High bendability press-hardened steel part and method of manufacturing the same

Abstract

The invention deals with a high bendability press hardened steel part having a composition comprising, by weight percent: C : 0.05 - 0.14 %, Mn : 1.4 - 2.5 %, Si : 0.1 - 1.5 %, Al : 0.02 - 0.1 %, Cr : 0.01 - 1.0 %, B : 0.0005 - 0.004 %, Ti : 0.01 - 0.1 %, P ≤ 0.020 %, S ≤ 0.010 %, N ≤ 0.02 % and comprising optionally one or more of the following elements, by weight percent: Ni ≤ 0.4%, Mo ≤ 0.40 %, Nb ≤ 0.08 %, 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, 85% or more of martensite the rest being bainite, an interdiffusion layer having a thickness thinter, a coating layer having a thickness thcoat and based on aluminium, the ratio thinter / (thinter + thcoat) being above or equal to 70%.

Description

[0001] High bendability press-hardened steel part and method of manufacturing the same

[0002] The present invention relates to a high strength press hardened steel part with high bendability.

[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. Moreover, one of major challenges in the automotive industry is to decrease the weight of vehicles to improve their fuel efficiency in view of the global environmental conservation, without neglecting the safety requirements, including the harshest environments.

[0005] This weight reduction can be achieved thanks to the use of steel parts with a martensitic or bainitic / martensitic microstructure.

[0006] The publication WO2017006159 relates to a press hardened steel part with a thickness comprised from 0,8 and 4 mm and a yield stress YS comprised from 700 and 950 MPa, a tensile stress TS comprised from 950 and 1200 MPa, and a high ductility characterized by a bending angle higher to 75°. Nevertheless, to increase properties as bendability, the steel part must contain at least 5% of selftempered martensite, which are obtained by the two steps cooling process of the steel, and bainite. Moreover, the average size of TiN must be controlled and to be limited in the zone near the surface to improve bendability. Such TiN control to obtain a high bendability can be tedious.

[0007] The purpose of the invention therefore is to solve the above-mentioned problem and to provide a press hardened steel part having a high bendability, with a bending angle above or equal to 85°, combined with a high yield strength (YS) above or equal to 830MPa, and easily processable on conventional process route.

[0008] Preferably the yield strength is above or equal to 880MPa.

[0009] Preferably, the tensile strength TS (MPa) and YS satisfy (TS + YS) I (%C*% Mn) > 17500, %C and %Mn being the nominal carbon and manganese value. 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 6. Another object is achieved by providing the method according to any one of claims 7 to 8.

[0010] The invention will now be described in detail and illustrated by examples without introducing limitations.

[0011] The composition of the press hardened steel part according to the invention will now be described, the content being expressed in weight percent (wt. %).

[0012] According to the invention the carbon content is from 0.05% to 0.14% to ensure a satisfactory strength. Above 0.14% of carbon, weldability and bendability of the steel are reduced. If the carbon content is lower than 0.05%, the tensile strength will be too low. Preferably, the carbon content is from 0.05% to 0.10%, more preferably from 0.06% to 0.10%

[0013] The manganese content is from 1.4% to 2.5 %. Above 2.5% of addition, the risk of central segregation increases to the detriment of the bendability. Below 1 .4% 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 1.4% to 1.9%. Preferably the manganese content is from 1.5% to 1.9%, more preferably from 1 .5% to 1 .8%.

[0014] According to the invention, silicon content is from 0.1 % to 1 .5%. Silicon is an element participating in the hardening in solid solution. Silicon is added to limit carbides formation. Above 1 .5%, 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. In a preferred embodiment of the invention, the silicon content is from 0.1 % to 1 %. Preferably the silicon content is from 0.1 % to 0.8% more preferably from 0.1 % to 0.7%, even more preferably from 0.1 % to 0.6%.

[0015] The aluminium content is from 0.02% 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%.

[0016] According to the invention, the chromium content is from 0.01 % 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 0.01 % to 0.5%, more preferably from 0.05% to 0.5%, even more preferably from 0.05% to 0.3%.

[0017] 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%.

[0018] The titanium content is from 0.01 % to 0.1 % 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.

[0019] Some elements can optionally be added.

[0020] Nickel can be added up to 0.4% to limit hydrogen intake of the steel during its production and limiting the risk of delayed fracture due to hydrogen embrittlement. The nickel content is considered as a residual element up to 0.020%.

[0021] Molybdenum content can optionally be added up to 0.40%. As boron, molybdenum improves the hardenability of the steel. Molybdenum is not higher than 0.40% to limit cost.

[0022] Niobium can optionally be added up to 0.08% to improve ductility 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 below or equal to 0.05%.

[0023] Calcium may be also added as an optional element up to 0.1 % and preferably in a minimal amount of 0.0001 %. 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. 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 using a blast furnace, the level of unavoidable impurities is very low. In the case of a production route using an Electric Arc Furnace loaded with scraps, the steel sheet can further comprise residual elements coming from such scraps such as copper, antimony, arsenic, and lead, up to 0.03% which are considered as unavoidable impurities.

[0024] 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.

[0025] The microstructure of the press hardened steel part according to the invention will now be described.

[0026] The steel part comprises successively from the bulk to the surface of the steel part:

[0027] - a bulk having a microstructure comprising, in surface fraction, at least 85% of martensite the rest being optional bainite,

[0028] - an interdiffusion layer having a thickness thinter,

[0029] - a coating layer having a thickness thcoat and based on aluminium,

[0030] - the ratio between the interdiffusion layer thickness thinter and the sum of the coating layer and the interdiffusion thicknesses (thinter + thcoat) being above or equal to 70%.

[0031] The interdiffusion layer is formed during the reheating above 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.

[0032] During the heating above Ac3 of the steel blank, all microstructural elements are transformed into austenite, which is then transformed into at least 85% of martensite, the rest being optional bainite, during the die-quenching.

[0033] The rerolling step of the coated steel sheet decreases both the thickness of the steel sheet and of the coating in the said steel sheet, but it increases the alloying rate of the coating in the steel part during austenitization. The ratio between the interdiffusion layer thickness thinter and the sum of the coating layer and the interdiffusion thicknesses (thinter + thcoat) is thus above or equal to 70%, which improves the bendability of the steel part.

[0034] Below 70%, the coating is not alloyed enough to improve bendability.

[0035] Preferably, the ratio between the interdiffusion layer thickness and the sum of the coating layer and the interdiffusion thicknesses is above or equal to 75%.

[0036] The press hardened steel part according to the invention has a bending angle higher than 85°.

[0037] The press hardened steel part according to the invention has a yield strength YS above or equal to 830 MPa. Preferably YS is above or equal to 880 MPa.

[0038] Preferably, the tensile strength TS and YS (%) satisfy (TS + YS) I (%C*%Mn) > 17500, %C and %Mn being the nominal carbon and manganese value.

[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 partly, with a reduction rate from 20 to 60%, more preferably from 30% to 60%, even more preferably from 40% to 60%, to refine the microstructure and to obtain a better alloying of the coating. The rerolling can be done to obtain multiple sheet thicknesses by differential rolling during the steel sheet production process.

[0041] Below 20% of rerolling, the thickness of the steel sheet coating is not decreased enough to obtain after hot forming a high alloyed coating and a ratio thinter / (thinter + thcoat) above or equal to 70%. Above 60% of rerolling, the hydrogen intake during the reheating of the rerolled area may be to high leading to delayed fracture of the part.

[0042] The coated and rerolled steel sheet is cut to a predetermined shape, to obtain a steel blank. The steel blank is heated to a temperature Ti higher than Ac3 in a furnace having a dew point DP from -30°C to + 20°C and maintaining at said Ti temperature during a dwell time ti of 10s to 900s to obtain a heated steel blank.

[0043] The heated blank is then transferred to a forming press, and hot formed. After hot forming, the steel part is then die-quenched.

[0044] Ac3 is defined by the following formula:

[0045] 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.

[0046] The aluminium based coated steel sheet to be used in such process can be produced as follows.

[0047] 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.

[0048] The semi product is obtained by casting liquid steel, which can be produced by a steelmaking process using for example the Basic Oxygen Furnace (BOF) route. In BOF route, hot metal or pig iron obtained for example in a blast furnace or a smelting furnace, is decarburized to be turned into liquid steel. Optionally, ferrous scraps comprising elements such as copper, nickel, chromium, molybdenum, tin, arsenic, antimony, or lead are loaded in the furnace together with this hot metal or pig iron. Direct Reduced Iron (DRI) may also be charged.

[0049] The liquid steel can also be produced in an Electric Arc Furnace (EAF) by melting ferrous scraps to directly produce liquid steel. DRI may also be charged together with ferrous scrap in the EAF. In the case of a production route using an Electric Arc Furnace loaded with scraps, the steel sheet can comprise residual elements coming from such scraps such as Copper, Antimony, Arsenic, and Lead, up to 0.03% which are considered as unavoidable impurities.

[0050] P, S and N are also part of the unavoidable impurities whatever the process route. 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 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, before being coated with an aluminium-based coating and cooled to room temperature. The coated steel sheet is then submitted to the rerolling operations described above.

[0051] Preferably, the aluminum-based coating can comprise only aluminium and impurities inherent in processing. The aluminium-based coating can comprise 8 to 1 1 % by weight of silicon, the rest being aluminium and impurities inherent in processing. The aluminium based coating can also comprise 8% to 1 1 % silicon, from 2% to 4% iron, the rest being aluminum and impurities inherent in processing.

[0052] The microstructure of the aluminium based coated and rerolled steel sheet used for the press hardened steel part according to the invention will now be described.

[0053] The coated and rerolled steel sheet comprises successively from the bulk to the surface of the steel sheet:

[0054] - a bulk having a microstructure comprising, in surface fraction, from 10% to 40% of islands of martensite-austenite, the rest being ferrite

[0055] - an intermetallic layer,

[0056] - a coating layer based on aluminium,

[0057] The intermetallic layer is located at the interface between the bulk and the coating and is formed during the cooling of the coated steel sheet at room temperature. This intermetallic layer has a ferritic structure and is enriched with aluminium in solid solution, it may also include silicon in solid solution. 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.

[0058] The invention will be now illustrated by the following examples, which are by no way limitative.

[0059] 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.

[0060] Table 1 - Compositions The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent (wt.%).

[0061] Steel A is according to the invention, B is a reference. Underlined values: not corresponding to the invention

[0062] 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 pickled and cold rolled with a reduction rate of 48 %. The steel sheets are reheated to a temperature TH before being hot dip coated with an aluminium-silicon coating comprising 10% of silicon, in a bath at 660°C.

[0063] In trials 1 , 3, 5 and 7 the coated steel sheets are rerolled with a reduction rate of R (%), to obtain a thickness of 1 mm.

[0064] In trials 2, 4, 6 and 8 the coated steel sheets are not rerolled, as it can be seen with the value R of 0%. Nevertheless, the hot rolling and cold rolling steps are done to obtain a coated steel sheet with 1 mm thickness.

[0065] The steel sheets are cut to obtain a steel blank, heated to a temperature Ti of 920°C in a furnace having a dew point DP detailed in Table 2 and maintained at said temperature for a dwell time of 300 s, before being hot-formed and diequenched.

[0066] The following specific conditions were applied:

[0067] Table 2 - Process parameters

[0068] Underlined values: not corresponding to the invention

[0069] The steel parts were analyzed, and the corresponding microstructure is gathered in table 3. Mechanical properties are gathered in Table 4.

[0070] Table 3 - Microstructure of the press hardened steel part 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. 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.

[0071] Table 4 - Mechanical properties of the press hardened steel part

[0072] The tensile strength TS, the yield strength YS and the total elongation TE of the press hardened steel part have been measured according to ISO standard ISO 6892-1. The bendability a is measured according to the method VDA238-100 bending Standard (with normalizing to a thickness of 1 .5 mm).

[0073] Underlined values: do not match the targeted value

[0074] The examples show that the steel parts according to the invention, namely trials 1 and 3 are the only ones to show an improved bendability, combined to high yield strength, thanks to their specific compositions and microstructures.

[0075] The steel part according to trials 2 and 4 have been formed with similar process parameters as trials 1 and 3 respectively without the sheet being rerolled. It appears that the bendability of the part cut out from rerolled coated steel sheet are improved. Moreover, the tensile strength TS and the yield strength YS are also improved. The steel part of trials 5-8 have a chemical composition which is not according to the invention. The carbon content is too high, and the manganese content too low to obtain a compromise between an improved bendability according to the invention and a high strength. The rerolling step of trials 5 and 7 slightly improved the values of bending angles and YS and TS, but not enough to obtain values according to the invention.

Claims

CLAIMS1 . A press hardened steel part made of a steel having a composition comprising, by weight percent:C : 0.05 - 0.14 %Mn : 1 .4 - 2.5 %Si : 0.1 - 1.5 %Al : 0.02 - 0.1 %Cr : 0.01 - 1.0 %B : 0.0005 - 0.004 %Ti : 0.01 - 0.1 %P < 0.020 %S < 0.010 %N < 0.02 % and comprising optionally one or more of the following elements, by weight percent:Ni < 0.4%Mo < 0.40 %Nb < 0.08 %Ca < 0.1 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel part comprising successively from the bulk to the surface of the steel part:- a bulk having a microstructure comprising, in surface fraction, 85% or more of martensite the rest being optional bainite,- an interdiffusion layer having a thickness thinter,- a coating layer having a thickness thcoat and based on aluminium,- the ratio between the interdiffusion layer thickness thinter and the sum of the coating layer and the interdiffusion thicknesses (thinter + thcoat) being above or equal to 70%.

2. A press hardened steel part according to claim 1 , wherein the press hardened steel part has a manganese content from 1 .4% to 1 .9%.

3. A press hardened steel part according to claims 1 and 2, wherein the press hardened steel part has a silicon content from 0.1 % to 1 %.

4. A press hardened steel part according to any one of claims 1 to 3, wherein the press hardened steel part has a bending angle above or equal to 85°.

5. A press hardened steel part according to any one of claims 1 to 4, wherein the press hardened steel part has a yield strength YS above or equal to 830MPa.

6. A press hardened steel part according to any one of claims 1 to 5, wherein the press hardened steel part has a tensile strength TS and a yield strength YS satisfying (TS + YS ) / (%C*% Mn) > 17500, %C and %Mn being the nominal carbon and manganese value expressed in weight percent.

7. A process for manufacturing a press hardened steel part according to claim 1 comprising the following successive steps:- providing an aluminium based coated steel sheet, said steel having a chemical composition according to claim 1- rerolling at least part of said coated steel sheet with a reduction rate from 20 to 60% to obtain a coated and rerolled steel sheet,- cutting said coated and rerolled steel sheet to a predetermined shape, to obtain a steel blank,- heating the steel blank to a temperature Ti higher than Ac3 and maintaining at said Ti temperature during a dwell time ti of 10s to 900s to obtain a heated steel blank,- transferring the heated steel blank to a forming press,- hot-forming the heated steel blank in the forming press to obtain a formed part,- die-quenching the formed part.

8. A process for manufacturing a press hardened steel part according to claim 7, wherein said an aluminium based coated steel sheet is provided following the successive steps: - casting a steel to obtain a semi-product, said steel having a composition according to claim 1 ,- heating the semi-product at a temperature from 1100°C to 1300°C,- hot rolling the heated semi-product at a finish hot rolling temperature from 800°C to 950°C, - coiling the hot rolled steel sheet at a coiling temperature lower than 670°C, pickling the steel sheet,- cold rolling the steel sheet, with a reduction rate from 20% to 80%,- heating the steel sheet to a temperature T H from 700°C to 900°C,- coating the steel sheet with an aluminium base coating, - cooling the aluminium based coated steel sheet to room temperature.

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