US3412781A - Process of using a low carbon steel composition in a continuous casting process - Google Patents

Description

United States Patent 3,412,781 PROCESS OF USING A LOW CARBON STEEL COMPOSITION IN A CONTINUOUS CASTING PROCESS John H. Richards, Penn Hills Township, Allegheny County, Pa., assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Sept. 21, 1965, Ser. No. 489,060 8 Claims. (Cl. 164-76) ABSTRACT OF THE DISCLOSURE A process of using a low carbon steel composition in a method for the continuous casting of steel Which comprises introducing to a casting mold a steel whose composition is adjusted to contain 0.01 to 0.08% carbon, 0.20 to 0.60% manganese, 0.03 to 0.08% silicon, not over 0.015% aluminum, and the balance essentially iron and incidental impurities and continuously casting same.

This invention relates to processes for the continuous casting of steel and more particularly to processes in which the composition of the molten steel is adjusted prior to casting in order to give cast slabs having good mechanical properties and especially suited for flat rolling.

A large part of the demand for flat rolled steel, such as sheet and tinplate, has been satisfied by rimmed drawing quality steel produced in conventional ingot molds. Steel of this type has customarily had a low carbon content of about 0.03 to about 0.06% and a silicon content not over 0.02%. Violent rimming action in the ingot mold with evolution of large quantities of gas is characteristic of rimmed steel casting.

The steel compositions customarily used for the production of rimmed steel do not lend themselves to the continuous casting process because of violent rimming action. Evolution of more than minute quantities of gas in a continuous casting mold results in blow holes and cavities within the casting, because the gas does not have an opportunity to escape from a continuously formed casting as it has in the case of a conventional ingot. Hence it is desirable to form continuous steel castings from steel having a composition which Will cause little or no gas evolution in the mold. At the same time it is imperative that the steel produced in the continuous casting mold have mechanical properties at least as good as those of rimmed drawing quality steel. Furthermore, the quantity of metal oxides, such as iron oxide, alumina, and silica, must be held to a minimum because these oxides tend to accumulate along the surfaces of continuously formed castings and produce castings of inferior quality which require considerable conditioning before they can be rolled.

It is an object of this invention to provide a steel composition especially adapted for the continuous casting of slabs which are suitable for rolling into flat rolled products.

It is a further object of this invention to provide a continuous casting process in which the liberation of gas in the mold is kept to a minimum.

It is a further object of this invention to provide a continuous process for casting slabs having mechanical properties equal to or better than those of rimmed drawing quality steel.

It is a still further object of this invention to provide a continuous process for casting slabs having mechanical properties equal to or better than those of rimmed drawing quality steel.

It is a still further object of this invention to provide a continuous casting process in which the quantity of metal oxides introduced into the mold is minimized.

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These and other objects will be apparent from the disclosure which follows.

According to this invention the composition of a furnace melt from a steel making furnace is adjusted by the addition of manganese and silicon, and aluminum if desired, to give molten steel having a composition as follows:

Percent C .01.08 Mn .20-.60 Si .O3-.08 Al Not over .015

Fe and incidental impurities balance.

The molten steel of the above composition is then introduced into a continuous casting mold. Best results are obtained with a molten steel of the above composition in which the sum of the silicon content and 0.1 times the manganese content is not less than the carbon content.

The carbon content of the molten steel should not be less than .01%, because the oxygen content of steel is excessively high for continuous casting when the carbon content drops below 01%. Also, the lining life of a steel making furnace is shortened when the carbon content is below 01%. The carbon content of molten steel should not exceed 08%, because sheet material rolled from eastings containing more than 08% carbon become excessively hard and therefore unsuited for deep drawing applications when annealed according to standard procedures.

The manganese and silicon ranges for the molten steel are chosen because of the synergistic effect of these amounts in preventing pinhole porosity of steel whose carbon content is in the range of .01% to .08%. Furthermore, the oxygen content of the steel can be estimated and controlled more reliably when the manganese content is in the range of this invention than when the manganese content is less.

The amount of acid soluble aluminum in the steel is preferably not greater than 015%, because larger amounts tend to cause the formation of excessive quantities of non-metallic inclusions. Furthermore the presence of alumina in large amounts in the non-metallic inclusions is particularly undesirable because metal oxide inclusions containing large amounts of alumina tend to form massive agglomerates rather than glassy films along the side walls of the mold as the casting descends. These massive agglomerates are very difficult to remove by the action of the cooling water sprays below the mold, and mar an excessively large portion of the surface area of the casting so that extensive conditioning of the slab is required.

A preferred molten steel composition for continuous casting according to this invention is as follows:

Percent C .03-.06 Mn .35-.45 Si .03-.08 Al Not over .015

Fe and incidental impurities balance.

The sum of the silicon content and 0.1 times the manganese content is not less than the carbon content in the preferred molten steel compositions of this invention.

Castings formed from molten steel compositions within the above indicated preferred range give outstanding mechanical properties when finished according to standard mill finishing treatments.

Carbon levels of at least 03% are desirable because the amounts of oxygen in steel containing less than .03% carbon are frequently so high as to cause undue gas evolution in the mold and to cause the oxidation of steel and of deoxidizer elements such as manganese, aluminum and silicon, forming excessive quantities of non-metallic inclusions. Carbon contents in the range of 03% to .06% are also desirable for best results in annealing of fiat rolled products such as strip obtained after the casting is rolled.

Molten steel for the present process may be obtained from any steel making furnace, such as a basic oxygen process (BOP) furnace or an electric furnace, the former being preferred. The composition of a furnace melt from a basic oxygen process furnace used for making steel for the instant process is customarily as follows:

C percent .03-.06 Si do .02 Mn do 0. 1-0.25 O p.p.rn 600900 S "percent" .02 P do 0.015

The standard basic oxygen furnace practice for making low carbon steel may be used without modifiaction. However, it is frequently advantageous to modify the customary BOP furnace practice by charging enough manganese to the furnace to obtain a residual manganese con tentof at least 0.1% in the furnace melt. It is essential that the residual manganese content in the furnace melt be at least 0.10% when the sulfur content of the iron supplied to the furnace is in a normal range of from about 0.025% to 0.050%, in order to keep the sulfur content in the furnace melt down to an acceptable amount not greater than 0.02%. Residual manganese contents of over 0.1% are obtained by the addition of a manganese ore to the furnace charge, or by the addition of hot metal (iron from the blast furnace) containing enough manganese to give the residual manganese content of at least 0.10%. The use of manganese ore is preferred, since high manganese hot metal usually contains so much phosphorus as to raise the quantity of phosphorus in the steel casting above acceptable limits. The use of manganese ore makes it possible to obtain the desired residual manganese content in the furnace melt without also obtaining an excessively high phosphorus content. Either a high grade or low grade manganese ores may be used. The amount of ore added is at least about 0.1% by weight of Mn, based on the total weight of the furnace charge. Generally larger quantities are required because a large part of the manganese is lost to the furnace slag.

The temperature in the furnace is customarily held within the range of 2850 to 3000 F. Temperatures above 3000 F. are to be avoided, because these high tempera tures cause rapid deterioration of the furnace lining, resulting in the present of excessive quantities of refractory oxide slag in the furnace melt.

It is impossible to produce a furnace melt having the desired composition for introduction into a continuous casting mold according to the conventional process. The equilibrium relationships existing between carbon and silicon at the usual furnace temperatures require that either the carbon content be above the acceptable maximum of 0.08% or that the silicon content be below the minimum of 0.03%. It is necessary to form a furnace melt having the desired carbon content (which cannot be satisfactorily reduced in the molten steel after it has been tapped from the furnace) and to add silicon as required to bring up the silicon content to the desired level. It is also necessary to add manganese to bring up the content to the level desired for the purposes of this invention. A large portion of the manganese content of the molten steel introduced into the mold is added after tapping of the furnace melt, because it is impractical to charge enough manganese to a basic oxygen process furnace to furnish the desired manganese content in view of the excessive losses of manganese to furnace slag.

It will be noted that the silicon content for steels to be used in the present invention is considerably higher than the silicon content of steels used for the production of rimmed drawing quality steel. The silicon content according to the present invention is in the range of .03.08%, while the silicon content and rimmed drawing quality steels intended for rolling into fiat rolled products is customarily not allowed to exceed .02%. Surprisingly the higher content according to this invention is not detrimental but actually beneficial to the mechanical properties of the rolled steel. Furthermore, the higher silicon content is necessary in order to avoid rimming action with the production of cavities and blow holes in the continuous casting.

Manganese may be added in the ladle in the form of silicomanganese, high or medium carbon ferromanganese, or electrolytic manganese. The addition of silicomanganese also supplies the entire quantity of silicon which must be added in order to bring the molten steel composition up to the desired silicon level of 0.030.08%. Customarily about 6 to 10 lbs. per ton of silicomanganese and about 2 to 4 lbs. per ton of medium carbon ferromanganese are added in order to supply the necessary manganese and silicon to the molten steel. Instead of adding medium carbon manganese, either high carbon manganese or electrolytic manganese may be added. Frequently the amounts of high carbon manganese required are somewhat less than the amounts of medium carbon ferromanganese normally required, being only about 1 to 2 lbs. per ton in most instances.

The silicomanganese and the ferromanganese are most conveniently added to the molten steel during the filling of the tapping ladle with the furnace melt obtained in the steel making furnace. Best results are obtained when the silicomanganese and ferromanganese are added during the filling of the middle third of the ladle.

In addition to manganese and silicon, it is frequently desirable to add small amounts of aluminum to the tapping ladle in order to improve the characteristics of the non-metallic inclusions so as to minimize conditioning of continuously cast slabs formed in the present process. To this end it has been found that the addition of about /2 to about 1% lbs. per ton of aluminum in the ladle simplifies the problems of inclusions from the solidified slabs.

After the composition of the steel has been adjusted so that it is within either the broad or preferred range described above, the steel is then poured into the upper end of an open-ended tubular water cooled continuous casting mold. solidification of the steel is initiated in the mold. A casting having a solidified skin surrounding a liquid metal core is withdrawn downwardly from the mold. solidification of the entire cross-sectional area is accomplished by means of water sprays located below the mold, as is conventional in the art.

The solidified slab may be further treated as for example by hot rolling, in order to obtain fiat rolled products such as strip. These fiat rolled products may be annealed and coiled and may be finished according to conventional metal Working techniques.

This invention will now be described with reference to specific embodiments thereof as illustrated in the examples which follow.

EXAMPLE The following standard procedure was used in all of the heats described in this example:

A steel furnace melt was tapped from a basic oxygen process (BOP) furnace into a tapping ladle. The steel furnace melt was made according to a modified standard BOP furnace practice for making low carbon steel, using by weight of hot metal (iron from a blast furnace) and 25% by weight of scrap, plus 10 to 20 pounds per ton of a manganese ore containing about 50% Mn. The furnace practice was standard except for the addition of the manganese ore. The carbon and manganese contents of the steel as tapped from the furnace were determined. The furnace melt contained insufiicient manganese and silicon, to give a steelhaving the desired composition for molding in a continuous casting mold. Hence manganese and silicon were added to the steel in the tapping ladle. Manganese was added in the form of silicomanganese and either medium carbon ferromangaremove surface imperfections, most of which resulted from mechanical defects resulting from handling the last slab. The rather small percentage of the surface area which required conditioning is indicated in Table 11 below.

TABLE II Steel composition, percent (slab analysis) Percent Heat No Slab condi- N o. C M11 Si Total 02, tioning Al p.p.m.

1 2 0. 038 0. 47 0. 032 0. 004 166 2 5 0. 048 O. 33 0. 040 0. 006 197 8 3 1 0. 060 0. 45 0. 038 0. 005 297 5 4 5 0. 050 0. 36 0. 024 O. 011 212 5 5 1 0. 055 0. 48 0. 040 0. 008 171 2 2 0. 055 0. 48 0. 040 0. 008 1 3 0. 055 0. 48 0. 040 0. 008 1 4 0. 055 0. 48 0.040 0.008 2 5 0. 055 0. 48 0. 040 0. 008 3 6 1 (l. 050 0. 40 0. 040 O. 005 217 1 2 0. 050 0. 40 0. 040 0. 005 2 3 0. 050 0. 40 0. 040 0. 005 3 4 0.050 0. 40 0. 040 0. 005 8 5 0. 050 0. 40 0. 040 0. 005 4 6 0. 050 0. 4O 0. 040 0. 005 2 nese, high carbon, ferromanganese, or electrolytic manganese as indicated in each heat. The silicomanganese also supplied the silicon required. Aluminum was also added to the tapping ladle in the amounts indicated. The addition of manganese, silicon and aluminum to the tapping ladle deoxidizes the steel sufliciently to avoid blow holes and other evidences of an open or unkilled steel in the mold. The carbon and manganese contents of the furnace melt, and the amounts of silicomanganese, ferromanganese, and aluminum added to the tapping ladle are indicated in Table I below.

TABLE I Tap composition, Ladle addition, lbs/ton percent Heat Slab No. No. Medium Silico- 0 Mn carbon manga- Alumiierronese num manganese Electrolytic manganese.

Molten steel was poured from the ladle into the upper end of an open-ended water cooled tubular continuous casting mold while the lower end of the mold was closed with a starter bar. Partial solidification of the steel was initiated in the mold. When the mold was substantially filled with molten steel, the starter bar and the attached casting having a solidified shell and a molten core were lowered below the mold as pouring of molten metal from the ladle was continued. The cast was cooled by spraying water on its surface as it descended below the mold until it was completely solidified. The casting was cut into slabs of predetermined length. These slabs were allowed to cool to room temperature. Samples of these slabs were taken for analysis of carbon, manganese, silicon, aluminum and oxygen. These analyses are reported in Table 11 below.

A small amount of scum consisting essentially of iron oxide and of the oxides of the deoxidizing elements (manganese, silicon and aluminum) was observed on the molten metal surface in the mold in each heat, the amount varying somewhat from heat to heat. Most of this scum descended with the casting along the side walls thereof,

metal slabs were conditioned by scarfing in order to The cold slabs were reheated and hot rolled in a hot strip mill. This mill included a plurality of roughing stands and a plurality of finishing stands followed by a coiler. Temperatures of the steel were taken immediately after the last roughing stand, immedaitely after the last finishing stand, and immediately before the coiler. These temperatures were reported in Table 'III below as roughing, finishing, and coiling temperatures respectively.

The hot strip coil was pickled and then cold rolled in a multiple stand cold mill. The percentage reduction in thickness (which is the difference between the initial and the final thickness divided by the initial thickness and multlplied by was 61.4% in all cases.

The cold rolled strip was annealed in an inert atmosphere for the soak time and at the average soak temperature indicated in Table III below.

TABLE III Hot rolling data, temp., F. Annealing data l-Ieat No. Slab No. Rongh- Soak Average ing Finish Coiling time, soak hrs. temp., F.

Lower yield point, tensile strength, and elongation determinatlons were made in the longitudinal direction according to standard ASTM procedures. Results are reported in Table -IV below.

TABLE IV Heat Slab Lower yield, Tensile strength, Elong. in 2 No. No. p.s.1., long1- p.s.i., longiinches, percent,

tudinal tudinal longitudinal The above steels are equal to or superior to rimmed DQ steel in their ability to be drawn into shapes without Fe and incidental impurities balance.

which comprises the steps of adjusting a steel furnace melt to said composition and casting said composition continuously.

2. A process according to claim 1 in which the sum of the silicon content and 0.1 times the manganese content of said melt is not less than the carbon content.

3. A process of using a steel composition containing Percent C .03.06 Mn .3 5.45

Si .O3.08 Al 0.004.O 1 5 balance substantially iron and incidental impurities, comprising the steps of adjusting a steel furnace melt to said composition and casting said steel continuously.

4. A process according to claim 1 in which the residual manganese content of said furnace melt is not less than 0.10%.

5. A process according to claim 1 in which said furnace melt is tapped from a basic oxygen process steel making furnace.

6. A process according to claim 5 in which manganese ore in an amount of at least 0.1% by weight of Mn, based on the total weight of the furnace charge, is charged to said furnace.

7. A process according to claim 1 in which said melt contains not over 02% sulfur.

8. A process according to claim 1 including the steps of forming a solidified steel slab in said mold and rolling said slab into a fiat rolled product.

References Cited UNITED STATES PATENTS 2,983,598 5/1961 Wheatley l64-57 3,215,567 11/1965 Yoshida l4831 3,262,821 7/1966 Yoshida l48l2.ll

OTHER REFERENCES Metals Handbook, 1939 edition, pp. 1243 and 1244.

JOHN F. CAMPBELL, Primary Examiner.

P. M. COHEN, Assistant Examiner.