US3724528A - Method for manufacturing of sound killed steel ingots - Google Patents

Abstract

A high quality homogeneous sound killed steel ingot having a reduced quantity of negative segregation and of inverted V-shaped segregation zones is produced by adding a stirring agent to the molten steel during the solidification process and at a time period between the period during which the negative segregation section and the inverted V-shaped segregation zones are formed, and the period during which the V-shaped segregation zone is formed wherein the stirring agent is preferably an element, alloy, compound or mixture thereof of an element selected from the group of elements of Groups I-a or II-a of the Periodic Table or zinc, or the halides of an element of Groups I-b, III-b, IV-a, IV-b, VI-a, VII-a, or VIII.

Description

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[451 Apr. 3, 1973 I METHOD FOR MANUFACTURING OF SOUND KILLED STEEL INGOTS [75] Inventors; Kilehl Nat-its, ltarni; Tnkanlke Mori, Ashiya; Takamlehl Itoo, Kobe, all of Japan [73] Assignee: Kobe Steel, Ltd, Kobe-city, Japan [22] Filed: (May 8, 1970 211 Appl. No.: 35,842

[30] Foreign Application Priority Date May 8, 1969- .lspsn ..44l35364 [52] US. Cl. "164/58 [511 Ill. Cl. ..B22d 27l20 [58] Hello! Search l64/55-59 [56] Relerelees cm UNITED STATES PATENTS 2,837,800 6/1958 lhchiya et al 164/56 3,208,117

9/1965 Goedeeke eta] ..l64/56 FOREIGN PATENTS OR APPLICATIONS Primary Examiner-Robert D. Baldwin Assistant Examiner-John E. Roethel Attorney-Oblon, Fisher & Spivak [57] ABSTRACT A high quality homogeneous sound killed steel ingot having a reduced quantity of negative segregation and of inverted V-shaped segregation zones is produced by adding a stirring agent to the molten steel during thep 4 Claims, 6 Drawing Figures 1/1964 France ..l64/56 6/l95l Belgium ..l64/56 p PATENTEDAPR 3 I375 FIG. 2b

INVENTORS KHCHI NARITA TAKASUKE MORI TAKAMICHI T00 04w, Au 4 spww ATTORNEYS METHOD FOR MANUFACTURING OF SOUND KILLED STEEL INGOTS BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to a method for manufacturing a sound killed steel ingot and to products produced therefrom.

2. Description Of Prior Art Killed steel excels rimmed steel or semi-killed steel due to its greater degree of interior homogeneity which permits its use as a high grade structural steel or a forging steel which requires severe standards. However, although killed steel has excellent homogeneity, except for the presence of solidification shrinkage holes occurring at the upper portion of the ingot, segregation defects, resulting from a solidification segregation phenomena are often detected in the ingot. For instance, negative segregation sections may occur at the center of the bottom side of the ingot. Inverted V- shaped segregation zones may occur near the center portion of the upper half ofthe ingot and V-shaped segregation zones may occur at the center of the ingot.

' Oxide type inclusions of relatively large size are often found in the negative segregation sections and it is these inclusions which are the frequent cause of defective steel. Sulphide type inclusions of relatively large size are frequently found in the inverted V-shaped segregation zones which has the effect of deteriorating the workability and toughness of the steel.

In recent years, the demand for large size high quality structural steels have increased considerably. This increased demand has significantly magnified the segregation problem, since large sizes necessitate longer solidification periods and the longer the period of solidification, the greater is the degree of segregation and the larger the defects.

Various attempts have been made by the prior art to minimize the solidification segregation phenomena. For instance, it has been suggested to accelerate solidification by increasing the pressure on the molten steel during solidification so that initial solidification will occur at a higher temperature. Heat is removed rapidly so as to homogenize the structure in the steel ingot and reduce segregation. Another technique suggested in the prior art is homogeneous casting (HOC Method) or ultrasonic casting wherein the ingot is reduced and the structure is homogenized. Still another prior art method is supersonic casting, wherein the crystal grains within the ingot is pulverized, the hardness of the interior structure is increased and the cavities are reduced. Another method is the scraper method, wherein after casting the molten steel, it is solidified while stirring so that the boundary between the dendride section and the free crystal section can be defined without formation of V-shaped or inverted V- shaped regeneration. By this latter technique, a fine interior structure steel ingot can be obtained, the segregation of phosphorus, sulphur, oxygen, etc. is reduced and the mechanical properties of the central section of the steel ingot is improved. Another technique is electromagnetic stirring, wherein the steel ingot is solidified in a rotating magnetic field and the inverted V-shaped segregation is removed to the center by the buoyance in the centrifugal field. The crystals are then pulverized. Another method is the rotary casting method, wherein the growth of culumnar structure section is suppressed and the tesseral system zone is increased to provide a homogeneous ingot. Still another method is vacuum casting wherein casting is carried out under vacuum to reduce gasses in molten steel and thereby minimize the extent of defects growing in the steel ingot, etc.

All of these ingot-producing methods for producing killed steel ingots of homogeneousquality are in principle simply variations of one or more of the following techniques:

l. Controlling macroscopic mass transfer in molten steel during solidification so as to homogenize and compact the structure of the ingot;

2. Quicken solidification so as to reduce solidification segregation and compact the structure; and,

3. Reduce the absolute amount of H and O which often cause solidification defects when the degree of segregation is great. The most industrially successful technique, however, has been the vacuum casting method, which uses the principle (3) of reducing the quantity of H and 0. All of the other methods cause difficulties for existing plant facilities and accordingly are only used for specialty purposes.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a high quality killed steel in large quantities.

It is another object of this invention to provide a high quality killed steel which has a reduced quantity of segregation sections.

Another object of this invention is to reduce the quantity of negative segregation sections in killed steel.

Still another object of this invention is to reduce the quantity of inverted V-shaped segregation zones in killed steel ingots.

These and other objects have :now herein been attained by adding a stirring agent to the molten steel during the solidification process between the time of the negative segregation section and inverted V-shaped segregation zones are formed, and the time that the V- shaped segregation zone is formed. Suitable stirring agents include alloys, compounds or mixtures of ele' ments of Groups I-a, or II-a of the Periodic Table or zinc or the halides of an element: of Groups I-b, III-b, IV-a, IV-b, VI-a, VII-a, or VIII.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In this invention, the molten steel stirring agent must have the following properties:

l. The boiling temperature must lie between 450" and 1,500 C. It the boiling temperature is lower than 450 C., the stirring action is too vigorous which is dangerous in practical application. If the boiling tempera- The alloy as the molten steel stirring agent comprises two groups, one of which is alloys of the mutual alloys among the elemental substances mentioned above and the other is alloys containing other elements such as Cu, Al, Si, Ti, Zr, Nb, Ta, Mn, Fe, and Ni. For instance, the alloy belonging to the first group may be any of the following or mixture thereof:

Ca-Zn, Ca-Mg, Ca-Li, Ca-Na, Li-Mg,

K-Na, Li-Na, K-Mg, Ca-Mg-Zn K-Mg-Na and the second group are as follows:

Al-Ca, Ca-Cu, Ca-Si, Ca-S, Ca-Ni,

Al-Mg, Cu-Mg, Ni-Mg, C-Mg, Mg-Pb,

Mg-Mn, Mg-Ti, Mg-Si, Mg-S, Mg-Zr,

La-Mg, Ce-Mg, Cr-Mg, Al-Zn, Cu-Zn,

Ti-Zn, AlLi, Al-Na, Na-Si, Fe-K,

Cu-K, Cu-Na, Cr-Na, Al-K, Al-Cu-Mg,

ALMg-Si, K-Si-Mg, Al-Mg-Zn, Fe-Si-Mg, Mg-Si-Ca,

K-Si-Na, Mg-Si-Ni, Mg-Si-Cu, Al-Cu-Zn, Mg-Ni-Ca,

CuNi-Z-n, Mg-Si-Ca-Fe, Mg-Si-Ni-Fe. The compound as the molten steel stirring agent comprises inorganic halides, hydroxides, carbonates, bicarbonates and oxides of the La group and halides of the lb, lI-a, II-b, III-b, lV-a, IV-b, Vl-a, VII-a, and VIII Groups. Halides:

KCl, CsCl, NaCl, LiCl, Kl, Csl, Nal, CsBr, NaNr, LiBr, KF, CsF, CuCl, CuBr, CuBr BaCl Becl tends to form oxides to a high degree at this stage which cannot move as freely as carbon and sulphur. The oxides, therefore, accompany the settling ferrous crystals downward. At the lower half of the steel ingot, which corresponds to the negative segregation section, the rate of solidification from the bottom is reduced and solidification is apparently stagnant. Ferrous crystals of high purity and the oxides descend from the upper part and are accumulated in this portion. The oxides grow by aggregation in this area, and part of the oxides will tend to float upward. At the upper half of the steel ingot, molten steel of a relatively low specific gravity, which is rich in the light elements, produced in the solidification transition layer, move with the aggregation toward the zone of lower solidification phase rate, threading through the dendrites. Upon reaching the zone of sufficiently small solidification phase rate, the

MgCl CaBr MgBr Cal Bel ZnCl,, lnF AlF LaCL- crcla, CfCl CIFz, PbClz, PbF2, Pblz,

M00 MoCl MoCl MnCl c001,, recs, NiCl,,

- close to equilibrium so that molten elements are briskly separated in molten steel. Accordingly, at this period in the solidification process, relatively high purity ferrous crystals are produced in a solidification transistion layer. Carbon, phosphorous, sulphur and like substances are separated in the remaining molten steel causing a local increase in concentration of these like elements. Ferrous crystals of relatively high purity have a large specific gravity which will settle downward, while molten steel rich in carbon, phosphorous, sulphur, etc. aggregate and will tend to float upward due to the difference in specific gravity. in the middle of solidification, the sedimentation phenomena of ferrous crystals and floatation phenomena of molten steel rich in impurities occur. Oxygen within the molten steel low specific gravity molten steel ascends substantially along the solidification line, partially being arrested by the crystals, forming the inverted V-shaped segregation line. As solidification advances further, the molten steel temperature is lowered still further, the viscosity increases, and the percentage of the solid phase is increased. During this period, macroscopic-mass transfer in the remaining molten steel almost disappears. During this period of advanced solidification, the V-shaped segregation line which corresponds substantially to the shape of the solidification line is formed.

In order to reduce the segregation sections such as the negative segregation section and the inverted V- shaped segregation zone, it has been found that homogenization can be attained by stirring the remaining molten steel, after the middle stage of solidification, when the segregation sections are formed.

In the present invention, negative segregation and inverted V segregation occurs, then the stirring agent is inserted so that these segregation zones are reduced and eliminated by homogenization.

This can be accomplished by using the fairly high vapor pressure in the solidification temperature range of the stirring agent. In practice, a suitable stirring agent in the form of an element, alloy, compoundor mixture thereof, is added to the molten steel so that the vaporization of the stirring agent will stir and homogenize the molten steel.

Although the stirring agent may be any substance containing the above-mentioned elements, for practical application, a Mg alloy contaning less than 50% Mg, Ca and the alloy thereof are desirable. The stirring agent can be easily and effectively added to molten steel by attaching a suitable capsule containing the stirring agent to an iron rod, which is then inserted to the predetermined depth in the mold. The amount to be added depends on the size of the steel ingot, but the use of amounts in excess of about 1% is apt to dangerously scatter molten steel. An amount of less than about 0.000196 will not exhibit a sufficient effect. The

- preferred amount is such that when the stirring agent is added, the surface of molten steel at the feeder head section will shake'slightly. Excellent resultsare obtainable within the range of about 0.0005 to 0.02 percent.

The metal element inserted into the molten steel will quickly evaporate and the vapors produced will stir and homogenize the molten steel, reduce the segregation and compact the cast structure. It will also have the effect of accelerating solidification by removing evaporation heat from molten steel when the metal is vaporized. It will have the further effect of scattering the fine deoxidated products within the steel ingot through the evaporation of the metal producing numerous solidifications cores to produce a finer cast sturcture. As can be readily appreciated, therefore, the process of the present invention provides very desirable advantages.

For best results, the molten steel stirring agent should be added between the time slightly before the complete formation of the columnar structure of the steel ingot and the time of formation of V-shaped segregation zone is completed.

If the stirring agent is added too soon after casting, the fluidity of molten steel will be too high and the segregation will be rather small, so that the desired effects will not be obtained. Moreover, if added too soon, the rather thin initial shell on the surface of the steel ingot can easily be broken, allowing the molten steel to exude through to form a double layer. On the other hand, if the stirring agent is added close to the complete solidification point, it will have very undesirable effects. In fact, even greater solidification shrinkage holes may be formed and segregation and rolling up of scums may actually increase.

The amount of the molten steel stirring agent and the number of doses depend on the kind of stirring agent used, its composition, the chemical composition of steel, the size of steel ingot, solidification time, etc., but in the ordinary ingot-making, one to five doses is sufficient for this purpose.

FIGS. 1, 2, and 3 are macrographs for comparison of corroded structures of steel ingots. In each FIG., a shows a comparative example and b shows the example of the invention.

Having generally described the invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting in any manner.

EXAMPLE 1 835C steel (C; 0.37 percent, Si:'0.32 percent, Mn: 0.65 percent, S: 0.018 percent, P: 0.013 percent, 0.0071 percent) which is melted in a basic highfrequency furnace was top-poured into 100 kg sand molds which were lined with an exothermic heat conservative. The casting temperature was l,569 C. and the pour rate wasl ton/3.0 min. Upon completion of casting, the exothermic heat conservative was added to the feeder head at the rate of Skg/t. One of the ingots cast as above was allowed to solidify as it was for comparison purposes while to the other ingot was added Fe- Si-Mg alloy containing 20% Mg, packed in-a capsule. The ingot was treated three times, 9 min., 14 min., and '19 min. after casting at the rate of 200 g/t each time, by inserting the capsule through a guide into the center of the mold. Where sand molds are used for casting, the negative segregation section and the inverted V-shaped segregation zone were formed inthe steel ingot within from about min. to about 23 min. after casting. The

time to add the alloy was therefore predetermined to match the time at which these segregation zones were formed. V

The macroscopic corroded structures in the longitudinal section of these steel ingots are shown in FIG. 1, wherein a shows a macrograph of the corroded structure of the steel ingot solidified without any addition. The branched dendrite zone is observed around the periphery of the steel ingot, and several stripes of the inverted V-shaped segregation lines are recognized in this zone. On the other hand, b shows macroscopic corroded structure of the steel ingot to which was added the Fe-Si-Mg alloy during the solidification process thereof, and substantially the whole surface, except the surface layer section consists of fine tesseral system. The inverted V-shaped segregation line is not recog nized. The cast structure is highly dense and homogeneous.

The segregation of various elements in said two steel ingots are shown in comparison in Table 1 below:

TABLE 1 Comparison of Segregation Percenta e In 100 kg; Steel 8 C S P I 0 Control Steel Ingot 12.5 15.2 13.9 20.2 Steel Ingot of Invention 5.5 5.0 4.3 9.1 I

Segregation Percentage {Maximum analytic value (average) in steel ingotl/Average value As evident from Table 1 above, by application of the method of the'invention, steel ingots having a lesser degree of segregation of carbon, sulphur, phosphorus, oxygen, etc. can be obtained.

EXAMPLE 2 i S40C steel (C: 0.39 percent, Si: 0.27 percent, Mn:

0.71 percent, S: 0.023 percent, P: 0.019 percent, 0: 0.00065 percent) which had been melted ina basic high-frequency furnace was top-poured into the kg sand molds. The pouring temperature was l, 555 C. and the pour rate was 1. ton /3 min. 20 sec. Upon completion of pouring the exothermic heat conservative was added to the feeder head at the rate of 5 kg/t. One of the steel ingots cast as above was left to solidify as it was, as a control while an Al-Zn alloy containing 20% Zn, packed in a capsule, was added to the other ingot in two doses at 5 min. and 10min. after casting at the rates of 200 g/t and g/t, respectively. The alloy was inserted in the form of a capsule which was atare shown in FIG. 2, wherein a shows the steel ingot which was solidified without addition of the alloy, in which the inverted V-shaped segregation lines though slight are observed in the branched dendride section, and the V-shapedsegregation lines are observed at the central section. At the center of the lower half section of the steel ingot there exists a tesseral system showing the negative segregation of carbon, sulphur, phosphoorus, etc. and the positive segregation of oxygen. b shows the macroscopic corroded structure of the steel ingot to which was added the Al-Zn alloy during the solidification process thereof, wherein substantially the whole surface of the steel ingot except the peripheral section is observed to consist of fine tesseral system. Neither the inverted V-shaped segregation lines nor V-shaped segregation lines are observed. The cast structure is notably dense and homogeneous. The segregation percentages of various elements in the two steel ingots are compared in Table 2 below:

TABLE 2 Comparison of Segregation Percenta es in 100 kg Steel Ingots S P Control Steel Ingot 11.1 20.8 14.3 17.5 Steel Ingot of Invention 2.5 7.5 10.6 8.2

Segregation Percentage [Maximum analytic value (average) in steel ingot]/Average value The steel ingot of the invention above had a smaller S30C steel (C: 0.29 percent, Si: 0.27 percent, Mn: 0.68 percent, S: 0.016 percent, P: 0.012 percent, 0: 0.0057 percent) which had been melted in a basic highfrequency furnace was top-poured into 1 ton sand molds. The pouring temperature was 1,510 C. and the pouring rate was 1 ton/1 min. 50 sec. After casting 5 kg of the exothermic heat conservative was added to the feeder head. The cast steel was in a round shape, 45 cm average diameter and 95 cm long. The feeder head weighed aBout 1.2 tons. Two steel ingots were cast under the same conditions. One of the ingots was left to solidify per se for preparing a control steel ingot. Metallic Ca was inserted into the other ingot in the form of a capsule attached to the end ofan iron rod to the depth of cm above the upper solidifying surface in the mold. The ingot was treated three times during the solidivication process, in doses of 200 g, 150 g and 100 g, min., 26 min., and 38 min. after casting, respectively. The negative segregation section and inverted V-shaped segregation zone were formed in the steel ingot between from about 10 min. to about 90 min. after pouring. Accordingly, the Ca was added during the first half of the period in which these segregation zones were formed.

In the mascroscopic corroded structure of the steel ingot which was solidified without any metal addition was recognized the inverted V-shaped segregation lines, V-shaped segregation lines and negative segregation section. In the negative segregation section, a max- I imum size of silicate of about 350 p. was observed to exist. On the other hand, in the steel ingot to which Ca was added, the inverted V-shaped segregation lines were reduced to a minimum, the V-shaped segregation lines disappeared and the oxides present in the position corresponding to the negative segregation section were dispersed in a minute form. The maximum size thereof being about 50 u.

As shown in Table 3, it is evident that the steel ingot to which the method of the invention was applied shows a lesser degree of segregation of various elements than'the ordinary steel ingots.

TABLE 3 Comparison of Segregation Percenta es o 1 on Steel Ing0ts( C S P 0 Control Steel Ingot 76.2 93.8 81.0 69.2 Steel Ingot Of Invention 29.1 13.5 26.3 27.1

Segregation Percentage [Maximum anal tic value (average) in steel ingotl/Average alue EXAMPLE 4 S40C steel (C: 0.44 percent, Si: 0.26 percent, Mn: 0.67 percent, P: 0.009 percent, S: 0.019 percent, 0: 0.0028 percent) had been melted in a basic open hearth was bottom-poured into 4-ton metal molds. One of the steel ingots of the same surface plate was solidified per se, as a control while to the other steel ingot was treated according to the methods of this invention. Fe-Si-Mg alloy containing 30% Mg was packed in a capsule which was attached to an iron rod and was inserted through a guide to the center of the mold in doses of 250g, 200g, and 150g, 15 min., 25 min., and 35 min. after casting, respectively. The alloy was inserted to 15 cm above the solidifying surface at the time of the addition.

The macroscopic corroded structues of the two steel ingots are compared in FIG. 3, wherein'a shows the control steel ingot and b shows the steel ingot to which the methods of this invention were applied. In a, the inverted V-shaped segregation lines and V-shaped segregation lines are distinctly observed, whereas in the steelingot b of the invention, the inverted V-shaped segregation lines are reduced to minimum, and the V- shaped segregation lines have completely disappeared. The density of thecast structure of the steel ingot is increased throughout the ingot.

EXAMPLE 5 S40C steel (C: 0.38 percent, Si: 0.32 percent, Mn: 0.65 percent, S: 0.014 percent, P: 0.011 percent, 0: 0.0042 percent) had been melted in a basic electric furnace was bottom-poured into 3 ton molds. The pouring temperature was l,550 C. and the pouring time was 4 min. 30 sec., four ingots were prepared per each surface plate. One of the steel ingots was solidified without any addition as a control. To seven other steel ingots the method of the invention was applied, and upon completion of pouring, NaCl, ZnCl,, MgCl,, KF, Al-Ca alloy of 50% Ca, Fe-Si-Mg alloy of 3.0% Mg and a mixture of Cu-Zn alloy of Zn and NaCl, respectively, packed in capsules were inserted into the molds in three doses at 15 min., 25 min., and 36 min. after pouring. These steel ingots were rolled into blooms, 200 mm. dia., and sectional samples were taken from positions corresponding to the head, center and bottom of the steel ingot, .and the macroscopic corroded structures of the respective sections and the segregation of the components thereof were examined.

As is evident from the examples, when using the methods of this invention, the inverted V-shaped segregation zone and V-shaped segregation zoneare reduced or eliminated, and large oxide type inclusions occurring in the negative segregations section are turned into finer form. A markedly dense and more homogeneous cast structure can be obtained; also, the segregation of the component elements is reduced and the segregation condition of oxygen is notably improved.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and the scope of the invention. Accordingly,

What is claimed and intended to be covered by letters patent is:

1. In a method for reducing a sound killed steel ingot,

the improvement comprising adding a stirring agent to the molten steel during the solidification process of the killed steel ingot at a time period between the period before the V-shaped segregation zone is formed and a after the negative segregation section and the inverted V-shaped segregation zones are formed, wherein said stirring agentis characterized by a boiling temperature of between 450,and 1,500 C., such that when introduced into the'molten steel, it will vaporize under,

the process conditions whereby the vaporsproduced willstir the molten steel and thereby reduce the extent of negative and inverted V-shaped segregation zones in said steel ingot.

2. The method of claim 1, wherein said stirring agent is selected from the Group consisting of:

1. Elements, alloys, compounds or mixtures of at least one element selected from the Group consisting of elements of Groups I-a, ll-a, or zinc,

2 Halide compounds of elements selected from the Group consisting of those elements in Groups I-b,

III-b, IV-a, IV-b, Vl-a, VII-a and VIII of the

Claims (3)

1. 2. The method of claim 1, wherein said stirring agent is selected from the Group consisting of:
2. 3. The method of claim 2, wherein the stirring agent is a magnesium alloy containing less than 50 percent magnesium.
3. 4. The method of claim 1, wherein the stirring agent is inserted directly into the ingot in capsule form in an amount of between 1 and 0.01 percent.

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