US 3132025 A
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- quenched and tempered constructional alloy steels.
3,132,025 ALLOY STEEL John L. Hurley, Bloomfield, N..l., assignor to The International Nickel Company, Inc, New York, N.Y., a corporation of Delaware Filed Dec. 3, 1962, Ser. No. 241,535 7 Claims. (Cl. 75-124) The present invention relates to low alloy steels and, more particularly, to low alloy constructional steels in the hot-rolled condition which manifest an excellent combination of metallurgical properties including strength, formability, toughness, weldability, and corrosion resistance which render the steels eminently suitable for divers applications.
A hot-rolled steel from a commercial viewpoint should be capable of use in a tremendous number of diiierent applications as opposed to finding possible recourse in the utilization of many difi'erent and varying steel compositions (or those requiring heat treatments) for the same applications. This requires a steel of low cost which can be easily processed and which is characterized by a combination of properties, each of which does not fall below a minimum requirement. A particular steel may possess certain properties which are quite good, but at the same time be afiiicted with one or more inferior properties which would limit its application. If a hot-rolled steel is to be versatile of application, as in accordance with the present invention, it should possess (1) a yield strength of at least 70,000 p.s.i., (2) good impact properties at relatively low temperature as well as at room temperature and above, e.g., over 15 foot-pounds (ft-lbs.) at 50 F. and over at least 75 ft.-lbs. at room temperature as measured by the Charpy V-notch test, (3) a tensile elongation of over 20%, (4) good formability, including a reduction of area of at least about 60%, (5) and (6) good corrosion resistance characteristics. In addition, since structural applications are a primary use of steels in the as-rolled condition, such steels should possess a ratio of yield to tensile strength of at least 0.8 and preferably at least 0.85. Such ratios assure a designer that lesser amounts of material can be safely employed than otherwise might be the case.
If the prior art structural steels (other than the low strength plain carbon steels) could be. categorized, at least two major classes would evolve, to wit, (1) the hotrolled, high strength low alloy steels, and (2) the water The former, as marketed commercially, generally have a yield strength on the order of about 50,000 to 55,000 pounds per square inch (p.s.i.) in the hot-rolled condition, a strength considerably superior to that of the plain hotrolled carbon steels, but quite insufiicient for far too many commercial applications. Higher strengths, e.g., 65,000 p.s.i., have been attained through hardening. (precipitation) treatments but it has been found necessary to exercise various precautions to avoid theoccurrence of undesirable results. For example, embrittlement is an ever present obstacle attendant the precipitation treatment and to minimize this problem, molybdenum with or without aluminum has been considered necessary. Further, it has been common practice to normalize prior hot-rolled steels to efifect improved properties, e.g., impact strength,
. but this treatment is tedious and expensive and it would as is well known, high silicon contents result in the so United States Patent 0 "ice termed dirty steels which, from the commercial viewpoint, are difiicuit and undesirable to process. Moreover, high silicon contents can adversely affect impact strength and the tensile elongation of such steels is marginal (15 %20%) in the hot-rolled condition. To improve such properties, normalizing and stress relieving operations have been proposed.
Regarding the quenched and tempered structural steels, yield strengths of 80,000 psi. upwards to 110,000 p.s.i. have been attained but the processing of such steels is expensive and attended by much difiiculty. For example, the cost of such steels is increased by the necessity of installing and operating equipment capable of affording strict operational control in the austenitizing, quenching, and tempering operations. Further, liquid quenching, being a severe and drastic type of quench, is causative of or promotes buckling and distortion, thus undesirably limiting the shapes that can be produced or necessitating the utilization of elaborate jigs for quenching. Still further, ductility (tensile elongation), for example, is about 20% for such alloys and while this may be adequate for various forming operations, it is at best marginal for many others. Formability is of tremendous importance to exploit the strength of the steels, e.g., the buckling strength of a formed structural member in large measure is dependent upon the smallest radius of bending that can be used. In addition, in welding such steels, special lowhydrogen fluxes are necessary to avoid underbead cracking. Should the most economical submerged arc welding methods be used, lower yield strengths result and embrittlement of the weld heat-affected zone is encountered.
It has now been discovered that special low alloy steels of particular composition can be provided in the hotrolled condition and which are characterized by a highly satisfactory combination of properties including each of those specified hereinbefore. Thus, the steels are especially suitable as structural steels for general application.
It is an object of the present invention to provide a new and improved low alloy constructional steel in the hot-rolled condition and characterized by the combination of properties referred to hereinbefore.
Other objects and advantages Will become apparent from the following description.
Generally speaking and in accordance with the present invention, the combination of properties referred to herein is readily obtained with hot-rolled, low alloy steels of the following most advantageous composition based on weight percent: carbon in as small amounts as feasible up to 0.08%, e.g., about 0.01% to 0.06% carbon, about 0.2% to about 0.75% manganese, silicon in an amount suflicient for deoxidation purposes, e.g., 0.05%, and up to 0.35%, about 1% to about 1.7% copper, about 0.7% to about 1.6% nickel, about 0.01% to about 0.16% columbium, and the balance essentially iron. Aluminum in amounts to achieve good deoxidation, e.g., up to 0.1%, as is consistent with commercial steelmaking practice, is
advantageous. Up to 0.5% aluminum can be employed but because such amounts can lead to high inclusion content, it is preferred that the aluminum content not exceed 0.1%. Chromium is not required, and, in any event, it
' should not be present in an amount higher than 0.2% since higher amounts, e.g., 0.3%, impair impact strength. Sim ilarly, molybdenum should be kept below 0.2% since higher amounts can adversely aiiect both the ratio of yield to tensile strength and impact properties in the as-rolled condition. Sulfur and phosphorus should be kept as low as is commercially practical.
In the hot-rolled condition, steels within the foregoing ranges aiford yield strengths well in excess of 70,000 psi. together with yield to tensile strength ratios of at least 0.8 and up to well over 0.9, high tensile elongations (1.4" gage length) of at least as high as and up to at least exceptional formability as evidenced by reductions of area of over and excellent impact toughness as measured by the standard Charpy V-notch tests, i.e., impact strengths of at least 15 ft.-lbs. at 50 F. and ft.-lbs. to as high as over 200 ft.-lbs. at room temperature. Further, the steels are very readily weldable and manifest good corrosion resistance. In addition, the alloy steels are easily processable and are of relatively low cost. For example, normalizing and/or stress relieving treatments are not at all required as has been the case with many prior art steels. These additional considerations render the alloy steels particularly attractive from a com mercial viewpoint.
A further advantage of the alloy steels within the instant invention is that they can be hardened (as will be shown hereinafter) by a very simple precipitation hardening treatment. This treatment results in a higher yield strength level, a strength level which would be of particular advantage for certain applications.
For continuously achieving optimum results, it is important that the foregoing compositional ranges be observed. However, satisfactory results can be achieved with alloy steels having the following ranges: carbon in an amount up to 0.08%, about 0.1% to about 1% manganese, silicon from about 0.03% to less than 0.5%, about 0.9% to about 2% copper, about 0.5% to about 2.5% nickel, about 0.005% to about 0.25% columbium and the balance essentially iron.
in accordance with the invention, it has been found that the yield strength of the alloy steels is virtually insensitive to carbon content. That is to say, the alloy steels of the present invention are not dependent upon carbon content for strength as is the case with many other prior art steels. A small amount of carbon, e.g., 0.01%, is necessary to combine with the Columbium upon cooling from the hotworking operation in order to achieve a small ferrite grain size. While it is most advantageous that the carbon content not exceed 0.06%, there are commercial applications which do not require optimum formability, impact toughness, and weldability (which are afforded by alloy steels within the invention in which the carbon content advantageously does not exceed about 0.06%). In such cases, the carbon content can be extended up to about 0.19%. Because of the extremely low carbon content that can be employed and because of the virtual insensitivity of yield strength to carbon content, the steels can be decarburized during processing without incurring a detrimental loss of strength.
With respect to the manganese content of the alloy steels, at least 0.1% should be present to obviate fabricating diificulties attributable to such elements as sulfur and to avoid cracking during hot rolling. It is preferred that the manganese be less than about 1% since it has been found that higher amounts adversely affect impact properties. Silicon should be kept as low as is consistent with commercial steelmaking practice. As mentioned hereinbefore, high amounts of silicon, e.g., 0.75%, lead to a steel which contains an undesirably high number of inclusions which can adversely affect certain desired properties. Copper is a most essential element in accordance with the invention and where precipitation hardening is desired, at least 1% copper should be present. However, the presence of copper in appreciable amounts above 1.7% tends to cause cracking during hot rolling. Nickel has been found to be very beneficial in preventing cracking during hot rolling, but if the nickel content appreciably exceeds about 2.5%, i.e., over 3%, the alloy steels of the invention would have a tendency to form transformation products other than ferrite (and possibly small amounts of pearlite) during cooling from the austenitic state. This can lead to undesirable difiiculties. Further, to avoid cracking during hot working, the ratio of copper to nickel should not appreciably exceed a ratio of 2 to 1.
To achieve optimum strength, columbium is important since hot-rolled steels devoid of columbium are quite inferior in various properties, particularly yield strength. Columbium is not present in the hot-rolled steels of the invention to repress the growth of austenite grain size by carbide action as would be the case, for example, in carburizing steels. Rather, columbium must first, in order to obtain the enhanced properties in accordance with the invention, be dissolved in the steel during heating prior to hot rolling. It will be appreciated that heating of the steels to high temperatures, e.g., over about 2000 F, is necessary in accordance with the present invention in order for the hot rolling and/ or hot working operations to be carried out successfully. The austenite grain size increases when the steels are heated to rolling temperature but it is broken down during hot working. It is considered that the beneficial effects of columbium are actually imparted upon the transformation of the steel from austenite to ferrite during cooling of the hot-rolled steels from austenitizing temperatures. The transformation reaction occurs so rapidly upon air-cooling that it acts somewhat as a triggering mechanism for columbium in its role of providing a small ferrite grain size, e.g., A.S.T.M.
I No. 9 or 10 or smaller, and of imparting increased strength and toughness to the steels. The following conditions are necessary to achieve this rapidly occurring transformation: the steels must be in the austenitic condition, the Columbium must be dissolved in the austenite, the austenite must be subjected to heavy plastic deformation, e.g., by hot rolling, hot working, forging, etc., and the steels, after the hot rolling operation, must be cooled through the transformation range at a rate approximating that employed in the air-cooling of a steel plate. The more rapid the rate of air-cooling, the lower is the tempera ture at which ferrite forms and this, in turn, provides a finer grain size. Further, with low carbon contents the transformation temperature of the alloy steels is compar-atively high. Since it is common to employ a finishing pass at, say, about 1650 F. in the production of hotrolled steels, initiation of the transformation to ferrite is shortened with steels having high transformation temperatures. While it is much preferred to employ columbium, vanadium can be used to replace Columbium in whole or in part.
For the purpose of giving those skilled in the art a better understanding of the invention and/or a better appreciation of the advantages of the invention, the following illustrative description and data are given:
Several alloy steels having compositions within the ranges set forth above were prepared and are identified in Table I.
Table 1 Alloy No 0, Mn, Si, Ni, On, A], Ob,
percent percent percent percent percent percent percent;
The above steels were melted (about 30 1b. melts) in an a air induction furnace and subsequently forged to plate 3 precipitation hardened condition, the latter involving a a heating for one hour at 1-000 F. and air-cooling (after hot rolling and cooling). The tensile properties given in Table II represent the average of duplicate 0.357-inch diameter tensile bars which were cut from the /2-inch HOT ROLLED-AIR COOLED severely during hot rolling. As indicated previously herein, the nickel content should be present in amounts of at least about 0.5% and, most advantageously, at least 0.7%. In both the copper-free and columbium-free com- Yield '1. S.,1 Red. Oharpy V-notch Impact, Ft.-lb. Alloy N0. Point, 191., in
10, p.s.i. Percent Area, p.si Percent 70 F. 0 F. 50 F.
77.5 30.9 31. 0 75.2 177, 230 145, 221 174, 134 77. 1 7s. 2 55.2 77. 0 155, 201 155, 199 124, 178 75.4 80.7 27. 5 70. 5 149,140 155,101 112, 59 79. s 91. 2 25. 0 53. s 135, 39 54, 74 21,44 75.8 80. 9 33. 2 71. 5 13s, 13s 95, 100 73, 44 75.5 27.9 30.5 54. 5 73, 2 91,50 53, 50 74.9 91. 7 23. 5 59. 0 s2, 54 29, 55 22, 2s 73. 5 s2. 5 30. 5 67.8 173,137 129,133 107, 95 as. 9 s9. 5 26.8 59. 3 122 84, 70 45, 55 73.4 84.0 29. 5 71.8 123 55 87 42,101
HOT ROLLED 1,000 F., 1 HOUR 95.9 30.1 74. 5 130,85 g s3, 32 94. 3 a5. 0 74.2 157,150 124, 58 75, 52 95. 5 24. 9 57.3 151,102 32, 54 7, 7 98.6 25. 7 51.5 54, 45 22, 35 14, 7 98.5 23. 5 57.3 110,45 28 17,14 101.2 25. 2 51. 5 70, 57 21, 15, 7 104. 3 25. 5 57. 4 49, 50 23, 21 9, s
1 Tensile strength. 2 Elongation in 1.4 gage length.
Table II illustrates that each of the alloy steels manifested a yield point of well over 70,000 p.s.i. in the hotrolled condition, a strength that could be further im .proved by the application of the simple precipitation .Thus, for optimum results it is advantageous that the carbon content hot exceed about 0.06%.
The data in Table II' are further illustrative of the excellent elongation, reduction of area, and impact strength properties of the alloy steels in the hot-rolled condition. Apart from alloy No. 7, each of the alloy steels mani tested a ratio of yield to tensile strength of greater than 0.85, an elongation of at least 25%, a reduction of area of at least 60%, andla Charpy impact resistance of at least 15 ft-lbs. (over 20 ft.-lbs.) at 50 and at least 75 -t.-lbs. at room temperature. Actually, the reduction of area data is a better indicator of tensile ductility rather than elongation, because at least one of the pairs of specimens tested (except alloys No. 4, 5 and 6) deformed extensively outside the gage limits and, thus, the actual elongation data given in Table II for these alloys, while extremely good, are below the true values. As will be appreciated, hot-rolled alloy steels within the invention are exceedingly tough as illustrated by alloys No. 1 and 2 which are characterized by elongations of over reductions of area of over 70%, and impact resistances of well over 100 ft.-lbs. at 50 F.
In addition to the foregoing data, alloys outside the scope of the invention were prepared and tested under the same conditions outlined hereinabove. Three of such alloys had compositions within the most advantageous range except that either nickel or copper or columibum was not added to the steels. The nickel-free steel cracked parable steels, it Was found that the yield strength was less than the required minimum of 70,000 p.s.i. In addition, a chromium-containing steel otherwise within the composition limits of the invention except that it contained 0.33% chromium, was prepared and tested. At 50 F. this steel had Charpy V-notch impact values of 7 and 14 lbs. This illustrates the importance (as referred to hereinbefore) of maintaining the chromium content (if added or introduced by Way of scrap) low.
That the alloy steels are readily weldable is demonstrated by the following submerged arc weld test: a plate specimen to be Welded was prepared with the top edges .being hand ground to a bevel inch wide with a 30 to 40 degree angle. A nickel-copper alloy steel wire inch in diameter was used as the welding wire together with a suitable flux. Using a current of about 600 amperes, a voltage of about 30 volts and a deposition rate of 12 inches per minute, one pass on each side of the plate was made. Inspection showed the welds to be crack free.
For applications requiring a hardened alloy steel, the alloy steels of the present invention can be hardened by subjecting them to a temperature of between about 850 F. to about 1150" F., e.g., 950 F. to about 1050 F., for a period sufiicient to effect the required degree of precipitation hardening. For example, one hour at 1000 F. is quite satisfactory. A shorter period of time can be employed at the higher levels of temperature (1050- F. to 1150 F.). Conversely, if the alloy steels are heat treated at, say, 850 F. to 950 F., a period greater than one hour might be necessary. A particular benefit of the precipitation hardened steel is that a tenacious scale is formed during the precipitation treatment and this adherent scale can form an excellent base for a protective coating, e.g., paint.
In view of the combination of metallurgical properties characteristic of the alloy steels within the present invention, the steels can be used in a wide variety of applications such as automobile bumpers, truck frames, tubular vessels, railroad cars, structural components including I-beams, rolled form shapes, etc. The alloy steels are also suitable for use in applications for moderate temperature use, i.e., above room temperature and up to about 400 F. or 500 F.
As will be readily understood by those skilled in the art, the term balance when used to indicate the amount of iron in the alloy steels does not exclude the presence of other elements commonly present as incidental elements, e.g., deoxidizing and cleaning elements, and impurities ordinarily associated therewith in small amounts which do not adversely afiect the basic characteristics of the steels. In addition, the terms hot-rolled or asrolled as used herein are intended to include, as those skilled in the art will readily understand, such operations as the application of finishing passes or temper rolling.
Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to Without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.
1. An alloy steel characterized in the hot-rolled condition by a yield strength of at least 70,000 p.s.i., a yield to tensile strength ratio of at least 0.85, a Charpy V-notch impact strength of at least 15 ft.-lbs. at 50 F. and at least 75 ft.-lbs. at room temperature, a tensile elongation of over 20%, a reduction of area of at least 60% and good weldability and corrosion resistance, said alloy steel consisting essentially of carbon in an amount up to 0.06% about 0.2% to about 0.75% manganese, silicon in an amount up to 0.35%, about 1% to about 1.7% copper, about 0.7% to about 1.6% nickel, the ratio of copper to nickel not exceeding about 2 to 1, about 0.01% to about 0.16% columbium, aluminum in a small but effective amount up to about 0.1% sufiicient to insure good deoxidation, and the balance essentially iron.
2. An alloy steel characterized in the hot-rolled condition by a yield strength of at least 70,000 p.s.i., a yield to tensile strength ratio of at least 0.85, a Charpy V-notch impact strength of at least 15 ft.-lbs. at 50 F. and at least 75 ft.-lbs. at room temperature, a tensile elongation of over 20%, a reduction of area of at least 60%, good weldability and corrosion resistance, said alloy steel consisting essentially of carbon in an amount up to 0.08%, about 0.2% to about 0.75% manganese, silicon in an amount up to about 0.35%, about 1% to about 1.7% copper, about 0.7% to about 1.6% nickel, the ratio of copper to nickel not exceeding about 2 to 1, about 0.01% to about 0.16% columbium, aluminum in a small but eifective amount up to 0.1% sufiicient to insure good deoxidation, and the balance essentially iron.
3. An alloy steel in the hot-rolled condition characterized by a yield strength E at least 70,000 p.s.i. together with a yield to tensile strength ratio of at least 0.85, a Charpy V-notch impact strength of at least ft.lbs. at
F. and at least 75 ft.-lbs. at room temperature, a tensile elongation of over 20%, a reduction of area of at least good weldability and corrosion resistance, said alloy steel consisting essentially of carbon in an amount up to 0.08%, about 0.1% to about 1% manganese, silicon from about 0.03% to less than 0.5 about 0.9% to about 2% copper, about 0.5% to about 2.5% nickel, the ratio of copper to nickel not exceeding about 2 to 1, about 0.00-5% to about 0.25% columbium, aluminum in a small but effective amount up to 0.5% sufficient to insure good deoxidation, and the balance essentially Iron.
4-. An alloy steel consisting essentially of carbon in an amount up to 0.19%, about 0.1% to about 1% manganese, silicon from about 0.03% to less than 0.5%, about 0.9% to about 2% copper, about 0.5% to about 2.5% nickel, the ratio of copper to nickel not exceeding about 2 to 1, about 0.005% to about 0.25% columbium, alu minum in a small but effective amount up to 0.5 suflicient to insure good deoxidation, and the balance essentially iron.
5. A hardened alloy steel consisting essentially of carbon in an amount up to 0.06%, about 0.2% to about 0.75% manganese, silicon in an amount up to 0.35%, about 1% to about 1.7% copper, about 0.7% to about 1.6% nickel, the ratio of copper to nickel not exceeding about 2 to 1, about 0.01% to about 0.16% columbium, aluminum in a small but effective amount up to about 0.1% sufiicient to insure good deoxidation, and the balance essentially iron.
6. A hardened alloy steel consisting essentially of carbon in an amount up to 0.08%, about 0.1% to about 1% manganese, silicon from about 0.03% to less than 0.5%, about 0.9% to about 2% copper, about 0.5% to about 2.5 nickel, the ratio of copper to nickel not exceeding about 2 to 1, about 0.005% to about 0.25 columbium, up to 0.5 aluminum, and the balance essentially iron.
7. A hardened alloy steel consisting essentially of carbon in an amount up to 0.19%, about 0.1% to about 1% manganese, silicon from about 0.03% to less than 0.5%,
about 0.9% to about 2% copper, about 0.5 to about 2.5 nickel, the .ratio of copper to nickel not exceeding about 2 to 1, about 0.005 to about 0.25 columbium, up to 0.5 aluminum, and the balance essentially iron.
References Cited in the file of this patent UNITED STATES PATENTS 2,046,168 Kinzel et al June 30, 1936 2,158,651 Becket et'al May 16, 1939 2,182,135 Reinhardt Dec. 5, 1939 2,443,932 Rofi et al June 22, 1948 3,010,822 Altenburger Nov. 28, 1961 OTHER REFERENCES Alloys of Iron and Copper, pages 97 and 160, by Gregg and Daniloif, published in 1934 by the McGraw-Hill Book Company, New York.