JP2005273004A - Low specific gravity steel plate excellent in ductility and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low specific gravity steel plate excellent in ductility and a method for producing the same.
SOLUTION By mass%, C: 0.0035% or less, Si: 0.01 to 7.0% or less, Mn: 0.01 to 3.0%, P: 0.01% or less, S: 0 .05% or less, Al: 2.0 to 10.0%, N: 0.001 to 0.05%, the balance being made of Fe and inevitable impurities, and the Fe-Al ordering ratio being 0.00. A low specific gravity steel sheet excellent in ductility, characterized by 3 or less, specific gravity <7.50, and uniform elongation of 20% or more.
[Selection figure] None

Description

  The present invention relates to a low specific gravity steel plate excellent in ductility 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. To reduce the weight of automobiles, increasing the strength of steel is an effective means, but when 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. In addition, there are drawbacks such as difficulty in welding with steel plates, and therefore, application to automobile members is limited.

  Then, as what has the merit of a steel plate and an aluminum alloy plate, the high Al content steel plate which added a large amount of aluminum to iron is considered, for example, in patent documents 1, C: 0.002-0.1%, 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. 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 a high Al content steel sheet is applied as a steel sheet for automobiles because of (a) inferior productivity (particularly, cracking occurs during rolling) and (b) low ductility. It was difficult.

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

  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 : 0.1 to 0.8%, C: 0.01 to 0.5%, and a technology related to an aluminum-containing iron-based alloy containing residual Fe has been proposed, but there is no mention of low specific gravity, heavy elements Mo and Zr are essential, and it cannot be said that low specific gravity is taken into consideration.

  In addition, for manufacturability, forging and warm rolling are carried out, so-called so-called widely industrialized manufacturing methods and manufacturing equipment from melting to hot rolling and cold rolling were used. It is different from the manufacturing method. In addition, the inventors' tests have not led to a significant improvement in ductility.

  Patent Document 3 discloses oxidation resistance containing C: 0.05% or less, Si: 0.1 to 1%, Al: 2 to 8%, Y: 0.01 to 1%, and residual Fe. Although an iron alloy has been proposed, there is no mention of a low specific gravity, and in order to improve oxidation resistance, Y as a heavy element is essential, and it cannot be said that low specific gravity is taken into consideration.

  In addition, there is no mention of strength and ductility, and in the tests of the present inventors, no significant improvement in ductility has been achieved.

  As described above, it has been difficult to produce a low specific gravity steel plate excellent in ductility on an industrial scale with the conventional technology.

JP-A-3-140439 JP-A-8-253844 U.S. Pat. No. 4,334,923

  The present invention is intended to solve the above problems, and an object of the present invention is to provide a steel sheet that secures ductility comparable to that of ordinary steel in addition to low specific gravity, and a method for producing the same.

  The inventors of the present invention have both a component composition and a manufacturing method for a method for improving ductility, hot workability and cold workability for various materials containing a large amount of Al based on iron. As a result of repeated research, we found a method to achieve high ductility comparable to carbon steel in addition to low specific gravity.

  So far, the deterioration of hot workability and cold workability is due to grain boundary embrittlement, and after making the Al content 5.0 to 10.0%, S and P are extremely reduced, In addition, carbon nitride that precipitates in the grains is reduced by extremely low C to reduce the difference in strength between the grain boundaries and the grains, and the recrystallization of ferrite is promoted during hot rolling by optimizing the hot rolling conditions. It has been found that the grain boundary strength can be improved and the ductility, hot workability, and cold workability can be greatly improved by making the particles finer.

  However, it has also been found that as the Al content increases, it tends to have high strength and embrittlement. This time, I discovered a new reason why Al, which should have a small solid solution strengthening ability, tends to have high strength and embrittlement. That is, it has been found that the local ordering of Fe—Al is one factor that causes high strength and embrittlement. The present inventors have found a method for suppressing the local ordering of Fe—Al as much as possible.

The present invention is configured based on such knowledge, the gist of which is
(1) In mass%,
C: 0.0035% or less,
Si: 0.01 to 7.0% or less,
Mn: 0.01 to 3.0%,
P: 0.01% or less,
S: 0.005% or less,
Al: 2.0-10.0%,
N: 0.001 to 0.05%
The balance is Fe and inevitable impurities, the Fe-Al ordering ratio is 0.3% or less, the specific gravity <7.50, and the uniform elongation is 20% or more. Excellent low specific gravity steel plate.

(2) Furthermore, in mass%,
Ti: 0.005 to 0.3%,
Nb: 0.005-0.3%
V: 0.01 to 0.5%
The low specific gravity steel sheet excellent in ductility according to the above (1), characterized by containing one or two of the above.

(3) Furthermore, in mass%,
Cr: 0.01 to 5.0%,
Ni: 0.01 to 5.0%,
Mo: 0.01 to 3.0%,
W: 0.01-3.0%,
Cu: 0.1 to 3.0%
The low specific gravity steel sheet excellent in ductility as described in (1) or (2) above, comprising one or more of the above.

(4) Furthermore, in mass%,
B: The low specific gravity steel plate excellent in ductility according to any one of (1) to (3), characterized by containing 0.0003 to 0.01%.

(5) Furthermore, in 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%
The low specific gravity steel sheet excellent in ductility according to any one of the above (1) to (4), characterized by containing one or more of the above.

(6) In mass%,
C: 0.20% or less,
Si: 0.01 to 0.5% or less,
Mn: 0.01 to 3.0%,
P: 0.01% or less,
S: 0.05% or less,
Al: 4.0-6.0%,
N: 0.001 to 0.05%
With the balance being Fe and inevitable impurities, Fe-Al ordering ratio of 0.3 or less, specific gravity <7.50, uniform elongation x tensile strength of 8000 MPa% or more Excellent low specific gravity steel plate.

(7) Each component represented by the mass%,
1.0 <Al- (21.43 × C + 1.43 × Mn) (1)
The low specific gravity steel sheet excellent in ductility according to the above (6), characterized in that:

  (8) A method for producing a low specific gravity steel plate excellent in ductility according to any one of (1) to (7), wherein the steel is composed of the component according to any one of (1) to (7). The slab is heated to a temperature range of 1000 to 1250 ° C, and includes at least one pass of a large reduction with a reduction rate of 30% or more in the temperature range of 950 to 1150 ° C, and a finish rolling temperature of 850 ° C or more, until winding. A method for producing a low specific gravity steel sheet excellent in ductility, characterized by cooling at 10 ° C./s or more and winding at less than 550 ° C.

  (9) A method for producing a low specific gravity steel sheet having excellent ductility according to any one of (1) to (7), wherein the steel is composed of the component according to any one of (1) to (7). The slab is heated to a temperature range of 1000 to 1250 ° C., and includes at least one pass of a large reduction of 30% or more in the temperature range of 950 to 1150 ° C., and then at least 1 second in the temperature range of 950 to 1100 ° C. After stopping, at least one pass of large reduction with a reduction rate of 30% or more in a temperature range of 900 ° C. or more and less than 1000 ° C. and a finish rolling temperature of 650 ° C. or more and less than 850 ° C., and winding up to 10 ° C. / The manufacturing method of the low specific gravity steel plate excellent in ductility characterized by cooling at s or more and winding up at 600 to 750 degreeC.

  (10) After annealing, a hot-rolled steel sheet produced by the method for producing a low specific gravity steel sheet having excellent ductility described in (8) or (9) is annealed for 1 to 1000 seconds in a temperature range of 700 ° C. to 1100 ° C. The manufacturing method of the low specific gravity steel plate excellent in the ductility characterized by cooling to the temperature range of 400 degrees C or less with the average cooling rate of 10 degrees C / s or more.

  (11) The hot-rolled steel sheet produced by the method for producing a low specific gravity steel sheet having excellent ductility described in (8) or (9) is pickled, cold-rolled, and a temperature of 600 ° C. or higher and 1100 ° C. or lower. A method for producing a low specific gravity steel sheet excellent in ductility, characterized by annealing in an area for 1 to 1000 seconds and cooling to a temperature range of 400 ° C. or lower at an average cooling rate of 10 ° C./s or higher after annealing.

  According to this invention, the low specific gravity steel plate excellent in ductility is obtained.

  Below, the meaning of each requirement in this invention and the reason for limitation are demonstrated concretely. First, the reasons for limiting the components of the high-strength low specific gravity steel sheet having excellent ductility in the present invention will be described.

  “%” Means “% by mass”.

  C: C improves the strength but causes a significant decrease in toughness and ductility. Therefore, the lower the C, the more desirable, and 0.0035% or less. On the other hand, extremely low is disadvantageous in terms of cost. As a lower limit of small ductility deterioration in consideration of economy, 0.0010% or more is desirable.

  Furthermore, the combination with Al and Mn can increase the addition amount to 0.2% while suppressing the deterioration of the strength-ductility balance and the occurrence of surface scratches and cracks on the steel sheet. The material can be secured without lowering.

  Specifically, Al: 4.0 to 6.0% is effective, and Mn: more than 0.2% to 3.0% or less, or 1.0 <Al- ( 21.43 × C + 1.43 × Mn) is preferable in terms of structure control and material control.

  Si: Si is an element useful for increasing the strength of a steel sheet by solid solution strengthening. However, excessive addition exceeding 7.0% decreases the hot workability and reduces the scale produced by hot rolling. The peelability and chemical conversion processability are significantly degraded. On the other hand, since extremely low is disadvantageous in terms of cost, 0.01% was made the lower limit as a level that does not deteriorate workability and chemical conversion treatment.

  Mn: Mn is an element effective for forming MnS and suppressing grain boundary embrittlement due to S. If it is less than 0.01%, the effect is not exhibited, and an excessive addition exceeding 3.0% deteriorates toughness and ductility. Therefore, the Mn content is set to 0.01 to 3.0%.

  From the viewpoint of improving toughness and ductility by inclusion control, it is desirable that the amount of S be 20 times or more, or more than 0.2% and 3.0% or less. Further, in the case of avoiding extremely low C from the viewpoint of refining cost and from the viewpoint of increasing strength, and adding more than 0.0035% carbon, 1.0 <Al− (21 .43 × C + 1.43 × Mn) is effective for suppressing the deterioration of the strength-ductility balance and the occurrence of surface scratches and cracks in the steel sheet.

  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, the upper limit is considered in consideration of the reachable level and cost of current refining technology. Was set to 0.01%.

  S: S is an impurity element that deteriorates hot workability and toughness, and is preferably as low as possible. However, the upper limit is set to 0.05% in consideration of the reachable level and cost of the current refining technology. Preferably it is 0.005%.

  Al: Al is an essential element for achieving a low specific gravity. If less than 2.0%, the effect of lowering the specific gravity is small, so 2.0% was made the lower limit. If it exceeds 10.0%, precipitation of intermetallic compounds and cluster formation become prominent, and the ordering rate of Fe—Al increases rapidly. Since it becomes difficult to ensure ductility comparable to that of ordinary steel, the content is set to 2.0 to 10.0%.

  In addition, as a range that suppresses regularization and cluster formation and achieves both low specific gravity and high ductility, a range of 7.0% or less is desirable, and in addition, in order to obtain manufacturability and workability comparable to that of ordinary steel , Less than 5.0% is desirable.

  In particular, from the viewpoint of refining costs and high strength, when avoiding extremely low C and adding more than 0.0035% carbon, 4.0% or more and 6.0% or less, preferably 5. Less than 0%, and more preferably 1.0 <Al− (21.43 × C + 1.43 × Mn) in combination with the amounts of Mn and C satisfies the deterioration of the strength-ductility balance and the surface damage of the steel sheet. It is effective in suppressing the occurrence of cracks.

  N: N is preferably low because ductility is deteriorated. However, considering the reachability level and cost of the current refining technology, the lower limit was set to 0.001%.

  The present inventors have found that a major cause of the decrease in ductility is local ordering of Fe—Al, and by establishing this, ductility as high as that of ordinary steel has been ensured. In press molding, since uniform elongation is particularly important, the object is to ensure uniform elongation of 20% or more, or uniform elongation × tensile strength of 8000 MPa% or more.

  For this reason, about the regularization rate, it was decided to ensure high ductility by setting it as 0.3 or less.

  Here, the ordering of Fe—Al is defined as when the structure of all the materials is an intermetallic compound. Not only perfect ordering but also local ordering (cluster formation) contributes to ductility deterioration. By limiting and securing this ordering rate, both low specific gravity and high ductility are achieved. Is.

  In addition, the ordering rate can be experimentally measured by preparing a sufficiently heat-treated sample and a random sample and obtaining the peak ratio and the electric resistance ratio in X-ray diffraction.

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

  Ti: Ti forms carbides, nitrides or carbonitrides, and reduces the amount of dissolved carbon and nitrogen. Therefore, in addition to contributing to the improvement of ductility, it has the effect of suppressing grain coarsening. If it is less than 0.005%, those effects are not manifested, and if over 0.3% is added excessively, ductility deteriorates, so the Ti content was made 0.005 to 0.3%.

  Nb: Like Ti, Nb has the effect of forming fine carbonitrides and suppressing crystal grain coarsening, but if it is less than 0.005%, the effect is not manifested, and over 0.3% is added excessively. Then, in addition to deterioration of ductility, the element is heavier than iron, so the content of Nb is set to 0.005 to 0.3%.

  Cr: Cr is an effective element that improves ductility and toughness. This effect is not manifested at less than 0.01%, and excessive addition exceeding 5% degrades toughness. Therefore, the content of Cr is set to 0.01 to 5.0%.

  Ni: Ni is an effective element that improves ductility and toughness. This effect is not manifested at less than 0.01%, and excessive addition exceeding 5.0% degrades toughness. Therefore, the Ni content is set to 0.01 to 5.0%.

  Mo: Mo is an effective element that improves ductility and toughness. This effect is not manifested at less than 0.01%, and excessive addition of more than 3.0% degrades toughness, and is a heavier element than iron, so the Mo content is reduced to 0.01-3. 0.0%.

  W: W is an effective element for improving ductility and toughness. This effect is not manifested at less than 0.01%, and excessive addition of more than 3.0% degrades toughness, and is a heavier element than iron, so the Mo content is reduced to 0.01-3. 0.0%.

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

  B: B segregates at the grain boundary by itself, thereby improving the grain boundary bonding force, suppressing the grain boundary segregation of P and S, increasing the grain boundary strength, suppressing ductility, secondary work cracking, improving toughness, And it is an element effective in improving hot workability. These effects are not manifested at less than 0.0003%, and excessive addition over 0.01% produces coarse precipitates at the grain boundaries and deteriorates hot workability. .0003-0.01%. Further, in order to suppress nitride precipitation, composite addition with Ti is desirable.

  Ca, Mg, Zr, and REM: Ca, Mg, Zr, and REM are all effective elements that suppress hot workability and toughness deterioration due to S. This effect is not exhibited when Ca is less than 0.001%, Mg is less than 0.0005%, Zr is less than 0.001%, and REM is less than 0.001%, while Ca is 0.01%, Mg 0.01%, Zr 0.05%, and REM over 0.05% will deteriorate toughness.

  Therefore, the Ca content is 0.001 to 0.01%, the Mg content is 0.0005 to 0.01%, the Zr content is 0.001 to 0.05%, and the REM content is 0. 0.001 to 0.05%. Here, REM is an abbreviation for Rare Earth Metal and is a general term for Y and lanthanoid elements.

  Next, the reason for limiting the characteristic value will be described. When the specific gravity is 7.50 or more, the effect of weight reduction is small compared to the specific gravity of steel plates normally used as automotive steel plates (same as the specific gravity of iron of 7.86), and the thinness accompanying the increase in strength so far Since it is difficult to reduce the weight more than that, the specific gravity should be less than 7.50.

  In consideration of characteristics required for a steel sheet for automobiles, the ductility was set to a uniform elongation of 20% or more, or a uniform elongation × tensile strength of 8000 MPa% or more.

  Next, the reasons for limiting the manufacturing conditions will be described.

  In the present invention, weakening due to the addition of a large amount of Al is prevented, and the material is secured as a lightweight high-strength steel sheet for automobiles (uniform elongation is 20% or more, or uniform elongation × tensile strength is 8000 MPa% or more). It is important to control not only the chemical components but also the structure control in hot rolling, that is, the suppression of local ordering rate increase due to the promotion of recrystallization.

  It decided to heat the steel slab which consists of the said chemical component to 1000-1250 degreeC temperature range. The upper limit was set from the viewpoint of the problem of oxide scale and the coarsening of the crystal grain size, and the lower limit was set from the viewpoint of increasing deformation resistance and preventing heat cracking.

  In addition, by including at least one pass of a large reduction with a reduction ratio of 30% or more in a temperature range of 950 to 1150 ° C., recrystallization during hot rolling is promoted to ensure ductility in a hot rolled sheet state or during cold rolling. The toughness was secured. If the rolling reduction is low or the rolling temperature is too high, recrystallization does not occur, and if the rolling temperature is too low, there is a concern of hot cracking, so the range is limited to the above range.

  Further, when the finishing temperature is 850 ° C. or higher, recrystallization is promoted by cooling at a cooling rate of 10 ° C./S or more after rolling and winding in a temperature range of less than 550 ° C., preferably 500 ° C. or lower. And cluster formation can be suppressed. The upper limit of the finishing temperature is not particularly defined, but is desirably 950 ° C. or lower in order to suppress the coarsening of crystal grains.

  On the other hand, in order to promote recrystallization, it is effective to lower the finishing temperature and raise the winding temperature. In this case, the finishing temperature is set to 650 ° C. or more and less than 850 ° C., and after completion of rolling, cooling is performed at a cooling rate of 10 ° C./S or more, and winding is performed in a temperature range of 600 to 750 ° C. Can be promoted.

  In hot rolling, in order to sufficiently recrystallize and homogenize, and to suppress cluster formation together, a large reduction with a reduction ratio of 30% or more is performed at least one pass or more in a temperature range of 950 to 1150 ° C. To do.

  Thereafter, the film is retained in a temperature range of 950 to 1100 ° C. for at least 1 second for sufficient recrystallization. Further, in order to promote recrystallization and reduce the ordering rate, at least one pass of large reduction with a reduction rate of 30% or more is included in a temperature range of 900 ° C. or more and less than 1000 ° C.

  Thereafter, cooling from a finish rolling temperature of 650 ° C. or more to less than 850 ° C. to winding is effective at keeping the ordering rate low by cooling at a cooling rate of 10 ° C./s or more.

  In order to improve the ductility of each hot-rolled sheet manufactured in any of the above-described steps, from the viewpoint of recrystallization or carbide precipitation control, after rolling the hot-rolled sheet, annealing is performed at a temperature of 700 ° C. or higher and 1100 ° C. or lower. May be. At this time, from the viewpoint of suppressing the formation of clusters as much as possible, the annealing time was set to 1 second to 1000 seconds and further cooled to a temperature range of 400 ° C. or lower at 10 ° C./S or higher.

  When manufacturing a cold-rolled steel sheet, the hot-rolled steel sheet is wound, pickled, cold-rolled, and annealed at a temperature of 600 ° C. or higher and 1100 ° C. or lower. If the annealing temperature is less than 600 ° C, it will not be recrystallized / recovered and sufficient effect will not be obtained. If it exceeds 1100 ° C, the crystal grains will become coarse and grain boundary embrittlement will be promoted. The temperature range was 600 ° C to 1100 ° C.

  Further, from the viewpoint of preventing clusters, the annealing time is set to 1 second to 1000 seconds and further, cooling is performed at a temperature of 10 ° C./S or higher to a temperature range of 400 ° C. or lower.

  Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

  Steel having the composition shown in Table 1 was hot-rolled under the conditions shown in Tables 2 to 5 and cold-rolled, and then annealed under the conditions shown in Tables 2 to 5. After hot rolling and after cold rolling, the occurrence of cracks in the hot rolled sheet and the cold rolled sheet was observed, respectively. The results are shown in Tables 2 to 5.

  The specific gravity and mechanical properties of the plate after annealing were evaluated. The specific gravity was measured using a pycnometer. Further, the ordering rate was obtained by measuring a peak ratio in X-ray diffraction or measuring an electric resistance by preparing a sufficiently heat-treated sample and a random sample.

  For details, refer to Document: D. Chipman and BE Warren, Journal of Applied Physics vol.21 (1950), p.696, and Document: DDT Keating and BE Warren, Journal of Applied Physics vol.22 (1951), p. 286.

  Specific gravity, uniform elongation, tensile strength and ordering rate are shown in Tables 2 to 5.

  In the present invention examples according to the inventions (1) to (5), no cracks are generated in the hot-rolled sheet and the cold-rolled sheet, and a uniform elongation of 20% or more can be secured. Moreover, in the invention examples according to the inventions (6) to (8) in which the carbon content exceeds 0.0035%, there is no occurrence of cracks in the hot-rolled sheet and the cold-rolled sheet, and uniform elongation × tensile A strength of 8000 MPa% or more can be secured.

  On the other hand, in any of the comparative examples in which either the component or the production condition deviates from the component limitation range of the present invention, the uniform elongation is less than 20%, and the uniform elongation × tensile strength is less than 8000 MPa%, In some cases, cracks in the hot-rolled sheet also occur, indicating that the productivity is poor.

  From the above, it is clear that a low specific gravity steel sheet 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.

  As described above, according to the present invention, a low specific gravity steel sheet having excellent ductility can be obtained. Since this low specific gravity steel sheet can be used effectively for reducing the weight of automobiles, the present invention has a great industrial applicability.

Claims (11)

% By mass
C: 0.0035% or less,
Si: 0.01 to 7.0% or less,
Mn: 0.01 to 3.0%,
P: 0.01% or less,
S: 0.05% or less,
Al: 2.0-10.0%,
N: 0.001 to 0.05%
With the balance being Fe and inevitable impurities, Fe-Al ordering ratio of 0.3 or less, specific gravity <7.50, uniform elongation of 20% or more, and excellent ductility Low specific gravity steel plate. Furthermore, in mass%,
Ti: 0.005 to 0.3%,
Nb: 0.005-0.3%
V: 0.01 to 0.5%
The low specific gravity steel plate excellent in ductility according to claim 1, comprising one or two of the following. Furthermore, in mass%,
Cr: 0.01 to 5.0%,
Ni: 0.01 to 5.0%,
Mo: 0.01 to 3.0%,
W: 0.01-3.0%,
Cu: 0.1 to 3.0%
The low specific gravity steel sheet excellent in ductility according to claim 1 or 2, characterized by containing one or more of the following. Furthermore, in mass%,
B: 0.0003 to 0.01%
The low specific gravity steel plate excellent in ductility according to any one of claims 1 to 3. Furthermore, in 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%
The low specific gravity steel plate excellent in ductility of any one of Claims 1-4 characterized by containing 1 type, or 2 or more types of these. % By mass
C: 0.20% or less,
Si: 0.01 to 0.5% or less,
Mn: 0.01 to 3.0%,
P: 0.01% or less,
S: 0.05% or less,
Al: 4.0-6.0%,
N: 0.001 to 0.05%
With the balance being Fe and inevitable impurities, Fe-Al ordering ratio of 0.3 or less, specific gravity <7.50, uniform elongation x tensile strength of 8000 MPa% or more Excellent low specific gravity steel plate. Each component represented by the mass%,
1.0 <Al- (21.43 × C + 1.43 × Mn) (1)
The low specific gravity steel sheet excellent in ductility according to claim 6, wherein:   It is a method of manufacturing the low specific gravity steel plate excellent in ductility of any one of Claims 1-7, Comprising: The steel slab which consists of a component of any one of Claims 1-7 is 1000-1250. Heating to a temperature range of ℃, including at least one pass of large reduction with a reduction rate of 30% or more in a temperature range of 950 to 1150 ° C, and a finish rolling temperature of 850 ° C or more, and winding up to 10 ° C / s or more The method for producing a low specific gravity steel plate excellent in ductility, characterized by being cooled at 550 ° C. and wound up at less than 550 ° C.   It is a method of manufacturing the low specific gravity steel plate excellent in ductility of any one of Claims 1-7, Comprising: The steel slab which consists of a component of any one of Claims 1-7 is 1000-1250. After heating to a temperature range of ℃, including at least one pass or more of a large reduction of 30% or more in a temperature range of 950 to 1150 ° C., and then retaining at a temperature range of 950 to 1100 ° C. for at least 1 second, In a temperature range of 900 ° C. or higher and lower than 1000 ° C., at least one pass of large reduction with a reduction rate of 30% or higher is set to a finish rolling temperature of 650 ° C. or higher and lower than 850 ° C. The manufacturing method of the low specific gravity steel plate excellent in ductility characterized by winding at 600 degreeC or more and 750 degrees C or less.   A hot-rolled steel sheet produced by the method for producing a low specific gravity steel sheet excellent in ductility according to claim 8 or 9 is annealed for 1 to 1000 seconds in a temperature range of 700 ° C or higher and 1100 ° C or lower, and 10 ° C / s or higher after annealing. A method for producing a low specific gravity steel sheet excellent in ductility, characterized by cooling to a temperature range of 400 ° C. or lower at an average cooling rate.   The hot-rolled steel sheet produced by the method for producing a low specific gravity steel sheet excellent in ductility according to claim 8 or 9, is pickled, cold-rolled, and is subjected to a temperature range of 600 ° C to 1100 ° C for 1 to 1000 seconds. A method for producing a low specific gravity steel sheet excellent in ductility, characterized by annealing and cooling to a temperature range of 400 ° C. or lower at an average cooling rate of 10 ° C./s or higher after annealing.