ES3055855T3 - Coated steel sheet and high strength press hardened steel part and method of manufacturing the same - Google Patents

Abstract

The invention relates to a coated steel sheet and a pressure-hardened steel part having a composition comprising, in weight percentage: C 0.26-0.40%, Mn 0.5-1.8%, Si 0.1-1.25%, Al 0.01-0.1%, Cr 0.1-1.0%, Ti 0.01-0.1%, B 0.001-0.004%, P <= 0.020%, S <= 0.010%, N <= 0.010%, the remainder of the composition being iron and unavoidable impurities resulting from smelting. The pressure-hardened steel part comprises a bulk having a microstructure comprising, as a surface fraction, more than 95% martensite and less than 5% bainite, a coating layer on the surface of the steel part, a ferritic interdiffusion layer between the coating layer and the bulk, and a ratio of the ferritic grain width in the interdiffusion layer GWint to the austenite grain size above in the bulk PAGSbulk, satisfying the following equation (GWint/PAGSbulk) - 1 >= 30%.

Description

[0001] DESCRIPTION

[0002] Coated steel sheet and high-strength pressure-hardened steel part and its manufacturing process

[0003] The present invention relates to coated steel sheets and to high-strength pressure-hardened steel parts that have good bending capacity properties.

[0004] High-strength pressure-hardened parts can be used as structural elements in motor vehicles for anti-intrusion or energy-absorbing functions.

[0005] In such applications, it is desirable to produce steel parts that combine high mechanical strength, high impact resistance, and good corrosion resistance. Furthermore, one of the main challenges in the automotive industry is to reduce vehicle weight to improve fuel efficiency in light of global environmental conservation, without neglecting safety requirements.

[0006] This weight reduction can be achieved in particular through the use of steel parts with a martensitic or bainitic/martensitic microstructure.

[0007] Publication WO2016104881 relates to a hot-formed part used as a structural component of a vehicle or similar, requiring impact resistance characteristics and, more particularly, having a tensile strength of 1300 MPa or higher, and a manufacturing process involving heating a steel material to a temperature at which a single austenite phase can be formed, and quenching and hot-forming the same using a die. To obtain such properties, the base steel sheet comprises a thin ferrite layer less than 50 μm thick on the surface, and the size and density of the carbides must be controlled. This ferrite layer on the substrate inhibits the propagation of fine cracks formed in the plating layer to the base, but results in low bending capacity with a bending angle of less than 70°.

[0008] Publication WO2018179839 relates to a hot-pressed part obtained by hot-pressing a steel sheet having a microstructure that changes in the thickness direction, with a soft layer made of at least 90% ferrite, a transition layer made of ferrite and martensite, and a hard, primarily martensitic layer, exhibiting high strength and high bending capacity. To obtain such properties, the cold-rolled steel sheet is annealed in an atmosphere with a dew point temperature ranging from 50°C to 90°C, which could be detrimental to the aluminum alloy coating. Further prior art is described in documents US2017/260599A1 and WO2018/220540A1.

[0009] Therefore, the object of the invention is to solve the aforementioned problem and provide a pressure-hardened steel part having a combination of high mechanical properties with a tensile strength TS greater than or equal to 1500 MPa and a bending angle greater than 70°. The pressure-hardened steel part according to the invention has a yield strength YS greater than or equal to 1250 MPa.

[0010] Another object of the invention is to obtain a coated steel sheet that can be transformed by hot forming into said pressure-hardened steel piece.

[0011] The object of the present invention is achieved by providing a steel sheet according to claim 1. Another object is achieved by providing the process according to claim 2. Another object of the present invention is achieved by providing a pressure-hardened steel piece according to claim 3. The steel piece may also comprise the features of claim 4. Another object is achieved by providing the process according to claim 5.

[0012] The invention will now be described in detail and illustrated by examples without introducing limitations, with reference to the accompanying figures:

[0013] - Figure 1a illustrates a schematic section of the coated steel sheet of test 4, which is not according to the invention

[0014] - Figure 1b represents a schematic section of the pressure-hardened steel piece of test 4, which is not according to the invention

[0015] - Figure 2a illustrates a schematic section of the coated steel sheet of test 3, which is not according to the invention

[0016] - Figure 2b represents a schematic section of the pressure-hardened steel piece of test 3, which is not according to the invention

[0017] - Figure 3a illustrates a schematic section of the coated steel sheet of test 2, which is according to the invention - Figure 3b represents a schematic section of the pressure-hardened steel piece of test 2, which is according to the invention

[0018] - Figure 4a illustrates a schematic section of the coated steel sheet of test 1, which is according to the invention - Figure 4b represents a schematic section of the pressure-hardened steel piece of test 1, which is according to the invention

[0019] - Figure 5a illustrates a schematic section of the coated steel sheet of test 5, which is not according to the invention

[0020] - Figure 5b represents a schematic section of the pressure-hardened steel piece of test 5, which is not according to the invention

[0021] The composition of the steel according to the invention will now be described, with the content expressed as a percentage by weight.

[0022] According to the invention, the carbon content is between 0.26% and 0.40% to ensure satisfactory strength. Above 0.40% carbon, the weldability and bending capacity of the steel sheet may be reduced. If the carbon content is less than 0.26%, the tensile strength will not reach the target value.

[0023] The manganese content is between 0.5% and 1.8%. Above 1.8% addition, the risk of central segregation increases, impairing flexural strength. Below 0.5%, the hardenability of the steel sheet is reduced. Preferably, the manganese content is between 0.5% and 1.3%.

[0024] According to the invention, the silicon content is between 0.1% and 1.25%. Silicon is an element involved in solid solution hardening. Silicon is added to limit carbide formation. Above 1.25%, silicon oxides form on the surface, which impairs the steel's coating capacity. Furthermore, the weldability of the steel sheet may be reduced. Preferably, the silicon content is between 0.2% and 1.25%. More preferably, the silicon content is from 0.3% to 1.25%. More preferably, the silicon content is from 0.3% to 1%.

[0025] The aluminum content is between 0.01% and 0.1%, as it is a very effective element for deoxidizing steel in the liquid phase during its processing. Aluminum can protect boron if the titanium content is insufficient. The aluminum content is less than 0.1% to avoid oxidation problems and ferrite formation during pressure hardening. Preferably, the aluminum content is between 0.01% and 0.05%.

[0026] According to the invention, the chromium content is between 0.1% and 1.0%. Chromium is an element that participates in solid solution hardening and must be greater than 0.1%. The chromium content is less than 1.0% to limit processability problems and cost.

[0027] The titanium content is between 0.01% and 0.1% in order to protect boron from the formation of BN. The titanium content is limited to 0.1% to prevent the formation of TiN.

[0028] According to the invention, the boron content is between 0.001% and 0.004%. Boron improves the hardenability of the steel. The boron content does not exceed 0.004% to avoid the risk of breakage of the flat roughing during continuous casting.

[0029] Some elements can be added optionally.

[0030] Nickel up to 0.5% can be added as an optional element, as it can substantially reduce sensitivity to delayed fracture.

[0031] A molybdenum content of up to 0.40% may be added optionally. Like boron, molybdenum improves the hardenability of the steel. The molybdenum content is not greater than 0.40% to limit cost.

[0032] According to the invention, up to 0.08% niobium may be optionally added to improve the ductility of the steel. Above 0.08% addition, the risk of NbC or Nb(C,N) carbide formation increases, impairing the bending capacity. Preferably, the niobium content is less than or equal to 0.05%.

[0033] Calcium can also be added up to 0.1% as an optional element. The addition of Ca in the liquid stage allows the creation of fine oxides that promote the castability of the continuous casting.

[0034] The remainder of the steel composition is iron and impurities resulting from smelting. In this respect, P, S, and N are at least considered residual elements that are unavoidable impurities. Their content is less than 0.010% for S, less than 0.020% for P, and less than 0.010% for N.

[0035] The microstructure of the coated steel sheet according to the invention will now be described.

[0036] A section of a coated steel sheet of the invention is schematically represented in Fig. 3a and Fig. 4a. The coated steel sheet comprises a mass (2) covered by a decarburized layer (3) comprising, on top, a ferrite layer having a thickness of 1 μm to 100 μm (4), and a coating layer (1). Preferably, the thickness of the ferrite layer is between 20 μm and 100 μm. More preferably, the thickness of the ferrite layer is from 25 μm to 100 μm. More preferably, the thickness of the ferrite layer is from 30 μm to 80 μm.

[0037] The mass of the coated steel sheet (2) has a microstructure comprising, in a surface fraction, 60% to 90% ferrite, the remainder being islands of martensite-austenite, pearlite or bainite.

[0038] This ferrite forms during the intercritical annealing of the cold-rolled steel sheet. The remainder of the microstructure is austenite at the end of homogenization, which transforms into islands of martensite-austenite, pearlite, or bainite during cooling of the steel sheet.

[0039] The decarburized layer present on the top of the mass is obtained during the annealing of the cold-rolled steel sheet by controlling the atmosphere in the furnace to establish a dew point temperature strictly above -10 °C and below or equal to 20 °C.

[0040] The coated steel sheet according to the invention can be produced by any suitable manufacturing process, and one can be defined by a person skilled in the art. However, the process according to the invention is preferred, comprising the following steps:

[0041] A semi-finished product that can be further hot-rolled is provided with the steel composition described above. The semi-finished product is reheated to a temperature between 1150 °C and 1300 °C.

[0042] Next, the steel sheet is hot rolled at a finishing hot rolling temperature of between 800 °C and 950 °C.

[0043] Next, the hot-rolled steel is cooled and coiled to a temperature T<coil> below 670 °C and, optionally, pickled to remove rust.

[0044] The coiled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling reduction ratio is preferably between 20% and 80%. Below 20%, recrystallization is not favored during subsequent heat treatment, which may impair the ductility of the steel sheet. Above 80%, there is a risk of edge cracking during cold rolling.

[0045] The steel sheet is then annealed in an HNx atmosphere with 0% to 15% H2, at an annealing temperature T<A> between 700 °C and 850 °C and held at that annealing temperature T<A> for a holding time t<A> between 10 s and 1200 s, in order to obtain an annealed steel sheet. Below 700 °C, the kinetics of decarburized layer formation are too slow to obtain a ferrite layer on top. The holding time t<A> is greater than or equal to 10 s to allow the ferrite layer to form, and less than or equal to 1200 s to limit the thickness of this ferrite layer.

[0046] During this annealing process, the atmosphere in the furnace is controlled to have a dew point temperature T<DP1> strictly above -10 °C and below or equal to 20 °C in order to form a decarburized layer according to the invention. If T<DP1> is below or equal to -10 °C, the formation of the decarburized layer is slowed, and the ferrite layer does not form on top. The bending capacity of the steel part will be too low. If T<DP1> is above 20 °C, the surface of the steel sheet may be completely oxidized, which impairs the coating capacity and mechanical properties of the sheet.

[0047] In one embodiment of the invention, the annealed steel sheet is heated to an annealing temperature T2 between 700 °C and 850 °C and held at said temperature T2 for a holding time t2 between 10 s and 1200 s, the atmosphere having a dew point T<DP2> strictly above -10 °C and less than or equal to 20 °C.

[0048] Next, the steel sheet is coated with an aluminum alloy coating.

[0049] The microstructure of the pressure-hardened steel part according to the invention will now be described. A section of the pressure-hardened steel part is shown schematically in Fig. 3b and Fig. 4b.

[0050] The steel part comprises successively from the mass to the surface of the steel part: - a mass (7) having a microstructure comprising, in the surface fraction, more than 95% martensite and less than 5% bainite,

[0051] - a ferritic interdiffusion layer (6),

[0052] - an aluminum-based coating layer (5).

[0053] During the heating of the steel blank cut from the steel sheet according to the invention, all the microstructural elements of the bulk material transform into austenite, and the ferrite of the decarburized layer transforms into austenite with a grain size wider than the austenite of the bulk material. After hot forming, the steel blank is die-quenched. The interdiffusion layer grows from the previous broad-grained austenite layer, and thus has a grain width greater than the previous austenitic grain size in the bulk material. The ratio of the ferritic grain width in the interdiffusion layer GW<int> to the previous austenite grain size PAGS<bulk> satisfies the following equation:

[0056]

[0058] in order to improve the bending capacity of the steel sheet, without deteriorating the mechanical properties.

[0059] The ferritic grain width is the average distance between two parallel grain boundaries, with the grain boundaries oriented in the direction of the sheet thickness. The combination of annealing temperature T<A>, annealing time t<A>, and dew point temperature T<DP1> according to the invention promotes the formation of a large grain width GW<int> in the interdiffusion layer. Furthermore, the heat treatment of the steel blank prior to press forming regulates the growth of the austenitic grain and thus the PAGS in the bulk.

[0060] In one embodiment, the pressure-hardened steel part may further comprise a martensite layer with a carbon gradient between the bulk and the interdiffusion layer, as represented by (8) in Fig. 4b. During heating of the steel blank, carbon diffuses from the bulk to the surface. The ferrite upper portion of the decarburized layer then transforms into an austenite layer with a carbon gradient. During die quenching, this austenite layer with a carbon gradient transforms into a martensite layer with a carbon gradient.

[0061] The pressure-hardened steel part according to the invention has a tensile strength TS greater than or equal to 1500 MPa and a bending angle greater than 70°. The bending angle was determined on pressure-hardened parts according to the bending standard of procedure VDA238-100 (normalized to a thickness of 1.5 mm).

[0062] According to the invention, the elastic limit YS is greater than or equal to 1250 MPa. TS and YS are measured according to ISO standard, ISO 6892-1.

[0063] The pressure-hardened steel part according to the invention can be produced by any suitable manufacturing process, and one can be defined by a person skilled in the art. However, the process according to the invention is preferred, comprising the following steps:

[0064] A coated steel sheet according to the invention is cut to a predetermined shape to obtain a steel blank. The steel blank is then heated to a temperature between 880 °C and 950 °C for 10 s to 900 s to obtain a heated steel blank.

[0065] Next, the heated blank is transferred to a forming press before being hot formed and die-hardened.

[0066] The invention will now be illustrated by the following examples, which are in no way limiting. Example

[0067] 6 qualities, whose compositions are listed in Table 1, were melted into semi-finished products and processed into steel sheets, then steel parts, following the process parameters listed in Table 2.

[0068] Table 1 - Compositions

[0069] The analyzed compositions are grouped in the following table, where the content of the element is expressed as a percentage by weight.

[0071]

[0072]

[0074] Table 2 - Process parameters

[0075] The steel semi-finished products, as melted, were reheated to 1200 °C, hot-rolled to a final hot-rolling temperature between 800 and 950 °C, coiled at 550 °C, and cold-rolled at a reduction rate of 60%. The steel sheets were then heated to a temperature T<A> and held at that temperature for a holding time t<A>, in an atmosphere of HNx with 5% H<2>, having a controlled dew point. The steel sheets were then cooled to a temperature of 560 to 700 °C and subsequently hot-dip coated with an aluminum-silicon coating comprising 10% silicon.

[0076] Samples 1, 2, 5, and 6 were subjected to a second annealing at a temperature T<2> prior to coating, with the steel sheet being held at this temperature T<2> for a holding time t<2>, in an HNx atmosphere with 5% H<2> and a controlled dew point. The following specific conditions were applied:

[0079]

[0081] Underlined values: do not correspond to the invention

[0082] The coated steel sheets were analyzed and the corresponding properties of the decarburized layer are collected in Table 3.

[0083] T l - Pr i l r r l l min r r i r

[0086]

[0088] [0052]Next, the coated steel sheets were cut to obtain a steel blank, heated to 900 °C for 6 minutes and hot-formed. The steel blanks were analyzed and the corresponding microstructure, ferritic grain width in the interdiffusion layer GW<int> and the previous austenite grain size in the PAGS<mass> are collected in Table 4. The mechanical properties are collected in Table 5.

[0089] T l 4 - Mi r r r l i z r n r i r r i n

[0092]

[0094] The surface fractions, ferritic grain width in the interdiffusion layer, and PAGS are determined by the following procedure: a sample is cut from the pressure-hardened steel part, polished, and etched with a known reagent to reveal the microstructure. Subsequently, the section is examined using an optical or scanning electron microscope, for example, a scanning electron microscope with a field emission gun (“FEG-SEM”) at a magnification greater than 5000x, coupled to a back-scattered electron (BSE) device.

[0095] T l - Pr i m ni l i z r n r i r r i n

[0098]

[0100] The examples show that the steel parts according to the invention, namely examples 1-2, are the only ones that exhibit all the target properties thanks to their specific compositions and microstructures.

[0101] Figure 3a represents a schematic section of the coated steel sheet from test 2. The combination of process parameters of the invention, annealing temperature T<A>, annealing time t<A> and dew point temperature T<DP1>, allows obtaining a decarburized layer (3), where a ferrite layer is formed on top (4).

[0102] Next, the coated steel sheet is hot-formed. Fig. 3b shows a schematic section of the pressure-hardened steel part from test 2.

[0103] The grain width of the ferrite formed in the interdiffusion layer (5) is inherited from the pure ferrite layer where austenite formation occurs during heating, with a larger grain size. The interdiffusion layer grows within this larger austenite grain. The ferrite grain width in the interdiffusion layer (6) is therefore greater than the previous austenite grain size in the mass (7), leading to good bending capacity with a bending angle greater than 70°.

[0104] Figure 4a represents a schematic section of the coated steel sheet from test 1. The combination of the process parameters of the invention, the annealing temperature T<A>, the annealing time t<A> and the dew point temperature T<DP1> allow obtaining a decarburized layer (3), where a ferrite layer is formed on the top (4), thicker than in test 1 due to the higher C content.

[0105] Next, the coated steel sheet is hot-formed. Fig. 4b shows a schematic section of the pressure-hardened steel part from test 1.

[0106] The ferrite grain width formed in the interdiffusion layer (6) is inherited from the pure ferrite layer where austenite formation occurs during heating, resulting in a larger grain size. The interdiffusion layer grows within this larger austenite grain. The ferrite grain width in the interdiffusion layer (6) is therefore greater than the austenite grain size in the bulk (7), leading to good bending capacity with a bending angle greater than 70°. Furthermore, due to the thick ferrite layer (4) in the coated steel sheet, a martensite layer with a carbon gradient forms between the bulk and the interdiffusion layer in the pressure-hardened steel part, resulting in a tensile strength exceeding 1500 MPa.

[0107] In test 3, the coated steel sheet has a decarburized layer, without a ferrite layer on its top, as shown schematically in Fig. 2a. The absence of a ferrite layer is due to the low dew point temperature T<DP1> of -10° C, which slows down the decarburization kinetics.

[0108] Next, the coated steel sheet is hot-formed. Figure 2b represents a schematic section of the pressure-hardened steel part from test 3. Due to the absence of the ferrite layer, the ferritic grain width in the interdiffusion layer (6) is then equivalent to the previous austenite grain size in the bulk (7), leading to a low bending angle below 70°.

[0109] In test 4, the low dew point temperature T<DP1> of -40 °C implies absence of the decarburized layer and ferrite layer on the coated steel sheet.

[0110] Fig.1a represents a schematic section of the coated steel sheet from this test, with the coating layer (1) and the mass (2).

[0111] Next, the coated steel sheet is hot-formed. Figure 1b represents a schematic section of the pressure-hardened steel part from test 4. Due to the absence of the ferrite layer, the ferritic grain width in the interdiffusion layer (6) is then equivalent to the previous austenite grain size in the bulk (7), leading to a low bending angle below 70°.

[0112] In test 5, the steel sheet is held for 10,800 s at the homogenization temperature, which forms a thicker ferrite layer in the decarburized layer of the coated steel sheet than in the previous tests. Fig. 5a shows a schematic section of the coated steel sheet from test 5, with the coating layer (1), the decarburized layer (3), the thicker ferrite layer (4) with a coarser grain size, and the mass (2).

[0113] Next, the coated steel sheet is hot-formed, and Fig. 5b shows a schematic section of the pressure-hardened steel part from test 5. During heating of the steel part, the bulk microstructure is austenitic, and the thick ferrite layer transforms into a carbon-gradient austenite layer. However, due to the ferrite layer thickness exceeding 100 µm, a ferrite layer remains between the interdiffusion layer and the carbon-gradient austenite layer.

[0114] During die hardening of the steel part, the ferrite layer is still present and the carbon-gradient austenite layer transforms into a carbon-gradient martensite layer, resulting in a multiphase layer. This triggers a decrease in the yield strength.

[0115] In test 6, the steel sheet has a low carbon content of 0.21%. This low carbon content, combined with the process parameters, produces a decarburized layer on the steel sheet coated with the ferrite layer. However, due to the low carbon level, the yield strength and tensile strength of the pressure-hardened steel part are not achieved.

Claims (5)

1. CLAIMS 1. A coated steel sheet made of a steel having a composition comprising, in percentage by weight: and optionally comprising one or more of the following elements, as a percentage by weight: The remainder of the composition being iron and unavoidable impurities resulting from the smelting process, said coated steel sheet comprising from the mass to the surface of the coated steel sheet: - comprising a mass with a microstructure, in the surface fraction, of 60% to 90% ferrite, the remainder being islands of martensite-austenite, pearlite or bainite, - said mass being covered by a decarburized layer comprising in the upper part a ferrite layer having a thickness of 1 μm to 100 μm - a coating layer made of aluminum or aluminum alloy. 2. A process for producing a coated steel sheet according to claim 1, said process comprising the following successive steps: - melting a steel to obtain a flat roughing, said steel having a composition according to claim 1, - reheat the flat grinding to a temperature T<reheating> between 1100°C and 1300°C, - hot roll the reheated flat roughing at a finishing hot rolling temperature between 800 °C and 950 °C - winding the hot-rolled steel sheet at a temperature T<coil> below 670 °C to obtain a wound steel sheet, - Optionally, pickle the coiled steel sheet, - cold roll the coiled steel sheet to obtain a cold rolled steel sheet - heating the cold-rolled steel sheet to an annealing temperature T<A> between 700 °C and 850 °C and holding the steel sheet at said temperature T<A> for a holding time t<A> between 10 s and 1200 s, to obtain an annealed steel sheet, the atmosphere comprising between 0% and 15% H2 and having a dew point T<DP1> strictly above -10 °C and less than or equal to 20 °C - cool said annealed steel sheet to a temperature in the range between 560°C and 700°C, - Coat the annealed steel sheet with aluminum or an aluminum alloy coating - cooling the coated steel sheet to room temperature to obtain a steel sheet having a microstructure according to claim 1. 3. A pressure-hardened steel part, the steel part having a composition comprising, in percentage by weight: and optionally comprising one or more of the following elements, as a percentage by weight: The remainder of the composition being iron and unavoidable impurities resulting from the smelting process, said steel piece comprising successively from the mass to the surface of the steel piece: - a mass having a microstructure comprising, in surface fraction, more than 95% martensite and less than 5% bainite, - a ferritic interdiffusion layer, - an aluminum-based coating layer, where the relationship between the ferritic grain width in said interdiffusion layer GW<int> and the previous austenite grain size in the mass PAGS<mass>, satisfies the following equation: and having a tensile strength TS greater than or equal to 1500 MPa, an elastic limit YS greater than or equal to 1250 MPa and a bending angle greater than 70°, TS and YS being measured according to ISO standard, ISO 6892-1, and the bending angle according to the bending standard of procedure VDA238-100 (with normalization to a thickness of 1.5 mm). 4. A pressure-hardened steel part according to claim 3, wherein the steel part pressure-hardened comprises a martensite layer with a carbon gradient between said mass and said ferritic interdiffusion layer. 5. A manufacturing process for a pressure-hardened steel part according to any of claims 3 to 4, comprising the following successive steps: - providing a steel sheet according to claim 1, or produced by a process according to claim 2, - cut said steel sheet into a predetermined shape to obtain a steel blank, - Heat the steel blank to a temperature between 880 °C and 950 °C for 10 s to 900 s to obtain a heated steel blank, -transfer the heated blank to a forming press, - hot-form the heated raw piece in the forming press to obtain a shaped piece, - temper the shaped piece in a die.