EP4092141A1 - Flat steel product with an al coating, method for producing the same, steel component and method for producing the same - Google Patents

Abstract

The invention relates to a steel flat product for hot forming, consisting of a steel substrate (13) consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and a The protective coating (15) applied to the steel substrate (13) is based on Al. The non-ferrous mass fraction of the protective coating (15) of Mg as an additional alloy component is less than 2.50% Mg in total. In addition, the non-ferrous mass fraction of the protective coating (15) Mn as an additional alloy component totals more than 0.30% Mn and the non-ferrous mass fraction of the protective coating (15) of Si as an additional alloy component totals less than 1.80%.

Description

The invention relates to a steel flat product for hot forming consisting of a steel substrate composed of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B and a protective coating applied to the steel substrate based on Al, which optionally contains a total of up to 20% by weight of other alloying elements.

The invention also relates to a method for producing a flat steel product according to the invention. The term "flat steel product" includes all rolled products whose length is much greater than their thickness. This includes steel strips and sheets as well as blanks and blanks made from them.

Furthermore, the invention relates to a steel component produced by hot forming.

During hot press hardening, also known as hot forming, steel blanks, which are separated from cold- or hot-rolled steel strip, are heated to a deformation temperature that is generally above the austenitization temperature of the respective steel and placed in the heated state in the tool of a forming press. In the course of the subsequent forming, the sheet metal blank or the component formed from it experiences rapid cooling through contact with the cool tool. The cooling rates are set in such a way that a hardened structure results in the component. The structure is transformed into a martensitic structure.

Finally, the invention also relates to a method for producing such a steel component.

Typical steels suitable for hot press hardening are steels A-E, the chemical composition of which is listed in Table 2.

For hot-rolled MnB steel sheets provided with an Al coating, which are intended for the production of steel components by hot press form hardening, is in EP 0 971 044 B1 an alloy specification is given according to which, in addition to iron and unavoidable impurities (in % by weight), MnB steel has a carbon content of more than 0.20% but less than 0.5%, a manganese content of more than 0.5%, but less than 3%, a silicon content of More than 0.1% but less than 0.5% Chromium more than 0.01% but less than 1% Titanium less than 0.2% Aluminum less than 0.1% , a phosphorus content of less than 0.1%, a sulfur content of less than 0.05% and a boron content of more than 0.0005% but less than 0.08%. The Al coating is a so-called AISi coating, which consists of 9-10% by weight Si, 2-3.5% by weight iron and the remainder aluminum. The steel flat products thus obtained and coated are annealed at a heating temperature of more than 700 °C. During this annealing process, the protective coating melts and the protective coating is alloyed through. Here, iron diffuses from the steel substrate into the protective coating, so that phases are formed that have a higher temperature stability. The melted protective coating thus solidifies. A solidified, temperature-stable protective coating is a prerequisite for the subsequent forming step. In the forming step, the flat steel product is placed in a press-forming tool, hot-formed into the steel component and cooled so quickly that a hardened structure is created in the steel substrate of the flat steel product.

The AISi coating described has the disadvantage that, in comparison to uncoated material, long annealing times are required for thorough alloying.

The object of the present invention is therefore to provide a hot-formed flat steel product that can be further processed in a shorter time.

This object is achieved by a steel flat product for hot forming, which consists of a steel substrate consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and a Steel substrate applied protective coating based on Al. The non-ferrous mass fraction of additional alloy components of the protective coating is optionally up to 10% in total. The non-ferrous mass fraction of the protective coating of Mg as an additional alloy component is less than 2.50% Mg, preferably less than 1.50%, in particular the proportion is 0.10-0.50% Mg of the protective coating of Mn as an additional alloy component in total more than 0.30% Mn, preferably more than 0.60% Mn, particularly preferably more than 0.80% Mn. In addition, the non-ferrous mass fraction of the protective coating of Si as an additional alloy component is less than 1.80% Si, preferably less than 1.20% Si, preferably less than 0.80% Si, particularly preferably less than 0.60% Si.

Within the meaning of this application, an iron-free mass fraction of an alloy component in the protective coating is understood to mean the fraction of the total mass of this alloy component in the total mass of all elements in the protective coating except iron. The protective coating thus comprises a proportion of up to 2.5% by weight magnesium, more than 0.30% by weight manganese and less than 1.80% by weight silicon based on the total mass of all elements except iron in the protective coating . Using the non-ferrous mass fraction to characterize the protective coating has the advantage that the numerical values do not change as a result of iron diffusing in from the steel substrate.

It has been shown that the addition of manganese can lead to faster alloying and thus a reduction in the annealing time. Surprisingly, this effect only occurs if the protective coating has a low silicon content. The effect decreases with increasing silicon content. The effect is no longer observable from an ironless mass fraction of 2.00% silicon.

Magnesium has proven to be an advantageous, additional alloying component, which can be easily alloyed into Al protective coatings of the type in question here. The amount of Mg added is adjusted in such a way that the total iron-free mass fraction of magnesium is less than 2.50%, in particular less than 1.50%.

The non-ferrous mass fraction of magnesium in the protective coating is preferably at least 0.10% and at most 0.50% Mg, with Mg contents of less than 0.50%, in particular less than 0.45% or up to 0.40% or up to 0.35%, have proven to be particularly favorable in practice. The small amounts of magnesium added to the Al coating are characterized by a higher affinity for oxygen than the main component, aluminum, of the protective coating. Even with the presence of such small amounts of magnesium, a thin oxide layer forms on the surface of the protective coating, covering the aluminum lying between it and the steel substrate. During the heating required for hot forming of the flat steel product, this thin layer prevents the aluminum from reacting with the moisture present in the atmosphere of the furnace used for heating the flat steel product. This effectively prevents oxidation of the aluminum in the coating and the associated release of hydrogen, which could diffuse into the coating and the steel substrate of the flat steel product. Surprisingly, this also applies in particular when the Al-based coating becomes locally molten as a result of heating and its surface cracks, so that molten coating material comes into contact with the furnace atmosphere. In the case of longer annealing times in particular, the hydrogen concentration in the component hot-formed from the flat steel product is lower in the case of flat steel products according to the invention than in flat steel products conventionally provided with an Al coating.

The advantage of alloying magnesium is that it reduces the amount of hydrogen entering the steel substrate. If a very high local concentration of hydrogen is reached, this weakens the bond at the grain boundaries of the steel substrate structure to such an extent that a crack occurs along the grain boundary during use as a result of the resulting stress.

The layer thickness of the protective coating is typically in the range of 5-35 μm, in particular 10-25 μm.

The faster alloying of the protective coating has several advantages. On the one hand, the duration of the hot forming process can be shortened, which makes the production process more efficient. In addition, energy savings can be achieved through the shortened annealing time. Incidentally, other furnaces can also be used for heating and keeping at the heating temperature. Roller hearth furnaces, for example, are used for this process step. When using the flat steel product according to the invention, shorter roller hearth furnaces can be used because of the shortened annealing time. In particular, furnaces that were originally designed for the process security of uncoated material can be used.

In a further developed embodiment of the flat steel product, the nonferrous mass fraction of the protective coating of Mn as an additional alloy component is more than 1.00% Mn, in particular more than 1.30% Mn. It has been shown that from a non-ferrous mass fraction of 1.00% manganese, the annealing time is significantly reduced.

A special design variant of the steel flat product is characterized in that the non-ferrous mass fraction of the protective coating of Mn as an additional alloy component is less than 1.80% manganese, preferably less than 1.60% manganese. As the Mn content increases, the melting point increases, making the protective coating more difficult to apply by hot dip coating. In addition, high manganese levels promote Slag formation in the melt and are therefore also disadvantageous. Although the exemplary embodiments shown with 1.60% manganese have the advantages according to the invention, they exhibited major slag problems, which made production difficult.

In a further developed embodiment of the flat steel product, the ironless mass fraction of the protective coating of Si as an additional alloy component is less than 1.50% Si, in particular less than 1.00% Si, preferably less than 0.80% Si. The lower the silicon content, the more the manganese content affects the alloying time. Therefore, the silicon content is preferably very low. However, melts with a silicon content well below 0.50% are technically difficult to implement, since contamination with silicon is difficult to avoid. In order to ensure efficient production, the silicon content is therefore in particular more than 0.03%, preferably 0.05%, in particular 0.10%. With these silicon contents, the effect according to the invention still occurs significantly, but the coating process can be implemented much more cost-effectively, since it is not necessary to pay so much attention to silicon impurities.

The Al-based protective coating can be applied to the flat steel product particularly economically by hot-dip coating, also known as "hot-dip aluminizing" in technical jargon.

The object according to the invention is also achieved by a steel component produced by hot-press forming a flat steel product as described above.
The steel component comprises in particular a steel substrate consisting of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and a protective coating based on Al applied to the steel substrate. The non-ferrous mass fraction of additional alloy components of the protective coating is optionally up to 10% in total. The non-ferrous mass fraction of the protective coating of Mg as an additional alloy component is less than 2.5% Mg in total. Furthermore, the non-ferrous mass fraction of the protective coating of Mn as an additional alloy component is more than 0.30% Mn in total. In addition, the non-ferrous mass fraction of the protective coating of Si as an additional alloying component is less than 1.80% Si in total. The steel component has the same advantages that are explained above in relation to the flat steel product. Likewise, with reference to the steel flat product, preferred non-ferrous mass fractions of the various elements (eg manganese, silicon and magnesium) are mentioned. These preferred non-ferrous mass fractions with their advantages also apply to the steel component.

• Bereitstellen eines Stahlsubstrats aus einem Stahl, der 0,1 - 3 Gew. % Mn und optional bis zu 0,01 Gew.-% B aufweist;
• Beschichten des Stahlsubstrats mit einem Schutzüberzug auf Basis von Al, , der optional in Summe einen eisenlosen Massenanteil von bis zu 10 % an zusätzlichen Legierungsbestandteilen aufweist, wobei der eisenlose Massenanteil des Schutzüberzugs von Mg als zusätzlichem Legierungsbestandteil in Summe weniger als 2,50 % Mg beträgt und wobei der eisenlose Massenanteil des Schutzüberzugs von Mn als zusätzlichem Legierungsbestandteil in Summe mehr als 0,30 % Mn beträgt und der eisenlose Massenanteil des Schutzüberzugs von Si als zusätzlichem Legierungsbestandteil in Summe weniger als 1,80 % Si beträgt.

The object according to the invention is also achieved by a method for producing a flat steel product as described above, comprising the following work steps:

• providing a steel substrate of a steel comprising 0.1-3 wt% Mn and optionally up to 0.01 wt% B;
• Coating of the steel substrate with a protective coating based on Al, which optionally has a non-ferrous mass fraction of up to 10% of additional alloy components in total, with the non-ferrous mass fraction of the protective coating of Mg as an additional alloy component totaling less than 2.50% Mg and wherein the nonferrous mass fraction of the protective coating of Mn as an additional alloying component is more than 0.30% Mn in total and the nonferrous mass fraction of the protective coating of Si as an additional alloying component is less than 1.80% Si in total.

In particular, in this process the protective coating is applied to the steel substrate by hot dip coating. Hot dip coating, also called "hot-dip aluminizing" in technical jargon, is a particularly economical process for applying a protective coating.

In the case of hot-dip coating, a melt is used consisting of aluminum with an optional admixture of up to 10% non-ferrous mass fraction of additional alloy components. The non-ferrous mass fraction of magnesium as an additional alloy component in the melt is less than 2.50% magnesium in total. Furthermore, the non-ferrous mass fraction of manganese as an additional alloy component in the melt totals more than 0.30% manganese and the non-ferrous mass fraction of silicon as an additional alloy component in the melt totals less than 1.80% silicon. Hot-dip coating results in the protective coating being made up of an alloy layer which adjoins the steel substrate and a top layer which adjoins the alloy layer. The composition of the top layer essentially corresponds to the composition of the melt, whereas the alloy layer already contains an iron content of typically more than 30% by weight, since the steel substrate and the adjacent melt mix during the hot dipping process. Because the steel substrate essentially comprises iron, this mixing in the alloy layer does not change the iron-free mass fractions. The melt and the protective coating therefore have the same non-ferrous mass fractions of the alloying elements.

The layer thickness of the protective coating is typically in the range of 5-35 μm, in particular 10-25 μm.

With regard to the steel flat product, preferred non-ferrous mass fractions of the various elements (e.g. manganese, silicon and magnesium) are mentioned above. These preferred non-ferrous mass fractions and their advantages also apply to the process for producing the flat steel product and in particular to the composition of the melt if production is carried out by means of hot-dip coating.

In a further variant of the method, the steel flat product is pre-alloyed immediately after coating by being kept at a pre-alloying temperature of 500°C-600°C for a pre-alloying time of 15-30 seconds. In the context of this application, "immediately" is to be understood as meaning that the flat steel product does not cool after coating until the protective coating has completely solidified. In practice, depending on the design of the coating system, there can be up to 10 seconds between coating and pre-alloying.

The pre-alloying step leads to increased diffusion, so that iron is already diffusing from the substrate into the protective coating and increased iron-containing phases are beginning to form there. As a result, the subsequent annealing process for the through-alloying can be further shortened. According to the invention, the manganese content alone shortens the annealing time in the annealing process (see below). The master alloy enables this annealing time to be shortened even further.

• Herstellen eines Stahlflachprodukts durch Anwendung zuvor erläuterten Verfahrens;
• Glühen des Stahlflachproduktes in einem auf eine Temperatur T vorgeheizten Ofen für eine Glühzeit t definiert durch das Polygon ABCD nach Figur 11 für Stahlflachprodukte mit einer Dicke zwischen 0,7mm und 1,5mm oder in einem auf eine Temperatur T vorgeheizten Ofen für eine Glühzeit t definiert durch das Polygon EFGH nach Figur 11 für Stahlflachprodukte mit einer Dicke zwischen 1,5mm und 3,0mm
• Warmformen des Stahlflachprodukts zu dem Stahlbauteil.

Furthermore, the object of the invention is achieved by a method for producing a steel component as described above, comprising the following work steps:

• producing a steel flat product by using the method explained above;
• Annealing of the steel flat product in a furnace preheated to a temperature T for an annealing time t defined by the polygon ABCD figure 11 for steel flat products with a thickness between 0.7mm and 1.5mm or in a furnace preheated to a temperature T for a glow time t defined by the polygon EFGH figure 11 for steel flat products with a thickness between 1.5mm and 3.0mm
• Hot forming of the steel flat product to form the steel component.

The annealing of the steel flat product in a furnace preheated to a temperature T for an annealing time t defined by a polygon ABCD is to be understood within the meaning of this application that the value pair of temperature T and annealing time t is within the polygon formed by the points ABCD becomes.

In the figure 11 The points AH shown have the following pairs of values: designation Temperature [°C] Glow time t [minutes] A 930 1.5 B 930 7 C 880 12 D 880 2.5 E 940 2.5 f 940 9 G 900 13 H 900 4

It goes without saying that the same degree of full alloying is achieved more quickly at higher temperatures (ie with a shorter annealing time) than at lower temperatures. In addition, the thickness of the flat steel product must also be taken into account, since thicker flat steel products take longer (or higher temperatures are required) to reach the necessary core temperature for the formation of austenite inside the flat steel product.

Other preferred ranges for the thickness of the steel flat product, temperature T and annealing time t are given in the table below: Thickness d [mm] Temperature [°C] Glow time [minutes] 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

• Herstellen eines Stahlflachprodukts durch Anwendung zuvor erläuterten Verfahrens;
• Glühen des Stahlflachproduktes mit einer Dicke d in einem auf eine Temperatur T vorgeheizten Ofen für eine Glühzeit t gemäß einer Varianten aus der voranstehenden Tabelle, beispielsweise also Glühen des Stahlflachproduktes mit einer Dicke von 0,7 - 0,9 mm in einem auf eine Temperatur von 910-930°C vorgeheizten Ofen für eine Glühzeit von 1,5 - 5 Minuten.
• Warmformen des Stahlflachprodukts zu dem Stahlbauteil.

The object according to the invention is thus also achieved by a method for producing a steel component as described above, comprising the following work steps:

• producing a steel flat product by using the method explained above;
• Annealing of the flat steel product with a thickness d in a furnace preheated to a temperature T for an annealing time t according to a variant from the table above, for example annealing of the flat steel product with a thickness of 0.7-0.9 mm in a temperature of 910-930°C preheated oven for an annealing time of 1.5 - 5 minutes.
• Hot forming of the steel flat product to form the steel component.

As already explained, the addition of manganese while simultaneously limiting the silicon content accelerates the diffusion process of the iron from the steel substrate into the protective coating, ie the time for full alloying is shortened. Therefore, the annealing time compared to a standard process, such as that in EP2086755 is described, can be significantly shortened.

During the annealing process, the so-called Fe seam forms in the protective coating. This is a high ferrous phase in the protective coating at the interface with the steel substrate. The thickness of the Fe seam is a measure of the degree of alloying penetration of the protective coating. the The thickness of the Fe seam after a certain annealing time is therefore a measure of the alloying penetration rate of the coating.

In particular, the method is developed in such a way that when the steel flat product is annealed in a furnace that has been preheated to a temperature T for an annealing time t defined by the polygon ABCD figure 11 for flat steel products with a thickness between 0.7mm and 1.5mm or in a furnace preheated to a temperature T for an annealing time t defined by the polygon EFGH figure 11 for flat steel products with a thickness between 1.5 mm and 3.0 mm, an Fe seam results with a thickness greater than 2.5 μm, in particular greater than 8 μm, preferably greater than 10 μm. The thickness of the Fe-seam is adjusted according to the other requirements. A sufficiently thick Fe seam ensures that no liquid phases (e.g. liquid aluminium) occur in the protective coating during hot forming. This has several advantages. On the one hand, liquid phases would lead to contamination of furnace rollers or the forming tool. On the other hand, the more rapid alloying causes the surface of the steel flat product to discolour. While the steel flat product shows a shiny, metallic surface before annealing, the surface of the steel flat product is dark and dull after the protective coating has been thoroughly alloyed. The faster the protective coating alloys through, the faster the surface discolours. The dark, matt surface means that the heat coupling is significantly improved. The necessary core temperature for hot forming is therefore also reached more quickly. This results in a self-reinforcing effect of the manganese content according to the invention. On the one hand, the alloying time of the protective coating is reduced. On the other hand, the faster alloying leads to an increased heating rate in the steel substrate and thus to a faster annealing process.

In a special development, the heating temperature is so high that the flat steel product has a hot forming temperature at the start of forming at which the structure of the steel substrate is completely or partially converted into an austenitic structure, and that the flat steel product is quenched after forming or in the course of forming is so that hardened structure is formed in the structure of the steel substrate of the steel flat product. In particular, the heating temperature is at least 700°C, in particular 880°C to 950°C.

The steel substrate is of a steel containing 0.1-3 wt% Mn and optionally up to 0.01 wt% B. In particular, the microstructure of the steel is martensitic by hot working or partially martensitic structure convertible. The microstructure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic microstructure, since this has a particularly high degree of hardness.

The steel substrate is particularly preferably a steel which, in addition to iron and unavoidable impurities (in % by weight), consists of 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 Approx: ≤ 0.005% by weight and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V" in the following contents CR: 0.08-1.0% by weight, B: 0.001 - 0.005% by weight Mon: ≤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 consists.

The elements P, S, N, Sn, As, Ca are impurities that cannot be completely avoided in steel production. In addition to these elements, you can also other elements may be present as impurities in the steel. These other elements are summarized under the "unavoidable impurities". The total content of unavoidable impurities is preferably not more than 0.2% by weight, preferably not more than 0.1% by weight. The optional alloying elements Cr, B, Nb, Ti, for which a lower limit is given, can also occur in contents below the respective lower limit as unavoidable impurities in the steel substrate. In that case, they are also counted among the unavoidable impurities, the total content of which is limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The individual upper limits for the respective contamination of these elements are preferably as follows: CR: ≤ 0.050% by weight, B: ≤ 0.0005% by weight Nb: ≤ 0.005% by weight, Ti: ≤ 0.005% by weight

These preferred upper limits are to be considered as alternatives or together. Preferred variants of the steel therefore meet one or more of these four conditions.

In a preferred embodiment, the carbon content of the steel is at most 0.37% by weight and/or at least 0.06% by weight. In particularly preferred variants, the C content is in the range from 0.06 to 0.09% by weight or in the range from 0.12 to 0.25% by weight or in the range from 0.33 to 0.37% by weight %.

In a preferred embodiment, the Si content of the steel is at most 1.00% by weight and/or at least 0.06% by weight.

In a preferred variant, the Mn content of the steel is at most 2.4% by weight and/or at least 0.75% by weight. In particularly preferred embodiment variants, the Mn content is in the range of 0.75-0.85% by weight or in the range of 1.0-1.6% by weight.

In a preferred variant, the Al content of the steel is at most 0.75% by weight, in particular at most 0.5% by weight, preferably at most 0.25% by weight. Alternatively or additionally, the Al content is preferably at least 0.02%.

It has also been shown that it can be helpful if the sum of the silicon and aluminum contents is limited. In a preferred variant, the sum of the contents of Si and Al (usually referred to as Si+Al) is therefore at most 1.5% by weight, preferably at most 1.2% by weight. Additionally or alternatively, the sum of the contents of Si and Al is at least 0.06% by weight, preferably at least 0.08% by weight.

The elements P, S, N are typical impurities that cannot be completely avoided in steel production. In preferred variants, the maximum P content is 0.03% by weight. Regardless, the S content is preferably at most 0.012%. In addition or as a supplement, the N content is preferably at most 0.009% by weight.

Optionally, the steel also contains chromium with a content of 0.08 - 1.0% by weight. The Cr content is preferably at most 0.75% by weight, in particular at most 0.5% by weight.

In the case of an optional addition of chromium, the sum of the chromium and manganese contents is preferably limited. The total is at most 3.3% by weight, in particular at most 3.15% by weight. Furthermore, the sum is at least 0.5% by weight, preferably at least 0.75% by weight.

The steel preferably also optionally contains boron with a content of 0.001-0.005% by weight. In particular, the B content is at most 0.004% by weight.

The steel can optionally contain molybdenum with a content of at most 0.5% by weight, in particular at most 0.1% by weight.

Furthermore, the steel can optionally contain nickel with a content of at most 0.5% by weight, preferably at most 0.15% by weight.

Optionally, the steel can also contain copper with a content of at most 0.2% by weight, preferably at most 0.15% by weight.

In addition, the steel can optionally contain one or more of the micro-alloying elements Nb, Ti and V. The optional Nb content is at least 0.02% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional Ti content is at least 0.01% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional V content is at most 0.1% by weight, preferably at most 0.05% by weight.

In the case of an optional addition of several of the elements Nb, Ti and V, the sum of the contents of Nb, Ti and V is preferably limited. The total is at most 0.1% by weight, in particular at most 0.068% by weight. Furthermore, the sum is preferably at least 0.015% by weight.

The above explanations of preferred steel substrates also apply to the steel substrate of the flat steel product, as well as to the steel component and to the production methods described.

In a preferred variant of the steel flat product and the steel component and the two processes, the steel substrate is a steel from the group of steels A-E, the chemical analysis of which is given in Table 2. Table 2 is to be understood in such a way that for each steel from the group of steels A-E the element proportions are given in percent by weight. A minimum and a maximum weight percentage is given here. For example, steel A therefore has a carbon content C: 0.05% by weight-0.10% by weight.

Fig. 1

ein Querschliffbild eines Stahlbauteils mit einem Vergleichsschutzüberzug;

Fig. 2

ein Querschliffbild eines erfindungsgemäßen Stahlbauteils mit einem Schutzüberzug;

Fig. 3

ein Querschliffbild eines Stahlbauteils mit einem alternativen Vergleichsschutzüberzug;

Fig.4

ein Querschliffbild eines erfindungsgemäßen Stahlbauteils mit einem alternativen Schutzüberzug;

Fig.5

die Dicke des Fe-Saums in Abhängigkeit der Glühzeit bei drei verschiedenen Varianten für die eisenlosen Massenanteile von Mangan;

Fig.6

die Dicke des Fe-Saums in Abhängigkeit der Glühzeit bei drei verschiedenen Varianten für die eisenlosen Massenanteile von Mangan bei gleichzeitiger Anwesenheit von Si;

Figur 7

die Dicke des Fe-Saums bei einer Glühzeit von 3 Minuten in Abhängigkeit des eisenlosen Massenanteils von Mangan;

Figur 8

die Dicke des Fe-Saums bei einer Glühzeit von 3 Minuten in Abhängigkeit des eisenlosen Massenanteils von Silizium.

Figur 9

die Dicke des Fe-Saums in Abhängigkeit der Glühzeit bei drei verschiedenen Varianten ohne Magnesium;

Figur 10

die Dicke des Fe-Saums in Abhängigkeit der Glühzeit bei drei verschiedenen Varianten ohne Magnesium nach einer Vorlegierung;

Figur 11

geeignete Glühparameter für das Verfahren zum Herstellen eines Stahlbauteils.

Figur 12

eine schematische Darstellung eines Stahlflachproduktes

The invention is explained in more detail using the following exemplary embodiments in conjunction with the figures. show:

1

a cross-section of a steel component with a comparative protective coating;

2

a cross section of a steel component according to the invention with a protective coating;

3

a cross-section of a steel component with an alternative comparative protective coating;

Fig.4

a cross section of a steel component according to the invention with an alternative protective coating;

Fig.5

the thickness of the Fe seam as a function of the annealing time for three different variants for the non-ferrous mass fractions of manganese;

Fig.6

the thickness of the Fe seam as a function of the annealing time for three different variants for the non-ferrous mass fractions of manganese with the simultaneous presence of Si;

figure 7

the thickness of the Fe seam with an annealing time of 3 minutes as a function of the non-ferrous mass fraction of manganese;

figure 8

the thickness of the Fe seam with an annealing time of 3 minutes as a function of the non-ferrous mass fraction of silicon.

figure 9

the thickness of the Fe seam as a function of the annealing time for three different variants without magnesium;

figure 10

the thickness of the Fe seam as a function of the annealing time for three different variants without magnesium after a master alloy;

figure 11

suitable annealing parameters for the process of manufacturing a steel component.

figure 12

a schematic representation of a steel flat product

the Figures 1-4 show cross-section images of steel components that were produced with the same steel substrate and with the same forming process. Only the composition of the protective coating was varied.

Shaped blanks were cut from a 1.5 mm thick strip of steel grade D according to Table 2 with a 25 μm thick aluminum-based protective coating on both sides. Both a punching tool and a laser were used as cutting methods. The detailed chemical composition of the substrate 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%.

These blanks were annealed in a roller hearth furnace at 920° C. for an annealing time t. This heating temperature is above the Ac3 temperature, which is around 860°C for this steel grade. Thus, an at least partially austenitic structure was formed in the steel substrate. The blanks were then formed in a forming tool and quenched in the process.

Table 1 shows the thickness of the Fe seam for different variants of the non-ferrous mass fractions of the elements Mg, Mn and Si and for different annealing times t.

the Figures 1-4 show examples of cross-sections of the steel component produced in this way with different compositions. the Figures 5-8 show diagrams used to explain the various effects.

figure 1 shows a steel component 21 with a protective coating 15 based on aluminum on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The melt consisted of aluminum with an additional alloy of 0.4% by weight Mg, 0.8% by weight Mn and 2.0% by weight Si. The steel flat product 11 (see figure 12 ) accordingly had a protective coating 15 with a thickness of 25 μm after the coating, the protective coating 15 having an iron-free mass fraction of 0.4% Mg, 0.8% Mn and 2.0% Si. After the explained hot forming, the steel component 21 shown in the cross section with the protective coating 15 based on aluminum resulted, the protective coating 15 having an iron-free mass fraction of 0.4% Mg, 0.8% Mn and 2.0% Si.

figure 2 shows a steel component 21 with a protective coating 15 based on aluminum on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The melt consisted of aluminum with an additional alloy of 0.4% by weight Mg, 1.6% by weight Mn and 2.0% by weight Si. The steel flat product 11 (see figure 12 ) accordingly had a protective coating 15 with a thickness of 25 μm after the coating, the protective coating 15 having an iron-free mass fraction of 0.4% Mg, 0.8% Mn and 2.0% Si. After the explained hot forming, the steel component 21 shown in the cross-section resulted with the protective coating 15 based on aluminum, the protective coating 15 having an iron-free mass fraction of 0.4% Mg, 1.6% Mn and 2.0% Si.

The two steel components 21 in Figure 1 and Figure 2 only show hints of an Fe-fringe. The thickness of the Fe seam is less than 1 µm. The protective coating 15 is therefore not sufficiently alloyed in the 3 minutes at 920°C.

figure 3 shows a steel component 21 with a protective coating 15 based on aluminum on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The melt consisted of aluminum with an additional alloy of 0.4% by weight Mg and 0.8% by weight Mn. Apart from impurities in the range of 0.2% by weight, the melt contained no silicon. The steel flat product 11 (see figure 12 ) accordingly had a protective coating 15 with a thickness of 25 μm after the coating, the protective coating 15 having an ironless mass fraction 0.4% Mg, 0.8% Mn and less than 0.50% Si. After the explained hot forming, the steel component 21 shown in the cross section image resulted with the protective coating 15 based on aluminum, the protective coating 15 having an iron-free mass fraction of 0.4% Mg, 0.8% Mn and less than 0.50% Si.

In figure 3 it can be clearly seen that an Fe seam 17 has formed in the protective coating. This has a thickness of 3 µm. The alloying of the protective coating 15 is therefore complete after 3 minutes at 920°C. Although the in figure 1 and figure 3 The steel components 21 shown have gone through an identical manufacturing process apart from the silicon content of the melt, as shown in figure 3 a pronounced Fe hem. The reduction in the silicon content means that the process time can be significantly reduced.

figure 4 shows a steel component 21 with a protective coating 15 based on aluminum on a steel substrate 13. The protective coating 15 was applied by hot dip coating. The melt consisted of aluminum with an additional alloy of 0.4% by weight Mg and 1.6% by weight Mn. Apart from impurities in the range of 0.2% by weight, the melt contained no silicon. Accordingly, after the coating, the flat steel product had a protective coating 15 with a thickness of 25 μm, the protective coating 15 having an iron-free mass fraction of 0.4% Mg, 1.6% Mn and less than 0.50% Si. After the explained hot forming, the steel component 21 shown in the cross section with the aluminum-based protective coating resulted, the protective coating 15 having an iron-free mass fraction of 0.4% Mg, 1.6% Mn and less than 0.50% Si.

In figure 4 a clear Fe seam 17 with a thickness of 7 μm can be seen. The thorough alloying of the protective coating 15 is therefore complete after 3 minutes at 920° C. and is also more advanced than in figure 3 . Increasing the manganese content further shortens the process time. At the same time, the comparison shows figure 4 With figure 2 that this effect of manganese only occurs with a low silicon content. Although in the variant after figure 2 the manganese content compared to the variant figure 1 is increased, there is no stronger Fe seam in due to the higher silicon content figure 2 compared to figure 1 .

• Aluminium mit 0,4% Magnesium, 0,2% Silizium
• Aluminium mit 0,4% Magnesium, 0,8% Mangan, 0,2% Silizium
• Aluminium mit 0,4% Magnesium, 1,6% Mangan, 0,2% Silizium

figure 5 shows the thickness of the Fe seam as a function of the annealing time for three different variants for the non-ferrous mass fractions:

• Aluminum with 0.4% magnesium, 0.2% silicon
• Aluminum with 0.4% magnesium, 0.8% manganese, 0.2% silicon
• Aluminum with 0.4% magnesium, 1.6% manganese, 0.2% silicon

The effect of the manganese can be clearly seen here. While there is virtually no difference with longer annealing times, the addition of manganese results in an Fe fringe of 7 µm resulting even with the short annealing time of 3 minutes, which demonstrates adequate alloying.

• Aluminium mit 0,4% Magnesium, 0% Mangan, 2% Silizium
• Aluminium mit 0,4% Magnesium, 0,8% Mangan, 2% Silizium
• Aluminium mit 0,4% Magnesium, 1,6% Mangan, 2% Silizium

figure 6 shows the same representation as figure 5 . With the variants in figure 6 However, the silicon content was also increased to an iron-free mass fraction of 2%. The following variants are therefore shown (the last two curves are identical and therefore cannot be distinguished in the diagram):

• Aluminum with 0.4% magnesium, 0% manganese, 2% silicon
• Aluminum with 0.4% magnesium, 0.8% manganese, 2% silicon
• Aluminum with 0.4% magnesium, 1.6% manganese, 2% silicon

It can be clearly seen that the effect of the manganese no longer occurs. After 3 minutes of annealing, no significant Fe seam can be seen in any of the three variants.

figure 7 shows the thickness of the Fe seam with an annealing time of 3 minutes as a function of the non-ferrous mass fraction of manganese. The ironless mass fraction of silicon is about 0.2%. From an iron-free mass fraction of manganese of about 0.3%, a thicker Fe rim results, with the thickness of the Fe rim increasing with the manganese content.

figure 8 shows the effects of silicon. The thickness of the Fe seam with an annealing time of 3 minutes is plotted as a function of the non-ferrous mass fraction of silicon. In all cases, the non-ferrous mass fraction of manganese was 1.6%. Below 1.8% silicon, the manganese effect is still significant, with the smaller the massless fraction of silicon, the greater the effect.

• Aluminium ohne Beilegierung
• Aluminium mit 1,6% Mangan, 0,0% Silizium
• Aluminium mit 1,6% Mangan, 0,5% Silizium

figure 9 shows the described effects again for variants without magnesium. The thickness of the Fe seam is shown as a function of the annealing time for three different variants for the non-ferrous mass fractions:

• Aluminum without additives
• Aluminum with 1.6% manganese, 0.0% silicon
• Aluminum with 1.6% manganese, 0.5% silicon

It can be clearly seen that a significant Fe fringe forms much earlier in the variants with 1.6% manganese. In addition, it also becomes clear that the same effect occurs both with 0.5% silicon and with 0% silicon. In the tests without magnesium alloying, the silicon contamination was reduced to less than 0.05%. In these tests, 0% silicon is therefore to be understood as up to 0.05% Si.

figure 10 shows the thickness of the Fe seam as a function of the annealing time for the same layer compositions as in figure 9 . In these variants, however, pre-alloying was carried out before annealing, in which the mold blanks were held at a pre-alloying temperature of 680 °C for a pre-alloying time of 13 seconds. Compared to figure 9 In all cases, the Fe seam forms much earlier and is thicker than without a master alloy for the same annealing time. Furthermore, the manganese effect can be seen that a significant Fe fringe forms faster with manganese alloying. This is the case with both 0% Si and 0.5% silicon.

The last three lines of Table 1 are not plotted. In this series of tests, it was examined again whether there are any significant effects of the magnesium content. It has been shown that the magnesium content has no impact on the effect.

figure 12 shows a schematic representation of a steel flat product 11 for hot forming, which consists of a steel substrate 13, which consists of a steel that has 0.1-3% by weight Mn and optionally up to 0.01% by weight B, and a protective coating 15 applied to the steel substrate 13 based on Al. <b>Table 1</b> mg [%] Mn [%] Si [%] Glow time [minutes] Fe border [µm] pre-alloyed? 0.4 0 0.2 3 1.0 no 0.4 0 0.2 5 11.0 no 0.4 0 0.2 10 20.0 no 0.4 0.8 0.2 3 3.0 no 0.4 0.8 0.2 5 12.0 no 0.4 0.8 0.2 10 21.0 no 0.4 1.6 0.2 3 7.0 no 0.4 1.6 0.2 5 11.0 no 0.4 1.6 0.2 10 21.0 no 0.4 0.8 2.0 3 1.0 no 0.4 0.8 2.0 5 9.0 no 0.4 0.8 2.0 10 17.0 no 0.4 1.6 2.0 3 1.0 no 0.4 1.6 2.0 5 9.0 no 0.4 1.6 2.0 10 17.0 no 0.4 0 2.0 3 1.0 no 0.4 0 2.0 5 8.0 no 0.4 0 2.0 10 19.0 no 0.4 0.4 2.0 3 2.0 no 0.4 0.4 2.0 5 11.0 no 0.4 0.4 2.0 10 20.0 no 0.4 1.2 2.0 3 5.0 no 0.4 1.2 2.0 5 11.0 no 0.4 1.2 2.0 10 20.0 no 0.4 1.6 0.8 3 5.0 no 0.4 1.6 0.8 5 11.0 no 0.4 1.6 0.8 10 19.0 no 0.4 1.6 1.6 3 3.0 no 0.4 1.6 1.6 5 10.0 no 0.4 1.6 1.6 10 18.0 no 0 0 0 1 0.0 no 0 0 0 1.5 0.0 no 0 0 0 2 0.0 no 0 0 0 2.5 0.0 no 0 0 0 3 1.0 no 0 0 0 5 10.2 no 0 1.6 0 1 0.0 no 0 1.6 0 1.5 0.0 no 0 1.6 0 2 3.0 no 0 1.6 0 2.5 6.1 no 0 1.6 0 3 7.0 no 0 1.6 0 5 12.5 no 0 1.6 0.5 1 0.0 no 0 1.6 0.5 1.5 0.0 no 0 1.6 0.5 2 3.2 no 0 1.6 0.5 2.5 5.9 no 0 1.6 0.5 3 7.2 no 0 1.6 0.5 5 12.2 no 0 0 0 1 0.0 Yes 0 0 0 1.5 0.0 Yes 0 0 0 2 5.2 Yes 0 0 0 2.5 7.8 Yes 0 0 0 3 9.2 Yes 0 0 0 5 13.0 Yes 0 1.6 0 1 0.0 Yes 0 1.6 0 1.5 5.2 Yes 0 1.6 0 2 6.1 Yes 0 1.6 0 2.5 8.6 Yes 0 1.6 0 3 9.2 Yes 0 1.6 0 5 14.0 Yes 0 1.6 0.5 1 0.0 Yes 0 1.6 0.5 1.5 4.5 Yes 0 1.6 0.5 2 6.0 Yes 0 1.6 0.5 2.5 8.0 Yes 0 1.6 0.5 3 8.8 Yes 0 1.6 0.5 5 13.2 Yes 0.4 1.6 0.8 3 5.0 no 1 1.6 0.8 3 6.0 no 1.5 1.6 0.8 3 5.0 no steel grade min / max C si Mn P S Al Nb Ti Cr+Mo B A at least 0.05 0.05 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 Max 0.10 0.35 1.00 0.030 0.025 0.075 0.100 0.150 0.0050 B at least 0.05 0.03 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 Max 0.10 0.50 2.00 0.030 0.025 0.075 0.100 0.150 0.0050 C at least 0.05 0.05 1.00 0.000 0.000 0.015 0.005 0.000 0.00 0.0010 Max 0.16 0.40 1.40 0.025 0.010 0.150 0.050 0.050 0.50 0.0050 D at least 0.10 0.05 1.00 0.000 0.000 0.005 0.000 0.00 0.0010 Max 0.30 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0050 E at least 0.250 0.10 1.00 0.000 0.000 0.015 0.000 0.00 0.0010 Max 0.380 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0500

Claims (14)

Steel flat product (11) for hot forming, consisting of a steel substrate (13) consisting of a steel containing 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and a Steel substrate (13) applied protective coating (15) based on Al, which optionally has a non-ferrous mass fraction of up to 10% of additional alloy components, with non-ferrous mass fraction of the protective coating (15) of Mg as an additional alloy component in total less than 2 50% Mg, in particular 0.10 - 0.50% by weight Mg, is characterized in that the nonferrous mass fraction of the protective coating (15) of Mn as an additional alloy component totals more than 0.30% Mn and the nonferrous Mass fraction of the protective coating (15) of Si as an additional alloying component is less than 1.80% in total. Flat steel product (11) according to Claim 1, characterized in that the non-ferrous mass fraction of Mn in the protective coating (15) as an additional alloy component is more than 0.60% Mn, preferably more than 0.80% Mn. Flat steel product (11) according to one of the preceding claims, characterized in that the non-ferrous mass fraction of the protective coating (15) of Si as an additional alloy component is less than 1.20% Si, preferably less than 0.80% Si. Flat steel product (11) according to one of the preceding claims, characterized in that the non-ferrous mass fraction of Mn in the protective coating (15) as an additional alloy component is less than 1.80% Mn in total. Flat steel product (11) according to one of the preceding claims, characterized in that the protective coating (15) is applied by hot dip coating. A steel member (21) produced by hot press forming a flat steel product (11) as claimed in any one of the preceding claims. A steel component (21) according to claim 6 comprising a steel substrate (13) selected from a steel comprising 0.1-3 wt% Mn and optionally up to 0.01 wt% B and one coated on the steel substrate (13) applied protective coating (15) based on Al, which is optionally in Total has a non-ferrous mass fraction of up to 10% of additional alloy components, with the non-ferrous mass fraction of the protective coating (15) of Mg as an additional alloy component totaling less than 2.5% Mg, in particular 0.10-0.50% Mg characterized in that the non-ferrous mass fraction of the protective coating (15) of Mn as an additional alloy component is more than 0.30% Mn in total and the non-ferrous mass fraction of the protective coating (15) of Si as an additional alloy component is less than 1.80% in total amounts to. Steel component according to one of Claims 6 to 7 , characterized in that the steel component is an automobile component, in particular a bumper support, a bumper reinforcement, a door reinforcement, a B-pillar reinforcement, an A-pillar reinforcement, a roof frame or a sill. Method for producing a steel flat product (11) according to one of Claims 1 to 5, comprising the following work steps: - providing a steel substrate (13) made of a steel containing 0.1-3% by weight Mn and optionally up to 0.01% by weight B; - Coating of the steel substrate (13) with a protective coating (15) based on Al, which optionally has a non-ferrous mass fraction of up to 10% in total of additional alloy components, the non-ferrous mass fraction of the protective coating (15) of Mg as an additional alloy component in The total is less than 2.5% Mg, in particular 0.10-0.50% Mg, and the nonferrous mass fraction of the protective coating (15) of Mn as an additional alloy component totals more than 0.30% Mn and the nonferrous mass fraction of the protective coating (15) of Si as an additional alloy component is less than 1.80% in total. A method according to claim 9, characterized in that the protective coating (15) is applied to the steel substrate (13) by hot dip coating. Method according to one of Claims 9 to 10, characterized in that the steel flat product is pre-alloyed immediately after the coating by being kept at a pre-alloying temperature of 500°C-600°C for a pre-alloying time of 15-30 seconds. Method for producing a steel component (21) according to one of Claims 6 to 8, comprising the following steps: - producing a steel flat product (11) by using one of the methods specified in claims 9 or 10; - Annealing of the flat steel product (11) in an oven preheated to a temperature T for an annealing time t defined by the polygon ABCD according to Figure 11 for flat steel products with a thickness between 0.7 mm and 1.5 mm or in an oven preheated to a temperature T for an annealing time t defined by the polygon EFGH according to FIG. 11 for flat steel products with a thickness between 1.5 mm and 3.0 mm - Hot forming of the steel flat product to form the steel component (21). Method according to Claim 12, characterized in that the heating temperature is so high that the steel flat product (11) has a hot forming temperature at the beginning of the forming at which the structure of the steel substrate (13) is completely or partially converted into an austenitic structure, and that the Steel flat product (11) is quenched after forming or in the course of forming, so that hardened structure forms in the structure of the steel substrate (13) of the steel flat product. Process according to Claim 13, characterized in that the heating temperature is at least 700°C, in particular 880°C to 950°C.

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