DK1013785T4

DK1013785T4 - A method for producing an item from a strip of hot rolled steel sheet

Info

Publication number: DK1013785T4
Application number: DK99403227.4T
Authority: DK
Inventors: Jean-Pierre Laurent; Jean-Paul Hennechart; Dominique Spehner; Jacques Devroc
Original assignee: Arcelormittal France
Priority date: 1998-12-24
Filing date: 1999-12-21
Publication date: 2015-11-23
Also published as: FR2787735A1; ES2274609T5; FR2787735B1; DE69933751T3; EP1013785A1; DK1013785T3; DE69933751T2; EP1013785B2; PT1013785E; ES2274609T3; EP1013785B1; DE69933751D1; ATE343658T2

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

Sheet steels that must undergo forming at high temperature and/or thermal treatment are not supplied in coated form for reasons of resistance of the coating during the thermal treatment. Therefore, coating is conducted on the finished part and this requires careful cleaning of the surfaces and hollow parts. This cleaning requires the use of acids and/or bases whose recycling and storage are a significant financial burden and pose risks for operators and the environment. Moreover, the thermal treatment must be conducted in a controlled atmosphere to prevent any decarburisation and oxidation of the steel. Then, in the case of hot working, mill scale damages the forming tools because of its abrasive power and this reduces the quality of the parts obtained from a dimensional and aesthetic viewpoint, or requires frequent and costly tool repairs to be conducted. The parts thus obtained must then undergo a costly aftertreatment to increase their corrosion resistance, and this is difficult or even impossible to perform, particularly in the case of parts that have cavities .

The aim of the invention is to propose rolled steel sheets 0.2 mm to about 20 mm thick coated after hot rolling that must undergo a hot or cold working operation followed by a thermal treatment, wherein the increase in temperature must be assured without decarburisation of the steel of the sheet and without any oxidation of the surface of said sheet before, during and after the hot working and/or the thermal treatment.

The invention relates to a process according to claim 1 and a part according to claim 2.

The invention will be better understood from the following description .

The process according to the invention consists of coating a hot-rolled steel for thermal treatment and/or hot working with a metal or a metal alloy such as aluminium or an aluminium alloy composed, for example, of aluminium and silicon. The coating of metal or metal alloy is selected to provide protection against corrosion and to withstand a high temperature. During a thermal treatment or an increase in temperature for the forming operation the coating forms an alloyed layer that has a high resistance to corrosion, abrasion, wear and fatigue. The coating does not change the formability properties of the steel and thus allows a wide variety of cold and hot forming operations.

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

The rolled sheet can be coated, for example, with the two following types of metal alloy:

Alloy No. 1: Si 9% to 10%; Fe 2% to 3.5%; remainder A1 and impurities .

Alloy No. 2: F 2% to 4%; remainder A1 and impurities.

In the case of alloy No. 1 the thickness of the coating is in the range of between 5 pm and 100 pm and preferably between 10 pm and 25 pm. In the case of alloy No. 2 the thickness of the coating is in the range of between 15 pm and 100 pm and preferably between 30 pm and 50 pm.

For the forming or thermal treatment the sheet is subjected to an increase in temperature preferably in the range of between 750°C and 1200°C in a furnace having an atmosphere that no longer needs to be controlled because of the barrier to oxidation formed by the coating. During the increase in temperature the aluminium-based coating is transformed into an alloyed surface layer that has different phases depending on the temperature treatment and a high hardness that may exceed 600 HV lOOg.

Sheets with a thickness in the range of between 0.2 mm and 20 mm that have good forming properties as well as a high resistance to corrosion can be produced using the process of the invention.

The sheets delivered in coated state have a significant resistance to corrosion during increases in temperature, forming, thermal treatments and during use of the finished formed parts.

Moreover, corrosion as well as decarburisation of the base steel can be prevented because of the presence of the coating during thermal treatments and hot forming operations. This has an undeniable advantage in the case of hot working in a drawing press. In fact, the intermetallic alloy formed prevents the formation of mill scale and wear of the tools as a result of mill scale, and as a result of this allows an extension to the average service life of said tools. It has been observed that the hot formed intermetallic alloy has a lubricating function at high temperature. Moreover, the effect of protecting against decarburisation of the intermetallic alloy allows the use of a furnace at high temperature above 900°C with a non-controlled atmosphere, even in the case of heating periods of several hours.

On exiting the furnace it is no longer necessary to pickle the part obtained, which results in a saving as no pickling bath is required for finished parts.

By virtue of the characteristics of the coating after temperature increase, the parts obtained have an increased resistance to fatigue, wear, abrasion and corrosion. Moreover, the coating is weldable before and after temperature increase.

As a result of hardening on cooling, the steel of the sheet assures that the part obtained has high mechanical properties after forming and, as a result of its lubrication qualities and friction resistance, the coating transformed into hot worked intermetallic alloy itself assures an improvement of the forming operation in hot drawing. A strip of hot-rolled sheet steel of the following composition by weight is used in one embodiment: 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%, aluminium: less than 0.1%, phosphorus: less than 0.05%, sulphur: less than 0.03%, boron: 0.0005% to 0.01%; the strip of sheet metal is coated with a coating of alloy No. 1 or 2, the sheet is austenitised at 950°C before forming and hardening in the tool, wherein the coating assures a lubricating function during the forming operation in addition to its functions of protecting against cold corrosion, hot corrosion and against decarburisation. During hardening the alloyed coating does not interfere with the extraction of heat by the tool and can benefit this. It is no longer necessary after forming and tempering to pickle the part or to protect it, as the base coating ensures protection throughout the process .

Claims

1. A method for producing an item from a strip of rolled and hot-rolled steel sheet, characterized in that:) - the rolled sheet is coated with an aluminum alloy, comprising 9-10% silicon and 2 to 3.5% iron, with the the balance being aluminum and impurities, in a layer having a thickness of between 5 and 100 microns, or of an aluminum alloy comprising 2-4% iron, the balance being aluminum and impurities, in a layer with a thickness of between 15 and 100 microns, the coating with the aluminum alloy is carried out by hot dipping immediately after the hot rolling and pickling, - the coated sheet is subjected to a temperature greater than 700 ° C before the shaping, so thereby forming one) intermetallic alloy compound based on aluminum, iron and silicon on the surface, providing a protection against corrosion and against decarburisation of the steel, since the intermetallic compound can fulfill a lubrication function - the topic is hot pressed in - formed the subject cooled by tempering at a rate above the critical hardening speed.

2.

Subject obtained by hot pressing by a hot-rolled steel sheet, which comprises a coating which is formed by a) intermetallic alloy compound based on aluminum, iron and silicon on the surface, which coating is obtained by varmebehanlding of the plate, which has previously coated with an aluminum alloy, comprising 9-10% silicon and 2 to 3.5% iron, the balance being aluminum and impurities, in a layer in a thickness of between 5 and 100 microns, or of an aluminum alloy comprising 2-4% iron, the balance being aluminum and impurities, in a layer having a thickness of between 15 and 100 microns, the coating is deposited by hot dipping immediately after the hot rolling and pickling, and by) the heat treatment is carried out at a temperature of more than 700 ° C, the hot pressing is followed by a tempering at a rate above the critical hardening temperature.