Process for manufacturing stamped products, and stamped products prepared from the same

[0001] The present invention relates to methods for producing hot stamped products obtained from coated steels and various applications of the products according to the invention, such as in spot welding.

BACKGROUND OF THE INVENTION [0002] In recent years, the use of coated steels in hot stamping processes for forming parts has become important, especially in the automotive industry. The production of such parts or products may involve consecutive main stages:

- Coating of steel strips or sheets

- Cutting or cutting of obtained blanks

- Heating of semi-finished products to achieve alloying of a steel substrate with a pre-coating, as well as steel austenization

- Hot forming, followed by rapid cooling of the parts to obtain predominantly martensitic structures

This is illustrated e.g. in US 6296805, which is hereby incorporated by reference.

By alloying the precoat with a steel substrate that creates high melting inter-matrix alloys, blanks having such a coating can be heated to a temperature range in which the austenization of the metal substrate occurs, allowing further hardening by quenching.

[0003] Heat treatment of semi-finished products, due to inter-matrix alloying of the coating and austenization of the substrate, is most often carried out in furnaces. The thermal cycles that the blanks undergo first include the heating phase, whose speed is a function of parameters such as furnace temperature setpoint, transmission speed, workpiece thickness, heating method, and coating reflectance. After this heating phase, thermal cycles typically include a holding phase whose temperature is the oven control temperature.

[0004] Parts or articles obtained after heating, hot pressing and rapid cooling show very high mechanical resistance and can be used in structural / structural applications, for example in applications in the automotive industry. Such parts must often be welded to others and high weldability is required. This means that:

- The welding operation should be able to be carried out in a sufficiently wide operational range to ensure that any shift in nominal welding parameters will not affect the quality of the weld. In the case of resistance welding, which is very common in the automotive industry, the operating range of welding is determined by a combination of parameters: among which the most important are the welding current I and the force F applied to the part during welding. The correct combination of these parameters makes it easier to ensure that an insufficient weld core diameter (caused by insufficient strength or insufficient force) cannot be obtained, and that no metal eczema occurs during welding.

- The welding operation should also be carried out in such a way as to obtain a large od1

Mechanical resistance of the weld. Such mechanical resistance can be assessed in tests such as shear tensile tests or cross tensile tests.

EP1380666 also discloses a method comprising hot stamping Al-coated steel sheets for the production of welded components. However, the weldability needs further improvement.

There remains a need for a production method that allows the production of stamped parts or products that are very useful for spot welding, which are easy to paint and which show good corrosion resistance.

SUMMARY OF THE INVENTION [0005] The inventors have found that certain coated steels in which the primary steel strip or sheet is at least partially coated (sometimes referred to as "pre-coated," wherein the term pre-means that the transformation of the nature of the precoating will occur during heat treatment) before hot stamping or forming) on ​​at least one side with an aluminum or aluminum alloy coating and where the coating has a certain thickness, it is conveniently formed into shaped parts after processing under certain conditions, and thus shows a certain increased weldability.

[0006] The inventors have also found that particularly good weldability of aluminized and hot stamped parts is associated with the special order of the coating layers on the parts, starting from the steel substrate outwards, and with the controlled proportion of pores in these layers.

[0007] The inventors have also found that this special arrangement of layers is associated with specific heating conditions.

OBJECT OF THE INVENTION [0008] The object of the present invention is to provide new hot stamped parts that are made of precoated steel.

[0009] Another object of the present invention is to provide new production products, such as a motor vehicle, which includes such stamped parts.

[0010] Another object of the present invention is to provide new methods for producing stamped parts having high weldability.

[0011] These and other objectives will be apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS [0012]

Figure 1 shows furnace temperature conditions as a function of total oven residence time for sheets with a total thickness of 0.7-1.5 mm and 1.5-3 mm, which provide particularly convenient coatings for welding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0013] The invention is carried out with some precoated steel strips that include a primary steel strip and an aluminum or aluminum alloy precoat on at least part of one side of the primary steel strip. For many applications, the basic steel strip or sheet metal can contain any type of steel, which can be coated with aluminum or aluminum alloy

EP 2 242 863 B1. However, for certain applications, such as the car's structural part, it is preferred that the basic steel strip contains steel providing ultra high strength parts, over 1000 MPa. In such cases, it is preferred that the primary steel strip contains boron steel.

[0014] The strip may originate, due to its processing, from a hot rolling mill, and may be cold rolled again depending on the desired final thickness. Preferred thicknesses are 0.7 to 3 mm. Typically, the basic steel strip will be stored and transported in a coil, both before and after the coating.

[0015] An example of a preferred steel for basic steel strips is steel having the following weight composition:

0.10% <carbon <0.5%

0.5% <manganese <3%

0.1% <silicon <1%

0.01% <chromium <1% nickel <0.1% copper <0.1% titanium <0.2% aluminum <0.1% phosphorus <0.1% sulfur <0.05%

0.0005% <boron <0.010% and the rest includes, consists essentially of, or consists of iron and unavoidable impurities from treatment. The use of such steel ensures very high mechanical resistance after heat treatment, and the aluminum-based coating ensures high corrosion resistance.

[0016] Particularly preferably the weight composition of the steel in the strip of basic steel is as follows: 0.15% <carbon <0.25%

0.8% <manganese <1.8%

0.1% <silicon <0.35%

0.01% <chromium <0.5% nickel <0.1% copper <0.1% titanium <0.1% aluminum <0.1% phosphorus <0.1% sulfur <0.05%

0.002% <boron <0.005% and the rest includes, consists essentially of, or consists of iron and unavoidable impurities from treatment.

An example of a preferred commercially available steel strip for basic steel is 22MnB5.

[0017] Chromium, manganese, boron and carbon may be added to the steel composition of the invention because of

Their effect on hardenability. In addition, carbon enables high mechanical characteristics to be achieved due to its effect on martensite hardness.

[0018] Aluminum (aluminum) is introduced into the composition to carry out liquid deoxidation and to protect the effectiveness of boron.

[0019] Titanium, the ratio of which, with respect to the nitrogen content, should exceed 3.42, is introduced, for example, to prevent boron and nitrogen joining, as the nitrogen will combine with titanium.

[0020] The alloying elements, Mn, Cr, B, allow hardenability to allow hardening in stamping tools or to use mild hardening fluids to limit deformation of parts during heat treatment. In addition, the composition according to the invention is optimized in terms of weldability. You can also add Ni and Cu in an amount of up to 0.1%.

[0021] Steel can be treated to globulate sulfides with calcium, which improves sheet fatigue strength.

[0022] The basic steel strip is coated (or precoated, the term pre-mean that the transformation of the nature of the precoating will occur during heat treatment prior to pressing) using aluminum or an aluminum alloy, preferably by hot dip coating. A typical metal bath for an Al-Si coating usually contains, in its basic composition based on the weight of the composition, from 8% to 11% silicon, from 2% to 4% iron, the rest being aluminum or aluminum alloy, and inevitable impurities from processing. Silicon is present to prevent the formation of an inter-matrix iron-metal layer that reduces adhesion and shaping ability. Other alloying elements useful according to the invention together with aluminum include iron and calcium in an amount between 15 and 30 ppm by weight, including combinations of two or more of them with aluminum. A typical composition of the Al-Si coating is as follows: Al-9.3% Si-2.8% Fe. However, the coatings of the invention are not limited to such compositions.

Without being bound by any particular theory of operation, the inventors believe that a number of advantages of the invention are associated first of all with a specific range of precoating thickness from 20 to 33 microns:

- In the case of a pre-coating less than 20 microns thick, the alloy layer that forms when the blank is heated has insufficient roughness. Therefore, the adhesion of the subsequent paint to the surface is low and the corrosion resistance is reduced.

- If the thickness of the precoat is above 33 micrometers at a given place on the baking sheet, the risk is that the difference in thickness between this place and some other places where the precoat is thinner becomes too important and stopping when heating the blank becomes uneven. The inventors have also shown that adjusting the thickness of the pre-coating within the narrow range described above contributes to the formation of coatings after alloying, the thickness of which is also regulated in the exact range. It is also a factor ensuring that the range of resistance welding parameters to which parts are subjected after alloying does not change.

The pre-coated steel sheets or strips are then cut into semi-finished products

In an oven before hot stamping, to obtain products or parts. The inventors have found that very good welding properties are obtained when the coating obtained on parts or products made of semi-finished products which have been subjected to intermetallic alloying, austenization and hot pressing, exhibits distinctive features. It should be noted that such a coating differs from the initial precoat because the heat treatment causes a alloying reaction with the steel substrate that modifies both the physicochemical character and the geometry of the precoat: in this aspect the inventors found that particularly good weldability of aluminized and extruded hot parts is related to the following order of coating layers on the parts ranging from the steel substrate to the outside:

- (a) Interdiffusion layer,

- (b) Intermediate layer,

- (c) The inter-matrix layer,

- (d) Surface layer

The inventors have further found that particularly good weldability is obtained with a limited content / number of pores in the coating layers, as will be detailed below.

In a preferred embodiment, the layers are as follows:

- (a) Interdiffusion layer, preferably of medium hardness (e.g. HV50g between 290 and 410, where HV50g is the hardness measured under a load of 50 grams) In a preferred embodiment, this layer has the following composition, by weight: 86-95% Fe, 4-10 % Al, 0-5% Si

- (b) Intermediate layer (HV50g e.g. around 900 - 1000, +/- 10%)) In a preferred embodiment, this layer has the following composition, by weight: 39-47% Fe, 53-61% Al, 0-2% Si

- (c) An inter-matrix layer, with a hardness of HV50g e.g. around 580-650, +/- 10%) In a preferred embodiment, this layer has the following composition, by weight: 62-67% Fe, 30-34% Al, 2-6% si

- (d) Surface layer (HV50g e.g. around 900 - 1000, +/- 10%)) In a preferred embodiment, this layer has the following composition, by weight: 39-47% Fe, 53-61% Al, 0-2% Si

In a preferred embodiment, the total thickness of the layers (a) to (d) is over 30 microns.

In another preferred embodiment, the thickness of the layer (a) is below 15 microns.

[0023] The inventors have further found that high weldability is particularly achieved when layers (c) and (d) are substantially continuous; the nature of the essential continuity of these layers is defined as follows: the layers can be completely continuous. However, they may be fragmented in some areas due to parts of the layers coming from the lower or upper levels. According to the invention, this fragmentation must be limited, i.e. layers (c) and (d) must occupy at least 90% of their respective level. High weldability is achieved when less than 10% of layer (c) is present on the outer surface of the part. Without wishing to be bound by theory, it is believed that this arrangement of specific layers, in particular layer (a) and layers (c) and (d) affects the resistivity of the coating both due to its natural characteristics and as a result of roughness. Therefore, this specific arrangement affects the flow of current, heat release on surfaces, and the formation of weld nuclei at the initial stage of spot welding. Such advantageous arrangement of layers is obtained, for example, when steel sheets are initially 5

EP 2 242 863 B1 coated with aluminum or an aluminum alloy whose thickness is e.g. 0.7 to 3 mm, heated for 3 to 13 minutes (this residence time includes the heating phase and holding time) in an oven without a special atmosphere, heated to temperature 880 to 940 ° C. The invention does not require a furnace with a controlled atmosphere. Other conditions leading to such an advantageous distribution of layers are given in Figure 1 and below.

Particularly favorable conditions are as follows:

- for 0.7-1.5 mm thick

- 930 ° C, from 3 minutes to 6 minutes;

- 880 ° C, from 4 minutes 30 seconds to 13 minutes

- for a thickness of 1.5 to 3 mm

- 940 ° C, from 4 minutes to 8 minutes;

- 900 ° C, from 6 minutes 30 seconds to 13 minutes

For sheets with a total thickness greater than or equal to 0.7 mm, and less than or equal to 1.5 mm, favorable processing conditions: (oven temperature, total oven time) are illustrated in figure 1 as being within the "ABCD plot "For sheets with an overall thickness greater than 1.5 mm and less than or equal to 3 mm, favorable processing conditions: (oven temperature, total oven time) are illustrated in figure 1 by the diagram" EFGH ". The heating rate Vc is between 4 and 12 ° C / s when achieving a favorable distribution of the melt layer. Vc, depending on the specific furnace settings, is defined as the average heating rate between 20 and 700 ° C of a pre-coated steel blank in a preheated furnace. The inventors have found that regulation of Vc in this specific range allows influencing the nature and morphology of the resulting alloy layers. It is emphasized that the heating rate Vc differs from the average heating rate, which is the heating rate between room temperature and the oven holding temperature.

The inventors have surprisingly found that special heating conditions favorably affect the formation of alloy layers, resulting in less porosity. Without wishing to be bound by the theory of the invention, it is believed that the formation of preferred alloy layers occurs over a defined temperature range due to the specific kinetics of alloying in this range: in this aspect, it has been found that controlling the heating rate over a specified temperature range between 500 and 700 ° C ( referred to in the description as Vc ') is particularly important and that the value of Vc' should be between 1.5 and 6 ° C / s.

When Vc 'is lower than 1.5 ° C / s, there is a risk that oxidation kinetics, resulting from the interaction of oxygen from the furnace atmosphere with the surface of the precoat, competes with the kinetics of melt formation between the steel substrate and the precoat. Accordingly, the desired distribution of the molten layer will not be obtained. In addition, the low heating rate V'c causes too many pores in the coating.

When Vc 'is greater than 6 ° C / s, there is a tendency that the inter-matrix layer (c) will occupy more than 10% of the outer surface of the part, resulting in poor weldability.

When Vc is between 1.5 and 6 ° C / s, the nature of the substantial continuity of layers (c) and (d) is fully ensured.

EP 2 242 863 B1

Without being bound by theory, it is believed that the formation of porosity and its effect on weldability can be explained as follows:

Porosity mainly occurs during interdiffusion of the precoat with a steel substrate due to differences in diffusion fluxes. This creates a stream of vacancies with the formation of Kirkendal defects. It appears that the occurrence of vacancies in the form of porosity is optimized when the heating rate V'c is between 1.5 and 6 ° C / s.

During spot welding of welding products, the current initially flows around the pores, which gradually collapse due to the increase in pressure and temperature. Therefore, the current flows through the coating, which certain properties may change discontinuously, which in turn can lead to increased sparking of metal splashes during welding operations.

Increased spot welding is observed when the coating obtained as a result of interdiffusion contains, in the surface part, less than 10% porosity. For a given representative area of ​​the coating, this share is the total area occupied by the pores, relative to the surface of the coating.

Particularly good weldability is achieved when the surface layer has controlled compactness, which means that the surface layer (d) has less than 20% of pores: this proportion is the surface occupied by the pores in the surface layer (d), relative to the surface of the surface layer.

A particular advantage is due to the pre-coatings, whose thickness is between 20 and 33 micrometers, because with this thickness a favorable distribution of layers is obtained, and because the uniformity of the thickness of the pre-coating is associated with the uniformity of the coating formed after alloying.

[0024] The heated blanks are then transferred from the furnace to the die, hot pressed in the press to obtain a part or product and cooled at a rate Vr over 30 ° C / s. The cooling rate Vr is defined in the description as the average speed between removing the heated blank from the oven, up to 400 ° C. Under such conditions, austenite formed at high temperature is mainly transformed into martensitic or martensitic-bainitic structures of high strength.

In a preferred embodiment, the time that elapses between removing the heated intermediate and entering the intermediate into the hot stamping press is no more than 10 seconds. Otherwise, partial transformation to austenite easily occurs: if it is desired to achieve a completely martensitic structure, the transfer time between leaving the furnace and pressing should be less than 10 s.

The operation of the obtained coating consists in particular in protecting the base sheet from corrosion in various conditions. During the heat treatment of the finished part or in the hot shaping process, the coating forms a layer that exhibits basic resistance to abrasion, wear, fatigue, shock, as well as good corrosion resistance and good paint and glue. The coating avoids various surface preparation operations such as those for heat treated steel sheets without any coating.

The heat treatment that occurs during the hot shaping process or after shaping allows high mechanical characteristics, which may exceed 1500 MPa, to be achieved7

EP 2 242 863 B1 to mechanical strength and 1,200 MPa with respect to the yield strength. The final mechanical characteristics can be adjusted and depend in particular on the proportion of martensite in the structure, on the carbon content in steel and on heat treatment.

The invention also relates to the use of hot rolled steel sheet, which can then be cold rolled and coated, for structural / structural and / or non-impact or substructural parts for land motor vehicles, such as, for example, a bumper rod, door reinforcement, wheel spokes, etc.

The present invention will now be further described by means of certain example forms which are not intended to be limiting.

EXAMPLES [0025]

i) - Conditions according to the invention: in the embodiment, cold-rolled steel sheet 1.2 mm thick was produced: it contains: 0.23% carbon, 1.25% manganese, 0.017% phosphorus, 0.002% sulfur, 0, 27% silicon, 0.062% aluminum, 0.021% copper, 0.019% nickel, 0208% chromium, 0.005% nitrogen, 0.038% titanium, 0.004% boron, 0.003% calcium. The sheet was pre-coated with an alloy based on 9.3% silicon, 2.8% iron, the rest being aluminum and unavoidable impurities. The thickness on each side of the sheet was adjusted to be in the range (20-33) microns.

The sheets were then cut into blanks, which were heated at 920 ° C for 6 min, this time comprising the heating phase and holding time. The heating rate Vc between 20 and 700 ° C was 10 ° C / s. The heating rate Vc 'between 500 and 700 ° C was 5 ° C / s. No special furnace atmosphere control was used. The blanks were transferred from the furnace to the press in less than 10 s, hot pressed and hardened to obtain fully martensitic structures.

The parts obtained after hot stamping are covered with a coating of 40 microns thickness, which was a four-layer structure. Starting from the steel substrate, the layers were as follows:

- (a) Interdiffusion layer, i.e. an inter-matrix layer, 17 microns thick. This layer itself consists of two sub-layers. HV50g hardness is from 295 to 407, and the average weight composition is as follows: 90% Fe, 7% Al, 3% Si.

- (b) Intermediate layer, 8 microns thick. This layer has a hardness of 940 HV50g and the average weight composition is as follows: 43% Fe, 57% Al, 1% Si,

- (c) An inter-matrix layer, 8 microns thick, showing a hardness of 610 HV50g, and the average weight composition is as follows: 65% Fe, 31% Al, 4% Si

- (d) Surface layer, 7 microns thick, 950 HV50g, and the average weight composition is as follows: 45% Fe, 54% Al, 1% Si

Layers (c) and (d) are pseudo-continuous, i.e. they occupy at least 90% of the level corresponding to the appropriate layer. In particular, layer (c) does not reach the outer surface with very few exceptions. In any case, this layer (c) takes up less than 10% of the outer surface.

EP 2 242 863 B1

A small number of pores were observed in the coating, their surface proportion in this coating is below 10%. The surface proportion of porosity in the outer layer (d) is below 20%.

ii) Comparative conditions: blanks of the same basic material with the pre-coating were heated in an oven under different conditions: Blanks were heated to 950 ° C for 7 minutes, this time including the heating phase. The heating rate Vc was 11 ° C / s. The heating rate Vc 'between 500 and 700 ° C was 7 ° C / s. These conditions correspond to a degree of alloying which is more important than under conditions (i)

- In this coating, the inter-matrix layer (c) is not continuous and appears to be dispersed in the coating. About 50% of this layer is present on the outer surface of the part. The 10-micrometer interdiffusion layer in contact with the steel substrate is thinner than in the previous case. In addition, the pores are much more numerous than under conditions (i), since their share in the surface of the coating exceeds 10%. Such pores are in particular more numerous in the surface layer (d), where their share in the surface exceeds 20%.

- Spot resistance welding was carried out in two situations, i) and ii):

- (i): Coating with pseudo-layered layers (c) and (d), with layer (c) occupying less than 10% of the outer surface and with a low surface porosity

- (ii): Coating with mixed and discontinuous layers, with layer (c) covering more than 10% of the outer surface and with a greater surface proportion of porosity

Spot resistance welding was carried out by superimposing two parts and joining them under the following conditions:

- Compression force and welding force: 4000 N

- Compression time: 50 cycles

- Welding and holding time: 18 cycles respectively

In each case, an appropriate intensity range was established to achieve:

- No spraying during welding

- Permissible weld testicle size.

Tensile tests were also performed to assess the weldability.

- For condition i), the weldability range, expressed by current, is 1.4 kA. In condition (ii) the weldability range is extremely small. Sparks and coating spattering are associated with a higher proportion of porosity and layer distribution.

In this connection, it can be concluded that the coating according to the invention provides much more satisfactory results.

[0026] Although the above description is clear with respect to the understanding of the invention, in order to avoid any confusion the following terms used in the following statement in the preferred embodiments and claims have the following meanings:

precoat: - material (Al or Al alloy) coating or applied to at least part of the strip or sheet etc., of basic steel, to form a composite coating

Pre-base / base, wherein the composite has not been subjected to an alloying reaction between the coating of an Al or Al-alloy material and the primary steel alloying or alloying: - the reaction between the initial coating and the base steel to form at least one layer intermediate composition differing from both the basic steel and the initial coating. The alloying reaction occurs during the heat treatment immediately preceding hot stamping. The alloying reaction affects the overall thickness of the precoat. In a highly preferred embodiment, the following layers are formed in the alloying reaction: (a) interdiffusion, (b) intermediate, (c) intermetallic and (d) surface as described above;

precoated steel: - composite precoat / base, not subjected to an alloying reaction between the coating material and the base steel; Coating: - Precoat after having undergone an alloying reaction between the precoat and base steel. In a highly preferred embodiment, the coating comprises (a) interdiffusion, (b) intermediate, (c) intermetallic and (d) surface layers as described above; coated steel or product: - pre-coated steel or product, subjected to an alloying reaction between the precoat and the base steel. In a highly preferred embodiment, the coated steel is a strip or sheet etc. of a base steel with a coating according to the invention comprising (a) interdiffusion, (b) intermediate, (c) intermetallic and (d) surface layers as described above;

semi-finished product: - shape cut from the tape. product: - hot pressed blank

The above description of the invention provides a method and process for its implementation and application, so that any person skilled in the art is capable of its implementation and application.