US2979394A - Method for preparing high grade rimmed steel with wide ferritic peripheral zone - Google Patents

Description

April 11, 1961 METHOD FOR PREPARING HIGH GRADE RIMMED STEEL W. WERZ WITH WIDE FERRITIC PERIPHERAL ZONE Filed Nov. 115, 1958 2 Sheets-Sheet 1 W/LHELM WERZ INVENTOR.

ATTOR N EYS Aprll 11, 1961 w. WERZ 2,979,394

METHOD FOR PREPARING HIGH GRADE RIMMED STEEL WITH WIDE FERRITIC PERIPHERAL ZONE 2 Sheets-Sheet 2 Filed NOV. 13, 1958 INVENTOR.

W/LHELM WERZ ATTOR N EYS METHOD FOR PREPARING HIGH GRADE RIMMED STEEL WITH WIDE FERRITIC PE- RIPHERAL ZONE 1 Wilhelm Werz, Rheinhausen, Germany, assignor to Niederrheinische I-liitte Aktiengesellschaft, Duisburg, a corporation of Germany Filed Nov. 13, 1958, Ser. No. 773,797 Claims priority, application Germany Nov. 14, 1957 7 Claims. (Cl. 75-57) 'of the above mentioned type which may be used advantageously for welding, cold drawing and cold upsetting or heading purposes.

It is still another object of the invention to provide a method for preparing wide ferritic peripheral rimmed steel within wider limits of casting temperature, casting time and equilibrium ratio of (C)-(O) content of the melt.

Other and further objects will become apparent from a study of the specification and accompanying drawings in which: i

Figure 1 is a schematic cross-section of a melt bar produced by conventional methods showingthe dimensions of the peripheral layer and central zone in millifmeters;

Figure 2 is a schematic cross-section of a melt bar produced in accordance with the method of the invention showing the dimensions of the peripheral layer and central zone in millimeters;

Figures 3, 4, and 5 show sectional photographicviews of Baumann prints taken of the head, middle and foot of an ingot block, respectively, said block having been produced by conventional methods;

Figures 6, 7, and 8 show sectional photographic views of Baumann prints taken of the head, middle and foot of an ingot block, respectively, said block having been produced in accordance with the method of the invention, and

Figure 9 shows a photographic view of the fracture of a head slice of an ingot block produced in accordance with the method of the invention.

In conventional operation, molten rimmed steel separates during solidification in the ingot mold, with strong evolution of gases, into a peripheral layer or rim made up almost completely of pure ferrite crystals and a segregated central zone in which the far greater proportion of the accompanying elements is contained. The relatively pure, soft ferritic peripheral layer, which is free i of surface defects, imparts to the rimmed steel its special qualities, such as good welding properties and properties which render the steel readily suitable for drawing and cold upsetting or heading operations. 7

For thesepurposes, the steel must have a sufiiciently thick peripheral layer, so that sections of steel which are to be welded, rolled wire rods which are to be drawn and steel which is to be subjected to cold upsetting or heading finishing may be worked to their best advantage.

ted States Patent 0 2,979,394 Patented Apr. 11, 1961 Hence, in the production of high grade rimmed steel, it is of particular importance that the casting temperature, as well as the speed of casting and solidification of the melt are so adapted to the heat content of a particular sized ingot that the cast ingot poured will boil as long as possible in the mold, enabling a comparatively wide ferritic peripheral zone to form and a correspondingly narrow segregated central zone to solidify. In such cases, the ring of gas pockets remaining between the two Zones is seated so deeply in the steel that it readily welds within the steel upon work-up on the blooming roll and is thus prevented from causing any surface defects on the surfaces of the resulting bars or slabs.

To attaina long boiling period in the mold under conventionalconditions, in addition to the above mentioned factors, the carbon and oxygen content must be far from the equilibrium product of (C) (O), and a vigorous corresponding carbon monoxide generation must be pro duced which will last for a sufiiciently long period. These various conditions are only obtained in practice where the melt is cast with an 0.06 to 0.08% carbon content and an extremely high oxygen content. In this way, after solidification, a larger or smaller proportion of residual oxygen may remain in the steel. On the other hand, melts which are produced with less than or more than 0.06 to 0.08% carbon,whether the oxygen content is low or not, have shorter boiling periods in the mold and accordingly solidify with narrower peripheral layers and wider segregated central zones. Steels made in this way cannotbe used for the special purposes desired, since they do not possess the required properties of wide ferritic peripheral zone high grade rimmed steel.

It is evident from the foregoing considerations that the conventional production of high grade rimmed steel having a wide ferritic peripheral zone, useful for welding, drawing and cold upsetting or heading purposes, is beset with a substantial degree of uncertainty, and is possible only within comparatively narrow limits of casting temperature, speed of casting and solidification, and ratio of carbon-oxygen content.

It has been found in accordance with the method of the present invention, that the uncertainty involved in the production of high grade rimmed steel having a wide ferritic peripheral zone may be controlled and minimized, and that because of its versatility adherence to the previously prescribed narrow limits need not be followed. In accordance with the method of the invention, rimmed molten steel may be charged with nascent hydrogen up to the uppermost saturation limit with the result that the molten mass boils for a considerably longer period of time in the mold, and thereafter solidifies with a very wide, almost purely ferritic, peripheral zone.

In order to attain such results, hydrogen in the form of calcium hydride may be added in small portions to the melt as it enters the ladle. From the oxothermal decomposition of the calcium hydride, about 0.4 cubic meter of hydrogen is liberated per kilogram of calcium hydride, together with a corresponding quantity of heat. The heat which is so liberated increases the casting temperature of the melt, so that the melt becomes more thinly fluid and is charged to the uppermost limit of saturation with hydrogen. After the casting has been completed, the dissolved hydrogen emerges from the melt during the solidification period in the mold. Since this hydrogen evolution and accompanying increase in heat occurs simultaneously with the carbon monoxide reaction, the boiling and solidification periods of the melt in the mold is thereby correspondingly increased. In the case of a 5-ton ingot, these periods may be extended to almo double their normal periods.

or" the (C)'(O) ratio.

In this connection, the hydrogen given off reinforces the purifying action of the carbon monoxide during the boiling period, while residual oxygen which would otherwise remain in the melts is'deoxidized by the hydrogen via the gas phase and is driven oti. ingots formed from melts treated in accordance with the method of the invention, therefore, solidify with a wide, almost pure,

peripheral layer, a correspondingly smaller segregated central zone and a ring of gas bubbles located deeply below the ingot surface.

three minutes, the immersion pyrometer indicated a melt ing temperature of 1620 C. in the casting ladle. The melt was then cast into four spans of four 5-ton ingots each, Without hot top. The melt was highly fluid and boiled in the ingot molds for about 27 to 32 minutes without rising noticeably. After each span was cast, a small specimen block was correspondingly cast (test specimens 2 to 5 of Table 1).

Table 1, which follows, contains the analysis values of Due to the fact that the dis the furnace specimen and of the span specimens:

TABLE '1 Test Specimen O Si Mn P S As Cu Sn Cr N 1. percent" 0. 013 0.026 0. 023 0.20 0. 025 0 07 0.004 2. d 0.012 0. 024 0.022 0.20 0 024 0 07 0.004 3. 0.012 0.023 0.022 0.20 0 023 0 00 0.004 4. 0.013 0. 025 0.021 0. 0 024 0 07 0.003 5. 0.012 0. 022 0. 022 0.20 0 023 0 07 0. 003

solved hydrogen almost completely boils out of the melt together with the carbon monoxide during the solidification period, steel made in this manner is not susceptible to flaking.

The calcium hydride used in accordance with the method of the invention may be any technical grade of calcium hydride, such as approximately 65% calcium hydride, the remaining being made up of about 5% calcium carbide, 2% iron, silicon and aluminum, and 28% calcium oxide. This technical calcium hydride may be used in the proportion of from 0.3 to 0.5 kilogram per ton of steel.

Therefore, in accordance with the method of the invention, rimmed steel may now be produced with a carbon content of more than or less than 0.06 to 0.08% and an oxygen content Which is closer to equilibrium in terms Heretofore, the product of the (C)-(O) ratio content, closer to equilibrium, was insufficient to sustain the longer boiling period in the mold necessary to produce a rimmed steel having a wide ferritic peripheral layer and good welding, drawing and cold upsetting or heading properties.

The invention is further described by the following example, but it is to be understood that the invention is not to be limited thereto.

Example For one melt SAE 1010, a charge of about 70% scrap iron and 30% liquid pig steel was melted in an 80-ton Siemens-Martin furnace; and in the customary manner this charge was boiled down with ore without change of slag to 0.13% carbon. Seven minutes before drawing the melt by tapping, a melting temperature of 1630 C. was measur d in the furnace by means of an immersion pyrometer, and subsequently a last ladle specimen was scooped from the melt (test specimen 1 of Table l).

The molten product was then drawn from the furnace into a casting ladle, the running time of the steel amounting to eight minutes. After the first minute, at five equal one minute intervals up to butnot including the seventr minute, five paper bags containing 5 kilograms each of technical calcium hydride (65%) were added, one at each interval, to the molten steel. Twenty-five kilograms in all were used. The remaining content of the technical product was made up of about 5% calcium carbide, 2% Fe, Si and Al, and 28% calcium oxide. The addition product disintegrates at about 1100 C. exothermic temperature, and from the kilograms of technical calcium hydride, about 10 cubic meters of nascent hydrogen and 50 cubic meters of calcium vapor were liberated, which insofar as they were not absorbed by the molten steel, were combusted in the atmosphere upon their emergence from the melt.

When the furnace slag had followed during the next The comparative oxygen and hydrogen values were obtained by means of test rods from the furnace specimen and by means of 90 mm. square ingot slices from the foot,

H5 middle and head portions of a rolled-out block from the second span specimen. Thus, the test rods of the ingot slices lay half Within the range of the ferritic peripheral zone and half within the range of the liquation nucleus.

The mean values for oxygen and hydrogen obtained from each of these three portions of the rolled-out block as well as from the furnace specimen test rods are shown in Table 2:

TABLE 2 The flocculation test was made in the customary manner with ingot slices 12 mm. in thickness from the foot, middle and head portions, respectively, of the block using the hardness test technique. Accordingly, the slices were each notched 2 mm. deep in the middle of the flat surface, then hardened at 950 C. after a holding time of 15 minutes in 10% soda-lye and subsequently broken. Figure 9 of the accompanying drawing shows the head slices free from flocculation at the point of fracture.

As illustrated by Figure 1 of the drawing, conventional melts which are not charged with hydrogen, and which contain about .10% carbon and the customary amount of accompanying elements, after the rolling out of a 5-ton ingot thereof to a 95 mm. square bar, have in cross-section, a peripheral zone area of about and a segregated central zone area of about 45%. In contrast thereto, as illustrated by Figure 2 of the drawing, this ratio increases in melts produced in accordance with the invention which are charged with hydrogen to the uppermost limit of saturation. Thus, after the rolling out of a 5-ton ingot made from a melt produced in accordance with the invention, using 25 kilograms of calcium hydride added in 5 kilogram portions, the resulting 95 mm. square bars had in cross-section, a peripheral zone area of about 75% and a segregated central zone area of about 25%.

The characteristic differences between conventionally produced melts and melts produced in accordance with the method of the invention may be further seen from Table 3 which follows, wherein approximately 80-ton melts from a basic open-hearth furnace were cast with group teeming into S-ton ingots and thereafter rolled into 95 mm. square bars:

6 melt with hydrogen up to the extreme nascent hydrogen saturation limit prior to casting.

Melt A corresponds to the bar produced in the conventional manner as shown schematically in Figure 1, wherein the width of the peripheral layer is 16 mm, occupying 5O cm. or 55% of the cross-sectional area, and the width of the central zone is 63 mm., occupying 40 cm. or 45% of the cross-sectional area.

Melt B corresponds to the bar produced in accordance with the method of the invention as shown schematically in Figure 2, wherein the width of the peripheral layer is 24 mm., occupying 68 crn. or 75 of the cross-sectional area, and the width of the central zone is 47 mm., occupying 22 cm. or 25% of the cross-sectional area.

Figures 3, 4 and 5 show Baumann prints of the head, middle and foot, respectively, of a block produced by conventional methods while Figures 6, 7 and 8 show Baumann prints of the head, middle and foot, respectively, a block produced in accordance with the method of the invention. It is obvious from these views that the head, middle and foot of the block produced in accordance with the method of the invention contain a desirably wide peripheral layer and a narrow central zone while the conventionally produced block has a comparatively narrow peripheral layer and a wide central zone.

While the foregoing specification and accompanying drawings have been set forth for the purpose of illustration, it is to be understood that various changes and modifications may be made without departing from the spirit and scope of the invention, which is to be limited only by the scope of the appended claims.

I claim:

1. A method for the production of high grade rimmed steel having a wide ferritic peripheral layer and a correspondingly narrow central zone, consisting of saturating the melt with nascent hydrogen prior to casting then allowing said hydrogen to boil off during the solidification stage.

2. A method for the production of high grade rimmed steel having a wide ferritic peripheral layer substantially pure and free of surface defects and a correspondingly narrow central zone which comprises substantially all the accompanying elements, consisting of charging the steel 3. A method as in claim 2, wherein the hydrogen charged is in the form of calcium hydride.

4. A method as in claim 2, wherein the hydrogen charged is in the form of calcium hydride used within the proportion of 0.3 to 0.5 kilogram per ton of steel.

5. A method for the production of high grade rimmed steel having a wide ferritic peripheral layer free of surface defects and a correspondingly narrow central zone consisting of charging the melt prior to casting with calcium hydride within the proportion of 0.3 to 0.5 kilogram per ton of steel melt, whereby to generate additional heat and saturate said melt with nascent hydrogen to the uppermost limit, extend the period of boiling and the period of solidification of the melt in the mold, and purify said melt during said periods.

6. A method as in claim 5, wherein said calcium hydride is at least pure.

7. The method of claim 5, consisting of adding 25 kgs. of 65 calcium hydride to 80 tons of a molten charge constituted of scrap iron and 30% liquid pig steel, at a temperature above 1100 0., whereby said calcium hydride disintegrates, liberating 10 cubic meters of nascent hydrogen and allowing said liberation of hydrogen from said molten mass to continue for 27-32 minutes, whereby the resulting rimmed steel bars have a peripheral zone constituting about of the cross-sectional area and a narrow central zone constituting about 25% of said cross-sectional area of said bars.

References Cited in the file of this patent UNITED STATES PATENTS 1,888,132 Kinzel Nov. 15, 1932 2,874,038 Ruhenbeck et al. Feb. 17, 1959 FOREIGN PATENTS 2,314 Great Britain 1879 OTHER REFERENCES The Journal of Iron and Steel Institute, Sixth Report on the Heterogeneity of Steel Ingot (Section VIII, pages 137-151, plates XIII-XVI; pp. -146, plate XV relied on), 1935, London, England.

Claims (1)

1. A METHOD FOR THE PRODUCTION OF HIGH GRADE RIMMED STEEL HAVING A WIDE FERRITIC PERIPHERAL LAYER AND A CORRESPONDINGLY NARROW CENTRAL ZONE, CONSISTING OF SATURATING THE MELT WITH NASCENT HYDROGEN PRIOR TO CASTING THEN ALLOWING SAID HYDROGEN TO BOIL OFF DURING THE SOLIDIFICATION STAGE.

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