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 having excellent rigidity that is preferably and mainly used for automobile body parts, and a method for manufacturing the same.
• the high-strength thin steel sheet of the present invention which is preferably applicable as structural members having a columnar or nearly columnar cross-sectional shape with a rigid sensitivity index of sheet thickness of approximately 1, such as center pillars, side sills, side frames and cross members of automobiles, has a tensile strength of 780 MPa or higher and shows excellent ductility.
• Young's modulus of members used for these parts In the case of steel having body-centered cubic lattice, it is known that Young's modulus, which is strongly dependent on texture, has the highest value in ⁇ 111> direction in which atoms are most densely packed, while having the smallest value in ⁇ 100> direction in which atoms are less dense. It is widely known that Young's modulus of normal iron which is less anisotropic in crystal orientation is approximately 210 GPa. However, if the crystal orientation is anisotropic and the atomic density can be increased in a particular direction, Young's modulus can be increased in that direction.
• JP 5-255804 A discloses a technique that uses steel resulting from adding Nb or Ti to ultra low carbon steel and involves controlling, in a hot rolling step, the rolling reduction ratio to be 85 % or more in a temperature range of Ar 3 to (Ar 3 + 150 °C) and thereby facilitating transformation of non-recrystallized austenite to ferrite, so that ferrite in ⁇ 311 ⁇ 011> and ⁇ 332 ⁇ 113> orientations is allowed to grow at the stage of hot-rolled sheet, and Young's modulus is increased in a direction perpendicular to the rolling direction through the subsequent cold rolling and recrystallization annealing whereby ⁇ 211 ⁇ 011> orientation is made into the primary orientation.
• JP 8-311541 A discloses a method for manufacturing a hot-rolled steel sheet with an increased Young's modulus by adding Nb, Mo and B to low carbon steel having C content of 0.02 % to 0.15 % and controlling the rolling reduction ratio to be 50 % or more in a temperature range of Ar 3 to 950 °C, thereby causing growth in ⁇ 211 ⁇ 011> orientation.
• JP 2006-183131 A (PTL 3) and JP 2005-314792 A (PTL4) disclose techniques that use steel resulting from adding Nb to low carbon steel, define the content of C that is not fixed as carbonitride, and involve controlling, in a hot rolling step, the total rolling reduction ratio to be 30 % or more at 950 °C or lower to facilitate transformation of non-recrystallized austenite to ferrite so that ferrite in ⁇ 113 ⁇ 110> orientation is allowed to grow at the stage of hot-rolled sheet and Young's modulus is increased in a direction perpendicular to the rolling direction through the subsequent cold rolling and recrystallization annealing whereby ⁇ 112 ⁇ 110> orientation is made into the primary orientation.
• the technique disclosed in PTL 2 has a problem that it cannot utilize texture control by cold working since the target steel sheet is a hot-rolled steel sheet, where it is difficult not only to achieve even higher Young's modulus, but also to manufacture such a high strength steel sheet that has a sheet thickness of less than 2.0 mm in a stable manner by low temperature finish rolling.
• the conventional techniques are directed to increasing Young's modulus of steel sheets, such as hot-rolled steel sheets or mild steel sheets, having a large sheet thickness, materials having high strength but poor ductility, or materials difficult to produce.
• steel sheets such as hot-rolled steel sheets or mild steel sheets
• An object of the present invention is to provide a high-strength thin steel sheet having excellent rigidity that has a sheet thickness as small as 1.6 mm or less, but a tensile strength as high as 780 MPa or higher, more preferably 980 MPa or higher, in a transverse direction perpendicular to the rolling direction (hereinafter, also referred to as the "transverse direction"), and satisfies a condition that Young's modulus in the transverse direction is 240 GPa or higher, as well as an advantageous method for manufacturing the same.
• C is an element that stabilizes austenite and may improve quench hardenability and greatly facilitate formation of a low temperature transformation phase during a cooling step at the time of annealing after cold rolling, thereby making a significant contribution to enhancement of strength.
• C content needs be 0.06 % or more, more preferably 0.08 % or more.
• C content exceeding 0.12 % leads to an increased volume fraction of a hard, low temperature transformation phase, which results in not only an excessive increase in strength of steel, but also a deterioration in workability.
• such high C content inhibits recrystallization in an orientation in which Young's modulus is advantageously improved in an annealing process after cold rolling. Further, such high C content also leads to a deterioration in weldability.
• C content should be not more than 0.12 %.
• Si is one of the important elements in the present invention. Since Si raises the Ar 3 transformation point in hot rolling, it facilitates recrystallization of worked austenite when rolling is performed at a temperature immediately above Ar 3 . Thus, if Si content is excessively high exceeding 1.5 %, a crystal orientation necessary for increasing Young's modulus can no longer be obtained. Moreover, addition of a large amount of Si not only deteriorates weldability of a steel sheet, but also advances formation of fayalite on a surface of a slab during heating in a hot rolling process, thereby facilitating the occurrence of a surface pattern, which is referred to as so-called red scales.
• Si content should be not more than 1.5 %.
• Si content is preferably 1.2 % or less.
• Si is an element that stabilizes ferrite and is able to stabilize austenite and facilitate formation of a low temperature transformation phase by facilitating transformation to ferrite and concentrating C in austenite during a cooling step subsequent to soaking in two-phase region in an annealing process after cold rolling. Further, Si may enhance the strength of steel by solid solution strengthening. To obtain this effect, Si content should be 0.5 % or more, preferably 0.7 % or more.
• Mn is also one of the important elements in the present invention.
• Mn is an austenite-stabilizing element that may, during a heating step in an annealing process after cold rolling, lower the Ac 1 transformation point, facilitate transformation of non-recrystallized ferrite to austenite, and allow a low temperature transformation phase that is formed during a cooling step after soaking to grow in an orientation in which Young's modulus is advantageously improved, thereby inhibiting a decrease in Young's modulus associated with the formation of the low temperature transformation phase.
• Mn may also improve quench hardenability and greatly facilitate formation of a low temperature transformation phase during a cooling step after soaking annealing in an annealing process, thereby making a significant contribution to enhancement of strength. Further, Mn acts as a solid-solution-strengthening element, which also contributes to enhancement of strength of steel. To obtain this effect, Mn content should be 1.0 % or more.
• Mn content is to be 3.0 % or less, more preferably 2.5 % or less.
• P is an element that segregates at grain boundaries, which results in a deterioration in not only ductility and toughness, but also in weldability of a steel sheet. In addition, P causes an inconvenience that alloying is delayed when the steel sheet is used as a hot-dip galvannealed steel sheet. Thus, P content is to be 0.05 % or less.
• S is an element that significantly reduces ductility in hot rolling to induce hot cracking, and severely deteriorates surface texture.
• it is desirable to minimize S content because S deteriorates ductility and hole expansion formability by forming coarse MnS as an impurity element. These problems become more pronounced when S content exceeds 0.01 %.
• S content is to be 0.01 % or less. From the viewpoint of improvement of particularly hole expansion formability, S content is preferably 0.005 % or less.
• Al is a ferrite-stabilizing element that significantly raises the Ac 3 point in annealing and thus inhibits transformation of non-recrystallized ferrite to austenite, thereby interfering with the growth in an orientation in which Young's modulus is advantageously improved when ferrite is generated from austenite during cooling.
• Al content is to be 0.5 % or less, preferably 0.1 % or less.
• Al content is preferably 0.01 % or more.
• N content brings about slab cracking during hot rolling and may cause surface defects.
• N content should be 0.01 % or less.
• Ti is the most important element in the present invention. That is, Ti inhibits recrystallization of worked ferrite during a heating step in an annealing process so that transformation of non-recrystallized ferrite to austenite is facilitated, while allowing growth of ferrite, which is generated during a cooling step after annealing, in an orientation in which Young's modulus is advantageously improved.
• fine precipitates of Ti contribute to enhancement of strength, and furthermore, have an advantageous effect on refinement of ferrite and martensite. To obtain this effect, Ti content should be 0.02 % or more, preferably 0.04 % or more.
• the amount of C that is not fixed as carbide should be not more than 0.10 %, preferably not more than 0.09 %.
• the amount of C that is not fixed as carbide should be not less than 0.05 %, preferably not less than 0.06 %.
• the present invention may also contain the following elements as appropriate.
• Nb is also an important element of the present invention.
• Nb inhibits recrystallization of worked ferrite during a heating step in an annealing process after cold rolling so that transformation of non-recrystallized ferrite to austenite is facilitated and coarsening of austenite grains is inhibited, while allowing growth of ferrite, which is generated during a cooling step after annealing soaking, in an orientation in which Young's modulus is advantageously improved.
• fine carbonitrides of Nb effectively contribute to enhancement of strength, and furthermore, have an advantageous effect on refinement of ferrite and martensite.
• Nb content is preferably 0.02 % or more.
• Nb content is preferably not more than 0.10 %, more preferably not more than 0.08 %.
• Nb is also contained along with Ti
• the value of ([%C] - (12/92.9) x [%Nb] - (12/47.9) ⁇ [%Ti*]) is controlled within a range of 0.05 % to 0.10 %, preferably 0.06 % to 0.09 %.
• Cr is an element that inhibits formation of cementite, thereby improving quench hardenability. Cr has an effect of greatly facilitating formation of martensite phase during a cooling step after soaking in an annealing process. To obtain this effect, Cr content is preferably 0.1 % or more. However, if a large amount of Cr is added, the effect attained by the addition will be saturated and alloy cost will also increase. Thus, Cr is preferably added in an amount of 1.0 % or less. In addition, when a steel sheet is used as a hot-dip galvanized steel sheet, oxides of Cr formed on a surface of the steel sheet induces absence of zinc coating. Thus, Cr content is preferably 0.5 % or less.
• Ni is an element that improves quench hardenability and may facilitate formation of martensite phase during a cooling step after soaking in an annealing process.
• Ni effectively contributes to enhancement of strength of steel as a solid-solution-strengthening element.
• surface defects are induced during hot rolling due to cracking associated with a reduction in hot ductility.
• Ni content is preferably 0.1 % or more.
• addition of a large amount of Ni interferes with formation of ferrite, which is necessary for increasing Young's modulus, during a cooling step after soaking, and furthermore, results in an increase in alloy cost.
• Ni content is preferably 1.0 % or less.
• Mo is an element that improves quench hardenability and may facilitate formation of martensite phase during a cooling step after soaking in an annealing process, thereby contributing to enhancement of strength.
• Mo content is preferably 0.1 % or more. However, if a large amount of Mo is added, the effect attained by the addition will be saturated at some point and alloy cost will also increase. Thus, Mo content is preferably 1.0 % or less, more preferably 0.5 % or less.
• Cu is an element that improves quench hardenability and facilitates formation of martensite phase during a cooling step after soaking in an annealing process, thereby contributing to enhancement of strength.
• Cu content is preferably 0.1 % or more.
• excessive addition of Cu deteriorates hot ductility and induces surface defects associated with cracking during hot rolling.
• Cu content is preferably 2.0 % or less.
• B is an element that improves quench hardenability by inhibiting transformation from austenite to ferrite and facilitates formation of martensite during a cooling step after soaking in an annealing process, thereby contributing to enhancement of strength.
• B content is preferably 0.0005 % or more.
• B content is preferably 0.0030 % or less.
• the steel sheet of the present invention has a microstructure in which ferrite phase is the primary phase, including, in area ratio, 60 % or more of ferrite phase and 15 % to 35 % of martensite phase.
• the area ratio of ferrite phase should be 60 % or more since ferrite phase is effective in causing growth of texture which is advantageous for improving Young's modulus.
• the area ratio of martensite phase should be 15 % or more.
• the area ratio of martensite phase should be not more than 35 %.
• a total of the area ratios of ferrite phase and martensite phase should be 95 % or more.
• Phases other than the ferrite phase and the martensite phase may include pearlite, bainite and cementite phases, which are not problematic if contained in an amount of not more than 5 %, preferably not more than 3 %, more preferably not more than 1 %.
• average grain size of ferrite exceeding 4.0 ⁇ m leads to a reduction in strength, which necessitates increasing the volume fraction of martensite phase and adding more elements, and results in a decrease in Young's modulus and an increase in manufacturing cost.
• average grain size of ferrite should be 4.0 ⁇ m or less.
• the average grain size of ferrite is preferably 3.5 ⁇ m or less.
• an average grain size of martensite exceeding 1.5 ⁇ m increases the potential of progress in void linking upon working/deformation, which results in a reduction in ductility of the steel sheet.
• the average grain size of martensite should be not more than 1.5 ⁇ m, more preferably not more than 1.0 p,m.
• Area ratios of ferrite phase and martensite phase were determined by subjecting a cross-section of the steel sheet to nital etching, observing the cross-section with scanning electron microscope (SEM), taking three images of 25 ⁇ m ⁇ 30 ⁇ m regions, analyzing these images by image processing and measuring the areas of ferrite phase and martensite phase.
• SEM scanning electron microscope
• the average grain size was calculated by dividing a total of respective areas of ferrite phase and martensite phase within the field of view by the number of grains in these phases to determine an average area of the grains, the value of which average area is then raised to the power of 1/2.
• steel having a chemical composition in accordance with the above-described composition is prepared by steelmaking, depending on the target strength level. Any appropriate steelmaking process may be applied, such as normal converter steelmaking process or electric furnace steelmaking process.
• the steel prepared by steelmaking is cast into a slab, which in turn is directly subjected to hot rolling, or alternatively subjected to cooling and subsequent heating before hot rolling, under a condition of finisher delivery temperature of 850 °C to 950 °C to obtain a hot-rolled sheet. Then, the sheet is subjected to coiling at 650 °C or lower, followed by pickling and subsequent cold rolling at a rolling reduction ratio of 60 % or more.
• the sheet is heated at an average heating rate of 15 °C/s or higher within a temperature range of (Ac 1 - 100 °C) to Ac 1 , held at a soaking temperature of 780 °C to 880 °C for a duration of 150 seconds or less, and then cooled to 350 °C or lower at an average cooling rate until at least 350 °C of 5 °C/s to 50 °C/s.
• finisher delivery temperature By controlling finisher delivery temperature to be 950 °C or lower, transformation from non-recrystallized austenite to ferrite advances to provide fine ferrite phase, and furthermore, the degree of accumulation of the crystal grains in (112)[1-10] orientation may be increased through cold rolling and annealing. However, if the finisher delivery temperature is below 850 °C, it may more likely fall below the Ar 3 transformation point, which results in mixing hot-rolled phase with worked phase, thereby disturbing accumulation in (112)[1-10] orientation after the cold rolling and annealing. This also poses difficulty in manufacture, such as a significant increase in rolling load due to increased transformation resistance. Thus, the finisher delivery temperature should be within a range of 850 °C to 950 °C.
• the coiling temperature after the finish rolling exceeds 650 °C, carbonitrides of Ti and Nb coarsen and thus the effects of inhibiting recrystallization of ferrite and inhibiting coarsening of austenite grains are weakened during heating stage of the annealing process after the cold rolling.
• the coiling temperature is to be not higher than 650 °C.
• the coiling temperature is lower than 400 °C, many hard, low temperature transformation phases are generated, which causes non-uniform deformation in the subsequent cold rolling, thereby disturbing accumulation in an orientation in which Young's modulus is advantageously improved. This results in no growth of the texture after annealing, which makes it difficult to improve Young's modulus.
• the coiling temperature is preferably not lower than 400 °C.
• the sheet is subjected to pickling, followed by cold rolling at a rolling reduction ratio of 60 % or more.
• This cold rolling causes accumulation in (112)[1-10] orientation in which Young's modulus is effectively improved. That is, growth in (112)[1-10] orientation is caused by cold rolling to provide more ferrite grains having (112)[1-10] orientation even in the microstructure after the subsequent annealing process and improve Young's modulus.
• the rolling reduction ratio during cold rolling should be 60 % or more, more preferably 65 % or more. However, the rolling load becomes larger with higher rolling reduction ratio during cold rolling, so that manufacture becomes more difficult.
• the upper limit of the rolling reduction ratio during cold rolling is preferably 85 %.
• the soaking temperature should be 780 °C or higher.
• an excessively high soaking temperature coarsens austenite grains, which makes it difficult for ferrite, which results from the retransformation during cooling after annealing, to accumulate in (112)[1-10] orientation.
• the soaking temperature should be 880 °C or lower.
• the soaking duration should be 150 seconds or less.
• the soaking duration is preferably 15 seconds or more.
• ferrite during cooling after soaking allows for growth of texture which is advantageous for improving Young's modulus. Accordingly, ferrite is to be formed at an area ratio of 60 % or more during this cooling step. To this end, the upper limit of the cooling rate should be 50 °C/s. On the other hand, an excessively slow cooling rate hampers formation of martensite. Thus, the cooling rate should be not lower than 5 °C/s, preferably not lower than 10 °C/s.
• a high cooling stop temperature causes formation of bainite and pearlite instead of martensite, which leads to a reduction in strength and an increase in YS/TS ratio.
• the hardness of martensite is reduced by tempering during cooling and thus the contribution to enhancement of strength becomes small, which hampers provision of good TS-El balance.
• the steel sheet may be subjected to the process where the steel sheet is passed through an overaging zone.
• the steel sheet may be passed through molten zinc, or alternatively, when manufactured as a hot-dip galvannealed steel sheet, the steel sheet may be subjected to an alloying process.
• the steel sheet may be subjected to temper rolling for adjusting the shape of the steel sheet, in which case there is no significant change in Young's modulus or tensile properties if the percent elongation is not more than 0.8 %, preferably not more than 0.6 %.
• Steel A having a chemical composition as shown in Table 1 was prepared by steelmaking in a vacuum melting furnace. Then, Steel A was subjected to hot rolling, pickling, cold rolling and subsequent annealing to produce a cold-rolled steel sheet.
• the following basic conditions were set - heating condition prior to hot rolling: 1250 °C for one hour; finisher delivery temperature of hot rolling: 880 °C; sheet thickness after hot rolling: 4.4 mm; coiling condition: process corresponding to coiling where furnace cooling was conducted after a holding time of one hour at 600 °C; rolling reduction ratio during cold rolling: 68 %, sheet thickness after cold rolling: 1.4 mm, average heating rate from (Ac 1 - 100 °C) to Ac 1 : 20 °C/s, duration at soaking temperature of 830 °C: 60 seconds, average cooling rate until 300 °C: 15 °C/s, and subsequent cooling to room temperature: air cooling.
• Table 2 Table 2
• test specimens of 10 mm x 50 mm were cut from the steel sheets in a direction perpendicular to the rolling direction of the steel sheets. Then, a resonance frequency measuring device of lateral vibration type was used to measure Young's modulus (Ec) in accordance with the standard (C1259) of American Society to Testing Materials. In addition, JIS No. 5 tensile test specimens were cut from the cold-rolled steel sheets, which had been subjected to temper rolling with percent elongation of 0.5 %, in a direction perpendicular to the rolling direction for measuring their tensile properties (tensile strength TS and elongation EI).
• Step Sheet A1 A cold-rolled steel sheet (Steel Sheet A1), which was produced in accordance with the basic conditions, exhibited good strength-elongation balance and high Young's modulus, as shown in Table 2, such that TS: 1064 MPa; EI: 16.3 %; TS x EI: 17343 MPa ⁇ %; Ec: 252 GPa; area ratio of ferrite: 67 %; area ratio of martensite: 33 %; ferrite grain size: 2.9 ⁇ m; and martensite grain size: 0.8 ⁇ m.
• Step Sheets A3, A4 and A7 such as TS of 780 MPa or higher, TS x EI of 16500 or higher and Ec of 240 GPa or higher.
• Steels B to N having chemical compositions shown in Table 4 were prepared by steelmaking in a vacuum melting furnace. Then, these steels were subjected to hot rolling, pickling, cold rolling and annealing sequentially under the conditions shown in Table 5.
• each of the steel sheets (Steel Sheets B to G and L to N) obtained according to the present invention exhibited excellent properties, such as TS of 780 MPa or higher, TS x EI of 16500 or higher and Ec of 240 GPa or higher.
• Comparative Examples having chemical compositions out of an appropriate range according to the present invention are inferior in at least one of tensile strength (TS), strength-elongation balance (TS x EI) and Young's modulus (Ec).
• TS tensile strength
• TS x EI strength-elongation balance
• Ec Young's modulus
• a thin steel sheet having both high strength and high rigidity with a tensile strength of 780 MPa or higher and Young's modulus of 240 GPa or higher.

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• 2012
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