Patent Description:
The press hardening process has been disclosed in the publication <CIT>. A hardened steel part is obtained by heating a steel blank to a temperature at which the steel is transformed into austenite and then hot formed in a press. The blank is simultaneously rapidly cooled in the press tool and held so to prevent distortion thus obtaining a martensitic and/or bainitic microstructure. The steel used may have the following composition: C<<NUM>%, <NUM>-<NUM>%Mn, S and P<<NUM>, <NUM>-<NUM>% Cr, <NUM>-<NUM>% Mo, <<NUM>% Ti, <NUM>-<NUM>% B, <<NUM>%Al. However, this publication does not provide a solution for obtaining simultaneously high mechanical resistance and elongation, good bendability and weldability.

The fabrication of parts with good corrosion resistance and tensile strength higher than <NUM> MPa is disclosed by the publication <CIT> : an aluminized steel sheet with <NUM>-<NUM>% C, <NUM>-<NUM>% Mn, <NUM>-<NUM>% Si, <NUM>-<NUM>% Cr, <<NUM>% Ti, <NUM>% Al and P, <<NUM>% S, <NUM>-<NUM>% B, is heated, formed and rapidly cooled. However, due to the high tensile strength level, the total elongation in tensile test is lower than <NUM>%.

The publication <CIT> discloses the press hardening of a steel blank with a composition containing: <NUM>-<NUM>% C, <NUM>-<NUM>% Mn, <<NUM>% Si, <<NUM>% S, <<NUM>% P, <NUM>-<NUM>% Al, <NUM>-<NUM>% Al, <NUM>-<NUM>% Ti, <<NUM>% N, <<NUM>% Cu, Ni, Mo, <<NUM>% Ca. After press hardening, a tensile strength higher than <NUM> MPa can be obtained. However, due to the nature of the microstructure, which is equiaxed ferrite, it is not possible to achieve very high tensile strength level.

The document <CIT> discloses a steel composition comprising <NUM>-<NUM>% C, <NUM>-<NUM>% Si, <NUM>-<NUM>% Mn, <NUM>-<NUM>% Ni, ≤<NUM>% P, ≤<NUM>% S, <NUM>-<NUM>% B, optionally <NUM>-<NUM>% Ti, optionally <NUM>-<NUM>% Al, optionally <NUM>-<NUM>% N. This composition makes it possible to manufacture a press hardened part with a tensile strength higher than <NUM> MPa and with elongation higher than <NUM>%. However, due to its high nickel content, this steel is costly to manufacture.

The document <CIT> discloses a press hardened part made from a steel alloy containing <NUM>-<NUM>% C, <NUM>-<NUM>% Si, <NUM>-<NUM>% Mn, <<NUM>% P, < <NUM>% S, <NUM>-<NUM>% Cr, <<NUM>% N, <NUM>-<NUM>% Nb, <NUM>-<NUM>%B, <NUM>-<NUM>%Ti. The part has a tensile strength higher than <NUM> MPa and a total elongation more than <NUM> %. However, due to its high chromium content, this steel is costly to manufacture.

Thus, it is desired to have a press hardened part and a manufacturing process that would not have the previous limitations. It is more particularly desired to have a press hardened steel part with a thickness comprised between <NUM>,<NUM> and <NUM> and a yield stress YS comprised between <NUM> and <NUM> MPa, a tensile stress TS comprised between <NUM> and <NUM> MPa, and a high ductility characterized by a bending angle higher to <NUM>°.

It is also desired to have a press hardened part with a fracture strain under plane strain condition, higher than <NUM>.

As heavily deformed areas in the press hardened parts, such as for example the radii zones, are subjected to high stress concentration during further service conditions or during vehicles collisions, it is also desirable to have press hardened parts which would display higher ductility in these deformed zones.

It is also desirable to have press hardened parts which would be easily weldable, and press hardened welded joints with high ductility and free from significant softening in the Heat Affected Zones.

It is also desirable to have steel sheets that would be suitable for Laser welding: this process is very sensitive to misalignment defects that can be due to insufficient flatness: thus, sheets with very good flatness properties are required for Laser welding.

It is also desirable to have a steel sheet that could be easily weldable either in a homogeneous process (i.e. welding of two sheets with the same composition) or in heterogeneous process (welding of two sheets with different steel compositions) and further press hardened, and that these press hardened welds have high mechanical properties.

It is also desired to have a steel composition for press hardening that could be available either in uncoated state or with a metallic coating providing to the steel substrate a corrosion resistance after press hardening.

To this end, a first object of the present invention is a press hardened steel part with a steel chemical composition comprising, in weight: <NUM>% ≤ C ≤ <NUM>%, <NUM>% ≤ Mn ≤ <NUM>%, <NUM>% ≤ Si ≤ <NUM>%, <NUM>% ≤ Al ≤ <NUM>%, <NUM>% ≤ Cr ≤ <NUM>%, wherein <NUM>% ≤(C+Mn+Si+Cr) ≤ <NUM>%, <NUM>% ≤ Nb ≤ <NUM>%, <NUM> × N ≤Ti ≤ <NUM> × N, wherein: <NUM>% ≤ (Nb + Ti) ≤<NUM>,<NUM>%, <NUM> ≤ B ≤ <NUM>%, <NUM>% ≤N < <NUM>%, <NUM>% ≤ S ≤ <NUM>%, <NUM>% ≤ P ≤ <NUM>% optionally: <NUM>% ≤ Ca ≤<NUM>%, the remainder being Fe and unavoidable impurities, and wherein the microstructure comprises, in the majority of the part, in surface fractions: less than <NUM>% of bainite, less than <NUM>% of austenite, less than <NUM>% of ferrite, the remainder being martensite, said martensite consisting of fresh martensite and of self-tempered martensite.

Preferably, the composition is such that: <NUM> ≤ B ≤ <NUM>%.

Preferably, the composition is such that: <NUM>% ≤ (C + Mn+ Si+ Cr) ≤ <NUM>%.

In a preferred mode, the C content of the steel part is such that: <NUM>% ≤ C ≤ <NUM>%.

Preferably, the microstructure comprises at least <NUM>% in surface fraction of self-tempered martensite.

The sum of fresh martensite and of self-tempered martensite surface fractions is preferably comprised between <NUM> and <NUM>%.

According to a preferred mode, the average size of titanium nitrides is less than <NUM> micrometers in the outer zones comprised between one quarter thickness of the part, and the closest surface of the part.

Preferably, the average length of sulfides is less than <NUM> micrometers in the outer zones comprised between one quarter thickness of the part, and the closest surface of the part.

According to a preferred mode, the press hardened steel part comprises at least one hot deformed zone (A) with a deformation quantity higher than <NUM>, and at least one zone (B) having experienced the same cooling cycle in press hardening than zone (A), wherein the deformation quantity is less than <NUM>.

The difference in hardness between the zone (B) and the hot deformed zone (A) is preferably more than <NUM> HV.

Preferably, the average lath width of the martensitic-bainitic structure in the hot deformed zone (A) is reduced by more than <NUM>% as compared to the lath width of the martensitic-bainitic structure in the zone (B).

In a preferred mode, the average lath width of the martensitic-bainitic structure in the hot deformed zone (A) is less than <NUM>.

The average lath width of the martensitic-bainitic structure in the zone (B) is preferably comprised between <NUM> and <NUM>.

According to one mode of the invention, the press hardened steel part is coated with a metallic coating.

The metallic coating is preferably zinc-based alloy or zinc alloy.

Preferably, the metallic coating is aluminum-based alloy or aluminum alloy.

In a preferred mode, the press hardened part has a yield stress comprised between <NUM> and <NUM> MPa, a tensile stress TS comprised between <NUM> and <NUM> MPa, and a bending angle higher than <NUM>°.

According to a preferred mode, the press hardened steel part has a variable thickness.

Very preferably, the variable thickness is produced by a continuous flexible rolling process.

Another object of the invention is a press hardened Laser welded steel part, wherein at least one first steel part of the weld is an Al coated part as described above, welded with at least at least one second steel part, the composition of which contains from <NUM> to <NUM>% of carbon in weight, and wherein the weld metal between the first steel part and the second steel part has an aluminum content less than <NUM>% in weight, and wherein the first steel part, the second steel part, and the weld metal, are press hardened in the same operation.

The invention has also for object a process for manufacturing a press hardened steel part comprising the following and successive steps:.

Preferably, the cold rolling ratio is comprised between <NUM> and <NUM>%.

The annealing temperature Ta is preferably comprised between <NUM> and <NUM>, and very preferably between <NUM> and <NUM>.

In a particular mode, the blank is cold formed before heating said blank at said temperature Tm.

Preferably, the hot forming is performed with a deformation quantity higher than <NUM> in at least one hot deformed zone of the part,.

In a preferred mode, the annealed steel sheet is precoated with metallic precoating, before cutting the annealed steel blank to a predetermined shape.

The metallic precoating is preferably zinc, or zinc-based alloy, or zinc alloy.

Preferably, the metallic precoating is aluminum, or aluminum-based alloy, or aluminum alloy.

According to a preferred mode, the sheet is precoated with at least one intermetallic layer containing Al and iron, and optionally silicon, and the precoating contains neither free Al, nor τ <NUM> phase of Fe3Si2Al12 type , nor τ <NUM> phase of Fe2Si2Al9 type.

In another preferred mode, the metallic precoating comprises a layer of aluminum or an aluminum-based alloy or an aluminum alloy, topped by a layer of zinc or zinc-based alloy or a zinc alloy.

The invention has also for object a process for manufacturing a press hardened Laser welded steel part, comprising the successive following steps of:.

The invention has also for object the use of a part as described above, or manufactured according to a process as described above, for the manufacturing of structural or safety parts of vehicles.

The invention will now be described in more details but without limitations in view of the following figures, wherein:.

The press hardened steel parts are manufactured from a steel sheet having a specific composition, the elements being expressed in weight percentage:.

Thus, the majority of the hardened part contains more than <NUM>% of martensite in surface fraction. The surface fraction is determined through the following method: a specimen is cut from the press hardened part, polished and etched with a reagent known per se, so as to reveal the microstructure. The section is afterwards examined through optical or scanning electron microscope. The determination of the surface fraction of each constituent (martensite, bainite, ferrite, austenite) is performed with image analysis through a method known per se.

Martensite is present as fine elongated laths, oriented within the prior austenite grains.

According to the cooling rate in the press hardening process and to the transformation temperature Ms of austenite into martensite, martensite may be present as fresh martensite and/or as self-tempered martensite. The specific features of these sub-constituents can be determined through electron microscope observations:.

According to a preferred mode of the invention, the sum of fresh martensite and of self-tempered martensite surface fractions in the press hardened part is comprised between <NUM> and <NUM>%. Such condition contributes to achieve a tensile strength of at least <NUM> MPa, when the cooling rate in press hardening is comprised between <NUM> and <NUM>/s in a temperature range between <NUM> and <NUM>.

According to another preferred mode, the microstructure of the press hardened parts contains, in surface fraction, at least <NUM>% of self-tempered martensite. Thus, increased ductility and bendability is obtained, as compared to the situation wherein the microstructure contains only fresh martensite.

As the press hardened part must have high bendability properties, it has been found that the average size of titanium nitrides must be controlled to this end. The average size of TiN may be determined through observations by Scanning or Transmission Electron Microscopy observations. More specifically, it has been determined that the average size of TiN must be limited in the outer zones near the surface of the press hardened part, which are the most strained zones during bending. These zones are comprised between one quarter thickness of the part, and the closest surface of the part. Such outer zones, parallel to the main surfaces of the press hardened parts, are illustrated as an example on a schematic drawing of a press hardened Hat-shaped part (or "omega" shaped) on <FIG>, wherein they are referred to as (A) and (B). It will be understood that such illustration of the outer zones is not limited to this specific hat-shaped geometry, but applies to any press hardened part geometry.

If the average size of TiN is not less than <NUM> micrometers, damage is initiated at the boundaries between the rectangular-shaped titanium nitrides and the matrix, and the bending angle may be less than <NUM>°.

In these outer zones, there is also a risk that damage initiation results from the presence of elongated sulfides: these constituents can be present when sulfur content is sufficiently high to combine, mainly with manganese, under the form of coarse precipitates. As their plasticity is high at elevated temperatures, they are easily elongated by hot rolling and during hot deformation in press hardening. Thus, when the average length of sulfides is higher than <NUM> micrometers in the outer zones (i.e. from one quarter thickness to the closest surface), the fracture strain can be less than <NUM> due to ductile initiation on these sulfides. This press hardened part described above may be uncoated or optionally coated. The coating may be aluminum-based alloy or aluminum alloy. The coating may be also zinc-based alloy or zinc alloy.

In a particular embodiment, the press hardened steel part of the invention can have a thickness which is not uniform but which can vary. Thus, it is possible to achieve the desired mechanical resistance level in the zones which are the most subjected to external stresses, and to save weight in the other zones of the press hardened part, thus contributing to the vehicle weight reduction. In particular, the parts with non-uniform thickness can be produced by continuous flexible rolling, i.e. by a process wherein the sheet thickness obtained after rolling is variable in the rolling direction, in relationship with the load which has been applied through the rollers to the sheet during the rolling process. Thus, within the conditions of the invention, it is possible to manufacture advantageously vehicle parts with varying thickness such as front and rear rails, seat cross members, tunnel arches, pillars, dash panel cross members, or door rings.

The process for manufacturing the press hardened part will be now explained.

A semi-product in the form of cast slab or ingot, able to be further hot-rolled, is provided with the steel composition described above. The thickness of this semi-product is typically comprised between <NUM> and <NUM>.

This semi-product is hot-rolled so to obtain a hot-rolled steel sheet and coiled at a temperature Tc. The coiling temperature must not be higher than <NUM>, otherwise a too important precipitation of niobium carbonitrides occurs, which induces hardening and increases difficulties for the further cold rolling step. When Tc does not exceed <NUM>, at least <NUM>% of free niobium remains in the steel sheet. Tc must also not be lower than Ms so to avoid martensite formation which makes the cold rolling step more difficult.

At this stage, the thickness of the hot rolled steel sheet may be in the typical range of <NUM>- <NUM>. For applications wherein the desired final thickness in the range of <NUM>-<NUM>, the steel sheets may be directly annealed with the process described below. For applications in the range of <NUM>-<NUM>, the hot-rolled sheets are pickled in usual conditions and further cold rolled. The cold rolling ratio is defined in the following manner: if t<NUM> designates the thickness before cold rolling, and tf the thickness after cold rolling, the rolling ratio is : (ti - tf)/ti. In order to obtain a high fraction of recrystallization during the ulterior annealing, the cold rolling ratio is typically comprised between <NUM> and <NUM>%.

Then, the hot rolled, or hot rolled and further cold rolled sheet, is annealed in the intercritical range Ac1-Ac3, at a temperature Ta selected so to obtain less than <NUM>% of unrecrystallized fraction. When the unrecrystalllized fraction is less than <NUM>%, it has been put into evidence that the flatness of the steel sheet after annealing was especially good, which makes it possible to produce sheets or blanks that can be used in Laser welding. Laser welding requests blanks with strict flatness tolerances, otherwise geometrical defects can occur in welding due to gaps. An annealing temperature Ta comprised between <NUM> and <NUM> makes it possible to obtain this result. An annealing temperature in the preferred range of <NUM>-<NUM> makes it possible to achieve very stable results.

After the holding step at the temperature Ta, the immediate further steps of the process depend upon the type of sheet which is to be manufactured:.

The annealed steel sheet, either uncoated or precoated, is then cut to a predetermined shape so to obtain a flat blank that is able to be hot formed in a further step.

As an option, before the heating and hot forming steps in press, the blank can be cold formed so to obtain a predeformed blank. This cold predeformation makes it possible to reduce the amount of deformation in the next hot forming step.

Then, the blank, either flat or cold predeformed, is heated at a temperature Tm comprised between <NUM> and <NUM>. The heating means are not limited and can be radiation, induction, or resistance-based. The heated blank is held at Tm for a duration Dm comprised between <NUM> and <NUM> minutes. These (temperature-duration) ranges make it possible to obtain the full transformation of the steel into austenite. If the blank is precoated, this treatment causes the interdiffusion of the precoating with the steel substrate. Thus, during the heating, intermetallic phases are temporarily or definitively created by interdiffusion, which make it possible to facilitate further deformation in the hot press and to prevent decarburization and oxidation of the steel surface. For increased process efficiency, the duration Dm is comprised between <NUM> and <NUM> minutes.

After the heating and holding steps, the heated blank is extracted from the heating device, which can be for example a heating furnace. The heated blank is transferred into a forming press, the transfer duration Dt being less than <NUM>. This transfer must be fast enough so to avoid the formation of polygonal ferrite before the hot deformation in the press, otherwise there is a risk that the tensile strength of the press hardened part becomes less than <NUM> MPa.

The heated blank is thereafter hot formed in a forming press, so to obtain a formed part. During the forming step, the modes and quantities of deformation differ from one place to another because of the geometry of the final part and of the forming tools. For example, some zones may be in expansion, while other are deformed in restraint. Whatever the deformation mode, an equivalent deformation εb can be defined at each location in the press hardened part, as <MAT>, wherein ε<NUM> and ε<NUM> are the principal deformations. Thus, εb expresses the amount of strain introduced by the hot forming process in each zone of the press hardened part.

The part is then kept within the tooling of the forming press so as to ensure a proper cooling rate and to avoid part distortion due to shrinkage and phase transformations.

The part mainly cools by conduction through heat transfer with the tools. The tools may include coolant circulation so as to increase the cooling rate, or heating cartridges so as to lower cooling rates. Thus, the cooling rates can be adjusted through the implementation of such means.

For obtaining a press hardened part according to the invention, the formed part is first cooled in a temperature range between <NUM> and <NUM>, at a cooling rate CR1 comprised between <NUM> and <NUM>/s. Within this range, a transformation of austenite into martensite, eventually of bainite, occurs.

In a further step, the part is cooled in a temperature range comprised <NUM> and <NUM>, at a cooling rate CR2 between <NUM> to <NUM>/s, slower than the cooling rate CR1, i.e. than CR2<CR1. Within this range, the self-tempering of the martensite may occur at a certain degree, i.e. fine carbides precipitate. Toughness is increased through this self-tempering step.

Parts obtained through the described method have a thickness which is typically comprised between <NUM> and <NUM>.

The inventors have found a method to obtain high ductility in the zones of the press hardened part wherein high stress concentration is experienced during the use of the part: when the zones in the forming press are deformed with an equivalent strain εb higher than <NUM>, the inventors have shown that the structure of these deformed zones is finer and that softer and more ductile phases, can be obtained in these regions.

The inventors have compared not deformed or little deformed zones (the later designating zones wherein εb <<NUM>) with zones wherein strain has been applied with an amount higher than <NUM>. The hardness of the highly strained zones, decreases of more than <NUM> HV1 (HV1 being the Vickers Hardness measured under 1kgf load) as compared to unstrained or little strained zones in the press hardened part. This local softening corresponds to a toughness increase. However, the amount of the softening remains limited which means that the yield stress and tensile strength requirements are fulfilled in these deformed zones.

The average martensitic (fresh or self-tempered)/bainitic lath size Ls has been measured in little or highly deformed zones. After proper etching to reveal the microstructure, the lath size is determined by the intercept method which is known per se. In the zones wherein the applied strain is higher than <NUM>, the average bainitic/martensitic lath size width is less than <NUM>. By comparison, the average lath size Ls in little deformed zones is in the range of <NUM>-<NUM>. It has been also put into evidence that the application of strain level higher than <NUM> reduces the lath size of more than <NUM>%, as compared to little deformed zones. This reduction of the lath size increases the resistance to eventual crack initiation and propagation.

Thus, the combination of the steel composition and of the press hardening parameters, makes it possible to achieve high-ductility in targeted zones of the parts. In automobile applications, the formed parts display higher ductility in case of collisions. Another object of the invention is a press hardened welded steel part with aluminum coating, which takes advantage of the high mechanical properties of the press hardened part: for manufacturing such part, at least one first steel sheet with the composition above, coated with Al, or Al-based alloy, or Al alloy, is provided. Together with this first sheet, at least a second steel sheet, also precoated with Al, or Al-based alloy, or Al alloy, is provided. The sheets may have the same compositions or different compositions, and the same thickness or different thicknesses. In the case of different compositions, it has been put into evidence that the carbon content of the second steel has to be comprised between <NUM> and <NUM>% in weight, so to create a weld having the desired ductility properties.

The first and second sheets are welded along one of their respective peripheral sides. On these peripheral sides, a part of the thickness of the Al precoating is removed. This removal can be performed through pulsed Laser ablation, or through mechanical ablation. The aim of this ablation is to avoid that a too high quantity of Al of the precoating is molten and incorporated in the weld metal. According to the initial Al precoating thickness, and to the sheet thicknesses, the quantity of Al to be removed by ablation may be more of less high. The inventors have shown that the ablation conditions must be adapted so that the Al content in the weld metal created between the first and the second sheet, is less than <NUM>% in weight. Otherwise, either brittle intermetallics may precipitate in the weld, or the high Al content could prevent the transformation into austenite when heating before press forming, due to the alphagene character of aluminum.

Thus, when these conditions are fulfilled, the first and second sheets may be press hardened in the conditions described above, without the risk of cracks during hot forming. The press hardened welded part thus obtained, wherein the weld metal and the first and second sheet have been press hardened in the same operation, displays high mechanical resistance and ductility properties.

The invention will be now illustrated by the following examples, which are by no way limitative.

Steels with compositions according to table <NUM>, expressed in weight percentage, have been provided under the form of slabs. These slabs have been heated at <NUM>, hot-rolled and coiled at <NUM>. After pickling, the hot-rolled sheets have been cold rolled down to a thickness of <NUM>, with a rolling ratio of <NUM>%. The sheets were thereafter annealed at <NUM>, so to obtain an unrecrystallized surface fraction less than <NUM>%, and precoated with Al-Si by continuous hot-dipping in a bath at <NUM>. The precoating thickness is <NUM> on both sides. These precoated sheets have been cut into blanks which have been further press hardened.

Table <NUM> details the press hardening conditions, i.e. the heating temperature Tm, the heating duration Dm, the transfer duration Dt, and the cooling rates CR1 and CR2, which have been applied.

Yield stress YS and tensile strength TS have been determined on the press hardened parts, using 20x <NUM><NUM> specimens according to Standard ISO (EN <NUM>-<NUM>).

Critical bending angle has been determined on press hardened parts of <NUM>×<NUM><NUM> supported by two rollers, according to VDA-<NUM> bending Standard. The bending effort is exerted by a <NUM> radius sharp punch. The spacing between the rollers and the punch is equal to the thickness of the tested parts, a clearance of <NUM> being added. The crack apparition is detected since it coincides with a load decrease in the load-displacement curve. Tests are interrupted when the load decreases more than 30N of its maximal value. The bending angle (α) of each sample is measured after unloading and thus after specimen spring-back. Five samples along each direction (rolling direction and transverse direction) are bent so to obtain an average value αA of the bending angle.

The fracture strain is determined through bending specimens in plane strain conditions, which is the most severe condition in view of vehicle collision. From these tests, it is possible to determine the critical displacement of the specimens when fracture occurs. On the other hand, Finite Element Analysis allows modeling the bending of such specimen, i.e. to know the strain level which is present in the bent zone for such critical displacement. This strain in such critical conditions is the fracture strain of the material.

The results of such mechanical tests are presented in Table <NUM>. By convention, the test conditions associate the steel composition and the press hardening compositions. Thus, I1B refers for example to the steel composition <NUM> tested with the condition B.

The table <NUM> presents also some microstructural features of the press hardened parts. The surface fractions of the different constituents have been determined by polishing and etching the specimens with different reagents (Nital, Picral, Bechet-Beaujard, sodium metabisulfite and LePera) so to reveal the specific constituents. Quantification of the surface fractions have been performed through image analysis and Aphelion™ software, on more than ten representative zones of at least <NUM> × <NUM><NUM>.

The determination of TiN and sulfides has been performed by using optical micrography, Scanning Electron Microscopy associated to X-microanalysis. These observations have been performed in the zones located near the surfaces of the specimens, wherein the strain is the most important in bending mode. These sub-surface zones are located between one quarter thickness, and the closest surface of the parts. In each case, it was determined if the average size of TiN exceeded <NUM>, and if the average size of the sulfides exceeded <NUM>.

In the trials I1B, I2A, I3A, I4E, compositions and press hardening conditions correspond to the invention and the desired microstructural features are obtained. As a consequence, high tensile properties, high ductility and impact resistance are achieved. The microstructure of the parts I1B and I2A, as observed by Scanning Electron Microscope, is illustrated respectively on the <FIG>. Some details concerning the constituents have been highlighted on the micrographs.

In the trial R1A, the C, Mn, Cr, Nb contents do not fulfill the conditions of invention. Even if the press hardening conditions are in accordance with the ranges of the invention, the quantity of self-tempered martensite is insufficient and the bending angle and fracture strain do not meet the requested values.

In the trial I2C, even if the composition corresponds to the element ranges of the invention, the heating temperature Tm is insufficient. As a consequence, the ferrite surface fraction is too high and the martensite surface fraction is too low. Thus, the yield stress of <NUM> MPa cannot be reached.

In the trial R2D, due to the high cooling rates CR1 and CR2, the amount of self-tempered martensite is insufficient.

In the trial R3B, the C, Cr and B contents are too low. Thus, as the hardenability is insufficient, the ferrite content is too high and yield stress and tensile stress cannot be reached. The microstructure of R3B is illustrated on <FIG>. For a given treatment (B), the influence of the steel composition can be seen through the comparison of microstructures of parts I1B (according to the invention) and R3B (not according to the invention) Furthermore, the high Ti content causes the formation of TiN with an average size higher than <NUM>. In fracture tests, some cleavage areas have been observed. <FIG> illustrates that these brittle areas correspond to the presence of TiN (pointed out in <FIG> by arrows) which act as initiation sites for cleavage. These coarse TiN are located near the surface of the press hardened part, i.e. in the outer zones comprised between one quarter thickness and the closest surface of the part. As a consequence, the fracture strain is insufficient.

In the trial R4A, the Nb and the S contents do not fulfill the conditions of the invention. The microstructure of the part R4A is illustrated on the <FIG>. Compositions of steels I4A and R4A are very similar, except for Nb and S contents. From the comparison of <FIG> and <FIG>, it may be seen that the prior austenitic grain size is larger in the absence of Nb, which in turns causes the formation of martensite laths with increased length, which offer less resistance to crack propagation. Furthermore, R4A has higher sulfur content, thus causing the formation of elongated MnS as illustrated on <FIG>. These elongated sulfides are located near the outer zones comprised between one quarter thickness and the closest surface of the part. As a consequence, the critical bending angle and the fracture strain are too low.

Resistance spot welding tests have been performed on the press hardened parts produced in the conditions I2A and R1A above. The welding parameters are: intensity: <NUM>,2kA, welding force: <NUM> daN. Hardness tests have been performed on cut and polished spot welds in order to determine an eventual softening in the Heat Affected Zones near the weld metal. The thermal cycle associated to the welding induces a temperature gradient ranging from room temperature up to steel liquidus. Heating at temperature in the range of Ac1-Ac3 may cause a softening of the microstructure of the press hardened part. This softening is measured by the difference between the base metal hardness and the minimum hardness value in the Heat Affected Zone. When softening is too important, an external applied stress can be concentrated in the softened zone, thus causing a premature failure by strain concentration. Tensile tests have been performed on resistance spot welds, and the total elongation of the welds has been measured. As compared to the base metal elongation, the welds cause an elongation variation which may be more or less pronounced as compared to the one of the base metal. Thus, the relative elongation variation is defined by: (base metal elongation- weld elongation)/base metal elongation. Results are presented in the table <NUM>.

The amount of HAZ softening is less pronounced in the press hardened part I2A, manufactured according to the invention, than in the reference part R1A. Even in spite of the presence of this softened zone, no elongation loss is measured for the condition of invention I2A, while the elongation loss is significant for reference part R1A.

Precoated Al-Si steel sheets with the compositions I2 and R1 were provided. As explained above, the fabrication process makes it possible to produce blanks with strict flatness tolerances which allow Laser welding.

Furthermore, a steel sheet, <NUM> thick, precoated with <NUM> thick Al-Si, having the composition of table <NUM>, was also provided.

When press hardened in condition A, this steel makes it possible to obtain tensile strength UTS of about <NUM> MPa.

All these Al-Si precoated steel blanks were ablated on one of their peripheral sides. The metallic portion of the Al-Si coating was removed, while the intermetallic layer between the steel substrate and the precoating was left in place. This ablation was performed through a YAG Laser, 4kW, with a focus spot of <NUM><NUM>, on the upper and lower sides on the precoated sheets.

Afterwards, Laser welding was performed with a 4kW YAG Laser, with a welding speed of <NUM>/Mn, under Helium protection. Different configurations were tested:.

In all cases, the ablation performed before welding made it possible to achieve aluminum content in the weld metal lower than <NUM>%. Thus, formation of intermetallic compounds was avoided and the complete transformation of the weld metal into austenite, before press hardening, was achieved. All the welded joints were heated and press hardened according to condition A of table <NUM>, so to manufacture press hardened Laser welded steel parts. Thus, the different elements of the welded joints (base steels sheets surrounding the weld, and the weld itself) were press hardened in the same operation. Tensile specimens were machined in the direction transverse to the welds and in the adjacent base steels. Results of the welds have been compared with the ones of the adjacent base steels.

Thus, provided that the Al-Si welds contain less than <NUM>% Al, the steel sheet according to the invention can be welded to steel sheets with C content up to <NUM>% without risk of embrittlement.

The steel <NUM> has been provided and press hardened in the condition B of table <NUM>, so to manufacture parts having various "omega" shapes. This made it possible to obtain zones with a small deformation amount (εb <<NUM>), and zones wherein εb=<NUM>. The latter zones correspond to stress concentration in use conditions.

Specimens have been cut out from these parts and etched with Nital so to reveal the microstructure. These specimens have been observed by Electron Microscope with a Field Emission Gun, at magnification of <NUM> and <NUM>. The observed zones are mainly composed of martensite (fresh or self-tempered) and bainite. The average size of the lath width of the martensite and bainite (i.e. without distinguishing these two constituents) was determined by the method of intercepts. Furthermore, Vickers Hardness measurements have been performed in the different zones.

The results are presented in table <NUM>.

The strained zone shows hardness decrease of <NUM> HV. As estimated from this hardness value, the UTS of this strained zone is about <NUM> MPa, which fulfills the requested value.

Regarding average lath width, the strained zone displays a reduction of more than <NUM>% as compared to the little strained or unstrained zones. Thus, the finer lath structure in the deformed zones makes it possible to achieve increased toughness in the zones that are the most critical during the use of the part.

Claim 1:
A press hardened steel part wherein the chemical composition of the steel comprises, in weight: <MAT> <MAT> <MAT> <MAT> <MAT> wherein: <MAT> <MAT> <MAT> wherein: <MAT> <MAT> <MAT> <MAT> <MAT> optionally: <MAT> the remainder being Fe and unavoidable impurities,
and wherein the microstructure comprises, in at least <NUM>% of the volume of said part, in surface fractions: less than <NUM>% of bainite, less than <NUM>% of austenite, less than <NUM>% of ferrite, the remainder being martensite, said martensite consisting of fresh martensite and of self-tempered martensite.