JP2012107339A - Aluminum alloy sheet for automobile and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture an Al-Mg series alloy sheet excellent in a formability and a shape-fixability without applying a stabilizing treatment.SOLUTION: In a method for manufacturing the aluminum alloy sheet for automobile, excellent in a press-formability, surface roughness resistance and shape-fixability; molten metal composed, by mass, of 3.0-3.5% Mg, 0.05-0.3% Fe, 0.05-0.15% Si, and further, regulated to <0.1% Mn and the balance Al with inevitable impurities, is cast into a thin slab having 5-15 mm thickness with a twin-belt type caster so that the cooling speed at 1/4 thickness becomes 20-200°C/sec, and after winding into a coil, a cold-rolling is applied at 60-98% cold-rolling ratio with the rolls having 0.2-0.7 μm Ra of the roll surface roughness, and after applying the finish-annealing at 400-520°C holding temperature with CAL (a continuous annealing line) or 300-400°C holding temperature with a butch-type annealing furnace, this stress-correction is performed with a leveller.

Description

  The present invention relates to an aluminum alloy plate for automobiles and a method for producing the same, and more particularly to an aluminum alloy plate suitable for forming automobile body sheets and the like and a method for producing the same.

  Conventionally, for example, a cold-rolled steel sheet is mainly used for an outer plate for an automobile. Recently, however, the use of aluminum alloy plates such as Al—Mg and Al—Mg—Si has been studied along with the weight reduction of automobile bodies. In particular, Al—Mg alloy plates have been proposed as automobile body sheets because of their excellent strength, formability, and corrosion resistance.

  As a method for producing these aluminum alloy sheets, conventionally, slabs are cast by DC casting, both sides of the slab are chamfered, and then homogenized by a soaking furnace, hot rolling, cold rolling, intermediate annealing, cold rolling. A method of performing final annealing and finishing to a predetermined plate thickness is employed (Patent Document 1).

  On the other hand, a method has been proposed in which a thin slab is continuously cast by a belt-type casting machine, wound directly on a coil, subjected to cold rolling and final annealing, and finished to a predetermined thickness. For example, Mg: 3.3 to 3.5 wt%, Mn: 0.1 to 0.2 wt%, Fe: 0.3 wt% or less, Si: 0.15 wt% or less, any one or two Including the above, the remaining ordinary impurities and Al molten metal, a thin slab having a thickness of 5 to 10 mm to a cooling rate of 40 to 90 ° C./sec. Press formability and stress corrosion cracking resistance, characterized by casting at 15 m / min and winding on a roll, followed by cold rolling with a roll having a roll surface roughness Ra of 0.2 to 0.7 μm and annealing. A method for manufacturing an aluminum alloy sheet for automobiles, which is characterized by excellent performance, is disclosed (Patent Document 2).

  However, in this method, for the purpose of recrystallized grain refinement, as the chemical composition of the molten metal, 0.1 to 0.2 wt% of Mn is contained, and since the solidification cooling rate is relatively fast, Al— (Fe · Mn ) Although the size of intermetallic compounds such as -Si is reduced and the moldability is excellent, the solid solution amount of Mn in the matrix becomes too high, so the problem is that the yield strength is high and the springback after molding becomes large. is there.

  In order to solve this problem, for example, after annealing a continuous cast rolled sheet of aluminum alloy containing Mg: 3 to 6%, the distortion is corrected, and is performed at a predetermined temperature of 240 to 340 ° C. for 1 hour or more. A so-called stabilization process of slow cooling is also proposed (Patent Document 3).

Japanese Patent No. 3155678 WO2006 / 011242 JP-A-11-80913

  However, since it is disadvantageous from the viewpoint of cost to perform the stabilization treatment on the continuously cast and rolled plate, it is possible to produce an Al-Mg alloy plate excellent in formability and shape freezing property without performing the stabilization treatment. It was an issue.

  In order to solve the above problems, the present invention includes Mg: 3.0 to 3.5 mass%, Fe: 0.05 to 0.3 mass%, Si: 0.05 to 0.15 mass%, and Mn : The thickness is controlled to be less than 0.1 mass%, and the remaining substantially inevitable impurities and Al are melted to a thickness of 5 so that the cooling rate at 1/4 thickness is 20 to 200 ° C./sec by a twin belt type casting machine. After casting a thin slab of ˜15 mm and winding it on a coil, it is cold-rolled with a roll roughness Ra of 0.2 to 0.7 μm and a cold rolling rate of 60 to 98%, and a holding temperature of 400 by CAL. A method for producing an aluminum alloy plate for automobiles excellent in press formability, rough surface property and shape freezing property, characterized by performing final annealing continuously at ˜520 ° C. and then correcting the strain with a leveler, was adopted. The final annealing may be performed at a holding temperature of 300 to 400 ° C. using a batch annealing furnace.

  By adopting such a production method, Mg: 3.0 to 3.5 mass%, Fe: 0.05 to 0.3 mass%, Si: 0.05 to 0.15 mass%, and Mn: 0 0.1% by mass, the balance is substantially inevitable impurities and Al, and the maximum equivalent circle diameter of the intermetallic compound in the region of 1/4 thickness is 5 μm or less, and the average recrystallized grain size is It is 15 μm or less, has a surface roughness Ra of 0.2 to 0.6 μm, a proof stress of 145 MPa or less, and a tensile strength of 225 MPa or more, and has excellent press formability, rough skin property and shape freezing property. It became possible to provide an aluminum alloy sheet.

  ADVANTAGE OF THE INVENTION According to this invention, the Al-Mg type | system | group alloy board excellent in the moldability and the shape freezing property can be manufactured, without performing a stabilization process to a continuous cast rolling board.

  The reason why the chemical composition of the alloy is limited in the present invention will be described. In the present specification, “%” representing the chemical composition means “% by mass” unless otherwise specified.

[Mg: 3.0-3.5%]
Mg is an element that increases the strength by solid solution strengthening, and if it is less than 3.0%, this effect cannot be exhibited, and the tensile strength decreases. When the Mg content exceeds 3.5%, the yield strength becomes too high and the shape freezing property is lowered.

[Fe: 0.05 to 0.3%]
Fe crystallizes as fine particles of an intermetallic compound such as Al-Fe-Si during casting, and functions as a recrystallization nucleation site during annealing after cold rolling. Therefore, the larger the number of particles of these intermetallic compounds, the more recrystallized nuclei that are generated. As a result, a large number of fine recrystallized grains are formed. In addition, the fine particles of the intermetallic compound pin the grain boundaries of the generated recrystallized grains, suppress the growth due to the coalescence of the crystal grains, and stably maintain the fine recrystallized grains. In order to exhibit this effect, the Fe content needs to be 0.05% or more. However, if the Fe content exceeds 0.3%, the tendency of the intermetallic compound to crystallize becomes strong, and voids are formed starting from this intermetallic compound during molding, resulting in poor moldability. Therefore, the Fe content is limited to 0.05 to 0.3%. A preferable range is 0.05 to 0.25%.

[Si: 0.05 to 0.15%]
Si crystallizes as fine particles of an intermetallic compound such as Al-Fe-Si during casting, and functions as a recrystallization nucleation site during annealing after cold rolling. Therefore, the larger the number of particles of these intermetallic compounds, the more recrystallized nuclei that are generated. As a result, a large number of fine recrystallized grains are formed. In addition, the fine particles of the intermetallic compound pin the grain boundaries of the generated recrystallized grains, suppress the growth due to the coalescence of the crystal grains, and stably maintain the fine recrystallized grains. In order to exhibit this effect, the Si content needs to be 0.05% or more. However, if the Si content exceeds 0.15%, the tendency of the intermetallic compound to crystallize becomes strong, and voids are formed starting from this intermetallic compound during molding, resulting in poor moldability. Therefore, the Si content is limited to 0.05 to 0.15%. A preferable range is 0.05 to 0.1%.

[Mn: less than 0.1%]
When the Mn content is 0.1% or more, the solidification cooling rate at the time of casting is high, so the Mn solid solution amount in the matrix becomes large, the yield strength in the final plate becomes too high, and the shape freezing property decreases. Further, the upper limit is preferably limited to less than 0.08%, more preferably less than 0.06%.

[Ti of optional component: 0.001 to 0.1%]
In the present invention, it is preferable to contain Ti in the range of 0.001 to 0.1% in order to refine the crystal grains of the cast ingot. In order to exhibit this effect, the Ti content needs to be 0.001% or more. However, if the Ti content exceeds 0.1%, coarse intermetallic compounds such as TiAl ₃ are generated, voids are formed during molding, and moldability is lowered. A more preferable range of the Ti content is 0.001 to 0.05%. Ti may be added as a master alloy such as Al-10% Ti, or as a grain refiner (rod hardener) such as Al-5% Ti-1% B, Al-10% Ti-1% B. It may be added.

[B as an optional component: 0.0005 to 0.01%]
In the present invention, it is preferable to contain B in the range of 0.0005 to 0.01% in order to refine the crystal grains of the cast ingot. The effect of B is to co-exist with Ti and to generate nuclei (TiBx) that are the starting points of αAl crystal grain generation in the molten metal. A more preferable range of the B content is in the range of 0.0005 to 0.005%. B may be added as a master alloy such as Al-5% B, or as a grain refiner (rod hardener) such as Al-5% Ti-1% B, Al-10% Ti-1% B. It may be added.

  Although the manufacturing method of the aluminum alloy plate by this invention is not limited to the method demonstrated below, there exist casting conditions and final annealing conditions, These significance and the reason for limitation are demonstrated below.

[Thin slab casting conditions]
In the double belt casting method, molten metal is poured between rotating belts facing each other up and down, and the molten metal is solidified by cooling from the belt surface to form a slab. Then, it is a continuous casting method that is drawn out and wound up in a coil shape.

  In the present invention, the thickness of the slab to be cast is preferably 5 to 15 mm. When the thickness of the thin slab is less than 5 mm, the amount of aluminum passing through the casting machine per unit time becomes too small and casting becomes difficult. On the contrary, if the thickness exceeds 15 mm, winding with a roll becomes impossible, so the slab thickness range is limited to 5 to 15 mm. With this thickness, the solidification cooling rate at a quarter thickness during slab casting is 20 to 200 ° C./sec, and the maximum equivalent circle diameter of the intermetallic compound can be controlled to 5 μm or less.

[Cold rolling roll surface roughness Ra 0.2-0.7 μm]
Furthermore, the reason for limiting the roughness of the surface of the cold rolling roll to Ra 0.2 to 0.7 μm is to adjust the surface roughness of the final annealed plate. Since the shape of the roll surface is transferred to the rolled plate surface by the cold rolling process, the surface roughness of the final annealed plate is Ra 0.2 to 0.6 μm. If the surface roughness of the final annealed plate is within the range of Ra 0.2 to 0.6 μm, the surface shape of the final plate serves as a micropool that uniformly holds the low-viscosity lubricant used during molding, and press A plate with excellent formability. In addition, it is preferable that the roughness of the surface of a cold rolling roll is Ra0.3-0.7 micrometer, and the surface roughness of the final annealing board is Ra0.3-0.6 micrometer in this case. The roughness of the surface of the cold rolling roll is more preferably Ra 0.4 to 0.7 μm. In this case, the surface roughness of the final annealed plate is Ra 0.4 to 0.6 μm.

[The maximum equivalent circle diameter of intermetallic compounds is 5 μm or less]
About the intermetallic compound in the metal structure in the area | region of thickness 1/4 of the aluminum alloy plate by this invention, it is limited to 5 micrometers or less of the maximum diameter of a circle equivalent diameter. In this way, the dispersion of very fine intermetallic compounds in the matrix suppresses the movement of dislocations during the formation of aluminum plates, increases the tensile strength by solid solution strengthening with Mg, and excels in formability. It becomes a plate.

[Average recrystallized grain size of 15 μm or less]
The average recrystallized grain size in the region of the thickness 1/4 of the final annealed plate is limited to 15 μm or less. If it exceeds this, the level | step difference which arises in a crystal grain boundary at the time of material deformation | transformation will become large too much, the orange peel after a deformation | transformation will become remarkable, and rough skin property will fall.

[Reason for limiting cold rolling rate of 60% to 98%]
The rolling reduction during cold rolling is preferably 60% to 98%, and the dislocations generated by plastic working by rolling are accumulated around these fine crystals, so that the fine re- Necessary for obtaining a crystal structure. When the rolling reduction during cold rolling is less than 60%, the accumulation of dislocations is not sufficient and a fine recrystallized structure cannot be obtained. If the rolling reduction during cold rolling exceeds 98%, the ear cracks during rolling become remarkable and the yield decreases. A more preferable cold rolling rate is in the range of 70% to 96%.

[Final annealing conditions with continuous annealing furnace]
The temperature of the final annealing by a continuous annealing furnace is limited to 400-520 degreeC. When the temperature is lower than 400 ° C., the energy required for recrystallization is insufficient, so that a fine recrystallization structure cannot be obtained. When the holding temperature exceeds 520 ° C., the growth of recrystallized grains becomes remarkable, the average recrystallized grain size exceeds 15 μm, and the moldability and the rough skin property deteriorate.

  The holding time for continuous annealing is preferably within 5 minutes. When the holding time of continuous annealing exceeds 5 min, the growth of recrystallized grains becomes remarkable, the average recrystallized grain size exceeds 15 μm, and the formability and rough skin property are deteriorated.

  The heating rate and cooling rate during the continuous annealing treatment are preferably 100 ° C./min or more for the heating rate. When the rate of temperature increase during the continuous annealing treatment is less than 100 ° C./min, a fine recrystallized structure cannot be obtained, and the formability and the rough skin property are lowered.

[Final annealing conditions in a batch furnace]
The temperature of final annealing by a batch furnace is limited to 300-400 degreeC. When the temperature is lower than 300 ° C., the energy required for recrystallization is insufficient, so that a fine recrystallization structure cannot be obtained. If the holding temperature exceeds 400 ° C., the growth of recrystallization becomes remarkable, the average grain size of the recrystallized grains exceeds 15 μm, and the moldability and the rough skin property deteriorate.

  Although the holding time of the final annealing by a batch furnace is not specifically limited, 1-8 hours are preferable. If it is less than 1 hour, the coil may not be heated uniformly. When holding time exceeds 8 hours, the average particle diameter of a recrystallized grain will exceed 15 micrometers, and a moldability and skin roughening property will fall.

[Strain correction by leveler]
After the final annealing, since the plate is deformed by thermal strain, correction processing such as repeated bending with a leveler roll is performed in a coil or plate state, the shape is corrected, and the flatness is restored. By this distortion correction, it becomes possible to obtain predetermined tensile strength and yield strength, and it is possible to obtain an aluminum alloy plate excellent in formability, rough skin property, and shape freezing property.

  Examples according to the present invention will be described below in comparison with comparative examples. After degassing the molten metal of the chemical composition shown in Table 1 (alloys A, B, C, D, E, F, I), a 10 mm thick thin slab is continuously cast by a twin belt casting machine and directly coiled Rolled up. Similarly, after degassing the molten metal having the chemical composition (alloy G) shown in Table 1, a slab of (width) 1000 mm × (thickness) 500 mm × (length) 4000 mm is cast by DC casting, After chamfering, homogenization was performed at 450 ° C. for 8 hours in a soaking furnace, hot rolling was performed, and the coil was wound as a 6 mm thick hot rolled plate. Similarly, after degassing the molten metal having the chemical composition shown in Table 1 (alloy H), a 6 mm thick thin slab was continuously cast by a twin roll casting machine and directly wound on a coil.

  Next, these thin slabs and hot-rolled plates are cold-rolled with a cold-rolling roll finished to a predetermined surface roughness (Ra 0.6 μm, 1.0 μm) to form a plate having a thickness of 1 mm. The plate was passed through CAL and continuously annealed at a holding temperature of 460 ° C. Further, in order to remove the thermal strain of the final annealed plate, the strain was corrected through a leveler, and then cut to obtain a test material. In addition, Table 2 has shown the manufacturing conditions of the test material in each manufacturing process about an Example and a comparative example.

  Next, the recrystallized grain size of this test material, the maximum equivalent circle diameter of the intermetallic compound, surface roughness, 0.2% proof stress (0.2% YS), tensile strength (UTS), and elongation (EL) The overhang height was measured.

The recrystallized grain size (D) of the test material was measured by a cross cut method. The test material was cut out, embedded and polished in a resin, anodized in an aqueous borofluoric acid solution, and an anodized film was applied to the surface of the cross section of the test material. Take a crystal grain photograph (200x) of the cross section of the test material using a polarizing microscope, draw three lines in the vertical and horizontal directions, count the number (n) of crystal grain boundaries that cross the line, The average value (D) of the particle sizes obtained by dividing the total length (L) by (n-1) was taken as the average recrystallized particle size of the test material. The maximum equivalent circle diameter of the intermetallic compound was measured using an image analyzer (trade name: Luzex).
D = L / (n-1)

  The surface roughness of the test material is measured according to JISB0601 using a surface roughness meter, the measurement direction is the direction perpendicular to the rolling direction, the measurement area is 4 mm, and the cutoff is 0.8 mm. The average roughness Ra was set. The roll surface roughness was measured in accordance with JISB0601, using a surface roughness meter, similarly to the surface roughness of the test material, the measurement direction was the roll lateral direction, the measurement area was 4 mm, and the cut-off was 0.00. The average roughness Ra was set to 8 mm.

The overhang height indicates the limit molding height at break using the following molds.
(Punch: 100mmφ, shoulder R: 50mm, die: 105mmφ, shoulder R: 4mm)

  The skin roughness was evaluated in three stages (O: excellent, Δ: slightly inferior, x: inferior) by visually observing the surface state in the vicinity of the fractured portion of the test piece after the tensile test.

  Table 3 shows the results of Examples and Comparative Examples measured as described above.

  In Examples 1 and 2, since the Mg content is moderate and the Mn content is also suppressed to less than 0.1%, the yield strength is 145 MPa or less, the shape freezing property is excellent, and fine recrystallized grains are used. It has excellent skin roughness, has a fine intermetallic compound, and the surface shows an appropriate surface roughness of 0.35, 0.41 μm, so the overhang height is 29 mm or more and excellent in moldability. Yes.

  On the other hand, in Comparative Example 1, since the Mg content is as high as 3.75%, the 0.2% proof stress is too high and the shape freezing property is lowered. Since Comparative Example 2 has a low Mg content of 2.5%, both the tensile strength and the elongation are insufficient.

  Although the comparative example 3 is moderate Mg content, since Mn content is as high as 0.2%, 0.2% yield strength becomes too high and shape freezing property is falling.

  Since the comparative example 4 is high with Mg content of 4.0% and Mn content of 0.3%, the 0.2% proof stress is too high and the shape freezing property is lowered.

  In Comparative Example 5, since the solidification cooling rate at the time of slab casting by the DC casting method is low, the maximum diameter of the intermetallic compound is too large, the recrystallized grain size is too large, the tensile strength is reduced, and the rough surface is rough. Has fallen.

  In Comparative Example 6, since the solidification cooling rate of the cast and rolled plate by the twin roll method is high, the number of intermetallic compounds that become the core of the recrystallized grains during the final annealing, the so-called pinning effect that hinders the movement of the grain boundaries of the recrystallized grains Insufficient number of intermetallic compounds having, the recrystallized grain size becomes too large, the tensile strength and elongation are insufficient, and the rough skin property is lowered.

  In Comparative Example 7, since the surface roughness of the cold rolling roll is Ra 1.0 μm and the surface roughness of the test material is Ra 0.8 μm, the overhang height is 28 mm and the formability is lowered.

Claims (4)

  Mg: 3.0 to 3.5 mass%, Fe: 0.05 to 0.3 mass%, Si: 0.05 to 0.15 mass%, further Mn: regulated to less than 0.06 mass%, the remainder substantially The maximum equivalent circle diameter of the intermetallic compound in the region of ¼ thickness is 5 μm or less, the average recrystallized grain size is 15 μm or less, and the surface roughness is Ra. : Aluminum alloy for automobiles excellent in press formability, rough surface property and shape freezing property, characterized by being 0.3-0.6 μm, proof stress 145 MPa or less, tensile strength 225 MPa or more, and overhang forming height 29 mm or more Board.   Furthermore, Ti: 0.001-0.1% is contained, The aluminum alloy plate for motor vehicles excellent in press-formability, skin roughening property, and shape freezing property of Claim 1 characterized by the above-mentioned.   Mg: 3.0 to 3.5 mass%, Fe: 0.05 to 0.3 mass%, Si: 0.05 to 0.15 mass%, further Mn: regulated to less than 0.06 mass%, the remainder substantially A thin slab with a thickness of 5 to 15 mm is casted on a coil by casting a molten metal composed of inevitable impurities and Al with a twin-belt casting machine so that the cooling rate at a quarter thickness is 20 to 200 ° C./sec. After that, roll rolling with a roll surface roughness Ra of 0.3 to 0.7 μm was subjected to cold rolling with a cold rolling rate of 60 to 98%, and finally subjected to final annealing at a holding temperature of 400 to 520 ° C. with CAL. Then, the manufacturing method of the aluminum alloy plate for motor vehicles excellent in press-formability, rough skin property, and shape freezing property characterized by carrying out distortion correction with a leveler. 4. The method for producing an aluminum alloy plate for automobiles having excellent press formability, rough surface property and shape freezing property according to claim 3, wherein the final annealing is performed at a holding temperature of 300 to 400 [deg.] C. by a batch annealing furnace.