EP0435968B1 - Process for manufacturing coil-break-free hot strip and age-resistant hot-galvanized cold strip - Google Patents

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 DEG 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

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 a low carbon and manganese content tend to form flow figures. Lines appear on the strip surface that run transversely to the direction of tension, that is to say transversely to the rolling direction. These lines are irregularities in the thickness of the hot strip, ranging from slight shadows on the surface of the hot strip to severe creases. They are generally referred to as coil breaks because, under certain conditions, they occur only during the unwinding process of the hot strip coiled into coils during its further processing. 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 as serious damage to the belt surface, especially when running in a small arc and when there is a frequent change between leading and staying behind, which leads to severe blows in the belt. 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 is distributed unevenly, so that flow can start at different loads.

Coil breaks can no longer be eliminated by subsequent straightening, bending, pickling or skin-dressing. The tendency towards coil breaks increases the softer, thinner and narrower the hot strip to be unwound. The same applies to smaller coil diameters.

It is known that flow figures do not occur in so-called dual-phase steels because these steels have no pronounced yield point (no yield point elongation). It is also known that skin passaging can also remove the pronounced yield point, thereby avoiding the occurrence 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 dressing to remove pronounced maxima and minima of the yield point cannot be a permanent measure to avoid coil breaks, since the increasing yield point reappears at room temperature (see archive for the iron and steel industry) , 1955, page 72).

• 0,015 - 0,04 % C
• max. 0,04 % Mn
• 0,02 - 0,05 % S
• max. 0,01 % N

und Titangehalte von etwa 0,1 bis 0,4 % in Abhängigkeit von den hohen Schwefelgehalten sowie den Kohlenstoff- und Stickstoffgehalten, die vollständig abgebunden werden müssen, aufweist (EP-A2-0 231 864) .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 and the carbon and nitrogen contents which have to be set completely (EP-A2-0 231 864).

• - N, der durch Bildung von Aluminiumnitrid vermeidbar sei sowie
• - hoher Haspeltemperatur,
• - Abschreckalterung durch die Feuerverzinkung aufgrund des Kohlenstoffgehaltes beschrieben.

Technical reports, Thyssen Stahl AG, issue 2/89, pages 197-211 report in detail on the development of hot-dip galvanized, high-strength steels with good cold formability. In particular, the possibility of dressing the hot-dip galvanized strip for the temporary, non-permanent conditioning with regard to coil breaks is mentioned. 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.

Tests with niobium, titanium and phosphorus as alloying elements with contents up to 0.1% and C contents 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 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 amounts of approximately 0.1%, and the steel thereby becomes resistant to aging.

All of these processes or steels are relatively expensive because of high titanium or niobium contents or because of additional process steps.

• 0,001 - 0,06 % Kohlenstoff
• 0,01 - 0,40 % Silizium
• 0,10 - 0,80 % Mangan
• 0,005 - 0,08 % Phosphor
• 0,005 - 0,02 % Schwefel
• max. 0,009 % Stickstoff
• 0,015 - 0,08 % Aluminium
• 0,01 - 0_"04 % Titan
• max. 0,15 % von einem oder mehreren der Elemente Kupfer, Vanadium, Nickel

DE 38 03 064 C1 discloses a steel strip with the composition in% by weight:

• 0.001 - 0.06% carbon
• 0.01-0.40% silicon
• 0.10-0.80% manganese
• 0.005-0.08% phosphorus
• 0.005 - 0.02% sulfur
• Max. 0.009% nitrogen
• 0.015 - 0.08% aluminum
• 0.01-0 _" 04% titanium
• Max. 0.15% of one or more of the elements copper, vanadium, nickel

• aus im Strang vergossenen Brammen von 210 mm Dicke durch Erwärmung im Stoßofen auf 1250 °C und Walzen auf 3 mm Dicke herzustellen. Die Walzendtemperaturen betragen zum Beispiel 860, 870, 880, 890 und 900 °C.

Balance iron and unavoidable impurities

• from slabs cast in the strand of 210 mm thickness by heating in the pusher furnace to 1250 ° C and rollers to 3 mm thickness. The final roll temperatures are, for example, 860, 870, 880, 890 and 900 ° C.

The tape was coiled. the reel temperatures were, for example, 490, 500, 450, 430, 710 and 500 ° C.

Thereafter, the mixture was cooled to room temperature and, after pickling, strips were reduced by cold rolling in various stages from 10% to 80% to sheet metal thickness and reeled. The coil was heated to 700 _° C in the bell annealing furnace, annealed to a crystallization rate of 1.1 t / h to 1.9 t / h and then cooled to 120 _° C in the furnace.

The tape was then trained in order to avoid the appearance of flow figures (coil breaks) in the subsequent assembly into sheet metal. This material is not resistant to aging.

It is therefore an object of the invention to provide a method for producing soft coilbreak-free steel strip with permanent yield point elongation and good mechanical properties with the lowest possible consumption of alloying elements, and a method for producing a hot-dip galvanized steel strip which 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 process according to the invention for the hot strip provides for a defined titanium content in the steel to be made more uniform, which has the following analysis values in% by weight:

Remainder iron and unavoidable impurities, whereby the steel is cast into slabs in the continuous casting process and then finished from a slab heated from room temperature to over 1100 ° C, to 2 to 6 mm thick hot strip at temperatures above the Ars point, at temperatures from 550 to To reel at 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 prolonged aging.

According to the invention, the titanium content of the steel for the production of coilbreak-free steel strip with permanent yield point elongation is set so that it is stoichiometrically necessary for the setting of nitrogen but less than for the additional setting of carbon.

The ultimate cause of the advantageous effect of the titanium composition in connection with the other alloying elements in the production of a band by the specified method is not known. However, it is clear from the literature that the nitrogen is completely set during the steel melting process, so that precipitation-hardening titanium nitrides cannot form when the slab is heated to at least 1100 ° C and during the subsequent hot rolling, which would otherwise lead to undesirable increases in the strength of the soft steel strip .

The carbon, on the other hand, is only partially bound with the remaining amount of titanium; the remaining carbon is obviously kept in solution. The treatment and titanium metering according to the invention can probably achieve a considerable delay in the rate of diffusion of the carbon.

An addition of niobium and / or vanadium as an alloying element would also have resulted in a nitrogen setting in the steel, but only if a considerable hardening of the steel strip was accepted.

Completely surprisingly, it was found that, in the case of hot strip made of soft steel, which tends to form a pronounced yield point, it is not important, as was previously assumed, that the yield point elongation 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 the stress-strain diagram or force-extension diagram in the region of the yield point elongation the tension has no alternating maxima and minima, that is to say one that is as smooth as possible Has course. This overcomes the previous prejudice that to avoid flow figures or coil breaks it is necessary to avoid the occurrence of a pronounced yield point. The hot strip according to the invention remains coilbreak-free even after prolonged cold or warm aging, has good mechanical properties and can be produced with a low consumption of alloying elements.

These findings from the production of coil break-free hot strip can be used according to the invention in the production of aging-resistant hot-dip galvanized cold strip.

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 can be solved by using the specified steel if the specified steps and annealing temperatures are observed during the galvanizing process. Throughput speeds for the strip are set which are approx. 30-40% slower than the throughput speeds in the annealing furnace for titanium-free steels. Such through speeds as used in the invention are normally also used for the IF steels in the specified temperature range.

Due to the permanent stretching limit elongation achieved by the treatment described, without alternating minima and maxima in the course of the force, the hot strip shows practically no signs of aging. Surprisingly, it was found that the subsequent treatment steps cold rolling with z. B. about 50 to 90% degree of deformation has no negative impact on the yield strength, provided that the recrystallizing annealing in the continuous furnace before the galvanizing bath provides a complete recrystallization of the structure in the steel strip. The quenching of the strip directly behind the galvanizing bath initially leads to a pronounced elongation limit with alternating minima and maxima in the course of the force. 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. As a result, the force curve is again homogenized in the area of the elastic limit. The force curve remains after aging.

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

Examples of the coil break-free steel strip produced according to the invention and comparison strips are shown on the basis of the hot strip samples characterized in Table 1 according to their composition (extract from the melt analysis), finish rolling temperature and reel temperature.

• B 1/4 der Warmbandlänge R_eH obere Streckgrenze
• M 1/2 der Warmbandlänge R_m Zugfestigkeit
• G 3/4 der Warmbandlänge
• E Ende des Warmbandes
• Tabelle 3 zeigt weitere technologische Werte für erfindungsgemäß hergestelltes Warmband unterschiedlicher Dicke, gehaspelt bei unterschiedlichen Temperaturen.

Table 2 shows the mechanical properties measured for the hot strip U2 with a thickness of 2.9 mm in the fresh rolled state, the samples being taken from the hot strip at various points (sample position). It means:

• B 1/4 of the hot strip _length R _eH upper yield point
• M 1/2 of the hot strip length R _m tensile strength
• G 3/4 of the hot strip length
• E end of hot strip
• Table 3 shows further technological values for hot strip of different thickness produced according to the invention, coiled at different temperatures.

As a rule, after cooling in the coil in still air in a relatively fresh state, the steel strip is further processed into cold strip or assembled as hot strip or machined into cutting, stamped or bent parts after decoiling. As described at the beginning, coil breaks can be done Avoid if the hot strip has been previously trained or if a special additional, and therefore costly, wrapping process was used with the help of a so-called McKay roller system, with which consciously minimal, technically ineffective coil breaks can be generated.

The processing of such tapes becomes critical when they are used for a longer period of time, e.g. B. were stored for three to twelve months before they are processed. Coil breaks are then created. For this reason, the steel strips produced according to the invention were subjected to two tests under practical conditions.

For some of the hot strip samples given in the tables, only the force extension diagrams for the initial state of the hot strip sample and for others after artificial aging were recorded. In the force-extension diagrams, the extension is plotted on the abscissa and the force is plotted on the ordinate. The aging took place artificially by aging the sample material at 250 _° C for 8 hours.

• Fig. 1 die gemessenen Kraft-Verlängerungs-Diagramme der Warmbandprobe Z im Ausgangszustand (linkes Teilbild) und im Zustand nach Auslagerung (rechtes Teilbild);
• Fig. 2 die gemessenen Kraft-Verlängerungs-Diagramme der Warmbandprobe Y im Ausgangszustand (linkes Teilbild) und im Zustand nach Auslagerung (rechtes Teilbild);
• Fig. 3 die gemessenen Kraft-Verlängerungs-Diagramme der Warmbandprobe X im Ausgangszustand (linkes Teilbild) und im Zustand nach Auslagerung (rechtes Teilbild);
• Fig. 4 die gemessenen Kraft-Verlängerungs-Diagramme der Warmbandprobe W im Ausgangszustand (linkes Teilbild) und im Zustand nach Auslagerung (rechtes Teilbild);
• Fig. 5 die gemessenen Kraft-Verlängerungs-Diagramme der Warmbandprobe V im Ausgangszustand (linkes Teilbild) und im Zustand nach Auslagerung (rechtes Teilbild);
• Fig. 6 die gemessenen Kraft-Verlängerungs-Diagramme der Warmbandprobe U1 im Ausgangszustand (linkes Teilbild) und im Zustand nach Auslagerung (rechtes Teilbild);
• Fig. 7 die gemessenen Kraft-Verlängerungs-Diagramme der Warmbandprobe U2 im Ausgangszustand (linkes Teilbild) und im Zustand nach Auslagerung (rechtes Teilbild) für die Probenlagen B, M, G und E.

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

The force-extension diagrams shown in FIGS. 1 and 2 for the hot strip samples Z and Y made of a conventional steel, which are intended as a comparative example, show pronounced yield point strains with a highly uneven course, both in the initial state and after aging. A comparison between the hot strip samples Z and Y shows that a low reel temperature alone does not lead to an equalization of the yield point elongation.

The force-elongation diagrams shown in FIGS. 3 to 6 for the hot strip samples X, W, V and U1 produced, which have titanium contents increasing from 0.01% by weight to 0.04% by weight, show that with increasing titanium content, relatively low reel temperature, a substantial homogenization of the force curve is achieved in the area of the elongation limit.

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 force curve, both in the freshly rolled and in the aged state 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 TQ melts with thicknesses of 2.09 to 4.20 mm at approximately the same final rolling temperatures of 870-880 _° C and reel temperatures of 490 to 535 _° C while 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. As can be seen from Table 2, in all sample layers with a steel with 0.033% by weight carbon, 0.20% by weight manganese and 0.04% by weight titanium and a reel temperature of 450 _° C., the yield strength and tensile strength values according to the method according to the invention achieved that correspond to those of the known soft, non-microalloyed grades according to US standard SAE 1006/1008. In addition, this material is characterized by high elongation values of around 45% (standard As sample).

Oil filter covers with such a flat surface were produced from this tape that they did not need to be reworked. In addition, formed parts can advantageously be produced from the coilbreak-free hot strip, the surface of which has to meet increased requirements.

The production of aging-resistant hot-dip galvanized thin sheet according to the invention is explained in more detail with the aid of further examples.

Table 4 shows an analysis extract for the steel N and the steel L according to the invention as well as comparatively for the titanium-free steels P, K and an IF steel O.

Table 5 shows a graphic 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 months of aging.

Table 6 shows comparison parameters of steels L, K for freshly rolled galvanized cold strip and a condition after three months of aging.

Table 7 shows the temperatures in the furnace and on the strip in the course of a galvanizing process.

• Fig. 8 ein Kraft-Verlängerungs-Diagramm eines frisch verzinkten, eines dressierten und eines gealterten Stahlbandes L,
• Fig. 9 ein Kraft-Verlängerungs-Diagramm eines gealterten feuerverzinkten Stahlbandes K.

Continue to show

• 8 is a force extension diagram of a freshly galvanized, a trained and an aged steel strip L,
• 9 is a force extension diagram of an aged hot-dip galvanized steel strip K.

While the strip L galvanized according to the invention has a yield point of 298 N / mm ² immediately after hot-dip galvanizing and hardly changes after a three-month period of external storage (302 N / mm ² ), the comparative strip K, which has also been outsourced, shows an increase in the yield point from 231 to 309 N / mm ^2.

The effect of the invention is also very clear from the illustrations in FIGS. 8 and 9.

In both figures, the extension is plotted on the abscissa and the force on the ordinate. The force curve for a hot-dip galvanized undressed strip L is shown on the left in FIG. 8, in the middle after a skin pass degree of 0.7% and on the right after three months of aging.

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 an inhomogeneous force curve in the region of the yield point for the grade K galvanized strip after three months of aging, which is permanently noticeable in the form of coil break lines on the part produced from the strip after sheet metal processing.

Table 6 also clearly shows that the good forming behavior (Aso-elongation) is practically retained in the steel according to the invention. The elongation decreases with the band L by 1.5% points, whereas the elongation with the steel K is reduced by 6.6% points after aging.

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

The steel L according to the invention (analysis see Table 4) was melted with 0.06% titanium content in the oxygen inflation process 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 coiled after water cooling at 520 ° C. (T _Ha ).

After cooling in still air, the coil was rolled out with a degree of deformation of 70% to form a strip 1.2 mm thick and 1175 mm wide after it had previously been pickled. The cold strip was then run at a speed of 41 m / min. corresponding to 27.3 t / h throughput, preheated to 690 ° C in a continuous annealing furnace before the galvanizing bath and annealed at 900 ° C in the annealing zone (see table 7).

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 obtained the desired surface roughness through light skin-pass treatment with a 0.7% degree of deformation.

The hot-dip galvanized strips treated in this way can be used for shaped parts such. B. Seat frames of cars and trim panels can be used.

Claims (3)

1. Method of producing a coilbreak-free strip of steel, having a thickness of between 2 and 6 mm and a yield point of less than 350 N/mm², with a durable yield point expansion without alternating minima and maxima, produced from steel of the following composition in percentages by weight:

the remainder being iron and unavoidable impurities, said steel being extruded, a separated slab being heated from substantially ambient temperature to at least 1100°C, then finish-rolled to form a hot strip at temperatures above the Ar₃ point and finally wound in the temperature range of between 550 and 400 _° C.

2. Method according to claim 1, characterised in that the titanium content of the steel is set to at least the quantity stoichiometrically required for the bonding of the nitrogen and set to less than the quantity stoichiometrically required for the additional complete bonding of the carbon.

3. Use of a steel strip according to one of claims 1 or 2, for producing a hot-galvanised strip by means of blanching, cold-rolling and recrystallising continuous annealing processes at furnace temperatures of between 800 and 940 _° C and by subsequently hot-galvanising and finishing with a conversion degree of between 0.5 and 2 %.

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