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    • 专利摘要:
    It is a method for manufacturing a steel product comprising the steps of: providing a steel starting product heated to a temperature of between 380 °C and 700 °C, which has a fully austenitic metastable structure, with a composition comprising, in weight percentage: 0.15% = C = 0.40%, 1.5% = Mn = 4.0%, 0.5% = Si = 2.5%, 0.005% = Al = 1.5%, with 0.8% = Si+Al = 2.5%, S = 0.05%, P = 0.1%, at least one element chosen from Cr and Mo, so that: 0% = Cr = 4.0%, 0% = Mo = 0.5% and 2.7% = Mn + Cr + 3 Mo = 5.7% and, optionally, one or several elements chosen from: Nb = 0.1 %, Ti = 0.1%, Ni = 3.0%, 0.0005% = B = 0.005%, 0.0005% = Ca = 0.005%, with the balance of the composition consisting of iron and unavoidable impurities that result of the foundry; subjecting said starting product to a hot forming step at a temperature comprising between 700°C and 380°C, with an accumulated strain deb between 0.1 and 0.7, in at least one location of said product heated steel starting material to obtain a fully austenitic hot-formed steel product; then, abruptly cool the hot formed steel product by cooling it, at a cooling rate VR2 higher than the critical martensitic cooling rate, for a sudden cooling temperature QT less than Ms in order to obtain (...) .
    公开号:BR112017010093B1
    申请号:R112017010093-2
    申请日:2015-11-17
    公开日:2021-05-18
    发明作者:Artem Arlazarov；Kangying Zhu
    申请人:Arcelormittal；
    IPC主号:
    专利说明:

    FIELD OF THE INVENTION
    [001] The present invention relates to a method for manufacturing a high-resistance steel product and a high-resistance steel product obtained through this method. BACKGROUND OF THE INVENTION
    [002] More specifically, the present invention relates to a method for manufacturing a steel product, for example, a sheet of steel or a piece of steel, which combines good elongation properties and a high tensile strength.
    [003] High resistivity steel sheets produced from DP steels (two-phase) or TRIP steels (Transform-Induced Plasticity) are currently used to manufacture various parts in the automotive industry, in cars and trucks.
    [004] In order to reduce the weight of equipment produced from these steels, it is very important to increase the tensile strength and the yield strength without reducing the elongation that is necessary to have a good processability and without reducing weldability.
    [005] For this purpose, it was proposed in the document in WO 2012/153008 to use CMnSi steels that contain from 0.15% to 0.4% of C, from 1.5% to 3% of Mn and from 0.005% to 2% Si, such steels being heat treated to have a fully martensitic structure.
    [006] The document in WO 2012/153008 therefore discloses a method for manufacturing a sheet or piece of steel in which the steel is heated to a temperature between 1,050 °C and 1,250 °C, then subjected to a rough rolling to a temperature between 1150 °C and 900 °C, thereafter cooled to a temperature between 380 °C and 600 °C, subjected to a final hot rolling at that temperature and subsequently directly cooled directly to room temperature.
    [007] This manufacturing method allows to obtain a sheet or piece of steel with a tensile strength higher than the tensile strength of steel sheets that are manufactured by austenitizing steel and then sudden cooling to obtain a complete martensitic hardening.
    [008] However, although this method does not harm the elongation properties of steel, it does not improve these properties. The total elongation TE of steel sheets obtained by such a method is generally limited to less than 7% for a tensile strength of about 1600 MPa.
    [009] Thus, it remains desirable to be able to produce a sheet or piece of steel that has a YS yield strength of more than 1,000 MPa up to 1,700 MPa, a TS tensile strength of more than 1,300 MPa, up to 2,000 MPa, an elongation uniform EU of more than 7%, a total elongation TE of more than 10%, a tensile strength x total product elongation (TS X TE) higher than 18,000% in MPa, and a tensile strength x uniform product elongation ( TS x EU) higher than 13,000% in MPa. These properties are measured in accordance with the ISO 68921 standard, published in October 2009. It needs to be emphasized that, due to differences in measurement methods, in particular due to differences in the specimen size used, the values of the total elongation in accordance with the ISO standard are quite different, in particular lower than the full elongation values according to the JIS Z 2201-05 standard. DESCRIPTION OF THE INVENTION
    [010] To this end, the invention relates to a method for manufacturing a steel product comprising the successive steps of: - providing a starting steel product heated to a temperature comprising between 380°C and 700°C, wherein said heated steel starting product has a fully austenitic metastable structure, said heated steel starting product having a composition comprising, by weight percentage: 0.15% < C < 0.40%, 1.5% < Mn < 4.0%, 0.5 < Si < 2.5%, 0.005% < Al < 1.5%, with 0.8% < Si+Al < 2.5%, S < 0.05%, P < 0.1%, at least one element chosen from Cr and Mo, so that: 0% < Cr < 4.0%, 0% < Mo < 0.5%, and 2.7 % < Mn + Cr + 3 Mo < 5.7% and, optionally, one or several elements chosen from: Nb < 0.1%, Ti < 0.1%, Ni < 3.0%, 0.0005% < B < 0.005%, 0.0005% < Ca < 0.005%, the balance of the composition being iron and unavoidable impurities resulting from the casting, - subjecting said heated steel starting product to a step d and hot forming at a temperature comprising between 700°C and 380°C, with an accumulated strain εb between 0.1 and 0.7, in at least one location of said heated steel starting product, to obtain a product of hot-formed steel, with the steel structure remaining completely austenitic, then abruptly cool the hot-formed steel product by cooling it, at a cooling rate VR2 higher than the critical martensitic cooling rate, to a temperature of quench QT lower than the martensite Ms starting temperature of the steel in order to obtain a structure containing between 40% and 90% martensite, the rest of the structure being austenite, so - keep in, or reheat the product to a holding temperature PT between QT and 470°C and holding it at said temperature PT for a duration Pt between 5 s and 600 s.
    [011] According to other advantageous aspects of the invention, the method comprises one or more resources, considered alone or according to any technically possible combination: - the method further comprises a step of cooling the retained product to room temperature at a rate cooling greater than 0.005 °C/s in order to obtain fresh martensite; the heated steel starting product is a hot rolled steel sheet and the steel product is a steel sheet, and wherein said hot forming step is a rolling step; - the step of providing a heated steel starting product comprises: • heating a steel semi-product, with a composition as defined in claim 1, to a temperature higher than the AC3 temperature of the steel in order to obtain a fully austenitic structure , • subjecting said steel semi-product to a rough rolling step at a temperature above a temperature T2 between 1200 and 850°C, with an accumulated reduction strain a greater than 1, in order to obtain said starting product heated steel; - said heated steel starting product has an average austenitic grain size of less than 30 µm; - the starting product is a steel blank, the steel product being a steel piece and the step of providing a heated steel starting product comprises heating said steel blank to a temperature higher than the AC3 temperature of the steel, in order to obtain a completely austenitic structure; - said steel blank has a thickness between 1.0 mm and 4.0 mm; - said hot forming step is a hot rolling step; - said hot forming step is a hot stamping step; - said hot forming step is a hot drawing step; - said hot forming step is a roll forming step; - said steel blank comprises at least one coating layer; - the coating layer is applied on said starting product before heating and the coating layer is aluminum coating or aluminum based, or zinc coating or zinc based.
    [012] The invention also relates to a steel product that has a composition comprising, in percentage by weight: 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 4.0%, 0 .5 ≤ Si ≤ 2.5%, 0.005% ≤ Al ≤ 1.5%, with 0.8% ≤ Si+Al ≤ 2.5%, S < 0.05%, P ≤ 0.1%, by the minus one element chosen from Cr and Mo, so that: 0% ≤ Cr ≤ 4.0%, 0% ≤ Mo ≤ 0.5%, and 2.7% ≤ Mn + Cr + 3 Mo ≤ 5.7% and, optionally, one or several elements chosen from Nb < 0.1% Ti < 0.1%, Ni < 3.0% 0.0005% ≤ B ≤ 0.005% 0.0005% ≤ Ca ≤ 0.005%, where the balance of the composition consists of iron and unavoidable impurities resulting from the smelting, the structure of at least one location of the steel product being: - hardened martensite or carbide-free martensite slats, with a surface percentage of at least 40 %, - fresh martensite, in the form of islands or films, in which the surface percentage of said fresh martensite comprises between 5% and 30%, and - austenite, with a surface percentage of 5% to 35%.
    [013] According to other advantageous aspects of the invention, the steel product comprises one or more of the following functions, considered alone or according to any technically possible combination: - the product of the tensile strength TS of the steel by the uniform elongation EU of the steel is greater than or equal to 13,000% in MPa; - the martensite strips have an average size of less than 1 µm, the aspect ratio of said martensite strips being between 2 and 5; - the maximum size of islands of said fresh martensite with an aspect ratio of less than 3 is less than 3 μm; - the average size of the previous austenitic grain is less than 30 μm; - the aspect ratio of the previous austenitic grain is higher than 1.3; - said austenite is in the form of films or islands, with the smallest dimension of said films or islands having a value of less than 0.3 μm, and the largest dimension of said films or islands having an average value of less than 2 μm ; - said tempered martensite comprises, in surface percentage, less than 0.5% of carbides, compared to the surface of said tempered martensite and said carbides have an average size of less than 50 nm; - said steel product is a steel sheet and the structure of the entire steel sheet consists of: - hardened martensite or carbide-free martensite slats, with a surface percentage of at least 40%, - fresh martensite, in the form of islands or films, in which the surface percentage of said fresh martensite comprises between 5% and 30%, and - austenite, with a surface percentage of 5% to 35%; - said steel product is a hot-stamped steel part and the structure of at least 20% of the volume of said hot-stamped part consists of: - hardened martensite or carbide-free martensite slats, with a percentage of pile surface minus 40%, - fresh martensite, in the form of islands or films, where the surface percentage of said fresh martensite comprises between 5% and 30%, and - austenite, with a surface percentage of 5% to 35%; - said steel product comprises at least one coating layer; - said at least one coating layer is zinc or zinc-based alloy, or aluminum or aluminum-based alloy; and - said at least one coating layer is applied before hot stamping. BRIEF DESCRIPTION OF THE DRAWINGS
    [014] The invention will now be described in detail without introducing limitations and illustrated by examples, and the attached Figures among which: - Figure 1 is a scanning electron micrograph (SEM) that illustrates the microstructure of a steel product according to the invention; Figures 2 and 3 are SEM illustrating the microstructure of steel products obtained by manufacturing methods that are not in accordance with the present invention; and - Figures 4, 5 and 6 are graphs comparing mechanical properties of steel products obtained by manufacturing methods that are or are not in accordance with the present invention. DESCRIPTION OF ACHIEVEMENTS OF THE INVENTION
    [015] The steel product according to the present invention has the following composition: - 0.15% < C < 0.40% to ensure a satisfactory resistibility and improve the stability of the retained austenite. In particular, with a carbon content of less than 0.15%, the sudden cooling capacity of steel is not good enough, which does not allow sufficient martensite to form with the manufacturing method used. With a C content greater than 0.40%, the weldability of steel is reduced. In fact, welded joints produced from the sheets would have insufficient hardness. Preferably, the carbon content is greater than or equal to 0.25%. Preferably, the carbon content is not higher than 0.33%; - 1.5% < Mn < 4.0%. Manganese lowers the temperatures Ac1, Ac3 and Ms, that is, respectively, the temperature at which austenite begins to form under heating (Ac1), the temperature at which the austenite transformation is complete under heating (Ac3) and the temperature in which the transformation from austenite to martensite starts under cooling (Ms). Therefore, Mn improves the stability of retained austenite by further chemically enriching austenite in Mn and decreasing the austenite grain size. Refinement of austenite grain size leads to a decrease in the diffusion distance and therefore arrests the diffusion of C and Mn during a temperature retention step that can be performed during the heat treatment cooling cycle. In order to obtain a stabilizing effect sufficient to allow deformation of the steel in the temperature range of 700 to 380°C during cooling, the Mn content needs to be at least 1.5%. Furthermore, when the Mn content is greater than 4%, segregated zones appear, which are detrimental to the stretch flangability and impair the implantation of the invention. Preferably, the Mn content is greater than 1.8%. Preferably, the Mn content is not higher than 2.5%; - 0.5% < Si < 2.5% and 0.005% < Al < 1.5%, and the silicon and aluminum contents additionally satisfy the following ratio: 0.8% < Si+Al < 2.5% . According to the invention, Si and Al together are of great importance:
    [016] Silicon delays the precipitation of cementite upon cooling below the equilibrium transformation temperature Ae3. Therefore, an addition of Si helps to stabilize a sufficient amount of residual austenite in the form of islands. Si additionally provides solid solution reinforcement and retards carbide formation during carbon redistribution from martensite to austenite which results from an immediate reheat and retention step performed after a partial martensitic transformation. At a very high content, silicon oxides form on the surface, which impairs the steel's ability to coat. Therefore, the Si content is preferably less than or equal to 2.5%.
    [017] Aluminum is a very effective element to deoxidize steel in the liquid phase during elaboration. The Al content is not less than 0.005% in order to obtain sufficient deoxidization of the steel in the liquid state. Additionally, like Si, Al stabilizes residual austenite. The Al content is not higher than 1.5% in order to avoid the occurrence of inclusions, to avoid oxidation problems and to ensure the hardenability of the material.
    [018] The effects of Si and Al in stabilizing austenite are similar. When the Si and Al contents are such that 0.8% < Si+Al < 2.5%, satisfactory stabilization of the austenite is obtained, thereby making it possible to form the desired microstructures. - Sulfur and phosphorus must be kept at low levels, ie, S < 0.05% and P < 0.1%, so as not to deteriorate too much the ductility and hardness of the pieces. As achieving extremely low sulfur is costly, a sulfur content higher than 0.0005% is preferred for economic reasons. Similarly, a phosphorus content higher than 0.0005% is preferred.
    [019] The steel according to the invention contains at least one element chosen from molybdenum and chromium. Cr and Mo are very effective in delaying the transformation of austenite and preventing the formation of proeutectoid or bainite ferrite, and can be used to implement the invention. In particular, these elements have an influence on the isothermal transformation diagram under cooling (also known as time-temperature transformation diagram (TTT)): additions of Cr and Mo separate the ferrite-perlite transformation domain from the domain of bainite transformation, the ferrite-perlite transformation that occurs at higher temperatures than the bainite transformation. Therefore, these transformation domains appear as two distinct “noses” in the TTT diagram, which opens a “bay” that allows deforming the steel under cooling between these two noses, without causing unwanted transformation of austenite into ferrite, perlite and/or bainite. For the compositions of the invention, this temperature range for deformation comprises between 380 and 700°C. Hot forming of metastable austenite in this range is known as “ausforming”.
    [020] If the steel composition comprises Cr, the Cr content need not be higher than 4.0%. In fact, above this value, the Cr effect is saturated and increasing its content would be onerous, without providing any beneficial effect.
    [021] If the steel composition comprises Mo, the Mo content is not higher than 0.5%, due to its high cost.
    [022] Additionally, according to the invention, the contents of Mn, Cr and Mo are such that 2.7% < Mn + Cr + 3 Mo < 5.7%. The Mn, Cr and Mo factors in this relationship reflect their respective abilities to prevent austenite transformation and to provide hardening to obtain sufficient mechanical properties.
    [023] The steel according to the invention optionally contains niobium and/or titanium.
    [024] When Nb is present in the composition, the Nb content should not be higher than 0.1% and preferably higher than 0.025%. When Ti is present in the composition, the Ti content should not be higher than 0.1% and preferably higher than 0.01%.
    [025] In these quantities, Nb has a strong synergy effect with B to improve the hardenability of steel and Ti can protect B against BN formation. Furthermore, the addition of Nb and Ti can increase the softening strength of martensite during quenching.
    [026] This effect of Nb and Ti appears visibly with contents in Nb and Ti, respectively, higher than 0.025% and 0.01%.
    [027] The contents of Nb and Ti are each not higher than 0.1% in order to limit the hardening of steel at high temperatures provided by these elements, which could make it difficult to produce thin plates due to the increase of hot rolling forces.
    [028] Optionally, the composition may comprise nickel, in an amount less than or equal to 3.0% and preferably higher than 0.001%.
    [029] Steel can optionally contain boron in an amount comprising between 0.0005% and 0.005%, so as to increase the capacity for sudden cooling of the steel. In fact, an important deformation of austenite can result in the accelerated transformation of austenite to ferrite during cooling. An addition of B, in an amount comprising between 0.0005% and 0.005%, helps to avoid this initial ferritic transformation.
    [030] Optionally, steel can comprise calcium in an amount comprising between 0.0005% and 0.005%: combining with O and S, Ca helps to prevent the formation of large size inclusions that negatively impair the ductility of the sheets .
    [031] The remainder of the steel composition is iron and impurities that result from the casting. Impurities can include nitrogen, the N content being no higher than 0.010%.
    [032] The method for manufacturing a steel product according to the invention seeks to manufacture a steel product that has, in at least one location of the product, a microstructure consisting of tempered martensite or carbide-free martensite slats, with a percentage of at least 40% surface, fresh martensite, present as islands or films, the surface percentage of said fresh martensite comprising between 5% and 30% and retained austenite with a surface percentage of 5% to 35%.
    [033] These microstructural functions can be present throughout the product, or only in some places, in order to resist locally rigorous fatigue deformations. In the latter case, these microstructural functions need to be present in at least 20% of the product's volume in order to obtain significant strength resistance.
    [034] The fabrication method will now be described. The method comprises a step of providing a heated steel starting product at a temperature comprising between 380°C and 700°C, said heated starting steel product having a completely austenitic structure. With reference to this temperature range and the steel composition below, it is understood that this austenitic structure is in a metastable state, that is, that this heated steel starting product is obtained from a heating step in the austenitic range , followed by cooling at a rate that is high enough so that the austenite does not have time to transform.
    [035] Said heated starting product additionally has a composition comprising, by weight percentage: 0.15% < C < 0.40%, 1.5% < Mn < 4.0%, 0.5 < Si < 2.5%, 0.005% < Al < 1.5%, with 0.8% < Si+Al < 2.5%, S < 0.05%, P < 0.1%, at least one element chosen from Cr and Mo, so that: 0% < Cr < 4%, 0% < Mo < 2%, and 2.7% < Mn + Cr + 3 Mo < 5.7% and optionally one or several elements chosen from: Nb < 0.1%, Ni < 3.0%, Ti < 0.1%, 0.0005% < B < 0.005%, 0.0005% < Ca < 0.005%, with the balance The composition consists of iron and unavoidable impurities that result from the smelting.
    [036] Said heated starting product is, for example, a semi-product or a blank.
    [037] A semi-product is defined as a sheet that has undergone a hot rolling step, however, in which the thickness is greater at this stage than the desired final thickness.
    [038] A blank is defined as the result of cutting a sheet of steel or rolling it to a shape related to the final desired geometry of the product to be produced.
    [039] According to the invention, the heated starting product is subjected, in at least one location of the starting product, to a hot forming step, at a temperature comprising between 700 °C and 380 °C, with an accumulated deformation between 0.1 and 0.7, whereby the steel structure remains completely austenitic, ie ausforming is performed.
    [040] The hot forming step can be performed in one or several successive stages. Since strain modes may differ from one product location to another due to product geometry and local fatigue strain modes, an equivalent cumulative strain εb is defined at each location in the product as
    accumulated in all stages of deformation.
    [041] If the hot forming is performed through hot rolling, the accumulated deformation εb is defined from the initial sheet thickness ti before hot rolling and the final sheet thickness tf after hot rolling, by:

    [042] Under these conditions, a plastically deformed austenite structure, in which recrystallization does not occur, is obtained.
    [043] The hot forming step is conducted between temperatures T3 and T3', both of which comprise between 380 °C and 700 °C, for example, between 550 °C and 450 °C, in order to allow austenite refinement , to avoid recrystallization of the deformed austenite and to avoid transformation of the austenite during the hot forming step. In particular, due to the composition of the steel, the formation of ferrite, perlite and/or bainite during this hot forming step is avoided.
    [044] In fact, as revealed above, Mn improves the stability of retained austenite.
    [045] Furthermore, Cr and Mo delay the austenite transformation and prevent the formation of proeutectoid or bainite ferrite, separating the ferrite-perlite transformation domain from the bainite transformation domain. These transformation domains therefore appear as two distinct “noses” in an isothermal transformation diagram (also known as a time-temperature transformation diagram (TTT)), thus opening a “window” that allows steel to deform under cooling. between these two noses without forming ferrite, perlite and/or bainite. Therefore, the ausforming step is preferably carried out at a temperature within this window.
    [046] The hot forming step leads to an increase in the tensile strength TS and the yield strength YS of the steel, compared to a steel not subjected to such a hot forming step. In particular, the hot forming step leads to a ΔTS increase in tensile strength of at least 150 MPa and a ΔYS increase in yield strength of at least 150 MPa.
    [047] At this point, the hot-formed product has a structure consisting of deformed austenite, with the deformation ratio of the austenite being between 0.1 and 0.7, and where the average size of the austenite grains is smaller at 30 µm, preferably less than 10 µm.
    [048] According to the invention, the hot-formed product is then quenched by cooling it, at a cooling rate VR2 higher than the critical martensitic cooling rate, to a sharp cooling temperature QT lower at the starting temperature of martensite Ms of the steel, in order to obtain a structure that contains between 40% and 90% of martensite, with the remainder of the structure being austenite.
    [049] As it is desired to have a final structure that contains a significant amount of retained austenite, that is, between 5% and 35%, the QT temperature need not be too low and needs to be chosen according to the desired amount of austenite retained, in any case higher than the Mf transformation temperature of steel, that is, the temperature at which martensite transformation is complete. More specifically, it is possible to determine for each chemical composition of steel an ideal sudden cooling temperature QTop that achieves the desired residual austenite content. A person skilled in the art knows how to determine this hypothetical QTop burst cooling temperature.
    [050] Due to the fact that the martensite transformation occurs from a deformed and finer austenite grain, the refinement of martensite slats is higher than in the previous technique, as will be explained below.
    [051] To safely ensure that the structure contains between 40% and 90% martensite for a composition conforming to the ranges indicated above, the quench temperature QT is preferably below Ms-20 °C and comprises, preferably between 100 °C and 350 °C.
    [052] Without additional cooling, the product, whose microstructure consists essentially in this moment of retained austenite and martensite, is then immediately maintained or reheated to a retention temperature PT that comprises between QT and 470 °C.
    [053] For example, the product is reheated to a PT holding temperature higher than Ms.
    [054] Then, the product is kept at PT temperature for a duration Pt, where Pt comprises between 5 s and 600 s.
    [055] During this retention step, carbon is divided between martensite and austenite, that is, it diffuses from martensite to austenite, which leads to an improvement in the ductility of the martensite and an increase in the content of austenite carbon without the appearance of significant amounts of bainite and/or carbides. The enriched austenite makes it possible to obtain a TRIP (transformation-induced plasticity) effect on the final product.
    [056] The degree of division increases with the duration of the hold step. Therefore, the retention duration Pt is chosen long enough to provide the most complete division possible. The retention duration Pt needs to be greater than 5 s and preferably greater than 20 s, in order to optimize the enrichment of the austenite in carbon.
    [057] However, too long a duration can cause the decomposition of austenite and the very high splitting of martensite and thus a reduction in mechanical properties. Therefore, the duration is limited in order to avoid the formation of ferrite as much as possible. Therefore, the Pt retention duration must be less than 600 s. The product is finally cooled to room temperature at a cooling rate required to create 5% to 30% of fresh martensite, and to obtain a percentage of retained austenite surface of 5% to 35%. Preferably, the cooling rate should be greater than 0.005 °C/s.
    [058] The quench and hold steps are defined as a “dark cool and split” (“Q-P”) step.
    [059] Therefore, the steel product obtained is characterized, at the site subjected to the hot forming step, by a microstructure consisting of tempered martensite or carbide-free martensite slats, with a surface percentage of at least 40%, fresh martensite , in the form of islands or films, where the surface percentage of said fresh martensite comprises between 5% and 30% and retained austenite, with a surface percentage of 5% to 35%.
    [060] Martensite slats are very thin. Preferably, these martensite laths, as characterized by EBSD, have an average size of at most 1 µm.
    [061] Additionally, the average aspect ratio of these martensite slats preferably comprises between 2 and 5.
    [062] These functions are determined, for example, by observing the microstructure with a scanning electron microscope with a field emission gun ("FEG-SEM") at a magnification greater than 1,200x, coupled to a diffraction device backscattering (“EBSD”). Two adjoining slats are defined as distinct slats when their disorientation is at least 5°. The morphology of the individualized slats is then determined by image analysis with conventional software known to a person skilled in the art. The largest dimension lmax, the smallest dimension lmin and the lmax aspect ratio lmin of each batten are therefore determined. This determination is carried out on a sample of at least 1,000 slats. The 1 max 1min average aspect ratio, which is then determined for that sample, preferably comprises between 2 and 5.
    [063] Tempered martensite and martensite slats comprise less than 0.5% of carbides in surface percentage compared to the surface of said quenched martensite and slats. These carbides have an average size of less than 50 nm.
    [064] The largest dimension of fresh martensite islands with an aspect ratio of less than 3 is less than 3 µm.
    [065] Retained austenite is needed particularly to enhance ductility. As seen above, the retained austenite is deformed, with a deformation ratio comprising between 0.1 and 0.7.
    [066] Preferably, the retained austenite is in the form of films or islands. The smallest dimension of such films or islands has a value of less than 0.3 µm and the largest dimension of such films or islands has an average value of less than 2 µm. Refining the retained austenite improves its stability so that during deformation, the retained austenite turns to martensite over a wide strain range. Retained austenite is also stabilized by splitting carbon from martensite to austenite.
    [067] The average previous austenitic grain size, which is the average size of the austenite just before the transformation of the same under cooling, that is, in the present case, the average size of the austenite further on to the hot forming step, is less than 30 µm, preferably less than 10 µm. Additionally, the aspect ratio of the previous austenitic grain is higher than 1.3.
    [068] To determine this aspect ratio, the previous austenitic grains are revealed in the final product by a suitable method known to a person skilled in the art, for example, by pickling with a pickling reagent of picric acid. Previous austenitic grains are observed under an optical microscope or a scanning electron microscope. The aspect ratio of the previous austenitic grains is then determined by image analysis with conventional software known to a person skilled in the art. In a sample of at least 300 grains, the largest and smallest dimensions of the previous austenitic grains are determined and the aspect ratio of the grains is determined as the ratio between the largest dimension and the smallest dimension. The aspect ratio which is then determined, as the mean of the values obtained on the samples, is higher than 1.3.
    [069] With this method of fabrication, it is possible to obtain a high-resistance steel product that has a yield strength YS of more than 1,000 MPa up to 1,700 MPa and a tensile strength TS of more than 1,300 MPa up to 2,000 MPa, in together with a uniform elongation UE of at least 7% and a total elongation TE of at least 10%, where the product TS X TE is higher than 18,000% in MPa and the product TS x UE is higher than 13,000% in MPa.
    [070] In fact, even if the sudden cooling to QT temperature, followed by the retention step at PT temperature, results in a decrease in the percentage of martensite surface in the steel microstructure, which could lead to a decrease in tensile strength TS, this treatment increases the ductility of martensite through structure refinement, ensures the absence of carbide precipitates and leads to the formation of carbon-enriched austenite, so that this treatment results in an increase in the yield strength YS, resistance to TS traction and uniform and total elongations.
    [071] According to a first embodiment of the invention, the manufacturing method is performed to manufacture a steel sheet.
    [072] According to this first embodiment, the heated starting product is a hot rolled steel sheet with a composition according to the invention and the hot forming step is a hot rolling step.
    [073] The step of providing a heated starting product with a fully austenitic structure comprises providing a semi-product with a composition according to the invention, heating the semi-product to a temperature T1 higher than the AC3 temperature of the steel in order to obtain a completely austenitic structure and subjecting the semi-product to a rough rolling step, with an accumulated reduction strain εa greater than 1, in order to obtain said hot rolled steel sheet.
    [074] The semi-product is obtained by molding a steel with a composition according to the invention. The molding can be carried out in the form of ingots or continuous molding plates, with a thickness of approximately 200 mm. The molding can also be carried out in order to obtain thin slabs with a thickness of a few tens of millimeters, for example between 50 mm and 80 mm.
    [075] The semi-product is subjected to a total austenitization by heating at a temperature T1 that comprises between 1,050 and 1,250 °C, for a duration t1 sufficient to allow a complete austenitization. The temperature T1 is therefore above the temperature AC3 at which the transformation from ferrite to austenite is completed under heating. This heating therefore results in a complete austenitization of the steel and the dissolution of Nb carbonitrides that may be present in the starting product. In addition, the temperature T1 is high enough to allow a subsequent rough rolling step above Ar3 to be carried out.
    [076] The semi-product is then subjected to a rough rolling at a temperature that comprises between 1,200 °C and 850 °C, with a finishing rolling temperature T2 above Ar3, so that the steel structure remains completely austenitic at this stage .
    [077] The accumulated strain εa of the rough rolling is greater than 1. When indicating by ti the thickness of the semiproduct before the rough rolling and by tf_a thickness of the semiproduct after the end of the rough rolling, εa is
    calculated through:
    [078] Therefore the average austenitic grain size obtained is less than 30 µm. At this stage, this average austenitic grain size can be measured by tests in which the steel specimen is directly quenched after the rough rolling step. The sample is then cut along a direction parallel to a rolling direction to obtain a cut surface. The cut surface is polished and etched with a reagent known to a person skilled in the art, eg a Béchet-Beaujard reagent, which reveals the ancient austenitic grain boundaries.
    [079] The hot rolled sheet is then cooled down to a temperature T3 that comprises between 380 °C and 700 °C, at a cooling rate VR1 greater than 2 °C/s, in order to avoid austenite transformation.
    [080] The hot rolled sheet is then subjected to a final hot rolling step with an accumulated reduction strain εb comprising between 0.1 and 0.7. The final hot rolling is carried out in the temperature range between 380 °C and 700 °C.
    [081] Therefore, the hot rolled steel sheet obtained has a structure that still consists of austenite, with an austenitic grain size of less than 30 μm, preferably less than 10 μm. Therefore, the hot rolled sheet is subjected to ausforming.
    [082] The hot rolled steel sheet is then cooled to a cooling rate VR2 greater than the critical martensitic cooling rate, to a sharp cooling temperature QT in order to obtain a percentage of martensite surface comprising between 40% and 90%, the rest being unprocessed austenite. The temperature QT is preferably below Ms-20°C and above Mf, for example, comprises between 100°C and 350°C. Without further cooling, the sheet is then held immediately or reheated from temperature QT to a holding temperature PT that comprises between QT and 470 °C, and held at temperature PT for a duration of Pt, where Pt comprises between 5 s and 600 s. During this retention step, carbon splits between martensite and austenite, that is, it diffuses from martensite to austenite without creating carbides. The degree of division increases with the duration of the retention step. Therefore, the duration is chosen to be long enough to provide the most complete division possible. However, too long a duration can cause austenite to decompose and martensite to split too high and thus a reduction in mechanical properties. Therefore, the duration is limited in order to avoid the formation of ferrite as much as possible. The sheet is finally cooled to room temperature at a cooling rate greater than 0.005 °C/s in order to obtain 5% to 30% of fresh martensite and thus obtain a retained austenite surface percentage of 5% to 35%.
    [083] According to a second embodiment of the invention, the fabrication method is performed to fabricate a steel part.
    [084] According to this second embodiment, the starting product is a steel blank with a composition according to the invention.
    [085] The step of providing a heated starting product comprises providing a steel blank with a composition according to the invention and heating the steel blank to a temperature higher than the AC3 temperature of the steel, so as to obtain a completely austenitic structure.
    [086] The steel blank has a thickness between 1.0 mm and 4.0 mm, for example.
    [087] This steel blank is obtained by cutting a sheet of steel or by rolling it to a shape related to the desired final geometry of the part to be produced.
    [088] This steel blank can be uncoated or optionally pre-coated. The precoat can be aluminum or an aluminum-based alloy. In the latter case, the precoat can be obtained by dipping the board into a bath of Si-Al alloy, which comprises, by weight, from 5% to 11% Si, from 2% to 4% Fe, optionally, from 15ppm to 30ppm of Ca, with the remainder consisting of Al and impurities resulting from the smelting.
    [089] The precoat can also be zinc or a zinc based alloy. Precoating can be achieved by continuous hot dip galvanizing or post-galvanizing annealing.
    [090] The steel blank is first heated to a temperature T1 above the steel temperature Ac3, preferably between 900 °C and 950 °C, at a heating rate, for example, higher than 2 °C/s , in order to obtain a completely austenitic structure. The blank is kept at temperature T1 in order to obtain a homogeneous temperature inside the blank. Depending on the thickness of the blank, which comprises between 1.0 mm and 4.0 mm, the retention time at temperature T1 is from 3 minutes to 10 minutes.
    [091] This heating step, which is preferably carried out in an oven, results in a complete austenitization of the steel.
    [092] The heated steel blank is then extracted from the furnace, transferred into a hot forming device, eg a hot stamping press, and cooled to a temperature T3 between 380 °C and 700 ° C, at a cooling rate VR1 greater than 2 °C/s, in order to avoid an austenite transformation. The blank transfer can be conducted before or after the blank has cooled to temperature T3. In any case, this transfer needs to be fast enough to avoid austenite transformation. The steel blank is then subjected to a hot forming step in the temperature range comprising between 380 °C and 700 °C, for example, comprising between 450 °C and 550 °C, in order to allow hardening of the austenite, to avoid recrystallization of the deformed austenite and to avoid transformation of the austenite during the hot forming step. Therefore, this hot forming step is carried out through ausforming.
    [093] Deformation can be performed by methods such as hot rolling, or hot stamping in a press, roll forming or hot drawing.
    [094] The hot forming step can be carried out in one or several stages. The blank is deformed with a deformation εb comprising between 0.1 and 0.7 at at least one location in the blank.
    [095] According to an embodiment, the deformation mode is chosen so that the accumulated deformation εb comprises between 0.1 and 0.7 in the entire blank.
    [096] Optionally, the deformation is conducted so that this condition is only satisfied in some particular places of the blank, which correspond to the most pressured places, being that, particularly, high mechanical properties are desired. The location of the thus deformed blank represents at least 20% of the blank volume, so as to obtain a significant increase in mechanical properties in the final piece.
    [097] According to this realization, a product with different mechanical properties from one place to another of the part is obtained.
    [098] Therefore, the steel part obtained, in the places subjected to the hot forming step, has a structure consisting of austenite, with an austenitic grain size of less than 30 μm, preferably less than 10 μm.
    [099] The steel part thus obtained is then cooled to a cooling rate VR2 greater than the critical martensitic cooling rate, to a sudden cooling temperature QT, preferably below Ms-20 °C, for example, which it comprises between 100 °C and 350 °C, in order to obtain a percentage of martensite surface that comprises between 40% and 90%, the rest being austenite.
    [0100] The steel part is then reheated to or maintained at a retention temperature PT that comprises between QT and 470 °C, and maintained at the PT temperature for a duration Pt, where Pt comprises between 5 s and 600 s.
    [0101] The piece is finally cooled to room temperature at a cooling rate greater than 0.005 °C/s in order to obtain 5% to 30% of fresh martensite and to have 5% to 35% of retained austenite.
    [0102] By way of example and comparison, sheets produced from steels having the compositions that are reported in Table I were produced by various manufacturing methods. EXAMPLES TABLE I STEEL COMPOSITIONS

    [0103] A first series of sheets (Tests 1 to 7 in Tables II and III) was produced according to the first embodiment of the invention, heating semi-products with the above compositions at a temperature T1 for a duration of t1, then submitting the semi-product is heated to a rough rolling mill at a temperature T2 between 1,200 °C and 850 °C, with an accumulated reduction strain of 2.
    [0104] The sheets were then cooled to a temperature T3, at a cooling rate VR1 greater than 20 °C/s, then subjected to a final hot rolling step, starting at said temperature T3 and ending at a temperature T3', with an accumulated reducing strain εb.
    [0105] The sheets were then cooled to a QT temperature, then immediately reheated to a PT holding temperature and held at PT temperature for a Pt duration (Tests 3 to 6 in Table II below).
    [0106] The sheets were finally cooled to room temperature at a cooling rate greater than 0.1 °C/s.
    [0107] A second series of sheets (Tests 8 to 14 in Tables II and III) was produced according to the second realization.
    [0108] Steel blanks with the given compositions, in this case, steel sheets with a thickness of 3 mm, were heated to a temperature T1, at a heating rate greater than 2 °C/s and kept at a temperature T1 by a duration t1.
    [0109] The steel blanks were then cooled to a temperature T3, at a cooling rate VR1 greater than 2 °C/s, then subjected to a hot forming step, starting at said temperature T3 and which ends at a temperature T3', with an accumulated reducing strain εb. Under the conditions of the invention, the hot formed sheets were still completely austenitic after this hot forming step.
    [0110] The sheets were then cooled to a QT temperature, then reheated to a PT holding temperature and held at PT temperature for a Pt duration.
    [0111] The sheets were finally cooled to room temperature at a cooling rate greater than 0.1 °C/s.
    [0112] For comparative purposes, a third series of sheets was manufactured using manufacturing processes that are not in accordance with the invention (Tests 15 to 18 in Tables II and III).
    [0113] The fabrication methods of Tests 15 and 17 differ from the fabrication methods used for the first and second series of examples in that they do not include a hot forming step at a temperature comprising between 700 °C and 380 °C.
    [0114] The manufacturing methods of Tests 16 and 18 differ from the manufacturing methods used for the first and second series of examples in that the sheets were cooled to room temperature immediately after the final lamination step, without any retention step, that is, without any “sudden cooling and splitting” step.
    [0115] The manufacturing parameters for the first, second and third series of sheets are reported in Table II and the structures and mechanical properties obtained are reported in Table III.


    na: not determined
    [0116] The microstructures of steel according to examples 1 to 13 comprise more than 40% of tempered martensite or carbide-free ferrite slats, 5 to 30% of islands or fresh martensite film and between 5 and 35% austenite. The microstructures of steel according to examples 1 to 13 are such that the martensite slats have an average size of less than 1 µm and the aspect ratio of the martensite slats comprises between 2 and 5. In addition, the aspect ratio of the previous austenitic grain is higher than 1.3 for examples 1 to 13.
    [0117] These examples have a yield stress YS that comprises between 1,000 MPa and 1,700 MPa, a tensile strength TS that comprises between 1,300 MPa and 2,000 MPa, a uniform elongation greater than 7%, an overall elongation greater than 10 %, one product (tensile strength x total elongation) greater than 18,000% in MPa and one product (tensile strength x uniform elongation) greater than 13,000% in MPa.
    [0118] Tests 11, 17 and 18 have the same composition. Test 11 was obtained by a manufacturing method according to the invention, which comprises both a hot forming step at a temperature comprising between 700 °C and 380 °C and a retention step, while Test 17 was obtained with a manufacturing method that does not comprise any hot forming step at a temperature that comprises between 700 °C and 380 °C and Test 18 was obtained with a manufacturing method that does not comprise any retention step that allows division of carbon into martensite.
    [0119] In other words: - Test 11, according to the invention, comprises an ausforming and a step of "sudden cooling and division"; - Test 17, which is not in accordance with the invention, comprises only one step of "sudden cooling and splitting", without ausforming; - Test 18, which is not in accordance with the invention, comprises only one ausforming step, without a “sudden cooling and splitting” step.
    [0120] Figures 1, 2 and 3 show a comparison of the structure of Tests 11, 17 and 18, respectively. In these Figures, austenite (A) appears as white or completely light gray zones, fresh martensite (M) appears as light gray zones, and temperate martensite (Mt) appears as dark gray zones with or without small particles representing carbides. MA refers to austenite/martensite islands.
    [0121] Comparison of the structure of Test 11 (illustrated in Figure 1) with the structure of Test 17 (illustrated in Figure 2) shows that the combination of a hot forming step at a temperature comprising between 700 °C and 380 °C and a retention step at a PT temperature between QT and 470 °C, according to the invention, provides a finer and more homogeneous structure than a method comprising a retention step, but no shaping step at hot at a temperature of between 700 °C and 380 °C.
    [0122] The structure of Test 18, illustrated in Figure 3, comprises essentially fresh martensite. This result shows that in the absence of a retention step that allows the breakdown of carbon into martensite, the austenite turns almost entirely into fresh martensite upon cooling.
    [0123] The consequences of these differences in structures on the mechanical properties of sheets are emphasized by comparing the mechanical properties of Tests 3, 9, 15 and 16.
    [0124] Similar to Tests 11, 17 and 18, Tests 3, 9, 15 and 16 have the same composition, and were obtained by various manufacturing methods.
    [0125] Tests 3 and 9 were obtained by a manufacturing method according to the invention, which comprises both a hot forming step at a temperature comprising between 700 °C and 380 °C as a retention step. Tests 3 and 9 both had a yield strength higher than 100 MPa, a tensile strength higher than 1600 MPa, a uniform elongation higher than 7%, an overall elongation higher than 10% and a tensile strength product. tensile x total elongation greater than 18,000% in MPa.
    [0126] On the contrary, Test 15 was obtained with a manufacturing method that does not comprise any hot forming step at a temperature that comprises between 380 °C and 700 °C. Test 15, although it has good elongation properties, has an insufficient tensile strength, which is much less than 1,600 MPa, so that the tensile strength x total elongation product of the same is less than 18,000% in MPa and the product of tensile strength x uniform elongation thereof is less than 13,000% in MPa. In particular, due to the absence of a hot forming step at a temperature comprising between 380 °C and 700 °C during the fabrication of Test 15, the microstructure of Test 15 does not have martensite slats that have an average size of minus than 1 µm and an aspect ratio between 2 and 5.
    [0127] Additionally, Test 16, obtained with a manufacturing method that does not comprise any retention step that allows division of carbon into martensite, although it has a high yield strength and tensile strength, has insufficient uniform and total elongations, so that the tensile strength x total elongation product thereof is much less than 18,000% MPa and the tensile strength x uniform elongation product thereof is much less than 13,000% MPa.
    [0128] These examples show that, surprisingly, applying both a hot forming step at a temperature that comprises between 700 °C and 380 °C and a retention step leads to a better doubling of ductility and resistibility properties than average elongations and resistivities obtained with a hot forming step at a temperature comprising between 380 °C and 700 °C or a retention step.
    [0129] This effect is illustrated in Figures 4, 5 and 6.
    [0130] Figure 4 is a graph that represents the total TE elongation of Tests 3, 9, 15 and 16 as a function of their TS tensile strength. The domain of the invention is delimited by lines L1 (TS = 1300 MPa), L2 (TS = 2000 MPa), L3 (TE = 10%) and L4 (TS x TE = 18,000% in MPa).
    [0131] Figure 4 shows that the double total elongation/tensile strength obtained by a manufacturing method according to the invention, which comprises both a hot forming step at a temperature comprising between 700 °C and 380 °C as a retention step, it is much better than the double full elongation/tensile strength obtained by a manufacturing method that comprises only a hot rolling step at a temperature comprising between 700 °C and 380 °C (Test 15) and the total elongation/tensile strength obtained by a manufacturing method comprising only one retention step (Test 16). This total elongation/intermediate yield point is illustrated in Figure 4 by line I1.
    [0132] Additionally, these results show that, surprisingly, the method according to the invention provides a tensile strength x total elongation product higher than 18,000% in MPa, while such a high value is not obtained along line I1.
    [0133] Figure 5 is a graph that represents the uniform UE elongation of Tests 3, 9, 15 and 16 as a function of their yield strength YS. The domain of the invention is delimited by lines L5 (YS = 1,000 MPa), L6 (YS = 1,700 MPa) and L7 (UE = 7%).
    [0134] Similar to Figure 4, Figure 5 shows that the uniform elongation and yield point obtained by a manufacturing method according to the invention are much better than the uniform elongation/ yield point obtained by a method of fabrication that comprises only one retention step (Test 16).
    [0135] Figure 6 is a graph that represents the uniform UE elongation of Tests 3, 9, 15 and 16 as a function of their TS tensile strength. The domain of the invention is delimited by lines L8 (TS = 1300 MPa), L9 (TS = 2000 MPa), L10 (UE = 7%) and L11 (TS x UE = 13,000% in MPa).
    [0136] Figure 6 shows that the double uniform elongation/tensile strength obtained by a manufacturing method according to the invention, which comprises both a hot forming step at a temperature comprising between 700 °C and 380 °C as a retention step, it is much better than the double full elongation/tensile strength obtained by a manufacturing method that comprises only a hot rolling step at a temperature comprising between 700 °C and 380 °C (Test 15) and the total elongation/tensile strength obtained by a manufacturing method comprising only one retention step (Test 16). This intermediate elasticity limit/uniform elongation is illustrated in Figure 6 by line I2.
    [0137] Additionally, these results show that, surprisingly, the method according to the invention provides a product of tensile strength x uniform elongation higher than 13,000% in MPa, while such a high value is not obtained along the line I2.
    [0138] Therefore, the sheets or parts produced can be used to manufacture automotive parts, such as front or rear crossmembers, columns, bumper beams.
    权利要求:
    Claims (27)
    [0001]
    1. METHOD FOR MANUFACTURING A STEEL PRODUCT characterized by comprising the successive steps of: - providing a steel starting product heated to a temperature that comprises between 380°C and 700°C, the heated steel starting product having a fully austenitic metastable structure, the heated steel starting product having a composition comprising, by weight percentage: 0.15% < C < 0.40%, 1.5% < Mn < 4.0%, 0.5 < Si < 2.5%, 0.005% < Al < 1.5%, with 0.8% < Si+Al < 2.5%, S < 0.05%, P < 0.1%, at least one element chosen from Cr and Mo, so that: 0% < Cr < 4.0%, 0% < Mo < 0.5%, and 2.7% < Mn + Cr + 3 Mo < 5.7 % where the balance of the composition consists of iron and unavoidable impurities resulting from the casting, - subjecting the heated steel starting product to a hot forming step at a temperature comprising between 700 °C and 380 °C, with a accumulated deformation εb between 0.1 and 0.7, in at least one location of the hot steel starting product to obtain a hot-formed steel product, with the steel structure remaining completely austenitic, then - abruptly cool the hot-formed steel product by cooling it, at a cooling rate VR2 higher than the cooling rate critical martensite, up to a quench temperature QT lower than the starting temperature of martensite Ms of steel in order to obtain a structure containing between 40% and 90% martensite, the rest of the structure being austenite, then - keep in , or reheat the quenched hot-formed product to a PT holding temperature between QT and 470 °C and retain the quenched hot-formed product at the PT holding temperature for a Pt duration of between 5 s and 600 s.
    [0002]
    2. METHOD, according to claim 1, characterized in that the composition additionally comprises one or several elements chosen from: Nb < 0.1%, Ti < 0.1%, Ni < 3.0%, 0.0005% < B < 0.005%, 0.0005% < Ca < 0.005%.
    [0003]
    3. METHOD, according to any one of claims 1 to 2, characterized in that it additionally comprises, after retention, a step of cooling the product to room temperature at a cooling rate greater than 0.005°C/s in order to obtain fresh martensite.
    [0004]
    4. METHOD according to any one of claims 1 to 3, characterized in that the heated steel starting product is a hot rolled steel sheet and the steel product is a steel sheet, and in which the shaping step is hot is a lamination step.
    [0005]
    5. METHOD according to claim 4, characterized in that the step of supplying the heated steel starting product comprises: - heating a steel semi-product with the composition to a temperature higher than the AC3 temperature of the steel in order to obtain a completely austenitic structure, - subjecting the steel semi-product to a rough rolling step at a temperature above a temperature T2 between 1200 and 850 °C, with an accumulated reduction strain εa greater than 1, in order to obtain the starting product of heated steel.
    [0006]
    6. METHOD according to claim 5, characterized in that the heated steel starting product has an average austenitic grain size of less than 30 µm.
    [0007]
    7. METHOD, according to any one of claims 1 to 3, characterized in that the heated metal starting product is a heated raw steel piece, the steel product being a steel piece and the step of supplying the product of Heated steel starting comprises heating a steel blank to a temperature higher than the AC3 temperature of the steel, so as to obtain a completely austenitic structure.
    [0008]
    8. METHOD, according to claim 7, characterized in that the steel blank has a thickness between 1.0 mm and 4.0 mm.
    [0009]
    9. METHOD, according to any one of claims 7 to 8, characterized in that the hot forming step is a hot rolling step.
    [0010]
    10. METHOD according to any one of claims 7 to 8, characterized in that the hot forming step is a hot stamping step.
    [0011]
    11. METHOD, according to any one of claims 7 to 8, characterized in that the hot forming step is a hot drawing step.
    [0012]
    12. METHOD, according to any one of claims 7 to 8, characterized in that the hot forming step is a roll forming step.
    [0013]
    13. METHOD according to any one of claims 7 to 12, characterized in that the steel blank comprises at least one coating layer.
    [0014]
    14. METHOD according to claim 13, characterized in that the coating layer is applied to the steel blank before heating and in which the coating layer is aluminum coating or based on aluminum, or zinc coating or with base in zinc.
    [0015]
    15. STEEL PRODUCT characterized by having a composition comprising, in percentage by weight: 0.15% < C < 0.40 %, 1.5% < Mn < 4.0 %, 0.5 < Si < 2, 5%, 0.005% < Al < 1.5%, with 0.8% < Si+Al < 2.5%, S < 0.05%, P < 0.1%, at least one element chosen from Cr and Mo, so that: 0% < Cr < 4.0%, 0% < Mo < 0.5%, and 2.7% < Mn + Cr + 3 Mo < 5.7% and the composition balance consists of in iron and unavoidable impurities resulting from the casting, the structure of at least one location of the steel product being: - hardened martensite or carbide-free martensite slats, with a surface percentage of at least 40%, with the martensite slats have an average size of less than 1 μm, the aspect ratio of martensite slats being between 2 and 5, - fresh martensite, in the form of islands or films, where the percentage of fresh martensite surface comprises between 5% and 30%, and - austenite, with a surface percentage of 5% to 35%.
    [0016]
    16. STEEL PRODUCT according to claim 15, characterized in that the composition additionally comprises one or several elements chosen from: Nb < 0.1%, Ti < 0.1%, Ni < 3.0%, 0.0005% < B < 0.005%, 0.0005% < Ca < 0.005%.
    [0017]
    17. STEEL PRODUCT according to any one of claims 15 to 16, characterized in that the product of the tensile strength TS of the steel by the uniform EU elongation of the steel is greater than or equal to 13,000% in MPa.
    [0018]
    18. STEEL PRODUCT according to any one of claims 15 to 17, characterized in that the maximum size of the islands of fresh martensite with an aspect ratio of less than 3 is less than 3 µm.
    [0019]
    19. STEEL PRODUCT, according to any one of claims 15 to 18, characterized in that the average size of the previous austenitic grains is less than 30 µm.
    [0020]
    20. STEEL PRODUCT, according to any one of claims 15 to 19, characterized in that the aspect ratio of the previous austenitic grains is higher than 1.3.
    [0021]
    21. STEEL PRODUCT, according to any one of claims 15 to 20, characterized in that the austenite is in the form of films or islands, with the smallest dimension of the films or islands having a value of less than 0.3 µm, with the the largest dimension of the films or islands has an average value of less than 2 µm.
    [0022]
    22. STEEL PRODUCT according to any one of claims 15 to 21, characterized in that the tempered martensite comprises, as a percentage of surface, less than 0.5% of carbides, compared to the surface of the tempered martensite, and in which the carbides have an average size of less than 50 nm.
    [0023]
    23. STEEL PRODUCT according to any one of claims 15 to 22, characterized in that the steel product is a steel sheet and the structure of the entire steel sheet consists of: - hardened martensite or carbide-free martensite slats, with a surface percentage of at least 40%, - fresh martensite, in the form of islands or films, where the surface percentage of the fresh martensite comprises between 5% and 30%, and - austenite, with a surface percentage of 5% to 35%.
    [0024]
    24. STEEL PRODUCT, according to any one of claims 15 to 22, characterized in that the steel product is a hot-stamped steel part and the structure of at least 20% of the volume of the hot-stamped part consists of: - martensite hardened or carbide-free martensite slats, with a surface percentage of at least 40%, - fresh martensite, in the form of islands or films, where the surface percentage of fresh martensite comprises between 5% and 30%, and - austenite , with a surface percentage of 5% to 35%.
    [0025]
    25. STEEL PRODUCT according to claim 24, characterized in that the steel product comprises at least one coating layer.
    [0026]
    26. STEEL PRODUCT according to claim 25, characterized in that the coating layer is zinc or zinc-based alloy, or aluminum alloy or aluminum-based.
    [0027]
    27. STEEL PRODUCT according to claim 26, characterized in that the coating layer is applied before hot stamping.
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    EP3221476B1|2020-05-13|
    EP3221476A1|2017-09-27|
    JP2018500465A|2018-01-11|
    BR112017010093A2|2018-02-14|
    CN107002155B|2019-06-11|
    HUE050200T2|2020-11-30|
    CN110144439A|2019-08-20|
    KR20170086035A|2017-07-25|
    PL3221476T3|2020-11-02|
    WO2016079675A1|2016-05-26|
    CN110218845B|2021-12-17|
    CN107002155A|2017-08-01|
    RU2017117067A|2018-11-21|
    WO2016079565A1|2016-05-26|
    RU2017117067A3|2019-03-26|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    DE4219336C2|1992-06-10|1995-10-12|Mannesmann Ag|Use of a steel to manufacture construction pipes|
    JPH09111408A|1995-10-11|1997-04-28|Toa Steel Co Ltd|Low strain type carburized and quenched steel stock for gear|
    JP3757537B2|1997-05-01|2006-03-22|大同特殊鋼株式会社|Manufacturing method of connecting rod|
    US6254698B1|1997-12-19|2001-07-03|Exxonmobile Upstream Research Company|Ultra-high strength ausaged steels with excellent cryogenic temperature toughness and method of making thereof|
    US20070068607A1|2005-09-29|2007-03-29|Huff Philip A|Method for heat treating thick-walled forgings|
    PL2086755T3|2006-10-30|2018-05-30|Arcelormittal|Coated steel strips, methods of making the same, methods of using the same, stamping blanks prepared from the same, stamped products prepared from the same, and articles of manufacture which contain such a stamped product|
    DE102006053819A1|2006-11-14|2008-05-15|Thyssenkrupp Steel Ag|Production of a steel component used in the chassis construction comprises heating a sheet metal part and hot press quenching the heated sheet metal part|
    JP5214905B2|2007-04-17|2013-06-19|株式会社中山製鋼所|High strength hot rolled steel sheet and method for producing the same|
    CN100560772C|2007-04-24|2009-11-18|西安交通大学|The preparation method of granule carbonide reinforced ferritic steel|
    WO2009090443A1|2008-01-15|2009-07-23|Arcelormittal France|Process for manufacturing stamped products, and stamped products prepared from the same|
    JP5327106B2|2010-03-09|2013-10-30|Ｊｆｅスチール株式会社|Press member and manufacturing method thereof|
    KR101253885B1|2010-12-27|2013-04-16|주식회사 포스코|Steel sheet fir formed member, formed member having excellent ductility and method for manufacturing the same|
    CN102080192B|2011-01-07|2012-07-25|南京钢铁股份有限公司|Low-yield ratio, high-plasticity, ultrafine-grain and high-strength steel and manufacturing method thereof|
    WO2012153008A1|2011-05-12|2012-11-15|Arcelormittal Investigación Y Desarrollo Sl|Method for the production of very-high-strength martensitic steel and sheet or part thus obtained|
    EP2524970A1|2011-05-18|2012-11-21|ThyssenKrupp Steel Europe AG|Extremely stable steel flat product and method for its production|
    FI20115702A|2011-07-01|2013-01-02|Rautaruukki Oyj|PROCEDURE FOR MANUFACTURING HIGH-STRENGTH STRUCTURAL STEEL AND HIGH-STRENGTH STRUCTURAL STEEL PRODUCT|
    WO2014037627A1|2012-09-06|2014-03-13|Arcelormittal Investigación Y Desarrollo Sl|Process for manufacturing press-hardened coated steel parts and precoated sheets allowing these parts to be manufactured|
    CN102943169B|2012-12-10|2015-01-07|北京科技大学|Quenching and annealing preparation method of ultrahigh-strength thin steel plate for automobiles|
    MX368319B|2013-01-18|2019-09-27|Kobe Steel Ltd|Manufacturing method for hot press formed steel member.|
    CN104630647B|2015-02-02|2016-08-31|大连理工大学|A kind of preparation method of high-strength hot-dip zinc-coated Q ＆ P steel|
    CN104674119B|2015-02-10|2017-08-11|广东坚宜佳五金制品有限公司|The preparation method and high strength steel of high strength steel|US10619223B2|2016-04-28|2020-04-14|GM Global Technology Operations LLC|Zinc-coated hot formed steel component with tailored property|
    WO2018115914A1|2016-12-19|2018-06-28|Arcelormittal|A manufacturing process of hot press formed aluminized steel parts|
    WO2018134874A1|2017-01-17|2018-07-26|新日鐵住金株式会社|Hot stamp molded body and method for producing same|
    WO2018215813A1|2017-05-22|2018-11-29|Arcelormittal|Method for producing a steel part and corresponding steel part|
    US20210087661A1|2017-12-28|2021-03-25|GM Global Technology Operations LLC|Steel for hot stamping with enhanced oxidation resistance|
    EP3775311A1|2018-03-30|2021-02-17|AK Steel Properties, Inc.|Low alloy third generation advanced high strength steel and process for making|
    WO2019222950A1|2018-05-24|2019-11-28|GM Global Technology Operations LLC|A method for improving both strength and ductility of a press-hardening steel|
    CN108754319B|2018-06-08|2020-08-04|武汉钢铁有限公司|Hot forming steel with tensile strength of more than or equal to 1800MPa produced by ESP production line and method|
    CN112534078A|2018-06-19|2021-03-19|通用汽车环球科技运作有限责任公司|Low density press hardened steel with enhanced mechanical properties|
    US20220056543A1|2018-09-20|2022-02-24|Arcelormittal|Hot rolled steel sheet with high hole expansion ratio and manufacturing process thereof|
    DE102019201883A1|2019-02-13|2020-08-13|Thyssenkrupp Steel Europe Ag|Method for producing a sheet steel component|
    CN110129670B|2019-04-25|2020-12-15|首钢集团有限公司|1300 MPa-grade high-strength high-plasticity steel for hot stamping and preparation method thereof|
    CN112251679A|2020-09-18|2021-01-22|东南大学|Double-phase high-strength steel and preparation method thereof|
    CN112575256B|2020-11-26|2021-12-31|博耀能源科技有限公司|High-strength and high-toughness large-diameter wind power bolt with shell/horse complex phase structure and preparation method thereof|
    法律状态:
    2019-10-01| B06U| Preliminary requirement: requests with searches performed by other patent offices: procedure suspended [chapter 6.21 patent gazette]|
    2021-04-06| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-05-18| B16A| Patent or certificate of addition of invention granted [chapter 16.1 patent gazette]|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 17/11/2015, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    PCT/IB2014/066128|WO2016079565A1|2014-11-18|2014-11-18|Method for manufacturing a high strength steel product and steel product thereby obtained|
    IBPCT/IB2014/066128|2014-11-18|
    PCT/IB2015/058887|WO2016079675A1|2014-11-18|2015-11-17|Method for manufacturing a high strength steel product and steel product thereby obtained|
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