WO2018092735A1 - Tôle d'acier à haute résistance, procédé de production associé, et tôle d'acier galvanisée à haute résistance - Google Patents

Tôle d'acier à haute résistance, procédé de production associé, et tôle d'acier galvanisée à haute résistance Download PDF

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WO2018092735A1
WO2018092735A1 PCT/JP2017/040814 JP2017040814W WO2018092735A1 WO 2018092735 A1 WO2018092735 A1 WO 2018092735A1 JP 2017040814 W JP2017040814 W JP 2017040814W WO 2018092735 A1 WO2018092735 A1 WO 2018092735A1
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steel sheet
temperature
ferrite
strength
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PCT/JP2017/040814
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Japanese (ja)
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秀和 南
由康 川崎
金子　真次郎
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Ｊｆｅスチール株式会社
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Publication of WO2018092735A1 publication Critical patent/WO2018092735A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc

Definitions

  • the present invention relates to a high-strength steel sheet excellent in formability, which is mainly suitable as a structural member of an automobile, and a method for producing the same, and in particular, has a tensile strength (TS) of 780 MPa or more and is excellent not only in ductility but also in rigidity. Furthermore, the present invention seeks to obtain a high-strength steel sheet that is also excellent in deep drawability.
  • the high-strength steel sheet of the present invention includes a high-strength galvanized steel sheet having a galvanized layer on its surface.
  • the Young's modulus is largely governed by the texture of the steel sheet.
  • the Young's modulus In the case of iron having a body-centered cubic lattice, the Young's modulus is high in the ⁇ 111> direction, which is the atomic dense direction, and conversely low in the ⁇ 100> direction where the atomic density is small It has been known. It is known that the Young's modulus of normal iron with no anisotropy in crystal orientation is about 206 GPa, but by giving anisotropy to the crystal orientation and increasing the atomic density in a specific direction, Young's modulus can be increased. However, when considering the rigidity of an automobile body, since a load is applied from various directions, a steel sheet having a high Young's modulus in each direction is required in addition to a specific direction.
  • Patent Document 1 in mass%, C: 0.02 to 0.15%, Si: 0.3% or less, Mn: 1.0 to 3.5%, P : 0.05% or less, S: 0.01% or less, Al: 1.0% or less, N: 0.01% or less, and Ti: 0.1 to 1.0%, the balance being Fe and inevitable
  • a slab composed of mechanical impurities is hot-rolled, cold-rolled at a rolling reduction of 20 to 85%, and then recrystallized and annealed to have a ferrite single-phase microstructure, TS of 590 MPa or more, and the rolling direction.
  • Patent Document 2 discloses that in mass%, C: 0.05 to 0.15%, Si: 1.5% or less, Mn: 1.5 to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Nb: 0.02 to 0.15% and Ti: 0.01 to 0.15%, the balance A slab composed of Fe and inevitable impurities is hot-rolled, cold-rolled at a rolling reduction of 40 to 75%, and then recrystallized to have a mixed structure of ferrite and martensite, and TS is 590 MPa. As described above, a method for producing a high-rigidity and high-strength steel sheet having excellent workability and having a Young's modulus in a direction perpendicular to the rolling direction of 230 GPa or more has been proposed.
  • Patent Document 3 by mass%, C: 0.010 to 0.050%, Si: 1.0% or less, Mn: 1.0 to 3.0%, P: 0.005 to 0.00. 1%, S: 0.01% or less, Al: 0.005 to 0.5%, N: 0.01% or less and Nb: 0.03 to 0.3%, the balance being Fe and inevitable
  • a slab made of impurities is cold-rolled after hot rolling, and recrystallized and annealed to have a steel structure including an area ratio of the ferrite phase of 50% or more and an area ratio of the martensite phase of 1% or more.
  • a method for producing a high-strength steel sheet having a Young's modulus in the direction perpendicular to the rolling of 225 GPa or more and an average r value of 1.3 or more has been proposed.
  • Patent Document 3 discloses that the rigidity and workability are excellent, and among the workability, it discloses that the deep drawability is particularly excellent, but TS is as low as about 660 MPa at most. Further, the techniques described in Patent Documents 1 to 3 do not necessarily have the feature of being excellent not only in ductility and rigidity but also in deep drawability.
  • the present invention has been developed in view of such circumstances, and has developed a ferrite texture to ⁇ -fiber (a fiber texture in which the ⁇ 111> axis is parallel to the normal direction of the rolling surface) and utilizes bainite transformation. Then, by dispersing an appropriate amount of retained austenite, an object is to obtain a high-strength steel sheet having not only ductility but also rigidity and further excellent deep drawability while having a TS of 780 MPa or more.
  • excellent ductility that is, El (total elongation) means that the value of TS ⁇ El is 15000 MPa ⁇ % or more.
  • excellent rigidity that is, Young's modulus means that the Young's modulus in the rolling direction and the 45 ° direction with respect to the rolling direction is 205 GPa or more, and the Young's modulus in the direction perpendicular to the rolling direction is 220 GPa or more.
  • being excellent in deep drawability means that the average r value, which is an index of deep drawability, is 0.95 or more regardless of the strength of the steel sheet.
  • the interstitial elements C and N are added due to the effect of promoting precipitation of the added Ti and / or Nb by making the coiling temperature (CT) of the hot rolling relatively high. It is important to deposit as carbides and nitrides with high thermal stability.
  • CT coiling temperature
  • heat treatment is performed to soften the hot-rolled sheet, and then the cold rolling is performed to increase the reduction ratio as much as possible to obtain ⁇ -fiber ( ⁇ 110> axis). It is important to develop a fiber texture that is parallel to the rolling direction) and a ⁇ -fiber texture.
  • the gist configuration of the present invention is as follows. 1. Ingredient composition is mass%, C: 0.08% to 0.35%, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 3.00% or less, P: 0.001% to 0.100%, S: 0.0001% or more and 0.0200% or less and N: 0.0005% or more and 0.0100% or less, Ti: 0.001% or more and 0.200% or less and Nb: 0.001% or more and 0.200% or less containing one or two selected from the balance, the balance consists of Fe and inevitable impurities, Steel structure is area ratio, Ferrite is 20% or more, the average grain size of the ferrite is 10 ⁇ m or more and 20 ⁇ m or less, and bainite is 5% or more, Martensite is 5% or more, The total area ratio of bainite and martensite is 15% or more, In volume ratio, residual austenite is 5% or more, Further, the ferrite texture has a microstructure in which the inverse strength
  • Al 0.01% or more and 1.00% or less
  • V 0.005% or more and 0.100% or less
  • B 0.0001% to 0.0050%
  • Cr 0.05% or more and 1.00% or less
  • Cu 0.05% or more and 1.00% or less
  • Sb 0.0020% or more and 0.2000% or less
  • Sn 0.0020% or more and 0.2000% or less
  • Ta 0.0010% or more and 0.1000% or less
  • Ca 0.0003% or more and 0.0050% or less
  • the method for producing a high-strength steel sheet according to 1 or 2 The steel slab having the component composition of 1 or 2 is heated to 1100 ° C. or higher and 1300 ° C. or lower, hot rolled at a finish rolling exit temperature: 800 ° C. or higher and 1000 ° C. or lower, and a coiling temperature: 300 ° C. or higher. Winding at 800 ° C. or lower, after pickling treatment, as it is or after holding in the temperature range of 450 ° C. or higher and 800 ° C. or lower for 900 s or more and 36000 s or less, cold rolling is performed at a rolling reduction of 40% or more, Subsequently, the obtained cold-rolled sheet is heated to a temperature range of 450 ° C.
  • T1 temperature (° C.) 720 + 29 ⁇ [% Si] -21 ⁇ [% Mn] + 17 ⁇ [% Cr]
  • T2 temperature (° C.) 946 ⁇ 203 ⁇ [% C] 1/2 + 45 ⁇ [% Si] ⁇ 30 ⁇ [% Mn] + 150 ⁇ [% Al] ⁇ 20 ⁇ [% Cu] + 11 ⁇ [% Cr] +350 ⁇ [% Ti] + 104 ⁇ [% V] [% X] is the mass% of the component element X of the steel sheet, and zero for the component elements not contained.
  • the present invention it is possible to effectively obtain a high-strength steel sheet having a TS of 780 MPa or more and excellent not only in ductility but also in rigidity and also in deep drawability. Therefore, by applying the high-strength steel plate obtained by the present invention to, for example, an automobile structural member, fuel efficiency can be improved by reducing the weight of the vehicle body, and the industrial utility value is extremely large.
  • C 0.08% to 0.35%
  • C is an element indispensable for increasing the strength of a steel sheet and ensuring a stable amount of retained austenite, and is an element necessary for ensuring the amount of bainite and martensite and for retaining austenite at room temperature. If the C content is less than 0.08%, it is difficult to secure desired TS and El.
  • the C content is 0.08% or more and 0.35% or less.
  • they are 0.12% or more and 0.33% or less, More preferably, they are 0.15% or more and 0.31% or less, More preferably, they are 0.20% or more and 0.30% or less.
  • Si 0.50% to 2.50%
  • Si is an element useful for improving the El of the steel sheet by suppressing the formation of carbides and promoting the formation of retained austenite. It is also effective in suppressing the formation of carbides due to decomposition of retained austenite. Furthermore, since it has a high solid solution strengthening ability in ferrite, it contributes to improving the strength of steel. Further, Si dissolved in ferrite has an effect of improving work hardening ability and increasing the ductility of the ferrite itself. In order to obtain such an effect, the Si amount needs to be 0.50% or more.
  • the Si amount is set to 0.50% or more and 2.50% or less.
  • they are 0.80% or more and 2.00% or less, More preferably, they are 1.00% or more and 1.80% or less, More preferably, they are 1.10% or more and 1.70% or less.
  • Mn is effective for securing the strength of the steel sheet.
  • the hardenability is improved to facilitate complex organization.
  • Mn acts to suppress the formation of pearlite during the cooling process, and facilitates transformation from austenite to bainite and martensite.
  • the amount of Mn needs to be 1.50% or more.
  • the Mn content is 1.50% or more and 3.00% or less. Preferably they are 1.50% or more and 2.70% or less, More preferably, they are 1.80% or more and 2.50% or less.
  • P 0.001% to 0.100%
  • P is an element that has a solid solution strengthening action and can be added according to a desired strength.
  • it is an element effective for complex organization in order to promote ferrite transformation.
  • P amount 0.001% or more.
  • the amount of P exceeds 0.100%, weldability is deteriorated and, when galvanizing is alloyed, the alloying speed is greatly delayed to impair the quality of galvanizing.
  • impact resistance is deteriorated by embrittlement due to grain boundary segregation. Therefore, the P amount is set to 0.001% or more and 0.100% or less. Preferably it is 0.005% or more and 0.050% or less.
  • the amount of S needs to be 0.0001% or more due to restrictions in production technology. Therefore, the S amount is set to 0.0001% or more and 0.0200% or less. Preferably it is 0.0001% or more and 0.0050% or less.
  • N is an element that greatly deteriorates the aging resistance of steel.
  • the amount of N exceeds 0.0100%, deterioration of aging resistance becomes remarkable, so the amount is preferably as small as possible.
  • the amount of N needs to be 0.0005% or more due to restrictions on production technology. There is. Therefore, the N amount is set to 0.0005% or more and 0.0100% or less. Preferably it is 0.0005% or more and 0.0070% or less.
  • Ti in addition to the above components, in order to obtain a ferrite having an orientation that is advantageous for improving the Young's modulus, Ti: 0.001% to 0.200% and Nb: 0.001% to 0.200 It is necessary to contain any 1 type or 2 types in% or less. [Ti: 0.001% or more and 0.200% or less] Ti forms precipitates with C, S, and N, and not only generates ferrite with an orientation that is advantageous for improving rigidity and deep drawability during annealing, but also suppresses coarsening of recrystallized grains. It also contributes to the improvement of strength. Moreover, when B is added, since N is precipitated as TiN, precipitation of BN is suppressed, and the effect of B described later is effectively expressed.
  • the Ti amount needs to be 0.001% or more.
  • the amount of Ti exceeds 0.200%, carbonitrides cannot be completely dissolved during normal slab reheating, and coarse carbonitrides remain, thereby increasing strength and suppressing recrystallization. The effect is not obtained.
  • the Ti amount is set to 0.001% or more and 0.200% or less. Preferably they are 0.030% or more and 0.170% or less, More preferably, they are 0.050% or more and 0.150% or less.
  • Nb forms fine precipitates at the time of hot rolling or annealing, and not only generates ferrite with an orientation that is advantageous for improving rigidity and deep drawability at the time of annealing, but also coarsens the recrystallized grains. Suppressing and contributing to the improvement of strength.
  • Nb is added in an appropriate amount, the austenite phase generated by reverse transformation at the time of annealing is refined, so that the microstructure after annealing is also refined to increase the strength. In order to obtain such an effect, the Nb amount needs to be 0.001% or more.
  • the Nb amount is set to be 0.001% or more and 0.200% or less. Preferably they are 0.030% or more and 0.170% or less, More preferably, they are 0.050% or more and 0.150% or less.
  • the high-strength steel sheet of the present invention is at least one selected from Al, V, B, Cr, Cu, Sb, Sn, Ta, Ca, Mg, and REM, if necessary, in addition to the above basic components. These elements can be contained alone or in combination.
  • the balance of the component composition of the steel sheet is Fe and inevitable impurities.
  • Al 0.01% or more and 1.00% or less
  • Al is an element effective for suppressing the formation of carbides and promoting the formation of retained austenite.
  • it is an element added as a deoxidizer in the steel making process. In order to obtain such effects, the Al amount needs to be 0.01% or more.
  • the Al content is set to 0.01% or more and 1.00% or less. Preferably they are 0.03% or more and 0.50% or less.
  • V forms fine precipitates during hot rolling or annealing, thereby suppressing coarsening of recrystallized grains, contributing to an increase in strength, and advantageous for improving rigidity and deep drawability during annealing. Produces a well-oriented ferrite. In order to obtain such an effect, the amount of V needs to be added by 0.005% or more. On the other hand, if the amount of V exceeds 0.100%, the moldability deteriorates. Therefore, when adding V, the content is made 0.005% or more and 0.100% or less.
  • B 0.0001% or more and 0.0050% or less
  • B is an element effective for strengthening steel, and the effect of addition is obtained at 0.0001% or more.
  • the B amount is set to 0.0001% or more and 0.0050% or less. Preferably it is 0.0005% or more and 0.0030% or less.
  • [Cu: 0.05% to 1.00%] Cr and Cu not only serve as solid solution strengthening elements, but also stabilize austenite and facilitate complex organization in the cooling process during annealing.
  • the Cr content and the Cu content must each be 0.05% or more.
  • the Cr content and the Cu content exceed 1.00%, the formability of the steel sheet is lowered. Therefore, when adding Cr and Cu, their contents are 0.05% or more and 1.00% or less, respectively.
  • Sb and Sn are added as necessary from the viewpoint of suppressing decarburization in the region of several tens of ⁇ m of the steel sheet surface layer caused by nitriding and oxidation of the steel sheet surface. This is because suppressing such nitriding and oxidation prevents the martensite generation amount on the steel sheet surface from decreasing and is effective in ensuring the strength and material stability of the steel sheet. On the other hand, if any of these elements is added excessively exceeding 0.2000%, the toughness is reduced. Accordingly, when Sb and Sn are added, their contents are within the range of 0.0020% or more and 0.2000% or less, respectively.
  • Ta like Ti and Nb, generates carbides and carbonitrides and contributes to high strength.
  • a part of the Nb carbide or Nb carbonitride is solid-solved to form a composite precipitate such as (Nb, Ta) (C, N), which significantly suppresses the coarsening of the precipitate. It is thought that it contributes effectively to the strength improvement of a steel plate by the object stabilization effect. Therefore, it is preferable to contain Ta.
  • the above-mentioned precipitate stabilization effect can be obtained by setting the Ta content to 0.0010% or more.
  • the precipitate stabilization effect is saturated. , Alloy costs increase. Therefore, when Ta is added, the content is within the range of 0.0010% to 0.1000%.
  • Ca, Mg, and REM are elements used for deoxidation, and are effective elements for spheroidizing the shape of the sulfide and improving the adverse effect of the sulfide on local ductility. In order to obtain these effects, 0.0003% or more must be added. However, when Ca, Mg and REM are added in excess of 0.0050%, inclusions and the like are increased to cause defects on the surface and inside. Therefore, when Ca, Mg and REM are added, their contents are 0.0003% or more and 0.0050% or less, respectively.
  • the high-strength steel sheet of the present invention comprises a composite structure in which retained austenite responsible for ductility and bainite and martensite responsible for strength are dispersed in soft ferrite that contributes to higher Young's modulus and higher r-value.
  • the area ratio of ferrite generated in the second annealing and cooling process needs to be 20% or more.
  • the upper limit of the area ratio of ferrite is not particularly limited, but is preferably 80% or less, more preferably 70% or less, and still more preferably 60% or less for securing the strength.
  • it is important that the average crystal grain size of ferrite is 10 ⁇ m or more and 20 ⁇ m or less. If the average crystal grain size of ferrite is less than 10 ⁇ m, a desired texture cannot be obtained, and a desired Young's modulus and average r value cannot be ensured. On the other hand, if the average crystal grain size of ferrite exceeds 20 ⁇ m, the desired area ratio of bainite and martensite cannot be obtained, and the desired strength cannot be ensured.
  • the average crystal grain size of ferrite is 10 ⁇ m or more and 20 ⁇ m or less. Preferably they are 11 micrometers or more and 17 micrometers or less.
  • Such control of the ferrite grain size can be achieved by appropriately controlling the annealing temperature and the holding time in the manufacturing process, particularly in the first annealing treatment.
  • bainite is necessary for concentrating C in untransformed austenite and obtaining retained austenite that can exhibit the TRIP effect in a high strain region during processing. In order to achieve both high strength and high ductility, it is effective to increase the amount of retained austenite produced.
  • C concentration to austenite in the holding process after the second annealing, when the amount of bainite produced is less than 5%, C concentration to austenite does not sufficiently proceed, so that a residual that exhibits a TRIP effect in a high strain region during processing. The amount of austenite decreases.
  • the area ratio of bainite needs to be 5% or more in terms of the area ratio with respect to the entire steel sheet structure.
  • the upper limit of the area ratio of bainite is not particularly limited, but is preferably 60% or less, more preferably 50%, in order to ensure the area ratio of ferrite that is advantageous for increasing the Young's modulus and the r value. % Or less.
  • the martensite area ratio needs to be 5% or more in order to ensure the strength of the steel sheet.
  • the upper limit of the martensite area ratio is not particularly limited, but in order to ensure a good ductility at the same time as securing an area ratio of ferrite advantageous for increasing the Young's modulus and r value, martensite
  • the area ratio is preferably 50% or less, more preferably 40% or less.
  • the upper limit of the total area ratio of bainite and martensite is not particularly limited, but is preferably 70% or less for securing the area ratio of ferrite that is advantageous for increasing the Young's modulus and increasing the r value. More preferably, it is 60% or less, More preferably, it is 55% or less.
  • the area ratio of ferrite, bainite, and martensite is 1 vol.
  • the area ratio of ferrite, bainite, and martensite was calculated for the three visual fields using Adobe Photoshop from Adobe Systems, and the values were averaged. Can be sought.
  • ferrite has a gray structure (base structure)
  • martensite has a white structure
  • bainite has a structure in which a white structure is mixed with a gray base.
  • the average crystal grain size of ferrite can be obtained as follows. Using the above-mentioned Adobe Photoshop, the value obtained by correcting the length of the line segment drawn on the image to the actual length was divided by the number of crystal grains passing through the line segment drawn on the image.
  • the amount of retained austenite in order to ensure a good ductility and strength-ductility balance, the amount of retained austenite needs to be 5% or more by volume ratio. In order to secure a better ductility and strength-ductility balance, the amount of retained austenite is preferably 8% or more, more preferably 11% or more in terms of volume ratio.
  • the upper limit of the volume ratio of retained austenite is not particularly limited, but is preferably 20% or less. The volume fraction of retained austenite was determined by measuring the X-ray diffraction intensity after grinding and polishing the steel plate to 1 ⁇ 4 of the plate thickness in the plate thickness direction.
  • Co—K ⁇ is used, and the amount of retained austenite is calculated from the intensity ratio of each surface of (200), (220), (311) of austenite to the diffraction intensity of each surface of (200), (211) of ferrite. Calculated.
  • microstructure according to the present invention in addition to the above-described ferrite, bainite, martensite, and retained austenite, carbides such as tempered martensite, tempered bainite, pearlite, cementite, and other known structures may be included.
  • carbides such as tempered martensite, tempered bainite, pearlite, cementite, and other known structures may be included.
  • the effect of the present invention is not impaired even if these total amounts are within the range of 15% or less in terms of area ratio.
  • ⁇ -fiber is a fiber texture whose ⁇ 110> axis is parallel to the rolling direction
  • ⁇ -fiber is a fiber texture whose ⁇ 111> axis is parallel to the normal direction of the rolling surface.
  • the body-centered cubic metal is characterized in that ⁇ -fiber and ⁇ -fiber are strongly developed by rolling deformation and a texture belonging to them is formed even by recrystallization annealing.
  • the inverse strength ratio of ⁇ -fiber to ⁇ -fiber in the ferrite texture when the inverse strength ratio of ⁇ -fiber to ⁇ -fiber in the ferrite texture is 3.0 or less, the degree of integration of ⁇ -fiber suitable for high Young's modulus and high r-value is low. It is difficult to ensure the Young's modulus and the average r value. Therefore, the inverse strength ratio of ⁇ -fiber to ⁇ -fiber in the ferrite texture needs to exceed 3.0.
  • the upper limit of the inverse intensity ratio is not particularly limited, but is preferably 8.0 or less. In the high-strength steel sheet obtained by the conventional general manufacturing method, the inverse strength ratio of ⁇ -fiber to ⁇ -fiber is about 1.0 to 2.5.
  • the inverse strength ratio of ⁇ -fiber to ⁇ -fiber in the ferrite texture was smoothed by wet polishing and buffing using a colloidal silica solution on the plate thickness section (L section) parallel to the rolling direction of the steel sheet. Thereafter, 0.1 vol. Corrosion with% nital reduces asperities on the sample surface as much as possible, and completely removes the work-affected layer, and then corresponds to 1/4 position of the plate thickness (1/4 of the plate thickness in the depth direction from the steel plate surface).
  • the crystal orientation is measured using SEM-EBSD (Electron Back-Scatter Diffraction).
  • any one or two elements of Ti and Nb are added, and the steel slab in which the composition of other alloy elements is appropriately controlled is heated and subjected to hot rolling.
  • the hot rolling coiling temperature (CT) is set to a relatively high temperature, so that the interstitial elements C and N are highly thermally stable due to the precipitation promoting effect of the added Ti and / or Nb. It is important to deposit as carbides and nitrides.
  • heat treatment is performed to soften the hot-rolled sheet. Thereafter, when cold rolling is performed, it is important to develop the texture of ⁇ -fiber and ⁇ -fiber by increasing the reduction ratio as much as possible.
  • the steel sheet structure before the annealing treatment is a structure in which solute C and N are precipitated as carbides and nitrides having high thermal stability, and a texture of ⁇ -fiber and ⁇ -fiber is developed. Then, by recrystallizing the ferrite in the first heating step (first annealing treatment) in the subsequent ferrite single phase region, the texture of the ferrite is developed into ⁇ -fiber and ⁇ -fiber, particularly ⁇ -fiber. As a result, the Young's modulus in all directions can be improved, and the average r value can be improved.
  • the second heating step (second annealing process) in the ferrite + austenite two-phase region, a certain amount of austenite is generated while maintaining the ferrite texture, and bainite and residual austenite are generated in the subsequent cooling process.
  • a high strength steel sheet having a TS of 780 MPa or more excellent in not only ductility but also rigidity, and also excellent in deep drawability is obtained. Is possible.
  • the high-strength galvanized steel sheet of the present invention can be manufactured by subjecting the above-described high-strength steel sheet to a publicly known galvanizing treatment.
  • Heating temperature of steel slab 1100 ° C or higher and 1300 ° C or lower
  • Precipitates present in the heating stage of the steel slab exist as coarse precipitates in the finally obtained steel sheet and do not contribute to strength, so the Ti and Nb-based precipitates precipitated during casting are redissolved.
  • the heating temperature of the steel slab is less than 1100 ° C., not only is it difficult to sufficiently dissolve the precipitate, but there is a problem that the risk of trouble occurring during hot rolling due to an increase in rolling load increases. .
  • the heating temperature of the steel slab of the present invention needs to be 1100 ° C. or higher.
  • the heating temperature of the steel slab exceeds 1300 ° C., the scale loss increases as the oxidation amount increases.
  • the heating temperature of the steel slab needs to be 1300 ° C. or lower. Therefore, the heating temperature of the steel slab is set to 1100 ° C. or higher and 1300 ° C. or lower. Preferably they are 1150 degreeC or more and 1280 degrees C or less, More preferably, they are 1150 degreeC or more and 1250 degrees C or less.
  • the finish rolling exit temperature of hot rolling needs to be 800 ° C. or higher and 1000 ° C. or lower. Preferably they are 820 degreeC or more and 950 degrees C or less.
  • the steel slab is preferably manufactured by a continuous casting method in order to prevent macro segregation, but it can also be manufactured by an ingot-making method or a thin slab casting method.
  • the steel slab is not cooled to room temperature.
  • Energy-saving processes such as direct feed rolling and direct rolling that are rolled immediately after application can also be applied without problems.
  • the slab is made into a sheet bar by rough rolling under normal conditions. However, if the heating temperature is lowered, the sheet is heated using a bar heater before finishing rolling in order to prevent problems during hot rolling. It is preferred to heat the bar.
  • Winding temperature after hot rolling 300 ° C or higher and 800 ° C or lower
  • the coiling temperature after hot rolling exceeds 800 ° C
  • the crystal grain size of ferrite in the hot-rolled sheet structure increases, and the carbonitrides of Ti and Nb become coarse, so that during cold rolling and annealing Of ⁇ -fiber becomes weak, and it becomes difficult to secure a desired Young's modulus and average r value.
  • the coiling temperature after hot rolling is less than 300 ° C., the hot rolled sheet strength increases, the rolling load in cold rolling increases, and the productivity decreases.
  • the coiling temperature after hot rolling needs to be 300 ° C. or higher and 800 ° C. or lower. Preferably they are 350 degreeC or more and 700 degrees C or less, More preferably, they are 380 degreeC or more and 650 degrees C or less.
  • rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once.
  • part or all of the finish rolling may be lubricated rolling.
  • Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material.
  • the hot-rolled steel sheet thus manufactured is pickled. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion properties and plating quality in the final high-strength steel sheet. Moreover, pickling may be performed once or may be divided into a plurality of times. After the above pickling treatment, it is kept as it is or in a temperature range of 450 ° C. to 800 ° C. for a time of 900 s to 36000 s, and then cold-rolled at a reduction ratio of 40% or more.
  • Heat treatment temperature range and holding time after hot-rolled plate pickling treatment Hold for 900 s to 36000 s in a temperature range of 450 ° C. to 800 ° C.
  • tempering after hot rolling is insufficient, and at least one of ferrite, pearlite, bainite and martensite is mixed during the subsequent cold rolling.
  • the resulting structure becomes uneven, and the uniform refinement becomes insufficient under the influence of the hot-rolled sheet structure.
  • the ratio of the coarse low temperature transformation phase increases in the structure of the final annealed plate, resulting in a non-uniform structure, and the El, Young's modulus, and average r value of the final annealed plate may decrease.
  • productivity may be adversely affected.
  • the heat treatment temperature range exceeds 800 ° C., it becomes a non-uniform and hardened coarse two-phase structure of ferrite and martensite or pearlite, and becomes a non-uniform structure before cold rolling.
  • the ratio of coarse martensite may increase, and the El, Young's modulus, and average r value of the final annealed sheet may also decrease. Therefore, when heat treatment is performed after the hot-rolled sheet pickling treatment, the temperature range needs to be 450 ° C. or higher and 800 ° C. or lower, and the holding time needs to be 900 s or higher and 36000 s or lower.
  • Cold rolling is performed after the hot rolling step to accumulate ⁇ -fiber and ⁇ -fiber effective in improving Young's modulus and average r value. That is, by developing ⁇ -fiber and ⁇ -fiber by cold rolling, the ferrite having ⁇ -fiber and ⁇ -fiber, especially ⁇ -fiber, is increased in the structure after the subsequent annealing process, and Young's modulus and average Increase the r value.
  • the rolling reduction during cold rolling needs to be 40% or more.
  • the rolling reduction is preferably 45% or more, more preferably 50% or more.
  • count of a rolling pass and the rolling reduction for every pass the effect of this invention can be acquired, without being specifically limited.
  • it is about 80% industrially.
  • Temporal range of first annealing treatment 450 ° C. or higher and T1 temperature or lower
  • this is a very important invention constituent element.
  • the annealing temperature range of the first firing treatment is less than 450 ° C., a large amount of unrecrystallized ferrite remains, and the amount of ferrite having ⁇ -fiber formed during recrystallization of ferrite decreases, and the Young's modulus and average r in each direction The value drops.
  • austenite is first nucleated from the nucleation site of recrystallized ferrite having ⁇ -fiber, which is suitable for improvement of Young's modulus and average r value.
  • the area ratio of ferrite having ⁇ -fiber is reduced.
  • the volume fraction of austenite generated during annealing increases, and the volume fraction of ferrite accumulated in ⁇ -fiber and ⁇ -fiber, especially ⁇ -fiber, decreases, resulting in a decrease in Young's modulus and average r value in each direction. To do.
  • the temperature range of the first annealing process needs to be 450 ° C. or higher and T1 temperature or lower.
  • the temperature range of the first annealing treatment is preferably 500 ° C. or higher and T1 temperature or lower, more preferably 550 ° C. or higher and T1 temperature or lower.
  • the T1 temperature means the Ac 1 point.
  • the holding time in the first annealing treatment is less than 300 s, unrecrystallized ferrite remains, and the degree of accumulation in ⁇ -fiber decreases, so that the Young's modulus and average r value in each direction decrease. For this reason, holding time shall be 300 s or more. Further, there is no particular limitation, but if the holding time exceeds 100,000 s, the recrystallized ferrite grain size becomes coarse and it becomes difficult to secure a desired TS. Therefore, the holding time is preferably 100,000 s or less. . Accordingly, the holding time is 300 s or longer. Preferably it is 300 s or more and 100,000 or less, More preferably, it is 300 or more and 36000 s or less, More preferably, it is 300 or more and 21600 s or less.
  • the heat treatment method may be any of continuous annealing and batch annealing. Moreover, when implementing a cooling process after the 1st annealing process, you may cool to room temperature and you may perform the process which passes an overaging zone.
  • the cooling method and cooling rate in the cooling step are not particularly defined, and any cooling such as furnace cooling in batch annealing, air cooling, and gas jet cooling, mist cooling, and water cooling in continuous annealing may be used.
  • the pickling may be performed according to a conventional method. Although there is no particular limitation, since the steel sheet shape may be deteriorated when the average cooling rate to room temperature or overaging zone exceeds 80 ° C./s, the average cooling rate is 80 ° C./s or less. It is preferable to do.
  • the temperature range of the second annealing treatment is set to T1 temperature or more and T2 temperature or less.
  • the holding time of the second annealing treatment is not particularly limited, but is preferably 10 s or more and 1000 s or less.
  • the T2 temperature means the Ac 3 point.
  • the average cooling rate to at least 550 ° C. in the cooling step after reheating is set to 5 ° C./s or more.
  • they are 5 degreeC / s or more and 200 degrees C / s or less, More preferably, they are 8 degreeC / s or more and 80 degrees C / s or less, More preferably, they are 10 degreeC / s or more and 50 degrees C / s or less.
  • the cooling stop temperature after the second annealing treatment is set to 300 ° C. or more and 500 ° C. or less. Furthermore, from the viewpoint of improving the balance between strength and ductility, the cooling stop temperature after the second annealing treatment is preferably set to 300 ° C. or higher and 480 ° C. or lower. More preferably, it is 350 degreeC or more and 460 degreeC or less.
  • the holding time in the reheating temperature region is set to 10 s or more. Preferably, it is 10 seconds or more and 1000 seconds or less.
  • the cooling after the holding does not need to be specified, and may be cooled to a desired temperature by any method.
  • the desired temperature is preferably about room temperature.
  • the galvanizing alloying treatment when the alloying treatment is performed at a temperature exceeding 600 ° C., untransformed austenite is transformed into pearlite, and a desired volume ratio of retained austenite cannot be secured, and El may be lowered. Therefore, when the galvanizing alloying treatment is performed, it is preferable to perform the galvanizing alloying treatment in a temperature range of 470 ° C. or more and 600 ° C. or less. Moreover, you may perform an electrogalvanization process.
  • the plating adhesion amount is preferably 20 to 80 g / m 2 per side (double-sided plating), and the alloyed hot-dip galvanized steel sheet (GA) is subjected to alloying treatment so that the Fe concentration in the plating layer is 7 to 15 mass. % Is preferable.
  • the reduction ratio of the skin pass rolling after the heat treatment is preferably in the range of 0.1% to 2.0%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit of the good range. Moreover, since productivity will fall remarkably when it exceeds 2.0%, this is made the upper limit of a favorable range.
  • Skin pass rolling may be performed online or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.
  • Other manufacturing method conditions are not particularly limited, but from the viewpoint of productivity, a series of treatments such as annealing, hot dip galvanization, alloying treatment of galvanization, etc. are performed by CGL (Continuous Galvanizing) which is a hot dip galvanizing line. Line). After hot dip galvanization, wiping is possible to adjust the amount of plating.
  • conditions, such as plating other than the above-mentioned conditions can depend on the conventional method of hot dip galvanization.
  • Example 1 Steel having the component composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in a converter and made into a slab by a continuous casting method.
  • the obtained slab was heated under the conditions shown in Table 2 and hot-rolled, and then pickled.
  • Nos. 1 to 11, 13 to 23, 25, 27, 29, 30, 32 to 37, 39, and 41 were subjected to hot-rolled sheet heat treatment.
  • Nos. 29, 30, 32 to 37, 39, and 41 were subjected to pickling treatment after the heat treatment of the plate.
  • annealing was performed twice under the conditions shown in Table 2 to obtain a high-strength cold-rolled steel sheet (CR).
  • GI hot-dip galvanized steel sheets
  • GA galvannealed steel sheets
  • EG electrogalvanized steel sheets
  • the hot dip galvanizing bath uses a zinc bath containing Al: 0.14% by mass or 0.19% by mass in GI, and uses a zinc bath containing Al: 0.14% by mass in GA.
  • GA 470 ° C.
  • GA made Fe density
  • the amount of EG plating adhered was 50 g / m 2 per side (double-sided plating).
  • T1 temperature (degreeC) was calculated
  • T1 temperature (° C.) 720 + 29 ⁇ [% Si] -21 ⁇ [% Mn] + 17 ⁇ [% Cr]
  • [% X] is the mass% of the component element X of the steel sheet, and zero for the component elements not contained.
  • T1 means Ac 1 point and T2 means Ac 3 point.
  • the high-strength cold-rolled steel sheet (CR), hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA) and electrogalvanized steel sheet (EG) obtained as described above were used as test steels. Characteristics were evaluated. The mechanical properties were evaluated by performing a tensile test and Young's modulus measurement as follows. The results are shown in Table 3. Table 3 also shows the thickness of each steel plate as the test steel.
  • the tensile test is based on JIS Z 2241 (2011) using a JIS No. 5 test piece obtained by taking a sample so that the length of the tensile test piece is perpendicular to the rolling direction of the steel sheet (C direction). And TS (tensile strength) and El (total elongation) were measured.
  • TS tensile strength
  • El total elongation
  • Young's modulus measurement is 10 mm ⁇ 50 mm from three directions, ie, the rolling direction of the steel sheet (L direction), the 45 ° direction (D direction) with respect to the rolling direction of the steel sheet, and the direction perpendicular to the rolling direction of the steel sheet (C direction).
  • the test piece was cut out and the Young's modulus was measured using a transverse vibration type resonance frequency measuring device according to the American Society to Testing Materials standard (C1259).
  • the rigidity that is, the Young's modulus is excellent when the Young's modulus in the 45 ° direction with respect to the rolling direction and the rolling direction is 205 GPa or more and the Young's modulus in the direction perpendicular to the rolling direction is 220 GPa or more. is there.
  • the average r-value measurement was taken from three directions, ie, the rolling direction (L direction) of the steel sheet, the 45 ° direction (D direction) with respect to the rolling direction of the steel sheet, and the direction perpendicular to the rolling direction of the steel sheet (C direction).
  • each plastic strain ratio r L , r D , r C was determined according to JIS Z 2254 (2008), and the average r value was calculated by the following formula.
  • Average r value (r L + 2r D + r C ) / 4 Note that the present invention is excellent in deep drawability when the average r value, which is an index of deep drawability, is 0.95 or more regardless of the strength of the steel sheet.
  • TS is 780 MPa or more, excellent in ductility, has a balance between high strength and ductility, and is excellent in rigidity and deep drawability.
  • the comparative example one or more of strength, ductility, balance between strength and ductility, rigidity, and deep drawability was inferior.
  • this invention is not limited by the description which makes a part of indication of this invention by this embodiment. That is, all other embodiments, examples, operation techniques, and the like made by those skilled in the art based on the present embodiment are all included in the technical scope of the present invention.
  • the equipment for performing the heat treatment on the steel sheet is not particularly limited.
  • the present invention it is possible to produce a high-strength steel sheet having a TS of 780 MPa or more and excellent not only in ductility but also in rigidity and also in deep drawability. Therefore, by applying the high-strength steel plate obtained by the present invention to, for example, an automobile structural member, fuel efficiency can be improved by reducing the weight of the vehicle body, and the industrial utility value is extremely large.

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Abstract

L'invention concerne une tôle d'acier à haute résistance qui a une plage de composition comprenant de 0,001 à 0,200 % de Ti et/ou de 0,001 à 0,200 % de Nb parmi d'autres constituants et ayant une structure d'acier dans laquelle de la ferrite est présente selon un rapport de surface supérieur ou égal à 20 %, la ferrite ayant une taille de grain cristallin moyenne de 10 à 20 µm, de la bainite est présente selon un rapport de surface supérieur ou égal à 5 %, de la martensite est présente selon un rapport de surface supérieur ou égal à 5 %, le rapport de surface total de bainite et de martensite est supérieur ou égal à 15 %, et de l'austénite résiduelle est présente selon un rapport volumique supérieur ou égal à 5 %. La tôle d'acier à haute résistance a une TS supérieure ou égale à 780 MPa, qui est obtenue par une texture de ferrite se présentant sous forme de microstructure ayant un rapport d'intensité inverse entre des fibres γ et des fibres α de plus de 3,0, et présente une ductilité et rigidité excellentes ainsi qu'une aptitude à l'emboutissage profond.
PCT/JP2017/040814 2016-11-15 2017-11-13 Tôle d'acier à haute résistance, procédé de production associé, et tôle d'acier galvanisée à haute résistance WO2018092735A1 (fr)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2021205943A1 (fr) * 2020-04-07 2021-10-14 日本製鉄株式会社 Tôle d'acier
EP4166685A4 (fr) * 2020-06-11 2023-11-22 Baoshan Iron & Steel Co., Ltd. Acier à ultra-haute résistance présentant une excellente plasticité et son procédé de fabrication

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008024980A (ja) * 2006-07-20 2008-02-07 Nippon Steel Corp 高強度合金化溶融亜鉛めっき鋼板及びその製造方法
WO2013047836A1 (fr) * 2011-09-30 2013-04-04 新日鐵住金株式会社 Feuille d'acier galvanisée et son procédé de fabrication
JP2015145518A (ja) * 2014-02-03 2015-08-13 Ｊｆｅスチール株式会社 高強度熱延鋼板およびその製造方法
WO2016125463A1 (fr) * 2015-02-03 2016-08-11 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance, et procédé de fabrication de celle-ci

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008024980A (ja) * 2006-07-20 2008-02-07 Nippon Steel Corp 高強度合金化溶融亜鉛めっき鋼板及びその製造方法
WO2013047836A1 (fr) * 2011-09-30 2013-04-04 新日鐵住金株式会社 Feuille d'acier galvanisée et son procédé de fabrication
JP2015145518A (ja) * 2014-02-03 2015-08-13 Ｊｆｅスチール株式会社 高強度熱延鋼板およびその製造方法
WO2016125463A1 (fr) * 2015-02-03 2016-08-11 Ｊｆｅスチール株式会社 Tôle d'acier à haute résistance, et procédé de fabrication de celle-ci

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2021205943A1 (fr) * 2020-04-07 2021-10-14 日本製鉄株式会社 Tôle d'acier
EP4134464A4 (fr) * 2020-04-07 2023-08-23 Nippon Steel Corporation Tôle d'acier
JP7425359B2 (ja) 2020-04-07 2024-01-31 日本製鉄株式会社 鋼板
EP4166685A4 (fr) * 2020-06-11 2023-11-22 Baoshan Iron & Steel Co., Ltd. Acier à ultra-haute résistance présentant une excellente plasticité et son procédé de fabrication

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