JP3063020B2 - Aluminum alloy plate excellent in strength and deep drawability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy sheet suitable for forming a sheet of a molded article requiring strength and formability, such as an automobile body panel, an air cleaner, and an oil tank, and a method for producing the same. It is.

[0002]

2. Description of the Related Art In general, cold-rolled steel sheets have been frequently used as sheet materials for forming body panels for automobiles. However, recently, in order to reduce the weight of automobile bodies and improve fuel efficiency, they are replaced with cold-rolled steel sheets. There is an increasing demand for using aluminum alloy plates. Conventionally, as an aluminum alloy sheet for forming requiring strength and formability, Al-Mg based 5052 alloy (Al-2.5wt% Mg-0.25wt% Cr alloy) O material and 5
182 alloy (Al-4.5wt% Mg-0.35wt% Mn alloy)
O material or Al-Cu 2036 alloy (Al-2.6wt
% Cu-0.25 wt% Mn-0.45 wt% Mg) T4 material and the like are known. Of these, the Al-Mg alloy sheet is superior in both deep drawing formability and strength compared to other alloy sheets,
It is often used for deep drawing products such as inner members.

[0003] Al-Mg-based alloy sheets for forming are usually manufactured in the following steps: production of ingots for rolling → homogenization → hot rolling → cold rolling → final annealing. In addition, if necessary, intermediate annealing is performed during the cold rolling. When flatness of the sheet material is particularly required, straightening correction may be performed after annealing by means such as a tension leveler, a roller leveler, and a skin pass rolling.

[0004] The conventional Al-Mg-based alloy produced as described above has excellent formability as compared with other aluminum alloys, but is inferior as compared with cold-rolled steel sheets. For this reason, there is a problem that cracks are more likely to occur during press forming than with cold-rolled steel sheets. Further, since the strength is inferior to that of the cold-rolled steel sheet, it is difficult to reduce the wall thickness, and there is a problem that the effect of reducing the weight of products such as a vehicle body cannot always be sufficiently achieved.

[0005] By the way, it is already known that the elongation of an Al-Mg based alloy sheet increases in proportion to the Mg content. Therefore, in order to improve elongation, a conventional Al-Mg alloy (Mg
Aluminum alloys with a higher Mg content than 2.5-5 wt%) have been studied. For example, JP-A-4-14793
In No. 6, Mg 4-8 wt%, Cu 0.05-0.7 wt%, Mn
An aluminum alloy plate containing 0.01 to 0.3 wt% and 0.002 to 0.01 wt% Be and having a crystal grain size of 30 to 100 μm has been proposed. Such a high content of Mg
Since the Al-high Mg alloy sheet has a large elongation, points such as stretch formability, bend formability, and flange formability, which are highly correlated with elongation, are improved.

[0006]

However, as described above, the conventional Al-high Mg alloy plate having a high Mg content has the following problems. That is, deep drawability is inferior to cold-rolled steel sheets, and in particular, in press forming under conditions where lubrication is not very good, such as in molding of automotive parts, cracks are likely to occur at the time of forming and productivity is low. bad. The strength is higher than that of other aluminum alloy sheets due to the high Mg content, but is still inferior to that of the cold-rolled steel sheet, so that it is difficult to reduce the thickness.

As a result of detailed examination of the above-mentioned problems of the conventional Al-high Mg alloy sheet, the inventors have found that the deep drawability of an aluminum alloy sheet is better as the material strength is higher. And, it has been found that an alloy plate obtained by appropriately dispersing a Cr-based intermetallic compound in an Al-Mg-based alloy and recrystallizing finely has extremely high strength and excellent deep drawing formability, and completed the present invention. It is a thing.

An object of the present invention is to improve the metal structure of an Al-high Mg alloy sheet and to provide an aluminum alloy sheet having strength and deep drawability almost comparable to those of a cold rolled steel sheet. Another object of the present invention is to provide a method for producing an aluminum alloy having strength and deep drawability as described above.

[0009]

That is, one aluminum alloy plate according to the present invention has a Mg content of 5 to 10% by weight and a Be0.
0001-0.01 wt%, Cr 0.01-0.05 wt%,
And 0.005 to 0.1 wt% of Ti or 0.005 to 0.
1 wt% and B 0.00001-0.05 wt%, Fe and Si as impurities are each regulated to less than 0.2 wt%,
The remainder is an aluminum alloy plate composed of Al and other unavoidable impurities, and its metal structure is such that a Cr-based intermetallic compound having an average diameter of 0.2 μm or less is dispersed by 0.1 to 0.5 vol%, and The particle size is 9 to 25 μm. Another one of the aluminum alloy sheets according to the present invention is characterized in that the first aluminum alloy sheet contains 0.05 to 1.0 wt% of Cu in addition to the above-mentioned compositional elements.

The method for manufacturing an aluminum alloy sheet according to the present invention is characterized in that an aluminum alloy ingot having the same composition as the aluminum alloy sheet according to the above-mentioned invention is formed at 450 to 540 ° C.
After homogenizing under the condition of 4 hours or less, hot rolling is performed. Immediately after the hot rolling, or in the middle of the cold rolling following the hot rolling, at a temperature of 230 to 330 ° C. for 1 to 100 hours, the Cr-based Performing at least one precipitation treatment of an intermetallic compound;
After the final cold rolling to a predetermined thickness, 400-500
The heat treatment is performed at a temperature of 120 ° C. for 120 seconds or less.

[0011]

The reasons for selecting the elements other than aluminum in the composition of the aluminum alloy sheet according to the present invention and the reasons for limiting the contents thereof will be described. Mg is added to improve the strength and deep drawability of the aluminum alloy sheet to be manufactured. If the Mg content is less than 5% by weight, the effect is insufficient. On the other hand, if the Mg content exceeds 10% by weight, the hot-rolling property of the alloy rapidly decreases, and it is difficult to manufacture an alloy sheet. become.

[0012] Be is added to prevent casting cracks and oxidation of the molten metal during melting and casting, and to prevent loss of Mg due to oxidation of the ingot during homogenization. Be content 0.0001wt
%, The effect is insufficient, and the content is 0.1%.
If it exceeds 01 wt%, toxicity becomes a problem.

[0013] Cr is added to improve the strength and deep drawability of the alloy sheet without reducing the elongation of the alloy sheet. Cr is converted into a Cr-based intermetallic compound (AL ₇ Cr or Al ₁₈ Mg ₃ Cr ₂ ) in the metal structure of the alloy sheet by a precipitation process described below, with an average diameter of 0.2 μm or less, in the range of 0.1 to 0.5 vol%. When it is dispersed by the method, the crystal grains of the alloy sheet are refined to improve the strength and the deep drawability. When the average diameter of the Cr-based intermetallic compound exceeds 0.2 μm or the amount of dispersion is less than 0.1 vol%, the effect is small, and the amount of dispersion exceeds 0.5 vol%. And the elongation of the alloy plate decreases. If the addition amount of Cr is less than 0.01 wt%, the dispersion amount of the Cr-based intermetallic compound cannot be increased to 0.1 vol% or more. Conversely, if the addition amount of Cr exceeds 0.05 wt%, the Cr-based The dispersion amount of the intermetallic compound exceeds 0.5 vol%.

[0014] Ti, or Ti and B, are added to improve the hot rollability and reduce the variation in strength and formability after final annealing by making the ingot structure of the alloy uniform and fine. Is done. If the Ti content is less than 0.005 wt%, the effect is small. If the Ti content exceeds 0.1 wt%, a coarse intermetallic compound is formed, and the elongation of the alloy plate is reduced. On the other hand, B
Since the coexistence with Ti further enhances the refining effect of the alloy ingot structure, it is desirable to add 0.00001 to 0.05 wt%. If the B content is less than 0.00001 wt%, the effect is small. If the B content exceeds 0.05 wt%, coarse TiB ₂ particles are formed, and the elongation of the alloy plate is reduced.

[0015] Fe and Si are impurities in this alloy, and each is restricted to less than 0.2 wt%. If the content of each of them exceeds 0.2 wt%, Fe and Si form a coarse intermetallic compound, so that the elongation of the alloy plate decreases. In addition, the hot rollability of the alloy is also reduced (cracking occurs).

Cu is added in the range of 0.05 to 1.0 wt% in order to further improve the deep drawability and strength of the alloy sheet. If the Cu content is less than 0.05 wt%, the above effect is insufficient, and if the Cu content exceeds 1.0 wt%, the hot rollability of the alloy is sharply reduced. If Mn, Zr, and V are added in amounts of 0.2 wt% or less, the strength can be slightly improved without significantly reducing the elongation of the alloy sheet. Note that there is no particular problem in achieving the effects of the present invention as long as the content of Zn and other unavoidable impurities is 0.3 wt% or less in total.

Next, in the manufacturing method according to the present invention,
The reason why the manufacturing conditions are selected as described above will be described. First, the aluminum alloy ingot having the above-described composition is subjected to a homogenization treatment at 450 to 540 ° C. for 24 hours or less. This homogenization treatment is performed to make the solute atom distribution of the alloy ingot uniform, to make the structure after annealing uniform, and to improve the strength and elongation of the alloy sheet. If the temperature of the homogenization treatment is less than 450 ° C, the effect is insufficient. If the treatment temperature exceeds 540 ° C or the treatment time exceeds 24 hours, the loss of Mg due to oxidation becomes remarkable, Hot rolling cracks easily occur.

The aluminum alloy ingot after the homogenization treatment as described above is then hot-rolled. In hot rolling, it is desirable to reduce the rolling reduction of at least the first three rolling passes (preferably 3% or less) in order to prevent hot rolling cracks. Further, in order to prevent hot rolling cracks, the crystal grain size of the alloy ingot after the homogenization treatment is set to 1000
μm or less, and the hot rolling start temperature is desirably 320 to 470 ° C.

Immediately after the above-mentioned hot rolling or during the cold rolling following the hot rolling, 230 to 330
The precipitation treatment of the Cr-based intermetallic compound is performed one or more times at 1 ° C. for 1 to 100 hours. By the precipitation treatment under these conditions, the structure of the alloy plate contains 0.1 to 0.5 vol of a Cr-based intermetallic compound (Al ₇ Cr or Al ₁₈ Mg ₃ Cr ₂ ) having an average diameter of 0.2 μm or less.
% Is dispersed and precipitated. Cr in dispersed state as described above
The intermetallic compound suppresses grain boundary movement of recrystallized grains in the final annealing of the alloy plate and suppresses grain growth, thereby making crystal grains after the final annealing finer. Therefore, the strength and deep drawability of the alloy plate can be improved. In the above-mentioned deposition treatment, the treatment temperature is 230
C. or less than 1 hour, the above effect is insufficient, and when the processing temperature exceeds 330 ° C.,
When the intermetallic compound is coarsened, the effect of refining the crystal grains in the final annealing is lost, and the strength and the deep drawability of the alloy sheet are reduced.

The alloy plate after the treatment as described above includes:
For example, 400 to 500 using a continuous annealing furnace (CAL) or the like.
High-temperature, short-time annealing is performed at a temperature of 120 ° C. for 120 ° C. or less, so that the average grain size of the metal structure is 9 to 25 μm. As described above, the alloy plate manufactured as described above has higher strength and deep drawability as the crystal grains are finer. However, when the average crystal grain size is less than 9 μm, the elongation is significantly reduced. And the deep drawability also decreases. If the average crystal grain size of the alloy plate structure exceeds 25 μm, the strength of the alloy plate decreases, and the deep drawability also decreases.

By controlling the average crystal grain size of the metal structure of the aluminum alloy sheet to be in the range of 9 to 25 μm, the alloy sheet has improved strength and deep drawability as described above, and also has the following characteristics. Also occurs. That is, it is possible to prevent the occurrence of the Lüdersline (strain pattern on the surface) during the deep drawing of the alloy sheet. In addition, the work brittleness is greatly improved in a wide range of temperature environments (for example, -100 ° C. to room temperature). As a result, the material does not become brittle and crack even when pressed in a low temperature environment. Does not become brittle during use in a low-temperature environment and is not broken by a weak impact.

When the above-mentioned high temperature and short time annealing temperature is lower than 400 ° C., recrystallization is insufficient, or even if recrystallization is performed, the temperature becomes less than 9 μm. On the other hand, when the annealing temperature exceeds 500 ° C., the average crystal grain size exceeds 25 μm, and in any case, the deep drawing formability of the alloy sheet is reduced. According to the high-temperature short-time annealing under the aforementioned conditions, the alloy plate structure before and after the annealing
Since the distribution state of the Cr-based intermetallic compound does not change, the distribution state of the Cr-based intermetallic compound before annealing is preserved as it is. Further, according to the above-mentioned annealing conditions, the recrystallized grains are equiaxed grains, so that the crystal grain size is measured equally when observed from the plate surface or from the plate cross section. When the above-described annealing is performed in a stationary batch furnace, the crystal grain size is temporarily 9 to 25 μm.
Even if it is, anisotropy occurs in the strength, and the alloy plate tends to have low elongation and low formability.

The alloy sheet subjected to the final annealing as described above is
Tension leveler, roller leveler, as required
Straightening can be performed by means such as skin pass rolling. Further, the surface may be washed with an acid or an alkali as needed. The aluminum alloy plate of the present invention manufactured as described above has excellent strength and deep drawability as compared with other aluminum alloy plates, and is suitable for forming automotive body panels, air cleaners, oil tanks, and the like. Further, the occurrence of the Luders line during the deep drawing is suppressed. Further, the aluminum alloy sheet of the present invention exhibits excellent work brittleness resistance in a wide range of temperature environments (for example, -100 ° C to normal temperature).

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The aluminum alloy plate and the method for manufacturing the same according to the present invention will be described in more detail with reference to the following examples. Example 1 Alloy sample No. 1 shown in Table 1 was used. 1 to No. An aluminum alloy having a composition of 16 was DC cast (40 mm thick) by a conventional method.
(0 mm, width 1650 mm, length 4500 mm). The alloy ingot was homogenized at 490 ° C. for 3 hours, and then hot-rolled to a thickness of 5 mm under the following conditions. Rolling conditions Rolling start temperature = 460 ° C Rolling rate of each of the first three rolling passes = 2% Rolling rate of rolling passes after 4 times = Gradual increase in the range of 3 to 45% Total number of passes = 28 times Table 1 Alloy sample No. The alloys Nos. 1 to 5 have compositions corresponding to claim 1 of the aluminum alloy sheet according to the present invention and claim 3 of the method for producing the same. The alloys of Nos. 6 to 8 are aluminum alloy sheets according to the present invention.
And a composition corresponding to claim 4 of the method for producing the same.
o. Alloys 9 to 16 are comparative examples having compositions outside the scope of the present invention. In each alloy sample of Table 1, Cu having a content of less than 0.05 wt% is an impurity.

The alloy sheet hot-rolled as described above is then cold-rolled to a thickness of 2 mm, precipitated at 300 ° C. for 8 hours, and finally cold-rolled to a thickness of 1 mm. Recrystallize by heating at 480 ° C for 20 seconds in an annealing furnace,
O material was manufactured.

The cross section of the alloy plate thus manufactured was observed (photographed) at a magnification of 100 times with an optical microscope, and the average crystal grain size of the metal structure was measured by a cross-cut method.
In addition to measuring the tensile strength, proof stress, and elongation of the alloy plate by a tensile test, a deep drawing test was performed using a deep drawing tester under the following conditions, and the critical drawing height was measured to determine the deep drawability. evaluated. Deep drawing conditions Blank dimensions and shape: 100 mm □ (100 × 100
mm) Punch size and shape: 50mm □ (50 × 50mm) Die size and shape: 52mm □ (52 × 52mm) Punch shoulder radius: 5mm Die shoulder radius: 3mm Wrinkle holding force: 2500kg Further, nitric acid-methanol mixed solution (1: 2 volume ratio)
To prepare a thin film sample (thickness of 2800 to 3500 °) of the alloy plate after the final annealing by a jet polishing method,
This thin film sample was subjected to an accelerating voltage of 200 using a transmission electron microscope.
Observation was performed at a KV of 40,000 times, and the photograph (30 visual fields) was analyzed by an image analyzer to calculate the average diameter and the amount of dispersion of the Cr-based intermetallic compound. Further, it was confirmed by the above-described analysis method that the dispersion state of the Cr-based intermetallic compound in the alloy sheet after the completion of the above-described precipitation treatment and after the final annealing was the same. The results of these measurements, observations and calculations are shown in Table 2.

[0027]

[Table 1]

[0028]

[Table 2]

As is clear from the results shown in Table 2, the plates made of the alloys of Samples No. 1 to No. 8 having the composition of the present invention have high strength and excellent deep drawability. In contrast, Mg
The plate made of the alloy of Sample No. 9 having a small content and a slightly large content of Fe and Si is inferior in both deep drawability and elongation. The alloy of sample No. 10 having a low content of Ti and B and a high content of Fe had large crystal grains after casting, so that hot rolling cracks occurred and production was impossible. The plate made of the alloy of sample No. 11 having a small Cr content has a low strength and a low deep drawability because the dispersion amount of the Cr-based intermetallic compound is small and the crystal grains after the final annealing are large. The plate made of the alloy of Sample No. 12 having a high Cr content has low elongation and poor deep drawability. Sample N with high Cu content
o. Sample No. 14 with 13 alloys and high Mg content
The alloy of No. suffered from hot rolling cracks and could not be manufactured. Be
The alloy of Sample No. 15 having a small content cracked at the time of casting and could not be manufactured. The plate made of the alloy of Sample No. 16 having a high Ti content has low elongation and poor deep drawability.

Example 2 A hot-rolled sheet of the alloy of sample No. 4 in Table 1 (sheet thickness 5 mm)
For case No. in Table 3. As shown in 17 to 29, cold rolling, precipitation treatment, cold rolling,
Annealing was sequentially performed to produce an aluminum alloy sheet having a thickness of 1 mm. The average crystal grain size of the aluminum alloy plate was measured, and the tensile strength, proof stress, and elongation were measured by a tensile test. Further, a deep drawing test was performed under the same conditions as in Example 1 to determine the critical drawing height. The deep drawability was evaluated by measurement. The results are shown in Table 4. The case No. in Table 3 The manufacturing conditions of Nos. 17 to 21 are the manufacturing conditions within the range of the manufacturing method according to the present invention, and the case Nos. Manufacturing conditions 22 to 29 are manufacturing conditions other than the manufacturing method according to the present invention.

[0031]

[Table 3]

[0032]

[Table 4]

As is clear from Tables 3 and 4, cases No. 17 to No. 21 depending on the manufacturing conditions of the manufacturing method of the present invention.
Is excellent in both elongation and strength and good in deep drawability. On the other hand, in case no. Nos. 22 and 26 and the temperature of the precipitation treatment were lower than the conditions of the present invention, or the time of the precipitation treatment was shorter than the conditions of the present invention. 23, 2
In each alloy plate of No. 4, the amount of dispersion of the Cr-based intermetallic compound in the metal structure is small. Case No. in which the temperature of the precipitation treatment was higher than the condition of the present invention. In each of the alloy plates 25 and 27, the Cr-based intermetallic compound of the metal structure is coarse and the amount of dispersion is too large. As a result, the average crystal grain size of each of these alloy sheets after annealing exceeded 25 μm, and both the strength and the deep drawability were Case No. 17-N
o. 21 is inferior to each alloy plate. Also, the alloy plates of Cases 28 and 29 whose final annealing temperature is higher than the conditions of the present invention or whose annealing time is longer than the conditions of the present invention also have an average crystal grain size after annealing exceeding 25 μm, The strength and deep drawability were the case No. It is inferior to each alloy plate of Nos. 17-21.

Case No. 19 and Case No. Transmission electron micrographs of a thin film sample (0.28 μm thickness) of each of the alloy plates No. 22 are shown in FIGS. 1 and 2. FIG. 1 is a transmission electron microscope image of the metal structure of the alloy plate of Case No. 19 of the embodiment of the present invention after final annealing, in which particles of the Cr-based intermetallic compound having an average diameter of 0.08 μm are dispersed by 0.23 vol%. ing. On the other hand, in case No. In the alloy plate No. 22, the average diameter of the Cr-based intermetallic compound in the structure was 0.11 μm.
m, and the amount of dispersion is 0.06 vol%.

[0035]

The aluminum alloy plate according to the present invention has the following features.
In addition to being excellent in strength and deep drawability almost comparable to those of cold rolled steel sheets, Lüdersline hardly occurs during deep drawing. Further, the alloy sheet and the molded part thereof according to the present invention are excellent in resistance to work brittleness. According to the manufacturing method of the present invention, an aluminum alloy plate having the above-described characteristics can be manufactured industrially.

[Brief description of the drawings]

FIG. 1 is an enlarged photograph of a metal structure of an aluminum alloy plate according to an embodiment of the present invention.

FIG. 2 is a diagram showing an enlarged photograph of a metal structure of an aluminum alloy plate as a comparative example.

────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI C22F 1/00 630 C22F 1/00 630A 630K 686 686B 691 691B 691C (72) Inventor Yoichiro Toji 2-6, Marunouchi, Chiyoda-ku, Tokyo No. 1 Inside Furukawa Electric Co., Ltd. (56) References JP-A-3-287739 (JP, A) JP-A-3-107439 (JP, A) JP-A-4-318145 (JP, A) JP-A-5 −345962 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 21/00-21/18 C22F 1/04-1/057

Claims (4)

(57) [Claims] 1. Mg 5-10 wt%, Be 0.0001-0.
01 wt%, Cr 0.01-0.05 wt%, and Ti0.00
5 to 0.1 wt% or 0.005 to 0.1 wt% of Ti and B0.
0001-0.05wt%, and F as an impurity
e and Si are each regulated to less than 0.2 wt%, and the balance is an aluminum alloy plate composed of Al and other unavoidable impurities, and the metal structure of the aluminum alloy plate has an average diameter of 0.2 μm.
An aluminum alloy plate excellent in strength and deep drawability, characterized in that 0.1 to 0.5 vol% of a Cr-based intermetallic compound having a particle size of m or less is dispersed and the average crystal grain size is 9 to 25 μm. 2. Mg 5-10 wt%, Be 0.0001-0.
01wt%, Cr0.01 ~ 0.05wt%, Cu0.05 ~
1.0 wt%, and Ti 0.005 to 0.1 wt% or Ti0.
005-0.1wt% and B0.00001-0.05wt%
Fe and Si as impurities are regulated to less than 0.2 wt% each, and the balance is an aluminum alloy plate composed of other unavoidable impurities and Al, and the metal structure of the aluminum alloy plate has an average diameter. 0.2μ
An aluminum alloy plate excellent in strength and deep drawability, characterized in that 0.1 to 0.5 vol% of a Cr-based intermetallic compound having a particle size of m or less is dispersed and the average crystal grain size is 9 to 25 μm. 3. Mg 5-10 wt%, Be 0.0001-0.
01 wt%, Cr 0.01-0.05 wt%, and Ti0.00
5 to 0.1 wt% or 0.005 to 0.1 wt% of Ti and B0.
0001-0.05wt%, and F as an impurity
e and Si are regulated to less than 0.2 wt% each, and the remainder is aluminum alloy ingot consisting of Al and other unavoidable impurities, after homogenizing at 450-540 ° C for 24 hours or less. Immediately after hot rolling, or in the course of cold rolling subsequent to the hot rolling, a precipitation treatment of a Cr-based intermetallic compound is performed at least once at 230 to 330 ° C. for 1 to 100 hours. After final cold rolling to a thickness of 400 to 500
A method for producing an aluminum alloy sheet having excellent strength and deep drawability, wherein the heat treatment is performed at a temperature of 120 ° C. for 120 seconds or less. 4. Mg 5-10 wt%, Be 0.0001-0.
01wt%, Cr0.01 ~ 0.05wt%, Cu0.05 ~
1.0 wt%, and Ti 0.005 to 0.1 wt% or Ti0.
005-0.1wt% and B0.00001-0.05wt%
Fe and Si as impurities are regulated to less than 0.2 wt% each, and the balance is made of an aluminum alloy ingot consisting of Al and other unavoidable impurities at 450-540 ° C for 24 hours or less. Hot rolling, immediately after the hot rolling, or during the cold rolling following the hot rolling, precipitation treatment of the Cr-based intermetallic compound at 230 to 330 ° C. for 1 to 100 hours. At least once, and after final cold rolling to a predetermined thickness, 400 to 500
A method for producing an aluminum alloy sheet having excellent strength and deep drawability, wherein the heat treatment is performed at a temperature of 120 ° C. for 120 seconds or less.