METHOD OF PRODUCING HIGH STRENGTH CARBON STEEL PLATE
United States Patent 3601885
A method of making high strength readily weldable carbon steel plates for railroad tank cars in the "as rolled" condition and in the normalized condition for pressure vessels, including tank cars which are intended for moderate and low temperature service, wherein steel is melted in a furnace to include, in addition to iron and incidental impurities, 0.15-0.50 percent silicon, 0.01-0.47 percent copper, 0.05-0.35 percent nickel, 0.05-0.25 percent chromium, and 0.01-0.15 percent molybdenum, the nickel and chromium content totaling at least about .20 percent; including 0.008 to 0.015 percent aluminum in the melt; adding sufficient manganese whereby the manganese content will fall within a range of a maximum of about 1.50 percent, and a minimum of 1.00 percent or 5.0 to 6.5 times the desired carbon content at pour, whichever is greater; refining the melt to a carbon content within a range of a maximum of 0.25 percent and a minimum of 0.20 percent or forty percent of the predetermined gauge in inches of the plate to be produced from the melt, whichever is lesser; pouring the melt into a mold to form an ingot; and rolling the ingot into plate of the predetermined gauge. Considerable economy can be realized through inclusion of a high percentage of scrap metal in the melt to attain the desired levels of copper, nickel, chromium and molybdenum. For normalizing, the rolled plates are heated to a temperature of 1525-1650° F., held at such temperature forty-five minutes for each inch of plate thickness and air cooled.
US Patent References:
Heat-resistant low alloy steels
Ripich - April 1949 - 2467701
Submerged arc weld metal composition
Keay - November 1963 - 3110798
Heat-resistant low alloy steels
Ripich - April 1949 - 2467701
Submerged arc weld metal composition
Keay - November 1963 - 3110798
Application Number:
05/864245
Publication Date:
08/31/1971
Filing Date:
08/07/1969
View Patent Images:
Export Citation:
Assignee:
Lukens Steel Company (Coatesville, PA)
Primary Class:
Other Classes:
164/76.100
International Classes:
C22C38/44; B23K19/00
Field of Search:
75/124,125,128.9 164/76 29/527.5,527.7
Other References:
volume I of Metals Handbook, eighth edition, 1961, pages 87-91..
Primary Examiner:
Campbell, John F.
Assistant Examiner:
Reiley D. C.
Parent Case Data:
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a divisional application of Ser. No. 785,730 filed Dec. 20, 1968 now Pat. No. 3,499,757, which in turn is a continuation-in-part of application Ser. No. 642,324, filed May 31, 1967, now abandoned; a continuation-in-part of application Ser. No. 415,184, filed Dec. 1, 1964, now abandoned; a continuation-in-part of application Ser. No. 354,764, filed Mar. 25, 1964, now abandoned.
In application Ser. No. 293,576, filed July 3, 1963, now included in U.S. Pat. No. 3,310,441, issued Mar. 21, 1967, an improvement in heat-treated carbon steel plates was disclosed wherein the notch toughness qualities are improved at low temperatures particularly in the -75.degree. F. and under range. The present invention is concerned with steel plates for a different purpose having generally narrower ranges and a different balance of elements within, however, a similar but, as will be seen, distinctive chemistry. The steels described herein are not hot-rolled high-strength steels of the type which are classified as structural steels. Instead the steel plates are in the as-rolled condition, not having been heat treated as distinguished from the steel plates disclosed in the aforementioned application; the properties of the instant steel plates thus being derived principally from their chemistry.
Claims:
I claim
1. A method for making high strength readily weldable carbon steel plate for railroad tank cars and the like which comprises the steps of
2. A method of making steel plate in accordance with claim 1 wherein scrap iron is included in the melt, said scrap iron including therein all copper, nickel, chromium and molybdenum required for the said specified ranges thereof.
3. A method of making steel plate in accordance with claim 1 which comprises the additional step of normalizing plate rolled from said ingot by raising the temperature of said plate to 1525-1650° F., holding said plate at said temperature for at least 45 minutes for each inch of its thickness, and air cooling said plate.
4. A method in accordance with claim 1 wherein said carbon content is refined to be at pour about 15-19 percent of the manganese content.
5. A method in accordance with claim 1 wherein said carbon content is refined to be at pour about 16-20 percent of the manganese content.
6. A method in accordance with claim 1 wherein sufficient nickel and chromium is included in the melt to total over 0.25 percent.
7. A method of producing a readily weldable carbon steel plate having a yield point of at least 50,000 p.s.i. and a tensile strength of at least 81,000 p.s.i., for railroad tank cars in the as-rolled condition, said method comprising the steps of:
8. A method of producing a readily weldable carbon steel plate having a yield point in excess of 50,000 p.s.i., a tensile strength in excess of 81,000 p.s.i. and minimum Charpy V-notch impact strength of 20 foot-pounds at -50° F. in the normalized condition which comprises the steps of:
1. A method for making high strength readily weldable carbon steel plate for railroad tank cars and the like which comprises the steps of
2. A method of making steel plate in accordance with claim 1 wherein scrap iron is included in the melt, said scrap iron including therein all copper, nickel, chromium and molybdenum required for the said specified ranges thereof.
3. A method of making steel plate in accordance with claim 1 which comprises the additional step of normalizing plate rolled from said ingot by raising the temperature of said plate to 1525-1650° F., holding said plate at said temperature for at least 45 minutes for each inch of its thickness, and air cooling said plate.
4. A method in accordance with claim 1 wherein said carbon content is refined to be at pour about 15-19 percent of the manganese content.
5. A method in accordance with claim 1 wherein said carbon content is refined to be at pour about 16-20 percent of the manganese content.
6. A method in accordance with claim 1 wherein sufficient nickel and chromium is included in the melt to total over 0.25 percent.
7. A method of producing a readily weldable carbon steel plate having a yield point of at least 50,000 p.s.i. and a tensile strength of at least 81,000 p.s.i., for railroad tank cars in the as-rolled condition, said method comprising the steps of:
8. A method of producing a readily weldable carbon steel plate having a yield point in excess of 50,000 p.s.i., a tensile strength in excess of 81,000 p.s.i. and minimum Charpy V-notch impact strength of 20 foot-pounds at -50° F. in the normalized condition which comprises the steps of:
Description:
BACKGROUND OF THE INVENTION
This invention relates to an improvement in carbon steel plates. In particular, the invention relates to improvements to the strength characteristics of hot-rolled carbon steel plates.
The present invention is also concerned with "as rolled" flange and firebox grade steel plate having unexpectedly high tensile strengths. The unusual properties of the flange and firebox grade steels as well as the properties of the structural grade steels are achieved through chemical control and balance.
In the steelmaking industry, steel is considered to be carbon steel when, within the limits of the recognized field of alloy steels, no minimum content of any element is added to obtain a desired alloying effect, when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: Manganese 1.65, silicon 0.60, copper 0.60. The composition disclosed herein is considered to fall within the foregoing definition of carbon steel in view of the means of producing same. However, the claims of invention as to composition should not necessarily be interpreted as so limited.
Before the development of the steel plates disclosed in U.S. Pat. No. 3,310,441, the only commercial steel available which provided plate with a minimum tensile strength of 70,000 p.s.i. and notch toughness at -75° F. of 15 foot pound in normalized condition was a steel which contained 2.25 percent nickel, and was thus a comparatively expensive alloy steel. It was, however, discovered that by controlling the residual elements, which almost inevitably occur in carbon steel plate produced from scrap, i.e., copper, nickel, chromium and molybdenum, to comparatively high levels, it was possible to produce a heat-treated carbon steel plate as disclosed in U.S. Pat. No. 3,310,441 which served the same uses as the known alloy steel plates and could be produced at substantially lower cost. Such steel plate is used primarily for offshore drilling rigs in frigid regions such as in the Alaskan shore area, for the storage of refrigerated liquified gas such as propane and ammonia, and in general, for cryogenic uses. All the steel plates for such use and otherwise disclosed in the aforesaid U.S. Pat. No. 3,310,441, are heat-treated steels, however. Tests run on such steel plates in the as-rolled condition prior to the application of heat treatment failed to disclose the high strength levels of the instant invention. But, on the other hand these tests did indicate that high levels of the so-called residuals produced unexpectedly high strength levels as well as improved weldability and good toughness in the as-rolled plates as well as in the heat-treated plates.
For many years railroad tank cars have been fabricated from steel plate in the as-rolled condition. This has been and currently is a specification of the Interstate Commerce Commission. Until the instant invention, the highest strength carbon steel plate commercially available for tank car use was one having 75,000 p.s.i. minimum tensile strength. It was theorized, however, that a steel plate having a tensile strength level about 81,000 p.s.i. or over would permit a substantial reduction of weight in railroad tank cars constructed from standard gauge steel plate. In larger tank cars the hypothesised reduction of weight would be as high as 15 percent and even more. Needless to say such a reduction of weight would significantly add to the cargo-carrying capacity of the tank car.
SUMMARY OF THE INVENTION
Although no one would have seriously considered it possible to achieve such physical properties in carbon steel plate of the type involved, with the knowledge that surprising increases in strength levels of carbon steel could be obtained through controlling the residual elements to high levels without sacrificing other essential qualities of the steel, it occurred to me that unexpected further increases in strength levels might be consistently obtainable by further modification in the chemistry to permit commercial production of a tough, readily weldable steel having exceptionally high strength levels in the as-rolled condition suitable for tank car use. Because of variations in strength characteristics which occur even in plate produced from the same melt it would be necessary to find a chemistry which would produce plate having about 87,000 p.s.i. tensile strength to insure that all the plate from a given melt exceeded 81,000 p.s.i. tensile strength. Now, for the first time carbon steel plate has been produced acceptable for railroad tank cars guaranteed to minimum tensile strengths above 81,000 p.s.i. in the as-rolled condition which at the same time has the necessary characteristics of good weldability and toughness. The steel has already experienced a significant commercial success with 50 to 100,000 tons per year presently being produced.
Initially, it was thought that a low-alloy steel, produced to the same chemical analysis as the instant invention, except utilizing a minimum of 0.02 percent vanadium in lieu of the high residual levels, would be the equivalent of the invention. Although such steel could be produced to over 81,000 p.s.i. tensile strength in the as-rolled condition, in practice it proved inferior in weldability and toughness and has not experienced general commercial acceptance. The necessary chemistry will be described in more detail in the subsequent description of the invention. But it should be understood that the steel plate of this invention differs from the steel plates taught by U.S. Pat. No. 3,310,441 not only in application as noted above and also in the particular chemical balance, notably in a comparatively higher carbon content and lower manganese to carbon ratio.
I have recently further discovered that plates produced to about the chemistry taught herein which are normalized by being held at a temperature of about 1550° F. for 1 hour per inch thickness and air-cooled, provide surprisingly good toughness to temperatures down to -50° F. and are superior as high-strength carbon steel plates for use as pressure vessels both in moderate and lower temperature service. Also, it will be appreciated from the subsequent description of the invention, optimum results are produced for this particular type of steel through a somewhat further modified chemical balance.
In view of the foregoing, it is an object of the invention to provide a high-strength carbon steel plate in the as-rolled condition which may be priced significantly lower than steel plates of comparable capabilities having suitable properties for use in structural applications--particularly for fabrication of railroad tank cars.
It is also an object of the instant invention to provide a flange and firebox grade steel having a minimum tensile strength of 81,000 p.s.i.
It is another object of this invention to provide a high-strength carbon steel of such a type which is readily weldable.
It is a further object of this invention to provide carbon-steel plates of the aforementioned improved qualities and thicknesses up to 2 inches for structural steel.
It is a yet further object of this invention to provide a tough, weldable, high-strength carbon steel which is superior for use in pressure vessels exposed to moderate and lower temperatures.
A still further object of the invention relates to the production of the foregoing steel, largely from less expensive scrap steel in a melting furnace by the cold-charging process and in the particular chemistry balance achieved through this means of production.
DESCRIPTION OF PREFERRED EMBODIMENTS
It has been discovered that the unusual properties of carbon steel plate according to the invention can be achieved by carefully balancing the carbon and manganese levels together with the inclusion of high levels of nickel, chromium, molybdenum and copper within what is presently considered the industrial standards of maximums for residual elements through the careful control of scrap added to the furnace producing the steel. The relative amounts of carbon and manganese have an effect on the yield point, tensile strength, ductility, etc. It has been generally found that the strength level is improved as the ratio of manganese to carbon is increased. However, with the instant invention, increased strength levels are obtained with a lowering of the manganese-to-carbon ratio.
The refinement of the grain structure of these steels is further improved as a result of treating the steel with aluminum. However, the addition of aluminum is optional for structural grade steels although it is preferred for flange and firebox grade steels. Still further, improvement results when the steel contains high levels of the elements nickel, chromium, molybdenum and copper, even though these elements are maintained within the industrial standards of maximums for residual elements. This latter factor not only improves essential physical characteristics of the resulting steel plates, but also permits the use of 80 percent or more scrap in the production of carbon steel plate according to the invention. This is an important factor since, at least currently, the market value of scrap is less than one-half that of pig iron.
Steel in accordance with this invention is prepared in a melting furnace of the open-hearth or electric-arc type. Approximately 80 percent or more of the charge is scrap steel with about one-half of same being purchased scrap and the remaining being scrap generated at the mill from trimmings, diversions, etc. It is important that the chemical constituents of such scrap be known as accurately as possible. However, by experience it has been learned that the copper-nickel-chromium-molybdenum contents in purchased scrap fall in a ratio of roughly 7-5-3-1. Thus by careful control of scrap in accordance with one of the chemical constituents--say, copper--the other constituents are also controlled within limits. But it should be needless to say that purchased scrap must be carefully inspected and chemically checked by individuals skilled in this art for segregation into identified areas of the scrap yards. Scrap generated in the mill is similarly identified and segregated. At the present time, with such close inspection and segregation, steel plate having characteristics in accordance with the invention may be produced with substantially less than 5 percent diversions.
It will be appreciated from the foregoing that by controlling the type and amount of scrap, the levels of copper, nickel, chromium and molybdenum contents are similarly controlled. With knowledge of desired amounts of such elements, the object is to employ various grades of scrap in the most economical ratios. Of course, in so doing, the inventory of scrap on hand at the time must be kept in mind. An example of such control is a mixture as follows: ------------------------------------------------------------ --------------- TABLE I
Source Percentages ____________________________________________________________ ______________ Molds 10.3 Casts 8.7 Total Pure Iron 19.0 No. 1 Busheling 8.0 No. 1 Bundles 3.3 No. 2 Heavy Melting 8.4 No. 2 Bundles 12.0 Machine Shop Turnings 9.3 Total Purchased Scrap 41.0 Common Plate 33.3 Pit (Sprues and Runners) 6.7 Total Plant Scrap 40.0 ____________________________________________________________ ______________
The foregoing produced steel which included 0.30 percent copper, 0.19 percent nickel, 0.13 percent chromium and 0.04 percent molybdenum. The cost of a comparable charge in pig iron for the elements of copper, nickel, chromium and molybdenum added is approximately two and one-half times the foregoing.
It has been found that to produce steel plate economically in accordance with the invention, the preferred minimum percentages of copper, nickel, chromium and molybdenum should be 0.11, 0.10, 0.07, 0.02, respectively. On an individual basis, the respective minimum percentages should be about 0.10, 0.08, 0.04 and 0.01, and the sum of the percentages of copper, chromium and molybdenum should exceed 0.20 but generally should not exceed 0.60. The sum of the nickel and chromium content percentages should be at least 0.15 and preferably at least 0.20. The preferred maximum percentages of copper, nickel, chromium and molybdenum are 0.35, 0.15, 0.15 and 0.05, respectively, and the desired percentage ranges for these constituents are about 0.11-0.27, 0.10-0.15, 0.07-0.09 and 0.02-0.03, respectively. However, as will be noted from subsequent tables, one or several of such elements may extend out of these desired ranges.
For the invention herein, the presence of copper follows almost inevitably from economical use of scrap for the charge. But it is not believed that the presence of copper, within the prescribed residual levels, significantly affects as a matter of commercial practicality, the desired characteristics of the steel plate produced. More critical are the nickel and chromium contents which, as indicated above, should preferably total 0.20 percent or more.
Charged into the melting furnace just prior to the tap or into the ladle is standard ferro manganese or other manganese additive whereby the manganese percentage by weight will be 0.70 to 1.60, preferably 0.80 to 1.50 for structural steels, and 0.80 to 1.20 for flange and firebox grade steels. Where plate from the steel is intended for tank cars, the manganese percentage range will be 1.00-1.50, the amount depending upon the thickness of the plate to be produced and being on the lower side for thinner plates, say of 3/8-inch thickness, and on the higher side for thicker plates, say of 3/4 to 1 inch or more in thickness. For the desired quality of steel plate, it has been found that in general the higher the total of copper, nickel, chromium and molybdenum, within the limits of the invention, the less manganese or carbon is required. Ferro silicon or other silicon-containing additive is added at approximately the same time to deoxidize or quiet the molten steel. The amount is somewhat lesser for thinner plates than for thicker plates.
In the cold-charging production of steel, the charges are kept in a heated or molten condition for approximately two to four times as long as with the hot-charge method. During the molten period, the carbon content tends to decrease. As noted above, less carbon and manganese are required for strength in the thinner gauges than in the thicker gauges. For this reason, different ranges of carbon and manganese and percentage by weight are specified for different gauges of structural steel plate approximately as shown in the following table: ##SPC1##
The chromium content also tends to become lower while the steel is molten, and for this reason a slightly high chromium content in the initial charge is not considered deleterious.
The specific chemistry of the resulting carbon steel is as follows: ------------------------------------------------------------ --------------- TABLE III
Element Percentage by Weight ____________________________________________________________ ______________ Carbon 0.14-0.25 Manganese 1.00-1.60 Phosphorus 0.010-0.040 Sulfur 0.010-0.050 Silicon 0.10-0.60 Aluminum 0.00-0.055 Copper 0.01-0.47 Nickel 0.05-0.35 Chromium 0.05-0.25 Molybdenum 0.01-0.15 Iron and incidental impurities Balance ____________________________________________________________ ______________
Of course, for the above steels, it will be understood that the carbon content should preferably be maintained the ranges indicated before, depending upon the gauge of the plates. Manganese range for such steels is preferably held between 0.80 to 1.50 percentage by weight for structural steel.
Structural carbon steel plate having the above-disclosed chemistry and produced by conventional hot-rolled processes has properties heretofore only obtainable in steels containing vanadium, columbium, nitrogen or combinations thereof. These unusual properties are disclosed in Table IV. ##SPC2##
In carbon steels, it is common for purchasers to specify a certain maximum or minimum copper content. For example, maximum copper content of 0.25 or 0.35 percent may be specified, or a 0.20 percent minimum is usual. It is known that the inclusion of copper in amounts from 0.20-0.60 percent increases the resistance of the steel to atmospheric corrosion and slightly increases the yield point and tensile strength, but adversely affects the ductility. In filling such orders with controlled production, as disclosed herein, the range variations of copper, nickel, chromium and molybdenum, as disclosed in Table III, can easily be obtained in a number of these structural steels as specifically disclosed in Table V. ##SPC3##
As-rolled carbon steel plate in thicknesses up to 1 inch having the chemistry of Table III when made according to fine grain melting practice with the inclusion of about 0.02-0.055 percent aluminum, have a minimum tensile and other qualities of flange and firebox grades. Such steels are suitable for boiler shells tank car tanks and other pressure vessels. These flange and firebox grade steels are also nonheat treated. Preferably the manganese content of the flange and firebox grades is about 0.8-1.2 percent and the silicon content is 0.15-0.3 percent. Also, the maximum phosphorus and sulfur content should be about 0.035 and 0.04, respectively.
A 3/4-inch plate was prepared having the following chemistry and qualities: ##SPC4##
The welding properties of the above 3/4-inch plate were determined to be excellent in terms of transverse and longitudinal tensile strengths. Also, no evidence of cracking was observed on a 180° bend made in the as-welded and stress-relieved conditions.
The toughness of this steel was also determined with excellent results. Impact low-temperature V-notch Charpy test results were better than conventional structural steels.
Table VII shows a number of typical melts for the production of plates designed for use in tank cars. Plates of this steel uniformly had tensile strengths between 81,000 and 101,000 p.s.i. yield points above 50,000 p.s.i. and an elongation in 8 inches of at least 18 percent. Bend test specimens of such steels can stand being bent cold through 180° without cracking on the outside of the bent portion through an inside diameter of twice the thickness of the specimen. It will be noted that such steel plates are made to fine grain practice. The carbon content is about 18 percent of the manganese content in the examples and, should preferably be 16-20 percent. It is of primary importance that steel plates for tank car end use be readily weldable through standard welding procedures. From experience it has been found that the plates are, in fact, readily weldable.
It has been found that plates having acceptable minimum yield strengths and tensile points can be manufactured without the high residual levels specified for the steel plates of the invention through the addition of vanadium in amounts exceeding 0.02 percent. However, such plates have not been broadly accepted because they do not present the advantage and weldability of the plates of the invention and the degree of toughness is less. ##SPC5## ##SPC6## ##SPC7## ##SPC8##
For the production of tank-car steel plate, the chemical analysis should be as set forth in Table VIII below:
Element Percentage by Weight ____________________________________________________________ ______________ Carbon 0.15-0.25 Manganese 1.00-1.50 Silicon 0.15-0.50 Aluminum 0.008-0.015 Copper 0.01-0.47 Nickel 0.05-0.35 Chromium 0.05-0.25 Molybdenum 0.01-0.15 Iron balance ____________________________________________________________ ______________
The nickel and chromium contents must, it has been found total at least 0.15 percent and preferably should be at least 0.20 percent. The carbon, manganese and silicon contents are increased as the thickness of the plate increases. Thus, a 3/8-inch thickness plate should have its carbon content in the range of 0.15 to 0.20 percent, its manganese content 1.00-1.20 percent and its silicon content 0.15-0.30 percent For a 7/16-inch thickness plate, the carbon content should be 0.18-0.25 percent, the manganese content should be 1.10-1.35 percent and the silicon content 0.15-0.30 percent. For plates 9/16-inch thick, the carbon content should be 0.20-0.25 percent, the manganese content 1.10-1.35 percent, and the silicon content 0.15-0.30 percent. For thicknesses of 3/4-inch and above, the carbon content should be 0.20-0.25 percent, the manganese content 1.20-1.50 percent, and the silicon content 0.15-0.50 percent. As the thickness is further increased, the manganese carbon and silicon content levels should be preferably in the higher portions of the described ranges. From a commercial standpoint, the most important thicknesses are from 7/16-inch to 1 inch. However, the steel plates can be produced with the desired characteristics up to 2 inches in thickness. In producing the tank car steel plate, the so-called residuals of copper, nickel, chromium, and molybdenum are controlled through careful selection of the scrap charge, as previously described, and the manganese, carbon and silicon are controlled through additions to the melt, the amounts of each depending upon the thickness required of the plates to be produced from the melt. As noted with reference to Table VII, the carbon content is between 16 and 20 percent of the manganese content and preferably about 18 percent. It is to be appreciated that the physical properties of the steel plate are achieved by carefully controlling the chemistry of the plate rather than through any subsequent heat treatment of the plate.
The success achieved with the tank car plate suggested the possibility that a similar chemistry might be utilized to produce unique superior high-strength carbon steel plates for pressure vessels or tank cars in moderate and lower-temperature service. Table IX shows the chemical analysis and physical properties of a number of plates which have been produced to a chemistry very similar to that of the above-described tank car steel which have been heat-treated by normalizing them at temperatures of 1525-1650° F., held 45 minutes for each inch thickness, and air cooled. Plates so produced have tensile strengths consistently 10 to 20 percent ##SPC9## ##SPC10## ##SPC11## ##SPC12## ##SPC13## higher than normalized plates disclosed in my previously mentioned U.S. Pat. No. 3,310,441. In toughness at -50° F. specimens from such plates show consistent impact strengths above 20 foot-pounds with standard Charpy V-notch tests. The preferred chemistry ranges for the normalized plates are as follows: ------------------------------------------------------------ --------------- TABLE VIII
Element Percentage by Weight ____________________________________________________________ ______________ Carbon 0.22-0.25 Manganese 1.15-1.50 Silicon 0.20-0.30 Aluminum 0.020 min. Copper 0.01-0.35 Nickel 0.05-0.25 Chromium 0.05-0.25 Molybdenum 0.01-0.08 ____________________________________________________________ ______________
The nickel and chromium percentage totals should be at least 0.15 and preferably at least 0.25. The carbon content percentage should be preferably 0.23 percent. For plates up to 3/4-inch thickness, the manganese range should preferably be 1.15-1.35, aiming at a percentage of 1.25. For plates over 3/4-inch, the manganese content is preferably 1.25-1.50, aiming at a 1.40 percent. The steel plates are made to fine-grain practice and in this connection, the desired percentage of aluminum is 0.025. The carbon content is preferably about 16 to 17 percent of the manganese content and, in the examples, ranges 15 to 19 percent. Plate produced in accordance with these specifications is intended for fusion welding and by such techniques is readily weldable. In bend tests, specimens of such steel plates may be bent cold through 180° without cracking on the outside of the bent portion to an inside diameter which is twice the thickness of the specimen. If the test is made on a specimen reduced in thickness, the rolled surface is on the outer curve of the bend.
In the foregoing disclosure and in the claims, percentages are expressed by weight unless indicated otherwise.
Plates from the steels disclosed herein are characterized by high strength characteristics normally associated only with alloy steels containing significant amounts of vanadium, columbium, nitrogen, etc. From the values listed above for tensile strengths, yield points and other characteristics, it will be apparent to one skilled in the art that these higher strength steels exceed standard specifications so that considerable savings are realized.
This invention relates to an improvement in carbon steel plates. In particular, the invention relates to improvements to the strength characteristics of hot-rolled carbon steel plates.
The present invention is also concerned with "as rolled" flange and firebox grade steel plate having unexpectedly high tensile strengths. The unusual properties of the flange and firebox grade steels as well as the properties of the structural grade steels are achieved through chemical control and balance.
In the steelmaking industry, steel is considered to be carbon steel when, within the limits of the recognized field of alloy steels, no minimum content of any element is added to obtain a desired alloying effect, when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: Manganese 1.65, silicon 0.60, copper 0.60. The composition disclosed herein is considered to fall within the foregoing definition of carbon steel in view of the means of producing same. However, the claims of invention as to composition should not necessarily be interpreted as so limited.
Before the development of the steel plates disclosed in U.S. Pat. No. 3,310,441, the only commercial steel available which provided plate with a minimum tensile strength of 70,000 p.s.i. and notch toughness at -75° F. of 15 foot pound in normalized condition was a steel which contained 2.25 percent nickel, and was thus a comparatively expensive alloy steel. It was, however, discovered that by controlling the residual elements, which almost inevitably occur in carbon steel plate produced from scrap, i.e., copper, nickel, chromium and molybdenum, to comparatively high levels, it was possible to produce a heat-treated carbon steel plate as disclosed in U.S. Pat. No. 3,310,441 which served the same uses as the known alloy steel plates and could be produced at substantially lower cost. Such steel plate is used primarily for offshore drilling rigs in frigid regions such as in the Alaskan shore area, for the storage of refrigerated liquified gas such as propane and ammonia, and in general, for cryogenic uses. All the steel plates for such use and otherwise disclosed in the aforesaid U.S. Pat. No. 3,310,441, are heat-treated steels, however. Tests run on such steel plates in the as-rolled condition prior to the application of heat treatment failed to disclose the high strength levels of the instant invention. But, on the other hand these tests did indicate that high levels of the so-called residuals produced unexpectedly high strength levels as well as improved weldability and good toughness in the as-rolled plates as well as in the heat-treated plates.
For many years railroad tank cars have been fabricated from steel plate in the as-rolled condition. This has been and currently is a specification of the Interstate Commerce Commission. Until the instant invention, the highest strength carbon steel plate commercially available for tank car use was one having 75,000 p.s.i. minimum tensile strength. It was theorized, however, that a steel plate having a tensile strength level about 81,000 p.s.i. or over would permit a substantial reduction of weight in railroad tank cars constructed from standard gauge steel plate. In larger tank cars the hypothesised reduction of weight would be as high as 15 percent and even more. Needless to say such a reduction of weight would significantly add to the cargo-carrying capacity of the tank car.
SUMMARY OF THE INVENTION
Although no one would have seriously considered it possible to achieve such physical properties in carbon steel plate of the type involved, with the knowledge that surprising increases in strength levels of carbon steel could be obtained through controlling the residual elements to high levels without sacrificing other essential qualities of the steel, it occurred to me that unexpected further increases in strength levels might be consistently obtainable by further modification in the chemistry to permit commercial production of a tough, readily weldable steel having exceptionally high strength levels in the as-rolled condition suitable for tank car use. Because of variations in strength characteristics which occur even in plate produced from the same melt it would be necessary to find a chemistry which would produce plate having about 87,000 p.s.i. tensile strength to insure that all the plate from a given melt exceeded 81,000 p.s.i. tensile strength. Now, for the first time carbon steel plate has been produced acceptable for railroad tank cars guaranteed to minimum tensile strengths above 81,000 p.s.i. in the as-rolled condition which at the same time has the necessary characteristics of good weldability and toughness. The steel has already experienced a significant commercial success with 50 to 100,000 tons per year presently being produced.
Initially, it was thought that a low-alloy steel, produced to the same chemical analysis as the instant invention, except utilizing a minimum of 0.02 percent vanadium in lieu of the high residual levels, would be the equivalent of the invention. Although such steel could be produced to over 81,000 p.s.i. tensile strength in the as-rolled condition, in practice it proved inferior in weldability and toughness and has not experienced general commercial acceptance. The necessary chemistry will be described in more detail in the subsequent description of the invention. But it should be understood that the steel plate of this invention differs from the steel plates taught by U.S. Pat. No. 3,310,441 not only in application as noted above and also in the particular chemical balance, notably in a comparatively higher carbon content and lower manganese to carbon ratio.
I have recently further discovered that plates produced to about the chemistry taught herein which are normalized by being held at a temperature of about 1550° F. for 1 hour per inch thickness and air-cooled, provide surprisingly good toughness to temperatures down to -50° F. and are superior as high-strength carbon steel plates for use as pressure vessels both in moderate and lower temperature service. Also, it will be appreciated from the subsequent description of the invention, optimum results are produced for this particular type of steel through a somewhat further modified chemical balance.
In view of the foregoing, it is an object of the invention to provide a high-strength carbon steel plate in the as-rolled condition which may be priced significantly lower than steel plates of comparable capabilities having suitable properties for use in structural applications--particularly for fabrication of railroad tank cars.
It is also an object of the instant invention to provide a flange and firebox grade steel having a minimum tensile strength of 81,000 p.s.i.
It is another object of this invention to provide a high-strength carbon steel of such a type which is readily weldable.
It is a further object of this invention to provide carbon-steel plates of the aforementioned improved qualities and thicknesses up to 2 inches for structural steel.
It is a yet further object of this invention to provide a tough, weldable, high-strength carbon steel which is superior for use in pressure vessels exposed to moderate and lower temperatures.
A still further object of the invention relates to the production of the foregoing steel, largely from less expensive scrap steel in a melting furnace by the cold-charging process and in the particular chemistry balance achieved through this means of production.
DESCRIPTION OF PREFERRED EMBODIMENTS
It has been discovered that the unusual properties of carbon steel plate according to the invention can be achieved by carefully balancing the carbon and manganese levels together with the inclusion of high levels of nickel, chromium, molybdenum and copper within what is presently considered the industrial standards of maximums for residual elements through the careful control of scrap added to the furnace producing the steel. The relative amounts of carbon and manganese have an effect on the yield point, tensile strength, ductility, etc. It has been generally found that the strength level is improved as the ratio of manganese to carbon is increased. However, with the instant invention, increased strength levels are obtained with a lowering of the manganese-to-carbon ratio.
The refinement of the grain structure of these steels is further improved as a result of treating the steel with aluminum. However, the addition of aluminum is optional for structural grade steels although it is preferred for flange and firebox grade steels. Still further, improvement results when the steel contains high levels of the elements nickel, chromium, molybdenum and copper, even though these elements are maintained within the industrial standards of maximums for residual elements. This latter factor not only improves essential physical characteristics of the resulting steel plates, but also permits the use of 80 percent or more scrap in the production of carbon steel plate according to the invention. This is an important factor since, at least currently, the market value of scrap is less than one-half that of pig iron.
Steel in accordance with this invention is prepared in a melting furnace of the open-hearth or electric-arc type. Approximately 80 percent or more of the charge is scrap steel with about one-half of same being purchased scrap and the remaining being scrap generated at the mill from trimmings, diversions, etc. It is important that the chemical constituents of such scrap be known as accurately as possible. However, by experience it has been learned that the copper-nickel-chromium-molybdenum contents in purchased scrap fall in a ratio of roughly 7-5-3-1. Thus by careful control of scrap in accordance with one of the chemical constituents--say, copper--the other constituents are also controlled within limits. But it should be needless to say that purchased scrap must be carefully inspected and chemically checked by individuals skilled in this art for segregation into identified areas of the scrap yards. Scrap generated in the mill is similarly identified and segregated. At the present time, with such close inspection and segregation, steel plate having characteristics in accordance with the invention may be produced with substantially less than 5 percent diversions.
It will be appreciated from the foregoing that by controlling the type and amount of scrap, the levels of copper, nickel, chromium and molybdenum contents are similarly controlled. With knowledge of desired amounts of such elements, the object is to employ various grades of scrap in the most economical ratios. Of course, in so doing, the inventory of scrap on hand at the time must be kept in mind. An example of such control is a mixture as follows: ------------------------------------------------------------ --------------- TABLE I
Source Percentages ____________________________________________________________ ______________ Molds 10.3 Casts 8.7 Total Pure Iron 19.0 No. 1 Busheling 8.0 No. 1 Bundles 3.3 No. 2 Heavy Melting 8.4 No. 2 Bundles 12.0 Machine Shop Turnings 9.3 Total Purchased Scrap 41.0 Common Plate 33.3 Pit (Sprues and Runners) 6.7 Total Plant Scrap 40.0 ____________________________________________________________ ______________
The foregoing produced steel which included 0.30 percent copper, 0.19 percent nickel, 0.13 percent chromium and 0.04 percent molybdenum. The cost of a comparable charge in pig iron for the elements of copper, nickel, chromium and molybdenum added is approximately two and one-half times the foregoing.
It has been found that to produce steel plate economically in accordance with the invention, the preferred minimum percentages of copper, nickel, chromium and molybdenum should be 0.11, 0.10, 0.07, 0.02, respectively. On an individual basis, the respective minimum percentages should be about 0.10, 0.08, 0.04 and 0.01, and the sum of the percentages of copper, chromium and molybdenum should exceed 0.20 but generally should not exceed 0.60. The sum of the nickel and chromium content percentages should be at least 0.15 and preferably at least 0.20. The preferred maximum percentages of copper, nickel, chromium and molybdenum are 0.35, 0.15, 0.15 and 0.05, respectively, and the desired percentage ranges for these constituents are about 0.11-0.27, 0.10-0.15, 0.07-0.09 and 0.02-0.03, respectively. However, as will be noted from subsequent tables, one or several of such elements may extend out of these desired ranges.
For the invention herein, the presence of copper follows almost inevitably from economical use of scrap for the charge. But it is not believed that the presence of copper, within the prescribed residual levels, significantly affects as a matter of commercial practicality, the desired characteristics of the steel plate produced. More critical are the nickel and chromium contents which, as indicated above, should preferably total 0.20 percent or more.
Charged into the melting furnace just prior to the tap or into the ladle is standard ferro manganese or other manganese additive whereby the manganese percentage by weight will be 0.70 to 1.60, preferably 0.80 to 1.50 for structural steels, and 0.80 to 1.20 for flange and firebox grade steels. Where plate from the steel is intended for tank cars, the manganese percentage range will be 1.00-1.50, the amount depending upon the thickness of the plate to be produced and being on the lower side for thinner plates, say of 3/8-inch thickness, and on the higher side for thicker plates, say of 3/4 to 1 inch or more in thickness. For the desired quality of steel plate, it has been found that in general the higher the total of copper, nickel, chromium and molybdenum, within the limits of the invention, the less manganese or carbon is required. Ferro silicon or other silicon-containing additive is added at approximately the same time to deoxidize or quiet the molten steel. The amount is somewhat lesser for thinner plates than for thicker plates.
In the cold-charging production of steel, the charges are kept in a heated or molten condition for approximately two to four times as long as with the hot-charge method. During the molten period, the carbon content tends to decrease. As noted above, less carbon and manganese are required for strength in the thinner gauges than in the thicker gauges. For this reason, different ranges of carbon and manganese and percentage by weight are specified for different gauges of structural steel plate approximately as shown in the following table: ##SPC1##
The chromium content also tends to become lower while the steel is molten, and for this reason a slightly high chromium content in the initial charge is not considered deleterious.
The specific chemistry of the resulting carbon steel is as follows: ------------------------------------------------------------ --------------- TABLE III
Element Percentage by Weight ____________________________________________________________ ______________ Carbon 0.14-0.25 Manganese 1.00-1.60 Phosphorus 0.010-0.040 Sulfur 0.010-0.050 Silicon 0.10-0.60 Aluminum 0.00-0.055 Copper 0.01-0.47 Nickel 0.05-0.35 Chromium 0.05-0.25 Molybdenum 0.01-0.15 Iron and incidental impurities Balance ____________________________________________________________ ______________
Of course, for the above steels, it will be understood that the carbon content should preferably be maintained the ranges indicated before, depending upon the gauge of the plates. Manganese range for such steels is preferably held between 0.80 to 1.50 percentage by weight for structural steel.
Structural carbon steel plate having the above-disclosed chemistry and produced by conventional hot-rolled processes has properties heretofore only obtainable in steels containing vanadium, columbium, nitrogen or combinations thereof. These unusual properties are disclosed in Table IV. ##SPC2##
In carbon steels, it is common for purchasers to specify a certain maximum or minimum copper content. For example, maximum copper content of 0.25 or 0.35 percent may be specified, or a 0.20 percent minimum is usual. It is known that the inclusion of copper in amounts from 0.20-0.60 percent increases the resistance of the steel to atmospheric corrosion and slightly increases the yield point and tensile strength, but adversely affects the ductility. In filling such orders with controlled production, as disclosed herein, the range variations of copper, nickel, chromium and molybdenum, as disclosed in Table III, can easily be obtained in a number of these structural steels as specifically disclosed in Table V. ##SPC3##
As-rolled carbon steel plate in thicknesses up to 1 inch having the chemistry of Table III when made according to fine grain melting practice with the inclusion of about 0.02-0.055 percent aluminum, have a minimum tensile and other qualities of flange and firebox grades. Such steels are suitable for boiler shells tank car tanks and other pressure vessels. These flange and firebox grade steels are also nonheat treated. Preferably the manganese content of the flange and firebox grades is about 0.8-1.2 percent and the silicon content is 0.15-0.3 percent. Also, the maximum phosphorus and sulfur content should be about 0.035 and 0.04, respectively.
A 3/4-inch plate was prepared having the following chemistry and qualities: ##SPC4##
The welding properties of the above 3/4-inch plate were determined to be excellent in terms of transverse and longitudinal tensile strengths. Also, no evidence of cracking was observed on a 180° bend made in the as-welded and stress-relieved conditions.
The toughness of this steel was also determined with excellent results. Impact low-temperature V-notch Charpy test results were better than conventional structural steels.
Table VII shows a number of typical melts for the production of plates designed for use in tank cars. Plates of this steel uniformly had tensile strengths between 81,000 and 101,000 p.s.i. yield points above 50,000 p.s.i. and an elongation in 8 inches of at least 18 percent. Bend test specimens of such steels can stand being bent cold through 180° without cracking on the outside of the bent portion through an inside diameter of twice the thickness of the specimen. It will be noted that such steel plates are made to fine grain practice. The carbon content is about 18 percent of the manganese content in the examples and, should preferably be 16-20 percent. It is of primary importance that steel plates for tank car end use be readily weldable through standard welding procedures. From experience it has been found that the plates are, in fact, readily weldable.
It has been found that plates having acceptable minimum yield strengths and tensile points can be manufactured without the high residual levels specified for the steel plates of the invention through the addition of vanadium in amounts exceeding 0.02 percent. However, such plates have not been broadly accepted because they do not present the advantage and weldability of the plates of the invention and the degree of toughness is less. ##SPC5## ##SPC6## ##SPC7## ##SPC8##
For the production of tank-car steel plate, the chemical analysis should be as set forth in Table VIII below:
Element Percentage by Weight ____________________________________________________________ ______________ Carbon 0.15-0.25 Manganese 1.00-1.50 Silicon 0.15-0.50 Aluminum 0.008-0.015 Copper 0.01-0.47 Nickel 0.05-0.35 Chromium 0.05-0.25 Molybdenum 0.01-0.15 Iron balance ____________________________________________________________ ______________
The nickel and chromium contents must, it has been found total at least 0.15 percent and preferably should be at least 0.20 percent. The carbon, manganese and silicon contents are increased as the thickness of the plate increases. Thus, a 3/8-inch thickness plate should have its carbon content in the range of 0.15 to 0.20 percent, its manganese content 1.00-1.20 percent and its silicon content 0.15-0.30 percent For a 7/16-inch thickness plate, the carbon content should be 0.18-0.25 percent, the manganese content should be 1.10-1.35 percent and the silicon content 0.15-0.30 percent. For plates 9/16-inch thick, the carbon content should be 0.20-0.25 percent, the manganese content 1.10-1.35 percent, and the silicon content 0.15-0.30 percent. For thicknesses of 3/4-inch and above, the carbon content should be 0.20-0.25 percent, the manganese content 1.20-1.50 percent, and the silicon content 0.15-0.50 percent. As the thickness is further increased, the manganese carbon and silicon content levels should be preferably in the higher portions of the described ranges. From a commercial standpoint, the most important thicknesses are from 7/16-inch to 1 inch. However, the steel plates can be produced with the desired characteristics up to 2 inches in thickness. In producing the tank car steel plate, the so-called residuals of copper, nickel, chromium, and molybdenum are controlled through careful selection of the scrap charge, as previously described, and the manganese, carbon and silicon are controlled through additions to the melt, the amounts of each depending upon the thickness required of the plates to be produced from the melt. As noted with reference to Table VII, the carbon content is between 16 and 20 percent of the manganese content and preferably about 18 percent. It is to be appreciated that the physical properties of the steel plate are achieved by carefully controlling the chemistry of the plate rather than through any subsequent heat treatment of the plate.
The success achieved with the tank car plate suggested the possibility that a similar chemistry might be utilized to produce unique superior high-strength carbon steel plates for pressure vessels or tank cars in moderate and lower-temperature service. Table IX shows the chemical analysis and physical properties of a number of plates which have been produced to a chemistry very similar to that of the above-described tank car steel which have been heat-treated by normalizing them at temperatures of 1525-1650° F., held 45 minutes for each inch thickness, and air cooled. Plates so produced have tensile strengths consistently 10 to 20 percent ##SPC9## ##SPC10## ##SPC11## ##SPC12## ##SPC13## higher than normalized plates disclosed in my previously mentioned U.S. Pat. No. 3,310,441. In toughness at -50° F. specimens from such plates show consistent impact strengths above 20 foot-pounds with standard Charpy V-notch tests. The preferred chemistry ranges for the normalized plates are as follows: ------------------------------------------------------------ --------------- TABLE VIII
Element Percentage by Weight ____________________________________________________________ ______________ Carbon 0.22-0.25 Manganese 1.15-1.50 Silicon 0.20-0.30 Aluminum 0.020 min. Copper 0.01-0.35 Nickel 0.05-0.25 Chromium 0.05-0.25 Molybdenum 0.01-0.08 ____________________________________________________________ ______________
The nickel and chromium percentage totals should be at least 0.15 and preferably at least 0.25. The carbon content percentage should be preferably 0.23 percent. For plates up to 3/4-inch thickness, the manganese range should preferably be 1.15-1.35, aiming at a percentage of 1.25. For plates over 3/4-inch, the manganese content is preferably 1.25-1.50, aiming at a 1.40 percent. The steel plates are made to fine-grain practice and in this connection, the desired percentage of aluminum is 0.025. The carbon content is preferably about 16 to 17 percent of the manganese content and, in the examples, ranges 15 to 19 percent. Plate produced in accordance with these specifications is intended for fusion welding and by such techniques is readily weldable. In bend tests, specimens of such steel plates may be bent cold through 180° without cracking on the outside of the bent portion to an inside diameter which is twice the thickness of the specimen. If the test is made on a specimen reduced in thickness, the rolled surface is on the outer curve of the bend.
In the foregoing disclosure and in the claims, percentages are expressed by weight unless indicated otherwise.
Plates from the steels disclosed herein are characterized by high strength characteristics normally associated only with alloy steels containing significant amounts of vanadium, columbium, nitrogen, etc. From the values listed above for tensile strengths, yield points and other characteristics, it will be apparent to one skilled in the art that these higher strength steels exceed standard specifications so that considerable savings are realized.