US3861906A

US3861906A - Calcium deoxidized, fine grain steels

Info

Publication number: US3861906A
Application number: US319676A
Authority: US
Inventors: Vijayakumar A Tipnis; James Harmon Doubrava; Roger A Joseph
Original assignee: Republic Steel Corp
Current assignee: Ltv Steel Co Inc
Priority date: 1972-12-29
Filing date: 1972-12-29
Publication date: 1975-01-21
Anticipated expiration: 1992-01-21
Also published as: CA995496A; JPS506518A; DE2364872A1; FR2212432A1; IT1000764B; FR2212432B3

Abstract

Low carbon and low alloy fine grain steels exhibiting improved machinability, especially by high speed steel tools used in gear cutting operations, are produced by a new calcium deoxidation practice in which: (1) the amount of calcium added to the molten steel in pounds per ton is at least 0.25 plus 13.33 times the oxygen content (percent by weight) of the steel at the time of addition, (2) the temperature of the steel at the time of the calcium addition is from 30* to 120* F. above the calculated liquidus temperature, and (3) the calcium addition is at least in part made to the ladle when it is from about one-third to twothirds full. The calcium deoxidized steels are characterized by calcium aluminate instead of alumina inclusions and by type I instead of type II sulfides, with the sulfides being present as manganese sulfide and/or calcium manganese sulfide coatings which surround the calcium aluminate particles.

Description

United States Patent Tipnis et al.

[451 Jan. 21, 1975 CALCIUM DEOXIDIZED, FINE GRAIN STEELS [75] Inventors: Vijayakumar A. Tipnis, Parma;

James Harmon Doubrava, South Euclid; Roger A. Joseph, Cleveland, all of Ohio [73] Assignee: Republic Steel Corporation,

Cleveland, Ohio [22] Filed: Dec. 29, 1972 [21] Appl. No.: 319,676

[52] US. Cl. 75/124, 75/58 [51] Int. Cl. C22c 37/10, C2lc 7/00 [58] Field of Search 75/124, 58

[56] References Cited UNITED STATES PATENTS 2,127,245 8/1938 Breeler 75/124 2,687,954 8/1954 Lohr 75/124 2,853,379 9/1958 Althouse.. 75/124 2,901,346 8/1969 Huddle 75/124 2,950,187 8/1960 Ototani.... 75/124 3,110,798 11/1963 Keay 75/124 3,115,406 12/1963 Ballass 75/124 3,130,045 4/1964 Richards 75/124 3,132,938 5/1964 Decker 75/124 3,314,828 4/1967 Harrison 75/124 Primary Examiner--C. Lovell Assistant ExaminerPeter D. Rosenberg Attorney, Agent, or FirmWatts, Hoffman, Fisher & Co.

[57] ABSTRACT Low carbon and low alloy fine grain steels exhibiting improved machinability, especially by high speed steel tools used in gear cutting operations, are produced by a new calcium deoxidation practice in which: (1) the amount of calcium added to the molten steel in pounds per ton is at least 0.25 plus 13.33 times the oxygen content (percent by weight) of the steel at the time of addition, (2) the temperature of the steel at the time of the calcium addition is from 30 to 120 F. above the calculated liquidus temperature, and (3) the calcium addition is at least in part made to the ladle when it is from about one-third to two-thirds full. The calcium deoxidized steels are characterized by calcium aluminate instead of alumina inclusions and by type 1 instead of type II sulfides, with the sulfides being present as manganese sulfide and/or calcium manganese sulfide coatings which surround the calcium aluminate particles.

8 Claims, 17 Drawing Figures PATENTEI] JANZI I975 SHEET 10F 7 PATENTEDJANZI I975 SHEET P. [1F 7 w wE 50 PATENTEDJANZI 1975 SHEET 3 BF 7 M QE z 24 u x0 :5 T EWZ w.) 2 @Zw Q oE 2 55025 35:

PATENIED JAN ZI I975 SHEET 5 OF 7 00 com o8 oo. 0 O 68 .8 85-2 0m 0281 zzm oNww m On CALCIUM DEOXIDIZED, FINE GRAIN STEELS BACKGROUND OF THE INVENTION This invention relates generally to improved machining steels, and more specifically to improved machining steels produced by fine grain, calcium deoxidation practice.

The invention is particularly concerned with the provision of fine grain, low carbon and low alloy steels for the production of gears, pinions, worms, etc. used in differential, transmission and steering mechanisms of automobiles, trucks, tractors and off-the-road equipment. Automotive pinions, gears, worms, and the like are structural power-transmitting members. Since the power transmission in these members is done through the gear teeth, the gear teeth must (a) withstand high static and dynamic contact and bending loads, (b) resist bending and contact fatigue, and provide wear resistant, score-free smooth contact surfaces for quiet and efficient operation. It has been the usual practice to use carburizing grade, low alloy or carbon steels in order to provide the combination of required core and surface properties. These carburizing grade steels have been conventionally made by fine grain, aluminumsilicon deoxidizing practice to achieve consistent case and core properties after carburizing.

Conventional aluminum-silicon deoxidation practice produces numerous small, very hard oxides and occasionally large alumina clusters which are particularly harmful to the high speed steel tools used for cutting gear teeth. AS a result, gear cutter wear and failure has been a serious problem. The most common gear cutter wear characteristics are (a) nose or tip wear which occurs at the tip of the cutter blades, (b) nicking or chipping of the cutting edge, (e) wearland which affects the overall surface finish and dimensional tolerance of gear tooth profiles, (d) chip wash wear which is caused by the chips flowing acorss the rake face and down the backside of the tool, and (e) a built-up deposit of highly deformed workpiece material on the cutting edge. All of these characteristic wear features of high speed steel tools take place primarily as abrasive wear at the clearance face of the tools.

It has been the usual practice to keep the sulfur content of conventionally produced, fine grain gear steels under about 0.035% in order to avoid degradation of mechanical properties. More recently, mildly resulfurized steels having sulfur contents up to about 0.09% have been used for some gear applications. Nevertheless, the free-machining ability of sulfur has not been fully realized because the aluminum deoxidation produces eutectic phase stringy sulfides (type II), instead of globular sulfides (type I) which are beneficial to machiriability. The stringy sulfides are also harmful to transverse mechanical properties.

Free-machining additives such as lead, selenium, tellurium, nitrogen or phosphorous are generally not added to gear steels because of the danger of degrading hot working and mechanical properties and also because of the cost. Moreover, the use of such freemachining additives in aluminum-silicon deoxidized steels is not especially effective in preventing abrasive wear of high speed steel tools which typically operate with relatively low cutting temperatures in the range of from about 300 to 800F.

The use of calcium in the form of calcium bearing deoxidizers such as calcium-silicon or calciummanganese-silicon for deoxidizing cast steels dates back many years. Although the deoxidizing effect of calcium was erratic, it was found that whenever the calcium effect was achieved, there was some improvement in the mechanical properties of steels, especially impact strength. These improvements on mechanical properties were attributed to the effect of calcium on the morphology of the non-metallic inclusions in steel. In general, it was recognized that calcium additions tended to produce smaller, more globular oxide inclusions instead of the larger clusters of alumina galaxies typically found in steels deoxidized with aluminum and silicon alone. Also, the sulfides in steels treated with calcium bearing deoxidizers were found to be globular (type I) as opposed to the stringy type II sulfides.

In recent years, steels deoxidized with various calcium bearing compounds have received attention from research workers in the field of machinability. This work has shown that improvements in life of carbide tools which contain titanium carbide could be obtained in connection with laboratory size heats of some calcium deoxidized steels. Adhesive wear, as distinguished from the abrasive wear of high speed steel tools, is an important factor in the case of carbide tools which are operated at high cutting speeds and temperatures. The improvement in the life of certain carbide tools has been attributed to the fact that the high cutting temperatures, e.g., from about l,000 to 2,700F., are sufficient to soften the inclusions resulting from calcium deoxidation. The softened inclusions are deposited on the carbide tools in the form of layers which retard adhesive wear.

The beneficial effects of calcium deoxidation on the life of carbide tools is not indicative that similar improvements could be achieved in the case of high speed tools. This is because the two classes of tools operate at different cutting speeds and temperatures and are subject to distinctive types of wear. In the case of high speed steel tools, the cutting temperatures generally are not high enough to soften the calcium bearing oxide inclusions and produce wear retarding layers on the tools. In fact, the few investigations which have been conducted by others with high speed steel tools on calcium deoxidized steels have shown a significant decrease in drill life and gear hobber life. Prior to the present invention, there has not been any known large scale steel making practice which made it possible to realize the benefits of calcium deoxidation in the production of fine grain gear steels so as to improve the life of the expensive high speed steel tools employed in gear cutting operations.

SUMMARY OF THE INVENTION An object of the present invention is to provide a calcium deoxidized steel which is characterized by superior machinability.

Another object of the present invention is to provide a fine grain gear steel produced by a calcium deoxidation practice.

A further object of the present invention is to provide a calcium deoxidation practice which makes it possible in the large scale, commercial production of steel to achieve the desired effects of calcium on morphology of the non-metallic inclusions.

Still another object of the present invention is to provide a calcium deoxidation practice which is especially useful in the large scale, commercial production of fine grain, machining steels for gears and the like.

The preferred embodiments of the present invention contemplate a production method of calcium deoxidation which has been specially developed for making fine grain steels, particularly improved machining gear steels. According to this method, calcium in the form of one or more calcium bearing alloys is added to the molten steel when it is at a temperature in the range of from 30 to 120F. above the calculated liquidus temperature, the amount of the calcium addition in pounds per ton of steel being at least equal to 0.25 plus 13.33 times the oxygen content of the steel at the time of addition. The preferred method is carried out by adding the calcium to the ladle between the time it is from onethird to two-thirds full, and by adding the aluminum required for grain refinement no later than simultaneously with the calcium.

The preferred method as generally described above makes it possible consistently to achieve the desired calcium effects on morphology of the non-metallic inclusions, whereby the new calcium deoxidized steels are characterized by the absence of harmful alumina inclusions and type II sulfides. The calcium and aluminum are present in the calcium aluminate phase, while the sulfides are present in globular or type I form. Usually, the sulfides are combined in a calcium-manganese sulfide phase which forms coatings around the calcium aluminate particles.

A new family of improved machining, low carbon and low alloy steels having the characteristics described above has been developed using the new calcium deoxidation practice of the invention. Using the new steels, the life of gear cutting tools, such as rougher and finisher cutters, has been improved by 70% to 100% or more. These improvements in tool life have resulted in substantial cost savings and productivity gains in the manufacture of gears and the like.

In addition to exhibiting improved machining properties, the new calcium deoxidized steels of the invention exhibit hardenability, carburizing and heat treatment response which is at least equivalent to that of conventionally aluminum-silicon deoxidized fine grain steels. The mechanical properties such as impact strength, tensile strength, ductility and fatigue resistance of the new calcium deoxidized steels are essentially the same as conventionally aluminum-silicon deoxidized steels.

Other objects, advantages and a fuller understanding of the invention will be had from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 6 are electron microprobe analyses of a non-metallic inclusion in a calcium deoxidized steel of the present invention;

FIGS. 7 through 12 are electron microprobe analyses of non-metallic inclusions in a conventionally aluminum-silicon deoxidized steel;

FIG. 13 is a graph showing case and core hardenability of a calcium deoxidized steel of the present invention versus a conventionally aluminum-silicon deoxidized steel;

FIG. 14 is a plot of tool life against cutting speed in connection with a calcium deoxidized steel of the present invention and a conventionally aluminum-silicon deoxidized steel;

FIG. 15 is a plot of tool wear against the volume of metal removed in connection with a calcium deoxidized steel of the present invention and a conventionally aluminum-silicon deoxidized steel;

FIG. 16 shows typical results of ring gear production performance test using calcium deoxidized steels ofthe present invention and conventionally aluminum-silicon deoxidized steels; and,

FIG. 17 shows typical results of pinion gear production performance tests using calcium deoxidized steels of the present invention and conventionally aluminumsilicon deoxidized steels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the invention have particular applicability to and are described below in connection with low carbon and low alloy steels to which aluminum is added for grain refinement. Typical steels include carburizing grade steels of the S.A.E.- /A.I.S.I. 1500 series, such as grades 1524-1527; the S.A.E./A.I.S.I. 4000 series, such as

grades

4023, 4027, 4028, 4118, 4422, 4427 and 4620; the S.A.E./A.I.S.I. 5000 series, such as grade 5130; the S.A.E./A.I.S.I. 6000 series, such as grade 6118; the S.A.E./A.I.S.I. 8000 series, such as grades 8615, 8620, 8622, 8625, and 8630; the S.A.E./A.I.S.I. 9000 series, such as grade 9310; S.A.E. EX33; and the like. The invention is also applicable to through-hardening steels such as S.A.E.- /A.I.S.I. grades 4340, 8640, moderately alloyed hot worked die steels such as A.I.S.I. L6, and the like. Still other steels which benefit from the calcium deoxidizing treatment of the invention are standard boron steels such as 94Bl7 and the like.

As will be familiar to those experienced in the art, fine grain steels have been made by aluminum-silicon deoxidation practices using sufficient aluminum to obtain an A.S.T.M. grain size of from about 5 to about 8 (regular McQuaid-Ehn), with a size of about 7 or 8 being the most typical. Vanadium may be substituted for aluminum, but is not preferred because of the higher cost. In the usual practice, the aluminum is added in an amount of from about 0.015 to 0.1% by weight with the preferred range being from about 0.03 to 0.06% by weight. The aluminum or low aluminum plus vanadium as conventionally added for grain refinement produces hard oxide inclusions (vanadium predominately precipitates as a carbide phase after heat treatment) and type II sulfides of which the former has been found to be the primary cause of abrasive wear of high speed steel tools. This problem has been particularly severe in the case of continuous or strand cast steels where the large oxide inclusions have greater tendency to be trapped in the solidifying metal because of the faster freezing times as compared to ingot cast steels.

The present invention provides a method of calcium deoxidation which effectively reduces or eliminates the occurrence of aluminum oxide inclusions and type II sulfides in ingot cast and strand cast fine grain steels produced by open hearth, electric furnace and basic oxygen furnace melting. Instead of the hard alumina inclusions in conventionally aluminum-silicon deoxidized steels, the calcium deoxidized steels of the invention are characterized by the relatively softer calcium aluminate phase which is generally surrounded by an even softer calcium-manganese sulfide phase. The improved inclusion morphology is further characterized by the more desirable globular or type I manganese sulfides as distinguished from type II sulfides.

The invention is carried out by adding calcium to the molten steel in the form of calcium bearing alloys. Suitable alloys include calcium-silicon alloys, calciumbarium-silicon alloys, calcium-barium-silicon aluminum alloys, calcium-barium-silicon-manganese alloys, calcium-barium-silicon-manganese-aluminum alloys, and the like, as well as selected combinations. Strontium may be substituted in part or full for barium. When the selected calcium bearing alloy includes aluminum, it may be used to provide the entire amount of aluminum needed for grain refinement, thereby avoiding the necessity of a separate aluminum addition. The more preferred alloys are those which contain barium. Barium is believed to lower the vapor pressure of calcium and to modify its reactivity during addition to liquid steel so that sufficient calcium is present to combine with aluminum, silicon and oxygen for producing the desired modification of oxide inclusions. The most preferred alloy which has been found to produce consistent calcium deoxidizing effects is an alloy of calcium, aluminum, barium and silicon.

Although calcium deoxidizing alloys of the type employed in the method of this invention are known and, in fact, are commercially available, the prior art has not provided a practice of using such alloys in the large scale production of steel which is effective to obtain consistent calcium deoxidation effects. In order to consistently produce the desired effects of calcium on morphology of the non-metallic inclusions, it has been discovered that it is essential to control (1 the amount of calcium used, (2) the time and manner of adding the calcium deoxidizing agent, and (3) the temperature of the steel at the time of addition. Each of these critical aspects of the preferred practice of this invention is more fully described below.

1. Amount of Calcium The amount of calcium which is required to obtain the desired deoxidizing effects and to produce a fine grain steel characterized by the absence of alumina inclusions depends upon the oxygen content of the steel as determined by the tap carbon. Lower carbon steels require larger amounts of calcium, and higher carbon steels require smaller amounts of calcium. In order to achieve consistent and effective inclusion control in fine grain steels, it has been determined that the minimum calcium addition in pounds per ton of steel should be 0.25 plus 13.33 times the oxygen content (percent by weight) of the steel at the time of addition. Increasing benefits of calcium can be obtained by additions up to a maximum amount per ton of steel of about 0.85 plus 13.33 times the oxygen content (percent by weight) ofthe steel at the time of addition. While larger amounts of calcium can be added, it has been found that amounts in excess of the preferred maximum do not produce any substantial benefits and are therefore uneconomic. 2. Time and Manner of Calcium Addition The practice of the invention is add at least part of the required amount of calcium to the molten steel when it is tapped or poured into the ladle. The calcium bearing alloy or alloys should be added to the ladle when it is between about one-third to two-thirds full in order to assure good distribution in the metal. A portion of the calcium also can be added to the tundish and/or the ingot molds. According to the preferred practice, the calculated amount of calcium in the form of one or more alloys is added to the ladle in the described manner and subsequent additions are made to the tundish and/or ingot molds to make up for ladle losses. Effective inclusion control is achieved by adding the calcium to the molten steel simultaneously with or subsequent to the aluminum addition. Aluminum additions subsequent to calcium deoxidation have been found to promote alumina inclusions.

3. Temperature of the Steel The temperature of the molten steel at the time when the calcium bearing alloy is introduced should be within the range of from 30F to 120F. above the calculated liquidus temperature. Depending upon the particular steel making process, the temperature may vary slightly within the indicated range with electric furnace steels being deoxidized at somewhat higher temperatures than basic oxygen furnace and open hearth steels.

The following Table I sets forth 15 examples of calcium-aluminum deoxidized fine grain steels produced in accordance with the preferred practice of the invention as described above by basic oxygen furnace, electric furnace and open hearth steel making processes. The table lists the amount of calcium added, the ladle temperature at the time of addition, the oxygen content, and the final chemistry. In each instance, the aluminum and calcium were added as a calcium-barium-siliconaluminum alloy having the following nominal composition in percent by weight: calcium 12.0%, barium 10.0%, silicon 40.0%, aluminum 20.0% iron balance.

TABLE I Calcium Ladle tem- Final chemistry (in wt. percent) Steel added perature Oxygen (wt. N 0. Grade Practice (lbs/ton) percent) Mn 1 S Si Ni (Jr Mo Cu Al 1 The calcium deoxidized steels of the invention are generally characterized by a composition comprising in percent by weight up to 0.70 carbon, more preferably from 0.13 to 0.70 carbon, up to 1.65 manganese, preferably from 0.45 to 1.65 manganese, up to about 0.09 sulfur, up to 0.45 silicon, from .015 to 0.1 aluminum, preferably from 0.03 to 0.06 aluminum, at least 0.0002 calcium, and normal amounts of phosphorous, copper and nitrogen. The preferred upper limit of calcium is about 0.003. While amounts of calcium in excess of 0.003 may be included in the preferred compositions, such excess amounts have not been found to provide substantial benefits. The preferred steels also may include one or more alloying additions including in percent by weight up to 3.50 nickel, up to 3.50 chromium, up to 0.35 vanadium, up to 0.80 molybdenum, and the usual amounts of boron present in standard boron steels, i.e., from about 0.0005 to 0.0015 boron.

The disclosed calcium content of steels produced in accordance with the invention has been determined by known spectrographic methods using solid samples in order to reduce contamination and sample preparation time. The spectrographic method employed was based on primary control standards published by the National Bureau of Standards as SRM 1261, 1262 and 1264 and was supplemented by suitable secondary control standards in order to obtain a calibration curve effective to determine residual calcium in a range of from 0.000l% to 0.02%\ In the case of ingot cast steels, the calcium content of a heat has been determined from samples taken from the top, middle and bottom portions of the first, middle and last ingots in the heat. In the case of strand cast steels, the calcium content of a heat has been determined from samples taken from the first, middle and last billets from each strand. In all cases, the samples were taken from the outer 20% of the ingots and billets.

Selenium and/or lead may be included to advantage in the steels of this invention. Selenium represents a partial substitute for sulfur with the result that the inclusions in the steel tend to be more globular. When se lenium is present, it combines with the sulfur to form Mn Ca (S Se) coatings on the calcium aluminate particles which enhance machinability. Amounts of selenium in the range of from 0.01 to 0.15 percent by weight are sufficient to promote the desired effect. When lead is added in conjunction with selenium, the

percent by weight in leaded steels. Equivalent amounts of tellurium and bismuth can be substituted in part or in full for selenium and lead, respectively.

As generally described above, the steels of this invention are characterized by the absence of hard alumina inclusions and by the presence of the softer calcium aluminate phase. This is shown by FIGS. I-6 which are electron microprobe analyses of a non-metallic inclusion in a S.A.E./A.I.S.l. I526 grade steel produced according to the invention (steel No. 12 of Table I). As will be apparent from these figures, calcium and aluminum co-exist as a calcium aluminate phase. By way of comparison, FIGS. 7l2 are electron microprobe analyses of an S.A.E./A.I.S.l. 1526 grade steel produced by conventional aluminum-silicon deoxidation without any calcium addition. These latter figures show the conventional type of alumina inclusion. The calcium aluminate inclusions of the type shown in FIGS. l-6 are significantly softer than the alumina inclusion of FIGS. 7l2. Moreover, the calcium aluminate inclusions are surrounded by a still softer calcium, manganese sulfide phase, making the composite inclusion substantially less hard and less abrasive than the alumina inclusions.

An important aspect of the invention is that the heat treat response to annealing, carburizing, and the hardenability response for the new calcium deoxidized steels is similar to that of conventional aluminumsilicon deoxidized steels. In addition, the mechanical working properties and the longitudinal mechanical properties such as tensile strength and yield strength, elongation, reduction in area and impact strength of the new calcium deoxidized steels are equivalent to those of aluminum-silicon deoxidized steels.

FIG. 13 is a plot of the core and ease (up to 1.0% carbon) hardenability of an A.I.S.I. 1526 calcium deoxidized steel and an equivalent grade steel produced by aluminum-silicon deoxidation. As can be seen, there is no difference between the steels. Thus, calcium deoxidation does not adversely affect the hardenability of gear steels.

The following Table II shows the results of subjecting samples of A.I.S.I. 1526 and 8620 steels produced by calcium deoxidation and aluminum-silicon deoxidation to carburizing cycles similar to those used in automotive gear plants. The results indicate no difference in the case and core grain size and the effective ease depths.

TABLE II CARBURIZING RESPONSE Grade AlSl Regular McQuaid-Ehn** Gas Carburization*** or Deoxi- G5. 0.5. Case Case G3. OS. Case SAE dation Case Core Eff. Total Case Core Eff. Total 1526 Al-Si 78 8*7 .060 .070 8-7 7-8 .050 .065 I526 Cal DeOx 8 8-7 .060 .075 8 8-7 .050 .065 8620 Al Si 78 8-7 .060 .070 8 8 .050 .075 8620 Cal-DeOx 78 8-7 .060 .070 7-8 76 .050 .060

Underlined grain size is the predominant size.

" Regular McQuaid-Ehn Box carburized in carburizing compound [or eight hours at I700 F plus furnace cool to I200 F, then box cool to 900 F followed by air cool.

' Gas Carhurization Gas carburized in a batch type furnace at a carbon potential of0.90 for eight hours at 1700 F and cooled under carburizing compound.

lead-selenide phase tends to form which results in Mn Ca (S Pb Se) coatings on the calcium aluminate particles. When lead is added without selenium, the lead is present in the steel as isolated lead particles or as lead particles associated with the sulfide phase. Preferred lead additions range from 0.01 to 0.15 percent by weight in non-leaded steels and up to as high as 0.30

Results of carburizing studies are listed in the following Table III. These results show that calcium deoxidized steels of the invention essentially respond in the same manner as aluminum-silicon deoxidized steels to prior forging and/or normalizing treatments. No grain coarsening was observed in the carburized case structures. Thus, the calcium deoxidized steels and conventionally aluminum-silicon deoxidized gear steels have the same heat treat response with respect to annealing and carburizing treatments.

TABLE 111 The improved machinability of calcium deoxidizcd steels produced according to the invention has been demonstrated by tool life and tool wear tests using M2 CARBURlZlNG RESPONSE AFTER EXPOSURE TO FORGlNG AND NORMALlZlNG TEMPERATURES Underlined grain size is the predominant size.

" Normalized one hour at 1800 F, then box carburized in carhurizing comp'ound for eight hours at 1700 F plus furnace cooled to 1200 F plus box cooled to 900 F plus air cooled.

Held at 2150 F for one-half hour plus air cooled. Held at 1750 F for four hours plus air cooled. Box carburized in carburizing compound for eight hours at 1700 F plus furnace cooled to 1200 F plus box cooled to 900 F plus air cooled.

Tensile tests giving the results on tensile strength, high speed steel tools. Single point turning tests were yield strength, elongation and reduction in area on samples of calcium deoxidized and aluminum-silicon deoxidized S.A.E./A.I.S.l. grades 1526 and EX33 steels are listed in Table IV. As can be seen from this table,

carried out on a precision continuously variable speed lathe without the use of any cutting fluid. The test samples were obtained from the first, middle, and the last ingots of the heats. Each test sample was about 4% the mechanical properties of calcium deoxidized steels inches in diameter and 18 inches long. Prior to the and aluminum-silicon deoxidized steels are essentially the same. The longitudinal room temperature impact strength data listed in Table V for the same steels also show little difference.

TABLE IV tests, the samples were annealed to give a hardness of 140 to 180 BHN. During the tests a speed range of from 100 sfpm to 400 sfpm was explored; the feed and depth of out were held constant at 0.0107 ipr and 0.050

MECHANICAL PROPERTIES (1526 Normalized and EX33 Hot Rolled) Grade Typical Composition Tensile Strength Deoxidation (psi) Yield Strength (psi) Percent Reduction in Area Percent Elongation Al-Si 84.500

Cal-DeOx 87,000

Al-Si 88.300

Cal-DeOx 87,500

1526 samples were forged to 1 V4 in. round and normalized at 1650 F for one hour (140-180 RHN).

EX33 samples were hot rolled to 1 174 in. round (160-207 BHN).

TABLE V LONGlTUDlNAL IMPACT PROPERTIES Grade Full Size Charpy Composition ft-lbs Deoxidation Al-Si 14.00

Cal-DeOx 18.00

Al-Si 20.00

Cal-DeOx 18.00

All Charpy lmpact Tests were run on pseudocarburized samples. 1700 F for six hours in salt; furnace cooled to 1550 F for 10 minutes in salt and water quenched; tempered at 350 F. Hardness: R -43 (1526); R 46-47 (8620).

inches, respectively. Time to total tool failure was considered as the tool life criterion.

The turning tool life tests on various grades of steels followed a similar pattern with calcium deoxidized steels showing a significantly longer tool life than aluminum-silicon deoxidized steels of corresponding grades. Typical comparative results on a 4027 grade steel are plotted in FIG. 14. As shown, the calcium deoxidized steel had a significantly longer tool life at all cutting speeds. The chip characteristics of the two steels were found to be similar.

The multiple cutter milling tests were carried out on 4 inch square blocks having a length of 12 inches. The tests were conducted using straight oil as the coolant. The wearland wear on the tools was measured. During the milling tests, the speed and feed used were sfpm and 0.001 ipr, respectively. These tests were carried out on several carbon and alloy grades of calcium deoxidized and aluminum-silicon deoxidized gear steels. In all cases, the tool wear increased rapidly for aluminum-silicon deoxidized steels as compared to the calcium deoxidized steels of the invention. Typical results on a 8620 grade steel are plotted in FIG. 15. As can be seen from this figure, almost twice as much volume of the calcium deoxidized steel can be removed as compared to that of the corresponding grade aluminum-silicon deoxidized steel prior to reaching the stage of rapid rise of wearland wear.

The improved machinability of the calcium deoxidized steels of the invention has been further proven under day-to-day machining conditions in the actual production of automotive differential pinions, ring gears, side gears, and pinion mates. The drive ring gear tests were carried out on ring gears made from forgings annealed to about 140 to 180 BHN hardness prior to rough turning or blanking. The gear cutting operations were carried out on 606/607 Gleason gear cutting machines. The 606 Gleason rougher had Formate type cutting to rough cut the required gear teeth profiles on the blank. The rough gear cutting was carried out at a speed of about 150 sfpm using a feed of about 4.0 seconds per tooth. The 607 Gleason finisher had a helixform type cutter. The finish gear cutting was carried out at about sfpm using a feed of about 4.0 seconds per tooth.

The ring gear tests were conducted on several grades of calcium deoxidized and aluminum-silicon deoxidized steels. Each trial test was comprised of about 10,000 ring gears. The typical results of these tests on calcium deoxidized and aluminum-silicon deoxidized S.A.E./A.l.S.1. 4023, 1527M, 1527 and 1526 steels are plotted in FIG. 16. As evidenced by this figure, the rougher and finisher tool life was increased from about 70 to 100% when calcium deoxidized steels of the in- 3 vention were used instead of aluminum-silicon deoxidized steels.

Drive pinion tests were conducted on drive pinions made from forgings which had been annealed, rough turned, and made ready for gear cutting in a manner similar to that used for drive ring gears. The pinion gear cutting was carried out on 1 16 Gleason hypoid generators. One Gleason rougher and two Gleason finishers, one for the drive side and the other for the coast side, were used to complete the tooth profile. The cutting speed used for the 116 Gleason was about 150 sfpm with a feed of about 22 seconds per tooth. The tests were conducted on several grades of calcium deoxidized and aluminum-silicon deoxidized steels, and each test was comprised of about 10,000 pinions. Typical results of these tests on calcium deoxidized and aluminum-silicon deoxidized S.A.E./A.I.S.1.

grades

4027, 4028 and 4427 steels are plotted in FIG. 17. As shown in this figure, the rougher and finisher tool life was increased from 70 to 100% when calcium deoxidized gear steels were used instead of aluminum-silicon deoxidized steels.

Side gear and pinion mate gear tests were carried out on 8625 grade steels. The side gears were made from forgings and the pinion mates were made from either hot rolled or cold drawn bar product. Both the side gear forgings and the bars for the pinion mate were machined in an automatic multispindle machine to make gear blanks which were then fed through a Gleason Revacycle rougher and finisher to cut the teeth. Each trial test was comprised of about 2,000 tons of steel. During the trial tests, the tool wear of the high speed steel form tool used on the multispindle automatics was measured and the Revacycle tool life and tool wear were evaluated. In comparison to aluminum-silicon deoxidized steels of the same grade, the calcium deoxidized steels produced only one-third as much wear on the high speed steel form tools. The Revacycle tool life was raised about 30%. The as-machined surface finish and dimensional tolerance of the calcium deoxidized steel also was improved on the automatics.

It will be apparent from the foregoing that the objective of providing a calcium deoxidized, fine grain gear steel characterizied by the absence of oxide inclusions and improved machinability has been achieved. It will similarly be seen that the invention achieves the objective of providing a calcium deoxidation practice which in the large scale production of gear steels results in a consistent calcium effect in modifying the non-metallic inclusions in the desired manner, thereby improving the machinability by high speed steel tools. More particularly, the calcium deoxidized steels of the invention provide a substantial to improvement in the tool life of the expensive high speed steel gear cutters used for making differential drive pinions, ring gears, side gears and pinion mates. This improvement in the tool life of expensive gear cutters not only results in a substantial savings in tool regrind and tool material costs, but also increases the productivity by reducing down time spent in changing and setting up fresh cutters. For example, it has been possible using the calcium deoxidized steels of the invention to increase productivity by 25% without any drop in tool life of gear cutters. At the same time, the calcium deoxidized steels of the invention do not necessitate any change in heat treat operations such as annealing and carburizing, and

the new steels also adequately meet the mechanical and service properties required of gear steels.

Many modifications and variations of the invention will be apparent to those skilled in the art in light of the foregoing, detailed disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically shown and described.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A calcium deoxidized steel comprising by weight from 0.13 to 0.70% carbon, from 0.45 to 1.65% manganese, from 0.015 to 0.10% aluminum, at least 0.0002% calcium, up to 0.09% sulfur, up to 3.5% nickel, up to 3.5% chromium, up to 0.35% vanadium, up to 0.80% molybdenum, and up to 0.0015% boron, said steel being characterized by calcium aluminate inclusions and by the absence of alumina inclusions.

2. A steel as claimed in claim 1 including from 0.01 to 0.15% by weight of at least one member selected from the group consisting of selenium and tellurium, said steel being further characterized by the presence of manganese and calcium sulfide (with partial substitution of tellurium and/or selenium for sulfur) coatings on the calcium aluminate inclusions.

3. A steel as claimed in claim 1 including from 0.01 to 0.30% by weight of at least one member selected from the group consisting of lead and bismuth.

4. A steel as claimed in claim 1 including from 0.01 to 0.15% by weight of at least one member selected from the group consisting of selenium and tellurium and from 0.01 to 0.30% by weight of at least one member selected from the group consisting of lead and hismuth, the selenium and/or tellurium and the lead and- /or bismuth being combined with sulfur and present in calcium manganese sulfide coatings which surround the calcium aluminate inclusions.

5. A calcium deoxidized, fine grain steel containing from 0.13 to 0.70% by weight carbon, at least 0.015% by weight aluminum, and at least 0.0002% by weight calcium, and characterized by the presence of calcium aluminate inclusions and the absence of alumina inclusions.

6. A steel as claimed in claim made by adding the calcium to the molten steel when at a temperature in the range of from 30 to 120F. above the calculated liquidus temperature, the amount of the calcium addition in pounds per ton of steel being at least equal to 0.25 plus 13.33 times the oxygen content (percent by weight) of the molten steel at the time of the calcium addition.

7. A steel as claimed in claim 6 in which the molten steel is melted in a furnace and tapped or poured into a ladle, at least part of the calcium being added to the steel at a time when the ladle is from about one-third to two-thirds full.

8. A steel as claimed in claim 7 in which the aluminum is added no later than simultaneously with the calcium addition.

Claims

0.13 TO 0.70% CARBON, FROM 0.45 TO 1.65% MANGANESE, FROM 0.015 TO 0.10% ALUMINUM, AT LEAST 0.0002% CALCIUM, UP TO 0.09% SULFUR, UP TO 3.5% NICKEL, UP TO 3.5% CHROMIUM, UP TO 0.35% VANADIUM, UP TO 0.80% MOLYBDENUM, AND UP TO 0.0015% BORON, SAID STEEL BEING CHARACTERIZED BY CALCIUM ALUMINATE INCLUSIONS AND BY THE ABSENCE OF ALUMINA INCLUSIONS.
2. A steel as claimed in claim 1 including from 0.01 to 0.15% by weight of at least one member selected from the group consisting of selenium and tellurium, said steel being further characterized by the presence of manganese and calcium sulfide (with partial substitution of tellurium and/or selenium for sulfur) coatings on the calcium aluminate inclusions.
3. A steel as claimed in claim 1 including from 0.01 to 0.30% by weight of at least one member selected from the group consisting of lead and bismuth.
4. A steel as claimed in claim 1 including from 0.01 to 0.15% by weight of at least one member selected from the group consisting of selenium and tellurium and from 0.01 to 0.30% by weight of at least one member selected from the group consisting of lead and bismuth, the selenium and/or tellurium and the lead and/or bismuth being combined with sulfur and present in calcium manganese sulfide coatings which surround the calcium aluminate inclusions.
5. A calcium deoxidized, fine grain steel containing from 0.13 to 0.70% by weight carbon, at least 0.015% by weight aluminum, and at least 0.0002% by weight calcium, and characterized by the presence of calcium aluminate inclusions and the absence of alumina inclusions.
6. A steel as claimed in claim 5 made by adding the calcium to the molten steel when at a temperature in the range of from 30* to 120*F. above the calculated liquidus temperature, the amount of the calcium addition in pounds per ton of steel being at least equal to 0.25 plus 13.33 times the oxygen content (percent by weight) of the molten steel at the time of the calcium addition.
7. A steel as claimed in claim 6 in which the molten steel is melted in a furnace and tapped or poured into a ladle, at least part of the calcium being added to the steel at a time when the ladle is from about one-third to two-thirds full.
8. A steel as claimed in claim 7 in which the aluminum is added no later than simultaneously with the calcium addition.