Patent Application
20240229214
The invention relates to a flat steel product for hot forming, consisting of a steel substrate which consists of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B, and of an Al-based protective coating applied to the steel substrate, optionally containing a total of up to 20 wt % of other alloy elements.
The invention likewise relates to a method of producing a flat steel product of the invention. The term “flat steel product” here includes all rolled products having a length very much greater than their thickness. These include steel strips and sheets, and (precut) blanks obtained therefrom.
The invention further relates to a steel component produced by hot forming.
In hot press hardening, also called hot forming, steel blanks that have been divided from a cold-or hot-rolled steel strip are heated to a forming temperature which is generally above the austenitization temperature of the respective steel and placed in the heated state into the forming tool of a forming press. In the course of the forming conducted subsequently, the sheet metal blank, or the component formed therefrom, undergoes rapid cooling as a result of contact with the cold forming tool. The cooling rates are adjusted so as to result in a hard microstructure in the component. The microstructure is transformed to a martensitic microstructure.
Finally, the invention also relates to a method of producing such a steel component.
Typical steels that are suitable for hot press hardening are steels A-E, the chemical composition of which is listed in table 2.
For hot-rolled MnB sheet steels provided with an Al coating that are intended for production of steel components by hot press hardening, EP 0 971 044 B1 specifies an alloying method in which an MnB steel, as well as iron and unavoidable impurities, has (in wt %) 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%, a chromium content of more than 0.01% but less than 1%, a titanium content of less than 0.2%, an aluminum content of 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 what is called an AlSi coating, consisting of 9-10 wt % of Si, 2-3.5 wt % of iron, and of aluminum as the balance. The coated flat steel products with these characteristics are annealed at a heating temperature of more than 700° C. During this annealing operation, the protective coating is melted and the protective coating forms a metallurgical bond. There is diffusion here of iron from the steel substrate into the protective coating, so as to form phases having higher thermal stability. Thus, the molten protective coating solidifies. A solidified, thermally stable protective coating is a prerequisite for the subsequent forming step. In the forming step, the flat steel product is placed into a press-forming tool, where it is hot-formed to give the steel component and at the same time cooled at such a rate as to form hard microstructure in the steel substrate of the flat steel product.
The AlSi coating described has the disadvantage that long annealing times are required for metallurgical bonding compared to uncoated material.
It is therefore an object of the present invention to provide a flat steel products for hot forming that can be processed further within a shorter time.
This object is achieved by a flat steel product for hot forming, consisting of a steel substrate which consists of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B, and of an Al-based protective coating applied to the steel substrate. The iron-free mass fraction of additional alloy constituents of the protective coating optionally adds up to 10%. The remaining iron-free mass fraction is formed from aluminum. Therefore, the protective coating consists of iron, aluminum, and optionally of additional alloy constituents other than iron. In this case, the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 2.50% Mg, preferably less than 1.50%; in particular, the proportion is 0.10-0.50% Mg. In addition, the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn, preferably more than 0.60% Mn, more preferably more than 0.80% Mn, especially preferably 1.35% Mn, especially preferably 1.40% Mn. In addition, the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80% Si, preferably less than 1.20% Si, preferably less than 0.80% Si, more preferably less than 0.60% Si.
An iron-free mass fraction of an alloy constituent in the protective coating, in the context of this application, is understood to mean the proportion of the total mass of this alloy constituent in the total mass of all elements in the protective coating except for iron. The protective coating thus comprises a proportion of up to 2.5 wt % of magnesium, more than 0.30 wt % of manganese and less than 1.80 wt % of silicon, based on the total mass of all elements except for iron in the protective coating. The use of the iron-free mass fraction for characterization of the protective coating has the advantage that the numerical values do not change as a result of inward diffusion of iron from the steel substrate.
It has been found that the inclusion of manganese in the alloy can achieve faster metallurgical bonding and hence shortening of the annealing time. Surprisingly, this effect occurs only in the case of low silicon contents of the protective coating. The effect decreases with rising silicon contents. Even over and above an iron-free mass fraction of 2.00% silicon, the effect is no longer observable.
An advantageous additional alloy constituent has been found to be magnesium, which can be readily included in Al protective coatings of the type in question here. The amount of Mg added is adjusted such that the iron-free mass fraction of magnesium adds up to less than 2.50%, especially less than 1.50%.
The iron-free mass fraction of magnesium in the protective coating is preferably at least 0.10% and at most 0.50% Mg, and particularly favorable Mg contents in practice have been found to be less than 0.50%, especially less than 0.45% or to 0.40% or to 0.35%. Magnesium, which is added to the Al coating in small amounts, is notable for higher oxygen affinity than the main aluminum constituent of the protective coating. Even in the presence of such small amounts of magnesium, a thin oxide layer is formed on the surface of the protective coating, and covers the aluminum between it and the steel substrate. This thin layer, in the heating required for the hot forming of the flat steel product, hinders reaction of the aluminum with the moisture present in the atmosphere of the furnace used for the heating of the flat steel product. Oxidation of the aluminum in the coating and associated release of hydrogen that could diffuse into the coating and the steel substrate of the flat steel product are thus effectively prevented. In particular, this is surprisingly true even when the Al-based coating becomes locally molten as a result of the heating and its surface breaks, such that molten coating material comes into contact with the furnace atmosphere. Especially in the case of longer annealing times, in flat steel products of the invention, a lower hydrogen concentration compared to flat steel products provided conventionally with an Al coating is found in the component hot-formed from the flat steel product.
The inclusion of magnesium in the alloy has the advantage that this reduces the input of hydrogen into the steel substrate. If a locally very high hydrogen concentration is attained, this weakens the binding at the grain boundaries of the steel substrate microstructure such that a crack forms along the grain boundary as a result of the stress that occurs in use.
The layer thickness of the protective coating is typically in the range of 5-35 μm, especially 10-25 μm.
The faster metallurgical bonding for the protective coating has multiple advantages. Firstly, the process duration of the hot forming process can be shortened, which makes the production process more efficient. Moreover, the shortened annealing time can achieve energy savings. In addition, it is also possible to use different furnaces for the heating and the holding at the heating temperature. For this process step, for example, roller hearth furnaces are used. When the flat steel product of the invention is used, it is possible to use shorter roller hearth furnaces because of the reduced annealing time. In particular, it is possible to make use of furnaces that were originally designed for process control of uncoated material.
In a further-developed embodiment of the flat steel product, the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 1.00% Mn, especially more than 1.30% Mn. It has been found that another distinct shortening of the annealing time arises over and above an iron-free mass fraction of 1.00% manganese.
It is a feature of a specific execution variant of the flat steel product that the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to less than 1.80% manganese, preferably less than 1.60% manganese. The melting point increases with rising Mn content, and so the protective coating is more difficult to apply by hot dip coating. Moreover, high manganese contents promote slag formation in the melt and are therefore likewise disadvantageous. Although the working examples shown with 1.60% manganese have the advantages of the invention, major slag problems were apparent, which makes production difficult.
In a further-developed embodiment of the flat steel product, the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.50% Si, especially less than 1.00% Si, preferably less than 0.80% Si. The lower the silicon content, the greater the effect of the manganese content on the metallurgical bonding time. The silicon content is therefore preferably very low. However, melts having a silicon content well below 0.50% are difficult to implement industrially since it is possible only with difficulty to avoid silicon impurities. In order to assure efficient production, the silicon content is therefore especially more than 0.03%, preferably 0.05%, especially 0.10%. With these silicon contents, the effect of the invention is still significant, but the coating process can be implemented much less expensively since it is not necessary to pay such great attention to silicon impurities.
The Al-based protective coating can be applied particularly economically to the flat steel product by hot dip coating, also referred to in the jargon as “hot dip aluminizing”.
The object of the invention is likewise achieved by a steel component produced by hot press forming of an above-described flat steel product.
The steel component especially comprises a steel substrate consisting of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B, and an Al-based protective coating applied to the steel substrate. The iron-free mass fraction of additional alloy constituents of the protective coating optionally adds up to 10%. In this case, the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 2.5% Mg. In addition, the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn. Moreover, the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80% Si. The steel component has the same advantages elucidated above in relation to the flat steel product. Likewise specified are iron-free mass fractions of the different elements (for example of manganese, silicon and magnesium) that are preferred with regard to the flat steel product. These preferred iron-free mass fractions with their benefits are likewise applicable to the steel component.
The object of the invention is likewise achieved by a method of producing an above-described flat steel product, comprising the following steps:
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 the jargon, is a particularly economically viable method of applying a protective coating.
Hot dip coating is accomplished using a melt which consists of aluminum having an optional addition comprising an iron-free mass fraction of up to 10% of additional alloy constituents. The iron-free mass fraction of magnesium as additional alloy constituent in the melt adds up to less than 2.50% magnesium. In addition, the iron-free mass fraction of manganese as additional alloy constituent in the melt adds up to more than 0.30% manganese, and iron-free mass fraction of silicon as additional alloy constituent in the melt adds up to less than 1.80% silicon. The hot dip coating results in a structure of the protective coating composed of an alloy layer adjoining the steel substrate, and an outer layer adjoining the alloy layer. The composition of the outer layer corresponds essentially to the composition of the melt, whereas the alloy layer already contains an iron content of typically more than 30 wt %, since there is mixing between the steel substrate and the adjoining melt during the hot dip operation. Since the steel substrate comprises essentially iron, this mixing in the alloy layer does not alter the iron-free mass fractions. The melt and protective coating thus have the same iron-free mass fractions of the alloy elements.
The layer thickness of the protective coating is typically in the range of 5-35 μm, especially 10-25 μm.
With reference to the flat steel product, preferred iron-free mass fractions of the different elements (for example of manganese, silicon and magnesium) are specified above. These preferred iron-free mass fractions with their advantages are likewise applicable to the method of producing the flat steel product and especially to the composition of the melt if production is effected by hot dip coating.
In a further variant of the method, the flat steel product is prealloyed immediately after coating by keeping it at a prealloying temperature of 500° C.-600° C. for a prealloying time of 15-30 seconds. What is meant by “immediately” in the context of this application is that the flat steel product, after being coated, does not cool down to the extent of full solidification of the protective coating. In practice, depending on the configuration of the coating plant, there may be up to 10 seconds between the coating and the prealloying.
The prealloying step results in increased diffusion, such that iron already diffuses from the substrate into the protective coating, where it already begins to form iron-containing phases to an increased degree. The effect of this is that the subsequent annealing process for the metallurgical bonding can additionally be shortened. According to the invention, as a result of the manganese content, the annealing time in the annealing process is already shortened (see below). The prealloying makes it possible to further shorten this annealing time.
In addition, the object of the invention is achieved by a method of producing an above-described steel component, comprising the following steps:
What is meant in the context of this application by the annealing of the flat steel product in a furnace preheated to a temperature T for an annealing time t defined by a polygon ABCD is that the pair of values of temperature T and annealing time t is within the polygon formed by the points ABCD.
The points A-H shown in
It will be apparent that the same degree of metallurgical bonding is achieved more quickly at higher temperatures (i.e. with shorter annealing time) than at lower temperatures. In addition, the thickness of the flat steel product also has to be taken into account since it takes longer in the case of thicker flat steel products (or higher temperatures are required) to attain the necessary core temperature for the formation of austenite within the flat steel product.
Further preferred ranges for thickness of the flat steel product, temperature T and annealing time t are specified in the following table:
1-1.4
1-1.4
The object of the invention is thus likewise achieved by a method of producing an above-described steel component, comprising the following steps:
As already elucidated, the inclusion of manganese in the alloy, with simultaneous limitation of the silicon content, accelerates the process of diffusion of the iron from the steel substrate into the protective coating, i.e. shortens the time for metallurgical bonding. Therefore, the annealing time can be distinctly shortened compared to a standard process as described, for example, in EP2086755.
During the annealing operation, what is called the Fe seam is formed in the protective coating. This is a phase with a high iron content in the protective coating at the boundary to the steel substrate. In this context, the thickness of the Fe seam is a measure of the degree of metallurgical bonding of the protective coating. In the present invention, “metallurgical bonding” means the thickness of the Fe seam is greater than 2.5 μm, especially greater the 8 μm, preferably greater than 10 μm. The thickness of the Fe seam at a particular annealing time is therefore a measure of the rate of metallurgical bonding of the coating.
In particular, in a development of the method, annealing of the flat steel product in a furnace preheated to a temperature T for an annealing time t defined by the polygon ABCD according to
After the annealing time t, the flat steel product is taken from the furnace at a heating temperature. In particular, the heating temperature corresponds to the temperature T of the preheated furnace. In a specific development, the heating temperature is sufficiently high that the flat steel product at the start of forming has a hot forming temperature at which the microstructure of the steel substrate has been fully or partly transformed to austenitic microstructure, and that the flat steel product is quenched after forming or in the course of forming, such that hard microstructure forms in the microstructure of the steel substrate of the flat steel product. In particular, the heating temperature is at least 700° C., especially 880° C. to 950° C.
The steel substrate is composed of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B. In particular, the microstructure of the steel is convertible to a martensitic or partly martensitic microstructure by hot forming. The microstructure of the steel substrate of the steel component is thus preferably a martensitic or at least partly martensitic microstructure, since this has particularly high hardness.
More preferably, the steel substrate is a steel which, as well as iron and unavoidable impurities, consists (in wt %) of
and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents:
The elements P, S, N, Sn, As, Ca are impurities that cannot be avoided completely in steelmaking. As well as these elements, it is also possible for further elements to be present as impurities in the steel. These further elements are referred to collectively under the category of “unavoidable impurities”. The content of unavoidable impurities preferably adds up to not more than 0.2 wt %, preferably not more than 0.1 wt %. The optional alloy elements Cr, B, Nb, Ti for which a lower limit is specified may also occur in contents below the respective lower limit as unavoidable impurities in the steel substrate. In that case, they are likewise counted among the unavoidable impurities, the total content of which is limited to not more than 0.2 wt %, preferably not more than 0.1 wt %. The individual upper limits for the respective impurity among these elements are preferably as follows:
These preferred upper limits should be considered alternatively or collectively. Preferred variants of steel thus meet one or more of these four conditions.
In a preferred embodiment, the C content of the steel is not more than 0.37 wt % and/or not less than 0.06 wt %. In particularly preferred execution variants, the C content is in the range of 0.06-0.09 wt % or in the range of 0.12-0.25 wt % or in the range of 0.33-0.37 wt %.
In a preferred embodiment, the Si content of the steel is not more than 1.00 wt % and/or not less than 0.06 wt %.
The Mn content of the steel, in a preferred variant, is not more than 2.4 wt % and/or not less than 0.75 wt %. In particularly preferred execution variants, the Mn content is in the range of 0.75-0.85 wt % or in the range of 1.0-1.6 wt %.
The Al content of the steel, in a preferred variant, is not more than 0.75 wt %, especially not more than 0.5 wt %, preferably not more than 0.25 wt %. Alternatively or supplementarily, the Al content is preferably not less than 0.02%.
In addition, it has been found that it can be helpful when the sum total of the contents of silicon and aluminum is limited. In a preferred variant, therefore, the sum total of the contents of Si and Al (typically referred to as Si+Al) is not more than 1.5 wt %, preferably not more than 1.2 wt %. Supplementarily or additionally, the sum total of the contents of Si and Al is not less than 0.06 wt %, preferably not less than 0.08 wt %.
The elements P, S, N are typical impurities that cannot be avoided completely in steelmaking. In preferred variants, the P content is not more than 0.03 wt %. Independently thereof, the S content is preferably not more than 0.012%. Additionally or supplementarily, the N content is preferably not more than 0.009 wt %.
The steel optionally additionally contains chromium with a content of 0.08-1.0 wt %. The Cr content is preferably not more than 0.75 wt %, especially not more than 0.5 wt %.
In the case of optional inclusion of chromium in the alloy, the sum total of the contents of chromium and manganese is preferably limited. The sum total is not more than 3.3 wt %, especially not more than 3.15 wt %. In addition, the sum total is not less than 0.5 wt %, preferably not less than 0.75 wt %.
The steel preferably optionally additionally contains boron with a content of 0.001-0.005 wt %. In particular, the B content is not more than 0.004 wt %.
Optionally, the steel may contain molybdenum with a content of not more than 0.5 wt %, especially not more than 0.1 wt %.
In addition, the steel may optionally contain nickel with a content of not more than 0.5 wt %, preferably not more than 0.15 wt %.
Optionally, the steel may additionally contain copper with a content of not more than 0.2 wt %, preferably not more than 0.15 wt %.
In addition, the steel may optionally contain one or more of the micro alloy elements Nb, Ti and V. The optional Nb content is at least 0.02 wt % and at most 0.08 wt %, preferably at most 0.04 wt %. The optional Ti content is at least 0.01 wt % and at most 0.08 wt %, preferably at most 0.04 wt %. The optional V content is at most 0.1 wt %, preferably at most 0.05 wt %.
In the case of optional inclusion of two or more of the elements Nb, Ti and V in the alloy, the sum total of the contents of Nb, Ti and V is preferably limited. The sum total is not more than 0.1 wt %, especially not more than 0.068 wt %. In addition, the sum total is preferably not less than 0.015 wt %.
The above elucidations relating to preferred steel substrates are likewise applicable to the steel substrate of the flat steel product, and also to the steel component and to the production method described.
In a preferred variant of the flat steel product and of the steel component and of the two methods, 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 should be understood such that the proportions of the elements in percent by weight are given for each steel from the group of steels A-E. A minimum and maximum proportion by weight are specified here. For example, steel A thus comprises a carbon content C: 0.05 wt %-0.10 wt %.
The invention is elucidated in detail with reference to the working examples that follow, in conjunction with the figures. The figures show:
Blanks were cut from a 1.5 mm-thick strip of steel type D according to table 2 with a double-sided 25 μm-thick aluminum-based protective coating. The cutting method employed was either a punching tool or a laser. The exact 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 about 860° C. for this type of steel. Thus, at least a partly austenitic microstructure was formed in the steel substrate. Subsequently, the blanks were formed and quenched in a forming tool.
Table 1 shows the thickness of the Fe seam for various variants of the iron-free mass fractions of the elements Mg, Mn and Si, and for various annealing times t.
The two steel components 21 in
It is clearly apparent in
In
The effect of the manganese is clearly apparent here. While there is as good as no difference in the case of longer annealing times, the addition of manganese to the alloy has the effect that even the short annealing time of 3 minutes results in an Fe seam of 7 μm, which confirms sufficient metallurgical bonding.
It is clearly apparent that the effect of the manganese no longer occurs. After an annealing time of 3 minutes, no significant Fe seam is apparent in any of the three variants.
It is clearly apparent that, in the variants with 1.6% manganese, a significant Fe seam is formed at a much earlier stage. In addition, it likewise becomes clear that the same effect occurs both at 0.5% silicon and at 0% silicon. In the experiments without magnesium addition, it was possible to lower the silicon contamination to below 0.05%. 0% silicon in these experiments should therefore be regarded as up to 0.05% Si.
The last three rows of table 1 are not shown in the graph. This test series examined once again whether there are significant effects of the magnesium content. It was found that the magnesium content has no effects on the effect.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21175294.4 | May 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/063495 | 5/18/2022 | WO |
