EP3872206B1 - Post-treated cold rolled steel sheet product and method of manufacturing a post-treated cold rolled steel sheet product - Google Patents

Description

The invention relates to a cold-rolled flat steel product which has been post-treated to increase strength and has an increased yield strength and increased tensile strength, and a method for its manufacture.

Flat steel products of the type in question are rolled products obtained by cold rolling, such as steel strips or sheets, as well as blanks and blanks made from them.

All information on contents of the steel compositions given in the present application are based on the weight, unless expressly stated otherwise. All "% figures" in connection with a steel alloy that are not specified in more detail are therefore to be understood as figures in "% by weight".

Information on structural components refers to percentages by volume (“% by volume”), unless expressly stated otherwise.

High-strength flat steel products are becoming increasingly important, especially in the field of vehicle construction, since they enable the vehicle's dead weight to be reduced and the payload to be increased. A low weight not only contributes to the optimal use of the technical performance of the respective drive unit, but also supports resource efficiency, cost optimization and climate protection.

A significant reduction in the dead weight of sheet steel constructions can be achieved by increasing the mechanical properties, in particular the strength of the processed flat steel product.

In addition to high strength, modern flat steel products intended for vehicle construction are expected to have good toughness properties, good resistance to brittle fracture and optimum suitability for cold forming and welding.

In particular, there are applications in vehicle construction for which a particularly high forming resistance is primarily required. This is reflected in the properties determined in the tensile test in the form of a high yield point R _p02 .

A large number of attempts are known to meet these requirements through alloying or process engineering measures. What these tests have in common is that they are based on a so-called two-phase or multi-phase steel, the structure of which consists of at least two dominant phases, with multi-phase steels being able to contain smaller proportions of other phases.

For example, from the WO 2015/158731 A1 a steel flat product consisting of a two-phase steel is known. The steel flat product is manufactured by hot and cold rolling. After cold rolling, it goes through an additional heat treatment to increase the yield strength.

From the WO 2016/177420 A1 a steel flat product is known which has a tensile strength Rm ≥ 950 MPa, a yield point ≥ 800 MPa and an elongation at break A50 ≥ 8%. The steel flat product consists of a steel which (in % by weight) contains 0.05 - 0.20% C, 0.2 - 1.5% Si, 0.01 - 1.5% Al, 1.0 - 3.0% Mn, ≤ 0.02% P, ≤ 0.005% S, ≤ 0.008% N, and optionally 0.05 - 1.0%, 0.05 - 0.2% Mo, 0.005 - 0.2 % Ti, 0.001 - 0.05% Nb, 0.0001 - 0.005% B, balance Fe and unavoidable impurities. The following applies: 1.5 ≤ ψ ≤ 3, with ψ=(%C+%Mn/5+%Cr/6)/(%Al+%Si) and %C, %Mn, %Cr, %Al, %Si of the respective C -, Mn, Cr, Al or Si content of the steel. At the same time, the flat steel product has a microstructure consisting of (in area %) ≦5% bainite, ≦5% polygonal ferrite, ≧90% martensite and ≦2% by volume residual austenite, with at least half of the martensite being tempered martensite. Likewise in the WO 2016/177420 A1 shown a special heat treatment to achieve the mechanical properties and structure.

Against the background of the prior art explained above, the object of the invention was to specify a method for producing a flat steel product with a high yield point and a high tensile strength R _m that can be carried out reliably and thereby leads to an optimal combination of properties of the flat steel product obtained.

For the purposes of this application, the yield point of a flat steel product is understood to mean the lower yield point R _el if the flat steel product has a pronounced yield point. Otherwise (ie for flat steel products without a pronounced yield point), the yield point of the flat steel product is understood to mean the yield point R _p02 for the purposes of this application.

For the purposes of this application, tensile strength and yield point are determined in accordance with DIN-EN ISO 6982-1, specimen form 2 (Annex B Tab. B1).

A steel flat product of the same quality should be created in the same way.

With regard to the method, this object has been achieved by the invention in that the work steps specified in claim 1 are carried out when producing a cold-rolled flat steel product with a high yield point and a high tensile strength R _m .

A flat steel product that achieves the above-mentioned object according to the invention has the features specified in claim 8 .

Advantageous refinements of the invention are specified in the dependent claims and are explained in detail below, along with the general idea of the invention.

• C: 0,05-0,25 %,
• Si: 0,05-0,6 %,
• Mn: 1,0-3,0 %,
• Al: 0,02-1,5 %,
• N: weniger als 0,02 %,
• P: 0,005-0,2 %,
• S: weniger als 0,05 %
• einem oder mehrere Elemente aus der Gruppe "Cr, Mo" mit der Maßgabe:
  • Cr: 0,2-1,5 %,
  • Mo: 0,003-1,0 %,
• optional einem oder mehrere Elemente aus der Gruppe "Ti, Nb, B" mit der Maßgabe:
  • B: weniger als 0,005 %
  • Ti+Nb+15-B : 0,02-0,15 %
• sowie optional einem oder mehrere Elemente aus der Gruppe "V, Cu, Ni, Ca" mit der Maßgabe:
  • V: 0,0005-0,05 %
  • Cu: 0,0001-0,5 %
  • Ni: 0,002-0,2 %
  • Ca: 0,0005-0,007 %
• Rest Eisen und unvermeidbare Verunreinigungen.

In the method according to the invention for producing a cold-rolled flat steel product that has been post-treated to increase strength, a cold-rolled flat steel product is made from a steel and has the following composition (in % by weight):

• C: 0.05-0.25%,
• Si: 0.05-0.6%,
• Mn: 1.0-3.0%,
• Al: 0.02-1.5%,
• N: less than 0.02%,
• P: 0.005-0.2%,
• S: less than 0.05%
• one or more elements from the group "Cr, Mo" with the proviso:
  • Cr: 0.2-1.5%,
  • Mon: 0.003-1.0%,
• optionally one or more elements from the group "Ti, Nb, B" with the proviso:
  • B: less than 0.005%
  • Ti+Nb+15-B : 0.02-0.15%
• and optionally one or more elements from the group "V, Cu, Ni, Ca" with the proviso:
  • V: 0.0005-0.05%
  • Cu: 0.0001-0.5%
  • Ni: 0.002-0.2%
  • Approx: 0.0005-0.007%
• remainder iron and unavoidable impurities.

• Nachwalzen des kaltgewalzten Stahlflachproduktes, wobei der über das Nachwalzen erzielte Walzgrad W_G2 insgesamt 8 - 40 % beträgt;
• Anlassglühen des nachgewalzten Stahlflachproduktes bei einer Nachglühtemperatur T_G2 von 100-400°C über eine Glühdauer von 0,2-25 Stunden.

According to the invention, the cold-rolled flat steel product provided in this way is post-treated to increase strength, with the following work steps being carried out

• skin-pass rolling of the cold-rolled flat steel product, the rolling degree W _G2 achieved via the skin-pass rolling being 8-40% in total;
• Tempering of the re-rolled steel flat product at a post-annealing temperature T _G2 of 100-400°C over an annealing period of 0.2-25 hours.

The temper rolling takes place at room temperature, although the flat steel product usually heats up to a certain extent as a result of the temper rolling.

Basically, it is known that the yield point can be increased both by plastic deformation during re-rolling and by tempering. During plastic deformation, new dislocations occur in the lattice structure, which contribute to the increase in strength. Tempering, on the other hand, leads to the formation and growth of precipitations that prevent dislocations from sliding. One speaks of "pinning" the dislocations.

Surprisingly, it has been shown that an additional effect occurs when temper rolling is combined with subsequent tempering, which leads to a drastic increase in the yield point. The increase in yield point goes beyond the sum of the individual effects of temper rolling and tempering. There is an unexpected synergy effect. Numerous tests have shown that the combination of both post-treatment steps results in an increase in the yield point that is at least 50 MPa greater than the sum of the individual effects, with an increase of 100 MPa, sometimes even more than 200 MPa, compared to the sum of the individual effects is achieved.

When the cold-rolled steel flat product is re-rolled, dislocations are introduced into the structure as a result of the deformation. During the subsequent tempering, these dislocations lead then during recovery processes to form sub-grains so that the yield strength increases similar to grain refinement. It has been shown that a significant synergetic effect occurs from a degree of rolling of 8%. On the other hand, if the degree of rolling is too high, the damage to the structure is so great that the subsequent product no longer meets the requirements in further processing. Optimal degrees of rolling during re-rolling are therefore at most 40%, preferably at most 30% and at least 8%, preferably at least 20%.

The subsequent tempering must be designed in such a way that sufficient thermal energy is introduced to enable the local recovery processes, but not too much thermal energy, otherwise global microstructure formation will occur. For this purpose, afterglow temperatures in the range of 100° to 400°C have proven to be appropriate. The afterglow temperature is preferably greater than 130°C and/or less than 330°C. The annealing time is expediently 0.2-25 hours.

The mechanical properties of flat steel products according to the invention, which are specified here in general and in relation to the exemplary embodiments presented below, are each determined on flat tensile specimens in accordance with DIN EN ISO6892-1.

In practice, the tempering provided according to the invention after the skin-pass rolling is carried out as batch annealing.

The alloy of the steel from which the flat steel products to be processed according to the invention are made is selected in such a way that optimal mechanical properties are achieved under the influence of the additional post-treatment step.

C is present in the steel of a cold rolled steel flat product processed in accordance with the present invention at levels of 0.05-0.25% by weight to produce sufficient martensite to increase strength. At higher C contents, too little ferrite occurs. In addition, too high a C content has a negative effect on weldability. The C content is preferably at most 0.20% by weight, particularly preferably at most 0.18%. On the other hand, if the C content is less than 0.05%, the desired strength is not obtained. The C content is preferably at least 0.08% by weight, particularly preferably at least 0.12% by weight.

Si is present in the steel of a cold-rolled flat steel product processed according to the invention in contents of 0.05-0.6% in order to increase the strength by solid solution hardening without impairing the ductility. In addition, Si serves as a ferrite former. Excessively high Si contents can impair the surface finish, for example as a result of adherent scale or grain boundary oxidation. In order to rule this out with certainty, the Si content can be limited to a maximum of 0.6% by weight. The Si content is preferably at most 0.42%. On the other hand, if the Si content is too low, the strength-increasing effect of solid solution hardening in the ferrite phase is insufficient. If the desired effect of Si is to be available with particular certainty, the Si content can be set to at least 0.24% by weight.

Mn is present in the steel of a cold-rolled flat steel product processed according to the invention in amounts of 1.0-3.0% by weight in order to promote solid solution strengthening as well as martensite formation to increase strength. This is done by Mn stabilizing the austenite from which the martensite is formed. The volume fraction of the martensite is therefore adjusted by targeted adjustment of the Mn content. The Mn content is preferably at least 1.5% by weight, in particular at least 1.7% by weight. On the other hand, an excessive addition of Mn leads to an insufficient proportion of the martensite phase. Therefore, the Mn content is preferably at most 2.4% by weight.

Al is present in the steel of a cold-rolled flat steel product processed according to the invention in amounts of 0.02-1.5% by weight, on the one hand to serve as a deoxidizing agent and to bind nitrogen during melting and on the other hand to ensure the sufficient amount of ferrite and thus the ductility increase. However, in order to avoid an unfavorable influence on the casting quality and the coatability, the maximum content of 1.5% by weight should not be exceeded. Compliance with an upper limit of 0.9% by weight has proven to be particularly advantageous.

N is an undesirable alloying component attributable to unavoidable impurities. Its content in the steel of a cold-rolled flat steel product processed according to the invention must therefore be at most 0.02% by weight. Too high an N content impairs the workability and, if B and/or Al is also present, can lead to the formation of harmful nitrides and thus prevent the effectiveness of these elements. The N content is preferably at most 0.01% by weight. Optimally, it is limited to at most 0.008% by weight, especially at most 0.006% by weight.

P is an undesirable alloying component attributable to unavoidable impurities. Excessive addition of P can lead to embrittlement and thus to reduced crash properties. In addition, the weldability is impaired by the P content. For these reasons, the P content should not exceed 0.2% by weight. The P content is preferably at most 0.05%, in particular at most 0.03%.

S is an undesirable alloying component attributable to unavoidable impurities. Its content in the steel of a cold-rolled flat steel product processed according to the invention may therefore not be more than 0.05% by weight. In order to ensure good ductility of the steel product, the formation of MnS or (Mn,Fe)S must be kept as low as possible. The S content is preferably at most 0.01% by weight, particularly preferably at most 0.005% by weight.

In the steel of a steel flat product processed according to the invention, Cr and Mo contribute to increasing the strength. They favor the formation of martensite by shifting the ferrite-pearlite transformation zones during heat treatment. In order to achieve this effect, the Mo content is at least 0.005% by weight, preferably at least 0.005% by weight. The Cr content is at least 0.2% by weight, preferably at least 0.3% by weight. If the Cr or Mo content is too high, however, undesirable carbides can form. In addition, the alloy cost increases excessively. The Mo content is therefore at most 1.0% by weight, preferably at most 0.3% by weight. The Cr content is at most 1.5% by weight, preferably at most 0.8% by weight.

In the steel of the cold-rolled flat steel product processed according to the invention, Ti, B and Nb contribute to the increase in strength and lead to a finer microstructure. B enables a higher proportion of martensite by suppressing the formation of ferrite and bainite, but can only develop its full effect through the additional addition of Ti, which prevents the formation of unwanted boron nitrides by forming fine Ti(C,N) precipitations. This increase in strength due to the formation of precipitates is favored or reinforced by the additional addition of Nb. It has been shown that the sum of the contents of Ti, Nb and 15 times the content of B should be at least 0.02% by weight in order to achieve these properties (i.e. Ti+Nb+15·B >_ 0 .02% by weight). In order to avoid a negative influence on the processability, however, a maximum content of 0.15% by weight for the sum mentioned must not be exceeded (i.e Ti+Nb+15·B ≤ 0.15% by weight). At the same time, the B content is at most 0.005% by weight. Excessive addition of boron leads to increased brittleness. Therefore, the boron content is less than 0.005% by weight, preferably less than 0.003% by weight.

The addition of V in the steel of the cold-rolled steel flat product processed according to the present invention results in an improvement in workability and an improved resistance to delayed cracking through a finer microstructure. In order to make optimal use of these advantages, a V content in the range of 0.0005-0.05% by weight should be chosen, in particular it should be at least 0.005% by weight.

Cu and Ni contribute to strengthening in the steel of the cold-rolled flat steel product processed according to the present invention, and may be added singly or in combination. The Cu content is at least 0.0001% by weight, preferably at least 0.001% by weight. However, the Cu content should not exceed 0.5% by weight, preferably 0.08% by weight. The Ni content is at least 0.002% by weight, preferably at least 0.01% by weight. At maximum, the Ni content should be no greater than 0.2% by weight, preferably no greater than 0.1% by weight.

The addition of Ca in the steel of the cold-rolled flat steel product processed according to the invention leads to a finer distribution of inclusions in the steel and forms spherical sulfides, which can reduce disadvantages of other harmful sulfides in further processing. For this purpose, the Ca content should be at least 0.0005% by weight. However, since too high a Ca content can have adverse effects on castability and hot workability, it should be at most 0.007% by weight, preferably at most 0.005% by weight.

In a specific embodiment, the steel has a carbon equivalent C _eq of between 0.3% and 1.3%. The carbon equivalent is defined as the weighted sum of the following element contents: $${\mathrm{C}}_{\mathrm{eq}}=\mathrm{C}+\frac{1}{6}\mathrm{Mn}+\frac{1}{5}\mathrm{Mon}+\frac{1}{15}\mathrm{no}+\frac{1}{5}\mathrm{Cr}+\frac{1}{5}\mathrm{V}+\frac{1}{15}\mathrm{Cu},$$

It has been shown that the carbon equivalent is well suited to characterizing the subsequent workability of the steel flat product. With values in the range of 0.3% to 1.3%, the steel flat product can both be welded and coated without any problems compared to other steel alloys with a similar strength and a higher proportion of alloying elements. The carbon equivalent is preferably at most 0.7% for this. More preferably, the carbon equivalent is at least 0.3%.

mit

• T_G2: Nachglühtemperatur in der Einheit °C
• W_G2: Walzgrad beim Nachwalzen in %
• C_äq: Kohlenstoffäquivalent in %
• K : Konstante mit dem Wert 10 °C

In particular, the method is designed in such a way that the production index P _WG is between 0.1 and 2.7. The production index P _WG is defined as follows: $${\mathrm{P}}_{\mathrm{flat\; share}}=\frac{{\mathrm{T}}_{\mathrm{G}2}}{\mathrm{K}\cdot {\mathrm{W}}_{\mathrm{G}2}}\cdot {\mathrm{C}}_{\mathrm{eq}}$$

with

• T _G2 : Afterglow temperature in °C
• W _G2 : Degree of rolling during temper rolling in %
• C _eq : carbon equivalent in %
• K : constant with the value 10 °C

Extensive tests have shown that a particularly stable process with reliable post-treatment to increase strength results when the production index is in the specified range.

A cold-rolled flat steel product is preferably used as the starting material for the post-treatment process according to the invention, the structure of which consists of at least two phases, of which martensite and ferrite are the dominant phases, with more than 10% by volume martensite and more than 60% by volume ferrite available. The ferrite content is preferably more than 70% by volume, in particular more than 80%. The remainder may contain bainite or precipitates. The structure of the steel flat product should contain at least 60% by volume of ferrite in order to be able to set the necessary elongation. At least 10% by volume of martensite should also be present in the structure of the flat steel product according to the invention in order to achieve the strength and to enable a tempering effect. The post-treated microstructure consists of at least two phases, of which ferrite and martensite are the dominant phases. As a result of the post-treatment, the martensite is now tempered martensite. The ferrite phase shows slightly stretched grains, any previously present residual austenite has disintegrated. The other phase components are unchanged compared to the starting product. The post-treated flat steel product thus has a structure consisting of at least two phases which (in vol%) more than 10% tempered martensite and more than 60% ferrite. The ferrite content is preferably more than 70% by volume, in particular more than 80%.

In a preferred embodiment, the cold-rolled flat steel product is coated between temper rolling and tempering. Coating has the advantage that protection against corrosion is guaranteed. In a particularly advantageous development, the cold-rolled flat steel product is coated, in particular electrolytically coated, between re-rolling and tempering. The advantage of a coating between re-rolling and tempering is that any hydrogen absorbed during the coating is removed again during tempering. Hydrogen can lead to hydrogen embrittlement and should therefore be avoided if possible. An electrolytic coating has the advantage that the flat steel product is not overheated, for example in comparison to hot-dip coating. Excessive heating during coating could affect the structure and thus the mechanical properties.

• Vergießen eines Stahls mit der vorgenannten Zusammensetzung zu einer Bramme;
• Wiedererwärmen der Bramme auf eine 1200-1500 ° C betragende Wiedererwärmungstemperatur;
• Warmwalzen der wiedererwärmten Bramme zu einem Warmband, wobei die Warmwalzendtemperatur des Warmbands bei Beendigung des Warmwalzens 800-1000°C beträgt;
• Haspeln des Warmbands bei einer Haspeltemperatur von 400-700 °C;
• Beizen des Warmbands
• Kaltwalzen des Warmbands in einem oder mehreren Kaltwalzschritten zu einem kaltgewalzten Stahlflachprodukt, wobei der über das Kaltwalzen erzielte Kaltwalzgrad insgesamt 20-80 % beträgt;
• Durchlaufglühen des kaltgewalzten Stahlflachprodukts bei einer Durchlaufglühtemperatur von 700-950 °C; Dabei kann das Durchlaufglühen auch durch ein Feuerbeschichten realisiert werden.
• Abkühlen des kaltgewalzten Stahlflachprodukts auf Raumtemperatur

The cold-rolled flat steel products provided for carrying out the method according to the invention can be produced starting from a steel with the composition explained above. To do this, the following work steps can be carried out when creating the steel flat product provided:

• casting a steel having the aforesaid composition into a slab;
• reheating the slab to a reheating temperature of 1200-1500°C;
• hot rolling the reheated slab into a hot strip, wherein the hot rolling finish temperature of the hot strip at the completion of the hot rolling is 800-1000°C;
• coiling of the hot strip at a coiling temperature of 400-700 °C;
• Pickling the hot strip
• cold-rolling the hot strip in one or more cold-rolling steps to form a cold-rolled flat steel product, the degree of cold-rolling achieved via the cold-rolling being 20-80% in total;
• continuous annealing of the cold-rolled steel flat product at a continuous annealing temperature of 700-950 °C; The continuous annealing can also be implemented by hot-dip coating.
• Cooling the cold-rolled flat steel product to room temperature

und für die Haltezeiten t₁, t₂ gilt: $$0\mathrm{s}\le t_1\le 60\mathrm{s}\mathrm{und}0\mathrm{s}\le t_2\le 900\mathrm{s}$$

In a particularly preferred variant of the method, the cooling of the cold-rolled flat steel product to room temperature has two intermediate steps. In this case, the cold-rolled flat steel product is cooled to a first cooling temperature T ₁ in the first intermediate step and is held at the first cooling temperature T ₁ for a first holding time t ₁ . The cold-rolled flat steel product is then cooled to a second cooling temperature T ₂ in the second intermediate step and is held at the second cooling temperature T ₂ for a second holding time t ₂ . The following applies to the cooling temperatures T ₁ , T ₂ : $$T_1>{\mathrm{<figure-callout\; id="450"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}_2,450°\mathrm{C}\le T_1\le 80°\mathrm{C}\mathrm{and}400°\mathrm{C}\le T_2\le 600°\mathrm{C}$$

and for the holding times t ₁ , t ₂ applies: $$0\mathrm{s}\le t_1\le 60\mathrm{s}\mathrm{and}0\mathrm{s}\le t_2\le 900\mathrm{s}$$

This two-stage cooling process has the advantage that ferrite is formed in the first intermediate step and the proportion of bainite and residual austenite is adjusted in the second intermediate step.

Alternatively, however, the cooling can also take place in a single cooling step to room temperature.

Optionally, the cold-rolled flat steel product that has been post-treated to increase strength can be provided with a metallic protective coating. This is example useful if components are made from the steel flat product that are exposed to a corrosive environment in practical use. The metallic coating can be applied in any suitable manner, application by hot-dip coating being particularly suitable here, for example in a continuous hot-dip coating plant.

• aus einem Stahl mit der nachfolgend angegebenen Zusammensetzung besteht (in Gew.-%)
  • C: 0,05-0,25 %,
  • Si: 0,05-0,6 %,
  • Mn: 1,0-3,0 %,
  • AI: 0,02-1,5 %,
  • N: weniger als 0,02 %,
  • P: 0,005-0,2 %,
  • S: weniger als 0,05 %
  • einem oder mehrere Elemente aus der Gruppe "Cr, Mo" mit der Maßgabe:
    • Cr: 0,2-1,5 %,
    • Mo: 0,003-1,0 %,
  • optional einem oder mehrere Elemente aus der Gruppe "Ti, Nb, B" mit der Maßgabe:
    • B: weniger als 0,005 %
    • Ti+Nb+15*B: 0,02-0,15 %
  • sowie optional einem oder mehrere Elemente aus der Gruppe "V, Cu, Ni, Ca" mit der Maßgabe:
    • V: 0,0005-0,05 %
    • Cu: 0,0001-0,5 %
    • Ni: 0,002-0,2 %
    • Ca: 0,0005-0,007 %
  • Rest Eisen und unvermeidbare Verunreinigungen,
  und
• eine Dehngrenze von mindestens 1000 MPa und eine Zugfestigkeit R_m von mindestens 1100 MPa aufweist,
• wobei die legierungsunabhängige Zugfestigkeit R̃_m mindestens 400 MPa beträgt, mit $${\tilde{\mathrm{R}}}_{\mathrm{m}}=\frac{{\mathrm{R}}_{\mathrm{m}}}{\mathrm{C}+\mathrm{Si}+\mathrm{Mn}+\mathrm{Cr}+\mathrm{Mo}},$$
  
  wobei C, Si, Mn, Cr und Mo die jeweiligen Elementgehalte in Gewichtsprozent sind.

The object of the invention is also achieved by a cold-rolled flat steel product that has been post-treated to increase strength

• consists of a steel with the following composition (in % by weight)
  • C: 0.05-0.25%,
  • Si: 0.05-0.6%,
  • Mn: 1.0-3.0%,
  • AI: 0.02-1.5%,
  • N: less than 0.02%,
  • P: 0.005-0.2%,
  • S: less than 0.05%
  • one or more elements from the group "Cr, Mo" with the proviso:
    • Cr: 0.2-1.5%,
    • Mon: 0.003-1.0%,
  • optionally one or more elements from the group "Ti, Nb, B" with the proviso:
    • B: less than 0.005%
    • Ti+Nb+15*B: 0.02-0.15%
  • and optionally one or more elements from the group "V, Cu, Ni, Ca" with the proviso:
    • V: 0.0005-0.05%
    • Cu: 0.0001-0.5%
    • Ni: 0.002-0.2%
    • Approx: 0.0005-0.007%
  • remainder iron and unavoidable impurities,
  and
• has a yield strength of at least 1000 MPa and a tensile strength R _m of at least 1100 MPa,
• where the alloy-independent tensile strength R̃ _m is at least 400 MPa, with $${\tilde{\mathrm{R}}}_{\mathrm{m}}=\frac{{\mathrm{R}}_{\mathrm{m}}}{\mathrm{C}+\mathrm{si}+\mathrm{Mn}+\mathrm{Cr}+\mathrm{Mon}},$$
  
  where C, Si, Mn, Cr and Mo are the respective element contents in percent by weight.

For the purposes of this application, a post-treated, cold-rolled flat steel product has a yield strength of at least 1000 MPa if the yield strength is at least 1000 MPa in at least one direction (ie, for example, transversely or longitudinally to the rolling direction). The same applies to the tensile strength and the alloy-independent tensile strength.

With regard to the element contents and the microstructure details, the above explanations regarding the advantages and preferred embodiment variants apply.

The after-treatment steps according to the invention, namely re-rolling and tempering, regularly result in a yield strength of at least 1000 MPa; preferred variants have a yield strength of at least 1200 MPa, in particular at least 1400 MPa. A tensile strength of at least 1100 MPa is also achieved, with preferred embodiment variants having a tensile strength of at least 1200 MPa, in particular at least 1400 MPa.

In addition, the alloy-independent tensile strength R _m is at least 400 MPa, preferably at least 450 MPa. The high tensile strength is therefore not achieved by high alloying with elements that contribute to hardening (C, Si, Mn, Cr, Mo), but rather by the post-treatment steps of temper rolling and tempering according to the invention. The cold-rolled steel flat product, which has been post-treated to increase strength, has the advantage that high strength can be achieved without excessive alloying. It is therefore correspondingly cheaper to produce. In addition, the negative effects of the high alloy content on later processing steps such as welding or coating are eliminated. In this regard, low-alloy steels are easier to process.

In a preferred variant of the cold-rolled steel flat product post-treated to increase strength, the sum of the grain boundary lengths for small-angle grain boundaries of a square measuring field of 50 μm*50 μm in a longitudinal section is greater than 10 mm, preferably greater than 15 mm, particularly preferably greater than 20 mm.

Differences in orientation of the lattice of less than 15° are referred to as small-angle grain boundaries. The sum of the grain boundary lengths is determined using the EBSD method. The EBSD method (electron backscattering diffraction) is one of the electron microscopic examination methods. The information from the electrons backscattered by the sample is used. The electron beam scans the surface of the sample during an analysis. The impinging electrons are scattered in the sample. Some of these hit lattice surfaces of the examined grain under Bragg conditions and are diffracted. The resulting diffraction pattern (Kikuchi pattern) is recorded using a phosphor screen and processed and interpreted by software. The Kikuchi patterns contain information about the existing crystal symmetries, which allow conclusions to be drawn about the investigated crystallographic phases and the orientation of the examined grain, as well as lattice distortions, misorientation of grain boundaries, etc. If you now look at a square measuring field of 50µm*50µm on the surface of a section taken along the rolling direction (longitudinal section), it is possible to add up the total length of the small-angle grain boundaries which separate orientation differences of the lattice of <15°.

The invention is explained in more detail below using exemplary embodiments.

To test the invention, seventeen steel melts 1-17 were melted, the composition of which is given in Table 1. Table 1 also shows the carbon equivalent C _eq determined from the composition.

The steel melts 1-17 have been cast into slabs for the subsequent tests 1-17. The slabs cast from the steel melts were reheated to a reheating temperature of 1260-1300°C and then hot-rolled in a conventional manner at a hot-rolling finish temperature of 880-990°C, each into a hot strip having a thickness of 2-3 mm.

The hot strips obtained were cooled to a coiling temperature of 525-585° C. and coiled at this coiling temperature to form a coil.

After cooling, the hot strips were cold-rolled in a similarly conventional manner with an overall degree of cold-rolling of 20-60% on average, which was achieved by cold-rolling, to form cold-rolled steel strips. The degree of cold rolling KWG achieved via cold rolling is determined here in the usual manner using the formula KWG = 100% * (dV-dN)/dV, where dV is the thickness of the hot strip before cold rolling and dN is the thickness of the cold strip obtained after referred to as cold rolling. The cold-rolled steel strips then underwent continuous annealing at an annealing temperature of 816-916°C.

The steel strips were cooled to room temperature in two intermediate steps. In the first intermediate step, the steel strips were cooled to a first cooling temperature T ₁ with 650° C.≦ T ₁ ≦800° C. and held at the first cooling temperature for a first holding time t ₁ with 0s≦ t ₁ ≦20 s. The steel strips were then cooled to a second cooling temperature T ₂ and held at the second cooling temperature T ₂ for a second holding time t ₂ . The following applied to the second cooling temperature T ₂ and the second holding time t ₂ : $$450°\mathrm{C}\le T_2\le 550°\mathrm{C}\mathrm{and}60\mathrm{s}\le t_2\le 500\mathrm{s}$$

All of the steel strips produced in this way had a structure with more than 10% martensite and more than 60% ferrite.

Each of the cold-rolled steel strips obtained in the tests described above was then first subjected to temper rolling with a temper rolling degree W _G2 and then to an additional tempering anneal carried out as a batch annealing, during which it was held at a temperature T _G2 for more than 20 minutes .

After the tempering, the yield point and the tensile strength R _m were measured for the steel strips obtained, both longitudinally and transversely to the rolling direction. The results are shown in Table 2. The production index PWG and the alloy-independent tensile strength R̃ _m were also determined from these measurement results.

It can be seen that a very high yield point and a very high tensile strength R _m were consistently achieved by the additional temper rolling carried out according to the invention with subsequent tempering. Tensile strength and yield point were determined according to DIN-EN ISO 6982-1, specimen form 2 (Annex B Tab. B1). Only the two non-inventive examples 6 and 11, which have a low degree of rolling W _G2 of 5%, do not achieve such high strength values.

With this combination of properties, cold-rolled steel strips according to the invention are optimally suited for the production of components which have high strength but do not have the high-alloy chemical analysis typical of this strength. This reduces the associated welding problems and the cost of the alloying components.

The Figures 1 and 2 show, as an example for the steel from example no. 13 described above (see Table 1), the increase in yield strength through temper rolling without tempering ( figure 1 ) and by tempering without prior temper rolling ( figure 2 ). The difference in yield strength between the condition after re-rolling or tempering and the initial condition is plotted in each case. In all cases, the yield point was determined perpendicular to the rolling direction. figure 1 shows this difference as a function of the degree of rolling. figure 2 shows the difference as a function of the glow temperature during the initial glow. The annealing time was 20 minutes in each case. Both figures show a clear increase in the yield strength due to the respective post-treatment. Through tempering at 300°C, 400°C and 500°C ( figure 2 ) steel no. 13 had developed a pronounced yield point, so that R _el was determined as the yield point and used to calculate the difference. In all other cases, the measurement did not result in a pronounced yield point, so that the value R _p02 was determined as the yield point and used in further evaluation.

figure 5 Figure 13 shows the synergistic effect of temper rolling and tempering on strength for steel #13. The difference in yield strength between the condition after temper rolling and tempering and the condition after temper rolling without tempering is plotted. (A degree of rolling of 0% means the case without temper rolling). If the two effects (tempering and tempering) on the strength were independent of one another, there should not be any dependency on the degree of rolling, since the effect of rolling has just been subtracted. For all three afterglow temperatures (200°C, 300°C and 400°C) there should be a curve parallel to the x-axis. Instead, however, an increase with increasing degree of rolling can be seen for all three afterglow temperatures. The overall effect therefore goes beyond the sum of the two individual effects. During tempering at 300°C and 400°C without previous skin-pass rolling and during tempering at 300°C and 400°C with previous skin-pass rolling with a rolling degree of 10%, steel no. 13 developed a pronounced yield point, so that the yield strength R _el determined and used in the evaluation. So this affects the four data points in the top left figure 3 . In all other cases, the measurement did not result in a pronounced yield point, so that the value R _p02 was determined as the yield point and used in further evaluation. Here, too, the yield point was determined in all cases perpendicular to the rolling direction. The same behavior can also be seen in measurements along the rolling direction.

The Figures 4 and 5 show light microscopic longitudinal sections of steel no. 13 after nital etching. The high ferrite content of more than 60% by volume can be clearly seen in both figures. figure 4 shows the steel in its initial state without post-treatment. In figure 5 On the other hand, steel No. 13 is shown after post-treatment to increase strength as shown in Table 2, in which the steel was first temper rolled to a degree of rolling of 30% and then tempered at 300° C. for more than 20 minutes. The rolling direction is included figure 5 in the plane of the drawing and runs horizontally. In figure 5 the slightly stretched grains of the ferrite phase can be clearly seen.

Based on the Figures 4 and 5 In the longitudinal sections shown, the sums of the grain boundary lengths for small-angle grain boundaries of a square measuring field of 50 µm*50 µm were also determined using the EBSD method. The sum of the small-angle grain boundaries in the initial state (i.e figure 4 ) 4.27mm. Due to the post-treatment, this value increases to 23.04mm (condition according to figure 5 ). Corresponding measurements result in 13.1 mm (No. 16) and 17 mm (No. 17) for the post-treated steel flat products No. 16 and No. 17 according to Tables 1 and 2.

Claims (9)

1. Method for producing a cold-rolled flat steel product aftertreated for strength enhancement, wherein a cold-rolled flat steel product is provided, with the flat steel product provided being generated by means of the following operating steps:

   - casting a steel having the composition as indicated below in % by weight to form a slab:

   - C: 0.05 - 0.25%,

   - Si: 0.05 - 0.6%,

   - Mn: 1.0 - 3.0%,

   - Al: 0.02 - 1.5%,

   - N: less than 0.02%,

   - P: 0.005 - 0.2%,

   - S: less than 0.05%

   - one or more elements from the group of "Cr, Mo", subject to the following provisos:

   - Cr: 0.2 - 1.5%,

   - Mo: 0.003 - 1.0%,

   - optionally one or more elements from the group of "Ti, Nb, B", subject to the following provisos:

   - B: less than 0.005%

   - Ti + Nb + 15*B: 0.02 - 0.15%

   - and optionally one or more elements from the group of "V, Cu, Ni, Ca", subject to the following provisos:

   - V: 0.0005 - 0.05%

   - Cu: 0.0001- 0.5%

   - Ni: 0.002 - 0.2%

   - Ca: 0.0005 - 0.007%

   - balance: iron and unavoidable impurities;

   - reheating the slab to a 1200 - 1300°C reheat temperature;

   - hot-rolling the reheated slab to give a hot strip, with the final hot-rolling temperature of the hot strip at the end of hot-rolling being 800 - 1000°C;

   - winding the hot strip at a winding temperature of 400 - 700°C;

   - pickling the hot strip;

   - cold-rolling the hot strip in one or more cold rolling steps to give a cold-rolled flat steel product, with the degree of cold-rolling achieved over the cold-rolling amounting in total to 20 - 80%;

   - subjecting the cold-rolled flat steel product to continuous annealing at a continuous-annealing temperature of 700 - 950°C;

   - cooling the cold-rolled flat steel product to room temperature;

   characterized in that the provided cold-rolled flat steel product is aftertreated for strength enhancement, wherein the following operating steps are traversed:

   - rerolling of the cold-rolled flat steel product, with the degree of rolling W_G2 achieved over the rerolling amounting in total to 8 - 40%;

   - temper-annealing the rerolled flat steel product at a reannealing temperature T_G2 of 100 - 400°C over an annealing time of 0.2 - 25 hours.
2. Method according to Claim 1, characterized in that the steel has a carbon equivalent C_eq where $${\mathrm{C}}_{\mathrm{eq}}=\mathrm{C}+\frac{1}{6}\mathrm{Mn}+\frac{1}{5}\mathrm{Mo}+\frac{1}{15}\mathrm{Ni}+\frac{1}{5}\mathrm{Cr}+\frac{1}{5}\mathrm{V}+\frac{1}{15}\mathrm{Cu},$$

   

   and the production index PWG where $${\mathrm{P}}_{\mathrm{WG}}=\frac{{\mathrm{T}}_{\mathrm{G}2}}{\mathrm{K}\cdot {\mathrm{W}}_{\mathrm{G}2}}\cdot {\mathrm{C}}_{\mathrm{eq}}$$

   

   is between 0.1 and 2.7, with:

   T_G2: reannealing temperature in the unit °C

   W_G2: degree of rolling on rerolling in %

   C_eq: carbon equivalent in %

   K: constant with a value of 10°C.
3. Method according to either of Claims 1 and 2, characterized in that the steel has a structure consisting of at least two phases and containing in % by volume more than 10% tempered martensite and more than 60% ferrite.
4. Method according to any of Claims 1 to 3, characterized in that the cold-rolled flat steel product is coated, more particularly coated electrolytically, between rerolling and temper-annealing.
5. Method according to any of Claims 1 to 4, characterized in that the cooling of the cold-rolled flat steel product to room temperature comprises two intermediate steps, with the cold-rolled flat steel product being cooled in the first intermediate step to a first cooling temperature T₁ and being held at the first cooling temperature T₁ for a first holding time t₁, and with cold-rolled flat steel product being cooled in the second intermediate step to a second cooling temperature T₂ and being held at the second cooling temperature T₂ for a second holding time t₂, with the cooling temperatures T₁, T₂ obeying the following conditions: $$T_1>T_2,{450}^{\circ }\mathrm{C}\le T_1\le {800}^{\circ }\mathrm{C}\mathrm{and}{400}^{\circ }\mathrm{C}\le T_2\le {600}^{\circ }\mathrm{C}$$

   

   and with the holding times t₁, t₂ obeying the following conditions: $$0\mathrm{s}\le t_1\le 20\mathrm{s}\mathrm{and}0\mathrm{s}\le t_2\le 900\mathrm{s}$$

   
6. Method according to any of Claims 1 to 5, characterized in that the cold-rolled flat steel product provided for hardening bears a metallic protective coating which is applied in particular by melt dip coating.
7. Cold-rolled flat steel product aftertreated for strength enhancement and consisting of

   - a steel having the composition as indicated below in % by weight:

   - C: 0.05 - 0.25%,

   - Si: 0.05 - 0.6%,

   - Mn: 1.0 - 3.0%,

   - Al: 0.02 - 1.5%,

   - N: less than 0.02%,

   - P: 0.005 - 0.2%,

   - S: less than 0.05%

   - one or more elements from the group of "Cr, Mo", subject to the following provisos:

   - Cr: 0.2 - 1.5%,

   - Mo: 0.003 - 1.0%,

   - optionally one or more elements from the group of "Ti, Nb, B", subject to the following provisos:

   - B: less than 0.005%

   - Ti + Nb + 15*B: 0.02 - 0.15%

   - and optionally one or more elements from the group of "V, Cu, Ni, Ca", subject to the following provisos:

   - V: 0.0005 - 0.05%

   - Cu: 0.0001- 0.5%

   - Ni: 0.002 - 0.2%

   - Ca: 0.0005 - 0.007%

   - balance: iron and unavoidable impurities,

   and having

   - a yield point of at least 1000 MPa and a tensile strength R_m of at least 1100 MPa, determined according to DIN-EN ISO 6982-1, sample shape 2 (Annex B tab. B1),

   - where the alloy-independent tensile strength R̃_m is at least 400 MPa, where $${\tilde{\mathrm{R}}}_{\mathrm{m}}=\frac{\mathrm{Rm}}{\mathrm{C}+\mathrm{Si}+\mathrm{Mn}+\mathrm{Cr}+\mathrm{Mo}},$$

   

   where C, Si, Mn, Cr and Mo are the respective element contents in per cent by weight,

   - and where the steel has a structure consisting of at least two phases and comprising in % by volume more than 10% tempered martensite and more than 60% ferrite.
8. Cold-rolled flat steel product aftertreated for strength enhancement according to Claim 7, characterized in that the steel has a carbon equivalent C_eq where $${\mathrm{C}}_{\mathrm{eq}}=\mathrm{C}+\frac{1}{6}\mathrm{Mn}+\frac{1}{5}\mathrm{Mo}+\frac{1}{15}\mathrm{Ni}+\frac{1}{5}\mathrm{Cr}+\frac{1}{5}\mathrm{V}+\frac{1}{15}\mathrm{Cu},$$

   

   which is in the 0.3% to 1.3% range.
9. Cold-rolled flat steel product aftertreated for strength enhancement according to either of Claims 7 and 8, characterized in that the sum total of the grain boundary lengths for small-angle grain boundaries in a square measuring field of 50 µm * 50 µm in a polished longitudinal section is greater than 10 mm, preferably greater than 15 mm, more preferably greater than 20 mm.

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