High strength press hardened steel part and method of manufacturing the same

Abstract

The invention deals with a press hardened steel part having a composition comprising, by weight percent: C 0.2 - 0.34 %, Mn 0.50 – 1.24 %, Si 0.5 – 2 %, P ≤ 0.020 %, S ≤ 0.010 %, N ≤ 0.010 %, and comprising optionally one or more of the following elements: Al: ≤0.2 %, Cr ≤ 0.8 %, Nb ≤ 0.06 %, Ti ≤ 0.06 %, B ≤ 0.005% Mo ≤ 0.35% the remainder of the composition being iron and unavoidable impurities resulting from the smelting. The press hardened steel part has a microstructure comprising, in surface fraction, 95% or more of tempered martensite and 5% or less of bainite, austenite or ferrite.

Description

1 High strength press hardened steel part and method of manufacturing the same The present invention relates to high strength press hardened steel parts having good bendability and weldability properties. High strength press-hardened parts can be used as structural elements in automotive vehicles for anti-intrusion or energy absorption functions. 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 in order to improve their fuel efficiency in view of the global environmental conservation, without neglecting the safety requirements. This weight reduction can be achieved in particular thanks to the use of steel parts with a tempered martensitic or bainitic / martensitic microstructure. Such types of parts can be welded, and the motor vehicle manufacturers prescribe that the weld joint should not constitute the weakest zone of the welded steel part. Indeed, the presence of spot welds on structural components in the car body can result in failure during crash, due to the localisation of the strain in the softened heat affected zone (HAZ). The purpose of the invention therefore is to solve the above-mentioned problem and to provide a press hardened steel part having a combination of high mechanical properties with tensile strength TS above or equal to 1000 MPa, a uniform elongation loss ΔUEI in spot welded areas below or equal to 25% and a bending angle above or equal to 55 °. Preferably, the press hardened steel part according to the invention has a fracture strain above or equal to 0.50. Preferably, the press hardened steel part according to the invention has a yield strength YS above or equal to 980 MPa. According to one aspect, the object of the present invention is achieved by providing a steel part of the type described herein. According to another aspect, the object is achieved by providing a method of the type described herein. 2 The invention will now be described in detail and illustrated by examples without introducing limitations. The composition of the steel according to the invention will now be described, the content being expressed in weight percent. According to the invention the carbon content is from 0.2% to 0.34% to ensure a satisfactory strength. Above 0.34% of carbon, fracture strain and bending angle of the steel sheet do not achieved the targeted values. Moreover, the weldability of the steel sheet may be reduced. If the carbon content is lower than 0.2%, the tensile and yield strengths will not reach the targeted value. The manganese content is from 0.50% to 1.24%. Above 1.24% of addition, the risk of central segregation increases to the detriment of the bendability, and the fracture strain may be reduced. Below 0.50% the hardenability of the steel sheet is reduced, and the tensile and yield strengths will not reach the targeted value. The silicon content is from 0.5% to 2%. Silicon is an element participating in the hardening in solid solution. Silicon is added to limit carbides formation and to ensure high level of tensile strength. Above 2%, silicon oxides form at the surface, which impairs the coatability of the steel. Moreover, the weldability of the steel sheet may be reduced. Preferably, the silicon content is from 0.5% to 1.8%. More preferably the silicon content is from 0.6% to 1.8%, more preferably from 0.6% to 1.6%. Some elements can optionally be added. The aluminium content can optionally be added up to 0.2% as it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Preferably, the aluminium content is below or equal to 0.1%. More preferably, the aluminium content is below or equal to 0.06%. Optionally, the chromium content can be added up to 0.8% to improve hardening in solid solution. The chromium content is below or equal to 0.8% to limit processability issues and cost. Preferably, the chromium content is below or equal to 0.6%. Niobium content can optionally be added up to 0.06% for prior austenitic grain size refinement and to improve ductility of the steel. Above 0.06% of addition, the risk of formation of NbC or Nb(C,N) carbides increases to the detriment of the bendability. The titanium content can optionally be added up to 0.06% in order to protect boron from formation of BN. Preferably the titanium content is higher than 0.01%. The boron content can optionally be added up to 0.005%. Boron improves the hardenability of the steel. The boron content is not higher than 0.005% to avoid a risk of breaking the slab during continuous casting. Molybdenum can optionally be added up to 0.35%. As boron, molybdenum improves the hardenability of the steel. Molybdenum is not higher than 0.35% to limit cost. The remainder of the composition of the steel is iron and impurities resulting from the smelting. In this respect, P, S and N at least are considered as residual elements which are unavoidable impurities. Their content below or equal to 0.020 % for P, below or equal to 0.010 % for S, and below or equal to 0.010 % for N. The microstructure of the press hardened steel part according to the invention will now be described. The press hardened steel part has a microstructure comprising, in surface fraction, 95% or more of tempered martensite. This tempered martensite is formed during the heating of the steel part to a temperature Ttemp comprised from 390°C to 510°C, for a holding time ttemp comprised from 1s to 1000s. Some bainite, ferrite and austenite can optionally be present, the sum of which being, in surface fraction, of 5% or less. Preferably the microstructure of the press hardened steel part is 100% made of tempered martensite. The press hardened 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: A steel sheet having a composition according to the invention is provided and cut to a predetermined shape, so as to obtain a steel blank. The steel blank is heated to a temperature THF comprised from 810°C to 960°C, preferably from 850°C to 950°C and more preferably from 880°C to 950°C, and is maintained at said The temperature for a holding time the comprised from 5 s to 1200s, to obtain a heated steel blank with a fully austenitic microstructure. The said heated steel blank is transferred to a forming press and hot forming in order to obtain a steel part. The steel part is then die-quenched until reaching a temperature below or equal to 200°C. The steel part is reheated to a temperature Ttemp comprised from 390°C to 510°C, and maintained at said temperature Ttemp for a holding time ttemp comprised from 1s to 1000s, to obtain a tempered steel part, in order to ensure temperature homogeneity on all the steel part. Above 510°C, the tensile strength of the steel part is reduced. Below 390°C, the uniform elongation loss <semantics>ΔUEI<annotation encoding="application / x-tex">\Delta UEI< / annotation>< / semantics> in spot welded areas is above 25%. The tempered steel part is then cooled to room temperature. For each tempered product, the HAZ sensitivity is assessed through the uniform elongation loss of JIS tensile specimen with weld compared to a reference without weld. The uniform elongation loss <semantics>ΔUEI<annotation encoding="application / x-tex">\Delta UEI< / annotation>< / semantics> is calculated as follows: The uniform elongation UEI of the steel is measured according to standard JIS Z2241 on a tensile test specimen. A welded spot is done on a tensile test specimen, centred on the deformation area of the specimen. The uniform elongation UElw of this welded tensile test specimen is measured according to standard JIS Z2241. The uniform elongation loss <semantics>ΔUEI<annotation encoding="application / x-tex">\Delta UEI< / annotation>< / semantics> is determined by the formula : [Image disponible dans le document PDF, Image available in the PDF document] In a first preferred embodiment of the invention, the steel sheet provided to manufacture the steel part is produced by the following successive steps: A steel slab having a composition described above is cast and reheated to a temperature Treheat comprised from 1100°C to 1300°C before to be hot rolled at a finish hot rolling temperature comprised from 800°C to 950°C to obtain a hot rolled steel sheet. The hot rolled steel sheet is then coiled to a temperature Tcoil lower than 670°C. The hot rolled steel sheet can optionally be pickled to remove oxidation. The hot rolled steel sheet can optionally be heated to a temperature Theat comprised from 500°C to 750°C, and maintained at said Thba temperature for a holding time the comprised from 300s to 50h. The steel sheet is then cold rolled to obtain a cold rolled steel sheet. The cold-rolling reduction ratio is preferably comprised 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. The cold rolled steel sheet is optionally annealed to an annealing temperature TA comprised from 650°C to 900°C and maintained at said temperature TA for a holding time tA comprised from 10s to 1200s, to obtain an annealed steel sheet, in order to reduce the tensile strength to facilitate the cut of the steel. The steel sheet is finally cooled to room temperature. Preferably, the said annealed steel sheet is coated with aluminium or aluminium alloy coating or with zinc or zinc alloy coating before being cooled to room temperature. In a second embodiment of the invention, the steel sheet provided to manufacture the steel part is produced by the following successive step: A steel slab having a composition according to the invention is cast and reheated to a temperature Treheat comprised from 1100°C to 1300°C before being hot rolled at a finish hot rolling temperature comprised from 800°C to 950°C to obtain a hot rolled steel sheet. The hot rolled steel sheet is then coiled to a temperature Tcoil lower than 670°C. The hot rolled steel sheet can optionally be pickled to remove oxidation. The hot rolled steel sheet can optionally be heated to a temperature Thba from 500°C to 750°C, and maintained at said Theat temperature for a holding time theat from 300s to 50h. The steel sheet is then cold rolled to obtain a cold rolled steel sheet. The cold-rolling reduction ratio is preferably 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. The cold rolled steel sheet is optionally annealed to an annealing temperature TA comprised from <semantics>500∘<annotation encoding="application / x-tex">500^{\circ}< / annotation>< / semantics>C to <semantics>750∘<annotation encoding="application / x-tex">750^{\circ}< / annotation>< / semantics>C and maintained at said temperature TA for a holding time tA comprised from 300s to 50h, to obtain an annealed steel sheet, in order to reduce the tensile strength to facilitate the cut of the steel. The steel sheet is finally cooled to room temperature. The press hardened steel part according to the invention has a tensile strength TS above or equal to 1000 MPa, a uniform elongation loss ΔUEI in spot welded areas below or equal to 25%, and a bending angle above or equal to 55°. In a preferred embodiment of the invention, the press hardened steel part has a yield strength YS above or equal to 980MPa. In another preferred embodiment, the press hardened steel part according to the invention has a fracture strain above or equal to 0.50. The invention will be now illustrated by the following examples, which are by no way limitative. Example 8 grades, which compositions are gathered in table 1, were cast in semi- products and processed into steel sheets, then steel parts, following the process parameters gathered in table 2. Table 1 - Compositions The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent. [Image disponible dans le document PDF, Image available in the PDF document] Steels A-E are according to the invention, steels F-H are out of the invention Underlined values: not corresponding to the invention Table 2 – Process parameters Steel semi-products, as cast, were reheated at 1250°C, hot rolled with a finish hot rolling temperature comprised from 800 to 950°C, coiled at 580°C and cold rolled with a reduction rate of 58%. Steel sheets are then heated to a temperature TA of 790°C and maintained at said temperature <semantics>TA<annotation encoding="application / x-tex">T_A< / annotation>< / semantics> for a holding time <semantics>tA<annotation encoding="application / x-tex">t_A< / annotation>< / semantics> of 180s. The steel sheets were cut to a predetermined shape, so as to obtain a steel blank. The steel blanks were then heated to a temperature The for a holding time the of 120s, before being transferred to a forming press. The heated blanks were hot- formed in the forming press to obtain a steel part, before being die-quenched until reaching a temperature of 80°C. The steel parts were then reheated to a temperature Ttemp from 390°C to 510°C, and maintained at said Ttemp temperature for a holding time ttemp from 1s to 1000s, before being cooled to room temperature. [Image disponible dans le document PDF, Image available in the PDF document] Underlined values: not corresponding to the invention The steel parts were analyzed, and the corresponding microstructure and properties are gathered in table 3 and in table 4, respectively. Table 3 – Microstructure of the steel part [Image disponible dans le document PDF, Image available in the PDF document] 9 [Image disponible dans le document PDF, Image available in the PDF document] Underlined values: not according to the invention 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, for example Nital reagent, 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 an Electron Backscatter Diffraction (EBSD) device. Tempered martensite can be distinguished from martensite thanks to its low dislocation density compared to martensite. Table 4 – Properties of the steel parts TS and YS are measured according to ISO standard ISO 6892-1. The bending angle has been determined on press hardened parts according to the method VDA238-100 bending Standard (with normalizing to a thickness of 1.5 mm). The term fracture strain refers to the fracture strain criterion defined by Pascal Dietsch et al. in "Methodology to assess fracture during crash simulation: fracture strain criteria and their calibration”, in Metallurgical Research Technology Volume 114, Number 6, 2017. The fracture strain is the equivalent strain within the material at the deformation point when the critical bending angle has been reached. The fracture strain values have been determined in plane strain conditions, which is the most severe condition in view of vehicle collision, and are obtained thank to finite elements analysis. [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] Underlined values: not corresponding to the invention Table 5 – Spot welding properties of the press hardened steel part 5 A welded spot is done on the tensile test specimen, centred on the deformation area of the specimen. The corresponding uniform elongation loss ΔUEI of the resistance spot weld are gathered in table 5. [Image disponible dans le document PDF, Image available in the PDF document] Underlined values: do not match the targeted values nd: non determined value Thanks to their specific compositions and process parameters used, the examples according to the invention, namely examples 1-7 are the only one to show combination of high mechanical properties, with TS higher than 1000MPa, a bending angle above or equal to 55°, and an uniform elongation loss lower than 25%. Moreover examples 1-7 have a fracture strain higher than 0.50. The tempering temperature applied on steel part of trials 8 and 9 is too low to limit the detrimental impact of HAZ softening on the uniform elongation, as shown by the uniform elongation loss higher than 25%. Moreover, in comparison to trial 2 with the same steel composition, the low temperature of tempering of trial 8 leads to a higher uniform elongation loss and lower fracture strain value than trial 2. No tempering is done on steel part of trial 10, which implies a uniform elongation loss higher than 25%. In trial 11, the carbon content of the steel part is too high to achieve the targeted fracture strain and bendability values.

Claims

<pat:ClaimStatement>CLAIMS< / pat:ClaimStatement> <pat:Claims com:id="claims"> <pat:Claim com:id="CLM-00001"> <pat:ClaimNumber>1< / pat:ClaimNumber> <pat:ClaimText>1. A press hardened steel part made of a steel having a composition comprising, by weight percent: [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document] the remainder of the composition being iron and unavoidable impurities resulting from smelting, wherein the steel: (i) has a microstructure comprising, in surface fraction: 95% or more of tempered martensite, and 5 % or less of the sum of bainite, austenite and ferrite, (ii) has a tensile strength TS above or equal to 1000 MPa, a uniform elongation loss ΔUEI in spot welded areas below or equal to 25% and a bending angle above or equal to 55°, - the tensile strength TS being measured according to ISO standard ISO 6892-1, - the uniform elongation loss ΔUEI being measured through [(UEI- UElw) / UEI]*100, UEI being a uniform elongation of the steel on a tensile test specimen and UEIw being a uniform elongation of a welded tensile test specimen, UEI and UEIw being measured according to standard JIS Z2241, and _ the bending angle being determined according to method VDA238. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00002"> <pat:ClaimNumber>2< / pat:ClaimNumber> <pat:ClaimText>2. The press hardened steel part according to claim 1, having a fracture strain above or equal to 0.50, wherein the fracture is an equivalent strain within the steel at a deformation point when a critical bending angle has been reached, defined in Metallurgical Research Technology Volume 114, Number 6, 2017. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00003"> <pat:ClaimNumber>3< / pat:ClaimNumber> <pat:ClaimText>3. The press hardened steel part according to any one of claims 1 and 2, wherein the press hardened steel part has a yield strength YS above or equal to 980MPa, the yield strength YS being measured according to ISO standard ISO 6892-1. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00004"> <pat:ClaimNumber>4< / pat:ClaimNumber> <pat:ClaimText>4. A method for producing a press hardened steel part, said method comprising the following successive steps: a) providing a steel sheet having the composition according to claim 1, b) cutting said steel sheet to a predetermined shape to obtain a steel blank, C) heating the steel blank to a temperature The comprised from 810°C to 960°C and maintaining the steel blank at said THF temperature for a holding time the comprised from 5s to 1200s to obtain a heated steel blank, d) transferring the heated steel blank to a forming press, e) hot forming the heated steel blank in the forming press to obtain a steel part, f) die-quenching the steel part until reaching a temperature below or equal to 200°C, g) reheating the steel part to a temperature Ttemp comprised from 390°C to 510°C, and maintaining the steel part at said temperature Ttemp for a holding time ttemp comprised from 1s to 1000s, to obtain a tempered steel part, and h) cooling the tempered steel part to room temperature. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00005"> <pat:ClaimNumber>5< / pat:ClaimNumber> <pat:ClaimText>5. The method according to claim 4, wherein the steel sheet provided at step a) is produced by the following successive steps: i) casting a steel to obtain a slab, said steel having the composition according to claim 1, ii) reheating the slab at a temperature Treheat comprised from 1100°C to 1300°C to obtain a reheated slab, 14 iii) hot rolling the reheated slab at a finish hot rolling temperature comprised from 800°C to 950°C, to obtain a hot rolled steel sheet, iv) coiling the hot rolled steel sheet at a coiling temperature Tcoil lower than 670°C to obtain a coiled steel sheet, and V) cold rolling the coiled steel sheet to obtain a cold rolled steel sheet. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00006"> <pat:ClaimNumber>6< / pat:ClaimNumber> <pat:ClaimText>6. The method according to claim 5, comprising a step of pickling the coiled steel sheet after the step of coiling the hot rolled steel sheet and prior to the step of cold rolling the coiled steel sheet. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00007"> <pat:ClaimNumber>7< / pat:ClaimNumber> <pat:ClaimText>7. The method according to claim 5 or claim 6, comprising a step of heating the hot rolled steel sheet to a temperature THBA comprised from 500°C to 750°C, and maintaining the hot rolled steel sheet at said temperature Theat for a holding time the comprised from 300s to 50h, the step of heating the hot rolled steel sheet accruing after the step of coiling the hot rolled steel sheet and prior to the step of cold rolling. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00008"> <pat:ClaimNumber>8< / pat:ClaimNumber> <pat:ClaimText>8. The method according to claim 5, comprising a step of heating the cold rolled steel sheet to an annealing temperature TA comprised from 650°C to 900°C and maintaining the cold rolled steel sheet at said temperature TA for a holding time tA comprised from 10s to 1200s to obtain an annealed steel sheet, the step of heating the cold rolled steel sheet occurring after the step of cold rolling the coiled steel sheet. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00009"> <pat:ClaimNumber>9< / pat:ClaimNumber> <pat:ClaimText>9. The method according to claim 5, wherein the cold rolled steel sheet is heated to an annealing temperature TA comprised from 500°C to 750°C and maintained at said temperature TA for a holding time tA comprised from 300s to 50h, to obtain an annealed steel sheet, and cooled to room temperature. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00010"> <pat:ClaimNumber>10< / pat:ClaimNumber> <pat:ClaimText>10. The method for producing the press hardened steel part according to claim 9, wherein the said annealed steel sheet is coated with aluminium or with an aluminium alloy coating or with zinc or zinc alloy coating. < / pat:ClaimText> < / pat:Claim> < / pat:Claims>