EP0985054B1 - Method for continuous manufacture of a steel band for drawing with improved surface properties - Google Patents

Description

The present invention relates to a continuous manufacturing process for a strip of stamping steel with improved surface properties, in particular greater surface hardness according to Brinell, Vickers, ... tests, better resistance to indentation and higher suitability for coating with a layer protective metal such as during hot dip galvanizing.

Steel is undoubtedly a material used in a very large number of fields manufacturers to manufacture elements of variable shapes and dimensions, in particular using deformation and shaping techniques by stamping deep and extra-deep.

In this regard, we will mention more specifically and without this constituting a limitation of the fields of application of the present invention, that the automobile industry requires steel strips with increasing mechanical properties large while wishing to maintain deformation capabilities in accordance with stamping processes generally used to manufacture body parts. We know that these requirements are often conflicting and that manufacturing steels current can not meet them while the steel strips manufactured according to the process of the invention meet these requirements and can be shaped by deep and extra deep stamping.

In this context, two complementary paths have been developed, on the one hand research of new grades of steel and on the other hand the development of various treatment methods, mainly of the thermal type, to give the strips steel, including the new grades mentioned above, sufficient deformability for their use in deep and extra deep drawing operations.

Special attention will be paid to explaining the processes implementing a heat treatment called annealing, operated continuously, framework in which the method of the present invention.

Among the grades of steels capable of undergoing deep drawing operations or extra deep, we will mention mild low carbon steels of the ELC type (ELC = Extra Low Carbon, i.e. with C <0.1%) and of the ULC type (ULC = Ultra Low Carbon, i.e. with C <0.01%).

In particular, subsequent developments have given rise to other grades of steels called "Interstitial Free" or "IF". They are characterized by their very low content of interstitial elements (C, N) and by the presence of elements with high affinity for carbon, and nitrogen such as Ti and / or Nb. These items are added in sufficient quantity to be able to react with all of the C and N.

To obtain mechanical properties allowing deep drawing, the strips steel must have a particular texture which is the result of a certain number of constraints in their manufacturing process. Among other things, we must respect a certain range of winding temperatures after hot rolling, a range of cold rolling reduction rate values and various parameters defining the continuous annealing cycle operated later.

Many patents and scientific articles describe in detail the procedure which is well known to all manufacturers of automotive sheet metal. In particular, document EP-A-0 559 225 describes a process as claimed herein in claim 1, except for the H ₂ O, H ₂ and CO conditions of the gaseous atmosphere of the reactive annealing and the carburization rate. In addition, it is known that it is important to avoid having free interstitial elements during the recrystallization at the start of continuous annealing to form the texture (111) favorable for stamping.

Adequate choice of steel composition combined with optimization in heat treatments performed on the steel strip in question, allows obtaining of a product with mechanical properties which are consistent with what requires deep drawing, i.e. a Lankford coefficient r> 2, a high work hardening coefficient n, a high elongation at break El (El = Elongation), a low elastic limit YS (YS = Yield Strength), a limit plateau weak YPE elastic and preferably zero (YPE = Yield Point Elongation).

However, the use of this type of IF steel is not without posing many problems in terms of job properties. This is how we see fragility intergranular CWE increased during cold deformation (CWE = Cold Working Embrittlement), weak mechanical properties such as YS elastic limit, breaking load TS (TS = Tensile Strength), or the hardness, therefore a weak aptitude at cutting and low friction properties, as well as more difficult in galvanizing by the appearance of defects at the grain boundaries of the steel (outburst structure) and less resistance in service.

• l'ajout de bore à la composition chimique de l'acier pour diminuer la fragilité à froid du joint de grains GB (GB = Grain Boundaries),
• l'ajout de niobium comme élément piégeant le C, associé à un recuit à plus haute température (> 820°C) et un refroidissement accéléré permettant de dissoudre une partie des carbures et de garder quelques ppm de C en solution; ces tôles d'acier étant généralement peintes, la présence de ce C permet également de regagner quelques MPa de limite élastique lors de la cuisson de la peinture BH (BH = Bake-Hardening). La température de recuit nécessaire à la remise en solution du NbC devient néanmoins excessive pour les fours de recuit en vertu d'une limitation de productivité ainsi que de l'apparition de problèmes de guidage de la bande (heat buckles),
• l'introduction de C en quantités modérées après la recristallisation par des procédés de recarburation pour diminuer la fragilisation lors du formage à froid CWE et donner des possibilités de durcissement lors du traitement de cuisson de la peinture (effet BH).

To remedy some of these problems, solutions have been proposed such as:

• the addition of boron to the chemical composition of the steel to reduce the cold brittleness of the grain boundary GB (GB = Grain Boundaries),
• the addition of niobium as an element trapping C, associated with annealing at a higher temperature (> 820 ° C) and accelerated cooling making it possible to dissolve part of the carbides and to keep a few ppm of C in solution; these steel sheets being generally painted, the presence of this C also makes it possible to regain a few MPa of elastic limit during the curing of the BH paint (BH = Bake-Hardening). The annealing temperature necessary for re-solution of the NbC nevertheless becomes excessive for the annealing furnaces by virtue of a limitation of productivity as well as the appearance of problems of guiding the strip (heat buckles),
• the introduction of C in moderate quantities after recrystallization by recarburization processes to reduce embrittlement during cold CWE forming and give possibilities of hardening during the baking treatment of the paint (BH effect).

In this regard, we will mention the existence of a part of a carburetion process in continuous in a non-soaking atmosphere, therefore with low carbon potential, and on the other hand a process of controlled recarburization of IF steel initially having a carbon concentration [C] ≤ 0.01%.

These processes give rise to steels with improved properties within the framework of the volume but not surface mechanical properties, while being suitable for coating.

It is also known that rephosphorous shades have been developed for increase the mechanical strength of steels for stamping, in particular in order to make thinner body sheets. However, the phosphorus included in this type of steel has the defect of reducing the Lankford coefficient r and of increase intergranular fragility (CWE), while greatly reducing reactivity so-called steels during the annealing of Zn-Fe layers during galvanizing soaking resulting in a decrease in the productivity of the lines of galvanizing.

In summary, the various manufacturing processes mentioned above lead either to obtaining steel grades suitable for deep drawing operations EDDQ (EDDQ = Extra Deep Drawing Quality) but with weak properties mechanical (YS, TS), surface hardness and resistance during cold forming low, either with improved mechanical properties (YS, TS, CWE) but obtained at the expense of stamping properties.

In the current state of the art, only the introduction of Nb to steel chemistry, subject problems mentioned above, or moderate carburetion processes allow obtain a texture favorable to deep drawing (EDDQ) associated with paint annealing (BH) hardening possibilities. In all cases, however, this hardening is deliberately limited, either to avoid soaking during carburation treatment, either to limit the aging favored by an excess of C not precipitated, or finally to limit the annealing temperature, case of ULC-Ti-Nb or ULC-Nb.

Consequently, the mechanical properties of these steel strips are low, particularly their surface hardness, which leads to problems when cutting and stamping (friction) as well as during their subsequent use, especially because of their low resistance to indentation.

The process of the present invention makes it possible to manufacture steel strips from low carbon grades that have deep draw characteristics satisfactory, even for deep or extra deep drawing, while having surface properties without the above limitations, and it can be executed continuously, which is an advantage for its profitability in the context of a economical industrial application.

The process for the continuous production of a steel strip for stamping with improved surface properties, object of the present invention, in which a strip, having undergone hot rolling with end-of-rolling winding at a temperature between 500 and 800 ° C. and a cold rolling with a reduction rate of at least 30% and preferably 75%, undergoes a heating step, then a heat treatment according to the present invention, hereinafter called reactive annealing, making it possible on the one hand to recrystallize, that is to say regeneration of the crystal lattice following the deformation, and on the other hand to carburize the strip at a temperature T, the two possibly being simultaneous or not depending on the type of final properties of the targeted strip, and finally said strip being subjected to cooling, is essentially characterized in that the reactive annealing in question is carried out completely or partially in a gaseous atmosphere comprising at least CO and H2 in volume concentrations satisfying the following relationship: [% vol H2] + 2 [% vol CO] - 80 ≥ 0, at least H2O and possibly CO2 with concentrations less than or equal to 3% by volume, optionally another gas which has the object of simultaneously carrying out a surface enrichment treatment, a balance in a neutral gas such as N2, in that the carburetion temperature T is between 650 and 950 ° C, in that the total duration t of the aforementioned reactive annealing heat treatment is between 0.1 and 300 seconds, preferably between 0.1 and 20 seconds , and in that the carburetion speed is greater than or equal to 10 ^(-10) .exp (T / 100) g.cm (-2) .s (-1), where T is the carburetion temperature expressed in degrees Kelvin.

According to a preferred variant of the process of the aforementioned invention, the atmosphere gas also includes another gas which aims to simultaneously operate a surface enrichment treatment such as nitriding, boriding, sulfurization, oxidation, ....

This is how we can associate the carburetion process simultaneously or successively a step of nitriding, sulfurizing, boriding and / or of oxidation to improve the properties of the layer from a resistance point of view corrosion, reduced coefficient of friction and / or improved reactivity during subsequent finishing treatments.

According to a first embodiment of the process of the present invention, the strip is made of a steel of the "Interstitial Free" or "IF" type containing titanium contents with or without niobium respecting the following relationships: Ti * = total [Ti] - ((48/32) [S] + (48/14) [N]) 1≤ ((Ti * / 48) + ([Nb] / 93)) / ([C] / 12).

According to a particular embodiment of the process of the present invention, the strip is made of a steel of the "Interstitial Free" or "IF" type, the Nb content of which respects the following relationship: 1≤ ([Nb] / 93)) / ([C] / 12).

In the context of the aforementioned steel grades, it was surprisingly found that obtained perfectly clean strip surfaces even with atmospheres deemed to be soapy.

According to another embodiment of the process of the invention, a surface layer of carbides is formed on the strip, the thickness of which is between 0.01 and 50 μm .

The advantage of high fuel potentials is the realization at moderate temperature (eg 810 ° C.) of an austenitic phase in skin during the carburetion process while retaining a ferritic structure at heart.
The austenitic phase is characterized by a greater solubility of the interstitial elements (C, N, B) and by a lower diffusion coefficient.
Consequently, by favoring the formation of the austenitic phase in skin by a rapid carburization of this one, one can achieve a hard skin very rich in carbon with a soft heart having developed a microstructure favorable to stamping (Lankford coefficient> 2) by annealing in the ferritic phase. Indeed, the austenitic skin hardens by enriching itself in carbon while limiting the diffusion of this one towards the core of the steel.
This way of proceeding makes it possible to obtain surface properties clearly different from those of the core of the steel and differs from existing products resulting from treatment processes using either atmospheres with lower carbon potential, or aiming for a lower carburation and more homogeneous.

According to a preferred variant of the preceding modality, the microstructure and surface hardness by cooling the strip after reactive annealing at a speed of between 4 and 1000 ° C / second, preferably between 4 and 100 ° C / second.

According to yet another method of implementing the method of the invention, the steel strip undergoes diffusion heat treatment after carburetion low or non-fuel atmosphere before final cooling.

The aforementioned diffusion treatment in a controlled atmosphere allows both to homogenize the properties of the skin and restore surface cleanliness sufficient for possible subsequent finishing treatments such as galvanizing by dipping or by electrodeposition, phosphating-painting, ....

According to yet another method of implementing the method of the invention, the carburation is carried out using an ionized carburation gas in the form of plasma.

According to a preferential alternative of the preceding modality, the two sides of the steel strip undergo differentiated treatments.

The two previous methods have the effect, on the one hand, of increasing the possibilities of application of the process by allowing several surface treatments to be carried out via separate plasmas, and on the other hand to provide an attractive solution in the case of differentiated treatments according to the faces of the steel strip depending specific subsequent applications.

According to another preferred variant of implementation of the process of the present invention, an overaging treatment is carried out after the cooling of the steel strip.

According to yet another method of implementing the method of the invention, the carburation is operated, totally or in part, in at least one demarcated area, called an independent zone, which is either juxtaposed, or partially integrated or completely in the structure of one of the traditional strip preheating ovens, for heating, holding at temperature or subsequent overaging as is known to proceed commonly during the manufacture of a steel strip.

According to a different method of implementing the method of the invention, the carburetion is carried out in whole or in part during the cooling of the strip.

In the context of using independent zones for reactive annealing, that is to say zones forming part of a succession of treatment zones thermal in which one or more of these zones are explicitly allocated to operate a carburetion, we will note the advantage that this provision has if we wishes to carry out several types of treatment successively by injecting different concentrations even at the level of a different treatment of the two tape faces.

According to a preferred method of implementing the method of the invention, the steel strip passes first in a preheating zone of the strip, then in an independent carburation treatment zone, then a heating zone with temperature maintenance, then undergoes cooling and finally a treatment of aging, the existence of which is linked to the desired final properties.

According to another preferred method of implementing the method of the invention, the steel strip passes first into a preheating zone, then into a zone heating to temperature, then in an independent treatment area carburetion, then a temperature holding zone followed by cooling and finally an overaging treatment whose existence is linked to the final properties desired.

According to yet another preferred method of implementing the method of the invention, the steel strip first passes through a preheating zone of the strip, then in a temperature heating zone, then in a first zone temperature maintenance followed by an independent carburation treatment zone, then in a second temperature holding zone followed by cooling and finally an overaging treatment whose existence is linked to the final properties desired.

According to yet another preferred method of implementing the method of the invention, the steel strip enters a heat treatment unit in which it passes first in a recrystallization zone, then in an area independent of carburation treatment, then in a treatment zone of diffusion, then leaves the heat treatment unit in question.

Figure 1 shows schematically the treatment path followed by the steel strip (B) under the previous modality. We distinguish the treatment unit thermal (1), the latter being maintained under a determined atmosphere, the strip (B) first passes through a recrystallization zone (2), then into a zone independent of treatment (3) by means of a reactive gas injected at the flow rate (Q), then passes into a diffusive treatment zone (4), and then leaves the heat treatment (1) in question.

t(s) :

durée du traitement en secondes,

HV1 :

dureté Vickers superficielle (charge 1 kg),

HV3 :

dureté Vickers superficielle (charge 3 kg),

HV10 :

dureté Vickers superficielle (charge 10 kg),

YS (MPa) :

limite élastique exprimée en mégapascals,

TS (MPa) :

charge de rupture exprimée en mégapascals,

r :

coefficient de Lankford,

[C]total :

concentration moyenne en carbone exprimée en ppm.

By way of example of the results obtained by applying the method of the present invention, the values below have been collected in the table below of certain mechanical properties measured on four treated steels as well as on an untreated steel serving as a reference.
The four steels T1, T2, T3 and T4 underwent reactive annealing at a temperature of 810 ° C in an atmosphere composed of 45% CO, 0.25% H2O, 25% H2, balance N2, for periods varying from 1 to 300 seconds.
The different symbols used are:

t (s):

duration of treatment in seconds,

HV1:

Vickers surface hardness (load 1 kg),

HV3:

Vickers surface hardness (load 3 kg),

HV10:

Vickers surface hardness (load 10 kg),

YS (MPa):

elastic limit expressed in megapascals,

TS (MPa):

breaking load expressed in megapascals,

r:

Lankford coefficient,

[C] total:

average carbon concentration expressed in ppm.

It appears that the layer of perlite formed in skin increases with the duration of the treatment and that the steels treated have values of YS, TS and hardness of significantly greater surface area than that of reference steel.

• réalisation de propriétés de surface nettement différentes des propriétés du coeur comme par exemple l'obtention d'une dureté de surface élevée associée à un coeur ductile;
• amélioration de la résistance à l'indentation, de la résistance à la corrosion et diminution de la résistance au frottement;
• possibilité de modifier la réactivité de la tôle au cours des traitements de finition;
• possibilité de réaliser le traitement de carburation dans des zones indépendantes, c'est-à-dire sans modification majeure des lignes de recuit continu existantes;
• possibilité de réaliser un traitement différencié suivant les faces de la bande en acier.

In conclusion, we will mention all the advantages associated with the process, namely:

• achievement of surface properties clearly different from the properties of the core such as, for example, obtaining a high surface hardness associated with a ductile core;
• improvement in resistance to indentation, resistance to corrosion and reduction in resistance to friction;
• possibility of modifying the reactivity of the sheet during finishing treatments;
• possibility of carrying out the carburetion treatment in independent zones, that is to say without major modification of the existing continuous annealing lines;
• possibility of carrying out a differentiated treatment according to the faces of the steel strip.

Claims (24)

1. A method for continuously producing a steel strip for deep drawing having improved surface properties, wherein a strip, after been submitted to a hot rolling step with a winding step at the rolling end at a temperature in the range between 500 and 800°C, as well as a cold rolling step with a reduction rate of at least 30%, preferably 75%, is submitted to a heating step, followed by a thermal treatment according to the present invention, hereafter so-called reactive annealing step, making it possible to carry out, on the one hand, a re-crystallization, i.e., a regeneration of the crystalline network as a result of deformation and, on the other hand, a strip carburization at a temperature T, both being able to be simultaneous or not depending upon the type of expected final properties in the strip and finally said strip being submitted to a cooling step, characterized in that said reactive annealing step is completely or partially performed under a gaseous atmosphere comprising at least CO and H₂ in volume concentrations according to the following formula : [% vol H2] + 2[% vol CO]-80 ≥ O, at least some H₂O and possibly CO₂ having concentrations of less than or equal to 3% in volume, optionally another gas, the object of which is to perform simultaneously a surface enrichment treatment, with a neutral gas balance such as N₂, in that the carburization temperature T is in the range between 650 and 950°C, in that the total duration t of the thermal treatment in said reactive annealing step is in the range between 0.1 and 300 seconds and in that the carburization rate is higher than or equal to 10^(-10).exp (T/100) g.cm(-2).s(-1), where T is the carburization temperature expressed in Kelvin degrees.
2. A method according to claim 1, characterized in that the total duration t of the thermal treatment in said reactive annealing step is in the range between 0.1 and 20 seconds.
3. A method according to claim 1 or claim 2, characterized in that the gaseous atmosphere also comprises another gas, the object of which is to perform simultaneously a surface enrichment treatment.
4. A method according to claim 3, characterized in that the surface treatment is a nitride hardening.
5. A method according to claim 3, characterized in that the surface treatment is a boride hardening.
6. A method according to claim 3, characterized in that the surface treatment is a sulphuration.
7. A method according to claim 3, characterized in that the surface treatment is an oxidation.
8. A method according to any of claims 1-7, characterized in that the strip is comprised of an "Interstitial Free" or "IF" type steel, containing either titanium, either niobium or both.
9. A method according to claim 8, characterized in that the titanium content meets the following relationships : Ti* = total[Ti]-((48/32) [S]+(48/14) [N]) 1 ≤ ((Ti*/48)+([Nb)/93))/([C)/12) wherein the contents are expressed in weight percentage.
10. A method according to claim 8, characterized in that the niobium content meets the following relationship : 1 ≤ ([Nb]/93)/([C]/12) wherein the contents are expressed in weight percentage.
11. A method according to one or more of claims 1-10, characterized in that, on the strip, a surface carbon layer is formed, the thickness of which is within the range between 0.01 and 50 µm.
12. A method according to one or more of claims 1-11, characterized in that the carbide treatment is performed through an ionised carburization gas in the form of plasma.
13. A method according to one or more of claims 1-12, characterized in that the surface microstructure and hardness are modulated by performing strip cooling step after the reactive annealing step at a rate in the range between 4 and 1000°C/second.
14. A method according to claim 13, characterized in that the strip cooling step is performed after the reactive annealing step at a rate in the range between 4 and 100°C/second.
15. A method according to any of claims 1-14, characterized in that the steel strip is subjected, after carburization, to a thermal diffusion treatment under a slightly carburizing atmosphere before the final cooling.
16. A method according to any of claims 1-14, characterized in that the steel strip is subjected, after carburization, to a thermal diffusion treatment under a non carburizing atmosphere before the final cooling.
17. A method according to any of claims 1-16, characterized in that both sides of the steel strip are subjected to different treatments.
18. A method according to one or more of claims 1-17, characterized in that an over-ageing treatment is performed after cooling the steel strip.
19. A method according to one or more of claims 1-18, characterized in that the carburization step is performed, completely or partially, in at least a bordered area, called independent area, which is either superimposed with, or partially or completely integrated within the structure of one of the conventional strip ovens for preheating, heating, temperature holding or subsequent over-ageing steps, as it is known to proceed usually for producing a steel strip.
20. A method according to one or more of claims 1-19, characterized in that said carburization is performed completely or partially when cooling the strip.
21. A method according to any one of claims 1-19, characterized in that the steel strip first goes through a strip preheating area, followed by an independent carburization treatment area and then a heating zone with the temperature being held, then is subjected to a cooling step and finally to an over-ageing treatment.
22. A method according to any one of claims 1-19, characterized in that the steel strip first goes through a preheating area, followed by a temperature heating zone, then in an independent carburization treatment area, followed by a temperature holding area, followed by a cooling step and finally an over-ageing treatment, the existence of which is dependent upon the desired final properties.
23. A method according to any of claims 1-19, characterized in that the steel strip first goes through a strip preheating area, then in a temperature heating area, then in a first temperature holding area followed by an independent carburization treatment area, then in a second temperature holding area followed by a cooling step, and finally an over-ageing treatment the existence of which is dependent upon the desired final properties.
24. A method according to one or more of claims 1-19, characterized in that the steel strip enters a thermal treatment unit, in which it first goes through a re-crystallisation area, then in an independent carburization treatment area, then in a treatment diffusion area, leaving finally through the thermal treatment unit.

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