Method for producing a steel strip from a high-strength multiphase steel, and the corresponding steel strip

Abstract

The invention relates to a method for producing a steel strip from a high-strength multiphase steel, which steel strip has a tensile strength of at least 700 MPa in the longitudinal direction, in which method a rolled wide strip made of steel consisting of the following elements in % by weight: C ≥ 0.080 to ≤ 0.350, Mn ≥ 0.80 to ≤ 3.50, Mo ≥ 0.10 to ≤ 1.00, N ≥ 0.0020 to ≤ 0.0160, S < 0.020, optionally one or more of the following elements: Cr ≥ 0.050 to ≤ 1.0, P < 0.050, Cu ≥ 0.001 to ≤ 1.0, Si ≥ 0.05 to ≤ 1.5, AI ≥ 0.0030 to ≤ 1.0, Ni ≥ 0.03 to ≤ 1.50, Nb ≥ 0.005 to ≤ 0.150, Ti ≥ 0.005 to ≤ 0.150, V ≥ 0.001 to ≤ 0.30, B ≥ 0.0005 to ≤ 0.0050, and Ca ≥ 0.0005 to v 0.0060, remainder iron, including the usual smelting-induced impurities associated with steel, and having a carbon equivalent CEV which is greater than 0.570 and smaller than 0.900, the carbon equivalent CEV resulting, according to the formula: CEV = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5, from the contents of the corresponding elements in % by weight, is heat-treated as a whole - in particular rolled up to form a coil, in such a way that it has a temperature above 770°C for a period of > 30 minutes, and is cooled, after this heat treatment, to a temperature below 100°C, the cooling process being performed between 750°C and 200°C (i) at an average cooling rate of greater than 1 K / h and smaller than 300 K / h and (ii) in the temperature range from a martensite start temperature Ms to 100°C in a period of more than 10 h, the numerical value [Ms] of the martensite start temperature Ms in °C being determined on the basis of the numerical values of the respective alloy contents of the elements C, Mn, Ni, Cr, Si and Mo in % by weight according to the following formula: [M s ] = 539 - 423 [C] - 30.4[Mn] - 17.7[Ni] - 12.1[Cr] - 11.0[Si] - 7.0[Mo]. The invention also relates to a corresponding steel strip.

Description

[0001] Method for producing a steel strip from a high-strength multi-phase steel and corresponding steel strip

[0002] The invention relates to a method for producing a steel strip from a high-strength multi-phase steel which has a tensile strength of at least 700 MPa in the longitudinal direction.

[0003] The invention further relates to a corresponding steel strip made of a high-strength multi-phase steel, which has a tensile strength of at least 700 MPa in the longitudinal direction.

[0004] Multiphase steels possess an excellent combination of strength, formability, and ductility due to their multiphase microstructure. In particular, phase fractions of martensite and / or bainite exceeding 25% by volume are an essential microstructural component for achieving high tensile strengths (e.g., > 600 MPa). With increasing fractions, e.g., 50, 70, or 100% by volume, tensile strengths exceeding 1300 MPa are possible, depending on the chemical composition. Especially for annealing treatments with slow cooling rates and / or a coarse microstructure resulting from annealing at high annealing temperatures, the proportion of hard phase components (martensite or bainite, possibly also tempered) must be higher to achieve higher strength.For a low-alloy steel to form sufficient phase fractions of bainite and martensite, annealing treatment well above the steel's characteristic transformation temperature (Ai) followed by sufficiently high cooling rates is necessary. For the large-scale production of low-alloy multi-phase steels, continuous annealing plants such as "continuous annealing" or hot-dip galvanizing lines are required, where cooling rates are well above 1 K / s, meaning the steel strip is cooled from temperatures above the Ai temperature to room temperature within seconds or minutes.

[0005] Steel strip is defined below as hot-rolled or cold-rolled and annealed steel strip. Typical thicknesses of hot-rolled steel strip, also known as hot-rolled strip, are between 1.8 mm and 18 mm. Cold-rolled, annealed steel strip is referred to as cold-rolled strip or thin sheet and typically has thicknesses in the range of 0.5 mm to 2.5 mm. Through targeted processing, the strip thickness can be adjusted to varying degrees of flexibility, even within a cold-rolled strip or thin sheet. In addition to large-scale heat treatment by continuous annealing in a continuous annealing furnace, coiled strip is also heat-treated "as a whole" on an industrial scale in furnaces such as so-called batch annealing plants (discontinuous annealing / discontinuous annealing plants). Batch annealing treatments of low-alloyed strip are carried out either as recovery annealing or recrystallization / soft annealing.During recovery annealing, a usually cold-formed strip is annealed at temperatures below 700 °C in order to achieve high tensile strength with a high yield strength and low ductility in the steel strip produced by annealing. Typically, recovery-annealed steel exhibits a pronounced yield strength, moderate ductility, and a high yield strength / tensile strength ratio of > 0.8, which can be critical for further processing of the steel strip. The materials-related mechanism of recovery, which determines the technological parameters after batch annealing, is highly dependent on the annealing temperature, annealing time, and previous cold working of the strip (e.g., the degree of cold rolling during cold rolling). During softening / recrystallization annealing, the strip is annealed at temperatures around the Ai transformation temperature for several hours to days.The tensile strength after the previously described annealing treatment is below 600 MPa and is significantly lower than the strength before annealing. However, ductility increases significantly with recrystallization annealing compared to the unannealed and cold-rolled material.

[0006] The highly competitive automotive market is forcing manufacturers to constantly find solutions to reduce fleet fuel consumption and CO2 emissions while maintaining maximum comfort and occupant protection. On the one hand, weight savings of all vehicle components play a crucial role, but on the other hand, the best possible behavior of the individual components under high static and dynamic loads during operation and in the event of a crash is also important. For safety-relevant components, in addition to high strength and good ductility, good toughness is particularly essential when selecting the material to ensure safety in the event of a crash. Steel manufacturers are contributing to solving this problem by providing high-strength steels.In addition, by providing high-strength steels with lower sheet thickness, the weight of vehicle components can be reduced while maintaining or possibly even improving component behavior.

[0007] In addition to the required weight reduction, these newly developed multi-phase steels must meet the high material requirements regarding yield strength, tensile strength, impact energy and elongation at break.

[0008] Toughness is determined using the Charpy impact test according to DIN EN ISO 148-1 on solid specimens. Structural steels typically achieve an impact energy of 27 J at +20 °C. Toughness decreases with decreasing temperature, so a material exhibits improved – and correspondingly good – toughness if a significantly higher impact energy of 40 J is determined at a lower temperature of -20 °C, or if an impact energy of over 27 J is achieved at a temperature of -40 °C.

[0009] Multiphase steels are known, for example, from the published specifications

[0010] DE 10 2017 131 247 A1, DE 10 2017 130 237 A1, and DE 10 2015 111 177 A1. The material properties disclosed therein result from a high phase content of bainite and / or martensite, which require sufficiently rapid cooling conditions. Large-scale processing of such multi-phase steels is carried out using continuous annealing systems.

[0011] Document EP 1 431 407 A1 describes a method for producing a steel sheet from a multi-phase steel, wherein a rolled strip sheet is heat-treated as a whole such that it assumes a maximum temperature between 600°C and 800°C for a period of one hour, and after this heat treatment is cooled to a temperature below 100°C, wherein the cooling preferably takes place at an average cooling rate between 5 K / h and 100 K / h. The maximum temperature during the heat treatment is selected such that, although complete recrystallization occurs, only enough austenite is formed to ensure that the deep-drawability is not impaired. The steel sheet microstructure is adjusted by a subsequent additional heat treatment.

[0012] The object of the invention is to provide measures for providing steel strips made of a high-strength multiphase steel that is also characterized by a certain degree of toughness. This object is achieved according to the invention by a method for producing a steel strip having the features of independent claim 1 and a steel strip having the features of independent claim 7. Preferred embodiments of the invention are specified in the subclaims, each of which may represent an aspect of the invention individually or in combination.

[0013] A first aspect of the invention relates to a method for producing a steel strip from a high-strength multi-phase steel which has a tensile strength of at least 700

[0014] MPa in the longitudinal direction, whereby in this process a rolled strip sheet of

[0015] Steel consisting of the following elements in weight%:

[0016] C > 0.080 to < 0.350,

[0017] Mn > 0.80 to < 3.50,

[0018] Mo > 0.10 to < 1.00,

[0019] N > 0.0020 to < 0.0160,

[0020] S < 0.020,

[0021] Optionally one or more of the following elements:

[0022] Cr > 0.050 to < 1.0,

[0023] P < 0.050,

[0024] Cu > 0.001 to < 1.0,

[0025] Si > 0.05 to < 1.5,

[0026] AI > 0.0030 to < 1.0,

[0027] Ni > 0.03 to < 1.50,

[0028] Nb > 0.005 to < 0.150,

[0029] Ti > 0.005 to < 0.150,

[0030] V > 0.001 to < 0.300,

[0031] B > 0.0005 to < 0.0050 and

[0032] Ca > 0.0005 to < 0.0060,

[0033] The remainder being iron, including usual impurities associated with steel melting, and having a carbon equivalent CEV which is greater than 0.570 and less than 0.900, wherein the carbon equivalent CEV is calculated according to the following formula

[0034] CEV = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 from the contents of the corresponding elements in weight%, as a whole - in particular wound into a coil - is heat-treated in such a way that it assumes a temperature for a period > 30 minutes above 770°C, and after this heat treatment is cooled to a temperature below 100°C, wherein the cooling

[0035] (i) between 750°C and 200°C with an average cooling rate greater than 1 K / h and less than 300 K / h and

[0036] (ii) in the temperature range from a martensite start temperature M sup to 100°C in a time greater than 10 hours, where the numerical value [M s ] the martensite start temperature M s in °C based on the numerical values ​​of the respective alloying proportions of the elements C, Mn, Ni, Cr, Si, and Mo in wt.%, determined according to the following formula:

[0037] [M s ] = 539 - 423 [C] - 30.4[Mn] - 17.7[ / Vi] - 12.1[Cr] - 11.0[Si] - 7.0[Mo]

[0038] The phase components bainite and / or martensite, which are characteristic of multi-phase steels, form from austenitic phase components when the steel cools from a temperature above the Ai temperature. To prevent the austenite from transforming—or only transforming to a small extent—into the ferrite and / or pearlite phases, the material must have sufficient through-hardenability, corresponding to the technically feasible cooling rate. The through-hardenability of a steel depends on its chemical composition and can be approximately described by the following carbon equivalent (CEV):

[0039] CEV = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5

[0040] The heat treatment is also referred to as "annealing" and can be carried out, for example, in the bell-type annealing plant mentioned above. However, an excessively high proportion of alloying elements such as manganese (Mn), chromium (Cr), carbon (C), vanadium (V), molybdenum (Mo), copper (Cu), and / or nickel (Ni) is not tolerable for the preceding process steps such as continuous casting, hot rolling, or cold rolling, and the subsequent joining operations such as welding. For steels with lower proportions of alloying elements, a low annealing temperature is preferably selected in order to locally enrich the alloying elements in the austenite and thus achieve better through-hardenability in the austenite. For the reasons stated above, the CEV is limited to 0.570 to a maximum of 0.900. For cost reasons, the proportion of alloying elements, and thus also the CEV, should be kept low.For the aforementioned reason (formation of sufficient bainite and / or martensite from the austenitic phase components), the average cooling rate should ideally be between 1 K / h and 300 K / h in the critical temperature range of 750 °C to 200 °C. The strength-promoting microstructural components of multiphase steels, such as bainite and / or martensite, form from the austenitic phase components during cooling at temperatures below 570 °C. At temperatures above room temperature, especially above 200 °C, the locally high strengths of the martensite and bainite phases are reduced by a process known as tempering or self-tempering.In this materials science mechanism, the precipitation of forced carbon into carbides and the relief of transformation-induced stresses lead to a reduction in the strength of the hard phases bainite and martensite, and thus also to a decrease in strength, but also to an improvement in the toughness and ductility of the annealed steel strip. This tempering mechanism is thermally activated. The strength loss due to tempering / self-tempering increases accordingly with longer residence times at higher temperatures, especially at temperatures above 100 °C. To limit the strength loss, it is necessary to counteract the tempering mechanisms with alloying concepts after the formation of the hard phases. Multi-phase steels with untempered martensite have lower toughness.To significantly improve, i.e., increase, the toughness of the material, a sufficient holding time above 100°C is necessary after martensite formation, i.e., after the temperature has initially fallen below the martensite formation temperature Ms. Below 100°C, the tempering effects of martensite are negligible. Therefore, a tempering time above 100°C of more than ten hours (10 h) is recommended to ensure a sufficient tempering effect even at low martensite start temperatures (Ms temperatures).

[0041] Since the Ms temperature depends particularly on the chemical composition, it must be estimated individually for each steel, which is done here by the empirical formula [M s ] = 539 - 423[C] - 30.4[Mn] - 17.7[IVi] - 12.1[Cr] - 11.0[Si] - 7.0[Mo] for the numerical value [M s] of the Ms temperature in °C, where the numerical values ​​[C], [Mn], [Ni], [Cr], [Si] and [Mo] result from the existing alloy contents C, Mn, Ni, Cr, Si and Mo in wt.%.

[0042] To ensure that sufficient amounts of austenite form during annealing, temperatures above 770 °C, preferably above 780 °C, must be maintained. However, annealing temperatures that are too high lead to undesirable grain growth and should be avoided for reasons of energy conservation and with regard to the temperature resistance of furnace linings, so that the maximum annealing temperature should preferably not be above 950 °C, and in particular not above 850 °C. The maximum annealing temperature during heat treatment is preferably in a range > 800 °C and < 950 °C. To ensure that the steel strip has homogeneous properties across the strip length and width, resulting from a homogeneous temperature distribution, a residence time of more than 30 minutes above 770 °C must be maintained.

[0043] The product produced by the production method according to the invention, i.e. the hot-rolled and / or cold-rolled steel strip made of high-strength multi-phase steel with a tensile strength of at least 700 MPa in the longitudinal direction, has, in particular, a microstructure per steel thickness in which the sum of the volume fractions of the microstructure components martensite, tempered martensite, retained austenite, M / A phase, upper bainite and lower bainite is at least 25.0 volume% and the residual microstructure consists of ferrite and pearlite.

[0044] To determine the technological parameters, tensile tests were conducted in accordance with DIN EN ISO 6892-1:2020-06, which were taken longitudinally to the rolling direction. For a sample thickness < 5 mm, a gauge length of 80 mm was specified ("A80mm sample"); for a sample thickness > 5 mm, the gauge length was specified with a proportional factor of 5 ("As sample").

[0045] Due to the method according to the invention for producing a steel strip, continuous annealing is not necessary in large-scale production and the final product steel strip still has a high tensile strength > 700 MPa, with good ductility Aso> 8% and a low yield strength ratio R p o,2 / Rm < 0.70, preferably below 0.60, and the technological parameters after heat treatment are not significantly influenced by the microstructure or the cold deformation before heat treatment.

[0046] As already mentioned, the process can be implemented on an industrial scale using a batch annealing plant. For low-alloy steels, this is only possible using the manufacturing process according to the invention, as the process involves targeted phase transformations during heat treatment. For this purpose, it is imperative, similar to heat treatment during continuous annealing, to anneal in a temperature range above the Ai temperature, although the temperature does not necessarily have to be above the A3 temperature. The required annealing temperatures can vary depending on the chemical composition of the steel strip. To maintain constant strength across the strip thickness, heat treatment is preferably carried out in a non-decarburizing atmosphere.

[0047] According to a preferred embodiment of the invention, the strip sheet is heated during heat treatment from 200°C to a temperature of 750°C at an average heating rate between 1 K / h and 150 K / h. An accelerated heating rate > 150 K / h should be avoided due to internal stresses and homogeneous material properties, particularly in a coil wound into a steel strip as a whole. However, a heating rate slower than 1 K / h is not recommended for energy savings reasons.

[0048] To ensure homogeneous heating of the coiled steel strip, a heating period of at least 30 minutes is preferred. Longer holding times are beneficial for more homogeneous heating, but are not recommended due to the associated grain growth, which in turn causes a loss of strength.

[0049] By processing a coiled steel strip in a bell-type annealing furnace, for example, significantly lower heating rates are technically feasible than in a continuous annealing furnace. However, it is still recommended to heat the entire annealing cycle at the highest possible heating rates to avoid undesirable thermodynamically stable precipitates and unwanted grain growth during heating. Heating too quickly, in turn, hinders uniform heating of the steel strip, so average heating rates in the critical temperature range of 200 °C until an annealing temperature of 750 °C is reached should be between 1 K / h and 150 K / h.

[0050] According to yet another preferred embodiment of the invention, it is provided that the steel strip reaches a maximum temperature of at least 780°C and at most 950°C, preferably of at least 790°C and at most 850°C, during the heat treatment.

[0051] Furthermore, it is advantageously provided that the steel strip is provided with a surface coating in the form of a metallic coating, organic coating, or paint after cooling. The steel strip is then a coated steel strip, which, in addition to the actual steel strip made of high-strength multi-phase steel, also includes the coating on one or both sides.

[0052] According to a further preferred embodiment of the invention, it is provided that the quotient R p o.2 / R mof the heat-treated steel strip is increased by at least 0.05 by a plastic deformation such as stretch-bend straightening, bending straightening or tempering and the steel strip has a ratio of yield strength to tensile strength R p o,2 / Rm between 0.50 and 0.90.

[0053] According to a further preferred embodiment of the invention, the chemical composition of the rolled strip sheet comprises at least one of the following elements in % by weight:

[0054] Cr > 0.300 to < 0.700,

[0055] Mo > 0.150 to < 0.60

[0056] Mn < 2.70

[0057] Si > 0.25 < 0.80 and

[0058] Cu < 0.20.

[0059] A further aspect of the invention relates to a steel strip made of a high-strength multi-phase steel, which has a tensile strength of at least 700 MPa in the longitudinal direction, good toughness and a continuous yield strength, the multi-phase steel consisting of the elements in weight%:

[0060] C > 0.080 to < 0.350,

[0061] Mn > 0.80 to < 3.50,

[0062] Mo > 0.10 to < 1.00,

[0063] N > 0.0020 to < 0.0160,

[0064] S < 0.020,

[0065] Optionally one or more of the following elements: Cr 0.050 to 1 ,0,

[0066] P 0.050,

[0067] Cu 0.001 to 1 ,0,

[0068] Si 0.05 to 1 ,5,

[0069] AI 0.0030 to 1.0,

[0070] Ni 0.03 to 1.50,

[0071] Nb 0.005 to 0.150,

[0072] Ti 0.005 to 0.150, V > 0.001 to < 0.300,

[0073] B > 0.0005 to < 0.0050 and

[0074] Ca > 0.0005 to < 0.0060,

[0075] The remainder being iron, including usual impurities associated with steel melting, and having a carbon equivalent CEV which is greater than 0.570 and less than 0.900, wherein the carbon equivalent CEV is calculated according to the following formula

[0076] CEV = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 from the contents of the corresponding elements in weight%, whereby the multi-phase steel has a structure in which the sum of the volume fractions of the structural components martensite, tempered martensite, retained austenite, M / A phase, upper bainite and lower bainite is at least 25.0 volume% and the residual structure consists of ferrite and pearlite.

[0077] Such a steel strip can be produced from a rolled strip of steel of the corresponding composition by heat-treating this strip—in particular, rolled into a coil—"as a whole" using the manufacturing process for producing a steel strip described above. This heat treatment is also referred to as "annealing" and can be carried out, for example, using the batch annealing system mentioned above.

[0078] The invention therefore makes it possible to provide a steel strip which has a high tensile strength > 700 MPa, in particular with a good ductility Aso > 8%, a low yield strength ratio R po,2 / Rm < 0.70 and a continuous yield strength, and in which these technological parameters are not significantly influenced by the microstructure after heat treatment or by cold deformation prior to heat treatment. In other words, it is specifically intended that the ratio of yield strength to tensile strength R p o,2 / Rm is below 0.70 and the elongation at break Aso is > 8%.

[0079] A continuous yield strength is particularly preferable for subsequent forming, as it prevents, for example, the formation of flow patterns during deep drawing.

[0080] To determine the technological parameters, tensile tests were conducted in accordance with DIN EN ISO 6892-1:2020-06, which were taken longitudinally to the rolling direction. For a sample thickness < 5 mm, a gauge length of 80 mm was specified ("A80mm sample"). For a sample thickness > 5 mm, the gauge length was specified with a proportional factor of 5 ("As sample").

[0081] According to a preferred embodiment of the invention, the proportion of residual austenite is below 1% by volume and the pearlite content is below 2% by volume.

[0082] According to a particular embodiment of the invention, the ratio of yield strength to tensile strength R p o,2 / Rm is below 0.70 and the elongation at break Aso is > 8% in the longitudinal direction.

[0083] According to yet another preferred embodiment of the invention, the steel strip has at least one of the following properties: a notched bar impact energy at a test temperature of -20°C > 40 J, with the test being carried out on impact bending specimens according to DIN EN ISO 148-1 and along the rolling direction, and a notched bar impact energy at a test temperature of -40°C > 27 J, with the test being carried out on impact bending specimens according to DIN EN ISO 148-1 and along the rolling direction. The notched bar impact energy determined in this way, i.e., according to the Charpy notched bar impact test, is a measure of toughness.

[0084] In particular, the steel strip has a constant thickness, whereby the term "constant thickness" is to be understood in the sense of the usual standard tolerance (e.g., according to EN 10051). Alternatively, it is envisaged that the steel strip has a deliberately varying thickness along its length.

[0085] The effect of the elements in the steel strip according to the invention with a multiphase microstructure is described in more detail below. Multiphase steels are typically chemically structured in such a way that alloying elements are combined with and without microalloying elements. Accompanying elements are unavoidable and, where necessary, are taken into account in the analysis concept with regard to their effect.

[0086] Tramp elements are elements that are already present in the iron ore or that enter the steel during production. Due to their predominantly negative effects, they are generally undesirable. Attempts are made to remove them down to a tolerable level or to convert them into less harmful forms. Hydrogen (H) is the only element that can diffuse through the iron lattice without causing lattice strain. This means that hydrogen is relatively mobile in the iron lattice and can be absorbed relatively easily during production. Hydrogen can only be absorbed into the iron lattice in atomic (ionic) form. Hydrogen has a strong embrittling effect and diffuses preferentially to energetically favorable locations (defects, grain boundaries, etc.). Defects act as hydrogen traps and can significantly increase the residence time of hydrogen in the material.Cold cracks can develop through recombination to molecular hydrogen. This behavior occurs in hydrogen embrittlement or hydrogen-induced stress corrosion cracking. Hydrogen is also often cited as a cause of delayed fractures, which occur without external stresses. Therefore, the hydrogen content in steel should be as low as possible.

[0087] Oxygen (O): In its molten state, steel has a relatively high capacity to absorb gases; however, at room temperature, oxygen is only soluble in very small quantities. Similar to hydrogen, oxygen can only diffuse into the material in atomic form. Due to its highly embrittling effect and the negative impact on aging resistance, attempts are made to reduce the oxygen content as much as possible during production. There are two ways to reduce oxygen: one is process engineering, such as vacuum treatment, and the other is analytical. By adding certain alloying elements, the oxygen can be converted into less hazardous states. For example, binding of oxygen via manganese, silicon, and / or aluminum is generally common. However, the resulting oxides can cause negative properties as defects in the material.However, with fine precipitation, especially of aluminum oxides, grain refinement can also occur. For the reasons stated above, the oxygen content in the steel should be as low as possible.

[0088] Nitrogen (N) is also a byproduct of steel production. Steels containing free nitrogen are prone to severe aging. Even at low temperatures, the nitrogen diffuses around dislocations and blocks them. This causes an increase in strength combined with a rapid loss of toughness. The nitrogen can be bound in the form of nitrides by alloying with aluminum or titanium, for example. For the reasons stated above, the nitrogen content is limited to < 0.0160 wt.% or to amounts unavoidable during steel production.

[0089] Sulfur (S), like phosphorus, is bound as a trace element in iron ore. It is undesirable in steel (with the exception of free-cutting steels) because it is prone to strong segregation and has a strong embrittling effect. Attempts are therefore made to keep the amount of sulfur in the melt as low as possible (e.g. by deep vacuum treatment). Furthermore, the existing sulfur is converted into the relatively harmless compound manganese sulfide (MnS) by adding manganese. The manganese sulfides are often rolled out in rows during the rolling process and act as nuclei for the transformation. This leads to a pronounced row-like microstructure, particularly in the case of diffusion-controlled transformation, and in the case of pronounced rowing can lead to impaired mechanical properties (e.g. pronounced martensite rows instead of distributed martensite islands, anisotropic material behavior, reduced elongation at break). For the reasons stated above, the sulfur content is limited to < 0.020 wt.% orlimited to quantities unavoidable in steel production.

[0090] Phosphorus (P) is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. However, attempts are generally made to reduce the phosphorus content as much as possible because, among other things, its low diffusion rate leads to a strong tendency to segregation and significantly reduces toughness. The accumulation of phosphorus at grain boundaries causes grain boundary fractures. Phosphorus also raises the transition temperature from tough to brittle behavior to as much as 300°C. During hot rolling, near-surface phosphorus oxides at the grain boundaries can lead to fracture cracking. The negative effects of phosphorus can be partially compensated by alloying with small amounts of boron. It is assumed that boron increases grain boundary cohesion and reduces phosphorus segregation at the grain boundaries.However, in some steels, P is used in small amounts (<0.1%) as a microalloying element due to its low cost and high strength enhancement, for example, in higher-strength IF (interstitial-free) steels. For the reasons stated above, the optional phosphorus content is limited to <0.050% or to amounts unavoidable during steelmaking.

[0091] Alloying elements are usually added to steel to specifically influence certain properties. A single alloying element can influence different properties in different steels. The relationships are diverse and complex. The effect of alloying elements will be discussed in more detail below.

[0092] Carbon (C) is considered the most important alloying element in steel. Its targeted addition of up to 2.06% iron is what turns iron into steel. The carbon content is often drastically reduced during steelmaking. In the multiphase steel according to the invention, its content ranges from 0.080 to 0.350 weight percent. Due to its comparatively small atomic radius, carbon is dissolved interstitially in the iron lattice. Its solubility is a maximum of 0.02% in alpha iron and a maximum of 2.06% in y-iron. In dissolved form, carbon significantly increases the hardenability of steel. Due to the varying solubility, pronounced diffusion processes are necessary during phase transformation, which can lead to very different kinetic conditions. Furthermore, carbon increases the thermodynamic stability of austenite, which is reflected in the phase diagram by an expansion of the austenite region to lower temperatures.As the forced carbon content in martensite increases, so does the lattice distortion and, with it, the strength of the diffusion-free phase. Carbon is also required for carbide formation. Cementite (FeS₁C) is a representative of this carbides, which is found in almost every steel. However, much harder special carbides can also form with other metals such as chromium, titanium, niobium, and vanadium. Not only the type but also the distribution and size of the precipitates are crucial for the resulting increase in strength. To ensure sufficient strength on the one hand and good weldability on the other, the minimum C content is set at 0.080 wt.% and the maximum C content at 0.350 wt.%.

[0093] Aluminum (Al) is typically added to steel to bind the oxygen and nitrogen dissolved in iron. The oxygen and nitrogen are thus converted into aluminum oxides and aluminum nitrides. These precipitates can cause grain refinement by increasing the nucleation sites, thus improving toughness and strength. Aluminum nitride is not precipitated if titanium is present in sufficient quantities. Titanium nitrides have a lower enthalpy of formation and are formed at higher temperatures. In the dissolved state, aluminum, like silicon, shifts ferrite formation to shorter times, thus allowing the formation of sufficient ferrite. It also suppresses carbide formation, thus delaying the transformation of austenite. For this reason, Al is also used as an alloying element in retained austenitic steels to replace some of the silicon with aluminum.The reason for this approach is that Al is somewhat less critical for the galvanizing reaction than Si. By suppressing carbides (especially MsC carbides, where M stands for a metallic alloying element) in bainitic microstructure components or tempered martensite, Al prevents a reduction in strength of the aforementioned hard phases martensite and / or bainite and results in a less severe decrease in strength after annealing. The optional Al content is therefore set at 0.0030 wt% to 1.0 wt%.

[0094] Silicon (Si) binds oxygen during casting and thus reduces segregation and impurities in the steel. In addition, silicon increases the strength of the ferrite through solid solution strengthening with only a slight decrease in elongation at break. Another important effect is that silicon shifts the formation of ferrite to shorter times and thus enables the formation of sufficient ferrite before quenching in continuously annealed material. This effect is particularly advantageous when using low-alloy steels in the inventive batch annealing treatment of multi-phase steels. Ferrite formation enriches and stabilizes the austenite with carbon. At higher contents, silicon noticeably stabilizes the austenite in the lower temperature range, especially in the area of ​​bainite formation, by preventing carbide formation.During hot rolling, high silicon contents can cause highly adhesive scale to form, which can impair further processing. By suppressing carbides (particularly M3C carbides, where M stands for a metallic alloying element) in bainitic microstructure components or tempered martensite, Al prevents a reduction in strength of the aforementioned hard phases martensite and / or bainite and leads to a less severe decrease in strength after annealing. The optional Si content is therefore set at 0.050 wt% to 1.5 wt%, preferably 0.25 wt% to 0.80 wt%.

[0095] Manganese (Mn) is added to almost all steels for desulfurization to convert the harmful sulfur into manganese sulfides. Furthermore, manganese increases the strength of the ferrite through solid solution strengthening and shifts the transformation to lower temperatures. A main reason for alloying with manganese is the significant improvement in hardenability. Due to the diffusion hindrance, the pearlite and bainite transformation is shifted to longer times and the martensite initiation temperature is lowered. Like silicon, manganese tends to form oxides on the steel surface during annealing. Depending on the annealing parameters and the contents of other alloying elements (particularly Si and Al), manganese oxides (e.g., MnO) and / or mixed Mn oxides (e.g., Mn2SiO4) can occur. However, manganese is considered less critical at a low Si / Mn or Al / Mn ratio, since globular oxides rather than oxide films are more likely to form.The Mn content is therefore set at 0.80 wt% to 3.50 wt%, preferably up to 2.70 wt%.

[0096] Molybdenum (Mo): Molybdenum is added in a similar way to chromium to improve hardenability. The pearlite and bainite transformation is delayed, and the martensite initiation temperature is lowered. In particular, the delay in pearlite formation by Mo is essential for producing a steel according to the invention. Molybdenum also significantly increases tempering resistance and, through solid solution strengthening, increases the strength of the ferrite. The Mo content is alloyed depending on the dimensions, the plant configuration, and the microstructure. For the reasons stated above, the Mo content is set at 0.10 to 1.00 wt. %, preferably from 0.150 to a maximum of 0.60 wt. %, particularly for cost reasons.

[0097] Chromium (Cr): The addition of chromium primarily improves hardenability. In the dissolved state, chromium shifts the pearlite and bainite transformation to longer times and simultaneously lowers the martensite initiation temperature. Another important effect is that chromium significantly increases tempering resistance. Chromium is also a carbide former. If chromium is present in carbide form, the austenitizing temperature before hardening must be high enough to dissolve the chromium carbides. Otherwise, the increased nucleus count can impair hardenability. Chromium also tends to form oxides on the steel surface during annealing, which can impair the galvanizing quality. The optional Cr content is therefore set at values ​​of 0.05 to 1.0 wt.%, preferably 0.30 to 0.70.

[0098] Copper (Cu): The addition of copper can increase tensile strength and hardenability. In combination with nickel, chromium, and phosphorus, copper can form a protective oxide layer on the surface, which can significantly reduce the corrosion rate. In combination with oxygen, copper can form harmful oxides at the grain boundaries, which can have negative effects, especially in hot forming processes. The optional copper content is therefore limited to 1.00, preferably 0.20, weight percent.

[0099] Calcium (Ca): Calcium is used in the production of high-strength steels for deoxidation, desulfurization, and to control the size and shape of oxides and sulfides. Especially in high-strength steels, this improves ductility and toughness. Furthermore, steels with calcium additions are less prone to hot cracking, for example, during hot rolling. For the reasons stated above and due to the very low solubility of calcium in steel, the optional calcium content is limited to 0.0005 to 0.0060 wt.%, if necessary.

[0100] Nickel (Ni): In combination with oxygen, nickel can form harmful oxides at the grain boundaries, which can have negative effects, especially on hot forming processes. However, nickel also increases hardenability and lowers the transformation temperature. For the aforementioned reasons and cost reasons, the optional nickel content is limited to 0.03 to 1.50 weight percent.

[0101] Microalloying elements are generally added in very small quantities (< 0.1%). Typical microalloying elements are aluminum, vanadium, titanium, niobium, and boron. Unlike the alloying elements, they act primarily through precipitation, but can also influence properties in dissolved form. Despite the small amounts added, microalloying elements strongly influence the manufacturing conditions as well as the processing and final properties. Carbide and nitride formers that are soluble in the iron lattice are generally used as microalloying elements. The formation of carbonitrides is also possible due to the complete solubility of nitrides and carbides in each other. The tendency to form oxides and sulfides is generally most pronounced with the microalloying elements, but is usually deliberately prevented by the addition of other alloying elements.This property can be used positively by binding the generally harmful elements sulfur and oxygen. However, binding can also have negative effects if it results in insufficient microalloying elements being available for carbides to form.

[0102] Titanium (Ti) forms very stable nitrides (TiN) and sulfides (TiS2) even at high temperatures. Depending on the nitrogen content, some of these only dissolve in the melt. If the resulting precipitates are not removed with the slag, the high temperature at which they form causes them to form coarse particles in the material, which are generally not beneficial for the mechanical properties. A positive effect on toughness is achieved through the binding of free nitrogen and oxygen. Titanium protects other dissolved microalloying elements, such as niobium, from binding by nitrogen, allowing them to optimally develop their effect. Nitrides, which only form at lower temperatures due to the decrease in oxygen and nitrogen content, can also effectively hinder austenite grain growth.Unbound titanium forms titanium carbides at temperatures above 1150 °C and can thus cause grain refinement (inhibition of austenite grain growth, grain refinement through delayed recrystallization, and / or increased nuclei during o-Zy transformation) as well as precipitation hardening. The optional Ti content therefore ranges from 0.005 to 0.150 wt.%.

[0103] Niobium (Nb) causes strong grain refinement because, of all microalloying elements, it is the most effective at delaying recrystallization and also inhibiting austenite grain growth. Its strength-enhancing effect is qualitatively superior to that of titanium, evident from the increased grain refinement effect and the larger quantity of strength-enhancing particles (titanium bonding to coarse TiN at high temperatures). Niobium carbides form at temperatures below 1200 °C. During nitrogen bonding with titanium, niobium can increase its strength-enhancing effect by forming small, effective carbides in the lower temperature range (smaller carbide sizes). A further effect of niobium is the delay of the a-Zy transformation and the lowering of the martensite start temperature in the dissolved state. This occurs partly through the solute drag effect and partly through grain refinement.This increases the strength of the microstructure and thus also increases resistance to volume expansion during martensite formation. In principle, the addition of niobium to alloys is limited until its solubility limit is reached. While this limits the amount of precipitation, exceeding it primarily results in early precipitation formation with relatively coarse particles. Precipitation hardening can therefore be particularly effective in steels with a low C content (higher supersaturation is possible) and in hot forming processes (deformation-induced precipitation). The optional Nb content is therefore limited to values ​​of 0.005 to 0.150 wt.%. Vanadium (V): Carbide and nitride formation in vanadium only begins at temperatures around 1000 °C or even after the a / y transformation, i.e., much later than in titanium and niobium. Vanadium therefore has hardly any grain refining effect due to the small number of precipitates present in the austenite.Austenite grain growth is also not inhibited by the late precipitation of vanadium carbides. Thus, the strength-enhancing effect is based almost exclusively on precipitation hardening. One advantage of vanadium is its high solubility in austenite and the high volume fraction of fine precipitates caused by the low precipitation temperature. The optional V content is therefore limited to values ​​of 0.001 to 0.300 wt.%.

[0104] Boron (B) forms nitrides or carbides with nitrogen as well as with carbon; however, this is generally not the desired result. Firstly, due to their low solubility, only a small amount of precipitates forms, and secondly, these are mostly precipitated at the grain boundaries. An increase in surface hardness is not achieved (with the exception of boronizing, which forms FeB and Fe2B in the surface zone of a workpiece). To prevent nitride formation, attempts are generally made to bind the nitrogen with more affine elements. Titanium, in particular, can ensure the binding of all the nitrogen. In very small amounts in the dissolved state, boron leads to a significant improvement in hardenability. The mechanism of action of boron can be described as follows: with suitable temperature control, boron atoms accumulate at the grain boundaries and, by reducing the grain boundary energy, significantly impede the formation of growth-capable ferrite nuclei.When controlling the temperature, care must be taken to ensure that boron is predominantly distributed atomically within the grain boundaries and not present in the form of precipitates due to excessively high temperatures. The effectiveness of boron decreases with increasing grain size and rising carbon content (> 0.8%). A quantity above 60 ppm also causes a decrease in hardenability, as boron carbides act as nuclei at the grain boundaries. Due to its small atomic diameter, boron diffuses extremely well and has a very high affinity for oxygen, which can lead to a reduction in the boron content in areas close to the surface (up to 0.5 mm). In this context, annealing above 1000 °C is not recommended. This is also recommended because boron can lead to severe coarse grain formation at annealing temperatures above 1000 °C. For the reasons stated above, the optional B content is limited to values ​​of 0.0005 to 0.0050 wt.%.In the following, embodiments of the invention are explained using examples by means of figures and tables.

[0105] It shows:

[0106] Fig. 1 is a graphic representation of the temperature profile of a rolled steel strip and a plant heat-treating this strip during a heat treatment according to a preferred embodiment of the invention in a temperature-time diagram and

[0107] Fig. 2 The stress-strain curves of a steel strip designed according to the invention and the stress-strain curves of a comparison steel strip with a different steel composition.

[0108] In principle, the annealing treatments according to the invention can be performed in multiple stages, or additional annealing treatments can be provided as part of the overall process. An exemplary time-temperature cycle illustrating the characteristic temperature ranges for holding times, cooling rates, and heating rates is shown in Fig. 1. Accordingly, the temperature-time diagram in Fig. 1 plots temperature T (in °C) against time t in hours (h).

[0109] For this purpose, a rolled strip of steel of appropriate composition is formed into a compact shape, in particular rolled into a coil, which allows the strip to be transferred as a whole into a heat treatment apparatus (keywords: discontinuous annealing treatment / discontinuous annealing plant). There, in a first phase or first step S1, the sheet metal strip is heated to a temperature T > 750 °C within approximately 22 hours, i.e., at an average heating rate of approximately 25 K / h, whereby a total maximum temperature of at least 780 °C and at most 950 °C, preferably at most 850 °C, is reached during the heat treatment. Subsequently, in a second phase / second step S2, the strip is held at a temperature above 770 °C for approximately 15.5 hours using the apparatus. The strip is then cooled (overlapping phases / steps S3 and S4).During this cooling, the temperature range from 750°C to 200°C is traversed over a period of approximately 16 hours. This results in a third step S3 of cooling from 750°C to 200°C with an average cooling rate of approximately 35 K / h. As soon as the steel falls below the martensite start temperature, which depends particularly on the chemical composition of the steel, the phase transformation of austenite to martensite begins. To ensure that this freshly formed austenite is sufficiently tempered, which in turn results in good toughness, the strip sheet is held in the temperature range between the martensite start temperature Ms and 100°C for approximately 17 hours in a fourth step S4.

[0110] When the steel strip is cooled, the desired microstructure is achieved, resulting in a steel strip made of high-strength multiphase steel. Cooling occurs down to a certain temperature, preferably in a heat treatment facility. This is, for example, a bell-type annealing system. The example shown, at approximately 35 K / h, falls within a preferred cooling range of 20 K / h to 80 K / h.

[0111] Table 1 below lists exemplary material concepts, more specifically steel concepts, and their chemical composition in weight percent. Where an alloying element or the CEV value is not in accordance with the invention, this is indicated. In addition to the steel concepts according to the invention, which serve as input material for the inventive production of a product according to the invention in the form of a hot- or cold-rolled strip sheet, steel concepts that are not in accordance with the invention are also listed for comparison.

[0112] In particular, steels with Mo contents < 0.10 wt.% are not in accordance with the invention, as they form excessively high volumetric pearlite upon cooling. However, steel 8 contains Mo, but is nevertheless not in accordance with the invention, as it contains too low a total proportion of microalloying elements, as evidenced by the low CEV value of 0.55. This is particularly evident in steel 28, as it has the highest Mo content at 0.586, but also only has a non-inventive CEV of 0.55.

[0113] The parameters of a manufacturing process according to the invention are listed in Table 2. Process parameters that are not according to the invention are marked. Steels with a thickness of less than 2.0 mm were cold rolled in particular before annealing. Steels with a thickness of greater than 2.0 mm were not cold rolled. A forming operation carried out before heat treatment, such as cold rolling, is therefore not necessary for production according to the invention. According to Table 2, in particular those steels with manufacturing processes whose chemical composition is not according to the invention are not according to the invention. In process cycles 34, 45 and 57, the maximum annealing temperature of 766°C is not sufficient to form sufficient austenite, so that these process cycles are also not according to the invention.

[0114] The characteristics of the product according to the invention produced according to a manufacturing process according to the invention are listed in Table 3.

[0115] Chemical compositions according to the invention, combined with the corresponding process cycles according to the invention, also result in the steels according to the invention with the properties according to the invention. In particular, the combination of high tensile strength > 700 MPa and high toughness with a notched bar impact energy > 40 J at a test temperature of 20°C, the continuous yield strength R po.2 and the good ductility > 8% characterize the steels according to the invention. By way of example, the corresponding stress-strain curves for the inventive steel 0, produced according to production process 0, and the non-inventive steel 2, produced according to production process 3, can be seen in Fig. 2. In the stress-strain curve diagram in Fig. 2, the stress o in MPa is plotted against the strain E in %. The curve A with a dash-dot notation results from steel 2 and process 3 and is therefore an example of a comparative example. The curve B noted with a solid line results from steel 0 and process 0 and is therefore an example of an inventive example. The inventive steel 0 has in particular a high tensile strength > 700 MPa, a continuous yield strength, good ductility and a low yield strength ratio Rp0.2 / Rm.The non-inventive steel 2 has an undesirably pronounced yield strength and a low tensile strength of approximately 600 MPa.

[0116] Table 1 *Underlined values ​​indicate properties not according to the invention

[0117] Table 1 (continued) *Underlined values ​​indicate properties not according to the invention

[0118] Table 1 (continued) *Underlined values ​​indicate properties not according to the invention

[0119]

[0120] Table 2 (continued)

[0121] Table 2 (continued)

[0122] Table 3

[0123] *Underlined values ​​indicate properties not according to the invention

[0124] Table 3 (continued)

[0125] *Underlined values ​​indicate properties not according to the invention, ** Instead of an Asomm- an As-tensile specimen was tested

[0126] Table 3 (continued) *Underlined values ​​indicate properties not according to the invention

Claims

Patent claims 1. A process for producing a steel strip from a high-strength multi-phase steel having a tensile strength of at least 700 MPa in the longitudinal direction, wherein a rolled strip sheet made of steel having the following chemical composition in weight %: C > 0.080 to < 0.350, Mn > 0.80 to < 3.50, Mo > 0.10 to < 1.00, N > 0.0020 to < 0.0160, S < 0.020, Optionally one or more of the following elements: Cr > 0.050 to < 1.0, P < 0.050, Cu > 0.001 to < 1.0, Si > 0.05 to < 1.5, Al > 0.0030 to < 1.0, Ni > 0.03 to < 1.50, Nb > 0.005 to < 0.150, Ti > 0.005 to < 0.150, V > 0.001 to < 0.300, B > 0.0005 to < 0.0050 and Ca > 0.0005 to < 0.0060, The remainder being iron, including usual impurities associated with steel melting, and having a carbon equivalent CEV which is greater than 0.570 and less than 0.900, wherein the carbon equivalent CEV is calculated according to the following formula CEV = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 from the contents of the corresponding elements in weight%, as a whole - in particular wound into a coil - is heat-treated (S1 - S4) in such a way that it assumes a temperature above 770°C for a period of more than 30 minutes (S2), and after this heat treatment is cooled to a temperature below 100°C, wherein the cooling (S3, S4) takes place between 750°C and 200°C with an average cooling rate of more than 1 K / h and less than 300 K / h (S3) and in the temperature range from a martensite start temperature M s up to 100°C in a time greater than 10 h (S4), where the numerical value [M s] the martensite start temperature M s in °C based on the numerical values ​​of the respective alloying proportions of the elements C, Mn, Ni, Cr, Si, and Mo in wt.%, determined according to the following formula: [M s ] = 539 - 423 [C] - 30.4[Mn] - 17.7[JVi] - 12.1[Cr] - 11.0[Si] - 7.0[Mo] 2. Method according to claim 1, characterized in that the strip sheet is heated during the heat treatment with an average heating rate between 1 K / h and 150 K / h from 200°C to a temperature of 750°C (S1).

3. A method according to claim 1 or 2, characterized in that the steel strip sheet reaches a maximum temperature of at least 780°C and at most 950°C, preferably at most 850°C, during the heat treatment and the heat treatment preferably takes place in a non-decarburizing atmosphere (S2).

4. Method according to one of claims 1 to 3, characterized in that the steel strip is provided with a surface coating in the form of a metallic coating, organic coating or paint after cooling.

5. Method according to one of claims 1 to 4, characterized in that the steel strip is plastically deformed after cooling - in particular by stretch-bend straightening, bend-straightening or temper-passing - in such a way that the quotient R p o,2 / Rm from the yield strength and tensile strength of the steel strip is increased by at least 0.05 and the steel strip then has a resulting ratio of yield strength R p o,2 to tensile strength R m between 0.50 and 0.

90.

6. A method according to any one of claims 1 to 5, wherein the chemical composition of the steel of the rolled strip sheet corresponds to at least one of the following: Elements in weight%: Cr > 0.300 to < 0.700, Mo > 0.150 to < 0.60 Mn < 2.70 Si > 0.25 < 0.80 and Cu < 0.

20.

7. Steel strip made of a high-strength multi-phase steel, which has a tensile strength of at least 700 MPa in the longitudinal direction, good toughness and a continuous yield strength, in particular manufactured according to at least one or more Process according to claims 1 to 7, wherein the multi-phase steel has the following chemical Composition in weight%: C > 0.080 to < 0.350, Mn > 0.80 to < 3.50, Mo > 0.10 to < 1.00, N > 0.0020 to < 0.0160, S < 0.020, Optionally one or more of the following elements: Cr > 0.050 to < 1.0, P < 0.050, Cu > 0.001 to < 1.0, Si > 0.05 to < 1.5, Al > 0.0030 to < 1.0, Ni > 0.03 to < 1.50, Nb > 0.005 to < 0.150, Ti > 0.005 to < 0.150, V > 0.001 to < 0.300, B > 0.0005 to < 0.0050 and Ca > 0.0005 to < 0.0060, The remainder being iron, including usual impurities associated with steel melting, and having a carbon equivalent CEV which is greater than 0.570 and less than 0.900, wherein the carbon equivalent CEV is calculated according to the following formula CEV = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 from the contents of the corresponding elements in weight%, whereby the multi-phase steel has a structure per % steel thickness in which the sum of the volume fractions of the structural components martensite, tempered martensite, retained austenite, M / A phase, upper bainite and / or lower bainite is at least 25.0 volume% and the residual structure consists of ferrite and pearlite.

8. Steel strip according to claim 7, characterized in that the proportion of residual austenite is less than 1% by volume and the pearlite content is below 2% by volume.

9. Steel strip according to claim 7 or 8, characterized in that the ratio of yield strength to tensile strength R p o.2 / R m is below 0.70 and the elongation at break Aso is > 8% in the longitudinal direction.

10. Steel strip according to one of claims 7 to 9, characterized in that the Steel strip has at least one of the following properties: a notched bar impact energy at a test temperature of -20°C > 40 J, whereby the test is carried out on impact bending specimens according to DIN EN ISO 148-1 and longitudinal to the rolling direction, and a notched bar impact energy at a test temperature of -40°C > 27 J, whereby the test is carried out on Notched bar impact tests are carried out according to DIN EN ISO 148-1 and along the rolling direction.

Images