Press hardened steel part and method of manufacturing the same

Abstract

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

Description

Technical Field

[0001] This invention relates to high-strength and high-flexural-resistance press-hardened steel components with improved resistance to delayed fracture. Background Technology

[0002] High-strength pressed and hardened components can be used as structural elements in motor vehicles for intrusion prevention or energy absorption functions.

[0003] In such applications, it is desirable to produce steel components that combine high mechanical strength, high impact resistance, and good corrosion resistance.

[0004] However, it is well known that the susceptibility to delayed cracking increases with increasing mechanical strength, especially after cold forming or hot forming operations, because high residual stress is likely to remain after deformation and is applied during vehicle assembly, such as during spot welding.

[0005] In practice, hydrogen atoms can gradually accumulate in lattice defects, such as dislocations and grain boundaries, through diffusion. When hydrogen reaches a critical concentration after a certain period, it can become detrimental. This delay is caused by the residual stress distribution field and hydrogen diffusion kinetics, due to the low hydrogen diffusion coefficient at room temperature. This hydrogen can lead to crack formation, for example, along grain boundaries and / or dislocation slip planes. Furthermore, hydrogen accumulated at grain boundaries weakens their cohesion and promotes the occurrence of delayed intergranular cracks.

[0006] Some parts produced by hot forming exhibit very poor performance in terms of hydrogen absorption during hot stamping. For example, parts made from custom-rolled steel billets are known to absorb large amounts of hydrogen in the furnace before the hot forming step, which can lead to crack formation.

[0007] Furthermore, it is known that the risk of delayed fracture increases during vehicle assembly, especially after the paint is applied to the parts, where low-temperature heating occurs and thus less hydrogen degassing is performed.

[0008] To limit hydrogen absorption and crack formation, it is known to pre-coat steel sheets with a barrier pre-coating that includes elements to prevent hydrogen absorption into the steel during austenitizing heat treatment. For example, publication WO2021 / 084379 uses a nickel-chromium barrier layer. However, nickel is an expensive element.

[0009] Publication No. WO2022129994 relates to a coated steel plate and a high-strength press-hardened steel component with good bending properties, having a bending angle greater than 70° and a yield strength YS greater than or equal to 1000 MPa.

[0010] To achieve such excellent bending properties in steel components, the steel sheet is heated in an atmosphere with water injection and a controlled dew point to create a decarburized layer on top of a ferrite layer at the top of the sheet. This decarburized layer contributes to improved bending properties after forming; the bending properties are further enhanced when the decarburized layer of the steel sheet includes this upper ferrite layer. However, heating the billet in a furnace with a high dew point may increase hydrogen intake, thereby increasing the risk of delayed fracture. Summary of the Invention

[0011] Therefore, the object of the present invention is to solve the above-mentioned problems and to provide a press-hardened steel component made of re-rolled steel sheet, which has both high strength and good resistance to delayed fracture, even under conditions that generate a large amount of hydrogen intake, such as heating the billet in a furnace with a high dew point temperature or by applying a large amount of re-rolling reduction.

[0012] The object of the present invention is achieved by providing a steel component according to claim 1. This steel component may further include the features of any one of claims 2 to 5. Another object is achieved by providing a method according to claim 6. This method may further include the features of any one of claims 7 to 9. Detailed Implementation

[0013] The invention will now be described in detail and illustrated by examples, without introducing limitations.

[0014] The composition of the press-hardened steel component according to the present invention will now be described, the content of which is expressed as a weight percentage (wt.%).

[0015] According to the present invention, the carbon content is 0.15% to 0.22% to ensure satisfactory strength. When the carbon content exceeds 0.22%, the weldability and bendability of the steel decrease. Furthermore, a large amount of carbon lowers the Ms temperature, which may result in a high-stress martensitic microstructure.

[0016] If the carbon content is below 0.15%, the yield strength and tensile strength will be too low. In a preferred embodiment of the invention, the carbon content is 0.15% to 0.20%.

[0017] The manganese content is from 0.5% to 2.5%. When the addition exceeds 2.5%, the risk of center segregation increases, thereby impairing flexibility. Below 0.5%, the hardenability of the steel decreases, and the tensile strength and yield strength become too low. In a preferred embodiment of the invention, the manganese content is from 0.5% to 1.8%. Preferably, the manganese content is from 0.8% to 1.5%, more preferably from 0.8% to 1.3%.

[0018] According to the present invention, the silicon content is from 0.1% to 1.25%. Silicon is an element involved in solid solution hardening. Silicon is added to limit carbide formation. Above 1.25%, silicon is detrimental to toughness. Furthermore, silicon oxides can form on the surface, which can impair the coatability of the steel and may reduce the weldability of the steel sheet and steel parts. Preferably, the silicon content is from 0.1% to 1.0%, more preferably from 0.3% to 1.0%, and even more preferably from 0.5% to 1.0%.

[0019] The aluminum content is 0.01% to 0.1% because it is a very effective element for deoxidizing steel in the liquid phase during refining. Aluminum can protect boron if the titanium content is insufficient. The aluminum content is below 0.1% to avoid oxidation problems and ferrite formation during press hardening. Preferably, the aluminum content is 0.02% to 0.07%, more preferably 0.02% to 0.06%, and even more preferably 0.02% to 0.05%.

[0020] According to the present invention, the chromium content is from 0.1% to 1.0%. Chromium is an element involved in the hardenability of the steel sheet and must be higher than 0.01%. The chromium content is lower than 1.0% to limit machinability issues and costs. Preferably, the chromium content is from 0.3% to 1.0%, more preferably from 0.4% to 1.0%, and even more preferably from 0.5% to 1.0%.

[0021] According to the present invention, the boron content is from 0.0005% to 0.004%. Boron improves the hardenability of steel. The boron content is not higher than 0.004% to avoid the risk of slab fracture during continuous casting. Preferably, the boron content is from 0.001% to 0.004%.

[0022] The titanium content is 0.01% to 0.1% to protect boron from BN formation. The titanium content is limited to 0.1% to avoid TiN formation. In a preferred embodiment, the Ti / N ratio is > 3.42 to protect boron. Preferably, the titanium content is 0.01% to 0.05%.

[0023] The molybdenum content is added at a level of 0.05% to 0.40%, increasing resistance to delayed fracture. Like boron, molybdenum improves the hardenability of steel. The molybdenum content is not higher than 0.40% to limit costs. Preferably, the molybdenum content is 0.05% to 0.30%, more preferably 0.05% to 0.25%, and even more preferably 0.10% to 0.25%.

[0024] The niobium content is 0.01% to 0.08% to refine the microstructure and improve the ductility and bendability of the steel. When the addition exceeds 0.08%, the risk of forming NbC or Nb(C,N) carbides increases, thereby impairing bendability. Preferably, the niobium content is 0.01% to 0.07%, more preferably 0.01% to 0.06%, and even more preferably 0.01% to 0.05%.

[0025] You can add elements as you like.

[0026] Calcium can also be added as an optional element at a maximum of 0.1%. Adding Ca in the liquid phase enables the formation of fine oxides, which improves the castability of continuous casting. Furthermore, calcium can help limit the formation of harmful MnS by promoting the formation of CaO-CaS.

[0027] The remaining component of steel is iron and unavoidable impurities produced during the smelting process and depending on the process route.

[0028] In production routes that do not use scrap steel, as is typically the case in blast furnace-basic oxygen converter (BF-BOF) routes, the level of unavoidable impurities is very low.

[0029] In production routes that use scrap steel, such as in electric arc furnaces (EAF) or converters charged with BF-BOF, the steel plates may also contain residual elements from such scrap steel, such as copper, antimony, arsenic, tin, and lead, up to 0.03% of each, which are considered unavoidable impurities.

[0030] Regardless of the process route, P, S, and N are unavoidable impurities. Their content is: S less than or equal to 0.010%, P less than or equal to 0.020%, and N less than or equal to 0.02%.

[0031] The microstructure of the press-hardened steel component according to the present invention will now be described.

[0032] The steel component comprises, from its base to its surface, the following:

[0033] - A matrix having a microstructure comprising 95% or more martensite by surface fraction, with the remainder being optional bainite.

[0034] - Interdiffusion layer,

[0035] - Aluminum-based coating,

[0036] The interdiffusion layer forms during the reheating of the steel billet to a temperature above or equal to Ac3, and consists of iron from the matrix and aluminum from the solid solution of the coating, and may contain other elements from the matrix, such as silicon, chromium, or manganese. The interdiffusion layer has a thickness of th. inter Preferably, it is 2 µm to 30 µm.

[0037] The coating has a thickness of th coatPreferably, the thickness is from 2 µm to 45 µm. Preferably, the aluminum-based coating comprises pure aluminum and impurities inherent in the processing. Preferably, the aluminum-based coating comprises 8% to 11% silicon by weight, the remainder being aluminum and impurities inherent in the processing. Preferably, the aluminum-based coating comprises 8% to 11% silicon, 2% to 4% iron, and the remainder being aluminum.

[0038] During the heating of the billet cut from the coated and rerolled steel sheet to a temperature of Ac3 or higher, all microstructure elements are transformed into austenite, which is then transformed into at least 95% martensite during die hardening, with the remainder being optional bainite. In a preferred embodiment, the amount of martensite is at least 97%, and even more preferably at least 98%.

[0039] The re-rolling step of the coated steel sheet reduces both the thickness of the steel sheet and the thickness of the coating in the steel sheet. coat However, it increases the hydrogen intake in steel during austenitization at temperatures above or equal to Ac3.

[0040] The press-hardened steel components according to the invention possess both high strength and good resistance to delayed fracture, even under conditions that generate a large amount of hydrogen intake, such as heating steel billets in a furnace with a high dew point temperature, or by applying a large amount of re-rolling reduction.

[0041] The press-hardened steel component according to the present invention exhibits good resistance to delayed fracture even at hydrogen content up to 1.48 ppm. After the press-hardened steel component has undergone a four-point bending test according to ASTM G33-99 and held at room temperature for 96 hours, no cracks are found on the press-hardened steel component.

[0042] Preferably, the hydrogen content H of the pressed hardened steel component is... diff It is 0.95 ppm or higher, more preferably 1.00 ppm or higher, and even more preferably 1.05 ppm or higher.

[0043] Preferably, the tensile strength TS of the press-hardened steel component is greater than or equal to 1200 MPa. Preferably, the yield strength YS of the press-hardened steel component is greater than or equal to 1000 MPa.

[0044] Preferably, the press-hardened steel component according to the invention has a bending angle greater than 60°.

[0045] The steel component according to the invention can be produced by any suitable manufacturing method, and one method can be defined by those skilled in the art. However, the method according to the invention is preferred, which includes the following steps.

[0046] Aluminum-based coated steel sheet is provided. The coated steel sheet is then rolled at least once with a reduction rate of 5% to 80%, preferably 10% to 80%, more preferably 20% to 80%, and even more preferably 20% to 70% or 30% to 70%, to refine the thickness of the coated steel sheet and the coating thickness. coat At rerolling rates below 5%, the thickness reduction of the steel sheet is insufficient. Furthermore, at reduction rates below 5%, guiding the steel sheet in a flexible mill can be difficult, and the sheet may cease contact with the mill altogether. At rerolling rates above 80%, the hydrogen intake during reheating of the rerolled steel sheet may be too high, leading to delayed component fracture.

[0047] In a first embodiment of the invention, steel plates can be re-rolled during the steel plate production process by differential rolling to obtain various steel plate thicknesses, in order to further produce customized rolled steel billets, for example by a continuous flexible rolling process.

[0048] In a second embodiment of the invention, rerolling can be performed by a continuous uniform rerolling process to obtain a uniform thickness on the steel plate.

[0049] The coated and re-rolled steel sheet is cut into a predetermined shape to obtain a steel billet.

[0050] In a first embodiment of the invention, the billet is a custom-rolled billet having more than one steel plate thickness obtained by differential rolling during the steel plate production process.

[0051] In a second embodiment of the invention, the billet is a billet with a uniform thickness, which is lower than the thickness obtained after hot rolling and cold rolling steps, and is obtained by a continuous uniform rerolling process.

[0052] The steel billet is heated in a furnace with a dew point DP of -40°C to +30°C to a temperature T1 higher than Ac3, and held at said temperature T1 for a residence time t1 of 10 seconds to 900 seconds to obtain a heated steel billet. Preferably, the dew point DP is -20°C to +30°C, more preferably -10°C to +30°C, and even more preferably 0°C to +30°C.

[0053] The heated steel billet is then transferred to a forming press for hot forming. After hot forming, the steel part is then die-hardened.

[0054] Ac3 is defined by the following formula:

[0055] All elements are expressed as a weight percentage.

[0056] For example, aluminum-based coated steel sheets that can be used in such a process can be produced as follows.

[0057] A semi-finished product capable of further hot rolling is provided, having the aforementioned steel composition. Such a semi-finished product may, for example, be a slab.

[0058] This semi-finished product is obtained by casting molten steel, which can be produced using steelmaking processes with or without scrap steel.

[0059] The semi-finished product is heated to a temperature of 1100°C to 1300°C. The steel sheet is then hot-rolled at a finishing hot-rolling temperature of 800°C to 950°C. The hot-rolled steel is then cooled and coiled at a temperature below 670°C and pickled to remove surface oxidation. The steel sheet is then cold-rolled with a reduction rate of 20% to 80%. The steel sheet is then reheated to a temperature T of 700°C to 900°C. H and in the T H The holding time t at the temperature ranges from 10 seconds to 600 seconds. H The atmosphere in the furnace has a dew point T strictly above -10°C and below or equal to +20°C. DP1 This process forms a decarburized layer on the top of the steel sheet, which is then coated with an aluminum-based coating and cooled to room temperature. The aluminum-coated steel sheet is then subjected to the aforementioned re-rolling operation.

[0060] Preferably, the aluminum-based coating comprises pure aluminum and impurities inherent in the processing. Preferably, the aluminum-based coating comprises 8% to 11% silicon by weight, the remainder being aluminum and impurities inherent in the processing. Preferably, the aluminum-based coating comprises 8% to 11% silicon, 2% to 4% iron, and the remainder being aluminum.

[0061] The microstructure of a coated steel sheet, for example, that can be used to manufacture a press-hardened steel component according to the invention, will now be described.

[0062] The coated steel sheet consists of the following components from the substrate to the surface:

[0063] - A matrix having a microstructure comprising 60% or more ferrite by surface fraction, with the remainder being optional cementite, pearlite, or martensite-austenite islands.

[0064] - Decarburization layer,

[0065] - Aluminum-based coating,

[0066] The decarburized layer is located at the interface between the substrate and the coating. It is at a dew point T DP1 The coated steel sheet is heated to temperature T in a furnace with a specific atmosphere. HThis decarburized layer is formed during the rolling process. It persists even after the re-rolling step. The decarburized layer contains, on its upper part, an intermetallic compound layer with a ferritic structure, rich in aluminum from the solid solution, and may also contain silicon from the solid solution. During the re-rolling step, the intermetallic compound layer is broken up. The intermetallic compound layer may, for example, contain 15 at.% to 25 at.% iron and 5 at.% to 20 at.% silicon, with the remainder being aluminum.

[0067] The invention will now be described through the following embodiments, which are by no means limiting.

[0068] Example

[0069] The two types of steel, whose composition is summarized in Table 1, are cast into semi-finished products, processed into steel plates according to the process parameters summarized in Table 2, and then processed into steel components.

[0070] Table 1 - Composition

[0071] The composition of the test is summarized in the table below, where the element content is expressed as a weight percentage (wt.%).

[0072]

[0073] Steel A conforms to this invention, and B is a reference.

[0074] Underlined values: do not conform to this invention.

[0075] The as-cast steel semi-finished product is reheated at 1200°C, hot-rolled at a finishing hot-rolling temperature of 895°C, and coiled at 550°C. The steel sheet is then cold-rolled with a reduction rate of 48%. The steel sheet is then heated to a dew point temperature T. DP1 Reheated in the furnace to temperature T H and in T H The heat preservation time t H Then, it is hot-dip coated with an aluminum-silicon coating containing 10% silicon in a bath at 660°C.

[0076] Aluminum-based coated steel sheets made of steel A include:

[0077] - The matrix has a microstructure comprising 92% ferrite, 3% cementite, and 5% martensite-austenite islands by surface fraction.

[0078] - A decarburized layer, topped with an intermetallic compound layer.

[0079] - And aluminum-based coating.

[0080] Aluminum-based coated steel sheets made of steel B include:

[0081] - The matrix has a microstructure consisting of 95% ferrite and the remainder cementite.

[0082] - Coating,

[0083] And it does not include the decarburized layer.

[0084] The aluminum-coated steel sheet is re-rolled at a reduction rate of R (%).

[0085] Aluminum-coated and re-rolled steel sheets are cut to obtain billets, heated to temperature T1 in a furnace with dew point DP as detailed in Table 2, held at said temperature for a dwell time t1, and then hot-formed and die-hardened.

[0086] The following specific conditions were applied:

[0087] Table 2 – Process Parameters

[0088]

[0089] Underlined values: do not conform to this invention.

[0090] The steel components were analyzed, and the corresponding microstructures are summarized in Table 3. The mechanical properties are summarized in Table 4.

[0091] Table 3 – Microstructure of Press-hardened Steel Components

[0092]

[0093] The surface fraction was determined by cutting a specimen from a press-hardened steel component, polishing and etching it with reagents known per se to expose the microstructure. The sections were then examined by optical or scanning electron microscopy, for example using a scanning electron microscope with a field emission electron gun (“FEG-SEM”) at a magnification greater than 5000x, combined with EBSD (Electron Backscattered Diffraction) equipment.

[0094] Analysis of the coating and interdiffusion layer was performed using microprobe EDXMA (energy-dispersive X-ray microscopy) or SEM-EDXA (energy-dispersive X-ray ablation analysis). The thickness of the interdiffusion layer was [th]. inter and coating thickness th coat Measurements are taken through a cross-section.

[0095] At the end of the manufacturing process, the diffusible hydrogen content H was measured using TDA (Thermal Desorption Analysis) experiments. diff .

[0096] The TDA apparatus consists of a heating chamber in which a flat sample, 10 mm wide and 50 mm long, is heated to 900 °C in an infrared furnace at a constant heating rate of 1200 °C / hour under a constant flow of pure nitrogen. The hydrogen gas released by the nitrogen is detected by a quadrupole mass spectrometer.

[0097] Table 4 – Mechanical Properties of Press-Hardened Steel Components

[0098] The tensile strength TS and yield strength YS of press-hardened steel components were measured according to ISO standard ISO 6892-1.

[0099] Delayed fracture was assessed using a four-point bending test according to standard ASTM G33-99: a sample cut from the part was deformed by bending it between two upper rollers and two lower rollers, with the deflection corresponding to the applied stress level of 100% yield strength. The part was then held at room temperature for 96 hours and visually inspected for cracks.

[0100]

[0101] Underlined values: do not meet target values

[0102] According to the steel components of the present invention, namely Tests 1 and 2, even in high diffusible hydrogen content H diff It also has improved resistance to delayed fracture and high strength.

[0103] In fact, it is known that hydrogen intake during the reheating of rerolled steel sheets can be high because the coating thickness is reduced, and as from high H... diff The values ​​can be seen. However, the steel components according to the invention, even in these high H values... diff No cracks were observed at these values. In fact, the steel components according to the invention can withstand up to 1.48 ppm of diffusible hydrogen without cracking, which is remarkable.

[0104] Conversely, for the steel component of Experiment 3, the coated steel sheet having a chemical composition that does not conform to the present invention was re-rolled at the same reduction rate as in Experiment 3, and cracks appeared at the end of the process.

Claims

1. A press-hardened steel component, the press-hardened steel component comprising, by weight percentage, C：0.15% - 0.22% Mn: 0.5% - 2.5% Si: 0.1% - 1.25% Al：0.01% - 0.1% Cr：0.1% - 1.0 % B：0.0005% - 0.004% Ti: 0.01% - 0.1% Mo: 0.05% - 0.40% Nb: 0.01% - 0.08% P ≤ 0.020% S ≤ 0.010% N ≤ 0.02% And optionally include one or more of the following elements: by weight percentage, Ca ≤ 0.1% The balance of the composition is iron and unavoidable impurities produced during smelting. The steel component comprises, from its base to its surface, the following components in sequence: - A matrix having a microstructure comprising 95% or more martensite by surface fraction, with the remainder being optional bainite. - Interdiffusion layer, - Aluminum-based coating, The steel component is made of re-rolled steel sheet.

2. The press-hardened steel component according to claim 1, wherein, The carbon content is 0.15% to 0.20%.

3. The press-hardened steel component according to any one of claims 1 and 2, wherein, The manganese content is 0.5% to 1.8%.

4. The press-hardened steel component according to any one of claims 1 to 3, wherein, The pressed and hardened steel component has a bending angle greater than or equal to 60°.

5. The press-hardened steel component according to any one of claims 1 to 4, wherein, After the press-hardened steel component underwent a four-point bending test according to ASTM G33-99 and was held at room temperature for 96 hours, the press-hardened steel component was subjected to a hydrogen content H... diff It exhibits good resistance to delayed fracture at concentrations up to 1.48 ppm.

6. A method for manufacturing a press-hardened steel component according to any one of claims 1 to 5, comprising the following sequential steps: - Provides an aluminum-based coated steel sheet, said steel having a chemical composition according to any one of claims 1 to 3. - The aluminum-based coated steel sheet is re-rolled at least once with a reduction rate of 5% to 80% to obtain a coated and re-rolled steel sheet. - The coated and re-rolled steel sheet is cut into a predetermined shape to obtain a steel billet. - The steel billet is heated in a furnace with an atmosphere dew point DP of -40°C to +30°C to a temperature T1 higher than or equal to Ac3, and held at said temperature T1 for a residence time t1 of 10 seconds to 900 seconds to obtain a heated steel billet. - The heated steel billet is transferred to a forming press. - The heated steel billet is thermoformed in the forming press to obtain the formed part. - The molded part is subjected to compression molding and quenching.

7. A method for manufacturing a press-hardened steel component according to claim 5, wherein, The aluminum-based coated steel sheet is provided in the following sequence of steps: - Cast steel to obtain a semi-finished product, said steel having the composition according to any one of claims 1 to 3. - The semi-finished product is heated at a temperature of 1100°C to 1300°C. - The heated semi-finished product is hot-rolled at a fine hot-rolling temperature of 800°C to 950°C. - Hot-rolled steel sheets are coiled at a coiling temperature below 670°C. - Pickling of the steel plate is optional. - The steel sheet is cold-rolled with a reduction rate of 20% to 80%. - Heat the steel plate to a temperature T of 700°C to 900°C. H and in T H The heat preservation time is maintained from 10 seconds to 600 seconds. H The atmosphere in the furnace has a dew point T strictly above -10°C and below or equal to +20°C. DP1 , - The steel plate is coated with an aluminum-based coating. - Cool the coated steel sheet to room temperature.

8. The method for manufacturing press-hardened steel components according to any one of claims 5 to 6, wherein, The re-rolling step is completed through a continuous flexible rolling process.

9. The method for manufacturing a press-hardened steel component according to any one of claims 5 to 6, wherein, The re-rolling step is completed through a continuous uniform rolling process.

Images