Flat steel product with an aluminum coating, method for its production, steel component and method for its production

Abstract

The invention relates to a flat steel product for hot forming, consisting of a steel base body (13) and an aluminum-based protective coating (15) applied to the steel base body (13), the steel base body consisting of a steel having 0.1 to 3% by weight of Mn and optionally up to 0.01% by weight of B. Here, the sum of the non-ferrous mass fractions of Mg as additional alloying components in the protective coating (15) is less than 2.50% Mg. Furthermore, the sum of the non-ferrous mass fractions of Mn as additional alloying components in the protective coating (15) is greater than 0.30% Mn, and the sum of the non-ferrous mass fractions of Si as additional alloying components in the protective coating (15) is less than 1.80%.

Description

Technical Field

[0001] The present invention relates to a flat steel product for hot forming, comprising a steel substrate and an aluminum-based protective coating applied to the steel substrate, the steel substrate being composed of steel having 0.1-3 wt% Mn and selectively up to 0.01 wt% B, the protective coating selectively containing other alloying elements in total up to 20 wt%.

[0002] The present invention also relates to a method for producing the flat steel products of the present invention. "Flat steel products" herein includes all rolled products whose length is much greater than their thickness. These products include steel strips and plates, and the resulting cut pieces and slabs.

[0003] The present invention also relates to a steel component produced by thermoforming.

[0004] In hot pressing, also known as hot forming, steel slabs cut from cold-rolled or hot-rolled steel strips are heated to a deformation temperature typically above the austenitizing temperature of the corresponding steel and placed in the tooling of a forming press while still heated. During the subsequent forming process, the sheet metal cut or the part formed therefrom undergoes rapid cooling due to contact with the cold tooling. This cooling rate is designed to create a hard microstructure within the part. This microstructure then transforms into a martensitic microstructure.

[0005] Finally, the present invention also relates to a method for producing such steel components. Background Technology

[0006] A typical steel suitable for hot pressing is AE steel, whose chemical composition is listed in Table 2.

[0007] For hot-rolled MnB steel sheets with an Al coating used in the production of steel components by hot pressing hardening, EP 0971 044 B1 provides an alloying method in which the MnB steel, excluding iron and unavoidable impurities, has (in weight percent) a carbon content greater than 0.20% but less than 0.5%, a manganese content greater than 0.5% but less than 3%, a silicon content greater than 0.1% but less than 0.5%, a chromium content greater than 0.01% but less than 1%, a titanium content less than 0.2%, an aluminum content less than 0.1%, a phosphorus content less than 0.1%, a sulfur content less than 0.05%, and a boron content greater than 0.0005% but less than 0.08%. The Al coating is a so-called AlSi coating, which consists of 9-10% by weight Si, 2-3.5% by weight iron, and the balance aluminum. The resulting coated flat steel product is annealed at a heating temperature above 700°C. During annealing, the protective coating melts, and complete alloying of the protective coating occurs. Here, iron from the steel matrix diffuses into the protective coating, forming a phase with higher thermal stability. The molten protective coating then solidifies. This solidified, thermally stable protective coating is a prerequisite for subsequent forming steps. In the forming process, the flat steel product is placed in a stamping tool, where it is formed into a steel component while still hot, and simultaneously cooled rapidly, resulting in a hard microstructure within the steel matrix of the flat steel product.

[0008] The disadvantage of the AlSi coating is that, compared to uncoated materials, it requires a longer annealing time to achieve complete alloying. Summary of the Invention

[0009] Therefore, the object of the present invention is to provide a flat steel product for thermoforming that can be further processed in a shorter time.

[0010] This objective is achieved by a flat steel product for hot forming, comprising a steel substrate and an aluminum-based protective coating applied to the steel substrate, the steel substrate being composed of steel having 0.1-3 wt% Mn and selectively up to 0.01 wt% B. Here, the total iron-free mass fraction of the additional alloying components in the protective coating selectively totals up to 10%. The remaining iron-free mass fraction is composed of aluminum. Therefore, the protective coating is composed of iron, aluminum, and selectively other alloying components besides iron. In this case, the total iron-free mass fraction of Mg as an additional alloying component in the protective coating is less than 2.50% Mg, preferably less than 1.50%, and particularly 0.10-0.50% Mg. Furthermore, the total iron-free mass fraction of manganese as an additional alloying component in the protective coating is greater than 0.30% Mn, preferably greater than 0.60% Mn, more preferably greater than 0.80% Mn, particularly preferably 1.35% Mn, and particularly preferably 1.40% Mn. Furthermore, the total iron-free mass fraction of silicon as an additional alloying component in the protective coating is less than 1.80% Si, preferably less than 1.20% Si, more preferably less than 0.80% Si, and particularly preferably less than 0.60% Si.

[0011] In the context of this application, the iron-free mass fraction of the alloy composition in the protective coating refers to the proportion of the total mass of that alloy composition to the total mass of all elements in the protective coating except iron. Therefore, based on the total mass of all elements in the protective coating except iron, the protective coating includes a maximum of 2.5 wt% magnesium, greater than 0.30 wt% manganese, and less than 1.80 wt% silicon. The advantage of using the iron-free mass fraction to characterize the protective coating is that the value does not change due to iron diffusion from the steel matrix.

[0012] The study found that adding manganese through alloying can achieve faster full alloying, thereby shortening the annealing time. Surprisingly, this effect only occurs when the silicon content of the protective coating is low. The effect diminishes as the silicon content increases. This effect is no longer observable when the iron-free mass fraction of silicon exceeds 2.00%.

[0013] Magnesium has been shown to be a favorable alloying component that can be readily incorporated into aluminum protective coatings of the type described herein. The amount of magnesium added should be set to less than 2.50% of the total iron-free mass fraction of magnesium, and especially less than 1.50%.

[0014] The iron-free mass fraction of magnesium in the protective coating is preferably at least 0.10% and at most 0.50%, with amounts below 0.50%, particularly below 0.45%, or at most 0.40% or 0.35%, proving particularly advantageous in practice. Magnesium added in small amounts to the Al coating is characterized by a higher oxygen affinity than aluminum, the main component of the protective coating. Even with such a small amount of magnesium, a thin oxide layer forms on the surface of the protective coating, covering the aluminum between this oxide layer and the steel substrate. During the heating process required for hot forming of the flat steel product, this thin oxide layer prevents the aluminum from reacting with moisture in the furnace atmosphere used to heat the flat steel product. This effectively prevents the oxidation of aluminum in the coating and the resulting release of hydrogen that could diffuse into the coating and the steel substrate of the flat steel product. This is particularly true if the aluminum-based coating partially melts during heating and its surface cracks, allowing the molten coating material to come into contact with the furnace atmosphere. Especially when the annealing time is long, the components thermoformed from the flat steel product of the present invention have a lower hydrogen concentration compared with conventional flat steel products with aluminum coating.

[0015] The advantage of adding magnesium in alloying is that it reduces the input of hydrogen into the steel matrix. If the local hydrogen concentration is very high, it will weaken the bonding force at the grain boundaries of the steel matrix, thus causing cracks to form along the grain boundaries under the stress generated during use.

[0016] The thickness of the protective coating is typically between 5 and 35 μm, especially 10 to 25 μm.

[0017] Rapid and complete alloying of the protective coating offers several advantages. First, it shortens the process time of hot forming, making the production process more efficient. Furthermore, energy savings are achieved through the shortened annealing time. Additionally, different furnaces can be used for heating and holding at the heating temperature. For example, a roller hearth furnace can be used for this process step. When using the flat steel products of this invention, a shorter roller hearth furnace can be used due to the shortened annealing time. In particular, furnaces originally designed for ensuring the process of uncoated materials can be used.

[0018] In the implementation plan for further development of flat steel products, the total iron-free manganese mass fraction as an additional alloying component in the protective coating is greater than 1.00% Mn, and especially greater than 1.30% Mn. It has been demonstrated that when the iron-free manganese mass fraction reaches 1.00% or more, the annealing time is shortened more significantly.

[0019] A specific embodiment of the flat steel product is characterized by a total iron-free mass fraction of manganese as an additional alloying component in the protective coating of less than 1.80% manganese, preferably less than 1.60% manganese. The melting point increases with increasing manganese content, making the protective coating more difficult to apply by hot-dip galvanizing. Furthermore, high manganese content promotes slag formation in the melt, which is also disadvantageous. While the illustrated embodiment with a manganese content of 1.60% possesses the advantages of the invention, it exhibits a serious slag problem, which poses difficulties for production.

[0020] In a further developed embodiment of the flat steel product, the total iron-free mass fraction of silicon as an additional alloying component in the protective coating is less than 1.50% Si, especially less than 1.00% Si, and preferably less than 0.80% Si. The lower the silicon content, the greater the influence of manganese content on the full alloying time. Therefore, a very low silicon content is preferred. However, melts with silicon contents far below 0.50% are technically difficult to achieve because it is difficult to avoid silicon impurities. Therefore, to ensure efficient production, the silicon content is particularly greater than 0.03%, preferably 0.05%, and especially 0.10%. At these silicon contents, the effects of the invention remain significant, but the coating process can be implemented at a significantly lower cost because silicon impurities are not as important.

[0021] Al-based protective coatings can be applied to flat steel products particularly economically through hot-dip galvanizing, also known in technical terms as "flame aluminizing".

[0022] The objective of this invention can also be achieved by producing steel components by hot pressing the aforementioned flat steel products.

[0023] This steel component specifically comprises a steel substrate and an Al-based protective coating applied to the steel substrate, the steel substrate being composed of steel having 0.1-3 wt% Mn and selectively up to 0.01 wt% B. The total iron-free mass fraction of the additional alloying components in the protective coating selectively totals up to 10%. Here, the total iron-free mass fraction of Mg as an additional alloying component in the protective coating is less than 2.50% Mg. Furthermore, the total iron-free mass fraction of Mn as an additional alloying component in the protective coating is greater than 0.30% Mn. Furthermore, the total iron-free mass fraction of Si as an additional alloying component in the protective coating is less than 1.80% Si. This steel component has the same advantages as the aforementioned flat steel product. Similarly, preferred iron-free mass fractions of different elements (e.g., manganese, silicon, and magnesium) are also mentioned in relation to the flat steel product. These preferred iron-free mass fractions and their advantages also apply to the steel component.

[0024] The objective of this invention can also be achieved by a method for producing the above-mentioned flat steel products, the method comprising the following steps:

[0025] - Provides a steel matrix consisting of steel having 0.1-3 wt% Mn and selectively up to 0.01 wt% B;

[0026] - An aluminum-based protective coating is applied to a steel substrate. The protective coating selectively includes additional alloying elements with a total of up to 10% by mass fraction of iron-free components, wherein the total iron-free mass fraction of Mg as an additional alloying element in the protective coating is less than 2.50%Mg, and wherein the total iron-free mass fraction of Mn as an additional alloying element in the protective coating is greater than 0.30%Mn, and the total iron-free mass fraction of Si as an additional alloying element in the protective coating is less than 1.80%Si.

[0027] In particular, in this method, the protective coating is applied to the steel substrate by hot-dip galvanizing. Hot-dip galvanizing, also known in technical terms as "flame galvanizing," is a particularly economical and feasible method for applying protective coatings.

[0028] The melt used in hot-dip galvanizing is composed of aluminum, selectively blended with additional alloying elements of up to 10% by weight of iron-free mass fraction. Here, the total iron-free mass fraction of Mg as an additional alloying element in the melt is less than 2.50% magnesium. Furthermore, the total iron-free mass fraction of Mn as an additional alloying element in the melt is greater than 0.30% manganese, and the total iron-free mass fraction of Si as an additional alloying element in the melt is less than 1.80% silicon. Through hot-dip galvanizing, a protective coating structure is obtained consisting of an alloy layer adjacent to the steel substrate and a capping layer adjacent to the alloy layer. The composition of the capping layer is essentially the same as that of the melt, while the iron content in the alloy layer typically exceeds 30% by weight because mixing occurs between the steel substrate and the adjacent melt during the hot-dip galvanizing operation. Since the steel substrate mainly consists of iron, this thorough mixing in the alloy layer does not change the iron-free mass fraction. Therefore, the melt and the protective coating have the same iron-free mass fraction of alloying elements.

[0029] The thickness of the protective coating is typically between 5 and 35 μm, especially 10 to 25 μm.

[0030] Regarding flat steel products, the preferred iron-free mass fractions of different elements (e.g., manganese, silicon, and magnesium) are as described above. These preferred iron-free mass fractions and their advantages also apply to the production methods of flat steel products, especially to the composition of the melt when using hot-dip coating.

[0031] In another variation of this method, the flat steel product is pre-alloyed directly after coating by holding it at a pre-alloying temperature of 500°C-600°C for 15-30 seconds. In the context of this application, "directly" means that the flat steel product is not cooled to the point where the protective coating is fully solidified after coating. In practice, depending on the design of the coating equipment, the time between coating and pre-alloying may be at most 10 seconds.

[0032] Enhanced diffusion is achieved through the pre-alloying step, allowing iron to diffuse from the matrix into the protective coating and initiating the formation of more iron-containing phases within the coating. This results in an additional reduction in the subsequent annealing process for full alloying. According to the invention, the annealing time in the annealing process has been reduced by adjusting the manganese content (see below). Pre-alloying can further shorten the annealing time.

[0033] Furthermore, the objective of this invention can also be achieved by a method for producing the aforementioned steel components, the method comprising the following steps:

[0034] - Flat steel products are produced using the above method;

[0035] - For flat steel products with a thickness between 0.7mm and 1.5mm, the flat steel products are annealed in a furnace preheated to temperature T. Figure 11 The annealing time t is defined by the polygon ABCD in the diagram; or for flat steel products with a thickness between 1.5 mm and 3.0 mm, the flat steel product is annealed in a furnace preheated to temperature T. Figure 11 The annealing time t is defined by the polygon EFGH in the middle;

[0036] - Hot-form flat steel products into steel components.

[0037] In the sense of this application, annealing flat steel products in a furnace preheated to temperature T for an annealing time t defined by polygon ABCD means that the values ​​of temperature T and annealing time t are within the polygon formed by points ABCD.

[0038] Figure 11 The point AH shown has the following value pairs:

[0039]

[0040]

[0041] It is obvious that at higher temperatures (i.e., shorter annealing times), the same degree of complete alloying is achieved faster than at lower temperatures. Furthermore, the thickness of the flat steel product must also be taken into account, as thicker flat steel products require longer times (or higher temperatures) to reach the core temperature necessary for austenite formation within the flat steel product.

[0042] The table below further provides the preferred ranges for the thickness, temperature T, and annealing time t of the flat steel products:

[0043] 0.7-0.9 910-930 1.5-5 0.7-0.9 880-900 2.5-7 1-1.4 910-930 1.8-6 1-1.4 880-900 3-8.5 1.5-1.8 910-930 2.5-7 1.5-1.8 880-900 3.5-10 1.9-2.4 910-930 3.5-10 1.9-2.4 880-900 4-11 2.5-3.5 910-930 4-11 2.5-3.5 880-900 4.5-12

[0044] Therefore, the object of the present invention is also achieved by a method for producing the above-mentioned steel components, the method comprising the following steps:

[0045] - Flat steel products are produced using the above method;

[0046] - Annealing flat steel products with a thickness of d in a furnace preheated to temperature T for an annealing time t that can be determined according to one of the variants in the table above, for example, annealing flat steel products with a thickness of 0.7-0.9 mm in a furnace preheated to 910-930°C for 1.5-5 minutes.

[0047] - Hot-form flat steel products into steel components.

[0048] As mentioned earlier, by limiting the silicon content while adding manganese to the alloy, the diffusion process of iron from the steel matrix to the protective coating is accelerated, thus shortening the time for complete alloying. Therefore, the annealing time is significantly reduced compared to the standard process described in documents such as EP2086755.

[0049] During the annealing process, so-called "iron seams" form in the protective coating. These are iron-rich phases that form at the interface between the protective coating and the steel substrate. In this case, the thickness of the iron seams is a measure of the degree of complete alloying of the protective coating. In this invention, "complete alloying" means an iron seam thickness greater than 2.5 μm, particularly greater than 8 μm, and preferably greater than 10 μm. Therefore, the iron seam thickness after a specific annealing time can measure the rate of complete alloying of the coating.

[0050] This method is further extended in this way, especially for flat steel products with a thickness between 0.7 mm and 1.5 mm, where the flat steel products are annealed in a furnace preheated to temperature T by... Figure 11 The annealing time t is defined by the polygon ABCD in the figure, or for flat steel products with a thickness between 1.5 mm and 3.0 mm, annealing in a furnace preheated to temperature T by... Figure 11When the annealing time t is defined by the polygon EFGH, a seam thickness greater than 2.5 μm, especially greater than 8 μm, and preferably greater than 10 μm, is formed. The thickness of the seam can be adjusted according to other requirements. A sufficiently thick seam ensures that no liquid phase (e.g., liquid aluminum) appears in the protective coating during hot forming. This has several advantages. First, the liquid phase can contaminate the furnace rolls or forming tools. Second, rapid full alloying leads to a change in the surface color of the flat steel product. The flat steel product before annealing has a bright metallic luster, while the flat steel product after full alloying of the protective coating has a dark and matte surface. The faster the full alloying of the protective coating, the faster the surface color changes. The effect of the dark and matte surface is a significant improvement in heat transfer. Therefore, the core temperature required for hot forming can be reached more quickly. This achieves the self-enhancing effect of the manganese content according to the invention. First, the full alloying time of the protective coating is shortened. Second, faster full alloying leads to an increased heating rate of the steel substrate, thereby accelerating the annealing operation.

[0051] After the annealing time t, the flat steel product is removed from the furnace at a heating temperature. This heating temperature corresponds specifically to the preheated furnace temperature T. In a particular extended embodiment, the heating temperature is high enough that the flat steel product has a hot-forming temperature at the start of forming, at which the microstructure of the steel matrix has been wholly or partially transformed into an austenitic microstructure, and the flat steel product is quenched after forming or during forming, thereby forming a hard microstructure within the steel matrix of the flat steel product. The heating temperature is particularly at least 700°C, especially 880°C to 950°C.

[0052] The steel matrix consists of steel having 0.1-3 wt% Mn and, selectively, up to 0.01 wt% B. The microstructure of this steel can be transformed into a martensitic or partially martensitic microstructure, particularly through hot forming. Therefore, the microstructure of the steel matrix in steel components is preferably martensitic or at least partially martensitic, because this microstructure has particularly high hardness.

[0053] The preferred steel matrix is ​​a steel whose composition (by weight%), excluding iron and unavoidable impurities, is as follows:

[0054] C: 0.04-0.45% by weight

[0055] Si: 0.02-1.2% by weight

[0056] Mn: 0.5-2.6% by weight

[0057] A1: 0.02-1.0% by weight

[0058] P: ≤0.05% by weight

[0059] S: ≤0.02% by weight

[0060] N: ≤0.02% by weight

[0061] Sn: ≤0.03% by weight

[0062] As: ≤0.01% by weight

[0063] Ca: ≤0.005% by weight

[0064] And selectively containing one or more of the following elements: chromium, boron, molybdenum, nickel, copper, niobium, titanium, and vanadium:

[0065] Cr: 0.08-1.0% by weight

[0066] B: 0.001-0.005% by weight

[0067] Mo: ≤0.5% by weight

[0068] Ni: ≤0.5% by weight

[0069] Cu: ≤0.2% by weight

[0070] Nb: 0.02-0.08% by weight

[0071] Ti: 0.01-0.08% by weight

[0072] V: ≤0.1% by weight.

[0073] Elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided during steel production. In addition to these elements, other elements may also be present in steel as impurities. These other elements are collectively referred to as "unavoidable impurities." The total content of unavoidable impurities is preferably a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. Selective alloying elements Cr, B, Nb, and Ti, for which lower limits are specified, may also appear in the steel matrix as unavoidable impurities at levels below their respective lower limits. In this case, they are also considered unavoidable impurities, with their total content limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The upper limits for the respective impurities of these elements are preferably as follows:

[0074] Chromium: ≤0.050% by weight

[0075] Boron: ≤0.0005% by weight

[0076] Niobium: ≤0.005% by weight

[0077] Titanium: ≤0.005% by weight.

[0078] These preferred upper limits should be considered as alternatives or common. Therefore, preferred steel variants satisfy one or more of the above four conditions.

[0079] In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt% and / or a minimum of 0.06 wt%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt%, 0.12-0.25 wt%, or 0.33-0.37 wt%.

[0080] In a preferred embodiment, the Si content of the steel is a maximum of 1.00% by weight and / or a minimum of 0.06% by weight.

[0081] In a preferred variant, the Mn content of the steel is a maximum of 2.4 wt% and / or a minimum of 0.75 wt%. In particularly preferred embodiments, the Mn content is in the range of 0.75-0.85 wt% or 1.0-1.6 wt%.

[0082] In preferred variants, the aluminum content of the steel is a maximum of 0.75% by weight, particularly a maximum of 0.5% by weight, and preferably a maximum of 0.25% by weight. Alternatively or additionally, the aluminum content is preferably a minimum of 0.02%.

[0083] Furthermore, it has been found that limiting the total content of silicon and aluminum is also helpful. Therefore, in a preferred variant, the total content of silicon and aluminum (commonly referred to as Si+Al) is a maximum of 1.5% by weight, preferably a maximum of 1.2% by weight. Alternatively, the total content of Si and Al is at least 0.06% by weight, preferably at least 0.08% by weight.

[0084] P, S, and N are typical impurities that cannot be completely avoided during steelmaking. In preferred variants, the P content is a maximum of 0.03% by weight. In contrast, the S content is preferably a maximum of 0.012%. Additionally, or alternatively, the N content is preferably a maximum of 0.009% by weight.

[0085] In addition, the steel selectively contains 0.08-1.0% by weight of chromium. The chromium content is preferably a maximum of 0.75% by weight, and especially a maximum of 0.5% by weight.

[0086] When chromium is selectively added to an alloy, the total content of chromium and manganese is preferably limited. This total is a maximum of 3.3% by weight, and more particularly a maximum of 3.15% by weight. Furthermore, this total is at least 0.5% by weight, preferably at least 0.75% by weight.

[0087] Selectively, the steel preferably contains boron in a content of 0.001-0.005% by weight. The boron content is particularly high, with a maximum of 0.004% by weight.

[0088] Steel may selectively contain molybdenum in a maximum amount of 0.5% by weight, and especially a maximum amount of 0.1% by weight.

[0089] In addition, the steel may selectively contain nickel, with a maximum content of 0.5% by weight, preferably a maximum of 0.15% by weight.

[0090] In addition, the steel may selectively contain copper, with a maximum content of 0.2% by weight, preferably a maximum of 0.15% by weight.

[0091] In addition, the steel may selectively contain one or more of the microalloying elements Nb, Ti, and V. The selective Nb content is at least 0.02% by weight, and at most 0.08% by weight, preferably at most 0.04% by weight. The selective titanium content is at least 0.01% by weight, and at most 0.08% by weight, preferably at most 0.04% by weight. The selective V content is at most 0.1% by weight, preferably at most 0.05% by weight.

[0092] In cases where Nb, Ti, and V are selectively alloyed, the total content of Nb, Ti, and V is preferably limited. The total is at most 0.1% by weight, and particularly at most 0.068% by weight. Furthermore, this total is preferably at least 0.015% by weight.

[0093] The above description of the preferred steel substrate also applies to the steel substrate of flat steel products, as well as to steel components and the aforementioned production method.

[0094] In the flat steel products, steel components, and preferred variants of both methods, the steel matrix is ​​a steel from group AE, whose chemical analysis is shown in Table 2. Table 2 is interpreted as giving the elemental proportions of each steel in group AE in weight percentage. Minimum and maximum weight percentages are given here. For example, the carbon content (C) of steel A is 0.05 wt%–0.10 wt%. Attached Figure Description

[0095] The present invention will now be described in detail with reference to the accompanying drawings and the following embodiments.

[0096] In the picture:

[0097] Figure 1 A cross-sectional grinding disc image of a steel member with a contrasting protective coating is shown;

[0098] Figure 2 A cross-sectional grinding disc image of a steel member with a protective coating according to the present invention is shown;

[0099] Figure 3 A cross-sectional grinding disc image of a steel member with a different contrasting protective coating is shown;

[0100] Figure 4A cross-sectional grinding disc image of a steel member with an alternative protective coating according to the present invention is shown;

[0101] Figure 5 The Fe seam thickness, depending on annealing time, is shown for three different iron-free manganese fractions.

[0102] Figure 6 The Fe gap thickness, depending on annealing time, is shown for three different iron-free manganese fractions in the presence of silicon.

[0103] Figure 7 The Fe seam thickness, depending on the iron-free manganese mass fraction, is shown when the annealing time is 3 minutes;

[0104] Figure 8 The Fe gap thickness, depending on the iron-free mass fraction of silicon, is shown when the annealing time is 3 minutes;

[0105] Figure 9 The Fe seam thickness, depending on the annealing time, is shown in three different magnesium-free variant schemes;

[0106] Figure 10 The Fe gap thickness, depending on the annealing time, is shown in three different magnesium-free variant schemes after pre-alloying.

[0107] Figure 11 Suitable annealing parameters for a method of producing steel components are shown;

[0108] Figure 12 A schematic diagram of the flat steel product is shown. Detailed Implementation

[0109] Figure 1-4 Cross-sectional grinding disc images of steel components produced using the same steel substrate and the same forming method are shown. Only the composition of the protective coating varies.

[0110] Formed slabs were cut from 1.5 mm thick strip of steel grade D, coated on both sides with a 25 μm thick aluminum-based protective layer according to Table 2. The cutting methods employed were punching tools and laser cutting. The precise chemical composition of the matrix was: C: 0.223 wt%, Si: 0.294 wt%, Mn: 1.275 wt%, P: 0.008 wt%, S: 0.002 wt%, Al: 0.046 wt%, Cr: 0.181 wt%, Cu: 0.054 wt%, Mo: 0.001 wt%, N: 0.001 wt%, Ni: 0.035 wt%, Nb: 0.002 wt%, Ti: 0.033 wt%, V: 0.007 wt%, B: 0.0033 wt%, Sn: 0.002 wt%.

[0111] These formed slabs were annealed in a roller hearth furnace at 920°C for a time of t. This heating temperature is higher than the Ac3 temperature, which is approximately 860°C for this steel grade. Therefore, at least a portion of the austenitic microstructure was formed in the steel matrix. Subsequently, the formed slabs were shaped in forming tools and quenched there.

[0112] Table 1 shows different variants of Mg, Mn and Si with no iron mass fraction and Fe gap thickness at different annealing times t.

[0113] Figure 1-4 Examples of cross-sectional grinding disc diagrams of steel components produced in this way under different compositions are shown as examples. Figure 5-8 A chart is shown to explain the different effects.

[0114] Figure 1 A steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13 is shown. The protective coating 15 is applied by hot-dip galvanizing. Here, the melt consists of an aluminum alloy with added 0.4 wt% Mg, 0.8 wt% Mn, and 2.0 wt% Si. Thus, the flat steel product 11 (see...) Figure 12 A protective coating 15 with a thickness of 25 μm is formed after the coating process, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 0.8% Mn, and 2.0% Si. After the hot forming, a steel component 21 with an aluminum-based protective coating 15, as shown in the cross-sectional grinding disc diagram, is obtained, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 0.8% Mn, and 2.0% Si.

[0115] Figure 2 A steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13 is shown. The protective coating 15 is applied by hot-dip galvanizing. The melt consists of an aluminum-added alloy with 0.4 wt% Mg, 1.6 wt% Mn, and 2.0 wt% Si added. Thus, the flat steel product 11 (see...) Figure 12 After coating, a protective coating 15 with a thickness of 25 μm is formed, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 0.8% Mn, and 2.0% Si. Following the hot forming, a steel component 21 with an aluminum-based protective coating 15, as shown in the cross-sectional grinding disc diagram, is obtained, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 1.6% Mn, and 2.0% Si.

[0116] Figure 1 and Figure 2 The two steel components 21 in the diagram only indicate the Fe seam. The thickness of the Fe seam is less than 1 μm. Therefore, the protective coating 15 was not fully alloyed within 3 minutes at 920°C.

[0117] Figure 3 A steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13 is shown. The protective coating 15 is applied by hot-dip galvanizing. The melt consists of an aluminum-added alloy with 0.4 wt% Mg and 0.8 wt% Mn added. The melt is free of silicon except for impurities in the range of 0.2 wt%. Thus, the flat steel product 11 (see...) Figure 12 After coating, a protective coating 15 with a thickness of 25 μm is formed, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 0.8% Mn, and less than 0.50% Si. Following the hot forming, a steel component 21 with an aluminum-based protective coating 15, as shown in the cross-sectional grinding disc diagram, is obtained, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 0.8% Mn, and less than 0.50% Si.

[0118] from Figure 3 As can be clearly seen, Fe cracks 17 were formed in the protective coating. Its thickness is 3 μm. Therefore, the complete alloying of the protective coating 15 was completed after 3 minutes at 920°C. Although... Figure 1 and Figure 3 The steel components 21 shown are produced using the same method except for the silicon content in the melt. Figure 3 However, a significant Fe gap was obtained. Therefore, reducing the silicon content can greatly shorten the process time.

[0119] Figure 4 A steel component 21 with an aluminum-based protective coating 15 on a steel substrate 13 is shown. The protective coating 15 is applied by hot-dip galvanizing. Here, the melt consists of an aluminum-added alloy with 0.4 wt% Mg and 1.6 wt% Mn added. The melt is free of silicon except for impurities in the range of 0.2 wt%. Thus, the flat steel product has a protective coating 15 with a thickness of 25 μm after coating, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 1.6% Mn, and less than 0.50% Si. After the hot forming, the steel component 21 with the aluminum-based protective coating, shown in the cross-sectional grinding disc diagram, is obtained, wherein the protective coating 15 has an iron-free mass fraction of 0.4% Mg, 1.6% Mn, and less than 0.50% Si.

[0120] exist Figure 4 In the image, a 7μm thick Fe seam 17 can be observed. Therefore, after 3 minutes at 920℃, the complete alloying of the protective coating 15 is achieved, and due to its higher manganese content, it is more efficient than... Figure 3 Furthermore, the increase in manganese content further shortened the process duration. Meanwhile, compared to... Figure 4 and Figure 2It can be observed that manganese only has this effect when the silicon content is low. Although relative to... Figure 1 variants, Figure 2 The manganese content in the variant was increased, but compared with... Figure 1 compared to, Figure 2 Because of the high silicon content, stronger Fe gaps did not appear.

[0121] Figure 5 The Fe seam thickness, depending on annealing time, is shown in three different variants with no iron mass fraction:

[0122] - Aluminum and 0.4% magnesium, 0.2% silicon

[0123] - Aluminum and 0.4% magnesium, 0.8% manganese, 0.2% silicon

[0124] - Aluminum and 0.4% magnesium, 1.6% manganese, 0.2% silicon

[0125] The role of manganese is evident here. Although there is little difference with longer annealing times, the addition of manganese to the alloy produces 7 μm Fe gaps even with a short annealing time of 3 minutes, demonstrating adequate and complete alloying.

[0126] Figure 6 It shows the relationship with Figure 5 The same diagram. However, in Figure 6 In this variant, the silicon content is further increased to 2% iron-free mass fraction. Therefore, the following variant is shown (the last two curves are identical and therefore indistinguishable in the graph):

[0127] - Aluminum and 0.4% magnesium, 0% manganese, 2% silicon

[0128] - Aluminum and 0.4% magnesium, 0.8% manganese, 2% silicon

[0129] - Aluminum and 0.4% magnesium, 1.6% manganese, 2% silicon

[0130] It is clear that the effect of manganese is no longer present. After annealing for 3 minutes, no obvious Fe gaps were observed in any of the three variants.

[0131] Figure 7 The figure shows the Fe gap thickness depending on the iron-free mass fraction of manganese when the annealing time is 3 minutes. The iron-free mass fraction of silicon is approximately 0.2%. When the iron-free mass fraction of manganese exceeds approximately 0.3%, a thicker Fe gap is produced, and the thickness of the Fe gap increases with increasing manganese content.

[0132] Figure 8The effect of silicon is illustrated. The figure shows the iron gap thickness depending on the iron-free mass fraction of silicon when the annealing time is 3 minutes. In all cases, the iron-free mass fraction of manganese is 1.6%. The effect of manganese remains significant even with silicon content below 1.8%, with the effect increasing as the iron-free mass fraction of silicon decreases.

[0133] Figure 9 The aforementioned effects of the magnesium-free variant are illustrated again. The figure shows the Fe seam thickness depending on annealing time for three different variants with no iron mass fraction:

[0134] - Aluminum, no alloying added.

[0135] - Aluminum and 1.6% manganese, 0.0% silicon

[0136] - Aluminum and 1.6% manganese, 0.5% silicon

[0137] It is evident that in the variant with a manganese content of 1.6%, significant Fe cracks formed at an earlier stage. Furthermore, it is equally clear that the same effect is produced at silicon contents of 0.5% and 0%. In the tests without added magnesium, silicon impurities could be reduced to below 0.05%. Therefore, 0% silicon in these tests should be understood as a maximum of 0.05% silicon.

[0138] Figure 10 It shows the relationship with Figure 9 The same coating composition is used, depending on the Fe seam thickness at the annealing time. However, in these variations, pre-alloying is performed before annealing, where the shaped slab is held at a pre-alloying temperature of 680°C for 13 seconds. Figure 9 In comparison, Fe seams formed significantly earlier in all cases, and were thicker than those without pre-alloying within the same annealing time. Furthermore, the effect of manganese was also evident; the addition of manganese to the alloy resulted in more pronounced Fe seams forming more quickly. This was the case at both silicon contents of 0% and 0.5%.

[0139] The last three rows of Table 1 are not shown in the figure. This experimental series re-examined whether magnesium content had a significant effect. The results showed that magnesium content had no effect on the results.

[0140] Figure 12 A schematic diagram of a flat steel product 11 for hot forming is shown, which consists of a steel substrate 13 and an aluminum-based protective coating 15 applied to the steel substrate, wherein the steel substrate is composed of steel having 0.1-3 wt% Mn and a selectivity of up to 0.01 wt% B.

[0141] Table 1

[0142]

[0143]

[0144]

Claims

1. A flat steel product (11) for hot forming, the flat steel product comprising a steel substrate (13) and an aluminum-based protective coating (15) applied to the steel substrate (13), the steel substrate (13) comprising, by weight percentage, steel of the following composition excluding iron and unavoidable impurities: C: 0.04 - 0.45% by weight Si: 0.02 - 1.2% by weight Mn: 0.5 - 2.6% by weight Al: 0.02 - 1.0% by weight P: ≤ 0.05% by weight S: ≤ 0.02% by weight N: ≤ 0.02% by weight Sn: ≤ 0.03% by weight As: ≤ 0.01% by weight Ca: ≤ 0.005% by weight And selectively containing one or more of the following elements: chromium, boron, molybdenum, nickel, copper, niobium, titanium, vanadium: Cr: 0.08-1.0% by weight B: 0.001-0.005% by weight Mo: ≤0.5% by weight Ni: ≤0.5% by weight Cu: ≤0.2% by weight Nb: 0.02-0.08% by weight Ti: 0.01-0.08% by weight V: ≤0.1% by weight The selective total iron-free mass fraction of the additional alloying components in the aluminum-based protective coating is up to 10%, and the remaining iron-free mass fraction is formed by aluminum, wherein the total iron-free mass fraction of Mg as an additional alloying component in the protective coating (15) is less than 2.50% Mg, characterized in that, The total iron-free mass fraction of Mn, which is an additional alloying component in the protective coating (15), is greater than 0.30%Mn and less than 1.80%Mn, and the total iron-free mass fraction of Si, which is an additional alloying component in the protective coating (15), is less than 1.00%.

2. The flat steel product (11) as described in claim 1, characterized in that, The total iron-free mass fraction of Mg as an additional alloying component in the protective coating (15) is 0.10-0.50% Mg.

3. The flat steel product (11) as described in claim 1, characterized in that, The total iron-free mass fraction of Mn, which is an additional alloying component in the protective coating (15), is greater than 0.60% Mn and less than 1.80% Mn.

4. The flat steel product (11) as described in claim 3, characterized in that, The total iron-free mass fraction of Mn, which is an additional alloying component in the protective coating (15), is greater than 0.80% Mn and less than 1.80% Mn.

5. The flat steel product (11) as described in any one of the preceding claims, characterized in that, The total iron-free mass fraction of Si, which is an additional alloying component in the protective coating (15), is less than 0.80% Si.

6. A fully alloyed steel component (21) having an Fe seam thickness greater than 2.5 μm, the steel component comprising a steel substrate (13) and an aluminum-based protective cladding (15) applied to the steel substrate (13), the steel substrate being composed of steel with the following composition (by weight percentage), excluding iron and unavoidable impurities: C: 0.04 - 0.45% by weight Si: 0.02 - 1.2% by weight Mn: 0.5 - 2.6% by weight Al: 0.02 - 1.0% by weight P: ≤ 0.05% by weight S: ≤ 0.02% by weight N: ≤ 0.02% by weight Sn: ≤ 0.03% by weight As: ≤ 0.01% by weight Ca: ≤ 0.005% by weight And selectively containing one or more of the following elements: chromium, boron, molybdenum, nickel, copper, niobium, titanium, vanadium: Cr: 0.08-1.0% by weight B: 0.001-0.005% by weight Mo: ≤0.5% by weight Ni: ≤0.5% by weight Cu: ≤0.2% by weight Nb: 0.02-0.08% by weight Ti: 0.01-0.08% by weight V: ≤0.1% by weight The selective total iron-free mass fraction of the additional alloying components in the aluminum-based protective coating is up to 10%, and the remaining iron-free mass fraction is formed by aluminum, wherein the total iron-free mass fraction of Mg as an additional alloying component in the protective coating (15) is less than 2.50% Mg, characterized in that, The total iron-free mass fraction of Mn, which is an additional alloying component in the protective coating (15), is greater than 0.30%Mn and less than 1.80%Mn, and the total iron-free mass fraction of Si, which is an additional alloying component in the protective coating (15), is less than 1.00%.

7. The steel component as described in claim 6, characterized in that, The iron-free mass fraction of Mg as an additional alloying component in the protective coating (15) is 0.10-0.50%.

8. The steel component as claimed in claim 6, characterized in that, The steel component is an automotive part.

9. The steel component as claimed in claim 8, characterized in that, The automotive components mentioned are bumper beams, bumper reinforcements, door reinforcements, B-pillar reinforcements, A-pillar reinforcements, roof frames, or door sills.

10. A method for producing the flat steel product (11) according to any one of claims 1 to 5, comprising the following steps: - The steel matrix is ​​provided by steel with the following composition (by weight percentage), excluding iron and unavoidable impurities: (13) C: 0.04 - 0.45% by weight Si: 0.02 - 1.2% by weight Mn: 0.5 - 2.6% by weight Al: 0.02 - 1.0% by weight P: ≤ 0.05% by weight S: ≤ 0.02% by weight N: ≤ 0.02% by weight Sn: ≤ 0.03% by weight As: ≤ 0.01% by weight Ca: ≤ 0.005% by weight And selectively containing one or more of the following elements: chromium, boron, molybdenum, nickel, copper, niobium, titanium, vanadium: Cr: 0.08-1.0% by weight B: 0.001-0.005% by weight Mo: ≤0.5% by weight Ni: ≤0.5% by weight Cu: ≤0.2% by weight Nb: 0.02-0.08% by weight Ti: 0.01-0.08% by weight V: ≤0.1% by weight; - A protective coating (15) of aluminum base is applied to the steel substrate (13), wherein the total iron-free mass fraction of additional alloying components in the protective coating of aluminum base is selectively summed to a maximum of 10%, and the remaining iron-free mass fraction is formed of aluminum, wherein the total iron-free mass fraction of Mg as an additional alloying component in the protective coating (15) is less than 2.50%Mg, and wherein the total iron-free mass fraction of Mn as an additional alloying component in the protective coating (15) is greater than 0.30%Mn and less than 1.80%Mn, and the total iron-free mass fraction of Si as an additional alloying component in the protective coating (15) is less than 1.00%.

11. The method as described in claim 10, characterized in that, The total iron-free mass fraction of Mg as an additional alloying component in the protective coating (15) is 0.10-0.50% Mg.

12. The method as described in claim 10, characterized in that, The protective coating (15) is applied to the steel substrate (13) by hot-dip galvanizing.

13. The method according to any one of claims 10 to 12, characterized in that, Flat steel products are pre-alloyed directly after coating by holding them at a pre-alloying temperature of 500℃-600℃ for 15-30 seconds.

14. A method for producing a steel member (21) according to any one of claims 6 to 9, comprising the following steps: - Producing flat steel products (11) by using the method of any one of claims 10 to 12; - The flat steel product (11) is annealed in a furnace preheated to temperature T for annealing time t, for flat steel products with a thickness between 0.7 mm and 1.5 mm, the annealing time being within the polygon formed by points ABCD, or annealed in a furnace preheated to temperature T for annealing time t, for flat steel products with a thickness between 1.5 mm and 3.0 mm, the annealing time being within the polygon formed by points EFGH, to form an Fe seam with a thickness greater than 2.5 µm, wherein points ABCD / EFGH are as follows: - The flat steel product is thermoformed into steel components (21).

15. The method as described in claim 14, characterized in that, The flat steel product is taken out of the furnace at a heating temperature after annealing time t, wherein the heating temperature is high enough that the flat steel product (11) has a hot forming temperature at the start of forming, wherein the microstructure of the steel matrix (13) has been wholly or partially transformed into an austenitic microstructure, and the flat steel product (11) is quenched after forming or during forming, thereby forming a hard microstructure in the microstructure of the steel matrix (13) of the flat steel product.

16. The method as described in claim 15, characterized in that, The heating temperature is at least 700°C.

17. The method as described in claim 16, characterized in that, The heating temperature is 880°C to 950°C.

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