Flat steel products, methods for manufacturing the same, and uses of such flat steel products

Abstract

The present invention relates to a cold rolled flat steel product, the steel base material of which has a tensile strength of 750-940 MPa, high strength, improved weldability and optimized forming properties, and can be manufactured at low cost at the same time. The cold rolled flat steel product according to the present invention has the following composition in mass percent: C: 0.040-0.100%, Mn: 2.10-2.50%, Si: 0.10-0.40%, Cr: 0.30-0.90%, Ti: 0.020-0.080%, B: 0.0005-0.0020%, N: 0.003-0.010%, Al: max. 0.10%, Ca: max. 0.005%, P: max. 0.025%, S: max 0.010%; possibly one or more of the following elements: Mo: max 0.20%; Nb: max 0.050%; Cu: max 0.10%; V: max 0.020%; Ni: max 0.10%, the balance being iron and unavoidable impurities, the total content of impurities being limited to 0.5% by mass or less, the content of phosphorus ("P") and sulfur ("S") belonging to the impurities. Furthermore, the steel substrate has a two-phase structure consisting of 10-40% by volume of martensite, 30-90% by volume of ferrite, including bainitic ferrite, not more than 5% of retained austenite, the balance being other structural components unavoidable related to the production.

Description

Technical Field

[0001] The present invention relates to cold-rolled flat steel products, a method for manufacturing the same, and use of the flat steel products according to the present invention.

Background Art

[0002] "Flat steel products" are understood here as rolled products, each of whose length and width is substantially larger than its thickness. These include, in particular, steel strips, steel plates, and pre-cut parts obtained therefrom, such as blanks and the like.

[0003] In this specification, unless otherwise specified, information regarding alloying components is always given in mass%.

[0004] When an equation or condition is referred to in the text and the values therein are calculated or found based on the proportions of specific alloying elements, unless otherwise specified, the conceivable proportions of the alloying elements are indicated in mass% in each case in these equations or conditions.

[0005] Especially in the field of automobile body structures, there is a demand for high-strength steel, and at the same time, it must have good formability. In particular, in the manufacture of parts formed into complex shapes, for example, there are high demands regarding local shape change ability and edge crack resistance, which can be quantified by good values in a hole expansion test.

[0006] Cold-rolled flat steel products, known from European Patent No. 2031081B1, can be hot-dip plated with a zinc-based anticorrosion coating and have a structure consisting of 20-70% martensite, up to 8% retained austenite, with the remainder being ferrite and / or bainite. The flat steel products have a tensile strength of at least 950 MPa and are made of steel consisting of C: 0.050-0.105%, Si: 0.10-0.60%, Mn: 2.10-2.80%, Cr: 0.20-0.80%, Ti: 0.02-0.10%, B: <0.0020%, Mo: <0.25%, Al: <0.10%, Cu: up to 0.20%, Ni: up to 0.10%, Ca: up to 0.005%, P: up to 0.2%, S: up to 0.01%, N: up to 0.012% (by weight), with the remainder being iron and unavoidable impurities.

[0007] The concept of this type of steel is characterized by a low elastic limit ratio resulting from significant strength differences between its structural components.

[0008] In practice, while the type of flat steel products described above possess particularly high strength, they have been found to be prone to forming edge cracks that impair their workability. Furthermore, such flat steel products require further improvement in their suitability for welding. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] European Patent No. 2031081B1 [Overview of the project] [Problems that the invention aims to solve]

[0010] Against this backdrop, there is a need to develop flat steel products that have a steel base material with high strength, improved weldability, and optimized forming properties, and that can be manufactured cost-effectively.

[0011] Furthermore, methods for manufacturing and using such flat steel products are also specified, and the flat steel products according to the present invention are particularly suitable for such use. [Means for solving the problem]

[0012] A product proposed to address this need has at least the features described in claim 1.

[0013] A method for enabling the cost-effective manufacture of the product according to the present invention is described in claim 9. Such a product can be provided by the method shown in claim 8, particularly using a zinc ("Zn")-based anticorrosion coating.

[0014] In this case, when implementing the method according to the present invention and the possible variations and extensions thereof as described herein, it should be obvious to those skilled in the art that additional work steps not explicitly mentioned in this example are known, based on their practical experience, to be commonly applied when implementing such a method.

[0015] Therefore, the steel base material for the flat steel product according to the present invention is manufactured from steel consisting of C: 0.040-0.100%, Mn: 2.10-2.50%, Si: 0.10-0.40%, Cr: 0.30-0.90%, Ti: 0.020-0.080%, B: 0.0005-0.0020%, N: 0.003-0.010%, Al: up to 0.10%, Ca: up to 0.005%, P: up to 0.025%, S: up to 0.010%, Mo: up to 0.20%, Nb: up to 0.050%, Cu: up to 0.10%, V: up to 0.020%, Ni: up to 0.10% (mass%), with the remainder being iron and unavoidable impurities.

[0016] In this case, the steel base material of the flat steel product according to the present invention has a two-phase structure consisting of 10 to 40 volume percent martensite, 30 to 90 volume percent ferrite (including bainite ferrite), 5% or less retained austenite, and the remainder being other structural components unavoidable due to the manufacturing process, and such other structural components are present only if the sum of the fractions of the other structural components is less than 100%.

[0017] The fractions of each alloying component provided in accordance with the present invention are determined as follows, and each of the following descriptions also refers to the composition of the steel base material of the flat steel product according to the present invention when only one flat steel product according to the present invention is being discussed.

[0018] The flat steel products according to the present invention contain 0.040 to 0.100 mass% of carbon ("C"). If the C fraction is less than 0.040 mass%, the strength is significantly reduced. The maximum carbon fraction of 0.100 wt% provided according to the present invention was selected in relation to the good weldability of the steel. Furthermore, if the carbon fraction exceeds 0.100 wt%, it leads to the formation of a harder carbon-rich martensite phase, thereby significantly increasing the hardness difference between martensite and ferrite. This adversely affects the hole-expanding behavior and weldability of the flat steel products according to the present invention. The positive effect of the presence of C in the steel of the flat steel products according to the present invention can be particularly fully utilized when the C fraction is between 0.05 mass% and 0.08 mass%.

[0019] Silicon ("Si") is included in the flat steel product according to the present invention at a fraction of 0.10 to 0.40 mass% in order to increase strength through the hardening effect that Si has on ferrite. The upper limit of the Si fraction is 0.40 mass% in order to avoid intergranular oxidation, which can adversely affect the coating properties and surface characteristics of the steel.

[0020] A manganese fraction ("Mn") of 2.10 to 2.50 mass% reliably prevents the formation of pearlite in the flat steel product according to the present invention during cooling of the annealing line. At the same time, the Mn fraction determined according to the present invention mediates the formation of martensite in the structure and thus substantially contributes to an increase in strength. In particular, the Mn fraction provided according to the present invention compensates for the strength loss that would otherwise be expected as a result of the C fraction, which is set to a relatively low value according to the present invention. The Mn fraction is preferably 2.20 mass% or more and 2.40 mass% or less.

[0021] Deoxidation during steel production requires aluminum ("Al") at a fraction of up to 0.10 mass%.

[0022] To deoxidize the steel during steel production, calcium ("Ca") can be similarly added to the steel of the flat steel product according to the present invention at a fraction of up to 0.005% by mass. This effect can be achieved by adding at least 0.0005% by mass of Ca.

[0023] Chromium ("Cr") also helps to increase the strength in the steel of the flat steel product according to the present invention. For this purpose, a Cr fraction of at least 0.30 mass%, and in particular at least 0.40 mass%, is required. To reduce the risk of significant intergranular oxidation, the upper limit of the range specified for the Cr fraction according to the present invention is limited to up to 0.90 mass%, and in particular to up to 0.80 mass%. If the chromium fraction exceeds 0.80 mass%, the method for manufacturing the flat steel product obtained according to the present invention should be carried out so that an annealing temperature GT of at least 840°C is set in order to reliably obtain the desired two-phase structure and desired mechanical properties of the flat steel product according to the present invention.

[0024] Similarly, to improve strength by forming fine Ti precipitates, such as TiC or Ti(C,N) precipitates, and to obtain a particulate structure, titanium ("Ti") is provided in the steel of the flat steel product according to the present invention at a fraction of 0.020 to 0.080% by mass. To particularly surely achieve this effect, a Ti fraction of at least 0.030% by mass can be provided. The precipitation amount made possible by the Ti fraction provided according to the present invention contributes, inter alia, to an optimal combination of mechanical properties characterizing the steel according to the present invention. The positive influence of the presence of Ti in the steel of the flat steel product according to the present invention can be particularly effectively utilized when the Ti fraction is 低于0.07% by mass.

[0025] The effect of Ti in the steel of the flat steel product according to the present invention can be further assisted by adding Ti in an amount corresponding to 11 times or less of the respective N fraction and B fraction of the steel of the flat steel product according to the present invention. Thus, in this embodiment, the following %Ti of the Ti fraction is applied: %Ti≦11×(%N+%B) where %N is the given N fraction and %B is the given B fraction. By restricting the Ti fraction in this way, an optimal amount of Ti precipitates is obtained, and at the same time, the formation of boron nitride, which has an adverse effect on formability, is prevented.

[0026] Boron ("B") is present in the steel of the flat steel product according to the present invention at a fraction of 0.0005 to 0.0020% by mass in order to increase the strength on the one hand and not to reduce the formability on the other hand of the flat steel product according to the present invention.

[0027] The nitrogen ("N") fraction is limited to 0.010% by mass or less in the steel of the flat steel product according to the present invention so that Ti acts as an alloying element in the structure and does not completely bind with N. To ensure a sufficient amount of Ti(C,N) precipitates in the structure, a fraction of N of at least 0.003% by mass is provided.

[0028] Impurities are permissible in the steel of the flat steel products according to the present invention. Although these are technically unavoidable in the practical and economical manufacture of the flat steel products according to the present invention, they are kept in very small amounts and do not adversely affect the desired properties of the flat steel products according to the present invention.

[0029] Impurities include fractions of phosphorus ("P") and sulfur ("S"). The fraction of P is limited to 0.025 mass%, and especially less than 0.015 mass%, to avoid a decrease in weldability. The fraction of S is limited to a maximum of 0.010 mass%, to avoid the formation of MnS and / or (Mn / Fe)S, which adversely affect the tensile properties of the steel according to the present invention.

[0030] The total percentage of impurities in the flat steel product according to the present invention is limited to 0.5% by mass or less, and when the total amount of impurities is 0.3% by mass or less, a deterioration in the properties of the flat steel product is particularly reliably avoided.

[0031] The steel according to the present invention may optionally contain up to 0.20 mass% molybdenum ("Mo"), up to 0.050 mass% niobium ("Nb"), up to 0.10 mass% copper ("Cu"), up to 0.020 mass% vanadium ("V"), and up to 0.10 mass% nickel ("Ni"). The fractions of these elements are limited so as to have only a slight effect on the properties of the flat steel product according to the present invention. Therefore, they may also be as low as "0%" in a technical sense, i.e., so low that they can be considered impurities and have no effect whatsoever on the flat steel product according to the present invention.

[0032] As a result of the adjustments to the steel composition of the flat steel product according to the present invention, it is possible to provide a flat steel product made of duplex steel that has a tensile strength Rm of 750 to 940 MPa, an elastic limit of 440 to 650 MPa, and a fracture elongation A80 of more than 13%, and is characterized by particularly good formability with minimal edge cracking tendency, and similarly good weldability. The tensile strength Rm, elastic limit Rp0.2, and fracture elongation A80 are determined according to DIN ISO 6892, respectively (longitudinal tensile direction; sample form 2).

[0033] The essential difference between the present invention and the prior art described at the beginning, for example, European Patent No. 2031081B1, lies in one precipitation state of the structure of the flat steel product according to the present invention, characterized by the distribution of hardness values ​​to the martensite and ferrite phases, and a large amount of fine precipitates. This structural state can be achieved mainly by the addition of a carbon fraction limited according to the present invention, as well as constant amounts of Ti and B. In this way, above-average rigid behavior is achieved, with an increased deformation gradient in hole expansion tests.

[0034] The fraction of martensite and ferrite, including bainite ferrite, in the structure of the flat steel product according to the present invention is quantified by image analysis.

[0035] As a result of the alloy selection according to the present invention, the martensite fraction in the structure of the flat steel product according to the present invention is limited to 40 volume% or less, and at least 10 volume% of martensite is present to ensure the required strength.

[0036] The remainder of the structure of the flat steel product according to the present invention consists mainly of ferrite, with a retained austenite fraction of 5 volume% or less, which may be 90 volume% or less and includes at least 30 volume% of bainite ferrite.

[0037] The flat steel products according to the present invention exhibit particularly good forming properties, as evidenced by high values ​​for hole expansion ratio HER (measured according to DIN ISO 16630) exceeding 20% ​​and maximum draw depth exceeding 33 mm (measured in a limiting dome height (LDH) test using a 100 mm hemispherical die). These are achieved by early local quenching, which is superior to comparable products in this strength class and is reflected in a tensile strain hardening index n of at least 0.22%, measured in the elastic interval of 0.2% to 2.2% according to DIN EN ISO 10275:2014.

[0038] Due to their unique properties, the flat steel products according to the present invention are particularly suitable for the manufacture of axial load components, such as longitudinal and transverse members, or bending load resistant components, such as B-pillars, B-pillar reinforcements, or sills of automobile bodies.

[0039] According to the present invention, the cold-rolled flat steel product obtained by the present invention can be manufactured by performing at least the following steps: a) A process of melting a molten steel containing C: 0.040-0.100 mass%, Mn: 2.10-2.50 mass%, Si: 0.10-0.40 mass%, Al: up to 0.10 mass%, Cr: 0.30-0.90 mass%, Ti: 0.020-0.080 mass%, B: 0.0005-0.0020 mass%, Ca: up to 0.005 mass%, P: up to 0.025 mass%, S: up to 0.010 mass%, N: 0.003-0.010 mass%, and optionally up to 0.20 mass% Mo, up to 0.050 mass% Nb, up to 0.10 mass% Cu, up to 0.020 mass% V, and up to 0.10 mass% Ni, as well as iron and unavoidable impurities as a remainder; b) A process of casting the molten material to produce a precursor such as a slab or thin slab, c) A process of producing a hot-rolled strip by hot-rolling a precursor at a hot-rolling completion temperature of 850-980°C; d) A process of coiling a hot-rolled strip at a coiling temperature of 480-650°C; e) The process of pickling the hot-rolled strip with acid; f) A process of cold-rolling a hot-rolled strip to form a cold-rolled flat steel product with a total cold-rolling ratio of 25-70%; g) A process of annealing cold-rolled flat steel products in a continuous furnace at an annealing temperature GT of 780-920°C; h) A step of cooling a cold-rolled flat steel product heated to an annealing temperature GT to a cooling completion temperature KET of 380 to 500°C, A cold-rolled flat steel product heated to an annealing temperature GT is cooled to a final cooling temperature KET in two steps. In the first step of the cooling process, the cold-rolled flat steel product is cooled from a given annealing temperature GT to an intermediate temperature ZT in the range of 750-620°C at a cooling rate AR1 exceeding 1.5 K / s. In the second step, it is cooled from the intermediate temperature ZT to a given final cooling temperature KET at a cooling rate AR2, in which case AR2 > 4 × AR1 is applied. or A cold-rolled flat steel product heated to an annealing temperature GT is cooled to a final cooling temperature KET in two steps: in the first step of the cooling process, the cold-rolled flat steel product is cooled from a given annealing temperature GT to an intermediate temperature ZT in the range of 700-450°C at a cooling rate AR1 exceeding 5K / s; and in the second step, it is cooled from the intermediate temperature ZT to a given final cooling temperature KET at a cooling rate AR2, in which case AR2 < (AR1) / 3 is applied; i) Optionally, a step of cooling or heating a cold-rolled flat steel product from a cooling end temperature KET to a bath inlet temperature BT of 450-490°C, and a step of conveying it through a molten bath made of zinc or a zinc alloy having a Zn fraction of at least 75% by weight; j) A step of cooling the resulting cold-rolled flat steel product to room temperature, and / or a step of cooling the cold-rolled flat steel product from the cooling end temperature KET to room temperature; k) Optionally, a step of skin-pass rolling a cold-rolled flat steel product with a skin-pass ratio of up to 2%, preferably 0.2 to 0.7%.

[0040] The melting of the alloyed molten material according to the present invention can be carried out in a conventional manner so that the molten material can be cast to produce a precursor, which is typically a slab or a thin slab (work steps a) and b). In this case, the slab typically has a thickness of 180 mm to 260 mm, while the thin slab typically has a thickness of about 40 mm to 60 mm.

[0041] The hot rolling of the precursor can similarly be carried out in a conventional manner on an assembly known from the prior art. The hot rolling completion temperature is set to 850-980°C, preferably 880-950°C.

[0042] After hot rolling, the resulting hot-rolled strip is cooled to a coiling temperature of 480-650°C and wound into a coil at this temperature. The most reliable coiling temperature range is limited to between 500°C and 600°C. Above 600°C, the risk of intergranular oxidation, which degrades the surface quality of the flat steel product, increases. Below 500°C, the strength of the hot-rolled strip is significantly reduced, leading to difficulties in subsequent forming. The coiled hot-rolled flat steel product is then cooled to room temperature in the coil.

[0043] Subsequently, the flat steel products can be optionally descaled. For this purpose, for example, they can be passed through a pickling apparatus to remove scale adhering to them.

[0044] Next, the optionally descaled hot-rolled strip is cold-rolled to form cold-rolled flat steel products, and the total cold-rolling ratio KG achieved during the cold-rolling process ([thickness of flat steel product before cold-rolling - thickness of flat steel product after cold-rolling] / thickness of flat steel product before cold-rolling] × 100%) is 25-70%.

[0045] When the flat steel product according to the present invention is coated with a zinc-based corrosion-resistant layer by hot-dip galvanizing, the cold-rolled flat steel product can be manufactured according to work processes a) to f), and then the following work processes can be completed in a continuous flow: g) A step of annealing the cold-rolled flat steel product in a continuous furnace at an annealing temperature GT of 780-920°C to achieve a sufficient amount of recrystallization after cold forming. Here, setting the annealing temperature to 810-890°C yields optimal annealing results. The typical annealing time Gt in which the flat steel product is held in the annealing furnace at the annealing temperature GT is 10-1000 seconds.

[0046] h) A process of cooling a cold-rolled flat steel product heated to an annealing temperature GT to a cooling completion temperature KET of 380-500°C.

[0047] This cooling process is carried out in the following two steps: According to a variation of the first method, a cold-rolled flat steel product is cooled in a first step of cooling from a given annealing temperature GT to an intermediate temperature ZT in the range of 750 to 620°C at a cooling rate AR1 greater than 1.5 K / s, and in a second step, it is cooled from the intermediate temperature ZT to a given cooling completion temperature KET at a cooling rate AR2, in which case AR2 > 4 × AR1 is applied.

[0048] In a variation of the second method, the cold-rolled flat steel product is cooled in a first step of cooling from a given annealing temperature GT to an intermediate temperature ZT in the range of 700 to 450°C at a cooling rate AR1 exceeding 5K / s, and in a second step, it is cooled from the intermediate temperature ZT to a given cooling completion temperature KET at a cooling rate AR2, in which case AR2 < (AR1) / 3 is applied.

[0049] The selection of the cooling rates in the first and second steps achieves the desired structural formation of the flat steel product according to the present invention.

[0050] i) A step of cooling and / or heating a cold-rolled flat steel product from a cooling end temperature KET to a bath inlet temperature BT of 450-490°C, conveying the cold-rolled flat steel product through a molten bath made of zinc or a zinc alloy, and adjusting the thickness of the layer formed on the flat steel product when it exits the molten bath. The composition of the molten bath can be selected by conventional methods, and the molten bath may be pure zinc molten or consist of at least 75% by weight of Zn.

[0051] j) A process of cooling the cold-rolled flat steel products that emerge from the molten bath to room temperature.

[0052] When the cold-rolled flat steel product according to the present invention is left uncoated or electrolytically coated, the annealing process is performed in a continuous furnace at an annealing temperature in the range of 780 to 920°C for an annealing time Gt of 10 to 1000 seconds. Subsequently, the heated cold-rolled flat steel product is cooled to a cooling end temperature KET in the range of 380 to 500°C, so that the cooling of the cold-rolled flat steel product heated to the annealing temperature GT is performed in two steps. In the first step of the cooling process, the cold-rolled flat steel product is cooled from a given annealing temperature GT to an intermediate temperature ZT in the range of 700 to 450°C at a cooling rate AR1 exceeding 5K / s, and in the second step, it is cooled from the intermediate temperature ZT to a given cooling end temperature KET at a cooling rate AR2, in which case AR2 < (AR1) / 3 is applied. After this, the cold-rolled flat steel product is cooled to room temperature.

[0053] Optionally, the resulting cold-rolled flat steel products, with or without anticorrosive coating, can be subjected to further skin-pass rolling to optimize their mechanical properties, surface properties, and dimensional accuracy. For this purpose, success has been demonstrated with forming rates ("skin-pass rates") of up to 2%, particularly 0.2–0.7%. [Brief explanation of the drawing]

[0054] [Figure 1] This diagram illustrates how material failure can occur. [Figure 2] This figure shows that material flow from the flange region is completely prevented. [Figure 3] This figure shows the hole widening achieved according to the transformation as a function of the opening angle of the forming punch used with respect to the center plane in each case. [Modes for carrying out the invention]

[0055] [Examples] The present invention will be described in more detail below with reference to exemplary embodiments.

[0056] To test the present invention, ten molten materials A to J were melted, and their compositions are shown in Table 1. Molten materials A to J were cast into slabs in a conventional continuous casting plant, then hot-rolled to form hot-rolled strips, coiled into coils, and cooled to room temperature. Subsequently, the hot-rolled strips were pickled and cold-rolled at a total cold-rolling rate of KG to form cold-rolled flat steel products that exist as cold strips.

[0057] To coat the cold-rolled flat steel products obtained in this manner with a Zn-based anticorrosion coating, the cold-rolled flat steel products were annealed at a given annealing temperature GT for a given annealing time Gt. Starting from the annealing temperature GT, the cold-rolled flat steel products were cooled to the final cooling temperature KET. For this purpose, the cooling of the flat steel products was carried out in one or two steps, with the first cooling step proceeding at a cooling rate AR1 to an intermediate temperature ZT, and then the second cooling step starting from the intermediate temperature ZT and proceeding at a cooling rate AR2 to the final cooling temperature KET (Table 2).

[0058] The cooled cold-rolled flat steel product is then heated or cooled to the bath inlet temperature BT and transported through a molten bath consisting of at least 75% Zn. The thickness of the anticorrosive coating thus applied to the cold-rolled flat steel product by hot-dip plating was adjusted in the conventional way by blowing off excess coating material as the flat steel product exits the molten bath.

[0059] After being cooled to room temperature using water or air as in the conventional method, cold-rolled flat steel products with an anti-corrosion coating were subjected to skin-pass rolling, and these were skin-pass rolled with a skin-pass rate of 0.2 to 0.7% (skin-pass rate = [(thickness of flat steel product before skin-pass rolling - thickness of flat steel product after skin-pass rolling) / thickness of flat steel product before skin-pass rolling] × 100%).

[0060] For the cold-rolled flat steel products obtained in this manner, the tensile strength Rm, elastic limit Rp0.2, elongation A80, and hole expansion ratio HER according to DIN ISO 16630 were determined according to DIN ISO 6892 (longitudinal tensile direction, sample form 2). The structural fractions of ferrite F and martensite M were determined using an optical microscope according to DIN 50601:1985-08. The remaining structure consisted of small amounts of bainite and retained austenite, where present. The latter was determined by standard quantitative phase analysis using the Rietveld method according to DIN EN 13925 (2003.07). These properties are shown in Table 3.

[0061] In order to demonstrate the specific effects of the present invention on formability and hole expansion behavior, the following tests are performed in addition to the hole expansion ratio HER test conducted in accordance with DIN ISO 16630.

[0062] A steel strip having a tensile strength Rm of at least 750 MPa, manufactured using the alloy concept according to the present invention, is characterized in that, in a hole expansion test that reduces the angle of the cone, when the test is performed by changing the angle of the cone in the range of 180° to 50°, an increase above average is achieved in the measured hole expansion, in order to affect the shape change distribution in the region of drilling with a width close to 0 mm to 5 mm in a target manner.

[0063] In these tests, perforation is achieved by mechanical shear cutting. The same cutting parameters are set for all samples. The width of the cut gap is in the range of 9–15% of the thickness of the flat steel product being tested. By using perforations punched in the same way, the influence of the cutting process is eliminated, and the same conditions are achieved for all punched shapes.

[0064] Material failure is characterized by constriction or cracking across the entire sheet thickness in the cutting edge region. At a significantly larger hole diameter of 20 mm compared to testing according to DIN ISO 16630, the effect of sheet thickness in the typical sheet thickness range of 1.0–2.0 mm is relatively low. The achieved hole expansion values ​​for various punches are more easily compared by geometric transformation to the center plane of the metal sheet. Assuming a "uniaxial tensile force" on the edge and using the measured hole expansion, the sheet thickness reduction can be found according to the relationship shown in Table 4.

[0065] Sheet thickness, edge [mm] = original sheet thickness × e (-0.5*LN((HER / 100)+1) ), Center plane [mm] O = Drilled edge O at time of breakage + 2 × COS (cone angle / 2) × Sheet metal thickness, edge / 2 Center plane HER[%]=[(Center plane O HER-Exit O) / Exit O]×100% (See also Figure 1).

[0066] The effects that occur in the hole widening experiment performed using the method described above can be detected by FE analysis. The moment of breakage and / or the possible maximum expansion are determined by video analysis. For this purpose, the process is observed from above and centered by a camera. By using a telecentric lens, the hole expansion and / or the diameter of the inner edge defining a given hole can be measured before the moment of breakage and calculated as the percentage of hole expansion relative to the exit diameter. For this purpose, the image frequency of the video film is at least 10 images per 1 mm of punch trajectory for a punch speed of 1 mm / s.

[0067] Furthermore, to evaluate the overall formability in the stretch forming region, the pull-out depth was analyzed using a limit dome height test (LDH test). In this test, as schematically shown in Figure 2, material flow from the flange region was completely prevented during formation, and the material was formed with a 0.100 mm hemispherical punch (Nakazima tool) that led to material failure (see Figure 1). The holding force was set to 400 kN and the pull-out speed to 1.0 mm / sec (+ / - 0.2).

[0068] Figure 3 shows the hole expansion achieved according to the transformation described above, as a function of the opening angle of the forming punch used with respect to the center plane in each case. The metal sheets tested were each 1.5 mm thick. One group consisted of steel constructed according to melt analysis A in Table 1 in accordance with the present invention (the relevant values ​​are copied in Figure 2 by circles connected to each other by dotted lines). The other group consists of conventional steel available under the name "DP800-DH," which, in mass%, comprises 0.157% C, 1.98% Mn, 0.114% Si, 0.324% Al, 0.106% Cr, 0.004% Ti, 0.0002% B, 0.012% P, 0.001% S, 0.0038% N, 0.02% Mo, 0.022% Nb, 0.01% Cu, 0.001% V, 0.02% Ni, and the remainder being iron and unavoidable impurities. Hole expansion achieved in sheet metal samples made of the material according to the present invention was clearly better than in sheet metal samples made of conventional steel. [Table 1] [Table 2] [Table 3] TIFF0007844498000004.tif236119

Claims

1. A cold-rolled flat steel product having a tensile strength of 750 to 940 MPa, an elastic limit Rp0.2 of 440 to 650 MPa, and a fracture elongation A80 of more than 13% (in all cases determined according to DIN ISO 6892 (longitudinal tensile direction, sample form 2)), and the steel base material is, by mass % C: 0.040-0.100%, Mn: 2.10-2.50%, Si: 0.10-0.40%, Cr: 0.30-0.90%, Ti: 0.020-0.080%, B: 0.0005-0.0020%, N: 0.003-0.010%, Al: maximum 0.10%, Ca: maximum 0.005%, P: maximum 0.025%, S: maximum 0.010%, Depending on the case, one or more of the following elements: Mo: Maximum 0.20%, Nb: maximum 0.050%, Cu: maximum 0.10%, V: maximum 0.020%, Ni: 0.10% maximum It consists of steel comprising iron and unavoidable impurities as the remainder, the total fraction of impurities being limited to 0.5% by mass or less, and the fractions of phosphorus ("P") and sulfur ("S") belonging to the said impurities, For the fraction of Ti at %Ti, the equation %Ti ≤ 11 × (%N + %B) applies, where %N = a given fraction of N and %B = a given fraction of B. It has a two-phase structure consisting of 10-40 area percent martensite, 30-90 area percent ferrite including bainitic ferrite, less than 5% retained austenite, and other structural components unavoidable due to the manufacturing process. Here, the structural fractions of ferrite and martensite were determined using an optical microscope according to DIN 50601:1985-08, and the remaining structure was determined by standard quantitative phase analysis using the Rietveld method according to DIN EN13925 (2003.07). Cold-rolled flat steel products.

2. The flat steel product according to claim 1, characterized in that the strain hardening index n, measured in the expansion range of 0.2 to 2.2% in accordance with DIN EN ISO 10275:2014, is at least 0.22%.

3. The flat steel product according to claim 1 or 2, characterized in that its tensile strength Rm is 780 to 900 MPa as determined according to DIN ISO 6892 (longitudinal tensile direction, sample form 2).

4. A flat steel product according to any one of claims 1 to 3, characterized in that it has a hole expansion ratio HER of more than 20% as determined in accordance with DIN ISO 16630.

5. The flat steel product according to claim 4, characterized in that the hole expansion ratio HER with a 180° conical punch is at least 15%, and the hole expansion ratio HER with a 50° conical punch is at least 25%.

6. A flat steel product according to any one of claims 1 to 5, characterized in that it has a draw depth of more than 33 mm as determined by an LDH test.

7. A flat steel product according to any one of claims 1 to 6, characterized in that it is coated with a corrosion-preventive layer applied by hot-dip galvanizing or electrolytic coating.

8. A method for manufacturing a cold-rolled flat steel product according to any one of claims 1 to 7, comprising the following steps: a) A process of melting a molten steel containing C: 0.040 to 0.100 mass%, Mn: 2.10 to 2.50 mass%, Si: 0.10 to 0.40 mass%, Al: up to 0.10 mass%, Cr: 0.30 to 0.90 mass%, Ti: 0.020 to 0.080 mass%, B: 0.0005 to 0.0020 mass%, Ca: up to 0.005 mass%, P: up to 0.025 mass%, S: up to 0.010 mass%, N: 0.003 to 0.010 mass%, up to 0.20 mass% Mo, up to 0.050 mass% Nb, up to 0.10 mass% Cu, up to 0.020 mass% V, and up to 0.10 mass% Ni, with the remainder being iron and unavoidable impurities. However, for the fraction of Ti in %Ti, the equation %Ti ≤ 11 × (%N + %B) applies, where %N = a given fraction of N and %B = a given fraction of B; b) A step of casting the molten material to produce a precursor such as a slab or thin slab; c) A step of producing a hot-rolled strip by hot-rolling the precursor at a hot-rolling completion temperature of 850 to 980°C; d) A step of coiling the hot-rolled strip at a coiling temperature of 480 to 650°C; e) A step of pickling the hot-rolled strip with acid; f) A step of cold-rolling the hot-rolled strip to form a cold-rolled flat steel product with a total cold-rolling ratio of 25 to 70%; g) A step of annealing the cold-rolled flat steel product in a continuous furnace at an annealing temperature GT of 780 to 920°C, provided that if the chromium content in the molten steel exceeds 0.80% by mass, the annealing temperature GT is at least 840°C; h) A step of cooling the cold-rolled flat steel product heated to the annealing temperature GT to a cooling completion temperature KET of 380 to 500°C, The cold-rolled flat steel product, heated to the annealing temperature GT, is cooled to a cooling completion temperature KET in two steps: in the first step of cooling, the cold-rolled flat steel product is cooled from the given annealing temperature GT to an intermediate temperature ZT in the range of 750 to 620°C at a cooling rate AR1 exceeding 1.5 K / s; and in the second step, it is cooled from the intermediate temperature ZT to the given cooling completion temperature KET at a cooling rate AR2, in which case AR2 > 4 × AR1 is applied. or The cold-rolled flat steel product, heated to the annealing temperature GT, is cooled to a cooling completion temperature KET in two steps: in the first step of the cooling process, the cold-rolled flat steel product is cooled from the given annealing temperature GT to an intermediate temperature ZT in the range of 700 to 450°C at a cooling rate AR1 exceeding 5 K / s; and in the second step, it is cooled from the intermediate temperature ZT to the given cooling completion temperature KET at a cooling rate AR2, in which case AR2 < (AR1) / 3 is applied; i) Optionally, a step of cooling or heating the cold-rolled flat steel product from the cooling completion temperature KET to a bath inlet temperature BT of 450 to 490°C, and a step of conveying it through a molten bath made of zinc or a zinc alloy having a Zn fraction of at least 75% by weight; j) A step of cooling the resulting cold-rolled flat steel product to room temperature, and / or a step of cooling the cold-rolled flat steel product from the cooling completion temperature KET to room temperature; k) Optionally, a step of skin-pass rolling the cold-rolled flat steel product with a skin-pass rate of up to 2%.

9. The method according to claim 8, wherein the precursor is a slab or a thin slab.

10. The method according to claim 8 or 9, characterized in that the coiling temperature is 500 to 600°C.

11. The method according to any one of claims 8 to 10, characterized in that the annealing temperature GT is 810 to 890°C.

12. The method according to any one of claims 8 to 11, wherein in step k), the cold-rolled flat steel product is skin-pass rolled with a skin-pass ratio of 0.2 to 0.7%.

13. Use of the cold-rolled flat steel product according to any one of claims 1 to 7 for manufacturing a component subjected to axial stress or a component subjected to bending stress.

14. The use according to claim 13, wherein the component subjected to axial stress is a longitudinal member or a transverse member.

15. The use according to claim 13 or 14, wherein the component subjected to bending stress is a B-pillar, B-pillar reinforcement, or sill of an automobile body.

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