GB2087428A - Process and Installation for the Continuous Heat Treatment of Steel Sheet - Google Patents

Abstract

For the purpose of providing sheet with both a high elasticity limit and a good suitability for stamping, a steel containing at most 0.16% of C, 2.0% of Mn, 0.55% of Si and at least 0.03% of Al, and also, if appropriate, up to 0.05% of Nb and 0.1%of V, is used. The sheet is heated rapidly to a temperature above the Ac3 point, that is to say between 700 and 900 DEG C, and the sheet is kept at this temperature for as long as possible without the grain coarsening. The sheet is then cooled at 100 DEG C/second to a temperature below the Ar1 point and the sheet is left to cool at a rate of about 1 DEG C/second. The tempering level is between 550 and 650 DEG C, preferably at 600 DEG C, and it is produced by immersing the sheet either in a liquid medium for example a bath of salts or of metal, or in a gaseous medium. Apparatus includes a heating zone 4, soaking zone 5, and gas cooling unit 6 or metal or salt bath 7, followed by slow cooling zone 8. Before undergoing slow cooling the sheet may be chromium-plated. <IMAGE>

Description

SPECIFICATION Process and Installation for the Continuous Heat Treatment of Steel Sheet The present invention relates to a process and an installation for the continuous heat treatment of steel sheet, in particular for the purpose of providing sheet with both a high elasticity limit and a good stability for stamping.

An appropriate heat treatment can provide steel sheet with either a high elasticity limit or a good suitability for stamping. Thus, treatments, especially continuous treatments, have been proposed which consist in principle in heating the sheet to a temperature above its conversion temperature and subsequently cooling it in a controlled manner.

In one of these treatments, the object of which is to obtain mild steel sheet suitable for stamping, the sheet is heated at 700-9000C, preferably at about 7000C, for a holding time of 40-60 seconds, then abruptly cooled by quenching in boiling water, and then subjected to overageing for 60 seconds at a temperature of 300-4000C.

This treatment provides the sheet with a high degree of homogeneity of mechanical properties over the whole width of the strip; at the same time, as the cooling becomes more intense, these properties become very sensitive to variations in the contents of carbon, phosphorus and nitrogen.

In another process, to produce mild steels for stamping, the sheet is cooled at a rate of about 100C per second to a temperature of 4500C, at which temperature overageing takes place for 1 80 seconds.

When extremely intense cooling in water is used (1 ,0000C/second) for the purpose of cooling the steel to 4500C, the overageing is shortened to 60 seconds.

To produce high-strength steel sheet in accordance with the first process described, the heat cycle is modified in such a way that the sheet is annealed at a temperature considerably above the conversion point, hence at about 8500 C, and is then cooled by quenching in boiling water; a subsequent treatment is not envisaged.

The second and third processes described necessarily use overageing treatments.

Thus, it seems that the current state of the art indeed makes it possible to produce sheet either with a high elasticity limit or with a good suitability for stamping, but a combination of these two properties can only be obtained with difficulty. It would be desirable to have available sheets which possess these two properties simultaneously, in particular in the field of manufacture of motor vehicle components, and the present invention provides a continuous heat treatment process for steel sheet permitting the continuous production of sheet of improved combined properties in this regard.

The process according to the invention is characterised in that a steel containing at most 0.16% of C. 2.0% of Mn and 0.55% of Si and at least 0.03% of Al, and also, if appropriate, up to 0.05% of Nb and 0.1% of V is used; in that the sheet is heated rapidly to a temperature above the Ac3 point, that is to say between 700 and 9000C; in that the sheet is kept at this temperature for as long a period as possible without thereby allowing the grain to coarsen; in that the sheet is then cooled at about 1 000C/second to a temperature below the Ar1 point; and in that the sheet is left to cool at a rate of the order of 1 OC/second.

The tempering level is preferably established between 550 and 6500C, e.g. at about 6000 C.

In a particularly advantageous embodiment of the process according to the invention, the tempering level is produced by immersing the sheet, when the heating has ended, in a liquid medium, e.g. a bath of molten salts or of molten metal (which may be molten alloy), heated to the required temperature, ensuring that thermal equilibrium in the sheet is established rapidly.

Alternatively, provision can be made for cooling by means of a gas (e.g. air).

The sheet ieaving the rapid cooling stage can then be subjected to a coating operation and then to controlled cooling.

As regards the chemical composition of the steel used, numerous experiments have shown that a manganese content of the order of 1.7% and a silicon content of 0.20% give rise to the desired strength, whilst improving the suitability for stamping. Niobium and/or vanadium can be present, if appropriate, in order to ensure a higher elasticity limit. The experimental conversion diagram of steel, Fig. 2 of the accompanying drawings, illustrates the above.

An installation for carrying out the process according to the invention is characterised in that it comprises, in particular, a furnace for heating the sheet to the required temperature for a given period, and a rapid cooling system of adjustable temperature, which is installed downstream of the said furnace and which is followed by means for cooling the sheet slowly to a low temperature.

Fig. 1 of the accompanying drawings shows diagrammatically, without implying any limitation, an embodiment of the installation according to the invention.

In Fig. 1 the installation and the resulting heat cycle are shown juxtaposed and in register. The sheet is heated to a temperature T1, that is to say a temperature above its conversion temperature, and kept between T1 and T2, the latter temperature corresponding to the start of the rapid cooling which takes place between time t3 and time t4. The sheet is thus cooled abruptly to the temperature T3 and it can then enter the controlled slow cooling stage.

The following are shown in particular: a feed device (1) for the sheet, comprising an unwinder and a welder; an electrolytic degreasing chamber (2); an input accumulator (3); a heating furnace (4); a chamber (5) for the zone for keeping the sheet at a temperature close to that chosen as the starting point of rapid cooling; a rapid cooling system which can be either a system of gas jets (6), or a vat (7) for immersing the sheet, which serves to keep the sheet at the tempering temperature; a slow cooling system (8); an output accumulator (9); and a take-up device (10) comprising a winder, a flattening machine, a skin-pass and a packaging line.

The vat (7) can contain either a bath of molten salt or a bath of molten metal. This depends on the preferred method of temperature adjustment, appropriate techniques existing for both salt and metal baths.

In an advantageous embodiment, a vat in which an additional operation, for example chromium plating, can take place is intercalated between the rapid cooling system (6) pr (7) and the slow cooling system (8).

Claims (14)

Claims

1. A process for the continuous heat treatment of steel sheet in which sheet of steel containing at most 0.16% of C, 2.0% of Mn and 0.55% of Si, at least 0.03% of Al, and optionally up to 0.05% of Nb and 0.1% of V is heated rapidly to a temperature above the Ac3 point, that is to say between 700 and 9000C, kept at this temperature for as long a period as possible without thereby allowing the grain to coarsen, then cooled at about 1 000C/second to a temperature below the Ar1 point, and then left to cool at a rate of the order of 1 OC/second.

2. A process according to claim 1 wherein the tempering level is established between 550 and 6500C, preferably at 6000C.

3. A process according to claim 2 wherein the tempering level is at about 6000C.

4. A process according to any preceding claim wherein the tempering level is produced by immersing the sheet in a liquid medium.

5. A process according to claim 4 wherein the liquid medium is a salt bath.

6. A process according to claim 4 wherein the liquid medium is a metal bath.

7. A process according to claim 2 wherein the tempering level is produced by subjecting the sheet to the action of a gas.

8. A process according to any preceding claim wherein, before the slow cooling, the sheet is subjected to a coating operation.

9. A process according to claim 8 wherein the coating operation is chromium plating.

1 0. An installation for carrying out a process according to claim 1 and comprising a furnace for heating the sheet to the required temperature for a given period, and a rapid cooling system of adjustable temperature, which is installed downstream of the said furnace and which is followed by means for cooling the sheet slowly to a low temperature.

1 An installation according to claim 10 wherein the rapid cooling system is a vat for containing a salt or metal bath.

12. An installation according to claim 10 wherein the rapid cooling system is a system of gas jets.

1 3. An installation according to any of claims 10 to 1 2 wherein a vat for containing a coating bath is arranged between the rapid cooling system and the slow cooling means.

14. An installation according to claim 13 wherein the coating bath is a chromium plating bath.

1 5. A method for the heat treatment of steel sheet, the method being substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 6. A method for the heat treatment of steel sheet, the method being substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

1 7. An installation for the heat treatment of steel sheet, the installation being substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 8. Steel sheet, of high elastic limit and suitable for stamping, obtained by means of a method or installation according to any preceding claim.