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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0421—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
  • C21D8/0436—Cold rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B2201/00—Special rolling modes
  • B21B2201/04—Ferritic rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0421—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
  • C21D8/0426—Hot rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0421—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
  • C21D8/0431—Warm rolling

Definitions

• the present invention relates to a method for manufacturing a thin strip of cold-rolled mild steel for stamping.
• a mild steel slab is used, continuously cast, the thickness of which is generally greater than 150 mm, for example between 150 mm and 300 mm.
• This slab is rolled at high temperature in a roughing rolling mill, in order to produce a blank whose thickness is generally between 20 mm and 40 mm.
• This blank is then rolled in a hot finishing train to the desired thickness, most often between 1.5 mm and 4 mm.
• the mild steels currently used are known under the names ELC, for “Extra Low Carbon” and ULC, for “Ultra Low Carbon”, whose carbon contents are less than 0.1% and 0.01% respectively.
• the rolling of the blanks to the desired thickness is carried out entirely in the austenitic field, with temperatures of end of hot rolling greater than 850 ° C.
• Such hot rolling in ferrite makes it possible to produce softer hot strips and more easily cold rollable.
• the hot strips are cooled and then cold rolled to the desired final thickness. They are then subjected to recrystallization annealing, either in coils in an oven or in a continuous annealing line comprising an overaging zone.
• Cold strips obtained from hot strips rolled in the ferritic field, for their part have a significantly lower drawing ability than cold strips of austenitic origin, under the same cold reduction conditions.
• the plastic anisotropy coefficient r ⁇ of these bands is hardly worth 1.3 for reduction rates between 70% and 75%.
• the subject of the present invention is a process for manufacturing a thin strip of cold-rolled steel, for stamping, which makes it possible to significantly improve the ability to stamp this strip while fully taking advantage of the advantages associated with a hot finish rolling of the strip in the ferritic field, with an end of rolling temperature below 780 ° C.
• a method of manufacturing a thin strip of cold-rolled mild steel for stamping in which a mild steel is hot rolled in a temperature range where said steel has a ferritic structure with a temperature end of rolling less than 780 ° C to form a hot strip, is characterized in that said hot strip is then cold rolled with a reduction in thickness giving the cold strip a degree of work hardening final corresponding to a cold reduction rate greater than 75%.
• said cold rolling is carried out with a thickness reduction rate greater than 75%, and preferably between 78% and 85%.
• the thickness of the hot strip can advantageously be determined as a function of the desired final thickness of the cold strip and of the reduction rate applied to cold rolling.
• the thickness of the hot strip of ferritic origin may be included in a range extended by example between 1 mm and 8 mm, without this resulting in difficulties in cold rolling.
• said hot rolling is carried out in the ferrite by means of lubricated cylinders, with an end of rolling temperature leaving at least a degree of work hardening in the hot strip equivalent to that which corresponds to an intermediate rate of cold reduction of 10% and cold rolling is then carried out with a reduction rate equal to the difference between the desired final reduction rate and said intermediate reduction rate.
• This latter implementation makes it possible to apply a total work hardening to the strip of mild steel ensuring a high drawing ability, that is to say a degree of work hardening corresponding to a cold rolling only with a thickness reduction rate of at least 75%, while performing said work hardening partially during hot rolling in the ferrite and partially during cold rolling with a thickness reduction adapted accordingly.
• This accumulation of hot and cold work hardening can be carried out on any type of ULC (IF and non-IF) and ELC steel.
• hot rolling in the ferrite must be carried out with lubricated cylinders, so that the surfaces of the strip are not deformed by shearing; moreover, the temperature at the end of hot rolling in the ferrite must be lowered to an extent such as to avoid complete recrystallization steel in order to leave a determined degree of work hardening in the hot strip.
• This end of rolling temperature depends in particular on the type of steel considered; in addition, it will advantageously be chosen as a function of the degree of work hardening to be maintained in the hot strip.
• the combination of hot rolling in ferrite and cold rolling with an increased or adapted reduction rate, according to the invention, makes it possible to substantially improve various properties of the cold strips.
• a slab of steel is continuously cast.
• the reheating of the slab can be limited to a temperature such that the nitride AlN is not dissolved.
• the reheating temperature is advantageously between (A c3 + 50 ° C) and 1150 ° C. Furthermore, this temperature is sufficient to avoid reheating in the intercritical phase, that is to say austenitic + ferritic, or in the ferritic phase, which would lead to the formation of very coarse cementite on the hot-rolled strip.
• the first part of the strips were hot rolled in the austenitic field, cold rolled with increasing reduction rates and then continuously annealed under usual conditions. These bands serve as a reference.
• a second part of the strips were hot rolled in the ferritic field, with a fully recrystallized structure, i.e. without residual work hardening, then cold rolled with reduction rates lower than 75% corresponding to the optimal rates of the austenitic bands, and finally annealed either in coil, or continuously.
• a third part of the bands were hot rolled in the ferritic field, also with a fully recrystallized structure, then cold rolled with reduction rates greater than 75% according to the present invention, and finally annealed either in coil or continuously.
• a fourth part of the strips were hot rolled in ferrite, with a partially recrystallized structure so as to maintain in these hot strips a degree of work hardening - estimated from the hardness of the strips - corresponding to a reduction rate d 'cold thickness greater than 10%, then cold-rolled so that the degree of total work hardening corresponds to a reduction rate in cold thickness greater than 75%.
• Table 1 illustrates the improvements observed on the cold strips treated by the method of the invention by comparison with the strips obtained by conventional methods.
• the designation “hot reduction *” is applied to the intermediate cold reduction rate, assimilated to a hot reduction rate because it is carried out during hot rolling in ferrite, and estimated from the hardness of the hot strip.
• the cold strips produced from hot strips rolled in ferrite develop their maximum properties for cold reduction rates greater than 75%. It is also found that the cold strips produced from hot strips rolled in ferrite, but having a structure which is still partially hardened, develop their maximum properties when the rate of reduction in cumulative thickness is greater than 75%, part of the required work hardening being carried out by hot rolling and the rest being carried out by cold rolling with lower thickness reduction rates, less than 75%.
• the cold strips obtained from hot strips laminated in austenite have a maximum drawing ability for a cold reduction rate of between 70% and 75%.
• the properties considered here are the plastic anisotropy coefficient r ⁇ and the planar anisotropy ⁇ r, which express in a known manner the ability to stamp.
• the process of the invention makes it possible to produce DDQ (Deep Drawing Quality) grades from ULC steel.
• Table 2 shows the improvement in the resistance to natural aging of continuously annealed cold strips obtained by the process of the invention (I), compared with reference strips (R) hot rolled in austenite. .
• a slab of ELC steel is heated to a temperature between A3 and 1150 ° C, performs the rough lamination of this slab in the austenitic domain, the finish lamination of this preform is carried out to a thickness of 1 mm to 8 mm with a finish lamination temperature below 780 ° C., to form a hot strip and this hot strip is subjected to at least one cold rolling step to the desired final thickness, with a total reduction rate greater than 75%.

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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)

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Publications (2)

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• 1993
  • 1993-12-20 BE BE9301421A patent/BE1007790A6/fr not_active IP Right Cessation
• 1994
  • 1994-12-14 EP EP94870193A patent/EP0659891A3/de not_active Withdrawn

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