AU2005227556A1 - High-rigidity high-strength thin steel sheet and method for producing same - Google Patents

Description

VERIFICATION OF TRANSLATION RE: INTERNATIONALAPPLICATION NO. PCT/JP2005/006288 I, Hiromitsu HARA, c/o Sugimura International Patent & Trademark Agency Bureau, of No. 2-4, Kasumigaseki 3-chome, Chiyoda-ku, Tokyo, Japan, am the translator of document and I state that the following is a true translation to the best of my knowledge and belief. (Signature of Translator) 2 6' / Hiromitsu HARA (Dated) April 26, 2006 SPECIFICATION HIGH-STIFFNESS HIGH-STRENGTH THIN STEEL SHEET AND METHOD FOR PRODUCING THE SAME TECHNICAL FIELD [0001] This invention relates to a high-stiffness high-strength thin steel sheet suitable mainly as a vehicle body for automobiles and a method for producing the same. Moreover, the high-stiffness high 5 strength thin steel sheet according to the invention is a column-shaped structural member having a thickness susceptibility index of the stiffness near to 1 such as a center pillar, locker, side flame, cross member or the like of the automobile and is widely suitable for applications requiring a stiffness. 10 RELATED ART [0002] As a result of recent heightened interest in global environ ment problems, the exhaust emission control is conducted even in the automobiles, and hence the weight reduction of the vehicle body in the automobile is a very important matter. For this end, it is effective to 15 attain the weight reduction of the vehicle body by increasing the strength of the steel sheet to reduce the thickness thereof. [0003] Recently, the increase of the strength in the steel sheet is considerably advanced, and hence the use of thin steel sheets having a thickness of less than 2.0 mm is increasing. In order to further 20 reduce the weight by the increase of the strength, it is indispensable to simultaneously control the deterioration of the stiffness in parts through the thinning of the thickness. Such a problem of deteriorating the stiffness of the parts through the thinning of the thickness in the steel sheet is actualized in steel sheets having a tensile strength of not less 25 than 590 MPa, and particularly this problem is serious in steel sheets having a tensile strength of not less than 700 MPa. [0004] In general, in order to increase the stiffness of the parts, it is effective to change the shape of the parts, or to increase the number of welding points or change the welding condition such as changeover to O1;RdAHH- (1/17) laser welding or the like in the spot-welded parts. However, when these parts are used in the automobile, there are problems that it is not easy to change the shape of the parts in a limited space inside the automobile, and the change of the welding conditions causes the 5 increase of the cost and the like. [0005] Consequently, in order to increase the stiffness of the parts without changing the shape of the parts or the welding conditions, it becomes effective to increase the Young's modulus of the material used in the parts. 10 [0006] In general, the stiffness of the parts under the same shape of parts and welding conditions is represented by a product of Young's modulus of the material and geometrical moment of inertia of the part. Further, the geometrical moment of inertia can be expressed so as to be approximately proportionate to t when the thickness of the material 15 is t. In this case, k is a thickness susceptibility index and is a value of 1-3 in accordance with the shape of the parts. For example, in case of one plate shape such as panel parts for the automobile, k is a value near to 3, while in case of column-shape such as structural parts, k is a value near to 1. 20 [0007] When k of the parts is 3, if the thickness is made small by 10% while equivalently maintaining the stiffness of the parts, it is required to increase the Young's modulus of the material by 37%, while when k of the parts is 1, if the thickness is made small by 10%, it may be enough to increase the Young's modulus by 11%. 25 [0008] That is, in case of the parts having k near to 1 such as column shaped parts, it is very effective to increase the Young's modulus of the steel sheet itself for the weight reduction. Particularly, in case of steel sheets having a high strength and a small thickness, it is strongly demanded to highly increase the Young's modulus of the steel sheet. 30 [0009] In general, the Young's modulus is largely dependent upon the texture and is known to become high in a closest direction of atom. Therefore, it is effective to develop {112}<110> in order to develop an orientation advantageous for the Young's modulus of steel being a 05846HH (2/37) body-centered cubic lattice in a steel making process comprising the rolling through rolls and the heat treatment, whereby the Young's modulus can be increased in a direction perpendicular to the rolling direction. 5 [0010] There have hitherto been variously examined steel sheets by controlling the texture to increase the Young's modulus. [0011] For example, the patent article 1 discloses a technique wherein a steel obtained by adding Nb or Ti to an extremely low carbon steel is hot-rolled at a rolling reduction at Ar 3 -(Ar 3 +150 0 C) of 10 not less than 85% to promote transformation from non-crystallized austenite to ferrite to thereby render the texture of ferrite at the stage of the hot-rolled sheet into {311}<011> and {332}<113>, which is an initial orientation and is subjected to a cold rolling and a recrystalliza tion annealing to render {211}<011> into a main orientation to thereby 15 increase the Young's modulus in a direction perpendicular to the rolling direction. [0012] Also, the patent article 2 discloses a method for producing a hot rolled steel sheet having an increased Young's modulus in which Nb, Mo and B are added to a low carbon steel having a C content of 20 0.02-0.15% and the rolling reduction at Ar 3 -950 0 C is made to not less than 50% to develop [211]<011>. [0013] Further, the patent article 3 discloses a method for producing a hot rolled steel sheet having a high stiffness in which Nb is added to a low carbon steel having a C content of not more than 0.05% and a 25 finish rolling start temperature is made to not higher than 950 0 C and a finish rolling end temperature is made to (Ar 3 -50 0 C)-(Ar 3 +100 0 C) to control the development of {100} decreasing the Young's modulus. [0014] Moreover, the patent article 4 discloses a method for producing a hot rolled steel sheet in which Si and Al are added to a low 30 carbon steel having a C content of not more than 0.05% to enhance Ar 3 transformation point and the rolling reduction below Ar 3 transformation point in the hot rolling is made to not less than 60% to increase Young's modulus in a direction perpendicular to the rolling direction.

- -T [0015] Patent article 1: JP-A-H05-255804 Patent article 2: JP-A-H08-311541 Patent Article 3: JP-A-H05-247530 Patent article 4: JP-A-H09-53118 5 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED IN THE INVENTION [0016] However, the aforementioned techniques have the following problems. In the technique disclosed in the patent article 1, the Young's 10 modulus of the steel sheet is increased by using the extremely low carbon steel having a C content of not more than 0.01% to control the texture, but the tensile strength is low as about 450 MPa at most, so that there is a problem in the increase of the strength by applying this technique. 15 [0017] In the technique disclosed in the patent article 2, since the C content is as high as 0.02-0.15%, it is possible to increase the strength, but as the target steel sheet is the hot rolled steel sheet, the control of the texture through cold working can not be utilized, and hence there are problems that it is difficult to further increase the Young's modulus 20 but also it is difficult to stably produce high-strength steel sheets having a thickness of less than 2.0 mm through low-temperature finish rolling. [0018] Also, the technique disclosed in the patent article 3 is the production of the hot rolled steel sheet, so that it has the same 25 problems as mentioned above. [0019] Further, in the technique disclosed in the patent article 4, the crystal grains are coarsened by conducting the rolling at the ferrite zone, so that there is a problem that the workability is considerably deteriorated. 30 [0020] Thus, the increase of the Young's modulus in the steel sheet by the conventional techniques is targeted to hot rolled steel sheets having a thick thickness or soft steel sheets, so that it is difficult to increase the Young's modulus of high-strength thin steel sheet having a 05846HH (4/37) thickness of not more than 2.0 mm by using the above conventional techniques. [0021] As a strengthening mechanism for increasing the tensile strength of the steel sheet to not less than 590 MPa, there are mainly a 5 precipitation strengthening mechanism and a transformation texture strengthening mechanism. [0022] When the precipitation strengthening mechanism is used as the strengthening mechanism, it is possible to increase the strength while suppressing the lowering of the Young's modulus of the steel 10 sheet as far as possible, but the following difficulty is accompanied. That is, when utilizing the precipitation strengthening mechanism for finely precipitating, for example, a carbonitride of Ti, Nb or the like, in the hot rolled steel sheet, the increase of the strength is attained by conducting the fine precipitation in the coiling after the hot rolling, 15 but in the cold rolled steel sheet, the coarsening of the precipitate can not be avoided at the step of recrystallization annealing after the cold rolling and it is difficult to increase the strength through the precipitation strengthening. [0023] When utilizing the transformation texture strengthening 20 mechanism as the strengthening mechanism, there is a problem that the Young's modulus of the steel sheet lowers due to strain included in a low-temperature transformation phase such as bainite phase, martensite phase or the like. [0024] It is, therefore, an object of the invention to solve the above 25 problems and to provide a high-stiffness high-strength thin steel sheet having a tensile strength of not less than 590 MPa, preferably not less than 700 MPa, a Young's modulus of not less than 225 GPa, preferably not less than 230 GPa, more preferably not less than 240 GPa and a thickness of not more than 2.0 mm as well as an advantageous method 30 for producing the same. MEANS FOR SOLVING PROBLEMS [0025] In order to achieve the above object, the gist and construction of the invention are as follows. 05846HH (5/37) - U (I) A high-stiffness high-strength thin steel sheet comprising C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.5-4.0%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more than 0.01% and Nb: 0.02-0.40% as mass%, provided that C, N 5 and Nb contents satisfy the relationships of the following equations (1) and (2): 0.01 sC+(12/14)xN-(12/92.9)xNb s 0.06 ..... . (1) N - (14/92.9)x(Nb-0.01) ..... (2) and the remainder being substantially iron and inevitable impurities, 10 and having a texture comprising a ferrite phase as a main phase and having a martensite phase at an area ratio of not less than 1%, and having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 225 GPa. [0026] (II) A high-stiffness high-strength thin steel sheet 15 according to the item (I), which further contains one or two of Ti: 0.01-0.50% and V: 0.01-0.50% as mass% in addition to the above composition and satisfy the relationships of the following equations (3) and (4) instead of the equations (1) and (2): 0.01 r C+(12/14)xN*-(12/92.9)x 20 Nb-(12/47.9)xTi*-(12/50.9)xV - 0.06 ..... . (3) N* (14/92.9)x(Nb-0.01) ..... . (4) provided that N* in the equations (3) and (4) is N* = N-(14/47.9)xTi at N-(14/47.9)xTi > 0 and N* = 0 at N-(14/47.9)xTi s 0, and Ti* in the equation (3) is Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS at Ti-(47.9/14)xN 25 (47.9/32.1)xS > 0 and Ti* = 0 at Ti-(47.9/14)xN-(47.9/32.1)xS 0. [0027] (III) A high-stiffness high-strength thin steel sheet according to the item (I) or (II), which further contains one or more of Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% as mass% in addition to the above composition. 30 [0028] (IV) A method for producing a high-stiffness high-strength thin steel sheet comprising subjecting a starting material of steel comprising C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.5-4.0%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, 05846HH (6/37) - I N: not more than 0.01% and Nb: 0.02-0.40% as mass%, provided that C, N and Nb contents satisfy the relationships of the following equations (1) and (2): 0.01 :C+(12/14)xN-(12/92.9)xNb s 0.06 ..... . (1) 5 N r (14/92.9)x(Nb-0.01) ..... (2) to a hot rolling step under conditions that a total rolling reduction below 950°C is not less than 30% and a finish rolling is terminated at Ar 3 -900 0 C, coiling the hot rolled sheet below 650 0 C, pickling, subjecting to a cold rolling at a rolling reduction of not less than 50%, 10 raising a temperature to 780-900 0 C at a temperature rising rate from 500 0 C of 1-40°C/s to conduct soaking, and then cooling at a cooling rate up to 500 0 C of not less than 5C/s to conduct annealing. [0029] (V) A method for producing a high-stiffness high-strength thin steel sheet according to the item (IV), wherein the starting 15 material of steel further contains one or two of Ti: 0.01-0.50% and V: 0.01-0.50% as mass% in addition to the above composition and satisfies the relationships of the following equations (3) and (4) instead of the equations (1) and (2): 0.01 r C+(12/14)xN*-(12/92.9) 20 xNb-(12/47.9)xTi*-(12/50.9)xV : 0.06 ..... . (3) N* :s (14/92.9)x(Nb-0.01) ..... . (4) provided that N* in the equations (3) and (4) is N* = N-(14/47.9)xTi at N-(14/47.9)xTi > 0 and N* = 0 at N-(14/47.9)xTi r 0, and Ti* in the equation (3) is Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS at Ti-(47.9/14)xN 25 (47.9/32.1)xS > 0 and Ti* = 0 at Ti-(47.9/14)xN-(47.9/32.1)xS S 0. [0030] (VI) A method for producing a high-stiffness high-strength thin steel sheet according to the item (IV) or (V), wherein the staring material of steel further contains one or more of Cr: 0.1-1.0%, Ni: 0.1 1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% as mass% 30 in addition to the above composition. EFFECT OF THE INVENTION [0031] According to the invention, it is possible to provide a high stiffness high-strength thin steel sheet having a tensile strength of not 05846HH (7/37) less than 590 MPa, preferably not less than 700 MPa and a Young's modulus of not less than 225 GPa, preferably not less than 230 GPa, more preferably not less than 240 GPa. [0032] That is, the starting material of low carbon steel added with 5 Mn and Nb is roll-reduced below 950'C, preferably below 900 0 C (strictly speaking, just above Ar 3 point) in the hot rolling to promote the transformation from non-recrystallized austenite to ferrite and then cold rolled to develop a crystal orientation useful for the improvement of Young's modulus and thereafter a low-temperature transformation 10 phase suppressing the lowering of the Young's modulus is produced and a greater amount of ferrite phase useful for the improvement of the Young's modulus is retained in the cooling stage by the control of the heating rate in the annealing step and the soaking at two-phase region, whereby the thin steel sheet satisfying higher strength and higher 15 Young's modulus can be produced, which develops an effective effect in industry. [0033] Further explaining in detail, the starting material of low carbon steel added with Mn and Nb is roll-reduced just above Ar 3 transformation point in the hot rolling to increase the non 20 recrystallized austenite texture having a crystal orientation of {112}<111>, and subsequently the transformation from the non recrystallized austenite of {112}<111> to ferrite is promoted in the cooling stage to develop ferrite orientation of {113}<110>. [0034] In the cold rolling after the coiling and pickling, the rolling 25 is carried out at a rolling reduction of not less than 50% to turn the crystal orientation of {113}<110> to {112}<110> useful for the improvement of the Young's modulus, and in the temperature rising stage at the subsequent annealing step, the temperature is raised from 500 0 C to the soaking temperature at a heating rate of 1-40°C/s to 30 promote the recrystallization of ferrite having an orientation of {112}<110> and provide a two-phase region at a state of partly retaining the non-recrystallized grains of {112}<110>, whereby the transformation from the non-recrystallized ferrite of {112}<110> to 05846HH (8/37) austenite can be promoted. [0035] Further, in the transformation from austenite phase to ferrite phase at the cooling after the soaking, ferrite grains having an orientation of {112}<110> is grown to enhance the Young's modulus, 5 while the steel enhancing the hardenability by the addition of Mn is cooled at a rate of not less than 5C/s to produce the low-temperature transformation phase, whereby it is attempted to increase the strength. [0036] Moreover, the low-temperature transformation phase is produced by retransforming the austenite phase transformed from 10 ferrite having an orientation of {112}<110> during the cooling, so that {112}<110> can be also developed even in the crystal orientation of the low-temperature transformation phase. [0037] Thus, the Young's modulus is enhanced by developing {112}<110> of ferrite phase, and particularly {112}<110> is increased 15 in the orientation of the low-temperature transformation phase largely exerting on the lowering of the Young's modulus, whereby the strength can be increased by the formation of the low-temperature transformation phase and the lowering of the Young's modulus accompanied with the formation of the low-temperature transformation phase can be largely 20 suppressed. BRIEF DESCRIPTION OF THE DRAWINGS [0038] FIG. 1 is a graph showing an influence of a total rolling reduction below 950 0 C or below 900 0 C on Young's modulus; FIG. 2 is a graph showing an influence of a final temperature 25 in hot finish rolling on Young's modulus; FIG. 3 is a graph showing an influence of a coiling temperature on Young's modulus; FIG. 4 is a graph showing an influence of a rolling reduction in cold rolling on Young's modulus; and 30 FIG. 5 is a graph showing an influence of an average temperature rising rate from 500 0 C to soaking temperature in annealing on Young's modulus. 05846HH (9/37) - IV BEST MODE FOR CARRYING OUT THE INVENTION [0039] The high-stiffness high-strength thin steel sheet according to the invention is a steel sheet having a tensile strength of not less than 590 MPa, preferably not less than 700 MPa, a Young's modulus of not 5 less than 225 GPa, preferably not less than 230 GPa, more preferably not less than 240 GPa, and a thickness of not more than 2.0 mm. Moreover, the steel sheet to be targeted in the invention includes steel sheets subjected to a surface treatment such as galvanization inclusive of alloying, zinc electroplating or the like in addition to the cold 10 rolled steel sheet. [0040] The reason of limiting the chemical composition in the steel sheet of the invention will be described below. Moreover, the unit for the content of each element in the chemical composition of the steel sheet is "% by mass", but it is simply shown by "%" unless otherwise 15 specified. [0041] C: 0.02-0.15% C is an element stabilizing austenite and can largely contribute to increase the strength by enhancing the hardenability at the cooling stage in the annealing after the cold rolling to largely 20 promote the formation of the low-temperature transformation phase. Further, the Ar 3 transformation point is lowered in the hot rolling and it is possible to conduct the rolling at a lower temperature region when the rolling is conducted just above Ar 3 , whereby the transformation from the non-recrystallized austenite to ferrite can be promoted to 25 develop {113}<110>, and the Young's modulus can be improved at the subsequent cold rolling and annealing steps. Moreover, C can contribute to increase the Young's modulus by promoting the transformation of ferrite grains having {112}<110> from the non recrystallized ferrite to austenite after the cold rolling. 30 [0042] In order to obtain such effects, the C content is required to be not less than 0.02%, preferably not less than 0.05%, more preferably not less than 0.06%. On the other hand, when the C content exceeds 0.15%, the fraction of hard low-temperature 05846HH (10/37) transformation phase becomes large, and the strength of the steel is extremely increased but also the workability is deteriorated. Also, the greater amount of C suppresses the recrystallization of the orientation useful for the increase of the Young's modulus at the 5 annealing step after the cold rolling. Further, the greater amount of C brings about the deterioration of the weldability. Therefore, the C content is required to be not more than 0.15%, preferably not more than 0.10%. [0043] Si: not more than 1.5% 10 Si raises the Ar 3 transformation point in the hot rolling, so that when the rolling is carried out just above Ar 3 , the recrystallization of worked austenite is promoted. Therefore, when Si is contained in an amount exceeding 1.5%, the crystal orientation required for the increase of the Young's modulus can not be obtained. Also, the 15 greater amount of Si deteriorates the weldability of the steel sheet but also promotes the formation of fayalite on a surface of a slab in the heating at the hot rolling step to accelerate the occurrence of surface pattern so-called as a red scale. Furthermore, in case of using as a cold rolled steel sheet, Si oxide produced on the surface deteriorates 20 the chemical conversion processability, while in case of using as a galvanized steel sheet, Si oxide produced on the surface induces non plating. Therefore, the Si content is required to be not more than 1.5%. Moreover, in case of steel sheets requiring the surface properties or the galvanized steel sheet, the Si content is preferable to 25 be not more than 0.5%. [0044] Also, Si is an element stabilizing ferrite and promotes the ferrite transformation at the cooling stage after the soaking of two phase region in the annealing step after the cold rolling to enrich C in austenite, whereby austenite can be stabilized to promote the formation 30 of the low-temperature transformation phase. For this end, the strength of steel can be increased, if necessary. In order to obtain such an effect, the Si content is desirable to be not less than 0.2%. 05846HH (11/37) [0045] Mn: 1.5-4.0% Mn is one of important elements in the invention. Mn is an element suppressing the recrystallization of worked austenite in the hot rolling and stabilizing austenite, and since Mn lowers the Ar 3 5 transformation point, when the rolling is carried out just above Ar 3 , it is possible to conduct the rolling at a lower temperature region, and further Mn has an action of suppressing the recrystallization of the worked austenite. Moreover, Mn can promote the transformation from the non-recrystallized austenite to ferrite to develop {113}<110> 10 and improve the Young's modulus in the subsequent cold rolling and annealing steps. [0046] Furthermore, Mn as an austenite stabilizing element lowers Ac, transformation point in the temperature rising stage at the annealing step after the cold rolling to promote the transformation 15 from the non-recrystallized ferrite to austenite, and can develop the orientation useful for the improvement of the Young's modulus to control the lowering of the Young's modulus accompanied with the formation of the low-temperature transformation phase with respect to the orientation of the low-temperature transformation phase produced 20 in the cooling stage after the soaking. [0047] Also, Mn enhances the hardenability in the cooling stage after the soaking and annealing at the annealing step to largely promote the formation of the low-temperature transformation phase, which can largely contribute to the increase of the strength. Further, Mn acts as a 25 solid-solution strengthening element, which can contribute to the increase of the strength in steel. In order to obtain such an effect, the Mn content is required to be not less than 1.5%. [0048] On the other hand, when the Mn content exceeds 4.0%, Ac 3 transformation point is excessively lowered in the temperature rising 30 stage at the annealing step after the cold rolling, so that the recrystal lization of ferrite phase at the two-phase region is difficult and it is required to raise the temperature up to an austenite single-phase region above Ac 3 transformation point. As a result, ferrite of {112}<110> 05846HH (12/37) - I.) orientation useful for the increase of the Young's modulus obtained by the recrystallization of worked ferrite can not be developed to bring about the lowering of the Young's modulus. Further, the greater amount of Mn deteriorates the weldability of the steel sheet. Therefore, 5 the Mn content is not more than 4.0%, preferably not more than 3.5%. [0049] P: not more than 0.05% Since P segregates in the grain boundary, if the P content exceeds 0.05%, the ductility and toughness of the steel sheet lower but also the weldability is deteriorated. In case of using the alloyed 10 galvanized steel sheet, the alloying rate is delayed by P. Therefore, the P content is required to be not more than 0.05%. On the other hand, P is an element effective for the increase of the strength as a solid-solution strengthening element and has an action of promoting the enrichment of C in austenite as a ferrite stabilizing element. 15 In the steel added with Si, it has also an action of suppressing the occurrence of red scale. In order to obtain these actions, the P content is preferable to be not less than 0.01%. [0050] S: not more than 0.01% S considerably lowers the hot ductility to induce hot tearing 20 and considerably deteriorate the surface properties. Further, S hardly contributes to the strength but also forms coarse MnS as an impurity element to lower the ductility and drill-spreading property. These problems become remarkable when the S content exceeds 0.01%, so that it is desirable to reduce the S content as far as possible. 25 Therefore, the S content is not more than 0.01%. From a viewpoint of improving the drill-spreading property, it is preferable to be not more than 0.005%. [0051] Al: not more than 1.5% It is an element useful for deoxidizing steel to improve the 30 cleanness of the steel. However, Al is a ferrite stabilizing element, and largely raises the Ar 3 transformation of the steel, so that when the rolling is carried out just above Ar 3 , the recrystallization of worked austenite is promoted to suppress the development of the crystal 05846HH (13/37) - I orientation required for the increase of the Young's modulus. Further, when the Al content exceeds 1.5%, the austenite single-phase region disappears and it is difficult to terminate the rolling at austenite region in the hot rolling step. Therefore, the Al content is required to be not 5 more than 1.5%. From this viewpoint, Al is preferable to be made lower, and further preferable to be limited to not more than 0.1%. On the other hand, Al as a ferrite forming element promotes the formation of ferrite in the cooling stage after the soaking at the two phase region in the annealing step after the cold rolling to enrich C in 10 austenite, whereby austenite can be stabilized to promote the formation of the low-temperature transformation phase. As a result, the strength of the steel can be enhanced, if necessary. In order to obtain such an effect, the Al content is desirable to be not less than 0.2%. [0052] N: not more than 0.01% 15 N is a harmful element because slab breakage is accompanied in the hot rolling to cause surface defect. When the N content exceeds 0.01%, the occurrence of slab breakage and surface defect becomes remarkable. Therefore, the N content is required to be not more than 0.01%. 20 [0053] Nb: 0.02-0.40% Nb is a most important element in the invention. That is, Nb suppresses the recrystallization of worked austenite at the finish rolling step in the hot rolling to promote the transformation from the non-recrystallized austenite to ferrite and develop {113}<110> and can 25 increase the Young's modulus at the subsequent cold rolling and annealing steps. Also, the recrystallization of worked ferrite is suppressed at the temperature rising stage in the annealing step after the cold rolling to promote the transformation from the non recrystallized ferrite to austenite. As to the orientation of the low 30 temperature transformation phase produced in the cooling stage after the soaking, the orientation useful for the increase of the Young's modulus can be developed to suppress the lowering of the Young's modulus accompanied with the formation of the low-temperature 05846HH (14/37) - I..) transformation phase. Also, a fine carbonitride of Nb can contribute to the increase of the strength. In order to obtain such an action, the Nb content is required to be not less than 0.02%, preferably not less than 0.05%. 5 [0054] On the other hand, when the Nb content exceeds 0.40%, the all carbonitride can not be solid-soluted in the re-heating at the usual hot rolling step and hence coarse carbonitride remains, so that the effect of suppressing the recrystallization of worked austenite in the hot rolling step and the effect of suppressing the recrystallization of 10 worked ferrite in the annealing step after the cold rolling can not be obtained. Also, even if the hot rolling of the slab after the continuous casting is started as it is without conducting the re-heating after the continuously cast slab is cooled, when Nb is included in an amount exceeding 0.40%, the improvement of the effect of suppressing the 15 recrystallization is not recognized and the increase of the alloy cost is caused. Therefore, the Nb content is 0.02-0.40%, preferably 0.05 0.40%. [0055] In the invention, the contents of C, N and Nb are required to satisfy the relationship of the following equations (1) and (2): 20 0.01 sC+(12/14)xN-(12/92.9)xNb r 0.06 ..... . (1) N r (14/92.9)x(Nb-0.01) ..... . (2) [0056] If C not fixed as a carbonitride is existent in an amount exceeding 0.06%, the introduction of strain in the cold rolling becomes non-uniform and further the recrystallization of the orientation useful 25 for the increase of the Young's modulus is suppressed, so that the C amount not fixed as the carbonitride calculated by (C+(12/14)xN (12/92.9)xNb) is required to be not more than 0.06%, preferably not more than 0.05%. At this moment, N is preferentially fixed and precipitated as compared with C, so that the C amount not fixed as the 30 carbonitride can be calculated by (C+(12/14)xN-(12/92.9)xNb). On the other hand, when the C amount not fixed as the carbonitride is less than 0.01%, the C content in austenite decreases in the annealing at the two-phase region after the cold rolling and the formation of 05846HH (15/37) - ilu martensite phase after the cooling is suppressed, so that it is difficult to increase the strength of the steel. Therefore, the amount of (C+(12/14)xN-(12/92.9)xNb), which is the C amount not fixed as the carbonitride, is 0.01-0.06%, preferably 0.01-0.05%. Further, N 5 coarsely precipitates a nitride of Nb at a high temperature, and hence the effect of suppressing the recrystallization by Nb is reduced. In order to control this action, the N content is required to be limited to N s (14/92.9)x(Nb-0.01) in relation with the Nb content, preferably N s (14/92.9)x(Nb-0.02). 10 [0057] Moreover, the term "the remainder being substantially iron and inevitable impurities" used herein means that steels containing slight amounts of other elements without damaging the action and effect of the invention are included within the scope of the invention. In case of further increasing the strength, one or two of Ti and V and 15 one or more of Cr, Ni, Mo, Cu and B may be added, if necessary, in addition to the above definition of the chemical composition. [0058] Ti: 0.01-0.50% Ti is an element contributing to the increase of the strength by forming a fine carbonitride. Also, it is an element contributing to 20 the increase of the Young's modulus by suppressing the recrystal lization of worked austenite in the finish rolling step of the hot rolling to promote the transformation from the non-recrystallized austenite to ferrite. Since Ti has the above actions, the content is preferable to be not less than 0.01%. On the other hand, when the Ti content exceeds 25 0.50%, all the carbonitride can not be solid-soluted in the re-heating at the usual hot rolling step and a coarse carbonitride remains, and hence the effect of increasing the strength and the effect of suppressing the recrystallization can not be obtained. Also, even if the hot rolling of the slab after the continuous casting is started as it is without 30 conducting the re-heating after the continuously cast slab is cooled, the Ti content exceeding 0.50% is small in the contribution to the effect of increasing the strength and the effect of suppressing the recrystallization and also the increase of the alloy cost is caused. 05846HH (16/37) Therefore, the Ti content is preferably not more than 0.50%, more preferably not more than 0.20%. [0059] V: 0.01-0.50% V is an element contributing to the increase of the strength 5 by forming a fine carbonitride. Since V has such an action, the V content is preferable to be not less than 0.01%. On the other hand, when the V content exceeds 0.50%, the effect of increasing the strength by the amount exceeding 0.50% is small and the increase of the alloy cost is caused. Therefore, the V content is preferably not 10 more than 0.50%, more preferably not more than 0.20%. [0060] In the invention, when Ti and/or V are included in addition to Nb, the contents of C, N, S, Nb, Ti and V are required to satisfy the relationship of the following equations (3) and (4) instead of the equations (1) and (2): 15 0.01 r C+(12/14)xN*-(12/92.9) xNb-(12/47.9)xTi*-(12/50.9)xV r 0.06 ..... . (3) N* s (14/92.9)x(Nb-0.01) ..... . (4) provided that N* in the equations (3) and (4) is N* = N-(14/47.9)xTi at N-(14/47.9)xTi > 0 and N* = 0 at N-(14/47.9)xTi < 0, and Ti* in the 20 equation (3) is Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS at Ti-(47.9/14)xN (47.9/32.1)xS > 0 and Ti* = 0 at Ti-(47.9/14)xN-(47.9/32.1)xS < 0. [0061] Further, N coarsely precipitates the nitride of Nb at a high temperature as previously mentioned, so that the effect of suppressing the recrystallization through Nb is decreased. In case of Ti 25 containing steel, N is preferentially fixed as a nitride of Ti, N* as a N amount not fixed as a nitride of Ti is required to be limited to N* S (14/92.9)x(Nb-0.01), preferably N* r (14/92.9)x((Nb-0.02). [0062] Ti and V form the carbonitride to decrease the C content not fixed as the carbonitride. Further, Ti is fixed by the formation of a 30 sulfide, so that the value of C+(12/14)xN*-(12/92.9)xNb (12/47.9)xTi*-(12/50.9)xV is required to be 0.01-0.06%, preferably 0.01-0.05% when Ti and/or V are added in order that the C content not fixed as the carbonitride is made to 0.01-0.06%. 05846HH (17/37) [0063] Cr: 0.1-1.0% Cr is an element enhancing the hardenability by suppressing the formation of cementite and can largely contribute to the increase of the strength by largely promoting the formation of the low-temperature 5 transformation phase in the cooling stage after the soaking at the annealing step. Further, the recrystallization of worked austenite is suppressed in the hot rolling step to promote the transformation from non-recrystallized austenite to ferrite and develop {113}<110>, and the Young's modulus can be increased at the subsequent cold rolling 10 and annealing steps. In order to obtain such an effect, Cr is preferable to be included in an amount of not less than 0.1%. On the other hand, when the Cr content exceeds 1.0%, the above effect is saturated and the alloy cost increases, so that Cr is preferable to be included in an amount of not more than 1.0%. Moreover, when the 15 thin steel sheet of the invention is used as a galvanized steel sheet, the oxide of Cr produced on the surface induces the non-plating, so that Cr is preferable to be included in an amount of not more than 0.5%. [0064] Ni: 0.1-1.0% Ni is an element stabilizing austenite to enhance the harden 20 ability, and can largely contribute to the increase of the strength by largely promoting the formation of the low-temperature transformation phase in the cooling stage after the soaking at the annealing step. Further, Ni as an austenite stabilizing element lowers Ac, trans formation point in the temperature rising stage at the annealing step 25 after the cold rolling to promote the transformation from the non recrystallized ferrite to austenite, and develops the orientation useful for the increase of the Young's modulus with respect to the orientation of the low-temperature transformation phase produced in the cooling stage after the soaking, whereby the lowering of the Young's modulus 30 accompanied with the formation of the low-temperature transformation phase can be suppressed. Since Ni is an element suppressing the recrystallization of worked austenite in the hot rolling and stabilizing austenite, when Ar 3 transformation point is lowered to conduct the 05846HH (18/37) rolling just above Ar 3 , it is possible to conduct the rolling at a lower temperature region to further suppress the recrystallization of worked austenite, and also the transformation from the non-recrystallized austenite to ferrite is promoted to develop {113}<110>, whereby the 5 Young's modulus can be increased at the subsequent cold rolling and annealing steps. In case of adding Cu, the surface defect is induced by cracking accompanied with the lowering of the hot ductility in the hot rolling, but the occurrence of the surface defect can be controlled by composite addition of Ni. In order to obtain such an action, Ni is 10 preferable to be included in an amount of not less than 0.1%. [0065] On the other hand, when the Ni content exceeds 1.0%, Ac 3 transformation point is extremely lowered in the temperature rising stage at the annealing step after the cold rolling and the recrystalliza tion of ferrite phase at the two-phase region is difficult, and hence it is 15 required to raise the temperature up to austenite single phase region above Ac 3 transformation point. As a result, ferrite of orientation obtained by the recrystallization of worked ferrite and useful for the increase of the Young's modulus can not be developed to bring about the decrease of the Young's modulus. And also, the alloy cost 20 increases. Therefore, Ni is preferable to be included in an amount of not more than 1.0%. [0066] Mo: 0.1-1.0% Mo is an element enhancing the harenability by making small the mobility of the interface, and can largely contribute to the increase 25 of the strength by largely promoting the formation of the low temperature transformation phase in the cooling stage at the annealing step after the cold rolling. Further, the recrystallization of worked austenite can be suppressed, and the transformation from the non recrystallized austenite to ferrite is promoted to develop {113}<110> 30 and the Young's modulus can be increased at the subsequent cold rolling and annealing steps. In order to obtain such an action, Mo is preferable to be included in an amount of not less than 0.1%. On the other hand, when the Mo content exceeds 1.0%, the above effect is 05846HH (19/37) saturated and the alloy cost increases, so that Mo is preferable to be included in an amount of not more than 1.0%. [0067] B: 0.0005-0.0030% B is an element suppressing the transformation from 5 austenite phase to ferrite phase to enhance the hardenability, and can largely contribute to the increase of the strength by largely promoting the formation of the low-temperature transformation phase in the cooling stage at the annealing step after the cold rolling. Further, the recrystallization of worked austenite can be suppressed, and the 10 transformation from the non-recrystallized austenite to ferrite is promoted to develop {113}<110> and the Young's modulus can be increased at the subsequent cold rolling and annealing steps. In order to obtain such an effect, B is preferable to be included in an amount of not less than 0.0005%. On the other hand, when the B content 15 exceeds 0.0030%, the above effect is saturated, so that B is preferable to be included in an amount of not more than 0.0030%. [0068] Cu: 0.1-2.0% Cu is an element enhancing the hardenability, and can largely contribute to the increase of the strength by largely promoting 20 the formation of the low-temperature transformation phase in the cooling stage at the annealing step after the cold rolling. In order to obtain such an effect, Cu is preferable to be included in an amount of not less than 0.1%. On the other hand, when the Cu content exceeds 2.0%, the hot ductility is lowered and the surface defect accompanied 25 with the cracking in the hot rolling is induced and the hardening effect by Cu is saturated, so that Cu is preferable to be included in an amount of not more than 2.0%. [0069] The reason on the limitation of the texture according to the invention will be described below. 30 In the thin steel sheet of the invention, it is required to have a texture comprising a ferrite phase as a main phase and having a martensite phase at an area ratio of not less than 1%. The term "ferrite phase as a main phase" used herein means 05846HH (20/37) - 1_1 that the area ratio of the ferrite phase is not less than 50%. [0070] Since the ferrite phase is less in the strain, useful for the increase of the Young's modulus, excellent in the ductility and good in the workability, the texture is required to be the ferrite phase as a main 5 phase. Also, in order to render the tensile strength of the steel sheet into not less than 590 MPa, it is required that the low-temperature transformation phase as a hard phase is formed in a portion other than the ferrite phase as a main phase or a so-called second phase to 10 provide a composite phase. At this moment, the feature that a hard martensite phase among the low-temperature transformation phases is particularly existent in the texture is advantageous because the fraction of the second phase for obtaining the target tensile strength level is made small and the fraction of ferrite phase is made large, 15 whereby the increase of the Young's modulus is attained and further the workability can be improved. For this end, the martensite phase is required to be not less than 1% as an area ratio to the whole of the texture. In order to obtain the strength of lot less than 700 MPa, the area ratio of the martensite phase is preferable to be not less than 16%. 20 [0071] The texture of the steel sheet according to the invention is preferable to be a texture comprising ferrite phase and martensite phase, but there is no problem that phases other than the ferrite phase and martensite phase such as bainite phase, residual austenite phase, pearlite phase, cementite phase and the like are existent at the area 25 ratio of not more than 10%, preferably not more than 5%. That is, the sum of area ratios of ferrite phase and martensite phase is preferably not less than 90%, more preferably not less than 95%. [0072] Next, the reason on the production conditions limited for obtaining the high-stiffness high-strength thin steel sheet according to 30 the invention and preferable production conditions will be explained. The composition of the starting material of steel used in the production method of the invention is the same as the composition of the aforementioned steel sheet, so that the description of the reason on 05846HH (21/37) the limitation of the starting material of steel is omitted. [0073] The thin steel sheet according to the invention can be produced by successively conducting a hot rolling step of subjecting the starting material of steel having the same composition as the 5 composition of the steel sheet to a hot rolling to obtain a hot rolled sheet, a cold rolling step of subjecting the hot rolled sheet after pickling to a cold rolling to obtain a cold rolled sheet, and an annealing step of attaining the recrystallization and composite texture in the cold rolled sheet. 10 [0074] (Hot rolling step) Finish rolling: total rolling reduction below 950 0 C is not less than 30%, and the rolling is terminated at Ar 3 -900 0 C. In the final rolling at the hot rolling step, the rolling is conducted just above Ar 3 transformation point to develop a non 15 recrystallized austenite texture having a crystal orientation of {112}<111>, and the {112}<111> non-recrystallized austenite can be transformed to ferrite in the subsequent cooling stage to develop ferrite orientation of {113}<110>. This orientation advantageously acts to the improvement of the Young's modulus in the formation of 20 the texture at the subsequent cold rolling and annealing steps. In order to obtain such an action, it is required that the total rolling reduction below 950 0 C (total rolling reduction) is not less than 30%, more preferably the total rolling reduction below 900 0 C is not less than 30%, and the finish rolling is terminated at a temperature region 25 of Ar 3 -900 0 C, preferably Ar 3 -850 0 C. [0075] Coiling temperature: not higher than 650'C When the coiling temperature after the finish rolling exceeds 650'C, the carbonitride of Nb is coarsened and the effect of suppressing the recrystallization of ferrite becomes small in the 30 temperature rising stage at the annealing step after the cold rolling and it is difficult to transform the non-recrystallized ferrite into austenite. As a result, the orientation of the low-temperature transformation phase transformed in the cooling stage after the soaking can not be 05846HH (22/37) controlled, and the Young's modulus is largely lowered by the low temperature transformation phase having such a strain. Therefore, the coiling temperature after the finish rolling is required to be not higher than 650'C. 5 Moreover, when the coiling temperature is too low, a great amount of the hard low-temperature transformation phase is produced and the subsequent cold rolling becomes difficult, so that it is preferable to be not lower than 400 0 C. [0076] (Cold rolling step) 10 Cold rolling is carried out at a rolling reduction of not less than 50% after the pickling. After the hot rolling step, the pickling is carried out for removing scale formed on the surface of the steel sheet. The pickling may be conducted according to the usual manner. Thereafter, the cold 15 rolling is conducted. By the cold rolling at a rolling reduction of not less than 50% can be turned the orientation of {113}<110> developed on the hot rolled steel sheet to an orientation of {112}<110> effective for the increase of the Young's modulus. Thus, as the orientation of {112}<110> is developed by the cold rolling, the orientation of 20 {112}<110> in ferrite is enhanced in the texture after the subsequent annealing step and further the orientation of {112}<110> is developed in the low-temperature transformation phase, whereby the Young's modulus can be increased. In order to obtain such an effect, the rolling reduction in the cold rolling is required to be not less than 50%. 25 [0077] (Annealing step) Temperature rising rate from 500 0 C to soaking temperature: 1-40oC/s, Soaking temperature: 780-900 0 C The temperature rising rate at the annealing step is an important process condition in the invention. In the course of raising 30 the temperature to a soaking temperature of two-phase region or a soaking temperature of 780-900 0 C at the annealing step, the recrystal lization of ferrite having an orientation of {112}<110> is promoted, while a part of ferrite grains having an orientation of {112}<110> is 05846HH (23/37) arrived to a two-phase region at a non-recrystallized state, whereby the transformation from the non-recrystallized ferrite having an orientation of {112}<110> can be promoted. Therefore, the Young's modulus can be increased by promoting the growth of ferrite grains 5 having an orientation of {112}<110> when austenite is transformed into ferrite in the cooling after the soaking. Further, when the strength is increased by producing the low-temperature transformation phase, austenite phase transformed from ferrite having an orientation of {112}<110> is re-transformed in the cooling, so that {112}<110> 10 can be also developed with respect to the crystal orientation of the low-temperature transformation phase. By developing {112}<110> of ferrite phase is increased the Young's modulus, while {112}<110> is particularly developed in the orientation of the low-temperature transformation phase largely influencing the lowering of the Young's 15 modulus, whereby the lowering of the Young's modulus accompanied with the formation of the low-temperature transformation phase can be suppressed while forming the low-temperature transformation phase. When austenite is transformed from the non-recrystallized ferrite while promoting the recrystallization of ferrite in the temperature 20 rising stage, an average temperature rising rate largely exerting on the recrystallization behavior from 500 0 C to 780-900 0 C as a soaking temperature is required to be 1-40 0 C/s, preferably 1-30 0 C/s. In this case, the reason why the soaking temperature is 780-900 0 C is due to the fact that when it is lower than 780 0 C, the 25 recrystallization is not completed, while when it exceeds 900 0 C, the fraction of austenite becomes large and ferrite having an orientation of {112}<110> reduces or disappears. Moreover, the soaking time is not particularly limited, but it is preferable to be not less than 30 seconds for forming austenite, while it is preferable to be not more than about 30 300 seconds because the production efficiency is deteriorated as the time is too long. [0078] Cooling rate to 500 0 C after soaking: not less than 5C/s In the cooling stage after the soaking, it is required to form 05846HH (24/37) the low-temperature transformation phase containing martensite for increasing the strength. Therefore, an average cooling rate to 500 0 C after the soaking is required to be not less than 5 0 C/s. [0079] In the invention, steel having a chemical composition in 5 accordance with the target strength level is first melted. As the melting method can be properly applied a usual converter process, an electric furnace process and the like. The molten steel is cast into a slab, which is subjected to a hot rolling as it is or after the cooling and heating. After the finish rolling under the aforementioned finish 10 conditions in the hot rolling, the steel sheet is coiled at the afore mentioned coiling temperature and then subjected to usual pickling and cold rolling. As to the annealing, the temperature is raised under the aforementioned condition, and in the cooling after the soaking, the cooling rate can be increased within a range of obtaining a target low 15 temperature transformation phase. Thereafter, the cold rolled steel sheet may be subjected to an overaging treatment, or may be passed through a hot dip zinc in case of producing as a galvanized steel sheet, or further in case of producing as an alloyed galvanized steel sheet, a re-heating may be conducted up to a temperature above 500 0 C for the 20 alloying treatment. EXAMPLES [0080] The following examples are given in illustration of the invention and are not intended as limitations thereof. At first, a steel A having a chemical composition shown in 25 Table 1 is melted in a vacuum melting furnace of a laboratory and cooled to room temperature to prepare a steel ingot (steel raw material). [0081] Table 1 Kind Chemical composition of X Y Remarks steel C Si Mn P S Al N Nb value value A 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 0.03 0.011 Acceptable 05846HH (25/37)example 05846HH (25/37) [0082] Thereafter, the hot rolling, pickling, cold rolling and annealing are successively conducted in the laboratory. The basic production conditions are as follows. After the steel ingot is heated at 1250°C for 1 hour, the hot rolling is conducted under conditions that 5 the total rolling reduction below 900 0 C, i.e. total rolling reduction ratio below 900 0 C is 40% and the final rolling temperature (corresponding to a final temperature of finish rolling) is 830 0 C to obtain a hot rolled sheet having a thickness of 4.0 mm. Thereafter, the coiling condition (corresponding to a coiling temperature of 600 0 C) is simulated by 10 leaving the hot rolled sheet up to 600 0 C and keeping in a furnace of 600 0 C for 1 hour and then cooling in the furnace. The thus obtained hot rolled sheet is pickled and cold-rolled at a rolling reduction of 60% to a thickness of 1.6 mm. Then, the temperature of the cold rolled sheet is raised at 10 0 C/s on average up to 500 0 C and further from 15 500 0 C to a soaking temperature of 820'C at 5 0 C/s on average. Next, the soaking is carried out at 820'C for 180 seconds, and thereafter the cooling is carried out at an average cooling rate of 10 0 C/s up to 500 0 C, and further the temperature of 500 0 C is kept for 80 seconds, and then the sheet is cooled in air. Moreover, Ar 3 transformation point of this 20 steel under the above production conditions is 730'C. [0083] In this experiment, the following conditions are further individually changed under the above production conditions as a basic condition. That is, the experiment is carried out under the basic condition except for the individual changed conditions that the total 25 rolling reduction below 950'C or total rolling reduction below 900 0 C is 20-65% and the final temperature of the hot finish rolling is 710-920 0 C and the coiling temperature is 500-670 0 C and the rolling reduction of the cold rolling is 40-75% (thickness: 2.4-1.0 mm) and the average temperature rising rate from 500'C to the soaking 30 temperature (820 0 C) in the annealing is 0.5-45 0 C/s. [0084] From the sample after the annealing is cut out a test specimen of 10 mm x 120 mm in a direction perpendicular to the rolling direction as a longitudinal direction, which is finished to a thickness 05846HH (26/37) of 0.8 mm by a mechanical polishing and a chemical polishing for removing strain, and thereafter a resonance frequency of the sample is measured by using a lateral vibration type internal friction measuring device to calculate a Young's modulus therefrom. With respect to the 5 sheet subjected to a temper rolling of 0.5%, a tensile test specimen of JIS No. 5 is cut out in the direction perpendicular to the rolling direction and subjected to a tensile test. Further, the sectional texture is observed by a scanning type electron microscope (SEM) after the corrosion with Nital to judge the kind of the texture, while three 10 photographs are shot at a visual region of 30 [tm x 30 pm and then area ratios of ferrite phase and martensite phase are measured by an image processing to determine an average value of each phase as an area ratio (fraction) of each phase. [0085] As a result, the values of the mechanical characteristics under 15 the basic condition in the experiment according to the production method of the invention are Young's modulus E: 245 GPa, TS: 800 MPa, El: 20%, fraction of ferrite phase: 70% and fraction of martensite phase: 25%, from which it is clear that the thin steel sheet has an excellent balance of strength-ductility and a high Young's 20 modulus. Moreover, the remainder of the texture other than ferrite phase and martensite phase is either of bainite phase, residual austenite phase, pearlite phase and cementite phase. [0086] Then, the relationship between the production conditions and Young's modulus is explained based on the above test results with 25 reference to the drawings. Even in any experimental conditions, the tensile strength is 750-850 MPa, and the fraction of ferrite phase is 80-60%, the fraction of martensite phase is 17-40%, and the remainder of the texture of the second phase other than martensite phase is either of bainite phase, residual austenite phase, pearlite phase and cementite 30 phase. [0087] In FIG. 1 is shown influences of the total rolling reduction below 950 0 C and the total rolling reduction below 900 0 C upon Young's modulus, respectively. When the total rolling reduction below 950 0 C 05846HH (27/37) is not less than 30% being the acceptable range of the invention, the Young's modulus indicates an excellent value of not less than 225 GPa, and further when the total rolling reduction below 900'C is not less than 30%, the Young's modulus indicates a more excellent value of not 5 less than 240 GPa. [0088] In FIG. 2 is shown an influence of the final temperature of the hot finish rolling upon the Young's modulus. When the final temper ature is Ar 3 -900°C being the acceptable range of the invention, the Young's modulus indicates an excellent value of not less than 225 GPa, 10 and further when the final temperature is Ar 3 -850 0 C, the Young's modulus indicates a more excellent value of not less than 240 GPa. [0089] In FIG. 3 is shown an influence of the coiling temperature upon the Young's modulus. When the coiling temperature is not higher than 650 0 C being the acceptable range of the invention, the 15 Young's modulus indicates an excellent value of not less than 225 GPa. [0090] In FIG. 4 is shown an influence of the rolling reduction of the cold rolling upon the Young's modulus. When the rolling reduction is not less than 50% being the acceptable range of the invention, the Young's modulus indicates an excellent value of not less than 225 GPa. 20 [0091] In FIG. 5 is shown an influence of the average temperature rising rate from 500 0 C to the soaking temperature of 820 0 C in the annealing upon the Young's modulus. When the temperature rising rate is 1-40 0 C/s being the acceptable range of the invention, the Young's modulus indicates an excellent value of not less than 225 GPa, 25 and further when the temperature rising rate is 1-30 0 C/s, the Young's modulus indicates a more excellent value of not less than 240 GPa. [0092] Furthermore, steels B-Z and AA-BF having a chemical composition as shown in Tables 2 and 3 are melted in a vacuum melting furnace of a laboratory and then successively subjected to the hot 30 rolling, pickling, cold rolling and annealing under the above basic condition, respectively. In Tables 4 and 5 are shown characteristics obtained by the aforementioned tests. Moreover, the Ar 3 transforma tion point in the steels B-Z and AA-BF under the above production 05846HH (28/37) conditions is 650-760'C. Also, the residual texture other than ferrite phase and martensite phase in the tables is either of bainite phase, residual austenite phase, pearlite phase and cementite phase. 05846HH (29/37) [0093] Table 2 Kind Chemical composition (mass %) of other X y Remarks steel C Si Mn P S Al N Nb compo- N* nents value value Acceptable B 0.02 0.2 2.5 0.02 0.001 0.03 0.002 0.07 - 0.01 - 0.009 Steel Comprtv C 0.02 0.2 2.5 0.02 0.001 0.03 0.002 0.14 - 0.00 - 0.020 Co ative Acceptable D 0.06 0.2 2.5 0.02 0.0010.03 0.002 0.12 - 0.05 - 0.017 Steel Acceptable E 0.07 0.2 2.5 0.02 0.001 0.03 0.002 0.08 - 0.06 - 0.011 StAccetbleel Steel F 0.04 0.2 2.5 0.02 0.001 0.030.0020.25 - 0.01 - 0.036 Steel S0.06 0.2 2.5 0.02 0.001 0.03 0.002 0.35 - 0.02 - 0.051 Acceptable ____ Steel H 0.05 0.2 2.5 0.02 0.001 0.03 0.002 0.05 - 0.05 - 0.006 Acceptable I 0.05 0.2 2.5 0.02 0.0010.03 0.002 0.04 - 0.05 - 0.005 StAccetbleel Comparative J 0.11 0.2 2.5 0.02 0.001 0.03 0.002 0.30 - 0.07 - 0.044 ComSativel K 0.04 0.2 1.4 0.02 0.001 0.03 0.002 0.08 - 0.03 - 0.011 Com arative S0.04 0.2 1.5 0.02 0.0010.03 0.002 0.08 - 0.03 - 0.011 AccStble M 0.04 0.2 2.0 0.02 0.001 0.03 0.002 0.08 - 0.03 - 0.011 Acceptable S0.04 0.2 3.5 0.02 0.0010.03 0.0020.00.03 .03 - 0.011 Steel 0.002 0.08 - 0.03 - 0.011 Acceptable P0.04 0.01 3.7 0.02 0.001 0.03 Stee.0 .1 .09Acale S0.02 0.2 2.5 0.01 0.001 0.03 0.002 0.07 - 0.01 - 0.011 Steel Acceptable S0.02 0.2 2.5 0.01 0.0010.03 0.002 0.07 - 0.01 - 0.011 Steel S0.04 0.2 3.7 0.02 0.0010.03 0.002 0.07 - 0.01 - 0.01109 Acceptable 0.02 0.2 2.5 0.01 0.001 .5 Acceptable U 0.02 0.01 2.5 0.01 0.001 1.0 0.002 0.07 - 0.01 - 0.009 StAcctbleel Acceptable Q 0.02 1.5 2.5 0.01 0.0010.03 0.002 0.07 - 0.01 - 0.009 Steel R 0.02 0.2 2.5 0.01 0.001 0.5 0.002 0.07 - 0.01 - 0.009 Acceptable I Steel 0.01 .009Acceptable 0.02 0.2 2.510.0110.001 1.0 0.002 0.07 - .1 - 009 Steel U 0.02 0.2 2.5 0.01 0.001 1.05 0.002 0.07 - 0.01 - 0.009 Acceptable Steel ~Acceptable S0.02 1.5 2.5 0.01 0.001 10.0 0.002 0.08 -:0.0 0.01 - 0.009 0.011 AccStble W 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Ti:0.01 0.02 0.0000.011 Acceptable Steel Acceptable W 0.04 0.2 2.5 0.02 0.0010.03 0.002 0.08 Ti:0.05 0.02 0.0000.011 Steel Acceptable Y 0.04 0.2 2.5 0.02 0.0010.00.0.002 0.08 V:0.05 0.02 0.0020.011 acceptable I_ _ _ Steel Z 0.08 0.2 2.5 0.02 0.001 0.03 0.002 0.08 V:0.20 0.02 0.0020.011 Acceptable L _ I__ I_ _ _ ____ _ Steel Note) In case of adding no Ti or V, X value = C+(12/14)xN-(12/92.9)xNb In case of adding Ti or V, X value = C+(12/14)xN*-(12/92.9)x Nb-(12/47.9)xTi*-(12/50.9)xV Y value = (14/92.9)x(Nb-0.01) provided that N*=N-(14/47.9)xTi at N-(14/47.9)xTi>0, N*=0 at N-(14/47.9)xTi0, Ti*=Ti-(47.9/14)xN-(47.9/32.1)xS at Ti-(47.9/14)xN-(47.9/32.1)xS>0, Ti*=0 at Ti-(47.9/14)xN-(47.9/32.1)xS0. 05846HH (30/37) [0094] Table 3 Kind Chemical composition (mass %) of other X y Remarks steel C Si Mn P S Al N Nb compo- N* nents value value 7 02 2. - 0 - i:0 10, Acceptable AA 0.07 0.2 2.5 0.02 0.001 0.03 0.002 0.08 V:0.10 0.01 0.000 0.011 AccSteel 0.01, ec lalle AB 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Cr:0.1 0.03 - 0.011 Acctetble AD 0.04 0.2 2.50.02 0.0 1 02 Acceptable AC 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Cr:1.0 0.03 - 0.011 StAccetbleel Acceptable AD 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Ni:0.2 0.03 - 0.011 AccSteble S0.001 0.03 0.002 0.08 :1.0 0.03 - 0.011 Acceptable AE 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Ni:1.0 0.03 - 0.011 Steel AF 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Mo:0.2 0.03 - 0.011 AStce ble A 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 :.0 0.03 - 0.011 Accept ble AG 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Mo:1.0 0.03 - 0.011 Accptble ANcceptable AH 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Cu:0.3 0.03 - 0.011 AccStetble ___ ________ 00020.08:0.0100.03 - 0011Acceptable AK 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Cu:2.0 0.03 - 0.011 A tccetble Ni:0.1e Acceptable AJ 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 :0.0010 0.03 - 0.011 Steel Accept able AK 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0

.

0 8 3:0.0030 0.03 - 0.011 Steel Cur:0.1 AL 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 U 2:0.1' 0.03 - 0.011 Acceptable r:0.1 eptable Cr:0.1, N:0.0010 *0 0~l Acceptable AO 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Mo:0.12' 0.01 0.000 0.011 AccSteble Mo:0.1, te B:0.0010 Ti:0.1, AP 0.06 0.2 2.5 0.02 0.001 0.03 0.002 0.08 0.0 ' 0.06 0.000 0.011 Accettable A:0.31 007 0000 1 o el i1:0.0 00 V:0.02, C r:0 .2 , c p t l e AQ 0.04 0.2 2.5 0.02 0.001 0.03 0.002 0.08 Ni:0.2, 0.01 0.000 0.011 Ac Steel Mo :0.1, ie Cu:0 1, B.0005 1i:0.15, c e ta l AR 0.13 0.01 2.0 0.02 0.001 0.03 0.003 0.06V: 0

.

1 0 0.06 0.000 0.008 AccSeptable BC 0.0 0.1 .9 0.1 0.00 0.010.0020.05 i:0.254, 1000000 c~tedl AS 0.15 0.1 2.1 0.03 0.002 0.02 0.002 0.03 :0.10 0.06 0.000 0.003 Aceptbleel Comnarative AT 0.16 0.1 2.2 0.01 0.001 0.03 0.001 0.09 i:0.31 0.07 0.000 0.012 cSrteeIl AX 0.06 0.2 3.9 0.01 0.001 0.03 0.002 0.02 i:0.02 0.05 0.000 0.002 Acceptable eSteel In csofadnnTioV, vaue=+(2/4)-12/2.000 Cornarative AY 0.070.01 4.2 0.02 0.002 0.05 0.001 0.05 i:0.01 0.06 0.000 0.006 eel AZ 0.050.01 2.3 0.03 0.001 0.04 0.002 - - 0.05 - 0.002 arte - 0000Com arqtive BA 0.06 0.01 1.8 0.01 0.001 0.03 0.001 0.01 - 0.06 - 0.000 teI Acceptable BB 0.040.01 1.5 0.01 0.001 0.02 0.003 0.02 i:0.02 0.04 0.000 0.002 Sce t l at ~~~ ~~ 000 N-14479)Pi0 N*0awN(44.9xi0 BC 0.08 0.1 1.9 0.01 0.001 0.01 0.002 0.05 i:0.25 0.01 0.000 0.006 tee Acceptable BD 0.14 0.1 1.8 0.02 0.002 0.0210.002 0.05 Ti:0.45 0.02 0.000 0.006 steel Acceptable BE 0.10 0.1 1.9 0.01 0.001 0.03 0.002 0.05 .2 0 0.000 0.006 Steel -- Acceptble BF 0.12 0.01 1.8 0.02 0.002 0.04 0.001 0.05 V:0.40 0.02 0.000 0.006 St e tl Note) In case of adding no Ti or V, X value = C+(12/14)xN-(12/92.9)xNb In case of adding Ti or V, X value = C+(12/14)xN*-(12/92.9)x Nb-(12/47.9)xTi*-(12/50.9)xV Y value = (14/92.9)x(Nb-0.01) provided that N*=N-(14/47.9)xTi at N-(14/47.9)xTi>0, N*=0 at N-(14/47.9)xTir0, Ti*=Ti-(47.9/14)xN-(47.9/32.1)xS at Ti-(47.9/14)xN-(47.9/32.1)xS>0, Ti*=0 at Ti-(47.9/14)xN-(47.9/32.1)xS<0. 05846HH (31/37) [0095] Table 4 Mechanical Steel texture erie properties Kind of Fraction Fraction of Remarks steel offerrite martensite TS El E phase phase (MPa) (%) (GPa) (%) (%) B 95 4 600 30 252 Invention Example C 100 0 530 32 255 Comparative Example D 50 46 990 15 240 Invention Example E 50 55 1060 12 235 Invention Example F 75 20 860 18 243 Invention Example G 80 19 890 16 242 Invention Example H 70 26 750 21 240 Invention Example I 70 25 750 22 235 Invention Example J 30 68 1180 10 220 Comparative Example K 90 8 570 30 231 Comparative Example L 85 12 590 29 241 Invention Example M 80 17 650 28 242 Invention Example N 60 35 860 17 242 Invention Example O 50 50 890 16 235 Invention Example P 98 1 590 30 253 Invention Example Q 90 7 630 30 248 Invention Example R 94 3 620 29 242 Invention Example S 94 3 630 29 241 Invention Example T 93 4 640 28 240 Invention Example U 92 4 650 27 240 Invention Example V 70 25 810 20 246 Invention Example W 75 23 780 21 247 Invention Example X 73 24 810 19 245 Invention Example Y 72 22 800 20 246 Invention Example Z 68 28 890 15 243 Invention Example 05846HH (32/37) [0096] Table 5 Mechanical Steel texture erie properties Kind of Fraction Fraction of Remarks steel offerrite martensite TS El E phase phase (MPa) (%) (GPa) (%) (%) AA 85 13 780 20 248 Invention Example AB 65 30 810 19 245 Invention Example AC 60 36 850 17 242 Invention Example AD 64 30 810 19 245 Invention Example AE 58 37 860 17 241 Invention Example AF 65 31 820 18 243 Invention Example AG 59 37 870 17 241 Invention Example AH 67 29 810 20 243 Invention Example AI 60 33 840 17 242 Invention Example AJ 60 34 850 17 243 Invention Example AK 50 43 900 15 241 Invention Example AL 63 34 820 18 242 Invention Example AM 61 34 820 18 241 Invention Example AN 59 37 860 17 242 Invention Example AO 57 38 870 16 240 Invention Example AP 82 15 760 22 248 Invention Example AQ 84 13 800 20 247 Invention Example AR 70 25 900 15 235 Invention Example AS 68 30 950 13 226 Invention Example AT 45 55 920 14 213 Comparative Example AX 60 35 850 16 225 Invention Example AY 40 60 1000 13 208 Comparative Example AZ 75 20 760 21 210 Comparative Example BA 73 21 770 20 215 Comparative Example BB 80 18 700 24 226 Invention Example BC 70 28 820 17 245 Invention Example BD 73 25 920 14 240 Invention Example BE 80 20 800 20 241 Invention Example BF 70 28 890 17 243 Invention Example 05846HH (33/37) [0097] In the steel C, the C content (X-value) not fixed as a carbonitride is as small as 0.00%, and the ferrite phase is 100%, and the fraction of the second phase is 0%, and TS is smaller than the acceptable range of the invention. In the steel J, the X-value is as 5 high as 0.07%, and the Young's modulus is smaller than the acceptable range of the invention. In the steel K, the Mn content is as low as 1.4%, and TS is smaller than the acceptable range of the invention. In the steel AT, the C content is as high as 0.16%, and the X-value is as high as 0.07, and the Young's modulus is smaller than the acceptable 10 range of the invention. In the steel AZ, the Mn content is as large as 4.2%, and the Young's modulus is smaller than the acceptable range of the invention. In the steel AZ, Nb is not contained, while in the steel BA, the Mb content is as small as 0.01%, so that the Young's modulus is smaller than the acceptable range of the invention. 15 With respect to the other steels, all items are within the acceptable range of the invention, and TS and Young's modulus satisfy the acceptable range of the invention. INDUSTRIAL APPLICABILITY [0098] According to the invention, it is possible to provide high 20 stiffness high-strength thin steel sheets having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 225 GPa. 05846HH (34/37)

Claims (6)

1. A high-stiffness high-strength thin steel sheet comprising C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.5-4.0%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not 5 more than 0.01% and Nb: 0.02-0.40% as mass%, provided that C, N and Nb contents satisfy the relationships of the following equations (1) and (2): 0.01 sC+(12/14)xN-(12/92.9)xNb r 0.06 ..... (1) N r (14/92.9)x(Nb-0.01) ....

(2) 10 and the remainder being substantially iron and inevitable impurities, and having a texture comprising a ferrite phase as a main phase and having a martensite phase at an area ratio of not less than 1%, and having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 225 GPa. 15 2. A high-stiffness high-strength thin steel sheet according to claim 1, which further contains one or two of Ti: 0.01-0.50% and V: 0.01-0.50% as mass% in addition to the above composition and satisfy the relationships of the following equations (3) and (4) instead of the equations (1) and (2): 20 0.01 C+(12/14)xN*-(12/92.9) xNb-(12/47.9)xTi*-(12/50.9)xV r 0.06 ..... . (3) N* : (14/92.9)x(Nb-0.01) .. . . (4) provided that N* in the equations (3) and (4) is N* = N-(14/47.9)xTi at N-(14/47.9)xTi > 0 and N* = 0 at N-(14/47.9)xTi 5 0, and Ti* in the 25 equation (3) is Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS at Ti-(47.9/14)xN (47.9/32.1)xS > 0 and Ti* = 0 at Ti-(47.9/14)xN-(47.9/32.1)xS r 0.

3. A high-stiffness high-strength thin steel sheet according to claim 1 or 2, which further contains one or more of Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% as 30 mass% in addition to the above composition.

4. A method for producing a high-stiffness high-strength thin steel sheet comprising subjecting a starting material of steel comprising C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.5-4.0%, 05846HH (35/37) P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more than 0.01% and Nb: 0.02-0.40% as mass%, provided that C, N and Nb contents satisfy the relationships of the following equations (1) and (2): 5 0.01 <C+(12/14)xN-(12/92.9)xNb s 0.06 ..... . (1) N - (14/92.9)x(Nb-0.01) ..... . (2) to a hot rolling step under conditions that a total rolling reduction below 950 0 C is not less than 30% and a finish rolling is terminated at Ar 3 -900°C, coiling the hot rolled sheet below 650'C, pickling, 10 subjecting to a cold rolling at a rolling reduction of not less than 50%, raising a temperature to 780-900 0 C at a temperature rising rate from 500'C of 1-40°C/s to conduct soaking, and then cooling at a cooling rate up to 500 0 C of not less than 5C/s to conduct annealing.

5. A method for producing a high-stiffness high-strength thin 15 steel sheet according to claim 4, wherein the starting material of steel further contains one or two of Ti: 0.01-0.50% and V: 0.01-0.50% as mass% in addition to the above composition and satisfies the relation ships of the following equations (3) and (4) instead of the equations (1) and (2): 20 0.01 r C+(12/14)xN*-(12/92.9) xNb-(12/47.9)xTi*-(12/50.9)xV r 0.06 ..... . (3) N* - (14/92.9)x(Nb-0.01) ..... . (4) provided that N* in the equations (3) and (4) is N* = N-(14/47.9)xTi at N-(14/47.9)xTi > 0 and N* = 0 at N-(14/47.9)xTi r 0, and Ti* in the 25 equation (3) is Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS at Ti-(47.9/14)xN (47.9/32.1)xS > 0 and Ti* = 0 at Ti-(47.9/14)xN-(47.9/32.1)xS r 0.

6. A method for producing a high-stiffness high-strength thin steel sheet according to claim 4 or 5, wherein the staring material of steel further contains one or more of Cr: 0.1-1.0%, Ni: 0.1-1.0%, 30 Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% as mass% in addition to the above composition. 05846HH (36/37)