is in the range of 0.36 to 0.50, the balance is made of Fe and inevitable impurities, and the (211) plane X-ray intensity ratio in the plane parallel to the plate surface of the steel plate surface is 1.2. As described above, the (211) plane X-ray intensity ratio is 1.5 or more and the (222) plane X-ray intensity ratio is 2 in a plane parallel to the plate surface at the center of the thickness. 5 or less, The Charpy absorbed energy (vE ⁇ 40 ) at ⁇ 40 ° C.
- [3] The high-strength thick steel plate according to [1] or [2], wherein a fracture surface transition temperature (vTrs) at a position 5 mm from the steel plate surface in the thickness direction by a Charpy impact test is ⁇ 60 ° C. or lower.
- the component composition is further in terms of mass%, Nb: 0.003 to 0.040%, Cu: 0.01 to 0.5%, Ni: 0.01 to 2.0%, Cr: 0.00.
- the component composition is further, in mass%, V: 0.001 to 0.10%, Mg: 0.0005 to 0.0050%, Zr: 0.0005 to 0.0200%, REM: 0.00.
- the center is hot-rolled in the austenite recrystallization temperature range, and then in the austenite non-recrystallization temperature range, and at least a part of the hot rolling in the austenite non-recrystallization temperature range is performed at the temperature-sheet thickness center of the steel sheet surface.
- the temperature distribution in the sheet thickness direction is controlled so as to be performed under the condition of the temperature of ⁇ ⁇ 40 ° C., the temperature difference between the steel sheet surface temperature and the center thickness at the end of hot rolling is within 5 ° C., and A method for producing a high-strength thick steel plate, wherein the temperature at the center of the plate thickness is Ar 3 ° C. to (Ar 3 +30) ° C.
- the heat input can be stably stabilized by an industrially simple process that can optimize the rolling conditions and control the texture in the plate thickness direction. It is possible to provide a high-strength thick steel plate excellent in toughness and brittle crack propagation stopping characteristics of a high heat input weld zone exceeding 300 kJ / cm.
- the texture and toughness value are appropriately controlled according to the respective positions in the plate thickness direction, so that it has excellent brittle crack propagation stopping characteristics.
- a strong deck of a container ship and a bulk carrier for example, a strong deck of a container ship and a bulk carrier.
- the application to the deck member joined to the hatch side combing in the part structure is very useful industrially because it contributes to improving the safety of the ship.
- the present invention will be specifically described.
- the component composition and the texture inside the steel sheet are defined.
- C 0.03 to 0.20%
- the C content is set to 0.03% or more in order to ensure a desired strength.
- the C content is specified in the range of 0.03 to 0.20%.
- the C content is preferably 0.04% or more, and more preferably 0.05% or more. Further, the C content is preferably 0.15% or less, more preferably 0.12% or less.
- Si 0.01 to 0.30% Si is effective as a deoxidizing element and as a steel strengthening element, but if the content is less than 0.01%, the effect cannot be sufficiently obtained. On the other hand, if it exceeds 0.30%, not only the surface properties of the steel sheet are impaired, but also the toughness of the bond portion of the base material and the high heat input welded joint is extremely deteriorated. Therefore, the content is made 0.01 to 0.30%.
- the upper limit of the Si content is preferably 0.20%, and more preferably 0.10%.
- Mn 1.5 to 3.0% Mn is contained as a strengthening element and a quenching element. If the Mn content is less than 1.5%, the effect is not sufficient. On the other hand, if it exceeds 3.0%, the weldability deteriorates and the steel material cost also increases. Therefore, the Mn content is 1.5 to 3.0%.
- the Mn content is preferably 1.7% or more, and more preferably 1.9% or more. Further, the Mn content is preferably 2.7% or less, and more preferably 2.5% or less.
- P 0.02% or less P increases the toughness as the content increases.
- Thickness In order to maintain good toughness with respect to steel sheets exceeding 50 mm, the P content is controlled to 0.02% or less. Preferably, the P content is controlled to 0.01% or less, more preferably 0.006% or less.
- S 0.0005 to 0.01% If S content increases, toughness will deteriorate. Therefore, S is suppressed to 0.01% or less. On the other hand, in order to obtain excellent toughness at the bond part of the high heat input welded joint, it is necessary to contain 0.0005% or more of S.
- the content of S is preferably 0.0010% or more. Further, the content of S is preferably 0.0050% or less, and more preferably 0.0030% or less.
- Ti 0.005 to 0.030%
- Ti has the effect of forming nitrides, carbides, or carbonitrides by adding a trace amount, and making the crystal grains finer to improve the base material toughness. Further, Ti precipitates as TiN during solidification and contributes to high toughness by suppressing the austenite coarsening in the weld and ferrite transformation nuclei.
- the Ti content is less than 0.005%, the effect is small.
- the Ti content exceeds 0.030%, the effect cannot be obtained due to the coarsening of the TiN particles, so the Ti content is 0.005 to 0. 0.030%.
- the Ti content is preferably 0.008% or more. Further, the Ti content is preferably 0.020% or less.
- Al acts as a deoxidizer, and for this purpose, it needs to contain 0.005% or more. However, if it contains more than 0.080%, the toughness is lowered and, when welded, weld metal Reduce the toughness of the part. For this reason, the Al content is specified in the range of 0.005 to 0.080%.
- the Al content is preferably 0.02% or more. Further, the Al content is preferably 0.060% or less.
- N 0.0025 to 0.0075%
- N is an element necessary for securing the necessary amount of TiN. If it is less than 0.0025%, a sufficient amount of TiN cannot be obtained, and if it exceeds 0.0075%, it is solidified in the region where TiN is dissolved by the welding heat cycle. Since the amount of dissolved N is increased and the toughness is remarkably lowered, the content is made 0.0025 to 0.0075%.
- the N content is preferably 0.0030% or more.
- the N content is preferably 0.0070% or less.
- Ca 0.0003% to 0.0030%
- Ca is an element having an effect of improving toughness by fixing S. In order to exert such an effect, it is necessary to contain at least 0.0003%, but even if contained over 0.0030%, the effect is saturated, so 0.0003 to 0.0030%
- the Ca content is preferably 0.0010% or more. Further, the Ca content is preferably 0.0025% or less.
- B 0.0003 to 0.0030% B fixes N due to dissolution of TiN as BN in the weld heat affected zone, and suppresses the deterioration of the weld toughness. Moreover, it improves hardenability and contributes effectively to securing the strength of the base material. Such an effect is exhibited by addition of 0.0003% or more, and even if it is added in an amount of more than 0.0030%, the effect is saturated, so the B content is 0.0003 to 0.0030%. To do.
- the content of B is preferably 0.0008% or more. Further, the content of B is preferably 0.0025% or less.
- O 0.0030% or less O decreases in cleanliness as the content increases. For this reason, it is desirable to reduce as much as possible in the present invention. In particular, when the O content exceeds 0.0030%, CaO-based inclusions become coarse and lower the base material toughness, so the content is made 0.0030% or less.
- This parameter formula defines the content of Ca, S, and O in the steel in order to form a composite sulfide in which MnS is precipitated on CaS.
- the value of this parameter formula is greater than 0 and less than 1, CaS crystallizes in the solidification stage when melting steel, and the amount of solid solution S is secured after crystallization of CaS, and MnS is formed on the surface of CaS. Precipitates.
- MnS has a ferrite nucleation ability, and forms a Mn dilute band around it to promote ferrite transformation and improve the toughness of the weld heat affected zone. Ferrite transformation is further promoted by precipitation of ferrite-forming nuclei such as TiN, BN, and AlN on MnS.
- ferrite-forming nuclei such as TiN, BN, and AlN on MnS.
- the value of this parameter formula is 0 or less, CaS does not crystallize, S precipitates in the form of MnS alone, and the composite sulfide cannot be finely dispersed in the weld heat affected zone.
- the value of this parameter formula is 1 or more, S is completely fixed by Ca, and MnS acting as a ferrite nucleation does not precipitate on CaS. Cannot be dispersed.
- Ceq C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 is set to 0.36 to 0.50.
- each element symbol in the above formula represents the content (% by mass) of each element, and the element not contained is 0.
- Ceq is 0.38 to 0.48.
- the above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities. Furthermore, in order to improve the characteristics, it is possible to contain one or more of Nb, Cu, Ni, Cr, Mo, V, Mg, Zr, and REM.
- Nb 0.003 to 0.040%
- Nb precipitates as NbC during ferrite transformation or reheating, and contributes to increasing the strength.
- it has the effect of expanding the non-recrystallized region in the rolling of the austenite region and contributes to the refinement of ferrite, so it is also effective in improving toughness.
- the effect is exhibited by the content of 0.003% or more, but if it exceeds 0.040%, coarse NbC precipitates and conversely causes a decrease in toughness.
- the upper limit is preferably 0.040%.
- the Nb content is more preferably 0.010% or more. Further, the Nb content is more preferably 0.030% or less.
- Cu 0.01 to 0.5%
- Ni 0.01 to 2.0%
- Cu and Ni are elements that enhance the hardenability of steel. It contributes directly to strength improvement after rolling, and can be contained for improving functions such as toughness, high-temperature strength, or weather resistance. All of these effects are exhibited by inclusion of 0.01% or more. However, excessive inclusion deteriorates toughness and weldability. Further, since the cost of the alloy becomes high, when Cu and / or Ni is contained, the respective ranges are 0.01 to 0.5% for Cu and 0.01 to 2.0% for Ni. And
- Cr 0.01-0.5%
- Mo 0.01-0.5%
- Cr and Mo are both elements that enhance the hardenability of steel. It contributes directly to strength improvement after rolling, and can be contained for improving functions such as toughness, high-temperature strength, or weather resistance. All of these effects are exhibited by inclusion of 0.01% or more. However, excessive inclusion deteriorates toughness and weldability. As a range in which the toughness and weldability are not deteriorated, each range in the case of containing Cr and / or Mo is preferably 0.01 to 0.5%.
- V 0.001 to 0.10%
- V is an element that improves the strength of the steel by precipitation strengthening that precipitates as V (CN), and this effect is exhibited by containing V in an amount of 0.001% or more.
- V is contained exceeding 0.10%, toughness is reduced.
- the V content is preferably in the range of 0.001 to 0.10%.
- Mg, Zr, and REM are all elements that have an effect of improving toughness due to oxide dispersion.
- Mg, Zr, and REM are Mg: 0.0005 to 0.0050%, Zr: 0.0005 to 0.0200%, and REM: 0.0005 to 0.0200, respectively.
- Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc.
- Sb 0.01% or less
- Sn 0.01% or less
- Zn 0.
- the range is 01% or less
- Co 0.1% or less.
- the (211) plane X in the plane parallel to the plate surface at the center of the plate thickness is used in order to improve the crack propagation stop characteristic for cracks propagating in the direction parallel to the plate surface such as the rolling direction or the direction perpendicular to the rolling direction.
- a line intensity ratio, a (222) plane X-ray intensity ratio, and a (211) plane X-ray intensity ratio in a plane parallel to the plate surface of the steel sheet surface are defined.
- the surface parallel to the plate surface (rolled surface) of the steel plate surface in the present invention is not only a simple surface of the steel plate but also a surface on which the X-ray intensity ratio (degree of integration of crystal planes) can be measured.
- Including the surface after processing For example, when the outermost surface of a steel plate is covered with a scale, it refers to the surface from which it has been removed. In addition, when the outermost surface of the steel plate is a mirror surface and the X-ray intensity ratio (degree of crystal plane integration) can be measured as it is, the surface of the steel plate (rolled surface) itself is referred to.
- the brittle cracks are prevented from progressing in a straight line.
- the crack propagation stop characteristic can be improved.
- Kca (-10 ° C), which is the target for ensuring structural safety, is a thick material with a thickness of 50 mm or more that has been used for hull outer plates such as recent container ships and bulk carriers.
- the center of the plate thickness meaning the plate thickness 1/2 position.
- the X-ray intensity ratio (the degree of crystal plane integration) is measured. In this case, a position error of several percent of the plate thickness from the position of the plate thickness 1/2 in the vertical direction (position error of more than 0% to 5% or less of the plate thickness) is allowed.
- the (211) plane X-ray intensity ratio is 1.5 or more, the (222) plane X-ray intensity ratio is 2.5 or less, and the (211) plane X-ray intensity ratio in a plane parallel to the plate surface of the steel sheet surface is 1 Need to be 2 or more.
- the (211) plane X-ray intensity ratio is 1.6 or more and the (222) plane X-ray intensity ratio is 2. It is preferable that the (211) plane X-ray intensity ratio in a plane parallel to the plate surface of the steel plate surface is 2 or more and 1.4 or more.
- the (211) plane X-ray intensity ratio is a numerical value indicating the degree of integration of the (211) crystal plane of the target material, and the (211) reflection X-ray diffraction intensity (I (211)) of the target material.
- the molten steel having the above component composition is melted in a converter or the like, made into a steel material (slab) by continuous casting or the like, heated to 1000 to 1200 ° C., and then hot-rolled.
- the heating temperature of the steel material is in the range of 1000 to 1200 ° C.
- the preferable heating temperature range is 1000 to 1150 ° C, and more preferably 1030 to 1130 ° C.
- hot rolling is performed.
- control is performed to reduce the temperature difference between the inside of the steel sheet and the front and back surfaces of the steel sheet by controlling the temperature distribution in the thickness direction by heating from the front and back surfaces of the steel sheet during rolling.
- This control is preferably performed after the end of rolling in the austenite recrystallization temperature range. More preferably, after completion of rolling in the austenite recrystallization temperature range, before rolling in the austenite non-recrystallization temperature range.
- the above control is performed so that the rolling in the austenite non-recrystallization temperature region is performed under the condition of the temperature of the steel sheet surface ⁇ the temperature at the center of the plate thickness ⁇ ⁇ 40 ° C.
- rolling is performed at a temperature of the steel sheet surface ⁇ a temperature at the center of the sheet thickness ⁇ ⁇ 20 ° C.
- the rolling in the austenite non-recrystallization temperature region is performed at least partly from the start to the end of rolling in the austenite non-recrystallization temperature region.
- the above-mentioned conditions may be maintained and the rolling may be performed.
- Rolling according to the above conditions may be performed at the time or part of the rolling (temporary point) between them.
- the rolling is performed so that the above condition is satisfied at the end of rolling in the austenite non-recrystallization temperature range.
- the properties of the steel sheet can be further improved by satisfying the above conditions at the end of rolling in the austenite non-recrystallization temperature region.
- the above control is performed before the start of rolling in the austenite non-recrystallization temperature range, from the start of rolling in the austenite non-recrystallization temperature range, or after a lapse of a certain time from the start of rolling.
- Roll by More preferably, the above control is performed before the start of rolling in the austenite non-recrystallization temperature range, and from the start of rolling in the austenite non-recrystallization temperature range, or after a lapse of a certain time from the start of rolling, the rolling is completed. Until that time, rolling is performed under the above conditions.
- the means for heating from the front and back surfaces of the steel sheet is not particularly limited, and examples thereof include heating by an atmospheric furnace and heating by high frequency.
- the upper limit of the temperature of the steel sheet surface-the center of the sheet thickness is not particularly limited, but is preferably 80 ° C. or less, more preferably 60 ° C. or less, from the viewpoint of securing the base material toughness and material uniformity.
- the cumulative rolling reduction is 40% or more when the temperature at the center of the sheet thickness is in the range of Ar 3 to Ar 3 + 30 ° C. More preferably, the cumulative rolling reduction in the range of Ar 3 to Ar 3 + 20 ° C. is 40% or more, and still more preferably, the cumulative rolling reduction in the range of Ar 3 to Ar 3 + 20 ° C. is 50% or more.
- the temperature at the center of the plate thickness is just above the Ar 3 point.
- Ar 3 point (Ar 3 transformation point) is a value calculated using the following equation.
- Ar 3 points (° C.) 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo
- C, Si, Mn, Cu, Ni, Cr, and Mo in the above formulas represent the content (% by mass) of each of the above elements, and the elements that do not contain 0.
- the rolling in the austenite recrystallization temperature range is not particularly limited, but the cumulative rolling reduction is preferably 10% or more. More preferably, the cumulative rolling reduction is 15% or more.
- the temperature difference between the temperature of the steel sheet surface at the end of hot rolling and the temperature at the center of the sheet thickness is within 5 ° C.
- the temperature at the center of the sheet thickness at the end of hot rolling is preferably Ar 3 ° C to (Ar 3 +30) ° C.
- the rolled steel sheet is cooled from a temperature of 3 or higher Ar to 600 ° C. or lower at a cooling rate of 2 ° C./s or higher.
- the cooling rate By setting the cooling rate to 2 ° C./s or more, the texture strength developed during rolling can be maintained.
- the temperature at the center of the plate thickness is obtained by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer.
- the temperature condition in the cooling condition after rolling is the temperature at the center of the plate thickness.
- the high strength thick steel plate in the present invention refers to a thick steel plate having a tensile strength of 580 MPa or more.
- the high-strength thick steel plate in the present invention preferably has a tensile strength of 600 MPa or more.
- the thickness of the high-strength thick steel plate in the present invention is preferably more than 50 mm, more preferably 55 mm or more, and further preferably 60 mm or more.
- the fracture surface transition temperature at the center of the plate thickness by the Charpy impact test is preferably ⁇ 60 ° C. or lower (vTrs ⁇ ⁇ 60 ° C.). Furthermore, the high-strength thick steel plate in the present invention preferably has a fracture surface transition temperature of ⁇ 60 ° C. or less (vTrs ⁇ ⁇ 60 ° C.) at a position 5 mm from the steel plate surface in the plate thickness direction according to the Charpy impact test. Thereby, the characteristic of a steel plate is improved more.
- the temperature at the center of the plate thickness described above is Ar 3.
- the cumulative rolling reduction in the range of ⁇ Ar 3 + 30 ° C. is 45% or more, or the cumulative rolling reduction in the range of the thickness center of Ar 3 to Ar 3 + 30 ° C. is less than 45%
- the temperature at the center of the plate thickness at the end of hot rolling is preferably 740 ° C. or lower.
- Molten steel of each component composition shown in Table 1 is melted in a converter, made into a steel material by a continuous casting method, heated at the heating temperature shown in Table 2, hot rolled to a plate thickness of 55 to 100 mm, and then cooled. A thick steel plate was obtained.
- Table 2 shows hot rolling conditions and cooling conditions. In the case of heating from the front and back surfaces of the steel sheet during hot rolling and controlling the temperature distribution in the plate thickness direction, the presence or absence of heating during rolling in Table 2 was indicated as ⁇ . The heating was performed after the end of rolling in the austenite recrystallization temperature range and before the start of rolling in the austenite non-recrystallization temperature range.
- a JIS 14A test piece of ⁇ 14 was taken from the position of the thickness 1 ⁇ 4, a tensile test was performed, and yield strength (YS) and tensile strength (TS) were measured.
- vTrs fracture surface transition temperature
- a JIS No. 4 impact test piece was collected so that the direction of the longitudinal axis of the test piece was parallel to the rolling direction
- a Charpy impact test was performed to measure Charpy absorption energy at ⁇ 20 ° C. to ⁇ 100 ° C. and its brittle fracture surface ratio, and a brittle-ductile fracture surface transition temperature (vTrs) (° C.) was obtained.
- vTrs fracture surface transition temperature
- surface layer for the fracture surface transition temperature (vTrs) at a position 5 mm from the steel sheet surface in the thickness direction (described as “surface layer” in Table 3)
- a JIS No. 4 impact test piece was collected so that the direction of the longitudinal axis of the test piece was parallel to the rolling direction. That is, since the thickness of the test piece is 10 mm, the measurement position on the test piece is the center position in the thickness direction of the test piece, that is, the position of 5 mm from the steel plate surface to the plate thickness direction.
- a Charpy impact test was performed on this specimen to measure the Charpy absorbed energy at -20 ° C to -100 ° C and its brittle fracture surface ratio, and the brittle-ductile fracture surface at a position 5 mm from the steel sheet surface in the thickness direction.
- the transition temperature (vTrs) (° C.) was determined.
- the (211) plane X-ray intensity ratio and the (222) plane X-ray intensity ratio in a plane parallel to the plate surface at the center of the plate thickness, and the plate surface of the steel plate surface are parallel.
- the (211) plane X-ray intensity ratio in the plane was measured.
- the measurement method is as follows. First, a sample with a surface thickness of 0.5 mm or a thickness of 1 mm is taken from the center of the plate thickness, and a surface parallel to the plate surface is mechanically polished / electropolished to prepare a test piece for X-ray diffraction. prepare. In the case of the plate thickness surface layer portion, the surface closest to the outermost surface is polished.
- Kca ( ⁇ 10 ° C.) N / mm 3/2 Kca ( ⁇ 10 ° C.) N / mm 3/2
- Kca (-10 °C) N / mm 3/2 can be evaluated with excellent brittle crack propagation stop characteristics if 6000 N / mm 3/2 or more.
- V groove was given to the test plate for joint extract
- the heat input (kJ / cm) at this time is shown in Table 3.
- a JIS No. 4 impact test piece with the notch position as the bond part was taken from the obtained high heat input welded joint, and a Charpy impact test was conducted at a test temperature of ⁇ 40 ° C.
- the average value of absorbed energy was determined as absorbed energy vE ⁇ 40 (J). If this absorbed energy vE ⁇ 40 (J) is 80 J or more, it can be evaluated that the joint has excellent toughness.
- the (211) plane X-ray intensity ratio in the plane parallel to the plate surface at the center of the plate thickness is 1.5 or more, and in the plane parallel to the plate surface of the steel plate surface ( 211)
- the plane X-ray intensity ratio is 1.2 or more
- the (222) plane X-ray intensity ratio in a plane parallel to the plate surface at the center of the sheet thickness is 2.5 or less
- Kca ( ⁇ 10 ° C.) N It can be seen that an excellent brittle crack propagation stop characteristic is obtained when / mm 3/2 is 6000 N / mm 3/2 or more, and that the Charpy absorbed energy of the weld joint bond portion is 80 J or more and has excellent joint toughness.
- the component composition, the (211) plane X-ray intensity ratio in the plane parallel to the plane of the plate thickness, and the (222) plane X Any one or more of the line strength ratio, the (211) plane X-ray intensity ratio in the plane parallel to the plate surface of the steel sheet, and the tensile strength do not satisfy the provisions of the present invention, and Kca ( ⁇ 10 ° C.) N Necessary characteristics are not obtained at any one of / mm 3/2 and absorbed energy vE ⁇ 40 (J). Specifically, Kca ( ⁇ 10 ° C.) is less than 6000 N / mm 3/2 or vE ⁇ 40 (J) is less than 80 J, and necessary characteristics are not obtained.
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Abstract
Description
0<[(Ca-(0.18+130×Ca)×O)/1.25]/S<1 ・・・(1)
すなわち、鋼を溶製する際の凝固段階でCaSを晶出させるにあたり、(1)式を満足するようにCa、Sの添加量および添加時の溶鋼中の溶存酸素量を制御することによって、CaSの晶出後の固溶S量を確保すれば、CaSの表面上にMnSが析出する。MnSはフェライト核生成能を有し、その周囲にMnの希薄帯が形成されるとフェライト変態が促進され、溶接熱影響部の靭性を向上させる。MnS上にTiN、BN、AlN等のフェライト生成核が析出することによって、より一層、フェライト変態が促進される。
[1]成分組成が、質量%で、C:0.03~0.20%、Si:0.01~0.30%、Mn:1.5~3.0%、P:0.02%以下、S:0.0005~0.01%、Ti:0.005~0.030%、Al:0.005~0.080%、N:0.0025~0.0075%、Ca:0.0003~0.0030%、B:0.0003~0.0030%、O:0.0030%以下を含有し、かつ、Ca、O、Sが下記(1)式を満たし、下記(2)式で定義されるCeqが0.36~0.50の範囲にあり、残部がFeおよび不可避的不純物からなり、鋼板表面の板面に平行な面における(211)面X線強度比が1.2以上、板厚中央の板面に平行な面における(211)面X線強度比が1.5以上かつ(222)面X線強度比が2.5以下であり、
300kJ/cm超の入熱量で溶接した溶接継手ボンド部の-40℃におけるシャルピー吸収エネルギー(vE-40)が80J以上であり、ESSO試験による-10℃におけるKca値(Kca(-10℃))が6000N/mm3/2以上であり、引張り強さが580MPa以上であり、板厚が50mm超である、高強度厚鋼板。
0<[(Ca-(0.18+130×Ca)×O)/1.25]/S<1 ・・・(1)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ・・・(2)
ただし、上記(1)式および(2)式における各元素記号は各元素の含有量(質量%)を表し、含有しない元素は0とする。
[2]シャルピー衝撃試験による板厚中央部の破面遷移温度(vTrs)が-60℃以下である、[1]に記載の高強度厚鋼板。
[3]シャルピー衝撃試験による鋼板表面から板厚方向に5mmの位置の破面遷移温度(vTrs)が-60℃以下である、[1]または[2]に記載の高強度厚鋼板。
[4]成分組成が、さらに、質量%で、Nb:0.003~0.040%、Cu:0.01~0.5%、Ni:0.01~2.0%、Cr:0.01~0.5%、Mo:0.01~0.5%のなかから選ばれる1種または2種以上を含有する、[1]~[3]のいずれかに記載の高強度厚鋼板。
[5]成分組成が、さらに、質量%で、V:0.001~0.10%、Mg:0.0005~0.0050%、Zr:0.0005~0.0200%、REM:0.0005~0.0200%のなかから選ばれる1種または2種以上を含有する、[1]~[4]のいずれかに記載の高強度厚鋼板。
[6]前記[1]~[5]のいずれかに記載の高強度厚鋼板の製造方法であって、前記成分組成を有する鋼素材を、1000~1200℃の温度に加熱したのち、板厚中央が、オーステナイト再結晶温度域、次いで、オーステナイト未再結晶温度域で熱間圧延を行い、少なくとも前記オーステナイト未再結晶温度域での熱間圧延の一部を、鋼板表面の温度-板厚中央の温度≧-40℃となる条件で行うように板厚方向の温度分布を制御し、熱間圧延終了時における鋼板表面の温度と板厚中央の温度との温度差を5℃以内とし、かつ、板厚中央の温度をAr3℃~(Ar3+30)℃とする、高強度厚鋼板の製造方法。
本発明では、成分組成および鋼板内部の集合組織を規定する。
以下の説明において、鋼板成分における%は、すべて質量%を意味する。
Cは、鋼の強度を向上する元素であり、本発明では、所望の強度を確保するために、Cの含有量を0.03%以上とする。一方、Cの含有量が0.20%を超えると、溶接性が劣化するばかりか靭性にも悪影響がある。このため、Cの含有量は、0.03~0.20%の範囲に規定する。なお、Cの含有量は、好ましくは0.04%以上であり、より好ましくは0.05%以上である。また、Cの含有量は、好ましくは0.15%以下であり、より好ましくは0.12%以下である。
Siは、脱酸元素として、また、鋼の強化元素として有効であるが、0.01%未満の含有量ではその効果が十分に得られない。一方、0.30%を超えると鋼板の表面性状を損なうばかりか、母材および大入熱溶接継手のボンド部の靭性が極端に劣化する。従って、その含有量を0.01~0.30%の範囲とする。Siの含有量の上限値としては、0.20%が好ましく、0.10%がより好ましい。
Mnは、強化元素および焼入れ元素として含有する。Mnの含有量が、1.5%より少ないとその効果が十分でない一方で、3.0%を超えると溶接性が劣化し、鋼材コストも上昇する。そのため、Mnの含有量は、1.5~3.0%とする。Mnの含有量は、好ましくは1.7%以上であり、より好ましくは1.9%以上である。また、Mnの含有量は、好ましくは2.7%以下であり、より好ましくは2.5%以下である。
Pは、含有量が多くなると靭性が劣化してしまう。板厚:50mm超の鋼板に対して、良好な靭性を保つためには、Pの含有量を0.02%以下に制御する。好ましくは、Pの含有量を0.01%以下、さらに好ましくは0.006%以下に制御する。
Sは、含有量が多くなると靭性が劣化してしまう。そのため、Sは0.01%以下に抑制する。一方、大入熱溶接継手のボンド部おいて優れた靱性を得るためには、Sを0.0005%以上含有することが必要である。Sの含有量は、好ましくは0.0010%以上である。また、Sの含有量は、好ましくは0.0050%以下であり、より好ましくは0.0030%以下である。
Tiは、微量の含有により、窒化物、炭化物、あるいは炭窒化物を形成し、結晶粒を微細化して母材靭性を向上させる効果を有する。また、Tiは、凝固時にTiNとなって析出し、溶接部でのオーステナイトの粗大化抑制やフェライト変態核となって高靭性化に寄与する。Tiの含有量が、0.005%未満ではその効果が少なく、一方、0.030%を超えるとTiN粒子の粗大化によってその効果が得られなくなるため、Tiの含有量は0.005~0.030%とする。Tiの含有量は、好ましくは0.008%以上である。また、Tiの含有量は、好ましくは0.020%以下である。
Alは、脱酸剤として作用し、このためには0.005%以上の含有を必要とするが、0.080%を超えて含有すると、靭性を低下させるとともに、溶接した場合に、溶接金属部の靭性を低下させる。このため、Alの含有量は、0.005~0.080%の範囲に規定した。なお、Alの含有量は、好ましくは0.02%以上である。また、Alの含有量は、好ましくは0.060%以下である。
Nは、TiNの必要量を確保するために必要な元素で、0.0025%未満では十分なTiN量が得られず、0.0075%を超えると溶接熱サイクルによってTiNが溶解する領域において固溶N量が増加して靭性を著しく低下させるため、0.0025~0.0075%とする。Nの含有量は、好ましくは0.0030%以上である。また、Nの含有量は、好ましくは0.0070%以下である。
Caは、Sの固定による靭性改善効果を有する元素である。このような効果を発揮させるためには少なくとも0.0003%は含有することが必要であるが、0.0030%を超えて含有しても効果が飽和するため、0.0003~0.0030%とする。Caの含有量は、好ましくは0.0010%以上である。また、Caの含有量は、好ましくは0.0025%以下である。
Bは、溶接熱影響部でTiNの溶解によるNをBNとして固定し、溶接部靭性の劣化を抑制する。また、焼入性を向上させ母材の強度確保に有効に寄与する。このような効果は0.0003%以上の添加で発揮され、また、0.0030%よりも多く添加してもその効果は飽和するため、Bの含有量は0.0003~0.0030%とする。Bの含有量は、好ましくは0.0008%以上である。また、Bの含有量は、好ましくは0.0025%以下である。
Oは、含有量が多くなると清浄度を低下させる。このため本発明ではできるだけ低減することが望ましい。特に、O含有量が0.0030%を超えるとCaO系介在物が粗大化して母材靭性を低下させてしまうため、0.0030%以下とする。
但し、Ca、O、Sは、各元素の含有量(質量%)を表す。
本パラメータ式はCaS上にMnSが析出した複合硫化物の形態とするため、鋼中のCa、S、Oの含有量を規定するものである。
本パラメータ式の値が、0超え、1未満の場合、鋼を溶製する際の凝固段階でCaSが晶出し、CaSの晶出後に固溶S量が確保されて、CaSの表面上にMnSが析出する。
MnSはフェライト核生成能を有し、その周囲にMnの希薄帯を形成してフェライト変態を促進し、溶接熱影響部の靭性を向上させる。MnS上にTiN、BN、AlN等のフェライト生成核が析出することによって、より一層、フェライト変態が促進される。
本パラメータ式の値が、0以下の場合には、CaSが晶出せず、SはMnS単独の形態で析出し、溶接熱影響部において複合硫化物を微細分散させることができない。
一方、本パラメータ式の値が1以上の場合には、SがCaによって完全に固定され、フェライト生成核として作用するMnSが、CaS上に析出しないため、溶接熱影響部において複合硫化物を微細分散させることができない。
なお、本発明では、CaをCaSとして晶出させるために、Caと結合力の強いO量をCa添加前に低減させておくことが必要であり、Ca添加前の残存酸素量は、0.0030%以下であることが好ましい。残存酸素量の低減方法としては、脱ガスを強化する、あるいは、脱酸剤を投入する、などの方法をとることができる。
板厚50mmを超える厚鋼板の強度および集合組織強度を保つためには、Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5で定義されるCeqを0.36~0.50とする。ただし、前記式における各元素記号は各元素の含有量(質量%)を表し、含有しない元素は0とする。好ましくはCeqが0.38~0.48である。
Nbは、NbCとしてフェライト変態時あるいは再加熱時に析出し、高強度化に寄与する。また、オーステナイト域の圧延において未再結晶域を拡大させる効果を有し、フェライトの細粒化に寄与するので、靭性の改善にも有効である。その効果は0.003%以上の含有により発揮されるが、0.040%を超えて含有すると、粗大なNbCが析出して逆に、靭性の低下を招くので、Nbを含有する場合、その上限は0.040%とするのが好ましい。Nbの含有量は、より好ましくは0.010%以上である。また、Nbの含有量は、より好ましくは0.030%以下である。
CuおよびNiは、鋼の焼入れ性を高める元素である。圧延後の強度向上に直接寄与するとともに、靭性、高温強度、あるいは耐候性などの機能向上のために含有させることができ、これらの効果は、いずれも、0.01%以上の含有によって発揮されるものの、過度の含有は靭性や溶接性を劣化させる。また、合金のコストも高くなってしまうため、Cu及び/又はNiを含有する場合には、それぞれの範囲を、Cuは0.01~0.5%、Niは0.01~2.0%とする。
CrおよびMoは、いずれも鋼の焼入れ性を高める元素である。圧延後の強度向上に直接寄与するとともに、靭性、高温強度、あるいは耐候性などの機能向上のために含有させることができ、これらの効果は、いずれも、0.01%以上の含有によって発揮されるものの、過度の含有は靭性や溶接性を劣化させる。靭性や溶接性を劣化させない範囲としては、Cr及び/又はMoを含有する場合のそれぞれの範囲を、0.01~0.5%とすることが好ましい。
Vは、V(CN)として析出する析出強化によって、鋼の強度を向上させる元素であり、この効果はVを0.001%以上含有させることにより発揮される。しかし、Vを0.10%を超えて含有すると、靭性を低下させる。このため、Vを含有させる場合には、Vの含有量を0.001~0.10%の範囲とすることが好ましい。
Mg、ZrおよびREM(希土類金属)はいずれも、酸化物の分散による靱性改善効果を有する元素である。このような効果を発現させるには、Mg、Zr、REMのうちの少なくとも1種を0.0005%以上含有させることが好ましい。一方、Mgは0.0050%超、ZrおよびREMは0.0200%超を添加しても、その効果は飽和するだけである。よって、これらの元素を含有する場合は、Mg、Zr、REMをそれぞれ、Mg:0.0005~0.0050%、Zr:0.0005~0.0200%、REM:0.0005~0.0200%の範囲とすることが好ましい。
本発明では、圧延方向または圧延直角方向など板面に平行な方向に伝播する亀裂に対して亀裂伝播停止特性を向上させるために、板厚中央の板面に平行な面における(211)面X線強度比および(222)面X線強度比、鋼板表面の板面に平行な面における(211)面X線強度比を規定する。なお、本発明における鋼板表面の板面(圧延面)に平行な面とは、鋼板の単純な表面だけではなく、鋼板表面をX線強度比(結晶面の集積度)が測定可能な面に処理した後の面を含む。例えば、鋼板の最表面がスケールで覆われている時などは、それを取り除いた面を言う。また、鋼板の最表面が鏡面になっており、そのままX線強度比(結晶面の集積度)を測定できる場合などは、鋼板の表面(圧延面)そのものをいう。
上記成分組成になる溶鋼を、転炉等で溶製し、連続鋳造等で鋼素材(スラブ)とし、1000~1200℃に加熱後、熱間圧延を行う。
熱間圧延はまず、板厚中央の温度がオーステナイト再結晶温度域で圧延を行い、ついで板厚中央の温度がオーステナイト未再結晶温度域で圧延を行う。このとき、圧延途中で鋼板の表裏面から加熱を行い板厚方向の温度分布を制御することにより、鋼板内部と鋼板の表裏面の温度差を小さくする制御を行う。これにより目標とする板厚中央と鋼板表面における集合組織強度が得られる。この制御は、前記オーステナイト再結晶温度域での圧延終了後に行われるのが好ましい。より好ましくは、前記オーステナイト再結晶温度域での圧延終了後、オーステナイト未再結晶温度域での圧延開始前である。
また、好ましくは、上記制御をオーステナイト未再結晶温度域での圧延開始前に行い、オーステナイト未再結晶温度域での圧延開始時から、或いは、前記圧延開始時より一定時間経過後から、前記条件による圧延を行う。より好ましくは、上記制御をオーステナイト未再結晶温度域での圧延開始前に行い、オーステナイト未再結晶温度域での圧延開始時から、或いは、前記圧延開始時より一定時間経過後から、前記圧延終了時までの間、前記条件による圧延を行う。
なお、前記鋼板の表裏面から加熱を行うための手段としては、特に制限されないが、例えば、雰囲気炉による加熱や、高周波による加熱等が挙げられる。また、前記鋼板表面の温度-板厚中央の温度の上限は、特に制限されないが、母材靭性の確保および材質均一性の観点からは、80℃以下が好ましく、60℃以下がより好ましい。
Ar3点(℃)=910-310C-80Mn-20Cu-55Ni-15Cr-80Mo
ただし、上記式中のC、Si、Mn、Cu、Ni、Cr、Moは、前記各元素の含有量(質量%)を表し、含有しない元素は0とする。
また、上記オーステナイト再結晶温度域での圧延は、特に制限されないが、累積圧下率が10%以上であることが好ましい。より好ましくは、累積圧下率が15%以上である。
表1に示す各成分組成の溶鋼を、転炉で溶製し、連続鋳造法で鋼素材とし、表2に示す加熱温度で加熱し、板厚55~100mmに熱間圧延後、冷却を行い厚鋼板を得た。表2に熱間圧延条件と冷却条件を示す。熱間圧延中に、鋼板の表裏面から加熱を行い板厚方向の温度分布制御を行ったものについては、表2の圧延中の加熱有無に○を示した。前記加熱は、オーステナイト再結晶温度域での圧延終了後、オーステナイト未再結晶温度域での圧延開始前に行い、前記加熱後、30秒以内にオーステナイト未再結晶温度域での圧延を開始した。なお、前記加熱を行わなかったものについては、表2の圧延中の加熱有無に×を示した。熱間圧延後は直ちに板厚中央の冷却速度が2~10℃/sで350~500℃の範囲まで冷却し、その後、放冷した。なお、前記鋼板の表裏面からの加熱は、雰囲気炉加熱装置により行った。また、表1のAr3は、前述したものと同様の計算式により求めた。
鋼板表面から板厚方向に5mmの位置(表3中に「表層部」と記載)の破面遷移温度(vTrs)については、鋼板の表面に形成されているスケール(黒皮)を除去した後、該鋼板の表面から、JIS 4号衝撃試験片を試験片の長手軸の方向が圧延方向と平行となるように採取した。すなわち、前記試験片の厚さは10mmであるから、前記試験片における測定位置は、該試験片の厚さ方向の中心位置、すなわち、鋼板表面から板厚方向に5mmの位置となる。この試験片に対してシャルピー衝撃試験を行って、-20℃~-100℃におけるシャルピー吸収エネルギーおよびその脆性破面率を測定し、鋼板表面から板厚方向に5mmの位置の脆性-延性破面遷移温度(vTrs)(℃)を求めた。
測定方法はまず、板厚表層下0.5mmあるいは板厚中央から板厚1mmのサンプルを採取し、板面に平行な面を機械研磨・電解研磨することにより、X線回折用の試験片を用意する。なお、板厚表層部の場合には、最表面に近い方の面を研磨するものとする。この試験片を用いて、Mo線源を用いてX線回折装置を使用して、X線回折測定を実施し、(211)面X線強度比および(222)面X線強度比を求めた。
Claims (6)
- 成分組成が、質量%で、
C:0.03~0.20%、
Si:0.01~0.30%、
Mn:1.5~3.0%、
P:0.02%以下、
S:0.0005~0.01%、
Ti:0.005~0.030%、
Al:0.005~0.080%、
N:0.0025~0.0075%、
Ca:0.0003~0.0030%、
B:0.0003~0.0030%、
O:0.0030%以下を含有し、かつ、Ca、O、Sが下記(1)式を満足し、下記(2)式で定義されるCeqが0.36~0.50の範囲にあり、残部がFeおよび不可避的不純物からなり、
鋼板表面の板面に平行な面における(211)面X線強度比が1.2以上、板厚中央の板面に平行な面における(211)面X線強度比が1.5以上かつ(222)面X線強度比が2.5以下であり、
300kJ/cm超の入熱量で溶接した溶接継手ボンド部の-40℃におけるシャルピー吸収エネルギー(vE-40)が80J以上であり、ESSO試験による-10℃におけるKca値(Kca(-10℃))が6000N/mm3/2以上であり、引張り強さが580MPa以上であり、板厚が50mm超である、高強度厚鋼板。
0<[(Ca-(0.18+130×Ca)×O)/1.25]/S<1 ・・・(1)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ・・・(2)
ただし、上記(1)式および(2)式における各元素記号は各元素の含有量(質量%)を表し、含有しない元素は0とする。 - シャルピー衝撃試験による板厚中央の破面遷移温度(vTrs)が-60℃以下である、請求項1に記載の高強度厚鋼板。
- シャルピー衝撃試験による鋼板表面から板厚方向に5mmの位置の破面遷移温度(vTrs)が-60℃以下である、請求項1または2に記載の高強度厚鋼板。
- 成分組成が、さらに、質量%で、
Nb:0.003~0.040%、
Cu:0.01~0.5%、
Ni:0.01~2.0%、
Cr:0.01~0.5%、
Mo:0.01~0.5%のなかから選ばれる1種または2種以上を含有する、請求項1~3のいずれか一項に記載の高強度厚鋼板。 - 成分組成が、さらに、質量%で、
V:0.001~0.10%、
Mg:0.0005~0.0050%、
Zr:0.0005~0.0200%、
REM:0.0005~0.0200%のなかから選ばれる1種または2種以上を含有する、請求項1~4のいずれか一項に記載の高強度厚鋼板。 - 請求項1~5のいずれか一項に記載の高強度厚鋼板の製造方法であって、前記成分組成を有する鋼素材を、1000~1200℃の温度に加熱したのち、板厚中央が、オーステナイト再結晶温度域、次いで、オーステナイト未再結晶温度域で熱間圧延を行い、前記熱間圧延の途中で、鋼板の表裏面から加熱を行い、少なくとも前記オーステナイト未再結晶温度域での熱間圧延の一部を、鋼板表面の温度-板厚中央の温度≧-40℃となる条件で行うように板厚方向の温度分布を制御し、
熱間圧延終了時における鋼板表面の温度と板厚中央の温度との温度差を5℃以内とし、かつ、板厚中央の温度をAr3℃~(Ar3+30)℃とする、高強度厚鋼板の製造方法。
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