Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D1/00—Straightening, restoring form or removing local distortions of sheet metal or specific articles made therefrom; Stretching sheet metal combined with rolling
  • B21D1/05—Stretching combined with rolling

Definitions

• ABSTRACT A high-strength, low alloy steel containing about 0.01 to about 0.1 percent carbon, about 1.5 to about 2.5 percent manganese, about 0.1 to about 0.5 percent molybdenum, about 0.05 to about 0.2 percent niobium, and the balance iron, as the essential alloying constituents along with the usual impurities in conventional amounts.
• the low alloy steel is further characterized as having a predominantly acicular-ferrite microstructure contributing toward its excellent combination of strength and impact resistance in the hot rolled and hot rolled plus aged conditions.
• novel low alloy steel comprising the present invention which is of a controlled chemical composition, enabling the attainment of an optimum combination of physical properties and which can be manufactured in the form of plate and strip stock employing conventional mass production steel mill facilities.
• the resultant hot-rolled plate and strip is characterized as having a predominantly acicular-ferrite micro-structure as distinguished from polygonal ferrite microstructures characteristic of prior art low alloy structural steels.
• the benefits and advantages of the present invention are based on the discovery that by carefully controlling the amount of carbon, manganese, molybdenum and niobium, as the essential alloying constituents, a low alloy steel is provided which is readily adaptable to hot rolling for forming plate and coiled strip stock which is characterized by a microstructure having niobium carbonitride in the form of extremely fine-sized particles distributed through a predominantly acicular-ferrite matrix.
• the chemistry of the low alloy steel comprising the present invention is controlled so as to provide a carbon content of from about 0.01 to about 0.1 percent, manganese from about 1.5 to about 2.5 percent, molybdenum from about 0.1 to about 0.5 percent, niobium from about 0.05 to about 0.2 percent, silicon up to about 0.6 percent, sulfur .up to 0.04 percent maximum, phosphorous up to 0.04 percent maximum, nitrogen up to 0.015 percent maximum, zirconium present in stoichiometric proportions for all nitrogen present in excess of about 0.008 percent to'form the corresponding zirconium nitride, withthe balance consisting essentially of iron along with conventional impurities present inamounts which do not significantly affect the physical properties and microstructure of the steel alloy.
• the low alloy steel comprising the present discovery is particularly suitable for forming hot-rolled plate stock usually ranging from about inch to about one inch thick, it is also suitable for making steel strip of a thickness usually of i i-inch or less.
• the slab preliminary to hot rolling, is heated to a temperature sufficient to effect a solid solution of the niobium in the austenite.
• This temperature in accordance with the compositions usable in the practice of the present invention, conventionally ranges from about 2250 F. to about 2350 F.
• the finishing temperature for rolling the plates is not critical but in the production of coil strip, it is important that the coiling temperature, that is, the temperature of the steel strip as it enters the coiler at the end of the runout table, should not exceed about 1150 F. to about 1 F. due to the adverse effect of higher temperatures on the attainment of the appropriate acicular-ferrite microstructure.
• the combination of optimum physical properties of the low alloy steel comprising the present invention in the form of hot-rolled plate and coil strip stock is attained employing controlled proportions of carbon, manganese, molybdenum and niobium as the essential alloying constituents which are present in amounts expressed in terms of percentages by weight.
• the carbon content of the alloy may broadly range from about 0.01 to about 0.1 percent, and preferably is controlled within a range of from about 0.02 up to about 0.07 percent.
• Amounts of carbon in excess of about 0.1 percent are undesirable because of the formation of excessive amounts of a brittle martensite phase in the final rolled steel product which adversely affects the toughness and formability properties of the alloy, whereas amounts less than about 0.01 percent are economically impractical to attain and detract from the formation of sufficient amounts of precipitated niobium carbonitride in the final rolled stock.
• the quantity of manganese is controlled within a range of about 1.5 to about 2.5 percent in order to suppress the formation of polygonal ferrite during cooling of the hot-rolled plate.
• the presence of manganese also inhibits the premature precipitation of niobium carbonitride in the austenite prior to and during hot rolling of the slab or ingot.
• the manganese content is preferably maintained within a range of about 1.8 to about 2.2 percent; whereas in the manufacture of coiled strip, the manganese content is preferably controlled within the lower end of the permissible range,
• niobium in the specific amounts indicated has a grain refining effect on the austenite during hot-rolling operations.
• the niobium is controlled within a range of about 0.06 to about 0.1 percent.
• the foregoing alloying constituents employed within the amounts indicated in combination with iron, along with conventional impurities in usual amounts, provides a low alloy steel which is predominantly of an acicular-ferrite microstructure avoiding the formation of excessive amounts of polygonal ferrite and further avoiding a retension of prior austenite grain boundaries when fabricated into hot-rolled plate up to one inch in thickness employing conventional air cooling practices.
• the acicular-ferrite substructure of the alloy is believed still further strengthened by a partial precipitation of niobium c'arbonitride during the cooling of the hotrolled stock.
• Still further improvement in strength can be attained without any appreciable loss in toughness by effecting an additional precipitation of niobium carbonitride either by reducing the cooling rateafter the transformation as in the case of coiled strip production or,alternat ively, by stress relieving the. rolled plates by reheating in the case of plate stock produced on a conventional plate mill.
• the chemistry of the low alloy steel comprising the present invention enables a melting thereof by conventional open hearth, electric or basic oxygen processes.
• the melting and/or handling of the alloy is controlled so as to "maintain the net nitrogen content thereof at a level less than about 0.015 percent'and preferably at a level of 0.007 percent.
• the nitrogen content of the alloy is present in an amount above about 0.008 percent, it is control the nitride form in the austenite.
• alloy steel comprising the, present invention may further include up to about 0.08 percent aluminum to achieve good-deoxidation in accordancewith conventional steel making practice, while amounts ranging from about 0.02 to about 0.05 percent are usually preferred. Sulfur and phosphorous are not desired and should be maintained atlevels as low as commercially feasible; generally below 0.04 and preferably below 0.03 percent maximum. Silicon may also be present as an optional constituent in amounts up to about 0.35 percent, and preferably iskept as low as economically feasible.
• the niobium is in solid solution in the austenite at the initiation of the hot-rolling operation, which requires ingot or slab reheat temperatures usually of about 2250 F. to about 2350" F.
• the reheat temperature of the slab is preferably controlled ata minimum above that level at which the 4s
• the-low niobium is present in a solid solution in the austenite since further heating to higher temperatures promotes grain growth in the slab.
• the temperature atwhich the finishing operation is performed on the hot-rolled plates is not critical.
• the hot rolling of the preheated ingot or slab to a coiled strip is performed under controlled cooling conditions to avoid any appreciable formation of polygonal ferrite in the final product.
• cooling rates corresponding to air cooling rates conventionally encountered in the fabrication of hot-rolled plate can be satisfactorily employed. Such air cooling rates are in the order of about 3 F. per second as measured on a 2&- inch thick steel plate at a temperature of 1300 F.
• the finishing temperature is important to the extent that it should be sufficiently low such that the coiling temperature should not exceed about 1150" F. to about 1175" F. due to an adverse effect on the mechanical properties and microstructure of the resultant strip.
• each steel sample consisted essentially of iron with trace amounts of other impurities.
• the sample steels were prepared in laboratory quantities and processed in a manner simulating typical commercial production techniques.
• steel samples 1-4, inclusive have been generally categorized as compositions typical of those having a low niobium content; whereas steel samplesS-IZ, inclusive, have been categorized as being typical of compositions having a high niobium content.
• Tensile test data including yield strength (Y.S. ultimate tensile strength (U.T.S.), percent elongation'in one inch Elong.) and percent reduction of area Red), and Charpy V-notch impact test data are set forth in Tables 2A and 2B on test specimens prepared Table 3 provides a comparison of tensile test and impact data of sample 6 steel as a function of plate thickness.
• the plate was finish-rolled at 1600 F. and air cooled, and'the test specimens were oriented in a direction parallel to the rolling direction.
• the tensile specimens from the 0.375-inch plates were of a diameter of 0.188 inch, and all the others were 0.250-inch in diameter.
• the specimens for Charpy impact evaluation from the 0.375-inch plates were of a width of 0.295-inch, and all the others were 0.394-inch wide.
• Table 4 provides tensile test data and impact data on several of the steel samples processed in a manner to simulate commercial production of coiled strip having a nominal thickness of /4-inch in which the coiling temperature was about 1150 F. Finish rolling of the strip was performed at l600 F.
• the thermal efi'ect of coiling TABLE 2A Tensile test data Cliarpy V-noteh impact data Steel 0.2% ofiset U.”I.S. Percent Percent Ft.-lb- Ft.lb. 201t.-lb. 50% shear number Y.S. (Ks.i.) (K s.i.) elong. red. at 75 F. at -50 F. temp. (F.) fracture (F.)
• the alloy steel article as defined in claim 1 consisting essentially of about 0.02 to about 0.07 percent carbon, about 1.8 to about 2.2 percent manganese, about 0.18 to about 0.4 percent molybdenum, about 0.06 to I I TABLE r
• Tensile test data Half-size eharpy V-notch data 0.2% i ofiset; Fh-lb. Ft.-lb. Ft.-lb. fin-lb; 50% shear Steel Y.S. U.T.S. Percent at at at temp. fracture number (K s.i.) (K s.i.) elong. 75 F. 0 F. 50 F. (F.) (R) 70. 8 84.
• specimens of steel strip derived from samples 1 and 5 were observed to exhibit excellent bendability, enabling transverse specimens to be bent through an angularity of 180 around a ⁇ 4-inch diameter mandrel and thereafter further flattened without evidencing any cracking or fracture thereof.
• Specimens of steel samples 1 and 5 where also welded utilizing a manuai arc welding technique which formed a sound weld of sufiicient ductility to enablebending thereof through an angularity-of 90 degrees without fracture. A traverse taken across a polished section of the weld to determine the microhardness thereof evidenced the absence of any hard or soft zones adjacent to the weld line.
• a hot rolled alloy steel article consisting essentially of about 0.01 to about 0.1 percent carbon, greater than 1.5 up to about 2.5 percent manganese,
• said steel further characterized as having a microstructure comprised of a predominantly acicular-ferrite matrix, and substantially devoid of any-prior austenite grain boundaries, said alloy steel article in an as-rolled condition having a Charpy V-notch impact value of at least about 100 tit-lb at 75. F. as measured on a standard. test specimen and a 0.2 percent offset yield strength of at least" about 60 ksi.
• alloy steel article as defined in claim 1 further characterized by precipitated niobium carbonitride particles dispersed throughout the predominantly. acicular-ferrit e matrix.
• niobium from about 0.02'to about 0.05 percent aluminum, up to about 0.007 percent nitrogen, up to a maximum of 0.03 percent sulfur and lb at F. as measured on a half-size tesE and a 0.2 percent offset yield strength of at 67 ksi.
• the method of making a low-alloy steel plate which comprises the steps of forming a solidified mass of an alloy consisting essentially of about 0.01 to about 0.1 percent carbon, greater than 1.5 up to about 2.5 percent manganese, about 0.1 to about 0.5 percent molybdenum, about 0.05 to about 0.2 percent niobium, up to about 0.08 percent aluminum, up to about 0.015 percent nitrogen and zirconium present in a stoichiometric amount to form the corresponding zirconium nitride with that amount of nitrogen present in excess of about 0.008 percent up to a maximum of 0.04 percent sulfur, up to a maximum of 0.04 percent phosphorous and the balance iron; heatingsaid mass-to an elevated temperature sufficient to form a solid'solution of substantially all of the niobium in an austenitic structure, deforming said mass while at said elevated temperature, air cooling the.

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• Heat Treatment Of Sheet Steel (AREA)

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Citations (6)

• 1970
  • 1970-05-11 US US00036391A patent/US3726723A/en not_active Expired - Lifetime
• 1971
  • 1971-04-15 CA CA110,449A patent/CA942542A/en not_active Expired
  • 1971-04-27 DE DE2120618A patent/DE2120618C3/de not_active Expired
  • 1971-05-07 FR FR7116595A patent/FR2091353A5/fr not_active Expired
  • 1971-05-11 GB GB1422971*[A patent/GB1308092A/en not_active Expired

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