EP0971044B1 - Clad hot-rolled and cold-rolled steel sheet, presenting a very high resistance after thermal treatment - Google Patents

Abstract

A hot rolled steel sheet, consisting of a hot dip aluminized, shaped and heat treated sheet of low alloy steel of specified composition, is new. A novel hot rolled steel sheet consists of a steel of composition (by wt.) 0.15-0.5% (exclusive) C, 0.5-3% (exclusive) Mn, 0.1-0.5% (exclusive) Si, 0.01-1% (exclusive) Cr, less than 0.2% Ti, less than 0.1% Al, less than 0.1% P, less than 0.05% S, 0.0005-0.08% (exclusive) B, balance Fe and impurities. An Independent claim is also included for forming a part from the above steel sheet having a hot dip aluminized coating which is heated at more than 5 to 600 degrees C/s. or more after sheet forming and before heat treatment. Preferred Features: The sheet is hot dip coated in a bath of composition 2-3.5% Fe and balance Al, optionally with addition of 9-10% Si.

Description

The invention relates to a hot-rolled and cold-rolled steel sheet coated with and having a very high resistance after heat treatment.

In this technical field, the proposed solutions mechanical characteristics are exerted to the detriment of the formatting properties. There is a solution of separating shaping properties and those needed for use. The characteristics necessary for the use are obtained by a treatment thermal posterior or concomitant to the shaping. In this case, the sheets offered are not delivered coated for problems with coating during heat treatment. The coating is therefore carried out on finished parts which requires careful cleaning of surfaces and parts hollow. In addition the heat treatment must be carried out under atmosphere controlled to avoid decarburization and oxidation of the sheet metal. The steel sheets for heat treatment do not have pre-coating, which requires post-treatments of degreasing, stripping and coating.

During the continuous coating of hot-rolled flat products and cold, pre-annealing and cooling before or after the operation zinc or aluminum, are only used for bring the sheet to a temperature close to that of the bath or to restore the mechanical properties of the sheet degraded during cold rolling. These thermal cycles are chosen according to the composition of the steel so that no allotropic transformation takes place during the thermal cycle, the objective being to obtain mechanical characteristics similar to those measured on uncoated steel sheets delivered. Thus, JP 01-176,056 discloses a high-strength steel composition having good resistance to fatigue and containing 0.30-0.55% C, 0.15-0.35% Si, 1.00-2.00% Mn, less than 0.040% S, 0.0010-0.0040% B, 0102-0.05% Ti, less than 0.008% N, less than 0.003% O and 0.02-0.04% Al and optionally less than 0.40% Cr, the Ti / N ratio being greater than or equal to 3.

The object of the invention is to produce a hot rolled steel sheet or cold of a desired thickness, coated, and presenting possibilities of setting in extended form and which, after heat treatment performed on the finished part, allows to obtain a mechanical strength greater than 1000 MPa, a resistance to shocks, fatigue, abrasion and heavy wear, all maintaining good corrosion resistance and good suitability painting and collage. It must also be possible to implement the hot form with quenching in the tool by obtaining the same properties.

0,15% < carbone < 0,5%

0,5% < manganèse < 3%

0,1% < silicium < 0.5%

0,01% < chrome < 1%,

titane < 0,2%,

aluminium < 0,1%,

phosphore < 0,1%,

soufre < 0,05%,

0,0005% < bore < 0,08%, le solde étant du fer et des impuretés inhérentes à l'élaboration, ledit revêtement étant à base d'aluminium ou d'alliage d'aluminium.

The subject of the invention is a hot-rolled steel sheet, which may optionally be subsequently cold-rolled, said sheet being coated and treated thermally, the steel of the sheet having the following weight composition:

0.15% <carbon <0.5%

0.5% <manganese <3%

0.1% <silicon <0.5%

0.01% <chromium <1%,

titanium <0.2%,

aluminum <0.1%,

phosphorus <0.1%,

sulfur <0.05%,

0.0005% <boron <0.08%, the balance being iron and impurities inherent in the development, said coating being based on aluminum or aluminum alloy.

• la composition pondérale de la tôle est de préférence la suivante:
• 0,20% < carbone < 0,5%
• 0,8% < manganèse < 1,5%
• 0,1% < silicium < 0,35%
• 0,01% < chrome < 1%,
• titane < 0,1%,
• aluminium < 0,1%,
• phosphore < 0,05%,
• soufre < 0,03%,
• 0,0005% < bore < 0,01%, le solde étant du fer et des impuretés inhérentes à l'élaboration.
• dans la composition pondérale de la tôle, la teneur en titane par rapport à la teneur en azote est supérieure à 3,42, le bore ne pouvant plus être combiné à l'azote .
• le bain métallique pour le revêtement comprend dans sa composition pondérale de base, de 9% à 10% de silicium, de 2% à 3,5% de fer, le solde étant de l'aluminium.
• le bain métallique pour le revêtement comprend dans sa composition pondérale de base, de 2% à 4% de fer, le solde étant de l'aluminium.

The other features of the invention are:

• the weight composition of the sheet is preferably as follows:
• 0.20% <carbon <0.5%
• 0.8% <manganese <1.5%
• 0.1% <silicon <0.35%
• 0.01% <chromium <1%,
• titanium <0.1%,
• aluminum <0.1%,
• phosphorus <0.05%,
• sulfur <0.03%,
• 0.0005% <boron <0.01%, the balance being iron and impurities inherent in the elaboration.
• in the weight composition of the sheet, the titanium content with respect to the nitrogen content is greater than 3.42, boron can no longer be combined with nitrogen.
• the metal bath for the coating comprises in its basic composition by weight, from 9% to 10% silicon, from 2% to 3.5% iron, the balance being aluminum.
• the metal bath for the coating comprises in its basic composition by weight, from 2% to 4% iron, the balance being aluminum.

The invention also relates to a method for producing a part to from the coated sheet in which after shaping and before treatment thermal, the coating of the part is subjected to an elevation of temperature exceeding 5 ° C / second which may exceed 600 ° C / second.

• le traitement thermique consiste à réchauffer le revêtement et la pièce à une température supérieure à 750°C.

Another characteristic of the process is:

• the heat treatment consists in heating the coating and the part at a temperature above 750 ° C.

The invention also relates to the use of hot-rolled steel sheet and can then be cold-rolled and coated, for structural parts and / or anti-intrusion or underbody for a land motor vehicle such as a bumper beam, a door reinforcement, an arm wheel ...

The following description will make the invention clear.

The sheet according to the invention which arises, because of its elaboration, of a hot rolling mill, can optionally be cold-rolled in depending on the desired final thickness. It is then coated based on aluminum, for example by dipping in a bath further comprising from 8% to 11% of silicon, from 2% to 4% of iron, the sheet having a high mechanical strength after heat treatment and high resistance to corrosion, as well as a good aptitude for painting and gluing.

The function of the coating is notably to protect the sheet metal base against corrosion hot and cold. Characteristics in the state of delivery of the sheet according to the invention allow a wide variety of shaping especially a deep stamping. The heat treatment applied in a hot forming process or after the shaping makes it possible to obtain high mechanical characteristics may exceed 1500 MPa for strength and 1200 MPa for the yield strength. The final mechanical characteristics are adaptable and depend on the carbon content of the steel and the heat treatment.

During heat treatment carried out on finished part or during a process hot forming, the coating forms a layer having a high resistance to abrasion, wear, fatigue, shock as well as good resistance to corrosion and a good aptitude for painting and gluing.

0,15% < carbone < 0,5%

0,5% < manganèse < 3%

0,1% < silicium < 0,5%

0,01% < chrome < 1%,

titane < 0,2%,

aluminium < 0,1%,

phosphore < 0,1%,

soufre < 0,05%,

0,0005% < bore < 0,08%, le solde étant du fer et des impuretés inhérentes à l'élaboration est élaboré sous la forme d'une tôle laminée à chaud et éventuellement laminée à froid pour obtenir l'épaisseur désirée. La tôle d'acier est ensuite revêtue au trempé, après décapage, dans un bain d'aluminium contenant, soit de 8% à 11% de silicium et 2% à 4% de fer, soit de 2% à 4% de fer sans silicium, ou encore, dans un bain d'aluminium contenant de préférence de 9% à 10% de silicium et 2% à 3,5% de fer.

According to the invention, the steel whose weight composition is as follows:

0.15% <carbon <0.5%

0.5% <manganese <3%

0.1% <silicon <0.5%

0.01% <chromium <1%,

titanium <0.2%,

aluminum <0.1%,

phosphorus <0.1%,

sulfur <0.05%,

0.0005% <boron <0.08%, the balance being iron and impurities inherent in the preparation is developed in the form of a hot rolled sheet and possibly cold rolled to obtain the desired thickness. The steel sheet is then dip-coated, after pickling, in an aluminum bath containing either 8% to 11% silicon and 2% to 4% iron, or 2% to 4% iron without silicon, or in an aluminum bath preferably containing from 9% to 10% silicon and 2% to 3.5% iron.

In an exemplary embodiment, a coating of the sheet by dipping in a metal bath containing an aluminum alloy comprising a proportion of approximately 90% aluminum, the coating layer has a first layer of alloy in contact with the surface of the steel. This layer, directly in contact with the surface of the sheet is strongly allied in iron.

A second layer of coating, above the first, contains approximately 90% aluminum and may contain, depending on the composition of the bath, silicon and a little iron.

The first alloy layer may crack when the sheet is form for the manufacture of parts.

According to a form of the invention, after the shaping of the piece, the coating is subjected to a temperature rise at a speed greater than 5 ° C / second which may exceed 600 ° C / second. This climb in temperature allows rapid reflow of the aluminum which clogs the cracks caused by the operation of shaping the workpiece.

Another advantage of the invention lies in the fact that the diffusion iron in the coating will be initiated at high temperature. We will have a better cohesion between coating and the steel of the sheet. In another form of the invention, the heat treatment can be carried out locally, in strongly deformed areas.

In an exemplary embodiment, the steel sheet according to the invention containing 0.21% carbon; 1.14% manganese; 0.020% phosphorus; 0.0038% sulfur; 0.25% silicon; 0.040% aluminum; 0.009% copper; 0.020% nickel; 0.18% chromium; 0.0040% nitrogen; 0.032% titanium; 0.003% boron; 0.0050% calcium is coated with a base layer aluminum having about 20 microns thick.

According to the invention, the sheet, in the state of delivery in a reel or in sheets, whose thickness can be between 0.25 mm and 15 mm has good shaping properties as well as good corrosion resistance as well as a good aptitude for painting and gluing.

Sheet metal, a coated steel product, has significant resistance Corrosion in the delivery state, during shaping and processing as well as during the use of the finished part. After treatment thermally, a significant mechanical strength is obtained that can exceed 1500 MPa. The presence of the coating during the heat treatment of the parts prevents any decarburization of the base metal as well as any oxidation. This is an undeniable advantage, especially in the case of formatting hot. In addition, the heating of the treated part does not require an oven having a controlled atmosphere to avoid decarburization.

The heat treatment of the metal of the sheet consists of heating at a temperature between Ac1, transformation start temperature austenitic, for example, 750 ° C, and 1200 ° C, in an oven, for a time which is a function of the temperature to be reached and the thickness of the sheet of the room. The composition is optimized to limit the magnification of the grains during heat treatment. If the targeted structure is totally martensitic, the holding temperature must be greater than Ac3, for example 840 ° C, end of austenitic formation temperature. The temperature maintenance must followed by a cooling adapted to the final structure aimed. For a totally martensitic structure and for a steel having the composition of for example, the cooling rate must be greater than the critical speed of quenching which is 27 ° C / s for austenitization at 900 ° C for 5 min, the sheet having a thickness of about 1 mm.

It is also possible to obtain structures, in particular ferritobainitic, or ferrito-martensitic, by heating at a temperature between Ac1, for example 750 ° C and Ac3, for example 840 ° C followed by a appropriate cooling. Depending on the level of resistance to be achieved and the applied heat treatment, one or more of these phases are present in varying proportions. For the highest resistance levels, the structure is mainly composed of martensite.

Chromium, manganese, boron and carbon are added, in the composition of the steel according to the invention, for their influence on hardenability. In addition, carbon achieves high mechanical characteristics thanks to its effect on the hardness of martensite.

Aluminum is introduced into the composition in order to trap oxygen and to protect the effectiveness of boron.

Titanium whose ratio of the content to the nitrogen content must be greater than 3.42 is introduced in order to avoid the combination of boron with nitrogen, nitrogen being combined with titanium.

The alloy elements, Mn, Cr, B, allow a hardenability permitting quenching in stamping tools or the use of low hardness limiting the deformation of parts during treatment thermal. In addition, the composition according to the invention is optimized from the point of view of view of weldability.

The steel of the sheet can undergo a treatment of globulization of sulphides made with calcium which improves fatigue resistance sheet metal.

Steel is particularly suitable for the production of structure and anti intrusion.

The proposed coating avoids various operations of surface preparation as for steel sheets for processing thermal having no coating.

The modulation of the heat treatment parameters allows to achieve, with a given composition, different levels of resistance in hot and cold sheet metal depending on the thickness.

During the heat treatment, the coating based for example of aluminum is transformed into a layer allied with iron and having different phases depending on the heat treatment and having a high hardness that can exceed 600HV100g.

The following table shows an example of maximum strength of the steel sheet according to the invention after heat treatment. Heat treatment Rm (Mpa) 850 ° C / 5 minutes 1695 900 ° C / 5 minutes 1675 950 ° C / 5 minutes 1665

Claims (8)

1. Hot rolled steel sheet, which could afterwards eventually be cold rolled, said sheet being coated and thermally treated, the steel of the sheet having the following ponderal composition:

   0,15% < carbon < 0,5%

   0,5% < manganese < 3%

   0,1% < silicon < 0,5%

   0,01% < chromium < 1%

   titanium < 0,2%

   aluminium < 0,1%

   phosphorus < 0,1%

   sulphur < 0,05%

   0,0005% < boron < 0,08%, the rest being iron and impurities from steel making, said coating being based on aluminium or aluminium alloy.
2. Coated sheet according to claim 1, characterised in that the ponderal composition of the sheet is preferably the following:

   0,20% < carbon < 0,5%

   0,8% < manganese < 1,5%

   0,1% < silicon < 0,35%

   0,01% < chromium < 1%

   titanium < 0,1%

   aluminium < 0,1%

   phosphorus < 0,05%

   sulphur < 0,03%

   0,0005% < boron < 0,01%, the rest being iron and impurities from steel making.
3. Coated sheet according to claim 1, characterised in that, in the ponderal composition of the sheet, the content in titanium with respect to the content in nitrogen is greater than 3,42, wherein the boron cannot combine anymore with the nitrogen.
4. Coated sheet according to claim 1, characterised in that the metallic bath for the coating comprises in its basic ponderal composition, from 9% to 10 % silicon, from 2% to 3,5 % iron, the rest being aluminium.
5. Sheet according to claim 1, characterised in that the metallic bath for the coating comprises in its basic ponderal composition, from 2% to 4 % iron, the rest being aluminium.
6. Method for the realisation of a piece, from the coated sheet according to claims 1 to 5, characterised in that, after forming and before thermal treatment, the coating of the piece is subjected to a temperature rise, with a speed greater than 5°C/second, but which could also exceed 600°C/second.
7. Method according to claim 6, characterised in that the thermal treatment consists in heating up the coating and the piece to a temperature greater than 750°C.
8. Use of the hot rolled steel sheet which could afterwards be cold rolled and coated, according to claims 1 to 5, for structural pieces and/or anti-intrusion parts or substructures of motor-driven land vehicles.