JP4464811B2 - Manufacturing method of high strength and low specific gravity steel sheet with excellent ductility - Google Patents

Description

The present invention relates to method of producing a high strength low density steel sheet excellent in ductility to be used in automobiles parts.

  In recent years, in order to cope with environmental problems, it is 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. However, if the plate thickness is limited by the rigidity of the member, the plate thickness cannot be reduced even if the strength is increased. , It was difficult to reduce the weight.

  As a means to achieve weight reduction in the above case, it is conceivable to use an aluminum alloy plate having a specific gravity lower than that of steel, but in addition to being expensive, aluminum alloy plate is inferior in workability compared to steel. However, application to automobile members has been limited due to the disadvantages such as being difficult to weld with steel plates.

  Therefore, a steel plate and an aluminum alloy plate that have the advantages of a high Al content steel plate in which a large amount of aluminum is added to iron is considered. For example, Patent Document 1 discloses that C: 0.002 to 0.1%, Al : 3 to 10%, one or more of Ni, Co, and Cu is 0.01 to 7%, Mn is 5% or less, and 2% or less of one or more of Si and Ti is 0.1 to 2%. A vibration-absorbing alloy having a low specific gravity composed of 6%, O: 0.0005 to 0.04%, N: 0.0002 to 0.05%, residual Fe and inevitable impurities is disclosed. However, such high Al-containing steel sheets can be used as automotive steel sheets because of (i) poor manufacturability (particularly cracking during rolling) and (ii) low ductility. It was difficult.

  In addition, if a large amount of Al is contained, ductility, hot workability and cold workability are greatly deteriorated, and as disclosed in Patent Document 1, annealing at a relatively high temperature for a long time (heating at 650 to 1200 for 5 to 600 minutes). It is difficult to produce a high Al-containing steel plate by a normal thin steel plate production process, for example, continuous annealing, and to ensure a good strength and ductility level.

  As a technique for improving the ductility of a high Al-containing steel sheet, for example, in Patent Document 2, Al: 4 to 9.5%, Ti: 0.5 to 2.0%, Mo: 0.5 to 2%, Zr: The technology of an aluminum-containing iron-base alloy containing 0.1 to 0.8%, C: 0.01 to 0.5% and the remaining Fe has been proposed, but there is no mention of low specific gravity, and it is a heavy element Mo and Zr are essential, and it cannot be said that low specific gravity is considered.

  In addition, for manufacturability, forging and warm rolling are performed. A wide range of industrial manufacturing methods, from so-called melting to hot rolling and cold rolling, and methods using manufacturing equipment, Is different. Further, according to the tests by the present inventors, the proposal of Patent Document 2 has not led to a significant improvement in ductility.

  Patent Document 3 discloses oxidation resistant iron containing C: 0.05% or less, Si: 0.1 to 1%, Al: 2 to 8%, Y: 0.01 to 1%, and the residual Fe. Alloys have been proposed, but there is no mention of low specific gravity, and Y, which is a heavy element, is essential to improve oxidation resistance, and it cannot be said that low specific gravity is considered. Further, there is no mention of strength and ductility, and according to the tests of the present inventors, the proposal of Patent Document 3 has not led to a significant improvement in ductility.

  In Patent Document 4, C: 0.02 to 0.1%, Si ≦ 0.5, Mn: 0.2 to 2.0%, P: ≦ 0.05, S: ≦ 0.01, Al : Steel plates containing 0.5 to 5% and the remaining Fe have been proposed, but since the Al content is as small as 5% or less, the effect of reducing the specific gravity is small. Further, it is described that when Al is added in excess of 5%, the formability and the cold workability are greatly deteriorated, so that the production is difficult.

  In Patent Document 5, Si <0.2%, Mn: 0.03 to 0.2%, Al: 5 to 9%, a total of 1% or less of Cu + Mo + W + Co + Cr + Ni, A steel sheet containing a total of 0.1% or less of Sc + Y + REM and residual Fe has been proposed. Patent Document 6 includes C: 0.0036 to 0.1%, Si <0.2%, Mn: 0.03 to 0.2%, Al: 7 to 9%, total of Cu + Mo + W + Co + Cr + Ni of 1% or less, Sc + Y + REM of 0.1% or less and Steel sheets containing residual Fe have been proposed, but none of the manufacturing techniques for improving formability and manufacturability have been proposed, and according to the tests of the present inventors, steel sheets of these components have been proposed. It has been difficult to manufacture the sheet by a normal sheet steel manufacturing process.

  Further, Patent Document 7 proposes a damping alloy material containing Al: 6 to 10% and residual Fe and having an average crystal grain size in the range of 300 to 700 μm. If it is large, a surface defect (rough skin) called orange peel occurs during press working, so that it is difficult to apply to automobile members. In addition, no manufacturing technique has been proposed for improving moldability and manufacturability. As described above, it has been difficult to produce high-strength, low-specific gravity steel plates with excellent ductility on an industrial scale with the conventional technology.

JP-A-3-140439 JP-A-8-253844 U.S. Pat. No. 4,334,923 Japanese Patent No. 2517492 US Pat. No. 6,383,662B1 Japanese Patent No. 3457331 JP 2001-59139 A

The present invention has been made to solve the above-mentioned problems, and an object thereof is to provide a method of producing a high strength low density steel sheet excellent in ductility.

The inventors of the present invention have studied a method for improving ductility of various materials having a large amount of Al on an iron base and having different components. Even when the amount is as high as 8.0 to 12.0%, 0.8 to 1.2% C and 10.0 to 30.0% Mn are added to make the base structure austenite, and the manufacturing conditions are appropriate. It was found that the ductility can be greatly improved by suppressing the precipitation of ferrite and (Fe, Mn) ₃ AlC (perovskite carbide) phase as much as possible.

(1)
The present invention is configured based on such knowledge, and the gist thereof is an area ratio, austenite is 90% or more, ferrite is 5% or less, and (Fe, Mn) ₃ AlC (perovskite carbide) phase. Is a method of producing a high strength low specific gravity steel sheet having excellent ductility, wherein the specific gravity is less than 1%, the specific gravity <6.8, the tensile strength is 700 MPa or more, and the elongation is 50% or more. 0.8-1.2%, Si: 3.0% or less, Mn: 10.0-30.0%, P: 0.02% or less, S: 0.02% or less, Al: 8.0- A steel slab containing 12.0%, N: 0.001 to 0.05%, the balance being Fe and inevitable impurities is heated to a temperature of 1100 ° C. or higher and 1250 ° C. or lower, and at a temperature of 1050 ° C. or higher. After rough rolling with a total rolling reduction of 85% or higher, reheat to a temperature of 1100 ° C or higher and continue Hot rolled at a finish rolling temperature of 900 ° C or higher, cooled to a temperature of 200 ° C or lower at a cooling rate of 20 ° C / second or higher, and rolled up. It is a manufacturing method .

(2)
After winding the steel plate, it may be annealed at a temperature of 800 ° C. or higher and 1200 ° C. or lower and cooled to a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher .

(3)
In addition, the steel sheet is wound, pickled, cold rolled, annealed at a temperature of 800 ° C. or higher and 1200 ° C. or lower, and cooled to a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher after annealing. You may do it.

(4)
In addition, the steel sheet is wound and annealed, then pickled, cold rolled, annealed at a temperature of 800 ° C. or higher and 1200 ° C. or lower, and after annealing, a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher. You may cool to.

(5)
The steel slab may further contain one or two of Ti: 0.005 to 0.3% and Nb: 0.005 to 0.3% in mass%.

(6)
The steel slab is further in mass%, Cr: 0.05 to 3.0%, Ni: 0.05 to 15.0%, Mo: 0.05 to 3.0%, Cu: 0.1 to 3 0.0%, B: 0.0003 to 0.01%, V: 0.01 to 0.5%, or one or more may be contained .

(7)
Further, the steel slab is further in mass%, Ca: 0.001 to 0.01%, Mg: 0.0005 to 0.01%, Zr: 0.001 to 0.05%, REM: 0.001. You may contain 1 type, or 2 or more types of -0.05% .

  According to the present invention, a high-strength low specific gravity steel plate excellent in ductility can be obtained.

Below, the significance of each requirement in the present invention and the reason for limitation will be specifically described.
First, the reasons for limiting the components of the high strength and low specific gravity steel sheet excellent in ductility in the present invention will be described.

C: C is an austenite stabilizing element and is an essential element for forming an austenite structure in a component system in which a large amount of Al is added. If the mass% is less than 0.8%, even if a large amount of other austenite stabilizing elements such as Mn is added, the formation of ferrite cannot be suppressed and the ductility is lowered. In addition, if it is added in excess of 1.2% by mass, ductility decreases because precipitation of the (Fe, Mn) ₃ AlC (perovskite carbide) phase cannot be suppressed even if the production conditions are optimized. Therefore, the C content is 0.8 to 1.2% by mass.

  Si: Si is an element useful for increasing the strength of a steel sheet by solid solution strengthening, but excessive addition exceeding 3.0% by mass reduces the hot workability and the scale generated by hot rolling. In order to remarkably deteriorate the peelability and chemical conversion processability, the Si content was set to 3.0% or less by mass%.

  Mn: Like M, Mn is an austenite stabilizing element, and is an essential element for forming an austenite structure in a component system in which a large amount of Al is added. If the mass% is less than 10%, the ferrite formation cannot be suppressed even if a large amount of other austenite stabilizing elements such as C is added, and the ductility is lowered. Moreover, when it adds excessively exceeding 30% by mass%, ductility will deteriorate significantly. Therefore, the Mn content is 10 to 30% by mass.

  P: P is an impurity element that segregates at the grain boundary to lower the grain boundary strength and deteriorates toughness, and is preferably as low as possible. However, considering the reachable level and cost of current refining technology, Was 0.02% by mass.

  S: S is an impurity element that deteriorates hot workability and toughness, and is preferably as low as possible. However, considering the reachable level and cost of current refining technology, the upper limit is 0.02% by mass%. It was.

Al: Al is an essential element for achieving a low specific gravity. If the mass% is less than 8%, the effect of lowering the specific gravity is small, so the lower limit was made 8%. Al is a strong ferrite stabilizing element, and if it exceeds 12.0% by mass, the formation of ferrite cannot be suppressed even if a large amount of C or Mn is added, and ductility is lowered. If Al exceeds 12.0%, (Fe, Mn) ₃ AlC (perovskite)
Since precipitation of the carbide) phase cannot be suppressed, ductility is reduced. Therefore, the Al content is set to 8.0 to 12.0% by mass.

  N: N has the effect of forming nitrides and suppressing crystal grain coarsening, but the effect is not manifested at less than 0.001% by mass, and toughness when added at more than 0.05% by mass Therefore, the N content was set to 0.001 to 0.05% by mass.

  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, Ti, Nb, Cr, Ni, Mo, Cu, B , V, Ca, Mg, Zr, or REM may be added.

  Ti: Ti has the effect of suppressing the coarsening of grains by forming TiN, but if the mass% is less than 0.005%, those effects are not manifested. Since toughness deteriorates, the content of Ti is set to 0.005 to 0.3% by mass%.

  Nb: Nb has the effect of forming fine carbonitrides and suppressing grain coarsening, but the effect is not manifested at less than 0.005% by mass, exceeding 0.3% by mass Since the toughness deteriorates when excessively added, the Nb content is set to 0.005 to 0.3% by mass.

  Cr: Cr is an effective element that improves ductility and toughness. This effect is not manifested at less than 0.05% by mass, and excessive addition exceeding 3% by mass degrades toughness. Therefore, the Cr content is 0.05 to 3.0% by mass.

  Ni: Ni is an effective element for improving ductility and toughness, but since this effect is not manifested in less than 0.05% by mass, the lower limit was made 0.05%. Ni is an austenite stabilizing element and is an effective element for forming an austenite structure. However, if the mass percentage exceeds 15%, the ductility is significantly deteriorated. Therefore, the Ni content is set to 0.05 to 15.0% by mass.

  Mo: Mo is an effective element that improves ductility and toughness. This effect is not manifested at less than 0.05% by mass, and excessive addition exceeding 3% by mass degrades toughness. Therefore, the Mo content is set to 0.05 to 3.0% by mass%.

  Cu: Cu is an effective element that improves ductility and toughness. This effect is not manifested at less than 0.1% by mass, and excessive addition exceeding 3% by mass degrades toughness. Therefore, the Cu content is 0.1 to 3.0% by mass.

  B: B segregates at the grain boundary itself, thereby improving the grain boundary bonding force and suppressing the grain boundary segregation of P and S, increasing the grain boundary strength, and improving ductility, toughness, and hot workability. Is an effective element. These effects are not manifested at less than 0.0003% by mass, and excessive addition of more than 0.01% by mass results in the formation of coarse precipitates at the grain boundaries and deteriorates hot workability. The content of was 0.0003 to 0.01% by mass%.

  V: V has the effect of forming fine carbonitrides and suppressing grain coarsening, but the effect is not manifested at less than 0.01% by mass, exceeding 0.5% by mass. Since the toughness deteriorates when excessively added, the V content is set to 0.01 to 0.5% by mass.

  Ca, Mg, Zr, REM: Ca, Mg, Zr, and REM are all effective elements that suppress hot workability and toughness deterioration due to S. This effect is exhibited when Ca is less than 0.001% by mass, Mg is less than 0.0005% by mass, Zr is less than 0.001% by mass, and REM is less than 0.001% by mass. Ca is 0.01% by mass, Mg is 0.01% by mass, Zr is 0.05% by mass, REM is over 0.05% by mass and excessive toughness deteriorates. . Therefore, the Ca content is 0.001 to 0.01% by mass, the Mg content is 0.0005 to 0.01% by mass, and the Zr content is 0.001 to 0.00% by mass. The content of REM was set to 0.001 to 0.05% by mass%.

Next, the structure of the high strength and low specific gravity steel sheet in the present invention will be described.
The steel sheet according to the present invention has an area ratio of 90% or more of austenite, 5% or less of ferrite, and 1% or less of (Fe, Mn) ₃ AlC (perovskite carbide) phase.

  The austenite content is 90% or more in terms of area ratio when the second phase content of ferrite, bainite, martensite, tempered martensite, etc. is 10% or more in total. This is because proper deformation is hindered and ductility is greatly reduced. The reason why the ferrite content is 5% or less in terms of area ratio is that when the ferrite content exceeds 5%, deformation concentrates on the ferrite portion and the ductility is significantly reduced.

Further, (Fe, Mn) ₃ AlC to 1% or less in (perovskite determined carbide) phase area ratio of the content of, in the austenite (Fe, Mn) ₃ When AlC (perovskite carbide) phase is precipitated deformed austenite This is because the strength is increased and the ductility is lowered, but the ductility is drastically lowered when the content of the (Fe, Mn) ₃ AlC (perovskite carbide) phase exceeds 1%.

  The steel sheet of the present invention may contain one or more of bainite, martensite, tempered martensite, and pearlite as the remaining structure. These tissues can be identified by observing with an optical microscope or a scanning electron microscope.

In the present invention, the area ratio of each structure of austenite, ferrite, (Fe, Mn) ₃ AlC (perovskite carbide) phase is C (width direction) cross-section t (plate thickness) / 4 part of the steel plate. It is defined as the average value when 10 visual fields are observed at 200 to 5000 times with a scanning electron microscope.

Next, the reason for limiting the characteristic value will be described.
When the specific gravity is 6.8 or more, the weight reduction effect is small as compared with the specific gravity of steel plates normally used as automotive steel plates (same as the specific gravity of iron of 7.86).

  The strength and ductility are set to a tensile strength of 700 MPa or more and an elongation of 50% or more in consideration of characteristics necessary for an automobile steel plate.

Next, the reasons for limiting the manufacturing conditions will be described.
In the present invention, the steel slab composed of the component according to any one of (1) to (4) is heated to a temperature of 1100 ° C. or higher and 1250 ° C. or lower, and the total rolling reduction is 85% or higher at a temperature of 1050 ° C. or higher. After rough rolling, reheat to a temperature of 1100 ° C or higher, then hot-roll at a finish rolling temperature of 900 ° C or higher, and cool to a temperature of 200 ° C or lower at a cooling rate of 20 ° C / second or higher. Wind up.

  If the slab heating temperature is less than 1100 ° C., the carbonitride is not sufficiently dissolved and the required strength and ductility cannot be obtained, so the lower limit of the slab heating temperature is 1100 ° C. When the slab heating temperature is higher than 1250 ° C., hot workability is significantly deteriorated due to melt embrittlement at the segregation part, so the upper limit of the slab heating temperature is 1250 ° C.

  The reason for reheating to a temperature of 1100 ° C. or higher after performing rough rolling at a temperature of 1050 ° C. or higher at a total rolling reduction of 85% or higher is to reduce segregation in the rough rolled sheet. If these conditions are not satisfied, segregation during casting remains, so that a large amount of ferrite precipitates in the segregated part, and deformation concentrates on the ferrite part, resulting in a significant decrease in ductility. The method of reheating the rough rolled plate may be a method using a heater or a method of inserting it into a reheating furnace, and there is no need to define it.

(Fe, Mn) ₃ AlC (perovskite carbide) phase if the finish rolling temperature is less than 900 ° C, the cooling rate after hot rolling is less than 20 ° C / second, or the cooling stop temperature exceeds 200 ° C Precipitates and the ductility decreases significantly. For this reason, the lower limit of the finish rolling temperature is set to 900 ° C., and after hot rolling, it is cooled to a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or more and wound.

  In order to improve the ductility of the hot-rolled sheet, from the viewpoint of recrystallization and carbide precipitation control, after winding the hot-rolled sheet, it is annealed at a temperature of 800 ° C. or higher and 1200 ° C. or lower, and a cooling rate of 20 ° C./second or higher. May be cooled to a temperature of 200 ° C. or lower.

  Here, if the annealing temperature is less than 800 ° C., the effect is small, and if it exceeds 1200 ° C., the crystal grains become coarse and grain boundary embrittlement is promoted, so the annealing temperature of the hot-rolled sheet is 800 ° C. or more and 1200 ° C. or less. The range.

In addition, if the cooling rate after annealing is less than 20 ° C / second or the cooling stop temperature exceeds 200 ° C, the (Fe, Mn) ₃ AlC (perovskite varbide) phase precipitates and the ductility decreases significantly. It was decided to cool to a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher after annealing.

  When manufacturing a cold-rolled steel sheet, the steel sheet is wound, pickled, cold-rolled, annealed at a temperature of 800 ° C. or higher and 1200 ° C. or lower, and 20 ° C./second or higher after annealing. You may cool to the temperature of 200 degrees C or less with a cooling rate.

  If the annealing temperature is less than 800 ° C, it will not be recrystallized and unrecovered, and a sufficient effect will not be obtained. If it exceeds 1200 ° C, the crystal grains will become coarse and grain boundary embrittlement will be promoted. The temperature range was from ° C to 1200 ° C.

If the cooling rate after annealing is less than 20 ° C / second or the cooling stop temperature exceeds 200 ° C, the (Fe, Mn) ₃ AlC (perovskite varbide) phase precipitates during cooling and the ductility is significantly reduced. Therefore, it was decided to cool to a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher after annealing.

  The conditions for cold rolling are not particularly limited, but a rolling reduction of 40% or more is desirable for sufficient recrystallization during annealing.

  In addition, the steel sheet is wound and annealed, then pickled, cold rolled, annealed at a temperature of 800 ° C. or higher and 1200 ° C. or lower, and after annealing, a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher. You may cool to.

  As before, even in this case, if the annealing temperature is less than 800 ° C, it will not be recrystallized and unrecovered, and a sufficient effect will not be obtained. If it exceeds 1200 ° C, the crystal grains become coarse and grain boundary embrittlement is promoted. The annealing temperature of the cold rolled sheet was set to a temperature range of 800 ° C. or higher and 1200 ° C. or lower.

Also in this case, if the cooling rate after annealing is less than 20 ° C / second or the cooling stop temperature exceeds 200 ° C, the (Fe, Mn) ₃ AlC (perovskite varbide) phase precipitates during cooling. Since the ductility is greatly lowered, it was decided to cool to a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher after annealing.

  The conditions for cold rolling are not particularly limited, but in this case as well, it is desirable that the rolling reduction be 40% or more in order to sufficiently recrystallize during annealing.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
Example 1
Steels (No. 1 to 11) containing each chemical component shown in Table 1 (shown by mass%) and the balance being Fe and inevitable impurities are hot-rolled under the conditions shown in Table 2 and a reduction rate of 50 %, And then annealed under the conditions shown in Table 2. The specific gravity and mechanical properties of the plate after annealing were evaluated. The specific gravity was measured using a pycnometer. Specific gravity, yield stress, tensile strength and elongation are shown in Table 2.

  In Examples (Nos. 1 to 5) of the present invention, the specific gravity <6.8 is satisfied, the tensile strength is 700 MPa or more, and a high elongation of 50% or more is obtained with respect to ductility. On the other hand, in the comparative examples (Nos. 6, 7, and 8) in which any one or more of the components deviate from the component range of the present invention, the elongation is 20% or less, indicating that the ductility is inferior. Moreover, in the comparative examples (Nos. 9, 10, and 11) in which the manufacturing conditions deviate from the scope of the present invention, the elongation is 20% or less, indicating that the ductility is inferior.

(Example 2)
In addition, steel (No. 1 to 11) containing each chemical component (shown in mass%) shown in Table 1 and the balance being Fe and inevitable impurities was hot rolled under the conditions shown in Table 2. The plates were evaluated for specific gravity and mechanical properties (yield stress, tensile strength and elongation). Table 3 shows the specific gravity, yield stress, tensile strength and elongation of the hot-rolled sheet.

  Further, the specific gravity and mechanical properties (yield stress, tensile strength and elongation) of the hot-rolled sheet annealed material that was subjected to hot-rolled sheet annealing under the conditions shown in Table 4 were also evaluated. Table 4 shows the specific gravity, yield stress, tensile strength and elongation of the annealed hot-rolled sheet. In Examples (Nos. 1 to 5) of the present invention, both the hot-rolled sheet and the hot-rolled sheet annealed material satisfy the specific gravity <6.8, the tensile strength is 700 MPa or more, and the ductility is 50% or more. The high elongation of is obtained. On the other hand, in the comparative examples (Nos. 6, 7, and 8) in which any one or more of the components deviate from the component range of the present invention, the elongation is 20% or less, indicating that the ductility is inferior. Moreover, in the comparative examples (Nos. 9, 10, and 11) in which the hot rolling conditions deviate from the scope of the present invention, the elongation is 20% or less, indicating that the ductility is inferior.

(Example 3)
Furthermore, steel (No. 1 to 11) containing each chemical component (shown by mass%) shown in Table 1 and the balance consisting of Fe and inevitable impurities was hot-rolled under the conditions shown in Table 2, Hot-rolled sheet annealing was performed under the conditions shown in Fig. 4, cold-rolled at 50% reduction after pickling, and the specific gravity and mechanical properties (yield stress) of the cold-rolled sheet annealed and cooled under the conditions shown in Table 2 , Tensile strength and elongation). Table 5 shows the specific gravity, yield stress, tensile strength and elongation of the cold rolled sheet.

  In Examples (Nos. 1 to 5) of the present invention, specific gravity <6.8 is satisfied, tensile strength is 700 MPa or more, and high elongation of 50% or more is obtained with respect to ductility. On the other hand, in the comparative examples (Nos. 6, 7, and 8) in which any one or more of the components deviate from the component range of the present invention, the elongation is 20% or less, indicating that the ductility is inferior. Moreover, in the comparative examples (Nos. 9, 10, and 11) in which the hot rolling conditions deviate from the scope of the present invention, the elongation is 20% or less, indicating that the ductility is inferior.

  From the above, it is clear that a high strength low specific gravity steel plate excellent in ductility can be obtained by specifying the steel components in the range shown in the present invention and producing them under the conditions shown in the present invention.

  The steel plate of the present invention is used for automobile parts, for example.

Claims (7)

% By mass
C: 0.8 to 1.2%,
Si: 3.0% or less,
Mn: 10.0-30.0%,
P: 0.02% or less,
S: 0.02% or less,
Al: 8.0 to 12.0%,
N: 0.001 to 0.05%
The steel slab containing Fe and the balance consisting of Fe and inevitable impurities is heated to a temperature of 1100 ° C. or higher and 1250 ° C. or lower and subjected to rough rolling at a temperature of 1050 ° C. or higher and a total rolling reduction of 85% or higher. It is reheated to a temperature of ℃ or higher, subsequently hot rolled at a finish rolling temperature of 900 ℃ or higher, cooled to a temperature of 200 ℃ or lower at a cooling rate of 20 ℃ / second or more, and wound up In the area ratio, austenite is 90% or more, ferrite is 5% or less, and (Fe, Mn) ₃ AlC (perovskite carbide) phase is 1% or less, specific gravity <6.8, tensile strength is 700 MPa or more, A method for producing a high-strength, low-specific gravity steel sheet with an elongation of 50% or more and excellent ductility.   2. The ductility according to claim 1, wherein the steel sheet is rolled up, annealed at a temperature of 800 ° C. or more and 1200 ° C. or less, and cooled to a temperature of 200 ° C. or less at a cooling rate of 20 ° C./second or more. A method for producing excellent high strength and low specific gravity steel sheets.   After winding the steel plate, pickling, cold rolling, annealing at a temperature of 800 ° C or higher and 1200 ° C or lower, and cooling to a temperature of 200 ° C or lower at a cooling rate of 20 ° C / second or higher after annealing. The manufacturing method of the high strength low specific gravity steel plate excellent in ductility of Claim 1 characterized by these.   The steel sheet is wound and annealed, then pickled, cold rolled, annealed at a temperature of 800 ° C to 1200 ° C, and cooled to a temperature of 200 ° C or lower at a cooling rate of 20 ° C / second or higher after annealing. The manufacturing method of the high strength low specific gravity steel plate excellent in ductility of Claim 2 characterized by performing. The steel slab is further mass%,
Ti: 0.005 to 0.3%,
Nb: 0.005 to 0.3%,
The manufacturing method of the high strength low specific gravity steel plate excellent in ductility in any one of Claims 1-4 characterized by containing 1 type or 2 types of these. The steel slab is further mass%,
Cr: 0.05 to 3.0%,
Ni: 0.05 to 15.0%,
Mo: 0.05 to 3.0%,
Cu: 0.1 to 3.0%,
B: 0.0003 to 0.01%,
V: 0.01 to 0.5%
The manufacturing method of the high strength low specific gravity steel plate excellent in ductility in any one of Claims 1-5 characterized by containing 1 type, or 2 or more types of these. The steel slab is further mass%,
Ca: 0.001 to 0.01%,
Mg: 0.0005 to 0.01%,
Zr: 0.001 to 0.05%,
REM: 0.001 to 0.05%,
1 or 2 types or more of these, The manufacturing method of the high strength low specific gravity steel plate excellent in ductility in any one of Claims 1-6 characterized by the above-mentioned.