EP0400031A1

EP0400031A1 - Cold-rolled sheet or strip and process for manufacturing them.

Info

Publication number: EP0400031A1
Application number: EP19890901844
Authority: EP
Inventors: Klaus Freier; Walter Zimnik
Original assignee: Preussag Stahl AG
Current assignee: Salzgitter AG
Priority date: 1988-01-29
Filing date: 1989-01-27
Publication date: 1990-12-05
Anticipated expiration: 2009-01-27
Also published as: ES2018975A6; DE3843732A1; DD285298B5; DE3803064C1; JPH0814003B2; DE58906176D1; DE3843732C2; DD285298A5; EP0400031B1; US5139580A; JPH03503185A; WO1989007158A1; GR1000537B; DE3803064C2; EP0400031B2

Abstract

Pour produire une tôle présentant de bonnes caractéristiques de formage, notamment pour l'emboutissage profond à symétrie de rotation, on allie un acier pauvre en carbone contenant au maximum 0,009 % de N, avec 0,01-0,04 % de Ti et dans certains cas également avec 0,01 à 0,06 % de Niob, on effectue une coulée continue, on réchauffe les brames au-dessus de 1120 degrés Celsius, on lamine pour obtenir un feuillard à chaud au-dessus du point d'Ar3 et on bobine à 520 U 100 degrés Celsius. Après laminage à froid pour obtenir l'épaisseur de tôle mince souhaitée, le feuillard acier subit un recuit de recristallisation avant de subir pour finir une passe de dressage et d'être confectionné en tôles.To produce sheet with good forming characteristics, especially for rotationally symmetric deep drawing, a low carbon steel containing a maximum of 0.009% N is alloyed with 0.01-0.04% Ti and in some cases also with 0.01 to 0.06% Niob, continuous casting is carried out, the slabs are heated above 1120 degrees Celsius, rolled to obtain a hot strip above the point of Ar3 and we coil at 520 U 100 degrees Celsius. After cold rolling to obtain the desired thin sheet thickness, the steel strip undergoes recrystallization annealing before finally undergoing a straightening pass and being made into sheets.

Description

Cold rolled sheet or strip and process for its manufacture

description

The invention relates to a method for producing a sheet or strip and a sheet or strip suitable for deep-drawing according to the preambles of claims 1 and 6.

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

A perfect tip freedom can only be expected from isotropic material without segregations, without non-metallic inclusions, without pearlite-like cementite precipitates and with a pan-cake-free structure. Therefore, in the following description, only the term "low-corner" is also used for "corner-free" tape according to the prior art.

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

The value for the flat anisotropy is calculated from the anisotropy r for different expansion behavior of the material in the rolling direction as well as at 45 degrees and 90 degrees. Different r values can be set for different deep-drawing properties. For the steels mentioned in the publication, tip-free material can only be achieved by normalizing the cold-rolled strip - in a continuous annealing at about 1000 degrees Celsius, the sheet in the final state having an ASTM 8 grain size with a relative tip height of about 0.3 to 0. 4% and Delta r approx. ± 0.1.

For non-normalizing annealed strip, only a low-tip condition can be achieved through compromises in the process control in sheet metal production. The final rolling temperatures should be approximately 750 degrees Celsius and the cold rolling degrees should either be below 25% or above 80% and are said to be unfavorable for the cornering

Recrystallization temperatures of over 600 degrees Celsius can be worked.

It is also described that normalization cannot take place in the bundle, but only in a continuous annealing line, because at the high temperatures the strips would stick together.

DE-OS 3234 574 discloses a generic cold-rolled steel sheet or steel strip suitable for deep drawing. Depending on the content of carbon, oxygen, sulfur and nitrogen, the titanium content should rise to values of up to 0.15%, the reel temperature should be over 700 degrees Celsius or at least 580 degrees Celsius with subsequent hot strip heating to over 700 degrees Celsius . We also recommend a cold rolling degree of 70 - 85% and a continuous annealing at 700 - 900 degrees Celsius with a maximum holding time of two minutes. No information is given on the formation of the material. EP-A1-101 740 recommends a slab heating temperature of less than 1100 degrees Celsius, a final rolling temperature of less than Ar ₃ , coiling 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 should 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 tricky after deep-drawing.

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

It is generally known that hot-rolled strip has good quasi-isotropic deformability, but has an inadequate surface quality and tolerances that are too large, and is also not produced in thicknesses below 1.2 mm.

The invention is therefore based on the object of proposing a sheet-free or at least low-corner deep-drawing suitable sheet made of steel strip and a corresponding manufacturing 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 and 6.

Advantageous developments of the invention are in the

Subclaims recorded.

Surprisingly, it has been shown that when the slab, annealing, rolling and coiling temperatures according to the invention are used, a recrystallizing annealing of a coil in the bell-type furnace is sufficient to give the steel strip or the prefabricated steel sheet excellent deep-drawing properties, in particular an extreme low angle.

The values of the grain size 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 method according to the invention, wherein additional low yield strength values can be maintained by choosing appropriate ones Cold rolling degrees depending on the titanium content. This has the advantage that high investments in continuous annealing for normalizing treatment can be dispensed with.

By varying the addition of titanium within the specified limits, practically any desired degree of cold rolling can be set for the production of tip-free material and / or likewise a yield strength between 175 and 450 N / mm ² with tensile strengths of 310 to 520 N / mm ² .

One of the reasons for the favorable properties of the sheet produced is the early formation of titanium nitride, so that a pan-cake structure cannot arise during the recrystallizing annealing due to the aluminum nitride precipitates. By choosing low reel temperatures of around 520 degrees Celsius, hot strip qualities were surprisingly achieved, which ensured cold-rolled material and made it possible to further refine the grain.

A particular advantage of the hot strip produced in this way is that there is in principle no restriction with regard to 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, which leads to coarse grain during recrystallization annealing. So far, the production of almost strip-free cold strip was tied to certain cold rolling degrees, unless normal annealing was to be carried out.

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

The variation of the cold rolling degrees as a function of the amount of titanium alloyed is limited to cold rolling degrees of 45 to 85% with the simultaneous addition of niobium within the specified limits.

The addition of niobium does not hinder the early formation of titanium nitride, so that even with this steel alloy according to the invention, a pan-cake structure cannot arise during the recrystallizing annealing. A serious technical and economic significance of the invention lies in the use of the thin sheet for rotationally symmetrical deep-drawn parts such as needle bearing cages, pulley halves etc. The sheet according to the invention can be used in these cases without substantial reworking such as cutting off the tips. The low-end point also prevents the formation of sectoral wall weaknesses during deep drawing, so that the drawn parts do not have any imbalance during rotation. Further advantages of low-lobe or lobe-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 the pickling, strips were reduced by cold rolling in various stages from 10% to 80% to the thickness of the sheet and rewound. The coil was heated to 700 degrees Celsius in a bell annealer of the Ludwig company, recrystallized at a throughput of 1.1 t / h to 1.9 t / h and then cooled in the furnace to 120 degrees Celsius. 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 at 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 which, for intermediate values of the alloying element titles, for example 0.025% Ti - starting from steel B - tip-free cell pulling at cold rolling grades of up to 15% or 20% and up to 85% can be expected 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 photos of FIGS. 3 to 7 of deep-drawn cells corresponding to the steels according to FIG. 10 and Tables 1 and 2 clearly illustrate the result.

Surprisingly, it was found that a certain level of tensile strength and yield strength could be assigned to the "corner-free" degrees of deformation (FIG. 11) and that the greatest cornering was found at the same time with the lowest yield strength / tensile strength.

Example: Steel B a) Point free at cold rolling degree 10% - 15% ≙ yield point level R _p 0.2 = 400 - 350 N / mm ² tensile strength level R _m = 450 - 400 N / mm ²

b) Pointedness at the degree of cold rolling 30% ≙ Rp _0.2 = 180 N / mm ² and R _m = 320 N / mm ²

c) _{Point freedom} with cold rolling _degree 50 - 80% ≙ R _{p0, 2} = 250 - 280 N / mm ² and R _m = 360 - 370 N / mm ²

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

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 art is considerable and extends up 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. 12.

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 flat anisotropy Delta r, an increase in the reel temperatures to 710 degrees Celsius with roughly the same roll temperatures resulted in grain coarsening and deterioration the plane anisotropy. FIGS. 2a, 2b, 2c show corresponding results on cups made from 180 mm round blanks, which were deep-drawn using 100 mm stamps at a retention force of 50 kN.

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 added, but listed without titanium content.

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 hot strip of 4 mm thickness 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 different 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 rounds of 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 contains no titanium in the alloy, otherwise belongs to the generic steel grade, FIG. 16 clearly shows that deep-drawing without tipping was not possible with any of the tried and tested degrees of cold rolling. When using the steels G to I rolled and annealed according to the invention, 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 area of epsilon = 55 to 85% almost tip free in

Range from 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 the deep-drawn sheet, 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.

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 0) and after every further quarter of the length of the tape up to the end of the tape (position 1).

Table 2

Steel Tw Th K figure ° C ° C min / max

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

Table 3

Steel Tw Th Pg K Δ r figure

° C ° C t / h min / max

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 tables 2 and 3 mean

Tw final roll temperature

Th reel temperature

K grain size according to ASTM

Pg annealing throughput

Δr flat anisotropy

Claims

Cold rolled sheet or strip and process for its manufacture

Claims

1. Method for producing a cold-rolled sheet or strip with good ductility from steel with the following composition in percent by weight:

Max. 0.10% carbon max. 0.40% silicon 0.10 to 1.0% manganese max. 0.08% phosphorus max. 0.02% sulfur max. 0.009% nitrogen

0.015 to 0.08% aluminum

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 unavoidable impurities,

which is annealed after hot rolling and cold rolling, characterized in that the slab is heated to above 1120 degrees Celsius and rolled into hot strip at a final rolling temperature above the Ar ₃ point and coiled at 520 ± 100 degrees Celsius and annealed in the coil after cold rolling becomes.

2. A process for producing a cold-rolled sheet or strip according to claim 1, characterized in that it is cold-rolled depending 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%

about 0.04% titanium: epsilon 15-25%, preferably 20% or

Epsilon 55 - 80%, preferably 60 - 70%

and then annealed at temperatures below A _1, and then tempered with a degree of deformation of approximately 1%.

Method according to claim 1, characterized in that a steel is used which additionally contains 0.01-0.06% niobium.

A process for producing a cold-rolled sheet or strip according to claim 1, characterized in that it is cold-rolled depending 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%

5. The method according to any one of claims 1 to 4, characterized in that the steel is annealed in the tight coil after cold rolling.

6. Suitable for deep drawing sheet or strip of steel in the specified composition and produced by one of the methods specified in claims 1 to 5, characterized by a recrystallized structure with a ferrite grain size finer than ASTM 7 for a titanium content of 0.01% and finer as ASTM 9 for titanium contents from 0.015 to 0.04%.

7. Suitable for deep drawing sheet or strip according to claim 6, characterized in that the titanium content corresponds to at least 3.5 times the nitrogen content.

8. Use of a sheet or strip produced according to one of the methods according to claims 1 to 5 for low-corner deep drawing, preferably of rotationally symmetrical

Share.

9. Use of a steel according to claim 1 or 3 for the production of deep-drawn, preferably rotationally symmetrical parts.