EP0400031B1 - Cold-rolled sheet or strip and process for manufacturing them - Google Patents

Description

The invention relates to a method for producing a Sheets or strips as well as a sheet suitable for deep drawing or tape according to the preambles of claims 1 and 5.

For deep drawing of rotationally symmetrical steel parts cold-rolled strip or sheet as free of texture as possible, quasi-isotropic forming is possible and the drawn part is flawless. This means that a z. B. cylindrical deep-drawn part has no wavy edge.

Complete tip freedom is only of isotropic material without segregations, without non-metallic inclusions, without pearl cord-like cementite precipitates and pan-cake-free Expected structure. Therefore, the following description only the term "low-tip" also for according to the prior art "zip-free" tape used.

Details are given in "Blech, Rohr, Profiles" 9/1977, pp. 341 - 346 the cause of the tip formation is described and a measure of the relative tip height Z and the flat anisotropy delta r Are defined. Results with a value of zero would be ideal (tip-free material).

The value for the plane anisotropy is calculated from the Anisotropy r for different expansion behavior of the Material in the rolling direction as well as under 45 degrees and 90 degrees. Different are for different deep-drawing properties r values adjustable.  

For the steels mentioned in the publication, Tip-free material only by normalizing the cold-rolled Band in a continuous annealing at about 1000 degrees Celsius reach, 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 reach approx. ± 0.1.

For a strip that has not been normalized, it is only a low-corner Condition due to compromises in the conduct of the process To achieve sheet metal production. The should Final rolling temperatures approx. 750 degrees Celsius and the cold rolling degrees either below 25% or above 80% and with than for Pointed unfavorably designated Recrystallization temperatures of over 600 degrees Celsius be worked.

It is also described that normalization is not in the federal government, but can only be done in a continuous annealing, because with the high temperatures the tapes would stick together.

DE-OS 32 34 574 is a generic for deep drawing suitable cold-rolled steel sheet or steel strip is known. The Titanium content should, depending on the carbon content, Oxygen, sulfur and nitrogen, to values up to 0.15% can, the reel temperature over 700 degrees Celsius or at least 580 degrees Celsius with subsequent Hot strip heating to over 700 degrees Celsius. Farther a cold rolling degree of 70 - 85% and a continuous annealing 700 - 900 degrees Celsius with a maximum holding time of two minutes recommended. There will be no information on the formation of the material given.  

US Pat. No. 4,125,416 discloses, inter alia, a process 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. With a degree of deformation of 67%, a high-strength cold strip with a yield strength greater than 48.1 kg / mm² is produced, which is then recrystallized in the coil or in a continuous annealing.

From EP-A1-101 740 for a generic cold rolled steel a slab heating temperature less than 1100 degrees Celsius, a final roll temperature of below Ar₃, Coil temperatures of 320 - 600 degrees Celsius and cold rolling degrees 50 - 95% and recrystallizing continuous annealing recommended. A steel with a maximum of 0.005% carbon, maximum 0.004% nitrogen and maximum 0.02% niobium in combination with one or more of the elements aluminum, chrome, boron or Find tungsten. High average r values are achieved above 1.2. Indications of the rickety of the material after the Deep drawing is not disclosed.

Another process for producing deep-drawing steels with slab annealing temperature less than 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 furnaces are disclosed in EP-B1-120 976. The The process should achieve r values around 2; Values for tip formation are not disclosed.

It is well known that hot strip is a good quasi-isotropic Formability has, but not sufficient Surface quality and too large tolerances and also not is produced in thicknesses below 1.2 mm.

The invention is therefore based on the object Corner-free or at least low-corner deep-drawn sheet from steel strip and a corresponding manufacturing process to propose in the case of continuous annealing at temperatures dispensed above A₁, but still produced inexpensively can be.  

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 according to the invention a recrystallizing annealing for the mentioned steel Federal in the hood furnace is sufficient to the steel belt or the assembled steel sheet excellent deep drawing properties, especially extreme extreme poverty.

The usually in the prior art for the steel St 4 NZ or RSt 14 grain size values achieved by normalizing at best ASTM 8 corresponding to 490 µm² can by Process according to the invention by recrystallizing annealing fall below, with additionally low Yield strength values can be maintained by choice corresponding degrees of cold rolling depending on the titanium content. This gives the advantage that high investments for a Continuous annealing for a normal annealing treatment can be dispensed with can.

By varying the addition of titanium in the specified Virtually any desired degree of cold rolling for the Set generation of tip-free material and / or exactly the same also a yield strength between 175 and 450 N / mm² Tensile strengths from 310 to 520 N / mm².

One of the causes of the favorable properties of the produced Bleches can be seen in the early formation of titanium nitride, so that a pan-cake structure during the recrystallizing Annealing caused by the aluminum nitride precipitates can.  

By choosing low reel temperatures around 520 degrees Celsius surprisingly hot strip qualities were achieved, which after the Cold rolling ensured a tip-free material and one enabled additional grain refinement.

A particular advantage of the hot strip produced in this way is in that in principle no restriction with regard to the subsequent cold rolling, provided the degree of cold rolling is at least about 5%, i.e. above the known critical weak cold deformation that remains at Recrystallization annealing leads to coarse grain. So far you were in the production of approximately strip-free cold strip to certain Cold rolling grades bound, unless normal annealing should be carried out.

It was surprisingly found that a certain titanium content in the steel alloy is essential to the invention To be able to carry out methods and inventive Achieve material properties, but these process parameters then at least with regard to the degree of cold rolling, if the steel alloy contains the strength-increasing element niobium will be added.

The variation of the cold rolling degrees depending on the amount of alloyed titanium is combined with the addition of niobium in the specified limits on cold rolling grades of 45 to 85% limited.

The addition of niobium does not hinder early education of titanium nitride, so that even in this invention Steel alloy a pan-cake structure during the recrystallizing glow cannot arise.  

A serious technical and economic importance of the Invention lies in the use of sheet metal for rotationally symmetrical deep-drawn parts such as needle bearing cages, Pulley halves, etc. The sheet according to the invention can in these cases without substantial rework such as cutting off the Tip are used. The point poorness prevents Thermoforming also creates sectoral wall weaknesses, so that the drawn parts are unbalanced when rotating. Other advantages of low-corner or corner-free cold strip are known, so that a further description is unnecessary.

Some embodiments are the result of Clarify the inventive method.

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 the pickling, strips were reduced by cold rolling in various stages from 10% to 80% to the thickness of the sheet and rewound. The bundle 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 wells, 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 of the corners with the conventional corner measuring devices, in particular of corner-poor and Tip-free wells with small height differences are problematic even with the smallest deep-drawing 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 in the range of 30 - 50% cold rolling degrees can at best be described as flake-free. For the comparative steel F, which was coiled at 500 degrees Celsius according to the state of the art, cornering was found at degrees of cold rolling greater than 30%.
The photos in Figures 8 and 9 demonstrate this impressively.

When using the steels A - D rolled and annealed according to the invention, the cells showed a different deep-drawing result 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 degrees of Epsilon = 30 - 50%, while cold rolling degrees of 20% or 60% only made it possible to pull wells 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 with Epsilon = 60 - 70% or 20%
Pointed arm at Epsilon = 15%, 25%; 55%; 80%
From the comparison of the curves for steels A - D, tendencies can be read that can be expected for intermediate values of the alloying element titanium, e.g. 0.025% Ti - starting from steel B - cup-free drawing at cold rolling grades of up to 15% or 20% and up to 85% a curve shift to the right; conversely, values between 0.01% and 0.02% suggest a shift in the "corner-free" cold rolling degrees to lower forming ratios.

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

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

Example: Steel B

a) Point-free with cold rolling degree 10% - 15% ≙
Yield strength level R _p0.2 = 400 - 350 N / mm²
Tensile strength level R _m = 450 - 400 N / mm²

b) Pointedness at the degree of cold rolling 30%
R _p0.2 = 180 N / mm² and R _m = 320 N / mm²

c) Point-free with a cold rolling degree of 50 - 80% ≙
R _p0.2 = 250 - 280 N / mm² and R _m = 360 - 370 N / mm²

This knowledge enables a component or function adapted Choice of 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 invention achieved grain size in ASTM units; the achievable Grain refinement compared to steels without titanium addition after The state of the art is considerable 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). By comparison, the hot strip values for the grain size (ASTM 9-10) were included in FIG. 12 for steels AD.
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 negatively affect the grain size or the level anisotropy Delta r, an increase in the reel temperatures 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 Cups made of 180 mm rounds with 100 mm stamps at 50 kN Retention force were deep drawn.

Table 1 also lists the melt analyzes of the steel G with 0.01% titanium, H with 0.02% titanium and I with 0.03% titanium with 0.05% and 0.06% addition of niobium in the process , a comparative steel K with 0.05% niobium addition, but without titanium content was listed.
Slabs 220 mm thick were cast in the strand from the melts G - I according to the invention and the comparative melt K. After heating in the pusher furnace to 1250 degrees Celsius, the slab was rolled out into 4 mm thick hot strip and coiled and cooled to room temperature. The final roll temperature was 880 degrees Celsius and the reel temperature was 510 degrees Celsius. After pickling, the strips were reduced by cold rolling in various stages from 10 to 80% to sheet thickness and reeled. After coiling, the tightly wound coil was heated to 700 degrees Celsius in the Ludwig annealing furnace and recrystallized annealing at throughput rates of 1.1 tons or 1.8 tons per hour, then cooled to 120 degrees Celsius in the annealing furnace. After tempering with a degree of deformation of 1.1%, the strip was made into sheet metal. Sheet blanks 90 mm in diameter were deep-drawn into cups using drawing dies of 50 mm in diameter (FIGS. 13-16).

For the comparative steel K, which is not titanium in the alloy contains, otherwise belongs to the generic type of steel, shows Fig. 16 clearly shows that none of the tried cold rolling degrees Tip-free deep drawing was possible.  

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 area = 55 to 85% almost tip-free in the area 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 steels produced according to the invention, for example with a titanium content of 0.01% on Ready-to-draw sheet metal yield and tensile strength values are found to be more than 50 N / mm² above the characteristic values of the only titanium alloy material.

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 tape length to the end of the tape (position 1). stole Tw ° C Th ° C K min / max Figure A 860 490 10/7 3rd B 870 500 11/9 4th 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 stole 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

Tw

Walzendtemperatur

Th

Haspeltemperatur

K

Korngröße nach ASTM

Pg

Glühdurchsatz

Δr

ebene Anisotropie

In tables 2 and 3 mean

Tw

End of roll temperature

Th

Reel temperature

K

Grain size according to ASTM

Pg

Annealing throughput

Δr

flat anisotropy

Claims (8)

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 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 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 %.
6. Sheet or strip which is suitable for deep-drawing according to claim 5, characterised in that the titanium content corresponds to at least 3.5 times the nitrogen content.
7. 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.
8. Use of a steel according to claim 1 or 3 for the production of deep-drawn, preferably rotationally symmetrical parts.