EP0922777A1 - Flat product, such as sheet, made from ductile high-yield steel and process for manufacturing the same - Google Patents

Abstract

The steel product is obtained by hot rolling, tempering to form a carbide forming and/or gamma forming element enriched phase in a ferrite matrix, heat treating to form islands of austenite and/or enriched with gamma forming elements in already formed austenite and cooling to obtain a microstructure with ferrite matrix and retained austenite, bainite and/or martensite. A flat steel product, e.g. sheet, has a multiphase structure obtained by (a) hot rolling a steel containing 0.05-0.8% C, 0.2-3.0% Mn and ≤ 1% Si at a stable austenitic phase temperature; (b) tempering at between 300 degrees C and 50 degrees C above the eutectoid (A1) temperature for ≥ 4 hr. and optionally cooling to ambient temperature to form a phase enriched in carbide forming and/or austenite forming elements such as carbon and manganese in a ferrite matrix; (c) heat treating at above the A1 temperature and below the austenite forming (A3) temperature to form islands of austenite and/or enriched with austenite forming elements such as Mn in the already formed austenite; and (d) cooling to ambient temperature to obtain a final product having a ferrite matrix containing islands of retained austenite, bainite and/or martensite. An Independent claim is also included for production of the above flat steel product.

Description

The present invention relates to a flat product, such than a multi-phase steel sheet containing Mn with a ferrite matrix.

High strength steels like steels rephosphorus, microalloyed steels, bake-hardening steels, are widely used for auto parts. Sheets made in such steel require sufficient strength to meet safety automobiles and must also have excellent properties of shaping.

It is also known that the strength and ductility of a multiphase steel can be improved by a microstructure multiphase possibly combined with an induced transformation of plasticity ("transformation induced plasticity," TRIP ") of austenite residual martensite.

The "TRIP" effect was first discovered by Zackay et al in steels containing large amounts of nickel and of chrome.

However, the presence in these steels of large quantities of such alloying elements pose problems for the manufacture of steels under economically profitable conditions.

It should also be noted that a steel containing a significant amount of residual austenite can be obtained by adding silicon and manganese and by hot cycle rolling controlled thermal or cold rolling followed by annealing intercritical combined with bainitic maintenance producing a structure formed of several phases and with isolated areas or islands of residual austenite which is stable at room temperature.

Such a steel is known in particular from document US-A-4,544,422 and articles by Gregory H. Haidemonopoulos "Austenite stabilization from direct cementite conversation in low-alloy steels ", in Steel Research 67 (1996) n ° 3, p. 93 to 99, and "Modeling of austenite stability in low-alloy triple-phase steels "Steel Research 67 (1996) n ° 11, p. 513 to 519.

These known steels and their production, however, have some significant drawbacks. Indeed, these steels require usually large amounts of additives.

In addition, the fact that in these known steels the content of silicon is generally relatively high, around 1.25 to 1.50% Si, gives a steel sheet with certain defects in surface called "cat tongues", which are created when rolling to hot. Furthermore, these known steels pose problems during the metallic dip coating, such as galvanizing, due to embrittlement of the coating and problems with the wettability of the metal liquid intended to form the coating.

One of the essential aims of the present invention is to offer a steel sheet to overcome the drawbacks mentioned above and which is therefore particularly suitable for galvanizing while being able to be manufactured according to a very economically process favorable.

The steel according to the invention has properties improved strength and ductility, while having a content extremely reduced in silicon, so it suits everything particularly for shaping and surface treatment in automobile industry.

To this end, the steel sheet according to the invention has a structure and properties that can be obtained by a process as defined in claim 1.

Advantageously, the steel according to the invention has the following chemical composition: 0.05 to 0.8% C; 0.2 to 3.0% Mn; If ≤ 1.0%; B ≤ 0.100%; Ti, Nb, Zr and V each ≤ 0.200%, Al ≤ 0.400%; N ≤ 0.100%; P ≤ 0.100%; Cr, Ni and Cu each ≤ 2,000%; Mo ≤ 0.500%.

The invention also relates to the above method particular for the manufacture of the flat steel product according to the invention.

According to a particular embodiment of this process, the abovementioned tempering is carried out at a temperature below the eutectoid temperature (A ₁ ), so as to form cementite and to cause the diffusion of carburogenic elements, such as Mn in this cementite.

According to a preferred embodiment, the method according to the invention comprises a cooling step until the ambient temperature after the above-mentioned maintenance or tempering step, this step is then followed by a cold rolling step before the aforementioned heat treatment stage.

Other details and features of the invention will emerge of the description given below, by way of nonlimiting example, of some particular embodiments of the invention with reference to the accompanying drawings.

Figures 1 to 15 relate to graphs schematics of the treatment temperature in ° C, depending on the steel treatment time, illustrating the different stages of the process according to the invention.

Figures 16 to 23 are schematic representations of a cross section of different microstructures of steel according to the invention.

In the different figures, the same figures of reference designate analogous or identical elements.

In general, the invention relates to a steel multiphase preferably in the form of sheet metal and comprising a ferrite matrix in which islands are distributed at least one of the following phases: bainite, martensite or austenite which is also stable at room temperature, and which possibly shows an induced plasticity transformation ("TRIP"). In addition, the ferrite matrix can be reinforced by precipitates secondary of micro-alloy elements.

Chemically, this steel can contain from 0.05 to 0.8% carbon, 0.2-3.0% manganese and less than 1% carbon silicon. However, according to the invention, preference is given to steels containing as little silicon as possible, for example less 0.5%, preferably at most 0.4% or even less than 0.2%.

In addition to the above, this steel may have a deliberately adjusted low content of boron, titanium, niobium, zirconium, vanadium, aluminum, nitrogen, phosphorus, chromium, nickel, copper, molybdenum and traces of impurities generally unavoidable during the steel preparation, the rest being iron.

More particularly, for the steel according to the invention, the concentration of the various aforementioned elements is advantageously like follows: B ≤ 0.100%; Ti, Nb, Zr and V each ≤ 0.200%, Al ≤ 0.400%; N ≤ 0.100%; P ≤ 0.100%; Cr, Ni and Cu ≤ 2,000% each; Mo ≤ 0.500%.

• le laminage à chaud de cet acier à une température à laquelle la phase austénitique est stable, c'est-à-dire à une température supérieure à la température de transformation de l'austénite (A₃),
• un revenu ou maintien de la tôle bobinée à une température comprise entre 300°C et une température de 50°C au-dessus de la température eutectoïde (A₁) pendant au moins 4 heures, suivi éventuellement d'un refroidissement jusqu'à la température ambiante, de manière à former une phase enrichie en éléments carburigènes et/ou gammagènes, tels que du C et Mn dans une matrice de ferrite,
• un traitement thermique à une température supérieure à la température eutectoïde susdite (A₁) et inférieure à la température de formation d'austénite (A₃) de manière à former des ílots d'austénite etlou d'enrichir en éléments gammagènes, tels que du Mn de l'austénite déjà formée, et
• un refroidissement subséquent jusqu'à la température ambiante d'une manière telle à obtenir un produit final présentant une matrice de ferrite contenant des ílots d'au moins une des phases suivantes : austénite résiduelle, bainite et martensite.

Le traitement thermique précité peut être précédé ou suivi d'un laminage à froid.As already mentioned above, the invention also relates to a process for the manufacture of a multi-phase steel sheet corresponding to the above chemical composition, according to which this steel is subjected, for example in the form of a slab, to the successive stages following:

• hot rolling of this steel at a temperature at which the austenitic phase is stable, that is to say at a temperature above the transformation temperature of austenite (A ₃ ),
• tempering or maintaining the wound sheet at a temperature between 300 ° C and a temperature of 50 ° C above the eutectoid temperature (A ₁ ) for at least 4 hours, possibly followed by cooling to the ambient temperature, so as to form a phase enriched with carburogenic and / or gamma-genic elements, such as C and Mn in a ferrite matrix,
• a heat treatment at a temperature higher than the above eutectoid temperature (A ₁ ) and lower than the austenite formation temperature (A ₃ ) so as to form islets of austenite and / or to enrich with gamma elements, such as Mn of the austenite already formed, and
• subsequent cooling to room temperature in such a way as to obtain a final product having a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite.

The above heat treatment can be preceded or followed by cold rolling.

In fact, the invention consists in presenting a steel containing simultaneously at least two phases such as ferrite / austenite residual or ferrite / bainite, ferrite / martensite, ferrite / bainite / martensite with or without residual austenite and a very reduced silicon content, having, in addition, a structure and properties of a steel obtained by the process as defined above.

In this process, we can generally detect three important successive steps.

In the first stage, a hot rolling of a steel takes place, corresponding to the above-mentioned chemical composition, above the temperature A ₃ , which is the temperature for transforming austenite into ferrite and which depends on the chemical composition steel. The heating of the bram takes place at temperatures of 1100 ° C to 1350 ° C, for 130 to 250 minutes. This step can be considered as the austenization step during which the formation of ferrite and perlite is avoided and where C and Mn are kept in solution. Hot rolling comprises a roughing rolling followed by a finishing rolling which takes place at a temperature higher than the temperature A ₃ .

This hot rolling can then optionally be followed from cooling to room temperature and this from a so as to form a microstructure of the bainite and / or the martensite, while avoiding the formation of ferrite and perlite.

This cooling preferably has at least partially place on a cooling table called "Runout table" or by a accelerated cooling tool called "Ultra Fast Cooling".

In the second step, after the winding of the sheet obtained by the aforementioned rolling, during which the sheet continues to cool, and after pickling, the tempering takes place which can, for example, be achieved by heating in an oven bell annealing up to a temperature between 300 ° C and 50 ° C above the eutectoid temperature A ₁ . This temperature is maintained for at least 4 hours and preferably up to at most 200 hours.

For some thermomechanical paths developed, it there is a combination of hot rolling directly followed by holding or tempering without cooling the sheet to temperature ambient.

The flat product, when held or tempered, is still covered on the surface by an iron oxide called "calamine".

The advantages of calamine encountered during maintenance or income are as follows: no problem sticking the turns during maintenance or income, and no problem of decarburization of the sheet surface even without the use of a reducing atmosphere in the isolation bell or in the annealing bell.

In the third step, heat treatment takes place at a temperature higher than the eutectoid temperature A ₁ and lower than the austenite formation temperature A ₃ .

This heat treatment can be carried out either directly after tempering, or after possibly combined cooling with cold rolling. It is in any case followed by cooling controlled up to room temperature. The main purpose of this operation is to obtain a final product having a ferrite matrix containing islets from at least one of the following phases: austenite residual, bainite and martensite. The relative quantity of these phases is essentially a function of the starting chemical composition of the steel treaty and special conditions under which the different steps are performed.

More particularly, the C content determines the maximum volume of ferrite and makes it possible to significantly decrease the starting martensite temperature Ms according to the formula: M s = 539 - 423 x C - 30.4 x Mn -7.5 x Si

(Andrews formula)

The C content controls the start time in a continuous cooling diagram and the formation kinetics of ferrite and bainite.

Thus, the choice of the C content makes it possible to obtain a martensite microstructure and / or bainite during cooling on a wide band train table called "runout table" or by a accelerated cooling device called "Ultra Fast Cooling".

Carbon is an interstitial element which hardens the phases ferritics in steel. In an income microstructure, a phase rich in carbon called "iron carbide" or "cementite" is formed. Through the carbon content, the fraction of this phase formed can be controlled. Manganese is known as an element that increases the hardenability of a steel and as a substitute which hardens ferritic phases in steel. Manganese can be enriched by exchange reaction in iron carbides (substitution of iron by manganese) during income processing.

The choice of the manganese content makes it possible to influence the bainite formation zone in a cooling diagram continuous and in the isothermal maintenance diagram. Manganese reduces the starting temperature of the formation of martensite and improve, therefore, the stability of the phase austenitic.

Silicon is a substitute element which hardens ferritic phases in steel. It is alpha-gene, that means it increases the temperature of ferrite formation during cooling. At a higher Si content, certain defects in surface called "cat's tongue", which are created when rolling to hot, may appear. Furthermore, these known steels pose problems with metallic dip coating, such as galvanization, due to embrittlement of the coating and problems of wettability of the liquid metal intended to form the coating.

Silicon stabilizes carbon in solution in austenite and in bainite by inhibiting precipitation of cementite. This effect is explained by the fact that silicon is relatively poorly soluble in cementite, which requires controlled ejection, by diffusion, of silicon in the transformation front. This causes inhibition of growth of cementite embryos.

The elements, consisting of niobium, vanadium, zirconium and titanium, alone or in combination, are used in low amount to form carbides, nitrides or carbonitrides so to be able to block the growth of grains during the heating of the slab and to be able to increase resistance by the precipitation effect

During hot rolling, during cooling on the cooling table and / or in the coil after winding, an effect increase in resistance by the precipitation effect may have location.

In the non-precipitated state, this means in the state in solution in the crystalline ferrite mesh, the microalloy elements can lead to an effect of increasing resistance when treating returned by secondary precipitation.

Aluminum is used to fix nitrogen in solution in forming aluminum nitrides. Aluminum nitrides also have a positive effect by reducing the growth of austenite grains up to a temperature of around 1150 ° C during heating steel during hot rolling.

Aluminum is a substitution element which hardens ferritic phases in steel and which stabilizes carbon in solution in austenite and bainite by inhibiting precipitation of cementite. This effect is explained by the fact that aluminum is relatively poorly soluble in cementite, which requires ejection controlled, by diffusion, of aluminum in the transformation front. This causes growth inhibition of the cementite embryos.

Phosphorus has a positive effect on the resistance of steel, but should also be kept small enough to avoid the effects of embrittlement. Nitrogen is an impurity the content of which must be maintained as small as possible.

Nitrogen is an interstitial element which hardens the phases ferritics in steel and, in solution in steel, it increases the sensitivity to aging.

In very small quantities, boron is deposited in the joints grain and thus increases ductility. Boron also harms the formation of phases, which are obtained by diffusion, such as ferrite and perlite phases. Boron increases the hardenability of steel during hot rolling. Boron forms precipitates with nitrogen in solution.

At a temperature for which the bainite formation has instead, boron decreases the kinetics of bainite formation. Boron can be enriched by exchange reaction in iron carbides (substitution carbon by boron) or by coupled precipitation of borocarbons of iron and iron carbides during an income treatment at a sufficient activation temperature.

Molybdenum is a substitute element, it hardens ferritic phases in steel. Molybdenum is known as an element which increases the hardenability of a steel. Molybdenum can be enriched by exchange reaction in iron carbides (substitution of iron by molybdenum) or by coupled precipitation of iron carbides with molybdenum carbides during tempering treatment at a temperature sufficient activation.

Chromium is a substitution element which hardens ferritic phases in steel. Chromium is known as an element which increases the hardenability of a steel and can be enriched by reaction exchange in iron carbides (substitution of iron for chromium) or by coupled precipitation of iron carbides with carbides of chromium during tempering treatment at activation temperature sufficient.

Nickel and copper are substitutes which harden the ferritic phases in steel and which are known as elements which increase the hardenability of a steel.

It should also be noted that a heat treatment, such as tempering or annealing, at a temperature between the temperatures A ₁ and A ₃ is called "intercritical tempering or annealing".

In some of the process steps described above as well as in some of the particular embodiments of this process described below, mention is made of so-called "slow" cooling, said cooling "fast" and / or cooling said "accelerated".

Generally speaking, "slow cooling" corresponds generally at a decrease in temperature from 10 ° to 40 ° C by hour, depending on the thickness of the sheet, the composition of the steel and the initial temperature of the sheet.

By cons, by "rapid cooling", which generally place by means of a fluid generally corresponds to a temperature decrease from 10 to 300 ° C per second, depending on sheet thickness, sheet composition and temperature sheet metal initial.

Accelerated cooling by quenching the coil wrapped in liquid generally corresponds to a decrease in the temperature with a speed greater than 10 ° C per minute.

The graphs shown schematically in Figures 1 to 15 illustrate some particular embodiments of this process, which is applied to a steel slab or ingot meeting the aforementioned composition and having reheated to temperatures from 1100 ° C to 1350 ° C for 130 to 250 minutes.

In the first embodiment according to FIG. 1, after the first step, comprising hot rolling in the austenitic region above the temperature A3, represented by the line segment 1, followed by cooling, represented by the segment of straight 2, to obtain a tempered structure of bainite and / or martensite, and the second step formed by tempering, represented by reference 3, which takes place by annealing "bell" at a temperature between 300 ° C and the temperature At ₁ , for at least 4 hours, a sheet is obtained comprising iron carbides (cementite) in which the carburogenic elements, such as Mn has diffused, more particularly cementite enriched with manganese, chromium, molybdenum and / or boron and optionally secondary precipitates formed of micro-alloy of these elements in the ferrite matrix.

By "bell" annealing, there is in fact meant an annealing discontinuous on a sheet rolled up in a coil in an oven maintained at the desired temperature.

After cooling 5, slow or accelerated by quenching, the sheet was subjected to the third step, which comprises the aforementioned heat treatment 4, formed either by continuous annealing in the temperature zone situated between the temperature A ₁ and A ₃ , preferably at most 900 ° C, for 1 second to 5 minutes, or annealing "bell" intercritical in the above temperature zone. During this annealing, the carbides (cementite) are transformed into austenite, the chemical composition of which will be enriched by the elements contained and the carbides with germination of the austenite at the joints of the cementite / ferrite grain. This annealing is then followed by rapid cooling, linked to the conventional technology of continuous annealing which is an annealing of an unrolled sheet, or slow or accelerated cooling by quenching, which, in this case, can be carried out. on a coil sheet.

In the case of continuous annealing, strips of sheet metal are butt welded so as to obtain in practice a endless band. So in such a case, the annealing time is extremely limited and is generally between a few seconds and a few minutes, while the annealing bell takes usually several hours. In the latter case, if necessary, a accelerated quench cooling can also be applied.

Furthermore, if necessary, a pickling of the scale can be applied before or after the second step mentioned above. The pickling can be carried out either by a chemical attack on the scale in an acid bath either by a reduction of the scale during a annealing in a reducing atmosphere rich in hydrogen.

Finally, the sheet is either uncoated or coated by deposition electrochemical metal, or galvanized for example after putting in shape.

FIG. 2 relates to a second embodiment of the method according to the invention which differs from that of FIG. 1 by the fact that the annealing 3 by bell annealing is carried out at a temperature below the temperature A1 and is immediately followed by the heat treatment 4 formed by an intercritical tempering by bell annealing between the temperature A ₁ and a temperature of 50 ° C. above this temperature A ₁ .

In this case, relatively slow heating can take place between tempering 3 and heat treatment 4, during which also residual austenite. This heat treatment 4 is then followed by cooling 5, which can be slow cooling typically related to conventional bell annealing technology or a accelerated cooling by quenching.

The third embodiment, illustrated by FIG. 3, differs from the previous two by the fact that the tempering 3, like the aforementioned heat treatment 4, takes place at the same temperature which is higher than the temperature A ₁ and lower at the temperature A ₁ + 50 ° C, so as to constitute a single operation.

During heating, which precedes this operation, begins the formation of a microstructure of fine particles of cementite with nuclei for austenite germs during austenization at tempering temperature 3 and heat treatment 4. As in the previous embodiment, this operation is then followed by slow or accelerated cooling 5, as in two previous embodiments.

The fourth embodiment, illustrated in FIG. 4, differs from the third embodiment in that the hot rolling 1 is first followed by cooling 2 and by winding at a temperature t ₂ , below the temperature at the start of the bainite formation, and then directly from the income 3, in particular from a "bell" annealing of the black coil, that is to say not pickled. Pickling can take place after cooling 5.

The fifth embodiment, illustrated in FIG. 5, differs from the previous one in that the winding, after hot rolling 1, takes place at a temperature t ₂ for which the microstructure is composed of ferrite / perlite or ferrite / perlite / bainite.

As an alternative to the embodiments of FIGS. 4 and 5, it is possible, as in the second embodiment and as indicated by dashed lines 3 ′ in these FIGS. 4 and 5, producing the tempering 3 at a temperature below the temperature A ₁ .

The sixth embodiment, illustrated in FIG. 6, differs from the previous one in that, on the one hand, the winding of the sheet, after hot rolling, takes place at a temperature higher than the temperature At ₁ and below the temperature At ₁ + 50 ° C., this in order to keep a fraction of untransformed austenite and that, on the other hand, this winding is followed by maintenance substantially at this temperature under an isolation bell for at least 4 hours to enrich the unprocessed austenite with gamma elements, such as manganese and / or boron.

The previous embodiments provide a hot rolled product (LAC) which can have several types of microstructures, the nature of which is linked to the chemical composition of the austenite formed and at the final cooling rate (diagram TRC). It is therefore a ferrite matrix with islands of at least one of the following phases: residual austenite, bainite and / or martensite.

Advantageously, hot rolled steel can still undergo, as already mentioned above, after pickling either by a acid bath either by annealing in a reducing atmosphere, a stage of galvanization and / or electrochemical metallization, so as to obtain a sheet of galvanized or electrogalvanized steel, by example.

The embodiments of the method according to the invention illustrated schematically by the graphs of FIGS. 7 to 15 are relating to a coil which has been substantially cooled to room temperature, possibly followed by rolling to cold with, for example, a reduction rate between 40% and 90%.

Thus, the seventh embodiment according to FIG. 7 is distinguished from that of FIG. 1 by the fact that a cooling 5 says "slow" or accelerated by quenching with possibly a conventional cold rolling 6, is interposed between the income 3 and the heat treatment 4, which in this embodiment is formed continuous intercritical annealing.

During this cold rolling 6, generally a reduction in sheet thickness between 40 and 90% and a hardened microstructure, which recrystallizes during subsequent annealing 4. If such cold rolling is planned, this is preceded by a pickling which can be done before or after the income transaction 3.

It should also be noted that the sheet metal subject to cold rolling 5 is, before annealing 4, essentially consisting of ferrite and cementite rich in carburogenic elements, such as Mn, Cr, Mo and / or B.

The embodiment according to Figure 8 is a variant from the previous one, which is distinguished by the fact that the treatment thermal 4 is formed by discontinuous intercritical annealing, again called "intercritical bell annealing".

The embodiments according to FIGS. 9 and 10 are distinguished from that of FIG. 8 by the fact that the income 3 is dissociated into a partial income 3a at a temperature below the temperature A1 and an income 3b between the temperature A ₁ and temperature A ₁ + 50 ° C. In the embodiment according to FIG. 9, the heat treatment 4 is a continuous intercritical annealing, while in the embodiment according to FIG. 10, it is an intercritical annealing by bell annealing.

The two embodiments according to FIGS. 11 and 12 are distinguished from the preceding embodiments by the fact that both the tempering 3 and the heat treatment 4 take place at a temperature higher than the temperature A ₁ and that, from moreover, these two operations 3 and 4 are separated by cooling 5 and cold rolling 6.

In the embodiment according to FIG. 11, it is continuous intercritical annealing 3, and, in that of FIG. 12, it is an intercritical annealing by annealing bell 3.

In both of these embodiments, the austenite formed is enriched with so-called "gammagenic" elements, such as Mn, B andlou C, during the income 3.

In the embodiment illustrated in FIG. 13, the hot rolling 1 is followed by a slight cooling 2 and a winding below the temperature A ₁ and above the temperature t ₂ of the start of the bainite formation to obtain a ferrite / perlite microstructure. This temperature of the coil is then maintained by thermal insulation in a bell for at least 4 hours, as in the embodiment illustrated in FIG. 6.

The embodiment according to Figure 14 is comparable to that of Figure 4 or to that of Figure 5, except that a cooling 5, said to be "slow" or accelerated by quenching, possibly with cold rolling 6, is provided between tempering 3 and processing 4. The heat treatment 4 can be carried out by annealing. continuous intercritical or by an intercritical annealing bell.

The embodiment according to FIG. 15 differs from that of FIG. 13 by the fact that the hot rolling 1 is immediately followed by keeping the insulating bell at a temperature between the temperature A ₁ and the temperature A ₁ + 50 ° C to keep unprocessed austenite in the microstructure.

In the embodiments illustrated by the various figures, when the heat treatment 4 is formed by continuous intercritical annealing, this generally takes place at a temperature between the temperature A ₁ and 900 ° C. for 1 second to 5 minutes, optionally combined either with metallization by quenching, or followed by metallization by electrochemical deposition. If the heat treatment 4 is formed by a bell intercritical annealing, this preferably takes place at a temperature lying between the temperature A ₁ and 50 ° C. above this temperature.

• une meilleure qualité de surface et d'aptitude à la galvanisation comparés aux aciers multiphasés contenant du Si,
• un bon rapport entre la limite d'élasticité et la charge de rupture,
• de bonnes propriétés de formage, un allongement uniforme élevé,
• une augmentation de la limite d'élasticité et de la charge de rupture par effet de la précipitation secondaire des éléments de micro-alliages lors du revenu par rapport aux aciers multiphasés classiques,
• un coefficient d'écrouissage élevé lors d'une déformation,
• une protection contre la striction, grâce aux valeurs n élevées et un allongement uniforme élevé,
• un palier de vermiculure ("yield point elongation") faible ou non existant permettant des allongements de "skinpass" faibles,
• un bon comportement à la fatigue mécanique par une combinaison de phases dures formées de bainite et de martensite, et de phases douces, de ferrite et austénite résiduelle,
• une valeur d'absorption d'énergie élevée pendant la déformation à haute vitesse par suite des mécanismes de multiplication des dislocations.

It has been found that the steel obtained according to the process described above, more particularly the particular embodiments illustrated by FIGS. 1 to 15, have in particular the following properties:

• better surface quality and suitability for galvanizing compared to multi-phase steels containing Si,
• a good relationship between the yield strength and the breaking load,
• good forming properties, high uniform elongation,
• an increase in the elastic limit and the breaking load by the effect of the secondary precipitation of the elements of micro-alloys during tempering compared to conventional multi-phase steels,
• a high work hardening coefficient during a deformation,
• protection against necking, thanks to the high n values and high uniform elongation,
• a low or non-existent yield point elongation ("yield point elongation") allowing low "skinpass" elongations,
• good behavior under mechanical fatigue by a combination of hard phases formed from bainite and martensite, and from soft phases, ferrite and residual austenite,
• a high energy absorption value during high speed deformation as a result of dislocation multiplication mechanisms.

In order to further illustrate the purpose of this invention, below are given some concrete examples of chemical compositions of a multiphase steel according to the invention and of parameters of the different stages of development applied to this steel.

Example 1

This example was made according to the embodiment particular of the process illustrated by the graph in FIG. 7.

Chemical composition :

0.16% C

1.60% Mn

0.40% If

1)

Reheating :

temperature: 1280 ° C
time: 150 min.

2) roughing rolling: Final temperature: 1100 ° C (Step ¹⁾

3) Finishing rolling: final temperature: 850 ° C

4) Cooling table: cooling rate: 50 ° C / s

5) Winding temperature:
below 400 ° C, natural air cooling

6) Stripping:

7) Annealing bell ^(Step 2) heating rate 40 ° C / h holding temperature 640 ° C hold time 40 hrs cooling rate 15 ° C / h

8) Cold rolling: reduction rate 60%

9) Continuous Annealing: ^(Step 3) heating rate 15 ° C / s holding temperature 740 ° C hold time 50 sec cooling rate 40 ° C / s skinpass lengthenings 0.2%

10) Galvanization by passage through a bath of liquid zinc.

Figure 16 shows schematically a section transverse of the steel according to this example provided with a coating 8 of zinc obtained by galvanization. This steel includes a ferrite matrix 9 in which the islands 10 are distributed substantially uniformly residual austenite of substantially equivalent dimensions.

Mechanical properties obtained during an industrial test on a sheet sample 1.5 mm thick taken from the center line of the sheet in the direction of rolling subjected to a tensile test according to ISO standard with test piece, base 80 mm, width 20 mm. Yield strength Rp 0.2% (MPa) 370 Breaking load Rm (MPa) 580 Ratio Rp 0.2% / Rm (%) 63.8 Bearing elongation (%) 1.1 uniform elongation (%) 18.8 Total elongation (%) 31.2 Work hardening coefficient n 0.262 Anisotropy coefficient r 1.32

Example 2

This example was made according to the embodiment particular of the process illustrated by the graph in FIG. 3.

Chemical composition :

0.10% C

0.20% Mn

1.00% If

0.06% Ti

0.050% B

1)

Reheating :

temperature: 1300 ° C
time: 200 min.

2) Coating for roughing: final temperature:
1100 ° C (Step ¹⁾

3) Finishing rolling: final temperature: 860 ° C

4) Cooling table: cooling rate: 25 ° C / s

5) Winding temperature:
below 300 ° C, natural air cooling

6) Stripping:

7) Annealing bell: heating rate 100 ° C / h holding temperature 740 ° C hold time 200 h cooling rate 25 ° C / h

Figure 17 schematically shows the structure of steel obtained following this example.

This steel comprises a ferrite matrix 9 in which are distributed substantially uniformly the islands 10 of bainite substantially equivalent dimensions.

Example 3

This example was made according to the embodiment particular of the process illustrated by the graph of Figure 7

Chemical composition :

0.80% C

3.00% Mn

0.2% If

1)

Reheating :

temperature: 1150 ° C
time: 135 min.

2) Coating for roughing: final temperature:
1000 ° C (Step ¹⁾

3) Finishing rolling: final temperature: 750 ° C

4) Cooling table: cooling rate: 40 ° C / s

5) Winding temperature:
below 200 ° C, natural air cooling

6) Stripping

7) Annealing bell ^(Step 2) heating rate 60 ° C / h holding temperature 300 ° C hold time 10 a.m. cooling rate 50 ° C / h

8) Cold rolling: reduction rate 40%

9) Continuous annealing: heating rate 10 ° C / s holding temperature 730 ° C hold time 240 sec cooling rate 100 ° C skinpass lengthenings ≥ 0.2%

10) Electro-zinc plating

Figure 18 shows schematically the microstructure of the steel obtained according to this example provided with a zinc coating 8 formed by electrodeposition. This steel has a ferrite matrix 9 in which are distributed substantially uniformly islands 10 of martensite of very reduced dimensions.

Example 4

This example was made according to the embodiment particular of the process illustrated by the graph of Figure 7

Chemical composition

0.170% C

1,800% Mn

0.020% Ti

0.080% V

0.003% B

1) Reheating at 1280 ° C, for 150 min

2) Coarse rolling: final temperature of 1100 ° C

3) Finishing rolling: final temperature of 910 ° C up to a thickness of 2.0 mm

4) Cooling table: cooling rate of 50 ° C / s

5) Winding temperature: temperature of 350 ° C, cooling of the air coil to room temperature

6) Classic pickling, i.e. with the coil unwound in hot HCl

7)

Basic annealing:

40 ° C / hour rise
holding temperature of 520 ° C
40 h hold time
cooling rate of 15 ° C / hour to room temperature

8) Cold rolling with a reduction rate of 60%

9)

Continuous annealing:

15 ° C / s rise
holding temperature of 820 ° C
hold time 46 s
30 ° C / s cooling rate
0.6% skinpass level

10) Metallization: classic galvanization combined with annealing continued

The mechanical properties obtained during an industrial test on a sheet sample of a thickness of 1.5 mm taken from the center line of the sheet in the direction of rolling subjected to a traction of 20/80 (ISO standard) are as follows : Yield strength Rp0.2% (MPa): 471 Breaking load Rm (MPa): 649 Ratio Rp0.2% / Rm (%) 72.6 Bearing elongation (%) 1.0 Uniform elongation (%) 14.1 Total elongation (%) 28.3 Work hardening coefficient n 0.221

Figure 19 shows schematically the structure of steel obtained according to this example provided with a zinc coating 8 formed by galvanizing. This steel has a ferrite matrix 9 in which are distributed substantially uniformly islands 10 of austenite residue and precipitates 11 of vanadium carbides.

Example 5

This example was made according to the embodiment particular of the process illustrated by the graph in FIG. 13.

Chemical composition

0.800% C

3,000% Mn

1) Reheating at 1100 ° C for 200 min

2) Coarse rolling: final temperature of 1000 ° C

3) Finishing rolling: final temperature of 740 ° C up to 1.5 mm thick

4) Cooling table: cooling rate of 10 ° C / s

5) Winding temperature: temperature of 620 ° C, maintained by insulation bell

6)

Insulation bell:

holding temperature of 620 ° C
168 h hold time
25 ° C / hour cooling rate

7) Cold rolling with a reduction rate of 50%

8)

Continuous annealing:

10 ° C / s rise
holding temperature of 850 ° C
hold time 40 s
25 ° C / s cooling rate
without skinpass

9) Metallization: electro-zinc coating.

Figure 20 shows schematically the structure of steel obtained according to this example provided with a zinc coating 8 formed by plating. This steel includes a ferrite matrix 9 in which are distributed substantially uniformly with islets of martensite 10.

Example 6

This example was made according to the embodiment particular of the process illustrated by the graph in FIG. 14.

Chemical composition

0.100% C

0.200% Mn

0.400% If

0.040% Ti

0.050% Nb

0.200% Cr

0.200% MB

1) Reheating at 1300 ° C for 135 min

2) Coarse rolling: final temperature of 1100 ° C

3) Finishing rolling: final temperature of 890 ° C up to a thickness of 3.0 mm

4) Cooling table: cooling rate of 50 ° C / s

5) Winding temperature: temperature of 400 ° C, direct application basic annealing

6)

Base annealing:

60 ° C / h rise
holding temperature of 580 ° C
20 h hold time
cooling rate of 50 ° C / hour

7) Classic stripping

8) Cold rolling with a reduction rate of 70%

9)

Continuous annealing:

10 ° C / s rise
holding temperature of 740 ° C
hold time 240 s
cooling rate of 70 ° C / s
0.6% skinpass level

Figure 21 shows schematically the structure of steel obtained following this example. This steel includes a ferrite matrix 8 in which are distributed substantially uniformly islands 10 of martensite and bainite as well as precipitates 11 of titanium carbides and niobium carbides.

Example 7

This example was made according to the embodiment particular of the process illustrated by the graph in FIG. 6.

Chemical composition

0.150% C

2,000% Mn

0.030% B

1) Reheating at 1280 ° C for 150 min

2) Coarse rolling: final temperature of 1100 ° C

3) Finishing rolling: final temperature of 840 ° C up to a thickness of 6.0 mm

4) Cooling table: cooling rate of 15 ° C / s

5) Winding temperature: temperature of 740 ° C, maintained by insulation bell

6)

Insulation bell:

holding temperature of 740 ° C
40 h hold time
accelerated cooling rate of 5 ° C / s

Figure 22 shows schematically the structure of steel obtained according to this example provided with a calamine film 8. This steel presents a ferrite matrix 9 in which are distributed substantially uniform islets 10 of martensite and austenite residual.

Example 8

This example was made according to the embodiment particular of the process illustrated by the graph in FIG. 5.

Chemical composition

0.400% C

0.600% Mn

0.020% Ti

0.002% B

0.400% AI

0.400% If

1) Reheating at 1250 ° C for 200 min

2) Coarse rolling: final temperature of 1080 ° C

3) Finishing rolling: final temperature of 800 ° C up to 2.5 mm thick

4) Cooling table: cooling rate of 20 ° C / s

5) Winding temperature: temperature of 600 ° C, application direct from base annealing

6)

Base annealing:

rise of 30 ° C / h
holding temperature of 750 ° C
100 h hold time
natural air cooling

7) Classic pickling.

Figure 23 shows schematically the structure of steel obtained according to this example comprising a ferrite matrix 9 in which are distributed substantially uniformly of bainite islands.

It is understood that the invention is not limited to the embodiment of the particular method of preparing a steel, but, on the other hand, extends to any steel having substantially the same structure and morphology as the multiphase steel obtained by the process specific described above and illustrated by the appended figures.

Claims (25)

Flat product, such as sheet metal, made of multiphase steel whose chemical composition contains carbon and manganese, characterized in that this steel has a structure capable of being obtained by a process comprising the following steps: hot rolling, at a temperature at which the austenitic phase is stable, of a steel containing from 0.05 to 0.8% of C, 0.2 to 3.0% of Mn and Si ≤ 1%, maintaining or returning to a temperature between 300 ° C and a temperature of 50 ° C above the eutectoid temperature (A ₁ ) for at least 4 hours, possibly followed by cooling to room temperature, so as to form a phase enriched in carburigenic and / or gammagenic elements, such as carbon and manganese in a ferrite matrix, a heat treatment at a temperature higher than the above eutectoid temperature (A ₁ ) and lower than the austenite formation temperature (A ₃ ) so as to form islets of austenite and / or to enrich with gamma elements such as Mn austenite already formed, and subsequent cooling to room temperature in such a way as to obtain a final product having a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite. Product according to claim 1, characterized in that the aforementioned method comprises a step of pickling after rolling hot. Product according to claim 1, characterized in that the above-mentioned method comprises a step of pickling after the step of maintaining or tempering at a temperature between 300 ° C and a temperature of 50 ° C above the eutectoid temperature ( A ₁ ) or after the above heat treatment. Product according to any one of claims 1 to 3, characterized in that the method comprises a step of cooling after hot rolling at a speed and to a temperature below the temperature A ₁ which are such as to obtain a microstructure containing either bainite and / or martensite, or perlite with possibly ferrite. Product according to any one of Claims 1 to 3, characterized in that, when the aforementioned hot rolling makes it possible to obtain a rollable sheet, this rolling is followed by a winding of the sheet obtained at a temperature between the eutectoid temperature ( A ₁ ) and a temperature of 50 ° C. above this eutectoid temperature, so as to keep unprocessed austenite in the microstructure obtained. Product according to any one of claims 1 to 3, characterized in that the steel subjected to the aforementioned hot rolling also includes at least one of the following: B, Ti, Nb, Zr, V, AI, N, S, P, Cr, Ni, Cu, Mo in the following concentrations: B ≤ 0.100%; Ti, Nb, Zr or V each ≤ 0.200 ° 16, Al ≤ 0.400%; N ≤ 0.100%; P ≤ 0.100%; Cr, Ni or Cu each ≤ 2,000%; Mo ≤ 0.500%. Product according to any one of claims 1 to 6, characterized in that the steel subjected to the aforementioned hot rolling contains at most 0.5% Si, more particularly less than 0.2% of Si, and is preferably substantially free of Si. Product according to either of Claims 6 or 7, characterized in that the following elements: Al, N, B have respectively the following concentrations: AI ≤ 0.100%; N ≤ 0.05%; B ≤ 0.060%; Ti, Nb, Zr or V each ≤ 0.100%; Cr ≤ 0.500%. Product according to any one of claims 1 to 8, characterized in that it has a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite, the content of this phase in the ferrite matrix being less than 40% by volume. Product according to any of claims 1 and 9, characterized in that the aforementioned method comprises a step of cooling to room temperature after the maintenance or income mentioned above, this last step then being possibly followed by a cold rolling step before the step of aforementioned heat treatment. Process for the manufacture of a flat product, such as a steel sheet, according to any one of Claims 1 to 10, characterized in that it comprises the following steps: hot rolling, at a temperature at which the austenitic phase is stable, of a steel containing from 0.05 to 0.8% of C, 0.2 to 3.0% of Mn and Si ≤ 1%, tempering or maintaining at a temperature between 300 ° C and a temperature of 50 ° C above the eutectoid temperature (A ₁ ) for at least 4 hours, possibly followed by cooling to room temperature, so as to form a phase enriched in gammagenic and / or carburogenic elements, such as C and Mn in a ferrite matrix, a heat treatment higher than the above eutectoid temperature (A ₁ ) and lower than the austenite formation temperature (A ₃ ) so as to form islets of austenite and / or to enrich with gamma elements, such as Mn of l austenite already formed, and subsequent cooling to room temperature in such a way as to obtain a final product having a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite. Process according to Claim 11, characterized in that the above-mentioned tempering is carried out at a temperature below the eutectoid temperature (A ₁ ) so as to form cementite and to cause the diffusion of carburogenic elements, such as Mn in this cementite. Process according to Claim 11, characterized in that the aforementioned tempering is carried out at a temperature above the eutectoid temperature (A ₁ ) so as to form austenite and cause gamma elements to diffuse in this austenite. Method according to claim 11 or 12, characterized in that the aforementioned income is followed, before the heat treatment mentioned above, from cooling to room temperature for one in such a way as to slow down the enrichment of the phase already enriched in gamagenic and / or carburogenic elements. Method according to claim 13 or 14, characterized in that the heat treatment is formed by continuous annealing at a temperature of at most 900 ° C for at most 5 minutes, so as to transform the enriched phase into elements gammagènes etlou carburigènes in austenite whose composition chemical corresponds substantially to that of the phase enriched in gammagenic and / or carburogenic elements, this annealing then being followed rapid cooling, for example of the order of 10 ° C to 300 ° C per second. Method according to claim 11, characterized in that the aforementioned income is directly followed, without cooling intermediate, of the aforementioned heat treatment, formed of an annealing bell for at least 2 hours, in such a way as to transform from cementite to austenite, this heat treatment then being followed cooling for example of the order of 10 ° C to 40 ° C per hour or accelerated cooling by quenching, for example of the order of 20 ° C per minute. Method according to any of claims 11 to 16, characterized in that it comprises a cooling step between hot rolling and the above maintenance or tempering. Method according to claim 17, characterized in that that the aforementioned cooling is followed by a pickling and possibly cold rolling before the aforementioned heat treatment. Method according to any of claims 11 to 18, characterized in that the heat treatment consists of a bell annealing and is followed by slow cooling; for example from around 10 ° to 40 ° C per hour or accelerated cooling by quenching, for example of the order of 20 ° C per minute. Process according to claim 18 or 19, characterized in that the cold rolling is followed by annealing at a temperature between the eutectoid temperature (A ₁ ) and the austenite formation temperature (A ₃ ), this annealing being then followed by rapid cooling, for example of the order of 10 ° C to 300 ° C per second. Process according to any one of Claims 11 to 20, characterized in that the abovementioned tempering is carried out in two successive stages, a first stage at a temperature between 300 ° C and the eutectoid temperature (A ₁ ) and a second stage at a temperature between at the eutectoid temperature (A ₁ ) and a temperature of 50 ° C above this temperature A ₁ . Process according to any one of Claims 11 to 20, characterized in that the aforementioned tempering or maintenance and the aforementioned heat treatment are carried out in a single operation at a temperature A ₁ and 50 ° C above this temperature A ₁ . Process according to any one of Claims 11 to 16 and 18 to 22, characterized in that the hot rolling is directly followed by tempering either at a temperature above the temperature A ₁ and below the temperature A ₁ + 50 ° C either at a temperature below the temperature A ₁ . Process according to any one of Claims 11 to 22, characterized in that it comprises, between the hot rolling and the aforementioned holding or tempering, cooling and winding either at a temperature at which the microstructure is composed by bainite etlou martensite, ie at a temperature below the temperature A ₁ at which the microstructure is composed of ferrite / perlite or ferrite / perlite / bainite. Method according to any of claims 11 to 24, characterized in that the heat treatment is combined or followed by a step of forming a metallic coating, such as galvanizing or electrochemical metallization.

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