JP2005029889A - High strength low specific gravity steel plate excellent in ductility and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength low specific gravity steel plate excellent in ductility and a method for producing the same.
SOLUTION By mass%, C: 0.001 to 0.01%, Si: 3.0% or less, Mn: 0.01 to 3.0%, P: 0.02% or less, S: 0.01% or less, Al: 5.0 to 10.0%, N: 0.001 -0.05%, Ti: 0.005-0.3%, Nb: 0.005-0.3%, Cr: 0.05-3.0%, Ni: 0.05-5.0%, Mo: 0.05-3.0%, Cu: 0.1 -3.0%, B: 0.0003-0.01%, V: 0.01-0.5%, Ca: 0.001-0.01%, Mg: 0.0005-0.01%, Zr: 0.001-0.05%, REM: 0.001-0.05% High strength with excellent ductility, characterized by containing more than seeds, the balance being Fe and inevitable impurities, specific gravity <7.2, tensile strength of 440 MPa or more, elongation of 25% or more Low specific gravity steel plate.
[Selection figure] None

Description

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

  In recent years, in order to cope with environmental problems, it has been desired to reduce the weight of automobiles for the purpose of reducing carbon dioxide emissions and reducing fuel consumption. Increasing the strength of steel is an effective means for reducing the weight of automobiles, but if the plate thickness is limited by the rigidity of the member, the plate thickness can be reduced even if the strength is increased. Therefore, 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 are 0.01 to 7%, Mn: 5% or less, 2% or less of one or more of Si and Ti are 0 0.1-6%, O: 0.0005-0.04%, N: 0.0002-0.05%, low specific gravity vibration-absorbing alloy composed of residual Fe and inevitable impurities is disclosed.

  However, such a high Al-containing steel sheet is applied as a steel sheet for automobiles because of (i) inferior productivity (particularly cracking during rolling) and (ii) low ductility. Was difficult.

  In addition, when 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 (650 to 1200 ° C. for 5 to 600 minutes). It is necessary to produce a steel plate by heating), and it has been 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 : 0.1 to 0.8%, C: 0.01 to 0.5%, and the technology of an aluminum-containing iron-based alloy containing Fe is proposed, but there is no mention of low specific gravity, Certain 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 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. Moreover, in the test of the present inventors, the ductility has not been improved significantly.

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

  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: A steel sheet containing 0.5 to 5% and the remaining Fe has been proposed, but since the Al content is as small as 5% or less, the effect of reducing the specific gravity is small. Moreover, when Al is added exceeding 5%, it is described that manufacture is difficult because formability and cold workability are significantly deteriorated.

  In Patent Document 5, Si <0.2%, Mn: 0.03 to 0.2%, Al: 5 to 9%, Cu + Mo + W + Co + Cr + Ni of 1% or less in total, Sc + Y + REM of 0.1% or less in total And a steel sheet containing the remaining Fe has been proposed, and in Patent Document 6, C: 0.0036 to 0.1%, Si <0.2%, Mn: 0.03 to 0.2%, Al: Steel sheets containing 7 to 9%, Cu + Mo + W + Co + Cr + Ni of 1% or less in total, and 0.1% or less of Sc + Y + REM and residual Fe in total have been proposed, both of which are manufactured to improve formability and manufacturability No technology has been proposed, and it has been difficult to manufacture steel sheets having these components by a normal thin steel sheet manufacturing process in the tests of the present inventors.

  Patent Document 7 proposes a vibration-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 so large, a surface defect (rough skin) called orange peel occurs at the time of press working, so that it is difficult to apply to automobile members. In addition, no manufacturing technique for improving moldability and manufacturability has been proposed.

  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-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 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 manufacturing the same.

  The inventors of the present invention have found a method for improving ductility, hot workability, and cold workability for various materials containing a large amount of Al on an iron base and having different components from both the component and production methods. As a result of repeated research, the deterioration of the ductility, hot workability and cold workability of the high Al content steel is due to grain boundary embrittlement, and the Al content is set to 5.0 to 10.0%. In addition, S and P are extremely reduced, and carbon nitrides precipitated in the grains are reduced by extremely low C, thereby reducing the difference in strength between the grain boundaries and the grains, and further by optimizing the hot rolling conditions. It was found that the grain boundary strength can be improved and the ductility, hot workability and cold workability can be greatly improved by promoting recrystallization of ferrite during hot rolling and making it finer.

  As a result of further research, even when the S content is large, the addition amounts of Mn and S are limited to Mn: more than 0.2 to 3.0% and S: 0.02% or less, respectively. The addition ratio of S (Mn / S) exceeds 20, and in combination with the optimization of hot rolling conditions, the dissolution / precipitation behavior of MnS is controlled, so that MnS is formed and grain boundary embrittlement due to solid solution S is caused. It was found that the inhibitory effect was remarkably exhibited and the ductility, hot workability and cold workability could be greatly improved.

  On the other hand, when the S content can be extremely reduced, the addition amounts of Mn and S are limited to Mn: less than 0.03% and S: less than 0.005%, respectively, so that MnS is not precipitated as much as possible. It has been found that the ductility, hot workability and cold workability can be greatly improved.

  This invention is comprised based on such knowledge, and the summary is as follows.

  (1) By mass%, C: 0.001 to 0.01%, Si: 3.0% or less, Mn: 0.01 to 3.0%, P: 0.02% or less, S: 0.01 % Or less, Al: 5.0 to 10.0%, N: 0.001 to 0.05%, the balance is made of Fe and inevitable impurities, the specific gravity is <7.2, and the tensile strength Is a high strength low specific gravity steel sheet excellent in ductility, characterized by having an elongation of 440 MPa or more and an elongation of 25% or more.

  (2) Contains the component described in (1) above, and further contains one or two of Ti: 0.005 to 0.3% and Nb: 0.005 to 0.3% by mass%. A high-strength, low-specific gravity steel sheet with excellent ductility.

  (3) It contains the component described in the above (1) or (2), and further, in mass%, Cr: 0.05 to 3.0%, Ni: 0.05 to 5.0%, Mo: 0.00. Contains one or more of 05-3.0%, Cu: 0.1-3.0%, B: 0.0003-0.01%, V: 0.01-0.5% High strength and low specific gravity steel sheet with excellent ductility.

  (4) It contains the component according to any one of (1) to (3), and further, by mass%, Ca: 0.001 to 0.01%, Mg: 0.0005 to 0.01%, A high strength and low specific gravity steel sheet excellent in ductility, characterized by containing one or more of Zr: 0.001 to 0.05% and REM: 0.001 to 0.05%.

  (5) Instead of Mn and S, by mass%, Mn: more than 0.2 to 3.0%, S: 0.02% or less, and 20 <(Mn / S) is satisfied The high strength low specific gravity steel sheet excellent in ductility according to any one of the above (1) to (4).

  (6) In any one of the above (1) to (4), Mn: less than 0.03% and S: less than 0.005% are contained in mass% instead of Mn and S. High strength low specific gravity steel sheet with excellent ductility as described.

  (7) Instead of Mn and Ni, it contains one or two of Mn: more than 3.0 to 30.0% and Ni: more than 5.0 to 15.0% by mass%. The high strength low specific gravity steel sheet excellent in ductility according to any one of (1) to (4).

  (8) A method for producing a high-strength low-specific gravity steel sheet having excellent ductility according to any one of (1) to (4), wherein the component according to any one of (1) to (4) is used. The steel slab is heated to a temperature of 1100 ° C. or higher, and is hot-rolled at a temperature of 1000 ° C. or higher, including at least one pass of large reduction with a reduction rate of 30% or higher, and a finish rolling temperature of 800 ° C. or higher. A method for producing a high-strength, low-specific gravity steel sheet with excellent ductility.

  (9) The method for producing a high strength and low specific gravity steel sheet having excellent ductility according to (8), wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C. or higher and 1100 ° C. or lower.

  (10) The steel sheet is wound, pickled, cold-rolled, and annealed at a temperature of 600 ° C. or higher and 1100 ° C. or lower. High strength and low specific gravity excellent in ductility as described in (8) above A method of manufacturing a steel sheet.

  (11) A method for producing a high strength and low specific gravity steel sheet having excellent ductility as described in (5) above, wherein the steel slab comprising the component as described in (5) is heated to a temperature of 1100 ° C. or higher and 1150 ° C. or lower. And hot rolling at a finish rolling temperature of 800 ° C. or higher and 850 ° C. or lower, including at least one pass at a temperature of 1000 ° C. or higher and 1100 ° C. or lower at a rolling reduction of 30% or higher, and a temperature of 600 ° C. or higher and 700 ° C. or lower. A method for producing a high-strength, low-specific gravity steel sheet excellent in ductility, characterized by being wound up by a roll.

  (12) The method for producing a high strength and low specific gravity steel sheet having excellent ductility according to (11), wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C. or higher and 1100 ° C. or lower.

  (13) After winding the steel plate, pickling, cold rolling to reduce the reduction ratio of the first pass to 20% or less, annealing at a temperature of 600 ° C. to 1100 ° C., and after annealing, 20 ° C. The method for producing a high strength and low specific gravity steel sheet having excellent ductility according to the above (11), wherein the steel sheet is cooled to a temperature of 200 ° C. or less at a cooling rate of at least / sec.

  (14) A method for producing a high-strength, low-specific gravity steel sheet having excellent ductility as described in (6) above, wherein a steel slab composed of the component as described in (6) is heated to a temperature of 1100 ° C. or higher and 1150 ° C. or lower. And hot-rolling at a finish rolling temperature of 700 ° C. or higher and 850 ° C. or lower, including at least one pass at a temperature of 1000 ° C. or higher and 1100 ° C. or lower and a high rolling reduction of 40% or higher. A method for producing a high-strength, low-specific gravity steel sheet excellent in ductility, characterized by being wound up by a roll.

  (15) The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to the above (14), wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C. or higher and 1100 ° C. or lower.

  (16) After winding the steel plate, pickling, cold rolling to reduce the first-pass reduction rate to 30% or less, annealing at a temperature of 600 ° C. to 1100 ° C., and after annealing, 20 ° C. The method for producing a high strength and low specific gravity steel sheet having excellent ductility according to the above (14), wherein the steel sheet is cooled to a temperature of 200 ° C. or less at a cooling rate of at least / sec.

  (17) A method for producing a high-strength low specific gravity steel sheet having excellent ductility as described in (7), wherein the steel slab composed of the component as described in (7) is heated to a temperature of 1050 ° C. or higher and 1150 ° C. or lower, Hot at a finishing rolling temperature of 800 ° C. or higher and 850 ° C. or lower, including at least one pass at a temperature of 1000 ° C. or higher and 1100 ° C. or lower and a large rolling reduction of 25% or higher, and a total rolling reduction at 1000 ° C. or lower of 70% or higher. A method for producing a high strength and low specific gravity steel sheet excellent in ductility, characterized by rolling and winding at a temperature of 600 ° C. or higher and 750 ° C. or lower.

  (18) The method for producing a high strength and low specific gravity steel sheet having excellent ductility according to the above (17), wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C. or higher and 1100 ° C. or lower.

  (19) After winding the steel plate, pickling, cold rolling to a reduction rate of 15% or less in the first pass is performed at 30 ° C. or higher, and annealing is performed at a temperature of 600 ° C. or higher and 1100 ° C. or lower. Then, it cools to the temperature of 200 degrees C or less with the cooling rate of 20 degrees C / second or more, The manufacturing method of the high strength low specific gravity steel plate excellent in ductility as described in said (17) characterized by the above-mentioned.

  According to the present invention, it is possible to obtain a high-strength, low-specific gravity steel plate having excellent ductility.

  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 low specific gravity steel sheet having excellent ductility in the present invention will be described. In addition,% means the mass%.

  C: C is an essential element for improving the strength. However, if it is less than 0.001%, the effect is not expressed. On the other hand, excessive addition exceeding 0.01% is caused by precipitation of carbide in the grains. Grain boundary embrittlement is promoted because the difference in strength between the grain boundaries and the grains increases. Therefore, the C content is set to 0.001 to 0.01%.

  Si: Si is an element useful for increasing the strength of a steel sheet by solid solution strengthening. However, excessive addition exceeding 3.0% decreases the hot workability and reduces the scale produced by hot rolling. The peelability and chemical conversion processability are significantly degraded. Therefore, the Si content is set to 3.0% or less.

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

  Even when the S content is large, in order to sufficiently exhibit the effect of suppressing grain boundary embrittlement of Mn, it is more preferably more than 0.2% to 3.0%. On the other hand, when the S content can be extremely reduced to less than 0.005%, on the contrary, ductility and workability can be improved by suppressing the precipitation of MnS as much as possible, so the Mn content is less than 0.03%. It is desirable.

  However, the addition of a large amount of Mn is particularly effective for generating austenite and martensite, and is effective for increasing the strength by strengthening the structure when the amount of Al is relatively large. If it is 3.0% or less, the effect is small, and if it exceeds 30.0%, the ductility is significantly deteriorated. For this reason, 30.0% can be added as an upper limit.

  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, but considering the reachable level and cost of the current refining technology, The upper limit was made 0.02%.

  S: S is an impurity element that degrades hot workability and toughness, and is preferably as low as possible, but the upper limit was set to 0.01% in consideration of the reachable level and cost of the current refining technology . However, when Mn: more than 0.2 to 3.0% is included and the addition ratio of Mn and S (Mn / S) is more than 20, even when the S content is large, the grain boundary brittleness of Mn Since the effect of suppressing crystallization is sufficiently exhibited, the upper limit of the S content can be increased to 0.02%.

  In addition, when the S content can be extremely reduced to less than 0.005%, at the same time, the Mn content is reduced to less than 0.03% to suppress the precipitation of MnS as much as possible, thereby reducing ductility and workability. Since this can be improved, the upper limit of the S content may be less than 0.005%.

  Al: Al is an essential element for achieving a low specific gravity. If it is less than 5.0%, the effect of lowering the specific gravity is small, so the lower limit was made 5.0%. On the other hand, if it exceeds 10.0%, precipitation of intermetallic compounds becomes remarkable, and ductility, hot workability and cold workability deteriorate, so the Al content is set to 5.0 to 10.0%. . In order to maximize the effect of lowering the specific gravity, the Al content is desirably more than 9.0 to 10.0%.

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

  The above are the basic components of the present invention, and 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 it is less than 0.005%, those effects are not expressed. On the other hand, if over 0.3% is added, toughness deteriorates. Therefore, the Ti content is set to 0.005 to 0.3%.

  Nb: Nb has the effect of forming fine carbonitrides and suppressing crystal grain coarsening, but the effect is not expressed at less than 0.005%, on the other hand, excessive addition exceeding 0.3%, Since toughness deteriorates, the Nb content 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 0.05%, while excessive addition exceeding 3.0% degrades toughness. Therefore, the Cr content is set to 0.05 to 3.0%.

  Ni: Ni is an effective element that improves ductility and toughness. This effect is not manifested at less than 0.05%, while excessive addition exceeding 5.0% degrades toughness. Therefore, the Ni content is set to 0.05 to 5.0%.

  However, the addition of Ni is particularly effective for austenite and martensite generation, and, like Mn, when the amount of Al is relatively large, it is effective for increasing the strength by strengthening the structure. If the content is less than 5.0%, the effect is small. On the other hand, if it exceeds 15.0%, the ductility is significantly deteriorated. For this reason, 15.0% can be added as an upper limit.

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

  Cu: Cu is an effective element that improves ductility and toughness. This effect does not appear at less than 0.1%, while 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, and improving ductility, toughness, and hot workability. It is an effective element to improve. These effects are not manifested at less than 0.0003%. On the other hand, excessive addition of more than 0.01% produces coarse precipitates at the grain boundaries and deteriorates hot workability. Was 0.0003 to 0.01%.

  V: V has the effect of forming fine carbonitrides and suppressing coarsening of crystal grains, but if less than 0.01%, the effect is not manifested. On the other hand, if over 0.5% is added, toughness Therefore, the content of V is set to 0.01 to 0.5%.

  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 manifested 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 is Excess addition exceeding 0.01%, Zr 0.05% and REM 0.05% deteriorates 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%.

  Next, the reason for limiting the characteristic value will be described. When the specific gravity is 7.2 or more, the weight reduction effect is small compared to the specific gravity of a steel plate normally used as a steel plate for automobiles (same as the specific gravity of iron of 7.86), so the specific gravity is set to less than 7.2. Regarding the strength and ductility, the tensile strength is set to 440 MPa or more and the elongation is set to 25% or more in consideration of characteristics necessary for an automobile steel plate.

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

  In this invention which concerns on said (8), the steel slab which consists of a component in any one of said (1)-(4) is heated to the temperature of 1100 degreeC or more, and the reduction rate is 30% at the temperature of 1000 degreeC or more. Hot rolling is performed at a finish rolling temperature of 800 ° C. or more, including at least one pass of the above-described large reduction.

  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. The upper limit of the heating temperature is not particularly defined, but is preferably 1250 ° C. or lower in order to suppress coarsening of crystal grains.

  At the time of hot rolling, in order to promote recrystallization of ferrite and make it finer, it is necessary to include at least one pass of a large reduction at a reduction rate of 30% or more at a temperature of 1000 ° C. or more. If this condition is not satisfied, the ferrite recrystallization does not proceed and coarse ferrite grains remain, and good ductility, hot workability and cold workability cannot be obtained.

  If the finish rolling temperature is less than 800 ° C., the hot workability deteriorates and cracking occurs during hot rolling, so the lower limit of the finish rolling temperature is set to 800 ° C. 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.

  The winding temperature is not particularly limited, and there is no problem if it is carried out in the temperature range from the finishing temperature to room temperature.

  In this invention which concerns on said (9), in order to improve the ductility of a hot-rolled sheet, from a viewpoint of recrystallization and carbide precipitation control, after winding a hot-rolled sheet, it anneals at the temperature of 700 to 1100 degreeC. May be.

  Here, if the annealing temperature is less than 700 ° C, the effect is small, and if it exceeds 1100 ° C, the crystal grains become coarse and grain boundary embrittlement is promoted, so the annealing temperature of the hot-rolled sheet is a temperature of 700 ° C to 1100 ° C. The range.

  In this invention which concerns on said (10), when manufacturing a cold-rolled steel plate, after winding up a steel plate, it pickles, cold-rolls, and anneals at the temperature of 600 to 1100 degreeC.

  When the annealing temperature is less than 600 ° C., sufficient effects cannot be obtained because it is not recrystallized / recovered. On the other hand, when it exceeds 1100 ° C., the crystal grains become coarse and grain boundary embrittlement is promoted. The annealing temperature was set to a temperature range of 600 ° C. to 1100 ° C.

  On the other hand, ductility, hot workability, and cold workability can be further improved by further limiting the contents of Mn, S, Al, and the like and producing them under optimum conditions according to the components. In such a case, it manufactures on the following manufacturing conditions [this invention which concerns on said (11)-(19)].

  In this invention which concerns on said (11), the steel slab which consists of a component as described in said (5) is heated to the temperature of 1100 degreeC or more and 1150 degrees C or less, and the reduction rate is 30% or more at the temperature of 1000 degreeC or more and 1100 degrees C or less. And hot rolling at a finish rolling temperature of 800 ° C. or higher and 850 ° C. or lower, and winding at a temperature of 600 ° C. or higher and 700 ° C. or lower.

  In the component described in (5), the contents of Mn and S are limited to Mn: more than 0.2 to 3.0% and S: 0.02% or less, respectively, and the addition ratio of Mn and S (Mn Since / S) is more than 20, MnS can be formed by optimizing the hot rolling conditions, and grain boundary embrittlement due to the solid solution S can be suppressed.

  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 heating temperature exceeds 1150 ° C., MnS re-dissolves and grain boundary embrittlement occurs due to the solid solution S, so the upper limit of the slab heating temperature is 1150 ° C. By setting the upper limit of the slab heating temperature to 1150 ° C., coarsening of crystal grains can be prevented.

  At the time of hot rolling, in order to promote recrystallization of ferrite and make it finer, it is necessary to include at least one pass of a large reduction with a reduction ratio of 30% or more at a temperature of 1000 ° C. or more and 1100 ° C. or less. If the rolling temperature at the time of large reduction is less than 1000 ° C. or the reduction ratio is less than 30%, the ferrite recrystallization does not proceed and coarse ferrite grains remain, and good ductility, hot workability and cold Workability cannot be obtained. On the other hand, when the rolling temperature under large pressure exceeds 1100 ° C., the recrystallized ferrite crystal grains are coarsened, so that good ductility, hot workability and cold workability cannot be obtained.

  If the finish rolling temperature is less than 800 ° C., the hot workability deteriorates and cracking occurs during hot rolling, so the lower limit of the finish rolling temperature is set to 800 ° C. When the finishing temperature exceeds 850 ° C., the accumulation of strain during rolling is not sufficient, and recovery / recrystallization in subsequent winding is suppressed, so the upper limit of the finishing temperature was set to 850 ° C.

  If the winding temperature is less than 600 ° C., ferrite recovery / recrystallization does not proceed, so the lower limit of the winding temperature was set to 600 ° C. When the coiling temperature exceeds 700 ° C., the recrystallized ferrite crystal grains become coarse, and good ductility, hot workability and cold workability cannot be obtained. Therefore, the upper limit of the coiling temperature is 700 ° C. did.

  In this invention which concerns on said (12), in order to improve the ductility of a hot-rolled sheet, from a viewpoint of recrystallization and carbide precipitation control, after winding a hot-rolled sheet, it anneals at the temperature of 700 to 1100 degreeC. May be.

  Here, if the annealing temperature is less than 700 ° C., the effect is small, and if it exceeds 1100 ° C., the crystal grains are coarsened and grain boundary embrittlement is promoted, so the annealing temperature of the hot-rolled sheet is 700 ° C. or more and 1100 ° C. or less. The temperature range.

  In this invention which concerns on said (13), when manufacturing a cold-rolled steel plate, after winding up a steel plate, it pickles and performs cold rolling which makes the rolling reduction of the 1st pass 20% or less, 600 Annealing is performed at a temperature of not lower than 1 ° C. and not higher than 1100 ° C., and after annealing, it is cooled to a temperature of not higher than 200 ° C. at a cooling rate of 20 ° C./second or higher.

  In order to prevent cracking during cold rolling, the rolling reduction in the first pass was set to 20% or less.

  When the annealing temperature is less than 600 ° C., sufficient effects cannot be obtained because it is not recrystallized / recovered. On the other hand, when it exceeds 1100 ° C., the crystal grains become coarse and grain boundary embrittlement is promoted. The annealing temperature was set to a temperature range of 600 ° C. or higher and 1100 ° C. or lower.

  If the cooling rate after annealing is less than 20 ° C./second or the cooling stop temperature is higher than 200 ° C., grain growth occurs during cooling and the crystal grains become coarse, and impurity elements such as P enter the grain boundary. Segregates, grain boundary embrittlement occurs, and ductility deteriorates. Therefore, after annealing, it was decided to cool to 200 ° C. or lower at a cooling rate of 20 ° C./second or higher.

  In this invention which concerns on said (14), the steel slab which consists of a component as described in said (6) is heated to the temperature of 1100 degreeC or more and 1150 degrees C or less, and the reduction rate is 40% or more at the temperature of 1000 degreeC or more and 1100 degrees C or less. Is rolled at a finish rolling temperature of 700 ° C. or higher and 850 ° C. or lower, and wound at a temperature of 550 ° C. or higher and 700 ° C. or lower.

  In the component described in (6), the contents of Mn and S are extremely reduced to Mn: less than 0.03% and S: less than 0.005%, respectively, and MnS is prevented from being precipitated as much as possible. It is excellent in hot workability and cold workability, and the manufacturing conditions can be relaxed.

  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 heating temperature exceeds 1150 ° C., the crystal grains become coarse, so the upper limit of the slab heating temperature is set to 1150 ° C.

  At the time of hot rolling, in order to promote recrystallization of the ferrite and make it finer, it is necessary to include at least one pass of a large reduction with a reduction ratio of 40% or more at a temperature of 1000 ° C. or more and 1100 ° C. or less.

  If the rolling temperature at the time of large reduction is less than 1000 ° C. or the reduction ratio is less than 40%, ferrite recrystallization does not proceed and coarse ferrite grains remain, and good ductility, hot workability, and cold Interworkability is not obtained. Since the steel is excellent in hot workability, cracking can be prevented even when a large reduction of 40% or more is performed, and it is very effective in promoting recrystallization.

  On the other hand, when the rolling temperature under large pressure exceeds 1100 ° C., the recrystallized ferrite crystal grains are coarsened, so that good ductility, hot workability and cold workability cannot be obtained.

  Since the above steel is excellent in hot workability, cracking can be prevented even if the finishing temperature is lowered to 700 ° C. However, if the finishing rolling temperature is less than 700 ° C, the hot workability deteriorates and hot rolling Since cracks occurred inside, the lower limit of the finish rolling temperature was set to 700 ° C. When the finishing temperature exceeds 850 ° C., the accumulation of strain during rolling is not sufficient, and recovery / recrystallization in subsequent winding is suppressed, so the upper limit of the finishing temperature was set to 850 ° C.

  Since the above steel has good hot workability and can be subjected to relatively low temperature and large pressure, recovery and recrystallization of ferrite can be promoted even if the coiling temperature is lowered to 550 ° C., but the coiling temperature is 550. If the temperature is less than ℃, ferrite recovery / recrystallization does not proceed, so the lower limit of the coiling temperature is 550 ℃. When the coiling temperature exceeds 700 ° C., the recrystallized ferrite crystal grains become coarse, so that good ductility, hot workability and cold workability cannot be obtained. Therefore, the upper limit of the coiling temperature is 700 ° C. It was.

  In this invention which concerns on said (15), in order to improve the ductility of a hot-rolled sheet, from a viewpoint of recrystallization and carbide precipitation control, after winding a hot-rolled sheet, it anneals at the temperature of 700 to 1100 degreeC. May be.

  Here, if the annealing temperature is less than 700 ° C., the effect is small, and if it exceeds 1100 ° C., the crystal grains are coarsened and grain boundary embrittlement is promoted, so the annealing temperature of the hot-rolled sheet is 700 ° C. or more and 1100 ° C. or less. The temperature range.

  In this invention which concerns on said (16), when manufacturing a cold-rolled steel plate, after winding up a steel plate, it pickles and performs cold rolling which makes the rolling reduction of the 1st pass 30% or less, 600 Annealing is performed at a temperature of not lower than 1 ° C. and not higher than 1100 ° C. After the annealing, cooling is performed to a temperature of not higher than 200 ° C. at a cooling rate of 20 ° C./second or higher.

  In order to prevent cracking during cold rolling, the rolling reduction in the first pass was set to 30% or less.

  When the annealing temperature is less than 600 ° C., sufficient effects cannot be obtained because it is not recrystallized / recovered. On the other hand, when it exceeds 1100 ° C., the crystal grains become coarse and grain boundary embrittlement is promoted. The annealing temperature was set to a temperature range of 600 ° C. to 1100 ° C.

  If the cooling rate after annealing is less than 20 ° C./second or the cooling stop temperature exceeds 200 ° C., grain growth occurs during cooling, the crystal grains become coarse, and impurities such as P enter the grain boundary. Since element segregation causes grain boundary embrittlement and ductility deteriorates, after annealing, it was decided to cool to a temperature of 200 ° C. or lower at a cooling rate of 20 ° C./second or higher.

  In this invention which concerns on said (17), the steel slab which consists of a component of said (7) is heated to the temperature of 1050 degreeC or more and 1150 degrees C or less, and the reduction rate is 25% or more at the temperature of 1000 degreeC or more and 1100 degrees C or less. Hot rolling is carried out at a finish rolling temperature of 800 ° C. or higher and 850 ° C. or lower, and wound at a temperature of 600 ° C. or higher and 750 ° C. or lower.

  In the component described in (7), the contents of Mn and Ni are increased to more than 3.0 to 30.0% and more than 5.0 to 15.0%, respectively. Because the amount is very high, the alloy itself is fragile, and cracks may occur when excessively cold and hot rolling during hot working or cold rolling at low temperatures, so it is necessary to specify appropriate manufacturing conditions There is.

  The slab heating temperature is desirably as low as possible from the viewpoint of preventing crystal grain coarsening. However, if the slab heating temperature is less than 1050 ° C., the required strength and ductility can be obtained without sufficiently dissolving the carbonitride. Therefore, the lower limit of the slab heating temperature was 1050 ° C. When the heating temperature exceeds 1150 ° C., the crystal grains become coarse, so the upper limit of the slab heating temperature is 1150 ° C.

  At the time of hot rolling, in order to promote recrystallization of ferrite and refine the grain size, the total rolling reduction at 1000 ° C. or less including at least one pass of large rolling at a rolling reduction of 25% or more at a temperature of 1000 ° C. or more and 1100 ° C. or less. It is necessary to make it 70% or more.

  If the rolling temperature at the time of large reduction is less than 1000 ° C. or the reduction ratio is less than 25%, ferrite recrystallization does not proceed and coarse ferrite grains remain, and good ductility, hot workability, and cold Interworkability is not obtained.

  On the other hand, when the rolling temperature under large pressure exceeds 1100 ° C., the recrystallized ferrite crystal grains are coarsened, so that good ductility, hot workability and cold workability cannot be obtained. If the total rolling reduction at 1000 ° C. or lower is less than 70%, accumulation of strain during rolling is not sufficient, and recovery / recrystallization in subsequent winding is suppressed.

  If the finish rolling temperature is less than 800 ° C., the hot workability deteriorates and cracking occurs during hot rolling, so the lower limit of the finish rolling temperature is set to 800 ° C. When the finishing temperature exceeds 850 ° C., the accumulation of strain during rolling is not sufficient, and recovery / recrystallization in subsequent winding is suppressed, so the upper limit of the finishing temperature was set to 850 ° C.

  If the winding temperature is less than 600 ° C., ferrite recovery / recrystallization does not proceed, so the lower limit of the winding temperature was set to 600 ° C. When the coiling temperature exceeds 750 ° C., the recrystallized ferrite crystal grains become coarse, and good ductility, hot workability and cold workability cannot be obtained. Therefore, the upper limit of the coiling temperature is 750 ° C. did.

  In this invention which concerns on said (18), in order to improve the ductility of a hot-rolled sheet, from a viewpoint of recrystallization and carbide precipitation control, after winding a hot-rolled sheet, it anneals at the temperature of 700 to 1100 degreeC. May be.

  Here, if the annealing temperature is less than 700 ° C., the effect is small, and if it exceeds 1100 ° C., the crystal grains are coarsened and grain boundary embrittlement is promoted, so the annealing temperature of the hot-rolled sheet is 700 ° C. or more and 1100 ° C. or less. The temperature range.

  In this invention which concerns on said (19), when manufacturing a cold-rolled steel plate, after rolling up a steel plate, it pickles and cold rolling which makes the rolling reduction of the 1st pass 15% or less is 30 degreeC or more It anneals at the temperature of 600 degreeC or more and 1100 degrees C or less, and it cools to the temperature of 200 degrees C or less at a cooling rate of 20 degrees C / sec or more after annealing.

  In order to prevent cracking during cold rolling, the rolling reduction in the first pass was set to 15% or less, and the cold rolling temperature was set to 30 ° C. or higher.

  When the annealing temperature is less than 600 ° C., sufficient effects cannot be obtained because it is not recrystallized / recovered. On the other hand, when it exceeds 1100 ° C., the crystal grains become coarse and grain boundary embrittlement is promoted. The annealing temperature was set to a temperature range of 600 ° C. to 1100 ° C.

  If the cooling rate after annealing is less than 20 ° C./second or the cooling stop temperature is higher than 200 ° C., grain growth occurs during cooling and the crystal grains become coarse, and impurity elements such as P enter the grain boundary. Since segregation occurs, grain boundary embrittlement occurs and ductility deteriorates. Therefore, after annealing, it was decided to cool to 200 ° C. or lower at a cooling rate of 20 ° C./second or higher.

  Hereinafter, the present invention and its effects will be described more specifically by way of examples.

  Steel having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 2, cold-rolled, and then annealed under the conditions shown in Table 2.

  After hot rolling and after cold rolling, the occurrence of cracks in the hot rolled sheet and cold rolled sheet was observed, respectively. The results are also 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 also shown in Table 2.

  In the present invention examples (No. 1 to 5, No. 12 to 14), the specific gravity <7.2 is satisfied, the tensile strength is 440 MPa or more, and a high elongation of 30% or more is obtained with respect to ductility. In addition, cracks in the hot rolled sheet and the cold rolled sheet have not occurred.

  On the other hand, in the comparative examples (No. 6, 7, 8) in which any one or more of the components deviate from the component-limited range of the present invention, the elongation is 20% or less and the ductility is inferior. Recognize. Moreover, in these comparative examples, the hot-rolled plate and the cold-rolled plate are also cracked, indicating that the hot workability and the cold workability are inferior.

  Further, in the comparative examples (Nos. 9, 10, and 11) in which the manufacturing conditions deviate from the limited range of the present invention, the elongation is 20% or less, and the hot rolled sheet and the cold rolled sheet are cracked. It can be seen that ductility, hot workability and cold workability are poor.

  In addition, the specific gravity and mechanical properties of a hot-rolled sheet obtained by hot rolling steel having the composition shown in Table 1 under the conditions shown in Table 2 were also evaluated. Table 3 shows the specific gravity, yield stress, tensile strength and elongation of the hot-rolled sheet. Furthermore, this hot-rolled sheet was subjected to hot-rolled sheet annealing under the conditions shown in Table 4, and the specific gravity and mechanical properties of the hot-rolled sheet annealed material were also evaluated. Table 4 shows the specific gravity, yield stress, tensile strength and elongation of the hot-rolled sheet annealed material.

  In the present invention examples (No. 1-5, No. 12-14), both the hot-rolled sheet and the hot-rolled sheet annealed material satisfy the specific gravity <7.2, the tensile strength is 440 MPa or more, and the ductility With regard to, high elongation of 30% or more is obtained.

  On the other hand, in the comparative examples (No. 6, 7, 8) in which any one or more of the components deviate from the component-limited range of the present invention, the elongation is 20% or less and the ductility is inferior. Recognize. Moreover, in the comparative examples (No. 9, 10, 11) in which the hot rolling conditions deviate from the limited range of the present invention, it is understood that the elongation is 20% or less and 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.

  As described above, according to the present invention, it is possible to provide a high strength and low specific gravity steel plate having excellent ductility. Therefore, the present invention has high applicability in the steel plate utilization industry.

Claims (19)

% By mass
C: 0.001 to 0.01%,
Si: 3.0% or less,
Mn: 0.01 to 3.0%,
P: 0.02% or less,
S: 0.01% or less,
Al: 5.0 to 10.0%,
N: 0.001 to 0.05%
And the balance is Fe and inevitable impurities, the specific gravity <7.2, the tensile strength is 440 MPa or more, and the elongation is 25% or more. Low specific gravity steel plate. Furthermore, in mass%,
Ti: 0.005 to 0.3%,
Nb: 0.005-0.3%
The high strength low specific gravity steel plate excellent in ductility according to claim 1, comprising one or two of the following. Furthermore, in mass%,
Cr: 0.05-3.0%,
Ni: 0.05-5.0%,
Mo: 0.05-3.0%
Cu: 0.1 to 3.0%,
B: 0.0003 to 0.01%
V: 0.01 to 0.5%
The high strength 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%,
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 high strength low specific gravity steel plate excellent in ductility according to any one of claims 1 to 3, wherein one or more of the above are contained. In mass%, instead of Mn and S,
Mn: more than 0.2 to 3.0%,
S: 0.02% or less, and
20 <(Mn / S)
The high-strength, low-specific gravity steel sheet excellent in ductility according to any one of claims 1 to 4, wherein: In mass%, instead of Mn and S,
Mn: less than 0.03%,
S: Less than 0.005% is contained, The high strength low specific gravity steel plate excellent in ductility of any one of Claims 1-4 characterized by the above-mentioned. In mass%, instead of Mn and Ni,
Mn: more than 3.0 to 30.0%,
Ni: more than 5.0 to 15.0%
1 type or 2 types of these are included, The high strength low specific gravity steel plate excellent in ductility of any one of Claims 1-4 characterized by the above-mentioned.   It is a method of manufacturing the high strength low specific gravity steel plate excellent in ductility of any one of Claims 1-4, Comprising: The steel slab which consists of a component of any one of Claims 1-4 is 1100. Heating to a temperature of ℃ or higher, excellent rolling ductility characterized by including at least one pass of a large reduction with a rolling reduction of 30% or more at a temperature of 1000 ℃ or more and hot rolling at a finish rolling temperature of 800 ℃ or more Manufacturing method of high strength low specific gravity steel plate.   The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to claim 8, wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C or higher and 1100 ° C or lower.   The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to claim 8, wherein the steel sheet is wound, pickled, cold rolled, and annealed at a temperature of 600 ° C to 1100 ° C. .   A method for producing a high strength and low specific gravity steel sheet having excellent ductility according to claim 5, wherein the steel slab comprising the component according to claim 5 is heated to a temperature of 1100 ° C or higher and 1150 ° C or lower, and 1000 ° C or higher. Hot rolling at a temperature of 1100 ° C. or less and hot rolling at a finish rolling temperature of 800 ° C. or more and 850 ° C. or less including at least one pass of a large reduction with a reduction rate of 30% or more and winding at a temperature of 600 ° C. or more and 700 ° C. or less. A method for producing a high strength and low specific gravity steel sheet having excellent ductility.   The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to claim 11, wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C or higher and 1100 ° C or lower.   After winding the steel plate, it is pickled, cold-rolled with a reduction rate of the first pass of 20% or less, annealed at a temperature of 600 ° C. or higher and 1100 ° C. or lower, and after annealing, 20 ° C./second or higher. The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to claim 11, wherein the steel sheet is cooled to a temperature of 200 ° C. or less at a cooling rate of 10 ° C.   A method for producing a high strength and low specific gravity steel sheet having excellent ductility according to claim 6, wherein the steel slab comprising the component according to claim 6 is heated to a temperature of 1100 ° C. or higher and 1150 ° C. or lower, and 1000 ° C. or higher. Hot rolling at a temperature of 1100 ° C. or less and hot rolling at a finish rolling temperature of 700 ° C. or more and 850 ° C. or less, including at least one pass of a large reduction of 40% or more, and winding at a temperature of 550 ° C. or more and 700 ° C. or less. A method for producing a high strength and low specific gravity steel sheet having excellent ductility.   The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to claim 14, wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C or higher and 1100 ° C or lower.   After winding the steel plate, it is pickled, cold rolled to a reduction rate of 30% or less in the first pass, annealed at a temperature of 600 ° C. or higher and 1100 ° C. or lower, and after annealing, 20 ° C./second or higher. The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to claim 14, wherein the steel sheet is cooled to a temperature of 200 ° C. or less at a cooling rate of 15 ° C.   A method for producing a high-strength steel low specific gravity plate excellent in ductility according to claim 7, wherein the steel slab comprising the component according to claim 7 is heated to a temperature of 1050 ° C to 1150 ° C, and 1000 ° C to 1100 And hot rolling at a finish rolling temperature of 800 ° C. or higher and 850 ° C. or lower with a total rolling reduction of at least 1 pass including a large reduction of 25% or higher at a temperature of ≦ ° C. A method for producing a high-strength, low-specific gravity steel sheet excellent in ductility, characterized by winding at a temperature of from ℃ to 750 ℃.   The method for producing a high strength and low specific gravity steel sheet excellent in ductility according to claim 17, wherein the steel sheet is rolled up and then annealed at a temperature of 700 ° C or higher and 1100 ° C or lower.   After winding the steel plate, it is pickled and cold-rolled at a first pass reduction of 15% or less at 30 ° C. or higher, annealed at a temperature of 600 ° C. or higher and 1100 ° C. or lower, and after annealing, 20 18. The method for producing a high strength and low specific gravity steel sheet having excellent ductility according to claim 17, wherein the steel sheet is cooled to a temperature of 200 [deg.] C. or lower at a cooling rate of [deg.] C./second or higher.