Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment

Definitions

• ABSTRACT A ferritic hot-rolled high-strength low-alloy steel having a yield strength in excess of 65 ksi and excellent subzero impact properties which contains 0.03 to 0.15% carbon, 05 to 2.0% magnanese, 0.1 to 0.40%
• the steel is strengthened by the combined effects of grain refinement, precipitation hardening and a high dislocation density which are effected by partially hot rolling the steel above the Ar transition temperature and then hot rolling to effect a 10 to 40% thickness reduction at intercritical temperatures between the Ar and Ar transition temperatures.
• Some embodiments of this steel may be further strengthened by a subcritical temper, or a small amount of cold working.
• This invention is predicated on my development of a new improved hot-rolled, high-strength, low-alloy steel having yield strengths of 65 to 100 ksi in combination with subzero Charpy V-notch (CVN), 50 percent shear fracture appearance transition temperatures (FATT), which are developed by combining the traditional precipitation-strengthening and grain-refining mechanisms with dislocation strengthening.
• CVN subzero Charpy V-notch
• FATT percent shear fracture appearance transition temperatures
• Another object of this invention is to provide a new and improved hot-rolled, high-strength, low-alloy steel having a yield strength of 65 to 100 ksi and exceptional toughness at subzero temperatures characterized by 50 percent shear FATT values at temperatures as low as 80F.
• a further object of this invention is to provide a lowcarbon, low-alloy steel that can be processed by a special hot working technique to provide a combination of high-strength and good toughness in the as hot-worked condition, and which may be tempered or cold worked in some instances to produce further strengthening without attendant severe reductions in ductility or impact notch toughness.
• Another object of this invention is to provide a highstrength, low-alloy steel containing manganese. molybdenum and columbium which can be processed to provide an exceptional combination of strength and toughness through the combined mechanisms of precipitation strengthening, grain refining and dislocation strengthening.
• Still another object of this invention is to provide a process for producing a hot-rolled, high-strength, lowalloy steel having an exceptional combination of strength and toughness which is achieved by controlling alloy composition and controlled rolling practices.
• Yet another object of this invention is to provide a process for producing a hot-rolled, high-strength, lowalloy steel utilizing a controlled rolling practice that combines the traditional precipitation-strengthening and grain-refining mechanisms with dislocationstrengthening that is introduced by continuing the hotrolling operation below the upper critical (Ar temperature, and which may further involve tempering to improve yield strength without severe reductions inductility or toughness.
• Still a further object of this invention is to provide a new and improved steel line-pipe having a yield strength of 65-100 ksi and exceptional toughness at subzero temperatures which is therefore ideally suited for Arctic applications.
• Another object of this invention is to provide a lowcarbon, low-alloy steel plate which can be fabricated into line-pipe using the conventional U and 0 process which will exhibit an increase in yield strength after such fabrication.
• this invention concerns a new and improved low-carbon, low-alloy steel which is processed by a special hot working technique to provide a combination of high strength and toughness, which results from a combination of precipitationstrengthening, grain-refining and dislocationstrengthening.
• the alloy of this invention has the following composition by weight:
• the steel to be hot rolled is heated to a temperature sufficient to dissolve all carbides and nitrides into an austenite matrix.
• this will require heating the steel to a temperature above 2000F.
• hot rolling of the steel is com menced either at the maximum heating temperature above 2000F or at any other temperature reasonably above the Ar; austenite-to-ferrite transformation temperature which is sufficient to retain the carbides and nitrides in solution.
• the crux of this inventive process resides in not completing the hot rolling at these temperatures, but rather completing no more than about 90% of the intended hot reduction above the Ar transformation temperature.
• the partially hot rolled steel is allowed to cool to a temperature below the Ar transformation temperature, but above the Ar transformation temperature so that a portion but not all of the'austenite is transformed to ferrite.
• the final hot rolling is performed on the partially transformed metal to effect at least thickness reduction but not enough to cause any recrystallization and/or grain growth of the ferrite grains, i.e., usually not more than about a 40% thickness reduction.
• this intercritical final deformation should provide a thickness reduction within the range to 30%. Thickness reductions of less than 10% will not usually uniformly strain the metal throughout its cross-sections and, therefore, the deformed ferrite grains and the strengthening effect caused thereby will not be uniformly distributed throughout.
• the upper limit of 40% thickness reductions is somewhat arbitrary, with the actual limit being dependent upon strength of the rolling mill and the ability of the steel to withstand deformation without recrystallization. My experience has shown however that a maximum limit of about 40% thickness reductions is reasonable from practical and metallurgical view points.
• the strengthening effect caused by the deformed ferrite will be a function of the amount of such ferrite present in the steel. Therefore, it is preferred that the steel be cooled to a temperature at least about F below the Ar transition temperature for the intercritical hot rolling, in order to assure a significant proportion of ferrite in the steel. The lower the temperature the steel is cooled below the Ar transition temperature, the greater the amount of ferrite formed, and therefore, the higher the resulting yield strength will be.
• the steel Upon completion of the intercritical hot rolling between the Ar and Ar transition temperatures, the steel is allowed to cool to ambient temperatures where the microstructure is characterized by the presence of both equiaxed and cold-worked grains of ferrite in a proportion depending upon the extent of austenite-toferrite transformation prior to the final deformation at temperatures below the Ar temperature, plus a small amount of pearlite and/or bainite.
• the equiaxed grains of course result from the transformation of austenite after the rolling is completed and the steel cooled.
• the cold-worked ferrite grains are elongated in appearance and contain a high dislocation density, and carbide and nitride precipitates appear uniformly distributed within both types of ferrite grains.
• the steel will exhibit yield strengths of at least 65 ksi, and Charpy V-notch 15 mil lateral expansion transition temperatures (LETT) of from to l50F, and 50% shear fracture appearance transition temperatures (FATT) of from 45 to 90F.
• TERT Charpy V-notch 15 mil lateral expansion transition temperatures
• FATT 50% shear fracture appearance transition temperatures
• the carbide and nitride forming elements molybdemum and columbium are of course essential ingredients for conventional precipitation strengthening.
• the carbide and nitride precipitate particles must be formed during hot rolling and/or during transformation so that the dislocations and subgrain boundaries in the deformed ferrite grain structure can be pinned and stabilized thereby preventing recrystallization and grain growth to retain the strengthening effect of the dislocations.
• the mere presence of the carbide and nitride formers alone is not enough however to prevent recrystallization and grain growth.
• the carbon and manganese contents, and to some extent the amounts of the carbide and nitride formers must be critically controlled in order to con-' trol the austenite-to-ferrite transformation temperature, Ar so it is not so low as to make processing difficult at temperatures therebelow for the final hot rolling step, or not so high as to cause the deformed ferrite grains to recrystallize and grow in spite of the carbide and nitride precipitates.
• the composition limits must be adjusted to provide an Ar 3 transition temperature within the range l350 to 1500F, and preferably between l400 and l475F.
• vanadium Inorder to optimize both strength and toughness in this steel the use of vanadium should be avoided. However, in applications where toughness is not critical, up to 0.20% vanadium can be added to the steel to ordinarily increase yield strength by up to ksi for an addition of 0.08%, or up to ksi for an addition of 0.20% vanadium. It should be understood however that while vanadium will increase yield strength, it will impair the impact transition temperature consistent with known effects of precipitation strengthening.
• a further unexpected feature of this invention steel is that tempering the steel, subsequent to the hotrolling, will in some cases increase the yield strength by about 10 ksi with no or very little impairment of the impact properties.
• Those skilled in the art would normally think that the relatively high Ar transition temperature, as characteristic of these steels, would cause complete precipitation strengthening of the steel in the ashot-worked condition, and that no secondary hardening response would be expected.
• secondary hardening of such magnitude in other steels is normally accompanied by a 50 to 70F increase (impairment) in impact-transition temperature. This is not observed however in the steel of this invention.
• tempering response of the steels can be very rapid, but over-aging is slow so that a wide variety of tempering time and temperature, times from one minute to two hours and temperatures within a range of about 1 100 to 1250F can be used to attain uniform mechanical properties without risk of over-aging. Although this mechanism is not completely understood I believe this secondary hardening results not from precipitation strengthening but from relief of associated residual microstresses.
• this unusual tempering response is noted only in some cases. Specifically, it can be realized only with those alloys wherein the manganese and- /or molybdenum are on the higher side of the recited range. Although these limits are not well defined, the tempering improvement will not be effective on alloys having less than 1.20% manganese and less than 0.20% molybdenum, and will be effective if manganese contents exceed about 1.30% and/or molybdenum contents exceed about 0.25%. In the area therebetween, further strengthening with a tempering treatment may or may not be effective depending upon the combined amounts of manganese and molybdenum.
• an optimum composition for the steel of this invention to be used in the as-hot-rolled condition is 0.05 to 0.10% carbon, 1.0 to 1.3% manganese, 0.15 to 0.25% molybdenum and 0.02 to 0.05% columbium. If this composition is hot rolled at a temperature above 1500F, cooled to about l400F and then hot rolled again at this intercritical temperature to effect a thickness reduction of 20 to 30%, yield strengths of about ksi can be achieved in combination with excellent notch toughness, i.e. with FATT values of about F.
• an optimum composition of the steel of this invention that may be strengthened by tempering would differ from the above composition in requiring 1.3 to 1.6% manganese and 0.20 to 0.40% molybdenum. If this latter composition is hot rolled at temperatures above 1500F, cooled to about 1400F and then hot rolled at this intercritical temperature to effect a thickness reduction of 20 to 30%, yield strengths of about 70 ksi can be achieved in the as-hot-rolled condition, and about 80 ksi in the tempered condition, in either case, combined with excellent notch toughness, i.e. FATT values of about 80F.
• steel plates of the present invention when made into pipe using the U and process may exhibit an increase in yield strength in the unexpanded condition, and may exhibit pronounced further strengthening when expanded.
• the final pipe strength is essentially equal to or considerably greater than that of the original plate.
• those steels which exhibit the largest increases in yield strength are those rolled steels containing the higher manganese and/or molybdenum contents, i.e. those responsive to tempering, and the increase in yield strengths achieved by pipe forming correspond closely with yield strength increases achieved by tempering. It appears therefore that the two strengthening mechanisms are closely related, and I believe that both are due to elimination of residual stresses in the as-hot-rolled plates.
• plates which have compositions which would not normally show an increase in yield strength if tempered, and plates which have been tempered, will, after fabrication into pipe by the U and 0 process and expanded, exhibit yield strengths that remain substantially constant or increase only slightly.
• those steel plates having compositions which are responsive to tempering are fabricated into line-pipe by the U and 0 process and expanded will exhibit an increase in yield strength thereby eliminating any need for tempering the steel before or after pipe fabrication.
• EXAMPLES In one test, a number of steels were made as 100- pound air-induction-melted heats and cast as slab ingots. These slabs were heated to about 2250 to 2300F, and then hot-rolled in fourteen reduction passes to %-inch thick plates with the first pass at about 2200F, the last pass either at I540F or at 1400F, and the remaining passes distributed more or less uniformly arrest) produced by the heat of transformation.
• Table I gives the chemical compositions and Table ll the mechanical properties of these steels. Some of the 10 steels were finish-rolled at 1540F which is above their upper critical-transformation temperature, and the others were finish-rolled at 1400F, which for all of these steels is below their upper-critical temperature.
• microstructures of the steels showed the character istic veined and deformed ferrite grains with the volume fraction of this deformed ferrite varying with alloy content in a manner consistent with the effects of carbon, manganese, molybdenum, columbium, and vana- 2O dium on changing the upper-critical transformation temperature.
• compositions arc in weight percent.
• Table III below exemplifies the increase in yield ture; strength which can be achieved by forming steel plates 0. hot rolling said heated slab at a temperature above of this invention into line-pipe using the conventional the Ar transition temperature sufficient to effect U and 0 process and expanding.
• Steels 1 and 2 are con- 20 no more than 90% of the intended hot reduction; ventional prior art line-pipe steels, while steels 3-12 are d. cooling the partially hot-rolled slab to a temperasteels according to this invention. The steels were fabriture 'below the Ar;, transition temperature but cated into 30, 36 or 42 inch diameter line-pipe.
• Table lll Determined by strip tension specimens. All steels were Si-Al-killed. Steel 2 also contained 018% Cu, 018% Cr and 0.08% Ni.
• the steel slab formed further contains small additions of strengthening elements selected from the group consisting of nickel, copper and chromium which are added as strengtheners and to impart corrosion resistance.
• a ferritic hot-rolled high-strength low-alloy steel according to claim 10 consisting essentially of the following composition by weight:
• a ferritic hot-rolled high-strength low-alloy steel according to claim consisting essentially of the following composition by weight:
• the Ar transition temperature sufficient to effect no more than 90% of the intended hot reduction

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• 1973
  • 1973-05-31 US US365777A patent/US3860456A/en not_active Expired - Lifetime
• 1974
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