Shaped Sheet Metal Part with Improved Processing Properties

The invention relates to a shaped sheet metal part having improved processing properties and to a process for producing such a shaped sheet metal part from a flat steel product.

Where a “flat steel product” or else a “sheet metal product” is discussed hereinafter, this means rolled products such as steel strips or sheets, from which “sheet metal blanks” (also called blanks) are divided for the production of bodywork components, for example. “Shaped sheet metal parts” or “sheet metal components” of the type according to the invention have been produced from such sheet metal blanks; the terms “shaped sheet metal part”, “sheet metal component” and “component” are used synonymously here.

All figures relating to contents of the steel compositions that are reported in the present application are based on weight, unless explicitly stated otherwise. All indeterminate percentage figures associated with a steel alloy should therefore be regarded as figures in “% by weight”. With the exception of the figures for the residual austenite content of the microstructure of a shaped sheet metal part of the invention that are based on volume (reported in “% by volume”), figures relating to the contents of the different microstructure constituents are each based on the area of a section of a sample of the respective product (reported in area percent, “area %”), unless explicitly stated otherwise. Figures given in this text for the contents of the constituents of an atmosphere are based on volume (reported in “% by volume”).

Mechanical properties, such as tensile strength, yield point, elongation, that are reported here have been ascertained by the tensile test according to DIN EN ISO 6982-1, sample form 2 (Annex B Tab. B1) (version of 2020-06), unless explicitly stated otherwise. Bending angle is determined according to VDA Standard 238-100 for the force maximum. Vickers hardness HV5 was determined to DIN EN ISO 6507 (2018.07). Yield point in the context of this application, in the case of a pronounced yield point, means the yield point Re. In the case of a continuous yield point, by contrast, yield point means the value for the Rp0.2 yield point.

The microstructure was determined on longitudinal sections that had been subjected to etching with 3% Nital (alcoholic nitric acid). The proportion of residual austenite was determined by x-ray diffractometry.

WO 2019/223854 A1 discloses a shaped sheet metal part and a process for producing such a shaped sheet metal part, which has a tensile strength of at least 1000 MPa. This shaped sheet metal part consists of a steel composed of, as well as iron and unavoidable impurities, (in % by weight) 0.10-0.30% C, 0.5-2.0% Si, 0.5-2.4% Mn, 0.01-0.2% Al, 0.005-1.5% Cr, 0.01-0.1% P and any further optional elements, especially 0.005-0.1% Nb. Moreover, the shaped sheet metal component comprises an anticorrosion coating containing aluminum.

WO 2006/128821 and WO 2007/122230 A1 disclose processes for producing shaped sheet metal parts having improved processing properties. These involve using forming tools having different temperature zones.

Against the background of the prior art, the problem addressed was that of further developing a shaped sheet metal part such that improved processing properties are achieved in conjunction with an aluminum-based anticorrosion coating. Furthermore, the intention was to specify a process by which such shaped sheet metal parts can be produced practically.

The invention solves this problem by a process for producing a shaped sheet metal part having at least one first zone and one second zone having different material properties, comprising the following steps:

Compared to known flat steel products, the steel substrate of the flat steel product used in accordance with the invention has an aluminum content of at least 0.10% by weight, more preferably at least 0.11% by weight, especially at least 0.12% by weight, preferably at least 0.16% by weight. The maximum aluminum content is 1.0% by weight, especially not more than 0.8% by weight.

In a first developed variant, the aluminum content is at least 0.10% by weight, more preferably at least 0.11% by weight, especially at least 0.12% by weight, preferably at least 0.16% by weight. The maximum aluminum content in this variant is not more than 0.50% by weight, especially not more than 0.35% by weight, preferably not more than 0.25% by weight, especially not more than 0.24% by weight.

In a second developed variant, the aluminum content is at least 0.50% by weight, preferably at least 0.60% by weight, preferably at least 0.70% by weight. The maximum aluminum content in this variant is not more than 1.0% by weight, especially not more than 0.9% by weight, preferably not more than 0.80% by weight.

It is well known that aluminum (“Al”) is added as deoxidant in the production of steel. Reliable binding of the oxygen present in the steel melt requires at least 0.01% by weight of Al. Furthermore, Al may additionally be used for binding of contents of N that are unwanted but unavoidable for production-related reasons. Comparatively high aluminum contents have been avoided to date since the Ac3 temperature also moves to higher temperatures with the aluminum content. This has an adverse effect on austenitization, which is important for hot forming. However, it has been found that elevated aluminum contents surprisingly lead to positive effects in conjunction with an aluminum-based anticorrosion coating.

In the coating of the flat steel product with an aluminum-based anticorrosion coating and in the subsequent hot forming of sheet metal blanks divided therefrom to give shaped sheet metal parts, there is diffusion of iron from the steel substrate into the liquid anticorrosion coating. This forms, in the interdiffusion zone, iron aluminide compounds having relatively high density via a multistage phase transformation (Fe2Al5→Fe2Al→FeAl→Fe3Al). The formation of such denser phases is associated with higher consumption of aluminum than in the case of lower-density phases. This locally higher aluminum consumption leads to formation of pores (vacancies) in the resultant phase. These pores are preferably formed in the transition region between steel substrate and anticorrosion coating, where the proportion of aluminum available is shaped to a significant degree by the aluminum content of the steel substrate. In particular, there can be an accumulation of pores in the form of a band in the transition region.

Such pores, and in particular a band of pores, cause a variety of problems:

It has been found that, surprisingly, increasing the aluminum content (“Al”) in the steel substrate to the lower limits described or higher can achieve a distinct reduction in pore formation on coating with an aluminum-based anticorrosion coating and subsequent hot forming. Especially in the transition region between steel substrate and anticorrosion coating, the locally higher aluminum consumption in the case of formation of denser iron aluminide compounds can be at least partly compensated for by the aluminum content of the steel substrate, such that the formation of pores, especially a band of pores, is suppressed.

In the case of an excessively high Al content, especially in the case of contents of more than 1.0% by weight of Al, there is a risk that Al oxides will form at the surface of a product manufactured from a steel material alloyed in accordance with the invention, which would worsen the wetting characteristics in the hot dip coating operation. Moreover, in the case of relatively high Al contents, the formation of nonmetallic Al-based inclusions is favored, which, as coarse inclusions, have an adverse effect on crash characteristics. Therefore, the Al content chosen is preferably below the upper limits already mentioned.

The bending characteristics of the sheet metal component are supported in particular by the inventive niobium content (“Nb”) of at least 0.001% by weight. The niobium content is preferably at least 0.005% by weight, especially at least 0.010% by weight, preferably at least 0.015% by weight, more preferably at least 0.020% by weight, especially at least 0.024% by weight, preferably at least 0.025% by weight.

The niobium content specified leads more particularly, in the process described hereinafter for production of a flat steel product for hot forming with an anticorrosion coating, to a distribution of niobium carbonitrides that leads to a particularly fine hardening microstructure in the subsequent hot forming operation. During the cooling after the hot dip coating, the coated flat steel product is kept within a temperature range of 400° C. and 300° C. for a certain period of time. Within the temperature range, there is still a certain diffusion rate of carbon in the steel substrate, while thermodynamic stability is very low. Carbon thus diffuses to and accumulates at lattice defects. Lattice defects are caused in particular by dissolved niobium atoms which widen out the atomic lattice by virtue of their much higher atomic volume, and hence increase the size of the tetrahedral and octahedral gaps in the atomic lattice, such that the local solubility of C is increased. Consequently, clusters of C and Nb arise in the steel substrate, which are then transformed to very fine precipitates in the subsequent austenitization step of hot forming and act as additional austenite seeds. The result is therefore a refined austenite microstructure with relatively small austenite grains and hence also a refined hardening microstructure.

This also relates in particular to the ferritic interdiffusion layer that forms in the hot forming operation. The refined ferritic microstructure in the interdiffusion layer promotes the reduction of tendencies to initiate cracking under flexural loads.

However, too high an Nb content leads to worsened recrystallizability. Therefore, the Nb content is not more than 0.2% by weight. Further preferably, the niobium content is not more than 0.20% by weight, especially not more than 0.15% by weight, preferably not more than 0.10% by weight, especially not more than 0.05% by weight.

Aluminum and niobium both have an influence on grain refining in austenitization in the hot forming process. It has been found that Al, as well as Nb, especially refines grain growth at elevated temperatures in austenite (for example at more than 1200° C.) via comparatively early formation (i.e. taking place at relatively high temperatures) of AlN. The formation of AlN is thermodynamically favored over the formation of NbN or NbC. The precipitation of AlN has a grain-refining effect here in austenite and hence a toughness-improving effect. Rising Al/Nb ratios improve this effect. It is therefore optionally the case that, for the Al/Nb ratio of Al content to Nb content:

the Al/Nb ratio is preferably ≥2, especially ≥3. At the same time, an excessively high ratio of Al/Nb has the effect that AlN formation is no longer as advantageously fine, and that increasingly coarser AlN particles occur, which again reduces the grain refining effect. It has been found that this effect occurs earlier in the case of low manganese contents than in the case of higher manganese contents since the Ac3 temperature decreases with rising manganese content. It is therefore advantageous, optionally in the case of low manganese contents of not more than 1.6% by weight, to establish an Al/Nb ratio for which:

which corresponds roughly to an atomic ratio of the two elements of ≤6. Preferably, when Mn≤1.6% by weight, the Al/Nb ratio is ≤18.0, especially ≤16.0, preferably ≤14.0, more preferably ≤12.0, especially ≤10.0, preferably ≤9.0, especially ≤8.0, preferably ≤7.0.

In the case of higher manganese contents of Mn≥1.7% by weight, by contrast, higher ratios are also possible. It is therefore advantageous, optionally in the case of higher manganese contents of 1.7% by weight or more, to establish a ratio of Al/Nb for which:

Preferably, when Mn≥1.7% by weight, the Al/Nb ratio is ≤28.0, especially ≤26.0, preferably ≤24.0, more preferably ≤22.0, preferably ≤20.0, especially ≤18.0, especially ≤16.0, preferably ≤14.0, more preferably ≤12.0, especially ≤10.0, preferably ≤9.0, especially ≤8.0, preferably ≤7.0.

Irrespective of the manganese content, it is thus optionally preferable to establish a ratio of Al/Nb for which:

The Al/Nb ratio is preferably ≤18.0, especially ≤16.0, preferably ≤14.0, more preferably ≤12.0, especially ≤10.0, preferably ≤9.0, especially ≤8.0, preferably ≤7.0.

Carbon (“C”) is present in the steel substrate of the flat steel product in contents of 0.27-0.5% by weight. C contents set at such a level contribute to the hardenability of the steel in that they delay ferrite and bainite formation and stabilize the residual austenite in the microstructure.

However, high C contents can adversely affect weldability. In order to improve weldability, the carbon content can be adjusted to 0.50% by weight, preferably to not more than 0.45% by weight, more preferably 0.40% by weight, preferably not more than 0.38% by weight, especially not more than 0.35% by weight.

In order to be able to utilize the positive effects of the presence of C particularly reliably, C contents of at least 0.30% by weight, preferably 0.32% by weight, especially at least 0.33% by weight, especially at least 0.34% by weight, preferably at least 0.35% by weight, may be provided. With these contents, taking account of the further provisions of the invention, it is possible to reliably achieve tensile strengths of the shaped sheet metal part of at least 1700 MPa, especially at least 1800 MPa, after hot press forming.

Silicon (“Si”) is used to further increase the hardenability of the flat steel product and the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silico-manganese as alloying agent, which has a beneficial effect on production costs. A hardening effect is already established over and above an Si content of 0.05% by weight. A significant rise in strength occurs over and above an Si content of at least 0.15% by weight, especially at least 0.20% by weight. Si contents above 0.6% by weight have a disadvantageous effect on coating characteristics, especially in the case of Al-based coatings. Si contents of not more than 0.50% by weight, especially not more than 0.30% by weight, are preferably established in order to improve the surface quality of the coated flat steel product.

Manganese (“Mn”) acts as a hardening element in that it significantly delays ferrite and bainite formation. In the case of manganese contents of less than 0.4% by weight, during press hardening, significant proportions of ferrite and bainite are formed even in the case of very rapid cooling rates, which should be avoided. Mn contents of at least 0.5% by weight, especially at least 0.8% by weight, preferably of at least 0.9% by weight, more preferably of at least 1.10% by weight, are advantageous when a martensitic microstructure is to be ensured, especially in regions of relatively high forming. Manganese contents of more than 3.0% by weight have an adverse effect on processing properties, and therefore the Mn content of flat steel products of the invention is limited to not more than 3.0% by weight, preferably not more than 2.5% by weight. Weldability in particular is greatly restricted, and therefore the Mn content is limited preferably to not more than 1.6% by weight and especially to 1.30% by weight, especially to not more than 1.20% by weight. Manganese contents of not more than 1.6% by weight are additionally also preferred for economic reasons.

Titanium (“Ti”) is a microalloy element which is included in the alloy in order to contribute to grain refining, and at least 0.001% by weight of Ti, especially at least 0.004% by weight, preferably at least 0.010% by weight of Ti, should be added for sufficient availability. There is a distinct deterioration in cold rollability and recrystallizability over and above 0.10% by weight of Ti, and therefore any greater Ti contents should be avoided. In order to improve cold rollability, the Ti content may be restricted preferably to 0.08% by weight, especially to 0.038% by weight, more preferably to 0.020% by weight, especially 0.015% by weight. Titanium additionally has the effect of binding nitrogen and hence making it possible for boron to display its greatly ferrite-inhibiting effect. Therefore, in a preferred development, the titanium content is more than 3.42 times the nitrogen content in order to achieve sufficient binding of nitrogen.

Boron (“B”) is included in the alloy in order to improve the hardenability of the flat steel product in that boron atoms or boron precipitates adjoining austenite grain boundaries reduce the grain boundary energy, which suppresses the nucleation of ferrite during press hardening. A distinct effect on hardenability occurs in the case of contents of at least 0.0005% by weight, preferably at least 0.0007% by weight, especially at least 0.0010% by weight, especially at least 0.0020% by weight. In the case of contents exceeding 0.01% by weight, by contrast, there is increased formation of boron carbides, boron nitrides or boron nitrocarbides, which in turn constitute preferred nucleation sites for the nucleation of ferrite and lower the hardening effect again. For that reason, the boron content is limited to not more than 0.01% by weight, preferably not more than 0.0100% by weight, preferably not more than 0.0050% by weight, especially not more than 0.0035% by weight, especially not more than 0.0030% by weight, preferably not more than 0.0025% by weight.

Phosphorus (“P”) and sulfur (“S”) are elements that are introduced into the steel as impurities by iron ore and cannot be eliminated entirely in the industrial scale steelworks process. The P content and the S content should be kept as low as possible due to the residual deterioration in mechanical properties, for example notched impact resistance, with increasing P content or S content. Moreover, there is incipient embrittlement of the martensite over and above P contents of 0.03% by weight, and therefore the P content of a flat steel product of the invention is limited to not more than 0.03% by weight, especially not more than 0.02% by weight. The S content of a flat steel product of the invention is limited to not more than 0.02% by weight, preferably not more than 0.0010% by weight, especially not more than 0.005% by weight.

Nitrogen (“N”) is likewise present as an impurity in the steel in small amounts owing to the steel manufacturing process. The N content should be kept as low as possible and should be not more than 0.02% by weight. Especially in the case of alloys containing boron, nitrogen is harmful since the formation of boron nitrides prevents the transformation-retarding effect of boron, and therefore the nitrogen content in this case should preferably be not more than 0.010% by weight, especially not more than 0.007% by weight.

Further typical impurities are tin (“Sn”) and arsenic (“As”). The Sn content is not more than 0.03% by weight, preferably not more than 0.02% by weight. The As content is not more than 0.01% by weight, especially not more than 0.005% by weight.

As well as the above-elucidated impurities P, S, N, Sn and As, it is also possible for further elements to be present in the steel as impurities. These further elements are combined under the “unavoidable impurities”. The content of these “unavoidable impurities” preferably adds up to not more than 0.2% by weight, preferably not more than 0.1% by weight. The optional alloy elements Cr, Cu, Mo, Ni, V, Ca and W described hereinafter for which a lower limit is specified may also be present in the steel substrate as unavoidable impurities in contents below the respective lower limit. In that case, they are likewise counted among the “unavoidable impurities”, the total content of which is limited to not more than 0.2% by weight, preferably not more than 0.1% by weight.

Chromium, copper, molybdenum, nickel, vanadium, calcium and tungsten may optionally each be included in the alloy individually or in combination with one another as part of the steel of a flat steel product of the invention.

Chromium (“Cr”) suppresses the formation of ferrite and perlite during accelerated cooling of a flat steel product of the invention and enables complete martensite formation even in the case of relatively low cooling rates, which achieves an increase in hardenability.

These stated effects are established over and above a content of 0.01% by weight, and a content of at least 0.10% by weight, preferably at least 0.15% by weight, has been found to be useful for reliable processing in practice. However, excessively high contents of Cr impair the coatability of the steel. Therefore, the Cr content of the steel of a steel substrate is limited to not more than 1.0% by weight, preferably not more than 0.80% by weight, especially not more than 0.75% by weight, preferably not more than 0.50% by weight.

Vanadium (“V”) may optionally be included in the alloy in contents of 0.001-1.0% by weight. The vanadium content is preferably not more than 0.3% by weight. For reasons of cost, not more than 0.2% by weight of vanadium is included in the alloy.

Copper (“Cu”) may optionally be included in the alloy in order to increase hardenability in the case of additions of at least 0.01% by weight, preferably at least 0.010% by weight, especially at least 0.015% by weight. In addition, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges. In the case of an excessively high Cu content, there is a distinct deterioration in hot rollability owing to low-melting Cu phases at the surface, and therefore the Cu content is limited to not more than 0.2% by weight, preferably not more than 0.1% by weight, especially not more than 0.10% by weight.

Molybdenum (“Mo”) may optionally be added in order to improve process stability, since it distinctly slows ferrite formation. Over and above contents of 0.002% by weight, there is dynamic formation of molybdenum-carbon clusters up to and including ultrafine molybdenum carbides at the grain boundaries, which distinctly slow the mobility of the grain boundary and hence diffusive phase transformations. Moreover, molybdenum reduces grain boundary energy, which reduces the nucleation rate of ferrite. The Mo content is preferably at least 0.004% by weight, especially at least 0.01% by weight. Because of the high costs associated with alloying of molybdenum, the content should be not more than 0.3% by weight, especially not more than 0.10% by weight, preferably not more than 0.08% by weight.

Nickel (“Ni”) stabilizes the austenitic phase and may optionally be included in the alloy in order to reduce the Ac3 temperature and to suppress the formation of ferrite and bainite. Nickel additionally has a positive influence on hot rollability, especially when the steel contains copper. Copper worsens hot rollability. In order to counter the adverse effect of copper on hot rollability, it is possible to include 0.01% by weight of nickel in the alloy as part of the steel; the Ni content is preferably at least 0.020% by weight. For economic reasons, the nickel content should remain limited to not more than 0.5% by weight, especially not more than 0.20% by weight. The Ni content is preferably not more than 0.10% by weight.