JP5257239B2 - High strength low specific gravity steel plate excellent in ductility, workability and toughness, and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength low specific gravity steel plate excellent in ductility, workability and toughness used for automobile parts and the like, and a method for producing the same.

  In recent years, in order to cope with environmental problems, it has been desired to reduce the weight of automobiles for the purpose of reducing carbon dioxide emissions and reducing fuel consumption. Increasing the strength of steel is an effective means for reducing the weight of automobiles, but if the lower limit of the plate thickness is limited by the rigidity of the member, the plate thickness can be reduced even if the strength is increased. It was not possible to reduce the weight.

  Therefore, some of the present inventors have proposed a high Al-containing steel sheet in which the specific gravity is reduced by adding a large amount of aluminum to iron (see, for example, Patent Documents 1 to 5). These solve the problems of the high Al-containing steel sheet that (a) the productivity is inferior (particularly, cracking occurs during rolling) and (b) the ductility is low. Furthermore, the present inventors have proposed a method for forming a solidified structure after casting into a fine equiaxed crystal structure in order to improve the ductility, hot workability and cold workability of the high Al content steel (for example, patents). Reference 6,).

JP 2005-15909 A JP 2005-29889 A JP 2005-273004 A JP 2006-176843 A JP 2006-176844 A JP 2008-261023 A

  Recently, it is becoming possible to produce a high Al-containing steel sheet excellent in ductility and workability on an industrial scale. However, the high Al-containing steel sheet has lower toughness than a general automotive steel sheet having the same strength, and has limited applications. Therefore, improvement of toughness is an important issue in order to expand the application range of high Al-containing steel sheets to automobile parts.

  In particular, when the ductile brittle transition temperature (vTrs) of the Charpy impact test is −60 ° C. or lower, application to many structural members for automobiles becomes possible. In view of such circumstances, the present invention improves the toughness of a low specific gravity steel sheet to which Al is added, and provides a high strength low specific gravity steel sheet excellent in ductility, workability and toughness, and a method for producing the same.

  In order to increase the toughness of the high Al-containing steel, the present inventors have examined elements that lower the toughness in addition to equiaxed crystallization of the solidified structure. As a result, the toughness of high Al content steel with a fine equiaxed crystal structure in the cast structure is greatly affected by Al content and Si content, Si content is reduced, and the product of Al content and Si content is reduced. It has been found that toughness can be significantly improved by limiting the above. The gist of the present invention is as follows.

(1) In mass%,
C: more than 0.100%, 0.500% or less,
Si: less than 0.20%,
Mn: more than 0.20%, 3.00% or less,
Al: 3.0 to 10.0%,
N: 0.0030 to 0.0100%,
Ti: more than 0.100%, containing 1.000% or less,
P: 0.0200% or less,
S: limited to 0.0100% or less, the content of C and Ti (% by mass)
0.200 <C + Ti ≦ 1.500
And the product of the content (mass%) of Al and Si is
Al × Si ≦ 0.8
A high-strength, low-specific gravity steel sheet excellent in ductility, workability and toughness, characterized in that the balance consists of Fe and inevitable impurities, and the specific gravity is less than 7.5.

(2) Furthermore, in mass%,
Nb: 0.300% or less,
V: A high strength low specific gravity steel plate excellent in ductility, workability and toughness as described in (1) above, containing one or both of 0.50% or less.

(3) Furthermore, in mass%,
Cr: 3.00% or less,
Mo: 3.00% or less,
Ni: 5.00% or less,
Cu: A high strength low specific gravity steel sheet excellent in ductility, workability and toughness as described in (1) or (2) above, containing one or more of 3.00% or less.

(4) Furthermore, in mass%,
B: The high strength low specific gravity steel sheet excellent in ductility, workability and toughness according to any one of the above (1) to (3), characterized by containing 0.0100% or less.

(5) Furthermore, in mass%,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
Zr: 0.0500% or less,
REM: 0.0500% or less of one type or two or more types, (1) to (4) above, high strength and low excellent in ductility, workability and toughness Specific gravity steel plate.

(6) High strength excellent in ductility, workability and toughness according to any one of (1) to (5) above, wherein the tensile strength is 440 MPa or more and the elongation is 25% or more Low specific gravity steel plate.

(7) A method for producing the high-strength low specific gravity steel sheet according to any one of (1) to (6) above, wherein the component according to any one of (1) to (5) is used. A method for producing a high strength low specific gravity steel sheet excellent in ductility, workability and toughness, characterized in that the resulting steel is cast at a molten steel superheat degree of 50 ° C. or less and the obtained steel slab is hot rolled.

(8) Ductility characterized by heating the steel slab produced by the method described in (7) above to 1100 ° C or higher, hot rolling at a finish rolling temperature of 800 ° C or higher, and winding at 600 to 750 ° C. The manufacturing method of the high strength low specific gravity steel plate excellent in workability and toughness.

(9) Manufacture of a high strength low specific gravity steel sheet excellent in ductility, workability and toughness, characterized by annealing the steel sheet manufactured by the method described in (7) or (8) above at 700 to 1100 ° C. Method.

(10) After pickling the steel plate manufactured by the method of any one of said (7)-(9), annealing at 600-1100 degreeC which makes a cold rolling rate 20-95%, 20 degreeC The manufacturing method of the high strength low specific gravity steel plate excellent in ductility, workability, and toughness characterized by cooling to 200 degrees C or less with the cooling rate of / s or more.

  According to the present invention, a high-strength low specific gravity steel plate having good manufacturability and excellent ductility, workability and toughness can be obtained, and the industrial contribution is extremely remarkable.

The relationship between the amount of C + Ti (mass%) and the equiaxed crystal ratio is shown.

The relationship between the product of Al content (mass%) and Si content (mass%) and the brittle ductile fracture surface transition temperature vTrs (° C.) is shown.

The inventors of the present invention have Fe-0.09% Si-1.30% Mn-0.0091% P-0.0021% S-4% Al-0.006% N as basic components, and C and Ti. Steels with various addition amounts were melted and cast in the laboratory, and the equiaxed crystal ratio was measured by observing the solidified structure. The degree of superheated molten steel during casting was set to 50 ° C. or less. Here, the molten steel superheat degree [° C.] is a value obtained by subtracting the molten steel temperature [° C.] during casting from the liquidus temperature [° C.] determined from the component composition,
Molten steel superheat degree [° C.] = Molten steel temperature [° C.] − Liquidus temperature [° C.]
It is.

  FIG. 1 shows the relationship between the C content [% by mass] and the Ti content [% by mass], and the relationship between the C + Ti content [% by mass] and the equiaxed crystal ratio. From FIG. 1, when C + Ti is in the range of more than 0.200% and 1.500% or less, the equiaxed crystal ratio is 80% or more, especially when C + Ti is in the range of 0.5 to 1.0% or less. It can be seen that the axial ratio is 100%. However, if either one of C and Ti is 0.1% or less, the equiaxed crystal ratio is significantly reduced. Further, it has been found that when C + Ti exceeds 1.5%, cracks are likely to occur in the slab. Therefore, C + Ti is limited to a range of more than 0.2 to 1.5%.

  Next, the present inventors have made 0.11% C-0.12% Si-1.5% Mn-0.0086% P-0.0057% S-0.4% Ti-0.0065% N As a basic component, hot rolled sheets were manufactured in a laboratory for various steels with different amounts of Al and Si added, and toughness was evaluated. The hot-rolled sheet has a tensile strength of about 500 MPa and a sheet thickness of 3.6 mm. The toughness was evaluated based on the brittle ductile fracture surface transition temperature vTrs [° C.] in accordance with JIS Z 2242. The Charpy impact test was performed using a sub-size V-notch test piece having a thickness of 2.5 mm.

  The results are shown in FIG. FIG. 2 shows the brittle ductile fracture surface transition temperature vTrs [° C.] arranged by the product of Al content and Si content, Al × Si. As shown in FIG. 2, it was found that the brittle ductile fracture surface temperature decreased and the toughness improved as the Al content [mass%] × Si content [mass%] was decreased. In particular, in order to set vTrs [° C.] to −60 ° C. or less, it is necessary to set the upper limit of Al content [mass%] × Si content [mass%] to 0.8 or less.

  Next, the reasons for limiting the components of the high-strength low-specific gravity steel sheet having excellent toughness in the present invention will be described.

  C: C is an essential element for making the solidified structure a fine equiaxed crystal structure, and needs to be added in an amount exceeding 0.100%. On the other hand, addition of C exceeding 0.500% deteriorates toughness and weldability. Therefore, the C content is set to more than 0.100% to 0.500% or less.

  Ti: Ti is also an essential element for making the solidified structure into a fine equiaxed crystal structure, and needs to be added in an amount exceeding 0.100%. On the other hand, addition of Ti exceeding 1.000% degrades toughness. Therefore, the Ti content is set to more than 0.100% to 1.000% or less. In order to obtain a finer equiaxed crystal structure, the lower limit of the Ti content is preferably set to 0.300%.

  As described above, the total amount of addition of C and Ti, that is, C + Ti is made more than 0.200% and not more than 1.500%, so that the solidified structure can be made into a fine equiaxed crystal structure. it can.

  Al: Al is an essential element for achieving low specific gravity. If the Al content is less than 3.0%, the effect of lowering the specific gravity is insufficient, and the specific gravity cannot be less than 7.5. On the other hand, if the Al content exceeds 10.0%, precipitation of intermetallic compounds becomes remarkable, and ductility, hot workability, and cold workability deteriorate. Therefore, the content of Al is set to 3.0 to 10.0%. In order to obtain better ductility, the upper limit of the Al content is preferably 6.0%.

  Si: Si is an element that degrades the toughness of high Al-containing steel, and it is desirable to reduce it. The upper limit of the Si content is limited to less than 0.2% in consideration of the current refining technology and manufacturing costs. Although the lower limit of the Si content is not specified, it is preferable to contain 0.01% or more of Si from the current refining technology and manufacturing cost.

  As described above, by setting the product of the addition amounts of Al and Si, that is, Al × Si, to 0.8 or less, extremely good toughness can be obtained. The Al content [mass%] × Si content [mass%] is desirably as low as possible, and the lower limit is not specified. However, like the lower limit of Si, 0.03 It is preferable to make it above.

  Mn: Mn is an element effective for forming MnS and suppressing grain boundary embrittlement due to solute S. However, if the amount of Mn is 0.20% or less, the effect is not exhibited, and if it exceeds 3.00% and added excessively, the toughness deteriorates. Therefore, the Mn content is set to more than 0.20% and not more than 3.00%.

  P: P is an impurity element that segregates at the grain boundary to lower the grain boundary strength and degrade the toughness, and is desirably reduced. The upper limit of the P content is limited to 0.0200% in consideration of the current refining technology and manufacturing costs.

  S: S is an impurity element that degrades hot workability and toughness, and is desirably reduced. The upper limit of the S content is limited to 0.0100% in consideration of the current refining technology and manufacturing costs.

  N: N is an essential element for forming Ti nitrides and carbonitrides, that is, TiN and Ti (C, N), and making the solidified structure a fine equiaxed crystal structure. This effect is not exhibited when the N content is less than 0.0030%, and when N exceeding 0.0100% is added, the toughness deteriorates due to the formation of coarse TiN. Therefore, the N content is set to 0.0030 to 0.0100%.

  The above are the basic components of the present invention, which are usually composed of Fe and unavoidable impurities other than the above, but depending on the desired strength level and other necessary characteristics, Nb, V, Cr, Ni, Mo, Cu, B , Ca, Mg, Zr, or REM may be added.

  Nb: Nb is an element that forms fine carbonitrides and is effective in suppressing the coarsening of crystal grains. In order to increase toughness, it is preferable to add 0.005% or more of Nb. However, when Nb is added excessively, the precipitate becomes coarse and the toughness may be deteriorated. Therefore, the Nb content is preferably 0.300% or less.

  V: V, like Nb, is an element that forms fine carbonitrides. In order to suppress coarsening of crystal grains and increase toughness, it is preferable to add 0.01% or more of V. If the V content exceeds 0.50%, the toughness may deteriorate, so the upper limit of the V content is preferably 0.50% or less.

  Cr, Mo, Ni, Cu: Cr, Mo, Ni, Cu are effective elements that improve ductility and toughness. However, if the Cr, Mo, and Cu contents are each 3.00% and the Ni content exceeds 5.00%, the toughness may be impaired due to the increase in strength. Therefore, the upper limit of the Cr amount is preferably 3.00% or less, the upper limit of the Mo amount is 3.00% or less, the upper limit of the Ni amount is 5.00% or less, and the upper limit of the Cu amount is preferably 3.00% or less. In order to improve ductility and toughness, the Cr content is preferably 0.05 or more, the Mo content is 0.05% or more, the Ni content is 0.05% or more, and the Cu content is preferably 0.10% or more.

  B: B is an element that segregates at the grain boundaries and suppresses the grain boundary segregation of P and S. However, when the amount of B exceeds 0.0100%, precipitates are formed, and hot workability may be impaired. Therefore, the B content is set to 0.0100% or less. In order to improve ductility, toughness, and hot workability by strengthening grain boundaries, 0.0003% or more of B is preferably added.

  Ca, Mg, Zr, REM: Ca, Mg, Zr, and REM are elements that control the form of sulfide and are effective in suppressing hot workability and toughness deterioration due to S. However, since the effect is saturated even if it is added in excess, Ca may be added at 0.0100% or less, Mg at 0.0100% or less, Zr at 0.0500% or less, and REM at 0.0500% or less. preferable. In order to improve toughness, it is preferable to add 0.0010% or more of Ca, 0.0005% or more of Mg, 0.0010% or more of Zr, and 0.0010% or more of REM.

  Next, the characteristics of the high strength and low specific gravity steel sheet of the present invention will be described.

  The specific gravity is less than 7.5 when the specific gravity is 7.5 or more, because the effect of weight reduction is small compared to the specific gravity of steel plates normally used as automotive steel plates (same as iron specific gravity of 7.86). The specific gravity of the steel sheet is determined by the component composition, and it is preferable to increase the content of Al that contributes to weight reduction.

  The strength and ductility are preferably a tensile strength of 440 MPa or more and an elongation of 25% or more in consideration of characteristics necessary for an automobile steel plate.

  Next, a manufacturing method will be described.

  In the present invention, steel composed of the above components is cast at a molten steel superheat degree of 50 ° C. or less, and the obtained steel slab is hot-rolled. Further, pickling, cold rolling and annealing may be performed.

  If the molten steel overheating temperature exceeds 50 ° C, TiN or Ti (C, N) crystallized in the liquid phase aggregates and coarsens, so it does not function effectively as a solidification nucleus of ferrite, and the component range of the present invention Even within, the solidified structure becomes a coarse columnar crystal structure. Therefore, the superheat degree of the molten steel is set to 50 ° C. or less. Although the lower limit of the molten steel superheat degree is not specified, it is usually 10 ° C. or higher.

  When the heating temperature of the hot rolling is less than 1100 ° C., the carbonitride is not sufficiently dissolved, and the required strength and ductility may not be obtained. Therefore, the lower limit of the heating temperature is preferably 1100 ° C. The upper limit of the heating temperature is not particularly defined, but if it exceeds 1250 ° C., the particle size becomes large and the hot workability may be lowered, so that it is preferably 1250 ° C. or less.

  When the finish rolling temperature is less than 800 ° C., the hot workability deteriorates and cracks may occur during hot rolling. Therefore, the lower limit of the finish rolling temperature is preferably 800 ° C. The upper limit of the finishing temperature is not particularly specified, but if it exceeds 1000 ° C, the particle size becomes large, and cracking may occur during cold rolling.

  When the coiling temperature is less than 600 ° C., the recovery and recrystallization of ferrite become insufficient, and the workability may be impaired. Therefore, the lower limit of the winding temperature is preferably 600 ° C. On the other hand, when the coiling temperature exceeds 750 ° C., the recrystallized ferrite crystal grains become coarse, and ductility, hot workability, and cold workability may deteriorate. Therefore, the upper limit of the winding temperature is preferably 750 ° C.

  In order to improve the ductility of the hot-rolled sheet, it is preferable to anneal after hot rolling. The annealing temperature of the hot-rolled sheet is preferably 700 ° C. or higher in order to control the form of the precipitate and improve the ductility. Moreover, when the annealing temperature of a hot-rolled sheet exceeds 1100 degreeC, a crystal grain may coarsen and grain boundary embrittlement may be promoted. Therefore, the upper limit of the annealing temperature of the hot-rolled sheet is preferably 1100 ° C. or less.

  Cold rolling and annealing may be performed on the hot-rolled steel sheet to produce a cold-rolled steel sheet. Below, the preferable manufacturing conditions of a cold-rolled steel plate are described.

  The cold rolling rate of cold rolling is preferably 20% or more from the viewpoint of productivity. In order to promote recrystallization during annealing, the cold rolling rate is preferably 50% or more. If the cold rolling rate exceeds 95%, cracks may occur during cold rolling. Therefore, the upper limit of the cold rolling rate is preferably 95% or less.

  The annealing temperature after cold rolling is preferably set to 600 ° C. or higher in order to sufficiently advance recrystallization and recovery. On the other hand, when the annealing temperature after cold rolling exceeds 1100 ° C., the crystal grains are coarsened and grain boundary embrittlement may be promoted. Therefore, the upper limit of the annealing temperature of the cold-rolled sheet is preferably 1100 ° C. or less.

  The cooling rate after annealing of the cold-rolled steel sheet is preferably 20 ° C./s or more, and the cooling stop temperature is preferably 200 ° C. or less. This is to prevent grain coarsening due to grain growth during cooling and grain boundary embrittlement due to segregation of impurity elements such as P to the grain boundaries, thereby improving ductility. Although the upper limit of the cooling rate is not specified, it is technically difficult to exceed 500 ° C./s. Further, the lower limit of the cooling stop temperature depends on the temperature of the refrigerant, and it is difficult to make it lower than room temperature.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
[Example 1]
Steel having the composition shown in Table 1 was cast at the molten steel superheat degree shown in Table 1. The solidified structure of the obtained steel piece was observed and the equiaxed crystal ratio was measured. The steel slab was hot-rolled under the conditions shown in Table 2, and the crack occurrence state of the hot-rolled sheet was observed after hot rolling. The crack occurrence of hot-rolled sheets is evaluated as “◯” when no cracks occur, “△” when micro-ear cracks occur, and “X” when large cracks are observed. did. Furthermore, the specific gravity, mechanical properties, and toughness of the plate after hot rolling were evaluated.

  The specific gravity was measured using a pycnometer. The tensile test was conducted in accordance with JIS Z 2241. The toughness was evaluated by a Charpy impact test in accordance with JIS Z 2242 and by the brittle ductile fracture surface transition temperature (vTrs [° C.]). The test piece used was a sub-size V-notch test piece having a thickness of 2.5 mm conforming to JIS Z 2202 (produced by grinding the upper and lower surfaces of the steel sheet when the steel sheet thickness is greater than 2.5 mm). Table 2 shows specific gravity, yield stress, tensile strength, elongation, brittle ductile fracture surface transition temperature, and crack occurrence.

  Hot rolling No. 1 to 11 are examples of the present invention, satisfying specific gravity <7.5, tensile strength is 440 MPa or more, high elongation of 25% or more is obtained with respect to ductility, and brittle ductile fracture surface transition temperature is It is -60 degrees C or less, and the crack of a hot-rolled sheet does not generate | occur | produce. Moreover, hot rolling No. 1 differs only in manufacturing conditions. No. 1 and hot rolled No. 1 When 9 to 11 are compared, it can be seen that better ductility and toughness can be obtained under preferable production conditions.

On the other hand, hot rolling No. using steel IM whose components are outside the scope of the present invention. In any of 12 to 16, the elongation is 20% or less, the brittle ductile fracture surface transition temperature is 20 ° C. or more, and it is understood that the ductility and toughness are inferior. Moreover, in these comparative examples, the hot-rolled sheet is also cracked and inferior in hot workability.
[Example 2]

  The hot-rolled sheet manufactured under the conditions shown in Table 2 was further annealed under the conditions shown in Table 3. These hot-rolled annealed plates were also evaluated for specific gravity, mechanical properties, and toughness in the same manner as in Example 1. Table 3 shows the specific gravity, yield stress, tensile strength, elongation, brittle ductile fracture surface transition temperature, and crack generation status of the hot-rolled annealed sheet.

  Annealing No. 1 to 11 are examples of the present invention, satisfying specific gravity <7.5, tensile strength is 440 MPa or more, high elongation of 25% or more is obtained with respect to ductility, and brittle ductile fracture surface transition temperature is It is -60 degrees C or less. Moreover, annealing No. which has the same composition and differs only in manufacturing conditions. 1 and annealing no. Comparing 9 to 11, it can be seen that the ductility and toughness are further improved by the preferable production conditions.

On the other hand, annealing No. 1 using steels I to M whose components are outside the scope of the present invention. In any of 12 to 16, the elongation is 20% or less, the brittle ductile fracture surface transition temperature is 20 ° C. or more, and it is understood that the ductility and toughness are inferior.
[Example 3]

  About the hot-rolled sheet which hot-rolled the steel which has the composition shown in Table 1 on the conditions shown in Table 2, it cold-rolled on the conditions shown in Table 4, and observed the crack generation condition of the cold-rolled sheet. As for the occurrence of cracks in cold-rolled sheets, “○” indicates that no crack has occurred, “△” indicates that a minute ear crack has occurred, and “×” indicates that some large cracks have been observed. did. Further, the cold-rolled sheet was annealed, and the specific gravity, mechanical properties, and toughness of the cold-rolled sheet were also evaluated in the same manner as in Example 1. Table 4 shows the specific gravity, yield stress, tensile strength, elongation, brittle ductile fracture surface transition temperature, and crack generation status of the cold-rolled annealed sheet.

  Cold rolled No. 1 to 11 satisfy specific gravity <7.5, tensile strength is 440 MPa or more, high elongation of 25% or more is obtained with respect to ductility, and brittle ductile fracture surface transition temperature is −60 ° C. or less. The cold-rolled plate is not cracked. Moreover, it has cold rolling No. which has the same composition and differs only in manufacturing conditions. 1 and cold rolled No. 1 When comparing 9 to 11, it can be seen that better ductility and toughness can be obtained by satisfying preferable production conditions.

On the other hand, cold rolling No. using steel FJ whose component is outside the scope of the present invention. In any of 12 to 16, the elongation is 20% or less, the brittle ductile fracture surface transition temperature is 20 ° C. or more, and it is understood that the ductility and toughness are inferior. Moreover, the crack of the cold-rolled board has also generate | occur | produced and it turns out that it is inferior to cold workability.

  INDUSTRIAL APPLICABILITY The present invention can provide a high-strength, low-specific gravity steel plate that has good manufacturability and is excellent in ductility, workability, and toughness, can provide a steel plate that contributes to weight reduction of an automobile, and has extremely high industrial applicability. high.

Claims (10)

% By mass
C: more than 0.100%, 0.500% or less,
Si: less than 0.20%,
Mn: more than 0.20%, 3.00% or less,
Al: 3.0 to 10.0%,
N: 0.0030 to 0.0100%,
Ti: more than 0.100%, containing 1.000% or less,
P: 0.0200% or less,
S: limited to 0.0100% or less, the content of C and Ti (% by mass)
0.200 <C + Ti ≦ 1.500
And the product of the content (mass%) of Al and Si is
Al × Si ≦ 0.8
A high-strength, low-specific gravity steel sheet excellent in ductility, workability and toughness, characterized in that the balance consists of Fe and inevitable impurities, and the specific gravity is less than 7.5. Furthermore, in mass%,
Nb: 0.300% or less,
The high strength low specific gravity steel sheet excellent in ductility, workability, and toughness according to claim 1, wherein one or both of V: 0.50% or less are contained. Furthermore, in mass%,
Cr: 3.00% or less,
Mo: 3.00% or less,
Ni: 5.00% or less,
Cu: 3.00% or less of 1 type, or 2 or more types, The high strength low specific gravity steel plate excellent in ductility, workability, and toughness of Claim 1 or 2 characterized by the above-mentioned. Furthermore, in mass%,
B: 0.0100% or less is contained, The high strength low specific gravity steel plate excellent in ductility, workability, and toughness of any one of Claims 1-3 characterized by the above-mentioned. Furthermore, in mass%,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
Zr: 0.0500% or less,
REM: 0.0500% or less of 1 type or 2 types or more, The high strength low specific gravity steel plate excellent in ductility, workability, and toughness of any one of Claims 1-4 characterized by the above-mentioned.   The high strength and low specific gravity steel sheet having excellent ductility, workability and toughness according to any one of claims 1 to 5, wherein the tensile strength is 440 MPa or more and the elongation is 25% or more.   A method for producing the high strength and low specific gravity steel sheet according to any one of claims 1 to 6, wherein the steel comprising the component according to any one of claims 1 to 5 has a molten steel superheat degree of 50. A method for producing a high-strength, low-specific gravity steel sheet excellent in ductility, workability and toughness, characterized by casting at a temperature of ℃ or less and hot rolling the obtained steel slab.   The steel slab produced by the method according to claim 7 is heated to 1100 ° C. or higher, hot rolled at a finish rolling temperature of 800 ° C. or higher, and wound at 600 to 750 ° C. A method for producing a high-strength, low-specific gravity steel sheet with excellent toughness.   A method for producing a high strength and low specific gravity steel sheet excellent in ductility, workability and toughness, characterized by annealing the steel sheet produced by the method according to claim 7 or 8 at 700 to 1100 ° C.   After pickling the steel plate manufactured by the method of any one of Claims 7-9, performing cold rolling which makes a cold rolling rate 20-95%, annealing at 600-1100 degreeC, then 20 degreeC The manufacturing method of the high strength low specific gravity steel plate excellent in ductility, workability, and toughness characterized by cooling to 200 degrees C or less with the cooling rate of / s or more.

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