US3513036A - Process for producing coiled,hotrolled,pickled steel strip - Google Patents

Description

United States Patent 3,513,036 PROCESS FOR PRODUCING COILED, HOT- ROLLED, PICKLED STEEL STRIP William H. McFarland, Hobart, and Henry M. Tietz,

Gary, Ind., assignors t0 Inland Steel Company, Chlcago, Ill., a corporation of Delaware N0 Drawing. Filed May 2, 1967, Ser. No. 635,371

Int. Cl. C21f US. Cl. 148-12 '14 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention relates generally to hot-rolled, pickled steel strip, in coiled form, and more particularly to a soft, ductile, readily drawable, hot-rolled, pickled steel strip in coiled form, and to a process for producing this product.

As used herein, the term strip refers to flat steel having a thickness between 0.060 and 0.25 inch, in all widths.

Hot-rolled, pickled strip having the desired softness, ductility and drawability has heretofore been available in relatively short, separate, discrete pieces. More recently, there has been a demand for hot-rolled, pickled strip in coil form, for convenience in materials handling and in fabricating the ultimate product. In the manufacture of hot-rolled, pickled strip in coil form, problems arise, in connection with producing the desired softness, ductility and drawability, which do not arise when this strip is provided as relatively short, separate, discrete pieces. These problems can best be understood from the following description of a conventional process for producing hot-rolled, pickled strip in coil form.

Steel slabs to be hot-rolled into strip are conventionally heated to an elevated temperature above the A for the steel (e.g., around 2300" F.) and subjected to hot-rolling. The strip undergoing rolling arrives at the finishing train of rolls (which imparts the desired final thickness to the strip) at a conventional finishing temperature of 1950'- 2100 F. After the finishing train, the strip moves on rollers along a table upon which the strip is cooled by water sprays following which the strip is coiled at a conventional strip-coiling temperature of 1100-1250 F. The coils of hot-rolled strip are then cooled to room temperature. Thereafter, the hot-rolled strip is uncoiled and run through a pickling bath of acid to remove scale from the surface of the steel. Following pickling, the strip is re- Patented May 19, 1970 ice coiled. The steps of uncoiling, pickling and recoiling are continuous.

Unless precautions are taken, a strip will undergo coldworking during the continuous steps of uncoiling, pickling and recoiling; and it is not uncommon for the strip to undergo 3% cold deformation during these steps.

As rolled, the steel strip is not ductile enough to be drawn, and requires annealing to improve softness and drawability. A steel strip which was cold-worked during the pickling operation will soften during annealing, but the grain size of the steel will become relatively large (e.g., 4 or larger on the ASTM scale). This is undesirable because, in a steel undergoing drawing, relatively large grains product undesirable surface imperfections which resemble orange peel.

Grain coarsening can be prevented by annealing at a relatively low temperature (e.g., 1100-1150 F.), but

the resulting product is less ductile than if annealed at a higher temperature.

Conventional processes, for producing hot-rolled strip in coiled form, took precautions to minirnze cold-working before annealing. Temper-rolling the strip before annealing, was avoided; and the use of rollers, around which the strip could be run through the pickling bath, was minimized. Abrading of the steel strip in connection with pickling (e.g., by passing the strip through pinch rolls or by striking the strip), to remove scale, was avoided.

In addition to minimizing cold-working of the strip during the pickling operation, the rec'oiled strip was annealed at a relatively low temperature to minimize recrystallization of such grains as were cold-worked. In the conventional processes, recrystallization caused coarse grains. The problems of recrystallization and coarse grains were particularly severe at the surface of the strip.

SUMMARY OF THE INVENTION In accordance with the present invention, there is produced a hot-rolled strip, in coiled form, having the desired softness and ductility, a relatively small grain size and improved drawability. No precautions are taken to avoid cold-working during pickling or recoiling, no precautions are taken to prevent recrystallization during annealing; and annealing may be conducted at relatively high temperatures.

Essentially, the process for producing the product of the present invention involves finishing hot-rolling at a relatively low temperature (e.g., 1450 F. or lower) and coiling at a relatively low temperature (e.g., 950 F. or lower).

Because cold-working during the pickling operation need not be avoided, the strip may be abraded in connection with pickling, to facilitate the removal of scale from the surface of the strip; and the strip may be temper-rolled before annealing.

Because the strip may be annealed at higher temperatures, without worrying about the effects of recrystallization, shorter annealing times may be employed, thus increasing the utilization of annealing equipment.

A hot-rolled strip produced by the process of the present invention is softer and has a smaller grain size, without coarse grains on the surface, than the strip produced in accordance with conventional processes. Accordingly,

DESCRIPTION OF THE PREFERRED EMBODIMENTS A hot-rolled steel strip in coil form, in accordance with the present invention, has a composition consisting essentially of 0.02-0.25 wt. percent carbon, 0.04-0.60 wt. percent manganese, phosphorus and sulphur in. residual amounts and the balance consisting essentially of iron. Preferably, suflicient manganese is included to prevent hot shortness. A preferable embodiment has between 0.050.l Wt. percent carbon and 0.20-0.40 wt. percent manganese. The steel may be rolled from rimmed, semikilled, killed or mechanically capped ingots. If the ingot has been killed, aluminum and/or silicon are present in amounts up to 0.08 wt. percent aluminum and 0.35 wt. percent silicon.

A steel slab, having a composition described above, is heated to an elevated temperature, above the A for the steel, e.g., to 2300 F., and subjected to hot-rolling. The slab is substantially reduced in thickness before it is subjected to the finishing train of rolls in the hot-rolling mill. The temperature of the strip is controlled, as it leaves the finishing train, to a temperature no greater than 1450 F. In a more preferable embodiment, the finishing temperature for the hot-rolling operation is controlled within the range l300-l400 F.

The finishing temperature may be controlled by reducing the initial temperature of hot-rolling; or by holding the strip, before finishing, on a run-out table until the temperature of the strip drops to the desired level.

Because the friction between the strip and the rolling mill increases as the temperature of the strip drops, it

may be desirable to apply lubricant to the strip before it enters the finishing train of the hot-rolling mill, to decrease the friction and decrease the work load on the mill.

Between the finishing train and the coiling reel, the strip is cooled by water sprays until the temperature of the strip, before coiling, is reduced to 950 F. or less. In a more preferable embodiment, the temperature is reduced to 650900 F.

The temperature at which the strip passes through the finishing train of the hot-rolling mill is a temperature at which ferrite grains are cold-Worked. The temperature at which the strip is coiled is a temperature below the recrystallization temperature for cold-worked ferrite. Between the hot-roll finishing step and the coiling step, the strip is cooled at a rate which prevents significant recrystallization of ferrite.

Following coiling, the hot-rolled strip of steel, in coiled form, is allowed to cool (e.g. to room temperature) before pickling. After cooling, the strip is uncoiled and run continuously through a pickling bath, and then recoiled. During pickling, no precautions are taken to prevent cold-working. Typically, during the uncoiling, pickling and recoiling steps, the strip may undergo 3 percent elongation or cold-working and at least 1 percent coldworking.

The uncoiled strip may be abraded prior to entering the pickling bath and while in the bath, .to facilitate removal of scale from the strip. Abrading includes passing the strip through pinch rolls or striking the strip. Moreover, because the hot roll finishing temperature is relatively low, the scale is more readily removable from the strip than would be the case if the finishing temperature was relatively high.

After the strip has been recoiled, it is annealed utilizing, for example, conventional box-annealing apparatus. If desired, the strip may be temper-rolled, before annealing, which gives an improved surface with maximum softness compared to a strip temper-rolled after annealing.

Annealing may be conducted at a temperature of 1000 14,00 F. for a time of 148 hours. The lower the temperature, the longer the annealing time and vice versa. In a more preferable embodiment, the annealing temperature is 1100-1300 F. and the time is 624 hours.

A hot-rolled, pickled strip, in coiled form, in accordance with the present invention, subjected to an annealing operation as described above, has a final grain size no larger than 5 on the ASTM scale; and this is true of both the interior and the surface grains. The same strip has a final Rockwell B hardness no greater than 55. A preferable embodiment of hot-rolled, pickled strip, in coiled form, in accordance with the present invention, has a grain size between 6 and 8 onthe ASTM scale and has'a Rockwell B hardness no greater than 50.

Following annealing, the strip may be. subjected to a temper-roll, which gives an improved surface to the strip and suppresses the yield point in the steel, thereby eliminating what are known as stretcher strains (surface defects manifest by unsightlysurface lines). Temper-rolling after annealing produces a product which is slightly harder than one which has been temper-rolled before annealing.

The following tables reflect the properties of hot-rolled, pickled steel strip, in coiled form, in accordance with the present invention, compared to essentially the same steel prepared in accordance with conventional processes. In each table, a coil identified as A undergoes conven tional processing while coils identified as B, C and D undergo processing in accordance with the present invention.

Table I gives the composition and dimensions of the respective coils.

Table II reflects the mechanical properties of the respective coils after pickling but before annealing.

Table III reflects the hardness and grain size of the strip in the respective coils, as a function of annealing temperature, for an annealing time of 6 hours.

Table IV reflects the hardness and grain size of the strip in the respective coils, as a function of annealing temperature, for an annealing time of 10 hours.

Table V reflects the hardness and grain size of the strip in the respective coils, as a function of annealing temperature, for an annealing time of 14 hours.

Table VI reflects the hardness and grain size of the strip in the respective coils, as a function of annealing temperature, for an annealing time of 24 hours.

Table VII reflects the mechanical properties of the strip in the respective coils after a given anneal.

TABLE I.CHEMICAL ANALYSES AND DIMENSIONS C, Mn, P, S, Thickwt. Wt. wt. Wt. ness, Width, percent percent percent percent inches inches Coil A2 Head 06 38 008 024 114 35 Tail O5 38 008 017 Coil B:

Head .04 .38 .009 .017 .115 35 Tail 04 38 008 012 Coll C: 1

Head 05 39 008 025 115 35 Tail 06 39 009 027 Coil D: i

Hea 06 40 009 027 .122 35 Tail 06 38 008 023 TABLE IL-MECHANICAL PROPERTIESAFTER PICKLIN G AND WITHOUT ANNEALIN G Hardness Rockwell Rb-average Yield strength, Tensile strength, Percent elongation Percent yield As hot p.513 p.s.i. in 2" 2 elongation 2 rolled Aged Average Coil and hot mill practice and 35 Trans- Trans- Trans- Trans- Olsen, pickled days Long'l. verse Longl. verse Long 1. verse Longl. verse inches 2 Flnlsh, 1,560 F., Head 59 60 34, 800 39, 900 46, 200 47, 600 29 36 Coll, 1,140 F., (Standard Practice) Tall 60 62 35, 500 44, 500 48, 900 50, 300 39 37 0. 1

. Finish, 1,460 F., Head 61 64 38,700 45, 600 51,400 50, 300 36 34 0. Coil, 850 F., Tail 69 64 39, 000 42, 500 50, 900 52, 600 82 31 0 Finish, 1,410 F., Head 68 61 34, 300 45, 100 47, 000 54,300 33 l 17 0 Coil, 785 F., Tail 72 68 45, 300 50, 800 56, 100 59, 100 1 22 1 13 0 D:

Finish, 1,340 F., Head. 78 80 52,100 60,800 58,600 68,600 18 18 0 Coil, 660 F., Tail 83 81 54, 600 62, 400 60, 400 69, 300 1 11 12 0 1 Fracture outside gauge lines. 2 Aged days.

TABLE III.HARDNESS AND GRAIN SIZE VERSUS ANNEALING TEMPERATURE ANNEALING TIME 6 HOURS PACK COOLED, TAIL TESTS Hardrirlests Annealed 1,150 F. Annealed 1,225 F. Annealed 1,300 F. Annealed 1,350 F.

after 0 rolling, Hardness, ASTM, Hardness, ASTM, Hardness, ASTM, Hardness, ASTM. Rb R grain size Rb grain size Rb grain size Rb grain size 1 Coarse surface grain. 2 Coarse grain through sample thlckness. 3 Material showed elongated cold-worked grain.

TABLE IV.HARDNESS AND GRAIN SIZE VERSUS ANNEALING TEMPERATURE AN- NEALING TIME 10 HOURS PACK COOLED, TAIL TESTS Annealed 1,150 F. Annealed 1,225 F. Annealed 1,350" F. Hardness after hot Hard- ASTM, Hard- ASTM, Hard- ASTM, rolling, ness, grain ness, grain ness, grain Rb Rb size R size Rb size 1 Coarse surface grain. I 2 Coarse grain through sample thickness.

TABLE V.HARDNESS AND GRAIN SIZE VERSUS ANNEALING TEMPERATURE ANNEALING TIME 14 HOURS PACK COOLED, TAIL TESTS Hardness Annealed 1,150 F. Annealed 1,225 F. Annealed 1,300 F. Annealed 1,350 F.

after 01: rolling, Hardness, ASTM, Hardness, ASTM, Hardness, ASTM, Hardness, ASTM Rb Rb grain size Rb grain size Rb grain size Rb grain size 1 Coarse surface grain. 2 Coarse grain through sample thickness.

TABLE VI.HAR1)NESS AND GRAIN SIZE VERSUS ANNEALING TEMPERATURE ANNEALING TIME 24 HOURS PACK OOOLED, TAIL TESTS Hardness Annealed 1,150 F. Annealed 1,225 F. Annealed 1,300 F. Annealed 1,350 F.

a ter 0 rolling, Hardness, ASTM, Hardness, ASTM, Hardness, ASTM, Hardness, ASTM. Rb Rb grain size Rb grain size R grain size Rb grain size l Coarse surface grain. 1 Coarse grain through sample thrckness.

TABLE VII.-MECHANICAL PROPERTIES AFTER ANNEALING Process tem- Annealing peratures, F. cycle 1 Rockwell Average Temp, Time, hardness, Olsen, Grain Finish Coil (hrs) Rb inches size Yield Strength, Tensile Strength, Percent Elonga- Yield Point p.s.i. p.s.i. tion in 2 inches Elongation Trans- Trans Trans- Trans- Coil Longl verse Longl. verse Longl. verse Longl. verse A 31, 300 30, 300 42, 800 43, 800 38 2 31 3. 6 3. 5 B 34, 700 36, 300 42, 300 44, 600 44 24 5. 5 5.0 o 31, 100 31, 300 41,800 44, 200 42 2 36 3. 5 2. 5 D 28, 200 32, 350 39, 500 43, 800 44 36 3. 4 3. 0

l Furnace cooled. 2 Fractured outside gauge marks.

The foregoing tables illustrate the improvement in softness and decrease in grain size resulting from a process in accordance with the present invention, compared to conventional processing. Improved softness and decreased grain size result in improved drawing properties for the coil of hot-rolled, pickled strip.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

We claim: 1. In a proces for producing a coil of hot-rolled strip steel, said process comprising the steps of, in sequence, hot-rolling a strip, cooling said strip, coiling the strip, cooling the coil, uncoiling, pickling, recoiling and annealing, the features of:

finishing said hot-rolling step at an elevated temperature no greater than 1450 F.;

cooling said strip, between said finishing of said hotrolling step and said coiling step, at a cooling rate sufficiently rapid to prevent substantial recrystallization of ferrite; and performing said coiling step at a temperature no greater than 950 F. and which is below the recrystallization temperature for cold-worked ferrite;

whereby there is produced a coil of soft, ductile strip steel having a relatively fine grain size both in the interior and at the surface of the steel.

2. In a process as recited in claim 1 wherein:

said annealing step comprises heating said steel at a temperature and for a time which gives a Rockwell B hardness no greater than 55 with a grain size no larger than 5 on the ASTM scale.

3. In a process as recited in claim 2 wherein:

said annealing step is performed at a temperature between 1000 F. and 1400 F. for a time between one hour and 48 hours. 4. In a process as recited in claim 2 wherein: said annealing step is performed at a temperature greater than 1150 F. and up to 1300" F.

5. In a process as recited in claim 2 wherein:

said steel, after said annealing step, has a Rockwell B hardness no greater than 50, and a grain size between 6 and 8 on the ASTM scale.

6. In a process as recited in claim 1 wherein:

said hot-rolling step is finished at a temperature between 1300 F. and 1400 F. 7. In a process as recited in claim 1 wherein:

said coiling step is performed at a temperature between 650 F. and 900 F. I Y 8. In a process as recited in claim 1 wherein: said hot-rolling step is finished at a temperature between 1300 F. and 1400 F.; V and said coiling step is performed at a temperature between 650 F. and 900 F. 9. In a process as recited in claim 1 wherein said steel has a composition consisting essentially of:

0.02-0.25 wt. percent carbon; 0.04-0.60 wt. percent manganese; and the balance consisting essentially of iron. 10. In a process as recited in claim 1 wherein: said steel is subjected to 1%-3% cold working following said hot-rolling step. 11. In a process as recited in claim 1 wherein said steel is temper-rolled after pickling and before annealing. 12. In a process as recited in claim 1 wherein said steel undergoes an abrading step in connection with pickling and before annealing, to remove scale.

13. In a process as recited in claim 1 wherein: said annealing step is performed without avoiding recrystallization. 14. In a process as recited in claim 1 wherein: the steps following said hot-rolling step and preceding said annealing step are performed without minimizing cold working.

References Cited UNITED STATES PATENTS 2,597,979 5/1952 Darmara 148-42 2,606,848 8/1952 Farling et a1 148-12 3,245,844 .4/1966 Weber 148l2 3,248,270 4/1966 Laidman et al. 148-12 3,320,099 5/1967 Weber 148--12 X FOREIGN PATENTS 762,127 11/1956 Great Britain. 773,071 4/ 1957 Great Britain.

OTHER REFERENCES aking, Shaping and Treating of Steel, 1957, pp. 593- 94.

G. K. WHITE, Assistant Examiner U.S. c1. X.R.

mum UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION p 3 513, U30 Dated May 19 1970 Inventor(s) William H. McFarland and Henry M. Tietz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 14, "product" should be -produce- Column 6, Table 2, 7th column of numbers, 4 rows down,

"82" should be 28.

Column 7, Table 7, last column of numbers [Coil B) "5.0" should be 4.0--.

SIGNED SEAIED (SEAL) Attest:

Edward M. Fletcher, Jl.

Ccmlsaioner of Patents