WO2008044936A1

WO2008044936A1 - Magnesium alloy sheet process

Info

Publication number: WO2008044936A1
Application number: PCT/NO2007/000346
Authority: WO
Inventors: Lothar LÕCHTE; Håkon WESTENGEN; John RØDSETH
Original assignee: Norsk Hydro Asa
Priority date: 2006-10-11
Filing date: 2007-10-03
Publication date: 2008-04-17
Also published as: NO20064605L

Abstract

A process for the production of a magnesium alloy thin sheet, where the alloy includes by wt. up to 2.0% Al, up to 2.0% Zn, up to 1.0% Mn, between 0.1 - 1.0 % RE, up to 0.05% of each of the elements Fe, Cu and Ni, bal.Mg, wherein the alloy is continuously cast in a twin roll casting process forming a strip or cast sheet near net shape with a cooling rate of between 100 and 1000 K/s and subsequent plastic deformation in the roll gap or bite with an exit temperature of < 350 °C, the cast alloy sheet or strip is preheated to a hot rolling temperature of between 350 - 450 0C to obtain required re-crystallization, and the preheated cast alloy sheet or strip is further hot-rolled in one or more steps to its final thickness and with an exit temperature > 150 °C and with a grain size < 30 microns.

Description

Magnesium alloy sheet process

The present invention relates to a process for producing magnesium alloy thin sheet.

Magnesium sheet is a product that is currently produced in a similar manner as Aluminium sheet. The main process route is the semi-continuous direct chill (DC) casting process in combination with a multi-step hot rolling process. Due to the hexagonal crystal structure, cold rolling is not possible as such. In addition there are several technological properties that become important in the context of sheet production creating difficulties, even though Aluminium and Magnesium show chemically some similarities. Two important aspects are the differences with regard to the heat capacity (Al: 2.2 kJ/(dm^Λ3*K), Mg: 1.5 kJ/(dm^Λ3*K)) and the ability for grain refinement during casting.

Due to the hexagonal crystal structure of Mg needs to be hot rolled down to final gauge instead of cold rolling. While for Al hot rolling is a multi-pass process, but carried out in one heat Mg needs to be reheated several times, because of the lower heat capacity and the thereby significant loss of temperature during rolling. Mg and Mg alloys need to be reheated several times in order to hot roll DC cast slabs down to final gauges, suitable for applications like automotive, with sheet thicknesses in the range 1-4 mm. In addition Mg and Mg alloys are more sensitive to hot cracking during hot deformation of slabs having an as-cast microstructure. Hot cracking is mainly due to the coarse grained microstructure in as-cast condition. While for Al standardized grain refiners like Titianiumboride are common for DC casting of either rolling slabs or extrusion billets, there is no technical and commercial efficient grain refiner available for Mg. Due to this lack of suitable grain refiners in combination with the relative slow cooling rates during DC casting (0.5 ... 20 K/s) normally a grain size up to 1 mm can be formed.

WO 05007320 A1 shows a continuous casting method for a magnesium alloy, comprising the steps of shielding a molten metal surface in a mould with inert gas, and controlling the metal level with micro wave by using lubricating oil with high flash point, wherein the mould having a thickness gradually decreasing in the casting direction, and pressing down is performed just below the mould so as to manufacture magnesium sheets as product without hot rolling. The magnesium sheets produced according to this method has obvious limitations with regard to sheet thickness.

WO 04020126 A1 describes a process for the production of magnesium alloy strip using twin roll casting equipment where the alloy is held at the metal source at a temperature sufficient to maintain the alloy in the feed device at about 5 mm - 20 mm above a centreline of the bite in a plane containing the axis of the rolls, and where heat energy extraction by the cooled rolls is maintained at below 400 ⁰C. No information is given in this reference as the alloy composition, which is crucial to the quality of the cast product.

WO 05009638 relates to a process for casting magnesium alloy thin sheet using a vertical mould where the magnesium alloy is solidifying while vibrating the mould and immediately thereafter the solidified magnesium alloy is hot rolled by means of a pair of rolls under pressure into a given thickness. Thus, the process as described in this WO reference is not based on twin roll casting and no ally composition is mentioned therein.

EP 01330556 A1 concerns a method for producing a magnesium hot strip where a melt form a magnesium alloy is continuously cast to form a roughed strip with a maximum thickness of 50 mm, and where the cast strip is hot-rolled directly from the casting heat at a temperature of at least 250 ⁰C and maximum of 500 ⁰C with a final thickness of 4 mm and a first hot-rolling pass provides a reduction in thickness of at least 15 %. The method specifies wide ranges of wrought alloy elements of up to 10 % aluminium, up to 10% lithium, up to 2% zinc, up to 2% manganese, up to 1 % zirconium and up to 1% cerium

This EP patent application combines twin roll casting and hot rolling with several rolling stands. A major disadvantage with such combination is a significant temperature loss due to the long contact time when hot rolling is performed at the same temperature as the casting temperature. The typical casting speed is in the area of 1 - 2m/min while the typical rolling speed is up to 150m/min.

With the present invention is provided method for the production of magnesium thin sheet which is not emcumbered with the above disadvantages, but which is based on selecting the best suitable alloy in combination with the most effective process for producing magnesium sheet with high efficiency, at low cost and with high quality The selected alloy composition ensures a short freezing range in combination with improved yield strength and formability, e.g. in terms of tensile test elongation. In particular the short freezing range, in conjunction with the absence of a tendency to sticking of the liquid metal to the rolls, makes the alloy suitable for twin roll casting. The selected alloy shows a very small volume fraction of eutectic segregation, and especially the centreline segregations are avoided. Furthermore, the alloy composition in conjunction with the fast cooling in the casting bite and the plastic deformation in the bite secures a fine grain size after homogenisation and after inter-annealing as well as final annealing. This grain structure provides excellent mechanical properties, reduces the tendency for texture formation and improves the formability of the final sheet. The twin-roll casting ensures a near net shape prior to hot-rolling and therefore a technical and commercial efficient process route to final gauges needed for applications e.g. in the automotive area. The method according to the invention is characterized by the features defined in the attached independent claim 1.

Dependent claims 2 - 6 define preferred embodiments of the invention.

The invention will be further described in the following by way of example and with reference to the attached figures, where:

Fig. 1 shows a principle sketch of a known process route of DC casting and hot rolling of a magnesium alloy sheet,

Fig. 2 shows a principle sketch of another known magnesium sheet process route,

Fig. 3 shows a principle sketch of a magnesium sheet process route according to the invention,

Fig. 4 a principle sketch (detail) on the arrangement of a casting nozzle and the rolls in a twin roll casting equipment and the freezing of the magnesium alloy (metal) in the gap between the rolls,

Fig. 5 depicts the microstructure of a Mg alloy being tested according to the invention, after casting, Fig. 6 shows a microstructure the same alloy after re-crystallisation according to the invention,

Fig. 7 shows a final microstructure of the same alloy after hot-rolling and annealing according to the invention,

Fig. 8 shows the tensile test properties for the final alloy depicted in Fig. 7.

Fig. 1 shows a principle sketch of a known process route of producing magnesium sheet 5 where relatively thick magnesium slabs 2 initially are produced in a semi-continuous direct chill (DC) casting process 1. The slabs 2 are further homogenized in a homogenizing process 3, and after homogenisation each slab 2 is rolled to final sheet thickness in a multi-step hot rolling process 4. A couple of the major disadvantages with such process is, as discussed above, that the process is very cumbersome and expensive due to the need for the alloy to be reheated several times in order to hot roll the DC cast slabs down to final thickness, and the problem that Mg and Mg alloys are more sensitive to hot cracking during hot deformation of slabs having an as-cast microstructure.

Fig. 2 shows another known process, as for instance described in EP 01330556 A1 , where a magnesium alloy 6 is continuously cast in twin roll casting process 8 to form a magnesium roughed strip or cast sheet 9 which is directly feed, in hot condition, to a hot rolling process equipment 10 for producing sheet with the final thickness. With this solution there is significant temperature loss due to the long contact time when hot rolling is performed at the same temperature as the casting temperature.

The present invention is based on selecting the best suitable alloy in combination with the most effective process for producing magnesium sheet with high efficiency, at low cost and with high quality. The process according to the invention is shown in Fig. 3 and includes, beyond the selection of the alloy, the process steps of continuously casting the selected alloy in a twin roll casting process 11 to produce a near net shape strip or cast sheet 12 at a cooling rate of between 100 and 1000 K/s and subsequent plastic deformation in the roll gap or bite 13 with an exit temperature of < 350 ⁰C. The alloy sheet is further preheated in a preheating process 14 to a hot rolling temperature of between 350 - 450 ⁰C to obtain required re-crystallization, and the preheated cast alloy sheet or strip is finally hot-rolled in one or more steps to its final sheet thickness and with an exit temperature > 150 ⁰C and with a grain size < 30 microns. Preferably the exit temperature from the roll gap is < 300 ⁰C and the grain size after hot-rolling is < 20 microns. The selected alloy according to the invention should have the following composition (chemical):

The term "others" in the table may include other allying elements and impurities up to 0,05.

Fe, Cu and Ni according to ASM standard for high purity Mg alloys is chosen in order to achieve high corrosion resistance.

Zn enhances the strength and formability and improves the corrosion resistance at the same time. At a higher Zn level the sheet/alloy becomes brittle again. A minimum Zn level is preferred in order to lower the reactivity of the liquid metal during contact with any casting equipment as well as to increase the strength of the final product. Rare earths (RE) are either in solid solution or form small particles, having the function of pinning grain boundaries and thus preventing the grain structure from growing to undesired large average grain size. Higher amounts of RE promotes the tendency to sticking. A minimum amount is needed for stabilization of the grain size. The elements Al and Mn enable the precipitation of Fe in the liquid state and thus removing Fe away from the melt either by taking it away together with the dross or let the Fe-containing particles sink to the bottom of the furnace or crucible. At the same time Mn leads to further grain refeinmenet and Al promotes the forming of a dense oxide layer that slows down any kind of oxidation or corrosion.

The amount of the sum of both elements Al and Zn need to be chosen in a way that the freezing range (e.g. characterized by differential scanning calorimeter (DSC) preferably is equal to or below 65K. Fig. 4 shows a principle sketch (detail) on the arrangement of a casting nozzle and the rolls in a hot-rolling equipment and the freezing of the magnesium alloy (metal) in the gap or bite between the rolls. The upper picture in Fig. 4 indicates that a shorter freezing range is line with a higher degree of plastic deformation in the twin roll casting gap or bite. A small freezing range lowers the tendency of forming segregation lines and enables to increase the casting speed compared to alloys with a longer freezing range. The higher the plastic deformation in the casting bite the easier is the recrystallization during pre-heating prior to hot rolling. At the same time the grain size achieved by that recyrystallization process is smaller. Equilibrium freezing ranges are given for a variety of alloys in the following table (as example):

AZ31B (with 3%AI, 1%Zn, 0.4%Mn): about 8OK ZE10 (with 1.2%Zn, 0.2%Ce): about 58K Mod. ZE10 (with 1.2%Zn, 0.2%Ce, 0.2%AI): about 55K Mod. ZE10 (with 1.2%Zn, 0.2%Ce, 0.4%Mn): about 50

As can be seen, the uppermost of these alloys, having an Al content of 3% and Zn amount of 1.0%, are not below the preferred freezing range according to the invention.

Roll casting is, as indicated above and shown in the figures, a casting process which solidifies liquid metal in-between 2 rolls being made of a metal having a high thermal conductivity and strength to withstand the high pressure in the roll bite. In many cases the arrangement of the rolls is such that the metal flow is horizontal, but it can be in any other direction as well. The roll material could be e.g. either steel or adapted Cu-alloys. The liquid metal solidifies within a short distance of some em's having contact with the top and bottom roll under high pressure.

Due to the high conductivity of the rolls and the high pressure within the roll gap the liquid metal solidifies fast, typically with cooling rates of 100 up to 1000K/s. During and immediately after solidification the metal is subjected to plastic deformation in the casting gap between the rolls, creating an enhanced density of dislocations. When pre- heating the alloy sheet prior to hot rolling, the stored energy in terms of dislocations is high enough to enable recrystallization of the microstructure and thereby a grain refinement in the solid state. A grain size of below 100μm can be achieved without additional substances for grain refinement during solidification. In order to meet that small grain size and keep it low, a minimum amount of rare earth are necessary in addition to the high cooling rate typical for the twin roll casting process. The small RE- containing particles pin the grain boundaries and prevent them from moving and thereby coarsening the grain structure.

After the hot-rolling, in one or more passes depending on the thickness, the sheet product is annealed (not show in the figures) to obtain desired grain size.

For the final product, after hot-rolling and after annealing, a grain size of 30μm and below can be achieved without additional grain refiners like Zr, carbon containing substances etc.

Example.

An Mg alloy sheet was produced according to the invention having the following alloy composition:

Fe: 0.004%

Cu: <0.001 %

Mn: 0.028%

Cr: <0.001%

Zn: 1.24%

Al: 0.053%

RE: 0.188% The alloy was heated to a temperature of 720⁰C and cast in a twin roll casting machine to form a magnesium strip with a thickness of 5mm. Fig. 5 shows the micro structure of the alloy after the casting operation.

The Mg strip was further re-heated at a temperature of 400°C to obtain required re- crystallisation of the alloy. Fig. 6 shows the grains structure of the alloy after casting and after re-crystallisation, prior to rolling.

The re-crystallised Mg strip was then hot-rolled in 4 passes and then annealed at a temperature 350⁰C for 1 h to its final grain structure. The final Mg sheet had no cracks or physical defects, and as can be seen from Fig. 7 the final grain size obtained was in the order of 18 microns.

The alloy was further mechanically tested according to European ISO standard. Fig. 8 shows the tensile strength properties for the alloy after the finally annealed variant of the alloy. On the x-axis the particular process routes are indicated by a number in addition to the final gauge. The number for the process routes indicates rolling reductions chosen between 10% and 40% thickness reduction per pass. As can be seen the final properties are not very sensitive with regard to the rolling scheme. For any of the chosen rolling schemes a yield strength RpO, 2 of >100MPa and an elongation to fracture A50mm of >15% can be achieved in the direction parallel to the rolling direction.

Claims

1. A process for the production of a magnesium alloy thin sheet, the alloy including by wt. up to 2.0% Al, up to 2.0% Zn, up to 1.0% Mn, between 0.1-1.0 % RE, up to 0.05% of each of the elements Fe, Cu and Ni, bal. Mg, and other alloying elements including any impurities up to 0,05% wherein

- the alloy is continuously cast in a twin roll casting process forming a strip or cast sheet near net shape with a cooling rate of between 100 and 1000 K/s and subsequent plastic deformation in the roll gap or bite with an exit temperature of < 350⁰C,

- the cast alloy sheet or strip is preheated to a hot rolling temperature of between 350-450⁰C to obtain required re-crystallization, and

- the preheated cast alloy sheet or strip is further hot-rolled in one or more steps to its final thickness and with an exit temperature > 150⁰C and with a grain size < 30 microns.

2. Method according to claim 1 , characterised in that the alloy contains by wt.0.30 % Al, between 0.6 an 1.6 % Zn, up to 0.5 % Mn, between 0.1 - 0.5 % RE, up to 0.05% of each of the elements Fe, Cu and Ni.

3. Method according to claims 1 and 2, characterised in that the exit temperature of the Mg alloy strip or sheet from the roll gap is < 300⁰C

4. Method according to claims 1-3, characterised in that the Mg sheet after hot-rolling is annealed at a temperature of <400°C for >5min.

5. Method according to claims 4, characterised in that the Mg sheet after hot-rolling is annealed at a temperature of 350⁰C for 30min.

6. Method according to claims 1 -3, characterised in that the grain size is < 20 microns.