Classifications

• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature

Definitions

• This invention relates to a novel high strength steel and to a process for making such steel.
• U.S.P.S. 4,050,959 discloses a process of making a cold reduced two phase steel in which the chemical composition of the steel is controlled within the following range in the steel making stage: the steel is hot rolled, pickled and cold reduced, and the cold reduced steel strip so obtained is subjected to the following full continuous annealing process:
• a primary object of the present invention is to provide, without the use of costly alloy additions, a novel and improved steel having good ductility which is suitable for use in sheet form for automotive applications and the like and which is not subject to room temperature aging but which has a high work hardening rate and age hardening response so that stamping and paint baking result in high strength in the finished part.
• a more specific object of the invention is to provide a high strength low alloy steel of the foregoing character which exhibits a minimum total elongation of 18%, is free from ductility loss due to room temperature aging, and which, in the final fabricated part, exhibits a yield strength of at least 550 MPa (80 ksi).
• a cold rolled strip of aluminium-killed steel containing added phosphorous and silicon as ferrite strengtheners said steel comprising from 0.05 to 0.15 wt % carbon, from 0.30 to 0.60 wt % manganese, from 0.04 to 0.10 wt. % phosphorous, from 0.10 to 0.50 wt. % silicon, from 0.02 to 0.08 wt. % aluminium sufficient to tie up all the nitrogen present and to provide a fully killed steel and the balance essentially iron, and usual incidental elements in amounts not exceeding the normal amounts resulting from the steel making process;
• tempering said strip by reheating said strip to a sub-critical temperature to effect tempering of the martensite;
• the resultant steel having a minimum total elongation of 18% and a yield strength of from 400 to 500 MPa (58-73 ksi) and being free from ductility loss due to room temperature aging, and after straining and heating incident to fabrication of a finished part, said steel having a minimum yield strength of 550 MPa (80 ksi).
• the resultant sheet or strip has good formability; it is free from ductility loss due to room temperature ageing; and after straining and aging during fabrication of a formed part, the desired high strength level is developed.
• the steel product exhibits a minimum of 18% total elongation prior to stamping and a minimum yield strength of 550 MPa (80 Ksi) after stamping followed by a typical automotive paint bake cycle. This combination of properties allows design engineers to take advantage of potential weight reductions by utilising the high strength of the steel while permitting part fabrication without extensive modification of existing dies.
• High strength levels can be obtained in a ferrite-martensite microstructure by increasing the relative proportion of martensite, but the ductility and formability of the product suffers.
• the martensite content is kept low enough so that the cold rolled product has a minimum total elongation of 18%, but a moderately high strength level of from 400 to 500 MPa (58-73 Ksi), dependent upon the carbon content, is realised by including silicon and phosphorous in the steel as ferrite strengtheners.
• the dual phase product has a high strain hardening capacity, and the final fabricated part exhibits the desired minimum yield strength of 550 KPa (80 Ksi).
• the dual phase microstructure is obtained by rapid quenching from an inter-critical temperature within a controlled narrow range at a cooling rate which is in excess of the critical cooling rate and which is high enough so that phosphorous does not migrate and relocate at the grain boundaries where it would cause poor ductility. Instead, the phosphorous remains within the ferrite grains, and the desired stiffening effect of phosphorous is realised while retaining acceptable ductility. Moreover, the cold rolled sheet product of the present invention is not subject to detrimental room temperature ageing due to nitrogen since the steel is aluminium-killled.
• the steel composition of the present invention consists of the following with the balance essentially iron:
• the maximum silicon and manganese contents are such that rapid quenching with water, rather than air cooling, is necessary in order to obtain the desired ferrite-martensite microstructure.
• the steel is fully killed with aluminum, as reflected by the above-listed range of aluminum content in the steel.
• the steel is still subject to carbon strain aging so that the desired high strength levels are obtained in automotive applications and the like after stamping and heating, as in paint baking.
• the hot metal from the blast furnace is refined in a basic oxygen converter.
• the hot metal may be subjected to conventional desulfurization, e.g. by calcium carbide injection, prior to being charged to the basic oxygen converter.
• desulfurization e.g. by calcium carbide injection
• the required additions of aluminum, silicon, and phosphorus may be carried out in the ladle prior to ingot casting or continuous casting.
• the usual hot rolling and cold rolling practices may be used to provide cold rolled coils for subsequent continuous annealing in accordance with the invention.
• the finishing temperature may be from about 785°C to about 955°C, and the coiling temperature may be from about 480°C to about 705°C.
• the percent cold reduction may range from about 40% to about 80%, but a relatively high degree of cold reduction of from about 50% to about 75% is preferred in order to obtain a fine grain size after the annealing step.
• the thickness of the cold rolled strip may be from about 0.3 mm to about 3 mm.
• the cold rolled strip is processed, in accordance with the invention, in a continuous annealing line in which the strip is (1) heated in a soak section to a temperature between the A 1 and A3 critical points, (2) water quenched in a quench section at a rapid rate to obtain a dual phase ferrite-martensite microstructure, and (3) reheated in a tempering section to a subcritical temperature and cooled to ambient temperature.
• the strip is temper rolled for flatness.
• the inter-critical soak temperature must be carefully controlled in the soak section of the continuous annealing line, preferably within ⁇ 10°C, for partially austenitizing the steel to the desired extent and thereby realizing the aim ductility and yield strength in the quenched and tempered product.
• the cold rolled strip is heated to a narrow temperature range between the A 1 and the A3 critical points such that from 5% to 25% austenite is present. In general, the strip is heated at a soak temperature of from 745°C to 845°C for a period of from about 20 to about 120 seconds.
• the partially austenitized strip passes from the soak furnace into a water quench zone of any suitable design capable of rapidly quenching the strip at a rate in excess of the critical cooling rate so that all of the austenite present is converted into martensite which is uniformly distributed in fine grain polygonal ferrite.
• the average volume fraction of martensite present in the quenched product is from about 5% to about 25%.
• a preferred quench system utilizes submerged nozzles such as disclosed in Taylor et al U.S. Patent Nos. 3,360,202 and 3,410,734.
• the cooling rate will ordinarily be in excess of 1000°C/sec.
• the rapid quench rate also has the advantage of avoiding relocation of phosphorus to the grain boundaries which would impair the ductility of the product.
• the quenched strip is reheated to a sub- critical temperature, e.g. from 150°C to 480°C for a period of from 5 to 300 seconds, in order to effect tempering of the relatively high carbon content martensite.
• the strip is then cooled to substantially ambient temperature and, when necessary, temper rolled for flatness in the conventional manner to obtain a cold reduction not in excess of 2%.
• the resultant steel sheet or strip product has a minimum total elongation of 18% and a yield strength of from 400 to 500 MPa (58-73 ksi), dependent upon the carbon content.
• the product has excellent formability for automotive and other applications, and after stamping and a typical paint bake cycle the yield strength of the fabricated part exceeds the required minimum of 550 MPa (80 ksi) and in most cases is in excess of 620 MPa (90 ksi).
• the excess aluminum present in the killed steel ties up all the nitrogen present so that there is no loss of ductility due to room temperature aging.
• the uniformity of properties within a coil is excellent, e.g. the variation in yield strength being less than 40 MPa (6 ksi). The latter is an important property for a stamping die which is set up for springback control.
• the product has excellent spot weldability and compares favorably in this respect with aluminum-killed AISI 1006 steel.
• the test coils were then processed in a continuous annealing line having in sequence a soak furnace, a water quench system with submerged nozzles, a tempering furnace, a final water cooling tank for cooling the strip to room temperature, and a temper mill.
• the line was operated at speeds ranging from 90 metres/min. for the 1.2 mm thick strip to 130 metres/min. for the 0.5 mm strip, with the heating times varying inversely with the line speed. For example, at a line speed of 110 metres/min. for the 0.8 mm strip, the heating times in the soak furnace and the tempering furnace were 70 and 65 seconds, respectively.
• the strip temperature in the soak furnace and in the tempering furnace as determined from radiation pyrometer measurements, were about 788° and 260°C, respectively, and these temperatures were controlled within about ⁇ 10°C.
• Hole expansion tests which determine the percent expansion of a 12.7 mm original diameter hole after the appearance of the first through-thickness crack, were performed using the same apparatus with a self-aligning punch to go through the initial hole.
• Stretch-bend tests consisted of applying force to the center of an edge clamped 50.8x209.5 mm test sample with a round end steel punch mounted in the movable platen of a hydraulic testing machine to produce a V-shaped specimen.
• the hole expansion and stretch-bend tests are measures of edge formability.
• An unsupported sample length of 139.7 mm with an R/t ratio of about 1.6 was employed for all stretch-bend tests.
• Tensile testing was performed using 50.8 mm gauge length specimens which were pulled at a cross head speed of 12.7 mm per minute on an Instron testing device.
• Tables III and IV The results of the tensile tests, averaged for each coil, are presented in Tables III and IV for the 0.07% carbon and the 0.10% carbon steels, respectively.
• A.R refers to the "as-received” steel condition following the quench, temper and temper-roll treatment performed on the continuous annealing line
• S&A refers to the steel properties which resulted from straining the as-received steel 2% in tension and aging for one hour at 204°C, which is a simulation of a stamping and pain-baking treatment.
• Formability and microstructural characterization parameters are presented in Table V.
• the attainment of the high part strength following forming and paint-baking is dependent on the ability of the steel to respond to the aging treatment. Since all portions of a formed part may not receive 2% strain and the paint cycle may not always consist of one hour at 204°C (400°F), the effects of lesser amounts of pre-strain and lower aging temperatures on strained and aged properties of the 0.8 mm test steel were also investigated. It was found that for an aging temperature of 204°C, the minimum strength of 550 MPa (80 ksi) is attained without any prestrain. This indicates that the strain imparted by the temper rolling is sufficient to produce the minimum required strain aging response in any stamping.
• the test steel attains the minimum strength as long as the tensile pre-strain is in excess of 0.6%. If the tensile prestrain level is maintained at 2%, the minimum strength requirement is met even at aging temperatures as low as 121 °C. It was concluded from the data that the steel of the present invention is quite versatile and that, with proper die design to put additional strain into the flat areas of a part, even lower temperature paint-bake cycles are usable.
• the weldability of the test steel was also evaluated along with that of AISI 1006 using a range of weld evaluation criteria.
• the welding lobe curves showed that the steel of the present invention is quite similar to plain carbon steel in terms of weld time-weld current flexibility, with no hold-time restrictions being required.
• mechanical testing of the welds showed, in most cases, high strength levels commensurate with the base metal strength.

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• Chemical & Material Sciences (AREA)
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• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
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• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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• 1979
  • 1979-02-07 JP JP50048579A patent/JPS55500221A/ja active Pending
  • 1979-02-07 DE DE7979900260T patent/DE2965340D1/de not_active Expired
  • 1979-02-07 WO PCT/US1979/000064 patent/WO1979000644A1/en unknown
  • 1979-09-11 EP EP19790900260 patent/EP0009050B1/en not_active Expired

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