EP0985054A1

EP0985054A1 - Method for continuous manufacture of a steel band for drawing with improved surface properties

Info

Publication number: EP0985054A1
Application number: EP98916676A
Authority: EP
Inventors: Jean-François WILLEM; Jean Crahay; Vincent Leroy
Original assignee: Centre de Recherches Metallurgiques CRM ASBL
Current assignee: Centre de Recherches Metallurgiques CRM ASBL
Priority date: 1997-05-27
Filing date: 1998-04-28
Publication date: 2000-03-15
Anticipated expiration: 2018-04-28
Also published as: DE69802525T2; DE69802525D1; WO1998054371A1; BE1011178A3; EP0985054B1

Abstract

The invention concerns a method for continuous manufacture of a steel band for drawing with improved surface properties, whereby a band, after being hot rolled and coiled at the end of the rolling process at a temperature ranging between 500 and 800 °C, and cold rolled with a reduction ratio of at least 30 % and preferably 75 %, is subjected to a heating step, then a thermal treatment hereafter called reactive annealing, for producing re-crystallisation, i.e. regeneration of the crystal lattice following deformation, and carburizing of the band at a temperature T, simultaneously or not depending on the final properties required for the band, and finally said band is cooled. The invention is characterised in that said reactive annealing is wholly or partially carried out under a gaseous atmosphere comprising at least CO and H2 in concentrations by volume corresponding to the following relationship: [ % vol H2] + 2 [ % vol CO] - 80 ≥0, at least H2O and CO2 in concentrations not more than 3 % by volume, a remainder of a neutral gas such as N2, and the carburizing temperature T ranges between 650 and 950 °C, and the total duration t of said reactive annealing thermal treatment is between 0.1 and 300 seconds and the carburizing rate is not less than 10(-10).exp(T/100)g.cm(-2).s(-1), in which T is the carburizing temperature expressed in Kelvin's.

Description

Process for the continuous production of a steel strip for stamping with improved surface properties.

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

Steel is, without question, a material used in a very large number of industrial fields to manufacture elements of variable shapes and dimensions, in particular by using deformation and shaping techniques by deep and extra-deep drawing.

In this regard, it will be mentioned more particularly and without this constituting a limitation of the fields of application of the present invention, that the automobile industry requires steel strips having increasingly large mechanical properties while wishing to retain aptitudes. deformation in accordance with the stamping processes generally used to manufacture bodywork parts. It is known that these requirements are often antagonistic and that the steels of current manufacture cannot satisfy them whereas the steel strips manufactured according to the process of the invention meet these requirements and can be formed by deep and extra-deep drawing .

In this context, two complementary ways have been developed, on the one hand the search for new steel grades and on the other hand the development of various treatment processes, mainly of the thermal type, to give steel strips, including the aforementioned new grades, 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, the framework in which the process of the present invention is inscribed.

Among the grades of steels capable of undergoing deep or extra-deep drawing operations, there will be mentioned low carbon mild steels of the ELC type (ELC = Extra Low Carbon, that is to say with C <0, 1%) and of the ULC type (ULC = Ultra Low Carbon, that is to say with C <0.01%).

More particularly, subsequent developments have given rise to other grades of steel known as "Interstitial Free" or "IF". They are characterized by their very low content of interstitial elements (C, N) and by the presence of elements with great affinity for carbon and nitrogen such as Ti and / or Nb. These elements 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 steel strips must have a particular texture which is the result of a certain number of constraints in their manufacturing process. Among other things, a certain range of coiling temperatures after hot rolling must be observed, a range of values of the reduction rate in cold rolling as well as various parameters defining the continuous annealing cycle operated subsequently.

Many patents and scientific articles describe in detail the procedure which is well known to all manufacturers of automotive sheet metal. In particular, it is known that it is important to avoid having free interstitial elements at the time of recrystallization at the start of continuous annealing to form the texture (1 1 1) favorable for stamping.

An adequate choice of the composition of the steel associated with an optimization in the heat treatments operated on the steel strip in question, allows the production of a product having mechanical properties which are in accordance with what deep drawing requires. , namely 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 level of elastic limit YPE weak 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 properties of use. This is how we observe increased CWE intergranular brittleness during cold deformation (CWE = Cold Working Embrittlement), weak mechanical properties such as YS elastic limit, TS breaking load (TS = Tensile Strength) , or hardness, therefore a low cutting ability and low friction properties, as well as a more difficult behavior in galvanizing by the appearance of defects at the grain boundaries of the steel (outburst structure) and less resistance in service.

To remedy some of these problems, solutions have been proposed such as: - adding 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 paint BH (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, mention will be made of the existence, on the one hand, of a process of continuous carburetion in a non-soaking atmosphere, therefore with a 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 methods give rise to steels having improved properties within the framework of volume mechanical properties but not of surface, while being suitable for coating.

It is also known that rephosphorous grades have been developed to increase the mechanical strength of steels for stamping, and this in particular in order to produce sheets for finer bodywork. However, the phosphorus included in this type of steel has the defect of reducing the Lankford coefficient r and of increasing its intergranular brittleness (CWE), while strongly reducing the reactivity of said steels during the formation annealing of the Zn-Fe layers during hot dip galvanizing, consequently reducing the productivity of the galvanizing lines.

In summary, the various manufacturing processes mentioned above lead either to obtaining grades of steels suitable for deep drawing operations EDDQ (EDDQ = Extra Deep Drawing Quality) but having low mechanical properties (YS, TS), a low surface hardness and resistance during cold forming, ie to 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 to the problems mentioned above, or moderate carburetion methods allow a texture favorable to drawing to be obtained. deep (EDDQ) associated with possibilities of hardening during paint annealing (BH). In all cases, however, this hardening is deliberately limited either to avoid sweating during the carburetion treatment, or to limit the aging favored by an excess of non-precipitated C, 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 during cutting and stamping (friction) as well as during their subsequent use, in particular because of their low resistance to indentation.

The process of the present invention makes it possible to manufacture steel strips from low carbon grades which have satisfactory drawing characteristics, even for deep or extra-deep drawing, while possessing surface properties without limitations. mentioned above, and it can be executed continuously, which is an advantage for its profitability in the context of an 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 the 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 H20 and C02 of concentrations less than or equal to 3% by volume, a balance in a neutral gas such as N2, in that the temperature T of carburetion 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 carburetor speed is greater than or equal to 10 ^M0 Mexp (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 gaseous atmosphere also comprises another gas which has the object of simultaneously operating a surface enrichment treatment such as nitriding, boriding, sulfurization, oxidation, etc. .

This is how the carburetion process can be combined simultaneously or successively with a nitriding, sulfurizing, boriding and / or oxidation step to improve the properties of the layer from the point of view of corrosion resistance. , decrease in the 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] / 1 2).

According to a particular method of implementing 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 complies with the following relationship: 1 ≤ ([Nbl / 93) ) / ([Cl / 1 2).

In the context of the abovementioned steel grades, it was surprisingly found that perfectly clean strip surfaces were obtained even with atmospheres reputed to be soaking.

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 carburation 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 preferential variant of the previous modality, the microstructure and the surface hardness are modulated 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 embodiment of the method of the invention, the steel strip undergoes after carburization a heat treatment of diffusion in a low or non-fuel atmosphere before final cooling.

The aforementioned diffusion treatment under controlled atmosphere makes it possible both to homogenize the properties of the skin and to restore sufficient surface cleanliness for possible subsequent finishing treatments such as dip galvanization or by electroplating, phosphating-painting. , ....

According to yet another embodiment of the process of the invention, the carburization is carried out using a carburetted gas ionized in the form of plasma. According to a preferential alternative to the previous method, the two faces of the steel strip undergo differentiated treatments.

The two preceding methods have the effect, on the one hand of increasing the possibilities of application of the method by making it possible to operate several surface treatments via separate plasmas, and on the other hand of providing an attractive solution in the case of differentiated treatments according to the faces of the steel strip according to specific subsequent applications.

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

According to yet another modality of implementation of the method of the invention, the carburetion is operated, totally or in part, in at least one delimited zone, called independent zone, which is either juxtaposed, or integrated partially or totally in the structure of one of the traditional furnaces for preheating the strip, for heating, for maintaining at temperature or for subsequent overaging, as is known in the art of proceeding during the manufacture of a steel strip.

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

In the context of the use of independent zones for reactive annealing, that is to say zones forming part of a succession of heat treatment zones in which one or more of these zones are explicitly assigned to operate a carburation, we will note the advantage that this arrangement has if one wishes to carry out several types of treatment successively by injecting gases of different concentrations and this even at the level of a differentiated treatment of the two faces of the strip.

According to a preferred embodiment of the process of the invention, the steel strip passes first into a zone for preheating the strip, then into an independent carburetion treatment zone, then a heating zone with temperature maintenance, then undergoes cooling and finally an aging treatment whose existence is linked to the desired final properties.

According to another preferred embodiment of the process of the invention, the steel strip passes first into a preheating zone, then into a temperature heating zone, then into an independent carburization treatment zone, then a temperature maintenance zone followed by cooling and finally an overaging treatment, the existence of which is linked to the desired final properties.

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

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

Figure 1 shows schematically the treatment path followed by the steel strip (B) in the context of the previous modality. One distinguishes there the heat treatment unit (1), this being maintained under a determined atmosphere, the strip (B) passes first into a recrystallization zone (2), then into an independent treatment zone (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 unit (1) in question. By way of example of the results obtained by application of the process of the present invention, the values below have been collated for 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, 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,

[CJtotal: average carbon concentration expressed in ppm.

BOARD

Atmosphere 45% CO 0.25% H ₂ 0 25% H ₂ Balance: N ₂

Temperature 810 ° C

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

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

- Realization 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

1. Process for the continuous production of a steel strip for stamping with improved surface properties, in which a strip, having undergone hot rolling with end-of-rolling winding at a temperature between 500 and 800 ° C, as well as cold rolling with a reduction rate of at least 30% and preferably 75%, undergoes a heating step, then thereafter a heat treatment according to the present invention, hereinafter called reactive annealing, making it possible to produce a on the one hand recrystallization, that is to say the regeneration of the crystal lattice following the deformation, and on the other hand the carburetion of the strip at a temperature T, the two being able to be simultaneous or not depending on the type of final properties of the targeted strip, and finally said strip being subjected to cooling, characterized in that the reactive annealing in question is carried out totally or partially under a gaseous atmosphere comprising at least s CO and H2 in volume concentrations satisfying the following relationship:

[% vol H2] + 2 [% vol CO] - 80 ≥ 0, at least H20 and C02 of concentrations less than or equal to 3% by volume, a balance in a neutral gas such as N2, in that the temperature T of carburetion 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 and in that the carburization speed is greater than or equal to 10 ^{M 0} Mexp (T / 100) g.cm (-2) .s (-1), where T is the carburetion temperature expressed in degrees Kelvin.

2. Method according to claim 1, characterized in that the total duration t of the aforementioned reactive annealing heat treatment is between 0.1 and 20 seconds.

3. Method according to claims 1 or 2, characterized in that the gaseous atmosphere also comprises another gas which has the object of simultaneously operating a surface enrichment treatment.

4. Method according to claim 3, characterized in that the surface treatment is a nitriding.

5. Method according to claim 3, characterized in that the surface treatment is a boriding.

6. Method according to claim 3, characterized in that the surface treatment is a sulfurization.

7. Method according to claim 3, characterized in that the surface treatment is an oxidation.

8. Method according to either of claims 1 to 7, characterized in that the strip consists of a steel of the "Interstitial Free" or "IF" type containing either titanium, niobium, or both .

9. Method according to claim 8, characterized in that the titanium content satisfies the following relationships:

Ti * = total [Ti] - ((48 / 32HS] + (48/14) [N])

1 ≤ ((Ti * / 48) + (tNb] / 93)) / ([C] / 12) in which the contents are expressed as a percentage by weight.

10. Method according to claim 8, characterized in that the niobium content satisfies the following relationship:

1 <= ([Nb] / 93) / ([C] / 12) in which the contents are expressed as a percentage by weight.

1 1. Method according to one or more of Claims 1 to 10, characterized in that a surface layer of carbides is formed on the strip, the thickness of which is between 0.01 and 50 μm.

12. Method according to one or more of claims 1 to 1 1, characterized in that the carburization is carried out using a carburetion gas ionized in the form of plasma.

1 3. Method according to one or more of claims 1 to 1 2, characterized in that the microstructure and the surface hardness are modulated by cooling the strip after reactive annealing at a speed of between 4 and 1000 ° C / second .

14. Method according to claims 1 3, characterized in that the strip is cooled after reactive annealing at a speed of between 4 and 100 ° C / second.

15. Method according to either of claims 1 to 14, characterized in that the steel strip undergoes after carburization a diffusion heat treatment under a low-fuel atmosphere before final cooling.

16. Method according to either of claims 1 to 14, characterized in that the steel strip undergoes after carburization a heat treatment of diffusion in a non-fuel atmosphere before final cooling.

17. Method according to either of claims 1 to 16, characterized in that the two faces of the steel strip undergo differentiated treatments.

1 8. Process according to one or more of claims 1 to 1 7, characterized in that an overaging treatment is carried out after the steel strip has cooled.

19. Method according to one or more of claims 1 to 18, characterized in that the carburetion is carried out, totally or in part, in at least one delimited zone, called independent zone, which is either juxtaposed, or integrated partially or totally in the structure of one of the traditional furnaces for preheating the strip, heating, maintaining at temperature or subsequent overaging as is known in the art when manufacturing a steel strip.

20. Method according to one or more of claims 1 to 19, characterized in that the carburetion is carried out wholly or in part during the cooling of the strip.

21. Process according to either of Claims 1 to 19, characterized in that the steel strip passes first through a zone for preheating the strip, then through an independent zone for the treatment of carburetion, then a zone of heating with temperature maintenance, then undergoes cooling and finally an aging treatment.

22. Method according to either of claims 1 to 19, characterized in that the steel strip passes first into a preheating zone, then into a temperature heating zone, then into a zone independent of carburation treatment, then a temperature maintenance zone followed by cooling and finally an aging treatment whose existence is linked to the desired final properties.

23. Method according to either of claims 1 to 19, characterized in that the steel strip passes first through a zone for preheating the strip, then through a zone of heating at temperature, then through a first temperature maintenance zone followed by an independent carburation treatment zone, then in a second temperature maintenance zone followed by cooling and finally an aging treatment, the existence of which is linked to the desired final properties.

24. Method according to one or more of claims 1 to 19, characterized in that the steel strip enters a heat treatment unit in which it passes first into a recrystallization zone, then into an independent carburation treatment zone , then in a diffusion treatment zone, then leaves the heat treatment unit.