US2835570A - Zinc die casting steels - Google Patents

Description

May 20, 1958 v. KLAYBOR ZINC DIE CASTING STEELS Filed July 2, 1957 INVENTOR Leonard V Klaybor ZINC DIE CASTING STEELS Leonard V. Klaybor, Dunkirk, N. Y., assignor to Allegheny Ludlum Steel Corporation, Breckenridge, Pa., a corporation of Pennsylvania Application July 2, 1957, Serial No. 673,550 Claims. (Cl. 75-124) This invention relates to steel and in particular to a die casting steel suitable for use in the die casting of molten zinc.

Heretofore there have been many steels that have been made and used as dies in the die casting of zinc. While special emphasis has been placed on abrasion and thermal shock characteristics in these steels, a fundamental problem has existed for some time in using the known die casting steels for such applications. In the past, where the known steels have been used in contact with molten zinc, the molten zinc appears to attack the material of the die combining with the iron thereof to form a coating of a zinc-iron compound. This zinc-iron intermetallic compound is extremely brittle and when formed on the surface of the die causes portions of this coating to flake off removing a portion of the die surface thereby producing premature die failure. This difiiculty is most apparent when the steel is used as a sprue for die casting zine, an application of the die steel where the hottest material under the highest pressure is applied to the die and where the worst die life is usually obtained. In such application, the known dies have a maximum life as measured by the number of shots of molten zinc passing through the sprue of only about 100,000 shots.

An object of this invention is to provide a steel for use in die casting molten zinc and alloys thereof.

A specific object of this invention is to provide a steel suitable for use in die casting molten zinc and alloys thereof and comprising from about 0.25% to about 0.75% carbon, from about 1.10% to about 5.00% manganese, from about 0.10% to about 1.00% silicon, from about 0.10% to about 1.00% chromium, from about 2.0% to about 4.0% aluminum and the balance iron with incidental impurities, said steel being characterized by its resistance to formation of brittle intermetallic zinciron compound in the presence of molten zinc.

These and other objects of this invention Will-become apparent to one skilled in the art when taken in conjunction with the following description and the drawing, the single figure of which is a photomiorograph taken at a magnification of 500 times of a die casting steel embodying the teachings of this invention.

In its broader aspects the alloy of this invention comprises about 0.25% to 0.75% carbon, about 1.10% to about 5.00% manganese, about 0.10% to about 1.00% silicon, about 0.10% to about 1.00% chromium, about 2.0% to about 4.0% aluminum, and the balance substantially all iron with incidental impurities, each of the alloying elements performing a specific function in the steel. The carbon, manganese and chromium are the principal hardening agents within this alloy, it being found that a minimum of 0.25 carbon is required in order to obtain a hardness of 300 Brinell or greater as will be ref-e red to hereinafter. Manganese and chromium are the principal elements for providing the hardenability required in the alloy of this invention. The manganese content is extremely critical in the alloy of this invention in that at least 1.10% manganese is necessary in order to obtain the minimum hardenability based on an oil quenching medium used to quench the alloy from the heat treatment temperatures which will be more fully described hereinafter. The aluminum is believed to function in a manner also to be described hereinafter.

The alloy of this invention may be made in any of the well-known manners, for example, by electric furnace arc melting. In making the steel, predetermined quantities of scrap and/or hot metal are placed in an electric furnace together with sufiicient alloying elements and fluxing agents in order to produce the steel of desired analysis. This practice is common in the art and will not be described in detail. The molten steel of the prescribed chemical composition is cast into ingots which, when solidified, are then forged at a temperature in the range between 1900 F. and 2100" F. While forging is preferred, it will be appreciated that hot pressing or any other operation usually used in the fabrication of steel from ingot to a semi-finished mill product, for example; billet or bar or other similar shape, may be used. The forging is followed by an annealing treatment at a temperature of about 1500 F. from which temperature the. semi-finished mill product is permitted to cool slowly.

Thereafter the annealed material is hardened by heating at a temperature in the range between 1800 F. and 1900 F. for a time period ranging between one-half hour and four hours, and then quenched in air or oil, depending upon the size and composition as will be described more fully hereinafter, and tempered at a temperature between 900 F. and 1200" F. for a time period ranging between 3 and 10 hours. This results in a steel having a hardness between 300 and 375 Brinell. The steel may then be machined to the shape and size of the die, still retaining the hardness characteristics.

Referring to Table I, the composition of the steel of this invention is set forth in the form of the general range, the optimum (1) range for an air quenching steel, the optimum (2) range for an oil quenching steel together with a preferred air hardening composition as alloy A.

Table I General Optimum Optimum AlloyA Range 0.a0-0.50 0.37 1. 10-3.25 I 3.09 0. 20-030 0.27 0.40-0.50 0.45 2. -3. 25 3. 05 Bal. B81.

From Table I it can be seen that the manganese content determines the quenching medium of this steel. It has been found that a manganese content between, 2.75% and 5.00% is needed in order to impart the characteristic to the steel of this invention of attaining a desired hardness and microstructure by air quenching. On the other hand, if an oil quenching medium is to be employed, then the manganese content of the steel may be reduced to as little as 1.10% without impairing the resulting hardness and microstructure.

In order to more clearly illustrate the highly critical nature of the manganese content, reference is directed to Table II which lists the chemical composition of a series of alloys having a varying manganese content. It

is to be noted in Table II that the composition of the.

Since the effect of the manganese is highly critical from the standpoint of hardenability, the hardenability characteristic of the subject alloy was measured using the standard Jominy end quench test. The criterion used to determine the acceptable hardenability characteristic of the alloy was a hardness of at least 32 R at a distance of twelve sixteenths from the quenched end. The alloys were preheated to 1500 F. and thereafter the tempera ture was raised to 1800 F. and held for 10 minutes at that temperature. The alloys were then transferred to the Iominy test stand and quenched according to the standard practice.

Referenceis directed to Table 111 which illustrates the efifect of manganese on the hardenability of the alloys of Table II.

to such an extent that the alloys have a minimum hardness penetration of 32 R at a depth of thirty and twentyfour sixteenths, respectively.

Substantially similar results were obtained in alloys Nos. 6, 7, 8, 9 and 10 which are nominally a 2% aluminum alloy. Thus the data for alloys Nos. 6 and 7 having a manganese content of 0.53% and 0.71% show that the alloys have a shallow hardenability. However, for alloys Nos. 8, 9 and 10, containing 1.12%, 1.26% and 1.6% manganese, respectively, it is clear that these alloys possess sufiicient hardenability to be used in the dies for die casting zinc alloys.

It is significant to point out that alloys of this invention are hardenable to such an extent as to possess a hardness of at least 300 BHN. Since the hardening takes place by means of a phase transformation phenomena of austenite to martensite, the data of Table III clearly substantiating this, it follows that the addition of about a minimum of 2% aluminum, a strong ferrite forming element, does not affect the ability of this alloy to form austenite upon heat treatment. Heretofore, it was believed that the addition of 2% or more of aluminum to an alloy containing up to 04% carbon, up to 0.8% silicon, up to 0.8% chromium, up to 0.8% manganese and the balance iron would completely suppress the gamma loop and eliminate the austenite phase thus making the alloy incapable of hardening through phase transformation. The data contained in Table III clearly refutes such contention.

Table III [Hardness (B0)] Alloy No 1 2 3 4 5 6 7 8 9 10 Mn Content 0.45 0.76 1.05 1.57 1.5 0.53 0.71 1.12 1.26 1.60

Distance from Quenehed End in 1612115 of an Inch:

1 47 37 43 44 2 51 48.5 50.5 53 51 34 34 43 51 53 49 53 53. 5 53. 5 53. 5 29 41 51 51 42 53. 5 54 53. 5 53. 5 23 25. 5 3s 4s 4s 50 52 54 54 26 24. 5 46. 5 46 46 29 54 53 24. 5 23 35 45 45 26 40 4s 52. 5 53 22. 5 1 34 43. 5 42 25 36 44. 5 50 53 21 20 5 33.5 41 41 23. 5 33 42.5 47 52 20 19 31. 5 40. 5 40. 5 23 31 40. 5 44 51. 5 19 17. 5 31 41 4o 22 20 39. 5 41 49 13 16. 5 29. 5 40 39. 5 22 27 37 40 47 17 17 23. 5 39. 5 3s. 5 21. 5 25. 5 34. 5 38 45 18 16.5 28 39 38 21 25. 5 32 36. 5 44 17. 5 15. 5 27. 5 3s. 5 26. 5 20. 5 23. 5 33 34. 5 44 17. 5 16 26 37. 5 37 19 25 31. 5 33 42 17 16 25.5 37.5 37 19 24 31 31. 5 41 16 16 27 36 35 18. 5 21.5 29' 30 39 16 16 26 35. 5 34 17 21 28 29 38 16 15. 5 26 35 33 16 19 27 29 36. 5 v16 15 23 34. 5 32 15. 5 1s 26 30 35. 5 16 15 19. 5 33 29. 5 15 19. 5 24. 5 31 35. 5 16. 5 15 1s. 5 32. 5 2s 15 19. 5 24 2s 35 16 14. 5 19 32 2s 15 1s. 5 24 26. 5 32. 5 16. 5 14. 5 20 32 26. 5 13. 5 18 19. 5 26 32. 5 16.5 14.5 21 30 25 13.5 18 21 25 32 14. 5 14 2o 23 24. 5 14 17 13 23. 5 32 15. 5 14 20 27. 5 23. 5 13. 5 15 18. 5 24 32 14. 5 14 21 27 23. 5 14 16 18 23 31 15 14 21 27 24 15 17 19.5 21.5 29. 5 15 14 21. 5 26. 5 24 14. 5 15 21 24 28 17 15 2o 27 25 14. 5 18 24 25.5 27 15.5 14 20 2s 25. 5 15 13 25 2 26 From the data contalned 1n Table III 1t 1s clear that Examinanon of steel dies having a composition within manganese exerts a pronounced effect on the alloy of this invention. By comparing the test results for alloys Nos.'1, 2 and 3, it is seen that increasing the manganese content from 0.45% to 1.05% in a nominal composition containing about 3% aluminum is eifective for increasing hardenability. However, even with 1.05% manganese present, the alloy does not possess suflicient hardenability to obtain a hardness of 32 R (297 BHN) at a distance of twelve sixteenths from the quenched end. Since this is the criterion of acceptability for the alloy of this invention it follows that the manganese content must exceed 1.05%. The data for alloys Nos. 4 and 5 having a manganese content of 1.57% and 1.5%, respectively, clearly illustrate that a manganese content of about 1.5% is sufficient for increasing the hardenability the general range given hereinbefore reveals that the aluminum content in the steel cooperates with the carbon content in the steel to form very small islands of ferrite in the resulting steel. These islands or" ferrite are evident in the figure, which its a photornicrograph taken at a magnification of 500 times of the die casting steel having thecomposition of alloy A as given in Table I which was forged, annealed at 1500 R, and thereafter slowly cooled, hardened at temperature of 1850 F. and thereafter tempered at a temperature of 1100 C. In the photomicrograph the structure of the steel is seen to comprise a matrix of tempered martensite 10 having islands of ferrite 12 contained therein. It is believed that these small islands of ferrite 12 function to improve the machineability of the heat treated, hardened composi- I tion of the steels of this invention without producing any adverse effect in their use as die steels.

In practice, it is preferred to increase the aluminum content proportionally within the broad range given as the carbon content is increased within the given broad range to thereby insure the presence of the small islands of ferrite 12 in the matrix of tempered martensite 10. While the composition of the steel is varied within the broad range given in Table I hereinbefore, it is preferred that in the optimum range the aluminum content be increased from 2.75% to 3.25% in direct proportion to and as the carbon content increases from 0.30% to 0.50%. When such relationship is maintained between the aluminum and carbon contents, it is found that the very small islands of ferrite 12 are always present as illustrated in the figure and that the resulting steel has excellent machineability.

In use it is believed that the aluminum content of the steel forms a very thin transparent microscopic protective film of aluminum oxide on the surface of the die which functions to protect the die from the formation of the brittle intermetallic zinc-iron compound which is formed on the known die steels of the prior art when employed in zinc die casting applications. This thin protective coating of aluminum oxide is adherent to the surface of the die and clearly increases the operating life of the die.

As a specific example of the exceptionally long die life found with dies formed from the steel of this invention, reference may be had to the specific composition of alloy A given in Table I. When such steel is formed into a die and used as the sprue in the die casting of zinc, it has been found that 280,000 shots of molten zinc have been passed therethrough without any sign of die failure. Such die life is exceptional when compared to the die life of the prior art die steels used in such applications, and it is believed that the presence of the aluminum in the alloy of this invention makes possible such an in-v crease in the die life. When dies formed of the steel described hereinbefore are examined after use as the sprue for die casting zinc, it is found that such dies are entirely free of the brittle intermetallic zinc-iron coatings found on the surfaces of the prior art dies after such use.

The steel of this invention requires no special skills or apparatus in its preparation and use. The heat treatments are relatively simple and the alloy composition can be readily reproduced by anyone skilled in the art.

Further, it is to be noted that the alloying content of the steel is low and there is a significant absence of strategic alloying elements.

This application is a continuation-in-part of application Serial No. 499,445, filed April 5, 1955, now abandoned.

I claim:

1. A die steel for use in die casting molten zinc and alloys thereof consisting of, from about 0.25% to about 0.75% carbon, from about 1.10% to about 5.0% manganese, from about 0.10% to about 1.00% silicon, from about 0.10% to about 1.00% chromium, from about 2.0% to about 4.0% aluminum, and the balance substantially all iron with incidental impurities.

2. A die steel for use in die casting molten zinc and alloys thereof, consisting of, about 0.30% to 0.50% carbon, about 2.75% to 3.25% manganese, about 0.20% to 0.30% silicon, about 0.40% to 0.50% chromium, about 2.75 to 3.25 aluminum, and the balance iron with incidental impurities, said steel being characterized by being hardened in air to a hardness of at least 300 Brinell Hardness Number.

3. A die steel for use in die casting molten zinc and alloys thereof, consisting of, about 0.30% to 0.50% carbon, about 1.10% to 3.25 manganese, about 0.20% to 0.30% silicon, about 0.40% to 0.50% chromium, about 2.75% to 3.25% aluminum, and the balance iron with incidental impurities, said steel being characterized by being hardened by quenching in oil to a hardness of at least 300 Brinell Hardness Number.

4. A die steel for use in die casting molten zinc and alloys thereof, consisting of, about 0.37% carbon, about 3.09% manganese, about 0.27% silicon, about 0.45% chromium, about 3.05% aluminum, and the balance iron with incidental impurities.

5. A die steel for use in die casting molten zinc and alloys thereof, consisting of, from 0.30% to 0.50% carbon, about 1.10% to 3.25% manganese, about 0.10% to 1.00% silicon, about 0.10% to 1.00% chromium, about 2.75% to 3.25% aluminum, and the balance iron with incidental impurities, said carbon content varying in direct proportion with respect to a variation in the aluminum content with the ranges given.

References Cited in the file of this patent FOREIGN PATENTS 304,303 Great Britain May 21, 1930

Claims (1)

1. A DIE STEEL FOR USE IN DIE CASTING MOLTEN ZINC AND ALLOYS THEREOF CONSISTING OF, FROM ABOUT 0.25% TO MANGANESE, FROM ABOUT 0.10% TO ABOUT 1.00% SILICON, FROM ABOUT 0.10% TO ABOUT 1.00% CHROMIUM, FROM ABOUT 2.0% TO ABOUT 4./% ALUMINUM, AND THE BALANCE SUBSTANTIALLY ALL IRON WITH INCIDENTAL IMPURTITIES.

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