High strength steel part and associated hot stamping process

Abstract

Steel part having the following characteristics: -a chemical composition comprising, by weight % 0.26 ≤ C ≤ 0.33 0.15 ≤ Si ≤ 0.40 1.0 ≤ Mn ≤ 1.8 0.10 ≤ Cr ≤ 0.40 0.02 ≤ Al ≤ 0.10 0.01 ≤ Ti ≤ 0.06 0.0010 ≤ B ≤ 0.0050 the remainder of the composition being iron and unavoidable impurities resulting from the elaboration process, -a microstructure comprising, in surface fraction, 70% or more of auto-tempered martensite, from 1 to 20% of bainite, the rest of the microstructure comprising optionally retained austenite and ferrite.

Description

[0001] High strength steel part and associated hot stamping process

[0002]

[0001] The present invention relates to a high strength steel part and a process to produce the same.

[0003]

[0002] High strength steel parts can be used as structural elements in automotive vehicles for anti-intrusion or energy absorption functions.

[0004]

[0003] In such type of applications, it is desirable to produce steel parts that combine high mechanical strength and high impact resistance. Moreover, one of the major challenges in the automotive industry is to decrease the weight of vehicles in order to improve their fuel efficiency without neglecting the safety requirements. This weight reduction can be achieved in particular thanks to the use of steel parts with a predominantly martensitic microstructure.

[0005]

[0004] It is well known to produce high strength steel parts using a hot stamping process in which a steel blank is heated to above its austenitizing temperature Ac3 and then hot formed and quenched in the same tool to reach a fully martensitic micro-structure, as for example described in PCT / IB2006 / 004019.

[0006]

[0005] Currently, the mainstream hot stamping steel grade is 22MnB5 and closely related metallurgies. This grade offers very good mechanical resistance after hot stamping, reaching an ultimate tensile strength of 1500MPa or more. It has been widely adopted in the automotive industry for safety parts. The ultimate tensile strength range of this one shot die quenched grade can be translated in Vickers hardness to a range from 470Hv to 580Hv, preferably from 480Hv to 550Hv.

[0007]

[0006] Using 22MnB5 and closely related grades, it is necessary to quench the part at a very high cooling rate, typically above 50°C / s, to obtain the desired microstructure and mechanical properties. It is also necessary to keep the formed part in the stamping and quenching tool down to a temperature of 200°C or less. These two limitations impose a one shot simple hot stamping process, leaving out the possibility of performing more complex operations such as the use of different forming tools for parts having complex shapes or mechanical trimming at high temperature, known as hot trimming. Furthermore, opening the die at 200°C or less means that the part has to stay in the die for a long time to reach the final temperature, which leads to long cycle times and reduced productivity.

[0008]

[0007] Additionally, die quenched 22MnB5 has a microstructure which consists approximately of 10% to 20% of fresh martensite and more than 80% of auto-tempered martensite. While this allows for very high strength, the significant amount of fresh martensite leads to relatively low levels of deformability, as measured by a bending test or crash ductility tests such as the fracture strain measurement or notch tensile tests. This drawback of die quenched 22MnB5 is usually mitigated by the integration of a paint baking step in the production process of automotive parts, during which the part is heated to a temperature from 150°C to 250°C for a time comprised from 10 minutes to 2 hours. This paint baking step has the effect to temper the remaining fresh martensite, which increases the bendability and crash ductility. However, current evolutions in automotive paint-shop, driven by environmental and economic considerations, tend to reduce or even fully dispose of the paint baking step. The relatively low bendability and crash ductility of fresh 22MnB5 die quenched part could become a serious issue depending on the evolution of paint shop technologies.

[0009]

[0008] The purpose of the current invention is to provide a hot stamped part and an associated manufacturing process which overcomes the above- mentioned challenges.

[0010]

[0009] The object of the present invention is achieved by providing a steel part according to claim 1 , optionally having the features of claim 2. Another object of the present invention is achieved by applying a process to produce such a steel part according to claim 3.

[0011]

[0010] By applying the current invention, it is possible to apply a complex hot stamping process to produce a high strength hot stamped part having similar mechanical resistance as a 22MnB5 hot stamped part in a traditional one shot hot stamping process. This offers much more flexibility to the hot stamper in terms of stamping process design and for example the possibility of integrating a trimming operation during hot stamping. Furthermore, the stamping tool can be opened at a much higher temperature, which allows to decrease the overall cycle time and increase productivity. Additionally, the microstructure of the hot stamped part according to the invention leads to similar bending and crash ductility behavior as a die quenched and paint baked 22MnB5 part, without applying the paint baking step.

[0012]

[0011] The invention will now be described in detail and illustrated by examples without introducing limitations. Figures 1 and 2 are temperature profiles of hot stamping processes according to the invention, figure 3 is a temperature profile of a hot stamping process outside of the invention. Figures 1 , 2 and 3 relate to the experimental trials that are detailed in the examples.

[0013]

[0012] A blank of steel refers to a flat sheet of steel, which has been cut to any shape suitable for its use. A blank has a top and 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 blank. 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]

[0013] A hot stamped steel part refers to a part that was formed by hot stamping a steel blank.

[0015]

[0014] By average thickness of a part, or of a portion of a part, it is meant the overall average thickness of the material making up the part after it has been formed into a 3-dimensional part from an initially flat sheet.

[0016]

[0015] When referring to the thickness of a steel part, one refers to the local thickness measured using for example the above-described spindle and angle or else using for example a micrograph of a cross section, paying attention to the correct perpendicularity of the sample.

[0017]

[0016] Hot stamping is a forming technology which involves heating a blank up to a temperature at which the microstructure of the steel has at least partially transformed to austenite, forming the blank at high temperature by stamping it and quenching the formed part to obtain a microstructure having a very high strength. Hot stamping allows to obtain very high strength parts with complex shapes and presents many technical advantages

[0017] The yield strength and ultimate tensile strength are measured according to ISO standard ISO 6892-1 , published in October 2009. The tensile test specimens are cut-out from flat areas of the hot stamped part. If necessary, small size tensile test samples are taken to accommodate for the total available flat area on the part.

[0018]

[0018] 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 Hv.

[0019]

[0019] The bending angle is measured according to the VDA-238 bending standard. The bending angle of a part measures its bendability, which is representative of the ability of the part to resist deformation without cracking.

[0020]

[0020] Ac3 designates the equilibrium temperature at which a given steel grade is fully austenitic. It can be estimated from the chemical composition using the following formula, in which the result is a temperature in °C and the chemical elements are expressed in weight%:

[0021] Ac3 = 935-257*C-25*Mn+32*Si-17*Cr-83*Ni+17*AI+129*Mo+156*Nb-2*B

[0022]

[0021] Ms designates the temperature under which martensite starts to form. It can be estimated from the chemical composition using the following formula, in which the result is a temperature in °C and the chemical elements are expressed in weight%:

[0023] MS = 545 - 601.2 * (1 - e“°'868*c) - 34.4 * Mn - 9.2 * Cr - 17.3

[0024] * Ni — 13.7 * Si — 15.4 * Mo — 1.4 * Al — 16.3 * Cu

[0025] - 361 * Nb - 2.44 * Ti - 3448 * B

[0026]

[0022] The composition of the steel part according to the invention will now be described.

[0027]

[0023] The chemical compositions are given in terms of a lower and upper limit of the composition range, said limits being comprised within the possible composition range according to the invention. In the case when preferred ranges for a given element are disclosed, the present invention also discloses all possible combinations of these preferred ranges for each individual element.

[0028]

[0024] According to the invention the carbon ranges from 0.26% to 0.33% to ensure a satisfactory strength. Above 0.33% of carbon, weldability issues may arise, the hardness of the steel will be too high and the deformability and bendability of the part will be deteriorated. If the carbon content is lower than 0.26%, the hardness will not reach the targeted value.

[0029]

[0025] The silicon content ranges from 0.15% to 0.40%. Silicon is an element participating in the hardening in solid solution and limiting carbides formation. Above 0.40%, silicon oxides form at the surface, which impairs the coatability of the steel. Below 0.15% the targeted hardness is not met.

[0030]

[0026] The manganese content ranges from 1 .0% to 1 .8 %. Above 1 .8%, the risk of MnS formation is increased to the detriment of the bendability. Below 1.0% the hardenability of the steel sheet during the hot stamping process is reduced and the targeted hardness is not reached. The maximum Mn content is preferably limited to 1 .4% to further limit the risk of MnS formation.

[0031]

[0027] The chromium content ranges from 0.10% to 0.40%. Chromium is used to provide strength by solid solution hardening and to improve the hardenability of the steel sheet during hot stamping. Chromium is limited to 0.40% to limit costs and avoid processing issues. Below 0.10% the targeted hardness is not met.

[0032]

[0028] The aluminum content ranges from 0.02% to 0.10% as it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Aluminum can protect boron if titanium content is not sufficient. The aluminum content is lower than 0.10% to avoid oxidation problems and ferrite formation during press hardening. Below 0.02% the desired deoxidizing properties of aluminum in the liquid phase are not reached.

[0033]

[0029] The titanium content ranges from 0.01 % to 0.06 % in order to protect boron, which would otherwise be trapped within BN precipitates. Titanium content is limited to 0.06% to avoid excess TiN formation. Below 0.01 %, the desired boron protection properties of titanium are not reached.

[0030] The boron content ranges from 0.0010% to 0.0050%. Boron improves the hardenability of the steel. The boron content is not higher than 0.0050% to avoid semi-product breaking issues right after casting. Below 0.0015%, the targeted hardness is not met.

[0034]

[0031] Sulphur is controlled to below or equal to 0.005%, preferably 0.003%, because the presence of Sulphur in the liquid steel can lead to the formation of MnS precipitates which are detrimental to bendability.

[0035]

[0032] Phosphorous is controlled to below or equal to 0.04%, because it leads to fragility and weldability issues. In a specific embodiment, the P content is controlled to below or equal to 0.02% to further avoid fragility and weldability issues.

[0036]

[0033] Nitrogen is controlled to below 0.010%, 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.

[0037]

[0034] Nickel is optionally added, up to a level of 0.5%, preferably 0.3%. Nickel is a hardening element and can also be used to protect the steel from delayed cracking caused by hydrogen embrittlement.

[0038]

[0035] Molybdenum is optionally added up to 0.3%. Molybdenum improves the hardenability of the steel. Molybdenum is limited to 0.3% to limit costs and avoid processing issues.

[0039]

[0036] Niobium is optionally added up to 0.1 %. Niobium improves ductility of the steel. Niobium is limited to 0.1 % to limit costs and avoid processing issues.

[0040]

[0037] Vanadium is optionally added up to 0.3%. Vanadium improves the hardenability of the steel. Vanadium is limited to 0.3% to limit costs and avoid processing issues.

[0041]

[0038] In case one or several of the above elements are added, the following formula is further verified: Cr + Mo + Nb + V < 0.5% in order to limit costs and avoid processing issues.

[0042]

[0039] Copper is optionally added, up to a level of 0.4%. Copper can improve the hydrogen embrittlement cracking resistance of steel and is a strengthening element of steel. Copper is limited to 0.4% 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.

[0043]

[0040] 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 and the level of scrap used in the steel melt. For example, when using a Basic oxygen furnace route with a low level of steel scrap (recycled steel), the level of impurities will remain very low. It is however also possible to add a high amount of scrap in the converter to the pig iron produced in the basic oxygen furnace, which will increase the level of impurities. Furthermore, when elaborating the steel using an electric furnace for example, with a very high ratio of recycled scrap steel, the level of impurities will be significantly increased. When using a high level of scrap, the level of Sn can reach 0.05%, As can reach 0.03%, Sb can reach 0.03% and Pb can reach 0.03%.

[0044]

[0041] The steel blank 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:

[0045]

[0042] -A semi-product, typically a slab, able to be further hot-rolled, is provided with the steel composition described above.

[0046]

[0043] -The semi product is then optionally reheated at a temperature comprised from 1150°C to 1300°C.

[0047]

[0044] -The steel sheet is then hot rolled at a finish hot rolling temperature comprised from 800°C to 950°C.

[0048]

[0045] -The hot-rolled steel is then cooled and coiled at a temperature lower than 670°C, and optionally pickled to remove oxidation.

[0049]

[0046] -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.

[0047] -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.

[0050]

[0048] -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.

[0051]

[0049] 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.

[0052]

[0050] 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.

[0053]

[0051] Said steel sheet is then cut into steel blanks, having the desired shape for the subsequent hot stamping operation. The cutting step can be performed mechanically using a shear. The cutting step can be performed using laser cutting or any other technical mean.

[0054]

[0052] In a particular embodiment the steel blank to be hot stamped is a tailor welded steel blank. Tailor welded steel blanks are made by assembling together, for example by laser welding together, several 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).

[0055]

[0053] In a particular embodiment the steel blank to be hot stamped is a tailor rolled steel blank. A tailor rolled steel blank is a steel blank having multiple sheet thicknesses obtained by differential rolling during the steel sheet production process.

[0054] The hot stamping process according to the invention will now be descried.

[0056]

[0055] A steel blank having the above-described chemical composition is heated to a temperature from Ac3 to 950°C and held above Ac3 for a duration comprised from 10 seconds to 15 minutes.

[0057]

[0056] The steel blank is subsequently transferred to a hot stamping equipment and processed into a steel part while controlling the temperature profile of the steel part in the following ranges:

[0058] - the average cooling rate from Ac3 to Ms+50°C is equal to or greater than 25°C / s, preferably 30°C / s,

[0059] -the average cooling rate from Ms+50°C to Ms-150°C is comprised from 2°C / s to 15°C / s,

[0060] -there is no limitation on the cooling rate from Ms-150°C to room temperature,

[0061] -The heating profile, after reaching room temperature, can include optionally a paint baking step of said steel part at a temperature from 150°C to 250°C for a time comprised from 10 minutes to 2 hours.

[0062]

[0057] Thanks to the lowered cooling rate from Ms+50°C to Ms-150°C, it is possible for example to integrate several complex operations within the hot stamping production cycle to produce parts with complex shapes and with hot trimming.

[0063]

[0058] The lower cooling rates below Ms+50°C and the absence of cooling rate limitation below Ms-150°C is compatible with air cooling, which means that the part can be directly unloaded from the hot stamping tool at temperatures higher than 200°C, thus reducing cycle time and increasing productivity.

[0064]

[0059] The microstructure of the steel part according to the invention will now be described:

[0065]

[0060] The steel part comprises in surface fraction, 75% or more of autotempered martensite, from 1 to 20% of bainite, <5% of retained austenite and ferrite. The microstructure does not contain any fresh martensite.

[0061] The bainite content is ensured to be at least 1 %, which improves the ductility of the steel part while maintaining a high level of mechanical strength (contrary to ferrite, which greatly diminishes the hardness of the steel). The bainite content should not exceed 20% in order to guarantee the desired hardness level of the steel part.

[0066]

[0062] The auto-tempered martensite can be differentiated from fresh martensite by SEM analysis using the following protocol:

[0067] -Martensite grains are first identified,

[0068] -within the martensite, carbide particles are detected, using a minimum particle size detection threshold of 10nm,

[0069] -the density of particles per square micron is computed

[0070] -the martensite grains are classified into fresh martensite and autotempered martensite according to their carbide density: fresh martensite has a carbide density comprised from 0 to 4 carbides per square micron, auto-tempered martensite has a carbide density comprised from 5 to 100 carbide per square micron.

[0071]

[0063] The inventors have found that the above-described hot stamped parts have a hardness which is comprised from 470Hv to 580Hv, preferably from 480Hv to 550Hv, which corresponds to the targeted mechanical properties of mainstream hot stamped part applications using one shot hot stamping on 22MnB5. The inventors have also found that the above-described microstructure, comprising mainly auto-tempered martensite and no fresh martensite, allows to also reach very good bending and crash ductility behaviors, which would require a paint baking step when using one shot die quenched 22MnB5.

[0072]

[0064] The invention will now be illustrated by way of examples, which are in no way limitative.

[0073]

[0065] Table 1a summarizes the chemical compositions of the samples, expressed in wt%, as well as the computed Ac3 temperature (in °C). The underlined values are outside the range of the current invention.

[0074] Table 1a: Chemical composition

[0075]

[0066] Table 1 b summarizes the process parameters that were used to produce the steel blanks to be hot stamped using the above-described steel grades.

[0076] Table 1b: steel blanks production process parameters

[0077]

[0067] Three different temperature profiles were tested on the abovedescribed steel grades, profile 1 , profile 2 and profile 3. The first two profiles correspond to a first rapid quenching and then slower cooling, as in the current invention, while the third one is a simple one-shot hot stamping profile, not according to the invention but provided as a reference.

[0078]

[0068] Tables 2a, 3a and 4a summarize respectively the characteristics of the hot-stamping temperature profile of profile 1 , 2 and 3 - the temperature evolution is linear between two listed points in the tables. The first column indicates the temperature. The second column indicates the time spent between the temperature of the previous line and the temperature of the current line. The third column indicates the cumulated time spent in total since the first line of the table. All temperatures are indicated in °C, all times are indicated in seconds, all cooling speeds in °C / s. Figures 1 , 2 and 3 are graphic representations of respectively temperature profiles 1 , 2 and 3.

[0079]

[0069] Tables 2b, 3b and 4b summarize the holding time above Ac3, and the cooling rates between Ac3 and Ms + 50°C, as well as between Ms + 50°C and Ms - 150°C for each of the three steel grades that were tested respectively for temperature profiles 1 , 2 and 3. In the case of profile 3, the values which are outside of the invention are underlined.

[0080]

[0081] Table 2a: Profile 1 description

[0082]

[0083] Table 3a: Profile 2 description

[0084]

[0085] Table 4a: Profile 3 description

[0086]

[0070] Table 5 summarizes the properties of the hot stamped samples.

[0087] References 11 to I2 are according to the invention while R1 to R7 are outside of the invention. The microstructure is expressed in surface fraction.

[0088] Table 5: sample properties

[0071] Thanks to the combination of steel grade A, which is according to the invention, and to the application of hot stamping temperature profiles according to the invention, samples 11 and I2 have the desired predominantly auto-tempered martensitic microstructure, without fresh martensite, and

[0089] 5 hardness values in the desired range of 470Hv to 580Hv.

[0090]

[0072] On the other hand, as in the case of R1 and R3, when applying the same temperature profiles 1 and 2 to grade B, which is 22MnB5, the desired mechanical strength is not reached, with hardness values below 470Hv.

[0091]

[0073] When applying profiles 1 and 2 to grade C, as in the case of R2 and w R4, the produced part’s mechanical strength is above the desired range, with hardness values above 580Hv.

[0092]

[0074] Profile 3 is a one-shot rapid die quenching process down to room temperature, which is not according to the invention and provided as reference. When applying said temperature profile to grades A, B and C, as 15 in the case of R5, R6 and R7, the microstructure contains significant amounts of fresh martensite, from 10% to 20%, which will result in deteriorated bending and crash ductility results.

Claims

CLAIMS1 ) Steel part having the following characteristics:-a chemical composition comprising, by weight %0.26 < C < 0.330.15 < Si <0.401.0<Mn<1.40.10 <Cr< 0.400.02 < Al <0.100.01 < Ti <0.060.0010 <B <0.00500 < S < 0.0050 < P < 0.040< N <0.0100< Ni <0.50< Mo <0.30< Nb<0.10< V < 0.30 < Cu < 0.4Cr + Mo + Nb + V < 0.5%, the remainder of the composition being iron and unavoidable impurities resulting from the elaboration process,-a microstructure comprising, in surface fraction, 75% or more of autotempered martensite, from 1 to 20% of bainite, optionally less than 5% of retained austenite and ferrite, the microstructure does not contain any fresh martensite,-a hardness comprised from 470Hv to 580Hv.2) Steel part according to claim 1, wherein said steel part is topped with a metallic coating comprising an aluminum content, expressed in weight%, equal to or higher than 50%.3) Process for producing a steel part comprising the following steps: -Providing a steel blank having a composition according to claim 1 or 2,-heating it to a temperature from Ac3 to 950°C and holding it above Ac3 for a duration comprised from 10 seconds to 15 minutes,-Transferring said heated steel blank to a hot stamping equipment and processing it into a steel part while controlling the temperature profile of the steel part in the following ranges:-the average cooling rate from Ac3 to Ms+50°C is equal to or greater than 25°C / s,-the average cooling rate from Ms+50°C to Ms-150°C is comprised from 2°C / s to 15°C / s.

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