Method for manufacturing a hot stamped part and associated hot stamped part

Abstract

Method for manufacturing a hot stamped part and hot stamped, wherein the hot stamped part is manufactured by hot stamping a steel sheet comprising C : 0.10 - 0.4 %, Mn : 1.0 – 11 %, Si : 0.2 – 2.0 %, Cr ≤ 2.0%, Mo ≤ 0.5 %, Nb ≤ 0.1 %, Al ≤ 0.10 %, Ti : 0.01 - 0.1 %, B : 0.0005 - 0.005 %, P ≤ 0.02 %, S ≤ 0.04 %, N ≤ 0.01 %, Ni ≤ 1.0%, Cu ≤ 1.0%, heating said steel sheet above Ac3, stamping said steel sheet above MS-start, the temperature at which martensite starts to form for said steel sheet, applying to said sheet a deformation, expressed in equivalent plastic strain, which does not exceed 1.0 and which locally at least is greater than or equal to 0.4.

Description

[0001] Method for manufacturing a hot stamped part and associated hot stamped part

[0002] The present invention relates to a method for manufacturing a hot stamped part and to the associated hot stamped part.

[0003] Hot stamping is a forming technology for steel which involves heating a blank of steel, or a preformed part made from a blank of steel, up to a temperature at which the microstructure of the steel has at least partially transformed to austenite, forming the blank or preformed part at high temperature by stamping it and simultaneously quenching the formed part to obtain a microstructure having a very high strength, possibly with an additional partitioning or tempering step in the heat treatment.

[0004] In the current description and claims, the term hot stamping should be understood as comprising also complex hot stamping processes during which for example several separate stamping operations are performed, or during which for example hot trimming is performed.

[0005] Hot stamping allows to obtain parts having very high mechanical resistance and complex shapes. As such, it addresses the challenges of producing vehicles which are safer and more environmentally friendly in their production process and during their entire life cycle.

[0006] The current trend is to apply hot stamping to a growing number of parts and to ever more complex part shapes. To do so, hot stampers are devising ever more complex tools and hot stamping processes.

[0007] For example, it is known to trim the edges of the part or punch holes in the part during the hot stamping process - an operation known as hot trimming or in-die trimming, which involves mechanically cutting a steel sheet at high temperature. This allows to produce part which are ready to use directly through the hot stamping process, without costly post processing steps, such as laser trimming.

[0008] For example, it is known to apply a hot stamping process in which there are several process stages to stamp the parts according to different directions in order to produce complex shapes that could not be reached in a one step process.

[0009] These developments in hot stamping generally go in the direction of increasing the applied deformations to the steel sheet, to reach more complex shapes or to perform in die trimming, and of increasing the temperature range over which said steel sheet is deformed, to accommodate for the more complex and therefore lengthy stamping processes.

[0010] The thus obtained hot stamped part needs to retain high and homogeneous mechanical properties despite the increased deformation and process temperature ranges.

[0011] It is a purpose of the current invention to address this issue by providing a hot stamping process allowing for high deformations of the part and a wide deformation temperature range, while preserving high and stable mechanical properties of the hot stamped part.

[0012] A further purpose of the current invention is to provide a hot stamped part which has been manufactured using a large deformation range and retains high and stable mechanical properties.

[0013] A steel sheet refers to a flat sheet of steel having a top and a bottom face, which are also referred to as a top and bottom side or as a top and bottom surface. The distance between said faces is designated as the thickness of the sheet. The thickness can be measured for example using a micrometer, the spindle and anvil of which are placed on the top and bottom faces. In a similar way, the thickness can also be measured on a formed part.

[0014] Hardness is a measure of the resistance to localized plastic deformation induced by mechanical indentation. It is well correlated to the mechanical properties of a material and is a useful local measurement method which does not require to cut out a sample for tensile testing. In the current invention, the hardness measurements are made using a Vickers indenter according to standard ISO 6507- 1 . The Vickers hardness is expressed using the unit Hv1 .

[0015] The composition of the steel according to the invention will now be described, the content being expressed in weight percent. The chemical compositions are given in terms of a lower and upper limit of the composition range, said limits being themselves included within the possible composition range according to the invention.

[0016] According to the invention the carbon ranges from 0.10% to 0.4% to ensure a satisfactory strength. Above 0.4% of carbon, weldability and bendability of the steel sheet may be reduced. If the carbon content is lower than 0.10%, the tensile strength after hot stamping will not reach the targeted value. In a specific embodiment, the minimum carbon content is 0.15% in order to further guarantee the strength of the hot stamped part.

[0017] The manganese content ranges from 1 .0% to 11 %. Above 11 % of addition, the risk of central segregation increases to the detriment of processability and the risk of crack formation during hot stamping and subsequent use of the part will be increased. Below 1.0% the hardenability of the steel sheet is reduced and the required strength after hot stamping will not be reached. In a specific embodiment, the maximum manganese content is 4% in order to further limit the risk of segregation and cracking.

[0018] The silicon content ranges from 0.2% to 2.0%. Silicon is an element participating in the hardening in solid solution. Silicon is added to limit carbides formation. Above 2.0%, silicon oxides form at the surface, which impairs the coatability of the steel. Moreover, the weldability of the steel sheet may be reduced. In a specific embodiment, the minimum silicon content is 0.5% in order to further have a hardening effect.

[0019] The chromium content does not exceed 2.0%. Chromium is an element participating in the hardening in solid solution. The chromium content is limited to below 2.0% to limit processability issues and cost. In a particular embodiment, a minimum chromium content is 0.15%, preferably 0.3% in order to further ensure a hardening effect.

[0020] The molybdenum content does not exceed 0.5%. Molybdenum improves the hardenability of the steel. Molybdenum is not higher than 0.5% to limit costs.

[0021] The niobium content is limited to 0.1 %. Niobium improves ductility of the steel. Above 0.1 % the risk of formation of coarse NbC or Nb(C,N) precipitates increases to the detriment of processability.

[0022] According to the invention, the aluminum is limited to 0.10%. Aluminum is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Aluminium can protect boron if titanium content is not sufficient. The aluminium content is lower than 0.10% to avoid oxidation problems and ferrite formation during press hardening. According to the invention, the titanium content ranges from 0.01 % to 0.1 %. Titanium protects boron, which can be trapped within BN precipitates. Titanium content is limited to 0.1 % to avoid excess TiN formation.

[0023] According to the invention, the boron content ranges from 0.0005% and 0.005%. Boron improves the hardenability of the steel. The boron content is not higher than 0.005% to avoid slab breaking issues during continuous casting. In a particular embodiment, the minimum boron content is 0.001 %, in order to further increase hardenability.

[0024] Phosphorous is controlled to below 0.02%, because it leads to fragility and weldability issues.

[0025] Sulphur is controlled to below 0.04% because the presence of Sulphur in the liquid steel can lead to the formation of MnS precipitates which are detrimental to bendability.

[0026] Nitrogen is controlled to below 0.01 %, preferably below 0.004% even more preferably below 0.003%. The presence of Nitrogen can lead to the formation of precipitates such as TiN or TiNbCN, which are detrimental to the bendability.

[0027] Nickel is optionally added, up to a level of 1.0%. Nickel contributes to increasing the hardenability and strength by solid solution while simultaneously providing higher toughness to the martensite.

[0028] Copper is optionally added, up to a level of 1.0%. Copper can improve the hydrogen embrittlement cracking resistance of steel and is a strengthening element of steel. In a particular embodiment, a minimum amount of 0.15% of Copper is added. Copper is limited to 1.0% in order to limit costs, hot shortness issues and because it does not exhibit the desired strengthening and hydrogen embrittlement cracking resistance above this value.

[0029] The remainder of the composition of the steel is iron and impurities resulting from the elaboration process. The level of impurities resulting from the elaboration process will depend on the production route used. For example, when using a Blast Furnace route with a low level of scrap, the level of impurities will remain very low. On the other hand, when elaborating the steel using an electric furnace, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. In this latter processing route, for example, the level of Cu can go up to 0.25% (even if no copper is voluntarily added), Sn can go up to 0.05%, As can go up to 0.03%, Sb can go up to 0.03% and Pb can go up to 0.03%.

[0030] The chemical composition of the steel sheet according to the invention further satisfies the following formula (the elements are expressed in weight %):

[0031] Q < 25

[0032] Wherein Q = 114 - 68*C - 18*Mn + 20*Si - 56*Cr - 61 *Ni - 37*AI + 39*Mo + 79*Nb - 46 *Cu - 17691 *B

[0033] Verifying this formula ensures that the steel sheets can withstand low quenching speeds while still retaining high mechanical properties. For example, when using complex hot stamping processes, the steel sheet, or at least part of the steel sheet, can be cooled mostly by air cooling. By verifying the above formula using factor Q, it is possible to keep very high mechanical properties even when considering the areas of the part which were not quenched rapidly (e.g. at least partially air cooled) during the hot stamping process.

[0034] Preferably, the steel even has a lower factor Q, with Q < 16.

[0035] In a particular embodiment, the steel sheet composition comprises the following elements expressed in weight%:

[0036] C: 0.15% - 0.25%

[0037] Mn: 1.5% - 2.5%

[0038] Si: 0.7% - 2.0%

[0039] Cr O.5% - 1.5%

[0040] Al: 0.03% - 1 %

[0041] Ti : 0.02% - 0.1 %

[0042] B: 0.0015% - 0.0050%

[0043] P < 0.012% the remainder of the composition being iron and unavoidable impurities resulting from the smelting.

[0044] In a particular embodiment, the steel sheet composition comprises the following elements expressed in weight%: C: 0.10-0.3%

[0045] Mn: 3-4.2%

[0046] Si : 0.7 - 2%

[0047] CrO.5% - 1.5%

[0048] Al : 0.1 - 1%

[0049] Mo : 0.1 -0.5%

[0050] Nb : 0.01 - 0.05%

[0051] Ti : 0.01 - 0.05%

[0052] B : 0.001 - 0.005% the remainder of the composition being iron and unavoidable impurities resulting from the smelting.

[0053] In a particular embodiment, the steel sheet chemical composition is (in weight percent)

[0054] C : 0.15-0.25%

[0055] Mn : 0.5 -1.8%

[0056] Si : 0.1 - 1.25 %

[0057] Cr: 0.1 - 1.0 %

[0058] Al : 0.01 -0.1 %

[0059] Ti: 0.01-0.1 %

[0060] B: 0.001 - 0.004 %

[0061] P < 0.020 %

[0062] S <0.010 %

[0063] N <0.010 % and comprising optionally one or more of the following elements, by weight percent:

[0064] Mo < 0.40 %

[0065] Nb < 0.08 %

[0066] Ca<0.1 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting. In a particular embodiment, the steel sheet chemical composition is (in weight percent)

[0067] C : 0.26 - 0.40 %

[0068] Mn : 0.5 -1.8%

[0069] Si : 0.1 - 1.25 %

[0070] Cr: 0.1 - 1.0 %

[0071] Al : 0.01 -0.1 %

[0072] Ti: 0.01 -0.1 %

[0073] B: 0.001 - 0.004 %

[0074] P < 0.020 %

[0075] S <0.010 %

[0076] N <0.010 % and comprising optionally one or more of the following elements, by weight percent:

[0077] Ni < 0.5 %

[0078] Mo < 0.40 %

[0079] Nb < 0.08 %

[0080] Ca<0.1 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting.

[0081] In a particular embodiment, the steel sheet chemical composition is (in weight percent)

[0082] C: 0.24 - 0.38%

[0083] Mn: 0.40 - 3%

[0084] Si: 0.10-0.70%

[0085] Cr: 0 - 2%

[0086] Al: 0.015-0.070%

[0087] Nb < 0.060%

[0088] Ti : 0.015 -0.10%

[0089] N: 0.003-0.010%

[0090] S: 0.0001 - 0.005% P: 0.0001 - 0.025%

[0091] Ni: 0.25 - 1 %

[0092] And wherein:

[0093] Ti / N >3,42,

[0094] . Mn 2.6C + -

[0095] 5.3

[0096] And comprising optionally:

[0097] Mo: 0.05 - 0.65%

[0098] Ca: 0.0005 - 0.005%

[0099] W: 0.001 - 0.30% the remainder of the composition being iron and unavoidable impurities resulting from the smelting,

[0100] In a particular embodiment, the steel sheet chemical composition is (in weight percent)

[0101] C: 0.25% to 0.4%, Mn: 0.3% to 3.0%,

[0102] Si: 0.25% to 2.0%, P < 0.050%,

[0103] S < 0.010%,

[0104] N < 0.010%, Ti: 0.010% to 0.1 %,

[0105] B: 0.0005% to 0.050%, Cu: 0.15 to 1.0%,

[0106] Mo: 0.10 to 1.00%, Cr < 1.00%, Ni <1.00%, Al < 0.1 %, Nb < 0.10%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, In a particular embodiment, the material used to manufacture at least a portion of the high slenderness part or the entire high slenderness part is steel comprising the following elements expressed in weight% :

[0107] C: 0.10 - 0.18 %

[0108] Mn: 6.0 - 11.0 %

[0109] Mo: 0.05 - 0.5 %

[0110] B: 0.0005 - 0.005%

[0111] S < 0.010 %

[0112] P < 0.020 %

[0113] N < 0.008 % and comprising optionally one or more of the following elements, in weight percentage:

[0114] Al < 3%

[0115] Si < 1 .20 %

[0116] Ti < 0.050 %

[0117] Nb < 0.050 %

[0118] Cr < 0.5%

[0119] In a particular embodiment, the steel sheet composition comprises the following elements expressed in weight%:

[0120] C: 0.15- 0.4%

[0121] Mn: 1 - 3.5%

[0122] Si: 1.0 - 1.65%

[0123] Cr < 2%

[0124] Al < 0.5%

[0125] Ti < 0.1 %

[0126] B < 0.005% the remainder of the composition being iron and unavoidable impurities resulting from the smelting.

[0127] The method according to the invention will now be explained in further detail. The coated steel sheet to be used in the process according to the invention can be produced by any appropriate manufacturing method and the man skilled in the art can define one. It is however preferred to use the method according to the invention comprising the below described steps:

[0128] -A semi-product able to be further hot-rolled, is provided with the steel composition described above.

[0129] -The semi product is then optionally reheated at a temperature comprised from 1150°C to 1300°C.

[0130] -The steel sheet is then hot rolled at a finish hot rolling temperature comprised from 800°C to 950°C.

[0131] -The hot-rolled steel is then cooled and coiled at a temperature lower than 670°C, and optionally pickled to remove oxidation.

[0132] -The coiled steel sheet is then optionally cold rolled to obtain a cold rolled steel sheet. The cold-rolling reduction ratio preferably ranges from 20% to 80%. Below 20%, the recrystallization during subsequent heat-treatment is not favored, which may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold-rolling.

[0133] -In an embodiment of the invention the steel sheet undergoes an annealing step in an annealing furnace during which it is heated to an annealing temperature comprised from 700°C to 850°C and maintained in a soaking section of said annealing furnace at said annealing temperature for a holding time comprised from 10 seconds to 20 minutes to produce an annealed steel sheet.

[0134] -In an embodiment of the invention, said annealed steel sheet is cooled to a temperature range from 400°C to 700°C and further coated with a metallic coating.

[0135] In a particular embodiment, the steel sheet used is coated on at least one side with a metallic coating comprising at least 50% Al in weight percent. This offers both protection against scale formation during hot stamping, and corrosion protection to the part when it is in use.

[0136] In a particular embodiment the steel sheet is coated on at least one side with a metallic coating comprising at least 50% Zn in weight percent. This offers both protection against scale formation during hot stamping, and corrosion protection to the part when it is in use. In a particular embodiment the steel sheet is a tailor welded steel sheet. Tailor welded steel sheets are made by assembling together, for example by laser welding together, several sheets or cut-out blanks of steel, known as sub-blanks, in order to optimize the performance of the part in its different areas, to reduce overall part weight and to reduce overall part cost. The sub-blanks forming the tailor welded sheets can be assembled with or without overlap, for example they can be laser butt-welded (no overlap), or they can be spot-welded to one another (with overlap).

[0137] In a particular embodiment the steel sheet is a tailor rolled steel sheet. A tailor rolled steel sheet is a steel having multiple sheet thicknesses obtained by differential rolling during the steel sheet production process.

[0138] A steel sheet corresponding to the above description, the chemical compositions, optional coatings and other particular characteristics being taken alone or according to any possible combination, is first heated to above Ac3, the temperature at which it fully transforms to an austenitic microstructure. For example, the steel sheet is heated to above 870°C, preferably 900°C, preferably 930°C.

[0139] The heated steel sheet is then hot stamped. During this operation, which can comprise several sub-operations, every area of the steel sheet undergoes a local deformation, which will be expressed using the equivalent plastic strain in the current description and claims.

[0140] The equivalent plastic strain is a well-known value to mechanical engineers, used to estimate the global plastic deformation undergone by a solid. It is an invariant of the plastic strain tensor and has de following definition:

[0141] Where seqpis the equivalent plastic strain, and EP. EPis the contracted product of the plastic deformation tensor EPby itself.

[0142] The equivalent plastic strain field of a given part is calculated by all the commercially available Computer Assisted Design (CAD) softwares, such as for example Pamstamp®, Abaqus® or LS-Dyna®. Detailed definitions of said equivalent plastic strain are given in reference manuals for mechanical engineers, such as for example “Lemaitre, J., Chaboche, J., Benallal, A., Desmorat, R. (2020). Mecanique des materiaux solides - 3e ed.. France: Dunod”.

[0143] The equivalent plastic strain field of a given formed part can be determined in the following way, the procedure described below is one example of a method to determine the plastic strain field and is in no way limiting - other methods also exist: -A numerical model of the physical part is acquired using a 3D camera. The output of this first operation is a CAD file representing the physical part.

[0144] -The CAD file is then processed by a reverse forming software, such as for example Pamstamp® Onestep, which computes the deformation field which was necessary to stamp the shape of the part starting from a flat blank.

[0145] -Said deformation field is then expressed in a corresponding equivalent plastic strain field, using any of the commercial softwares listed above (for example Pamstamp®, Abaqus® or LS-Dyna®).

[0146] When the above-described method cannot be applied, for example because only a portion of the full formed part is available or in order to assess deformation very locally in specific areas such as for example on the edges, Electron Back Scattered Diffraction (EBSD) measurements in conjunction with Scanning Electron Microscope (SEM) observations can be done. It relies on the correlation which exists between deformation and local crystal misorientation. The following reference for example gives an example of such measurement: “Kamaya M. Assessment of local deformation using EBSD: quantification of accuracy of measurement and definition of local gradient. Ultramicroscopy. 2011 Jul;111 (8):1189-99. doi:

[0147] 10.1016 / j.ultramic.2O11.02.004. Epub 2011 Feb 21. PMID: 21763236.”

[0148] Another methodology that can be applied to determine the equivalent plastic strain on a formed part is to measure the thickness of the deformed material in a formed area and compare it with the thickness of an undeformed area.

[0149] In the current invention, the steel sheet is deformed during the hot stamping operation and the local equivalent plastic strain can go up to but not exceed 1.0. Above this value, the inventors have found that it was not possible to efficiently guarantee homogeneous and high mechanical properties on the resulting hot stamped part - furthermore there is a serious risk that the part might locally crack when applying such high amounts of deformation.

[0150] On the other hand, the method according to the invention provides for areas of high deformation, having a local equivalent plastic strain equal to or greater than 0.4, preferably equal to or greater than 0.5, even more preferably equal to or greater than 0.7. This allows to produce complex parts having complex shapes and also allows to perform hot trimming during the stamping process. For example, in the case of hot trimming, the inventors have found that the applied equivalent plastic strain is typically in the range of 0.7 - 0.9.

[0151] Furthermore, the equivalent plastic strain applied during hot stamping according to the invention is applied above the MS-start temperature, which is the temperature below which martensite formation starts.

[0152] For example, MS-start is computed using the following formula (wherein all the elements are expressed in weight %):

[0153] MS - start = 545 - 601.2 * (1 -e-°'868*c) - 34.4 * Mn - 9.2 * Cr - 17.3 *

[0154] Ni - 13.7 * Si - 15.4 * Mo - 1.4 * Al - 16.3 * Cu - 361 * Nb - 2.44 * Ti - 3448 * B

[0155] Preferably, the deformation is applied above 500°C, preferably above 700°C. The inventors have found that it was not possible to guarantee stable and high mechanical properties on the entire hot stamped part when deforming it at a temperature which is too low. Indeed, when the forming temperature is too low, the material is significantly harder and less ductile, which means that it is either not possible to reach the desired amount of deformation on the part or that imposing very high deformation on the parts results in failures such as cracks.

[0156] In a particular embodiment, the hot stamping process according to the invention involves applying a deformation rate with important differences according to the area of the steel sheet which is being deformed. This is the case for example when manufacturing parts which have flat areas, which are not or practically not deformed, alongside curved areas with higher deformations or hot trimmed areas or hot punched areas or hot flanged areas, in which the hot trimming or hot punching or hot flanging operation induces very high deformations on the edges, typically in the range of 0.7 - 0.9 equivalent plastic strain. For example, the difference in equivalent plastic strain between the area of the hot stamped part having the highest equivalent plastic strain and the area having the lowest equivalent plastic strain, is equal to or greater than 0.3, preferentially 0.4, preferentially 0.5, preferentially 0.6. Even with these very high differences in deformation rates on the part, the inventors have found that it is possible to reach stable mechanical properties on the part.

[0157] The hot stamped part according to the invention will now be described. Said hot stamped part is produced using a steel sheet corresponding to the above description, the chemical compositions, optional coatings and other optional characteristics, being taken alone or according to any possible combination.

[0158] The hot stamping process applies to said steel sheet a total deformation, expressed in equivalent plastic strain, which is less than or equal to 1.0 and at least locally equal to or greater than 0.4, preferably equal to or greater than 0.5, even more preferably equal to or greater than 0.7. At the same time, despite this important amount of deformation, at least locally, the mechanical properties of the part remain very high and stable thanks to the microstructure of said part, which comprises 80% or more martensite and 20% or less of the sum of ferrite, bainite, residual austenite and perlite, preferentially 85% or more martensite and 15% or less of the sum of ferrite, bainite, residual austenite and perlite. The high amount of martensite and the limited amount of the softer phases ferrite, bainite, residual austenite and perlite ensure very high and stable mechanical properties to the part.

[0159] In a particular embodiment, there exists on the part important differences in deformation rate, expressed in equivalent plastic strain. For example, the difference in equivalent plastic strain between the area of the hot stamped part having the highest equivalent plastic strain and the area having the lowest equivalent plastic strain, is equal to or greater than 0.3, preferentially 0.4, preferentially 0.5, preferentially 0.6. Even with these very high differences in deformation rates on the part, the inventors have found that it is possible to reach stable mechanical properties on the part.

[0160] The hot stamped part according to the invention has very stable mechanical properties over the entire area of the part, despite having undergone high deformation rates and possibly having undergone deformation rates which are significantly different in different areas of the part. One way to estimate this stability is to measure the Vickers hardness in different areas of the part. Indeed, it is well known that the Vickers hardness is well correlated to the mechanical properties, in particular to the ultimate tensile strength.

[0161] For example, the difference in Vickers hardness indentation at all points of the part does not exceed 25%, preferentially 20%, even more preferentially 15%.

[0162] Such stable mechanical properties on the part yield numerous advantages: the part has better overall performance because it does not have any weak spots, it is easy to predict its behavior thanks to its homogeneity and the part performance is overall robust. It particular, it avoids an early strain localization during crash or fatigue which can lead to premature failure of the part. It also allows to limit the amount of soft phase I hard phase interfaces which can be damaged during mechanical solicitation such as hot trimming, hot punching, hot flanging etc. This high and stable mechanical performance is particularly remarkable and necessary in the area of parts with thinner sheet thickness which are by nature already prone to strain / stress concentration.

[0163] The invention will be further illustrated by means of examples resulting from trials conducted by the inventors.

[0164] 8 different steel grades were tested, the chemical compositions of said grades are listed in table 1a. The corresponding values of the compound value Q computed according to the above described formula and the MS-start and Ac3 values are listed in table 1 b. Values that are outside of the range of the invention are underlined.

[0165] Grades A - D are according to the invention, because their chemical composition in terms of individual elements and in terms of their Q factor is within the range of the invention. Grades E - H are not according to the invention, because their factor Q is strictly greater than 25, and in the case of grade H the composition in carbon is furthermore below the range of the invention. All compositions are expressed in weight %:

[0166] Table 1a: chemical compositions (individual elements)

[0167] Table 1b: chemical compositions (formula combining several elements)

[0168] The samples were produced by hot rolling at a finish hot rolling temperature of 900°C a first solidified intermediate product.

[0169] The thus produced samples were then submitted to the following hot stamping process:

[0170] -the samples were heated up to 900°C in an austenitizing furnace during 6 minutes,

[0171] -the samples were then cooled down to a set hot forming temperature and deformed at a given total equivalent plastic strain at said hot forming temperature - the details of said hot forming temperatures and equivalent plastic strain levels are given in table 2.

[0172] -the samples were then cooled down to room temperature at 100°C / s.

[0173] Samples 11 to I8 are according to the invention, whereas samples R1 to R7 are reference samples, outside of the invention. For each sample, the Heat reference, indicated in table 1 , is given. The deformation temperature and equivalent plastic strain applied during the hot stamping process are indicated. It should be noted that all the deformation temperatures are above MS-start for each of the heat references.

[0174] The reference hardness corresponds to hardness measurements that were performed on samples that were quenched to room temperature from a temperature of 750°C without applying any deformation. It should be noted that for all samples, the microstructure of the samples used to measure the reference hardness, i.e. quenched from 750°C to room temperature without deformation, is 100% martensite.

[0175] The deformed hardness corresponds to the hardness measured on the particular hot stamped sample. The column labeled 1 - (HVdef / HVref) corresponds to the relative difference between the reference hardness and the deformed hardness and is expressed in %. When said value is positive, it means that the deformed hardness is lower than the reference hardness and conversely when said value is negative it implies that the deformed hardness is higher than the reference hardness. The amount of martensite and of the phases ferrite + bainite + perlite + residual austenite is reported in the last columns. Values that are underlined correspond to cases outside of the range of the invention.

[0176] Table 2: hot stamped samples characteristics

[0177] The samples according to the invention all exhibit a very stable hardness value, regardless of the applied deformation rate and deformation temperature. The difference between the reference hardness and the deformed hardness stays within an absolute value range of less than 25%, more particularly less than 20% and less than 15%. This is obtained thanks to a stable microstructure even when applying high deformation rates and varying the deformation temperature. Indeed, as was explained previously, the samples which were produced for the reference hardness measurements, quenched from 750°C to room temperature without deformation, all have a fully martensitic microstructure. However, in the case of the samples that are outside of the invention, the deformed samples exhibit significantly lower amounts of martensite, because of the high deformation rates applied. This lower martensite content is associated with a drop in hardness. Conversely, the inventive samples all retain a very high amount of martensite, above 80% and in most cases above 85%, which ensures very stable and high hardness.

[0178] The difference between the samples used to measure the reference hardness and the deformed samples is representative of the differences in hot stamping temperature and deformation rates that can occur on a part. The reference hardness samples represent the conditions that occur no flat areas of a part, for which there is no deformation and the contact between the tool and the steel sheet during the hot stamping process is excellent, ensuring good quenching conditions from a high temperature down to room temperature. On the other hand, the deformed samples are representative of what occurs in the areas of the part where there is a high deformation and I or in cases when the deformation occurs later on in the overall hot stamping process, in which case the steel sheet has cooled down significantly.

[0179] Thus the samples according to the invention correspond to a case in which the difference in deformation rate between the non-deformed area (sample used to measure the reference hardness) and the deformed area is 0.5, as expressed in equivalent plastic strain, and the difference in hardness between these areas remains below 15%.

Claims

CLAIMS1 . Method for manufacturing a hot stamped part comprising the following steps: -providing a steel sheet having the following chemical composition, by weight percent:C : 0.10 - 0.4 %Mn : 1 .0 - 11 %Si : 0.2 - 2.0 %Cr < 2.0%Mo < 0.5 %Nb < 0.1 %Al < 0.10 %Ti : 0.01 - 0.1 %B : 0.0005 - 0.005 %P < 0.02 %S < 0.04 %N < 0.01 %Ni < 1.0%Cu < 1.0% and Q < 25Wherein Q = 114 - 68*C - 18*Mn + 20*Si - 56*Cr - 61 *Ni - 37*AI + 39*Mo + 79*Nb - 46 *Cu - 17691 *B, the remainder of the composition being iron and unavoidable impurities resulting from the smelting,-heating said steel sheet above Ac3,-stamping said steel sheet above MS-start, the temperature at which martensite starts to form for said steel sheet-applying to said sheet a deformation, expressed in equivalent plastic strain, which does not exceed 1 .0 and which locally at least is greater than or equal to 0.4.

2. Method for manufacturing a hot stamped part according to claim 1 , wherein said steel sheet is stamped above 500°C.

3. Method for manufacturing a hot stamped part according to claim 2, wherein said steel sheet is stamped above 700°C.

4. Method according to any one of claims 1 to 3, wherein the deformation applied during stamping, expressed in equivalent plastic strain, is at least locally equal to or greater than 0.5.

5. Method according to any one of claims 1 to 4, wherein the difference in equivalent plastic strain between the area of the hot stamped part having the highest equivalent plastic strain and the area having the lowest equivalent plastic strain, is equal to or greater than 0.3.

6. Method according to claim 5, wherein the difference in equivalent plastic strain between the area of the hot stamped part having the highest equivalent plastic strain and the area having the lowest equivalent plastic strain, is equal to or greater than 0.5.

7. Method according to any one of claims 1 to 6, wherein the hot stamping operation includes a hot trimming operation, in which part of the material of the provided steel sheet is removed from the final hot stamped part by mechanically cutting it at high temperature.

8. Method according to any one of claims 1 to 7, wherein the provided steel sheet is coated on at least one side with a metallic coating comprising at least 50% Al in weight percent.

9. Method according to any one of claims 1 to 7, wherein the provided steel sheet is coated on at least one side with a metallic coating comprising at least 50% Zn in weight percent.

10. Method according to any one of claims 1 to 9, wherein the provided steel sheet is a tailor welded steel sheet.11 . Method according to any one of claims 1 to 9, wherein the provided steel sheet is a tailor rolled steel sheet.

12. Hot stamped part obtained by hot stamping a steel sheet having the following chemical composition, by weight percent:C : 0.10 - 0.4 %Mn : 1 .0 - 11 %Si : 0.2 - 2.0 %Cr < 2.0%Mo < 0.5 %Nb < 0.1 %Al < 0.10 %Ti : 0.01 - 0.1 %B : 0.0005 - 0.005 %P < 0.02 %S < 0.04 %N < 0.01 %Ni < 1.0%Cu < 1 .0% and Q < 25Wherein Q = 114 - 68*C - 18*Mn + 20*Si - 56*Cr - 61 *Ni - 37*AI + 39*Mo + 79*Nb - 46 *Cu - 17691 *B, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, wherein:-the total deformation applied in all areas of said steel sheet during the hot stamping process, expressed in equivalent plastic strain, is less than or equal to 1 .0 and wherein at least locally a deformation equal to or greater than 0.4 is applied,-the hot stamped part has a microstructure comprising 80% or more martensite and 20% or less of the sum of ferrite, bainite, residual austenite and perlite.

13. Hot stamped part according to claim 12, wherein the hot stamping operation includes a hot trimming operation, in which part of the material of the steel sheet is removed from the final hot stamped part by mechanically cutting it at high temperature.

14. Hot stamped part according to claim 12 or 13, wherein the difference in equivalent plastic strain between the area of the hot stamped part having the highest equivalent plastic strain and the area having the lowest equivalent plastic strain, is equal to or greater than 0.3.

15. Hot stamped part according to claim 14, wherein the difference in equivalent plastic strain between the area of the hot stamped part having the highest equivalent plastic strain and the area having the lowest equivalent plastic strain, is equal to or greater than 0.5.

16. Hot stamped part according to any one of claims 12 to 15, wherein the deformation, expressed in equivalent plastic strain, is at least locally equal to or greater than 0.5.

17. Hot stamped part according to any one of claims 12 to 16, having at all points a microstructure comprising 85% or more martensite and 15% or less of the sum of ferrite, bainite, residual austenite and perlite.

18. Hot stamped part according to any one of claims 12 to 17, wherein the difference in Vickers hardness indentation at all points of the part does not exceed 25%.

19. Hot stamped part according to claim 18, wherein the difference in Vickers hardness indentation at all points of the part does not exceed 20%.

20. Hot stamped part according to claim 19, wherein the difference in Vickers hardness indentation at all points of the part does not exceed 15%.21 . Hot stamped part according to any one of claims 12 to 20, wherein the steel sheet used to manufacture said hot stamped part is coated on at least one side with a metallic coating comprising at least 50% Al in weight percent.

22. Hot stamped part according to any one of claims 12 to 20, wherein the steel sheet used to manufacture said hot stamped part is coated on at least one side with a metallic coating comprising at least 50% Zn in weight percent.

23. Hot stamped part according to any one of claims 12 to 22, wherein the steel sheet used to manufacture said hot stamped part is a tailor welded steel sheet.

24. Hot stamped part according to any one of claims 12 to 22, wherein the steel sheet used to manufacture said hot stamped part is a tailor rolled steel sheet.

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