MX2008003770A - Method for making a steel part of multiphase microstructure - Google Patents

Abstract

The invention concerns a method for making a steel part ofmultiphase microstructure, said microstructure comprising ferrite and being homogeneous in each of the zones of said part, including the following steps:cutting a blank in a steel strip whereof the composition is typical of that of multiphase microstructure steel;heating said blank until a holding temperature T1 higher than Ac1 but lower than Ac3 is reached, and maintaining said holding temperature T1 for a dwell time M adjusted so that the steel after the blank has been heated includes an austenite proportion not less than 25%of the surface;transferring said heated blank into a shaping equipment so as to shape by heat process said part;and cooling the part inside the equipment at a cooling speed V such that the steel microstructure after the part has been cooled is a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each of the zones of said part.

Description

METHOD FOR MAKING A PIECE OF STEEL OF MULTIPLE PHASE MICROSTRUCTURE DESCRIPTION OF THE INVENTION The present invention relates to a process for manufacturing a piece made of steel having a homogeneous multi-phase microstructure in each of the regions of the piece, and having high mechanical properties. To meet the lightening requirements of automobile structures, it is known to use either TRIP steels (the term TRIP means transformation induced plasticity) or dual phase steels which combine a very high tensile strength with very high deformability. TRIP steels have a microstructure composed of ferrite, residual austenite and optionally bainite and martensite, which allows them to reach tensile strengths ranging from 600 to 1000 MPa. The dual phase steels have a microstructure composed of ferrite and martensite, which allows them to reach tensile strengths ranging from 400 MPa to more than 1200 MPa. These types of steels are widely used to produce energy absorbing parts, for example structural and safety parts such as longitudinal members, cross members and reinforcements. Ref. 191193 To manufacture such parts, it is usual for a blank, cut from a cold-rolled strip of dual-phase steel or TRIP steel, to undergo a cold forming process, for example, deep drawing between tools. However, the development of pieces made of dual-phase steel or TRIP steel is limited due to the difficulty of controlling the expansion of the formed piece, the expansion is greater than the tensile strength Rm of the steel. This is because, in order to alleviate the effect of the expansion, car manufacturers are obliged to incorporate this parameter into the design of new parts, which, on the one hand, require numerous developments and, on the other hand, eliminate the Formation interval that can be produced. In addition, in the case of large deformation, the microstructure of the steel is no longer homogeneous in each of the regions of the piece, and the behavior of the piece in service is difficult to predict. For example, when a sheet of TRIP steel is cold formed, the residual austenite is converted to martensite under the effect of deformation. Since the deformation is not homogeneous throughout the piece, certain regions of the piece will still contain residual austenite that has not been converted to martensite, the regions will therefore have a high residual ductility, while other regions of the piece that have undergone deformation large will have a ferritic-martensitic structure, possibly containing bainite, which is of low ductility. The object of the present invention is, therefore, to remedy the aforementioned disadvantages and to propose a process for manufacturing a piece made of steel comprising ferrite and having a multi-phase microstructure which is homogeneous in each of the regions of the piece, and that exhibits no expansion after a blank has been formed, obtained from a steel strip whose composition is typical of that of steels having a multi-phase microstructure. For this purpose, a first object of the invention is a process for manufacturing a piece made of steel having a multi-phase microstructure, the microstructure comprises ferrite and is homogeneous in each of the regions of the piece, the process comprises the steps which consist of: cutting a blank from a steel strip, the composition of which consists, in% by weight, of: 0.01 < C < 0.50% 0.50 < Mn < 3.0% 0.001 < Yes < 3.0% 0.005 < To < 3.0% Mo < 1.0% Cr < 1.5% 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 rest of the composition is iron and impurities resulting from the fusion; - optionally, the blank undergoes previous cold deformation; the blank is heated to reach an impregnation temperature Ts above Acl but below Ac3 and is maintained at this impregnation temperature Ts for an impregnation time ts adjusted so that the steel, after the blank has been heated , has an austenite content equal to or greater than 25% per area; - the hot blank is transferred in a forming tool to hot form the piece; and - the piece is cooled inside the tool at a cooling speed V so that the microstructure of the steel, after the piece has cooled, is a multi-phase microstructure, the microstructure comprises ferrite and is homogeneous in each of the regions of the piece. To determine the% of contents per area of the various phases present in a microstructure (ferrite phase, austenite phase, etc.), the area of the various phases is measured in a section produced along a plane perpendicular to the plane of the strip (this plane may be parallel to the rolling direction or parallel in the cross direction of the laminate). The various phases sought are revealed by the proper chemical attack according to their nature. Within the context of the present invention, the term "forming tool" is understood to mean any tool that allows a part to be obtained from a blank, such as for example a deep drawing tool. Therefore, this excludes cold rolled or hot rolled tools. The inventors have shown that, by heating the blank at an impregnation temperature Ts between Acl and Ac3, a multi-phase microstructure comprising ferrite exhibiting homogeneous mechanical properties, regardless of the cooling rate of the blank between the tools, it is obtained provided that the cooling speed is quite high. The homogeneity of the mechanical properties is defined within the context of the invention by a dispersion in the tensile strength Rra within a range of cooling speed ranging from 10 to 100 ° C / s of less than 25%. This is because the inventors have found that, by subjecting the blank to a heat treatment in the inter-critical range, then Rm (100 ° C / s) -Rm (10 ° C / s) / Rm (100 ° C) C / s) is less than 0.25, Rm (100 ° C / s) is the tensile strength of the piece cooled to 100 ° C / s and Rm (10 ° C / s) is the tensile strength of the piece cooled to 10 ° / s. The second object of the invention is a piece made of steel, which comprises ferrite and has a multi-phase microstructure that is homogeneous in each of the regions of the piece, which can be obtained by the process. Finally, the third object of the invention is a land vehicle that includes the piece. The features and advantages of the present invention will become more readily apparent during the course of the following description, given by way of non-limiting example, with reference to the accompanying Figure 1 in which: Figure 1 is a photograph of a piece obtained by cold forming (reference G) and of a piece obtained by hot forming (reference A). The process according to the invention consists in hot forming, within a certain temperature range, a blank cut before a strip of steel whose composition is typical of that of steels having a multi-phase microstructure, which at Start does not necessarily have a multiple phase structure, to form a piece of steel that acquires a multi-phase microstructure that cools between the training tools. The inventors have also shown that, provided that the cooling rate is quite high, a homogenous multi-phase microstructure can be obtained whatever the cooling rate of the blank between the tools. The benefit of this invention is based on the fact that there is no need for the multi-phase microstructure to be formed during the manufacturing stage of the hot-rolled sheet or its coating and in the fact that the formation of the microstructure in the manufacturing stage of the piece, by hot forming, makes it possible to guarantee that the final multi-phase microstructure is homogeneous in each of the regions of the piece. This is advantageous in the case of its use for energy absorbing parts, since the microstructure is not altered as is the case when the pieces made of dual phase steel or TRIP steel are formed in cold. The inventors have in fact confirmed that the energy absorption capacity of a piece, determined by the tensile strength multiplied by the elongation (Rm x A), is greater when the piece has been obtained according to the invention than when has obtained by cold forming a blank made of dual phase steel or TRIP steel. This is because the cold forming operation consumes some of the energy absorbing capacity. In addition, by performing a hot forming operation, the expansion of the piece becomes negligible, while it is very large in the case of a cold forming operation. It is also larger than the higher tensile strength Rm. This puts a brake on the use of very high strength steels. Another advantage of the invention is based on the fact that the hot forming operation results in formability appreciably greater than with cold forming. Accordingly, it is possible to obtain a wider variety of shapes and to imagine new part designs while still maintaining steel compositions whose characteristics, such as, for example, weldability, are known. The obtained piece has a multi-phase microstructure comprising ferrite preferably with a content equal to or greater than 25% per area, and at least one of the following phases: martensite, bainite, residual austenite. This is because a ferrite content of at least 25% per area yields sufficient ductility to the steel so that the formed pieces have a high energy absorption capacity. A steel blank proposed to be formed, for example by deep drawing, is cut before either a hot-rolled steel strip or a cold-rolled steel strip, the steel consists of the following elements: - carbon with a content between 0.01 and 0.50% by weight. This element is essential to obtain good mechanical properties, but it must not be present in too large a quantity in order not to deteriorate the weldability. To promote hardenability and obtain a sufficient elastic limit Re, the carbon content must be equal to or greater than 0.01% by weight; - manganese with a content between 0.50 and 3.0% by weight. Manganese promotes hardenability, making it possible for a high elastic limit Re to be achieved. However, it is necessary that the steel does not comprise much manganese, to avoid segregation which can be demonstrated in the heat treatments that will be mentioned later in the description. In addition, excess manganese prevents spark welding if the amount of silicon is insufficient, and the ability of the steel to be galvanized deteriorates. Manganese also plays a role in the inter-diffusion of iron and aluminum in the case in which the steel is coated with aluminum or an aluminum alloy; - silicon with a content between 0.001 and 3.0% by weight. Silicon improves the elastic limit Re of the steel. However, up to 3.0% by weight, the hot dip galvanization of the steel becomes difficult and the appearance of the zinc coating is unsatisfactory; - aluminum with a content between 0.005 and 3.0% by weight. Aluminum stabilizes ferrite. Its content must remain below 3.0% by weight to avoid degradation of the weldability due to the presence of aluminum oxide in the welding zone. However, a minimum amount of aluminum is required to deoxidize the steel; - molybdenum with a content equal to or less than 1.0% by weight. Molybdenum promotes the formation of martensite and increases resistance to corrosion. However, excess molybdenum can promote the phenomenon of cold cracking in the weld zones and reduce the toughness of the steel. - chromium with a content equal to or less than 1.5% by weight. The chromium content should be limited to avoid problems of surface appearance in the case of steel galvanization; - phosphorus with a content equal to or less than 0.10% by weight. The phosphorus is added to allow the amount of carbon to be reduced and improve the weldability, while still having an equivalent level of elastic limit Re of the steel. However, above 0.10% by weight, it makes the steel brittle due to the increased risk of segregation defects, and the weldability deteriorates; - titanium with a content equal to or less than 0.20% by weight. Titanium improves the elastic limit Re, however, its content should be limited to 0.20% by weight to avoid degradation of toughness; - vanadium with a content equal to or less than 1.0% by weight. Vanadium improves the elastic limit Re by refining grain and promotes the weldability of steel. However, above 1.0% by weight, the tenacity of the steel deteriorates and there is a risk of cracks appearing in the welded zones; - optionally, nickel with a content equal to or less than 2.0% by weight. Nickel increases the elastic limit Re. In general, its content is limited to 2.0% by weight due to its high cost; - optionally, copper with a content equal to or less than 2.0% by weight. Copper increases the elastic limit Re, however, excess copper promotes the appearance of cracks during hot rolling and degrades the hot formability of the steel; - optionally, sulfur with a content equal to or less than 0.05% by weight. Sulfur is an element of segregation, the content of which must be limited to avoid cracking during hot rolling; and - optionally, niobium with a content equal to or less than 0.15% by weight. Niobium promotes the precipitation of carbonitrides, increasing the elastic limit Re. However, above 0.15% by weight, the weldability and hot formability are degraded. The rest of the composition consists of iron and other elements that are usually expected to be found as impurities resulting from the fusion of the steel, in proportions that do not affect the desired properties. In general, before they are cut into blanks, the steel strips are protected against corrosion by a metallic coating. Depending on the final use of the part, this metallic coating is chosen from zinc or zinc alloy coatings (eg zinc-aluminum) and, if good heat resistance is also desired, aluminum or aluminum alloy coatings (eg aluminum). -silicon). These coatings are deposited conventionally, either by hot dip coating in a liquid metal bath, or by electrodeposition, or by vacuum coating.

To implement the manufacturing process according to the invention, the steel blank is heated to raise it to an impregnation temperature Ts above Acl but below Ac3 and is maintained at this temperature Ts for a time of impregnation which is it is adjusted so that the steel, after the blank has been heated, has an austenite content equal to or greater than 25% per area. Immediately after this heating operation of the steel blank and keeping it at the temperature, the hot blank is transferred in a forming tool to form a piece and cooled in it. The cooling of the piece inside the forming tool is done at a quite high cooling rate V to prevent all the austenite from being transformed into ferrite and so that the microstructure of the steel after the piece has cooled is a microstructure of multiple phases that includes ferrite, the microstructure is homogeneous in each of the regions of the piece. The expression "homogenous multi-phase microstructure in each of the regions of the piece" is understood to mean a microstructure which is constant in terms of contents and morphology in each of the regions of the piece, and in which several phases they are uniformly distributed. So that the cooling speeds V are quite high, the training tools can be cooled, for example, by circulation of a fluid. In addition, the clamping force of the forming tool must be sufficient to ensure intimate contact between the blank and the tool and ensure effective and homogeneous cooling of the part. Optionally, after the blank has been cut from the steel strip and before the blank is heated, it may optionally undergo previous cold deformation. The prior cold deformation of the blank, for example by cold forming or light stretching of the blank, before the hot forming operation is advantageous as long as it allows the resulting pieces to have a more complex geometry. Furthermore, to obtain certain geometries in a single forming operation, it is possible only if two blanks are butt welded together. A previous cold deformation can therefore allow a piece to be obtained as a single piece, that is to say a piece obtained by the formation of a single blank. In a first preferred implementation of the invention, the process according to the invention is performed to manufacture a piece made of steel having a multi-phase microstructure comprising either ferrite and martensite or ferrite and bainite, or also ferrite, martensite and bainita To form this microstructure, the multi-phase composition mentioned above, in particular the carbon, silicon and aluminum contents, of the steel are adapted. Accordingly, the steel comprises the following elements: carbon with a content preferably between 0. 01 and 0.25%, more preferably between 0.08 and 0.15%, by weight. The carbon content is limited to 0.25% by weight to limit the formation of martensite and thus prevent the ductility and formability from deteriorating; - manganese with a content preferably between 0. 50 and 2.50% by weight and more preferably between 1.20 and 2.00% by weight; silicon with a content preferably between 0.01 and 2.0% by weight and more preferably between 0.01 and 0.50% by weight; - aluminum with 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 be less than 1.5% by weight to avoid the degradation of the spark weldability due to the formation of aluminum oxide inclusions A1203; - molybdenum with a content preferably between 0.001 and 0.50% by weight and more preferably between 0.001 and 0.10% by weight; - chromium with a content preferably equal to or less than 1.0% by weight and more preferably equal to or less than 0.50% by weight; - phosphorus with a content preferably equal to or less than 0.10% by weight; - titanium with a content preferably equal to or less than 0.15% by weight; - niobium with a content preferably equal to or less than 0.15% by weight; and - vanadium with a content preferably equal to or less than 0.25% by weight. The rest of the composition consists of iron and other elements that are usually expected to be found as impurities resulting from the fusion of the steel, in contents that do not affect the desired properties. To form a piece made of multi-phase steel comprising ferrite and martensite and / or bainite according to the invention, the blank is heated to an impregnation temperature Ts above Acl but below Ac3 to control the austenite content formed during heating of the blank and not to exceed the preferred upper limit of 75% austenite per area. An austenite content in hot steel at an impregnation temperature Ts for an impregnation time of between 25 and 75% per area offers a good compromise in terms of tensile strength of the steel after formation and uniformity of properties steel mechanics thanks to the robustness of the process. This is because above 25% austenite per area, the hardening phases, such as for example martensite and / or bainite, are formed with sufficient amount during the cooling of the steel for the elastic limit Re of the steel after the training is enough. However, up to 75% austenite per. area, it is difficult to control the austenite content in the steel and there is a risk of forming an excess number of phases of enrage during the cooling of the steel and consequently forming a piece of steel that has an insufficient elongation in the rupture A , deteriorating the capacity of energy absorption of the piece. The impregnation time of the steel blank at the impregnation temperature Ts essentially depends on the thickness of the strip. Within the context of the present invention, the thickness of the strip is typically between 0.3 and 3 mm. Accordingly, to form an austenite content between 25 and 75% per area, the impregnation time ts is preferably between 10 and 1000 s. If the steel blank is maintained at an impregnation temperature Ts for an impregnation time ts longer than 1000 s, the coarser austenite grains and the elastic limit Re of the steel after formation will be limited. In addition, the hardenability of the steel is reduced and the surface of the steel is oxidized. However, if the blank is maintained for a time of impregnation ts shorter than 10 s, the content of austenite formed will be insufficient and the content of martensite and / or bainite formed during cooling in the tool of the piece will be insufficient for that the elastic limit Re of the steel is quite high. The cooling speed 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. Nevertheless, the cooling rate V must be quite high so that the desired multi-phase microstructure is obtained, and is preferably greater than 10 ° C / s. For a cooling speed V equal to or less than 10 ° C / s, there is a risk of carbide formation that will contribute to the degradation of the mechanical properties of the part. Under these conditions, what is formed after cooling is a piece made of multi-phase steel that comprises more than 25% ferrite per area, the rest is martensite and / or bainite, and the various phases are homogeneously distributed in each of the regions of the piece. In a preferred implementation of the invention, 25 to 75% ferrite per area and 25 to 75% martensite and / or bainite per area are formed. In a second preferred implementation of the invention, the process according to the invention is used to manufacture a piece made of TRIP steel. Within the context of the invention, the term "TRIP steel" is understood to mean one having a multi-phase microstructure comprising ferrite, residual asutenite and optionally martensite and / or bainite. To form this multi-phase TRIP microstructure, the composition mentioned above and in particular the carbon, silicon and aluminum contents of the multi-phase steel are adapted. Accordingly, the steel comprises the following elements: carbon with a content preferably between 0.05 and 0.50% by weight and more preferably between 0.10 and 0.30% by weight. To form the stabilized residual austenite, it is preferable that this element be present with a content equal to or greater than 0.05% by weight, this is due to the fact that carbon plays a very important role in the formation of the microstructure and the mechanical properties: according to the invention, a bainite transformation takes place starting from an austenitic structure formed at high temperature, and bainite ferrite strips are formed. Due to the very low solubility of the carbon in ferrite compared with austenite, the carbon of the austenite is discarded between the slats. Thanks to certain alloying elements of the steel composition according to the invention, in particular silicon and manganese, carbide, especially cementite, very little precipitation occurs. Consequently, the austenite of internal slats becomes progressively enriched with carbon without precipitation of carbides. This enrichment is such that austenite is stabilized, ie the martensite transformation of this austenite does not take place during cooling below room temperature.; - manganese with 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 and helps to lower the temperature of the beginning of transformation of Ms martensite and stabilize austenite. This addition of manganese also contributes to the hardening of effective solid solution and therefore to a high yield strength Re being achieved. However, since an excess of manganese prevents sufficient ferrite from being formed during cooling, the carbon concentration in the residual austenite is insufficient to be stable. The manganese content is more preferably between 0.60 and 2.0% by weight. In this way, the desired effects above are obtained without the risk of forming a harmful banded structure that could result from any segregation of manganese during solidification; silicon with a content preferably between 0.001 and 3.0% by weight and more preferably between 0.01 and 2.0% by weight. The silicon stabilizes the ferrite and stabilizes the residual austenite at room temperature. Silicon inhibits the precipitation of austenite cementite during cooling, considerably reducing the growth of carbides. This is derived from the fact that the solubility of silicon in cementite is very low and that this element increases the carbon activity in austenite. Consequently, any cementite point formation will be surrounded by an austenitic zone rich in silicon that will have to be discarded at the precipitate / matrix interface. This austenite enriched in silicon is also more carbon rich, and cementite growth is encouraged due to the lower diffusion resulting from the reduced carbon gradient between the cementite and the adjacent austenitic zone. This addition of silicon helps to stabilize a sufficient amount of residual austenite to obtain a TRIP effect. This addition of silicon also helps increase the elastic limit Re thanks to the hardening of solid solution. However, an excessive addition of silicon causes the formation of highly adherent oxides, which are difficult to remove during a pickling operation, and the possible appearance of surface defects due, in particular, to a lack of wettability in the operations of hot dip galvanization. To stabilize a sufficient amount of austenite while still reducing the risk of surface defects, the silicon content is preferably between 0.01 and 2.0% by weight; - aluminum with a content preferably between 0. 005 and 3.0% by weight. Similar to silicon, aluminum stabilizes ferrite and increases ferrite formation during cooling of the blank. It has a very low solubility in cementite and can be used for this purpose to prevent the cementite from precipitating during impregnation at a bainite transformation temperature and to stabilize the residual austenite; - molybdenum with a content preferably equal to or less than 1.0% by weight and more preferably equal to or less than 0.60% by weight; - chromium with a content preferably equal to or less than 1.50% by weight. The chromium content is limited to avoid problems of surface appearance in the case of steel galvanization; - nickel with a content preferably equal to or less than 2.0% by weight; - copper with a content equal to or less than 2.0% by weight; - phosphorus with a content preferably equal to or less than 0.10% by weight. Phosphorus in combination with silicon increases the stability of residual austenite by suppressing the precipitation of carbides; - sulfur with a content preferably equal to or less than 0.05% by weight; - titanium with a content preferably equal to or less than 0.20% by weight; and - vanadium with a content preferably equal to or less than 1.0% by weight and more preferably equal to or less than 0.60% by weight. The rest of the composition consists of iron and other elements that are usually expected to be found as impurities resulting from the melting of the steel, in contents that do not affect the desired properties. The impregnation time of the steel blank at an impregnation temperature Ts above ACl but below Ac3 essentially depends on the thickness of the strip. Within the context of the present invention, the thickness of the strip is typically between 0.3 and 3 mm. Accordingly, to form an austenite content equal to or greater than 25% per area, the impregnation time ts is preferably between 10 and 1000 s. If the steel blank is maintained at an impregnation temperature Ts for an impregnation time ts longer than 1000 s, the coarser austenite grains and the yield strength Re of the steel after formation will be limited. In addition, the hardenability of the steel is reduced and the surface of the steel is oxidized. However, if the blank is maintained for a time of impregnation ts shorter than 10 s, the content of austenite formed will be insufficient and residual austenite and bainite will not form substantially during tool cooling of the part. The cooling speed 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 piece made of steel having a multi-phase TRIP microstructure, it is preferable that the cooling rate V be between 10 ° C / s and 200 ° C / s. This is because below 10 ° C / s essentially ferrite and carbides will be formed, but insufficient residual austenite and martensite, while above 200 ° C / s martensite will be formed essentially with insufficient residual austenite. It is essential to form austenite with a content equal to or greater than 25% per area during heating of the blank so that, in the cooling of the steel in the forming tool, sufficient residual austenite remains and the desired TRIP effect can therefore be achieved. get. Under these conditions, what is obtained after cooling is a piece made of multi-phase steel which consists, in% per area, of ferrite with a content equal to or greater than 25%, from 3 to 30% residual austenite and optionally of martensite and / or bainite. The TRIP effect can advantageously be put to good use to absorb the energy in the case of a high speed impact. This is because during a large deformation of a TRIP steel part, the residual austenite is progressively transformed to martensite, while the orientation of the martensite is selected. This has the effect of reducing the residual stresses in the martensite, reducing the internal stresses in the piece and finally limiting the damage of the piece, since the latter will fracture to a greater elongation A if it were not made of a steel TRIP. The invention will now be illustrated by the examples given by way of indication but without implying limitation, with reference to the single attached figure, which is a photograph of a piece obtained by cold forming (reference G) and of a piece obtained by training hot (reference A). The inventors carried out tests both on steels having, on the one hand, a composition typical of that of steels having a multi-phase multi-structure comprising ferrite and martensite and / or bainite (point 1) and, on the other hand, a composition typical of that of steels that have multiple phase TRIP microstructure (point 2). 1 - . 1 - Steel with a composition typical of that of steels having a multi-phase microstructure comprising ferrite and martensite 1.1 Evaluation of the influence of heating and cooling rates Blanks measuring 400 x 600 mm were cut from a steel strip , the composition of which, given in table I, is that of a DP780 grade steel (Dual Phase 780). The strip had a thickness of 1.2 mm. The Acl temperature of the steel was 705 ° C and the Ac3 temperature was 815 ° C. The blanks were heated to a variable impregnation temperature Ts and maintained for an impregnation time of 5 min. Then they were immediately transferred to a deep drawing tool in which they were both formed and cooled at variable cooling speeds V, keeping them in the tool for a period of 60 s. The deep drawn pieces had a structure similar to the shape of an omega.

After the pieces were completely cooled, their elastic limit Re, their tensile strength Rm and their elongation at break A were measured and the microstructure of the steel was determined. With respect to the microstructure, F denotes ferrite, M denotes martensite and B denotes bainite. The results are given in table II.

Table I: Chemical composition of the steel according to the invention, expressed in% by weight, the rest is iron or impurities Table II. Mechanical properties and microstructure of deep-drawn parts * according to the invention, The results of this test clearly show that only by heating the steel to a temperature between Acl and Ac3 it is possible to obtain a multi-phase microstructure comprising ferrite, whatever the rate of cooling of the steel in the forming tool. This is because when the steel is heated to a temperature above Ac3, then it is necessary that the cooling rate V be strictly controlled during the formation, to obtain a steel having a multi-phase microstructure comprising more than 25% of ferrite per area, and preferably between 25% and 75% of ferrite per area. In addition to a small variation in the mechanical properties according to the cooling rate for the parts as claimed according to the invention, their energy absorption capacity is higher than that of the pieces obtained with heating at a temperature above Ac3. . 1. 2 Evaluation of expansion The purpose of this trial was to show the benefit of hot forming compared to cold forming, and to assess expansion. For this purpose, a piece made of grade steel DP780 was manufactured by cold deep drawing of a blank cut from a strip of steel 1.2 mm thick, the composition of the steel is indicated in table I but which, different from the strip used in point 1, had already , before deep drawing, a multi-phase microstructure comprising 70% ferrite per area, 15% martensite per area and 15% bainite per area. Figure 1 clearly shows that the piece formed by cold deep drawing (indicated in the figure by the letter G) had a high expansion compared to piece A (see table II) formed by hot deep drawing (identified by the letter A ). 2 - . 2 - Steel with a composition typical of that of TRIP steels. Raw parts measuring 200 x 500 mm were cut from a steel strip, the composition of which, given in Table III, was that of a TRIP 800 grade steel. strip had a thickness of 1.2 mm. The Acl temperature of this steel was 751 ° C and the Ac3 temperature was 875 ° C. The blanks were heated to a variable impregnation temperature Ts for an impregnation time of 5 min. and then they were immediately transferred to a deep drawing tool in which they were both formed and cooled with a cooling speed of 45 ° C / s, keeping them in the tool for a period of 60 s. The deep drawn pieces had a structure similar to that of an omega shape.

After the pieces had cooled completely, their elastic limit Re, their tensile strength Rm and their elongation at break A were measured and the microstructure of the steel was determined. With respect to the microstructure, F denotes ferrite, A denotes residual austenite, M denotes martensite and B denotes bainite. The results are given in table IV.

Table III: Chemical composition of the steel according to the invention, expressed in% by weight, the rest is iron or impurities Table IV. Mechanical properties and microstructure of deep-drawn parts * according to the invention.

The tests carried out clearly show that by deep drawing the blanks produced according to the invention it is possible to obtain parts having very high mechanical properties and also a small variation in the mechanical properties whatever the cooling temperature. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (19)

CLAIMS Having described the invention as above, the contents of the following claims are claimed as property:

1. Process for manufacturing a piece made of steel having a multi-phase microstructure, the microstructure comprises ferrite and is homogeneous in each of the regions of the piece, characterized in that it comprises the steps that consist of: - cutting a blank from a strip of steel, the composition of which consists, in% by weight, of: 0.01 < C < 0.50% 0.50 < Mn < 3.0% 0.001 < Yes < 3.0% 0.005 < To < 3.0% Mo < 1.0% Cr < 1.5% 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 rest of the composition is iron and impurities resulting from the fusion; - optionally, the blank undergoes previous cold deformation; the blank is heated to reach an impregnation temperature Ts above Acl but below Ac3 and is maintained at this impregnation temperature Ts for an impregnation time ts adjusted so that the steel, after the blank has been heated , has an austenite content equal to or greater than 25% per area; the hot blank is transferred in a forming tool to hot form the piece; and - the piece is cooled inside the tool at a cooling speed V so that the microstructure of the steel, after the piece has cooled, is a multi-phase microstructure, the microstructure comprises ferrite and is homogeneous in each of the regions of the piece.

2. Process according to claim 1, characterized in that additionally the microstructure of the steel, after the piece has been cooled, is a multi-phase microstructure with a ferrite content equal to or greater than 25% per area. Process according to any of claims 1 and 2, characterized in that the composition of the steel comprises, in% by weight: 0.01 = C < 0.25% 0.50 < Mn < 2.50% 0.01 < Yes < 2.0% 0.005 < To < 1.5% 0.001 < Mo < 0.50% Cr < 1.0% P < 0.10% Ti < 0.15% Nb < 0.15% V = 0.25%, the rest of the composition is iron and impurities resulting from the fusion; the blank remains at the impregnation temperature T? for a time of impregnation ts adjusted so that the steel, after heating, has an austenite content between 25 and 75% per area; and the microstructure of the steel, after the piece has cooled, is a multi-phase microstructure comprising ferrite and either martensite, or bainite, or else both martensite and bainite. Process according to claim 3, characterized in that the steel additionally comprises, in% by weight: 0.08 = C < 0.15%

1. 20 < Mn < 2.00% 0.01 < Yes = 0.50% 0.005 < To < 1.0% 0.001 < Mo < 0.10% Cr = 0.50% P < 0.10% Ti = 0.15% Nb = 0.15% V < 0.25%, the rest of the composition is iron and impurities resulting from the fusion. Process according to any of claims 3 and 4, characterized in that the impregnation time ts is between 10 and 1000 s. 6. Process according to any of claims 3 and 5, characterized in that the cooling rate V is greater than 10 ° C / s. Process according to any of claims 3 to 6, characterized in that the multi-phase structure of the steel, after the piece has been cooled, comprises 25 to 75% of ferrite per area and 25 to 75% of martensite and / or bainita by area. Process according to any of claims 1 and 2, characterized in that the steel comprises, in% by weight:

0. 05 < C = 0.50% 0.50 < Mn < 3.0% 0.001 < Yes < 3.0% 0.005 < To < 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 rest of the composition is iron and impurities resulting from the fusion; The microstructure of the steel, after the piece has been cooled, is a TRIP multi-phase microstructure comprising ferrite, residual austenite and optionally martensite and / or bainite. 9. Process according to claim 8, characterized in that the steel additionally comprises, in% by weight: 0.10 < C < 0.30% 0.60 < Mn < 2.0% 0.01 < Yes < 2.0% 0.005 < To < 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 rest of the composition is iron and impurities resulting from the fusion. 10. Process according to any of claims 8 and 9, characterized in that the impregnation time ts is between 10 and 1000 s. 11. Process according to any of claims 8 to 10, characterized in that the cooling rate V is between 10 and 200 ° C / s. Process according to any of claims 8 to 11, characterized in that additionally, after the piece has cooled, the multi-phase microstructure of TRIP steel consists, in% per area, of ferrite with a content equal to or greater 25%, from 3 to 30% residual austenite and optionally martensite and / or bainite. 13. Process according to any of claims 1 to 12, characterized in that the forming operation is a deep drawing operation. 14. Process according to any of claims 1 to 13, characterized in that the steel strip is coated before with a metallic coating, before being cut to form a blank. 15. Process according to claim 14, characterized in that the metallic coating is a coating based on zinc or zinc alloy. Process according to claim 14, characterized in that the metallic coating is a coating based on aluminum or an aluminum alloy. Piece made of steel having a homogenous multi-phase microstructure in each of the regions of the piece, characterized in that the microstructure comprises ferrite, which can be obtained by the process according to any of claims 1 to 16. 18. Use of the steel part according to claim 17 to absorb energy. 19. Land vehicle, characterized in that it includes the steel part according to claim 17.