US3639117A

US3639117A - Method for producing bearing grade alloy steels

Info

Publication number: US3639117A
Application number: US37297A
Authority: US
Inventors: Albert L Lehman
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1970-05-14
Filing date: 1970-05-14
Publication date: 1972-02-01
Anticipated expiration: 1989-02-01

Abstract

This invention is an improvement in a method for producing alloy steel which may contain, for example about 1.00% carbon, about 0.45% manganese, about 0.25% silicon and 1.50% chromium. The steel has a lower incidence of nonmetallic inclusions than steel produced in a conventional method, for example an air-melt electric furnace, open hearth. The steel is melted and tapped and then vacuum treated by any one of several well-known methods. The steel is deoxidized and a fluoride-containing acid slag is added to the steel. The steel and the slag are intimately mixed. The steel is then teemed into molds which can be relatively large.

Description

United States Patent Lehman Feb, 1, 1972 [54] METHOD FOR PRODUCING BEARING GRADE ALLOY STEELS [21] Appl. No.: 37,297

[52] US. Cl.. ..75/49, 75/58, 75/94 [51] Int. Cl. ..C2lb 7/10, C21b 7/06 [58] Field of Search ..75/49,58,94; 164/55, 56, 164/61 [56] References Cited UNITED STATES PATENTS 2,510,155 6/1950 Tanczyn .164/56 2,848,317 8/1958 Coupette et a1. .....75/49 2,993,780 7/1961 Allard ..75/49 3,052,936 9/1962 Hamilton.. .164/56 3,158,466 11/1964 Muller ..75/58 3,201,224 8/1965 Grim ..75/49 X 3,251,680 5/1966 Goss et al ..75/5l X 3,269,828 8/1966 Hale ..75/58 X 3,314,782 4/1967 Amaud ..75/58 X 3,389,989 6/1968 Finkl ..75/49 3,501,290 3/1970 Finkl et al ..75/49 X Primary Examiner-Henry W. Tarring, ll Attorney-Joseph J. OKeefe [5 7] ABSTRACT This invention is an improvement in a method for producing alloy steel which may contain, for example about 1.00% carbon, about 0.45% manganese, about 0.25% silicon and 1.50% chromium. The steel has a lower incidence of nonmetallic inclusions than steel produced in a conventional method, for example an air-melt electric furnace, open hearth. The steel is melted and tapped and then vacuum treated by any one of several well-known methods. The steel is deoxidized and a fluoride-containing acid slag is added to the steel. The steel and the slag are intimately mixed. The steel is then teemed into molds which can be relatively large.

2 Claims, No Drawings METHOD FOR PRODUCING BEARING GRADE ALLOY STEELS BACKGROUND OF THE INVENTION Alloy steels intended for use in many highly stressed applications, for example, bearing races, are produced to exacting cleanliness specifications. Production of steels of this type usually include the addition of a'solid deoxidant, such as aluminum, silicon, calcium-silicon, calcium-manganese-silicon, aluminum-silicon alloys or aluminum-silicon-manganese alloys, to the liquid steel to deoxidize the steels prior to teeming into molds. The solid deoxidants react with oxygen to form nonmetallic inclusions, for example, alumina, silica and com plex alumino-silicates which may become entrapped in the steel as it solidifies. The nonmetallics so entrapped become foci for fatigue failure, thereby making these steels susceptible to early failure by fatigue.

Several methods have been used to produce steels having a lower incidence of nonmetallic inclusions therein. One such method includes melting the steels in an initial air refining process and a remelting and refining process in a vacuum arc furnace. Another method which has been used is vacuum treatment of the steel after air refining. Both of these methods have been partially successful. However, the cleanliness of steels produced by these processes has been erratic.

Steels of the bearing grade type were then teemed into relatively small ingot molds to prevent the formation of coarse carbides in the grain boundaries, to prevent carbon segregation in the top and center portions of the ingot and to increase the rate of solidification of the steels to thereby decrease the size of nonmetallic inclusions formed in the ingots. When teemed into larger molds the carbides and carbon segregation cause weaknesses in the ingots and may cause the ingots to overheat in the top and center portions of the ingot when heated for hot working, making the ingot susceptible to bursting during hot working.

It is an object of this invention to provide an improved method for producing substantially clean alloy steels in which the coarse carbides and carbon segregation are reduced to a minimum of nonmetallic inclusions in the ingots.

It is also an object of this invention to provide an improved method for producing alloy steels which will permit the steels to be teemed into relatively large ingot molds.

SUMMARY OF THE INVENTION Broadly, the method of the invention includes melting alloy steels in an air-melt furnace, for example, electric furnace, by well-known methods, vacuum treating the steel, adding a fluoride-containing acid slag to the steel after vacuum treatment, intimately mixing the steel and the slag and teeming the steel into molds ofany size and type.

PREFERRED EMBODIMENT OF THE INVENTION l have found that alloy steel and in particular bearing grade alloy steel, having a decreased incidence of nonmetallic inclusions therein may be produced by melting and refining the steel in a conventional air-melt metallurgical furnace, such as a basic electric furnace. The liquid steel is tapped into a conventional ladle. The liquid steel is then vacuum treated by any one of several conventional methods. An acid slag is added to the liquid steel after vacuum treatment. The liquid steel and the acid slag are intimately mixed so that substantially all the liquid steel will come into contact with the acid slag. At least one of several strong well-known solid deoxidants is added to the steel to remove substantially all ofthe oxygen remaining in the liquid steel. Steels produced by this method may be teemed into relatively large ingot molds.

An alloy steel, for example a bearing grade steel containing about 0.90 to about 1. percent carbon, about 0.30 to about 0.60 percent manganese, up to about 0.25 percent silicon, about 0.75 to about 2.00 percent chromium, the remainder incidental impurities, such as phosphorus and sulfur, may be refined in a conventional furnace by conventional refining methods. The liquid steel is tapped from the furnace into a furnace ladle in the usual manner. The liquid steel is exposed to a vacuum of less than about 1 mm. of mercury. The method for vacuum treating the'steel may be any one of several well known methods. One such method is described in US. Pat. No. 3,019,496 entitled Vacuum Casting Apparaus" issued Feb. 6, 1962 to H. C. Bigge. Of course, it is possible to vacuum treat the liquid steel in the furnace ladle by placing an appropriate apparatus atop the furnace ladle and applying a vacuum thereto. Vacuum treatment decreases the oxygen level in the liquid steel. After melting, the oxygen in the liquid steel may be about 0.003% to about 0. l5% Vacuum treatment will reduce the oxygen content to between about 0.004% to about 0.0063%. Because some oxygen remains in the liquid steel after treatment by the above described method, a solid deoxidant, such as aluminum or silicon or alloys and mixtures thereof, may be added to the liquid steel to remove a substantial portion if not all of the oxygen remaining in the steel. The solid deoxidant reacts with oxygen in the steel to form nonmetallic inclusions such as alumina, silica and complex alumino-silicates.

In order to reduce the number of the aforementioned nonmetallic inclusions in the steel to a minimum, an acid slag containing silica and a fluorine-containing compound, for exam ple cryolite (Na AlF or sodium fluoride (NaF) is added to the liquid steel in the ladle. The acid slag may contain, for example 3 parts of silica to one part of the fluorine-containing compound on a weight basis. It is preferred to use cryolite in the acid slag: The steel and acid slag are intimately mixed. The mixing may be accomplished by bubbling an inert gas, for example argon, upwardly through the liquid steel after vacuum treatment. The ladle may be equipped with an electrical induction coil whereby the liquid steel is stirred by a flow of magnetic flux in the liquid steel. The acid slag absorbs a substantial portion, if not all of the nonmetallic inclusions in the liquid steel.

Mixing may also be accomplished by placing the acid slag on the bottom of a teeming ladle placed in the vacuum chamber prior to the vacuum treatment. The liquid steel is poured from the furnace ladle into the vacuum chamber where it is received in the teeming ladle. The simple expedient of allowing the liquid steel to drop into the teeming ladle after it has been exposed to the vacuum near the top of the vacuum chamber, causes the liquid steel to be intimately mixed with the slag which absorbs a substantial portion of the nonmetallic inclusions in the liquid steel.

Alloy grade bearing steel is usually teemed into 24 inch diameter molds. However, steel made according to the instant invention may be teemed into ingot molds as large as 32 inches in diameter with no detrimental effects due to carbide coarsening or network formation in the grain boundaries, carbon segregation in the top and center portions of the ingot and formation of unusually large nonmetallic inclusions formed because of decreased rate of solidification. Because of the fewer number of molds required, mill yields are increased and the time required for mold preparation and ingot stripping are kept at a minimum.

Whenever percentages are noted in this specification and claims, such percentages are on a weight basis unless otherwise noted.

In a specific example of the invention an alloy steel was prepared in a conventional manner in an electric furnace.

The steel had the following chemical compositions:

Percent 0 Mn P S Si Or A nace ladle. About 20 pounds of CaMnSi alloy, 150 pounds of 5 Sio and 50 pounds of cryolite were then added to the liquid steel in the teeming ladle. The steel and slag were intimately mixed for 6 minutes by bubbling about l0 cu. ft. per minute or argon gas upwardly through the liquid steel. The liquid steel was removed from the vacuum chamber and teemed into 32- inch :75 molds. The steel was hot worked into bars and billets suitable for shipment.

I claim:

1. In a method for reducing the amount of nonmetallic inclusions in an alloy steel wherein, sequentially, said alloy steel is refined under a basic slag in an air-melt furnace, said alloy steel and said basic slag are tapped into a furnace ladle and said alloy steel is vacuum treated by placing a teeming ladle into a vacuum chamber sealing said vacuum chamber by placing an airtight cover seal thereon, said airtight cover seal having an entry port into said vacuum chamber covered by a meltable material, placing said furnace ladle containing said alloy steel and basic slag atop said airtight cover seal in a manner that will allow said alloy steel to flow downwardly into said teeming ladle in said vacuum chamber when a vacuum is formed in said vacuum chamber, the improvement comprising: placing an acid slag consisting of 3 parts of silica to l part of cryolite into said teeming ladle prior to sealing said vacuum chamber whereby when said vacuum is formed in said vacuum chamber, said alloy steel will flow downwardly from said furnace ladle into said teeming ladle and contact said acid slag to cause the nonmetallic inclusions in said alloy steel to be adsorbed by said acid slag and terminating vacuum treatment of said alloy steel in said vacuum chamber prior to the passage of said basic slag in said furnace ladle into said teeming ladle.

2. In a method for manufacturing an alloy steel in which said steel is refined under a basic slag in an air-melt furnace, said steel is tapped into a furnace ladle, said furnace ladle is placed in a vacuum chamber which is sealed by placing a sealing cover over said vacuum chamber, said sealing cover having a port covered by a removable cover through which materials can be charged into said steel in said vacuum chamber, forming a vacuum in said vacuum chamber for a time to reduce the gaseous contents of said steel while said steel is being agitated, the improvement comprising: terminating the vacuum treatment and charging an acid slag consisting of 3 parts of silica to 1 part of cryolite said steel through said port in said sealing cover while said refined steel is being agitated to cause said steel and said acid slag to be in intimate contact with each other and nonmetalic inclusions in said steel to be adsorbed by said acid slag.

- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 39,117 Dated Februar 1, 1972 Albert L.- Lehman Inventor(s) It is certified thst error appears in the above-identified patent and that said Letters Patentar'e hereby corrected as shown below:

T Column 1, line l9, "lower't' should read --l0w--.

Column 2, line 12, "0.15%" should read "015%". Column 2, 1121913, "0.00%" should read -.OOl Column 3-, line, 6, "Siog" should read --SiO Column 3, line 8, "or"should read --of-.

Column claim 2, line 21, after "cryol'ite" insert the word -into- Signed and. sealed this Ilth day of July 1972;

(SEAL) Attest:

EDWARD M.FLETGHER, JR. ROBERT GOTTSCHALK Attesting Officer 1 Commissioner of Patents FORM PO-1050 (10-69) v USCOMM DC 6376 P69 m tr u.s. eovsnumzm' PRINTING OFFICE 1989 0-366-334.