EP1013785B2 - 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.

The subject of the invention is a method according to claim 1 and a part according to claim 2.
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 does not modify the formability properties of 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 the coating thickness is between 15 microns and 100 microns, and preferably between 30 microns and 50 microns.

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 provides, by quenching effect on cooling, high mechanical characteristics of the part obtained after shaping, 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 (2)

1. Process for manufacturing a part from a strip of rolled and hot-rolled sheet steel, characterised in that:

   • the rolled sheet is coated with an aluminium alloy comprising 9 to 10% silicon and 2 to 3.5% iron, the remainder being made up of aluminium and impurities, in a layer with a thickness in the range of between 5 and 100 µm, or with an aluminium alloy comprising 2 to 4% iron, the remainder being made up of aluminium and impurities, in a layer with a thickness in the range of between 15 and 100 µm, wherein the coating with aluminium alloy is conducted by hot dipping immediately after hot-rolling and pickling;

   • the coated sheet is subjected to an increase in temperature higher than 700°C before forming until thus producing a surface compound of intermetallic alloy based on aluminium, iron and silicon that assures protection against corrosion, decarburisation of the steel, wherein the intermetallic compound can assure a lubricating function;

   • the part is hot drawn;

   • the part formed by hardening is cooled at a rate higher than the critical hardening rate.
2. Part obtained by hot drawing a hot-rolled sheet steel comprising a coating formed from a compound of intermetallic alloy based on aluminium, iron and silicon, wherein said coating is obtained by thermal treatment of said sheet that has firstly been coated with an aluminium alloy comprising 9 to 10% silicon and 2 to 3.5% iron, the remainder being made up of aluminium and impurities, in a layer with a thickness in the range of between 5 and 100 µm, or with an aluminium alloy comprising 2 to 4% iron, the remainder being made up of aluminium and impurities, in a layer with a thickness in the range of between 15 and 100 µm, wherein said coating is deposited or hot dipped immediately after hot rolling and pickling, and said thermal treatment is conducted at a temperature higher than 700°C, wherein the hot drawing is followed by a hardening operation at a rate higher than the critical hardening rate.