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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • 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
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
• • 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
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• This invention is directed to a method for the production of dual-phase steel sheet or strip products with improved deep drawability.
• undesirable grain growth e.g. U. S. Patents 3,914,135 or 4,033,789
• the formability of dual-phase steels, coupled with their relatively high-strength to weight ratios, have led to the adoption of such steels for the production of automotive parts such as bumpers, wheel racks, brackets--primarily as hot-rolled products.
• cold-rolled, dual-phase steels have not been employed for the production of automobile body panels, one reason being that steel sheets for body panel applications must have good deep-drawing capabilities.
• r m value is the ratio of true width strain to true thickness strain when the sheet is strained in tension.
• r m value is the ratio of true width strain to true thickness strain when the sheet is strained in tension.
• r m value is the ratio of true width strain to true thickness strain when the sheet is strained in tension.
• the r values of dual-phase steel sheets are invariably poor, i.e. around unity (see, for example, Hayami et al., Formable HSLA and Dual-Phase Steels, Conference Proceedings, TMS-AIME, 1979, pages 167 to 180).
• deep-drawing steels having r values of the order of 1.6 to 2.0 and yield strengths of the order of 40 ksi the presently available, dual-phase steels are much inferior.
• crystallographic texture must be strongly (111) with very little (100) and other undesirable orientations.
• strong (111) textures can be produced successfully in various low-carbon sheet steels, development of an equally strong (111) texture in the ferrite + martensite aggregate would be much more difficult to accomplish. This is because it is difficult to achieve a sharp texture in martensite due to the nature of the transformation variance. It has now been found that if proper orientations [e.g.
• the predominant (111) texture] are first developed or provided to the steel, and if the steel is thereafter briefly intercritically annealed so that local regions with a high carbon content could become austenite pools at the intercritical temperature, these austenized regions will, upon rapid cooling, transform to martensite or bainite without unduly affecting the ferrite matrix - if the amount of martensite formed is small.
• Patent 3,827,924 the disclosures of which patents are hereby incorporated by reference. Examples of three such steels were treated in the laboratory to provide an illustration of the properties which could be achieved. The chemical composition of these illustrative steels is provided in Table I below.
• samples were given a simulated box-annealing treatment at 780°C for a period of four hours. Although the use of such a shortened soak period will result in some sacrifice of deep-drawing texture, it was deemed to be preferential, in order to reduce grain size and thereby attain a more uniform distribution of martensite during the subsequent transformation. After this four-hour hold at temperature, the specimens were cooled by removing them from the hot zone to the colder zone of the furnace. This simulated an accelerated cooling rate attainable in box-annealing and was intended further to reduce the size of the carbides or pearlite colonies at the grain boundaries.
• the change in initial strain hardening rate, resulting from the intercritical heat treatment can be noted by the increase in flow stress after 4% elongation (column labeled "o0.04"). With the presence of yield point elongation of about 1 to 2%, the increase in flow strength at 4% elongation ranged from 10 to 14 ksi. When the yield point elongation was completely eliminated, the corresponding increase in flow stress ranged from 17 to 20 ksi.
• both ductility and initial strain-hardening rate could be improved by changing the steel composition to increase the hardenability, for example, by utilizing elements such as nickel, which would have little or no effect on the annealing texture or the rm value.
• Such increased hardenability would enable the employment of less severe cooling rates from the intercritical anneal, thereby minimizing supersaturation and quenched aging of the ferrite matrix.
• box annealing has most widely been employed for the development of good deep-drawing properties in low carbon sheet steels; annealing with relatively rapid heating rates, such as in continuous annealing, can produce equally satisfactory results if (i) the prior hot processing conditions (such as the finishing and coiling temperature which influence the carbide size and the distribution in the hot rolled band), and (ii) the amount of subsequent cold rolling i reduction are appropriately adjusted.
• the prior hot processing conditions such as the finishing and coiling temperature which influence the carbide size and the distribution in the hot rolled band
• the amount of subsequent cold rolling i reduction are appropriately adjusted.
• annealing to produce a high r m value and the subsequent intercritical heat treatment for the production of from about 2 to 10% austenite can be accomplished in one continuous anneal -- eliminating the separate intercritical heat treatment utilized in the aforementioned examples.
• the steel sheet will be annealed to provide a crystallographic texture capable of yielding an r m value greater than 1.5 and preferably greater than 1.7.
• the next heating phase whether performed as part of the initial anneal for texture formation o: as a discrete step, will be conducted for a time and temperature (preferably within the range A 1 to A 3 ) sufficient to produce from about 2 to 10% austenite, preferably less than 7% austenite.
• the sheet will be cooled at a rate sufficient to transform all or a major portion of the austenite to martensite or bainite--the most preferred range of such decomposition products being about 3 to 5%.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Applications Claiming Priority (2)

Publications (1)

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Cited By (5)

Citations (8)

• 1981
  • 1981-11-20 CA CA000390588A patent/CA1182387A/fr not_active Expired
  • 1981-12-02 BR BR8107848A patent/BR8107848A/pt unknown
  • 1981-12-03 EP EP81305697A patent/EP0053913A1/fr not_active Withdrawn
  • 1981-12-03 ES ES507684A patent/ES507684A0/es active Granted
  • 1981-12-04 JP JP19466681A patent/JPS57140828A/ja active Pending

Patent Citations (12)

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