FR2546908A1 - Ferritic stainless steel sheets for producing deep-drawn and polished articles such as saucepans - Google Patents

Abstract

A grade of ferritic stainless steel sheet making it possible to avoid or very greatly limit the polishing work after deep-drawing of a receptacle such as a saucepan. The sheet of the invention, 0.4 to 1.5 mm in thickness, has a crystalline orientation texture of cubic type on top close to {111} < 112 > in the case of which the crystals in most cases have a normal to the unit face of the crystal lattice basic cube situated in the plane normal to the sheet and containing the rolling direction. Its composition is preferably (% by weight): C </= 0.06, Si </= 0.8, Mn </= 1.0, Zr 0.15 to 0.9, Cr 14.0 to 19.0 and other impurities. The sheet of the invention is employed for varied deep-drawn and polished receptacles and in particular for kitchen receptacles with or without a brazed diffuser bottom.

Description

FERRITIC STAINLESS STEEL SHEETS FOR CONSTRUCTION
Stamped and polished parts such as casseroles
The present invention relates to the field of rolled sheets and their ability to draw as well as the use of these sheets for the manufacture of pressed and polished containers such as saucepans.

The present invention relates more precisely to ferritic stainless steel sheets at approximately 17 Cr and a particular quality of these sheets making it possible to avoid or very greatly limit the polishing work usually necessary after stamping of a container intended for sale. , for example a saucepan.

I1 is already known to use for the manufacture of deep-drawn containers and in particular saucepans, provided or not with a brazed diffuser bottom optionally protected by a brazed lower plate of stainless steel sheets of austenitic stainless steel or ferritic stainless steel. 17 Ó Cr approx. Ferritic stainless steel sheets at 17 Ó
Cr approximately of the AISI 430 type may be preferred over austenitic stainless steel sheets, for example of the AISI 304 type, for reasons of cost and / or thermal conductivity reasons. However, stainless steel sheets at approximately 17 Ó Cr of type AISI 430 give on the body of the stamping very apparent corrugations on the outside or "crumpling". I1 is then necessary to introduce into the manufacturing process a very important polishing work when one wants to obtain a product of uniform appearance.

Ferritic 17 Ó stainless steel sheets have also been used for the manufacture of deep-drawn vessels and in particular saucepans.
Cr + Ti of the type usually called "430 Ti". Compared to sheets of the AISI 430 type, these sheets of the “430 Ti” type in fact have, with superior corrosion resistance, an improved ability to draw. However, the use of "430 Ti" sheets is made expensive by the appearance of particular micro-corrugations on the barrel of the stamping, which necessitate a polishing work considered to be prohibitive.

The high polishing due to the corrugations or micro-corrugations mentioned for the two types of grades "AI SI 430" and "430 Ti" is particularly troublesome for the organization and economy of the manufacture of stamped containers in ferritic stainless steel and especially pots and pans.

A light polishing is carried out regardless of the stamped stainless steel container manufactured to obtain the polished surface finish usual for sale and favorable for the maintenance of the containers, and in the case of a cooking container fitted of a brazed diffuser base - the brazing operation typically comprising a stay of the brazed base and the neighboring metal between 650 and 8500C for 1 to 2 minutes - the polishing required depends not only on the corrugations or micro-corrugations of the barrel. stamping but still oxidation from the brazing operation.

The present invention relates to a quality of ferritic stainless steel sheet, the use of which for the manufacture of stamped containers such as saucepans, with or without a brazed diffuser base, makes it possible to very significantly reduce the importance of the polishing (s) required. when using the usual grades of ferritic stainless steel at 17 v Cr, such as sheets of the type AISI 430 or "430 Ti". This quality of sheet metal, usually between 0.4 and 1.5 mm thick, is first identified by its orientation texture, this particular texture being strongly responsible for minimizing the surface ripples or microwaves that occur. during deformation and especially during stamping. Such undulations, corresponding to the phenomenon of "stringing" or "crumpling" are difficult to polish because they in fact require a real work of abrasion, while roughness or micro-surface roughness is easier to polish.

The quality of ferritic stainless steel sheet that is easy to polish after stamping is thus identified by its orientation texture, the invention then relates to such a sheet of particular composition - which we will call "Z" hereinafter - this sheet having the type of favorable texture for ease of polishing after stamping and further resulting in "total waviness depths or deviations Wt" according to the standard
NF E 05-015 particularly weak. This composition "z" is (by weight)
c # 0.06 P # 0.045
If 4 # 0.08 MB # 0.8
Mn # 1.0 Cu # 0.4
Zr 0.15 to 0.9 Ni # 0.3
N 4 0.03 Cr 14.0 to 19.0
S <0.03 sum of other elements except
Fe: # 0.2
Fe balance with preferably: Zr 0.25 + 6 (C + N) and 4 0.9, the stainless steel then being stabilized against intergranular corrosion by Zr, and by further Cr = 15.0 to 18.0. "Other elements" denote impurities or minor additions.

The manufacture by rolling of cold-rolled ferritic stainless steel sheets of composition "Z" begins with hot rolling, carried out as is known for common ferritic stainless steels at 17 Ó Cr, from a preheating to about 1050 and 120D0C, on a continuous train, the end of hot rolling temperature of the product being of the order of 9000C -
The hot-rolled strip, with a thickness of between 2 and 6 mm, preferably between 2.5 and 4 mm, is annealed between 800 and 10000C under low-oxidizing conditions, then shot-peened and pickled. The presence of zirconium nitrides in the steel contributes to obtaining fine grains
Cold rolling reduces the thickness of the annealed strip, with or without intermediate annealing, down to the delivery thickness between 0.4 and 1.5 mm and the strip then undergoes a final annealing on the run followed by a " Skin-pass "- finishing and work hardening pass - producing an elongation of less than 1 o. Stripping can optionally be carried out at the intermediate and final stages.

The tests which follow and the drawings which illustrate them make it possible to describe the invention in more detail.

FIG. 1 represents the figure of poles (, 203 obtained on a sample of composition of the type "430 Ti" cold rolled from 3 mm to 0.5 mm thick, with intermediate annealing at a thickness of 1 mm, and annealing. in final.

FIG. 2 represents the pole figure obtained on another sample of the same composition, cold rolled from 3 mm to 0.5 mm thick without intermediate annealing and final annealing.

FIG. 3 represents the axial half-section of a test stamping of the pan type.

FIG. 4 represents the orientations of the planes containing generators along which the ripple measurements are located.

FIG. 5 gathers the measurements of undulations "Wt" according to the generatrices of the various stamped containers of the tests, according to the orientations of FIG. 4.

FIG. 6 groups together the curves of undulations "Ut" along the generatrix through these various containers as a function of the level.

FIG. 7 represents the pole figure 12009 obtained on a sample of composition "Z" cold rolled from 3 mm to 0.5 mm thick, with intermediate annealing at a thickness of 1 mm, and annealing at the end.

FIG. 8 represents the figure of blades obtained on another sample of the same composition, cold rolled, from 3 mm to 0.5 mm in thickness without intermediate annealing, and annealing at the end.

1st test: a comparison was made in tensile deformation of samples of sheet of the type ex430 Ti "cold-rolled 3 mm thick to 0.5 mm thick and annealed at the end, with or without intermediate annealing.

The traction was carried out along an axis located 300 from the rolling direction, on an ISO specimen of width x useful length = 12.5 x 50 mm, with an elongation of 15 n. It is known that such a disorientation with respect to the rolling direction accentuates the deformation roughness.

The test pieces marked A and B were observed qualitatively by eye and with a magnifying glass.
A - rolled from 3 to 1 mm and from 1 to 0.5 mm with an intermediate annealing at a thickness of 1 mm: "medium corrugations.

B - rolled from 3 to 0.5 mm without intermediate annealing: "a lot of undulations".

We then took up non-traction sample portions, that is to say as rolled + annealed, and polished specimens were prepared after removing 1/4 of the thickness for texture examination.

We obtained the pole figures a2003 reproduced in figure 1 (sample A) and in figure 2 (sample B), on which the vertical axis marked "D.L." represents the rolling direction and the horizontal axis marked "D.T." represents the transverse direction, 900 from the previous one.

In Figure 1, for 11 sample A rolled with intermediate annealing, as shown in Figure 1, for example, the contours of the levels of the distribution of the orientations of the elementary crystals of the family (1) are observed, a texture of crystalline orientation of the cube type on vertex close to {111} (112), for which the crystals most often have a normal to an elementary face of the cube dB base of the crystal lattice located in the plane normal to the sheet and - containing the rolling direction.

Sample A gives less ripples than B after traction.

In FIG. 2, for the sample A rolled without intermediate annealing, as shown in particular by the level contours of the family (2,2 '), the texture splits, the crystals being arranged with a certain tilting relative to the previous arrangement, which results in the existence of sets of crystals whose normals are tilted either to the left or to the right of the previous plane. This arrangement is at the origin of the phenomenon of undulations or micro-corrugations, and B gives distinctly more undulations than A after traction.

The final thickness reduction rates or thickness reduction rates between the last two "rf" anneals are defined by the relation
rf = li - lf x 100 in%
1.

where 1 is the thickness before rolling which takes place after the penultimate annealing, and where "lf" is the thickness at the end of cold rolling, before final annealing.

The reduction rate "rf" of A is 50 Ó while that of B is 83 "0. B further deformed before final annealing gives significantly more ripples than A in tensile strain, and as has been shown texture examinations B has a split texture (Fig. 2).

2nd test: deep-drawn parts of the pan type, with a directrix shown in figure 3, were made from three grades of sheet metal:
C - type AISI 430 - thickness 0.5 mm - cold rolled from 3 mm to 0.5 mm thick with intermediate annealing at a thickness of 1 mm (rif = 50 o)
D - type "430 Ti" - thickness 0.5 mm - same rolling range (rf = 50%).

E - type "430 Ti" - thickness 0.8 mm - cold rolled from 3 mm to 0.8 mm thick without intermediate annealing (r f = 73).

These stamped parts (3) had a diameter D = 160 mm and a height
H = 78 mm, they had at their upper part a border or collar (4). Measurements of undulations according to the generator were carried out on lengths (5) of 5 mm, the upper ends (6) of which are located 10 mm below the border (4) (figure 3), in 4 planes passing by the axis of symmetry of the stamping, whose orientations at 300 - 450 - 600 and 900 of an origin plane "0" corresponding to the rolling direction "L", are shown diagrammatically in FIG. 4. The plane at "900" corresponds to the direction perpendicular to the rolling direction or transverse direction and is marked with the letter "T".

In all cases, the micro-roughness measurements were carried out in accordance with standard NF E 05-1115, with a constant low-pass filter, in accordance with standard NF E 05-017, and the comparisons relate to the total undulation depth or deviation t defined by NF E 05-015.

In addition to the measurements on the lengths (5), the results of which are shown in the graph of FIG. 5, measurements of corrugation were carried out in the transverse direction of the sheet over lengths of 10 mm, along the generator. from plane "T" to 900 (figure 4), this at levels "d" (figure 3 of 25 - 50 and 70 mm above the base of the container. The results of these measurements are represented by the curves of the figure 6.

In FIG. 5, the curves of undulations "W" according to the generatrices of the container as a function of the orientation are marked as follows "17/50" = type AISI 430 or 17 Z Cr thickness 0.5 mm with rf = 50 m
(sheet C). "T / 50" = type "430 Ti" 0.5 mm thick with rf = 50 OD (sheet D). "T / 73" = type "430 Ti" thickness 0.8 mm with rf = 73 á (sheet E).

In FIG. 6, the curves of undulations "Wt" following the generatrix across the container as a function of the level "d" are marked in the same way.

By considering the results of the two series of corrugation measurements (figures 5 and 6) and in particular the maximum corrugation values, it is concluded that the sheet D (curve "T / 50"), rolled according to the same process as sheet C (curve "17 / 50'D) seems to have an aptitude for polishing after stamping close to that of the latter sheet, and being differentiated above all by a less heterogeneity. In addition, the difference in thickness of E ( curve "T / 73") (thickness 0.8 mm) and of D pu 1, T / 50 "(thickness 0.5 mm) having only a relatively small influence on the corrugation depths" Wt "compared , it is concluded that the increase in "rf" from 508Ó to 73 S resulted for the type sheet 't430 Ti "by a notable increase in the depths of undulation, particularly important in the case of measurements according to the generators reported on figure 5.

This degradation of the ability to polish after stamping of the sheet E linked to the increase in “rf” corresponds to the degradation qualitatively observed in the first test, going from A to e, degradation which was also observed on the texture.

3rd test: we compared in tensile deformation, as in the first test, samples of sheet of composition "Z" cold-rolled 3 mm thick to 0.5 mm thick and-annealed in the end, with or without intermediate annealing during this rolling
F - rolled from 3 to 1 mm and from 1 to 0.5 mm with an intermediate annealing to the thickness of 1 mm (rf = 50 S).

and G - rolled from 3 to 0.5 mm without intermediate annealing (rf = 83 m).

After traction, the qualitative observation gave as a notation "few undulations, but a certain roughness", that for F and for G, this qualitative observation not distinguishing them.

Texture examinations were then carried out on portions of samples.
F and G not towed, as for A and B (first test) We obtained the pole figures t200} reproduced in figure 7 (sample F) and in figure 8 (sample G) The evolution between the two pole figures is relatively little marked, unlike the change between the pole figures obtained on samples A and B (figures 1 and 2). And despite the slight splitting (6,6 ') (figure 7), the two crystalline orientation textures are of the cube type on vertex close to {111} <112>, of which all the crystals have a normal to an elementary face of the base cube of the crystal lattice located in the plane normal to the sheet and containing the rolling direction. In addition, the texture of figure 7 (sample F) appears more favorable than that of figure 1 (sample A), the anisotropy being little marked.

According to these initial reviews, the. Cold-rolled sheets of "Z" composition have better post-deformation polishing ability than "430 Ti" type plates, and this ability deteriorates less as "rc" increases.

4th test: stamped parts of the same geometry as previously were produced (2nd test, FIG. 3) and measurements of “Wt” undulations were made under the same conditions and at the same locations.

The results of these measurements are shown in Figures 5 and 6 to facilitate comparisons. The qualities and markings of the curves and the sheets are as follows. "Z / 50": sheet H of composition "Z" and 0.5 mm thick - rolled to
cold 3 mm thick with intermediate annealing at 1 mm thickness
(rf = 50 Ó).

. 'Z / 73 ": cold rolled sheet J of composition" Z "and 0.8 mm thick
from 3 mm to 0.8 mm thick without intermediate annealing (rf = 73 Ó).

Sheet H (curve "Z / 50") was rolled like sheets D (curve "T / 50") and C (curve "17/50").

Sheet J (curve "Z / 73") was rolled like sheet E (curve "T / 73").

The measurements of undulations "Ut" of FIG. 5 show that the results obtained from the sheets H (curve "Z / 50") and J (curve "Z / 73") are both close and better than those obtained at from sheet D (curve "T / 50"), these results are close to those obtained from sheet C (curve "17/50") for orientations 30 to 60 ", they are much better between 600 and 90.

The "Wt" ripple measurements in figure 6 show a little more difference between the H ("Z / 50" curve) and J ("Z / 73" curve) plates, but the "Z / 73" curve is still below the "T / 50" curve.

For all the measurements made on the containers stamped from sheets H and J (curves "Z / 50" and "Z / 73" of figures 5 and 6) the corrugations "Wt" are between 4 and 7 jjm and the importance of the undulations is little affected by the orientation (figure 5) or by the level (figure 6).

Three surprising advantages are therefore observed in these tests of cold-rolled sheets of composition "Z" with regard to their aptitude for polishing after stamping: - the corrugations are clearly weaker than those of the sheets of com
parison.

- the importance of the undulations varies relatively little on the same part,
it varies less than for the comparison plates.

- the increase of "rf" from 50 Ó to 73% only leads to an increase
low of the maximum value observed for "Ut": 7.4 m instead of
6.8 m. In other words, the ability to polish after stamping of
sheets of composition "Z" is only slightly affected by the rate of
final reduction in thickness by cold rolling, while the ability
corresponding to the composition sheets of the type "430 Ti" is strongly
affected (see curves "b / 50" and ~ "Tof731 'of la.fwger6-5).

This latter advantage - relating to insensitivity to an essential factor in the cold rolling procedure - is important to both the laminator and the manufacturer of stamped and polished vessels.

As it was observed on the samples of plates A and B of composition type "430 Ti" (first test) and F and G of composition "Z" (third test), the favorable behavior of plates of composition "Z" is linked to a type of texture that is found only on the "A" sheet of "430 Ti" having the lowest final reduction rate of thickness (rf = 50 m).

Sheets of composition "Z" are thus of particular interest for the manufacture of stamped and polished containers, and in particular saucepans, the more or less advanced level of polishing being in particular set as a function of aesthetic exterior appearance requirements. Another advantage of sheets of "Z" composition, in the case of culinary containers with a brazed diffuser base, is their good resistance to hot oxidation. The improvement in the resistance to hot oxidation of this type of shade, improvement due to the presence of Zr, has been described in the patent application
J 55/024 - 901.

According to the applicant's studies, this improvement thus allows a continuous or cyclic service temperature more than 100 ° C. above the service temperature corresponding to the sheets of the AISI 430 type. The tests carried out with the sheets of composition "Z" for the production of brazed bases have shown that this difference in oxidation behavior in their favor had a very significant effect in the base brazing operation: the oxidation is very slight, limited to pale yellow, with a sheet of composition "Z "- and then easy to remove by polishing - while the oxidation is greater in the case where an AISI 430 or" 430 Ti "sheet is used.

The sheets of composition "Z" cold-rolled with a final deformation rate "rif" "preferably between 40 and 80 ', therefore have a double advantage from the point of view of the polishing of deep-drawn cooking utensils with brazed diffuser base: that the virtual absence of micro-corrugations making polishing of the barrel easier, and that of the very low oxidability making polishing of the heated zone during the brazing operation also easier.

The sheets of composition "Z" of the invention are used for various stamped and polished containers, and in particular for culinary containers with or without a brazed diffuser base. In the case of these stamped containers with or without brazed diffuser bottom, the thickness of the stainless steel sheets of composition "Z" is between 0.4 mm and 1.5 mm and preferably between 0.5 mm and 1 mm. .

Claims (3)

CLAIMS 1 / Stainless steel sheet 0.4 to 1.5 mm thick for making stamped and polished parts with aesthetic exterior appearances, characterized in that it is made of ferritic stainless steel and in that it has a texture of crystalline orientation of cube type on vertex close to {111} <112>, for which the crystals most often have a normal to an elementary face of the base cube of the crystal lattice located in the plane normal to the sheet and containing the rolling direction. 2 / Sheet according to claim 1, characterized in that it has as composition (% by weight): C 40.06 - Si 40.8 - Mn 6: 1.0 - Zr 0.15 to 0.9 - N s 0.03 - S s 0.03 P 40.045 - Mo <- 0.8 - Cu <0.4 - Ni 5 0.3 - Cr 14.0 to 19.0 - sum of other elements except Fe: # 0.2 - Fe balance. 30 / sheet according to claim 2, characterized in that it preferably contains (by weight): Zr>, 0.25 + 6 (C + N) and 0.9 Cr 15.0 to 18.0 40 / Sheet according to claim 1 to 3, characterized in that it has a thickness of 0.5 to 1 mm. 50 / A method of manufacturing a sheet according to claims 1 and 2, characterized in that the hot-rolled strip with a thickness of between 2.5 and 4 mm is annealed between 800 and 10,000 C under low oxidizing conditions and then shot peened and pickled, and in that it is then cold rolled to the delivery thickness of between 0.4 and 1.5 mm, with or without intermediate annealing, the rate of final reduction in thickness by rolling being included between 40 and 80%, the strip then undergoing a final annealing at the pass followed by a "skin pass" - finishing and work hardening pass - producing an elongation of less than 1 Dó 60 / Application of a sheet according to the any one of claims 1 to 4, to the manufacture of a stamped and polished container of aesthetic exterior appearance. 70 / Application according to claim 6, characterized in that the container is used in a saucepan or a cooking utensil. 8 / The application of claim 5 or 6, characterized in that the container is used in a pan or a cooking utensil with a brazed diffuser base.