EP1929053B1 - Method for making a steel part of multiphase microstructure - Google Patents

Abstract

Making a steel part having a multi-phased microstructure having ferrite comprises: cutting a steel blank in to a steel strip of a composition having e.g. carbon (C), manganese (Mn), silicon (Si), aluminum (Al), molybdenum (Mo), chromium (Cr), phosphorus (P), titanium (Ti), and vanadium (V) (all at specific weight percentage); optionally pre-deforming the blank in cold; heating the blank to a temperature greater than the steel temperature and keeping the part under this temperature; transferring the heated blank within a working tool to make the part hard; and cooling the part within the tools. Manufacturing a steel part having a multi-phased microstructure containing ferrite, which is homogeneously distributed in each area of the part, comprises: cutting a steel blank in to a steel strip of a composition containing carbon (C) at 0.01-0.50 wt.%, manganese (Mn) at 0.5-3 wt.%, silicon (Si) at 0.001-3 wt.%, aluminum (Al) at 0.005-3 wt.%, molybdenum (Mo) =1 wt.%, chromium (Cr) at =1.5 wt.%, phosphorus (P) at =0.1 wt.%, titanium (Ti) at =0.2 wt.%, vanadium (V) at =1 wt.%, optionally elements like nickel at =2 wt.%, copper at =2 and sulfur (S) at =0.05 wt.% and rest iron and other impurities; optionally pre-deforming the blank in cold; heating the blank to a temperature greater than the steel temperature and maintaining the part under this temperature such that the part after heating comprises austenite of >=25%; transferring the heated blank within a working tool to make the part hard; and cooling the part within the tools to give the multi-phased microstructure Independent claims are included for: (1) a steel part obtained by the process; and (2) an automobile engine comprising the steel part.

Description

The present invention relates to a method of manufacturing a multi-phased microstructure steel piece homogeneous in each of the zones of said part, and having high mechanical characteristics.

In order to meet the requirements for lightening automotive structures, it is known to use either TRIP steels (the term meaning transformation induced plasticity) or dual phase steels which combine a very high mechanical strength with very high possibilities of deformation. . TRIP steels have a microstructure composed of ferrite, residual austenite, and possibly bainite and martensite, which enables them to reach tensile strengths ranging from 600 to 1000 MPa. The dual-phase steels have a microstructure composed of ferrite and martensite, which enables them to reach tensile strengths ranging from 400 MPa to more than 1200 MPa.

These types of steels are widely used for the realization of energy absorbing parts, such as structural and safety parts such as longitudinal members, sleepers and reinforcements.

Usually to manufacture this type of parts, it is cold forming, for example by stamping between tools, a blank cut in a cold rolled strip of dual phase steel or TRIP steel.

Typical processes by stamping are disclosed in FR 2671749 , EP 1013785 or US 2004/0163739 .

However, the development of dual phase steel or TRIP steel parts is limited because of the difficulty in controlling the elastic return of the shaped part, elastic return which is even more important than the tensile strength Rm of steel is important. Indeed, to overcome the effect of springback, car manufacturers are obliged to integrate this parameter when designing new parts, which on the one hand, requires many developments, and on the other hand, limits extent of achievable forms.

In addition, in case of significant deformation, the microstructure of the steel is no longer homogeneous in each of the areas of the room, and the behavior of the part in use is difficult to predict. For example, during the cold forming of a TRIP steel sheet, the residual austenite is transformed into martensite under the effect of the deformation. The deformation is not homogeneous throughout the room, some areas of the room will still contain residual austenite not transformed into martensite and therefore having a significant residual ductility, while other areas of the room having undergone significant deformation will present a ferritic-martensitic structure optionally comprising ductile bainite.

The object of the present invention is therefore to overcome the aforementioned drawbacks, and to propose a method of manufacturing a steel part comprising ferrite and having a homogeneous multi-phased microstructure in each of the zones of said part, and not exhibiting resilient return after forming a blank from a steel strip whose composition is typical of that of multi-phase microstructure steels.

• découper un flan dans une bande en acier dont la composition est constituée en % en poids :
  • 0,01 ≤ C ≤ 0,50 %
  • 0,50 ≤ Mn ≤ 3,0 %
  • 0,001 ≤ Si ≤ 3,0 %
  • 0, 005 ≤ Al ≤ 3,0 %
  • Mo ≤ 1,0 %
  • Cr ≤ 1,5 %
  • P ≤ 0,10%
  • Ti≤0,15%
  • V≤1,0%,
  à titre optionnel, un ou plusieurs éléments tels que
  • Ni ≤ 2,0%
  • Cu≤2,0%
  • S≤0,05%
  • Nb≤0,15%
  le reste de la composition étant du fer et des impuretés résultant de l'élaboration,
• éventuellement pré-déformer à froid ledit flan,
• chauffer ledit flan jusqu'à atteindre une température de maintien T1 supérieure à Ac1 mais inférieure à Ac3, et le maintenir à cette température de maintien T1 pendant un temps de maintien M ajusté de manière à ce que l'acier après chauffage du flan comprenne une proportion d'austénite supérieure ou égale à 25 % surfacique,
• transférer ledit flan chauffé au sein d'un outillage de mise en forme de manière à former à chaud ladite pièce, et
• refroidir la pièce au sein de l'outillage avec une vitesse de refroidissement V telle que la microstructure de l'acier après refroidissement de la pièce soit une microstructure multi-phasée, ladite microstructure comprenant de la ferrite et étant homogène dans chacune des zones de ladite pièce.

For this purpose, the invention defined according to claim 1 has for its first object a method of manufacturing a steel part having a multi-phased microstructure, said microstructure comprising ferrite and being homogeneous in each of the zones of said part, comprising the steps of:

• cutting a blank into a steel strip whose composition consists of% by weight:
  • 0.01 ≤ C ≤ 0.50%
  • 0.50 ≤ Mn ≤ 3.0%
  • 0.001 ≤ If ≤ 3.0%
  • 0, 005 ≤ Al ≤ 3.0%
  • Mo ≤ 1.0%
  • Cr ≤ 1.5%
  • P ≤ 0.10%
  • Ti≤0,15%
  • V≤1,0%
  optional, one or more elements such as
  • Ni ≤ 2.0%
  • Cu≤2,0%
  • S≤0,05%
  • Nb≤0,15%
  the rest of the composition being iron and impurities resulting from the preparation,
• optionally pre-deforming said blank,
• heating said blank until a holding temperature T1 higher than Ac1 but lower than Ac3, and maintain it at this holding temperature T1 for a hold time M adjusted so that the steel after heating the blank comprises a proportion of austenite greater than or equal to 25% by surface,
• transferring said heated blank into a shaping tool so as to heat said workpiece, and
• cooling the part within the tooling with a cooling rate V such that the microstructure of the steel after cooling of the part is a multi-phase microstructure, said microstructure comprising ferrite and being homogeneous in each of the zones of said room.

To determine the surface% of the various phases present in a microstructure (ferritic phase, austenitic phase, etc.), the area of the different phases is measured in a section made along a plane perpendicular to the plane of the strip (this plane may be parallel to to the rolling direction, or parallel to the direction transverse to the rolling). The different phases sought are revealed by a chemical attack adapted according to their nature.

For the purposes of the present invention, the term "forming tool" means any tool that makes it possible to obtain a part from a blank, such as for example a stamping tool. This excludes cold or hot rolling tools.

The inventors have demonstrated that heating the blank to a holding temperature T1 between Ac1 and Ac3 gives, provided that the cooling rate is sufficient, a multi-phase microstructure comprising ferrite having mechanical properties. homogeneous regardless of the cooling rate of the blank between the tools. The homogeneity of the mechanical properties is defined in the sense of the invention by a dispersion of the tensile strength Rm in a range of cooling rates ranging from 10 to 100 ° C./s less than 25%. Indeed, the inventors have found that by subjecting the blank to heat treatment in the intercritical range, then Rm (100 ° C / s) - Rm (10 ° C / sec) / Rm (100 ° C / sec) ) <0.25, Rm (100 ° C / s) being the tensile strength of the piece cooled to 100 ° C / s, and Rm (10 ° C / s) being the tensile strength of the cooled part at 10 ° C / s.

The invention has as its second object a steel part comprising ferrite and having a homogeneous multi-phased microstructure in each zone of said part, obtainable by said method.

Finally, the third object of the invention is a motorized land vehicle comprising said part.

• la figure 1 est une photographie d'une pièce obtenue par mise en forme à froid (référence G) et d'une pièce obtenue par mise en forme à chaud (référence A).

The features and advantages of the present invention will become more apparent from the following description, given by way of non-limiting example, with reference to the appended FIG. 1 in which:

• Figure 1 is a photograph of a part obtained by cold forming (reference G) and a part obtained by hot forming (reference A).

The method according to the invention consists in shaping hot, in a certain temperature range, a blank previously cut in a steel strip whose composition is typical of that of multi-phase microstructure steels, but which initially does not does not necessarily have a multi-phased structure, to form a steel part that acquires a multi-phase microstructure during its cooling between the formatting tools. The inventors have furthermore demonstrated that, provided that the cooling rate is sufficient, a homogeneous multi-phased microstructure could be obtained whatever the rate of cooling of the blank between the tools.

The advantage of this invention lies in the fact that it is not necessary to form the multi-phased microstructure at the stage of manufacture of the hot sheet, or of its coating, and that forming it at The stage of manufacture of the part, by hot forming, ensures a homogeneous final multi-phased microstructure in each of the zones of the part, which is advantageous in the case of a use for absorption parts. energy, because the microstructure is not altered as is the case when cold forming of dual-phase steel or TRIP steel parts.

The inventors have indeed verified that the energy absorption capacity of a part, determined by the tensile strength multiplied by the elongation (Rm × A), is greater when the part has been obtained. according to the invention than when it was obtained by cold forming of a dual phase steel blank or TRIP steel. Indeed, cold forming consumes some of the energy absorption capacity.

In addition, by carrying out a hot shaping, the elastic return of the part becomes negligible, while it is very important in the context of cold forming. It is also more important that the tensile strength Rm of steel increases, which is a brake on the use of very high strength steels.

Another advantage of the invention lies in the fact that the hot shaping leads to a much higher shaping ability than cold. Thus, a variety of wider shapes can be accessed and new designs of parts can be envisaged while retaining steel compositions whose characteristics, such as weldability, are known.

The part obtained has a multi-phase microstructure comprising ferrite at a proportion preferably greater than or equal to 25% by surface, and at least one of the following phases: martensite, bainite, residual austenite. In fact, a proportion of at least 25% ferrite surface area makes it possible to give the steel ductility sufficient for the formed parts to have a high energy absorption capacity.

• du carbone à une teneur comprise entre 0,01 et 0,50 % en poids. Cet élément est essentiel à l'obtention de bonnes caractéristiques mécaniques, mais ne doit pas être présent en quantité trop importante pour ne pas léser la soudabilité. Pour favoriser la trempabilité, et obtenir une limite d'élasticité Re suffisante, la teneur en carbone doit être supérieure ou égale à 0,01 % en poids.
• du manganèse à une teneur comprise entre 0,50 et 3,0 % en poids. Le manganèse favorise la trempabilité, ce qui permet d'atteindre une limite d'élasticité Re élevée. Cependant, il faut éviter que l'acier ne comprenne trop de manganèse, pour éviter la ségrégation qui peut être mise en évidence dans les traitements thermiques qu'on évoquera ultérieurement dans la description. En outre, un excès de manganèse empêche le soudage par étincelage si la quantité de silicium est insuffisante, et détériore l'aptitude à la galvanisation de l'acier. Le manganèse joue également un rôle dans l'inter-diffusion du fer et de l'aluminium, en cas de revêtement de l'acier par de l'aluminium ou un alliage d'aluminium.
• du silicium à une teneur comprise entre 0,001 et 3,0 % en poids. Le silicium améliore la limite d'élasticité Re de l'acier. Cependant au-delà de 3,0 % en poids, la galvanisation au trempé à chaud de l'acier devient difficile, et l'aspect du revêtement de zinc n'est pas satisfaisant.
• de l'aluminium à une teneur comprise entre 0,005 et 3,0 % en poids. L'aluminium stabilise la ferrite. Sa teneur doit rester inférieure à 3,0 % en poids pour éviter de détériorer la soudabilité due à la présence d'oxyde d'aluminium dans la zone soudée. Cependant, un minimum d'aluminium est requis pour désoxyder l'acier.
• du molybdène à une teneur inférieure ou égale à 1,0 % en poids. Le molybdène favorise la formation de martensite et, augmente la résistance à la corrosion. Cependant, un excès de molybdène peut favoriser le phénomène de fissuration à froid dans les zones soudées, et réduire la ténacité de l'acier.
• du chrome à une teneur inférieure ou égale à 1,5 % en poids. La teneur en chrome doit être limitée pour éviter les problèmes d'aspect de surface en cas de galvanisation de l'acier.
• du phosphore à une teneur inférieure ou égale à 0,10 % en poids. Le phosphore est ajouté pour permettre de réduire la quantité de carbone et améliorer la soudabilité, tout en maintenant un niveau équivalent de limite d'élasticité Re de l'acier. Cependant, au-delà de 0,10 % en poids, il fragilise l'acier en raison de l'augmentation du risque de défauts de ségrégation, et la soudabilité est détériorée.
• du titane à une teneur inférieure ou égale à 0,20 % en poids. Le titane améliore la limite d'élasticité Re, cependant sa teneur doit être limitée à 0,20 % en poids pour éviter la dégradation de la ténacité.
• du vanadium à une teneur inférieure ou égale à 1,0 % en poids. Le vanadium améliore la limite d'élasticité Re par affinement du grain et favorise la soudabilité de l'acier. Cependant, au delà de 1,0 % en poids, la ténacité de l'acier est détériorée et des fissures risquent d'apparaître dans les zones soudées.
• à titre optionnel, du nickel à une teneur inférieure ou égale à 2,0 % en poids. Le nickel augmente la limite d'élasticité Re. On limite généralement sa teneur à 2,0 % en poids en raison de son coût élevé.
• à titre optionnel, du cuivre à une teneur inférieure ou égale à 2,0 % en poids. Le cuivre augmente la limite d'élasticité Re, cependant un excès de cuivre favorise l'apparition de fissures lors du laminage à chaud, et dégrade la formabilité à chaud de l'acier.
• à titre optionnel, du soufre à une teneur inférieure ou égale à 0,05 % en poids. Le soufre est un élément ségrégeant dont la teneur doit être limitée afin d'éviter les fissures lors du laminage à chaud.
• à titre optionnel, du niobium à une teneur inférieure ou égale à 0,15 % en poids. Le niobium favorise la précipitation de carbonitrure, ce qui augmente la limite d'élasticité Re. Cependant, au-delà de 0,15 % en poids, la soudabilité et la formabilité à chaud sont dégradées.

The steel blank to be shaped, for example by stamping, is previously cut either in a hot-rolled steel strip or in a cold rolled steel strip, the steel consisting of the following elements:

• carbon at a content of between 0.01 and 0.50% by weight. This element is essential to obtain good mechanical properties, but must not be present in too large a quantity not to damage the weldability. To promote quenchability, and obtain a sufficient elastic limit Re, the carbon content must be greater than or equal to 0.01% by weight.
• manganese at a content of between 0.50 and 3.0% by weight. Manganese promotes hardenability, which allows to achieve a yield strength Re high. However, it is necessary to avoid that the steel understands too much manganese, to avoid the segregation which can be highlighted in the heat treatments which will be evoked later in the description. In addition, an excess of manganese prevents spark welding if the amount of silicon is insufficient, and deteriorates the galvanizing ability of the steel. Manganese also plays a role in the inter-diffusion of iron and aluminum, when steel is coated with aluminum or an aluminum alloy.
• silicon at a content of between 0.001 and 3.0% by weight. Silicon improves the elasticity limit Re of steel. However, above 3.0% by weight, the hot dipping of the steel becomes difficult, and the appearance of the zinc coating is unsatisfactory.
• aluminum at a content of between 0.005 and 3.0% by weight. Aluminum stabilizes ferrite. Its content must remain below 3.0% by weight to avoid damaging the weldability due to the presence of aluminum oxide in the welded zone. However, a minimum of aluminum is required to deoxidize the steel.
• molybdenum at a content of not more than 1.0% by weight. Molybdenum promotes the formation of martensite and increases the resistance to corrosion. However, an excess of molybdenum can promote the phenomenon of cold cracking in welded areas, and reduce the toughness of the steel.
• chromium at a content less than or equal to 1.5% by weight. The chromium content must be limited to avoid surface appearance problems when galvanizing the steel.
• phosphorus at a content less than or equal to 0.10% by weight. Phosphorus is added to reduce the amount of carbon and improve weldability, while maintaining an equivalent level of steel yield strength Re. However, beyond 0.10% by weight, it weakens the steel because of the increased risk of segregation defects, and the weldability is deteriorated.
• titanium at a content less than or equal to 0.20% by weight. Titanium improves the yield strength Re, however its content must be limited to 0.20% by weight to avoid the degradation of toughness.
• vanadium at a content of less than or equal to 1.0% by weight. Vanadium improves the yield strength Re by grain refinement and promotes the weldability of steel. However, beyond 1.0% by weight, the toughness of the steel is deteriorated and cracks may appear in the welded areas.
• optionally, nickel at a content less than or equal to 2.0% by weight. Nickel increases the yield strength Re. Its content is generally limited to 2.0% by weight because of its high cost.
• optionally copper less than or equal to 2.0% by weight. Copper increases the yield strength Re, however an excess of copper promotes the appearance of cracks during hot rolling, and degrades the hot formability of the steel.
• optionally, sulfur at a content of less than or equal to 0.05% by weight. Sulfur is a segregating element whose content must be limited in order to avoid cracks during hot rolling.
• optionally, niobium at a content less than or equal to 0.15% by weight. Niobium promotes the precipitation of carbonitride, which increases the yield strength Re. However, beyond 0.15% by weight, the weldability and hot formability are degraded.

The remainder of the composition is iron and other elements that are usually expected to be found as impurities resulting from steel making, in proportions that do not affect the properties of the steel. sought.

Generally, before being cut into blanks, the steel strips are protected against corrosion by a metal coating. Depending on the final destination of the part, this metal coating is chosen from zinc or zinc alloy coatings (zinc-aluminum for example), and if it is also desired to withstand good heat resistance, the coatings of aluminum or aluminum alloy (aluminum-silicon for example). These coatings are deposited in a conventional manner either by hot dipping in a bath of liquid metal, by electrodeposition, or under vacuum.

To implement the manufacturing method according to the invention, the steel blank is heated to bring it to a holding temperature T1 greater than Ac1 but lower than Ac3, and is maintained at this temperature T1 for a holding time M that is adjusted so that steel, after heating the blank, comprises a proportion of austenite greater than or equal to 25% by surface.

Immediately after this operation of heating and maintaining the temperature of the steel blank, the heated blank is transferred into a forming tool to form a part, and cool it. The cooling of the workpiece within the shaping tool is performed with a cooling rate V sufficient to prevent all of the austenite from becoming ferrite, and so that the microstructure of the steel after cooling the piece is a multi-phase microstructure comprising ferrite, and which is homogeneous in each of the areas of the room.

Homogeneous multi-phased microstructure in each of the zones of the part is understood to mean a microstructure having constancy in terms of proportion and morphology in each zone of the part, and in which the different phases are uniformly distributed.

In order for the cooling speeds V to be sufficient, the shaping tools can be cooled, for example by fluid circulation.

In addition, the clamping force of the shaping tool must be sufficient to ensure intimate contact between the blank and the tool, and ensure efficient and homogeneous cooling of the room.

Optionally, after having cut the blank in the steel strip, and before heating it, it is possible to carry out a cold pre-deformation of the blank.

Cold pre-deformation of the blank, for example by profiling or cold stamping of the blank, before hot forming is advantageous insofar as it allows access to parts that may have a more complex geometry .

Moreover, obtaining certain geometries in a single shaping operation is possible only if two blanks are folded between them. Cold pre-deformation can thus make it possible to obtain a part in one piece, that is to say a part obtained by forming a single blank.

In a first preferred embodiment of the invention, the method according to the invention is used to manufacture a steel part having a multi-phase microstructure comprising either ferrite and martensite, either ferrite and bainite, or ferrite, martensite and bainite.

• du carbone à une teneur de préférence comprise entre 0,01 et 0,25 % en poids, et plus préférentiellement comprise entre 0,08 et 0,15 %. La teneur en carbone est limitée à 0,25 % en poids pour limiter la formation de martensite et éviter ainsi la détérioration de la ductilité et de la formabilité.
• du manganèse à une teneur comprise de préférence entre 0,50 et 2,50 % en poids, et plus préférentiellement comprise entre 1,20 et 2,00 % en poids.
• du silicium à une teneur de préférence comprise entre 0,01 et 2,0 % en poids, et plus préférentiellement comprise entre 0,01 et 0,50 % en poids.
• de l'aluminium à une teneur de préférence comprise entre 0,005 et 1,5 % en poids, et plus préférentiellement comprise entre 0,005 et 1,0 % en poids. Il est préférable que la teneur en aluminium soit inférieure à 1,5 % en poids, de manière à éviter la dégradation de la soudabilité par étincelage due à la formation d'inclusions d'oxyde d'aluminium Al₂O₃.
• du molybdène à une teneur comprise de préférence entre 0,001 et 0,50 % en poids, et plus préférentiellement comprise entre 0,001 et 0,10 % en poids.
• du chrome à une teneur de préférence inférieure ou égale à 1,0 % en poids, et plus préférentiellement inférieure ou égale à 0,50 % en poids.
• du phosphore à une teneur de préférence inférieur ou égale à 0,10 % en poids.
• du titane à une teneur de préférence inférieure ou égale à 0,15 % en poids.
• du niobium à une teneur de préférence inférieure ou égale à 0,15 % en poids.
• du vanadium à une teneur de préférence inférieure ou égale à 0,25 % en poids.

To form this microstructure, the composition of the multiphase steel described above, and in particular the carbon content, of silicon and aluminum is adapted. Thus, steel includes the following elements:

• carbon at a content preferably between 0.01 and 0.25% by weight, and more preferably between 0.08 and 0.15%. The carbon content is limited to 0.25% by weight to limit the formation of martensite and thus avoid deterioration of ductility and formability.
• manganese at a content preferably between 0.50 and 2.50% by weight, and more preferably between 1.20 and 2.00% by weight.
• silicon at a content preferably between 0.01 and 2.0% by weight, and more preferably between 0.01 and 0.50% by weight.
• aluminum at a content preferably between 0.005 and 1.5% by weight, and more preferably between 0.005 and 1.0% by weight. It is preferable that the aluminum content is less than 1.5% by weight, so as to avoid the degradation of the spark weldability due to the formation of Al ₂ O ₃ aluminum oxide inclusions.
• molybdenum at a content preferably between 0.001 and 0.50% by weight, and more preferably between 0.001 and 0.10% by weight.
• chromium at a content preferably less than or equal to 1.0% by weight, and more preferably less than or equal to 0.50% by weight.
• phosphorus at a content preferably less than or equal to 0.10% by weight.
• titanium at a content preferably less than or equal to 0.15% by weight.
• niobium at a content preferably less than or equal to 0.15% by weight.
• vanadium at a content preferably less than or equal to 0.25% by weight.

The remainder of the composition is iron and other elements that are usually expected to be found as impurities resulting from steel making, in proportions that do not affect the properties of the steel. sought.

To form a multi-phase steel piece comprising ferrite, and martensite and / or bainite according to the invention, the blank is heated to a holding temperature T1 greater than Ac1 but less than Ac3, so as to to control the proportion of austenite formed during the heating of the blank, and not to exceed the preferential upper limit of 75% of austenite surface area.

A proportion of austenite in the steel heated to a holding temperature T1 during a holding time M of between 25 and 75% by weight offers a good compromise in terms of the mechanical strength of the steel after shaping and regularity. mechanical characteristics of the steel thanks to the robustness of the process. Indeed, beyond 25% of austenite surface, sufficient hardening phases, such as for example martensite and / or bainite, are formed during the cooling of the steel so that the yield strength Re of the steel after shaping is sufficient. On the other hand, above 75% of austenite surface, it is difficult to control the proportion of austenite in the steel, and one risks forming too many hardening phases during the cooling of the steel and consequently, of forming a steel part with insufficient elongation at break A, which will impair the energy absorption capacity of the part.

The holding time of the steel blank at the holding temperature T1 depends essentially on the thickness of the strip. In the context of the present invention, the thickness of the strip is typically between 0.3 and 3 mm. Therefore, to form a proportion of austenite between 25 and 75% by surface, the holding time M is preferably between 10 and 1000 s. If the steel blank is maintained at a holding temperature T1 for a holding time M greater than 1000 s, the austenite grains increase and the elastic limit Re of the steel after forming will be limited. In addition, the hardenability of the steel is reduced and the surface of the steel oxidizes. On the other hand, if the blank is held for a holding time M less than 10 s, the proportion of austenite formed will be insufficient, and the proportion of martensite and / or bainite formed during the cooling of the part between tool, will be insufficient for the elastic limit Re of the steel is sufficient.

The cooling rate V of the steel part in the forming tool depends on the deformation and the quality of the contact between the tool and the steel blank. However, the cooling rate V must be sufficiently high for the desired multi-phased microstructure to be obtained, and is preferably greater than 10 ° C./s. With a cooling rate V less than or equal to 10 ° C / s, it is likely to form carbides that will contribute to degrade the mechanical characteristics of the part.

Under these conditions, after cooling, a multi-phase steel piece comprising more than 25% ferrite surface area is formed, the rest being martensite and / or bainite, the various phases being homogeneously distributed in each of the zones of the In a preferred embodiment of the invention, 25 to 75% ferrite surface area and 25 to 75% surface area of martensite and / or bainite are preferably formed.

In a second preferred embodiment of the invention, the method according to the invention is used to manufacture a TRIP steel part. In the context of the invention is meant TRIP steel, a multiphase microstructure comprising ferrite, residual austenite, and possibly martensite and / or bainite.

• du carbone à une teneur comprise de préférence entre 0,05 et 0,50 % en poids, et plus préférentiellement comprise entre 0,10 et 0,30 % en poids. Pour former de l'austénite résiduelle stabilisée, il est préférable que cet élément soit présent à une teneur supérieure ou égale à 0,05 % en poids. En effet, le carbone joue un rôle très important sur la formation de la microstructure et les propriétés mécaniques : selon l'invention, une transformation bainitique intervient à partir d'une structure austénitique formée à haute température, et des lattes de ferrite bainitique sont formées. Compte tenu de la solubilité très inférieure du carbone dans la ferrite par rapport à l'austénite, le carbone de l'austénite est rejeté entre les lattes. Grâce à certains éléments d'alliage de la composition d'acier selon l'invention, en particulier le silicium et le manganèse, la précipitation de carbures, notamment de cémentite, intervient très peu. Ainsi, l'austénite interlattes s'enrichit progressivement en carbone sans que la précipitation de carbures n'intervienne. Cet enrichissement est tel que l'austénite est stabilisée, c'est à dire que la transformation martensitique de cette austénite n'intervient pas lors du refroidissement jusqu'à la température ambiante.
• du manganèse à une teneur de préférence comprise entre 0,50 et 3,0 % en poids, et plus préférentiellement entre 0,60 et 2,0 % en poids. Le manganèse favorise la formation d'austénite, contribue à diminuer la température de début de transformation martensitique Ms et à stabiliser l'austénite. Cette addition de manganèse participe également à un durcissement efficace en solution solide et donc à l'obtention d'une limite d'élasticité Re élevée. Cependant, un excès de manganèse ne permettant pas de former suffisamment de ferrite lors du refroidissement, la concentration de carbone dans l'austénite résiduelle est insuffisante pour qu'elle soit stable. La teneur en manganèse est plus préférentiellement comprise entre 0,60 et 2,0 % en poids. De la sorte, les effets recherchés ci-dessus sont obtenus sans risque de formation d'une structure en bandes néfaste qui proviendrait d'une ségrégation éventuelle du manganèse lors de la solidification.
• du silicium à une teneur de préférence comprise entre 0,001 et 3,0 % en poids, et plus préférentiellement comprise entre 0,01 et 2,0 % en poids. Le silicium stabilise la ferrite et stabilise l'austénite résiduelle à température ambiante. Le silicium inhibe la précipitation de la cémentite lors du refroidissement à partir de l'austénite en retardant considérablement la croissance des carbures : ceci provient du fait que la solubilité du silicium dans la cémentite est très faible et que cet élément augmente l'activité du carbone dans l'austénite. De la sorte, un germe éventuel de cémentite se formant sera environné d'une zone austénitique riche en silicium qui aura été rejeté à l'interface précipité-matrice. Cette austénite enrichie en silicium est également plus riche en carbone et la croissance de la cémentite est ralentie en raison de la diffusion peu importante résultant du gradient réduit de carbone entre la cémentite et la zone austénitique avoisinante. Cette addition de silicium contribue donc à stabiliser une quantité suffisante d'austénite résiduelle pour obtenir un effet TRIP. De plus, cette addition de silicium permet d'augmenter la limite d'élasticité Re grâce à un durcissement en solution solide. Cependant, une addition excessive de silicium provoque la formation d'oxydes fortement adhérents, difficilement éliminables lors d'une opération de décapage, et l'apparition éventuelle de défauts de surface dus notamment à un manque de mouillabilité dans les opérations de galvanisation au trempé. Afin d'obtenir la stabilisation d'une quantité suffisante d'austénite tout en réduisant le risque de défauts de surface, la teneur en silicium est préférentiellement comprise entre 0,01 et 2,0 % en poids.
• de l'aluminium à une teneur de préférence comprise entre 0,005 et 3,0 % en poids. Comme le silicium, l'aluminium stabilise la ferrite et accroît la formation de ferrite lors du refroidissement du flan. Il est très peu soluble dans la cémentite et peut être utilisé à ce titre pour éviter la précipitation de la cémentite lors d'un maintien à une température de transformation bainitique et stabiliser l'austénite résiduelle.
• du molybdène à une teneur de préférence inférieure ou égale à 1,0 % en poids, et plus préférentiellement inférieure ou égale à 0,60 % en poids.
• du chrome à une teneur de préférence inférieure ou égale à 1,50 % en poids. La teneur en chrome est limitée pour éviter les problèmes d'aspect de surface en cas de galvanisation de l'acier.
• du nickel à une teneur de préférence inférieure ou égale à 2,0 % en poids.
• du cuivre à une teneur de préférence inférieure ou égale à 2,0 % en poids.
• du phosphore à une teneur de préférence inférieure ou égale à 0,10 % en poids. Le phosphore en combinaison avec le silicium augmente la stabilité de l'austénite résiduelle en supprimant la précipitation des carbures.

• du soufre à une teneur de préférence inférieure ou égale à 0,05 % en poids.
• du titane à une teneur de préférence inférieure ou égale à 0,20 % en poids.
• du vanadium à une teneur de préférence inférieure ou égale à 1,0 % en poids, et plus préférentiellement inférieure ou égale à 0,60 % en poids.

To form this multi-phased microstructure TRIP, the composition of the multiphase steel described above, and in particular the carbon content, of silicon, of aluminum, is adapted. Thus, steel includes the following elements:

• carbon at a content preferably between 0.05 and 0.50% by weight, and more preferably between 0.10 and 0.30% by weight. To form stabilized residual austenite, it is preferable that this element is present at a content greater than or equal to 0.05% by weight. Indeed, the carbon plays a very important role on the formation of the microstructure and the mechanical properties: according to the invention, a bainitic transformation occurs from a high temperature austenitic structure, and bainitic ferrite slats are formed . Given the very solubility lower carbon in ferrite compared to austenite, the carbon of the austenite is rejected between the slats. Thanks to certain alloying elements of the steel composition according to the invention, in particular silicon and manganese, the precipitation of carbides, in particular of cementite, intervenes very little. Thus, the austenite interlatte is progressively enriched in carbon without the precipitation of carbides intervening. This enrichment is such that the austenite is stabilized, that is to say that the martensitic transformation of this austenite does not occur during cooling to room temperature.
• manganese at a content preferably between 0.50 and 3.0% by weight, and more preferably between 0.60 and 2.0% by weight. Manganese promotes the formation of austenite, helps to reduce the martensitic transformation start temperature Ms and to stabilize the austenite. This addition of manganese also contributes to an effective hardening in solid solution and thus to obtaining a yield strength Re high. However, since an excess of manganese does not make it possible to form enough ferrite during cooling, the carbon concentration in the residual austenite is insufficient for it to be stable. The manganese content is more preferably between 0.60 and 2.0% by weight. In this way, the effects sought above are obtained without risk of formation of a harmful band structure that would come from a possible segregation of manganese during solidification.
• silicon at a content preferably between 0.001 and 3.0% by weight, and more preferably between 0.01 and 2.0% by weight. Silicon stabilizes the ferrite and stabilizes the residual austenite at room temperature. Silicon inhibits the precipitation of cementite during cooling from austenite by considerably delaying the growth of carbides: this is due to the fact that the solubility of silicon in cementite is very low and that this element increases the activity of carbon in the austenite. In this way, an eventual germ of cementite forming will be surrounded by a zone austenitic silicon rich that will have been rejected at the precipitated-matrix interface. This silicon-enriched austenite is also richer in carbon and the growth of cementite is slowed down because of the small diffusion resulting from the reduced carbon gradient between the cementite and the surrounding austenitic zone. This addition of silicon thus contributes to stabilizing a sufficient amount of residual austenite to obtain a TRIP effect. In addition, this addition of silicon makes it possible to increase the yield strength Re by means of hardening in solid solution. However, an excessive addition of silicon causes the formation of strongly adherent oxides, which are difficult to eliminate during a stripping operation, and the possible appearance of surface defects due in particular to a lack of wettability in dip galvanizing operations. In order to obtain the stabilization of a sufficient amount of austenite while reducing the risk of surface defects, the silicon content is preferably between 0.01 and 2.0% by weight.
• aluminum at a content preferably between 0.005 and 3.0% by weight. Like silicon, aluminum stabilizes ferrite and increases the formation of ferrite during the cooling of the blank. It is very slightly soluble in cementite and can be used as such to prevent the precipitation of cementite during maintenance at bainitic transformation temperature and stabilize the residual austenite.
• molybdenum at a content preferably less than or equal to 1.0% by weight, and more preferably less than or equal to 0.60% by weight.
• chromium at a content preferably less than or equal to 1.50% by weight. The chromium content is limited to avoid surface appearance problems when galvanizing steel.
• nickel at a content preferably less than or equal to 2.0% by weight.
• copper at a content preferably less than or equal to 2.0% by weight.
• phosphorus at a content preferably less than or equal to 0.10% by weight. Phosphorus in combination with silicon increases the stability of residual austenite by suppressing the precipitation of carbides.

• sulfur at a content preferably less than or equal to 0.05% by weight.
• titanium at a content preferably less than or equal to 0.20% by weight.
• vanadium at a content preferably less than or equal to 1.0% by weight, and more preferably less than or equal to 0.60% by weight.

The remainder of the composition is iron and other elements that are usually expected to be found as impurities resulting from steel making, in proportions that do not affect the properties of the steel. sought.

The holding time of the steel blank at a holding temperature T1 greater than Ac1 but less than Ac3 essentially depends on the thickness of the strip. In the context of the present invention, the thickness of the strip is typically between 0.3 and 3 mm. Therefore, to form a proportion of austenite greater than or equal to 25% by surface, the holding time M is preferably between 10 and 1000 s. If the steel blank is maintained at a holding temperature T1 for a holding time M greater than 1000 s, the austenite grains increase and the elastic limit Re of the steel after forming will be limited. In addition, the hardenability of the steel is reduced and the surface of the steel oxidizes. On the other hand, if the blank is held for a holding time M less than 10 s, the proportion of austenite formed will be insufficient, and sufficient residual austenite and bainite will not be formed during the cooling of the part between the tool.

The cooling rate V of the steel part in the forming tool depends on the deformation and the quality of the contact between the tool and the steel blank. To obtain a steel part having a multi-phased microstructure TRIP, it is preferable that the cooling rate V is between 10 ° C./s and 200 ° C./s. In fact, below 10 ° C / s, ferrite and carbide will be essentially formed, and insufficient residual austenite and martensite, and above 200 ° C / s, essentially martensite will be formed. insufficient residual austenite.

It is essential to form a proportion of austenite greater than or equal to 25% by surface during heating of the blank, so that when cooling the steel between the forming tool, there is enough residual austenite and that the desired TRIP effect can thus be obtained.

Under these conditions, after cooling, a multiphase steel part consisting, in% by surface, of ferrite at a proportion greater than or equal to 25%, of 3 to 30% of residual austenite, and possibly of martensite, is formed. / or bainite.

The TRIP effect can advantageously be used to absorb energy in the event of high speed shocks. Indeed, during a significant deformation of a TRIP steel part, the residual austenite is gradually transformed into martensite by selecting the orientation of the martensite. This has the effect of reducing the residual stresses in the martensite, reducing the internal stresses in the part, and finally limiting the damage of the part, because the rupture thereof will take place for an elongation A more important than if it was not TRIP steel.

The invention will now be illustrated by examples given by way of nonlimiting indication and with reference to the single appended figure which is a photograph of a coin obtained by cold forming (reference G) and a part obtained by hot forming (reference A).

The inventors have carried out tests both on steels presenting on the one hand a composition typical of that of mutli-phased microstructure steels comprising ferrite and martensite and / or bainite (point 1), and of on the other hand a composition typical of that of TRIP mutli-phased microstructure steels (point 2).

1- Steel of composition typical of that of multi-phase microstructure steels comprising ferrite and martensite 1.1 Evaluation of the influence of heating and cooling speeds

Blanks 400 x 600 mm in size are cut from a steel strip whose composition, indicated in Table I, is that of a steel grade DP780 (Dual Phase 780). The strip has a thickness of 1.2 mm. The Ac1 temperature of this steel is 705 ° C and the Ac3 temperature is 815 ° C. The blanks are brought to a variable holding temperature T1, during a holding period of 5 minutes. Then, they are immediately transferred to a stamping tool in which they are both shaped and cooled with variable cooling rates V, keeping them in the tool for a period of 60 s. The stamped parts are similar to an Omega shape structure

After complete cooling of the parts, their yield strength Re, their tensile strength Rm, and their elongation at break A are measured, and the microstructure of the steel is determined. With regard to the microstructure, F represents ferrite, M martensite, and B bainite. The results are shown in Table II. Table I: chemical composition of the steel according to the invention, expressed in% by weight, the balance being iron or impurities. VS mn Yes al MB Cr P Ti Nb V 0.15 1.91 0.21 0.37 0.005 0.19 0.01 0.03 0,001 - T1 (° C) V (° C / s) Room Re (MPa) Rm (MPa) AT (%) Rm x A Microstructure (% surface area) * 800 10 AT 354 803 18.2 14615 86% F + 14% M 35 B 502 982 13.8 13552 72% F + 28% M 100 VS 530 1046 13.3 13912 55% F + 5% B + 40% M 900 10 D 441 723 14.3 10339 50% F + 42% B + 8% M 35 E 724 1100 8 8800 90% B + 10% M 100 F 890 1285 4.6 5911 100% M * according to the invention

The results of this test show that only heating the steel at a temperature between Ac1 and Ac3 makes it possible to obtain a multi-phase microstructure comprising ferrite, whatever the speed of cooling of the steel in the shaping tool. Indeed, when the steel is heated to a temperature above Ac3, it is then necessary to strictly control the cooling rate V during shaping, to obtain a multi-phase microstructure steel comprising more than 25% per unit area. ferrite, and preferably between 25% and 75% ferrite surface.

In addition, a low dispersion of the mechanical characteristics as a function of the cooling rate for the parts obtained according to the invention, their energy absorption capacity is greater than that of the parts obtained with heating at a temperature above Ac3.

1.2 Evaluation of the elastic return

The purpose of this test is to show the interest of a hot shaping compared to a cold shaping, and to evaluate the elastic return.

For this purpose, a piece of DP780 grade steel is manufactured by cold stamping a blank cut from a 1.2 mm thick steel strip, the composition of which is indicated in Table I, but which, unlike the strip used in point 1, already has before stamping a multi-phased microstructure comprising 70% ferrite surface, 15% martensite surface, and 15% bainite surface. FIG. 1 clearly shows that the part formed by cold stamping (marked in the figure by the letter G) has a strong springback, with respect to the piece A (see Table II) formed by hot stamping (marked by the letter AT).

2- Steel of composition typical of that of TRIP steels

Blanks measuring 200 × 500 mm are cut from a steel strip whose composition, indicated in Table III, is that of a TRIP 800 grade steel. The strip has a thickness of 1.2 mm. The Ac1 temperature of this steel is 751 ° C and the Ac3 temperature is 875 ° C. The blanks are brought to a variable holding temperature T1, during a hold time of 5 minutes, and then immediately transferred to a stamping tool in which they are both shaped and cooled with a cooling rate V of 45 ° C / s, keeping them in the tool for 60 s. The stamped parts are similar to an Omega shape structure.

After complete cooling of the parts, their yield strength Re, their tensile strength Rm, and their elongation at break A are measured, and the microstructure of the steel is determined. With regard to the microstructure, F represents ferrite, residual austenite, martensite, and bainite. The results are shown in Table IV. Table III: chemical composition of the steel according to the invention, expressed in% by weight, the balance being iron or impurities VS mn Yes al MB Cr P Ti Nb V 0.2 1.5 1.5 0.05 0,007 0.01 0,011 0.005 - - T1 (° C) Room Re (MPa) Rm (MPa) AT (%) Rm x A Microstructure (% surface area) * 760 H 541 1174 12.4 14558 35% F + 17% A + 48% M * 800 I 485 1171 12.8 14989 45% F + 11% A + 44% M * 840 J 454 1110 14.3 15873 45% F + 15% A + 38% M + 2% B * according to the invention

The tests carried out show that the stamping of the blanks made according to the invention makes it possible to obtain parts having very high mechanical characteristics, as well as a small variation of the mechanical characteristics whatever the cooling temperature.

Claims (17)

1. Process for manufacturing a part made of steel having a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each of the regions of said part, which process comprises the steps consisting in:

   - cutting a blank from a strip of steel, the composition of which consists, in % by weight, of:

   0.01 ≤ C ≤ 0.500

   0.50 ≤ Mn ≤ 3.0%

   0.001 ≤ Si ≤ 3.0%

   0.005 ≤ Al ≤ 3.0%

   Mo ≤ 1.0%

   Cr ≤ 1.50

   P ≤ 0.10%

   Ti ≤ 0.20%

   V ≤ 1.0% and

   optionally, one or more elements such as:

   Ni ≤ 2.0%

   Cu ≤ 2.0%

   S ≤ 0.05%

   Nb ≤ 0.15%,

   the balance of the composition being iron and impurities resulting from the smelting;

   - optionally, said blank undergoes prior cold deformation;

   - said blank is heated so as to reach a soak temperature T_s above Ac1 but below Ac3 and held at this soak temperature T_s for a soak time t_s adjusted so that the steel, after the blank has been heated, has an austenite content equal to or greater than 25% by area;

   - said heated blank is transferred into a deep-drawing tool so as to hot-stamp said part; and

   - the part is cooled within the tool at a cooling rate V such that the microstructure of the steel, after the part has been cooled, is a multiphase microstructure, said microstructure comprising ferrite with a content equal to or greater than 25% by area and being homogeneous in each of the regions of said part.
2. Process according to Claim 1, in which the composition of the steel comprises, in % by weight:

   0.01 ≤ C ≤ 0.250

   0.50 ≤ Mn ≤ 2.50%

   0.01 ≤ Si ≤ 2.0%

   0.005 ≤ Al ≤ 1.5%

   0.001 ≤ Mo ≤ 0.50%

   Cr ≤ 1.0%

   P ≤ 0.10%

   Ti ≤ 0.15%

   Nb ≤ 0.15%

   V ≤ 0.25%,

   the balance of the composition being iron and impurities resulting from the smelting; the blank is held at the soak temperature T_s for a soak time t_s adjusted so that the steel, after heating, has an austenite content between 25 and 75% by area; and the microstructure of the steel, after the part has been cooled, is a multiphase microstructure comprising ferrite and either martensite, or bainite, or else both martensite and bainite.
3. Process according to Claim 2, characterized futher that the steel comprises, in % by weight:

   0.08 ≤ C ≤ 0.15%

   1.20 ≤ Mn ≤ 2.00%

   0.01 ≤ Si ≤ 0.50%

   0.005 ≤ Al ≤ 1.0%

   0.001 ≤ Mo ≤ 0.10%

   Cr ≤ 0.50%

   P ≤ 0.10%

   Ti ≤ 0.15%

   Nb ≤ 0.15%

   V ≤ 0.25%,

   the balance of the composition being iron and impurities resulting from the smelting.
4. Process according to either of Claims 2 and 3, characterized in that the soak time t_s is between 10 and 1000 s.
5. Process according to any one of Claims 2 to 4, characterized in that the cooling rate V is greater than 10°C/s.
6. Process according to any one of Claims 2 to 5, characterized in that the multiphase structure of the steel, after said part has been cooled, comprises 25 to 75% ferrite by area and 25 to 75% martensite and/or bainite by area.
7. Process according to Claim 1, in which the steel comprises, in % by weight:

   0.05 ≤ C ≤ 0.50%

   0.50 ≤ Mn ≤ 3.0%

   0.001 ≤ Si ≤ 3.0%

   0.005 ≤ Al ≤ 3.0%

   Mo ≤ 1.0%

   Cr ≤ 1.50%

   Ni ≤ 2.0%

   Cu ≤ 2.0%

   P ≤ 0.10%

   S ≤ 0.05%

   Ti ≤ 0.20%

   V ≤ 1.0%,

   the balance of the composition being iron and impurities resulting from the smelting; the microstructure of the steel, after the part has been cooled, is a TRIP multiphase microstructure comprising ferrite, residual austenite and optionally martensite and/or bainite.
8. Process according to Claim 7, characterized futher that the steel comprises, in % by weight:

   0.10 ≤ C ≤ 0.30%

   0.60 ≤ Mn ≤ 2.0%

   0.01 ≤ Si ≤ 2.0%

   0.005 ≤ Al ≤ 3.0%

   Mo ≤ 0.60%

   Cr ≤ 1.50%

   Ni ≤ 0.20%

   Cu ≤ 0.20%

   P ≤ 0.10%

   S ≤ 0.05%

   Ti ≤ 0.20%

   V ≤ 0.60%,

   the balance of the composition being iron and impurities resulting from the smelting.
9. Process according to either of Claims 7 and 8, characterized in that the soak time t_s is between 10 and 1000 s.
10. Process according to any one of Claims 7 to 9, characterized in that the cooling rate V is between 10 and 200°C/s.
11. Process according to any one of Claims 7 to 10, characterized further in that, after the part has been cooled, the multiphase microstructure of the TRIP steel consists, in % by area, of ferrite with a content equal to or greater than 25%, of 3 to 30% residual austenite and optionally of martensite and/or bainite.
12. Process according to any one of Claims 1 to 11, characterized in that the steel strip is coated beforehand with a metal coating, before being cut to form a blank.
13. Process according to Claim 12, characterized in that the metal coating is a coating based on zinc or a zinc alloy.
14. Process according to Claim 12, characterized in that the metal coating is a coating based on aluminum or an aluminum alloy.
15. Hot-stamped part having a homogeneous multiphase microstructure in each of the regions of said part, said microstructure comprising ferrite with a content equal to or greater than 25% by area, which is obtained by the process according to any one of Claims 1 to 14.
16. Use of the steel part according to Claim 15 for absorbing energy.
17. Land motor vehicle that includes the steel part according to Claim 15.

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