JP2006118000A - Lightweight high strength steel having excellent ductility and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength low specific gravity steel sheet having excellent ductility, and to provide its production method. <P>SOLUTION: The lightweight high strength steel having excellent ductility has a composition containing, by mass, 0.1 to 1.0% C, ≤3.0% Si, 10.0 to 50.0% Mn, ≤0.01% P, ≤0.01% S, 5.0 to 15.0% Al and 0.001 to 0.05% N, and the balance Fe with inevitable impurities, and in which the mass% of each component satisfies the following inequality (1), has a specific gravity of ≤7.0, has a structure comprising ferrite and austenite where the area ratio of any phase is the maximum therein, and in which the product TS×El of tensile strength: TS(MPa) and fracture elongation: El(%), satisfies ≥20,000 (MPa×%). C≤-0.020×Mn+Al/15+0.53 (1). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high strength low specific gravity steel 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 possible to use an aluminum alloy plate having a lower specific gravity than steel, but the aluminum alloy plate is inferior to steel in addition to being expensive. In addition, there are drawbacks such as difficulty in welding with steel plates, and therefore, application to automobile members is limited. Therefore, a high Al content steel plate in which a large amount of aluminum is added to iron can be considered as having the advantages of a steel plate and an aluminum alloy plate. However, such a high Al-containing steel sheet can be applied as a steel sheet for automobiles because of (1) inferior productivity (particularly cracking during rolling) and (2) low ductility. It was difficult. In particular, when the Al content is increased, ductility, hot workability and cold workability are greatly deteriorated due to precipitation of intermetallic compounds such as Fe ₃ Al and FeAl. It was extremely difficult to produce high-strength steel and to ensure good strength and ductility level.

  As a technique for improving the ductility of a high Al-containing steel sheet, for example, in Patent Document 1, Al: 4 to 9.5%, Ti: 0.5 to 2.0%, Mo: 0.5 to 2%, Zr: A technology of an aluminum-containing iron-based 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 taken into consideration. In addition, for manufacturability, forging and warm rolling are carried out, so-called melting, hot rolling, cold rolling and widely used manufacturing methods, manufacturing methods using manufacturing equipment, Is different. In addition, the inventors' tests have not led to a significant improvement in ductility.

Patent Document 2 discloses an aluminum-containing iron-based alloy containing Al: 10 to 19%, Ti: 2 to 8%, Mo: 0.5 to 10%, Hf: 0.1 to 1% and the remaining Fe. Although technology has been proposed, Hf, which is a heavy element, is essential, and emphasis is placed on heat resistance rather than low specific gravity. In addition, the production is based on the production from powder, and the manufacturing method widely used industrially, from so-called melting to hot rolling and cold rolling, and the manufacturing method of the thin steel plate using the manufacturing equipment Different. In addition, the inventors' tests have not led to a significant improvement in ductility.
As described above, it has been difficult to produce a high-strength, low-specific gravity steel plate excellent in ductility on an industrial scale with the conventional technology.
JP-A-8-253844 US Pat. No. 4,684,505

  The present invention is intended to solve the above-described problems, and an object of the present invention is to provide a high-strength low-specific gravity steel plate excellent in ductility and a method for producing the same.

  In order to improve ductility, hot workability and cold workability for various materials containing a large amount of Al on an iron basis, the present inventors have conducted research from both the component and manufacturing methods. As a result, it has been found that the ductility, hot workability and cold workability of the high Al content steel can be dramatically improved by the structure control even in the steel having a high Al content. That is, complex organization.

It has been reported that the deterioration of ductility and toughness of high Al-containing steels so far is due to grain boundary embrittlement due to precipitation of intermetallic compounds such as Fe ₃ Al, FeAl, etc., and mainly measures to suppress grain boundary embrittlement. Met.
In the Japanese Patent Application No. 2003-354060, the present inventors made Al content more than 10 to 22.0%, extremely reduced S and P, and further refined using fine carbonitride. By suppressing the embrittlement behavior of the steel plate and by optimizing the hot rolling conditions, the precipitation of intermetallic compounds such as Fe ₃ Al and FeAl during hot rolling, cooling and winding is suppressed as much as possible. It has been disclosed that workability and cold workability can be greatly improved. That is, about 13000M (Pa *%) was achieved by TS (MPa) x El (%) about the strength-ductility balance.
As a result of diligent research to further differentiate the steel from 590 to 780 MPa class, the present inventors have reduced the ferrite phase ratio, which is the maximum area ratio of steel so far, by controlling the fraction of the metal structure. It was found that the strength-ductility balance can be drastically improved by forming a composite structure with austenite.

The present invention is configured based on such knowledge, and the gist thereof is as follows.
(1) By mass%, C: 0.1 to 1.0%, Si: 3.0% or less, Mn: 10.0 to 50.0%, P: 0.01% or less, S: 0.01 % Or less, Al: 5.0 to 15.0%, N: 0.001 to 0.05%, the balance is composed of Fe and inevitable impurities, and the mass% of each component is represented by the following formula (1) Specific gravity: 7.0 or less, ferrite and austenite in the structure, and any area ratio of the structure is maximum in the structure, tensile strength: TS (MPa) and elongation at break: El (%) Value of product: A lightweight high-strength steel excellent in ductility, characterized in that TS × El is 20000 (MPa ·%) or more.
C ≦ −0.020 × Mn + Al / 15 + 0.53 (1)
(2) Further, by mass%, Cr: 0.01 to 5.0%, Ni: 0.01 to 15.0%, Mo: 0.01 to 5.0%, Co: 0.01 to 5.0 %, Cu: 0.01 to 5.0% of one or two or more kinds, the lightweight high-strength steel excellent in ductility according to (1).
(3) Further, by mass%, it contains one or more of Ti: 0.005 to 1%, V: 0.005 to 1%, Nb: 0.005 to 0.5%, The lightweight high-strength steel excellent in ductility as described in (1) or (2).
(4) Further, by mass%, Ca: 0.001-0.01%, Mg: 0.0005-0.3%, REM: 0.001-0.5%, Y: 0.001-0.1 % High-strength steel excellent in ductility according to any one of (1) to (3).
(5) The lightweight high-strength steel excellent in ductility according to any one of (1) to (4), further containing, by mass%, B: 0.0002 to 0.1%.
(6) The lightweight high-strength steel excellent in ductility according to any one of (1) to (5), wherein the area ratio of ferrite is 10 to 50%.

（８）前記（７）記載の方法にて製造した熱延鋼板を９００〜１２００℃で１分〜５時間の焼鈍を施し、その後３０℃／秒以上で６００℃以下に冷却することを特徴とする延性に優れた軽量高強度鋼の製造方法。
（９）前記（７）記載の方法にて製造した熱延鋼板を酸洗した後、１パス当たりの圧延率が５％以上２０％以下で全圧下率が３０％以上の冷延を２０℃以上で行い、８００〜１２００℃で２０秒〜５時間の焼鈍を施し、その後３０℃／秒以上で６００℃以下の温度域に冷却することを特徴とする延性に優れた軽量高強度鋼。 (7) A method for producing the high-strength steel sheet according to any one of (1) to (6), wherein the steel slab is composed of the component according to any one of (1) to (5). In a holding temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and holding temperature; T (K), holding time; t (h), heating and holding are performed under conditions satisfying the following formula (2): 1100 ° C. In the above temperature range, at least one pass reduction ratio of 30% or more is reduced for one pass or more, hot rolled at a finish rolling temperature of 850 ° C. or more, and cooled to a temperature range of 600 ° C. or less at 30 ° C./second or more. And the manufacturing method of the lightweight high strength steel excellent in ductility characterized by winding up in the temperature range of 600 degrees C or less.

(8) The hot-rolled steel sheet produced by the method described in (7) above is annealed at 900 to 1200 ° C. for 1 minute to 5 hours, and then cooled to 30 ° C./second or more and 600 ° C. or less. A method for producing lightweight high-strength steel with excellent ductility.
(9) After pickling the hot-rolled steel sheet produced by the method described in (7) above, cold rolling with a rolling reduction per pass of 5% to 20% and a total rolling reduction of 30% or more is 20 ° C. A lightweight high-strength steel excellent in ductility, characterized in that it is annealed at 800 to 1200 ° C. for 20 seconds to 5 hours and then cooled to a temperature range of 30 ° C./second or more and 600 ° C. or less.

  According to the present invention, a high strength and low specific gravity steel sheet having excellent ductility can be 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 according to (1) will be described.
C: C is an important additive element that improves strength and lowers specific gravity. Furthermore, it is an austenite forming element that is easier to diffuse than other elements, and is an additive element important for controlling the structure in the manufacturing process. On the other hand, since excessive addition may cause manufacturability deterioration due to excessive graphite production, it was set to 1.0% or less. Further, in order to control the structure of ferrite and austenite, which is the aim of the present invention, the addition is limited to the range satisfying C ≦ −0.020 × Mn + Al / 15 + 0.53 together with Mn and Al.

  Si: Si is an element that increases the strength of the steel sheet by solid solution strengthening and is useful for lowering the specific gravity. However, the combined addition with Al decreases the toughness, ductility and hot workability, and occurs in hot rolling. In order to remarkably deteriorate the peelability of the scale and the chemical conversion treatment, it is preferably low, and it is preferably 3.0% or less, particularly 0.1% or less when toughness is important. On the other hand, since extremely low significantly reduces refining productivity, it is desirable that the amount be 0.01% or more so that deterioration of toughness does not appear remarkably.

  Mn: Since Mn is an austenite forming element and is effective for strengthening, it is added in an amount of 10.0% or more. Further, in combination with C and Al, in order to form a ferrite-austenite composite structure, the addition was limited to the range satisfying C ≦ −0.020 × Mn + Al / 15 + 0.53. On the other hand, excessive addition exceeding 50.0% deteriorates ductility and toughness. Therefore, the Mn content is set to 10.0 to 50.0%.

  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%. However, when the amount of carbon and Al added is large, 0.0030% or less is desirable.

  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 0.01% in consideration of the reachable level and cost of the current refining technology. However, when the amount of carbon and Al added is large, 0.0030% or less is desirable.

Al: Al is an essential element for achieving a low specific gravity, and is added at 5.0% or more. Moreover, addition of 8.0% or more is desirable from the viewpoint of low specific gravity. On the other hand, in the manufacturing process, in order to significantly promote ferrite transformation, from the viewpoint of structure control, in combination with C and Mn which are austenite forming elements, C ≦ −0.020 × Mn + Al / 15 + 0.53 is satisfied. When the content is limited to 15% and exceeds 15%, the embrittlement behavior due to the intermetallic compound becomes remarkable, so this was made the upper limit.

N: N has the effect of forming nitrides and suppressing crystal grain coarsening, but if less than 0.001%, the effect is not manifested, and if added over 0.05%, the toughness deteriorates. The content was 0.001 to 0.05%.

The above are the basic components of the present invention, usually consisting of Fe and unavoidable impurities other than the above, but in the inventions according to (2) to (5), depending on the desired strength level and other necessary characteristics, One or more of Cr, Ni, Mo, Co, Cu, Ti, V, Nb, Ca, Mg, REM, Y, and B may be added.
Cr: Since Cr is an additive element for improving the balance between strength and ductility, it was added in an amount of 0.01% or more. On the other hand, in order to prevent deterioration of ductility and toughness, 5.0% was made the upper limit. Also, Fe 3 _Al, if the intermetallic compound such as FeAl are precipitated is preferably added to contribute to the production improvement.

  Ni: Ni is an effective element that improves ductility and toughness. Further, like Mn, it is an austenite-forming element and is effective for the composite formation of ferrite and austenite. Therefore, the content of 0.01% or more and 15.0% or less can prevent ductility deterioration. Therefore, the Ni content is set to 0.01 to 15.0%.

  Mo: Mo is an effective element that improves ductility and toughness. This effect is manifested at 0.01% or more, and by making it 5% or less, deterioration of toughness can be prevented and further reduction in specific gravity can be promoted. Therefore, the Mo content is set to 0.01 to 5.0%.

  Co: Co is an effective element that improves ductility and toughness. This effect is manifested at 0.01% or more, and by setting it to 5% or less, deterioration of toughness can be prevented and reduction in specific gravity can be promoted. Therefore, the Mo content is set to 0.01 to 5.0%.

  Cu: Cu is an effective element that improves ductility and toughness. This effect is manifested at 0.01% or more, and toughness can be prevented by setting it to 5% or less. Therefore, the Cu content is set to 0.01 to 5.0%.

  Ti: Ti forms TiN or TiC and has the effect of suppressing crystal grain coarsening. However, these effects are manifested at 0.005% or more, and toughness can be prevented at 1% or less. Therefore, the Ti content is set to 0.005 to 1%.

  V: V has the effect of forming fine carbonitrides and suppressing crystal grain coarsening, but the effect is manifested at 0.005% or more, and toughness deterioration can be prevented by making it 1% or less. . Therefore, the content of V is set to 0.005 to 1%.

  Nb: Nb has the effect of forming fine carbonitrides and suppressing crystal grain coarsening, but the effect is manifested at 0.005% or more, and the specific gravity is reduced by making it 0.5% or less. Promote and prevent deterioration of toughness. Therefore, the Nb content is set to 0.005 to 0.5%.

  Ca, Mg, REM (abbreviation of Rare Earth Metal, indicating a lanthanoid element), Y: These are all effective elements that suppress the deterioration of hot workability and toughness due to segregation elements such as S. This effect is exhibited when Ca is 0.001% or more, Mg is 0.0005% or more, REM is 0.001% or more, Y is 0.001% or more, Ca is 0.01% or less, and Mg is 0.00%. Deterioration of toughness can be prevented by setting 3% or less, REM 0.5% or less, and Y 0.1% or less. Moreover, reduction in specific gravity can be promoted by making the addition amount of REM and Y below the said upper limit. Therefore, Ca: 0.001 to 0.01%, Mg: 0.0005 to 0.3%, REM: 0.001 to 0.5%, and Y: 0.001 to 0.1%.

  B: B is added in a small amount to promote toughness improvement and hard second generation. Therefore, the addition amount is 0.0002% or more. On the other hand, 0.1% was made the upper limit in order to prevent deterioration of hot workability, ductility and toughness.

Next, the reason for limiting the characteristic value will be described.
If the specific gravity exceeds 7.0, the weight reduction effect is small compared with the specific gravity of steel plates normally used as automotive steel plates (same as the specific gravity of iron 7.86), so the specific gravity is set to 7.0 or less.
The strength and ductility are considered to be necessary characteristics that the steel plate for automobiles for processing is at least as high as the current steel plate of 590 to 780 MPa class, and the product of tensile strength: TS (MPa) and elongation at break: El (%) Value: TS × El is set to 20000 MPa ·% or more.

Next, the reason for limiting the tissue fraction will be described.
Since the ferrite of the high Al content steel contains a large amount of Al, the productivity and the material are deteriorated by the formation of Fe-Al intermetallic compounds and their clusters. Therefore, manufacturability and material deterioration can be suppressed by appropriately controlling the composite organization. Here, a hard layer such as martensite and bainite can be considered as a structure to be combined with ferrite, but ferrite containing Al is not sufficiently soft and tough unlike those not containing Al, so its deformability is small. Martensite and bainite are not suitable for the purpose of the present invention. Therefore, by combining with austenite having high deformability to some extent, the metal microstructure of the steel has ferrite and austenite, and one of them is the phase with the largest area ratio. Austenite formation requires a large amount of austenite-forming elements such as C and Mn, and a good structure control is performed in combination with an appropriate composite with these and production conditions described later. That is, by satisfying C ≦ −0.020 × Mn + Al / 15 + 0.53, the composite with austenite is achieved, and more preferably, the area ratio of ferrite is 50% or less and welding is performed in addition to a better material. Sex is also improved. On the other hand, in order to obtain the above-described effects, particularly weldability and relatively high yield strength, ferrite needs to be 10% or more in terms of area ratio.

  In addition to the above, as the remaining microstructure of the microstructure, 1 or 2 or more of carbides, nitrides, sulfides, oxides, etc. are contained in an area ratio of 1% or less, or martensite and bainite are also contained in an area ratio of 3% or less. However, it can be used in the present invention because it does not significantly degrade manufacturability and materials. In addition, identification of each phase of the above microstructure, ferrite, bainite, austenite, martensite, interfacial oxidation phase and remaining structure, observation of the existing position and measurement of area ratio are described in Nital reagent and JP-A-59-219473. The steel plate rolling direction cross section or the rolling perpendicular direction cross section is corroded with the disclosed reagent, and quantification is possible by observation with an optical microscope of 500 to 1000 times and an electron microscope (scanning type and transmission type) of 1000 to 100,000 times. It is possible to obtain an area ratio of each tissue by observing 20 fields of view or more and using a point counting method or image analysis. The austenite amount can be determined by X-ray diffraction. The total of each phase of the microstructure is 100%, but phases that cannot be observed and identified with an optical microscope such as carbides, oxides, and sulfides are included in the phase with the largest area ratio.

Next, the reasons for limiting the manufacturing conditions will be described.
In this invention which concerns on said (7), first, the steel slab which consists of a chemical component of any one of said (1)-(5) is heated by heating in the temperature range of 1100 degreeC or more and 1300 degrees C or less. Hold. At that time, the heating and holding are performed under the condition satisfying the following formula (3) when the heating temperature is T (K) and the heating time is t (h).
Next, at least 1 pass reduction in the temperature range of 1100 ° C. or higher, preferably 40% or higher, 1 pass or higher, hot rolled at a finish rolling temperature of 50 ° C. or higher, preferably 900 ° C. or higher, Hot rolling is performed by cooling to a temperature range of 600 ° C. or less, preferably 500 ° C. or less, at 30 ° C./second or more, preferably 50 ° C./second or more, and winding in a temperature range of 600 ° C. or less.

When the slab heating temperature at the time of heating and holding is less than 1100 ° C. or the holding time in this temperature range does not satisfy the above formula (3) with the heating temperature; T (K), heating time; t (h) In addition, segregation etc. cannot be sufficiently homogenized due to the high alloy, carbon nitride does not dissolve sufficiently, the required strength and ductility cannot be obtained, hot toughness is insufficient, hot cracking, etc. Therefore, the lower limit of the slab heating temperature is 1100 ° C. The upper limit of the heating temperature is particularly important in the case of a high carbon component, and is set to 1300 ° C. or less from the viewpoint of ensuring hot workability.
Further, from the viewpoint of promoting recrystallization during hot rolling, at least 1 pass reduction rate is 30% or more, desirably 40% or more, and 1 pass or more is reduced at a temperature range of 1100 ° C. or more, and 850 ° C. or more, preferably 900 ° C. or more. It is necessary to perform hot rolling at a finish rolling temperature of 600 ° C. or less after hot finish rolling in order to suppress embrittlement phase precipitation such as FeAl, Fe ₃ Al and (Fe, Mn) ₃ AlC phase, preferably 500 ° C. Cooling is performed at 30 ° C./second, preferably 50 ° C./second or more, to the following temperature range, and the winding process is performed at a temperature range of 600 ° C. or less, preferably 500 ° C. or less. From the viewpoint of suppressing the embrittlement phase precipitation, the conditions of high-temperature finishing (900 ° C. or higher) -quenching-low-temperature winding (500 ° C. or lower) are desirable. The upper limit of the cooling rate after hot finish rolling is not particularly defined, but if it exceeds 1000 ° C / second, a special cooling device is often required on the equipment, which is not economically preferable, and is 1000 ° C / second or less. Is desirable. Moreover, the lower limit of the cooling stop temperature and the coiling temperature is not particularly defined, but is set to room temperature or higher from the viewpoint of productivity.

In this invention which concerns on said (8), in order to improve the ductility of a hot-rolled sheet, from a viewpoint of recrystallization, carbon, or carbide precipitation control, a hot-rolled steel sheet is annealed at 900-1200 degreeC for 1 minute-5 hours. And then cooled to 30 ° C./second or more and 600 ° C. or less. In order to suppress precipitation of embrittled phases such as FeAl, Fe ₃ Al and (Fe, Mn) ₃ AlC phase during annealing and cooling, the annealing temperature is set to 900 ° C. or higher, and 30 ° C./second after annealing, preferably 50 ° C./second. The temperature is lowered to 600 ° C. or lower, preferably 500 ° C. or lower. Further, the annealing temperature was set to 1200 ° C. or less in order to prevent the coarsening of the grains. For the purpose of sufficient solution, the annealing time was set to 1 minute or longer, and 5 hours or shorter to prevent grain coarsening. However, it is economically effective to use a continuous annealing step, and it is desirable to keep it for 1 hour or less. On the other hand, when the box annealing process is used, the holding time is set to 5 hours or less.

When manufacturing a cold-rolled steel sheet in the invention according to (9), after rolling the steel sheet, pickling, rolling rate per pass is 5% or more and 20% or less, and total rolling reduction is 30% or more. After performing cold rolling at 20 ° C., annealing is performed at 800 to 1200 ° C. for 20 seconds to 5 hours, and then cooled to 600 ° C. or less at 30 ° C./second. In order to prevent cracking during cold rolling, the rolling reduction per pass and the rolling temperature were defined as described above. In order to recrystallize, the annealing temperature was set to 30% or more and a condition of 800 ° C. or more and 20 seconds or more, and from the viewpoint of preventing grain coarsening, 1200 ° C. or less and 5 hours or less. However, it is economically effective to use a continuous annealing step, and it is desirable to keep it for 1 hour or less. On the other hand, when the box annealing process is used, the holding time is set to 5 hours or less. Further, the cooling after annealing is performed at 30 ° C./second, preferably 50 ° C./second or more and 600 ° C. or less, preferably 500 ° C. or less, thereby embrittlement of FeAl, Fe ₃ Al, (Fe, Mn) ₃ AlC phase, etc. Phase precipitation can be suppressed.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
Steel slabs A to I and CA to CE having the compositions shown in Table 1 were prepared and hot-rolled under the conditions shown in Table 2. Some steel slabs were hot-rolled under the conditions shown in Table 2 and then annealed under the conditions shown in Table 3. Further, some other steel slabs were hot-rolled under the conditions shown in Table 2 and then cold-rolled and annealed under the conditions shown in Table 3.

After these treatments, the occurrence of cracks in the hot rolled sheet and cold rolled sheet was observed. The hot-rolled sheet and hot-rolled annealed sheet had a thickness of 2.3 to 3.2 mm, and the cold-rolled annealed sheet was subjected to a test by collecting a JIS No. 5 test piece from a steel sheet having a thickness of 1.0 to 1.8 mm. The results are shown in Tables 2 and 3. Moreover, the specific gravity measurement sample was extract | collected from the obtained steel plate or steel ingot, and specific gravity was measured. Further, the mechanical properties of the obtained steel sheet were evaluated. The specific gravity was measured using a pycnometer. Specific gravity, yield stress, tensile strength (TS) and elongation at break (El) are shown in Table 4.
Furthermore, the ferrite area ratio, the austenite area ratio, and the area ratio of other structures were measured by performing corrosion treatment on the cross section in the rolling direction of each steel plate and performing microscopic observation. The results are shown in Table 4.

As shown in Table 4, it can be seen that the steel according to the present invention satisfies the specific gravity of 7.0 or less and satisfies TS × El ≧ 20000 MPa ·%. Moreover, in the comparative steel, cracks in the hot rolled and cold rolled sheets are also generated, and it is understood that the hot workability and the cold workability are inferior. It can also be seen that all the comparative steels whose manufacturing conditions deviate from the limited range of the present invention have a large specific gravity.
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.

Claims (9)

% By mass
C: 0.1 to 1.0%
Si: 3.0% or less,
Mn: 10.0-50.0%,
P: 0.01% or less,
S: 0.01% or less,
Al: 5.0 to 15.0%,
N: 0.001 to 0.05%
And the balance consists of Fe and inevitable impurities, the mass% of each component satisfies the following formula (1), the specific gravity is 7.0 or less, and either of them has ferrite and austenite in the structure The area ratio is the largest in the structure, and the product of tensile strength: TS (MPa) and elongation at break: El (%): TS × El is 20000 (MPa ·%) or more. Excellent lightweight high strength steel.
C ≦ −0.020 × Mn + Al / 15 + 0.53 (1) In addition,
Cr: 0.01 to 5.0%,
Ni: 0.01 to 15.0%,
Mo: 0.01 to 5.0%,
Co: 0.01-5.0%
Cu: 0.01 to 5.0%
The lightweight high-strength steel excellent in ductility according to claim 1, comprising one or more of the following. In addition,
Ti: 0.005 to 1%
V: 0.005 to 1%,
Nb: 0.005 to 0.5%
The lightweight high-strength steel excellent in ductility according to claim 1 or 2, characterized by containing at least one of the following. In addition,
Ca: 0.001 to 0.01%,
Mg: 0.0005 to 0.3%,
REM: 0.001 to 0.5%,
Y: 0.001 to 0.1%
The lightweight high-strength steel excellent in ductility according to any one of claims 1 to 3, characterized by containing one or more of the following.   The lightweight high-strength steel excellent in ductility according to any one of claims 1 to 4, further comprising, in mass%, B: 0.0002 to 0.1%. The lightweight high-strength steel excellent in ductility according to any one of claims 1 to 5, wherein the area ratio of ferrite is 10 to 50%. It is a method of manufacturing the high-strength steel plate according to any one of claims 1 to 6, wherein the steel slab composed of the component according to any one of claims 1 to 5 is not less than 1100 ° C and not more than 1300 ° C. In the holding temperature range, the holding temperature; T (K), holding time; t (h), and heating and holding under the condition satisfying the following formula (2), at least 1 pass in the temperature range of 1100 ° C. or higher A rolling reduction of 30% or more and a reduction of 1 pass or more, hot rolling at a finish rolling temperature of 850 ° C. or more, cooling to a temperature range of 600 ° C. or less at 30 ° C./second or more, and a temperature range of 600 ° C. or less A method for producing a lightweight, high-strength steel excellent in ductility, characterized in that it is wound up by a roll.

  The hot-rolled steel sheet produced by the method according to claim 7 is annealed at 900 to 1200 ° C for 1 minute to 5 hours and then cooled to 30 ° C / second or more and 600 ° C or less, and has excellent ductility. A lightweight, high-strength steel manufacturing method. After pickling the hot-rolled steel sheet produced by the method according to claim 7, cold rolling at a rolling reduction per pass of 5% or more and 20% or less and a total rolling reduction of 30% or more is performed at 20 ° C. or more, A lightweight high-strength steel excellent in ductility, characterized by annealing at 800 to 1200 ° C. for 20 seconds to 5 hours and then cooling to a temperature range of 30 ° C./second to 600 ° C.