EP3204530B2 - Cold rolled steel sheet and recrystallisation-annealed steel flat product and method for producing the same - Google Patents

Description

The invention relates to a method for producing a cold-rolled and recrystallization-annealed flat steel product with a ferritic microstructure.

Flat steel products of this type are used in particular in the field of automobile body construction, where particularly high demands are placed on the formability and optical appearance of the components formed from such flat steel products.

When we talk about flat steel products here, we are talking about rolled products such as steel strips or sheets as well as cut pieces and blanks made from them.

Where information on alloy contents is provided below, this always refers to weight unless otherwise stated. In contrast, information on the composition of atmospheres always refers to the volume in question unless otherwise stated.

Flat steel products intended for car body construction or similar applications are typically provided with a surface structure that is characterized by a defined roughness and an equally defined peak distribution in order to meet customer-specific requirements with regard to formability and surface appearance (paintability and paint gloss). A typical example of corresponding specifications from the automotive industry is an arithmetic mean roughness (hereinafter referred to as "roughness") Ra of 1.1 - 1.6 µm with a peak number RPc of at least 60 1/cm. The roughness Ra and the peak number RPc are determined in accordance with steel iron test sheet SEP 1940 using a stylus device in accordance with ISO 3274.

Another criterion for determining the surface quality to be achieved for optimum paintability and optimum paint gloss is the so-called "waviness value Wsa(1 - 5)", hereinafter referred to as "Wsa", which is determined in accordance with steel-iron test sheet SEP 1941:2012-05 after 5% plastic elongation in the Marciniak cupping test. Typical requirements are Wsa values of 0.35 µm to 0.40 µm. Particularly good paint gloss is achieved with Wsa values of ≤ 0.35 µm, in particular < 0.30 µm. In order to achieve such low Wsa values, peak numbers RPc of at least 75 1/cm and roughnesses Ra of 0.9 - 1.4 µm are required.

The adjustment of the material properties Ra and RPc in the production of cold-rolled flat steel products is typically carried out by skin passing after the recrystallization annealing that the flat steel products undergo after cold rolling in order to ensure their optimum formability.

"Skin-passing" is understood here as a pre-rolling or finishing process carried out after recrystallizing annealing, in which the flat steel product is subjected to a slight deformation of approx. 0.2 - 2.0%, which is referred to here as the "skin-passing degree". The skin-passing degree is determined by comparing the peripheral speeds of the deflection rollers, which are equipped with position sensors, before and after the rolling mill in which the flat steel product is skin-passed. The skin-passing degree D follows from the path difference of the deflection rollers (inlet path s1, outlet path s2) as D = [(s2-s1)/s1]*100.

The combined requirement of "high peak number RPc" and "high roughness Ra" represents a complex manufacturing task that is generally valid. This is due to the fact that a high roll roughness required to achieve high Ra values generally results in a low peak number RPc, since the increasing surface fissures (= roughness) of the roll push the distance from wave crest to wave crest on the roll surface apart and thus reduces the number of peaks that can be reproduced on the flat steel product. To make matters worse, even with dry skin passing, a peak transfer loss of around 20% occurs when the peaks on the roll surface are transferred to the rolled flat steel product.

In addition, if the skin pass degree D is set too high, the roughness Ra will be too high. If, on the other hand, the skin pass degree D is set too low, the edges of the strip may not be dressed, particularly with wide strip dimensions. In these cases, the Ra and RPc values achieved are then too low.

The skin pass degree D cannot be varied arbitrarily with regard to the mechanical properties of the steel substrate. A skin pass degree D that is too low does not adequately counteract a pronounced yield point. On the other hand, a skin pass degree D that is too high can result in the strength of the steel substrate being uncorrectably high due to excessive work hardening.

The challenges of skin-pass rolling become more severe the softer, wider and thinner the flat steel product to be produced is. "Soft" here means a steel that, in the recrystallized state and after skin-pass rolling, has a yield strength Rp0.2 of no more than 180 N/mm2 and a tensile strength Rm of no more than 340 N/mm2. In practice, this means that flat steel products of the type in question here with dimensions typical for automobiles can currently only be produced with the desired operational reliability at great expense. Steels with a yield strength Rp0.2 of max. 150 MPa and a tensile strength Rm of no more than 310 MPa are particularly critical.

There are various proposals to make this effort manageable in practice and to produce flat steel products that offer optimal conditions for painting with even the most stringent requirements sufficient gloss.

An example of this is the EP 0 234 698 B1 known method for producing a steel sheet suitable for painting. This method provides for a regular pattern of depressions to be created in the surface of a skin-pass roll using an energy beam. The flat steel product to be processed is skin-pass rolled using two work rolls, at least one of which is processed in the manner described above. The reduction in cross-section achieved by skin-pass rolling should not be less than 0.3% in order to transfer the pattern from the work roll to the surface of the steel sheet. In this way, a steel sheet is to be obtained which has an average surface roughness Ra within the range of 0.3 to 3.0 µm and a microscopic shape forming the surface roughness, which consists of trapezoidal raised regions with a flat upper surface, groove-like recessed regions formed in such a way that they completely or partially surround a raised region, and flat central regions formed between the raised regions outside the recessed regions in such a way that they are higher than the bottom of the recessed regions and lower or of the same height as the upper surfaces of the raised regions. At the same time, the raised regions and recessed regions are to have certain geometric dependencies, inter alia, on the diameter of the recesses formed in the skin-pass roll.

A similar proposal is in the DE 36 86 816 T2 There, too, it was proposed to introduce a uniform surface roughness pattern into the surface of a cold-rolled flat steel product, resulting in a surface roughness Ra of 0.3 - 2.0 µm.

From the WO 2011/162135 A1 Finally, a thin cold-rolled steel sheet and a process for its production are known. The steel sheet consists of a steel with, in % by weight, 0.10% or less C, 0.05% or less Si, 0.1 - 1.0% Mn, 0.05% or less P, 0.02% or less S, 0.02 - 0.10% Al, less than 0.005% N and the remainder being Fe and unavoidable impurities. The steel sheet thus obtained is subjected to an annealing treatment in which it is annealed for at least 30 s at an annealing temperature of 730 - 850 °C and then cooled to a maximum temperature of 600 °C at a cooling rate of at least 5 °C/s. The cold-rolled annealed steel flat product obtained thereafter has a structure consisting mainly of ferrite, which has an average crystal grain diameter of 5 - 30 µm. Finally, the steel flat product is skin-pass rolled using a roll whose surface roughness Ra is at most 2 µm. The stretch ratio achieved by skin-pass rolling is adjusted depending on the average crystal grain diameter of the thin cold-rolled annealed sheet.

From the EP 2 508 629 A1 A process is known for producing a non-grain-oriented steel which contains 0.1-1% Si, 0.005-1.0% Al, not more than 0.004% C, 0.10-1.50% Mn, not more than 0.2% P, not more than 0.05% S, not more than 0.002% N, not more than 0.006% Nb+V+Ti and the remainder being iron and unavoidable impurities.

From the DE 197 01 443 A1 A sheet or strip having a yield strength Rp0.2 > 200 N/mm ² and a method for producing such a sheet or strip are known.

From the EP 0 484 960 A2 A cold-rolled steel strip having an r-value in the 45° direction of at least 1.90 and a method for producing such a strip are known.

Out of EP 1 111 081 A1 A Nb-alloyed ultra-low carbon steel containing almost no niobium carbides and a process for its production are known.

Out of EP 2 700 731 A1 A steel sheet alloyed with boron, which has a boron-nitrogen ratio B/N=(B (mass%)/10.81)/(N (mass%)/14.01) of less than or equal to 3.0, and a process for its production are known.

Out of DE 196 22 164 C1 A process for producing a cold-rolled steel sheet or strip containing 0.01-0.08% C is known.

Out of DE 102012017703A1 A flat product made of a metal material having a surface structure with an arithmetic mean roughness of 0.3-3.6 µm and a peak number of 45-180 1/cm, and a method for its production are known.

Against the background of the prior art explained above, the object of the invention was to provide a method for producing a flat steel product.

A method which allows the reliable production of a flat steel product is specified in claim 1.

Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general inventive concept.

• C: 0,0001 - 0,003 %,
• Si: 0,001 - 0,025 %,
• Mn: 0,05 - 0,20 %,
• P: 0,001 - 0,015 %,
• Al: 0,02 - 0,055 %,
• Ti: 0,01 - 0,1 %,

A cold-rolled and recrystallization-annealed flat steel product with a ferritic microstructure that can be produced by the process according to the invention therefore consists of a steel with the following composition (in wt. %):

• C: 0.0001 - 0.003%,
• Si: 0.001 - 0.025%,
• Mn: 0.05 - 0.20%,
• P: 0.001 - 0.015%,
• Al: 0.02 - 0.055%,
• Ti: 0.01 - 0.1%,

• Cr: 0, 001 - 0, 05 %,
• V: bis zu 0,005 %,
• Mo: bis zu 0,015 %,
• N: 0,001 - 0,004 %,

• wobei zu den unvermeidbaren Verunreinigungen B, Cu, Nb, Ni, Sb, Sn und S zählen, deren Anteil in Summe höchstens 0,2 Gew.-% ist, wobei im Fall der Anwesenheit von Nb, B oder Sb für diese Verunreinigungen gilt: Sb-Gehalt höchstens 0,001 Gew.-%, Nb-Gehalt höchstens 0,002 Gew.-% und B-Gehalt höchstens 0,0005 Gew.-%, und weist
  • eine Dehngrenze Rp0,2 von bis zu 180 MPa,
  • eine Zugfestigkeit Rm von bis zu 340 MPa,
  • eine Bruchdehnung A80 von mindestens 40 %,
  • einen n-Wert von mindestens 0,23
    sowie an mindestens einer seiner Oberflächen
  • eine arithmetische Mittenrauheit Ra von 0,8 - 1,6 µm und
  • eine Spitzenzahl RPc von mindestens 75 1/cm auf.

The remainder is iron and unavoidable impurities, whereby the steel may additionally contain the following optional alloying elements:

• Cr: 0.001 - 0.05%,
• V: up to 0.005%,
• Mo: up to 0.015%,
• N: 0.001 - 0.004%,

• where the unavoidable impurities include B, Cu, Nb, Ni, Sb, Sn and S, the total amount of which is not more than 0.2% by weight, where in the case of the presence of Nb, B or Sb the following applies to these impurities: Sb content not more than 0.001% by weight, Nb content not more than 0.002% by weight and B content not more than 0.0005% by weight, and has
  • a yield strength Rp0.2 of up to 180 MPa,
  • a tensile strength Rm of up to 340 MPa,
  • an elongation at break A80 of at least 40%
  • an n-value of at least 0.23
    and at least one of its surfaces
  • an arithmetic mean roughness Ra of 0.8 - 1.6 µm and
  • a peak number RPc of at least 75 1/cm.

The depressions and peaks formed in the surface, which determine the mean roughness Ra and the peak number RPc, are distributed stochastically.

A flat steel product that can be produced by the method according to the invention thus consists of a soft steel that has a yield strength Rp0.2 of up to 180 MPa, in particular less than 150 MPa, a tensile strength Rm of up to 340 MPa, in particular less than 310 MPa, and at the same time has a high elongation with a breaking elongation A80 of at least 40% and a high n-value of at least 0.23. With this combination of properties, it is optimally suited for forming, in particular for deep drawing.

At the same time, a steel flat product that can be produced by the process according to the invention has a surface quality characterized by an arithmetic mean roughness Ra of 0.8 - 1.6 µm and a peak number RPc of at least 75 1/cm, which makes it extremely suitable for painting with optimized paint gloss. Surface structures according to the invention thus reliably achieve Wsa values of at most 0.40 µm, typically at most 0.35 µm, in particular less than 0.30 µm, in particular even when the steel flat products that can be produced by the process according to the invention are in a range of dimensions typical for automotive applications with thicknesses of up to 1.0 mm and widths of at least 1000 mm.

A steel flac product that can be produced by the process according to the invention is particularly suitable for forming and painting in the uncoated state or in the state covered with a metallic protective layer.

If such a metallic coating is intended, it should be applied by electrolytic coating. By using known electrolytic processes, it is ensured that the surface structure of the steel strip that has been skin-rolled according to the invention is retained on the surface of the flat steel product covered with the metallic coating. An electrolytically applied layer based on zinc is particularly suitable as a metallic protective layer.

As an alternative or in addition to a metallic protective coating of the type mentioned above, the flat steel product that can be produced by the method according to the invention can also be coated with an inorganic or organic coating. Inorganic coating means a passive layer typical for strip processes, e.g. as phosphating or chromating. Organic coating means a thick-layer passivation typical for strip processes, e.g. based on Cr(III)-containing compounds. Known coating agents can also be used here, which are usually used to improve paint adhesion, friction behavior in the forming tool and the like.

The surface texture formed on the surface of a flat steel product produced by the method according to the invention is characterized by a stochastic distribution of depressions and peaks which Determine the roughness value Ra according to the invention and the peak number RPc according to the invention.

Stochastic surface textures, as prescribed according to the invention, are irregular surface textures that are characterized by an irregular statistical distribution of design features such as depressions, which in turn can vary in distance, shape and size from one another. Deterministic surface textures, on the other hand, are regular surface textures that are characterized by a regular distribution of similar design features.

According to the invention, a stochastic surface texturing is aimed at in order to optimize the friction behavior between the steel surface and the tool during forming processes in the oiled or greased state. In a tool-based forming process, in particular during deep drawing or stretch drawing, a stochastic surface structure is characterized by the fact that under high pressure loads the lubricant can flow out of the stress zone via microchannels that open up between the peaks and valleys of the surface texture. Compared to the more isolated lubrication pockets of a deterministic surface texturing, this more finely structured network of microchannels allows a more even distribution of the lubricant over the entire surface where contact occurs between the tool and the flat steel product during the forming process. Furthermore, a stochastic basic structure ensures flow and adhesion properties for organic or metallic coatings, which can, if necessary, also be applied to the flat steel product that can be produced using the method according to the invention.

The roughness value Ra should not be less than 0.8 µm for the surface according to the invention of a flat steel product such as the one before, because otherwise the surface is too smooth. However, the roughness value Ra should not be greater than 1.6 µm either, because the surface is then too rough to achieve optimized forming properties. In order to be able to use the advantages of the invention reliably, roughness values Ra of 0.9 -1.4 µm can be provided.

The peak number RPc should not be less than 75 per cm because this would have a negative effect on the Wsa value. By setting the peak number at at least 75 1/cm, it is ensured that the Wsa value of a flat steel product that can be produced by the process according to the invention does not rise above 0.40 µm, in particular not above 0.35 µm, and that a coating achieves an optimal paint gloss. Higher peak numbers lead to further improved Wsa values of the surface of a flat steel product that can be produced by the process according to the invention. In this way, the Wsa values of flat steel products that can be produced by the process according to the invention of less than 0.30 µm can be achieved. Wsa values of at most 0.40 µm are reliably achieved if the peak number RPc for the surface produced according to the invention is set at at least 75 per cm. Wsa values of 0.35 µm or less are achieved if the peak number RPc for the flat steel product surface created according to the invention is set at at least 80 per cm. Finally, Wsa values of less than 0.30 µm can be ensured by setting a minimum value of 90 per cm for the peak number RPc.

A flat steel product that can be produced using the method according to the invention contains C, Si, Mn, P, Al and Ti as mandatory alloying elements with the following proviso: The C content of the flat steel product that can be produced using the method according to the invention is 0.0001 - 0.003 wt.%. C is unavoidably contained in the steel melt, so that C contents of at least 0.0001 wt.% can always be found in a steel according to the invention. A C content above 0.003 wt.%, however, impairs the desired formability due to an excessively strong strengthening contribution from carbon. This can be reliably prevented by reducing the C content to 0.002 wt.% or less.

Si is present in a flat steel product such as the one above in amounts of 0.001 - 0.025 wt.%. Si is also unavoidably present in the steel melt. However, a Si content above the limit of 0.025 wt.% according to the invention impairs the formability due to an excessive contribution to hardening. In order to avoid negative effects of the presence of Si, the Si content of a flat steel product such as the one above can be limited to a maximum of 0.015 wt.%.

Mn is present in a flat steel product such as the one before in amounts of 0.05 - 0.20 wt.%. Mn contents in this range contribute optimally to the formability of a flat steel product such as the one before. If the Mn contents lie outside the range specified by the invention, the amount is too low or too high due to solid solution strengthening. An optimal influence of the presence of Mn in the flat steel product such as the one before can be ensured by limiting the Mn content to a maximum of 0.15 wt.%.

P is provided in a flat steel product as before in amounts of 0.001 - 0.015 wt.%. P is also unavoidably contained in the steel melt and contributes to solid solution strengthening. However, a P content above the limit according to the invention impairs the desired formability and has negative effects on the desired painting result. In order to utilize the positive influences of the presence of P through solid solution strengthening and at the same time safely exclude negative influences, the P content can be limited to a maximum of 0.012 wt.%.

Al is present in a flat steel product as before in contents of 0.02 - 0.055 wt.%. Al is used in steel production to calm the steel melt and must therefore be within the limits of the invention However, an Al content above the upper limit of the Al content provided for in the invention impairs the desired formability. The positive influence of Al in the alloy of a flat steel product such as the one above can be optimally utilized by limiting the Al content to a maximum of 0.03 wt.%.

Ti is present in a flat steel product like the one before in amounts of 0.01 - 0.1 wt.%. Ti serves to bind interstitial alloying elements and thus contributes to precipitation strengthening. With a Ti content of less than 0.01 wt.%, interstitial alloying elements are still dissolved in the crystal lattice, which has a negative effect on the desired formability. Ti contents above 0.1 wt.% do not further improve the formability. The positive effects of the presence of Ti can be used with a high degree of certainty when the Ti content is 0.05 - 0.09 wt.%.

In addition to the alloying elements mentioned above which are always present in a flat steel product which can be produced by the process according to the invention, a flat steel product which can be produced by the process according to the invention can optionally additionally contain the following alloying elements in order to achieve or adjust certain properties:
Cr can be added to a flat steel product that can be produced by the process according to the invention in amounts of 0.001 - 0.05 wt.%, so that the presence of Cr at such low amounts has a positive effect on the mechanical properties of the flat steel product that can be produced by the process according to the invention, in particular its yield strength and tensile strength. However, a Cr content above the range provided for in the invention impairs the desired formability.

In the same way, V can optionally be added to the steel melt to also contribute to the binding of interstitial alloying elements and thus to precipitation strengthening. For this purpose, V can be present in the flat steel product as before in contents of up to 0.005 wt.%.

Mo can optionally be present in the flat steel product as before in amounts of up to 0.015 wt.% to serve for solid solution strengthening. However, a Mo content above the limit according to the invention impairs the desired formability.

Basically, N contents in the flat steel product, as before, are to be classified as technically unavoidable impurities. However, in contents of 0.001 - 0.004 wt.%, N can also serve as precipitation strengthening through the formation of TiN. If the N content is greater than 0.004 wt.%, there is a risk that nitrogen will be dissolved in the crystal lattice and cause a pronounced yield point, which results in poor deep-drawing formability. Therefore, the optional N content is optimally limited to a maximum of 0.003 wt.% in order to ensure the desired forming properties.

In addition to the alloying elements mentioned above and iron as the main component of a steel according to the invention, technically unavoidable impurities can be present in the flat steel product as before. These include B, Cu, Nb, Ni, Sb, Sn and S, the total amount of which is no more than 0.2% by weight, whereby in the case of the presence of Nb, B or Sb the following special requirements apply to these impurities: Sb content no more than 0.001% by weight, Nb content no more than 0.002% by weight and B content no more than 0.0005% by weight.

Flat steel products can, for example, be manufactured reliably using the manufacturing method according to the invention.

The method according to the invention is described in patent claim 1.

In step b) of the method according to the invention, the respective sub-steps intended for the heat treatment of the flat steel product are carried out in a continuous furnace. The heat treatment process takes place as a continuous annealing process because in this way the individual sub-steps of the heat treatment fit together homogeneously. The uninterrupted process results in a significantly lower scatter in the mechanical properties of the flat steel product over its length and width.

In the continuous furnace intended for continuous heat treatment in practice, individual sections can be heated directly in a manner known per se, for example in the manner of a DFF (Direct Fired Furnace), a DFI (Direct Flame Impingement) or an NOF (Non Oxidizing Furnace) furnace, or indirectly, for example in the manner of an RTF (Radiant Tube Furnace).

The cooling of the flat steel product to the overaging start temperature T2 and the final cooling of the flat steel product to room temperature can be carried out in a conventional manner by blowing gas, e.g. N2, H2 or a mixture thereof, by applying water, mist or by cooling by contact with cooling rolls, whereby each of these measures can also be carried out in combination with one or more of the other cooling measures.

A holding temperature T1 is provided for the recrystallizing annealing, which lies in the temperature range of 750 - 860 °C. At annealing temperatures below 750 °C, complete recrystallization of the structure of the flat steel product can no longer be reliably achieved. At temperatures above 860 °C, however, there is a risk of coarse grain formation. Both would have a negative effect on the forming properties. Optimum results from recrystallizing annealing are obtained when the temperature T1 is 800 - 850 °C.

The duration t1 for which the steel flat product is held at the holding temperature T1 during recrystallization annealing is 30 - 90 seconds in order to ensure optimum forming properties of the steel flat product produced according to the invention. If t1 were less than 30 seconds, complete recrystallization of the structure could no longer be achieved in an operationally reliable manner. If the holding time t1 is longer than 90 seconds, there would again be a risk of coarse grain formation.

After holding at the holding temperature T1, the flat steel product is cooled to the overaging start temperature T2 at a cooling rate CR1 of 2 -100 °C/s. The cooling rate CR1 is selected so that a flat steel product with optimal forming properties is obtained. A minimum cooling rate CR1 of 2 °C/s is required to avoid coarse grain formation. If, on the other hand, the cooling rate CR1 is above 100 °C/s, the grain would be too fine, which would also be contrary to the desired good formability.

The overaging start temperature T2 is at least 400 °C, because at temperatures below this the cooling capacity required for cooling to the overaging start temperature T2 would be high, but the material properties would no longer be positively influenced. If the overaging start temperature T2 were above 600 °C, however, the recrystallization would not be stopped sustainably enough and there would be a risk of coarse grain formation. With an overaging start temperature T2 of 400 - 600 °C, in particular 400 - 550 °C, optimized forming properties can be achieved.

Starting from the overaging start temperature, the flat steel product is subjected to an overaging treatment for a period t2 of 30 - 400 seconds, during which it is cooled to the overaging end temperature T3 at a cooling rate CR2 of 0.5 - 12 °C/s. If the time t2 were less than 30 seconds, the time in which the interstitial alloy atoms could distribute themselves evenly by diffusion in the recrystallized structure of the flat steel product would be too short. This would have a negative effect on the forming properties. An overaging treatment lasting longer than 400 seconds would not have any additional positive effect. A cooling rate CR2 of at least 0.5 °C/s is set in order to complete the overaging treatment within a practical time. If, on the other hand, a cooling rate CR2 above 12 °C/s were set, the duration t2 of the overaging treatment would be too short. There would then be too little time for the diffusion of the interstitial alloying elements, which in turn would impair the forming properties.

According to the invention, the final temperature T3 of the overaging treatment is 250 - 350 °C. If the final overaging temperature T3 were above 350 °C, the flat steel product would be too hot when it enters the final cooling stage, which would have a negative effect on the surface quality and thus the painting properties of the flat steel product as before. On the other hand, an final overaging temperature T3 below 250 °C would have no additional positive effect.

The partial work steps of work step b) are carried out in a protective gas annealing atmosphere that has a hydrogen content of 1 - 7 vol.% and also consists of nitrogen and technically unavoidable impurities. With an H2 content of less than 1.0 vol.% there is a risk of oxide formation on the surface of the flat steel product, which would impair its surface quality and thus its painting properties. An H2 content of the annealing atmosphere above 7.0 vol.%, on the other hand, would have no additional positive effect and would also be problematic from the point of view of operational safety.

According to the invention, the dew point of the annealing atmosphere is between -10 °C and -60 °C. If the dew point of the annealing atmosphere were above -10 °C, there would also be a risk of oxide formation on the surface of the flat steel product, which is undesirable with regard to the desired surface. A dew point below -60 °C would only be possible on a large scale with great effort and would not have any additional positive effect. Optimum operating conditions are achieved when the dew point of the annealing atmosphere is between -15 °C and -50 °C.

The cooling of the flat steel product, which begins after the end of the overaging treatment, takes place in the protective gas atmosphere already explained. A cooling rate CR3 of 1.5 - 5.0 °C/s is provided. This cooling rate CR3 is selected in such a way that deterioration of the surface quality due to oxide formation, which could occur if the cooling is too slow, is avoided in an economical manner.

The step c) of the method according to the invention is essential for the particularly good suitability of flat steel products for painting with optimized paint gloss. This special suitability results from a Wsa value of at most 0.40 µm, typically at most 0.35 µm, in particular less than 0.30 µm, which represents a minimized waviness of the flat steel product surface.

The above-defined degree of skin passing D for the skin passing rolls (work step c)) provided according to the invention after the heat treatment (work steps b.)) is 0.4 - 0.7%. With a degree of skin passing D of less than 0.4%, the deformation of the flat steel product would be insufficient for optimal forming properties. With such low degrees of skin passing, the values specified according to the invention for the roughness Ra and the peak number RPc could also not be achieved. With a degree of skin passing D of more than 0.7%, however, there would be a risk that too much hardening would be introduced into the steel strip, which in turn would have a negative effect on the forming properties. Furthermore, degrees of skin passing D of more than 0.7% could lead to a roughness Ra that would lie outside the range of roughnesses specified according to the invention with regard to the desired surface properties. In order to achieve the desired surface properties with particularly wide flat steel products, i.e. In order to produce the surface structure specified according to the invention with a high level of operational reliability in flat steel products with a width of typically 1500 mm and more, the skin pass degree D can be set to at least 0.5%. If any negative effect of skin pass rolling is to be avoided, the skin pass degree D can be limited to a maximum of 0.6%. The latter is particularly useful if the alloying components of the steel from which a flat steel product is made, such as the one above, are each present in contents that are in the ranges highlighted above as being particularly advantageous.

In order to ensure that the skin-pass rolling process imprints a surface structure on the surface of the flat steel product that corresponds to the specifications according to the invention optimized with regard to the coating properties, the skin-pass work roll acting on the relevant surface of the flat steel product has a roughness Ra of .0 - 2.5 µm and a peak number RPc of at least 100 per cm. If the roughness Ra of the work roll were less than 1.0 µm or greater than 2.5 µm, the values of Ra and RPc according to the invention cannot be applied to the flat steel product within the limits of the invention. Forming and painting properties would deteriorate accordingly. In order to ensure in practice that the roughness values Ra required by the invention are reliably achieved on the flat steel product, the roughness Ra of the skin-pass work roll can be set to 1.2 - 2.3 µm.

The peak number RPc of the skin-pass work roll surface is at least 100 per cm, with higher peak numbers RPc, such as peak numbers RPc of the work roll of at least 110 per cm, in particular more than 130 per cm, being particularly advantageous. By providing high peak numbers RPc of 100 per cm and more on the peripheral surface of the skin-pass work roll that comes into contact with the flat steel product, it is ensured that the required peak number RPc is transferred to the respective skin-pass rolled flat steel product using the skin-pass parameters explained above that correspond to the specifications according to the invention.

In order to ensure that a surface structure with a stochastic distribution of peaks and valleys is formed on the respective surface of the flat steel product, the surface structure of the peripheral surface of the skin-pass work roll that comes into contact with the flat steel product is also formed stochastically.

The surface structure provided according to the invention can be produced, for example, in a manner known per se using the EDT technique ("EDT" = Electro Discharge Texturing) established for the targeted roughening of skin pass rolls in the cap(-) or pulse(+) process. A detailed explanation of these processes can be found in the dissertation by Henning Meier, "On the roughening of roller surfaces with spark discharges", TU Braunschweig 1999, Shaker Verlag 1999 .

The EDT technique is based on roughening the roll surface by spark erosion. For this purpose, the skin-pass work roll is moved past an electrode in a tank containing a dielectric. Sparks are created in the roll surface by sparking. If the electrode is connected as an anode (+) (i.e. the current flows away from the roll towards the electrode), very inhomogeneous craters are created on the roll, which is associated with a higher number of peaks. In the opposite case (i.e. connecting the electrode as a cathode (-)), the current flows towards the roll. The results are smooth craters.

The cap(-) variant of the EDT technique is based on a capacitor discharge that occurs as soon as the electrode is close enough to the roll. The cap process produces a stochastic texture on the work rolls because the capacitor capacitance fluctuates to different degrees (between 30% and 100%) and thus holes of different sizes are shot into the roll material.

The pulse (+) variant of the EDT technique is based on a principle in which the same amount of energy is always applied to the roller to be textured. This creates a stochastic surface texture with greater regularity, which nevertheless offers a sufficiently stochastic distribution of the depressions and peaks for the purposes of the invention.

Following roughening, the work roll according to the invention can optionally undergo a post-treatment. In this process, strongly protruding peaks of the surface structure are ground off in order to reduce contamination of the steel flat product surface by broken-off peaks. The post-treatment can be carried out as a SuperFinish treatment. This is a fine machining process with the aim of removing peaks that protrude above the average roughness depth or reducing their number to a minimum. Possibilities for the practical implementation of the SuperFinish process are, for example, from the DE 10 2004 013 031 A1 or the EP 2 006 037 B1 known. The number of peaks changes negligibly as a result of the respective post-treatment. However, the surface is made more uniform and the contact area is increased. This is reflected in a negative Rsk value (= skewness of the roughness distribution). With high Rsk values, the roughness is therefore unevenly distributed, whereas low or negative Rsk values are associated with a very even roughness distribution.

Finally, the skin-pass work rolls can be hard-chrome plated in the usual way before use in order to optimize their wear resistance.

From an operational point of view, it is advantageous to complete work steps b) and c) of the method according to the invention in a continuous run without interruption. For this purpose, the heat treatment device (work step b)) and the skin-pass rolling stand required for work step c) are set up in a line. The steel flat product cooled after work step b) and emerging from the heat treatment device according to the skin-pass rolling according to work step c) is then processed in a single skin-pass pass. If, however, the skin-pass rolling is to be carried out offline, i.e. independently of the heat treatment process, several skin-pass rolling passes can also be carried out, whereby it can also be seen here that optimal results are achieved when the offline skin-pass rolling is carried out in just one pass.

The optional use of a skin-pass medium (wet skin-pass rolling) can have advantages in terms of a cleaning or lubricating effect during skin-pass rolling. Dry skin-pass rolling, on the other hand, can have the advantage that the flat steel product does not come into contact with any wetting medium and, as a result, the risk of corrosion formation during subsequent storage or further processing of the flat steel product is minimized.

By using the method according to the invention, it is possible to produce a flat steel product with the above-mentioned mechanical material properties according to the invention, which at the same time has the surface structure according to the invention over the entire bandwidth (fully dressed). The surface texturing according to the invention, which is characterized by roughness values Ra and peak numbers RPc corresponding to the specifications according to the invention, allows a significantly better paint gloss to be produced compared to a comparative product with surface texturing not according to the invention.

Fig. 1

einen Ausschnitt einer lackierten Oberfläche eines aus einem wie zuvor Stahlflachprodukt geformten Automobil-Karosseriebauteils;

Fig. 2

einen Ausschnitt einer lackierten Oberfläche eines aus einem nicht erfindungsgemäß hergestellten Stahlflachprodukt geformten Automobil-Karosseriebauteils;

Fig. 3

den schematischen Verlauf einer erfindungsgemäßen Wärmebehandlung (Arbeitsschritt b)).

This will be explained in more detail below using examples.

Fig. 1

a section of a painted surface of an automobile body component formed from a flat steel product as before;

Fig. 2

a section of a painted surface of an automobile body component formed from a flat steel product not manufactured according to the invention;

Fig. 3

the schematic course of a heat treatment according to the invention (working step b)).

Cold-rolled, hardened steel flat products in the form of steel strips B1 - B12 made of steels S1 - S6 were provided, which had the composition given in Table 1.

The steel flat products were heat treated in various dimensions in a continuously operating RTF heat treatment furnace, then cooled to room temperature and subsequently skin pass rolled in-line.

The heat treatment comprises a recrystallization annealing in which the steel strips B1 - B12 were heated to a holding temperature T1 of 835 °C ± 15 °C, at which they were held for a holding time T1 of 60 s.

After recrystallization annealing, steel strips B1 - B12 were subjected to an overaging treatment. For this purpose, they were cooled from the holding temperature T1 at a cooling rate CR1 of 8.5 °C/s to an overaging start temperature T2 of 530 ± 15 °C.

Based on this, the steel strips B1 - B12 were then cooled over an aging period t2 of 302 seconds to an aging end temperature T3 of 280 ± 15 °C. The cooling rate CR2 with which the steel strips B1 - B12 were cooled from the aging start temperature T2 to the aging end temperature T3 was 0.82 °C/s.

During the entire heat treatment, the steel strips B1 - B12 were kept under an annealing atmosphere consisting of 4 vol.% H2 and the remainder N2 and unavoidable impurities. Their dew point was set at -45 °C ± 2 °C.

After the end of the over-aging treatment and before leaving the continuous furnace, the steel strips B1 - B12 were cooled to room temperature under the protective gas atmosphere at a cooling rate CR3 of 3.5 °C/s and were fed in a continuous flow into a four-high rolling stand with backup rolls and skin-pass work rolls intended for skin-pass rolling. The skin-pass work rolls of the skin-pass rolling stand were always roughened in cap(-) mode using EDT technology and subjected to hard chrome plating in a conventional manner. All skin-pass rolling tests were carried out without the use of a skin-pass agent (dry skin-passing).

The parameters of the skin-pass rolling (skin-pass degree D, roughness Ra_W and peak number RPc_W of the circumferential surface of the skin-pass work rolls that comes into contact with the steel strips) as well as the width b, thickness d, yield strength Rp0.2, tensile strength Rm, elongation A80 and the n-value determined for the steel strips B1 - B12 are given in Table 2. The mechanical properties were determined in a quasi-static tensile test according to DIN 6892 with the sample positioned longitudinally to the rolling direction.

The roughness Ra and peak count RPc determined for the surfaces of steel strips B1 - B12 are also listed in Table 2. The arithmetic mean roughnesses Ra, Ra_W and peak count RPc, RPc_W were always measured according to the Steel-Iron Test Sheet (SEP) 1940 using an electrical stylus device according to ISO 3274.

The properties of steel strips B1 and B9 show that better Wsa values can be achieved by increasing peak numbers RPc.

The steel strips B11 and B12, which were not manufactured according to the invention, demonstrate the importance of the degree of skin-passing for the success of the invention.

In addition, the Wsa values were determined for the surfaces of the steel strips B1 - B12. The results are also entered in Table 2. They confirm that the embodiments according to the invention achieve a Wsa value of < 0.40 µm and thus offer optimal conditions for a particularly good paint gloss. The waviness characteristic value Wsa was measured in accordance with the Steel-Iron Test Sheet (SEP) 1941, measured on a steel sample that experienced 5% plastic elongation in the Marciniak cupping test.

Fig. 1 and Fig. 2 illustrate this by comparing components which were produced from a flat steel product according to the invention and a flat steel product not produced according to the invention by forming and painting. The flat steel product not produced according to the invention, in Fig. 2 The embodiment shown, which was produced from the steel strip B3 not fulfilling the requirements of the invention, shows a significantly poorer paint gloss after painting than the one in Fig. 1 example shown, which has been formed from the steel strip B1 produced according to the invention. Table 1 Steel C Si Mn P Al Ti S Cr Nb V Mo N Cu Ni B Sn S1 0.0019 0.005 0.11 0.010 0.029 0.072 0.007 0.032 0.001 0.001 0.004 0.0017 0.014 0.021 0.0002 0.004 S2 0.0015 0.006 0.13 0.010 0.026 0.069 0.009 0.045 0.001 0.001 0.006 0.0026 0.017 0.022 0.0002 0.007 S3 0.0023 0.005 0.09 0.008 0.024 0.075 0.005 0.030 0.001 0.001 0.009 0.0027 0.017 0.027 0.0002 0.004 S4 0.0025 0.006 0.09 0.008 0.024 0.073 0.007 0.024 0.001 0.002 0.004 0.0037 0.010 0.016 0.0002 0.005 S5 0.0020 0.005 0.11 0.010 0.026 0.072 0.007 0.028 0.001 0.003 0.003 0.0025 0.011 0.015 0.0002 0.006 S6 0.0016 0.007 0.11 0.006 0.029 0.073 0.006 - - - - 0.0020 0.011 0.017 0.0002 0.004 Data in wt.%, remainder iron and unavoidable impurities steel band Steel d (mm) b (mm) Rp0.2 (MPa) Rm (MPa) A80 (%) n Ra (µm) RPc (1/cm) D (%) Ra_W (µm) RPc_W (1/cm) Wsa [µm] According to the invention? B1 S1 0.85 1608 136 289 47.6 0.245 0.94 92 0.4 1.4 139 0.27 Yes B2 S1 0.85 1608 139 285 46.5 0.245 1.30 79 0.5 1.4 105 0.37 Yes B3 S2 0.74 1651 142 287 47.8 0.240 1.31 68 0.5 3.0 95 0.44 no B4 S3 0.85 1573 142 294 47.5 0.241 1.35 84 0.6 2.2 115 0.33 Yes B5 S3 0.85 1573 138 290 47.2 0.247 1.29 83 0.5 2.2 115 0.32 Yes B6 S4 0.69 1551 147 302 44.0 0.238 1.16 85 0.6 2.2 114 0.34 Yes B7 S4 0.69 1551 146 303 45.4 0.236 1.18 80 0.6 2.2 110 0.35 Yes B8 S5 0.82 1610 160 297 45.8 0.230 1.27 53 0.7 1.5 90 0.47 no B9 S6 0.85 1573 130 278 48.5 0.246 1.25 93 0.5 1.4 139 0.26 Yes B10 S6 0.85 1573 133 281 47.9 0.245 1.19 87 0.6 2.2 124 0.31 Yes B11 S6 0.85 1573 129*) 275 48.1 0.214 0.71 65 0.3 1.1 110 0.43 no B12 S4 0.69 1551 187 352 38.2 0.238 1.72 70 0.8 2.2 104 0.41 no *) In example B11, the flat steel product showed a pronounced yield strength Reh, the value of which is given here.

Claims (11)

1. Method for producing a cold-rolled and recrystallisation-annealed flat steel product having a ferritic microstructure, which has a yield strength Rp0.2 of up to 180 MPa, a tensile strength Rm of up to 340 MPa, an elongation at break A80 of at least 40%, an n-value of at least 0.23 and, on at least one of its surfaces, an arithmetic mean roughness Ra of 0.8 - 1.6 µm and a relative peak count RPc of at least 75 1/cm, wherein the valleys and peaks formed in the surface due to the mean roughness Ra and the relative peak count RPc are stochastically distributed, comprising the following working steps:

   a) providing a roll-hardened, cold-rolled flat steel product having ferritic microstructure, consisting of a steel having the following composition (in wt%) C: 0.0001- 0.003%, Si: 0.001 - 0.025%, Mn: 0.05 - 0.20%, P: 0.001 - 0.015%, Al: 0.02 - 0.055%, Ti: 0.01 - 0.1%,

   the remainder being iron and unavoidable impurities, wherein the steel may additionally contain the following optional alloy elements: Cr: 0.001 - 0.05%, V: up to 0.005%, Mo: up to 0.015 %, N: 0.001 - 0.004%,

   wherein the unavoidable impurities include B, Cu, Nb, Ni, Sb, Sn and S, which amount to at most 0.2% by weight; and wherein if Nb, B or Sb are present, the following applies for these impurities: Sb content at most 0.001 wt%, Nb content at most 0.002 wt% and B content at most 0.0005 wt%,

   b) heat treating the flat steel product in a continuous run through an annealing furnace under an annealing atmosphere which, at a dew point of -10 °C to -60 °C, consists of 1% - 7% by volume of H2, the remainder being N2 and unavoidable impurities,

   - wherein the flat steel product, for the recrystallisation annealing,

   - is heated up to a hold temperature T1 of 750 - 860 °C,

   - is kept at the hold temperature T1 for a period t1 of 30 - 90 s,

   - wherein the flat steel product, for a subsequent overaging treatment,

   - is cooled from the hold temperature T1 at a cooling rate CR1 of 2 - 100°C/s to an overaging start temperature T2 of 400 - 600°C,

   - after the cooling to the overaging start temperature T2, is cooled over a period t2 of 30 - 400 s at a cooling rate CR2 of 0.5 - 12°C/s to an overaging end temperature T3 of 250 - 350 °C, and

   - wherein the flat steel product, after cooling to the overaging end temperature T3, is cooled at a cooling rate CR3 of 1.5 - 5.0°C/s to room temperature;

   c) temper rolling the recrystallisation-annealed flat steel product with a temper reduction D of 0.4 - 0.7% using a working temper roll having a circumferential area that comes into contact with the flat steel product having an arithmetic mean roughness Ra of 1.0 - 2.5 µm and a peak count RPc of at least 100 1/cm, wherein the depressions and peaks shaped into the surface of the working temper roll that account for the mean roughness Ra and the peak count RPc are in stochastic distribution.
2. Method according to Claim 1, characterised in that the hold temperature T1 is 800 - 850°C.
3. Method according to one of Claims 1 and 2, characterised in that the overaging temperature T2 is 400 - 550°C.
4. Method according to one of Claims 1 to 3, characterised in that the dew point of the annealing atmosphere is -15°C to -50°C.
5. Method according to one of Claims 1 to 4, characterised in that the temper rolling is executed in the form of wet temper rolling in which, upstream of the working temper roll in conveying direction of the flat steel product, a temper rolling fluid is applied at least to the surface of the flat steel product on which the working temper roll acts.
6. Method according to one of Claims 1 to 5, characterised in that the temper reduction D is 0.5 - 0.6%.
7. Method according to any one of Claims 1 to 6, characterised in that the arithmetic mean roughness Ra of the circumferential area of the working temper roll that comes into contact with the flat steel product is 1.2 - 2.3 µm.
8. Method according to any one of Claims 1 to 7, characterised in that the peak count RPc of the circumferential area of the working temper roll that comes in contact with the flat steel product is at least 130 1/cm.
9. Method according to one of Claims 1 to 8, characterised in that operating steps b) and c) are performed in an uninterrupted sequence.
10. Method according to one of Claims 1 to 9, characterised in that the flat steel product, after the temper rolling, is covered with a metallic coating based on Zn.
11. Method according to Claim 9, characterised in that the metallic coating is applied to the flat steel product by electrolytic galvanisation.

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