JP7510046B2 - Clad Material - Google Patents

Description

The present invention relates to a clad material composed of an austenitic stainless steel plate and an aluminum alloy plate.

Clad materials, which are made by laminating and diffusion-bonding metal sheets such as stainless steel sheets and aluminum alloy sheets, have the properties of both the stainless steel sheets and metal sheets that make up each layer, and have composite properties that cannot be obtained with either material alone, so they are used in a variety of parts. In particular, clad materials used in press-formed parts for electronic devices, such as the housings of mobile electronic devices, require excellent formability.

Patent Document 1 discloses an invention that uses the rolling texture created by cold working to make the anisotropy of the pure aluminum plate and the stainless steel strip different, thereby offsetting the anisotropy of the pure aluminum plate and the anisotropy of the stainless steel strip, thereby producing a stainless steel/aluminum clad plate with excellent workability.

Patent Document 2 discloses an invention in which a stainless steel plate is joined to an aluminum or aluminum alloy plate produced by a continuous thin plate casting method, and then the two are pressure welded together by hot or cold working under conditions in which the aluminum or aluminum alloy plate is worked at a working ratio of 20 to 60% and the stainless steel plate is worked at 4% or less, and then annealed at a temperature of 230 to 450°C, thereby obtaining a clad plate with excellent formability.

Patent document 3 discloses an invention for a deep-drawable clad plate made of ferritic stainless steel with a grain size number adjusted to the range of 7 to 10 and aluminum.

Patent document 4 discloses an invention for an aluminum alloy and stainless steel clad plate with an insert material of an aluminum alloy containing Mg.

Japanese Patent Application Laid-Open No. 10-244620 Japanese Patent Application Laid-Open No. 8-309561 Japanese Patent Application Laid-Open No. 11-319970 Japanese Patent Application Laid-Open No. 9-103891

However, the above conventional technologies do not have excellent peel strength and there is a risk of peeling during processing into various parts.

The present invention has been made to solve the above problems, and aims to provide a clad material consisting of an austenitic stainless steel plate and an aluminum alloy plate that has excellent peel strength.

As a result of extensive research, the inventors have discovered that high peel strength can be achieved by aligning the crystal orientation in the surface layer of austenitic stainless steel sheets and aluminum alloy sheets to a specific orientation. Specifically, it is important to increase the concentration of the {110}<112> orientation in the surface layer of austenitic stainless steel sheets and aluminum alloy sheets.

Since such austenitic stainless steel sheets and aluminum alloy sheets have many crystal grains with similar crystal arrangements on the surface, there is a high probability that crystal grains with similar arrangements will come into contact when the two sheets are bonded together. As a result, it is believed that diffusion bonding can be achieved sufficiently even with short heating times at low temperatures.

The present invention has been made based on the above findings, and provides a clad material of an austenitic stainless steel plate and an aluminum alloy plate,
In a surface layer portion of the austenitic stainless steel plate close to an interface with the aluminum alloy plate, an area ratio of martensite is 5.0% or less, and an X-ray random intensity ratio of {110}<112> orientation is 8.0 or more,
In a surface layer portion of the aluminum alloy plate close to the interface with the austenitic stainless steel plate, an X-ray random intensity ratio in a {110}<112> orientation is 4.0 or more.
Clad material."

According to the present invention, it is possible to obtain a clad material having excellent peel strength, which is composed of an austenitic stainless steel plate and an aluminum alloy plate.

FIG. 13 is a diagram showing an ODF of a φ2=45° cross section in which the crystal orientation of the present invention is displayed.

[Clad material]
The clad material of the present invention is a clad material comprising an austenitic stainless steel plate layer and an aluminum alloy plate layer, and it is important to increase the concentration of {110}<112> orientation in the surface layer portion close to the interface between each layer.

There are no restrictions on the total thickness of the clad material, but it is preferably 0.1 to 0.6 mm. There are also no restrictions on the thickness of each layer, but it is preferable that the thickness of the austenitic stainless steel plate layer is 0.05 to 0.55 mm, and the thickness of the aluminum alloy plate layer is 0.05 to 0.55 mm.

In the following description, the surface layer close to the interface of each layer is also called the "surface layer", specifically, when the thickness of each layer is t, it means the region up to 20 μm from the interface. There are no particular restrictions on the surface layer close to the surface opposite the interface of each layer, and it may be the same as the surface layer close to the interface, or it may be different. In the following description, a two-layer clad material is described, but it goes without saying that a clad material with three or more layers may also be used. Each layer is described below.

<Austenitic stainless steel plate>
The austenitic stainless steel sheet constituting the clad material of the present invention has a predetermined metal structure. The metal structure, chemical composition, and manufacturing method of the austenitic stainless steel sheet will be described below in that order.

(Metal structure)
As described above, in order to obtain excellent peel strength, it is important to control the metal structure in the surface layer close to the contact point (interface) of the austenitic stainless steel sheet with the aluminum alloy sheet. Specifically, it is necessary to adjust the area ratio of martensite in the surface layer of the steel sheet, the average grain size of austenite grains, and the X-ray random intensity ratio of the {110}<112> orientation within the ranges shown below. Each provision will be explained in detail.

Area ratio of martensite: 5.0% or less If there is a lot of martensite in the surface layer of the steel sheet, it will transform into austenite phase when heat is applied by diffusion bonding or laser processing, etc., and the flatness of the steel sheet will decrease, thereby reducing the peel strength. In addition, since the area ratio of the austenite phase is reduced, the proportion of {110}<112> oriented grains in the entire structure will also decrease. Therefore, the area ratio of martensite in the surface layer of the steel sheet is set to 5.0% or less.

The area ratio of martensite in the surface layer is determined by the following procedure. First, for a surface parallel to the plate surface with an area of 100 μm x 100 μm or more after electrolytic polishing of the material, fcc and bcc structures are selected as the crystal structures and measurements are performed using EBSD (Electron Back Scattering Diffraction). Then, areas that are not identified as fcc structures, i.e., have a bcc crystal structure or are highly strained and cannot be measured, are considered to be martensite, and their area ratio is determined.

In addition, when preparing the sample, if a polishing agent such as colloidal silica is used for finish polishing, the austenite phase of the surface layer may undergo processing-induced martensitic transformation, so the sample must be prepared by electrolytic polishing or chemical polishing. The amount of polishing is limited to 1/8 of the plate thickness. If it is difficult to measure on a surface parallel to the plate surface, it is also possible to substitute a measurement from a plate thickness cross section. In this case, the same measurement is performed on a region from the outermost layer to 20 μm on the side forming the interface with the aluminum alloy plate, with an area of ^10,000 μm2 or more.

X-ray random intensity ratio of {110}<112> orientation: 8.0 or more The {110}<112> orientation is a typical main orientation of the rolling texture of austenite. By making the accumulation in the same orientation in the surface layer (joint surface) of the steel sheet 8.0 or more, high peel strength is ensured. For this reason, the lower limit of the X-ray random intensity ratio of this orientation is set to 8.0. The X-ray random intensity ratio of the {110}<112> orientation is preferably 8.5 or more, more preferably 9.0 or more. There is no particular upper limit, but if the X-ray random intensity ratio exceeds 20.0, it will no longer be possible to satisfy the orientation difference of 15° or more with the adjacent crystal grains, and will no longer function as an effective crystal grain boundary, so it is desirable to set this value as the upper limit.

The random intensity ratio of the {110}<112> orientation can be determined from the crystal orientation distribution function (ODF) that represents the three-dimensional texture, calculated by series expansion based on multiple pole figures from among the {200}, {311}, and {220} pole figures measured by X-ray diffraction. Note that the X-ray random intensity ratio in this invention is a value obtained by measuring the X-ray intensity of a standard sample that does not have accumulation in a specific orientation and a test material under the same conditions using X-ray diffraction or the like, and dividing the obtained X-ray intensity of the test material by the X-ray intensity of the standard sample.

Figure 1 shows the ODF of a φ2=45° cross section, which displays the crystal orientation of the present invention. Strictly speaking, the {110}<112> orientation refers to the orientation expressed as φ1=55°, Φ=90°. However, since measurement errors may occur due to the processing of the test piece and the setting of the sample, the maximum value in the range of φ1=50-60°, Φ=85-90° is used to represent the intensity ratio of this orientation. Here, the crystal orientation is usually expressed as (hkl) or {hkl} for the orientation perpendicular to the plate surface, and [uvw] or <uvw> for the orientation parallel to the rolling direction.

{hkl} and <uvw> are generic names for equivalent planes, and (hkl) and [uvw] refer to individual crystal planes. In other words, since the present invention is directed to an fcc structure, for example, the (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1), (-11-1), (1-1-1), and (-1-1-1) planes are equivalent and indistinguishable. In such cases, these orientations are collectively referred to as {111}.

Note that since ODF is also used to display the orientation of crystal structures with low symmetry, it is generally expressed as φ1 = 0 to 360°, Φ = 0 to 180°, φ2 = 0 to 360°, and each orientation is displayed as (hkl) [uvw]. However, in the present invention, since the target is a highly symmetrical fcc crystal structure, Φ and φ2 are expressed in the range of 0 to 90°. In addition, the range of φ1 changes depending on whether or not symmetry due to deformation is considered when performing calculations, but in the present invention, it is expressed as φ1 = 0 to 90° taking symmetry into consideration. In other words, a method is selected in which the average value of the same orientation at φ1 = 0 to 360° is displayed on the ODF of 0 to 90°. In this case, (hkl) [uvw] and {hkl} <uvw> are synonymous. Therefore, for example, the X-ray random intensity ratio of the (110)[1-12] direction in the ODF on the φ2=45° cross section shown in Figure 1 is synonymous with the X-ray random intensity ratio of the {110}<112> direction.

The X-ray diffraction sample is prepared by mechanically polishing and buffing the surface to be bonded, followed by electrolytic polishing to remove distortion and expose a surface 20 μm deep from the bonded surface. This exposed surface is used as the measurement surface.

If it is not possible to measure by X-ray diffraction, it may be measured by the EBSD method. In this case, measurements are made on the exposed surface at 10 μm pitches within a field of view of 400 μm x 400 μm or more. If it is difficult to measure on a surface parallel to the plate surface, the plate thickness cross section may be used as the measurement surface. In this case, an imaginary line parallel to the joint interface is drawn at a position 20 μm thick from the joint interface, and measurements are made on the imaginary line at 10 μm pitches over a length of 8 mm. The above mainly describes the method for measuring the X-ray random intensity ratio in austenitic stainless steel plates, but the X-ray random intensity ratio in aluminum alloy plates can also be measured by the same method.

Average grain size of austenite grains: 5 μm or less By setting the average grain size of the austenite grains in the surface layer to 5 μm or less, the number of crystal grains per unit area increases and the frequency of the {110}<112> oriented grains is averaged, which is considered to improve the peel strength. Therefore, in the present invention, it is preferable to set the upper limit of the average grain size of the austenite grains in the surface layer of the steel sheet to 5 μm. In addition, when etching or the like is performed, there is also the advantage that the processed surface becomes smooth.

The average grain size of austenite grains is calculated by the following procedure. First, in the surface layer of the material produced by the above-mentioned method, an area of 100 μm × 100 μm or more is measured by EBSD, and among the areas determined to have an fcc structure, an area surrounded by a boundary with an orientation difference of 15° or more is regarded as one crystal grain, and the average area S per crystal grain is calculated from the number of crystal grains contained in the specified area. Then, the average grain size D of the austenite grains is calculated from the average area by the following formula (iii).
D = (2S/π) ^0.5 ... (iii)

(Chemical Composition)
The chemical composition of the austenitic stainless steel sheet constituting the clad material is not particularly limited as long as it has the above-mentioned metal structure. For example, in mass %,
C: 0.005 to 0.15%,
Si: 1.0% or less,
Mn: 1.5% or less,
Cr: 15.0 to 20.0%,
Ni: 6.0 to 15.0%,
N: 0.005 to 0.15%,
Nb: 0.01 to 0.50%,
Mo: 0 to 2.0%,
Cu: 0 to 1.5%,
V: 0 to 0.15%,
Ti: 0 to 0.30%,
B: 0 to 0.010%,
The balance is Fe and impurities.
Examples of chemical compositions include those having an Md30 value of 20 to 60° C., as calculated by the following formula (i):
Md30 value = 497 - 462 x (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 20 x (Ni + Cu) - 18.7Mo ... (i)
In the above formula, the element symbols represent the content (mass%) of each element in the steel, and 0 is substituted when the element is not contained.

The reasons for limiting the amount of each element in the chemical composition shown are as follows. In the following explanation, "%" for the content means "mass %."

C: 0.005 to 0.15%
C is a strong solid solution strengthening element that inexpensively increases the strength of steel sheets. It also combines with Nb to precipitate fine Nb compounds, which has the effect of suppressing recrystallization and grain growth. However, C is also a strong austenite stabilizing element, and if its content is excessive, the processing-induced transformation required for grain refinement will not occur. In addition, during final annealing at low temperature for the purpose of grain refinement, coarse Cr carbides precipitate at the grain boundaries, forming a Cr-deficient phase, making it impossible to maintain the corrosion resistance required for stainless steel. Therefore, the C content should be 0.005 to 0.15%. The C content is preferably 0.01% or more. The C content is preferably 0.13% or less, and more preferably 0.12% or less.

Si: 1.0% or less Si is used as a deoxidizer during smelting and contributes to strengthening steel. However, if the Si content is excessive, there is a high possibility that coarse oxides will be formed, which reduces workability. Therefore, the Si content should be 1.0% or less. The Si content is preferably 0.6% or less. If the above effects are to be obtained, the Si content is preferably 0.1% or more.

Mn: 1.5% or less Mn contributes to preventing brittle fracture during hot working and strengthening the steel. However, since Mn is a strong austenite-forming element, if the Mn content is excessive, the amount of processing-induced martensite formed during cold rolling is reduced, and fine crystal grains cannot be obtained by final annealing. Therefore, the Mn content is preferably 1.5% or less. The Mn content is preferably 1.2% or less. In addition, if the above effect is to be obtained, the Mn content is preferably 0.1% or more.

Cr: 15.0 to 20.0%
Cr is a basic element of stainless steel, and forms an oxide layer on the surface of the steel material, enhancing corrosion resistance. However, Cr is a strong ferrite stabilizing element, so if the Cr content is excessive, delta ferrite will be formed. This delta ferrite will deteriorate the hot workability of the material. Therefore, the Cr content should be 15.0 to 20.0%. The Cr content is preferably 16.0% or more and 19.0% or less.

Ni: 6.0 to 15.0%
Ni is an austenite-forming element and has the effect of stabilizing the austenite phase at room temperature. However, if the Ni content is excessive, the austenite phase becomes too stabilized, and processing-induced martensitic transformation during cold rolling does not occur. Furthermore, Ni is an expensive element, and an excessive increase in the content leads to a significant increase in costs. Therefore, the Ni content is preferably 6.0 to 15.0%. The Ni content is preferably 6.5% or more. Moreover, the Ni content is preferably 11.0% or less, and more preferably 9.0% or less.

N: 0.005 to 0.15%
N, like C, is a strong solid solution strengthening element and contributes to improving the strength of steel. It also combines with Nb to precipitate as fine Nb compounds during hot rolling or annealing, and has the effect of suppressing recrystallization and grain growth. However, if the N content is excessive and a large number of coarse nitrides are generated during the manufacturing process of the steel sheet, it significantly deteriorates hot workability and makes manufacturing difficult. In addition, like C, N is also a strong austenite stabilizing element, and if the N content is excessive, the processing-induced transformation required for grain refinement will not occur. Therefore, the N content should be 0.005 to 0.15%. The N content is preferably 0.01% or more. The N content is preferably 0.13% or less, and more preferably 0.12% or less.

Nb: 0.01 to 0.50%
Nb is an effective element for refining the grains of a material because it generates fine carbides or nitrides during annealing and inhibits the grain growth of crystals by the pinning effect. In addition, Nb develops the processed texture of austenite by suppressing recrystallization during hot working as a solid solution or as a carbonitride. However, if the Nb content is excessive, recrystallization is inhibited, a large amount of unrecrystallized parts remain after annealing, and hot workability is also deteriorated. In addition, Nb is an extremely expensive element, and an excessive increase in the content leads to a significant increase in cost. Therefore, the Nb content is preferably 0.01 to 0.50%. The Nb content is preferably 0.03% or more, more preferably 0.04% or more. In addition, the Nb content is preferably 0.30% or less, more preferably 0.20% or less.

Mo: 0 to 2.0%
Mo is an element that improves the corrosion resistance of the material, so it may be contained as necessary. However, Mo is extremely expensive, and an excessive increase in the content leads to a significant increase in costs. Therefore, the Mo content should be 2.0% or less. The Mo content is preferably 1.0% or less. If the above effects are to be obtained, the Mo content is preferably 0.1% or more.

Cu: 0 to 1.5%
Cu is an austenite generating element and is an effective element for adjusting the stability of the austenite phase, so it may be contained as necessary. However, if the Cu content is excessive, it will segregate at grain boundaries during the manufacturing process. Such grain boundary segregation significantly deteriorates the warm workability and makes manufacturing difficult. Therefore, the Cu content is preferably 1.5% or less. The Cu content is preferably 1.0% or less. In addition, if it is desired to obtain the above effects, the Cu content is preferably 0.1% or more.

V: 0 to 0.15%
Ti: 0 to 0.30%
Like Nb, V and Ti are elements that suppress recrystallization, strengthen desirable texture, and are effective in refining crystal grains, so one or two selected from these elements may be contained as necessary. However, excessive inclusion of the above elements leads to deterioration of workability. Therefore, it is preferable to set V to 0.15% or less and Ti to 0.30% or less. On the other hand, in order to obtain the above effects, it is preferable to contain one or more selected from V: 0.01% or more and Ti: 0.01% or more.

B: 0 to 0.010%
B is an element that strengthens grain boundaries and contributes to improving hot workability, so it may be contained as necessary. However, excessive B content leads to deterioration of workability. Therefore, the B content is preferably 0.010% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.001% or more.

In the chemical composition of the steel plate of the present invention, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel, and are acceptable within a range that does not adversely affect the effects of the present invention. The total amount of impurities is preferably 0.8% or less.

Md30 value: 20 to 60°C
In the metastable austenitic stainless steel of the present invention, fine crystal grains can be obtained by utilizing the transformation from austenite to stress-induced martensite (martensite) during cold rolling and the reverse transformation from stress-induced martensite to austenite during subsequent heat treatment.

If the Md30 value is less than 20°C and the austenite stability is too high, there is a risk that sufficient work-induced martensite will not be generated during cold rolling. In addition, cold working of austenite promotes static recrystallization during annealing, which develops unfavorable orientations such as {100}<001>, resulting in a relative reduction in {110}<112>. On the other hand, if the Md30 value exceeds 60°C, the austenite stability becomes too low, and austenite that is reverse transformed during heat treatment may transform back into martensite during cooling. In this case, the amount of austenite is reduced, and as a result, the number of grains with {110}<112> orientation is also reduced.

Therefore, the Md30 value calculated by the following formula (i) is preferably 20 to 60° C. The Md30 value is preferably 25° C. or higher, and more preferably 30° C. or higher. The Md30 value is preferably 55° C. or lower, and more preferably 50° C. or lower.
Md30 value = 497 - 462 x (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 20 x (Ni + Cu) - 18.7Mo ... (i)
In the above formula, the element symbols represent the content (mass%) of each element in the steel, and 0 is substituted when the element is not contained.

(Production method)
There is no particular limitation on the method for producing the austenitic stainless steel sheet of the present invention, but it can be produced by the method shown below. In the production method of the present invention, the steel is melted and cast by a conventional method to obtain a steel slab to be subjected to hot rolling. This steel slab may be obtained by forging or rolling a steel ingot, but from the viewpoint of productivity, it is preferable to produce the steel slab by continuous casting. It may also be produced by using a thin slab caster or the like.

(a) Heating step Usually, the steel slab is cooled after casting and heated again to perform hot rolling. In this case, the heating temperature of the steel slab when hot rolling is 1150 ° C or higher. This is because if the heating temperature is less than 1150 ° C, coarse Nb carbonitrides may remain unmelted and become the starting point of cracks during hot working, and promote randomization of the texture during hot rolling (suppressing the formation of a desirable texture). The heating temperature is preferably 1170 ° C or higher. Although there is no particular upper limit for the heating temperature, heating to more than 1400 ° C may lead to a decrease in productivity and may cause the growth of orientations that do not develop in normal rolling, so this temperature is preferably set as the upper limit. In addition, a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting of the molten steel may be adopted.

(b) Hot rolling process In the method for producing a steel sheet of the present invention, hot rolling is terminated in a temperature range of 880 to 1000 ° C. If rolling is performed at a finishing temperature of less than 880 ° C, the deformation resistance becomes too high, significantly impairing productivity, and promoting the development of a shear layer in the surface layer of the hot-rolled sheet. The finishing temperature is preferably 900 ° C or higher. On the other hand, if the finishing temperature of hot rolling exceeds 1000 ° C, recrystallization occurs in all rolling passes, randomizing the texture of the hot-rolled sheet and reducing the {110} <112> in the surface layer. The finishing temperature is preferably 980 ° C or lower, more preferably 950 ° C or lower.

In addition, in the final two passes of hot rolling, the shape ratio L calculated by the following formula (ii) is set to 4.5 or less. If at least one of the shape ratios of the final two passes exceeds 4.5, a layer called a shear layer, which has a different crystal orientation from the central layer of the plate thickness, is formed in the surface layer of the hot-rolled plate due to friction between the steel plate and the rolling roll. Since the shear layer does not include the {110}<112> orientation, when the shear layer develops in the hot rolling stage, the {110}<112> of the surface layer is also reduced. The shape ratio L is preferably set to 4.2 or less, and more preferably less than 4.0. There is no particular lower limit for the shape ratio L, but if it is set to less than 2.5, the thickness of the hot-rolled plate becomes thick and the load of cold rolling becomes high, so this value is set as the lower limit. The shape ratio L is preferably set to 2.8 or more, and more preferably to 3.0 or more.
L = (√(R × (t _in -t _out ))) / ((2t _out +t _in )/3) ... (ii)
In the formula, the symbols have the following meanings:
L: Shape ratio at each pass R: Roll radius at each pass (mm)
t _in : Inlet plate thickness at each pass (mm)
t _out : Outlet thickness at each pass (mm)

(c) Coiling process The steel sheet hot-rolled under the above conditions is coiled at a temperature range of 900°C or less. If the coiling temperature exceeds 900°C, recrystallization will proceed during coiling, weakening the desired texture. The coiling temperature is preferably 880°C or less, and more preferably 850°C or less. There is no particular lower limit for the coiling temperature, but a temperature below 550°C not only has no particular effect, but also increases the strength of the coil, making uncoiling difficult, so this temperature is preferably set as the lower limit.

After completing the above hot rolling, the steel plate is manufactured by repeating cold rolling and annealing once or multiple times, as in the general process. In this case, only the final process is limited.

(d) Final cold rolling process When the rolling ratio of the final cold rolling is less than 40%, martensite transformation during cold rolling does not occur sufficiently, and grain refinement due to reverse transformation during the subsequent annealing does not occur. Furthermore, since a processed texture is not formed, {110}<112> does not develop. On the other hand, when the rolling ratio exceeds 90%, an orientation different from that of normal rolling develops, and in addition to reducing {110}<112>, the load on the equipment also becomes extremely high. Therefore, the rolling ratio is set to 40 to 90%. The rolling ratio is preferably 45% or more, and more preferably 50% or more. Moreover, the rolling ratio is preferably 85% or less, and more preferably 80% or less.

(e) Final annealing process When the final annealing temperature is less than 600°C, reverse transformation does not occur and a grain size of 5 μm or less cannot be achieved. On the other hand, when the final annealing temperature exceeds 1000°C, grain growth is promoted, the grain size is coarsened, and orientations other than {110}<112> develop. Therefore, the final annealing temperature is set to 600 to 1000°C. The final annealing temperature is preferably 650°C or higher, more preferably 700°C or higher. In addition, the final annealing temperature is preferably 980°C or lower, more preferably 970°C or lower.

In addition to the above manufacturing conditions, the hot-rolled sheet may be annealed before cold rolling. The annealing temperature before cold rolling is preferably 600 to 1000°C. This is because if the temperature is less than 600°C, the hot-rolled sheet does not soften sufficiently, which increases the processing load during cold rolling, and if the temperature exceeds 1000°C, the grain size becomes coarse and static recrystallization progresses.

In addition, after the final annealing, cold rolling (temper rolling) may be performed to adjust the mechanical properties of the steel sheet, followed by heat treatment to reduce residual stress (strain relief) that causes changes in the shape of the sheet and reverse transformation to the gamma parent phase. The temper rolling ratio is preferably 50% or less. This is because at 50% or less, it is possible to adjust the required mechanical properties as specified in JIS standards (G4305) and the like. The heat treatment temperature is preferably 600°C to 900°C, and more preferably 650°C to 850°C. This is because the strain relief effect cannot be obtained at temperatures below 600°C, and reverse transformation does not occur. In addition, at temperatures above 900°C, the effect of performance adjustment by cold rolling disappears.

<Aluminum alloy plate>
The aluminum alloy sheet constituting the clad material of the present invention has a predetermined metal structure. The metal structure, chemical composition, and manufacturing method of the aluminum alloy sheet will be described below in that order.

(Metal structure)
X-ray random intensity ratio of {110}<112> orientation: 4.0 or more In a surface layer portion of the aluminum alloy plate close to the interface with the austenitic stainless steel plate, the X-ray random intensity ratio of {110}<112> orientation must be 4.0 or more. This surface layer portion of the aluminum alloy plate has many metal surfaces ({110}<112> orientation) the same as the surface layer portion of the austenitic stainless steel plate, and therefore can ensure high peel strength.

It is desirable that the grain size of the aluminum alloy sheet after it has been made into a clad material by crimping or rolling be 100 μm or less. If the grain size exceeds 100 μm, the surface properties may deteriorate, and the appearance may be impaired.

(Chemical Composition)
The chemical composition of the aluminum alloy plate is not particularly limited as long as it has the above-mentioned metal structure. However, if it is a precipitation-strengthened aluminum alloy, it is easy to increase the X-ray random intensity ratio of the {110}<112> orientation. Examples of the precipitation-strengthened aluminum alloy include 2000 series aluminum alloys (aluminum alloys with elements such as Cu added), 6000 series aluminum alloys (aluminum alloys with elements such as Mg and Si added), etc.

(Production method)
Although there is no particular restriction on the manufacturing method of the aluminum alloy sheet, it is preferable to perform cold rolling of 60% or more in order to develop a strong {110}<112> orientation. In addition, from the viewpoint of developing {110}<112> in the surface layer, it is preferable that the roll diameter when performing cold rolling is 100 mm or more.

[Method of manufacturing clad material]
The clad material of the present invention can be manufactured by stacking austenitic stainless steel sheets and aluminum alloy sheets that satisfy the above-mentioned conditions, rolling or pressing them, and further performing heat treatment to cause diffusion bonding. Since oxides may form at the interface between the layers and adversely affect the bonding strength, it is preferable to perform the rolling or pressing and heat treatment under conditions that make it difficult for oxides to form on the surface of each layer, such as in a vacuum. In addition, it is preferable that the rolling or pressing and heat treatment satisfy the following conditions.

The rolling or pressure bonding is preferably carried out at a temperature of 10 to 450° C. A temperature below 10° C. not only fails to provide any particular effect but also leads to an increase in deformation resistance and an increase in costs, so this is set as the lower limit. On the other hand, a temperature above 450° C. may lead to rapid grain growth of the aluminum alloy sheet, resulting in deterioration of the surface properties and a decrease in the degree of integration in the {110}<112> orientation.
It is preferable to apply a pressure of 20 to 200 MPa during compression bonding. If the pressure is less than 20 MPa, sufficient peel strength cannot be obtained. On the other hand, if a pressure of more than 200 MPa is applied, no particular effect is obtained and costs increase, so it is desirable to set this value as the upper limit.

The reduction ratio (reduction of area) during rolling should be in the range of 5 to 30%. It is industrially difficult to control the reduction ratio at less than 5%, and there is a possibility that sufficient peel strength may not be obtained, so this value is set as the lower limit. On the other hand, if the reduction ratio is more than 30%, shear deformation occurs at the interface between the austenitic stainless steel plate and the aluminum alloy plate, and crystal rotation occurs in the surface layer of the soft aluminum alloy plate, reducing the accumulation in the {110}<112> orientation. For this reason, it is desirable to set the upper limit of the reduction ratio to 30%.

After rolling or bonding, a further heat treatment may be performed. In this case, the heat treatment temperature should be 150°C to 450°C. If the heat treatment temperature is less than 150°C, sufficient diffusion does not occur and the properties do not change, so this temperature is the lower limit, and so it is desirable to set the heat treatment temperature at this temperature. On the other hand, if the heat treatment temperature exceeds 450°C, the grains of the aluminum alloy plate will grow significantly and the surface properties will deteriorate, so it is desirable to set the heat treatment temperature at 450°C or less.

In addition, clad materials with three or more layers can be manufactured, for example, by manufacturing a two-layer clad material using the above method, and then laminating and diffusion bonding other austenitic stainless steel sheets and aluminum alloy sheets.

The types of austenitic stainless steel shown in Table 1 were melted and produced using the process shown in Table 2 to produce austenitic stainless steel sheets having the metal structure shown in Table 2. In Table 2, "SRT" means the heating temperature before rolling, "L1" means the shape ratio in the pass immediately before the final pass, "L2" means the shape ratio in the final pass, "FT" means the rolling end temperature, "CT" means the coiling temperature, "CR" means the rolling ratio in the final cold rolling, and "AT" means the final annealing temperature.

For the aluminum alloys, commercially available alloy sheets shown in Table 3 were procured. Note that for B4-2 in Table 3, a commercially available alloy (0.4 mm thick) was cold-rolled at a rolling reduction of 70% and then heat-treated at 500°C for 30 minutes.

The average grain size of austenite grains and the area ratio of martensite in the surface layer of the austenitic stainless steel sheet were measured as follows. First, the surface parallel to the surface of the surface layer of the steel sheet was measured by EBSD. Then, among the regions determined to have an fcc structure, the region surrounded by a boundary with an orientation difference of 15° or more was regarded as one crystal grain, and the average area S per crystal grain was calculated from the number of crystal grains contained in a given area. Then, the average grain size D of the austenite grains was calculated from the average area using the above formula (iii). Next, the regions that were not determined to have an fcc structure, i.e., had a bcc crystal structure, or were highly strained and therefore unmeasurable, were regarded as martensite, and their area ratio was calculated.

The random intensity ratio of the {110}<112> orientation in the surface layer of the austenitic stainless steel sheet and the aluminum alloy sheet was measured as follows. First, the sheet was mechanically polished and buffed, and then electrolytically polished to remove distortion, and X-ray diffraction was performed using a sample adjusted so that the measurement surface was 15 to 20 μm from the surface. X-ray diffraction was also performed under the same conditions on a standard sample that did not have accumulation in a specific orientation.

Next, an ODF was obtained by series expansion based on the {200}, {311}, and {220} pole figures obtained by X-ray diffraction. The random intensity ratio was then determined from this ODF. The X-ray diffraction of the surface layer was measured for the surface layer close to the interface of the clad plate. The X-ray diffraction in the examples was measured for the austenitic stainless steel plate and aluminum alloy plate before rolling or bonding, and it was confirmed that the measured values were equivalent to the measured values of the austenitic stainless steel plate and aluminum alloy plate for the clad plate.

The resulting austenitic stainless steel sheets and aluminum alloy sheets were combined in various ways and then rolled or pressed to produce clad sheets.

The rolling was performed by heating the material to 350°C in an Ar atmosphere furnace, then removing it from the atmosphere and rolling it at a reduction rate of 20%. It was then heated again to 350°C and heat-treated by holding it at that temperature for 30 minutes.

The bonding was performed by stacking a 50 mm x 50 mm austenitic stainless steel plate and an aluminum alloy plate, applying a stress of 50 MPa, heating to 350°C in a vacuum, and holding at that temperature for 30 seconds. Table 4 shows the combination of clad plates, the peel strength obtained in a peel test conducted after heat treatment, and the grain size of the aluminum alloy.

The peel test was carried out according to the T-peel test method described in JIS K 6854:1999. A peel test piece measuring 10 mm in width and 150 mm in length was used, and the test was carried out at a tensile speed of 180 mm/min. The target peel strength was 20 MPa or more.

The grain size of the aluminum alloy in the clad plate was measured by EBSD in an area of 1000 μm × total thickness of the aluminum alloy plate from the plate thickness cross section of the clad plate. At this time, among the areas determined to have an fcc structure, an area surrounded by a boundary with an orientation difference of 15° or more was considered as one crystal grain, and the average area S per crystal grain was calculated from the number of crystal grains contained in a given area. The average grain size D of the aluminum alloy was then calculated from the average area using the above formula (iii).

As shown in Table 4, clad plates No. 1 to 8, which satisfy the conditions of the present invention, had sufficient peel strength, but No. 9 to 13, in which the random strength ratio of the {110}<112> orientation of the surface layer of the austenitic stainless steel plate or aluminum alloy plate is outside the range of the present invention, did not have sufficient peel strength.

According to the present invention, it is possible to obtain a clad material having excellent peel strength, which is composed of an austenitic stainless steel plate and a metal plate made of aluminum or an aluminum alloy. This clad material is suitable as a material for press-molded parts for electronic devices, such as the housings of mobile electronic devices.

Claims (4)

A clad material of an austenitic stainless steel plate and an aluminum alloy plate,
The austenitic stainless steel plate contains Nb: 0.01 to 0.50%, and has an Md30 value calculated from the following formula of 20 to 60 ° C.,
In a surface layer portion of the austenitic stainless steel plate 15 to 20 μm from the interface with the aluminum alloy plate, an area ratio of martensite is 5.0% or less, and an X-ray random intensity ratio of {110}<112> orientation is 8.0 or more,
In a surface layer portion of the aluminum alloy plate 15 to 20 μm from the interface with the austenitic stainless steel plate, an X-ray random intensity ratio in a {110}<112> orientation is 4.0 or more.
Clad material.
Md30 value = 497 - 462 x (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 20 x (Ni + Cu) - 18.7Mo
In the above formula, the element symbols represent the content (mass%) of each element in the steel, and 0 is substituted when the element is not contained. The chemical composition of the austenitic stainless steel sheet is
In mass percent,
C: 0.005 to 0.15%,
Si: 1.0% or less,
Mn: 1.5% or less,
Cr: 15.0 to 20.0%,
Ni: 6.0 to 15.0%,
N: 0.005 to 0.15%,
Nb: 0.01 to 0.50%,
Mo: 0 to 2.0%,
Cu: 0 to 1.5%,
V: 0 to 0.15%,
Ti: 0 to 0.30%,
B: 0 to 0.010%,
The balance is Fe and impurities.
The clad material according to claim 1 . The total thickness of the clad material is 0.6 mm or less.
The clad material according to claim 1 or 2 . A clad material of an austenitic stainless steel plate and an aluminum alloy plate,
In a surface layer portion of the austenitic stainless steel plate 15 to 20 μm from the interface with the aluminum alloy plate, an area ratio of martensite is 5.0% or less, and an X-ray random intensity ratio of {110}<112> orientation is 8.0 or more,
In a surface layer portion of the aluminum alloy plate 15 to 20 μm from the interface with the austenitic stainless steel plate, an X-ray random intensity ratio in a {110}<112> orientation is 4.0 or more,
The peel strength measured according to the T-peel test method described in JIS K 6854:1999 using a peel test piece having a width of 10 mm and a length of 150 mm at a tensile speed of 180 mm/min is 20 MPa or more.
Clad material.