US3692514A - Alloy steel containing copper and nickel adapted for production of line pipe - Google Patents

Abstract

A LOW ALLOY STEEL COMBINING STRENGTH AND TOUGHNESS AND ADAPTED FOR LINE PIPE PRODUCTION. STEEL CONTAINS CARBON, NICKEL, MANAGNESE, COPPER, CHROMIUM, MOLYBDENUM, COLUMBIUM AND USUALLY SILICON, THE BALANCE BEING ESSENTIALLY IRON. CHROMIUM AND MOLYBDENUM ARE CORRELATED SUCH THAT THE TOTAL THEREOF IS AT LEAST ABOUT 0.5%. UNNECESSARY TO LIQUID QUENCH.

Description

United States Patent 3,692,514 ALLOY STEEL CONTAINING COPPER AND NICKEL ADAPTED FOR PRODUCTION OF LINE PIPE Peter Paul Hydrean, Mahwah, N.J., assignor to The International Nickel Company, Inc., New York, N.Y. No Drawing. Continuation-impart of application Ser. No. 721,086, Apr. 12, 1968. This application Dec. 13, 1968, Ser. No. 783,746

Int. Cl. C22c 37/10 US. Cl. 75124 Claims ABSTRACT OF THE DISCLOSURE A low alloy steel combining strength and toughness and adapted for line pipe production. Steel contains carbon, nickel, manganese, copper, chromium, molybdenum, columbium and usually silicon, the balance being essentially iron. Chromium and molybdenum are correlated such that the total thereof is at least about 0.5%. Unnecessary to liquid quench.

This application is a continuation-in-part of application Ser. No. 721,086, filed Apr. 12, 1968, now abandoned.

As is generally known, there are many commercial operations which require structural type steels capable of affording a combination of relatively high yield strength together with good toughness. The more demanding applications necessitate steels which not only exhibit satisfactory toughness characteristics (as determined by the Charpy V-notch, CVN, test) at room temperatures but at sub-zero temperatures as well. It is not at all unusual to find a steel satisfactorily tough at the higher tempera ture but undesirably brittle at a somewhat lower temperature level. Viewed against this background steels capable of consistently delivering a yield strength of at least about 80,000 p.s.i. and possessing the ability to absorb an energy impact of at least about 70 foot-pounds at room temperature, upwards of about 50 foot-pounds at 0 F. and not less than about foot-pounds at minus F. would fulfill the demands imposed by any number of commercial areas of utility.

Now, the above requirements might be met by various steels containing substantial amounts of alloying elements or by some low alloy steels in the quenched and tempered condition. However, steels containing substantial amounts of alloying elements are economically unattractive, particularly where large tonnage is contemplated. With regard to steels of the quenched and tempered type, the indispensable necessity to quench in order to reach a sufficiently high strength plateau too often renders such steels impractical for many applications since quenching, as is well documented, may result in distortion or warping and requires facilities which frequently are not avail able.

An illustrative application requiring a low cost steel characterized by high strength, good toughness, including low temperature toughness, in the air cooled (as opposed to liquid quenched) and aged condition, and which is weldable is, for example, line pipe used in the transmission of natural gas and other products. In this connection and in order to accommodate increased pressures under which natural gas and other products are transmitted, it is not surprising to find industry confronted with the vexing problem of whether to use line pipe formed from a steel of a given (known) composition but of greater wall thickness than previously used or of finding a different but still economical pipe material characterized by a higher yield strength such that small thicknesses can be utilized. Pipe of large wall thickness is undesirable since 3,692,514 Patented Sept. 19, 1972 the weight per unit length increases substantially, thus causing problems in material handling and installation. Higher strength but thinner wall line pipe is considerably more attractive in several respects, particularly in view of less weight per unit length and greater ease of installation.

But to use a steel of higher strength, however, generally envisages a loss in impact toughness, especially at low temperatures. As will become clear herein, for line pipe to be used in the colder geographical locations it is not enough to develop a nonquenched steel of high strength onlya marked ability to absorb a high level of impact energy at temperatures down to about, say, minus 30 F. is a prerequisite. The reason for this is illustrated by the following:

As natural gas leaves a pumping station, it is under pressure, a pressure of about 900 p.s.i. being not uncommon, the temperature being in the neighborhood of about F. Somewhat distant therefrom the pipe mayemerge from the ground and the pressure inside the pipe will have dropped considerably, say, to 650 p.s.i., due to the distance from the station. (Temperature will also have dropped.) However, in the event of a shutdown the exposed pipe, of course, approaches ambient temperature. Conceivably, in the relatively cold regions, temperatures as low as minus 30 F. or lower might be encountered. Upon start-up it is considered that the pipe is particularly susceptible to fracture since it is this point in time during which gas pressure builds up before the pipe has had an opportunity to become heated by the gas. This condition is conducive to failure and thus reflects the need for subzero toughness.

Another consideration in selecting a material for line pipe is mode of fracture at low temperature. A material is more acceptable as line pipe if the mode of fracture is primarily shear (sometimes referred to as fibrous or duetile fracture) rather than cleavage (sometimes referred to as brittle fracture). The velocity of a crack having a fracture mode which is 100% fibrous is only about 500 to 700 feet per second, as compared to about 1500 to 2000 feet per second for a 100% brittle fracture mode. The greater the crack velocity, the longer the size of crack which forms if a pipe ruptures while in service. Should a crack develop, the crack length determines the length of pipe which must be replaced. Thus, the trend in specifying line pipe material is to require a material having as low a crack propagation velocity as possible.

The direct measurement, however, of crack propagation velocity requires elaborate testing procedures; consequently, indirect methods of testing have been developed. One such measure is the temperature at which the fracture surface indicates that the mode of fracture consists of 50% fibrous and 50% brittle fracture. This temperature is hereafter designated the ductile-brittle transition temperature for the material. The lower the ductilebrittle transition temperature, the more desirable is the material for line pipe applications.

It is also noteworthy to further point out that a material for use as line pipe should develop high yield strength and good impact toughness at low temperature after hot rolling at finishing temperatures of not less than about 1550 F. Final rolling temperatures of 1550 F. and above are commercially necessary to avoid mass production problems encountered in steel mills. Hot rolling at temperatures substantially below about 1550 F. results in a lower level of production since the hot rolling mills must be operated at a reduced speed. Moreover, the roll separating forces are greater at lower temperatures and consequently more powerful hot mills may be required especially when wide plates are being produced.

With the foregoing in mind, material primarily intended for line pipe should have a yield strength (as measured by the yield point or 0.2% offset) of at least about 85,000 p.s.i., an impact toughness of at least. about (a) 80 foot-pounds CVN at 70 F., (b) 50 foot-pounds at F., and (c) 20, preferably at least 40, foot-pounds at minus 50 F., a ductile-brittle transition temperature based on 50% fibrous-50% brittle fracture of not greater than about 20 F. to 30 F. when the line pipe is buried and less than about minus 30 F. when the pipe is to be used at or near the earths surface. In addition the steel should be weldable under field conditions and not prone to form porous welds which, for example, result from excess amounts of aluminum (i.e., in excess of about 0.15%), nitrogen (e.g., in excess of about 50 p.p.m.), or precipitated aluminum nitrides (e.g., 0.03% and above). These requirements should be met by steels in the form of pipe in the nonquenched and aged condition as well as in the form of plat in the same condition.

It has now been discovered that the foregoing objectives can be achieved with new low alloy steels containing controlled amounts of selected alloying elements, including carbon, nickel, manganese, copper, chromium, molybdenum and columbium. Recourse to liquid quenching is completely unnecessary since the desired properties are attained in the air cooled and aged condition. Similarly, the need for large wall thicknesses in the production of line pipe is also obviated.

It is an object of this invention to provide a low cost steel having a high yield strength, good toughness at low temperatures as well as at temperatures on the order of 60 F. to 100 F., a low ductile-brittle transition temperature, and which is suitable in a wide variety of applications.

The invention also contemplates providing a weldable steel especially suited for use as line pipe for the transmission of gases, e.g., natural gas, liquids, and other products.

Generally speaking, steels in accordance with the present invention contain (percent by weight) about 0.01% to 0.12% carbon, about 0.6% to 1.3% nickel, about 0.2% to 0.8% manganese, about 0.8% to 1.6% copper, provided that the nickel content is at least about 55% of the copper content, about 0.3% to 1.4% chromium, about 0.1% to 0.9% molybdenum, with the proviso that the sum of the chromium plus molybdenum totals at least about 05%, about 0.1% to 0.65% silicon, about 0.02% to 0.12% columbium, up to about 0.008% boron, up to about 0.15% aluminum, and the balance essentially iron. The use of the expression balance or balance essentially in referring to the iron content of the alloys, as will be understood by those skilled in the art, does not exclude the presence of other elements commonly present as incidental constituents, e.g., deoxidizing and cleansing elements, and impurities normally associated therewith in small amounts which do not adversely affect the basic characteristics of the alloys. In this regard, elements such as oxygen, nitrogen, phosphorus, sulfur, and the like should be kept to a minimum in accordance with good steelmaking practice. For line pipe applications particularly, the nitrogen should be maintained low, i.e., not exceeding 50 parts per million (p.p.m.).

In carrying the invention into practice, the carbon should not be less than about 0.01% in order to maintain adequate strength. Carbon contents above about 0.12% lower impact toughness at low temperatures and are detrimental to weldability. Beneficially, carbon should be maintained below about 0.07%, e.g., not in excess of about 0.04%, to develop the best impact toughness at low temperatures and excellent weldability. The copper content should be at least about 0.8%, and advantageously at least about 1%, e.g., about 1.3%, to contribute to strength by means of precipitation hardening. Copper in amounts in excess of about 1.6% does not impart any further appreciable increase in yield strength for the nickel levels contemplated but can bring about a lowering of the low temperature impact toughness. Ad-

4 vantageously, the copper level should not exceed about 1.4%.

At least about 0.6% nickel, and advantageously at least about 0.8%, e.g., about 0.9%, is necessary to prevent hot-shortness due to the copper. And in this regard the minimum amount of nickel should be at least about 55% of the copper content. Nickel also improves tough ness and provides a significant contribution to strength due to solution strengthening and grain refinement, but amounts above about 1.3% result in an unnecessary increase in cost without a corresponding improvement in properties. A nickel range of 0.8% to about 1% is particularly satisfactory. At least about 0.2% manganese, and advantageously at least about 0.35% should be present to maintain adequate strength. However, manganese in excess of about 0.8% detracts from low temperature toughness. For best results, it should be maintained below about 0.6%.

Molybdenum in amounts of at least about 0.1%, and advantageously in amounts of at least about 0.25%, e.g., about 0.35%, contribute to strength through solid solution strengthening. Above about 0.9%, molybdenum can result in impairment of impact toughness. Where impact toughness is of paramount importance, molybdenum should not exceed about 0.6%. At least 0.3% chromium should be present in the subject steels and beneficially at least about 0.5%, e.g., about 0.7%, in order to significantly contribute to solid solution strengthening. Preferably, it does not exceed 1.2%.

The roles of molybdenum and chromium are deemed somewhat unexpected since the prior art indicates that these constituents individually exert a detrimental infiuence in respect of low temperature impact toughness in otherwise similar steels. Nonetheless, in accordance with the instant invention and provided the molybdenum and chromium contents, as set forth above, are carefully controlled and correlated, an increase in strength through solid solution strengthening is not only obtained but, through mutual interaction in a manner not completely understood, a marked improvement in impact toughness (especially at low temperatures) and a ductile-brittle transition temperature significantly below that heretofore characteristic of commercially used line pipe steels is attained without the necessity of quenching or the necessity of rolling at low finishing temperatures, e.g., as low as 1200 F.

It is believed that through the coaction of chromium and molybdenum, higher aging temperatures can be employed than otherwise might be the case and to advantage. By reason of this, it is thought that the steels of this invention can develop higher impact toughness at low temperatures, lower ductile-brittle transition temperatures, and lower crack propagation velocity after aging at high temperatures, i.e., about 1050 F. to 1150 F., e.g., about 1125 F. Temperatures up to the Ac, temperature of the steels (circa 1340 F.) can be employed and, in this connection, exceptional results have been attained upon aging at temperatures, for example, of 1225 F. By way of explanation and generally speaking, aging at about 1050 F. for approximately one hour is very near the peak aging condition (in achieving the highest strength levels). Aging at 1050 F. for longer periods of time, e.g., 3 hours or more, or at the higher temperatures is, in effect, an overaging treatment. However, no loss in strength of any consequence is brought on by overaging, but enhanced toughness obtains. Thus, in accordance with the invention, it is highly desirable that the steels be overaged. Moreover, as indicated previously, the sum of the chromium and molybdenum con tents should total at least about 0.5%, and in consistently achieving a highly desirable combination of properties the total should not be less than about 0.6%.

A possible explanation as to the role played by chromium and molybdenum resides in their ostensible role of suppressing the auto-aging reaction attributable to the For the purpose of giving those skilled in the art a copper during cooling from the hot rolling operation. better appreciation of the advantages of the invention, Various data suggest this to be the case since in the there is given herein data illustrative of the markedly presence of chromium and molybdenum the unaged yield improved combination of properties characteristic of the strength is lower than that obtained in their absence. Upsteels of this invention. In Table I there is presented a on subsequent aging the yield strength is increased to series of steel compositions in weight percent, Steels 1 values as high as 92,000 p.s.i. and above. This represents through 7 being within the invention whereas A through a very substantial increase, particularly when the relatively D are outside the range thereof but which are included high aging temperatures are taken into consideration. Put for comparative purposes.

TABLE I Percent C Mn 81 N1 Cr M0 Al Cb Cu B Fe 0.41 0.28 0.88 0.07 0.37 0.04 0.07 1.30 Balance.

another way, chromium and molybdenum are thought Steels 1 to 7, A, B and C were air melted and then to cause copper to remain in solution during cooling from cast into ingots. After soaking for five hours, the ingots hot rolling and thus a greater amount of copper is availwere forged at about 2150 F. to 2250 F. to 2-inch secable to come out of solution upon aging. This would tend tions, reheated to 2250 F. and then rolled to l-inch thick to minimize the loss in strength that might otherwise be plate. The plates were then air cooled to 1550 F., rolled expected to occur. to /2-inch thick (Alloy 7 was %-inch in thickness) and Columbium should be present in amounts of at least thereafter air cooled. Steels l to 7, A and B were aged at about 0.02%, advantageously in amounts of at least about 1125 F. (Alloy 7 was also aged at 1225 F.) for about 0.03%, e.g., about 0.07%, thereby contributing about 3 hours while Steel C was stress relieved at about to strength by suppressing the austenite to ferrite trans- 1000 F. Steel D is a prior art steel which was rolled from formation and promoting finer grain size. It should not l /s-inch square bar to A-inch thick strip at 1750 F., exceed about 0.12% to avoid impairment of impact then rolled at 1550" F. to Az-inch thick by 1 /2-inch wide toughness, particularly at lower temperatures. Moreover, strip followed by aging at 1050 F. for 4 hours.

when higher amounts of columbium are copresent with The data given in Table II were obtained from tests excess manganese, subzero impact toughness is substanconducted on the steels set forth in Table I (average tially adversely affected. A columbium content of 0.03% values being reported where more than one determination to 0.1% is most beneficial in promoting strength without made) the yield point (or yield strength) and ultimate substantially decreasing impact toughtness. tensile strength being reported in thousands of pounds per Boron can be present in amounts up to about 0.008% square inch (K s.i.), the tensile elongation (EL), taken on and contributes to the strength of the steel. In this rea one inch gage length, and reduction of area (R..A.), gard, boron contents of about 0.0005 to 0.005%, e.g., being given in percent and the impact toughness beabout 0.002%, are beneficial. Aluminum, if present, ing set forth in foot-pounds (ft.-lbs.). Where it was not should not exceed about 0.15%, and advantageously 4 possible to determine the yield point of the steel, the yield should be maintained below about 0.08%. Nitrogen, in strength was determined by the 0.2% offset method. The the production of line pipe steels should not exceed about transition temperature was taken as the temperature at p.p.m. to avoid weld porosity, particularly when weldwhich the Charpy V-notch specimen displayed a 50% ing with a covered electrode, due to the formation of fibrous-50% brittle fracture appearance.

TABLE II Percent CVN (ft-lbs.) Transition U.I.S. temp. (K at.) El RA. F. 0 F. 50 F. F.)

90.1 25.5 77.5 140 95 40 90.7 25.5 72.0 90 00 38 90.7 20.0 77.0 N.d. N.d. N.d. N (1 103.5 25.0 75.0 100 95 80 N d 1 Alloy 7 aged at 1.125 F. 2 Alloy 7 aged at 1,225 F. NorE.N.d.=Not determined. aluminum nitrides. Moreover, excess nitrogen can cause The data shown in Table II indicate that the steels of strain aging in the steel since the manufacture of line this invention display an excellent combination of strength, pipe requires internal expansion of the pipe to increase ductility and toughness, coupled with a low ductile-brittle strength and assure circumferential uniformity, i.e. roundfracture transition temperature. For example, Steel 1, a

ness. Silicon in amounts up to about 0.65% promotes 70 steel contemplated by this invention, in the air cooled and strength. For best results, however, the silicon content is aged condition has a high yield strength 87.4 K s.i.), ex-

kept below about 0.45%, a range of 0.2% to 0.45%, cellent impact toughness even at temperatures as low as e.g., about 0.28% being exceptionally satisfactory in conminus 50 F. (95 foot-pounds) and lower, and a ductiletributing to the best combination of strength and toughbrittle transition temperature of minus 40 F. which is ness. 75 considerably below normal operating temperatures for line pipe thus qualifying this steel as an excellent line pipe material. Particular mention should be directed to Alloy 7B which was overaged at the temperature of 1225 F. An excellent set properties were obtained including a remarkably low transition temperature of about minus 128 F. Steel 6, also Within the scope of this invention, in the air cooled and aged condition has a high yield strength 103.0 K s.i.) and good impact toughness even at temperatures as low as minus 50 F. (33 foot-pounds) thus making this steel suitable in a wide variety of structural applications although its relatively high transition temperature (60 F.) makes it less desirable than other steels within this invention for line pipe applications, e.g., Steel 1. It might be further added that aging times of less than 3 hours, e.g., 1 hour, have been found quite suitable, particularly over the temperature range 1200 F. to 1280" F.

Steel A, which is outside the compositional scope of this invention because it does not contain chromium and molybdenum, offers a reasonably high yield point along with a high room temperature toughness but is characterized by a low temperature toughness of only 6 ft.-lbs. at minus 50 F. and a relatively high ductile-brittle transition temperature of 28 F. Steel B, also devoid of chromium and molybdenum, has an acceptable ductile-brittle transition temperature, but has an unsatisfactory low yield strength. The sum of the molybdenum of chromium contents of Steel C is less than 0.5 and there is not enough copper to develop sutficient strength. Steel D is noted by its poor impact toughness at low temperature, a result reflecting that the steel is chromium-free and the nickel content is too low.

The steels of the present invention are weldable under field conditions and are suitable for use in a wide variety of applications such, for example, as truck frames, railroad cars, tubular products, chains, structural components including l-beams, channel irons, angle irons, rolled form shapes, etc. In particular, these steels are adaptable for use as line pipe in the transmission of natural gas and other products. In addition, the steels of this invention develop high strength and room temperature toughness in the quenched and aged condition (e.g., Steel 1 has a yield strength of about 107,300 psi. and a room temperature toughness of about 85 foot-pounds CVN). The point of emphasis, however, is that liquid quenching, while not excluded from the invention, is not a necessity. In the quenched and aged condition these steels are suitable for a variety of structural applications such, for example, as truck frames, tubular products, chains, structural components including l-beams, channel irons, angle irons, rolled form shapes, etc.

While the specific steels described herein were prepared and tested in the killed condition, steels in the rimmed, semi-killed and capped conditions are also contemplated. In this connection, the silicon content of the steels would be lowered to that consistent with commercial practice, e.g., about 0.01% to 0.02% silicon and up to an amount short of killing the steel, e.g., 0.1% or 0.12%, due consideration, of course, being given to processing and the presence or absence of other strong deoxidizing agents, notably aluminum. It also should be mentoined that any given range of an alloying constituent contemplated herein can be used in conjunction with any other given range for any other constituent.

The steels of this invention can be prepared by conventional methods. These include the basic and acid open hearth process, the basic and acid converter process oxygen blown, the basic and acid converter process air blown, duplex process, basic and acid electric furnace process, basic and acid induction melting process, etc. When the steel is employed as line pipe, it would be necessary to maintain the nitrogen at levels less than 50 p.p.m. and in this regard the best of the foregoing steelmaking processes that can achieve this are: the basic and acid open hearth,

the basic and acid converter process oxygen blown, duplex process oxygen blown, basic and acid electric furnace process, and the basic and acid induction melting process.

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.

I claim:

1. A strong tough steel adapted for use in the fabrication of line pipe and consisting essentially of about 0.01% to 0.12% carbon, about 0.6% to 1.3% nickel, about 0.2% to 0.8% manganese, about 0.8% to 1.6% copper, the percentages of copper and nickel being correlated such that the nickel content is at least about 55% of the copper content, about 0.3% to 1.4% chromium, about 0.1% to 0.9% molybdenum, the sum of the chromium and molybdenum being at least 0.5%, up to about 0.65% silicon, about 0.02% to 0.12% columbiurn, up to about 0.008% boron, up to about 0.15% aluminum and the balance essentially iron.

2. A steel in accordance with claim 1 in which the sum of the chromium and molybdenum is at least 0.6%.

3. As a new article of manufacture, a steel line pipe fabricated from an alloy steel in accordance with claim 1.

4. A steel in accordance with claim 1 having a yield strength of at least about 85,000 p.s.i., a room temperature toughness of at least about 80 foot-pounds CVN, a low temperature toughness of at least about 50 foot-pounds CVN at 0 F. and at least about 20 foot-pounds CVN at minus 50 F. and a ductile-brittle transition temperature based upon 50% fibrous-50% brittle fracture of not more than about 10 F.

5. A steel in accordance with claim 1 containing about 0.01% to 0.07% carbon, about 0.8% to 1.2% nickel, about 0.35% to 0.6% manganese, about 0.25% to 0.6% molybdenum, about 1% to 1.4% copper, about 0.5% to 1.2% chromium, about 0.2% to 0.45% silicon and about 0.03% to 0.1% columbium.

6. A steel in accordance with claim 1 containing about 0.01% to 0.04% carbon, about 0.8% to 1% nickel, about 0.35% to 0.6% manganese, about 0.35% to 0.6% molybdenum, about 1% to 1.3% copper, about 0.5% to 1.2% chromium, about 0.2% to 0.45 silicon, and about 0.03 to 0.1% columbium.

'7. A steel in accordance with claim 1 which contains about 0.03% carbon, about 0.9% nickel, about 0.45% manganese, about 0.35% molybdenum, about 1.3% copper, about 0.7% chromium, about 0.28% silicon, and about 0.07% columbium.

8. A steel in accordance with claim 1 in the air cooled quenched and aged condition.

9. A steel in accordance with claim 1 in the averaged condition.

10. A steel in accordance with claim 5 in the air cooled quenched and overaged condition.

1 References Cited UNITED STATES PATENTS 2,150,342 3/1939 Saklatwalla -125 3,110,586 11/1963 Gulya 75124 3,288,600 11/1966 Johnsen 75l25 3,303,061 2/ 1967 Wilson 75124 3,110,798 11/ 1963 Keay 75--125 3,328,211 6/ 1967 Nakamura 75l25 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75125, 128 T 353 UNITED STA ES PATE NT- OFFHQE QERTIFICATE 0F QORREQ'HON Patent NO 3,692,514 t ed Senjember 1077 hwentorm PETER PAUL HYDREAN 2' appears in the above-identified patenfi It is certified that erro y corrected as shown below:

and that said Letters Patent are hereb Column 3, line 17, for "p1at read "plate";

Table II, for Y.P. (Kg.s.i)" read "Y.P. (K.s.i.);

Column 7, line 4, after "set" insert "of";

line 26, de 2nd occurrence and insert and Claim 8, line 2, delete quenchedF';

Claim 10, line 2 delete "quenched" Signed end sealed this 22nd day of Jahuary l97 L.

(SEAL) Attest:

EDWARD M. FLETCHER JR RENE D TEG'I'MEY I I o ER Attestlng Offlcer Acting Commissioner of Patents