EP1147235A1

EP1147235A1 - Method for making a cold rolled steel strip for deep-drawing

Info

Publication number: EP1147235A1
Application number: EP00969113A
Authority: EP
Inventors: Annick De Paepe; Jean-Claude Herman; Stéphan Wilmotte
Original assignee: Centre de Recherches Metallurgiques CRM ASBL
Current assignee: Centre de Recherches Metallurgiques CRM ASBL
Priority date: 1999-10-13
Filing date: 2000-10-03
Publication date: 2001-10-24
Also published as: KR20010080760A; US6638380B1; BE1012934A3; WO2001027340A1

Abstract

The invention concerns a method for making a low carbon and low manganese cold rolled steel strip for deep-drawing whereby a steel strip hot rolled in the austenistic region coiled at the end of the high temperature hot rolling process (680 DEG C < T < 750 DEG C) is subjected to a cold rolling process with a reduction rate between 65 and 80 %, and finally subjected to an annealing and overageing heat treatment. It consists in heating the steel strip at a heating rate Vh ranging between 150 DEG C/s and 1000 DEG C up to the annealing temperature Ta ranging between 650 DEG C and 750 DEG C, and in maintaining it at said annealing temperature for a time interval between 1 and 20 seconds, then in cooling it at a cooling rate Vc between 100 DEG C/s and 500 DEG C/s up to an overageing temperature Toa ranging between 150 DEG C and 450 DEG C.

Description

Method of manufacturing a cold rolled steel strip for deep drawing.

Technical area

The present invention relates to a method of manufacturing a cold rolled steel strip for deep drawing.

State of the art. At the present time, the steel strips intended for stamping operations are generally cold-rolled steel strips, which have very favorable properties in this respect. The manufacture of these cold strips, however, involves various thickness reduction operations and heat treatment which increase the cost.

The use of hot-rolled steel strips for stamping operations, replacing the traditional cold-rolled strips, is therefore arousing growing interest, both in manufacturing and in users.

It is well known that the steels intended for deep drawing are mild steels, that is to say steels whose carbon content is between 0.02 and 0.08% by weight and that of manganese between 0, 1 and 0.4% by weight.

According to usual practice, mild steels are hot rolled in the austenitic range and the end of rolling temperature is higher than the transformation temperature Ar3, that is to say a temperature generally between 820 ° C and 880 ° C. The possibilities of using these conventional hot strips are however very limited, due to their random texture and their poor drawing ability. In addition, it is impossible in practice to manufacture thin hot-rolled strips by this conventional method. Indeed, the small thickness of the strips results in rapid cooling thereof, even during hot rolling, so that it is not possible to carry out the finishing rolling in the austenitic field in order to obtain a microstructure favorable to subsequent shaping operations by deep drawing. Classic method currently used

At present, deep drawing steels of the type FeP01 and FeP03, the designations being relative to the European standard EN 101 30, use low carbon steels (0.02 <C <0.08% by weight ), low manganese (0.1 <Mn <0.4% by weight), which undergo hot rolling in the austenitic region and are wound at high temperature (680 ° C <T <750 ° C). These steel strips are then cold rolled with a reduction rate of between 65 and 80% and undergo continuous annealing.

Table 1 shows the minimum mechanical properties required for the two types of commercial deep-drawing steel FeP01 and FeP03.

Table 1: Mechanical properties guaranteed for commercial steels for deep drawing.

FeP01 and FeP03 are the types of steels as defined in the European standard EN 101 30 relating to the qualities of commercial steels for deep drawing; YS (MPa) is the elastic limit expressed in megapascals; TS (MPa) is the breaking load expressed in megapascals; Eltot (%) is the total elongation at break expressed in%; R90 is the Lankford parameter measured at 90 ° to the rolling direction.

During the aforementioned hot rolling, the winding of the steel strip at a high temperature, that is to say between 680 ° C and 750 ° C is operated in order to obtain in the hot strip the total precipitation N in the form of coarse nitrides, a condition which favors the control of the texture of the strip during re-installation annealing.

After cold rolling, the strip undergoes continuous annealing comprising heating at a speed of the order of 10 ° C / s to an annealing temperature located in the ferπtique region, that is to say lower or equal to 720 ° C and maintained at this temperature for about 1 minute, followed by cooling at a speed between 10 and 20 ° C / s up to the aging temperature. This overaging treatment is necessary to obtain a strip whose microstructure contains a sufficiently low quantity of soluble carbon in order to present a reduced aging index (Al = Aging Index). In general, the time required to maintain an aging temperature between 350 ° C. and 500 ° C. essential in order to obtain an adequate precipitation of carbides is several minutes.

Disadvantages of the aforementioned conventional method.

• A relatively low annealing temperature (<720 ° C) which gives rise to a fairly fine-grained microstructure and thus favors the presence of nucleation sites for Fe3C during the primary slow cooling, ie from the annealing temperature up to the aging temperature. The conventional values of the cooling rates in the case of steel strips with a thickness of 0.8 mm subjected to conventional cooling by gas jets are between 5 and 15 ° C / s. Consequently, a non-negligible amount of C is already precipitated at the start of the overaging treatment and this results in a negative effect of weaker supersaturation and therefore a slower kinetics of the precipitation of carbides at the overaging temperature.

• An obligation to maintain a relatively long aging temperature of the order of 3 to 5 minutes which is the consequence of the previous remark but necessary in order to reduce the amount of interstitial carbon present in the final product below a sufficiently low value to avoid any subsequent aging.

• After a conventional overaging treatment, that is to say with maintenance at 400 ° C. for 3 to 5 minutes, in a continuous annealing line implementing the conventional technique of cooling by gas jets, the index of aging of the product obtained is around 50-60 Mpa. Given that a product is defined as being non-sensitive to aging (aging-free) when its aging index is less than 30 Mpa (See following: K. Ushioda et al., Metallurgical investigation for producing non-aging deep- drawable LC AK-steel sheets by continuous annealing, Developments in the annealing of sheet steels, ed. by R. Pradhan and I. Gupta, 1 992, pg. 261 -285), which corresponds to an absence of flow level (Luders strain) in an aging simulation in the form of a maintenance at 1 00 ° C for 1 hour. The above operation of keeping at 100 ° C for 1 hour is representative of a 3 month storage operation at 30 ° C usually carried out in reality by the sheet producer before shipment to the user. This means that the products thus obtained, and therefore also the commercial types FeP01 and FeP03 for deep drawing thus obtained, are sensitive to aging after straining (strain aging).

Presentation of the invention

To avoid the aforementioned drawbacks, the present invention provides a method of manufacturing a hot-rolled steel strip for deep drawing of the FeP01 and FeP03 type.

In accordance with the invention, a method of manufacturing a cold rolled steel strip for deep drawing, of a thickness of between 0.3 mm and 1 mm, for application to low carbon steels (0, 02 <C <0.08% by weight), low manganese (0.1 <Mn <0.4% by weight), S <0.01 5% by weight, Si <0.1% by weight, P < 0.08% by weight, Al <0.05% by weight, Nb <0.02% by weight and Ti <

0.03% by weight, in which a steel slab of the above type is subjected to hot rolling in the austenitic region with winding at the end of hot rolling at high temperature (680 ° C <T <750 ° C) , said hot-rolled strip being subsequently subjected to cold rolling with a reduction rate of between 65 and 80%, and finally undergoing a heat treatment of annealing and over-aging, is essentially characterized in that the strip is heated. steel at a heating rate Vh between 1 50 ° C / s and 1000 ° C / s up to the annealing temperature Ta between 650 ° C and 750 ° C, in that said strip is kept at the annealing temperature for a time ta between 1 and 20 seconds, in that said strip is cooled at a cooling rate Vc between

100 ° C / s and 500 ° C / s up to an overaging temperature Toa between

1 50 ° C and 450 ° C.

According to a mode of implementation of the process, object of the present invention, the steel strip is heated, until the annealing temperature Ta is reached, by induction, preferably by creating a longitudinal induced flux.

This way of heating the steel strip has the advantage of great flexibility in the choice of the temperature Ta, as well as the feasibility in terms of speed of very high Vh heating. In addition, this type of induction heating improves the productivity of the process and extends its practical field of application.

According to another method of implementing the method, which is the subject of the present invention, the cooling of the strip from the annealing temperature Ta to the aging temperature Toa comprises at least one spraying of liquid or a spray of cooling gas onto the strip or bringing it into contact with a cooling roller.

The cooling procedure is particularly advantageous if it is desired to achieve a sufficiently high cooling rate so that the implementation of the whole process can be carried out in compact lines with high productivity.

According to a method of implementing the process which is the subject of the present invention, after annealing, a continuous overaging treatment is carried out by cooling said strip to an overaging temperature Toa of between 350 ° C and 450 ° C and by keeping the strip at the overaging temperature Toa for a period of between 40 seconds and 2 minutes and finally cooling it to a temperature below 100 ° C.

According to another method of implementing the process which is the subject of the present invention, after annealing, said strip is cooled to an aging temperature of between 150 ° C. and 250 ° C., said strip is wound to form coils, which are introduced, at a temperature between 1 30 ° C and 230 ° C, in a tunnel oven in a protective atmosphere to prevent oxidation of said coils and said coils are kept in said tunnel oven until the latter cool at a temperature below 100 ° C.

The preceding modality makes it possible to replace the conventional overaging treatment operated continuously, that is to say with isothermal maintenance of the moving band for 40 seconds to 2 minutes at a temperature between 350 ° C and 450 ° C, by a non-isothermal alternative treatment of the strip in the form of a coil placed in a tunnel oven with a non-oxidizing protective atmosphere in which it undergoes slow cooling down to the temperature below 100 ° C. during which the soluble carbon precipitates as carbide.

According to another mode, after the cooling which follows the operation respectively either of overaging with maintenance at the temperature Toa, or the passage in the form of coils in a tunnel oven, the strip is subjected to a rolling operation known as skin pass with a reduction rate between 0.5% and 2.5%.

Description of the example. The example below presents a comparison between the mechanical properties obtained in the case of a steel strip which has undergone on the one hand a conventional continuous annealing treatment and on the other hand an ultra short continuous annealing treatment USA (USA = Ultra Short Annealing) according to the process of the present invention. The strip is made of ELC steel, calmed with aluminum and having a total carbon percentage of 0.032%. Said strip was first subjected to hot rolling, then cold rolling with a reduction rate of 75%, and finally a continuous annealing followed by overaging.

Table 2 summarizes the data characterizing each of the anneals, namely the conventional continuous annealing and the USA ultra short annealing continuous annealing according to the invention.

Table 2: Characterizing data of continuous annealing.

USA = Ultra Short Annealing

Vh = Heating rate expressed in ° C / s

Ta = Annealing temperature expressed in ° C ta = holding time at the annealing temperature expressed in seconds

Vc = Cooling rate after annealing expressed in ° C / s oa = Overage temperature expressed in ° C toa = time to maintain overage temperature expressed in seconds

Table 3 below gives the mechanical properties of the steel strip after treatment.

Table 3: Mechanical properties of the steel strip after annealing and overaging

USA = Ultra Short Annealing

YSI = lower yield strength = lower yield strength expressed in megapascals

TS = Tensile Strength = breaking load expressed in megapascals

YPel = Yield Point elongation = length of the plastic bearing expressed in%

Elto = total Elongation = total elongation at break expressed in% R90 = the Lankford parameter measured at 90 ° relative to the rolling direction

GS = Grain Size = grain size expressed in micrometers

Ci = interstitial Carbon content = quantity of interstitial carbon expressed in ppm

It appears from the values presented in table 3 that the ultra short USA annealing according to the invention is softer and it follows that the strip, after ultra short annealing, has lower values for the YSI (YSI = lower yield strength = lower elastic limit) and TS (TS = Tensile Strength = breaking load) than in the case of a conventional annealing treatment. It follows that the strip treated with USA annealing meets the FeP03 quality standards for drawing ability, while that treated with conventional annealing barely meets the standards for lower quality FePOl in the same context. The steel strip treated with ultra short USA annealing according to the process of the present invention is therefore better suited to undergo deep drawing operations.

In addition, it should be noted that the interstitial carbon content is lower in the strip treated with USA annealing, which ensures a lower aging index, which is an advantage when using said steel strip. This improvement in the aging index is linked to a cooling rate from the annealing temperature in the USA case much faster than in a conventional annealing, with the effect that a greater amount of carbon is precipitated during the 40 seconds 5 maintenance during aging while softer cooling in conventional annealing leads to a lower amount of precipitated carbon even if the duration of maintenance during aging is significantly longer, here 180 seconds.

o Conclusions.

The method of the invention makes it possible to manufacture steel strips for deep drawing which satisfy the criteria required to be suitable for stamping operations in the commercial quality FeP03, with the following additional advantages:

• The steel strip has a lower aging coefficient than during the implementation of conventional continuous annealing processes;

• The treatment times for both annealing and over-aging are shorter, which constitutes a significant economic advantage in the context of the sizing of industrial installations. In addition, both the rationalization and the flexibility of the overaging operations can be improved, since they can be physically separated from the continuous process including annealing and simultaneous treatment of several coils in the same tunnel oven.

Claims

R E V E N D I C A T I O N S

. Method for manufacturing a cold rolled steel strip for deep drawing, of a thickness of between 0.3 mm and 1 mm, for application to low carbon steels (0.02 <C <0.08 % by weight), low manganese (0.1 <Mn <0.4% by weight), S <0.01 5% by weight, Si <0.1% by weight, P <0.08% by weight, Al <0.05% by weight, Nb <0.02% by weight and Ti <0.03% by weight, in which a steel slab of the above type is subjected to hot rolling in the austenitic region with winding at the end of hot rolling at high temperature (680 ° C <T <750 ° C), said hot rolled strip being subsequently subjected to cold rolling with a reduction rate of between 65 and 80%, and finally undergoing a annealing and overaging heat treatment, characterized in that the steel strip is heated to a heating rate Vh between

1 50 ° C / s and 1000 ° C / s up to the annealing temperature Ta between 650 ° C and 750 ° C, in that said strip is maintained at the annealing temperature for a time ta between 1 and 20 seconds, in that said strip is cooled at a cooling rate Vc of between 100 ° C / s and 500 ° C / s to an overaging temperature Toa of between

1 50 ° C and 450 ° C.

2. Method according to claim 1, characterized in that the heating of the steel strip is carried out until reaching the annealing temperature Ta by induction.

3. Method according to claim 2, characterized in that the induced flow is longitudinal.

4. Method according to either of claims 1 to 3, characterized in that the cooling of the strip from the annealing temperature Ta to the aging temperature Toa comprises at least one spraying of liquid onto the strip .

5. Method according to either of claims 1 to 3, characterized in that the cooling of the strip from the annealing temperature Ta to the overaging temperature Toa comprises at least one projection of cooling gas on the bandaged.

6. Method according to either of claims 1 to 3, characterized in that the cooling of the strip from the annealing temperature Ta to the aging temperature Toa comprises at least one contacting of the strip with a cooling roller.

7. Method according to one or more of claims 1 to 6, in which, after annealing, a continuous overaging treatment is carried out, characterized in that said strip is cooled to an overaging temperature Toa of between 350 ° C and 450 ° C., in that the strip is kept at the overaging temperature Toa for a period of between 40 seconds and 2 minutes and in that it is finally cooled to a temperature below 100 ° C.

8. Method according to one or more of claims 1 to 6, characterized in that after the annealing, said strip is cooled to an overaging temperature between 1 50 ° C and 250 ° C, in that it is wound said strip for forming coils, in that one introduces, at a temperature between 1 30 ° C and 230 ° C, said coils in a tunnel oven in a protective atmosphere to prevent oxidation of said coils and in that one maintains said coils in said tunnel furnace until the latter has cooled to a temperature below 100 ° C.

9. Method according to claims 7 or 8, characterized in that after the cooling which follows the operation respectively either continuous overaging with maintenance at the temperature Toa, or the passage in the form of coils in a tunnel oven, the strip in a rolling operation known as skin pass with a reduction rate of between 0.5% and 2.5%.