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
  • C21D8/0426—Hot rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
  • B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
  • B21B1/26—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
• • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0231—Warm rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B2201/00—Special rolling modes
  • B21B2201/02—Austenitic 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B27/00—Rolls, roll alloys or roll fabrication; Lubricating, cooling or heating rolls while in use
  • B21B27/06—Lubricating, cooling or heating rolls
  • B21B27/10—Lubricating, cooling or heating rolls externally
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
  • B21B45/004—Heating the product
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
  • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
  • B21B45/0239—Lubricating
  • B21B45/0242—Lubricants
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
  • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
  • B21B45/0239—Lubricating
  • B21B45/0245—Lubricating devices
  • B21B45/0263—Lubricating devices using solid lubricants
• • 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
  • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0226—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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0236—Cold 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
  • C21D8/0247—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 characterised by the heat treatment
  • C21D8/0263—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 characterised by the heat treatment following 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
• • 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
  • C21D8/0447—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 heat treatment
  • C21D8/0463—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 heat treatment following hot rolling

Definitions

• the present invention relates to a method for manufacturing a thin strip of mild steel, that is to say having a carbon content of less than 0.2%, and preferably less than 0.1%.
• the thin strip has a thickness less than or equal to 2 mm.
• a thin strip, coated or not is used in particular in automobile bodywork and in household appliances.
• the strip of the invention can be used for example for the manufacture of tinplate.
• a slab of continuously cast mild steel is most often used, the thickness of which is generally between 150 mm and 300 mm.
• the slab is loaded into a reheating oven where it is homogenized at a high temperature, generally above 1000 ° C, before being hot rolled.
• the slab Before the homogenization reheating, the slab may have been cooled to a relatively low temperature, possibly up to ambient temperature; the so-called cold oven is then used.
• the solidified slab may also have been cooled only to an intermediate temperature, for example between 600 ° C and 900 ° C; in this case, the so-called hot charging is practiced in the reheating oven.
• the slab could have undergone only partial cooling, to a temperature which remains above 1000 ° C, and which is preferably between 1000 ° C and 1200 ° C; in this case, the slab is ready to be laminated directly, without passing through a reheating oven.
• the slab is rolled at high temperature in a roughing rolling mill, in order to produce a blank having a thickness generally between 20 mm and 40 mm.
• This blank is then rolled in a hot finishing train to the desired final thickness, most often between 1.5 mm and 4 mm.
• the rolled strip At the exit from the hot finishing train, the rolled strip generally has a temperature above 850 ° C., in the case of mild steels.
• the strip is finally cooled on the exit table of the finishing train, then it is wound at a temperature generally equal to or higher than 550 ° C.
• This conventional hot rolling practice makes it possible to manufacture a hot strip, ductile and formable, perfectly recrystallized, which can be used directly after pickling or which can be subsequently cold rolled and subjected to recrystallization annealing to produce a thin strip Cold.
• a second cold rolling step may then prove necessary to give the thin strip the required mechanical characteristics.
• This cold manufacturing cycle is frequently called "cold double reduction treatment" with reference to the two cold rolling operations.
• the level of resistance results from the whole manufacturing process, and in particular from the reduction rate of the second operation of cold rolling; this generally does not exceed 50%.
• the ductility also depends on the reduction rate during the last cold rolling, as well as on the aging sensitivity of the final product.
• a first method consists in limiting the heating temperature of the slabs to a level such that the dissolution of the nitride AlN does not occur.
• the disadvantage of this method is that it no longer makes it possible to guarantee hot rolling entirely in the austenitic domain, which inevitably leads to heterogeneous structures and to irregularities in the dimensions of the hot strips.
• the winding temperature of the hot strips can be increased above 700 ° C., so as to precipitate in the form of the nitride AlN all the nitrogen previously dissolved during the initial heating.
• excessively high winding temperatures lead to heterogeneities along and across the strips and to the formation of an oxide layer of excessive thickness.
• winding temperatures close to 700 ° C give rise to the formation of very coarse cementite, which deteriorates the surface condition of very thin products.
• the current procedure therefore results from a compromise considered hitherto optimal: the slabs are heated to a temperature sufficient to carry out hot rolling entirely in the austenitic domain, and the strips are hot-rolled at a temperature of 550 ° C to 650 ° C so as to precipitate part of the nitrogen in the form of the nitride AlN without coalescence of the cementite occurring beyond a certain size.
• the present invention provides a method which makes it possible to remedy the various aforementioned drawbacks and to manufacture a hot-rolled strip free from defects and perfectly homogeneous and whose thickness is equal to or less than 2 mm.
• the hot strip obtained by the process of the invention has the stamping quality, which is all the more desirable as the thickness of the strip is reduced and approaches the usual range of thickness of a cold rolled strip.
• the present invention also provides a method for manufacturing a cold-rolled, non-aging strip, the dimensional tolerances of which are strictly observed and in which the cementite is finely dispersed.
• a method of manufacturing a thin strip of mild steel which comprises a step of hot finishing rolling, is characterized in that said finishing rolling is carried out in a temperature range where the steel has a ferritic structure, with an end-of-rolling temperature of 750 ° C or less, in that the cylinders used for said finishing rolling are lubricated, and in that said hot-rolled strip is subjected to recrystallization annealing.
• the position and extent of the ferritic range of a steel depends in particular on its chemical composition, namely in particular its carbon and alloying content.
• the temperature of engagement of the finish rolling in the ferrite can therefore vary depending on the steel used.
• the symbol IF indicates "Interstitial Free" steels, that is to say which do not contain interstitial atoms.
• the engagement temperature of the finish rolling is equal to or less than 800 ° C, and it is preferably between 750 ° C and 400 ° C.
• ELC and ULC steels are sensitive to the phenomenon of dynamic aging, well known to specialists; it is an increase in resistance to deformation in certain temperature ranges, due in particular to the presence of C and N atoms in the structure of steel. It is important that the rolling in the ferritic phase is carried out so as to avoid significant increases in the rolling load, due precisely to this dynamic aging. Indeed, these increases in load could lead to undesirable mechanical overloads on the stands of the rolling mill.
• the temperature of engagement of the finish rolling is either between 750 ° C and 550 ° C, and preferably between 700 ° C and 600 ° C, or between 450 ° C and 250 ° C, and preferably between 400 ° C and 300 ° C.
• the purpose of finishing hot rolling in the ferritic phase is to produce thin hot strips which have a work hardened structure.
• the work hardening of these strips must be high, so that the hardness measured on the hot strips is at least equal to half the hardness that is measured on the same strips after cold rolling with rates of reduction greater than 50%.
• the hot-rolled strip in ferritic phase which has a work hardened structure is then recrystallized.
• the strip is recrystallized by continuous annealing carried out directly after the finishing rolling; this annealing can then reduce the rolling oxides.
• the hot-rolled strip is wound and pickled, and then recrystallized either by continuous annealing or by static annealing on a coil.
• An additional variant of the invention consists in carrying out the re-recrystallization of the hot strip which has been hardened during rolling in the ferritic phase.
• the high winding temperatures necessary to ensure recrystallization can be obtained by adapting the cooling on the output table and taking advantage of the temperature increase which occurs in the last stands of the finishing mill. This rise in temperature, caused by the deformation of the strip, is all the more important as the work hardening in the last stands of the finishing rolling mill is high.
• the rolling speeds in the ferritic phase are the usual rolling speeds; they are therefore compatible with the possibilities of existing finishing trains.
• the winding temperatures if it is required, are determined as a function of the temperatures at the end of rolling in the ferritic phase.
• the maximum temperatures reached during direct annealing of the work hardened strips are also adapted as a function of the nature of the steel to ensure total recrystallization of the latter.
• the thin hot strips can then be cold rolled with a reduction rate of at least 20%, to produce thin cold strips of reduced thickness, that is to say say less than or equal to 1.0 mm, and preferably less than or equal to 0.5 mm. These are then recrystallized by continuous annealing or from a coil, to produce stamping quality strips. Cold strips of reduced thickness, that is to say less than or equal to 0.5 mm, having a stamping quality can be obtained with cold reduction rates of less than 90% and up to 40% .
• ultra-thin non-aging cold-rolled strips intended for example for the manufacture of tinplate.
• the steel is subjected to a hot rolling comprising a roughing rolling and a finishing rolling, and to at least one cold rolling step, said finishing rolling is carried out in the ferritic field up to a thickness from 0.5 to 2 mm with an end-of-finish-rolling temperature between 750 ° C and 300 ° C, to form a hot strip, said hot strip is spooled at a temperature between 750 ° C and 200 ° C, and this strip is subjected to hot at least one cold rolling step to the desired final thickness, with a total reduction rate less than or equal to 95%.
• the heating of the slab can here be limited to a temperature such that the nitride AlN is not dissolved.
• the reheating temperature is advantageously between (A c3 + 50 ° C) and 1050 ° 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 roughing rolling is carried out in the austenitic field, to form a blank whose thickness varies according to the thickness of the final product to be manufactured.
• the thickness of the blank is generally between 20 mm and 50 mm.
• the blank is cooled so as to effect the phase transformation of the austenite into ferrite.
• the cooling rate conditions, in known manner, the size of the ferritic grain, the size and the dispersion of the cementite islands or of the residual austenite islands.
• An accelerated cooling of the blank is therefore carried out, in order to obtain a fine and dispersed cementite and therefore a homogeneous ferritic structure.
• the duration of this cooling is advantageously less than 30 seconds.
• This duration depends on the temperature to which the blank has been cooled and which will be the temperature at the start of the finish rolling in the ferritic field. Depending on the value of this temperature, a finely dispersed residual ferrite + austenite mixed structure is obtained or a ferritic structure containing finely dispersed cementite.
• the finishing rolling is carried out in the ferritic field, to produce a hot strip having a thickness of between 0.5 mm and 4 mm, and preferably between 0.5 mm and 2 , 5 mm.
• the finish rolling is carried out without waiting for the coalescence of the islands of residual austenite or cementite.
• the strip is cooled while hot to the winding temperature. This is preferably between 750 ° C and 200 ° C.
• the thin hot strip is then cold rolled, generally in two operations separated by continuous annealing or in coil, to a final thickness of less than 0.50 mm, and preferably less than 0.20 mm.
• the strip thickness is reduced to less than 0.5 mm, and preferably to less than 0.35 mm, with a reduction rate of less than 90%.
• the second cold rolling operation gives the strip its thickness and its final properties, with a reduction rate of less than 50% and preferably less than 30%.
• a first series of examples illustrate the improvements in the properties of hot strips obtained by the method of the invention.
• Example 2 relates to thin hot strips of ELC steel. It is illustrated by Table 2, the columns of which have the same objects as in Table 1.
• the hot strips No. 4, 5 and 6 underwent direct recrystallization annealing at 720 ° C for 60 s .
• the hot strips produced by the process of the invention have better deformability than conventional strips: marked improvement in the anisotropy coefficients, without any decisive modification of the limit of elasticity, breaking load and elongation.
• Example 3 illustrated by Table 3, relates to cold strips of steels of the ULC-Ti and ELC types, cold rolled from hot recrystallized strips.
• the conditions for direct recrystallization annealing are the same as for the corresponding steel grades presented in Tables 1 and 2 respectively.
• the process of the invention has improved the deformation ability compared to conventional cold bands: lower yield strength and breaking load, slight increase in elongation and coefficients d more favorable anisotropy.
• the cold strip produced by the process of the invention has been found to be insensitive to aging.
• This characteristic due to the limited heating temperature of the slab, results in a remarkable stability of the mechanical properties, in particular of the elongation and therefore of the ductility of the strip. This result is not achieved by the prior method.
• the ductility is significantly higher by the method of the invention than by the previous method, which promotes the use of the cold strip produced according to the invention.
• Example 4 relates to the particular case of a strip which has undergone only one cold rolling operation from a hot strip recrystallized by continuous direct annealing.
• the cold strip obtained also has very favorable properties, as regards strength, ductility and aging. It thus appears that, by using a very thin hot strip with a work hardened structure, the process of the invention makes it possible, for certain ranges of final thicknesses, to eliminate a cold rolling operation without harming the properties of the strip to be cold obtained.
• a steel strip hot rolled by the process of the invention is distinguished by the absence of aging, as well as by a very low yield strength and breaking load in the case where the laminated structure in the ferritic domain completely recrystallizes, that is to say if the rolling temperature (T F ⁇ 700 ° C) and / or if the winding temperature (T B ⁇ 550 ° C) are sufficiently high.
• steel, also not aging is characterized by a ferritic microstructure with elongated grain, partially or totally restored. Whatever the state of the structure of the hot strip, the cementite is finely dispersed there.

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

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Cited By (15)

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• 1992
  • 1992-07-13 EP EP92870104A patent/EP0524162B1/de not_active Revoked
  • 1992-07-13 DE DE1992627548 patent/DE69227548T2/de not_active Revoked

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