Cold rolled flat steel product for packaging and method for producing flat steel product

Abstract

To provide a cold rolled flat steel product for packaging made of a low carbon steel having a thickness of less than 0.49 mm and a method of producing the same.SOLUTION: A cold rolled flat steel product has a martensite-free microstructure and has a flatness of 5 I-units or less. The flat steel product is a standard grade for packaging with tensile strengths from 300 MPa to 550 MPa, which can be produced from a cold rolled steel sheet with a carbon content from 0.01 wt.% to 0.1 wt.% by inductive annealing of the steel sheet and subsequent water cooling for quenching the recrystallization-annealed steel sheet. To achieve flatness of 5 I-units or less, the induction annealed steel sheet is first primarily cooled in the manufacturing process to a take-off temperature at a primary cooling rate of less than 1000 K / s, with the take-off temperature being below the transformation temperature of 723°C, and preferably below 690°C, and thereafter the primarily cooled induction annealed steel sheet is secondary cooled by water cooling with a water temperature of less than 80°C at a secondary cooling rate of less than 1000 K / s.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a cold rolled flat steel product for packaging purposes having a thickness of less than 0.49 mm, and to a method for producing the same. [Background technology]

[0002] In the production of flat steel products, such as cold rolled steel sheets and strips for packaging, the flat steel products are annealed above the recrystallization temperature after (first) cold rolling in order to restore the original structure and formability of the steel. In particular for flat steel products for packaging production, such as beverage or food cans, good formability is an important material parameter, since the flat steel products undergo severe deformations during production by deep drawing and stretch forming processes. In order to withstand these deformations during packaging production, the flat steel products also require high strength.

[0003] In order to reduce weight and conserve resources, thinner flat steel products are increasingly being used to manufacture packaging materials. For very thin flat steel products with a thickness of less than 0.2 mm, it is necessary to further increase the strength of the steel used to ensure sufficient stability of the packaging material using them. For this reason, concepts have been developed in the prior art for manufacturing higher strength flat steel products with tensile strengths of more than 550 MPa. For example, Patent Document 1 discloses a method for manufacturing steel sheets for use as packaging steel, which allows the manufacture of cold-rolled steel sheets with tensile strengths of more than 500 MPa with elongations at break of more than 5%. The high tensile strength is achieved by forming a multiphase microstructure containing a hard martensite phase. In order to form a multiphase microstructure, the recrystallization annealing of cold-rolled flat steel products involves first rapid heating by magnetic induction to an annealing temperature above the Ac1 temperature, followed by rapid cooling by water cooling, thereby converting the austenite phase formed in the steel during induction heating to the annealing temperature into martensite.

[0004] The induction heating of flat steel products during recrystallization annealing, as known from patent document 1, is also advantageous from the point of view of reducing climate-damaging waste gases. In conventional heating methods, flat steel products are heated and annealed by heat conduction or heat radiation in bell-type or continuous annealing furnaces. This method generally involves operating the annealing furnaces using fossil fuels, which generates climate-damaging emissions, in particular carbon dioxide and nitrogen oxides. However, induction furnaces are supplied with electrical energy, so that they can also be operated climate-friendly using electricity generated from renewable energies.

[0005] However, the installation of induction furnaces entails high investment costs. High-strength flat steel products with tensile strengths of over 550 MPa are only niche products in the field of packaging steel, and their share of the overall market for packaging steel products is small. Therefore, for economic reasons, it is necessary to be able to manufacture not only high-strength flat steel products, but also those in the so-called standard grade range, with tensile strengths in the range of 300 MPa to 550 MPa, by annealing equipment equipped with induction furnaces and water cooling in the post-process for quenching recrystallized annealed steel sheets.

[0006] However, for such low strength standard grade flat steel products, the temperature change between the rapid heating in the induction furnace and the rapid cooling in the water cooler causes problems with the flatness of the flat steel products after annealing. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] DE102011056847A1 Summary of the Invention

[0008] The present invention is therefore based on the object of providing a standard grade flat steel product that can be subjected to a fast, cost-effective and energy-efficient recrystallization annealing in a climate-friendly manner with reduced generation of climate-harmful emissions and that has a flatness sufficient for the production of packaging materials.

[0009] This object is solved by a flat steel product according to claim 1 and by a manufacturing method according to claim 8. Preferred embodiments of the flat steel product and the manufacturing method thereof are disclosed in the respective dependent claims.

[0010] The flat steel packaging product according to the invention is made using steel having the following composition by weight: Carbon (C): 0.01%~0.1% Silicon (Si): Less than 0.03% Manganese (Mn): Less than 0.6% Phosphorus (P): Less than 0.1% Sulfur (S): Less than 0.03% Aluminum (Al): Less than 0.1% Nitrogen (N): Less than 0.07% The balance is iron and unavoidable impurities. The flat steel product for packaging is produced by casting a slab from a steel melt having the above-mentioned composition, producing a hot strip from said slab by hot rolling, cold rolling said hot strip with a reduction ratio of at least 80% to produce a cold rolled steel strip with a thickness of less than 0.49 mm, subjecting said cold rolled steel strip to a recrystallization annealing in a continuous annealing furnace at an annealing temperature above the recrystallization temperature, in which said cold rolled steel strip is induction heated to the annealing temperature and held at the annealing temperature for a given holding time. After the holding time, the annealed steel strip is cooled in a primary cooling step at a primary cooling rate of up to 1000 K / s to a take-off temperature (T q ) and then immediately in a secondary cooling step, the steel strip is cooled to a temperature below 80°C at a secondary cooling rate of more than 1000 K / s. q ) is below the transformation temperature (Ac1) of 723 ° C, preferably below the recrystallization temperature, particularly preferably below 690 ° C, in particular below 600 ° C. The primary cooling rate is preferably below 400 K / s, in particular below 200 K / s. The secondary cooling rate is preferably above 1500 K / s, in particular above 2000 K / s.

[0011] In the manufacturing method according to the invention, the steel strip is inductively heated to the annealing temperature for annealing in a continuous annealing furnace, preferably with a heating rate of more than 100 K / s. Preferably, the heating rate is higher than 300 K / s, in particular between 400 K / s and 600 K / s.

[0012] In the primary cooling, which is carried out at a much lower cooling rate than the secondary cooling, the steel strip is cooled to a temperature below the transformation temperature (Ac1) of 723 ° C, so that martensite does not form in the steel. However, since there is no equilibrium state in the real steel structure, martensite islands may be detected even below the transformation temperature Ac1. In order to avoid martensite phases, the primary cooling is preferably carried out to a starting temperature below 690 ° C, particularly preferably below 650 ° C, in particular in the range of 500 ° C to 600 ° C. In principle, starting temperatures below 500 ° C are also possible, but are not preferred since they require higher cooling capacities during the primary cooling of the annealed steel strip. The flat steel products produced by the production method according to the invention are characterized in that they have a martensite-free microstructure when the starting temperature is below the transformation temperature Ac1, preferably below 690 ° C. The steel microstructure of the flat steel products according to the invention is at least substantially composed only of ferrite, i.e. preferably more than 90%, more preferably more than 95% ferrite. In some cases, in addition to ferrite, components of cementite and / or pearlite and / or retained austenite may be present. Preferably, the steel microstructure of the flat steel product of the invention does not contain martensite, bainite or austenite, these preferred microstructures being ensured in the manufacturing method according to the invention by the cooling conditions, in particular when cooling the cold rolled steel strip after annealing.

[0013] In order to minimize the cooling time, the steel strip is cooled from the annealing temperature to the starting temperature (T q) is preferably at least 25 K / s. In order to enable the primary cooling to be performed completely by gas cooling, the primary cooling rate is preferably limited to a maximum of 1000 K / s, more preferably less than 400 K / s, particularly less than 200 K / s. In order to enable the primary cooling to be performed by nitrogen gas cooling, the primary cooling rate is preferably in the range of 25 K / s to 400 K / s for the convenience of the equipment. With regard to the appropriate selection of a cooling device for primary cooling, the product of the thickness of the steel strip and the primary cooling rate is preferably less than 5 K / s. -1 mm~200Ks -1 mm.

[0014] In comparison with flat steel products of comparable composition, in particular with a low carbon content (low carbon, LC) in the range of 0.01% to 0.1% by weight, and with a multiphase steel structure including a hard martensite phase, the flat steel products according to the invention exhibit lower tensile strength and lower yield strength. In the aged state, the flat steel products according to the invention exhibit, for example, a 0.2% yield strength (0.2% proof stress) (Rp0.2) in the range of 300 MPa to 700 MPa. The breaking elongation (A) in the aged state of the flat steel products according to the invention is preferably in the range of 10% to 35%, more preferably in the range of 20% to 30%. The aged state of the flat steel products is either naturally obtained by storage and / or painting of the flat steel products and subsequent drying, or artificially obtained by heating the flat steel products to a temperature in the range of 200°C to 210°C for 20 minutes. The mechanical parameters of the stress-strain curves of the flat steel products according to the invention are therefore comparable to the values ​​of standard grade steel sheets for packaging production.

[0015] The strength of the flat steel product depends on the mechanical post-treatment after cooling. In the case of a once-rolled (SR) steel sheet, which after cooling is skin-passed (i.e. cold re-rolled) to a rolling reduction of up to 5%, the yield strength of the flat steel product according to the invention is in the range of 300 MPa to 500 MPa. In the case of a twice-rolled (DR) steel sheet, after ageing treatment, the yield strength of the flat steel product according to the invention is in the range of 430 MPa to 700 MPa. In both cases the tensile strength is about 25 MPa higher than the value of the yield strength.

[0016] In order to maintain the tensile strength or yield strength of the flat steel product according to the invention within these preferred ranges, it is preferred to maintain the starting temperature during cooling of the recrystallized annealed steel strip below 600°C, in particular in the most preferred range of 500°C to 600°C.

[0017] Thus, the method according to the invention allows the production of flat steel packaging products with strength and elongation at break within the range of standard grades, due to the fast induction heating and the fast secondary cooling, which experience a large temperature difference in a short time, but which are cooled in two stages, slow primary cooling and fast secondary cooling, after the recrystallization annealing. The method according to the invention therefore allows the production of flat steel packaging products within the range of standard grades in a cost-efficient, environmentally and climate-friendly manner, despite the very fast, energy-efficient and low-emission heat treatment consisting of a combination of induction heating and quenching with water (water quenching). The advantages of induction heating are, in particular, the formation of very short continuous annealing furnaces (ensuring a very fast and efficient process control, and low investment costs), low operating and maintenance costs (compared to conventional annealing furnaces operating on fossil fuels using conventional radiant heating tubes), low-emission operation (due to the fact that induction furnaces can be operated with renewable electricity and no nitrogen oxides are produced), as well as the possibility of fast start-up and stop-up of the strip annealing system without heating or cooling stages and of rapid temperature changes during the annealing process. Finally, since induction furnaces are much more efficient than conventional annealing furnaces using radiant heating tubes, the method according to the invention is very energy efficient.

[0018] The total annealing time that the cold rolled steel strip remains in the continuous annealing furnace during annealing consists of the heating time to heat the steel strip to the annealing temperature and the holding time, which can be made very short by induction heating. Depending on the heating rate, the heating time ranges from 1 to 6 seconds. Depending on the annealing and recrystallization temperatures, annealing times of less than 5 seconds, often less than 1 second, are sufficient to ensure complete recrystallization of the steel structure. During the holding time, the steel strip is kept above the recrystallization temperature. In the method according to the invention, the holding time is preferably set to between 0.5 and 2.0 seconds, more preferably less than 1 second, in particular between 0.60 and 0.95 seconds.

[0019] The annealing temperature is preferably between 690°C and 769°C. This upper limit of 769°C is determined by the Curie temperature and cannot be exceeded during induction heating of the steel. An annealing temperature of 690°C is sufficient to completely recrystallize the steel structure within the preferred annealing time of less than 5 seconds, and therefore represents an advantageous lower limit for the annealing temperature. As the annealing temperature increases, the annealing time required for complete recrystallization decreases. Annealing times of less than 1 second require an annealing temperature of 725°C, so that a particularly preferred range of annealing temperatures is between 725°C and 769°C.

[0020] The primary cooling of the steel strip after full recrystallization in the induction furnace is preferably carried out by gas cooling, in particular by gas jets. The secondary cooling is preferably carried out by water cooling, in particular by water quenching, by immersing the steel strip in a water bath having a water temperature below 95°C, preferably below 80°C. For energy reasons, a maximum water temperature of up to 95°C is advantageous, since at these temperatures the cooling water can be reused with optimal efficiency as a heating medium in other processes. However, water temperatures above 80°C should be avoided, since excessive evaporation of the cooling water when the steel strip is immersed in the water bath and thus long cooling times may occur due to the Leidenfrost phenomenon caused by the vapor layer between the steel strip and the cooling water. Also, low water temperatures below 25°C are preferred in order to optimize the flatness of the steel strip. The preferred water temperature range is therefore between 10°C and 80°C, more preferably between 15°C and 50°C.

[0021] Moreover, surprisingly, the flat steel product according to the invention also exhibits a very good flatness. The flatness of the flat steel product according to the invention can be quantified by the measurement method specified in the ASTM standard A1030 / A1030M-11 "Standard Practice for Measuring Flatness Characteristics of Steel Sheet Products". Assuming that the irregularities of the flat steel product (also known as steel plate, steel strip) essentially follow a sinusoidal curve, the flatness of the flat steel product can be expressed in "I-units" by calculation according to said standard. Generally, rolling mills for cold rolling or skin-passing flat steel products are equipped with measuring devices which directly record the flatness of the cold rolled or skin-passed flat steel product in I-units (even in the case of flatness defects of different shapes). The flat steel product according to the invention has a flatness of less than or equal to 5 I-units.

[0022] The good flatness of the flat steel product according to the invention is also due to the two-stage cooling after the recrystallization annealing. Due to the primary cooling with a primary cooling rate that is significantly lower than the secondary cooling rate, preferably less than 400 K / s, the recrystallization annealed steel strip is pre-cooled and then finally cooled in the secondary cooling with a very high secondary cooling rate, preferably more than 1000 K / s, to a temperature below 80° C. As a result, the stresses on the steel structure are reduced, especially immediately after the steel strip leaves the continuous annealing furnace and during the primary cooling. Due to the lower stresses, less irregularities are formed. Since the steel strip is cooled to a temperature below the recrystallization temperature, in particular below 600° C. during the primary cooling, the steel structure is already formed after the primary cooling. Therefore, even if the secondary cooling is performed with a very high secondary cooling rate, no significant stresses are generated in the steel strip and almost no irregularities are generated.

[0023] The flatness of the flat steel product according to the invention may also be characterised by an edge corrugation having a maximum wave height (h) of 1.5 mm, preferably a maximum wave height (h) of less than 1.0 mm. Another characteristic of good flatness can be attributed to the number of waves along a given length of the steel strip. In a flat steel product according to the invention, the number of waves having a wave height (h) of more than 1.0 mm per metre length of the steel strip in the rolling direction is preferably less than 6.

[0024] An important parameter of the method according to the invention, which influences the flatness of the flat steel product, is the starting temperature (T q ) at this starting temperature (T q The critical starting temperature (T) of about 715°C is determined by the annealing temperature (during the primary cooling, the steel strip is cooled from the annealing temperature) and the cooling rate and cooling time of the primary cooling. q), the flat steel product according to the invention shows a good flatness in the range of not more than 5 I-units. Above this limiting starting temperature, a linear increase in flatness defects expressed in I-units is observed at the steel strip ends. Therefore, the starting temperature is preferably below 715°C. Furthermore, at starting temperatures above 700°C, the formation of a multiphase microstructure may be observed, comprising martensite and / or retained austenite in a ferritic matrix. Since such multiphase structures should be excluded in the flat steel product according to the invention, and since flat steel products with a multiphase structure show a high tempering sensitivity with a decrease in tensile strength after ageing of up to 50 MPa, the starting temperature is preferably below 690°C, in particular between 500°C and 650°C, most preferably between 550°C and 600°C.

[0025] Water quenching, especially water bath quenching, can cause corrosion resistance problems on the surface of the steel strip due to the (iron) oxide layer. To avoid corrosion problems, the oxide coverage on the surface of the flat steel product should be a maximum of 300 C / m 2 , preferably 100 C / m 2 It is desirable that the quenching water used for the water quenching during the secondary cooling is adjusted in the method according to the invention in order to achieve this. In particular, for this purpose, the dissolved oxygen content of the quenching water is reduced. In particular, in order to achieve this, the quenching water can be depressurized, for example by injecting an inert gas (such as N2 and / or HNx).

[0026] Furthermore, in order to minimize the oxide layer on the surface of the steel strip, the oxygen content of the atmosphere in the continuous annealing furnace can be minimized and the quenching water temperature can be set to 20°C or less. If the quenching water is adjusted by reducing the dissolved oxygen concentration, a low quenching water temperature of 20°C or less can be set. By adjusting the quenching water, for example with inert gas (N2, HNx), the dissolved oxygen concentration in the quenching medium can be selectively adjusted almost independently of the temperature. This means that a predetermined constant dissolved oxygen concentration can be set at various water temperatures. In fact, the higher the water temperature, the thicker the vapor film between the steel strip surface and the cooling water, and therefore the longer it remains stable. The longer the vapor film remains stable, the longer the reaction time of the three-phase reaction (water, steam, steel) and therefore the higher the oxide loading that can be measured on the steel strip surface, since the oxidation of the steel strip surface occurs in the evaporation zone of the stable vapor film where the three-phase reaction takes place. Oxygen dissolved in the cooling water passes through the steam film due to the (local) temperature rise caused by the steel strip during secondary cooling, and is used to oxidize the steel strip surface, increasing the amount of oxide carried. Therefore, in order to prevent the amount of oxide carried during secondary cooling from becoming high, it is preferable that the temperature of the cooling water is 20°C or less, and particularly a low temperature in the range of 15°C to 20°C. As a result, the temperature of the cooling water is 300 C / m on the steel strip surface. 2 An oxide coverage of less than 100 nm can be achieved.

[0027] In the method according to the present invention, the recrystallized annealed steel strip can be skin-passed after cooling with a skin-pass degree of 0.2% to 5% or can be secondarily cold-rolled (re-rolled) with a re-rolling reduction ratio of more than 5% to 45%. In skin-pass rolling (single roll, SR), the yield strength is in the range of 300 MPa to 500 MPa, and in secondary re-rolling (double roll, DR), the yield strength is in the range of 430 MPa to 700 MPa. In skin-pass or re-rolling, the flatness is further improved, especially across the width of the steel strip.

[0028] After the second cold rolling (re-rolling) or skin pass rolling, the flat steel product according to the invention can also be coated with a corrosion resistant layer, such as a tin layer applied by electrolytic tin plating (for tinplate production) and / or a chromium / chromium oxide coating applied by electrolytic chromium plating (for the production of electrolytic chromium coated steel: ECCS). Additionally or instead, chromium-free passivation layers and organic coatings, such as polymer layers (especially in the form of polymer coatings or films), can also be applied to the surface.

[0029] These and other features and advantages of the flat steel product and its manufacturing method according to the invention are disclosed in the following embodiments and examples with reference to the drawings. [Brief description of the drawings]

[0030] [Figure 1] FIG. 1 is a typical curve of a stress-strain diagram obtained in a tensile test of an aged flat steel product according to the invention, where ε is strain, σ is stress, ReL is lower yield strength, ReH is upper yield strength and Rm is tensile strength. [Diagram 2] 2A and 2B are graphs showing the strength and breaking elongation of flat steel products as a function of the starting temperature (Tq) for the method according to the invention, where FIG. 2a shows the influence of the starting temperature (Tq) on the strength (Rp2, ReL, ReH and Rm) and FIG. 2b shows the influence of the starting temperature (Tq) on the breaking elongation A. [Diagram 3] FIG. 3 is a graph showing the tensile strength and yield strength of flat steel products as a function of the degree of re-rolling during secondary cold rolling and skin pass rolling (skin pass), respectively. FIG. 3a shows the effect of the re-rolling reduction ratio (NWG) on tensile strength (Rm), and FIG. 3b shows the effect of the re-rolling reduction ratio (NWG) on upper yield strength (ReH). [Figure 4] 1 is a graph illustrating a temperature-time diagram of a typical annealing cycle performed in a method according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The production of flat steel products according to the invention is carried out by casting slabs from a steel melt with a low carbon content (low carbon steel, LC) and hot rolling them to obtain hot strips. In the following, the composition of the steels from which the flat steel products according to the invention can be produced is explained in detail.

[0032] Steel Composition

[0033] Carbon (C): 0.01% or more, 0.1% or less, preferably less than 0.085% Carbon increases hardness and strength. The object of the present invention is to produce cold-rolled flat steel products for packaging with moderate strength, for example with a yield strength (ReL) in the range of 300 MPa to 700 MPa. Therefore, in order to achieve the desired strength even with a low rerolling or skin pass degree of less than 5%, the steel contains at least 0.01 wt.% carbon. In order to ensure the rollability of the flat steel product during the first cold rolling and, if necessary, during the second cold rolling (rerolling or skin pass) step without reducing the breaking elongation, the carbon content must not be too high. Furthermore, as the carbon content increases, a noticeable anisotropy occurs during the manufacture and processing of the flat steel product in the form of latency, since carbon is mainly present in the form of cementite due to its low solubility in the ferrite lattice of the steel. Also, as the carbon content increases, the surface quality deteriorates and, as the peritectic point is approached, the risk of slab cracking increases. Therefore, during the heat treatment of the cold-rolled flat steel product, flatness defects may occur. Therefore, in order to avoid the occurrence of slab cracks and the associated flatness defects, it is preferable to limit the carbon content to a maximum of 0.1% by weight. In order to achieve good flatness and isotropic material properties of the cold-rolled steel sheet, a carbon content of less than 0.085% by weight is particularly preferred.

[0034] Manganese (Mn): 0.6% or less, preferably more than 0.17% Manganese also increases hardness and strength. Moreover, manganese improves the weldability and wear resistance of the steel. Moreover, the addition of manganese can reduce the tendency to red brittleness during hot rolling by binding sulfur to the less harmful MnS. Moreover, manganese brings about grain refinement and it can increase the nitrogen solubility in the iron lattice and prevent carbon diffusion to the slab surface. Therefore, the manganese content is preferably 0.17% by weight or more. To obtain the desired strength, the manganese content is preferably more than 0.2% by weight, in particular 0.30% by weight or more. However, if the manganese content is too high, the corrosion resistance of the steel is sacrificed and food suitability is no longer guaranteed. Moreover, if the manganese content is too high, it means that the strength of the hot strip is too high and it is no longer possible to cold roll the hot strip. Therefore, the upper limit of the manganese content is 0.6% by weight.

[0035] Phosphorus (P): Less than 0.1% Phosphorus is an undesirable by-product in steel, and high phosphorus contents, among other things, lead to embrittlement of the steel and poor formability of flat steel products, so the upper limit for phosphorus content is 0.1% by weight.

[0036] Sulfur (S): less than 0.03%, preferably more than 0.001% Sulfur is an undesirable incidental element that deteriorates the ductility and corrosion resistance of steel. Therefore, the sulfur content in steel should be 0.03% by weight or less. On the other hand, since complex and costly measures are required to desulfurize steel, it is not reasonable from an economical point of view to set the sulfur content to less than 0.001% by weight. Therefore, the sulfur content is within the range of 0.001% to 0.03% by weight, and particularly preferably within the range of 0.005% to 0.01% by weight.

[0037] Aluminum (Al): less than 0.1%, preferably more than 0.002% Aluminum is required in steel manufacturing as a deoxidizer when hardening steel. It also improves scale resistance and formability. Therefore, the aluminum content is preferably more than 0.002% by weight. However, aluminum has the disadvantage that it forms aluminum nitride with nitrogen, reducing the amount of free nitrogen, thereby reducing the strength that can be achieved by solid solution strengthening with nitrogen. Furthermore, if the aluminum concentration is too high, surface defects will occur in the form of aluminum clusters, which may also cause flatness defects. Therefore, aluminum is used at a concentration of less than 0.1% by weight.

[0038] Silicon (Si): Less than 0.03% Silicon is a solution hardener that improves the scale resistance of steel. In steel production, silicon plays the role of a deoxidizer. Silicon also has the positive effect of improving the tensile strength and yield strength of steel. Therefore, in order to obtain the desired strength, the silicon content is preferably 0.003 wt.% or more. However, if the silicon content is too high, especially above 0.03 wt.%, the corrosion resistance of the steel may decrease, making surface treatment, especially by electrolytic coating, difficult.

[0039] Nitrogen (N): less than 0.07%, preferably more than 0.001% In alloy steels, nitrogen acts as a solid solution strengthener to increase hardness and strength, and also has a positive effect on formability, which is expressed, for example, by a high value of the Erichsen depth (Erichsen index). Therefore, in order to obtain the desired strength and formability, it is preferable that the nitrogen content is more than 0.001 wt%. However, if the nitrogen concentration in the molten steel is too high, the cold rollability of the hot strip produced from the molten steel decreases. This is particularly noticeable when the nitrogen content exceeds 0.016 wt%. Furthermore, if the nitrogen concentration in the molten steel is high, the risk of defects occurring in the hot rolled strip increases, since the hot workability decreases at very high nitrogen concentrations. If there are defects in the hot rolled strip, the flatness of the cold rolled steel sheet is impaired. For this reason, the upper limit of the nitrogen content to achieve good flatness is 0.07 wt%, preferably not more than 0.016 wt%.

[0040] Optional: nitride formers, in particular niobium, titanium, boron, molybdenum, chromium Nitride-forming elements such as aluminum, titanium, niobium, boron, molybdenum and chromium are disadvantageous for the steel of the flat steel product according to the invention, because they reduce the proportion of free nitrogen by the formation of nitrides. Moreover, these elements are expensive, which increases the production costs. On the other hand, elements such as niobium, titanium and boron have the effect of increasing the strength by grain refinement as micro-alloying components, without reducing the toughness. The aforementioned nitride-forming agents can therefore be advantageously added within certain limits as alloying components of the steel melt. The steel can therefore (optionally) contain the following alloying components by weight: Titanium (Ti): Less than 0.01% Boron (B): Less than 0.005% Niobium (Nb): Less than 0.01% Chromium (Cr): preferably more than 0.01% to allow the use of scrap in the production of molten steel and to prevent the diffusion of carbon at the slab surface, and less than 0.1% to avoid carbides and nitrides. Molybdenum (Mo): Less than 0.02% to prevent excessive increase in recrystallization temperature.

[0041] Other optional ingredients

[0042] Besides residual iron (Fe) and unavoidable impurities, molten steel may contain optional components such as: · Optional Copper (Cu): More than 0.002% for convenience, to allow scrap to be used in molten steel production, but less than 0.1% to ensure food suitability. Optional Nickel (Ni): Expediently more than 0.01% to allow scrap use in molten steel production and to improve toughness, but less than 0.1% to ensure food suitability. · Optional Tin (Sn): preferably less than 0.03%.

[0043] Manufacturing method of flat steel products

[0044] With the above mentioned steel composition, a steel melt for the manufacture of flat steel products according to the invention is produced which is first continuously cast and, after cooling, cut into slabs which are then reheated to a preheat temperature above 1100°C, in particular 1200°C, and hot rolled to produce hot rolled strips with a thickness in the range of 1-4 mm.

[0045] The final rolling temperature during hot rolling is preferably higher than the Ar3 temperature, particularly preferably 800°C to 920°C, in order to maintain austenite.

[0046] The hot-rolled strip is coiled at a predetermined and suitable constant coiling temperature (reel temperature, HT), which is preferably below Ar1 in order to maintain the ferritic range and is preferably in the range of 500°C to 750°C, particularly preferably below 640°C, in order to achieve a homogeneous distribution of carbides and the precipitation of cementite as fine as possible.

[0047] To produce packaging steel in the form of thin flat steel products (very thin plates) in the range of thicknesses up to 0.49 mm, the hot rolled strip is cold rolled with a thickness reduction (rolling degree or cold rolling degree) of at least 80%, preferably in the range of 85% to 98%. The width of the steel strip is preferably up to 1200 mm, particularly preferably in the range of 700 mm to 1100 mm. In order to restore the crystal structure of the steel destroyed during cold rolling, the cold rolled steel strip is then recrystallization annealed in an annealing furnace at an annealing temperature above the recrystallization temperature. This is done, for example, by passing the flat steel product in the form of cold rolled steel strip through a continuous annealing furnace in which the steel strip is induction heated to the annealing temperature and held at the annealing temperature for a given holding time. When the cold rolled steel strip is annealed in a continuous annealing furnace, the heating rate for induction heating to the annealing temperature is more than 100 K / s, preferably more than 300 K / s, in particular 400 K / s to 600 K / s.

[0048] The annealing temperature is preferably 725° C. to 769° C., and the holding time is less than 1 second, particularly 0.3 to 0.9 seconds, more preferably less than 0.7 seconds, particularly 0.50 to 0.60 seconds.

[0049] After the holding time has elapsed, the recrystallized annealed steel strip is cooled in the primary cooling step to a starting temperature (T q ), preferably below 715° C., in particular below 600° C. q ). Preferably, the primary cooling rate is less than 400 K / s, for example in the range of 100 K / s to 400 K / s. Immediately after the primary cooling, a secondary cooling is performed to a temperature of the steel strip below 80° C., with a secondary cooling rate of more than 1000 K / s. Preferably, the secondary cooling rate is more than 1500 K / s, for example in the range of 1600 K / s to 3000 K / s. Preferably, the primary cooling is performed by gas cooling, in particular by a gas jet, and preferably the secondary cooling is performed by water cooling, in particular by immersing the steel strip in a water bath with a water temperature of up to 80° C.

[0050] A typical annealing cycle for annealing and cooling a steel strip is shown in Figure 4, where Tm is the (maximum) annealing temperature to which the steel strip is heated in the annealing furnace, T q is the starting temperature for the secondary cooling, T f indicates the water temperature of the water cooling device for performing the secondary cooling. The steel strip is induction heated to the annealing temperature within a short heating time t1, preferably less than 2 seconds, for example between 1.0 and 2.0 seconds. The steel strip is induction heated to the annealing temperature within a holding time t H = t2-t1, and then held at the annealing temperature until t2, after which it is cooled. The cooling is carried out in two stages, i.e. the steel strip is cooled at a slow rate to the starting temperature T q A primary cooling step (up to time t3) by gas cooling, in which the steel strip is cooled to a water temperature T f and a secondary cooling process in which the melt is cooled to room temperature.

[0051] Hereinafter, examples of the embodiment of the present invention and comparative examples will be described.

[0052] Example of an embodiment of the present invention

[0053] Steel melts of the alloy compositions listed in Table 1 were hot rolled and then (primary) cold rolled to produce flat steel products (steel plates).

[0054] The cold rolled flat steel product was then subjected to recrystallization annealing in a continuous annealing furnace by induction heating at a high heating rate to an annealing temperature above the recrystallization temperature and holding at the annealing temperature for a holding time. At the end of the holding time, the steel strip was cooled in two stages: by gas cooling at a slow cooling rate to the starting temperature T q After primary cooling to a specified water temperature T f The steel strip was then subjected to a rapid quenching in a water bath at 0.5 °C and a secondary cooling. After cooling, the steel strip was cold re-rolled with a re-rolling reduction of 5% to 45% or skin-passed with a skin-pass reduction of 0.2% to a maximum of 5%. The process parameters during annealing and cooling and the subsequent secondary cold rolling or skin-pass are given in Table 2 and the resulting material parameters are given in Table 3, where: T m is the annealing temperature (unit: °C), tH is the residence time (unit: seconds), dT / dt_K1 is the cooling rate during the first cooling period (unit: K / s), T q is the starting temperature at the end of the first cooling period (unit: °C), T f is the water temperature during secondary cooling (unit: °C), D is the thickness of the steel strip after rerolling or skin pass (unit: mm), NWG is the degree of re-rolling during the second cold rolling (unit: %) or the degree of skin-pass during skin-pass (unit: %). Rp0.2 or Rp2 is the 0.2% yield strength (in the rolling direction) (unit: MPa), Rp0.5 or Rp5 is the 0.5% yield strength (in the rolling direction) (unit: MPa), ReL is the lower yield strength (in the rolling direction) (unit: MPa), ReH is the upper yield strength (in the rolling direction) (unit: MPa), A is the elongation at break (in the rolling direction) (unit: %), Rm is the tensile strength (in the rolling direction) (unit: MPa), · P is the flatness measured according to the standard defined in "ASTM A1030 / A1030M-11" (unit: I-unit).

[0055] Comparative Example

[0056] For comparison, steel sheets with alloy compositions according to Table 1 were induction heated in a continuous annealing furnace to an annealing temperature above the recrystallization temperature at a heating rate of more than 100 K / s, held at the annealing temperature for a holding time to perform recrystallization annealing, and then cooled in a single step by water cooling at a cooling rate of more than 1000 K / s. The comparative flat steel products were then subjected to a single step cooling process at a cooling rate of more than 1000 K / s. After cooling, the steel strips were cold rerolled with a rerolling reduction of 5% to 45% or skin-passed with a skin-pass reduction of 0.2% to a maximum of 5%. The process parameters for annealing and cooling and the subsequent secondary cold rolling or skin-pass for these comparative examples are given in Table 2, and the resulting material parameters are given in Table 3.

[0057] The test specimens of the examples and comparative examples according to the invention were artificially aged after skin pass or secondary re-rolling by holding them at an ageing temperature of about 200°C to 210°C for more than 20 minutes. Tensile tests were carried out on the aged specimens to determine the mechanical properties (according to DIN EN102020 and EN ISO 6892). Figure 1 shows a typical stress-strain diagram of the test specimens after ageing.

[0058] For skin-pass specimens with a re-rolling reduction of less than 5%, ReL is specified as the yield strength in the aged state. For re-rolled specimens with a re-rolling reduction of 5% or more, ReL cannot always be determined, so ReH or 0.5% proof stress (Rp0.5) is specified as the yield strength in the aged state. In general, to determine the yield strength, if ReL can be determined, this parameter should be used; otherwise, if the plateau region of the Luders strain range is evident, it is recommended to use Rp0.2% yield strength as the appropriate parameter, and otherwise the maximum ReH value and / or maximum Rm value should be used.

[0059] As can be seen in Table 3, in the comparative examples (Comparative Examples 1, 6-10, 15-17, 20-22), a multiphase microstructure with martensite phase is formed, and therefore these comparative examples cannot be qualified as being within the scope of the present invention. This can be explained by the high starting temperature and the rapid cooling during the secondary cooling. Therefore, the process control during cooling of the recrystallized annealed steel strip should be adjusted to a starting temperature T qIt is preferable to set the temperature to less than 690°C. Due to the multiphase microstructure, these comparative examples have higher yield strength and tensile strength compared to the examples according to the invention, but the yield strength is outside the preferred range of up to 500 MPa (for single pass cold rolled steel strip with skin pass degree less than 5%). Furthermore, Comparative Examples 6-10, 12, 15-17 and Comparative Examples 20-22 show flatness defects represented by high flatness values ​​of more than 5 I-Unit, and therefore these comparative examples are considered to be outside the scope of the invention. In Comparative Example 12, no martensite phase is formed. However, due to the high water temperature during secondary cooling, this sample shows flatness defects represented by a flatness value of 17.3 I-Unit, which is outside the scope of the invention, compared to Examples 2-5 and 11 according to the invention (which are the same No. 6 steel).

[0060] Starting temperature T q In order to clarify the effect of on the strength and breaking elongation, different starting temperatures T q Tensile tests at 2000 K and 2000 K were performed on the aged specimens. The results of these tests are shown graphically in Figure 2, where Figure 2a shows the yield strength (Rp2, ReL, ReH and Rm) versus starting temperature T q Figure 2b shows the effect of the starting temperature T on the breaking elongation A. q From Fig. 2a, the effect of the starting temperature T q As can be seen in Fig. 2b, an increase in the intensity can be observed at the starting temperature T q has almost no effect on the breaking elongation.

[0061] Tests were carried out on specimens according to the invention and comparative specimens with different degrees of re-rolling (NWG) to determine the change in strength upon re-rolling (secondary cold rolling, or skin pass). The results of these tests are shown graphically in FIG. 3. FIG. 3a shows the effect of the degree of re-rolling (NWG) on the tensile strength (Rm) and FIG. 3b shows the effect of the degree of re-rolling (NWG) on the upper yield strength (ReH). From FIG. 3a and FIG. 3b, it can be seen that the strength increase with the degree of re-rolling is smaller for the specimens according to the invention than for the comparative specimens. Thus, the flat steel product according to the invention can be maintained in a medium strength range, with a yield strength of 430 MPa to 700 MPa, even for twice-rolled (DR) specimens with a re-rolling degree of 5% to 45%. This allows flat steel products to be formed by twice-cold rolling with a thickness of 0.21 mm or less while having a medium strength. At a skin pass degree of 5% or less, the flat steel product according to the invention has a yield strength in the range of 300 MPa to 500 MPa.

[0062] [Table 1]

[0063] [Table 2] JPEG2025032252000004.jpg244149

[0064] [Table 3] JPEG2025032252000006.jpg247136

Claims

1. 1. A cold rolled flat steel product for packaging having a thickness of less than 0.49 mm, comprising, by weight: C: 0.01-0.1%, Si: less than 0.03%; Mn: less than 0.6%; P: less than 0.1%, S: less than 0.03%, Al: less than 0.1% N: less than 0.02%; Optionally, Cr: less than 0.1%; Optionally, Ni: less than 0.1%; Optionally, Cu: less than 0.1%; Optionally, Ti: less than 0.01%; Optionally, B: less than 0.005%; Optionally, Nb: less than 0.01%; In some cases, Mo: less than 0.02%; Optionally, Sn: less than 0.03%; The balance is iron and unavoidable impurities, The composition is The flat steel product has a martensite-free microstructure and has a flatness of 5 I-units or less. Flat steel products.

2. 2. The flat steel product according to claim 1, characterized in that the flat steel product has a yield strength of 300 MPa to 700 MPa in an aged state, the aged state of the flat steel product being obtained naturally by storage at room temperature and / or painting followed by drying, or artificially by heating the flat steel product at a temperature in the range of 200°C to 210°C for 20 minutes.

3. 3. A flat steel product according to claim 1 or 2, characterized in that the flat steel product, in its aged state, has an elongation at break (A) of at least 10%, preferably up to 35%, more preferably at least 15% and in particular up to 30%, said aged state of the flat steel product being obtained naturally by storage at room temperature and / or painting and subsequent drying or artificially by heating the flat steel product at a temperature in the range of 200°C to 210°C for 20 minutes.

4. 3. Flat steel product according to claim 1 or 2, characterized in that the steel microstructure of the flat steel product contains neither martensite nor bainite, and also contains no austenite.

5. 5. A flat steel product according to claim 4, characterized in that the steel microstructure of the flat steel product comprises at least substantially only ferrite or comprises ferrite and one or more of the microstructural components cementite and pearlite.

6. 3. A flat steel product according to claim 1 or 2, characterized in that the flat steel product is skin-passed with a skin-pass reduction of 5% or less and has a yield strength of 300 MPa to 500 MPa, or the flat steel product is re-rolled with a re-rolling reduction of more than 5% to 45% or less and has a yield strength of 430 MPa to 700 MPa.

7. 3. A flat steel product according to claim 1 or 2, characterized in that the flat steel product has an edge corrugation with a maximum wave height (h) of 1.5 mm and / or has less than 6 waves with a wave height (h) of more than 1.0 mm over a length of 1 m in the rolling direction of the flat steel product.

8. 2. A method for producing a flat steel product according to claim 1, wherein the flat steel product comprises: hot rolling a slab made from said steel to obtain a hot rolled strip; cold rolling the hot rolled strip at a reduction ratio of at least 80% to obtain a cold rolled steel strip having a thickness of less than 0.49 mm; annealing the cold rolled steel strip in a continuous annealing furnace, in which the cold rolled steel strip is induction heated to an annealing temperature higher than a recrystallization temperature in the continuous annealing furnace and held at the annealing temperature for a predetermined holding time to obtain a recrystallized annealed steel strip; The recrystallized annealed steel strip is cooled to a starting temperature (T ｑ ) a step of primary cooling; secondary cooling of the steel strip at a secondary cooling rate higher than 1000 K / s to a temperature below 80° C.; 2. A method for producing a steel having the composition according to claim 1, comprising the steps of:

9. 9. The method of claim 8, wherein the primary cooling is accomplished by gas cooling and the secondary cooling is accomplished by water cooling.

10. 10. The method according to claim 8 or 9, wherein the steel strip is skin-passed with a skin-pass degree of 0.2% to 5% after cooling, and the yield strength of the skin-passed steel strip is between 300 MPa and 500 MPa.

11. 10. The method according to claim 8 or 9, wherein the steel strip is skin-passed after cooling with a skin-pass reduction of 0.2% to 5% and has a yield strength after skin-pass of 300 MPa to 500 MPa, or wherein the steel strip is cold re-rolled after cooling with a re-rolling reduction of more than 5% to 45% and has a yield strength after re-rolling of 430 MPa to 700 MPa.

12. 10. The method according to claim 8 or 9, wherein the cold rolled steel strip is inductively heated to the annealing temperature during annealing in the continuous annealing furnace with a heating rate of more than 100 K / s.

13. 10. The method according to claim 8 or 9, wherein the cold rolled steel strip is maintained at the annealing temperature during annealing in the continuous annealing furnace with a holding time of less than 1 second.

14. 10. The method according to claim 8 or 9, wherein the annealing temperature is between 725°C and 769°C.

15. The cold rolled steel strip is cooled during primary cooling from the annealing temperature to the starting temperature (T ｑ 10. The method according to claim 8 or 9, wherein the mixture is cooled to a temperature of 300° C.

16. The surface of the flat steel product immediately after secondary cooling has a maximum oxide coverage of 300 C / m ２ 10. The method according to claim 8 or 9, having a surface oxidation of

17. The starting temperature (T ｑ 10. The method according to claim 8 or 9, wherein the heating temperature is up to 600°C.

18. 10. The method according to claim 8 or 9, wherein after the secondary cooling, at least one surface of the flat steel product is provided with a tin and / or a chromium / chromium oxide coating and / or an organic coating.

Images