Method of manufacturing a steel strip having a multiphase structure and steel strip

Abstract

To provide a method for manufacturing a steel strip having a multiphase structure, using which complex component geometries having a high edge-crack resistance at the same time having a high energy absorption capacity can be manufactured, in particular using which a drop in the lower elastic limit is compensated and thus a combination of a high lower elastic limit or a high lower elastic limit ratio with a high tensile at break is achieved, it is proposed that the method comprises the following steps: - manufacturing a hot-rolled or cold-rolled steel strip from a steel consisting of the following elements in percent by weight: C: 0.085 to 0.149; Al: 0.005 to 0.1 ; Si: 0.2 to 0.75; Mn: 1.6 to 2.9; P: < 0.02; S: < 0.005; and optionally from one or more of the following elements in percent by weight: Cr: 0.05 to 0.5; Mo: 0.05 to 0.5; Ti: 0.005 to 0.060; Nb: 0.005 to 0.060; V: 0.001 to 0.060; B: 0.0001 to 0.0060; N: 0.0001 to 0.016; Ni: 0.01 to 0.5; Cu: 0.01 to 0.3; the remainder being iron, including the usual elements accompanying the steel, - first annealing at a temperature between 750 °C to 950 °C inclusive for a total duration of 10 s to 1200 s, in particular 50 s to 650 s, in particular continuous annealing of the steel strip, in particular of a cold-rolled steel strip, and then first cooling the steel strip at an average cooling rate of 2 K / s to 150 K / s, in particular 5 K / s to 100 K / s, to a temperature between 200 °C to 500 °C inclusive, - further cooling the steel strip to a subcooling temperature below 100 °C at an average cooling rate of 1 K / s to 50 K / s, - final annealing, in particular continuous annealing, of the steel strip in Hollomon-Jaffe parameters, wherein the maximum temperature T H is 100 °C to 470 °C inclusive in K, the total duration is 2 s to 1000 s inclusive in h, - final cooling of the steel strip to room temperature at an average cooling rate of 1 K / s to 160 K / s, in particular 1 K / s to 30 K / s. The invention also relates to a steel strip having a multiphase structure manufactured by the method.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing steel strip with a multiphase structure and the steel strip having a multiphase structure. Background Technology

[0002] Steel strip is understood below as hot-rolled or cold-rolled and annealed steel strip. Common thicknesses for hot-rolled steel strip (also known as hot-rolled strip) are between 2 mm and 8 mm. Cold-rolled and annealed steel strip is called cold strip or sheet and has common thicknesses between 0.5 mm and 2.5 mm.

[0003] The fiercely competitive automotive market constantly forces manufacturers to find solutions to reduce fleet fuel consumption and CO2 emissions while maintaining maximum possible comfort and occupant protection. In this regard, weight reduction of all vehicle components plays a crucial role, but so does the optimal performance of each component under high static and dynamic loads during operation and in the event of a collision.

[0004] Steel suppliers are addressing these challenges by providing high-strength steel. Furthermore, by offering high-strength steel with lower plate thicknesses, the weight of vehicle components can be reduced while maintaining or even improving component performance.

[0005] In addition to the required weight reduction, these newly developed steels must also meet high material requirements in terms of elastic limit, tensile strength and elongation at break, and bake hardening, as well as high component requirements in terms of toughness, edge cracking sensitivity, improved bending angle and bending radius, energy absorption, and hardening as defined by the work hardening effect.

[0006] In addition, good machinability must be ensured. This affects both the processes performed by automakers, such as stamping and forming, optional hot tempering and subsequent optional tempering, welding and / or surface finishing, such as phosphating and cathodic immersion coating, and the manufacturing processes performed by raw material suppliers, such as surface refining through metal or organic coatings.

[0007] There is also an increasing demand for improved joint suitability, such as better general weldability (e.g., a larger usable weld area in resistance spot welding), improved failure characteristics of the weld (fracture mode) under mechanical stress, and high resistance to liquid metal embrittlement (LME). Furthermore, sufficient resistance to delayed hydrogen embrittlement (i.e., delayed fracture-free) is sought. This also applies to the weldability of high-strength steels in pipe manufacturing, for example, using high-frequency induction welding (HFI) methods.

[0008] Depending on the application, the automotive industry uses yield strength (R) e Or elastic limit R p0.2 With tensile strength R m In terms of ratios, increasingly significant different requirements are being placed on steel types.

[0009] The required combination of properties for steel ultimately represents a trade-off for the specific individual characteristics of a component. However, with the increasing complexity of component geometries, these properties are often no longer sufficient.

[0010] For duplex steels, a typical characteristic is a low yield strength ratio (Ri) of, for example, less than 0.6. e / R m It has high tensile strength, strong strain hardening, and good cold formability. These properties are mainly used for formability in stretching and deep drawing processes.

[0011] Dual-phase steels consist primarily of ferrite with embedded martensitic second phases. It has been found that, in the case of low-carbon microalloyed steels, small proportions of other phases (e.g., bainite and retained austenite) advantageously influence characteristics such as porosity, bending properties, and hydrogen-induced brittle fracture properties. In this case, bainite can exist in different morphologies, such as upper bainite and lower bainite.

[0012] For complex or multiphase steels, a typically high yield strength ratio R is present. e / R m They are particularly outstanding for their high resistance to edge cracking. This can be attributed to the small strength differences among the various microstructure components, which advantageously influences uniform deformation in the cut edge region. These steels also exhibit high energy absorption capacity under impact conditions, thus these complex or multiphase steels are increasingly used in automobile manufacturing. The multiphase microstructure is characterized by a predominantly ferrite-bainite matrix, within which a certain proportion of martensite, tempered martensite, retained austenite, and / or pearlite may also be present. Delayed recrystallization or precipitation of microalloying elements leads to intense grain refinement (i.e., fine-grained microstructure) and thus high strength.

[0013] Compared to dual-phase steels, these complex or multiphase steels exhibit higher yield strength, higher yield strength or elastic limit ratio, lower strain hardening, and higher pore-expanding capacity. Therefore, these steels are well-suited for manufacturing components with complex geometries, particularly those subjected to impact loads requiring high energy absorption capacity.

[0014] Multiphase steels are known, for example, from published documents DE 10 2012 002 079 A1 and DE 10 2015 111 177 A1. While the material properties disclosed there are already representative of relatively complex component geometries, a higher elastic limit ratio is needed to achieve even more complex component geometries, while also possessing high resistance to edge cracking and high energy absorption capacity.

[0015] Patent specification JP 5 741456 B2 and published documents US 2019 / 003002 A1, EP 3 476 963A1 and DE 10 2017 123236 A1 also describe the production of cold-rolled or hot-rolled flat steel products from multiphase steels with defined tensile strength and target improved elongation.

[0016] To produce thin sheets, cold-rolled steel strip is typically annealed in a continuous annealing process to recrystallize and become easily formable sheets for economic reasons. Depending on the alloy composition and strip cross-section, process parameters such as running speed, annealing temperature, and cooling rate must be set to correspond to the required mechanical properties and the microstructure needed for this purpose.

[0017] In order to obtain a fine-grained microstructure after a continuous annealing process, it is known to set a minimum cold rolling degree based on the recrystallization temperature in order to set a corresponding dislocation density for recrystallization annealing.

[0018] If the cold rolling degree is too low—even in localized areas—the critical threshold for recrystallization cannot be overcome, making it impossible to achieve a fine grain and relatively uniform microstructure. Due to the varying grain sizes in the cold zone, different grain sizes will appear in the final microstructure even after recrystallization, leading to fluctuations in eigenvalues. Grains of different sizes can transform into different phase compositions after cooling from the furnace temperature, providing further inhomogeneity.

[0019] To achieve the desired microstructure, the cold strip is heated in a continuous annealing furnace to a temperature at which the desired microstructure (e.g., a duplex or complex phase microstructure) is produced during cooling.

[0020] If the surface of the cold strip should be hot-dip galvanized due to high corrosion resistance requirements, the annealing treatment is usually carried out in a continuous hot-dip galvanizing facility, where heat treatment or annealing and subsequent galvanizing are carried out in a continuous process.

[0021] In tropical conditions, the required microstructure is sometimes set during annealing in a continuous furnace, depending on the alloy concept, to achieve the desired mechanical properties.

[0022] A drawback of these multiphase or complex-phase steels has been found to be that, although a high elastic limit ratio can be achieved by austenitizing annealing of the hot or cold zones in a continuous furnace, this comes at the cost of a lower tensile strength at break (A) compared to dual-phase steels. 80 This is achieved at a cost. If a high tensile strength at break (A) is required... 80 If this is not achieved, a high elastic limit ratio cannot be reliably set in the process. This is because, during large-scale continuous annealing, depending on the alloy concept, the austenite-to-bainite transformation does not fully occur. At temperatures between 200°C and 500°C, the retained austenite is carbon-rich in the holding region and thus stabilized. Then, by finally cooling to a temperature below 100°C, the remaining austenite transforms into martensite (fresh martensite). Due to the formation of fresh martensite and the resulting shear deformation, slippery dislocations are generated in the surrounding microstructure, which, from a technical perspective, manifest as R... p0.2 The elastic limit is reduced and the sensitivity to edge cracking is increased. Summary of the Invention

[0023] Therefore, the objective of this invention is to describe a method for manufacturing steel strip with a multiphase microstructure and a steel strip with a multiphase microstructure, which enables the manufacture of complex component geometries with high energy absorption capacity and high edge crack protection. Specifically, the method should compensate for the decrease in elastic limit and thus achieve a combination of high elastic limit or high elastic limit ratio and high tensile strength at break. The corresponding cold-rolled or hot-rolled steel strips should also be described.

[0024] This task is solved by the method for manufacturing steel strip with a multiphase microstructure according to claim 1 and the method for manufacturing steel strip with a multiphase microstructure according to claim 21. Advantageous designs of the invention are described in the dependent claims.

[0025] According to the teachings of the present invention, a method for manufacturing steel strip having a multiphase structure includes the following steps:

[0026] - Hot-rolled or cold-rolled steel strip is manufactured from steel, said steel being composed of the following elements by weight percentage: C: 0.085 to 0.149; Al: 0.005 to 0.1; Si: 0.2 to 0.75; Mn: 1.6 to 2.9; P: ≤0.02; S: ≤0.005; and optionally composed of one or more of the following elements by weight percentage: Cr: 0.05 to 0.5; Mo: 0.05 to 0.5; Ti: 0.005 to 0.060; Nb: 0.005 to 0.060; V: 0.001 to 0.060; B: 0.0001 to 0.0060; N: 0.0001 to 0.016; Ni: 0.01 to 0.5; Cu: 0.01 to 0.3; the remainder being iron, including common elements associated with steel.

[0027] - The steel strip, particularly cold-rolled steel strip, is first annealed, particularly continuously annealed, at a temperature between 750°C and 950°C (inclusive) for a total duration of 10 to 1200 seconds, particularly 50 to 650 seconds, and then the steel strip is first cooled to a temperature between 200°C and 500°C (inclusive) at an average cooling rate of 2 K / s to 150 K / s, particularly 5 K / s to 100 K / s.

[0028] - The steel strip is further cooled to a supercooled temperature below 100°C at an average cooling rate of 1 K / s to 50 K / s.

[0029] - Using Hollomon-Jaffe parameters The steel strip is subjected to final annealing, particularly continuous annealing, wherein the highest temperature T, expressed in K, is... H The total duration in hours, ranging from 100℃ to 470℃ (inclusive). The duration is from 2 seconds to 1000 seconds (inclusive).

[0030] - By cooling the steel strip to room temperature at an average cooling rate of 1 K / s to 160 K / s, particularly 1 K / s to 30 K / s, a high-strength and high-ductility steel strip made of multiphase steel is achieved according to the present invention.

[0031] Advantageously, the elastic limit can be variably adjusted according to process parameters through the final annealing and the final cooling, and the R of the steel strip after final annealing can be achieved. p0.2 Elastic limit and tensile strength R of the steel strip after final annealing m A high percentage.

[0032] Furthermore, the steel strip according to the invention exhibits good weldability and low susceptibility to liquid metal and hydrogen embrittlement. These and other advantages of the steel strip according to the invention are achieved through alloying concepts and specialized processes. This steel strip is particularly suitable for manufacturing components that thereby possess improved formability, enhanced energy absorption capacity, and improved weldability.

[0033] In the method for manufacturing steel strip according to the present invention, the two process steps of "final annealing and final cooling" can be directly adjacent in terms of time and location, or they can be staggered by hours or days or occur in different locations, depending on the circumstances.

[0034] The reference steel is shown in Table 1 below. A I and B II With the example steel C according to the invention III ,、D IV ,、D V E VI F VII To G VIIIA comparison of the corresponding alloy compositions. Example steel D. IV The alloy compositions of Dv are the same, only differing indices are provided for later description. The essential difference between the example steel according to the invention and the reference steel is the lower carbon content, which improves weldability and minimizes sensitivity to liquid metal and hydrogen embrittlement. Reference Steel A I and B II This is not according to the present invention because the carbon content is too high. This results in poor weldability. Furthermore, the tensile strength is too low (less than 920 MPa).

[0035] Reference steel A I and B II It also does not react as effectively to the treatment according to the present invention.

[0036] The role of each element in the multiphase steel strip according to the present invention is described in more detail below. Multiphase steels are typically chemically constructed such that alloying elements, with or without microalloying elements, are combined. Associated elements are unavoidable, and their influence is considered in the analytical concept when necessary.

[0037] Associated elements are elements that are already present in iron ore or enter steel due to manufacturing processes. They are generally undesirable due to their primary negative impacts. Attempts are made to remove associated elements to tolerable levels or convert them to harmless forms.

[0038] Hydrogen (H) is the only element that can diffuse through an iron lattice without inducing lattice strain. This allows hydrogen to move relatively freely within the iron lattice and makes it relatively easy to absorb during manufacturing. Here, hydrogen can only be absorbed into the iron lattice in atomic (ionic) form. Hydrogen has a strong embrittlement effect and preferentially diffuses to energy-favorable sites (defects, grain boundaries, etc.). Here, defects act as hydrogen traps and can significantly increase the residence time of hydrogen in the material. Cold cracking can occur by recombination into molecular hydrogen. This behavior occurs in cases of hydrogen embrittlement or hydrogen-induced stress corrosion cracking. Hydrogen is also frequently considered a cause of delayed-fracture, which occurs in the absence of external stress. Therefore, the hydrogen content in steel should be as low as possible.

[0039] Oxygen (O): In the molten state, steel has a relatively high absorption capacity for gases, but only a very small amount of oxygen dissolves at room temperature. Similar to hydrogen, oxygen can only diffuse into the material in atomic form. Due to its strong embrittlement effect and negative impact on aging resistance, efforts are made to minimize oxygen content during manufacturing. To reduce oxygen, methods such as vacuum treatment and analytical techniques exist. Oxygen can be converted into a harmless state by adding specific alloying elements. Therefore, combining oxygen with manganese, silicon, and / or aluminum is common practice. However, the resulting oxides can act as defects in the material, leading to negative properties. Conversely, in the case of fine precipitation, especially fine precipitation of alumina, grain refinement may also occur. Therefore, for these reasons, the oxygen content in steel should be as low as possible.

[0040] Nitrogen (N) is also a byproduct of steelmaking. Steel containing free nitrogen tends to have a strong aging effect. At low temperatures, nitrogen diffuses into dislocations and blocks them. Therefore, nitrogen leads to increased strength but a rapid loss of toughness. Nitrogen can be incorporated into aluminum or titanium alloys in the form of nitrides. For the reasons mentioned above, the optional nitrogen content is limited to ≤0.016% by weight or an unavoidable amount in steelmaking.

[0041] Like phosphorus, sulfur (S) is bound as a trace element in iron ore. Sulfur is undesirable in steel (except for free-machining steels) because it tends to segregate strongly and has a strong embrittlement effect. Therefore, efforts are made to achieve the lowest possible sulfur content in the melt (e.g., through deep vacuum processing). Furthermore, by adding manganese, existing sulfur is converted into the relatively harmless compound manganese sulfide (MnS). Manganese sulfide is typically rolled out in rows during rolling and acts as nuclei for transformation. Especially in the case of diffusion-controlled transformation, this results in a distinct linear microstructure, and in cases of extremely pronounced linear structures, it can lead to deterioration of mechanical properties (e.g., prominent martensitic lines instead of distributed martensitic islands, anisotropic material properties, reduced tensile strength at break). For the above reasons, sulfur content is limited to ≤0.005% by weight or an unavoidable amount in steel manufacturing.

[0042] Phosphorus (P) is a trace element in iron ore and dissolves as a substitutional atom in the iron lattice. Phosphorus enhances hardness and hardenability through solid solution strengthening. However, it is generally desirable to minimize phosphorus content as much as possible because phosphorus tends to segregate strongly, especially due to its low diffusivity, and significantly reduces toughness. Grain boundary fracture occurs due to phosphorus accumulation at grain boundaries. Furthermore, phosphorus can raise the transition temperature from ductile to brittle properties to up to 300°C. During hot rolling, near-surface phosphorus oxides can cause fracture tearing at grain boundaries. The negative effects of phosphorus can be partially compensated by adding small amounts of boron through alloying. Boron is believed to improve grain boundary cohesion and reduce phosphorus segregation at grain boundaries. However, in some steels, boron is used in small amounts (<0.1%) as a microalloying element due to its low cost and high strength increase, such as in high-strength interstitial-free (IF) steels. For the reasons mentioned above, phosphorus content is limited to ≤0.020% or an unavoidable amount in steel manufacturing.

[0043] Alloying elements are typically added to steel to specifically influence particular properties. Here, alloying elements can affect different properties in different types of steel. The relationships are diverse and complex. The roles of alloying elements should be discussed in more detail below.

[0044] Carbon (C) is considered the most important alloying element in steel. Iron can only be transformed into steel by the targeted introduction of up to 2.06% carbon. During steelmaking, the carbon content typically decreases sharply. In the case of the multiphase steel according to the invention, particularly for continuous hot-dip refining, the carbon proportion is from 0.085% to 0.149% by weight, preferably up to 0.115% by weight. Due to its relatively small atomic radius, carbon dissolves intermittently in the iron lattice. Here, the maximum solubility is 0.02% in α-iron and 2.06% in γ-iron. Carbon significantly improves the hardenability of steel in its dissolved form. Due to the different solubilities, a significant diffusion process is required during the phase transformation, which can lead to very different kinetic conditions. Furthermore, carbon improves the thermodynamic stability of austenite, which is manifested in the phase diagram as the austenitic region extending to lower temperatures. With the increase of the carbon content forcibly dissolved in martensite, the lattice distortion and the resulting intensity of diffusionless phase formation increase. Carbon is also required to form carbides. A representative example found in almost every type of steel is cementite (Fe3C). However, it can also form significantly harder special carbides with other metals such as chromium, titanium, niobium, and vanadium. Here, not only the type of precipitate, but also its distribution and size, are of decisive importance to the resulting increase in strength. Therefore, to ensure both sufficient strength and good weldability, a minimum C content is set at 0.085% by weight, and a maximum C content is set at 0.149% by weight, preferably 0.115% by weight.

[0045] Aluminum (Al) is typically added to steel through alloying to combine with oxygen and nitrogen dissolved in the iron. The oxygen and nitrogen are then converted into aluminum oxide and aluminum nitride. These precipitates can lead to grain refinement by increasing the number of nuclei, thereby improving toughness and strength values. Aluminum nitride does not precipitate when a sufficient amount of titanium is present. Titanium nitride has a lower enthalpy of formation and forms at higher temperatures. In the dissolved state, aluminum, like silicon, shortens the ferrite formation time, allowing sufficient ferrite to form. Aluminum also inhibits carbide formation, resulting in a delayed austenite transformation. For this reason, Al is also used as an alloying element in retained austenitic steels to replace some of the silicon. This approach is adopted because Al is slightly less important for the galvanizing reaction than Si. Therefore, the Al content is limited to 0.005% by weight to a maximum of 0.1% by weight.

[0046] Silicon (Si) binds to oxygen during casting, thereby reducing segregation and contamination in steel. Furthermore, silicon enhances the strength and yield strength ratio of ferrite through solid solution strengthening, while only slightly reducing the tensile strength at break. Another important effect is that silicon shortens the ferrite formation time, allowing sufficient ferrite to be generated before quenching. Through ferrite formation, austenite becomes carbon-enriched and stabilized. At higher content levels, silicon significantly stabilizes austenite in the low-temperature range, particularly in the bainite formation region, by preventing carbide formation. During hot rolling, a highly adhesive oxide scale may form at high silicon content, which can affect further processing. In continuous galvanizing, silicon may diffuse to the surface during annealing and form film-like oxides, alone or with manganese. When the steel strip is immersed in molten zinc, these oxides worsen galvanizability by weakening the galvanizing reaction (dissolving iron and forming an inhibitory layer). This manifests as poor zinc adhesion and ungalvanized areas. However, good galvanizability and good zinc adhesion of the steel strip can be ensured through proper furnace operation, appropriate moisture content in the annealing gas, and / or through a low Si / Mn ratio and / or by using an appropriate amount of silicon. For the above reasons, the minimum Si content is set at 0.200% by weight, and the maximum Si content is set at 0.750% by weight.

[0047] Manganese (Mn) is added to almost all steels for desulfurization, converting harmful sulfur into manganese sulfide. Furthermore, manganese enhances the strength of ferrite through solid solution strengthening and lowers the transformation temperature. The primary reason for adding manganese through alloying is the significant improvement in hardenability. Due to hindered diffusion, the transformation times of pearlite and bainite are prolonged, while 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 content of other alloying elements (especially Si and Al), oxides of manganese (e.g., MnO) and / or mixed oxides of Mn (e.g., Mn₂SiO₄) may occur. However, at low Si / Mn or Al / Mn ratios, manganese is considered less important because spherical oxides are formed rather than an oxide film. However, high manganese content negatively impacts the appearance and zinc adhesion of the zinc layer. Therefore, the Mn content is set from 1.6 wt% to 2.9 wt%, preferably up to 2.6 wt%.

[0048] Chromium (Cr): The addition of chromium primarily improves hardenability. In its molten state, chromium prolongs the transformation times of pearlite and bainite while lowering the martensite initiation temperature. Another important effect is that chromium significantly increases tempering resistance, resulting in almost no strength loss in the zinc bath. Chromium is also a carbide former. If chromium is present in carbide form, the austenitizing temperature before hardening must be chosen high enough to dissolve the chromium carbides. Otherwise, hardenability may deteriorate due to the increased number of crystal nuclei. During annealing, chromium also tends to form oxides on the steel surface, which can reduce the quality of zinc plating. Therefore, the optional Cr content is set to a value between 0.05 wt% and 0.500 wt%.

[0049] Molybdenum (Mo): The addition of molybdenum is similar to that of chromium to improve hardenability. It prolongs the transformation time of pearlite and bainite and lowers the martensite initiation temperature. Molybdenum also significantly increases tempering resistance, thus preventing strength loss in the zinc bath, and improves the strength of ferrite through solid solution strengthening. The Mo content is added based on size, facility configuration, and microstructure. For cost reasons, an optional Mo content of 0.05 to 0.5% by weight is set.

[0050] Copper (Cu): The addition of copper can increase tensile strength and hardenability. Copper, when combined with nickel, chromium, and phosphorus, can form a protective oxide layer on the surface, which can significantly reduce corrosion rates. Copper combined with oxygen can form harmful oxides at grain boundaries, which can have negative effects, particularly on hot forming processes. Therefore, the optional copper content is limited to 0.01 to 0.3% by weight.

[0051] Nickel (Ni): Nickel can combine with oxygen to form harmful oxides at grain boundaries, which can have negative effects, particularly on hot forming processes. Therefore, the optional nickel content is limited to 0.01 to 0.050% by weight.

[0052] Microalloying elements are typically added in very small amounts (<0.1%). Unlike alloying elements, microalloying elements primarily function by forming precipitates, but can also affect properties in the dissolved state. Despite their small addition, microalloying elements strongly influence manufacturing conditions, as well as processing characteristics and final properties. Carbide and nitride formations soluble in the iron lattice are commonly used as microalloying elements. Carbonitrides can also form due to the complete solubility of nitrides and carbides in each other. The tendency to form oxides and sulfides is usually most pronounced in microalloying elements, but is often selectively inhibited by other alloying elements. This property can be actively utilized in such cases that sulfur and oxygen, which are typically detrimental, can combine. However, this combination can also have negative effects if there are no longer sufficient microalloying elements available for carbide formation. Typical microalloying elements are aluminum, vanadium, titanium, niobium, and boron. These elements can dissolve in the iron lattice and form carbides and nitrides with carbon and nitrogen.

[0053] Titanium (Ti) forms very stable nitrides (TiN) and sulfides (TiS2) at high temperatures. These nitrides (TiN) and sulfides (TiS2) are only partially dissolved in the melt, depending on the nitrogen content. If these precipitates are not removed with the slag, they form coarse particles in the material due to the high temperatures, which are generally detrimental to mechanical properties. The combination of free nitrogen and oxygen has a positive effect on toughness. Thus, titanium protects other dissolved microalloying elements (such as niobium) from combining with nitrogen. These microalloying elements can then exert their effects optimally. Nitrides that form at lower temperatures due to the reduced oxygen and nitrogen content can also effectively inhibit austenite grain growth. Unbonded titanium forms titanium carbide at temperatures above 1150°C, which can lead to grain refinement (inhibiting austenite grain growth by delaying recrystallization and / or increasing the number of nuclei during the α / γ transformation) and precipitate hardening. Therefore, the optional Ti content has a value of 0.005 to 0.060% by weight.

[0054] Niobium (Nb) causes strong grain refinement because it is the most effective of all microalloying elements in delaying recrystallization and inhibiting austenite grain growth. The strength increase effect is estimated to be qualitatively greater than that caused by titanium, which is evident in the increased grain refinement effect and the larger number of strength-increasing particles (titanium combines to form TiN at high temperatures). Niobium carbides form at temperatures below 1200 °C. When nitrogen combines with titanium, niobium can enhance its strength-increasing effect by forming smaller carbides (smaller carbide sizes), the effect of which is effective in the low-temperature range. Another effect of niobium is the delay of the α / γ transformation and a lowering of the martensite initiation temperature in the dissolved state. This is due, on the one hand, to the solute dragging effect, and on the other hand, to grain refinement. Grain refinement leads to increased strength of the microstructure, thereby also resulting in greater resistance to volumetric expansion during martensite formation. In principle, the alloying addition of niobium is limited until its solubility limit is reached. While this solubility limit restricts the amount of precipitates, exceeding it can lead to the early formation of precipitates with very coarse particles. Therefore, precipitate hardening can become effective, particularly for steels with low carbon content (potentially with greater supersaturation) and during hot forming processes (deformation-induced precipitates). Thus, the Nb content is limited to a value of 0.005 to 0.060% by weight.

[0055] Vanadium (V): Vanadium carbides and nitrides begin to form at temperatures around 1000 °C or after the α / γ transformation, much later than in the case of titanium and niobium. Therefore, due to the small amount of precipitates present in austenite, vanadium has almost no grain-refining effect. Austenite grain growth is also not inhibited by the late precipitation of vanadium carbides. Thus, the strength-increasing effect is almost entirely based on precipitation hardening. One advantage of vanadium is its high solubility in austenite and the large volumetric proportion of fine precipitates due to its low precipitation temperature. Therefore, the optional V content is limited to values ​​from 0.001 to 0.060 wt%.

[0056] Boron (B) forms nitrides with nitrogen and carbides with carbon; however, this is generally not the goal. On the one hand, only a small amount of precipitates form due to its low solubility; on the other hand, most of these precipitates form at grain boundaries. No increase in surface hardness is achieved (except for the formation of FeB and Fe2B borides in the edge regions of the workpiece). To prevent nitride formation, attempts are usually made to combine nitrogen with elements that have a higher affinity. Titanium, in particular, can guarantee the binding of all nitrogen. Boron, in its dissolved state, leads to a significant improvement in hardenability in very small amounts. The mechanism of boron's action can be described as boron atoms accumulating at grain boundaries under appropriate temperature control, making the formation of ferrite nuclei that can grow more difficult by lowering the grain boundary energy. At this temperature control, care must be taken that boron is distributed primarily in atomic form at the grain boundaries and does not exist as precipitates due to excessively high temperatures. The effectiveness of boron decreases with increasing grain size and carbon content (>0.8%). Furthermore, amounts exceeding 60 ppm lead to reduced hardenability because boron carbide acts as nucleation sites at grain boundaries. Due to its small atomic diameter, boron diffuses very well and has a very high affinity for oxygen, which can result in reduced boron content in areas near the surface (up to 0.5 mm). In this regard, annealing at temperatures exceeding 1000°C is not recommended. Moreover, this is also discouraged because annealing temperatures above 1000°C can drastically lead to the formation of coarse grains. Boron is an extremely critical element in the continuous hot-dip refining process of zinc because even in minimal amounts, boron, alone or together with manganese, forms a thin film of oxide on the steel surface during annealing. These oxides passivate the steel strip surface and inhibit the zinc plating reaction (dissolving iron and forming an inhibitory layer). Whether a thin film of oxide forms depends both on the amount of free boron and manganese and on the annealing parameters used (e.g., moisture content in the annealing gas, annealing temperature, annealing time). Higher manganese content and longer annealing times tend to result in the formation of spherical and less significant oxides. Increasing the moisture content in the annealing gas can also reduce the amount of boron oxides on the steel surface. For the reasons mentioned above, the boron content is limited to a value of 0.0001 to 0.0060% by weight.

[0057] The specification states that, by means of the method according to the invention, the R of the steel strip after final annealing and final cooling... p0.2 The elastic limit value relative to the R of the steel strip before final annealing p0.2 The elastic limit value should be increased by at least 5%, especially 10%.

[0058] According to the present invention, the R of the steel strip is restored by final annealing and final cooling. p0.2 The elastic limit is determined using one or more of the following conditions:

[0059] (1) In-situ deformation of surrounding tissues through martensite and / or lower bainite

[0060] (2)Optional additional ex-situ deformation by tempering cold rolling and / or stretching of the steel strip

[0061] (3) Sufficiently high temperature and time for carbon diffusion during final annealing.

[0062] (4) Sufficient carbon concentration in a supersaturated solution, for example, by inhibiting cementite precipitation.

[0063] (5) Small grain size and optional structure of component dispersion / short carbon diffusion path.

[0064] Furthermore, it is stipulated that the R of the steel strip after final annealing and final cooling... p0.2 The elastic limit value relative to the R of the steel strip before final annealing p0.2 The elastic limit value increases by at least 5% to 50% (inclusive), and particularly to 40% (inclusive).

[0065] In this case, it is particularly advantageous to use Hollomon-Jaffe parameters Hp = 9 × 10. 3 The steel strip that is finally annealed and then finally cooled has an R-value after final cooling. p0.2 The elastic limit value, which is relative to the R of the steel strip before final annealing. p0.2 The elastic limit value has increased by at least 15%.

[0066] Hollomon-Jaffe parameters are defined as follows: Where T H In units of K, The unit is h. This parameter represents the highest temperature T of the final annealing. H Logically related to the total duration (see, for example, A. Kamp, S. Celotto, DNHanlon; Mater. Sci. Eng. A538 (2012) 35-41). The Hollomon-Jaffe parameter includes the natural logarithm ln(x).

[0067] When calculating Hp according to the present invention, the highest temperature reached on the surface of the steel strip during the final annealing is set as the highest temperature T. H The highest temperature T H The effect of increased Hp value and elastic limit, or the metal physical processes occurring according to the present invention, are decisive. Therefore, the lower temperatures during the heating phase of the final annealing are neglected. Total duration In this case, the duration is defined as the final annealing duration. Therefore, final cooling is not considered in the total duration. If the final annealing occurs in the furnace, the total duration begins when the anneal enters the furnace and ends when it exits. The final annealing can also be performed, either inductively or conductively, in a known manner.

[0068] As a process parameter, the Hollomon-Jaffe parameter Hp represents additional process conditions during final annealing, in addition to temperature and total duration. Hp limits the maximum temperature T. H and total duration The possible combinations of , such that 12 × 10 should be satisfied. 3 >Hp>7.5×10 3 10.5×10 is preferred. 3 >Hp>8×10 3 .

[0069] According to the present invention, the steel strip is finally annealed in such a manner that the steel strip after final annealing and final cooling has the tensile strength R of the steel strip after final cooling. m This value represents the tensile strength compared to the tensile strength R of the steel strip before final annealing. m The value has increased and / or the steel strip after final annealing and final cooling has the tensile strength R of the steel strip after final cooling. m The value, which is the tensile strength value relative to the tensile strength R of the steel strip before final annealing. m The value remains unchanged in the sense that it is not less than that before the final annealing.

[0070] Advantageously, the steel strip, after final annealing and final cooling, has a tensile strength R of at least 920 MPa. m And an elastic limit R of at least 720 MPa p0.2 Therefore, this steel strip has high strength.

[0071] The method was optimized by optimizing the steel strip at its highest temperature T. H and total duration Finally, annealing is performed, in which case... Where T H In units of K, In h, and 12 × 10 3 >Hp>7.5×10 3 10.5×10 is preferred. 3 >Hp>8×10 3 .

[0072] It is particularly advantageous that the steel strip is finally annealed at a maximum temperature above 200°C and / or up to 400°C and / or for a total duration of 10s to 500s.

[0073] Additionally, it can be specified that the steel strip, particularly after the first annealing and the first cooling, undergoes intermediate annealing at a temperature between 200°C and 500°C (inclusive) for a total duration of 10 to 430 seconds before further cooling, especially continuous annealing.

[0074] Advantageously, the steel strip is cooled to a supercooled temperature below 50°C and selectively cooled all the way to room temperature.

[0075] One variant specifies that the steel strip is intermediately cooled to an intermediate temperature greater than 600°C after the first annealing and before the first cooling. In this case, it is preferably specified that the steel strip is intermediately cooled over a period of 5 to 300 seconds at an average cooling rate of 0.1 K / s to 30 K / s.

[0076] Alternatively, the steel strip can also undergo final annealing in multiple stages (e.g., in multiple consecutive furnaces). If the final annealing is performed in n stages, then T H , The Hp value should be calculated as follows: the highest temperature of final annealing, T H The maximum value involving all n stages, i.e., T H =max(T) Hi ), where T Hi It is the highest temperature of the i-th level.

[0077] Total duration of n-stage annealing The calculation is as follows: in It is the annealing duration of stage i.

[0078] Therefore, the Hp value of the multi-stage final annealing can be derived from the known form:

[0079] An advantageous application of the method according to the invention is to perform intermediate annealing on the steel strip in conjunction with the hot-dip coating, particularly hot-dip galvanizing, of the steel strip.

[0080] It has been shown that hot-rolled or cold-rolled steel strips are preferably made from steel having an alloy of Cr and Mo, wherein Mn+Cr+4×Mo>2.5 wt% and 0.1 wt%≤Mo≤0.5 wt%.

[0081] In this case, it is advantageous to specify that the hot-rolled or cold-rolled steel strip is made of the aforementioned steel, but with a C content of 0.085% to 0.115% by weight and / or is made of the aforementioned steel, but with a Mn content of 1.6% to 2.6% by weight.

[0082] Advantageously, the steel strip is tempered and cold rolled with a rolling force F[N]>(0.5×β) and a maximum rolling degree of 1.5% before final annealing, where β is the width of the steel strip in millimeters.

[0083] According to the teachings of the present invention, a steel strip having a multiphase microstructure is also provided, the steel strip being composed of the following elements in weight percentage: C: 0.085 to 0.149; Al: 0.005 to 0.1; Si: 0.2 to 0.75; Mn: 1.6 to 2.9; P: ≤0.02; S: ≤0.005; and optionally composed of one or more of the following elements in weight percentage: Cr: 0.05 to 0.5; Mo: 0.05 to 0.5; Ti: 0.005 to 0.060; Nb: 0.005 to 0.060; V: 0.001 to 0.060; B: 0.0001 to 0.0060; N: 0.0001 to 0.016; Ni: 0.01 to 0.5; Cu: 0.01 to 0.3; the remainder being iron, including common elements associated with steel, characterized in that the elastic limit R of the steel strip is... p0.2 The product of the tensile strength at break (A80) and the tensile strength at break (A80) is greater than 5600 MPa%, particularly greater than 7200 MPa%. The advantages described above in relation to the manufacturing method also apply to the steel strip according to the invention. Advantageously, the steel strip is manufactured according to the above-described manufacturing method.

[0084] Advantageously, Cr and Mo are added to the steel by alloying, wherein Mn + Cr + 4 × Mo > 2.5 wt% and 0.1 wt% ≤ Mo ≤ 0.5 wt%.

[0085] Particularly preferably, the steel strip has a minimum tensile strength of 920 MPa, particularly 980 MPa, and / or a bake hardening value BH2 of ≥25 MPa and / or a residual austenite content of less than 10%, particularly less than 5%.

[0086] Advantageously, the R of the steel strip after final annealing and final cooling is specified. p0.2 The elastic limit and the tensile strength R of the steel strip after final annealing and final cooling m The ratio is greater than 0.68 and between 0.97 (inclusive).

[0087] Advantageously, the microstructure of the steel strip after final annealing and final cooling has the following composition: ferrite: less than 60%; bainite + martensite: 30% to 98%; retained austenite: less than 10%, especially less than 5%. The percentages given for the microstructure composition refer to area percentages, and volume percentages are also commonly used.

[0088] Preferably, at least 1% fresh martensite is present in the microstructure of the steel strip prior to final annealing. The invention is particularly effective due to the presence of fresh martensite, as it ensures a reduction in the elastic limit, which is compensated for by the heat treatment according to the invention. The more fresh martensite present, the stronger this beneficial effect of the heat treatment.

[0089] Furthermore, an advantageous feature of the microstructure of the steel strip after final annealing and final cooling is that the KG5 characteristic value of the microstructure is less than 0.4, and in particular less than 0.3.

[0090] In connection with the present invention, room temperature should be understood as a temperature between 10°C and 40°C, preferably between 15°C and 25°C.

[0091] The method for manufacturing high-strength steel strip with a multiphase microstructure according to the present invention is explained in more detail below. This manufacturing is carried out by a continuous annealing facility or optionally by a hot-dip galvanizing facility using cold-rolled or hot-rolled steel strips of varying thicknesses. In this case, during the first annealing, the cold-rolled or hot-rolled strip is continuously annealed at a temperature between 750°C and 950°C for a total duration of 10 s to 1200 s to set the desired degree of austenitization. Depending on the degree of austenitization, the phase proportion of recovered and / or recrystallized ferrite is maintained. The tendency for recovery and / or recrystallization can be controlled by optional elements such as Mo, Ni, Ti, and V, where higher contents of these elements result in a delay in recrystallization kinetics. After a first cooling to a temperature of 200°C to 500°C at an average cooling rate of 2 K / s to 150 K / s, an intermediate annealing is performed for a total duration of 10 s to 430 s within a temperature range of 200°C to 500°C (inclusive), with the aim of transforming austenite into bainite. Optionally, hot-dip refining can be performed. To suppress the transformation to ferrite or coarser bainite at higher temperatures during the cooling stage and to achieve a sufficiently large process window, Mn, Mo, Cr, Ni, Nb, and B can be added, particularly through alloying. During intermediate annealing in the temperature range of 200°C to 500°C, the austenite transformation does not occur completely because the residual austenite is enriched in carbon and thus stabilized. The remaining austenite must be cooled to a supercooling temperature of less than 100°C, preferably less than 50°C, at an average cooling rate of 1 K / s to 50 K / s to transform into martensite. Through the formation of martensite and the resulting shear deformation, slipperable dislocations are generated in the surrounding structure, which, technically speaking, manifest as R p0.2The elastic limit is reduced. To restore the high elastic limit and high elastic limit ratio of >0.68 to 0.97 (inclusive) of the steel according to the invention, heat treatment is required after cooling to below 100°C, preferably below 50°C. During the final annealing, the tetragonality of the martensite's tetragonal core phase disappears due to carbon diffusion into the surrounding microstructure and the transformation of slippery dislocations into fixed (immobile) dislocations due to Cotrell clouds. Related to this is the restoration of the high elastic limit that occurs during this process, and the reduction of edge cracking sensitivity by transforming martensite with a hard tetragonal core structure into a cubic core structure. To increase tempering resistance and prevent tensile strength loss, Mo or V can optionally be added by alloying. The elastic limit ratio can be set variable depending on the temperature and time of the final annealing. To achieve a significant increase in the elastic limit, it has been shown that using the highest temperature T during mass production... H A final annealing of at least 100°C is advantageous. The final microstructure of the multiphase steel according to the invention consists of <60% ferrite, 30 to 98% bainite and martensite (fresh or tempered before and after final annealing), and a small amount of retained austenite of less than 10%, preferably less than 5%, wherein at least 1% fresh martensite is present before final annealing.

[0092] In principle, each annealing process can be constructed in multiple stages, or additional annealing processes can be provided for the entire process.

[0093] Tables 2a and 2b list the relevant process parameters for continuous annealing, exemplarily used to select temperature cycles 1a to VII for continuous annealing, said process parameters for manufacturing the steel strip according to the invention. The following process parameters are listed in Tables 2a and 2b:

[0094] T IA The highest annealing temperature in the critical region (first annealing).

[0095] t IA : Duration of annealing (first annealing)

[0096] T m Intermediate temperature

[0097] CR1: From T IA Cool to T m Average cooling rate at time

[0098] T OA Cooling stop temperature

[0099] CR2: From T m Cool to T OA Average cooling rate at time

[0100] t OA: Keep until T OA Time

[0101] T HD Hot-dip refining temperature (intermediate annealing)

[0102] T0: Supercooling temperature after hot-dip refining

[0103] CR3: Average cooling rate after hot-dip refining

[0104] T H The highest final annealing temperature to T0.

[0105] Final annealing duration

[0106] Hp: Hollomon-Jaffe parameters Where T H In units of K, Units are in h.

[0107] The final annealing is described as the final step of continuous annealing using the Hollomon-Jaffe parameter Hp, as previously described. Laboratory and large-scale trials were conducted using the temperature cycles illustrated in Tables 2a and 2b, and the resulting steel strips were then characterized in terms of mechanical properties. Each laboratory trial was associated with the final step of final annealing after reaching T0, and the previously produced steel strips were simulated during a laboratory-scale continuous annealing process to determine the dependence of the final properties on the Hp value.

[0108] Table 3 below—divided into Tables 3a and 3b—describes the reference steel. A I and B II With the example steel C according to the invention III D IV D V E VI F VII and G VIII Mechanical characteristic values ​​in the longitudinal direction (rolling direction) before and after final annealing, and R caused by final annealing at the corresponding Hp value. p0.2 The relative change of the elastic limit. The following mechanical characteristic values ​​are listed in Tables 3a and 3b:

[0109] R p0.2 0 Elastic limit before final annealing

[0110] R m 0 Tensile strength before final annealing

[0111] A 80 0Tensile strength at break before final annealing

[0112] Rp 0.2 f Elastic limit after a complete temperature cycle

[0113] R m f Tensile strength after a complete temperature cycle

[0114] R p0.2 f / Rm f Elastic limit ratio after complete temperature cycling

[0115] A 80 f Tensile strength at break after complete temperature cycling

[0116] ΔR p0.2 Changes in the elastic limit caused by final annealing

[0117] ΔR m Changes in tensile strength due to final annealing

[0118] ΔR p0.2 / R p0.2 0 The relative increase in the elastic limit due to final annealing

[0119] In this case, the reference steel and the example steel according to the invention have comparable R values ​​before final annealing. p0.2 Elastic limit (R) p0.2 0 According to temperature cycling, an elastic limit ratio R of 0.93 can be achieved in the example steel according to the invention. p0.2 f / R m f (See, for example, temperature cycling IIIa). Here, the example steel according to the invention maintains a high tensile strength at break >9%. A high Hp value is required to obtain a high elastic limit (see...). Figure 1 A very low Hp value does not lead to a significant increase in the elastic limit; for example, at Hp = 6.5, the increase is only 1% in temperature cycle IIIf (Table 2). The tensile strength of the steel according to the invention also increases due to the final annealing, thereby achieving a final tensile strength R > 920 MPa. m f This is significantly higher than the reference steel. A I and B II tensile strength R m f In this case, the example steel C is post-treated with temperature cycles IIIf, IVe, Vg, and Vh. IIID IV and D V It has been assessed as lacking in creativity because the Hp value is less than or equal to 7.5 and the increase in yield strength is less than 5%.

[0120] exist Figure 1 The diagram illustrates the R-value of the steel plate achieved through final annealing according to the present invention. p0.2 The elastic limit is determined based on the relative increase of the Hollomon-Jaffe parameter Hp. Therefore, the elastic limit due to final annealing is plotted on the y-axis in an x / y graph with values ​​from 0 to 0.5. p0.2 Elastic limit change (ΔR) p0.2 ) and the R of the steel strip before final annealing p0.2 Elastic limit (R) p0.2 0 The ratio ΔR p0.2 / R p0.2 0 And on the x-axis from 6 to 11

[10] 3 The values ​​of [] plot the Hollomon-Jaffe parameters Where T H The unit is K (the highest final annealing temperature after cooling to the supercooling temperature T0). The unit is h. Using the Hollomon-Jaffe parameter Hp, the final annealing duration can be determined. and maximum final annealing temperature T H The conditions for final annealing are used to characterize this. Five curves are plotted in this graph for reference steel A with temperature cycling groups la-f. I Reference steel with temperature cycling groups IIa-e B II Example steel C according to the invention, having temperature cycling groups IIIa-f III and the example steel D according to the invention, having temperature cycling groups IVa-e and Va-h. IV and D V These curves were fitted using the adapted Johnson-Mehl-Avrami-Kolmogorow equation (see, for example, A. Kolmogoroff; Izv. Akad. Nauk SSSR Ser. Mat. 1 (1937) 355-359) based on experimental measurements (see Table 3), which describes the dynamics of the transformation of martensite from a tetragonal to a cubical phase during annealing and the resulting simultaneous increase in the elastic limit. Reference steel A treated during temperature cycle I... I This shows that at Hp = 11 × 10 3 The minimum increase in elastic limit is approximately 20%. Reference steel B treated during temperature cycling II...II It shows a higher relative increase in the elastic limit compared to reference steel A, but at 9×10 3 Higher Hp values ​​are only possible at higher values. Higher Hp values ​​are classified as more difficult to implement technically because they require higher final annealing temperatures and / or final annealing times. Higher final annealing temperatures can cause undesirable changes in the coating, while longer final annealing times can lead to reduced productivity in mass production. Therefore, lower Hp values ​​should be pursued.

[0121] In the example steel C according to the invention III D IV and D V It can be seen from the reference steel A I and B II The increase is significantly higher than that of the elastic limit. At an Hp value of 9 × 10⁻⁶, the increase is even greater. 3 At that time, for steel C treated during temperature cycling 111a-f III For steel D treated during temperature cycling IVa-e IV And for steel D treated during temperature cycling Va-h V Regarding R p0.2 The increase in the elastic limit has exceeded 20%, while the reference steel A I and B II <10%. Therefore, the example steels according to the invention exhibit a significant increase in elastic limit even at lower Hp values, due to their composition, particularly the increased Si content, which avoids cementite precipitation and keeps the carbon required to increase the elastic limit dissolved. Although reference steels A and B have significantly higher C contents, the increase in elastic limit is significantly lower compared to the steels according to the invention.

[0122] Steel A is listed in Table 4. I -G vIII The organizational components.

[0123] The microstructure was determined by electron backscatter diffraction measurements in a longitudinal section perpendicular to the roll surface using Kikuchi band contrast and optical imaging. Furthermore, the grain diameter was also determined by electron backscatter diffraction measurements, where grains were defined as grain boundaries with an azimuth angle ≥15° (so-called large-angle grain boundaries - GWKG, see G. Gottstein, Physikalische Grundlagen der Materialkunde, Springer-Verlag Berlin Heidelberg, 2007).

[0124] According to the present invention, steel C III -G VIIIThe microstructure consists of <60% ferrite, 30% to 98% bainite and martensite (fresh or tempered before final annealing and tempered after final annealing), and less than 10%, particularly less than 5%, of retained austenite, wherein at least 1% fresh martensite is present before final annealing. Furthermore, the steel C according to the invention... III To G VIII The tissue has a KG5 eigenvalue of <0.4, preferably <0.3.

[0125] Fresh martensite exhibits high dislocation density and high hardness due to its formation mechanism. In electron backscatter diffraction, such regions appear darker than other microstructures in the Kikuchi band contrast because the diffraction conditions are disrupted by the disturbed lattice. Therefore, the proportion of fresh martensite can be quantitatively determined. Alternatively, the formation of fresh martensite can be determined by means of the dilatation method based on the volume change during sample cooling.

[0126] The KG5 eigenvalue does not change during the final annealing. Percentages used to describe microstructure composition refer to area percentages, but volume percentages are also commonly used.

[0127] In the present case, if fresh martensite is subsequently annealed again at a minimum temperature of at least 100°C after its formation, martensite is defined as tempered martensite. The minimum temperature of 100°C here corresponds to the minimum temperature of the final annealing according to the invention. In the present case, fresh martensite prior to final annealing should therefore be understood as tempered martensite after final annealing. Thus, fresh martensite is a transformation product of austenite, which is formed during cooling and has not undergone tempering.

[0128] For reference steel A I In terms of materials science, the microstructure does not change due to different temperature cycles of the final annealing under investigation (the temperature cycles before the final annealing are the same). Therefore, Table 4 for steel A... I The microstructure described applies to all temperature cycles la-f. This also applies to steel B in Table 4. II and D IV The organizational components.

[0129] As previously stated, the temperature cycling according to the invention requires an Hp value > 7.5. Therefore, a KG5 value < 0.3 does not necessarily guarantee the success of any temperature cycling, but it represents a favorable criterion for eventual annealing success starting from Hp > 7.5.

[0130] The KG5 eigenvalue describes the area ratio of grains with an equivalent diameter d = √(4A / π) > 5 μm and a shape factor F < 3, where A is the area of ​​the grain.

[0131] The formula for calculating the shape factor is as follows: Where P is the perimeter and A is the area of ​​the grain. The shape factor of a round grain is close to 1 (spherical), while elongated grains or grains with irregular grain boundaries have a higher shape factor >1.

[0132] The KG5 value does not change during the final annealing process.

[0133] By limiting the shape factor to F < 3, the composition of irregular microstructures that are strongly elongated during rolling is not important when considering grain size. Therefore, the KG5 eigenvalue is related to the composition of the newly formed coarse microstructure during cooling after the first annealing.

[0134] The newly formed microstructure after the first annealing plays a decisive role in the elastic limit, as the formation of new martensite in these regions reduces the elastic limit. Short diffusion paths are necessary for subsequent successful final annealing and a strong increase / recovery of the elastic limit, preferably with the smallest possible grain size and therefore a low KG5 eigenvalue <0.4, advantageously <0.3.

[0135] Reference steel B with a characteristic value of 0.58 for KG5 II (Left-side tissue image) and example steel D with a KG5 eigenvalue of 0.1. IV An exemplary comparison of tissue structures in the (right-hand tissue image) Figure 2 As shown in the image.

[0136] Grains with an equivalent diameter d > 5 μm and a shape factor F < 3 in Figure 2 The middle section is marked in gray, while the remaining fine structure is shown in white. The lowest possible proportion of grains shown in gray is advantageous to this invention, as reflected by the KG5 characteristic value.

[0137]

[0138] Table 1

[0139]

[0140] Table 2a

[0141]

[0142] Table 2b

[0143]

[0144] Table 3a

[0145]

[0146] Table 3b

[0147]

[0148] Table 4

Claims

1. A method for manufacturing a steel strip having a multiphase structure, comprising the following steps: - manufacturing a hot-rolled or cold-rolled steel strip from a steel consisting of the following elements in percent by weight: C: 0.085 to 0.149 Al: 0.005 to 0.1 Si: 0.200 to 0.750 Mn: 1.6 to 2.9 P：≤0.02 S：≤0.005 and optionally consisting of one or more of the following elements in percent by weight: Cr: 0.05 to 0.5 Mo: 0.05 to 0.5 Ti: 0.005 to 0.060 Nb: 0.005 to 0.060 V: 0.001 to 0.060 B: 0.0001 to 0.0060 N: 0.0001 to 0.016 Ni: 0.01 to 0.5 Cu: 0.01 to 0.3 with the remainder being iron and unavoidable impurities, - first annealing the steel strip at a temperature between 750 °C to 950 °C inclusive for a total duration of 10 s to 1200 s and then first cooling the steel strip to a temperature between 200 °C to 370 °C inclusive at an average cooling rate of 2 K / s to 150 K / s, - further cooling the steel strip to a subcooling temperature below 100 °C at an average cooling rate of 1 K / s to 50 K / s, - a final annealing of the steel strip with a maximum temperature T 3 in K of 1000°C to 470°C inclusive, a total duration in h of 2s to 1000s inclusive, H (ln ( ) +20) where the Hollomon-Jaffe parameters H H p = T p0.2 and n are such that the steel strip has a yield strength of 700 MPa to 900 MPa inclusive, a tensile strength of 800 MPa to 1000 MPa inclusive, an elongation at break of 5% to 15% inclusive, and a total elongation of 10% to 20% inclusive. - final cooling of the steel strip to room temperature at an average cooling rate of 1 K / s to 160 K / s, wherein the R p0.2 elastical limit value of the steel strip after the final cooling is at least 5% higher than the R p0.2 elastical limit value of the steel strip before the final annealing, so that for the steel strip after the final annealing and the final cooling a R p0.2 elastical limit and a tensile strength R 80 of at least 920 MPa are obtained, and a R m elastical limit of at least 720 MPa is obtained, and the ratio R p0.2 elastical limit R p0.2 to the tensile strength R m tensile strength R p0.2 / R m is greater than 0.68 to 0.97, inclusive, and the microstructure of the steel strip after the final annealing and the final cooling has the following composition: ferrite: less than 60 vol%; bainite + martensite: 30 vol% to 98 vol%; residual austenite: less than 10 vol%, wherein at least 1% of fresh martensite is present in the microstructure before the final annealing.

2. The method of claim 1, wherein, the first annealing being a continuous annealing of the steel strip.

3. The method of claim 2, wherein, the steel strip being a cold-rolled steel strip.

4. The method of claim 1, wherein, the total duration of the first annealing being 50 s to 650 s.

5. The method of claim 1, wherein, the first cooling having an average cooling rate of 5 K / s to 100 K / s.

6. The method of claim 1, wherein, the first cooling of the steel strip being a cooling to a temperature between 320 °C to 370 °C.

7. The method of claim 1, wherein, the final cooling having an average cooling rate of 1 K / s to 30 K / s.

8. The method of claim 1, wherein, the R value of the final cooled steel strip p0.2 the yield limit value relative to the R value of the final annealed steel strip p0.2 the yield limit value is increased by at least 10%.

9. The method of claim 1, wherein, the final annealing being continuous.

10. The method of claim 1, wherein, the structure having less than 5 vol% residual austenite.

11. The method of claim 1, wherein, the R value of the final cooled steel strip p0.2 the yield limit value relative to the R value of the final annealed steel strip p0.2 the yield limit value is increased by at least 5% to 50% of the value comprised.

12. The method of claim 11, wherein, the R value of the final cooled steel strip p0.2 the yield limit value relative to the R value of the final annealed steel strip p0.2 the yield limit value is increased by at least 5% to 40% inclusive.

13. The method of claim 1, wherein, with Hollomon-Jaffe parameters Hp = 9 x 10 3 The steel strip after final annealing and then final cooling has a R p0.2 The steel strip after final annealing and then final cooling has a R p0.2 The steel strip after final annealing and then final cooling has a R p0.2 The steel strip after final annealing and then final cooling has a R 14. The method of claim 1, wherein, The steel strip after the final annealing and the final cooling has a tensile strength R m value, which tensile strength value is increased relative to the tensile strength R m value of the steel strip before the final annealing.

15. The method of claim 1, wherein, at least one of the following listed conditions is fulfilled with respect to the final annealing of the steel strip: at the maximum temperature T H and total duration with a final annealing at the maximum temperature T 3 > Hp > 7.5 x 10 3 ; the steel strip is final annealed at a maximum temperature above 200 °C or at a maximum temperature up to 400 °C; the steel strip is final annealed for a total duration of 10 s to 500 s.

16. The method of claim 15, wherein, at the maximum temperature T H and total duration with a final annealing at the maximum temperature T 3 > Hp > 8 x 10 3 .

17. The method of claim 1, wherein, the steel strip is interannealed after the first annealing and the first cooling at a temperature between 200 °C to 500 °C inclusive for a total duration of 10 s to 430 s, wherein the steel strip is cooled to a subcooling temperature below 50 °C.

18. The method of claim 17, wherein, the interannealing is continuous.

19. The method of claim 1, wherein, the steel strip is intercooled after the first annealing and before the first cooling to an intermediate temperature greater than 600 °C, wherein the steel strip is intercooled at an average cooling rate of 0.1 K / s to 30 K / s over a time of 5 s to 300 s.

20. The method of claim 1, wherein, the steel strip is final annealed in multiple stages, and / or the steel strip is interannealed in connection with a hot dip coating of the steel strip.

21. The method of claim 1, wherein, manufacturing a hot-rolled or cold-rolled steel strip from a steel to which Cr and Mo are added by alloying, wherein Mn + Cr + 4 x Mo > 2.5 wt% and 0.1 wt% < Mo < 0.5 wt%, and / or the C content is 0.085 wt% to 0.115 wt%, and / or the steel strip is interannealed after the first annealing and the first cooling at a temperature between 200 °C to 500 °C inclusive for a total duration of 10 s to 430 s, Mn content is 1.6 to 2.6 wt%.

22. The method of claim 1, wherein, The steel strip is cold rolled in temper rolling with a rolling force F [N] > (0.5 x β) and a maximum reduction of 1.5% before the final annealing, where β is the strip width in mm.

23. A steel strip having a multi-phase structure, the steel strip consisting of the following elements in weight percent: C： 0.085 to 0.149 Al: 0.005 to 0.1 Si: 0.200 to 0.750 Mn: 1.6 to 2.9 P：≤0.02 S：≤0.005 and optionally one or more of the following elements in weight percent: Cr: 0.05 to 0.5 Mo: 0.05 to 0.5 Ti: 0.005 to 0.060 Nb: 0.005 to 0.060 V: 0.001 to 0.060 B: 0.0001 to 0.0060 N: 0.0001 to 0.016 Ni: 0.01 to 0.5 Cu: 0.01 to 0.3 wherein at least 1% fresh martensite is present in the structure before the final annealing. The remainder includes iron, including elements commonly found in steel, characterized by the R of the steel strip. p0.2 Elastic limit and tensile strength at break A 80 The product is greater than 5600 MPa%, and the tensile strength R is at least 920 MPa. m And having an elastic limit R of at least 720 MPa p0.2 And the R of the steel strip after final annealing and final cooling p0.2 The elastic limit and the tensile strength R of the steel strip after final annealing and final cooling m The ratio is greater than 0.68 to 0.97 including the end value, wherein the microstructure of the steel strip (i) after final annealing and final cooling has the following composition: ferrite: less than 60% vol%; bainite + martensite: 30% vol% to 98% vol%; retained austenite: less than 10% vol%, wherein grains defined by large-angle grain boundaries can be identified in the microstructure of the steel strip after final annealing and final cooling, and the KG5 characteristic value of the microstructure is less than 0.4, wherein the KG5 characteristic value describes the area ratio of grains with equivalent diameter d and shape factor F, wherein - and - where P is the perimeter and A is the area of the respective grain and its determination is made by means of electron backscattered diffraction, The structure has less than 5 vol% retained austenite.

24. The steel strip defined in claim 23, characterized in that The product of the R p0.2 elastic limit and the breaking elongation A 80 is greater than 7200 MPa %.

25. The steel strip defined in claim 23, characterized in that The structure has a KG5 characteristic value of less than 0.

3.

26. The steel strip defined in claim 23 wherein Cr and Mo are added to the steel by alloying, wherein Mn + Cr + 4 x Mo > 2.5 wt% and 0.1 wt% < Mo < 0.5 wt%.

27. The steel strip defined in claim 23 wherein The steel strip has a minimum tensile strength of 980 MPa, and / or 28. The steel strip defined in claim 23 wherein The steel strip has a bake hardening value BH2 of > 25 MPa. ​

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