Method for producing a steel component provided with a metallic coating protecting against corrosion

Abstract

The present invention relates to a method for manufacturing a steel component comprising a substrate and a coating, a corresponding steel component and its use in the automotive sector.

Description

Technical field

[0001] The present invention relates to a method for manufacturing a steel component comprising a substrate and a coating, a corresponding steel component and its use in the automotive sector. Technical background

[0002] To offer the combination of low weight, maximum strength, and protective properties required in modern car body construction, components made of high-strength steels are now used in areas of the body that can be subjected to particularly high loads in the event of a crash. In hot forming, also known as hot press hardening, steel blanks, which are cut from cold- or hot-rolled steel strip, are heated to a forming temperature that is generally above the austenitizing temperature of the respective steel, and placed in the die of a forming press while still heated. During the subsequent forming process, the sheet metal blank, or the component formed from it, experiences rapid cooling through contact with the cool die. The cooling rates are set so that a hard microstructure results in the component.

[0003] WO 2015 / 036151 A1 discloses a method for producing a steel component with a metallic, corrosion-protective coating and a corresponding steel component. The method according to this document comprises coating a flat steel product with an alloy of aluminium, zinc, magnesium and optionally silicon and iron, cutting a blank from the flat steel product, heating the blank and forming the blank to obtain the desired steel component.

[0004] DE 699 07 816 T2 discloses a process for producing a coated hot- and cold-rolled steel sheet with very high strength after thermal treatment. For this purpose, a flat steel product is coated and thermally treated. During the thermal treatment, the workpiece is heated to a temperature of over 750 °C.

[0005] EP 2 993 248 A1 discloses a steel flat product with an aluminum-containing coating, wherein this coating contains 0.005 to 0.7 wt.% of at least one alkali and / or alkaline earth metal, and a process for its production. In this process, the coated steel flat product is heated to a temperature of 700 to 900 °C for 360 s, 600 s or 800 s and subsequently formed.

[0006] When the sheet metal blanks, consisting of a steel substrate and an aluminum-based metallic corrosion protection coating, are heated, hydrogen diffuses through the metallic coating into the steel substrate as a result of the surface reaction of the moisture present in the furnace with the aluminum coating. After press hardening, the hydrogen can no longer escape from the steel substrate because the metallic coating acts as a barrier to diffusible hydrogen (Hdiff) at room temperature. The Hdiff content reduces the stresses that the steel can withstand over time, and spontaneous "hydrogen-induced" fractures can occur under tensile stresses in the sheet metal. To prevent cracking under the stresses typically present during body-in-white construction, the diffusible hydrogen content should be kept below a component-specific value.This value depends, among other things, on the complexity of the hot forming operation, post-processing (e.g., laser cutting, punching, mechanical cutting, or hot trimming), the installation situation and joining concept, and thus the stress state in the car body. The amount of H diff remaining after processing should preferably be ≤ 0.4 ppm (parts per million) before critical body-in-white processes, depending on the processing method.

[0007] Furthermore, there are manufacturing processes in which areas of coated steel strips are rolled to a thinner sheet thickness than other areas, and corresponding sheet blanks with different rolling degrees are then taken from these.

[0008] This allows for the production of weight-optimized and load-adapted components. The ratio of thickness reduction due to rolling to the initial thickness is called the degree of reduction. According to the invention, the degree of reduction applies only to a rolling process in which the coating is already present on the substrate. The rolled areas with a lower sheet thickness compared to the sheet thickness present before rolling exhibit a significantly higher density of defects in the steel substrate due to the rolling process. As a result, diffusible hydrogen can accumulate more readily in the rolled areas than in the unrolled areas, leading to a higher diffusible hydrogen content after hot forming and press hardening. Consequently, hydrogen-induced cracking can occur significantly more quickly in material rolled after coating following hot forming and press hardening.A well-known method for reducing the diffusible hydrogen content in a component is to lower the dew point in the furnace where the steel sheet is heated before forming. This reduces the formation of diffusible hydrogen from the existing moisture in the furnace atmosphere during the oxidation of the substrate, thereby also lowering the H₂ diffusion uptake of the steel component. However, lowering the dew point becomes increasingly complex the more it needs to be set. Therefore, it is desirable to avoid influencing the dew point as much as possible and, if necessary, not to lower it too drastically.

[0009] The present invention is therefore based on the objective of providing a method for manufacturing steel components comprising a substrate and a coating, with which corresponding steel components can be obtained that have the lowest possible hydrogen diffusion content in order to minimize the risk of hydrogen-induced cracking after hot forming and during subsequent use. Furthermore, it is an objective of the present invention to provide a method with which it is possible not to exceed a specific hydrogen diffusion content in a hot-formed component by selecting various furnace parameters depending on the degree of rolling and the sheet thickness of the flat steel product used.

[0010] This problem is solved by the inventive method according to claim 1 for producing a steel component with a diffusible hydrogen content H diff of up to 0.4 ppm .

[0011] Furthermore, these tasks are also solved by a corresponding steel component according to claim 9 and by the use according to claim 12 of the steel component according to the invention in the automotive sector, in particular as a bumper support / reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame or sill.

[0012] The method according to the invention is described in detail below.

[0013] The process according to the invention serves to produce a steel component with a diffusible hydrogen content Hdiff of up to 0.4 ppm, preferably 0.01 to 0.4 ppm, particularly preferably 0.05 to 0.4 ppm, for example 0.1, 0.2, 0.3, or 0.4 ppm, in each case in the material after hot forming. Hdiff describes the amount of hydrogen atoms that are present in dissolved form in the steel substrate after hot forming. Methods for determining the Hdiff content are known to those skilled in the art, for example thermal desorption mass spectrometry (TDMS) with heated samples.

[0014] Step (A) of the process according to the invention comprises providing a steel flat product with a coating containing (all values ​​in wt.%) 3 to 15 Si, 1 to 3.5 Fe, 0.05 to 5.0 alkali and / or alkaline earth metals, balance Al and unavoidable impurities, which has a rolling degree-sheet thickness ratio of greater than 0.8 to 200.

[0015] According to the invention, in step (A) of the process according to the invention, any flat steel product with a suitable coating that appears suitable to a person skilled in the art can be used. According to the invention, a flat steel product consisting of (all values ​​in wt.%) is used in the process according to the invention. 0.06 to 0.50, preferably 0.18 to 0.37, particularly preferably 0.20 to 0.25 C, 0.50 to 3.0, preferably 0.80 to 2.00, particularly preferably 1.00 to 1.60 Mn, 0.10 to 0.50, preferably 0.15 to 0.40, particularly preferably 0.20 to 0.30 Si, 0.01 to 1.00, preferably 0.10 to 0.5, particularly preferably 0.10 to 0.40 Cr, up to 0.20, preferably 0.01 to 0.10, particularly preferably 0.01 to 0.05 Ti, up to 0.10, preferably 0.01 to 0.05, particularly preferably 0.02 to 0.05 Al, up to 0.10, preferably 0.00 to 0.05, particularly preferably 0.00 to 0.02 P, up to 0.1, preferably 0.001 to 0.1 Nb, up to 0.01 N, up to 0.05, preferably 0.00 to 0.005, particularly preferably 0.00 to 0.003 S and up to 0.1, preferably 0.001 to 0.05, particularly preferably 0.002 to 0.0035 B, balance Fe and unavoidable impurities, with a coating containing (all values ​​in wt.-%) 3 to 15 Si, 1 to 3.5 Fe, 0.05 to 5.0, preferably 0.05 to 1.5, particularly preferably 0.11 to 0.6 alkali and / or alkaline earth metals, balance Al and unavoidable impurities.

[0016] According to the invention, unavoidable impurities in the substrate include, for example, Cu, Mo, V, Ni and / or Sn.

[0017] The steel flat product used is a strip, in particular a hot-rolled or cold-rolled strip, a sheet, i.e., a piece of a hot-rolled or cold-rolled strip, or a blank from a hot-rolled or cold-rolled strip. The present invention preferably relates to the method according to the invention, wherein the steel flat product is a blank from a hot-rolled or cold-rolled strip. Methods for producing a hot-rolled or cold-rolled strip are known per se to those skilled in the art and are described, for example, in (Hoffmann, Hartmut; Neugebauer, Reimund; Spur, Günter (2012): Handbuch Umformen. Munich: Carl Hanser Verlag GmbH & Co. KG. Pages 109 to 165 and pages 196 to 207).

[0018] The steel substrate used according to the invention preferably has a hard structure, for example at least 80% martensite, the remainder bainite, ferrite and retained austenite.

[0019] The flat steel product according to the invention is provided with a coating, wherein the coating preferably contains 3 to 15, more preferably 7 to 12, most preferably 9 to 10 silicon, 1 to 3.5, more preferably 2 to 3.5 iron, 0.05 to 5.0, more preferably 0.05 to 1.5, more preferably 0.11 to 0.6 alkali and / or alkaline earth metals, the remainder being aluminum, and unavoidable impurities (all values ​​in wt.%). Within the scope of the present invention, alkali and / or alkaline earth metals are preferably magnesium, calcium, and / or lithium, particularly preferably magnesium.

[0020] Methods for producing a corresponding coated steel flat product are known to those skilled in the art; for example, the coating can be applied by hot-dip coating, electrolytic coating, or by a batch coating process. The present invention therefore preferably relates to the method according to the invention, wherein the coating is applied by hot-dip coating, electrolytic coating, or by a batch coating process.

[0021] Preferably, the aluminum-silicon-iron alloy is applied by means of a continuous hot-dip coating process. Preferably, the temperature of the aluminum melt bath during coating is between 660 °C and 720 °C.

[0022] Silicon in the coating acts as a diffusion blocker and serves to calm the molten pool during the application of the coating formed from the aluminum alloy by means of hot-dip coating.

[0023] According to the invention, the coating thickness is preferably 5 to 60 µm, preferably 10 to 40 µm. This results in a coating weight according to the invention of 20 to 240 g / m², preferably 40 to 200 g / m², particularly preferably 50 to 180 g / m², for example 60, 80 or 150 g / m². The present invention therefore preferably relates to the method according to the invention, wherein the coating weight of the coating on both sides is 20 to 240 g / m².

[0024] According to the invention, the coating can be present on one side of the flat steel product or on both sides of the flat steel product. The present invention therefore preferably relates to the method according to the invention, wherein the coating is present on one side of the flat steel product or on both sides of the flat steel product.

[0025] The steel flat product provided in step (A) of the inventive method has a rolling degree-sheet thickness ratio of 0.8 to 200, preferably greater than 0.8 to 180, particularly preferably greater than 0.8 to 150.

[0026] The steel flat product provided according to the invention preferably has a reduction in thickness of 0.5 to 75%, particularly preferably 2.5 to 60%. According to the invention, the reduction in thickness is expressed as a percentage. Within the scope of the present invention, reduction in thickness means the ratio of the thickness reduction due to rolling to the initial thickness of the steel flat product; in particular, the reduction in thickness is determined according to the following formula (2): Abwalzgrad = Δ h h o with h equals the thickness reduction due to rolling, i.e., starting thickness - final thickness ( Δh = h 0 - h1) and h0 equal to the initial thickness of the steel flat product, each in mm. In a preferred embodiment of the method according to the invention, a steel flat product is used in step (A) which has areas that are rolled to a smaller sheet thickness than other areas. In this preferred case according to the invention, the largest existing degree of rolling is used as the basis for the respective component.

[0027] The dimensionless rolling ratio-sheet thickness ratio (WGB) is determined according to the invention using the following formula (3): WGB = 1 , 5 ⋅ 1 + Abwalzgrad ⋅ 100 1 2 ⋅ 1 + Blechdicke / mm where the sheet thickness is specified in mm and is identical to h 1 , the final thickness of the steel flat product after rolling.

[0028] According to the invention, the steel flat products used in step (A) of the inventive method preferably have a sheet thickness (final thickness). h 1 ) of 0.5 to 6 mm, particularly preferably 0.8 to 3 mm.

[0029] Preferably, according to the invention, the coated steel flat product from step (A), after process step (B) has been carried out, is transferred directly to process step (C) according to the invention. However, it is also possible that further steps are carried out between steps (A) and (B) or (C), for example, separating areas, in particular sheets or blanks of the steel flat product, for example by shearing or laser cutting, introducing holes by laser processing or punching, and / or prior heat treatments to change the properties of the coating or the substrate.

[0030] Step (B) of the method according to the invention comprises the determination of a WOP value as a function of the degree of rolling-sheet thickness ratio WGB within an area spanned by straight connecting lines between the points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100), and P51 (WGB 150, WOP 100). 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP value is plotted on the y-axis and the degree of rolling-sheet thickness ratio is plotted on the x-axis, as preferably in Figure 1As illustrated, a suitable WOP value range is determined according to the invention, from which a WOP value can then be selected. However, according to the invention, all WOP values ​​lying within the determined WOP value range fulfill the condition that a steel component with a diffusible hydrogen content of no more than 0.4 ppm is obtained.

[0031] Step (B) of the inventive method serves to determine a WOP value as a function of the rolling degree-sheet thickness ratio of the steel flat product used, where WOP means "hydrogen-related furnace parameter" and is dimensionless. The WOP value then indicates which process parameters should be used for the heat treatment in step (C) so that steel components with diffusible hydrogen contents of a maximum of 0.4 ppm are obtained.

[0032] When determining the WOP value according to the present invention, a range of suitable WOP values ​​is determined via the rolling ratio to the sheet thickness. A WOP value can then preferably be selected from this range and used to determine corresponding values ​​for Toven, toven, and Tdew point using the equation of the general formula (I). In general, however, all values ​​within the correspondingly determined range of WOP values ​​are suitable for insertion into the equation of the general formula (I) to determine corresponding values ​​for Toven, toven, and Tdew point.

[0033] Step (B) of the method according to the invention is preferably carried out by determining the WOP value within an area spanned by straight connecting lines between the points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100), as well as P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100). 100) in a coordinate system where the WOP value is plotted on the y-axis and the degree-of-rolling-sheet-thickness ratio is plotted on the x-axis, is graphically determined for a given degree-of-rolling-sheet-thickness ratio (area A). The corresponding diagram is shown in Figure 1 As shown, area A results from a union of the shown sub-areas "3", "4" and "5" in Figure 1 .

[0034] In a preferred embodiment of the method according to the invention, the WOP value is determined according to step (B) of the method according to the invention within an area spanned by straight connecting lines between the points P12 (WGB 0.8, WOP 300) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P52 (WGB 150, WOP 200), P52 (WGB 150, WOP 200) and P32 (WGB 50, WOP 200), P32 (WGB 50, WOP 200) and P33 (WGB 50, WOP 300) as well as P33 (WGB 50, WOP 300) and P12 (WGB 0.8, WOP 300) in a coordinate system in which the WOP value is plotted on the y-axis and the degree of rolling-sheet thickness ratio (WGB) is plotted on the x-axis (area B). The corresponding diagram is in Figure 1 As shown, area B is the depicted sub-area "5" excluding sub-areas "3" and "4". Figure 1 .

[0035] With the WOP value determined in step (B) of the method according to the invention, it can then be determined according to the invention at which dew point temperature of the oven atmosphere T dew point, at which mean oven temperature T oven and for which duration t oven step (C) of the method according to the invention is carried out.

[0036] Step (C) of the method according to the invention comprises treating the steel flat product at a mean furnace temperature T furnace (in K) for a duration t furnace (in h), wherein the dew point temperature of the furnace atmosphere T dew point (in K), the mean furnace temperature T furnace (in K) and the duration t furnace (in h) according to the following equation of the general formula (1) WOP = T Ofen K ⋅ log t Ofen h + 1 , 15 + T Taupunkt K − 243 , 15 1 , 6 so that the WOP value is adjusted using Figure 1 the set interval between the minimum and maximum WOP value.

[0037] The furnace temperature Tfurnace (in K) is the average temperature prevailing in the furnace during step (C) of the process according to the invention. According to the invention, Tfurnace can assume any value that a person skilled in the art considers suitable. Preferably, in the process according to the invention, Tfurnace AC1 is up to 1373 K, more preferably 1113 to 1253 K, particularly preferably 1133 to 1223 K, and most preferably 1153 to 1193 K. Here, AC1 represents the first austenitizing temperature, which depends on the alloy composition.

[0038] The duration toven (in h) is the time over which the specified oven temperature Toven prevails in step (C). According to the invention, toven can take any value that a person skilled in the art considers suitable. In the method according to the invention, toven particularly describes the period in which the flat steel product is moved through a continuous furnace or remains in a stationary furnace. Preferably, in the method according to the invention, toven is 0.05 to 0.5 h, more preferably 0.067 to 0.25 h, and particularly preferably 0.067 to 0.4 h.

[0039] In one embodiment, the oven temperature Toven, the oven duration toven, and the WOP value are used to calculate and then adjust the dew point temperature of the oven atmosphere Tdew point using equation (1). The dew point temperature of the oven Tdew point (in K) is, for example, 243.15 to 333.15 K, preferably 253.15 to 303.15 K, and particularly preferably 263.15 to 293.15 K.

[0040] In another preferred embodiment, the dew point temperature of the oven atmosphere Tdew point, the oven duration toven, and the WOP value are used to calculate and then adjust the oven temperature Toven using equation (1).

[0041] In another preferred embodiment, the dew point temperature of the oven atmosphere Tdew point, the oven temperature Toven and the WOP value are used to calculate and then set the oven duration t using equation (1).

[0042] Step (C) of the method according to the invention can generally be carried out in any furnace known to the person skilled in the art, for example roller hearth furnaces, chamber furnaces, multi-layer chamber furnaces, walking beam furnaces.

[0043] Step (D) of the inventive method comprises forming the heated steel flat product from step (C) in a forming tool while simultaneously cooling it to obtain the steel component.

[0044] In general, in step (D) of the method according to the invention, all methods for hot forming known to those skilled in the art can be used, for example described in Hot Forming in Automotive Engineering - Processes, Materials, Surfaces, Landsberg / Lech: Verl. Moderne Industrie, 2012, The Library of Technology.

[0045] In step (D) of the process according to the invention, the desired steel component is obtained from the flat steel product from step (C) by forming. To ensure that the desired microstructure, for example at least 80% martensite, the remainder bainite, ferrite, and retained austenite, is formed in the steel component, the forming is carried out with simultaneous cooling. The cooling in step (C) of the process according to the invention preferably takes place at a rate of 27 to 1000 K / s, particularly preferably 50 to 500 K / s. The present invention therefore preferably relates to the process according to the invention, wherein the cooling in step (D) takes place at a cooling rate of 27 to 500 K / s.

[0046] The present invention also relates to a steel component consisting of (all values ​​in wt.%) 0.06 to 0.50, preferably 0.18 to 0.37, particularly preferably 0.20 to 0.25 C, 0.50 to 3.0, preferably 0.80 to 2.00, particularly preferably 1.00 to 1.60 Mn, 0.10 to 0.50, preferably 0.15 to 0.40, particularly preferably 0.20 to 0.30 Si, 0.01 to 1.00, preferably 0.10 to 0.5, particularly preferably 0.10 to 0.40 Cr, up to 0.20, preferably 0.01 to 0.10, particularly preferably 0.01 to 0.05 Ti, up to 0.10, preferably 0.01 to 0.05, particularly preferably 0.02 to 0.05 Al, up to 0.10, preferably 0.00 to 0.05, particularly preferably 0.00 to 0.02 P, up to 0.1, preferably 0.001 to 0.1 Nb, up to 0.01 N, up to 0.05, preferably 0.00 to 0.005, particularly preferably 0.00 to 0.003 S and up to 0.1, preferably 0.001 to 0.05, particularly preferably 0.002 to 0.0035 B, balance Fe and unavoidable impurities, with a coating containing (all values ​​in wt.-%) 3 to 15 Si, 1 to 3.5 Fe, 0.05 to 5.0, preferably 0.05 to 1.5, particularly preferably 0.11 to 0.6, alkali and / or alkaline earth metals, balance Al and unavoidable impurities, produced by the inventive process. Preferably, the coating weight on both sides of the steel component according to the invention is 20 to 240 g / m².

[0047] The steel component according to the invention preferably has a through-alloy layer between the steel substrate and the aluminum-based coating. Preferably, the steel component according to the invention has a through-alloy layer with a thickness of 5 to 60 µm, more preferably 10 to 45 µm. The thickness of the alloy layer can be measured using methods known to those skilled in the art (e.g., according to DIN EN ISO 1463).

[0048] The details and preferred embodiments mentioned with regard to the method according to the invention apply accordingly to the steel component according to the invention.

[0049] The present invention also relates to the use of a coated steel component according to the invention in the automotive sector, in particular as a bumper support / reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame or sill.

[0050] Regarding the individual features of the use according to the invention and the preferred embodiments, what has been said regarding the method according to the invention applies accordingly.

[0051] According to the invention, a method according to claim 1 is provided for the production of a steel component with a diffusible hydrogen content Hdiff of up to 0.4 ppm, comprising at least the following steps: (A) Providing a steel flat product with the composition of claim 1, comprising a coating containing (all values ​​in wt.%) 3 to 15 Si, 1 to 3.5 Fe, 0.05 to 5.0 alkali and / or alkaline earth metals, balance Al and unavoidable impurities, which has a degree-of-rolling-sheet-thickness ratio (DFR) of greater than 0.8 to 200, wherein the DFR is determined according to claim 1, and wherein the steel flat product has areas rolled to a lesser sheet thickness than other areas, the highest degree of roll is taken as the basis, and the degree of roll applies to a rolling operation in which the coating is already present on the substrate, (B) Determining a WOP value as a function of the degree-of-rolling-sheet-thickness ratio DFR within an area spanned by straight connecting lines between points P11 (DFR 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26,WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) and P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP value is plotted on the y-axis and the degree of rolling-sheet thickness ratio is plotted on the x-axis, (C) Treatment of the steel flat product at a mean furnace temperature T furnace (in K) for a duration t furnace (in h), wherein the dew point temperature of the furnace atmosphere of the furnace T dew point (in K), the mean furnace temperature T furnace (in K) and the duration t furnace (in h) according to the following equation of the general formula (1) , WOP = T Ofen K ⋅ log t Ofen h + 1 , 15 + T Taupunkt K − 243 , 15 1 , 6 (D) the heated steel flat product from step (C) is set up, and (D) the reshaping of the heated steel flat product from step (C) in a forming tool while simultaneously cooling to obtain the steel component.

[0052] Preferably, the WOP value is determined according to step (B) within an area spanned by straight connecting lines between points P12 (WGB 0.8, WOP 300) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P52 (WGB 150, WOP 200), P52 (WGB 150, WOP 200) and P32 (WGB 50, WOP 200). P32 (WGB 50, WOP 200) and P33 (WGB 50, WOP 300) as well as P33 (WGB 50, WOP 300) and P12 (WGB 0.8, WOP 300) in a coordinate system in which the WOP value is plotted on the y-axis and the degree of rolling-sheet thickness ratio (WGB) is plotted on the x-axis.

[0053] According to an advantageous further development, t of the furnace is 0.05 to 0.5 h, preferably 0.060 to 0.4 h, particularly preferably 0.067 to 0.25 h.

[0054] According to an advantageous further development, the steel flat product is a blank made from hot-rolled strip or a blank made from cold-rolled strip.

[0055] According to an advantageous further development, the coating is carried out by fire coating, electrolytic coating or by means of a piece coating process.

[0056] According to an advantageous further development, the surface weight of the double-sided coating is 20 to 240 g / m².

[0057] According to an advantageous further development, cooling in step (D) takes place at a cooling rate of 10 to 500 K / s, preferably above 27 K / s.

[0058] According to an advantageous further development, the content of diffusible hydrogen H diff is 0.1, 0.2, 0.3 or 0.4 ppm in the material after hot forming.

[0059] One aspect of the invention relates to a steel component comprising a substrate consisting of (all values ​​in wt.%) 0.06 to 0.50 C, 0.50 to 3.0 Mn, 0.10 to 0.50 Si, 0.01 to 1.00 Cr, up to 0.20 Ti, up to 0.10, preferably 0.01 to 0.05, particularly preferably 0.02 to 0.05 Al, up to 0.10, preferably 0.00 to 0.05, particularly preferably 0.00 to 0.02 P, up to 0.1, preferably 0.001 to 0.1 Nb, up to 0.01 N, up to 0.05, preferably 0.00 to 0.005, particularly preferably 0.00 to 0.003 S and up to 0.1, preferably 0.001 to 0.05, particularly preferably 0.002 to 0.0035 B, balance Fe and unavoidable impurities, with a coating containing (all values ​​in wt.%) 3 to 15 Si, 1 to 3.5 Fe, 0.05 to 5.0 alkali and / or alkaline earth metals, balance Al and unavoidable impurities, produced by the method specified in claim 1 or one of its advantageous embodiments.

[0060] According to an advantageous further development, the surface weight of the double-sided coating is 20 to 240 g / m².

[0061] According to an advantageous further development, the steel component has a through-alloyed layer with a thickness of 5 to 60 µm, preferably 10 to 45 µm.

[0062] One aspect of the invention is the use according to claim 12 of a coated steel component of the above-mentioned manner in the automotive sector, in particular as a bumper support / reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame or sill. Figures

[0063] Figure 1 The diagram shows the WOP value plotted against the degree of rolling to sheet thickness ratio. In this diagram, 1 WOP value (hydrogen-related furnace parameter value), 2 WGB (degree of rolling to sheet thickness ratio), 3 partial area "3", 4 partial area "4", 5 partial area "5". Figure 2 shows by way of example how the WOP value is determined according to the invention with a known rolling degree-sheet thickness ratio, where E1: Degree of reduction 0.5%, initial sheet thickness 3.0 mm, degree of reduction to sheet thickness ratio 1.6, resulting in a WOP value of 300 to 790. E2: Degree of reduction 2.5%, initial sheet thickness 3.0 mm, degree of reduction to sheet thickness ratio 3.8, resulting in a WOP value of 300 to 780. E3: Degree of reduction 30%, initial sheet thickness 1.5 mm, degree of reduction to sheet thickness ratio 41.8, resulting in a WOP value of 300 to 630. E4: Degree of reduction 50%, initial sheet thickness 1.98 mm, degree of reduction to sheet thickness ratio 63.6 or degree of reduction 47%, initial sheet thickness 1.5 mm, degree of reduction to sheet thickness ratio 64.7, resulting in the following values: WOP value from 200 to 600. Examples Example 1

[0064] The following exemplary embodiments serve to explain the invention in more detail.

[0065] Circuit boards are used that have been obtained from melts containing the alloy components according to Table 1. Table 1: Melt composition of the steel flat products used Alloying elements Alloy component in wt.% Melt A Melt B Melt C Melt D C 0,224 0,212 0,219 0.212 to 0.225 Si 0,23 0,22 0,26 0.21 to 0.27 Mn 1,20 1,11 1,14 1.11 to 1.20 P 0,014 0,009 0,013 0.009 to 0.016 S 0,0029 0,0013 0,0023 0.0006 to 0.0029 Total 0,035 0,027 0,032 0.026 to 0.038 Cr 0,190 0,187 0,183 0.180 to 0.190 Note 0,001 0,001 0,001 0.001 to 0.001 Mon 0,0055 0,0018 0,0040 0.0016 to 0.0055 Ti 0,028 0,029 0,025 0.020 to 0.033 B 0,0022 0,0024 0,0026 0.0021 to 0.0028 All values ​​in wt.%, balance Fe and unavoidable impurities

[0066] The steel flat products used have a coating containing 9 to 10 wt.% Si, 2 to 3.5 wt.% iron, the remainder aluminum, and the amount of Mg specified in Table 2. The coating weight, sheet thickness, and degree of rolling of the steel flat products used are also listed in Table 2. The degree-of-rolling-sheet-thickness ratio (Formula 3) is then used in the diagram according to... Figure 1The corresponding WOP value is determined, and then, using formula (1), Toven, toven, and Tdew point of the oven atmosphere are determined and set. The heated steel flat product is then removed from the oven and, after a transport time of 6 seconds, placed into a mold. After placement in the mold, the mold immediately closes and remains closed for approximately 20 seconds to cool the component to <80°C through contact with the cooled tools. Samples are taken from the manufactured steel components and analyzed using thermal desorption mass spectrometry (TDMS) to determine the amount of diffusible hydrogen they contain. H diff ). Table 2: Serial No. melt Mg content [wt.%] Print weight on both sides [g / m²< ] Sheet thickness [mm] Degree of rolling [%] WG B Stove [K] t oven [h] T dew point [K] H diff [ppm] WOP value Area V1 A 0,3 80 1,50 0 1,3 1193,15 0,100 288,15 0,12 557 - V2 A 0,3 80 1,50 0 1,3 1193,15 0,167 288,15 0,17 584 - V3 A 0,3 140 1,50 0 1,3 1193,15 0,222 288,15 0,13 606 - V4 A 0,3 140 1,50 0 1,3 1193,15 0,100 288,15 0,14 557 - V5 A 0 140 1,50 0 1,3 1193,15 0,222 288,15 0,45 606 - 6 A 0,3 140 1,10 27 41,0 1193,15 0,100 248,15 0,07 129 A 7 A 0,3 140 1,10 27 41,0 1193,15 0,167 248,15 0,05 156 A 8 A 0,3 140 1,10 27 41,0 1193,15 0,222 248,15 0,05 177 A 9 A 0,3 140 1,10 27 41,0 1193,15 0,100 268,15 0,26 288 A 10 A 0,3 140 1,10 27 41,0 1193,15 0,167 268,15 0,14 315 B 11 A 0,3 140 1,10 27 41,0 1193,15 0,222 268,15 0,12 336 B 12 A 0,3 140 0,80 47 76,0 1193,15 0,100 248,15 0,15 129 A 13 A 0,3 140 0,80 47 76,0 1193,15 0,167 248,15 0,13 156 A 14 A 0,3 140 0,80 47 76,0 1193,15 0,222 248,15 0,05 177 A 15 A 0,3 140 0,80 47 76,0 1193,15 0,100 268,15 0,34 288 A 16 A 0,3 140 0,80 47 76,0 1193,15 0,167 268,15 0,29 315 B 17 A 0,3 140 0,80 47 76,0 1193,15 0,222 268,15 0,22 336 B V18 A 0,3 140 0,80 47 76,0 1193,15 0,222 288,15 0,89 606 - V19 C 0 140 1,50 0 1,3 1193,15 0,083 288,15 0,47 550 - V20 C 0 140 1,50 0 1,3 1193,15 0,167 288,15 0,59 584 - V21 C 0 140 1,50 0 1,3 1193,15 0,083 268,15 0,20 281 - V22 C 0 140 1,50 0 1,3 1193,15 0,083 248,15 0,10 122 - 23 B 0,4 140 1,35 30 43,0 1193,15 0,083 268,15 0,20 281 A 24 B 0,4 140 1,35 30 43,0 1193,15 0,167 268,15 0,17 315 B 25 B 0,4 140 1,35 30 43,0 1193,15 0,083 288,15 0,22 550 B 26 B 0,4 140 1,35 30 43,0 1193,15 0,167 288,15 0,31 584 B 27 B 0,4 140 1,00 50 76,5 1193,15 0,083 268,15 0,11 281 A 28 B 0,4 140 1,00 50 76,5 1193,15 0,083 268,15 0,13 281 A 29 B 0,4 140 1,00 50 76,5 1193,15 0,167 268,15 0,11 315 B 30 B 0,4 140 1,00 50 76,5 1193,15 0,167 268,15 0,12 315 B 31 B 0,4 140 1,00 50 76,5 1193,15 0,083 288,15 0,26 550 B 32 B 0,4 140 1,00 50 76,5 1193,15 0,167 288,15 0,28 584 B V33 D 0,3 140 1,50 0 1,3 1253,15 0,083 288,15 0,27 554 - V34 D 0,3 140 1,50 0 1,3 1153,15 0,167 288,15 0,27 579 - V35 D 0 140 1,50 0 1,3 1153,15 0,250 288,15 0,47 610 - 36 D 0 140 1,50 25 35,1 1193,15 0,083 268,15 0,29 281 V37 D 0 140 1,50 25 35,1 1193,15 0,050 288,15 0,52 536 - 38 D 0 140 1,50 25 35,1 1193,15 0,083 258,15 0,05 185 A 39 D 0 140 1,50 25 35,1 1193,15 0,167 258,15 0,22 219 A 40 D 0,5 140 1,00 0 1,5 1193,15 0,167 288,15 0,27 584 B 41 D 0,5 140 1,97 0 1,2 1193,15 0,083 288,15 0,10 550 B 42 D 0,5 140 1,97 0 1,2 1193,15 0,250 288,15 0,30 616 B V43 D 0 140 1,50 25 35,1 1193,15 0,083 288,15 0,85 550 - V44 D 0 140 1,50 25 35,1 1193,15 0,167 268,15 0,49 315 - 45 D 0,3 140 1,50 0 1,3 1193,15 0,083 298,15 0,24 718 B 46 D 0 140 1,30 30 43,5 1193,15 0,083 268,15 0,27 281 A 47 D 0 140 1,30 30 43,5 1193,15 0,083 268,15 0,27 281 A V48 D 0 140 1,30 30 43,5 1193,15 0,083 288,15 0,57 550 - V49 D 0 140 0,95 50 77,5 1193,15 0,083 268,15 0,58 281 - V50 D 0 140 0,95 50 77,5 1193,15 0,167 268,15 0,47 315 - V. Comparative example Example 2

[0067] Exemplary determination of permissible values ​​for Toven, toven and Tdew point to maintain an Hdiff value of 0.4 ppm in manufactured components made from flat steel products.

[0068] Example E3 from Figure 2 : h 0 = 2 , 143 mm ; h 1 = Blechdicke = 1 , 5 mm ; Δh = 0 , 643 mm ; Beschichtung mit Mg 0 , 35 Gew . − % Abwalzgrad = Δh h 0 = 0 , 643 mm 2 , 143 mm = 0 , 3 = 30 % WGB = 1 , 5 ⋅ 1 + Abwalzgrad ⋅ 100 1 2 ⋅ 1 + Blechdicke / mm = 1 , 5 ⋅ 1 + 0 , 3 ⋅ 100 1 2 ⋅ 1 + 1 , 5 mm mm = 41 , 8

[0069] For a WGB value of 41.8, a WOP value of 300 to 630 can be expected. Figure 1The values ​​can be read off or calculated using the given points. The three parameters Toven, toven, and Tdew point can now be set such that a WOP value of 300 ≤ is obtained. WOP ≤ 630, for example: T oven = 930 °C = 1203.15 K; t Oven = 400 s = 0.111 h; and T Dew point = 10 °C = 283.15 K 300 ≤ WOP ≤ 630 ⇔ 300 ≤ T Ofen K ⋅ log t Ofen h + 1 , 15 + T Taupunkt K − 243 , 15 1 , 6 ≤ 630 ⇔ 300 ≤ 1203 , 15 K K ⋅ log 0 , 111 h h + 1 , 15 + 283 , 15 K K − 243 , 15 1 , 6 ≤ 630 ⇔ 300 ≤ 487 ≤ 630 ⇔ wahre Aussage

[0070] Since the calculated WOP value of 487 lies between 300 and 630, a maximum H diff value of 0.4 ppm in the component can be maintained through the selected parameters. Commercial applicability

[0071] The steel component produced according to the invention exhibits a low tendency to hydrogen-induced fractures under load stresses and can therefore be advantageously used in the automotive sector, aircraft construction or railway vehicle construction.

Claims

1. Method for producing a steel component with a diffusible hydrogen content Hdiff of up to 0.4 ppm, comprising at least the following steps: (A) providing a flat steel product consisting of (all data in wt.%): 0.06 to 0.50 C, 0.50 to 3.0 Mn, 0.10 to 0.50 Si, 0.01 to 1.00 Cr, up to 0.20 Ti, up to 0.10 Al, up to 0.10 P, up to 0.1 Nb, up to 0.01 N, up to 0.05 S, and up to 0.1 B, residual Fe and unavoidable impurities, with a coating comprising (all figures in wt.%) 3 to 15 Si, 1 to 3.5 Fe, 0.05 to 5.0 alkali and / or alkaline earth metals, remainder Al and unavoidable impurities, which has a rolling degree-sheet thickness ratio (WGB) of greater than 0.8 and up to 200, wherein WGB is a dimensionless value determined by WGB = 1 , 5 ⋅ 1 + rolling degree ⋅ 100 1 2 ⋅ 1 + sheet thickness / mm where the sheet thickness is set in mm and is identical to h1 , the final thickness of the flat steel product after rolling, wherein the flat steel product has areas that are rolled to a lower sheet thickness than other areas, wherein the largest existing reduction ratio is taken as a basis, and the reduction ratio applies to a rolling operation in which the coating is already present on the substrate, (B) Determining a WOP value as a function of the rolling degree-sheet thickness ratio WGB within an area spanned by straight line segments between points P11 (WGB 0.8, WOP 100) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P51 (WGB 150, WOP 100) as well as P51 (WGB 150, WOP 100) and P11 (WGB 0.8, WOP 100) in a coordinate system in which the WOP value is plotted on the y-axis and the rolling degree-sheet thickness ratio is plotted on the x-axis, (C) Treating the flat steel product at an average furnace temperature Tfurnace(in K) for a duration tfurnace(in h), where the dew point temperature of the furnace atmosphere Tdew point (in K), the average furnace temperature Tfurnace (in K) and the duration tfurnace (in h) are set in accordance with the following equation of the general formula (1) WOP = T furnace K ⋅ log t furnace h + 1 , 15 + T dew point K − 243 , 15 1 , 6 and (D) forming the heated flat steel product from step (C) in a forming tool while simultaneously cooling it to obtain the steel component.

2. Method according to claim 1, characterized in that the determination of the WOP value according to step (B) is performed within an area spanned by straight line segments between points P12 (WGB 0.8, WOP 300) and P13 (WGB 0.8, WOP 800), P13 (WGB 0.8, WOP 800) and P21 (WGB 26, WOP 650), P21 (WGB 26, WOP 650) and P41 (WGB 74, WOP 590), P41 (WGB 74, WOP 590) and P53 (WGB 150, WOP 520), P53 (WGB 150, WOP 520) and P52 (WGB 150, WOP 200), P52 (WGB 150, WOP 200) and P32 (WGB 50, WOP 200), P32 (WGB 50, WOP 200) and P33 (WGB 50, WOP 300) as well as P33 (WGB 50, WOP 300) and P12 (WGB 0.8, WOP 300) in a coordinate system in which the WOP value is plotted on the y-axis and the rolling degree-sheet thickness ratio (WGB) is plotted on the x-axis.

3. Method according to one of claims 1 to 2, characterized in that tfurnace is 0.05 to 0.5 h, preferably 0.060 to 0.4 h, particularly preferably 0.067 to 0.25 h.

4. Method according to one of claims 1 to 3, characterized in that the flat steel product is a blank from a hot strip or a blank from a cold strip.

5. Method according to one of claims 1 to 4, characterized in that the coating is applied by hot-dip coating, electrolytic coating or by means of a piece coating process.

6. Method according to one of claims 1 to 5, characterized in that the coating weight of the double-sided coating is 20 to 240 g / m2.

7. Method according to one of claims 1 to 6, characterized in that the cooling in step (D) is carried out at a cooling rate of 10 to 500 K / s, preferably above 27 K / s.

8. Method according to one of claims 1 to 7, characterized in that the content of diffusible hydrogen Hdiff is 0.1, 0.2, 0.3, or 0.4 ppm in the material after hot forming.

9. Steel component manufactured by a method according to one of claims 1 to 8, wherein in step (A) a flat steel product is used which has areas that are rolled to a lower sheet thickness than other areas, the largest existing reduction ratio being taken as a basis, and the reduction ratio applies to a rolling operation in which the coating is already present on the substrate, and wherein the manufactured steel component has a diffusible hydrogen content Hdiff of up to 0.4 ppm.

10. Steel component according to claim 9, characterized in that the coating weight of the double-sided coating is 20 to 240 g / m2.

11. Steel component according to claim 9 or 10, characterized in that it has a through-alloyed alloy layer with a thickness of 5 to 60 µm, preferably 10 to 45 µm.

12. Use of a coated steel component according to one of claims 9 to 11 in the automotive industry, in particular as a bumper support / reinforcement, door reinforcement, B-pillar reinforcement, A-pillar reinforcement, roof frame, or sill.

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