EP1013785B1 - Process for manufacturing of a part from a hot-rolled sheet - Google Patents

Abstract

A component is produced from a strip of rolled (especially hot-rolled) sheet steel, by coating with aluminum or its alloy and pressing operations. The process involves: (a) coating the rolled sheet with a metal or an alloy assuring protection of the surface and the steel; (b) raising the temperature of the coated sheet for forming; (c) producing, at the surface, an intermetallic alloy composite assuring protection against corrosion and decarburation of the steel, with the intermetallic composite able to assure a lubrication function; (d) forming the coated sheet, notably by pressing; (e) cooling the formed component in order to confer increased hardness of the steel and increased surface hardness in the coating. The metal used for coating the steel sheet is aluminum or an aluminum alloy, so that the intermetallic alloy composite is made up of aluminum, iron and silicon.

Description

The invention relates to a method for producing a workpiece from a strip of hot-rolled steel sheet.

Steel sheets to be subjected to high temperature forming and / or heat treatment are not delivered coated for coating considerations during heat treatment. The coating is therefore performed on finished part which requires a careful cleaning of surfaces and hollow parts. This cleaning requires the use of acids and / or bases whose recycling and storage are a significant financial burden and present risks for operators and the environment. In addition, the heat treatment must be carried out under a controlled atmosphere in order to avoid decarburization and oxidation of the steel. Then, in the case of hot forming, calamine, by virtue of its abrasive power, damages the forming tools, which reduces the quality of the parts obtained from the dimensional and aesthetic point of view, or forces frequent and expensive tool repairs. Finally, to increase their resistance to corrosion, the parts thus obtained must receive an expensive post-processing whose application is difficult or impossible, especially in the case of parts with depressions.

The object of the invention is to propose sheet steel rolled from 0.2 mm to about 20 mm thick, coated after hot rolling, and to be shaped, either hot or cold followed a heat treatment, the temperature rise to be ensured without decarburization of the sheet steel, without oxidation of the surface of said sheet, before, during and after hot forming and or heat treatment.

• on revêt la tôle laminée d'un métal ou d'un alliage métallique assurant une protection de la surface et de l'acier,
• on soumet la tôle revêtue à une élévation en température pour sa mise en forme,
• on réalise, de ce fait un composé allié intermétallique, en surface, assurant une protection contre la corrosion, contre la décarburation de l'acier, le composé intermétallique pouvant assurer une fonction de lubrification,
• on effectue la mise en forme par emboutissage,
• on refroidit la pièce formée pour conférer des caractéristiques mécaniques de dureté élevées de l'acier et une dureté superficielle élevée du revêtement.

The subject of the invention is a method for producing a workpiece from a strip of hot-rolled steel sheet and characterized in that:

• the sheet metal is coated with a metal or a metal alloy providing protection of the surface and steel,
• the coated sheet is subjected to a rise in temperature for its shaping,
• an intermetallic alloy compound is thus produced, at the surface, providing protection against corrosion, against the decarburization of the steel, the intermetallic compound being able to provide a lubricating function,
• the shaping is carried out by stamping,
• the formed part is cooled to impart high mechanical hardness characteristics of the steel and a high surface hardness of the coating.

• on revêt la tôle laminée à chaud après laminage à chaud et décapage,
• le métal ou l'alliage métallique de revêtement est de l'aluminium ou un alliage d'aluminium.
• l'alliage intermétallique est un composé à base principalement d'aluminium, de fer et de silicium,
• le revêtement de métal ou d'alliage métallique et effectué au trempé à chaud.
• la tôle revêtue est soumise à une élévation de température supérieure à 700°C avant la mise en forme,
• la tôle revêtue est emboutie à chaud.
• la pièce obtenue par emboutissage est refroidie pour subir une trempe, à une vitesse supérieure à la vitesse critique de trempe.

The other features of the invention are:

• the hot-rolled sheet is coated with hot rolling and pickling,
• the metal or metal coating alloy is aluminum or an aluminum alloy.
• the intermetallic alloy is a compound based mainly on aluminum, iron and silicon,
• the coating of metal or metal alloy and carried out by hot dipping.
• the coated sheet is subjected to a temperature rise above 700 ° C before forming,
• the coated sheet is hot stamped.
• the piece obtained by stamping is cooled to quench at a speed greater than the critical quenching speed.

The invention also relates to a hot rolled sheet having a coating of metal or metal alloy deposited on the surface, for hot forming an intermetallic compound, the intermetallic compound providing, a protection against corrosion, a barrier against decarburization of the sheet steel, lubrication during the hot shaping of the sheet, and on the finished part, after shaping and cooling, the mechanical characteristics of the hardness of the sheet, the elevated steel as well as a high surface hardness of the surface of said finished part.

• la tôle comporte un dépôt en surface d'aluminium ou d'un alliage d'aluminium.

Another feature of the invention is:

• the sheet has a surface deposit of aluminum or an aluminum alloy.

The invention also relates to the use of sheet metal in hot forming by stamping parts of high mechanical properties in hardness and high surface hardness characteristics ensuring very good resistance to abrasion.
The following description will make the invention clear.

The method according to the invention consists in coating a hot rolled steel, for heat treatment and hot forming, with a metal or a metal alloy such as aluminum or an aluminum alloy composed for example of aluminum and of silicon. The metal or metal alloy coating is selected to have corrosion protection and to withstand a high temperature. During a heat treatment, or a temperature rise for forming, the coating forms an alloy layer having a high resistance to corrosion, abrasion, wear and fatigue. The coating does not alter the formability properties of the steel and thus allows a wide variety of cold and hot forming.

After hot rolling the strip can be pickled and cold rolled before being coated.

• Alliage N°1: Si: 9% à 10%; Fe: 2% à 3,5%; Solde: Al et des impuretés.
• Alliage N°2: Fe: 2% à 4%; Solde: Al et des impuretés.

Pour l'alliage N°1 l'épaisseur du revêtement est compris entre 5 µm et 100 µm, et de préférence entre 10 µm et 25 µm. Pour l'alliage N°2 l'épaisseur du revêtement est compris entre 15 µm et 100 µm, et de préférence entre 30 µm et 50 µm.The laminated sheet can be coated with, for example, the following two types of metal alloy:

• Alloy No. 1: Si: 9% to 10%; Fe: 2% to 3.5%; Balance: Al and impurities.
• Alloy No. 2: Fe: 2% to 4%; Balance: Al and impurities.

For alloy No. 1 the coating thickness is between 5 microns and 100 microns, and preferably between 10 microns and 25 microns. For alloy No. 2 thickness the coating is between 15 μm and 100 μm, and preferably between 30 μm and 50 μm.

For shaping or heat treatment, the sheet is subjected to a temperature rise preferably between 750 ° C and 1200 ° C in an oven with an atmosphere no longer requiring control because of the barrier to oxidation formed by the coating. When raising the temperature, the aluminum-based coating is transformed into an alloyed surface layer having different phases depending on the temperature treatment and having a high hardness of more than 600 HV 100g.

According to the process of the invention, it is possible to produce sheets having a thickness of between 0.2 mm and 20 mm, having good shaping properties as well as good resistance to corrosion.

The sheets delivered coated, have a significant resistance to corrosion during temperature rises, shaping, heat treatments, and during the use of finished formed parts.

The presence of the coating during heat treatments or hot forming avoids in addition to corrosion, the decarburization of the base steel. This has an undeniable advantage in the case of hot forming in a stamping press. Indeed, the formed intermetallic alloy avoids the formation of calamine, the wear of the tools by calamine, and thereby allows an extension of the average life of said tools. It has been noticed that the hot formed intermetallic alloy has a high temperature lubricant function. In addition, the effect of protection against decarburization of the intermetallic alloy allows the use of high temperature furnace exceeding 900 ° C having an uncontrolled atmosphere, and this, even for heating times of several hours.

Out of the oven, it is no longer necessary to strip the piece obtained, resulting in a saving due to the removal of the pickling bath of finished parts.

Due to the characteristics of the coating after temperature rise, the resulting pieces have increased resistance to fatigue, wear, abrasion and corrosion. In addition, the coating is weldable before and after temperature rise.

The steel of the sheet ensures, by cooling quenching effect, high mechanical characteristics of the part obtained after forming, the coating converted into a hot intermetallic alloy, for its part, because of its lubricant qualities and resistance to friction, an improvement in shaping, in the field of hot stamping.

• carbone: 0,15% à 0,25%,
• manganèse: 0,8% à 1,5%,
• silicium: 0,1% à 0,35%,
• chrome: 0,01% à 0,2%,
• titane: moins de 0, 1 %,
• aluminium: moins de 0,1 %,
• phosphore: moins de 0,05%,
• soufre: moins de 0,03%,
• bore : 0,0005% à 0,01 %,

on revêt la bande de tôle d'un revêtement de type alliage N°1 ou 2,
on austénitise la tôle à 950°C avant formage et trempe dans l'outil, le revêtement assurant un rôle de lubrifiant lors de la mise en forme, en plus de ses fonctions de protection contre la corrosion à froid, à chaud et contre la décarburation. Lors de la trempe, le revêtement allié ne gène pas l'extraction de la chaleur par l'outil et peut la favoriser. Après formage et trempe, il n'est plus nécessaire de décaper la pièce ou de la protéger, le revêtement de base assurant la protection tout au long du procédé.In an exemplary embodiment, a sheet of hot-rolled steel sheet of the following weight composition is used:

• carbon: 0.15% to 0.25%,
• manganese: 0.8% to 1.5%,
• silicon: 0.1% to 0.35%,
• chromium: 0.01% to 0.2%,
• titanium: less than 0, 1%,
• aluminum: less than 0.1%,
• phosphorus: less than 0.05%,
• sulfur: less than 0.03%,
• boron: 0.0005% to 0.01%,

the sheet metal strip is coated with an alloy coating No. 1 or 2,
the sheet is austenitized at 950 ° C. before forming and quenching in the tool, the coating acting as a lubricant during shaping, in addition to its functions of protection against cold corrosion, hot corrosion and against decarburization . During quenching, the alloy coating does not interfere with the extraction of heat by the tool and can promote it. After forming and quenching, it is no longer necessary to strip the part or to protect it, the base coating providing protection throughout the process.

Claims (3)

1. Process of producing a part from a strip of a hot-rolled steel sheet, characterized in that:

   - the rolled sheet is coated with an aluminium alloy containing 9 to 10% silicon and 2 to 3.5% iron, the balance consisting of aluminium and impurities, as a layer having a thickness between 5 and 100 µm, or with an aluminium alloy containing 2 to 4% iron, the balance consisting of aluminium and impurities, as a layer having a thickness between 15 and 100 µm, the coating with the aluminium alloy being carried out by hot-dip coating, immediately after hot rolling and pickling;

   - the coated sheet is heated to a temperature above 700°C before the forming operation until consequently producing an alloyed intermetallic compound based on aluminium, iron and silicon on the surface, protecting the steel from corrosion and from decarburation, it being possible for said intermetallic compound to provide a lubricating function;

   - the part is hot-stamped; and

   - the formed part is quench-cooled at a rate above the critical quench rate.
2. Hot-rolled steel sheet having a coating consisting of an alloyed intermetallic compound, based on aluminium, iron and silicon, on the surface, said coating being obtained by the heat treatment of said sheet precoated with an aluminium alloy containing 9 to 10% silicon and 2 to 3.5% iron, the balance consisting of aluminium and impurities, as a layer having a thickness between 5 and 100 µm, or with an aluminium alloy containing 2 to 4% iron, the balance consisting of aluminium and impurities, as a layer having a thickness between 15 and 100 µm, said coating being deposited by hot-dip coating, immediately after the hot rolling and pickling, and said heat treatment being carried out at a temperature above 700°C.
3. Part obtained by hot-stamping the sheet according to Claim 2, followed by a quench at a rate above the critical quench rate.