US3897245A

US3897245A - Low carbon steels having cold workability

Info

Publication number: US3897245A
Application number: US308930A
Authority: US
Inventors: John Savas
Original assignee: Republic Steel Corp
Current assignee: Ltv Steel Co Inc
Priority date: 1970-04-16
Filing date: 1972-11-24
Publication date: 1975-07-29
Anticipated expiration: 1992-07-29

Abstract

Improved low carbon steels adapted for cold working are compositionally characterized by a particularly low content of manganese and a content of aluminum in metallic, e.g. alloying form. The contents of oxygen and nitrogen in the steel are advantageously very low, with avoidance of significant hard inclusions such as aluminum oxide. The steels are advantageously produced by procedure wherein a special deoxidation of the melt, preferably using a vacuum technique, is effected before addition of the aluminum. Economy being favored by the saving of manganese, the compositional features coact to provide low carbon steels having desirable cold workability, e.g. as measured by one or more properties such as reduction of work hardening and avoidance of cracking or the like under cold deformation.

Description

United States Patent 1 91 Savas 1 July 29, 1975 LOW CARBON STEELS HAVING COLD [73] Assignee: Republic Steel Corporation,

Cleveland, Ohio [22] Filed: Nov. 24, 1972 [21] Appl. No.: 308,930

Related US. Application Data [63] Continuation of Ser. No. 29,183, April 16, 1970,

abandoned.

[52] US. Cl 75/124; 75/124 [51] Int. Cl. C22c 37/10 [58] Field of Search 75/124 [56] References Cited UNlTED STATES PATENTS 1,680,007 8/1928 Boegehold 75/124 2,768,892 10/1956 Shoenberger. 75/124 3,304,174 2/1967 Ototani 75/124 3,496,034 2/1970 Alger 75/124 3,508,911 4/1970 Riedel.... 75/124 3,671,334 6/1972 Buechev 75/124 3,708,280 1/1973 Mimino 75/124 Satoh 75/124 Davies 75/124 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or Firm-Cooper, Dunham, Clark, Griffin & Moran [57] ABSTRACT Improved low carbon steels adapted for cold working are compositionally characterized by a particularly low content of manganese and a content of aluminum in metallic, e.g. alloying form. The contents of oxygen and nitrogen in the steel are advantageously very low, with avoidance of significant hard inclusions such as aluminum oxide. The steels are advantageously produced by procedure wherein a special deoxidation of the melt, preferably using a vacuum technique, is effected before addition of the aluminum. Economy being favored by the saving of manganese, the compositional features coact to provide low carbon steels having desirable cold workability, e.g. as measured by one or more properties such as reduction of work hardening and avoidance of cracking or the like under cold deformation.

12 Claims, 2 Drawing Figures PATENTEI] JUL 2 9 I975 SHEET 1 Tia. l.

B/ Q a U o g A A KEV vsv A VOV O 1010 RIMMED I 101OAK El /40 20 4o 60 so PERCENT cow REDUCTION INVENTOR. OHN 541/45 A TTO/QA/EY PATENTEDJULZSIQYS 3,897, 245

SHEET 2 105 KEY z24ov I I I i 0 2o 40 so so 100 PERCENT cow REDUCTION VENTOR JOHN S it A5 BY .1 1 E W S. W

ATT'O RME I LOW CARBON STEELS HAVING COLD WORKABILITY This is a continuation of application Ser. No. 29,183 filed Apr. 16, 1970, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to low carbon steels and is particularly concerned with improvement in low carbon, non-alloy steels which are particularly adapted or intended for cold working, and with methods of producing and treating such steels. Such steel may be conveniently identified as of non-alloy type, thereby meaning, in general, steels which do not have the content of alloying elements usually recognized as constituting alloy steel, i.e. a substantial content of metals other than manganese. Thus for example the steels herein described ordinarily need not contain more than about 1% of metallic elements other than iron; notably useful embodiments may have no more than a total of about 0.65% of such elements, and preferably much less. In an alternative or specific sense, the products herein described can be classed as low carbon, low manganese steels. lndeed an important aim of the invention is to afford improvement in cold workability of low carbon steels with a concomitant decrease in such alloying content as usually characterized them, i.e. in a direction favoring economy of production, as further explained below.

Examples of cold working, for which the steels produced in accordance with the present invention are generally adapted, include operations such as cold heading, cold upsetting, cold extrusion and other types of cold forging and the like, including swaging, coining and roll threading. Thus references herein to improvement in cold'workability are intended, in a somewhat generic sense, to mean improvement in at least one property of the exemplified cold workability, for instance improvement by depressing work hardening or decreasing the degree of same for a given extent of cold reduction, or improvement by reducing cracking or other tendency to failure of the workpiece upon substantial deformation, or improvement by reason of reduced load or pressure required for a selected degree of deformation. Cold working as here contemplated can be alternatively described as cold deformation, i.e. in all cases involving permanent plastic deformation of the metal, and sometimes the term cold forging has been used generically to embrace the various cold forming operations mentioned above.

In a'special sense the present improvements are notably recognized in cold deformation under compressive force, being the situation chiefly encountered in the stated types of cold working, but such characterization of the steel does not exclude other utility, as for analogous cold forming. On the other hand, a steel having deep drawing properties in sheet form is'not by any means necessarily of special use for cold deformation as primarily contemplated for the steels of this invention, nor have the latter been specially designed for deep drawability in that commonly understood sense. Moreover, although some properties of steel according to the invention may be conveniently recognizedor tested by cold rolling (which is a deformation under compressive force) and the use of such steel for fabrication by cold rolling is not necessarily excluded, it is presently contemplated that the unusual superiority or LII improvement of the steels described below is most significantly realized in their classification of use for producing discrete parts-or articles by more or less direct compression or impact, as distinguished from the production of conventional cold-rolled strip.

Heretofore a number of steels have been employed for cold working or cold forming, including conventional low carbon, non-alloy steels such as in the AlSl series from numbers 1006 to I041, as also a variety of alloy steels, an object of the present invention being to provide improved results in all cases, particularly with respect to one or more properties of cold workability as compared with the low carbon. steels, and also 'with respect to various'alloy steels, notably in attaining a good cold working product at a relatively low cost of production. I

Among previous improvements in steels for this purpose it may be noted that useful advantage has been realized by modified carbon steels with extremely low silicon but moderately large manganese content (e.g. in the range of 0.8 to 1.4%), such steel being borontreated and preferably produced with use of vacuum carbon deoxidation techniques and with a corresponding reduction of the normal aluminum addition for killing. As will be understood, these boron-treated carbon grades have involved an increase in cost, in part due to the manganese content, which is significantly higher than in the corresponding ordinary AISl compositions.

It does not appear that much, if any attention has been heretofore paid to minor alloying additions for possible effect in cold forming, or particularly in relation to economic aspects of the production of such steel, as for minimizing the need for a conventional alloying element. Thus, as noted above, boron has been used mainly for its advantages in hardening or hardenability, although indirectly promoting cold forming by permitting use of a lower carbon content. Small quantities of aluminum are employed for killing steel, i.e. deoxidizing it, in ladle or ingot, and also for control or refinement of grainsize at the same time or by similar addition. Other or related effects have been ascribed to aluminum, but it appears that in general these have not been deemed to have any particular relation to cold forming in the sense of the present invention, and in any event such additions have usually been made in circumstances where the killing or deoxidizing action has been necessarily significant or the actual resulting composition uncertain, as for instance an early mention of melting steel and a small amount of aluminum together for assertedly retaining malleability. In the case of aluminumkilled steel for deep drawing, aluminum in excess may have a special further purpose of combining with nitrogen; indeed various proposals have been made for the formation or precipitation of aluminum nitride, in steels to have ductility. As indicated, aluminum tends and is indeed usually intended, to combine with an element such as oxygen normally present at the time of addition, and there is corresponding tendency to produce minute hard inclusions, eg of aluminum oxide,.in the steel.

As will become apparent, the present invention is designed to afford significant betterment in the production of low carbon steel for cold working, e.g. rod, bar, and like stock, whether in coil or piece form, and to do so with advantages of economy, i.e. at least without substantial increase of cost.

SUMMARY or THE INVENTION To the above and other ends, it has been discovered that a low carbon steel exhibiting significant improvement in cold workability can be achieved .relatively inexpensively by a compositionwhich includes aluminum in metallic state and wherein manganese. a conventional element in low carbon non-alloy steels. is markedly reduced in amount, and indeed to special advantage is reduced to a very low value. An essential feature is that the aluminum is understood to be present at least predominantly in the nature of an alloying element, i.e. definable as inmetallic state distinguished from common use of aluminum ineffectuating the deoxidation or killing of steel, it beingfurther understood to be important for the invention that aluminum oxide inclusions and indeed also appreciable amounts of aluminum nitride be avoided, and likewise that the oxygen content of the steel and preferably nitrogen, too, be

low. I

Thus in a special aspect of the invention, the method of producing the novel steel product having the above stated compositional characteristics involves deoxidation, or more generally degassing, of the melt, by an operation which does not yield unwanted inclusions and which is advantageously effected prior to the addition of aluminum. To this end, the desired melt is very preferably produced or treated with the aid of vacuum conditions, e.g. by utilizing vacuum carbon deoxidation techniques. Thus, for instance, the melt may be prepared under vacuum, as by heating in an induction furnace where vacuum conditions are maintained or applied. Alternatively and with good practicability, the melt as produced in a more conventional furnace operation, such as of the so-called basic oxygen type, is subjected to vac uum degassing in the ladle, i.e. vacuum carbon deoxidationf In any case, reduction of the oxygen content is effectively achieved without introducing aluminum or other elements which produce hard oxides, and indeed quite preferably, the nitrogen content of the steel is also reduced to a very low value. The aluminum is only thereafter added, in amounts indicated hereinbelow.

As stated, an important aspect of the composition is the incorporation of only a limited content of manganese. Not only is it found that the formulation accommodates itself to the lower amount of manganese without sacrificeof properties heretofore thought to require a higher content, but also that the manganese reduction coacts with the aluminum addition in promoting cold deformation properties. A particularly important consequence is that although the need for special degassing or deoxidizing treatment and the use of special (though still modest) amounts of aluminum may increase the cost of production, eg as compared with ordinary aluminum killed steel or rimmed steel, the employment of only a low percentage of manganese represents a saving of cost, which at least substantially offsets or balances the foregoing increase of'expense. At least preferably, the manganese is also added only after the vacuum deoxidizing or like treatment, so as to take further advantage of the latter for maximum saving, in that unnecessary loss of manganese in chemical combination or otherwise, and its tying-up of nitrogen (against removal by vacuum) can be minimized.

It may be noted that manganese is' normally employed in carbon steels for one or more of 'three purposes, i.e. as a deoxidant, as an alloy addition to improve hardenability, and as an agent to tie up sulfur in the form of manganese sulfide with corresponding improvement in hot workability, In the steels of the present invention, the deoxidant function of manganese is essentially dispensed with, while it is found that its addition for aid of hardenability or for accommodating sulfur is either of less consequence or is sufficiently realized with much less amounts of the element. Indeed itis found convenient to control sulfur, and likewise phosphorus, in such fashion (as by melting technique) that there is a basic avoidance of anything greater than rather low percentage contents of these elements. Hardenability, moreover, to the extent. (if any) demanded in a cold working steel, was found inherently attainable in the stated compositions and could be enhanced by theexpedient of boron addition if desired.

In anyevent, the stated steels of the invention are found, notably by reason of the aluminum addition under the described circumstances, and in cooperation with the low content of manganese, to provide significantly improved cold working properties, for example as against ordinary low carbon steels of the AISI grades, while providing for relatively inexpensive production, i.e. economic feasibility despite the cost of aluminum addition and special deoxidation. Indeed the present compositions are understood to compare favorably with steels heretofore specially produced, atsignificantly greater cost, for cold forming.

In a more specific sense the improved cold working non-alloy steels are characterized by compositions consisting essentially (in weight percent) of up to 0.5% carbon, very advantageously carbon not greater than about 0. 15%, manganese in the range up to 0.45%, with verysatisfactory results ofunusually economical character at manganese levels of not more than 0.2%, and finally, an aluminum content of 0.05 to 0.25%, most effectively as a rule,in the lower part of such range, the balance being iron with incidental impurities and minor special-purpose additions that may sometimes be desired. As more explicitly set forth below, siliconis preferably kept low, the phosphorus and sulfur contents are suitably limited, and the nitrogen and oxygen present in the ultimate steelare-preferably reduced to very low values. v

Steels of these compositions, which are readily produced by procedure as explained above, have shown marked. improvement in a number of respects in cold forming operations. Thus, by way of example and without limitation: work hardening is reduced, as measured by the hardness attained after a given, considerable extent of working reduction; failure by cracking or the like is likewise minimized, as compared with other steels, upon substantial cold deformation; and in at least some circumstances, the compressive load or pressure required for effecting a given degree or character of deformation is measurably less.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a graph showing significantly reduced propensity to work hardening, of steels according to the present invention, in comparison with standard, commercial, low carbon steels.

FIG. 2 is a similar graph showing the lower work hardening propensity of a certain other steel of the invention, compared with a steel which lacks the reduced manganese characteristic.

DETAILED DESCRIPTION As stated, the present invention providessteels for cold working, in which one or more properties of cold workability are improved, e.g. relative to standard low carbon steels, without significant increase in cost. An essential feature of the new steels isthe inclusion of aluminum in the nature of an alloying element, with the oxygen and indeed also preferably the nitrogen content of the steel reduced to a low value, such reduction being attained by vacuum treatment ofthe melt, and the aluminum being added only after such treatment so that it is present substantially or at leastpredominantly in metallic state (or not combined as oxide or nitride) and does not function significantly in themanner of conventional aluminum additions for killing or the like. In cooperation with the stated aluminum addition, the manganese content is established at a relatively low value, i.e. not more than 0.45% and advantageously at a substantially lower value, ina given steel, than is conventionally utilized for a corresponding standard steel of the same, selected low carbon composition, there being unusual advantages with manganese not more than 0.2%. The 'coacting function of the low manganese is twofold, in that such characteristic is found to contribute affirmatively to the improvement in cold workability, while it permits a significant reduction in cost which balances such increase in cost as may be due to the vacuum treatment and the aluminum addition.

The nature and results of the invention have been demonstrated by an extensive testing'program, including the representative experimental heats set forth by their respective compositions in the following table, the balance of the melt .in each case being essentially iron, except for incidental impurities. Y

TABLEl of these was preparedin an induction furnace, in amount of about 3.5 pounds of melt or more in each case, utilizing conventional techniques for producing heats of low carbonsteel to have analyses of the sort noted in the table, eg as to carbon and manganese content and with respect to concentrations of phosphorus and sulfur, that were relatively low in most instances, as set forth. For each heat the aluminum was added at the end of thesteel making cycle, i.e. added in metallic state. In thisfirst set of six heats, attention was focused on -very low carbon compositions, i.e. of about 0.171 or less, and notably also on very low manganese contents, of the order of 0.10% for four of these heats. Of the two heats lacking this characteristic of greatly reduced manganese, heat Z26had a particu- -larly high manganese content, not uncommon in ordinary low carbon steels of thistype, while heat B8 10 was produced in a normal manner for such steels, i.e. without vacuum treatment and with no more than normal aluminum addition such as for deoxidation.

Heats V758, V850 and Z26 were made by vacuum induct ion operation, i.e. by induction heating under vacuum conditions such that the melt was subjected to vacuum carbon deoxidation in the furnace. Heats B810, B811 and B812 were produced by air induction, i.e. melting in air, without vacuum treatment either in the furnace or thereafter. In consequence the first three heats were characterized by extremely low contents of oxygen and nitrogen, the vacuum conditions serving to reduce nitrogen materially as well asto achieve deoxidation. In contrast the three air induction heats showed much largericoncentrations of both oxygen and nitrogen, being comparable to ordinary steel production where no vacuum operation is employed and before any special deoxidation. For example, one specimen of HEATS TESTED Composition (Weight 7 Exper. v No. Type C Mn P v S' Y Al B Other V758 1 0.09 0.11 0.007 0.008 0.08 V850 l 0.08 0.11 0.020 0.016 0.07 Z26 2 0.07 0.48 i 0.020 0.020 0.057 B810 3 0.05 0.35 0.008 0.015 0.031 B811 3 0.04 0.10 0.021 0.018 0.075 B812 3 0.04 0.09 0.034 0.015 0.05 Z228 1 0.08 0.11 0.018 0.014 0.05 Z229 l 0.08 0.11 0.018 0.014 0.12 Z230 4 0.07 0.10 0.014 0.014 0.05 Zr-0.()4 Z231 4 0.07 0.10 0.015 0.015 0.12 Zr-0.05 Z232 4 0.07 0.10 0.018 0.015 0.11 V 0.06 Z233 4 0.07 0.10 0.020 0.017 0.11 MO-0.05 Z236 4 0.08 0.09 0.015 0.013 0.12 0.0016 Zr-(l.()3 Z237 5 0.19 0.39 0.019 0.014 0.12 0.0016 Zr-().()4 Z238 5 0.21 0.36 0.021 0.014 0.05 0.0020 Z239 2 0.21 0.80 0.020 0.015 0.05 0.0020 Z240 2 0.39 0.65 0.020 0.015 0.05 Z241 (i 0.40 0.10 0.020 0.015 0.21 00016 For convenient reference these heats are here classified by experimental types, numbered from 1 to 6 inclusive,'

heat V850 showed oxygen at only 30 ppm (parts per million) and nitrogen less than 20 ppm, whereas a corresponding specimen of heat B811 revealed oxygen at 120 ppm and nitrogen at ppm.

In'the case of the above and other experimental heats described below, the cast ingot material from each heat was divided into appropriate pieces and reduced to shapes suitable for testing, by hot forging in the temperature range of 2lO0 F...to 1700 F. More specifically, the ingots from the heats were first hot forged into billets having a, 2 /2 inch square dimension in section, at

the stated temperature, and these squares were then hot forged, in the same temperature range, to 1% inch square bars.

In the case of all heats at least some of these bars were cooled to room temperature from the stated hot condition while embedded in vermiculite, which is a finely particulate, refractory material having thermal insulating properties. i.e. providing a heat-transferretarding environment. ln such case, the bar cooled at a much slower rate than would have been achieved by air cooling. In all instances than heat V758, there were also a number of the hot forged bars that were simply cooled in air, thus at a substantially faster rate, for com parative evaluation of both types of cooling. The respectively different cooling procedures are denoted by letters V or A following the heat number in subsequent references to tests. Thus No. V850V means the part of heat V850 that was vermiculite cooled, while No. V850A identifies the air cooled portion of the same heat. This mode ofidentification is followed in the case of other heats described below, the final letter V or A at the end of the heat number representing the type of cooling from the temperature of hot production, i.e. in vermiculite (very slow) or in air (more rapid).

In terms of specific cooling rates, and by way of example for purposes of comparison, the vermiculite and air cooling of the 1% inch bars can be represented as being at about 100 to 200 F. per hour and about 50 to 100 F. per minute respectively.

A first set of tests on the six heats mentioned above involved cold reduction, not only to determine and confirm general suitability for cold deforming, but particularly to appraise the propensity of material to work hardening. In each case sections of 10 to 12 inch length were cut from the bars and were ground to 1.1 inch squares. These square section pieces were then cold reduced on a rolling mill in increments from about 10 to about 80%. At various percentages of reduction, pieces were cut off and tested for hardness, i.e. by standard instrumentation to determine Rockwell B values. The results of these tests at percentage-reduction stages representative in significance, made with respect to heat No. V758 (bars of vermiculite-cooled state) and both types of bars from heats V850, Z26 and B810, are shown in the following table.

terial V758V and V850V reached values of 90.5 to 92.5, as against 94.5 and 95.5 for the high manganese and air-induction products respectively (226V and B8l0V) that had also been vermiculite-cooled. In the situation of air-cooled metal at the highest percent reduction, V850A showed hardness of 94.5 whereas 226A and V81OA reached values of 96.5 and 97.5.

Similar tests were made with heats B81 1 and B812 and were relatively poor or at best inconclusive in comparison with heats V758 and V850 (of the invention), a rather high work hardening result in the case of heat B812 being ascribed in part to its substantial phosphorus content, well over 0.030%. As noted below, however, these air-induction-melted heats B81 1 and B812 showed to notable disadvantage in another set of tests and thus were demonstrated to lack the overall improved suitability for cold working which characterized V758 and V850.

As will be noted, heat B810 was essentially a standard grade, low carbon steel, of very low carbon content, the amount of added aluminum being no more than might be conventionally supplied for killing. lndeed it became apparent that in the case of all three heats B810, B811 and B812, the aluminum addition was unquestionably consumed to a large extent in a deoxidizing function, thus precluding its appearance in substantially metallic state or even predominantly metallic state, while at the same time undoubtedly leaving a considerable amount of aluminum oxide inclusions distributed in the steel.

In another set of tests. pieces having a 1.00 inch square section, and a height of 1% inch, were cut and ground from the various forged bars. These specimens were then upset on a hydraulic press using a 0.450 inch stop-block, the compressive force in each case being sufficient to reduce the specimen, in its longer dimension, by about The upset pieces were flat blocks of somewhat square contour, but having arcuate, bulging sides as expected from such upsetting treatment, the final vertical thickness in each case being about 0.46 to 0.47 inch. The results of this cold compacting test are given in the following table, the various heats and nature of cooling operation being designated as explained above.

TABLE II WORK HARDENING PROPENSlTY OF HEATS, NOTED BY NO.

No. No. Nov No. No. No. No. vsv V850A vssov 226A 226V B810A B810V 71 /7 "A '7! 71 Red. R Rcd. R Red. R,, Rod. R,, Red. R Red. R Red. R,

0 50.5 0 41.5 0 36 0 47 0 42 0 49 0 42 10 65 10 71.5 11 65 8.3 73.5 8.4 65.5 8.3 76.5 8.4 71.5 41 80.5 35 84.5 35 x0 39 87 39 a4 39 88.5 39 86.5 58 x7 54 54 x5 57 9o 57 89 57 94.5 58 91 79 92.5 79 94.5 79 90.5 78 96.5 78 94.5 78 97.5 79 95.5

In each instance the hardness, i.e. R reading, for the 60 TABLE 111 article before cold reduction is noted (zero 7: Red.) and the R,, values for the various stages are noted. RESULTS OF COLD COMPACTING It will be seen at once that at the relatively large reil HlGH Q FROM BARSU en 1 y z pset set duction of about 79% the heats representative of the 5 f B Atcemer Raging present Invention (V758 and V850) showed significantly less hardness than corresponding specimens of xgggx l heats Z26'and B810. Thus the vermiculite-cooled ma- V850V 71:4 5

TABLE Ill-Continued RESULTS OF COLD COMPACTlNG 1" Sq X 1 /12" HlGH SECTIONS FROM BARS Identity 7! Upset Upset of Bars At Center Rating B810A 71.3 3 B81OV 71.2 4 B81 1A 71.3 6 B81 1V 71.4 7 3812A 70.8 7 B812V 71.1 8 226A 71.1 2 226V 71.? 2.5

Smooth surface; no cracks.

2. Slight roughness; no cracks.

2.5. Considerable roughness; no cracks. 3 to 5. Slight to intermediate cracking.

6 to 8. Bad to extremely bad cracking.

9 to 10. Split partly to wholly through.

As will be seen, the bars from heats V758 to V850 performed excellently, while all of the air induction heats showed more or less deterioration. Heats B811 and B812 revealed very serious failure, thus demonstrating (as intimated above) their relative unsuitability for cold working, while heat B810 showed measurably less satisfactory results than the heats of the present invention, in this as well as in the work-hardening tests. The pieces from heat Z26 showed relatively good results but it will be remembered that this heat exhibited a definitely greater work hardening propensity. As will be appreciated, even though it was prepared by vacuum induction treatment and received a significant aluminum addition, the composition was characterized by a relatively high manganese content.

From forged bars of the same heats as identified in the headings of Table 11, respectively characterized by cooling treatments as there indicated, a number of specimens having a cylindrical shape, 1 inch in diameter and 1 inch high, were prepared by cutting and grinding, and were subjected to compression tests in a ZOO-ton tensile machine, to determine loads required for various percentages of reduction. These tests re-, vealed that whereas the bars from heats V75 8V, V850A and V850V showed higher values of upper yield point than heats Z26A, Z26V and B810V, and such values comparable to that of 3810A, the compressive loads necessary to deform the test specimens to' about 30 to 40% of their original height were appreciably less, for the steels of the invention, than the standard low carbon specimens (B810) and at least compared favorably with the experimental, relatively high manganese heat Z26'. Measured in thousands of pounds (pounds X 1000), the vermiculite cooled specimens V758V and V850V both showed load values of approximately 78 and 100 for 30 and 40% compression respectively, whereas B810V and Z26V both showed values of about 80 and 102. Similarly the air cooled specimen V850A required values of 81 and 104 for 30 and 40% respectively, while the air cooled B810A required loads of 83 and 105.5, the values for Z26A being 82 and 104] Further comparison of the work hardening propenwithout vacuum techniques and without aluminum addition except where needed in limited conventional amount, e.g. for killing, were subjected to progressive reduction as in the case of the above tests, and hardness determinations were made at various percentages of such reduction. The results are shown in FIG. 1, where Rockewll B hardness is plotted against percent cold reduction. The upper curve represents the successive hardnesses of the commercial products, with the solid squares being the 1010 rimmed and the open squares the 1010 AK. The lower curve represents the values for products of the present invention, the triangles being heat V758V and the circles being heat V850V. As will be seen, the work hardening propensity was significantly less in the case of the present, improved steels.

As noted above, the air induction heats B811 and B812, where no special deoxidation was employed prior to aluminum addition, fell short of the improved properties of the heats of the present invention, particularly in respect to tendency to cracking or other deterioration upon direct reduction under compressive load. In addition, heat B812 showed, in rolling reduction tests, by comparison of its air cooled samples with air cooled samples of V850, and likewise by comparison of its vermiculite cooled samples with vermiculite cooled samples of V758 and V850 that it was characterized by a significantly greater propensity to work hardening. This is attributed, in part, to the substantially higher phosphorus content of heat B812 (Table 1, above), it being presently understood that the level of phosphorus should not be over 0.030% and very advantageously not more than about 002571.

For further demonstration of the nature and properties of the improved cold working steels a number of additional heats were prepared by vacuum induction heating, including degassing by vacuum carbon deoxidation as before, with varying compositions as reported in Table 1 above, these being heats Z228 and following. In each instance the procedure was essentially as described in connection with heats V758, V850 and Z26, including forging of ingots from the several heats, to suitable square section billets at 2100 F. to 1700 F. and further forging of portions of the ingots to smaller square section bars, in the same temperature range. 1n all cases, hot, forged bars were cooled separately by vermiculite cooling and by air cooling, the specimens being correspondingly identified by a final letter V or A after the heat number in references thereto below. Work hardening tests were performed in the same manner as for heats V758 and others, i.e. utilizing 10 to 12 inch long sections from the bars, after grinding to 1.1 inch square cross-sectional shape. Hardness was tested, to determine Rockwell B values, at successive increments of cold reduction.

For six of these heats, being Z228 to Z233 inclusive from Table l, and represented by a total of twelve different samples respectively in the air cooled (A) and vermiculite cooled (V) state, the results of the cold rolling tests are shown in the following table:

TABLE IV WORK HARDENING PROPENSITY OF HEATS, NOTED BY NO.

No. No. No. No. No. No. 2228A 2229A 2230A 2231A 2232A 2233A "/1 '2; '7: I "/1 71 Red. R Red. R Red. R Red. R Rod. R Red. R r

Z228\' Z229; 7230\' 7.231\' 7.232. ZIHV 0 54.5 0 54 0 52.5 0 53.5 0 56 0 55 19.7 76.5 19.8 76.5 19.5 75.5 19.8 77 19.5 77 19.5 77.5 69.3 89.5 69.3 90.5 69.2 89.5 68.8 92.5 68.7 91.5 69.1 90.5

For simplicity only a lesser number of percent reduction stages are included in the table, but in fact the same comparability of results was shown at other, e.g. intermediate points of reduction. Heats Z228 and Z229 were essentially of the same class as heats V758 and V850, as to carbon composition and manganese content, and it will be apparent that the results in work hardening were essentially similar between corresponding types of Z228 and Z229, even though there was considerable difference in aluminum content. Heats Z230 to Z233 inclusive clearly reveal that the addition of minor alloying ingredients, e.g. zirconium, vanadium and molybdenum did not adversely affect the cold working characteristics, this being particularly true of the vermiculite cooled specimens. Hence it is apparent that minor additions of such elements may be safely ineluded for properties that they may contribute, e.g. in

that have about 0.2% carbon. While in both the air cooled and vermiculite cooled states, heats Z237 and Z238 showed higher values of Rockwell B hardness after successive percentage reductions in cold rolling, than preceding, lower carbon heats in Tables IV and V, the results with heat Z239, having a manganese content of 0.80%, showed significantly greater work hardening. A similar comparison in the higher carbon heats Z240 and Z241 (e.g. carbon about 0.4%) revealed a like, very marked difference, i.e. attributable in at least substantial measure to the relatively high manganese content (0.65%) in heat Z240 as against a very low manganese content (0.10%) in heat Z241. The superiority of 2241A and Z241V (of the invention) as to lower work hardening propensity is graphically shown in FIG. 2, plotting the hardness test results for various percentages of cold reduction, in comparison with 2240A and Z240V.

The results of the tests in Table V, taken further in comparison with tests of Table IV, demonstrate not only that the content of manganese may be reduced for advantages of cost, but also that in the steels having the compositional characteristics of the invention (including the stated aluminum percentages, present essentially in metallic state), a low manganese content, e.g. not more than 0.45%, preferably 0.4% or less and very advantageously not higher than about 0.2%, contrib- TABLE V WORK HARDENING PROPENSlTY OF HEATS. NOTED BY N0.

No. No. No. No. No. No. 2236A 2237A 2238A 1 2239A 2240A 2241A '71 "/1 /7 S1 "/7 7 Red. R" Rcd. R Red. R Red. R Red. R Red. R

No. No. No. No. No. No. Z"36V 7M7 223m Z239\' 7 4m 7 41\' 0 52.5 o 61.5 0 65 0 67.5 0 211.5 (1 68 19.8 76.5 19.5 84.5 19.2 85.5 18.9 88 21.6 97.5 22.4 89.5 68.5 92 69.0 95.5 68.8 96.5 68.6 98 66.8 101.5 67.6 96

In this instance it will be observed, from a comparison utes materially to desirable cold working characterisof heat Z236 with heat Z231 (Table IV), for example, tics. It is specifically apparent from all the data in Tathat the addition of boron does not increase the work bles IV and V.that all of these heats with the exception hardening propensity. In consequence, as also indiof Z239 and Z240 afforded good'properties, with recated by other heats of the series tested in Table V. spect to reduced work hardening at the selected carbon boron may well be incorporated for its known effect in levels, whereas tests with heats Z239 and Z240 improvmg hardenability. (deemed outside the scope'of the invention) showed Heats Z237 and Z238 represent larger contents of the greatest work hardening propensity of all the comboth carbon and manganese, within the broader limits positions reported.

of the invention as presently understood, while for comparison heat Z239, otherwise similar to heat Z238, includes a much higher level of manganese. i.e. as might characterize some of the ordinary carbon steels vA review of Tables ll, 1V and V further shows that in almost all cases the vermiculite cooled, i.e. slower cooled specimens, had a definitely lesspropensity to work hardening than those where the hot formed bars were cooled in air, this being most consistently true of the very low carbon compositions, e.g. up to about 0.10%.

Although in many cases steels used for cold working are not heat treated and need not meet high requirements of hardenability, tests of the present composi tions, i.e. specimens of representative steels of the invention from Table 1, reveal some degree of hardenability, and particularly show that with the inclusion of boron a relatively very good hardenability can he achieved. These determinations were made by a standard Jominy test, involving end quenching ofa one inch diameter rod that was at a selected temperature in the range of 1650 F. to 1700 F. After grinding off about 0.020 to 0.025 inch from the surface, measurements of Rockwell C hardness were made at sixteenth of an inch increments from the quenched end. The tests showed, for example, that specimens of Z241A and Z24lV had good hardenability, essentially comparable to Z240A and Z250V. In other words, even with a very low content of manganese, the inclusion of the indicated small percentage of boron was sufficient to promote hardenability, as compared with the high manganese heat, Z240.

Useful values of hardenability were also determined for heats Z237, Z238 and Z239, further showing that the lower-than-normal manganese contentof the first two of these in comparison with the high manganese of the third did not substantially alter thhe hardenability, at least within about one-fourth of an inch from the quenched surface. The effect of boron was further demonstrated in Jominy tests of samples from heats Z231 and Z236, with significantly higher Rockwell C values being attained at very close distances from the end in the case of the boron-treated heat Z236. Moreover, hardenability curves plotted for boron containing steels of the present invention, in the carbon range upwards from about 0.2%, showed definite superiority over conventional band plots for standard, so-called carbon H steels of comparable carbon contents.

Calculations were further made to determine the boron hardenability factors of the 0.2% carbon heats, Z237, Z238, and Z239. In comparison with prior art production steels which develop boron hardenability factors of from 1.5 to 3.0 maximum, the boron factors for the above noted heats were 4.6, 4.6 and 3.4 respectively. It has thus been found that the steels of the present invention, Z237 and Z238, show a particularly significant improvement in the boron factor of about 100%, from an average of about 2.3 to about 4.6. The special processing of these three experimental heats even resulted in an improvement of the boron factor for the Z239 heat, which was of standard analysis except for the boron addition. With a factor of 3.4, even this heat surpassed the 3.0 boron factor set as a possible maximum for production materials produced according to the present state of the art.

The heats of steel set forth in Table [were all prepared to have essentially no silicon, i.e. less than about 0.01%, and it is of special advantage that the compositions of the invention contain little or no silicon, e.g. not more than 0.05%. Although in most cases optimum cold workability is believed to be attained with the lowest possible content of silicon, it is conceived that circumstances of production or even some special requirement for the presence of silicon may counterbalance any corresponding diminution of coldforming characteristics, a convenient upper limit for this element being about 0.30%, and the preferable silicon range being from 0 to not more than 0.15%.

As explained, the compositions should be very low in oxygen content, e.g. up to 40 ppm, advantageously not more than 35 ppm and preferably not higher than 30 ppm. it is particularly desirable that the steel be very low in content of oxygen which is combined (or combinable) with aluminum or which is present in hard inclusions. whether of aluminum oxide or other compound. As stated, the procedure of making the steel advantageously employs a suitable vacuum degassing technique. e.g. so-called vacuum carbon deoxidation wherein the effect of appropriately high vacuum and reaction of carbon present is understood to remove oxygen, presumably as gaseous carbon monoxide. The use of other special deoxidation or degassing procedures is not, however, excluded from the broader contemplation of the invention, asfor example so-called argon or other inert gas operations utilized for degassing, the use of electroslag or other special melting techniques, or even special alkaline-earth metal compounds employed fpr deoxidizing, all being understood to degas or deoxidize without leaving hard inclusions or undesired amounts of oxygen in any form in which it could combine with aluminum.

Addition of the aluminum only after degassing is a highly effective procedure for attaining the desired compositional characteristics, both to low oxygen and as to presence of aluminum in predominantly or indeed substantially all metallic state, i.e. uncombined (especially with oxygen or nitrogen). Vacuum degas sing is also of effect in reducing the nitrogen content to 30 ppm or below; it should most advantageously be no more than 25 ppm, and preferably not over 20 ppm, e.g. for avoidance of nitrides, particularly precipitated aluminum nitride. As indicated, it is preferred to add the desired manganese, or as much of it as possible, only after degassing, the same being true of other nitride formers such as zirconium and indeed also any other minor additions, e.g. vanadium, molybdenum or boron. Upper permissible limits of phosphorus and sulfur appear to be 0.030% each, preferred limits being from 0 to 0.025% and very preferably not more than 0.020%.

In Table 1 experimental type- 1 represents presently preferred compositions, with carbon from 0.01 to 0.15%, a more convenient lower limit in practice being about 0.03%, and manganese in the very low range of 0.25% and below, e.g. from 0 to 0.2%; a presently preferred range for manganese, having in mind its desired functions, is 0.05 to 0.15%. Aluminum is added in amounts of 0.05 to a general upper limit ofO. 15%, most advantageously not more than 0.12%. With the total content of oxygen and-nitrogen generally less than 0.01%, and in a specific, preferred sense not more than 0.007%, there is not even theoretical possibility of tying much aluminum in undesired non-metallic state, so that such element can be deemed to be present substantially as an alloying constituent.

Steels designated as type 4 in Table l are compositionally similar to type 1, except for inclusion of one or more optional additions, zirconium, vanadium and molybdenum being conceived to be appropriate in amountsfrom O to 0.1% each and boron from 0 to 0.1%, a preferred upper limit of boron being 0.006%. It isnoted, however, that for many purposes of a cold 15 working steel, optional elements of this character have not been found necessary in the present compositions and their omission. as in the type 1 heats, correspond ingly represents a specific characteristic of notable advantage.

Steels identified types 5 and 6 relate to higher carbon contents, and in type 5 to higher manganese contents. as where for practical reasons of production or for special reasons, e.g. strength. hardness or hardenability, greater levels of one or both of these elements may be desired. Type 5 can be defined as including carbon up to 0.25% and demonstrates the tolerance of the steels for manganese in the upper portion of the broader range of the invention, while type 6 has a special virtue of combined economy and effectiveness in steels for which heat treatment and corresponding high qualities of hardenability may be desired, it being also conceived that a lower percentage of aluminum (than in heat Z241), e.g. about 0.15%, may even be prefera ble in such steel. lt presently appears that in most cases the carbon content need not be greater than 0.4%.

As will now be appreciated, the improved steel is sufficiently characterized by not more than low contents of manganese, nitrogen and oxygen, by substantial freedom from hard aluminum oxide inclusions, and by the presence of aluminum in at least predominantly metallic state, for providing effective cold workability, and indeed cold workability improved in one or more properties relative to ordinary low carbon steel. Stated in another way, the characteristics of aluminum content and of not more than low oxygen, for example as produced by the described practice of deoxidizing and adding the aluminum, and the characteristic of low manganese content, particularly coact (in nature and amount) to provide effective cold workability.

Not only is the manganese content limited to numerically low values, including unusually low proportions in the preferred compositions, but it appears that a further characterization of the invention is that considering the selected carbon level, the manganese is kept at what would be regarded as an abnormally low level for ordinary steel of such carbon percentage; thus generally, there can be both special superiority and a marked saving in manganese, in comparison with selected ordinary low carbon compositions. Ordinarily, for example, manganese and aluminum need not respectively be over 0.2 and 0.12% at most, for carbon contents up to 0.15%.

It is also noted that optimum values of aluminum may vary with carbon content, e.g. as indicated above for the lowest range of carbon. For carbon from 0.15 to 0.25%, an aluminum range of 0.08 to 0.15% is contemplated, preferably not more than about 0.12%, while in the carbon range of 0.25 to 0.5%, the aluminum can again range up to about 0.20%, or higher, with some preference for values of at least 0.12%.

All proportions herein are expressed by weight, and in all examplification of complete compositions the bal ance is, of course iron, with the exception of incidental impurities such as may be present in industrial practice. It will be appreciated that there can be instances where one or more residual element or elements of the group of nickel, chromium and copper may be present in production heats of the standard carbon steel grades, or where residual or like small quantities of elements such as titanium or columbium may be included in carbon steelmelts, e.g. as a result of the use of various deoxidizing materials or the use of alloy additions such as boron-containing master alloys, or as a result of intentional addition such as for grain refinement or improving hardenability. In such cases the present invention is nevertheless deemed applicable in' a generic sense, in aid of cold workability and even though an incidental element or the like might perhaps of itself be less than desirable for such workability yet might be unavoidable, or required for another'purpose. Hence it is conceived that the inclusion of a total of 0 to 0.5% or possibly 1% of other element or elements individually occurring in no more than about the following maximum percentage amounts from the group consisting of 0.3 max. nickel, 0.3 max. chromium, 0.3 max. copper, 0.1 max. titanium, and 0.1 max. columbium, should not be considered as being a deparature from the spirit of the invention, i.e. within its general definition of lowcarbon, non-alloy steels.

As will now be seen, the improvements of the present invention are capable of realization in an economical manner, to achieve a steel of superiority in one or more properties of cold working (at least as compared with the basic low carbon grade selected), and at the same time considerable flexibility of chemistry is deemed available, to suit various requirements of production or of special properties in the metal.

It is to be understood that the invention is not limited to the specific compositons and procedures hereinabove described but may be carried out in other ways without departure from its spirit.

1 claim:

l. A non-aluminum killed, low carbon, non-alloy steel having cold workability, consisting essentially of up to 0.5% carbon 0.05 to 0.4% manganese and 0.05 to 0.25% aluminum, phosphorus and sulfur each not over 0.030%, silicon not over 0.30%, 0 to 0.01% boron, 0 to 0.1% each of one or more elements selected from the-group consisting of zirconium, molybdenum and vanadium, balance iron except for incidental impurities, not more than 25 ppm of nitrogen and 35 ppm of oxygen, substantial freedom from hard oxide inclusions, and the presence of the aforesaid aluminum substantially all in metallic state rather than appreciably in combined state that occurs after use of aluminum for killing, so that said composition and characteristics provide improved cold workability of the steel in at least one of the respects of reduced cold work hardening and of avoidance of cracking on compressive cold deformation, in comparison with low carbon, nonalloy, aluminum-killed steels.

2. A steelas defined in claim 1, in which silicon is not over 0.15%.

3. A steel as defined in claim 1, in which the carbon is not over 0.25%, manganese is 0.05 to 0.25%, and aluminum is not over 0.15%.

4. A steel as defined in claim 3, in which silicon is not over 0.15%.

5. A steel as defined in claim 4, in which phosphorus and sulfur are each not over 0.025% and nitrogen and oxygen are not more than 25 ppm and 35 ppm respectively.

6. A steel as defined in claim 1, in which carbon is 0.01 to 0.15% and aluminum is 0.05 to 0.12%.

7. A steel as defined in claim 1, in which carbon is 0.15% to 0.25% and aluminum is 0.08 to 0.15%.

8. A steel as defined in claim 1, in which carbon is 0.25% to 0.5% and aluminum is at least 0.12%.

9. A non-aluminum killed low carbon, non-alloy steel having cold workability, consisting essentially of up to 0.2% carbon, to 0.2% manganese, and 0.05 to 0.15% aluminum, phosphorus and sulfur each not over 0.025%. silicon not over 0.15%, 0 to 0.15% boron, 0 to 01% each of one or more elements selected from the group consisting of zirconium, molybdenum and vanadium, nitrogen and oxygen not over ppm and 35 ppm respectively, and balance iron except for incidental impurities, said aluminum being present substantially all in metallic state rather than in combined state that occurs after use of aluminum for killing, said stecl being sufficiently characterized by not more thn low content of manganese, not more than said low contents of nitrogen and oxygen, substantial freedom from hard oxide inclusions, and said aluminum in metallic state only, so that said composition and characteristics provide improved cold workability of the steel in at least one of the respects of reduced work hardening and reduced tendency to cracking on compressive cold deformation, in comparison with low-carbon, non-alloy, aluminum-killed steel.

10. A steel as defined in claim 9, in which carbon is not over 0.15%, silicon is not over 0.05%, and nitrogen and oxygen are not over 20 ppm and 30 ppm respectively.

11. A steel as defined in claim 2, in which carbon is not over 0.25%. manganese not over 0.25%. phosphorus not over 0.025%, sulfur not over 0.02%. and aluminum not over 0.20%, and oxygen is not more than 30 12. A steel as defined in claim 11, in which silicon is not over 0.05%. aluminum not over 0.15% and nitrogen is present at not more than 20 ppm.

% UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 897 ,245

. DATED July 29, 1975 VENT0R S JOHN SAVAS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column 1, line 27, "characterized" should read characterizes Column 5, line 23, "advantages" should read advantage Column 13, line 29, "thhe" should read the Column 14, line 24 "fpr" should read for Column 16, line 34, Claim 1, should be inserted after carbon Colunm 17, line 5, Claim 9, before boron "0.15%" should read 0.01%

Column 17, line 13, Claim 9, "thn" should read than q Signed and Scaled this sum D2) 0f March 1979 [SEAL] Arrest:

RUTH C MASON DONALD W. BANNER Arresting Officer Commissioner of Patents and Trademark

Claims

1. A NON-ALUMINUM KILLED, LOW CARBON, NON-ALLOY STEEL HAVING ING COLD WORKABILITY, CONSISTING ESSENTIALLY OF UP TO 0.5% CARBON 0.05 TO 0.4% MANGANESE AND 0.05 TO 0.25% ALUMINUM, PHOSPHORUS AND SULFUR EACH NOT OVER 0.030%, SILICON NOT OVER 0.05%, 0 TO 0.01% BORON, 0 TO 0.1% EACH OF ONE OR MORE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF ZICRONIUM, MOLYBDENUM AND VANADIUM, BALANCE IRON EXCEPT FOR INCIDENTAL IMPURITIES, NOT MORE THAN 25 PPM OF NITROGEN AND 35 PPM OF OXYGEN, SUBSTANTIAL FREEDOM FROM HARD OXIDE INCLUSIONS, AND THE PRESENCE OF THE AFORESAID ALUMINUM SUBSTANTIALLY ALL IN METALLIC STATE RATHER THAN APPRECIABLY IN COMBINED STATE THAT OCCURS AFTER USE OF ALUMINUM FOR KILLING, SO THAT SAID COMPOSITION AND CHARACTERISTICS PROVIDE IMPROVED COLD WORKABILITY OF THE STEEL IN AT LEAST ONE OF THE RESPECTS OF REDUCED COLD WORK HARDENING AND OF AVOIDANCE OF CRACKING ON COMPRESSIVE COLD DEFORMATION, IN COMPARISON WITH LOW CARBON NON-ALLOY, ALUMINUM-KILLED STEELS.

2. A steel as defined in claim 1, in which silicon is not over 0.15%.

4. A steel as defined in claim 3, in which silicon is not over 0.15%.

9. A non-aluminum killed low carbon, non-alloy steel having cold workability, consisting essentially of up to 0.2% carbon, 0 to 0.2% manganese, and 0.05 to 0.15% aluminum, phosphorus and sulfur each not over 0.025%, silicon not over 0.15%, 0 to 0.15% boron, 0 to 0.1% each of one or more elements selected from the group consisting of zirconium, molybdenum and vanadium, nitrogen and oxygen not over 25 ppm and 35 ppm respectively, and balance iron except for incidental impurities, said aluminum being present substantially all in metallic state rather than in combined state that occurs after use of aluminum for killing, said steel being sufficiently characterized by not more thn low content of manganese, not more than said low contents of nitrogen and oxygen, substantial freedom from hard oxide inclusions, and said aluminum in metallic state only, so that said composition and characteristics provide improved cold workability of the steel in at least one of the respects of reduced work hardening and reduced tendency to cracking on compressive cold deformation, in comparison with low-carbon, non-alloy, aluminum-killed steel.

11. A steel as defined in claim 2, in which carbon is not over 0.25%, manganese not over 0.25%, phosphorus not over 0.025%, sulfur not over 0.02%, and aluminum not over 0.20%, and oxygen is not more than 30 ppm.

12. A steel as defined in claim 11, in which silicon is not over 0.05%, aluminum not over 0.15% and nitrogen is present at not more than 20 ppm.