Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0436—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • 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

Definitions

• the present invention relates to a high-strength thin steel sheet excellent in rigidity suitable for use mainly in automobile bodies and a method for producing the same.
• the high-strength thin steel sheet according to the present invention is suitable for application to structural members having a column thickness sensitivity index close to 1, such as a center pillar, a side sill, a side frame and a cross member of an automobile, or a cross-sectional shape close to it. It has a tensile strength of 780 MPa or more and is excellent in ductility.
• the weight of the car body has been reduced by reducing the plate thickness by increasing the strength of the steel plate, but recently, as a result of remarkable progress in increasing the strength of the steel plate, the plate thickness is less than 1.6 mm.
• the use of new steel plates is increasing.
• the tensile strength of 780MPa class and 980MPa class steel plates is increasing year by year, and in order to reduce the weight by increasing the strength, it is indispensable to simultaneously improve the decrease in the rigidity of the parts due to the reduction in thickness. It has become to.
• the problem of reduced part rigidity due to the thinning of the steel sheet has become apparent with steel sheets having a tensile strength of 590 MPa or more.
• the Young's modulus is strongly governed by the texture, and in the case of steel with a body-centered cubic lattice, the ⁇ 111> direction, which is the closest dense direction of atoms, is the highest, and conversely, the ⁇ 100> direction where the atomic density is low is the lowest. It is known. It is well known that the Young's modulus of ordinary iron with a small anisotropy in the crystal orientation is about 210 GPa, but if the anisotropy is given to the crystal orientation and the atomic density in a specific direction can be increased. The Young's modulus in that direction can be increased.
• Patent Document 1 uses a steel in which Nb or Ti is added to an extremely low carbon steel, and the reduction rate in the temperature range of Ar 3 to (Ar 3 + 150 ° C.) is 85% or more in the hot rolling process.
• Patent Document 2 describes that Nb, Mo, and B are added to a low carbon steel having a C content of 0.02 to 0.15%, and the rolling reduction in the temperature range of Ar 3 to 950 ° C. is 50% or more.
• a method for producing a hot-rolled steel sheet with an increased Young's modulus by developing the ⁇ 211 ⁇ ⁇ 011> orientation is disclosed.
• Patent Documents 3 and 4 use steel obtained by adding Nb to low carbon steel, specify the amount of C that is not fixed as carbonitride, and set the total reduction amount at 950 ° C. or lower in the hot rolling process to 30%. % Or more, by promoting ferrite transformation from unrecrystallized austenite, the ferrite of ⁇ 113 ⁇ ⁇ 110> orientation is developed in the hot-rolled sheet stage, and then ⁇ 112 ⁇ ⁇ A technique for increasing the Young's modulus in the direction perpendicular to the rolling direction with 110> as the main orientation is disclosed.
• Patent Document 1 uses an extremely low carbon steel having a C content of 0.01% or less and controls the texture to increase the Young's modulus of the steel sheet, but the tensile strength obtained is 450 MPa at most. However, there was a limit to further increase the strength by applying this technology.
• Patent Document 2 The technique disclosed in Patent Document 2 is a hot-rolled steel sheet, so that it is not possible to use texture control by cold working, and it is difficult to achieve higher Young's modulus, There is a problem that it is difficult to stably produce a high-strength steel plate having a thickness of less than 2.0 mm by low-temperature finish rolling.
• Patent Document 3 increases the tensile strength by increasing the alloy addition amount and increasing the martensite fraction, but the total elongation is low, and the strength-elongation balance (TS ⁇ El ) Also decreases, it was difficult to improve the workability along with the increase in strength.
• the techniques disclosed in Patent Documents 3 and 4 increase the Young's modulus by setting the total reduction amount at 950 ° C. or lower to 30% or higher in the hot rolling process. There was a problem that it was difficult to ensure a total reduction amount of 30% or more due to the high load.
• the conventional technology is intended to increase the Young's modulus for hot-rolled steel sheets and soft steel sheets with a large thickness, and even high-strength materials have poor ductility and are difficult to manufacture. Therefore, it has been difficult to increase the Young's modulus while maintaining high ductility in a high-strength steel sheet having a plate thickness of 1.6 mm or less and a TS of 780 MPa or more using such a conventional technique.
• the present invention has solved the above problems, and even when the plate thickness is 1.6 mm or less, the tensile strength in the direction perpendicular to the rolling is 780 MPa or higher, more preferably 980 MPa or higher, and the Young's modulus in the direction perpendicular to the rolling is high.
• the purpose of this paper is to propose a high-strength steel sheet with excellent rigidity that satisfies 240 GPa and more, together with its advantageous manufacturing method.
• the Young's modulus of steel depends greatly on the texture, and in the case of plain steel with a body-centered cubic lattice, it is high in the ⁇ 111> direction, which is the close-packed direction of atoms, and conversely, the atomic density is small ⁇ 100> Since the (112) [1-10] orientation is developed, the ⁇ 111> direction is aligned in the direction perpendicular to the rolling direction of the steel sheet, so that the Young's modulus in this direction can be increased.
• DP steel obtained by strengthening a soft ferrite phase with a hard martensite phase is known to have generally good ductility.
• the volume ratio of the martensite phase tends to increase as a whole, which is effective not only in reducing ductility but also in increasing the Young's modulus in the direction perpendicular to the rolling (112) [ 1-10] It was difficult to develop the direction.
• the inventors examined the Young's modulus in the direction perpendicular to the rolling direction in a high-strength thin steel sheet having a TS of 780 MPa or more in order to solve the above-mentioned problems.
• the strengthening it becomes possible to keep the volume ratio of martensite low even at an ultra-high strength of TS of 780 MPa or more, and by increasing the integration of ferrite into (112) [1-10] It was found that both high strength and high rigidity can be achieved while being ductile.
• the present invention is based on the above findings.
• the gist configuration of the present invention is as follows. 1. In mass%, C: 0.06 to 0.12%, Si: 0.5 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, and Ti : 0.02 to 0.20%, satisfying the relationship shown in the following formulas (1) and (2), the balance has a composition consisting of Fe and inevitable impurities, By area ratio, ferrite phase: 60% or more, martensite phase: 15-35%, and the total of ferrite phase and martensite phase is 95% or more, and the average grain size of ferrite is 4.0 ⁇ m or less.
• the steel sheet further comprises, in mass%, Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Cu: 0.1 to 2.0%, and B: 0.0005 to 0.0030%. 3.
• C 0.06 to 0.12%
• Si 0.5 to 1.5%
• Mn 1.0 to 3.0%
• P 0.05% or less
• S 0.01% or less
• Al 0.5% or less
• N 0.01% or less
• Ti 0.02 to 0.20% steel
• the contents of C, N, S and Ti satisfy the relationship shown in the following formulas (1) and (2), with the balance being the composition of Fe and inevitable impurities
• the hot rolling process after finishing rolling at 850-950 ° C in the hot rolling process, the material is wound at 650 ° C or lower, pickled, and then cold rolled at a reduction rate of 60% or higher, and then in the annealing process.
• (Ac 1 -100 ° C) to Ac 1 average heating rate heated to a soaking temperature of 780 to 880 ° C at a rate of 15 ° C / s or more, and held at the soaking temperature for 150 s or less
• a method for producing a thin steel sheet wherein the steel sheet is cooled to 350 ° C. or lower at an average cooling rate of at least 350 ° C. at 5 to 50 ° C./s.
• the steel material further includes, in mass%, Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Cu: 0.1 to 2.0%, and B: 0.0005 to 0.0030%
• the manufacturing method of the thin steel plate of said 4 or 5 containing 1 type or 2 types or more selected from among.
• the tensile strength is 780 MPa or more, more preferably 980 MPa or more
• the Young's modulus in the direction perpendicular to the rolling is 240 GPa or more, more preferably 245 GPa or more
• TS ⁇ El satisfies 16500 MPa ⁇ % or more.
• C 0.06-0.12% C is an element that stabilizes austenite, and in the cooling process during annealing after cold rolling, it enhances hardenability and greatly promotes the generation of low-temperature transformation phase, which can greatly contribute to high strength. it can.
• the C content needs to be 0.06% or more. More preferably, it is 0.08% or more.
• the C content exceeds 0.12%, the fraction of the hard low-temperature transformation phase becomes large, and not only the steel is extremely strengthened but also the workability is deteriorated.
• a large amount of C content suppresses recrystallization in an orientation advantageous for increasing the Young's modulus in the annealing process after cold rolling.
• a large amount of C content causes deterioration of weldability. For this reason, C content needs to be 0.12% or less.
• Si 0.5-1.5% Si is one of the important elements in the present invention. Since Si raises the Ar 3 transformation point in hot rolling, recrystallization of processed austenite is promoted when rolling immediately above Ar 3 . For this reason, when a large amount of Si exceeding 1.5% is contained, the crystal orientation necessary for increasing the Young's modulus cannot be obtained. In addition, the addition of a large amount of Si not only deteriorates the weldability of the steel sheet but also promotes the generation of firelite on the surface of the slab during heating in the hot rolling process, thereby promoting the generation of a surface pattern called the so-called red scale. To do.
• the Si oxide produced on the surface deteriorates the chemical conversion treatment property, and when used as a hot-dip galvanized steel sheet, the Si oxide produced on the surface is not present. Induces plating. Therefore, the Si content needs to be 1.5% or less. In the case of a steel sheet or hot dip galvanized steel sheet that requires surface properties, the Si content is preferably 1.2% or less.
• Si is an element that stabilizes ferrite, and in the cooling process after two-phase region soaking in the annealing process after cold rolling, promotes ferrite transformation and concentrates C in austenite. Austenite can be stabilized and the generation of a low temperature transformation phase can be promoted. Further, Si can increase the strength of steel by solid solution strengthening. In order to obtain such an effect, the Si content needs to be 0.5% or more. Preferably it is 0.7% or more.
• Mn 1.0-3.0% Mn is also one of the important elements in the present invention.
• Mn is an austenite stabilizing element that lowers the Ac 1 transformation point in the heating process in the annealing process after cold rolling, promotes the austenite transformation from unrecrystallized ferrite, and forms in the cooling process after soaking.
• an orientation that is advantageous for improving the Young's modulus can be developed, and a decrease in Young's modulus associated with the generation of the low-temperature transformation phase can be suppressed.
• Mn can greatly contribute to high strength by enhancing the hardenability and greatly promoting the generation of the low temperature transformation phase in the cooling process after soaking in the annealing process.
• the Mn content needs to be 1.0% or more.
• a large Mn content exceeding 3.0% remarkably suppresses the formation of ferrite during cooling after annealing, and a large Mn content also deteriorates the weldability of the steel sheet. Therefore, the Mn content is 3.0% or less. More preferably, it is 2.5% or less.
• P 0.05% or less P not only segregates at the grain boundaries to lower the ductility and toughness of the steel sheet, but also deteriorates the weldability. Moreover, when using it as an alloying hot-dip galvanized steel plate, the problem which delays alloying speed arises. Therefore, the P content is set to 0.05% or less.
• S 0.01% or less S significantly reduces the ductility in hot rolling, induces hot cracking, and significantly deteriorates the surface properties.
• S is desirably reduced as much as possible because it forms coarse MnS as an impurity element, thereby reducing ductility and hole expansibility. Since these problems become significant when the S content exceeds 0.01%, the S content is set to 0.01% or less.
• the S amount is preferably 0.005% or less.
• Al 0.5% or less
• Al is a ferrite stabilizing element and greatly increases the Ac 3 point during annealing. Therefore, by suppressing the austenite transformation from unrecrystallized ferrite, ferrite is generated from austenite during cooling. In doing so, it will hinder the development of orientation that is advantageous to the Young's modulus. For this reason, the Al content is set to 0.5% or less. Preferably it is 0.1% or less. On the other hand, since Al is useful as a deoxidizing element for steel, the Al content is preferably 0.01% or more.
• N 0.01% or less
• Ti 0.02-0.20%
• Ti is the most important element in the present invention. That is, Ti promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite in the heating process in the annealing process, and improves Young's modulus with respect to ferrite generated in the cooling process after annealing. It is possible to develop a dominant orientation. Ti fine precipitates contribute to an increase in strength, and also have an advantageous effect on the refinement of ferrite and martensite. In order to obtain such an effect, the Ti content needs to be 0.02% or more. Preferably it is 0.04% or more.
• the carbonitride cannot be completely dissolved at the time of reheating in the normal hot rolling process, and coarse carbonitride remains. And the recrystallization suppression effect is inhibited.
• the hot rolling is started after continuous casting without going through the process of reheating after once cooling the slab from continuous casting, the effect of increasing the strength when the added amount of Ti exceeds 0.20% Further, the contribution to the recrystallization suppressing effect is small, and the alloy cost is also increased. Therefore, the Ti content needs to be 0.20% or less.
• the amount of C not fixed as a carbide calculated by the formula (1) needs to be 0.10% or less. Preferably it is 0.09% or less.
• the amount of C not fixed as carbide is less than 0.05%, the amount of C in austenite decreases during annealing in the two-phase region after cold rolling, and consequently the martensite phase generated after cooling decreases. It is difficult to increase the strength. For this reason, the amount of C which is not fixed as a carbide needs to be 0.05% or more. Preferably it is 0.06% or more.
• Nb 0.02 to 0.10% Nb, like Ti, is an important element in the present invention.
• the austenite transformation from the unrecrystallized ferrite is promoted, and the coarsening of the austenite grains is suppressed, and after annealing soaking
• the fine carbon nitride of Nb contributes effectively to an increase in strength. Furthermore, it works advantageously for miniaturization of ferrite and martensite.
• the Nb content is preferably 0.02% or more.
• the carbonitride cannot be completely dissolved at the time of reheating in the normal hot rolling process, and coarse carbonitride remains, so in the hot rolling process
• the effect of suppressing recrystallization of processed austenite and the effect of suppressing recrystallization of processed ferrite in the annealing process after cold rolling cannot be obtained.
• the recrystallization suppression is as much as the amount of Nb added exceeds 0.10%. The contribution to the effect is small, and the alloy cost is also increased. Therefore, the Nb content is preferably 0.10% or less. More preferably, it is 0.08% or less.
• Nb When Nb is contained in addition to Ti, the following relational expression (3) is satisfied instead of the above expression (1).
• Nb reduces the amount of C that is not fixed as carbide by forming carbide. Therefore, in order to set the amount of C not fixed as carbide to 0.05 to 0.10%, when Nb is added, [% C] ⁇ (12 / 92.9) ⁇ [% Nb] ⁇ (12 / 47.9) ⁇ [% Ti Set the value of * ] to 0.05-0.10%. Preferably it is 0.06 to 0.09%.
• Cr 0.1-1.0% Cr is an element that improves hardenability by suppressing the formation of cementite, and has the effect of greatly promoting the formation of martensite phase in the cooling process after soaking in the annealing process. In order to acquire this effect, it is preferable to contain Cr 0.1% or more. On the other hand, addition of a large amount of Cr not only saturates the effect, but also increases the alloy cost, so Cr is preferably added at 1.0% or less. Further, when used as a hot dip galvanized steel sheet, the Cr content is preferably 0.5% or less because the Cr oxide formed on the surface induces non-plating.
• Ni 0.1-1.0%
• Ni is an element that enhances hardenability and can promote the formation of a martensite phase in the cooling process after soaking in the annealing process.
• Ni also contributes effectively to increasing the strength of steel as a solid solution strengthening element.
• the Ni content is preferably 0.1% or more.
• addition of a large amount of Ni hinders the formation of ferrite necessary for increasing the Young's modulus in the cooling process after soaking, and increases the alloy cost. Therefore, Ni is preferably contained at 1.0% or less.
• Mo 0.1-1.0%
• Mo is an element that enhances hardenability, and can contribute to high strength by promoting the formation of martensite phase in the cooling process after soaking in the annealing process.
• the Mo content is preferably 0.1% or more.
• Mo is preferably contained at 1.0% or less. More preferably, it is 0.5% or less.
• Cu 0.1-2.0%
• Cu is an element that enhances hardenability, and contributes to high strength by promoting the formation of martensite phase in the cooling process after soaking in the annealing process.
• the Cu content is preferably 0.1% or more.
• excessive Cu addition reduces hot ductility and induces surface defects accompanying cracks during hot rolling, so the Cu content is preferably 2.0% or less.
• B 0.0005-0.0030%
• B is an element that enhances hardenability by suppressing transformation from austenite to ferrite, and contributes to high strength by promoting the formation of martensite in the cooling process after soaking in the annealing process.
• the B content is preferably 0.0005% or more.
• excessive addition of B significantly inhibits the formation of ferrite during cooling after soaking, and lowers the Young's modulus. Therefore, it is preferably contained at 0.0030% or less.
• the steel sheet of the present invention has a structure having a ferrite phase as a main phase, a ferrite phase having an area ratio of 60% or more, and a martensite phase at 15 to 35%. Since the ferrite phase is effective for the development of a texture that is advantageous for improving the Young's modulus, the area ratio needs to be 60% or more. In addition, the inclusion of the martensite phase improves the strength and strength-elongation balance, and therefore it is necessary to include a martensite phase with an area ratio of 15% or more.
• the area ratio of the martensite phase if the area ratio of the martensite phase exceeds 35%, the Young's modulus in the direction perpendicular to the rolling cannot be secured, so the area ratio of the martensite phase needs to be 35% or less. Further, in order to improve the strength-elongation balance, the total area ratio of the ferrite phase and the area ratio of the martensite phase needs to be 95% or more.
• phases other than the ferrite phase and the martensite phase include pearlite, bainite, and cementite. However, these phases may be included if they are 5% or less. Preferably it is 3% or less, more preferably 1% or less.
• the average grain size of ferrite needs to be 4.0 ⁇ m or less.
• the thickness is preferably 3.5 ⁇ m or less.
• the average particle size of martensite exceeds 1.5 ⁇ m, the voids are more easily connected when subjected to processing and deformation, resulting in a decrease in the ductility of the steel sheet.
• the area ratio of the ferrite phase and martensite phase was measured by scanning electron microscope (SEM) observation after taking a nital corrosion of the cross section of the steel sheet, and taking three photos of the 25 ⁇ m ⁇ 30 ⁇ m region. Then, it was determined by measuring the areas of the ferrite phase and the martensite phase. The average particle size was determined by dividing the sum of the areas of the ferrite phase and martensite phase in the field of view from the SEM photograph by the number of the phases, and obtaining the average area, which was a value of the 1/2 power.
• the tensile strength (TS) in the direction perpendicular to the rolling direction is 780 MPa or more
• the Young's modulus is 240 a GPa or more
• the strength-elongation balance (TS ⁇ El) is 16500 MPa ⁇ % or more.
• steel of chemical composition according to the above-described composition is melted according to the target strength level.
• a melting method a normal converter method, an electric furnace method, or the like can be applied as appropriate.
• the molten steel is cast into a slab and then heated as it is or after being cooled, and hot-rolled at a finishing temperature of 850 to 950 ° C. Next, it is wound at 650 ° C. or lower, pickled, and then cold-rolled at a rolling reduction of 60% or more.
• the temperature range from (Ac 1 -100 ° C.) to Ac 1 is heated at an average rate of temperature rise of 15 ° C./s or more and at a soaking temperature of 780 to 880 ° C. for 150 s or less
• the average cooling rate up to at least 350 ° C. is set to 5 to 50 ° C./s and then cooled down to 350 ° C. or lower.
• Winding temperature 650 ° C or less
• the coiling temperature after finish rolling exceeds 650 ° C.
• Ti and Nb carbonitrides become coarse, and in the heating stage in the annealing process after cold rolling, the effect of suppressing recrystallization of ferrite and austenite
• the coiling temperature is set to 650 ° C. or lower because the effect of suppressing the coarsening of the grains is reduced.
• the coiling temperature is lower than 400 ° C, a lot of hard low-temperature transformation phase is generated, the deformation in the subsequent cold rolling becomes non-uniform, and the accumulation in the direction advantageous for Young's modulus is hindered.
• the texture after annealing does not develop and it is difficult to improve the Young's modulus.
• the winding temperature is preferably 400 ° C. or higher.
• the rolling reduction during cold rolling needs to be 60% or more. More preferably, it is 65% or more.
• the upper limit of the rolling reduction during cold rolling is preferably 85%.
• the soaking time needs to be 150 s or less.
• the soaking time is preferably set to 15 seconds or more.
• a process of passing the overaging zone may be performed.
• you may let it pass in hot dip zinc and when manufacturing as an alloying hot dip galvanized steel plate, you may perform an alloying process.
• you may perform temper rolling for the shape adjustment of a steel plate and if an elongation rate is 0.8% or less, there will be no big change in a Young's modulus and a tensile characteristic. Preferably it is 0.6% or less.
• Example 1 steel A having the composition shown in Table 1 was melted in a vacuum melting furnace, hot-rolled, pickled, cold-rolled, and then annealed to produce a cold-rolled steel sheet.
• heating conditions prior to hot rolling 1 hour at 1250 ° C, finishing temperature of hot rolling: 880 ° C, plate thickness after hot rolling: 4.4 mm, winding conditions: furnace after holding at 600 ° C for 1 hour winding corresponding processing of cooling, the reduction ratio of cold rolling: 68%, thickness after cold rolling: 1.4 mm, average heating rate from (Ac 1 -100 °C) to Ac 1: 20 °C / s, Soaking temperature: Holding time at 830 ° C .: 60 s, average cooling rate up to 300 ° C .: 15 ° C./s, then cooling to room temperature: air cooling was the basic condition. Table 2 shows these basic conditions.
• a 10 mm x 50 mm test piece was cut out from a direction perpendicular to the rolling direction of the steel sheet, and a Young's modulus (Ec) according to the American Society to Testing Materials standard (C1259) using a transverse vibration type resonance frequency measuring device. ) was measured. Further, a JIS No. 5 tensile test piece was cut out from a cold rolled steel sheet subjected to temper rolling with an elongation of 0.5% and perpendicular to the rolling direction, and tensile properties (tensile strength TS and elongation El) were measured. The area ratio ( ⁇ ) of the ferrite phase, the area ratio (M) of the martensite phase, and the average crystal grain size of each phase were determined by the method described above. The obtained results are shown in Tables 2 and 3.
• the cold-rolled steel sheet (steel sheet: A1) produced according to the basic conditions was TS: 1064 MPa, El: 16.3%, TS ⁇ El: 17343 MPa ⁇ %, Ec: 252 GPa, ferrite area ratio: 67 %, Martensite area ratio: 33%, ferrite grain size: 2.9 ⁇ m, martensite grain size: 0.8 ⁇ m, a good strength-elongation balance and a high Young's modulus.
• TS is 780 MPa or more
• Example 2 Further, steels B to N having the components shown in Table 4 were melted in a vacuum melting furnace, and hot rolling, pickling, cold rolling and annealing were sequentially performed under the conditions shown in Table 5. The cold rolled steel sheet thus obtained was examined in the same manner as in Example 1. The obtained results are also shown in Table 5.
• a thin steel sheet having a high strength such as a tensile strength of 780 MPa or more and a Young's modulus of 240 GPa or more.

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