DE102021104584A1 - High-strength, hot-rolled flat steel product with high local cold workability and a method for producing such a flat steel product - Google Patents

Abstract

The present invention is based on the object of creating a high-strength, hot-rolled flat steel product and a method for producing such a flat steel product, and thus, in relation to the steel, to achieve a combination of high strength with simultaneous high local cold workability and high cost-effectiveness. This is achieved by a high-strength, hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8 and a hole expansion ratio of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, a Elongation at break of at least 10%, preferably at least 16%, a degree of cold workability of at least 0.12, advantageously at least 0.17 and a ratio of local and global cold workability of at least 5 and at most 13, and a structure consisting of more than 50 volumes -% bainite, balance precipitation strengthened ferrite, with the following chemical composition of the steel (in % by weight): C: 0.04 to 0.08; Si: 0.1 to 0.6; Mn: 1.0 to 2.0; P: max 0.06; S: 0.01 or less; N: 0.012 or less; AI: up to 0.06; Ti: up to 0.18 and/or Nb: up to 0.08; Mon: up to 0.35; with Ti+Nb more than 0.06, with an over-stoichiometric proportion of carbon and nitrogen according to the following formula: 1.0 < (C/12+N/14) /(Ti/48+Nb/93+Mo/96) , balance iron including unavoidable steel-accompanying elements.

Description

The invention relates to a high-strength, hot-rolled flat steel product with high local cold workability. Furthermore, the invention relates to a method for producing such a flat steel product.

In the present case, cold formability is understood to mean formability at a temperature in the range from 10° C. to 700° C., preferably between 10° C. and 200° C., particularly preferably between 10 and 80° C. and particularly preferably at room temperature between 15 and 40° C .

In particular, the invention relates to a high-strength, micro-alloyed, predominantly bainitic hot strip with an optimized alloy composition and microstructure, which is used, for example, as a chassis component in the automotive industry.

The invention also relates to high-strength hot strip with tensile strengths of at least 760 MPa and at the same time high cold workability.

1. 1. Bruchdehnung (A)
2. 2. Lochaufweitungsverhältnis (LA)
3. 3. Maß der Kaltumformbarkeit (FL)
4. 4. Verhältnis von lokaler zu globaler Kaltumformbarkeit (LFR)

The description of cold formability is complex and can only be adequately quantified by an AND-linked combination of characteristic values.
The following characteristic values are therefore used to describe the cold formability of the flat steel product according to the invention:

1. 1. Elongation at break (A)
2. 2. Hole Expansion Ratio (LA)
3. 3. Degree of cold formability (FL)
4. 4. Ratio of local to global cold formability (LFR)

Parameters such as elongation at break, uniform elongation and hole expansion ratio are established parameters for describing cold formability.

In order to quantify high global cold workability and high local cold workability, it is necessary to use both a local and a global cold workability parameter. Within the scope of the invention, the hole expansion ratio characteristic value is selected as a representative of the local formability and the uniform elongation characteristic value is selected as a representative of the global cold formability. In this case, the true sizes are given and not the technical (percentage) sizes, the determination of which is given in the description of the exemplary embodiments.

The invention particularly includes flat steel products made of steels with a multi-phase structure, which essentially contains, ie a proportion of more than 50% by volume, bainite and which have a yield point ratio of at least 0.8. In addition to a high tensile strength of at least 760 MPa and an elongation at break A of at least 10%, the flat steel product also has a high hole expansion capacity with a hole expansion ratio LA of at least 30%, a degree of cold workability FL of at least 0.12 and a local to global ratio Cold workability LFR in the range of at least 5 and at most 13.

As is known, bainitic steels are steels which are characterized by a comparatively high yield point and tensile strength with a sufficiently high elongation for cold forming processes. Good weldability is given due to the chemical composition. The microstructure typically consists of bainite as the predominant component with portions of ferrite. The microstructure may occasionally contain small amounts of other phases, such as martensite and retained austenite.

Such a steel is, for example, in the published application DE 10 2012 002 079 A1 disclosed. The disadvantage here, however, is that the hole expansion capacity is not yet sufficiently high.

The highly competitive automotive market is forcing manufacturers to constantly find solutions to reduce fleet consumption and CO2 emissions while maintaining the greatest possible level of comfort and occupant protection. On the one hand, the weight reduction of all vehicle components plays a decisive role, but on the other hand, the best possible behavior of the individual components under the high static and dynamic loads both during use of an automobile and in the event of a crash.

By providing high-strength to ultra-high-strength steels with strengths of up to 1050 MPa or more and by reducing the sheet thickness that can be achieved with these steels, the weight of the vehicles can be reduced while at the same time improving the forming behavior of the steels used during production and operation.

High-strength steels must therefore meet comparatively high requirements in terms of their strength, ductility and energy absorption, without their processing, such as stamping, hot and cold forming, thermal hardening (e.g. air hardening, press hardening), welding and/or surface treatment, e.g. a metallic refinement, organic coating or painting, disadvantages occur compared to conventional steels.

Newly developed steels therefore have to meet the increasing material requirements for yield strength, tensile strength, hardening behavior and elongation at break with good processing properties such as formability and weldability, in addition to the required weight reduction through reduced sheet thicknesses.

To ensure the required reduction in sheet thickness, a high-strength steel with a single- or multi-phase structure must therefore be used to ensure sufficient strength of the motor vehicle components and to meet the high forming and component requirements in terms of toughness, edge crack resistance, improved bending angle, bending radius and energy absorption.

Improved joint suitability in the form of better general weldability, expressed by a larger usable welding area in resistance spot welding and improved failure behavior of the weld seam (fracture pattern) under mechanical stress, as well as sufficient resistance to delayed cracking due to hydrogen embrittlement, is also increasingly required.

The hole expansion capacity is a material property that describes the resistance of the material to crack initiation and crack propagation during forming operations in areas close to edges and previously sheared, such as when collaring.

The hole expansion test is normatively regulated in ISO 16630, for example.

According to this, holes punched in a metal sheet are widened by means of a mandrel. The measured variable is the change in the hole diameter, based on the initial diameter, to the diameter at which the first crack through the sheet occurs at the edge of the hole.

Improved resistance to edge cracks means an increased formability of the sheet edges and is described by an increased hole expansion capacity. This fact is known under the synonyms "Low Edge Crack" (LEC) or under "High Hole Expansion" (HHE) as well as xpand®.

• - Gießen einer Bramme, gefolgt von dem Schritt eines erneuten Erhitzens der erstarrten Bramme auf eine Temperatur zwischen 1050 und 1260 °C;
• - Warmwalzen der Stahlbramme mit einer Eintrittstemperatur in der letzten Endwalzgerüst zwischen 980 und 1100 °C;
• - Fertigwalzen mit einer Fertigwalztemperatur zwischen 950 und 1080 °C;
• - Abkühlen des warmgewalzten Stahlbands mit einer Primärkühlrate zwischen 50 und 150 °C/s zu einer Zwischentemperatur auf dem ROT (Auslaufrollenbahn) zwischen 600 und 720 °C und gefolgt von:
• - milder Erwärmung des Stahls zwischen 0 und +10 ° C/s durch latente Wärme resultierend aus der Phasenumwandlung von Austenit zu Ferrit oder;
• - isothermes Halten des Stahls, oder;
• - dumildes Abkühlen des Stahls, was insgesamt zu einer Temperaturände-rungsrate führt in die Sekundärstufe der ROT von -20 bis 0 °C/s;- um die Wickeltemperatur zwischen 580 und 660 °C zu erreichen.

From the patent EP 3 516 085 B1 discloses a method for producing a high-strength, hot-rolled steel strip with a tensile strength of at least 570 MPa, preferably at least 780 MPa, with which good cold-formability of the steel strip is to be achieved. The procedure comprises the following steps:

• - casting a slab, followed by the step of reheating the solidified slab to a temperature between 1050 and 1260°C;
• - hot rolling of the steel slab with an entry temperature in the last finishing stand between 980 and 1100 °C;
• - finish rolling with a finish rolling temperature between 950 and 1080 °C;
• - Cooling of the hot-rolled steel strip with a primary cooling rate between 50 and 150 °C/s to an intermediate temperature on the ROT (outlet roller conveyor) between 600 and 720 °C and followed by:
• - mild heating of the steel between 0 and +10 °C/s by latent heat resulting from the phase transformation from austenite to ferrite or;
• - holding the steel isothermally, or;
• - gentle cooling of the steel, resulting in an overall rate of temperature change in the secondary stage of the ROT of -20 to 0 °C/s; - to reach the coiling temperature between 580 and 660 °C.

The steel known from this consists of (in weight %): between 0.015 and 0.15 C; at most 0.5 Si; between 1.0 and 2.0 Mn; at most 0.06 P; at most 0.008S; at most 0.1 Al sol; at most 0.02N; between 0.02 and 0.45V; optionally one or more of: at least 0.05 and/or at most
0.7 Mo; at least 0.15 and/or at most 1.2 Cr; at least 0.01 and/or at most 0.1 Nb; optionally Ca in an amount consistent with calcium treatment
inclusion control matches; remainder Fe and unavoidable impurities; and wherein the steel has a substantially single-phase ferritic microstructure containing a mixture of polygonal ferrite (PF) and acicular/bainitic ferrite (AF/BF) and wherein the total volume fraction of the sum of the ferrite constituents is at least 95% and wherein the ferrite constituents are mixed with fine Composite carbides and/or carbo-nitrides of V and optionally of Mo and/or Nb are precipitation strengthened.

However, it has been found that the cold-formability, in particular the local cold-formability, of this flat steel product is not yet sufficiently large. In addition, the alloy concept is comparatively expensive.

• - Erhitzen von Stahl mit der chemischen Zusammensetzung auf eine Temperatur von zumindest 1250 °C,
• - Warmwalzen des Stahls bei einer abschließenden Walztemperatur von 850 - 930 °C,
• - Abschrecken des Stahls auf eine Haspeltemperatur von 450 - 575 °C,
• - Haspeln des Stahls bei der Haspeltemperatur,
• - Kühlen des Stahls, und
• - Kaltnachwalzen

Furthermore, from the patent specification EP 3 492 611 B1 a method for producing hot-rolled steel is known, having a tensile strength of at least 950 MPa and a microstructure comprising bainite with an area ratio of 70% or more, the difference being one or both of the following: martensite with an area ratio of 30% or less, and optionally ferrite with an area ratio of 20% or less, the method comprising the steps of:

• - heating steel with the chemical composition to a temperature of at least 1250 °C,
• - hot rolling of the steel at a final rolling temperature of 850 - 930 °C,
• - quenching the steel to a coiling temperature of 450 - 575 °C,
• - steel coiling at coiling temperature,
• - cooling the steel, and
• - skin pass rolling

The steel has the following alloy composition in % by mass,
C 0.07-0.10, Si 0.01-0.25, Mn 1.5-2.0, Cr 0.5-1.0, Ni 0.1-0.5, Cu 0.1- 0.3, Mo 0.01-0.2, Al 0.01-0.05, Nb 0.015-0.04, V 0-0.1, Ti 0-0.1, difference Fe and unavoidable impurities. The steel used is comparatively expensive due to the addition of chromium, copper and nickel and also does not yet have sufficiently high cold workability. Local cold formability is not discussed.

Proceeding from this, the present invention is based on the object of creating a high-strength, hot-rolled flat steel product and a method for producing such a flat steel product, and thus, in relation to the steel, a combination of high strength with simultaneous high local cold workability and high economic efficiency reach.

This object is achieved by a high-strength, hot-rolled flat steel product having the features of claim 1 and a method for producing a flat steel product having the features of claim 13. Advantageous refinements of the invention are specified in the dependent claims.

• C: 0,04 bis 0,08
• Si: 0,1 bis 0,6
• Mn: 1,0 bis 2,0
• P: max. 0,06
• S: max. 0,01
• N: max. 0,012
• AI: bis zu 0,06
• Ti: bis zu 0,18 und/oder Nb: bis zu 0,08
• Mo: bis zu 0,35
• mit Ti+Nb mehr als 0,06 Gewichts-%, wobei ein überstöchiometrischer Anteil an Kohlenstoff und Stickstoff gemäß nachfolgender Formel 1 vorliegt: $$\mathrm{1,0}<\left(\text{C}/12+\text{N}/14\right)/\left(\text{Ti}/48+\text{Nb}/93+\text{Mo}/96\right),$$
  

According to the invention, a high-strength, hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, has an elongation at break of at least 10%, preferably at least 16%, a degree of cold workability of at least 0.12, advantageously at least 0.17 and a ratio of local to global cold workability of at least 5 and at most 13, and a structure consisting of more than 50% by volume Bainite, remainder precipitation-hardened ferrite, with the following chemical composition of the steel (in % by weight):

• C: 0.04 to 0.08
• Si: 0.1 to 0.6
• Mn: 1.0 to 2.0
• P: 0.06 max
• S: 0.01 max
• N: 0.012 or less
• AI: up to 0.06
• Ti: up to 0.18 and/or Nb: up to 0.08
• Mon: up to 0.35
• with Ti+Nb more than 0.06% by weight, with a superstoichiometric proportion of carbon and nitrogen according to the following formula 1: $$1.0<\left(\text{C}/12+\text{N}/14\right)/\left(\text{Ti}/48+\text{Nb}/93+\text{Mon}/96\right),$$
  

• - eine absolute Abweichung von maximal 12, vorteilhaft maximal 7 Volumen-% des Anteils an Ferrit in den Bereichen oberflächennah oder 1/4 Dicke des Stahlflachproduktes, in Bezug auf den Bereich 1/2 Dicke des Stahlflachproduktes, beträgt und/oder
• - die Abweichung im Aspektverhältnis in Walzrichtung in den drei Bereichen des Stahlflachproduktes, zum Mittelwert weniger als 0,3 in jeder der drei Bereiche beträgt, und/oder
• - in den drei Bereichen der Härteunterschied HV0,1 jeweils maximal 20 HV 0,1, vorteilhaft maximal 15 HV 0,1, noch vorteilhafter maximal 10 HV 0,1 im Vergleich zum Mittelwert über die gesamte Dicke des Stahlflachproduktes beträgt, eine ausgezeichnete Kombination von Festigkeits-, Dehnungs- und Umformeigenschaften.

remainder iron including unavoidable steel-accompanying elements, with optional addition of one or more elements of Cr, Ni, V, B or Ca,
where the structure distribution over the thickness of the steel flat product in the three areas close to the surface, 1/4 thickness and ½ thickness of the steel flat product, is characterized by

• - an absolute deviation of a maximum of 12, advantageously a maximum of 7% by volume of the proportion of ferrite in the areas close to the surface or 1/4 thickness of the flat steel product, in relation to the area 1/2 thickness of the flat steel product, and/or
• - the deviation in the aspect ratio in the rolling direction in the three areas of the steel flat product, from the mean, is less than 0.3 in each of the three areas, and/or
• - in the three areas the hardness difference HV0.1 is a maximum of 20 HV 0.1, advantageously a maximum of 15 HV 0.1, even more advantageously a maximum of 10 HV 0.1 compared to the mean value over the entire thickness of the steel flat product, an excellent combination of Strength, elongation and forming properties.

For the measurements of microstructure distribution and hardness across the thickness of the steel flat product, it is irrelevant from which side of the surface these are carried out.

In particular, the flat steel product is characterized by a combination of high strength and excellent cold formability. In addition, the production of this flat steel product according to the invention based on the alloying elements C, Si, Mn, Nb and/or Ti is comparatively inexpensive.

The steel flat product according to the invention is specifically characterized by a high elongation at break A of at least 10%, a high hole expansion ratio (LA) of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, a measure of cold workability (FL) of at least 0, 12, advantageously at least 0.17 and a ratio of local to global cold workability (LFR) of at least 5 and at most 13, while at the same time having a high tensile strength of at least 760 MPa.

In an advantageous development of the invention, the steel alloy also optionally has one or more elements of Cr, Ni, V or B with the following contents in % by weight in order to set optimized combinations of properties: Cr: more than 0.1 up to 0.6, Ni : more than 0.1 up to 0.6, V: more than 0.01 up to 0.2 and B: more than 0.0005 up to 0.01, with a superstoichiometric proportion of carbon and nitrogen according to the following formula 2 exists: $$1.0<\left(\text{C}/12+\text{N}/14\right)/\left(\text{Ti}/48+\text{Nb}/93+\text{Mon}/96+\text{V}/51\right)$$

In a further advantageous development of the invention, the steel is alloyed with Ca for inclusion control. As a result, the inclusions of MnS and Al2O3, which are unfavorable with regard to the final properties, are replaced by inclusions containing Ca that are less harmful, particularly with regard to the morphology. The addition to the steel is a maximum of 0.01% by weight.

• Ti: mindestens 0,02, vorteilhaft mindestens 0,04, noch vorteilhafter mindestens 0,06, Nb:
  • mindestens 0,01, Mo: mindestens 0,05 und Ti + Nb: bis zu 0,2.

In a further advantageous development of the invention, the flat steel product contains the following alloy composition in % by weight in order to achieve particularly favorable combinations of properties:

• Ti: at least 0.02, advantageously at least 0.04, more advantageously at least 0.06, Nb:
  • at least 0.01, Mo: at least 0.05, and Ti + Nb: up to 0.2.

The microstructure consists mainly of bainite and, to a lesser extent, of ferrite. The bainite is a mixture of constituents characterized by a main constituent of at least 50% by volume and minor constituents, the main constituent being bainitic ferrite formed by precipitations of (Ti, Nb, Mo)(C,N) or V( C,N) is hardened and the secondary components consist of carbon-rich components such as martensite, retained austenite, lower bainite and pearlite. The structure advantageously consists of more than 75% by volume of bainite.

In addition, the structure may contain carbon-rich structural components. Particularly favorable properties are only achieved if the microstructure contains a maximum of 10%, preferably a maximum of 5%, carbon-rich microstructure components (e.g. martensite, retained austenite, pearlite).

It has also proven to be advantageous if the grain elongation of all microstructure components in the rolling direction, measured at a position 1/2 the thickness of the flat steel product below the surface of the flat steel product, characterized by the area-average aspect ratio of all microstructure components in the rolling direction of at most 2.0, and/or the mean value over the three areas close to the surface, 1/4 thickness and 1/2 thickness of the steel flat product is at most 2.0, advantageously 1.6.

It has also proven to be advantageous for high cold workability if, on average, half of the precipitations of (Ti, Nb, Mo)(C,N) or V(C,N), which strengthen the ferrite and the main component of bainitic ferrite a diameter of less than 10 nm and/or the precipitates have a mean distance of less than 750 nm.

• - oberflächennah: mindestens 0,9
• - 1/2-Dicke des Stahlflachproduktes: maximal 0,1

It is also advantageous if the ratio of shear texture components to rolling texture components increases towards the surface and has the following values:

• - near the surface: at least 0.9
• - 1/2 thickness of the steel flat product: maximum 0.1

The hot-rolled flat steel product according to the invention can be provided with a metallic or non-metallic coating. The metallic coating can be applied to the flat steel product electrolytically or by hot dipping and is advantageously zinc-based.

Such a hot-rolled flat steel product is advantageously used in the automobile industry for the production of components, in particular chassis components. Hot-rolled flat steel products according to the invention have thicknesses of 1.6 to 6.0 mm. However, thicknesses less than 1.6 mm or greater than 6.0 mm are also covered by the invention.

The flat steel product according to the invention advantageously has a tensile strength Rm of at least 760 MPa along the rolling direction, a yield point ratio of at least 0.8, an elongation at break A of at least 10%, preferably at least 16%, a hole expansion ratio of at least 30%, advantageously at least 40% or even at least 50%. The degree of cold workability is at least 0.12, advantageously at least 0.17, with a ratio of local and global cold workability of at least 5 and at most 13.

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 effect and interaction generally depends heavily on the amount, the presence of other alloying elements and the state of the solution in the material. The connections are varied and complex. The effect of the alloying elements in the alloy according to the invention will be discussed in more detail below.

In the case of the figures given below and in the claims for alloying element contents and all other figures, the figures should be included as basic values. The use of the term "up to" in the content ranges, e.g. 0.01 to 1% by weight means that the basic values, here 0.01 and 1 are included.

Carbon C: Needed for the formation of carbides, especially in connection with the so-called micro-alloying elements Nb, V and Ti, promotes the formation of martensite and bainite, stabilizes austenite and generally increases strength. Higher contents of C deteriorate the welding properties and lead to the deterioration of the elongation and toughness properties, which is why a maximum content of at most 0.08% by weight is specified. In order to achieve sufficient material strength, a minimum addition of 0.04% by weight is required.

Manganese Mn: Stabilizes austenite, increases strength and toughness. Higher contents of > 2.0% by weight Mn increase the risk of central segregation, which significantly reduces ductility and thus product quality. Lower contents <1.0% by weight do not allow the required strength and toughness to be achieved at the desired moderate analysis costs. Therefore, the content of Mn is specified to be 1.0 to 2.0% by weight.

Aluminum Al: Used for deoxidation in the steelworks process. The amount of Al used depends on the process. Therefore, no minimum Al content is specified. An Al content of more than 0.06% by weight significantly impairs the casting behavior in continuous casting. This results in greater effort when casting. Therefore, the content of Al is set to 0.06% by weight at maximum

Silicon Si: Is one of the elements that enable steel to be strengthened by solid solution strengthening in a cost-effective manner. However, Si reduces the surface quality of the hot strip by promoting firmly adhering scale on the reheated slabs, which can only be removed with great effort or only insufficiently if the Si content is high. This is particularly disadvantageous during subsequent galvanizing. Therefore, the Si content is limited to a maximum of 0.6% by weight. For the effectiveness of Si, a lower limit of 0.1% by weight is to be regarded as sensible.

Calcium Ca: is added to the steel to control inclusions, to prevent the formation of unfavorable inclusions of MnS and Al2O3 and to form inclusions containing Ca, which are less harmful in terms of morphology, with these elements. The addition to the steel is a maximum of 0.01% by weight.

Micro-alloying elements are usually only added in very small amounts (<0.2% by weight per element). In contrast to the alloying elements, they mainly act through the formation of precipitates, but can also influence the properties in the dissolved state. Despite the small amounts added, micro-alloying elements have a strong influence on the targeted manufacturing conditions as well as the processing and end properties of the product.

Typical micro-alloying elements are, for example, niobium and titanium. These elements can be dissolved in the iron lattice and form carbides, nitrides and carbonitrides with carbon and nitrogen. Since the micro-alloying elements are comparatively expensive, the alloyed proportion is kept as low as possible. On the other hand, the carbon that is over-stoichiometric and therefore not bound in precipitations of the micro-alloying elements in carbon-rich structural components contributes to the cost-effective and necessary increase in strength. Therefore, the over-stoichiometric proportion of carbon and nitrogen is calculated according to $$\left(\text{C}/12+\text{N}/14\right)/\left(\text{Ti}/48+\text{Nb}/93+\text{Mon}/96\right)\text{on}>1\text{fixed}.$$

The effect of Nb and Ti depends in particular on the process control during hot rolling and the subsequent cooling process. With the addition of micro-alloying elements, the aim is to achieve grain refinement in the course of the process and to produce precipitations in the nanometer size range. Therefore, an Nb+Ti content of more than 0.06% by weight is a prerequisite for achieving the desired strength and good elongation properties. A cumulative value of more than 0.2% by weight, on the other hand, no longer improves the properties of the steel, since contents above the specified cumulative value in the specified analysis and when using conventional furnaces can no longer be dissolved when the slabs are reheated and thus have no positive effect.

Niobium Nb: The addition of niobium has a grain-refining effect, in particular due to the formation of carbides in the rolling process, which at the same time improves strength, toughness and elongation properties. In addition, very fine Nb-containing precipitations can be formed after the phase transformation, which contribute significantly to the strength of the product. With contents of more than 0.08% by weight saturation occurs, which is why a maximum content of less than or equal to 0.08% by weight is provided. A minimum content of 0.01% by weight is provided for sufficient effectiveness.

Titanium Ti: As a carbide former, it has a grain-refining effect, which improves strength, toughness and elongation properties at the same time. Contents of Ti of more than 0.18% by weight impair the ductility and the hole expansion capacity by the formation of very coarse, primary TiN precipitates, which is why a maximum content of 0.18% by weight is specified. A minimum content of 0.02, advantageously 0.04, even more advantageously 0.06% by weight is provided for sufficient effectiveness.

Molybdenum Mo: Increases hardenability and reduces the critical cooling rate, thus promoting the formation of fine, bainitic structures. In addition, even the use of small amounts of Mo delays the coarsening of fine precipitates, which should be as fine as possible to increase the strength of micro-alloyed structures. A minimum content of 0.05% by weight is provided for sufficient effectiveness and is limited to a maximum of 0.35% by weight for cost reasons.

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, among other things, it is very susceptible to segregation and greatly reduces toughness. The accumulation of phosphorus at the grain boundaries can cause cracks to appear along the grain boundaries during hot rolling. In addition, phosphorus increases the transition temperature from tough to brittle behavior by up to 300 °C. However, the use of small amounts of P can also be used to increase the strength at low cost through targeted measures that are precisely controlled on the process side. For the above reasons, the phosphorus content is limited to a maximum of 0.06% by weight.

Sulfur S: Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel because it leads to undesirable inclusions of MnS, which degrades elongation and toughness properties. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt and, if necessary, to convert the elongated inclusions into a more favorable geometric shape by means of a so-called Ca treatment. For the above reasons, the sulfur content is limited to a maximum of 0.01% by weight.

Nitrogen N: Is also a secondary 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, for example, by alloying aluminum, niobium or titanium. As a result, however, the alloying elements mentioned are no longer available for the new formation of small precipitations, which are very efficient in terms of strength, in the later process. For the above reasons, the nitrogen content is limited to a maximum of 0.012% by weight.

Chromium Cr: As an optional alloyed element, Cr improves strength and reduces corrosion rate and retards ferrite and pearlite formation. The maximum content is set at a maximum of 0.6% by weight, since higher contents result in a deterioration in ductility. A content of more than 0.1% by weight is intended for sufficient effectiveness.

Nickel Ni: The optional use of even small amounts of Ni promotes ductility while maintaining strength. Because of the comparatively high cost, the content of Ni is limited to at most 0.6% by weight. A content of more than 0.1% by weight is intended for sufficient effectiveness.

Vanadium V: With the present alloy concept, the addition of vanadium is not absolutely necessary. For cost reasons, the vanadium content is limited to a maximum of 0.2% by weight. If an addition of V is nevertheless intended, the over-stoichiometric proportion of carbon and nitrogen is calculated according to formula 2: (C/12+N/14)/(Ti/48+Nb/93+Mo/96+V/51) to > 1 fixed. A V content of more than 0.01% by weight is then also provided for sufficient effectiveness.

Boron B: B is an effective hardenability enhancing element that is effective in very small amounts. The martensite start temperature remains unaffected. To be effective, boron must be in solid solution. Since it has a high affinity for nitrogen, the nitrogen must first be bound are, preferably by the stoichiometrically necessary amount of titanium. Due to its low solubility in iron, the dissolved boron tends to accumulate at the austenite grain boundaries. There it partially forms Fe-B carbides, which are coherent and lower the grain boundary energy. Both effects delay the formation of ferrite and pearlite and thus increase the hardenability of the steel. Excessively high levels of boron are harmful, however, since iron boride can form, which has a negative effect on the hardenability, formability and toughness of the material.

For the above reasons, the boron content for the alloy concept according to the invention is limited to values of a maximum of 0.01% by weight. A content of more than 0.0005% by weight is intended for sufficient effectiveness.

• - Erschmelzen einer Stahlschmelze enthaltend (in Gewichts-%):
  • C: 0,04 bis 0,08
  • Si: 0,1 bis 0,6
  • Mn: 1,0 bis 2,0
  • P: max. 0,06
  • S: max. 0,01
  • N: max. 0,012
  • Al: bis zu 0,06
  • Ti: bis zu 0,18 und/oder
  • Nb: bis zu 0,08
  • Mo: bis zu 0,35 mit Ti+Nb mehr als 0,06 und wobei ein überstöchiometrisches Verhältnis an Kohlenstoff und Stickstoff gemäß nachfolgender Formel 1 vorliegt: 1,0 < (C/12+N/14) / (Ti/48+Nb/93+Mo/96) eingestellt wird, mit optionaler Zulegierung eines oder mehrerer Elemente von Cr, Ni, V, B oder Ca, Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente
• - Vergießen der Stahlschmelze zu einer Bramme oder Dünnbramme mittels eines horizontalen oder vertikalen Brammen- oder Dünnbrammengießverfahrens,
• - Wiedererwärmen der Bramme oder Dünnbramme auf 1100 °C bis 1270 °C und anschließendes Warmwalzen der Bramme oder Dünnbramme mit folgenden direkt aufeinanderfolgenden Schritten:
  • - Walzen im letzten Walzstich zu einem Warmband auf die geforderte Enddicke bei einer Endwalztemperatur EWT wobei gilt: $$\begin{array}{l}\text{EWT}\ge \text{EWTmin}=682°\text{C}+464\text{ C}+6445\text{ Nb}-644\times \text{Nb}\mathrm{0,5}+732\text{ V}-230\text{ V}\mathrm{0,5}+890\text{ Ti}\\ +363\text{ Al}-36\text{ Si}\end{array}$$
    
• - Abkühlen mit einer mittleren Abkühlgeschwindigkeit von 30 K/s bis 150 K/s
• - Aufhaspeln des Warmbandes zu einem Coil bei einer Haspeltemperatur HT, die gering genug ist, um die vorteilhaften Gefügebestandteile einzustellen mit $$\text{HT}\le \text{HTmax}=761°\text{C}-217\times \text{C}-77\times \text{Mn}+97\times \text{Si}-47\times \text{Mo}-53\times \text{Cr}-34\times \text{Ni}-21\times \text{V}$$
  
  und andererseits geeignet ist, um ausreichend Ausscheidungsverfestigung im anschließendem zeitabhängigen Abkühlprozess T(t) zu erbringen, definiert durch (Formel 5) 17000 ≤ HP ≤ 18800 mit HP(T, t) = T(t) × (In(t) + 20), wobei die Temperatur T in K und die Dauer t in h angegeben wird.
• - Abkühlen in einem Abkühlprozess T(t) mit einer mittleren Abkühlrate von 5 K/h bis 50 K/h zwischen Haspeltemperatur und 100°C, mit anschließendem Abkühlen an ruhender Luft auf Raumtemperatur.

A method according to the invention for producing a hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of over 30%, advantageously at least 40%, particularly advantageously at least 50%, a measure the cold formability of at least 0.12, advantageously at least 0.17 and a ratio of local and global cold formability of at least 5 and at most 13, comprising the steps:

• - Melting of a steel melt containing (in weight %):
  • C: 0.04 to 0.08
  • Si: 0.1 to 0.6
  • Mn: 1.0 to 2.0
  • P: 0.06 max
  • S: 0.01 max
  • N: 0.012 or less
  • Al: up to 0.06
  • Ti: up to 0.18 and/or
  • Nb: up to 0.08
  • Mo: up to 0.35 with Ti+Nb greater than 0.06 and with a superstoichiometric ratio of carbon and nitrogen according to Formula 1 below: 1.0<(C/12+N/14)/(Ti/48+ Nb/93+Mo/96) with optional alloying of one or more elements of Cr, Ni, V, B or Ca, balance iron including unavoidable steel-accompanying elements
• - Casting of the molten steel into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,
• - Reheating of the slab or thin slab to 1100 °C to 1270 °C and subsequent hot rolling of the slab or thin slab with the following consecutive steps:
  • - Rolling in the last rolling pass to a hot strip to the required final thickness at a final rolling temperature EWT where the following applies: $$\begin{array}{l}\text{EWT}\ge \text{EWTmin}=682°\text{C}+464\text{C}+6445\text{Nb}-644\times \text{Nb}0.5+732\text{V}-230\text{V}0.5+890\text{Ti}\\ +363\text{Al}-36\text{si}\end{array}$$
    
• - Cooling with an average cooling speed of 30 K/s to 150 K/s
• - Coiling the hot strip into a coil at a coiling temperature HT that is low enough to set the advantageous structural components with $$\text{HT}\le \text{HT max}=761°\text{C}-217\times \text{C}-77\times \text{Mn}+97\times \text{si}-47\times \text{Mon}-53\times \text{Cr}-34\times \text{no}-21\times \text{V}$$
  
  and on the other hand is suitable to provide sufficient precipitation strengthening in the subsequent time-dependent cooling process T(t), defined by (Formula 5) 17000 ≤ HP ≤ 18800 with HP(T, t) = T(t) × (In(t) + 20 ), where the temperature T is given in K and the duration t in h.
• - Cooling in a cooling process T(t) with an average cooling rate of 5 K/h to 50 K/h between coiling temperature and 100°C, with subsequent cooling in still air to room temperature.

The idea on which the invention is based is explained below and described in more detail using examples.

Today, thermomechanical rolling is usually used to produce high-strength, micro-alloyed hot strip. The finish rolling takes place in a low temperature range of less than EWTmin, in which the austenite no longer recrystallizes and as a result the accumulated dislocations lead to a high nucleus density at the beginning of the phase transformation and thus produce a fine hot strip structure. A key goal of thermomechanical rolling is to increase strength and ductility by reducing the grain size of the hot strip structure.

In alloys with a superstoichiometric ratio of carbon and nitrogen to microalloy elements, the carbon, in contrast to nitrogen, is not completely precipitated in the form of strength-increasing microalloy precipitates. The carbon that is not precipitated in microalloy precipitates leads to the formation of carbon-rich structural components and different carbon-rich components of bainite. It is crucial for cold workability that the carbon-rich structural components and the carbon-rich components of the bainite are advantageously present in terms of size and distribution. Advantageously means that the size is small and the distribution is as uniform as possible.

In addition to different alloy compositions, different processes are also used to achieve a balanced relationship between local and global cold workability.

There are essentially three process paths that can be distinguished in which the type and distribution of the carbon-rich structural components and the state of precipitation of the micro-alloying elements are adjusted. The type and distribution of the carbon-rich structural components influences the cold formability and the state of precipitation of the micro-alloying elements influences the strength.

1. 1. Niedrige Haspeltemperaturen von z.B. 450 < HT < 550 °C führen zur Einstellung eines Tieftemperaturbainits mit Bestandteilen von kohlenstoffreichen Bestandteilen, die sehr fein verteilt sind, z.B. unterer Bainit. Das resultierende Produkt weist eine hohe Kaltumformbarkeit mit einem ausgeprägten Anteil von lokaler Kaltumformbarkeit auf („hohe lokale Kaltumformbarkeit“). Die Festigkeit ist allerdings vergleichsweise gering, da bei den niedrigen Haspeltemperaturen nur ein geringerer Anteil von Mikrolegierungselementen zur Ausscheidung gebracht werden und entsprechend der Beitrag zu Ausscheidungsverfestigung gering ist.
2. 2. Hohe Haspeltemperatur von z.B. HT > 650 °C zur Einstellung eines ferritischen Gefüges. Der Kohlenstoff liegt in Form harter Gefügebestandteile wie Karbide, Perlit oder Martensit vor. Das resultierende Produkt weist eine hohe Kaltumformbarkeit mit einem geringeren Anteil von lokaler Kaltumformbarkeit auf. Die Festigkeit ist höher, weil ein höherer Anteil von Mikrolegierungselemente zur Ausscheidung gebracht werden.
3. 3. Eine mittlere Haspeltemperatur von z.B. 550 < HT < 650 °C zur Erzeugung eines Mischgefüges bestehend aus Hochtemperaturbainit (z.B. oberer Bainit und granularer Bainit) und Ferrit mit sowohl hoher lokaler Kaltumformbarkeit als auch hoher Festigkeit durch einen hohen Ausscheidungsanteil war bislang nicht zielführend. Entweder wurde nur eine hohe Kaltumformbarkeit oder es wurde nur eine hoher Festigkeit durch einen hohen Ausscheidungsanteil erreicht.

The process paths are

1. 1. Low coiling temperatures of eg 450 < HT < 550 °C lead to the setting of a low-temperature bainite with components of carbon-rich components that are very finely distributed, eg lower bainite. The resulting product has high cold formability with a pronounced proportion of local cold formability (“high local cold formability”). However, the strength is comparatively low, since at the low coiling temperatures only a small proportion of micro-alloying elements are precipitated and the contribution to precipitation hardening is correspondingly small.
2. 2. High coiling temperature of eg HT > 650 °C to set a ferritic structure. The carbon is in the form of hard structural components such as carbides, pearlite or martensite. The resulting product has high cold formability with a lower proportion of local cold formability. The strength is higher because a higher proportion of micro-alloying elements are precipitated.
3. 3. An average coiling temperature of e.g. 550 < HT < 650 °C to produce a mixed structure consisting of high-temperature bainite (e.g. upper bainite and granular bainite) and ferrite with both high local cold workability and high strength due to a high proportion of precipitation has not been expedient up to now. Either only high cold formability was achieved or only high strength was achieved through a high proportion of precipitation.

Within the scope of the present investigations, it was found to be essential that the predominantly bainitic, micro-alloyed hot strip exhibits both high strength and high local cold formability when, in combination with the alloy composition and a super-stoichiometric ratio of 1.0 < (C/12+N /14) / (Ti/48+Nb/93+Mo/96), the steel flat product is finish-rolled with a final rolling temperature of at least EWTmin according to formula 3 and then coiled and cooled in a temperature time window that is characterized by a maximum coiling temperature HTmax according to formula 4 and characterized by 17000 ≤ HP ≤ 18800, where HP is calculated according to Formula 5.

During investigations, it was found that the hot-rolled steel flat product has a strength contribution to the tensile strength by precipitation of at least 80 MPa or more when in coiled and cooled over a temperature time window characterized by 17000 ≤ HP ≤ 18800 compared to a temperature time window characterized by HP ≤ 15990. The strength contribution is necessary to cost-effectively achieve high tensile strength and high yield strength ratio. At the same time, if HTmax is observed within the temperature-time window, the structure that is favorable for local formability and strength is formed.

The flat steel product according to the invention is characterized by compliance with the temperature-time window mentioned in that half of the precipitations of (Ti, Nb, Mo) (C,N) and / or V (C,N), the ferrite and the main component of bainitic Reinforce ferrite, have a diameter of less than 10 nm and/or the precipitates have an average spacing of less than 750 nm.

Surprisingly, it was found that when the specified temperature-time window is observed in combination with a final rolling temperature of at least EWTmin, the local cold workability is high, while when the specified temperature-time window is observed in combination with a final rolling temperature of less than EWTmin, the local cold workability is comparatively low.

The flat steel product produced according to the invention has, in addition to a cost-effective alloy concept, high strength and, at the same time, high local cold workability. In addition, the manufacturing method according to the invention is characterized by high process stability.

In contrast to ferrite, bainite usually consists of different components. The different components of the bainite are formed from the austenitic phase during the production of the hot strip after the final rolling with decreasing temperature. Compared to ferrite, bainite forms at lower temperatures and bainite has a higher average dislocation density.

Both the high strength and the high local cold formability can only be achieved with a predominantly bainitic structure. The reason is that the bainitic structure has a high density of dislocations and a small grain size. With a predominantly ferritic structure, the high local cold formability is not achieved. The reason is that the grain size of the ferrite is comparatively large and the over-stoichiometric carbon is precipitated in the form of very hard and comparatively coarse carbides at phase boundaries. During cold forming, these carbides lead to early material failure due to local stress concentrations.

With previously known solutions, hot strip can be achieved with either the property combination of comparatively low strength with comparatively high local cold workability or the property combination of comparatively high strength with comparatively low local cold workability.

• - Definition wahre Gleichmaßdehnung: In (1+Ag/100), wobei Ag die technische Gleichmaßdehnung ist
• - Definition wahres Lochaufweitungsverhältnis: In (1+LA/100), wobei LA das technische Lochaufweitungsverhältnis ist

The invention, on the other hand, allows the property combination of high strength and high local cold workability to be achieved.
The reason is that the accumulated local damage during the forming process, especially with large differences in strain, is only adequately taken into account when the true values are considered.

• - Definition of true uniform elongation: In (1+Ag/100), where Ag is the technical uniform elongation
• - Definition of true hole expansion ratio: In (1+LA/100), where LA is the technical hole expansion ratio

• - Definition Maß der Kaltumformbarkeit (Formability Level „FL“): (wahre Gleichmaßdehnung × wahres Lochaufweitverhältnis)^0,5

The degree of cold formability is described by the geometric mean of local and global formability:

• - Definition of degree of cold formability (Formability Level "FL"): (true uniform elongation × true hole expansion ratio) ^0.5

The ratio of local and global cold formability is defined as: Local Formability Ratio (LFR) = (true hole expansion ratio / true uniform strain)

• - A ≥ 10 %
• - LA ≥ 30 %
• - Maß der Kaltumformbarkeit ≥ 0,12
• - 5 ≤ Verhältnis von lokaler und globaler Kaltumformbarkeit ≤ 13

The following criteria are required for high local cold formability, in particular for the area of application of the flat steel product according to the invention:

• - A ≥ 10%
• - LA ≥ 30%
• - Degree of cold formability ≥ 0.12
• - 5 ≤ ratio of local and global cold formability ≤ 13

• - A ≥ 16 % LA ≥ 50 %
• - Maß der Kaltumformbarkeit ≥ 0,17
• - 5 ≤ Verhältnis von lokaler zu globaler Kaltumformbarkeit ≤ 13

The test results presented in the appendix extend to exemplary embodiments with a tensile strength of at least 760 MPa. For the exemplary embodiments, the following criteria are required for high local cold formability, especially for the area of application:

• - A ≥ 16% LA ≥ 50%
• - Degree of cold formability ≥ 0.17
• - 5 ≤ ratio of local to global cold workability ≤ 13

As part of the investigations, the mechanical-technological properties and the microstructure of the hot-rolled steel flat products were examined. In addition to tensile tests according to ISO 6892-1 to determine the tensile strength Rm, yield point Rp0.2 and elongation at break A and uniform elongation Ag, hole expansion tests according to ISO 16630 were carried out.

• EWT = Endwalztemperatur
• HT = Haspeltemperatur
• MW = Mittelwert
• Leg. = Legierung
• GOS = Grain Orientation Spread, (deutsch: Kornorientierungsstreuung)
• KAM = Kernel Average Misorientation, (deutsch: Mittlere Kernel-Missorientierung)
• IQ = Image Quality (deutsch: Beugungsmusterqualität)
• AR = Aspect Ratio (deutsch: Aspektverhältnis)
• SGV= Streckgrenzverhältnis
• S_P = Festigkeitsbeitrag durch Ausscheidungsbildung
• S_M = Festigkeit in einem überwiegend bainitischen Gefüge, das aufgrund des geringen Wertes des Parameters HP keine Ausscheidungen aufweist

The following abbreviations are used in the tables in the appendix and in the following description:

• EWT = finish rolling temperature
• HT = reel temperature
• MV = mean
• legs = alloy
• GOS = Grain Orientation Spread
• KAM = Kernel Average Misorientation
• IQ = Image Quality (German: diffraction pattern quality)
• AR = Aspect Ratio
• SGV= yield ratio
• S _P = Contribution to strength through the formation of precipitates
• S _M = strength in a predominantly bainitic structure that does not show any precipitates due to the low value of the HP parameter

For hot strip thicknesses > 3 mm, the proportional specimen shape with the designation A for elongation at break was used. In the case of hot strip thicknesses of ≤ 3 mm, the non-proportional specimen shape with an initial gauge length of 80 mm was used. For better comparability, when using the non-proportional sample shape, the values for the elongation at break were converted from the uniform elongation according to A=AG×b, b having previously been determined at 2.254 on the basis of comparison samples. In the case of the hole expansion test, the mean value from at least 3 individual tests is always given.

• - Oberflächennah: Messfeld mit 100 µm × 100 µm mit einem Abstand von der Probenoberfläche von 0,1 mm
• - 1/4 Dicke: Messfeld mit 100 µm × 100 µm mittig zwischen Oberfläche und Probenmitte
• - 1/2 Dicke: Messfeld mit 100 µm × 100 µm mit Abstand von der Probenmitte von 0,1 mm Die Positionierung der Messfelder lässt sich der Skizze in 2 entnehmen.

For the metallographic assessment of the structure of the hot strip, the following areas were defined in different thickness ranges of the sample:

• - Near-surface: measuring field with 100 µm × 100 µm at a distance of 0.1 mm from the sample surface
• - 1/4 thickness: measuring field with 100 µm × 100 µm in the middle between the surface and the middle of the sample
• - 1/2 thickness: measuring field with 100 µm × 100 µm at a distance of 0.1 mm from the center of the sample The positioning of the measuring fields can be seen in the sketch in 2 remove.

The metallographic investigations were carried out on samples along the direction of rolling.

• GOS (Grain Orientation Spread): Mittlere Missorientierung aller Messpunkte innerhalb eines Kornes zur mittleren Orientierung des Korns. Zur Bestimmung der Körner wird ein Segmentierungswinkel von 15° verwendet.

To characterize the microstructure, electron backscatter images (EBSD) were taken in the measuring fields defined above. For this purpose, longitudinal sections were created which were ground mechanically and polished to 1 µm. The samples were then polished with OP-S for approx. 10 minutes in order to create a surface that was as free of deformation as possible. An EDAX DigiView 5 EBSD camera with a binning of 10 × 10 and a recording rate of 140 Hz was used for the measurements, the acceleration voltage was 15 kV. The increment between the individual measuring points was 0.1 µm in each case. The parameters relevant to the invention were determined as follows:

• GOS (Grain Orientation Spread): Mean disorientation of all measuring points within a grain to the mean orientation of the grain. A segmentation angle of 15° is used to determine the grains.

KAM (Kernel Average Misorientation) and GKAM: To calculate the KAM values, the average misorientation of an EBSD measuring point to its next-but-one neighboring measuring points is determined. The maximum permissible misorientation is 4°. For the GKAM, the KAM values of all measuring points of a grain are averaged, whereby a segmentation angle of 15° is used to determine the grains.

• - Ferrit besteht aus polygonalem und quasipolygonalem Ferrit und die Körner werden durch Korngrenzen mit Misorientierungswinkeln > 15° abgegrenzt. Im Korninneren des Ferrits treten keine Kleinwinkelkorngrenzen < 15° auf, die Werte des Grain Orientation Spreads (GOS) betragen < 2° und die Werte der Grain Kernel Average Misorientation (GKAM) sind typischerweise < 0,4°. TEM-Aufnahmen zeigen eine hohe Dichte von (Ti, Nb, Mo)(C,N)- Ausscheidungen im Korninneren. Insbesondere im Bereich der Kornzwickel können (Fe,Mn)-Karbide vorliegen.
• - Die Körner des granularen Bainit, werden durch Korngrenzen von > 15° abgegrenzt. Aufgrund der displaziven Phasenumwandlung des Austenits in den Bainit treten im Korninneren Kleinwinkelkorngrenzen auf, die GOS-Werte betragen ≥ 2° und die GKAM-Werte sind typischerweise ≥ 0,4°. In der EBSD IPF (Inverse Pole Figure) -Map sind typischerweise Lanzetten unterschiedlicher Orientierung im Korninneren zu erkennen. Lanzetten, die in der EBSD Image Quality Map keine Zweitphase zeigen, werden im Folgenden mit „bainitischem Ferrit“ bezeichnet. Zwischen den Körnern des bainitischen Ferrits ist eine kohlenstoffreiche Zweitphase in Form von Martensit, MA-Phase, unteren Bainit oder Perlit eingelagert. Insbesondere im Bereich der Kornzwickel können (Fe,Mn)-Karbide vorliegen. Der Flächenanteil der Zweitphase nimmt mit zunehmender Haspeltemperatur ab und kann 0 - 10 % betragen.

The types of structure that occur are defined metallographically as follows:

• - Ferrite consists of polygonal and quasi-polygonal ferrite and the grains are delimited by grain boundaries with misorientation angles > 15°. There are no small-angle grain boundaries < 15° in the grain interior of the ferrite, the values of the grain orientation spread (GOS) are < 2° and the values of the grain kernel average misorientation (GKAM) are typically < 0.4°. TEM images show a high density of (Ti, Nb, Mo)(C,N) precipitations in the interior of the grain. (Fe,Mn)-carbides can be present in particular in the area of the grain interstices.
• - The grains of granular bainite are delimited by grain boundaries of > 15°. Due to the displacive phase transformation of austenite into bainite, small-angle grain boundaries occur inside the grain, the GOS values are ≥ 2° and the GKAM values are typically ≥ 0.4°. In the EBSD IPF (Inverse Pole Figure) map, lancets of different orientation can typically be seen in the interior of the grain. Lancets that do not show a second phase in the EBSD Image Quality Map are referred to as "bainitic ferrite" in the following. Interspersed between the grains of bainitic ferrite is a carbon-rich second phase in the form of martensite, MA phase, lower bainite, or pearlite. (Fe,Mn)-carbides can be present in particular in the area of the grain interstices. The surface area of the second phase decreases with increasing coiling temperature and can be 0 - 10%.

Aspect ratio: For the EBSD measurement in the electron microscope, the samples were aligned in such a way that the rolling direction corresponds to the Y-direction of the measuring field. With the help of the Matlab Toolbox MTEX, ellipses were adapted to the shape of the individual grains (segmentation angle 15°) and parameterized via their long and short semi-axes, as well as the orientation of the long axis. The ellipses were calculated from these parameters for each grain and then the intersection of these ellipses with the X and Y axes of the coordinate system was determined. The ratio of the X-axis intersections of the grain ellipses to the Y-axis intersections of the grain ellipses corresponds to the aspect ratio of the grains in the rolling direction to the sheet normal. This calculation method ensures that the stretching of grains whose long axis does not point exactly in the direction of rolling is only determined in the normal direction of rolling or sheet metal.

• Die Härtewerte werden als Mittelwert aus 6 Einzelmessungen angegeben.

Jeweils 3 Härteeindrücke für die oberflächennahe Position sind zwischen 0 % und 10 % sowie 90% und 100% Abstand von der Oberfläche bezogen auf die Dicke des Blechs positioniert.The HV0.1 hardness test was carried out on the microsection in points at different distances from the surfaces. No measurement is made at a distance from the surfaces and the center of 0.1 mm. In addition:

• The hardness values are given as the average of 6 individual measurements.

3 hardness indentations for the near-surface position are positioned between 0% and 10% and 90% and 100% distance from the surface based on the thickness of the sheet.

3 hardness impressions each for the 1/4 position are positioned between 20% and 30% and 70% and 80% distance from the surface based on the thickness of the sheet. 3 hardness impressions each for the 1/2 position are positioned between 40% and 50% and 50% and 60% distance from the surface based on the thickness of the sheet. The positioning of the hardness indentations can be seen in the sketch 2 remove.

The alloy compositions of two exemplary embodiments are summarized in Table 1. Alloys A and B are single casts, so all samples A1-A14 and B1-B20 have the same compositions. Table 1 also shows the calculated values for the superstoichiometric ratio of carbon and nitrogen to microalloying elements (formula 2), ie $$1.0<\left(\text{C}/12+\text{N}/14\right)/\left(\text{Ti}/48+\text{Nb}/93+\text{Mon}/96+\text{V}/51\right)$$

Table 1 shows the alloy compositions of two examples.

Tables 2 and 3 show the results from different exemplary embodiments. Also shown is an evaluation of the results with regard to achieving the required characteristic values with J (achieved) and N (not achieved). If the specifications according to the invention, according to the 2nd line in the tables, are not met, they are marked with an underscore. The listed values are commercially rounded.

For alloys A and B, Table 2 lists the results for the mechanical characteristics with different process conditions. The underlined values are outside the required mechanical properties or outside the targeted process conditions.

With regard to the final rolling temperature (EWT), it must be ensured that complete recrystallization is achieved across the strip thickness in every thickness range. This is the case when EWT - EWTmin ≥ 0, where EWTmin = 682 °C + 464 C + 6445 Nb - 644 × Nb ^0.5 + 732 V - 230 V ^0.5 + 890 Ti +363 AI - 36 Si (formula 3). All element data are given in % by weight.

During the subsequent coiling of the strip, it must be ensured that a structure consisting of more than 50% by volume of bainite is created. This is given when the HT - HTmax s 0, where HTmax = 761 °C - 217 × C - 77 × Mn + 97 × Si - 47 × Mo - 53 × Cr-34 × Ni - 21 × V (Formula 4) . All elements are given in weight percent. If the conditions for EWT and HT are met, the characteristic values for formability given in Table 2 can be achieved. It turns out that a cost-effective increase in strength can only be achieved if a sufficient contribution to the tensile strength Rm is made by precipitation hardening _SP after coiling during the subsequent cooling process. For this it is necessary that during the cooling process T(t) a suitable temperature T prevails for a suitable duration t. This is given (Formula 5) when 17000 ≤ HP ≤ 18800 with HP(T, t) = T(t) × (In(t) + 20), where in the calculation of HP we always have T in K and t in h is specified.

1. 1. Der Abkühlprozess T(t) wird in n gleiche Zeitabschnitte t_i mit den zugehörigen Temperaturen T_i aufgeteilt, wobei n ausreichend groß zu wählen ist, so dass bei Aufteilung in deutlich mehr Zeitabschnitte das Ergebnis nahezu gleich bleibt.
2. 2. Berechnung der einzelnen Parameter HP_i = HP_i (t_i, T_i) = T_i × (In (t_i) + 20).
3. 3. Berechnung der Zeitabschnitte t_i* = exp (HP_i/T*-20), wobei T* eine beliebige Temperatur darstellt, zum Beispiel die Haspeltemperatur HT.4. Berechnung des Parameters HP mit HP = T* × (ln(t₁*+t₂*+...+t_n*)+20)

The procedure for calculating the HP parameter is as follows:

1. 1. The cooling process T(t) is divided into n equal periods of time t _i with the associated temperatures T _i , where n must be selected sufficiently large so that the result remains almost the same when divided into significantly more periods of time.
2. 2. Calculation of the individual parameters HP _i =HP _i (t _i , T _i )=T _i ×(In (t _i )+20).
3. 3. Calculation of the time periods t _i * = exp (HP _i /T*-20), where T* represents any temperature, for example the coiler temperature HT.4. Calculation of the parameter HP with HP = T* × (ln(t ₁ *+t ₂ *+...+t _n *)+20)

1. 1. Ermittlung der Festigkeit S_M in einem überwiegend bainitischen Gefüge, das aufgrund des geringen Wertes des Parameters HP keine Ausscheidungen aufweist. Mit Hilfe von TEM-Untersuchungen wurde festgestellt, dass der Zustand unabhängig von der Legierung bei HP = 15990 vorliegt. Als erster Schritt werden für alle Anwendungsbeispiele die Daten Rm über HP im Bereich 16080 < HP < 18000 aufgetragen und anschließend eine lineare Regression vorgenommen. im zweiten Schritt wird die Festigkeit bei HP = 15990 mit Hilfe der Regressionsgeraden bestimmt. Im vorliegenden Fall ist das für Legierung A der Festigkeitsbeitrag S_M,A (15990) = 804 MPa und für Legierung B der Festigkeitsbeitrag S_M,B (15990) = 762 MPa.
2. 2. Berechnung der theoretischen Festigkeit eines ausscheidungsfreien Gefüges in Abhängigkeit von HP durch S_M (HP) = S_M (15990) - 0,0495 (HP-15990)
3. 3. Berechnung des Festigkeitsbeitrags S_P (HP) durch S_P (HP) = Rm (HP) - S_M (HP)

The strength contribution S _P due to the formation of precipitates is determined in the following steps:

1. 1. Determination of the strength S _M in a predominantly bainitic structure which, due to the low value of the parameter HP, does not exhibit any precipitations. With the help of TEM investigations, it was found that the state exists at HP = 15990, regardless of the alloy. As a first step, the data Rm over HP in the range 16080 < HP < 18000 are plotted for all application examples and then a linear regression is carried out. in the second step, the strength at HP = 15990 is determined using the regression line. In the present case, this is the strength contribution S _M,A (15990) = 804 MPa for alloy A and the strength contribution S _M,B (15990) = 762 MPa for alloy B.
2. 2. Calculation of the theoretical strength of a precipitation-free structure as a function of HP by SM (HP) = SM ( ₁₅₉₉₀ ) - _0.0495 (HP-15990)
3. 3. Calculation of the strength contribution S _P (HP) by S _P (HP) = Rm (HP) - S _M (HP)

In the case of alloy compositions A and B, high strengths Rm are achieved in a particularly cost-effective manner, since the cooling process with the specified ranges of HP allows a strength contribution S _P through precipitation of ≥ 80 MPa.

In addition, high local cold formability with high strength can only be observed at 17000 s HP ≤ 18800, but not at HP > 18800 or HP < 17000 (Table 2).

The material-related causes for the different local cold formability at high strength were analyzed using microstructure components and features in longitudinal sections.

The bainitic structure of the hot strip produced according to the invention consists of a main component of ≧50% and secondary components, the main component being bainitic ferrite, which is strengthened by precipitations of (Ti, Nb, Mo)(C,N). Transmission electronic studies of the precipitates on individual representative samples have shown that half of the precipitates of (Ti,Nb,Mo)(C,N) strengthening the main bainitic ferrite constituent are <10 nm in diameter and/or the precipitates unite have an average distance of less than 750 nm. The minor components consist of higher carbon components such as martensite, MA phase, lower bainite, and pearlite. Since the main component has a higher formability than the secondary components, a minimum proportion of the main component of ≧50% is advantageous.

Table 3 shows the results of the microstructure investigations for alloy A for different final rolling temperatures according to formula 3, different coiling temperatures according to formula 4 and HP values according to formula 5.

1. a. Die Proben bestehen aus einem ferritisch-bainitischem Gefüge. Der Ferritanteil beträgt 48 % und 66%.
2. b. Die Abweichung im Anteil des Ferrits in der oberflächennahen Position und der Position ¼ Dicke in Bezug auf die Position ½ Dicke beträgt maximal 59 % und 17 %.
3. c. Die Gefügestreckung ist vergleichsweise stark ausgeprägt. Dies gilt insbesondere für die Position ½ Dicke; hier beträgt das Aspektverhältnis 2,9 und 2,5.
4. d. Die Härte über Dicke variiert vergleichsweise stark. Insbesondere an der Oberfläche ist die Härte geringer und weicht um -24 HV0,1 und -26 HV0,1 vom Mittelwert über die Probendicke ab.
5. e. Die Schertexturkomponenten variieren vergleichsweise stark und betragen in der oberflächennahen Position 0,92 und 0,96 und in der Position ½ Dicke 0,01 und 0,01.

The following applies to samples not processed according to the invention: The structure of the hot strip samples is inhomogeneous and anisotropic across the strip thickness. The inhomogeneity and the anisotropy can be described as follows for the two samples A2 and A6 with the HP values 17232 and 18380:

1. a. The samples consist of a ferritic-bainitic structure. The ferrite content is 48% and 66%.
2. b. The deviation in the proportion of ferrite in the near-surface position and the ¼ thickness position with respect to the ½ thickness position is 59% and 17% at maximum.
3. c. The stretching of the structure is comparatively pronounced. This is especially true for the ½ thickness position; here the aspect ratio is 2.9 and 2.5.
4. i.e. The hardness over thickness varies comparatively strongly. The hardness on the surface in particular is lower and deviates by -24 HV0.1 and -26 HV0.1 from the mean value over the sample thickness.
5. e. The shear texture components vary relatively widely, being 0.92 and 0.96 in the near-surface position and 0.01 and 0.01 in the ½ thickness position.

It is known that the local material behavior during cold forming with its high demands on local cold forming capacity is negatively influenced by microstructural inhomogeneities, since the damage is localized early and leads to material failure. In the present case, the in a. - i.e. listed features directly and indirectly to elongated areas of reduced formability, e.g. areas with an increased proportion of carbon-rich second phase of the granular bainite. The influence of the in e. However, the feature listed on the local cold formability is not known.

The different completed recrystallization of the austenite over the thickness of the strip immediately after the last rolling step and before cooling was identified as the cause of the structural inhomogeneity across the hot strip thickness. With an increased temperature range for finish rolling to more than EWTmin but with the average coiling temperature remaining the same, it has been possible within the scope of the invention to achieve complete recrystallization across the strip thickness in every thickness range and thus to set a ferritic-bainitic structure that is homogeneous across the strip thickness. The result is surprising, since the absence of recrystallization in the last passes of the rolling process with over-stoichiometric bainitic hot strip grades leads to a coarser structure, as expected, but the local cold formability is positively influenced, contrary to expectations.

An EWT of at least EWTmin according to formula 3 is necessary for complete recrystallization across the strip thickness in every thickness range.

• - Die Gefüge der Warmbandproben sind über Banddicke vergleichsweise homogen und isotrop. Die Homogenität und die Isotropie lassen sich bei den zwei Proben mit den HP-Werten 18380 (Probe A7) und 17232 (Probe A9) wie folgt beschreiben:
  1. a. Die Proben bestehen aus einem überwiegend bainitischem Gefüge. Der Ferritanteil beträgt 22 % und 49 %.
  2. b. Die Abweichung im Anteil des Ferrits in der oberflächennahen Position und der Position ¼ Dicke in Bezug auf die Position ½ Dicke beträgt maximal 7 % und -2 %.
  3. c. Die Gefügestreckung ist vergleichsweise gering ausgeprägt. Dies gilt insbesondere für die Position ½ Dicke; hier beträgt das Aspektverhältnis 1,5 und 1,5.
  4. d. Die Härte über Dicke variiert vergleichsweise wenig. Dies gilt insbesondere für die oberflächennahe Position. Die Härte weicht an der Oberfläche um -10 HV0,1 und 0 HV0.1 vom Mittelwert ab.
  5. e. Die Schertexturkomponenten betragen in der oberflächennahen Position 0,98 und 0,98 und in der Position ½ Dicke 0,01 und 0,03.

The material-related causes for the high local cold formability were analyzed using microstructure components and features in longitudinal sections. The result of the structural analysis of samples A7 and A9 according to Table 3, which were processed according to the invention, is:

• - The structure of the hot-rolled strip samples is comparatively homogeneous and isotropic across the strip thickness. The homogeneity and isotropy of the two samples with the HP values 18380 (sample A7) and 17232 (sample A9) can be described as follows:
  1. a. The samples consist of a predominantly bainitic structure. The ferrite content is 22% and 49%.
  2. b. The deviation in the proportion of ferrite in the near-surface position and the ¼ thickness position with respect to the ½ thickness position is maximum 7% and -2%.
  3. c. The structural elongation is comparatively low. This is especially true for the ½ thickness position; here the aspect ratio is 1.5 and 1.5.
  4. i.e. Hardness over thickness varies comparatively little. This applies in particular to the position close to the surface. The hardness deviates from the mean value by -10 HV0.1 and 0 HV0.1 on the surface.
  5. e. The shear texture components are 0.98 and 0.98 in the near-surface position and 0.01 and 0.03 in the ½ thickness position.

• Mindestens die Hälfte der Ausscheidungen von (Ti, Nb, Mo)(C,N), die den Hauptbestandteil aus bainitischem Ferrit verfestigen, weisen einen Durchmesser von < 10 nm auf und/oder die Ausscheidungen weisen einen mittleren Abstand von weniger als 750 nm auf.

Through further investigations, the target-oriented structure was further characterized in the following point:

• At least half of the precipitates of (Ti,Nb,Mo)(C,N) strengthening the main bainitic ferrite component are <10 nm in diameter and/or the precipitates have an average spacing of less than 750 nm .

• 1 shows once again in summary form the range of cold formability claimed according to the invention, which is limited by the data for FL and LFR.
• 2 shows the positioning of the hardness indentations with a distance from the surfaces (0% and 100%): 0.1 mm and distance from the center (50%): 0.1 mm, as well as the EBSD measuring fields with a distance from the surface (0 %): 0.1mm Distance from center (50%): 0.1mm.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102012002079 A1 [0010]
• EP 3516085 B1 [0021]
• EP 3492611 B1 [0024]

Claims (19)

High-strength, hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, an elongation at break of at least 10% , preferably at least 16%, a degree of cold workability of at least 0.12, advantageously at least 0.17 and a ratio of local and global cold workability of at least 5 and at most 13, and a structure consisting of more than 50% by volume bainite, remainder Precipitation strengthened ferrite, with the following steel chemical composition (in % by weight): C: 0.04 to 0.08 Si: 0.1 to 0.6 Mn: 1.0 to 2.0 P: 0.06 max S: max 0.01 N: max 0.012 AI: up to 0.06 Ti: up to 0.18 and/or Nb: up to 0.08 Mo: up to 0.35 with Ti+Nb more than 0 .06, with a superstoichiometric proportion of carbon and nitrogen according to below he formula is present: $$1.0<\left(\text{C}/12+\text{N}/14\right)/\left(\text{Ti}/48+\text{Nb}/93+\text{Mon}/96\right),$$

Rest iron including unavoidable steel-accompanying elements, with optional addition of one or more elements of Cr, Ni, V, B or Ca, whereby the structure distribution over the thickness of the steel flat product in the three areas near the surface, 1/4 thickness and 1/2 thickness of the steel flat product, is characterized by - an absolute deviation of a maximum of 12% by volume, advantageously a maximum of 7% by volume, of the proportion of ferrite in the areas close to the surface or 1/4 thickness of the flat steel product, in relation to the area 1/2 thickness of the steel flat product and/or - the deviation in the aspect ratio of the grains in the rolling direction in the three areas of the steel flat product from the mean is less than 0.3 in each of the three areas, and/or - in the three areas the hardness difference HV0.1 is a maximum of 20 in each case HV 0.1, advantageously a maximum of 15 HV 0.1, even more advantageously a maximum of 10 HV 0.1, compared to the mean value over the entire thickness of the steel flat product. flat steel product claim 1 , characterized by an addition of Ca of max. 0.01% by weight. flat steel product claim 1 or 2 , characterized in that the steel contains (in weight %): Ti + Nb: max. 0.2 Steel flat product according to at least one of Claims 1 until 3 , characterized in that the steel contains (in weight %): Ti min.: 0.02, advantageously 0.04, more advantageously 0.06 Nb min.: 0.01 Mo min.: 0.05 Steel flat product according to at least one of Claims 1 until 4 , characterized in that the steel contains (in weight %): Cr: up to 0.6 Ni: up to 0.6 V: up to 0.2 B: up to 0.01 with a superstoichiometric proportion of carbon and Nitrogen is present according to the following formula: $$1.0<\left(\text{C}/12+\text{N}/14\right)/\left(\text{Ti}/48+\text{Nb}/93+\text{Mon}/96+\text{V}/51\right)$$

flat steel product claim 5 , characterized in that the steel contains (in weight %): Cr: more than 0.1 Ni: more than 0.1 V: more than 0.01 B: more than 0.0005 Steel flat product according to at least one of Claims 1 until 6 , characterized in that the bainite is a mixture of components which is characterized by a main component of at least 50% by volume and secondary components, the main component consisting of bainitic ferrite which is formed by precipitations of (Ti, Nb, Mo)(C, N) and/or V(C;N) is strengthened and the secondary components consist of carbon-rich components such as martensite, retained austenite, lower bainite and pearlite. Steel flat product according to at least one of Claims 1 until 7 , characterized in that the structure consists of more than 75% by volume of bainite and the rest is ferrite. Steel flat product according to at least one of Claims 1 until 8th , characterized in that the structure contains, in addition to bainite and ferrite, up to 10% by volume, advantageously up to 5% by volume, carbon-rich structural components such as martensite, retained austenite, pearlite, retained austenite. Steel flat product according to at least one of Claims 1 until 9 , characterized in that the grain elongation of all structural components in a position 1/2 thickness of the steel flat product is characterized by the aspect ratio of all structural components in the rolling direction of at most 2.0 and/or the mean value over the three areas near the surface, 1/4 thickness and 1/ 2 thickness of the flat steel product is at most 2.0, advantageously 1.6. Steel flat product according to at least one of Claims 1 until 10 , characterized in that half of the precipitates of (Ti, Nb, Mo)(C,N) and/or V(C,N) strengthening the ferrite and the main component of bainitic ferrite have a diameter of less than 10 nm and/or the precipitates have an average spacing of less than 750 nm. Steel flat product according to at least one of Claims 1 until 11 , characterized in that the ratio of shear texture components to rolling texture components increases towards the surface and has the following values: - close to the surface: min. 0.9 - 1/2 thickness: max. 0.1 Method for producing a hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of over 30%, advantageously at least 40%, particularly advantageously at least 50%, a measure of cold formability of at least 0.12, advantageously at least 0.17 and a ratio of local and global cold workability of at least 5 and at most 13, comprising the steps: - melting a steel melt containing (in weight %): C: 0.04 to 0, 08 Si: 0.1 to 0.6 Mn: 1.0 to 2.0 P: 0.06 max. S: 0.01 max. N: 0.012 max. AI: up to 0.06 Ti: up to 0 ,18 and/or Nb: up to 0.08 Mo: up to 0.35 with Ti+Nb more than 0.06 and where there is a superstoichiometric ratio of carbon and nitrogen according to the following formula: 1.0 < (C/12 +N/14) / (Ti/48+Nb/93+Mo/96) with optional addition of one or more elements of Cr, Ni, V, B or Ca, remainder iron including unavoidable steel-accompanying elements - Casting of the molten steel into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process, - Reheating of the slab or thin slab to 1100 °C to 1270 °C and subsequent hot rolling of the slab or thin slab with the following directly consecutive steps: - Rolling in the last Roll pass to a hot strip to the required final thickness at a final rolling temperature EWT where: $$\begin{array}{l}\text{EWT}\ge \text{EWTmin}=682°\text{C}+464\text{C}+6445\text{Nb}-644\times {\text{Nb}}^{0.5}+732\text{V}-230{\text{V}}^{0.5}+890\text{Ti}\\ +363\text{Al}-36\text{si}\end{array}$$

- Cooling at an average cooling rate of 30 K/s to 150 K/s - Coiling the hot strip into a coil at a coiling temperature HT, which is low enough to set the advantageous structural components with $$\text{HT}\le \text{HT max}=761°\text{C}-217\times \text{C}-77\times \text{Mn}+97\times \text{si}-47\times \text{Mon}-53\times \text{Cr}-34\times \text{no}-21\times \text{V}$$

and on the other hand is suitable to provide sufficient precipitation strengthening in the subsequent cooling process T(t), defined by 17000 ≤ HP ≤ 18800 with HP(T, t) = T(t) × (In(t) + 20), where T in K and t are given in h, - Cooling in a cooling process T(t) at an average cooling rate of 5 K/h to 50 K/h between the coiling temperature and 100 °C, with subsequent cooling in still air to room temperature. procedure after Claim 13 characterized in that the slab is hot rolled into a steel flat product with a thickness of 1.6 mm to 6.0 mm. procedure after Claim 13 and 14 , characterized in that the steel flat product is provided with a metallic coating electrolytically or by hot dipping. procedure after claim 15 , characterized in that the metallic coating is zinc-based. Process according to at least one of Claims 13 until 16 , for the production of a steel flat product according to claims 1 until 12 . Use of a steel flat product according to at least one of Claims 1 until 12 for the manufacture of components in the automotive industry. Use of a steel flat product Claim 18 for the production of chassis components.

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