US3392062A - Process of producing heat-treatable strips and sheets from heat-treatable aluminum alloys with a copper content of less than 1% - Google Patents

Description

United States Patent ABSTRACT OF THE DISCLOSURE In a process of producing thin material from an initially-rolled member of aluminum alloy with a copper content of less than 1% which material can reach its maximum strength by subsequent simple aging, the immediate and rapid quenching of a hot rolled aluminum alloy from a temperature of 350 C., preferably 400 C., or more down to a maximum of 200 C. in less than two minutes, preferably less than one minute.

The present invention refers to the production of strips and sheets from heat-treatable aluminum alloys, containing less than 1% copper which strips and sheets can reach their maximum strength by subsequent simple aging treatment.

The process of the present invention is especially suitable for aluminum-magnesium-silicon alloys, but is also suitable for aluminum-zinc-magnesium alloys with low magnesium content.

It is to be understood that the designation aluminummagnesium-silicon alloy herein refers to all aluminum alloys with a chemical composition lying between the following limits:

Percent by wt. Magnesium 0.3-1.5 Silicon 0.2-1.6 Copper 1 Aluminum Balance These alloys may contain one or more of the following additions:

Manganese up to 1% Chrome up to 0.4% Iron up to 0.5% Titanium up to 0.2% Zinc up to 0.2% Copper up to 1% The basic metal can be aluminum of commercial purity (i.e. according to ASTN B209) as well as aluminum of high or highest purity (refined aluminum). As a few examples, there could be mentioned AA6011, AA6061 or AA6063 according to the Registration Record of Aluminum Association Alloy Designations and Chemical Composition Limites for Wrought Aluminum Alloys, published by The American Association, 420 Lexington Ave., N.Y., N.Y., 10017.

The aluminum-zinc-magnesium alloys referred to in this invention may contain Percent Zinc 3.5-5 Magnesium 0.5-1.5 Copper 1 Aluminum Balance Furthermore, these alloys may contain additions like manganese, chromium or other elements. Because of their 3,392,062 Patented July 9, 1968 ice relatively low magnesium content compared to materials like e.g. AA7075, AA7079, AA7178 etc. these alloys are designated as low alloyed.

Today, the heat-treating of light metal sheets and strips tends to follow continuous procedures. In other words, continuous annealing furnaces are used for the heat-treatment of rolled metal products. With a given capacity, the length of the heating track is governed by the soaking time of the metal to be heated at the required temperature. From an economic point of view, a short heating track is more desirable since the longer such a heating track is, the more costly it becomes.

The demand for the smallest possible length of heating track in a furnace also depends on technical reasons, because the handling of material in long furnaces is more difiicult, and this leads to increased danger to the surfaces of the heat-treated material and to increased liability of these surfaces becoming damaged. Every possible provision resulting in reducing the soaking time necessary to obtain the required annealing effect is therefore a considerable technical progress.

An important item in the heat-treatment of sheets and strips from heat-treatable aluminum alloys is the solution heat-treatment followed by quenching. To achieve the required mechanical strength values in the end-condition, the product must be subjected to a solution heattreatment long enough to put into solution practically all alloy components producing the hardening effect.

According to the well-known common practice [Y. Bresson and P. Guesne, Rev. Metallurg. (1961), p. 345- 350], cold-rolled sheets or strips of aluminum-1% magnesium-1% silicon alloy with a thickness of 3 mm. and less, have to be soaked in a continuous furnace at solution heat-treatment temperature of (545 C.:l0 C.) for at least 2 minutes before it is quenched. Such a procedure requires a heating chamber of somewhat more than 10 m. in length and with sheets of 3 mm. and less in thickness, the transit speed ranges between 3 and 5 m./ minute. Compared to the speeds normally attainable in coldrolling and hot-rolling, such a transit speed of 5 m./ minute and less is very slow. The diflierence becomes especially unfavorable when handling strips.

The present invention allows a noticeable increase in the transit speed, thus permitting an economic production of sheets and strips in essentially copper-free, heat-treatable aluminnum alloys suitable for artificially aging.

According to the invention, the initial slab or plate has to be homogenized and hot-rolled while still in the warm condition. The hot-rolled strip which should have a temperature of more than 350 C., preferably more than 400 C., then immediately has to be cooled down to a temperature of 200 C., or less preferably quenched to room temperature in less than 2 minutes, preferably less than 1 minute. Subsequently the material is coldrolled with a thickness reduction of at least 20%, then solution-treated in a continuous furnace for less than 1 minute, preferably less than 30 seconds and immediately quenched after leaving the furnace. Such sheets and strips can, at least, be naturally aged or preferably artificially aged.

Hot-rolling speeds for the subsequent passes have to be chosen individually in order to take advantage of the heat produced by rolling down the material, to maintain the metal temperature above 350 C., preferably above 400 C.

The process of the invention has the advantage of maintaining the homogenized condition of the initial slab or plate during hot rolling and subsequently in the strip leaving the hot mill by a quench. This results in the cold-rolled strip with a thickness of 3 mm. or less, preferably between 1.5 and 0.1 mm., needing only a few seconds (e.g. 5-10 seconds) to stay in the continuous furnace in order to become heated up to the homogenizing temperature and to reach that homogeneous condition which offers the same mechanical strength properties normally reached after quenching and naturally aging or artifically aging. In other words, sheets or strips manufactured according to the present invention can pass a homogenization furnace of given length with a speed which is many times greater than that necessary with the common methods. This rise in efiiciency, therefore, correspondingly achieves an economical increase.

The procedure according to the invention is especially suitable for the continuous production of strips. For the use of the material on other continuous manufacturing lines, it is also possible to cut the strips into any required lengths before cold-rolling as Well as after solution treatment. The finished products in this case are sheets instead of strips.

In the following table, the characteristic values of the common methods are compared with those resulting from the procedure of the present invention.

With the Common Method procedure of the present invention Time for the hot-rolled plate 5 to 50 rninutes Less than 2 after cold-rolling (referred to aluminum-1% magnesium- 1% silicon alloy).

It is also possible to coil and store the material quenched after hot-rolling before starting with coldrolling.

The following example illustrates a certain way in which the principle of the invention can be applied, but it is not to be construed as limiting the broader aspects of the invention.

To manufacture an aluminum-1% magnesium-1% silicon alloy, the rolling slab is soaked for 6 hours at 530 C. (metal temperature). This is followed by hotrolling it down to a thickness of 7 mm., for instance, starting with a metal temperature of 500 C. The plate temperature after hot-rolling was not less than 380 C. The last hot-rolling pass is followed by an immediate quench of the plate in water, the temperature of the plate falling down to 50-80 C. Subsequent cold-rolling results in a strip of e.g. 1 mm. in thickness, which then is solution-treated at a temperature of e.=g. 530 C. (metal temperature) in a continuous furnace with a heating track of all together 10 m. in length. The transit speed is 30 m./minute, i.e. the total heating cycle takes 20 seconds: 10 seconds are used for heating up the metal, Whilst the remaining 10 seconds count as soaking time. For the subsequent quenching, air blast or water .is used. Finally the strip or sheet is aged by reheating the material for 14 hours at a temperature of 160 C. The strip produced in this way has the following mechanical strength properties:

Tensile strength p.s.i 46,400 Yield strength p.s.i 40,100 Elongation percent 14.0

Brinell No. (IO/2.5) 101 An aluminum plant is known where the hot-rolled strip is cooled with water spray before coiling. However, cooling the strip at the run-out of the rolling mill in this plant is not done to maintain the homogeneous structure, but to facilitate the handling of the strip only. They do not roll any heat-treatable aluminum alloys at all in this plant, but exclusively pure aluminum and the non-heattreatable aluminum-manganese alloys.

Publications about this plant (for example the leaflet entitled Continuous Strip Mill at Rogerstone, issued by the former called Northern Aluminum Company Ltd.,

Banbury, Oxfordshire, a few years ago) did not suggest the use of strip from heat-treatable aluminum alloys immediately quenched after hot-rolling as a basic material for the production of heat-treatable sheet and strip from aluminum alloys by cold-rolling followed by a heat-treatment in a continuous furnace. The present invention therefore is based on a special treatment combination resulting in a distinct increased effect compared with that attainable by common procedures.

What is claimed is: 1. In a process of producing strip-like material with a thickness of about 3 mm. and less from an initially rolled member made of a heat-treatable aluminum alloy with a copper content of less than 1%, which material can reach its maximum strength by subsequent simple aging, and which process comprises the steps of:

(1) homogenizing'the initially rolled member, (2) hot-rolling the member in homogenized condition, (3) subsequentlycold rolling the member with at least 20% reduction in the thickness (4) subjecting the cold roller material to a solution heat-treatment in a continuous furnace for a period of less than one minute, and

(5) quenching the product after leaving the continuous furnace, the improvement comprising:

maintaining a temperature up from a minimum of 350 C. during hot rolling, and immediately thereafter quenching the hot-rolled member from said temperature down to a maximum of 200 C. in less than 2 minutes.

2. The process as described in claim 1, wherein in step (2) the hot-rolling temperature is maintained at not less than 400 C. and in step (3) the quenching is done from a temperature of at least 400 C. down to a temperature below 200 C. in less than 2 minutes.

3. The process as described in claim 1 wherein in step (3) the quenching is done in order that the temperature of the hot-rolled member falls below 200 C. in less than 1 minute.

4. The process as described in claim 1, wherein in step (4) the duration of solution-heat-treatment is less than 30 seconds.

5. The process as described in claim 1, wherein in step (2) the hot-rolling temperature is maintained at not less than 400 C., the quenching of the rot rolled :member is done from the temperature of at least 400 C. down to below 200 C. in less than 1 minute, and in step (4) the duration of the solution-heat-treatment is of less than 30 seconds. 3

6. The process as described in claim 1, wherein in step (2) the hot-rolling temperature is maintained at not less than 400 C., the quenching of the hot rolled member is done from a temperature of at least 400 C. down to 5080 C. in less than one minute, and in step (4) the duration of the solution-heat-treatment is of less than 30 seconds.

7. The process as described in claim 1, wherein in step (2) the hot-rolling temperature is maintained at not less than 400 C., the quenching of the hot rolled member is done from the temperature of at least 400 C. down to room temperature in less than 1 minute, and in step (4) the duration of the solution-heat-treatment is of less than 30 seconds.

8. The process as described in claim 1, wherein the hot-rolled material in strip form is coiled and stored after quenching and before cold-rolling.

9. The process as described in claim 1, wherein the successive hot-rolling passes are chosen individually and maintained at a level which makes the heat produced by the hot-rolling operation such as to maintain the temperature of the hot-rolled material at a minimum of 350 C.

10. The process as described in claim 1, wherein the successive hot-rolling passes are chosen individually and maintained at a level which makes the heat produced by 'the hot-rolling operation such as to maintain the temperature of the hot-rolled material at a minimum of 400 C.

References Cited UNITED STATES PATENTS 2,887,422 5/1959 Stone et a1 148156 X 3,194,545 7/1965 Smith 143-159 X 3,093,459 6/1963 Sieoel et a1. 14811.5

6 3,264,143 8/1966 Turner 14811.5 3,312,576 4/1967 Palik 14811.5

OTHER REFERENCES Metal Progress, October 1951, pp. 8892.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

W. W. STALLARD, H. F. SAITO, Assistant Examiners.