Process for producing an aluminum alloy sheet for motor vehicle

Abstract

An aluminum alloy sheet for motor vehicles is produced by casting a melt, containing 3.0-3.5 mass % Mg, 0.05-0.3 mass % Fe, 0.05-0.15 mass % Si, and less than 0.1 mass % Mn, a balance substantially being inevitable impurities and Al, into a slab having a thickness of 5 to 15 mm in a twin-belt caster so that cooling rate at depth of thickness of the slab is 20 to 200 C./sec; winding the cast thin slab into a coiled thin slab subjected to cold rolling with a roll having a surface roughness of 0.2 to 0.7 m Ra at a cold rolling reduction of 50 to 98%; subjecting the cold rolled sheet to final annealing either continuously in a CAL at a holding temperature of 400 to 520 C. within 5 minutes or in a batch annealing furnace at a holding temperature of 300 to 400 C. for 1 to 8 hours; and subjecting the resulting sheet to straightening with a leveler.

Claims

1. A process for producing a sheet of an aluminum alloy for motor vehicles, the process comprising: casting a melt of the aluminum alloy comprising 3.0-3.35 mass % Mg, 0.05-0.3 mass % Fe, 0.05-0.15 mass % Si, 0.001-0.1% mass % Ti, 0.0005-0.01% B, further a limited amount of less than 0.08 mass % Mn, and a balance substantially being inevitable impurities and Al, into a thin slab having a thickness of 5 to 15 mm in a twin-belt caster so that the cooling rate at depth of a thickness of the thin slab is 20 to 200 degree Celsius/sec; winding the cast thin slab into a coiled thin slab; subjecting the coiled thin slab to cold rolling with a roll having a surface roughness of 0.2 to 0.7 micro-meters Ra at a cold rolling reduction of 50 to 98% to produce a cold rolled thin sheet; subjecting the cold rolled thin sheet to final annealing continuously in a CAL at a holding temperature of 400 to 520 degree Celsius within 5 minutes; and subjecting the resulting sheet to straightening with a leveler, producing the aluminum alloy sheet having an intermetallic compound maximum size of 5 micro-meters or less by circle-equivalent diameter in a region at depth of the sheet thickness, an average recrystallized grain size of 11-15 micro-meters, a surface roughness of 0.2 to 0.6 micro-meters Ra, and a yield strength of 134-145 MPa.

2. The process for producing an aluminum alloy sheet for motor vehicles, according to claim 1, wherein the amount of Mn is limited to less than 0.06 mass %.

3. A process for producing a sheet of an aluminum alloy for motor vehicles, the process comprising: casting a melt of the aluminum alloy comprising 3.0-3.35 mass % Mg, 0.05-0.3 mass % Fe, 0.05-0.15 mass % Si, 0.001-0.1% mass % Ti, 0.0005-0.01% B, further a limited amount of less than 0.08 mass % Mn, and a balance substantially being inevitable impurities and Al, into a thin slab having a thickness of 5 to 15 mm in a twin-belt caster so that the cooling rate at depth of a thickness of the thin slab is 20 to 200 degree Celsius/sec; winding the cast thin slab into a coiled thin slab; subjecting the coiled thin slab to cold rolling with a roll having a surface roughness of 0.2 to 0.7 micro-meter Ra at a cold rolling reduction of 50 to 98% to produce a cold rolled thin sheet; subjecting the cold rolled thin sheet to final annealing in a batch annealing furnace at a holding temperature of 300 to 400 degree Celsius for 1 to 8 hours; and subjecting the resulting sheet to straightening with a leveler, producing the aluminum alloy sheet having an intermetallic compound maximum size of 5 micro-meters or less by circle-equivalent diameter in a region at depth of the sheet thickness, an average recrystallized grain size of 11-15 micro-meters, a surface roughness of 0.2 to 0.6 micro-meter Ra, and a yield strength of 134-145 MPa.

4. The process for producing an aluminum alloy sheet for motor vehicles, according to claim 3, wherein the amount of Mn is limited to less than 0.06 mass %.

Description

EXAMPLES

(1) Hereinafter, Examples according to the present invention will be described as compared with Comparative Examples. A melt each having a chemical composition shown in Table 1 (alloy A, B, C, D, E, F, I) was degassed and settled, and the resulting melt was then fed to a twin-belt caster to continuously cast a thin slab having a thickness of 10 mm, which was directly wound into a coil. Similarly, a melt having the chemical composition shown in Table 1 (alloy G) was degassed and settled, and the resulting melt was then subjected to DC casting process to cast a slab of 1000 mm (width)500 mm (thickness)4000 mm (length). The slab was subjected to face milling of both surfaces thereof and then subjected to homogenization of 450 C.8 hours in a soaking furnace followed by hot rolling to produce a hot-rolled sheet having a thickness of 6 mm, which was wound into a coil. Similarly, a melt having the chemical composition shown in Table 1 (alloy H) was degassed and settled, and the resulting melt was then fed to a twin-belt caster to continuously cast a thin slab having a thickness of 6 mm, which was directly wound into a coil.

(2) TABLE-US-00001 TABLE 1 Chemical composition of alloy Composition (mass %) Alloy symbol Mg Mn Fe Si A 3.35 0.00 0.2 0.08 B 3.25 0.06 0.2 0.08 C 3.75 0.05 0.2 0.08 D 2.50 0.07 0.2 0.08 E 3.45 0.20 0.2 0.08 F 4.00 0.30 0.2 0.08 G 3.35 0.00 0.2 0.08 H 3.35 0.00 0.2 0.08 I 3.25 0.06 0.2 0.08

(3) Next, these thin slabs and the hot rolled sheets were cold rolled with cold rolling rolls which were finished to a predetermined surface roughness (0.6 m, 1.0 m Ra) to form sheets having a thickness of 1 mm. Then, these sheets were passed through a CAL to undergo continuous annealing at a holding temperature of 460 C. Further, the finally annealed sheets were passed through a leveler to undergo straightening to remove thermal strain therefrom followed by cutting to obtain test specimens. Note that Table 2 shows production conditions of the test specimens in each production step in Examples and Comparative Examples.

(4) TABLE-US-00002 TABLE 2 Production conditions Cold rolling Cooling roll surface Alloy Casting process/ rate roughness Thickness Annealing symbol thickness (mm) ( C./s) Hot rolling Ra (m) (mm) temperature Example 1 A Twin belt/10 100 None 0.6 1.0 460 C. Example 2 B Twin belt/10 78 None 0.6 1.0 460 C. Comparative C Twin belt/10 85 None 0.6 1.0 460 C. Example 1 Comparative D Twin belt/10 75 None 0.6 1.0 460 C. Example 2 Comparative E Twin belt/10 76 None 0.6 1.0 460 C. Example 3 Comparative F Twin belt/10 74 None 0.6 1.0 460 C. Example 4 Comparative G DC/500 3 6 mm 0.6 1.0 460 C. Example 5 Comparative H Twin roll/6 300 None 0.6 1.0 460 C. Example 6 Comparative I Twin belt/10 78 None 1.0 1.0 460 C. Example 7

(5) Next, these test specimens were evaluated for the recrystallized grain size, the intermetallic compound maximum size by circle-equivalent diameter, surface roughness, 0.2% yield strength (0.2% YS), tensile strength (UTS), elongation (EL), and punch stretch height.

(6) The recrystallized grain size (D) of a test specimen was measured by an cross-cut method. The test specimen was cut, embedded in a resin, polished, and subjected to anodic coating in an aqueous fluoroboric acid solution to apply an anodic oxide film to the surface of the section of the test specimen. A photograph (200 times) of grains in the section of the test specimen was taken with a polarizing microscope. On the photograph, three lines were drawn both in the vertical direction and in the horizontal direction. The number (n) of crystal grain boundaries crossing these lines was counted. The average value (D) of the grain sizes determined by dividing the total length (L) of the lines by (n1) was defined as the average recrystallized grain size of the test specimen. The intermetallic compound maximum size by circle-equivalent diameter was measured with an image analyzer (trade name: LUZEX).

(7) D=L/(n1)

(8) The surface roughness of the test specimen was measured using a surface roughness meter according to JIS B0601, wherein the direction of measurement was perpendicular to the rolling direction; the measurement region was 4 mm; and the cutoff was 0.8 mm. The resulting surface roughness was defined as the average roughness Ra. Note that surface roughness of the roll was measured in the same manner as in the measurement of the surface roughness of the test specimen using a surface roughness meter according to JIS B0601, wherein the direction of measurement was in the transverse direction of the roll; the measurement region was 4 mm; and the cutoff was 0.8 mm. The resulting surface roughness was defined as the average roughness Ra.

(9) The punch stretch height was measured using the following die assembly and indicates the critical forming height at break. (Punch: 100 mm in diameter, shoulder R: 50 mm, die: 105 mm in diameter, shoulder R: 4 mm)

(10) The resistance to surface roughening was evaluated at three stages (: excellent, : a little poor, X: poor) by visually observing the surface condition near the broken part of the test piece after the tensile test.

(11) The results of Examples and Comparative Examples measured as described above are shown in Table 3.

(12) TABLE-US-00003 TABLE 3 Evaluation results of properties Punch Evaluation of Average size of Maximum size Surface stretch resistance recrystallized of crystallized roughness YS UTS height to surface grains (m) products (m) Ra (m) (Mpa) (Mpa) EL (%) (mm) roughening Example 1 12 3.7 0.35 133 234 28 30 Example 2 11 3.5 0.41 134 233 27 30 Comparative 10 4 0.37 146 248 27 29 Example 1 Comparative 11 4.2 0.42 121 209 26 30 Example 2 Comparative 9 4.1 0.39 148 244 27 29 Example 3 Comparative 8 4.5 0.38 155 265 27 28 Example 4 Comparative 25 15 0.45 120 224 28 27 Example 5 Comparative 54 2 0.35 115 222 26 26 X Example 6 Comparative 13 3.7 0.80 132 232 28 28 Example 7

(13) In Examples 1 and 2, the Mg content is proper, and in addition, the Mn content is suppressed to less than 0.1%. As a result, the test specimens in Examples 1 and 2 are excellent in shape fixability since they have a yield strength of 145 MPa or less; they are excellent in resistance to surface roughening since they have fine recrystallized grains; and they are excellent in formability to an extent of a punch stretch height of 29 mm or more since they have fine intermetallic compounds and have a proper surface roughness of 0.35 and 0.41 m, respectively.

(14) On the other hand, in Comparative Example 1, since the Mg content is as high as 3.75%, the 0.2% yield strength is excessively increased to result in reduction of shape fixability. In Comparative Example 2, since the Mg content is as low as 2.5%, both the tensile strength and elongation are insufficient.

(15) In Comparative Example 3, the Mg content is proper, but the Mn content is as high as 0.2%. As a result, the 0.2% yield strength is excessively increased to result in reduction of shape fixability.

(16) In Comparative Example 4, since the Mg content and the Mn content are as high as 4.0% and 0.3%, respectively, the 0.2% yield strength is excessively increased to result in reduction of shape fixability.

(17) In Comparative Example 5, since the solidification cooling rate during the slab casting by a DC casting process is low, the maximum size of the intermetallic compounds is excessively large, and the recrystallized grain size is also excessively large. As a result, the tensile strength is reduced, and the resistance to surface roughening and punch stretch formability are also reduced.

(18) In Comparative Example 6, since the solidification cooling rate of the cast rolled sheet by a twin-roll process is high, the number of the intermetallic compounds which serve as the nuclei of recrystallized grains during the final annealing is insufficient, and the number of the intermetallic compounds having so-called pinning effect that prevents the motion of the grain boundaries of recrystallized grains is also insufficient, thereby excessively increasing the size of the recrystallized grains. As a result, the tensile strength and elongation are insufficient, and the resistance to surface roughening and punch stretch formability are reduced.

(19) In Comparative Examples 7, the cold rolling roll has a surface roughness of 1.0 m Ra, and the test specimen has a surface roughness of 0.8 m Ra. As a result, the punch stretch height is 28 mm, indicating a reduced formability.