Hot work forging die block and method of manufacture thereof
United States Patent 3929423
A low alloy hot work forging die block characterized by (1) a final low non-metallic inclusion content, especially with respect to silicates and aluminates, due to very low final oxygen and aluminum levels, (2) improved machinability and (3) excellent hardenability, and a method of manufacture thereof which emphasizes oxygen control.
US Patent References:
Process for the manufacture of cast iron having an eutectic graphite formation
Ruff - May 1964 - 3133813
Superior white cast iron
Moore - November 1966 - 3282683
Purification of molten ferrous base metals
Kanter - July 1967 - 3332772
Magnesium alloy for cast iron
Newitt - August 1967 - 3336118
/3695865.html
Wolker - October 1972 - 3695865
Process for the manufacture of cast iron having an eutectic graphite formation
Ruff - May 1964 - 3133813
Superior white cast iron
Moore - November 1966 - 3282683
Purification of molten ferrous base metals
Kanter - July 1967 - 3332772
Magnesium alloy for cast iron
Newitt - August 1967 - 3336118
/3695865.html
Wolker - October 1972 - 3695865
Application Number:
05/358827
Publication Date:
12/30/1975
Filing Date:
05/09/1973
View Patent Images:
Export Citation:
Assignee:
A. Finkl & Sons Company (Chicago, IL)
Primary Class:
Other Classes:
420/91, 75/237, 420/109, 75/246
International Classes:
C22C38/46; B22F9/00; C22C33/00; C22C38/40
Field of Search:
75/128,129,130.5,53 29/180
Primary Examiner:
Rosenberg, Peter D.
Attorney, Agent or Firm:
Staples, James G.
Claims:
I claim
1. A low alloy, hot work, forging steel die block, said steel block being characterized by a low-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said die block consisting of the following composition by weight percent:
2. The steel die block of claim 1 further characterized in that
3. A low alloy, hot work, forging steel die block, said steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
4. The steel die block of claim 3 further characterized
5. A low alloy, hot work, forging steel die block, said steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
6. The steel die block of claim 5 further characterized
7. A forged, low alloy, hot work, steel die block, said forged steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said die block consisting of the following composition by weight percent:
8. The steel die block of claim 7 further characterized in that
9. A forged, low alloy, hot work, steel die block, said forged steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
10. The steel die block of claim 9 further characterized
11. A forged, low alloy, hot work, steel die block, said forged steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
12. The steel die block of claim 11 further characterized
1. A low alloy, hot work, forging steel die block, said steel block being characterized by a low-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said die block consisting of the following composition by weight percent:
2. The steel die block of claim 1 further characterized in that
3. A low alloy, hot work, forging steel die block, said steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
4. The steel die block of claim 3 further characterized
5. A low alloy, hot work, forging steel die block, said steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
6. The steel die block of claim 5 further characterized
7. A forged, low alloy, hot work, steel die block, said forged steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said die block consisting of the following composition by weight percent:
8. The steel die block of claim 7 further characterized in that
9. A forged, low alloy, hot work, steel die block, said forged steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
10. The steel die block of claim 9 further characterized
11. A forged, low alloy, hot work, steel die block, said forged steel die block being characterized by a low non-metallic inclusion content, including silicates and aluminates, high machinability in all conditions including final hardened condition, and substantially uniform through hardness in heavy sections, said steel die block consisting of the following composition by weight percent:
12. The steel die block of claim 11 further characterized
Description:
This invention relates generally to low alloy hot work forging steel die blocks and associated parts having substantially improved characteristics over present commercially used products, including, particularly, improved cleaniness, machinability and hardenability, and a method of manufacture thereof. The above mentioned "associated parts" include inserts, guide pins for dies, tie plates, sow blocks, ram guides and rams for drop hammers, and bolster plates for presses, all of which will hereafter be referred to collectively as "die blocks."
BACKGROUND OF THE INVENTION
Die blocks of the type to which this invention is directed are subjected to unusually severe operating conditions during normal use since, among other things, they are subjected to intermittent heating and cooling from temperatures of, for example, about 300°F to about 1100°F and more, heavy impact loads and severe abrasion. Under abnormal working conditions, which invariably occur from time to time in any operation, the die blocks may be subjected to prolonged exposure at high temperatures as where a high temperature work-piece becomes stuck in the dies, or where a large forging is not lifted from the lower cavity between blows. Concurrently with the aforesaid operating conditions the die blocks must be relatively easily machinable, mainly after final heat treatment. And finally the die block must be of substantially uniform hardness at all depths to maximise production after one or more resinkings of the cavity.
SUMMARY OF THE INVENTION
Accordingly, a primary object of this invention is to provide a die block having, as contrasted to current products, improved cleanliness, machinability, and hardenability.
Another object is to provide a die block as above described in which the quantity of non-metallic inclusions, primarily silicates and aluminates, present in the final product is substantially decreased over current products, and those that are present tend to be more widely dispersed and therefore less objectionable in the final product.
Another object is to provide a die block having the following broad composition:
C .45 - .60 Mn .65 - 1.25 S .020 - .045 P .040 max. Si up to .35 Ni .75 - 1.25 Cr .60 - 1.75 Mo .30 - .45 V .03 - .10 Al .015 max. O 2 , ppm 60 max. H 2 , ppm 2.4 max.
More preferably it is an object to provide a die block having the following preferred composition:
C .50 - .60 Mn .75 - 1.10 S .030 - .040 P .025 max. Si .20 - .35 Ni .85 - 1.15 Cr .75 - 1.40 Mo .33 - .43 V .04 - .06 Al less than .005 O 2 , ppm 50 max. H 2 , ppm 2.2 max.
Another object is to provide a die block having the following most preferred composition:
C .53 - .57 Mn .75 - .95 S .035 aim P .020 max. Si .20 - .35 Ni .85 - 1.05 Cr .85 - 1.15 Mo .36 - .43 V .05 aim Al less than .005 O 2 , ppm 30 max. H 2 , ppm 2.0 max.
Yet another object is to provide a method of making die blocks of the above described chemical composition.
Other objects and advantages of the invention will become apparent from a reading of the foregoing exemplary description of the invention.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The product of the invention may be defined by the weight percent of chemical composition as follows:
Broad Preferred Most Preferred ______________________________________ C .45 - .60 .50 - .60 .53 - .57 Mn .65 - 1.25 .75 - 1.10 .75 - .95 S .020 - .045 .030 - .040 .035 aim P .040 max. .025 max. .020 max. Si up to .35 .20 - .35 .20 - .35 Ni .75 - 1.25 .85 - 1.15 .85 - 1.05 Cr .60 - 1.75 .75 - 1.40 .85 - 1.15 Mo .30 - .45 .33 - .43 .36 - .43 V .02 - .10 .04 - .06 .05 aim Al .015 max. less than .005 less than .005 O 2 , ppm 60 max. 50 max. 30 max. H 2 , ppm 2.4 max. 2.2 max. 2.0 max. ______________________________________
Carbon is necessary to provide the required wear resistance and hardness. If the carbon is significantly higher than 0.60 the die blocks will be subject to breakage in the field. If substantially less than 0.45 carbon is used wear resistance will not be suitable for the extremely strenuous field applications to which the die blocks are subjected. Preferably a minimum of 0.50 carbon is used to ensure good wear resistance and hardness, and maximum production. Most preferably carbon in the range of 0.53 to 0.57 with an aim of 0.55 is used.
Manganese is necessary for hardenability and as a deoxidizer in the steelmaking process. It also functions to control sulphides in forging operations. If significantly more than 1.25% manganese is present there is a danger that retained austenite will be present. If substantially less than 0.65% manganese is present the hardenability of the die block may be deleteriously affected. In addition manganese contributes to wear resistance, although to a lesser extent than other carbide formers. Preferably manganese is present in the range of 0.75% to 1.10%, and most preferably from 0.75% to 0.95%. Manganese should be present in an amount at least 20 times the sulphur content to ensure sulphur control.
Sulphur is essential for machinability. If the sulphur is significantly above 0.045, or 0.050 at most, an undesirably high quantity of sulphides may be present in the final product. Excess sulphides will deleteriously affect transverse properties. If significantly less than about 0.020 sulphur is present machinability will be adversely affected. Machinability, particularly in the final, hardened condition, is a primary requirement of a die block. Accordingly, a preferred range, of 0.030 to 0.040 sulphur is desired, with an aim of 0.035. Those skilled in the art will recognize the preferred range and aim as at or above the extreme upper limit for this general type of low alloy steel, electric furnace grade.
Phosphorus can exert a beneficial effect on machinability. However, the deleterious effects of phosphorus in this general type of steel, such as an increase in the transition temperature, outweight any beneficial effects and accordingly the phosphorus content should be kept as low as possible. Under no circumstances should more than 0.040% be present, preferably no more than 0.025%, and most preferably no more than 0.020%. It will be noted that the intentional combination of high sulphur and low phosphorus in this type of steel is unusual.
Silicon is specified for its deoxidizing ability in the steelmaking process. If silicon is present in substantially greater quantities than that specified there is a tendency towards embrittlement of the final product. If the die blocks are made by conventional steelmaking processes the silicon may be in the range of 0.20 to 0.35. However if the molten steel from which the die blocks are made is subjected to carbon deoxidation treatment silicon levels at the lower end of the range may be quite acceptable.
Nickel is required to impart toughness to the die block and strengthen the ferrite. If substantially more than 1.25% nickel is present there is a danger of retained austenite and decreased machinability. Excess nickel may also promote hairline cracking which requires scarfing and/or conditioning at the press. If substantially less than 0.75% nickel is present than that specified the die block will lack toughness, and hardenability will be reduced, thereby adversely affecting die life in large sections by washing. Preferably nickel should be present in the range of about 0.85 - 1.15%, and most preferably in the range of 0.85 - 1.05%.
It has been discovered that copper is substantially interchangeable with nickel up to about 0.5% for the die blocks with which this invention is concerned. Accordingly copper may be substituted for nickel up to about 0.5% copper. However, experience has shown that the nickel content should always be higher than the copper to avoid problems in forging the die blocks. Therefore, the nickel and copper content may be expressed as nickel and/or copper of from about 0.75% to about 1.75%, or other specified range, but not to exceed 0.50% copper, with the nickel content always greater than the copper content.
Chromium is necessary for carbide formation, for hardenability, and for wear resistance. If substantially more than the maximum specified amount of chromium is present the hardening temperature will be too high for normal production heat treatment, and heavy sections will be subject to loose or weak centers. If substantially less than the minimum specified quantity of chromium is present the die block will be deficient in wear resistance and hardenability. Preferably chromium is present in the amount of 0.75% to 1.40%, and most preferably from 0.85% to 1.15%.
Molybdenum is one of the most important elements. It is a potent carbide former and contributes to hardenability and wear resistance. Preferably the molybdenum is maintained between 0.33 to 0.43 since this range appears to yield optimum results, although a range of 0.30 to 0.45 may be tolerable. For thick sections it may be desirable to work near the upper end of the broad range, and preferably in the range of 0.36 to 0.43. If the final product is to be a die block of substantial cross-sectional thickness it may be advantageous to increase the molybdenum to a minimum of 0.36 to ensure thorough response to the hardening process.
Vanadium is specified for its grain refining properties. If a significantly greater quantity of vanadium is present than that specified the hardenability of the die block may be decreased due to the insolubility of vanadium carbide at normal treat temperatures. If significantly less vanadium is present than that specified the necessary grain refinement may not be achieved. Preferably vanadium is present in the range of 0.03 to 0.10, and most preferably in the range of 0.04 to 0.06, with an aim of 0.05.
Since the amount of aluminum present will also have a significant effect on the quantity of aluminates formed it is desirable to control the melting procedures so as to minimize the amount of aluminum present. The steel should contain, therefore, a maximum of 0.015 aluminum, and preferably less than 0.005 aluminum.
As those skilled in the art will appreciate, there are essentially four types of non-metallic inclusions which, in this type of steel, are considered to be impurities, namely silicates, aluminates, complex oxides and sulphides.
The quantity of silicates and aluminates formed will be substantially proportional to the amount of available oxygen in the steel. The complex oxides are thought to be formed largely during tapping and teeming. The amount of sulphides formed will, of course, be proportional to the sulphur or sulphur containing materials in the steel, including sulphur from such sources as scrap and oil in turnings and other scrap materials in the shop, the degree to which furnace or vacuum ladle refining is carried out, and intentional additions such as ladle additions of pyrites to meet the desired sulphur specification. Teeming techniques to reduce oxygen pick up may be employed such as the use of a vacuum or inert atmosphere during teeming and/or elimination of splash through the use of splash pads, no dribble teeming techniques or bottom pouring.
The silicates and aluminates are formed as the oxygen comes out of solution due to temperature drop. It is believed that if, at the time the silicates and aluminates are formed, a condition of oxygen starvation in the molten steel exists the oxide and sulphide formation can be very significantly decreased. Accordingly it is essential that steps be taken to ensure low oxygen levels in the steel as will be further described in detail hereafter. For purposes of chemical definition of the die blocks of this invention the quantity of oxygen present in the final product must therefore be less than 60 ppm, preferably less than 50 ppm, and most preferably less than 30 ppm. It will be understood that it is difficult and costly to consistently achieve oxygen levels much below 15 ppm at a carbon content of 0.55% on a commercial production basis, although by careful attention to the practice set out herein such extremely low levels have been achieved infrequently.
Single point tool life tests were conducted on a sample, hereafter referred to as steel B, having the following composition:
C Mn P S Si Ni Cr Mo V Al O 2 H 2 ____________________________________________________________ ______________ .56 .84 .018 .035 .30 .94 .93 .36 .048 .005 29 ppm 1.5 ppm ____________________________________________________________ ______________ The sample was tested in the annealed condition and had a BHN of 212-222. The test was conducted using M-2 HSS tools, 0.0107 ipr feed, 0.0500 in. depth of cut, and no coolant. Total destruction of the tool was used as the criterion for tool life, and the results were compared to results obtained under identical test conditions with a steel, hereafter referred to as Steel A, having the identical composition except a sulphur content of 0.017.
______________________________________ High Speed Steel Tool Steel Life at 125 SFPM, Seconds BHN ______________________________________ A 15 217 B 55 212 ______________________________________
After the two steels were quenched and tempered to BHN 444, single point tests were conducted using C7 grade carbide inserts with 5° negative rake angle. Flank wear after 548 seconds at 200 SFPM was measured, and the following results were obtained:
Carbide Tools, Flank Wear Steel at 548 Seconds BHN ______________________________________ A .0064 444 B .0044 444 ______________________________________
From the foregoing it can be seen that the steel of this invention possesses substantially improved machinability.
In the manufacture of the die blocks of this invention by electric furnace practice it is desirable to make maximum use of melting scrap of similar composition. Conventional electric furnace processing steps may then be followed, except as noted below.
Phosphorus removal may be accomplished by utilization of mill scale at low temperatures, that is, in the range of about 2750°F to 2840F, for example. The mill scale formed during forging has the desirable ability to yield substantial quantities of oxygen to the bath while keeping the bath relatively cool. Slag-off of this initial oxidizing slag is preferably carried out at low temperatures, that is, under about 2840°F, to remove the P 2 O 5 from the molten metal into the slag system.
After furnace processing the steel may be further deoxidized in the tapping ladle by tapping onto V, FeSi, and/or CaSi or other metallic deoxiders.
If the sulphur content is below specification at this point resulphurization to reach an aim of 0.035, for example, may be carried out by the addition of stick sulphur, pyrities, or other appropriate sulphur addition materials.
After furnace and tapping ladle treatment the tap ladle should be subjected to refining under vacuum conditions, preferably by subjection to the simultaneous effect of vacuum and purging as illustrated and described in greater detail in U.S. Pat. No. 2,236,635 to which reference is here made for a more detailed understanding. Such vacuum degassing treatment is carried out for a period of time which will enable the oxygen level in the steel to be lowered to under 60 ppm, preferably under 50 ppm, and most preferably under about 30 - 35 ppm. In a heat of approximately 65 tons the vacuum degassing may for example be carried out for about 10 to 30 minutes, the exact time depending upon the usual process variables encountered in commercial melt shop practice including temperature, amount of slag present, and starting gas values.
The vacuum degassing treatment causes flotation of undesirable large non-metallic inclusions into whatever slag may be present on the surface. It will be understood that these inclusions are particularly deleterious in the final product since they adversely affect transverse properties and can function as focal points for stress raisers.
The vacuum degassing treatment further ensures reduction of hydrogen into the flake-free region. Experience has shown that for this type of steel the bulk of a large number of heats will be flake free at the 2.5 ppm level. However, since the failure attributable to flaking, usually termed "thermal rupture" is a serious defect it is desirable that the possibility of failure due to flaking be minimized as much as possible, and accordingly a maximum of 2.4 ppm is specified. Flaking failures have occurred even in the 2.3 to 2.4 ppm range and accordingly a maximum of 2.2 ppm is preferred. Recently rare cases of flaking have been experienced at the 2.2 ppm level and accordingly 2.0 ppm maximum is the most preferred limit.
To ensure that a thoroughly uniform and controllable teeming temperature of about 2830°F in the ladle will be obtained at the conclusion of the vacuum degassing treatment, the steel may be subject to intermittent or continuous alternating current arc heating under vacuum, as more fully described in U.S. Pat. Nos. 3,501,289 and 3,501,290, to which reference is here made for a more detailed understanding.
Other processing steps such as inert gas shroud or vacuum teeming, and/or bottom pouring may be employed as necessary to ensure a sound ingot having good surface qualities.
The silicates and aluminates will be substantially randomly dispersed in such fashion as to minimize their deleterious effects on transverse properties. Further, there will be a reduction in the number of especially large sized inclusions in the center of the ingot and a decreased concentration of inclusions near the surface of the ingot.
A series of commercial 65-ton (nominal size) heats containing examples of heats of the aforementioned broad, preferred, and most preferred ranges are compiled in the following table:
HEAT TABLE ____________________________________________________________ ______________ Heat No. C Mn S Si Ni Cr Mo V Al O 2 1 H 2 1 ____________________________________________________________ ______________ 138,789 .60 .92 .028 .34 1.02 .98 .39 .051 .005 18 1.5 238,495 .59 .86 .041 .33 .94 1.05 .37 .044 " 24 1.7 238,500 .53 .94 .022 .22 .91 1.01 .36 .048 " N.A. 2 2.4 138,791 .56 .95 .027 .26 .95 1.06 .37 .052 " 26 1.9 138,772 .54 .90 .028 .28 .93 .87 .36 .045 " 27 1.7 138,770 .57 .87 .034 .34 .96 .87 .36 .043 " 40 1.9 138,782 .58 .95 .032 .29 .93 1.05 .40 .052 " 23 1.9 238,498 .56 .94 .031 .31 .93 .98 .36 .053 " 15 1.4 138,773 .57 .90 .033 .34 .90 .96 .36 .048 " N.A. 1.2 ____________________________________________________________ ______________ 1 In ppm 2 Not available.
Of especial significance is the very low final Al content and the unusually low oxygen content, which ranges down to the 15 ppm level with respect to heat 238,498, for example.
From the foregoing description it will be appreciated that a unique die block having a low non-metallic inclusion content, particularly inclusions of silicates and aluminates, good machinability and improved hardenability, has been disclosed. Various modifications will of course at once occur to those skilled in the art. Accordingly, the scope of the invention should be limited not by the scope of the foregoing exemplary description, but only by the scope of the hereinafter appended claims when interpreted in light of the pertinent prior art.
BACKGROUND OF THE INVENTION
Die blocks of the type to which this invention is directed are subjected to unusually severe operating conditions during normal use since, among other things, they are subjected to intermittent heating and cooling from temperatures of, for example, about 300°F to about 1100°F and more, heavy impact loads and severe abrasion. Under abnormal working conditions, which invariably occur from time to time in any operation, the die blocks may be subjected to prolonged exposure at high temperatures as where a high temperature work-piece becomes stuck in the dies, or where a large forging is not lifted from the lower cavity between blows. Concurrently with the aforesaid operating conditions the die blocks must be relatively easily machinable, mainly after final heat treatment. And finally the die block must be of substantially uniform hardness at all depths to maximise production after one or more resinkings of the cavity.
SUMMARY OF THE INVENTION
Accordingly, a primary object of this invention is to provide a die block having, as contrasted to current products, improved cleanliness, machinability, and hardenability.
Another object is to provide a die block as above described in which the quantity of non-metallic inclusions, primarily silicates and aluminates, present in the final product is substantially decreased over current products, and those that are present tend to be more widely dispersed and therefore less objectionable in the final product.
Another object is to provide a die block having the following broad composition:
C .45 - .60 Mn .65 - 1.25 S .020 - .045 P .040 max. Si up to .35 Ni .75 - 1.25 Cr .60 - 1.75 Mo .30 - .45 V .03 - .10 Al .015 max. O 2 , ppm 60 max. H 2 , ppm 2.4 max.
More preferably it is an object to provide a die block having the following preferred composition:
C .50 - .60 Mn .75 - 1.10 S .030 - .040 P .025 max. Si .20 - .35 Ni .85 - 1.15 Cr .75 - 1.40 Mo .33 - .43 V .04 - .06 Al less than .005 O 2 , ppm 50 max. H 2 , ppm 2.2 max.
Another object is to provide a die block having the following most preferred composition:
C .53 - .57 Mn .75 - .95 S .035 aim P .020 max. Si .20 - .35 Ni .85 - 1.05 Cr .85 - 1.15 Mo .36 - .43 V .05 aim Al less than .005 O 2 , ppm 30 max. H 2 , ppm 2.0 max.
Yet another object is to provide a method of making die blocks of the above described chemical composition.
Other objects and advantages of the invention will become apparent from a reading of the foregoing exemplary description of the invention.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The product of the invention may be defined by the weight percent of chemical composition as follows:
Broad Preferred Most Preferred ______________________________________ C .45 - .60 .50 - .60 .53 - .57 Mn .65 - 1.25 .75 - 1.10 .75 - .95 S .020 - .045 .030 - .040 .035 aim P .040 max. .025 max. .020 max. Si up to .35 .20 - .35 .20 - .35 Ni .75 - 1.25 .85 - 1.15 .85 - 1.05 Cr .60 - 1.75 .75 - 1.40 .85 - 1.15 Mo .30 - .45 .33 - .43 .36 - .43 V .02 - .10 .04 - .06 .05 aim Al .015 max. less than .005 less than .005 O 2 , ppm 60 max. 50 max. 30 max. H 2 , ppm 2.4 max. 2.2 max. 2.0 max. ______________________________________
Carbon is necessary to provide the required wear resistance and hardness. If the carbon is significantly higher than 0.60 the die blocks will be subject to breakage in the field. If substantially less than 0.45 carbon is used wear resistance will not be suitable for the extremely strenuous field applications to which the die blocks are subjected. Preferably a minimum of 0.50 carbon is used to ensure good wear resistance and hardness, and maximum production. Most preferably carbon in the range of 0.53 to 0.57 with an aim of 0.55 is used.
Manganese is necessary for hardenability and as a deoxidizer in the steelmaking process. It also functions to control sulphides in forging operations. If significantly more than 1.25% manganese is present there is a danger that retained austenite will be present. If substantially less than 0.65% manganese is present the hardenability of the die block may be deleteriously affected. In addition manganese contributes to wear resistance, although to a lesser extent than other carbide formers. Preferably manganese is present in the range of 0.75% to 1.10%, and most preferably from 0.75% to 0.95%. Manganese should be present in an amount at least 20 times the sulphur content to ensure sulphur control.
Sulphur is essential for machinability. If the sulphur is significantly above 0.045, or 0.050 at most, an undesirably high quantity of sulphides may be present in the final product. Excess sulphides will deleteriously affect transverse properties. If significantly less than about 0.020 sulphur is present machinability will be adversely affected. Machinability, particularly in the final, hardened condition, is a primary requirement of a die block. Accordingly, a preferred range, of 0.030 to 0.040 sulphur is desired, with an aim of 0.035. Those skilled in the art will recognize the preferred range and aim as at or above the extreme upper limit for this general type of low alloy steel, electric furnace grade.
Phosphorus can exert a beneficial effect on machinability. However, the deleterious effects of phosphorus in this general type of steel, such as an increase in the transition temperature, outweight any beneficial effects and accordingly the phosphorus content should be kept as low as possible. Under no circumstances should more than 0.040% be present, preferably no more than 0.025%, and most preferably no more than 0.020%. It will be noted that the intentional combination of high sulphur and low phosphorus in this type of steel is unusual.
Silicon is specified for its deoxidizing ability in the steelmaking process. If silicon is present in substantially greater quantities than that specified there is a tendency towards embrittlement of the final product. If the die blocks are made by conventional steelmaking processes the silicon may be in the range of 0.20 to 0.35. However if the molten steel from which the die blocks are made is subjected to carbon deoxidation treatment silicon levels at the lower end of the range may be quite acceptable.
Nickel is required to impart toughness to the die block and strengthen the ferrite. If substantially more than 1.25% nickel is present there is a danger of retained austenite and decreased machinability. Excess nickel may also promote hairline cracking which requires scarfing and/or conditioning at the press. If substantially less than 0.75% nickel is present than that specified the die block will lack toughness, and hardenability will be reduced, thereby adversely affecting die life in large sections by washing. Preferably nickel should be present in the range of about 0.85 - 1.15%, and most preferably in the range of 0.85 - 1.05%.
It has been discovered that copper is substantially interchangeable with nickel up to about 0.5% for the die blocks with which this invention is concerned. Accordingly copper may be substituted for nickel up to about 0.5% copper. However, experience has shown that the nickel content should always be higher than the copper to avoid problems in forging the die blocks. Therefore, the nickel and copper content may be expressed as nickel and/or copper of from about 0.75% to about 1.75%, or other specified range, but not to exceed 0.50% copper, with the nickel content always greater than the copper content.
Chromium is necessary for carbide formation, for hardenability, and for wear resistance. If substantially more than the maximum specified amount of chromium is present the hardening temperature will be too high for normal production heat treatment, and heavy sections will be subject to loose or weak centers. If substantially less than the minimum specified quantity of chromium is present the die block will be deficient in wear resistance and hardenability. Preferably chromium is present in the amount of 0.75% to 1.40%, and most preferably from 0.85% to 1.15%.
Molybdenum is one of the most important elements. It is a potent carbide former and contributes to hardenability and wear resistance. Preferably the molybdenum is maintained between 0.33 to 0.43 since this range appears to yield optimum results, although a range of 0.30 to 0.45 may be tolerable. For thick sections it may be desirable to work near the upper end of the broad range, and preferably in the range of 0.36 to 0.43. If the final product is to be a die block of substantial cross-sectional thickness it may be advantageous to increase the molybdenum to a minimum of 0.36 to ensure thorough response to the hardening process.
Vanadium is specified for its grain refining properties. If a significantly greater quantity of vanadium is present than that specified the hardenability of the die block may be decreased due to the insolubility of vanadium carbide at normal treat temperatures. If significantly less vanadium is present than that specified the necessary grain refinement may not be achieved. Preferably vanadium is present in the range of 0.03 to 0.10, and most preferably in the range of 0.04 to 0.06, with an aim of 0.05.
Since the amount of aluminum present will also have a significant effect on the quantity of aluminates formed it is desirable to control the melting procedures so as to minimize the amount of aluminum present. The steel should contain, therefore, a maximum of 0.015 aluminum, and preferably less than 0.005 aluminum.
As those skilled in the art will appreciate, there are essentially four types of non-metallic inclusions which, in this type of steel, are considered to be impurities, namely silicates, aluminates, complex oxides and sulphides.
The quantity of silicates and aluminates formed will be substantially proportional to the amount of available oxygen in the steel. The complex oxides are thought to be formed largely during tapping and teeming. The amount of sulphides formed will, of course, be proportional to the sulphur or sulphur containing materials in the steel, including sulphur from such sources as scrap and oil in turnings and other scrap materials in the shop, the degree to which furnace or vacuum ladle refining is carried out, and intentional additions such as ladle additions of pyrites to meet the desired sulphur specification. Teeming techniques to reduce oxygen pick up may be employed such as the use of a vacuum or inert atmosphere during teeming and/or elimination of splash through the use of splash pads, no dribble teeming techniques or bottom pouring.
The silicates and aluminates are formed as the oxygen comes out of solution due to temperature drop. It is believed that if, at the time the silicates and aluminates are formed, a condition of oxygen starvation in the molten steel exists the oxide and sulphide formation can be very significantly decreased. Accordingly it is essential that steps be taken to ensure low oxygen levels in the steel as will be further described in detail hereafter. For purposes of chemical definition of the die blocks of this invention the quantity of oxygen present in the final product must therefore be less than 60 ppm, preferably less than 50 ppm, and most preferably less than 30 ppm. It will be understood that it is difficult and costly to consistently achieve oxygen levels much below 15 ppm at a carbon content of 0.55% on a commercial production basis, although by careful attention to the practice set out herein such extremely low levels have been achieved infrequently.
Single point tool life tests were conducted on a sample, hereafter referred to as steel B, having the following composition:
C Mn P S Si Ni Cr Mo V Al O 2 H 2 ____________________________________________________________ ______________ .56 .84 .018 .035 .30 .94 .93 .36 .048 .005 29 ppm 1.5 ppm ____________________________________________________________ ______________ The sample was tested in the annealed condition and had a BHN of 212-222. The test was conducted using M-2 HSS tools, 0.0107 ipr feed, 0.0500 in. depth of cut, and no coolant. Total destruction of the tool was used as the criterion for tool life, and the results were compared to results obtained under identical test conditions with a steel, hereafter referred to as Steel A, having the identical composition except a sulphur content of 0.017.
______________________________________ High Speed Steel Tool Steel Life at 125 SFPM, Seconds BHN ______________________________________ A 15 217 B 55 212 ______________________________________
After the two steels were quenched and tempered to BHN 444, single point tests were conducted using C7 grade carbide inserts with 5° negative rake angle. Flank wear after 548 seconds at 200 SFPM was measured, and the following results were obtained:
Carbide Tools, Flank Wear Steel at 548 Seconds BHN ______________________________________ A .0064 444 B .0044 444 ______________________________________
From the foregoing it can be seen that the steel of this invention possesses substantially improved machinability.
In the manufacture of the die blocks of this invention by electric furnace practice it is desirable to make maximum use of melting scrap of similar composition. Conventional electric furnace processing steps may then be followed, except as noted below.
Phosphorus removal may be accomplished by utilization of mill scale at low temperatures, that is, in the range of about 2750°F to 2840F, for example. The mill scale formed during forging has the desirable ability to yield substantial quantities of oxygen to the bath while keeping the bath relatively cool. Slag-off of this initial oxidizing slag is preferably carried out at low temperatures, that is, under about 2840°F, to remove the P 2 O 5 from the molten metal into the slag system.
After furnace processing the steel may be further deoxidized in the tapping ladle by tapping onto V, FeSi, and/or CaSi or other metallic deoxiders.
If the sulphur content is below specification at this point resulphurization to reach an aim of 0.035, for example, may be carried out by the addition of stick sulphur, pyrities, or other appropriate sulphur addition materials.
After furnace and tapping ladle treatment the tap ladle should be subjected to refining under vacuum conditions, preferably by subjection to the simultaneous effect of vacuum and purging as illustrated and described in greater detail in U.S. Pat. No. 2,236,635 to which reference is here made for a more detailed understanding. Such vacuum degassing treatment is carried out for a period of time which will enable the oxygen level in the steel to be lowered to under 60 ppm, preferably under 50 ppm, and most preferably under about 30 - 35 ppm. In a heat of approximately 65 tons the vacuum degassing may for example be carried out for about 10 to 30 minutes, the exact time depending upon the usual process variables encountered in commercial melt shop practice including temperature, amount of slag present, and starting gas values.
The vacuum degassing treatment causes flotation of undesirable large non-metallic inclusions into whatever slag may be present on the surface. It will be understood that these inclusions are particularly deleterious in the final product since they adversely affect transverse properties and can function as focal points for stress raisers.
The vacuum degassing treatment further ensures reduction of hydrogen into the flake-free region. Experience has shown that for this type of steel the bulk of a large number of heats will be flake free at the 2.5 ppm level. However, since the failure attributable to flaking, usually termed "thermal rupture" is a serious defect it is desirable that the possibility of failure due to flaking be minimized as much as possible, and accordingly a maximum of 2.4 ppm is specified. Flaking failures have occurred even in the 2.3 to 2.4 ppm range and accordingly a maximum of 2.2 ppm is preferred. Recently rare cases of flaking have been experienced at the 2.2 ppm level and accordingly 2.0 ppm maximum is the most preferred limit.
To ensure that a thoroughly uniform and controllable teeming temperature of about 2830°F in the ladle will be obtained at the conclusion of the vacuum degassing treatment, the steel may be subject to intermittent or continuous alternating current arc heating under vacuum, as more fully described in U.S. Pat. Nos. 3,501,289 and 3,501,290, to which reference is here made for a more detailed understanding.
Other processing steps such as inert gas shroud or vacuum teeming, and/or bottom pouring may be employed as necessary to ensure a sound ingot having good surface qualities.
The silicates and aluminates will be substantially randomly dispersed in such fashion as to minimize their deleterious effects on transverse properties. Further, there will be a reduction in the number of especially large sized inclusions in the center of the ingot and a decreased concentration of inclusions near the surface of the ingot.
A series of commercial 65-ton (nominal size) heats containing examples of heats of the aforementioned broad, preferred, and most preferred ranges are compiled in the following table:
HEAT TABLE ____________________________________________________________ ______________ Heat No. C Mn S Si Ni Cr Mo V Al O 2 1 H 2 1 ____________________________________________________________ ______________ 138,789 .60 .92 .028 .34 1.02 .98 .39 .051 .005 18 1.5 238,495 .59 .86 .041 .33 .94 1.05 .37 .044 " 24 1.7 238,500 .53 .94 .022 .22 .91 1.01 .36 .048 " N.A. 2 2.4 138,791 .56 .95 .027 .26 .95 1.06 .37 .052 " 26 1.9 138,772 .54 .90 .028 .28 .93 .87 .36 .045 " 27 1.7 138,770 .57 .87 .034 .34 .96 .87 .36 .043 " 40 1.9 138,782 .58 .95 .032 .29 .93 1.05 .40 .052 " 23 1.9 238,498 .56 .94 .031 .31 .93 .98 .36 .053 " 15 1.4 138,773 .57 .90 .033 .34 .90 .96 .36 .048 " N.A. 1.2 ____________________________________________________________ ______________ 1 In ppm 2 Not available.
Of especial significance is the very low final Al content and the unusually low oxygen content, which ranges down to the 15 ppm level with respect to heat 238,498, for example.
From the foregoing description it will be appreciated that a unique die block having a low non-metallic inclusion content, particularly inclusions of silicates and aluminates, good machinability and improved hardenability, has been disclosed. Various modifications will of course at once occur to those skilled in the art. Accordingly, the scope of the invention should be limited not by the scope of the foregoing exemplary description, but only by the scope of the hereinafter appended claims when interpreted in light of the pertinent prior art.