Flat steel product, method for the production thereof, and use of such a flat steel product

Abstract

To provide a flat steel product which has a steel base material having high strength, improved weldability and optimized forming characteristics and can be produced at a low cost, and to provide a method for producing the same.SOLUTION: The cold-rolled flat steel product consists, in percent by mass, of one or more of C: 0.040 to 0. 100%; Mn: 2.10 to 2.50%; Si: 0.10 to 0.40%; Cr: 0.30 to 0.90%; Ti: 0.020 to 0.080%, B: 0.0005 to 0.0020%; N: 0.003 to 0.010%; Al: ≤ 0.10%; Ca: ≤ 0.005%; P: ≤ 0.025%; S: ≤ 0.010%; optionally Mo: ≤ 0.20%; Nb: ≤ 0.050%; Cu: ≤ 0.10%; V: ≤ 0.020%; Ni: ≤ 0.10%, the balance being iron and unavoidable impurities, and further wherein the steel substrate has a duplex structure consisting of 10-40 vol.% martensite, 30-90 vol.% ferrite, and up to 5% retained austenite.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled flat steel product, a method for its manufacture and the use of a flat steel product according to the invention. do. [Background technology]

[0002] "Flat steel products" are understood here as rolled products, the length and width of which are respectively: These include, in particular, steel strips, steel plates and the materials obtained therefrom. This includes pre-cut parts, such as blanks, that can be cut into shapes.

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

[0004] When formulas or conditions are mentioned in the text, the values ​​therein are based on the proportions of specific alloying elements. If calculated or found, the assumed percentage of alloying elements unless otherwise stated. is given in each case as a percentage by weight in these formulas or conditions.

[0005] Particularly in the field of automobile body structures, there is a demand for high strength steels that at the same time have good formability. In particular, in the manufacture of parts that are formed into complex shapes, e.g. , local shape change ability and energy, which can be quantified by a good value in the hole expansion test. There are high demands on crack resistance.

[0006] A cold rolled flat steel product is known from EP 2031081 B1, which is It can be hot-dipped with a lead-based anti-corrosion coating and contains 20-70% martensitic consisting of up to 8% retained austenite and the remainder of ferrite and / or bainite The flat steel product has a tensile strength of at least 950 MPa and is 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: max. 0.20%, Ni: max. 0.10 %, Ca: max. 0.005%, P: max. 0.2%, S; max. 0.01%, N: max. 0.0 12% (by weight), and the remainder being iron and unavoidable impurities.

[0007] This type of steel concept is characterized by a low elastic limit ratio resulting from significant strength differences between the structural components. be assigned.

[0008] In fact, flat steel products of the type described above have particularly high strength, but their processing It has been found that such materials tend to form edge cracks that impair their performance. Such flat steel products need to be further improved in terms of their suitability for welding. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] European Patent No. 2031081B1 Summary of the Invention [Problem to be solved by the invention]

[0010] Against this background, there are new products with high strength, improved weldability and optimized forming properties. There is a need to develop a flat steel product that has a steel base material that can be manufactured cost effectively.

[0011] Furthermore, methods for producing and using such flat steel products are also specified, and the flat steel products according to the invention The product shall be particularly suitable for them. [Means for solving the problem]

[0012] To address this need, the proposed product has at least the features of claim 1. do.

[0013] A method allowing for the cost-effective production of the product according to the invention is described in claim 9. Such products, in particular those using zinc ("Zn")-based anticorrosion coatings, It can be provided by the method set forth in claim 8.

[0014] In this case, the method according to the invention as described herein and its possible variations and extensions are When implementing this feature, a person skilled in the art may need to add additional steps not explicitly mentioned in this example. It should be clear that, based on their practical experience, It is known to be commonly applied in such cases.

[0015] Therefore, the steel substrate of the flat steel product according to the present invention contains C: 0.040 to 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: 0 0.10%, V: up to 0.020%, Ni: up to 0.10% (mass%), and the balance It is made from steel consisting of iron as a raw material and inevitable impurities.

[0016] In this case, the steel substrate of the flat steel product according to the invention contains 10 to 40% by volume of martensite, 3 0 to 90% by volume of ferrite (including bainite ferrite), 5% or less of residual austenite The remainder consists of other structural components that are unavoidable due to the manufacturing process. It has a two-phase structure, and such other structural components are present in the structure when the sum of the fractions of the other components is less than 100%. Present only if full.

[0017] The fraction of each alloying component provided in accordance with the present invention is determined as follows: Each of the following descriptions applies to the present invention when only one flat steel product according to the present invention is discussed. It also refers to the composition of the steel substrate of the flat steel product.

[0018] The flat steel product according to the present invention comprises 0.040 to 0.100 mass % carbon ("C"). If the fraction is less than 0.040% by mass, the strength is significantly reduced. The maximum carbon fraction of 0.100 wt.% was selected for good weldability of the steel. Carbon fractions above 0.100 wt.% also lead to the formation of a harder carbon-rich martensite phase. This significantly increases the difference in hardness between martensite and ferrite. This has a negative effect on the hole expansion behavior and weldability of the flat steel product according to the invention. The positive effect of the presence of C in the steel of flat steel products is seen when the C fraction is 0.05 mass% or more and 0.08 mass% or more. It can be particularly well utilized when the amount is 50% or less.

[0019] Silicon ("Si") increases strength due to the hardening effect that Si has on ferrite The Si content in the flat steel product according to the present invention is 0.10 to 0.40 mass %. The upper limit of Cr is to avoid intergranular oxidation, which can adversely affect the coatability and surface properties of the steel. Therefore, the content is 0.40 mass %.

[0020] Manganese ("Mn") in a fraction of 2.10 to 2.50 mass % is This ensures that the formation of pearlite in the flat steel product according to the invention is prevented during cooling. The determined fraction of Mn therefore mediates the formation of martensite in the structure and thus strengthens it. In particular, the Mn fraction provided in accordance with the present invention contributes substantially to the increase in the degree of This compensates for the loss of strength that would otherwise be expected as a result of the C fraction being set to a relatively low value according to The Mn fraction is preferably 2.20 mass % or more and 2.40 mass % or less.

[0021] Aluminum ("Al") is required for deoxidation during steelmaking at a maximum fraction of 0.10% by mass .

[0022] The steel of the flat steel product according to the present invention may be doped with calcium ("Ca") to deoxidize the steel during steel production. can be added in a fraction of up to 0.005% by weight. This effect is at least This can be achieved by adding 0.0005% by weight of Ca.

[0023] Chromium ("Cr") also serves to increase the strength of the steel in the flat steel products of the present invention. For this purpose, at least 0.30% by weight, in particular at least 0.40% by weight, of C In order to reduce the risk of significant grain boundary oxidation, the Cr fraction is required according to the present invention. The upper limit of the range specified for the content is limited to 0.90% by mass, especially 0.80% by mass. If the chromium fraction exceeds 0.80 mass%, the flat steel product obtained by the present invention The manufacturing method is such that the desired two-phase structure and the desired mechanical properties of the flat steel product according to the invention are ensured. To obtain this, an annealing temperature GT of at least 840°C is set. It should be.

[0024] Similarly, the formation of fine Ti precipitates, e.g., TiC or Ti(C,N) precipitates, increases the strength. In order to improve the hardness and obtain a fine-grained structure, titanium ("Ti") is added to the steel of the flat steel product according to the invention. ") is provided in a fraction of 0.020 to 0.080% by mass. This effect is particularly reliably achieved In order to achieve this, a Ti fraction of at least 0.030 mass % may be provided. The amount of precipitation made possible by the Ti fraction selected is, inter alia, the function that characterizes the steel according to the invention. The presence of Ti in the steel of the flat steel product according to the invention contributes to an optimum combination of mechanical properties. The effect of the lath can be utilized particularly effectively when the Ti fraction is 0.07 mass % or less.

[0025] The effect of Ti in the steel of the flat steel product according to the present invention is compared with that of the steel of the flat steel product according to the present invention. Further support is provided by adding an amount equivalent to 11 times or less of the N and B fractions. Therefore, in this embodiment, the following Ti fractions %Ti apply: %Ti≦11×(%N+%B) where %N = given N fraction, and %B = given B fraction. The Ti fraction is calculated in this way: By limiting the amount of Ti precipitates, the optimum amount can be obtained, but at the same time, the formability is adversely affected. The formation of boron nitride is prevented.

[0026] Boron ("B") increases the strength of the flat steel product according to the invention on the one hand, but also on the other hand. In order to prevent deterioration of formability, the steel of the flat steel product according to the present invention contains 0.0005 to 0.0020 It is present in mass percent fractions.

[0027] Nitrogen ( The "N" fraction is limited to 0.010% by mass in the steel of the flat steel product according to the invention. In order to ensure a sufficient amount of Ti(C,N) precipitates, at least 0.003 mass% of A rate of N is provided.

[0028] Impurities are permitted in the steel of the flat steel product according to the invention. Although technically unavoidable in the practical and economical production of Therefore, the desired properties of the flat steel product according to the invention are not adversely affected.

[0029] Impurities include fractions of phosphorus ("P") and sulfur ("S"). The P fraction is Limited to 0.025% by weight, especially less than 0.015% by weight, to avoid deterioration of adhesion The S fraction is preferably MnS and / or ( It is limited to a maximum of 0.010 wt. % to avoid the formation of Mn / Fe)S.

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

[0031] The steel according to the invention may optionally contain up to 0.20% by weight of molybdenum ("Mo"), up to 0.050% by weight niobium ("Nb"), max 0.10% by weight copper ("Cu"), max 0 0.020% by weight of Vanadium ("V") and up to 0.10% by weight of Nickel ("Ni") ) can be added. The fractions of these elements have a significant effect on the properties of the flat steel product according to the invention. Therefore, they are also technically " 0%”, i.e., it may be so low that it can be considered an impurity, and the flat steel according to the present invention It has no effect on the product.

[0032] As a result of the described adjustment according to the invention of the steel composition of the flat steel product according to the invention, 75 Tensile strength Rm of 0 to 940 MPa, elastic limit of 440 to 650 MPa, and It has an elongation at break of A80 and has particularly good forming properties with minimal edge crack tendency. It is therefore possible to provide flat steel products made of duplex stainless steel which are characterized by good weldability as well as good weldability. The tensile strength Rm, elastic limit Rp0.2, and breaking elongation A80 are D Determined in accordance with IN ISO 6892 (longitudinal tensile direction; specimen type 2).

[0033] The present invention and the method described at the beginning, which are known for example from EP 2031081 B1, The essential difference from the prior art is the division of hardness values ​​into martensite and ferrite phases. One analysis of the structure of the flat steel product according to the invention is characterized by a large amount of fine precipitates, as well as a thin film of the fabric. This structural state is mainly determined by the carbon fraction defined in accordance with the present invention, as well as the T This can be achieved by adding a certain amount of i and B. In this way, the hole expansion test As the deformation gradient at increases, an above-average robust behavior is achieved.

[0034] The flat steel product according to the present invention has a structure containing martensite and bainitic ferrite. The fraction of ferrite is quantified by image analysis.

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

[0036] The remainder of the structure of the flat steel product according to the invention is comprised of a retained austenite fraction of not more than 5% by volume. Predominantly ferrite, which may be up to 90% by volume and is at least 30% by volume Contains bainitic ferrite.

[0037] The flat steel products according to the invention have a hole expansion ratio HER (DIN ISO 1663) of more than 20%. 0) and a maximum drawing depth of more than 33 mm (using a 100 mm hemispherical die) The particularly good forming properties are manifested in the high values ​​of limiting dome height (LDH) These are achieved by early localized quenching, which is comparable to the quotient of this strength class. It is more precise than the standard and has a 0.2% to 100% content according to DIN EN ISO 10275:2014. reflected in a tensile strain hardening exponent n of at least 0.22% measured in the 2.2% elastic interval .

[0038] Due to their specific properties, the flat steel products according to the invention are particularly suitable for axially loaded components. for example the manufacture of longitudinal and transverse beams, or bending load-bearing components, e.g. B-pillars, It is suitable for the manufacture of 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 is at least It can be manufactured by carrying out the following industrial process: a)C:0.040~0.100% by mass, Mn:2.10~2.50% by mass, Si:0 .10~0.40% by mass, Al: max. 0.10% by mass, Cr: 0.30~0.90% by mass , Ti:0.020~0.080% by mass, B:0.0005~0.0020% by mass, Ca : Maximum 0.005 mass%, P: Maximum 0.025 mass%, S; Maximum 0.010 mass%, N: 0.003-0.010 wt.%, and possibly up to 0.20 wt.% Mo, max. 0. 0.50% by mass Nb, max. 0.10% by mass Cu, max. 0.020% by mass V, and max. Steel melt containing at most 0.10% by mass of Ni, and the remainder iron and unavoidable impurities melting the b) casting the melt to produce a precursor such as a slab or thin slab; c) hot rolling the precursor at a hot rolling finish temperature of 850 to 980 ° C to produce a hot rolled strip; the process of d) coiling the hot-rolled strip at a coiling temperature of 480 to 650°C; e) pickling the hot rolled strip; f) The hot rolled strip is cold rolled to produce cold rolled flat steel products with a total cold rolling reduction of 25 to 70%. forming process; g) Annealing cold-rolled flat steel products in a continuous furnace with an annealing temperature of 780-920°C. Ringing process; h) The cold-rolled flat steel product heated to the annealing temperature GT is cooled to 380-500°C. cooling to a temperature of 1000 KET; The cold-rolled flat steel product heated to the annealing temperature GT is cooled to the finishing temperature K in two processes. ET, and the cold rolled flat steel product is cooled to a given annealing temperature in the first stage of its cooling. From the GT temperature to the intermediate temperature ZT in the range of 750 to 620°C, the In the second step, the material is cooled from the intermediate temperature ZT to a given cooling end temperature to KET at a cooling rate of AR2, where AR2 > 4 × AR1; or The cold-rolled flat steel product heated to the annealing temperature GT is cooled to the finishing temperature K in two processes. ET, and the cold rolled flat steel product is cooled to a given annealing temperature during the first stage of its cooling. From the GT to the intermediate temperature ZT in the range of 700 to 450°C, the The cooling rate is AR1, and in the second step, the intermediate temperature ZT is cooled to a given cooling end temperature Cooling to KET at a cooling rate of AR2, where AR2<(AR1) / 3 applies. process; i) Optionally, the cold-rolled flat steel product is cooled from the end temperature KET to the bath entrance temperature of 450-490°C. and cooling or heating the zinc or zinc alloy to a temperature BT. conveying the mixture through a molten bath having a Zn fraction of 75% by weight; j) cooling the emerging cold rolled flat steel product to room temperature and / or cooling the product from the cooling end temperature KET to room temperature; k) Optionally, cold-rolled flat steel with a skin pass rate of up to 2%, preferably 0.2-0.7% This is the process of skin pass rolling the product.

[0040] Melting of the alloyed melt according to the present invention involves casting the melt into a slab or alloy. This can be done in a conventional manner so that precursors that are thin slabs or slabs can be produced. (Work steps a) and b). In this case, the slab is typically 180mm to 260mm thick. The thickness of the thin slab is typically about 40 mm to 60 mm.

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

[0042] After hot rolling, the resulting hot-rolled strip is cooled to a coiling temperature of 480 to 650°C. , and wound into a coil at this temperature. A particularly reliable coiling temperature range is 500°C. If the coiling temperature exceeds 600°C, the flat steel product The risk of grain boundary oxidation, which deteriorates the surface quality, increases. If this occurs, the strength of the hot rolled strip will be significantly reduced, causing difficulties in the subsequent forming of the coil. The hot rolled flat steel product is cooled to room temperature in the coil.

[0043] Thereafter, the flat steel product can optionally be descaled. For example, the flat steel product may be passed through a pickling unit which removes any scale adhering to the product.

[0044] The optionally descaled hot rolled strip is then cold rolled to produce a cold rolled flat steel product. The total cold rolling reduction KG achieved during the cold rolling process (thickness of the flat steel product before cold rolling) [Thickness - Thickness of flat steel product after cold rolling] / [Thickness of flat steel product before cold rolling] × 100%) is 2 5 to 70%.

[0045] The flat steel product according to the present invention is coated with a zinc-based anticorrosion layer by hot dip coating. When cold rolled flat steel products are manufactured, they can be produced according to steps a) to f). The following steps can then be completed in a continuous stream: g) To achieve a sufficient amount of recrystallization after cold forming, cold rolled flat steel products are Annealing in a continuous furnace at an annealing temperature GT of 20°C. Optimum annealing results are achieved by setting the annealing temperature between 810 and 890°C. Typical annealing time G during which the product is held in the annealing furnace at the annealing temperature GT t is 10 to 1000 seconds.

[0046] h) The cold-rolled flat steel product heated to the annealing temperature GT is cooled to 380-500°C. and cooling to a temperature of 1000K.

[0047] This cooling is done in two steps: According to a variant of the first method, the cold-rolled flat steel product is cooled in the first stage of its cooling. , from a given annealing temperature GT to an intermediate temperature ZT in the range of 750-620°C, In the second step, the material is cooled at a cooling rate AR1 exceeding 1.5 K / s, and then cooled from the intermediate temperature ZT to The material is cooled at a cooling rate AR2 from the temperature of the material to a given cooling end temperature KET, where AR2>4 ×AR1 is applied.

[0048] According to a variant of the second method, on the other hand, the cold-rolled flat steel product is cooled in the first stage of its cooling. In this case, the intermediate temperature ZT is in the range of 700 to 450°C from the given annealing temperature GT. In a second step, the alloy is cooled at a cooling rate AR1 exceeding 5 K / s to an intermediate temperature ZT to a given cooling end temperature KET at a cooling rate AR2, where AR2< (AR1) / 3 is applied.

[0049] The selection of the cooling rates in the first and second steps is important for the quality of the flat steel product according to the present invention. Achieve the desired structure formation.

[0050] i) From the cooling end temperature KET of the cold rolled flat steel product to the bath entrance temperature BT of 450 to 490 ° C. and / or heated to a temperature of 1000°C, and then cold-rolled flat through a molten bath of zinc or a zinc alloy. The process of transporting the steel product and adjusting the thickness of the layer formed on the flat steel product as it leaves the molten bath. The composition of the molten bath can be selected in a conventional manner, and the molten bath is a pure zinc melt. It consists of or comprises at least 75% by weight of Zn.

[0051] j) Cooling the cold rolled flat steel product emerging from the molten bath to room temperature.

[0052] The cold rolled flat steel product according to the invention may be left uncoated or may be electrolytically coated. When annealing, the annealing temperature is in the range of 780 to 920°C in a continuous furnace for 10 to 100 minutes. The annealing process is performed with an annealing time Gt of 0 seconds. The flat steel product is the cold rolled flat steel product heated to the annealing temperature GT and cooled to the end temperature K. The cooling end temperature K is set in the range of 380-500°C so that the cooling to ET is carried out in two steps. The cold rolled flat steel product is cooled to a given annealing temperature in the first stage of its cooling. From the cooling temperature GT to the intermediate temperature ZT in the range of 700 to 450°C, the cooling rate exceeds 5 K / s. In the second step, the temperature is cooled from the intermediate temperature ZT to a given cooling end temperature K It is cooled to ET at a cooling rate AR2, where AR2<(AR1) / 3 applies. The cold rolled flat steel product is then cooled to room temperature.

[0053] Optionally, the resulting cold rolled flat steel product, with or without a corrosion protection coating, Further skinning is performed to optimize its mechanical properties, its surface properties, and its dimensional accuracy. For this purpose, a maximum of 2%, especially 0.2-0.7%, of Cu can be used. Forming rates ("skin pass rates") have proven successful. [Brief explanation of the drawings]

[0054] [Figure 1] FIG. 10 illustrates the material reaching material failure. [Figure 2] FIG. 10 shows that material flow from the flange area is completely prevented. [Figure 3] FIG. 10 shows the hole expansion achieved according to the transformation as a function of the opening angle of the forming punch used relative to the central plane in each case. DETAILED DESCRIPTION OF THE INVENTION

[0055] [Example] The invention is explained in more detail below with reference to exemplary embodiments.

[0056] To test the present invention, ten melts A to J were melted, the compositions of which are shown in Table 1. Melts A to J are cast into slabs in a conventional continuous casting plant and subsequently hot rolled to produce hot rolled strip. The hot rolled strip was then formed into a coil and cooled to room temperature. and cold rolled at a total cold reduction of KG to form a cold rolled flat product present as a cold strip. do.

[0057] The cold-rolled flat steel products thus obtained are coated with a Zn-based anticorrosion coating. To test the properties of the cold rolled flat steel products, the annealing temperature GT is set at a given annealing time. The cold rolled flat steel was annealed for a period of Gt. The product was cooled to the cooling end temperature KET. For this purpose, the cooling of the flat steel product was carried out in one The cooling is carried out in one or two steps, and the cooling is carried out at a cooling rate of AR1 in the first step of the cooling. Then in a second step of cooling, the cooling starts from the intermediate temperature ZT and continues until the The cooling proceeded at a rate of AR2 to the end temperature KET (Table 2).

[0058] The cooled cold rolled flat steel product is then heated or cooled to the bath entrance temperature BT. The metal is then transported through a molten bath consisting of at least 75% Zn. The thickness of the corrosion protection coating applied to cold rolled steel flat products is determined by the thickness of the coating applied to the flat products when they leave the molten bath. The coating was conditioned in the conventional manner by blowing off excess coating material as it was applied.

[0059] After conventional cooling to room temperature with water or air, cold pressed steel with a corrosion-protective coating is The flat steel products are subjected to skin pass rolling and then skinned at a skin pass rate of 0.2 to 0.7%. Skin pass rate = [(thickness of flat steel product before skin pass rolling - thickness after skin pass rolling [Thickness of flat steel product after skin pass rolling / Thickness of flat steel product before skin pass rolling] × 100%).

[0060] The cold-rolled flat steel products obtained in this way were measured for tensile strength Rm and elastic limit Rp0. 2 and elongation A80, and the hole expansion ratio HER according to DIN ISO 16630. Determined in accordance with IN ISO 6892 (longitudinal tensile direction, specimen type 2). Ferrite F and the structural fraction of martensite M is determined optically according to DIN 50601:1985-08. The remaining structure, if present, is composed of small amounts of bainite and The latter consisted of retained austenite. These properties were determined by standard quantitative phase analysis in accordance with 13925 (2003.07). is shown in Table 3.

[0061] To demonstrate the specific effects of the present invention on formability and hole expansion behavior, In addition to the hole expansion ratio (HER) test carried out in accordance with ISO 16630, the following tests are carried out: will be done.

[0062] The alloys manufactured using the alloy concept according to the present invention have a tensile strength Rm of at least 750 MPa. The steel strip to be used is subjected to a hole expansion test in which the cone angle is reduced to a width of approximately 0 mm to 5 mm. In order to affect the shape change distribution of the hole area in a targeted manner, the range of 180° to 50° When tests were conducted with varying cone angles in the range, the measured hole expansion was above average. The invention is characterized by the fact that an increase in

[0063] In these tests, perforation is achieved by mechanical shear cutting. For all samples The same cutting parameters are set. The width of the cutting gap is 9 to 10 times the thickness of the flat steel product to be tested. 15%. By using the same punched perforations, the cutting process The influence of the temperature is eliminated and the same conditions are achieved for all punched shapes.

[0064] Material failure occurs by narrowing or cracking through the entire sheet thickness in the area of ​​the cutting edge. It is characterized by a significantly larger 20% reduction compared to the test according to DIN ISO 16630. For perforation diameters of mm, sheet thicknesses in the typical sheet thickness range of 1.0-2.0 mm The effect of the hole size on the center plane of the metal sheet is relatively low. The comparison is more easily made by a geometric transformation of the Using the specified hole expansion, the sheet thickness reduction can be found according to the relationship shown in Table 4. can.

[0065] Sheet thickness, edge [mm] = original sheet thickness × e (-0.5*LN((HER / 1 00)+1) ), Center plane [mm] O = Perforation edge at break O + 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 occurring in the hole expansion experiments performed in the above manner can be detected by FE analysis. The moment of breakage and / or the maximum possible extension can be determined by video analysis. For this purpose, the process is observed centrally from above by a camera. Using a centric lens to enlarge and / or define a given hole The diameter of the inner edge is measured before the moment of failure and calculated as the percentage of the hole expansion with respect to the exit diameter. For this purpose, the image frequency of the video film is set to 1 mm / s. The punch speed is at least 10 images per mm of punch trajectory.

[0067] In addition, to evaluate the overall formability in the stretch forming region, a limit dome height test (LD The pull-out depth was analyzed using a test (H test). In this test, the pull-out depth during the formation process was analyzed as shown diagrammatically in Figure 2. The flow of material from the flange area is completely prevented and the material reaches the material failure (see Figure 1). ) 0100mm hemispherical punch (Nakazima tool) is used. The pulling force was set to 400 kN and the pulling speed was set to 1.0 mm / sec (+ / - 0.2).

[0068] Figure 3 shows the hole expansion achieved following the transformation described above, in each case relative to the center plane. 1 shows a diagram showing the strength of the metal sheet as a function of the opening angle of the forming punch used for the test. The samples were each 1.5 mm thick. One group was subjected to melt analysis A in Table 1 in accordance with the present invention. (Relevant values ​​are shown in the diagram by circles connected to each other by dotted lines.) (Reproduced in Figure 2). The other group is a conventional model available under the designation "DP800-DH." It consists of steel, which is composed of, by mass%, 0.157% C, 1.98% Mn, and 0.114% S. i, 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 0.022% Nb, 0.01% Cu, 0.001% V, 0.02% Ni, and The balance consists of iron and unavoidable impurities. The hole expansion achieved was clearly better than that of the conventional steel sheet metal specimens. [Table 1] [Table 2] [Table 3] TIFF2026016678000005.tif236119

Claims

1. A cold-rolled flat steel product having a tensile strength of 750 to 940 MPa, the steel substrate comprising: 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%, Optionally, one or more of the following elements: Mo: max 0.20% Nb: maximum 0.050%, Cu: maximum 0.10%, V: maximum 0.020%, Ni: 0.10% maximum and the balance being iron and unavoidable impurities, the total fraction of impurities being 0 5% by mass or less, and the fraction of phosphorus ("P") and sulfur ("S") in said impurities Belongs to - 30 to 90% by volume containing 10 to 40% by volume of martensite and bainite ferrite % ferrite, 5% or less retained austenite, and other unavoidable defects due to the manufacturing process. and the remainder of the structural components of the cold rolled flat steel product.

2. The strain hardening exponent n measured in the expansion range of 0.2 to 2.2% is at least 0.22%.

2. A flat steel product according to claim 1 , characterized in that it is

3. For the Ti fraction in %Ti, %Ti≦11×(%N+%B) where %N = given N fraction and %B = given B fraction. The flat steel product according to any one of claims 1 to 2.

4. Its tensile strength Rm is 780 to 900 MPa, and its elastic limit Rp0.2 is 440 to 650 MPa and its breaking elongation A80 is greater than 13% (in both cases DIN I 10. The method of claim 1, wherein the tensile strength is determined in accordance with ISO 6892 (longitudinal tensile direction, specimen form 2).

4. A flat steel product according to any one of claims 1 to 3.

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

6. The hole expansion ratio HER with a 180° conical punch is at least 15%, and 6. The method according to claim 5, wherein the hole expansion ratio HER with the drill punch is at least 25%. Flat steel products listed.

7. characterized by having a pull-out depth of greater than 33 mm as determined by LDH testing; A flat steel product according to any one of claims 1 to 6.

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

9. A cold rolled flat steel formed according to any one of claims 1 to 8, comprising the following working steps: Product manufacturing method: a) C: 0.040 to 0.100% by mass, Mn: 2.10 to 2.50% by mass, Si: 0 .. 10 to 0.40% by mass, Al: maximum 0.10% by mass, Cr: 0.30 to 0.90% by mass , Ti: 0.020 to 0.080% by mass, B: 0.0005 to 0.0020% by mass, Ca : maximum 0.005% by mass, P: maximum 0.025% by mass, S; maximum 0.010% by mass, N: 0.003 to 0.010 mass%, maximum 0.20 mass% Mo, maximum 0.050 mass% N b, max. 0.10 wt. % Cu, max. 0.020 wt. % V, and max. 0.10 wt. % melting a steel melt containing Ni and the balance being iron and unavoidable impurities; b) casting the melt to produce a precursor such as a slab or thin slab; c) hot-rolling the precursor at a hot-rolling finish temperature of 850 to 980°C to produce a hot-rolled strip; a step of manufacturing the d) coiling the hot-rolled strip at a coiling temperature of 480 to 650°C; e) pickling the hot rolled strip; f) The hot-rolled strip is cold-rolled to produce a cold-rolled flat steel having a total cold-rolling reduction of 25 to 70%. forming the article; g) annealing the cold rolled flat steel product in a continuous furnace at an annealing temperature GT of 780 to 920 ° C. kneeling; h) The cold-rolled flat steel product heated to the annealing temperature GT is 380 to 500 ° C. a step of cooling to a cooling end temperature KET of The cold rolled flat steel product heated to the annealing temperature GT is cooled in two steps. and the cold rolled flat steel product is cooled to a finishing temperature KET in the first stage of its cooling. From a given annealing temperature GT to an intermediate temperature ZT in the range of 750 to 620°C, .5 K / s, and in the second step, the intermediate temperature ZT is cooled at a cooling rate AR1 exceeding .5 K / s. to the given cooling end temperature KET at a cooling rate AR2, where AR2 > 4×AR1 is applied, or The cold rolled flat steel product heated to the annealing temperature GT is cooled in two steps. The cold rolled flat steel product is cooled to a finishing temperature KET during the first stage of its cooling. From a given annealing temperature G T to an intermediate temperature Z T ranging from 700 to 450° C. In a second step, the intermediate temperature ZT is cooled at a cooling rate AR1 exceeding 5 K / s. The cooling rate AR2 is then increased to the given cooling end temperature KET, where AR2<( AR1) / 3 is applied; i) optionally, the cold rolled flat steel product is cooled to a temperature of 450 to 490°C from the cooling end temperature KET; and cooling or heating the bath to a bath inlet temperature BT of zinc or a zinc alloy. conveying the molten metal through a molten bath having a Zn fraction of at least 75% by weight; j) cooling the emerging cold rolled flat steel product to room temperature, and / or cooling the rolled flat steel product from said cooling end temperature KET to room temperature; k) optionally, the cold rolled flat sheet with a skin pass rate of up to 2%, preferably 0.2 to 0.7%. A process in which steel products are skin-pass rolled.

10. The method according to claim 9, wherein the coiling temperature is 500 to 600°C. Law.

11. Claim 9 or claim 10, characterized in that the annealing temperature GT is 810 to 890°C.

10. The method according to claim 10.

12. for manufacturing components that are subject to axial stress, e.g. longitudinal and transverse members, or Components subject to bending stress, e.g. B-pillars of automobile bodies, B-pillar reinforcements, Cold-rolled flat steel obtained according to any one of claims 1 to 8 for producing steel plates or sills. Product Use.

Images