PRESS-HARDENED STEEL PART AND ITS MANUFACTURING METHOD

Abstract

The invention deals with a high strength and high bendability press hardened steel part made of a rerolled steel sheet having a composition comprising, by weight percent: C : 0.15 - 0.22 %, Mn : 0.5 – 2.5 %, Si : 0.1 – 1.25 %, Al : 0.01 – 0.1 %, Cr : 0.1 – 1.0 %, B : 0.0005 - 0.004 %, Ti : 0.01 - 0.1 %, Mo : 0.05 - 0.40 %, Nb : 0.01 - 0.08 %, P ≤ 0.020 %, S ≤ 0.010 %, N ≤ 0.02 % and comprising optionally one or more of the following elements, by weight percent: Ca ≤ 0.1 %, the remainder of the composition being iron and unavoidable impurities resulting from the smelting. The press hardened steel part comprises a bulk having a microstructure comprising, in surface fraction, 95% or more of martensite the rest being bainite, an interdiffusion layer, and a coating layer based on aluminium.

Description

[0001] Press-hardened steel part and method of manufacturing the same

[0002] The present invention relates to a high strength and high bendability press hardened steel part having improved resistance to delayed fracture.

[0003] High strength press-hardened parts can be used as structural elements in automotive vehicles for anti-intrusion or energy absorption functions.

[0004] In such type of applications, it is desirable to produce steel parts that combine high mechanical strength, high impact resistance and good corrosion resistance.

[0005] However, it is known that the sensitivity to delayed cracking increases with the mechanical strength, in particular after cold-forming or hot-forming operations since high residual stresses are liable to remain after deformation and are applied during the assembly of vehicles such as during the spot welding.

[0006] Indeed, hydrogen atoms may progressively build up by diffusion into the crystal lattice defects, such as dislocations and grain boundaries. Hydrogen may become harmful when it reaches a critical concentration after a certain time. This delay results from the residual stress distribution field and from the kinetics of hydrogen diffusion, the hydrogen diffusion coefficient at room temperature being low. This hydrogen could lead for example, to cracks formation along grain boundaries and / or dislocations gliding planes. In addition, hydrogen localized at the grain boundaries weakens their cohesion and favors the appearance of delayed intergranular cracks. Some parts produced by hot-forming have really bad behavior concerning the hydrogen absorption during the hot-stamping. For example, the parts made from tailored rolled blanks are known to absorb a high amount of hydrogen in the furnace before the hot forming step, which can lead to formation of cracks.

[0007] Moreover, the risk of delayed fracture is known to be increased during the assembly of vehicles, in particular after paint baking of the parts, in which a low temperature heating is done, with consequently less hydrogen degassing.

[0008] To limit the hydrogen absorption, and formation of cracks, it is known to pre-coat a steel sheet with a barrier pre-coating comprising to prevent the absorption of hydrogen into the steel during the austenitization thermal treatment. For example, the publication WO2021 / 084379 used a nickel chromium barrier. Nevertheless, nickel is an expensive element. The publication WO2022129994 relates to a coated steel sheet and to a high strength press hardened steel part having good bendability properties, with a bending angle higher than 70°, and a yield strength YS above or equal to 1000 MPa. To obtain such a good bendability property of the steel part, the steel sheet is heated in an atmosphere with a water injection and a controlled dew point, in order to obtain at the upper part of the sheet, a decarburized layer topped by a ferritic layer. After forming, this decarburized layer helps to increase the bendability, which is improved when the decarburized layer of the steel sheet contains this upper ferrite layer. Nevertheless, the heating of the blank in a furnace having a high dew point can increase the hydrogen intake and consequently the risk of delayed fracture.

[0009] The purpose of the invention therefore is to solve the above-mentioned problems and to provide a press hardened steel part made from rerolled steel sheet having a combination of high strength and good resistance to delayed fracture, even in conditions generating high amount of hydrogen intake, as for example heating the blank in a furnace having a high dew point temperature, or by applying high amount of rerolling reduction.

[0010] The object of the present invention is achieved by providing a steel part according to claim 1 . The steel part can also comprise characteristics of anyone of claims 2 to 5. Another object is achieved by providing the method according to claim 6. The method can also comprise characteristics of anyone of claims 7 to 9.

[0011] The invention will now be described in detail and illustrated by examples without introducing limitations.

[0012] The composition of the press hardened steel part according to the invention will now be described, the content being expressed in weight percent (wt. %).

[0013] According to the invention the carbon content is from 0.15% to 0.22% to ensure a satisfactory strength. Above 0.22% of carbon, weldability and bendability of the steel are reduced. Moreover, a high amount of carbon reduces the Ms temperature, which can lead to highly stressed martensitic microstructure. If the carbon content is lower than 0.15%, the yield strength and the tensile strength will be too low. In a preferred embodiment of the invention, the carbon content is from 0.15% to 0.20%.

[0014] The manganese content is from 0.5% to 2.5 %. Above 2.5% of addition, the risk of central segregation increases to the detriment of the bendability. Below 0.5% the hardenability of the steel is reduced, and the tensile and yield strengths will be too low. In a preferred embodiment of the invention, the manganese content is from 0.5% to 1.8%. Preferably the manganese content is from 0.8% to 1.5%, more preferably from 0.8% to 1 .3%

[0015] According to the invention, silicon content is from 0.1 % to 1.25%. Silicon is an element participating in the hardening in solid solution. Silicon is added to limit carbides formation. Above 1 .25%, silicon is detrimental for toughness. Moreover, silicon oxides form at the surface, which impairs the coatability of the steel, and the weldability of the steel sheet and steel part may be reduced. Preferably the silicon content is from 0.1 % to 1.0%, more preferably from 0.3% to 1.0%, even more preferably from 0.5% to 1 .0%.

[0016] The aluminium content is from 0.01 % and 0.1 % as it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Aluminium can protect boron if titanium content is not enough. The aluminium content is lower than 0.1 % to avoid oxidation problems and ferrite formation during press hardening. Preferably the aluminium content is from 0.02% to 0.07%, more preferably from 0.02% to 0.06%, even more preferably from 0.02% to 0.05%.

[0017] According to the invention, the chromium content is from 0.1 % to 1.0 %. Chromium is an element participating in the hardenability of the steel sheet and must be higher than 0.01 %. The chromium content is below 1.0% to limit processability issues and cost. Preferably, the chromium content is from 0.3% to 1.0%, more preferably from 0.4% to 1 .0%, even more preferably from 0.5% to 1 .0%.

[0018] According to the invention, the boron content is from 0.0005% to 0.004%. Boron improves the hardenability of the steel. The boron content is not higher than 0.004% to avoid a risk of breaking the slab during continuous casting. Preferably the boron content is from 0.001 % to 0.004%.

[0019] The titanium content is from 0.01 % to 0.1 % in order to protect boron from formation of BN. Titanium content is limited to 0.1 % to avoid TiN formation. In a preferred embodiment, Ti / N >3.42 for the boron protection. Preferably, the titianium content is from 0.01 % to 0.05%.

[0020] Molybdenum content is added up to a content from 0.05% to 0.40% and increases the resistance to delayed fracture. As boron, molybdenum improves the hardenability of the steel. Molybdenum is not higher than 0.40% to limit cost. Preferably, the molybdenum content is from 0.05% to 0.30%, more preferably from 0.05% to 0.25%, even more preferably from 0.10% to 0.25%.

[0021] The niobium content is from 0.01 % to 0.08% to refine the microstructure and to improve ductility and bendability of the steel. Above 0.08% of addition, the risk of formation of NbC or Nb(C,N) carbides increases to the detriment of the bendability. Preferably the niobium content is from 0.01 % to 0.07%, more preferably from 0.01 % to 0.06%, even more preferably from 0.01 % to 0.05%.

[0022] Some elements can optionally be added.

[0023] Calcium may be also added as an optional element up to 0.1 % Addition of Ca at the liquid stage makes it possible to create fine oxides which promote castability of continuous casting. Moreover, calcium can help to limit the formation of detrimental MnS by promoting the formation of CaO-CaS.

[0024] The remainder of the composition of the steel is iron and unavoidable impurities resulting from the smelting process and depending on the process route. In the case of a production route without the use of scraps, as it is generally the case in the Blast Furnace-Basic Oxygen Furnace (BF-BOF) route, the level of unavoidable impurities is very low.

[0025] In the case of a production route using scraps, as in an Electric Arc Furnace (EAF) or loaded in a converter in a BF BOF,the steel sheet can further comprise residual elements coming from such scraps such as copper, Antimony, Arsenic, Tin and Lead, each up to 0.03% which are considered as unavoidable impurities.

[0026] P, S and N are also part of the unavoidable impurities whatever the process route. Their content is below or equal to 0.010 % for S, below or equal to 0.020 % for P and below or equal to 0.02 % for N. The microstructure of the press hardened steel part according to the invention will now be described.

[0027] The steel part comprises successively from the bulk to the surface of the steel part:

[0028] - a bulk having a microstructure comprising, in surface fraction, 95% or more of martensite the rest being optional bainite,

[0029] - an interdiffusion layer,

[0030] - a coating layer based on aluminium,

[0031] The interdiffusion layer is formed during the reheating above or equal to Ac3 of the blank, and is composed of iron, coming from the bulk, and aluminium in solid solution coming from the coating, and may include other elements coming from the bulk like silicon, chromium, or manganese. The interdiffusion layer has a thickness thinter, preferably from 2 pm to 30 pm.

[0032] The coating layer has a thickness thcoat , preferably comprised from 2 pm to 45 pm. Preferably, the aluminum-based coating comprised pure aluminium and impurities inherent in processing. Preferably, the aluminum-based coating comprised 8 to 1 1 % by weight of silicon, the rest being aluminium and impurities inherent in processing. Preferably the aluminium based coating comprised 8% to 1 1 % silicon, from 2% to 4% iron, the rest being aluminum.

[0033] During the heating above or equal to Ac3 of the steel blank cut out from the coated and rerolled steel sheet, all microstructural elements are transformed into austenite, which is then transformed during the die-quenching into at least 95% of martensite, the rest being optional bainite. In a preferred embodiment, the amount of martensite is at least 97% and even better of at least 98%.

[0034] The rerolling step of the coated steel sheet decreases both the thickness of the steel sheet and of the coating thcoat in the said steel sheet, but it increases the hydrogen intake in the steel during the austenitization at temperature above or equal to Ac3.

[0035] The press hardened steel part according to the invention has a combination of high strength and good resistance to delayed fracture, even in conditions generating high amount of hydrogen intake, as for example heating the blank in a furnace having a high dew point temperature, or by applying high amount of rerolling reduction. The press hardened steel part according to the invention has indeed a good resistance to delayed fracture up to a hydrogen content of 1 .48ppm, with no cracks on the press hardened steel part, after the said press hardened steel part has been subjected to a four-points bending test according to standard ASTM G33-99 and kept for 96 hours at room temperature.

[0036] Preferably, the press hardened steel part has a hydrogen content Hditf of 0.95ppm or more, more preferably of 1 .00 ppm or more, even more preferably of 1 .05 ppm or more.

[0037] Preferably, the tensile strength TS of the press hardened steel part is above or equal to 1200MPa. Preferably the yield strength YS of the press hardened steel part is above or equal to 1000MPa.

[0038] Preferably the press hardened steel part according to the invention has a bending angle higher than 60°.

[0039] The steel part 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 following steps.

[0040] An aluminium based coated steel sheet is provided. This coated steel sheet is rerolled at least once with a reduction rate from 5 to 80%, preferably from 10% to 80%, more preferably from 20% to 80% and even more preferably from 20 to 70% or 30 to 70%, to refine the thickness of the coated steel sheet and the thickness of the coating thcoat. Below 5% of rerolling, the thickness of the steel sheet is not decreased enough. Moreover, the guiding of the sheet in the flexible rolling mill can be difficult at a rate below 5%, the sheet may no longer be in contact with the rolling mill. Above 80% of rerolling, the hydrogen intake during the reheating of the rerolled sheet may be too high leading to delayed fracture of the part.

[0041] In a first embodiment of the invention, the rerolling can be done to obtain multiple sheet thicknesses by differential rolling during the steel sheet production process, in order to produce further a tailored rolled blank, for example by continuous flexible rolling process. In a second embodiment of the invention, the rerolling can be done to obtain a uniform thickness on the sheet, by a continuous uniform rerolling process.

[0042] The coated and rerolled steel sheet is cut to a predetermined shape, to obtain a steel blank.

[0043] In the first embodiment of the invention, this blank is a tailor rolled blank having more than one sheet thickness obtained by differential rolling during the steel sheet production process.

[0044] In the second embodiment of the invention, this blank is a blank having a uniform thickness, lower than the thickness obtained after the hot rolling and cold-rolling steps, and obtained by continuous uniform rerolling process.

[0045] The steel blank is heated to a temperature T 1 higher than Ac3 in a furnace having a dew point DP from -40°C to + 30°C and maintaining at said Ti temperature during a dwell time ti of 10s to 900s to obtain a heated steel blank. Preferably, the dew point DP is from -20°C to +30°C, more preferably from -10°C to +30°C, even more preferably from 0°C to +30°C.

[0046] The heated blank is then transferred to a forming press, and hot formed. After hot forming, the steel part is then die-quenched.

[0047] Ac3 is defined by the following formula:

[0048] Ac3(°C) = 910 -203 %C+ 44.7%Si-15.2%Ni+31 ,5%Mo +104.4%V+13.1 %W, the elements being expressed in weight percent.

[0049] The aluminium based coated steel sheet that can be used, for example, in such process can be produced as follows.

[0050] A semi-product able to be further hot rolled, is provided with the steel composition described above. Such semi-product can for example be a slab.

[0051] The semi product is obtained by casting liquid steel, which can be produced by a steelmaking process with or without the use of scraps.

[0052] The semi product is heated to a temperature from 1 100°C to 1300°C. The steel sheet is then hot rolled at a finish hot rolling temperature from 800°C to 950°C. The hot-rolled steel is then cooled and coiled at a temperature lower than 670°C and pickled to remove surface oxidation. The steel sheet is then cold rolled, with a reduction rate from 20% to 80%. The steel sheet is then reheated to a temperature TH comprised from 700°C to 900°C and maintained at said TH temperature for a holding time tH from 10s to 600s,, the atmosphere in the furnace having a dew point TDPI strictly higher than -10°C and below or equal to +20°C, thus forming a decarburized layer at the top of the sheet, before being coated with an aluminium- based coating and cooled to room temperature. The aluminium based coated steel sheet is then submitted to the rerolling operations described above.

[0053] Preferably, the aluminum-based coating comprised pure aluminium and impurities inherent in processing. Preferably, the aluminum-based coating comprised 8 to 1 1 % by weight of silicon, the rest being aluminium and impurities inherent in processing. Preferably the aluminium based coating comprised 8% to 1 1 % silicon, from 2% to 4% iron, the rest being aluminum.

[0054] The microstructure of the coated steel sheet that can be used, for example, for the manufacturing of the press hardened steel part according to the invention will now be described.

[0055] The coated steel sheet comprises successively from the bulk to the surface of the steel sheet:

[0056] - a bulk having a microstructure comprising, in surface fraction, 60% or more of ferrite, the rest being optional cementite, pearlite or martensiteaustenite islands,

[0057] - a decarburized layer,

[0058] - a coating layer based on aluminium,

[0059] The decarburized layer is located at the interface between the bulk and the coating. It is formed during the heating up to a temperature TH of the coated steel sheet in a furnace having an atmosphere with a dew point T DPI . This decarburized layer is still present during the rerolling step. This decarburized layer comprises in its upper part an intermetallic layer with a ferritic structure which is enriched with aluminium in solid solution, it may also include silicon in solid solution. Upon the rerolling step, the intermetallic layer is fragmented. The intermetallic layer can comprise for example from 15 at.% to 25 at.%. of iron and from 5 at.% to 20 at.% of silicon, the rest being aluminium. The invention will be now illustrated by the following examples, which are by no way limitative.

[0060] 5 Example

[0061] 2 grades, which compositions are gathered in table 1 , were cast in semiproducts and processed into steel sheets, then steel parts, following the process parameters gathered in table 2. 0 Table 1 - Compositions

[0062] The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent (wt.%).

[0063] Steel A is according to the invention, B is a reference. 5 Underlined values: not corresponding to the invention

[0064] Steel semi-products, as cast, were reheated at 1200 °C, hot rolled with a finish hot rolling temperature of 895°C and coiled at 550°C. The steel sheets are cold rolled with a reduction rate of 48 %. The steel sheets are reheated to a0 temperature T H and maintained at TH for a holding time tH in a furnace having a dew point temperature TDPI, before being hot dip coated with an aluminium-silicon coating comprising 10% of silicon, in a bath at 660°C.

[0065] The aluminium based coated steel sheet made of steel A comprises:

[0066] - a bulk having a microstructure comprising, in surface fraction, 92% of5 ferrite, 3% of cementite and 5% of martensite-austenite islands,

[0067] - a decarburized layer, with an intermetallic layer at the top,

[0068] - and a coating layer based on aluminium.

[0069] The aluminium based coated steel sheet made of steel B comprises:

[0070] - a bulk having a microstructure comprising 95% of ferrite, the rest being0 cementite - a coating layer, and does not comprise a decarburized layer.

[0071] The aluminium based coated steel sheets are rerolled with a reduction rate of R (%).

[0072] The aluminium based coated and rerolled steel sheets are cut to obtain a steel blank, heated to a temperature of Ti in a furnace having a dew point DP detailed in Table 2 and maintained at said temperature for a dwell time of ti and before being hot-formed and die-quenched.

[0073] The following specific conditions were applied:

[0074] Table 2 - Process parameters

[0075] Underlined values: not corresponding to the invention

[0076] The steel parts were analyzed, and the corresponding microstructure is gathered in table 3. Mechanical properties are gathered in Table 4.

[0077] Table 3 - Microstructure of the press hardened steel part

[0078] The surface fractions are determined through the following method: a specimen is cut from the press hardened steel part, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through optical or scanning electron microscope, for example with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification greater than 5000x, coupled to a EBSD (Electron Back Scattered Diffraction) device.

[0079] The analysis of the coating and interdiffusion layers is done through a micro-probe EDXMA (Energy Dispersive X-Ray micro analysis) or with SEM-EDXA (Energy dispersive X-ray analysis). The thickness of the interdiffusion layer thinter and of the coating thCOat are measured through a cross section.

[0080] The diffusible hydrogen content Hdiff is measured thanks to TDA (Thermal Desorption Analysis) experiments at the end of the manufacturing process.

[0081] The TDA set-up consists of a heating room in which a flat sample of 10 mm width and 50 mm length is heated in an infra-red furnace at a constant heating rate of 1200°C / h under a constant flow of pure nitrogen up to 900°C. Released hydrogen carried by nitrogen is detected by a quadrupole mass spectrometer.

[0082] Table 4 - Mechanical properties of the press hardened steel part

[0083] The tensile strength TS and the yield strength YS of the press hardened steel part have been measured, according to ISO standard ISO 6892-1 .

[0084] The delayed fracture is evaluated by using a four-points bending test according to standard ASTM G33-99: A sample cut out from the part is deformed by bending it in between two upper rolls and two lower rolls, with a deflection corresponding to a level of applied stress of 100% of the yield strength value. The parts are then kept for 96 hours at room temperature, and the presence or not of cracks is then visually observed.

[0085] Underlined values: do not match the targeted value

[0086] The steel parts according to the invention, namely trials 1 and 2, have an improved resistance to the delayed fracture combined with high strength, even with a high diffusible hydrogen content Hdiff.

[0087] Indeed, it is known that hydrogen intake during the reheating of the rerolled sheet may be high, because of the decreased thickness of the coating, and as it can be seen with the high value of Hdiff. Nevertheless, the steel part according to the invention presents no cracks, even with these high values of Hdiff. Indeed, the steel part according to the invention can accept up to 1 .48ppm of diffusible hydrogen without formation of cracks, which is remarkable.

[0088] On the opposite, the steel part of trial 3, for which a coated steel sheet having a chemical composition not according to the invention, is rerolled to the same rate of trial 3, and presents cracks at the end of the process.

Claims

1. A press-hardened steel part having a composition including, by weight %: C - 0.15-0.22 Mn - 0.5-2.5 Si - 0.1-1.25 Al - 0.01-0.1 Cr - 0.1-1.0 B - 0.0005-0.004 Ti - 0.01-0.1 Mo - 0.05-0.40 Nb - 0.01-0.08 P≤0.020 S≤0.010 N≤0.02 and optionally including one or more of the following elements, wt.%: Ca≤0.1, the rest of the composition is made up of iron and inevitable impurities resulting from smelting, the said steel part, containing in sequence from the main part to the surface of the steel part: a major part having a microstructure containing in surface fractions 95% or more martensite, the remainder being optionally bainite, interdiffusion layer, aluminum-based coating layer, wherein the steel part is made from re-rolled steel sheet.

2. A hardened part according to claim 1, in which the carbon content is 0.15-0.20%.

3. A hardened part according to item 1 or 2, in which the manganese content is 0.5-1.8%.

4. A hardened part according to any one of paragraphs 1-3, wherein the press-hardened steel part has a bending angle greater than or equal to 60°.

5. A hardened part according to any one of paragraphs 1-4, wherein the press-hardened steel part has the corresponding resistance to delayed fracture at a hydrogen content H diff to 1.48 ppm after the specified press-hardened steel part was subjected to a four-point bend test in accordance with ASTM G33-99 and aged for 96 hours at room temperature.

6. A method for manufacturing a press-hardened steel part according to any one of paragraphs 1-5, comprising the following sequential stages: obtaining a steel sheet with an aluminum-based coating, with a chemical composition according to any of paragraphs 1-3; at least once re-rolling said steel sheet with an aluminum-based coating with a degree of reduction of from 5 to 80% to obtain a re-rolled steel sheet with a coating, cutting said re-rolled coated steel sheet to a given shape to obtain a steel blank; heating a steel workpiece to a temperature T1 exceeding or equal to Ac3 in a furnace atmosphere with a dew point DP ranging from -40°C to +30°C, and maintaining said temperature T1 for a holding time t110-900 s to obtain a heated steel workpiece; moving the heated steel blank into the molding press; hot forming of a heated steel blank in a forming press to produce a formed part; hardening of a molded part under a press.

7. The method according to claim 6, wherein said steel sheet with an aluminum-based coating is obtained in the following stages: casting steel to obtain a semi-finished product, wherein said steel has a composition in accordance with that specified in any of paragraphs 1-3, heating the semi-finished product at a temperature of 1100-1300°C, hot rolling of a heated semi-finished product at a hot rolling end temperature of 800-950°C, winding of hot-rolled steel sheet at a winding temperature below 670°C, etching of steel sheet is not necessary, cold rolling of steel sheet with a degree of compression of 20-80%, heating the steel sheet to temperature T H , amounting to 700-900°C, and holding at temperature T H during the holding time t H , amounting to 10-600 s, while the atmosphere in the furnace has a dew point T DP1 strictly above -10°C and below or equal to +20°C application of an aluminum-based coating to a steel sheet, cooling the coated steel sheet to room temperature.

8. The method according to claim 6 or 7, wherein said re-rolling step is carried out by a continuous rolling process to obtain a variable thickness.

9. The method according to claim 6 or 7, wherein said re-rolling step is carried out by a continuous rolling process to obtain a constant thickness.