WO1990013672A1

WO1990013672A1 - Process for manufacturing coil-break-free hot strip and age-resistant hot-galvanized cold strip

Info

Publication number: WO1990013672A1
Application number: PCT/DE1990/000338
Authority: WO
Inventors: Klaus Freier; Klaus-Dieter Hufenbach; Stefan Hussy; Walter Zimnik
Original assignee: Stahlwerke Peine-Salzgitter Ag
Priority date: 1989-05-09
Filing date: 1990-05-09
Publication date: 1990-11-15
Also published as: DK0435968T3; DE59009505D1; ATE126276T1; JPH04500541A; EP0435968B1; EP0435968B2; DE4015248A1; EP0435968A1; ES2075899T3

Abstract

Process for manufacturing coil-break-free hot strip and age-resistant hot-galvanized strip with constant long-term elongation at yield and low yield point. Conventionally, coil breaks in hot strip are prevented either by adequate alloying or by dressing the hot strip to prevent elongation at yield. The steel strip of the invention, on the other hand, is manufactured with a minimal quantity of alloying titanium and a low reel temperature between 550 and 400°C. The quantity of alloying titanium chosen is equal to the quantity stoichiometrically necessary to bind the nitrogen but less than that required to bind the carbon and the nitrogen. In addition, the coil-break-free, age-resistant steel used for the hot-galvanized strips is a vacuum steel with a titanium content of 0.02 to 0.04 % or an LD steel with a titanium content of 0.03 to 0.08 %.

Description

Process for the production of coilbreak-free hot strip and aging-resistant hot-dip galvanized cold strip

description

The invention relates to a method for producing steel strip.

Under certain deformation conditions, such as. B. when reeling or straightening hot strip, strips made of mild steel with low carbon and manganese tend to form

Flow figures. Doing so step on the belt surface

Lines that run transversely to the direction of tension, i.e. transversely to the rolling direction. These lines are

Irregularities in the hot strip thickness, which range from slight shadows on the hot strip surface to severe creases. They are commonly referred to as coil breaks because they are under certain

Preconditions only during the unwinding process of the hot strip coiled into coils

Further processing occur. In unwinding systems that work without braking or without sufficient tension, the hot strip to be unwound between the unwinding system and the infeed rollers of a processing system can run both in a large arc and in a narrow arc, depending on whether it is leading or lagging behind. Coil breaks occur particularly when running in a small arc and with frequent changes between leading and staying behind, which leads to strong blows in the belt

serious damage to the belt surface. Fast uncoiling results in particularly strong beating and correspondingly strong coil breaks. Coil breaks between the straightening rolls can also occur. Due to the sinusoidal continuous bending of the band, the tension in the different

Band sections distributed unevenly so that flow can start at different loads.

Coil breaks are due to subsequent straightening,

Bending, pickling or skin training no longer

remove. The tendency towards coil breaks increases the softer, thinner and narrower the hot strip to be unwound. The same applies to those who are getting smaller

Coil diameter.

It is known that flow figures in so-called

Dual-phase steels do not occur because these steels have no pronounced yield strength (none

Yield strength). It is also known that a skin pass is also the most pronounced

Eliminate yield point, thereby avoiding the appearance of flow lines. These measures are e.g. B. specified in Materials Science Steel, Volume 2, pages 97, 86, Springer Verlag, Berlin-Heidelberg-New-York-Tokyo, 1985, Verlag Stahl Eisen mbH, Düsseldorf.

On the other hand, it is known from an investigation of the aging behavior of soft steels that the skin passivation to remove pronounced maxima and minima

Yield strength cannot be a permanent measure to avoid coil breaks, since with increasing storage time the pronounced yield strength reappears at room temperature (see archive for the

Eisenhüttenwesen, 1955, page 72). It is also known to design an age-free, cold-rolled steel strip with good forming properties for a later metallic coating in the weld pool, which among other things

0.015 - 0.04% C

Max. 0.04% Mn

0.02 - 0.05% S

Max. 0.01% N

and titanium contents of about 0.1 to 0.4% depending on the high sulfur contents as well as the carbon and nitrogen contents which have to be set completely (EP-A2-0 231 864).

Technical reports, Thyssen Stahl AG, Issue 2/89, pages 197-211 provide detailed information on the development of hot-dip galvanized, high-strength steels with good quality

Cold formability reported. In particular, the possibility of dressing the hot-dip galvanized strip for the temporary, not permanent

Conditioning mentioned regarding coil breaks.

With regard to aging resistance, the negative influence of:

- N, which can be avoided by the formation of aluminum nitride and

- high reel temperature,

- Aging described by hot-dip galvanizing due to the carbon content.

Trials with niobium, titanium and phosphorus as

Alloy elements with contents up to 0.1% as well

C content below 0.1% for steels with an upper yield strength of max. about 350 N / mm ² would have shown that, in contrast to niobium, titanium did not have the desired nucleating effect for interstitially dissolved Have carbon. It was therefore recommended to subject these steels to an aging treatment in a bell annealing furnace at 200 to 300 ° C in order to make them more resistant to aging.

It is also known that carbon and nitrogen can be stably bound in IF steels by adding niobium and / or titanium in a content of about 0.1%, and the steel is thereby resistant to aging.

All of these processes or steels are relatively expensive because of high titanium or niobium contents or because of

additional procedural steps.

The object of the invention is therefore to provide a process for the production of soft coilbreak-free steel strip with permanent yield point elongation and good mechanical properties with the lowest possible consumption of alloying elements, and a process for the production of a hot-dip galvanized

Steel strip that avoids the disadvantages of the prior art.

The solution to this problem is specified in claim 1. The subclaims indicate expedient developments of the method according to the invention.

The manufacturing method according to the invention for the hot strip provides for the equalization of the

Elongation at yield point to set a defined titanium content in the steel, which has the following analytical values in% by weight: C: 0.02 - 0.08%

Si: 0.01 - 0.40%

Mn: 0.05 - 1.00%

P: 0.002 - 0.025%

S: 0.001 - 0.015%

AI: 0.015 - 0.080%

N: ≤ 0.0080%

Ti: 0.03 - 0.080%

Remainder iron and unavoidable impurities, the steel being cast into slabs in the continuous casting process and then from a slab heated from room temperature to over 1100 ° C. to a thickness of 2 to 6 mm

Finish rolling hot strip at temperatures above the Ar ₃ point, reeling at temperatures from 550 to 400 ° C. The hot strip has an almost constant yield strength of less than 350 N / mm ² in the freshly rolled state and after long aging.

The titanium content of the steel used to manufacture

Coilbreak-free steel strip with permanent yield point elongation is according to the invention

set that it is larger than stoichiometrically necessary for the setting of nitrogen, but less than for the additional setting of carbon.

The concrete cause of the advantageous effect of the titanium composition in connection with the others

Alloy elements in the production of a tape by the specified method is not in the latter

Consequence known. However, it is clear from the literature that the nitrogen is completely set during the steel melting process, so that the

Slab heating to at least 1100 ° C and during the subsequent hot rolling therefore none

can form precipitation-hardening titanium nitrides, which would otherwise lead to undesirable increases in strength of the soft steel strip.

The carbon, on the other hand, will match the rest

Amount of titanium only partially set; the remaining carbon is obviously kept in solution.

By the treatment according to the invention and

Titanium dosing can likely significantly delay the rate of diffusion of the

Carbon can be achieved.

An addition of niobium and / or vanadium as

Alloy element would also have one

Nitrogen binding in the steel is achieved, however, only with the acceptance of a considerable solidification of the

Steel strip.

So it was completely surprising that at

Hot strip made of soft steel, which tends to form a pronounced yield strength, it does not matter, as was previously assumed, the

Elongation at stretch z. B. to avoid or change by higher alloying the steel or tempering the strip.

Rather, it is important according to the invention that the hot strip has a very uniform yield point elongation when there is a pronounced yield point, or in

Stress-strain diagram or

Force extension diagram in the area of

Elongation at stretch limit the stress has no alternating maxima and minima, i.e. has a smooth course. This is the prejudice that existed so far overcome that it was necessary to avoid flow figures or coil breaks, the occurrence of a

to avoid pronounced yield strength. The

The hot strip according to the invention remains coilbreak-free even after prolonged cold or warm aging, has good mechanical properties and can be used

low consumption of alloying elements can be produced.

These findings from the manufacture of

According to the invention, coil break-free hot strip can be used in the production of aging-resistant

hot-dip galvanized cold strip can be used.

In the following, hot-dip galvanizing is understood to mean the metallic molten bath coating of steel strip with zinc and its mixtures such as Galfan and Galvalume. Galfan and Galvalume are registered trademarks of ILZRO, New York.

Practical tests have shown that the problems of the prior art in the production of

hot-dip galvanized strip by using the

specified steel can be solved if the specified steps and annealing temperatures during the

Galvanizing process are observed. Throughput speeds for the belt are set which are approx. 30-40% slower than that

Throughput speeds in the annealing furnace for titanium-free steels. Such flow rates as used in the invention are in

specified temperature range is normally also used for the IF steels. Because of the treatment described

The hot strip shows practically no stretching limit elongation achieved without alternating minima and maxima in the course of the force

Signs of aging. It was surprising

found that the following

Treatment steps cold rolling with z. B. about 50 to 90% degree of deformation no negative impact on the

Has yield strength, provided that

recrystallizing annealing in a continuous furnace in front of the galvanizing bath results in a complete recrystallization of the structure in the steel strip. Quenching the strip directly behind the galvanizing bath initially leads to a pronounced elongation limit

alternating minima and maxima in the force curve.

Subsequently, this band is trained to form a desired surface roughness and possibly better band flatness and, if necessary, stretch-oriented, eg. B. with 0.5 to 2% degree of deformation.

This will force the

Yield strength stretch range homogenized again. The force curve remains after aging.

With this method, an age-free and flow figure-free hot-dip galvanized sheet can be produced.

Surprisingly, it was also found that a vacuum steel of the composition in% by weight:

C: 0.004-0.02%

Si: 0.01-0.40%

Mn: 0.05-0.50% P: 0.002 - 0.025%

S: 0.001 - 0.015%

AI: 0.015 - 0.080%

N: ≤ 0.0080%

Ti: 0.02 - 0.04%

Remainder iron and inevitable impurities, is resistant to aging if it, as described above with the steel with higher carbon, too

Hot strip is rolled out. This steel also retains the same treatment during pickling,

Cold rolling and in the galvanizing plant its good aging properties. Because of its technological values, it is also easy to cold-form.

Examples of the manufactured according to the invention

Coilbreak-free steel strip and comparison strips are based on the composition in Table 1 (extract from the melt analysis), finish rolling temperature and

Coil temperature characterized hot strip samples shown.

Table 2 shows that for the hot strip U2 with 2.9 mm

Thickness mechanical measured in fresh rolled condition

Properties, with the samples at different

Places (sample position) of the hot strip was removed. It means:

B 1/4 of the hot strip _length R _eH upper yield point

M 1/2 of the hot strip length R tensile strength

G 3/4 of the hot strip length

E end of hot strip

Table 3 shows further technological values for hot strip produced according to the invention different thickness, coiled at

different temperatures.

As a rule, after cooling in the coil in still air, the steel strip is further processed into cold strip in cold rolled condition or as hot strip

assembled or for cutting, punching or

Bent parts machined after decoiling. As described at the beginning, coil breaks can be avoided if the hot strip is previously treated or if a special, and therefore costly, wrapping process is carried out using a so-called

McKay roller system, with the deliberately minimal,

technically ineffective coil breaks can be used.

The processing of such tapes becomes critical when they are used for a longer period of time, e.g. B. three to twelve

Were stored for months before being processed. Coil breaks are then created.

For this reason, the invention

manufactured steel strips under two tests

Subjected to practical conditions.

For some of those listed in the tables

Hot strip samples were only those

Force extension diagrams for the initial state of the hot strip test and for others also after

artificial aging. In the

Force-extension diagrams are the extension on the abscissa and the force on the ordinate. The aging took place in an artificial way Storage of the sample material at 250 ° C for 8

Hours.

Show it:

1 shows the measured force extension diagrams of the hot strip sample Z in the initial state (left part) and in the state after aging (right part);

2 shows the measured force extension diagrams of the hot strip sample Y in the initial state (left partial image) and in the state after aging (right partial image);

3 shows the measured force extension diagrams of the hot strip specimen X in the initial state (left partial image) and in the state after aging (right partial image);

4 shows the measured force extension diagrams of the hot strip specimen W in the initial state (left partial image) and in the state after aging (right partial image);

5 shows the measured force-extension diagrams of the hot strip sample V in the initial state (left part) and in the state after aging (right part);

6 shows the measured force extension diagrams of the hot strip sample U1 in the initial state (left drawing) and in the state after

Outsourcing (right drawing);

7 shows the measured force extension diagrams of the hot strip specimen U2 in the initial state (left partial image) and in the state after aging (right partial image) for the sample layers B, M, G and E.

The shown in Figures 1 and 2

Strength Extension Diagrams for As

Comparative example imagined hot strip samples Z and Y from a conventional steel show, both in

Initial state as after outsourcing, pronounced

Yield strength strains with very uneven

Course. A comparison between the hot strip samples Z and Y shows that a low reel temperature alone does not equalize the

Yield stress expansion.

Based on that shown in Figures 3 to 6

Force elongation diagrams for the hot strip samples X, W, V and UI produced according to the invention, which have titanium contents increasing from 0.01% by weight to 0.04% by weight, show that with increasing titanium content at a relatively low reel temperature, a substantial equalization of the force curve is achieved in the area of the yield strength expansion.

The qualitative representation clearly shows that only from 0.03% titanium (FIG. 5) is a yield point elongation achieved without pronounced maxima and minima in an alternating sequence. Such a steady course of force, both in the freshly rolled and in the aged condition of undressed strips, was previously completely unknown.

The hot strips showed the same behavior with 0.04 to 0.05% by weight of Ti from the melts T-Q with thicknesses of

2.09 to 4.20 mm with approximately the same

Final rolling temperatures of 870-880 ° C and

Reel temperatures from 490 to 535 ° C during the samples

V, U1, U2 were coiled at 450 and 430 ° C, respectively. Despite varying C, Ti, N contents, the yield strength was approx. 300 ± 20 N / mm ² , the tensile strength approx. 390 ± 15 N / mm ² . Partial aging hardly changed the values and the strips could be processed excellently without showing coil breaks.

As shown in FIG. 7, the uniformity of the yield point elongation occurs in all examined

Sample layers on. As can be seen from Table 2, steel with 0.033% by weight of carbon, 0.20% by weight of manganese and 0.04% by weight of titanium as well as a reel temperature of 450 ° C after the

method according to the yield strength and

Tensile strength values achieved that correspond to those of the known soft, non-microalloyed grades according to US standard SAE 1006/1008. This also stands out

Material due to high elongation values of around 45%

(A ₅ standard sample).

Oil filter covers with such a flat surface were produced from this tape that they did not need to be reworked. In addition, the coil break-free hot strip can advantageously be used to produce shaped parts whose surface area is increased

Requirements must meet. Using further examples, the

Production of aging-resistant hot-dip galvanized sheet according to the invention explained in more detail.

Table 4 shows an analysis extract for the

Steels N, L according to the invention and comparatively for the titanium-free steels P, K and an IF steel O.

Table 5 shows a graphical representation of the

Yield strengths for the galvanized grades P, O, N before and after six months of aging and specific individual values for strips made of steels K, L before and after

three month aging.

Table 6 shows comparative parameters of the steels L, K for fresh-rolled galvanized cold strip and a state after three months of aging.

Table 7 shows the temperatures in the furnace and on the conveyor in

Course of a galvanizing process.

Continue to show

8 is a force extension diagram of a freshly galvanized, a trained and an aged steel strip L,

Fig. 9 is a force extension diagram of a

aged hot-dip galvanized steel strip K.

With the vacuum steel P, which contains no titanium, the significant increase in yield strength falls by more than

50 N / mm ² after six months of aging (table

5) while the IF steel O is known to have no aging. The steel N according to the invention shows no aging, just like the much more expensive IF steel O.

A similar behavior is with the concrete

specified individual analyzes (bands) K, L

ascertain.

While the strip L galvanized according to the invention has a yield strength of 298 N / mm ² immediately after hot-dip galvanizing and hardly changes after three months of external storage (302 N / mm ² ), this also shows

outsourced reference tape K one

Yield strength increase from 231 to 309 N / mm ² .

The effect of the invention is also very clear on the

Representations in Figures 8 and 9.

In both figures, the one on the abscissa is

Extension and on the ordinate the force

applied.

The force curve for a is on the left in FIG

hot-dip galvanized undressed tape L, in the middle with a skin pass of 0.7% and to the right

three-month outsourcing.

The belt according to the invention maintains the homogeneous force curve after the skin pass, so that no disruptive flow figures become recognizable after processing.

In contrast to this, FIG. 9 shows one for the galvanized strip of grade K after three months of aging

inhomogeneous force curve in the area of the yield point, which is after the

Sheet metal processing makes permanent noticeable in the form of coil break lines.

Table 6 also clearly shows that the good

Forming behavior (A ₈₀ elongation) in the invention Steel remains practically intact. The elongation decreases by 1.5% points for the strip L, whereas for the steel K the elongation is reduced by 6.6% points after aging.

The manufacturing process will be illustrated using the example of steel L.

The steel L

(Analysis see Table 4) melted with 0.06% titanium content and then cast in the strand. A separated slab was heated from room temperature to 1220 ° C and rolled to a hot strip of 4 mm thickness with a final rolling temperature (T _We ) of 870 ° C and after a

Water cooling coiled at 520 ° C (T _Ha ).

After cooling in still air, the coil was 70%

Degree of forming into a strip 1.2 mm thick and 1175 mm

Rolled out width after it was previously pickled. The

Cold strip was then at a speed of

41 m / min. corresponding to 27.3 t / h throughput, in

Continuous annealing furnace in front of the galvanizing bath preheated to 690 ° C and in the annealing zone (see table 7)

Annealed at 900 ° C.

The belt still had a temperature of 510 ° C in the trunk area before the bath inlet. At this temperature it was drawn through the galvanizing bath (460 ° C) and then quenched.

At the end of the galvanizing line, the strip was given the desired surface roughness by a light one

Dressing with 0.7% degree of deformation. In the same way, the vacuum steel of grade N with a content of 0.03% titanium was treated and processed into 0.8 mm thick Galfan-coated strip.

The hot-dip galvanized strips treated in this way can be used for shaped parts such. B. seat frame of cars and

Decorative panels are used.

t \ 3

Claims

1. Process for making coilbreak-free

Steel tape from 2 to 6 mm thick with a

Yield point of less than 350 N / mm ² with permanent yield point elongation without alternating

Minima and Maxima made of steel following

Composition in percent by weight:

C: 0.02 - 0.08%

Si: 0.01 - 0.40%

Mn: 0.05 - 1.00%

P: 0.002 - 0.025%

S: 0.001 - 0.015%

AI: 0.015 - 0.080%

N: ≤ 0.0080%

Ti: 0.03 - 0.080%

Remainder iron and inevitable impurities, which shed in the strand, heated a separated slab from about room temperature to at least 1100 ° C, then at temperatures above the

Ar ₃ point rolled to hot strip and

finally in the temperature range from 550 to 400 ° C.

2. The method according to claim 1, characterized in that the titanium content of the steel to at least that for setting the nitrogen and less than for additional complete setting of the

Carbon stoichiometrically necessary amount is adjusted.

3. Use a steel band according to one of the

Claims 1 or 2 for the manufacture of a

hot-dip galvanized strip by pickling, cold rolling and recrystallizing continuous annealing

Oven temperatures from 800 to 940 ° C as well

Subsequent hot-dip galvanizing and tempering with 0.5 to 2% degree of forming.

4. Process for producing a hot-dip galvanized

Band with a yield strength of less than 280 N / mm ² according to claim 3, characterized by the

Use of a steel melted under vacuum with 0.004 to 0.02% by weight of C and 0.02 to 0.04% by weight

Ti and 0.05 to 0.50% by weight of Mn.