EP4314356A1

EP4314356A1 - Steel strip made of a high-strength multiphase steel and process for producing such a steel strip

Info

Publication number: EP4314356A1
Application number: EP22722412.8A
Authority: EP
Inventors: Jan ROIK; Manuel Maikranz-Valentin; Konstantin MOLODOV
Original assignee: Salzgitter Flachstahl GmbH
Current assignee: Salzgitter Flachstahl GmbH
Priority date: 2021-04-01
Filing date: 2022-04-01
Publication date: 2024-02-07
Also published as: WO2022207913A1; DE102021108448A1; KR20230164098A; CN117222754A

Abstract

The invention relates to a steel strip made of a high-strength multiphase steel which has a tensile strength of at least 780 MPa in the longitudinal direction, the multiphase steel consisting of the following elements in % by weight: C ≥ 0.08 to ≤ 0.23, Mn ≥ 1.5 to ≤ 3.5, Si + Al ≥ 0.25 to ≤ 2, N ≥ 0.0020 to ≤ 0.0160, P < 0.05, S < 0.01, Cu < 0.20, optionally one or more of the following elements: Ca ≥ 0.0005 to ≤ 0.0060, Cr ≥ 0.05 to ≤ 1.0, Mo ≥ 0.05 to ≤ 1.0, Ni ≥ 0.05 to ≤ 0.50, Nb ≥ 0.005 to ≤ 0.15, Ti ≥ 0.005 to ≤ 0.15, V ≥ 0.001 to ≤ 0.30 and B ≥ 0.0005 to ≤ 0.0050, balance: iron, including customary steel-accompanying impurities resulting from melting, and having a carbon equivalent CEV which is greater than 0.49 and smaller than 0.9, preferably greater than 0.49 and smaller than 0.75, wherein the carbon equivalent CEV results from the contents of the corresponding elements in % by weight according to the following formula: CEV = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 and wherein the ratio of the carbon equivalent CEV and the sum of the contents of Si and Al in % by weight is less than 2.3, wherein the multiphase steel has a microstructure where the sum of the volume fractions of the microstructure constituents martensite, tempered martensite, residual austenite, upper bainite and/or lower bainite is at least 30% by volume, and the residual microstructure consists of ferrite and perlite. The invention further relates to a process for producing such a steel strip.

Description

Steel strip made from a high-strength multi-phase steel and method for producing such a steel strip

The invention relates to a steel strip made from a high-strength multi-phase steel which has a tensile strength of at least 780 MPa in the longitudinal direction.

The invention also relates to a method for producing a steel strip from a high-strength multi-phase steel which has a tensile strength of at least 780 MPa.

Due to their multi-phase structure, multi-phase steels have an excellent combination of strength, formability and ductility. In particular, phase fractions of more than 30% by volume of martensite and/or bainite are an essential microstructural component in order to achieve high tensile strengths (e.g. > 600 MPa). With an increasing proportion, e.g. of 50 or 70% by volume, strength classes up to over 980 MPa are also possible, depending on the chemical composition. Particularly in the case of annealing treatments with slow cooling rates and/or a coarse structure resulting from annealing treatment at high annealing temperatures, the proportion of hard phase components (martensite or bainite, possibly also tempered) must be higher in order to achieve greater strength. In order for a low-alloy steel to be able to form sufficient phase proportions of bainite and martensite, an annealing treatment well above the transformation temperature Ai, which is characteristic of steel, with subsequent sufficiently high cooling rates is necessary. For the large-scale production of low-alloy multi-phase steels, continuous annealing systems such as so-called "continuous annealing" or hot-dip galvanizing lines are necessary, in which the cooling rates are well above 1 K/s, i.e. the steel strip is cooled from temperatures above the Ai temperature to room temperature within seconds or minutes.

Steel strip is understood below to mean a hot-rolled or cold-rolled and annealed steel strip. Typical thicknesses of a hot-rolled steel strip, also referred to as hot strip, are between 1.8 mm and 18 mm. Cold-rolled, annealed steel strips are referred to as cold strip or thin sheet and usually have thicknesses in the range from 0.5 mm to 2.5 mm, with the strip thickness also being able to be adjusted in different flexible ways by specific processing, even within a cold strip or thin sheet. In addition to the large-scale heat treatment by continuous annealing in a continuous annealing furnace, strip sheets are also used on an industrial scale as coiled strips in furnaces such as e.g. B. so-called batch annealing plants heat treated "as a whole". Batch annealing of low alloy strip is done either as a recovery anneal or as a recrystallization/soft anneal. During recovery annealing, a strip sheet that is usually cold-worked is annealed at temperatures below 700 °C in order to achieve high tensile strength with a simultaneously high yield point and low ductility in the steel strip produced by the annealing. A recovery-annealed steel usually has a pronounced yield point, moderate ductility and a high yield point/tensile strength ratio > 0.8, which can be critical for the further processing of the steel strip. The material-related mechanism of recovery, which is the cause of the technological parameters after batch annealing, is very much dependent on the annealing temperature, the annealing time and the previous cold deformation (e.g. degree of cold rolling) of the strip. During soft/recrystallization annealing, the strip is annealed at temperatures around the Ai transformation temperature for several hours to days. The tensile strength after the annealing treatment described above is below 600 MPa and is significantly lower compared to the strength before the annealing treatment. However, the ductility increases significantly with a recrystallization anneal compared to the unannealed and cold-rolled material.

The hotly contested automotive market is forcing manufacturers to constantly find solutions to reduce fleet fuel consumption and CO2 emissions while maintaining the greatest possible comfort and occupant protection. On the one hand, the weight saving of all vehicle components plays a decisive 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 also plays a role. The steel manufacturers contribute to the solution of this task by providing high-strength steels. In addition, by providing high-strength steels with a lower sheet thickness, the weight of the vehicle components can be reduced with the same and possibly even improved component behavior.

In addition to the required weight reduction, these newly developed multi-phase steels have to meet the high material requirements in terms of yield strength, tensile strength and elongation at break. Multiphase steels are known, for example, from the published patent applications DE 102017 131 247 A1, DE 102017 130237 A1 and DE 102015 111 177 A1. The material properties disclosed there result from a high phase proportion of bainite and/or martensite, which require sufficiently rapid cooling conditions. Large-scale processing of such multi-phase steels takes place with continuous annealing systems.

It is the object of the invention to specify alternative measures for providing steel strips made from high-strength multi-phase steel.

The object is achieved according to the invention by a steel strip having the features of independent claim 1 and a method for producing a steel strip having the features of independent claim 9. Preferred embodiments of the invention are specified in the dependent claims, which individually or in combination define an aspect of Invention can represent.

A first 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 780 MPa in the longitudinal direction, the multi-phase steel consisting of the following elements in % by weight:

C > 0.08 to < 0.23,

Mn > 1.5 to < 3.5,

Si + Al > 0.25 to < 2,

N > 0.0020 to < 0.0160,

P<0.05,

S<0.01,

Cu < 0.20, and optionally one or more of the following:

Ca > 0.0005 to < 0.0060,

Cr > 0.05 to < 1.0,

Mo > 0.05 to < 1.0,

Ni > 0.05 to < 0.50,

N b > 0.005 to < 0.15,

Ti > 0.005 to < 0.15,

V > 0.001 to < 0.30 and

B > 0.0005 to < 0.0050,

Remainder iron, including the usual steel-related ones caused by smelting consists of impurities and has a carbon equivalent CEV which is greater than 0.49 and less than 0.9, the carbon equivalent CEV changing 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 (i.e. the mass fractions of these elements) and where the ratio from the carbon equivalent CEV and the sum of the contents of Si and Al in % by weight is less than 2.3 (CEV/ (Si+Al)

< 2.3), the multi-phase steel having a microstructure in which the sum of the volume fractions of the microstructure components martensite, tempered martensite, retained austenite, upper bainite and/or lower bainite is at least 30% by volume, and the remaining microstructure consists of ferrite and pearlite . A steel strip of this type can be produced from a rolled strip of steel of the appropriate composition by heat treatment of this strip – in particular rolled up into a coil – “as a whole” using the method for producing a steel strip described below. Said heat treatment is also referred to as "annealing" and can be carried out, for example, by the batch-type annealing system mentioned at the beginning.

The invention makes it possible to provide a steel strip which has a high tensile strength> 780 MPa, in particular with a good ductility Aso> 5% and a low yield strength ratio R _p o _, 2 / Rm <0.8, and in which these technological Post heat treatment characteristics are not significantly affected by microstructure or by cold working prior to heat treatment. In other words, it is provided in particular that the ratio of yield point to tensile strength R _p o _, 2/Rm is below 0.8 and the elongation at break Aso >5%.

According to a preferred embodiment of the invention, it is provided that the content in % by weight of the element C is between 0.09 and 0.2 and/or the content in % by weight of the element Mo is less than 0.4.

According to another preferred embodiment of the invention, it is provided that the content in % by weight of the element Mn is between 1.8 and 2.5 and/or that the content in % by weight of the sum of the elements Si + Al is between 0.25 and 1 is

According to a particular embodiment of the invention, the carbon equivalent CEV is less than 0.7. Such a steel can be processed particularly well. According to yet another preferred embodiment of the invention, it is provided that the proportion by volume of the common structural components martensite, tempered martensite, residual austenite, upper bainite and/or lower bainite in the structure of the multiphase steel is at least 50% by volume, particularly preferably at least 70% by volume , and the remaining structure consists of ferrite and pearlite.

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 advantageously provided that the steel strip has a thickness that is purposefully different in the longitudinal extent. The ratio between the maximum thickness and the minimum thickness is in particular between 1.16 and 3. For nominal strip thicknesses of 2.0 mm or more, this ratio lies outside the usual standard tolerance. Such a steel strip with a thickness that is purposefully different in the longitudinal extension is, in particular, a flexibly rolled steel strip for so-called “tailor rolled blanks”. The flexibly rolled steel strip is flexibly rolled as sheet metal before heat treatment, with the rollers producing different sheet thicknesses by moving them up and down. The advantage is the homogeneous transition between different thicknesses.

According to yet another preferred embodiment of the invention, it is provided that the steel strip has a thickness of between 4 mm and 18 mm, which is not readily possible in industrial production in continuous annealing furnaces.

A second aspect of the invention relates to a method for producing a steel strip from a high-strength multiphase steel, in particular a steel strip mentioned above, which has a tensile strength of at least 780 MPa in the longitudinal direction, a rolled steel strip consisting of the elements in % by weight:

C > 0.08 to < 0.23,

Mn > 1.5 to < 3.5,

Si + Al > 0.25 to < 2,

N > 0.0020 to < 0.0160,

P<0.05,

S<0.01,

Cu < 0.20, optionally one or more of the following:

Ca > 0.0005 to < 0.0060,

Cr > 0.05 to < 1.0,

Mo > 0.05 to < 1.0,

Ni > 0.05 to < 0.50,

N b > 0.005 to < 0.15,

Ti > 0.005 to < 0.15,

V > 0.001 to < 0.30 and

B > 0.0005 to < 0.0050

The remainder is iron, including the usual impurities associated with steel, and having a carbon equivalent CEV which is greater than 0.49 and less than 0.9, preferably greater than 0.49 and less than 0.75, with the carbon equivalent CEV corresponding to the following Formula CEV = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 from the contents of the corresponding elements in % by weight and where the ratio of the carbon equivalent CEV and the sum of the contents of Si and Al in % by weight is less than 2.3, as a whole, in particular rolled up into a coil, is heat-treated in such a way that it assumes a temperature above 750° C. and after this heat treatment to a temperature below 200° C is cooled, the cooling taking place between 750 °C and 200 °C at an average cooling rate of more than 1 K/h and less than 300 K/h.

Thanks to the method according to the invention for the production of a steel strip, large-scale production does not depend on continuous annealing and the final product, steel strip, still has a high tensile strength > 780 MPa, with good ductility Aso > 5% and a low yield strength ratio R o _, 2 /Rm < 0.8 and the technological characteristics after the heat treatment are not significantly affected by the microstructure or the cold working before the heat treatment.

On an industrial scale, the process z. B. be implemented with a batch-type annealing system. In terms of materials, this is only possible with low-alloy steels using the production method according to the invention, since the method includes specific phase transformations during the heat treatment. For this it is absolutely necessary, similar to a heat treatment during continuous annealing, to anneal in a temperature range above the Ai temperature, with the temperature but does not necessarily have to be above the A ₃ temperature. The necessary annealing temperatures can vary depending on the chemical composition of the steel strip.

The corresponding product is a hot-rolled and/or cold-rolled steel strip with a multi-phase structure and the associated characteristic technological properties of the aforementioned multi-phase steels.

The phase components bainite and/or martensite, which are characteristic of multi-phase steels, are formed from austenitic phase components when cooling a steel from temperatures above the Ai temperature. To ensure that the austenite does not transform into the phases ferrite and/or pearlite, the material must be sufficiently hardened in accordance with the technically possible cooling rate. The hardenability of a steel depends on the chemical composition and can be approximately described by the following carbon equivalent CEV:

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

However, too high a proportion of alloying elements such as manganese (Mn), chromium (Cr), carbon (C), vanadium (V), molybdenum (Mo), copper (Cu) and/or nickel (Ni) is for the previous process steps like Continuous casting, hot rolling or cold rolling and the following joining operations such as welding cannot be tolerated. In the case of steels with lower proportions of alloying elements, a low annealing temperature should preferably be selected in order to locally enrich the alloying elements in the austenite and thus achieve better through-hardenability locally in the austenite. For the above reasons, the CEV is limited to 0.49 to a maximum of 0.9, preferably to 0.75, preferably to a maximum of 0.7. For cost reasons, the proportion of alloying elements and thus the CEV should be kept low. For the aforementioned reason, the average cooling rate should be between 1 K/h and 300 K/h in the critical temperature range of 750 °C to 200 °C.

The strength-enhancing microstructure components of multi-phase steels, such as bainite and/or martensite, are formed from the austenitic phase components during cooling at temperatures below 570 °C. At temperatures above room temperature, in particular above 200° C., the local high strengths of the martensite and bainite phases are reduced by so-called tempering or self-tempering. at This material-scientific mechanism leads in particular to the precipitation of forcibly dissolved carbon to form carbides and the reduction of transformation-related stresses to a reduction in strength of the hard phases bainite and martensite and thus also to a reduction in strength of the annealed steel strip. This starting mechanism is thermally activated. The drop in strength due to tempering/self-tempering increases accordingly with longer residence times at higher temperatures, especially at temperatures above 200 °C. In order to keep the drop in strength to a minimum, it is imperative to cool down to temperatures below 200 °C at an accelerated rate after the formation of the hard phases and to counteract the tempering mechanisms with alloy concepts.

The elements Si and Al in particular are helpful in delaying the kinetics of carbide formation and thus in stabilizing the hard phases. Lowering the transformation temperatures of bainite and martensite also leads to fewer tempering effects with the same process control. The elements described in the CEV also bring about a reduction in the transformation temperatures of bainite and in particular martensite, which is positive for the invention. However, high proportions of Cr or Mn can also lead to the formation of additional carbides during the annealing treatment, which can also lead to a lower maximum strength. For the aforementioned reasons, it is necessary to limit the total content of the alloying elements Si and Al to greater than 0.25% by weight and also to limit the ratio of the CEV to the aforementioned total content of Si and Al to a maximum value of 2.3.

Temperatures above 750° C., preferably above 780° C., even more preferably above 790° C., must be maintained so that sufficient proportions of austenite are formed during the annealing treatment. However, excessively high annealing temperatures lead to undesirable grain growth, so that the maximum annealing temperature is preferably not above 70° C.+A _r 3 temperature. The A _r 3 temperature depends on the chemical composition and can be estimated using the following formula

Ar ₃ = 910 - 203 c - 30 Mn + 44.7 Si - 11 Cr + 31.5 Mo - 15.2 Ni

Accordingly, according to a preferred embodiment of the method according to the invention, it is provided that the sheet metal strip is heated from 100° C. to a temperature of 750°C, and in which the strip sheet remains in the temperature range from 750°C to Ar3 + 70°C for at least 1 h, the numerical value of the temperature Ar3 being calculated using the above formula from the contents of the corresponding elements in % by weight is calculated.

In order to save energy and with regard to the temperature resistance of furnace linings, it is recommended to limit the temperatures to a maximum of 900 °C or, even better, to 850 °C. Accordingly, the sheet steel strip preferably reaches a maximum temperature of at least 780° C. and at most 900° C., preferably at least 790° C. and at most 850° C., during the heat treatment.

In order to ensure homogeneous heating of the coiled steel strip, heating of at least 1 hour is preferably provided. Longer holding times are beneficial for more homogeneous heating, but are not recommended due to the associated grain growth, which in turn causes a drop in strength.

By processing a coiled steel strip in a bell-type annealing furnace, for example, significantly lower heating rates than in a continuous annealing furnace are technically feasible. However, it is recommended to heat up the entire annealing cycle at the highest possible heating rates in order to avoid undesirable thermodynamically stable precipitations and undesirable grain growth during heating. In turn, heating up too quickly prevents the steel strip from being evenly heated, so that the average heating rates in the critical temperature range of 100 °C until the minimum annealing temperature of 750 °C is reached should be between 1 K/h and 300 K/h.

Furthermore, it is preferably provided that after cooling, the steel strip is provided with a surface coating in the form of a metallic coating, organic coating or paint finish.

The strip sheet provided for the heat treatment has in particular a constant thickness, the term “constant thickness” being to be understood in the sense of the usual standard tolerance (eg according to EN 10051). Alternatively, the points for the Heat treatment provided strip sheet with advantage in the longitudinal extent specifically different thicknesses, the ratio between maximum thickness and minimum thickness is in particular between 1.16 and 3. For nominal strip thicknesses of 2.0 mm or more, this ratio lies outside the usual standard tolerance. Such a sheet metal strip with a thickness that is purposefully different in the longitudinal extension is, in particular, a flexibly rolled sheet metal strip for so-called “tailor rolled blanks”. The flexibly rolled strip is rolled again before heat treatment, with the rolls producing different sheet thicknesses by moving them up and down. The advantage is the homogeneous transition between different thicknesses. The resulting steel strip is then a flexibly rolled steel strip made from a high-strength multi-phase steel.

The effect of the elements in the steel strip according to the invention with a multi-phase structure is described in more detail below. The multi-phase steels are typically chemically structured in such a way that alloying elements are combined with and without micro-alloying elements. Accompanying elements are unavoidable and, if necessary, are taken into account in the analysis concept with regard to their effect.

Tramp elements are elements that are already present in the iron ore or that get into the steel as a result of the production process. Due to their predominantly negative influences, they are generally undesirable. Attempts are being made to remove them 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 creating lattice strains. As a result, the hydrogen in the iron lattice is relatively mobile and relatively easy to absorb during manufacture. Hydrogen can only be absorbed into the iron lattice in atomic (ionic) form. Hydrogen has a strong embrittling effect and preferentially diffuses to energetically favorable points (vacancies, grain boundaries, etc.). Defects act as hydrogen traps and can significantly increase the residence time of the hydrogen in the material. Cold cracks can occur as a result of recombination to form molecular hydrogen. This behavior occurs in hydrogen embrittlement or hydrogen-induced stress corrosion cracking. Hydrogen is also often cited as the reason for a delayed crack, the so-called delayed fracture, which occurs without external stresses. Therefore, the hydrogen content in the steel should be as low as possible. Oxygen (O): In the molten state, steel has a relatively high absorption capacity for gases, but oxygen is only soluble in very small amounts at room temperature. Analogous to hydrogen, oxygen can only diffuse into the material in atomic form. Due to the strong embrittling effect and the negative effects on the aging resistance, attempts are made during production to reduce the oxygen content as much as possible. To reduce the oxygen there are, on the one hand, process engineering approaches such as vacuum treatment and, on the other hand, analytical approaches. By adding certain alloying elements, the oxygen can be converted into less dangerous states. Binding of the oxygen via manganese, silicon and/or aluminum is usually common. However, the resulting oxides can cause negative properties as defects in the material. However, in the case of a fine precipitation, especially of aluminum oxides, grain refinement can also occur. For the above reasons, the oxygen content in the steel should be as low as possible.

Nitrogen (N) is also a by-element from steel production. Steels with free nitrogen tend to have a strong aging effect. Even at low temperatures, the nitrogen diffuses at dislocations and blocks them. It thus causes an increase in strength combined with a rapid loss of toughness. Binding of the nitrogen in the form of nitrides is possible by alloying, for example, aluminum or titanium. For the reasons mentioned above, the nitrogen content is limited to <0.016% by weight or to the amounts unavoidable in steel production.

Like phosphorus, sulfur (S) is bound as a trace element in iron ore. It is undesirable in steel (with the exception of free-cutting steels) because it tends to segregate and is very brittle. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt (e.g. by means of deep vacuum treatment). Furthermore, the sulfur present is converted into the relatively harmless compound manganese sulfide (MnS) by adding manganese. The manganese sulphides are often rolled out in rows during the rolling process and act as nuclei for the transformation. In the case of diffusion-controlled transformation in particular, this leads to a cellular structure and, in the case of pronounced cellularity, can lead to deteriorated mechanical properties (e.g. pronounced martensite lines instead of distributed martensite islands, anisotropic material behavior, reduced elongation at break). For the above reasons, the sulfur content is <0.01% by weight or at the steel production limited to unavoidable quantities.

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, since its low diffusion rate, among other things, has a strong tendency to segregate and greatly reduces toughness. Grain boundary fractures occur as a result of the accumulation of phosphorus at the grain boundaries. In addition, phosphorus increases the transition temperature from tough to brittle behavior by up to 300 °C. During hot rolling, near-surface phosphorus oxides at the grain boundaries can lead to cracking. By alloying small amounts of boron, the negative effects of phosphorus can be partially compensated. It is believed that boron increases grain boundary cohesion and decreases phosphorus segregation at grain boundaries. In some steels, however, P is used in small amounts (< 0.1%) as a micro-alloying element due to the low cost and the high increase in strength, for example in higher-strength IF steels (interstitial free). For the above reasons, the phosphorus content is limited to < 0.050% or to the amounts unavoidable in steel production.

Alloying elements are usually added to the steel in order to specifically influence certain properties. An alloying element can influence different properties in different steels. The connections are varied and complex. The effect of the alloying elements will be discussed in more detail below.

Carbon (C) is considered the most important alloying element in steel. Iron only becomes steel through its targeted introduction of up to 2.06%. The carbon content is often drastically reduced during steel production. In the multi-phase steel according to the invention, its proportion is 0.08% by weight to 0.23% by weight. Due to its comparatively small atomic radius, carbon is dissolved interstitially in the iron lattice. The solubility is a maximum of 0.02% in a-iron and a maximum of 2.06% in g-iron. In dissolved form, carbon increases the hardenability of steel considerably. Due to the different solubility, pronounced diffusion processes are necessary during the phase transformation, which can lead to very different kinetic conditions. In addition, carbon increases the thermodynamic stability of austenite, which is shown in the phase diagram in an expansion of the austenite area towards lower temperatures. With increasing forced dissolved carbon content in the martensite, the lattice distortions and the associated strength of the non-diffusing phase increase. Carbon is also required for carbide formation. A representative found in almost every steel is cementite (FesC). However, significantly 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 is of decisive importance for the resulting increase in strength. Therefore, in order to ensure sufficient strength on the one hand and good weldability on the other hand, the minimum C content is set to 0.08% by weight and the maximum C content to 0.23% by weight, preferably between 0.09 and 0.2% by weight -%, fixed.

Aluminum (AI) is usually alloyed with steel to bind the oxygen and nitrogen dissolved in the iron. The oxygen and nitrogen are thus converted into aluminum oxides and aluminum nitrides. These precipitations can cause grain refinement by increasing the nucleation sites and thus increase the toughness and strength values. Aluminum nitride is not precipitated when titanium is present in sufficient quantity. 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 sufficient ferrite to form. It also suppresses carbide formation and thus leads to delayed austenite transformation. For this reason, Al is also used as an alloying element in retained austenitic steels in order to replace some of the silicon with aluminum. The reason for this approach is that Al is slightly less critical to the galvanizing reaction than Si.

Silicon (Si) binds oxygen during casting and thus reduces segregation and contamination in the steel. In addition, silicon increases the strength of the ferrite through solid solution strengthening with only a slightly decreasing elongation at break. Another important effect is that silicon shifts the formation of ferrite to shorter times, thus allowing sufficient ferrite to form before quenching in continuously annealed material. An effect that is particularly advantageous when using low-alloy steels in the batch annealing treatment of multi-phase steels according to the invention. The formation of ferrite enriches and stabilizes the austenite with carbon. At higher levels, silicon stabilizes the austenite noticeably in the lower temperature range, especially in the area of bainite formation by preventing carbide formation. During hot rolling with high silicon contents, highly adhering scale can form, which can impair further processing.

By suppressing carbides (particularly M _ß C-carbides, where M stands for a metallic alloying element) in bainitic microstructural constituents, or tempered martensite, additions of both Si and Al prevent and lead to strength reduction of the aforementioned hard phases martensite and/or bainite that the strength decreases less after an annealing treatment. For the above reasons, a total content of Al and Si is set at 0.25 to 2% by weight, preferably up to a maximum of 1% by weight.

Manganese (Mn) is added to almost all steels for desulfurization in order to convert the harmful sulfur into manganese sulfides. In addition, manganese increases the strength of the ferrite through solid solution strengthening and shifts the transformation to lower temperatures. A main reason for alloying manganese is the significant improvement in hardenability. Due to the hindrance to diffusion, the pearlite and bainitic transformation is shifted to longer times and the martensite start 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 (in particular Si and Al), manganese oxides (eg MnO) and/or Mn mixed oxides (eg Mn ₂ Si0 ₄ ) can occur. However, manganese is to be considered less critical with a low Si/Mn or Al/Mn ratio, since globular oxides rather than oxide films are formed. The Mn content is therefore set at 1.5% to 3.5% by weight, preferably 1.8 to 2.5% by weight.

Chromium (Cr): The addition of chromium mainly improves hardenability. In the dissolved state, chromium shifts the pearlite and bainitic transformation to longer times and at the same time lowers the martensite start 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 hardenability can deteriorate due to the increased number of germs. Chrome also tends to during the annealing treatment to form oxides on the steel surface, which can degrade the galvanizing quality. The optional Cr content is therefore set at values of 0.05 to 1.0% by weight.

Molybdenum (Mo): The addition of molybdenum is similar to chromium to improve hardenability. The pearlite and bainitic transformation is pushed to longer times and the martensite start temperature is lowered. Molybdenum also increases the tempering resistance considerably and increases the strength of the ferrite through solid solution strengthening. The Mo content is added depending on the dimensions, the system configuration and the microstructure. For cost reasons, the optional Mo content is set at 0.05 to 1.0% by weight, preferably up to a maximum of 0.4% by weight.

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 rate of corrosion. In combination with oxygen, copper can form harmful oxides at the grain boundaries, which can have negative effects, especially for hot forming processes. The maximum copper content is therefore limited to 0.2% by weight.

Calcium (Ca): Calcium is used in the manufacture of high-strength steels for deoxidation, desulfurization and to control the size and shape of oxides and sulfides. In the case of high-strength steels in particular, this results in improved ductility and toughness. In addition, steels with additions of calcium are less prone to hot cracking, e.g. during hot rolling. For the above reasons and because of the very low solubility of calcium in steel - if required - the calcium content is therefore limited to 0.0005 to 0.0060% by weight.

Nickel (Ni): In combination with oxygen, nickel can form harmful oxides at the grain boundaries, which can have negative effects, especially for hot forming processes. However, nickel also increases hardenability and lowers the AC 3 transformation _temperature . For the reasons mentioned above and for reasons of cost, the optional content of nickel is therefore limited to 0.05 to 0.50% by weight.

Micro-alloying elements are usually only added in very small amounts (<0.1%). Typical micro-alloying elements are aluminum, vanadium, titanium, and niobium Boron. In contrast to the alloying elements, they mainly act through the formation of precipitates, but they can also influence the properties in the dissolved state. Despite the small amounts added, micro-alloying 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. A 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 usually most pronounced with the micro-alloying elements, but is usually specifically prevented due to other alloying elements. This property can be used positively in that the elements sulfur and oxygen, which are generally harmful, can be bound. However, setting can also have negative effects if there are no longer enough micro-alloying elements available for the formation of carbides.

Titanium (Ti) forms very stable nitrides (TiN) and sulfides (T1S2) even at high temperatures. Depending on the nitrogen content, some of these only dissolve in the melt. If the precipitates produced in this way are not removed with the slag, they form coarse particles in the material due to the high temperature at which they form, which are generally not conducive to the mechanical properties. The binding of free nitrogen and oxygen has a positive effect on toughness. For example, titanium protects other dissolved micro-alloying elements, such as niobium, from being bound by nitrogen. These can then optimally develop their effect. Nitrides, which only form at lower temperatures due to the drop in oxygen and nitrogen content, can also effectively impede austenite grain growth. Unset 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 increase in the number of nuclei during α/Y transformation) and precipitation hardening. The optional Ti content therefore has values of 0.005 to 0.150% by weight.

Niobium (Nb) causes strong grain refinement, as it is the most effective of all micro-alloying elements in delaying recrystallization and also inhibiting austenite grain growth. The strength-increasing effect is qualitatively higher than that of titanium, evident from the increased grain refinement effect and the larger quantity of strength-increasing particles (bonding of the titanium to form coarse TiN at high temperatures). Niobium carbides form at temperatures below 1200 °C. When nitrogen is bonded with titanium, niobium can increase its strength-increasing effect through the formation of small carbides that are effective in terms of their effect in the lower temperature range (smaller carbide sizes). Another effect of the niobium is the delay in the α/Y transformation and the lowering of the martensite start temperature in the dissolved state. On the one hand, this is due to the solute drag effect and, on the other hand, to grain refinement. This causes an increase in the strength of the structure and thus also a higher resistance to the increase in volume during martensite formation. In principle, the alloying of niobium is limited until its solubility limit is reached. Although this limits the amount of precipitation, if it is exceeded it primarily causes early formation of precipitation with very coarse particles. Precipitation hardening can thus be effective primarily in steels with a low carbon content (higher oversaturation possible) and in hot forming processes (deformation-induced precipitation). The Nb content is therefore limited to values of 0.005 to 0.150% by weight.

Vanadium (V): Carbide and nitride formation of vanadium only begins at temperatures around 1000 °C or after the a/g transformation, i.e. much later than with titanium and niobium. Due to the small number of precipitations present in austenite, vanadium has hardly any grain-refining effect. The austenite grain growth is also not inhibited by the late precipitation of the vanadium carbides. Thus, the strength-increasing effect is based almost entirely on precipitation hardening. An advantage of vanadium is its high solubility in austenite and the large 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% by weight.

Boron (B) forms nitrides or carbides with nitrogen as well as with carbon; as a rule, however, this is not the aim. On the one hand, due to the low solubility, only a small amount of precipitation forms and, on the other hand, these are mostly precipitated at the grain boundaries. An increase in hardness on the surface is not achieved (except for boriding with the formation of FeB and Fe2B in the edge zone of a workpiece). In order to prevent nitride formation, an attempt is usually made to bind the nitrogen with elements with a higher affinity. Titanium in particular can ensure that all of the nitrogen is bound. In the dissolved state, boron in very small amounts leads to a significant improvement in hardenability. The mechanism of action of boron can be described in such a way that boron atoms accumulate at the grain boundaries with suitable temperature control and, by reducing the grain boundary energy, make the formation of viable ferrite nuclei significantly more difficult. When controlling the temperature, care must be taken to ensure that boron is predominantly atomically distributed in the grain boundary and is not present in the form of precipitates due to excessively high temperatures. The effectiveness of boron decreases with increasing grain size and increasing carbon content (> 0.8%). In addition, an amount exceeding 60 ppm causes a decrease in hardenability since boron carbides act as nuclei on the grain boundaries. Due to the 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 at over 1000 °C is not recommended. This is also recommended, since boron can lead to strong coarse grain formation at annealing temperatures of over 1000 °C. For the above reasons, the B content is limited to values of 0.0005 to 0.0050% by weight.

Finally, the invention also relates to the use of an above-mentioned steel strip to produce a motor vehicle component.

Embodiments of the invention are explained below on the basis of examples using figures and tables.

It shows:

1 shows a graphical representation of the temperature profile over time of a rolled steel strip and a plant that heat-treats this strip during a heat treatment according to a preferred embodiment of the invention in a temperature-time diagram and

2 shows the stress-strain curves of a steel strip designed according to the invention and the stress-strain curves of a comparative steel strip with a different composition of the steel.

In principle, the annealing treatments according to the invention can be multi-stage or additional annealing treatments can be provided in relation to the overall process. An example time-temperature cycle showing the characteristic Temperature ranges for holding times, cooling rates and heating rates are shown in FIG.

For this purpose, a rolled strip of steel of the appropriate composition is brought into a compact form, in particular rolled up into a coil, which makes it possible to place the strip as a whole in an apparatus for heat treatment. There, in a first step S1, the sheet metal strip is heated to a temperature T > 750° C. within about 3 hours. In a second step S2, the sheet metal strip is then kept at a temperature above 750° C. for about 8 hours by means of the apparatus. The following applies to the maximum temperature reached in the second step S2: T max < _Ar3 +70 K. The strip sheet is then cooled. During this cooling, the temperature range from 750 °C to 200 °C is covered in a period of about 14 hours. This results in a third step S3 of cooling from 750° C. to 200° C. with an average cooling rate of approximately 40 K/h. When the sheet steel made of steel with a suitable steel concept, i.e. a suitable composition, cools down, the desired microstructure results and the steel strip made of high-strength multiphase steel is produced. Cooling down to a certain temperature is preferably carried out in the heat treatment apparatus. This is, for example, a batch-type annealing system. At around 40 K/h, the example shown is in a preferred cooling range of 20 K/h to 80 K/h.

Table 1 below shows examples of material concepts, more precisely steel concepts, and their chemical composition in % by weight. Steel concepts according to the invention are marked accordingly. In addition to the steel concepts according to the invention, which are used in the form of a hot-rolled or cold-rolled sheet metal strip as the input material for manufacturing a product according to the invention, steel concepts that are not according to the invention are also given as a comparison.

The parameters of a production method according to the invention and the characteristics of the product according to the invention of this production method, ie the steel strip made of high-strength multiphase steel, are listed in Table 2. The exemplary material concepts are explained below. These are referred to as "Steel A", "Steel B", "Steel C" and "Steel D".

Steel A is not according to the invention, since the sum of hardenability-increasing alloying elements described by the CEV is below the required value of 0.49 is. After a heat treatment with the process parameters according to the invention, steel A has formed a microstructure consisting of ferrite and pearlite and no portions of bainite and/or martensite. The associated stress-strain curve can be seen in FIG. The heat treated steel A from has a tensile strength of 540 MPa and an undesirably high yield point.

Steel B from Table 2 is also not according to the invention, although steel B with a CEV value of 2.34 has sufficient through-hardenability. However, the ratio of CEV/(Si+Al) > 2.34 and thus the content of Si and Al is not sufficient in relation to the use of through-hardenability-increasing elements. This can also be seen from the achievable tensile strength of a maximum of 762 MPa.

Steels C and D are exemplary material concepts that are suitable for an annealing treatment according to the invention and the production of steel strips designed according to the invention. After a heat treatment with the process parameters according to the invention, the steels C and D have a martensite and/or bainite content of more than 30%. Due to the microstructure adjusted in this way, the steels also have the material properties characteristic of multiphase steels _, such as a yield point-tensile strength ratio (R _p o .2/Rm) between 0.45 and 0.6, a high tensile strength R _m above 780 MPa and a high elongation at break So> 8%.

Table 1

Table 2

Claims

patent claims

1. Steel strip made of high-strength multi-phase steel, which has a tensile strength of at least 780 MPa in the longitudinal direction, the multi-phase steel consisting of the elements in % by weight:

C > 0.08 to < 0.23,

Mn > 1.5 to < 3.5,

Si + Al > 0.25 to < 2,

N > 0.0020 to < 0.0160,

P<0.05,

S<0.01,

Cu < 0.20,

Optionally, one or more of the following:

Ca > 0.0005 to < 0.0060,

Cr > 0.05 to < 1.0,

Mo > 0.05 to < 1.0,

Ni > 0.05 to < 0.50,

N b > 0.005 to < 0.15,

Ti > 0.005 to < 0.15,

V > 0.001 to < 0.30 and

B > 0.0005 to < 0.0050

The remainder is iron, including the usual impurities associated with steel, and having a carbon equivalent CEV which is greater than 0.49 and less than 0.9, preferably greater than 0.49 and less than 0.75, with the carbon equivalent CEV corresponding to the following Formula CEV = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 from the contents of the corresponding elements in % by weight and where the ratio of the carbon equivalent CEV and the sum of the contents of Si and Al in % by weight is less than 2.3, the multiphase steel having a microstructure in which the sum of the volume fractions of the microstructure components martensite, tempered martensite, retained austenite, upper bainite and/or lower bainite is at least 30% by volume , and the remaining structure consists of ferrite and pearlite.

2. Steel strip according to claim 1, characterized in that the ratio of Yield strength to tensile strength R _p o _, 2/Rm is below 0.8 and the elongation at break Aso> 5%.

3. Steel strip according to claim 1 or 2, characterized in that the content in % by weight of the element C is between 0.09 and 0.2 and/or the content in % by weight of the element Mo is less than 0.4.

4. Steel strip according to one of claims 1 to 3, characterized in that the content in % by weight of the element Mn is between 1.8 and 2.5 and/or that the content in % by weight of the sum of the elements Si + Al is between 0.25 and 1.

5. Steel strip according to one of claims 1 to 4, characterized in that the carbon equivalent CEV is less than 0.7.

6. Steel strip according to one of Claims 1 to 5, characterized in that the sum of the volume fractions of the microstructure components martensite, tempered martensite, residual austenite, upper bainite and/or lower bainite in the structure of the multiphase steel is at least 50% by volume, preferably at least 70 % by volume, and the remaining structure consists of ferrite and pearlite.

7. Steel strip according to one of claims 1 to 6, characterized in that the steel strip has a deliberately varying thickness in the longitudinal extent, the ratio between maximum thickness and minimum thickness being in particular between 1.16 and 3.

8. Steel strip according to one of claims 1 to 7, characterized in that the thickness D of the steel strip is in the range >4 mm to <18 mm.

9. A method for producing a steel strip from a high-strength multiphase steel, in particular a steel strip according to one of the preceding claims, which has a tensile strength of at least 780 MPa in the longitudinal direction, with a rolled steel strip consisting of the elements in % by weight:

C > 0.08 to < 0.23,

Mn > 1.5 to < 3.5, Si + Al > 0.25 to < 2,

N > 0.0020 to < 0.0160,

P<0.05,

S<0.01,

Cu < 0.20,

Optionally, one or more of the following:

Ca > 0.0005 to < 0.0060,

Cr > 0.05 to < 1.0,

Mo > 0.05 to < 1.0,

Ni > 0.05 to < 0.50,

N b > 0.005 to < 0.15,

Ti > 0.005 to < 0.15,

V > 0.001 to < 0.30 and

B > 0.0005 to < 0.0050

The remainder is iron, including the usual impurities associated with steel, and having a carbon equivalent CEV which is greater than 0.49 and less than 0.9, preferably greater than 0.49 and less than 0.75, with the carbon equivalent CEV corresponding to the following Formula CEV = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 from the contents of the corresponding elements in % by weight and where the ratio of the carbon equivalent CEV and the sum of the contents of Si and Al in % by weight is less than 2.3, as a whole, in particular rolled up into a coil, is heat-treated in such a way that it assumes a temperature above 750° C. and after this heat treatment to a temperature below 200° C is cooled, the cooling taking place between 750°C and 200°C at an average cooling rate greater than 1 K/h and less than 300 K/h.

10. The method according to claim 9, characterized in that the sheet metal strip is heated from 100°C to a temperature of 750°C during the heat treatment at an average heating rate of between 1 K/h and 300 K/h.

11. The method according to claim 9 or 10, characterized in that the strip sheet for at least 1 hour in the temperature range from 750 ° C to Ar3 + 70 ° C dwells, whereby the numerical value of the temperature Ar3 is calculated using the following formula from the contents of the corresponding elements in % by weight:

Ar ₃ = 910- 203VC- 30 Mn + 44.7 Si -11 Cr + 31.5 Mo - 15.2 Ni 12. The method according to at least one of claims 9 to 11, characterized in that the strip sheet is made of steel during the heat treatment a maximum temperature of at least 780°C and at most 900°C, preferably at least 790°C and at most 850°C. 13. The method according to at least one of claims 9 to 12, 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. 14. The method according to at least one of claims 9 to 13, characterized in that the sheet metal strip has a specifically varying thickness in the longitudinal extent, the ratio between maximum thickness and minimum thickness being in particular between 1.16 and 3. 15. Use of a steel strip according to any one of claims 1 to 8 for

Manufacture of a motor vehicle component.