EP0400031B2 - Cold-rolled sheet or strip and process for manufacturing them - Google Patents

Description

The invention relates to a method for producing a sheet or strip and to Deep-drawing suitable sheet or strip according to the preambles of claims 1 and 5.

Texture-free cold-rolled strip is used to deep-draw rotationally symmetrical steel parts or sheet metal is used, so that quasi-isotropic forming is possible and the drawn part is free of corners. This means that a z. B. cylindrical deep-drawn part has no wavy edge.

A perfect tip freedom is only of isotropic material without segregation, without non-metallic Inclusions without expected pearlite-like cementite deposits and with a pan-cake-free structure. Therefore, in the following description, only the term "low-tip" is also used according to the state of the art Technique "corner-free" tape used.

In "Blech, Rohr, Profiles" 9/1977, pp. 341 - 346 the cause of the tip formation is described in detail and defines a measure for the relative tip height Z and the flat anisotropy Delta r. ideal would be results with the value zero (tip-free material).

The value for the plane anisotropy is calculated from the anisotropy r for different things Expansion behavior of the material in the rolling direction as well as at 45 degrees and 90 degrees. For Different deep-drawing properties, different r-values can be set.

For the steels mentioned in the publication, tip-free material can only be obtained by normalizing of the cold-rolled strip in a continuous annealing at about 1000 degrees Celsius, the sheet in the final state a grain size ASTM 8 with a relative tip height of approx. 0.3 to 0.4% and delta r approx. Reach ± 0.1.

For non-normalizing annealed strip is only a low-tip condition due to compromises in the To achieve process control in sheet metal production. The final roll temperatures should be approx. 750 Degrees Celsius and the cold rolling degrees are either below 25% or above 80% and with than for Pointed unfavorable recrystallization temperatures of over 600 degrees Celsius worked become.

It is also described that normalization is not carried out in a bundle, but only in a continuous annealing line can take place because the tapes would stick together at the high temperatures.

DE-OS 32 34 574 is a generic cold-rolled steel sheet suitable for deep drawing or steel band known. The titanium content should, depending on the carbon, oxygen, Sulfur and nitrogen, can rise to values up to 0.15%, the reel temperature over 700 degrees Celsius or at least 580 degrees Celsius with subsequent hot strip heating to over 700 degrees Celsius. Furthermore, a cold rolling degree of 70 - 85% and a continuous annealing at 700 - 900 Degrees Celsius with a maximum of two minutes holding time recommended. Notes on the formation of corners of the material are not given.

US Pat. No. 4,125,416 discloses, inter alia, a method in which hot-rolled slabs at temperatures above 830 ° C. made of alloy steel, which 0.06-0.20% C, 0.5-2.0% Mn, 0.30- May contain 0.50% Si as well as Nb and Ti in quantities of 0.01-0.10%, heated to temperatures above 980 ° C, then rolled into hot strip and coiled at 450-650 ° C.
C contents of 0.10-0.11%, a Ti content of 0.1% and an Nb content of 0.04% are specified for the production of high-strength cold strip from this hot strip. At 67% degree of deformation, a high-strength cold strip with a yield strength greater than 48.1 kg / mm ^{2 is} produced, which is then recrystallized annealed in a bundle or in a continuous anneal.

EP-A1-101 740 recommends a slab heating temperature of less than 1100 degrees Celsius, a final roll temperature of less than Ar ₃ , reel temperatures of 320-600 degrees Celsius and cold rolling degrees of 50-95% and recrystallizing continuous annealing for a generic cold-rolled steel. A steel with a maximum of 0.005% carbon, a maximum of 0.004% nitrogen and a maximum of 0.02% niobium is to be used in combination with one or more of the elements aluminum, chromium, boron or tungsten. High average r values above 1.2 are achieved. There are no indications that the material is rough after deep-drawing.

Another method for producing deep-drawing steels with a slab annealing temperature lower 1100 degrees Celsius, final rolling temperature max. 780 degrees Celsius and reel temperatures of at least 450 degrees Celsius and cold strip annealing in the hood or continuous annealing furnace are in EP-B1-120 976 disclosed. The method should achieve r values around 2; Values for tip formation are not disclosed.

It is generally known that hot strip has good quasi-isotropic formability, but one insufficient surface quality and tolerances, and also not in thicknesses below 1.2 mm is produced.

The invention is therefore based on the object of proposing a sheet-free steel sheet which is suitable for deep drawing or at least poor in drawing and a corresponding production process in which continuous annealing at temperatures above A _{1 is} dispensed with, but can nevertheless be produced inexpensively.

The object is achieved by claims 1, 3 and 5.
Advantageous developments of the invention are covered in the subclaims.

Surprisingly, it has been shown that when using the slab, annealing, Rolling and coiling temperatures for the steel mentioned a recrystallizing annealing of a coil in the Hood furnace is sufficient to give the steel strip or the assembled steel sheet excellent deep-drawing properties, in particular extreme extreme poverty.

The grain size values of at best ASTM 8 corresponding to 490 µm ² , which are usually achieved in the state of the art for steel St 4 NZ or RSt 14 by normal annealing, can be undercut by recrystallizing annealing using the process according to the invention, with additional low yield strength values being able to be maintained by selecting appropriate ones Cold rolling degrees depending on the titanium content. This has the advantage that high investments for a continuous annealing for a normal annealing treatment can be dispensed with.

By varying the addition of titanium within the specified limits, practically everyone can be Set the desired degree of cold rolling for the production of material free of corners.

One of the reasons for the favorable properties of the sheet produced is early Formation of titanium nitride can be seen, causing a pan-cake structure during recrystallizing annealing through which aluminum nitride precipitates cannot arise.

The choice of low reel temperatures around 520 degrees Celsius surprisingly made hot strip qualities achieved, which ensured a tip-free material after cold rolling and an additional Grain refinement enabled.

A particular advantage of the hot strip produced in this way is that, in principle, none There is a restriction regarding the subsequent cold rolling provided that the degree of cold rolling is at least approx. 5 %, i.e. remains above the known critical weak cold deformation that occurs during recrystallization annealing leads to coarse grain. So far, the production of almost strip-free cold strip was on certain degrees of cold rolling are bound, unless normal annealing is used.

It has surprisingly been found that a certain titanium content is essential in the steel alloy in order to be able to carry out the method according to the invention and material properties according to the invention to achieve, but then these process parameters at least with regard to the degree of cold rolling must be adjusted if the strength-increasing element niobium is added to the steel alloy.

The variation of the cold rolling degrees depending on the amount of the alloyed titanium is at simultaneous addition of niobium within the specified limits to cold rolling degrees of 45 to 85% limited.

The addition of niobium does not hinder the early formation of titanium nitride, so that this too Steel alloy according to the invention does not have a pan-cake structure during recrystallizing annealing can arise.

A serious technical and economic importance of the invention lies in the use of the Thin sheets for rotationally symmetrical deep-drawn parts such as needle bearing cages, pulley halves etc. The sheet metal according to the invention can in these cases without substantial rework such as cutting the Tip are used. The low-headedness also prevents the emergence of sectoral thermoforming Wall weaknesses, so that the drawn parts do not show any imbalance when rotating. Additional advantages Low-end or corner-free cold strip are known, so that a further description is unnecessary.

Some exemplary embodiments are intended to clarify the result of the method according to the invention.

From the melts A - D according to the invention and the comparative melts E - F (Table 1), slabs 210 mm thick are cast in the strand. After heating in the pusher furnace to 1250 degrees Celsius, the slab was rolled out into 3 mm thick hot strip, coiled and cooled to room temperature. The final roll temperatures and reel temperatures are shown in Table 2. After pickling, strips were reduced by cold rolling in various stages from 10% to 80% to thin sheet thickness and reeled. The coil was heated to 700 degrees Celsius in the Ludwig annealing furnace, recrystallized at a throughput of 1.1 t / h to 1.9 t / h and then cooled to 120 degrees Celsius in the furnace. After the tempering with degrees of deformation of 1 - 1.2%, the strip was made into sheet metal.
Sheet rounds of 90 or 180 mm in diameter were deep-drawn into cups using drawing dies of 50 or 100 mm in diameter and holding forces of 50 kN.

Figure 1 shows three different cups, which are to define the terms used in the following, pointed (Fig. 1a), low-pointed (Fig. 1b) and free (Fig. 1c), since the measurement of the height with the commercially available corner measuring devices, in particular low-cornered and Tip-free wells with small height differences are problematic even with the smallest deep-drawn burrs on the edge of the well.
This definition was adopted for Figure 10 to represent the lobes of wells from the different melts. The finding was confirmed that the steel E coiled at 710 degrees Celsius is free of flakes only at cold rolling degrees of less than approx. 25% and can only be described as low in the range 30 - 50% of cold rolling. For the comparative steel F which was coiled at 500 degrees Celsius according to the state of the art, cornering was found at cold rolling degrees greater than 30%.
The photos in Figures 8 and 9 demonstrate this impressively.

Stahl A mit 0,01 % Ti:
Die Näpfchen waren bei Kaltwalzgraden von Epsilon = 30 - 50 % absolut zipfelfrei, während Kaltwalzgrade von 20 % bzw. 60 % nur zipfelarmes Näpfchen-Ziehen ermöglichte.

Stahl B mit 0,02 % Ti:
Zipfelfrei bei Epsilon = 10 % sowie 50 - 80 %
Zipfelarm bei Epsilon = 20 %; 40 %

Stähle C1/C2 mit 0,03 % Ti, wobei C1 mit 500 Grad Celsius und C2 mit 450 Grad Celsius gehaspelt wurde:
Zipfelfrei bei Epsilon = 10 - 20 % sowie 60 - 80 %
Zipfelarm bei Epsilon = 30 %; 50 %

Stahl D mit 0,04 % Ti:
Zipfelfrei bei Epsilon = 60 - 70 % bzw. 20 %
Zipfelarm bei Epsilon = 15 %, 25 %; 55 %; 80 %

Aus dem Vergleich der Kurven für die Stähle A - D lassen sich Tendenzen ablesen, die für Zwischenwerte des Legierungselementes Titan beispielsweise 0,025 % Ti - ausgehend von Stahl B - zipfelfreies Näpfchen-ziehen bei Kaltwalzgraden bis 15 % oder 20 % und bis 85 % erwarten lassen, also eine Kurvenverschiebung nach rechts; bei Werten zwischen 0,01 % und 0,02 % umgekehrt eine Verschiebung der "zipfelfreien" Kaltwalzgrade zu niedrigeren Umformverhältnissen nahelegen.When using the steels A - D rolled and annealed according to the invention, the cells showed different deep-drawing results depending on the titanium content at different degrees of cold rolling:

Steel A with 0.01% Ti:
The cups were absolutely spot-free at cold rolling grades of epsilon = 30 - 50%, while cold rolling grades of 20% or 60% only made it possible to pull the cups with little tips.

Steel B with 0.02% Ti:
Point-free at Epsilon = 10% and 50 - 80%
Tip arm at Epsilon = 20%; 40%

Steels C1 / C2 with 0.03% Ti, whereby C1 was coiled at 500 degrees Celsius and C2 at 450 degrees Celsius:
Point-free at Epsilon = 10 - 20% and 60 - 80%
Tip arm at Epsilon = 30%; 50%

Steel D with 0.04% Ti:
Point-free at Epsilon = 60 - 70% or 20%
Pointed arm at Epsilon = 15%, 25%; 55%; 80%

The comparison of the curves for steels A - D shows tendencies that can be expected for intermediate values of the alloying element titanium, for example 0.025% Ti - starting from steel B - cup-free drawing at cold rolling grades of up to 15% or 20% and up to 85% , ie a curve shift to the right; conversely, values between 0.01% and 0.02% suggest a shift of the "corner-free" cold rolling degrees to lower forming ratios.

The photos of FIGS. 3 to 7 corresponding to the steels according to FIG. 10 and Table 1 or 2 of deep-drawn cells clearly illustrate the result.

Surprisingly, it was found that the "tip-free" degrees of deformation each have a certain tensile strength and yield point level could be assigned (Figure 11) and the greatest lobes at the same time the lowest yield strength / tensile strength was found.

Example: Steel B

a) Zipfelfreiheit beim Kaltwalzgrad 10 % - 15 % ≙

Streckgrenzenniveau R_p0,2 = 400 - 350 N/mm²

Zugfestigkeitsniveau R_m = 450 - 400 N/mm²

b) Zipfeligkeit beim Kaltwalzgrad 30 % ≙

R_p0,2= 180 N/mm² und R_m = 320 N/mm²

c) Zipfelfreiheit beim Kaltwalzgrad 50 - 80 % ≙

R_p0,2 = 250 - 280 N/mm² und R_m = 360 - 370 N/mm²

Diese Erkenntnis ermöglicht eine bauteil- oder funktionsangepaßte Wahl der Festigkeit für ein und dasselbe Bauteil durch Änderung der Parameter Titangehalt und Kaltwalzgrad.

a) Point-free with cold rolling degree 10% - 15% ≙

Yield strength level R _p0.2 = 400 - 350 N / mm ²

Tensile strength level R _m = 450 - 400 N / mm ²

b) Pointedness at the degree of cold rolling 30% ≙

R _p0.2 = 180 N / mm ² and R _m = 320 N / mm ²

c) Point-free with a cold rolling degree of 50 - 80% ≙

R _p0.2 = 250 - 280 N / mm ² and R _m = 360 - 370 N / mm ²

This knowledge enables component or function-specific selection of the strength for one and the same component by changing the parameters titanium content and degree of cold rolling.

Corresponding to FIG. 12, Table 2 shows the grain size achieved according to the invention in ASTM units; the achievable grain refinement compared to steels without titanium addition according to the state of the Technology is significant and extends to ASTM 11.

The coarsest grain was obtained with a small addition of Ti and a low degree of cold rolling (ASTM 7). For steel A-D, the hot strip values for the grain size (ASTM 9-10) were compared in FIG.
For a steel C (variants C3 - C5) tests were carried out with variable coiling temperature Th and annealing throughput Pg (Table 3). While fluctuations in the throughput of the bell annealer from 1.1 - 1.9 t / h did not have a negative effect on the grain size or the level anisotropy Delta r, an increase in the reel temperature to 710 degrees Celsius with roughly the same end temperatures resulted in coarsening and deterioration the plane anisotropy.

Figures 2a, 2b, 2c show corresponding results on wells made of 180 mm rounds, the with 100 mm stamps with 50 kN retention force were deep drawn.

Table 1 also shows the melt analyzes of the process to be used according to the invention Steel G with 0.01% titanium, H with 0.02% titanium and I with 0.03% titanium with 0.05% and 0.06% niobium addition A comparative steel K with 0.05% niobium addition but without titanium content was listed. From the melts G - I according to the invention and the comparative melt K, slabs of 220 mm thickness cast in the strand. After heating in the pusher furnace to 1250 degrees Celsius, the slab was Rolled into 4 mm thick hot strip and coiled and cooled to room temperature. The The final roll temperature was 880 degrees Celsius and the reel temperature was 510 degrees Celsius. After this The strips were pickled by cold rolling in different stages from 10 to 80% on thin sheet thickness reduced and reeled. After coiling, the tightly wrapped coil was placed in the bell annealer of the Ludwig construction type heated to 700 degrees Celsius and with a throughput rate of 1.1 tons or 1.8 Annealed tons per hour recrystallizing, then in the bell annealing furnace to 120 degrees Celsius cooled. After tempering with a degree of deformation of 1.1%, the strip became sheet metal assembled. Sheet rounds of 90 mm in diameter were closed with drawing dies of 50 mm in diameter Cups deep-drawn (Figures 13 - 16).

For the comparative steel K, which contains no titanium in the alloy, otherwise to the generic one 16 clearly shows that none of the tried-and-tested degrees of cold rolling have tip-free deep drawing was possible.

Stahl G mit 0,01 % Titan (Fig. 13):
Die Näpfchen waren bei Kaltwalzgraden von Epsilon = 45 bis 85 % in der Kategorie zipfelarm und bei etwa 60 bis 80 % Kaltwalzgraden sogar zipfelfrei.

Stahl H mit 0,02 % Titan (Fig. 14):
Zipfelarm im Bereich Epsilon = 55 bis 85 % fast zipfelfrei im Bereich von 60 bis 75 %.

Stahl I mit 0,03 % Titan (Fig. 15):
Zipfelarm im Bereich von 60 bis 70 % Kaltwalzgraden.

When the steels G to I rolled and annealed according to the invention were used, the wells showed a slightly different deep-drawing result depending on the titanium content at different degrees of cold rolling:

Steel G with 0.01% titanium (Fig. 13):
The cups were low in cold rolled grades of epsilon = 45 to 85% in the category "pointed" and even roughly 60 "to 80% cold rolled.

Steel H with 0.02% titanium (Fig. 14):
Tip arm in the epsilon range = 55 to 85% almost tip-free in the range of 60 to 75%.

Steel I with 0.03% titanium (Fig. 15):
Corner arm in the range of 60 to 70% cold rolling degrees.

In the case of the steels produced according to the invention, for example, with a titanium content of 0.01% on the deep-drawn sheet, the yield strength and tensile strength values were found to be more than 50 N / mm ² above the characteristic values of the only titanium-alloyed material.

Stahl Tw °C Th °C K min / max Figur A 860 490 10 / 7 3 B 870 500 11 / 9 4 C1 870 500 11 / 9 5 C2 880 450 11 / 9 6 D 890 430 11 / 9 7 E 900 710 9 / 4 8 F 890 500 9 / 6 9 Stahl Tw °C Th °C Pg t/h K Δr min /max Figur C3 880 520 1,1 9 - 10 -0,07/+0,06 2a C4 915 540 1,9 9 - 10 -0,04/+0,08 2b C5 870 710 1,9 8 - 9 +0,09/+0,17 2c In Tabelle 2 und 3 bedeuten

Tw

Walzendtemperatur

Th

Haspeltemperatur

K

Korngröße nach ASTM

Pg

Glühdurchsatz

Δr

ebene Anisotropie

The melts L and M according to the invention listed in Table 1 with phosphorus contents at the upper analysis limit were treated like steels A - F. The reel temperature was 510 and 500 degrees Celsius, respectively. At a cold rolling degree of 66%, the consistency of the results was checked over the entire strip length in order to confirm the effectiveness of the annealing. The wells from the deep-drawing test are shown in FIGS. 17 and 18, respectively. They show that tip-free material was produced both at the beginning of the tape (position O) and after every further quarter of the length of the tape up to the end of the tape (position 1).

steel Tw ° C Th ° C K min / max figure A 860 490 10/7 3 B 870 500 11/9 4 C1 870 500 11/9 5 C2 880 450 11/9 6 D 890 430 11/9 7 e 900 710 9/4 8th F 890 500 9/6 9 steel Tw ° C Th ° C Pg t / h K Δr min / max figure C3 880 520 1.1 9-10 -0.07 / + 0.06 2a C4 915 540 1.9 9-10 -0.04 / + 0.08 2 B C5 870 710 1.9 8 - 9 + 0.09 / + 0.17 2c In tables 2 and 3 mean

tw

rolling temperature

th

coiling temperature

K

Grain size according to ASTM

Pg

Glühdurchsatz

.delta..sub.R

flat anisotropy

Claims (7)

1. Method of producing a cold-rolled sheet or strip from steel, having good quasi-isotropic deformability and having the following composition in percentages by weight:

   0.03 - 0.08 % carbon

   max. 0.40 % silicon

   0.10 to 1.0 % manganese

   max. 0.08 % phosphorus

   max. 0.02 % sulphur

   max. 0.009 % nitrogen

   0.015 to 0.08 % aluminium

   0.01 to 0.04 % titanium

   max. 0.15 % of one or more of the elements from the group copper, vanadium, nickel, remainder: iron and inevitable impurities,

   wherein the titanium content corresponds to at least 3.5 times the nitrogen content and wherein the slab is heated to above 1120 degrees Celsius, rolled out to form a hot strip at a final rolling temperature above the Ar₃ point, wound on a reel at 520 ± 100 degrees Celsius and annealed in a recrystallising manner in the coil after the cold-rolling process.
2. Method of producing a cold-rolled sheet or strip according to claim 1, characterised in that it is cold-rolled in dependence on the titanium content with the following degrees of deformation (epsilon): approx. 0.01 % titanium epsilon 20 - 60 %, preferably 30 - 50 % approx. 0.02 % titanium epsilon 5 - 20 %, preferably 10 - 15 % or epsilon 40 - 85 %, preferably 50 - 80 % approx. 0.03 % titanium epsilon 5 - 25 %, preferably 10 - 20 % or epsilon 50 - 85 %, preferably 60 - 80 % approx. 0.04 % titanium epsilon 15 -25 %, preferably 20 % or epsilon 55 - 80 %, preferably 60 - 70 % and subsequently annealed in a recrystallising manner at temperatures below A₁ and thereafter finished with a degree of deformation of approx. 1 %.
3. Method of producing a cold-rolled sheet or strip from steel, having good quasi-isotropic deformability and having the following composition in percentages by weight:

   0.03 - 0.08 % carbon

   max. 0.40 % silicon

   0.10 to 1.0 % manganese

   max. 0.08 % phosphorus

   max 0.02 % sulphur

   max. 0.009 % nitrogen

   0.015 to 0.08 % aluminium

   0.01 to 0.04 % titanium

   max. 0.15 % of one or more of the elements from the group copper, vanadium, nickel

   0.01 to 0.06 % niobium

   remainder: iron and inevitable impurities,

   wherein the titanium content corresponds to at least 3.5 times the nitrogen content and wherein the slab is heated to above 1120 degrees Celsius, rolled out to form a hot strip at a final rolling temperature above the Ar₃ point and wound on a reel at 520 ± 100 degrees Celsius, then it is cold-rolled in dependence on the titanium content with the following degrees of deformation (epsilon): approx. 0.01 % titanium epsilon 45 to 85 %, approx. 0.02 % titanium epsilon 55 to 85 %, approx. 0.03 % titanium epsilon 60 to 70 %, and subsequently annealed in a recrystallising manner at temperatures below A₁ in the coil and thereafter finished with a degree of deformation of approx. 1 %.
4. Method according to one of claims 1 to 3, characterised in that the steel is annealed in the fixed coil after the cold-rolling process.
5. Sheet or strip, formed from steel in the above-mentioned composition, which is suitable for deep-drawing and is produced according to a method mentioned in claims 1 to 4, characterised by a recrystallised structure having a ferrite particle size finer than ASTM 7 for a titanium content of 0.01 % and finer than ASTM 9 for titanium contents of 0.015 to 0.04 % and by a titanium content which corresponds to at least 3.5 times the nitrogen content.
6. Use of a sheet or strip, which has been produced according to one of the methods according to claims 1 to 4, for the low-peak deep-drawing of, preferably, rotationally symmetrical parts.
7. Use of a steel according to claim 1 or 3 for the production of deep-drawn, preferably rotationally symmetrical parts.

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