DE4015248A1 - Mfg. coil-break-free hot rolled strip - Google Patents

Abstract

Coil-break-face steel strip of 2-6mm thickness with age-resistant constant yield print elongation it produced from a steel of compsn. in wt. % 0.002-0.08 C, 0.01-0.4 Si, 0.05-1.0 Mn, 0.002-0.025 P, 0.001-0.015 S, 0.015-0.08 Al, max. 0.008 N, 03-0.08 Ti balance Fe plus usual impurities. The steel is concast and slabs heated to at least 1100 deg.C before being rolled at above the Ar3 and coiled at 550-400 deg.C.

Description

The invention relates to a process for the preparation of coilbreak-free steel strip.

Under certain deformation conditions, such as e.g. when unwinding or straightening hot-rolled strip can tend, soft steel bands with low Carbon and manganese content to form Flow figures. This occurs on the belt surface Lines that are transverse to the direction of pull, ie across the Rolling direction run. These lines are Irregularities in hot strip thickness caused by slight shadow on the hot strip surface up to heavy kinks are enough. They are generally considered Coilbreaks, because they are under certain Prerequisites only during the unwinding process of the to coils coiled hot strip at the Further processing occur. In unwinding, the can work unrestrained or without enough train the unwound hot strip between rolling off and both the introduction rolls of a processing plant run in a large arc as well as in a tight arc, ever if it leads or lags behind. In particular when running in a small arc and with frequent change between fore-and-aft, which is too strong Blows in the band leads, Coilbreaks occur as severe damage to the tape surface. Fast unwinding has a particularly strong hitting and consequently strong coil breaks.  

In addition, coil breaks between the straightening rollers arise. Through the sinusoidal continuous bending of the band becomes the tension in the different ones Band sections unevenly distributed, so that a Use flow at different loads can.

Coilbreaks are by a subsequent judging, Bending, pickling or dressing no longer remove. The tendency to coil breaks increases, ever softer, thinner and narrower the unwinding hot strip is. The same applies to smaller ones Coil diameter.

It is known that flow figures in so-called Dual phase steels do not occur because these steels no pronounced yield strength (no Elongation limit strain). It is also known that a training also the pronounced Eliminate yield strength, thereby reducing the occurrence of Flow lines is avoided. These measures are e.g. specified in Materials Science Steel, Volume 2, pages 97, 86, Springer Verlag, Berlin-Heidelberg-New York-Tokyo, 1985, publishing house steel iron mbH, Duesseldorf.

Softer from a study of aging behavior On the other hand, steels are known to undergo tempering Elimination of pronounced maxima and minima Yield strength no permanent measure to avoid of coil breaks, as with increasing storage time even at room temperature the pronounced yield strength reappears (see archive for the Ironworks, 1955, page 72).

The object of the invention is therefore a method for producing soft coilbreak-free steel strip with time-constant yield stress elongation and with good mechanical properties at the lowest possible Specify consumption of alloying elements.

The solution to this problem is specified in claim 1. The dependent claim is an expedient development of the inventive method.

The production process according to the invention for the hot strip provides for the equalization of Yield limit elongation a defined titanium content in Steel, which has the following analysis values in% by weight, set:

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% al: from 0.015 to 0.080% N: 0.0080% Ti: 0.03 to 0.080%

The remainder iron and unavoidable impurities, wherein the steel is cast in a continuous casting process to slabs and then from a heated from room temperature to about 1100 ° C slab to roll 2 to 6 mm thick hot strip at temperatures above the Ar ₃ point, at temperatures of 550 to coarse up to 400 ° C. The hot strip has a nearly constant yield strength of less than 350 N / mm ² in the as-fresh state and after extended aging.

The titanium content of the steel for the production of coilbreak-free steel strip with time-stable Yield limit strain is inventively so set to be larger than to bind Nitrogen, but less than additional ligation of carbon is stoichiometrically necessary.

The concrete cause for the beneficial effect of Titanium composition in connection with the rest Alloy elements in the production of a band after the specified method is not in last Consequence known. However, it is clear from the literature, that during the steel fusion the nitrogen is fully tied, so that in the Slab heating to at least 1100 ° C and during the subsequent hot rolling is therefore no precipitation-hardening titanium nitrides can form what otherwise undesirable increases in strength of the soft steel strip would lead.

The carbon, in contrast, with the remaining Titan quantity only partially tied off; the rest Carbon is apparently kept in solution. By the treatment according to the invention and Titanium dosing can probably be a significant Delaying the diffusion rate of the Carbon can be achieved.

An addition of niobium and / or vanadium as Alloy element would indeed have a Nitrogen in the steel provided, but only below Acceptance of a substantial consolidation of the Steel strip.  

Quite surprisingly, it was found that at Hot strip made of soft steel, used to form a pronounced yield strength, it does not tend to arrives, as previously assumed, the Yield limit strain z. B. by higher alloying of To avoid steeling or dressing the tape or to to change.

Rather, it depends on the invention that the Hot strip in the presence of a pronounced yield strength as even as possible yield stress elongation has, or in the Stress-strain diagram or Force extension diagram in the field of Elongation limit stretching the tension no alternating Maxima and minima, so a smooth as possible course Has. This will replace the previously existing prejudice overcome that to avoid flow figures or Coilbreaks is necessary, the occurrence of a to avoid pronounced yield strength. The Hot strip according to the invention also remains longer cold or warm aging coilbreak-free, points good mechanical properties and can at low consumption of alloying elements become.

Examples of the inventively prepared Steel strapping and comparison straps are based on the in Table 1 by composition (extract from the Melting analysis), final rolling temperature and reel temperature characterized hot strip samples shown.

Table 2 shows the mechanical properties measured for the hot strip U 2 with 2.9 mm thickness in as-rolled condition, wherein the samples were taken at different points (sample position) of the hot strip. It means:

B 1/4 of the hot strip length
M 1/2 of the hot strip length
G 3/4 of the hot strip length
E end of the hot strip
R _eH upper yield strength
R _m tensile strength

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

In general, the steel strip after cooling in the coil in still air in a relatively fresh state Cold strip further processed or as a hot strip assembled or to cutting, punching or Machined by bending after bending off. As described at the beginning, coil breaks can be included avoid if the hot strip previously trained or a special additional, and therefore costly, Umwickelverfahren using a so-called McKay roller system, with the deliberate minimum, technically ineffective coil breaks can be generated was applied.

The processing of such tapes becomes critical if these are for a long time e.g. three to twelve Months were stored before further processing become. It then turn turn coil breaks. For this reason, the invention produced steel strips under two tests Subjected to practical conditions.  

For some of those given in the tables Hot strip samples were only the Force extension diagrams for the initial state the hot strip sample and for others also after artificial aging. In the Force Extension Diagrams is on the abscissa Extension, on the ordinate the strength applied. The aging was carried out by artificial way Storage of the sample material at 250 ° C for 8 Hours.

Show it:

FIG. 1 is the measured force-elongation diagrams of the hot strip sample Z in the initial state (left panel) and in the condition after aging (right frame);

Figure 2 shows the measured force-elongation diagrams of the hot strip sample Y in the initial state (left panel) and in the condition after aging (right frame).

Fig. 3 is the measured force-elongation diagrams of the hot strip sample X in the initial state (left panel) and in the condition after aging (right frame);

Figure 4 shows the measured force-elongation diagrams of the hot strip specimen W in the initial state (left panel) and in the condition after aging (right frame).

FIG. 5 shows the measured force extension diagrams of the hot strip sample V in the initial state (left partial image) and in the state after externalization (right partial image); FIG.

Figure 6 shows the measured force-elongation diagrams of the hot strip sample U 1 in the initial state (left panel) and in the condition after aging (right frame).

Fig. 7 shows the measured force-elongation diagrams of the hot strip sample U 2 in the initial state (left panel) and in the condition after aging (right partial image) for the sample positions B, F, G and E.

The force-elongation diagrams shown in FIGS. 1 and 2 for the comparative example hot strip samples Z and Y of a conventional steel, both in the initial state as well as after aging, show pronounced yield point expansions with a very uneven profile. A comparison between the hot strip samples Z and Y shows that a low coiler temperature alone does not result in equalization of the yield stress.

Based on the force-elongation diagrams shown in FIGS. 3 to 6 for the hot-rolled samples X, W, V and U 1 produced according to the invention, which have titanium contents of 0.01% by weight to 0.04% by weight, show that With increasing titanium content at a relatively low reeling temperature, a substantial equalization of the force curve in the region of the yield stress is achieved.

The qualitative representation clearly shows that it is only from 0.03% titanium ( FIG. 5) that a yield stress strain without pronounced maxima and minima is achieved in an alternating sequence.

Such a steady force course both in fresh-rolling as well as in the aged state of undressed ligaments so far completely unknown.

The hot tapes exhibited 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 reeling temperatures of 490 to 535 ° C while the samples V, U 1 , U 2 were coiled at 450 and 430 ° C, respectively. Despite varying C, Ti, N contents, the yield strength was about 300 ± + 20 N / mm ² , the tensile strength about 390 ± 15 N / mm ² . Partial aging hardly changed the values and the bands could be processed excellently without showing coil breaks.

As shown in Fig. 7, the equalization of the yield stress occurs in all the sample layers examined. As is apparent from Table 2, in all sample layers in a steel with 0.033 wt.% Carbon, 0.20 wt.% Manganese and 0.04 wt.% Titanium and a coiler temperature of 450 ° C by the inventive method yield strength and tensile strength values achieved, which 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 about 45% (A ₅ standard sample).

Table 1

Table 2

Table 3

Claims (2)

1. A method for producing coilbreak-free steel strip of 2 to 6 mm thickness with a yield strength of less than 350 N / mm ² with time-constant yield stress without alternating minima and maxima, made of steel of the following composition in weight percent: 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% al: from 0.015 to 0.080% N: 0.0080% Ti: 0.03 to 0.080% Remaining iron and inevitable impurities poured in the strand, heated a separated slab from about room temperature to at least 1100 ° C, then finish rolled at temperatures above the Ar ₃ point to hot strip and finally in the temperature range of 550 to 400 ° C is coiled. 2. The method according to claim 1, characterized that the titanium content of the steel on at least the to bind the nitrogen and less than to additional complete ligation of the Carbon stoichiometric amount required is set.