CN117480270A

CN117480270A - Strip made of 6xxx alloys and method of manufacture

Info

Publication number: CN117480270A
Application number: CN202280041990.6A
Authority: CN
Inventors: S·菲力浦; D·科舍尔
Original assignee: Constellium Neuf Brisach SAS
Current assignee: Constellium Neuf Brisach SAS
Priority date: 2021-06-17
Filing date: 2022-06-16
Publication date: 2024-01-30
Also published as: FR3124196B1; CA3221029A1; KR20240023116A; WO2022263782A1; FR3124196A1

Abstract

The present invention relates to the field of 6XXX series aluminum alloy strips, and to a method of making same. These strips are particularly useful for producing automotive body parts, as they provide a compromise between corrosion resistance and formability.

Description

Strip made of 6xxx alloys and method of manufacture

Technical Field

The invention relates to the field of aluminium alloy strips intended for the stamping manufacture of automotive body-in-white parts.

Background

Aluminum alloys are increasingly used in automotive construction to reduce vehicle weight and thereby reduce fuel consumption and greenhouse gas emissions.

Aluminum alloy strips are particularly useful for making many "body-in-white" components, including body skin components (or body skins) such as front fenders, front or body roofs, hood skins, trunk skins, or door skins.

Although many components have been made from aluminum alloy strip, the replacement from steel to aluminum is still tricky due to the poor formability of aluminum alloys compared to steel.

In fact, this type of application requires a set of characteristics, sometimes contradictory, such as:

high formability in the strip-fed state, T4 state, in particular for stamping operations,

Controlled yield strength in the strip-conveying state, T4 state, to control the rebound effect during forming,

good behaviour in various assembly methods for motor vehicle bodies, such as spot welding, laser welding, adhesive bonding, nailing or riveting,

having sufficient mechanical strength after electrophoresis and baking to achieve good in-use mechanical strength while minimizing the weight of the part,

good corrosion resistance, in particular filiform corrosion of the finished component,

not contradictory to the recycling requirements for manufacturing scrap or recycled vehicles,

acceptable costs in mass production.

Application WO2013/037919 discloses a method of manufacturing an AlMgSi alloy strip comprising casting a rolling slab from an AlMgSi alloy, homogenizing said rolling slab, bringing the rolling slab to a rolling temperature for hot rolling, and then optionally cold rolling to a final thickness. An improved method for producing aluminum strips from AlMgSi alloys is provided, which makes the method for producing AlMgSi aluminum strips with very good deformation properties more reliable, by producing hot strips which, when exiting from the last hot rolling pass, have a temperature of more than 130 ℃, preferably between 135 ℃ and up to 250 ℃, preferably not more than 230 ℃ and then rolling up the hot strips at this temperature.

Application JP11172390 discloses an alloy whose composition consists of one or more types (by weight) selected from the group consisting of: 0.2-2.0% Mg, 0.3-3.0% Si, < +0.8% Cu, 0.01-10.4% Mn, 0.01-0.4% Cr, 0.01-0.4% Zr, 0.01-0.4% V, 0.03-0.5% Fe, 0.005-0.2% Ti and 0.01-3.0% Zn, the balance being Al and unavoidable impurities. The alloy is rolled, and the obtained alloy sheet is subjected to dissolution heat treatment at > =480 ℃ for < +5min. Subsequently, the foil is first cooled at an average cooling rate of 150 ℃/min to a temperature of 50-150 ℃. Immediately after the end of the cooling, the second-step cooling was performed according to the inequality-1 < log (R) < (0.0178T-1.289), the temperature was lowered to 35 ℃, wherein R is the average cooling rate (. Degree. C./h) at the time of the second-step cooling, and T is the end temperature (. Degree. C.) of the first-step cooling.

Application JP10060567 discloses an aluminum alloy having a composition of 0.35 to 1.6% Mg and 0.35 to 1.6% Si (where Si/Mg > = 0.65) by weight, further containing at least one or more of < +0.8% Cu, <+0.1% Ti, <+0.3% Fe, <+0.3% Cr, <+0.8% Mn and < +0.15% Zr, the remainder being Al and unavoidable impurities (each < +0.05%). Then, the size of Si precipitates at the grain boundaries was adjusted to < +10 μm, the distance between the precipitates was adjusted to > =5 μm, and the conductivity thereof was adjusted to 40-45%.

Technical problem

The object of the present invention is to achieve an excellent compromise between all the required properties and in particular between formability and corrosion resistance. Formability of the strip in the T4 state after natural aging, which corresponds to the period of transportation and storage between quenching of the strip and its stamped parts, was evaluated. Corrosion is evaluated on the finished part, and therefore after strip stamping, painting and baking. Stoving enamels are also known to the person skilled in the art as "bake hardening" because they allow at the same time the punched strip to pass age hardening in order to obtain the characteristics required for the use of the component in an automotive vehicle.

Disclosure of Invention

One object of the present invention is an aluminium alloy strip comprising the following components in% by weight:

Si：1.2-1.5，

Fe：≤0.25，

Cu：≤0.05，

Mn：≤0.15，

Mg：0.20-0.45，

Cr：0.002-0.09，

Ni：≤0.15，

Zn：≤0.15，

Ti：≤0.15，

Zr：≤0.15，

unavoidable elements and impurities are each at most 0.05%, in total at most 0.15%, the balance being aluminum.

Another object of the invention is a method for manufacturing an aluminium alloy strip according to the invention, comprising the steps of:

a. the slab of the alloy according to the invention is cast, preferably by vertical semi-continuous casting,

b. preferably at a temperature of 500 to 600 c, more preferably 540 to 580 c, preferably 1 to 12 hours, optionally followed by a second stage at 420 to 550 c, for a maximum duration of 4 hours,

c. Cooling the slab to a hot rolling start temperature of 350 to 550 ℃ at a cooling rate preferably greater than 150 ℃/h, or cooling the slab to room temperature, then reheating the slab to a temperature of said hot rolling start temperature,

d. hot rolling the slab into a strip, the hot rolling ending temperature being between 250 ℃ and 450 ℃,

e. cold-rolled strip, optionally divided into two parts separated by an intermediate anneal, preferably coiled,

f. the solution heat treated strip, preferably at 500 c to 600 c, is preferably for 10s to 60s, and then quenched,

g. the strip is pre-aged by coiling it to a temperature of 50 c to 100 c, then cooled to room temperature,

h. the strip was naturally aged at room temperature for 72 hours to 6 months.

Another object of the invention is a vehicle body part

a. There is provided an aluminum alloy strip in accordance with the present invention,

b. the stamping is carried out,

c. the paint is applied to the surface of the glass,

d. baking the paint at a temperature of 120 ℃ to 200 ℃ for 15 minutes to 60 minutes,

characterized in that after corrosion testing according to EN 3665 the average length of filiform corrosion filaments of the sanded area is less than 2mm, preferably less than 1mm, or the average length of filiform corrosion filaments of the un-sanded area is less than 1mm, preferably less than 0.8mm.

Drawings

[ FIG. 1]: this figure is a photograph of a sample of the strip after streaking test.

[ FIG. 2]: this figure illustrates the dimensions (mm) of the tool used to determine the LDH (limit dome height) parameter values known to those skilled in the art, as an indication of the punching properties of the material.

[ FIG. 3]: this graph shows the change in elongation during the natural aging duration, data from table 5.

[ FIG. 4]: this graph shows the change in strain hardening coefficient during the natural aging duration, data from table 6.

[ FIG. 5]: this graph shows the change in yield strength and tensile strength during the natural aging duration, data from table 5.

[ FIG. 6]: this graph shows the change in bend angle during the natural aging duration, data from table 9.

Detailed Description

Unless otherwise indicated, all aluminum alloys referred to hereinafter are named according to the rules and names defined in the registration record series (Registration Record Series) by the aluminum association (Aluminum Association) in their periodic publications. Unless otherwise indicated, the components are expressed in weight percent. The expression 1.4Cu means the copper content in weight% times 1.4.

The metallurgical states involved are all named according to European standard EN-515.

The static tensile mechanical characteristics, in other words the ultimate tensile strength Rm, the conventional yield strength Rp0.2 at 0.2% elongation, the elongation at shrinkage Ag% and the elongation at break A%, are determined by the tensile test according to EN ISO 6892-1, the sampling and testing direction being defined by EN 485-1.

The strain hardening coefficient n is evaluated according to EN ISO 10275.

Modulus of elasticity is measured according to ASTM 1876.

The Lankford (Lankford) anisotropy coefficient is measured according to EN ISO 10113.

Filiform corrosion is characterized by EN 3665. The samples used for this characterization were prepared as follows: typical sanding for 2% pre-stretching, surface defect repair, and then surface treatment as usual in the automotive industry and baking finish using typical treatment at 180 ℃ for 20 minutes are performed in the rolling cross direction. Sand with P150 sandpaper for 10 seconds. Some samples were not sanded. Before the corrosion test, the painted sample was scraped to a width of 1mm, exposing the metal of the aluminum strip sample through the paint layer.

The bending angle, called alpha range (alpha norm), is determined by a three-point bending test according to NF EN ISO 7438 and the programs VDA 238-100 and VDA 239-200 version 2017.

Unless otherwise indicated, the definition of EN 12258 applies. Thin strips, or simply strips, are rolled products of rectangular transverse cross section, with a uniform thickness of between 0.20mm and 6mm.

LDH parameters are widely used to evaluate the punching properties of the strip. It has been the subject of a number of publications, in particular of R.Thompson, "The LDH test to evaluate sheet metal formability-Final Report of the LDH Committee of the North American Deep Drawing Research Group", SAE reference, detroit,1993, SAE Paper n. 930815. This is a punch test of a blank clamped around by a ring. The pressing force (blank holder pressure) is adjusted to prevent slip within the ring. Blanks having dimensions of 120mm by 160mm were loaded in a manner close to planar deformation. The punches used were hemispherical. Fig. 2 shows the dimensions of the tool used for this test. The radius of the hemispherical punch is 50.8mm. The ring diameter between the die and the blank holder was 132.6mm with its axis common to the punch axis and the axis of the hole of the blank holder and the die. The hole diameter of the blank holder and the die was 101.6mm. The die was provided with a chamfer of radius 6.3mm around the hole and opposite the blank holder. Lubrication between the ram and the strip is provided by graphite grease, such as shell HDM2 grease. The dropping rate of the punch was 50mm/min. The LDH value is a value of the displacement of the punch at the time of fracture, that is, the limit depth of punching. The break was detected by a 20daN drop in the pressure. The so-called LDH value is in fact the average of three experiments giving a 95% confidence interval for a 0.2mm measurement.

The invention is based on the fact that, thanks to a suitable composition that is tolerant of the presence of copper, a strip can be obtained that combines excellent punching properties after solution heat treatment, quenching and natural ageing at room temperature with very good corrosion resistance after a baking finish treatment. In particular, resistance to filiform corrosion is an important property for automotive body parts. These parts can be exposed to accidental or even malicious scratches or impacts. When scratches or impacts in the paint are deep enough, the metal is exposed to the external environment and filiform corrosion may occur. Filiform corrosion is a corrosion mode that begins with scraping or impact and spreads to the underlying metal surface. Thus, small scratches or impacts may result in large, very noticeable damaged surfaces.

In a preferred embodiment, the strip according to the invention has excellent filiform corrosion resistance after deformation, painting and baking. The deformation was 2% perpendicular to the rolling direction. Part of the surface of the sample is sanded, as this corresponds to the repair of surface defects during the production of the body part. These sanded surfaces are generally more susceptible to filiform corrosion. Painting includes surface treatment, electrophoresis and all operations known per se for painting. Baking lacquers, also known as bake hardens, can be simulated by a treatment at 180℃for 20 minutes. The average length of filiform corrosion filaments in the sanded area is less than 2mm, preferably 1mm. The average length of filiform corrosion filaments in the un-sanded area is less than 1mm, preferably less than 0.8mm.

In a preferred embodiment, the minimum LDH in the T4 state of an aluminium alloy strip according to the invention having a thickness of between 0.7mm and 1.0mm is at least 26.0mm. In another embodiment, the minimum LDH in the T4 state is at least 26.5mm for a strip according to the invention having a thickness of between 1.1mm and 1.5 mm. This feature is important when stamping complex geometries.

In a preferred embodiment, the strip in the T4 state according to the invention is characterized by a strain hardening coefficient of greater than 0.26 at a relatively high deformation of 14% to 16%.

In one embodiment, the average anisotropy rm= (r0+2r45+r90)/4 of the strip during natural ageing in the T4 state is between 0.54 and 0.66, the plane anisotropy Δr= (r0-2r45+r90)/2 being less than 0.25. This characteristic is important for stable stamping behaviour. The measurement is carried out according to ISO EN 10113 and at a deformation of 8% to 12%.

In one embodiment, the strip in the T4 state according to the invention has a bending angle TT of at least 100 °, preferably at least 120 °, or a bending angle TL of at least 120 °, preferably at least 145 °.

The concentration ranges imposed on the constituent elements of such alloys are described below:

si: along with magnesium, silicon is the intermetallic compound Mg formed in the aluminum-magnesium-silicon system (AA 6xxx family) ₂ Si or Mg ₅ Si ₆ This contributes to the structural hardening of these alloys during baking. The silicon content is between 1.2% and 1.5%. Higher contents reduce the bendability and mechanical strength after baking, because Si cannot be properly solution heat treated. When the Si content approaches the maximum value described above, it is necessary to increase the solution heat treatment duration, which reduces productivity, to ensure proper solution heat treatment of Si. The compromise between formability and productivity is a Si content of 1.25% to 1.45%, preferably 1.25% to 1.40%, more preferably 1.30% to 1.35%. In one embodiment, the minimum Si content is 1.25% and the maximum Si content is 1.50% or 1.45% or 1.40% or 1.35% or 1.30%. In another embodiment, the minimum Si content is 1.30% and the maximum Si content is 1.50% or 1.45% or 1.40% or 1.35%. In another embodiment, the minimum Si content is 1.35% and the maximum Si content is 1.50% or 1.45% or 1.40%. In another embodiment, the minimum Si content is 1.40% and the maximum Si content is 1.50% or 1.45%. In another embodiment, the minimum Si content1.44% and a maximum Si content of 1.50%. Preferably, the Si content exceeds the Mg content to achieve the desired formability. The amount of Si content exceeding the Mg content is the difference of Si content minus the Mg content. Preferably, the amount of Si content exceeding the Mg content is at least 0.95 wt%, preferably at least 1.00 wt%.

Fe: iron is generally considered an undesirable impurity. The presence of iron-containing intermetallic compounds is generally associated with reduced local formability. However, very pure alloys are expensive. One compromise is an Fe content of less than or equal to 0.25%, preferably less than or equal to 0.20% and preferably greater than or equal to 0.05%, more preferably greater than or equal to 0.10%. In one embodiment, the Fe content is at least 0.05% and at most 0.25% or 0.20% or 0.15% or 0.10%. In another embodiment, the Fe content is at least 0.10% and at most 0.25% or 0.20% or 0.15%. In another embodiment, the Fe content is at least 0.15%, at most 0.25% or 0.20%. In another embodiment, the Fe content is a minimum of 0.20% and a maximum of 0.25%.

Mn: manganese has a similar effect to iron by its contribution to the general intermetallic compound. The maximum Mn content was 0.15%. In one embodiment, the Mn content is at least 0.05%, and at most 0.15% or 0.10%. In another embodiment, the Mn content is at least 0.10% and at most 0.15%.

Mg: in general, the level of mechanical characteristics of AA6 xxx-family alloys increases with increasing magnesium content. Magnesium combines with silicon to form intermetallic compound Mg ₂ Si or Mg ₅ Si ₆ Which helps to improve mechanical properties, such as mechanical strength, after baking. The Mg content is between 0.20% and 0.45%. Too high Mg content reduces the solubility of Si during solution heat treatment, reducing the formability of the strip. The compromise between Si solubility after baking varnish and enhanced mechanical strength is a content of preferably less than or equal to 0.39%, more preferably 0.35%, more preferably 0.34%, more preferably 0.33%. In one embodiment, the Mg content is at least 0.25% and at most 0.45% or 0.40% or 0.35% or 0.30%. In one embodiment, the Mg content is at least 0.30% and at most 045% or 0.40% or 0.35%. In one embodiment, the Mg content is at least 0.35%, and at most 0.45% or 0.40%. In one embodiment, the Mg content is at least 0.40% and at most 0.45%.

The balance between Mg and Si is also important because, surprisingly, it allows the presence of Cu in the alloy, as described below.

Cu: copper is an element that participates in hardening precipitation in the AA 6000-family alloy, but is known to reduce corrosion resistance. The copper content is at most 0.05%. Allowing the presence of copper in the alloy is economically attractive because it enables recycling of the aluminum scrap and waste containing it. The presence of copper may come from the scrap and the waste itself, but may also be a result of accidental introduction. For example, during disassembly of a scrap vehicle, it is sufficient to leave the copper wire inadvertently to the aluminum part, contaminating the slabs obtained from the recycled aluminum alloy. In one embodiment, the Cu content is at least 0.01% and at most 0.05% or 0.04% or 0.03% or 0.02%. In one embodiment, the Cu content is at least 0.02% and at most 0.05% or 0.04% or 0.03%. In one embodiment, the Cu content is at least 0.03%, and at most 0.05% or 0.04%. In one embodiment, the Cu content is at least 0.04% and at most 0.05%.

Ti: this element can promote solution hardening to achieve the desired level of mechanical characteristics and also has a beneficial effect on ductility and corrosion resistance in use. On the other hand, in order to avoid the case of the formation of an initial phase during vertical casting, which has an adverse effect on all the characteristics claimed, a maximum Ti content of 0.15% is required. In one embodiment, the Ti content is at least 0.01% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02%. In another embodiment, the Ti content is at least 0.02% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03%. In another embodiment, the Ti content is at least 0.03% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04%. In another embodiment, the Ti content is at least 0.03% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04%. In another embodiment, the Ti content is at least 0.04% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06%. In another embodiment, the Ti content is at least 0.06% and at most 0.15% or 0.12% or 0.10% or 0.08%. In another embodiment, the Ti content is at least 0.08% and at most 0.15% or 0.12% or 0.10%. In another embodiment, the Ti content is at least 0.10% and at most 0.15% or 0.12%. In another embodiment, the Ti content is a minimum of 0.12% and a maximum of 0.15%.

Cr: since the Cr content is 0.002% at the minimum and 0.09% at the maximum, the Cr content is used as a hardening element. The addition of which can refine grains and stabilize the structure. In one embodiment, the Cr content is at least 0.002% and at most 0.09% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02% or 0.01%. In another embodiment, the Cr content is at least 0.01% and at most 0.09% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02%. In another embodiment, the Cr content is at least 0.02% and at most 0.09% or 0.08% or 0.06% or 0.04% or 0.03%. In another embodiment, the Cr content is at least 0.03% and at most 0.09% or 0.08% or 0.06% or 0.04%. In another embodiment, the Cr content is at least 0.04% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06%. In another embodiment, the Cr content is at least 0.06%, and at most 0.09% or 0.08%. In another embodiment, the Cr content is at least 0.08% and at most 0.09%.

Ni: the Ni content is 0.15% at maximum. The alloy is resistant to the presence of nickel, which can be introduced by recycling. In one embodiment, the Ni content is at least 0.002% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02% or 0.01% or 0.005%. In another embodiment, the Ni content is at least 0.005%, at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02% or 0.01%. In another embodiment, the Ni content is at least 0.01% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02%. In another embodiment, the Ni content is at least 0.02% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03%. In another embodiment, the Ni content is at least 0.03% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04%. In another embodiment, the Ni content is at least 0.04% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06%. In another embodiment, the Ni content is at least 0.06% and at most 0.15% or 0.12% or 0.10% or 0.08%. In another embodiment, the Ni content is at least 0.08% and at most 0.15% or 0.12% or 0.10%. In another embodiment, the Ni content is at least 0.10%, at most 0.15% or 0.12%. In another embodiment, the Ni content is at least 0.12% and at most 0.15%.

Zn: the content is 0.15% at maximum so as not to reduce corrosion resistance. Since Zn is an alloying element in aluminum alloys, it is attractive to accept Zn in order to recycle aluminum scrap or waste, especially from scrap vehicles. Indeed, zn is used in alloys for certain components, such as heat exchangers. In one embodiment, the Zn content is at least 0.001%, at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02% or 0.01% or 0.005% or 0.002%. In another embodiment, the Zn content is at least 0.002% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02% or 0.01% or 0.005%. In another embodiment, the Zn content is at least 0.005% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02% or 0.01%. In another embodiment, the Zn content is at least 0.01% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03% or 0.02%. In another embodiment, the Zn content is at least 0.02% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03%. In another embodiment, the Zn content is at least 0.03% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04%. In another embodiment, the Zn content is at least 0.04% and at most 0.15% or 0.12% or 0.10% or 0.08% or 0.06%. In another embodiment, the Zn content is at least 0.06% and at most 0.15% or 0.12% or 0.10% or 0.08%. In another embodiment, the Zn content is at least 0.08% and at most 0.15% or 0.12% or 0.10%. In another embodiment, the Zn content is at least 0.10%, at most 0.15% or 0.12%. In another embodiment, the Zn content is at least 0.12% and at most 0.15%.

Zr: the maximum Zr content is 0.15%. The content thereof must be limited in view of the influence on the particle diameter. Since Zr is an alloying element in some aluminum alloys, it is attractive to accept Zr for the purpose of recycling aluminum scrap or waste. In one embodiment, the minimum Zr content is 0.0005%, max 0.15% or 0.10% or 0.05% or 0.02% or 0.01% or 0.005% or 0.001%. In another embodiment, the minimum Zr content is 0.001%, and the maximum is 0.15% or 0.10% or 0.05% or 0.02% or 0.01% or 0.005%. In another embodiment, the minimum Zr content is 0.005% and the maximum is 0.15% or 0.10% or 0.05% or 0.02% or 0.01%. In another embodiment, the minimum Zr content is 0.01% and the maximum is 0.15% or 0.10% or 0.05% or 0.02%. In another embodiment, the minimum Zr content is 0.02% and the maximum is 0.15% or 0.10% or 0.05%. In another embodiment, the minimum Zr content is 0.05% and the maximum is 0.15% or 0.10%. In another embodiment, the minimum Zr content is 0.10% and the maximum is 0.15%.

Typically, the other elements are impurities, the content of which is kept below 0.05%, the total content being less than 0.15%; the balance being aluminum.

The method of manufacturing strip according to the invention typically comprises casting a slab, preferably by vertical semi-continuous casting, also known as direct chill casting or DC casting, which slab is preferably peeled to remove the cast skin (foundry cortical layer) and then homogenized.

The slab is cast from an alloy according to the composition previously described. The preferred dimensions of the slabs according to the invention are 200mm to 600mm thick, 1000mm to 3000mm wide, 2000mm to 8000mm long. The slabs are then cut to length and peeled.

The slab is then homogenized. Too low a homogenization temperature and too short a duration can force an increase in the solution heat treatment duration. Too long a duration may reduce productivity. Excessive temperatures may lead to incipient melting, which reduces both mechanical strength after baking and formability of the strip. The homogenization of the slab is carried out at a temperature of 500 to 600 ℃. The homogenization duration is advantageously at least 1 hour. An advantageous compromise is homogenization at 540 to 580 ℃ for a duration of 1 to 4 hours. In one embodiment, the homogenization temperature is between 520 ℃ and 600 ℃ or 580 ℃ or 560 ℃ or 540 ℃. In another embodiment, the homogenization temperature is between 540 ℃ and 600 ℃ or 580 ℃ or 560 ℃. In another embodiment, the homogenization temperature is between 560 ℃ and 600 ℃ or 580 ℃. In another embodiment, the homogenization temperature is between 560 ℃ and 600 ℃.

The homogenization duration is preferably at least 1 hour. In one embodiment, the maximum homogenization duration is 12 hours or 10 hours or 8 hours or 6 hours or 4 hours or 2 hours. In another embodiment, the longest homogenization duration is at least 2 hours and at most 12 hours or 10 hours or 8 hours or 6 hours or 4 hours. In another embodiment, the longest homogenization duration is at least 4 hours and at most 12 hours or 10 hours or 8 hours or 6 hours. In another embodiment, the longest homogenization duration is at least 6 hours and at most 12 hours or 10 hours or 8 hours. In another embodiment, the longest homogenization duration is at least 8 hours and at most 12 or 10 hours. In another embodiment, the longest homogenization duration is at least 10 hours and at most 12 hours.

Homogenization may optionally include a second stage at 420 ℃ to 550 ℃ for a maximum duration of 4 hours. This second stage enables the slab temperature to be reduced to its hot rolling temperature when the production accident reduces the production speed. In one embodiment, the maximum temperature of the second stage is 550 ℃ and 440 ℃ or 460 ℃ or 480 ℃ or 500 ℃ or 520 ℃ or 540 ℃. In another embodiment, the maximum temperature of the second stage is 540 ℃ and 440 ℃ or 460 ℃ or 480 ℃ or 500 ℃ or 520 ℃. In another embodiment, the maximum temperature of the second stage is 520 ℃ and 440 ℃ or 460 ℃ or 480 ℃ or 500 ℃. In another embodiment, the maximum temperature of the second stage is 500 ℃ and 440 ℃ or 460 ℃ or 480 ℃. In another embodiment, the maximum temperature of this second stage is 480 ℃ and 440 ℃ or 460 ℃. In another embodiment, the maximum temperature of the second stage is 460℃and 440 ℃. The purpose of this second stage is to avoid two passes through subsequent coolers, as described for example in patent application WO 2016012691.

Then, either the slab is cooled to room temperature and then heated to a hot rolling start temperature lower than the homogenization temperature, or the slab is directly cooled from the homogenization temperature to the hot rolling start temperature, which improves productivity because hot rolling can be started immediately. The direct cooling to the hot rolling start temperature is preferably carried out at a direct cooling rate of at least 150 ℃ per hour. Advantageously, the direct cooling rate is at most 500 ℃/h. Direct cooling can typically be performed by a machine as described in application WO 2016012691. Preferably, the direct cooling is performed in two steps, one step being sprinkling and the other step being homogenization. Optionally, the direct cooling may be performed in two passes through a machine as described in WO 2016012691.

The slab is then transferred to a hot rolling mill at a hot rolling start temperature. The hot rolling initiation temperature is between 350 ℃ and 550 ℃. Preferably, the hot rolling initiation temperature is between 500 ℃ and 400 ℃. Limiting the hot rolling initiation temperature too high leads to the risk of cracking of the slab during hot rolling, which can lead to slab rejection. An excessively low hot rolling start temperature may cause a shortage of hot rolling end temperature, making it difficult to roll a slab. In one embodiment, the hot rolling initiation temperature is at least 350 ℃ and at most 500 ℃ or 480 ℃ or 460 ℃ or 440 ℃ or 420 ℃ or 400 ℃ or 380 ℃. In another embodiment, the hot rolling initiation temperature is at least 380 ℃ and at most 550 ℃ or 500 ℃ or 480 ℃ or 460 ℃ or 440 ℃ or 420 ℃ or 400 ℃. In another embodiment, the hot rolling initiation temperature is at least 400 ℃ and at most 550 ℃ or 500 ℃ or 480 ℃ or 460 ℃ or 440 ℃ or 420 ℃. In another embodiment, the hot rolling initiation temperature is at least 420℃and at most 550℃or 500℃or 480℃or 460℃or 440 ℃. In another embodiment, the hot rolling initiation temperature is at least 440 ℃ and at most 550 ℃ or 500 ℃ or 480 ℃ or 460 ℃. In another embodiment, the hot rolling initiation temperature is at least 460℃and at most 550℃or 500℃or 480 ℃. In another embodiment, the hot rolling initiation temperature is at least 480 ℃ and at most 550 ℃ or 500 ℃. In another embodiment, the hot rolling initiation temperature is at least 500 ℃ and at most 550 ℃.

At the end of the hot rolling, the slab has been rolled into a strip, with a final hot rolling thickness of between 3mm and 10 mm. The hot rolling end temperature is between 250 ℃ and 450 ℃. The cooling between the start and end of hot rolling is the result of the usual heat exchange between the slab and strip and the air at room temperature of the mill, with hot rolling mill equipment such as, but not limited to, rollers or conveyor drums, and with usual lubricating or cooling fluids. In one embodiment, the hot rolling end temperature is 270 ℃ at a minimum and 450 ℃ or 400 ℃ or 380 ℃ or 360 ℃ or 340 ℃ or 320 ℃ or 300 ℃ at a maximum. In another embodiment, the hot rolling end temperature is at least 300 ℃ and at most 450 ℃ or 400 ℃ or 380 ℃ or 360 ℃ or 340 ℃ or 320 ℃. In another embodiment, the hot rolling end temperature is at least 320 ℃ and at most 450 ℃ or 400 ℃ or 380 ℃ or 360 ℃ or 340 ℃. In another embodiment, the hot rolling end temperature is at least 340 ℃ and at most 450 ℃ or 400 ℃ or 380 ℃ or 360 ℃. In another embodiment, the hot rolling end temperature is at least 360 ℃ and at most 450 ℃ or 400 ℃ or 380 ℃. In another embodiment, the hot rolling end temperature is at least 380 ℃ and at most 450 ℃ or 400 ℃. In another embodiment, the hot rolling end temperature is 400 ℃ at the minimum and 450 ℃ at the maximum.

The first embodiment is a combination of the following: the hot rolling start temperature is 400 ℃ to 450 ℃, preferably 400 ℃ to 430 ℃, the rolling end temperature is 350 ℃ to 450 ℃, preferably 350 ℃ to 420 ℃, the cooling during hot rolling is below 100 ℃, preferably 70 ℃, and no intermediate annealing is performed during cold rolling. This combination makes it possible to obtain a recrystallized state at the end of hot rolling, which recrystallizes during solution heat treatment to obtain good surface quality after painting.

The second embodiment is a combination of the following: the hot rolling start temperature is 450 ℃ to 500 ℃, preferably 460 ℃ to 500 ℃, the hot rolling end temperature is 250 ℃ to 350 ℃, preferably 260 ℃ to 320 ℃, the cooling during hot rolling is higher than 100 ℃, preferably higher than 125 ℃, more preferably higher than 150 ℃, and the intermediate annealing is performed during cold rolling. This bonding makes it possible to obtain a fibrous state at the end of hot rolling, which is recrystallized during solution heat treatment to obtain good surface quality after painting.

The first embodiment is preferred over the second embodiment because no intermediate annealing operation is performed, which is more economical.

The strip is then cold rolled to a final thickness of 0.8mm to 2 mm. Optionally, cold rolling is performed in two parts separated by an intermediate annealing operation of 300 ℃ to 500 ℃, preferably 300 ℃ to 400 ℃, more preferably 340 ℃ to 380 ℃. Such intermediate annealing is preferably performed on the coiled strip rather than in a continuous furnace, as the furnace for coil annealing is easier to build.

The strip is then solution heat treated and quenched in a continuous furnace. The solution heat treatment temperature is between 500 ℃ and 600 ℃. In one embodiment, the solution heat treatment temperature is at least 520 ℃ and at most 580 ℃ or 570 ℃ or 560 ℃ or 550 ℃ or 540 ℃. In another embodiment, the solution heat treatment temperature is at least 540 ℃ and at most 580 ℃ or 570 ℃ or 560 ℃ or 550 ℃. In another embodiment, the solution heat treatment temperature is at least 550 ℃, and at most 580 ℃ or 570 ℃ or 560 ℃. In another embodiment, the solution heat treatment temperature is at least 560 ℃ and at most 580 ℃ or 570 ℃. In another embodiment, the solution heat treatment temperature is at least 570 ℃ and at most 600 ℃. The duration of the solution heat treatment is between 10s and 60 s. The solution heat treatment duration is less than 10s, the strip cannot be subjected to sufficient solution heat treatment, and formability and mechanical strength characteristics of the strip after paint baking cannot be achieved. Too long a solution heat treatment duration may reduce productivity and thus production costs. Preferably by air quenching. Air-quench is advantageous for the surface quality of the strip, an important feature when used in automotive body skin components. Quenching with water results in a high cooling rate, which deforms the strip. The deformation caused by the water quench forces the use of levelers, which may impair the surface quality. The quenching rate up to a temperature of 100 ℃ is at least 15 ℃/s, preferably greater than 20 ℃/s, more preferably greater than 30 ℃/s. In view of the preferred air quenching, the maximum quenching rate is preferably 95 ℃/s.

The strip is then pre-aged. The pre-ageing is achieved by coiling the strip at a pre-ageing temperature of 50 ℃ to 100 ℃ and then cooling to room temperature. This pre-ageing serves to stabilize the mechanical properties and formability of the strip during natural ageing. In a preferred embodiment, the strip is reheated to a pre-ageing temperature and then coiled directly at said temperature. Such reheating is advantageous for controlling the winding temperature. In fact, on the one hand, the temperature after rapid cooling, for example quenching, is difficult to control, and reheating makes it possible to finely control the temperature of the strip. Furthermore, the solution heat treatment and quenching machine is usually separated from the machine that performs reheating by an accumulator in which the strip continues to cool, depending on the length of the accumulated strip. On the other hand, the steps of surface treatment known to the person skilled in the art and available to the automotive manufacturers for strip use often occur after quenching and before pre-ageing. The reheating then makes the pre-ageing temperature to be selected independent of the final surface treatment temperature. In one embodiment, the pre-ageing temperature is at least 60 ℃ and at most 100 ℃ or 95 ℃ or 90 ℃ or 85 ℃ or 80 ℃ or 75 ℃ or 70 ℃ or 65 ℃. In another embodiment, the pre-ageing temperature is at least 65℃and at most 100℃or 95℃or 90℃or 85℃or 80℃or 75℃or 70 ℃. In another embodiment, the pre-ageing temperature is at least 70℃and at most 100℃or 95℃or 90℃or 85℃or 80℃or 75 ℃. In another embodiment, the pre-ageing temperature is at least 75 ℃ and at most 100 ℃ or 95 ℃ or 90 ℃ or 85 ℃ or 80 ℃. In another embodiment, the pre-ageing temperature is at least 80℃and at most 100℃or 95℃or 90℃or 85 ℃. In another embodiment, the pre-ageing temperature is at least 85 ℃ and at most 100 ℃ or 95 ℃ or 90 ℃. In another embodiment, the pre-ageing temperature is at least 90 ℃ and at most 100 ℃ or 95 ℃. In another embodiment, the pre-ageing temperature is at least 95℃and at most 100 ℃. The pre-ageing is carried out during natural cooling of the rolls at room temperature in the workshop for a duration of 8 hours to 24 hours.

Room temperature is the temperature that is compatible with human activity. Room temperature is typically between 0 ℃ and 45 ℃. Cooling the roll to a temperature of 45 c at the pre-ageing temperature is advantageous because it does not require the use of a cooling medium, such as an air conditioner, during warmer seasons.

Thus, the strip is in the T4 state and naturally ages at room temperature for 72 hours to 6 months. This duration corresponds to the usual storage duration prior to the manufacture of the body part.

The strip is then used to manufacture body parts. Thus, the manufacturing method of the vehicle body part includes the following successive steps

According to the invention a strip is provided,

-stamping the strip

Painting, which includes all surface treatment, electrophoresis and painting operations known to the person skilled in the art,

stoving enamels, known to the person skilled in the art as "bake hardens", are carried out at temperatures of from 170℃to 200℃for from 15 minutes to 30 minutes.

At the end of the corrosion test according to EN 3665, the body parts have excellent filiform corrosion resistance. The average length of filiform corrosion filaments of the sanded area is less than 2mm, preferably less than 1. In the un-sanded area the filiform corrosion filaments have an average length of less than 1mm, preferably less than 0.8mm. Sanding is a typical repair of surface defects that occur during the production of a component. These sanding repairs are performed by workers in the production facility and are well known to those skilled in the art.

Examples

According to the aluminum alloy plate blank having the composition of the mass percentage in Table 1, casting was performed by vertical semi-continuous casting. Alloys C, D, E and F have a composition according to the invention. The slab size was 1820×520×3500.

TABLE 1

	Si	Fe	Cu	Mn	Mg	Cr	Ni	Zn	Ti	Zr
											A	1.27	0.19	0.089	0.09	0.29	0.012	0.004	0.006	0.03	0.0009
B	1.30	0.15	0.164	0.07	0.28	0.015	0.003	0.010	0.02	0.0009
											C	1.34	0.14	0.002	0.08	0.33	0.010	0.007	0.004	0.03	0.0006
D	1.34	0.14	0.002	0.08	0.33	0.010	0.007	0.004	0.03	0.0006
											E	1.33	0.15	0.029	0.06	0.32	0.011			0.03
F	1.33	0.15	0.029	0.06	0.32	0.011			0.03
											G	0.94	0.22	0.092	0.15	0.42	0.037	0.004	0.011	0.02	0.0013

The slabs were then cut to length, peeled and then homogenized at 560 ℃ for 2 hours. The homogenization oven was then set to 540 ℃. After 2 hours, the slab was taken out of the homogenization furnace at 540 ℃ and cooled to the hot rolling start temperature according to table 2. Cooling has been carried out by the machine described in application WO 2016012691. The slabs A, B and C require two passes, the others only passing through the machine once. The cooling rate was about 350 c/h. The cooling is carried out in two steps, the sprinkling step then being the homogenization step. The slab is then hot rolled into a strip. The rolling start temperatures of slabs A, B and C are between 400 ℃ and 450 ℃, while the rolling start temperatures of the other slabs are between 450 ℃ and 500 ℃. At the end of hot rolling, the slab A, B and C have temperatures between 350 ℃ and 400 ℃, while other slabs have hot rolling end temperatures below 300 ℃. The hot rolling end thickness of the strip is given in table 2. The strip is then cold rolled to an intermediate cold rolled thickness. Some of the strips were heat treated in rolls at 350 ℃ for 1 hour according to table 2. The strip was then rolled to the final thickness in table 2.

TABLE 2

The strip is then solution heat treated and air quenched in a continuous furnace. The solution heat treatment duration is given in table 3. The strip is then pre-aged. The pre-ageing is carried out by coiling the strip at a pre-ageing temperature, the resulting coil being naturally cooled to room temperature within 12 hours. Because the strip is produced under industrial conditions, the room temperature varies between 15 ℃ and 26 ℃. The pre-ageing temperatures are given in table 3. The rolls were then naturally aged at room temperature and sampled for different characterization.

TABLE 3

The strip in the T4 state was tested for punching properties using an LDH (limit dome height) test.

The sample size was 120 x 160mm, with the 160mm size being placed either in the longitudinal direction, i.e. in the rolling direction, or in the transverse direction, i.e. perpendicular to the rolling direction, or in the 45 ° direction between the first two directions. The results are shown in table 4.

Rolls E and F according to the invention exhibit better punching properties than roll G and the latter do not weaken with natural ageing duration.

TABLE 4

Formability can also be observed in the following analysis.

Tables 5 and 6 show the mechanical characterization results after different natural aging durations. These results demonstrate the stability of the mechanical properties during natural ageing, an essential feature of forming and in particular stamping, independently of the strip storage duration. Static tensile mechanical characteristics were determined by tensile test according to NF EN ISO 6892-1.

To ensure formability, these mechanical properties change little during natural aging. In particular, the strain hardening coefficient at high elongation between 14% and 16% varies by less than 0.04 during natural aging. The anisotropy of the strain hardening coefficient is also low.

Figure 3 shows the low time sensitivity of elongation during natural aging in table 5.

Fig. 4 shows the low time sensitivity of the strain hardening coefficient at high elongation between 14% and 16% in table 6.

Figure 5 shows that the tensile and yield strengths of table 5 are less sensitive to natural aging.

TABLE 5

TABLE 6

The anisotropy of the strip in the rolling direction, transverse to the rolling and 45% between said directions in the T4 state was evaluated using a lankford coefficient between 8% and 12%. The average anisotropy and the plane anisotropy during natural aging can thus be calculated. The results are shown in Table 8, and these characteristics remain stable throughout natural aging.

TABLE 8

Samples were taken for filiform corrosion testing. Corrosion was quantified in table 7 by measuring the average length of filiform corrosion filaments and the length of filiform corrosion. These results clearly show the effect of copper on filiform corrosion.

TABLE 7

The streaks were measured as follows. Samples of about 270mm (transverse to the rolling direction) x 50mm (rolling direction) were cut from the strip. A 15% tensile pre-deformation was then applied perpendicular to the rolling direction, i.e. along the length of the sample. The samples were then subjected to P800 sandpaper to reveal streaks. The streaks were measured on the sample D, and the results are shown in FIG. 1. The strip thus has a satisfactory surface quality, and the parts produced from the strip have a painted surface quality.

Strips D and F in the T4 state are characterized by bending. The strips D, E and F have a bending angle TT of at least 120 ° and a bending angle TL of at least 145 ° in the long rolling direction. Fig. 6 shows the low sensitivity of bend angles to natural aging with the data in table 9.

TABLE 9

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Claims

1. An aluminum alloy strip comprises the following components in percentage by weight:

Si：1.2-1.5，

Fe：≤0.25，

Cu：≤0.05，

Mn：≤0.15，

Mg：0.20-0.45，

Cr：0.002-0.09，

Ni：≤0.15，

Zn：≤0.15，

Ti：≤0.15，

Zr：≤0.15，

2. The aluminum alloy strip of claim 1, wherein Si:1.25 to 1.45, preferably 1.25 to 1.40.

3. The aluminum alloy strip according to claim 1 or 2, wherein Fe:0.10-0.20, or 0.10-0.15 or 0.15-0.20.

4. An aluminium alloy strip according to any one of claims 1 to 3, wherein Mn:0.05-0.10.

5. The aluminum alloy strip according to any one of claims 1 to 4, wherein Mg: less than or equal to 0.39%, more preferably 0.35%, more preferably 0.34%, more preferably 0.33%.

6. The aluminum alloy strip according to any one of claims 1 to 5, wherein Cu: up to 0.03, or Cu 0.01-0.04 or Cu 0.02-0.04.

7. The aluminum alloy strip according to any of claims 1 to 6, wherein Ti is 0.10 or less, or 0.06 or 0.02-0.08 or less.

8. The aluminium alloy strip according to any one of claims 1 to 7, wherein Cr 0.005-0.03 or Cr 0.01-0.05.

9. The aluminum alloy strip according to any of claims 1 to 8, wherein Ni 0.002-0.01 or Ni 0.005-0.02.

10. Aluminium alloy strip according to any one of claims 1 to 9, wherein the excess of Si content relative to Mg content is at least 0.95 wt%, preferably at least 1.00 wt%.

11. An aluminium alloy strip according to any one of claims 1 to 10, wherein the minimum LDH in the T4 state of the strip having a thickness of between 0.7mm and 1.0mm is at least 26.0mm, or the minimum LDH in the T4 state of the strip having a thickness of between 1.1mm and 1.5mm according to the invention is at least 26.5mm.

12. The aluminum alloy strip according to any of claims 1 to 11, characterized in that after the strip has been pre-deformed 2% in the rolling transverse direction, then painted, then baked at 180 ℃ for 20 minutes, the average length of filiform corrosion wires in the un-sanded area is less than 2mm, preferably less than 1mm, as characterized in accordance with EN 3665; alternatively, after the strip has been pre-deformed 2% in the rolling transverse direction and then painted, after 20 minutes of baking at 180 ℃, the filiform corrosion filaments are characterized according to EN 3665, the average length of the filiform corrosion filaments in the un-sanded area is less than 1mm, preferably less than 0.8mm.

13. A method of manufacturing an aluminium alloy strip according to any one of claims 1 to 12, comprising the steps of:

a. the slab is cast, preferably by vertical semi-continuous casting,

b. preferably at a temperature of 500 to 600 c, more preferably 540 to 580 c, preferably 1 to 12 hours, optionally including a second stage between 420 to 550 c, for a maximum duration of 4 hours,

c. cooling the slab to a hot rolling start temperature of between 350 ℃ and 550 ℃ at a cooling rate preferably greater than 150 ℃/h, or cooling the slab to room temperature, then reheating the slab to said hot rolling start temperature,

d. Hot rolling the slab into a strip, wherein the hot rolling is terminated at a temperature between 250 ℃ and 450 ℃,

e. cold-rolled strip, optionally divided into two parts separated by an intermediate anneal, said anneal preferably being carried out on the strip rolled up,

f. the strip is solution heat treated, preferably between 500 c and 600 c, preferably for 10s to 60s, and then quenched,

g. the strip is pre-aged by coiling the slab to a temperature of 50 c to 100 c, then cooled to room temperature,

h. the strip was naturally aged at room temperature for 72 hours to 6 months.

14. A vehicle body component is obtained by the steps of

a. There is provided an aluminum alloy strip as claimed in any one of claims 1 to 12,

b. the stamping is carried out,

c. the paint is applied to the surface of the glass,

characterized in that after corrosion testing according to EN 3665 the average length of filiform corrosion filaments in the sanded area is less than 2mm, preferably less than 1mm, or the average length of filiform corrosion filaments in the un-sanded area is less than 1mm, preferably less than 0.8mm.